Gen Luo

Dinitrogen Cleavage and Hydrogenation by a Trinuclear Titanium Polyhydride Complex

Titanium Cleaver A century after its discovery, the Haber Bosch process is still used to produce ammonia from nitrogen for fertilizer. Nonetheless, the process requires high temperature and pressure, and chemists continue to look for synthetic analogs to microbial nitrogenase enzymes, which have managed to slice through the N2 triple bond under ambient conditions for millennia. Most efforts in this vein have relied on a boost from the reducing power of alkali metals. Shima et al. (p. 1549; see the Perspective by Fryzuk) instead explored the reactivity of a titanium hydride cluster, which cleanly slices through N2 at room temperature and incorporates the separated N atoms into its framework. Though ammonia was not produced, the system offers hope in the search for mild nitrogen reduction catalysts. The collective reactivity of three hydride-bridged titanium centers cleaves dinitrogen under mild conditions. [Also see Perspective by Fryzuk] Both the Haber-Bosch and biological ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong N≡N triple bond and forming an N–H bond. This has spurred investigations of the reactivity of molecular multimetallic hydrides with dinitrogen. We report here the reaction of a trinuclear titanium polyhydride complex with dinitrogen, which induces dinitrogen cleavage and partial hydrogenation at ambient temperature and pressure. By 1H and 15N nuclear magnetic resonance, x-ray crystallographic, and computational studies of some key reaction steps and products, we have determined that the dinitrogen (N2) reduction proceeds sequentially through scission of a N2 molecule bonded to three Ti atoms in a μ-η1:η2:η2-end-on-side-on fashion to give a μ2-N/μ3-N dinitrido species, followed by intramolecular hydrogen migration from Ti to the μ2-N nitrido unit.

Homometallic Rare‐Earth Metal Phosphinidene Clusters: Synthesis and Reactivity

Two new trinuclear μ3 -bridged rare-earth metal phosphinidene complexes, [{L(Ln)(μ-Me)}3 (μ3 -Me)(μ3 -PPh)] (L=[PhC(NC6 H4 iPr2 -2,6)2 ](-) , Ln=Y (2 a), Lu (2 b)), were synthesized through methane elimination of the corresponding carbene precursors with phenylphosphine. Heating a toluene solution of 2 at 120 °C leads to an unprecedented ortho CH bond activation of the PhP ligand to form the bridged phosphinidene/phenyl complexes. Reactions of 2 with ketones, thione, or isothiocyanate show clear phospha-Wittig chemistry, giving the corresponding organic phosphinidenation products and oxide (sulfide) complexes. Reaction of 2 with CS2 leads to the formation of novel trinuclear rare-earth metal thione dianion clusters, for which a possible pathway was determined by DFT calculation.

Dinitrogen Activation by Dihydrogen and a PNP-Ligated Titanium Complex

The hydrogenolysis of the PNP-ligated titanium dialkyl complex {(PNP)Ti(CH2SiMe3)2} (1, PNP = N(C6H3-2-PiPr2-4-CH3)2) with H2 (1 atm) in the presence of N2 (1 atm) afforded a binuclear titanium side-on/end-on dinitrogen complex {[(PNP)Ti]2(μ2,η1,η2-N2)(μ2-H)2} (2) at room temperature, which upon heating at 60 °C with H2 gave a μ2-imido/μ2-nitrido/hydrido complex {[(PNP)Ti]2(μ2-NH)(μ2-N)H} (3) through the cleavage and partial hydrogenation of the N2 unit. The mechanistic aspects of the hydrogenation of the N2 unit in 2 with H2 have been elucidated by the density functional theory calculations.

