Coordinatively Unsaturated T-Shaped Platinum(II) Complexes Stabilized by Small N-Heterocyclic Carbene Ligands. Synthesis and Cyclometalation

Abstract

The reaction of the N-heterocyclic carbenes (NHCs) I i Pr 2 Me 2 and I i Pr 2 with [PtMe 3 I] 4 does not lead to the clean formation of complexes trans -[PtMeI(NHC) 2 ], as previously found for bulkier NHCs. However, these species can be prepared in high yields by an alternative procedure using the dimethyl species [PtMe 2 (NHC) 2 ] and iodomethane. The halogenated complexes trans -[PtMeI(NHC) 2 ] have been used as precursors for the formation of the coordinatively unsaturated Pt(II) derivatives [PtMe(NHC) 2 ][SbF 6 ] using AgSbF 6 as a halogen abstractor, whereas NaBAr F is not reactive enough to completely remove the iodide ligand, leading instead to the dinuclear species [Pt 2 (μ-I)(Me) 2 (NHC) 4 ][BAr F ]. Alternatively, low-coordinate T-shaped complexes [PtMe(NHC) 2 ][BAr F ] can be prepared by protonation of the dimethyl species [PtMe 2 (NHC) 2 ] with [H(OEt 2 ) 2 ][BAr F ]. The complex [PtMe(I i Pr 2 Me 2 ) 2 ][BAr F ] undergoes a cyclometalation process under mild heating, leading to the derivative [Pt(I i Pr 2 Me 2 ′)(I i Pr 2 Me 2 )][BAr F ] (where the prime denotes the cyclometalated ligand). This result is in contrast with the fast cyclometalation observed for the related Pt(II) complex bearing tert -butyl-substituted NHC (I t Bu). Different cyclometalation reaction mechanisms have been computationally addressed. DFT calculations indicate that the cyclometalation involving complexes with not very bulky NHCs ligands occurs via cis intermediates. Cyclometalation products bearing tert -butyl-substituted NHCs are kinetically and thermodynamically favored over those involving isopropyl groups.