Project

Metal-organic frameworks with hydrophobicity fine-tunned by using silicones chemistry

Goal: The project is devoted to design, synthesis and structural characterization of metal-organic frameworks (MOFs) with controlled hydrophobicity required for certain applications such as gas storage, drug delivery systems, self-compatibilizing fillers for special energy composites, supercapacitors, etc. Different from the approaches reported in literature consisting in attaching hydrophobic groups near coordination sites, or post-synthetic grafting of such groups onto linkers, here will be used mainly ligands with siloxane spacers having attached to the silicon atoms one of the highest hydrophobic group, methyl, but also some derivatives inserting more longer (octyl), more rigid (phenyl, diphenyl), more polar (chloropropyl) or more hydrophobe (trifluoropropyl) groups in order to fine tune moisture stability of the resulted MOFs but also their lipophilicity and crystallinity. The high flexibility of the siloxane backbone allows the organic groups to be arranged and presented to their best effect. In addition, metals in high oxidation state will be used. The key steps in achieving the project objectives consist in engineering the spacer by using new approaches in silicones chemistry (i.e., Piers-Rubinsztajn reaction), attaching coordination groups (by thiol-ene addition or nucleophilic substitution), construction of MOF's and their isolation in a form accessible to characterize accurately. Thus original polydentate ligands mainly consisting in polycarboxylic acids and N-donor heterocycles with controlled diorganosiloxane or silane spacers will be obtained and used to built MOFs. The rare examples of assembling using the flexible linker, apart from those published by the authors of this proposal, and limited investigation in the field opens the innovative perspective for new knowledge and unique properties of MOFs.
Website: silmofs.icmpp.ro

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Maria Cazacu
added 4 research items
Cyclotri- and tetrasiloxanes having a methyl group and a carboxylic acid coupled through thioether bridge at each silicon atom were used to coordinate copper(II). The amphiphilic character conferred by the co-existence of the two moieties with opposite behaviour on the silicon atom gives the molecule the ability to form aggregates in solution. When copper salt is added, the metal is quickly coordinated by carboxyl groups within these aggregates, as in situ taken microscopy images and video reveal, forming blue spheres. This morphology was confirmed by scanning and transmission electron microscopy, while the coordinating structures and their compositions were estimated by IR spectroscopy and UV–Vis diffuse reflectance spectroscopy, wide angle X-ray diffraction and energy-dispersive X-ray analysis. The wetting tests and moisture sorption analysis indicate hydrophobic materials, while thermal analysis shows a thermal stability of at least up to 180 °C. The compounds catalyze the decomposition of H2O2 in alkaline medium, the values found for the rate constant being between 0.8 and 3.8 × 10⁻³s⁻¹.
1,3-divinyltetramethyldisiloxane (V2), 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3), and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4) are used as hydrophobic substrates to attach carboxyl groups by thermal or photochemical activated thiol-ene addition of 3-mercaptopropionic acid (R1), thioglycolic acid (R2) and thioacetic acid (R3). The reactions occurrence is monitored by IR following the disappearance of specific absorption bands for -S-H and -CH=CH2 bonds. At the end, the structure of the compounds and thioetherification degree of vinyl groups are determined by NMR. Co-existence in the structure of the highly hydrophobic methyl group and the carboxyl or carbonyl group gives the compounds formed an amphiphilic character, as indicated by the calculated hydrophilic-lipophilic balance, making them capable of self-assembling in the solution, as the results of the dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) measurements also show. Under normal temperature conditions, these compounds are in the liquid state. The DSC measurements in the range (-150) – (+150) reveal only a glass transition in the negative field and no other thermal transitions or decomposition processes. This behavior could make these compounds containing polar groups useful as free-solvent liquid electrolytes. Dielectric spectra reveal an increase in both dielectric permittivity and conductivity of the thiol-ene addition products as compared with starting vinylsiloxanes. The increase is even more significant when the formers are doped with lithium, reaching the conductivity of order 10-4 S/cm.
