From NHC to Imidazolyl Ligand: Synthesis of Platinum and Palladium Complexes d10-[M(NHC)2] (M = Pd, Pt) of the NHC 1,3-Diisopropylimidazolin-2-ylidene

The widely held belief that N-heterocyclic,carbenes (NHCs) act only as innocent spectator ligands is not always accurate, even in the context of well-explored reactions. Ligand exchange in the conversion of [Pt-(PPh3)(2)(eta(2)-C2H4)] (3) to [Pt(iPr(2)Im)(2)] (2) depends critically on the particular reaction conditions employed, with slight changes leading to vastly different outcomes. In addition to [Pt(iPr(2)Im)(2)] (2), complexes [Pt(iPr(2)Im)(PPh3)(eta(2)-C2H4)] (5) and trans-[Pt((i)Pr(2)Im)(2)(Pr-i-Im*)(H)] (6) were isolated and in the case of 6 fully characterized. Complex 5 represents the first mixed-olefin complex in transition metal chemistry containing both an NHC and a phosphine ligand. Chemical degradation of the NHC was Shown to yield the new imidazole-2-yl iPr-Im* in 6. Therefore, the synthesis of [Pt(iPr(2)Im)(2)] (2) via metallic reduction of the ionic precursor [Pt(iPr(2)Im)(3)(Cl)]Cl-+(-) (9) is favorable, a procedure adaptable to analogous palladium compounds. While [Pd(iPr(2)Im)(3)(Cl)]Cl-+(-) (8) is the only product obtained from the reaction of iPr(2)Im and PdCl2, neutral [Pt((i)Pr(2)Im)(2)(Cl)(2)] (10), formed as a mixture of its two stereoisomers cis-10 and trans-10; is available through precise control of the Stoichiometry in the reaction of PtCl2 and exactly 2 equiv of iPr(2)Im.