3d4f heterobimetallic dinuclear and tetranuclear complexes involving tetrathiafulvalene as ligands: X-ray structures and magnetic and photophysical investigations

Article

pubs.acs.org/IC

3d4f Heterobimetallic Dinuclear and Tetranuclear Complexes

Involving Tetrathiafulvalene as Ligands: X-ray Structures and

Magnetic and Photophysical Investigations

Goulven Cosquer, Fabrice Pointillart,* Boris Le Guennic, Yann Le Gal, Stephane Golhen, Olivier Cador,

́

and Lahcene Ouahab*

̀

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Universite

Rennes Cedex, France

́ ́ ́

de Rennes 1, 263 Avenue du General Leclerc, 35042

S

* Supporting Information

ABSTRACT: Six new 3d4f heterobimetallic dinuclear complexes, [(L¹)-

MLn(hfac)₃] [M = Cu^II, Ni^II; Ln = Y^III, Er^III, Yb^III; L¹= 4,5-

bis(propylthio)tetrathiafulvalene-N,N′-phenylenebis(salicylideneimine) and

hfac⁻= 1,1,1,5,5,5-hexaﬂuoroacetylacetonate], and one tetranuclear

complex, [(L²)Cu(OH)Er(hfac)₃]₂(where L²= 4,5-bis(propylthio)-

tetrathiafulvalene-N,N′-phenyleneaminosalicylideneimine), have been syn-

thesized. All of the X-ray structures of the coordination complexes have

been resolved from single-crystal diﬀraction. A quantitative magnetic

approach has allowed one to determine the Cu−Ln ferromagnetic

interaction for Gd^III(1.29 cm⁻¹) and Tb^III(0.40 cm⁻¹) and the

antiferromagnetic interaction for Dy^III(−0.46 cm⁻¹) and Yb^III(−2.25

cm⁻¹), while in the case of Er^III, the magnetic interactions are negligible.

The UV−visible absorption properties have been studied in a chloroform

solution and rationalized by DFT and TD-DFT calculations. Upon

oxidation, intramolecular SOMO → LUMO (20800 cm⁻¹) and SOMO−n → SOMO (11350 cm⁻¹) charge transfers appear,

while the HOMO → LUMO charge transfers (20750 cm⁻¹) disappear. The reversibility of the oxidation has been conﬁrmed by

electrochemistry and absorption properties upon the addition of a reducing agent. Irradiation at the HOMO → LUMO charge-

transfer energy of the dinuclear complex [(L¹)NiY(hfac)₃] induces a ligand-centered ﬂuorescence at 14450 cm⁻¹.

Besides their magnetic properties, another advantage of

lanthanides lies in their speciﬁc luminescence properties with,

among others, potential applications in chelate lasers,¹⁰eﬃcient

organic light-emitting diodes (OLEDs),¹¹and polymer light-

emitting diodes (PLEDs).¹²

INTRODUCTION

■

Because of their strongly electron-donating and attractive

reversible redox properties, tetrathiafulvalene (TTF) and its

derivatives are used successfully as building blocks for

molecular conductors,¹molecular switches,²and solar-energy

systems.³Nowadays, the challenge of this chemistry is to

succeed the combination or the synergy between several

properties to reach the elaboration of multifunctional

materials.⁴Thus, one strategy consists of the combination of

TTF derivatives with 3d or 4f metal ions to take advantage of

their magnetic and optical properties, giving rise to the original

“π-3d”,⁵“π-4f”,⁶or “π-3d4f”⁷complexes. Such systems can be

obtained either by a “through-space” or a “through-bond”

approach. In the former, the metallic ion and the TTF core

interact through van der Waals contacts, while in the latter, the

metallic ion and the TTF core are covalently linked by a

chemical bridge. Using the second approach, the ﬁrst 3d4f

TTF-based heterobimetallic complexes were elaborated⁷to

combine the strong Ising-type magnetic anisotropy⁸of the Dy^III

and Tb^IIIions and the enhancement of the intramolecular

magnetic-exchange interactions when such ions are combined

with 3d ions.⁹

Following this strategy, we decided to extend the series of

3d4f coordination complexes with the near-infrared (NIR)-

emitters Er^IIIand Yb^IIIions associated with the [M(L¹)]

metallic precursors [where M = Cu^IIand Ni^IIand L¹= 4,5-

bis(propylthio)tetrathiafulvalene-N,N′-phenylenebis-

(salicylideneimine); Scheme 1]. The diamagnetic Y^IIIanalogue

has also been studied in order to perform quantum-chemical

calculations and to corroborate the absorption properties. The

obtained complexes have the following molecular formula:

[(L¹)]MLn(hfac)₃] [M = Cu^IIand Ln = Y^III(1), Er^III(2), Yb^III

(3); M = Ni^IIand Ln = Y^III(4), Er^III(5), Yb^III(6), where hfac⁻

= 1,1,1,5,5,5-hexaﬂuoroacetylacetonate anion]. The monosub-

stitution of the 4,5-bis(propylthio)tetrathiafulvalene-N,N′-

phenylene unit with a salicylideneimine leads to the new

ligand [L²= 4,5-bis(propylthio)tetrathiafulvalene-N,N′-phenyl-

Received: May 22, 2012

A

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Scheme 1. Synthetic Procedures for L¹and L²

Table 1. Crystallographic Data for Compounds 3, 4, and 7

(L¹)CuYb(hfac)₃(3)

(L¹)NiY(hfac)₃(4)

[(L²)Cu(OH)Er(hfac)₃]₂(7)

formula

C₄₅H₂₉CuF₁₈N₂O₈S₆Yb

1496.7

C₄₅H₂₉F₁₈N₂NiO₈S₆Y

1407.68

C₇₆H₅₂Cu₂Er₂F₃₆N₄O₁₆S₁₂

M/g mol⁻¹

cryst syst

space group

cell param

2805.6

triclinic

P1 (No. 2)

̅

P1 (No. 2)

̅

P1 (No. 2)

̅

a = 12.5872(8) Å

b = 13.7283(9) Å

c = 17.4659(8) Å

α = 87.316(4)°

β = 88.101(4)°

γ = 66.921(2)°

2773.2(3)

2

a = 13.2766(6) Å

b = 13.3127(10) Å

c = 16.5097(13) Å

α = 85.513(4)°

β = 77.716(4)°

γ = 84.331(4)°

2832.4(3)

2

a = 12.8981(13) Å

b = 13.7598(16) Å

c = 18.7360(20) Å

α = 93.903(4)°

β = 95.854(5)°

γ = 111.867(5)°

3049.5(6)

1

volume/Å³

Z

T/K

293(2)

2θ range/deg

ρ_calc/g cm⁻³

μ/mm⁻¹

no. of reﬂns

indep reﬂns

R_int

4.50 ≤ 2θ ≤ 50.78

1.791

5.92 ≤ 2θ ≤ 49.54

1.651

4.32 ≤ 2θ ≤ 50.90

1.527

2.397

1.682

2.017

15432

14453

17037

9464

8869

10434

0.0383

0.0486

0.0627

F_o> 2σ(F_o)²

2

6789

5091

5711

no. of variables

R1, wR2

905

899

646

0.0456, 0.1065

0.0517, 0.1149

0.0607, 0.1355

eneaminosalicylideneimine; Scheme 1]. The treatment of L²by

3d Cu^IIand 4f Er^IIIgives the tetranuclear complex of formula

[Cu(L²)(OH)Er(hfac)₃]₂(7).

The magnetic properties of the whole series of complexes

were studied and the nature of the interaction determined using

an quantitative method. The optical properties were measured

and discussed upon oxidation and reduction.

Crystallography. Single crystals were mounted on a Nonius four-

circle diﬀractometer equipped with a CCD camera and a graphite-

monochromated Mo Kα radiation source (λ = 0.71073 Å), from the

́ ́

Centre de Diﬀractometrie (CDIFX), Universite de Rennes 1, Rennes,

France. Data were collected at 293 K. Structures were solved with a

direct method using the SIR-97 program and reﬁned with a full-matrix

least-squares method on F²using the SHELXL-97 program.¹⁵

Crystallographic data are summarized in Table 1. Because of the

disorder and the large thermal factors of the propyl groups at 293 K,

the C atoms of these groups were reﬁned isotropically for 7. A

SQUEEZE procedure was realized. Data are listed for all voids in the

P1 unit cell, i.e., center of gravity, solvent-accessible volume, recovered

number of electrons in the void (90 electrons, corresponding to a n-

hexane molecule), and details about the squeezed material are given in

the CIF ﬁle.

