Tridentate facial ligation of tris(pyridine-2-aldoximato)nickel(II) and tris(imidazole-2-aldoximato)nickel(II) to generate NiIIFe IIINiII, MnIIINiII, Ni IINiII, and ZnIINiII and the electrooxidized MnIVNiII, NiIINiIII

Inorg. Chem. 2007, 46, 9003−9016

Tridentate Facial Ligation of Tris(pyridine-2-aldoximato)nickel(II) and

Tris(imidazole-2-aldoximato)nickel(II) To Generate Ni^IIFe^IIINi^II, Mn^IIINi^II,

Ni^IINi^II, and Zn^IINi^IIand the Electrooxidized Mn^IVNi^II, Ni^IINi^III, and Zn^IINi^III

Species: A Magnetostructural, Electrochemical, and EPR Spectroscopic

Study

Phalguni Chaudhuri,* Thomas Weyhermu1ller, Rita Wagner, Sumit Khanra, Biplab Biswas,

Eberhard Bothe, and Eckhard Bill

Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36,

D-45470 Mu¨lheim an der Ruhr, Germany

Received June 1, 2007

Eight hetero- and homometal complexes 1− −c

6, containing the metal centers Ni^IIFe^IIINi^II(1), Mn^IIINi^II(2), Ni^IINi^II(3a

and 4), Zn^IINi^II(5), and Zn^IIZn^II(6), are described. The tridentate ligation property of the metal complexes tris-

(pyridine-2-aldoximato)nickel(II) and tris(1-methylimidazole-2-aldoximato)nickel(II) with three facially disposed pendent

oxime O atoms has been utilized to generate the said complexes. Complex 1 contains metal centers in a linear

arrangement, as is revealed by X-ray diffraction. Complexes were characterized by various physical methods including

cyclic voltammetry (CV), variable-temperature (2

(EPR) measurements, and X-ray diffraction methods. Binuclear complexes 2

they all contain a metal ion in a distorted octahedral environment MN₃O₃and a second six-coordinated Ni^IIion in

−290 K) magnetic susceptibility, electron paramagnetic resonance

−6 are isostructural in the sense that

a trigonally distorted octahedral NiN₆geometry. Complexes 1−4 display antiferromagnetic exchange coupling of

the neighboring metal centers. The order of the strength of exchange coupling in the isostructural Ni^IIcomplexes,

2

3a−c, and 4, demonstrates the effects of the remote substituents on the spin coupling. The electrochemical

measurements CV and square wave voltammograms (SQW) reveal two reversible metal-centered oxidations, which

have been assigned to the Ni center ligated to the oxime N atoms, unless a Mn ion is present. Complex 2, Mn^IIINi^II,

exhibits a reduction of Mn^IIIto Mn^IIand two subsequent oxidations of Mn^IIIand Ni^IIto the corresponding higher

states. These assignments of the redox processes have been complemented by the X-band EPR measurements.

That the electrooxidized species [3a]⁺, [3b]⁺, [3c]⁺, and [4]⁺contain the localized mixed-valent Ni^IINi^IIIsystem resulting

from the spin coupling, a spin quartet ground state, S_t

)

³/₂, has been confirmed by the X-band EPR measurements.

Introduction

active research fields in inorganic chemistry because of its

relevance to different apparent dissimilar fields like bioi-

norganic chemistry¹and molecular magnetism.^2,3We have

been interested in such interactions, particularly involving

heterometal complexes^4-12with the help of suitably designed

The study of spin-spin interactions between paramagnetic

metal centers through multiatom bridges is one of the most

* To whom correspondence should be addressed. E-mail: chaudhuri@

mpi-muelheim.mpg.de.

(1) For example, see: (a) Holm, R. H., Solomon, E. I., Guest Eds. Chem.

ReV. 1996, 96 (7); 2004, 104 (2). (b) Lippard, S. J.; Berg, J. M.

Principles of Bioinorganic Chemistry; University Science Books: Mill

Valley, CA, 1994. (c) Kaim, W.; Schwederski, B. Bioanorganische

Chemie: B. G. Teubner: Stuttgart, Germany, 1991. (d) Bioinorganic

Chemistry of Copper; Karlin, K. D., Tyekla´r, Z., Eds.; Chapman &

Hall: New York, 1993. (e) Mechanistic Bioinorganic Chemistry;

Holden Thorp, H., Pecoraro, V. L., Eds.; American Chemical

Society: Washington, DC, 1995.

(2) (a) Magneto-Structural Correlations in Exchange Coupled Systems;

Willett, R. D., Gatteschi, D., Kahn, O., Eds.; Kluwer Academic

Publishers: Dordrecht, The Netherlands, 1985. (b) Magnetic Molecular

Materials; Gatteschi, D., Kahn, O., Miller, J. S., Palacio, F., Eds.;

Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (c)

A tribute to Olivier Kahn: J. Solid State Chem. 2001, 159 (No. 2),

July.

(3) Kahn, O. Molecular Magnetism; VCH-Verlagsgesellschaft: Weinheim,

Germany, 1993.

10.1021/ic701073j CCC: $37.00

Published on Web 08/25/2007

Inorganic Chemistry, Vol. 46, No. 21, 2007 9003

Chaudhuri et al.

Chart 1

soil bacteria and cyanobacteria. Of the redox-active nickel

enzymes, only NiSOD¹⁶and NiFe hydrogenases¹⁷are capable

of stabilizing the Ni³⁺(low-spin, S ) ¹/₂) oxidation state in

their active sites, although some catalytic cycles proposed

for acetyl coenzyme A synthase (ACS)¹⁸have invoked

transient intermediates with Ni³⁺centers. We report here the

synthesis, magnetic, electrochemical, spectroscopic, and other

physical properties including the structures of the following

compounds:

[Ni^II(PyA-H)₃Fe^III(PyA-H)₃Ni^II]ClO₄(1)

[LMn^III(PyA-H)₃Ni^II](ClO₄)₂(2)

[LNi^II(PyA-H)₃Ni^II]ClO₄(3a)

[LNi^II(PyA-Me)₃Ni^II]ClO₄(3b)

[LNi^II(PyA-Ph)₃Ni^II]ClO₄(3c)

[LNi^II(MeImA)₃Ni^II]ClO₄(4)

[LZn^II(PyA-H)₃Ni^II]ClO₄(5)

metal oximates as ligands. This work forms part of our

program on exchange-coupled oximate-based heteropoly-

metal systems¹³and deals specifically with the ligation

property of derivatives of tris(pyridine-2-aldoximato)nickel-

(II),⁴[Ni(PyA-R)₃]^-, and of tris(1-methylimidazole-2-aldoxi-

mato)nickel(II), [Ni(MeImA)₃]^-(Chart 1).

We were prompted to study the coordination chemistry

of these metal complexes as ligands because of the op-

portunity for their facile in situ formation dictated by the

thermodynamic stability of the resulting monoanion contain-

ing three facially disposed pendent oxime O atoms for

ligation.

In this paper, we explore the ability of [Ni(PyA-R)₃]^-and

[Ni(MeImA)₃]^-monoanions to generate such homo- and

heterometallic complexes, which will allow us to study

exchange-coupled interactions. Additionally, because the

oxime N atom has been established to stabilize the valence

states II+-IV+ of octahedral nickel,¹⁴we studied the

synthesized compounds electrochemically. Moreover, in-

volvement of the Ni^III/Ni^IIcouple has recently been reported¹⁵

for the catalytic reaction of the novel nickel-containing

superoxide dismutase (NiSOD), discovered in streptomyces

where PyA-R and MeImA represent the corresponding

aldoximato anion and L a tridentate macrocyclic amine 1,4,7-

trimethyl-1,4,7-triazacyclononane (Chart 1).

Additionally, to interpret the electrochemical results

unambiguously, we prepared complex [LZn^II(PyA-H)₃Zn^II]-

ClO₄(6), whose structural and electrochemical studies are

also included. Moreover, in an earlier paper,¹⁹we have

reported complexes, isostructural to 1,

[Ni^II(PyA-H)₃Mn^III(PyA-H)₃Ni^II]ClO₄(7)

and

[Ni^II(PyA-H)₃Cr^III(PyA-H)₃Ni^II]ClO₄(8)

(4) (a) Ross, S.; Weyhermu¨ller, T.; Bill, E.; Bothe, E.; Flo¨rke, U.;

Wieghardt, K.; Chaudhuri, P. Eur. J. Inorg. Chem. 2004, 984 and

references cited therein. (b) Ross, S.; Weyhermu¨ller, T.; Bill, E.;

Wieghardt, K.; Chaudhuri, P. Inorg. Chem. 2001, 40, 6656.

(5) Burdinski, D.; Birkelbach, F.; Weyhermu¨ller, T.; Flo¨rke, U.; Haupt,

H.-J.; Lengen, M.; Trautwein, A. X.; Bill, E.; Wieghardt, K.;

Chaudhuri, P. Inorg. Chem. 1998, 37, 1009.

(6) Birkelbach, F.; Flo¨rke, U.; Haupt, H.-J.; Butzlaff, C.; Trautwein, A.

X.; Wieghardt, K.; Chaudhuri, P. Inorg. Chem. 1998, 37, 2000.

(7) Chaudhuri, P.; Rentschler, E.; Birkelbach, F.; Krebs, C.; Bill, E.;

Weyhermu¨ller, T.; Flo¨rke, U. Eur. J. Inorg. Chem. 2003, 541.

(8) Chaudhuri, P.; Winter, M.; Fleischhauer, P.; Haase, W.; Flo¨rke, U.;

Haupt, H.-J. J. Chem. Soc., Chem. Commun. 1990, 1728.

(9) Chaudhuri, P.; Winter, M.; Della Ve´dova, B. P. C.; Fleischhauer, P.;

Haase, W.; Flo¨rke, U.; Haupt, H.-J. Inorg. Chem. 1991, 30, 4777.

(10) Krebs, C.; Winter, M.; Weyhermu¨ller, T.; Bill, E.; Wieghardt, K.;

Chaudhuri, P. J. Chem. Soc., Chem. Commun. 1995, 1913.

(11) Verani, C.; Weyhermu¨ller, T.; Rentschler, E.; Bill, E.; Chaudhuri, P.

Chem. Commun. 1998, 2475.

which will be on necessity mentioned here for comparison

purposes. Throughout this paper, the compounds are denoted

by the respective metal centers only, Ni^IIFe^IIINi^II(1), Mn^III-

Ni^II(2), Ni^IINi^II(3a-c and 4), Zn^IINi^II(5), Zn^IIZn^II(6), Ni^II-

Mn^IIINi^II(7), and Ni^IICr^IIINi^II(8), for the sake of clarity.

Experimental Section

Materials and Physical Measurements. Reagent- or analytical-

grade materials were obtained from commercial suppliers and used

without further purification, except those for electrochemical

measurements. Elemental analyses (C, H, N, and metal) were

performed by the Microanalytical Laboratory, Mu¨lheim, Germany.

(12) Verani, C.; Rentschler, E.; Weyhermu¨ller, T.; Bill, E.; Chaudhuri, P.

J. Chem. Soc., Dalton Trans. 2000, 4263.

(13) Chaudhuri, P. Coord. Chem. ReV. 2003, 243, 143.

(14) (a) Chakravorty, A. Isr. J. Chem. 1985, 25, 99. (b) Lappin, A. G.;

McAuley, A. AdV. Inorg. Chem. 1988, 32, 241. (c) Karmakar, S.;

Choudhury, S. B.; Ganguly, S.; Chakravorty, A. J. Chem. Soc., Dalton

Trans. 1997, 585.

(15) (a) Wuerges, J.; Lee, J.-W.; Yim, Y.-I.; Yim, H.-S.; Kang, S.-O.;

Carugo, K. D. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 8569. (b)

Barondeau, D. P.; Kassmann, C. J.; Brun, C. K.; Tainer, J. A.; Getzoff,

E. D. Biochemistry 2004, 43, 8038.

(16) Choudhury, S. B.; Lee, J. W.; Davidson, G.; Yim, Y. I.; Bose, K.;

Sharma, M. L.; Kang, S. O.; Cabelli, D. E.; Maroney, M. J.

