Water Molecule-Induced Reversible Magnetic Switching in a Bis-Terpyridine Cobalt(II) Complex Exhibiting Coexistence of Spin Crossover and Orbital Transition Behaviors

Abstract

The development of molecule-based switchable materials remains an important challenge in the field of molecular science. Achievement of a structural phase transition induced by adsorption/desorption of guest molecules in spin crossover (SCO) Co­(II) compounds is of significant interest because of the possibility that the spin state of the magnetic anisotropic high-spin (HS, S = 3/2) and low-spin (LS, S = 1/2) states can be switched via the induced changes in associated intermolecular interactions. In this study, we demonstrated a reversible magnetic switching associated with spin state conversion, along with a single-crystal to single-crystal (SCSC) phase transition induced by dehydration/rehydration. [Co­(terpy)2]­(BF4)2·H2O (1·H2O; terpy = 2,2′:6′,2′′-terpyridine) assembles in the solid state via π–π and CH−π interactions involving adjacent terpyridine cores along the ab direction to form two-dimensional (2D) layered domains. 1·H2O exhibits gradual and incomplete SCO, from fully HS to ca. 0.5 HS, and the field-induced single-molecule magnet (SMM) behavior attributed to the presence of the anisotropic partial high-spin Co­(II) species. 1·H2O undergoes a SCSC transformation accompanied by a change from the tetragonal space group I41/a to P42/n via a dehydration process. Dehydrated 1 exhibits a reverse thermal hysteresis behavior (T 1/2↑ = 287 K; T 1/2↓ = 270 K) in the gradual SCO region from fully HS to ca. 0.5 HS, followed by an ordinary thermal hysteresis (T′1/2↑ = 195 K; T′1/2↓ = 155 K) to fully LS Co­(II). A temperature-dependent single-crystal X-ray structural analysis revealed that the reverse hysteresis can be attributed to an order/disorder structural phase transition of the BF4 – anions involving a symmetry breaking to yield the monoclinic space group P21/n and orbital (angular momentum) transition (LT). Both the SCSC phase transition and magnetic behavior are switchable by dehydration/rehydration processes; thus 1 again adsorbs water at room temperature to give both the original structure and its magnetic behavior.

Spin transition triggered by desorption of crystal solvents for a two-dimensional cobalt(ii) complex with hydrogen bonding

“…3 represents a plot of the above two parameters based on the data from previous studies (Table 2). [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Compound 1 at 100 K has a d mean of 2.006 Å and (Σ) of 99.62°. The data are superimposed in Fig.…”

“…1a) has been utilised as a ligand and has contributed to the SCO behaviour in the Co 2+ ion. [8][9][10][11] In addition, a variety of functional groups (R = hydroxy, [12][13][14] alkoxy, 15,16 pyridyl 17,18 and others [19][20][21][22][23] ) may be easily introduced at the 4'position of the terpy ligand. This chemical modification allows the Co-terpy complexes to control the molecular and crystal structures.…”

Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands

“…Cooperative spin transitions are often associated with crystallographic phase transitions, which can lead to complex multiphase behavior. ,, However, incomplete SCO involving symmetry breaking from a spin-pure to a mixed-spin phase is less common . Moreover, this is the first example of this form of symmetry breaking in an SCO material, giving a 1:1 high/low-spin population distributed across three unique crystallographic sites.…”

Hydrogen-bonded cobalt(ii)-organic framework: normal and reverse spin-crossover behaviours

“…The rSCO behaviour has been reported in the iron(II) [46][47][48][49] and cobalt(II) complexes, 33,37,[50][51][52][53] and it is mainly explained as originating in the structural phase transition such as an order-disorder transition. For the rSCO behaviour of 1•desolv, we have focused on the intermolecular H-bond between carboxy and carboxylate sites.…”

“…Cobalt(II)-SCO compounds can exhibit a sensitive response toward the absorption/desorp-tion of guest molecules. [35][36][37][38][39][40] SCO-active Co-tpy complexes may act as switching components that are compatible with porous architectures such that functional materials may be developed.…”