The effect of anions in the synthesis and structure of pyrazolylamidino copper(II) complexes.

Six new pyrazolylamidino Cu(II) complexes are synthesized directly from the reactions of Cu(X)2 salts (X = ClO4-, BF4-, or Cl-) and pyrazole (pzH) in nitrile solution (RCN, R = Me or Et) at 298 K via the metal-mediated coupling of RCN with pzH: [Cu(HNC(R)pz)2(X)2] (X = ClO4- or BF4-, R = Me, 1 or 7 and Et, 2 or 8, respectively) and dichloro Cu(II) complexes [Cu2Cl2(µ-Cl)2(HNC(Me)pz)2] (3) and [CuCl2(HNC(Et)pz)] (4). Four more new complexes, [Cu2(µ-Cl)2(HNC(Me)pz)2(pzH)2][X]2 (X = ClO4-, 5 and BF4-, 9) and [Cu2(µ-Cl)2(HNC(Et)pz)2(pzH)2(X)2] (X = ClO4-, 6 and BF4-, 10), are obtained indirectly from the anion substitution reaction with Cl- ions in 1 and 7, and 2 and 8, respectively. All complexes are characterized by EA, FTIR, UV-vis and EPR spectroscopy and X-ray crystallographic analyses. HNC(Et)pz or pzH is unobserved in both the nitrile-exchange reaction of 2 to d6-1 and the anion-substitution reaction of 2 to d6-5 in the CD3CN solution. The 1H NMR results reveal that the pzH-RCN coupling is intramolecular and reversible on a Cu(II) center. The crystal structures of these complexes show diverse supramolecular assemblies through imino NH⋯anion hydrogen bonds and pyrazolylamidino pz-pz (π⋯π) and pz-Cu(II) (π⋯metal) interactions. EPR results suggest weak magnetic couplings between Cu(II) centers in the polynuclear Cu(II) complexes. The yield and rate of the formation of 1 are higher in the reaction of Cu(ClO4)2 with a 4-fold molar excess of pzH compared with a 2-fold excess, indicating that [Cu(pzH)4]2+ is the more active species for pzH-RCN coupling. The highest rate for the formation of 1 is achieved when [Cu(pzH)4(ClO4)2] is used in MeCN solution. Thus, a plausible synthetic path for synthesizing pyrazolylamidino Cu(II) complexes is established. An intermediate species, [Cu(HNC(Me)pz)2(pzH)2][ClO4]2 (1a), is proposed for the synthetic process based on spectroscopic studies and DFT calculations. The reaction of [Cu(pzH)4X2] (X = ClO4-, Cl-, NO3-, or BF4-) in MeCN solution suggests that the lability of coordinated anions upon nitrile substitution affects the rate of the formation of bis-pyrazolylamidino Cu(II) complexes.