A novel route to rhodaboratranes [Rh(CO)(PR₃){B(taz)₃}]⁺ via the redox activation of scorpionate complexes [RhLL'Tt]

The reaction of a mixture of the sodium salts of dihydrobis(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate, NaBt, and hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate, NaTt, with [{Rh(cod)(μ-Cl)}2] gave [Rh(cod)Bt] and [Rh(cod)Tt], which separately react with CO gas to give the unstable dicarbonyl [Rh(CO)2Bt] and an equilibrium mixture of two isomers of [Rh(CO)2Tt] and [(RhTt)2(μ-CO)3], respectively. Tertiary phosphorus donor ligands react with the mixture of [Rh(CO)2Tt] and [(RhTt)2(μ-CO)3] to give [Rh(CO)(PR3)Tt] (R = Cy, NMe2, Ph or OPh) and [Rh{P(OPh)3}2Tt] in which rhodium is bound to two sulfur atoms of the scorpionate ligand; the B–H bond is directed towards the metal to give an agostic-like B–Hcdots, three dots, centeredRh interaction. Dinuclear [(RhTt)2(μ-CO)3] has κ3[S3]-bound Tt ligands with a rhodium–rhodium bond bridged by three carbonyls. In solution the mononuclear Tt complexes undergo rapid dynamic interchange of the three thioxotriazolyl rings, probably viaκ3[S3]-coordinated intermediates. The monocarbonyls [Rh(CO)(PR3)Tt] (R = Cy, NMe2 or Ph) react with two equivalents of [Fe(η-C5H5)2][PF6] in the presence of triethylamine to give the monocationic rhodaboratranes [Rh(CO)(PR3){B(taz)3}]+, with boron NMR spectroscopy providing evidence for the boron–rhodium bond. In the solid state, rhodium is bound to the three sulfur atoms and the boron of the B(taz)3 fragment, forming a tricyclo[3.3.3.0] cage. The phosphine is trans to the Rh–B bond, the long Rh–P bond indicating a pronounced trans influence for the coordinated boron. The cation [Rh(CO)(PPh3){B(taz)3}]+ reacts with [NBun4]I to give [Rh(PPh3)I{B(taz)3}], in which the halide is trans to the Rh–B bond, and a second species, possibly [Rh(CO)I{B(taz)3}]. The dirhodaboratrane [Rh2(PCy3){B(taz)3}2][PF6]2, a minor byproduct in the synthesis of [Rh(CO)(PCy3){B(taz)3}][PF6], has a distorted square pyramidal rhodium atom with a vacant site trans to the Rh–B bond. The second metal has four coordination sites filled by the sulfur and boron atoms of a second B(taz)3 unit, the remaining octahedral sites occupied by two of the sulfur atoms of the first B(taz)3 unit which therefore bridges the two rhodium atoms.