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(pz^tBu₂)₃ reversibly inserts two molecules of CO₂ to afford Al(CO₂·pz^tBu₂)₂(pz^tBu₂), exhibiting CO₂ release only at elevated temperatures (>100 °C). In contrast, donor-stabilised Sc(pz^tBu₂)₃(thf) forms mono-inserted species [Sc(μ-CO₂·pz^tBu₂)(pz^tBu₂)₂]₂, which already releases CO₂ at ambient temperature and pressure and hence is isolable only at low temperature. For the yttrium complex Y(pz^tBu₂)₃(thf)₂, insertion of CO₂ is not observable at ambient temperature. The new homoleptic aluminium diisopropyl pyrazolate complex [Al(pz^iPr₂)₃]₂ shows exhaustive CO₂ insertion, while dimethyl pyrazolate could be isolated as the separated ion pair [Al(N,N′,N′′-Al{pz^Me₂}₃Me)₂][Al(pz^Me₂)₃Me]. The scandium complex Sc(pz^tBu₂)₃(thf) performed best in the catalytic cycloaddition reaction of CO₂ and epoxides, unveiling an inverse correlation of carboxophilicity ( CO₂ affinity) and catalytic activity. Carboxophilicity is assessed using CO₂-release temperature (via VT ¹H NMR spectroscopy and thermogravimetric analysis).