Coordination polymers of CdII and PbII derived from bipyridine–glycoluril: influence of metal‐ion size and counter‐ions

Two new one‐dimensional (1D) coordination polymers (CPs), namely catena‐poly[[[aquacadmium(II)]‐bis(μ‐4b,5,7,7a‐tetrahydro‐4b,7a‐epiminomethanoimino‐6H‐imidazo[4,5‐f][1,10]phenanthroline‐6,13‐dione)] bis(perchlorate) dihydrate], {[Cd(C₁₄H₁₀N₆O₂)₂(H₂O)](ClO₄)₂·2H₂O}_n or {[Cd(BPG)₂(H₂O)](ClO₄)₂·2H₂O}_n, 1 , and catena‐poly[[lead(II)‐bis(μ‐4b,5,7,7a‐tetrahydro‐4b,7a‐epiminomethanoimino‐6H‐imidazo[4,5‐f][1,10]phenanthroline‐6,13‐dione)] bis(perchlorate) dihydrate], {[Pb(C₁₄H₁₀N₆O₂)₂](ClO₄)₂·2H₂O}_n or {[Pb(BPG)₂](ClO₄)₂·2H₂O}_n, 2 , have been synthesized using bipyridine–glycoluril (BPG; systematic name: 4b,5,7,7a‐tetrahydro‐4b,7a‐epiminomethanoimino‐6H‐imidazo[4,5‐f][1,10]phenanthroline‐6,13‐dione), a urea‐fused tecton, in a mixed‐solvent system. The Cd^II ion in 1 is heptacoordinated and the Pb^II ion in 2 is hexacoordinated, with the Cd^II ion adopting a pentagonal bipyramidal geometry and the Pb^II ion adopting a distorted octahedral geometry. Both CPs form infinite linear chain structures which are hydrogen bonded to each other leading to the formation of three‐dimensional supramolecular network structures. Topological analysis of CPs 1 and 2 reveals that the structures exhibit 1D chain‐like arrangements in an AB–AB sequence and shows platonic uniform 2‐connected uninodal topologies. Furthermore, a comparative analysis of a series of structures based on the BPG ligand indicates that the size of the metal ion and the types of counter‐ions used have a great influence on the resulting frameworks and properties.     

Determination of the acid dissociation constants of p-benzohydroquinone by the INDO method

The stepwise acid dissociation constants for p-benzohydroquinone (QH₂) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF –MO method. We have optimized the geometries of QH₂, QH^?, and Q^2? and of the QH₂ · 6H₂O, QH^? · (H₃O⁺) · 5H₂O, and Q^2? · (H₃O⁺)₂ · 4H₂O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH₂ · 10H₂O, QH^? · (H₃O⁺) · 9H₂O, and QH^? · (H₃O⁺) · 8H₂O, have been determined. The standard free-energy changes calculated for the dissociation of QH₂ into QH^? and H⁺ is 0.0251 Hartree (65.9 kJ mol^?1) and that of QH^? into Q^2? and H⁺ is 0.0285 Hartree (74.8 kJ mol^?1). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol^?1) and 0.0248 Hartree (65.1 kJ mol^?1), respectively. © 1995 John Wiley & Sons, Inc.     

Near-Dirac–Hartree–Fock results for first-row atoms calculated with GTO basis sets

Relativistic basis sets for first-row atoms have been constructed by using the near-Hartree–Fock (nonrelativistic) eigenvectors calculated by Partridge. These bases generate results of near-Dirac–Hartree–Fock quality. Relativistic total and orbital energies, relativistic corrections to the total energy, and magnetic interaction energies for the first-row atoms have been presented. The smallest Gaussian expansions (13s8 p expansions) yield Dirac–Hartree–Fock total energies accurate through six significant digits, while the largest expansions (18s13p expansions) give these energies accurate through seven significant digits. These results are more accurate than some of the results reported earlier, particularly for the open-shell atoms, indicating that the basis employed is reasonably economical for relativistic calculations. © 1995 John Wiley & Sons, Inc.     

Selective recognition of fluoride and acetate by a newly designed ruthenium framework: experimental and theoretical investigations

