Spin crossover and polymorphism in a family of 1,2-bis(4-pyridyl)ethene-bridged binuclear iron(ii) complexes. A key role of structural distortions

Abstract

Two polymorphic modifications 1 and 3 of binuclear compound [{Fe(dpia)(NCS)(2)}(2)(bpe)] and pseudo-polymorphic modification [{Fe(dpia)(NCS)(2)}(2)(bpe)]·2CH(3)OH (2), where dpia = di-(2-picolyl)amine, bpe = 1,2-bis(4-pyridyl)ethene, were synthesized, and their structures, magnetic properties, and Mössbauer spectra were studied. Variable-temperature magnetic susceptibility measurements of three binuclear compounds show different types of magnetic behaviour. The complex 1 exhibits a gradual two-step spin crossover (SCO) suggesting the occurrence of the mixed [HS-LS] (HS: high spin, LS: low spin) pair at the plateau temperature (182 K), at which about 50% of the complexes undergoes a thermal spin conversion. The complex 2 displays an abrupt full one-step spin transition without hysteresis, centred at about 159 K. The complex 3 is paramagnetic over the temperature range 20-290 K. The single-crystal X-ray studies show that all three compounds are built up from the bpe-bridged binuclear molecules. The structure of 1 was solved for three spin isomers [HS-HS], [HS-LS], and [LS-LS] at three temperatures 300 K, 183 K, and 90 K. The crystal structures for 2 and 3 were determined for the [HS-HS] complexes at room temperature. The analysis of correlations between the structural characteristics and different types of magnetic behaviour for new 1-3 binuclear complexes, as well as for previously reported binuclear compounds, revealed that the SCO process (occurrence of full one-step, two-step, or partial (50%) SCO) is specified by the degree of distortion of the octahedral geometry of the [FeN(6)] core, caused by both packing and strain effects arising from terminal and/or bridging ligands. The comparison of the magnetic properties and the networks of intra- and inter-molecular interactions in the crystal lattice for the family of related SCO binuclear compounds suggests that the intermolecular interactions play a predominant role in the cooperativeness of the spin transition relative to the intramolecular interactions through the bridging ligand.

The Nature of Spin Crossover and Coordination Core Distortion in a Family of Binuclear Iron(II) Complexes with Bipyridyl‐Like Bridging Ligands

“…In addition to recently reported binuclear SCO complexes with the bis-monodentate 4,4Ј-bipyridine (4,4Ј-bpy) [15] and 1,2-bis(4-pyridyl) ethene (bpe) [18] bridging units, two new binuclear SCO compounds [{Fe(dpia)(NCS) 2 } 2 (bpac)] (1) and [{Fe(dpia)-(NCS) 2 } 2 (bpac)]·2CH 3 OH (2) [dpia = bis(2-picolyl)amine] with the 1,2-bis(4-pyridyl)ethyne (bpac) spacer ligand have been isolated. The new complexes show different types of SCO: 1 exhibits a two-step spin change, whereas 2 displays a relatively abrupt full one-step spin transition.…”

“…[15] It has been concluded that the three types of SCO in binuclear compounds are conditioned by the degree of distortion of the [FeN 6 ] octahedral geometry in different spin-state pairs, which is caused by both crystal packing and strain effects that arise from terminal and/or bridging ligands. [18] A full one-step SCO can be observed for weakly constrained [HS-HS] species and for systems involving a sterically constrained zone, common to both iron centers, that arises from a bridging ligand. The stabilization of the mixed [HS-LS] complex at the plateau temperature for the two-step or partial (50 %) SCO results from an additional distortion of the HS site in the mixed pair.…”

“…The only X-ray structural localization of the HS and LS sites in a mixed pair has been performed for a partial SCO complex, [13] whereas all studies for two-step compounds at the plateau temperature resulted in an averaged centrosymmetric structure. [15][16][17][18] These studies posed several problems of fundamental importance such as the intrinsic source of the three types of magnetic behavior and the cooperativity mechanism in binuclear compounds. [4,5] The general understanding of the origin of different types of SCO in binuclear complexes is an important challenge for the prediction of magnetic properties for new systems.…”

Two‐Step Spin Transition in an Iron(II) Coordination Network Based on Flexible Bitopic Ligand 1‐(Tetrazol‐1‐yl)‐3‐(1,2,3‐triazol‐1‐yl)propane

“…[25][26][27] It has been shown that serious differences in the degree of distortion of FeN 6 chromophore geometries can lead to various spin-crossover behaviors: One-step, twostep, or incomplete (50 %) transitions. On the other hand, our investigations of 3D network [Fe(qbtr) 3 ](ClO 4 ) 2 containing flexible ligand bridges did not reveal any influence of conformational alterations on the FeN 6 geometry.…”

“…[19] It is also assumed that in dinuclear complexes containing crystallographically unique iron(II) ions, intermolecular interactions influence spin-crossover properties leading to the occurrence of an intermediate phase comprising LS-HS or 50 % HS-HS and 50 % LS-LS pairs. [24] Investigations of dinuclear complexes based on 1,2-bis(4-pyridyl)ethane, [25] bis(2-picolyl)amine, [26] and 1,2-bis(4-pyridyl)ethyne [27] showed that a degree of distortion in the [FeN 6 ] chromo-phore caused by ligand strain and packing effects is responsible for spin-crossover (SCO) behavior. It is worth noting that the number of steps in the spin-transition curve does not necessarily match the number of crystallographically distinct iron(II) ions in a crystal lattice.…”