Co(II) and Cr(III) complexes of formate–formamide mixed ligands: synthesis, structures, single crystal-to-single crystal transformation and magnetic behaviour

Solvothermal treatment of an equimolar mixture of Co(NO₃)₂·6H₂O, HCONH₂ and NaN₃ in MeOH at 100 °C yielded a three-dimensional NaCl type network Co(HCOO)₂(HCONH₂)₂·HCONH₂ (1a) containing formamides in the pores of the structure. Solvated pink 1a undergoes single crystal-to-single crystal (SCSC) transformation at 215 °C to form the desolvated dark brown product Co(HCOO)₂(HCONH₂)₂ (1b) with the retention of the original framework. Reversible single crystal-to-single crystal transformation of 1b (brown) to 1a (pink) in the presence of excess formamide was also established at room temperature. The coordination environment around Co(II) in both 1a and 1b is octahedral with a CoN₂O₄ coordination composition. A similar reaction replacing Co(II) by Cr(III) produced a heterometallic 3D extended network Na[Cr(HCOO)₄(HCONH₂)₂]·2H₂O (2a) at 100 °C. An increase in reaction temperature to 150 °C produced a simple mononuclear complex Cr(HCOO)₃(HCONH₂)₃·3H₂O (2b). Variable temperature magnetic studies revealed the presence of a canting phenomena in both 1a and 1b, and hysteresis loop in the field dependent magnetisation plot at 2 K whereas complex 2a is simply paramagnetic in nature.