The relationship between the strength of hydrogen bonding and spin crossover behaviour in a series of iron(iii) Schiff base complexes

Abstract

X-ray crystal structures and magnetic properties of an isostructural series of iron(III) Schiff base complexes with the general formula [Fe(L(5))(NCX)]·Solv (where H2L(5) = N,N'-bis(2-hydroxy-naphthylidene)-1,6-diamino-4-azahexane, X = S, Solv = tetrahydrofuran, 1a; X = S, Solv = methanol and 0.5 pyrazine, 1b; X = S, Solv = butanone, 1c; Solv = N,N'-dimethylformamide, X = S (1d) or X = Se (1d'); X = S, Solv = dimethyl sulfoxide, 1e) are reported. In the crystals, the individual [Fe(L(5))(NCX)] molecules are connected through weak C-H···O, C-H···π or C-H···S non-covalent contacts into 2D supramolecular networks, while the guest-solvent (Solv) molecules are trapped in the cavities between two adjacent layers, which are furthermore stabilized by N-H···O hydrogen bonds connecting the Solv oxygen atom with the amine group of the [Fe(L(5))(NCX)] molecule, with the N···O distances varying from 2.921(6) Å (in 1d') to 3.295(2) Å (in 1a). The magnetic properties of the complexes were tuned by the different Solv molecules and as a result of this, four new spin crossover (SCO) compounds with cooperative spin transitions are reported, which are accompanied by thermal hysteresis in two cases (1d and 1e): , T1/2 = 84 K; 1d, T1/2↓ = 232 K, T1/2↑ = 235 K and 1e, T1/2↓ = 127 K, T1/2↑ = 138 K. The role of the N-H···O hydrogen bonding in the occurrence and tuning of SCO was also computationally studied using a topological analysis, and also by evaluation of non-covalent interaction (NCI) indexes. Both theoretical approaches showed a clear relationship between the strength of the N-H···O hydrogen bonds and T1/2, as already inferred from X-ray structural and magnetic data.

Solvent modified spin crossover in an iron(iii) complex: phase changes and an exceptionally wide hysteresis

“…In the case of Fe( iii ) there is only one study of solvent effects in the series [Fe(napet)NCS]·sol (napet = 1-[((2-{3-[2-hydroxy-1-naphthyl)methylideneamino]propylamino}ethylimino)methyl]-2-naphthol); sol = tetrahydrofuran, methanol and 0.5 pyrazine, butanone, DMF, DMSO). 37 Although, only three of the compounds were SCO active 37 the authors were able to demonstrate that an N–H···O hydrogen bond between the coordinated amine and an oxygen atom from the solvent was responsible for tuning the spin transition temperature. Despite this success only one of the structures was determined in both spin states limiting the conclusions that could be drawn regarding how supramolecular connectivity is altered by SCO.…”

Different Spin‐State Behaviors in Isostructural Solvates of a Molecular Iron(II) Complex

“…[24] At least two of these compounds undergo re-entrant symmetry breaking transitions as part of their SCO process, whichi sc onnectedt or earrangementso ft he disordered, included alcohol. [26] At hird example is [Fe(NCS) 2 (tpa)] (tpa = tris{pyrid-2-ylmethyl}amine), which interconverts between two crystal phasesu pona bsorptiono f an umber of different organic solvents in single-crystal to single-crystal fashion.T he effect of solvente xchange on SCO is smaller in this system,although the solvents had an interesting effect on the completeness of the SCO in one of the two phases. While these solvates are not all isostructural, the variation in their SCO temperature( T1 = 2 )c an be relatedt oh ydrogen bonding between the complex and the solvent.…”

Magnetic Bistability in Macrocycle‐Based FeII Spin‐Crossover Complexes: Counter Ion and Solvent Effects

“…Because this reorientation of the molecule is likely linked to the network of intermolecular interactions, and because it was not possible to obtain the LS crystal structure of 2 , a more detailed examination of the intermolecular contacts in 1 and 2 was performed to explain the differences observed in the magnetic behaviour of the two compounds ( T 1/2 = 136–145 K, abrupt and with 9 K hysteresis for 1 , versus T 1/2 = 83 K, abrupt and with no hysteresis for 2 ). It has been shown for a series of SCO compounds of the same family that hydrogen‐bonding interactions involving the coordinated N atoms of a SCO complex affect the spin transition temperature . Such interactions are present in compounds 1 and 2 and involve the N1 and N3 atoms of the amino groups of the ligands, which act as hydrogen donors, and two different acceptors (water O atoms in 1 and anion N atoms in 2 ; Tables and ).…”