The structure of dihydropyrroloindoles: A quantum-chemical estimation of conjugation in cyclohexadiene systems

A non-empirical quantum chemical calculation of isomeric 3,6-divinyl-3,4,5,6-tetrahydropyrrolo[3,2-e] indole 1 and 1,5-divinyl-1,4,5,8-tetrahydro[3,2-f]indole 2 structures carried out by DFT (B3LYP) method with 6-311++G(d, p) and 6-311++G(3df, p) basis sets showed the energy preference of 2 over 1 (1.33 kcal/mol and 1.47 kcal/mol, respectively). The structure of the molecule of 2 is planar while the molecule of 1 is non-planar due to the presence of sp ³-hybridized carbon atoms.     

Structure of aniline-containing Co(III) <Emphasis Type="Italic">trans</Emphasis>-dioximates with the [ZrF<Subscript>6</Subscript>]<Superscript>2−</Superscript> anion

Two new coordination compounds, trans-octahedral aniline-containing Co(III) dioximates [Co(DH)₂(Anil)₂]₂[ZrF₆]·2H₂O (I) and [Co(NioxH)₂(Anil)₂]₂[ZrF₆]·3H₂O (II), where DH^? is the dimethylglyoxime monoanion, and NioxH^? is the 1,2-cyclohexanedionedioxime monoanion, were synthesized, and their crystal structures were studied by XRD. In I, two centrosymmetric, crystallographically independent [Co(DH)₂(Anil)₂]⁺ complex cations (A and B) are linked by a system of hydrogen bonds, the [ZrF₆]^2? centrosymmetric anion, and the crystallization water molecules into a three-dimensional framework. The coordination polyhedron of the Co³⁺ atom in I and II is a slightly distorted N₆ octahedron formed by four nitrogen atoms of the two dioxime residues and two amine nitrogen atoms of the aniline fragment. The deviation of the cobalt atom from the plane of the metallocycle in II is up to 0.012 Å. The [ZrF₆]^2? outer-spheric anions and crystallization water molecules dominate in the crystal structures.     

Tetraphenylphosphonium carboxylates and sulfonates. Synthesis and structure

The reaction of the pentaphenyphosphorus solvate Ph₅P·1/2PhH (I) with carboxylic and sulfonic acids was used to synthesize tetraphenylphosphonium carboxylates Ph₄POC(O)R, R = C₆H₄(2-OH) (II), C₆H₄ (2-COOH) (III), H (IV), Me (V), CCl₃ (VI), Ph (VII), PhCH=CH (VIII), CH₂CH₂C(O)OH (IX), CH=CHC(O) OH(X), and CH₂C(O)OH (XI) and tetraphenylphosphonium sulfonates Ph₄POSO₂Ar, Ar = Ph (XII), C₆H₄Me₄ (XIII), and C₆H₃(-COOH)(4-OH) (XIV). Compound XII was also prepared from compound I and SO₃ in benzene. According to X-ray diffraction data, the crystals of I contain two types of crystallographically independent molecules with a slightly distorted trigonal-bipyramidal configuration [Ia, CaxPCax 178.44(8)°, P- C_ax 1.985(2), 1.987(2) Å, P-C_eq 1.854(2), 1.846(2), 1.840(2) Å; Ib, C_axPC_ax 178.45(9)°, P-C_ax 1.980(2), 1.975 (2) Å, P-C_eq 1.840(2), 1.846(2), 1.854(2) Å]. In the cations of compounds II, III and XIV, the coordination of the phosphorus atom is tetrahedral [CPC angle: II, 106.2(2)?111.6(1)°; III, 104.01(6)?113.03(6)°; XIV, 107.54 (6)?112.79(6)°]; the anions contain intramolecular O-H?O hydrogen bonds between the hydroxyl hydrogen atom and carboxyl oxygen atom (II, 1.34; III, 1.23; and XIV, 1.83 Å).     

