Fe and Co Complexes of Rigidly Planar Phosphino-Quinoline-Pyridine Ligands for Catalytic Hydrosilylation and Dehydrogenative Silylation

Co and Fe dihalide complexes of a new rigidly planar PNN ligand platform are prepared and examined as precatalysts for hydrosilylation of alkenes. Lithiation of Thummel's 8-bromo-2-(pyrid-2′-yl)quinoline followed by treatment with (i-Pr)₂PCl and (C₆F₅)₂PCl afforded the phosphine-quinoline-pyridine ligands, abbreviated ^RPQpy for R = i-Pr and C₆F₅, respectively. These ligands form 1:1 adducts with the dichlorides and dibromides of iron and cobalt. Crystallographic characterization of FeBr₂(^iPrPQpy), FeBr₂(^ArFPQpy), CoCl₂(^iPrPQpy), CoBr₂(^iPrPQpy), and CoCl₂(^ArFPQpy) confirmed that the M-P-C-C-N-C-C-N portion of these complexes is planar within 0.078 Å unlike previous generations of PNN complexes where deviations from planarity were ∼0.35 Å. Bond distances as well as magnetism indicate that the Fe complexes are high spin and the cobalt complexes are high spin or participate in spin equilibria. Also investigated were the NNN analogues of the ^RPQpy ligands, wherein the phosphine group was replaced by the mesityl ketimine. The complexes FeBr₂(^MesNQpy) and CoCl₂(^MesNQpy) were characterized crystallographically. Reduction of MX₂(^RPQpy) complexes with NaBHEt₃ generates catalysts active for anti-Markovnikov silylation of simple and complex 1-alkenes with a variety of hydrosilanes. Catalysts derived from ^MesNQpy exhibited low activity. Fe-^RPQpy derived catalysts favor hydrosilylation, whereas Co-^RPQpy based catalysts favor dehydrogenative silylation. Catalysts derived from CoX₂(^iPrPQpy) convert hydrosilanes and ethylene to vinylsilanes. Related experiments were conducted on propylene to give propenylsilanes.