Abstract/Summary

The 1 : 1 condensate of benzil and 2-hydrazinopyridine is the ligand (LH; H: a dissociable proton) here. Its reaction with CuCl2·2H2O in methanol at room temperature in equimolar proportion affords a mixture of two types of dark green (with metallic luster) single crystals—hexagonal (1a) and rectangular (1b). They are separated mechanically. The yield of 1a is higher. X-ray crystallography shows that 1a and 1b are penta-coordinate, dichloro-bridged dimers of the type Cu2L2Cl2 with very similar centrosymmetric structures. All the bonding parameters except for two mutually dependent bond angles in the N2OCl2 coordination sphere of Cu(II) are the same. Correspondingly, two different minima are located in DFT calculations on 1a and 1b. Energetically 1b is more stable than 1a in the gas phase by 3–4 kcal mol−1. Their X-band EPR spectra in the solid state at 77 K, which are axial, reveal that (dx2−y2)1 is the ground state in 1a (g∥ > g⊥) and (dz2)1 in 1b (g∥ < g⊥). In keeping with Ostwald ripening, the energetically less stable isomer 1a crystallizes first. As the crystallization time is allowed to be longer, more of 1b is formed. The transformation of 1a to 1b in methanol solution is found to follow the kinetics of a zero order reaction. The reverse transformation is not possible