Synthesis,anti-angiogenic and DNA cleavage studies of novel <Emphasis Type="Italic">N</Emphasis>-(4-methyl-3-((4-(pyridin-3-yl)pyrimidin-2-yl)amino)phenyl)piperidine-4-carboxamide derivatives

A series of novel N-(4-methyl-3-((4-(pyridin-3-yl)pyrimidin-2-yl)amino)phenyl)piperidine-4-carboxamide derivatives 10(a–f), 12(a–c) and 14(a–c) were synthesized and characterized by FTIR, ¹H-NMR, mass spectral and elemental analysis. The efficacy of these derivatives to inhibit in vivo angiogenesis was evaluated using chick chorioallantoic membrane (CAM) model and their DNA cleavage abilities were evaluated after incubating with calf thymus DNA followed by gel electrophoresis. These novel piperidine analogues efficiently blocked the formation of blood vessels in vivo in CAM model and exhibited differential migration and band intensities in DNA binding/cleavage assays. Among the tested compounds 10a, 10b, 10c, 12b, 14b and 14c showed significant anti-angiogenic and DNA cleavage activities compared to their respective controls and the other derivatives used in this study. These observations suggest that the presence of electron donating and withdrawing groups at positions 2, 3 and 4 of the phenyl ring of the side chain may determine their potency and as anticancer agents by exerting both anti-angiogenic and cytotoxic effects

Open image in new window

.

     

X-ray diffraction investigation of perfluoro-4-methylenecyclohexa-2,5-dienones

Perfluorinated 8-phenyl-7,8-diethylbicyclo[4.2.0]octa-1,4,6-trien-3-one (3), 2-(4-oxocyclohexa-2,5-dienylidene)-1,1-diethylbenzocyclobutene (4) and 2-(4-oxocyclohexa-2,5-dienylidene)-5-(2-phenyl-cis-1,2-diethylbenzocyclobuten-1-yloxy)-1,1-diethylbenzocyclobutene (5), including the 4-methylencyclohexa-2,5-dienone fragment, were prepared by the reaction of perfluoro-1,1-(1) and-1,2-diethylbenzocyclobutene (2) with pentafluorobenzene in SbF₅. Single crystal X-ray diffraction study of compounds 3–5 revealed that the oxygen atom of the C=O group participated in the formation of supramolecular architectures in all three compounds, and C=O…π bonding may be considered as the corresponding synthon (with increased separation in the case of compound 5). C-F…π bonding acts as a second synthon. F…F interactions in crystals 3–5 were classified as stabilizing or enforced.     

Synthesis of mixed metal oxide nanoparticles of SnO<Subscript>2</Subscript> with SrO via sol–gel technology: their structural,optical, and electrical properties

Mixed metal oxides of tin with strontium (xSnO₂.SrO) in different molar ratio {where x?=?1 (1), 2 (2), 4(3); SnO₂ doped with Sr⁺²(4), SnO₂ doped with Sr⁺² and co-doped with F^?(5)} have been prepared by sol–gel technology in basic medium using SnCl₂.2H₂O as precursor in isopropanol as solvent. Structural analysis by XRD patterns have shown formation of particles at nanoscale and phase separation of SnO₂ in tetragonal rutile framework in these mixed metal oxides. This fact was further supported by TEM. SEM images of all these samples have shown formation of various geometrical patterns ranging from spherical particles to nanorods. In the IR spectra of all these oxides, Sr–O absorption bands were present only in sample (1). UV–Vis spectroscopy has shown reduction in optical band gap in mixed metal oxides and the lowest value of band gap was observed for sample (3). Photoluminescence spectra of all these derivatives are found to be almost similar again indicated retention of tetragonal rutile SnO₂ framework. I–V curves of all these oxides are non-linear and lowest resistance was observed for sample (3). This fact was further supported by impedance measurements.

Open image in new window

     

Aluminum hydrides with radical-anionic and dianionic acenaphthene-1,2-diimine ligands

