John R. Chipperfield

The thermorchemistry of the di-η5-cyclopentadienyl derivatives of the first transition series and their unipositive ions

Abstract The heats of formation [△ H 0 f (MCp 2 , c, 298)] have been determined for the di-η 5 -cyclopentadienyl derivatives of magnesium, vanadium, chromium, manganese, iron, cobalt and nickel, by static-bomb calorimetry. They are (in kJ mol -1 ) (M =) Mg, 77.0 ± 3; V, 123 ± 4; Cr, 186 ± 3; Mn, 198 ± 2; Fe, 158 ± 4; Co, 205 ± 4; and Ni, 262 ± 3. the heats of formation [△ H 0 f (M(CO) x Cp, c, 298)] of tricarbonylcyclopentadienylmanganese and dicarbonylcyclopentadienylcobalt have also been determined as —478 ± 1 and —169 ± 10 kJ mol −1 , respectively. By using the measured heats of formation of the metallocenes of the first transition series and other enthalpy values from the literature in Hesss law cycles, the feasibility of the process MCp 2 → MCp + 2 + e (obtained from the photoelectron ionisation energies) has been analysed and shown to be the result of the interplay of exchange energy and ligand-field stabilisation energy terms in these low-spin complexes. The feasibility order, V Mn > Fe Ni is associated with the special stability associated with half-filled and filled shelss, which, because of the large ligand-field splitting, occur at d 3 , d 6 , d 8 and d 10 . It is shown that bond energy additivity does not occur for the above cyclopentadienylmetal carbonyls.

The first uranium based liquid crystals. Uranyl metallomesogens from β -diketone and tropolone ligands

Abstract A tropolone ligand (5-hexadecyloxytropolone) complexed with uranyl, produces a very narrow liquid crystal range at high temperatures. A β-diketone ligand (1-[4-decyloxyphenyl]-3-tridecylpropane-1,3-dione) designed for a more extensive liquid crystal range, produces a uranyl liquid crystal at low temperatures, which remains liquid crystalline while supercooling down to room temperature. This opens the way to a host of interesting new superheavy metallomesogens, uranium metallomesogens, low melting metallomesogens, and possible radiopharmaceuticals.

Reactivity of main-group—transition-metal bonds : VI. the kinetics of iodination of tetracarbonyl(trimethylstannyl)cobalt and pentacarbonyl(trimethylstannyl)rhenium

Abstract The kinetics of the iodine cleavage of the SnCo bond in [Me 3 SnCo(CO) 4 ] and of the SnRe bond in [Me 3 SnRe(CO) 5 ] have been measured. The order of rates of cleavage of the SnM bond in the compounds [Me 3 SnM(CO) x (cp) y ] (M = Mn, Re, x = 5, y = 0;M = Co, x = 4, y = 0; M = Cr, Mo, W, x = 3, y = 1; M = Fe, x = 2, y = 1; cp = η-cyclopentadienyl) indicates that the main factors determining reactivity towards iodine are the size of the metal atom (M) and the shielding of it by the other ligands.

Liquid crystal phase crossover (columnar↔calamitic) in polycatenar CuII 5-(3,4-dialkoxybenzylidine)aminotropololonates

Abstract The new tetracatenars bis[5-(3,4-dialkoxybenzylidine)aminotropolonato] copper(II) are mesogenic. As the 3,4-dialkoxy chains on the phenyl rings increase in length, the phase behavior shifts from calamitic to discotic ( columnar ), the first case of a metallomesogen series to exhibit both calamitic and columnar phases. When n ⩽12 the complexes melt to calamitic liquid crystals (specifically the smectic C phase), while for n ⩾13 the liquid crystal phases are columnar (specifically the phase columnar hexagonal phase)

Speciation in cobalt(III)-catalysed autoxidation: Studies using thin-layer chromatography

Abstract Thin-layer chromatography has been used to study the speciation of cobalt in the cobalt-catalysed autoxidation of benzaldehyde in acetic acid. The cobalt may be present as cobalt(II), a mixed-valency oxo-centred trimer, a cobalt(III) oxo-centred trimer, or as a hydroxo-bridged cobalt(III) dimer. During reaction the relative proportions of these species change, and the dimer is only present when reaction rate is at a maximum. At this time cobalt(II) and the mixed-valency trimer are absent.

Reactivity of Main-Grouptransition-metal bonds: IX. The kinetics of iodination of compounds containing two or more tintransition-metal bonds☆

Abstract Rate coefficients are reported for the cleavage by halogens of tintransition-metal bonds in compounds containing two or more such bonds. Bond reactivity with iodine is in the order SnCo ≈ Sn-Fe > Sn-Mo ≈ Sn-W > Sn-Mn. Rates of halogenation of compounds containing three tintransition-metal bonds show that a subtle balance between steric and electronic effects determines bond reactivity.

Mesogenic properties of copper(II) complexes formed from α-substituted β-dialdehydes and β-diketones

The relationship between molecular structure and mesogenic properties is explored for copper(II) complexes of α-substituted β-dialdehydes and β-diketones. The copper(II) complex of p-pentylphenylmalonaldehyde shows a nematic phase, but the presence of off-axial substituents or the removal of the aromatic ring precludes mesophase formation.

A simple automatic titrator based on a digital balance

Abstract A computer-controlled automatic titrator incorporating a weight burette is described. The titration vessel is mounted on the pan of a zero-displacement digital balance which records the weight of added sample as well as the weight of titrant added during the titration.

Reactivity of main-group–transition-metal bonds. Part 8. The kinetics of mercuration of compounds containing Group 4B elements bonded to manganese, iron, and molybdenum: effects of structure on reactivity

The kinetics of cleavage by HgBr2 of the main-group–transition-metal bonds in the following compounds are reported: [Mn(CO)5(MR3)](M = Sn, R = Et, Bun, or C6H11; M = Si or Ge, R = Me); [Fe(cp)(CO)2(MR3)](M = Sn, R = Bun or C6H11; M = Si or Ge, R = Me; cp =η-cyclopentadienyl); and [Mo(cp)(CO)3(SnBun3)Bun3]. The structure–reactivity patterns indicate that mercuration involves an SE2(open) transition state. Mercury(II)bromide does not cleave the tin–transition-metal bonds in [Mn(CO)5(SnPh3)] or [Fe(cp)(CO)2(SnPh3)] but cleaves the phenyl–tin bonds in these compounds.

Kinetics of organometallic reactions: a quick method to obtain rate coefficients for complex reactions from ‘pseudo-first-order’ rate coefficients and related information

Abstract Kinetic data from reactions which are more complicated than simple first order processes can give linear plots of ln |M∞ − Mt| against time (where M is a physical property of the reacting solution, such as absorbance or conductance). It is shown how these linear plots arise, and how the correct rate coefficients for complex reactions can be calculated from the pseudo-first-order rate coefficients, together with reagent concentrations and related information. This approach is useful for organometallic reactions, where there are often severe limitations on the experimental data that can be readily be obtained.