Template Synthesis of Tetracarbene Platinum(II) Complexes with Five- and Six-Membered-Ring N‑Heterocyclic Carbenes

Reaction
of 2 equiv of 2-azidoethyl isocyanide and the tetraphosphine
platinum­(II) complexes [1a–c]­(X)2 (a, PR3 = PMe3; b, PR3 = PMe2Ph; c, PR3 = PMePh2) yielded via phosphine substitution and
template-controlled cyclization of the isocyanide ligands the trans-di­(NH,NH-NHC) complexes [2a–c]­(X)2. Treatment of [2c]­(PF6)2 with an additional 2 equiv of 2-azidoethyl isocyanide
gave the tetracarbene complex [3]­(PF6)2, bearing four protic NH,NH-NHC ligands derived from a five-membered
heterocycle. The same template synthesis utilizing 3-azidopropyl isocyanide
and complex [1c]­(PF6)2 gave the
dicarbene complex [5]­(PF6)2 and,
after addition of 2 equiv more of 3-azidopropyl isocyanide, the tetracarbene
complex [6]­(PF6)2 with four protic
NH,NH-NHC ligands derived from a six-membered heterocycle. The intermediate trans-di­(NH,NH-NHC) platinum­(II) complex [5]­(PF6)2 also reacts with 2 equiv of 2-azidoethyl
isocyanide to give the tetracarbene complex [7]­(PF6)2, bearing two five-membered-ring NHCs and two
six-membered-ring NHCs.