Constantinos A. Tsipis

Discrepancy between the structural and magnetic dimensionality in the (μ-terephthalato)bis[(diethylenetriamine)Cu(II)]perchlorate complex

Abstract The crystal structure of [(dien)Cu(μ-tp)Cu(dien)](ClO4)2, where tp is the dianion of terephthalic acid and dien is diethylenetriamine, has been determined by direct X-ray methods. The complex crystallizes in the monoclinic space group P21/c with a=8.911(6), b=8.002(5), c=19.61(2) A, β=90.2(1)°, Z=2 and V=1398.3 A3. Solution of the structure of a twin crystal of the compound led to final values of R=0.069 and Rw=0.079 with 174 least-squares parameters for 2263 unique reflections with I>2σ(I). The compound is a perchlorate salt of a dicationic centrosymmetric binuclear copper(II) complex, [(dien)Cu(μ-tp)Cu(dien)]2+ — the crystallographic inversion center being located at the center of the benzene ring of the tp bridging ligand. Within the binuclear unit the copper(II) ions are bridged by tp ligand in a bis-unidentate fashion, the coordination geometry about each Cu(II) ion is distorted octahedral; the CuCu distance in each binuclear entity is 11.006(5) A. Moreover, two Cu(II) centers of two different dimeric units are bridged by an oxygen atom of a carboxylate group. Thus, chains along the b direction and layers parallel to (100) are formed. The variable-temperature (4.2–290 K) magnetic susceptibility data of the complex, interpreted with a dimer law with a molecular field approximation, yielded to J, g and zJ′ values of −3.66 cm−1 2.07 and −0.03 cm−1, respectively. An orbital interpretation of the coupling is proposed.

The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters.

The molecular and electronic structures, stabilities, bonding features and magnetic properties of prototypical planar isocyclic cyclo-U n X n ( n = 3, 4; X = O, NH) and heterocyclic cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH) clusters as well as the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] molecules including a planar tetracoordinate element E (ptE) and pentacoordinate U (ppU) at the ring centers, respectively, have been thoroughly investigated by means of electronic structure calculation methods at the DFT level. It was shown that 5f orbitals play a key role in the bonding of these f-block metal systems significantly contributing to the cyclic electron delocalization and the associated magnetic diatropic (magnetic aromaticity) response. The aromaticity of the perfectly planar cyclo-U n X n ( n = 3, 4; X = O, NH), cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH), E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters was verified by an efficient and simple criterion in probing the aromaticity/antiaromaticity of a molecule, that of the nucleus-independent chemical shift, NICS(0), NICS(1), NICS zz (0) and the most refined NICS zz (1) index in conjunction with the NICS scan profiles. Natural bond orbital analyses provided a clear picture of the bonding pattern in the planar isocyclic and heterocyclic uranium clusters and revealed the features that stabilize the ptEs inside the six- and eight-member uranacycle rings. The ptEs benefit from a considerable electron transfer from the surrounding uranium atoms in the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters justifying the high occupancy of the np orbitals of the central atom E.

Investigation of the factors controlling the regioselectivity of the hydrosilylation of alkynes catalysed by trans-di-μ-hydridobis(tricylcohexylphosphine)bis(silyl)diplatinum complexes

Abstract The hydrosilylation of terminal and internal alkynes and also of some hydroxy-alkynes catalysed by trans -di-μ-hydridobis(tricyclohexylphosphine)bis(silyl)diplatinum complexes is described. These diplatinum complexes are very efficient catalysts for such hydrosilylations; high yields (80–98%) can be obtained under mild conditions with a catalysts/reactant ratio of 10 −4 –10 −5 /1. Addition of hydrosilanes X 3 SiH to the alkynes RCCH, RCCR and RCCR′ proceeds in a cis fashion yielding trans -RCHCHSiX 3 , cis -RC(SiX 3 )CHR and mixtures of the regioisomers E -RC(SiX 3 )CHR′ and E -RCHC(SiX 3 )R′ respectively. In the case of terminal alkynes, minor amounts of internal adducts corresponding to Markownikow addition of the hydrosilanes are also formed. The effects of variation of the reaction conditions (concentration of reactants or catalyst and temperature) upon the rate and regioselectivity of the addition were assessed. The regioselectivity increases as the asymmetry of the electron density of the acetylenic π-bonds increases; the regioselectivity is explained in terms of a qualitative molecular orbital treatment of the transition state formed in the insertion step of the alkyne into the PtH bond.

