Detailed investigations of phase transitions and magnetic structure in Fe(iii), Mn(ii), Co(ii) and Ni(ii) 3,4,5-trihydroxybenzoate (gallate) dihydrates by neutron and X-ray diffraction

The effect of cation valency on the complex structures of divalent and trivalent transition metal gallates has been examined using a combination of neutron and synchrotron X-ray powder diffraction, single-crystal X-ray diffraction and XANES spectroscopy. In the divalent frameworks, M(C 7H4O5)·2H2O (M = Mn, Co and Ni), it was found that charge balance was achieved via the presence of protons on the meta-hydroxyl groups. It was also established that these compounds undergo a discontinuous phase transition at lower temperatures, which is driven by the position of the extra-framework water molecules in these materials. By contrast, in the trivalent Fe gallate, Fe(C7H3O 5)·2H2O, it was found that the stronger bonding between the meta-hydroxy oxygen and the cations leads to a weakening of the bond between this oxygen and its proton. This is turn is thought to lead to stronger hydrogen bonding with the extra-framework water. The lattice water is disordered in the Fe(iii) case, which prevents the phase transition found in the M(ii) gallates. Refinement against the neutron diffraction patterns also revealed that the relatively mild microwave synthesis of gallate frameworks in D2O led to an extensive deuteration of the ortho-hydrogen sites on the aromatic ring, which may suggest a more versatile method of deuterating aromatic organics. The antiferromagnetic structure of Co gallate has also been determined. © 2011 The Royal Society of Chemistry

The effect of pressure and substituents on the size of pseudo-macrocyclic cavities in salicylaldoxime ligands

The effect of pressure on the crystal structures of 3-chloro-, 3-methoxy-, 3-methyl- and 3-tert-butylsalicylaldoximes has been investigated. The compounds all form the dimeric structure found in salicylaldoxime form I at ambient pressure, which is based on intermolecular hydrogen bonds between the oximic hydrogen and the phenolic oxygen atoms across an inversion centre. These intermolecular interactions, along with intramolecular phenolic hydrogen to oximic nitrogen atom hydrogen bonds form a pseudo-macrocycle with a R-4(4)(10) ring motif. These hydrogen bonding motifs pre-organize an arrangement of four potential donor atoms for a metal cation which, when the phenol groups are deprotonated, provides an N2O22- pocket well suited to the binding of planar transition metal ions. The radius of the cavity defined by the donor atoms in the dimers is dependent on the nature of the 3-substituent, varying from 1.949 angstrom in the 3-methoxy- to 2.037 angstrom in the 3-Bu-t-derivative. Anisotropic compression of the crystals on the application of pressure results in significant changes in the radii of the cavities in the dimers which decrease by ca. 10% at 6 GPa. During compression of the 3-Bu-t-derivative a single crystal to single crystal phase transition was observed between 0.2 and 1.0 GPa to a new polymorph, 3-tert-butylsalicylaldoxime-II. The phase transition produces an increase in symmetry as the space group changes from P-1 to I2/a, but the intermolecular interactions remain essentially unchanged. No phase transitions were observed in the compression of 3-Cl-, 3-Me- or 3-MeO-salicylaldoxime up to 6.2 GPa

High-Pressure Study of Oxo-bridged Mixed-Valent Mn-III/Mn-IV Dimers

A combination of high-pressure single crystal X-ray diffraction and high-pressure SQUID magnetometry has been used to study two oxo-bridged mixed-valent Mn-III/Mn-IV dimers. [Mn2O2(bpy)(4)](ClO4)(3)center dot 3CH(3)CN, (1.3CH(3)CN; bpy = 2,2'-bipyridine) has been compressed to 2.0 GPa whilst [Mn2O2(bpy)(4)](PF6)(3)center dot 2CH(3)CN center dot 1H(2)O, (2.2CH(3)CN center dot 1H(2)O) could be measured crystallographically up to 4.55 GPa. The PF6 salt of [Mn2O2(bpy)(4)](3+) has never been reported before while 1 has been reported as a hydrate and in a different crystallographic space group. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of both complexes. In particular, in complex 1.3CH(3)CN the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond lengths, whilst in 2.2CH(3)CN center dot 1H(2)O the Mn-O Mn bridging angles and the Mn-O bond lengths are substantially unchanged. Interestingly 2.2CH(3)CN center dot 1H(2)O also shows a constant contraction in nearly all the Mn-N bonds. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 1.3CH(3)CN and 0.84 GPa for 2.2CH(3)CN center dot 1H(2)O

Polymerisation of a Cu(II) dimer into 1D chains using high pressure

Long intermolecular interactions are converted into covalent bonds when the complex [Cu-2(OH)(citrate)(Guanidine)2](-) is exposed to a pressure of 2.9 GPa; the coordination of the Cu centres changes from [4 + 1] to [4 + 2], but on increasing the pressure to 4.2 GPa some of the Cu centres become [4 + 1]-coordinate again

[Mn6] under pressure: a combined crystallographic and magnetic study

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior