Development of Transition-Metal Silicide Complexes
- Handford, Rex Charles Chao
- Advisor(s): Tilley, T. Don
Abstract
Chapter 1. Tetrelide compounds (i.e. binary materials of a metal (M) and a group 14 element E); MxEy) are of broad technological and industrial importance. Attention in the field has largely been devoted to the study of bulk tetrelide materials, particularly relating to catalytic transformations involving tetrelides, as well as their applications in nanomaterials and electronics applications. On the molecular scale, tetrelide compounds span a broad range of structures and bonding situations, ranging from high-nuclearity clusters to exotic molecules containing two-coordinate μ-E centers. As these molecules are readily amenable to detailed structural and spectroscopic interrogation, they provide insights toward the reactive units found in bulk tetrelide phases. This chapter summarizes historical and contemporary achievements in the development of molecular tetrelides, highlighting trends in the progression of the field towards rationally controlled, direct syntheses of tetrelides containing low-coordinate central μ-E atoms.
Chapter 2. A series of complexes, [BP3R]MX ([BP3R] = κ3-PhB(CH2PR2)3–, R = Ph, iPr; M = Ni, Co, Fe; X = halide), were explored as platforms for generation of first-row metal silylene complexes. Direct silylation of [BP3Ph]NiCl or [BP3iPr]CoCl with (THF)2LiSiHMes2 resulted in formation of the silylene complexes [BP3Ph]Ni(μ-H)(SiMes2) (2.1) and [BP3iPr]Co(μ-H)(SiMes2) (2.2), respectively. In contrast, treatment of [BP3iPr]FeBr with (THF)2LiSiHMes2 produced the iron-alkyl [BP3iPr]Fe(CH2-2-(SiH2Mes)-3,5-Me2C6H2) (2.3), a constitutional isomer of the expected silyl or silylene complex. Preparation of the nickel benzyl complex [BP3Ph]Ni(η2-Bn) (2.4) allowed for exploration of addition-elimination chemistry for access to silylene complexes from simple primary and secondary silanes. Heating toluene solutions of [BP3Ph]Ni(η2-Bn) in the presence of CySiH3 resulted in the formation of a dimeric μ-silylene complex [Ni(μ-BP2Ph)(μ-SiHCy)]2 (2.5). In the presence of 4-dimethylaminopyridine (DMAP), these conditions led to exclusive formation of the base-stabilized silylene complex [BP3Ph]Ni(μ-H)[SiHCy(DMAP)] (2.5).
Chapter 3. The [BP3iPr]CoI synthon Na(THF)6([BP3iPr]CoI) (3.1) reacts with PhSiH3 or SiH4 to form unusual [BP2iPr](SiH2R)(H)2Co=Si=Co(H)2[BP3iPr] species (R = Ph, 3.2a; R = H,3.2b; [BP2iPr] = κ2-PhB(CH2PiPr2)2) that result from activation of all Si–H and Si–C bonds in the starting silanes. Solution-spectroscopic data (multinuclear NMR, IR) for 3.2a,b, and the solid-state structure of 3.2a, indicate substantial Co=Si=Co multiple bonding and minimal interaction of the core silicon atom with nearby hydride ligands. In the presence of DMAP, 3.1 reacts with PhSiH3 to give [BP3iPr]Co(H)2SiHPh(DMAP) (3.3). Complexes 3.2a,b eliminate RSiH3 upon thermolysis in the presence of DMAP to generate [BP2iPr]Co(NC5H3NMe2)=Si=Co(H)2[BP3iPr] (3.4).
