Silanes, Silicon(II) and Germanium(II) Anions Bearing Nitrogen Heterocycles: Stepping Stones towards Novel Ligands for Base-Metals

Abstract

Low-valent silicon(II) compounds have recently emerged as a promising class of strong donor ligands for transition metals. N-heterocyclic silylenes (NHSis) are the heavier analogues of the widely-used N-heterocyclic carbenes (NHCs). Silylene ligands have been employed in a wide variety of catalytic reactions. In particular catalytic C–C cross-coupling reactions. Silylenes have shown to be promising alternatives to NHCs and phosphines. Related Si(II) ligands are anionic silanides (R3Si–), which are formally isoelectronic to phosphines and hence can be expected to find similar applications as stronger donor analogues. Recent studies have shown that electron-withdrawing N-heterocycles confer significant stability to the intrinsically reactive Si(II)– center by induction. In this thesis, the use of pyrrole-derivatives to stabilize silanides was investigated. Hydrosilanes bearing the dippIMP (dippIMPH = 2‑(N‑(2,6‑diisopropylphenyl)iminomethyl)pyrrole) substituent were synthesized. Somewhat surprisingly, these silanes undergo an intra‑molecular hydrosilylation reaction of the imine forming the bidentate aminomethylene (dippAMP) substituent, hampering their use as Si(II) precursors. However, this system allows for investigation of this catalyst-free hydrosilylation reaction. Detailed studies on the reaction of the monosubstituted (dippIMP)SiCl2H revealed distinct reaction steps allowing for the proposal of a reaction pathway involving a redistribution of substituents. A synthesis method that does not require hydrosilanes as a synthetic intermediate was sought in a method where the silicon atom is first bound to iron as an –SiCl3 ligand, followed by chloride substitution at silicon to form the ligand on the metal. Substitution for N-heterocycles afforded the pyr3Si–, (MI)3Si–, and (MP)2ClSi– ligands. Attempted substitution for tmim (tmimH3 = tris(3-methylindol-2-yl)methane) on Cl3Si–Fe(CO)4– and dippIMP on Cl3SiFp gave rise to unexpected complex reactions. The silanide (tmim)Si– was found difficult to access through the corresponding tetrahedral silanes (tmim)SiH and (tmim)SiCl intermediates. Alternatively, (tmim)Si– was synthesized through nucleophilic substitution for tmim on a Si(II)Cl2 precursor. (tmim)Si– undergoes direct complexation to the base-metal salts CuCl and FeCl2. The substitution on a Si(II)Cl2 precursor offers an interesting alternative method for the synthesis of free silanides and is anticipated to find more applications in this field in the future. The series (tmim)E (E = Si, P) was expanded with substitution for tmim on GeCl2·dioxane. In contrast to (tmim)Si–, (tmim)Ge– is reluctant to coordinate to FeCl2 to form the germyl dichloro iron complex. The reaction of (tmim)EK (E = Si, Ge) with Fe2(CO)9 afforded the germyl and silyl iron tetracarbonyl, akin to their phosphine analogue (tmim)PFe(CO)4. The electron donating properties of (tmim)Ge– were shown to be in between those of (tmim)P and (tmim)Si–. The knowledge obtained from this work is anticipated to further aid in the controlled synthesis of N-substituted silicon compounds. In particular the substitution on the Si(II)Cl2 precursor potentially allows for the formation of structures that would otherwise be difficult to access. The low-coordinate silyl iron chloride complex warrants research into its use in C–C cross coupling catalysis. Overall, the findings in this thesis contribute to the understanding of heavier group-14 analogues of carbenes and carbanions and promise to be instrumental for further development of this promising class of ligands.