NOVEL PLATINUM COMPLEXES SUPPORTED BY SULFONATED CNN PINCER LIGANDS RELEVANT TO AEROBIC METHANE FUNCTIONALIZATION CHEMISTRY

Abstract

Mild and selective aerobic methane functionalization has always been a challenge. Shilov developed a PtII based system that has a potential to solve the problem, but the non-practical PtIV oxidant needs to be substituted by more accessible oxidants, such as O2. On pursuing this goal, several series of PtII and PtIV methyl complexes derived from two pre-ligands, Ph-dpms [(6-phenylpyridin-2-yl)(pyridin-2-yl)methanesulfonate, HL1-] and Ph-pcpps [7-(6-phenylpyridin-2-yl)-6,7-dihydro-5H-cyclopenta[b]pyridine-7-sulfonate, HL2-] were synthesized. Their reactivity in O2 and X-H (X = C, Si) bond activation (PtIIMe), and in CH3-X (X = O, N, C) reductive elimination (PtIVMe), was studied. The three steps of the proposed catalytic cycle suitable for aerobic methane functionalization were investigated.The reverse of the first step, methane C-H activation, was probed by reacting K[(L1)PtIIMe] with acid H(Et2O)2BArF4 {BArF4 = tetrakis[3,5- bis(trifluoromethyl)phenyl]borate} in CH2Cl2 at -78 °C. Although methane and no stable PtIV(Me)H species were detected at both -78 °C and room temperature, the resulting solutions were shown to activate Si-H bonds of Et3SiH and Me3SiH to form Pt hydrido complexes. The second step, aerobic oxidation of PtIIMe complexes, was investigated using K[(L1)PtIIMe] and K[(L2)PtIIMe]. K[(L1)PtIIMe] reacts with O2 in MeOH solutions to form diastereomeric (L1)PtIVMe2 complexes, which are barely reactive in CH3-X reductive elimination. Notably, K[(L2)PtIIMe] reacted with O2 in MeOH or acetone / TFE to selectively form three out of four possible diastereomeric (L2)PtIV(Me)OH complexes with (L2)PtIVMe2 as a minor by-product. The remaining fourth diastereomer of (L2)PtIV(Me)OH was prepared using H2O2 as oxidant. The third step, CH3-X reductive elimination, was studied using a series of PtIVMe species supported by L1 and L2. The PtIVMe(Y) species (Y = Cl, I, OCH2CF3, OH) having methyl trans- to sulfonate and one (L2)PtIV(Me)OH complex having pyridyl trans- to methyl demonstrated facile CH3-X reductive elimination (X = Me2SO+, OH, O2CCF3, and Me2NPh+) using the corresponding nucleophiles with yields of the CH3-X products up to 99%. Two other (L2)PtIV(Me)OH complexes having methyl ligand trans- to pyridyl formed predominantly C-C coupling products in aqueous DMSO solutions of CF3CO2H. Overall, this work demonstrates the potential of our novel sulfonated pincer ligands to support aerobic functionalization of methane at a Pt center.