Spin Crossover in Nickel(II) Tetraphenylporphyrinate via Forced Axial Coordination at the Air/Water Interface

Abstract

Coordination-induced spin crossover (CISCO) in nickel(II) porphyrinates is an intriguing phenomenon that is interesting from both fundamental and practical standpoints. However, in most cases, realization of this effect requires extensive synthetic protocols or extreme concentrations of extra-ligands. Herein we show that CISCO effect can be prompted for the commonly available nickel(II) tetraphenylporphyrinate, NiTPP, upon deposition of this complex at the air/water interface together with a ruthenium(II) phthalocyaninate, CRPcRu(pyz)2, bearing two axial pyrazine ligands. The latter was used as a molecular guiderail to align Ni···Ru···Ni metal centers for pyrazine coordination upon lateral compression of the system, which helps bring the two macrocycles closer together and forces the formation of Ni–pyz bonds. The fact of Ni(II) porphyrinate switching from low- to high-spin state upon acquiring additional ligands can be conveniently observed in situ via reflection-absorption UV-vis spectroscopy. The reversible nature of this interaction allows for dissociation of Ni–pyz bonds, and thus, change of nickel cation spin state, upon expansion of the monolayer.

Monitoring the Substrate‐Induced Spin‐State Distribution in a Cobalt(II)‐Salen Complex by EPR and DFT

“…This substrate titration experiment at 120 K, therefore suggests that the 5‐coordinate [Co( 1 )( 3 ) n =1 ] species appears most abundant and this can exhibit a distribution between LS and HS states, whereas the [Co( 1 )] and 6‐coordinated adducts do not. It should also be noted that a temperature dependent equilibrium between the different coordination numbers 4, 5 and 6 ( n =0–2), likely exists and can inevitably alter the degree of coordination and spin state of the cobalt center, as indeed demonstrated in Ni(II) Schiff base complexes [47–49] . For example, Thies et al [47] .…”

“…It should also be noted that a temperature dependent equilibrium between the different coordination numbers 4, 5 and 6 (n = 0-2), likely exists and can inevitably alter the degree of coordination and spin state of the cobalt center, as indeed demonstrated in Ni(II) Schiff base complexes. [47][48][49] For example, Thies et al [47] showed that within the temperature range 298-328 K, the association constants for binding a single pyridyl ligand to a Ni(II)-porphyrin forming a 5-coordinate complex and a second pyridyl ligand forming a 6-coordinated complex decreases as the temperature increases. As such, at lower temperatures, more substrate binding is expected, and this in turn will alter the amount of the HS versus the LS states, as revealed by the different EPR signals for the two states.…”

Vapochromic effect in switchable molecular-based spin crossover compounds

“…Finally, it is worth mentioning the existence of the coordination-induced spin state switch (CISSS). [86][87][88][89][90][91][92][93][94][95] This phenomenon is defined as a switch in the spin state of the metal centre, due to a change of the coordination number of the central ion. In most cases, the switch in the spin state is achieved by light irradiation (ligand-driven coordinationinduced spin state switching, LD-CISSS) 87,88,90,[94][95][96][97] or takes place in solution [86][87][88][89]91,92,[94][95][96][97][98][99][100][101] or in thin films/monolayers.…”

Interface Asymmetry Induced and Surface Pressure Controlled Valence Tautomerism in Monolayers of bis-Phthalocyaninates of Lanthanides

“…Nevertheless, while the effect of pressure on the processes of redox isomerization is mentioned in the literature for bulk samples [16][17][18], for surface/interfacial systems this issue remains poorly investigated. Meanwhile, the intrinsically asymmetric nature of air/water and air/solid interfaces often enables processes which can be almost impossible in bulk media [19,20].…”