Indium(III) chloride explained

Indium(III) chloride is the chemical compound with the formula InCl₃ which forms a tetrahydrate. This salt is a white, flaky solid with applications in organic synthesis as a Lewis acid. It is also the most available soluble derivative of indium.^[1] This is one of three known indium chlorides.

Synthesis and structure

Being a relatively electropositive metal, indium reacts quickly with chlorine to give the trichloride. Indium trichloride is very soluble and deliquescent.^[2] A synthesis has been reported using an electrochemical cell in a mixed methanol-benzene solution.^[3]

Like AlCl₃ and TlCl₃, InCl₃ crystallizes as a layered structure consisting of a close-packed chloride arrangement containing layers of octahedrally coordinated In(III) centers,^[4] a structure akin to that seen in YCl₃.^[5] In contrast, GaCl₃ crystallizes as dimers containing Ga₂Cl₆.^[5] Molten InCl₃ conducts electricity,^[4] whereas AlCl₃ does not as it converts to the molecular dimer, Al₂Cl₆.

Reactions

InCl₃ is a Lewis acid and forms complexes with donor ligands, L, InCl₃L, InCl₃L₂, InCl₃L₃. For example, with the chloride ion it forms tetrahedral InCl₄⁻, trigonal bipyramidal InCl₅²⁻, and octahedral InCl₆³⁻.^[4]

In diethyl ether solution, InCl₃ reacts with lithium hydride, LiH, to form LiInH4. This unstable compound decomposes below 0 °C,^[6] and is reacted in situ in organic synthesis as a reducing agent^[7] and to prepare tertiary amine and phosphine complexes of InH₃.^[8]

Trimethylindium, InMe₃, can be produced by reacting InCl₃ in diethyl ether solution either with the Grignard reagent MeMgI or methyllithium, LiMe. Triethylindium can be prepared in a similar fashion but with the grignard reagent EtMgBr.^[9]

+ 3LiMe -> + 3LiCl

+ 3MeMgI -> + 3MgClI

+ 3EtMgBr -> + 3MgBr2

InCl₃ reacts with indium metal at high temperature to form the lower valent indium chlorides In₅Cl₉, In₂Cl₃ and InCl.^[4]

Catalyst in chemistry

Indium chloride is a Lewis acid catalyst in organic reactions such as Friedel-Crafts acylations and Diels-Alder reactions. As an example of the latter,^[10] the reaction proceeds at room temperature, with 1 mole% catalyst loading in an acetonitrile-water solvent mixture. The first step is a Knoevenagel condensation between the barbituric acid and the aldehyde; the second step is a reverse electron-demand Diels-Alder reaction, which is a multicomponent reaction of N,N'-dimethyl-barbituric acid, benzaldehyde and ethyl vinyl ether. With the catalyst, the reported chemical yield is 90% and the percentage trans isomer is 70%. Without the catalyst added, the yield drops to 65% with 50% trans product.

Notes and References

1. Araki, S.; Hirashita, T. "Indium trichloride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. .
2. http://www.indium.com/products/indiumcompounds.php#indiumtrichloride Indium Trichloride
3. Habeeb, J. J.; Tuck, D. G. "Electrochemical Synthesis of Indium(III) Complexes" Inorganic Syntheses, 1979, volume XIX,
4. Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier
5. Wells, A.F. Structural Inorganic Chemistry, Oxford: Clarendon Press, 1984. .
6. Book: Chemistry of aluminium, gallium, indium, and thallium. Anthony John Downs. Springer. 1993. 0-7514-0103-X.
7. Main Group Metals in Organic Synthesis vol 1, ed. Hisashi Yamamoto, Koichiro Oshima, Wiley VCH, 2004,
8. The Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities, Simon Aldridge, Anthony J. Downs, Wiley, 2011,
9. Main Group compounds in Inorganic Syntheses, vol 31, By Schultz, Neumayer, Marks; Ed., Alan H. Cowley, John Wiley & Sons, Inc., 1997,
10. An efficient synthesis of novel pyrano[2,3-d]- and furopyrano[2,3-d]pyrimidines via Indium-Catalyzed Multicomponent Domino Reaction Prajapati, D. Mukut Gohain, M. Beilstein Journal of Organic Chemistry 2006, 2:11