EXPERIMENTAL SECTION

■

General Procedures and Materials. All solvents were dried

using standard procedures. 4,5-Bis(propylthio)tetrathiafulvalene-N,N′-

phenylenebis(salicylideneimine) (L¹),¹³M-4,5-bis(propylthio)-

tetrathiafulvalene-N,N′-phenylenebis(salicylideneimine)¹³[M(L¹)]

(M = Cu, Ni), and Ln(hfac)₃·2H₂O¹⁴(Ln = Y, Er, Yb) compounds

were prepared according to literature procedures. All other reagents

were purchased from Aldrich Co. Ltd. and were used without further

puriﬁcation.

Physical Measurements. Cyclic voltammetry was carried out in a

CH₂Cl₂solution, containing 0.1 M (C₄H₉)₄NPF₆as the supporting

electrolyte. Voltammograms were recorded at 100 mV s⁻¹at a

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Table 2. Cell Parameters for Compounds 1, 2, 5, and 6

(L¹)CuY(hfac)₃(1)

(L¹)CuEr(hfac)₃(2)

(L¹)NiEr(hfac)₃(5)

(L¹)NiYb(hfac)₃(6)

formula

M/g mol⁻¹

C₄₅H₂₉CuF₁₈N₂O₈S₆Y

1411.4

C₄₅H₂₉CuErF₁₈N₂O₈S₆

1489.8

C₄₅H₂₉ErF₁₈N₂NiO₈S₆

1485.0

C₄₅H₂₉F₁₈N₂NiO₈S₆Yb

1490.7

cryst syst

triclinic

space group

cell parameters

P1 (No. 2)

̅

a = 12.66(5) Å

b = 13.75(5) Å

c = 17.51(7) Å

α = 86.55(4)°

β = 87.99(4)°

γ = 67.17(2)°

2804(19)

a = 12.636(50) Å

b = 13.828(50) Å

c = 17.495(70) Å

α = 86.99(4)°

β = 87.30(4)°

γ = 66.18(2)°

2791(2)

a = 13.00(5) Å

b = 13.52(5) Å

c = 16.50(6) Å

α = 77.316(3)°

β = 87.239(5)°

γ = 86.021(5)°

2821(18)

a = 13.11(4) Å

b = 13.37(2) Å

c = 16.51(6) Å

α = 77.23(4)°

β = 87.40(5)°

γ = 86.00(5)°

2814(14)

volume/Å³

T/K

293(2)

platinum disk electrode. The potentials were measured versus a

saturated calomel electrode (SCE; Table 4). Optical spectra were

measured using a KBr disk method on a Perkin-Elmer 1600 Series

Fourier transform infrared spectrometer (resolution 4 cm⁻¹) for IR.

Absorption spectra were recorded on a Varian Cary 5000 UV−

visible−NIR spectrometer. The luminescence spectra were measured

using a Horiba-Jobin Yvon Fluorolog-3 spectroﬂuorimeter, equipped

with a three-slit double-grating excitation and emission monochroma-

tor with dispersions of 2.1 nm mm⁻¹(1200 grooves mm⁻¹). The

steady-state luminescence was excited by unpolarized light from a 450

W xenon continuous-wave lamp and detected at an angle of 90° for

diluted solution measurements or at 22.5° for solid-state measurement

(front-face detection) by a Peltier-cooled red-sensitive Hamamatsu

R2658P photomultiplier tube (300−1010 nm). Spectra were

reference-corrected for both the excitation source light-intensity

variation (lamp and grating) and the emission spectral response

(detector and grating). Uncorrected NIR spectra were recorded at an

angle of 45° using a liquid-nitrogen-cooled, solid indium/gallium/

arsenic detector (850−1600 nm). The direct-current magnetic

susceptibility measurements were performed on a solid polycrystalline

sample with a Quantum Design MPMS-XL SQUID magnetometer

between 2 and 300 K in applied magnetic ﬁelds of 0.2 T for

temperatures of 2−20 K and 1 T for temperatures of 20−300 K. The

experimental data were corrected from the diamagnetism of the

sample holder, and the intrinsic diamagnetism of the materials was

evaluated with Pascal’s tables.

Computational Details. Density functional theory (DFT)

geometry optimizations and time-dependent DFT (TD-DFT)

excitation energy calculations of the ligand L¹, 3d metalloprecursor

[(L¹)Ni], and 3d4f heterobimetallic dinuclear analogue 4 were carried

out with the Gaussian 09 (revision A.02) package¹⁶employing the

PBE0 hybrid functional.¹⁷The “Stuttgart/Dresden” basis sets and

eﬀective-core potentials were used to describe the Y atom,¹⁸whereas

all other atoms were described with the SVP basis sets.¹⁹The

diﬀerences with a triple-ζ quality basis set were probed on the ligands,

and they are meaningless. The ﬁrst 50 monoelectronic excitations were

calculated for L¹and [(L¹)Ni], and 110 excitations were necessary for

complex 4. In all steps, a modeling of bulk solvent eﬀects (solvent =

chloroform) was included through the polarizable continuum model

(PCM),²⁰using a linear-response nonequilibrium approach for the

TD-DFT step.²¹Molecular orbitals (MOs) were sketched using the

Gabedit graphical interface.²²

for 1 h. n-Heptane was layered to the solution, and dark-red crystals of

1 were obtained after 1 week of slow evaporation. Yield: 11.7 mg

(39%). IR: 2965, 1652, 1609, 1531, 1510, 1458, 1370, 1257, 1202,

1144, 912, 800, 757, 660, 587 cm⁻¹.

[(L¹)CuEr(hfac)₃] (2). Single crystals of 2 were obtained by a method

similar to that for 1 starting from Er(hfac)₃·2H₂O (17.3 mg, 0.021

mmol) instead of Y(hfac)₃·2H₂O. Yield: 15.4 mg (49%). IR: 2969,

1652, 1608, 1530, 1509, 1459, 1370, 1256, 1200, 1143, 912, 799, 757,

659, 586 cm⁻¹.

[(L¹)CuYb(hfac)₃] (3). Single crystals of 3 were obtained by a

method similar to that for 1 starting from Yb(hfac)₃·2H₂O (17.4 mg,

0.021 mmol) instead of Y(hfac)₃·2H₂O. Yield: 6.2 mg (20%). IR:

2969, 1669, 1652, 1608, 1542, 1509, 1458, 1370, 1257, 1203, 1144,

799, 758, 660, 586 cm⁻¹.

[(L¹)NiY(hfac)₃] (4). A 10 mL CHCl₃solution of [(L¹)Ni] (14.6 mg,

0.021 mmol) and Y(hfac)₃·2H₂O (15.7 mg, 0.021 mmol) was stirred

for 1 h. n-Heptane was added to the solution, and crystals were

obtained after 1 week of slow evaporation. Yield: 8.1 mg (27%). IR:

2969, 1652, 1608, 1530, 1509, 1458, 1370, 1257, 1202, 1144, 799, 758,

660, 586 cm⁻¹.

[(L¹)NiEr(hfac)₃] (5). Single crystals of 5 were obtained by a method

similar to that for 4 starting from Er(hfac)₃·2H₂O (17.3 mg, 0.021

mmol) instead of Y(hfac)₃·2H₂O. Yield: 12.0 mg (38%). IR: 2967,

1669, 1652, 1608, 1531, 1508, 1458, 1365, 1292, 1257, 1203, 1145,

800, 757, 660, 587 cm⁻¹.

[(L¹)NiYb(hfac)₃] (6). Single crystals of 6 were obtained by a method

similar to that for 4 starting from Yb(hfac)₃·2H₂O (17.5 mg, 0.021

mmol) instead of Y(hfac)₃·2H₂O. Yield: 8.1 mg (27%). IR: 2967,

1669, 1653, 1608, 1531, 1506, 1458, 1365, 1292, 1257, 1205, 1145,

800, 757, 660, 587 cm⁻¹.

