John D. Wallis

Synthesis, structure and chemistry of a twisted olefinic bis-didentate proligand: 5,5 '-bi-5H-cyclopenta[2,1-b : 3,4-b ']dipyridinylidene

The elusive chiral twisted alkene, 5,5-bi-5H-cyclopenta[2.1-b : 3,4-b]dipyridinylidene, known also as 9,9-bi-4,5-diazafluorenylidene (BDAF), has been prepared in racemic form from 9-bromo-4,5-diazafluorene and from 9,9-bi-4,5-diazafluorenyl and fully characterised. X-Ray measurements show that there is a twist of 37.8 degrees about the double bond between the 4,5-diazafluorenylidene units. A 1:1 charge-transfer compound with 7,7,8,8-tetracyanoquinodimethane (TCNQ) contains an unusual packing arrangement which is centred around the formation of spiral stacks. Each BDAF molecule contributes one 4,5-diazofluorenylidene unit to the backbone of the stack, while the second half is involved in hydrogen-bonding interactions and additional stacking with TCNQ. Examples of complexes containing the axially symmetric tetradentate ligand binding to one and to two metal ions. [M(bdaf)Cl-2] and [(MCl2)(2)(bdaf)] (M = Co, Ni, Zn), are reported.

Crystal and molecular structures of tetracarbonyl(3,3′-dimethyl-2,2′-bipyridine)-chromium(0) and -molybdenum(0) benzene solvates and the unsolvated tungsten(0) analogue

Abstract The crystal structures of the complexes of 3,3-dimethyl- 2,2′-bipyridine (L), cis-[W(CO)4L], cis-[MO(CO)4L] · C4H6 and cis-[Cr(CO)4L] · C6H6 have been determined by X-ray diffraction. These structures show that repulsion between the methyl groups is principally responsible for the geometric distortion in each complex. Within the ligand, L, the two pyridine rings suffer distortion, in opposite directions, to pseudo-boat conformations so as to optimize metal—nitrogen binding. The benzene solvent molecules are arranged in a layer structure alternating with units of the metal complex. The metal complex molecule is beyond van der Waals contact with the benzene solvent molecule in each case.

Crystal and molecular structure of tetracarbonyl (6,6′-dimethyl-2,2′-bipyridine)molybdenum benzene solvate

The crystal structure of the complex cis-[Mo(CO) 4 (6.6-dimethyl-2,2-bipyridine)].C 6 H 6 , has been determined by X-ray diffraction. The structure shows that repulsion between the methyl groups and the carbonyl ligands in the equatorial plane is principally responsible for the geometric distortion observed. Within the asymmetrically bound ligand the two pyridine rings suffer distortions in the same direction to pseudo-boat conformations so as to optimise molybdenum-nitrogen binding. The benzene solvent molecules are in narrow channels which lie between the axial carbonyl ligands

Short intermolecular S...S contacts in a reaction product from the cyclic sulfate ester of dimethyl L-tartrate and 2-thioxo-1,3-dithiole-4,5-dithiolate.

Cyclic sulfate ester 6 reacts readily with dithiolate 4 to give meso and dl cyclic products 8 and 9. The crystal structure of 9 shows two short S...S intermolecular interactions (< 3.4 Angstrom) between trithiocarbonate groups, due to cooperative polarisations and the highly asymmetric shape of the bonded S atom. The latter effect is mainly responsible for the short S...S contacts in ETs radical cation salts. In contrast to similar systems, the sp(3) C atoms of the six-membered ring in 9 are displaced to the same side of the dithiole plane.

COMPLEXES OF 4,5-DIAZAFLUORENE AND 9,9′-BIS(4,5-DIAZAFLUORENYL) WITH NICKEL(II), COPPER(II) AND ZINC(II)

Abstract Complexes having the compositions [LMCl2], [L2MCl2], [L2Cu][PF6], [L′MCl2] and [L′(MCl2)2] of 4,5-diazafluorene, L, and 9,9′-bis(4,5-diazafluorenyl), L′, have been prepared from MCl2 (M = Ni, Cu, Zn) and characterised. Steric strain or the bite angle of the pro-ligand has no recognisable effect on its coordination chemistry with smaller 3d-metals.

Intramolecular interactions in ethyl bis-(2,4-dinitrophenyl)acetate

The crystal structure of ethyl bis(2,4-dinitrophenyl)acetate, determined by room temperature X-ray diffraction, shows that this crowded molecule contains two short intramolecular interactions between nitro oxygen atoms and electron deficient carbon atoms. One mimics an early stage of the nucleophilic addition to a carbonyl group (O...C, 2.730 Å), and the other mimics an early stage of a Michael reaction (O...C, 2.854 Å). Crystal data: C16H12N4O10, Mr = 420.29, a = 10.079(1), b = 11.139(1), c = 16.162(1) Å, β = 91.319(7)°, monoclinic, P21/c, Z = 4.

HETEROCYCLIC CATIONS WITH DELOCALISED PI-SYSTEMS AND SHORT INTRAMOLECULAR SULFUR… SULFUR CONTACTS: THE STRUCTURES OF TWO DERIVATIVES OF THE 2-(1,3-DITHIOL-2-YLIDENEMETHYL)-1,3-DITHIOLIUM CATION

Abstract The effective size of bonded divalent sulfur in S…S contacts is a function of the orientation of the groups, with the shortest contacts occurring when the groups are coplanar. A lower limit for the loner is indicated by the structures of the 2-(1,3-benzodithiol-2-ylidenemethyl)-1,3-benzodithiolium and 2-(1,3-dithiolan-2-ylidenemethyl)-1,3-dithiolanium cations, 2 and 3. These show almost mm symmetry with all four sulfur atoms involved in stabilisation of the positive charge. Short intramolecular sulfur… sulfur contact distances, 0.5–0.7 A within the sum of traditional van der Waals radii, and maximised by in-plane angular distortions, indicate a lower limit to the effective size of the bonded divalent sulfur atom of ca. 1.45 A.

Lone-Pair Lone - Pair Interactions

Abstract The lone-pair ionisation potentials of the halogens, disulphides and hydrazines are analysed by a simple MO procedure including orbital overlap. Calculations of Slater overlap integrals for adjacent sp 2 and sp 3 orbitals show that the energy of the cis isomer is lower than the energy of the trans in the former case but the reverse is true for sp 3 hybridised orbitals. Gaussian overlap integrals are greater for the trans form than for the cis, both for sp 3 and sp 2 nitrogen orbitals. Experimental evidence in support of these conclusions for sp 2 systems is to be found in the greater stability of the Z isomers of sulphonylamines and sulphur diimides.

Interaction between a di­methyl­amino group and an electron-deficient alkene in ethyl (E)-2-cyano-3-(8-di­methyl­amino­-1-naphthyl)­propenoate

The interaction between the peri substituents in the title compound, C18H18N2O2, measured at 150u2005K, represents an early stage in the addition reaction of an amino group to an electron-deficient alkene, and has an N⋯Csp2 separation of 2.531u2005(2)u2005A; comparison with related structures indicates that the nitrile group activates an alkene to nucleophilic attack more than a coplanar carboxylxadic ester group.

The relative stability of cis and trans isomers

Abstract By adopting an sp2 hybridised model, it is found that the overlap integral for two adjacent lone pairs is numerically greater for the trans isomer. On the basis of 1-electron Hiickel theory, this suggests that cis isomers are more stable than trans, contrary to the predictions of conformational analysis. Commonly, non-bonded interactions lead to the stabilisation of trans isomers, but when these repulsions are not dominant, the cis isomers are always the more stable.