“Crowned” Univalent Indium Complexes as Donors? Experimental and Computational Insights on the Valence Isomers of EE′X4 Species

The use of the univalent indium reagent [In([18]crown-6)][OTf] as a donor is investigated by its reactions with acceptors including InX3 (X=Cl, Br, I). The donor–acceptor complexes of the form [X([18]crown-6)InInX3] obtained in this manner represent the first new isomeric form of indium(II) halides identified for at least five decades. The formation of such complexes appears to be particularly favorable and they are isolated as products in many reactions involving low-valent indium, a halide source, and [18]crown-6. A convenient solution-phase synthesis of In[ECl4] salts is reported. This facile and direct syntheses of In[ECl4] (E=Al, Ga, In) salts allows for the in situ preparation and isolation of crown-ether complexes of the form [In([18]crown-6)][ECl4], whose existence had been postulated but never confirmed. Solution-phase and solid-state NMR experiments reveal that these compounds can exist as either donor–acceptor complexes or ionic salts, depending on the phase of the system, the nature of the solvent employed, and the identity of the metalate anion involved. Similar investigations into the effect of a smaller crown ether allow for the isolations of salts containing the cation [In([15]crown-5)]+. Computational investigations into the nature of the crowned univalent indium donor fragments, and on the donor–acceptor complexes produced, demonstrate the influence of anionic substituents on the reactivity of lone pair of electrons of the InI center. Natural bond orbital (NBO) analysis of donor–acceptor models shows that the composition of the EE bond MO should provide the ability to predict which models should form stable complexes.