Myrna H. Matus

A Hybrid Organic/Inorganic Benzene

Benzene (c-C6H6) is arguably one of the most fundamentally significant small molecules in chemistry. First discovered by Faraday in 1825, the study of benzene introduced the basic concept of aromaticity and delocalization. In addition to its fundamental importance, benzene and its derivatives (arenes) are ubiquitous in chemical research with numerous applications ranging from biomedical research to materials science. The inorganic isoelectronic relative of benzene, borazine (cB3N3H6), [4] has also played a pivotal role in fundamental as well as applied chemistry. The isoelectronic and isostructural relationship between the B N and C=C bond and its consequence on the aromaticity of borazine has been a topic of discussion. From a more applied perspective, borazine serves as a precursor to BN-based ceramic materials. More recently, borazine has been implicated in chemical hydrogen storage applications because it is formed as an intermediate in the hydrogen release from ammonia– borane. Both benzene and borazine have been known for more than 80 years, and consequently, their chemical and physical properties have been thoroughly investigated. The corresponding organic/inorganic (or organometalloidal) hybrid structure containing carbon, boron, and nitrogen, that is, 1,2-dihydro-1,2-azaborine 1, has thus far eluded characterization. The development of boron–nitrogen heterocycles such as 1,2-dihydro-1,2-azaborines (from hereon in, abbreviated as 1,2-azaborines) has been a relatively unexplored area of research. Dewar and White pioneered the chemistry of monocyclic and ring-fused polycyclic 1,2-azaborine derivatives in the 1960s. Recently, contributions by the groups of Ashe, Piers, and Paetzold, as well as our group have further advanced the preparation of novel BN heterocycles and sparked a renewed interest in the chemistry and properties of these compounds. Despite the advances achieved to date, and given the powerful tools made available by modern chemical synthesis, it is surprising that a simple heterocycle such as the parent 1,2-dihydro-1,2-azaborine 1 has remained elusive. Dewar attempted its synthesis and isolation in 1967 but ultimately concluded that it “seems to be a very reactive and chemically unstable system, prone to polymerization and other reactions.” Herein, we describe the first isolation and characterization of 1,2-dihydro-1,2-azaborine. Its successful preparation allows a direct comparison of the physical and spectroscopic properties of the series of an organic, inorganic, and now, an organometalloidal benzene. The present study demonstrates that 1,2-dihydro-1,2-azaborine 1 is not only isolable but it actually exhibits remarkable stability, consistent with substantial aromatic character. Our experimentally determined structural and spectroscopic properties are consistent with values derived from high-level computations. Scheme 1 illustrates our synthetic route to compound 1. Coupling of the in situ-generated allylboron dichloride with tert-butyldimethylsilyl allyl amine (TBS allyl amine) furnished diene 2. Ring-closing metathesis of this intermediate with the first-generation Grubbs catalyst produced an isomeric mixture of 3 and 3’ (60:40 ratio) in 82 % yield. Dehydrogenation of this mixture was carried out in the presence of catalytic amounts of Pd/C to generate 4. Treatment of heterocycle 4 with LiBHEt3 installed the B H functionality to give 5 in quantitative yield. Complexation of 1,2-azaborine 5 to {Cr(CO)3} produced the piano-stool adduct 6. Subsequent removal of the N-protecting group gave 7 in 76% yield. Finally, decomplexation of 1 from {Cr(CO)3} was accomplished using triphenylphosphine. The use of {Cr(CO)3} as a temporary “protecting group” was necessary because efforts toward cleaving the N TBS bond directly from 5 were unsuccessful. Compound 1 proved to be difficult to isolate, owing to its high volatility. However, we ultimately accomplished its isolation (10 % yield) by fractional vacuum transfer in the presence of a low-boiling [*] M. H. Matus, Prof. Dr. D. A. Dixon Department of Chemistry, University of Alabama Tuscaloosa, AL 35487 (USA) E-mail: dadixon@as.ua.edu

Coordination of aminoborane, NH2BH2, dictates selectivity and extent of H2 release in metal-catalysed ammonia borane dehydrogenation

In situ(11)B NMR monitoring, computational modeling, and external trapping studies show that selectivity and extent of H(2) release in metal-catalysed dehydrogenation of ammonia borane, NH(3)BH(3), are determined by coordination of reactive aminoborane, NH(2)BH(2), to the metal center.

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation.

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.

Thermochemistry and electronic structure of small boron and boron oxide clusters and their anions.

