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 [MoV(TpMe,Me)(O)Cl2] with 1,3-, 1,5-, 1,6-, 2,6-, and 2,7-dihydroxynaphthalene affords the dinuclear complexes [[Mo(TpMe,Me)(O)Cl]2(mu-C10H6O2)], abbreviated as 1,3-Mo2, 1,5-Mo2, 1,6-Mo2, 2,6-Mo2, and 2,7-Mo2, 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-Mo2, 1,6-Mo2, and 2,7-Mo2 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 from of the bridging ligand is a diradical. These complexes are classified as being "T-meta" (= topologically equivalent to meta). Complexes 1,5-Mo2 and 2,6-Mo2 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.