Biochemistry 1999, 38, 3744.

(17) Davidson, G.; Choudhury, S. B.; Gu, Z. J.; Bose, K.; Roseboom, W.;

Albracht, S. P. J.; Maroney, M. J. Biochemistry 2000, 39, 7468.

(18) Seravalli, J.; Kumar, M.; Ragsdale, S. W. Biochemistry 2002, 41, 1807.

(19) (a) Weyhermu¨ller, T.; Wagner, R.; Khanra, S.; Chaudhuri, P. Dalton

Trans. 2005, 2539. (b) Birkelbach, F.; Weyhermu¨ller, T.; Lengen, M.;

Gerdan, M.; Trautwein, A. X.; Wieghardt, K.; Chaudhuri, P. J. Chem.

Soc., Dalton Trans. 1997, 4529.

9004 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

Fourier transform IR spectra of the samples in KBr disks were

recorded with a Perkin-Elmer 2000 FT-IR instrument. Electronic

absorption spectra in solution were measured with a Perkin-Elmer

Lambda 19 spectrophotometer. Magnetic susceptibilities of pow-

dered samples were recorded with a SQUID magnetometer in the

temperature range 2-290 K with an applied field of 1 T.

Experimental susceptibility data were corrected for the underlying

diamagnetism using Pascal’s constants and for the temperature-

independent paramagnetism contributions. Cyclic voltammetry (CV)

and coulometric measurements were performed using EG&G

equipment (potentiostat/galvanostat model 273A). Mass spectra

were recorded with either a Finnigan MAT 8200 (electron ioniza-

tion, EI-MS) or a MAT 95 (electrospray ionization, ESI-MS)

instrument. A Bruker DRX 400 instrument was used for NMR

spectroscopy. X-band electron paramagnetic resonance (EPR)

spectra were recorded with a Bruker ELEXSYS E500 spectrometer

equipped with a helium-flow cryostat (Oxford Instruments ESR

910).

Preparations. The macrocycle 1,4,7-trimethyl-1,4,7-triazacy-

clononane (C₉H₂₁N₃dL) was prepared as described previously.²⁰

2-Acetylpyridinealdoxime [HPyA-Me]. To an ice-cold solution

of Na₂CO₃(1.06 g, 10 mmol) and NH₂OH‚HCl (1.4 g, 20 mmol)

in water (20 mL) was added 2-acetylpyridine (2.4 g, 20 mmol),

and the resulting suspension was stirred at ∼4 °C for 2 h. The

suspension was kept in a refrigerator overnight and then filtered to

procure the solid material, which was washed with a cold methanol/

water (1:3) solvent mixture. Yield: 2.4 g (∼88%). Purity checked

by gas chromatography (GC): ∼98%. EI-MS: m/z 136 [M]⁺.

1-Methyl-2-imidazolealdoxime [MeImAH]. A protocol very

similar to that for HPyA-Me was used. Yield: 69%. Mp: 168-

169 °C. Purity checked by GC: ∼99%. EI-MS: m/z 125 [M]⁺.

[Ni^II(PyA-H)₃Fe^III(PyA-H)₃Ni^II]ClO₄(1). To a light-green solu-

tion of NiCl₂‚6H₂O (0.47 g, 2 mmol) in distilled methanol (50 mL)

were added successively with stirring solid pyridine-2-aldoxime

(HPyA-H; 0.72 g, 6 mmol) and Fe(ClO₄)₂‚6H₂O (0.37 g, 1 mmol).

To the resulting solution was added 10 mL of [Bu₄N][OCH₃] (20%

in CH₃OH), which upon stirring yielded a deep-brown solution.

After 24 h, the precipitated brown microcrystalline substance was

filtered, washed thoroughly with methanol, and air-dried. Yield:

0.51 g (∼51%). X-ray-quality black-brown crystals were obtained

by diffusion of methanol to a dimethylformamide solution of 1.

Anal. Calcd for C₃₆H₃₀ClFeNi₂N₁₂O₁₀: C, 43.27; H, 3.03; N, 16.82;

Ni, 11.75; Fe, 5.59. Found: C, 43.2; H, 3.0; N, 16.8; Ni, 11.9; Fe,

5.7. IR (KBr, cm^-1): 1663m, 1602s, 1541m, 1473s, 1431m, 1341m,

1220m, 1121s, 1080s, 1013m, 889m, 778m, 686s, 640s, 623m,

522m. ESI-MS (positive mode) in CH₃OH/H₂O: m/z (%) 899.0

(100%) [M - ClO₄]⁺, 776.9 (20%) [M - ClO₄- C₆H₅N₂O]⁺.

[LMn^III(PyA-H)₃Ni^II](ClO₄)₂(2). Mn(ClO₄)₂‚6H₂O (0.36 g, 1

mmol) was added to a solution of 1,4,7-trimethyl-1,4,7-triazacy-

clononane (0.34 g, 2 mmol) in 25 mL of methanol under vigorous

stirring, resulting in a brown solution. Another red-brown solution

was produced by stirring NiCl₂‚6H₂O (0.24 g, 1 mmol) and

pyridinealdoxime (HPyA-H; 0.36 g, 3 mmol) in 25 mL of methanol.

The above-mentioned two solutions were mixed together and stirred

at room temperature for 15 min, to which sodium perchlorate

monohydrate (0.14 g) dissolved in methanol (5 mL) was added.

The solution yielded after 24 h red-brown needle-shaped crystals,

which were filtered and dried in air. Yield: 0.38 g (∼45%).

Parallelogram-shaped deep-brown crystals were grown from an

acetonitrile solution for the X-ray structural characterization. Anal.

Calcd for C₂₇H₃₆Cl₂MnNiN₉O₁₁(847.18): C, 38.28; H, 4.28; N,

14.88; Mn, 6.48; Ni, 6.93. Found: C, 38.1; H, 4.3; N, 15.0; Mn,

6.6; Ni, 6.7. IR (KBr, cm^-1): 1604m, 1551w, 1537w, 1476m,

1465m, 1384w, 1345w, 1302w, 1218w, 1121s, 1091vs, 1054s,

1010m, 783m, 749w, 742w, 688w, 624m, 520w. UV-vis in CH₃-

CN [λ_max, nm (ꢀ, M^-1cm^-1)]: 248 (29 150) 310 (2959), 498sh

(∼1578), 906 (34.7). ESI-MS (positive mode) in CH₂Cl₂: m/z (%)

764.1 (∼2) (MClO₄⁺), 323.7 (∼1) (M²⁺).

[LNi^II(PyA-H)₃Ni^II]ClO₄(3a). To a solution of NiCl₂‚6H₂O

(0.48 g, 2 mmol) in 25 mL of distilled methanol was added with

stirring 1,4,7-trimethyl-1,4,7-triazacyclononane (0.17 g, 1 mmol).

To the resulting greenish-blue solution were added pyridine-2-

aldoxime (HPyA-H; 0.36 g, 3 mmol) and triethylamine (1 mL).

The solution was stirred for 0.5 h to yield a dark-brown solution,

which was kept open overnight for evaporation. The solid was taken

up in acetonitrile, and the mixture was filtered to remove any solid

particles. The acetonitrile solution yielded after 1 week deep-brown

crystals, which were collected by filtration. Yield: 0.25 g (33%).

Deep-brown crystals were grown from an acetonitrile solution for

the X-ray structural characterization. Anal. Calcd for C₂₇H₃₆-

ClNi₂N₉O₇: C, 43.15; H, 4.83; N, 16.78; Ni, 15.62. Found: C,

43.1; H, 4.8; N, 16.6; Ni, 15.6. IR (KBr, cm^-1): 1601s, 1530m,

1474s, 1159m, 1138s, 1120s, 1096s, 1014m, 989m, 779m, 680m,

624m. UV-vis in CH₂Cl₂[λ_max, nm (ꢀ, M^-1cm^-1)]: 276 (38 965),

336 (27 280), 498 (569), 827 (31), 949 (41^∧). ESI-MS (positive

mode) in CH₃OH: m/z (%) 650.3 (100%) [M - ClO₄]⁺.

[LNi^II(PyA-Me)₃Ni^II]ClO₄(3b) and [LNi^II(PyA-Ph)₃Ni^II]ClO₄

(3c). Complexes 3b and 3c were prepared by the same protocol as

that for 3a using HPyA-Me for 3b and HPyA-Ph for 3c. Deep-

brown crystals were grown from a methanol solution for the X-ray

structural characterization. Complex 3b. Yield: 0.30 g (37%). Anal.

Calcd for C₃₀H₄₂ClN₉Ni₂O₇: C, 45.41; H, 5.34; N, 15.89; Ni, 14.79.

Found: C, 45.6; H, 5.4; N, 15.9; Ni, 14.8. IR (KBr, cm^-1): 3007,

2961, 2893, 2856, 2816m, 1594s, 1571m, 1524s, 1463s, 1203s,

1188s, 1180s, 1107s, 1086s, 1045m, 1018m, 990m, 780s, 750m,

700m, 638m, 622m. UV-vis in CH₂Cl₂[λ_max, nm (ꢀ, M^-1cm^-1)]:

285 (31 500), 341 (22 660), 483 (540). ESI-MS (positive mode) in

CH₃OH: m/z (%) 692.3 (100%) [M - ClO₄]⁺. Complex 3c.

Yield: 0.69 g (70%). Anal. Calcd for C₄₅H₄₈ClN₉Ni₂O₇: C, 55.17;

H, 4.94; N, 12.87; Ni, 11.98. Found: C, 55.2; H, 5.0; N, 12.8; Ni,

12.0. IR (KBr, cm^-1): 2886m, 2816m, 1590s, 1568m, 1456s, 1440s,

1263m, 1218s, 1142s, 1123s, 1107s, 1060s, 1017s, 975s, 792m,

780s, 745m, 708s, 638m, 622m. UV-vis in CH₂Cl₂[λ_max, nm (ꢀ,

M^-1cm^-1)]: 291 (31 500), 349 (19 300), 510 (580), 833 (35), 942

(40). ESI-MS (positive mode) in CH₃OH: m/z (%) 880.3 (100%)

[M - ClO₄]⁺.

[LNi^II(MeImA)₃Ni^II]ClO₄(4). The same protocol as that for 3a

was used for preparing complex 4 by replacing pyridine-2-aldoxime

with 1-methylimidazole-2-aldoxime. Deep-brown crystals were

obtained from a methanol solution for the X-ray structure deter-

mination. Yield: 0.59 g (77%). Anal. Calcd for C₂₄H₃₉ClN₁₂-

Ni₂O₇: C, 37.91; H, 5.17; N, 22.10; Ni, 15.43. Found: C, 38.0; H,

5.2; N, 22.0; Ni, 15.1. IR (KBr, cm^-1): 3552m, 3408m, 3237m,

3113m, 2890m, 2854m, 2813m, 1557s, 1458s, 1293m, 1282m,

1220s, 1137s, 1106s, 1057s, 1013s, 865m, 776m, 746m, 717m,

672m, 624m. UV-vis in CH₂Cl₂[λ_max, nm (ꢀ, M^-1cm^-1)]: 442

(340), 648 (30), 820 (30), 955 (44). ESI-MS (positive mode) in

CH₃OH: m/z (%) 659.3 (100%) [M - ClO₄]⁺.

[LZn^II(PyA)₃Ni^II]ClO₄(5). Zinc chloride (0.136 g, 1 mmol) was

added to a solution of 1,4,7-trimethyl-1,4,7-triazacyclononane (0.17

g, 1 mmol) in 25 mL of methanol under vigorous stirring, resulting

in a pale-yellow solution. Another orange-red solution was produced

by stirring Ni(ClO₄)₂‚6H₂O (0.37 g, 1 mmol), pyridine-2-aldoxime

(20) Wieghardt, K.; Chaudhuri, P.; Nuber, B.; Weiss, J. Inorg. Chem. 1982,

21, 3086.