Pairs of different substituted 3-mercapto-1,2,4-triazole units are coupled, through thioether bridges, to organic-inorganic substrates consisting in short hydrophobic silicone segment. A library of six compounds are isolated as crystalline solids and structurally characterized by X-ray single crystal diffraction, elemental, spectral and thermal analysis. The flexibility of the silicone spacer makes the small molecular compounds exhibit glass transition in the negative domain. The metal binding capacity is evaluated by quantum mechanics calculations, the results being in line with experimental data obtained by UV–vis spectroscopy titration. The results indicate that the prepared compounds can act as ligands for metal ions with high selectivity for Cu2+, an element of interest in biological processes, forming 1:1 stable mononuclear complexes with an association constant up to 8.87 × 103 M−1. The presence of the highly hydrophobic silicone spacer makes the behavior of bis-triazoles obtained more sensitive to the nature of the environment. The preliminary bioassay indicates lipophilic medium more suitable for biocide action of silicone-bridged bis-triazoles, which in some cases far exceeds that of reference. The mechanism of enzyme inhibition is demonstrated by molecular docking, and the results indicate that, in all docked complexes, the ligands are directly coordinated to the heme ferric iron.
Maria Cazacu
added 3 research items
A less common dicarboxylic acid with siloxane spacer, 1,3‐bis(carboxypropyl)tetramethyldisiloxane, whose structure is for the first time determined crystallographically, is used to coordinate aluminum through an environment‐friendly preparation process, resulting in a polymeric structure of amorphous metal–organic framework (aMOF) type. The high flexibility of the spacer induced by the siloxane bond gives the polymeric product a glass transition slightly below room temperature, while the long length of the former creates the premises for collapse of the network and consequently reduced porosity. At the same time, the high hydrophobicity of the tetramethyldisiloxane fragment and its low surface energy, which causes it to migrate to the air interface, gives the network low moisture sorption capacity guaranteeing the stability of its properties in a wet environment. The particulate complex, of the order of 1–200 nm, as generated by the synthesis, proves to be suitable as filler for silicone with outstanding reinforcement effect, without significantly affecting their transparency. The presence of the dimethylsiloxane units in the structure of both the matrix and the filler ensures good incorporation of the latter without the need for special compatibility treatments. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 47144.
Due to their high stability over a wide range of temperature or moisture, silicone-based dielectric elastomer transducers (DETs) are of great scientific interest. In this regard, highly stretchable silicone elastomer films were prepared on the basis of a high molecular weight (Mn = 380,000 gmol⁻¹) polydimethylsiloxane‑α,ω‑diol (PDMS) and stabilized by condensation crosslinking at room temperature, process which was monitored by IR spectroscopy. The obtained films were treated in oxygen radio-frequency (RF) plasma at 50 and 60 W for 3, 6 and 12 min. Depending on plasma treatment conditions, different surface morphologies were obtained, as evidenced by the atomic force microscopy (AFM), while chemical modifications were investigated by X-ray photoelectron spectroscopy (XPS). The elastomer films activated on both sides were further used as substrate for silver electrode deposition through Pulsed Laser Deposition (PLD) to obtain DETs. The deposited films were investigated from the point of view of mechanical and electrical behavior in normal state and after repeated mechanical stresses to assess the suitability of silver layer as stretchable electrode. Moreover, the achieved DETs were successfully tested in terms of actuation displacement.
Three dimanganese(III) complexes have been synthesised and fully characterised by standard spectroscopic methods and spectroelectrochemistry. Each Mn(III) ion is chelated by a salen type ligand (H2L), but there is variation in the bridging group: LMn(OOCCH=CHCOO)MnL, LMn(OOCC6H4COO)MnL, and LMn(OOCC6H4C6H4COO)MnL. X-ray diffraction revealed an axial compression of each six-coordinate high-spin d4 Mn(III) ion, which is a Jahn-Teller-active ion. Temperature dependent magnetic susceptibility and variable temperature-variable field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy were used to accurately describe the magnetic properties of the complexes, not only the single-ion spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E, but also the exchange interaction constant J between the two ions, which has been seldom determined for a di-Mn(III) complex. Quantum chemical calculations reproduced well the electronic and geometric structure of these unusual complexes, and, in particular, their electronic absorption spectra along with the spin Hamiltonian and exchange parameters.
Mihail Iacob
added 2 research items
Two metallo-supramolecular polymers {[Zn2(L)4(DMF)2]·0.8DMF}n (1) and {[Mn2L2(DMSO)4]·2DMSO}n (2) (H2L = bis(p-carboxyphenyl)diphenylsilane) have been synthetized by the reaction of Zn(NO3)2·6H2O and Mn(NO3)2·4H2O respectively, with the above mentioned acid as ligand under solvothermal conditions. Both coordination polymers were structurally characterized by single crystal X-ray diffraction, elemental analysis, FTIR and UV–Vis spectroscopy. Single crystal X-ray diffraction analysis reveals dinuclear Zn(II) and Mn(II) nodes bridged by mono or double deprotonated ligand molecules coordinated in syn-syn bidentate mode. Thermal and moisture stability and photophysical properties of the resulting coordination polymers were studied and correlated with their structure.
