Herbert Höpfl

The tetrahedral character of the boron atom newly defined—a useful tool to evaluate the N→B bond

Abstract The tetrahedral character at the boron atom in boron complexes with a coordinative N→B bond has been evaluated on the basis of 144 solid state and gas phase molecular structures in order to obtain additional information on the van der Waals versus covalent relationship of the boron–nitrogen bond. The tetrahedral character has been calculated by a new formula that includes all six bond angles at the boron atom. Thereby, the N→B bond length and the tetrahedral character are correlated and permit the description of the reaction path for the formation of the N→B bond.

New perspectives for boronic esters in macrocyclic chemistry

In the present contribution a tetrameric macrocyclic compound derived from 2,6-pyridinedimethanol and 3-nitrophenyl boronic acid, as well as 10 new dimeric boronates prepared from 2-salicylideneaminoethanol and different aryl boronic acids such as a 2-methylphenyl-, 3-methylphenyl-, 4-methylphenyl-, 3-methoxyphenyl-, 4-methoxyphenyl-, 3-chlorophenyl-, 4-chlorophenyl-, 3-nitrophenyl-, 3-trifluoromethylphenyl- and 4-fluorophenylboronic acid are described. The tetrameric and three of the dimeric structures have been analyzed by X-ray crystallography, and a series of parameters such as bond length, bond angles, deviation of the boron atom from the boronate mean plane and intermolecular interactions are discussed.

New macrocyclic oligoboronates

Abstract Two new macrocyclic oligoboronates were prepared in easy one step syntheses from aminodialcohols and phenylboronic acid. Reaction of 2,6-pyridinedimethanol gives a tetrameric species, while 2-(salicylideneamino)-1-hydroxyethane provides a cyclobisboronate. Both compounds are characterized by using X-ray structural analyses. Possible applications in the supramolecular host-guest chemistry are discussed.

Boron-nitrogen macrocycles: a new generation of calix[3]arenes.

The simple condensation reaction of 3,5-di-tert-butyl salicylaldehyde and 3-aminophenylboronic acid leads to a trimeric macrocyclic compound. The ability of this molecule to include small organic molecules was in a first approximation analyzed by (1)H NMR spectroscopy and X-ray crystallography.

Cytotoxic activity, X-ray crystal structures and spectroscopic characterization of cobalt(II), copper(II) and zinc(II) coordination compounds with 2-substituted benzimidazoles

Herein we present the synthesis, structural and spectroscopic characterization of coordination compounds of cobalt(II), copper(II) and zinc(II) with 2-methylbenzimidazole (2mbz), 2-phenylbenzimidazole (2phbz), 2-chlorobenzimidazole (2cbz), 2-benzimidazolecarbamate (2cmbz) and 2-guanidinobenzimidazole (2gbz). Their cytotoxic activity was evaluated using human cancer cell lines, PC3 (prostate), MCF-7 (breast), HCT-15 (colon), HeLa (cervic-uterine), SKLU-1 (lung) and U373 (glioblastoma), showing that the zinc(II) and copper(II) compounds [Zn(2mbz)(2)Cl(2)].0.5H(2)O, [Zn(2cmbz)(2)Cl(2)].EtOH, [Cu(2cmbz)Br(2)].0.7H(2)O and [Cu(2gbz)Br(2)] had significant cytotoxic activity. The isostructural cobalt(II) complexes showed not significant activity. The cytotoxic activity is related to the presence of halides in the coordination sphere of the metal ion. Recuperation experiments with HeLa cells, showed that the cells recuperated after removing the copper(II) compounds and, on the contrary, the cells treated with the zinc(II) compounds did not. These results indicate that the mode of action of the coordination compounds is different.

Macrocyclic Diorganotin Complexes of γ-Amino Acid Dithiocarbamates as Hosts for Ion-Pair Recognition

