Probing the equilibrium between mono- and di-nuclear nickel(II)-diamidate {[NiII(DQPD)]x, x = 1,2} complexes in chloroform solutions by combining acoustic and vibrational spectroscopies and molecular orbital calculations

Abstract

Abstract In this work a systematic study has been undertaken by combining vibrational and ultrasonic spectroscopies with molecular orbital calculations in an effort to gain a comprehensive understanding of the structural and dynamic characteristics of the [NiII(DQPD)]x complexes in the solid state and in chloroform solutions. Vibrational spectra revealed (i) the formation of a mono-nuclear NiII complex in the solid state and (ii) the formation of two distinct types of NiII complexes in chloroform solutions. The concentration-dependent vibrational spectroscopic measurements indicated that at low-concentrations mono-nuclear complex dominates, while in the high-concentration limit di-nuclear abounds in the structure. The temperature dependence of the spectra demonstrated the exact opposite behavior to that of concentration. We propose a dimerization equilibrium between mono- and di-nuclear complexes to account for the observed spectral changes. The energy profile of the dimerization reaction was theoretically determined exhibiting the [NiII2(DQPD)2] di-nuclear complex as the most thermodynamically favorable. Ultrasonic relaxation spectroscopy has been employed to establish the proposed dimerization equilibrium. The absorption results demonstrate the existence of a relaxation process which is attributed to the perturbation by the sound wave of an equilibrium between mono-nuclear and di-nuclear complexes. The enthalpy difference between the two states is also evaluated from the analysis of the acoustic results. The acoustically induced birefringence traces (stationary and transient) were also employed to examine the dynamic response of the NiII complexes in solutions, which directly reflects the structural modifications. From the transient birefringence signals the corresponding relaxation times have been estimated and correlated with the volume change associated to the structural reforms. Complementary computational methodologies have been implemented on isolated molecules in a state free of interactions and in solution with chloroform as solvent medium. The calculations performed provided information concerning the volume change between the two states.