The reaction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) with LiAlH₄ affords two products regardless of the solvent used (tetrahydrofuran or diethyl ether). These products were isolated as green and colorless crystals. Green crystals of the complex [(dpp-bian)Al(H)₂Li(THF)₃] (1) were obtained from tetrahydrofuran; colorless crystals of the complex [{dpp-bian(H₂)}Al(H)₂Li(Et₂O)₂] (2), from diethyl ether. The reactions of compound 1 with 2,6-di-tert-butyl-4-methylphenol and benzophenone gave monohydrides [(dpp-bian)Al(H)(OC₆H₂-2,6-Bu₂ ^t-4-Me)][Li(THF)₄] (3) and [(dpp-bian)Al(H)(OCHPh₂)- Li(THF)₂] (4), respectively. The diamagnetic aluminum hydride [(dpp-bian)AlH(THF)] (5) was synthesized by the reaction of dichloroalane HAlCl₂ (in situ) with the disodium salt of dpp-bian in THF; the paramagnetic hydride [(dpp-bian)AlH(Cl)] (6) containing the dpp-bian radical anion was synthesized by the reaction of the monosodium salt (dpp-bian)Na with monochloroalane H₂AlCl (in situ) in diethyl ether. The reaction of compound 6 with tert-butyllithium gives the complex [(dpp-bian)AlBu^t(Et₂O)] (7). Diamagnetic derivatives 1—5 and 7 were characterized by ¹Н NMR spectroscopy; paramagnetic compound 6, by ESR spectroscopy. The molecular structures of compounds 1—7 were determined by single-crystal X-ray diffraction.     

Syntheses,crystal structures and in vitro anticancer activities of oxovanadium(IV) complexes of amino acid Schiff base and 1,10-phenanthroline ligands

Four oxovanadium(IV) complexes, namely [VO(desa-met)(phen)]·MeOH·2H₂O (1) (desa-met = Schiff base derived from 4-(diethylamino)salicylaldehyde and dl-methionine, phen = 1,10-phenanthroline), [VO(o-van-met) (phen)]·MeOH·CH₂Cl₂·3H₂O (2) (o-van-met = Schiff base derived from o-vanillin and dl-methionine), [VO(dtbs-napa)(phen)]·2H₂O (3) (dtbs-napa = Schiff base derived from 3,5-di-tert-butyl salicylaldehyde and 3-(1-naphthyl)-l-alanine) and [VO(hyna-napa)(phen)]·1.5H₂O (4) (hyna-napa = Schiff base derived from 2-hydroxy-1-naphthaldehyde and 3-(1-naphthyl)-l-alanine), were synthesized and characterized by IR, HRMS, UV–vis spectra, molar conductance and single-crystal X-ray diffraction (XRD). X-ray structural analysis showed that the V(IV) atoms in all four complexes are six-coordinated in a distorted octahedral environment. In the crystals of complexes 1 and 2, π–π stacking interactions together with hydrogen bonds connect the molecular units into 2D networks. Meanwhile, CH–π stacking interactions are observed between the aromatic rings in the crystals of 1 and 4, while the π–π stacking interactions between aromatic rings in the crystals of 2 and 3 are arranged with a face-to-face mode. The in vitro anticancer activities of these complexes against A-549 and HeGp2 cells were tested by MTT assay.     

Microwave in glycosylation reaction: design,and synthesis of highly substituted nicotinonitrile nucleosides

An efficient and rapid regiospecific approach for the synthesis of cyclic and acyclic nucleosides of 2-oxonicotinonitriles was performed. Whereas, glycosylation of 2-oxonicotinonitriles 1a, b with peracetylated sugars (namely, peracetylated glucose, galactose and ribose) under MWI tolerated exclusively the desired N-nucleosides 2a, b, 4a, b and 6a, b in significant yields (75–86%) and in short reaction time (5–7 min.). The same products were obtained under the conventional conditions, using halo-sugar with low yields in hard conditions. Similarly, the acyclic nucleosides 8a, b and 9a, b were obtained under MWI and conventional conditions via reaction of 1a, b with 4-bromo butyl acetate and 2-acetoxyethoxymethyl bromide. Finally, deprotection of the latter blocked N-nucleosides was performed via treatment with aqueous methanolic solution containing a catalytic amount of triethyl amine to give the desired free nucleosides 3a, b, 5a, b, 7a, b, 10a, b and 11a, b, respectively. The free nucleosides (3a, b, 5a, b, 7a, b and 11a, b) were evaluated against Gram (+?ve) bacteria, Gram (–ve) bacteria and one pathogenic Fungi namely, Aspergillus flavus. Good results were obtained for compounds 7a, b and 11a, b compared with the used standard drugs (Cefotaxime and Dermatin).

