Tuning the Transition Temperature and Cooperativity of bapbpy‐Based Mononuclear Spin‐Crossover Compounds: Interplay between Molecular and Crystal Engineering

Abstract

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin-crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6'-di(pyridin-2-yl)-2,2'-bipyridine-6,6'-diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy-based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans-[Fe{R(2)(bapbpy)}(NCS)(2)] compounds were prepared, in which the R(2)bapbpy ligand bears picoline (9-12), quin-2-oline (13), isoquin-3-oline (14), or isoquin-1-oline (15) substituents. From this series, three compounds (12, 14, and 15) have SCO properties, one of which (15) occurs at 288 K. The crystal structures of compounds 11, 12, and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)(2)] (1), in which each iron complex interacts with its neighbors through weak N-H···S hydrogen bonding and π-π stacking. For compounds 12 and 15, hindering groups located near the N-H bridges weaken the N-S intermolecular interactions, which is correlated to non-cooperative SCO. For compound 14, the substitution is further away from the N-H bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio-temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1. Heat-capacity measurements were made for compounds 1, 12, 14, and 15 and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15. Finally, we found that although both pairs of compounds 11/12 and 14/15 are pairs of isomers their SCO properties are surprisingly different.

Effect of Metal Dilution on the Thermal Spin Transition of [FexZn1–x(bapbpy)(NCS)2]

“…For compound 1, cooperativity is mediated mainly by means of intermolecular N-H···S hydrogen-bonding networks. [5,10] With high iron fractions (x = 1.00 to x = 0.53) X-ray powder and infrared data of the mixed samples resemble those of compound 1, and one might conclude that in these samples the zinc complex adopts the coordination geometry found in the HS iron(II) phase. In such a phase the transition between a two-step SCO and a one-step SCO occurs for x = 0.76.…”

“…Remarkably, Xray crystallography established the long-range order of the [HS-LS-LS] motif in the IP, and the occurrence of strong supramolecular interactions (hydrogen bonds and π-π stacking interactions) along the crystallographic c axis of all three phases. Hydrogen-bonded intermolecular interactions seem to be of paramount importance for the cooperativity of compound 1, [9,10] since, for example, the solvate [Fe-(bapbpy)(NCS) 2 ]·2DMF (compound 2), which has two DMF molecules hydrogen-bonded to the NH bridges of the bapbpy ligand, shows a noncooperative SCO.…”

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…Although there have been few systematic studies, an example is provided by a series of iron(II) complexes of a series of hexadentate ligands Another example of the same effect occurs in some trans-[Fe(NCS) 2 L] complexes, where L represents a series of meridional tetrapyridyl ligands which adopt helically twisted conformations when coordinated to a metal ion. While not all were crystallographically characterized, examples that might be expected to adopt more twisted L ligands are high-spin, or exhibit significantly reduced SCO T ½ values, in the solid state [63,64]. Comparably distorted ligand conformations, imposed by intermolecular steric contacts rather than by ligand design, may also be responsible for stabilization of the high-spin state in some other crystalline metal complexes [65][66][67][68][69][70].…”

Synthesis, crystal structures and properties of Ni(NCS)2-4-(hydroxymethyl)pyridine coordination compounds

“…In this case the cations can be linked by pairs of anionic ligands into dimers or chains or can be connected by single anionic ligands into layers [8][9][10][11][12][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Even if for some compounds with terminal N-bonded ligands spin-crossover is observed [5][6][47][48][49][50], the coordination polymers with bridging anionic ligands show a more versatile magnetic behavior [32][33][34][35][36][37][38][39][40].…”