Polyiron oxides, oxyhydroxides and hydroxides as models for biomineralisation processes. In: Hill HAO, Sadler PJ, Thomson AJ, editors. Metal Sites in Proteins and Models. Berlin: Springer Berlin Heidelberg; 1997. pp. 1-38. [DOI: 10.1007/3-540-62870-3_1]

α-Pyridinyl Alcohols, α,α'-Pyridine Diols, α-Bipyridinyl Alcohols, and α,α'-Bipyridine Diols as Structure Motifs Towards Important Organic Molecules and Transition Metal Complexes

BACKGROUND

The preparation and use of pyridinyl alcohols as ligands showed incredible increment in the past three decades. Important property of pyridinyl alcoholato ligands is their strong basicity, which is mainly due to the lack of resonance stabilization of the corresponding anion. This strongly basic anionic nature gives them high ability to make bridges between metal centers rather than to bind to only one metal center in a terminal fashion. They are needed as ligands due to their ability to interact with transition metals both covalently (with oxygen) and hemilabile coordination (through nitrogen).

OBJECTIVE

The review focuses on the wide application of α-pyridinyl alcohols, α,α'-pyridine diols, α- bipyridinyl alcohols, and α,α'-bipyridine diols as structure motifs in the preparation of important organic molecules which is due to their strongly basic anionic nature.

CONCLUSION

It is clear from the review that in addition to their synthetic utility in the homogeneous and asymmetric catalytic reactions, the preparation of the crown ethers, cyclic and acyclic ethers, coordinated borates (boronic esters), pyridinyl-phosphine ligands, pyridinyl-phosphite ligands, and pyridinyl-phosphinite ligands is the other broad area of application of pyridinyl alcohols. In addition to the aforementioned applications they are used for modeling mode of action of enzymes and some therapeutic agents. Their strongly basic anionic nature gives them high ability to make bridges between metal centers rather than to bind to only one metal center in a terminal fashion in the synthesis of transition metal cluster complexes. Not least numbers of single molecule magnets that can be used as storage of high density information were the result of transition metal complexes of pyridinyl alcoholato ligands.

Binding of ligands containing carbonyl and phenol groups to iron(iii): new Fe6, Fe10 and Fe12 coordination clusters

The initial use of ligands 2'-hydroxyacetophenone (HL¹), 2-hydroxybenzophenone (HL²) and 2,2'-dihydroxybenzophenone (H₂L³) in iron(iii) chemistry is described. The syntheses and crystal structures are reported for five iron(iii) clusters: [Fe₁₀O₄(OMe)₁₄(L¹)₆(MeOH)₂](NO₃)₂·3MeOH (1·3MeOH), [Fe₁₂O₄(OH)(OMe)₁₇(L¹)₈](ClO₄)₂·2H₂O (2·2H₂O), [Fe₁₀O₄(OMe)₁₄Cl₄(L²)₄(MeOH)₂] (3), [Fe₁₀O₄(OMe)₁₄(L²)₆(py)₂](ClO₄)₂·MeOH (4·MeOH), where py = pyridine, and [Fe₆O₂(OEt)₆(O₂CMe)₂(L³)₂(HL³)₂] (5). The molecular structures of the decanuclear clusters 1, 3 and 4 are organized around a {Fe₁₀(μ₄-O)₄(μ₃-OMe)₂(μ-OMe)₁₂}⁸⁺ core consisting of ten {Fe₃O₄} face-sharing defective cubane units. The core of 2 consists of a {Fe₁₂(μ₄-O)₄(μ₃-OMe)₄(μ-OH)(μ-OMe)₁₃}¹⁰⁺ unit composed of twelve {Fe₃O₄} face-sharing defective cubanes. The ligands (L¹)^- and (L²)^- in 1-4 adopt the O,O'-bidentate chelating coordination mode and their roles are to terminate the further aggregation of the Fe^III/O^2-/RO^- cores. Complex 5 contains the {Fe₆(μ₄-O)₂(μ-OEt)₆(μ-O_carbonyl)₂}⁴⁺ core, where the μ-O_carbonyl atoms are the bridging carbonyl oxygens of the two η¹:η²:η¹:μ (L³)^2- ligands; the (HL³)^- groups behave as O_phenolate, O_carbonyl-bidentate chelating ligands with the neutral hydroxyl group being unbound to the Fe^III atoms. The core is composed of four {Fe₃O₄} face-sharing defective cubanes. The Fe^III atoms in 1-5 are all six-coordinate with distorted octahedral geometries. The IR spectra of the complexes are discussed in terms of the known coordination modes of the ligands and the ionic character of nitrates and perchlorates. Variable-temperature magnetic susceptibility and variable-field magnetization measurements establish that 2, 3 and 5 have S = 3, 0 and 5 ground states, respectively. The susceptibility data for 5 were fitted using a 3-J model indicating the simultaneous presence of both antiferromagnetic and ferromagnetic Fe^IIIFe^III exchange interactions. Known magnetostructural correlations in oxido-bridged iron(iii) systems have been applied to rationalise the magnetic behaviour of the three clusters. The results of the present study demonstrate the utility of HL¹, HL² and H₂L³ in the stabilisation of robust iron(iii)/oxido/alkoxido clusters.

Access to an unusual Fe9 core topology from the initial use of tricine in iron(III) cluster chemistry

We describe an enneanuclear Fe(III) complex with an unusual boat-shaped core topology and an S=11/2 ground state, assembled using the ligands tricine {N-(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)glycine, H(4)L} and 2-phenoxybenzoate.

Discrete tetrairon(III) cluster exhibiting a square-planar Fe4(mu4-O) core: structural and magnetic properties

The aerobic reaction of the Schiff-base ligand N-(benzimidazol-2-yl)salicylaldimine (Hbisi) with iron(II) perchlorate in methanol leads to the formation of the remarkable coordination compound [Fe(4)(mu(4)-O)(mu-MeO)(4)(bisi)(4)](ClO(4))(2) x 4 MeOH (1), whose single-crystal X-ray structure reveals the presence of a discrete Fe(III)(4)(mu(4)-O) core. Magnetic and Mossbauer studies both show that the exchange interaction within the square tetranuclear iron(III) unit is dominated by the central bridging mu(4)-oxido ligand, the involvement of the mu-methoxido bridges being negligible.

An alcoholysis route to a Cu16 cluster, and the influence of the alcohol

A systematic investigation of the alcoholysis of copper(II) pivalate (piv(-)) solutions has led to a series of Cu(2), Cu(6), and Cu(16) products, depending on the alcohol used as solvent. When Pr(n)OH or Bu(n)OH was employed, the products were the clusters [Cu(16)O(4)(OH)(4)(OR)(8)(piv)(12)(ROH)] (R = Pr(n), Bu(n)) that are almost isostructural. The Cu(16) clusters are antiferromagnetically-coupled with an S = 0 spin ground state.

Dissolution of insulating oxide materials at the molecular scale

Our understanding of mineral and glass dissolution has advanced from simple thermodynamic treatments to models that emphasize adsorbate structures. This evolution was driven by the idea that the best understanding is built at the molecular level. Now, it is clear that the molecular questions cannot be answered uniquely with dissolution experiments. At the surface it is unclear which functional groups are present, how they are arranged, and how they interact with each other and with solutes as the key bonds are activated. An alternative approach has developed whereby reactions are studied with nanometre-sized aqueous oxide ions that serve as models for the more complicated oxide interface. For these ions, establishing the structure is not a research problem in itself, and bond ruptures and dissociations can be followed with much confidence. We review the field from bulk-dissolution kinetics to the new isotope-exchange experiments in large oxide ions.

Modelling calcium carbonate biomineralisation processes

The structure-directing influence of the organic dicarboxylates malonate, succinate, glutarate and adipate as templating species on the hydrothermal formation of CaCO(3) was investigated at different temperatures (60, 80, 90, 120, 150 and 190 degrees C) and with a range of molar ratios of [Ca(2+)]/[templating species] (20, 14.3, 10, 7.7, 5, 1, 0.5 and 0.33). In the presence of the dicarboxylates, one, two or three polymorphs of CaCO(3) - calcite, aragonite and vaterite - could be formed, depending on the reaction conditions. In addition changes in crystal morphology were observed for the CaCO(3) polymorphs depending on the concentration of the template. In contrast, synthesis under ambient conditions of temperature and pressure resulted only in calcite formation, although template-dependent morphological changes were again observed. Crystalline products were all characterized by powder X-ray patterns and SEM (Scanning Electron Microscopy) micrographs. The ambient reactions with the chelating, dinucleating carboxylato ligands H(3)heidi and H(5)hpdta produce more profound changes in calcite morphology. With H(3)heidi rounded calcite crystals with shapes similar to that of otoliths are formed and with H(5)hpdta the formation of microtrumpets of constructed from bundles of nanocrystals of calcite is observed. The possible mode of action of these ligands on calcite formation is discussed in the context of known coordination chemistry with other metal ions.