As reported in the October 23, 2009, issue of the journal Science, Carolina chemists in collaboration with colleagues at the University of Washington have taken an important step in converting methane gas to a liquid, potentially making it more useful as a fuel and as a source for making other chemicals.
The carbon-hydrogen bonds of alkanes are weak ligands and thus reports of isolation or spectroscopic observation of alkane complexes in solution are extremely rare. Nevertheless, such complexes are postulated as intermediates that form prior to C-H bond scission in most oxidative addition reactions of alkanes. The Shilov system for catalytic conversion of methane to methanol is thought to involve a Pt(II) methane complex as a key intermediate. While a postdoctoral fellow in the Brookhart Group, Wes Bernskoetter, now on the faculty at Brown, succeeded in preparing the first solution-stable, NMR-observable transition metal complex of the simplest alkane, methane (CH4).
The methane complex was obtained by low temperature protonation of a pincer rhodium methyl complex and fully characterized by 1H, 13C and 31P NMR spectroscopy. Cindy Schauer, co-author of the study, carried out DFT calculations that suggest one C-H bond interacts preferentially with the Rh center to form a three-center, two-electron bond, as per the above figure. The Brookhart Group hopes that investigation of the properties of this and other methane complexes may lead to more efficient catalysts for functionalization of alkanes.
Published in JACS, researchers in the Brookhart Group discuss how several cationic (allyl)Pd(II) complexes were synthesized and shown to be highly active for (2,3)-vinyl addition polymerization of norbornene (NB) to yield polymers with low molecular weight distributions (MWDs) ranging from 1.2−1.4. Despite the low MWDs, slow initiation was followed by rapid propagation preventing molecular weight control of the poly(norbornene). Several intermediates in these polymerizations initiated with [(2-R-allyl)Pd(mesitylene)]+ complexes were fully characterized (NMR and X-ray diffraction). Consistent with previous observations the allyl and NB units couple in cis-exo fashion to yield a σ,π-complex capped by mesitylene. Mesitylene is readily displaced by NB to form an agostic intermediate in which NB acts as a bidentate ligand and binds to the cationic Pd center via the π-system and a γ-agostic interaction with the syn hydrogen at C7.
The identity of this complex was established by NMR spectroscopy and single-crystal X-ray diffraction. It is significant since it suggests bidentate binding of NB in the propagating species, which cannot be observed by NMR spectroscopy. The NMR studies suggest that the second insertion, i.e., insertion of NB in the agostic intermediate, is the slow initiation step and the subsequent insertions are extremely fast. Therefore, slow chelate opening is the major limitation preventing a living polymerization. This hypothesis was explored using a series of cationic substituted π-allyl complexes; significantly increased reactivity was observed when electron-withdrawing groups were introduced into the allyl moiety. However, despite these modifications initiation remained slow relative to chain propagation.