Organomagnesia: Reversibly High Carbon Dioxide Uptake by Magnesium Pyrazolates

A series of new pyrazolate and mixed pyrazolate/pyrazole magnesium complexes is described and their reactivity toward carbon dioxide is examined. The dimeric complex [Mg(pz^{t Bu, t Bu})₂]₂ inserts CO₂ instantly and quantitatively forming the tetrameric complex [Mg(CO₂·pz^{t Bu, t Bu})₂]₄ and monomeric donor-stabilized [Mg(CO₂·pz^{t Bu, t Bu})₂(thf)₂]. Complexes of the type [Mg_x(pz^R,R)_{2 x}(Hpz^R,R)_y]_n (R = iPr, tBu) engage in similar insertion reactions involving dissociation of the carbamic acid HOOCpz^R,R. Even solid polymeric derivatives [Mg(pz^R,R)₂]_n (R = Me, H) react instantaneously and exhaustively with CO₂, the resulting [Mg(CO₂·pz)₂]_m featuring a CO₂ capacity of 35.7 wt% (8.2 mmol g^-1). All described magnesium pyrazolates display completely reversible CO₂ uptake in solution and in the solid state, respectively, as monitored via VT ¹H NMR and in situ FTIR spectroscopy as well as thermogravimetric analysis. Fluorinated [Mg₂(pz^CF3,CF3)₄(thf)₃] does not yield any isolable CO₂ insertion product but exhibits the highest activity in the catalytic transformation of epoxides and CO₂ to cyclic carbonates.