Lanthanide podates with predetermined structural and photophysical properties: Strongly luminescent self-assembled heterodinuclear d-f complexes with a segmental ligand containing heterocyclic imines and carboxamide binding units

+

J. Am. Chem. Soc. 1996, 118, 6681-6697

6681

Lanthanide Podates with Predetermined Structural and

Photophysical Properties: Strongly Luminescent

Self-Assembled Heterodinuclear d-f Complexes with a

Segmental Ligand Containing Heterocyclic Imines and

Carboxamide Binding Units

Claude Piguet,*^,†Jean-Claude G. Bu1nzli,*^,‡Ge´rald Bernardinelli,^§

Ge´rard Hopfgartner, Ste´phane Petoud,^‡and Olivier Schaad^|

Contribution from the Department of Inorganic, Analytical and Applied Chemistry, the

Laboratory of X-ray Crystallography, and the Department of Biochemistry, UniVersity of GeneVa,

CH-1211 GeneVa 4, Switzerland, the Institute of Analytical and Inorganic Chemistry, UniVersity

of Lausanne, CH-1015 Lausanne, Switzerland, and Pharma DiVision, F. Hoffmann-La Roche Ltd.,

CH-4002 Basel, Switzerland

ReceiVed December 11, 1995^X

Abstract: The segmental ligand 2-{6-[N,N-diethylcarbamoyl]pyridin-2-yl}-1,1′-dimethyl-5,5′-methylene-2′-(5-

methylpyridin-2-yl)bis[1H-benzimidazole] (L²) reacts with stoichiometric amounts of Ln(III) (Ln ) La-Nd, Sm-

Tb, Tm-Lu, Y) and Zn(II) in acetonitrile to yield quantitatively and selectively the heterodinuclear triple-helical

complexes [LnZn(L²)₃]⁵⁺under thermodynamic control. The crystal structure of [EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃-

CN)₄(13; EuZnC₁₁₁H₁₁₁N₂₅O₁₉F₁₂S₄Cl, monoclinic, C2/c, Z ) 8) shows the wrapping of the three ligands L²about

a pseudo-C₃axis passing through the metal ions. Zn(II) occupies the distorted pseudooctahedral capping coordination

site defined by the three bidentate binding units while Eu(III) lies in the resulting “facial” pseudotricapped trigonal

prismatic site produced by the three remaining tridentate units as exemplified by luminescence measurements using

the Eu(III) structural probe. The separation of contact and pseudocontact contributions to the ¹H-NMR paramagnetic

shifts of the axial complexes [LnZn(L²)₃]⁵⁺(Ln ) Ce, Pr, Nd, Sm, Eu, Tm, Yb) establishes that the triple helical

structure is maintained in solution. Photophysical measurements and quantum yields in acetonitrile indicate that the

terminal N,N-diethylcarbamoyl group in L²favors efficient intramolecular L²f Eu(III) energy transfers leading to

strong Eu-centered red luminescence. Improved resistance toward hydrolysis also results from the use of carboxamide

groups, and no change in luminescence is observed for [EuZn(L²)₃]⁵⁺in moist acetonitrile. The preparation of the

segmental ligand L²from the new asymmetric synthon 6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic acid is described

together with its crystal and molecular structure (C₃₃H₃₃N₇O, monoclinic, P2₁/c, Z ) 4). The use of 3d metal ions

as a noncovalent tripodal spacer for lanthanide podates is discussed together with the crucial role played by carboxamide

groups for the control of structural, electronic, and photophysical properties.

Introduction

The design of organized molecular architectures containing

lanthanide metal ions Ln(III) and working as nanometric light-

converting devices^1,2and luminescent probes¹is a theme of

considerable current interest in supramolecular^1,2and analytical

chemistry,^1,3and biochemistry.⁴However, the selective intro-

duction of Ln(III) into supramolecular complexes with tailored

coordination sites and predetermined photophysical properties

represents a synthetic challenge⁵since Ln(III) displays large

and variable coordination numbers⁶with few stereochemical

preferences.^6,7The control of the coordination spheres around

Ln(III) thus mainly depends on the preorganization⁸of the

coordinating units which limits the structural flexibility and

increases the thermodynamic stability.⁹Macrocyclic¹⁰and

compartmental¹¹Schiff bases have been systematically inves-

tigated for the preparation of homo- and heteropolynuclear

lanthanide complexes, but improved structural control and

protection of Ln(III) are obtained with preorganized tri- to

(5) Piguet, C.; Bu¨nzli, J.-C. G.; Bernardinelli, G.; Hopfgartner, G.;

Williams, A. F. J. Alloys Compd. 1995, 225, 324-331.

(6) Choppin, G. R. In Lanthanide Probes in Life, Chemical and Earth

Sciences; Bu¨nzli, J.-C. G., Choppin, G. R., Eds.; Elsevier Publishing Co.:

Amsterdam, 1989; Chapter 1. Drew, M. G. B. Coord. Chem. ReV. 1977,

24, 179-275.

(7) Reisfeld, R.; Jørgensen, C. K. Lasers and Excited States of Rare

Earths. Inorganic Chemistry Concepts; Springer Verlag: Berlin-Heidel-

berg-New York, 1977; Vol. 1, Chapter 1.

^†Department of Inorganic, Analytical and Applied Chemistry, University

of Geneva.

^‡University of Lausanne.

^§Laboratory of X-ray Crystallography, University of Geneva.

F. Hoffmann-La Roche Ltd.

^|Department of Biochemistry, University of Geneva.

^XAbstract published in AdVance ACS Abstracts, July 1, 1996.

(1) Bu¨nzli, J.-C. G. in Lanthanide Probes in Life, Chemical and Earth

Sciences; Bu¨nzli, J.-C. G., Choppin, G. R., Eds.; Elsevier Publishing Co.:

Amsterdam, 1989; Chapter 7. Bu¨nzli, J.-C. G.; Froidevaux, P.; Piguet, C.

New J. Chem. 1995, 19, 661-668 and references therein.

(2) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993,

123, 201-228.

(3) Lehn, J.-M.; Regnouf de Vains, J.-B. HelV. Chim. Acta 1992, 75,

1221-1236. Richardson, F. S. Chem. ReV. 1982, 82, 541-552.

(4) Selvin, P. R.; Rana, T. M.; Hearst, J. E. J. Am. Chem. Soc. 1994,

116, 6029-6030. Coates, J.; Sammes, P. G.; West, R. M. J. Chem. Soc.,

Chem. Commun. 1995, 1107-1108. Saha, A. K.; Kross, K.; Kloszewski,

E. D.; Upson, D. A.; Toner, J. L.; Snow, R. A.; Black, C. D. V.; Desai, V.

C. J. Am. Chem. Soc. 1993, 115, 11032-11033.

(8) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112. Cram,

D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009-1027.

(9) Martell, A. E.; Hancock, R. D.; Motekaitis, R. J. Coord. Chem. ReV.

1994, 133, 39-65.

(10) Alexander, V. Chem. ReV. 1995, 95, 273-342 and references therein.

(11) Guerriero, P.; Tamburini, S.; Vigato, P. A. Coord. Chem. ReV. 1995,

139, 17-243 and references therein.

S0002-7863(95)04163-1 CCC: $12 00

+

6682 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

Chart 1

Lu) and Zn(II) in acetonitrile to give the heterodinuclear

complexes [LnZn(L¹)₃]⁵⁺, where Zn(II) is coordinated by the

three bidentate units in a facial pseudooctahedral arrangement,

producing a noncovalent tripodal spacer which controls the

orientation of the remaining tridentate units, leading to a “facial”

pseudotricapped trigonal prismatic site around Ln(III).²³The

fine-tuning of the structural, magnetic, and photophysical

properties results from the judicious choice of the d-block metal

ion,²⁴and the replacement of Zn(II) by Fe(II) leads to the

formation of spin-crossover heterodinuclear triple-helical com-

plexes [LnFe(L¹)₃]⁵⁺for Ln ) La to Eu.²⁴However, the

lipophilic tridentate unit of L¹gives only amorphous solids

during the isolation of the complexes, and the great similitudes

between the two binding units in L¹(same ligating atoms and

bite angles) limit the selectivity of the assembly process, leading

to the quantitative formation of the heterodinuclear complexes

[LnZn(L¹)₃]⁵⁺in solution under specific conditions: (i) con-

centration of the ligand >0.01 M, (ii) anhydrous acetonitrile as

solvent, and (iii) stoichiometric ratio Ln:Zn:L¹) 1:1:3.^23,24This,

together with the faint luminescence of [EuZn(L¹)₃]⁵⁺resulting

from the closely packed arrangement of the ligands,²¹has

prompted us to replace the terminal benzimidazole group with

a carboxamide group, leading to the new segmental ligand L²

which is expected (i) to improve the selective complexation of

3d- and 4f-block metal ions,²⁵(ii) to favor resonant L²f Ln

(Ln ) Eu, Tb) energy transfers,²⁶and (iii) to stabilize the

complexes in hydroxylic solvents.

hexapodal ligands possessing iminophenolate,¹²bipyridine,¹³

1,10-phenanthroline,¹⁴and hydroxypyridonate¹⁵coordinating

side arms connected to a capping covalent spacer. Further

developments have led to the syntheses of rigid and highly

preorganized macrobicyclic cryptands²possessing bihetero-

aryl^2,3,8and aliphatic polyamine¹⁶and polyglycol²binding units

connected to two terminal covalent tripods. Stable, strongly

luminescent, and often water resistant lanthanide complexes

result from the reaction of rigid cryptands with Ln(III), but the

fine-tuning of the coordination cavity and of the electronic

properties is severely limited by the tedious syntheses of

macropolycyclic receptors.¹⁷Inspired by the induced fit con-

cept,¹⁸recent studies aim to replace the covalent spacers in

preorganized receptors by noncovalent d-block metal complexes

which offer more synthetic and structural flexibilities and allow

the possible fine-tuning of the complexation properties of the

resulting receptors. For instance, R,R′-diimine binding units

have been connected by two inert facial pseudooctahedral

Cr(III) complexes to give an organometallic macrobicyclic

cryptand.¹⁹Recently, two segmental ligands possessing several

bidentate binding units have been assembled by a labile

pseudotetrahedral Cu(I), producing a new receptor suitable for

the complexation of aliphatic dicarboxylic acids.²⁰The ap-

plication of this concept to lanthanide chemistry has led to the

development of the segmental ligand L¹(Chart 1) which

possesses a tridentate binding unit coded for Ln(III)²¹and a

bidentate unit suitable for pseudooctahedral d-block metal ions.²²

L¹reacts with a stoichiometric amount of Ln(III) (Ln ) La to

In this paper, we present the synthesis of the new segmental

ligand L²and the thermodynamic assembly processes leading

to the strongly luminescent triple-helical heterodinuclear com-

plexes [LnZn(L²)₃]⁵⁺(Ln ) La to Lu) whose crystal structure

(Ln ) Eu) has been partially reported in a preliminary

communication.²⁷Detailed photophysical studies are reported,

and particular attention is focused on the structure and quantum

yields in solution since these points are crucial to the design of

lanthanide building blocks with predetermined structural and

photophysical properties.

Experimental Section

Solvents and starting materials were purchased from Fluka AG

(Buchs, Switzerland) and used without further purification, unless

otherwise stated. Acetonitrile, 1,2-dichloroethane, dichloromethane,

N,N-dimethylformamide (DMF), and triethylamine were distilled from

CaH₂, thionyl chloride was distilled from elemental sulfur, and N,N-

diethylamine was distilled from KOH. Silicagel (Merck 60, 0.040-

0.060 mm) was used for preparative column chromatography.

Preparation of the Ligand. 6-Methylpyridine-2-carboxylic acid

(1)²⁸and N,5-dimethyl-N-{4′-[4′′-(methylamino)-3′′-nitrobenzyl]-2′-

nitrophenyl}pyridine-2-carboxamide (4)²⁹were prepared according to

literature procedures.

(12) Yang, L. W.; Liu, S.; Wong, E.; Rettig, S. J.; Orvig, C. Inorg. Chem.

1995, 34, 2164-2178 and references therein.

(13) Balzani, V.; Berghmans, E.; Lehn, J.-M.; Sabbatini, N.; Tero¨rde,

R.; Ziessel, R. HelV. Chim. Acta 1990, 73, 2083-2089. Ziessel, R.; Maestri,

M.; Prodi, L.; Balzani, V.; Van Doersselaer, A. Inorg. Chem. 1993, 32,

1237-1241.

(22) Piguet, C.; Bernardinelli, G.; Bocquet, B.; Schaad, O.; Williams,

A. F. Inorg. Chem. 1994, 33, 4112-4121.

(23) Piguet, C.; Rivara-Minten, E.; Hopfgartner, G.; Bu¨nzli, J.-C. G. HelV.

Chim. Acta 1995, 78, 1541-1566. Piguet, C.; Hopfgartner, G.; Williams,

A. F.; Bu¨nzli, J.-C. G. J. Chem. Soc., Chem. Commun. 1995, 491-493.

(24) Piguet, C.; Rivara-Minten, E.; Hopfgartner, G.; Bu¨nzli, J.-C. G. HelV.

Chim. Acta 1995, 78, 1651-1672.

(25) Paul-Roth, C.; Raymond, K. N. Inorg. Chem. 1995, 34, 1408-1412.

Amin, S.; Voss, D. A.; Horrocks, W. de W., Jr.; Lake, C. H.; Churchill, M.

R.; Morrow, J. R. Inorg. Chem. 1995, 34, 3294-3300.

(26) (a) Sabbatini, N.; Guardigli, M.; Mecati, A.; Balzani, V.; Ungaro,

R.; Ghildini, E.; Casnati, A.; Pochini, A. J. Chem. Soc., Chem. Commun.

1990, 878-879; Hazenkamp, M. F.; Blasse, G.; Sabbatini, N.; Ungaro, R.

Inorg. Chim. Acta 1991, 172, 93-95; (b) Brayshaw, P. A.; Bu¨nzli, J.-C.

G.; Froidevaux, P.; Harrowfield, J. M.; Kim, Y.; Sobolev, A. N. Inorg.

Chem. 1995, 34, 2068-2076.

(27) Piguet, C.; Bernardinelli, G.; Bu¨nzli, J.-C. G.; Petoud, S.; Hopf-

gartner, G. J. Chem. Soc., Chem. Commun. 1995, 2575-2577.

(28) Black, G.; Depp, E.; Corson, B. B. J. Org. Chem. 1949, 14, 14-

21.

(14) Sabbatini, N.; Guardigli, M.; Manet, I.; Bolleta, F.; Ziessel, R. Inorg.

Chem. 1994, 33, 955-959.

(15) Xu, J.; Franklin, S. J.; Whisenhunt, D. W.; Raymond, K. N.; J.

Am. Chem. Soc. 1995, 117, 7245-7246.

(16) Smith, P. H.; Reyes, Z. E.; Lee, C. W.; Raymond, K. N. Inorg.

Chem. 1988, 27, 4154-4165.

(17) Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH:

Weinheim, New York, Basel, and Cambridge, 1992.

(18) Koshland, D. A. Angew. Chem., Int. Ed. Engl. 1994, 33, 2375-

2378.

(19) Burdinski, D.; Birkelbach, F.; Gerdan, M.; Trautwein, A. X.;

Wieghardt, K.; Chandhuri, P. J. Chem. Soc., Chem. Commun. 1995, 963-

964.

(20) Goodman, M. S.; Hamilton, A. D.; Weiss, J. J. Am. Chem. Soc.

1995, 117, 8447-8455.

(21) Piguet, C.; Bu¨nzli, J.-C. G.; Bernardinelli, G.; Bochet, C. G.;

Froidevaux, P. J. Chem. Soc., Dalton Trans. 1995, 83-97.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6683

Preparation of N,N-Diethyl-6-methylpyridine-2-carboxamide (2).

6-Methylpyridine-2-carboxylic acid (1; 20.0 g, 0.146 mol) was refluxed

in freshly distilled thionyl chloride (100 mL, 1.37 mol) with dry DMF

(500 µL) for 2 h. Excess thionyl chloride was evaporated and the crude

residue coevaporated with 1,2-dichloroethane (50 mL). The solid

residue was dried under vacuum and then dissolved in dichloromethane

(200 mL), and dry N,N-diethylamine (53.4 g, 0.73 mol) was added

dropwise under an inert atmosphere. The resulting solution was

refluxed for 2 h and evaporated. The brown residue was partitioned

between dichloromethane (200 mL) and half-saturated aqueous NH₄-

Cl solution (200 mL). The aqueous phase was extracted with

dichloromethane (2 × 100 mL), the combined organic phase dried (Na₂-

SO₄) and evaporated, and the resulting brown oil distilled (83 °C, 10^-2

Torr) to give 22.0 g (0.114 mol, yield 78%) of 2 as a colorless wax,

mp 45-46 °C. ¹H NMR in CDCl₃: δ 1.14 (3H, t, J³) 7 Hz), 1.24

(3H, t, J³) 7 Hz), 2.53 (3H, s), 3.30 (2H, q, J³) 7 Hz), 3.52 (2H, q,

J³) 7 Hz), 7.13 (1H, d, J³) 8 Hz), 7.29 (1H, t, J³) 8 Hz), 7.61 (1H,

d, J³) 8 Hz). EI-MS: m/z 192 (M⁺).

