Computational modeling of spin crossover phenomenon in adducts of iron <Emphasis Type="Italic">bis</Emphasis>-chelates with <Emphasis Type="Italic">o</Emphasis>-diiminobenzoquinones

A density functional B3LYP*/6-311++G(d,p) quantum chemical study of the interaction of Fe^II complexes with o-diiminobenzoquinones showed that adduct formation is accompanied by oxidation of the metal ion and conversion of the redox-active ligand to the semiquinonate form. Variation of substituents at nitrogen atoms of the bis-chelate and diimine made it possible to reveal the spin-crossover complexes. The nature and strength of the exchange interactions between the unpaired electrons of paramagnetic centers of the adducts studied depend on the spin state of their isomers and on the type of the iron complex.     

Synthesis and structure of polycrystalline adducts of Co(II) azomethine complexes with redox-active 2,4,6,8-tetrakis-(<Emphasis Type="Italic">tert</Emphasis>-butyl)phenoxazin-1-one

The six-coordinate cobalt complexes, C₅₇H_63.50N_4.50O₄Co (IIa), C₆₀H₆₉N₅O₄Co (IIb), C5₈H₆₇N₃O₈Co (IIc), C₅₆H₆₁N₅O₁₀Co (IId), C₅₆H₆₃N₃O₆Co (IIe), C₅₈H₆₆N₄O₆Co (IIf), and C₅₈H₆₃N₇O₈Co (IIg), adducts of high-spin tetrahedral Co(II) bis(salicylaldiminates) (C₂₉H_24.50N_3.50O₂Co (Ia), C₃₂H₃₀N₄O₂Co (Ib), C₃₀H₂₈N₂O₆Co (Ic), C₂₈H₂₂N₄O₈Co (Id), C₂₈H₂₄N₂O₄Co (Ie), C₃₀H₂₇N₃O₄Co (If), and C₃₀H₂₄N₆O₆Co (Ig)) and redox-active 2,4,6,8-tetrakis(tert-butyl)phenoxazin-1-one (L), were synthesized and studied for structure and magnetic properties. Complexes IIa–IIg have octahedral structure (CIF files CCDC nos. 1403920 (IIf), 1403922 (IIg)) and exist in the ground low-spin state (ls-Co^III-SQ), which arises upon intramolecular single-electron redox process in the ligand–metal system. The presence of substituents of different nature in the azomethine ligands of IIa–IIg does not induce any significant changes in their magnetic properties.     

[2+2] Cycloaddition of the Schrock titanium silylidene and acetylene

Russian Chemical Bulletin - [2+2] Cycloadduct of the titanium silylidene Cp2Ti(thf)=Si[Si3(SiMeBut 2)4] and acetylene was readily prepared, isolated, and characterized by NMR spectroscopy and...     

Quantum-chemical simulation of structure and conformational flexibility of 5,7-di(<Emphasis Type="Italic">tert</Emphasis>-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone

Density functional theory [DFT B3LYP/6-311++G(d,p)] simulation has revealed stable tautomers and conformers of polydentate ligand system based on 5,7-di(tert-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone with different structures of the coordination nodes, capable of formation of metal chelates. It has been shown that the tautomeric NH- and OH- forms with exo and endo location of the hydroxy group in the quinoline fragments (close in energy, ΔE_ZPE = 0.2–2.4 kcal/mol) are stabilized by intramolecular hydrogen bonds. Energy barriers of the interconversion of these forms via rotation about the C–OH bond of the phenolic fragment are of ΔE_ZPE^≠ = 2.1–4.2 kcal/mol, whereas the barrier of rotation about the bond between the quinoline and tropolone fragments is higher (ΔE_ZPE^≠ = 18.2 and 19.6 kcal/mol).     

<Emphasis Type="Italic">N,N′</Emphasis>-Bis(9-anthrylmethyl)diamines as fluorescent chemosensors for transition metal cations

N,N′-Bis(9-anthryl)-substituted diamines, imidazolidines, and hexahydropyrimidines were synthesized, and their luminescence properties and complexing ability were studied. N,N′-Bis(9-anthrylmethyl)ethane-1,2-diamine and N,N′-bis(9-anthrylmethyl)butane-1,4-diamine were found to be effective and selective PET chemosensors for Zn²⁺ and H⁺, respectively. 2-[1,3-Bis(9-anthrylmethyl)hexahydropyrimidin-2-yl]-6-methoxyphenol can be used as fluorescent chemosensor for mercury(II) cations.     

