Implication of a σ-Methane Complex en Route to Elimination of Methane from a Ruthenium Complex: An Experimental and Theoretical Investigation

The
five-coordinated 16-electron complex [Ru­(Me)­(dppe)2]­[OTf]
(3) undergoes methane elimination at room temperature
to afford the ortho-metalated species [(dppe)­{(C6H5)­(C6H4)­PCH2CH2P­(C6H5)2}­Ru]­[OTf] (7). Methane elimination, monitored using NMR spectroscopy,
revealed no intermediate throughout the reaction. The NOE between
Ru–Me protons and ortho phenyl protons and an agostic interaction
trans to the methyl group were found in complex 3 by
NMR spectroscopy, which form the basis for three plausible pathways
for methane elimination and ortho metalation: pathway I (through spatial
interaction), pathway II (through oxidative addition and reductive
elimination), and pathway III (through agostic interaction). Methane
elimination from complex 3 via pathway I was discounted,
since it involves interactions through space and not through bonds.
Moreover, the calculated energy barrier for the pathway I transition
state was quite high (71.3 kcal/mol), which also indicates that this
pathway is very unlikely. Furthermore, no spectroscopic evidence for
oxidatively added seven-coordinated Ru­(IV) species was found and the
computed energy barrier of the transition state for pathway II was
moderately high (41.1 kcal/mol), which suggests that this cannot be
the right pathway for methane elimination and ortho-metalation of
complex 3. On the other hand, indirect evidence in the
form of chemical reactions point to the most plausible pathway for
methane elimination, pathway III, via the intermediacy of a σ-CH4 complex that could not be found spectroscopically. DFT calculations
at several levels on this pathway showed an initial low-barrier rearrangement
through TS1 to a square-pyramidal intermediate wherein
methyl and agostic C–H are cis to each other. Migration of
hydrogen from agostic C–H and elimination of methane proceed
through the transition state TS2, which retains a weak
metal–H bonding through most parts of the reaction coordinate.
Upon comparison of all three pathways, pathway III was found to be
the most likely for methane elimination and ortho-metalation of complex 3.