Conformationally Flexible Dimeric Salphen Complexes for Bifunctional Catalysis

Appropriate modification of the salphen ligand allows an easy modular design of flexibly linked dimeric salphen species and their complexes, which can act as bifunctional catalysts. A series of chromium salphen systems including monomeric compound and dimers with different spacer lengths were tested for their catalytic performance in β-butyrolactone polymerization and CO2/propylene oxide copolymerization toward biodegradable materials. The results clearly show an enhancement in activity upon dimerization, thus underlining the role of bifunctional catalysis in the studied processes and extending the possible strategies for improvement of catalysts in these reactions

Mechanistic Insights into Heterogeneous Zinc Dicarboxylates and Theoretical Considerations for CO₂–Epoxide Copolymerization

Copolymerization of epoxides and CO2 with heterogeneous zinc dicarboxylates is prominent since the early days of this area of chemistry. However, in over 30 years of research, the efficiency of this catalyst system could not be improved significantly. Furthermore, a huge activity difference between zinc glutarate and its lower homologue zinc succinate exists, which could not be explained so far. A detailed investigation of the underlying copolymerization mechanisms on heterogeneous catalysts is therefore necessary. Such investigations are so far lacking, which renders logical improvements of the catalysts difficult. We therefore decided to conduct a detailed investigation on the different zinc-dicarboxylic catalysts, their copolymerization efficiency, solid state structure and supplemented the results with theoretical calculations. The results imply that the widely discussed bimetallic mechanism (for homogeneous catalysts) is in place for heterogeneous zinc dicarboxylates as well. Theoretical calculations conducted to identify an “ideal” Zn–Zn distance suggest an optimal separation of Zn atoms in the range of 4.3–5.0 Å. The combined copolymerization experiments and calculated models give a consistent explanation for the difference in activity of the different zinc-dicarboxylate catalysts and give a hint why the activity of the heterogeneous zinc-dicarboxylate system is limited

Mechanistic Insights into Heterogeneous Zinc Dicarboxylates and Theoretical Considerations for CO₂–Epoxide Copolymerization

Copolymerization of epoxides and CO2 with heterogeneous zinc dicarboxylates is prominent since the early days of this area of chemistry. However, in over 30 years of research, the efficiency of this catalyst system could not be improved significantly. Furthermore, a huge activity difference between zinc glutarate and its lower homologue zinc succinate exists, which could not be explained so far. A detailed investigation of the underlying copolymerization mechanisms on heterogeneous catalysts is therefore necessary. Such investigations are so far lacking, which renders logical improvements of the catalysts difficult. We therefore decided to conduct a detailed investigation on the different zinc-dicarboxylic catalysts, their copolymerization efficiency, solid state structure and supplemented the results with theoretical calculations. The results imply that the widely discussed bimetallic mechanism (for homogeneous catalysts) is in place for heterogeneous zinc dicarboxylates as well. Theoretical calculations conducted to identify an “ideal” Zn–Zn distance suggest an optimal separation of Zn atoms in the range of 4.3–5.0 Å. The combined copolymerization experiments and calculated models give a consistent explanation for the difference in activity of the different zinc-dicarboxylate catalysts and give a hint why the activity of the heterogeneous zinc-dicarboxylate system is limited

Kinetic and Mechanistic Investigation of Mononuclear and Flexibly Linked Dinuclear Complexes for Copolymerization of CO₂ and Epoxides

Mono- and dinuclear salphen-type complexes were developed and investigated in CO2/epoxide copolymerization reactions. Kinetic investigations indicate that the reaction occurs predominately in a bimetallic fashion in the absence of cocatalysts for both mono- and dinuclear complexes. The dinuclear system, therefore, maintains its activity even under highly diluted conditions of [PO]/[M] = 20000 at which the mononuclear system loses its efficiency. The effect of the nature and amount of added cocatalyst on catalytic performance was investigated as well, indicating a binary propagation mechanism both in mononuclear and dinuclear systems in the presence of cocatalysts