Vapochromic properties versus metal ion coordination of beta-bispyrazolato- copper(II) coordination polymers: a first-principles investigation

A series of monoadducts of the beta-bispyrazolato-copper(II) one-dimensional coordination polymer is investigated with plane-wave density functional theory-dispersive interactions (DFT-D) calculations. It was found that weak Lewis bases, such as H2O and CH3OH, prefer a symmetric bridging coordination to the Cu(II) ions, which in turn assume a highly distorted 4 + 2 octahedral configuration. Stronger Lewis bases, namely, NH3 and pyridine, prefer instead to bind to a single Cu(II) ion, which adopts a 5-fold coordination in a square-pyramidal environment. A semi-bridging coordination, corresponding to a 5 + 1 distorted octahedral Cu(II) environment, is finally predicted for molecules of intermediate Lewis basicity, such as CH3CN. The soundness of these results is corroborated by a good correlation found between the theoretical coordination number of the Cu(II) ions, the computed spin-down fundamental band gap, and the experimentally observed vapochromic effects.