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

Abstract

A series of dinuclear complexes has been prepared in which two {Mo^V(Tp^Me,Me)(O)Cl} fragments (abbreviated as Mo; Tp^Me,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.