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Single-electron oxidants are the primary reagents for investigations of the new oxidants and the development of electron-accepting materials for application in optoelectronics. Quinones are the well-known class of the neutral single-electron oxidants. Here, we present the properties of the strongest neutral electron acceptor of this class tetracyanoquinone (cyanil) and investigate its electron-accepting strength by analyzing the charge-transfer complex formations with the aromatic donor molecules. Charge-transfer complexes of tetracyanoquinone with aromatic electron donors are characterized spectroscopically in solution and isolated as the single crystals. Copyright © 2015 John Wiley & Sons, Ltd.

The direct irradiation of the charge-transfer (CT) absorption band of the 1/1 electron donor-acceptor complexes of dimethoxybenzenes with tetranitromethane leads to aromatic nitration with a high quantum yield. The analogous methoxytoluenes under the same photochemical conditions are converted in equally high yields to products of aromatic trinitromethylation. This dichotomy with different methoxybenzenes (ArH) is discussed within the context of the common reactive intermediates derived from the CT excitation of the complex. The subsequent interactions of the geminate fragments, i.e., by radical and ion-pair annihilation represent a unifying mechanism for the CT photochemistry leading to aromatic nitration and trinitromethylation.

Tetranitromethane (TNM) reacts spontaneously with N-vinylcarbazole to afford the product of 1,2-addition. Under the same conditions, 9-vinylanthracene and other olefins do not react with TNM until they are deliberately stimulated by a short pulse of visible light. Photo-activation under these conditions is associated with the charge-transfer excitation of the olefin-TNM complex to produce NO2. The subsequent homolytic addition of NO2 to the olefinic donor, followed by electron transfer of the radical adduct, is responsible for the novel chain mechanism for TNM addition in Scheme 1. Homolytic initiation can also be achieved chemically with various additives such as the electron-rich ferrocene, N,N,N′,N′-tetramethyl-1,4-phenylenediamine, and 5H-dibenz[b,f]azepine which generate NO2 by dissociative electron attachment to TNM. The limited quanta of light and traces of chemical initiators indicate that the chain mechanism for the addition of TNM occurs with relatively long kinetic chain lengths. The mechanistic implications of the chain mechanism for the formation of various types of products is discussed in the context of olefin structures.

Potassium-mirror reduction of tetracyanoethylene (TCNE) acceptor in tetrahydrofuran affords K(THF)2 TCNE salt (1) showing double TCNE/K chains assembled via unusual μ3-TCNE-bridging of potassium cations. These parallel ladder-type chains are further tethered by pairs of THF bridges between potassium centers and by intermolecular π-bonding in (TCNE)22− dimers, and this results in formation of quasi-2-D coordination networks. In the presence of crown-ether ligand, the same potassium-mirror reduction lead to formation of [K(18-crown-6)(THF)2]TCNE salt (2) in which monomeric tetracyanoethylene anion-radicals are positioned between bulky [K+(18-crown-6)(THF)2] counter-ions. In comparison, crystallization of tetracyanoethylene anion-radicals with K+(18-crown-6) counter-ions in dichloromethane affords K(18-crown-6)TCNE salt (3) consisting of 1-D chains with 1,2-(N,N’)-TCNE bindings of potassium cations (nested in the crown-ether cavities). Temperature-dependent magnetic susceptibility study revealed essentially isolated tetracyanoethylene anion-radicals (S = 1/2) in this 1-D coordination polymer.

The facile reduction of tetracyanopyrazine (TCP) by tetrakis(dimethylamino)ethylene (TDAE) leads to black crystals of the donor/acceptor salt: TDAE2+·(TCP)22−·2CH3CN in which the vertical anionic TCP stacks consisting of distinct π-(TCP)22− units are surrounded by TDAE dications. The monomers within these supramolecular π-(TCP)22− complexes are arranged co-facially at interplanar separation of ∼3.17 A characteristic for ion-radical π-dimers and show unusual ∼30° rotation relative to each other. The structural scrutiny of the π-dimers together with the X-ray crystallographic analysis of the neutral tetracyanopyrazine acceptor, its charge-transfer complexes, and the monomeric anion-radical TCP−˙ reveals the unique “quasi-quinonoidal” distortion of the π-bonded tetracyanopyrazine moieties. In contrast to the molecular bending and intermolecular charge-transfer that are commonly observed in various ion-radical π-dimers and in conventional donor/acceptor complexes, the specific electron density redistribution and bond length alternation in π-bonded (planar) tetracyanopyrazine moieties (relative to the neutral molecule and isolated anion-radical) can only be accommodated by the bonding molecular orbital (HOMO) of the (TCP)22− dimer that is notably different from the corresponding SOMO of the monomeric TCP−˙.

