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)

Abstract

AbstractTwo 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 pKa 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)2·2H2O were determined by X‐ray structure analyses. [1]+ and [2]+ undergo one, reversible reduction at E1/2 = −1.65 V and −1.51 V, respectively, and one irreversible reduction at Ep,c = −2.07 and Ep,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)