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 {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 Mo-2(C=C) (X=CH=CH), Mo-2(C=C)(2) (X=CH=CH-CH=CH), Mo-2(C=C)(3) (X=CH=CH-CH=CH-CH=CH), Mo-2(th) (X=2,5-thiophenediyl), Mo-2(th)(2) (X=2,5:2',5'-bithiophenediyl), Mo-2(th)(3) (X=2,5:2',5':2,5-terthiophenediyl), Mo-2(C=C) (X=C=C), Mo-2(N=N) (X=N=N), Mo-2(CO) [X=C(O)] and Mo-2(C(2)PhiC(2)) [X=CH=CH(1,4-C6H4)CH=CH]. 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 (N=N) 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)(mu -diolate)Mo(V)](0) --> [Mo(V)(mu -diolate)Mo(VI)](+)--> [Mo(V)(mu -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 Mo-2(C=C), Mo-2(th), Mo-2(N=N) and Mo-2(C=C) 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 Mo-2(th) affords the strongest electronic interaction but the weakest magnetic interaction.