Tetramethylpyrazine is effectively transformed via controlled N-protonation or N-methylation to afford a series of mono- and dicationic π-acceptors of particular use in the study of charge modulation during the self-assembly of various anionic donors and π-acid acceptors into ternary synthons and infinite linear chains (molecular wires). Thus, when the charges of the anionic donor (D) and the cationic acceptor (A) are unequal (either as 1:2 or as 2:1), the corresponding crystal structures of the donor/acceptor associates all contain structure motifs of discrete synthons consisting of (A···D···A) or (D···A···D) triads. On the other hand, when the anionic donor and cationic acceptor are of equal charge (either as 1:1 or as 2:2), the corresponding crystal structures all contain infinite chain arrangements: (···D···A···D···A···D···), which are the same as those we previously found in the cocrystallization of various anionic donors with neutral π-acids. Fully appreciated anion/π interactions are found in all o...

The self-assembly of various anions (e.g., thiocyanate, nitrite/nitrate, sulfite/sulfate, tetrahalometallates, etc.) with prototypical π-acids (consisting of cyano- and nitrosubstituted pyrazine and benzene, as well as tetracyanoethylene) occurs rapidly and selectively to yield a series of novel one-dimensional structures. The wire-like molecular chains all consist of parallel stacks of π-acids and alternate anions of different sizes and shapes that establish the dihedral angles α and φ sufficient to define these unique structures. Analogy of such linear arrays to nanoscopic wires is reinforced by the protective sheath of countercations that are completely arrayed around the linear cores. The critical feature of these efficient self-assemblies is shown to derive from anion/π-recognitions via charge-transfer forces between the electron-rich anions acting as electron donors and the electron-poor π-acids acting as electron acceptors to spontaneously generate synthons according to Mulliken theory.

The mechanistic conundrum is commonly posed by the intrinsic structural disconnect between a bimolecular (reactive) intermediate that is fleetingly detected spectroscopically in solution versus that rigorously defined by isolation and X-ray crystallography. We resolve this ambiguity by the combined experimental and theoretical application of the solvent media probe to the transient (1:1) precursor complex in the simplest chemical reaction involving direct adiabatic electron transfer (ET) among various donor/acceptor pairs. Of particular help in our resolution of such an important ET problem is the characterization of the bimolecular precursor complex as Robin-Day class II (localized) or class III (delocalized) from either the solvent-dependent or the solvent-independent response of the diagnostic intervalence absorption bands for the quantitative evaluation of the electronic coupling elements. The magnitudes of these intracomplex bindings are confirmed by theoretical (ab initio and DFT) computations that derive from X-ray structures and Marcus-Hush theories. Most importantly, the experimental solvent-induced ET barriers evaluated from the intervalence absorption bands are also quantitatively verified by the calculated outer-shell reorganization energies to establish unambiguously the intimate interconnection between the loosely bound bimolecular intermediate identified concurrently in solution and in the solid state.

Transient 1:1 precursor complexes for intermolecular self-exchange between various organic electron donors (D) and their paramagnetic cation radicals (D+*), as well as between different electron acceptors (A) paired with their anion radicals (A-*), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (pi-stacked) associates [D, D+*] and [A-*, A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA), which together with the Marcus reorganization energies (lambda) from the NIR spectral data are confirmed by molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents surrounding the redox centers. As a result, the steric modulation of the donor/acceptor separation (rDA) leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more open (unsubstituted) parents. The latter is discussed in the context of a continuous series of outer- and inner-sphere mechanisms for organic electron-transfer processes in a manner originally formulated by Taube and co-workers for inorganic (coordination) donor/acceptor dyads-with conciliatory attention paid to traditional organic versus inorganic concepts.

Tetrathiafulvalene (TTF) as the prototypical electron donor for solid-state (electronics) applications is converted to the unusual cation-radical salt, TTF+* CB- (where CB- is the non-coordinating closo-dodecamethylcarboranate), for crystallographic and spectral analyses. Near-IR studies establish the spontaneous self-association of TTF+* to form the diamagnetic [TTF+,TTF+] dication and to also undergo the equally rapid cross-association with its parent donor to form the mixed-valence [TTF+*,TTF] cation-radical. The latter, most importantly, represents the first (dyad) member of a series of p-doped tetrathiafulvalene (stacked) arrays, and the thorough scrutiny of its electronic structure with the aid of Mulliken-Hush (two-state) analysis of the diagnostic (intervalence) NIR band reveals Robin-Day Class II behavior. The theoretical consequences of the unique structure of the mixed-valence [TTF+*,TTF] dyad on (a) the electron-transfer mechanism for self-exchange, (b) the molecular-orbital analysis of the Marcus reorganization energy, and (c) the ab initio computation of the coupling element or transfer integral in p-doped (solid-state) arrays are discussed.