Graphical abstract

Open image in new window

     

Recovery of host crystals from inclusion crystals of <Emphasis Type="Italic">p-tert-</Emphasis>butylcalix[4]arene and <Emphasis Type="Italic">p-tert</Emphasis>-butylthiacalix[4]arene by the treatment with a solvent and/or supercritical CO<Subscript>2</Subscript>

Upon stirring inclusion crystals of p-tert-butylthiacalix[4]arene (2) in solvents with heating, guest compounds were efficiently desorbed to yield guest-free crystals. More specifically, upon treatment with methanol, the exchange of guest compounds with methanol in the crystals, followed by the desorption of the methanol afforded metastable host crystals 2β, whereas, upon treatment with heptane, the dissolution of the inclusion crystals and simultaneous crystallization of compound 2 afforded stable host crystals 2α. Further, a host crystal of p-tert-butylcalix[4]arene (1) was recovered by the treatment of 2:1 (host/guest) inclusion crystals of compound 1 with supercritical carbon dioxide (scCO₂), and through the combination of the guest exchange of 1:1 inclusion crystals of compound 1 with hexane and scCO₂ treatment of the resulting 2:1 inclusion crystals 1₂·hexane. Although the recovered host crystal of compound 1 contained a small amount of CO₂, it could be reused for the inclusion of organic compounds.     

Synthesis of novel 5,6,7,8,9,10-hexahydropyrimido[4,5-b]quinoline derivatives for antimicrobial and anti-oxidant evaluation

Because of the well-known chemotherapeutic activity of pyrimidoquinolines, a new series of the title compounds were synthesized and biologically screened for their antimicrobial and anti-oxidant activities. The intermediate compound, 2-amino-1-cyclohexyl-4-(3,4-dimethoxyphenyl)-1,4,5,6,7,8-hexahydroquinoline-3-carbonitrile (2), was prepared and allowed to react with oxalyl chloride to give the corresponding strategic starting material, 10-cyclohexyl-5-(3,4-dimethoxyphenyl)-4-oxo-3,4,5,6,7,8,9,10-octahydropyrimido[4,5-b]quinoline-2-carbonyl chloride 3. Compound 3 underwent diverse reactions with amines to give the corresponding carboxylic amides 4, with alcohols to give the carboxylic esters 5 and with hydrazine to give the carbohydrazide 8. Compound 8 reacted with aldoses to give the corresponding polyhydroxy(-)alkyl Schiff bases 9, which upon reaction with thioglycolic acid afforded the thiazolidinone-C-acyclic nucleosides 10. Several other pyrimidoquinolines incorporated to oxadiazole, iminothiazolidinones and thiadiazoles were synthesized starting from the 2-carbohydrazide derivative 8. Most of the newly prepared derivatives showed considerable antimicrobial activity against Gram +ve and Gram ?ve bacteria and anti-oxidant activity.     

Transition metal complexes of a hydrazone derived from hydralazine hydrochloride and 3,5-di-tert-butylsalicylaldehyde

Mononuclear Co(III), Ni(II) and Cu(II) coordination compounds of (E)-1-(3,5-di-tert-butyl-2-hydroxybenzylidene)-2-(phthalazin-1-yl)hydrazine (LH) were prepared and characterized by physicochemical and spectroscopic methods. The metal-to-ligand ratio was found to be 1:2 in [Co(L)₂]Cl·2H₂O (1) and [Ni(L)₂]·2H₂O (2), while it is 1:1 in [Cu(L)Cl]·2CH₃OH (3). The X-ray crystal structures of LH and complex 1 is are reported. LH shows monobasic behavior, coordinating through NNO donor atoms. The complexes were investigated for their antimicrobial properties. Complexes 1 and 3 show excellent antibacterial and antifungal activities, respectively.     

