Ti and Zr amino-tris(phenolate) catalysts for the synthesis of cyclic carbonates from CO2 and epoxides

Herein, we report the application of four amino-tris(phenolate)-based metal complexes incorporating Ti(IV) or
Zr(IV) centres (2a-3b) as homogeneous catalysts for the conversion of CO2 and epoxides into cyclic carbonates.
The four complexes were synthesised, characterised and then evaluated in combination with tetrabutylammo-
nium iodide, bromide or chloride as binary catalytic systems for the reaction of CO2 with 1,2-epoxyhexane as
epoxide substrate at 12 bar CO 2 pressure and 90 C for 2 h. The catalytic systems comprising the two Ti(IV)
complexes (2a and 2b) showed similar performance. One notable exception was the catalytic system consisting of
titanium complex 2b, bearing an axial Cl-ligand, and tetrabutylammonium chloride, which displayed higher
catalytic activity compared to other titanium-based systems. Even higher activity was achieved with Zr(IV)
complex 3a, bearing an axial isopropoxide ligand, which reached turnover numbers (TON metal) up to 1920 for the
reaction of CO2 with 1,2-epoxyhexane at 12 bar CO2 pressure and 90 C for 2 h. This performance is comparable
with that of state-of-the-art catalysts for this reaction. The catalytic system consisting of complex 3a and tetra-
butylammonium bromide was explored further by investigating its applicability with a broad substrate scope,
achieving quantitative conversion of several epoxides with CO 2 into cyclic carbonate products at 90 C and 12 bar
CO 2 pressure for 18 h. The selectivity towards the cyclic carbonate products was 98% for all studied terminal
epoxides and 80% for all examined cyclohexene-type epoxides.