Pavel A. Gunchenko

Overcoming lability of extremely long alkane carbon-carbon bonds through dispersion forces

Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes and the possibility of constructing hydrocarbons with extraordinarily long C–C single bonds through steric crowding. Although empirical bond distance/bond strength relationships have been established for C–C bonds (longer C–C bonds have smaller bond dissociation energies), these have no present theoretical basis. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry, as summarized by Pauling as early as 1960 and confirmed more recently. Here we report the preparation of hydrocarbons with extremely long C–C bonds (up to 1.704 Å), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with and without inclusion of dispersion corrections.

Stable Alkanes Containing Very Long Carbon–Carbon Bonds

The metal-induced coupling of tertiary diamondoid bromides gave highly sterically congested hydrocarbon (hetero)dimers with exceptionally long central C-C bonds of up to 1.71 Å in 2-(1-diamantyl)[121]tetramantane. Yet, these dimers are thermally very stable even at temperatures above 200 °C, which is not in line with common C-C bond length versus bond strengths correlations. We suggest that the extraordinary stabilization arises from numerous intramolecular van der Waals attractions between the neighboring H-terminated diamond-like surfaces. The C-C bond rotational dynamics of 1-(1-adamantyl)diamantane, 1-(1-diamantyl)diamantane, 2-(1-adamantyl)triamantane, 2-(1-diamantyl)triamantane, and 2-(1-diamantyl)[121]tetramantane were studied through variable-temperature (1)H- and (13)C NMR spectroscopies. The shapes of the inward (endo) CH surfaces determine the dynamic behavior, changing the central C-C bond rotation barriers from 7 to 33 kcal mol(-1). We probe the ability of popular density functional theory (DFT) approaches (including BLYP, B3LYP, B98, B3LYP-Dn, B97D, B3PW91, BHandHLYP, B3P86, PBE1PBE, wB97XD, and M06-2X) with 6-31G(d,p) and cc-pVDZ basis sets to describe such an unusual bonding situation. Only functionals accounting for dispersion are able to reproduce the experimental geometries, while most DFT functionals are able to reproduce the experimental rotational barriers due to error cancellations. Computations on larger diamondoids reveal that the interplay between the shapes and the sizes of the CH surfaces may even allow the preparation of open-shell alkyl radical dimers (and possibly polymers) that are strongly held together exclusively by dispersion forces.

Oxygen-Doped Nanodiamonds: Synthesis and Functionalizations†

Oxadiamondoids representing a new class of carbon nanoparticles were prepared from the respective diamondoid ketones via an effective two-step procedure involving addition of methyl magnesium iodide and oxidation with trifluoroperacetic acid in trifluoroacetic acid. The reactivities of the oxacages are determined by the position of the dopant and are in good agreement with computational predictions.

Selective Radical Reactions in Multiphase Systems: Phase-Transfer Halogenations of Alkanes

The present paper shows that selective radical reactions can be initiated and carried out in multiphase systems. This concept is applied to the selective functionalization of unactivated aliphatic hydrocarbons, which may be linear, branched, and (poly)cyclic, strained as well as unstrained. The phase-transfer system avoids overfunctionalization of the products and simplifies the workup; the selectivities are excellent and the yields are good. This is the only method for direct preparative iodination of alkanes applicable to large scale as well. We demonstrate that the reaction systems are indeed phase-transfer catalyzed through a systematic study of variations of the reactants, solvents, catalysts, and by measuring as well as computing the H/D kinetic isotope effects for the rate-limiting C-H abstraction step by *CHal3 radicals which are held responsible for the observed radical reactions. In the case of *CBr3, this key intermediate could also be trapped under otherwise very similar reaction conditions. To stimulate further work, the tolerance of some functional groups was tested as well.

