Spin-crossover and the LIESST effect in [Fe x Co 1‒ x (bpp) 2 ][BF 4 ] 2 (1.00 ≤ x ≤ 0.77). Comparison with bifunctional solid solutions of iron and cobalt spin-crossover centers

Activating a high-spin iron(II) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1-x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1-x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ± 0.1.

Slow magnetic relaxation and spin crossover behavior in two mixed-valence Co(ii)/Co(iii) complexes

Two mixed-valence Co(ii)/Co(iii) complexes of [CoII(phen)3][CoIII(HATD)2]2·3DMA·3.5H2O (1) with field induced single-molecule magnet behavior, and [CoII(terpy)2][CoIII(HATD)2]2·4DMA·H2O (2) with a gradual thermal spin crossover (SCO).

A multifunctional magnetic material based on a solid solution of Fe(ii)/Co(ii) complexes with a macrocyclic cyclam-based ligand

In order to prepare a multifunctional magnetic material combining spin crossover together with single-molecular magnetism, co-crystallization of Fe(ii) and Co(ii) complexes of the pyridine derivative of cyclam (Py₂-C = 1,8-bis(pyridin-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane) was performed. Complexes with the general formula [M^II(Py₂-C)](ClO₄)₂·H₂O (M^II = Fe (1), Co (2) or Fe_0.4Co_0.6 (3)) were prepared and thoroughly characterized. Based on X-ray molecular structures, they formed octahedral complexes with cis-arrangement of the coordinated pyridine moieties. Magnetic data revealed that the Fe(ii) complex 1 shows complete SCO with the transition temperature T_1/2 = 141 K, which is preserved also in the mixed Fe/Co system 3 (T_1/2 = 128 K). Co(ii) complex 2 behaves as a field-induced single-molecule magnet as well as the mixed system 3 with a direct and phonon bottleneck relaxation process, respectively. This is the first example of such Fe/Co solid solution providing SCO in combination with field-induced SMM properties. Unfortunately, the light-induced excited spin-state trapping (LIESST) effect was not observed either for the Fe(ii) complex 1 or the mixed system 3 and thus, the effect of SCO on SMM properties at low temperature could not be investigated in detail. Nevertheless, the obtained results clearly document the success of the solid solution methodology for the preparation of multifunctional magnetic materials.

Allosteric Spin Crossover Induced by Ligand-Based Molecular Alloying

The spin crossover (SCO) phenomenon represents a source of multistability at the molecular level, and dilution into a nonactive host was originally key to understand its cooperative nature and the parameters governing it in the solid state. Here, we devise a molecular alloying approach in which all components are SCO-active, but with significantly different characteristic temperatures. Thus, the molecular material [Fe(Mebpp)₂](ClO₄)₂ (2) has been doped with increasing amounts of the ligand Me2bpp (Mebpp and Me2bpp = methyl- and bis-methyl-substituted bis-pyrazolylpyridine ligands), yielding molecular alloys with the formula [Fe(Mebpp)_2-2x(Me2bpp)_2x](ClO₄)₂ (4x; 0.05 < x < 0.5). The effect of the composition on the SCO process is studied through single-crystal X-ray diffraction (SCXRD), magnetometry, and differential scanning calorimetry (DSC). While the attenuation of intermolecular interactions is shown to have a strong effect on the SCO cooperativity, the spin conversion was found to occur at intermediate temperatures and in one sole step for all components of the alloys, thus unveiling an unprecedented allosteric SCO process. This effect provides in turn a means of tuning the SCO temperature within a range of 42 K.