Rhodium(I) silyl complexes for C-F bond activation reactions of aromatic compounds: Experimental and computational studies

The rhodium(I) silyl complexes [Rh{Si(OEt)₃}(PEt ₃)₃] (2a) and [Rh{Si(OMe)₃}(PEt ₃)₃] (2b) were synthesized by treatment of [Rh(CH ₃)(PEt₃)₃] (1) with the corresponding silanes HSi(OEt)₃ and HSi(OMe)₃ at low temperature. The intermediate oxidative addition products fac-[Rh(H)(CH₃){Si(OR) ₃}(PEt₃)₃] (R = Et, 6a; R = Me, 6b) were observed by low-temperature NMR spectroscopy. A reaction of 2a with CO afforded trans-[Rh(CO){Si(OEt)₃}(PEt₃)₂] (7) by the replacement of the phosphine ligand in the position trans to the silyl group. Treatment of 2a,b with pentafluoropyridine led to C-F activation reactions at the 2-position, yielding [Rh(2-C₅F₄N)(PEt ₃)₃] (11). The silyl complexes [Rh{Si(OR) ₃}(PEt₃)₃] (2a,b) gave with 2,3,5,6-tetrafluoropyridine the C-F activation product [Rh(2-C₅F ₃HN)(PEt₃)₃] (10), whereas complex 7 reacted by C-H activation to furnish trans-[Rh(CO)(4-C₅F₄N)(PEt ₃)₂] (12). The C-F activation of pentafluoropyridine at 2b was studied with density functional theory calculations using a [Rh{Si(OMe)₃}(PMe₃)₃] model complex (2′). The calculations indicate that a silyl-assisted C-F activation mechanism, analogous to related ligand-assisted processes at metal-phosphine and metal-boryl bonds, is more accessible than a C-F oxidative addition/Si-F reductive elimination pathway. The silyl-assisted process also proceeds with a kinetic preference for activation at the 2-position, as the transition state in this case derives extra stabilization through a Rh⋯N interaction. The C-F oxidative addition transition states show a significant degree of phosphine-assisted character and are not only higher in energy than the silyl-assisted process but also favor activation at the 4-position.