Coordination Chemistry from Trigonally Coordinated Iron Platforms: Chemistry Relevant to Dinitrogen Reduction

Citation

Betley, Theodore Alexander (2005) Coordination Chemistry from Trigonally Coordinated Iron Platforms: Chemistry Relevant to Dinitrogen Reduction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PY23-ME66. https://resolver.caltech.edu/CaltechETD:etd-05272005-103515

Abstract

The synthesis for a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBP^iPr₃] ([PhBP^iPr₃] = [PhB(CH₂PⁱPr₂)₃]^-), is reported to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBP^iPr₃]Tl, was synthesized, characterized, and used to install the [PhBP^iPr₃] ligand onto complexes of Fe, Co, and Ru. The spectroscopic, electrochemical, magnetic, and structural features of these complexes are compared with similar examples.

Trigonally coordinated "[PhBP^iPr₃]M" platforms (M = Fe, Co) support both pi-acidic (N₂) and pi-basic (NR^2-) ligands at a fourth binding site. Methylation of monomeric [M⁰(N₂)^-] species successfully derivatizes the beta-N atom of the N₂ ligand and affords the diazenido product [M^II(N₂Me)]. Addition of RN₃ to M^I(N₂)M^I results in oxidative nitrene transfer to generate [PhBP^iPr₃]M≡NR with concomitant N₂ release.

A tetrahedrally coordinated L₃Fe-N_x platform that accommodates both terminal nitride (L₃Fe^IV≡N) and dinitrogen (L₃Fe^I-N₂-Fe^IL₃) functionalities is described. The diamagnetic L₃Fe^IV≡N species featured has been characterized in solution under ambient conditions by multinuclear NMR (¹H, ³¹P, and ¹⁵N) and infrared spectroscopy. The terminal nitride complex oxidatively couples to generate the previously reported L₃Fe^I-N₂-Fe^IL₃ species.

The [PhBP^iPr₃] ligand can support a single iron or cobalt center in a pseudo-tetrahedral environment in which dinitrogen is bound in the fourth coordination site. Zero-valent metal-dinitrogen complexes have the general formula, [[PhBP^iPr₃]M(mu-N₂)]₂[Mg²⁺], while bridging structures can also be obtained as neutral [M^I]—N₂—[M^I] or as anionic [(M)₂(N₂)]^- species. The nature of the structural distortions observed in both [M(mu-N₂)]₂[Mg²⁺] and [Mⁿ]—N₂—[Mⁿ] complexes are described. Magnetic characterization of the neutral and mixed-valence dimeric complexes reveal the complexes remain ferromagnetically coupled over all temperatures investigated.

The coordination chemistry of group VIII metals featuring the bis(8-quinolinyl)amine (HBQA) ligand is presented. The electrochemical behavior of Fe, Ru, and Os complexes bearing the BQA ligand is reported and compared to related ligand platforms. Halide and phosphine ligand exchange reactions are examined from complexes of the type (BQA)MX(PR₃)₂ (M = Ru, Os). Carbonyl and dinitrogen complexes of Ru and Os are prepared from halide abstraction from divalent Ru and Os precursors. The spectroscopic and structural features of these complexes are compared with similar examples.

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