Counterintuitive Interligand Angles in the Diaryls E{C6H3-2, 6-(C6H2-2,4,6-Pr-i(3))(2)}(2) (E = Ge, Sn, or Pb) and Related Species: The Role of London Dispersion Forces

The straightforward reaction of two equivalents of the lithium salt of the bulky terphenyl ligand Li(OEt2)C6H3-2,6-(C6H2-2,4,6-Pr-i(3))(2) with suspensions of GeCl(2)dioxane, SnCl2, or PbBr2 in diethyl ether resulted in the isolation of the very crowded sigma-bonded diaryl tetrylenes of formula E{C6H3-2,6-(C6H2-2,4,6-Pr-i(3))(2)}(2) (E = Ge (l), Sn (2), Pb (3)) as blue crystalline solids. Despite their high level of steric congestion, X-ray crystallography showed that compounds 1-3 possess C-ipso-E- C-ipso interligand bond angles in the range 107.61-112.55 degrees, which are narrower than those observed in analogous species with less bulky terphenyl substituents. Compounds 1-3 were characterized by H-1, C-13{H-1} (1-3), and Sn-119{H-1} (2) NMR spectroscopy, whereas solution Pb-207{H-1} NMR spectroscopy of 3 has not yet afforded a signal under ambient conditions. FT-IR and UV-vis spectra of 1-3 were also recorded. The relatively narrow interligand angles displayed by 1-3 are attributed in part to the increase in London dispersion force interactions between the two Ar-iPr6 (Ar-iPr6 = -C6H3-2,6-(C6H2-2,4,6-Pr-i(3))(2)) groups from carbon atoms in some of the isopropyl substituents and several carbon atoms from the flanking aryl rings. Density functional theory (DFT) calculations carried out at the PBE0/def2-QZVP level on the full series of diaryl tetrylenes, E(Ar-iPr6)(2), E(Ar-iPr4)(2) (Ar-iPr4 = -C6H3-2,6-(C6H3-2,6-Pr-i(2))(2)), and E(Ar-Me6)(2) (Ar-Me6 = -C6H3-2,6-(C6H2-2,4,6-Me-3)(2,) afford interaction energies as high as ca. 27 kcal mol(-1).