Boron rings enclosing planar hypercoordinate group 14 elements

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six pi and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A-B bonds is compensated by their unusually large number. Although a C7v pyramidal SiB8 structure is more stable than the D8h isomer, Born-Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization.     

The Planar CoB18− Cluster as a Motif for Metallo‐Borophenes

Monolayer‐boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B₃₆ cluster with a central six‐membered ring. Here we report a planar boron cluster doped with a transition‐metal atom in the boron network (CoB₁₈^?), suggesting the prospect of forming stable hetero‐borophenes. The CoB₁₈^? cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB₁₈^? is aromatic with ten π‐electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo‐borophenes, opening vast opportunities to design hetero‐borophenes with tunable chemical, magnetic, and optical properties.     

Sulphur-Bridged BAl5S5+ with 17 Counting Electrons: A Regular Planar Pentacoordinate Boron System

At present, most of the reported planar pentacoordinate clusters are similar to the isoelectronic substitution of CAl₅⁺, with 18 counting electrons. Meanwhile, the regular planar pentacoordinate boron systems are rarely reported. Hereby, a sulphur-bridged BAl₅S₅⁺ system with a five-pointed star configuration and 17 counting electrons is identified at the global energy minimum through the particle-swarm optimization method, based on the previous recognition on bridged sulphur as the peripheral tactics to the stable planar tetracoordinate carbon and boron. Its outstanding stability has been demonstrated by thermodynamic analysis at 900 K, electronic properties and chemical bonding analysis. This study provides adequately theoretical basis and referable data for its experimental capture and testing.     

Experimental and computational exploration of the dynamic behavior of (PNP)BF2, a boron compound supported by an amido/bis(phosphine) pincer ligand

The diarylamido/bis(phosphine) PNP pincer ligand (2-(i)Pr(2)P-4-MeC(6)H(3))(2)N has been evaluated as a scaffold for supporting a BF(2) fragment. Compound (PNP)BF(2) (6) was prepared by simple metathesis of (PNP)Li (5) with Me(2)SBF(3). NMR spectra of 6 in solution are of apparent C(2) symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF(2) coordination environment, with one phosphine donor in PNP remaining "free". Fortuitous formation of a single crystal of (PNP)BF(2)·HBF(4) (7), in which the "free" phosphine is protonated, furnished another structure containing the same PNBF(2) environment about boron for comparison and the two PNBF(2) environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF(2) systems were carried out and all displayed a similar four coordinate PNBF(2) environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C(1)-symmetric ground-state forms. Lineshape analysis of the (1)H and (19)F NMR spectra over a temperature range of 180-243 K yielded the activation parameters ΔH(?) = 8.1(3) kcal mol(-1) and ΔS(?) = -6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism.     

Indolocarbazole-based ligands for ladder-type four-coordinate boron complexes

A novel class of π-conjugated systems, which combine the indolo[3,2-b]carbazole unit with the formation of four-coordinate boron complexes, is presented. The resulting conjugated compounds have a double-laddered structure that provides interesting optical and electrochemical properties. The wide absorption range, covering most of the visible spectrum, along with the narrowing of the HOMO-LUMO energy gap, due to the presence of diphenylboryl centers, reinforces the potential of these molecules within the area of organic electronics.     

Na2B2S5 and Li2B2S5: Two Novel Perthioborates with Planar 1,2,4-Trithia-3,5-Diborolane Rings

For the first time perthioborates with trigonal planar coordination of boron were prepared. Na₂B₂S₅ (Pnma, a = 12.545(2) Å, b = 7.441(1) Å, c = 8.271(1) Å, Z = 4) and Li₂B₂S₅ (Cmcm, a = 15.864(1) Å, b = 6.433(1) Å, c = 6.862(1) Å, Z = 4) were obtained by reaction of the metal sulfides with stoichiometric amounts of boron and an excess of sulfur (effective molar ratio M:B:S = 1:1:4) at 600°C (650°C) and subsequent annealing. The non-isotypic structures contain exactly planar [B₂S₅]^2? groups consisting of five-membered B₂S₃ rings with one additional exocyclic sulfur on each of the boron atoms. The alkaline metal cations are four-coordinate (lithium) and (four + four)-coordinate (sodium) respectively.     

Crystal structure of [N,N'-bis(5-chlorosalicylidene)-1,3-diaminopropane]iron(II).

The crystal structure of the title compound has been determined. The coordination geometry about the iron(II) center is a tetrahedrally distorted square plane formed by the four-coordinate N2O2 donor set of the Schiff-base imine-phenol ligand. Molecules of the title compound are not planar. The two Schiff-base moieties, which themselves are reasonably planar, are inclined at an angle of 31.5(1) degrees.     

