Isomerism in rhodium(i) N,S-donor heteroscorpionates: ring substituent and ancillary ligand effects

Abstract

The heteroscorpionate ligands [HB(taz)₂(pz^R)]⁻ (pz^R = pz, pz^Me2, pz^Ph) and [HB(taz)(pz)₂]⁻, synthesised from the appropriate potassium hydrotris(pyrazolyl)borate salt and 4-ethyl-3-methyl-5-thioxo-1,2,4-triazole (Htaz), react with [{Rh(cod)(μ-Cl)}₂] to give [Rh(cod)Tx] {Tx = HB(taz)₂(pz), HB(taz)₂(pz^Me2), HB(taz)₂(pz^Ph), HB(taz)(pz)₂}; the heteroscorpionate rhodaboratrane [Rh{B(taz)₂(pz^Me2)}{HB(taz)₂(pz^Me2)}] is the only isolable product from the reaction of [{Rh(nbd)(μ-Cl)}₂] with K[HB(taz)₂(pz^Me2)]. Carbonylation of the cod complexes gave a mixture of [Rh(CO)₂Tx] and [(RhTx)₂(μ-CO)₃] which reacts with PR₃ to give [Rh(CO)(PR₃)Tx] (R = Cy, NMe₂, Ph, OPh). In the solid state the complexes are square planar with the particular structure dependent on the steric and/or electronic properties of the scorpionate and ancillary ligands. The complex [Rh(cod){HB(taz)(pz)₂}] has the heteroscorpionate κ²[N₂]-coordinated to rhodium with the B–H bond directed away from the rhodium square plane while [Rh(cod){HB(taz)₂(pz^Me2)}] is κ²[SN]-coordinated, with the B–H bond directed towards the metal. The complexes [Rh(CO)(PPh₃){HB(taz)₂(pz)}] and [Rh(CO)(PPh₃){HB(taz)₂(pz^Me2)}] are also κ²[SN]-coordinated but with the pyrazolyl ring cis to PPh₃; in the former the B–H bond is directed towards rhodium while in the latter the ring is pseudo-parallel to the rhodium square plane, as also found for [Rh(CO)₂{HB(taz)₂(pz^Me2)}]. The analogues [Rh(CO)(PR₃){HB(taz)₂(pz^Me2)}] (R = Cy, NMe₂) have the phosphines trans to the pyrazolyl ring. Uniquely, [Rh(CO)(PPh₃){HB(taz)₂(pz^Ph)}] is κ²[S₂]-coordinated. A qualitative mechanism is given for the rapid ring-exchange, and hence isomerisation, observed in solution.