Past, present and future of the clathrate inclusion compounds built of cyanometallate hosts

Incongruent Melting and Vitrification Behaviors of Anionic Coordination Polymers Incorporating Ionic Liquid Cations

Several meltable coordination polymers (CPs) that possess substantial advantages attributable to their high flexibility and processability have been developed recently; however, the melting mechanism and vitrification conditions of these materials are not yet fully understood. In this study, we synthesized meltable CPs [A][K(TCM)₂] (A = onium cation, TCM = C(CN)₃^-) incorporating ionic liquid components and investigated their crystal structures and melting behaviors in detail. These CPs feature two- or three-dimensional anionic [K(TCM)₂]_n^- frameworks incorporating onium cations. Each CP was found to undergo incongruent melting at a temperature between 73 and 192 °C to produce a heterogeneous mixture of the ionic liquid ([A][TCM]) and microcrystalline K[TCM]. Furthermore, they formed homogeneous liquids upon further heating to ∼240 °C. The melting points of these CPs were linearly correlated with those of their constituent ionic liquids. The vitrification of these materials upon rapid cooling from the molten state was further investigated. The cooling rates required for vitrification differed greatly between the CPs and were correlated with the cation flexibility.

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L_2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)₄] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)₄] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L_2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.

Highly emissive polymorphs of anhydrous cadmium tetracyanoplatinate and their solvated coordination networks

Two anhydrous polymorphs of cadmium cyanoplatinate Cd[Pt(CN)₄] coordination polymers have been synthesized and thermally, spectroscopically, and structurally characterized. α-Cd[Pt(CN)₄] and β-Cd[Pt(CN)₄] are densely packed, highly emissive 3-D solids, with quantum yields of 0.85 (λ_em = 520 nm) and 0.79 (λ_em = 448 nm) respectively. Their mutual hydrate, Cd(H₂O)[Pt(CN)₄]·2H₂O, forms a complex 3-D coordination polymer with Cd-O-Cd bridges and Pt-Pt interactions. Additionally, exposure of solid α-Cd[Pt(CN)₄] and β-Cd[Pt(CN)₄] to several solvent vapours results in the formation of 2-D cyanometallate sheets of the adduct compounds CdL₂[Pt(CN)₄] (L = DMSO, DMF, and pyridine). Cd(pyridine)₂[Pt(CN)₄] shows a significantly lower quantum yield (0.32) in comparison to the parent Cd[Pt(CN)₄] coordination polymers. Upon heating CdL₂[Pt(CN)₄] preferentially forms the kinetic product α-Cd[Pt(CN)₄].

Crystal structure of poly[[di-aqua-tetra-μ2-cyanido-iron(II)platinum(II)] acetone disolvate]

In the title polymeric complex, {[FePt(CN)4(H2O)2]·2C3H6O} n , the FeII cation has an octa-hedral [FeN4O2] geometry being coordinated by two water mol-ecules and four cyanide anions. The Pt cation is located on an inversion centre and has a square-planar coordination environment formed by four cyanide groups. The tetra-cyano-platinate anions bridge the FeII cations to form infinite two-dimensional layers that propagate in the bc plane. Two guest mol-ecules of acetone per FeII are located between the layers. These guest acetone mol-ecules inter-act with the coordinated water mol-ecules by O-H⋯O hydrogen bonds.

Synthesis, crystal structure, infrared spectrum, and thermal properties of [Ni(1,10-phenanthroline)3](fumarate)·9H2O complex with hydrogen bonded supramolecular layers involving fumarate anions

From the aqueous-ethanolic system Ni(OH)2—H2 fum—phen (H2fum = fumaric acid, phen = 1,10-phenanthroline), novel complex [Ni(phen)3](fum)·9H2O (1) was isolated and characterized by chemical analyses and FT-IR spectroscopy. Results of single crystal X-ray structure analysis have shown that the ionic crystal structure of 1 is built of [Ni(phen)3]2+ complex cations, fumarate dianions and nine crystallographically independent water molecules of crystallization. The Ni(II) atom exhibits hexa-coordination by three phen ligands with mean Ni-N bond length of 2.090 Å. Water molecules form hydrophilic supramolecular layers with fumarate dianions via extended network of O—H···O type hydrogen bonds with O···O distances from the range of 2.676(2)—2. 916(2) Å; hydrophobic complex cations are embedded between these layers. Thermal study of 1 has shown that endothermic dehydration in the temperature range of 95—195 °C takes at least two steps of the process.

Graphical Abstract

Crystal structure of [Ni(phen)3]fum·9H2O (phen = 1,10-phenanthroline; H2 fum = fumaric acid) which is built of supramolecular layers formed by hydrogen bonded water solvate molecules and fum dianions and between the supramolecular layers embedded [Ni(phen)3]2+ complex cations is described here.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Designing anisotropic cyanometallate coordination polymers with unidirectional thermal expansion (TE): 2D zero and 1D colossal positive TE

By design, zero thermal expansion of Cu(H₂O)₂[PtX₂(CN)₄] is observed in the two dimensional cyanometallate sheet (left) and colossal thermal expansion in the inter-sheet direction (right).

