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ABSTRACT: Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular "Venus flytrap". The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of bridged diarenes for multicentered interactions with a single NO moiety. The best fit of the electronic and structural parameters is provided by a calixarene host that allows the interacting centers to be arranged in a manner similar to those extant in [2:1] nitrosonium complexes with analogous (nonbridged) aromatic donors; this results in its very strong noncovalent binding with nitrosonium/nitric oxide with the formation constant of K(B) approximately 10(8) M(-)(1) and free-energy change of -DeltaG degrees = 45 kJ mol(-)(1). Such strong, selective, and reversible bindings of nitrosonium/nitric oxide by (cofacial) aromatic centers thus provide the basis for the development of efficient NO sensors/absorbents and also suggest their potential relevance to biochemical systems.
The Journal of Organic Chemistry 06/2003; 68(10):3947-57. · 4.45 Impact Factor
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ABSTRACT: Access to each C=C face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) is hindered by the hydrogenic canopy consisting of four beta-hydrogens; otherwise, these olefins have quite normal environments. X-ray crystallography and density functional (DFT) calculations show a 0.5 A larger annular opening in the protective cover of AA than that in SA. This contributes to the remarkable differences in reactivity toward various reagents, not only by limiting access to the olefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus, AA is subject to typical olefin-addition reactions with bromine, sulfuryl chloride, m-chloroperbenzoic acid, dioxygen, and so forth, albeit sometimes at attenuated rates. On the other hand, SA is singularly unreactive under identical reaction conditions, except for the notable exceptions that include Brønsted (protonic) acids, a nitrosonium cation, and dichlorine. The exceptions are characterized as three sterically limited (electrophilic) reagents whose unique reactivity patterns are shown to be strongly influenced by steric access to the C=C center. As such, the different degrees of steric encumbrance in the isomeric donors AA and SA shed considerable light on the diverse nature of olefinic reactions. In particular, they evoke mechanistic features in electrophilic addition versus electron transfer, which are otherwise not readily discernible with other less hindered olefinic donors. Transient structures of the olefinic-reaction intermediates such as the protonated carbocations AA-H+ and SA-H+ as well as the cation radicals AA*+ and SA*+ are probed by the combination of X-ray crystallographic analyses and density functional theoretical computations.
The Journal of Organic Chemistry 08/2002; 67(15):5106-16. · 4.45 Impact Factor
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04/2002;
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04/2002;
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ABSTRACT: [structure: see text]. Macromolecules 1c and 2c bearing multiple redox-active sites are synthesized by an efficient palladium-catalyzed coupling of 2,5-dimethoxytolylmagnesium bromide with readily available hexakis(4-bromophenyl)benzene and tetrakis(4-bromophenyl)methane. These macromolecular electron donors undergo reversible oxidation at a constant potential of 1.15 V vs SCE to yield robust, multiply charged cation radicals that are isolated in pure form using SbCl(5) as an oxidant. These nanometer-size cation-radical salts are shown to act as efficient "electron sponges" toward a variety of electron donors.
Organic Letters 10/2001; 3(18):2887-90. · 5.86 Impact Factor
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ABSTRACT: Silver(I) complexes with aromatic donors are thoroughly analyzed (with aid of the Cambridge Crystallographic Database) to identify the basic structural factors inherent to the bonding of an arene ligand. Most strikingly, the distance parameter d (which simply measures the normal separation of Ag from the mean aromatic plane) is singularly invariant at d = 2.41 +/- 0.05 A for all silver/arene complexes, independent of the hapticity (eta 1 or eta 2), hybridization, or multiple coordination. As such, a systematic series of stilbenoid ligands has been successfully designed to precisely modulate the penetration of silver(I) into the ligand cleft, and a multicentered poly(arene) ligand (X) designed to form a one-dimensional assembly of Ag/arene units. Simply stated, the depth penetration of silver(I) into the aromatic cavities of various cis-stilbenoid donors can be precisely predicted with a single parameter gamma that measures the separation of the two cofacial aryl groups comprising the cleft. This simple geometric consideration must be taken into account in any successful design of novel (poly)aromatic ligands for silver(I) complexation to constitute new molecular architectures.
Inorganic Chemistry 01/2001; 39(25):5707-16. · 4.60 Impact Factor
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ABSTRACT: Triethyloxonium hexachloroantimonate [Et(3)O(+)SbCl(6)(-)] is a selective oxidant of aromatic donors (ArH), and it allows the facile preparation and isolation of crystalline paramagnetic salts [ArH(+)(*), SbCl(6)(-)] for the X-ray structure determination of various aromatic cation radicals. The mechanistic relationship between the Meerwein salt [Et(3)O(+)SbCl(6)(-)] and the pure Lewis acid oxidant SbCl(5) is based on a prior ethyl transfer from oxygen to chlorine within the ion pair.
