A hypervalent pentacoordinate boron compound with an N-B-N three-center four-electron bond

For the purpose of synthesizing and characterizing hypervalent boron compounds with strong hypervalent interaction, we have prepared a boron compound with a tridentate ligand bearing two pyrimidine rings as nitrogen donors. X-ray analysis and molecular orbital calculations suggested that the boron compound was of hypervalent pentacoordinate structure with an N-B-N hypervalent bond. Thus, we have prepared the first hypervalent second row element compound with apical N coordination. A breakdown of energy contributions by DFT calculations revealed that the N-B-N bond energy of the pentacoordinate state ground state (13) was 2.8 kcal mol(-1). Implications were that the conjugation energy difference of 6.6 kcal mol(-1) (14.2-7.6 kcal mol(-1)) with the tetracoordinate state was a crucial factor for shifting stability toward the pentacoordinate structure.     

Synthesis and properties of hypervalent electron-rich pentacoordinate nitrogen compounds

Isolation and structural characterization of hypervalent electron-rich pentacoordinate nitrogen species have not been achieved despite continuous attempts for over a century. Herein we report the first synthesis and isolation of air stable hypervalent electron-rich pentacoordinate nitrogen cationic radical (11-N-5) species from oxidation of their corresponding neutral (12-N-5) species. In the cationic radical species, the nitrogen centers adopt a trigonal bipyramidal geometry featuring a 3-center-5-electron hypervalent attractive interaction. The combination of single crystal X-ray diffraction analysis and computational studies revealed weak N–O interactions between the central nitrogen cation and oxygen atoms. This successful design strategy and isolation of air-stable pentacoordinate hypervalent nitrogen species allow further investigations on reactivity and properties resulting from these unusually weakly coordinating interactions in nitrogen compounds.

Structural characterization of hypervalent electron-rich pentacoordinate nitrogen species has long been a synthetic challenge. Here we report the first nitrogen cationic radical (11-N-5) species featuring a weak hypervalent 3c-5e interaction.     

Synthesis and characterization of stable hypervalent carbon compounds (10-C-5) bearing a 2,6-bis(p-substituted phenyloxymethyl)benzene ligand

X-ray analysis of bis(p-fluorophenyl)methyl cation bearing a 2,6-bis(p-tolyloxymethyl)benzene ligand showed a symmetrical structure (10-C-5) where the two C-O distances are identical, although the distance (2.690(4) A) is longer than those (2.43(1) and 2.45(1) A) of 1,8-dimethoxy-9-dimethoxymethylanthracene monocation, which was recently reported by us. However, X-ray analysis of the more stable aromatic xanthylium cation with the same benzene ligand showed the tetracoordinate carbon structure where only one of the two oxygen ligands is coordinated with the central carbon atom. These results clearly indicate that the carbocations (10-C-5) bearing the sterically flexible benzene ligand were quite sensitive to the electronic effect on the central carbon atom. The electron distribution analysis by accurate X-ray measurements and the density functional calculation on the initially mentioned bis(p-fluorophenyl)methyl cation clearly show that the central carbon atom and the two oxygen atoms are bonded even if the bond is weak and ionic based on the small value of the electron density (rho(r)) and the small positive Laplacian value (nabla(2)rho(r)) at the bond critical points.     

Pentacoordinate 1H-phosphirenes: reactivity, bonding properties, and substituent effects on their structures and thermal stability

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a very high degree of p character, which results from their being both basal bonds in the SP structure and endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations, it can be considered that a sigma-pi interaction between the sigma orbital of the apical bond and the pi orbital of the C=C bond in the three-membered ring is operative in pentacoordinate phosphirenes. The sigma-pi interaction is found to lower the reactivity of the C=C bond of the three-membered ring. The reactivities of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-membered ring.     

The structure of a new isomer of dicarba-nido-undecaborane(13)

The intramolecular rearrangement involved in the electrophilic methylation of 7,8-B₉C₂H₁₁^2? leads after protonation to the methyl derivative of a new isomer of dicarba-nido-undecaborane(13). By an X-ray structure analysis the latter compound is shown to be 11-methyl-2,7-dicarba-nido-undecaborane(12). It has the structure of an icosahedron with one missing vertex, with the carbon atoms adjacent, one in the open face of the polyhedron and the other in its closed part. The methyl group is attached to the boron atom of the open face bonded to both carbon atoms. The open face has two BHB-bridges which are symmetric relative to its carbon atom. The main interatomic distances are: av. BB 1.794(8), av. BC 1.685(7), CC 1.606(6), BMe 1.551(8), av. BH (terminal) 1.08(3), av. BH (bridging) 1.26(4), av. CH 0.92(4) Å.     

