Organoiridium pyridonates and their role in the dehydrogenation of alcohols

New derivatives of 2-hydroxypyridine (2-hpH) and Cp*Ir(III) are described. Under conditions for catalytic dehydrogenation of 1-phenylethanol catalyzed by Cp*IrCl(Κ²-2-hp) (1), the main species observed are [Cp*₂Ir₂H₂(2-hp)]Cl ([2]Cl) and Cp*IrHCl(Κ¹-2-hpH) (3). Crystallographic analysis confirms that the cation in [2]PF₆ consists of a Cp* ₂Ir₂(μ-H)_x²⁺ core complemented by a pyridonate ligand that bridges via O and N centers. Although [2]Cl is catalytically highly active, the related salt [2]PF₆ is not. Addition of chloride sources reactivates [2]PF₆. Collectively, our experiments indicate that [2]Cl is a resting state that reverts to a more active species, which we propose is 1 itself. In situ NMR observations and PPh ₃ trapping experiments show that under catalytically relevant conditions 1 rapidly converts to 3, which can be observed spectroscopically. Compound 3 was independently generated by transfer hydrogenation of 1. In other experiments, 1 was found to ring-open upon treatment with PPh₃ to give Cp*IrCl(Κ¹-2-hp)(PPh₃), which in turn was found to react with AgPF₆ to give [Cp*Ir(Κ²- 2-hp)(PPh₃)]PF₆. Both PPh₃ derivatives proved catalytically inactive for dehydrogenation. Cp*IrCl(Κ²-2- hp-6-Me) was also prepared but could not be converted to Κ¹-2- hpH-6-Me derivatives. The complex Cp*IrCl(C₅H₃O ₂NH), nominally derived from the conjugate base of 2,6-dihydroxypyridine, features the novel ligand η³-C ₃H₃(CO)₂NH.