Michael P. Sigalas

A DFT Study on the Radical Scavenging Activity of Maritimetin and Related Aurones

The radical scavenging activity of maritimetin and a series of synthetic aurones has been studied by using density functional theory with the B3LYP exchange correlation functional. The computation of various molecular descriptors that could assist the elucidation of hydrogen atom and electron donating ability of the selected compounds was carried out in the gas phase and in the liquid phase (benzene, methanol, water) with the aid of IEF-PCM. For reasons of comparison a series of simple phenols of known activity were also included in the study. The results are discussed with regards to the structure-activity relationship principles of flavonoids and in particular to the capacity of the selected aurones to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH(*)) and superoxide anion (O(2)(-*)) radicals. The O-H bond dissociation enthalpy (BDE) seems to be the most proper parameter to characterize the antiradical properties of the studied compounds. The hydroxylation pattern in ring B defines the order of activity, while the extended conjugation and especially the presence of a catechol moiety in ring A are responsible for the high activity observed experimentally for the selected aurones.

Solution identification and solid state characterisation of a heterometallic polyoxometalate {Mo11V7}: (Mo11V5V)-V-VI (V2O52)-O-IV(mu(9)-SO3) (7-)

A polyoxomolybdenum/vanadium-sulfite {M(18)} cluster-based compound, [Mo(VI)(11)V(V)(5)V(IV)(2)O(52)(mu(9)-SO(3))](7-), is reported that exhibits a unique structural motif, arising from the incorporation of five V(V) and two V(IV) ions into a {M(18)} cluster framework templated by SO(3)(2-); this cluster compostion was first identified using cryospray mass spectrometry.

The effect of charged axial ligands on the EPR parameters in oxovanadium(IV) compounds: an unusual reduction of the Az (51V) values.

Two series of octahedral oxovanadium(IV) compounds, containing charged or neutral axial ligands, with the tetradentate amidate molecules Hcapca and H2capcah of the general formulae trans-[V(IV)OX(capca)]0/+ (where X = Cl- (1.CH2Cl2), SCN- (2), N3 (3), CH3COO- (4), PhCOO- (5), imidazole (6. CH3NO2), and eta-nBuNH2 (7)) and cis-[V(VI)OX(Hcapcah)]0/+ (where X = Cl- (8.0.5CH2Cl2), SCN (9), N3 (10.2CH3OH), and imidazole (11)), were synthesized and characterized by X-ray crystallography (1.CH3OH,8.CHCl3, 9.2CH3CN, 10.CH3CN and cis-[VO(imidazole)(Hcapcah)+) and continuous-wave electron paramagnetic resonance (cw EPR) spectroscopy. In addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8 CHCl3 and revealed that this isomer is more stable than the trans isomer, in good agreement with the experimental data. The cw EPR studies of compounds 1-5, that is, the V(IV)O2+ species containing monoanionic axial ligands, revealed a novel phenomenon of the reduction of their A, components by about 10% relative to the N4 reference compounds ([V(IV)O-(imidazole)4]2+ and [V(IV)O(2,2-bipyridine)2]2+). In marked contrast, such a reduction is not observed in compounds 6. CH3NO2-11, which contain neutral axial ligands. Based on the spin-Hamiltonian formalism a theoretical explanation is put forward according to which the observed reduction of Az is due to a reduction of the electron - nuclear dipolar coupling (P). The present findings bear strong relevance to cw EPR studies of oxovanadium(IV) in vanadoproteins, V(IV)O2+-substituted proteins, and in V(IV)O2+ model compounds, since the hyperfine coupling constant, Az, has been extensively used as a benchmark for identification of equatorial-donor-atom sets in oxovanadium(IV) complexes.

Interpretation of electron delocalization in benzene, cyclobutadiene, and borazine based on visualization of individual molecular orbital contributions to the induced magnetic field.

The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization.

Vanadium(V) Compounds with the Bis-(hydroxylamino)-1,3,5-triazine Ligand, H2bihyat : Synthetic, Structural, and Physical Studies of [V2VO3(bihyat)2] and of the Enhanced Hydrolytic Stability Species cis-[VVO2(bihyat)]

Reaction of the ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H(2)bihyat) with NaV(V)O(3) in aqueous solution followed by addition of either Ph(4)PCl or C(NH(2))(3)Cl, respectively, gave the mononuclear vanadium(V) compounds Ph(4)P[V(V)O(2)(bihyat)].1.5H(2)O (1) and C(NH(2))(3)[V(V)O(2)(bihyat)] (2). Treatment of V(IV)OSO(4).5H(2)O with the ligand H(2)bihyat in methyl alcohol under specific conditions gave the oxo-bridged dimer [V(V)(2)O(2)(mu(2)-O)(bihyat)(2)] (3). The structures for 1 and 3 were determined by X-ray crystallography and indicate that these compounds have distorted square-pyramidal arrangement around vanadium. The ligand bihyat(2-) is bonded to vanadium atom in a tridentate fashion at the pyridine-like nitrogen atom and the two deprotonated hydroxylamino oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms (Scheme 4 ), leads to very strong V-N bonds. The cis-[V(V)O(2)(bihyat)](-) species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 3.3-11.0, as it was evidenced by (1)H and (51)V NMR spectroscopy and potentiometry. The high affinity of the H(2)bihyat ligand for the V(V)O(2)(+) unit, its tridentate character, as well as its small size, paves the way for potential applications in medicine, analysis, and catalysis for the C(NH(2))(3)[V(V)O(2)(bihyat)] compound. The molecular structures, vibrational and electronic spectra, and the energetics of the metal-ligand interaction for compounds 1 and 3 have been studied by means of density functional calculations.

