Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO₂/epoxide copolymerization and cyclic ester polymerization

Abstract

The preparation of heterodinuclear complexes, especially those comprising early-late transition metals coordinated by a simple or symmetrical ancillary ligand, represents a fundamental challenge and an opportunity to prepare catalysts benefitting from synergic properties. Here, two new mixed titanium(iv)-zinc(ii) complexes, [LTi(OPr)ZnEt] and [LTi(OPr)ZnPh], both coordinated by a diphenolate tetra(amine) macrocyclic ligand (L), are prepared. The synthesis benefits from the discovery that reaction of the ligand with a single equivalent of titanium tetrakis(iso-propoxide) allows the efficient formation of a mono-Ti(iv) complex, [LTi(OPr)]. All new complexes are characterized by a combination of single crystal X-ray diffraction, multinuclear NMR spectroscopy and mass spectrometry techniques. The two heterobimetallic complexes, [LTi(OPr)ZnEt] and [LTi(OPr)ZnPh], feature trianionic coordination by the macrocyclic ligand and bridging alkoxide groups coordinate to both the different metal centres. The heterodinuclear catalysts are compared to the mono-titanium analogue, [LTi(OPr)], in various polymerization reactions. In the alternating copolymerizations of carbon dioxide and cyclohexene oxide, the mono-titanium complex is totally inactive whilst the heterodinuclear complexes show moderate activity (TOF = 3 h); it should be noted the activity is measured using just 1 bar pressure of carbon dioxide. In the ring opening polymerization of lactide and ε-caprolactone, the mono-Ti(iv) complex is totally inactive whilst the heterodinuclear complexes show moderate-high activities, qualified by comparison to other known titanium polymerization catalysts (l-lactide, k = 11 × 10 s at 70 °C, 1 M in [lactide]) and ε-caprolactone (k = 5 × 10 s at 70 °C, 0.9 M in [ε-caprolactone]).

Heterodinuclear zinc and magnesium catalysts for epoxide/CO2 ring opening copolymerizations

“…34 Recently some other hetero-catalysts have been reported including a series of zinc tris(lanthanide) complexes some of which show excellent activities at 10 bar pressure. [35][36][37] In general, there are very few reports of the synthesis or properties of well-dened heterodinuclear Zn(II)/Mg(II) complexes in the literature, perhaps because these metals are known to be labile, similarly sized and to show equivalent coordination chemistry. [38][39][40][41] In order to properly compare and understand the signicance of heterodinuclear synergic relationships it is important to use symmetrical ligands but this provides a further difficulty in terms of complex synthesis.…”

Groups 1, 2 and Zn(II) Heterodinuclear Catalysts for Epoxide/CO2 Ring-Opening Copolymerization

“…Next, the sequential metalation synthesis was investigated, using the previously reported monozincation strategy which involved reaction of the ligand with diethyl zinc to generate a mono-zinc intermediate. 19 This intermediate was not isolated but was reacted in situ with the appropriate metal (I) iodide precursor, in THF at 100 °C for 16 hours. In each case, the 1 H and 2D NMR spectra are identical to those obtained by the metal redistribution synthesis ( Fig.…”

“…[52][53] Building from the mechanistic hypothesis, a Zn(II)/Mg(II) heterodinuclear catalyst showed a 5-fold higher rate than either di-zinc or dimagnesium counterparts, or combinations of them. 19 This promising result highlights the potential to increasing performance using heterodinuclear complexes but so far there are very few examples of such complexes.…”

“…[1][2] Synergic interactions between two different metals has been proposed as responsible for enhanced activity in homogeneous catalysis ranging from metallation, 3 CH bond activation, [4][5][6] and CF bond activation, [7][8][9] lactone ring opening polymerization (ROP), [10][11][12] olefin polymerization [13][14][15][16][17] and, relevant to this work, the ring opening copolymerization (ROCOP) of CO2 and epoxides. [18][19][20][21][22] Here, new heterodinuclear complexes are applied as catalysts for the ring opening copolymerization (ROCOP) of epoxides with CO2. The product aliphatic polycarbonates contain from 30-50% carbon dioxide in the polymer backbone; the reaction is an efficient method to addvalue to waste CO2 to produce a commercially viable material.…”

The highly surprising behaviour of diphosphine ligands in iron-catalysed Negishi cross-coupling

“…On the other hand, the minor species proved to be an arylzinc adduct of a neutral iron dibromide (23). Such arylzinc-transition metal adducts are very rare, [59][60][61] and its formation and stability with respect to decomposition to Fe(0), even under forcing conditions, demonstrates that transmetallation from the zinc to neutral, let alone anionic iron centres is not at all facile.…”