A Thermal Spin Transition in [Co(bpy)3][LiCr(ox)3] (ox=C2O42−; bpy=2,2′-bipyridine)

Abstract

In the three-dimensional oxalate network structures [M(II)(bpy)3][M(I)-M(III)(ox)3] (ox= C2O4(2-); bpy = 2,2'-bipyridine) the negatively charged oxalate backbone provides perfect cavities for tris-bipyridyl complex cations. The size of the cavity can be adjusted by variation of the metal ions of the oxalate backbone. In [Co(bpy)3][NaCr(ox)3], the [Co(bpy)3]2 + complex is in its usual 4T1(t2g5e(g)2) high-spin ground state. Substituting Na+ by Li+ reduces the size of the cavity. The resulting chemical pressure destabilises the high-spin state of [Co(bpy)3]2+ to such an extent that the 2E(t2g6e(g)1) low-spin state becomes the actual ground state. As a result. [Co(bpy)3][LiCr(ox)3] becomes a spin-crossover system, as shown by temperature-dependent magnetic susceptibility measurements and single-crystal optical spectroscopy, as well as by an X-ray structure determination at 290 and 10 K.

Spin-crossover in cobalt(II) imine complexes

“…For iron(III) spin-crossover systems back bonding is of lesser importance and therefore the average bond length difference is with 0.13-0.16Å somewhat smaller [6]. For cobalt(II) complexes only one electron is transferred to the anti-bonding orbital upon spin-crossover, as a consequence the average bondlength difference is only 0.07 to 0.11Å [7,8]. In addition, the 2 E(t 6 2g e 1 g ) low-spin state is susceptible to a Jahn-Teller effect [9].…”

“…This supports the statement that the anisotropic bond length change in the terpy complex actively stabilises the lowspin state via a reinforced Jahn-Teller effect. However, even the [Co(bpy) 3 ] 2+ complex can be turned into spin-crossover complex when it is confined to the cavities of Zeolites [18] or embedded into a three-dimensional oxalate network [8]. In the Zeolites the thermal spin transition is very gradual, and at room temperature the high-spin fraction is still less than 50%.…”

“…But, as mentioned in the introduction, the effective ligand-field strength is quite close to the crossover point so that the lowspin 2 E state has to be at comparatively low energy. A small "chemical" pressure in the form a confining environment, for instance inside the cages of Zeolites or in the more tightly fitting cavity of the oxalate network with Li instead of Na, destabilises the high-spin state sufficiently for the low-spin state to become the ground state [8,19]. This is to be discussed in detail below.…”

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…In the last 20 years many efforts have been addressed to add in these materials a further physical property by playing with the functionality of the A + cations located between the bimetallic layers. This strategy produced a large series of multifunctional molecular materials where the magnetic ordering of the bimetallic layers coexists or even interacts with other properties arising from the cationic layers, such as paramagnetism [2,[76][77][78][79][80], non-linear optical properties [2,81,82], metal-like conductivity [83,84], photochromism [2,81,85,86], photoisomerism [87], spin crossover [88][89][90][91][92][93], chirality [94][95][96][97], or proton conductivity [2,98,99]. Moreover, it is well-established that the ordering temperatures of these layered magnets are not sensitive to the separation determined by the cations incorporated between the layers, which slightly affects the magnetic properties of the resulting hybrid material, by emphasizing its 2D magnetic character [2,[75][76][77][78][79][80]95,100,101].…”

Metal Ion Controlled Self-Assembly of a Chemically Reengineered Protein Drug Studied by Small-Angle X-ray Scattering

“…To generate the corresponding trimers, the superposition function in Maestro was employed to overlay the bipyridine moieties of three duplicates of the bipyridine functionalized InsX2 monomer with the bipyridine moieties in the crystal structure of a tris-(2,2′-bipyridine) cobalt(III) complex. 24 After substituting Fe(II) for Co(III), the resulting trimer models were subjected to energy minimization in MacroModel 25 using the OPLS_2005 26 force field and relaxed convergence criteria (gradient <1.0 kJ Å −1 mol −1 ). This allowed the removal of steric clashes while largely conserving the native insulin geometry.…”