Thermochemical properties of a set of small boron (B(n)) and boron oxide (B(n)O(m)) clusters, with n = 1-4 and m = 0-3, their anions, and the B(4)(2-) dianion, were calculated by using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, Q, 5) basis sets extrapolated to the complete basis set limit with additional corrections. Enthalpies of formation, bond dissociation energies, singlet-triplet or doublet-quartet separation gaps, adiabatic electron affinities (EA), and both vertical electron attachment and detachment energies were evaluated. The predicted heats of formation show agreement close to the error bars of the literature results for boron oxides with the largest error for OBO. Our calculated adiabatic EAs are in good agreement with recent experiments: B (calc, 0.26 eV; exptl, 0.28 eV), B(2) (1.95, 1.80), B(3) (2.88, 2.820 +/- 0.020), B(4) (1.68, 1.60 +/- 0.10), BO (2.50, 2.51), BO(2) (4.48, 4.51), BOB (0.07), B(2)O(2) (0.37), B(3)O (2.05), B(3)O(2) (2.94, 2.94), B(4)O (2.58), and B(4)O(2) (3.14, 3.160 +/- 0.015). The BO bond is strong, so this moiety is maintained in most of the clusters. Thermochemical parameters of clusters are not linearly additive with respect to the number of B atoms. The EA tends to be larger in the dioxides. The growth mechanism of small boron oxides should be determined by a number of factors: (i) formation of BO bonds, (ii) when possible, formation of a cyclic B(3) or B(4), and (iii) combination of a boron cycle and a BO bond. When these factors compete, the strength of the BO bonds tends to compensate the destabilization arising from a loss of binding in the cyclic boron clusters, in such a way that a linear boron oxide prevails. When the B(2) moiety is present in these linear clusters, the oxide derivatives prefer a high spin state.

Fundamental Thermochemical Properties of Amino Acids: Gas-Phase and Aqueous Acidities and Gas-Phase Heats of Formation

The gas-phase acidities of the 20 L-amino acids have been predicted at the composite G3(MP2) level. A broad range of structures of the neutral and anion were studied to determine the lowest energy conformer. Excellent agreement is found with the available experimental gas-phase deprotonation enthalpies, and the calculated values are within experimental error. We predict that tyrosine is deprotonated at the CO(2)H site. Cysteine is predicted to be deprotonated at the SH but the proton on the CO(2)H is shared with the S(-) site. Self-consistent reaction field (SCRF) calculations with the COSMO parametrization were used to predict the pK(a)s of the non-zwitterion form in aqueous solution. The differences in the non-zwitterion pK(a) values were used to estimate the free energy difference between the zwitterion and nonzwitterion forms in solution. The heats of formation of the neutral compounds were calculated from atomization energies and isodesmic reactions to provide the first reliable set of these values in the gas phase. Further calculations were performed on five rare amino acids to predict their heats of formation, acidities, and pK(a) values.

Bond Dissociation Energies in Second-Row Compounds

Heats of formation at 0 and 298 K are predicted for PF3, PF5, PF3O, SF2, SF4, SF6, SF2O, SF2O2, and SF4O as well as a number of radicals derived from these stable compounds on the basis of coupled cluster theory [CCSD(T)] calculations extrapolated to the complete basis set limit. In order to achieve near chemical accuracy (+/-1 kcal/mol), additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and vibrational zero-point energies. The calculated values substantially reduce the error limits for these species. A detailed comparison of adiabatic and diabatic bond dissociation energies (BDEs) is made and used to explain trends in the BDEs. Because the adiabatic BDEs of polyatomic molecules represent not only the energy required for breaking a specific bond but also contain any reorganization energies of the bonds in the resulting products, these BDEs can be quite different for each step in the stepwise loss of ligands in binary compounds. For example, the adiabatic BDE for the removal of one fluorine ligand from the very stable closed-shell SF6 molecule to give the unstable SF5 radical is 2.8 times the BDE needed for the removal of one fluorine ligand from the unstable SF5 radical to give the stable closed-shell SF4 molecule. Similarly, the BDE for the removal of one fluorine ligand from the stable closed-shell PF3O molecule to give the unstable PF2O radical is higher than the BDE needed to remove the oxygen atom to give the stable closed-shell PF3 molecule. The same principles govern the BDEs of the phosphorus fluorides and the sulfur oxofluorides. In polyatomic molecules, care must be exercised not to equate BDEs with the bond strengths of given bonds. The measurement of the bond strength or stiffness of a given bond represented by its force constant involves only a small displacement of the atoms near equilibrium and, therefore, does not involve any reorganization energies, i.e., it may be more appropriate to correlate with the diabatic product states.

Thermochemistry for the Dehydrogenation of Methyl-Substituted Ammonia Borane Compounds

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for (CH3)H2N-BH3, (CH3)HN=BH2, (BH3)HN=CH2, (CH3)H2B-NH3, (CH3)HB=NH2, and (NH3)HB=CH2, as well as various molecules involved in the different bond-breaking processes, from coupled cluster theory (CCSD(T)) calculations. In order to achieve near-chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core CCSD(T) energies, corrections for core-valence, scalar relativistic, and first-order atomic spin-orbit effects. Scaled vibrational zero-point energies were computed with the MP2 method. The heats of formation were predicted for the respective dimethyl- and trimethyl-substituted ammonia boranes, their dehydrogenated derivatives, and the various molecules involved in the different bond breaking processes, based on isodesmic reaction schemes calculated at the G3(MP2) level. Thermodynamics for dehydrogenation pathways in the monomethyl-substituted molecules were predicted. Dehydrogenation across the B-N bond is more favorable as opposed to dehydrogenation across the B-C and N-C bonds. Methylation at N reduces the exothermocity of the dehydrogenation reaction and makes the reaction more thermoneutral, while methylation at B moves it away from thermoneutral. Various mixtures of CH3NH2BH3 and NH3BH3 were made, and their melting points were measured. The lowest melting mixture contained approximately 35% NH3BH3 by weight and melted at 35-37 degrees C.