Low dimensional solids based on Mo6 cluster cyanides and Mn2+, Mn3+ or Cd2+ metal ions: crystal chemistry, magnetic and optical properties

Five new cluster compounds based on [Mo₆Brⁱ₈(CN)^a₆]²⁻ and [Mo₆Brⁱ₆Qⁱ₂(CN)^a₆]ⁿ⁻ (Q = S, Se, n = 3, 4) cluster units have been synthesized and characterized.

Poly[bis(trimethylammonium) [hexa-μ-cyanido-cadmium(II)dicopper(I)]]

The title compound, {(C3H10N)2[CdCu2(CN)6]}n, has been synthesized as an alternative to the high-emitting complexes containing more expensive metals. The CN−ligands make linkages between the CuIand CdIIions to form the coordination polymer, [CdCu2(CN)6]n2−, which is a three-dimensional framework classified as pyrite net (pyr). The net has a void space for accommodating a trimethylammonium ion located on a threefold rotation axis. The CdIIion lies on a special position with site symmetry -3 and is octahedrally coordinated by six N atoms. The CuIion is located on a threefold rotation axis and has a trigonal-planar coordination geometry formed by three C atoms. In the three-dimensional net, two CuIions are arranged closely [Cu...Cu = 3.9095 (5) Å], but the distance is not short enough to suggest a CuI–CuIinteraction. The crystal studied was a merohedral twin (twin operation 2[101]), the refined component ratio being 0.9202 (7):0.0798 (7). A powder of the title compound shows strong luminescence with an emission maximum at 509 nm and a quantum yield of 98% at room temperature.

Structural studies of metal–organic frameworks under high pressure

Over the last 10 years or so, the interest and number of high-pressure studies has increased substantially. One area of growth within this niche field is in the study of metal–organic frameworks (MOFs or coordination polymers). Here we present a review on the subject, where we look at the structural effects of both non-porous and porous MOFs, and discuss their mechanical and chemical response to elevated pressures.

Molecular iodine adsorption within Hofmann-type structures M(L)[M′(CN)4] (M = Ni, Co; M′ = Ni, Pd, Pt): impact of their composition

Hofmann-type clathrate frameworks MII(pz)[M′II(CN)₄] are able to efficiently and reversibly capture iodine in the gas phase and in solution.

Synthesis and characterization of two novel dicyanidoargentate(i) complexes containing N-(2-hydroxyethyl)ethylenediamine exhibiting significant biological activity

Two novel cyanido-complexes having very significant antibacterial, antifungal and anticancer activities in vitro were synthesized and characterized using various techniques.

Reversible reconstructive transition of the [CuZn(CN)4]− framework host induced by guest exchange

The framework host [CuZn(CN)₄]⁻ exhibits a reversible reconstructive transition in the solid-state between a cristobalite-like framework and a tridymite-like one induced by guest exchange.

Two dimensional cyano-bridged hetero-metallic coordination polymers containing metal⋅⋅⋅π interactions

Three cyano bridged hetero-metallic complexes of general formula, [Cu(NH3)2(μ-ampy)M(μ-CN)2(CN)2]n [ampy=4-aminomethylpyridine, M=Ni(II) (1), Pd(II) (2) and Pt(II) (3)] have been synthesized and characterized by vibrational (FT-IR and Raman) spectroscopy, single crystal X-ray diffraction, thermal analyses and elemental analyses. The complexes crystallize in triclinic system with space group P-1. In all complexes, M(II) ions are coordinated by four cyano ligands, and four N atoms in the equatorial plane around the Cu atom form a slightly distorted square-planar arrangement, while the slightly distorted octahedral coordination is completed by the cyanide N atoms in the axial positions. In one-dimensional structures of all the complexes, [Cu(ampy)](2+) cations and [M(CN)4](2-) anions are linked via bridging cyano ligands. The adjacent one-dimensional structures form a 2D network to connect by the μ-ampy bridging ligands. The 2D layers are further linked by metal⋯π and hydrogen bonding interactions to generate a three dimensional network.

Exploiting high pressures to generate porosity, polymorphism, and lattice expansion in the nonporous molecular framework Zn(CN)2