Ground and low-lying excited state properties of the first-row transition-metal oxide diatomics calculated by an improved ASED-MO model

Abstract The ground and selected low-lying electronic states of the entire series of the first-row transition-metal neutral oxides are investigated by the use of an improved atom superposition and electron delocalization molecular orbital (ASED-MO) approach. Two quantities are found to be very important in achieving agreement with experiment: these are the 〈κ, δ〉 parameter set-involved in the Wolfsberg-Helmholz constant, K — and the charge dependent Slater-type orbital exponents. For ScO through CrO the optimum 〈κ, δ〉 set of 〈0.87, 0.24 A −1 〉 yields ground states and spectroscopic constants ( R e , D 0 , ω e and μ ) in good agreement with experiment. The set 〈1.00, 0.35 A −1 ) works well for MnO to CuO and 〈0.75, 0.31 A −1 〉 for ZnO. Calculations are consistent with both ionic and covalent bonding contributions in the monoxides. The M-O bond strengths exhibit the “double humped” shape, characteristic of many periodic properties of the transition metals. Moreover, the sharp contrast in the calculated bond energies between the early and late transition-metal oxides accounts well for the pronounced differences in their reactivities.

Characterisation of Zeolitic Materials with a HEU‐Type Structure Modified by Transition Metal Elements: Definition of Acid Sites in Nickel‐Loaded Crystals in the Light of Experimental and Quantum‐Chemical Results

Nickel-loaded HEU-type zeolite crystals have been obtained by well-known synthetic procedures and characterised by X-ray fluorescence (XRF), scanning-electron microscopy/ energy-dispersive spectroscopy (SEM-EDS), FT-IR, diffuse reflectance UV/ Vis spectroscopy (DR(UV/Vis)S) and X-ray photoelectron spectroscopy (XPS) measurements as non-homoionic and non-stoichiometric substances containing exchangeable hydrated Ni2+ ions in the micropores and nickel hydroxide phases supported on the surface. Thermogravimetric analysis/differential gravimetry (TGA/DTG) and differential thermal analysis (DTA) demonstrated that full dehydration below approximately 400 degrees C follows a clearly endothermic process, whereas at higher temperatures the zeolite is amorphised and finally partially recrystallised to Ni(Al,Si) oxides, detected by powder X-ray powder diffraction (XRD). The solid acidity of NiHEU, initially determined by temperature-programmed desorption (TPD) of ammonia to be 8.93 mgg(-1) NH3, is attributed to the weak acid sites (fundamentally Lewis sites) resolved at approximately 183 degrees C, and to the strong acid sites (essentially Brønsted sites) resolved at approximately 461 degrees C in the TPD pattern. A more sophisticated study based on in situ/ex situ FT-IR with in situ/ex situ 27Al MAS NMR and pyridine (Py) as a probe molecule, revealed that the Lewis acid sites can be attributed primarily to Ni2+ ions, whereas the Brønsted ones can probably be associated with the surface-supported nickel hydroxide phases. The spectroscopic measurements in conjunction with powder XRD and 29Si MAS NMR data strongly suggest that distorted Al tetrahedra are formed during the dehydration process and Py chemisorption/complexation (NiHEU-Py), whereas the crystal structure is remarkably well preserved in the rehydrated material (NiHEU-Py/R). The structural, electronic, energetic and spectroscopic properties of all possible nickel(II) aqua and dihydroxy complexes absorbed in the zeolite micropores or supported on the zeolite surface were studied theoretically by density functional theory (DFT). The computed proton affinity, found to be in the range 182.0-210.0 kcalmol(-1), increases with increasing coordination number of the aqua and dihydroxy nickel(II) complexes.

Spectral and magnetic characterization of amine adducts of copper(II) valproate

Abstract The copper(II) complex of valproic acid and its corresponding pyridine and aniline adducts have been prepared and studied. The complexes were characterized on the basis of elemental analyses, molecular weight determinations, IR, electronic and EPR spectra, as well as variable temperature magnetic susceptibility measurements. All data are consistent with a binuclear structure for the complexes with four valproates as bridges and one donor amine ligand per copper atom in the case of adducts. Singlet— triplet energy separation values were found equal to 320, 344 and 300 cm−1 for the copper(II) valproate complex and its pyridine and aniline adducts, respectively.

