Bond Forming Reactions Mediated by Low-Coordinate, First-Row Transition Metal Complexes

Chapter 1. The two-coordinate compound (IPr)Fe[N(SiMe3)DIPP] catalyzes the cyclotrimerization of alkynes to arenes. Treatment of the Fe complex with one equivalent of diphenylacetylene results in formation of a bimetallic bridging alkyne complex, along with dissociation of IPr from Fe. At elevated temperatures, the bridging alkyne complex undergoes oxidative coupling to form a bimetallacyclopentadiene complex. This reaction occurs through a formal one electron oxidation of each metal center. Each complex catalyzes the cyclotrimerization of diphenylacetylene. Kinetic studies exhibit first order dependence on the bimetallic complexes, providing further support for the presence of these species in the catalytic cycle. DFT calculations support the experimental mechanistic data and suggest that the catalytic cycle is completed by binding of an alkyne to the diene complex, followed by insertion to form a hexatriene species, which then undergoes ring closure to form an inverse sandwich complex. The arene product is then displaced by alkyne to close the catalytic cycle./nChapter 2. The two-coordinate compound Ni[N(SiMe3)DIPP]2 catalyzes hydrosilation of primary and secondary silanes onto olefins. Initial attempts to determine the kinetic profile of this system proved difficult due to the presence of competing side reactions, such as silane redistribution, C–D activation of benzene-d6, and olefin isomerization. The identification and mitigation of these side reactions ultimately allowed for full kinetic analysis. Additionally, evidence of a Ni hydride was found during catalytic hydrosilation conditions. Such a species, if catalytically active, may be the basis for elucidation of a novel mechanism for hydrosilation./nChapter 3. The synthesis and reactivity of the first open-shell two-coordinate silyl complex, (IPr)Ni–Si(SiMe3)3, is reported. Reaction with CO2 results in a novel κ2-silylcarboxylate insertion product that possesses a distorted geometry. Oxidations of the Ni center, with [Cp2Fe][B(C6F5)4] and Ph3CCl, result in migratory rearrangements of the silyl ligand, apparently via related silylene intermediates. The distinct oxidation products are interconverted by reactions that introduce or remove chloride ([Bu4N]Cl and LiB(C6F5)4, respectively)./nChapter 4. The synthesis and reactivity of a rare Ni(I) methyl complex, [(IPr)Ni(μ-CH3)]2, is reported. Reaction with CO2 results in the formation of a bridging Ni(II) carbonate species. Additionally, the methyl complex activates a variety of heteroatom-H bonds to form methane. Specifically, treatment with SiH4 results in Si-H activation to forma bridging silylene complex, [(IPr)NiH(μ-SiH2)]2. The methyl complex is a competent catalyst for the coupling of CH3MgCl with aryl halides./nChapter 5. Sequential treatment of Fe[N(SiMe3)DIPP]2 (2) with Et3NHCl, IAd, and KC8 resulted in the formation of two new complexes, (IAd)Fe[N(SiMe3)DIPP]2 (3) and {(IAd)Fe[N(SiMe3)DIPP]}2(μ-N2) (4), which likely result from N2 trapping of an intermediate species (IAd)Fe[N(SiMe3)DIPP] (A). Routes to independently synthesize complexes 3 and 4 were also developed. These results differ from those observed by using the IPr ligand; similar sequential treatment of 2 with Et3NHCl, IPr, and KC8 resulted exclusively in the formation of two-coordinate complex (IPr)Fe[N(SiMe3)DIPP] (1). DFT calculations show that the (IAd)---Fe[N(SiMe3)DIPP] interaction is weaker than the analogous (IPr)---Fe[N(SiMe3)DIPP] interaction by 11.6 kJ/mol. This difference appears to stem largely from greater steric repulsion and diminished dispersion stabilization in the IAd-containing transient species A as compared to 1.

Advisor: Tilley, T. Don