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 CO 2 instantly and quantitatively forming the tetrameric complex [Mg(CO 2 ·pz t Bu, t Bu ) 2 ] 4 and monomeric donor-stabilized [Mg(CO 2 ·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 CO 2 , the resulting [Mg(CO 2 ·pz) 2 ] m featuring a CO 2 capacity of 35.7 wt% (8.2 mmol g -1 ). All described magnesium pyrazolates display completely reversible CO 2 uptake in solution and in the solid state, respectively, as monitored via VT 1 H NMR and in situ FTIR spectroscopy as well as thermogravimetric analysis. Fluorinated [Mg 2 (pz CF3,CF3 ) 4 (thf) 3 ] does not yield any isolable CO 2 insertion product but exhibits the highest activity in the catalytic transformation of epoxides and CO 2 to cyclic carbonates.