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)

Article abstract of DOI:10.1002/ejic.200400522

Two stereoisomers of [Ru(terpy)(bpyO)(CO)](PF₆)([1]⁺ and [2]⁺; terpy = 2,2′:6′,2″-terpyridine, bpyO = 2,2′-bipyridin-6-onato) 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](PF ₆)·H₂O and [2H](PF₆)₂· 2H₂O were determined by X-ray structure analyses. [1]⁺ and [2]⁺ undergo one, reversible reduction at E_1/2 = -1.65 V and -1.51 V, respectively, and one irreversible reduction at E_p,c = -2.07 and E_p,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 ν(C≡O) 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. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.