Structures and stability of I2 and ICl complexes with pyridine: Ab initio and DFT study.

Theoretical investigation of thermodynamic stability and bonding features of possible isomers of the molecular and ionic complexes of pyridine with molecular iodine and iodine monochloride IX (X = I,Cl) is presented. M06-2X DFT functional is found to provide bond distances and dissociation energies which are close to those obtained at high-level ab initio CCSD(T)/aug-cc-pvtz//CCSD/aug-cc-pvtz benchmark computations for the most stable isomers, formed via donation of a lone pair of nitrogen atom of pyridine to the iodine atom. These isomers are by 23-33 kJ mol-1 (in case of I2) and by 39-56 kJ mol-1 (in case of ICl) more stable than other molecular complexes. T-shaped π-σ* bonded isomers turn out to be energetically comparable with van der Waals bound compounds. Among the ionic isomers, structures featuring [IPy2]+ cation with I3 - or ICl2 - counterions are more stable. Oligomerization favors ionic isomers starting from the tetrameric clusters of the composition (IX)4Py4.

Lewis acid stabilized group 13-15 element analogs of ethylene.

Stabilization of hydrogen-substituted group 13-15 compounds H2 EE'H2 (E = B, Al, Ga; E' = P, As, Sb) by Lewis acids is considered at B3LYP/def2-TZVP, B3LYP-D3/def2-TZVP and M06-2X/def2-TZVP levels of theory. It is shown, that for many Lewis acids additional reactivity beyond the DA complex formation with H2 EE'H2 monomer is expected. In case of complexation with E(C6 F5 )3 , F/H exchange reactions with group 13 bound hydrides are predicted to be exothermic and accompanied by the activation energies which are smaller than dissociation of the complex into components. In case of complex formation with transition metal (TM) carbonyls, additional O → Al, TM-C → Al interactions are observed, which in several cases lead to cyclic structures. The most promising candidates for the experimental studies have been identified. Synthetic approaches to the most promising LA-only stabilized compounds are recommended.

Hydrogen Activation by Frustrated and Not So Frustrated Lewis Pairs Based on Pyramidal Lewis Acid 9-Boratriptycene: A Computational Study.

Structural features and reactivity of frustrated Lewis pairs (FLPs) formed by pyramidal group 13 Lewis acids based on 9-bora and 9-alatriptycene and bulky phosphines P t Bu3, PPh3, and PCy3 are considered at the M06-2X/def2-TZVP level of theory. Classic FLP is formed only in the B(C6Me4)3CH/P t Bu3 system, while both FLP and donor-acceptor (DA) complex are observed in the B(C6F4)3CF/P t Bu3 system. Formation of DA complexes was observed in other systems; the B(C6H4)3CH·P t Bu3 complex features an elongated DA bond and can be considered a "latent" FLP. Transition states and reaction pathways for molecular hydrogen activation have been obtained. Processes of heterolytic hydrogen splitting are energetically more favored in solution compared to the gas phase, while activation energies in the gas phase and in solution are close. The alternative processes of hydrogenation of B-C or Al-C bonds in the source pyramidal Lewis acids in the absence of a Lewis base are exergonic but have larger activation energies than those for heterolytic hydrogen splitting. The tuning of Lewis acidity of 9-boratriptycene by changing the substituents allows one to control its reactivity with respect to hydrogen activation. Interestingly, the most promising system from the practical point of view is the DA complex B(C6H4)3CH·P t Bu3, which is predicted to provide both low activation energy and thermodynamic reversibility of the heterolytic hydrogen splitting process. It appears that such "not so frustrated" or "latent" FLPs are the best candidates for reversible heterolytic hydrogen splitting.

Unusual molecular complexes of antimony fluoride dimers with acetonitrile and pyridine: structures and bonding.

The structures of two new molecular complexes of antimony pentafluoride with pyridine (Py) and acetonitrile (AN), SbF5·Py and Sb2F10·AN, and a molecular complex of antimony trifluoride Sb2F6·Py and its ionic derivative [HPy]+[Sb2F7]- in the solid state have been determined by single crystal X-ray structural analysis. The complexes Sb2F10·AN and Sb2F6·Py are the first structurally characterized compounds of dimeric antimony fluorides. To reveal the nature of bonding in the complexes and their stability, DFT computations of the electronic structure and thermodynamic characteristics were performed, in particular the analysis of the electrostatic potentials, the orbital interactions and the topology. The results indicate that the intermolecular Sb⋯F interactions can be described as a network of pnictogen bonds.

Influence of the solvent on the Lewis acidity of antimony pentahalides and group 13 Lewis acids toward acetonitrile and pyridine.