Inorganic Chemistry, Vol. 46, No. 21, 2007 9005

Chaudhuri et al.

H)₃]^-monoanion is less stable than the [Ni(PyA-H)₃]^-ion.

The color of complexes 1-5 is light to deep red-brown and

is dominated by the Ni^II(N_py)₃(N_azomethine)₃chromophore. They

are completely air-stable in the solid state and also in solution

for a few days.

Because the relevant bands in the IR spectra of comparable

pyridine-2-aldoximate-containing heteronuclear Cr^IIIM^IIand

Fe^IIIM^IIcomplexes have been reported earlier by us⁴and the

spectra of complexes 1-6, except for 4, are also very similar,

we refrain from discussing them again. The bands are given

in the Experimental Section.

The electronic spectral results indicate that complexes 2-5

are stable in solution. On the basis of their high extinction

coefficients and transitions similar to those reported in the

literature²¹and a comparison with the spectrum of only a

Zn^II-containing complex, 6 (see the Experimental Section),

all bands below 350 nm for complexes are assigned to π-π*

transitions of the oxime ligand. Judged on the basis of their

low extinction coefficients, we tentatively ascribe the bands

at 906 nm for 2, 498, 827, and 949 nm for 3a, 483 nm for

3b, 510, 833, and 942 nm for 3c, 442, 648, 820, and 955

nm for 4, and 774 nm for 5 to d-d transitions at the Ni^II

centers.

ESI-MS in the positive ion mode has been proven to be a

very useful analytical tool, except for 2, for characterizing

all complexes (see the Experimental Section), which show

the monopositively charged species [M - ClO₄]⁺as the base

peak (100%). The Mn-containing complex 2 does not provide

signals for unambiguous characterization, which is in ac-

cordance with our own earlier experience.¹⁹

Description of the Structures. The crystal structures of

complexes 1-6 have been determined by single-crystal X-ray

crystallography at 100(2) K. The X-ray structures confirm

that tri- and dinuclear complexes, Ni^IIFe^IIINi^II(1), Mn^IIINi^II

(2), Ni^IINi^II(3a-c and 4), Zn^IINi^II(5), and Zn^IIZn^II(6), have

indeed been formed in which two (for 2-6) or three (for 1)

pseudooctahedral polyhedra are joined face-to-face by three

oximato N-O groups. The N_ox-O bond lengths of average

) 1.347 ( 0.004 (1), 1.358 ( 0.01 (2), 1.331 ( 0.006 (3

and 4), 1.341 ( 0.007 (5), and 1.341 ( 0.004 Å (6) are

nearly identical with those for other comparable struc-

tures^4,9,19,22and significantly shorter than ∼1.40 Å in general

for free oxime ligands. The bond angle CdN_ox-O [average

117.7 ( 0.3° (1), 117.8 ( 0.8° (2), 119.3 ( 0.3° (3 and 4),

118.7 ( 0.4° (5), and 118.5 ( 0.3° (6)] and the bond distance

CdN_ox[average 1.296 ( 0.006 (1), 1.286 ( 0.004 (2), 1.298

( 0.005 (3 and 4), 1.288 ( 0.006 Å (5), and 1.287 ( 0.005

Å (6)] are, as expected, identical with those for other Fe^IIIM^II,

(0.36 g, 3 mmol), and 0.7 mL of Et₃N in 25 mL of methanol. The

above-mentioned two solutions were mixed together and stirred at

room temperature for 0.5 h, whereupon an orange solid precipitated

out. The solid was separated by filtration and crystallized from an

acetonitrile solution to yield X-ray-quality red-brown crystals of

4‚H₂O. Yield: 0.47 g (62%). Anal. Calcd for C₂₇H₃₆ClN₉NiO₇Zn:

C, 42.77; H, 4.79; N, 16.63; Zn, 8.62; Ni, 7.74. Found: C, 42.8;

H, 4.7; N, 16.6; Zn, 8.8; Ni, 7.7. IR (KBr, cm^-1): 1602s, 1533m,

1519m, 1475s, 1463m, 1230w, 1158m, 1131s, 1116s, 1091s,

1018m, 988m, 780m, 680m, 624m. UV-vis in CH₂Cl₂[λ_max, nm

(ꢀ, M^-1cm^-1)]: 425sh (∼21), 500sh (∼149), 774 (17.2). ESI-MS

(positive mode) in CH₂Cl₂: m/z (%) 656.2 (100%) [M - ClO₄]⁺.

[LZn^II(PyA)₃Zn^II]ClO₄(6). To a solution of 1,4,7-trimethyl-

1,4,7-triazacyclononane (0.17 g, 1 mmol), pyridine-2-aldoxime (0.36

g, 3 mmol), and Et₃N (1 mL) in methanol (50 mL) was added with

stirring zinc acetate dihydrate (0.44 g, 2 mmol) to provide a pale-

yellow solution. The solution kept overnight under a hood yielded

a very pale-yellow microcrystalline solid of 5, which was recrystal-

lized from methanol. Yield: 0.5 g (∼65%). Anal. Calcd for C₂₇H₃₆-

ClN₉O₇Zn₂: C, 42.40; H, 4.74; N, 16.48; Zn, 17.10. Found: C,

42.3; H, 4.8; N, 16.5; Zn, 17.2. IR (KBr, cm^-1): 3607m, 3383br,

1647m, 1603s, 1583m, 1541s, 1481s, 1462s, 1434m, 1342m,

1299m, 1227m, 1155m, 1085s, 1017s, 989m, 892m, 781m, 686m,

624m. UV-vis in CH₃CN [λ_max, nm (ꢀ, M^-1cm^-1)]: 272 (2.38 ×

10⁴), 321 (1.78 × 10⁴). ESI-MS (positive) in CH₂Cl₂: m/z (%) 664

(100%) [M - ClO₄]⁺.

Caution! Although we experienced no difficulties with the

compounds isolated as their perchlorate salts, the unpredictable

behaVior of perchlorate salts necessitates extreme caution in their

handling.

X-ray Crystallographic Data Collection and Refinement of

the Structures. The crystallographic data for 1-6 are summarized

in Table 1. Graphite-monochromated Mo KR radiation (λ )

0.710 73 Å) was used for 1-5, whereas a copper source (λ )

1.541 78 Å) was used for 6. Dark-red/brown to light-red crystals

of 1-5 or pale-yellow crystals of 6 were fixed with perfluoropoly-

ether onto glass fibers and mounted on a Bruker Kappa CCD for

1-5 or a Siemens SMART diffractometer for 6 equipped with a

cryogenic nitrogen cold stream, and intensity data were collected

at 100(2) K. Final cell constants were obtained from a least-squares

fit of all measured reflections. Intensity data were corrected for

Lorentz and polarization effects. The data sets were not corrected

for absorption, except for 4 and 6, for which the program SADABS

(version 2.1) was used for absorption corrections. The Siemens

ShelXTL software package (Sheldrick, G. M. Universita¨t Go¨ttingen,

Go¨ttingen, Germany) was used for solution, refinement, and artwork

of the structures; the neutral atom scattering factors of the program

were used.

Results and Discussion

The thermodynamic stability of the in situ generated [Ni-

(PyA-R)₃]^-monoanion containing three facially disposed

pendent oxime O atoms for ligation, together with the lability

of the first transition series divalent metal ions, is utilized

to synthesize homo- and heterometallic complexes. Although

no scrambling was observed, all attempts to strictly synthe-

size the cation [LNi^II(PyA-H)₃Zn^II]⁺, a constitution isomer

of complex 5, failed. On the other hand, complex 5

[LZn^II(PyA-H)₃Ni^II]⁺was readily isolated starting from its

respective materials. This can be attributed to the greater

thermodynamic stability of the [Ni(PyA-H)₃]^-anion than that

of the [LNi(solvent)₃]²⁺cation. In other words, the [Zn(PyA-

(21) Burger, K.; Ruff, I.; Ruff, F. J. Inorg. Nucl. Chem. 1965, 27, 179.

(22) (a) Beckett, R.; Hoskins, B. F. J. Chem. Soc., Dalton Trans. 1972,

291, 2527. (b) Nasakkala, M.; Saarinen, H.; Korvenranta, J.; Orama,

M. Acta Crystallogr., Sect. C 1989, 45, 1511. (c) Pearse, G. A.;

Raithby, P.; Lewis, J. Polyhedron 1989, 8, 301. (d) Schlemper, E. O.;

Stunkel, J.; Patterson, C. Acta Crystallogr., Sect. C 1990, 46, 1226.

(e) Nordquest, K. W.; Phelps, D. W.; Little, W. F.; Hodgson, D. J. J.

Am. Chem. Soc. 1976, 98, 1104. (f) Churchill, M. R.; Reis, A. H.

Inorg. Chem. 1972, 11, 1811. (g) Churchill, M. R.; Reis, R. H. Inorg.

Chem. 1972, 11, 2299. (h) Faller, J. W.; Blankenship, C.; Whitmore,

B.; Sena, S. Inorg. Chem. 1985, 24, 4483. (i) Churchill, M. R.; Reis,

A. H. Inorg. Chem. 1973, 12, 2280.

9006 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

Table 1. Crystallographic Data for 1, 2‚CH₃CN, 3a‚H₂O, 3b‚0.5CH₃OH, 3c, 4‚1.5H₂O, 5, and 6‚3CH₃OH‚3H₂O

Ni^IIFe^IIINi^II(1)

Mn^IIINi^II(2)

Ni^IINi^II(PyA-H) (3a)

Ni^IINi^II(PyA-Me) (3b)

empirical formula

fw

C₃₆H₃₀ClFeN₁₂Ni₂O₁₀

999.44

C₂₉H₃₉Cl₂MnN₁₀NiO₁₁

888.25

C₂₇H₃₈ClN₉Ni₂O₈

769.53

C_30.5H₄₄ClN₉Ni₂O_7.5

809.62

T, K

100(2)

wavelength, Å

cryst syst, space group

unit cell dimens

a, Å

b, Å

c, Å

0.710 73

trigonal, R3h

0.710 73

monoclinic, C2/c

0.710 73

monoclinic, P2₁

0.710 73

triclinic, P1h

13.835(2)

21.163(4)

90

120

3508.1(10); 3

1.419

1.223

19.9574(7)

24.6226(9)

14.9959(6)

90

90.73(1)

90

7368.4(5); 8

1.601

1.070

9.2784(3)

16.5598(9)

11.3633(6)

90

112.61(1)

90

1611.76(13); 2

1.586

1.314

11.6289(4)

13.8230(5)

13.8607(5)

119.548(3)

101.498(3)

102.176(3)

1770.03(11); 2

1.519

R, deg

â, deg

γ, deg

V, Å³; Z

calcd density, Mg/m³

abs coeff, mm^-1

F(000)

1.200

846

1527

3664

800

cryst size, mm

abs corrn

refinement method

0.09 × 0.08 × 0.06

not corrected

full-matrix least

squares on F ²

1875/19/112

1.122

0.04 × 0.03 × 0.02

not corrected

full-matrix least

squares on F ²

11 701/0/507

1.025

0.10 × 0.04 × 0.03

not corrected

full-matrix least

squares on F ²

10 193/2/433

1.053

0.30 × 0.26 × 0.18

not corrected

full-matrix least

squares on F ²

10 284/0/462

1.044

data/restraints/param

GOF on F ²

final R indices [I > 2σ(I)]

R1 ) 0.0626,

wR2 ) 0.1785

R1 ) 0.0920,

wR2 ) 0.2041

0.915 and -0.384

R1 ) 0.0500,

wR2 ) 0.0983

R1 ) 0.0860,

wR2 ) 0.1130

1.068 and -0.935

R1 ) 0.0484,

wR2 ) 0.0761

R1 ) 0.0673,

wR2 ) 0.0820

0.627 and -0.482

-0.001(9)

R1 ) 0.0289,

wR2 ) 0.0708

R1 ) 0.0314,

wR2 ) 0.0722

0.623 and -0.497

R indices (all data)

largest diff peak and hole, e A^-3

absolute structure param

Ni^IINi^II(PyA-Ph) (3c)

