Charge distribution within hypercarbon-halogenated 1-Ph-2-X-1,2-dicarba-closo-dodecaboranes, (X = F, Cl, Br, I): A dipole moment and computational study
Introduction
The o-, m- and p-carboranes (1,2-, 1,7- and 1,12-dicarba-closo-dodecaboranes, 1,2-, 1,7-, 1,12-closo-C2B10H10) represent relatively large, very symmetrical and remarkably stable molecules. Such high stabilities of these icosahedral systems have led to their use in such diverse fields as medicine (neutron capture therapy [1], molecular scaffolding (rigid building units [2]) and material science [3]), m-carboranes (1,7-) attracting much less attention in contrast to o- and p-carboranes. This is also valid in terms of researching these distorted icosahedra with an aim at their potential applications in nanotechnology [4].
Interest in capacity of these carboranes, regarded as three-dimensional aromatic systems [5], to communicate electronically with substituents resulted in our interest in acquiring data that describe both charge distribution within them and their ability to serve as models for simple electronic devices on the molecular level: the dipole moments and computational study of the p-carborane system with various substituents bonded to both carbon ends of the icosahedron [6] has revealed that the p-carborane cage behaves as a moderate conduit for electronic effects. The direction of the dipole moment in o-carborane [7], 1, was unambiguously confirmed by a well-established graphical method [8], based on geometrical solution of triangles, on a series of 1-(4-substituted-phenyl)-o-carboranes with the orientation of the positive end of the dipole towards the carbon atoms. As a consequence, the dipole moment vector of 1-Ph-o-carborane, 2a, lies at an angle of 28° with respect to the exohedral C–Car bond, i.e. practically in the symmetry axis of the carborane structure (an electron diffraction study of 2a yielded this angle to be 31° [9]).
The knowledge of the dipole moment value of o-carborane cage and its direction helps us to understand electronic interactions between substituents on carbon atoms and the o-carborane cage. Halogens are relatively easily bonded to the o-carborane carbons experimentally and fluorine was found to act as a weaker π-electron donor in 1-Ph-2-X-1,2-closo-C2B10H10 (2b, Fig. 1a) as revealed by NMR and computational data [10]. Interaction of halogens with benzene has been computed elsewhere [11] and it would therefore be interesting to know how such an interaction proceeds if halogen and benzene ring are attached to the two neighbouring cage carbons of o-carborane. Halogens are also prone to the so-called “halogen bonding” in terms of σ-hole (bonding towards nucleophiles with a region of positive electrostatic potential on the outermost portion of the surfaces of halogens [12]).
In order to understand this phenomenon further, we prepared a series of 1-Ph-2-X-1,2-closo-C2B10H10 (X = F, Cl, Br, I, 2b, 2c, 2d, 2e, respectively, Fig. 1a–d) and measured their dipole moments for quantifying the corresponding electron distribution. We also carried out theoretical calculations of these systems.
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Syntheses
Compounds 2b, 2c, 2d, and 2e were prepared by allowing to halogenate phenyl-ortho-carborane or lithiated 2a with N-fluorobenzenesulfonimide/benzene, Cl2/P4O10, Br2/toluene, and I2/tetrahydrofuran, respectively, according to known or slightly modified literature procedures [13]. The purity of all compounds was checked by analytical TLC and the structures of the molecules were confirmed on the basis of 1H{11B} and 11B{1H} NMR spectra recorded on a Varian Unity-500 instrument in CDCl3 solution.
Results and discussion
The electron deficiency of the o-carborane cage brings about its behavior both towards alkyl and aryl groups: the dipole moments are: o-carborane, 4.50 D [7], 1-methyl-o-carborane, 4.75 D [23], and 1-phenyl-o-carborane, 4.93 D [7]. From these values the carborane acts as an electron acceptor. The direction of the dipole in the latter was experimentally determined and, in essence of this approach, coincides within experimental error with that in o-carborane [7] and the same direction may be
Acknowledgements
Financial support from the Ministry of Education of the Czech Republic (Project LC523) is greatly appreciated. We thank Dr. M.A. Fox (University of Durham, UK) for providing MP2 geometries and helpful comments and Dr. Jan Macháček of the Institute of Inorganic Chemistry of the ASCR, v.v.i for technical assistance.
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