[(L²)Cu]. A 20 mL CH₃CN solution of Cu(OAc)₂·nH₂O (37.2 mg,

0,186 mmol) was added to a 20 mL CH₂Cl₂solution of L²(100 mg,

0.186 mmol). After 12 h of stirring, the precipitate was collected and

washed with ethanol. Yield: 109 mg (98%).

[(L²)Cu(OH)Er(hfac)₃]₂(7). A 5 mL CHCl₃solution of Er-

(hfac)₃·2H₂O (20.6 mg, 0,025 mmol) was added to a 10 mL CHCl₃

solution of [(L²)Cu] (15 mg, 0,025 mmol) and stirred for 1 h. n-

Hexane was layered to the solution, and red crystals of 7 were obtained

after 1 week of slow evaporation. Yield: 11.6 mg (31%).

RESULTS AND DISCUSSION

■

Synthesis of the Ligand L²and Complexes 1−7. 4,5-

Bis(propylthio)tetrathiafulvalene-N,N′-phenyleneaminosalicylide-

neimine (L²). Salicylaldehyde (0.256 g, 2.1 mmol) was slowly added to

a CH₂Cl₂/CH₃CN (1:3) solution of 5,6-diamino-2-[4,5-bis-

(propylthio)-1,3-dithio-2-ylidene]benzone[d]-1,3-dithiole (0.909 g,

2.1 mmol) and stirred overnight. The resulting precipitate was ﬁltered

and washed with ethanol. The ligand was obtained as an orange

powder and used without puriﬁcation for complexation with a Cu^II

ion. Yield: 1.05 g (93%).

Synthesis. The 3d4f heterobimetallic dinuclear complexes

1−6 were obtained following a previously published method,⁷

which consists of coordination of the Lewis acid Ln-

(hfac)₃·2H₂O (Ln = Y, Er, Yb) to the metallic Lewis base

[(L¹)M] (M = Cu, Ni). A modiﬁcation of the synthesis of L¹

consisting of the slow addition of 1 equiv of salycylaldehyde to

5,6-diamino-2-[4,5-bis(propylthio)-1,3-dithio-2-ylidene]-

benzone[d]-1,3-dithiole permits one to obtain the monosub-

stituted ligand L²(Scheme 1). The reaction between the Lewis

[(L¹)CuY(hfac)₃] (1). A 10 mL CHCl₃solution of [(L¹)Cu] (15 mg,

0.021 mmol) and Y(hfac)₃·2H₂O (15.7 mg, 0.021 mmol) was stirred

C

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Figure 1. ORTEP views of the asymmetric units of 3 (a) and 4 (b). Thermal ellipsoids are drawn at 30% probability. H atoms are omitted for clarity.

In the case of 3, only the three hfac⁻ions with the highest occupation factors are represented.

a

acid Er(hfac)₃·2H₂O and the Lewis base [(L²)Cu] leads to the

Table 3. Selected Bond Lengths (Å) for 3, 4, and 7

unprecedented 3d4f heterobimetallic tetranuclear complex 7.

3

4

7

Crystal-Structure Analysis of [(L¹)CuYb(hfac)₃] (3) and

[(L¹)NiY(hfac)₃] (4). Our previous work has shown that two

structural families built from the Cu^IIand Ni^IIions can be

obtained, respectively.⁷The reﬁnements of the X-ray structures

of 3 and 4 were done to be representative of each of these

families (Table 1). The structures of the other members of each

family were checked by determination of the cell parameters

(Table 2).

M−O1

M−O2

M−N1

M−N2

Ln−O1

Ln−O2

Ln−O2ⁱ

Ln−O3

Ln−O4

Ln−O5

Ln−O6

Ln−O7

Ln−O8

M1−Ln1

1.926(4)

1.916(4)

1.935(5)

1.920(5)

2.356(4)

2.346(4)

1.873(3)

1.865(3)

1.846(4)

1.853(4)

2.413(4)

2.416(3)

1.926(5)

1.934(5)

1.920(6)

1.989(7)

2.424(5)

2.431(5)

2.357(5)

2.443(7)

2.384(7)

2.387(6)

2.344(6)

2.427(6)

2.394(6)

3.262(11)

2.285(5)

2.287(5)

2.278(6)

2.315(5)

2.314(5)

2.345(5)

3.185(10)

2.358(4)

2.335(4)

2.325(4)

2.336(4)

2.321(4)

3.219(10)

Both compounds crystallize in the triclinic P1 (No. 2) space

̅

group (Table 1). The asymmetric unit is composed of one

[(L¹)M] and one Ln(hfac)₃moieties linked by two bridging

phenolate groups (Figure 1). The Cu and Ni ions are in a N₂O₂

inner coordination site, connected in a square-planar environ-

ment to the two amine N and two phenoxo O atoms from the

L¹ligand. The average values of Cu−O [1.921(4) Å] and Cu−

N [1.928(5) Å] bond lengths are longer than the

corresponding ones for Ni^II[1.869(3) and 1.850(4) Å] in

agreement with the electronic conﬁguration of the 3d ions

(Table 3). Ln ions are in a slightly distorted bicapped square-

faced trigonal-prismatic coordination sphere made of eight O

atoms that arise from three bis-chelating hfac⁻anions and one

bis-chelating [(L¹)M] metalloligand. The coordination poly-

hedron has a C_2vsymmetry.

a

i: −x, −y, −z.

Supporting Information), while in 4, a reminiscent organic

network is observed with S···S contacts (Figure S2 in the

Supporting Information). The Cu^IIion interacts with two O

and N atoms in the axial positions, while the Ni^IIion realizes

only one axial contact with one O atom.

Compound 7. 7 crystallizes in the triclinic P1 (No. 2) space

̅

The central C11−C12 bond lengths of the TTF fragment are

1.334(9) Å for 3 and 1.332(8) Å for 4. The arrangement of the

propyl groups is diﬀerent in 3 and 4 (Figure 1) and leads to the

formation of diﬀerent crystal packings. In 3, dimers of L¹

donors constitute the organic network (Figure S1 in the

group (Table 1). The asymmetric unit is composed of one

[(L²)Cu] metalloprecursor and one Er(hfac)₃unit. The two

moieties are linked by one bridging phenolate (O1) and one

μ₃-hydroxo (O2) groups. The inversion center generates a

heterobimetallic tetranuclear compound with the two Er^IIIions

D

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Figure 2. ORTEP view of the molecular structure of 7. Thermal ellipsoids are drawn at 30% probability. H atoms and thermal ellipsoids for the

propyl groups are omitted for clarity.

Figure 3. Crystal packing of 7 highlighting the formation of L²dimers (space-ﬁll representation) and inorganic networks (ball-and-stick

representation).

bridged by two μ₃-OH (Figure 2). The Cu ions are in a N₂O₂

inner coordination site, connected in a square-planar environ-

ment to two N atoms and one phenoxo O atom coming from

the L²ligand and one hydroxo O atom. The average Cu−O and

Cu−N distances are 1.930(5) and 1.955(7) Å, respectively

(Table 3). The Cu−O distances are homogeneous, while the

Cu−N2 bond length is longer than the Cu−N1 one because

N2 is doubly protonated (amine) and N1 is not protonated

(imine). The Cu^IIion performs short contacts with two O

atoms coming from hfac⁻anions in the axial positions [Cu1−

O5 = 2.575(6) Å and Cu1−O8 = 2.553(6) Å].

respective shortest intermolecular distances were found to be

equal to 9.123, 10.109, and 11.336 Å.

Large voids were observed between the dimers of donors

(Figure 3), giving rise to channels (Figure S3 in the Supporting

Information) that are ﬁlled by n-hexane molecules.

Electrochemical Properties. The redox properties of

complexes 2, 5, and 7 are investigated by cyclic voltammetry.

