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Low-Valent Cobalt Isocyanides : Analogues to Classical Carbonyls

  • Author(s): Carpenter, Alex Edward
  • et al.
Abstract

A synthetic and structural investigation of cobalt complexes stabilized by m-terphenyl isocyanides was conducted. These species were studied as isolable mimics to unsaturated cobalt carbonyls and as platforms to access coordination complexes with unusual dn/valence combinations. Through these studies, a critical assessment of isocyanide for carbonyl substitution is presented. This assessment includes a comparative study on the electronic influence of isocyanides (C[equivalence]N--R), as a function of R-group identity, in Cr(CNR)(CO)₅ complexes. Structural mimics to all three conformational isomers of Co₂(CO)₈ were prepared using the m-terphenyl isocyanides CNArMes² (ArMes² = 2,6-(2,4,6-Me₃C₆H₂)C₆H₃) and CNArDipp² (ArDipp² = 2,6-(2,6-i-Pr₂C₆H₃)C₆H₃). Solid-state X-ray diffraction data showed that 1,1-Co₂(CO)₆ (CNArMes²)₂ adopts a "D₂d-type" conformation, analogous to the elusive D₂d isomer of Co₂(CO)₈. The cobalt hydride HCo(CNArMes²)₄ was synthesized and structurally characterized. As a potential analogue to HCo(CO)₄, a broad survey of its reactivity was undertaken. In the course of this study, HCo(CNArMes²)₄ was found to mediate 1,1-hydrogenation and 1,1-hydrosilylation of CNArMes². Complementing this work, a complete series of mixed carbonyl-isocyanide cobalt metallates, [Co(CO)₄-n(CNArMes²)n]-, and hydrides, HCo(CO)₄-n(CNArMes²)n, were prepared. The synthesis and solution-phase properties of a zwitterionic cobalt metallate is presented. 2D EXSY NMR studies and isotopic labeling experiments demonstrate that [Eta]²- PPNCo(CNArMes²)₃, functions as a masked source of [Co(CNArMes²)₃]1-, a 16 valence electron isocyano metallate. Electrophillic functionalization of [Eta]²- PPNCo(CNArMes²)₃, with TMSCl, provided access to Co(SiMe₃)(CNArMes²)₃. This complex was structurally characterized by X-ray diffraction. The previously reported ability of Co(SiMe₃)(CNArMes²)₃ to form a [alpha] -complex with n-hexane at low-temperature in the solid state is contextualized with variable temperature IR and NMR studies. Reduction of Co(SiMe₃)(CNArMes²)₃ generated the highly reduced isocyano metallate K₂[Co(SiMe₃)(CNArMes²)₃]. This dissertation concludes with the synthesis and characterization of the first examples of terminal transition metal antimony trifluoride (SbF₃) and antimony difluoride (SbF₂) complexes

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