Co(III)/Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-Opening Copolymerization of CO2 and 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 (Đ < 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.


Experimental Section
General Procedures All experimental manipulations were performed using a dual-manifold nitrogen-vacuum Schlenk line or in a nitrogen filled glovebox. All solvents and reagents were obtained from commercial sources and used as received, unless stated otherwise. Acetonitrile, pentane and toluene were obtained from an SPS system, degassed by several freeze-pump-thaw cycles, further dried with 3 Å molecular sieves and stored under N 2 . The epoxide monomers were dried overnight over calcium hydride and purified by fractional distillation, followed by degassing with N2 and stored under N2. Research-grade carbon dioxide was dried by passing it through two drying columns (VICI Metronics carbon dioxide purifier) in series, at 50 bar pressure, before use at lower pressures in the copolymerizations. NMR analysis were performed using a Bruker AV 400 MHz spectrometer, at 298 K, unless stated. MALDI-ToF mass spectrometry was conducted on a Waters MALDI Micro MX in positive ion reflectron mode. The matrix for catalyst analysis was trans-1-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile and for polymers it was 1,8,9-anthracenetriol (dithranol). Gel permeation chromatrography analysis was conducted using a Shimadzu LC-20AD instrument, at 40 °C, with two mixed bed PSS SDV linear S columns in series, and with THF as eluent at a flow rate of 1 mL/min. Molar mass values were calibrated using narrow molar mass polystyrene standards. The macrocycle ligand was synthesized according to a literature procedure (see SI for full details). 1,2 General synthesis of catalysts 1-4: Under a nitrogen atmosphere, the appropriate metal acetate ([M(OAc)], where M = Na, K, Rb or Cs) (1.03 mmol) was added to solution of the ligand (0.40 g, 1.03 mmol) in acetonitrile (15 mL) and the solution was stirred for 30 mins, at 25 °C. Next, Co(OAc)2 (0.18 g, 1.03 mmol) was added and the reaction mixture was stirred for a further 2 h at 25 °C. Ethylene diamine (69 µL, 1.03 mmol) was added, dropwise, to the reaction mixture and it was stirred for 16 h, at 25 °C. The complexes were oxidized by exposure to air and by the addition of 2 equivalents of acetic acid (118 µL, 2.06 mmol) and the solution was stirred for up to 72 h with reaction progress being monitored using 1 H NMR spectroscopy. Once the oxidation was complete, the suspension was filtered and residual solvent volume reduced in vacuo. Excess acetic acid was removed by azeotropic distillation with toluene (3 x 25 mL). The resulting solid was washed with pentane (3 x 50 mL) and dried in vacuo to afford the target complexes as brown solids.  General procedure for PO/CO2 ROCOP A solution of catalyst (13 mg, 0.02 mmol), cyclohexene diol (46 mg, 0.4 mmol) and mesitylene (30 µL. 0.2 mmol, internal standard) in PO (6 mL, 85.7 mmol) was injected into a 100 mL Parr reactor, under a stream of dry CO 2 . The reactor was also fitted with a DiComp sentinel probe, attached to an ATR-IR spectrometer, which allowed for continual monitoring of PPC formation. The reactor was then pressurized with CO2 to the target reaction pressure and allowed to reach the required temperature. Upon reaction completion, the reactor vessel was cooled to room temperature and depressurized with the catalyst quenched by the addition of a 1 M solution of benzoic acid, in CHCl 3 . A sample of the crude reaction mixture was removed for NMR analysis. The polymer was precipitated by dropwise addition into methanol, the solution was decanted and polymer dried in vacuo.

Synthesis of Pro-Ligand:
To a suspension of NaH (2.19 g, 91.1 mmol) in DMSO (10 mL), a solution of 2,3-dihydroxybenzaldehyde (5.72 g, 41.4 mmol) in DMSO (20 mL) was added, dropwise, over the course of 2 h. Triethylene glycol ditosylate (9.72 g, 21.2 mmol) was then added in one portion and the S4 mixture stirred, for 48 h, under N 2 . Water (300 mL) was added, the mixture was washed with CHCl 3 (10 mL) and the organic layer was discarded. The aqueous layer was then acidified to pH 1 (using 6M HCl) and the product extracted with CHCl 3 (3 x 50 mL). The organic layer was washed with HCl (1M, 100 mL) and then dried (MgSO 4 ). The volatiles were removed under vacuum and the crude product was purified by column chromatography (silica gel, CH 2 Cl 2 (2 % methanol)) to give a pale yellow solid (4.40 g, 11.3 mmol, 55 %). 1         in THF, calibrated using narrow-M n polystyrene standards. dispersity in parentheses S14      (Table 3). S17 Figure S22: Illustrations of leading catalyst structures for each epoxide as used for comparison against catalyst 2.

General X-ray crystallography
Single crystal X-ray diffraction data were collected using a Rigaku Oxford Diffraction SuperNova inhouse diffractometer fitted with an Oxford CryoSystems 700 Series CryoStream. 3 Suitable crystals were chosen and mounted on a 200 m MiTeGen loop using perfluoropolyether oil (Fomblin®) at 298 K. Typically, a hemisphere of data was collected to resolution of 0.8 Å at 150 K.
The CrysAlisPro software was used for data collection, as well as peak hunting, indexing reflections in reciprocal space, integration of the raw frames and application of corrections including interframe scaling, Lorentz, flood field and dark current corrections.
The structures were solved using the SHELXT program 4 and least-square refined using the SHELXL program 5 within the Olex2 system suite. 6 The X-ray crystal structure of complex 1 Crystal The asymmetric unit cell contains one CH 2 Cl 2 molecule that was refined anisotropically, as well as additional disordered solvent that could not be satisfactorily modelled. The electron density associated with the disordered solvent (~70 electrons) is likely a disordered pentane molecule which was used during crystallisation, and was removed using the Olex2 BYPASS function.