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Electrochemical and Magnetic Exchange Interactions in Trinuclear Chain Complexes Containing Oxo-Mo(V) Fragments as a Function of the Topology of the Bridging Ligand

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posted on 05.01.1999, 00:00 by Van Ân Ung, Samantha M. Couchman, John C. Jeffery, Jon A. McCleverty, Michael D. Ward, Federico Totti, Dante Gatteschi
The trinuclear complexes [{Mo(O)(Tp*)Cl}(μ-1,n-C6H4O2){Mo(O)(Tp*)}(μ-1,n-C6H4O2){Mo(O)(Tp*)Cl}] (1, n = 4; 2, n = 3) have been prepared [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate], in which a chain of three paramagnetic oxo-Mo(V) fragments are linked by two 1,4-[OC6H4O]2- (for 1) or 1,3-[OC6H4O]2- (for 2) bridging ligands. The crystal structure of (CH2Cl2)3.5·(C6H14)0.5 was determined:  C63.5H88B3Cl9Mo3N18O7; triclinic, P1̄; a = 12.052(2) Å, b = 18.487(4) Å, c = 21.039(5) Å; α = 68.95(2)°, β = 86.12(2)°, γ = 78.637(13)°; V = 4289(2) Å3; Z = 2. It shows a V-shaped Mo−L−Mo−L−Mo array of three {(Tp*)Mo(O)} fragments with two 1,4-[OC6H4O]2- bridging ligands. The V-shape arises from the cis arrangement of the two bridging ligands at the central metal atom. Electrochemical measurements show the expected Mo(IV)/Mo(V) and Mo(V)/Mo(VI) couples at potentials consistent with significant electrochemical interactions between the terminal and central metal fragments but not between the two terminal metal fragments. Variable-temperature magnetic susceptibility measurements (1.17−225 K) show the occurrence of intramolecular antiferromagnetic (1) and ferromagnetic (2) exchange interactions between adjacent metal atoms with J = −44.0 (1) and +4.5 cm-1 (2) [based on H = −J(S1S2 + S1S3)] leading to S = 1/2 (1) and 3/2 (2) ground states. These results are in accord with the spin-polarization principle, which predicts that [1,4-C6H4O2]2- should be an antiferromagnetic linker whereas [1,3-C6H4O2]2- should be a ferromagnetic linker.