Ni^IINi^II(MeImA) (4)

Zn^IINi^II(5)

Zn^IIZn^II(6)

empirical formula

fw

C₄₅H₄₈ClN₉Ni₂O₇

979.79

C₂₄H₄₂ClN₁₂Ni₂O_8.5

787.57

C₂₇H₃₈ClN₉NiO₈Zn

776.19

C₃₀H₅₄ClN₉O₁₃Zn₂

915.01

T, K

100(2)

wavelength, Å

cryst syst, space group

unit cell dimens

a, Å

b, Å

c, Å

0.710 73

cubic, Pa3h

0.710 73

monoclinic, C2/c

0.710 73

monoclinic, P2₁

1.541 78

trigonal, R3c

20.9553(3)

90

9202.0(2); 8

1.414

0.936

17.8083(4)

15.7015(4)

24.5793(6)

90

104.911(4)

90

6641.4(3); 8

1.575

1.281

9.3049(3)

16.6595(6)

11.3975(4)

90

112.472(5)

90

1632.62(10); 2

1.579

1.455

11.9820(4)

47.0659(13)

90

120

5581.9(3); 6

1.558

2.775

R, deg

â, deg

γ, deg

V, Å³; Z

calcd density, Mg/m³

abs coeff, mm^-1

F(000)

4080

3288

804

2868

cryst size, mm

abs corrn

0.20 × 0.15 × 0.14

0.30 × 0.20 × 0.16

semiempirical from

equivalents

full-matrix least

squares on F ²

11 969/13/460

1.112

0.08 × 0.05 × 0.04

0.22 × 0.22 × 0.14

SADABS V2.1,

Bruker-Nonius

full-matrix least

squares on F ²

2395/4/174

1.076

not corrected

refinement method

full-matrix least

squares on F ²

5551/14/216

1.166

full-matrix least

squares on F ²

10 363/2/433

1.022

data/restraints/param

GOF on F ²

final R indices [I > 2σ(I)]

R1 ) 0.0464,

wR2 ) 0.1186

R1 ) 0.0521,

wR2 ) 0.1218

1.426 and -0.500

R1 ) 0.0414,

wR2 ) 0.0991

R1 ) 0.0468,

wR2 ) 0.1018

0.887 and -0.741

R1 ) 0.0254,

wR2 ) 0.0567

R1 ) 0.0287,

wR2 ) 0.0581

0.481 and -0.291

-0.001(5)

R1 ) 0.0523,

wR2 ) 0.1401

R1 ) 0.0526,

wR2 ) 0.1406

0.960 and -1.310

0.07(5)

R indices (all data)

largest diff peak and hole, e A^-3

absolute structure param

Mn^IIIM^II, and Cr^IIIM^IIcomplexes containing pyridine-2-

aldoximato reported earlier by us.^4,13

ligand through the pendent oximato O atoms to the centrally

placed Fe^IIIion. A view of the cation [Fe(PyA-H)₆Ni₂]⁺in

complex 1 is shown in Figure 1. Selected bond distances

and angles are listed in Table 2. The cation [Fe(PyA-H)₆Ni₂]⁺

having a crystallographic threefold inversion symmetry has

therefore a strictly linear arrangment of the Ni^IIFe^IIINi^IIarray,

X-ray Structure of 1. The lattice consists of discrete

trinuclear monocations and perchlorate anions. The heterot-

rinuclear complex, Ni^IIFe^IIINi^II(1), crystallizes in the space

group R3h, with threefold inversion symmetry and, as

expected, is isostructural with the analogous Ni^IIMn^IIINi^II(7)

and Ni^IICr^IIINi^II(8) complexes, described earlier by us.^19a

-

and the two Ni(PyA)₃units necessarily have opposite

chirality (∆ and Λ), making [Fe(PyA-H)₆Ni₂]⁺achiral.

The terminal Ni centers are sixfold-coordinated, yielding

a NiN₆core; coordination occurs facially through three

-

The trinuclear complex contains two Ni^II(PyA)₃moieties,

each having a NiN₆coordination sphere, acting as a tridentate

Inorganic Chemistry, Vol. 46, No. 21, 2007 9007

Chaudhuri et al.

Figure 1. ORTEP diagram of the cation in Ni^IIFe^IIINi^II(1) with 50% probability of the ellipsoids.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complex 1^a

complex 1 can be ascribed to both electronic (LFSE) and

size effects, as has been discussed earlier.^5,23

Ni(1)-N(2)

Ni(1)-N(9)

Fe(1)-O(1)

2.001(3) C(3)-N(2)

2.075(4) N(2)-O(1)

2.019(3)

1.296(6)

1.347(4)

The central iron atom Fe(1) is surrounded by an almost

perfect octahedron (with the deviation being less than 4°)

of six oximato oxygen atoms O(1), pendant from the two

Ni(PyA-H)₃^-fragments. All angles at the metal between cis

O atoms deviate from ideal 90°, being 86.78(11)° and 93.22-

(11)°; the cis-Fe(1)-O bond angle of 93.22(11)° represents

N(2)-Ni(1)-N(2)#2 89.14(14) N(9)#2-Ni(1)-N(9)

92.92(14)

N(2)-Ni(1)-N(9)#2 101.68(14) N(2)#3-Ni(1)-N(9)#3 78.54(14)

N(2)-Ni(1)-N(9)

N(2)#2-Ni(1)-N(9) 163.37(15)

78.55(14) N(9)#2-Ni(1)-N(9)#3 92.91(14)

-

N(9)-Ni(1)-N(9)#3 92.91(14) C(3)-N(2)-O(1)

117.7(3)

117.9(3)

124.3(3)

the O atoms originating from the same Ni(PyA-H)₃frag-

O(1)#1-Fe(1)-O(1) 180.0

C(3)-N(2)-Ni(1)

O(1)-N(2)-Ni(1)

ment, whereas the angle 86.78(11)° is exhibited between the

N(2)-O(1)-Fe(1)

115.8(2)

-

O atoms of two different Ni(PyA-H)₃fragments. The Fe-

Ni‚‚‚Fe

3.545

(1)-O(1) distance of 2.019(3) Å compares favorably with

that [2.021(7) Å] in a very similar oximato-bridged trinuclear

Fe^II(low-spin)Fe^III(high-spin)Fe^II(low-spin)²⁴as well as with

those in other Fe^IIIO₆species.^25,26That the central Fe(1) ion

should be ascribed to a III+ (d⁵high-spin) oxidation level

is borne out additionally by the facts that (i) a perchlorate

anion is present for maintaining the electroneutrality of the

monocationic [Fe(PyA-H)₆Ni₂]⁺complex and (ii) the mag-

netic data can only be simulated by considering an S_Fe) ⁵/₂

for the central Fe(1) center. We conclude that complex 1

^aSymmetry transformations used to generate equivalent atoms: #1, -x

+ ²/₃, -y + ⁴/₃, -z + ¹/₃; #2, -x + y, -x + 1, z; #3, -y + 1, x - y + 1,

2

1

z; #4, x - y + /₃, x + /₃, -z + /₃; #5, y - /₃; -x + y + /₃, -z + /₃;

1

2

1

2

#6, y - /₃, -x + y + /₃, -z - /₃; #7, x - y + /₃, x + /₃, -z - /₃.

pyridine N_py(9) atoms and three azomethine N_ox(2) atoms,

from the pyridine-2-aldoximate ligands (Figure 1). The Ni-

(1)-N_ox(2) bond length ) 2.001(3) Å is shorter than the

Ni(1)-N_py(9) ) 2.075(4) Å bond distance, as has been

observed earlier for comparable complexes.^4,19aThe Ni-N

bond lengths fall within the ranges that are considered as

normal covalent bonds for high-spin d⁸Ni^IIions. The facial

disposition of the three N_pyN_oxchelate rings at each Ni atom

is necessary for the ligation of the pendent oxime O atoms,

O(1) and its equivalents, to the central iron. The chelate rings

are planar. The average chelate bite angle on the two Ni

centers is 86.9°. This small but significant negative deviation

of the bite angle from 90° necessarily implies the presence

of substantial trigonal distortion. Indeed, the two Ni centers

can be considered to have distorted trigonal-antiprismatic

coordination, as is evident from the average twist angle ψ

of 40.0°, which deviates appreciably from the ideal 60° for

an octahedron. The trigonal twist angle ψ is defined as the

angle between the triangular faces comprising three pyridine

N atoms, N(9) and its equivalents, and three azomethine N

atoms, N(2) and its equivalents. That the array Ni(N-O)Fe

is not planar is shown by the dihedral angle θ of 38.4°

between the planes comprising Fe(O-N) and Ni(N-O)

atoms. These distortions of the six-coordinate d⁸Ni^IIion in

contains a Ni^IIFe^III(high-spin) core. Mo¨ssbauer data at 80

2

K and zero field also support the high-spin formulation of

the ferric center (δ_Fe) 0.57 mm‚s^-1; ∆E_Q) -0.05 mm‚s^-1).

Molecular Structure of 2‚CH₃CN. The structure of 2,

shown in Figure 2, clearly illustrates the heterodinuclear

nature of the complex. Selected bond lengths and angles for

complex 2 are given in Table 3. A [Ni(PyA-H)₃]^-anion

coordinates to a Mn^IIIcenter through its deprotonated oxime

O atoms with a Mn‚‚‚Ni separation of 3.506 Å. The Mn

(23) (a) Wentworth, R. A. D. Coord. Chem. ReV. 1972/73, 9, 171. (b)

Kirchner, R. M.; Meali, C.; Bailey, M.; Howe, N.; Torne, L. P.; Wilson,

L. J.; Andrews, L. C.; Rose, N. J.; Lingafelter, E. C. Coord. Chem.

ReV. 1987, 77, 89. (c) Kunow, S. A.; Takeuchi, K. J.; Grzybowski, J.

J.; Jircitano, A. J.; Goedken, V. L. Inorg. Chim. Acta 1996, 241, 21.

(24) Pal, S.; Melton, T.; Mukherjee, R.; Chakravorty, A. R.; Tomas, M.;

Falvello, L. R.; Chakravorty, A. Inorg. Chem. 1985, 24, 1250.

(25) ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Perga-

mon: England, 1987; Vol. 4.

(26) (a) Beattle, J. K.; Best, S. P.; Skelton, W. W.; White, A. H. J. Chem.

Soc., Dalton Trans. 1981, 2105. (b) Hair, N. J.; Beattle, J. K. Inorg.

Chem. 1977, 16, 245. (c) Linder, Von H. J.; Gottlicher, S. Acta

Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, B25,

832. (d) Iball, J.; Morgan, C. H. Acta Crystallogr. 1967, 23, 239.

9008 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

Figure 2. Molecular structure of the cation in complex Mn^IIINi^II(2).

with the macrocyclic amine;^6,25,27the N(1)-Mn(1)-O(41)

axis defines an elongated Jahn-Teller axis of a high-spin

d⁴ion in a distorted octahedral ligand field.

Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complex

2‚CH₃CN^a

Mn(1)-O(31)

Mn(1)-O(21)

Mn(1)-O(41)

Mn(1)-N(7)

Mn(1)-N(4)

Mn(1)-N(1)

1.932(2) Ni(2)-N(22)

1.996(3) Ni(2)-N(32)

2.029(2) Ni(2)-N(42)

2.106(3) Ni(2)-N(49)

2.168(3) Ni(2)-N(29)

2.195(3) Ni(2)-N(39)

2.043(3)

2.050(2)

2.058(3)

2.094(2)

2.100(2)

2.108(3)

The Ni center, Ni(2), in 2 is also sixfold-coordinated,

yielding a NiN₆core. Coordination occurs facially through

pyridine N atoms, N(29), N(39), and N(49), and three

azomethine N atoms, N(22), N(32), and N(42) from a

pyridine-2-aldoxime (PyA-H) ligand. The average Ni-N_py

) 2.113 ( 0.009 Å and average Ni-N_ox) 2.030 ( 0.01 Å

are nearly identical with those observed for 1, 3a-c, 4, 5,

and similar comparable structures.^4,19aThe Ni^IIcenter is

trigonally distorted with an average twist angle of 39.3°. Like

complex 1, the tris(pyridine-2-aldoximato)nickel(II) unit in

2 is identical with those reported earlier by us,⁴and hence

we will refrain from describing it again. In summary, the

crystal structure of 2 unequivocally establishes the presence

of the Mn^IIINi^IIcore in the cation of [LMn(PyA-H)₃Ni]-

(ClO₄)₂.

