From an Al4-Cluster to Al+ Complexes: Lessons on Metalloidal Clustering

Low-valent aluminium compounds are among the most reactive and widely researched main-group compounds. Since the isolation of [(AlCp*)4] in 1991 as the first stable, molecular AlI compound, a variety of highly reactive neutral or anionic low-valent aluminium complexes were developed. In particular, the strongly basic aluminyl anions allowed for nucleophilic activation of a large variety of small molecules and formation of elusive transition-metal complexes. By contrast, an accessible cationic, low-valent aluminium compound combining the nucleophilicity of low-valent compounds with the electrophilicity of aluminium is hitherto unknown. Here, we report the synthesis of [Al(AlCp*)3]+[Al(ORF)4]– (RF = C(CF3)3) via a simple metathesis route. Unexpectedly, the complex ion forms a dimer in the solid state and in concentrated solutions. Addition of Lewis bases results in monomerization and coordination to the unique formal Al+ atom giving [(L)xAl(AlCp*)3]+ salts with L = hexaphenylcarbodiphosporane (cdp; x = 1), tetramethylethylenediamine (tmeda; x = 1) and 4-dimethylamino-pyridine (dmap; x = 3). Depending on the donor strength of the ligand added, the Al+–AlCp* bonds in the [(L)xAl(AlCp*)3]+ cluster cations can be finely tuned between very strong (L = nothing) to very weak and approaching isolated [Al(L)3]+ ions (L = dmap). We anticipate our easily accessible low-valent aluminium cation salts to be the starting point for investigation and potential application of this unusual compound class. In particular, the ambiphilic reactivity of the cationic, low-valent compounds will be studied. Moreover, knowledge gained from the stabilization of the reported complex salts is expected to facilitate the isolation and application of novel cationic, low-valent Al complexes.

Modulating the Frontier Orbitals of an Aluminylene for Facile Dearomatization of Inert Arenes

Lewis bases are well known to stabilize electron-deficient species. We demonstrate herein that the redox property of a monocoordinated aluminylene 1 featuring only four valence electrons for the shell of Al can be boosted by a Lewis base. The coordination of 1 with an N-heterocyclic carbene (NHC) effectively shrinks the HOMO−LUMO gap, thereby enhancing the reactivity of the ensuing acyclic mono-NHC-stabilized aluminylene 2, which is isoelectronic with singlet carbenes. Moreover, such base coordination completely reverses the predominant chemical reactivity (i.e. electrophilicity/nucleophilicity) of aluminylenes. In marked contrast to 1, 2 readily undergoes a [4+1] cycloaddition reaction with naphthalene and biphenylene at room temperature. Remarkably, the enhanced ambiphilic nature of Al in 2 also enables facile cleavage of aromatic C−C bonds of inert arenes in both intra- and intermolecular fashion affording 3 and 5. The formation of 5 represents the first example of the cleavage of aromatic C(3)−C(4) bond in biphenylene by a single atom center.

Synthesis and Applications of Chiral Bicyclic Bisborane Catalysts

The development of chiral borane Lewis acid catalysts opened the door for transition-metal-free catalyzed asymmetric organic reactions. Herein, we have summarized our work on the preparation of two classes of novel chiral bicyclic bisborane Lewis acid catalysts derived from C2-symmetric [3.3.0] dienes and [4.4] dienes, respectively. These catalysts not only form frustrated Lewis pairs with Lewis bases to catalyze asymmetric hydrogenation reactions but also activate Lewis basic functional groups in traditional Lewis acid catalyzed asymmetric reactions.

Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and the bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We have decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species are the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results actually show that the enthalpy of BF3 adduct formation in DCM solution is clearly influenced by specific interactions, DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allows us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain because of identified reasons can be complemented by the computed values.

Modulating the Frontier Orbitals of an Aluminylene for Facile Dearomatization of Inert Arenes

Lewis bases are well known to stabilize electron-deficient species. We demonstrate herein that the redox property of a monocoordinated aluminylene 1 featuring only four valence electrons for the shell of Al can be boosted by a Lewis base. The coordination of 1 with an N-heterocyclic carbene (NHC) effectively shrinks the HOMO−LUMO gap, thereby enhancing the reactivity of the ensuing acyclic mono-NHC-stabilized aluminylene 2, which is isoelectronic with singlet carbenes. Moreover, such base coordination completely reverses the predominant chemical reactivity (i.e. electrophilicity/nucleophilicity) of aluminylenes. In marked contrast to 1, 2 readily undergoes a [4+1] cycloaddition reaction with naphthalene and biphenylene at room temperature. Remarkably, the enhanced ambiphilic nature of Al in 2 also enables facile cleavage of aromatic C−C bonds of inert arenes in both intra- and intermolecular fashion affording 3 and 5. The formation of 5 represents the first example of the cleavage of aromatic C(3)−C(4) bond in biphenylene by a single atom center.

Energy transfer in luminescence composite materials based on mesogenic europium (III) complexes and some semiconducting polymers according to quantum-chemical simulation

A TDDFT study of energy transfer processes in luminescence composite materials based on mesogenic europium (III) complexes with substituted β-diketones and Lewis bases and some semiconducting polymers is presented. The calculated energies of the lowest singlet and triplet excited states of ligands and polymers were used to construct the energy level diagrams and indentify the main channels of intramolecular energy transfer. A system of a complex and a polymer, which is most preferable for practical application in optoelectronics, was selected. The simulated data agree well with the experiment.

Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and the bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We have decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species are the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results actually show that the enthalpy of BF3 adduct formation in DCM solution is clearly influenced by specific interactions, DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allows us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain because of identified reasons can be complemented by the computed values.