Reactions of trimethinecyanines of the 1,3-dioxenium series with nucleophiles. The structures of the parent carbocation and of a product of its reaction with methanol

The chemistry and the regioselectivity of alcoholysis, aminolysis, and hydrolysis of symmetrical trimethinecyanine carbenium ions in 4-[3-(2,2-diorganyl-6-phenyl-4H-1,3-dioxen-4-ylidene) prop-1-enyl]-2,2-diorganyl-6-phenyl-1,3-dioxenium perchlorates (4a,b) were investigated. The structures of trimethinecyanine 4,5:4′,5′-bisannelated at the methine chain (3) and of a product of the reaction of nonannelated trimethinecyanine with the methoxide anion were established by X-ray diffraction analysis,¹H NMR spectroscopy, and quantum-chemical calculations (PM3). The nucleophile attacks the mesomeric π-conjugated system of the trimethinecyanine cation selectivily at the C(6) atom of one of the dioxenium fragrments, the second dioxenium ring being deactivated. Alcoholysis affords products of addition of the methoxy group to trimethinecyamines. In the course of aminolysis and hydrolysis, the dioxenium ring that is subjected to the attack eliminates acetone to form 1,3-dioxenylidenepropenylic derivatives of 1,3-enaminoketones and 1,3-ketoenols, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 521–529, March. 1999.     

Rearrangements of cyclopentadienyl cyanates,isocyanates and their thio-,seleno-, and telluro-analogs

Dynamic NMR spectroscopy revealed that pentaphenylcyclopentadienyl isoselenocyanate undergoes reversible hetero-Cope rearrangement (ΔG ^≠ _{408 K} ∼ 22 kcal mol⁻¹, C₆D₅CD₃) giving isomeric selenocyanate in which 1,5-sigmatropic shifts of the SeCN group along the perimeter of the cyclopentadiene ring occur (ΔG ^≠ _{298 K} = 16.7 kcal mol⁻¹, C₆D₅CD₃). On the contrary, pentaphenylcyclopentadienyl iso(thio)cyanates Ph₅C₅NCO and Ph₅C₅NCS are structurally rigid compounds on the NMR time scale. The energy barrier to the 3,3-shift of the isoselenocyanate group in pentaphenylcyclopentadienyl derivative Ph₅C₅NCSe (ΔG _{298 K} ^≠ = 17.9 kcal mol⁻¹) caclulated using the B3LYP/6-31G** method is 7.6 kcal mol⁻¹ lower than for the unsubstituted analog H₅C₅NCSe.     

Synthesis and structure of 1-[(3-hydroxybenzo[<Emphasis Type="Italic">b</Emphasis>]thiophen-2-yl)methylidene]-3-oxo-5-phenyl-1-pyrazolidinium-2-ide

Potentially tautomeric azomethine imine, 1-[(3-hydroxybenzo[b]thiophen-2-yl)methylidene]-3-oxo-5-phenyl-1-pyrazolidinium-2-ide has been prepared. According to X-ray diffraction, ¹H, ¹³C, and ¹⁵N NMR and electronic spectroscopy, the compound exists as 3-hydroxybenzothiophene structure containing intermolecular hydrogen bond between the hydroxy group and the carbonyl oxygen of the pyrazolidone ring in crystal state. Quantum chemical calculations predict the possibility of OH and NH tautomeric forms.     

Mechanisms of the hydrogen atom and the phenyl group migration in the molecule of heptaphenylcycloheptatriene

In terms of the density functional theory using the B3LYP functional, 1,2,3,4,5,6,7-heptaphenylcycloheptatriene was shown to be the most stable in the boat conformation of the cycloheptatriene ring with the H atom in the equatorial position. 1,5-Sigmatropic shifts of the H atom along the seven-membered ring perimeter take place when it is in the axial position through the asymmetric transition state with the barrier ΔE ^≠ _ZPE = 28.7 kcal mol^?1. The H atom can attain the axial position upon inversion of the seven-membered ring, which is accompanied by the orthogonal turn of the phenyl group at the sp³-hybridized C atom (ΔE ^≠ _ZPE = 22.6 kcal mol^?1). The energy barrier to the circular rearrangement of the H atom (ΔE ^≠ _ZPE = 32.2 kcal mol^?1) explains formation of isomers during the high-temperature synthesis of di(p-tolyl)pentaphenylcycloheptatriene. The barrier to the 1,5-sigmatropic shifts of the phenyl group is 19.7 kcal mol^?1 higher than that for the competing shifts of the H atom.