Metal-ligand cooperativity in first row transition metals: hydrosilylation catalysis and mechanistic insights

The utility of transition metal catalysts has greatly expanded the scope of organic reactions. Historically, homogenous catalysis is dominated by platinum group metals (PGMs) due to their relative high activity and stability to air when compared to the 1st row counterparts of PGMs. However, 1st row late transition metals such as Fe, Co, and Ni offer the advantage of being low cost and earth abundant. This is especially critical when considering that catalyst recycling technologies do not exist for some large scale industrial catalysis, such as the crosslinking of silicone polymers. Recent strive for more sustainable chemical processes has fueled the search for new modes of metal-promoted substrate activation that extend beyond classical organometallic transformation (ie. oxidative addition, reductive elimination, insertion etc.). To this end, metal-ligand cooperative (MLC) activation of small molecules has been implemented in a number of remarkably active 1st row transition metal catalysts. These MLC reactions involve either bond breakage/formation between substrates and ligand, or electronic transformations of the ligands during catalytic reactions. Hydrosilylation of alkenes is one of the most important reactions in the silicone industry, which is currently dominated by catalysts based on PGMs. 1st row transition metal catalysts for this process typically exhibit low turnover numbers and suffer from severe side reactions, such as dehydrogenative silylation and hydrogenation. However, recent progress has suggested 1st row transition metals employing ligands that assist in MLC activation of hydrosilanes have high potential in promoting PGM-like hydrosilylation activity. Chapter 1 gives an overview of MLC strategies and the classes of metal complexes that exhibit MLC activity, with emphasis on hydrosilylation. Chapter 2 presents the successful employment of one type of MLC strategy to a diphosphine-dialkoxide iron(II) system, Fe(P2O2)(CO)L (L = CO, NCMe, PMe3, py, acetamide). An unprecedented activation mode of silanes was observed. The iron-alkoxo functionality reacted with silanes to give hydridoiron(II) species with a pendent silyl-ether ligand. Similar activation modes have only been described for a few complexes containing metal-sulfur bonds. The Fe(P2O2)(CO)L complexes were also active for catalytic hydrosilylation of aldehydes, ketones, and styrene. The catalytic activity was observed to be dependent on the binding affinities of L. Imine ligands have received increased attention in the past decade due to their ability to assist in substrate activation via MLC pathways. Platforms incorporating both imine and phosphines have shown great promise in 1st row transition metal catalyzed hydrofunctionalization reactions. Chapter 3 describes the reactivity of a series of iron complexes containing a phosphine-imine ligand, 2-Ph2PC6H4CH=N(4-ClC6H4) (PCHNArCl). These complexes show rich redox chemistry that involve the ligand, including redox induced hapticity change and Cimine-Cimine bond coupling reactions. Chapter 4 expands on the theme of phosphine-imines to incorporate pyridines in a new ligand system. The resulting phosphine-imine-pyridine (PNpy) platform belongs to the large class of  diimine ligands. The modular synthesis of PNpy provides a convenient system to tune the sterics and electron richness of the ligands. Synthesis of several PNpy ligands and the corresponding metal-dihalide complexes are described. Chapter 5 focuses on Co-(PNpy) complexes obtained by formal 1e reduction of the corresponding Co(II) species. These compounds are active hydrosilylation catalysts, with rates and selectivity dependent on the PNpy ligands employed. In particular, the catalyst system CoCl2(iPr2PC3NHpy)/2NaBEt3H gave near quantitative anti-Markovnikov hydrosilylation of 1-octene using Ph2SiH2 after 15 mins with 1 mol% [Co] loading. DFT studies highlighted the redox non-innocence of PNpy, which is possibly relevant to the high catalytic activity observed. In-situ NMR studies revealed several details related to the mechanism of hydrosilylation: (1) alkene binding is observed in the form Co(SiR3)(PNpy)(styrene), (2) silanes protonolyze catalyst precursors to give s-silyl complexes, Co(SiR3)(PNpy)(PPh3), and (3) ethylene inserts into the Co-Si bond to give Co(CH2CH2SiR3)(PNpy)(PPh3). Chapter 6 presents the synthesis, reactivity and electrochemistry of reduced Fe-(PNpy) complexes. Some of these species exhibited high activity for selective anti-Markovnikov hydrosilylation. The electrochemical investigation of Fe(PNpy)(CO)2 complexes revealed unique redox properties when compared to currently known phosphine--diimine platforms. Chapter 7 describes preliminary studies on Fe, Co, and Ni complexes incorporating phosphine-amide-pyridine and phosphine-amine-pyridine ligands. These ligands were conveniently obtained by reduction of PNpy. The iron(II)-amido complex [Fe(Ph2PC6H4(amide)py)(CO)2]+ reacted with H2 to form iron(II)-hydrides. This represents a promising strategy for catalytic hydrogenation. The cationic [NiBr(Ph2PC6H4NHpy)]+ is a pre-catalyst for selective anti-Markovnikov hydrosilylation. It is proposed that the active species is a nickel(II)-hydride that forms by reaction of the precatalyst with HSiR3 to give BrSiR3 and [NiH(Ph2PC6H4NHpy)]+