Organoiridium Pyridonates and Their Role in the Dehydrogenation of Alcohols

Abstract

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 PPh3 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(C5H3O2NH), nominally derived from the conjugate base of 2,6-dihydroxypyridine, features the novel ligand η3-C3H3(CO)2NH.