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Model catalysts for CO2/epoxide copolymerization based on Co(III) complexes were studied, with focus on the preference of their alternative isomers, cisβ vs. trans. The systems range from model structures without the co‐catalyst, as derived from crystallographic data, to complex models with two (CH2)4N+R3 co‐catalyst chains (R = Me, Bu) grafted onto a Co(III)–salcy core. To explore the conformational space of the latter complexes, a computational protocol was developed, combining a systematic model‐building approach with static and molecular dynamics calculations, and multilevel energy assessment (PM7 and DFT). Results demonstrate an influence of the co‐catalyst on the relative stability of the isomers. The cisβ isomer is preferred for complexes without N+‐chains and the cisβ ↔ trans isomerization is feasible. Five‐coordinate species and open‐shell electronic configurations are energetically disfavored. The cisβ preference decreases with the introduction and enlargement of (CH2)4N+R3: both isomers can be populated for R = Me, while the trans isomer is visibly preferred for R = Bu. © 2018 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Carbon dioxide (CO2)/propylene oxide copolymerization using dual catalyst composed of Salen–cobalt(III) complex bearing four quaternary ammonium salts (a highly active poly(propylene carbonate) catalyst) and a double metal cyanide (DMC, a highly active poly(propylene oxide) catalyst) afforded poly(propylene carbonate-co-propylene oxide) through shuttling of the growing polymer chains between the two catalyst sites. Results of 1H NMR, 13C NMR, DSC studies supported formation of genuine poly(propylene carbonate-co-propylene oxide), not a mixture of poly(propylene carbonate) and poly(propylene oxide). The carbonate fraction was variable by tuning the ratio of the two catalysts and the CO2 pressure. [Copyright &y& Elsevier]

The synthesis and solid-state characterization of iron(III) salen complexes bearing monodentate and bidentate anionic oxygen donor ligands are reported. The synthesis of Fe(salen)OPh (OPh=2,6–diphenylphenoxide) (1) was accomplished using Fe[N(Si(CH3)3)2]3 with one equivalent of H2salen and the corresponding phenol. The complex was isolated as a crystalline sample by the slow diffusion of pentane into a concentrated solution of 1 at −20 °C. The solid-state structure of complex 1 reveals distorted square pyramidal geometry about the iron metal center. Alternatively the preparation of Fe(salen) complex 2 bearing the anionic, bidentate acetylacetonate ligand in the axial position was accomplished by thermally replacing two acetylacetonate ligands in Fe(acac)3 with one equivalent of H2salen. The complex was isolated and crystallographically characterized using X-ray quality crystals formed via the aforementioned protocol. The metal center adopts a very distorted octahedral geometry, indicating a preference for the square pyramidal species which is typical of other iron salen derivatives. Unfortunately, 1 and 2 were found to be inactive in the copolymerization of CO2 and cyclohexene oxide to afford polycarbonate in the presence of a Lewis base (i.e., N-MeIm and PCy3). The μ-[Fe(salen)]2O (3) derivative was also isolated via the protonation of Fe(salen)(N(Si(CH3)3)2). The complex is relatively air stable, forming large, dark crystals over a short period of time at −20 °C. The solid-state structure reveals a nearly linear Fe–O–Fe bond angle, which is due to the electronic repulsions associated with the sterically encumbering tert-butyl groups on the salen framework. [Copyright &y& Elsevier]

Propylene oxide (PO) binding and ring-opening reaction with the bifunctional CO2/epoxide copolymerization catalyst, based on the Co(III)-salcy complex including two quaternary ammonium salts with n-butyl substituents (N+-chains) were investigated by Density Functional Theory (DFT) calculations and compared with the model systems without the N+-chains. The importance of the different possible stereoisomers and the stereoselectivity of these processes for (S)- and (R)-enantiomers of PO were considered. To explore the conformational space for the real catalyst, a complex approach, developed previously was applied. The calculations for the model systems directly demonstrate that PO-ring opening proceeds preferentially in trans catalysts' configuration and no participation of cis-β isomers is viable; nucleophilic attack at the methylene-carbon atom is preferred over that at methine-carbon atom. For the real bifunctional catalyst, with the (S,S)-configuration of cyclohexane, the results indicate a preference of (R)-PO ring-opening over (S)-PO ring-opening (ca. 6:5). Concerning stereoisomers resulting from the orientation of N+-chains in the real catalyst, different groups of structures participate in the ring-opening reaction for (R)-PO, and different for (S)-PO. The high population of nonreactive complexes of (R)-PO may be the key factor responsible for decreasing the activity of the analyzed catalyst in the epoxide ring-opening reaction. [ABSTRACT FROM AUTHOR]

Based on our previous studies utilizing CrIII catalysts with trifluoroacetate as labile ligand and an ansa -bis-quinoline-olato ligand as a chemically more robust variant compared to known salen-type N 2 O 2 ligands, we synthesized and investigated a series of structurally similar quinoline-8-olato chromium complexes with different active (pseudo-)halogen ligands X in [(babhq)CrX], (babhq = 2,2′-(butyl-azanediyl)bis(quinolin-8-olate), X = Cl, NCO, and N 3. We demonstrate a high activity of these CrIII complexes in the solvent free carbon dioxide/cyclohexene oxide (CHO) copolymerization compared to the state of the art. In much deeper detail than in our previous communication, we altered the labile ligand X and undertook a catalyst screening on the polymerization parameters, such as pressure, temperature, and cocatalyst, i.e. [PPN]N 3 (bis (triphenylphosphine)iminiun azide). The activity of the chromium systems was found to much depend on the labile (pseudo-)halogen ligand X and increases in the order Cl < NCO < N 3. Especially the azide complex [(babhq)CrN 3 ] displays a high activity, displaying a TOF of 2165 h−1 after 1 h of reaction time along with an induction period of 30 min. At longer reaction times, up to 94% monomer conversion selectively into the respective polycarbonate (pCHC) is observed. [ABSTRACT FROM AUTHOR]