Synthesis,structure and fluxional behaviour of isomeric bis(trimethylstannyl)dihydropentalenes

Interaction of dihydropentalene (IV) with trimethylstannyldiethylamide in molar ratios of 1:1 or 1:2 leads to mono- or bis-organotin derivatives of IV, respectively. X-ray analysis of trans(E)-bis(trimethylstannyl)dihydropentalene (Va) has been carried out, R = 3.4%. Molecules of Va are centrosyommetric. The parmeters of monoclinic cell: a 8.680(1), b 7.322(1), c 13.073(2) Å, β 97.74(1)°, space group P2₁/c, Z = 2. Geometrical parameters of Va have been determined and their values are discussed in comparison with the same parameters for η¹-cyclopentadienyl compounds of elements. Chemical shifts ¹³C, ¹H and ¹¹⁹Sn and coupling constants ¹³C-¹¹⁹Sn and ¹¹⁷Sn-¹¹⁹Sn of bis- and tris-organotin derivatives of IV have been determined. Rapid intramolecular suprafacial metallotropic rearrangement, proceeding as a [1,5]-sigmatropic shift of SnMe₃ group, has been found in Va and cis-(Z)-bis(trimethylstannyl)dihydropentalene (Vb). Activation parameters have been evaluated by the analysis of temperature dependence of ¹³C NMR spectra within the framework of degenerate two-site exchange in the isomers Va and Vb, E_A: 42.2±0.9 and 31.2±0.6 kJ · mole⁻¹; ΔH^≠₂₉₈: 39.8±0.9 and 27.7±0.6 kJ · mole⁻¹; ΔS^≠₂₉₈: −6.2±4.0 and −87.0±3.1 J · mole⁻¹ · K⁻¹ and ΔG^≠₂₉₈: 41.6±1.5 and 54.6±1.1. kJ δ mole⁻¹, respectively.     

Propellanes—XXIV : Photochemistry of [4.4.3]propella-2,4-dienes

Several [4.4.3]propella-2,4-dienes upon irradiation afforded “open” trienes. These revert photochemically and thermally and afford the starting materials. In some cases cyclobutenes are obtained.     

Stereospecific synthesis of aryltetraline lignan analogues using 1,6-bis(dipropylboryl)hexa-2,4-diene

A novel convenient two-stage synthesis of aryltetraline lignin analogues from aromatic aldehydes and diallyl diboron derivatives on the basis of intramolecular Friedel–Crafts reaction has been developed.     

The synthesis, X-ray structure and fluxional behaviour of an ytterbium(II) phenalenide complex

The first lanthanoid complexes containing delocalized phenalenide anions are reported and the rapid migration of the eta(3)-bonded metal centre between the three rings of the pi-system is the first report of a degenerate haptotropic rearrangement in organolanthanoid chemistry.     

Molecular characterization and glassy-state properties of poly(spiro[2,4]hepta-4,6-diene)

Poly(spiro[2,4]hepta-4,6-diene) (PSHD) is a random copolymer composed of 1,2 and 1,4 units. Although PSHD has been reported to have a very large Sakurada–Mark–Houwink exponent (a = 1.71) characteristic of rigid rods, a new determination of the intrinsic viscosity–molecular-weight relation for fractionated samples with known copolymer composition gives a smaller value (0.81). The rigidity of the PSHD molecule was also investigated by calculations of the conformational energy associated with the main chain and the dynamic viscoelasticity associated with the molecular motion in a solid state. A sharp potential energy minimum occurs at a dihedral angle χ₁ = 260° in polymer composed of all trans 1,4 units. The dynamic loss tangent for the primary absorption decreases with increasing fraction of the 1,4 unit. These properties result from the characteristics of the stiff 1,4 unit sequences in the molecular chain. The density of PSHD decreases with increasing content of 1,4 units and was found to be considerably smaller than values for typical polymers, in relation to the occupied volume of the repeat unit. This fact can be explained by the peculiar aggregation state of molecular chains composed of rigid and flexible units.     

