Anna S. Lisovenko

Do solid-state structures reflect Lewis acidity trends of heavier group 13 trihalides? Experimental and theoretical case study.

Lewis acidity trends of aluminum and gallium halides have been considered on the basis of joint X-ray and density functional theory studies. Structures of complexes of heavier group 13 element trihalides MX(3) (M = Al, Ga; X = Cl, Br, I) with monodentate nitrogen-containing donors Py, pip, and NEt(3) as well as the structure of the AlCl(3)·PPh(3) adduct have been established for the first time by X-ray diffraction studies. Extensive theoretical studies (B3LYP/TZVP level of theory) of structurally characterized complexes between MX(3) and nitrogen-, phosphorus-, arsenic-, and oxygen-containing donor ligands have allowed us to establish the Lewis acidity trends Al > Ga, Cl ≈ Br > I. Analysis of the experimental and theoretical results points out that the solid state masks the Lewis acidity trend of aluminum halides. The difference in the Al-N bond distances between AlCl(3)·D and AlBr(3)·D complexes in the gas phase is small, while in the condensed phase, shorter Al-N distances for AlBr(3)·D complexes are observed with 9-fluorenone, mdta, and NEt(3) donors. The model based on intermolecular (H···X) interactions in solid adducts is proposed to explain this phenomenon. Thus, the donor-acceptor bond distance in the solid complexes cannot always be used as a criterion of Lewis acidity.

Donor-acceptor complexes of borazines.

Donor-acceptor complexes of borazine (BZ) and its substituted derivatives with Lewis acids (A = MCl(3), MBr(3); M = B, Al, Ga) and Lewis bases (D = NH(3), Py) have been theoretically studied at the B3LYP/TZVP level of theory. The calculations showed that complexes with Lewis bases only are unstable with respect to dissociation into their components, while complexes with Lewis acids only (such as aluminum and gallium trihalides) are stable. It was shown that formation of ternary D→BZ→A complexes may be achieved by subsequent introduction of the Lewis acid (acceptor A) and the Lewis base (donor D) to borazine. The nature of substituents in the borazine ring, their number, and position were shown to have only minor influence on the stability of ternary D→BZ→A complexes due to the compensation effect. Much weaker acceptor properties of borazine are explained in terms of large endothermic pyramidalization energy of the boron center in the borazine ring. In contrast to borazine, binary complexes of the isoelectronic benzene were predicted to be weakly bound even in the case of very strong Lewis acids; ternary DA complexes of benzene were predicted to be unbound. The donor-acceptor complex formation was predicted to significantly reduce both the endothermicity (by 70-95 kJ mol(-1)) and the activation energy (by 40-70 kJ mol(-1)) for the borazine hydrogenation. Thus, activation of the borazine ring by Lewis acids may be a facile way for the hydrogenation of borazines and polyborazines.

Structural and thermodynamic properties of molecular complexes of aluminum and gallium trihalides with bifunctional donor pyrazine: decisive role of Lewis acidity in 1D polymer formation

Solid state structures of group 13 metal halide complexes with pyrazine (pyz) of 2:1 and 1:1 composition have been established by X-ray structural analysis. Complexes of 2:1 composition adopt molecular structures MX3·pyz·MX3 with tetrahedral geometry of group 13 metals. Complexes of AlBr3 and GaCl3 of 1:1 composition are 1D polymers (MX3·pyz)∞ with trigonal bipyramidal geometry of the group 13 metal, while the weaker Lewis acid GaI3 forms the monomeric molecular complex GaI3·pyz, which is isostructural to its pyridine analog GaI3·py. Tensimetry studies of vaporization and thermal dissociation of AlBr3·pyz and AlBr3·pyz·AlBr3 complexes have been carried out using the static method with a glass membrane null-manometer. Thermodynamic characteristics of vaporization and equilibrium gas phase dissociation of the AlBr3·pyz complex have been determined. Comprehensive theoretical studies of (MX3)n·(pyz)m complexes (M = Al, Ga; X = Cl, Br, I; n = 1, 2; m = 1-3) have been carried out at the B3LYP/TZVP level of theory. Donor-acceptor bond energies were obtained taking into account reorganization energies of the fragments. Computational data indicate that the formation of (MX3·pyz)∞ polymers with coordination number 5 is only slightly more energetically favorable than the formation of molecular complexes of type MX3·pyz for X = Cl, Br. It is expected that on melting (MX3·pyz)∞ polymers dissociate into individual MX3·pyz molecules. This dovetails with low melting enthalpies of the (MX3·pyz)∞ complexes. Polymer stability decreases in the order AlCl3 > AlBr3 > GaCl3 > AlI3 > GaBr3 > GaI3. For MI3·pyz complexes computations predict that the monomeric structure motif is more energetically favorable compared to the catena polymer. These theoretical predictions agree well with the experimentally observed monomeric complex GaI3·pyz in the solid state. Thus, the Lewis acidity of the group 13 halides may play a decisive role in the formation of 1D polymeric networks.

