Evangelos G. Bakalbassis

Thegem-Diol Form of (py)2CO as a Ligand in Cobalt(II) Carboxylate Clusters: A Cubane Complex and a Novel Nonanuclear Species with a Vertex-Sharing Double Square Pyramidal Structure

A vertex-sharing double square pyramid of cobalt(II) ions (see picture) is present in the nonanuclear complex 1, obtained by treating cobalt acetate with 0.5 equivalents of (py2 )CO. The corresponding 1:1 reaction gave 2, which has a central core with a cubane structure. py=pyridine.

Ab initio and density functional theory studies for the explanation of the antioxidant activity of certain phenolic acids.

Ab initio and density functional theory molecular orbital calculations were carried out at both the HF/6-31+G(d) and B3LYP/6-31+G(d) levels for the four antioxidants, p-hydroxycinnamic acid derivatives, namely, the p-coumaric, caffeic, ferulic, and sinapinic acid and the corresponding radicals, in an attempt to explain the structural dependency of the antioxidant activity of these compounds. Optimized resulting geometries, vibrational frequencies, absolute infrared intensities, and electron-donating ability are discussed. Both the high degree of conjugation and the extended spin delocalization in the phenoxyl radicals offer explanation for the scavenging activity of the four acids. In structurally related compounds, the calculated heat of formation value in radical formation appears as a meaningful molecular descriptor of antioxidant activity in accordance with experimental data. This becomes more clear at the B3LYP level.

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.

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.

The [Cu2(O2CMe)4(btd)2] complex as a bridging unit: preparation, characterisation, X-ray structure and magnetism of the 2D coordination polymer {[Cu6(O2CMe)8(OMe)4(btd)2]}n (btd=2,1,3-benzothiadiazole)

A systematic investigation of the [Cu2(O2CMe)4(H2O)2]/btd reaction system is described, where btd = 2,1,3-benzothiadiazole. Reaction of [Cu2(O2CMe)4(H2O)2] with 5–8 equiv. of btd in both MeCN and MeOH yields [Cu2(O2CMe)4(btd)2 ]( 1) in 40–50% yields. Treatment of [Cu2(O2CMe)4(H2O)2] with 1.4 or 0.7 equiv. of btd in MeOH leads to the precipitation of the polymeric compound {[Cu6(O2CMe)8(OMe)4(btd)2]} (2) in 82% yield. Reaction of 1 with two equivalents of [Cu2(O2CMe)4(H2O)2] in MeOH under reflux provides an additional route to 2. The structure of 1 consists of centrosymmetric dinuclear [Cu2(O2CMe)4(btd)2] molecules of the paddle-wheel cage type. The two Cu II ions are bridged by four 1 : 1 :2 acetates, while a monodentate btd is at the apex of the square pyramid of each metal centre. The 2D structure of 2 consists of chains of tetranuclear, planar Cu4(OMe)4(O2CMe)4 repeating units running along the a axis, which are connected along the cell body diagonal via Cu2(O2CMe)4(btd)2 paddle-wheel dinuclear units. The btd molecules behave as bidentate bridging ligands. Within the tetranuclear units, each Cu II ion is connected via two acetate bridges with one neighbouring Cu II ion and via two methoxo bridges with the other neighbouring Cu II ion; intertetranuclear linking is provided by two monoatomic acetate bridges. The results of solid state magnetic susceptibility studies are described for complex 2 in the temperature range 3–300 K. The results reveal antiferromagnetic exchange interactions between the Cu II ions. The complicated structure of 2 does not permit an exact treatment for the determination of the various exchange interactions. However, an approximate 3-J magnetic model was constructed, resulting in an excellent fit. An orbital interpretation of the J-trend derived is also attempted.

2-(2′-pyridyl)quinoxaline (L) as a bidentate ligand: crystal structure, magnetic properties and quantum chemical study of [Cu2Cl4L2]

Abstract The X-ray crystal structure of the complex [Cu2Cl4L2], where L is 2-(2′-pyridyl)quinoxaline, has been determined. The structure consists of dimeric molecules [Cu2Cl4L2], in which copper ions are bridged by two chloride ligands. The geometry at copper can be described as either a distorted square-pyramidal or a distorted trigonalbipyramidal. Magnetic susceptibility and EPR data give evidence for exchange coupling with the best fit to the data of the dimeric expression for a pair of exchange-coupled S = 1 2 ions yielding g = 2.134, 2J = −10 cm−1 and zJ′ = −2 cm−1. The investigations of the ground-state electronic structure of the complex and its precursor ligand L, along with an orbital interpretation of the magnetic coupling based upon extended Huckel molecular orbital calculations, are also presented.

