Metal-metal interactions as a function of bridging ligand topology: An electrochemical, spectroelectrochemical, and magnetic study on dinuclear oxo-Mo(V) complexes with various isomers of dihydroxynaphthalene as bridging ligand

Reaction of [Mo-V(Tp(Me,Me))(O)Cl-2] with 1,3-, 1,5-, 1,6-, 2,6-, and 2,7-d ihydroxynaphthalene affords the dinuclear complexes [{Mo(Tp(Me,Me))(O)Cl}(2 )(mu-C10H6O2)], abbreviated as 1,3-Mo-2, 1,5-Mo-2, 1,6-Mo-2, 2,6-Mo-2, and 2,7-Mo-2, according to the substitution pattern of the bridging ligand. Ele ctrochemical, UV-vis/NIR spectroscopic, and variable-temperature magnetic s usceptibility studies have been used to probe the effects of the bridging-l igand topology on the metal-metal electronic and magnetic interactions. The complexes can be split into two classes according to the properties of the bridging ligands. Complexes 1,3-Mo-2, 1,6-Mo-2, and 2,7-Mo-2 all have brid ging ligands that are topologically equivalent to meta-substituted bridging ligands such as 1,3-dihydroxybenzene, in that (i) there is an odd number o f C atoms separating the two oxygen atoms, regardless of the pathway that i s taken through the ligand skeleton, and (ii) the doubly oxidized form of t he bridging ligand is a diradical. These complexes are classified as being "T-meta" (= topologically equivalent to meta). Complexes 1,5-Mo-2 and 2,6-M o-2 have bridging ligands that are topologically equivalent to para-substit uted groups such as 1,4-dihydroxybenzene, in that (i) there is an even numb er of C atoms separating the two oxygen atoms, whichever pathway is taken t hrough the ligand skeleton, and (ii) the doubly oxidized form of the bridgi ng ligand is a diamagnetic quinone. These complexes are classified as "T-pa ra". Electrochemical studies show that the comproportionation constants for the Mo(V)/Mo(IV) mixed-valence states of the T-meta complexes are smaller than those for the T-para complexes. Spectroelectrochemical studies show th at the Mo(V)/Mo(IV) mixed-valence states of the T-para complexes show prono unced Mo(IV)-->Mo(V) IVCT transitions, whereas those of the T-meta complexe s do not show these transitions. Magnetic susceptibility studies show that the T-meta complexes all display ferromagnetic exchange between the metal c enters, whereas the T-para complexes all display antiferromagnetic exchange . Thus, both the electronic and the magnetic properties of these complexes show a clear demarcation into two sets according to the bridging-ligand top ology.