Effects of the Counteranion on the Pyrazole−Nitrile Coupling Reaction Mediated by Nickel(II) Ions

The reaction of Ni(ClO4)2 with 4 equiv of pyrazole (pzH) in MeCN leads to [Ni(pzH)2(HNC(Me)pz)2](ClO4)2 via the intermediate [Ni(MeCN)2(pzH)4](ClO4)2, which is the first pyrazole−nitrile coupling reaction activated by a Ni(II) complex. The reaction is also successfully conducted with other nitriles RCN bearing an electron-donating group (R = Et, Bz) and is counteranion-specific. The reaction proceeds more quickly with NO3− as counteranion than with ClO4−, and BF4− is found to be unsatisfactory for the coupling reaction. As for the X-ray crystal structures, [Ni(MeCN)2(pzH)4](ClO4)2 possesses stronger pyrazolyl NH···anion hydrogen-bonding interactions than does [Ni(MeCN)2(pzH)4](BF4)2. The same trend is also observed by solution IR (pyrazolyl NH) analysis. The supramolecular structure of the complex [Ni(pzH)2(HNC(Me)pz)2](ClO4)2 displays a two-dimensional network dominated by cooperative face-to-face π−π stacking and edge-to-face CH···π bonding interactions. In addition, a mechanism based on anion-mediated/hydrogen-bonding-driven proton transfer for the coupling reaction is proposed.