New Schiff base-type products starting from 1-(3-aminopropyl)silatrane and three derivatives of salicylaldehyde having as substituents 3,5-dichloro- (1), 3-methoxy- (2) and 3,5-di-tert-butyl- (3), respectively were obtained and isolated with high yields (78-87%) in pure, crystalline forms and their structures were established by different methods. The molecular electronic transitions of the compounds in solvents with various polarities were investigated by UV-Vis spectral analysis. Their thermal behavior was studied by thermogravimetric analysis and differential scanning calorimmetry, results of the latter highlighting thermocromism of the compounds proved by the appearance of IR absorption bands specific for enolic form at temperature corresponding to each sample. The moisture sorption capacity and stability of the compounds in wet environment were investigated by vapor sorption analysis in dynamic regime and IR spectroscopy. The biological activity was assessed by specific tests. All results were discussed in correlation with the nature of substituents and structures formed. The chemical handling of the silatrane tail, by using different substituents on the silicon atom would allow fine tuning of the compounds properties.
Mihail Iacob
added 2 research items
A new high-spin d4 trigonal-bipyramidal (TBP) manganese(III) complex with a salen type ligand (H2L), namely MnL(NCS)·0.4H2O, has been synthesised and characterised by elemental analysis, ESI mass spectrometry, IR und UV‒vis spectroscopies and spectroelectrochemistry. X-ray diffraction revealed an axial compression of the TBP. Temperature dependent magnetic susceptibility and variable temperature-variable field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy were used to accurately describe the magnetic properties of this complex, and, in particular, the spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E. The HFEPR spectra allowed extraction of fourth order ZFS parameters. Quantum chemical calculations reproduced well the electronic and geometric structure of this unusual complex, and, in particular, its electronic absorption spectrum along with the spin Hamiltonian parameters.
A new bis(µ-chlorido) bridged cobalt(II) complex [Co2(µ-Cl)2(HL2)4][CoCl4] (1), where HL2 is a silyl-containing Schiff base, was synthesised. The structure of this compound was established by X-ray crystallography revealing a zwitterionic form adopted by the organic ligand. The temperature dependence of magnetic susceptibility and field dependence of magnetisation indicate the presence of ferromagnetic interactions between paramagnetic d7 cobalt(II) centres (SCo = 3/2). The exchange coupling parameter J(Co1-Co2) = +7.0 cm‒1 extracted from broken-symmetry (BS) DFT calculations fits well the value of +8.8 cm‒1 determined from experimental data and fitting them with the Hamiltonian . Electrochemical studies indicate that complex 1 is inefficient as catalyst in electrochemical reduction of protons. One of the reasons is the low stability of the complex in solution. In contrast, 1 was found to act as an effective homogeneous (pre)catalyst in the microwave-assisted net oxidation of cyclohexane with aqueous TBHP. The possible mechanism of catalytic oxidation and other advantages of using 1 in oxidation of cycloalkanes are discussed.
Mihail Iacob
added a project goal
The project is devoted to design, synthesis and structural characterization of metal-organic frameworks (MOFs) with controlled hydrophobicity required for certain applications such as gas storage, drug delivery systems, self-compatibilizing fillers for special energy composites, supercapacitors, etc. Different from the approaches reported in literature consisting in attaching hydrophobic groups near coordination sites, or post-synthetic grafting of such groups onto linkers, here will be used mainly ligands with siloxane spacers having attached to the silicon atoms one of the highest hydrophobic group, methyl, but also some derivatives inserting more longer (octyl), more rigid (phenyl, diphenyl), more polar (chloropropyl) or more hydrophobe (trifluoropropyl) groups in order to fine tune moisture stability of the resulted MOFs but also their lipophilicity and crystallinity. The high flexibility of the siloxane backbone allows the organic groups to be arranged and presented to their best effect. In addition, metals in high oxidation state will be used. The key steps in achieving the project objectives consist in engineering the spacer by using new approaches in silicones chemistry (i.e., Piers-Rubinsztajn reaction), attaching coordination groups (by thiol-ene addition or nucleophilic substitution), construction of MOF's and their isolation in a form accessible to characterize accurately. Thus original polydentate ligands mainly consisting in polycarboxylic acids and N-donor heterocycles with controlled diorganosiloxane or silane spacers will be obtained and used to built MOFs. The rare examples of assembling using the flexible linker, apart from those published by the authors of this proposal, and limited investigation in the field opens the innovative perspective for new knowledge and unique properties of MOFs.