Coordination-Induced Spin-State Switching with Nickel(II) salpn Complexes: Electronic versus Steric Effects and Influence of Intermolecular Interactions

Four nickel(II) salpn (salicylidenepropylene iminate) complexes are investigated concerning their capability to perform CISSS (coordination-induced spin-state switching). To this end, pyridine titration experiments are performed with the nickel salpn complexes [Ni(salpn)], [Ni(salpntBuCF(3))], [Ni(salpnCF(3))] and [Ni(salpnSbenz)]. The electronic-structural changes are monitored by UV/Vis spectroscopy and Evans NMR spectroscopy. Association constants (K-1s and K-2), as well as thermodynamic parameters (Delta H, Delta S, and Delta G), for the binding of pyridine are determined. In comparison with porphyrins and porphyrin-derived systems investigated earlier, the salpn systems exhibit quite different coordination properties. In particular, the fluorinated systems show the formation of dimers due to CH-pi and pi-pi stacking. On the other hand, the investigated compounds coordinate axial ligands with affinities comparable with those of the porphyrin-based analogs. This renders them eligible for the preparation of LD-CISSS (light-driven CISSS) systems, which, in turn, may be applied as functional MRI contrast agents.