The strong-field tripodal phosphine donor, [PhB(CH2PiPr2)3]-, provides access to electronically and coordinatively unsaturated transition metal complexes

This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBP(iPr)(3)] ([PhBP(iPr)(3)] = [PhB(CH(2)P(i)Pr(2))(3)](-)), to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBP(iPr)(3)]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 FeCl(2) and CoX(2) (X = Cl, I) to produce the monomeric, 4-coordinate, high-spin derivatives [PhBP(iPr)(3)]FeCl (2) and [PhBP(iPr)(3)]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(mu-Cl)](2) (5) and [[PhBP(3)]Ru(mu-Cl)](2) (6) were prepared and characterized ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)). The chloride complexes 2 and 3 reacted with excess CO to afford the divalent, monocarbonyl adducts [PhBP(iPr)(3)]FeCl(CO) (7) and [PhBP(iPr)(3)]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBP(iPr)(3)]Co(I)(CO)(2) (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBP(iPr)(3)]RuCl(CO)(2) (10) and [PhBP(3)]RuCl(CO)(2) (11), respectively. Dimers 5 and 6 were broken up by reaction with trimethylphosphine to produce the mono-PMe(3) adducts [PhBP(iPr)(3)]RuCl(PMe(3)) (12) and [PhBP(3)]RuCl(PMe(3)) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH(2)P(O)(i)Pr(2))(2)(CH(2)P(i)Pr(2))]CoCl (14), while exposure of 3 to excess oxygen results in the 6-electron oxidation product [PhB(CH(2)P(O)(i)Pr(2))(3)]CoCl (15). Complexes 2 and 4 were characterized via cyclic voltammetry to compare their redox behavior to their [PhBP(3)] 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, [PhBP(3)]CoI. The difference in spin states observed for these two systems reflects the conformational rigidity of the [PhBP(iPr)(3)] ligand by comparison to [PhBP(3)], leaving the former less able to accommodate a JT-distorted electronic ground state.