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Studies of the chemistry of indium I and II compounds.

My interest in the coordination chemistry of indium arose from studies of the solvent extraction behaviour of this and other elements with a convenient radio-trace.sup 114mIn, t .sub.l/2 50 d). We eventually realized the relatively poor extraction of indium(III) species from aqueous hydrohalic acids into basic solvents was due to the formation of aquo-complexes in solution. One way of confirming this was by preparing analogous species such as [InCl.sub 4(urea).sub.2] in the solid state. This we did, and then extended the work in one form or another over many years, to the point at which the coordination chemistry and organometallic chemistry of indium are now as thoroughly investigated and as well undersood as are those of many elements, and probably better than is the case for other Main Group metals(l-3)**. For the purposes of this article, it is sufficient to note that anionic, cationic and neutral indium(III) complexes are known with a wide range of monodentate and bidentate ligands, and that coordination numbers between 3 and 6 have been identified. Since an important step in any study of the low oxidation state species involves oxidation to the analogous indium(III) compounds, this knowledge has provided a firm base for the work we are not doing.

The applications of indium and its compounds are widespread, ranging from the use of the elements in alloys, bearing metals and solders, to the more recent interest in InP, InAs and similar materials in the electronics industry. The quantities involved are small, and supply and demand are apparently in equilibrium at present. indium(II) Compounds

One of the axioms of most textbooks of inorganic chemistry is that a monomeric M.sup II compound of a Group III element will have an unpaired electron and is unlikely to exist. Certainly no such compounds have ever been prepared for indium but see below). The indium(II) halides have been known for many years, and the structures of Inl.sub.2 and InBr.sub.2 in the solid state are those of the mixed oxidation state salts In.sup 1[In.sup III X.sub.4] X = Br,I), with the easily identifiable tetrahedral In X.sub.4 anion. The chloride structure is less easily explained, and it has even been suggested that the compound is actually ln.sub .5 Cl.sub. 9, formulated as ln.sup.I, sub.[sub In.sub3 L In'II2C'9 1, but in any case it is clear none of these diamagnetic "dihalides" involve In-In bonded species. A similar conclusion applies to In(CN)2, thought to be In f ln(CN)41 on the basis of its vibrational spectrum.

Compounds with In-In bonds can be prepared, however, and in one case, the adduct In2Br3l-2tmed has been shown by X-ray.crystallography to involve an In-In bond of length 2.775(2)A. The preparation of these compounds via the reaction of say InI with InBr3 is discussed below. Several such adducts have been reported, and Raman emissions in the 100-200 cm.sup.-1 region from v(In-In) are a useful diagnostic indication of the bonding. Another group of related compounds are the In.sub.2 X.sup.-2.sub.6 anions, readily prepared from InX.sub.2 + 2BU.sub.4NX, and isostructural with the corresponding Ga.sub.2 X.sup.-2.sub. 6 species. An interesting important property has been identified by .sup.115. In NMR studies, namely the disproportionation in solution. FORMULA OMITTED

This process can be described as an internal halide ion transfer via the intermediate FORMULA OMITTED

and a similar model for In.sub.2 X.sub.4 helps to explain the structural behaviour of indium(II) halides. Firstly, the In-In bond is fairly weak (94 +/- kJ mol.sup.-1 in In.sub.2(g)), and this must reflect both poor orbital overlap and strong electrostatic repulsion between two positive metal centres. Anything which lowers this repulsion, such as coordination by donor ligands, will strengthen the In-In bond against fission. Such coordination has a further effect, in that by saturating the indium atoms, the intramolecular ligand transfer process, (eq. (1)), is prevented, so that donor ligands exert both kinetic and thermodynamic effects in InX..sub.2 derivatives. In the absence of such ligands, an In.sub.2 X.sub.4 molecule will tend to rearrange to ln+ + InX.sub.4. No InR.sub.2 organometallic compounds have been reported, and the preparation and study of such species presents an interesting challenge.

Indium(I) Compounds For many years, the only known indium(I) species were the chalcogenides and the intractable monohalides, and the only derivatives salts of the anionic InX.sup-2.sub.3(X = CI,Br,I). Equally, only one organometallic compound was known, namely cyclopentadienylindium(l), InCp, first prepared by Fischer from InCl.sub 3 and NaCp, but also available via InCl + LiCp. This is a half-sandwich molecule 2, with five-fold symmetry, as shown by electron diffraction, and the bonding can be described in terms of interactions between the pir-orbitals of the ring carbon atoms and the metal 5s and 5p orbitals, similar to but quantitatively different from the model used in describing ferrocene. This approach predicts a lone pair of electrons above the indium, in keeping with the known dipole moment and with the donor activity of InCp. The chemistry of InCp can be summarized by the scheme. FORMULA OMITTED

Oxidation or donation changes the nature of the In-Cp bonding, and in the products shown the ligand is in the monohapto form. The mathetical replacement of the Cp ligand allowed the synthesis of Et.ub. 4NInX.sub.2 salts, and of derivatives of Beta -ketoenolates or similar compounds such as In .sup.1(8-hydroxyquinolate).

The electrochemical oxidation of anodic indium in nonaqueous solutions of RSH has been used to synthesize INSR compounds (R = C.sub.2 H.sub.5,n=C.sub.4 H.sub.9), although the corresponding arylthiolates are unstable and easily go to ln(SR).sub.3 Similar experiments with solutions of R(OH)sub 2 (R = C.sub.6 H.sub.4 etc.) gave the unusual In.sup.1 OR(OH) compounds, while with R(SH);sub.2 (R = alkane) the analogous products are InSR(SH). Treatment with (e.g.) Et.sub.3 N gives the salts Et.sub.3 NH [In.sup.l(0.sub.2 R) I or Et.sub.3NH[In.sup.II(S.sub.2R)], containing anionic indium(I) complexes, whose detailed structures have yet to be elucidated.

Oxidation of indium(I) Compounds

A simple test for the presence of indium(l) in a molecule is the quantitative oxidation by iodine to the appropriate diiodo-indium(III) species (c.f. Cpln above), and we have used this diagnostically with virtually all the indium(I) compounds noted in the previous section. A related heterogeneous reaction is that between InX X = Br,l) and an alkyl halide RX to give RIn.sup.III X.sup 2, a system extensively studied by Worrall and his co-workers.sup.1 This can be viewed as oxidative insertion into (or oxidative addition across) the C-X bond of RX, although the detailed mechanism has not been established. Indium(l) halides also react heterogeneously at the S-S bond of the dithiete 3 to give the indium(III) product 4, again by an oxidative insertion or addition. FORMULA OMITTED The study of such processes was much simplified by the discovery that indium(l) halides are soluble in mixtures of aromatic and basic solvents. For example, the solubility of InBr in toluene/tmed at -20 deg C is ca. 0.016 mol L.sup.-1, and the results suggest that the species in solution is the solvate InBr.3tmed. At room temperature, such solutions deposit indium metal because of the disproportionation FORMULA OMITTED but we were able to study a series of oxidative reactions such as FORMULA OMITTED

The details of these reactions, and of the isolation of the products have been discussed in the relevant publications; other related processes with transition metal and Main Group organometallic halides are presently being studied. An overall model of the reaction intermediate involves donation of indium(l) via its presumed lone pair (c.f. InCp above) followed by electron transfer as in 5 or 6 FORMULA OMITTED

It is worth noting that 6 is the reverse of the halide transfer invoked in 1, which explains why such processes are reversible. Models of this type are useful for counting electrons, but are certainly a gross over-simplification of the actual mechanism. Nevertheless, reactions such as eqs.(3)-(7) provide an extremely convenient route to some useful and interesting M-In compounds.

Reaction of indium(l) Halides with Ortho-quinones

In the development of the work on the oxidation of indium(I) halides, we studied the reactions with some substituted o-quinones in toluene/tmed and related media. With the strongly oxidizing Y.sub.4 C.sub.6 0.sub.2-0 (Y = Cl,Br), the reaction is the predictable formation of the corresponding dihalogeno (tetrahalogenocatecholato) indium(III) species. FORMULA OMITTED a process which is exactly parallel to the reaction of these same oxidants with SnX.sub.2 to give X.sub.2 SN.sup.IV(0.sub.2,C.sub.6,Y.sub.4). With 3,5-BU.sub.2.sup.t,H.sub.2,C.sub.6,-0(TBQ)in contrast, the reaction yields species which are ESR active and the overall scheme can be written as involving the semi-quinonate (TBSQ') complexes FORMULA OMITTED

The first step is then a one-electron transfer to give a mononuclear indium(II) species, which in the presence Of 1/2 I.sub.2 is oxidized to TBSO)InX(I). In the presence of a strong donor such as 1,10-phenanthroline, the second electron is transferred by an intramolecular process to give an indium(III)catecholato complex, and such compounds have been isolated and characterized. In the absence of a strong neutral donor, the indium(II) species will obviously dimerize, and this dimer can then disproportionate by halide transfer c.f. eq. (1)) to give a mixture of indium(I) and indium(III) semiquinonates. The ESR spectrum of this final solution then shows two different indium hyperfine constants, approximately 10.5 and 6.5 G, assigned to TBS(I)InI and (TBSQ)ln.sup.III, X.sub.2 respectively.

These conclusions have been confirmed by two sets of experiments. In the first, indium metal was refluxed with TBQ in toluene, to yield a solution of (TBSQ)In.sup.1, with A.sub In|11.0 G; the reactions of this compound are exactly those expected of an indium(I) compound (c.f. CpIn above) FORMULA OMITTED Oxidation is accompanied by a change in the Al. hyperfine constant to - 6.5 G. Secondly, we were able to prepare a stable derivative of TBS(Q)InX.sub.2 by the reaction sequence FORMULA OMITTED Such compounds were characterized analytically and spectroscopically, and we find A.sub.In| 6.7G, depending on X and donor ligand. More importantly, the structure of TBSQ)InBr.sub.2.2 pic was obtained by X-ray crystallography, which confirmed the presence of the semiquinonate ligand with its characteristic C-0 bond length of 1.27(2),k These experiments give a self-consistent picture of oxidative processes which involve a series of one-electron transfers, in sharp contrast to the two-electron processes normally proposed in Main Group chemistry. Analogous conclusions have been reached from studies of the reactions of various o- and p-quinones with SnX.sub.2, and more recently with phosphorus compounds. If the generality of such reaction mechanisms can be established, there is a real prospect of describing Main Group redox reactions in a completely new light. This work is obviously continuing with enthusiasm.

Acknowledgement

In the long and pleasant journey outlined in this article, I have been helped and encouraged by many graduate students, postdoctoral fellows, and colleagues. Their names are on the papers which record their achievements, but my gratitude to each of them can never be properly expressed. I must also thank the various universities and granting agencies which have provided facilities and funds over the years. References 1. D.G. Tuck, Comprehensive Organometallic Chemistry (Editors E.W. Abel, F.G.A. Stone and G. Wilkinson), Pergamon Press, Oxford, Vol. 1, Chap. 7 (1982). 2. D.G. Tuck, Pure Applied Chem., 55, 1477 (1983). 3. D.G. Tuck, Comprehensive Coordination Chemistry (Editors G. Wilkinson, R.D. Gillard and J.A. McCleverty) Pergamon Press, Oxford, Vol. 3, Chap. 25.2 (1987).
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Author:Tuck, Dennis G.
Publication:Canadian Chemical News
Date:Sep 1, 1990
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