Bidentate Iminophosphorane-NHC Ligand Derived from the Imidazo[1,5-a]pyridin-3-ylidene Scaffold

The synthesis of the bifunctional iminophosphorane-NHC (1) based on the imidazo[1,5-a]pyridin-3-ylidene (IPy) platform is reported. Its imidazo[1,5-a]pyridinium salt precursor [1 center dot H](X) was readily obtained by an efficient three-component coupling reaction between 5-bromoimidazo[1,5-a]pyridinium bromide, sodium azide, and triphenylphosphine according to a SN(Ar/)Staudinger reaction sequence. The stable free carbene 1 was generated by deprotonation of [1 center dot H](X) with potassium bis(trimethylsilyl)amide, and its coordination ability toward various transition-metals was evaluated, either upon direct metalation of the free carbene or by transmetalation from a silver(I) NHC complex. While the ligand 1 is singly bounded through the carbene carbon atom in the latter complex, it behaves as a chelating bidentate ligand in all other complexes that were prepared, including the cationic and neutral palladium(II) complexes [Pd(allyl)(kappa C-2,N -1)](OTf) ([5](OTf) and [PdCl2(kappa C-2,N -1)] (7), and the cationic rhodium(I) complexes [Rh(cod)(kappa C-2,N -1)](OTf) ([8](OTf)) and [Rh(CO)(2)(kappa C-2,N -1)](OTf) ([9](OTf)), generating stable 5-membered metallacycles. IR nu(CO) measurements carried out on the complex [Rh(CO)(2)(kappa C-2,N -1)](OTf) ([9](OTf)) show evidence of the strong donating character of the iminophosphorane-NHC ligand 1.