The Strong-Field Tripodal Phosphine Donor, [PhB(CH2PiPr2)3]-, Provides Access to Electronically and Coordinatively Unsaturated Transition Metal Complexes

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Abstract
This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBPiPr3] ([PhBPiPr3] = [PhB(CH2PiPr2)3]-), to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBPiPr3]Tl (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 [PhBPiPr3]FeCl (2) and [PhBPiPr3]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 {[PhBPiPr3]Ru(μ-Cl)}2 (5) and {[PhBP3]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 [PhBPiPr3]FeCl(CO) (7) and [PhBPiPr3]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBPiPr3]CoI(CO)2 (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBPiPr3]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 [PhBPiPr3]RuCl(PMe3) (12) and [PhBP3]RuCl(PMe3) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH2P(O)iPr2)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 [PhBPiPr3] ligand by comparison to [PhBP3], leaving the former less able to accommodate a JT-distorted electronic ground state.

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