Mechanistic Aspects of the Copolymerization of CO2 and Epoxides by Soluble Zinc Bis(phenoxide) Catalysts as Revealed by Their Cadmium Analogues

Single-site beta-diiminate zinc catalysts for the alternating copolymerization of CO2 and epoxides: catalyst synthesis and unprecedented polymerization activity

Synthetic routes to zinc beta-diiminate complexes are reported. The synthesis of 11 beta-diimine [(BDI)-H] ligands, with varying N-aryl substituents and bridging structures, is described. These ligands are converted to (BDI)ZnX complexes (X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr). X-ray structural data revealed that all zinc complexes examined exist as micro-X-bridged dimers in the solid state, with the exception of the zinc ethyl and amido complexes which were monomeric. Complexes of the form (BDI)ZnOR (R = alkyl, acyl) and (BDI)ZnN(SiMe3)2 are highly active catalysts for the alternating copolymerization of epoxides and CO2. Copolymerizations of cyclohexene oxide (CHO) and CO2 with (BDI-1)ZnX [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene)] and (BDI-2)ZnX [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene)], where X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr, were attempted at 50 degrees C and 100 psi CO2. Complexes with X = OAc, N(SiMe3)2, OMe, O(i)Pr all produced polycarbonate by the alternated insertion of CHO and CO2 with similar catalytic activities, comparable molecular weights, and narrow molecular weight distributions (MWD approximately 1.1), indicating the copolymerizations are living. Furthermore, ligand effects were shown to dramatically influence the polymerization activity as minor steric changes accelerated or terminated the polymerization activity.

Cs(2) fixation by carbonic anhydrase model systems-a new substrate in the catalytic cycle

The conversion of CS(2) with common carbonic anhydrase model systems has been studied using Hartree-Fock and density-functional theory methods employing the 6-311+G basis set. The calculated geometries and energetical parameters for [L(3)ZnOH](+)/CS(2) model systems (L = NH(3), imidazole) are compared with those obtained previously for the CO(2) hydration. While the same reaction mechanism applies for both heterocumulenes, the hypothetical conversion of CS(2) to give [L(3)ZnSC(O)SH](+) is characterized by a higher barrier and is much more exothermic than the corresponding CO(2) reaction cascade. Due to the increased number of heteroatoms, additional intermediates and product structures (compared with those involved in the CO(2) conversion) must be taken into account and have been analyzed in detail. The smaller electrophilicity of CS(2) is the reason for the higher activation energies, while the significantly increased exothermicity is due to the strong zinc(II)/sulfur interaction. The reversibility and therefore the existence of a catalytic cycle which could allow comparable CS(2) transformations must be questioned. Nevertheless, an interesting field of stoichiometric zinc-mediated CS(2) transformations is conceivable.

Solution and solid-state structures of phosphine adducts of monomeric zinc bisphenoxide complexes. Importance of these derivatives in CO2/epoxide copolymerization processes

Phosphine derivatives of the monomeric zinc phenoxide complexes, (phenoxide)2ZnLn, where phenoxide equals 2,6-di-tert-butylphenoxide, 2,4,6-tri-tert-butylphenoxide, and 2,6-diphenylphenoxide and n = 1 or 2, have been synthesized from the reaction of Zn[N(SiMe3)2]2 and the corresponding phenol followed by the addition of phosphine. The complexes have been characterized in solution by 31P NMR spectroscopy and in selected instances in the solid-state by X-ray crystallography. The small, basic phosphine, PMe3, provided the only case of an isolated complex possessing two phosphine ligands (i.e., n = 2). For all other larger phosphines only the monophosphine adducts were obtained. Furthermore, only fairly basic phosphines were found to bind to zinc, e.g., whereas PPh3 (pKa = 2.73) was ineffective, PPh2Me (pKa = 4.57) did form a strong bond to zinc. The solid-state structures of the monophosphine adducts consist of a near-trigonal planar geometry about the zinc center, where the average P-Zn-O angles are larger than the O-Zn-O angles. On the other hand, the bisphosphine adduct, Zn(O-2,4,6-tBu3C6H2)(2).2PMe3, is a distorted tetrahedral structure with O-Zn-O and P-Zn-P bond angles of 108.8(2) degrees and 107.1(9) degrees, respectively. Competitive phosphine binding studies monitored by 31P NMR spectroscopy provided a relative binding order of PPh3 approximately PtBu3 << PPh2Me < PCy3 < PMe2Ph < PnBu3 < PEt3 < PMe3. Hence, the relative binding of basic phosphine ligands at these congested zinc sites is largely determined by their steric requirements. All phosphine adducts, with the exception of PMe2Ph and PMe3, were found to undergo slow self-exchange (< 600 s-1) with free phosphine by 31P NMR spectroscopy. However, the two small phosphines, PMe2Ph (cone angle = 122 degrees) and PMe3 (cone angle = 118 degrees), were shown to undergo rapid exchange presumably via an associative mechanism. Although there was no kinetic preferences for PCy3 binding to cadmium vs zinc, cadmium was thermodynamically favored by about a factor of 2.5. The addition of up to 3 equiv of PCy3 to the Zn(O-2,6-tBu2C6H3)2 or Zn(O-2,4,6-tBu3C6H2)2 derivatives did not significantly alter the reactivity of these catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 to high-molecular weight poly(cyclohexene carbonate). However, the presence of PCy3 greatly retarded their ability to homopolymerize CHO to polyether or to afford polyether linkages during the copolymerization of CHO/CO2.