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

The reaction of potassium hydrotris(methimazolyl) borate {KTm = HB(mt)(3)} with [{Rh(cod)(mu-Cl)}(2)] gave [Rh(cod)Tm] while the complexes [Rh(CO)(PR3)Tm] (R = Ph or NMe2) and [Rh{P(OPh)(3)}(2)Tm] were isolated from light-sensitive [Rh(CO)(2)Tm], prepared in situ from KTm and [{Rh(CO)(2)(m-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 kappa(3)-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(eta-C5H5)(2)][PF6] in the presence of NHPr2i 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].