Mechanistic Insights into Ring Cleavage and Contraction of Benzene over a Titanium Hydride Cluster

Carbon-carbon bond cleavage of benzene by transition metals is of great fundamental interest and practical importance, as this transformation is involved in the production of fuels and other important chemicals in the industrial hydrocracking of naphtha on solid catalysts. Although this transformation is thought to rely on cooperation of multiple metal sites, molecular-level information on the reaction mechanism has remained scarce to date. Here, we report the DFT studies of the ring cleavage and contraction of benzene by a molecular trinuclear titanium hydride cluster. Our studies suggest that the reaction is initiated by benzene coordination, followed by H2 release, C6H6 hydrometalation, repeated C-C and C-H bond cleavage and formation to give a MeC5H4 unit, and insertion of a Ti atom into the MeC5H4 unit with release of H2 to give a metallacycle product. The C-C bond cleavage and ring contraction of toluene can also occur in a similar fashion, though some details are different due to the presence of the methyl substituent. Obviously, the facile release of H2 from the metal hydride cluster to provide electrons and to alter the charge population at the metal centers, in combination with the flexible metal-hydride connections and dynamic redox behavior of the trimetallic framework, has enabled this unusual transformation to occur. This work has not only provided unprecedented insights into the activation and transformation of benzene over a multimetallic framework but it may also offer help in the design of new molecular catalysts for the activation and transformation of inactive aromatics.

Direct nucleophilic trifluoromethylation using fluoroform: a theoretical mechanistic investigation and insight into the effect of alkali metal cations

Prakash and co-workers recently reported a direct trifluoromethylation of Si, B, S, and C centers using fluoroform (CF3H) and dramatic effects of alkali metal salts of hexamethyldisilazane in this reaction (Science, 2012, 338, 1324). Herein, the detailed mechanisms of trifluoromethylation of Si and C centers in the presence of (Me3Si)2NK as a base have been studied using the DFT method. It has been found that the origin of the dramatic effect of the alkali metals is the stability of an intermediate MCF3. More interestingly, a linear relationship has been found between the chemical hardness of M+ (M = Li, Na, K, Rb, Cs) and the difference between the values of ΔGdec and ΔGtfm (ΔGdec, decomposition energy of the key intermediate MCF3; ΔGtfm, relative energy barrier for the formation of a Si–CF3 or C–CF3 bond). These results may help to both theoretically and experimentally search for better bases to develop more atom-economic and environmentally benign protocols to achieve trifluoromethylation using CF3H.

Heteroatom-assisted olefin polymerization by rare-earth metal catalysts

Heteroatoms enhance polymerization and copolymerization of functional olefins. Heteroatom-functionalized polyolefins are of fundamental interest and practical importance. This has spurred investigations of the copolymerization of polar and nonpolar olefins. We report the first syndiospecific polymerization of a series of heteroatom-containing α-olefins and their copolymerization with ethylene catalyzed by half-sandwich rare-earth complexes. We have found that the interaction between a heteroatom in a functional α-olefin monomer and a rare-earth metal catalyst can significantly raise the olefin polymerization activity and thereby promote its copolymerization with ethylene. By using this heteroatom-assisted olefin polymerization (HOP) strategy, we have successfully synthesized a series of heteroatom (O, S, Se, N, and P)–functionalized polyolefins with high molecular weights and controllable functional monomer contents. The mechanistic aspect of the HOP process has been elucidated by computational studies. We expect that our findings will guide the design of new catalyst systems for the synthesis of various desired functional polyolefins.

Molecular understanding of the interaction of amino acids with sulfuric acid in the presence of water and the atmospheric implication

