Breaking Chemistry's Strongest Bond: Can Three-Coordinate [M{N(R)Ar} 3 ] Complexes Cleave Carbon Monoxide?

Abstract

The reaction pathway for the interaction of CO with three-coordinate TaIII, WIII and ReIII complexes (modelled on the experimental [M{N-(tBu)Ar}3] system) has been explored by using density functional methods. Calculations show that CO binds without a barrier to [Re(NH2)3], forming the encounter complex [OC-Re-(NH 2)3], which is stabilized by ≈ 280 kJ mol-1 relative to the reactants. The binding of [Ta(NH2)3] to the oxygen terminus of CO is inhibited by a barrier of only 20 kJ mol -1 and is followed by spontaneous cleavage of the C-O bond to form the sproducts [C-Re(NH2)3] and [O-Ta(NH2) 3]. The salient features of the potential energy surface are more favourable to CO cleavage than the analogous N2 cleavage by [Mo(NH2)3], which is less exothermic (335 vs. 467 kJ mol-1) and is impeded by a significant barrier (66 kJ mol -1). The ReIII/TaIII/CO system therefore appears to be an excellent candidate for cleaving the exceptionally strong C-O bond under mild laboratory conditions. The related WIII/Ta III dimer, which significantly weakens but does not cleave the CO bond, may be a suitable alternative when the chemistry is to be performed on activated CO rather than on the strongly bound oxide and carbide cleavage products.