The cyclic votammograms for the dinuclear and tetranuclear

complexes show two monoelectronic oxidations at 0.637 and

1.003 V for 2, 0.638 and 0.999 V for 5, and 0.621 and 0.988 V

for 7. They correspond successively to the formation of the

TTF fragment radical cation and dication (Figure S4 in the

Supporting Information). Oxidation potentials are shifted to

higher positive values relative to the parent TTF due to partial

electron transfer from the donor (TTF) to the N,N′-

phenylenbis(salicylideneimine) or N,N′-phenyleneaminosalicy-

lideneimine acceptor moiety. The values of the oxidation

potentials are in agreement with those previously found for the

[(L¹)M] (M = Cu^II, Ni^II)¹³and [(L¹)MLn(hfac)₃] (Ln = Gd^III,

Tb^III, Dy^III)⁷complexes. Only a very weak decrease of the

electron transfer from the TTF core to the acceptor is observed

in L²compared to L¹. Nevertheless, the electrochemical

properties attest to the redox activity of the complexes. Such

redox properties are required to play a role as molecular

precursors for 3d4f heterobimetallic conducting magnets.

The Er^IIIcenters are nine-coordinated, and the Er−O

distances range from 2.344(6) to 2.443(7) Å (Table 3). The

arrangement of the linked ligands leads to coordination

polyhedra, which can be described as a distorted four-capped

square antiprism with 14 triangular faces.²³The two propyl

groups of L²are oriented on the same side of the plane formed

by the TTF core. This arrangement leads to the formation of

“head-to-tail” dimers of L²(Figure 3), but no short S···S

contacts are found in these dimers because the shortest S2···S6

and S1···S5 contacts are found equal to 3.901 and 3.994 Å. This

situation is probably due to the big size of the Cu₂Er₂cluster

constituting the inorganic network (Figure 3). In the inorganic

cluster, the shortest Cu···Cu, Cu···Er, and Er···Er distances

were measured as 5.605, 3.263, and 3.965 Å, respectively. The

E

dx.doi.org/10.1021/ic3010689 | Inorg. Chem. XXXX, XXX, XXX−XXX

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Magnetic Properties. Dinuclear Complexes [(L¹)MLn-

(hfac)₃]. The temperature dependence of the χ_M, χ_MT being the

molar magnetic susceptibility, of compound 3 is represented in

Figure 4. At room temperature, χ_MT is equal to 2.74 cm³K

polynomials of the total angular momentum matrixes (J², J_z, J₊,

and J₋),²⁵whereas the following terms correspond to the

Zeeman eﬀect on the lanthanide and spin-only magnetic

moments, which are coupled through J. Parameters of interest

are the B^q_k’s (numerical parameters) and J. The magnetization is

ﬁrst calculated for three orientations (x, y, and z) of a given

magnetic ﬁeld; then the magnetic susceptibility is calculated by

dividing by the amplitude of the applied ﬁeld and averaging

over the three directions. Temperature dependences of χ_Mand

χ_MT can thus be ﬁtted. The best ﬁt was obtained with J =

−2.2537(1) cm⁻¹, g_s= 2.1 (ﬁxed), B₂⁰= 1.327(3) cm⁻¹, B₄⁰=

−0.4717(2) cm⁻¹, and χ_imp= 0.09669 cm³K mol⁻¹(see Figure

4). The agreement is very good, especially in the low-

temperature range, where we were able to reproduce T_max. A

better agreement was obtained in the high-temperature region

upon ﬁtting the χ_MT vs T curve [J = −2.1188(6) cm⁻¹, g_s= 2.1

(ﬁxed), B⁰₂= 4.982(3) cm⁻¹, B₄⁰= −0.34705(6) cm⁻¹, and χ_imp

= 0.042(4) cm³K mol⁻¹]. With these two sets of parameters,

the ground state of Yb^IIIunambiguously corresponds to M_J=

¹/₂, while excited states are located a few hundred

wavenumbers above (Table 4). The total splitting (∼600

Figure 4. Temperature dependence of the molar magnetic

susceptibility, χ_M, for a powdered sample of compound 3 with a

zoom on the low-temperature range in the inset. The best-ﬁtted curve

(see the text) is represented by the red line.

mol⁻¹. This value is slightly smaller than expected for two

uncoupled magnetic moments arising from Cu^II(0.413 cm³K

mol⁻¹with g_Cu= 2.1) and Yb^III(2.57 cm³K mol⁻¹for the

Table 4. Calculated Energy Levels (cm⁻¹) of the M_JStates

for Yb^IIIin Compound 3 with the Two Sets of Parameters

from the Fit of χ_Mand χ_MT vs T Curves

24a

2

8

ground-state multiplet F_7/2with g_7/2= /₇). Upon cooling,

χ_MT decreases slightly down to 20 K (∼1.6 cm³K mol⁻¹) and

more abruptly upon cooling further down to 2 K (0.34 cm³K

mol⁻¹). While the slight decrease upon cooling from room

temperature is commonly observed and is related to the

splitting of the ground-state multiplet of lanthanide due to the

local crystal ﬁeld, the abrupt decrease below 20 K can only be

due to interactions between the magnetic moments of the

M_J

B₂⁰= 1.327(3) cm⁻¹

B₂⁰= 4.982(3) cm⁻¹

B₄⁰= −0.4717(2) cm⁻¹

B₄⁰= −0.34705(6) cm⁻¹

1

/

0

2

7

/

104

347

647

221

280

547

3

/

5

/

2

1

ground-state multiplet F_7/2of Yb^IIIand the spin /₂of Cu^II.

Surprisingly, the molar magnetic susceptibility χ_Mpasses

through a broad maximum at T_max= 4.2 K (Figure 4), which

cannot be ascribed to an antiferromagnetic−paramagnetic

phase transition owing to the large shortest intermolecular

distance between magnetic moments (∼10 Å). Then, the

maximum arises from the compensation between the local

magnetic moments of the two metals in a dimer. Owing to the

presence of a large kinetic orbital moment, the quantitative

interpretation of the magnetism of lanthanide-containing

dimers is not straightforward, while a comparative method in

3d4f systems between the magnetic and nonmagnetic 3d-

substituted ion gives eﬃcient qualitative information on the

nature of magnetic interactions between 3d and 4f ions. We

thus wrote a program to ﬁt the magnetic susceptibility in 3d4f

systems. The complete Hamiltonian to be considered is

reported in the Supporting Information. The Hamiltonian,

with nine parameters to describe the zero-ﬁeld splitting of any

lanthanide, is written for a symmetry environment as low as C_2v.

Because it is unrealistic to ﬁt the data with nine parameters, we

decided then to ﬁt the temperature dependence of the magnetic

susceptibility considering the high D_4dsymmetry environment.

In this case, only four or ﬁve parameters, B⁰₂, B₄⁰(B₆⁰), and J, and

a small amount of paramagnetic impurities, χ_imp, have to be

taken into account (eq 1).

cm⁻¹) is in agreement with the splitting of the ground-state

multiplet observed in a low-symmetry environment by

luminescence.²⁶At room temperature, the thermal energy is

equal to 210 cm⁻¹; therefore, the uncertainty on the energies of

the highest M_Jstates is very large, and so they must be

considered with caution.

With the ﬁtted set of parameters, we can simulate the ﬁeld

dependence of the magnetization (Figure 5). The simulation

reproduces fairly well the experimental curve, especially the

change of curvature at high ﬁeld. The energies of the ﬁrst four

levels calculated for the dimer with J = −2.2537(1) cm⁻¹, B₂⁰=

1.327(3) cm⁻¹, and B₄⁰= −0.4717(2) cm⁻¹are represented in

Figure 6. The eigenvectors are linear combinations of vectors

0

̂

⃗

H = B₂O₂+ B₄O₄+ B₆O₆+ β(g_JJ + g_sS)·H − JJ ·S

(1)

Figure 5. Magnetic ﬁeld dependence of the magnetization of a

powdered sample of compound 3 measured at 2 K, with the simulated

curve obtained with the set of parameters given in the main text.