Preparation of 6-(N,N-Diethylcarbamoyl)pyridine-2-carboxylic

Acid (3). N,N-Diethyl-6-methylpyridine-2-carboxamide (2; 21.5 g,

0.112 mol) and selenium dioxide (55.8 g, 0.503 mol) were refluxed in

dry pyridine for 72 h under an inert atmosphere. After cooling, the

mixture was filtered to remove solid Se and evaporated to dryness.

The solid residue was suspended in water (700 mL) and the pH adjusted

to 9.5 with NaOH (5 M). The resulting clear solution was extracted

with dichloromethane (3 × 250 mL) and the aqueous phase filtered,

acidified to pH 3.0 with hydrochloric acid (37%), concentrated, and

cooled to -5 °C for 12 h. The resulting white crystals were collected

by filtration and crystallized from hot acetonitrile to give 13.5 g (61

mmol, yield 55%) of 3, mp 196-198 °C. ¹H NMR in CD₃OD: δ

1.18 (3H, t, J³) 7 Hz), 1.28 (3H, t, J³) 7 Hz), 3.32 (2H, q, J³) 7

Hz), 3.57 (2H, q, J³) 7 Hz), 4.91 (1H, s), 7.74 (1H, d.d, J³) 8 Hz,

J⁴) 1 Hz), 8.09 (1H, t, J³) 8 Hz), 8.20 (1H, d.d, J³) 8 Hz, J⁴) 1

Hz). EI-MS: m/z 222 (M⁺).

hot acetonitrile to give 0.537 g (0.988 mmol, yield 78%) of ligand L²

as colorless plates, mp 69-71 °C. ¹H NMR in CDCl₃: δ 1.13 (3H, t,

J³) 7 Hz), 1.30 (3H, t, J³) 7 Hz), 2.42 (3H, s), 3.35 (2H, q, J³) 7

Hz), 3.61 (2H, q, J³) 7 Hz), 4.21 (3H, s), 4.24 (3H, s), 4.29 (2H, s),

7.20 (1H, d.d, J³) 8 Hz, J⁴) 1 Hz), 7.23 (1H, d.d, J³) 8 Hz, J⁴)

1 Hz), 7.33 (1H, d, J³) 8 Hz), 7.34 (1H, d, J³) 8 Hz), 7.57 (1H, d,

J³) 8 Hz), 7.69 (1H, d, J⁴) 1 Hz), 7.71 (1H, d, J⁴) 1 Hz), 7.93

(1H, t, J³) 8 Hz), 8.20 (1H, d, J³) 8 Hz), 8.38 (1H, m), 8.51 (1H,

m). ¹³C NMR in CDCl₃: δ 12.79, 14.29, 18.42, 32.62 (primary C);

39.74, 42.20, 42.94 (secondary C); 109.69, 109.81, 119.72, 119.89,

122.59, 124.14, 124.44, 125.00, 125.62, 137.28, 137.83, 148.89 (tertiary

C); 133.41, 135.80, 136.16, 136.65, 142.77, 142.79, 148.02, 149.35,

149.73, 150.54, 154.18, 168.36 (quaternary C). EI-MS: m/z 543 (M⁺).

X-ray quality colorless prisms of L²were obtained by slow cooling (6

days) of a hot concentrated acetonitrile solution.

Preparation of the Complexes. The perchlorate salts Ln-

(ClO₄)₃‚nH₂O (Ln ) La, Pr, Nd, Sm, Eu, Tb, Tm, Yb, Lu, Y; n )

6-8) were prepared from the corresponding oxides (Glucydur, 99.99%)

according to a literature method.³⁰Ce(ClO₄)₃‚5.8H₂O was obtained

by metathesis of cerium(III) carbonate hydrate (Aldrich, 99.9%) with

aqueous perchloric acid.

Preparation of [Zn₂(L²)₂](ClO₄)₄(7). L²(54.4 mg, 0.1 mmol) in

1:1 dichloromethane/acetonitrile (4 mL) was slowly added to Zn-

(ClO₄)₂‚6H₂O (37.2 mg, 0.1 mmol) in acetonitrile (5 mL). The resulting

solution was evaporated, the solid residue dissolved in acetonitrile (3

mL), and diethyl ether slowly diffused into the solution for 2 days.

White microcrystals were separated by filtration to give 59 mg (36.5

µmol, yield 73%) of hygroscopic [Zn₂(L²)₂](ClO₄)₄(7).

Preparation of [LnZn(L²)₃](ClO₄)₅‚0.5C₄H₁₀O‚2H₂O (Ln ) La,

8; Nd, 9; Eu, 10; Gd, 11; Tb, 12). A solution of 61.3 µmol of Ln-

(ClO₄)₃‚nH₂O (Ln ) La, Nd, Eu, Gd, Tb) and 22.8 mg (61.3 µmol) of

Zn(ClO₄)₂‚6H₂O in acetonitrile (5 mL) was slowly added to a solution

of L²(0.1 g, 0.184 mmol) in 1:1 dichloromethane/acetonitrile (10 mL).

After being stirred at room temperature for 1 h, the solution was

evaporated, the solid residue dissolved in acetonitrile (4 mL), and diethyl

ether diffused into the solution for 2-3 days. The resulting white

microcrystalline aggregates were collected by filtration and dried to

give 84-92% of complexes [LnZn(L²)₃](ClO₄)₅‚0.5Et₂O‚2H₂O (Ln )

La, 8; Nd, 9; Eu, 10; Gd, 11; Tb, 12). X-ray quality crystals of [EuZn-

(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13) were obtained by slow diffusion

of diisopropyl ether into an acetonitrile solution of 10 containing 30

equiv of NBu₄CF₃SO₃. After 5 days, the resulting two separated liquid

phases were mixed using an ultrasonic bath to give a white emulsion

which gave fragile colorless prisms of 13 upon slow cooling at 277 K.

Complexes 7-12 were characterized by their IR spectra and give

satisfactory elemental analyses (Table SII in the supporting information).

For the purpose of the photophysical study, europium-doped La (8a)

and Gd (11a) complexes were prepared by replacing the lanthanide

solution by an Eu (2%)/Ln (98%) mixed solution.

Preparation of [LnZn(L²)₃](ClO₄)₅(Ln ) Ce, 14; Pr, 15; Sm,

16; Tm, 17; Yb, 18; Lu, 19; Y, 20). These complexes were prepared

in situ for ¹H NMR studies. A 263 µL (5.26 µmol) portion of an

equimolar 10^-2M solution of Ln(ClO₄)₃‚nH₂O (Ln ) Ce, Pr, Sm, Tm,

Yb, Lu, Y) and Zn(ClO₄)₂‚6H₂O in acetonitrile was added to L²(8.6

mg, 15.8 µmol), dissolved in 4 mL of 1:1 dichloromethane/acetonitrile.

After evaporation of the solution, the solid residue was dried under

vacuum and then dissolved in 700 µL of degassed CD₃CN to give a

7.5 mM solution of [LnZn(L²)₃](ClO₄)₅(Ln ) Ce, 14; Pr, 15; Sm, 16;

Tm, 17; Yb, 18; Lu, 19; Y, 20) which was used without further

purification. Caution! Perchlorate salts combined with organic ligands

are potentially explosive and should be handled with the necessary

precautions.³¹

Preparation of 6-(N,N-Diethylcarbamoyl)-N-methyl-N-{4′-{4′′-

{N-methyl-N-[(5′′′-methylpyridin-2′′′-yl)carbonyl]amino}-3′′-ni-

trobenzyl}-2′-nitrophenyl}pyridine-2-carboxamide (5). A mixture

of 3 (1.19 g, 5.34 mmol), freshly distilled thionyl chloride (6.35 g,

53.0 mmol), and DMF (100 µL) was refluxed for 90 min in dry

dichloromethane (50 mL). The mixture was evaporated and dried under

vacuum and the solid residue dissolved in dichloromethane (100 mL)

and added dropwise to a solution of N,5-dimethyl-N-{4′-[4′′-(methyl-

amino)-3′′-nitrobenzyl]-2′-nitrophenyl}pyridine-2-carboxamide (4; 0.775

g, 1.78 mmol) and triethylamine (0.9 g, 8.9 mmol) in dichloromethane

(50 mL). The solution was refluxed for 4 h under an inert atmosphere

and the crude product isolated according to the standard workup

procedure described for 2 and purified by column chromatography

(Silicagel, CH₂Cl₂/MeOH, 99.2:0.8 f 98:2) to give 0.807 g (1.26 mmol,

yield 71%) of 5 as a pale yellow solid, mp 90-93 °C dec. ¹H NMR

in CDCl₃: δ 0.8-1.3 (6H, m), 2.2-2.4 (3H, m), 2.8-3.7 (4H, m),

3.50 (6H, s), 4.0-4.2 (2H, m), 7.1-8.5 (12H, m). ES-MS: m/z 640.2

([M + H]⁺). EI-MS: m/z 593 ([M - NO₂]⁺).

Preparation of 2-{6-[N,N-Diethylcarbamoyl]pyridin-2-yl}-1,1′-

dimethyl-5,5′-methylene-2′-(5-methylpyridin-2-yl)bis[1H-benzimi-

dazole] (L²). To a solution of 5 (0.807 g, 1.26 mmol) in ethanol/

water (250 mL/64 mL), activated iron powder (2.11 g, 37.8 mmol),

and concentrated hydrochloric acid (37%, 7.9 mL, 94.8 mmol) were

added. The mixture was refluxed for 6 h under an inert atmosphere,

the excess of iron was filtered off, and ethanol was distilled under

vacuum. The resulting mixture was poured into dichloromethane (300

mL), Na₂H₂EDTA‚2H₂O (25 g) in water (150 mL) was added, and the

resulting stirred mixture was neutralized (pH 7.0) with concentrated

aqueous NH₄OH solution. Concentrated H₂O₂solution (30%, 1 mL)

was added under vigorous stirring, and the pH was adjusted to 8.5

with aqueous NH₄OH solution. After 15 min, the organic layer was

separated and the aqueous phase extracted with dichloromethane (3 ×

200 mL). The combined organic phases were dried (Na₂SO₄) and

evaporated and the crude residue purified by column chromatography

(Silicagel, CH₂Cl₂/MeOH 98.5:1.5 f 97:3) and then crystallized from

Physicochemical Measurements. Reflectance spectra were re-

corded as finely grounded powders dispersed in MgO (5%) with MgO

as the reference on a Perkin-Elmer Lambda 19 spectrophotometer

equipped with a Labsphere RSA-PE-19 integration sphere. Electronic

spectra in the UV-vis range were recorded at 20 °C from 10^-3

M

acetonitrile solutions with Perkin-Elmer Lambda 5 and Lambda 7

(30) Desreux, J. F. In Lanthanide Probes in Life, Chemical and Earth

Sciences, Bu¨nzli, J.-C. G., Choppin, G. R., Eds.; Elsevier Publishing Co.:

Amsterdam, 1989; Chapter 2, p 43.

(29) Piguet, C.; Bocquet, B.; Hopfgartner, G. HelV. Chim. Acta 1994,

77, 931-942.

(31) Wolsey, W. C. J. Chem. Educ. 1978, 55, A355.

+

6684 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

spectrometers using quartz cells of 0.1 and 0.01 cm path length.

Spectrophotometric titrations were performed with a Perkin-Elmer

Lambda 5 spectrophotometer connected to an external computer. In a

typical experiment, 50 mL of ligand (L²) in acetonitrile (10^-4M) was

titrated at 20 °C with an equimolar solution of Ln(ClO₄)₃‚nH₂O and

Zn(ClO₄)₂‚6H₂O (0.83 mM) in acetonitrile. After each addition of 0.20

mL, the absorbances at 10 different wavelengths were recorded using

a 0.1 cm quartz cell and transferred to the computer. Plots of extinction

as a function of the metal:ligand ratio gave a first indication of the

number and stoichiometry of the complexes formed; factor analysis³²

was then applied to the data to confirm the number of different

absorbing species. Finally, a model for the distribution of species was

fitted with a nonlinear least-squares algorithm to give stability constants

as previously described.³³IR spectra were obtained from KBr pellets

with a Perkin-Elmer 883 spectrometer. ¹H, ¹³C, and ¹¹³Cd NMR spectra

were recorded at 25 °C on a broad band Varian Gemini 300

spectrometer. Chemical shifts are given in parts per million with respect

to TMS. EI-MS (70 eV) spectra were recorded with VG-7000E and

Finnigan-4000 instruments. Pneumatically-assisted electrospray (ES-

MS) mass spectra were recorded from 10^-4M acetonitrile solutions

on an API III tandem mass spectrometer (PE Sciex) by infusion at

4-10 µL/min. The spectra were recorded under low up-front declus-

tering or collision-induced dissociation (CID) conditions, typically ∆V

) 0-30 V between the orifice and the first quadrupole of the

spectrometer.³⁴Determination of the total charge (z) of the complexes

was made by using the isotopic pattern (z e 3) or adduct ions with

perchlorate anions (z > 3).³⁵The experimental procedures for high-

resolution, laser-excited luminescence measurements have been pub-

lished previously.³⁶Solid state samples were finely powdered, and

low temperature (77 or 10 K) was achieved by means of a Cryodyne

Model 22 closed-cycle refrigerator from CTI Cryogenics. Lumines-

cence spectra were corrected for the instrumental function, but not

excitation spectra. Lifetimes are averages of at least 3-5 independent

determinations. Quantum yields, ligand excitation, and emission spectra

were recorded on a Perkin-Elmer LS-50 spectrometer equipped for low-

temperature measurements. The relative quantum yields were calcu-

squares refinements (on F) using weights of w ) 1/σ²(F_o) gave final

values of R ) 0.045 and R_w) 0.034 for 470 variables and 2886

contributing reflections. The mean shift/error on the last cycle was

0.81 × 10^-2, and the maximum was 0.25. H atoms were observed

and refined with fixed isotropic displacement parameters (U ) 0.05

Å²), and the 41 non-hydrogen atoms were refined with anisotropic

displacement parameters. The final Fourier difference synthesis showed

a maximum of +0.29 and a minimum of -0.33 e Å^-3

.

Crystal Structure Determination of [EuZn(L²)₃](ClO₄)(CF₃-

SO₃)₄(CH₃CN)₄(13). Crystal data for EuZnC₁₁₁H₁₁₁N₂₅O₁₉F₁₂S₄Cl: M

) 2708.1, monoclinic, C2/c, a ) 50.417(8) Å, b ) 20.822(4) Å, c )

23.498(5) Å, â ) 92.67(1)°, U ) 24641(8) Å³(by least-squares

refinement of 29 reflections, 38° e 2θ e 60°), Z ) 8, D_c) 1.46 g‚cm^-3

,

F(000) ) 11088, colorless prisms, crystal dimensions 0.10 × 0.25 ×

0.28 mm, µ(Cu KR) ) 5.472 mm^-1. Data collection and processing:

Nonius CAD4 diffractometer, T ) 200 K, ω-2θ scan, scan width 1.5

+ 0.14 tan θ, scan speed 0.137 deg‚s^-1, Cu KR radiation (λ ) 1.5418

Å); 13 029 reflections measured (4 e 2θ e 100°, -50 < h < 50, 0 <

k < 20, 0 < l < 23), 12 641 unique reflections (R_intfor equivalent

reflections 0.102) of which 7796 were observable (|F_o| > 4σ(F_o)). Two

reference reflections were measured every 30 min and showed a total

decrease in intensity of 5.4%. All intensities were corrected for this

drift. Structure analysis and refinement: Data were corrected for

Lorentz, polarization, and absorption effects³⁷(A*_min) 1.725, A*_max

) 4.387). The structure was solved by direct methods as described

for L². Full-matrix least-squares refinements (on F) gave final values

of R ) R_w) 0.102 (w ) 1) for 1520 variables and 7796 contributing

reflections. The non-H atoms of the solvent molecules were refined

with isotropic displacement parameters (12 atoms) and all the other

atoms (180) with anisotropic displacement parameters. The perchlorate,

the triflate anion g, and the acetonitrile molecules are disordered and

were refined with restraints on bond distances and angles. H atoms

were placed in calculated positions and contributed to F_ccalculations.