Synthesis of Tetracoordinate Boron‐Fused Benzoaceanthrylene Analogs via Tandem Electrophilic C−H Borylation

Benzoaceanthrylene analogs with tetracoordinate boron at the ring junction were synthesized through tandem electrophilic C−H borylation of 2,6‐dinaphthylpyridine followed by nucleophilic substitution. Notably, the [5,6]‐annulation occurred selectively over [6,6]‐annulation with the assistance of nitrogen coordination of the pyridine ring. The X‐ray crystallographic analysis revealed the polycyclic skeleton with a distorted tetracoordinate boron atom and a unique packing structure with intermolecular π–π interaction. The photophysical and electrochemical properties of these benzoaceanthrylene analogs were evaluated by UV‐vis spectroscopy and differential pulse voltammetry. The electron affinity of the fluorine‐substituted derivative is estimated to be 3.49 eV from the ionization potential and optical band gap. Thus, this derivative is expected to be a promising n‐type semiconducting material.     

Reversible photoswitching of the coordination numbers of silicon in organosilicon compounds bearing a 2-(phenylazo)phenyl group

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of 2-iodoazobenzene. Their structures were determined by (1)H, (13)C, (19)F, and (29)Si NMR spectra, UV-vis spectra, and X-ray crystallographic analyses. In the UV-vis spectra, silyl groups caused red shifts of both the n-pi and pi-pi transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the Z-isomers were found to be tetracoordinate in the absence of Si-N interactions by the (29)Si NMR spectra. The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]silicate.     

CRYSTAL STRUCTURE OF ORGANOLANTHANIDE [{Li(THF)_2}_2 (μ-Cl)_4(η~5-C_5H_5)Pr(THF)]

<正> The title complex crystallizes in the orthorhombic space group Pna21 with Mr = 570. 8, a = 16. 927(6), b = 18.942(7), c=10. 618(5)(?) and Z = 4. The structure was solved by Patterson and Fourier techniques and refined by block-diagonal least squares techniques to R = 0. 090 and Rw = 0. 093 for 1553 reflections with I>1. 5σ(I). The Pr3+ ion is bonded to five carbon atoms of cyclopentadi-enyl ring, four μ-chlorine atoms and one oxygen atom of THF in an octahedral geometry with the Pr-C distances in the range 2. 6.7 - 2. 79(?)(av. 2. 76(?)) , the Pr-Cl distances in the range 2. 81 - 2. 92(?) (av. 2. 86(?)) and the Pr -O distance of 2. 51 (?). Each Li+ ion is coordinated by two μ-chlorine atoms and two oxygen atoms from THF in a tetrahedral geometry with the Li-Cl distances in the range 2. 24 - 2. 61(?) (av. 2. 43(?)) and the Li -O distances in the range 1. 86-1. 94(?)(av. 1. 91(?)). The Pr atom and the two Li atoms are bridged asymmetrically by the chlorine atoms.     

Crystal and molecular structure of compounds containing a hypervalent O−Si(C3)−O fragment. Parametrization in the description of the coordination environment of the silicon atom

We have carried out an X-ray structural investigation of four pentacoordinated silicon compounds with a hypervalent O?Si(C₃)?O fragment. In their molecules, the axial Si?O bond lengths range from 1.711 to 2.785 Å. Analysis of the geometry of such fragments containing other atoms in axial positions shows that the main parameter determining the state of the hypervalent fragment is deviation of the Si atom from the plane of equatorial substituents. Some consequences of this study for structural modeling of nucleophilic substitution reactions are discussed.     

Esters of 2-iodoxybenzoic acid: hypervalent iodine oxidizing reagents with a pseudobenziodoxole structure

