Effect of Azide and Chloride Binding to Diamino-bis(phenolate) Chromium Complexes on CO₂/Cyclohexene Oxide Copolymerization

The affinity for nucleophile (azide, N₃^–, and chloride, Cl^–) binding significantly influences the catalytic activity of chromium complexes for the copolymerization of cyclohexene oxide (CHO) and CO₂. The binding of N₃^– and Cl^– to two amino-bis­(phenolate) Cr­(III) chloride complexes [L1Cr­(μ-Cl)]₂ (1) and [L2CrCl­(THF)] (2·THF), where L1 = methoxy­ethylamino-N,N-bis­(2-methylene-4,6-di-tert-butyl­phenolate) and L2 = dimethyl­amino­ethylamino-N,N-bis­(2-methylene-4,6-di-tert-butyl­phenolate), were studied by both matrix assisted laser desorption/ionization time-of-flight and electrospray ionization mass spectrometry (MALDI-TOF-MS and ESI-MS, respectively). Upon reaction with [PPN]­[N₃] ([PPN] = bis­(triphenyl­phosphine)­iminium), 1 exhibited greater ability to form six-coordinate bis-azide ions [L1Cr­(N₃)₂]⁻ and produced fewer five-coordinate species detected by MS than 2·THF. This corresponded to 2·THF having a significantly faster rate of polymerization with anionic nucleophile cocatalysts than 1. In the presence of 1 equiv of [PPN]­[N₃], 1 showed a slow reaction rate with an induction period of 20 min, whereas 2·THF showed a much faster reaction rate with no induction period. Also, 1 in the presence of 1 equiv of [PPN]­[Cl] showed faster copolymerization than in the presence of 2 equiv of [PPN]­[Cl]. The binding of chloride to 1 from addition of [PPN]­[Cl] was studied by UV–vis spectroscopy, which showed an equilibrium constant of 1.6 × 10³ M^–1 favoring the formation of [L1­Cr­(Cl)₂]⁻. The ring-opening process of CHO was observed in the mixture of 2·THF/​CHO/​[PPN]­[N₃] via ESI-MS. Combined with MALDI-TOF-MS of the polymer product obtained within the first 60 min, it was shown that N₃^– preferentially initiates ring-opening of the epoxide in CHO. Upon consumption of N₃^– through generation of active polymer chains, the chloride originating from the metal complex can serve as initiator of epoxide ring-opening. This was confirmed by the occurrence of chloride-containing polymers later in the reaction. Taken together, these studies provide mechanistic insight into the copolymerization of CO₂ and epoxide by diamino-bis­(phenolate) Cr­(III) chloride complexes.