59th Annual Report on Research 2014

We have been exploring each of the steps of the proposed Pt-catalyzed conversion of methane into toluene or ethane (depending on the other reactants). We are evaluating the best ligands for each step of the process, with the hope that one ligand will work in each fundamental step of the proposed catalytic cycle.

C-X Oxidative addition studies:

We have discovered, for the first time, that certain Pt(II) complexes are capable of intermolecular oxidative addition of aryl halide bonds. This is a crucial discovery for the eventual development of catalytic coupling of aryl halides with methane to generate toluenes.

C-C Reductive elimination studies:

One of the key steps in the proposed catalytic cycle is to dehalogenate a L_nPtR₃Br complex (where the R groups could be identical or different). This would result in R-R reductive elimination and would generate L_nPt, which could then activate a hydrocarbon. We have synthesized a variety of Pt(II) complexes bearing tridentate, monoanionic CNN ligands (where C = is an anionic or X-type ligand and both N donors are neutral or L-type ligands). These complexes react with relatively lipophilic Ag-salts (such as AgBF₄), which efficiently dehalogenate CNNPt^IV-Br complex (A) in solution to form a cationic 5-coordinate Pt^IV-complex (B), with BF₄^- as a counter-anion. We found that the fluxional behavior of A, observed at room temperature, can be modified by either (i) cooling the solution to -40 °C, (ii) by choosing a more coordinating counter-anion, such as OTf-, or (iii) by dissolving A in more coordinating solvents. We observed slow Csp3-Csp3 reductive elimination from A in non-coordinating solvents to form ethane and a Pt^II-solvento complex, C, as the only products. The exclusivity of the reaction products were ascribed to rigid geometric constraints imposed by the ligand-backbone. In order to demonstrate this hypothesis, we are currently fine-tuning the ligand geometry around the metal center to afford Csp2-Csp3 reductive elimination. We are currently working on a manuscript to detail these results.

H/D exchange of Pt-CH₃ groups:

We have also demonstrated facile and reversible H/D exchange in Pt^II-hydrocarbyl fragments of dipyridyl ketone (DPK)Pt^IIMe₂ (D) and DPKPt^IIPh₂ (E) complexes. Based on extensive computational and experimental investigations, we propose intermediacy of a Pt^IVR₂(H) complex (F; R=Me, Ph) formed upon assistance from addition of methanol across the C=O fragment of the parent complexes (D and E). Interestingly, while similar complexes result in concomitant loss of R-H fragments from the Pt^IV-center via reductive elimination, we found no evidence of reductive elimination even at higher temperature and is hence quite remarkable. We have conducted kinetic and mechanistic studies on the mechanism of exchange, including DFT studies. We are currently working on a manuscript to detail these results.