Abstract
The synthesis of three new dizinc(II) complexes bearing a macrocyclic [2 + 2] Schiff base ligand is reported. The bis(anilido)tetraimine macrocycle reacts with diethylzinc to form a bis(ethyl)dizinc(II) complex, [L(Et)Zn2Et2] (1). The reaction of complex 1 with isopropyl alcohol is reported, forming a bis(isopropyl alkoxide)dizinc complex, [L(Et)Zn2((i)PrO)2] (2). Furthermore, complex 1, with 2 equiv of alcohol, is applied as an initiator for racemic lactide ring-opening polymerization. It shows moderately high activity, resulting in a pseudo-first-order rate coefficient of 9.8 × 10(-3) min(-1), with [LA] = 1 M and [initiator] = 5 mM at 25 °C and in a tetrahydrofuran solvent. Polymerization occurs with good control, as evidenced by the linear fit to a plot of molecular weight versus conversion, the narrow dispersities, and the limited transesterification. The same initiating system is inactive for the ring-opening copolymerization of carbon dioxide (CO2) and cyclohexene oxide at 80 °C and 1 bar of CO2 pressure. However, stoichiometric reactions between complex 2 and CO2, at 1 bar pressure, result in the reversible formation of new dizinc carbonate species, [L(Et)Zn2((i)PrO)((i)PrOCO2)] (3a) and [L(Et)Zn2((i)PrOCO2)2] (3b), and the reaction was studied using density functional theory calculations. All of the new complexes, 1-3b, are fully characterized, including NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.
Highlights
The synthesis of dinuclear zinc(II) complexes is relevant for an understanding of the coordination chemistry and for applications as models for the active sites for various metalloenzymes or in catalysis.[1−7] In polymerization catalysis, dizinc complexes have proven to be highly effective for both ring-opening polymerization (ROP) of lactones and ringopening copolymerization with epoxides.[8−18] Both (ROCOP) processes of carbon dioxide (CO2) are societally relevant because they enable renewable resources to be used in the synthesis of useful, commodity materials
The reaction of the zinc ethyl complex 1 with excess isopropyl alcohol (10 equiv) and 1 bar of CO2 pressure leads to the same carbonate products as those obtained by reaction of the isolated alkoxide derivative 2 with CO2
The stoichiometric reaction between the dizinc bis(alkoxide) derivative and 1 bar of CO2 leads to the evolution of dizinc carbonate moieties (3a and 3b) as established by NMR
Summary
The synthesis of dinuclear zinc(II) complexes is relevant for an understanding of the coordination chemistry and for applications as models for the active sites for various metalloenzymes or in catalysis.[1−7] In polymerization catalysis, dizinc complexes have proven to be highly effective for both ring-opening polymerization (ROP) of lactones and ringopening copolymerization with epoxides.[8−18] Both (ROCOP) processes of carbon dioxide (CO2) are societally relevant because they enable renewable resources to be used in the synthesis of useful, commodity materials. We have investigated various other metal complexes of this macrocycle and shown that high TOF values of ∼5000 h−1 can be obtained using dimagnesium catalysts.[22] A number of researchers tethered together zinc β-diiminate complexes in order to overcome the entropy limitations of dimeric catalysts.[39−43] The geometry and structure of the tethering group were shown to be very important, with a number of well-characterized complexes being reported by various groups as unreactive,[44] while Rieger and co-workers proposed that using flexible tether groups was important.[39,41,42] earlier this year, Rieger and co-workers optimized the design strategy for such tethered β-diiminate catalysts, D (Chart 1), and were able to record a “record-breaking” TOF of 155000 h−1 (30 bar, 100 °C).[39] there is considerable interest and potential for the development of new dizinc catalysts and, in particular, to understand the requirements at both the metal ions and ancillary ligands to prepare highly active and selective catalysts. There is considerable scope for the development of new ligand scaffolds that direct dizinc coordination and that may yield further insight into the features required for successful catalysis
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