Formation of Irida-β-ketoimines and PCN ^amine-Ir(III) Complexes by Reacting Irida-β-diketones with Aliphatic Diamines: Catalytic Activity in Hydrogen Release by Methanolysis of H ₃N–BH ₃

Abstract

Aliphatic diamines [(H ₂N(CH ₂) _n NHR) ( a–d) n = 2: R = H ( a), R = CH ₃ ( b), R = C ₂H ₅ ( c), n = 3, R = H ( d) or rac-2-(aminomethyl)piperidine ( e)] react with [IrH(Cl){(PPh ₂( o-C ₆H ₄CO)) ₂H}] in THF to afford ketoimine complexes [IrH(Cl){(PPh ₂( o-C ₆H ₄CO))(PPh ₂( o-C ₆H ₄CN(CH ₂) _n NHR))H}] ( 2a– 2d) or [IrH(Cl){(PPh ₂( o-C ₆H ₄CO))(PPh ₂( o-C ₆H ₄CNCH ₂(C ₅H ₉NH)))H}] ( 2e), containing a bridging N–H···O hydrogen bond and a dangling amine. Complex 2e consists of an almost equimolar mixture of two diastereomers. In protic solvents, the dangling amine in complexes 2 displaces chloride to afford cationic acyl-iminium compounds, [IrH(PPh ₂( o-C ₆H ₄CO))(PPh ₂( o-C ₆H ₄CNH(CH ₂) _n NHR))]X ( 3a– 3d, X = Cl) or [IrH(PPh ₂( o-C ₆H ₄CO))(PPh ₂( o-C ₆H ₄CNHCH ₂(C ₅H ₉NH)))]Cl ( 3e) and ( 4a– 4b, X = ClO ₄), with new hemilabile terdentate PCN ^amine ligands adopting a facial disposition. Complexes 3 contain the corresponding phosphorus atom trans to hydride and the amine fragment trans to acyl, while complexes 4 contain the amine trans to hydride. 3b and 4b consist of 80:20 and 95:5 mixtures of diastereomers, respectively, while 3e contains a 65:35 mixture. In the presence of KOH, intermediate cationic acyl-iminium complexes 3 transform into neutral acyl-imine [IrH(PPh ₂( o-C ₆H ₄CO))(PPh ₂( o-C ₆H ₄CN(CH ₂) _n NHR))] derivatives ( 5) with retention of the stereochemistry. Single-crystal X-ray diffraction analysis was performed on 2a, [ 3a]Cl, [ 3b]Cl, [ 4a]ClO ₄, and 5b. Complexes 2, 3, and 5 catalyze the methanolysis of ammonia-borane under air to release hydrogen. The highest activity is observed for ketoimine complexes 2.