Studies of Carbon–Element Bond Forming Reactions Mediated by Complexes Featuring Unsaturated Group 8 – Group 14 Interactions

Chapter 1. Fundamental aspects of transition metal–Si chemistry are discussed, particularly in the context of silylene complexes. A discussion of the basic aspects of Si chemistry, namely the much lower electronegativity of Si vs. C, leads to the conclusion that particularly for late transition metal–Si bonds the bond may be polarized toward the metal, leading to a partial positive charge at Si in such molecules. This, along with the inherent weakness of π–bonding to Si, explains the high degree of electrophilicity often observed for silylene complexes. Electrophilic Si centers bound to transition metals have a tendency to interact with other, negatively charged ligands (in particular hydrides), forming nonclassical, delocalized bonding motifs. Because of this, Si—H bond activations often define a continuum, in contrast to the well–defined bond activations observed for C–based systems./nChapter 2. This Chapter details reactions of the iron allyl complex Cp∗(iPr2MeP)Fe(η3−C3H5) (2.1) with sterically demanding silanes. These reactions lead to stoichiometric hydrosilation of the allyl ligand, and dehydrocoupling reactions between the silane and the allyl group. Furthermore, this system has allowed access to a Si—H oxidative additionreductive elimination equilibrium involving Cp∗(iPr2MeP)FeH2(SiH2DMP) (2.5) and Cp∗(iPr2MeP)FeH(N2) (2.6), which was independently synthesized./nChapter 3. The iron mesityl dimer [FeMes2]2 has provided access to half–sandwich iron complexes using two strategies involving a formal protonolysis of one Mes ligand. In the first strategy, initial, in situ formation of a monometallic “(L)FeMes2” is proceeded by a reaction with Cp∗H to lose mesitylene and form Cp∗(L)FeMes (L = PiPr2Me, 3.2a; PPh3, 3.2b; dppe, 3.2c) complexes. In the second strategy, [FeMes2]2 is used to deprotonate IiPrHCl and form (IiPr)FeMesCl which then reacts with Cp∗K. These mesityl complexes are readily derivatized by E—H reagents (E = H, Cl, Si) to introduce the donor atom, E./nChapter 4. Two new base-free hydrosilylene complexes of iron were synthesized using the novel starting material Cp∗(iPr2MeP)FeMes (3.2a). These Cp∗(iPr2MeP)Fe(H)SiHR (R = DMP, 4.5; R = Trip, 4.4) complexes are in equilibrium with the corresponding iron silyl complexes, Cp∗(iPr2MeP)FeSiH2R, which for R = Trip can be trapped by N2 and characterized as Cp∗(iPr2MeP)Fe(N2)SiH2Trip (4.3). Unlike the Ru analogues, the Fe silylene complex with R = DMP is observed to undergo an intramolecular C—H activation involving formal addition of a benzylic C— bond across the Fe—Si bond. This increased activity for bond activations is also observed for reactions with hydrogen, where Fe reacts faster than a Ru analog to form the hydrogenation product, Cp∗(iPr2MeP)H2FeSiH2DMP (2.5)./nChapter 5. Cationic iron complexes [Cp∗(iPr2MeP)FeH2SiHR]+, generated and characterized in solution, are efficient catalysts for the hydrosilation of terminal alkenes and internal alkynes by primary silanes or SiH4 at low catalyst loading (0.1 mol %) and ambient temperature to yield only the corresponding secondary silane product. Mechanistic investigations indicate a mechanism similar to that of the homologous Ru–silylene system, with a lower–energy dissociative silane exchange (product release) accounting for higher rates of reaction for Fe relative to Ru./nChapter 6. The dihydridoruthenate, {[(solv)Na][Cp∗(iPr2MeP)RuH2]}2 (6.2, solv = THF, Et2O), was synthesized from Cp∗(iPr2MeP)RuCl (6.1) and sodium triethylborohydride. Compound 6.2 was used to generate Cp∗(iPr2MeP)RuH equivalents by salt metathesis with 6.1, which resonance Raman spectroscopy indicates is a mixture of the terminal dinitrogen complex, Cp∗(iPr2MeP)RuH(N2) (6.4), and diastereomers of the bridging dinitrogen complex, [Cp∗(iPr2MeP)RuH]2(µ−N2) (6.5 and 6.6). Compound 6.2 also reacted with the late transition metal chloride complexes [(COD)IrCl]2 and (IPr)CuCl to form novel hydride– bridged heterobimetallic complexes Cp∗(iPr2MeP)Ru(µ–H)2Ir(COD) (6.7) and Cp∗(iPr2MeP)Ru(µ–H)2CuIPr(6.8) which feature weakened Ru—H interactions relative to 6.2./nChapter 7. The hydridoruthenate {[(solv)Na][Cp∗(iPr2MeP)RuH2]}2 (6.2; solv = THF, Et2O) has provided access to Ru metallostannylene Cp∗(iPr2MeP)RuH2SnDMP (DMP = 2,6-dimesitylphenyl) (7.1) and metalloplumbylene Cp∗(iPr2MeP)RuH2PbArTrip2 (ArTrip2 = 2,6-bis(2,4,6-triisopropylphenyl)phenyl) (7.2) compounds by salt metathesis reactions with the corresponding [ArEX]2 precursors. The Sn complex 7.1 reacted cleanly with MeI as a nucleophile to generate the addition product Cp∗(iPr2MeP)RuH2SnI(Me)DMP (7.3) while a complex mixture was observed for Pb. A Ru monohydride synthon generated from 6.2 also provided access to the chlorostannylene and bromoplumbylene complexes Cp∗(iPr2MeP)RuH(SnClDMP) (7.4) and Cp∗(iPr2MeP)RuH(PbBrArTrip2) (7.5), respectively. These compounds have distorted trigonal planar geometries at Sn and Pb, with the Pb geometry very nearly T–shaped. The electronic structures of these molecules were investigated using density functional theory.

Advisor: Tilley, T. Don