Drastic Tuning of the Electronic Structures of Diruthenium Aryl Complexes by Isoelectronic Axial Ligands.

Reported herein is the use of aryls as axial ligands to manipulate reactivity at the distal metal site through metal-metal-ligand interactions in diruthenium paddlewheel complexes. The vacant ruthenium site in Ru2(ap)4(Ar) (1; ap = 2-anilinopyridinate and Ar = C6H4-4-NMe2), thus rendered reactive, is able to bind a series of isoelectronic ligands to afford three complexes of the form (Y)[Ru2(ap)4](Ar) [Y = CN- (2), HC≡C- (3), CO (4)], each of which exhibits a distinct electronic structure. While reactions with anionic ligands subsequently result in oxidation of the diruthenium core from Ru2(II,III) to Ru2(III,III), the reaction with CO yields a rare example of a Ru2(II,III)-COaxial adduct. The latter reaction is particularly interesting in its completely reversible change of the ground state from S = 3/2 in 1 to S = 1/2 in 4, the first of its kind seen in Ru2(II,III) species. In general, this work sheds light on the modulation of the electronic structure of diruthenium paddlewheel complexes using distinct coordination environments around each of the ruthenium centers.