An effective anion sensor, [Ru(II)(bpy)(2)(H(2)L(-))](+) (1(+)), based on a redox and photoactive {Ru(II)(bpy)(2)} moiety and a new ligand (H(3)L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid), has been developed for selective recognition of fluoride (F(-)) and acetate (OAc(-)) ions. Crystal structures of the free ligand, H(3)L and [1](ClO(4)) reveal the existence of strong intramolecular and intermolecular hydrogen bonding interactions. The structure of [1](ClO(4)) shows that the benzimidazole N-H of H(2)L(-) is hydrogen bonded with the pendant carboxylate oxygen while the imidazole N-H remains free for possible hydrogen bonding interaction with the anions. The potential anion sensing features of 1(+) have been studied by different experimental and theoretical (DFT) investigations using a wide variety of anions, such as F(-), Cl(-), Br(-), I(-), HSO(4)(-), H(2)PO(4)(-), OAc(-) and SCN(-). Cyclic voltammetry and differential pulse voltammetry established that 1(+) is an excellent electrochemical sensor for the selective recognition of F(-) and OAc(-) anions. 1(+) is also found to be a selective colorimetric sensor for F(-) or OAc(-) anions where the MLCT band of the receptor at 498 nm is red shifted to 538 nm in the presence of one equivalent of F(-) or OAc(-) with a distinct change in colour from reddish-orange to pink. The binding constant between 1(+) and F(-) or OAc(-) has been determined to be logK = 7.61 or 7.88, respectively, based on spectrophotometric titration in CH(3)CN. The quenching of the emission band of 1(+) at 716 nm (λ(ex) = 440 nm, Φ = 0.01 at 298 K in CH(3)CN) in the presence of one equivalent of F(-) or OAc(-), as well as two distinct lifetimes of the quenched and unquenched forms of the receptor 1(+), makes it also a suitable fluorescence-based sensor. All the above experiments, in combination with (1)H NMR, suggest the formation of a 1:1 adduct between the receptor (1(+)) and the anion (F(-) or OAc(-)). The formation of 1:1 adduct {[1(+)·F(-)] or [1(+)·OAc(-)]} has been further evidenced by in situ ESI-MS(+) in CH(3)CN. Though the receptor, 1(+), is comprised of two N-H protons associated with the coordinated H(2)L(-) ligand, only the free imidazole N-H proton participates in the hydrogen bonding interactions with the incoming anions, while the intramolecularly hydrogen bonded benzimidazole N-H proton remains intact as evidenced by the crystal structure of the final product (1). The hydrogen bond mediated anion sensing mechanism, over the direct deprotonation pathway, in 1(+) has been further justified by a DFT study and subsequent NBO analysis.     

Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.     

Acid-Catalyzed Hydrolysis of Some N,N-Dibenzylalkanesulfinamides in 50% Acetonitrile–Water

Abstract

The hydrolysis of some N,N-dibenzylalkanesulfinamides (RSONH(CH₂Ph)₂; 1, R = Me; 2, R = ⁱPr; 3, R = ^tBu; 4, R = 1-adamantyl) has been studied in 50% (v:v) acetonitrile–water solutions of hydrobromic and hydrochloric acids, mainly at 44.8 °C, using ultraviolet (UV) spectrophotometry to determine pseudo first-order rate constants. The compounds were found to hydrolyze by concurrent bimolecular neutral, acid-catalyzed, and acid-dependent nucleophilic (halide ion) catalysis pathways. The last-named is predominant in reactions in HBr solutions, but in HCl solutions, the acid-catalyzed pathway is predominant. The results indicate that both steric and electronic effects are important in these reactions. There appears to be no mechanistic switchover in the series 1→4.     

Cycloaddition profile of pentafulvenes with 3-oxidopyrylium betaine: experimental and theoretical investigations

A detailed investigation on the effect of solvent polarity, temperature, and microwave irradiation on periselectivity in cycloaddition reactions of pentafulvenes with 3-oxidopyrylium betaine is described. The base catalyzed generation of 3-oxidopyrylium betaine in CHCl₃ resulted in the exclusive formation of [6+3] adducts. With increase in solvent polarity and temperature, mixtures of [6+3] and [3+2] adducts were formed, where as under microwave irradiation, [3+2] adducts were formed exclusively. The experimental results have been rationalized on the basis of theoretical calculations.     

A Simple,Efficient and Green Procedure for the Knoevenagel Condensation of Aldehydes with N-Methylpiperazine at Room Temperature under Solvent-Free Conditions

Knoevenagel condensation of aromatic, aliphatic, and heteroaromatic aldehydes with active methylene compounds such as ethylcyanoacetate, malononitrile, and cyanoacetamide proceed very smoothly at room temperature by simply mixing the ingredients together under solvent-free conditions in the presence of N-methylpiperazine in excellent yields of the E-configured products.     

Octahedral CuII and NiII complexes manifesting with N′-[1-(pyridin-2-yl)ethylidene] acetohydrazide: Structural outlooks and spectral characteristics

A tridentate hydrazone precursor, N′-[1-(pyridin-2-yl)ethylidene]acetohydrazide (L) (1:1 refluxed product of acetichydrazide and 2-acetylpyridine), produced two octahedral Cu^II and Ni^II derivatives, [CuL₂]·NO₃ (1) and [NiL₂]·ClO₄·H₂O (2). Both are subjected to X-ray diffraction system, and structural investigation shows that the central metal atom (Cu^II or Ni^II) adopts a distorted octahedral geometry with N₄O₂ donor sets by coordination of a pair of independent hydrazone precursors. Besides X-ray study, IR and UV-vis spectra, thermal analysis and room temperature magnetic moments are utilized for establishing significant characteristics of both complexes. It is apparent that the M-N_pyridine bonds are slightly longer than the M-N_imino bonds, Cu1-N1 and Cu1-N4 [2.300(2) and 2.038(2) ?] for 1 and Ni1-N1 and Ni1-N4 [2.075(2) and 2.084(1) ?] for 2, Cu1-N2 and Cu1-N5 [2.062(1) and 1.932(1) ?] for 1 and Ni1-N2 and Ni1-N5 [2.008(2) and 1.975(2) ?] for 2, respectively. As per our observation, the effective magnetic moment value (μ_eff) is found to be 1.77 B.M. for 1 and 3.06 BM for 2, respectively.