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The first example of a heteropolyoxomolybdate containing palladium(IV) was isolated and characterized by X‐ray crystallography. The palladium(IV) hexamolybdate, K0.75Na3.75[PdMo6O24H3.5]·17H2O, was isolated from an aqueous solution at pH 4.5 in the space group P\bar{1}, a10.790(2), b12.244(3), c14.086(3) Å, α 113.77(1), β 90.41(1),γ 107.86(1)°, and the structure was determined using X‐ray diffraction methods, refining to a residual of 0.0301 for 5334 reflections. A formal “[PdMo6O24H3]5–” subunit exhibits the basic Anderson structure, with two [PdMo6O24H3]5–cluster anions in the structure bridged by a hydrogen atom (formally an H+) situated on a center of symmetry to give a “[Pd2Mo12O48H7]9–” dimeric anion. The palladium(IV) atom occupies a slightly distorted octahedral environment, with Pd–O distances ranging from 1.968 to 2.009 Å.

The onestep synthesis of a series of hybrid dibenzo Trögers base analogues bearing at least one halogen atom is described. The strategy involves a reaction between two different anilines and affords hybrid compounds in yields as high as 46 , together with the symmetric Trögers base products. This straightforward approach requires only one chromatographic step and has the potential to replace multistep approaches to hybrid Trögers base compounds. © WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

A new series of (diisopropylcarbamato)(oxido)zinc complexes were obtained by innovative synthetic approaches. The (carbamato)zinc clusters [Zn4(μ4-O)(O2CNiPr2)6] (1) and [Zn8(μ4-O)2(O2CNiPr2)12] (2) were synthesised by an identical reaction procedure involving protolysis of ZnEt2with iPr2NH/CO2and followed by stoichiometric hydrolysis. Recrystallisation of the reaction product from nitrile solvents yielded the tetrazinc complex 1. Conversely, recrystallisation from alkane solvents yielded crystals of the octazinc complex 2. The two structural isomers were readily interconverted simply by recrystallisation from the appropriate solvent. Both complexes were structurally characterised by X-ray crystallography. The tetrazinc complex 1has the quintessential basic zinc acetate type structure. Its dimer, the octazinc complex 2is a new derivative of the [M8(μ4-O)2(O2CNiPr2)12] class, examples of which are known for all other first-row transition metals. The solution chemistry was investigated using variable-temperature NMR spectroscopy and ESI-MS. The new synthetic approach offers a route to other similar complexes. Thus, using Et2NH in place of iPr2NH yielded the known complex [Zn4(μ4-O)(O2CNEt2)6] (3).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The copper(II) complexes [Cu(H2L1)Cl2]2[CuCl4]·CH3CN·H2O (HL1 = N-6-(4-hydroxyphenyl)-2-pyridylmethyl-N,N′,N′-trimethylethylenediamine), [Cu(HL2)Cl2] (HL2 = N-4-hydroxybenzylmethyl-N,N′,N′-trimethylethylenediamine)and [Cu2(μ-L3)2(H2O)(ClO4)]ClO4·H2O (HL3 = N-2-hydroxybenzylmethyl-N,N′,N′-trimethylethylenediamine) were prepared and characterised in acetonitrile solution (by ESI-MS, EPR and UV/Vis-NIR spectroscopy) and in the solid state (by X-ray crystallography). Each copper(II) complex is chiral and has a potential inter-complex hydrogen-bond donor group. Upon crystallisation, all three copper(II) complexes undergo spontaneous enantioselective self-assembly into hydrogen-bond-linked helices.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The complex cis-[PtBr2{PPh2(4-catechol)}2] 1has been synthesized by cleavage of the four methyl groups from cis-[PtCl2{PPh2(4-veratrole)}2] using BBr3, followed by work-up in the presence of excess bromide. An X-ray crystal structure of 1·(ethanol)2confirms that the two catechol rings are adjacent to each other and approximately parallel, and therefore well structured to act as double bidentate ligands for adjacent metal atoms on the surface of a nanocrystal. The crystal packing of 1·(ethanol)2involves intermolecular hydrogen-bonding interactions and a parallel fourfold phenyl embrace between PPh2moieties. Density functional calculations have demonstrated that conformational variability of the aryl rings in cis-[PtBr2{PPh2(4-catechol)}2] is energetically feasible, and two conformations of cis-[PtBr2{PPh2(4-catechol)}2] as a complex ligand for Ti atoms on the various surfaces of the anatase and rutile structures of TiO2have been assessed for geometrical commensurability. Three structural models for adsorbates of cis-[PtBr2{PPh2(4-catechol)}2] on TiO2are developed for anatase (110), anatase (101), and rutile (001).

To investigate the influence of a potentially N4-tripodal amine ligand on the structure and internal exchange processes of its complexes with late transition metals, five rhodium, six palladium and two platinum complexes have been prepared from seven alkyl-bridged N-heterocyclic amine tripodal ligands: tris(2-pyridylmethyl)amine, (2-(2-pyridylethyl))bis(2-pyridylmethyl)amine, bis(2-(2-pyridylethyl))-2-pyridylmethylamine, bis(2-(2-pyridylethyl))amine, ((6-(hydroxymethyl)-2-pyridyl)methyl)bis(2-pyridylmethyl)amine, tris(2-benzimidazolylmethyl)amine (tbima) and tris(3-ethyl-2-benzimidazolylmethyl)amine. Single-crystal X-ray diffraction studies were completed for ten complexes: the d6-rhodium(iii) complexes are octahedral with κ4N-bound ligands, whereas the d8-palladium(ii) and d8-platinum(ii) complexes are square planar, κ3N-bound by the tripodal ligand with a dangling N-donor leg, except for the unusual [Pd2(tbima)2Cl2]Cl2dimer in which each palladium(ii) ion is square planar and bound by two benzimidazole legs from one tbima ligand, one leg from the other tbima ligand and a chloride ancillary ligand. Cation bilayers are a common structural motif in the crystal structures. Variable-temperature 1H NMR studies reveal exchange occurs between the coordinated and dangling N-donor legs in the palladium and platinum complexes. Exchange free energy (ΔG‡c) values have been calculated and some general rules governing the favoured complex structures and exchange pathways elucidated. The palladium(ii) and platinum(ii) complexes of a ligand with an pyridylethyl leg are unstable with respect to elimination of vinylpyridine.

Abstract The diquinoline derivative 12 was found to yield the diquinolinium dibromide 13 trihydrate on crystallisation from 2-bromoethanol. X-Ray crystallography revealed that the diquinolinium dications pack as parallel (and anti-parallel) stacks, with the bromide ions and water molecules occupying inter-stack channels. The interplay of aromatic offset face-face, N+-H ·s O, bromide ·s H-O, C-H ·s bromide and bromine ·s bromide interactions determines this solid-state structure.

Copper(ii) bromide and chloride complexes of the new heptadentate ligand 2,6-bis(bis(2-pyridylmethyl)amino)methylpyridine (L) have been prepared. For the bromide complexes, chains of novel, approximately C₂-symmetric, chiral [Cu₂(L)Br₂]2+ ‘wedge-shaped’ tectons are found. The links between the dicopper tectons and the overall chirality and packing of the chains are dictated by the bromide ion content, not the counter anion. In contrast, the chloride complexes exhibit linked asymmetric [Cu₂(L)Cl₃]+ tectons with distinct N₃CuCl₂ and N₄CuCl₂ centres in the solid. The overall structures of the dicopper bromide and chloride units persist in solution irrespective of the halide. The redox chemistry of the various species is also described.

Copperii bromide and chloride complexes of the new heptadentate ligand 2,6-bisbis2-pyridylmethylaminomethylpyridine L have been prepared. For the bromide complexes, chains of novel, approximately C2-symmetric, chiral Cu2LBr22‘wedge-shaped’ tectons are found. The links between the dicopper tectons and the overall chirality and packing of the chains are dictated by the bromide ion content, not the counter anion. In contrast, the chloride complexes exhibit linked asymmetric Cu2LCl3tectons with distinct N3CuCl2and N4CuCl2centres in the solid. The overall structures of the dicopper bromide and chloride units persist in solution irrespective of the halide. The redox chemistry of the various species is also described.

Deprotonation of calix6arenes with barium in methanol followed by the addition of TiOPri4 or ZrOBun4 is effective in the formation of novel dimeric 2 1 barium–titaniumivzirconiumiv calix6arene complexes. In these complexes a central TiivZriv coordinated in the exo-position connects the two calix6arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titaniumzirconium centre and participating in cation–π interactions. A homometallic barium calix6arene dimer was also prepared wherein the calix6arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix6arenes through bridging methanol ligands. In this and the heterometallic complexes, cation–π complexation of the Ba2ion within the 1,3 alternate conformation of calix6arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr2ion did not form similar complexes in methanol, and the formation of an analogous 2 1 strontium–titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation–π interaction. The results show i that a subtle interplay of solvation strength, coordination array type and cavitycation size influences the accessibility of heterobimetallic complexes based on calix6arenes, and ii a synergistic endo–exobinding behaviour.

Deprotonation of calix[6]arenes with barium in methanol followed by the addition of [Ti(OPri)₄] or [Zr(OBun)₄] is effective in the formation of novel dimeric 2 ∶ 1 barium–titanium(iv)/zirconium(iv) calix[6]arene complexes. In these complexes a central Ti(iv)/Zr(iv) coordinated in the exo-position connects the two calix[6]arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium/zirconium centre and participating in cation–π interactions. A homometallic barium calix[6]arene dimer was also prepared wherein the calix[6]arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix[6]arenes through bridging methanol ligands. In this and the heterometallic complexes, cation–π complexation of the Ba2+ ion within the 1,3 alternate conformation of calix[6]arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr2+ ion did not form similar complexes in methanol, and the formation of an analogous 2 ∶ 1 strontium–titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation–π interaction. The results show (i) that a subtle interplay of solvation strength, coordination array type and cavity/cation size influences the accessibility of heterobimetallic complexes based on calix[6]arenes, and (ii) a synergistic endo–exo binding behaviour.

Reaction of calix[4]arene with alkaline earth metals (Ca/Sr/Ba) in methanol followed by the addition of various titanium(IV) sources afforded novel dimeric 1:1 alkaline earth/titanium(IV) calix[4]arene complexes. In each complex the central core of the crystallographically imposed centrosymmetric dimer consists of a rhombus of alternating alkaline earth metals and μ3-oxo-centres. Five terminal methanol ligands form seven-coordinate calcium and strontium centres, while the barium centres are bridged by two additional methanol ligands, making them nine-coordinate. Each oxo centre is the apical ligand of a square-pyramidal titanium(IV) centre, whose basal plane ligands are the four phenolate oxygens from a calix[4]arene in the symmetrical cone-conformation. Deprotonation of calix[4]arene with potassium metal, followed by the addition of a titanium(IV) source containing at least one acetylacetonate ligand, formed a mixed potassium/titanium complex based on one calix[4]arene. The potassium cation in this complex binds within the π-basic calix[4]arene cavity, whilst titanium is octahedrally complexed in the exo-position, by four phenolates and one acetylacetonate. The alkaline earth titanium dimers and potassium/titanium monomers could be interconverted merely by changing the solvent, with the appropriate cations and ligands present in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The crystallisation and crystal structures of three new crystal forms of MePh₃P+I₃− are described, bringing to four the number of polymorphs of this compound. Crystallisations were from alcohols, mixed alcohols, and dichloromethane, and are reproducible. There are strong similarities between all four of the crystal packing arrangements, which are solvent-free. In all structures the MePh₃P+ cations associate in two-dimensional nets using edge-to-face, offset-face-to-face, and methyl-to-face motifs, and the I₃− ions lie between these nets, with subtle variations in structure caused by sliding displacements of relatively constant cation-anion assemblies. The related compound EtPh₃P+I₃− has very similar crystal packing. The interactions between the I₃− and MePh₃P+ are mainly C–H⋯I, with some phenyl faces parallel to I₃−. The major electrostatic component of the lattice energy is associated with the occurrence of I₃− ions against the faces of nets of cations, consistent with this structural feature being the most invariant aspect of the crystal packing, while the structural variability is associated with the less stabilising cation⋯cation motifs. It appears that these crystals provide a good example of a system where shape is not a dominant feature in crystal packing, and that the tetramorphs represent almost equi-ergic local minima in a relatively deep section of the lattice energy hypersurface. The crystal packing of (MePh₃P+)₂I₈2− is similar.

A proposal for a redox-linked conformational gate to proton translocation—a proton pump gate—based upon a transition-metal redox-switchable hemilabile ligand (RHL) system is made. Consideration of the requirements for such a system reveals copper(II) to be the ideal metal centre. To test the proposal and, thereby, to provide an artificial proton pump gate, the copper coordination chemistry of three tris(pyridylmethyl)amine (tpa) ligands with one “leg” (PY*) substituted at the 6-position of the pyridine ring by a dimethoxyphenyl (L¹), a hydroquinone (H2L²) or a quinone (L³) substituent has been investigated. Crystal structures of sp-[Cu(κ⁴N-L¹)Cl]Cl·3 H2O (sp=square pyramidal), sp-[Cu(κ³N-H2L²)Cl2] and tbp-[Cu(κ⁴N,κO-HL²)][PF6] (tbp=trigonal bipyramidal) have been determined. The Cu^I complexes [Cu(L)(MeCN)n]⁺ (L=L¹, H2L²) display physicochemical properties consistent with a “dangling” PY* leg; from the NMR spectra, the barriers to inversion of the ligand amine donor for the Cu^I complexes are estimated to be within the range of about 30–45 kJ mol⁻¹. In the Cu^II complexes, coordination of the PY* leg is finely balanced and critically depends on the nature of the PY* substituent and the availability of potential co-ligand(s). For example, tbp-[Cu(κ⁴N-L¹)Cl]⁺ reacts cleanly with Cl⁻ ions to afford sp-[Cu(κ³N-L¹)Cl2]; Vis/NIR spectrophotometric titrations suggest the affinity of tbp-[Cu(κ⁴N-L¹)Cl]⁺ for Cl⁻ ion in dichloromethane is 9.7×10² and is at least 10⁴-fold greater than that of tbp-[Cu(κ⁴N-L³)Cl]⁺. The complex sp-[Cu(κ³N-H2L²)Cl2] has a “dangling” PY* leg, in which an intramolecular OH(hydroquinone)···N(pyridine) hydrogen bond “ties-up” the pyridyl nitrogen atom, and reacts with Brønsted bases to give tbp-[Cu(κ⁴N,κO-HL²)]⁺. Two-electron oxidation of sp-[Cu(κ³N-H2L²)Cl2] is linked to loss of two protons and a conformational change, and affords tbp-[Cu(κ⁴N-L³)Cl]⁺. The [Cu(κ³N-H2L²)Cl2]–[Cu(κ⁴N-L³)Cl]⁺ system provides a first demonstration of the critical step in the proposed proton pumping cycle, namely a redox-driven and proton-linked conformational change. The possible biological relevance of this work to proton pumping in cytochrome c oxidase is mentioned.

A proposal for a redox‐linked conformational gate to proton translocation—a proton pump gate—based upon a transition‐metal redox‐switchable hemilabile ligand (RHL) system is made. Consideration of the requirements for such a system reveals copper(II) to be the ideal metal centre. To test the proposal and, thereby, to provide an artificial proton pump gate, the copper coordination chemistry of three tris(pyridylmethyl)amine (tpa) ligands with one “leg” (PY*) substituted at the 6‐position of the pyridine ring by a dimethoxyphenyl (L1), a hydroquinone (H2L2) or a quinone (L3) substituent has been investigated. Crystal structures of sp‐[Cu(κ4N‐L1)Cl]Cl⋅3 H2O (sp=square pyramidal), sp‐[Cu(κ3N‐H2L2)Cl2] and tbp‐[Cu(κ4N,κO‐HL2)][PF6] (tbp=trigonal bipyramidal) have been determined. The CuIcomplexes [Cu(L)(MeCN)n]+(L=L1, H2L2) display physicochemical properties consistent with a “dangling” PY* leg; from the NMR spectra, the barriers to inversion of the ligand amine donor for the CuIcomplexes are estimated to be within the range of about 30–45 kJ mol−1. In the CuIIcomplexes, coordination of the PY* leg is finely balanced and critically depends on the nature of the PY* substituent and the availability of potential co‐ligand(s). For example, tbp‐[Cu(κ4N‐L1)Cl]+reacts cleanly with Cl−ions to afford sp‐[Cu(κ3N‐L1)Cl2]; Vis/NIR spectrophotometric titrations suggest the affinity of tbp‐[Cu(κ4N‐L1)Cl]+for Cl−ion in dichloromethane is 9.7×102and is at least 104‐fold greater than that of tbp‐[Cu(κ4N‐L3)Cl]+. The complex sp‐[Cu(κ3N‐H2L2)Cl2] has a “dangling” PY* leg, in which an intramolecular OH(hydroquinone)⋅⋅⋅N(pyridine) hydrogen bond “ties‐up” the pyridyl nitrogen atom, and reacts with Brønsted bases to give tbp‐[Cu(κ4N,κO‐HL2)]+. Two‐electron oxidation of sp‐[Cu(κ3N‐H2L2)Cl2] is linked to loss of two protons and a conformational change, and affords tbp‐[Cu(κ4N‐L3)Cl]+. The [Cu(κ3N‐H2L2)Cl2]–[Cu(κ4N‐L3)Cl]+system provides a first demonstration of the critical step in the proposed proton pumping cycle, namely a redox‐driven and proton‐linked conformational change. The possible biological relevance of this work to proton pumping in cytochrome coxidase is mentioned.

The preparation and properties of the racemic lattice inclusion host 6α,13α-dibromo-5bα,6,12bα,13-tetrahydropentaleno[1,2-b:4,5-b']diquinoline (8) are described. Strong hydrogen bonding interactions are not possible for this versatile host compound. Instead, weaker interactions (such as aryl offset face-face, aryl edge-face, halogen-halogen, halogen-π, and C−H···N synthons) compete against each other to generate the inclusion structure of lowest energy. The guests are contained within molecular pens formed by two hosts enclosing each guest. These host molecules are not directly linked at the corners of the pen, but rather assemble with their neighbours into layers of pens by means of aryl offset face-face interactions. The C−H···N weak hydrogen bond plays a dominant role in these structures by linking adjacent layers edge-edge through two distinct types of double motifs. One is the previously recognised aryl C−H···N dimer, while the other is a new bifurcated Ar−H···N···H−CBr−Ar interaction. The X-ray structures of nine inclusion compounds formed by 8 are reported. Seven of these in space group P21/c have isostructural host lattices where the layers of pens are stacked to produce guest channels. Several different guest arrangements are observed. In contrast, (8)2·(CH3CCl3) in space group C2/c and (8)2·(CF3C6H5) in P212121 have the pen layers stacked so as to enclose their guests in cages. The structure of the nonbrominated precursor 7, which undergoes self-resolution on crystallisation, is also described in crystal engineering terms. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Deprotonation of calix4arenes by alkali metal in methanol using simple benchtop procedures is effective in the formation of novel monomeric and dimeric titaniumiv complexes. For the larger alkali metal cations, Kand Cs, favourable complexation within the π-basic calix4arene cavity facilitates the formation of monomeric titaniumiv complexes when oligomer formation is inhibited by an acetylacetonate ligand. In contrast, the smaller Liand Naions preferentially form dimeric complexes with exo-bridging alkali and titaniumiv metal centres. A dimeric complex is also obtained for potassium in the absence of acetylacetonate. The solid state structures of the K–Ti and Na–Ti calix4arene dimer complexes show different structural characteristics depending on the nature of the alkali metal.

The complex Fe3-bpp2FeCN5(NO) 3-bpp 2,6-bispyrazol-3-ylpyridine undergoes an abrupt, thermally-induced 1A1↔ 5T2spin transition associated with a narrow ∼3 K hysteresis loop T12↓ 181 K; T12↑ 184 K. The transition is accompanied by a change in crystallographic space group P4nccat 294 K; Pncaat 100 K along with a change in the average Fe–N distance of 0.21 . The complex cation and anion are linked by hydrogen bonding involving each of the four pyrazolyl-NH moieties of each complex cation and the four equatorial cyano groups of each anion, resulting in layers of 4,4-nets. The hydrogen bonding remains essentially the same in both phases but the relative orientations of the cations and anions change significantly to allow for the change in the dimensions of the complex cation, accounting for the cooperativity associated with the spin change.

Deprotonation of calix[4]arenes by alkali metal in methanol using simple benchtop procedures is effective in the formation of novel monomeric and dimeric titanium(iv) complexes. For the larger alkali metal cations, K+ and Cs+, favourable complexation within the π-basic calix[4]arene cavity facilitates the formation of monomeric titanium(iv) complexes when oligomer formation is inhibited by an acetylacetonate ligand. In contrast, the smaller Li+ and Na+ ions preferentially form dimeric complexes with exo-bridging alkali and titanium(iv) metal centres. A dimeric complex is also obtained for potassium in the absence of acetylacetonate. The solid state structures of the K–Ti and Na–Ti calix[4]arene dimer complexes show different structural characteristics depending on the nature of the alkali metal.

A series of metal(ii) complexes of [6-(2',5'-dimethoxyphenyl)-2-pyridylmethyl]bis(2-pyridylmethyl)amine (L) have been prepared. X-Ray crystal structures have been determined for L and its metal(ii) chloride complexes for Mn, Fe, Co, Ni, Cu and Zn and the results compared. The preparation and crystal structure of the unusual carbonato-bridged copper-tetramer [Cu₄(L)₄(CO₃)₂][BF₄]₄·5.2H₂O is also described. The solution-state structures of the complexes are deduced from their physicochemical and spectroscopic properties. The Zn(ii) complex, [ZnCl₂(L)], shows inversion about the ligand amine group on the NMR timescale — results from a variable temperature NMR study are presented and allow estimation of the barrier to amine inversion as 56 ± 0.5 kJ mol−1. Overall, it is found that the intramolecular steric interactions introduced by substitution of the tris(2-pyridylmethyl)amine (tpa)-skeleton by a single 6-aryl group result in significant changes to the structures and properties of the resulting metal complexes: in particular the aryl-substitution in L causes (i) a weaker ligand-field compared to tpa favouring high-spin complexes, (ii) a tendency toward lower coordination numbers, and (iii) hemilability in the Ni(ii) and Cu(ii) complexes — the aryl-substituted leg of the ligand (L) is coordinatively labile.

With a view towards later studies of co-operativity in heteronuclear complexes with hard classical (oxygen-activating) and soft organometallic (organic-substrate binding) metal centres, four novel ditopic N₃P-donor ligands (L1–L4), each comprising triphenylphosphine tethered to an N,N'-bis(2-pyridyl-2-ethyl)amine (bpea) or a 1,4-diisopropyl-1,4,7-triazacyclononane (tacn*) N₃-donor group, have been designed and prepared by reductive aminations of ortho- and meta-(diphenylphosphino)benzaldehydes with bpea (for L1 and L3) and tacn* (for L2 and L4). A range of κN_n,κP-chelate mononuclear complexes have been isolated from the reactions of the ortho-substituted ligands, L1 and L2, with Cu(i), Zn(ii) and Pt(ii) sources, and the X-ray crystal structures of [Cu(L1)][PF₆], [Cu(L2)][PF₆] (communicated in: S. E. Watkins, D. C. Craig and S. B. Colbran, J. Chem. Soc., Dalton Trans., 1999, 1539) and [PtCl(L1)][PF₆] have been determined. Six complexes with the phosphine of L1–L4 co-ordinated to a softer [Pt(ii), Ir(i) or W(0)] metal centre and with dangling, metal-free N₃-donor domains have been prepared: for the ortho-substituted ligands L1 and L2, it was necessary to protect the hard, more basic N₃-donor domains by protonation (pH control) to prevent formation of κN_n,κP-chelate mononuclear complexes; for the meta-substituted ligands L3 and L4, pH control was unnecessary as the phosphine group selectively binds to the softer metal ions. The complex trans-[IrCl(CO)(L3)₂] reversibly forms a dioxygen adduct. An Ir(iii)Cu(ii)₂ and four Pt(ii)Cu(ii)₂ heterometallic complexes were prepared by adding hard Cu(ii) ions to the Ir(i) and Pt(ii) complexes with metal-free N₃-donor domains, and the full characterisation of these is described. The tungsten(0) carbonyl complex [W(CO)₅(L3)], with a metal-free N₃-bpea domain, was prepared for a study of metal ion recognition. No perturbation of the carbonyl region of the IR spectrum was observed when metal ions were added. The effect of submolar quantities of heterometallic complexes, obtained by adding a first d-series metal(ii) ion (2 equivalents) to [IrCl(CO)(L3)₂], on the oxidation of styrene by oxygen in methylethyl ketone has been assayed: inhibition of the oxidation is observed with the %conversion and the product selectivity dependant on the metal(ii) ion.

Three new nitrogen-donor ligands with ferrocenyl pendants, namely ferrocenylmethylbis(2-pyridylethyl)amine (L1), ferrocenylmethylbis(2-pyridylmethyl)amine (L2), and 1-ferrocenylmethyl-4,7-diisopropyl-1,4,7-triazacyclononane (L3), have been synthesised. Copper(ii) complexes of these ligands have been made and their physicochemical and redox properties characterised. Crystal structures of [Cu(L2)X₂·0.5Et₂O (X = Cl, Br), [Cu(L2)(OTf)₂(CH₃OH)], [Cu(L3)Cl₂], and [{(L3)Cu}₂(μ-OH)₂](OTf)₂·0.5Et₂O are reported. Oxidation of each complex with ceric ion affords the corresponding FeIII ~ CuII species. Variations in ligand design, the choice of co-ligand (chloride, bromide, trifluoromethanesulfonate (OTf) or solvent) and the choice of solvent allowed, in the case of acetonitrile solutions of [Cu(L1)(H₂O)₂](OTf)₂, the close matching of the FeIII–FeII and CuII–CuI electrochemical couples. [Cu(L1)(H₂O)₂](OTf)₂ reacts with dioxygen in acetonitrile, but is stable in other solvents such as tetrahydrofuran. Electronic spectra of the complex in acetonitrile exhibit a prominent ferrocenium ion band. These results are interpreted in terms of an intramolecular electron transfer equilibrium between FeII ~ CuII and FeIII ~ CuI tautomers for the complex that leads to it reacting with dioxygen; their possible biological relevance is discussed.

Certain diol molecules crystallize from solution as unidirectional double-stranded ladders. When these two strands assemble in-phase, a step-ladder arrangement results with the diols linked by (O–H)4 cycles of hydrogen bonds. Using racemic diol samples, the preferred arrangement has two enantiomerically pure strands of opposite handedness. If the two strands assemble out-of-phase, a staircase-ladder results where the molecules are connected by a hydrogen-bonded (O–H)n chain. The preferred arrangement contains enantiomerically pure diol molecules surrounding a 21 screw axis. If a racemic diol is used then either the crystals contain both (+)- and (–)-ladders or, alternatively, self-resolution occurs to yield a conglomerate. New diols (7)–(9) were synthesized and an X-ray investigation confirmed the prediction that these would produce further double-stranded ladders. These structures revealed new variants of the staircase-ladder where both enantiomers are present, inclusion compounds are formed, or crystallographically independent diol molecules are involved. Manuscript received: 27 August 2001. Final version: 5 November 2001.

The treatment of 3-(4-chlorophenyl)-4,6-dimethoxyindole with phosphoryl chloride in dimethylformamide generates the regioselective product 3-(4-chlorophenyl)-4,6-dimethoxyindole-7-carbaldehyde. This serves as a valuable ligand precursor and, on reaction with selected di- and tri-amines, yields a number of new acyclic ligands. Treatment of these ligands with selected divalent transition metal acetates in the presence of triethylamine in acetonitrile affords a range of neutral complexes, encompassing both tetrahedral and square-planar metal geometries, of differing nuclearities. In addition to the chemical and physical properties of the ligands and complexes reported, single crystal analyses were employed for geometric analysis in two ligand derivatives: the first, a mononuclear distorted tetrahedral racemic nickel(II) complex; the second, a dinickel heterochiral complex.

The di(1,8-naphthryridine) 7 and diquinoxaline 12 derivatives were synthesised as potential new lattice inclusion hosts where strong hydrogen bonding interactions would be absent. A number of potential supramolecular synthons (such as aryl face-face, aryl edge-face, halogen-halogen, C−H···N, nitrogen-halogen) were expected to be accessible, with competing combinations of these weak attractions providing the best (but probably different) type of host-guest structure in each case. While the former compound turned out to be unstable, the latter proved to be a versatile host which preferred to trap small polychloroalkane guests. The X-ray structures of 12·(chloroform)2, (12)2·(tetrahydrofuran), and (12)2·(1,1,2,2-tetrachloroethane) are reported and shown to have different lattice packing where the guests occupy layers, parallel tubes, and molecular boxes, respectively. The detailed interplay of the above synthons in forming these structures is described in crystal engineering terms. Most significantly, the C−H···N weak hydrogen bond plays a major role in all three inclusion structures. Both single linear and double cyclic interactions are involved in molecular edge-edge assembly of the host 12. Several new types of double cyclic interactions were discovered revealing that the C−H···N interaction is a key synthon for crystal engineering involving nitrogen heteroaromatic compounds.

The formation of anionic-cluster species from compounds containing the ‘open’ (i.e. non-cluster) [BiFe4(CO)16]3– (or [Bi{Fe(CO)4}4]3–) ion following loss of CO as a result of photolytic, pyrolytic and chemical means has been investigated. Photolysis of salts of the [BiFe4(CO)16]3– ion in acetonitrile, where the associated cations are [N(PPh3)2]+ or [NEt4]+, produce a range of species including [Bi2Fe5(CO)17]2–, [Bi2Fe4(CO)9]2–, [Bi2Fe3(CO)9]2–, [Fe2(CO)8]2– and [Fe4(CO)13]2–, with the distribution of species being cation dependent. The previously unknown [Bi2Fe5(CO)17]2– ion was obtained as the [N(PPh3)2]+ salt, crystallizing in the triclinic space group P 1, with a 15.851(7), b 17.199(8), c 20.020(8) Å, α 114.72(2), β 95.51(3), γ 113.01(2)o, Z 2 and V 4338(3) Å3. The structure was determined by X-ray diffraction methods to an R of 0.035 (Rw = 0.040) for 6778 independent observed reflections. The anion consists of a central square-based pyramidal ‘nido-Bi2[Fe(CO)3]3’ cluster unit, with two external Fe(CO)4 fragments attached to the two trans four-coordinate bismuth atoms located in the base of the pyramid. A thermogravimetric study of [NEt4]3[BiFe4(CO)16] indicated decomposition above 150˚C, with an apparent loss of three CO molecules per [BiFe4(CO)16]3– ion below 160˚C. Decomposition was complete by 205˚C and also involved pyrolysis of the [NEt4]+ cations. Pyrolysis of solid [NEt4]3[BiFe4(CO)16] at 160˚C for several hours under N2 implicated an intermediate species in the Bi/Fe carbonyl anion system, probably [Bi2Fe3(CO)9]2–, in addition to the [Fe2(CO)8]2– ion, although the final product obtained was found to be [Bi2Fe4(CO)13]2–. Reaction of [NEt4]3 [BiFe4(CO)16] with trimethylamine N-oxide in acetonitrile, in an attempt to selectively remove CO by chemical means, lead to the slow formation of [Bi2Fe4(CO)13]2– and a little [Bi2Fe5(CO)17]2–, although the [Bi2Fe5(CO)17]2– was observed to react slowly with (CH3)3NO to give [Bi2Fe4(CO)13]2– as the major product. Conversion of [Bi2Fe5(CO)17]2– to [Bi2Fe4(CO)13]2– is likely initiated through nucleophilic attack at the axial carbon site of a pendant Fe(CO)4 group, according to extended-Hückel molecular orbital calculations. Calculations also show that there is no instability introduced in having a second pendant Fe(CO)4 attached to the central cluster unit, and the anion can be produced by reaction of [Bi2Fe4(CO)13]2– with Fe2(CO)9 in tetrahydrofuran, through the addition of an Fe(CO)4 fragment to the three-coordinate bismuth atom in the anion. The other product of the reaction is volatile Fe(CO)5, which is easily separated from the desired product. Keywords: Anionic clusters; photolysis; pyrolysis; bismuth; organotransition metal compounds; group 15 elements; trimethylamine N-oxide.

The ligand tris[3-(4-methoxyphenyl)-5-methylpyrazol-1-ylmethyl]amine (L), the copper(II) complexes [CuLCl₂] 1, [CuLCl][SbF₆] 2 and [CuL(H₂O)][SbF₆]₂ 3, and the copper(I) complex [CuL][CF₃SO₃] 4 have been prepared. Complexes 1–3 are reduced by [BPh₄]⁻ ion in a range of solvents to afford [CuL]⁺ ion. The [CuL]⁺ ion does not react with dioxygen nor carbon monoxide but does react with chloride ion. Physicochemical and cyclic voltammetry data are reported for 1–4, along with crystal structure analyses of 1–3 and [CuL][CuCl₂]. There are some notable differences between the structures and properties of these copper complexes and those reported for analogues with pyridyl- or imidazolyl amine ligands.

The helical tubulands are a family of alicyclic diols which crystallise in chiral space group P3121 and whose lattices can contain guest molecules enclosed within parallel tubes. New examples can be designed following a series of rules based on molecular structure and symmetry. In order to probe the latter requirement, and also to increase host–guest interactions, the behaviour of 2,5,8-trimethyltricyclo[5.3.1.13,9]dodecane-syn-2,syn-8-diol 4was investigated using X-ray methods. Although molecules of 4have no C2symmetry they still form the helical tubuland lattice on crystallisation. In the pure apohost 4their symmetry-breaking C5 methyl groups protrude into the tube volumes where they randomly adopt one of two equivalent orientations. Although the pendant methyl groups reduce the volume available for guest inclusion, they create irregular tube wall surfaces and interact more effectively with guest molecules. The helical tubulate inclusion compounds of 4with diisopropyl ketone, benzene, toluene, and o-xylene show significant local methyl group ordering around a given guest molecule which performs a crucial templating role in formation of the product. In all cases, however, crystallographic C2symmetry results in the solid through overall disorder of these diol orientations. Hence this, rather than strict molecular symmetry, is the true symmetry requirement of the P3121 helical tubuland lattice.

Three potentially tripodal, tris(2-pyridylmethyl)amine (tpa) ligands with hydroxymethyl substituents, [6-(hydroxymethyl)-2-pyridylmethyl]bis(2-pyridylmethyl)amine (HL¹), bis[6-(hydroxymethyl)-2-pyridylmethyl]2-(pyridylmethyl)amine (H₂L²), tris[6-(hydroxymethyl)-2-pyridylmethyl]amine (H₃L³), have been prepared in good yields. From these ligands, the copper(II) co-ordination complexes [Cu(HL¹)Cl]Cl 1, [Cu(H₂L²)Cl]X (X = Cl 2 or p-toluenesulfonate 3), [Cu(L²BF₂)][BF₄] 4 a complex of a novel macrocyclic anion with a tpa core, [Cu(H₃L³)Br_0.43Cl_0.57]₂[Cu(Br_0.43Cl_0.57)₂(Br_0.97Cl_0.03)₂] 5 with compositionally disordered bromo and chloro co-ligands, [Cu(H₃L³)Br]Br 6 and the unusual trimer [Cu₃{H₃(L³)₂}Br][BF₄]₂ 7 have been synthesized and their spectroscopic and redox properties and crystal structures obtained.

Comparison of the Mössbauer spectral data for low-spin tris(ligand) iron(II) complexes of the series of bidentate N–N ligands 2,2'-bipyridine (bpy), 4-(pyridin-2-yl)thiazole (4pyt), 2-(pyridin-2-yl)thiazole (2pyt), 4,4'-bithiazole (4bt) and 2,2'-bithiazole (2bt) shows a progressive reduction in the p-acceptor character of the ligand with increase in the number of thiazole rings, the effect being greater for the 4-thiazolyl derivatives. The structure of [Fe(4pyt)3]2+ is remarkably similar to that of [Fe(bpy)3]2+ with an average Fe–N distance of 1.96 Å. Attempts to characterize structurally [Fe(2bt)3]2+ were not successful and led to crystallization of [Fe(2bt)2(H2O)2] [ClO4]2·(2bt)4. In the complex cation, which has the trans structure, the average Fe–N distance is 2.21 Å and the Fe–O distance is 2.08 Å. Mössbauer spectral data indicate a quintet ground state for this species. [Fe(4pyt)3] [BF4]2·H2O: monoclinic, space group P21/c, a 10.213(7), b 13.436(6), c 23.12(1) Å, b 109.65(3)˚, Z 4. [Fe(2bt)2(H2O)2] [ClO4]2·(2bt)4: triclinic, space group P–1, a 10.822(7), b 11.451(8), c 12.945(8) Å, a 70.31(5), b 69.92(5), g 63.32(6)˚, Z 1. Keywords. X-Ray crystallography; iron complexes; Mössbauer spectra; diimine ligands.

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 Æ low-spin Æ 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 Å, 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) Å, 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) Å, 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) Å, b 90.91(4)˚, Z 8. Uncoordinated bpp: orthorhombic, space group Pna21, a 8.075(3), b 22.479(9), c 5.525(1) Å, b Z 4. Keywords. Iron(II); thiocyanate; selenocyanate; spin transition; X-ray crystal structure.

The X-ray crystal structures of K6[CoII W12O40]·~16H2O and K5[CoIIIW12O40]·~16H2O are reported. The compounds are isostructural, hexagonal, space group P 6222, and have Z 3 with a 19.118(7), c 12.383(9) Å, V 3919.6(35) Å3 , and a 19.111(2), c 12.509(2) Å, V 3956.6(9) Å3 , respectively. Both anions exhibit the standard α-Keggin structure, which consists of a central tetrahedrally coordinated CoII or CoIII , surrounded by four groups of three edge-sharing octahedra (W3O13 subunits) which are linked in turn to each other and to the central CoO4 tetrahedron by shared oxygen atoms at the vertices. Although thermogravimetric analyses show that there are c. 16 water molecules per formula unit in both compounds, only six and three water molecules for the CoII and CoIII compounds, respectively, could be crystallographically located. The others are presumably zeolitic and highly disordered in nature. Structural differences between the anions on replacement of CoII by CoIII , as well as comparisons with the related [PW12O40]3– and [SiW12O40]4– ions, which contain (formally) PV and Si IV , respectively, are discussed. This comparison indicates that the W3O13 subunits become progressively more isolated with increasing size of the central heteroatom from PV to CoII , while the bonding within an individual W3O13 subunit becomes weaker. Extended-Hückel molecular orbital calculations are used to examine stability changes in the polyoxotungstate framework, using the actual polyoxotungstate geometries of the anions, on variation in size of the central heteroatom. These are compared to that in the [H3W12O40]5– ion, which has two centrally located H+ ions, but no steric effects. The studies show that no major changes occur in the overall stability of the framework, but that there is a redistribution in the contributions of the two types of bridging oxygen atoms to the stability of the framework, which parallels the progressive isolation and weaker bonding in the W3O13 subunits. Keywords. X-Ray crystallography; heteropolyoxotungstate complexes; tungsten.

The four 5-alkenyl-1-pyrroline 1-oxide-5-carboxylic esters1–4undergo regioselective intra molecular cycloaddition to give the cycloadducts5–8, which can be hydrogenolyzed to generate a range of azabicyclic compounds and unusual cyclic amino esters.

The crystal structures of bis(2,2′:6′,2″-terpyridine)ruthenium(II) perchlorate hydrate, bis(2,2′:6′,2″- terpyridine)osmium(II) perchlorate hemihydrate and bis((1,10-phenanthrolin-2-yl)(pyridin-2-yl)- amine)iron(II) tetrafluoroborate dihydrate are described. In the terpyridine complexes the ruthenium-nitrogen distances and the corresponding osmium-nitrogen distances are not significantly different. In both complexes the ligand geometry and the metal ion environment show the distortions usual for bis(terpyridine) systems. Distortions are less marked in the bis((1,10-phenanthrolin-2-yl)(pyridin-2-yl)amine)iron(II) cation in which each tridentate unit forms one five-membered and one six-membered chelate ring. [Ru(trpy)2] [ClO4]2.(H2O)1.1: tetragonal, space group I41/a, a, b12·527(2), c40·202(11) Å, Z8. [Os(trpy)2] [ClO4]2.(H2O)0·5: monoclinic, space group P21/n, a8·842(3), b8·861(1), c39·22(2) Å, β93·89(2)°, Z4. [Fe(phpyam)2] [BF4]2.(H2O)2: triclinic, space group P-1, a12·43(1), b12·45(1), c13·35(1) Å, α 62·70(10), β 78·55(8), γ 72·46(9)°, Z2.

A terminal pyridine ring in terpyridine has been replaced by a triazole or substituted triazole moiety to give the tridentates 6-(1,2,4-triazol-3-yl)-2,2´-bipyridine, 6-(1-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine, 6-(5-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine and 6-(1,5-dimethyl-1,2,4-triazol-3-yl)-2,2´-bipyridine. The effects of coordination of these on the electronic and structural properties of the [FeN6]2+species are considered. The primary effect is a reduction in the ligand field but salts of the [FeN6]2+derivatives are essentially low spin (singlet ground state) at room temperature. A small and temperature-dependent population of quintet state species is indicated by Mössbauer spectra but in no instance is a complete singlet → quintet transition observed up to 383 K. The electronic spectra of the [FeN6]2+species are similar to that of [Fe(trpy)2]2+but the energy of the charge-transfer transition is higher, indicating less effective metal ion → donor π-interaction than in the terpyridine complex. Consistent with this, the Mössbauer isomer-shift values are greater than for [Fe(trpy)2]2+and the E½values for the [FeN6]3+/[FeN6]2+couples are less positive than for [Fe(trpy)2]2+. Structural data reveal average Fe–N distances normal for low-spin iron(II) but with greater distortion of the coordination environment than in [Fe(trpy)2]2+. When present, the >NH group of the triazole moiety is hydrogen-bonded to solvate water and the anion in the hydrated tetrafluoroborate salts. Bis(6-(1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate tetrahydrate: monoclinic, space group P21/c, a15·917(7), b18·432(5), c10·845(5) Å, β 94·36(2)°, Z4; bis(6-(1-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate: triclinic, space group P−1, a11·68(1), b11·96(2), c12·46(2) Å, α 82·93(8), β 72·30(9), γ 68·49(10)°, Z2; bis(6-(5-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate monohydrate: monoclinic, space group P21/c, a10·715(4), b25·818(5), c11·039(4) Å, β 90·08(2)°, Z4; bis(6-(1,5-dimethyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) triflate tetrahydrate: orthorhombic, space group Pbcn; a20·654(6), b11·618(3), c16·080(6) Å, Z4.

Iron(II) and nickel(II) bis(ligand) complexes of 6-(5-methyl-1,2,4-oxadiazol-3-yl)-2,2′-bipyridine (L) are described. The ligand field in the iron complex is close to that at the singlet ( 1A1) ? quintet ( 5T2) crossover and magnetic and Mössbauer spectral evidence indicates that a spin transition occurs in salts of the iron complex but is centred above room temperature. The structures of [FeL2] [CF3SO3]2.CH3CN and [NiL2] [BF4]2.CH3CN were determined and both are very similar to the structures of the corresponding terpyridine complexes. Spectroscopic data indicate that for the iron complex π-interaction between the metal and the ligand is less than that in the terpyridine system. [FeL2] [CF3SO3]2.CH3CN is monoclinic, space group P21/c; a8 . 232(5), b25 . 273(10), c17 . 306(10) Å, β 92 . 37(3)°, Z4; [NiL2] [BF4]2.CH3CN is monoclinic, space group P21/c; a8 . 136(2), b17 . 558(2), c23 . 783(7) Å, β 109 . 32(1)°, Z4.

Reactions of several methyl imines with dimethyl acetylenedicarboxylate and with mercaptoacetic acid have been studied to investigate further the capricious behaviour of this functionality. The 1 : 1 adduct of imine (1) and dimethyl acetylenedicarboxylate is confirmed as the pyrroloisoquinoline structure (7) by X-ray methods [C17H17NO5, P-1, a9·037(2), b9·276(2), c10·033(2) Å, α 81·99(1), β 81·28(1), γ 64·9(2)°, Z2, R0·044]. New products from (2) and dimethyl acetylenedicarboxylate, the 1 : 1 adduct (10) which is readily converted into the known product (9), and the 1 : 2 adduct (12), are described. The structure of the last compound, which arises through a rearrangement process, was determined by X-ray crystallography [C27H36N2O9,P ca21, a19· 083(4), b15·217(3), c9·669(2) Å, Z4, R0·038]. Methyl imines (1) and (4) undergo cyclocondensation with mercaptoacetic acid yielding thiazolidinone products but others are inert under similar conditions.

The crystal structures of Na3[H6CoMo6O24].8H2O and K6[H4Co2Mo10O38].7H2O have been determined by X-ray diffraction. The monomer, Na3[H6CoMo6O24].8H2O, is triclinic, space group P-1, a6·451(1), b10·866(2), c10·922(2) Å, α 109·20(1), β 106·90(1), γ 95·43(1)°, V676·3(2) Å3, Z 1, and the structure was solved to an R1value of 0·0243 (wR20·0784) for 3761 independent observed reflections. The anion exhibits the well known Anderson structure with six octahedral MoO6edge-sharing units surrounding the central ‘CoO6’ octahedron, with all metals in a common plane. The dimer, K6[H4Co2Mo10O38].7H2O, is monoclinic, space group P21/c, a 11·795(5), b 11·626(2), c 29·731(13) Å, β 95·33(2)°, V4059(3) Å3, Z 4, and the structure was solved to an R1value of 0·0215 (wR20·1040) for 6546 independent observed reflections. The anion can be derived from the monomeric hexamolybdocobaltate(III) ion by removing one ‘MoO5’ unit (ignoring the hydrogen atoms) from each of two monomer anions, turning one 180° around a CoO6octahedral diagonal, and joining them to create two CoO6octahedra sharing an edge. Cyclic voltammetry shows that both anions are irreversibly reduced at low pH values (4·0–4·5), likely as a result of chemical reactions following the initial reduction steps. At higher pH values (4·5–5·4), a change in speciation occurs in both cases, most likely the result of the formation of less highly protonated species, which also display irreversible electrochemical behaviour.

Tris(ligand)iron(II) complexes of 2-(pyrazol-3-yl)pyridine (3ppH) and 3-(thiazol-2-yl)pyrazole (3tpH) undergo temperature-induced singlet (1A1) ? quintet (5T2) transitions. The transition in [Fe(3ppH)3] [CF3SO3]2.2H2O is continuous and centred above room temperature while that in the anhydrous triflate salt is discontinuous and is centred below room temperature. The latter transition occurs via a thermal hysteresis loop of width 12 K, Tc↓ and Tc↑ being 229 and 241 K, respectively. The displacement of the transition to lower temperature in the anhydrous salt is believed to be associated with the loss of hydrogen bonding involving the uncoordinated pyrazole >NH group and solvate water. In [Fe(3tpH)2(3tp)] [ClO4].2H2O and [Fe(3tpH)2(3tp)] [BF4].2H2O (3tp is the deprotonated ligand) continuous transitions are observed, centred below room temperature. In these instances the displacement is consistent with the intrinsically weaker field of the bidentate system containing two five-membered heterocycles. Structural data were obtained for [Fe(3ppH)3][CF3SO3]2.2H2O, [Fe(3tpH)3] [BF4]2.1·5H2O and [Ni(3tpH)3] [BF4]2.2(3tpH). The average metal–nitrogen distances in the complexes are 1·97, 2·18 and 2·09 Å, severally. The large difference in the distances for the two iron complexes arises from the different ground states: a singlet for the 3ppH complex and a quintet for the 3tpH complex. In all three salts there is extensive hydrogen bonding involving the pyrazole >NH groups, the anions and the solvate molecules. [Fe(3ppH)3] [CF3SO3]2.2H2O: monoclinic, space group P21/c, a12·33(1), b24·44(1), c12·55(1) Å, β 115·27(4)°, Z 4. [Fe(3tpH)3] [BF4]2.1·5H2O: monoclinic, space group C2/c, a41·56(2), b16·418(3), c18·154(7) Å, β 106·94(2)°, Z 8. [Ni(3tpH)3] [BF4]2.2(3tpH):Pbcn, a14·928(2), b15·310 (3), c17·882 (3) Å, Z4.

The X-ray crystal structures of 5b,6,12b,13-tetrahydropentaleno[1,2-b:4,5-b´]diquinoline 5,12-dioxide (5) and (6α,7α,14α,15α)-7,15-dibromo-6,7,14,15-tetrahydro-6,14-methanocycloocta[1,2-b:5,6-b´]diquinoline 5,13- dioxide (6) reveal two new packing modes of potential importance as supramolecular synthons for molecular assembly. Both involve intermolecular edge–edge association of an aryl ring and an N-oxide functionality. One is a bifurcated Ar–H···O(N)···H–Ar interaction which on repetition gives infinite chains of the molecules. The second is a 10-membered centrosymmetric dimer also involving Ar–H···O–N interactions but which merely links two neighbouring molecules of (6). The latter arrangement is a structural analogue of a recently reported dimeric centrosymmetric eight-membered Ar–H···N synthon.

2,2′-Bipyridine-6-carbothioamide (bpytm) and 2,2′-bipyridine-6-carboxamide (bpyam) are NNS and NNO donors, respectively, in their cationic bis(ligand)iron(II) and bis(ligand)nickel(II) complexes. The former ligand provides the stronger field and salts of [Fe(bpytm)2]2+have a singlet ground state, while those of [Fe(bpyam)2]2+have a quintet ground state. The magnetism and the electronic and Mössbauer spectra of salts of these cations have been measured. The low-temperature Mössbauer spectra of iron(II) complex salts of the carboxamide indicate, for the perchlorate and triflate salts, but not for the fluoroborate salt, a partial transition to singlet-state species. The mode of coordination of the ligands is indicated by infrared spectral data and has been confirmed by determination of the structures of [Ni(bpytm)2] Cl2.4H2O, [Ni(bpyam)2] [BF4]2.H2O and [Fe(bpyam)2] [BF4]2. In addition, the structures of the free ligands have been determined. Hydrogen bonding is present in the free ligands and their complexes. 2,2′-Bipyridine-6-carbothioamide: monoclinic, space group P21/c, a8·265(3), b11·175(2), c11·114(4) Å, β 94·47(2)°, Z4. 2,2′-Bipyridine-6-carboxamide: monoclinic, space group P21/c, a13·581(2), b9·926(1), c16·824(3) Å, β 116·481(7)°, Z8. [Ni(bpytm)2] Cl2.4H2O: triclinic, space group P-1, a9·291(5), b12·426(7), c13·425(7) Å, α 113·54(3), β 95·63(3), γ 94·43(3)°, Z2. [Ni(bpyam)2][BF4]2.H2O: triclinic, space group P-1, a10·663(5), b10·861(6), c12·799(6) Å, α 68·70(4), β 77·84(4), γ 78·47(4)°, Z2. [Fe(bpyam)2] [BF4]2: orthorhombic, space group P bcn, a12¡317(6), B12¡609(4), c16·644(8) Å, Z4.

Some 5-alkenyl-1-pyrroline-1-oxide-5-carboxylic esters have been prepared by alkylation of a related nitrone carboxylic ester. On thermal treatment, they undergo regioselective intramolecular 1,3-dipolar cycloaddition to give a range of cycloadducts, which can be hydrogenolysed to generate azabicyclic compounds and unusual cyclic amino esters.

The 1,5-dichloro-(1) and 1,5-dibromo-(2) naphthalene-2,6-diols form isostructural lattices incorporating ...O-H...O-H...O-H... hydrogen bonding surrounding 21 screw axes. Each phenolic hydroxy group participates in one donor and one acceptor hydrogen bond. When crystallised from dioxane, both compounds form new 1:1 co-crystalline materials whose lattices are closely related but not isostructural. All inter-phenolic hydrogen bonding is now absent with each hydroxy group acting instead as a donor to a dioxane oxygen atom. In consequence, the hydrogen bonded layer structures of pure1 or2 are now replaced by hydrogen bonded chain arrangements. All four crystal structures are analysed and compared in crystal engineering terms through discussion of the various types of other weak intermolecular attractions involved in their lattice constructions.