Mechanistic aspects of the complete set of hydrolysis and anation reactions of cis- and trans-DDP related to their antitumor activity modeled by an improved ASED-MO approach

Semi-empirical calculations at an improved modified atom superposition and electron delocalization molecular orbital model (ASED-MO) level of theory have been used to model both the hydrolysis and its microscopic reversible anation reactions of the cis-diammino-dichloro-platinum(II) (cis-DDP) antitumor drug and its biologically inactive trans-isomer. Optimized structures for the reactants and products are reported together with trigonal bipyramidal (TBP) structures relevant to an associative mechanism. The hydrolysis reactions of cis-DDP were found to be endothermic and have to surmount high activation barriers while the reverse anation reactions were exothermic exhibiting low activation barriers. The salient feature of the reaction profiles corresponding to an SN2 mechanism is the formation of weak adducts with square pyramidal (SP) geometries that precede and follow the formation of the near TBP transition states along the reaction coordinate. The computed activation energies and enthalpies of the reactions are in good agreement with available experimental and theoretical data. This provides persuasive support for the reliability of the structural, energetic and electronic data of the complex metal-containing systems delivered by the improved modified ASED-MO approach. The computed interaction energies for the formation of the SP weak adducts are reported for the first time thus being predictions as there are no experimental and other theoretical data available so far for comparisons to be made. Moreover, the reaction profile of the interaction of one and two guanine base molecules with all possible hydrolysis products of cis-DDP, modeling the formation of monofunctional and bifunctional adducts during the cis-DDP/DNA interactions was also investigated at the ASED-MO level of theory. The computed activation barriers of the reactions illustrated that the formation of a monofunctional adduct is mostly favored by the interaction of the cis-[Pt(NH3)2(H2O)Cl]+ hydrolysis product with guanine.

Density functional study of potential energy surfaces and relative stabilities of halonium cations of ethylene and cyclopentenes

Abstract Potential energy surfaces, hydride affinities and nuclear independent chemical shifts (NICS) of a variety of halonium cations of ethylene C 2 H 4 X + , cyclopentene C 5 H 8 X + and hydroxy substituted cyclopentenes C 5 H 8− n (OH) n X + ( n =1, 2), where X=Cl and Br were computed at the Becke3LYP/6-311++G(d,p) level of theory. The potential energy surfaces of all molecules under investigation have been scanned and the equilibrium geometries and their harmonic vibrational frequencies have been calculated. The computed hydride affinities of all conformers, as well as the NICS values for the 1,2-bridged cations indicate that the bromo cations are more stable than the analogous chloro cations.

Synthesis, x-ray structural study and magnetism of a novel binuclear copper(II) complex showing an unusual coordination mode for a squarato ligand

Abstract Reaction of the [N,N-diethyl,N′-ethylene (salicylideniminato)] (salicylato) copper(II) complex [Cu(SalNEt 2 )(Sal)] with squaric acid in CH 3 OH affords [Cu 2 (SalNEt 2 ) 2 (H 2 O)(C 4 O 4 )] · H 2 O, whose X-ray structure shows the first example of a binuclear copper(II) complex with an unusual μ-1,2-coordination mode for the bridging squarato (C 4 O 4 2− ) ligand. This type of linking leads to the shortest bimetallic distance, as well as to the largest antiferromagnetic interaction observed so far for squarato bridged copper(II) complexes.

Structure and stability of halonium cations of cycloalkenes. A theoretical study

Abstract A theoretical study of the chloronium and bromonium cations of cyclopropene, cyclobutene, cyclopentene and cyclohexene, C3H4X+, C4H6X+, C5H8X+ and C6H10X+, have been studied at the ab initio MP2 and density functional B3LYP levels of theory implementing 6-311++G(d,p) basis set. The potential energy surfaces of all molecules under investigation have been scanned and the equilibrium geometries and their harmonic vibrational frequencies have been calculated. The computed hydride affinities of all conformers found and the NICS for the bridged cations show that the bromo cations are more stable than the analogous chloro cations, with the bridged 1,4-bromocyclohexyl cation being the most stable cation species studied.

Study of Electron Delocalization in 1,2-, 1,3-, and 1,4-Azaborines Based on the Canonical Molecular Orbital Contributions to the Induced Magnetic Field and Polyelectron Population Analysis

The electron delocalization in 1,2-azaborine, 1,3-azaborine, and 1,4-azaborine is studied using canonical molecular orbital contributions to the induced magnetic field (CMO-IMF) method and polyelectron population analysis (PEPA). Contour maps of the out-of-plane component of the induced magnetic field (Bz(ind)) of the π system show that the three azaborines, in contrast with borazine, sustain much of benzenes π-aromatic character. Among them, 1,3-azaborine exhibits the strongest π delocalization, while 1,4-azaborine is the weakest. Contour maps of Bz(ind) for individual π orbitals reveal that the differentiation of the magnetic response among the three isomers originates from the π-HOMO orbitals, whose magnetic response is governed by rotational allowed transitions to unoccupied orbitals. The low symmetry of azaborines enables a paratropic response from HOMO to unoccupied orbitals excitations, with their magnitude depending on the shape of interacting orbitals. 1,3-Azaborine presents negligible paratropic contributions to Bz(ind) from HOMO to unoccupied orbitals transitions, where 1,2- and 1,4-azaborine present substantial paratropic contributions, which lead to reduced diatropic response. Natural bond orbital (NBO) analysis employing PEPA shows that only the 1,3-azaborine contains π-electron fully delocalized resonance structures.