Daniel B. Leznoff

The use of aurophilic and other metal–metal interactions as crystal engineering design elements to increase structural dimensionality

Research in the field of supramolecular chemistry has rapidly grown in recent years due to the generation of fascinating structural topologies and their associated physical properties. In order to rationally synthesize such high-dimensionality systems, several different classes of non-covalent intermolecular interactions in the crystal engineering toolbox can be utilized. Among these, attractive metallophilic interactions, such as those observed for d10 gold(I), have been increasingly harnessed as a design element to synthesize functional high-dimensional systems. This tutorial review will explore the methods by which gold(I) and other d10 and d8 metal centres have been employed to increase structural dimensionality via the formation of metal-metal interactions. Physical and optical properties associated with metallophilicity-based supramolecular structures will also be highlighted.

Polymorphism of Zn[Au(CN)2]2 and Its Luminescent Sensory Response to NH3 Vapor

Four polymorphic forms of the complex Zn[Au(CN)2]2 have been synthesized and both structurally and spectroscopically characterized. In each of the four polymorphs, a zinc center in a tetrahedral geometry with a Au(CN)2(-) unit at each tetrahedral vertex is observed. All four structures contain three-dimensional networks based on corner-sharing tetrahedra. Because of the long Au(CN)2(-) bridging unit, the extra space not occupied by one network is filled by two to five additional interpenetrated networks. Short gold-gold bonds with lengths ranging from 3.11 to 3.33 A hold the interpenetrated networks together. Three of the four polymorphs are luminescent, having solid-state emissions with wavelengths ranging from 390 to 480 nm. A linear correlation between the emission energy and the gold-gold distance was observed. Upon exposure to ammonia vapor, the polymers altered their structures and emission energies, with the emission wavelength shifting to 500 nm for {Zn(NH3)2[Au(CN)2]2}, which adopts a two-dimensional layer structure with octahedral, trans-oriented NH3 groups. The adsorption route is dependent on the polymorph used, with NH3 detection limits as low as 1 ppb. Desorption of the ammonia occurred over 30 min at room temperature.

Synthesis, structure and magnetic properties of 3D interpenetrating nets of M(pyrazine)[Au(CN)2]2 (M=Cu, Ni, Co) supported by aurophilic interactions

Abstract New inorganic coordination polymers of the form M(pyrazine)[Au(CN)2]2 (M=Cu (1); Ni (2); Co (3)) have been prepared and the copper(II) analogue structurally characterised. The X-ray analysis revealed two 3D interpenetrating α-polonium networks consisting of 1D chains of Cu–pyrazine units connected by [Au(CN)2] bridges. The two networks are connected via weak aurophilic interactions (AuAu: 3.4729(2) A). Thermogravimetric analysis of all three compounds indicated the robust nature of these 3D systems, with decomposition beginning at 260 (1), 406 (2) and 360°C (3). The magnetic susceptibility of 1 shows a maximum in χM at 5.0 K; these data could be fitted to the theoretical expressions for either a 1D or a 2D Heisenberg antiferromagnetic array (J=−2.74 cm−1, g=2.32 and J=−3.45 cm−1, g=2.33, respectively). The magnetic susceptibility versus temperature data for 2 and 3 showed effects primarily associated with single-ion zero-field splitting; any weak antiferromagnetic coupling in these systems is very weak and hence was not quantified. Although the aurophilic interactions between the [Au(CN)2] units may positively influence both the formation of an interpenetrating structure and its subsequent thermal stability, the incorporation of anionic [Au(CN)2] units into M(pyrazine) systems does not seem to significantly increase magnetic interactions.

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.

Phthalocyanine as a Chemically Inert, Redox-Active Ligand: Structural and Electronic Properties of a Nb(IV)-Oxo Complex Incorporating a Highly Reduced Phthalocyanine(4−) Anion

This report describes the reduction of a niobium(V) phthalocyanine complex and investigation of the electronic structure of the resulting products. The reduction of PcNbCl(3) (Pc = phthalocyanine dianion) with 5.5 equiv of potassium graphite in 1,2-dimethoxyethane (DME) resulted in the isolation of K(2)PcNbO.5DME (1a). Addition of 18-crown-6 to 1a gave [K(18-crown-6)](2)(mu-DME)PcNbO (1b). Both 1a and 1b were structurally characterized by single-crystal X-ray diffraction analysis. In both complexes, the niobium center adopts a square pyramidal geometry and is coordinated by four basal Pc nitrogen atoms and an apical oxo ligand. Notably, the Pc ligand in 1a is saddle-shaped, with significant bond length alternation, rather than flat with delocalized bonding. The production of ethylene during the reduction of PcNbCl(3), detected by gas chromatography/mass spectrometry (GC/MS), suggests that the oxo ligand likely results from double C-O bond activation of DME solvent. A combination of spectroscopic techniques and density functional theory (DFT) calculations were used to establish the electronic structure of 1a. The close correspondence of the electronic absorption spectrum of 1a to that of [PcZn](2-) with a di-reduced Pc(4-) ligand, indicates a similar electronic structure for the two complexes. Evaluation of the electronic transitions for 1a and [PcZn](2-) by time-dependent DFT calculations further suggests a similar electronic structure for both complexes, indicating that differences in symmetry between 1a and [PcZn](2-) do not significantly affect the nature of the electronic transitions. Electron paramagnetic resonance (EPR) spectroscopy of 1a in solution at room temperature gave a 10-line spectrum, while frozen-solution X- and Q-band EPR spectra are consistent with powder-pattern spectra defined by uniaxial g and (93)Nb hyperfine tensors: these imply the presence of a d(1) Nb(IV) metal center. EPR and electron nuclear double resonance spectroscopy suggests that the spin density in 1a is centered almost completely on the niobium, in agreement with the DFT calculations. These results illustrate the value of Pc as a chemically inert, redox-active ligand for stabilizing reactive metal centers.