Benzyl group conformation in 4-benzyl-4-hydroxypiperidines

The high-resolution ¹H and ¹³C NMR spectra of eight 4-benzyl-4-hydroxypiperidines 1–8 were recorded in CDCl₃ and analyzed. In 2, the conformation of the equatorial benzyl group at C(4) was established as an equilibrium mixture of A [the phenyl group is gauche with respect to OH and C(5)] and B [the phenyl group is gauche with respect to OH and C(3)], whereas in 3-alkyl-4-benzyl-4-hydroxypiperidines 3–8, the favored conformation of the benzyl group at C(4) is A. In 1, the axial benzyl group at C(4) adopts the gauche conformations A′ [the phenyl group is gauche with respect to OH and C(3)] and B′ [the phenyl group is gauche with respect to OH and C(5)], in which the phenyl ring of the benzyl group is gauche with respect to the OH group. The HF/DFT B3LYP/6-3G* hybrid calculations of model systems 1′–3′ also support these conformations. The ¹³C data reveal that the equatorial methyl group at C(3) exerts a shielding influence on the methyl-bearing carbon and the magnitude of the α effect was found to be approximately ?1.5 ppm. The parameters of the ¹³C substituent in the benzyl group show that the the α effect of the equatorial benzyl group is considerably higher in 3-ethyl tertiary alcohol 7 than in 3-methyl tertiary alcohol 3 and 4-benzyl-t(4)-hydroxypiperidine 2. This may be explained if we take into account the different conformations of the ethyl group in t(4)-hydroxy-3-ethyl-2,6-diphenylpiperidine 12 and 3-ethyl tertiary alcohol 7.     

Preparation,molecular and crystal structures of 3-(1-amino-2,2,2-trifluoroethylidene)-1,1,4,5,6,7-hexafluoroindan-2-one, 2-amino-1,1,4,5,6,7-hexafluoro-3-trifluoroacetylindene,and their complexes with dioxane and pyridine

3-(1-Amino-2,2,2-trifluoroethylidene)-1,1,4,5,6,7-hexafluoroindan-2-one (2) is synthesized by the interaction between 3-(1-amino-2,2,2-trifluoroethylidene)-2-imino-1,1,4,5,6,7-hexafluoroindan (1) and isopropylnitrite, and 2-amino-1,1,4,5,6,7-hexafluoro-3-trifluoroacetylindene (3) is prepared by hydrolysis. Single crystals are grown, and the molecular and crystal structure of enaminoketones obtained, the complex of compound 2 with 1,4-dioxane, and the complex of compound 3 with pyridine is studied. DFT calculations have been performed to find the complex formation energies of compounds 2 and 3 with dioxane and pyridine in the gas phase.     

Copper complexes of pyridyl–tetrazole ligands with pendant amide and hydrazide arms: synthesis,characterization, DNA-binding and antioxidant properties

Pyridyl–tetrazole ligands 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetamide (L1), 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetamide (L2), 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetohydrazide (L3) and 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetohydrazide (L4) have been prepared and coordinated with CuCl₂·2H₂O to furnish the corresponding complexes [Cu(L1) ₂ ]–[Cu(L4) ₂ ]. EPR spectra of the complexes are characteristic of square planar geometries, with nuclear hyperfine spin 3/2. DNA-binding studies using UV–Vis absorption spectroscopy, viscosity and thermal denature studies revealed that all of these complexes are avid binders of calf thymus DNA. The antioxidant properties of the free ligands and the Cu(II) complexes were investigated using the p-nitrosodimethyl aniline hydroxyl radical scavenging method, and [Cu(L4) ₂ ] was found to show the highest activity.     

Synthesis and crystal structures of (2.2.2-cryptand)rubidium chloride and bromide hydrates

Two complexes, (2.2.2-cryptand)rubidium chloride and bromide hydrates [Rb(Crypt-222]Hal · 3.5H₂O (Hal = Cl (I) and Br (II)), are synthesized. The structures of isomorphic crystals of compounds I and II are studied by X-ray diffraction analysis. The crystals are trigonal: space group P \(\overline 3 \), Z = 2; I: a = 11.810 Å, c = 11.302 Å; II: a = 11.890 Å, c = 11.402 Å. The structures are solved by a direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.060 (I) and 0.077 (II) for 2650 (I) and 2700 (II) independent reflections (CAD-4 automated diffractometer, λMoK _α radiation). In crystals of complexes I and II, the [Rb(Crypt-222)]⁺ cation of the host-guest type lies on the crystallographic axis 3 and has the approximate symmetry D ₃. In complexes I and II, the coordination polyhedron of the Rb⁺ cation is a two-base-centered trigonal prism somewhat distorted to an antiprism. The crystals of compounds I and II contain H-bonded disordered cubes of the water molecules and Cl^? or Br^? anions.     

