Paul D. Knight

Highly Active Dizinc Catalyst for the Copolymerization of Carbon Dioxide and Cyclohexene Oxide at One Atmosphere Pressure

A novel dizinc complex having a macrocyclic ancillary ligand shows remarkable activity at only one atmosphere of CO2 for the copolymerization of CO2 and cyclohexene oxide. Carbon dioxide is an attractive reagent for synthetic chemistry as it is abundant, inexpensive, of low toxicity, and is the waste product of many chemical processes. The copolymerization of carbon dioxide and epoxides, known for several decades, is a particularly promising route to activate and use CO2 as a renewable C-1 source. [1–5] Furthermore, if cyclohexene oxide is used, the resulting copolymer has a high glass transition temperature and tensile strength, but it is also degradable. The first report of this type of copolymerization came from Inoue et al. in 1969, and they used diethyl zinc and alcohols to produce poly(propylene carbonate), albeit with very low turnover numbers (TONs). Subsequently several research groups developed more active and controlled catalysts, and notable for their activity are the zinc phenoxide, zinc b-diiminate, and chromium(III)– or cobalt(III)–salen complexes. The zinc b-diiminate complexes show very high turnover frequencies (TOFs), as well as excellent control for the copolymerization of CO2 and cyclohexene oxide. Recent mechanistic studies by Coates and co-workers suggest that the most effective b-diiminate complexes are loosely associated dimers under the polymerization conditions. This proposal has led to the deliberate preparation of various dimetallic zinc catalysts, and among these dizinc catalysts, the anilido aniline complexes show particularly high TONs and TOFs because they can operate at low catalyst loadings. 24] However, all of the known high activity catalysts require substantial (> 7 atm) pressures of carbon dioxide, which significantly increases the overall energy requirement of the process. Although catalysts which operate at only one atmosphere of CO2 are known, 23, 26,27] so far the best reported TON was 20 and the highest TOF was 3.3 h . We report the preparation of a dimetallic zinc complex (Scheme 1) having a macrocyclic ancillary ligand, which shows very high activity for the copolymerization of cyclohexene oxide and carbon dioxide under mild pressures. The macrocyclic ligand H2L 1 was prepared in two steps with 84% overall yield from commercial reagents (see the Supporting Information) by using an adaptation of a synthetic route described previously. The dimetallic zinc complex, [LZn2(OAc)2] was synthesized by the deprotonation of H2L 1 using potassium hydride, and subsequent reaction with zinc acetate. The complex was isolated as a white solid in 70 % yield (Scheme 1). The stoichiometry of the complex was confirmed by elemental analysis, which was in agreement with the calculated values, and the identification of a fragment peak in the FAB mass spectrum for the molecular ion less an acetate group. The H NMR spectrum at 25 8C shows broadened resonances, consistent with several diastereoisomers being present, which are fluxional on the NMR timescale. When the sample was heated to 110 8C coalescence was observed (see Figure S1 in the Supporting Information). A single resonance was observed for the aromatic protons and the signal for the NH groups was a broadened resonance at d = 4.78 ppm. The methylene groups are diastereotopic, therefore four broadened resonances were observed from d = 3.32–2.46 ppm, each with an integral corresponding to 4H. The signals for the tertbutyl groups and the methyl group of the acetate resonate as singlets with integrals corresponding to 18H and 6H, respectively. The methyl groups on the ligand backbone are also diastereotopic and are observed as two singlets, each with a relative integral corresponding to 6H. The complex was tested at low pressures for the copolymerization of carbon dioxide and cyclohexene oxide (Table 1). Thus, at only one atmosphere of CO2, 80–100 8C, and a 0.1 mol% catalyst loading, poly(cyclohexene carbonate) was produced with a TON in the range of 430–530 and a TOF in the range of 18–25 h 1 (Table 1, entries 1–3). There are very few catalysts that are effective at such a low pressure, 23, 26,27] the most active of which is a dizinc Scheme 1. The synthesis of the dizinc complex [LZn2(OAc)2]. Reagents and conditions: a) KH, THF, 78 8C!RT, 1 h; b) Zn(OAc)2, THF, RT, 16 h.

