Allosteric Spin Crossover Induced by Ligand-Based Molecular Alloying

Abstract

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 ](ClO 4 ) 2 (2) has been doped with increasing amounts of the ligand Me2bpp (Mebpp and Me2bpp = methyl-and bis-methyl-substituted bis-pyrazolylpyridine ligands), y i e l d i n g m o l e c u l a r a l l o y s w i t h t h e f o r m u l a [ F e -(Mebpp) 2−2x (Me2bpp) 2x ](ClO 4 ) 2 (4x; 0.05 < x < 0.5). The effect of the composition on the SCO process is studied through singlecrystal 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.

Variations to Spin Crossover in [FexZn(1−x)(Me1,3bpp)2(ClO4)2] molecular alloys examined by Magnetometry and Single Crystal X-Ray Diffraction

“…An estimation of n using SCOH = 4.4 kJ/mol then actually shows that n is already close to unity for x = 0.10. A comparison of the value of nSCOH/RTSCO derived for the present series 3x with those for the analogue series 4x doping with [Fe(2Me1,3bpp)2](ClO4)2 [56] shows a surprisingly good agreement (Figure 7, right). The comparison allows one to realize that the effect of dilution in the range of x accessible for the 3x series is rather monotonous and relatively weak.…”

“…The model is based on heterophase fluctuations and gives a measure of cooperativity through the number of like-spin molecules (or here the SCO centers), n, per interacting domain, the larger the domain the more cooperative the transition [61,62]. This is the same model we have previously used to analyse the effect of doping complex 1 with another SCO-active analogue, [Fe(2Me1,3bpp)2](ClO4)2 (4) [56], except that here, the excess enthalpy associated with the SCO process, SCOH, has not been experimentally determined, and therefore we evaluate together the product nSCOH. Also the term has been included here to take into account the fraction of Fe(II) ions that remain in their HS state, something that was not necessary for the series 4x, in which a allosteric SCO occurred for all Fe(II) centers simultaneously in the solid solutions.…”

“…Both compounds exhibit abrupt SCO processes, at very different temperatures (at T1/2 of 184 and 378 K, respectively). In turn, the properties of crystalline solid solutions comprising complexes with both ligands were studied, unveiling an interesting allosteric effect to the SCO [56]. All these compounds being solvent free (i.e.…”

Di‐Iron(II) [2+2] Helicates of Bis‐(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

“…Three isomers of the bpp ligand are available: 2,6-di{pyrazol-1-yl}pyridine (1-bpp); [33][34][35][36] 2,6-di{1H-pyrazol-3-yl}pyridine (3bpp); [37,38] and the unsymmetric 2-{pyrazol-1-yl}-6-{1H-pyrazol-3-yl}pyridine (1,3-bpp). [34,[39][40][41] Hundreds of iron(II) complex salts supported by bpp derivatives are known, many of which exhibit SCO at accessible temperatures. [33][34][35][36][37][38][39][40][41] As a continuation of our long-standing interest in [Fe-(bpp) 2 ] 2 + chemistry, [33][34][35] we now report an investigation of ditopic ligands containing two 1,3-bpp domains linked by different spacers (Scheme 1).…”

“…[34,[39][40][41] Hundreds of iron(II) complex salts supported by bpp derivatives are known, many of which exhibit SCO at accessible temperatures. [33][34][35][36][37][38][39][40][41] As a continuation of our long-standing interest in [Fe-(bpp) 2 ] 2 + chemistry, [33][34][35] we now report an investigation of ditopic ligands containing two 1,3-bpp domains linked by different spacers (Scheme 1). [42][43][44] Some of these cleanly yielded dinuclear iron(II) helicate complexes.…”