Theoretical study of donor-acceptor ability of borazine, alumazine, and boraphosphinine

Donor-acceptor complexes of borazine, alumazine, and boraphosphinine were studied by a quantum-chemical method. Structural and thermodynamic characteristics of complexes with Lewis acids (BCl3 and AlCl3) and bases (NH3 and pyridine Py) were calculated by the B3LYP method with the TZVP basis set. Energies of donor-acceptor bonds and energies of reorganization of donors, acceptors, and heterocycles upon the complex formation were found. Analysis of the energy variations occurring at the complex formation has shown that the reorganization energies of acceptors (BCl3 and AlCl3) and heterocycles play a key role in the complex stabilizations, whereas the reorganization energies of donors (NH3 and Py) are small and do not bring essential contribution to the complex-formation energy. The stability of donor-acceptor complexes decreases in the sequence alumazine > boraphosphinine > borazine. High alumazine reactivity toward chlorine atoms of the acceptor molecules BCl3 and AlCl3 was noted.

Structures and stability of molecular InBr3Py(x) (x = 1-3) complexes: unexpected solid state stabilization of dimeric In2Br6Py4 as compared to valence-isoelectronic group 15 and 17 halogen bridging dimers.

Molecular structures of series of InBr3Py(x) complexes (x = 1-3) in the solid state have been determined by single crystal structure analysis. For x = 2, an unexpected dimeric In2Br6Py4 structure, which features a nearly planar In2Br6 unit, has been established. This structure completes the series of known valence-isoelectronic dimeric molecules of group 17 (I2Cl6) and group 15 elements (As2Cl6·2PMe3). Theoretical studies at the B3LYP/def2-TZVP level of theory reveal that all gaseous M2X6Py4 dimers (M = Al, Ga, In, Tl; X = Cl, Br) are energetically unstable with respect to dissociation into MX3Py2 monomers. This finding is in stark contrast to the valence-isoelectronic group 17 and 15 analogs, which are predicted to be energetically stable with respect to dissociation. Thus, additional interactions in the solid state play a crucial role in stabilization of the experimentally observed dimeric In2Br6Py4. Thermal stability and volatility of InBr3Py(x) complexes have been studied by tensimetry and mass spectrometry methods. Mass spectrometry data indicate that, in contrast to the lighter group 13 element halides, species with two In atoms, such as In2Br6Py2, are present in the gas phase. Thermodynamic characteristics for the heterogeneous dissociation processes of InBr3Py(x) (x = 2, 3) complexes with Py evolution have been determined.

Quantum chemical studies of hydrogenation of borazine and polyborazines in the presence of Lewis acids

Thermodynamic parameters and activation energies of hydrogenation processes of borazine, polyborazines, and their donor-acceptor complexes were calculated by the B3LYP/TZVP quantum chemical method. Formation of donor-acceptor complexes of borazine and polyborazines with Lewis acids leads to a considerable decrease in endothermicity and activation energy of their hydrogenation. This allows us to recommend Lewis acids for the use as catalysts in hydrogenation of borazine and polyborazines. Hydrogenation of polyborazines primarily occurs at the heterocycle periphery. The reactivity of polyborazines toward hydrogenation decreases with their increasing size.

