Selective C-H Functionalization of Methane and Ethane by a Molecular SbV Complex.

Owing to the strong nonpolar bonds involved, selective C-H functionalization of methane and ethane to esters remains a challenge for molecular homogeneous chemistry. We report that the computationally predicted main-group p-block SbV (TFA)5 complex selectively functionalizes the C-H bonds of methane and ethane to the corresponding mono and/or diol trifluoroacetate esters at 110-180 °C with yields for ethane of up to 60 % with over 90 % selectivity. Experimental and computational studies support a unique mechanism that involves SbV -mediated C-H activation followed by functionalization of a SbV -alkyl intermediate.

Supermetal: SbF5-mediated methane oxidation occurs by C-H activation and isobutane oxidation occurs by hydride transfer.

SbVF5 is generally assumed to oxidize methane through a methanium-to-methyl cation mechanism. However, experimentally no H2 is observed, and the mechanism of methane oxidation has remained unsolved for several decades. To solve this problem, density functional theory calculations with multiple chemical models (mononuclear and dinuclear) were used to examine methane oxidation by SbVF5 in the presence of CO leading to the methyl acylium cation ([CH3CO]+). While there is a low barrier for methane protonation by [SbVF6]-[H]+ (the combination of SbVF5 and HF) to give the [SbVF5]-[CH5]+ ion pair, H2 dissociation is a relatively high energy process, even with CO assistance, and so this protonation pathway is reversible. While Sb-mediated hydride transfer has a reasonable barrier, the C-H activation/σ-bond metathesis mechanism with the formation of an SbV-Me intermediate is lower in energy. This pathway leads to the acylium cation by functionalization of the SbV-Me intermediate with CO and is consistent with no observation of H2. Because this C-H activation/metal-alkyl functionalization pathway is higher in energy than methane protonation, it is also consistent with the experimentally observed methane hydrogen-to-deuterium exchange. This is the first time that evidence is presented demonstrating that SbVF5 acts beyond a Bronsted superacid and involves C-H activation with an organometallic intermediate. In contrast to methane, due to the much lower carbocation hydride affinity, isobutane significantly favors hydride transfer to give the tert-butyl carbocation with concomitant SbV to SbIII reduction. In this mechanism, the resulting highly acidic SbV-H intermediate provides a route to H2 through protonation of isobutane, which is consistent with experiments and resolves the longstanding enigma of different experimental results for methane versus isobutane.

Evidence for a synergistic salt-protein interaction -- complex patterns of activation vs. inhibition of nitrogenase by salt.

The molybdenum nitrogenase enzyme system, comprised of the MoFe protein and the Fe protein, catalyzes the reduction of atmospheric N(2) to NH(3). Interactions between these two proteins and between Fe protein and nucleotides (MgADP and MgATP) are crucial to catalysis. It is well established that salts are inhibitors of nitrogenase catalysis that target these interactions. However, the implications of salt effects are often overlooked. We have reexamined salt effects in light of a comprehensive framework for nitrogenase interactions to offer an in-depth analysis of the sources of salt inhibition and underlying apparent cooperativity. More importantly, we have identified patterns of salt activation of nitrogenase that correspond to at least two mechanisms. One of these mechanisms is that charge screening of MoFe protein-Fe protein interactions in the nitrogenase complex accelerates the rate of nitrogenase complex dissociation, which is the rate-limiting step of catalysis. This kind of salt activation operates under conditions of high catalytic activity and low salt concentrations that may resemble those found in vivo. While simple kinetic arguments are strong evidence for this kind of salt activation, further confirmation was sought by demonstrating that tight complexes that have previously displayed little or no activity due to the inability of Fe protein to dissociate from the complex are activated by the presence of salt. This occurs for the combination Azotobacter vinelandii MoFe protein with: (a) the L127Delta Fe protein; and (b) Clostridium pasteurianum Fe protein. The curvature of activation vs. salt implies a synergistic salt-protein interaction.