Molecular Structures of 3a‚H₂O, 3b‚0.5H₂O, 3c‚

1.5H₂O, 4, and 5. Because the structures of 3a-c and 4 are

very similar with respect to the atom connectivity and the

geometry of the Ni centers, we confine our structure

description to that of 3a; the crystal data for all four

compounds are listed in Table 1.

The structure of 3a consists of one distinct [LNi₂(PyA-

H)₃]⁺cation, one noncoordinatively bound perchlorate anion,

and one molecule of water of crystallization. A perspective

view of the cation in 3a is shown in Figure 3. Table 4 lists

selected bond distances and angles for complex 3a. Both Ni

centers, Ni(1) and Ni(2), are in a distorted octahedral

geometry with the core structure N₃Ni(1)(O-N_ox)₃Ni(2)-

(N_py)₃. The three atoms N(1), N(4), and N(7) of the facially

coordinated tridentate cyclic amine L and three atoms O(21),

O(31), and O(41), from the bridging oximate groups, are

O(21)-N(22)

N(22)-C(23)

O(31)-N(32)

1.346(3) N(32)-C(33)

1.287(4) O(41)-N(42)

1.359(3) N(42)-C(43)

1.278(4)

1.346(3)

1.286(4)

O(31)-Mn(1)-O(21)

O(31)-Mn(1)-O(41)

O(21)-Mn(1)-O(41)

O(31)-Mn(1)-N(7)

O(21)-Mn(1)-N(7)

O(41)-Mn(1)-N(7)

O(31)-Mn(1)-N(4)

O(21)-Mn(1)-N(4)

O(41)-Mn(1)-N(4)

N(7)-Mn(1)-N(4)

O(31)-Mn(1)-N(1)

O(21)-Mn(1)-N(1)

O(41)-Mn(1)-N(1)

N(7)-Mn(1)-N(1)

N(4)-Mn(1)-N(1)

98.67(10) N(22)-Ni(2)-N(32)

87.25(10)

88.61(11)

86.68(10)

95.08(9)

96.70(9)

N(22)-Ni(2)-N(42)

N(32)-Ni(2)-N(42)

169.32(10) N(22)-Ni(2)-N(49) 163.11(10)

89.34(11) N(32)-Ni(2)-N(49) 102.15(10)

90.96(9)

N(42)-Ni(2)-N(49)

78.09(10)

78.27(10)

88.75(10) N(22)-Ni(2)-N(29)

167.97(10) N(32)-Ni(2)-N(29) 161.48(10)

92.03(10) N(42)-Ni(2)-N(29) 104.25(10)

82.23(11) N(49)-Ni(2)-N(29)

94.81(10)

91.32(10) N(22)-Ni(2)-N(39) 101.63(11)

89.47(10) N(32)-Ni(2)-N(39) 77.89(11)

170.34(10) N(42)-Ni(2)-N(39) 160.93(10)

81.66(10) N(49)-Ni(2)-N(39)

80.85(11) N(29)-Ni(2)-N(39)

94.12(10)

93.65(11)

N(22)-O(21)-Mn(1) 113.2(2)

C(23)-N(22)-O(21) 118.3(3)

C(23)-N(22)-Ni(2) 116.9(2)

O(21)-N(22)-Ni(2) 124.6(2)

N(32)-O(31)-Mn(1) 111.7(2)

C(33)-N(32)-O(31) 117.2(3)

C(33)-N(32)-Ni(2) 117.3(2)

O(31)-N(32)-Ni(2) 125.3(2)

N(42)-O(41)-Mn(1) 112.6(2)

C(43)-N(42)-O(41) 118.1(3)

C(43)-N(42)-Ni(2) 116.9(2)

O(41)-N(42)-Ni(2) 124.6(2)

Mn(1)‚‚‚Ni(2)

3.506

^aSymmetry transformations used to generate equivalent atoms: #1, -x

1

3

+ 1, y, -z + /₂; #2, -x + 1, y, -z + /₂.

coordination geometry is distorted octahedral with three N

atoms from the facially coordinated tridentate amine and

three O atoms from three oximate groups. The observed Mn-

(1)-O and Mn(1)-N bond distances (Table 3) are in

agreement with those reported for high-spin Mn^IIIcenters

(27) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 1987, 35, 329.

Inorganic Chemistry, Vol. 46, No. 21, 2007 9009

Chaudhuri et al.

angles ranging between 82.9(1)° and 83.8(1)°, whereas the

O-Ni(1)-O angles in the six-membered rings fall between

94.2(1) and 95.9(1)°. The coordinated macrocyclic amine L

does not exhibit any unexpected features and hence does not

warrant further discussion. The Ni(2)(PyA-H)₃fragment of

complex 3a with a twist angle of 40.3° is very similar to

those of other complexes described here.

The molecular structures of the cations in 3b, 3c, and 4

are shown in Figures S1, S2, and S3 (Supporting Informa-

tion), respectively, and Table S1 (Supporting Information)

lists selected bond distances and angles for complex 4.

In the heterodinuclear complex Zn^IINi^II(5), Zn(1) is

coordinated to the macrocyclic amine and O atoms of the

tris(pyridine-2-aldoximato)nickel(II) monoanionic ligand,

yielding a six-coordinated ZnN₃O₃. The metrical parameters

for Zn(1)-O and Zn(1)-N distances with 2.083 and 2.215

Å, respectively, are very similar to those reported for

dinuclear Zn^IIcomplexes with this macrocyclic amine²⁹and

Figure 3. ORTEP diagram of complex [LNi^IINi^II(PyA-H)₃](ClO₄)‚H₂O

(3a).

-

for similar oxime-containing complexes.^4,30The Ni(PyA)₃

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

3a‚H₂O

unit with a twist angle of 41° is very similar to that described

before. Figure 4 shows the structure of the dinuclear

monocation in 5; Table 5 lists selected bond distances and

angles. The trigonal distortions of the Ni center in the

Ni(PyA)₃^-fragment, expressed as twist angle ψ and dihedral

angle θ, are listed in Table 6 for comparison purposes. Table

6 clearly exhibits the small but significant influence of the

heterometalatomonthetrigonaldistortionoftheNi(oximato)₃^-

unit.

Ni‚‚‚Ni

3.491

Ni(1)-O(41)

Ni(1)-O(21)

Ni(1)-O(31)

Ni(1)-N(7)

Ni(1)-N(4)

Ni(1)-N(1)

2.0284(19) Ni(2)-N(22)

2.023(2)

2.027(2)

2.040(2)

2.104(3)

2.114(2)

2.122(2)

2.059(2)

2.077(2)

2.117(3)

2.127(2)

2.137(2)

Ni(2)-N(42)

Ni(2)-N(32)

Ni(2)-N(49)

Ni(2)-N(29)

Ni(2)-N(39)

O(21)-N(22)

N(22)-C(23)

O(31)-N(32)

1.326(3)

1.297(4)

1.338(3)

N(32)-C(33)

O(41)-N(42)

N(42)-C(43)

1.289(4)

1.335(3)

1.289(3)

Molecular Structure of 6‚3CH₃OH‚3H₂O. Figure S4

(Supporting Information) displays the structure of the mono-

cation, and Table S2 (Supporting Information) summarizes

selected bond distances and angles.

O(41)-Ni(1)-O(21) 95.38(8)

O(41)-Ni(1)-O(31) 94.17(8)

O(21)-Ni(1)-O(31) 95.86(8)

O(41)-Ni(1)-N(7) 171.97(9)

N(22)-Ni(2)-N(29)

78.69(10)

N(42)-Ni(2)-N(29) 102.06(9)

N(32)-Ni(2)-N(29) 163.99(9)

N(49)-Ni(2)-N(29)

92.91(10)

Each Zn ion in the dinuclear monocation is in a distorted

octahedral ligand environment, one having a Zn(2)N₃O₃

coordination sphere and the other with a Zn(1)N₆coordina-

tion sphere, which are very similar to those reported²⁹for

Zn^IIcomplexes with this macrocyclic amine, and similar

analogous [LFe^III(PyA-H)₃Zn]²⁺and [LCr^III(PyA-H)₃Zn]²⁺

oxime-containing complexes⁴including complex 5 described

above. That the array Zn(1)(N-O)Zn(2) is not planar is

shown by the dihedral angle θ of 40.1° between the planes

comprising Zn(1)(N-O) and Zn(2)(O-N) atoms. The non-

bonding intramolecular Zn‚‚‚Zn distance in 6 is at 3.595 Å.

This metal‚‚‚metal distance is at 3.491 Å in the Ni^II₂dinuclear

complex 3a.

O(21)-Ni(1)-N(7)

O(31)-Ni(1)-N(7)

O(41)-Ni(1)-N(4)

O(21)-Ni(1)-N(4)

89.21(9)

91.91(9)

89.60(8)

90.10(9)

N(22)-Ni(2)-N(39) 100.90(10)

N(42)-Ni(2)-N(39) 163.22(9)

N(32)-Ni(2)-N(39)

N(49)-Ni(2)-N(39)

N(29)-Ni(2)-N(39)

78.19(9)

93.50(10)

92.96(8)

O(31)-Ni(1)-N(4) 172.62(9)

N(7)-Ni(1)-N(4)

O(41)-Ni(1)-N(1)

83.80(10)

91.10(9)

N(22)-O(21)-Ni(1) 112.14(16)

C(23)-N(22)-O(21) 119.5(3)

C(23)-N(22)-Ni(2) 117.0(2)

O(21)-N(22)-Ni(2) 123.48(17)

N(32)-O(31)-Ni(1) 110.10(15)

C(33)-N(32)-O(31) 119.1(2)

C(33)-N(32)-Ni(2) 116.6(2)

O(31)-N(32)-Ni(2) 124.27(17)

N(42)-O(41)-Ni(1) 113.20(15)

C(43)-N(42)-O(41) 119.2(2)

C(43)-N(42)-Ni(2) 117.1(2)

O(41)-N(42)-Ni(2) 123.53(17)

O(21)-Ni(1)-N(1) 170.39(9)

O(31)-Ni(1)-N(1)

N(7)-Ni(1)-N(1)

N(4)-Ni(1)-N(1)

90.72(9)

83.57(10)

82.85(10)

N(22)-Ni(2)-N(42) 89.47(9)

N(22)-Ni(2)-N(32) 89.80(9)

N(42)-Ni(2)-N(32) 88.79(9)

N(22)-Ni(2)-N(49) 163.62(9)

N(42)-Ni(2)-N(49) 78.46(9)

N(32)-Ni(2)-N(49) 100.85(9)

Magnetic Susceptibility and EPR Measurements. Mag-

netic susceptibility data for polycrystalline samples of the

complexes were collected in the temperature range 2-290

K in an applied magnetic field of 1 T. Because complex 5

contains only one paramagnetic center, viz., Ni^II, it is

magnetically mononuclear and accordingly exhibits es-

sentially temperature-independent µ_effvalues of 2.93 ( 0.03

the donor atoms for the six-coordinated Ni(1) center, NiN₃O₃

(Figure 3). The Ni(1)-N (average ) 2.127 ( 0.01 Å) and

Ni(1)-O (average ) 2.055 ( 0.02 Å) distances correspond

to those of known values for Ni^IIcomplexes with this

macrocyclic amine²⁸and are in agreement with a d⁸high-

spin electronic configuration, which is also fully supported

by the magnetochemical results. A deviation from the

idealized 90° interbond angles is found within the five-

membered N-Ni(1)-N chelate rings, with the N-Ni(1)-N

(29) Chaudhuri, P.; Stockheim, C.; Wieghardt, K.; Deck, W.; Gregorzik,

R.; Vahrenkamp, H.; Nuber, B.; Weiss, J. Inorg. Chem. 1992, 31,

1451.