A new self-penetrating uniform net, (8,4) (or 8(6)), containing planar four-coordinate nodes

Hydrothermal reaction of 3-(3-pyridyl)acrylic acid (pyaraH) with NiCl2 and NaOH results in formation of different polymorphs at different temperatures. alpha-Ni(pyara)2(H2O)2 forms at 180 degrees C, and has a 3D polymeric structure. The topology of the network is that of a self-penetrating, four-coordinate (8,4) network (Schl?fli symbol 86). This is the first uniform net to contain only planar four-coordinate nodes, and it is also the first four-coordinate uniform net not to have the Schl?fli symbol 66. Tauhis fundamental network topology has been previously unrecognized. Tauhe second polymorph, the beta phase, forms from an analogous reaction at 150 degrees C, and contains 1D chains hydrogen bonded into a 3D network.     

Structure determination of the 4d metal diborides: a quantum mechanical study

Metal diborides (MB(2)) often have interesting thermal, mechanical, and superconducting properties. MgB(2) was put into focus some years ago for its high transition temperature (39 K) in combination with its simple AlB(2) structure. The boron structure in MB(2) is assumed to be dependent on the electron transfer from the nearby positioned metal atoms. An electronic and structural comparison has been performed here for various initially planar and puckered transition-metal borides, using quantum mechanical density functional theory (DFT) calculations under periodic boundary conditions. In comparison to MgB(2), the experimentally planar transition-metal diborides (ZrB(2), NbB(2), and MoB(2)) and the experimentally puckered ones (TcB(2), RuB(2), RhB(2), and PdB(2)) have been examined. The results indicate that the energetic stability generally follows the experimentally obtained results. The metals that are less electronegative than boron donate electrons to boron, which in turn induce planar boron structures (graphitic-like). The metals that prefer to be planar donate more than one electron, while the trend for metals which favor puckered B structures is that they donate less than one electron per metal atom. Two donated electrons per metal atom (or very close to) will result in the most stable AlB(2) structure.     

Polymerization of coordinated monomers. XXII. Ab initio molecular orbital study on the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride as a model of growing radical end

The geometrical and electronic structures of the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride were determined by using an ab initio molecular orbital method. The 2-methoxycarbonyl propyl radical complex was a model of the growing radical end in the copolymerization of methyl methacrylate in the presence of boron halides. The most stable structure of the binary radical complex composed of 2-methoxycarbonyl propyl radical with boron trichloride was a twisted form in which the dihedral angle between the vinyl group and the ester group was 32°, while that of the binary radical complex composed of methyl methacrylate radical with boron trifluoride was a planar form as the free radical. The frontier orbital energy of 2-methoxycarbonyl propyl radical was lowered by 0.06 au by the coordination of boron trichloride, while that was lowered only by 0.02 au by the coordination of boron trifluoride. The polymerization mechanism was elucidated on the basis of these predictions. © 1993 John Wiley & Sons, Inc.     

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.     

Boron(III) Carbazosubphthalocyanines: Core‐Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues

Condensation of 1,8‐diamino‐3,6‐dichlorocarbazole with a series of disubstituted 1,3‐diiminoisoindolines, followed by treatment with BF₃?OEt₂ led to the formation of the corresponding core‐expanded boron(III) subphthalocyanine analogues. These air‐stable π‐conjugated boron(III) carbazosubphthalocyanines possess two boron‐containing seven‐membered‐ring units and a 16 π‐electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single‐crystal X‐ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone‐shaped structure, the π skeleton of this compound is almost planar.     

Nickel(II) and copper(II) complexes of allyl 2-(thiophen-2-ylmethylene)hydrazinecarbodithioate: synthesis,X-ray crystal structures,and theoretical study

We report allyl 2-(thiophen-2-ylmethylene)hydrazine-carbodithioate (HL) and its Ni(II) and Cu(II) complexes, [ML₂]. The compounds were fully characterized by elemental analysis, IR, ¹H-NMR, UV-Vis, and molar conductivity. The crystal structure analysis indicates that the metal is four-coordinate square planar and that a parallel stacking of the molecular planes is present in the crystals, with stacking distances of 3.642 and 3.676?Å for the Ni(II) and Cu(II) complexes, respectively. Gas phase DFT computations indicate that the thione tautomeric form of the free ligand is more stable than the thiol form by 14.52?kJ?mol^–1. For HL and ML₂, comparison between the computed and experimental data shows good agreement.     

Syntheses and crystal structures of nickel(II), copper(II), and zinc(II) complexes with a biphenyl-bridged bis(pyrrole-2-yl-methyleneamine) ligand

The reactions of nickel(II), copper(II), and zinc(II) acetate salts with a potentially tetradentate biphenyl-bridged bis(pyrrole-2-yl-methyleneamine) ligand yielded three complexes with different coordination geometries. X-ray crystal structural analysis reveals that in the nickel(II) complex each nickel is five-coordinate, distorted trigonal bipyramid. In the copper(II) complex, each copper is four-coordinate, between square planar and tetrahedral. In the zinc(II) complex, each zinc is four-coordinate with a distorted tetrahedral geometry and the molar ratio of the zinc and ligand is 1 : 2.     

