Thermally induced polymerization of binuclear [Ni2(en)2(H2O)6(pyr)]·4H2O complex

• [Ni2(en)2(H2O)6(pyr)]·4H2O undergoes dehydration accompanied by polymerization.
• Combination of experiment and DFT calculations identified two reaction mechanisms
• Polymerization occurs in parallel with dehydration process at lower heating rates.
• Polymerization occurs subsequent to dehydration process at higher heating rates.
• Reaction mechanism changes when slow diffusion of released water inhibits polymerization.

Binuclear [Ni2(en)2(H2O)6(pyr)]·4H2O complex undergoes dehydration in 325–400 K temperature region, which is accompanied by polymerization. Polymerized product is characterized by chelate coordination of carboxylate group to Ni, as identified by vibrational spectroscopy. XRD and spectroscopic measurements suggest that the resulting dehydration product is two-dimensional layered polymer with weak interconnectivity between the polymer layers. A combination of experimental measurements and DFT calculations was used to identify two reaction mechanisms, as well as the factors determining the change from one mechanism to the other. Reaction mechanism changes with increase in the heating rate, due to slow diffusion of released water inhibiting the polymerization. Polymerization occurs in parallel with dehydration at lower heating rates, while it follows dehydration at higher heating rates, leading to an increase in overall enthalpy of the reaction of around 50 kJ mol−1 and decrease in crystallinity of the polymeric product. Determined isokinetic temperature of the dehydration reaction corresponds to the vibrational frequency of NiOH2 bond.