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)}(PEt3)(3)] (2a) and [Rh{Si(OMe)(3)}(PEt3)(3)] (2b) were synthesized by treatment of [Rh(CH3)(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)(PEt3)(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-C5F3HN)(PEt3)(3)] (10), whereas complex 7 reacted by C-H activation to furnish trans-[Rh(CO)(4-C5F4N)(PEt3)(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 center dot center dot center dot 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.