Harold A. Goodwin

Spin crossover phenomena in Fe(II) complexes

The behaviour of spin crossover compounds is among the most striking and fascinating shown by relatively simple molecular species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in iron(II) complexes, to offer some rationalisation for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones in order to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.

Critical temperature of the LIESST effect in a series of hydrated and anhydrous complex salts [Fe(bpp)2]X2

Abstract The magnetic properties of a series of hydrated and dehydrated spin crossover compounds derived from the 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligand have been reinvestigated and the light-induced crossover studied at 10 K. The capacity of a compound to retain the light-induced HS information has been estimated through the determination of the T (LIESST). The position of each compound in the T (LIESST)– T 1/2 diagram has been analyzed and the effect of the nature of the salt, hydration degree and cooperativity have been discussed.

Crystal supramolecular motifs: two-dimensional grids of terpy embraces in [ML2]z complexes (L=terpy or aromatic N3-tridentate ligand)

By analysis of crystal packing we have identified a crystal supramolecular motif that is a two-dimensional net of terpy embraces formed by metal complexes [M(terpy)2]2+ (terpy=2,2′:6′,2″-terpyridyl) and similar meridional [M(N3-tridentate)2] complexes. The terpy embrace involves two complexes attracted by one offset-face-to-face (off) and two edge-to-face (ef) interactions by the outer pyridyl rings of the ligand. In many crystals containing small monoanions there is a two-dimensional net of these embraces, in which each complex forms eight ef and four off interactions with its neighbours. The principal axes of the complexes are normal to the layer, which is exactly or approximately planar, and can occur with high (tetragonal) or low crystal symmetry. Grooves that occur on the layer surfaces, formed between parallel central pyridyl rings of the ligands, run in orthogonal directions on the two surfaces of each layer. Anions and solvent molecules in the crystals are usually disordered, in or near the grooves. The net attractive energy of the terpy embrace for a pair of [M(terpy)2]2+ is calculated to be ca. 15 kJ mol-1: in the two-dimensional net the attractive cation···cation energy per cation is ca. 29 kJ mol-1. Inclusion of the anions associated with one layer increases the attractive energy per [M(terpy)2]2+ to the order of 130 kJ mol-1. A variety of ligands, which are minor or major modifications of terpy, also form this supramolecular motif. Hydrogen bonding involving NH functions of these ligands, solvent, and/or anions, does not in general disrupt the motif. In one instance where the [M(N3-tridentate)2] complex is uncharged there is mutual interpenetration of contiguous layers. These infinite two-dimensional nets of octahedral metal complex sites formed as crystal supramolecules are analogous to the two-dimensional gridlike supermolecules formed by extended oligo-chelating ligands. Opportunities for crystal engineering are discussed.

Electronic and Structural Properties of the Spin Crossover Systems Bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) Thiocyanate and Selenocyanate

Essentially high-spin [Fe(bpp)2][NCS]2·2H2O and [Fe(bpp)2][NCSe]2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) were isolated from an aqueous reaction mixture. Both salts undergo an abrupt transition to low spin below room temperature, that for the thiocyanate occurring in two steps and the high-spin AE low-spin AE high-spin cycle being accompanied by hysteresis in both steps. Recrystallization of the salts from nitromethane yielded a mixture from which bright yellow crystals were separated for structure determination. In addition, from the recrystallized selenocyanate, deep red-brown crystals of composition [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2 were obtained. Recrystallized [Fe(bpp)2][NCS]2·2H2O and [Fe(bpp)2][NCSe]2 were identified as high spin with average Fe–N distances of 2.16 and 2.17 A, respectively. In the unit cell of [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2, there are four independent iron atoms, three identified as low spin and the fourth as high spin. All salts crystallize in a layer-type array involving edge-to-face and face-to-face aryl–aryl-type interactions. Hydrogen bonding between pyrazole >NH groups, anions and solvate molecules is observed. The structure of the uncoordinated ligand was also determined, the molecule being found in a planar arrangement with thecis–cis configuration for the pyrazolyl groups relative to the central pyridyl and the >NH group being at the N 2 atom. Hydrogen bonding involving the >NH groups leads to stepped stacks of molecules. The principal difference in the geometry of coordinated and free bpp molecules is a contraction in the angles about the interannular bridges in the chelate rings. [Fe(bpp)2][NCS]2·2H2O: triclinic, space group P1–, a 8.302(6), b 8.446(6), c 21.531(13) A, a 78.78(5), b 82.80(5), g 89.85(4)˚, Z 2. [Fe(bpp)2][NCSe]2: triclinic, space group P1–, a 8.354(4), b 8.409(4), c 19.918(9) A, a 87.02(3), b 83.15(3), g 88.86(3)˚, Z 2. [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2: monoclinic, space group Pn, a 16.425(12), b 20.774(9), c 16.933(14) A, b 90.91(4)˚, Z 8. Uncoordinated bpp: orthorhombic, space group Pna21, a 8.075(3), b 22.479(9), c 5.525(1) A, b Z 4.