Synthesis and characterization of novel rhenium(I) complexes towards potential biological imaging applications

Background

Re(I) tricarbonyl complexes exhibit immense potential as fluorescence imaging agents. However, only a handful of rhenium complexes have been utilized in biological imaging. The present study describes the synthesis of four novel rhenium complexes, their characterization and preliminary biological studies to assess their potential as biological imaging agents.

Results

Four facial rhenium tricarbonyl complexes containing a pyridyl triazine core, (L1 = 5,5′(3-(2-pyridyl)-1,2,4-triazine-5,6-diyl)-bis-2-furansulfonic acid disodium salt and L2 = (3-(2- pyridyl)-5,6-diphenyl-1,2,4-triazine-4′,4′′-disulfonic acid sodium salt) have been synthesized by utililzing two different Re metal precursors, Re(CO)₅Br and [Re(CO)₃(H₂O)₃]OTf in an organic solvent mixture and water, respectively. The rhenium complexes [Re(CO)₃(H₂O)L1]⁺ (1), Re(CO)₃L1Br (2), [Re(CO)₃(H₂O)L2]⁺ (3), and Re(CO)₃L2Br (4), were obtained in 70–85% yield and characterized by ¹H NMR, IR, UV, and luminescence spectroscopy. In both H₂O and acetonitrile, complexes display a weak absorption band in the visible region which can be assigned to a metal to ligand charge transfer excitation and fluorescent emission lying in the 650–710 nm range. Cytotoxicity assays of complexes 1, 3, and 4 were carried out for rat peritoneal cells. Both plant cells (Allium cepa bulb cells) and rat peritoneal cells were stained using the maximum non-toxic concentration levels of the compounds, 20.00 mg ml^?1 for 1 and 3 and 5.00 mg ml^?1 for 4 to observe under the epifluorescence microscope. In both cell lines, compound concentrated specifically in the nuclei region. Hence, nuclei showed red fluorescence upon excitation at 550 nm.

Conclusions

Four novel rhenium complexes have been synthesized and characterized. Remarkable enhancement of fluorescence upon binding with cells and visible range excitability demonstrates the possibility of using the new complexes in biological applications.

Open image in new window

Graphical abstract Micrograph of rat peritoneal cells incubated with novel rhenium complex under epifluorescence microscope.

     

Importance of halide involving interactions at Hoogsteen sites in supramolecular architectures of some coordination metal complexes of N<Superscript>6</Superscript>-benzyl/furfuryl adenine

Background

Most of the benzyladenine and furfuryladenine derivatives inhibit tumor/cancer cell growth; their toxicity is lesser than the compounds used for the treatment of cancer now-a-days. Many cytokinin derivatives are tested for anticancer activity.

Results

A series of transition metal complexes containing N⁶-benzyl/furfuryl aminopurines of formula [Mn(FAH)₂(H₂O)(Cl₃)]₂.Cl₂(1), [Co(FAH)2(H₂O)(Cl₃)]₂.Cl₂(2), [Co(FAH)₂(Cl₄)]_{2 .}[Co(FAH)₂(H₃O)(Cl₃)].Cl₂(3), [Ni(FAH)₂(H₂O)(Cl₃)]₂.Cl_2. (H2O) (4), [Zn(BAH)Br₃] (5) and [Cd₂(BAH)₂(μ-Br)₄Br₂]_n(6) (where BAH and FAH benzyladeninium and furfuryladeninium cations respectively) have been synthesized and characterized. Crystal structures of (1-4) have similar distorted octahedral coordination geometry, while (5) and (6) have distorted tetrahedral geometry and octahedral geometries respectively. In (1-4) two halide ions and two cytokinin cations (BAH⁺/FAH⁺) are laterally coordinated to the metal ion. A water molecule and a halide ion are axially coordinated. But the coordination sphere of (5) consists of N7 coordinated benzyladeninium ion and three halide ions. The complex (6) is a coordination polymer bridged by bromide anions. A common notable feature in (1-4) is the presence of one or more lattice chloride anions. They help in a chain formation by N-H…Cl halide involving hydrogen bonding interactions in between the Hoogsteen site hydrogen.

