The strong-field tripodal phosphine donor, [PhB(CH2P iPr2)3]-, provides access to electronically and coordinatively unsaturated transition metal complexes

Article abstract of DOI:10.1021/ic0343096

This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBP^iPr₃] ([PhB^iPr₃] = [PhB(CH₂PⁱPr ₂)₃]^-), to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the TI(I) complex, [PhB^iPr ₃]TI (1), was synthesized and characterized in the solid-state by X-ray diffraction analysis. This precursor proves to be an effective transmetallating agent, as evidenced by its reaction with the divalent halides FeCl₂ and CoX₂ (X = Cl, I) to produce the monomeric, 4-coordinate, high-spin derivatives [PhBP ^iPr₃]FeCl (2) and [PhBP^iPr]CoX (X = Cl (3), I (4)) in good yield. Complexes 2-4 were each characterized by X-ray diffraction analysis and shown to be monomeric in the solid-state. For conformational and electronic comparison within a system exhibiting higher than 4-coordination, the 16-electron ruthenium complexes {[PhBP^iPr ₃]Ru(μ-Cl)}₂ (5) and {[PhBP ₃]Ru(μ-Cl)}₂ (6) were prepared and characterized ([PhBP₃] = [PhB(CH₂PPh₂)₃] ^-). The chloride complexes 2 and 3 reacted with excess CO to afford the divalent, monocarbonyl adducts [PhBP^iPr₃] ^-) FeCl(CO) (7) and [PhBP^iPr₃]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBP^iPr₃]CO^I(CO) ₂ (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBP^iPr₃]RUCl(CO) ₂ (10) and [PhBP₃] RuCl(CO)₂ (11), respectively. Dimers 5 and 6 were broken up by reaction with trimethylphosphine to produce the mono-PMe₃ adducts [PhBP^iPr ₃]RuCl(PMe₃) (12) and [PhBP₃]RuCl(PMe ₃) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH₂P(O)^IPr ₂)₂(CH₂PⁱPr₂)]CoCl (14), while exposure of 3 to excess oxygen results in the 6-electron oxidation product [PhB(CH₂P(O)ⁱPr₂)₃]CoCl (15). Complexes 2 and 4 were characterized via cyclic voltammetry to compare their redox behavior to their [PhBP₃] analogues. Complex 4 was also studied by SQUID magnetization and EPR spectroscopy to confirm its high-spin assignment, providing an interesting contrast to its previously described low-spin relative, [PhBP₃] CoI. The difference in spin states observed for these two systems reflects the conformational rigidity of the [PhBP^iPr3] ligand by comparison to [PhBP₃], leaving the former less able to accommodate a JT-distorted electronic ground state.