Zhen Hua Li

Heterolytic Cleavage of Dihydrogen by “Frustrated Lewis Pairs” Comprising Bis(2,4,6‐tris(trifluoromethyl)phenyl)borane and Amines: Stepwise versus Concerted Mechanism

Recently, the chemistry of “frustrated Lewis pairs” (FLPs), which was introduced by the research groups of Stephan and Erker, has received considerable attention. 2] One of the most remarkable applications of FLPs is in the heterolytic activation of H2 without the involvement of transition-metals. A variety of FLP systems have been shown to activate H2 under mild conditions, and have been applied as catalysts in metal-free hydrogenation reactions. By analogy to the transition metal chemistry, it was originally proposed by Stephan and co-workers that the activation of H2 is a stepwise process, in which H2 is initially activated by the Lewis acid, followed by proton transfer to the Lewis base (Scheme 1).

Metal‐Free HB(C6F5)2‐Catalyzed Hydrogenation of Unfunctionalized Olefins and Mechanism Study of Borane‐Mediated σ‐Bond Metathesis

Out with the metal: Metal-free hydrogenation of unfunctionalized olefins can be achieved by employing HB(C6F5)2 as the catalyst. The key step in the catalytic reaction is believed to involve a novel borane-mediated σ-bond metathesis, which has been investigated both experimentally and theoretically.

Organoborane Catalyzed Regioselective 1,4-Hydroboration of Pyridines

A bulky organoborane Ar(F)2BMe (Ar(F) = 2,4,6-tris(trifluoromethyl)phenyl, 1) has been synthesized. In C6D6 solution this organoborane and pyridine form a frustrated Lewis pair. Under mild conditions, 1 can efficiently catalyze 1,4-hydroboration of a series of pyridines. This reaction is highly chemo- and regioselective. The reaction intermediate, a boronium complex [Py2Bpin][Ar(F)2B(H)Me] (3), was characterized in solution by NMR spectroscopy, which was also confirmed by DFT calculation.

CO oxidation catalyzed by a single gold atom: benchmark calculations and the performance of DFT methods

Quantum chemical calculations were carried out on CO oxidation catalyzed by a single gold atom. To investigate the performance of density functional theory (DFT) methods, 42 DFT functionals have been evaluated and compared with high-level wavefunction based methods. It was found that in order to obtain accurate results the functionals used must treat long range interaction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionals with best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional, also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do not show good performance and the performance of the latter two are even at the bottom of the 42 functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. The reaction follows the following mechanism where CO and O(2) adsorb on Au atom forming an Au(OCOO) intermediate and subsequently O(2) transfer one oxygen atom to CO to form CO(2) and AuO. Then AuO reacts with CO to form another CO(2) to complete the catalytic cycle. The overall energy barrier at 0 K for the first CO oxidation step (Au + CO + O(2)→ AuO + CO(2)) is just 4.8 kcal mol(-1), and that for the second CO oxidation step (AuO + CO → Au + CO(2)) is just 1.6 kcal mol(-1).

Carbonyl Bonding on Oxophilic Metal Centers: Infrared Photodissociation Spectroscopy of Mononuclear and Dinuclear Titanium Carbonyl Cation Complexes

Mononuclear and dinuclear titanium carbonyl cation complexes including Ti(CO)(6)(+), Ti(CO)(7)(+), TiO(CO)(5)(+), Ti(2)(CO)(9)(+) and Ti(2)O(CO)(9)(+) are produced via a laser vaporization supersonic cluster source. The ions are mass selected in a tandem time-of-flight mass spectrometer and studied with infrared photodissociation spectroscopy in the CO stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the 15-electron Ti(CO)(6)(+) cation, which is characterized to have an octahedral O(h) structure. The Ti(CO)(7)(+) cation is determined to be a weakly bound complex involving a Ti(CO)(6)(+) core ion instead of the seventh coordinated ion. The TiO(CO)(5)(+) cation has a completed coordination sphere with a C(4v) structure. The Ti(2)(CO)(9)(+) cation is determined to have a doublet C(s) structure with two four-electron donor side-on bridging CO groups and one semibridging CO group. The Ti(2)O(CO)(9)(+) cation has a doublet C(s) structure involving a planar cyclic Ti(2)O(η(2)-μ-CO) core with a four electron donor side-on bridging CO. Bonding analysis indicates that the Ti(2)(CO)(9)(+) and Ti(2)O(CO)(9)(+) cations each have a Ti-Ti single bond. The results suggest that metal-metal multiple bonding is not favorable, and the oxophilic titanium centers failed to satisfy the 18-electron configuration in these metal carbonyl complexes.

Is the FeO2(-) anion bent or linear?

FeO(2)(-) anions were produced by co-condensation of laser-ablated iron atoms and electrons with dioxygen in excess argon at 6 K. A photosensitive absorption at 870.6 cm(-1) is assigned to the antisymmetric OFeO stretching vibration (nu(3)) of the inserted FeO(2)(-) anion trapped in solid argon. On the basis of the observed nu(3) vibrational frequencies for Fe(16)O(2) and Fe(18)O(2), the anion is estimated to be linear. Due to the severe symmetry-breaking problems of the reference wave function, calculations with single-reference methods, including various DFT and post-HF methods, are unreliable for this molecule. However, the state-averaged multireference MRCI method, which incorporates both dynamical and nondynamical correlations, predicted that the anion has a linear doublet ground state, consistent with the experimental observations.