(28) Chaudhuri, P.; Ku¨ppers, H.-J.; Wieghardt, K.; Gehring, S.; Haase, W.;

(30) Burdinski, D.; Bill, E.; Birkelbach, F.; Wieghardt, K.; Chaudhuri, P.

Inorg. Chem. 2001, 40, 1160.

Nuber, B.; Weiss, J. J. Chem. Soc., Dalton Trans. 1988, 1367.

9010 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

Figure 4. Molecular structure of the cation in complex [LZn^IINi^II(PyA-H)₃]⁺(5).

Table 5. Selected Bond Lengths (Å) and Angles (deg) for Complex 5

Table 6. Trigonal Distortion of the [Ni(oximate)₃]^-fragment in

Complexes 1-5, 7, and 8

Zn‚‚‚Ni

3.472

Fe^III‚‚‚Ni^II,

Ni^II‚‚‚Ni^II, or

Zn^II‚‚‚Ni^II(Å)

av twist

angle ψ

(deg)^a

dihedral

angle θ

(deg)^b

Zn(1)-O(41)

Zn(1)-O(21)

Zn(1)-O(31)

Zn(1)-N(7)

Zn(1)-N(4)

Zn(1)-N(1)

2.0547(12) Ni(1)-N(22)

2.0851(12) Ni(1)-N(42)

2.1078(12) Ni(1)-N(32)

2.2024(15) Ni(1)-N(49)

2.2163(16) Ni(1)-N(29)

2.2271(16) Ni(1)-N(39)

2.0325(14)

2.0324(14)

2.0504(14)

2.1088(14)

2.1164(13)

2.1193(14)

complex

Ni^IIFe^IIINi^II(1)

Mn^IIINi^II(2)

Ni^IINi^II(3a)

3.545

3.506

3.491

3.434

3.423

3.554

3.472

3.570

3.552

40.0

39.3

40.4

40.1

43.4

38.3

41.3

38.0

36.9

38.4

39.8/44.2/39.0

41.3/44.3/43.7

35.8/40.4/41.8

46.8/46.8/46.8

38.5/38.7/30.0

43.9/44.9/41.8

36.5

Ni^IINi^II(3b)

Ni^IINi^II(3c)

O(21)-N(22)

N(22)-C(23)

O(31)-N(32)

1.3354(17) N(32)-C(33)

1.283(2)

1.3395(17)

1.294(2)

Ni^IINi^II(4)

1.294(2)

1.3493(18) N(42)-C(43)

O(41)-N(42)

Zn^IINi^II(5)

Ni^IIMn^IIINi^II(7)¹⁶

Ni^IICr^IIINi^II(8)¹⁶

35.2

O(41)-Zn(1)-O(21) 96.52(5)

O(41)-Zn(1)-O(31) 95.44(5)

O(21)-Zn(1)-O(31) 97.07(5)

O(41)-Zn(1)-N(7) 169.31(6)

O(21)-Zn(1)-N(7)

O(31)-Zn(1)-N(7)

O(41)-Zn(1)-N(4)

O(21)-Zn(1)-N(4)

O(31)-Zn(1)-N(4) 169.93(5)

91.07(5)

^aThe trigonal twist angle ψ is the angle between the triangular faces

comprising N(29)-N(39)-N(49) (i.e., three pyridine N atoms) and N(22)-

N(32)-N(42) (i.e., azomethine N atoms) and has been calculated as the

mean of the Newman projection angles viewed along the centroid of focus.

For an ideal trigonal-prismatic arrangement, ψ is 0° and 60° for an

octahedron (or trigonal antiprism). ^bThat the core Ni(N-O)M is not planar

is shown by the dihedral angles θ between the planes comprising Ni(N-

O) and M(O-N) atoms.

N(7)-Zn(1)-N(4)

O(41)-Zn(1)-N(1)

81.47(6)

92.01(5)

89.40(5)

92.65(5)

89.50(5)

O(21)-Zn(1)-N(1) 167.82(5)

O(31)-Zn(1)-N(1)

90.74(5)

N(7)-Zn(1)-N(1)

N(4)-Zn(1)-N(1)

80.89(6)

80.30(6)

N(32)-Ni(1)-N(39) 78.14(5)

N(49)-Ni(1)-N(39) 93.74(6)

N(29)-Ni(1)-N(39) 93.08(5)

N(22)-O(21)-Zn(1) 110.15(9)

C(23)-N(22)-O(21) 118.98(14)

C(23)-N(22)-Ni(1) 117.24(12)

O(21)-N(22)-Ni(1) 123.77(10)

N(32)-O(31)-Zn(1) 107.82(9)

C(33)-N(32)-O(31) 118.77(14)

C(33)-N(32)-Ni(1) 116.59(12)

O(31)-N(32)-Ni(1) 124.63(10)

N(42)-O(41)-Zn(1) 111.11(9)

C(43)-N(42)-O(41) 118.42(14)

N(22)-Ni(1)-N(42) 89.92(6)

N(22)-Ni(1)-N(32) 90.23(5)

N(42)-Ni(1)-N(32) 89.40(6)

N(22)-Ni(1)-N(49) 164.48(6)

N(42)-Ni(1)-N(49) 78.57(5)

N(32)-Ni(1)-N(49) 99.92(5)

N(22)-Ni(1)-N(29) 78.43(6)

N(42)-Ni(1)-N(29) 101.21(5)

N(32)-Ni(1)-N(29) 164.35(6)

N(49)-Ni(1)-N(29) 93.50(6)

N(22)-Ni(1)-N(39) 99.86(6)

N(42)-Ni(1)-N(39) 164.10(5)

for isotropic exchange coupling with S₁) S₃) S_Ni) 1 and

S₂) S_Fe) /₂for 1, S₁) S_Mn) 2, S₂) S_Ni) 1, and S₃)

5

0 for 2, and S₁) S₂) S_Ni) 1 and S₃) 0 for 3a-c and 4.

The experimental data as the effective magnetic moments

(µ_eff) vs temperature (T) are displayed in Figures 5 and 6.

The experimental magnetic data were simulated using the

least-squares fitting computer program Julx³¹with a full-

matrix diagonalization approach, and the solid lines in

Figures 5 and 6 represent the simulations.

The magnetic moment µ_efffor 1, Ni^IIFe^IIINi^II, of 5.26 µ_B

(ø_MT ) 3.465 cm³‚K‚mol^-1) at 290 K decreases monotoni-

cally with decreasing temperature until it reaches a value of

1.77 µ_B(ø_MT ) 0.3916 cm³‚K‚mol^-1) at 30 K. Because of

normal field saturation, the magnetic moment decreases

further and reaches a value of 1.37 µ_B(ø_MT ) 0.2346

µ_Bfor T > 20 K. Simulations of the experimental magnetic

moment data yield a g_Nivalue of 2.062 with S_Ni) 1 and S_Zn

) 0 for 5.

For complexes 1-4, we use the Heisenberg spin Hamil-

tonian in the form

(31) Bill, E. Max-Planck-Institut fu¨r Bioanorganische Chemie: Mu¨lheim

H ) -2J(S ‚S + S ‚S ) + g µ S ‚B

∑

1

2

3

i B i

an der Ruhr, Germany, 2005.

Inorganic Chemistry, Vol. 46, No. 21, 2007 9011

Chaudhuri et al.

than 2.0, but without any success; all fittings were of poor

quality. Moreover, g values cannot be evaluated accurately

from the bulk susceptibility measurements. Hence, we prefer

the parameters reported above.

The magnetic behavior of Mn^IIINi^II(2) is characteristic of

an antiferromagnetic coupling of S_Mn) 2 and S_Ni) 1 in the

dinuclear complex. At 290 K, the µ_effvalue of 5.45 µ_B(ø_MT

) 3.707 cm³‚K‚mol^-1) decreases monotonically with de-

creasing temperature until it reaches a value of 2.85 µ_B(ø_MT

) 1.013 cm³‚K‚mol^-1) at 15 K, below which it drops rapidly

to 1.545 µ_B(ø_MT ) 0.2983 cm³‚K‚mol^-1) at 2 K. Below 15

K, the decrease in µ_effmay be due to the zero-field splitting

of the ground state S_t) 1, expected from an antiferromag-

netic coupling between two paramagnetic centers of S_Mn

)

2 and S_Ni) 1. The temperature dependence of µ_effwas well

simulated with parameters J_MnNi) -9.9 cm^-1, g_Mn) 1.99,

g_Ni) 2.17, and |D| ) 4.1 cm^-1, and the simulation is shown

as a solid line in Figure 5. It is to be noted that the

antiferromagnetic interaction operating in 2 is stronger than

that in Ni^IIMn^IIINi^II(7)^19airrespective of the nearly identical

Ni‚‚‚Mn distances [3.57 Å (7) vs 3.51 Å (2)]. We attribute

this to more covalence in the three Mn-N and three Mn-O

bonds together with the distortion of the Mn center for 2 in

comparison to that for only six Mn-O bonds in 7. This result

also underlines the importance of nonbridging ligands in

determining the strength of exchange-coupling interactions.

The magnetic moment µ_eff/molecule for Ni^IINi^II(PyA-H)₃

(3a) of 3.78 µ_B(ø_MT ) 1.787 cm³‚K‚mol^-1) at 290 K is

much smaller than the “spin-only” value of 4.00 µ_Bfor two

uncoupled Ni^IIions of spin S ) 1.0. The magnetic moment

decreases with decreasing temperature, reaching a value of

0.086 µ_B(ø_MT ) 0.937 × 10^-3cm³‚K‚mol^-1) at 2 K, which

is a clear indication of an antiferromagnetic exchange

coupling between two paramagnetic centers Ni^II(S_Ni) 1)

with a resulting diamagnetic ground state. The solid line in

Figure 6 represents the best fit with the following param-

eters: J ) -33.6 cm^-1, g_Ni) 2.156, and 0.2% PI,

paramagnetic impurity (S ) 1.0). Figure 6 also represents

the µ_effvs T plots for Ni^IINi^II(PyA-Me)₃(3b), Ni^IINi^II(PyA-

Ph)₃(3c), and Ni^IINi^II(MeImA)₃(4), together with their

simulations as the solid lines. The following best-fit param-

eters are evaluated: J ) -48.2 cm^-1, g_Ni) 2.198, and PI

) 0.3% for 3b, J ) -59.4 cm^-1, g_Ni) 2.211, PI ) 2.5%

for 3c, and J ) -27.0 cm^-1, g_Ni) 2.224, and PI ) 0.8%

for 4. Thus, the strength of the exchange coupling follows

the order 3c > 3b > 3a > 4, indicating that the substituents

on the aldehydic C can influence the strength of the exchange

coupling. Additionally, a comparison of the magnetic

behavior of 3a with that of 4 reveals the stronger σ-donor

ability of the pyridine N than that of the imidazole N. The

variation in the strength of the exchange couplings within

these four dinuclear Ni^IIcomplexes can be qualitatively

correlated to (i) the Ni-N_oximebond lengths, (ii) the non-

bonding Ni(1)‚‚‚Ni(2) distances, and (iii) the distortion of

the trigonal prism to the octahedron of the Ni^II(oximate)₃

unit, which enhances the antiferromagnetic exchange cou-

pling by making the magnetic orbitals’ overlap favorable.

Figure 5. Plots of µ_effvs T for complexes 1 and 2. The solid lines represent

the simulations using the parameters given in the text.

