Malcolm A. Halcrow

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands.

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

Crystal structure of the precursor of galactose oxidase: An unusual self-processing enzyme

Galactose oxidase (EC 1.1.3.9) is a monomeric enzyme that contains a single copper ion and catalyses the stereospecific oxidation of primary alcohols to their corresponding aldehydes. The protein contains an unusual covalent thioether bond between a tyrosine, which acts as a radical center during the two-electron reaction, and a cysteine. The enzyme is produced in a precursor form lacking the thioether bond and also possessing an additional 17-aa pro-sequence at the N terminus. Previous work has shown that the aerobic addition of Cu2+ to the precursor is sufficient to generate fully processed mature enzyme. The structure of the precursor protein has been determined to 1.4 Å, revealing the location of the pro-sequence and identifying structural differences between the precursor and the mature protein. Structural alignment of the precursor and mature forms of galactose oxidase shows that five regions of main chain and some key residues of the active site differ significantly between the two forms. The precursor structure provides a starting point for modeling the chemistry of thioether bond formation and pro-sequence cleavage.

The kinetics of crystal growth in the presence of tailor-made additives

Abstract The influence of tailor-made additives on the crystallisation of organic materials is examined from both a structural and kinetic viewpoint. In particular it seems that the adsorption mechanism of Cabrera and Vermilyea provides a convenient framework in which to discuss the observed kinetic effects. Some new data are reported for the {012} and {101} faces of L-asparagine crystals growing from aqueous solution in the presence of L-glutamic acid.

Interpreting and controlling the structures of six-coordinate copper(II) centres – When is a compression really a compression?

Many experimental observations of ‘inverse’ or ‘quenched’ Jahn–Teller effects in copper(II) compounds in fact reflect a normal Jahn–Teller-elongated configuration that is masked by structural disorder. On the other hand, a small number of copper(II) complexes are known that genuinely exhibit these unusual structures. It requires careful interpretation of spectroscopic and structural data to distinguish these two scenarios.

Antisymmetric exchange in two tricopper(II) complexes containing a [Cu3(μ3-OMe)]5+ core

Reaction of CuX2(X-=Cl- or Br-) with 2 molar equivalents of 3[5]-(2,4,6-trimethylphenyl)pyrazole (HpzMes) in MeOH in the presence of NaOH yields [Cu3X(HpzMes)2(micro-pzMes)3(micro3-OMe)]X (X-=Cl- or Br-). Crystal structures of these compounds show almost identical triangles of Cu(II) ions, centred by a triply bridging methoxide ligand and with three edge-bridging pyrazolide groups. The mesityl substituents on the bridging pyrazolide ligands are arranged in HT, HH, TT fashion. chi(M)T for both compounds decreases steadily with decreasing temperature, reaching 0.40 cm(3) mol(-1) K at 70 K before decreasing further below 40 K. This low temperature behaviour could not be interpreted using conventional superexchange Hamiltonians, but was reproduced by an alternative model that incorporated an additional antisymmetric exchange term. This interpretation was confirmed by the Q-band EPR spectra of the two compounds. NMR experiments show that the structures of these compounds are not retained in solution, in contrast to other closely related tricopper compounds. These are the first examples of triangular Cu(II) compounds bearing a [Cu3micro3-OR)]5+(R is not equal to H) core motif, and the first triangular compounds showing antisymmetric exchange to have been analysed by both susceptibility and EPR measurements.

Iron(II) complexes with a terpyridine embrace packing motif show remarkably consistent cooperative spin-transitions

Six structurally related iron(II) complexes show remarkably similar abrupt thermal spin-transitions.

An iron(II) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis

[FeL2][BF4]2·2H2O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1 : 1 high : low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1AA) or by thermal dehydration (phase 1BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.

A study of the thermal and light induced spin transition in [FeL2](BF4)2 and [FeL2](ClO4)2 L=2,6-di(3-methylpyrazol-1-yl)pyrazine.

The spin crossover compounds [FeL2](BF4)2, L=2,6-di(3-methylpyrazol-1-yl)pyrazine and [FeL2](ClO4)2 have very unusual two stage spin transitions which are initially steep and then become more gradual. A detailed variable temperature single crystal X-ray diffraction study has shown that the course of the spin transition is controlled by an order-disorder transition in the counter anions. The high and low spin states both crystallise in the tetragonal space group I4, the structures of the high and low spin states are presented at 290 and 30 K, respectively. The title compounds are shown to undergo LIESST (Light Induced Excited Spin State Trapping) under irradiation with either red or green laser light with wavelengths of 632.8 and 532.06 nm, respectively, at 30 K. The cell parameters for the tetragonal photo-induced metastable high spin state at this temperature are a= 9.169(6), c= 17.77(1) A for [FeL2](ClO4)2 with an increase in unit cell volume of 21 A3, and a= 9.11(1), c= 17.75(2) A and an increase in volume of 42.8 A3 for [FeL2](BF4)2.

Structural diversity in iron(II) complexes of 2,6-di(pyrazol-1-yl)pyridine and 2,6-di(3-methylpyrazol-1-yl)pyridine

The syntheses, magnetochemistry and crystallography of [Fe(L1)2]I0.5[I3]1.5 (1), [Fe(L1)2][Co(C2B9H11)2]2 (2) and [Fe(L2)2][SbF6]2 (3) (L1 = 2,6-di(pyrazol-1-yl)pyridine; L2 = 2,6-di(3-methylpyrazol-1-yl)pyridine) are described. Compounds 1 and 3 are high-spin between 5-300 K. For 1, this reflects a novel variation of an angular Jahn-Teller distortion at the iron centre, which traps the molecule in its high-spin state. No such distortion is present in 3; rather, the high-spin nature of this compound may reflect ligand conformational strain caused by an intermolecular steric contact in the crystal lattice. Compound 2 exhibits a gradual high --> low spin transition upon cooling with T(1/2) = 318 +/- 3 K, that is only 50% complete. This reflects the presence of two distinct, equally populated iron environments in the solid. One of these unique iron centres adopts the same angular structural distortion shown by 1 and so is trapped in its high-spin state, while the other, which undergoes the spin-crossover, has a more regular coordination geometry. In contrast with 3, the solvated salts [Fe(L2)2][BF4]2 x 4 CH3CN and [Fe(L2)2][ClO4]2 x (CH3)2CO both undergo gradual thermal spin-transitions centred at 175 +/- 3 K.

Steric effects on the electronic and molecular structures of nickel(II) and cobalt(II) 2,6-dipyrazol-1-ylpyridine complexes

Abstract The complexes [M(L1R)2](BF4)2 (M=Ni, Co; L1R=2,6-dipyrazol-1-ylpyridine [L1H], 2,6-bis-{3-iso-propylpyrazol-1-yl}pyridine [L1Pri], 2,6-bis-{3-phenylpyrazol-1-yl}pyridine [L1Ph], 2,6-bis-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine [L1Mes]) and [M(L2)2](BF4)2 (M=Ni, Co; L2=2-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}-6-{5-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine) have been prepared. Single crystal structure determinations of [M(L1H)2](BF4)2 (M=Ni, Co) and solvates of [Ni(L1Mes)2](BF4)2, [Co(L1Mes)2](ClO4)2 and [Co(L2)2](BF4)2 all show six-coordinate metal centres with local near-D2d symmetry. The L1Mes and L2 mesityl substituents have only a small effect on the MN{pyrazole} (M=Ni, Co) bond lengths in these compounds. The d–d spectra of the complexes show that L1Mes is a significantly better donor ligand than L1H, L1Pri or L1Ph, and that L1Pri is a weaker ligand than might be expected purely on inductive grounds. A combination of UV–Vis/NIR, EPR, NMR and magnetic measurements have demonstrated that all the Co(II) compounds are high-spin in the solid state and in solution at 290 K.