The ﬁrst three terms corresponds to the crystal-ﬁeld eﬀects

q

̂

with O_kthe operator equivalents, which can be expressed as

F

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Table 5. Best-Fitted Parameters for Compounds 5, 2, and 3

5 (NiEr)

⁶/₅(ﬁxed)

2 (CuEr)

⁶/₅(ﬁxed)

3 (CuYb)

⁸/₇(ﬁxed)

g_J

g_S

B₂⁰

B₄⁰

J

2.1 (ﬁxed)

2.1 ﬁxed

1.6898(6) cm⁻¹

0.02814(1) cm⁻¹

1.5096(2) cm⁻¹

1.327(3) cm⁻¹

0.02124(2) cm⁻¹

0.3955(3) cm⁻¹

−0.4717(2) cm⁻¹

−2.2537(1) cm⁻¹

the antiferromagnetic coupling found in 3. Analysis of the wave

functions reveals that the ground state of Er^IIIcorresponds to

4

the M_J= ⁹/₂state, while the highest state in the I_15/2

multiplet corresponds to M_J= ¹⁵/₂and is located about 600

cm⁻¹above the ground state (see Figure S6 in the Supporting

Information). Here again the uncertainty resides on the exact

position of the highest states (∼400 cm⁻¹above the ground

state), which are not accessible with thermal excitation at room

temperature.

Figure 6. First four energy levels calculated in zero ﬁeld for compound

3 with J = −2.2537 cm⁻¹, B₂⁰= 1.3271 cm⁻¹, and B⁰₄= −0.47176 cm⁻¹

with the eigenvectors on the basis of |M_J; M_S⟩.

|M_J; M_S⟩ with M_J= ⁷/₂, ⁵/₂, ³/₂, ¹/₂and M_S= ¹/₂for Ln

and Cu^II, respectively. These levels correspond only to coupling

between M_J= ¹/₂of Yb^IIIand M_S= ¹/₂of Cu^II. Therefore,

this energy diagram is identical with the energy diagram in the

We then decided to reinvestigate in a quantitative manner

the magnetism of the Dy and Tb analogues of compounds 2

and 5.⁷In ref 7, the comparative method was successfully

employed to determine the nature (ferro- or antiferromagnetic)

of the interaction between Cu^IIand Dy^IIIand between Cu^IIand

Tb^III. Antiferromagnetic interactions operate in the Cu−Dy

dimer and ferromagnetic in the Cu−Tb dimer. The ﬁtting of

the magnetic data with the model used for compounds 2 and 5

provides quantitative information on the amplitude of the

interaction (Figure S7 in the Supporting Information).

However, we have to add a third parameter, B⁰₆, to reproduce

satisfactorily both χ_MT vs T and M vs H curves. The procedure

is to ﬁt ﬁrst the Ni^IIanalogue with the zero-ﬁeld-splitting

parameters and then to inject these parameters (ﬁxed) in the

ﬁtting of the Cu^IIanalogue; then the only free parameter

remains the interaction between the magnetic centers.

1

case of anisotropic coupling between two spins /₂. With the

help of the Pauli matrixes, it is not diﬃcult to demonstrate that

the ﬁrst four levels can be reproduced with the Hamiltonian H

= −JS₁·S₂with J_z= J and J_x= J_y= 4J. The spin operators S₁and

S₂have their usual meaning. Of course, the simulation of the

magnetic data with this simple model, only valid when the

ground state is populated, requires the eﬀective Zeeman factors

g_z= g_J, g_x= g_y= 4g_Jfor Yb^IIIand g = 2.1 for Cu^II(Figure S5 in

the Supporting Information).

The χ_MT vs T curves for the Er^IIIcompounds 2 and 5 are

plotted in Figure 7. In the whole temperature range, χ_MT of 2 is

7

Unambiguously, ferromagnetic interactions operate in Gd^III

(+1.29 cm⁻¹) and Tb^III(+0.40 cm⁻¹) derivatives, while

antiferromagnetic interactions operate in Dy^III(−0.46 cm⁻¹)

and Yb^III(−2.25 cm⁻¹) compounds. In the Er^IIIderivative, the

interaction can be considered negligible.

Our quantitative approach conﬁrms the results obtained with

the qualitative empirical method previously reported, i.e., the

ferromagnetic interactions taking place between both Tb^IIIand

Cu^IIions.⁷In other heterobimetallic Cu−Dy complexes, it turns

out that the nature of the superexchange interaction, or at least

its amplitude, is related to the dihedral angle Dy−O1−O2−Cu:

the smaller this angle is, the smaller is the ferromagnetic

interaction in amplitude. The interaction appears to be very

small in amplitude for an angle close to 135°, while it is

unambiguously ferromagnetic for angle values higher than

150°.^27,28Nevertheless, in the latter case, the value of the

magnetic-exchange interaction depends not only on the

dihedral angle value but also on the Stevens operators

(symmetry around the Ln ion).²⁵Until now, no examples

close to our heterobimetallic compounds were published with a

similar angle value and symmetry, and consequently it is

diﬃcult to perform suitable comparisons. The magnetic studies

of the nature of the interaction between the Cu^IIand Yb^III/Er^III

ions are scarce. In the case of Yb^III, the reported examples deal

with dihedral angle values close to 180°. Even if the angle value

and symmetry seem to be comparable in the reported examples,

the nature of the interaction between the Cu^IIand Yb^IIIions

were found to be ferromagnetic^27cand antiferromagnetic.^27bIn

conclusion in our systems, the Cu−Ln interaction is

Figure 7. χ_MT vs T curves for compounds 5 (circles) and 2 (squares),

with the best-ﬁtted curves using the model described in the text (red

lines). The inset shows the ﬁeld dependences of the magnetization at 2

K for 5 (circles) and 2 (squares), with the simulated curves (red lines)

using parameters from the ﬁts.

stronger than that for 5. The substitution of the magnetic ion

Cu^IIby the diamagnetic Ni^IIdecreases, as expected, the eﬀective

moment. The model proposed here above can be applied to

these two systems: one (5) consists of a Er^IIIion (⁴I_15/2) in a

C_2venvironment, while the other (2) consists of a Er^IIIion in a

C_2venvironment coupled with a spin ¹/₂. The best-ﬁtted

parameters are grouped together in Table 5. Two points must

be raised: (1) Fitted parameters are close in 2 and 5 as a

consequence of identical coordination polyhedra around the

Er^IIIion in the two compounds; (2) the magnetization curves

are fairly well reproduced for both compounds. A small

ferromagnetic coupling exists in 2, about 5 times smaller than

G

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Inorganic Chemistry

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Figure 8. MO diagrams of L¹, [(L¹)Ni], and 4. The energy levels of the centered TTF, salphen, Ni^II, and hfac⁻orbitals are respectively represented

in orange, blue, black, and green.

ferromagnetic for Ln = Gd and Tb and antiferromagnetic for

Ln = Dy and Yb, while it is complicated to conclude in the case

of the Er^IIIion.

The experimental absorption curve of L¹was previously

deconvoluted in ﬁve bands.⁷The calculated UV−visible

absorption spectrum for L¹well reproduces the experimental

L¹curve (Figure 9). The lowest-energy band was calculated at

the average value of 23256 cm⁻¹(experimental value found at

25400 cm⁻¹) and attributed to π−π* HOMO → LUMO

(96%) (22783 cm⁻¹) + HOMO → LUMO +1 (81%) (23728

cm⁻¹) TTF donor to salphen acceptor charge transfers

(DACT¹; Table 6). The absorption band centered at 29900

cm⁻¹has been calculated at the average value of 30479 cm⁻¹

and attributed to the π−π* HOMO−1 → LUMO (81%)

(28943 cm⁻¹) + HOMO−1 → LUMO+1 (68%) (30554 cm⁻¹)

and HOMO−2 → LUMO+1 (46%) + HOMO−1 → LUMO

+1 (18%) (31939 cm⁻¹) intrasalphen transitions and salphen to

phenol CTs (Table 6). The next absorption band centered at

33000 cm⁻¹was identiﬁed as the HOMO → LUMO+4 (71%)

intra-TTF (ID) transition (32675 cm⁻¹; Table 6). The

Gaussian deconvolution centered at 36000 cm⁻¹was calculated

at 35320 cm⁻¹, attributed to several CTs involving the TTF

core, salphen acceptor, and phenyldiimine moiety, and mainly

described by the HOMO−1 → LUMO +3 (28%) + HOMO →

LUMO +5 (25%) transitions (Table 6). The highest-energy

large absorption band at 42400 cm⁻¹corresponds to the three

excitations calculated at 38081, 38537, and 40928 cm⁻¹, i.e., the

π−π* HOMO−7 → LUMO (38%) + HOMO−7→ LUMO+1

(26%) + HOMO−1→ LUMO+5 (53%) intrasalphen tran-

sitions (Table 6).

