The syntheses of threefold acceptor-substituted 1,3,5-triaminobenzene derivatives 2 ± 6 and their crystal structure analyses are described. As acceptors, nitro, trifluoromethylsulfone, and alkylsulfone groups are employed. The combination of hydrogen bonding, arene´´´arene and F´´´F contacts leads to remarkably similar solid-state layer structures for sterically quite dissimilar molecules.

The crystal structures of methyl 4‐methoxycubane‐1‐carboxylate (1), 1‐acetamido‐4‐fluorocubane (2), methyl 4‐acetoxycubane‐1‐carboxylate (3), 1,4‐difluorocubane (4), 1,4‐dichlorocubane (5), and N,N‐diisopropylcubane‐1,4‐dicarboxamide (6) have been investigated by means of X‐ray diffraction analysis. Fluorine and chlorine substituents cause a shortening of the vicinal bonds, as is seen in the 4‐halocubane‐1‐carboxylates. The cage bonds vicinal to the ester substituent, with a favorable orientation with regard to the π‐acceptor influence of this group become longer than the CH–CH bonds. Furthermore, the influence on bond length with respect to the orientation of this group relative to bonds within the cubane skeleton has been investigated experimentally. The effect of the methoxy group has also been found to depend on the orientation. The cage bond antiperiplanar to the methyl group is shortened, while the cage bonds in gauche orientation to this group are lengthened. As seen in the case of the halogen‐substituted derivatives, the bonds bearing the acetoxy substituent are shortened due to the σ‐acceptor property of this group. Ab initio calculations on compounds 1, 2, 4, and 5 performed at the 6‐31G* level confirm the experimental results.

The syntheses and the electrochemical behavior of the monomeric peralkylated hexaamino(1,3)metacyclophane 4, the dimeric dodecaamino(1,3)cyclophane 5a, and the dodecaamino(1,3,5)cyclophane 6 are described. Electrochemical measurements show that the hexaaminobenzene units in 4 and 5a undergo an unusually slow two-electron transfer attributed to the deformation of the rings into bis-cyanine cations when oxidized to the respective dication. Further oxidations to tri-, tetra-, and hexacationic units occur at more positive potentials. In the dimeric structures, no interaction between the rings can be seen in the (1,3)cyclophane, but strong interaction for the (1,3,5)cyclophane is observed.

The crystals of methyl 4-fluoro-1-cubanecarboxylate ( l ) , methyl 4-chloro-1-cubanecarboxylate (2) and methyl cubanecarboxylate (5) are isomorphous to each other as are the crystals of methyl 4-bromo-1-cubanecarboxylate (3) and methyl 4-iodo-1-cubanecarboxylate (4). As a result of the space groups P2,/m and Pnma, respectively, all molecules lie in a crystallographic mirror plane. Therefore, the methoxycarbony1 group is exactly planar, and the carbonyl fragment is in an eclipsed position to the cubane skeletal bond C2-C7.The electronic effects of the halogen atoms, in particular fluorine and chlorine, give rise to a shortening of the vicinal skeletal bonds. In contrast, the methoxycarbonyl groups causes a lengthening of the vicinal skeletal bonds C2-C6 and C2-C6A which are not in an eclipsed conformation. Ab initio calculations at the 6-31G* level of compounds 1, 2, and 5 confirm the experimental results.The structure of three-membered rings is influenced by substituent effects ['] which are increased significantly in polycyclic systems with bridging They are sensible probes for these kinds of effects. For cyclobutane derivatives the substituent effects are already too weak to be detected unambigu~usly [~]. Therefore, we selected the cubane system which is composed of four-membered subunits within a strained polycyclic system. It should be suitable to detect substituent effects on four-membered ring bonds.Since Eaton and Cole presented the synthesis of cubaneL4I many chemists were engaged in cubane and its derivative~ [~]. This paper deals with the geometric alterations of the rigid cubane skeleton caused by several different substituents particularly by fluorine, because of its strong electronic effect [']. Since Fleischer investigated the crystal structureL61 of cubane itself, a large number of X-ray crystalstructure investigations of the different derivatives have been carried out, but to our knowledge no derivative bearing a fluorine atom on the cubane skeleton has previously been studied. In this paper we describe the crystal structures of the compounds 2-5 and of methyl 4-fluoro-1-cubanecarboxylate (1) in particular to demonstrate the geometric distortions of the cubane skeleton caused by the different substituent effect of the halogen atoms and the ester group. / MeOOC I H SynthesesStarting from 1,4-cubanedicarboxylic acid (6, Scheme 1) which was synthesized according to the method of Chapman et and in the case of the Favorskii rearrangement according to the method of Luh and we obtained the central half ester 8 by partial saponification of dimethyl 1,4-~ubanedicarboxylate (7)L91. The latter could be synthesized by methanolysis of the diacid dichloride of 6 in 85% yield. Methyl 4-iodo-and 4-fluoro-1 -cubanecarboxylate 4 and 1 were prepared as described by Della and Head [lo]. Methyl 4-bromo-1-cubanecarboxylate (3) was obtained by a Hunsdiecker decarboxylative bromination described by Della et al.["I. A decarboxylative chlorination in the Kochi reaction[l2I led to methyl 4-chloro-1-cubanecarboxylate (2). T...

The rc system of la may adopt markedly different electronic structures with concomitant strong conformational preferences for the six-membered ring. Neutral la can exist either in a quinoid boat form or as a twist boat, formally resulting from the coupling of an anionic with a cationic trimethinecyanine. Deprotonation gives a rigid quinoid boat structure; protonation occurs at a ring atom to form an equally rigid cyanine.Knowledge of the electronic and geometric features of donor-acceptor-substituted n systems is of decisive importance to the rational design of molecules with nonlinearoptical propertiesr2]. In this respect, non-aromatic n systems seem to be the most They may result, for example, from the attachment of several strong donors and acceptors to the benzene nucleus (Jahn-Teller distortion). Qualitatively, these distortions are rationalized by the ,,cyanine c~ncept"~'~; exceptions, however, are known[3b1. With aid of this concept, no predictions are possible about the conformation of the distorted molecule.The non-aromatic["], C6lradialene-like trifold donor-substituted 1,3,5-trinitrobenzenes 1-3[*s4] are ideal objects for experimental studies of the interplay between electronic and geometric organization of the n system since they are conformationally flexible: mainly for electronic reasons['] they adopt strongly non-planar b~a t [ ' ,~" ,~,~] or twist boat conform a t i o n~[~~-~] which hardly differ energetically. The differences in geometries, however, are reflected by changes in the electronic structures of their n systems. So far, the direct comparison of both forms has been possible only for l a , which crystallizes in a boat form in the cosolvate with isopropylamine, but in a twist-boat form in the cosolvate with toluene. The strong alternation of the C-C bonds within the ring, and of the exocyclic C-N bonds found in the boat form[4a,b%d1 could be satisfactorily explained by invoking a quinoid resonance form (NQ in Schemes 1 and 2). The electronic structure of the n system in the twist-boat form could not be discussed in detail, because the numerical uncertainties of the geometric parameters were increased by the disorder of the cosolvated toluene[4a1. We therefore report on a more precise X-ray analysis of solvate-free l a . On the other hand, we were interested in the behavior of l a on [*I Permanent address: University of Ploiegti, Bd. Bucuresti 39, RO-2000 Romania.protonation and deprotonation. Its n system, again, may prefer an aromatic, a quinoid or a (cationic or anionic pentamethine-)cyanine-like structure (Scheme 1; PQ, PC and DQ, DC, respectively; the aromatic form is not shown), and all forms appear reasonable at first glance. Fortunately, 1 a behaves as an amphoteric molecule, and stable salts are formed in both cases. Thus, the rare possibility is given to study the different geometric and electronic forms of the n system with slight changes in steric effects [']. Therefore, we have not only determined the structure of solvate-free 1 a, but also those of its potassium and tetraflu...