Non-Heme Manganese Catalysts for On-Demand Production of Chlorine Dioxide in Water and Under Mild Conditions

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn^III(OH) species as the dominant form under catalytic conditions. A Mn^III(μ-O)­Mn^IV dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn^IV(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn^IV(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO₂ product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide

Quantitative Comparative Kinetics of 1‑Hexene Polymerization across Group IV Bis-Phenolate Catalysts

The kinetics of 1-hexene polymerization using a series of three Ti amine bis-phenolate catalysts, Ti­[^tBu-ON^XO]­Bn₂ (X = THF (<b>1</b>), pyridine (<b>2</b>), NMe₂ (<b>3</b>)), were investigated and compared to analogous Zr and Hf complexes. A model-based approach using a diverse set of data (including monomer consumption, molecular weight evolution, etc.) was employed to determine the reaction specific rate constants of the simplest mechanism. These catalysts exhibited similar mechanisms that include the elementary reaction steps of initiation, propagation via 1,2-insertion, misinsertion via 2,1-insertion, recovery from misinsertion by 1,2-insertion, and monomer independent chain transfer. Rate constants of the Ti catalysts are typically lower than those of the Hf and the Zr catalysts by 1 and 2 orders of magnitude, respectively. The percentage of regioerrors follows the trend of Ti > Hf > Zr for catalyst <b>1</b> while the trend of Ti > Zr > Hf occurs for catalysts <b>2</b> and <b>3</b>. The ratio of the propagation rate to the termination rate at a constant monomer concentration exhibits the trend Zr > Ti > Hf for catalysts with the same X. This relationship was developed further by computing <i>M</i>_n values from the determined rate constants under fixed reaction conditions. A quantitative structure–function relationship, similar to that found previously for Zr and Hf ,is observed between the logarithm of the chain transfer rate constant and the Ti–X bond distance. These findings underscore the importance of comprehensive quantitative kinetic modeling in establishing structure–function relationships

Comparison of Selected Zirconium and Hafnium Amine Bis(phenolate) Catalysts for 1‑Hexene Polymerization

The kinetics of 1-hexene polymerization using a family of three zirconium and hafnium amine bis-phenolate catalysts, M­[<i>t</i>-Bu-ON^XO]­Bn₂ (where M = Zr (<b>a</b>) or Hf (<b>b</b>), and X = THF (<b>1</b>), pyridine (<b>2</b>), or NMe₂ (<b>3</b>)), have been investigated to uncover the mechanistic effect of varying the metal center M. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction-specific rate constants involved in a given polymerization process. This study builds upon the mechanism of polymerization for <b>1a</b>–<b>3a</b>, which has been previously reported by applying the same methodology to the hafnium containing analogues, <b>1b</b>–<b>3b</b>. It has been observed that each elementary step-specific rate constant that involves the insertion of a monomer is reduced by an order of magnitude. As previously reported for catalysts <b>1a</b>–<b>3a</b>, a quantitative structure–activity relationship was uncovered between the logarithm of the monomer-independent chain transfer rate constants and the Hf–X bond distance for catalysts <b>1b</b>–<b>3b</b>. However, this dependence on the pendant ligand is 2.7 times weaker for the Hf-containing analogues versus those containing Zr. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure–activity relationships

Kinetic Modeling of 1-Hexene Polymerization Catalyzed by Zr(<i>t</i>Bu-ON^NMe₂O)Bn₂/B(C₆F₅)₃

Kinetic modeling using a population balance approach has been performed in order to identify a mechanism and a set of rate constants that describe the batch polymerization of 1-hexene by the homogeneous single-site catalyst Zr­(<i>t</i>Bu-ON^NMe₂O)­Bn₂ activated by B­(C₆F₅)₃ in toluene. The mechanism and rate constants were determined by making use of a multiresponse data set, including (i) monomer concentration versus time for various initial concentrations of monomer and catalyst, (ii) the time evolution of the molecular weight distribution, (iii) active site concentrations versus time, and (iv) vinyl end group concentrations versus time. The overall mechanism requires slow chain initiation compared to propagation, 2,1-misinsertion and recovery, and two chain transfer pathwaysone forming vinylidene end groups and the other forming vinylene end groups. The quantitative analysis of kinetic data clearly shows that a significant fraction of the catalyst does not participate in the chain growth process. The quantitative analysis is carefully detailed to provide a general procedure for kinetic model discrimination and the assignment of rate constants that can be used for other single-site catalysts

Interaction between Two Active Sites of the Same Catalyst for Macromonomer Enchained Olefin Polymerization

A zirconium amine bis­(phenolate) catalyst that is capable of simultaneously producing both oligomers and polymers of 1-hexene was investigated. It was found that the polymer produced has more branching than the commonly encountered poly­(1-hexene), suggesting an oligomer macromonomer enchainment. The polymer weight fraction and molecular weight are weakly dependent on conversion and are not affected by increasing the concentration of free oligomer macromonomers in solution. After considering a number of possible mechanisms, the most plausible supported by spectroscopic and kinetic data is a second-order reaction between an oligomer forming site and a polymer forming site, resulting in the transfer of oligomer macromonomers into the growing polymer chain. The ratio between oligomer and polymer products can be precisely tuned by varying the precatalyst activation conditions