Initial gas phase reactions between Al(CH3)3/AlH3 and ammonia: theoretical study.

Mechanisms of initial stages of gas phase reactions between trimethylaluminum and ammonia have been explored by DFT studies. Subsequent substitution of CH3 groups in AlMe3 by amido groups and substitution of hydrogen atoms in ammonia by AlMe2 groups have been considered. Structures of Al(CH3)x(NH2)3-x, NHx(Al(CH3)2)3-x (x = 0-3) and related donor-acceptor complexes, dimerization products, and reaction pathways for the methane elimination have been obtained. The transition state for the first methane elimination from Al(CH3)3NH3 adduct is the highest point on the reaction pathway; subsequent processes are exothermic and do not require additional activation energy. In excess ammonia, subsequent methane elimination reactions may lead to formation of [Al(NH2)3]2, while in excess trimethylaluminum, formation of N(AlMe2)3 is feasible. Formation of [AlMe2NH2]2 dimer is very favorable thermodynamically. Studies on model reactions between AlH3 and NH3 indicate that reaction barriers obtained for hydrogen-substituted species may serve as an upper estimate in studying the reactivity of methyl-substituted analogues in more complex systems.

Complex beryllium amidoboranes: Structures, stability, and evaluation of their potential as hydrogen storage materials

Complex beryllium amidoboranes Mx[Be(NH2BH3)x+2] (M = Li‐Cs, x = 1,2) have been computationally studied at M06‐2X/def2‐TZVPPD//B3LYP/def2‐TZVPPD level of theory. Compounds are predicted to be stable at room temperature but release H2 on heating. Agostic Be…HB bonds play an important role in stabilization of oligomeric beryllium imidoboranes. Polymeric imidoborane, hydrogen, and ammonia are expected as major thermal decomposition products of complex beryllium amidoboranes. Ammonia evolution is predicted to proceed at slightly higher temperatures than hydrogen evolution. Based on thermodynamic analysis, Li[Be(NH2BH3)3] and Li2[Be(NH2BH3)4] are the most perspective synthetic targets. Synthetic approaches to these potentially efficient hydrogen storage materials have been proposed.

Donor–acceptor complexes of inorganic analogs of benzene

GRAPHICAL ABSTRACT ABSTRACT Results of experimental and theoretical studies of inorganic benzene analogs: borazine, substituted borazines, polyborazines, alumazene, and their donor–acceptor complexes are summarized. Structural and energetic aspects of complex formation and thermal stability of heterocycles and their complexes are discussed. A mechanism for the gas-phase acetonitrile polymerization in the presence of alumazene is proposed on the basis of computational studies. It is experimentally shown that solution of B,B′,B″-tribromborazine in deuterobenzene undergoes fast (within minutes) H/D exchange in the presence of Lewis acid AlBr3. The proposed electrophilic substitution mechanism for the exchange is supported by quantum-chemical computations. In contrast, in the presence of AlBr3, unsubstituted borazine in deuterobenzene polymerizes with hydrogen evolution without H/D exchange. The absence of the H/D exchange may be explained by larger stability of the borazonium ion B3N3H7+ which prevents operation of the catalytic cycle. Quantum-chemical computations at B3LYP/TZVP level of theory indicate that upon complexation with AlCl3 both endothermicity and activation energies of hydrogenation processes of borazine and polyborazines are significantly reduced. The use of Lewis acids as catalysts in the processes of regeneration of spent hydrogen fuel is recommended.

Complexes of borazine and its analogs with Lewis acids and bases

The author’s review summarizes the results of B3LYP/TZVP quantum chemical calculations of complexes of borazine, substituted borazines, polyborazines, alumazine, and boraphosphabenzene with Lewis acids and bases. The effects of the nature of the heterocycle, substituents, and the donor and acceptor properties of the molecules on the thermodynamic characteristics of complex formation are considered. The reactivity of the complexes of heterocycles in hydrogenation and electrophilic substitution reactions was studied.