Hydrocarbon Activation with Cerium(IV) Ammonium Nitrate: Free Radical versus Oxidative Pathways

The reactivity of propellane C−C bonds towards cerium(IV) ammonium nitrate (CAN) was studied utilizing photochemical initiation in acetonitrile. Synthetic as well as computational (B3LYP/6-311+G** and MP2/6-31G*) data strongly suggest that the activation of the C−C bonds in cyclopropane and cyclobutane derivatives involves NO3 radical attack on the hydrocarbon ring and does not proceed through single electron transfer. The product structures are not consistent with the intermediate formation of propellane radical cations. These propellane systems could be generated independently by oxidation with charged electrophiles, by anodic oxidation, and through photo-oxidation with 1,2,4,5-tetracyanobenzene; the observed chemical behavior of radical cations is clearly different.

Exploring covalently bonded diamondoid particles with valence photoelectron spectroscopy

We investigated the valence electronic structure of diamondoid particles in the gas phase, utilizing valence photoelectron spectroscopy. The samples were singly or doubly covalently bonded dimers or trimers of the lower diamondoids. Both the bond type and the combination of bonding partners are shown to affect the overall electronic structure. For singly bonded particles, we observe a small impact of the bond on the electronic structure, whereas for doubly bonded particles, the connecting bond determines the electronic structure of the highest occupied orbitals. In the singly bonded particles a superposition of the bonding partner orbitals determines the overall electronic structure. The experimental findings are supported by density functional theory computations at the M06-2X/cc-pVDZ level of theory.

The Rearrangement of the Cubane Radical Cation in Solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.

Common Radical Cation Intermediates in Cage Hydrocarbon Activations

The oxidation of 3,6-dehydrohomoadamantane (1) was achieved under chemical (NO+BF4–/EtOAc, NO+OAc–/Ac2O, and NO+BF4–/CH3CN), photochemical (photoexcited 1,2,4,5-tetracyanobenzene), and electrochemical (Pt anode, CH3CN, NH4BF4) conditions. Supporting ab initio [density functional theory (BLYP) and Moller–Plesset perturbation theory (MP2)] computations utilizing standard basis sets, 6–31G* (optimizations) and 6–311+G* (single-point energy evaluations), agree with the experimental results implicating the involvement of the same radical cation intermediates in the activation processes. Isomeric radical cations formed from different precursors can equilibrate with low barriers (2.0–11.7 kcal mol–1) and lead to common products. The computed and experimental adiabatic ionization potential of adamantane shows that activation with NO+BF4– is also likely to occur through the adamantyl radical cation. Hence, the bonds need not be attacked directly by the electrophile in the C–H or C–C activation of alkanes with relatively low ionization potentials.

Comparative theoretical and experimental analysis of hydrocarbon σ-radical cations

The structures of σ-radical cations formed by ionization of adamantane, twistane, noradamantane, cubane, 2,4-dehydroadamantane, and protoadamantane were optimized at the B3LYP, B3LYP-D, M06-2X, B3PW91, and MP2 levels of theory using 6-31G(d), 6-311+G(d,p), 6-311+G(3df,2p), cc-PVDZ, and cc-PVTZ basis sets. On the whole, single-configuration approximations consistently describe the structure and transformations of the examined σ-radical cations. The best correlations (r = 0.97–0.98) between the calculated adiabatic ionization potentials and experimental oxidation (anodic) potentials of hydrocarbons were obtained in terms of B3PW91 approximation.

The Protoadamantane Radical Cation

DFT (B3LYP) and MP2 computations with a 6-31G* basis set show that a unique protoadamantane radical cation (1·+) structure with an elongated, half-broken C6−H bond prevails both in the gas phase and in solution. This is in agreement with the observed regioselectivity of the single electron-transfer oxidation of protoadamantane (1) with photoexcited 1,2,4,5-tetracyanobenzene, which only gives 5-(6-protoadamantyl)-1,2,4-tricyanobenzene (5), for which the X-ray crystal structure is reported. The regioselectivities for the functionalizations of 1 with electrophiles support this oxidation pathway. The H-coupled electron-transfer mechanism recently proposed for the C−H activation of alkanes with electrophilic oxidizers explains the high C6 positional selectivities in the functionalization of 1 in electrophilic media. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)