Stabilization and destabilization of the Ru-CO bond during the 2,2′-bipyridin-6-onato (bpyO)-localized redox reaction of [Ru(terpy)(bpyO)(CO)](PF6)

Two stereoisomers of [Ru(terpy)(bpyO)(CO)](PF6) ([1](+) and [2](+); terpy = 2,2':6',2-terpyridine, bpyO = 2,2'-bipyridin-6onato) were prepared. The pyridonato moiety in the bpyO ligand of [1](+) and [2](+) is located trans and cis, respectively, to CO. Treatment of [1](+) and [2](+) with HPF6 produced [1H](2+) and [2H](2+), both of which contain bpyOH (bpyOH = 6-hydroxy-2,2'-bipyridine). The difference in the pK(a) values of [1H](2+) (3.5) and [2H](2+) (3.9) reflects the stronger electronic interaction between CO and the pyridonato moiety in the bpyO ligand in the trans position compared with that in the cis position. The molecular structures of [1](PF6), [2](PF6).H2O and [2H](PF6)(2).2H(2)O were determined by X-ray structure analyses. [1]1 and [2]1 undergo one, reversible reduction at E-1/2 = -1.65 V and -1.51 V, respectively, and one irreversible reduction at E-p,E-c = -2.07 and E-p,E-c = -2.13 V, respectively. Both reductions are assigned to redox reactions localized at the terpy and bpyO ligands. Irreversible reduction of [1](0) results from reductive cleavage of the Ru-CO bond of [1](-). On the other hand, a two-electron oxidation of [2](-) almost regenerates [2](+) because of the depression of the reductive Ru-CO bond cleavage of [2]- due to cyclometalation formed by an attack of oxygen of bpyO to the carbon of the Ru-CO bond. An unusually large shift of the v(CdropO) band on going from [2](0) (1950 cm(-1)) to [2](-) (1587 cm(-1)) also supports a reversible cyclometalation driven by the bpyO-localized redox reaction. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).