Tetranuclear Complex 7. The room temperature χ_MT for

compound 7 is equal to 23.07 cm³K mol⁻¹, which is slightly

lower than the expected value for two ⁴I_15/2multiplets with g_J=

1

6/5 and two S = /₂(_gCu= 2.1). χ_MT decreases monotonically

upon cooling to reach 10.5 cm³K mol⁻¹at 2 K, while the

magnetization does not saturate under 50 kOe at 2 K (Figure

S8 in the Supporting Information). Owing to the structural

complexity of compound 7, it is diﬃcult to analyze precisely its

magnetism: (1) The distance between the Er^IIIions is so short

that we cannot neglect interactions between lanthanides, at

least of dipolar origin; (2) the coordination number and

therefore the polyhedron around lanthanide are diﬀerent with

respect to compounds 2 and 5 changing the zero-ﬁeld-splitting

parameters; (3) a superexchange interaction between Cu^IIand

Er^IIIions also exists.

Photophysical Properties. Absorption Properties. The

experimental absorption properties of the free ligand L¹and the

metallic precursors [(L¹)M] were previously studied in

solution.^7,13The MO diagrams were obtained by DFT (see

the Experimental Section) for L¹, [(L¹)Ni], and 4 (Figure 8 and

Table 6). To support and rationalize the experimental

attributions and observations, TD-DFT calculations (see the

Experimental Section) were also performed (Figure 9 and

Table 6). The experimental absorption spectrum of 4 is

depicted in Figure 10.

After metalation by the Ni^IIion, the lowest-energy CTs

(DACT¹) were calculated at the average energy of 20542 cm⁻¹

H

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Table 6. TD-DFT-Calculated Excitation Energies and Main Compositions of Low-Lying Electronic Transitions Associated with

a

an Oscillator Strength f > 0.10, 0.08, and 0.05, Respectively, for L¹, [(L¹)Ni], and 4

energy (cm⁻¹

)

exptl

theor

osc

type

assignment

transition

H → L (96%)

b

L¹

25400

22783

23728

28943

30554

31939

0.15 DACT¹

π_TTF→ π*_salphen

0.25

H → L+1 (81%)

H−1 → L (81%)

H−1 → L+1 (68%)

H−1 → L+1 (18%)

H−2 → L (46%)

H → L+4 (71%)

H−1 → L+3 (28%)

H → L+5 (25%)

H−7 → L (38%)

H−7 → L+1 (26%)

H−1 → L+5 (53%)

H → L (95%)

29900

0.22 IA

0.36

π_salphen→ π*_salphen+ π_salphen→ π*_phenol

0.12

b

33000

36000

32675

35320

0.84 ID

π_TTF→ π*_TTF

0.18 ILCT

π_salphen→ σ*_TTF

π_TTF→ π*_salphen

π_salphen→ π*_salphen

b

42400

38081

38537

40928

19688

0.16 IA

0.13 IA

0.11 IA

0.09 DACT¹

π_salphen→ π*_salphen

π_TTF→ π*_salphen

b

[(L¹)

Ni]

20200

22500

21396

22048

28410

27513

28898

31745

33373

34911

0.21

H → L+1 (85%)

0.49 IA

π_salphen→ π*_salphen

H−1 → L (85%)

H → L+2 (72%)

b

2

26900

29900

c

LMCT

π_TTF→ d_{x −y (Ni)}

π_salphen→ π*_salphen

0.29 IA

0.49

H−2 → L+1 (95%)

H−3 → L (76%)

H−7 → L (71%)

H → L+5/+6 (31/14%)

H → L+5 (15%)

0.57

b

32800

37500

0.19 ID

0.30 ID

π_TTF→ π*_TTF

MLCT

0.09 ID

ILCT

d_xz(Ni)→ π*_salphen

π_TTF→ π*_TTF

π_salphen/TTF→ σ*_TTF

π_salphen/TTF→ π*_TTF

π_salphen→ π*_salphen

π_TTF→ π*_salphen

H−9 → L (35%)

H → L+6 (31%)

35682

H−2 → L+3 (25%)

H−2 → L+4 (51%)

H−1 → L+7 (81%)

H → L (97%)

38158

40077

17568

19807

24080

25129

27815

28373

30302

31228

0.08 ILCT

0.09 IA

0.05 DACT¹

0.33

b

43100

19500

22000

26600

4

H → L+1 (98%)

0.27 IA

π_salphen→ π*_salphen

H−1 → L (88%)

H → L+5 (78%)

2

c

LMCT

π_TTF→ d_{x −y (Ni)}

30100

0.29 IA + ILCT

π_salphen→ π*_salphen+ π_salphen/TTF→ π*_salphen

H−2 → L+1 (81%)

H−3/−5 → L(36/21%)

H−4/−7 → L(32/41%)

0.11

0.54

0.14

H−4/−6/−7 → L

(27/18/33%)

32700

33927

0.08 ID + ILCT +

Ihfac

π_TTF→ π*_TTF+ π_TTF→ π*_salphen+ π_salphen→ σ*_TTF+ π_hfac→ π*_hfac

H−6 → L+1 (19%)

H → L+9 (27%)

35002

0.08

H−12 → L (17%)

H−8 → L+2 (25%)

H−5 → L+4 (13%)

H → L+9 (16%)

36097

37969

0.13

H−2 → L+6 (19%)

H−11 → L+1 (16%)

2

z

37600

0.06 ILCT + Ihfac

π_TTF→ π*_salphen+ π_hfac/d

→ π*_hfac+ π_hfac→ π*_hfac

(Ni)

H−10 → L+2 (12%)

H−9 → L+4 (27%)

H → L+11 (14%)

38721

0.11

H−10 → L+3/+4

(29/15%)

39202

39931

0.10

0.08

H−10 → L+4 (33%)

H−13/−15 → L+2

(21/28%)

42800

40365

0.06 IA

π_salphen→ π*_salphen

H−18 → L+1 (24%)

a

In addition, the charge-transfer and pure intramolecular transitions are reported. ID, IA, and Ihfac⁻and H, L, and nb represent respectively the

intramolecular TTF (donor), salphen (acceptor), and hfac⁻transitions and HOMO and LUMO; therefore, DACT and ILCT stand for the donor-to-

b

acceptor and intraligand charge transfers. The theoretical values are evaluated at the PCM(CHCl₃)−PBE0/SVP level of approximation. Data given

c

from this reference. The oscillator strength has been calculated to be very low because of the long-range character of these CT bands and the

absence of overlap between the TTF-centered orbitals and the Ni^II-centered d_{x −y}orbital.

2

I

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attributed to the d_xz(Ni)→ π*_salphenHOMO−9 → LUMO

transition. Finally, the highest energy absorption band was

again attributed to π−π* intrasalphen²⁻transitions (40077

cm⁻¹).

Coordination of the Y(hfac)₃unit leads to an additional 1850

cm⁻¹red shift of the DACT¹bands [experimental red

deconvolution in Figure 10; HOMO → LUMO (97%)

(17568 cm⁻¹) + HOMO → LUMO+1 (98%) (19807

cm⁻¹)]. The energy stabilization of the HOMO and LUMO

orbitals is equal to 0.13 and 0.40 eV, respectively (Figure 8). In

other words, the electronic withdrawing of the Y(hfac)₃unit is

weaker than the Ni^IIone. This can be explained by two reasons:

(i) the Y(hfac)₃unit is far from the TTF core and (ii) the Ni^II

ion is coordinated through the two phenolate groups and the

two imine groups, while the Y(hfac)₃unit is only coordinated

through the phenolate groups. Nevertheless, the energy

stabilization of the TTF-centered orbitals after 4f coordination

attests to the good electronic communication between both the

TTF core and Y^IIImetal center. The low-energy π−π* IA

transitions [HOMO−1 → LUMO (88%)] and LMCT

[HOMO → LUMO +5 (78%)] bands are now calculated at

24080 and 25129 cm⁻¹. The LMCT energy is calculated lower

in 4 than in [(L¹)Ni] because of the strong stabilization of the

Figure 9. Theoretical absorption spectra of compounds L¹(orange

line), [(L¹)Ni] (red line), and 4 (black line).