Predetermination of chirality at octahedral centres with tetradentate ligands: prospects for enantioselective catalysis

Abstract Methods for creation of chiral-at-metal architectures, principally at octahedral metal centres using tetradentate ligands are reviewed. Where achiral ligands are employed, one chiral racemic topography may be formed selectively, and complex resolution strategies have been successfully applied to furnish optically pure systems. The use of chiral ligands has allowed the diastereomeric formation of complexes with stereogenic metal centres, and this area is becoming quite well developed. A number of applications to stoichiometric and catalytic syntheses are described. In certain instances, chiral-at-metal systems outperform their traditional chiral ligand+metal counterparts as a result of the excellent expression of chirality in the active site(s). Prospects for future developments are indicated.

Dinuclear Zinc Complexes Using Pentadentate Phenolate Ligands

The synthesis and characterization of 15 dinuclear zinc complexes are reported, including the X-ray crystal structures of 5 complexes. The ligand motif utilizing p-cresol as a bridging unit between the two zinc centers and a set of three related ligands has been synthesized; 2,6-bis(R)-p-cresol (where R is CH(2)NMe(CH(2))(2)NMe(2) = L(1), CH=N(CH(2))(2)NMe(2) = L(2), and CH(2)NH(CH(2))(2)NMe(2) = L(3)). Dizinc trihalide complexes [L(n)Zn(2)(mu-X)X(2)] (where X = Cl, Br, I) have been prepared. The trihalide complexes were treated with potassium ethoxide, resulting in quantitative substitution of the bridging halide group to give [L(n)Zn(2)(mu-OEt)X(2)]. The dizinc ethoxide complexes were tested in situ as initiators for lactide ring opening polymerization. Dizinc triacetate complexes have also been synthesized [L(n)Zn(2)(OAc)(3)], as well as cationic diacetate species containing two bridging acetate groups, [L(n)Zn(2)(mu-OAc)(2)][BPh(4)]. Structural differences between complexes of the three ligands are discussed.

Biaryl-bridged Schiff base complexes of zirconium alkyls: synthesis structure and stability

Abstract Three substituted salicylaldimine derivatives H2L1–3 of 2,2′-diamino-6,6′-dimethylbiphenyl give, under appropriate conditions, isolable alkyls of zirconium [ZrL1–3R2] (R=CH2Ph, CH2But). Two molecular structures confirm their cis-α geometry (C2-symmetric with cis alkyl ligands). They decompose via 1,2-migratory insertion of an alkyl group to imine, followed in some instances by a second similar reaction. The dimeric molecular structure of one such doubly-inserted product is presented. The kinetics of decomposition by this process are studied briefly, and it is noted that the rate increases with increased steric demand of the salicylaldimine unit.

Problems and solutions for alkene polymerisation catalysts incorporating Schiff-bases; migratory insertion and radical mechanisms of catalyst deactivation

Steric blocking of an intramolecular 1,2-migratory insertion reaction of a zirconium salicylaldiminato complex leads to a long-lived catalyst for ethene polymerisation, but promotes a new radical catalyst decomposition mechanism in certain instances; kinetic and thermodynamic parameters for both pathways have been established.

Zirconium catalysed enantioselective hydroamination/cyclisation

A chiral zirconium alkyl cation catalyses the cyclisation of certain aminoalkenes with enantioselectivity up to 82%, the highest thus far observed for such a process.

Chiral-at-metal organolanthanides: enantioselective aminoalkene hydroamination/cyclisation with non-cyclopentadienyls

Chiral non-racemic complexes [ML{N(SiMe2H)2}(thf)] (M = Y, La, H2L = salicylaldimine ligands derived from 2,2′-diamino-6,6′-dimethylbiphenyl) are found not to be effective catalysts for the intramolecular hydroamination of aminoalkenes, but new amino/phenoxide ligand designs without reducible functional groups led to long-lived and enantioselective catalysts.