Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

Abstract

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K.

Rare-Earth (RE = Y, Gd, Tb, Dy, Ho, and Er) Chains Bridged with a Triplet Biradical and Magnetic Hysteresis Recorded for RE = Tb

“…Each RE 3+ ion is eight-coordinate, and the coordination structure is best described as a triangular dodecahedron from the SHAPE analysis (the CShM value 0.26–0.38 to the ideal TDD-8; Table S2). The N–O bond lengths are 1.282(5)–1.301(5) Å, typical of nitroxide complexes. ,, …”

“…Lanthanide ions are indispensable for conventional magnets and were recently proven to be a vital component for novel magnets on a single-molecule level. , The limited dimensionality affords an opportunity suitable for studying exchange coupling. − Exchange coupling is important for the development of 4f-based magnets because such an interaction provides a bias field on the 4f ion, reducing possible quantum tunneling of magnetization. , The O atom of nitroxide radicals can ligate to various metal ions, affording sizable 4f–2p couplings.…”

A Triplet/Singlet Ground-State Switch via the Steric Inhibition of Conjugation in 4,6-Bis(trifluoromethyl)-1,3-phenylene Bisnitroxide

“…The positions of three hfac groups cannot be symmetrically arranged and, accordingly, the two nitroxide groups are unequal. Double H bonds can be found in O1 The N1-O1 and N2-O2 bond lengths in 1 are 1.2842( 16) and 1.2850( 16) Å, respectively, typical of nitroxide radicals [50]. Now, we compare the geometries between the intact TF2PBN molecule [36] and the TF2PBN moiety in 1 (Table 1).…”

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction

“…As already noted, the narrow d-bands that are central to the properties of bistable materials usually are not described well by comparatively simple density functional approximations. That shortcoming has motivated extensive investigation of hybrid density functionals for the calculation of spin crossover in isolated molecules. − As a matter of context, we remark that proper Kohn–Sham evaluation of hybrid approximations requires use of the optimized effective potential. − That, however, is not the customary choice because of the significant computational resources required. Instead, the generalized Kohn–Sham procedure almost always is used with such exchange-correlation approximations.…”