Amino acids are important components of atmospheric aerosols. Despite the diversity of amino acids structures, however, the role of amino acids with additional non-characteristic functional groups in new particle formation (NPF) has almost remained unexplored. Herein, the interaction of serine (Ser) and threonine (Thr), which feature a hydroxyl group and differ by a methyl-substitution, with sulfuric acid (SA) and up to three water (W) molecules has been investigated at the M06-2X/6-311++G (3df, 3pd) level of theory. The effects of structural differences of amino acids on the structure and properties of clusters were also pointed out. Results show that serine may play more important role in stabilizing sulfuric acid to promote NPF in initial steps compared with threonine, glycine and alanine. Meanwhile, threonine may participate in ion-induced nucleation due to the high dipole moment of (Thr) (SA) isomers. Moreover, the effects of structure differences of amino acids can be seen in several aspects. Firstly, methyl substitution and hydroxyl group of amino acids have great influence on the structure of clusters. Secondly, hydrated (Ser) (SA) and (Tur) (SA) clusters could retain water even at low relative humidity, which may due to the hydroxyl group in serine and threonine. In addition, the Rayleigh light scattering intensities of amino acid-containing clusters are higher than trimethylamine, monoethanolamine and oxalic acid-involved counterparts. The effect of carboxyl group and methyl substitution on optical properties of clusters is also discussed. This study may bring new insight into the role of amino acids with additional non-characteristic functional groups in initial steps of NPF.

A molecular-scale study on the hydration of sulfuric acid-amide complexes and the atmospheric implication

Amides are ubiquitous in atmosphere. However, the role of amides in new particle formation (NPF) is poorly understood. Herein, the interaction of urea and formamide with sulfuric acid (SA) and up to four water (W) molecules has been studied at the M06-2X/6-311++G(3df,3pd) level of theory. The structures and properties of (Formamide)(SA)(W)n (n = 0-4) and (Urea)(SA)(W)n (n = 0-4) clusters were investigated. Results show that the interaction of SA with the CO group of amides plays a more important role in amide clusters compared with the NH2 group. Proton transfer to water molecule become dominant in highly hydrated amide clusters at lower temperatures. There is no proton transfer to CO group in formamide clusters. The Rayleigh light scattering intensities of amide clusters are comparable to that of amine and oxalic acid clusters reported previously. Moreover, unhydrated (Amide)(SA) clusters have similar or even higher ability than hydrated SA clusters to participate in ion-induced nucleation. In comparison with formamide, urea has more interacting sites and its clusters have higher Rayleigh light scattering intensities, larger dipole moment, stronger interaction with SA and lower water affinity. The intermolecular interaction in (Formamide)(SA) is slightly weaker than that of SA dimer, which may be compensated by the high concentration of formamide, thus enabling formamide to participate in initial steps of NPF. This study may bring new insight into the role of amides in initial steps of NPF from molecular scale and could help better understand the properties of amide-containing organic aerosol.

CCDC 1503342: Experimental Crystal Structure Determination

Related Article: Baoli Wang, Gen Luo, Masayoshi Nishiura, Shaowei Hu, Takanori Shima, Yi Luo, Zhaomin Hou|2017|J.Am.Chem.Soc.|139|1818|doi:10.1021/jacs.6b13323

Hydrodenitrogenation of pyridines and quinolines at a multinuclear titanium hydride framework

Investigation of the hydrodenitrogenation (HDN) of aromatic N-heterocycles such as pyridines and quinolines at the molecular level is of fundamental interest and practical importance, as this transformation is essential in the industrial petroleum refining on solid catalysts. Here, we report the HDN of pyridines and quinolines by a molecular trinuclear titanium polyhydride complex. Experimental and computational studies reveal that the denitrogenation of a pyridine or quinoline ring is easier than the ring-opening reaction at the trinuclear titanium hydride framework, which is in sharp contrast with what has been reported previously. Hydrolysis of the pyridine-derived nitrogen-free hydrocarbon skeleton at the titanium framework with H2O leads to recyclization to afford cyclopentadiene with the generation of ammonia, while treatment with HCl gives the corresponding linear hydrocarbon products and ammonium chloride. This work has provides insights into the mechanistic aspects of the hydrodenitrogenation of an aromatic N-heterocycle at the molecular level.Hydrodenitrogenation is a fundamental process in the petroleum refinery context. Here, the authors report the mild ring-opening and denitrogenation of pyridines and quinolines with a titanium hydride cluster and the unusual formation of denitrogenated cyclic hydrocarbon products.