Carbon dioxide affinity (“carboxophilicity”) of trivalent light metal pyrazolates

Abstract

Trivalent group 3 and 13 light metal pyrazolates were synthesised and their reactivity towards CO2 was investigated. The homoleptic complex Al(pztBu2)3 inserts reversibly two molecules of CO2 to afford Al(CO2·pztBu2)2(pztBu2), exhibiting CO2 release only at elevated temperatures (>100 °C). In contrast, donor-stabilised Sc(pztBu2)3(thf) forms the mono-inserted species [Sc(μ-CO2·pztBu2)(pztBu2)2]2, which already releases CO2 at ambient temperature and pressure, and hence, is isolable only at low temperature. For yttrium complex Y(pztBu2)3(thf)2, insertion of CO2 is not observable at ambient temperature. The new homoleptic aluminium diisopropyl pyrazolate [Al(pziPr2)3]2 shows exhaustive CO2 insertion, while the dimethyl pyrazolate could be isolated only as the separated ion pair [Al(N,N’,N’’-Al{pzMe2}3Me)2][Al(pzMe2)3Me]. The scandium complex Sc(pztBu2)3(thf) performed best in the catalytic cycloaddition reaction of CO2 and epoxides, unveiling an inverse correlation of carboxophilicity (=CO2 affinity) and catalytic activity. The carboxophilicity is assessed by the CO2-release temperature (via VT 1H NMR spectroscopy and thermogravimetric analysis).