Planar tetrakoordinierter Kohlenstoff in einem neutralen gesättigten Kohlenwasserstoff: theoretischer Entwurf und Charakterisierung

B-Heterocyclic Carbene Arising from Charge Shift: A Computational Verification

1-Borabicyclo[1.1.0]but-2(3)-ene (1BB) is a singlet biradical with two single electrons that can form an ionic resonance structure through a charge shift. The ionic resonance structure is a B-heterocyclic carbene (BHC), which can act as a carbene, Lewis base, or L- and Z-type ligand, to give adducts and complexes. Through a range of quantum methods, four types of stable compounds (A-D) derived from 1BB have been designed. These compounds retain the unique features of 1BB. As a consequence, the structures, stability, and Wiberg bond indices of the Lewis adducts of A-D with Lewis acids (BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , and C₁₃ BH₉ ) and Cu^I , Ag^I , and Au^I complexes have been investigated. Results show that A-D can indeed react as carbenes. Interestingly, compounds A-D, as L-type ligands, can attach to BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , C₁₃ BH₉ , and CuCl and form compounds with planar tetracoordinate carbon (ptC), whereas Z-type ligands A-D can bind to AgCl and AuCl to provide complexes with planar tetracoordinate boron (ptB). In addition, the binuclear complexes of ClX(1BB)CuCl (X=Ag, Au) have been studied and A-D behave as both L- and Z-type ligands, in which these complexes contain both ptC and ptB. Thus, a novel method for designing compounds with ptC and ptB is presented. These rationally designed compounds involve the elements of carbene, ptC, ptB, and L- and Z-type ligands, and are expected to be unique and useful in experimental chemistry once they are synthesized.

From Fenestrindane towards Saddle-Shaped Nanographenes Bearing a Tetracoordinate Carbon Atom

Two saddle-shaped polycyclic aromatic compounds (8 a and 8 b) bearing an all-cis-[5.5.5.5]fenestrane core surrounded by an o,p,o,p,o,p,o,p-cyclooctaphenylene belt were synthesized and characterized by NMR spectroscopy and mass spectrometry. The key step of this synthesis involves the formation of four cycloheptatriene rings from the corresponding electron-rich 1,4,9,12-tetraarylfenestrindane derivatives 7 a and 7 b in Scholl-type cyclizations. The structural details of the D_2d -symmetric saddle compound 8 a were determined by X-ray crystallography, and the properties of 8 a and 8 b were studied by UV/Vis and fluorescence spectroscopy and cyclic voltammetry.

Computationally designed families of flat, tubular, and cage molecules assembled with "starbenzene" building blocks through hydrogen-bridge bonds

Using density functional calculations, we demonstrate that the planarity of the nonclassical planar tetracoordinate carbon (ptC) arrangement can be utilized to construct new families of flat, tubular, and cage molecules which are geometrically akin to graphenes, carbon nanotubes, and fullerenes but have fundamentally different chemical bonds. These molecules are assembled with a single type of hexagonal blocks called starbenzene (D(6h) C(6)Be(6)H(6)) through hydrogen-bridge bonds that have an average bonding energy of 25.4-33.1 kcal mol(-1). Starbenzene is an aromatic molecule with six pi electrons, but its carbon atoms prefer ptC arrangements rather than the planar trigonal sp(2) arrangements like those in benzene. Various stability assessments indicate their excellent stabilities for experimental realization. For example, one starbenzene unit in an infinite two-dimensional molecular sheet lies on average 154.1 kcal mol(-1) below three isolated linear C(2)Be(2)H(2) (global minimum) monomers. This value is close to the energy lowering of 157.4 kcal mol(-1) of benzene relative to three acetylene molecules. The ptC bonding in starbenzene can be extended to give new series of starlike monocyclic aromatic molecules (D(4h) C(4)Be(4)H(4)(2-), D(5h) C(5)Be(5)H(5)(-), D(6h) C(6)Be(6)H(6), D(7h) C(7)Be(7)H(7)(+), D(8h) C(8)Be(8)H(8)(2-), and D(9h) C(9)Be(9)H(9)(-)), known as starenes. The starene isomers with classical trigonal carbon sp(2) bonding are all less stable than the corresponding starlike starenes. Similarly, lithiated C(5)Be(5)H(5) can be assembled into a C(60)-like molecule. The chemical bonding involved in the title molecules includes aromaticity, ptC arrangements, hydrogen-bridge bonds, ionic bonds, and covalent bonds, which, along with their unique geometric features, may result in new applications.

