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
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Organometallics 24 (8): 1966--1973 (2005)

Mono- and binuclear 2,2'-azobispyridine and 2,2'-bipyrimidine-bridged osmium arene complexes were prepd.; their redox reactions afford mixed-valence odd-electron intermediates, which structure was evaluated by EPR spectra. Electrochem. reactivity patterns were established by cyclic voltammetry, EPR, and UV/vis spectroelectrochem. 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 (μ-abpy)OsCl(C6Me6)2.bul.+ could be analyzed by X band and W band EPR with regard to 189Os hyperfine splitting and g anisotropy as an abpy anion radical species with significant contribution from the metal centers. The function of the p-conjugated acceptor ligand in mediating the interaction between two equiv. 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 sepg. the two chloride-dissociative processes, i.e., stabilizing the Os0OsII mixed-valent form (Me6C6)Os(m-abpy)OsCl(C6Me6)+. This result parallels the observations made for (C5Me5)Rh- and (C5Me5)Ir-contg. analogs and for the coupling of pure electron transfer centers through such bridging ligands. on SciFinder(R)
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