The effect of [CO] on acetyl-CoA synthesis activity of the isolated alpha subunit of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase from Moorella thermoacetica was determined. In contrast to the complete alpha(2)beta(2) enzyme where multiple CO molecules exhibit strong cooperative inhibition, alpha was weakly inhibited, apparently by a single CO with K(I) = 1.5 +/- 0.5 mM; other parameters include k(cat) = 11 +/- 1 min(-)(1) and K(M) = 30 +/- 10 microM. The alpha subunit lacked the previously described "majority" activity of the complete enzyme but possessed its "residual" activity. The site affording cooperative inhibition may be absent or inoperative in isolated alpha subunits. Ni-activated alpha rapidly and reversibly accepted a methyl group from CH(3)-Co(3+)FeSP affording the equilibrium constant K(MT) = 10 +/- 4, demonstrating the superior nucleophilicity of alpha(red) relative to Co(1+)FeSP. CO inhibited this reaction weakly (K(I) = 540 +/- 190 microM). NiFeC EPR intensity of alpha developed in accordance with an apparent K(d) = 30 microM, suggesting that the state exhibiting this signal is not responsible for inhibiting catalysis or methyl group transfer and that it may be a catalytic intermediate. At higher [CO], signal intensity declined slightly. Attenuation of catalysis, methyl group transfer, and the NiFeC signal might reflect the same weak CO binding process. Three mutant alpha(2)beta(2) proteins designed to block the tunnel between the A- and C-clusters exhibited little/no activity with CO(2) as a substrate and no evidence of cooperative CO inhibition. This suggests that the tunnel was blocked by these mutations and that cooperative CO inhibition is related to tunnel operation. Numerous CO molecules might bind cooperatively to some region associated with the tunnel and institute a conformational change that abolishes the majority activity. Alternatively, crowding of CO in the tunnel may control flow through the tunnel and deliver CO to the A-cluster at the appropriate step of catalysis. Residual activity may involve CO from the solvent binding directly to the A-cluster.

The chromium tetramethyltetraazaannulene catalyst system, Cr(tmtaa)Cl in the presence of a quaternary organic salt, has been employed to further investigate the mechanism of polycarbonate formation between cyclohexene oxide and carbon dioxide. Although the features of the proposed mechanism are generally well-accepted, the question of whether polymer chain propagation occurs from one side of the catalyst in a mono catalytic single-site fashion or from both sides of the N(4)-ligand plane in a dual catalytic manner. To probe this behavior, the ligand architecture was altered by the addition of a sterically encumbering strap (Cr(stmtaa)Cl), which effectively blocks one side of the complex and limits polymer chain growth to a single side of the catalyst. For direct comparison, an electronically similar catalyst, Cr(s(m)tmtaa)Cl, was prepared to mimic the strap complex while not possessing the site-blocking steric restraints of the full strap. Infrared spectroscopy demonstrated an uptake of 2 equiv of PPNN(3) cocatalyst by the strap mimic catalyst, while only 1 equiv of azide was able to bind to the catalyst containing the full strap, supporting the design function of both complexes. Monitoring the formation of poly(cyclohexylene carbonate) from the reaction of cyclohexene oxide and carbon dioxide via in situ infrared spectroscopy for both Cr(stmtaa)Cl and Cr(s(m)tmtaa)Cl, under identical conditions, revealed copolymer formation at essentially equivalent rates. Analysis of the polycarbonate products found that Cr(stmtaa)Cl and Cr(s(m)tmtaa)Cl produced copolymers with turnover frequencies of 806.6 and 797.3 h(-1), molecular weights of 11,431 and 12,003 Da, and polydispersities of 1.108 and 1.048, respectively. These results strongly support the idea that this and other catalysts systems presumed to operate by a similar process, such as Cr(salen)X, catalyze the copolymerization of epoxides with carbon dioxide through a mono catalytic single-site mechanism.

A tetramethyltetraazaannulene complex incorporating a chromium(III) metal center has been shown to be highly active toward the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexene carbonate) in the presence of [PPN]N3 [PPN+=bis(triphenylphosphoranylidene)ammonium] as a cocatalyst. An asymptotical rate increase was observed, leveling at 2 equiv of cocatalyst with a maximum turnover frequency of 1300 h(-1) at 80 degrees C. A benefit of this new catalyst system over that of the previously studied less-active (salen)CrX system is that the (tmtaa)CrCl catalyst has a much lower propensity toward the formation of a cyclic carbonate byproduct throughout the copolymerization reaction.

The high catalytic activity of a tetramethyltetraazaannulene (tmtaa) chromium complex toward the copolymerization of cyclohexene oxide and carbon dioxide to discriminatively provide poly(cyclohexylene carbonate) has directed further studies into the capabilities of the catalyst system. Various [PPN]X (PPN(+) = bis(triphenylphosphoranylidene)ammonium) cocatalysts, where X = Cl, N(3), Br, CN, and OBzF(5), in the presence of (tmtaa)CrCl were examined for catalytic reactivity and selectivity for polycarbonate formation, achieving turnover frequencies of 1500 h(-1) at 80 degrees C in the case of PPNCl. The catalyst system was examined under varied pressures and found to be active even at 1 bar of CO(2) pressure. In addition to cyclohexene oxide, the (tmtaa)CrCl complex was investigated for catalytic activity toward the coupling of carbon dioxide with propylene oxide, isobutylene oxide, 1,2-epoxyhexane, styrene oxide, and 4-vinyl cyclohexene oxide. Activation energies were found for the copolymerization reaction between cyclohexene oxide and carbon dioxide utilizing the tetramethyltetraazaannulene catalyst system to be 67.1 +/- 4.2 kJ.mol(-1) and 65.2 +/- 2.5 kJ.mol(-1) in neat epoxide and with methylene chloride cosolvent, respectively, upon monitoring these processes by in situ infrared spectroscopy. Supplementary to the studies involving (tmtaa)CrCl, electronic effects at the metal center on catalytic activity were examined through derivatization of the tmtaa ligand, resulting in increased activity as electron-donating substituents were added.

The synthesis of the metallodithiolate derivative of tungsten pentacarbonyl from the reaction of photogenerated W(CO)(5)THF and Ni-1 ((1,5-bis(2-mercapto-2-methylpropane)-1,5-diazacyclooctanato)nickel(II)) is described, along with its crystal structure. In N,N-dimethylformamide solution, the pentacarbonyl exists in equilibrium with its tetracarbonyl analogue and carbon monoxide. The pentacarbonyl complex stereoselectively loses cis carbonyl ligands, as is apparent from (13)CO-labeling studies, where the thus-formed tetracarbonyl tungsten complex resulting from chelate ring-closure is preferentially (13)CO-labeled among the two mutually trans CO groups. The kinetics of the addition of CO to the tetracarbonyl to afford the metal pentacarbonyl were monitored by means of in situ infrared spectroscopy in the nu(CO) region at CO pressures between 28 and 97 bar and temperatures over the range 45-60 degrees C. Under these conditions, there was no evidence for W-S bond cleavage in the pentacarbonyl complex with concomitant formation of W(CO)(6). These studies reveal that the tetracarbonyl complex and CO are only slightly unstable with respect to the formation of the pentacarbonyl complex, with an equilibrium constant at 50 degrees C of about 2.8 M(-1) or DeltaG degrees = -1.4 kJ/mol. The activation parameters determined for the ring-opening process (DeltaH = 89.1 kJ/mol and DeltaS = -37.2 J/mol.K) suggest a solvent-assisted concerted ring-opening mechanism.