Tetrathiafulvalene (TTF) spontaneously forms a series of unusual charge-transfer complexes with various quinonoid acceptors such as o-chloranil (CA) that show pronounced near-IR absorption (lambda(CT) = 1100 nm). The successful isolation of the corresponding [1 : 1] donor-acceptor complex from solution and X-ray crystallographic analysis at low temperatures reveal the polarized charge-transfer state: [TTF(q+),CA(q-)] with high degree of charge-transfer (q = 0.6), which is spectrally and crystallographically distinguished from the separate redox (ion-pair) state: [TTF(+*) + CA(-*) ]. The unique interconversion of charge-transfer and electron-transfer states is theoretically well-accommodated by Mulliken theory using semi-empirical valence-bond and molecular-orbital methodologies. Mechanistic implications are discussed of both the thermally activated and the photochemically promoted processes via fast (intracomplex) electron transfer followed by collapse of the adiabatic and the non-adiabatic (vibrationally-excited) ion-pairs, respectively, to the [2 + 4] cycloadduct of tetrathiafulvalene and o-chloranil.

The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the core of the basic understanding of electron movement and delocalization that is possible within p-doped aromatic (face-to-face) arrays. To this end, we now utilize a series of different aryl-donor groups (Ar) around a central platform to precisely evaluate the intramolecular electron movement among these tethered redox centers. As such, the unique charge-resonance (intervalence) absorption bands observed upon the one-electron oxidation or p-doping of various hexaarylbenzenoid arrays (Ar6C6) provide quantitative measures of the reorganization energy (lambda) and the electronic coupling element (H(ab)) that are required for the evaluation of the activation barrier (deltaG(ET)) for electron-transfer self-exchange according to Marcus-Hush theory. The extensive search for viable redox centers is considerably aided by the application of a voltammetric criterion that has led in this study to Ar = N,N-dialkyl-p-anilinyl, in which exceptionally low barriers are shown to lie in the range deltaG(ET) = 0.3-0.7 kcal mol(-1) for very fast electron hopping or peregrination around the hexagonal circuit among six equivalent Ar sites. Therefore, at transition temperatures T(t)0.5/R or roughly -20 degrees C, the electron-transfer dynamics become essentially barrierless since the whizzing occurs beyond the continuum of states and effectively achieves complete pi-delocalization.

The paramagnetic [1:1] encounter complex ( TCNE ) 2 - is established as the important precursor in the kinetics and mechanism of electron-transfer for the self-exchange between tetracyanoethylene acceptor (TCNE) and its radical-anion as the donor. Spectroscopic observation of the dimeric complex ( TCNE ) 2 - by its intervalence absorption band at the solvent-dependent wavelength of λIV ∼ 1500 nm facilitates the application of Mulliken–Hush theory which reveals the significant electronic interaction extant between the pair of cofacial TCNE moieties with the sizable coupling of HDA = 1000 cm−1. The transient existence of such an encounter complex provides the critical link in the electron-transfer kinetics by lowering the classical Marcus reorganization barrier by the amount of HDA in this strongly adiabatic system. Ab initio quantum-mechanical methods as applied to independent theoretical computations of both the reorganization energy (λ) and the electronic coupling element (HDA) confirm the essential correctness of the Mulliken–Hush formalism for fast electron transfer via strongly coupled donor/acceptor encounter complexes.

Carbon tetrabromide and bromoform are employed as prototypical electron acceptors to demonstrate the charge-transfer nature of various intermolecular complexes with three different structural types of electron donors represented by (1) halide and pseudohalide anions, (2) aromatic (π-bonding) hydrocarbons, and (3) aromatics with (n-bonding) oxygen or nitrogen centers. UV–Vis spectroscopy identifies the electronic transition inherent to such [1:1] complexes; and their Mulliken correlation with the donor/acceptor strength verifies the relevant charge-transfer character. X-ray crystallography of CBr4/HCBr3 complexes with different types of donors establishes the principal structural features of halogen bonding. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:449–459, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20264

[reaction: see text] Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or sigma-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2*) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1*) leads to only the sigma-dimer. Further encumbrances of 1* with a pair of alpha, beta- or beta, beta- tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3* - 6*) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-dimer by its intense near-IR band. The different temperature-dependent spectral (ESR and UV-vis) behaviors of these phenalenyl radicals allow the quantitative evaluation of the bond enthalpy of 12 +/- 2 kcal mol(-1) for sigma-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.