Chapter 4. The coordination chemistry of a bidentate ambiphilic ligand, PhB(CH2PtBu2)2 ([BP2tBu]) is established for a series of Co, Ni, and Cu compounds. The chloride starting materials [BP2tBu]MCl2 (M = Co (4.1Co), Ni (4.1Ni)) and [BP2tBu]CuCl (4.2Cu) are readily accessed in high yields by treatment of the corresponding metal chloride salts with [BP2tBu] in THF solution. Reduction of4.1Ni to [BP2tBu]NiCl (4.2Ni) is effected by treatment with KC8. In contrast, the corresponding CoI species was not isolable under similar reaction conditions. However, monovalent mesityl complexes of the form [BP2tBu]M(Mes) (M = Co (3Co), Ni (4.3Ni), Cu (4.3Cu)) were prepared by treatment of 4.1Co with one equiv of Mes2Mg(THF)2 or by reaction of 4.2M (M = Ni, Cu) with 0.5 equiv of Mes2Mg(THF)2. Modification of the borane moiety in 4.2Ni was explored by treatment of this complex with DMAP to produce [PhB(DMAP)(CH2PtBu2)2]NiCl (4.4), which displays distinct features in its UV-visible spectrum compared to 4.2Ni. The solid-state molecular structures of most complexes have been determined by single-crystal X-ray diffraction analysis.
Chapter 5. The synthesis of bimetallic molecular silicide complexes is reported, based on use of multiple Si—H bond activations in SiH4 at the metal centers of 14-electron LCoI fragments (L = Tp", HB(3,5-diisopropylpyrazolyl)3–; [BP2tBuPz], PhB(CH2PtBu2)2(pyrazolyl)). Upon exposure of (Tp"Co)2(μ-N2) (5.1) to SiH4, a mixture of (Tp"Co)2(μ-H) (5.2) and (Tp"Co)2(μ-H)2 (5.3) was formed and no evidence for Si—H oxidative addition products was observed. In contrast, [BP2tBuPz]-supported Co complexes led to Si—H oxidative additions with generation of silylene and silicide complexes as products. Notably, the reaction of ([BP2tBuPz]Co)2(μ-N2) (5.5) with SiH4 gave the dicobalt silicide complex [BP2tBuPz](H)2Co=Si=Co(H)2[BP2tBuPz] (5.8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP2tBuPz] ligand on Co—Si bonding in 5.7 and 5.8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP, 5.8 converted to the corresponding [BP2tBuPz]Co(L)x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH4, despite the lack of significant Si—H interactions in the starting complex. With heating to 60 °C, 5.8 underwent reaction with MeCl to produce small quantities of MexSiH4–x (x = 1–3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.
Chapter 6. A terminal silylene synthon, [BP3iPr](H)2CoSiH2(DMAP) (6.1) was accessed in high yields by double Si—H bond activation of SiH4 by [BP3iPr]Co(DMAP). Complex 6.1 is the first instance of a LM=SiH2(L')0,1 species, and was demonstrated by variable-temperature 1H NMR spectroscopic studies in the presence of added DMAP (one equiv) to exchange coordinated DMAP via an associative mechanism. Critically, the DMAP ligand of 6.1 is labile, and the Si—H bonds of the SiH2 moiety are prone to further activation to generate bimetallic {Co=Si=M'} (M' = Co, Fe) silicides. Thus, treatment of 6.1 with 0.5 equiv of (LCo)2(μ-N2) partners resulted in spontaneous formation of [BP3iPr](H)2Co=Si=Co(H)2L (L = [BP2tBuPz], (6.3); Tp", (6.4)) with concomitant release of DMAP. The symmetrical silicide [BP3iPr](H)2Co=Si=Co(H)2[BP3iPr] (6.5) was prepared by treatment of a mixture of 6.1 and [BP3iPr]Co(DMAP) with two equiv of Ph3B, extending the silicide-generating reactivity of 6.1 to coordinatively inert reaction partners. A heterobimetallic silicide, [BP3iPr](H)2Co=Si=Fe(H)2[SiP3iPr] (6.7; [SiP3iPr] = PhSi(CH2PiPr2)3), was accessed via in-situ KC8 reduction of [SiP3iPr]FeCl and subsequent addition of 6.1 and Ph3B. The solid-state molecular structures of most of the silicides, and their solution 29Si{1H} DEPT NMR spectra, have been obtained. These transformations demonstrate a fundamentally novel form of reactivity for a transition-metal silylene complex, and establishes the viability of LM=SiH2(L')0,1 species for accessing elaborate silicide structures.