Structural Characterization of Thermochromic and Spin Equilibria in Solid-State Ni(detu)₄Cl₂ (detu = N,N′-Diethylthiourea)

Abstract

Consecutive thermochromic lattice distortional and spin crossover equilibria in solid-state Ni(detu)4Cl2 (detu = N,N'-diethylthiourea) are investigated by variable-temperature X-ray crystallography (173-333 K), DFT calculations, and differential scanning calorimetry. Thermochromism and anomalous magnetism were reported previously (S. L. Holt, Jr., et al. J. Am. Chem. Soc. 1964, 86, 519-520); the latter was attributed to equilibration of a singlet ground state and a thermally accessible triplet state, but structural data were not obtained. A crystal structure at 173(2) K revealed [Ni(detu)4](2+) centers with distorted planar ligation of nickel(II) to the four sulfur atoms, with an average Ni-S bond length of 2.226(3) Å. The nickel ion was displaced out-of-plane by 0.334 Å toward a proximal apical chloride at a nonbonding distance of 3.134(1) Å. Asymmetry in the trans S-Ni-S angles was coupled to a monoclinic ↔ tetragonal lattice distortion (T(1/2) = 254 ± 11 K), resulting in thermochromism. Spin crossover occurs by tetragonal modulation of nickel(II) with approach of the proximal chloride at higher temperatures (T(1/2) = 383 ± 18 K), which is consistent with a contraction of -0.096(4) Å in the Ni···Cl separation observed at 293 K. A high-spin (S = 1) square-pyramidal [Ni(dmtu)4Cl](+) model (dmtu = N,N'-dimethylthiourea) was optimized by DFT calculations, which estimated limiting equatorial Ni-S bond lengths of 2.45 Å and an apical Ni-Cl bond of 2.43 Å. Electronic spectra of the spin isomers were calculated by TD-DFT methods. Assignment of the FTIR spectrum was assisted by frequency calculations and isotope substitution.

Versatility of Amide-Functionalized Co(II) and Ni(II) Coordination Polymers: From Thermochromic-Triggered Structural Transformations to Supercapacitors and Electrocatalysts for Water Splitting

“…220 °C ( 1′ and 2′ , respectively), we could not draw a clear conclusion about their structural transformations. However, on the basis of the UV studies and reported works, it is expected that the structural transformation accompanied by a reversible color change can be accounted for by the liberation and recoordination of two water molecules. , Removal of the coordinated water molecules generates desolvated purple crystals of 1 and green crystals of 2 by changing the coordination geometry of Co­(II) and Ni­(II), respectively, − ,− from octahedral to square-planar (Figure ), which can revert back to the octahedral geometry upon exposure to water.…”

“…There are also numerous reports on Ni­(II) complexes that illustrate a reversible thermochromic behavior, but the majority of these complexes are mononuclear in nature. − Contrarily, reports on Ni­(II)-based CPs with a reversible thermochromic behavior are extremely rare …”

Coexistence of Naked-Eye Mechanochromism, Vapochromism, and Thermochromism in a Soft Crystalline Layered Nickel(II) Coordination Polymer

“…Ni­(II) ion is one of the 3d transition metals that have a variety of coordination numbers and geometries, each with its own visible color. ,− Consequently, it is suitable to use a Ni­(II) center for fabrication of a new multistimuli responsive chromic CP. Vapochromism of some Ni­(II) complexes has been reported. ,, For instance, Baho and colleagues showed that the addition of CH 3 CN vapor can cause a change in coordination modes of nitrate ligands, leading to a shift in d–d bands of a Ni­(II) complex and a change in its color .…”

“…Vapochromism of some Ni­(II) complexes has been reported. ,, For instance, Baho and colleagues showed that the addition of CH 3 CN vapor can cause a change in coordination modes of nitrate ligands, leading to a shift in d–d bands of a Ni­(II) complex and a change in its color . In case of thermochromism, it has been extensively observed in many Ni­(II) systems. − Heat frequently induces the loss of lattice and coordinated solvent molecules, resulting in a structural transformation accompanying with the change in coordination environment. , Until now, there are many reports on mechanochromic behavior in coordination compounds. The reported mechanochromism generally occurs with a change in luminescent signal because most of them are constructed from 5d elements, such as Pt­(II) and Au­(I) ions, as well as some 3d 10 elements, such as Cu­(I) and Zn­(II) ions. ,,− The mechanical force can alter metallophilic interactions between metal centers, resulting in the luminescent color change.…”

Spin-Crossover Phenomena of Ni(cyclam)I2 Complex: Magnetostructural Correlations in Two Polymorphs

“…In clear contrast to the reverse SCO reported by Yamauchi et al, 1a and 1b exhibit eff gradually increasing with temperature, classified in normal SCO between ground LS and excited HS states, showing a similarity to previously reported behaviors in some nickel(II) complexes. 23,[32][33][34][35][36][37] In order to reproduce these behaviors, a van 't Hoff equation for the spin equilibrium is considered, instead of the conventional treatment like the Slichter-Drickamer model 38 to avoid overparametrization:…”