l,2-Diarylindano[a]indane: A Molecule with Skewed, Cofacially Disposed Aromatic Moieties and a Long C–C Single Bond

1-(4-Methoxyphenyl)-2-(4-nitrophenyl)-5,10-dimethoxyindano[1,2-a]indane (6) is a new system with a long carbon-carbon single bond arising from through-bond coupling. The versatility and simplicity of the aromatic molecular framework of this system suggests that it may be useful for the study of substituent effects on through-bond coupling, stacking between aromatic rings, charge-transfer, and carbon-carbon bond cleavage. Indanoindane 6 was prepared by bis-C-alkylation of 1-(4-methoxyphenyl)-2-(4-nitrophenyl)ethanone with 3-methoxybenzyl chloride, followed by double cyclization-dehydration. X-ray crystallography revealed that the indanoindane chassis of 6 was bent at the central single carbon-carbon single bond, with the two five-membered rings adopting a distorted envelope conformation. The priapic 4-methoxyphenyl and 4-nitrophenyl substituents attached to the central ethane unit are staggered with respect to each other, producing a skewed, cofacial array, with a centroid to centroid distance of 4.13 Å; these two rings are 2.93 Å apart at their closest point. The most remarkable structural feature of 6 is the elongated (1.618 Å) central carbon-carbon single bond. Molecular mechanics energy minimizations were unable to reproduce this value, whereas the AM1 molecular orbital method predicted a length of 1.614 Å. The Mulliken overlap population of the central single bond increased if methyl groups replaced either or both priapic aromatic substituents in the calculations. These results suggest that through-bond coupling is the origin of the central bond elongation. AM1 calculations indicated that the length of the central bond in 6 and in a model system is not significantly altered by the nature of the para substituents on the priapic aromatic rings. A survey of the crystallographic data on other indanoindanes demonstrated that central single-bond elongation is not a general characteristic of the indanoindane skeleton. Relative to a composite absorption spectrum of its constituent chromophores (p-alkylanisole, p-alkylnitrobenzene, and 5-methoxyindane), the absorption band of 6 is slightly more intense, slightly red-shifted, and wider and has a prominent red-edge tail. These changes in the UV spectrum of 6 are ascribed to a weak charge-transfer interaction between the priapic rings.