Distorted Molecules: Perturbation Design, Preparation and Structures

Abstract

The structure of a molecule can change considerably as its energy and thus its electron distribution within the time-domain of dynamic relaxation varies. Based on comparison of approriate measured data of related compounds and supported by quantum chemical calculations, therefore, charge-perturbed and/or sterically overcrowded molecules can be designed. Their preparation, handling, and structural characterization, frequently under extreme and especially largely aprotic conditions, provides some surprises. New structural principles become evident and old-fashioned ones are confirmed. Thus the contact-ion aggregates that form on ultrasonically supported reduction of unsaturated hydrocarbons with sodium metal partly contain dibenzene sodium sandwiches. Vicinal dimethylamino substituents or isoelectronic isopropyl groups cause steric overcrowding and facilitate oxidation to molecular cations by energetically favorable delocalization of the generated positive charge. Molecules and molecular ions in which an even number of II electrons are distributed over a D skeleton containing an odd number of centers preferentially form cyanine subunits. This is demonstrated by the novel ethene dication and dianion salts with central C-C single bonds and molecular halves twisted relative to each other. Altogether in two years well over 50 structures have been determined. Much has been learned from them, especially about electron transfer and contact ion-pair formation in aprotic solvents. Nevertheless, we had to realize that answers to many questions, above all "what crystallizes, how, and why", are still out of reach."ActuuNy, everything is more comjdicuted".calculation of an approximate one-dimensional MNDO energy hypersurface, in which only one of the total of (3n -6) = 30 degrees freedom for the molecule containing n = 12 centers, namely the dihedral angle w(C-SCCS-C) (Scheme 2), is considered. 550@ VCH Verluxsgesellschaft mbH, W-6940 Weinheinz. 1992 OS7O-OS33~92jOSOS-OSSO $3.50+ .2SjO Angew. Chem. Int. Ed. Engl. 31 (1992) 550-581 The Structures of Radical Cations R,NNR;@ in SolutionThe oxidation of silyl-substituted hydrazine derivatives by the oxygen-free and selective one-electron-transfer oxidation reagent AICI,/CH,Cl,[41 yields their radial cations['a' 4 9 61 in their doublet ground states [Eq. (a)].

Structural Distortions in Diiodine-Substituted Unsaturated Hydrocarbons

“…An extensive CSD structure search for related polyiodinesubstituted hydrocarbons revealed numerous derivatives with a planar skeleton analogous to prototype tetraiodoethylene (Scheme 1 B). [2] Selected examples are 1,1-diiododinitroethylene (Scheme 8 A), [17] 1,2-diiodocyclobutene-3,4-dione (Scheme 8 B), [18] 1,2,4,5-tetraiodo-p-benzoquinone (Scheme 8 C), [19] 3,4,5-triiodotoluolene (Scheme 8 D), [20] 2,3,4,5-tetraiodophthalic acid anhydride (Scheme 8 E), [21] and hexaiodobenzene (Scheme 8 F). [22] In 1,1-diiododinitroethylene, two elec- tron-withdrawing nitro groups shorten the distance I´´´I between the geminal iodine centers of 3.50 relative to those in tetraiodoethylene of 3.54 (Scheme 18) by only 0.04 .…”

“…Structural comparison based on density functional calculations: The rather large distortions detected in some cases in the structure determination of the selected diiodo-substituted six-membered ring prototype hydrocarbons phenanthrene, biphenyl, and naphthalene, which predominantly involve torsion angles, but also bond angles and even bond lengths, [2,5,11,14,15] stimulated an extensive search for related molecules in the Cambridge Structural Database. The most relevant information extracted concerns the structure of 1,12-diiodotriphenylene, hidden in a short communication on the dilithiation of polycyclic aromatic hydrocarbons (Scheme 4).…”

“…Despite the frequently demonstrated usefulness of interference radii for qualitative estimates of structures containing bulky groups, [2] some caution is advisable even for singlecenter substituents: The structure of planar tetraiodoethene (Scheme 1 B), determined by gas-phase electron diffraction, [8] has distances of 3.65 and 3.54 between vicinal and geminal iodine centers, which are considerably exceeded by the sum of their interference radii, 2r vdW I 2 Â 2.15 4.30 , and indicate that for groups CHal 2 , therefore, the assumption of spherical electron-density distributions around the individual halogen centers [7a,d] is obviously unjustified.…”

“…[2] Numerous such species have been reported, of which we have selected one p-and one electron-pair-perturbed example for this introduction. The recently published structure of octaphenyldibenzo[a,c]naphthacene (A) shows a 1058 twist along the long hydrocarbon axis, [3] and octachlorotetraradialene (B), structurally characterized in 1970, [4] is a severely distorted fourmembered ring.…”

Synthesis, spectroscopic, and structural studies of dimethyltellurium(IV) alkylene and diphenyl dithiophosphates. Crystal structures of and

“…As was found for non-cyclic dithiophosphates (1 1, 13, 18), the terminal sulfur atoms are oriented toward the tellurium atom at distances, 3.341(2) and 3.349(3) A in 1 and 3.490(3) and 3.516(2) A in 4, well within the sum of the van der Waals radii. The nature and importance of these secondary interactions is not clear so we introduce a more quantitative approach than hitherto attempted by estimating bond orders for these partial bonds based on proposals made recently in calculating a bond order for an unusually long Si-Si bond in a disilane derivative (34). The calculation was based on that devised by Pauling for partial bonds primarily involving hydrogen and first-row elements (31).…”