Gallium, indium and thallium

    Since 1980, interest in organometallic compounds of Ga, In and Tl has grown, mainly because of their potential use as precursors to semiconducting materials such as GaAs and InP. Volatile compounds are sought that can be used in the growth of thin films by MOCVD (metal organic chemical vapour deposition) or MOVPE (metal organic vapour phase epitaxy) techniques. Precursors include appropriate Lewis base adducts of metal alkyls, e.g. Me₃Ga.NMe₃ and Me₃In.PEt₃. Reaction 1.1 is an example of the thermal decomposition of gaseous precursors

to form a semiconductor which can be deposited in thin films (see Box 18.4).

                      (1.1)

   Gallium, indium and thallium trialkyls, R₃M, can be made by use of Grignard reagents (reaction 1.2), RLi (equation 1.3) or R₂Hg (equation 1.4), although a variation in strategy is usually needed to prepare triorganothallium derivatives (e.g. reaction 1.5) since R₂TlX is favoured in reactions 1.2 or 1.3. The Grignard route is valuable for the synthesis of triaryl derivatives. A disadvantage of the Grignard route is that R₃M.OEt₂ may be the isolated product.

                                     (1.2)

                         (1.3)

                 (1.4)

                                     (1.5)

   Trialkyls and triaryls of Ga, In and Tl are monomeric (trigonal planar metal centres) in solution and the gas phase. In the solid state, monomers are essentially present, but close intermolecular contacts are important in most structures. In trimethylindium, the formation of long In…..C interactions (Figure 18.10a) means that the structure can be described in terms of cyclic tetramers; further, each In centre forms an additional weak In…..C interaction (356pm) with the C atomof an adjacent tetramer to give an infinite network. The solid state structures of Me₃Ga and Me₃Tl resemble that of Me₃In.   

   Within the planar Me₃Ga and Me₃Tl molecules, the average Ga^_C and Tl^_C bond distances are 196 and 230 pm, respectively. Within the tetrameric units, the Ga_-----C and Tl_-----C separations are 315 and 316pm, respectively. Intermolecular interactions are also observed in, for example, crystalline Ph₃Ga, Ph₃In and (PhCH₂)₃In. Figure 18.10b shows one molecule of (PhCH₂(₃In, but each In atom interacts weakly with carbon atoms of phenyl rings of adjacent molecules.  Dimer formation is observed in Me₂Ga)µ-C≡CPh(₂GaMe₂ (Figure 18.10c), and the same bonding description that we outlined for R₂Al)PhC≡C(₂AlR₂ (18.9 and 18.10) is appropriate. Triorganogallium, indium and thallium compounds are airand moisture-sensitive. Hydrolysis initially yields the linear [R₂M]⁺‏ ion (which can be further hydrolysed), in contrast to the inertness of R₃B towards water and the formation of Al(OH)₃) from R₃Al. The [R₂Tl]⁺‏ cation is also present in R₂TlX (X = halide), and the ionic nature of this compound differs from the covalent character of R₂MX for the earlier group 13 elements. Numerous adducts R₃M^.L (L= Lewis base) are known in which the metal centre is tetrahedrally sited, e.g. Me₃Ga.NMe₃, Me₃ Ga.NCPh, Me₃In.OEt₂, Me₃In.SMe₂, Me₃Tl.PMe₃, [Me₄Tl]^- . In compound 1.1, donation of the lone pair comes from within the organic moiety; the GaC₃-unit is planar since the ligand is not flexible enough for the usual tetrahedral geometry to be adopted.