The final Fourier difference synthesis showed a maximum of +1.61

and a minimum of -2.08 e Å^-3located around the Eu atom.

lated using the following formula:²¹Q_x/Q_r

) A_r(λ_r)/A_x(λ_x) I(λ_r)/

Results

2

I(λ_x) n_x/n_r²D_x/D_rwhere the subscript r stands for the reference and

x for the samples, A is the absorbance at the excitation wavelength, I

is the intensity of the excitation light at the same wavelength, n is the

refractive index (1.341 for all solutions in acetonitrile), and D is the

measured integrated luminescence intensity.

Preparation of the Ligand L². The segmental ligand 2-{6-

[N,N-diethylcarbamoyl]pyridin-2-yl}-1,1′-dimethyl-5,5′-meth-

ylene-2′-(5-methylpyridin-2-yl)bis[1H-benzimidazole] (L²) is

obtained in two steps from the synthons 3 and 4 according to

a strategy based on a modified Phillips-type coupling reaction

as the key step for the synthesis of benzimidazole rings from

N-(2-nitroaryl)arenecarboxamide precursors (Scheme 1).²⁹The

asymmetric synthon 3 is obtained in good yield from 6-meth-

ylpyridine-2-carboxylic acid (1) using a usual amidation pro-

cedure²⁹followed by the oxidation of the methyl group bound

to the 6-position of the pyridine ring with anhydrous selenium

dioxide in pyridine.⁴¹This new approach was preferred to the

reported direct reaction of pyridine-2,6-dicarboxylic acid with

a stoichiometric amount of trifluoroacetic anhydride followed

by treatment with amines which gives only moderate yield of

asymmetric amides together with significant quantities of

unreacted starting materials and diamide byproducts.⁴²3 is

transformed into the acyl chloride with thionyl chloride²⁹and

then reacted with 4²⁹to give the asymmetric o-dinitro dicar-

boxamide compound 5. The key step, the reduction of 5 by

Fe, could lead to two possible isomers: the target ligand L²

and 6 resulting from the cyclization of the two carboxamide

groups attached to the 2- and 6-positions of the same pyridine

ring. TLC shows that only one isomer is formed, but NMR,

Crystal Structure Determination of L². Crystal data for

C₃₃H₃₃N₇O: M ) 543.7, monoclinic, P2₁/c, a ) 17.7172(7) Å, b )

7.4444(3) Å, c ) 21.229(1) Å, â ) 98.545(5)°, U ) 2768.9(2) Å³(by

least-squares refinement of 25 reflections, 44° e 2θ e 55°), Z ) 4, D_c

) 1.30 g‚cm^-3, F(000) ) 1152, colorless prisms, crystal dimensions

0.10 × 0.26 × 0.33 mm, µ(Cu KR) ) 0.651 mm^-1. Data collection

and processing: Nonius CAD4 diffractometer, T ) 170 K, ω-2θ scan,

scan width 1.5 + 0.14 tan θ, scan speed 0.09 deg‚s^-1, Cu KR radiation

(λ ) 1.5418 Å); 3554 reflections measured (4 e 2θ e 110°, -18 < h

< 18, 0 < k < 7, 0 < l < 22), 3452 unique reflections (R_intfor

equivalent reflections 0.024) of which 2886 were observable (|F_o| > 4

σ(F_o)). Two reference reflections were measured every 30 min and

showed a variation in intensity of <2.8 σ(I). Structure analysis and

refinement: Data were corrected for Lorentz, polarization, and absorp-

tion effects³⁷(A*_min) 1.066, A*_max) 1.186). The structure was solved

by direct methods using MULTAN 87;³⁸all other calculations used

the XTAL³⁹system and ORTEP II⁴⁰programs. Full-matrix least-

(32) Malinowski, E. R.; Howery, D. G. Factor Analysis in Chemistry, J.

Wiley: New York, Chichester, Brisbane, and Toronto, 1980.

(33) Piguet, C.; Bernardinelli, G.; Bocquet, B.; Quattropani, A.; Williams,

A. F. J. Am. Chem. Soc. 1992, 114, 7440-7451.

(34) Hopfgartner, G.; Piguet, C.; Henion, J. D.; Williams, A. F. HelV.

Chim. Acta 1993, 76, 1759-1766.

(35) Hopfgartner, G.; Piguet, C.; Henion, J. D. J. Am. Soc. Mass

Spectrom. 1994, 5, 748-756.

(39) Hall, S. R., Stewart, J. M., Eds. XTAL 3.2 User’s Manual;

Universities of Western Australia and Maryland, 1992.

(40) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National

Laboratory: Oak Ridge, TN, 1976.

(41) Piguet, C.; Bu¨nzli, J.-C. G.; Bernardinelli, G.; Hopfgartner, G.;

Williams, A. F. J. Am. Chem. Soc. 1993, 115, 8197-8206.

(42) Reddy, K. V.; Jin, S.-J.; Arora, P. K.; Sfeir, D. S.; Maloney, S. C.

F.; Urbach, F. L.; Sayre, L. M. J. Am. Chem. Soc. 1990, 112, 2332-2340.

(36) Piguet, C.; Williams, A. F.; Bernardinelli, G.; Moret, E.; Bu¨nzli,

J.-C. G. HelV. Chim. Acta 1992, 75, 1697-1717.

(37) Blanc, E.; Schwarzenbach, D.; Flack, H. D. J. Appl. Crystallogr.

1991, 24, 1035-1041.

(38) Main, P.; Fiske, S. J.; Hull, S. E.; Lessinger, L.; Germain, D.;

Declercq, J. P.; Woolfson, M. M. MULTAN 87; Universities of York,

England, and Louvain-La-Neuve, Belgium, 1987.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6685

and H¹, but no effects are detected between Me²and H³and

Me³and H¹², strongly suggesting that the pyridine and benz-

imidazole rings adopt similar transoid arrangement in solution

which is crucial for the coordination behavior of L²since the

transoid f cisoid conformational change associated with

lanthanide complexation controls the stability and the selectivity

of the final complexes.^21,46

Following the Assembly Processes of L²with d- and

f-Block Metal Ions. The ligand L²possesses two different

binding units connected by a methylene spacer which favors

the formation of polynuclear complexes.^33,41,47The bidentate

unit is coded for the coordination of pseudooctahedral d-block

metal ions²²while the tridentate unit is suitable for the

pseudotricapped trigonal prismatic coordination of Ln(III).²⁷We

thus expect the binding units of L²to simultaneously recognize

3d and 4f metal ions, leading to the selective formation of

heterodinuclear d-f complexes. Our strategy is based on the

following approach: (i) study of the homopolynuclear com-

plexes formed by L²with Zn(II) and La(III), (ii) detailed

characterization of the assembly processes leading to the

heteronuclear d-f complexes, and (iii) structural characterization

of the latter both in the solid state and in solution. In order to

limit the amount of ligand required for the studies, we have

resorted to electrospray mass spectrometry (ES-MS) for the

qualitative speciation,^34,35spectrophotometric titrations for the

quantitative analysis of the assembly processes,⁴⁷and ¹H NMR

and luminescence techniques for the structural investigations,²³

using Ln(III) as the shift reagent⁴⁸and luminescent probe.¹The

complexes were isolated as their perchlorate salts to avoid any

anion coordination.

Figure 1. Numbering scheme and ORTEP⁴⁰view of L²with ellipsoids

represented at 50% probability level.

Scheme 1

Homonuclear Complexes of L²with La(III). Upon com-

plexation to metal ions, the electronic π f π* transitions

centered on ligand L²(maximum of the band envelope at 33 320

cm^-1) are shifted toward lower energies, allowing the monitoring

of the complex formation. Titrations of L²with La(ClO₄)₃‚nH₂O

in acetonitrile have been performed for total ligand concentra-

tions of 10^-4-10^-5M and La:L²ratios in the range 0.1-1.8.

Complicated variations of the UV absorbances (Figure 2a) and

factor analysis³²suggest the existence of at least four absorbing

complexes. The spectrophotometric data can be fitted to

equilibria 1-4 where convergence is observed with a root-mean-

IR, and MS data do not allow the two possible isomers to be

clearly distinguished, even though we expect severe steric

constraints for the nine-membered ring in 6.⁴³The X-ray crystal

structure definitely establishes that L²is the only isomer formed

(Figure 1) and shows that the pyridine and the benzimidazole

rings within each coordinating unit adopt a transoid conforma-

tion (s-trans, i.e., N(1) trans to N(2) and N(4) trans to N(6);

dihedral angles between pyridine and benzimidazole least-

squares planes 14.0(1)° (bidentate unit) and 17.4(1)° (tridentate

unit)). The carboxamide group (C(25)-C(26)-O-N(7)) is

planar within experimental error and adopts a distorted transoid

conformation (O trans to N(6), dihedral angle between the least-

squares planes of the pyridine (N(6)) and the carboxamide group

49.8(1)°). All bond lengths and angles within the ligand are

typical⁴⁴and are given in the supporting information. As often

observed with polyaromatic heterocyclic ligands,⁴⁵intermo-

lecular π-stacking interactions occur between parallel bidentate

coordinating units (average distance between aromatic planes

3.6 Å), leading to closely packed pairs of ligands in the solid

state. Detailed NOEDIF measurements of L²in chloroform

show significant nuclear Overhauser effects (NOE) between Me²

La³⁺+ 3L²T [La(L²)₃]³⁺log(â₁₃^La) ) 19.6(5) (1)

2La³⁺+ 3L²T [La₂(L²)₃]⁶⁺log(â₂₃^La) ) 27.5(9) (2)

2La³⁺+ 2L²T [La₂(L²)₂]⁶⁺log(â₂₂^La) ) 22.3(9) (3)

3La³⁺+ 2L²T [La₃(L²)₂]⁹⁺log(â₃₂^La) ) 29.0(9) (4)

square (RMS) difference between calculated and observed

absorbances of 0.001 unit or less. However, the great similarity

between the reconstructed spectra of the four complexes prevents

an accurate determination of the stability constants which are

thus only mere estimations.

Unfortunately, homonuclear complexes of segmental ligands

with Ln(III) are known to give a weak ES-MS response^23,24as

a result of the poor matching between the ligand binding

(43) March, J. AdVanced Organic Chemistry, 4th ed.; J. Wiley & Sons:

New York, Chichester, Brisbane, Toronto, and Singapore, 1992; pp 155-

161.

(44) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.

G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2, 1987, S1-S19.

(45) Chisholm, M. H.; Huffman, J. C.; Rothwell, I. P.; Bradley, P. G.;

Kress, N.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 4945-4947.

Constable, E. C.; Elder, S. M.; Healy, J. A.; Tocher, D. A. J. Chem. Soc.,

Dalton Trans. 1990, 1669-1674. Constable, E. C.; Elder, S. M.; Walker,

J. V.; Wood, P. P.; Tocher, D. A. J. Chem. Soc., Chem. Commun. 1992,

229-231. Constable, E. C.; Cargill-Thompson, A. M. W.; Harveson, P.;

Macko, L.; Zehnder, M. Chem. Eur. J. 1995, 1, 360-367.

(46) Piguet, C.; Bu¨nzli, J.-C. G.; Bernardinelli, G.; Williams, A. F. Inorg.

Chem. 1993, 32, 4139-4149.

(47) Piguet, C.; Hopfgartner, G.; Bocquet, B.; Schaad, O.; Williams, A.

F. J. Am. Chem. Soc. 1994, 116, 9092-9102.

(48) Bertini, I.; Luchinat, C. NMR of Paramagnetic Molecules in

Biological Systems; Benjamin/Cummings Publishing Co.: Menlo Park, CA,

1986; Chapter 10. Bertini, I.; Turano, P.; Vila, A. J. Chem. ReV. 1993, 93,

2833-2932.

+

6686 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

we were unable to confirm the speciation established by

spectrophotometry. ¹H NMR titrations of L²with La(ClO₄)₃‚

6H₂O in CD₃CN lead to intricate mixtures of low-symmetry

complexes displaying slow chemical exchange processes on the

NMR time scale. This behavior strongly contrasts with the

selective and quantitative formation of the head-to-tail C₁-triple

helicate [La₂(L¹)₃]⁶⁺under similar conditions.²³The complexes

between L²and La(III) were not further investigated.

Homonuclear Complexes of L²with Zn(II). ES-MS

titrations of L²with Zn(ClO₄)₂‚6H₂O in acetonitrile for Zn:L²

ratios in the range 0.1-2.0 show the formation of four

successive complexes, [Zn(L²)₃]²⁺(m/z 847.8), [Zn(L²)₂]²⁺(m/z

576.4), [Zn₂(L²)₃]⁴⁺(m/z 440.4), and [Zn₂(L²)₂]⁴⁺(m/z 304.8),

and their adducts³⁵with perchlorate anions (Table 1). The

systematic observation of many different complexes for each

Zn:L²ratio suggests the formation of intricate mixtures in

solution. Spectrophotometric data obtained in the same condi-

tions display three end points for Zn:L²ratios of 0.3, 0.6, and

1.1 (Table 2, Figure 2b). Factor analysis³²points to three

absorbing complexes, and the data can be fitted to equilibria

5-7 (RMS ) 0.003). We were unable to satisfactorily

Zn²⁺+ 3L²T [Zn(L²)₃]²⁺log(â₁₃^Zn) ) 22(1) (5)

2Zn²⁺+ 3L²T [Zn₂(L²)₃]⁴⁺log(â₂₃^Zn) ) 28.4(8) (6)

2Zn²⁺+ 2L²T [Zn₂(L²)₂]⁴⁺log(â₂₂^Zn) ) 21.5(9) (7)

reproduce the data with a model including the species [Zn-

(L²)₂]²⁺observed by ES-MS, probably as a result of too similar

absorption spectra for the different complexes, which strongly

suggests that the reported stability constants are only estima-

tions.⁴⁷

1

Upon complexation to Zn(II), the H NMR spectrum of L²

is broadened by slow chemical exchanges on the NMR time

scale and is of no use for the structural characterization of the

homonuclear complexes. We observe a sharp end point for Zn:

L²) 1.0 and the formation of >90% (from integration of the

NMR signals) of the complex [Zn₂(L²)₂]⁴⁺which displays 23

well-resolved ¹H NMR signals fully assigned by using COSY,

NOESY, and NOEDIF techniques (Table 3). This feature

corresponds to two equivalent ligands L²coordinated to Zn(II)

and related by a C₂axis. The presence of the diastereotopic

methylene protons H^7,8, H^15,16, and H^17,18and the observation

of 33 signals in the ¹³C NMR spectrum preclude the existence

of mirror planes⁴⁹and point to C₂symmetry for the complex

[Zn₂(L²)₂]⁴⁺in solution. NOEs between Me²and H³, Me³and

H¹², and H^17,18and H¹⁴show that the bidentate and the tridentate

Figure 2. Variation of observed molar extinctions at 10 different

wavelengths for the spectrophotometric titrations of L²with (a) La-

(ClO₄)₃‚6H₂O, (b) Zn(ClO₄)₂‚6H₂O, and (c) an equimolar mixture of

La(ClO₄)₃‚6H₂O and Zn(ClO₄)₂‚6H₂O (metal concentration ) La(III)

concentration ) Zn(II) concentration) in CH₃CN at 293 K (total ligand

concentration 10^-4M).

possibilities and the large coordination numbers required by

Ln(III),⁴¹which lead to solvated species difficult to transfer into

the gaseous phase.^23,35Titrations of L²with La(III) show only

significant peaks for the free ligand (m/z 544.2 and 1087.6

corresponding to [L²+ H]⁺and [2L²+ H]⁺, respectively), and

Table 1. Molecular Peaks of Complexes of L²and Adduct Ions Observed by ES-MS

metal

Zn(II)

cation

m/z^a

metal

cation

[EuZn(L²)₃]⁵⁺

m/z^a

[Zn(L²)₃]²⁺

847.8

1795.8

575.8

1251.6

440.4

620.4

980.4

304.8

439.2

709.4

366.8

483.4

678.1

1066.8

Eu(III)/Zn(II)

369.6

487.0

682.2

1073.2

370.8

488.2

684.0

1075.8

371.0

488.3

684.6

1076.6

[Zn(L²)₃(ClO₄)]⁺

[Zn(L²)₂]²⁺

[EuZn(L²)₃(ClO₄)]⁴⁺

[EuZn(L²)₃(ClO₄)₂]³⁺

[EuZn(L²)₃(ClO₄)₃]²⁺

[GdZn(L²)₃]⁵⁺

[Zn(L²)₂(ClO₄)]⁺

[Zn₂(L²)₃]⁴⁺

Gd(III)/Zn(II)