Electronic-structure calculations for the self-association of phenalenyl radical (P*) predict the formation of dimeric species (sigma-P2) in which both moieties are connected by a sigma-bond with rP-P approximately 1.59 A and bond dissociation enthalpy of DeltaH(D) approximately 16 kcal mol(-1). Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approximately 34 kcal mol(-1) per phenalenyl moiety, largely owing to rehybridization. Ab initio calculations also reveal that the corresponding (one-electron) bond between phenalenyl radical and its closed-shell cation in sigma-P2+* is unstable relative to dissociation. Time-dependent DFT computations indicate the absence of any (strongly allowed) electronic transition in the visible region of the absorption spectrum of phenalenyl sigma-dimer. Such theoretical predictions are supported by experimental (ESR and UV-NIR) spectroscopic studies, in which the availability of a series of sterically hindered phenalenyl radicals allows definitive separations of the sigma-dimerization process from interference by pi-dimerization. As such, the thermodynamic parameters (determined from the temperature dependence of the ESR signals) with DeltaH(D) = 14 kcal mol(-1) and DeltaS(D) = 52 e.u. can be assigned to the formation of the colorless sigma-dimer. Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow sigma-bond formation) to confirm the energetic preference of sigma-dimerization over pi-dimerization.

Definitive X-ray structures of "separated" versus "contact" ion pairs, together with their spectral (UV-NIR, ESR) characterizations, provide the quantitative basis for evaluating the complex equilibria and intrinsic (self-exchange) electron-transfer rates for the potassium salts of p-dinitrobenzene radical anion (DNB(-)). Three principal types of ion pairs, K(L)(+)DNB(-), are designated as Classes S, M, and C via the specific ligation of K(+) with different macrocyclic polyether ligands (L). For Class S, the self-exchange rate constant for the separated ion pair (SIP) is essentially the same as that of the "free" anion, and we conclude that dinitrobenzenide reactivity is unaffected when the interionic distance in the separated ion pair is r(SIP)or =6 Angstroms. For Class M, the dynamic equilibrium between the contact ion pair (with r(CIP) = 2.7 Angstroms) and its separated ion pair is quantitatively evaluated, and the rather minor fraction of SIP is nonetheless the principal contributor to the overall electron-transfer kinetics. For Class C, the SIP rate is limited by the slow rate of CIP right arrow over left arrow SIP interconversion, and the self-exchange proceeds via the contact ion pair by default. Theoretically, the electron-transfer rate constant for the separated ion pair is well-accommodated by the Marcus/Sutin two-state formulation when the precursor in Scheme 2 is identified as the "separated" inner-sphere complex (IS(SIP)) of cofacial DNB(-)/DNB dyads. By contrast, the significantly slower rate of self-exchange via the contact ion pair requires an associative mechanism (Scheme 3) in which the electron-transfer rate is strongly governed by cationic mobility of K(L)(+) within the "contact" precursor complex (IS(CIP)) according to the kinetics in Scheme 4.

[reaction: see text] Reactive intermediates in the Friedel-Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV-vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies.

New carbocationic salts (via O-protonation of substituted benzoic acids) are prepared for the first time by controlled hydration of the corresponding benzoylium salts and isolated in pure crystalline form. Precise X-ray structural analyses reveal the rather unexpected (electronic) structure of the carboxylic-acidium functionality.

The high symmetry and stability of phenalenyl systems, both as the planar pi-radical (P*) and as the pi-cation (P+), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P* identifies its spontaneous self-association to the diamagnetic P2, previously characterized as the stacked pi-dimer by X-ray crystallography. Likewise, the rapid cross-association of P* with the closed-shell P+ leads to the stacked pi-dimer cation P2*+ with the "doubled" ESR spectrum diagnostic of complete (odd) electron delocalization. These pi-associations are confirmed by UV-vis studies that reveal diagnostic near-IR bands of both P2 and P2*+-strongly reminiscent of intermolecular charge-transfer absorptions in related aromatic (donor/acceptor) pi-associations. Ab initio molecular-orbital calculations for the pi-dimer P2 predict a binding energy of DeltaED = -11 kcal mol(-1), which is in accord with the experimental enthalpy change of DeltaHD = -9.5 kcal mol(-1) in dichloromethane solution. Most importantly, the calculations reproduce the intermonomer spacings and reveal the delicate interplay of attractive covalent and dispersion forces, balanced against the repulsions between filled orbitals. For comparison, the binding energy in the structurally related cationic pi-pimer P2*+ is calculated to be significantly larger with DeltaEP approximately -20 kcal mol(-1) (gas phase), owing to favorable electrostatic interactions not present in the neutral pi-dimer (which outweigh the partial loss of covalent interactions). As a result, our theoretical formulation can correctly account for the experimental enthalpy change in solution of DeltaHP = -6.5 kcal mol(-1) by the inclusion of differential ionic solvation in the formation of the pi-pimer.

The surprisingly uniform crystal packings of various arenediazonium salts (ArN2+X-) largely consist of two-dimensional ionic layers from various (N2+···X-) interactions coupled with a series of antiparallel arrays of pendant aryl groups (Ar). This crystalline architecture can be deliberately modified by the introduction of different (neutral) aromatic π-donors. On the basis of the exceptional electron-acceptor strength of the (N2+) group, the principal crystal-forming interactions of the arenediazonium tecton can be dissected into (a) strong coordination to anionic and electron-rich (neutral) σ-donors within the equatorial plane, (b) enhanced capacity for (C−H···X) hydrogen bonding, and (c) remarkable aptitude to form (noncovalent) charge-transfer bonds with aromatic π-donors. Thus, the detailed analysis of crystalline networks found in 3,5-dinitro and 4-carboethoxy derivatives of benzenediazonium salts recognizes them as remarkable intercalate and clathrate structures in which various charge-transfer int...

