Copper and Iron Complexes with Unusual Coordination Geometries Enforced by Phosphine Chelates

Citation

Mankad, Neal P. (2010) Copper and Iron Complexes with Unusual Coordination Geometries Enforced by Phosphine Chelates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6F4J-WE60. https://resolver.caltech.edu/CaltechTHESIS:03112010-172248190

Abstract

Chelating phosphine ligands were used to enforce targeted coordination geometries onto complexes of iron and copper, thereby imparting molecular properties distinct relative to species studied previously in other geometries. The bulky bis(phosphino)borate ligand [Ph₂B(CH₂P^tBu₂)₂]⁻ was used to provide trigonal planar complexes of Cu. This structural motif provided a rare opportunity for a single framework to stabilize Cu complexes in three discrete oxidation levels and allowed for the study of unique ligands including diazoalkanes, a diphenylcarbene, diarylamides, and a diarylaminyl radical. In the latter case, physical measurements (multiedge XAS and multifrequency EPR spectroscopy) and theoretical methods (DFT) were used to quantitate the delocalization of spin density between the Cu center and the NAr₂ unit, providing a comprehensive electronic structure picture for L₂CuER₂ (E = C or N) complexes in this system. In separate studies, trigonal bipyramidal Fe complexes were generated using the bulky, anionic tris(phosphino)silyl ligands [(2-R₂PC₆H₄)₃Si]⁻ (R = Ph or iPr). Low-valent Fe species in this system were found to activate dinitrogen, providing labile N₂ ligands trans to the silyl donor, including the first instance of a terminally bound N₂ ligated to a paramagnetic Fe center. Subsequent reactions involving these Fe^I-N₂ species and organoazides provided entry to unusual catalytic N-N coupling reactions. These reactions were found to involve reactive FeNAr intermediates, destabilized by virtue of the trigonal bipyramidal coordination geometry, which subsequently coupled bimolecularly in the N-N bond-forming step. The effects of perturbing previously studied C₃-symmetric pseudotetrahedral iron complexes to their trigonal bipyramidal analogues proved key to uncovering the chemistry of interest.

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