Esters of 2-iodoxybenzoic acid (IBX-esters) were prepared by the hypochlorite oxidation of the corresponding 2-iodobenzoate esters and isolated as chemically stable, microcrystalline products. These hypervalent iodine compounds are potentially valuable oxidizing reagents belonging to a new class of pentavalent iodine compounds with a pseudobenziodoxole structure. Methyl 2-iodoxybenzoate can be further converted to the diacetate or a bis(trifluoroacetate) derivative by treatment with acetic anhydride or trifluoroacetic anhydride, respectively. Single-crystal X-ray diffraction analysis of methyl 2-[(diacetoxy)iodosyl]benzoate 8a reveals a pseudobenziodoxole structure with three relatively weak intramolecular I...O interactions. The dimethyl and diisopropyl esters of 2-iodoxyisophthalic acid were prepared by oxidation of the respective iodoarenes with dimethyldioxirane. Single-crystal X-ray diffraction analysis of diisopropyl 2-iodoxyisophthalate 6b showed intramolecular I...O interaction with the carbonyl oxygen of only one of the two carboxylic groups, while NMR spectra in solution indicated equivalency of both ester groups. IBX-esters, methyl 2-[(diacetoxy)iodosyl]benzoate, and 2-iodoxyisophthalate esters can oxidize alcohols to the respective aldehydes or ketones in the presence of trifluoroacetic acid or boron trifluoride etherate. The bis(trifluoroacetate) derivative can oxidize alcohols to carbonyl compounds without acid catalyst.     

Pentacoordination of boron, carbon, aluminum, and silicon atoms in organic compounds: Anab initio study

Pentacoordination of boron, carbon, aluminum, and silicon atoms in bicyclic organic compounds of the pentalene type was studied using theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that the ability of the atom to form pentacoordinate structures increases on going from B to Al and from C to Si atom,i.e. as the number of the element of Groups IIIA and IVA of the periodic system increases. At the same time, the reverse tendencies are observed in the 2nd and 3rd periods of the periodic system,viz., the ability of the atom to form pentacoordinate structures increases on going from C to B and from Al to Si atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1238–1245, July, 1999.     

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.     

Optically active zwitterionic lambda(5)Si,lambda(5)Si'-disilicates: syntheses, crystal structures, and behavior in aqueous solution

The zwitterionic lambda(5)Si,lambda(5)Si'-disilicates 1-8 were synthesized and characterized by solid-state and solution NMR spectroscopy. In addition, compounds 26 H(2)O, 32 CH(3)CN, 45/2 CH(3)CN, 6CH(3)OH, 7, and 8CH(3)OHCH(3)CN were studied by single-crystal X-ray diffraction. The optically active (Delta,Delta,R,R,R,R)-configured compounds 1-8 contain two pentacoordinate (formally negatively charged) silicon atoms and two tetracoordinate (formally positively charged) nitrogen atoms. One (ammonio)alkyl group is bound to each of the two silicon centers, and two tetradentate (R,R)-tartrato(4-) ligands bridge the silicon atoms. Although these lambda(5)Si,lambda(5)Si'-disilicates contain SiO(4)C skeletons, some of them display a remarkable stability in aqueous solution as shown by NMR spectroscopy and ESI mass spectrometry.     

Novel 10-I-3 hypervalent iodine-based compounds for electrophilic trifluoromethylation

The synthesis of a new family of 10-I-3 hypervalent iodine compounds is described in which the CF3 functionality participates directly in the hypervalent bond. These materials are accessible by nucleophilic ligand substitution at iodine using Me3SiCF3 in the presence of a substoichiometric amount of fluoride. The expected T-shaped geometry at iodine was verified by X-ray crystallographic analyses of three of the products (1-trifluoromethyl-1,2-benziodoxol-3-(1 H)-one and two substituted 1-trifluoromethyl-1,3-dihydro-1,2-benziodoxoles). Preliminary results for the direct electrophilic transfer of the trifluoromethyl moiety onto organic nucleophiles show modest reactivity in polar aprotic solvents under relatively mild conditions. The overall process can be understood as a formal umpolung of the CF3 group.     

Intramolecular cyclization with nitrenium ions generated by treatment of N-acylaminophthalimides with hypervalent iodine compounds: formation of lactams and spiro-fused lactams

N-Phthalimido-N-acylnitrenium ions are generated from N-acylaminophthalimides, a new class of precursors, by treatment with hypervalent iodine compounds (PIFA and HTIB). In HFIP, the nitrenium ions undergo intramolecular electrophilic substitution reactions to afford N-aminonitrogen heterocycles in high yields. In TFEA, spirodienones bearing the 1-azaspiro[4.5]decane skeleton are obtained by treatment of N-phthalimido-3-(4-halogenophenyl)propanamides with HTIB as a result of ipso attack of the intermediate nitrenium ion. Similarly, using PIFA in TFEA, ipso cyclization of unactivated benzenoid compounds occurs to afford spirodiene derivatives. This involves loss of aromaticity despite the absence of other activating substituents on the phenyl group.     

