Electronic and magnetic metal–metal interactions in dinuclear oxomolybdenum(V) complexes across bis-phenolate bridging ligands with different spacers between the phenolate termini: ligand-centred vs. metal-centred redox activity

A series of dinuclear complexes has been prepared in which two {MoV(TpMe,Me)(O)Cl} fragments (abbreviated as Mo; TpMe,Me = tris(3,5-dimethylpyrazol-1-yl)hydroborate) are attached to either end of a bis-p-phenolate bridging ligand [(4,4′-OC6H4)–X–(4,4′-C6H4O)]2−. The complexes are Mo2(CC) (X = CHCH), Mo2(CC)2 (X = CHCH–CHCH), Mo2(CC)3 (X = CHCH–CHCH–CHCH), Mo2(th) (X = 2,5-thiophenediyl), Mo2(th)2 (X = 2,5:2′,5′-bithiophenediyl), Mo2(th)3 (X = 2,5:2′,5′:2″,5″-terthiophenediyl), Mo2(CC) (X = CC), Mo2(NN) (X = NN), Mo2(CO) [X = C(O)] and Mo2(C2ΦC2) [X = CHCH(1,4-C6H4)CHCH]. Electrochemical, UV/VIS/NIR spectroelectrochemical and magnetic measurements have been carried out in order to see how effectively the different spacer groups X mediate electronic and magnetic interactions between the two redox-active, paramagnetic, Mo centres. The electronic interactions were determined from the redox separation between the two successive one-electron oxidations which are formally Mo(VI)–Mo(V) couples; it was found that thienyl units in the bridging ligand are much more effective at maintaining electronic communication over long distances than p-phenylene or ethenyl spacers of comparable lengths. The azo (NN) linkage afforded a much weaker electronic interaction than the ethenyl or ethynyl spacers. UV/VIS/NIR spectroelectrochemical studies showed that whereas the first oxidation is metal-centred to give Mo(VI)–Mo(V) species with characteristic intense phenolate→Mo(VI) LMCT transitions in the near-IR region, the spectra of the doubly oxidised complexes are characteristic of quinones: thus, the sequence of species formed on oxidation is [Mo(V)(μ-diolate)Mo(V)]0 → [Mo(V)(μ-diolate)Mo(VI)]+ → [Mo(V)(μ-quinone)Mo(V)]2+, with an internal charge redistribution associated with the second oxidation. Semi-empirical ZINDO calculations provide some support for this. Magnetic susceptibility measurements on Mo2(CC), Mo2(th), Mo2(NN) and Mo2(CC) show that all are weakly antiferromagnetically coupled, as expected on the basis of a spin-polarisation picture, with the order of strength of the magnetic interaction being the reverse of the order for electronic coupling, such that Mo2(th) affords the strongest electronic interaction but the weakest magnetic interaction.