Ligand properties of boryl ligands in bis-boryl rhodium(iii) complexes: a case study

Abstract

The oxidative addition of five diborane(4) derivatives, symmetrical and unsymmetrical, to [Rh(PMe₃)₃Cl] was studied. Only for the more electron poor diboron derivatives, B₂cat₂, B₂pin₂ and catB–Bpin the resulting octahedral bis-boryl complexes [(PMe₃)₃Rh(boryl)₂Cl] were obtained, while for the more electron rich congeners only the equilibrium oxidative addition (catB–Bdmab) or no significant reaction (pinB–Bdmab) was observed (pin = (OCMe₂)₂, cat = 1,2-O₂C₆H₄, dmab = 1,2-(NMe)₂C₆H₄). By abstraction of the chlorido ligand with NaBArF (BArF = tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borat) in the presence of a neutral ligand (L = PMe₃, MeCN, MeNC) the corresponding cationic octahedral complexes [(PMe₃)₃Rh(boryl)₂L]⁺ were obtained. All isolated complexes were fully characterised including single crystal X-ray diffraction and heteronuclear, temperature dependent NMR spectroscopy. Whilst the complexes [(PMe₃)₃Rh(boryl)₂Cl] and [(PMe₃)₃Rh(boryl)₂L]⁺ show many similarities, their detailed structural and spectroscopic properties depend crucially on the properties of both boryl ligands.