A unique case of oxidative addition of interhalogens IX (X=Cl, Br) to organodiselone ligands: nature of the chemical bonding in asymmetric I-Se-X polarised hypervalent systems

The reactivity of the imidazoline-2-selone derivatives 1,1'-methylenebis(3-methyl-4-imidazoline-2-selone) (D1) and 1,2-ethylenebis(3-methyl-4-imidazoline-2-selone) (D2) towards the interhalogens IBr and ICl has been investigated in the solid state with the aim of synthesising "T-shaped" hypervalent chalcogen compounds featuring the extremely rare linear asymmetric I-E-X moieties (E=S, Se; X=Br, Cl). X-ray diffraction analysis and FT-Raman measurements provided a clear indication of the presence in the compounds obtained of discrete molecular adducts containing I-Se-Br and I-Se-Cl hypervalent moieties following a unique oxidative addition of interhalogens IX (X=Cl, Br) to the organoselone ligands. In all asymmetric hypervalent systems isolated, a strong polarisation was observed, with longer bond lengths at the selenium atom involving the most electronegative halogen. A topological electron density analysis on model compounds based on the quantum theory of atoms-in-molecules (QTAIM) and electron localisation function (ELF) established the three-centre-four-electron (3c-4e) nature of the bonding in these very polarised selenium hypervalent systems and new criteria were suggested to define and ascertain the hypervalency of the selenium atoms in these and related halogen and interhalogen adducts.     

(C5M2-n)(n-) (M = Li,Na, K,and n = 0, 1, 2). A new family of molecules containing planar tetracoordinate carbons

By highly correlated ab initio methods and DFT calculations, we have shown that alkaline metals can stabilize planar tetracoordinate carbon-containing molecules with the C(C4) skeleton. This family of molecules is C5M2, where M is an alkaline metal. The stability of these compounds is rationalized in terms of the delocalization of the p-orbital perpendicular to the molecular plane, the global hardness, and the electrophilicity. The analysis of several molecular scalar fields shows that the bonding between the C52- dianion and the metals is strongly ionic. The structures reported are the first examples with a planar tetracoordinate carbon, surrounded by carbon atoms, and stabilized, only, by electronic factors.     

Synthesis and structure of a hexacoordinate carbon compound

In an exploration of six coordination and hypervalence in carbon compounds, steric constraints have been employed to bring four ether O atoms in close proximity to an allenic carbon atom. The dimethylated dication 2 is confirmed to have hexacoordinate carbon by experimental charge density analysis and DFT calculations and is arguably hypervalent.     

Formation of hypervalent complexes of trifluorosilanes with pyridine and with 4-methoxypyridine, through intermolecular silicon...nitrogen interactions

Herein we report that trifluorohexylsilane (2), trifluorophenylsilane (3) and trifluoro(pentafluorophenyl)silane (4) form new hypervalent complexes with pyridine (py) and with 4-methoxypyridine (MeO-py), through intermolecular Si...N interactions. In general, stronger and more efficient binding is observed for the more electron poor (Si atom in) silane 4 and for the more electron rich (N atom in) 4-methoxypyridine. Binding constants of 15+/-2, 25+/-5, and 550+/-100 M(-1) at 25 degrees C in benzene were measured for the formation of the pentacoordinate 2.py, 3.py, and 3.MeO-py complexes, respectively. In addition, silane 3 also forms the hexacoordinate 3-2py and 3-2MeO-py complexes at low temperatures and silane 4 forms the 4-2py complex below room temperature and the 4-2MeO-py complex already at room temperature and in a high concentration. The various types of hypervalent complexes and different binding strengths described here for silanes 2-4 and previously for trifluoro(phenylethynyl)silane (1) and the possibility to modulate the binding modes (penta- vs hexacoordination) of these complexes (by the type of amine used, concentration, and the temperature applied) suggest that such new intermolecular Si...N interactions could be used as efficient and versatile binding motifs in supramolecular chemistry.     

Synthesis and Structure of Triphenylbismuth Difluoride

The reaction of triphenylbismuth dichloride with sodium fluoride in acetone leads to formation of triphenylbismuth difluoride in 73% yield. The X-ray diffraction data show that the bismuth atom in the two symmetrically independent molecules of bismuth difluoride has a trigonal-bipyramidal coordination with equatorial fluorine atoms. The Bi-F and Bi-C distances are 2.53(1)-2.59(1) and 2.10(3)-2.22(2) Å, respectively, and the FBiF angle is 175.1(5)°.