Diagnosis of magnetoresponsive aromatic and antiaromatic zones in three‐membered rings of d‐ and f‐block elements

Magnetoresponsive three‐membered rings of d‐ and f‐block elements have been thoroughly investigated with the help of electronic structure calculation methods. The magnetic response of the clusters was evaluated by the Nucleus Independent Chemical Shifts (NICS)zz‐scan curves, which in conjunction with symmetry‐based selection rules for the most significant translationally and rotationally allowed transitions helped rationalize and predict the orbital‐type of aromaticity/antiaromaticity of the clusters. The magnetoresponsive early (Groups 3, 4, and 5) transition metal M3 rings exhibit successive aromatic and antiaromatic zones separated by a nodal plane. The magnetoresponsive late (Groups 11 and 12) transition metal M3 rings exhibit long‐range aromatic zone with the NICSzz(R) values decaying rapidly and monotonically with respect to R. The magnetic response of Group 10 transition metal M3 rings is similar to that of the early transition metal M3 rings, but it is long‐range antiaromatic only for the [c‐Ni3] cluster. The NICSzz‐scan curve of the [(HtLa)3(μ2‐H)6] cluster is indicative of weak pure σ‐aromaticity due to the induced diatropic ring current from the translationally allowed a  1′ → e′ and e′ → a  1′ transitions. The aromatic–antiaromatic behavior of the [(HtCe)3(μ2‐H)6]+ and [(HtTm)3(μ2‐H)6]2− clusters is similar to that of the early d‐block elements. The magnetic response of [(HtYb)3(μ2‐H)6]3− is similar to that of [c‐Hg3]2−. The [(HtLu)3(μ2‐H)6] cluster can be considered as a doubly (σ + π) aromatic system, with the σ‐aromatic component being much stronger than the π‐aromatic one. Finally, the [(XtRe)3(μ2‐X)6] and [(XtRu)3(μ2‐X)6]+ (X = Cl, Br, I) clusters exhibit significant aromatic character with the greatest contribution to the induced diatropic ring currents coming from π‐type transitions.

Interaction of natrolite and thomsonite intergrowths with aqueous solutions of different initial pH values at 25°C in the presence of KCl : Reaction mechanisms

Natural zeolitic material composed of natrolite and thomsonite intergrowths (NAT/THO) was treated in solutions of different initial pH values at 25°C under N2 atmosphere and in 1M KCl as ionic modulator, until pH equilibration. The solid experimental products were studied by means of powder X-ray diffraction (XRD), scanning electron microscopy-energy dispersive system (SEM-EDS), Fourier-transformed infra-red (FTIR) and thermogravimetric analysis/differential thermal analysis (TGA/DTA). The liquid experimental products were analysed using atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES). The NAT/THO material exhibited an amphoteric character with a tendency to neutralise the reacting solutions. The pH equilibration was faster for the acidic region than for the basic one. The H+ ions are chemisorbed on the bulk material, whereas the OH− ions promote a proton detachment from the exchangable cation-water complexes. No Brœnsted acidity, possibly responsible for the neutralisation in the basic region, was found by temperature-programmed desorption (TPD) measurements. Reaction mechanisms involving hydrolysis and degradation-dissolution are proposed. Zeolite crystals remaining at the end of the experiments showed no loss of crystallinity, phase transformation nor even framework dealumination. The insertion of K+ into the zeolites is suggested here as the reason as to why no collapse of their crystal structure occurred in the most acidic solutions causing the zeolites to be more resistant to chemical weathering.

Mixed-ligand iron(III) dithiocarbamates. Calculation of ligand field parameters in electron spin crossover d5 systems by combination of spectral and magnetic data

Abstract New mixed-ligand iron(III) dithiocarbamate complexes of the general formula Fe(R 2 dtc) 2 [R 2 dtc] have been prepared and characterized. These complexes contain two different dithiocarbamate ligands with different ligand field strengths. Magnetic measurements show an electron spin crossover equilibrium that depends on the nature of the two dithiocarbamate ligands. Values of the ligand field parameters Δ, B and β 35 are given and were calculated from a combination of the spectral and magnetic properties of the eomplexes. These values seem more reasonable than those previously reported, which were calculated on the basis of crystal field bands. Assignments for the charge transfer bands in the complexes have also been made.

Relative reactivities of alkynes in hydrosilylation reactions catalysed by trans-di-μ-hydridobis(tricyclohexylphosphine)bis(silyl)diplatinum complexes

Abstract The relative rate constants ( k rel ) of the addition of triethylsilane to alkynes catalysed by trans -di-μ-hydridobis(tricyclohexylphosphine)bis(silyl or germyl)-diplatinum complexes were determined. It was found that the reactivities of alkynes, determined by competition reactions, increase as the π-acceptor capacity of alkynes increases. Generally, alkynes bearing an electron-withdrawing substituent adjacent to an acetylenic carbon atom are more reactive than unsubstituted alkynes in competition reactions. However, in simple hydrosilylation reactions these alkynes are less reactive than the unsubstituted alkynes and their reactivities decrease as the π-acceptor capacity increases. Also, in competition reactions terminal alkynes are more reactive than internal alkynes. The observed trends of the reactivities of alkynes were interpreted on the basis of the stability of the complexes formed between the alkynes and the central atom of the catalysts in the first step of the catalytic cycle.