<i>Ortho</i>-aryl substituted DPEphos ligands: rhodium complexes featuring C–H anagostic interactions and B–H agostic bonds

Abstract

The synthesis of new Schrock-Osborn Rh(i) pre-catalysts with <i>ortho</i>-substituted DPEphos ligands, [Rh(DPEphos-R)(NBD)][BAr^F ₄] [R = Me, OMe, ⁱPr; Ar^F = 3,5-(CF₃)₂C₆H₃], is described. Along with the previously reported R = H variant, variable temperature ¹H NMR spectroscopic and single-crystal X-ray diffraction studies show that these all have axial (C-H)⋯Rh anagostic interactions relative to the d⁸ pseudo square planar metal centres, that also result in corresponding downfield chemical shifts. Analysis by NBO, QTAIM and NCI methods shows these to be only very weak C-H⋯Rh bonding interactions, the magnitudes of which do not correlate with the observed chemical shifts. Instead, as informed by Scherer's approach, it is the topological positioning of the C-H bond with regard to the metal centre that is important. For [Rh(DPEphos-ⁱPr)(NBD)][BAr^F ₄] addition of H₂ results in a Rh(iii) ⁱPr-C-H activated product, [Rh(κ³,σ-P,O,P-DPEphos-ⁱPr')(H)][BAr^F ₄]. This undergoes H/D exchange with D₂ at the ⁱPr groups, reacts with CO or NBD to return Rh(i) products, and reaction with H₃B·NMe₃/<i>tert</i>-butylethene results in a dehydrogenative borylation to form a complex that shows both a non-classical B-H⋯Rh 3c-2e agostic bond and a C-H⋯Rh anagostic interaction at the same metal centre.