Ligand Strain and the Nature of Spin Crossover in Binuclear Complexes: Two‐Step Spin Crossover in a 4,4′‐Bipyridine‐Bridged Iron(II) Complex [{Fe(dpia)(NCS)₂}₂(4,4′‐bpy)] (dpia=di(2‐picolyl)amine; 4,4′‐bpy=4,4′‐bipyridine)

Abstract

The synthesis and detailed characterization of the new spin crossover (SCO) binuclear complex [{Fe(dpia)(NCS)(2)}(2)(4,4'-bpy)] (1; dpia=di(2-picolyl)amine, 4,4'-bpy = 4,4'-bipyridine) are reported. Variable-temperature magnetic susceptibility measurements show a relatively cooperative two-step spin transition suggesting the occurrence of three spin-state isomers: [HS-HS], [HS-LS], and [LS-LS] (HS: high spin, LS: low spin). A short plateau at 204 K separates the two steps and conforms with about 50% of the complexes having undergone a thermal spin conversion. Routine Mössbauer spectroscopy without applying a magnetic field clearly separates four iron(II) one-center spin states in three [HS-HS], [HS-LS], and [LS-LS] pairs and unambiguously confirms that the spin transition at the plateau temperature goes through the intermediate [HS-LS] state. The single-crystal X-ray structure was solved for three spin isomers at 293, 208, and 120 K. The structural study at the plateau temperature was unable to resolve the HS and LS sites in the [HS-LS] pair and only an average Fe-N bond length was obtained, which suggests that there is an intermediate [HS-LS] phase. The structural analysis at three temperatures revealed a dense three-dimensional network of both intra- and intermolecular interactions. The relative energies of the three spin-state isomers were evaluated by quantum-chemical DFT calculations. Comparison of compound 1 with previously known analogues, as well as the overall analysis of structural data for numerous binuclear complexes, allowed a conclusion to be reached on the crucial role of ligand strain effects in the SCO behavior of binuclear complexes. The suggested intramolecular mechanism explains the different types of SCO observed in binuclear complexes: one-step, two-step, and partial (50%) transitions.

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

“…7,8 Short-range correlations may also compete with long-range interactions and give rise to deviation from mean-field behaviors, 9 as evidenced in some cases by distinctive long relaxation tails from the LIESST state. 10 Although the vast majority of thermal spin transitions proceed in a single step, there has been an increasing number of two-step transitions reported for polymeric, [11][12][13][14][15][16][17] polynuclear, [18][19][20][21][22][23][24] or mononuclear SCO systems. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] Such transitions result most frequently from the existence of two crystallographically (symmetrically) independent SCO Fe I I sites with slightly different coordination environments, leading to dissimilar transition temperatures delimiting a plateau (or shoulder), which usually occurs with an approximately equal populations of HS and LS species.…”

The Development of Spin‐Crossover Research

“…Such differences in susceptibilities as these are not unusual in SCO chemistry and the authors were, of course, keen to find out why such differences occurred. The crystal structures of the dinuclear molecules, and how they packed in the crystal, were discussed in detail, as were the octahedral distortion parameters, (sum of deviations of the 12 cis N-Fe-N angles from 90 • ) for the HS-HS structures, around each Fe II centre, with polymorph 2 having two distinct Fe sites even at 293 K. The 2-step example 1 did not show structurally distinct Fe sites within each binuclear molecule at 183 78 and in other dinuclear SCO compounds having bipyrimidine, pyrazolate, triazolate and dicyanamide bridges. 64 The 'take home' message was that the nature of the spin-transition (full 1-step; full 2-step; half crossover) in dinuclear Fe II SCO compounds was primarily related to the degree of the octahedral distortions at the FeN 6 cores, that is intramolecular effects, these being influenced by packing and ligand strain arising from terminal and/or bridging ligands.…”

“…Finally, this thorough study showed that the [HS-HS] forms of phases I and (photoexcited/metastable) III * were different. However, the distortion arguments given for the phia analogues 78 4 ] derivative that we structurally characterised at around that time, with L = 1,2,4(N-picoline) remained HS-HS at all temperatures. 83 Recently, Garcia et al reported another such complex, with an imino-substituted triazole, 1,2,4(N=C(C 6 H 4 (2-OH)-triazole).…”

One-Dimensional Polymers Constructed with Paddle-Wheel Dinuclear Copper(II) Bridged by 4,4′-Bipyridine

“…The ligand 4,4 0 -bipyridine is an ideal connector between the transition metal atoms for the propagation of coordination networks due to the following features: it has two potential binding sites which are arranged in a divergent (exo) fashion; has a rigid structure which will help in the predictability of the network geometries; the length of the ligand is good enough to create the cavities of molecular dimensions upon the formation of the networks with metal atoms [18][19][20][21]. Herein, we report the synthesis, crystal structure, infrared spectrum, and thermal behavior of a new one-dimensional copper complex: {[Cu(NAP) 2 ](4,4 0 -bpy) 1/2 }Á DMF (NAPH = a-naphthoic acid, 4,4 0 -bpy = 4,4 0 -bipyridine), with neutral naphthalene-carboxylate paddle-wheel binuclear units [Cu 2 (NAP) 4 ] and the rigid linear bidentate bridging ligand 4,4 0 -bipyridine.…”