Structure and bonding in cyclic isomers of BAl2Hnm (n=3-6, m=-2 to +1): preference for planar tetracoordination, pyramidal tricoordination, and divalency

The structure and energetics of cyclic BAl2Hnm (n=3-6, m=-2 to +1), calculated at the B3LYP/6-311+G** and QCISD(T)/6-311++G** levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of BAl2H(3)2- and BSi2H3. The number of structures that contain hydrogens out of the BAl2 ring plane is found to increase from BAl2H3(2-) to BAl2H6+. Double bridging at one bond is common in BAl2H5 and BAl2H6+. Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of BAl2H(3)2-. BAl2H4- (2 b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the B--Al bond prefer not to be in the BAl2 plane so that the pi MO is stabilised by pi-sigma mixing. This stabilisation increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between B--Al bonds. The relative stabilisation energy (RSE) for the minimum energy structures of BAl2Hnm that contain pi-delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.

Hexa- and Octacoordinate Carbon in Hydrocarbon Cages: Theoretical Design and Characterization

A new series of hydrocarbon cages containing hexa- and octacoordinate carbon centers were designed theoretically by performing DFT calculations at the B3 LYP/6-311+G** level. Among these non-classical structures that were found to still obey the 8e rule, the two tetracations with octacoordinate carbons may be the first examples found in pure hydrocarbons. Structural characteristics, as well as thermodynamic and kinetic stabilities, were also investigated theoretically for these two octacoordinate tetracations. These hydrocarbon compounds containing hypercoordinate carbon centers provide a challenge for synthetic organic chemists.

Planar and Pyramidal Tetracoordinate Carbon in Organoboron Compounds

Using previously proposed C(BH)2(CH)2 (16, 17) and C(CH)2B2 (22) systems with a central planar tetracoordinate carbon (ptC) atom linking two three-membered rings as building blocks, a series of stable structures containing two and three ptC centers within a molecule have been designed and computationally studied with the DFT (B3LYP/6-311+G) method. Inclusion of a carbon atom ligated with pi-accepting and sigma-donating boron centers into at least one aromatic ring is critical for stabilization of a planar structure. A square pyramidal configuration at tetracoordinate carbon may be achieved in appropriately strained molecules such as [3.3.3.3]tetraborafenestrane 45 and others by surrounding the carbon with boron-centered ligands.

Benzoannelated cis,cis,cis,trans-[5.5.5.6]fenestranes: syntheses, base lability, and flattened molecular structure of strained epimers of the all-cis series

Tribenzofenestranes possessing the strained cis,cis,cis,trans-[5.5.5.6]-fenestrane skeleton have been synthesized from cis-2,6-diphenylspiro[cyclohexane-1,2'-indane]-1',3'-diols by two-fold cyclodehydration, in striking analogy to the strategy used previously to construct the stereoisomeric all-cis-tribenzo[5.5.5.6]fenestranes from the corresponding trans-diphenylspirodiols. In this manner, both of the parent hydrocarbons, all-cis-tribenzo[5.5.5.6]fenestrane 3 and cis,cis,cis,trans-tribenzo[5.5.5.6]fenestrane 4, have been made accessible from the spirodiketones 5 and 6, respectively. The C6-functionalized derivatives of 4-cis,cis,cis,trans-fenestranol 9 and cis,cis,cis,trans-fenestranone 12-were prepared through cis-diphenylspirotriol 8 and cis-diphenyldispiroacetaldiol 11, by using the same strategy. The cis,cis,cis,trans-[5.5.5.6]fenestrane framework readily epimerizes to the more stable all-cis isomers under basic conditions, but is stable under neutral or acidic conditions. For example, cis,cis,cis,trans-fenestranone 12 yielded all-cis fenestrane 3 under Wolff-Kishner conditions, but cis,cis,cis,trans-isomer 4 under Clemmensen conditions. Epimerization was also circumvented by radical-induced desulfurization of fenestrane dithiolane 15 with nBu3SnH/AIBN, producing 4 in excellent yields. A single-crystal X-ray structure analysis of 4 revealed that, in accordance with force field and semi-empirical MO calculations, the extra strain of the benzoannelated cis,cis,cis,trans-[5.5.5.6]fenestratriene framework [Estrain(4)-Estrain(3)=46 kJmol(-1)] is due both to the almost perfect boat conformation of the six-membered ring and to considerable bond angle widening at the central, non-bridged C4b-C15d-C11b unit (121 degrees). H/D exchange experiments with the cis,cis,cis,trans hydrocarbon 4 under basic conditions demonstrated that the strain-induced epimerization to 3 occurs through direct deprotonation of the "epimeric" benzylic bridgehead C7a-H bond, which was found to be more acidic than the two C-H bonds at the benzhydrylic bridgeheads.