Neil J. Henson

Importance of Out-of-State Spin–Orbit Coupling for Slow Magnetic Relaxation in Mononuclear FeII Complexes

Two mononuclear high-spin Fe(II) complexes with trigonal planar ([Fe(II)(N(TMS)(2))(2)(PCy(3))] (1) and distorted tetrahedral ([Fe(II)(N(TMS)(2))(2)(depe)] (2) geometries are reported (TMS = SiMe(3), Cy = cyclohexyl, depe = 1,2-bis(diethylphosphino)ethane). The magnetic properties of 1 and 2 reveal the profound effect of out-of-state spin-orbit coupling (SOC) on slow magnetic relaxation. Complex 1 exhibits slow relaxation of the magnetization under an applied optimal dc field of 600 Oe due to the presence of low-lying electronic excited states that mix with the ground electronic state. This mixing re-introduces orbital angular momentum into the electronic ground state via SOC, and 1 thus behaves as a field-induced single-molecule magnet. In complex 2, the lowest-energy excited states have higher energy due to the ligand field of the distorted tetrahedral geometry. This higher energy gap minimizes out-of-state SOC mixing and zero-field splitting, thus precluding slow relaxation of the magnetization for 2.

Iron Complex-Catalyzed Ammonia–Borane Dehydrogenation. A Potential Route toward B–N-Containing Polymer Motifs Using Earth-Abundant Metal Catalysts

Ammonia-borane (NH(3)BH(3), AB) has garnered interest as a hydrogen storage material due to its high weight percent hydrogen content and ease of H(2) release relative to metal hydrides. As a consequence of dehydrogenation, B-N-containing oligomeric/polymeric materials are formed. The ability to control this process and dictate the identity of the generated polymer opens up the possibility of the targeted synthesis of new materials. While precious metals have been used in this regard, the ability to construct such materials using earth-abundant metals such as Fe presents a more economical approach. Four Fe complexes containing amido and phosphine supporting ligands were synthesized, and their reactivity with AB was examined. Three-coordinate Fe(PCy(3))[N(SiMe(3))(2)](2) (1) and four-coordinate Fe(DEPE)[N(SiMe(3))(2)](2) (2) yield a mixture of (NH(2)BH(2))(n) and (NHBH)(n) products with up to 1.7 equiv of H(2) released per AB but cannot be recycled (DEPE = 1,2-bis(diethylphosphino)ethane). In contrast, Fe supported by a bidentate P-N ligand (4) can be used in a second cycle to afford a similar product mixture. Intriguingly, the symmetric analogue of 4 (Fe(N-N)(P-P), 3), only generates (NH(2)BH(2))(n) and does so in minutes at room temperature. This marked difference in reactivity may be the result of the chemistry of Fe(II) vs Fe(0).

Unravelling the Mechanism of the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)] Catalysts

The mechanism of catalytic hydrogenation of acetophenone by the chiral complex trans-[RuCl2{(S)-binap}{(S,S)-dpen}] and KO-t-C4H9 in propan-2-ol is revised on the basis of DFT computations carried out in dielectric continuum and the most recent experimental observations. The results of these collective studies suggest that neither a six-membered pericyclic transition state nor any multibond concerted transition states are involved. Instead, a hydride moiety is transferred in an outer-sphere manner to afford an ion-pair, and the corresponding transition state is both enantio- and rate-determining. Heterolytic dihydrogen cleavage proceeds neither by a (two-bond) concerted, four-membered transition state, nor by a (three-bond) concerted, six-membered transition state mediated by a solvent molecule. Instead, cleavage of the H-H bond is achieved via deprotonation of the η(2)-H2 ligand within a cationic Ru complex by the chiral conjugate base of (R)-1-phenylethanol. Thus, protonation of the generated (R)-1-phenylethoxide anion originates from the η(2)-H2 ligand of the cationic Ru complex and not from NH protons of a neutral Ru trans-dihydride complex, as initially suggested within the framework of a metal-ligand bifunctional mechanism. Detailed computational analysis reveals that the 16e(-) Ru amido complex [RuH{(S)-binap}{(S,S)-HN(CHPh)2NH2}] and the 18e(-) Ru alkoxo complex trans-[RuH{OCH(CH3)(R)}{(S)-binap}{(S,S)-dpen}] (R = CH3 or C6H5) are not intermediates within the catalytic cycle, but rather are off-loop species. The accelerative effect of KO-t-C4H9 is explained by the reversible formation of the potassium amidato complexes trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH2}] or trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH(K)}]. The three-dimensional (3D) cavity observed within these molecules results in a chiral pocket stabilized via several different noncovalent interactions, including neutral and ionic hydrogen bonding, cation-π interactions, and π-π stacking interactions. Cooperatively, these interactions modify the catalyst structure, in turn lowering the relative activation barrier of hydride transfer by ~1-2 kcal mol(-1) and the following H-H bond cleavage by ~10 kcal mol(-1), respectively. A combined computational study and analysis of recent experimental data of the reaction pool results in new mechanistic insight into the catalytic cycle for hydrogenation of acetophenone by Noyoris catalyst, in the presence or absence of KO-t-C4H9.

A Porous Metal−Organic Replica of α-PbO2 for Capture of Nerve Agent Surrogate

A novel metal-organic replica of α-PbO(2) exhibits high capacity for capture of nerve agent surrogate.

