Structural and Magnetic Properties of MnIII and CuII Tetranuclear Azido Polyoxometalate Complexes: Multifrequency High-Field EPR Spectroscopy of Cu4 Clusters with S=1 and S=2 Ground States
✍ Scribed by Pierre Mialane; Carole Duboc; Jérôme Marrot; Eric Rivière; Anne Dolbecq; Francis Sécheresse
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 303 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
Two new azido‐bridged polyoxometalate compounds were synthesized in acetonitrile/methanol media and their molecular structures have been determined by X‐ray crystallography. The [{(γ‐SiW~10~O~36~)Mn~2~(OH)~2~(N~3~)~0.5~(H~2~O)~0.5~}~2~(μ‐1,3‐N~3~)]^10−^ (1 a) tetranuclear Mn^III^ complex, in which an end‐to‐end N~3~^−^ ligand acts as a linker between two [(γ‐SiW~10~O~36~)Mn~2~(OH)~2~]^4−^ units, represents the first manganese–azido polyoxometalate. The magnetic properties have been studied considering the spin Hamiltonian ℋ︁=−J~1~(S~1~S~2~+S~1*~S~2*~)−J~2~(S~1~S~1*~), showing that antiferromagnetic interactions between the paramagnetic centers (g=1.98) occur both through the di‐(μ‐OH) bridge (J~1~=−25.5 cm^−1^) and the μ‐1,3‐azido bridge (J~2~=−19.6 cm^−1^). The [(γ‐SiW~10~O~36~)~2~Cu~4~(μ‐1,1,1‐N~3~)~2~(μ‐1,1‐N~3~)~2~]^12−^ (2 a) tetranuclear Cu^II^ complex consists of two [γ‐SiW~10~O~36~Cu~2~(N~3~)~2~]^6−^ subunits connected through the two μ‐1,1,1‐azido ligands, the four paramagnetic centers forming a lozenge. The magnetic susceptibility data have been fitted. This reveals ferromagnetic interactions between the four Cu^II^ centers, leading to an S=2 ground state (ℋ︁=−J~1~(S~1~S~2~+S~1*~S~2*~)−J~2~(S~2~S~2*~), J~1~= +294.5 cm^−1^, J~2~=+1.6 cm^−1^, g=2.085). The ferromagnetic coupling between the Cu^II^ centers in each subunit is the strongest ever observed either in a polyoxometalate compound or in a diazido‐bridged Cu^II^ complex. Considering complex 2 a and the previously reported basal–basal di‐(μ‐1,1‐N~3~)‐bridged Cu^II^ complexes in which the metallic centers are not connected by other magnetically coupling ligands, the linear correlation J~1~=2639.5−24.95*θ~av~ between the θ~av~ bridging angle and the J~1~ coupling parameter has been proposed. The electronic structure of complex 2 a has also been investigated by using multifrequency high‐field electron paramagnetic resonance (HF‐EPR) spectroscopy between 95 and 285 GHz. The spin Hamiltonian parameters of the S=2 ground state (D=−0.135(2) cm^−1^, E=−0.003(2) cm^−1^, g~x~=2.290(5), g~y~=2.135(10), g~z~=2.158(5)) as well as of the first excited spin state S=1 (D=−0.960(4) cm^−1^, E=−0.080(5) cm^−1^, g~x~=2.042(5), g~y~=2.335(5), g~z~=2.095(5)) have been determined, since the energy gap between these two spin states is very small (1.6 cm^−1^).