Oxygen Binding to [Pd(L)(L′)] (L= NHC, L′ = NHC or PR3, NHC = N-Heterocyclic Carbene). Synthesis and Structure of a Paramagnetic trans-[Pd(NHC)2(η1-O2)2] Complex

The reactivity of a number of two-coordinate [Pd(L)(L′)] (L = N-heterocyclic carbene (NHC) and L′ = NHC or PR3) complexes with O2 has been examined. Stopped-flow kinetic studies show that O2 binding to [Pd(IPr)(P(p-tolyl)3)] to form cis-[Pd(IPr)(P(p-tolyl)3)(η2-O2)] occurs in a rapid, second-order process. The enthalpy of O2 binding to the Pd(0) center has been determined by solution calorimetry to be −26.2(1.9) kcal/mol. Extension of this work to the bis-NHC complex [Pd(IPr)2], however, did not lead to the formation of the expected diamagnetic complex cis-[Pd(IPr)2(η2-O2)] but to paramagnetic trans-[(Pd(IPr)2(η1-O2)2], which has been fully characterized. Computational studies addressing the energetics of O2 binding have been performed and provide insight into reactivity changes as steric pressure is increased.