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

The affinity for nucleophile (azide, N-3(-) , and chloride, C- ) binding significantly influences the catalytic activity of chromium complexes for the copolymerization of cyclohexene oxide (CHO) and CO2 center dot The binding of N-3(-) and Cl- to two amino-bis(phenolate) Cr(III) chloride complexes [L1Cr(mu-Cl)](2) (1) and [L2CrCl(THF)] (2 center dot 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][N-3] ([PPN] = bis(triphenylphosphine)iminium), 1 exhibited greater ability to form six-coordinate bis-azide ions [L1Cr(N-3)(2)](-) and produced fewer five-coordinate species detected by MS than 2 center dot THF. This corresponded to 2 center dot THF having a significantly faster rate of polymerization with anionic nucleophile cocatalysts than 1. In the presence of 1 equiv of [PPN][N-3], 1 showed a slow reaction rate with an induction period of 20 min, whereas 2 center dot 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 X 10(3) M-1 favoring the formation of [L1Cr(Cl)(2)](-). The ring-opening process of CHO was observed in the mixture of 2 center dot THF/CHO/[PPN][N-3] via ESI-MS. Combined with MALDI-TOF-MS of the polymer product obtained within the first 60 min, it was shown that N-3(-') preferentially initiates ring-opening of the epoxide in CHO. Upon consumption of N-3(-) 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.