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.
C–F activation reactions for a silyl complex gave fluorosilane and Rh pyridyl complexes. In consecutive reactions, the fluorosilane can act as a fluoride source and a regeneration of the C–F bond occurs by Si–F bond cleavage. This sets back the C–F bond cleavage reaction with consequences for the overall chemoselectivity of the activation reactions.
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