Infrared Photodissociation Spectroscopic and Theoretical Study of Homoleptic Dinuclear Chromium Carbonyl Cluster Cations with a Linear Bridging Carbonyl Group

Infrared spectra of mass-selected homoleptic dinuclear chromium carbonyl cluster cations Cr(2)(CO)(n)(+) with n = 7-9 are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. The structures are established by comparison of the experimental spectra with the simulated spectra derived from density functional calculations. The Cr(2)(CO)(n)(+) cluster cations are characterized to have the (OC)(5)Cr-C-O-Cr(CO)(n-6)(+) structures with a linear bridging carbonyl group bonded to one chromium atom through its carbon atom and to the other chromium atom through its oxygen atom. The cluster cations all have a sextet ground state with the positive charge and the unpaired electrons located on the Cr(CO)(n-6) moiety. The formation of the linear bridging structures without Cr-Cr bonding can be rationalized that chromium forms strong Cr-CO bonds but weak Cr-Cr bonds.

Spontaneous Dihydrogen Activation by Neutral TaO4 Complex at Cryogenic Temperatures

Catalytic hydrogenation is one of the most important reactions in industry. The activation of dihydrogen by metal centers is a fundamental step in nearly all metal catalytic hydrogenation reactions. Great efforts have been made to split the H H bond at metal centers under mild conditions. Traditional homogeneous hydrogenation catalysts are based on precious metals, on which dihydrogen splitting often proceeds by hemolytic cleavage to form metal hydride complexes, which can then transfer hydrogen atoms to organic or other substrates. A second mechanism, heterolytic cleavage of dihydrogen into a hydridic and protic functionality, has also been observed for many transition-metal complexes. Transition-metal oxides are vital heterogeneous catalysts or supports in many processes involving H2. Considerable theoretical investigations have been focused on H2 adsorption and heterolytic dissociation on metal oxide surfaces. The reactions of bare transition-metal oxide cations with dihydrogen have been studied in the gas phase with mass spectrometric techniques to provide useful insight into the elementary steps of catalytic reactions and to characterize reactive intermediates that have previously not been within reach of condensed-phase techniques. In contrast to cationic reaction systems, neutral transition-metal oxide reactions have gained much less attention, in part because of the experimental challenges faced in detecting neutral species in the gas phase. Herein we report a joint matrixisolation infrared spectroscopic and theoretical study of dihydrogen activation by neutral tantalum oxide molecules. We found that, upon annealing, the ground-state TaO4 d 0 complex reacts spontaneously with dihydrogen to form [HTaO(OH)(h-O2)], which involves both the hydridic (H ) and the protic (H) functionalities. We studied the reactivity of tantalum oxides in different oxidation states (TaO (II), TaO2 (IV) and TaO4 (V)) by matrix-isolation infrared absorption spectroscopy, which has previously been described in detail. The tantalum oxide reactants were prepared either by pulsed laser evaporation of bulk Ta2O5 target or by the reactions of laser-evaporated tantalum atoms with dioxygen in solid argon. As has been reported previously, pulsed laser evaporation of bulk Ta2O5 target under controlled laser energy followed by condensation with pure argon formed only the TaO and TaO2 molecules. Figure 1 shows the spectra in a selected region

Matrix isolation spectroscopic and theoretical study of carbon dioxide activation by titanium oxide molecules.

The reactions of titanium monoxide and dioxide molecules with carbon dioxide were investigated by matrix isolation infrared spectroscopy. It was found that the titanium monoxide molecule is able to activate carbon dioxide to form the titanium dioxide-carbon monoxide complex upon visible light excitation via a weakly bound TiO(η(1)-OCO) intermediate in solid neon. In contrast, the titanium dioxide molecule reacted with carbon dioxide to form the titanium monoxide-carbonate complex spontaneously on annealing. Theoretical calculations predicted that both activation processes are thermodynamically exothermic and kinetically facile.

Infrared Photodissociation Spectroscopy of Mass-Selected Homoleptic Cobalt Carbonyl Cluster Cations in the Gas Phase

Infrared spectra of mass-selected homoleptic cobalt carbonyl cluster cations including dinuclear Co2(CO)8(+) and Co2(CO)9(+), trinuclear Co3(CO)10(+) and Co3(CO)11(+), as well as tetranuclear Co4(CO)12(+) are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures of these complexes are determined by comparison of the experimental spectra with those calculated by density functional theory. The Co2(CO)8(+) cation is characterized to have a Co-Co bonded structure with Cs symmetry involving a bridging CO ligand. The Co2(CO)9(+) cation is determined to be a mixture of the CO-tagged Co2(CO)8(+)-CO complex and the Co(CO)5(+)-Co(CO)4 ion-molecular complex. The Co3(CO)10(+) cation is the coordination-saturated trinuclear cluster, which is characterized to have a triangle Co3 core with C2 symmetry involving two edge-bridging and eight terminal CO ligands. The Co3(CO)11(+) cation is a weakly bound complex involving a Co3(CO)10(+) core ion. The Co4(CO)12(+) cluster cation is deduced to have a tetrahedral Co4(+) core structure with three edge-bridging and nine terminal carbonyls.