Systematic exploration of the molecular framework material Zn(CN)2 at high pressure has revealed several distinct series of transitions leading to five new phases: four crystalline and one amorphous. The structures of the new crystalline phases have been resolved through ab initio structural determination, combining charge flipping and direct space methods, based on synchrotron powder diffraction data. The specific transition activated under pressure depends principally on the pressure-transmitting fluid used. Without fluid or in large molecule fluids (e.g., isopropanol, ethanol, or fluorinert), the high-pressure behavior intrinsic to Zn(CN)2 is observed; the doubly interpenetrated diamondoid framework structure transforms to a distorted, orthorhombic polymorph, Zn(CN)2-II (Pbca) at ~1.50-1.58 GPa with asymmetric displacement of the bridging CN ligand and reorientation of the Zn(C/N)4 tetrahedra. In small molecule fluids (e.g., water, methanol, methanol-ethanol-water), the nonporous interpenetrated Zn(CN)2 framework can undergo reconstructive transitions to porous, non-interpenetrated polymorphs with different topologies: diamondoid (dia-Zn(CN)2, Fd3m, P(trans) ~ 1.2 GPa), londaleite (lon-Zn(CN)2, P6(3)/mmc, P(trans) ~ 0.9 GPa), and pyrite-like (pyr-Zn(CN)2, Pa3, P(trans) ~ 1.8 GPa). Remarkably, these pressure-induced transitions are associated with near 2-fold volume expansions. While an increase in volume with pressure is counterintuitive, the resulting new phases contain large fluid-filled pores, such that the combined solid + fluid volume is reduced and the inefficiencies in space filling by the interpenetrated parent phase are eliminated. That both dia-Zn(CN)2 and lon-Zn(CN)2 phases were retained upon release to ambient pressure demonstrates the potential for application of hydrostatic pressures to interpenetrated framework systems as a novel means to generate new porous materials.

FT-IR and Raman spectroscopic analysis of some Hofmann type complexes

Some new Hofmann type complexes with chemical formula of M(4 pypp)(2)Ni(CN)(4) (where 4 py pp=4-(1-pyrrolidinyl)piperidine and MNi or Co) were prepared and their FT-IR and Raman spectra were reported in the region of 4000-200 cm(-1) and 4000-100 cm(-1), respectively. The ligand molecule, polymeric sheet and metal-ligand bands of the compounds were assigned in detail. Vibrational spectra together with assignments of 4 pypp (C(9)H(18)N(2)) were experimentally and theoretically studied in the region of 400-100 cm(-1). Normal mode frequencies and corresponding vibrational assignments of Ni(CN)(4)(2-) were theoretically examined by standard quantum chemical technique. Reliable vibrational assignments were made on the basis of potential energy distribution (PED) analysis. The results suggest that these compounds are similar in structure to the Hofmann type complexes and the 4 phpy ligand molecule bonds to the metal (M) atom of |M-Ni(CN)(4)|(∞) polymeric layers.

Vibrational spectroscopic analysis of some Hofmann-Td type complexes

New Hofmann-T(d) type complexes in the form of Ni(4Chpy)(2)M(CN)(4) (M=Cd or Hg and 4chpy=4-(3-cyclohexen-1-yl)pyridine) have been prepared and their FT-IR and FT-Raman spectra have been reported. The results suggest that present compounds are similar in structure to the Hofmann-T(d) type complexes, in which the M atom is tetrahedrally coordinated to the carbon atoms of the four cyanide groups, while the Ni atom is octahedrally surrounded by six nitrogen atoms, two of which are from 4chpy ligands which have been coordinated as a unidentate ligand and the rest are from cyanide groups. In this host structure, the M(CN)(4) groups have been linked by the Ni(4chpy)(2) moieties to form a three-dimensional network.

Synthesis and vibrational spectroscopic analysis of some Hofmann type clathrates

New Hofmann type clathrates in the form of M(pp)(2)Ni(CN)(4)·2G (where pp = 1-phenylpiperazine, G = 1,4-dioxane and M = Ni, Co or Cd) have been prepared in powder form and their FT-IR and FT-Raman spectra are reported. The results suggest that the present compounds are similar in structure to the Hofmann type clathrates and their structures consist of conjugated polymeric layers of |M-Ni(CN)(4)|(∞) with the pp bound to the metal (M) atom.

Vibrational spectroscopic study on some Hofmann type clathrates: M(2-(1-cyclohexenyl)ethylamine)2Ni(CN)4·2benzene (M=Ni and Cd)

New Hofmann type benzene clathrates in the form of M(CyHEA)2Ni(CN)4·2benzene (where CyHEA=2-(1-cyclohexenyl)ethylamine and M=Ni or Cd) have been prepared in powder form and FT-IR and Raman spectra have been reported. The results suggest that title compounds are similar in structure to Hofmann type clathrates and their structures consist of polymeric layers of |M-Ni(CN)4|∞ with the CyHEA molecule bounded to the metal atoms (M).

Vibrational spectroscopic investigations of some Hofmann-T(d)-type complexes: Ni(2-(1-cyclohexenyl)ethylamine)(2)M(CN)(4) (M=Cd or Hg)

New Hofmann-T(d)- type complexes in the form of Ni(CyHEA)(2)M(CN)(4) (where CyHEA=2-(1-cyclohexenyl)ethylamine and M=Cd or Hg) have been prepared in powder form and their infrared (4000-100cm(-1)) and Raman (2800-1650cm(-1)) spectra have been reported. The thermal behaviour of these complexes has been investigated by differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA). The results suggest that these compounds are similar in structure to the Hofmann-T(d)-type complexes.