Figure 6. Simulated (solid lines) and experimental data for the µ_effvs T

plots of complexes 3a-c and 4.

cm³‚K‚mol^-1) at 2 K. This temperature dependence is in

agreement with a moderate antiferromagnetic coupling

between the neighboring Ni^IIand Fe^IIIions, resulting in an

1

S_t) /₂ground state for 1. The temperature dependence of

µ_effwas well simulated, shown as a solid line in Figure 5,

with parameters J ) -28.5 cm^-1and g_Fe) g_Ni) 2.00. The

observed antiferromagnetic coupling agrees well with the

comparable exchange-coupling constants reported earlier.^4,9,13

Because g_Niis generally expected to be greater than 2.0,

we attempted to fit the experimental data for 1 with g_Niother

For establishing magnetostructural correlations for such Ni^II

2

9012 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

complexes, more oximato-bridged paramagnetic complexes

of Ni^IIare thus warranted. To date, only a few structurally

characterized oximato-bridged homometal Ni^IIcomplexes

have been subjected to magnetic susceptibility measurements

to evaluate the nature of exchange coupling; the antiferro-

magnetic exchange-coupling constant exhibits a wide varia-

tion, -7 cm^-1> J > -80 cm^-1. Although 3a-c and 4 are

the only known triply oximato-bridged Ni^II₂complexes, their

J values lie well within the range reported in the literature.^13,32

Considering the O and N atoms of the bridging oximato

groups as sp²-hybridized in the network Ni(O-N)₃Ni or Mn-

(O-N)₃Ni, the nature of the exchange interactions between

the spin carriers can be rationalized on the basis of the

established Goodenough-Kanamori rules³³for superex-

change as presented concisely by Ginsberg³⁴and later by

Kahn³within the framework of localized spins. Thus, the

following dominant interactions of the sp²orbitals (oxime

N and O atoms) on either side of the bridging oximate can

be envisaged for the Ni^IINi^IIpair, present in complexes 3a-c

and 4

d_x₂_-y₂||σ ²(NO)||d′_x₂_-y₂

antiferromagnetic

weak antiferromagnetic

Figure 7. Cyclic voltammograms of complexes 2-6 recorded in CH₂Cl₂

solutions containing 0.1 M [(n-Bu)₄N]PF₆as the supporting electrolyte at

ambient temperatures and a scan rate of 100 mV s^-1. A glassy carbon

working electrode (0.03 cm²) was used, and potentials are referenced versus

the Fc⁺/Fc couple.

sp

d_z₂||σ ²(NO)||d′_z₂

sp

d_x₂_-y₂||σ ²(NO) d′_z₂

ferromagnetic

sp

Table 7. Formal Electrode Potentials for Oxidation and Reduction (in

V vs Fc⁺/Fc) of Complexes 2-5 [Measured at Ambient Temperatures in

CH₂Cl₂Solutions Containing 0.1 M [(n-Bu)₄N]PF₆]^a

which lead to an overall exchange interaction as antiferro-

magnetic and whose strength is expected to be much weaker

than that for the oximato-bridged Cu^IIcomplexes. This is

complex

oxidation oxidation oxidation reduction

2

[LMn^III(PyA-H)₃Ni^II]²⁺(2) +1.43 m

+0.38

+0.69

+0.59

+0.62

+0.58

+0.74

-0.30

[LNi^II(PyA-H)₃Ni^II]⁺(3a)

+1.29

+0.18

+0.055

+0.066

+0.11

+0.25

in complete accordance with the reported values for the

oximato-bridged Ni^{II 13,32}and Cu^{II 35}complexes. Similarly,

the superexchange pathway e_g(Ni)||σ_NO||e′_g(Fe) determines

the sign of J for the Fe^IIINi^IIpair in complex 1. On the other

[LNi^II(PyA-Me)₃Ni^II]⁺(3b) +1.14

[LNi^II(PyA-Ph)₃Ni^II]⁺(3c) +1.28

[LNi^II(MeImA)₃Ni^II]⁺(4)

[LZn^II(PyA-H)₃Ni^II]¹⁺(5)

+1.27 irr

[LZn^II(PyA-H)₃Zn^II]¹⁺(6) +0.90 irr

III

II

2

hand, d_z(Mn )||σ_NO||d′_z(Ni ) is determinant for the sign of

the exchange coupling for the Mn^IIINi^IIpair in complexes 2

and 7.

^am: measured in CH₃CN. irr: redox process is irreversible; peak

potentials are given.

M [(n-Bu)₄N]PF₆. A conventional three-electrode arrange-

ment was used, consisting of a glassy carbon working

electrode, a Ag/AgNO₃reference electrode, and a Pt wire

counter electrode. Ferrocene was added as an internal

standard after completion of a set of experiments, and

potentials are referenced vs the ferrocenium/ferrocene (Fc⁺/

Fc) couple. The cyclic voltammograms of the complexes are

shown in Figure 7, and the redox potentials obtained from

the voltammograms are presented in Table 7.

Electrochemistry. Cyclic voltammograms of complexes

2-6 have been recorded in CH₂Cl₂solutions containing 0.1

(32) (a) Agnus, Y.; Louis, R.; Jesser, R.; Weiss, R. Inorg. Nucl. Chem.

Lett. 1976, 12, 455. (b) Fauss, J.; Lloret, F.; Julve, M.; Clemente-

Juan, J. M.; Munoz, M.; Solans, X.; Font-Bardia, M. Angew. Chem.,

Int. Ed. Engl. 1996, 35, 1485. (c) Pavlishchuk, V. V.; Kolotilov, S.

V.; Addison, A. W.; Prushan, M. J.; Butcher, R. J.; Thompson, L. K.

Inorg. Chem. 1999, 38, 1759. (d) Pavlishchuk, V. V.; Kolotilov, S.

V.; Addison, A. W.; Prushan, M. J.; Schollmeyer, D.; Thompson, L.

K.; Goreshnik, E. A. Angew. Chem., Int. Ed. Engl. 2001, 40, 4734.

(e) Pavlishchuk, V. V.; Birkelbach, F.; Weyhermu¨ller, T.; Wieghardt,

K.; Chaudhuri, P. Inorg. Chem. 2002, 41, 4405. (f) Khanra, S.;

Weyhermu¨ller, T.; Rentschler, E.; Chaudhuri, P. Inorg. Chem. 2005,

44, 8176.

(33) (a) Goodenough, J. B. Magnetism and the Chemical Bond; Wiley:

New York, 1963. (b) Goodenough, J. B. J. Phys. Chem. Solids 1958,

6, 287. (c) Kanamori, J. J. Phys. Chem. Solids 1959, 10, 87.

(34) Ginsberg, A. P. Inorg. Chim. Acta ReV. 1971, 5, 45.

(35) A comprehensive list of J values for oximato-bridged polynuclear

copper(II) complexes are given in ref 13 (see Table 1).

Coulometric experiments for the reversible redox processes

were performed (at -25 °C) at appropriate potentials to

determine the number of electrons/molecule transferred in

the process. It was found that all reversible CV waves

represent one-electron oxidations (with 2, there is additionally

a one-electron reduction). For EPR analysis, after completion

of coulometry, samples of the fully oxidized or reduced forms

were transferred into EPR tubes and frozen to liquid-nitrogen

temperature.

Complex Zn^IIZn^II(6) contains only redox-inactive Zn^II

metal ions, and its cyclic voltammogram exhibits only one

irreversible oxidation at +0.93 vs Fc⁺/Fc. Clearly, this

Inorganic Chemistry, Vol. 46, No. 21, 2007 9013

Chaudhuri et al.

oxidation must occur at the ligand system. Because the

triazacyclononane ligand L is known not to be oxidizable in

this potential range,²⁷the oxidation must be due to the

(irreversible) oxidation of the pyridine-2-aldoxime moiety

in the PyA-H ligand.

Complex Zn^IINi^II(5) is composed of the Ni-containing

-

Ni^II(PyA-H)₃unit, ligated to the redox-inert LZn^II. Here,

an irreversible oxidation (at +1.27 V; again assigned to the

PyA oxidation) is preceded by two reversible oxidations at

+0.25 and +0.74 V vs Fc⁺/Fc. These additional reversible

oxidations must both arise from the Ni^IIion of the Ni^II(PyA)₃^-

ligand. Hence, they represent a successive, stepwise oxidation

of the Ni to its Ni^IIIform and then to its Ni^IVform. It is

known that oxime ligands¹⁴stabilize high-valent Ni oxidation

states, and the EPR spectrum (see the EPR of Electrogen-

erated Species section) confirms indeed that the reversible

oxidations are Ni-centered, unless a Mn ion is present.

Complex Ni^IINi^II(3a) carries an additional Ni ion in LNi^II

and thus a further oxidizable site. It shows two reversible

oxidations (0.18 and 0.69 V vs Fc⁺/Fc) at potentials similar

to those observed with 5. Clearly, the oxidations are Ni-

centered, and the similarity of the potentials of 3a-c and 4

suggests that they occur at (PyA)₃Ni^II, the common structural

element of 3a-c and 4. This is fully confirmed by the EPR

measurements (see the EPR of Electrogenerated Species

section). Therefore, the alternative mechanism of a Ni

oxidation at LNi^IIcan be ruled out, and the oxidations are

unambiguously assigned to Ni-based oxidations at (PyA)₃Ni^II,

yielding [LNi^II(oxime)₃Ni^III]²⁺and [LNi^II(oxime)₃Ni^IV]³⁺

species. This confirms once more that aldoximes stabilize

high-valent Ni centers better than other ligands, in our case

better than ligand L. An additional third oxidation is

discernible at +1.29 V, close to the border of the accessible

potential range of CH₂Cl₂. Because after oxidation a reverse,

re-reduction peak is clearly visible, this oxidation is (chemi-

cally) reversible, in contrast to the ligand-centered oxidation

observed with 5 and 6. Therefore, we assign it to the

oxidation of the Ni^IIion in LNi^IIto LNi^III. The symmetric

and narrow shape of the reverse wave of the oxidation is

typical for a “stripping” peak and indicates that complexes

3 and 4 after the third oxidation, being then a tetracation

[LNi^III(oxime)₃Ni^IV]⁴⁺, are no longer soluble in CH₂Cl₂and

rapidly deposit on the surface of the working electrode.

The Mn^III-containing complex 2 is a dication, in contrast

to the monocationic compounds 3-5. Therefore, if the first

oxidation of 2 is Ni-centered, it should occur at potentials

close to those of the second oxidation of 3a-c and 4, i.e.,

around +0.65 V. However, it occurs at significant lower

potentials, at +0.38 V. Therefore, we assign it to the

oxidation LMn^IIIf LMn^IV, which is again confirmed by the

EPR measurements (see the EPR of Electrogenerated Species

section). The reduction of 2 at -0.30 V is assigned to the

Mn^III/Mn^IIcouple because the alternative formation of Ni^I

does not appear feasible under such mild reducing potentials

and it was not observed with complexes 3a-c and 4. This

assignment is again in accordance with the EPR measure-

ments. One might expect a Ni-centered oxidation [(PyA)₃Ni^II

f (PyA)₃Ni^III] as a second oxidation step of 2, at potentials

Figure 8. X-band EPR spectrum of oxidized 2, [2]⁺, in CH₂Cl₂/toluene

(1:1) at 10 K. Experimental parameters: frequency 9.64 GHz, power 10

µW, modulation 1 mT. The dotted line is a spin Hamiltonian simulation

1

for total spin S_t) /₂with parameters given in the text.

where the third oxidation occurred with 3 (i.e., at around

+1.3 V). It was not found within the usable potential range

of CH₂Cl₂(<1.3 V vs Fc⁺/Fc); however, in CH₃CN, we

detected a reversible oxidation at +1.43 V, which is

assignable to the above Ni-centered oxidation process.

EPR of Electrogenerated Species. The one-electron-

oxidized product [2]⁺of Mn^IIINi^II(2) obtained by coulometric

experiments in CH₂Cl₂/toluene (1:1) was subjected to X-band

EPR spectroscopy at low temperatures. Figure 8 presents the

derivative spectrum of [2]⁺at 10 K and its simulation.

