(C∧C*)-cyclometalated platinum(II) imidazo[1,5-a]pyridine NHC complexes – Synthesis and characterization.

Imidazo[1,5- a ]pyridines have been used as monodentate ligands in late transition-metal N -heterocyclic carbene (NHC) complexes, but the N -aryl substituted heterocyclic ligands also allow for the formation of C ∧ C* cyclometalated complexes, which has previously only been shown for iridium(III) complexes. Here we present the first C ∧ C* platinum(II) NHC complexes of this class. A monoanionic bidentate ligand L – acetylacetonato (acac) or dipivaloylmethanato (dpm) – was used to generate heteroleptic compounds of the general formula [(C ∧ C*-NHC)Pt(L)]. Seven mononuclear complexes and one binuclear complex are presented. The square-planar coordination of the C ∧ C* cyclometalated species was confirmed by spectroscopy and additionally by three solid-state structures. The complexes have been fully characterized including 195 Pt NMR and extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY). From the experimental data we exclusively see the formation of five-membered metallacycles to the aryl substituent and no interaction with the pyridine part of the ligand, in good agreement with density functional theory calculations (B3LYP/6-31G(d)), which show that the five-membered metallacycles are significantly favoured over the four-membered metallacycles. [ABSTRACT FROM AUTHOR]