Correlation of thermochemical measurements and quantum chemical calculations of thermally unduced structural transformations of polynuclear complexes [(en)(H2O)3Ni-(pyr)-Ni(H2O)3(en)] 4H2O and [Cd(N-Boc-gly)2(H2O)2]n.

Ispitana je termička stabilnost i mehanizam stupnjevitog razlaganja polinuklearnih kompleksa [(en)(H2O)3Ni - (pyr) - Ni(H2O)3(en)] 4H2O i [Cd(N - Boc - gly)2(H2O)2]n u temperaturskom intervalu od 320 do 760 K. Utvrđeno je da su svi stupnjevi termički aktivirani. Iz površine endotermnih pikova određene su entalpije procesa, a primenom Kissinger-ove metode određena je ukupna vrednost prividne energije aktivacije svakog stupnja razlaganja. Analiza izotermalnih merenja kompleksa nikla ukazala je na sve značajniji uticaj difuzije sa napredovanjem reakcije. Utvrđena je zavisnost reakcionog puta od brzine zagrevanja kompleksa. Analizom promene oblika zavisnosti efektivne energije aktivacije od stepena napredovanja utvrđeno je postojanje reverzibilnog stupnja u procesu dehidratacije kompleksa nikla i promena ograničavaju¢eg stupnja uz sve značajniji uticaj difuzije. Porast, a zatim pad prividne enrgije aktivacije tokom dehidratacije kompleksa kadmijuma i stupnja razgradnje kompleksa nikla ukazuje na promenu ograničavaju¢eg stupnja. Neprekidni porast energije aktivacije razlaganja kompleksa kadmijuma ukazuje da se tokom razlaganjana odvijaju dva paralelna procesa. Vibracione frekvencije izračunate za izokinetičke temperature odgovaraju vibracijama Ni - OH2, Cd - OH2 i Cd - O veza, što je kvantnohemijskim proračunima i određeno. Korelacijom eksperimentalno određenih i izra£unatih termodinamičkih i kinetičkih parametara definisani su mehanizami dehidratacionih i stupnjeva razlaganja oba kompleksa. Proračuni su pokazali da se tokom dehidratacije kompleksa nikla odvija polimerizacija. Pri malim brzinama, polimerizacija pospešuje izlazak vode iz sistema, dok se pri višim brzinama zagrevanja ovaj proces odvija znatno sporije. Ovo uzrokuje postojanje dva reakciona puta na različitim brzinama zagrevanja. Na osnovu proračuna izračunata je konstanta ravnoteže povratnog stupnja na različitim temperaturama tokom dehidratacije polaznog kompleksa. Na ovaj način je pokazano da gubitak kristalne vode pospešuje odlazak koordinovane vode iz sistema. Pokazano je da niža energija aktivacije razlaganja kompleksa kadmijuma na početku procesa odgovara premeštanju (N - Boc - gly) liganda iz unutrašnjosti sistema ka površini, dok viša odgovara njegovom otkidanju sa same površine.Thermal stability and degradation mechanism of [(en)(H2O)3Ni - (pyr) - Ni(H2O)3(en)] 4H2O and [Cd(N - Boc - gly)2(H2O)2]n coordination polymers was investigated in 320-760 K temperature range. It was determined that all of the individual steps are thermally activated. Overall enthalpies of these processes were determined from the area of corresponding endothermic peaks, while the overall average value of activation energies of individual degradation processes were determined using the Kissinger method. The thermal degradation starts with multi-step dehydration process, which exhibits increased influence of diffusion on the reaction mechanism during the progress of the reaction, and its reaction path depends on the heating rate. Analysis of dependence of the value of the effective apparent activation energy on the reaction conversion degree indicates the existence of a reversible step in the dehydration process of Ni-coordination polymer and a change of rate-limiting step of this reaction, probably due to diffusion. Increase and then decrease in the value of the effective apparent activation energy for dehydration reaction of Cd-coordination polymer and degradation reaction of Ni-coordination polymer indicates a change of rate-limiting step, likely due to diffusion of released products. Continuous increase in the value of the effective apparent activation energy for degradation reaction of Cd-coordination polymer indicates that the reaction mechanism consists of two parallel processes. Vibrational frequencies corresponding to the determined values of isokinetic temperatures correspond to Ni - OH2,Cd - OH2 and Cd - O bonds, respectively, and this was confirmed using quantum-chemical calculations. Correlation of experimental and calculated thermodynamic and kinetic parameters has allowed the formulation of the corresponding mechanisms of thermal dehydration and degradation for both of these coordination polymers. DFT calculations indicate that Ni-coordination polymer undergoes polymerization during the dehydration process. At low heating rates, polymerization promotes release of water, while at higher heating rates the polymerization is much slower, resulting in two distinct reaction paths at different heating rates. DFT calculations were used to calculate the equilibrium constant of the reversible step of dehydration of Ni-coordination polymer, indicating that the release of uncoordinated water molecules promotes subsequent release of coordinated water molecules. The lower value of activation energy of thermal degradation of Cd-coordination polymer at the beginning of the process corresponds to the transfer of (N - Boc - gly) ligand from the bulk to the surface, while the higher value of activation energy at the end of process corresponds to release of (N - Boc - gl) ligand from the surface

