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 center dot center dot center dot 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 pi-pi stacking and edge-to-face CH center dot center dot center dot pi bonding interactions. In addition, a mechanism based on anion-mediated/hydrogen-bonding-driven proton transfer for the coupling reaction is proposed.