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···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.