The Journal of Organic Chemistry 09/1998; 63(17):5847-5856. · 4.45 Impact Factor
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ABSTRACT: The stepwise (one-electron) chemical oxidation of the tetraphenylethylene donor and its substituted analogues (D) can be carried out by electron exchange with aromatic cations or antimony(V) oxidants to selectively afford the cation radical (D+•) initially and then the dication (D2+). The ready interchange of the latter establishes the facile disproportionation (i.e., 2D+• D2+ + D) that was originally examined by only transient electrochemical techniques. The successful isolations of the crystalline salts of the tetraanisylethylene cation radical (1+•) as well as the tetraanisylethylene dication (12+) allow X-ray diffraction analysis (for the first time) to quantify the serial changes in the molecular structure upon successive oxidations. Five structural parameters (d, l, θ, φ, and q) are identified as quantitative measures of changes in bond (CαCβ, Cαanisyl) lengths, dihedral (CαCβ)/torsional (anisyl) angles, and quinoidal (anisyl) distortion attendant upon the removal of first one-electron and then another electron from the tetraanisylethylene framework. The linear variation of all five parameters in Chart 3 point to a strongly coupled relaxation of tetraanisylethylene (involving simultaneous changes of d, l, θ, φ, and q) to a severely twisted dication. Most noteworthy is the structure of the cation radical 1+• with d, l, θ, φ, and q values that are exactly one-half those of the dication. The complex molecular changes accompanying the transformation: D → D+• → D2+ bear directly on the donor properties and the disproportionation processes of various tetraarylethylenes.
06/1998;
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ABSTRACT: A versatile method for the high-yield synthesis of various tris-, bis-, and mono-annelated benzenes (as well as cyclooctatetraene) is based on the Pd-catalyzed coupling of three (or four) ethylenic units comprised of α,β-dibromoalkenes and α‘-alkenyl Grignard reagentsall carried out in a single pot. The particular application to tris(bicyclopentyl)-annelated benzene yields the syn isomer 1s in high purity; X-ray diffraction analysis confirms the aromatic bond alternation relevant to the Mills−Nixon effect. Most importantly, the efficient synthesis of 1s crystals of extraordinary quality allows us (for the first time) to make precise electron-density measurements of the “banana-type” distortion and the ellipticity (π-character) of the various aromatic C−C bondssufficient to identify the electronic origin of the classical Mills−Nixon effect. The unique electron-donor properties of tris-annelated benzenes also relate to their highly reversible one-electron oxidation potentials even in nonpolar solvents.
06/1998;
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ABSTRACT: The encounter complex between photoexcited quinones Q* and various aromatic donors (ArH) is observed directly by time-resolved ps spectroscopy immediately before it undergoes electron transfer to the ion-radical pair [Q•-, ArH•+]. The encounter complex (EC) is spectrally characterized by distinctive (near IR) absorption bands, and its temporal evolution is established by quantitative kinetics analysis. The structural characterization of the 1:1 encounter complex [Q*, ArH] identifies the cofacial juxtaposition of the donor and acceptor moieties for optimal overlap of their π-orbitals. Further comparisons of the (excited-state) encounter complex with the corresponding (ground-state) EDA complex of aromatic donors and quinones establish its charge-transfer character, which directly relates to electron transfer within the encounter complex. The mechanistic significance of the encounter complex to bimolecular electron transfer is discussed (Scheme 1).
11/1997;
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ABSTRACT: Molecular association of various aromatic hydrocarbons (D, including sterically hindered donors) with a representative group of diverse acceptors (A = quinone, trinitrobenzene, tetracyanoethylene, tropylium, tetranitromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors. Spectral (UV−vis) analyses of the colored solutions reveal their charge-transfer origin (λCT), and they provide quantitative information of the intermolecular association in the form of the KDA and εCT values for the formation and visualization, respectively, of different [D,A] complexes. Importantly, such measurements establish charge-transfer absorption to be a sensitive analytical tool for evaluating the steric inhibition of donor−acceptor association. For example, the steric differences among various hindered aromatic donors in their association with quinone are readily dramatized in their distinctive charge-transfer (color) absorptions and verified by X-ray crystallography of the charge-transfer crystals and/or QUANTA molecular modeling calculations of optimum intermolecular separations allowed by van der Waals contacts.
10/1997;
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ABSTRACT: Sterically hindered olefins are efficiently reduced to the corresponding alkanes by the borane−methyl sulfide (BMS) complex at room temperature (or below) in dichloromethane containing a mild one-electron oxidant (such as an aromatic cation radical) or by the passage of an anodic current. In an alternative procedure, the hydrogenation of the same (electron-rich) olefins with the BMS complex (in the absence of a one-electron oxidant) is also carried out in the presence of a strong Brønsted acid (such as HBF4). In the oxidative activation, olefin cation radicals are the first observable intermediates, and separate experiments (including transient electrochemistry) confirm the facile reduction of the olefin cation radical by borane to produce the corresponding alkane. The direct protonation of olefins produces carbocationic intermediates which are also efficiently reduced by borane to the corresponding alkanes. The intermediacy of both olefin cation radicals and carbocations in the hydrogenation procedure with borane is discussed.
08/1996;
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Tetrahedron. 50(23):6727-6758.
