Widely separated reduction processes of abpy-coupled areneosmium(II) reaction centers (abpy=2,2′-azobispyridine):: Stabilization of the radical intermediate and of the Os0OsII state

Electrochemical reactivity patterns have been established by cyclic voltammetry, EPR, and UV/vis spectroelectrochemistry for the transition [(C6Me6)ClOs](+)/[(C6Me6)Os] in mononuclear and dinuclear complexes with the 2,2'-azobispyridine (abpy) and 2,2'-bipyrimidine (bpym) bridging ligands. The isolated electron reservoir intermediate {[(mu-abpy)OsCl-(C6Me6)](2)}(center dot+) could be analyzed by X band and W band EPR with regard to Os-189 hyperfine splitting and g anisotropy as an abpy anion radical species with significant contribution from the metal centers. The function of the pi-conjugated acceptor ligand in mediating the interaction between two equivalent electron and atom transfer sites was analyzed through simulation of the cyclic voltammograms. In comparison with the system bridged by 2,2'-bipyrimidine, the dinuclear abpy complex displays a much stronger interaction between the two organometallic reaction centers, as illustrated by the 1.14 V vs 0.42 V splitting between the redox potentials separating the two chloride-dissociative processes, i.e., stabilizing the (OsOsII)-Os-0 mixed-valent form {[(Me6C6)Os(mu-abpy)OsCl(C6Me6)]}(+). This result parallels the observations made for (C5Me5)Rh- and (C5Me5)Ir-containing analogues and for the coupling of pure electron transfer centers through such bridging ligands.