Ligand Design, Coordination Chemistry, and Mechanistic Studies of (Phosphino)Borates and their Platinum, Nickel, and Copper Complexes

Citation

Thomas, John Christopher (2004) Ligand Design, Coordination Chemistry, and Mechanistic Studies of (Phosphino)Borates and their Platinum, Nickel, and Copper Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/GCAQ-8D59. https://resolver.caltech.edu/CaltechETD:etd-06012004-115510

Abstract

Synthetic methods are presented for the preparation of various substituted bis(phosphino)borates. A relatively general protocol based on the delivery of a nucleophilic phosphine-containing carbanion to a borane electrophile has been developed. Preparative methods for the synthesis of substituted diarylchloroboranes from dimethyldiaryltin reagents provide the borane electrophiles. Methyldialkyl- or methyldiarylphosphines are selectively deprotonated at the phosphine-methyl using alkyl lithium bases to form the carbanion nucleophiles. The reaction of diverse phosphine-containing carbanions with diarylchloroboranes results in bis(phosphino)borates selectively substituted at the borate, at the phosphine, or at both positions. In addition to the generated lithium salts of the bis(phosphino)borates, cation-exchange protocols provide methods for preparing ammonium and thallium bis(phosphino)borate salts. Structural data for some of these derivatives are presented.

The electronic properties of transition metals coordinated by bis(phosphino)borates are explored through NMR and IR spectroscopies. The spectroscopic features of platinum(II) dimethyl and methyl carbonyl complexes are examined for trends based on the substitution pattern of the (phosphino)borate ligand. These trends indicate that phosphine substituents have a more significant impact than borate substituents on electronics of the metal center. Structural and spectroscopic comparisons of structurally similar platinum(II) dimethyl and methyl carbonyl complexes indicate that the anionic bis(phosphino)borate ligand renders platinum(II) more electron-rich than structurally similar neutral phosphine donors. Related spectroscopic studies of anionic and neutral molybdenum(0) tetracarbonyl complexes provide results analogous to those found when comparing neutral and cationic platinum(II) systems.

Comparative studies on the ligand exchange and benzene C-H activation chemistry of structurally similar platinum(II) complexes convey the similarities and differences between zwitterionic and cationic systems. Examination of THF ligand self-exchange by magnetization transfer shows a change in mechanism between the neutral and cationic species. Both bis(phosphino)borate-ligated and neutral bis(phosphine) platinum methyl solvento complexes undergo a benzene C-H activation to form the corresponding phenyl solvento complex; however, the rates of reaction and ultimate products differ. Extensive isotopic studies indicate that the zwitterionic system forms observable intermediates prior to benzene C-H activation, some of which are attributable to ligand metalation processes.

Structural and spectroscopic studies of a phenyl-substituted tris(phosphino)borate on platinum are presented. Alkyl- and hydride-containing platinum(II) and platinum(IV) species have been synthesized. The structural and spectroscopic features of these complexes are compared to related tris(pyrazolyl)borate systems on platinum.

Coordination and reaction chemistry of an isopropyl-substituted tris(phosphino)borate on nickel are discussed. Complexes in the Ni(II), Ni(I), and Ni(0) oxidation states have been prepared. This system is compared through structural, spectroscopic, and electrochemical methods to related phenyl-substituted tris(phosphino)borate chemistry on nickel. Reactivity studies aimed at preparing Ni(III) and Ni(IV) complexes containing metal-ligand multiple bonds through group-transfer reactions are presented. Theoretical studies using density functional methods are used to probe several target species containing multiply-bonded ligands.

The coordination chemistry of copper(I) is explored using bis(phosphino)borates. Both aryl- and alkyl-substituted bis(phosphino)borates provide access to copper(I) complexes. A tert-butyl-substituted bis(phosphino)borate is particularly useful for preparing a family of three-coordinate compounds. The spectroscopic and structural features of these complexes are compared with similar, previously described examples.

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