A series of lithium complexes were prepared from 2(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-Rphenols ([ONN] RR′ ) and characterized through elemental analysis, 1 H and 13 C{ 1 H} NMR spectroscopy, and X-ray crystallography. Treatment of the ligands with n-butyllithium afforded {Li [ONN] RR′ } 3 [R = Me, R′ = t Bu, (1); R = R′ = t Bu (2); R = R′ = t Am, (3), t Am = C(CH 3 ) 2 CH 2 CH 3 ], with trimetallic structures in the solid-state as shown by single-crystal X-ray diffraction. The reactivity of these complexes in the ring-opening polymerization of ε-caprolactone (ε-CL), as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, was studied. Rates of polymerization were first order with respect to both monomer and lithium concentrations, and activation energies for the reactions were determined. MALDI-TOF MS analysis revealed that transesterification had occurred during the polymerization.

Abstract:A series of aluminum methyl and chloride complexes bearing 2(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-R-phenolate or 2(N-morpholinyl)-2-methylene-4-R′-6-R-phenolate ([ONE R1,R2 ]-) {[ R 1 = t Bu, R 2 = Me, E = NMe (L1); R 1 = R 2 = t Bu, E = NMe (L2); R 1 = R 2 = t Bu, 1 E = O (L3)} ligands were synthesized and characterized through elemental analysis, 1 H, 13 The catalytic activity of complexes 1-3 toward ring-opening polymerization (ROP) of ε-caprolactone was assessed. These complexes are more active than analogous Zn complexes for this reaction but less active than the Zn analogs for ROP of rac-lactide. Characteristics of the polymer as well as polymerization kinetics and mechanism were studied. Polymer end-group analyses were achieved using 1 H NMR spectroscopy and MALDI-TOF MS. Eyring analyses were performed and the activation energies for the reactions were determined, which were significantly lower for 1 and 2 compared with 3. This could be for several reasons: (1) the methylamine (NMe) group of 1 and 2, which is a stronger base than the ether (O) group of 3, might activate the incoming monomer via non-covalent interactions, and/or (2) the ether group is able to temporarily coordinate to the metal center and blocks the vacant coordination site towards incoming monomer, whilst the amine cannot do this. Preliminary studies using 4 and 5 towards copolymerization of cyclohexene oxide with carbon dioxide have been performed. 4 was inactive and 5 afforded polyethercarbonate (66.7% epoxide conversion, polymer contains 54.0% carbonate linkages).

A series of zinc complexes was prepared from 4,6‐disubstituted (R,R′) o‐[(4‐methyl‐1‐piperazinyl)methyl]phenols ([ONNR′,R]H) and characterized by elemental analysis, 1H and 13C{1H} NMR spectroscopy, and X‐ray crystallography. Reaction of these products with diethylzinc gave Zn[ONNtBu,tBu]2 (1) as a monometallic complex and {[μ‐ONNR′,R]ZnEt}2 [R′ = Me, R = tBu, (2); R′ = R = tBu (3); R′ = R = tAm (4)] bimetallic species that have distorted tetrahedral environments about the Zn centres. Reaction of 3 and 4 with alcohols gave {[ONNtAm,tAm]Zn(μ‐OR″)}2 [R″ = Bn (5); R″ = Et (6)] bimetallic species in which the Zn centres are bridged by benzyl alkoxide and ethoxide groups, respectively. A morpholinyl derived ligand was also synthesized and characterized (L4H), and its 1:1 stoichiometric reaction with ZnEt2 resulted in complex 7, {[μ‐ONOtBu,tBu]ZnEt}2. The reactivity of complexes 2–7 in the ring‐opening polymerization of rac‐lactide (LA) and ϵ‐caprolactone (ϵ‐CL) was studied. Reactions of carbon dioxide with cyclohexene oxide in the presence of 6 or 7/ROH (R = Bn, Et) afforded cyclohexene carbonate.

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Lithium and sodium compounds supported by tetradentate amino-bis(phenolato) ligands, [Li2(N2O2(BuBuPip))] (1), [Na2(N2O2(BuBuPip))] (2) (where [N2O2(BuBuPip)] = 2,2'-N,N'-homopiperazinyl-bis(2-methylene-4,6-tert-butylphenol), and [Li2(N2O2(BuMePip))] (3), [Na2(N2O2(BuMePip))] (4) (where [N2O2(BuMePip)] = 2,2'-N,N'-homopiperazinyl-bis(2-methylene-4-methyl-6-tert-butylphenol) were synthesized and characterized by NMR spectroscopy and MALDI-TOF mass spectrometry. Variable temperature NMR experiments were performed to understand solution-phase dynamics. The solid-state structures of 1 and 4 were determined by X-ray diffraction and reveal tetrametallic species. PGSE NMR spectroscopic data suggests that 1 maintains its aggregated structure in CD2Cl2. The complexes exhibit good activity for controlled ring-opening polymerization of rac-lactide (LA) both solvent free and in solution to yield PLA with low dispersities. Stoichiometric reactions suggest that the formation of PLA may proceed by the typical coordination-insertion mechanism. For example, (7)Li NMR experiments show growth of a new resonance when 1 is mixed with 1 equiv. LA and (1)H NMR data suggests formation of a Li-alkoxide species upon reaction of 1 with BnOH.

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The cover picture shows a butterfly‐shaped dimetallic alkylzinc complex reported by researchers at Memorial University, which is shown in the background. The complex, reported in the article by F. M. Kerton et al. on p. 5347 ff, can be readily converted to an alkoxide species. This initiates ring‐opening polymerization of rac‐lactide and ϵ‐caprolactone under conventional heating or upon microwave irradiation. Financial support from the Publication Subvention Program of Memorial University of Newfoundland is acknowledged.