Ni^II-centered d_{x −y}orbital after Y^IIIcoordination (Figure 8).

The decrease of the electron density on the salphen²⁻fragment

after 4f coordination favors the appearance of TTF →

salphen²⁻CT (ILCT) instead of IA and ID transitions (cf.

L¹and [(L¹)Ni] attributions) because salphen²⁻is a better

acceptor in 4 than in the metalloprecursor. Thus, the black

deconvolution (30100 cm⁻¹; Figure 10) was attributed to the

calculated HOMO−2 → LUMO+1 (81%) (27815 cm⁻¹) and

HOMO−3/−4/−5/−6/−7 → LUMO (average value of 29968

cm⁻¹) transitions (Table 6). The green deconvolution (Figure

10) involves ID + ILCT and the intra-hfac⁻bands, with their

calculated energies being in the range from 33927 to 36097

cm⁻¹(Table 6). The hfac⁻-centered transitions were calculated

at higher energy compared to the energy usually obtained

experimentally for such transitions (32700 cm⁻¹).^6lThe hfac⁻

contributions come from two kinds of transitions: (i) HOMO−

10 → LUMO+2/+3/+4 excitations (calculated energies from

37969 to 39202 cm⁻¹); (ii) HOMO−13/−15 → LUMO+2

excitations (39931 cm⁻¹). Concerning the HOMO−10 orbital,

2

Figure 10. Experimental dichloromethane solution (4 × 10⁻⁵M)

UV−visible absorption spectra (open gray circles), Gaussian

deconvolutions (dashed lines), best ﬁt (full black line; R = 0.9997),

and ﬂuorescence (open blue circles) with its extrapolation (full blue

line) for 4.

(experimental value found at 21350 cm⁻¹) and were attributed

to π−π* HOMO → LUMO (95%) (19688 cm⁻¹) + HOMO

→ LUMO+1 (85%) (21396 cm⁻¹) TTF-to-salphen²⁻CTs

(DACT¹; Table 6). The experimental energy stabilization of

4000 cm⁻¹due to Ni^IIcoordination was theoretically evaluated

to 2700 cm⁻¹. The red shift of DACT¹is due to a simultaneous

energy stabilization of 0.22 and 0.66 eV of the HOMO and

LUMO, which are centered on the TTF and salphen²⁻units

respectively (Figure 8). It is worth noticing that a strong π−π*

contribution centered on the salphen²⁻acceptor appeared after

metalation of L¹[HOMO−1 → LUMO (85%), 22048 cm⁻¹].

The expected ligand-to-metal CT (LMCT; 26900 cm⁻¹) was

II

2

a strong contribution of the ﬁlled d_zNi -centered orbital was

found (Figure 8). Finally, the highest-energy deconvolution was

attributed to IA transitions, which were associated with

HOMO−18 → LUMO+1 (24%) (40365 cm⁻¹).

The free ligand L¹(Figure S9 in the Supporting

Information), metallic precursors [ML¹] [M = Cu^II(Figure

S10 in the Supporting Information), Ni^II(Figure S11 in the

Supporting Information)], Cu^II- and Ni^II-based dinuclear

complexes [3 (Figure S12 in the Supporting Information)

and 4 (Figure 11)], and the tetranuclear complex 7 (Figure 12)

were studied upon the addition of diﬀerent amounts of NOPF₆

oxidizing and Fe^II(Cp*)₂reducing agents. The full description

of the behaviors upon oxidation and reduction are done for the

two representative complexes 4 and 7.

calculated at 28410 cm⁻¹and attributed to the π*_TTF

→

d_x2‑y2(Ni)HOMO → LUMO+2 (72%) transition. The π−π*

intrasalphen²⁻transitions for [(L¹)Ni] were calculated to the

average energy of 29395 cm⁻¹, which is almost the same energy

as that for the free ligand. The main responsible excitations for

this absorption band are HOMO−2 → LUMO+1 (95%)

(27513 cm⁻¹) + HOMO-3 → LUMO (76%) (28898 cm⁻¹)

and HOMO−7 → LUMO (71%) (31745 cm⁻¹; Table 6). The

following two experimental absorption bands (32800 and

37500 cm⁻¹) were attributed to the π−π* intra-TTF HOMO

→ LUMO+5/+6 excitations (34655 cm⁻¹) and π−σ*/π−π*

HOMO−2 → LUMO+3/+4 salphen²⁻-to-TTF CTs (36920

cm⁻¹). Compared to the free ligand, an additional metal-to-

ligand CT (MLCT) was identiﬁed at 34911 cm⁻¹and

Upon the addition of a NOPF₆oxidizing agent for 4, ﬁve

new bands appear simultaneously with the disappearance of the

HOMO → LUMO CT (DACT¹, 20750 cm⁻¹; Table 7 and

Figure 11a). The lowest-energy bands were attributed to

intramolecular CT (DACT², 11350 cm⁻¹; Table 7 and Figure

11c) but in the direction opposite to that of the neutral

complex 4. In analogy with previous work,²⁹the main

J

dx.doi.org/10.1021/ic3010689 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 11. (a) Experimental dichloromethane solution (4 × 10⁻⁵M) UV−visible absorption spectra upon the addition of a NOPF₆oxidizing agent.

Inset: intensity of the lowest-energy CT bands upon the addition of a NOPF₆oxidizing agent and a number of added NOPF₆equivalents. (b)

Experimental dichloromethane solution (4 × 10⁻⁵M) UV−visible absorption spectra of 4 (open gray circles), Gaussian deconvolutions (dashed

lines), and best ﬁt (full black line) (R = 0.9997). (c) Experimental dichloromethane solution (4 × 10⁻⁵M) UV−visible absorption spectra of 4^•+

(open gray circles), Gaussian deconvolutions (dashed lines), and best ﬁt (full black line) (R = 0.9997). (d) Positions and intensities of the Gaussian

deconvolutions for 4 (red sticks) and 4^•+(blue sticks).

contribution of DACT²is a monoelectronic SOMO−n →

SOMO excitation from the salphen²⁻moiety to the radical-

cation TTF^•+core (where SOMO−n is a lower-energy orbital

than SOMO). These CT excitations are allowed because of the

strong decrease of the donating character of the radical-cation

TTF^•+core compared to the neutral TTF core. In the same

energy range of DACT¹(red deconvolutions in the Figure 11b)

appear three new absorption bands, which are attributed to a π

→ π* intra-TTF^•+radical cation and to the SOMO → LUMO

intramolecular CT. The SOMO and LUMO are respectively

centered TTF^•+and salphen²⁻orbitals (DACT³, 20800 cm⁻¹;

Table 7 and Figure 11c). The SOMO → LUMO CT of 4^•+is

analogous to the HOMO → LUMO CT of 4. The LMCT

bands are red-shifted (1000 cm⁻¹) upon the addition of NOPF₆

(gray deconvolution; Table 7 and Figure 11a). This red shift

can be attributed to energy destabilization of the bonding-

centered ligand orbitals upon oxidation. The intrasalphen²⁻

excitation (IA, black deconvolution) remains almost at the

same energy in 4 and 4^•+and attests that the phenol groups are

not strongly aﬀected by oxidation.

oxidation is complete for 0.8−1 equiv of oxidizing agent, in

agreement with the formula of 4 and the formation of 4^•+. The

reversibility of the oxidation was veriﬁed by the addition of the

Fe^II(Cp*)₂reducing agent and the recovery of the neutral

species 4, in agreement with the electrochemical properties

(Figure S4 in the Supporting Information).

The behaviors upon oxidation and reduction of the metallic

precursors [(L¹)M] (Figures S10 and S11 in the Supporting

Information) and the corresponding 3d4f heterobimetallic

complexes (Figures 11 and S12 in the Supporting Information)

are very similar. This is also the case for the dinuclear and

tetranuclear complexes 4 and 7.

For 7, the reversible oxidation and reduction are complete

after the addition of 2 equiv of NOPF₆or Fe^II(Cp*)₂agents.

Consequently, both L²ligands are oxidized in the radical cation,

and the oxidized species of 7 can be written as 7^2(•+)with

simultaneous oxidation of both L²ligands, in agreement with

the electrochemical properties (Figure S4 in the Supporting

Information).