Dehydrogenation Reactions of Cyclic C2B2N2H12 and C4BNH12 Isomers

The energetics for different dehydrogenation pathways of C(2)B(2)N(2)H(12) and C(4)BNH(12) cycles were calculated at the B3LYP/DGDZVP2 and G3(MP2) levels with additional calculations at the CCSD(T)/complete basis set level. The heats of formation of the different isomers were calculated from the G3(MP2) relative energies and the heats of formation of the most stable isomers of c-C(2)B(2)N(2)H(6), c-C(2)B(2)N(2)H(12), and c-C(4)BNH(12) at the CCSD(T)/CBS including additional corrections together with the previously reported value for c-C(4)BNH(6). Different isomers were analyzed for c-C(2)B(2)N(2)H(x) and c-C(4)BNH(x) (x = 6 and 12), and the most stable cyclic structures were those with C-C-B-N-B-N and C-C-C-C-B-N sequences, respectively. The energetics for the stepwise loss of three H(2) were predicted, and the most feasible thermodynamic pathways were found. Dehydrogenation of the lowest energy c-C(2)B(2)N(2)H(12) isomer (6-H(12)) is almost thermoneutral with DeltaH(3dehydro) = 3.4 kcal/mol at the CCSD(T)/CBS level and -0.6 kcal/mol at the G3(MP2) level at 298 K. Dehydrogenation of the lowest energy c-C(4)BNH(12) isomer (7-H(12)) is endothermic with DeltaH(3dehydro) = 27.9 kcal/mol at the CCSD(T)/CBS level and 23.5 kcal/mol at the G3(MP2) level at 298 K. Dehydrogenation across the B-N bond is more favorable as opposed to dehydrogenation across the B-C, N-C, and C-C bonds. Resonance stabilization energies in relation to that of benzene are reported as are NICS NMR chemical shifts for correlating with the potential aromatic character of the rings.

Thermodynamic Properties of the XO2, X2O, XYO, X2O2, and XYO2 (X, Y = Cl, Br, and I) Isomers

High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolated to the complete basis set limit have been made for the thermodynamics of the BrBrO(2), IIO(2), ClBrO(2), ClIO(2), and BrIO(2) isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO(2) isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO(2) and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O-Br bond with a bond dissociation energy (BDE) of 15.9 kcal/mol, and in BrBrO(2), it is the Br-Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O-Br bond with a BDE of 12.6 kcal/mol. Of the IIO(2) isomers, IIO(2) is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO(2) is the I-I bond with a BDE of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O-I bonds with values of 19.0 and 5.2 kcal/mol, respectively.

Reactions of Diborane with Ammonia and Ammonia Borane: Catalytic Effects for Multiple Pathways for Hydrogen Release

High-level electronic structure calculations have been used to construct portions of the potential energy surfaces related to the reaction of diborane with ammonia and ammonia borane (B2H6 + NH3 and B2H6 + BH3NH3)to probe the molecular mechanism of H2 release. Geometries of stationary points were optimized at the MP2/aug-cc-pVTZ level. Total energies were computed at the coupled-cluster CCSD(T) theory level with the correlation-consistent basis sets. The results show a wide range of reaction pathways for H2 elimination. The initial interaction of B2H6 + NH3 leads to a weak preassociation complex, from which a B-H-B bridge bond is broken giving rise to a more stable H3BHBH2NH3 adduct. This intermediate, which is also formed from BH3NH3 + BH3, is connected with at least six transition states for H2 release with energies 18-93 kal/mol above the separated reactants. The lowest-lying transition state is a six-member cycle, in which BH3exerts a bifunctional catalytic effect accelerating H2 generation within a B-H-H-N framework. Diborane also induces a catalytic effect for H2 elimination from BH3NH3 via a three-step pathway with cyclic transition states. Following conformational changes, the rate-determining transition state for H2 release is approximately 27 kcal/mol above the B2H6 + BH3NH3 reactants, as compared with an energy barrier of approximately 37 kcal/mol for H2 release from BH3NH3. The behavior of two separated BH3 molecules is more complex and involves multiple reaction pathways. Channels from diborane or borane initially converge to a complex comprising the H3BHBH2NH3adduct plus BH3. The interaction of free BH3 with the BH3 moiety of BH3NH3 via a six-member transition state with diborane type of bonding leads to a lower-energy transition state. The corresponding energy barrier is approximately 8 kcal/mol, relative to the reference point H3BHBH2NH3 adduct + BH3. These transition states are 27-36 kcal/mol above BH3NH3 + B2H6, but 1-9 kcal/mol below the separated reactants BH3NH3 + 2 BH3. Upon chemical activation of B2H6 by forming 2 BH3, there should be sufficient internal energy to undergo spontaneous H2 release. Proceeding in the opposite direction, the H2 regeneration of the products of the B2H6 + BH3NH3reaction should be a feasible process under mild thermal conditions.