Unusual strength and directionality for the charge-transfer motif (established in solution) are shown to carry over into the solid state by the facile synthesis of a series of robust crystals of the [1:1] donor/acceptor complexes of carbon tetrabromide with the electron-rich halide anions (chloride, bromide, and iodide). X-ray crystallographic analyses identify the consistent formation of diamondoid networks, the dimensionality of which is dictated by the size of the tetraalkylammonium counterion. For the tetraethylammonium bromide/carbon tetrabromide dyad, the three-dimensional (diamondoid) network consists of donor (bromide) and acceptor (CBr(4)) nodes alternately populated to result in the effective annihilation of centers of symmetry in agreement with the sphaleroid structural subclass. Such inherently acentric networks exhibit intensive nonlinear optical properties in which the second harmonics generation in the extended charge-transfer system is augmented by the effective electronic (HOMO-LUMO) coupling between contiguous CBr(4)/halide centers.

Silver is an effective catalyst for the homo coupling of the Grignard reagent and alkyl halide when both alkyl groups are the same. Copper(I) catalyzes the cross coupling of Grignard reagents and alkyl bromides when carried out in tetrahydrofuran solutions at low temperatures. Facile vinylation of the Grignard reagent can be achieved with alkenyl bromides using an iron catalyst. Oxidation–reduction concepts are important in the formulation of these catalytic processes.

The monovalent cations of Na(+), K(+), Rb(+), and Cs(+) derived from the highly electropositive alkali metals represent prototypical charged spheres that are mainly subject to relatively simple electrostatic and solvation (hydration) forces. We now find that the largest of these Rb(+) and Cs(+) are involved in rather strong cation...pi(arene) interactions when they are suitably disposed with the ambifunctional hexasubstituted benzene C(6)E(6). The ether tentacles (E = methoxymethyl) allow these cations to effect eta(1)-bonding to the benzene center in a manner strongly reminiscent of the classical sigma-arene complexes with positively charged electrophiles where Z(+) = CH(3)(+), Br(+), Cl(+), Et(3)Si(+), etc. The somewhat smaller potassium cation is involved in a similar M(+)...pi(arene) interaction that leads to eta(2)-bonding with the aromatic center in the pi-mode previously defined in the well-known series of silver(I)/arene complexes. We can find no evidence for significant Na(+)... pi(arene) interaction under essentially the same conditions. As such, the sigma-structure of the Rb(+) and Cs(+) complexes and pi-structure of the K(+) complex are completely integrated into the continuum of sigma-pi bondings of various types of electrophilic (cationic) acceptors with arene donors that were initially identified by Mulliken as charge-transfer.

Intermolecular electron transfer from various arene donors (ArH) to the nitrosonium acceptor (NO+) proceeds via a series of unusual [1∶1] precursor complexes. Quantitative analysis of the accompanying pair of charge-transfer absorption bands (hνH and hνL) reveals the presence of strong electronic interactions between the donor/acceptor moieties in [ArH,NO+] complexes with the coupling elements lying in the range: Hab = 1.4 ± 0.5 eV. In the context of Sutin's development of Marcus–Hush theory, these dyads represent a Robin–Day Class III system for the 2-step transformation of the reactant diabatic state {ArH + NO+} to the final diabatic state {ArH+˙ + NO˙}, in which the unusually high values of Hab characterize a series of intermolecular (precursor) complexes that lie in a single (potential-energy) minimum. As such, the thermal electron transfer between the arene donor and the nitrosonium acceptor occurs first via the redistribution of a pair of electrons in the formation of the [ArH, NO]+ complex with Xmin = 1.0, and then followed by its homolytic dissociation to the {ArH+˙ and NO˙} products. This conclusion is experimentally confirmed by X-ray crystallographic and IR analyses of the precursor complexes that identify: (i) the strongly perturbed donor/acceptor moieties structurally akin to arene cation radicals (ArH+˙) and the reduced nitric oxide (NO˙), and (ii) the degree of charge transfer (Z) that is complete (100%). Optical electron transfer via the direct photoactivation of the precursor complex to the identical Franck–Condon state {ArH+˙, NO˙}* occurs independent of whether the high-energy (hνH) or the low-energy (hνL) absorption band is irradiated.