Tb(III)/Zn(II)

[Zn₂(L²)₃(ClO₄)]³⁺

[Zn₂(L²)₃(ClO₄)₂]²⁺

[Zn₂(L²)₂]⁴⁺

[GdZn(L²)₃(ClO₄)]⁴⁺

[GdZn(L²)₃(ClO₄)₂]³⁺

[GdZn(L²)₃(ClO₄)₃]²⁺

[TbZn(L²)₃]⁵⁺

[Zn₂(L²)₂(ClO₄)]³⁺

[Zn₂(L²)₂(ClO₄)₂]²⁺

[LaZn(L²)₃]⁵⁺

[TbZn(L²)₃(ClO₄)]⁴⁺

[TbZn(L²)₃(ClO₄)₂]³⁺

[TbZn(L²)₃(ClO₄)₃]²⁺

La(III)/Zn(II)

[LaZn(L²)₃(ClO₄)]⁴⁺

[LaZn(L²)₃(ClO₄)₂]³⁺

[LaZn(L²)₃(ClO₄)₃]²⁺

^am/z values given for the maximum of the peak.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6687

Table 2. Electronic Spectral Data for the Ligand L²and Its

Heterodinuclear Complexes in Acetonitrile at 293 K^a

compd

π f π*

compd

π f π*

L²

33 320 (50 690)

31 750 (39 090 sh)

[GdZn(L²)₃]⁵⁺32 050 (101 500)

30 490 (82 260 sh)

[LaZn(L²)₃]⁵⁺32 050 (101 330) [TbZn(L²)₃]⁵⁺32 050 (101 940)

30 490 (79 660 sh)

[CeZn(L²)₃]⁵⁺32 050 (101 900) [TmZn(L²)₃]⁵⁺32 050 (102 840)

30 490 (80 990 sh) 30 490 (86 230 sh)

[PrZn(L²)₃]⁵⁺32 050 (102 600) [YbZn(L²)₃]⁵⁺32 050 (101 300)

30 490 (82 790 sh) 30 490 (81 425 sh)

[NdZn(L²)₃]⁵⁺32 050 (101 440) [LuZn(L²)₃]⁵⁺32 050 (101 950)

30 490 (80 630 sh) 30 490 (85 500 sh)

[SmZn(L²)₃]⁵⁺32 050 (101 640) [YZn(L²)₃]⁵⁺32 050 (101 870)

30 490 (82 300 sh)

Figure 3. Possible structures for the complex [Zn₂(L²)₂]⁴⁺

.

30 490 (83 560 sh)

[EuZn(L²)₃]⁵⁺32 050 (102 050)

30 490 (81 880 sh)

30 490 (84 320 sh)

group of the second ligand (structure II), as reported for [Pd₂-

(quinquepyridine)₂]^{4+ 50}

Unfortunately, we were unable to

^aEnergies are given for the maximum of the band envelope in cm^-1

and ꢀ (in parentheses) in M^-1‚cm^-1; sh ) shoulder.

.

record dependable ⁶⁷Zn NMR signals to confirm this hypothesis

because the quadrupolar ⁶⁷Zn nucleus (I ) 5/2)⁴⁸induces very

broad lines for poorly symmetrical polyaza complexes.⁵¹The

replacement of Zn(II) by Cd(II) leads to a very similar behavior

in solution: (i) [Cd₂(L²)₂]⁴⁺(m/z 328.6) and its adduct ions³⁵

[Cd₂(L²)₂(ClO₄)_i]^(4-i)+(m/z 470.4, 756.2, and 1609.4 for i )

1-3, respectively) are observed in the ES-MS spectra, (ii) the

¹H NMR spectra are comparable (Table 3), and (iii) interstrand

NOE occurs between H¹and Me⁴. The ¹¹³Cd NMR spectrum

of [Cd₂(L²)₂]⁴⁺in acetonitrile displays only one singlet (δ 241.7

ppm with respect to Cd(ClO₄)₂(0.1 M) in D₂O) which is typical

of Cd(II) coordinated by four heterocyclic nitrogen atoms and

one oxygen atom⁵²and definitely establishes the helical head-

to-tail structure II for [Cd₂(L²)₂]⁴⁺. In view of the comparable

data for the Cd and Zn complexes, we infer that [Zn₂(L²)₂]⁴⁺

adopts structure II in solution, in contrast with the head-to-

head helical structure I found for the analogous complex [Zn₂-

binding units adopt cisoid conformations consecutive to their

coordination to Zn(II). Further structural information is obtained

from the intrastrand NOEs experienced by the proton of the

methylene spacer.^23,47The NOE map (H⁶-H⁹, H⁷-H⁹, H⁷-

H¹⁰, H⁸-H⁵, H⁸-H⁶) is typical of a helical twist of L²which

puts H⁹above the plane of the second benzimidazole ring

connected to the methylene spacer, leading to a significant

shielding effect for this proton (∆δ ) 1.63 ppm) as similarly

reported for the double helicate [Zn₂(L¹)₂]⁴⁺(∆δ ) 1.82 ppm).²³

These observations imply a comparable double-helical structure

for [Zn₂(L²)₂]⁴⁺and exclude the side-by-side structures III and

IV (Figure 3).⁴⁷However, the two remaining possible helical

structures I and II belong to the same C₂point group and yield

¹H NMR spectra with very similar patterns. A weak interstrand

NOE observed between H¹and H^15,16strongly suggests a head-

to-tail arrangement of the ligands in [Zn₂(L²)₂]⁴⁺which brings

the terminal pyridine ring of one ligand close to the carboxamide

Table 3. ¹H NMR Shifts (with Respect to TMS) for Ligand L²in CDCl₃and Its Complexes in CD₃CN at 298 K

Bidentate Binding Unit

compd

Me¹

Me²

H¹

H²

H³

H⁴

H⁵

H⁶

H^7,8

4.29

L²

2.42

2.21

2.36

2.16

2.15

1.89

1.75

1.96

2.11

2.37

3.33

2.59

4.23

4.45

4.34

4.21

3.81

3.58

3.91

4.13

4.55

6.12

4.92

8.51

7.77

8.18

7.74

7.24

6.97

7.37

7.65

8.14

9.95

8.56

7.64

7.99

8.04

7.86

7.85

7.84

7.56

7.40

7.64

7.80

8.09

9.16

8.33

8.26

8.28

8.34

8.17

8.18

8.17

7.73

7.50

7.85

8.09

8.52

10.12

8.89

7.33

7.36

7.53

7.63

7.61

7.22

6.96

7.31

7.55

7.99

9.82

8.46

7.20

7.80

7.12

7.25

7.22

6.91

6.67

6.97

7.18

7.56

9.20

8.02

7.71

7.20

6.56

5.34

5.36

5.42

3.10

1.77

3.70

4.98

7.31

15.76

9.24

[Zn₂(L²)₂]⁴⁺

4.11, 3.86

3.10, 3.80

3.54, 3.63

3.53, 3.64

3.04, 3.15

2.66, 2.87

3.18, 3.22

3.49, 3.53

3.99, 4.16

5.17, 6.27

4.15, 4.61

[Cd₂(L²)₂]⁴⁺

[LuZn(L²)₃]⁵⁺

[YZn(L²)₃]⁵⁺

[LaZn(L²)₃]⁵⁺

[CeZn(L²)₃]⁵⁺

[PrZn(L²)₃]⁵⁺

[NdZn(L²)₃]⁵⁺

[SmZn(L²)₃]⁵⁺

[EuZn(L²)₃]⁵⁺

[TmZn(L²)₃]⁵⁺

[YbZn(L²)₃]⁵⁺

Tridentate Binding Unit

compd

Me³

H⁹

H¹⁰

H¹¹

H¹²

H¹³

H¹⁴

H^15,16

3.61

3.62, 3.95

3.20, 3.40

2.60, 2.77

2.65, 2.78

2.84

2.26, 3.03

2.04, 3.27

2.55, 3.09

2.69, 2.75

2.59, 3.21

1.50, 3.24

1.93, 2.87

H^17,18

3.35

Me⁴

Me⁵

L²

4.21

4.20

4.31

4.38

4.37

4.33

5.27

5.92

5.00

4.55

3.75

-0.13

2.57

7.69

6.06

6.94

5.44

5.52

7.23

7.33

7.15

6.92

6.96

6.43

6.12

6.56

6.86

7.42

8.95

7.63

7.34

7.56

7.57

7.32

7.31

7.35

7.45

7.75

7.97

7.27

6.06

6.63

7.09

8.38

8.49

8.57

8.62

8.60

7.93

8.57

8.45

8.30

8.32

8.35

10.09

11.09

9.90

8.56

6.34

0.70

5.88

7.57

8.19

8.07

7.84

7.83

7.82

8.98

9.89

9.37

7.97

5.46

2.58

6.10

1.30

0.73

0.94

0.68

0.69

1.13

1.43

1.04

1.01

0.92

1.28

1.77

1.39

1.05

0.38

-1.29

-0.13

[Zn₂(L²)₂]⁴⁺

3.21, 3.42

3.50, 3.70

3.43, 3.45

3.41, 3.43

3.34, 3.42

3.47, 4.09

3.82, 4.83

3.69, 4.15

3.40, 3.55

2.54, 2.91

-0.31, 3.84

1.67, 3.31

[Cd₂(L²)₂]⁴⁺

[LuZn(L²)₃]⁵⁺

[YZn(L²)₃]⁵⁺

[LaZn(L²)₃]⁵⁺

[CeZn(L²)₃]⁵⁺

[PrZn(L²)₃]⁵⁺

[NdZn(L²)₃]⁵⁺

[SmZn(L²)₃]⁵⁺

[EuZn(L²)₃]⁵⁺

[TmZn(L²)₃]⁵⁺

[YbZn(L²)₃]⁵⁺

5.82

8.52

0.75

-0.12

-3.72

1.68

4.55

9.89

33.50

16.12

11.09

12.98

11.29

8.92

4.60

-3.19

4.52

-1.23

-2.72

0.90

0.32

2.67

11.86

5.20

+

6688 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

because the great similarity between the calculated absorption

spectra of the complexes prevents the simultaneous fit of the

eight equilibria 1-8. These data suggest however that the

assembly of the heterodinuclear complexes [LnZn(L²)₃]⁵⁺is

almost quantitative at 10^-4M, but competition with homo-

nuclear complexes occurs for lower concentrations. ES-MS

spectra confirm this statement and show the formation of

significant quantities of [Zn(L²)₃]²⁺(m/z 847.8) and [Zn(L²)₂]²⁺

(m/z 575.8) associated with increasing fractions of free ligand

for a total ligand concentration of 10^-5M (Figure 4b). At 10^-6

M, only significant peaks corresponding to the free ligands are

detected by ES-MS.

Slow diffusion of diethyl ether into a concentrated acetonitrile

solution of the complexes [LnZn(L²)₃]⁵⁺leads to the quantitative

isolation of polycrystalline aggregates whose elemental analyses

correspond to [LnZn(L²)₃](ClO₄)₅‚0.5Et₂O‚2H₂O (Ln ) La, 8;

Nd, 9; Eu, 10; Gd, 11; Tb, 12). The IR spectra are similar and

display the characteristic vibrations of the heterocyclic ligands

in the range 1570-600 cm^{-1 41,47}together with an intense

carbonyl stretching vibration assigned to the carboxamide group

(1595 cm^-1) which is red-shifted with respect to the free ligand

(1635 cm^-1). The ClO₄^-anions show the two vibrations (1095

and 625 cm^-1) typical of ionic perchlorates.⁵³

Crystal Structure of [EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃-

CN)₄(13). Fragile colorless single crystals of [EuZn-

(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13) suitable for X-ray dif-

fraction analysis were obtained by diffusion of diisopropyl ether

into an acetonitrile solution of 10 containing 30 equiv of NBu₄-

CF₃SO₃, followed by ultrasonic mixing and slow cooling to 277

K. A partial report of the crystal structure of this complex has

already been published.²⁷Selected bond lengths and angles are

given in Table 4. Figure 5 shows the atomic numbering scheme,

and Figure 6 gives an ORTEP⁴⁰stereoscopic view of the

complex perpendicular to the pseudo-C₃axis.

Figure 4. ES-MS spectra of [EuZn(L²)₃](ClO₄)₅(10) in acetonitrile

for total ligand concentrations of (a) 10^-4M and (b) 10^-5M.

(L¹)₂]⁴⁺under similar conditions.²³We have isolated [Zn₂-

(L²)₂](ClO₄)₄(7) by diffusion of diethyl ether into an acetonitrile

solution in the form of white microcrystals, but we were unable

to obtain crystals suitable for X-ray diffraction studies.

Heterodinuclear Complexes of L²with Zn(II) and Ln-

(III). ES-MS titrations of L²(10^-4M) with an equimolar

mixture of Ln(ClO₄)₃‚nH₂O (Ln ) La, Eu, Gd, Tb) and Zn-

(ClO₄)₂‚6H₂O in acetonitrile show the formation of only one

heterodinuclear complex [LnZn(L²)₃]⁵⁺and its adduct ions³⁵

with perchlorate (Table 1, Figure 4). These observations are

confirmed by spectrophotometric titrations under similar condi-

tions (total ligand concentration ) 10^-4M; Ln ) La, Eu; metal:

L²) 0.1-1.1; metal concentration ) Ln(III) concentration )

Zn(II) concentration) which display a sharp end point for metal:

L²) 0.33 combined with an isobestic point at 30 120 cm^-1

(Figure 2c). Factor analysis³²reveals the formation of only one

absorbing complex in significant quantity, and the fit with

equilibrium 8 readily converges with the following stability

The crystal structure of 13 consists of a [EuZn(L²)₃]⁵⁺cation,

five uncoordinated anions (one perchlorate and four triflates)

and 4 solvent molecules. The anions and solvent molecules

are disordered (see the Experimental Section). In [EuZn(L²)₃]⁵⁺

,

the three segmental ligands L²adopt a head-to-head arrange-

ment^23,47and are wrapped around a pseudo-C₃axis passing

through the metal ions (Eu‚‚‚Zn ) 8.960(3) Å). The three

bidentate binding units are coordinated to Zn(II) while the three

remaining tridentate units are bound to Eu(III), leading to a

heterodinuclear triple-helical structure. Within each tridentate

binding unit a, b, or c, the benzimidazole nitrogen atom N(4)

and the oxygen atom O(1) coordinated to Eu(III) lie on the same

side of the plane of the central pyridine ring (i.e., the dihedral

angles N(4)-C(20)-C(21)-N(6) and N(6)-C(25)-C(26)-

O(1) are of opposite sign), leading to a bent conformation similar

to that reported for L⁴in [Eu(L⁴)(NO₃)₃].³⁶This produces a

significant distortion of the screw thread for the tridentate units

coordinated to Eu(III) which strongly contrasts with the usual

regular wrapping observed for the analogous mononuclear [Eu-

(L³)₃]^{3+ 21,46}and dinuclear [Eu₂(L⁵)₃]^{6+ 41}complexes where the

coordinated nitrogen atoms of the two distal benzimidazole rings

lie systematically on the opposite side of the central pyridine

ring. The coordination polyhedron of the Eu atom is a slightly

distorted tricapped trigonal prism with the three N atoms of

the benzimidazole units and the three O atoms occupying the

vertices of the prism. The Eu-O and Eu-N bond distances

are close to the standard values reported for Eu-O(amide),^25,54

LnZn

Ln³⁺+ Zn²⁺+ 3L²T [LnZn(L²)₃]⁵⁺log(â₁₁₃

)

(8)

constants: log(â₁₁₃^LaZn) ) 29.0(4) and log(â₁₁₃^EuZn) ) 28.6(6)

(RMS ) 0.002). These values are only approximate since our

model did not take into account the possible formation of minor

quantities of the homonuclear complexes described in eqs 1-7.

This simplification is justified by the existence of one isobestic

point and by the ES-MS spectra which include peaks corre-

sponding to the free ligand and to [LnZn(L²)₃]⁵⁺only (Figure

4a). For a lower total ligand concentration (10^-5M), the

spectrophotometric data are modified, displaying a complicated

variation of the molar absorbances and two end points for metal:

L²) 0.2 and 0.6. Attempts to fit the data were unsuccessful

(49) Ru¨ttimann, S.; Piguet, C.; Bernardinelli, G.; Bocquet, B.; Williams,

A. F. J. Am. Chem. Soc. 1992, 114, 4230-4237.

(50) Constable, E. C.; Elder, S. M.; Healy, J.; Ward, M. D.; Tocher, D.

