Martyn A. Brown

Synthetic routes to lead(II) derivatives of aromatic 1,2-diols and orthoquinones

Abstract Complexes of lead(II) of the type Pb(O 2 R) where R is a substituted aromatic group, can be prepared by the direct electrochemical oxidation of a lead anode in a non-aqueous solution of the appropriate substituted catechol, R(OH) 2 . The products, which are colourless, air-stable, insoluble materials, and presumably cross-linked in the solid state, can be oxidised to the corresponding lead(IV) derivatives by iodine or tetrahalogeno- o -quinones. The direct reaction of elemental lead with an o -quinone in refluxing hexane or toluene yielded identifiable products in only two cases; with 2,5- tert -butyl-1,2-benzoquinone, the corresponding lead(II) catecholate was obtained, while with phenanthrenequinone the semiquinonate Pb(PSQ ) 2 was isolated. These latter reactions are shown by electron spin resonance spectroscopy to proceed via free radical intermediates, and the spectra are analysed to give information on the species present in the reaction media.

STUDIES OF ORGANOTIN(IV)-ORTHOQUINONE SYSTEMS

Abstract The primary process in the reaction of hexaphenylditin with various substituted orthoquinones (Q) is shown to involve attack by the quinone at a phenyl ligand. The intermediate thus formed decomposes to yield Ph3Sn(SQ·), where S(Q·−) is the corresponding semiquinonate. Rearrangement of these species in solution gives rise to biradicals, while intramolecular electron transfer may lead to the formation and precipitation of Ph2Sn(CAT), where CAT2− is the corresponding substituted catecholate. The identification of these processes depends in part on electron paramagnetic resonance spectroscopy. The reaction of Ph3SnCl or Ph2SnCl2 with Na(TBSQ·) (TBSQ·−=3,5-di-tert-butyl-orthobenzosemiquinonate) results in the formation of Ph2Sn(TBSQ·), which can undergo redistribution and intramolecular electron transfer, so that the solution chemistry of these latter systems is similar to that of the products of the Sn2Ph6+Q reaction.

Molecular structure of an indium(III) iodide-tris(4-methylpyridine) adduct

Abstract The six-coordinate adduct InI 3 (pic) 3 (pic = 4-methylpyridine) has been prepared, and studied by X-ray crystallography. The crystals are monoclinic, space group P 2 1 / n , with a =11.430(3), b =13.951(3), c =15.332(2) A, β =96.913(15)°, Z =4, R =0.062 for 4471 unique reflections. The InI bond distances are compared with those for a number of related compounds, as with other indium(III) systems, there is a pronounced relationship between r (InI) and the coordination number at indium.

Comparative studies of electron transfer in orthoquinone derivatives of gallium, indium and thallium

Abstract The interactions of gallium, indium and thallium with 3,5-di-tert-butyl-1,2-benzoquinone (TBQ) and its reduced derivatives TBSQ − (=3,5-di-tert-butyl-1,2-benzosemiquinonate anion) and TBCAT2− (=3,5-di-tert-butyl-catecholate) are discussed in terms of inter- and intramolecular electron transfer processes. The gallium(III) complex Ga( TBS Q )(TBCAT)(Bupy)2 (Bupy=3-n-butylpyridine), which is a rare example of a metal ion coordinated by both semiquinonate and catecholate ligands, has been characterized crystallographically; monoclinic, space group P21, a=10.3718(1), b=16.0047(1), c=14.8006(1) A, β=110.151(1)°, Z=2, Rw=0.069 for 5784 unique reflections. Electron spin resonance (ESR) spectroscopy shows that this substance undergoes intermolecular electron transfer in toluene. Gallium also forms Ga( TBS Q )I2, which is thermally unstable, releasing I2; the solid and solution properties of this, and related compounds, have been studied by ESR spectroscopy. In all the systems investigated, gallium is in the Ga(III) state. Attempts to prepare derivatives of In or Tl in this oxidation state were unsuccessful, and all products have the metal as M(I). For gallium, electron transfer is only detected as an inter-ligand process, while for indium and thallium, ligand↔metal transfer predominates.

Preparation, crystal structure determination and properties of adducts of indium methylene compounds with Group 15 donors

Abstract The reaction of InBr with CH2Br2 in 1,4-dioxane or acetonitrile yields the corresponding solvate of Br2InCH2Br, which on reaction with E(C6H5)3 (E=P, As, Sb) gives the indium(III)-Group 15 dimetallo-methane derivatives, Br3InCH2E(C6H5)3. The crystal structures of the two related compounds (E=As, Sb) have been determined by X-ray crystallography. For Br3InCH2As(C6H5)3, cell constants a=15.553(7), b=21.646(8), c=12.920 (10) A; space group Pbca, Z=8, R=0.064, Rw=0.054, and for Br3InCH2Sb(C6H5)3, a=15.439(6) A, b=22.016(4) c=13.138(5) A; space group Pbca, Z=8, R=0.062, Rw=0.049. Results are also reported for I3InCH2As(C6H5)3 (cell constants a=12.0122(2), b=15.8526(3), c=13.4180(3) A, β=109.933(1)°; space group P21/n, Z=4, R=0.0481, Rw=0.0431) and Br3InCH2N(C2H5)3 (a=7.362(2), b=14.700(2), c=13.049(1) A, β=98.90(1)°; space group P21/n, Z=4, R=0.0418, Rw=0.0368). The structural results are compared with those for other organoindium ylids with the same general structure Br3InCH2L (L=P(C6H5)3; 1,1,3,3,-tetramethyl-2-thiourea; N,N,N′,N′-tetramethylethanediamine). Semi-empirical quantum mechanical calculations, using the PM3 method, were carried out on the series Br3InCH2E(C6H5)3 (E=P, As, Sb), and the calculated structural parameters are compared with the values determined by X-ray crystallography. The presence of an ylid ligand H2Cδ−δ+E(C6H5)3 in the organoindium compounds is confirmed. Mass spectra and thermogravimetric analysis strongly suggest that the thermal decomposition of the compounds occurs via fission of the indiumcarbon bond, leading to the corresponding ylid, which can be trapped by reacting the X3InCH2E(C6H5)3 compounds with HBr to produce the onium derivative [CH3E(C6H5)3]+ [InX4]− (E=P, As; X=Br, I).

Indium(III) halide-3,5-di-tert-butyl-o-benzosemiquinone systems

Abstract The reaction of InX3 (X  Cl, Br, I) with 2 mol of Na+TBSQ·− (TBSQ·− =3,5-di-t-butyl-o-benzosemiquinonate anion) yields solutions of the diradicals InX(TBSQ)2, whose EPR spectra have been recorded. Addition of pyridine or γ-picoline (L) to such solutions produces adducts of the 3,5-di-t-butylcatecholate-indium(III) halide, of the type In(TBC)XLn. The mechanism of these reactions, and of the related ligand replacement and disproportionation equilibria, is discussed in terms of internal one-electron transfer processes. The compound In(TBC)(pic)2·DMF has been the subject of an X-ray crystallographic study. The substance forms monoclinic crystals, space group P21/n with a=12.992(5), b=13.923(4), c=18.491(4) A, β=98.82(2)o, Z=4, R=0.053 for 3244 unique reflections. The molecule is dimeric, with a central In2O2 ring, involving six-coordinate indium(III).

Spectroscopic and crystallographic studies of adducts of aluminium trichloride with cyclic ketones, para-quinones and ortho-quinones

Adducts of aluminium(III) chloride with 9-fluorenone, xanthenone and dibenzosuberenone have been characterised spectroscopically and by single crystal X-ray crystallography. These coloured pseudo-tetrahedral species, which are diamagnetic in both solid and solution states, serve as models for 1∶1 adducts of AlCl3 with substituted p-quinones. These latter compounds undergo intramolecular one-electron transfer in non-aqueous solution, giving rise to paramagnetic species which have been characterised by electron spin resonance spectroscopy. The 1∶1 adducts of AlCl3 with o-quinones show similar behaviour, although the solution chemistry reflects the differences between mono- and bi-dentate ligands.

The interaction of indium(III) iodide species with substituted ortho- and para-quinones

The interactions of substituted ortho- and para-quinones with indium(III) halides and the InI4– anion have been studied in non-aqueous solution. para-Quinones and InI3 give rise to stable 1∶1 adducts, which are diamagnetic in the solid state, but which decompose in solution to form (p-sq)InI2 derivatives, where p-sq˙– is the corresponding semiquinonate. With ortho-quinones, the reaction products are (o-sq)InI2 which react with 4-methylpyridine (pic) to form (o-sq)InI2pic2. The electron spin resonance spectra of these products, and their solution chemistry, are discussed. The reactions involve intramolecular one-electron transfer, resulting in oxidation of the iodide ligand. In contrast, the reaction of 3,5-di-tert-butyl-1,2-benzoquinone with InI4– apparently involves intermolecular electron transfer; in this case, the products are I3– and the corresponding catecholate (dbc), isolated as the solid InI(dbc)pic2. The mechanisms of these various processes are discussed.

Structure of lithium–p-semiquinonates in non-aqueous solution*

The reaction of phenyllithium with p-quinones (1,4-benzoquinone, 2,6-di-tert-butyl-1,4-benzoquinone, 1,4-naphthoquinone) in tetrahydrofuran gave the phenyl radical, which goes to biphenyl, and the corresponding lithium semiquinonate. The latter can also be obtained by direct reaction between lithium and the p-quinone. Electron spin resonance spectroscopy showed that a solution of the lithium derivative contains both monomeric and dimeric species. Adducts with 4-methylpyridine have also been prepared. Quantum-mechanical calculations were consistent with the ESR results. Related experiments with potassium derivatives were also performed.

Molecular structure of the unusual tris(tribromoindate)methane anion, [HC(InBr3)3]3–

The title molecule, which is obtained as the tetraphenylphosphonium salt following the reaction of InBr and HCBr3, is shown to involve pseudo-tetrahedral carbon and indium(III) sites; it is a member of a series of related [H4–nC(InBr3)n]n– anions.