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(κ 2 -2-hp) ( 1 ), the main species observed are [Cp* 2 Ir 2 H 2 (2-hp)]Cl ([ 2 ]Cl) and Cp*IrHCl(κ 1 -2-hpH) ( 3 ). Crystallographic analysis confirms that the cation in [ 2 ]PF 6 consists of a Cp* 2 Ir 2 (μ-H) x 2+ core complemented by a pyridonate ligand that bridges via O and N centers. Although [ 2 ]Cl is catalytically highly active, the related salt [ 2 ]PF 6 is not. Addition of chloride sources reactivates [ 2 ]PF 6 . 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 PPh 3 to give Cp*IrCl(κ 1 -2-hp)(PPh 3 ), which in turn was found to react with AgPF 6 to give [Cp*Ir(κ 2 -2-hp)(PPh 3 )]PF 6 . Both PPh 3 derivatives proved catalytically inactive for dehydrogenation. Cp*IrCl(κ 2 -2-hp-6-Me) was also prepared but could not be converted to κ 1 -2-hpH-6-Me derivatives. The complex Cp*IrCl(C 5 H 3 O 2 NH), nominally derived from the conjugate base of 2,6-dihydroxypyridine, features the novel ligand η 3 -C 3 H 3 (CO) 2 NH.