Conclusions

The observed crystal structures emphasize the role of the halide ions in developing the supramolecular architectures by halide involving hydrogen bonding interactions. Also most of the reported cobalt cytokinin complexes possess tetrahedral coordination geometry, but some cobalt complexes have distorted octahedral coordination geometry, which are discussed and compared.

Open image in new window

Graphical Abstract Supramolecular architectures of some coordination metal complexes of N6-benzyl/furfuryl adenine.

     

<Emphasis Type="Italic">cis</Emphasis>-Dioxomolybdenum(VI) complexes of sterically encumbered phenol-based tetradentate N<Subscript>2</Subscript>O<Subscript>2</Subscript> ligands: Structural,spectroscopic, and electrochemical studies

Two cis-dioxomolybdenum(VI) complexes [MoO₂L] (L: L ¹, 2 and L: L ², 3) in a phenol-based sterically encumbered N₂O₂ ligand environment have been synthesized, and their crystallographic characterizations are reported. The orange crystals of 2 are monoclinic, space group P2₁/a with unit cell dimensions as a=16.2407(17) Å, b=7.2857(8) Å, c=18.400(2) Å, β=98.002(9)°, Z=4, and d _cal=1.486 g cm^?3. The light orange crystals of 3, however, are orthorhombic, space group, Pbcn, with unit cell dimensions a=8.3110(12) Å, b=12.637(3) Å, c=34.673(5) Å, Z=4, and d _cal=1.187 g cm^?3. The structures were refined by a full-matrix least-squares procedure on F ² to a final R=0.046 (0.055 for 3) using 4944 (3677) all independent data. In both the cases, the Mo atom exists in a distorted octahedral geometry defined by a N₂O₄ donor set, which features a cis-Mo(–O)₂ and a trans-Mo(OPh)₂ arrangement. Compound 2 undergoes a quasireversible one-electron reduction at ?1.3 V vs Ag/AgCl reference due to Mo^VIO₂/Mo^VO₂ electron transfer and thus providing a rare example of steric solution to the comproportionation–dimerization problem encountered frequently in the development of valid biomimetic models for the active sites of oxomolybdenum enzymes.     

The influence of the negative hyperconjugation is relevant for the analysis of the π-π<Superscript>*</Superscript> conjugation with the mono-substitution and di-substitution of H<Subscript>2</Subscript>C= by O= and/or HN= in <Emphasis Type="Italic">trans</Emphasis>-buta-1,3-diene?

We have studied prop-2-en-1-imine (1), prop-2-enal (2), ethane-1,2-diimine (3), ethanedial (4), and 2-iminoacetaldehyde (5) to investigate the influence of the negative hyperconjugation in π-π^* interaction with the substitution of =CH₂ by =NH and/or =O in trans-buta-1,3-diene (6). The analyzes of the π-π^* interaction performed from evaluation of the π molecular orbital diagrams and electron localization function method demonstrated, that compared to 6, the substituted compounds 1-5 presented lower electron conjugation, especially in the structures bearing =O. The geometric parameters, natural bond orders, and topological analysis realized by quantum theory of atoms in molecules method indicated a predominant C-C and C=C character for the simple and double C-C bonds in the substituted compounds, 1-5, as compared to 6. Compound 4 had the highest enthalpy of formation, which reflected the lowest π-π^* interaction, maintained by the two =O conjugated groups. The natural bond orbital (NBO) and natural resonance theory (NRT) methods revealed that the π-π^* electron delocalization in substituted compounds, 1-5, is lower than in 6 from, firstly, of the less favorable interactions: π(X=C) ? π^*(C=C) and π(X=C) ? π^*(C=X), despite of the larger π(C=C) ? π^*(C=X) conjugation, with X = N and/or O, of 1-5 than π(C=C) ? π^*(C=C) of 6. But, most importantly, the weight of the interaction: n_π(O) ? σ^*(C-C), was determined from NBO and NRT methods as proportional to the π-π^* conjugation and thus demonstrating be decisive to establish the level of π electronic delocalization.