The dimethyl-, di-n-butyl-, and diphenyltin(IV) dithiocarbamate (dtc) complexes [{R2Sn(L-dtc)}x] 1-7 (1, L = L1, R = Me; 2, L = L1, R = n-Bu; 3, L = L2, R = Me, x = infinity; 4, L = L2, R = n-Bu; 5, L = L3, R = Me, x = 2; 6, L = L3, R = n-Bu, x = 2; 7, L = L3, R = Ph, x = 2) have been prepared from a series of secondary amino acid (AA) homologues as starting materials: N-benzylglycine (alpha-AA derivative = L1), N-benzyl-3-aminopropionic acid (beta-AA derivative = L2), and N-benzyl-4-aminobutyric acid (gamma-AA derivative = L3). The resulting compounds have been characterized by elemental analysis, mass spectrometry, IR and NMR ((1)H, (13)C, and (119)Sn) spectroscopy, thermogravimetric analysis, and X-ray crystallography, showing that in all complexes both functional groups of the heteroleptic ligands are coordinated to the tin atoms. By X-ray diffraction analysis, it could be shown that [{Me2Sn(L2-dtc)}x] (3) is polymeric in the solid state, while the complexes derived from L3 (5-7) have dinuclear 18-membered macrocyclic structures of the composition [{R2Sn(L3-dtc)}2]. For the remaining compounds, it could not be established with certainty whether the structures are macrocyclic or polymeric. A theoretical investigation at the B3LYP/SBKJC(d,p) level of theory indicated that the alpha-AA-dtc complexes might have trinuclear macrocyclic structures. The macrocyclic complexes 5-7 have a double-calix-shaped conformation with two cavities large enough for the inclusion of aliphatic and aromatic guest molecules. They are self-complementary for the formation of supramolecuar synthons that give rise to 1D molecular arrangements in the solid state. Preliminary recognition experiments with tetrabutylammonium acetate have shown that the [{R2Sn(L3-dtc)}2] macrocycles 6 and 7 might interact simultaneously with anions (AcO(-)), which coordinate to the tin atoms, and organic cations (TBA(+)), which accommodate within the hydrophobic cavity (ion-pair recognition).

Preparation and structural characterization of six new diorganotin(IV) complexes of the R2Sn(SaleanH2) and R2Sn(SalceanH2) type (R = Me, nBu, Ph)

Six new diorganotin(IV) complexes have been prepared from R2SnO (R� /Me, n Bu, Ph) and the Salan ligands SaleanH4 (SaleanH4� /N ,N ?-bis(o -hydroxybenzyl)-1,2-diaminoethane) and SalceanH4 (SalceanH4� /N ,N ?-bis(o -hydroxybenzyl)-1,2-diaminocyclohexane) in order to determine the preferred coordination mode of these ligands in dependence of their flexibility and basicity as well as the steric bulk of the organic groups attached to the tin(IV) atom. The present NMR spectroscopic and X-ray crystallographic study shows that the SaleanH2 and SalceanH2 ligands prefer a fac � /fac configuration when coordinated to a diorganotin(IV) fragment, in which the nitrogen atoms are located trans to the organic groups. Depending on the steric bulk of the organic substituents, the conformation of the coordinated ligand can vary with respect to the orientation of the benzyloxy phenyl rings. In the solution state there may exist a rapid equilibrium between these possible conformations. For related R2Sn(Salen) complexes so far only structures with trans configurations have been reported, however, for SalanH2 complexes with other metal ions both examples with fac � /fac and fac � /mer configurations are known. In the solid state structure of one of the complexes studied in here an interesting C� /H p intermolecular contact between a chloroform molecule and an aromatic ring has been detected. # 2002 Elsevier Science B.V. All rights reserved.

Facile preparation of [4.4]metacyclophane- and [5.5]paracyclophane-type macrocycles from arylboronic acids and salicylideneaminoaryl alcohols

Four different salicylideneaminoaryl alcohols have been treated with arylboronic acids in order to prepare air-stable cyclophane-type macrocyclic systems. In two cases, this objective could be realized with the high-yield formation of [4.4]metacyclophane and [5.5]paracyclophane derivatives. The skeleton in these macrocycles is held together by two chiral boron atoms. In the other two cases, monomeric boronates or polymeric material were obtained. The title structures were characterized by spectroscopic techniques and X-ray crystallography. They show transannular C-H...O hydrogen bonding, but no intramolecular pi-pi interactions. A synthetic strategy for the preparation of further boron macrocycles is presented.

X-ray crystallographic study of boroxazolidones obtained from l-ornithine, l-methionine, kainic acid and 2,6-pyridinedicarboxylic acid

Abstract In the present contribution, the boroxazolidones prepared from l -ornithine, l -methionine, kainic acid and 2,6-pyridinedicarboxylic acid have been studied by X-ray crystallography. A comparison of the structural data with corresponding boroxazolidines, bicyclic boronates and tricyclic borates has shown that in boron complexes with a boroxazolidone ring the B–O bond is longer in comparison to boron complexes with a boroxazolidine ring. At the same time the N→B bond length is shorter indicating that the hydrolytic stability of the complexes with boroxazolidone rings is enhanced.

Structure and Bonding in Boron-Containing Macrocycles and Cages — Comparison to Related Structures with Other Elements Including Organic Molecules

The present revision describes the generation of macrocyclic, cage-like and supramolecular structures incorporating three-or four-coordinate boron atoms. Preparative strategies are discussed and include, aside from traditional preparative methods known from organic chemistry, one-step syntheses by self-assembly and template syntheses. Emphasis is placed on chemical and structural analogies to well-known related structures from organic, coordination and organometallic chemistry. Applications of boron-containing hosts in ionic and molecular recognition, including chiral recognition, as well as selective molecular transport through lipophilic membranes are discussed.