(1.1)

   Species of type [E₂R₄] (single E^_E bond) and [E₂R₄]^- (E^_E bond order 1.5) can be prepared for Ga and In provided that R is especially bulky (e.g. R =)Me₃Si)₂CH, 2,4,6-ⁱPr₃C₆H₂), and reduction of [(2,4,6-ⁱPr₃C₆H₂)₄Ga₂] to [(2,4,6-ⁱPr₃C₆H₂(₄Ga₂]^- is accompanied by a shortening of the Ga^_Ga bond from 252 to 234 pm, consistent with an increase in bond order (1 to 1.5). By using even bulkier substituents, it is possible to prepare gallium(I) compounds, RGa (1.2) starting from gallium(I) iodide. No structural data are yet available for these monomers. However, 1.2 with R’ = H crystallizes as the weakly bound dimer 1.3, reverting to a monomer when dissolved in cyclohexane. The Ga^_Ga bond in 1.3 is considered to possess a bond order of less than 1. Reduction of 1.3 by Na leads to Na₂[RGaGaR], in which the dianion retains the trans-bent geometry of 1.2, and the Ga_Ga bond length is 235 pm. The salt Na₂[RGaGaR] can also be prepared from the reaction of RGaCl₂ and Na in Et₂O, and it has been claimed that [RGaGaR]^2-  contains a gallium–gallium triple bond.  However, recent results suggest that the Ga^_Ga interaction in Na₂[RGaGaR] is best described as consisting of a single bond, augmented by the weak interaction present in the precursor 1.2. Additionally, in the solid state, there are stabilizing interactions between the two Na‏ ions and the Ga^_Ga bond.

(1.2)                                         (1.3)

   The 2,6-dimesitylphenyl substituent is also extremely sterically demanding, and reduction of (2,6-Mes₂C₆H₃)GaCl₂ with Na yields Na₂[)2,6-Mes₂C₆H₃(₃Ga₃]; the [(2,6- Mes₂C₆H₃)₃Ga₃]^2- anion possesses the cyclic structure (1.4) and is a 2π-electron aromatic system.

(1.4)

In equation 12.47, we illustrated the use of the metastable GaBr as a precursor to multinuclear Ga-containing species. Gallium(I) bromide has also been used as a precursor to a number of organogallium clusters. For example, one of the products of the reaction of GaBr with (Me₃Si)₃CLi in toluene at 195K is 1.5.

(1.5)

Worked example : Reactions of {(Me₃Si)₃C}₄E₄ (E = Ga or In)

The reaction of the tetrahedral cluster {(Me3Si)3C}₄Ga₄ with I₂ in boiling hexane results in the formation of {(Me₃Si)₃CGaI}₂ and {(Me₃Si)₃CGaI₂}₂. In each compound there is only one Ga environment. Suggest structures for these compounds and state the oxidation state of Ga in the starting material and products. The starting cluster is a gallium(I) compound:

   I₂ oxidizes this compound and possible oxidation states are Ga(II) (e.g. in a compound of type R₂Ga^_GaR₂) and Ga(III). {(Me₃Si)₃CGaI}₂ is related to compounds of type R₂Ga^_GaR₂; steric factors may contribute towards a nonplanar conformation:

    Further oxidation by I₂ results in the formation of the Ga(III) compound {(Me₃Si)₃CGaI₂}₂ and a structure consistent with equivalent Ga centres is:

Self-study exercises

1. The Br2 oxidation of {(Me₃Si)₃C}₄In₄ leads to the formation of the In(II) compound {(Me₃Si)₃C}₄In₄Br₄ in which each In atom retains a tetrahedral environment. Suggest a structure for the product.

2. {(Me₃Si)₃CGaI}₂ represents a Ga(II) compound of type R₂Ga₂I₂. However, ‘Ga₂I₄’, which may appear to be a related compound, is ionic. Comment on this difference.

3. A staggered conformation is observed in the solid state for {(Me₃Si)₃CGaI}₂. It has been suggested that a contributing factor may be hyperconjugation involving Ga^_I bonding electrons.  What acceptor orbital is available for hyperconjugation, and how does this interaction operate?

   Cyclopentadienyl complexes illustrate the increase in stability of the M(I) oxidation state as group 13 is descended, a consequence of the thermodynamic 6s inert pair effect

Cyclopentadienyl derivatives of Ga(III) which have been prepared (equations 1.6 and 1.7) and structurally characterized include Cp₃Ga and CpGaMe₂.

                (1.6)

                   (1.7)      

   The structure of CpGaMe2 resembles that of CpAlMe₂ (Figure 18.9a), and Cp₃Ga is monomeric with three η¹-Cp groups bonded to trigonal planar Ga (Figure 1.1a).

Fig. 1.1 The solid state structures (X-ray diffraction) of (a) monomeric )η¹-Cp)₃Ga [O.T. Beachley et al. (1985) Organometallics, vol. 4, p. 751], (b) polymeric Cp₃In [F.W.B. Einstein et al. (1972) Inorg. Chem., vol. 11, p. 2832] and (c) polymeric CpIn [O.T. Beachley et al. (1988) Organometallics, vol. 7, p. 1051]; the zig-zag chain is emphasized by the red hashed line. Hydrogen atoms are omitted for clarity; colour code: Ga, yellow; In, green; C, grey.

   InCl₃, but is structurally different from Cp₃Ga; the solid contains polymeric chains in which each In atom is distorted tetrahedral (Figure 1.1b).  The reaction of (η⁵-C₅Me₅)₃Ga with HBF₄ results in the formation of [(C₅Me₅)₂Ga]⁺‏[BF₄]^-. In solution, the C5Me5 groups are fluxional down to 203 K, but in the solid state the complex is a dimer (1.6) containing [(η¹-C₅Me₅)(η³- C₅Me₅)Ga]⁺ ions. The structure of [(C₅Me₅)₂Ga]‏⁺ contrasts with that of [(C₅Me₅)₂Al]‏⁺, in which the C₅-rings are coparallel.

(1.6)

    We saw earlier that gallium(I) halides can be used to synthesize ArGa compounds. Similarly, metastable solutions of GaCl have been used to prepare (C₅Me₅)Ga by reactions with (C₅Me₅)Li or (C₅Me₅)₂Mg. An alternative route is the reductive dehalogenation of (C₅Me₅)GaI₂ using potassium with ultrasonic activation. In the gas phase and in solution,  (C₅Me₅)Ga is monomeric, but in the solid state, hexamers are present.    On moving down group 13, the number of M(I) cyclopentadienyl derivatives increases, with a wide range being known for Tl(I). The condensation of In vapour (at 77 K) onto C₅H₆ gives CpIn, and CpTl is readily prepared by reaction 1.8.

                     (1.8)

   Both CpIn and CpTl are monomeric in the gas phase, but in the solid, they possess the polymeric chain structure shown in  Figure 1.1c. The cyclopentadienyl derivatives(C₅R₅(M (M = In, Tl) are structurally diverse in the solid state, e.g. for R =PhCH₂  and M = In or Tl, ‘quasi-dimers’ 1.7 are present (there may or may not be a meaningful metal– metal interaction), and (η⁵-C₅Me₅(In forms hexameric clusters.

(1.7)

   One use of CpTl is as a cyclopentadienyl transfer reagent to d-block metal ions, but it can also act as an acceptor of Cp^-, reacting with Cp₂Mg to give [Cp₂Tl]^-. This can be isolated as the salt [CpMgL][Cp₂Tl] upon the addition of the chelating ligand L = Me₂NCH₂CH₂NMeCH₂CH₂NMe₂. The anion [Cp₂Tl]^­-  is isoelectronic with Cp₂Sn and possesses a structure in which the η⁵-Cp rings are mutually tilted. The structure is as shown in Figure 1.2c for Cp₂Si but with an angle α = 157⁰. Although this ring orientation implies the presence of a stereochemically active lone pair, it has been shown theoretically that there is only a small energy difference (3.5 kJmol^-1) between this structure and one in which the η⁵-Cp rings are parallel (i.e. as in Figure 1.2a). We return to this scenario at the end of the next section.