Successful crystallization and X-ray crystallographic analyses of the highly metastable (1∶1) complexes of bromine with benzene and toluene establish the unique (localized) structure B that differs in notable ways from the long-accepted (delocalized) structure A. Furthermore, we demonstrate the (highly structured) charge-transfer complexes [C6H6,Br2] and [CH3C6H5,Br2] to be the pre-reactive intermediates that are converted (via an overall Br+ transfer) to the Wheland intermediates in electrophilic aromatic bromination. The role of the dative ion pairs [C6H6˙+ Br2˙−] and [CH3C6H5˙+ Br2˙−] in the rate-limiting activation processes is underscored.

Intramolecular electron transfer in the organic mixed-valence cation radical D(ph)nD+• [where D = 2,5-dimethoxy-4-methylphenyl and (ph)n = poly-p-phenylene] is systematically probed by the structur...

Spontaneous formation of colored (1:1) complexes of various aromatic donors (ArH) with the nitrosonium acceptor (NO+) is accompanied by the appearance of two new (charge-transfer) absorption bands in the UV-vis spectrum. IR spectral and X-ray crystallographic analyses of the [ArH,NO+] complexes reveal their inner-sphere character by the ArH/NO+ separation that is substantially less than the van der Waals contact and by the significant enlargement of the aromatic chromophore. The reversible interchange between such an inner-sphere complex [ArH,NO+] and the redox product (ArH+.+ NO.) is quantitatively assessed for the first time to establish it as the critical intermediate in the overall electron-transfer process. Theoretical formulation of the NO+ binding to ArH is examined by LCAO-MO methodology sufficient to allow the unambiguous assignment of the pair of diagnostic (UV-vis) spectral bands. The MO treatment also provides quantitative insight into the high degree of charge-transfer extant in these inner-sphere complexes as a function of the HOMO-LUMO gap for the donor/acceptor pair. The relative stabilization of [ArH,NO+] is traced directly to the variation in the electronic coupling element H(AB), which is found to be substantially larger than the reorganization energy (lambda/2). In Sutin's development of Marcus-Hush theory, this inequality characterizes a completely delocalized Class III complex (which occupies a single potential well) according to the Robin-Day classification. The mechanistic relevance of such an unusual (precursor) complex to the inner-sphere mechanism for organic electron transfer is discussed.

To study the mechanism of electronic transduction along (poly)phenylene chains, a series of aromatic donors with general formula D–B–D has been synthesized [where D = 2,5-dimethoxy-4-methylphenyl donor and B = (poly)phenylene bridge]; and the corresponding cation–radical salts D–B–D+˙ SbCl6− have been isolated for X-ray crystallographic analyses. The magnitude of the electronic interaction between the D and D+˙ moieties through the various B bridges has been measured (i) as the difference between the first and the second oxidation potentials of D–B–D donors and (ii) as the structural changes induced in neutral D by the presence of the tethered D+˙ group in D–B–D+˙ cation–radicals. The intramolecular interaction of D and D+˙ groups was found to occur via π-conjugation of the bridging (poly)phenylene group. As such, the electronic interaction is highly dependent on the planarity of the (poly)phenylene bridge, and can be either inhibited or promoted by the deliberate modifications of the molecular conformation. Crystal structures of compounds A, B, B+˙+˙, 1, 1+˙, 2, 2+˙, 3+˙, 8 and 9+˙ are reported.

Aromatic donors (D) spontaneously form a series of colored intermolecular charge-resonance (CR) complexes with their cation radicals (D+˙) as well as charge-transfer (CT) complexes with various π-acceptors (A). Spectroscopic changes during the formation of these CR and CT complexes are both included in a single Mulliken plot of ECR,CTversus E°ox − E°red. Precise X-ray crystallographic studies now reveal almost identical structural features of the CR and CT complexes, especially with a prototypical strong electron donor such as octamethylbiphenylene (OMB). Whereas most weaker donors do not show perceptible alterations of their molecular geometry during complex formation, OMB exhibits pronounced bond elongation/contraction of almost 2 pm both in its CT complexes with strong acceptors (TCNE and TCNQ) and in its CR complex (OMB2)+˙. In both cases, the structural changes lie midway between geometries of the neutral OMB donor and its fully oxidized cation radical OMB+˙. The CT and CR complexes of OMB in the solid state also show similar crystal packing features that maintain interplanar separations as short as 3.1–3.2 A with significant distortion (∼6°) from planar molecular geometry. Together with the similar values of KCR = 350 M−1 and KEDA = 440–490 M−1, we acknowledge the common nature of the CT and CR complexes through an orbital consideration (Chart 1) that recognizes the antibonding component in CR complex formation.

Arene ligands experience significant ring expansion upon coordination with chromium tricarbonyl, as established by precise X-ray crystallographic analyses of various (η6-arene)Cr(CO)3 complexes. Su...

Spherical polyoxometalates (POMs) such as M6O192- and SiM12O404- (with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic “anchor” onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel charge-transfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19]2- and Keggin-type [SiM12O40]4- acceptors with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and π-dimeric forms). X-ray crystallographic studies reveal a unique “dimeric” arrangement of the cofacially oriented arene couples which show contact ...