The EPR spectrum exhibiting well-resolved sextets of the

characteristic ⁵⁵Mn hyperfine lines (I ) ⁵/₂) clearly indicates

a S ) ¹/₂powder pattern, resulting from the antiferromagnetic

3

interaction between the Mn^IV(S ) /₂) and Ni^II(S ) 1)

centers of the oxidized species, [2]⁺. This is in complete

accordance with the assignment of the Mn^III-centered oxida-

tion (see the Electrochemistry section) of the starting

compound 2 containing the Mn^IIINi^IIcore. The alternative

assignment of the Ni^II-centered oxidation should have led

to a ground state of S ) ³/₂, or ⁵/₂arising from spin-exchange

interactions between the spins of the oxidized species Mn^III-

Ni^IIIwith S_Mn) 2 and S_Ni)¹/₂. The simulation obtained

with the spin Hamiltonian parameters, g_x) 1.92, g_y) 1.94,

g_z) 1.936, A_x) 115.4 G, A_y) 104.7 G, and A_z) 111.2 G,

is shown as the dotted line in Figure 8. The g values smaller

than 2.00 indicate the dominance of g_Mnover g_Ni, as is also

in conformation with the spin-projection coefficients, g )

(5/3)g_Mn- (2/3)g_Ni(with g_Ni> 2).

III

The second oxidation of 2, [2]²⁺, performed in CH₃CN

leads to an EPR-silent species due to an integer-spin system

(S ) 1 or 2). No further characterization of the putative Ni^IV

in [2]²⁺was performed.

The electrooxidized species of 5, resulting in [LZn^II(PyA-

H)₃Ni^III]²⁺, provided EPR spectra at both X and Q bands,

shown in Figure 9.

The assignment from the electrochemical measurements

of the formation of the Ni^IIIcenter in [5]⁺is fully cor-

9014 Inorganic Chemistry, Vol. 46, No. 21, 2007

Tridentate Facial Ligation of [Ni(PyA-R)₃]^-and [Ni(MeImA)₃]^-

Figure 9. Q- and X-band (inset) EPR spectra of [5]⁺in CH₂Cl₂/toluene (1:1) at 10 K.

roborated with the EPR measurements. The EPR spectra

clearly show the low-spin character of Ni^IIId⁷in [5]⁺.³⁶The

Q-band spectrum is virtually axial with three principal g

values of 2.127, 2.125, and 2.03. At the X band, the spectrum

(inset of Figure 9) is rather broad, presumably because of

intermolecular interactions. The average g value, g ) 2.094,

indicates that the unpaired electron is associated primarily

with the Ni ion and not with the ligand. The present behavior

2

with g > g is consistent with a d_zground state in a

||

tetragonally distorted octahedral geometry with elongation

of the axial bonds. Thus, upon removal of an electron from

the Ni^IIcenter in 5, there is a structural change.

Figure 10. X-band EPR spectrum of one-electron-oxidized [3c]⁺measured

at 4.5 K (frequency 9.439924 GHz, power 200 µW, modulation 2 mT/200

kHz). The inset shows the absorption spectrum obtained by numerical

integration (dots). The thin solid lines are the result of a simulation for S_t

) ³/₂, with g ) (2.053, 2.053, 2.1), D ) 1 cm^-1, E/D ) 0.07, and a Gaussian

distribution of the rhombicity parameter with half-width σ(E/D) ) 0.09

and frequency-dependent line width Γ_ν) 12 mT (fwhm at g ) 2). The

intensity of the simulated absorption spectrum accounts for 65% of the total

intensity of the experimental absorption spectrum.

That the first electrochemical oxidation of 3a-c and 4

provides a mixed-valent Ni^IINi^IIIspecies, in which the Ni^III

center is associated only with the oxime N atoms, has also

been checked by the EPR measurements at helium temper-

atures. As a prototype of complexes 3 and 4, we have chosen

complex 3c, containing the phenyl derivative of the ligand

pyridine-2-aldoxime, PyA-Ph, for the coulometric experi-

ments, and the electrooxidized species [3c]⁺was subjected

to EPR measurements.

The X-band EPR spectrum of [3c]⁺measured in a frozen

solution at liquid-helium temperatures shows the superposi-

tion of an asymmetric derivative signal with shoulders and

satellite lines centered at g ≈ 2.2 and a broad low-field peak

at about B ) 165 mT (Figure 10).

actual derivative signal at g ) 4 as well as its contribution

in the absorption spectrum obtained by numerical integration

(inset of Figure 10) could be very well simulated with spin

3

S ) /₂and the parameters g ) (2.053, 2.053, 2.1), D ) 1

cm^-1(or any higher value), and E/D ) 0.07 (solid lines). A

Gaussian distribution of the rhombicity parameter with half-

width σ(E/D) ) 0.09 and a frequency-dependent line width

Γ_ν) 12 mT (fwhm at g ) 2) were applied to account for

the correct shape of the powder spectrum. This component

of the experimental spectrum must arise from the total spin

The high g factor of the latter signal, g ≈ 4, is typical of

a spin quartet species with large zero-field splitting and axial

symmetry (|D| . hν and E/D ≈ 0). In this situation, the

3

quartet state, S_t) /₂, of the coupled system Ni^II, S₁) 1,

1

and Ni^III, S₂) /₂. Because it is thermally populated at 4.7

3

effective g_x,yvalues of the |S ) /₂, m_s) (¹/₂Kramers

K, the exchange coupling constant apparently is either

positive or very weak.

doublet are equal to the value of 2S + 1, whereas g_zis at 2.

3

Moreover, the |S ) /₂, m_s) (³/₂Kramers doublet is then

One might speculate that the more complex signals

centered at g ≈ 2.2 represent the corresponding total spin

doublet, S_t) ³/₂, of the Ni^IINi^IIIsystem. This interpretation,

however, cannot be correct for the following reasons: (i)

The relative intensities of the g ≈ 2.2 signals are preparation-

extremely anisotropic and virtually EPR-silent. In fact, the

(36) (a) Chakravorty, A. Isr. J. Chem. 1985, 2599. (b) Lappin, A. G.;

McAuley, A. AdV. Inorg. Chem. 1988, 32, 241. (c) Salerno, J. C. In

The Bioinorganic Chemistry of Nickel; Lancaster, J. R., Ed.; VCH:

New York, 1988; p 53.

Inorganic Chemistry, Vol. 46, No. 21, 2007 9015

Chaudhuri et al.

dependent and scattered between 35% and more than 60%

at 4.2 K. (ii) The temperature dependence of its relative

intensity does not obey a Boltzmann law. Particularly, the

intensity ratio of the two components does not approach the

limit of equal population for T f 60 K. (iii) The effective g

tensor of the expected doublet species would not be at g ≈

2.2 for the actual ratio of exchange coupling and zero-field

interaction. The shape of the quartet spectrum proves the

presence of sizable zero-field splitting D > 1 cm^-1(as

expected from the partial parentship of spin from Ni^II),

whereas the appearance and thermal population of both

species at 4.7 K would be possible only for weak exchange

splitting on the order of kT (f|J| e 3 cm^-1). In this situation,

mixing of doublet and quartet wavefunctions due to compet-

ing zero-field splitting and exchange interaction would yield

more anisotropic g values for the putative doublet than was

observed, with all values below g ≈ 2.2 and at least one

component below g ) 2. [The same spin system and a similar

situation of intermediately strong spin coupling is encoun-

tered and was thoroughly studied for oxoferrylporphyrin

cation radical complexes for which the Fe^IVdO unit (S₁)

1) is weakly ferromagnetically coupled to the porphyrin

Ni^IINi^IIcomplexes. The thermodynamic stability of the in

-

situ generated monoanions Ni(PyA)₃and Ni(MeImA)₃,

with three facially disposed pendent oxime O atoms, makes

it feasible to use these complex anions as tridentate ligands

for other coordinatively unsaturated metal ions.^4,19aThe

exchange interaction between the central high-spin Fe^IIIand

neighboring Ni^IIis antiferromagnetic in nature, and the

exchange pathway e_g(Ni)||σ_sp²(NO)||e′_g(Fe) determines the

strength and sign of J for the Fe^IIINi^IIpair in complex 1. On

2

the other hand, the d_z(Mn)||σ_sp(NO)||d′_z(Ni) path is deter-

minant for the sign of the exchange coupling for the Mn^III-

Ni^IIpair in complexes 2 and 7.

The deliberately synthesized isostructural series of Ni^II

2

complexes 3a-c and 4 clearly demonstrate the influence of

the substitution on the aldehydic C on the strength of the

exchange coupling between the two Ni^IIcenters by changing

presumably the σ-donor property of the involved oxime N

atoms.

Electrochemical measurements indicate the reversible

oxidations of the Ni centers ligated to the oxime N atoms,

generating localized mixed-valent [LNi^IINi^III(oxime)₃]²⁺spe-

cies from 3a-c and 4. That the spin coupling in the Ni^IINi^III

1

radical, S₂) /₂].³⁷For these reasons, we have to infer that

1

II

III

system (S_Ni) 1 and S_Ni) /₂) results in a total spin quartet

3

the signals at g ≈ 2.2 do not originate from the excited total

spin doublet of the Ni^IINi^IIIspin system. We presume that

unknown intermolecular spin-spin interactions perturb our

EPR signal for a variable number of weakly attached

molecules in our frozen solution samples. However, the good

simulation of the low-field spectral component at g ≈ 4

shows that for the unperturbed subensemble the exchange

interaction is positive and gives rise to the observed quartet

EPR spectrum.

state, S_t) /₂, has been confirmed by the X-band EPR

measurements. On the contrary, for complex Mn^IIINi^II(2),

the first reversible oxidation occurs at the Mn^IIIcenter,

resulting in the [LMn^IVNi^II(oxime)₃]³⁺species. The second

oxidation is Ni-centered, generating the Mn^IVNi^IIIspecies.

These assignments are fully in conformation with the EPR

measurements, which indicate antiferromagnetic interactions

3

for the spin-coupled Mn^IVNi^II(S_Mn) /₂and S_Ni) 1)

IV

II

1

system, resulting in a doublet, S_t) /₂, ground state.

Acknowledgment. Financial support from the Max-

Planck Society and the German Research Council (DFG) is

gratefully acknowledged (Project: Priority Program “Mo-

lecular Magnetismus”, Ch111/3-3). The skillful technical

assistance of H. Schucht, A. Go¨bels, F. Reikowski, and U.

Pieper is thankfully acknowledged.

Concluding Remarks

To conclude, the following points of this study deserve

particular attention:

The ligation property of the metal complexes tris(pyridine-

2-aldoximato)nickel(II) and tris(1-methylimidazole-2-aldoxi-

mato)nickel(II) has been explored to generate heterometal

complexes such as Ni^IIFe^IIINi^II, Ni^IIMn^IIINi^II, Ni^IICr^IIINi^II,

Mn^IIINi^II, and Zn^IINi^II, together with a series of homometal

Supporting Information Available: Details of the X-ray

structural data for 1-6 and X-ray crystallographic data (CIF),

ORTEP diagrams of the cations in 3b, 3c, 4, and 6 as Figures S1-

S4, respectively, and Tables S1 and S2 listing selected bond angles

and lengths for 4 and 6, respectively. This material is available

free of charge via the Internet at http://pubs.acs.org.

(37) (a) Bill, E.; Ding, X. Q.; Bominaar, E. L.; Trautwein, A. X.; Winkler,

H.; Mandon, D.; Weiss, R.; Gold, A.; Jayaraj, K.; Hatfield, W. E.;

Kirk, M. L. Eur. J. Biochem. 1990, 188, 665. (b) Rutter, R.; Hager,

L. P.; Dhonau, H.; Hendrich, M.; Valentine, M.; Debrunner, P.

Biochemistry 1984, 23, 6809.

IC701073J

9016 Inorganic Chemistry, Vol. 46, No. 21, 2007

Article Doi

Tridentate facial ligation of tris(pyridine-2-aldoximato)nickel(II) and tris(imidazole-2-aldoximato)nickel(II) to generate NiIIFe IIINiII, MnIIINiII, Ni IINiII, and ZnIINiII and the electrooxidized MnIVNiII, NiIINiIII

DOI: 10.1021/ic701073j

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Article abstract of DOI:10.1021/ic701073j

Full text of DOI:10.1021/ic701073j

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