Topology, connectivity, and electronic structure of C and B cages and the corresponding nanotubes

After a brief discussion of the structural trends which appear with an increasing number of atoms in B cages, a one-to one correspondence between the connectivity of B cages and C cage structures will be proposed. The electronic level spectra of both systems from Hartree-Fock calculations is given and discussed. The relation of curvature introduced into an originally planar graphitic fragment to pentagonal "defects" such as are present in buckminsterfullerene is also briefly treated. A study of the structure and electronic properties of B nanotubes will then be introduced. We start by presenting a solution of the free-electron network approach for a "model boron" planar lattice with local coordination number 6. In particular the dispersion relation E(k) for the pi-electron bands, together with the corresponding electronic Density Of States (DOS), will be exhibited. This is then used within the zone-folding scheme to obtain information about the electronic DOS of different nanotubes obtained by folding this model boron sheet. To obtain the self-consistent potential in which the valence electrons move in a nanotube, "the March model" in its original form was invoked, and the results are reported for a carbon nanotube. Finally, heterostructures, such as BN cages and fluorinated buckminsterfullerene, will be briefly treated, the new feature here being electronegativity difference.     

An unprecedented fivefold interpenetrated lvt network containing the exceptional racemic motifs originated from nine interwoven helices

An unprecedented fivefold interpenetrated lvt network, containing the rare racemic motifs originated from nine interwoven helices, is reported, which represents the highest degree of interpenetration presently known for 3D nets containing only planar four-coordinate nodes.     

Redox-active ligand-mediated oxidative addition and reductive elimination at square planar cobalt(III): multielectron reactions for cross-coupling

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH(2)Cl(2), under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e(-) oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e(-) oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C-C coupling products. The net 2e(-) reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.     

Disk aromaticity of the planar and fluxional anionic boron clusters B20(-/2-)

The presence of excess electrons modifies the structural landscape and tends to extend the planarity of boron clusters. While the neutral B(20) is tubular, both the anion and dianion B(20)(-/2-) become planar. Geometrical features of the stable anions suggest the existence of a new type of cluster that is planar and doubly cyclic with one atom located at the center (see figure), as well as being fluxional.     

Comprehensive analysis of chemical bonding in boron clusters

We present a comprehensive analysis of chemical bonding in pure boron clusters. It is now established in joint experimental and theoretical studies that pure boron clusters are planar or quasi-planar at least up to twenty atoms. Their planarity or quasi-planarity was usually discussed in terms of pi-delocalization or pi-aromaticity. In the current article, we demonstrated that one cannot ignore sigma-electrons and that the presence of two-center two-electron (2c--2e) peripheral B--B bonds together with the globally delocalized sigma-electrons must be taken into consideration when the shape of pure boron cluster is discussed. The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n+2 (or 4n) electron counting rule for either pi- or sigma-electrons in the planar structures. We showed that pure boron clusters could have double (sigma- and pi-) aromaticity (B3-, B4, B5+, B6(2+), B7+, B7-, B8, B(8)2-, B9-, B10, B11+, B12, and B13+), double (sigma- and pi-) antiaromaticity (B6(2-), B15), or conflicting aromaticity (B5-,sigma-antiaromatic and pi-aromatic and B14, sigma-aromatic and pi-antiaromatic). Appropriate geometric fit is also an essential factor, which determines the shape of the most stable structures. In all the boron clusters considered here, the peripheral atoms form planar cycles. Peripheral 2c--2e B--B bonds are built up from s to p hybrid atomic orbitals and this enforces the planarity of the cycle. If the given number of central atoms (1, 2, 3, or 4) can perfectly fit the central cavity then the overall structure is planar. Otherwise, central atoms come out of the plane of the cycle and the overall structure is quasi-planar.     

Probing the planar tetra-, penta-, and hexacoordinate carbon in carbon-boron mixed clusters

The concept of planar hypercoordinate (e.g., penta- and hexacoordinate) carbons is intriguing [Exner, K.; Schleyer, P. v. R. Science 2000, 290, 1937] as it is neither compatible with the standard rule of three-dimensional chemical bonding nor with the maximum tetracoordination. Herein we undertake a comprehensive study of the planar tetra- (ptC), penta- (ppC), and hexacoordinate carbon (phC) by covering the whole family of carbon-boron mixed clusters C(m=1-4)B(n=4-8) and their anions. The potential energy surface of every carbon-boron cluster is sampled by using the basin-hopping global search algorithm coupled with ab initio geometry optimization. A large number of planar tetra-, penta-, and hexacoordinate carbon (local-minimum) structures are obtained. Several structures such as the phC consisting of C2B5, C2B5(-), etc. are reported for the first time. In particular, a ptC corresponding to the global minimum of CB4 is revealed, which appears to be highly stable for future synthesis. The boron-centered isomers are generally the more stable structures for planar multicoordinate carbons (ptC, ppC, and phC). The planar tetra-, penta-, and hexacoordinate boron are the prevalent structural motifs in low-lying isomers of the carbon-boron clusters. However, stability of the ptC and ppC units can be reinforced over the boron-centered isomers by attaching proper hydrocarbon unit -(CH)n- to form the so-called "hyparenes" [Wang, Z. X.; Schleyer, P. v. R. Science 2001, 292, 2465]). A new hyparene molecule is suggested for future synthesis of novel planar hypercoordinate carbon compounds.