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).

The Wheland intermediate in electrophilic aromatic nitrosation, viz. the nitrosoarenium σ-complex, is directly observed by transient absorption spectroscopy. Femtosecond time-resolved laser experiments based on charge-transfer photoexcitation of electron donor/acceptor (EDA) complexes of nitrosonium cation with various arenes reveal the ultrafast formation of nitrosobenzenium to occur in less than 10 ps via the radical/radical coupling of arene cation radicals and nitric oxide. The lifetimes of the σ-complexes in dichloromethane solution are strongly temperature dependentvarying from nanoseconds (T = 298 K) to microseconds (T = 195 K). Steady-state photolysis of arene/NO+ complexes in n-BuCl glasses at T = 77 K leads to nitrosoarenium σ-complexes which persist for several hours. Based on a reaction scheme that includes an ultrafast equilibrium between the [ArH+•,NO•] radical pair and the nitrosoarenium σ-complex, energy diagrams are constructed which establish the highly endergonic reaction profile of ele...

X-ray crystallographic structures of donor–acceptor complexes of aromatic hydrocarbons with transition metals are re-examined with the focus on the arene ligands. Thus, significant structural and electronic changes are revealed in the arene moiety due to coordination to the metal center including: (i) expansion of the aromatic six-carbon ring; (ii) endocyclic π-bond localization; (iii) distortion of the planarity (folding) of the arene ring; and (iv) shortening of the metalarene bond distances. All structural features are characteristic of metal–arene (π- or σ-) complexes that exhibit various degrees of (metal-to-ligand) charge transfer. The concept of charge-transfer bonding not only explains the structural details but also the various facets of chemical reactivity of metal-coordinated arenes including efficient carbonhydrogen bond activation and nucleophilic–electrophilic umpolung, both of which are critical factors in homogeneous metal catalysis.

Isolation of the unstable 1∶1 complex of 4-nitrosoanisole with NO+PF6− allows its precise X-ray structural characterization. The charge-transfer crystal is formed via strong N⋯N coordination [the distance of 1.938(5) A corresponding to a σ-bond order of ≈0.2] in the mean plane of the planar 4-nitrosoanisole donor. Thorough analysis of its molecular geometry in terms of valence resonance and MO schemes reveals a strong charge polarization with a local negative charge localized on the nitroso group and a local positive charge distributed over the adjacent p-methoxybenzyl moiety. Such a charge distribution accommodates the well-known passivation of nitrosoarenes to multiple nitrosation and explains the ease of demethylation of the complex. Comparison of a variety of nitroso- and nitroarene structures has shown that the nitrosoarene experiences a much stronger quinoidal distortion of the aromatic ring as compared with the latter. This indicates a stronger electron-withdrawing effect of the nitroso group relative to that of the nitro group. The weakened aromatic resonance in the nitrosoarenes could be responsible for the observed slower rate and the measurable isotope effect in electrophilic nitrosation as opposed to nitration.

Various electron-rich aromatic and olefinic donors (D) are readily converted to their cation radicals (D+˙) in the presence of strong protic acids, even in nonpolar solvents such as dichloromethane. By using the three prototypical organic donors octamethylbiphenylene (OMB), adamantylideneadamantane (AA) and the isomeric fused homoadamantene (FHA), we identify the protonated donor (D-H+) as the effective oxidant for electron transfer from the organic donor (i.e., D-H+ + D → D-H˙ + D+˙). The subsequent reduction of (D-H˙) to the dihydro product D-H2 is demonstrated by the isolation of 2,2′-biadamantane, the structure of which is established by X-ray crystallography.

The conformational preference of vicinal or 1,2-phenyl groups is probed in two classes of ring-substituted 1,2-diphenylbicyclooctene (stilbenoid) hydrocarbons 1a-1d and 2a-2c. UV-vis spectroscopy reveals, and X-ray crystallography verifies, the intramolecular (edge-to-face) orientation for the phenyl-phenyl interaction in stilbenoids 1a-1d. Most importantly, when two pairs of ortho-methyl substituents are present, the cofacial phenyl groups in the stilbenoid donors are established by X-ray crystallography and spectrally observed in the cation radicals (2a+.-2c+.) by the appearance of new bands with strong absorptions in the near IR with λmax = 1100-1315 nm, analogous to those previously observed in intermolecular (aromatic) interactions of aromatic cation radicals.Key words: stilbenoid hydrocarbon, cation radical, aryl-aryl interaction.

Substituted carbonylmanganese cations [Mn(CO) 5 L] + , where L=py, PPh 3 and PPh 2 Me, readily react with various organoborate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO) 5 (CH 3 CN)] + reacts with the same borate anions to afford the alkylated RMn(CO) 5 , where R=CH 3 and C 6 H 5 , as the sole carbonylmanganese product. As such, this alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the borate nucleophile. The two different pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity of the carbonylmanganese cation.

