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

Abstract

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(salpntBuCF3)], [Ni(salpnCF3)] and [Ni(salpnSbenz)]. The electronic‐structural changes are monitored by UV/Vis spectroscopy and Evans NMR spectroscopy. Association constants (K1s and K2), as well as thermodynamic parameters (ΔH, ΔS, and ΔG), 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–π and π–π 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.

Effect of Diamine Bridge on Reactivity of Tetradentate ONNO Nickel(II) Complexes

“…For a proper description of the spin‐state energetics, single point calculations at the S12 g/TZ2P level were additionally performed. As anticipated, the ground state of C2_core and C3_core is a singlet state (S=0), as for other square planar complexes of Ni(II) [12,13,48,49] . DFT calculations revealed large singlet‐triplet (S−T) energy gaps for both complexes.…”

“…Square planar nickel(II) complexes can change their coordination sphere by addition of donor ligands, this transformation can be detected by shifts in the UV‐Vis spectra [1,3,13] . Propensity of Ni(II) complexes towards formation of octahedral complexes depends on the nature of equatorial ligands and basicity of the axial ligand.…”

Improving the Light-Induced Spin Transition Efficiency in Ni(II)-Based Macrocyclic-Ligand Complexes

“…The square-planar coordinated Ni(II) metal-organic complex always prefers the singlet spin state, but its higher coordination complex turns over to the triplet spin state. This phenomenon is called coordination-induced spin-state switching (or CISSS) [44,45,46,47]. Due to the small energy difference between the equilibrium geometry for the singlet spin state and the MECP geometry, in the case of C5 (0.086 eV), it might have happened that the bistable character was lost and the system remained only in its triplet spin state.…”

Molecular Sensors Operating by a Spin‐State Change in Solution: Application to Magnetic Resonance Imaging

“…In search for an alternative to porphyrin ligands, the group led by Tuczek [55] explored classic tetradentate salicylidene‐iminate ligands (N2O2) which induce a square‐planar geometry in their Ni complexes (Scheme 4). As observed for the porphyrin chelates from above, electron‐withdrawing groups increase affinity of these Ni‐salpn complexes with pyridine, but thermodynamic parameters stayed low: the fraction of the two paramagnetic 5‐, and 6‐ccoordinate species in chloroform increased from 0 to 1 upon the addition of 8.7 equivalents of pyridine; only 2.2 eq were needed if each of the two phenol units received a para‐CF 3 substituent.…”