Energetic effects of solvation of SbF5 , SbCl5 , and 21 group 13 Lewis acids (LA) and their molecular complexes with acetonitrile and pyridine are evaluated using SMD approach. Compared to the gas phase, solvation increases the stability of boron- and aluminum-containing complexes but decreases the stability of gallium and indium-containing homologs due to larger solation energies of free LA. New Lewis acidity scales, based on the Gibbs energy of dissociation of the molecular complexes LA·pyridine and LA·acetonitrile in the gas phase, in benzene and acetonitrile solutions, are proposed.

Stability and Electronic Structure of Donor-Acceptor Stabilized Group 13/15 Oligomers.

Electronic structures and thermodynamic characteristics of chain inorganic group 13-15 oligomers [H2MEH2]n (M = B, Al, Ga, E = P, As; n = 4-15) are presented. Donor-acceptor interaction with both Lewis acids and Lewis bases effectively stabilizes chain isomers with respect to spontaneous cyclization and significantly changes their electronic structure.

Structures and Chemical Bonding in Antimony(III) Bromide Complexes with Pyridine.

Weakly or "partially" bonded molecules are an important link between the chemical and van der Waals interactions. Molecular structures of six new SbBr3 -Py complexes in the solid state have been determined by single-crystal X-ray diffraction analysis. In all complexes all Sb atoms adopt a pseudo-octahedral coordination geometry which is completed by additional Sb⋅⋅⋅Br contacts shorter than the sum of the van der Waals radii, with Br-Sb⋅⋅⋅Br angles close to 180°. To reveal the nature of Sb-Br and Sb-N interactions, the DFT calculations were performed followed by the analysis of the electrostatic potentials, the orbital interactions and the topological analysis. Based on Natural Bond Orbital (NBO) analysis, the Sb-Br interactions range from the covalent bonds to the pnictogen bonds. A simple structural parameter, non-covalence criterion (NCC) is defined as a ratio of the atom-atom distance to the linear combination of sums of covalent and van der Waals radii. NCC correlates with E(2) values for Sb-N, Sb-Cl and Sb-Br bonds, and appears to be useful criterion for a preliminary evaluation of the bonding situation.

Why Do B-P and Al-P Polymers Differ? Structures, Stability, and Electronic Properties of Chain and Ring [H2 PEH2 ]n Oligomers (E=B, Al; n=1-15).

Chain and ring [H2 PEH2 ]n oligomers (E=B, Al; n=2-15) have been computationally studied at the B3LYP/def2-TZVP level of theory. A strong tendency to cyclization was found for the chain Al-P oligomers with n>7, whereas analogous long B-P oligomers can exist as chain structures. Subsequent oligomerization enthalpies approach each other very closely for the ring and chain oligomers with an increase of n. However, these oligomers have very different electronic properties with band gaps below 0.2 eV for the chain [H2 PBH2 ]15 and more than 5 eV for the ring isomer [H2 PEH2 ]15 . The molecular orbitals in the ring oligomers are highly delocalized, whereas for the chain isomers, the HOMO and LUMO are localized at the different ends of the oligomeric chain.

Trimeric cluster of lithium amidoborane-the smallest unit for the modeling of hydrogen release mechanism.

A detailed first-principle DFT M06/6-311++G(d.p) study of dehydrogenation mechanism of trimeric cluster of lithium amidoborane is presented. The first step of the reaction is association of two LiNH2 BH3 molecules in the cluster. The dominant feature of the subsequent reaction pathway is activation of H atom of BH3 group by three Li atoms with formation of unique Li3 H moiety. This Li3 H moiety is destroyed prior to dehydrogenation in favor of formation of a triangular Li2 H moiety, which interacts with protic H atom of NH2 group. As a result of this interaction, Li2 H2 moiety is produced. It features N(-) H(+) H(-) group suited near the middle plane between two Li(+) in the transition state that leads to H2 release. The transition states of association and hydrogen release steps are similar in energy. It is concluded that the trimer, (LiNH2 BH3 )3 , is the smallest cluster that captures the essence of the hydrogen release reaction.

Mechanisms of Hydrogen Generation from Tetrameric Clusters of Lithium Amidoborane.

The first-principles study of dehydrogenation mechanism of tetrameric clusters of lithium amidoborane LiNH2BH3, (LiAB)4, is presented. The choice of tetramer is based on the suspicion that dimeric cluster models used in previous theoretical studies are too small to capture the essence of the reaction. Dehydrogenation pathways starting from three isomers of (LiAB)4 tetramers were explored by applying the artificial force induced reaction (AFIR) method at the M06 level of theory. All obtained reaction pathways feature initial dimerization of two LiAB molecules in the tetramer. Formation of intermediates containing the Li3H moiety is a very characteristic feature of all pathways. In the succeeding rate-limiting step of the release of H2 molecule, a hydridic H atom of the Li3H moiety activates a protic H atom of the NH2 group with formation of the Li2H2 moiety in transition state. The most kinetically favorable pathway has the activation enthalpy of 26.6 kcal mol(-1), substantially lower than that found for dimeric cluster. The obtained results suggest that only three LiAB molecules directly participate in the elementary reactions.