Dimethylaurate(I) has been prepared as the crystalline tetrabutylammonium salt for comparison with the known tetramethylaurate(III) analogue. The linear structure of dimethylaurate(I) and the square-planar structure of tetramethylaurate(III) have both been confirmed by X-ray crystallography. One-electron oxidation of dimethylaurate(I) by either ferrocenium or arenediazonium cations produces the metastable dimethylgold(II) intermediate, which can be trapped as the paramagnetic 9,10-phenanthrenequinone (PQ) adduct. Otherwise, dimethylgold(II) is subject to rapid reductive elimination of ethane and affords metallic gold (mirror). The analogous oxidation of tetramethylaurate(III) by ferrocenium, arenediazonium, or nitrosonium cations also proceeds via electron transfer to generate the putative tetramethylgold(IV) intermediate. The highly unstable (CH3)4AuIV spontaneously undergoes homolytic cleavage to produce methyl radical and the coordinately unsaturated trimethylgold(III), which can be intercepted by adde...

The photochemical coupling of various stilbenes (S) and chloranil (Q) is effected by the specific charge-transfer (CT) activation of the precursor electron donor−acceptor (EDA) complex [S, Q], and the [2+2] cycloaddition is established by X-ray structure elucidation of the crystalline trans-oxetanes formed selectively in high yields. Time-resolved (fs/ps) spectroscopy reveals the (singlet) ion-radical pair 1[S•+, Q•-] to be the primary reaction intermediate and thus unambiguously establishes for the first time the electron-transfer pathway for this typical Paterno−Buchi transformation. The alternative cycloaddition via the specific activation of the carbonyl component (as a commonly applied procedure in Paterno−Buchi couplings) leads to the same oxetane regioisomers in identical molar ratios. As such, we conclude that a common electron-transfer mechanism applies via the quenching of the photoactivated quinone acceptor by the stilbene donor to afford triplet ion-radical pairs 3[S•+, Q•-] which appear on th...

Photoexcited quinones (Q*) are efficiently quenched by polymethylbenzenes (ArH) via electron transfer (ET). However, the second-order rate constants (k2) exhibit Rehm−Weller (outer-sphere) dependence on the free energy (ΔGET), despite our new findings that the quenching occurs via a series of rather strong encounter complexes [Q*, ArH] with substantial (charge-transfer) bonding. The relatively high formation constants (KEC) of the encounter complexes indicate that any mechanistic interpretation of the driving-force dependence of the observed rate constants is highly ambiguous since k2 must be a composite of KEC and the intrinsic rate constant (kET) for electron transfer within the intermediate (inner-sphere) complex. As such, the reorganization energies extracted from Rehm−Weller plots lack thermodynamic significance. On the other hand, the unambiguous driving-force dependence of kET represents a unique example for the “normal” Marcus behavior of the endergonic electron transfer between the donor/acceptor...

The various aromatic hydrocarbons (Chart 2) constitute a sharply graded series of sterically encumbered (unhindered, partially hindered, and heavily hindered) donors in electron transfer (ET) to quinones (Chart 1). As such, steric effects provide the quantitative basis to modulate (and differentiate) outer-sphere and inner-sphere pathways provided by matched pairs of hindered and unhindered donors with otherwise identical electron-transfer properties. Thus the hindered donors are characterized by (a) bimolecular rate constants (k2) that are temperature dependent and well correlated by Marcus theory, (b) no evidence for the formation of (discrete) encounter complexes, (c) high dependency on solvent polarity, and (d) enhanced sensitivity to kinetic salt effectsall diagnostic of outer-sphere electron-transfer mechanisms. Contrastingly, the analogous unhindered donors are characterized by (a) temperature-independent rate constants (k2) that are 102 times faster and rather poorly correlated by Marcus theory, (...

Oxetanes are produced in the Paterno–Buchi cycloaddition of stilbene (S) to quinone (Q) via an efficient photoinduced electron transfer. Kinetics analysis of the time-resolved absorption spectra over three distinctive (ps, ns, µs) timescales establishes the coupling (kC) of the initially formed ion-radical pair 3[S+˙, Q–˙] to the 1,4-biradical ˙SQ˙ as the critical step toward oxetane formation. The (rather slow) rate constant of kC ≤ 107 s–1 in acetonitrile must compete with other faster decay pathways of the ion pair involving ionic separation, ion exchange (with added salt) and back electron transfer. As such, solvent polarity and donicity as well as added salts play an unusually prominent role in modulating the ion-pair microdynamics. Donor–acceptor complexation of the photoexcited quinone with the solvent and cis→trans isomerization of (Z)-stilbene must also be considered in the overall photodynamics of electron transfer.