Preparation, structural characterization and physical properties of palladium(II) and platinum(II) complexes of the biheteroaromatic ligand 2-(2′-pyridyl)quinoxaline (L)

Abstract The 1:1 and 1:2 reactions of [MX4]2− (M=Pd, Pt; X=Cl, Br) with the biheteroaromatic ligand 2-(2′-pyridyl)quinoxaline (L) were studied. The new complexes [PdCl2L] (1), [PdL2]Cl2 (2), [PtCl2L] (3), [PdBr2L] (4) and [PtBr2L] (5) were isolated and studied by a combination of conductivity measurements, X-ray powder patterns, thermal techniques (TG/DTG, DTA) and spectroscopy (IR, far-IR, Raman). Compounds 1 and 2 have also been characterized by using 1H and 13C NMR spectroscopies. The complex [PdCl2L] crystallizes in space group P21/c with a=9.842(1), b=10.223(1), c=15.130(1) A, β=122.70(1)°, V=1281.0(6) A3 and Z=4. Refinement of the structure gave a final R factor of 0.017 (Rw=0.024) for 2050 unique reflections having I>2.0σ(I). The X-ray crystal structure of 1 shows a slightly distorted square planar geometry around the PdII atom. The ligand L behaves as a bidentate chelate with the ligated atoms being the pyridine nitrogen and the nearest quinoxaline nitrogen. Similar monomeric, square planar structures are also assigned for 3, 4 and 5. A trans square planar structure is proposed for the [PdL2]2+ cation in 2. All data are discussed in terms of the nature of bonding and known and assigned structures. Compounds with the empirical formulae PtCl2L0.5 (3a) and PtBr2L0.25 (5c) were isolated during the thermal decomposition of 3 and 5; the vibrational studies of these thermally stable intermediates show that L acts as a tridentate bridging ligand and support complicated polymeric structures. The 1H NMR study of the 1:1 [PdI(dien)]I/L (dien=diethylenetriamine) reaction solution in DMSO-d6/MeOD reveals a monodentate quinoxaline coordination of L.

Antiferromagnetism tuned by the topology of the intervening bridging ligand. Crystal structure and magnetic properties of the (μ-phthalato){[bis(piperidine)]Cu(II)} chain complex

Abstract The crystal structure of the chain complex [(μ-phth)(pip) 2 Cu] n , where phth is the dianion of phthalic acid and pip is piperidine, has been determined by single crystal X-ray methods. The complex crystallizes in the monoclinic space group I 2/ a with α=15.853(2), b =10.785(1), c =11.629(1) A, β=111.321(3)°, Z =4 and V =1852.11 A 3 . Solution of the structure led to final values of R =0.0298 and R w =0.0330 with 167 least-squares parameters for 1578 unique reflections with F o >6.5σ( F o ). The compound is a chain copper(II) complex, [(pip) 2 Cu(μ-phth)] n being the polymeric entity. Within the chain the copper(II) ions are bridged by the phth ligand in an anisobidentate fashion, the coordination geometry about each Cu(II) ion is distorted octahedral; two nitrogen atoms belonging to two piperidine molecules along with two carboxylato oxygen atoms of two different phthalato bridges occupy the equatorial positions, whereas the two remaining oxygen atoms of the same carboxylato groups occupy the axial positions. Thus, chains along the c direction are formed. The intrachain metal-metal separation is 5.814 A. The variable-temperature (4.2-290 K) magnetic susceptibility data of the complex showed a perfect Curie law. The role of the topology of the intervening bridge on the strength of the antiferromagnetic coupling of the complex is discussed.

Copper(II) chloride–1-methylbenzotriazole chemistry: variation of product as a function of metal-to-ligand reaction ratio; synthesis, structure and properties of a dinuclear complex and a novel chain polymer with two alternating chromophores

The 1 : 2 and 1 : 1 reactions of CuCl2 with 1-methylbenzotriazole in EtOH–CH(OEt)3 gave the doubly chloro-bridged dimer [Cu2Cl4(C7H7N3)4]1 and the novel alternating linear-chain polymer [{Cu2Cl4(C7H7N3)2}n]2 respectively, crystal structures of which have been determined; magnetic susceptibility studies show that complex 1 possesses intramolecular- and complex 2 intrachain-ferromagnetic coupling.

Molecular orbital study of the bonding and reactivity of the diisocyanide complexes trans-[Mo(CNR)2(Ph2PCH2CH2PPh2)2] and derived aminocarbyne compounds

Abstract The bonding structure, properties and reaction patterns of the isocyanide and aminocarbyne-type complexes of molybdenum have been investigated with the aid of EHMO-SCCC calculations and of frontier orbital theory. The results indicate that (i) β-protonation of a ligating isocyanide, in either the trans -[Mo(CNMe) 2 (dppe) 2 ] ( 1 ) or trans -[Mo(CNHMe)(CNMe)(dppe) 2 ] + , is predominantly charge-controlled, whereas the α-nucleophilic attack at the dicarbene-type complex trans -[Mo(CNHMe) 2 (dppe) 2 ] 2+ is orbitally controlled, and (ii) electrophilic attack should preferentially occur at the Mo atom of a molybdenum complex involving linear isocyanide ligands but at the N atoms of a complex with bent isocyanide ligands. The conversion of the hydride into the aminocarbyne complex, the oxidation of 1 , and its oxidative-addition reactions with halogens are interpreted in terms of FMO theory.