ForceFit: A Code to Fit Classical Force Fields to Quantum Mechanical Potential Energy Surfaces

The ForceFit program package has been developed for fitting classical force field parameters based upon a force matching algorithm to quantum mechanical gradients of configurations that span the potential energy surface of the system. The program, which runs under UNIX and is written in C++, is an easy‐to‐use, nonproprietary platform that enables gradient fitting of a wide variety of functional force field forms to quantum mechanical information obtained from an array of common electronic structure codes. All aspects of the fitting process are run from a graphical user interface, from the parsing of quantum mechanical data, assembling of a potential energy surface database, setting the force field, and variables to be optimized, choosing a molecular mechanics code for comparison to the reference data, and finally, the initiation of a least squares minimization algorithm. Furthermore, the code is based on a modular templated code design that enables the facile addition of new functionality to the program.

Electro-reduction of nitrogen on molybdenum nitride: structure, energetics, and vibrational spectra from DFT

We used density functional theory to study the electrochemical conversion of nitrogen to ammonia on the (001), (100/010), (101), and (111) surfaces of γ-Mo2N. Based on the calculated free energy profiles for the reduction of nitrogen by the associative and dissociative mechanisms, reactivity was found to decrease in the order (111) > (101) > (100/010) ≈ (001). Namely, the cell potentials needed to drive the reduction to ammonia increase in the following order: -0.7 V on (111), -1.2 V on (101), and -1.4 V on (100/010) and (001) surfaces. The (111) surface was found to be the most reactive for nitrogen due to (i) its ability to adsorb the N2 in the side-on position which activates N-N bonding and (ii) its high affinity for N-adatoms which also prevents accumulation of H-adatoms on the catalytic surface at low cell potentials. We have also calculated vibrational frequencies of different NxHy species adsorbed on various γ-Mo2N surfaces. The frequencies are found to depend strongly on the type of the binding sites available on the crystal facets. A comparison of the calculated frequencies with the frequencies of the corresponding species in transition metal complexes and other metal surfaces shows that the frequencies of several signature modes fall in a similar region and might be used to assign the spectra of hydrogen and nitrogen containing surface species on different metal surfaces.

Kinetic hysteresis in gas adsorption behavior for a rigid MOF arising from zig-zag channel structures

A new porous MOF, Zn(TBC)2·{guest}, is synthesized and studied by the single crystallography, N2 isothermal adsorption and GC separation of CO2 from air. This MOF shows large hysteresis on N2 adsorption at 77 K up to a P/Po of 0.9, which arises from the unique zig-zag channel structures of the framework. The MOF shows promising separation ability for CO2 from air.

First-Principles Prediction of the Effects of Temperature and Solvent Selection on the Dimerization of Benzoic Acid

We introduce a procedure of quantum chemical calculations (B3P86/6-31G**) to study carboxylic acid dimerization and its correlation with temperature and properties of the solvent. Benzoic acid is chosen as a model system for studying dimerization via hydrogen bonding. Organic solvents are simulated using the self-consistent reaction field (SCRF) method with the polarized continuum model (PCM). The cyclic dimer is the most stable structure both in gas phase and solution. Dimer mono- and dihydrates could be found in the gas phase if acid molecules are in contact with water vapor. However, the formation of these hydrated conformers is very limited and cyclic dimer is the principal conformer to coexist with monomer acid in solution. Solvation of the cyclic dimer is more favorable compared to other complexes, partially due to the diminishing of hydrogen bonding capability and annihilation of dipole moments. Solvents have a strong effect on inducing dimer dissociation and this dependence is more pronounced at low dielectric constants. By accounting for selected terms in the total free energy of solvation, the solvation entropy could be incorporated to predict the dimer behavior at elevated temperatures. The temperature dependence of benzoic acid dimerization obtained by this technique is in good agreement with available experimental measurements, in which a tendency of dimer to dissociate is observed with increased temperatures. In addition, dimer breakup is more sensitive to temperature in low dielectric environments rather than in solvents with a higher dielectric constant.

Conformational analysis of the crystal structure for MDI/BDO hard segments of polyurethane elastomers

From conformational analysis, we have determined the two lowest energy crystal structures for the hard segments of 4,4′-diphenylmethane diisocyanate/1,4-butanediol (MDI/BDO)-based polyurethane elastomer. Both crystal forms give prominent X-ray scattering at ∼7.6 A. In one crystal form, (1), there is strong hydrogen bonding between linear chains with a density of 1.30 g/cm3, while in the other form, (2), van der Waals bonding gives rise to a double helix structure with a density of 1.22 g/cm3 and a formation energy 1.6 kJ/mol higher than form (1). The double helix crystal has a unit cell length of 18.8 A which is about half the 34.7 A unit cell length of the hydrogen-bonded crystal. The X-ray diffraction predicted for each crystal is presented and compared with experiment.

Investigation of formally zerovalent Triphos iron complexes

The reduction of Triphos [PhP(CH(2)CH(2)PPh(2))(2)] iron halide complexes has been explored, yielding formally zerovalent (κ(3)-Triphos)Fe(κ(2)-Triphos) and (κ(3)-Triphos)Fe(κ(2)-Bpy). Electrochemical analysis, coupled with the metrical parameters of (κ(3)-Triphos)Fe(κ(2)-Bpy), reveal an electronic structure consistent with a π-radical monoanion bipyridine chelate that is antiferromagnetically coupled to a low spin, Fe(I) metal center.