[M(dipicH2)(H2O)3]2+, M=Ni, Cu, Zn (dipicH2=dipicolinic acid) – A combined crystallographic, spectroscopic and computational study

Cationic metal complexes of dipicolinic acid (dipicH2) are stabilized by [Ce(dipic)3]2− ions in the three isomorphous crystals [M(dipicH2)(OH2)3][Ce(dipic)3] · 3H2O (M = Ni, 1; Cu, 2; Zn, 3). Magnetic dilution provided by the bulky anions leads to well-resolved EPR spectra in polycrystalline samples of 2. The cations have 4+2 coordination, the carbonyl atom of the carboxylic acid groups coordinating weakly from trans positions. In the case of 2 this steric distortion is augmented by Jahn–Teller distortion. All the three structures are satisfactorily modelled by calculations based on density functional theory (DFT). The switch of the Jahn–Teller axis upon deprotonation of the complex, leading to the neutral species Cu(dipic)(H2O)3, is also reproduced by DFT. Electronic transition energies as well as the g-tensor component of the d9 complex obtained are in good agreement with experiment. However, the calculated hyperfine coupling constants are in error. DFT also fails to satisfactorily account for the electronic transition in the d8 ion in 1.