Structures and fluxional behaviour of transition metal cluster carbonyls

Conclusion We have examined the structures of transition metal cluster carbonyls M_m(CO)n from the point of view of the geometrical arrangement of the carbonyl ligands, and, in particular, the polyhedra defined by their oxygen atoms. Provided that the metal atom cluster is reasonably spherical (which is presumed to be predetermined by electronic factors), these polyhedra correspond well to the ideal forms calculated by optimisation of mutual repulsions between points on a sphere. Thus the concepts of coordination polyhedra about single central atoms may be successfully applied to the coordination of carbonyl ligands about entire clusters of metal atoms. This is in contrast to the more usual emphasis on the environment of individual metal atoms in the cluster. In the fine balance of factors governing the structures of these molecules and anions, we believe that steric interaction between carbonyl groups is of greater importance than previously thought. The steric crowding of ligands around the central metal atom cluster has important implications for the reactivity and catalytic activity of cluster carbonyls. Additionally, a new approach to the fluxionality of these species, based on the idea that carbonyl mobility may occur by rearrangements of the entire ligand polyhedron, has led to new insights into the behaviour of a number of systems.     

Highly fluxional tetrahapto-cyclooctatetraene complexes of ruthenium(0)

In complexes of the type Ru(arene)(cot) (cot = cyclooctatetraene) the cyclooctatetraene ring is 1—4-η-bonded as shown by X-ray diffraction study of the hexamethylbenzene derivative, and the barrier to intramolecular exchange of the bound and unbound halves of the eight-membered ring is unusually low (< 6 kcal/mol).     

Theoretical analysis of the fluxional behaviour of cyclooctatetraene Ru and Ni complexes

Cyclooctatetraene's (COT) behaviour in some of its Ni and Ru complexes is studied by means of DFT methods (B3LYP and BLYP). The experimentally observed COT fragment conformational changes (tub-shaped or planar) are analysed, together with the different locations of metal–carbon bonds in the complexes. These phenomena are studied using both static and molecular dynamics calculations. The results allow us to predict changes in the COT fragment, which goes from aromatic to totally antiaromatic, depending on its environment. This situation favours the electronic flow through the metal atoms into the organometallic molecules, giving place to different geometries that generate the dynamic fluxional behaviour.     

On bromination of 2,4-pentanedionato-copper(II)

Treatment of bis-2,4-pentanedionato-copper(II), with liquid bromine, at room temperature, gave a low melting dark green reaction product containing weakly coordinated tribromide ion. Characterization of this product using elemental analysis, conductivity, electronic, IR and ESR-spectra was carried out and suggested that the composition of the product was [Cu(HacacBr)Br(Br₃)].     

Synthesis of Cationic 2,4-Dimethylpenta-2,4-dienylruthenium Complexes. Crystal Structure of Carbonyl(η4-cyclohexa-1,3-diene)-(η5-2,4-dimethylpenta-2,4-dienyl)ruthenium Tetrafluoroborate

The complex [Ru(η⁵-C₇H₁₁)₂H]BF₄ (C₇H₁₁ = 2,4-dimethylpenta-2,4-dienyl) is highly reactive towards two- and six-electron ligands. e.g. giving with CO complex [RuCO(η⁴-C₇H₁₂)(η⁵-C₇H₁₁)]BF₄. The 2,4-dimethylpenta-1,3-diene ligand (C₇H₁₂) of the latter complex is readily displaced giving, e.g. with excess cyclohexa-1,3-diene (C₆H₈) complex [RuCO(η⁴-C₆H₈)(η⁵-C₇H₁₁)]BF₄. These reactions provide a convenient entry into monopentadienylruthenium chemistry.     

A theoretical and experimental study of the fluxional behaviour of molybdenum dihydrobis- and hydrotris-pyrazolylborates

The fluxional barrier of (dicarbonyl)[dihydrobis(3,5-dimethylpyrazol-1-yl)borato][eta-(1,2,3)-2-methylpropen-1-yl]-molybdenum (1) has been measured and a complete assignment of its (1)H, (13)C and (15)N NMR signals has been carried out. Theoretical calculations at the B3LYP/LANL2DZ level including GIAO absolute shieldings (sigma) have allowed to analyze the molecular contributions to the barrier as well as to assign some signals involved in the fluxional process.     

Propellanes,LXXXVIII : Hydroxylation of 11,13-dioxo-12-methyl-12-aza [4,3,3]propella-3,8-diene with osmium tetroxide

The all-anti-tetrol 2 is the major product of OsO₄ -hydroxylation of the title compound 1.