Julie Lefebvre

Coordination polymers with cyanoaurate building blocks: Potential new industrial applications for gold

This report illustrates the concept that aurophilic interactions of gold-containing building blocks, particularly cyanoaurates, could be used as a tool to increase structural dimensionality in systems containingother metals in addition to gold(I). Such high-dimensionality systems may have useful optical, magnetic, conducting or porous materials properties. Recent successes from our group and others in using the neglected, luminescent [Au(CN)2]− building block to synthesize supramolecular coordination polymers with interesting and potentially commercially applicable physical properties will be surveyed. In most heterometallic [Au(CN)2]-based polymers, aurophilic interactions increase the structural dimensionality of the system and can impart increased thermal stability. The gold(I) ion can mediate significant magnetic interactions between transition-metal centres or influence iron(II) spin-transition behaviour in the polymers. The Cu[Au(CN)2]2(solvent)x polymer system is dynamically vapochromic, i.e., it shows large, reversible colour changes upon exposure to solvent vapours, thereby illustrating a sensor-type application. The related d8, square-planer [Au(CN)4]− building block, which has only recently been incorporated into coordination polymers, does not form any aurophilic interactions; weak Au-N(cyano) interactions control the intermolecular packing. Several structural examples of cyanoaurate-based coordination polymers are presented, including 2-D and 3-D arrays. The incorporation of cyanoaurates as components of advanced materials would provide a new utility and market for these key compounds of the gold mining and refining industry.

Magnetic Frustration and Spin Disorder in Isostructural M(μ-OH2)2[Au(CN)2]2 (M = Mn, Fe, Co) Coordination Polymers Containing Double Aqua-Bridged Chains: SQUID and μSR Studies

A series of isostructural M(mu-OH(2))(2)[Au(CN)(2)](2) (M = Co, Fe, and Mn) coordination polymers was synthesized from the reaction of M(II) with [(n)Bu(4)N][Au(CN)(2)]. The basic structural motif for these polymers is analogous to that of previously reported Cu(II)- and Ni(II)-containing polymers and contains repeating double aqua bridges between metal centers that yield a chain structure with pendant [Au(CN)(2)](-) units. The aqueous reaction with Fe(III) yields Fe(mu-OH(2))(mu-OH)[Au(CN)(2)](2), which has a similar structure. The magnetic properties of these polymers were investigated by a combination of SQUID magnetometry and zero-field muon spin relaxation. The double aqua bridges were found to mediate ferromagnetic interactions along the chains in the Co(II)-containing polymer, whereas intrachain antiferromagnetic interactions are present in the Fe(II)-, Fe(III)-, and Mn(II)-containing polymers. Weak magnetic interchain interactions mediated through hydrogen bonds, involving the bridging water molecules and the pendant cyanide groups, are also present. In zero field, the interchain interactions yield a phase transition to a disordered spin-frozen magnetic state below 2-5 K for every polymer. However, the degree of spin disorder varies considerably, depending on the metal center.

Vapochromic Behaviour of M[Au(CN)2]2-Based Coordination Polymers (M = Co, Ni)

A series of M[Au(CN)2]2(analyte)x coordination polymers (M = Co, Ni; analyte = dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), pyridine; x = 2 or 4) was prepared and characterized. Addition of analyte vapours to solid M(μ-OH2)[Au(CN)2]2 yielded visible vapochromic responses for M = Co but not M = Ni; the IR νCN spectral region changed in every case. A single crystal structure of Zn[Au(CN)2]2(DMSO)2 revealed a corrugated 2-D layer structure with cis-DMSO units. Reacting a Ni(II) salt and K[Au(CN)2] in DMSO yielded the isostructural Ni[Au(CN)2]2(DMSO)2 product. Co[Au(CN)2]2(DMSO)2 and M[Au(CN)2]2(DMF)2 (M = Co, Ni) complexes have flat 2-D square-grid layer structures with trans-bound DMSO or DMF units; they are formed via vapour absorption by solid M(μ-OH2)[Au(CN)2]2 and from DMSO or DMF solution synthesis. Co[Au(CN)2]2(pyridine)4 is generated via vapour absorption by Co(μ-OH2)[Au(CN)2]2; the analogous Ni complex is synthesized by immersion of Ni(μ-OH2)[Au(CN)2]2 in 4% aqueous pyridine. Similar immersion of Co(μ-OH2)[Au(CN)2]2 yielded Co[Au(CN)2]2(pyridine)2, which has a flat 2-D square-grid structure with trans-pyridine units. Absorption of pyridine vapour by solid Ni(μ-OH2)[Au(CN)2]2 was incomplete, generating a mixture of pyridine-bound complexes. Analyte-free Co[Au(CN)2]2 was prepared by dehydration of Co(μ-OH2)[Au(CN)2]2 at 145 °C; it has a 3-D diamondoid-type structure and absorbs DMSO, DMF and pyridine to give the same materials as by vapour absorption from the hydrate.

A Closer Look: Magnetic Behavior of a Three-Dimensional Cyanometalate Coordination Polymer Dominated by a Trace Amount of Nanoparticle Impurity

The structure and properties of the K{Ni[Au(CN)(2)](3)} coordination polymer, prepared as a powder at room temperature and recrystallized hydrothermally, are reported. The structure of K{Ni[Au(CN)(2)](3)} contains triply-interpenetrated Prussian Blue type pseudo-cubic arrays assembled from the alternation of octahedral Ni(II) centers and [Au(CN)(2)](-) bridging units. SQUID magnetometry studies have shown that K{Ni[Au(CN)(2)](3)} displays typical paramagnetic behavior for isolated Ni(II) centers down to 1.8 K. However, the magnetic behavior of the samples prepared under hydrothermal conditions suggests a superparamagnetic signature superimposed onto a paramagnetic background. After investigating the samples by transmission electron microscopy, it was determined that, in addition to K{Ni[Au(CN)(2)](3)}, the high-temperature (125, 135, and 165 degrees C) aqueous reaction of Ni(NO(3))(2)6 H(2)O with KAu(CN)(2) also led to the formation of nanoparticles of NiO and Au as minor side products, and that these dominated the magnetic behavior. Nanoparticles of various sizes and shapes were observed, depending on the reaction conditions. Samples containing nanoparticles were found to be superparamagnetic, exhibiting blocking temperatures of approximately 17-22 K, consistent with the behavior expected for NiO nanoparticles. These results illustrate the extreme care that must be taken when examining the physical properties of apparently analytically pure materials prepared by hydrothermal methods.