Ortho-aryl substituted DPEphos ligands: rhodium complexes featuring C-H anagostic interactions and B-H agostic bonds

The synthesis of new Schrock-Osborn Rh(i) pre-catalysts with ortho-substituted DPEphos ligands, [Rh(DPEphos-R)(NBD)][BAr4F] [R = Me, OMe, Pr-i; Ar-F = 3,5-(CF3)(2)C6H3], is described. Along with the previously reported R = H variant, variable temperature H-1 NMR spectroscopic and single-crystal X-ray diffraction studies show that these all have axial (C-H)MIDLINE HORIZONTAL ELLIPSISRh anagostic interactions relative to the d(8) 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-HMIDLINE HORIZONTAL ELLIPSISRh 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-Pr-i)(NBD)][BAr4F] addition of H-2 results in a Rh(iii) Pr-i-C-H activated product, [Rh(kappa(3),sigma-P,O,P-DPEphos-Pr-i ')(H)][BAr4F]. This undergoes H/D exchange with D-2 at the Pr-i groups, reacts with CO or NBD to return Rh(i) products, and reaction with H3B center dot NMe3/tert-butylethene results in a dehydrogenative borylation to form a complex that shows both a non-classical B-HMIDLINE HORIZONTAL ELLIPSISRh 3c-2e agostic bond and a C-HMIDLINE HORIZONTAL ELLIPSISRh anagostic interaction at the same metal centre.

Automated summary

The synthesis of new Schrock-Osborn Rh(i) pre-catalysts with ortho-substituted DPEphos ligands is described, and their interactions with the metal center are investigated. The experiments show axial C-HRh interactions in these complexes, leading to observed downfield chemical shifts.