Lattice trapping of metastable quintet state bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) bis(tetrafluoroborate), a spin crossover system, and kinetics of the quintet-singlet transformation

Abstract The 5 T 2 ⇌ 1 A 1 spin transition in [Fe(bpp) 2 ][BF 4 ] 2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) is abrupt and complete and shows hysteresis with the transition temperatures T c ↓ = 173 K and T c ↑ = 183 K. Rapid cooling of the sample causes the freezing-in of metastable quintet state species at low temperatures. Relaxation of the metastable quintet to singlet state species within the range 99–114 K follows simple first-order kinetics with an activation energy E a = 19.5 kJ mol −1 .

The influence of steric effects in substituted 2,2′-bipyridine on the spin state of iron(II) in [FeN6]2+ systems

Abstract Tris(ligand)iron(II) complexes of 6-methyl-2,2′- bipyridine (mbpy) and 2-(pyridin-2-yl)quinoline (pyq) display anomalous magnetic properties which are associated with a temperature-induced singlet (1A1) ⇋ quintet (5T2) transition. Two forms of [Fe(pyq)3][ClO4]2 were obtained: one which is essentially high spin over the range 89–300 K and the other which shows an almost complete, gradual transition in this range. In the three complex salts of [Fe(mbpy)3]2+ isolated, the perchlorate, fluoroborate and hexafluorophosphate, appreciable spin- pairing is observed at low temperatures but in no instance is the transition to singlet state species complete at 89 K. Data for solutions of both [Fe (mbpy)3] [ClO4]2 and [Fe(pyq)3] [BF4]2 indicate that negligible spin-pairing is observed down to 210 K. Mossbauer spectral data confirm the change in spin state populations in all solid salts and moreover indicate two sites for the high-spin species at low temperatures. It is believed that these may arise from ordering of anion sites in the lattice. In the room temperature structure of [Fe(mbpy)3] [ClO4]2· 1 2 mbpy disorder of one of the anions as well as the occluded free ligand occurs. In the structure of the complex cation considerable distortion is observed with the average FeN distance being 2.21 A. The ligand molecules are not planar, the pyridyl rings being twisted 4.3, 16.4 and 17.8° about the inter- ligand bridge. The methyl substituent induces a steric barrier to coordination and also greater inter-ligand repulsions than in complexes of bpy and this is presumably responsible for the accessibility of the quintet state for Fe(II). The steric effects of the fused benzene ring in pyq are predicted to be similar. [Fe(mbpy)3] [ClO4]2· 1 2 mbpy crystallises in space group P21/c (Z = 4) with cell parameters a = 19.769(10), b = 13.186(4), c = 15.043(8) A. A total of 2509 reflections with Io > 3σ(Io) were observed and gave a final R = 0.057.

Hydrogen bonding influences on the properties of heavily hydrated chloride salts of iron(II) and ruthenium(II) complexes of 2,6-bis(pyrazol-3-yl)pyridine, 2,6-bis(1,2,4-triazol-3-yl)pyridine and 2,2′∶6′,2″-terpyridine

The structures of a series of hydrated complex chlorides [Fe(btp)2]Cl2·6H2O, [M(bpp)2]Cl2·6.5H2O (M = Fe, Ru) and [Ru(terpy)2]Cl2·6H2O are described (btp = 2,6-bis(1,2,4-triazol-3-yl)pyridine; bpp = 2,6-bis(pyrazol-3-yl)pyridine; terpy = 2,2′∶6′,2″-terpyridine). The ligands are all of the terimine type. [Fe(btp)2]Cl2·6H2O is high spin while [Fe(bpp)2]Cl2·6.5H2O is low spin, the difference in the average Fe–N distance in the two complexes being 0.24 A. In all four complex salts there is extensive hydrogen bonding between the water and chloride ions. In addition, for the pyrazolyl and triazolyl ligands this involves the uncoordinated >NH groups. The arrangement of the cations in the four lattices is discussed in terms of the previously described “terpy embrace” adopted widely by bis(terimine)metal systems.