A. J. Am. Chem. Soc. 1990, 112, 4590-4592.

(51) Epperlein, B. W.; Kru¨ger, H.; Lutz, O.; Schwenk, A. Z. Naturforsch.

Teil A 1974, 29, 1553-1557.

(53) Nakamoto, K. Infrared and Raman Spectra of Inorganic and

Coordination Compounds, 3rd ed.; J. Wiley: New York, Chichester,

Brisbane, and Toronto, 1972; pp 142-154.

(54) Franklin, S. J.; Raymond, K. N. Inorg. Chem. 1994, 33, 5794-

5804.

(52) Goodfellow, R. J. In Multinuclear NMR; Mason, J., Ed.; Plenum

Press: New York and London, 1989; pp 566-577. Reger, D. L.; Myers,

S. M.; Mason, S. S.; Rheingold, A. L.; Haggerty, B. S.; Ellis, P. D. Inorg.

Chem. 1995, 34, 4996-5002.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6689

Table 4. Selected Bond Distances (Å) and Bond Angles (deg) for

[EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13)

Distances

ligand a

ligand b

ligand c

Eu‚‚‚Zn

8.960(3)

Eu-O(1)

Eu-N(4)

Eu-N(6)

Zn-N(1)

Zn-N(2)

2.40(2)

2.53(2)

2.59(2)

2.15(2)

2.12(2)

2.37(1)

2.58(2)

2.65(2)

2.19(2)

2.01(2)

2.38(1)

2.61(2)

2.55(2)

2.52(2)

2.03(2)

Angles

bite angles

ligand b

ligand a

ligand c

N(1)-Zn-N(2)

N(4)-Eu-N(6)

N(6)-Eu-O(1)

78.2(7)

63.7(5)

77.0(7)

62.9(5)

63.6(5)

74.0(7)

62.9(6)

63.9(5)

N-Zn-N

96.2(7)

159.1(7)

105.6(7)

96.4(7)

97.2(6)

174.1(6)

N(1a)-Zn-N(2b)

N(1a)-Zn-N(2c)

N(2a)-Zn-N(2b)

N(2a)-Zn-N(2c)

N(1b)-Zn-N(1c)

N(2b)-Zn-N(1c)

N(1a)-Zn-N(1c)

N(2a)-Zn-N(1b)

N(2a)-Zn-N(1c)

N(1a)-Zn-N(1b)

N(1b)-Zn-N(2c)

N(2b)-Zn-N(2c)

85.2(6)

174.1(7)

80.3(6)

96.4(6)

87.9(7)

104.7(7)

N-Eu-N

N(4a)-Eu-N(4b)

N(4b)-Eu-N(4c)

N(4a)-Eu-N(4c)

N(4a)-Eu-N(6c)

N(6a)-Eu-N(4b)

N(6b)-Eu-N(4c)

87.8(5)

88.9(5)

N(6a)-Eu-N(6b)

N(6b)-Eu-N(6c)

N(6a)-Eu-N(6c)

N(4a)-Eu-N(6b)

N(4b)-Eu-N(6c)

N(6a)-Eu-N(4c)

117.8(6)

120.7(6)

120.6(6)

74.2(5)

72.9(5)

75.0(6)

Figure 5. Atomic numbering scheme for [EuZn(L²)₃]⁵⁺(13).

85.3(6)

142.4(5)

147.9(5)

145.1(5)

O-Eu-N

N(4a)-Eu-O(1c)

N(6a)-Eu-O(1b)

N(6b)-Eu-O(1c)

O(1a)-Eu-N(6b)

O(1a)-Eu-N(4c)

O(1c)-Eu-N(4c)

O(1b)-Eu-N(4c)

O(1b)-Eu-N(4b)

143.9(5)

67.4(5)

N(4a)-Eu-O(1b)

N(6a)-Eu-O(1c)

N(4b)-Eu-O(1c)

O(1a)-Eu-N(4a)

O(1a)-Eu-N(6c)

O(1a)-Eu-N(4b)

O(1b)-Eu-N(6c)

82.7(5)

133.8(5)

78.0(5)

127.4(5)

69.1(5)

141.4(5)

134.8(5)

69.7(5)

134.8(5)

80.0(5)

126.8(6)

142.1(5)

126.3(5)

O-Eu-O

O(1a)-Eu-O(1b)

O(1a)-Eu-O(1c)

79.1(5)

79.5(5)

O(1b)-Eu-O(1c)

79.6(5)

Figure 6. ORTEP⁴⁰stereoview of the cation [EuZn(L²)₃]⁵⁺perpen-

dicular to the pseudo-C₃axis in 13.

Eu-N(pyridine), and Eu-N(benzimidazole).^21,41,46Zn(II) lies

in a severely distorted octahedral site produced by the six N

donor atoms of the three bidentate units. The Zn-N(1c) bond

length (2.52(2) Å) is much longer than the standard Zn-

N(pyridine) value (2.111 Å)⁵⁵and than the average Zn-N

distance in 13 (2.17 Å). As expected from the thermodynamic

“trans influence”,⁵⁶the Zn-N(2b) bond (trans to Zn-N(1c)) is

short (2.01(2) Å), which shifts N(1a) and N(2c) toward N(1c)

(N(1a)-Zn-N(2c) ) 159.1(7)°, Figure 6), leading to a Zn

coordination sphere close to a distorted trigonal bipyramid if

the loosely bond N(1c) atom is neglected. This distortion results

from a close intermolecular packing of the [EuZn(L²)₃]⁵⁺cations

by pairs in the unit cell. Within each pair, the cations are

disposed around an inversion center located on the side of the

Zn atom. This situation leads to particular structural features

between the two paired cations: (i) a parallel orientation of their

pseudo-C₃axes, (ii) a head-to-head arrangement with a short

intermolecular Zn‚‚‚Zn distance (8.315(4) Å), and (iii) a

staggered packing of the 5-methylpyridine rings bound to

Zn(II), leading to severe distortions of the bidentate coordinated

Figure 7. Packing of a pair of cations [EuZn(L²)₃]⁵⁺around an

inversion center showing their staggered arrangement in 13 (only the

5-methylpyridine groups bound to Zn(II) are represented).

units (Figure 7). The 5-methylpyridine ring of strand c is the

more constrained, and this may explain the long Zn-N(1c) bond

length. The pairs of cations (possessing opposite helicities) are

packed in the unit cell with their pseudo-C₃axis approximately

parallel to the ac plane and forming an angle of 27° with the c

axis.

(55) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.

G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1-S83.

(56) Appleton, T. G.; Clark, H. C.; Maurer, L. E. Coord. ReV. 1973, 10,

335-422. Enemark, E. J.; Stack, T. D. P. Angew. Chem., Int. Ed. Engl.

1995, 34, 996-998.

Photophysical Properties of [LnZn(L²)₃]⁵⁺(Ln ) La, 8;

La:Eu, 8a; Nd, 9; Eu, 10, 13; Gd, 11; Gd:Eu 11a; Tb, 12) in

the Solid State. The reflectance spectrum of the free ligand

+

6690 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Table 5. Ligand-Centered Absorption and Emission Properties in [LnZn(L²)₃]⁵⁺Complexes in the Solid State^a

Piguet et al.

π f π* (cm^-1

)

¹ππ* (cm^-1

)

³ππ* (cm^-1

)

τ(³ππ*) (ms)

compd

L^{1 b}

29 600

30 770

31 000

31 250

30 900

28 600 sh

26 200

25 000

22 600

24 940^c

22 600

21 480

22 800

19 200

19 700

20 040

19 960

c

18 650

18 900

18 870

19 050

c

719 ( 30

225 ( 32

560 ( 18

250 ( 4

11.7 ( 0.1

c

244 ( 43

84 ( 20

41 ( 2

36 ( 6

5.04 ( 0.03

c

[LaZn(L¹)₃]^{5+ b}

L²

17 860 sh

18 000 sh

c

[LaZn(L²)₃]⁵⁺

[GdZn(L²)₃]⁵⁺

[EuZn(L²)₃]⁵⁺

[TbZn(L²)₃]⁵⁺

32 800 sh

32 750 sh

26 300 sh

26 000 sh

26 400 sh

c

^aReflectance spectra recorded at 295 K, luminescence data at 77 K, and lifetime measurements at 10 K (λ_exc) 308 nm); sh ) shoulder. ^bFrom

ref 23. ^{c 3}ππ* luminescence quenched by transfer to Ln ion.

L²displays one band at 30 770 cm^-1, assigned to a π f π*

transition,⁵⁷which is slightly shifted upon complexation to yield

either a very broad transition for [LaZn(L²)₃]⁵⁺or a structured

band centered at ca. 31 000 cm^-1with two shoulders around

32 000 and 26000-27000 cm^-1for the other heteronuclear

complexes (Table 5, Figure F1 in the supporting information).

Ligand L¹displays somewhat similar spectra with a π f π*

transition split into two components separated by 1000 cm^-1

in the uncomplexed state and by 3400-4500 cm^-1in the

complexes. A weak luminescence is observed for L²at 77 K

upon excitation of its singlet states: a structured band with a

1

maximum at 24 900 cm^-1is assigned as arising from a ππ*

state while faint shoulders on the low-energy side (<20 000

cm^-1) are attributed to triplet state emissions. The latter

assignment is supported by time-resolved luminescence mea-

Figure 8. Emission spectrum of [EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃-

CN)₄(13) measured at 10 K under excitation through the ligand levels

(395 nm).

surements (Figure F2 in the supporting information): the singlet

state emission is strongly reduced or even disappears when the

delay time is set larger than 0.1 ms while the triplet state

emission is still observed. It appears as a structured band with

two maxima around 20 000 and 19 000 cm^-1and a shoulder

around 18 000 cm^-1. The two maxima could be thought as

originating from the two different nucleating parts of the ligand,

but they may also be simply part of a vibrational progression

(ca. 1000 cm^-1). The luminescence decay of the triplet state,

arising from the Eu(⁵D₀) and Tb(⁵D₄) excited levels are observed

at 77 and 10 K, while the metal-centered luminescence is totally

4

quenched in the Nd-containing complex, the small F_3/2-⁴I_15/2

gap (ca. 8000 cm^-1)⁷favoring radiationless deexcitation pro-

cesses.

In contrast to the complexes with L¹, the Eu- and Tb-

containing heteronuclear complexes with L²are strongly

luminescent both in the solid state (at room or low temperature)

and in solution. We have examined their properties in the solid

state at 295, 77, and 10 K for the purpose of investigating the

coordination environment around the luminescent ion, determin-

ing if the structure of the complexes changes along the series,

and analyzing the energy migration pathways. At 10 K, the

excitation spectrum of crystals of the Eu-containing compound

13 (Figure F3 in the supporting information) displays a broad

and intense band with a maximum at 25 700 cm^-1, correspond-

3

measured at 10 K, is biexponential and typical of such ππ*

states (41 and 560 ms as compared to 244 and 719 ms for L¹).²³

Assignment of these lifetimes to a particular component of the

triplet state emission was not possible because of the rather low

intensity of the emission and of the small separation of the two

components. There is however little doubt that these two

lifetimes are associated with the two different coordinating

segments of the ligand. Upon complexation to Zn(II) and

Ln(III), the singlet state emission band shifts to lower energies

(2200-3500 cm^-1) and a reduced intensity is observed for Eu

and Tb as a result of the energy transfer between the ligand

and the luminescent metal ions. For La and Gd, the triplet state

emission is not significantly altered by the complexation, except

for the lifetimes. Coordination of the metal ion perturbs the

electron density of the ligand, henceforth the triplet state,

facilitating nonradiative deexcitation paths and reducing the

³ππ* lifetimes. The effect in [LaZn(L²)₃]⁵⁺is similar to the

one observed in the corresponding L¹compounds.²³The

lifetime reduction measured for the paramagnetic [GdZn(L²)₃]⁵⁺

complex is substantially larger for both L¹and L²(Table 5)

1

ing to excitation through the ligand ππ* levels, and several

sharp features arising from direct excitation onto the Eu(III)

excited levels: ⁵D_J(J ) 0, 1, 2, 3) and ⁵L₆. The ligand

excitation band extends below 400 nm in the spectrum of 13

while there is a sharp cutoff at this wavelength in the spectrum

of the Tb-containing complex. Although the ligand-to-metal

charge-transfer (LMCT) transition could not be formally

evidenced, we note that the ¹ππ* ligand state in the Eu-

containing heteronuclear complex lies at lower energy (1200-

1400 cm^-1) than in the other complexes, indicating a possible

mixing with the LMCT state (and eventually with some of the

almost resonant f states). The emission bands obtained upon

excitation either through the ligand states, with subsequent

6

and is the consequence of the proximity of the P_7/2Gd(III)

level (ca. 32 000 cm^-1)⁷with the ligand singlet state.^21,23,41,58

Ligand L²displays a good antenna effect²since upon excitation

through the π f π* transition, the ligand-centered triplet state

5

3

energy transfer to the D_Jmanifold, or directly onto the D₀

level by use of a dye laser (Figure 8) are well resolved and

typical of an emitting species containing a single lanthanide

coordination environment with pseudotrigonal symmetry. In

luminescence is completely quenched by efficient ππ* f Ln

energy transfers to Nd, Eu, and Tb. Sharp emission bands

(57) Bochet, C. G.; Piguet, C.; Williams, A. F. HelV. Chim. Acta 1993,

76, 372-384.

(58) Alpha, B.; Ballardini, R.; Balzani, V.; Lehn, J.-M.; Perathoner, S.;

Sabbatini, N. Photochem. Photobiol. 1990, 52, 299-306.

5

7

particular, the weak D₀f F₀transition appears as a fairly

symmetrical band centered at 17 220 cm^-1and with a full width

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6691

Table 6. Identified Eu(⁷F_J) Crystal Field Levels (cm^-1, J ) 1-4) in [LnZn(L²)₃]⁵⁺Complexes, from Luminescence Spectra at 10 and 295 K,

5

under Excitation through the Ligand Levels (8a, 10, 11a) or to the D₀Level (13)

level

⁷F₁

13 (10 K)

10 (295 K)

8a (295 K)

11a (295 K)

level

⁷F₃

13 (10 K)

10 (295 K)

8a (295 K)

11a (295 K)

294

411

432

287

399

454

290

411

453

292

394

457

1824

1874

1931

2706

2717

2799

2860

2888

2990

1830

1881

1934

2700

2742

2801

2841

2901

2999

1834

1884

1927

2706

2736

2800

2837

2892

2998

1833

1888

1936

2700

2741

2800

2838

2900

2998

⁷F₂

984

967

976

970

⁷F₄

1002

1043

1085

1107

1016

1036

1094

1118

1016

1036

1091

1124

1019

1048

1090

1114

⁵D₀

17220

17229

17228

17226

5

at half-height (fwhh) of 10 cm^-1. The corresponding D₀r

⁷F₀excitation band is less symmetrical, and both its energy

(17220-17229 cm^-1) and fwhh (9-15 cm^-1) depend upon the

analyzing wavelength. This may reflect the simultaneous

are the six transitions observed to the ⁷F₄level (six allowed in

C₃symmetry¹).

When the [EuZn(L²)₃]⁵⁺complex is measured as a powdered

sample (10), the essential features of its luminescence spectrum

are unchanged, especially the relative intensities of the transi-

tions (Table SV in the supporting information), and the overall

shape of the emission bands. The ⁵D₀f ⁷F₀emission band is

broader (fwhh ) 20 cm^-1at 10 K) and displays a shoulder on

its low-energy side, 10 cm^-1from the maximum which occurs

at 17 231 cm^-1. Selective laser excitations within the profile

of the 0-0 transition yield emission spectra with similar overall

band shape and relative transition intensities, but differing in

the fine splitting of the bands. In particular, the splitting of the

⁷F₁(E) crystal field sublevels is larger than for sample 13,

ranging from 37 to 68 cm^-1. Increasing the temperature to 295

K tends to favor the species deviating most from the idealized

trigonal symmetry. Similar observations are made for the other

crystal field sublevels. The solid aggregates 10 differ in

chemical composition from the crystals 13 by their solvation

molecules and by the counteranions: 13 contains weaker

coordinating acetonitrile molecules, and in addition, four

perchlorate anions are replaced by triflates. The differences

observed between the emission spectra of 10 and 13 are

reminiscent of the effects evidenced for the homonuclear

complex with L⁵: single crystals grown with acetonitrile as

solvation molecules displayed highly symmetrical lanthanide

coordination sites while introduction of water molecules in the

lattice resulted in a lowering of the symmetry, as indicated by

excitation of a crystal phonon with an energy of about 10 cm^-1

.

The breadth of the 0-0 transition points to crystals of relatively

poor quality with defects resulting in different packing of the

molecules and henceforth in somewhat different environments

for the Eu(III) ions, as substantiated by the slightly different

emission spectra obtained when the excitation wavelength is

scanned through the 0-0 band profile. The low energy of the

⁵D₀level is typical of Eu(III) coordinated to several heterocyclic

N atoms^21,24,41,46and suggests a large nephelauxetic effect.⁷The

energy of the ⁵D₀f ⁷F₀transition depends upon the ability of

each coordinating atom to produce a nephelauxetic effect:⁵⁹

ν˜ - ν˜₀) C_CN∑_in_iδ_iwhere C_CNis a function of the total

coordination number of the Eu(III) ion (1.0 for CN ) 9), n is

the number of i-type atoms, δ_iis the ability of the coordinating

atom i to produce a nephelauxetic effect, and ν˜₀) 17 374 cm^-1

at 295 K (the temperature dependence of ν˜ is approximately 1

cm^-1per 24 K).¹The δ_ivalues are tabulated: -15.7 for an

amide O atom and -12.1 for an amine N atom.⁵⁹We have

however recently demonstrated that a heterocyclic N atom (HN)

tends to produce a larger nephelauxetic effect and have deduced

from our work on complexes with benzimidazole-containing

ligands a δ_HNparameter equal to -15.3. Taking the latter value

into account, we predict an energy of 17 223 cm^-1for the ⁵D₀

f ⁷F₀transition of 13 at 10 K, a value only marginally different

from the experimental finding, which confirms the correctness

of the two nephelauxetic parameters. In addition to the low

7

a larger F₁(E) splitting (41 cm^-1Vs 18 cm^-1).⁴¹Since water

molecules do not penetrate into the inner coordination sphere

of the Eu(III) ion, the lifetime of the Eu(⁵D₀) level remaining

unchanged, we concluded that the “solvation” effect in the

homonuclear complex with L⁵resides in hydrogen bonding with

the ligand strands, modifying the general coordination arrange-

ment around the metal ions.⁴¹Therefore, the recorded excitation

and emission spectra for 10 can be understood as reflecting the

presence of several species in the aggregates, differing es-

sentially by their solvation, henceforth by small distortions of

the lanthanide ion environment consecutive to interactions of

the ligand strands with the solvation molecules in the lattice.⁶⁰

Indeed, the Eu(⁵D₀) lifetimes are only marginally reduced (less

than 10%) in going from sample 13 to 10, while inner sphere

coordination of a single water molecule would result in a

dramatic decrease of this lifetime to less than 1 ms.¹It is

noteworthy that the luminescence measurements allow us to

detect second-sphere effects, which demonstrates the utility of

Eu(III) as a sensitive structural probe.

intensity of the ⁵D₀

f

⁷F₀transition, which is symmetry

forbidden in D₃but allowed in C₃symmetry, detailed analysis

of the ⁵D₀-excited emission spectrum of 13 (Figure 8, Table 6,

Table SV in the supporting information) confirms the pseudo-

trigonal symmetry unraveled by the X-ray diffraction study.

7

There are two main transitions to F₁, A f A and A f E, the

latter being split into two components, indicating a deviation

from the trigonal symmetry. The singlet/doublet pattern of the

⁷F₁level has separations of 117 and 21 cm^-1, similar to the

ones measured for [Eu(L³)₃]³⁺(110 and 17 cm^{-1 46}

) and for [Eu₂-

(L⁵)₃]⁶⁺(118 and 18 cm^-1).⁴¹In the latter two compounds,

the Eu(III) ion lies on a site with C₃symmetry or close to it,

meaning that the same situation prevails for [EuZn(L²)₃]⁵⁺while

the coordinating sites in [EuZn(L¹)₃]⁵⁺and [Eu₂(L⁵)₃]⁶⁺display

much larger distortions.²³Analysis of the ⁵D₀f ⁷F₂transition

is less straightforward considering possible interferences with

vibronic transitions.^1,41This transition may however be viewed

as comprised of two doublets (A f E, splittings 18 and 22 cm^-1

)

The luminescent properties of [LnZn(L²)₃]⁵⁺complexes

doped with 2% Eu(III) (Ln ) La, 8a; Gd, 11a) are unchanged

with respect to a pure Eu-containing powdered sample. At room

7

and of one, less intense singlet (A f A, F₂sublevel at 1043

cm^-1), which is coherent with a pseudotrigonal symmetry, as

(59) Frey, S. T.; Horrocks, W. de W., Jr. Inorg. Chim. Acta 1995, 229,

383-390.

(60) Ligand dissociation (or rearrangement) in the excited state may be

ruled out since it is not observed with crystals of 13.

+

6692 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

Table 7. Lifetimes of the Eu(⁵D₀) and Tb(⁵D₄) Excited Levels

(ms) in [LnZn(Lⁱ)₃]⁵⁺Complexes and Solutions under Various

Excitation Conditions (Analyzing Wavelength Set on the Maximum

to 10 K, but the increase amounts to a factor of 4 only for 10

while it is about 100-fold for 12. Moreover, the Eu-doped La-

and Gd-containing complexes 8a and 11a display an inverse

temperature dependence, the luminescence intensity decreasing

by a factor of 7-9 in going from 295 to 10 K. A temperature-

mediated energy migration process occurs in these compounds,

in which the energy taken up by the ligands migrates from one

molecule to the other until it reaches an Eu-containing site, a

phenomenon commonly observed in solid state samples doped

with lanthanide ions.⁶¹

5

7

5

7

of the D₀f F₂or D₄f F₅Transition)

compd

T (K)

λ

_exc(nm)

τ (ms)

[Eu₂(L⁵)₃]^{6+ a}

10

308

580.2

400

308

580.0

395

308

580.0

395

308

580.1

308

580.0

392

308

580.1

375

308

380

2.22 ( 0.07

2.30 ( 0.05

2.56 ( 0.10

2.53 ( 0.10

2.21 ( 0.30

2.44 ( 0.03

2.19 ( 0.01

2.35 ( 0.02

1.98 ( 0.01

1.67 ( 0.02

1.69 ( 0.01

2.29 ( 0.04^b

2.22 ( 0.01

1.97 ( 0.08

2.56 ( 0.02

2.59 ( 0.01

2.72 ( 0.03

2.88 ( 0.01

2.84 ( 0.05

2.90 ( 0.02

1.89 ( 0.06

1.64 ( 0.01

1.57 ( 0.02

[EuZn(L¹)₃]^{5+ a}

[EuZn(L²)₃]⁵⁺(13)

[EuZn(L²)₃]⁵⁺(10)

10

295

10

The Tb(⁵D₄) luminescence spectrum of complex 12 excited

through the ligand level is dominated by the ⁵D₄f ⁷F₅transition

(Figure F6 in the supporting information) and the relative,

5

7

corrected intensities of the D₄f F_Jtransitions at 10 K are

1.0, 2.4, 0.9, 0.4, 0.04, 0.03, and 0.03 for J ) 6, 5, 4, 3, 2, 1,

and 0, respectively. The ⁵D₄lifetime is shorter than the

europium ⁵D₀lifetime despite the larger ⁵D₄-⁷F₀gap compared

to ⁵D₀-⁷F₆, and it decreases with increasing temperature. This

is a common feature of macrocyclic Tb-containing com-

plexes^{21,23,41,46,58}and is interpreted as revealing a Tb f ligand

energy back-transfer, both the ¹ππ* and ³ππ* states of the ligand

[(Gd:Eu)Zn(L²)₃]⁵⁺(11a)

10, 10^-3M in CH₃CN

10, 10^-4M in CH₃CN

[TbZn(L²)₃]⁵⁺(12)

295

10

5

lying close in energy to the D₄terbium state.

Photophysical Properties of [EuZn(L²)₃]⁵⁺in Acetonitrile

Solution. The π f π* transition of the ligand in solution

488.0

appears at higher energy than in the solid state (2500 cm^-1

)

^aFrom ref 23. ^bThe decay is biexponential. A second lifetime of

7.6 ( 0.3 ms can be extracted; it corresponds to the mean lifetime

observed for the ligand-centered luminescence (see the text).

and is more red-shifted upon complexation (Table 2). All the

heteronuclear complexes present an absorption maximum at

32 050 cm^-1, including the europium compound, the π f π*

transition of which does not display a larger red shift as observed

in the solid state (Vide supra). The excitation spectrum of

[EuZn(L²)₃]⁵⁺10^-4M in acetonitrile displays a single, sym-

metrical, and relatively narrow band (fwhh ) 25 nm) assigned

to the ligand (Figure F3 in the supporting information). The

luminescent properties of this solution are similar to those of

the solid state samples: the overall shape of the spectrum and

5

7

temperature, one single D₀f F₀emission band appears at

17 228 (8a) and 17 226 (11a) cm^-1(Figure F5 in the supporting

information), the overall shape of the emission spectra (Figure

F4 in the supporting information) and the relative intensities of

the ⁵D₀f ⁷F_J(J ) 0-4) transitions (Table SV in the supporting

information) remain the same while the Eu(⁵D₀) lifetimes τ at

10 and 295 K in the Gd complex are identical to the ones

7

measured for compound 10 (Table 7). The F₁(E) splitting

5

7

the relative intensities of the D₀f F_Jtransitions (Table SV

in the supporting information) match those of compounds 10

and 13. Although the widths of the emission bands prevent a

site symmetry analysis, there is no doubt that the pseudotrigonal

arrangement of the triple helicate is maintained in solution, the

amounts to 42 cm^-1for 8a and to 63 cm^-1for 11a, compared

to an average of 55 cm^-1for 10 at room temperature. The

increase of this splitting with the atomic number reflects the

change in ionic radius, henceforth in Ln-N and -O distances,

since shorter bond lengths generate stronger ligand-field effects.

From these data, we conclude that [LnZn(L²)₃]⁵⁺complexes

(Ln ) La-Gd) behave essentially as an isostructural series of

compounds in which the lanthanide ions are well protected from

external interactions. The Eu(⁵D₀) lifetimes are long, 2.5 ms

at 10 K, in spite of the relative complexity of the ligand, which

points to fairly rigid coordination sites. The ligand-to-europium

energy transfer processes are rapid as a result of the good match

between the ligand ¹ππ* and the Eu(⁵L₆,⁵D₃) levels on one hand

7

⁵D₀f F₁transition appearing as one band with a shoulder

5

7

and the D₀f F₂transition as a doublet. The total emission

intensity of a 10^-3M solution decreases by 40% upon saturation

with oxygen (Figure F7 in the supporting information), which

proves the role played by the ligand triplet state in the ligand-

to-europium energy transfer, since dioxygen is known to quench

the ³ππ* state. The Eu(⁵D₀) lifetime is longer than in the solid

state and increases upon dilution to reach 2.9 ms at 10^-4

M

(Table 7) where the concentration quenching becomes less

important. The replacement of one benzimidazole unit by a

carboxamide group leads to [EuZn(L²)₃]⁵⁺solutions having a

quantum yield 10³times larger than that of [EuZn(L¹)₃]⁵⁺

solutions (Table 8), despite the rather similar photophysical

properties of ligands L¹and L². The only differences lie in a

blue shift (ca. 2000 cm^-1) of the π f π* transition, and in

³ππ* states extending at slightly lower energy for compounds

with L²(shoulders around 18 000 cm^-1, Table 5); this may

promote a better energy transfer to the metal ion since the energy

3

and the ligand ππ* and the Eu(⁵D₁,⁵D₀) levels on the other

hand, leading to Eu(⁵D₀) lifetimes almost independent of the

excitation mode. These lifetimes are somewhat longer than

those measured with L¹, 2.22 ms for the homonuclear EuEu

complex and 2.30 ms for the heteronuclear EuZn compound,²³

an effect consecutive to the coordination of the carboxamide

5

group. In addition, the lifetime of the D₀level and the total

luminescence intensity (Table S5 in the supporting information)

do not decrease much when the temperature is increased to 295

K. This points to a reduced influence of vibrational modes on

the radiationless decay processes, contrary to what was reported

for [Eu(L³)₃]³⁺and [Eu₂(L⁵)₃]⁶⁺for which τ decreases to ca.

0.3 ms at 295 K^41,46and for the complexes with L¹which are

almost nonluminescent at room temperature.²³Other interesting

temperature effects occur: the Eu- and Tb-containing hetero-

nuclear complexes 10 and 12 show the expected increase in

luminescence intensity upon lowering the temperature from 295

5

difference with D₀is reduced. Moreover, radiationless deex-

citation processes are less efficient in compounds with L², as

indicated by the long and temperature-independent Eu(⁵D₀)

lifetimes, a consequence of the more rigid metal ion environment

provided by the carboxamide coordination. In fact, the total

emission intensity, obtained by multiplying the quantum yield

(61) Blasse, G. Prog. Solid State Chem. 1988, 18, 79-171.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6693

Table 8. Quantum Yields at 295 K of Anhydrous Acetonitrile Solutions Relative to [Eu(terpy)₃]³⁺(terpy ) 2,2′:6′,2′′-Terpyridine)

compd

concn (mol/dm³) added H₂O concn (mol/dm³)

λ

_exc(nm) ꢀ(λ_exc) (M^-1cm^-1

)

rel quantum yield rel emission intensity^a

[Eu(terpy)₃]³⁺

10^-3

10^-4

10^-3

10^-4

10^-3

10^-4

0

390

380

442

414

395

380

71

119

530

1960

555

2750

1.00

0.87

0.27

1.00

0.87

0.18

0.93

0

0.93

0

0.27

0.60

1.00^b

[EuZn(L¹)₃]⁵⁺

[EuZn(L²)₃]⁵⁺

=1 × 10^-4

=3 × 10^-4

0.13

=7 × 10^-4

=5 × 10^-3

1.02

0.13

0.29

1.02

6.7

0.93

^aProduct of the relative quantum yield multiplied by the ratio of the molar absorption coefficients ꢀ(L)/ꢀ(terpy) at the excitation wavelength used

for measuring the emission of the heteronuclear complexes. ^bTaken as reference for 10^-4M solutions.

by the efficiency of light absorption (ratio of the molar

absorption coefficients), is 6-7 times larger for the hetero-

nuclear complex [EuZn(L²)₃]⁵⁺compared with [Eu(terpy)₃]³⁺

solutions. The efficient protection of the Eu(III) ion from

solvent interaction is demonstrated by the addition of water to

the solutions: the quantum yields remain constant up to a water

concentration of 0.93 M, representing a 10³-10⁴excess of H₂O,

while the same addition to [Eu(terpy)₃]³⁺solutions results in a

63% reduction of the luminescence intensity and in a total loss

of luminescence for [EuZn(L¹)₃]⁵⁺

.

