The catalysis of the reaction of carbon dioxide with epoxides (cyclohexene oxide or propylene oxide) using the (salen)Cr(III)Cl complex as catalyst, where H(2)salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediimine (1), to provide copolymer and cyclic carbonate has been investigated by in situ infrared spectroscopy. As previously demonstrated for the cyclohexene oxide/CO(2) reaction in the presence of complex 1, coupling of propylene oxide and carbon dioxide was found to occur by way of a pathway first-order in catalyst concentration. Unlike the cyclohexene oxide/carbon dioxide reaction catalyzed by complex 1, which affords completely alternating copolymer and only small quantities of trans-cyclic cyclohexyl carbonate, under similar conditions propylene oxide/carbon dioxide produces mostly cyclic propylene carbonate. Comparative kinetic measurements were performed as a function of reaction temperature to assess the activation barrier for production of cyclic carbonates and polycarbonates for the two different classes of epoxides, i.e., alicyclic (cyclohexene oxide) and aliphatic (propylene oxide). As anticipated in both instances the unimolecular pathway for cyclic carbonate formation has a larger energy of activation than the bimolecular enchainment pathway. That is, the energies of activation determined for cyclic propylene carbonate and poly(propylene carbonate) formation were 100.5 and 67.6 kJ.mol(-1), respectively, compared to the corresponding values for cyclic cyclohexyl carbonate and poly(cyclohexylene carbonate) production of 133 and 46.9 kJ.mol(-1). The small energy difference in the two concurrent reactions for the propylene oxide/CO(2) process (33 kJ.mol(-1)) accounts for the large quantity of cyclic carbonate produced at elevated temperatures in this instance.

The phosphine 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane, which we will condense to DAPTA, and its oxide have been fully characterized both in solution and in the solid state. These compounds were prepared by acylation of 1,3,5-triaza-7-phosphaadamantane (PTA) and its oxide with acetic anhydride. The nonionic compounds were found to be soluble in most common organic solvents, in addition to possessing extremely large molar solubilities in water. Indeed, the molar solubility of DAPTA was shown to be 7.4 M, which is 4 time more soluble than the commonly utilized water-soluble phosphine, triply meta-sulfonated triphenylphosphine (TPPTS). In the case of DAPTA this enhanced water solubility is attributed to a strong interaction of water with the amide nitrogen−CO bond dipole as revealed by a large red shift of the νC  O vibration on going from a weakly interacting solvent such as CH2Cl2 to water. This latter observation is supported by the short average amide nitrogen−carbonyl carbon bond distance of 1.375 Å as determined via X-ray crystallography, indicative of a strong Coulombic interaction between the nitrogen and carbon atoms. To assess the metal to phosphorus binding characteristics of DAPTA, several group 10 and group 6 complexes were prepared and their M−P bond distances were shown to be quite similar with those of their PTA analogues. For examples, the W−P bond distance in W(CO)5DAPTA of 2.492(3) Å is comparable to that previously reported for W(CO)5PTA of 2.4976(15) Å and slighter shorter than that found in W(CO)5PMe3 (2.516(2) Å). Accordingly, the PTA ligand has generally been characterized as possessing donor properties similar to that of PMe3. Consistent with these bonding parameters determined in the solid state, all three tungsten pentacarbonyl complexes have nearly identical ν(CO) frequencies in solution.

