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-dihydroxynaphthalene 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. Electrochemical, UV-vis/NIR spectroscopic, and variable-temperature magnetic susceptibility studies have been used to probe the effects of the bridging-ligand 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 bridging ligands that are topologically equivalent to meta-substituted bridging ligands such as 1,3-dihydroxybenzene, in that (i) there is an odd number of C atoms separating the two oxygen atoms, regardless of the pathway that is taken through the ligand skeleton, and (ii) the doubly oxidized form of the 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-Mo-2 have bridging ligands that are topologically equivalent to para-substituted groups such as 1,4-dihydroxybenzene, in that (i) there is an even number of C atoms separating the two oxygen atoms, whichever pathway is taken through the ligand skeleton, and (ii) the doubly oxidized form of the bridging ligand is a diamagnetic quinone. These complexes are classified as "T-para". 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 that the Mo(V)/Mo(IV) mixed-valence states of the T-para complexes show pronounced Mo(IV)-->Mo(V) IVCT transitions, whereas those of the T-meta complexes do not show these transitions. Magnetic susceptibility studies show that the T-meta complexes all display ferromagnetic exchange between the metal centers, 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 topology.