Structure of the Complexes [LnZn(L²)₃]⁵⁺(Ln ) La, 8;

Ce, 14; Pr, 15; Nd, 9; Sm, 16; Eu, 10; Tm, 17; Yb, 18; Lu,

19; Y, 20) in Acetonitrile. The ¹H NMR spectra of the

diamagnetic complexes [LnZn(L²)₃]⁵⁺(Ln ) La, Lu, Y) show

the quantitative formation of the heterodinuclear complexes in

solution for the complete lanthanide series and display 23 signals

arising from three equivalent ligands L²on the NMR time scale

and compatible with C₃or C_3Vsymmetry. The observed AB

spin systems for the diastereotopic methylene protons H^7,8

,

H^15,16, and H^17,18(Table 3) preclude the existence of mirror

planes⁴⁹leading to a C₃head-to-head arrangement of the ligands

in [LnZn(L²)₃]⁵⁺. The intrastrand NOEs observed for Me²-

H³, Me³-H¹², and H¹⁴-H^17,18(Figure 9) confirm that both the

bidentate and tridentate binding units adopt cisoid conformation

consecutive to their coordination to the metal ions while the

NOEs experienced by the protons close to the methylene spacer

(H⁷-H⁶, H⁷-H¹⁰, H⁸-H⁵, H⁸-H⁹, and H⁶-H⁹, Figure 9) are

typical of the helical conformation of the ligands L²previously

described for L¹in [LnZn(L¹)₃]⁵⁺(Ln ) La, Lu, Y).²³The

intramolecular closely packed triple-helical arrangement of the

ligands in [LnZn(L²)₃]⁵⁺(Ln ) La, Lu, Y) is evidenced by (i)

the significant upfield shift experienced by H⁶(∆δ ) 2.29 ppm,

Ln ) La) and H⁹(∆δ ) 1.87 ppm, Ln ) La) which lie in the

shielding region of the benzimidazole groups of the helically

twisted ligands^23,47and (ii) the weak interstrand NOEs observed

for Me⁴-Me³, H¹¹-H⁴, Me³-H⁵, and H¹⁰-Me²as previously

reported for [LnZn(L¹)₃]⁵⁺(Figure 9).²³A careful comparison

of the ¹H NMR spectra of [LnZn(L²)₃]⁵⁺(Ln ) La, Lu, Y, Table

3) shows that the contraction of the lanthanide radii only slightly

affects the structure of the heterodinuclear complexes except

for a small upfield shift of H⁹(∆δ ) 0.30 ppm for Y(III), 0.38

ppm for Lu(III)) associated with a small distortion of the

coordination sphere around Ln(III).²³

Figure 9. Selected (a) intrastrand and (b) interstrand NOEs observed

for [LnZn(L²)₃]⁵⁺(Ln ) La, Ce, Pr, Nd, Sm, Eu, Yb, Lu, Y) in CD₃-

CN. Contact distances were calculated from the crystal structure of

[EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13).

netic complexes [LaZn(L²)₃]⁵⁺for Ce-Nd, [YZn(L²)₃]⁵⁺for

Sm-Dy, and [LuZn(L²)₃]⁵⁺for Ho-Yb)⁶²and the isotropic

paramagnetic contribution δ_ij^isowhich has two origins, the Fermi

c 63

contact shift (δ_ij) and the pseudocontact (or dipolar) shift

(δ_ij^pc):⁶⁴

pc

δ_ij^iso) δ_ij^exp- δ_i^dia) δ_ij^c+ δ_ij

(9)

c

δ_ijresults from through-bond Fermi contact interactions and

is given by eq 10 where A_iis the hyperfine interaction parameter,

B₀the applied magnetic induction, and S_{z j}the expectation value

A_i‚ S

p‚γ‚B₀

c

δ_ij

)

^{z j}) F_i‚ S_{z j}

(10)

Separation of Contact and Pseudocontact Contributions

to the Paramagnetic Shifts in [LnZn(L²)₃]⁵⁺(Ln ) Ce, 14;

Pr, 15; Nd, 9; Sm, 16; Eu, 10; Tm, 17; Yb, 18). Further

structural and electronic information may be gained from the

use of paramagnetic lanthanide metal ions as shift reagents.⁴⁸

The binding of the paramagnetic lanthanide ion j in [LnZn-

(L²)₃]⁵⁺induces a frequency shift in the NMR signal of nucleus

i given by the sum of the diamagnetic contribution δ_i^dia(taken

from the chemical shift measured for the isostructural diamag-

of S_z.⁶³The pseudocontact shift δ_ij^pcdepends on the geometric

position of the nucleus i and results from the residual isotropic

(62) Kemple, M. D.; Ray, B. D.; Lipkowitz, K. B.; Prendergast, F. G.;

Rao, B. D. N. J. Am. Chem. Soc. 1988, 110, 8275-8287.

(63) Golding, R. M.; Halton, M. P. Aust. J. Chem. 1972, 25, 2577-

2581.

(64) Bleaney, B. J. J. Magn. Reson. 1972, 8, 91-100. Reuben, J.;

Rigavish, G. A. J. Magn. Reson. 1980, 39, 421-430.

+

6694 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

contribution associated with the anisotropic part of the molecular

magnetic susceptibility tensor given by eq 11 for axial com-

dicarboxylate)₃]^3-(F_i) 0.009-0.013),⁶⁵but similar to those

reported for the analogous triple-helical complexes [LnZn-

(L¹)₃]⁵⁺, demonstrating a significant spin delocalization onto

the tridentate unit²³induced by the coordination of the hetero-

cyclic N donor atoms of ligands L¹and L².

1 - 3 cos²θ_i

1

pc

δ_ij

)

(øh - ø^zz)_j‚

) G_i‚C_j(11)

r_i³

(

)

2Npγ

However, the agreement factors per Ln(III) (AF_j) are large

and not satisfying even though we have not taken Sm(III) into

account, since this ion induces negligible contact and pseudo-

contact shifts.^63,64Kemple et al.⁶²have shown that Bleaney’s

coefficients C_j⁶⁴are poor approximations for the experimental

molecular magnetic susceptibility tensors (ø) which must be

measured if a better separation of contact and pseudocontact

contributions is desired. We have therefore complied to the

following procedure: (1) Internal spherical coordinates of the

protons i were calculated from the crystal structure of [EuZn-

(L²)₃]⁵⁺(13) with the Eu atom at the origin and the z axis

corresponding to the pseudo-C₃axis passing through the metal

ions. According to the NMR data, axial C₃symmetry is required

in solution and C₃-averaged values of θ_iand r_iare used for the

calculation of the structural factors (1 - 3 cos²θ_i)/r_i³(Table

SIII in the supporting information). (2) Contact contributions

were limited to protons remote from Ln(III) by at most five

bonds, leading to only five significant contributions for H⁹and

H^11-14in agreement with the calculated F_ivalues (Table 9).

(3) The experimental paramagnetic isotropic shift of the aromatic

protons Hⁿ(n ) 1-6, 9-14) were fitted using multilinear least-

squares methods (12 × 6 fits)⁶²to an expression analogous to

eq 12 but containing only six parameters for each lanthanide j

(eq 14). êø_j^zzis the axial anisotropic susceptibility parameter

plexes.⁶²R_iand θ_iare the internal polar coordinates of the

nucleus i with respect to the ligand field axes, ø^zzis the axial

component of the magnetic susceptibility tensor of a sample

containing N paramagnetic ions, and øh ) (1/3)‚Tr(ø). Equations

10 and 11 show that the isotropic paramagnetic shift can be

expressed as the sum of two contributions constituted by the

product of two terms depending only on the lanthanide j and

the nucleus i considered (eq 12).⁶⁵

δ_ij^iso) F_i‚ S_{z j}+ G_i‚C_j

(12)

Theoretical values of S_{z j}and C_jhave been reported^63,64

which allow the straightforward separation of contact and

pseudocontact contributions using eq 12 and multilinear least-

squares analysis⁶⁵provided that (i) the system is axial (pos-

sessing at least a C₃axis),⁶²(ii) the various NMR signals of the

nucleus i may be safely assigned for more than two paramag-

netic Ln(III) ions, and (iii) the complexes [LnZn(L²)₃]⁵⁺are

isostructural. These conditions are fulfilled for Ln ) Ce, Pr,

Nd, Sm, Eu, Tm, and Yb since they possess a sufficiently short

electronic relaxation time⁴⁸to allow reliable 2D-COSY and

NOEs to be detected, leading to the dependable assignment of

1

the H NMR signals given in Table 3. Moreover, the hetero-

nuclear complexes possess a C₃axis in solution and are almost

isostructural as demonstrated by the similarity of the ¹H NMR

spectra for the diamagnetic complexes [LnZn(L²)₃]⁵⁺(Ln ) La,

Lu, Y) which cover the complete range of possible ionic radii

in the lanthanide series.⁶Least-squares values for F_iand G_i,

agreement factors for each proton (AF_i, eq 13)⁶⁵and for each

Ln(III) ion (AF_j, eq 13),⁶²correlation coefficients for S_{z j}versus

1 - 3 cos²θ_i

c

δ_ij^iso) ê‚ø_j^zz‚

+

δ_ij

(14)

∑

r_i³

(

)

i ) 9,11-14

c

of the lanthanide j in [LnZn(L²)₃]^{5+ 62}

significant contact contributions (Tables SVII and SVIII in the

supporting information).

,

and δ_ijare the five

1/2

calc 2

(δ ^exp- δ_ij

)

(δ ^exp- δ_ij

)

The agreement factors AF_jare significantly improved with

this method (0.007 < AF_j< 0.035; Table SVII in the supporting

information) and can be compared with those resulting from a

similar mathematical treatment applied to [Ln(texaphyrin)-

(NO₃)₂] (0.06 < AF_j< 0.26).⁶⁶Average F_iparameters may

∑

i

ij

j

AF_i)

AF_j)

exp 2

(δ

)

(δ

)

[

]

[

]

∑

i

ij

j

c

be calculated from the contact contributions (δ_ij) given in Table

(13)

SVII in the supporting information (excluding Sm(III)) and using

the spin expectation values tabulated by Golding and Halton.⁶³

They amount to F_i) -0.33 (H⁹), 0.18 (H¹¹), 0.11 (H¹²), 0.17

(H¹³), and 0.10 (H¹⁴), in qualitatively good agreement with the

values reported in Table 9, which confirms the unusual large

spin delocalization onto the coordinated tridentate binding units.

Theoretical calculations⁶⁴give estimates for the relative value

of the axial term (êø_j^zz) of the magnetic susceptibility tensor

for various Ln(III) with respect to Pr(III). Table 10 gives a

comparison of the theoretical ratios⁶⁴with the experimental

values obtained for [LnZn(L²)₃]⁵⁺(Ln ) Ce, Pr, Nd, Eu, Tm,

Yb) and using eq 11. Precise agreement is not expected due to

(i) insufficient knowledge of the ion crystal field parameters,

(ii) mixing of excited wave functions into the J manifold, (iii)

nonideal colinearity of the principal axis of the magnetic

susceptibility tensor and the molecular pseudo-C₃axis, and (iv)

possible distortions between the crystal structure and the

structure in solution,⁶²but we observe a qualitatively good

agreement for the larger Ln(III) ions (Ln ) Ce, Nd, Eu) and a

significant deviation for Tm and Yb. These observations

c

pc

δ_ij(σ^c) and C_jversus δ_ij(σ^pc),⁶⁵and calculated corrected⁶⁵

contact and pseudocontact contributions are given in Table 9.

The diastereotopic methylene protons have been excluded from

the fitting process since no reliable attribution is possible for

the separated signals of the AB spin systems.

The correlation coefficients and the agreement factors AF_i

reported in Table 9 are satisfying (0.07 < AF_i< 0.20) and can

be compared with those found for similar mathematical treat-

ments applied to [LnZn(L¹)₃]⁵⁺(0.08 < AF_i< 0.27)²³and [Ln-

(pyridine-2,6-dicarboxylate)₃]^3-(0.04 < AF_i< 0.27).⁶⁵As

expected,^62,65the contact contributions are negligible for spin

delocalization on more than five bonds, leading to significant

F_ivalues for H⁹and H¹¹to H¹⁴only, which implies that

Ln(III) is coordinated to the tridentate binding unit as found in

the crystal structure of 13. This is confirmed by the absolute

G_ivalues which are maximum for H⁶, H⁹, and Me⁴and which

point to short Ln-H distances (r_i) and large pseudocontact

contributions (eq 11). The F_ivalues of the pyridine protons

H¹²-H¹⁴are larger than those found for [Ln(pyridine-2,6-

(65) Reilley, C. N.; Good, B. W.; Desreux, J. F. Anal. Chem. 1975, 47,

2110-2116.

(66) Lisowski, J.; Sessler, J. L.; Lynch, V.; Mody, T. D. J. Am. Chem.

Soc. 1995, 117, 2273-2285.

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6695

1

Table 9. Corrected Computed Values for Contact (δ^c)^aand Pseudocontact (δ^pc)^aContributions to the Experimental Isotropic Paramagnetic H

NMR Shifts for [LnZn(L²)₃]⁵⁺Using Eq 12⁶⁵

Bidentate Binding Unit

Me¹

Me²

H¹

H²

H³

H⁴

H⁵

H⁶

AF_j^d

F_i^b

-0.015(8)

0.020(2)

0.14

-0.02(1)

0.033(3)

0.13

-0.03(1)

0.038(4)

0.15

-0.017(9)

0.022(2)

0.14

-0.02(1)

0.034(4)

0.15

-0.02(1)

0.038(2)

0.09

-0.019(6)

0.034(2)

0.07

-0.14(7)

0.17(2)

0.15

G_i^c

AF_i^d

σ^{c e}

-0.993

0.997

-0.03

-0.23

-0.07

-0.33

-0.09

-0.10

0.00

-0.05

0.14

0.07

0.12

-0.994

0.998

-0.04

-0.36

-0.10

-0.53

-0.13

-0.17

0.01

-0.09

0.23

0.11

0.20

-0.993

0.997

-0.05

-0.45

-0.13

-0.64

-0.17

-0.20

0.01

-0.10

0.27

0.13

0.24

-0.995

0.997

-0.03

-0.25

-0.08

-0.36

-0.09

-0.11

0.00

-0.05

0.16

0.08

0.14

-0.993

0.997

-0.05

-0.39

-0.11

-0.56

-0.14

-0.18

0.01

-0.10

0.23

0.11

0.20

-0.997

0.999

-0.03

-0.36

-0.09

-0.56

-0.12

-0.18

0.00

-0.08

0.22

0.14

0.19

-0.998

0.999

-0.02

-0.29

-0.07

-0.48

-0.09

-0.16

0.00

-0.07

0.18

0.13

0.15

-0.994

0.997

-0.26

-2.06

-0.66

-2.99

-0.80

-0.92

0.03

-0.41

1.33

0.62

1.16

σ^{pc e}

δ^c(CeZn)^f

δ^pc(CeZn)

δ^c(PrZn)

δ^pc(PrZn)

δ^c(NdZn)

δ^pc(NdZn)

δ^c(SmZn)

δ^pc(SmZn)

δ^c(EuZn)

δ^pc(EuZn)

δ^c(TmZn)

δ^pc(TmZn)

δ^c(YbZn)

δ^pc(YbZn)

0.44

0.33

0.18

0.67

0.14

0.02

0.11

1.05

0.04

0.39

1.71

0.06

0.65

1.97

0.07

0.75

1.16

0.04

0.43

1.75

0.06

0.66

2.00

0.06

0.77

1.80

0.05

0.72

9.26

0.34

3.56

Tridentate Binding Unit

Me³

Me⁴

Me⁵

H⁹

H¹⁰

H¹¹

H¹²

H¹³

H¹⁴

F_i^b

0.04(3)

-0.081(7)

0.14

0.990

-0.997

0.07

0.87

0.19

1.40

0.23

0.44

-0.01

0.19

-0.35

-0.27

-0.32

-4.19

-0.10

-1.71

-0.07(7)

0.20(2)

0.14

-0.931

0.992

-0.11

-1.87

-0.31

-3.16

0.04

0.11

0.01

-0.38

0.98

0.05(2)

-0.09(4)

0.13

0.994

-0.992

0.06

0.30

0.22

0.63

0.27

0.20

0.00

0.04

-0.49

-0.14

-0.38

-1.95

-0.15

-1.02

-0.29(9)

0.49(5)

0.14

-0.04(2)

0.033(5)

0.20

0.123(6)

0.005(1)

0.09

0.997

0.996

0.14

0.35(8)

-0.17(2)

0.15

0.996

-0.997

0.61

1.96

1.57

2.89

1.89

0.88

-0.07

0.39

-3.37

-0.63

-2.82

-8.99

-0.79

-3.31

0.17(6)

-0.12(2)

0.16

0.994

-0.996

0.31

1.43

0.76

1.98

0.93

0.62

-0.03

0.27

-1.57

-0.41

-1.37

-6.23

-0.35

-2.07

0.22(4)

-0.07(1)

0.15

0.996

-0.997

0.39

0.77

0.96

1.11

1.20

0.35

-0.05

0.19

-2.12

-0.25

-1.76

-3.50

-0.48

-1.26

G_i^c

AF_i^d

σ^{c e}

-0.993

0.997

-0.50

-5.44

-1.32

-8.22

-1.61

-2.53

0.05

-1.02

2.68

1.69

2.37

-0.991

0.993

-0.09

-0.44

-0.22

-0.62

-0.23

-0.17

0.01

-0.07

0.39

0.11

0.34

σ^{pc e}

δ^c(CeZn)^f

δ^pc(CeZn)

δ^c(PrZn)

δ^pc(PrZn)

δ^c(NdZn)

δ^pc(NdZn)

δ^c(SmZn)

δ^pc(SmZn)

δ^c(EuZn)

δ^pc(EuZn)

δ^c(TmZn)

δ^pc(TmZn)

δ^c(YbZn)

δ^pc(YbZn)

-0.04

0.48

-0.08

0.65

-0.03

-0.01

-1.27

0.02

-0.96

0.27

1.00

0.60

10.58

0.19

4.33

25.69

0.70

9.98

1.69

0.09

0.62

-0.37

0.14

^aCorrected contact and pseudocontact contributions are given in parts per million with [LnZn(L²)₃]⁵⁺(Ln ) La, Y, Lu) as diamagnetic references

(see the text). ^bContact term according to eq 12. ^cPseudocontact term according to eq 12. ^dAgreement factors as defined in eq 13. ^eCorrelation

coefficients for contact and pseudocontact contributions (see the text).⁶⁵^fδ^c(LnZn) is the corrected⁶⁵contact contribution in parts per million for the

complex [LnZn(L²)₃]⁵⁺, and δ^pcis the corresponding pseudocontact contribution.

