ABSTRACT
In this work, the synthesis of four new iron complexes with a Schiff base-like amphiphilic equatorial ligand (L) and dimethylaminopyridine (dmap) or 4,4'-bipyridine (bipy) as axial ligands is reported. Three of the complexes ([FeL(dmap)2] 1, [FeL(dmap)(MeOH)0.5]·MeOH 2, and [FeL(bipy)] n 3 have an iron(II) center, and two of those with an N4O2 coordination sphere (1 and 3) are spin crossover active. Both exhibit a thermal hysteresis (10 K with T1/2 = 131 K for 3 and 23 K with T1/2 = 161 K for 1) where the width depends on the velocity used for the measurement. Additionally, in both cases, the high spin state is trapped by rapid cooling, and a TTIESST was determined to be 121 K (1) and 101 K (3). Single crystals suitable for X-ray structure analysis were obtained for the three different complexes with dmap as axial ligand (1S, 2, and [µ-O-{FeL}2(dmap)] 4). The complex 1S has two dmap molecules in axial position, while the other two structures were obtained for a complex where a dmap ligand is exchanged by methanol and one where the iron(II) center is oxidized to iron(III) to form a dinuclear µ-O-complex. All three complexes were obtained under similar reaction conditions in the presence/absence of oxygen, and all three structures show the formation of lipid layer-like arrangements in the packing.
ABSTRACT
A new series of iron(II) 1D coordination polymers with the general formula [FeL1(pina)]·xsolvent with L1 being a tetradentate N2O2(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], and pina being a bridging axial ligand N-(pyrid-4-yl)isonicotinamide, are discussed. The X-ray crystal structure of [FeL1(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group P21/c, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1(pina)]·xH2O/MeOH. The 88 K wide thermal hysteresis loop (T1/2↑ = 328 K and T1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (T1/2↑ = 296 K and T1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1(pina)]·0.5H2O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (T1/2↑ = 321 K and T1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties.
ABSTRACT
Several new amphiphilic iron complexes were synthesised and characterised by single crystal X-ray structure analysis. The Schiff-base-like equatorial ligands contain long alkyl chains in their outer periphery with chain lengths of 8, 12, 16 and 22 carbon atoms. As axial ligands methanol, pyridine, 4-aminopyridine, 4-(dimethylamino)pyridine and 1,2-bis(4-pyridyl)ethane were used. X-ray structure analysis of the products reveals different coordination numbers, depending on the combination of equatorial and axial ligand. The driving force for this is the self-assembly to lipid-layer-like arrangements. This can be controlled through the chain lengths and the dimension of the axial ligands in a crystal-engineering-like approach. For this an empirical rule is introduced concerning the crystallisation behaviour of the complexes. The efficacy of this rule is confirmed with the crystallisation of an octahedral complex with two docosyl (C22) chains in the outer periphery. The rule is also applied to other ligand systems.
ABSTRACT
The novel hexadentate nitrogen based ligand N,N'-bis-(2-(1H-pyrazol-1-yl)pyridine-6-ylmethyl)-2,2'-biphenylenediamine (3) was synthesized and used for the preparation of iron Spin Crossover (SCO) complexes [Fe(3)][BF4]2 (4) and [Fe(3)][ClO4]2 (5), which differ only by the respective counter ion. These complex salts show different spin transition temperatures T(1/2) (135 and 157 K, respectively). This effect was studied by the investigation of the solid state structure of different low- and high-spin isomers. All complexes of this series show closely related crystal packing regardless of the counter ion, metal (Zn/Fe) and spin state. The isomer exhibiting the lower transition temperature (4) was also investigated in respect to its photomagnetic behaviour. The LIESST process could be monitored for this complex, but no reverse-LIESST was observed. The relaxation of the photo-induced state occurs at ca. 80 K, showing a complex, three-state relaxation mechanism.
ABSTRACT
Ladder type 1D coordination polymers were synthesised with the aim to improve the spin crossover properties of the iron(II) complexes following the concepts of crystal engineering. A wide hysteresis loop (34 K) was observed if rigid linkers were used. The first X-ray structure for a 1D iron(II) ladder is reported.
Subject(s)
Ferrous Compounds/chemistry , Polymers/chemistry , Ferrous Compounds/chemical synthesis , Models, Molecular , Molecular StructureABSTRACT
A novel iron(II) mononuclear spin transition complex [FeL(py)(2)] displays an abrupt spin transition around 225 K accompanied by a very wide thermal hysteresis loop (â¼50 K) that spreads out over 100 K. Crystal structure analysis in both low-spin and high-spin states reveals a lipid layer-like arrangement of the complex molecules and provides insights into the spin switching mechanism.
