Laboratory for Metal-organic Coordination Materials

Scientific results:

• The Laboratory has produced the porous metal-organic coordination polymer (H₃O)₂[Zn₄(ur)(Hfdc)₂(fdc)₄] (ur = urotropine, H₂fdc = 2,5-Furandicarboxylic acid) that contains in its structure  cryptand-like cavities that can be used to separate and detect Rb⁺ and Cs⁺ cations using optical methods in a solution. This is the firs example of the efficient use of porous coordination materials for the optical detection of  of these cations.
• The porous metal-organic coordination polymer [Zn₂(tdc)₂(dabco)] (H₂tdc = 2,5-Furandicarboxylic acid; dabco = 1,4-diazabicyclo octane) has demonstrated a significant increase in carbon dioxide (CO₂) absorption and selectivity with regard to dinitrogen (N₂) in comparison with the non-thiophene counterpart [Zn₂(bdc)₂(dabco)] (H₂bdc = terephthalic acid).  CO₂ absorption at 1 bar for [Zn₂(tdc)₂(dabco)] amounts to 67,4 ml/g (13,2 mass per cent) at 298 К and 153 ml/g (30,0 mass per cent) at 273 К. For [Zn₂(bdc)₂(dabco)] the equivalent values are 46 ml/g (9,0 mass per cent) and 122 ml/g (23,9 mass per cent) respectively. The isothermal heat of СО₂ absorption in [Zn₂(tdc)₂(dabco)] at zero coverage is not high (23,65 kJ/mol), which ensures easy regeneration of porous material. The strengthening of the segregation of CO₂/N₂ gas mixtures by a thiophene group has been confirmed both by computations with the ideal adsorbate solution theory and by experiments in the dynamic separation of gases in a flow. The preferable binding sites for absorbed CO₂ in [Zn₂(tdc)₂(dabco)] were unambiguously determinde using in situ X-ray structural analysis if the [Zn₂(tdc)₂(dabco)] monocrystal in a CO₂ atmosphere in combination with quantum-chemical computations. This research uncovers the role of  thiphene fragments in specific binding of СО₂ by induced dipole interaction between СО₂ and the sulfur atom, confirming that the increased capacity of СО₂ in [Zn₂(tdc)₂(dabco)] is achieved without the presence of open metal centers. Experimental data and theoretical conclusions suggest a strategy for improving the absorption properties of known materials by the inclusion of heterocycles based on sulfur into their porous structure.
• A number of new zinc(II)-thiophene-2,5-dicarboxylate (tdc) coordination polymers (NIIC-10), based novel wheel-shaped dodecanuclear building blocks. An X-ray structural analysis of   monocrystals has demonstrated 3D porous frameworks of a complex composition, [Zn₁₂(tdc)₆(glycolate)₆(dabco)₃] (glycolate is deprotonated multiatomic alcohol: ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol, glycerine; dabco = 1,4-diaza[2.2.2.]bicyclo octane). All the compounds are isostructural and differ in diol side groups decorating  the surface of the channels. For all the compounds we have thoroughly researched absorption of light gases (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) and larger hydrocarbons (benzene, cyclohexene) both in the liquid and in  the gaseous phase. We have computed the absorption enthalpies at zero coverage of the Henry constant and the selectivity ratios from various models. The flexible absorption functionality of the produced series of frameworks is attributed to the volatile nature of diol and can be adapted to a specific system of adsorbates. For instance, an ethylene glycolate framework demonstrates great selectivity of benzene in comparison with cyclohexane (20:1 for vapors, 92:1 for the liquid phase), while  a pentanedionate framework demonstrates an unprecedented advantage of adsorbing cyclohexane compared to benzene (a selectivity of up to 5:1). A compound with glycerine demonstrates a high selectivity of the absorption of СО₂/N₂ (up to 75,1), СО₂/СН₄ (до 7,7), С₂Н₂/СН₄ (up to 14,2) and С₂Н₄/СН₄ (up to 9,4). Moreover, due to the polar nature of the pores, this framework is characterised by the size-selective sorption  of  alkali metal cations in the order Li⁺ > Na⁺ > К⁺ > Cs⁺ as well as demonstrates prominent luminescence reaction to cesium atoms and urea.
• Porous metal-organic coordination polymers of the NIIC-10 family have been researched in the reaction of the carboxylation of mono-, di- and trisubstituted epoxides in the presence of tetrabutylammonium bromide (TBAB). The reactions were conducted in an environment without solvents in a СО₂ atmosphere at 80°С. All the produced mono- and disubstituted cyclic carbonates were synthesized with yields varying from good to excellent. Apart from that, the catalyst also demonstrated a good selectivity and the capability for reuse in five cycles without a significant loss of activity.
• We have produced a series of five isostructural cubic metal-organic frameworks (NIIC-20) based on dodecahedral carboxylate wheel-shaped construction blocks {Zn₁₂(RCOO)₁₂(glycolate)₆} similar to those present in NIIC-10. The crystalline structures contain large mesoporous cells with sizes of about 25 Å connected via rings to {Zn₁₂} whose internal diameter and chemical nature depend solely on the selected glycolate. The NIIC-20 compounds are distinguished by a large surface area and a rarely observed inverse adsorption affinity to saturated hydrocarbons (ethane) in comparison with unsaturated ones (ethylen, acetylene). The corresponding absorption selectivity ratios of IAST (ideal adsorbed solution theory) reach 15,4 for the C₂H₆/C₂H₄ gas mixtures and 10,9 for the C₂H₆/C₂H₂ gas mixtures in the conditions of the environment, which exceeds the values observed for any other porous frameworks produced earlier. The remarkable combination of a high absorption capability and a high absorption  selectivity makes the NIIC-20 series a new standard of porous materials intended for  ethylene separation.
• Our researchers have produced a series of five mesoporous metal-organic frameworks [Zn₁₂(i-bdc)₆(G)₆(dabco)₃] (i-bdc²⁻ = isophthalate, G = ethylenediolate, 1,2-propylenediolale, 1,2-buthylenediolate, 1,2-pentylenediolate, glycerolate) denoted as NIIC-20-G. The frameworks demonstrate gravimetric methane  adsorption uptakes at high pressure (56 atm), up to 210 cm³⋅g⁻¹ at 273 K and 156 cm³⋅g⁻¹ at 295 K. The corresponding volumetric values reach 175 ml/ml at 273 К and 130 ml/ ml at 295 К, which is close to the best values achieved for other metal-organic frameworks. The estimated maximum working capacity for methane at 295 К amounted to 73 ml/ml (in the pressure range from 35 to 5 atmospheres) and 109 ml/ ml (in the pressure range from 65 to 5 atmospheres). The NIIC-20-G compounds also demonstrate high absorption capability with respect to various volatile organic compounds (benzene, cyclohexane, xylene isomers) regardless of the chemical nature or the geometry of the substrate. The maximum values for the absorption of vapors at 298 К is 6,9 mmol/g for benzene, 5,4 mmol/g for cyclohexane, 4,4 mmol/g for о-xylene, 4,5 mmol/g for m-xylene. and 4,6 mmol/g for p-xylene. We have demonstrated that the NIIC-20-G compounds retain their crystal structure as well as adsorption properties in repeated experiments. It is worth noting that a complete regeneration of porous materials can be performed only under moderate heating (60 °С). The high absorption of various volatile hydrocarbons at low concentration over several cycles and the rather moderate energy expenditure for the regeneration of the material point to strong prospects of the NIIC-20-G compounds for air purification.
• Using a series of isoreticular metal-organic frameworks [Li₂Zn₂(bpy)(R-bdc)₃] (R = H, NH₂, Br and NO₂; bpy = 4,4′−dipyridyl) with a branched system of channels of various diameters, we have demonstrated the selective separation of benzene and cyclohexane both in the liquid and in the vapor phase. The propelling force of the highly efficient process of absorption is the formation of weak interactions between the absorbed benzene molecules and the hosting framework.
• The separation of hydrocarbon molecules, such as benzene/cyclohexane and o-xylene/m-xylene/p-xylene is important due to their wide use as a chemical raw material, but it is problematic due to the similar boiling temperatures and the close sizes of molecules. Physisorption separation could offer an energy-efficient solution to this problem, but the design and synthesis of sorbents demonstrating a high selectivity with respect to one of the hydrocarbons remain, in large part, an unsolved task. To solve this problem, we have produced a new poroous heterometallic coordination polymer, [Li₂Zn₂(bpy)(ndc)₃] (NIIC-30(Ph), ndc²⁻ = naphthalene-1,4-dicarbonate) with a unique sinuous shape of channels decorated with aromatic sorption sites and conducted a research of benzene/cyclohexane mixtures and xylene in the vapor and liquid phase. For an equimolar benzene/cyclohexane mxture it is possible to achieve a ten-fold excess of benzene in the absorbed phase. In the case of xylenes, the NIIC-30(Ph) microporous framework demonstrates outstanding selective sorption properties and becomes a new standard of m-/o-xylene separation. Moreover, NIIC-30(Ph) is sufficiently stable for conducting at least three cycles of separation of benzene/cyclohexane mixtures or triple o-xylene/m-xylene/p-xylene mixtures both in the liquid and in the vapor phase. A study of the crystal structure of inclusion compounds for each aromatic guest allowed to understand the peculiarities of the sorption behavior of NIIC-30(Ph) better.
• Hybrid organic-inorganic compounds based on CuI are viewed as promising luminophores for the lighting industry. Using N-monoalkylated hexamine salts [R-HMTA]X (R = Me, Et, Pr and propargyl; X = Cl and I) as multibridging ligands, we have produced a unique class of one-dimensional and two-dimensional hybrid CuI-materials. Reactions of [R-HMTA]X salts with CuI lead to compounds that combine ionic and dative bonds between inorganic and organic components. The latter is formed by structurally unique inorganic [Cu_xI_y]^(y−x)− clusters, chained or layered, bonded to each other via [R-HMTA]⁺ cations by multiple Cu–N bonds. Compounds constructed in this manner demonstrate adjustable luminescence in the range form dark-blue to red (λ_em = 430–625 nm) at the temperature of the environment with a microsecond lifetime and a quantum efficiency of up to 78 per cent. Notably, such materials possess nontrivial luminescence  that depends on the length of the excitation wave and the temperature, which allows to precisely regulate the color of luminescence from dark-red to red by changing the length of the excitation wave and (or) the temperature. On the basis of temperature-dependent emission spectroscopy and theoretical computations we have proposed a possible mechanism of luminescence. The highly interesting luminescence characteristics in combination with good thermal and photostability make these hybrid materials potential candidates for the use in energy-efficient lighting devices.
• Coordination polymers based on dynamic structural elements are of major fundamental and commercial interest for solving contemporary problems of controlled molecular separation, catalysis as well as data processing. In this case the wear resistance and fast structural dynamics of such materials in the conditions of the environment still pose a fundamental problem. We have produced a series of copper coordination polymers [Cu(bImB)Cl₂] and [Cu(bImB)₂Cl₂] based on the flexible ligand bImB (1, 4-bis (1 H-imidazol-1-yl) butane) packed into one- and two-dimensional (1D, 2D) structures demonstrate flexibility and reversible structural transformations. Using laser pulses as a fast source of activation energy, the heating of the coordination polymer is initiated, with subsequent anisotropic thermal expansion and a change of volume by 0,2-0,8 per cent with record-high speed of transformation from 2220 to 1640 s⁻¹ for 1D and 2D coordination polymers respectively. The compounds are stable during more than 103 cycles of structural transformations. 

Education and career development:

• One Doctor of Sciences and 8 Candidate of Sciences dissertations have been prepared and defended.
• Three new education courses for NSU students and postgraduate students of the Institute of Inorganic Chemistry of the Siberian Branch of the Russian Academy of Sciences: «Materials and their properties» (2014), «Modern aspects of the chemistry of metal-organic coordination polymers» (2015), «Modern aspects of the chemistry of cluster compounds and materials» (2016).
• Employees of the Laboratory participated in organizing and delivering lectures for scientific schools conducted on the basis of the Institute of Inorganic Chemistry of the Siberian Branch of the Russian Academy of Sciences, the NSU and the Buryat State University. 

Collaborations:

• University of Manchester, University of Nottingham (United Kingdom), 

• University of Science and Technology Liaoning (China PR), 

• Tohoku University (Japan), 

• Kurchatov Institute, Tomsk State University, Institute of General and Inorganic Chemistry of the Russian Academy of Sciences (Russia): joint research.