That Cys-X-Cys tripeptide linkages can serve as tetradentate N 2 S 2 ligands, utilizing carboxamido nitrogen and cysteinyl sulfur atoms as donors in metalloenzyme active sites, has recently been verified in several protein crystal structures. [1][2][3][4][5] It was further discovered that the nickel-bound Cys-Gly-Cys NiN 2 S 2 moiety of acetyl coA synthase binds through bidentate bridging thiolate groups to a second nickel center which mediates the organometallic reactions required of the biocatalyst (the assembly of CH 3 + , CO, and SR À into the acetyl coA thioester CH 3 C( = O)SR). [1,2] The (Cys-GlyCys)Ni unit joins a host of synthetic NiN 2 S 2 complexes that are known to form multimetal clusters through m-SR interactions. Natures control of binuclearity in the construction of an organometallic catalyst presents the intriguing possibility that the NiN 2 S 2 complexes might be suitable for development as a novel class of ligands for organometalllic chemistry and catalysis. To this end we have characterized a series of NiN 2 S 2 complexes, four of which are shown in Figure 1, according to their electron-donating ability and stereochemical fea- Figure 1. NiN 2 S 2 complexes used as S-donor ligands; bme-daco = 1,5-(1,5-diazacyclooctane)di(ethylthiolate), bme*-daco = di(2-methyl-2-propylthiolate, bme-Me 2 pda = N,N'-dimethyl-2,9-diazanonanedithiolate, ema 4À = 2,7-dioxo-3,6-diazaoctanedithiolate.

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The synthesis of nickel(II) and palladium(II) salicylaldiminato complexes incorporating the water-soluble phosphine 1,3,5-triaza-7-phosphaadamantane(PTA) has been achieved employing two preparative routes. Reaction of the original ethylene polymerization catalyst developed by Grubbs and co-workers (Organometallics 1998, 17, 3149), (salicylaldiminato)Ni(Ph)PPh(3), with PTA using a homogeneous methanol/toluene solvent system resulted in the formation of the PTA analogues in good yields. Alternatively, complexes of this type may be synthesized via a direct approach utilizing (tmeda)M(CH(3))(2) (M = Ni, Pd), the corresponding salicylaldimine, and PTA. Yields by this method were generally near quantitative. The complexes were characterized in solution by (1)H/(13)C/(31)P NMR spectroscopy and in the solid-state by X-ray crystallography. All derivatives exhibited square-planar geometry with the bulky isopropyl groups on the aniline being perpendicular to the plane formed by the metal center and its four ligands. Such orientation of these sterically encumbering groups is responsible for polymer chain growth during olefin polymerization in favor of chain termination via beta-hydride elimination. Polymerization reactions were attempted using the nickel-PTA complexes in a biphasic toluene/water mixture in an effort to initiate ethylene polymerization by trapping the dissociated phosphine ligand in the water layer, thereby eliminating the need for a phosphine scavenger. Unfortunately, because of the strong binding ability of the small, donating phosphine(PTA) as compared to PPh(3), phosphine dissociation did not occur at a temperature where the complexes are thermally stable.

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That Cys-X-Cys tripeptide linkages can serve as tetradentate N 2 S 2 ligands, utilizing carboxamido nitrogen and cysteinyl sulfur atoms as donors in metalloenzyme active sites, has recently been verified in several protein crystal structures. [1][2][3][4][5] It was further discovered that the nickel-bound Cys-Gly-Cys NiN 2 S 2 moiety of acetyl coA synthase binds through bidentate bridging thiolate groups to a second nickel center which mediates the organometallic reactions required of the biocatalyst (the assembly of CH 3 + , CO, and SR À into the acetyl coA thioester CH 3 C( = O)SR). [1,2] The (Cys-GlyCys)Ni unit joins a host of synthetic NiN 2 S 2 complexes that are known to form multimetal clusters through m-SR interactions. Natures control of binuclearity in the construction of an organometallic catalyst presents the intriguing possibility that the NiN 2 S 2 complexes might be suitable for development as a novel class of ligands for organometalllic chemistry and catalysis. To this end we have characterized a series of NiN 2 S 2 complexes, four of which are shown in Figure 1, according to their electron-donating ability and stereochemical fea- Figure 1. NiN 2 S 2 complexes used as S-donor ligands; bme-daco = 1,5-(1,5-diazacyclooctane)di(ethylthiolate), bme*-daco = di(2-methyl-2-propylthiolate, bme-Me 2 pda = N,N'-dimethyl-2,9-diazanonanedithiolate, ema 4À = 2,7-dioxo-3,6-diazaoctanedithiolate.

No abstract