Cobalt Phosphino-α-Iminopyridine-Catalyzed Hydrofunctionalization of Alkenes: Catalyst Development and Mechanistic Analysis

Abstract

A family of CoCl2(PNpy) complexes were prepared, where PNpy = 2-iminopyridyl-phosphine ligands derived from aminoalkyl and aminoaryl phosphines and 2-keto- and 2-formylpyridines. Reduction of CoCl2(PNpy) complexes in the presence of PPh3 gave CoH­(PNpy)­(PPh3) and CoMe­(PNpy)­(PPh3), which were active for hydrofunctionalization of alkenes. According to DFT calculations, the CoMe­(PNpy)­(PPh3) complexes are best described as Co­(II) derivatives of the anion [PNpy]−, with a labile PPh3 coligand. Metalation of Na­[Ph2PC2NHpy] gave the dimers [CoCl­(Ph2PC2NHpy)]2. Monomeric complexes catalyze hydrosilylation of 1-octene with Ph2SiH2, with the CoCl2(iPr2PC3NHpy)/2NaBEt3H system exhibiting the highest rate and selectivity for anti-Markovnikov product. In situ NMR studies established the following: (i) silanes protonolyze catalyst precursors to give the Co-silyl complexes Co­(SiR3)­(Ph2PC6H4NPhpy)­(PPh3), (ii) alkenes compete with PPh3 to give Co­(SiHPh2)­(Ph2PC6H4NPhpy)­(η2-alkene), (iii) ethylene inserts into the Co–Si bond to give Co­(CH2CH2SiR3)­(Ph2PC6H4NPhpy)­(PPh3).

Cobalt(II) and (I) Complexes of Diphosphine‐Ketone Ligands: Catalytic Activity in Hydrosilylation Reactions

“…Cobalt‐catalyzed hydrosilylation is a growing field with a number of highly efficient catalysts reported in recent years. The cobalt‐catalyzed hydrosilylation reactions reported by Holland, Huang and Fout (vide supra, Figure ) are proposed to follow the Chalk–Harrod mechanism, in agreement with an in‐depth investigation on mechanistic aspects, using a cobalt‐bound phosphine substituted 2‐iminopyridine ligand, as published by Rauchfuss, van Gastel and co‐workers . A rare example in which the specific role of a cooperative ligand is determined was shown by Peters and co‐workers with a diphosphine‐borane cobalt‐dinitrogen complex (Figure , right), where the silane substrate is activated between the metal and the ligand backbone by addition of –SiHPh 2 to Co and insertion of the hydride into the Co–B bond …”

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

“…The activity of the R PQpy complexes was higher, and these complexes, especially those based on cobalt, were the focus of catalytic studies. Together with our previous report on more flexible PNN ligands, 16,17 the present results show that catalytic activity is highly sensitive to the ligand backbone. The quinoline-based PQpy ligands vs PNN ligands based on aldimines both coordinate in a meridional manner, but the M(PQpy) cores are almost perfectly planar.…”

“…In the presence of 1-octene and triethoxysilane or heptamethyltrisiloxane, Co(PNpy)-based catalysts preferentially effected alkene isomerization, not hydrosilylation. 17 In this report, we redesigned the PNN platform in an effort to improve performance for hydrosilylation. We also were attentive to problems of autoionization, a process that deactivates precatalysts according to eq 1.…”

“…Cobalt and iron complexes of phosphine-2-iminopyridine (PNpy) ligands were recently reported by us as catalysts for hydrosilylation. , The phosphine-imine-pyridine (PNpy) platform features the redox noninnocent 2-iminopyridine module as seen in many hydrosilylation catalysts (Figure ). ,, The phosphine substituent provides a strongly anchoring substituent that is tunable electronically and sterically.…”

“…The Co-PNpy complexes exhibit excellent activity for the anti-Markovnikov hydrosilylation of 1-octene with Ph 2 SiH 2 . In the presence of 1-octene and triethoxysilane or heptamethyltrisiloxane, Co­(PNpy)-based catalysts preferentially effected alkene isomerization, not hydrosilylation …”