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

The
reaction of the N-heterocyclic carbenes (NHCs) IiPr2Me2 and IiPr2 with [PtMe3I]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 [PtMe2(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]­[SbF6] using AgSbF6 as a halogen abstractor, whereas NaBArF is not reactive enough to completely remove the iodide ligand,
leading instead to the dinuclear species [Pt2(μ-I)­(Me)2(NHC)4]­[BArF]. Alternatively, low-coordinate
T-shaped complexes [PtMe­(NHC)2]­[BArF] can be
prepared by protonation of the dimethyl species [PtMe2(NHC)2] with [H­(OEt2)2]­[BArF].
The complex [PtMe­(IiPr2Me2)2]­[BArF] undergoes a cyclometalation
process under mild heating, leading to the derivative [Pt­(IiPr2Me2′)­(IiPr2Me2)]­[BArF] (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 (ItBu). 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.