Charge Effects in PCP Pincer Complexes of NiII bearing Phosphinite and Imidazol(i)ophosphine Coordinating Jaws: From Synthesis to Catalysis through Bonding Analysis

This contribution reports on a new family of Ni-II pincer complexes featuring phosphinite and functional imidazolyl arms. The proligands (PIMCOPR')-P-R-O-H react at room temperature with Ni-II precursors to give the corresponding complexes [((PIMCOPR')-P-R)NiBr], where (PIMCOPR)-P-R=kappa(P),kappa(C)kappa(P)-{2-(R-2'PO),6-(R2PC3H2N2)C6H3}, R=iPr, R'=iPr (3b, 84%) or Ph (3c, 45%). Selective N-methylation of the imidazole imine moiety in 3b by MeOTf (OTf=OSO2CF3) gave the corresponding imidazoliophosphine [((PIMIOCOPiPr)-P-iPr)NiBr][OTf], 4b, in 89% yield ((PIMIOCOPiPr)-P-iPr=kappa(P),kappa(C),kappa(P)-{2-(iPr(2)PO),6-(iPr(2)PC(4)H(5)N(2))C6H3}). Treating 4b with NaOEt led to the NHC derivative [(NHCCOPiPr)NiBr], 5b, in 47% yield (NHCCOPiPr=kappa(P),kappa(C),kappa(C)-{2-(iPr(2)PO),6-(iPr(2)PC(4)H(5)N(2))C6H3}). The bromo derivatives 3-5 were then treated with AgOTf in acetonitrile to give the corresponding cationic species [((PIMCOPR)-P-R)Ni(MeCN)][OTf] [R = Ph, 6a (89%) or iPr, 6b (90%)], [((PIMIOCOPR)-P-R)Ni(MeCN)] [OTf](2) [R = Ph, 7a (79%) or iPr, 7b (88%)], and [(NHCCOPR)Ni(MeCN)][OTf] [R=Ph, 8a (85%) or iPr, 8b (84%)]. All new complexes have been characterized by NMR and IR spectroscopy, whereas 3b, 3c, 5b, 6b, and 8a were also subjected to X-ray diffraction studies. The acetonitrile adducts 6-8 were further studied by using various theoretical analysis tools. In the presence of excess nitrile and amine, the cationic acetonitrile adducts 6-8 catalyze hydroamination of nitriles to give unsymmetrical amidines with catalytic turnover numbers of up to 95.