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Relevant academic research and scientific papers

Co(III)/Alkali-Metal(I) heterodinuclear catalysts for the ring-opening copolymerization of CO2and propylene oxide

The ring-opening copolymerization of carbon dioxide and propene oxide is a useful means to valorize waste into commercially attractive poly(propylene carbonate) (PPC) polyols. The reaction is limited by low catalytic activities, poor tolerance to a large excess of chain transfer agent, and tendency to form byproducts. Here, a series of new catalysts are reported that comprise heterodinuclear Co(III)/M(I) macrocyclic complexes (where M(I) = Group 1 metal). These catalysts show highly efficient production of PPC polyols, outstanding yields (turnover numbers), quantitative carbon dioxide uptake (>99%), and high selectivity for polyol formation (>95%). The most active, a Co(III)/K(I) complex, shows a turnover frequency of 800 h-1 at low catalyst loading (0.025 mol %, 70 °C, 30 bar CO2). The copolymerizations are well controlled and produce hydroxyl telechelic PPC with predictable molar masses and narrow dispersity (D 1.15). The polymerization kinetics show a second order rate law, first order in both propylene oxide and catalyst concentrations, and zeroth order in CO2 pressure. An Eyring analysis, examining the effect of temperature on the propagation rate coefficient (kp), reveals the transition state barrier for polycarbonate formation: ΔG? = +92.6 ± 2.5 kJ mol-1. The Co(III)/K(I) catalyst is also highly active and selective in copolymerizations of other epoxides with carbon dioxide.

Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations

A series heterodinuclear catalysts, operating without co-catalyst, show good performances for the ring opening copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide. The complexes feature a macrocyclic ligand designed to coordinate metals such as Zn(II), Mg(II) or Co(III), in a Schiff base ‘pocket’, and Na(I) in a modified crown-ether binding ‘pocket’. The 11 new catalysts are used to explore the influences of the metal combinations and ligand backbones over catalytic activity and selectivity. The highest performance catalyst features the Co(III)Na(I) combination, [N,N′-bis(3,3’-triethylene glycol salicylidene)-1,2-ethylenediamino cobalt(III) di(acetate)]sodium (7), and it shows both excellent activity and selectivity at 1 bar carbon dioxide pressure (TOF=1590 h?1, >99 % polymer selectivity, 1 : 10: 4000, 100 °C), as well as high activity at higher carbon dioxide pressure (TOF=4343 h?1, 20 bar, 1 : 10 : 25000). Its rate law shows a first order dependence on both catalyst and cyclohexene oxide concentrations and a zeroth order for carbon dioxide pressure, over the range 10–40 bar. These new catalysts eliminate any need for ionic or Lewis base co-catalyst and instead exploit the coordination of earth-abundant and inexpensive Na(I) adjacent to a second metal to deliver efficient catalysis. They highlight the potential for well-designed ancillary ligands and inexpensive Group 1 metals to deliver high performance heterodinuclear catalysts for carbon dioxide copolymerizations and, in future, these catalysts may also show promise in other alternating copolymerization and carbon dioxide utilizations.

Selective Binding and Quantitation of Calcium with a Cobalt-Based Magnetic Resonance Probe

We report a cobalt-based paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance (MR) probe that is able to selectively bind and quantitate the concentration of Ca2+ ions under physiological conditions. The parent LCo complex features CEST-active carboxamide groups and an uncoordinated crown ether moiety in close proximity to a high-spin pseudo-octahedral CoII center. Addition of Na+, Mg2+, K+, and Ca2+ leads to binding of these metal ions within the crown ether. Single-crystal X-ray diffraction and solid-state magnetic measurements reveal the presence of a cation-specific coordination environment and magnetic anisotropy of CoII, with axial zero-field splitting parameters for the Na+- A nd Ca2+-bound complexes differing by over 90%. Owing to these differences, solution-based measurements under physiological conditions indicate reversible binding of Na+ and Ca2+ to give well-separated CEST peaks at 69 and 80 ppm for [LCoNa]+ and [LCoCa]2+, respectively. Dissociation constants for different cation-bound complexes of LCo, as determined by 1H NMR spectroscopy, demonstrate high selectivity toward Ca2+. This finding, in conjunction with the large excess of Na+ in physiological environments, minimizes interference from related cations, such as Mg2+ and K+. Finally, variable-[Ca2+] CEST spectra establish the ratio between the CEST peak intensities for the Ca2+- A nd Na+-bound probes (CEST80 ppm/CEST69 ppm) as a measure of [Ca2+], providing the first example of a ratiometric quantitation of Ca2+ concentration using PARACEST. Taken together, these results demonstrate the ability of transition metal PARACEST probes to afford a concentration-independent measure of [Ca2+] and provide a new approach for designing MR probes for cation sensing.

Trivalent Lewis Acidic Cations Govern the Electronic Properties and Stability of Heterobimetallic Complexes of Nickel

Assembly of heterobimetallic complexes is synthetically challenging due to the propensity of ditopic ligands to bind metals unselectively. Here, we employ a novel divergent approach for selective preparation of a variety of bimetallic complexes within a ditopic macrocyclic ligand platform. In our approach, nickel is readily coordinated to a Schiff base cavity, and then a range of redox-inactive cations (M=Na+, Ca2+, Nd3+, and Y3+) are installed in a pendant crown-ether-like site. This modular strategy allows access to complexes with the highly Lewis acidic trivalent cations Nd3+ and Y3+, a class of compounds that were previously inaccessible. Spectroscopic and electrochemical studies reveal wide variations in properties that are governed most strongly by the trivalent cations. Exposure to dimethylformamide drives loss of Nd3+ and Y3+ from the pendant crown-ether site, suggesting solvent effects must be carefully considered in future applications involving use of highly Lewis acidic metals.