N-heterocyclic carbene complexes of palladium in oxygen atom transfer reactions involving the making and breaking of N-O bonds

Abstract

Reaction of three equivalents of MesCNO (Mes=2,4,6-trimethylphenyl) with one equivalent of [Pd(IPr)(P(p-tolyl)3)] in toluene yields the solid complex [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))]. Three major steps are proposed to be involved in the reaction based on spectroscopic studies as well as literature precedents for related cycloadditions: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a CO bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product.Addition of two equivalents of NO at low temperature to the in situ generated peroxide complex [Pd(IPr)2(η2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2] in keeping with a literature precedent reported for similar complexes. Insight into the energetics of this reaction are probed by DFT calculations using the truncated ligand complex [Pd(IMe)2]. The computed enthalpy of binding of two moles of NO2 to form [Pd(IMe)2(NO2)2] is −112kcal/mol indicating that its preparation from [Pd(IMe)2], N2 and 2O2 is thermodynamically favorable by −96kcal/mol. Crystal structures of [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported.