Chris S. Hawes

Partial spin crossover behaviour in a dinuclear iron(II) triple helicate

Reported herein are the synthesis, structural, magnetic and Mössbauer spectroscopic characterisation of a dinuclear Fe(II) triple helicate complex [Fe(2)(L)(3)](ClO(4))(4).xH(2)O (x = 1-4), 1(H(2)O), where L is a bis-bidentate imidazolimine ligand. Low temperature structural analysis (150 K) and Mössbauer spectroscopy (4.5 K) are consistent with one of the Fe(II) centres within the helicate being in the low spin (LS) state with the other being in the high-spin (HS) state resulting in a [LS:HS] species. However, Mössbauer spectroscopy (295 K) and variable temperature magnetic susceptibility measurements (4.5-300 K) reveal that 1(H(2)O) undergoes a reversible single step spin crossover at one Fe(II) centre at higher temperatures resulting in a [HS:HS] species. Indeed, the T(1/2)(SCO) values at this Fe(II) centre also vary as the degree of hydration, x, within 1(H(2)O) changes from 1 to 4 and are centred between ca. 210 K-265 K, respectively. The dehydration/hydration cycle is reversible and the fully hydrated phase of 1(H(2)O) may be recovered on exposure to water vapour. This magnetic behaviour is in contrast to that observed in the related compound [Fe(2)(L)(3)](ClO(4))(4)·2MeCN, 1(MeCN), whereby fully reversible SCO was observed at each Fe(II) centre to give [LS:LS] species at low temperature and [HS:HS] species at higher temperatures. Reasons for this differing behaviour between 1(H(2)O) and 1(MeCN) are discussed.

Hysteretic carbon dioxide sorption in a novel copper(II)-indazole-carboxylate porous coordination polymer

The synthesis, structural and gas sorption studies of a porous Cu(II) coordination polymer featuring 1H-indazole-5-carboxylic acid (H(2)L) are presented. [Cu(HL)(2)] is a thermally and hydrolytically robust 4-connected 3D coordination polymer of NbO topology and is replete with 1D channels that permit selective and hysteretic sorption of CO(2).

Self-assembly of discrete and polymeric metallosupramolecular architectures from cyclen-derived ligands

Two divergent ligands 1,4,7,10-tetrakis-(4-cyanobenzyl)-1,4,7,10-tetraazacyclododecane L1 and 1,4,7,10-tetrakis-(4-carboxybenzyl)-1,4,7,10-tetraazacyclododecane dihydrochloride dihydrate H6L2·2Cl·2H2O have been prepared and structurally characterised, and the solid-state structures of four new metal complexes have been elucidated. Ligand L1 was found to form a dimeric complex of the form [Ag2(L1)2]·2ClO4 when reacted with silver perchlorate, in which a macrocycle-bound Ag(I) ion is coordinated by the nitrile group of an adjacent ligand, forming a tightly-bound pair of Ag(I) ions supported by intramolecular π–π interactions. Compound H6L2·2Cl·2H2O was prepared by a solvothermal recrystallization following hydrolysis of the ester precursor, and forms a densely packed three-dimensional structure containing numerous hydrogen bonding interactions. Complexes [Ni(H4L2)Cl]2·[NiCl4]·2EtOH 2 and [Co(H4L2)Cl]2·[CoCl4]·2EtOH 3 were characterised as isostructural discrete macrocycle-bound Ni(II) and Co(II) species where the formation of rare hydrogen-bonded carboxylic acid tetramers gives rise to a series of 2-dimensional sheets, containing checkerboard-type cavities encapsulating alternating tetrachloridometallate anions and regions containing disordered solvent molecules. Complex poly-{[Cu2Cl(H2L1)(HOEt)]2·[CuCl4]·5H2O} 4 consists of a series of two-dimensional sheets formed from a macrocycle-bound CuCl species linked by copper paddlewheel clusters and hydrogen-bonded carboxylic acid dimers. Charge balance is again achieved by tetrachloridometallate anions, and the sheets undergo parallel 2D → 2D interpenetration. The influence of the flexible geometry and donor ability of the methylene-linked pendant arms is clearly visible when comparing discrete complex 1 with hydrogen-bonded networks 2 and 3 and polymeric species 4. The presence of tetrachloridometallate counterions resulting from the presence of excess chloride ions in the reaction mixtures also provides powerful structure-directing effects in complexes 2, 3 and 4.

The influence of anion, ligand geometry and stoichiometry on the structure and dimensionality of a series of AgI-bis(cyanobenzyl)piperazine coordination polymers

Two new bis-(cyanobenzyl)piperazine ligands have been prepared, and have been used to prepare five Ag(I) coordination polymers which have been structurally characterised. Ligand N,N′-bis((4-cyanophenyl)methyl)piperazine L1 gave two structurally related two-dimensional coordination polymers poly-[Ag2(L1)(CF3SO3)2] 1 and poly-[Ag2(L1)(CF3SO3)2(C3H6O)1.5] 2 when reacted with silver trifluoromethanesulfonate in tetrahydrofuran and acetone, respectively. Compounds 1 and 2 display similar ligand geometries but form different extended networks, as a result of the inclusion of coordinating solvent molecules and the resulting influence on the flexible metal geometry. Reaction of N,N′-bis((3-cyanophenyl)methyl)piperazine L2 with silver ions gave rise to three new structurally characterised species. Complex poly-[Ag2(L2)(C3H6O)2]2(PF6) 3 forms a one-dimensional chain structurally related to 1. Complex poly-[Ag2(L2)(C3H8O)2(CF3SO3)2] 4 formed under similar conditions to 3, except with the inclusion of a coordinating anion which links equivalent chains to those found in 3 into a two-dimensional sheet. Finally, complex poly-[AgL2]CF3SO35 was formed simply by altering the reaction stoichiometry, producing a three-dimensional PtS network containing linear, acentric anion channels. Analysis of the structures reveals that the ligand geometries are largely consistent, leading to the extended structures being more influenced by the nature of the anions and solvent molecules.

Discrete and Polymeric Cu(II) Coordination Complexes with a Flexible bis-(pyridylpyrazole) Ligand: Structural Diversity and Unexpected Solvothermal Reactivity

Reported here is the synthesis and structural characterisation of five copper complexes derived from the bis-bidentate ligand 4,4′-methylenebis(1-(2-pyridyl)-3,5-dimethylpyrazole), L. Complex 1, [Cu2L(CH3COO)4(OH2)2]·6H2O, is a single stranded unsaturated helical species that forms a highly connected three-dimensional hydrogen-bonding network, whereas [Cu2L(NO3)4], 2, is a coordination polymer derived from [Cu2L] fragments linked together via bridging nitrate anions to yield undulating two-dimensional sheets with (6,3)-topology. Complexes 3, 4, and 5 co-crystallise within a single batch when L is reacted under solvothermal conditions with Cu(NO3)2·2.5H2O in acetonitrile, and each contains a co-ligand formed by either decomposition of the solvent or ligand. Complex 3, [Cu4(NO3)4(µ-CH3COO)2(µ-OH)2L2], forms an unusual discrete cyclic tetrameric species containing acetate co-ligands derived through acetonitrile hydrolysis; whereas complex 4, [CuL(C2O4)(NO3)], forms a one-dimensional coordination polymer containing bridging oxalate co-ligands, formed through hydrolysis and oxidation of acetonitrile. Complex 5, [Cu2L(µ-CN)2], is a two-dimensional coordination polymer with (6,3) topology where bridging between Cu(i) centres is furnished by cyanide co-ligands, suggesting a ligand decomposition pathway for its origin, and produced with concomitant reduction of the Cu(ii) starting reagent. Having initially obtained 3, 4, and 5 serendipitously each were then prepared as pure phases by careful adjustment and control of the reaction conditions (reactant stoichiometry, concentrations, and solvothermal temperature), details of which are discussed.

Synthesis of an Fe(II) dinuclear triple helicate from a novel -bis-(N-pyrazolyl)pyridine ligand, [Fe2L3]4+: solution and solid-state studies

Reported herein is the preparation of a new -bis-bidentate ligand 4,4′-methylene-bis-(1-(2-pyridyl)pyrazole), L, which forms a dinuclear triple helicate complex [Fe2L3]4+, 1, on reaction with Fe(BF4)2·6H2O in acetonitrile or nitromethane solution. The formation of 1 was studied by 1H NMR and UV–vis spectroscopic titration experiments, which identified the existence of dinuclear triple-stranded species in solution (most likely the rac-helicate), and this was corroborated by electrospray mass-spectrometry experiment which demonstrated the presence of an [Fe2L3]4+ parent ion. Careful choice of crystallisation solvents has allowed the isolation of two solvate species with different solid-state structures: [Fe2L3]·4(BF4)·1.3(H2O), 1A, in which the helicates pack efficiently via anion–π interactions along the helical axis and direct π–π stacking via parallel fourfold aryl embrace interactions, giving rise to narrow 1D solvent channels; and [Fe2L3]·4(BF4)·6MeNO2·2C6H6·4H2O, 1B, in which the inter-molecular interactions observed in 1A are disrupted by heavy solvation and the inclusion of several benzene molecules which interact by π–π overlap with the helicate complex, resulting in less efficient packing of the helicate and a more open lattice.

Coordination chemistry of N-picolyl-1,8-naphthalimides: colourful low molecular weight metallo-gelators and unique chelation behaviours

We have prepared a series of N-picolyl-1,8-naphthalimide derivatives and explored their coordination chemistry with d-block metal ions in the solid and solution states, and exploited the structural and chemical properties of these ligands to generate a family of robust metallogels. The 2-picolyl substituted ligands N-(2-picolyl)-1,8-naphthalimide L1 and N-(2-picolyl)-4-nitro-1,8-naphthalimide L2 showed interesting coordination behaviour in Mn, Co and Zn complexes 1–5, chelating through the pyridyl nitrogen atom and naphthalimide oxygen atom with subsequent bond localisation in the imide ring, the first example of such coordination behaviour to be structurally characterised. The 3-picolyl substituted ligands N-(3-picolyl)-1,8-naphthalimide L3 and N-(3-picolyl)-4-nitro-1,8-naphthalimide L4 vary in their coordination behaviour; the 4-unsubstituted ligand L3 acts as a simple monodentate donor giving crystalline assemblies, while the 4-nitro analogue L4 instead forms robust metallogels on reaction with MnII or CoII. The gels were found to be thermally and chemically reversible under various stimuli, and similar materials could be generated by installing a piperidinyl substituent at the naphthalimide 4-position, giving intensely coloured gels. These results describe previously unknown coordination chemistry and physical properties of the widely-used 1,8-naphthalimide backbone, and outline the important role played by subtle geometric variations in the physical characteristics of supramolecular metallogels.

The synthesis and characterisation of coordination and hydrogen-bonded networks based on 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid

The synthesis, structural and thermal characterisation of a number of coordination complexes featuring the N,O-heteroditopic ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate, HL are reported. The reaction of H2L with cobalt(II) and nickel(II) nitrates at room temperature in basic DMF/H2O solution gave discrete mononuclear coordination complexes with the general formula {[M(HL)2(H2O)4]·2DMF} (M = Co (1), Ni (2)), whereas the reaction with zinc(II) nitrate gave [Zn(HL)2]∞, 3, a coordination polymer with distorted diamondoid topology and fourfold interpenetration. Coordination about the tetrahedral Zn(II) nodes in 3 are furnished by two pyrazolyl nitrogen atoms and two carboxylate oxygen atoms to give a mixed N2O2 donor set. Isotopological coordination polymers of zinc(II), {[Zn(HL)2]·2CH3OH·H2O}∞, 4, and cobalt(II), [Co(HL)2]∞, 5, are formed when the reactions are carried out under solvothermal conditions in methanol (80 °C) and water (180 °C), respectively. The reaction of H2L with cadmium(II) nitrate at room temperature in methanol gives {[Cd(HL)2(MeOH)2]·1.8MeOH}∞6, a 2-D (4,4)-connected coordination polymer, whereas with copper(II) the formation of green crystals that transform into purple crystals is observed. The metastable green phase [Cu3(HL)4(μ2-SO4)(H2O)3]∞, 7, crystallises with conserved binding domains of the heteroditopic ligand and contains two different metal nodes: a dicopper carboxylate paddle wheel motif, and, a dicopper unit bridged by sulfate ions and coordinated by ligand pyrazolyl nitrogen atoms. The resultant purple phase {[Cu(HL)2]·4CH3OH·H2O}∞, 8, however, has single copper ion nodes coordinated by mixed N2O2 donor sets with trans-square planar geometry and is threefold interpenetrated. The desolvation of 8 was followed by powder X-ray diffraction and single crystal X-ray diffraction which show desolvation induces the transition to a more closely packed structure while the coordination geometry about the copper ions and the network topology is retained. Powder X-ray diffraction and microanalysis were used to characterise the bulk purity of the coordination materials 1–6 and 8. The thermal characteristics of 1–2, 4–6 and 8 were studied by TG-DTA. This led to the curious observation of small exothermic events in networks 4, 6, and 8 that appear to be linked to their decomposition. In addition, the solid state structures of H2L and that of its protonated salt, H2L·HNO3, were also determined and revealed that H2L forms a 2-D hydrogen bonded polymer incorporating helical chains formed through N–HO and O–HN interactions, and that [H3L]NO3 forms a 1-D hydrogen-bonded polymer.

Benzene-1,3,5-tricarboxamide n-alkyl ester and carboxylic acid derivatives: tuneable structural, morphological and thermal properties

A family of five benzene-1,3,5-tricarboxamide (BTA) compounds with varied side arm functionality [alkyl: n = 3, 4 and 5 chains; and ester and carboyxic acids] is reported. Investigations into their self-assembly behaviour revealed the extent to which minor changes in the side arm functionality can affect the structures formed. The carboxylic acid derivatives have a tendency to form gels, while the ester derivatives instead form crystalline phases with associated thermotropic phase transitions. The influence of the side chain length and terminal functional group is rationalised by a combination of single crystal X-ray diffraction studies and examinations of the bulk material properties.

Structural chemistry and selective CO2 uptake of a piperazine-derived porous coordination polymer

A new piperazine-derived ligand has been prepared and used to synthesise a porous coordination polymer which displays selective carbon dioxide uptake after solvent exchange and thermal activation. The ligand N,N′-bis(4-carboxyphenylmethylene)piperazine H2L1 was prepared from piperazine in three steps and good yield. A structure containing the deprotonated form K2L1·2H2O was determined and consists of a three-dimensional coordination polymer containing inorganic K2(COO)2(OH2) layers separated by the long organic bridging linker. The free compound H2L1 displays a one-dimensional hydrogen-bonded polymeric structure in the solid state with hydrogen bonding interactions between carboxylic acids and piperazine groups tightly linking molecules together. The two-dimensional polymeric complex [Zn3(L1)2(OH)2]·2DMF·0.5H2O 1 was prepared and analysed in the solid state to reveal tubular one-dimensional channels which, when activated by solvent exchange and evacuation, displayed selective affinity for CO2 over N2 and H2.