Poly(pyrazolyl)aluminate Complexes Containing Aluminum–Hydrogen Bonds

The treatment of LiAlH₄ with 2, 3, or 4 equiv of the 3,5-disubstituted pyrazoles Ph₂pzH or iPr₂pzH afforded [Li(THF)₂][AlH₂(Ph₂pz)₂] (97%), [Li(THF)][AlH(Ph₂pz)₃] (96%), [Li(THF)₄][Al(Ph₂pz)₄] (95%), and [Li(THF)][AlH(iPr₂pz)₃] (89%). The treatment of ZnCl₂ with [Li(THF)][AlH(Ph₂pz)₃] afforded Zn(AlH(Ph₂Pz)₃)H (70%). X-ray crystal structures of these complexes demonstrated κ² or κ³ coordination of the aluminum-based ligands to the Li or Zn ions. The treatment of [Li(THF)][AlH(Ph₂pz)₃] with MgBr₂ or CoCl₂ in THF/Et₂O solutions, by contrast, afforded the pyrazolate transfer products Mg₂Br₂(Ph₂pz)₂(THF)₃·2THF (25%) and Co₂Cl₂(Ph₂pz)₂(THF)₃·THF (23%) as colorless and blue crystalline solids, respectively. An analogous treatment of [Li(THF)][AlH(Ph₂pz)₃] with MCl₂ (M = Mn, Fe, Ni, Cu) afforded metal powders and H₂, illustrating hydride transfer from Al to M as a competing reaction path.