Hydrogen storage in a layered flexible [Ni2(btc)(en)2]n coordination polymer

[Ni2(btc)(en)2]n coordination polymer exhibits a layered two-dimensional structure with weak interaction between the layers. Correlation of experimental measurements, DFT calculations and molecular simulations demonstrated that its structural features, primarily the inherent flexibility of the layered polymeric structure, lead to improved hydrogen storage performance at room temperature, due to significant enhancement in isosteric heats of hydrogen adsorption. Volumetric measurements of hydrogen adsorption at room temperature show up to 0.3 wt.% hydrogen absorbed at 303 K and 2.63 bar of hydrogen pressure, with isosteric heats of adsorption of about 12.5 kJ mol−1. Predicted performance at room temperature is 1.8 wt.% at 48 bar and 3.5 wt.% at 100 bar, better than both MOF-5 and NU-100, with calculated values of isosteric heats for adsorption of hydrogen in 8–13 kJ mol−1 range at both 77 K and 303 K. Grand canonical Monte Carlo calculations show that this material, at 77 K, exhibits gravimetric hydrogen densities of more than 10 wt.% (up to 8.3 wt.% excess) with the corresponding volumetric density of at least 66 gL−1, which is comparable to MOF-5, but achieved with considerably smaller surface area of about 2500 m2 g−1. This study shows that layered two-dimensional MOFs could be a step towards MOF systems with significantly higher isosteric heats of adsorption, which could provide better room temperature hydrogen storage capabilities.This is the peer reviewed version of the following article: Blagojević, V.A., Lukić, V., Begović, N.N., Maričić, A.M., Minić, D.M., 2016, “Hydrogen storage in a layered flexible [Ni2(btc)(en)2]n coordination polymer”, International Journal of Hydrogen Energy, http://dx.doi.org/10.1016/j.ijhydene.2016.08.20

Thermal stability and degradation of binuclear hexaaqua-bis(ethylenediamine)-(μ 2-pyromellitato)dinickel(II) tetrahydrate

Thermal degradation of ternary transition metal complex containing tetraanion of pyromellitic acid, pyr, and ethylenediamine, en, [Ni2(en)2(H2O)6(pyr)]·4H2O, 1, was investigated under non-isothermal conditions. The mechanism of thermal degradation, which occurs in three steps, was clarified by TG/DSC measurements in conjunction with FT-IR spectroscopy and XRPD analysis. The complexity of all degradation steps has been revealed using isoconversional methods. Dehydration comprises the loss of ten water molecules in a relatively narrow temperature interval, resulting in a very complicated reaction mechanism. In addition, density functional theory calculations have been applied for better understanding of dehydration. The second degradation step, related to loss of en, was separated into two single-step processes with Fraser–Suzuki function. The obtained individual steps were described by Johnson–Mehl–Avrami A2 model and Šesták–Berggren model, respectively. Validation of the proposed kinetic triplets for individual steps was performed using master plot and Pérez-Maqueda criteria. The third degradation step is related to the fragmentation of pyr ion most likely followed with the release of a number of gaseous products

Synthesis and thermal stability of cis-dichloro[(E)-ethyl-2-(2-((8-hydroxyquinolin-2-il)methylene)hidrazinyl)acetate-kappa(2) N]-palladium(II) complex

The structure of new cis-dichloro[(E)-ethyl-2-(2-((8-hydroxyquinolin-2-il)methylene)hidrazinyl)acetate-kappa(2) N]-palladium(II) complex was determined using a combination of XRD and IR measurements and DFT calculations. Inherent flexibility of its structure is evident from the complexity of its IR spectrum, which could only be theoretically reproduced as a combination of several closely related structures, involving rotation around C-O bond and changes in hydrogen interactions of its -OH group. Its thermal stability and decomposition were studied non-isothermally, and the thermal decomposition mechanism was proposed using correlation with DFT calculations at the molecular level. It was determined that the initial degradation step consists of the release of Cl free radical, which then reacts with both the initial compound and the degradation products. Besides the endothermic steps, there are exothermic ones, contributing to the complex shape of the DSC curve, consisted of overlapping endothermic and exothermic peaks. Deconvolution of DTG curve allowed identification of primary fragments of the initial degradation process and, in conjunction with DFT calculations, construction of the most likely reaction mechanism