Janusz Lusztyk

On the molecular structure of dicyclopentadienylmagnesium

Abstract The molecular structure of (C 5 H 5 ) 2 Mg has been determined by gas phase electron diffraction. The best agreement between calculated and experimental intensity curves is obtained for a model with eclipsed (C 5 H 5 ) rings (symmetry D 5h ), but a model with staggered rings (symmetry D 5d ) cannot be ruled out. The MgC and CC bond distances are 2.339(4) and 1.423(2) A respectively. The nature of the metal-to-ring bonding is discussed and it is concluded that (C 5 H 5 ) 2 Mg is best regarded as a covalent rather than an ionic compound.

Photolysis of the carbon–hydrogen bond in pentamethylcyclopentadiene. Properties of the pentamethylcyclopentadienyl radical

Irradiation of pentamethylcyclopentadiene (A) in liquid solution with u.v. light results in homolysis of the ring C–H bond to give the pentamethylcyclopentadienyl radical (B)[a(15 H) 6.4, a(13C) 3.5 G], and atomic hydrogen which [graphic omitted] abstracts hydrogen from a second molecule of (A) to give molecular hydrogen and a second radical (B). The radicals (B) self-react at a diffusion-controlled rate (2kt 2 × 109 I mol–1 s–1 in hexane at 25°C) by two different routes. The first, which is thermally and photolytically reversible, is the combination to give the dehydrodimer (C), and the second, which is irreversible, is the disproportionation to give the parent cyclopentadiene (A) and the tetramethylfulvene (D).

Evidence for reversible ring-opening of the α-cyclopropylbenzyl radical

Kinetic absorption spectroscopy, EPR, and tributyistannane product data indicate that the α-cyclopropylbenzyl radical (1a) undergoes reversible ring-opening to the 4-phenylbut-3-enyl radical (2a) and that equilibrium favours the ring-closed form, (1a).

An electron spin resonance study of the methylcyclopentadienyl radicals, MenH5 –nC5·(n= 1–5)

The radicals Men C5H5 –n· have been generated by photolysis of the corresponding hydrocarbons (n= 3–5) or mercury derivatives (n= 0–5) and their e.s.r. spectra have been recorded. The spectra are interpreted in terms of thermal population of the ψA and ψS molecular orbitals, for which the predicted hyperfine coupling constants are calculated by the McLachlan equation, taking λ 0.75 and Q(C5H5·)–30 and Q(Me5C5·) 31.85 G. The energy separations between the ψA and ψS MOs can be rationalised on the assumption that a methyl substituent on a carbon atom of the cyclopentadienyl carrying a unit π-electron has a destablising interaction, within a factor of two, of 10 kJ mol–1.

Molecular structures of dicyclopentadienylmagnesium and dicyclopentadienylchromium by gas-phase electron diffraction

The molecular structures of (C5H5)2Mg and (C5H5)2Cr have been determined by gas-phase electron diffraction; the C–H bonds in (C5H5)2Cr are bent 2·9(1·1)° out of the plane of the C5 ring towards the metal atom.

The electron spin resonance spectra of the penta-alkylcyclopentadienyl radicals MeR4C5˙(R = Et, Pr, and Bu) and 1,3,5-Me3-2,4-Et2C5˙. Relative substituent effects of the alkyl groups

The penta-alkylcyclopentadienyl radicals Me4EtC5˙, Me4PrC5˙, Me4BuC5˙, and 1,3,5-Me3-2,4-Et2C5· have been prepared by photolysis of the corresponding penta-alkylcyclopentadienes. The e.s.r. spectra which are observed are strongly temperature-dependent. It is assumed that the unsymmetrical penta-alkylation breaks the degeneracy of the ψA and ψS molecular orbitals of the cyclopentadienyl radical, and that there is a Boltzmann distribution of the unpaired electron between these two levels. The e.s.r. spectra which are observed then represent a population-weighted average of the spectra associated with the radical in the ψA1ψ2 and ψS2ψA1 configurations. The McLachlan equation is used to give the spin-density distributions in these two configurations, and the induced separation between the ψA and ψS energy levels can then be derived. It is concluded that in the π-radical R–Ċ, the relative electron-releasing interaction between the alkyl group R and a;unit p-electron on the attached carbon atom falls in the sequence (kJ mol–1) Me (0) > Et (1.79)∼ Pr (1.84)∼ Bu (1.99).

Electronic effects in cyclobutadiene radical cations

The e.s.r. spectra of the cis-and trans-radical ions cyclo-Me2But2C4˙+, obtained from methyl-t-butylethyne, can be interpreted in terms of the breaking of the degeneracy of the molecular orbitals in the trans-isomer by more powerful electron release from the t-butyl groups than from the methyl groups.

The pentamethylcyclopentadienyl radical: generation, electron spin resonance spectrum, and reaction kinetics

The pentamethylcyclopentadienyl radical [a(15H) 6·4 G, a(13C) 3·5 G, g 2·0025], is formed by the photolysis of pentamethylcyclpentadiene, and under-goes self-reaction at a diffusion-controlled rate (2kt 2 × 109 l mol–1 s–1 at 25 °C) to give the dimer which redissociates on heating.

HEPTAMETHYLTRISILANE-2-THIOL-MEDIATED FREE-RADICAL-CHAIN REDUCTION OF ORGANIC HALIDES

Free-radical-chain reduction of organic halides is accomplished with heptamethyltrisilanethiol, a reducing agent combining the good hydrogen donating ability of a thiol and the excellent halogen abstracting properties of a silyl radical.

The electron spin resonances spectra of the [4]annulene (cyclobutadiene) radical cations, R4C4+˙

A series of simple (R = R′) and mixed (R ≠ R′)cyclobutadiene radical cations, R2R2′C4+˙, have been prepared by photolysis of the alkynes, RCCR′ or of mixtures of the alkynes RCCR and R′CCR′, in dichloromethane in the presence of aluminium chloride, and the e.s.r. spectra have been recorded.The magnitude of the 13C hyperfine coupling in Et4C4+˙ confirms that it is a π-radical, with no evidence for out-of-plane or in-plane (Jahn–Teller) distortion.The value of a(Hβ) for the radicals (RCH2)4 C4+˙ and (RCH2)2R′2C4+˙ indicate that, as the bulk of the alkyl substituents increases, the group R is pushed out of the plane of the ring. Some unusual temperature effects on a(Hβ) are ascribed to interaction of the radical cation with the solvent or with the counterion. As the bulk of the alkyl groups increases, the g-value decreases from 2.0030 to about 2.0022. cis- and trans-Isomers of the radicals Me2But2C4+˙, Et2But2C4+˙, Bui2But2C4+˙, and probably Me2Et2C4+˙ have been identified, and the spectra of cis- and trans-Me2But2C4+˙ are analysed in terms of breaking of the degeneracy of the molecular orbital of the cycrobutadiene system by differential electron release by the alkyl groups (But > Me).