Emission Properties. Upon excitation at 20850 cm⁻¹(480

nm) in the HOMO → LUMO CT transition, complex 4

exhibits a weak ﬂuorescence band centered at 14450 cm⁻¹(692

nm) (Figure 10). The energy of the CT excited state can be

estimated from the zero-phonon transition wavelength λ₀₋₀at

17850 cm⁻¹(560 nm) by averaging the energies of the lowest-

energy absorption maximum and the highest-energy emission

maximum. It is worth noting that CT excited states are known

to present signiﬁcant relaxation in their excited state, so the

energy of the donating state is better described by the CT

emission band taking into account its broadness.³⁰In the

The average energy of the DACT²bands is almost the same

for the free L¹(11300 cm⁻¹), the metallic precursor [(L¹)Ni]

(11850 cm⁻¹), and 4 (11350 cm⁻¹). Consequently, it is not

possible to conclude on the stability of the heterobimetallic

complexes under oxidation, in particular whether the 4f ion

remains coordinated or not. The presence of isosbestic points

(Figure 11a) attests the equilibrium between only two species

(4 and 4^•+) and may be a sign of the stability of the

heterobimetallic complex under oxidation. The evolution of the

intensity of DACT³upon the addition of NOPF₆shows that

K

dx.doi.org/10.1021/ic3010689 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 12. (a) Experimental dichloromethane solution (4 × 10⁻⁵M) UV−visible absorption spectra upon the addition of a NOPF₆oxidizing agent.

Inset: intensity of the lowest-energy CT bands upon the addition of an oxidizing NOPF₆(in blue) and a reducing Fe^II(Cp*)₂(in red) and number of

added NOPF₆equivalents. (b) Experimental dichloromethane solution (4 × 10⁻⁵M) UV−visible absorption spectra of 7 (open gray circles),

Gaussian deconvolutions (dashed lines), and best ﬁt (full black line) (R = 0.9997). (c) Experimental dichloromethane solution (4 × 10⁻⁵M) UV−

visible absorption spectra of 7^2(•+)(open gray circles), Gaussian deconvolutions (dashed lines), and best ﬁt (full black line) (R = 0.9997). (d)

Positions and intensities of the Gaussian deconvolutions for 7 (red sticks) and 7^2(•+)(blue sticks).

a

Table 7. Experimental Absorption Data for L¹, [Cu(L¹)], [Ni(L¹)], and Selected Complexes

Neutral form (cm⁻¹

)

Oxidized form (cm⁻¹

)

DACT¹

25400

LMCT

/

IA

DACT²

11300

ID + DACT³

LMCT

/

IA

L¹

29900

17300

24200

19000

21600

23200

17800

20600

22700

18500

21800

23800

18800

20700

22900

18900

21800

23700

30100

[Cu(L¹)]

19900

22800

26400

26900

26600

27000

29900

30000

30100

30000

10400

12100

26000

25200

25600

25400

29900

29700

30200

31100

29900

[Ni(L¹)]

(3)

20300

22600

10900

12800

19500

22300

10400

12300

(4)

19500

22000

10800

11900

(7)

20500

23600

10400

12000

a

LMCT = ligand-to-metal CTs, ID = intra-TTF donor excitations, IA = intra-salphen acceptor excitations, DACT¹= TTF donor to salphen²⁻

acceptor CTs, DACT²= salphen²⁻donor to TTF^•+acceptor CTs, and DACT³= TTF^•+donor to salphen²⁻acceptor CTs.

present case (Figure 10), the emission band starts with the

zero-phonon transition (17850 cm⁻¹) and presents a maximum

around 14450 cm⁻¹, and its red tail can be estimated at 9020

cm⁻¹using a rough deconvolution into the Gaussian function.

The energy of the CT donation is in the 17850−9020 cm⁻¹

range. Unfortunately, in solution and the solid state, at room

temperature no luminescence centered on the 4f ions is

observed in the case of 2, 3, 5, and 6.

CONCLUSIONS

■

Two easy synthetic procedures permitted access to two TTF-

based ligands, L¹and L². The former led to six heterobimetallic

dinuclear complexes of the formula [(L¹)MLn(hfac)₃] (where

L

dx.doi.org/10.1021/ic3010689 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

M = Cu^II, Ni^II; Ln = Y^III, Er^III, Yb^III), while the latter led to a

heterobimetallic tetranuclear complex of the formula [(L²)Cu-

(OH)Er(hfac)₃]₂. A quantitative magnetic approach was

AUTHOR INFORMATION

Corresponding Author

*E-mail: fabrice.pointillart@univ-rennes1.fr.

Notes

■

performed using an homemade program considering a

q

̂

Hamiltonian including the crystal-ﬁeld eﬀects with the O_k

The authors declare no competing ﬁnancial interest.

operator equivalents, the Zeeman eﬀect on the lanthanide,

and spin-only magnetic moments that are coupled through J.

Cu−Ln ferromagnetic interactions for Gd^III(1.29 cm⁻¹) and

Tb^III(0.40 cm⁻¹) and antiferromagnetic interactions for Dy^III

(−0.46 cm⁻¹) and Yb^III(−2.25 cm⁻¹) were determined, while

in the case of Er^III, the magnetic interactions are negligible.

The UV−visible absorption properties for the neutral

coordination complexes 1−7 were studied and rationalized by

DFT and TD-DFT calculations for free L¹, the metallic

precursor [(L¹)Ni], and 4. Thus, the lowest-energy absorption

bands have been attributed to CTs from the TTF core to the

salphen acceptor. Metalation by the Ni^IIion leads to

stabilization of both HOMO (0.22 eV) and LUMO/LUMO

+1 (0.66 eV) orbitals. Additional LMCTs and MLCTs

involving the LUMO+2 (d_{x −y}) and HOMO−9 (d_xz) orbitals

have appeared and have been identiﬁed. Finally, coordination of

the Y(hfac)₃moiety has provoked an additional red shift of the

CT bands (HOMO → LUMO and HOMO → LUMO+1;

1850 cm⁻¹), demonstrating the good electronic communication

along the whole ligand L¹.

Upon the addition of NOPF₆, the TTF core of the ligand

involved in L¹, [(L¹)M], and complexes 3, 4, and 7 is oxidized

in its radical-cation form. The 3^•+, 4^•+, and 7^2(•+)species show

new intramolecular CT bands in the energy ranges of 11350

cm⁻¹(SOMO−n → SOMO DACT²) and 20800 cm⁻¹(SOMO

→ LUMO DACT³), while the HOMO → LUMO CT bands

(DACT¹) of the respective neutral complexes disappear upon

oxidation. The lowest-energy CTs are in the direction opposite

to that of the neutral complexes, while the SOMO → LUMO

CT of the oxidized complexes is analogous to the HOMO →

LUMO CT of neutral ones. The reversibility of the oxidation

was studied by cyclic voltammetry and UV−visible−NIR

absorption properties upon the addition of a [Fe^II(Cp*)₂]

reducing agent.

ACKNOWLEDGMENTS

This work was supported by the CNRS, Rennes Met

■

́

ropole,

Universite

́

de Rennes 1, Reg

́

ion Bretagne, and FEDER. Dr. O.

Maury is acknowledged for access to the ﬂuorometer and his

help with the measurements.

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The irradiation of 4 at 20850 cm⁻¹induces the ﬂuorescence

of L¹at 14450 cm⁻¹with energy of the CT excited state of

17850 cm⁻¹.

Compound 7 is a promising system, and synthetic eﬀorts to

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ASSOCIATED CONTENT

■

S

* Supporting Information

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Inorg. Chem. 2004, 43, 3136. (n) Avarvari, N.; Fourmigue,

́

M.; Avarvari, N.

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Crystallographic information in CIF format, crystal packings for

3, 4, and 7 (Figures S1−S3), cyclic voltammetry for 2, 5, and 7

(Figure S4), χ_M(T) curve and best ﬁt for 3 (Figure S5), energy

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dx.doi.org/10.1021/ic3010689 | Inorg. Chem. XXXX, XXX, XXX−XXX

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3d4f heterobimetallic dinuclear and tetranuclear complexes involving tetrathiafulvalene as ligands: X-ray structures and magnetic and photophysical investigations

DOI: 10.1021/ic3010689

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