Efficient hydridoirida-β-diketone-catalyzed hydrolysis of ammonia- or amine-boranes for hydrogen generation in air

Abstract

The dihydridoirida-β-diketone [IrH₂{(PPh₂(o-C₆H₄CO))₂H}] (2) has been used as a homogeneous catalyst for the hydrolysis of ammonia- or amine-boranes to generate up to 3 equivalents of hydrogen in the presence of air. When using 0.5 mol% loading of 2, dimethylamine-borane is hydrolysed completely within 8 min at 30 °C and maintains its activity in consecutive runs. Ammonia-borane or tert-butylamine-borane is hydrolysed completely within 32 or 25 min respectively. Triethylamine-borane fails to be hydrolysed. Kinetic studies suggest a sequence of two consecutive first-order reactions, in which an intermediate builds up and finally falls, with the first step being the rate controlling step. ΔH^‡₁ are in the range 65–85 kJ mol⁻¹ and negative values of ΔS^‡₁ are obtained. A multinuclear NMR study of the catalyzed reaction shows the formation of a resting state (A) of the active catalyst proposed to be of the hydridodiacyl type [IrH(PPh₂(o-C₆H₄CO))₂(solvent)] with a hydride trans to the acyl group. In the absence of substrate a dormant species (B) is formed. By the reaction of hydridoirida-β-diketones with ammonia, the hydridoirida-β-ketoimine [IrHCl{(PPh₂(o-C₆H₄CO))(PPh₂(o-C₆H₄CNH))H}] (3) and the hydridobis(acylphosphane)aminoiridium(III) complex [IrH(PPh₂(o-C₆H₄CO))₂(NH₃)] (4), with a hydride trans to phosphane, are formed. Aromatic amines such as aniline or anisidines afford cationic [IrH{(PPh₂(o-C₆H₄CO))₂H}(C₆H₄RNH₂)]ClO₄ (R = H (6); p-MeO (7); o-MeO (8)) hydridoirida-β-diketones with a coordinated amine group trans to the hydride. The dormant species B is proposed to be of the hydridobis(acylphosphine)aminoiridium(III) type with a hydride trans to the amine group.