Ti(IV)–Tris(phenolate) Catalyst Systems for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide

Abstract

Titanium(IV) complexes of amino-tris(phenolate) ligands (LTiX, X = chloride, isopropoxide) together with bis-(triphenylphosphine)iminium chloride (PPNCl) are active catalyst systems for the ring-opening copolymerization of carbon dioxide and cyclohexene oxide. They show moderate activity, with turnover frequency values of ∼60 h −1 (0.02 mol % of catalyst, 80 °C, 40 bar of CO 2 ) and high selectivity (carbonate linkages >90%), but their absolute performances are lower than those of the most active Ti(IV) catalyst systems. The reactions proceed with linear evolution of polycarbonate (PCHC) molar mass with epoxide conversion, consistent with controlled polymerizations, and evolve bimodal molar mass distributions of PCHC (up to M n = 42 kg mol −1 ). The stoichiometric reaction between [LTiO i Pr] and tetraphenylphosphonium chloride, PPh 4 Cl, allows isolation of the putative catalytic intermediate [LTi(O i Pr)Cl] − , which is characterized using single-crystal X-ray diffraction techniques. The anionic titanium complex [LTi(OR)Cl] − is proposed as a model for the propagating alkoxide intermediates in the catalytic cycle.

Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

“…The trend in catalytic activity ( 1c > 1b-THF > 1a ) might originate from the inherent stability of the complexes and might also, at least in part, originate from the lability of the THF molecule and the ease with which this ligand is displaced to form the putative anionic six-coordinate intermediate upon activation with [PPN]Cl, as previously shown with the anionic catalysts [([ κ 3 - O,C,O]- Is NHC)HfCl 3 ] − [ 45 ] and [([ κ 4 - N,O,O,O]-ATP)TiCl(O i Pr)] − (ATP = amino-tris(phenolate)) [ 62 ]. The more active catalysts 1c and 1b-THF have, according to SCXRD and DFT, longer Ti-THF bonds than 1a , which indicate a weakly bonded THF ligand and a higher rate of formation of the active species and/or, by analogy to other catalytic systems [ 46 , 47 , 48 , 49 , 50 , 52 , 81 ], a faster dissociation of the growing polymer chains during the copolymerization.…”

“…Although most of the highly selective and active catalyst systems for the copolymerization of epoxides with CO 2 are based on divalent (Mg, Co, Zn) and trivalent (Cr, Co, Al) metals bearing ligands such as β-diketiminates, salens, porphyrins, and multidentate phenolate macrocycles [ 46 , 47 , 48 , 49 , 50 , 51 , 52 ], the group 4 metals have emerged as a new class of catalyst for this reaction, combining decent catalytic activity with high selectivity towards the formation of polycarbonates [ 41 , 42 , 43 , 44 , 45 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]. NHC-based catalysts of this kind have appeared to be particularly promising under relatively mild copolymerization conditions [ 41 , 42 , 43 , 44 , 45 ].…”

Conversion of CO2 into organic carbonates: a venture into catalysis, reaction design and product modification

“…39,40 A wide variety of homogeneous and heterogeneous catalytic systems has been developed for the reaction of CO2 with epoxides, with the purpose of increasing the activity but also of maximising the selectivity towards either the cyclic carbonate or the polycarbonate product. Numerous excellent reviews exist on the catalysis of CO2/epoxide reactions, 3,13,14,24,41,42 including few focussing only on the synthesis of cyclic carbonates 16,[43][44][45][46][47][48][49][50] or only on polycarbonates. 2,4,5,15,17,[51][52][53][54][55] The reader interested in a detailed overview of the numerous existing catalytic systems and/or in the various proposed reaction mechanisms is kindly referred to the abovementioned references.…”

“…105 In these systems, the protic functional groups can activate the epoxide substrate by interacting with its O atom through hydrogen bonding. 42,48,65,106 Table 1.5. Performance of selected homogeneous metal-free catalytic systems in the reaction of CO2 with epoxides a Catalytic loading in molar percentage of epoxide substrate, based on moles of Lewis acid sites/based on moles of nucleophilic species added separately.…”

“…In line with previous reports and based on our results (vide infra), we propose that during the catalytic cycle these complexes adopt a conformation in which the axial ligand remains coordinated to the metal centre. 48 A wide range of metal complexes in which the metal centre acts as Lewis acid active site have been studied, with the most common metals being zinc, chromium, cobalt, magnesium, aluminium and iron. [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63] Together with the metal species, the nature of the organic ligand plays a key role in determining the activity of the complex and the selectivity towards either the cyclic or the polymeric carbonate.…”