Zn(II) ([Zn(L²)₃]²⁺, [Zn(L²)₂]²⁺, [Zn₂(L²)₃]⁴⁺, and [Zn₂(L²)₂]⁴⁺

)

Table 10. Comparison of Theoretical and Experimental Values of

the Principal Value of The Magnetic Susceptibility (ø) for

contrasts with the formation of only two well-defined complexes

with the analogous ligand L¹: [Zn(L¹)₂]²⁺in which Zn(II) is

pseudooctahedrally coordinated by two tridentate binding units,

and the head-to-head C₂double helicate [Zn₂(L¹)₂]⁴⁺(structure

I, Figure 3).²³Moreover, the analogous complex [Zn₂(L²)₂]⁴⁺

adopts the alternating head-to-tail structure II (Figure 3) which

demonstrates the different coordination behaviors displayed by

ligands L¹and L². This is confirmed by the intricate mixtures

of homonuclear complexes which results from the reaction of

L²with La(III) while only one thermodynamically assembled

head-to-tail triple helicate [La₂(L¹)₃]⁶⁺is obtained under similar

conditions with L¹.²³Such features typically occur when the

stereochemical requirements of the metal ion do not match the

ligand binding capabilities.^33,67For L¹, the tridentate and the

bidentate binding units only differ in their denticity (the ligating

imine nitrogen atoms and the chelate bite angle being the same),

resulting in (i) a significant and similar affinity of both

[LnZn(L²)₃]^{5+ a}

theory^62,64experiment

CeZn

PrZn

NdZn

0.57

1.00

0.38

0.63

1.00

0.47

EuZn

TmZn

YbZn

-0.36

-4.82

-2.00

-0.54

-2.86

-1.07

^aRatios relative to Pr(III) are given.

suggest that the crystal structure of [EuZn(L²)₃]⁵⁺(13) is a

satisfying model for [LnZn(L²)₃]⁵⁺in solution for lanthanide

ions of similar sizes, but a contraction of the structure occurs

to accommodate the smaller Ln(III) ions, as previously deduced

from the upfield shift of H⁶and H⁹in the ¹H NMR spectrum of

[LuZn(L²)₃]⁵⁺

.

Discussion

The coordination behavior of L²with either d- or f-block

metal ions is very complicated, leading to intricate mixtures of

homonuclear complexes often difficult to characterize. The

formation of four different complexes upon reaction of L²with

(67) Constable, E. C. Tetrahedron 1992, 48, 10013-10059. Kra¨mer,

R.; Lehn, J.-M.; Marquis-Rigault, A. Proc. Natl. Acad. Sci. U.S.A. 1993,

90, 5394-5398.

+

6696 J. Am. Chem. Soc., Vol. 118, No. 28, 1996

Piguet et al.

Chart 2

between 47° and 54°, indicating a flattening along the pseudo-

C₃axis slightly more pronounced for the facial plane of the

prism defined by the three oxygen atoms (Table SIV in the

supporting information).

The structure is averaged in acetonitrile solution, in which

axial C₃triple-helical complexes [LnZn(L²)₃]⁵⁺are observed

on the NMR time scale with Zn(II) occupying the capping

position. As recently discussed,^66,68the separation of contact

and pseudocontact contributions to the isotropic paramagnetic

shifts for Ln(III) complexes is not an easy task, but crucial

structural and electronic information results from such analysis.

For the axial complexes [LnZn(L²)₃]⁵⁺, substantial simplifica-

tions occur,^62,65and the method proposed by Reilley and co-

workers⁶⁵(eq 12) gives an approximate separation of contact

and pseudocontact terms suitable for the dependable localization

of Ln(III) ions in the tridentate binding units and the evaluation

of spin delocalization onto the ligands. This first approach offers

two advantages: (i) no prior knowledge of the structure other

than axial symmetry around Ln(III) is needed and (ii) it allows

a straightforward prediction (using eq 12 and the theoretical

values of S_{z j}⁶³and C_j⁶⁴) of the paramagnetic shifts for Ln(III)

ions not included in the fitting process. For instance, the

assignment of the signals of the protons in [TmZn(L²)₃]⁵⁺was

confirmed using eq 12 and the parameters obtained from the

fit with Ln ) Ce, Pr, Nd, Sm, Eu, and Yb. However, we were

unable to obtain dependable assignment of the 23 signals of

[TbZn(L²)₃]⁵⁺with this technique since the large errors associ-

ated with the calculated parameters F_iand G_i(10-30%, Table

9) preclude a suitable prediction for Tb(III), which possesses a

large spin expectation value and a large Bleaney’s coefficient

compared to those used in the fitting process.⁶⁵A more accurate

separation of contact and pseudocontact contributions is obtained

using eq 14 and the method of Kemple and co-workers.⁶²It

confirms that the triple helical structure found in the solid state

for 13 is maintained in solution for the complete lanthanide

series [LnZn(L²)₃]⁵⁺(Ln ) La-Lu) with a minor contraction

for the heavy Ln(III) ions (Ln ) Tm, Yb, and Lu). Significant

contact contributions are observed for the aromatic protons

bound to the tridentate binding units, suggesting an unusually

large spin delocalization onto the ligands as reported previously

coordinating units for the same metal ion, (ii) the formation of

stable and well-defined homonuclear complexes with either d-

or f-block metal ions, and (iii) a limited thermodynamic

selectivity for the formation of heterodinuclear d-f complexes.

Only 45% of the heterodinuclear complex [LaZn(L¹)₃]⁵⁺is

formed in acetonitrile when a 10^-4M solution of L¹reacts with

an equimolar mixture of La(III) and Zn(II).²³The replacement

of the terminal benzimidazole group of L¹by an N,N-

diethylcarbamoyl group produces a segmental ligand L²with

two different binding units coded for one particular metal ion.²⁷

This leads to an opposite coordination behavior with the

formation of intricate mixtures of homonuclear complexes and

the quantitative formation of the heterodinuclear C₃triple

helicate [LnZn(L²)₃]⁵⁺for a total ligand concentration of 10^-4

M.

Although the assembly processes are strongly affected in

going from L¹to L², the heterodinuclear complexes [LnZn-

(Lⁱ)₃]⁵⁺(i ) 1, 2) display similar triple helical structures in

solution with the three ligands wrapped about the C₃axis passing

through the metal ions. The three bidentate units are pseu-

dooctahedrally coordinated to Zn(II), producing a noncovalent

tripod which organizes the strands for the coordination of

Ln(III). This thermodynamic control of the final structure

prevents the facial-meridional isomerization and leads to a pure

facial pseudotricapped trigonal prismatic arrangement of the

tridentate binding units around Ln(III). In contrast with the

systematic isolation of amorphous powders for the complexes

of the lipophilic ligand L¹,²³crystalline materials are obtained

for the complexes [LnZn(L²)₃](ClO₄)₅(Ln ) La-Lu), which

allowed us to determine the crystal structure of the heterodi-

nuclear complex [EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13).

The triple-helical structure in the latter is slightly distorted by

intermolecular packing interactions mainly affecting the coor-

dination sphere around Zn(II). A detailed geometrical analysis

of the coordination site occupied by Eu(III) was performed using

the previously described²¹angles φ, θ_i, and ω_i(Chart 2, Table

SIV in the supporting information; Rⁱis the sum of the vectors

defining the two tripods of the trigonal prism: R¹) Eu-N(4a)

+ Eu-N(4b) + Eu-N(4c) and R²) Eu-O(1a) + Eu-O(1b)

+ Eu-O(1c)²¹). The donor atoms O(1a), O(1b), and O(1c) of

the carboxamide groups do not significantly affect the trigonal

prismatic arrangement of the donor atoms of the tridentate units,

resulting in structurally similar, slightly distorted tricapped

trigonal prisms around Eu(III) for both [EuZn(L²)₃]⁵⁺and [Eu-

for [LnZn(L¹)₃]^{5+ 23}

and in good agreement with the large

,

nephelauxetic parameter associated with the heterocyclic nitro-

gen donor atoms coordinated to Eu(III) and determined from

the ⁵D₀f ⁷F₀excitation spectrum of [EuZn(L²)₃]^{5+ 23,64}

Despite

.

the improved agreement between experimental and calculated

paramagnetic isotropic shifts, this second approach suffers from

severe limitations: (i) prior knowledge of the molecular structure

of the complexes is necessary and (ii) the prediction of

paramagnetic isotropic shifts for the other Ln(III) ions is difficult

since the magnetic susceptibility parameters (êø^zz) and the

contact contributions are obtained by a least-squares fitting for

each Ln(III) studied and no reliable extrapolation is possible.^62,68

Analysis of the Eu(⁵D₀) luminescence in the solid samples

shows the site symmetry of the lanthanide ion surprisingly less

distorted with respect to a trigonal C₃symmetry in the

heteronuclear complexes with L²despite the replacement of one

imine coordinating group in L¹by an amide moiety, generating

a nonhomogeneous coordination set of atoms in the tridentate

binding unit. This relatively important structural difference

generates few variations in the photophysical properties of the

ligand L²with respect to L¹, but large changes in the luminescent

properties of the Eu-containing heteronuclear complex [EuZn-

(L²)₃]⁵⁺. Although such changes were expected on the basis

(L³)₃]^{3+ 21}as reflected in the similar ⁷F₁(E) crystal field splittings

,

(Vide supra). The angle φ amounts to 178° for [EuZn(L²)₃]⁵⁺

,

(68) Lisowski, J.; Sessler, J. L.; Mody, T. D. Inorg. Chem. 1995, 34,

4336-4342. Forsberg, J. H.; Delaney, R. M.; Zhao, Q.; Harakas, G.;

Chandran, R. Inorg. Chem. 1995, 34, 3705-3715.

which indicates that the two distal tripods are only slightly bent

(ideal trigonal prism: φ ) 180°), while the θ_iangles range

+

Lanthanide Podates with Predetermined Properties

J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6697

of literature data which reveal the beneficial influence of

carboxamide groups,^26athey exemplify the fact that the observed

emission results from second-order effects (f-f transitions are

spin forbidden) and therefore that a small shift in the ligand

electronic levels considerably influences the “resonant” energy

units to a d-block metal ion. The spectroscopically inert

Zn(II) is ideally suited for this purpose,²³leading to a nonco-

valent tripod in [LnZn(L²)₃]⁵⁺which controls the entropically-

driven^9,41formation of facial pseudotricapped trigonal prismatic

lanthanide podates. This approach is relevant to biological

recognition and regulation processes in which the tailored active

site results from the specific preorganization of the polypeptidic

backbone using various assembly processes based on nonco-

valent interactions such as hydrogen bonds, metal complexation,

or van der Waals interactions.⁷³The pseudotricapped trigonal

prismatic lanthanide building blocks in the podates [LnZn-

(L²)₃]⁵⁺are structurally similar to those found in [Eu(L³)₃]³⁺

and [LnZn(L¹)₃]⁵⁺, but the replacement of the terminal benz-

imidazole groups by carboxamide substituents in L²strongly

modifies the electronic properties of the tridentate binding units,

leading to (i) an increased thermodynamic stability, (ii) a

stronger luminescence of the Eu helicate [EuZn(L²)₃]⁵⁺(10³

more luminescent than [EuZn(L¹)₃]⁵⁺), and (iii) an improved

resistance toward hydrolysis. These points are crucial for the

development of tailored luminescent probes and bioanalytical

sensors,^1-3but the new lanthanide podates [LnZn(L²)₃]⁵⁺offer

further fascinating possibilities for the preparation of molecular

devices with predetermined physicochemical properties resulting

from the judicious choice of spectroscopically and magnetically

active d-block metal ions associated with Ln(III).^24,26

3

transfer from the ππ* state to the Eu(⁵D₀) level. In addition,

the latter may be facilitated by the presence of the ν(CdO)

vibration since the energy difference between the maximum of

3

the emission band originating from the ππ* state (ca. 19 000

cm^-1in the Gd-containing complex) and the ⁵D₀level at 17 220

cm^-1corresponds roughly to one ν(CdO) phonon: for instance,

an efficient ν(CdO) assisted radiationless energy transfer

between Eu(⁵D₁) and Eu(⁵D₀) has been evidenced in europium

perchlorate solutions in N,N-dimethylformamide.⁶⁹Finally, the

carboxamide group provides two additional features: (i) a more

rigid Ln(III) environment (LnsO distances are shorter than

LnsN bond lengths) favoring radiative deexcitation processes

and therefore a longer Eu(⁵D₀) lifetime and (ii) a possible blue

shift of the LMCT state, with respect to ligand L¹, as indicated

by the temperature-independent Eu(⁵D₀) lifetime, leading to

strong luminescence at room temperature, contrary to [EuZn-

(L¹)₃]⁵⁺which becomes nonluminescent when the temperature

is raised. A similar shift has been invoked to explain the

luminescence enhancement of Eu- and Tb-containing calix[4]-

arene complexes upon substitution of the phenol groups by

alkylcarbamoyl functions,^26aand of calix[8]arene⁷⁰and substi-

tuted benzimidazole-pyridine⁷¹complexes upon replacement

of the p-tert-butyl groups⁷⁰and the (4-X-phenyl)pyridine moiety

by electron-attracting nitro substituents. Finally, the resistance

of the complexes toward hydrolysis, testified by the insensitivity

of the luminescence total intensity with respect to water addition

makes L²a promising step toward the synthesis of water-soluble

and luminescent lanthanide triple helicates.

Acknowledgment. We gratefully acknowledge Dr. Elisabeth

Rivara-Minten, Ms. Ve´ronique Foiret, and Mr. Bernard Bocquet

for their technical assistance. C.P. thanks the Werner Founda-

tion for a fellowship, and J.-C.G.B. thanks the Fondation

Herbette (Lausanne) for the gift of spectroscopic equipment.

This work is supported through grants from the Swiss National

Science Foundation.

Conclusions

Supporting Information Available: Tables of atomic

coordinates, bond distances, and bond angles for L²and

[EuZn(L²)₃](ClO₄)(CF₃SO₃)₄(CH₃CN)₄(13), tables listing spheri-

cal coordinates for the protons in 13, structural data obtained

by a geometrical analysis of the europium coordination sphere,

table of elemental analyses for complexes 7-12, tables listing

the luminescence intensities of complexes 8-13 under various

experimental conditions, relative intensities of Eu(⁵D₀f⁷F_J) and

Tb(⁵D₄f⁷F_J) transitions, tables listing the computed values of

contact and pseudocontact contributions for aromatic protons

and axial anisotropic magnetic susceptibility parameters for

[LnZn(L²)₃]⁵⁺using eq 14, and Figures F1-F7 showing

reflectance, excitation, and emission spectra of complexes 8-13

(37 pages). See any current masthead page for ordering and

Internet access instructions.

Most of the synthetic and specific molecular receptors

developed so far for the selective complexation of Ln(III) ions

had a cyclic structure in order to maintain the functional groups

and the binding units in an appropriate orientation for them to

easily coordinate the metal ions according to the lock-and-key

concept.⁷²However, more flexible acyclic receptors offer

intrinsic advantages for the fine structural control often required

for the recognition of guests as versatile as the lanthanide ions.⁶

The new acyclic segmental ligand L²belongs to this second

category, and its tridentate binding unit is coded for the selective

coordination of Ln(III) ions^21,23when the three stands are

properly organized by the coordination of the bidentate binding

(69) Bu¨nzli, J.-C. G.; Yersin, J.-R. HelV. Chim. Acta 1982, 65, 2498-

2506.

(70) Bu¨nzli, J.-C. G.; Ihringer, F. Inorg. Chim. Acta 1996, 246, 1-11.

(71) Petoud, S.; Bu¨nzli, J.-C. G.; Piguet, C. Unpublished results.

(72) Lichtenthaler, F. W. Angew. Chem., Int. Ed. Engl. 1994, 33, 2364-

2374.

JA954163C

(73) Voyer, N.; Lamothe, J. Tetrahedron 1995, 51, 9241-9284.

Article Doi

Lanthanide podates with predetermined structural and photophysical properties: Strongly luminescent self-assembled heterodinuclear d-f complexes with a segmental ligand containing heterocyclic imines and carboxamide binding units

DOI: 10.1021/ja954163c

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Article abstract of DOI:10.1021/ja954163c

Full text of DOI:10.1021/ja954163c

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