Fluxional rhodium scorpionate complexes of the hydrotris(methimazolyl)borate (Tm) ligand and their static boratrane derivatives

Abstract

The reaction of potassium hydrotris(methimazolyl)borate {KTm = HB(mt)₃} with [{Rh(cod)(μ-Cl)}₂] gave [Rh(cod)Tm] while the complexes [Rh(CO)(PR₃)Tm] (R = Ph or NMe₂) and [Rh{P(OPh)₃}₂Tm] were isolated from light-sensitive [Rh(CO)₂Tm], prepared in situ from KTm and [{Rh(CO)₂(μ-Cl)}₂], and PR₃ or P(OPh)₃ under CO. The complexes [Rh(cod)Tm] and [Rh(CO)(PR₃)Tm] (R = Ph or NMe₂) adopt κ³-S₂H structures in the solid state but in all cases rapid dynamic exchange processes render the three mt rings equivalent in solution. Oxidation of [Rh(CO)(PPh₃)Tm] with [Fe(η-C₅H₅)₂][PF₆] in the presence of NHPrⁱ₂ gave a mixture containing two monocationic rhodaboratranes. One is assigned as [Rh(CO)(PPh₃){B(mt)₃}][PF₆] on the basis of IR and NMR spectroscopy, with boron trans to the phosphine ligand. The second, structurally characterised as [Rh(PPh₃){B(mt)₃}][PF₆], has boron trans to an empty coordination site, vacated by CO. Similar oxidation of [Rh(cod)Tm] gave small quantities of the boron-fluorinated bis(scorpionate) [Rh{FB(mt)₃}₂][PF₆].