Spin state behavior of iron(II)/dipyrazolylpyridine complexes. New insights from crystallographic and solution measurements

The isomeric complexes [Fe(1-bpp)2]2+ and [Fe(3-bpp)2]2+ (1-bpp=2,6-di[pyrazol-1-yl]pyridine; 3-bpp=2,6-di[1H-pyrazol-3-yl]pyridine) and their derivatives are some of the most widely investigated complexes in spin-crossover research. This article addresses two unique aspects of their spin-state chemistry. First, is an unusual structural distortion in the high-spin form that can inhibit spin-crossover in the solid state. A new analysis of these structures using continuous shape measures has explained this distortion in terms of its effect on the metal coordination geometry, and has shown that the most highly distorted structures are a consequence of crystal packing effects. Second, solution studies have quantified the influence of second-sphere hydrogen bonding on spin-crossover in [Fe(3-bpp)2]2+, which responds to the presence of different anions and solvents (especially water). Previously unpublished data from the unsymmetric isomer [Fe(1,3-bpp)2]2+ (1,3-bpp=2-[pyrazol-1-yl]-6-[1H-pyrazol-3-yl]pyridine) are presented for comparison. Modifications to the structure of [Fe(3-bpp)2]2+, intended to augment these supramolecular effects, are also described

Iron complexes of new dipyrazolylpyridine derivatives for spin-crossover applications

This thesis concerns the synthesis and characterisation of new di(pyrazolyl)pyridine and di(pyrazolyl)pyrazine derivatives, and their iron(II) complexes which exhibit spin crossover behaviour. The examination of their magnetic behaviour using NMR spectroscopy, X-ray crystallography and magnetic measurements is described. Chapter One introduces the background behind the spin crossover phenomenon, especially in the case of iron(H) compounds. The methods of identification and the applications of spin crossover are discussed. Finally the aims of the research are also detailed. Chapters Two and Three discuss the synthesis and characterisation of previously unreported 2,6-di(4-R-pyrazol-I-yl)pyrazine compounds (R ~ CI, Br, I, Me, NO" NH" CCPh). The synthesis of their iron(ll) complexes as BF, and CIO, salts is also detailed, and their magnetic behaviour is discussed and compared to previous 2,6- bis(pyrazol-l-yl)pyridine complexes. The crystal structures of the complexes are presented. Chapter Four is a study of novel di(pyrazol-l-yl)pyridine derivatives substituted at the pyrazolyl or pyridyl 4-positions, and their iron(Il) complexes as before. Magnetochemical data for three complexes are presented, together with crystallographic data and a discussion relating structure to magnetic behaviour. Chapter Five focuses on the study of di(pyrazolyl)pyridine derivatives substituted at the pyridyl 4-positions by azo, ethynyl or styryl groups, and their iron(ll) complexes. Examination of their magnetic behaviour and crystal structures demonstrates the difficulty of predicting the spin state of an iron(lI) complex.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Synthesis of 2,6-Di(pyrazol-1-yl)pyrazine derivatives and the spin-state behavior of their iron(II) complexes

Chlorination of 2,6-bis(pyrazol-1-yl)pyrazine (bppz) with NaClO in acetic acid afforded 2,6-bis(4-chloropyrazol-1-yl)pyrazine (L2Cl). 2,6-Bis(4-bromopyrazol-1-yl)pyrazine (L2Br), 2,6-bis(4-iodopyrazol-1-yl)pyrazine (L2I), 2,6-bis(4-methylpyrazol-1-yl)pyrazine (L2Me), and 2,6-bis(4-nitropyrazol-1-yl)pyrazine (L2NO2) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of L2NO2 with iron powder gave 2,6-bis(4-aminopyrazol-1-yl)pyrazine (L2NH2) and L2I was converted into 2,6-bis[4-(phenylethynyl)pyrazol-1-yl]pyrazine (L2CCPh) by a Sonogashira coupling reaction. The salts [Fe(L2Me)2]X2 (X- = BF4- and ClO4-) underwent thermal spin-crossover abruptly at around 200 K in one and two steps, respectively. The [Fe(L2Me)2]X2 salts exhibited different light-induced excited spin-state trapping (LIESST) behavior; the BF4- salt behaves classically [T(LIESST) = 93 K], but the ClO4- salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe(L2Cl)2][BF4]2 adopts a fixed 2:1 high/low-spin-state population that does not change with temperature below 300 K, whereas [Fe(L2Br)2][BF4]2 and [Fe(L2I)2][BF4]2 form low-spin solvated crystals that are transformed into high-spin powders on drying. The pyrazinyl group in the L2R ligands slightly stabilizes the low-spin state of the complexes, as determined by solution-phase magnetic measurements. The crystal structure of [Fe(L2CCPh)(OH2)z][BF4]2 contains a disordered mixture of six- (z = 3) and seven-coordinate (z = 4) iron centers

CCDC 899454: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 899458: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 899455: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 899459: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 899457: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 899456: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures