Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(mu-Cl)Ar'}2 [where Ar' indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr-Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar'CrCrAr' were isolated with greater than 40% yield. X-ray diffraction revealed a Cr-Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.
Single crystals of a lipophilic G-quadruplex formed by 5′-tert-butyl-dimethylsilyl-2′,3′,-di-Oisopropylidene G 2 were obtained from a CH 3 CN solution containing potassium picrate and cesium picrate. The X-ray structure showed that 16 units of G 2 and 4 equiv of alkali picrate form the lipophilic G-quadruplex. The quadruplex has a filled cation channel, with three K + ions and one Cs + ion located along its central axis. The quadruplex is formed by a pair of head-to-tail (G 2) 8 -K + octamers. Both octamers use eight carbonyl oxygens to coordinate K + . The two (G 2) 8 -K + octamers are of opposite polarity, being coaxially stacked in a head-to-head orientation. A Cs + cation, with an unusual coordination geometry, caps the cation channel. The Cs + is coordinated to four acetonitrile solvent molecules in an η 2 -fashion. Within an octamer the two tetramers are stacked so that they are 3.3 Å apart and twisted by 30°. A second stacking interaction is defined by the head-to-head arrangement between the two (G 2) 8 -K + octamers. This stacking, with a 90°twist, positions the exocyclic amines of the central two quartets so that both exocyclic NH2 B protons can hydrogen bond to the picrate anions that rim the quadruplex equator. The four picrates form an anionic belt that wraps around the cation channel. The sugars are well ordered in the structure. Circular dichroism spectra indicate that the G-quadruplex retains its helical structure in chlorinated solvents. Some diverse compounds have been proposed to form ion channels. These include magainin, cecropin, and gramicidin 1 and synthetic peptides that form bundles and nanotubes. 2,3 Various organic compounds also conduct ions across membranes. 4 Lipophilic ion pairs, modified phospholipids, bouquet molecules, unusual macrocycles, sterols, bolaamphiles, rigid rods, and crown-peptides all may form ion channels. 5-12 For many of these compounds, self-assembly in the membrane presumably gives channels with hydrophobic exteriors and hydrophilic interiors.Nucleobases self-associate via hydrogen bonding and base stacking. Thus, artificial ion channels are conceivable from lipophilic nucleobases. 13 Guanosine (G) is notorious for its propensity to aggregate. In a cation-templated process, G derivatives self-associate in water to give the G-quartet ( Figure 1). 14,15 The planar G-quartet is stabilized by hydrogen bonds between the NH1 amide and NH2 amino donors on one purine and the O6 and N7 acceptor atoms on a neighboring base. The G-quartet, with four oxygens surrounding a cavity, binds alkali cations with a selectivity of K + > Na + , Rb + . Cs + , Li + . 16 For example, K + forms a sandwich with two G-quartets. 17 These
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