Reactivity patterns and catalytic chemistry of iridium polyhydride complexes

Abstract

[IrH 5P 2] ( 1, P  PPr i 3) reacts autocatalytically with CF 3COOR (R  CH 2CF 3) in cyclo-C 6D 12 at 60°C according to: 1 + CF 3COOR → [IrH 2P 2(OR)] ( 2) + ROH ( 4) (eq. 1). The rate-law, − d[ 1] d t = k[ 1] 1 2 [CF 3COOR][ 2] 1 2 [ 4] − 1 2 ( k = 1.25 × 10 −4 M − 1 2 sec −1), is consistent with the mechanism, 1 + 2 ⇌ 2 [IrH 3P 2] ( 5) + 4 (rapid equilibrium); 5 + CF 3COOR → [IrH 2P 2{OCH(OR)CF 3}] ( 6) (rate determining); 6 → 2 + CF 3CHO; 5 + CF 3CHO → 2. 2 reacts rapidly with H 2 (25° C, 1 atm) according to: 2 + 2 H 2 → 1 + 4 (eq. 2). Although the combination of reactions 1 and 2 constitute a catalytic cycle for the hydrogenation of CF 3COOR (CF 3COOR + 2 H 2 → 2 ( 4), catalyzed by 1), such catalytic hydrogenation does not occur, presumably because H 2 suppresses reaction by rapidly converting the catalytic intermediates, 2 and 5, to 1. However, 1 was found to be effective as a catalyst or catalyst precursor for transfer hydrogenation, e.g. CH 2CHC(CH 3) 3 + (CH 3) 2CHOH → CH 3CH 2C(CH 3) 3 + (CH 3) 2CO. While not directly detected, IrH 3P 2 could be trapped at low temperatures by N 2 to yield the complexes [IrH 3P 2(N 2)] and [(IrH 3P 2) 2N 2] which are related through the labile equilibrium, [(IrH 3P 2) 2N 2] + N 2 ⇌ 2 [IrH 3P 2(N 2)] ( K eq ∼ 1.5 at 35° C).