The effect of pressure on the thermal hysteresis of the first‐order spin transition in bis(1,10‐phenanthroline‐2‐carbaldehyde phenylhydrazone) iron (II) complexes

The effect of pressure on the thermal hysteresis of the high‐spin (5T2) ∏ low‐spin (1A1) transition in [Fe(phy)2](BF4)2 and [Fe(phy)2](ClO4)2 has been studied (phy=1, 10‐phenanthroline‐2‐carbaldehyde phenylhydrazone). An increase of the hysteresis width ΔTc as well as of the residual fractions nres5T2 and (1−n5T2)res with pressure is observed. The quantity (ΔTc)1/2 shows a linear dependence on pressure p as expected from the Landau theory. The increase of the residual fractions is explained by the assumption of decreasing size of the cooperative region with pressure.

Coordination of the strong field di-imine 3,3′-bipyridazine. Structural, magnetic and spectroscopic properties of the Fe(II), Co(II) and Ni(II) complexes

Abstract The di-imine system 3,3′-bipyridazine (L) generates a particularly strong ligand field and this is revealed in the low-spin nature of [FeL2(NCS)2] and of salts of [FeL3]2+, the appearance of a doublet ⇄ quartet transition in [CoL3]2+, and the high energy (13 200 cm−1) of the 3A2g → 3T2g transition in the electronic spectrum of [NiL3]2+. In attempts to prepare bis(ligand) complexes the mixed low-spin cationic, high-spin anionic complexes [FeL3][FeCl4], [FeL3][Fe(NCS)4] and [FeL3][FeCl4O]2 were isolated. These were characterised by magnetic and Mossbauer spectral measurements. The structures of both [FeL3][ClO4]2 and [NiL3][ClO4]2 reveal unusually short FeN (1.927(3) A) and NiN (2.060 A) distances and these, and the high field strength, are believed to result in part from the absence of ortho-hydrogen atoms in the ligand molecules which cause considerable inter-ligand repulsion in complexes of the related di-imine, 2,2′-bipyridine. [FeL3][ClO4]2 crystallises in space group P 3 c1 (Z=2) with cell parameters a=10.281(2), b=10.281(2), c=15.863(4) A. A total of 430 reflections with Io>3σ(Io) was observed and a final value of R=0.045 was obtained. [NiL3][ClO4]2 crystallises in space group P321 (Z=1) with cell parameters a=10.225(3), b=10.225(3), c=8.049(3) A. Stacking faults and twinning create an apparent P 6 2c diffraction symmetry. A total of 266 reflections with Io>3σ(Io) was observed and a final value of R=0.042 was obtained.

The crystal and molecular structure of Cis-dichlorobis(2,2′-biimidazole)iron(III) chloride monohydrate

Possible cis-trans isomerism in octahedral dihalobis(2,2t-biimidazole)metal complexes has been investigated by determination of the crystal structure of the title compound, which crystallises with cisoctahedral coordination stereochemistry: FeN = 2.112(2), 2.209(2), 2.152(2), 2.108(2); FeCl = 2.290(1), 2.326(1). The two NH hydrogen atoms on each biimidazole ligand are hydrogen bonded to chloride and water in the crystal. Apparently ralated to this hydrogen-bonding is the fact that, within each imidazole ring, the CN bond distances to the iron-coordinated nitrogen atom are not significantly different from the CN distances to the hydrogen- bonded NH group. Crystal data: a = 7.625(1), b= 11.682(2), c = 21.533(3) A, β = 113.72(2)°; P21/c; Z = 4 × FeC12H14N8Cl3O; 2749 observed reflections, anisoptropic refinement, R = 0.027.