Article

Divalent carbon atom as the proton acceptor in hydrogen bonding.

Department of Quantum Chemistry, Faculty of Chemistry, Nicolaus Copernicus University, Gagarina 7, 87-100, Toruń, Poland.
Physical Chemistry Chemical Physics (Impact Factor: 4.2). 07/2009; 11(27):5711-9. DOI: 10.1039/B901968E
Source: PubMed

ABSTRACT Proton-accepting properties of the divalent carbon atom in carbodiphosphoranes and their simple derivatives as well as in carbenes have been investigated. Both these groups of chemical compounds may be characterized by the formula CL2, where L is a sigma electron donor. Therefore, the carbon atom within both these systems, being in its atomic state, can have one or two lone electron pairs and, as a result, it may form hydrogen bonds of the type D-H...CL2, where C acts as a proton acceptor. Complexes of C(NH3)2, C(PH3)2, C[P(CH3)3]2, CF2, CCl2, and imidazol-2-ylidene with such proton donors as H2O, HCF3, HCN and HCCH have been analyzed by means of high-level quantum chemical methods. Density functional theory (DFT) and second-order Møller-Plesset (MP2) approaches have been applied in conjunction with the aug-cc-pVTZ basis set. The electron density distribution calculated by means of the atoms in the molecules procedure has also been analyzed. Proton-accepting properties of the carbon atom are discussed in detail. It is shown that the divalent carbon atom in the group of chemical systems investigated should be treated as a normal proton acceptor, similar to the much more electronegative O or N atoms. Moreover, hydrogen bonds of the type D-H...CL2 within the complexes investigated have been found to be rather strong. The highest proton accepting ability of the carbon(0) atom found for the (NH3)2C derivative of carbodiphosphorane is explained on the basis of the Leffler-Hammond postulate. Within the group of carbenes, the strongest hydrogen bonds are formed by imidazol-2-ylidene. This is attributed to the significant aromatic character of the imidazol-2-ylidene ring that increases the proton-accepting properties of the carbene carbon atom.

0 Bookmarks
 · 
68 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh(3))(2), C(NHC(Me))(2), R(2) C=C=CR(2) (R = H, F, NMe(2)), C(3)O(2), C(CN)(2)(-) and N-methyl-substituted N-heterocyclic carbene (NHC(Me)). The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two AuCl moieties have been optimized and the strength and nature of the metal-ligand interactions in the mono- and diaurated complexes were investigated by means of energy decomposition analysis. The goal of the study is to elucidate the differences in the chemical behavior between carbones, allenes and carbenes. The results show that carbones bind one and two AuCl species in η(1) fashion, whereas allenes bind them in η(2) fashion. Compounds with latent divalent carbon(0) character can coordinate in more than one way, with the dominant mode indicating the degree of carbone or allene character. The calculated structures of the mono- and diaurated tetraaminoallenes (TAAs) reveal that TAAs exhibit a chameleon-like behavior: The bonding situation in the equilibrium structure is best described as allene [(R(2)N)(2)]C=C=C[(NR(2))(2)] in which the central carbon atom is a tetravalent C(IV) species, but the reactivity suggests that TAAs should be considered as divalent C(0) compounds C{C[(NR(2))(2)]}(2), that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η(1) mode, whereas the equilibrium structures of the η(1)- and η(2)-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN)(2)(2-) binds one and two AuCl very strongly in characteristic η(1) fashion. The N-heterocyclic carbene complex, [NHC(Me)(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHC(Me)(AuCl)(2)], has two η(1)-bonded AuCl moieties that exhibit aurophilic attraction, which yield a moderate bond strength that might be large enough for synthesizing the complex. The BDE for the second AuCl in [NHC(Me)(AuCl)(2)] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes.
    Chemistry 07/2011; 17(36):9944-56. · 5.93 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we report a novel synthesis of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H). An X-ray crystal structure and full characterization of the compound are included. Compared to hydrated TEMPO-H, its anhydrous form exhibits improved stability and a differing chemical reactivity. The reactions of anhydrous TEMPO-H with a variety of low-valent carbon centres are described. For example, anhydrous TEMPO-H was reacted with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), an unsaturated NHC. Crystals of [CHNC(6)H(2)(CH(3))(3)](2)C···HO(NC(5)H(6)(CH(3))(4)), IMes···TEMPO-H, were isolated and a crystal structure determined. The experimental structure is compared to the results of theoretical calculations on the hydrogen-bonded dimer. Anhydrous TEMPO-H was also reacted with the saturated NHC, 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), giving the product [CH(2)Ni-Pr(2)C(6)H(3)](2)CH···O(NC(5)H(6)(CH(3))(4)). In contrast, the reaction of hydrated TEMPO-H with 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene gave small amounts of the hydrolysis product, N-(2,6-diisopropylphenyl)-N-[2-(2,6-diisopropylphenylamino)ethyl]formamide. Finally, anhydrous TEMPO-H was reacted with (triphenylphosphoranylidene)ketene to generate Ph(3)PC(H)C(=O)O(NC(5)H(6)(CH(3))(4)). A full characterization of the product, including an X-ray crystal structure, is described.
    Organic & Biomolecular Chemistry 04/2011; 9(10):3672-80. · 3.57 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The present review is focused on the application of computational/theoretical methods to the wide and rich chemistry of allenes. Special emphasis is made on the interplay and synergy between experimental and computational methodologies, rather than on recent developments in methods and algorithms. Therefore, this review covers the state-of-the-art applications of computational chemistry to understand and rationalize the bonding situation and vast reactivity of allenes. Thus, the contents of this review span from the most fundamental studies on the equilibrium structure and chirality of allenes to recent advances in the study of complex reaction mechanisms involving allene derivatives in organic and organometallic chemistry.
    Chemical Society Reviews 02/2014; · 24.89 Impact Factor