Using HgX2 units (X = Cl, Cn) to increase structural and magnetic dimensionality in conjunction with (2,2'-bipyridyl)copper(II) building blocks

Abstract A series of complexes containing (bipy)nCuCl2 units (bipy=2,2′-bipyridyl; n=1,2) and linear, neutral Hg(CN)2 or HgCl2 building blocks have been synthesized and structurally characterized. Generally, the Lewis acidic HgX2 moieties accept chloride ligands from the copper(II) center, in some cases increasing the structural and magnetic dimensionality of the system as a result. [Cu(bipy)2(μ-Cl)2Hg(CN)2] (1) is a molecular complex in which the chloride ligands bridge the copper(II) and mercury(II) centers. [Cu(bipy)2Hg2Cl6]2 (2) contains two copper(II) centers connected by an [Hg4Cl12]4− bridge, that was generated by chloride migration from the harder copper(II) to the softer, highly Lewis acidic HgCl2 group. N-cyano coordination from Hg(CN)2 in {[Cu(bipy)Hg(CN)2Cl2]2Hg(CN)2} (3) generates a “Chinese-kite” type Cu2Hg2 rectangular cluster. The clusters are connected to form a 1D chain by Hg(CN)2 groups, that accept bridging chloride ligands from adjoining clusters. The crystallization of (bipy)CuCl2/Hg(CN)2-containing complexes from aqueous NH4OH yields 1D {[Cu(bipy)(OH)(Cl)]2Hg(CN)2}·2H2O (4) which is composed of [(bipy)Cu(OH)(Cl)]2 units bridged by Hg(CN)2 moieties. The variable temperature magnetic susceptibility of 3 can be fitted to the theoretical expression for a 1D antiferromagnetic chain of S=1/2 centers with alternating interaction pathways (J=−0.92 cm−1, α=0.17, g=2.26). For 4, μeff increases with decreasing temperature to a maximum and then decreases; fitting with the Bleaney–Bowers model for copper(II) dimers with weak antiferromagnetic interdimer interactions yielded J=56.3 cm−1, zJ′=−0.06 cm−1 and g=2.23. Complexes 3 and 4 are examples of coordination polymers with the rarely used Hg(CN)2 building block.

An Extremely Air-Stable 19π Porphyrinoid

A one-electron reduced species of a cationic phosphorus(V) tetraazaporphyrin complex has been isolated as an air-stable solid. Cyclic voltammetry and magnetic measurements showed the species to be a neutral π radical, and the solid state structure was elucidated by single crystal X-ray diffraction analysis. Magnetic circular dichroism spectroscopy and theoretical calculations also support a 19π electron conjugated electronic circuit.

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.

Synthesis and structural characterization of a magnesium phthalocyanine(3–) anion

The reduction of magnesium phthalocyanine (MgPc) with 2.2 equivalents of potassium graphite in 1,2-dimethoxyethane (DME) gives [K2(DME)4]PcMg(OH)(1) in 67% yield. Compound 1 was structurally characterized using single crystal X-ray crystallography and was found to be a monomeric, heterometallic complex consisting of a μ3-OH ligand that bridges a [MgIIPc3-]- anion to two potassium cations solvated by four DME molecules. An absorption spectrum of 1 confirms the Pc ligand is singly reduced and has a 3–charge. The solid-state structure of 1 does not indicate breaking of the aromaticity of the Pc ligand. Compound 1 is only the second Pc3- complex and the first reduced MgPc to be isolated and structurally characterized.

Magnetic, electrochemical and spectroscopic properties of iron(III) amine–bis(phenolate) halide complexes

Eight new iron(III) amine-bis(phenolate) complexes are reported. The reaction of anhydrous FeX(3) salts (where X = Cl or Br) with the diprotonated tripodal tetradentate ligands 2-tetrahydrofurfurylamino-N,N-bis(2-methylene-4,6-di-tert-butylphenol), H(2)L1, 2-tetrahydrofurfurylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), H(2)L2, and 2-methoxyethylamino-N,N-bis(2-methylene-4,6-di-tert-butylphenol), H(2)L3, 2-methoxyethylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), H(2)L4 produces the trigonal bipyramidal iron(III) complexes, L1FeCl (1a), L1FeBr (1b), L2FeCl (2a), L2FeBr (2b), L3FeCl (3a), L3FeBr (3b), L4FeCl (4a), and L4FeBr (4b). All complexes have been characterized using electronic absorption spectroscopy, cyclic voltammetry and room temperature magnetic measurements. Variable temperature magnetic data were acquired for complexes 2b, 3a and 4b. Variable temperature Mössbauer spectra were obtained for 2b, 3a and 4b. Single crystal X-ray molecular structures have been determined for proligand H(2)L4 and complexes 1b, 2b, and 4b.