Coordination and Activation of AlH and GaH Bonds

The modes of interaction of donor‐stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14‐ and 16‐electron transition‐metal fragments. More electron‐rich N‐heterocyclic carbene‐stabilized alanes/gallanes of the type NHC?EH₃ (E=Al or Ga) exclusively generate κ² complexes of the type [M(CO)₄(κ²‐H₃E?NHC)] with [M(CO)₄(COD)] (M=Cr, Mo), including the first κ² σ‐gallane complexes. β‐Diketiminato (′nacnac′)‐stabilized systems, {HC(MeCNDipp)₂}EH₂, show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)₂}AlH₂, both κ¹ and κ² complexes were isolated, while [Cr(CO)₄(κ²‐H₂Ga{(NDippCMe)₂CH})] is the only simple κ² adduct of the nacnac‐stabilized gallane which can be trapped, albeit as a co‐crystallite with the (dehydrogenated) gallylene system [Cr(CO)₅(Ga{(NDippCMe)₂CH})]. Reaction of [Co₂(CO)₈] with {HC(MeCDippN)₂}AlH₂ generates [(OC)₃Co(μ‐H)₂Al{(NdippCme)₂CH}][Co(CO)₄] ( 12 ), which while retaining direct Al?H interactions, features a hitherto unprecedented degree of bond activation in a σ‐alane complex.     

Activation of Homolytic SiZn and SiHg Bond Cleavage,Mediated by a Pt0 Complex,via Novel PtZn and PtHg Compounds

The thermally stable [(tBuMe₂Si)₂M] (M=Zn, Hg) generate R₃Si^. radicals in the presence of [(dmpe)Pt(PEt₃)₂] at 60–80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn ( 2 a and 2 b ), M=Hg ( 3 a and 3 b )) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt? MSiR₃ bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn ( 5 ), Hg ( 6 )) whose structures were determined by EPR spectroscopy and DFT calculations.     

Unusual Nitrile–Nitrile and Nitrile–Alkyne Coupling of FcCN and FcCCCN

The reactions of the Group 4 metallocene alkyne complexes, [Cp*₂M(η²‐Me₃SiC₂SiMe₃)] ( 1 a : M=Ti, 1 b : M=Zr, Cp*=η⁵‐pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc?C?N and Fc?C?C?C?N (Fc=Fe(η⁵‐C₅H₅)(η⁵‐C₅H₄)), is described. In case of Fc?C?N an unusual nitrile–nitrile C?C homocoupling was observed and 1‐metalla‐2,5‐diaza‐cyclopenta‐2,4‐dienes ( 3 a , b ) were obtained. As the first step of the reaction with 1 b , the nitrile was coordinated to give [Cp*₂Zr(η²‐Me₃SiC₂SiMe₃)(N?C‐Fc)] ( 2 b ). The reactions with the 3‐ferrocenyl‐2‐propyne‐nitrile Fc?C?C?C?N lead to an alkyne–nitrile C?C coupling of two substrates and the formation of 1‐metalla‐2‐aza‐cyclopenta‐2,4‐dienes ( 4 a , b ). For M=Zr, the compound is stabilized by dimerization as evidenced by single‐crystal X‐ray structure analysis. The electrochemical behavior of 3 a , b and 4 a , b was investigated, showing decomposition after oxidation, leading to different redox‐active products.     

A d10 Ni–(H2) Adduct as an Intermediate in HH Oxidative Addition across a NiB Bond

Bifunctional E? H activation offers a promising approach for the design of two‐electron‐reduction catalysts with late first‐row metals, such as Ni. To this end, we have been pursuing H₂ activation reactions at late‐metal boratranes and herein describe a diphosphine–borane‐supported Ni—(H₂) complex, [(^PhDPB^iPr)Ni(H₂)], which has been characterized in solution. ¹H NMR spectroscopy confirms the presence of an intact H₂ ligand. A range of data, including electronic‐structure calculations, suggests a d¹⁰ configuration for [(^PhDPB^iPr)Ni(H₂)] as most appropriate. Such a configuration is highly unusual among transition‐metal H₂ adducts. The nonclassical H₂ adduct is an intermediate in the complete activation of H₂ across the Ni? B interaction. Reaction‐coordinate analysis suggests synergistic activation of the H₂ ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d¹⁰ Ni—(H₂) interaction and in the H—H cleavage step.     

Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Ruthenium‐Catalyzed CC Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C? C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C? C bond cleavage was proposed in line with experimental data and DFT calculations.     

BH,CH,and BC Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L～BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C? H and ε‐B_sp2? H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B? H, C? H, and B? C bond activation as illustrated by reactivity studies with H₂ and boranes.     

Contrasting electronic requirements for CH binding and CH activation in d6 half‐sandwich complexes of rhenium and tungsten

A computational study of the interaction half‐sandwich metal fragments (metal = Re/W, electron count = d⁶), containing linear nitrosyl (NO⁺), carbon monoxide (CO), trifluorophosphine (PF₃), N‐heterocyclic carbene (NHC) ligands with alkanes are conducted using density functional theory employing the hybrid meta‐GGA functional (M06). Electron deficiency on the metal increases with the ligand in the order NHC < CO < PF₃ < NO⁺. Electron‐withdrawing ligands like NO⁺ lead to more stable alkane complexes than NHC, a strong electron donor. Energy decomposition analysis shows that stabilization is due to orbital interaction involving charge transfer from the alkane to the metal. Reactivity and dynamics of the alkane fragment are facilitated by electron donors on the metal. These results match most of the experimental results known for CO and PF₃ complexes. The study suggests activation of alkane in metal complexes to be facile with strong donor ligands like NHC. © 2015 Wiley Periodicals, Inc.     

Converting Ag2SCdS and Ag2SZnS into AgCdS and AgZnS Nanoheterostructures by Selective Extraction of Sulfur

A mild three‐step solution strategy is developed to prepare Ag? MS (M=Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag₂S? MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag₂S nanoparticles as catalysts, then the Ag₂S sections of the heterostructures are converted into silver nanoparticles by selective extraction of sulfur. Notably, for the prepared Ag? CdS heterostructures, the localized surface plasmon resonance of silver remarkably intensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light‐emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal–semiconductor hybrid nanostructures with desirable performances.     

Preferential Formation of SiOC over SiC Linkage upon Thermal Grafting on Hydrogen‐Terminated Silicon (111)

In a stringent and near oxygen‐free environment, Si?H surfaces were introduced to a trifluoroalkyne, an alcohol‐derivatized alkyne, as well as an equal mixture of both alkynes at a temperature of 130 °C. Contact angle measurements, high‐resolution X‐ray photoelectron spectroscopy (XPS), and angle‐resolved XPS were performed to examine the system. Si?H surfaces were found to have a strong preference towards the formation of Si?O?C rather than Si?C bonds when the alcohol and alkyne reactivities were compared.     

Tris(3,5‐dimethylpyrazolyl)methane‐Based Heterobimetallic Complexes that Contain Zn and CdTransition‐Metal Bonds: Synthesis,Structures, and Quantum Chemical Calculations

Reactions of the tris(3,5‐dimethylpyrazolyl)methanide amido complexes [M′{C(3,5‐Me₂pz)₃}{N(SiMe₃)₂}] (M′=Mg ( 1 a ), Zn ( 1 b ), Cd ( 1 c ); 3,5‐Me₂pz=3,5‐dimethylpyrazolyl) with two equivalents of the acidic Group 6 cyclopentadienyl (Cp) tricarbonyl hydrides [MCp(CO)₃H] (M=Cr ( 2 a ), Mo ( 2 b )) gave different types of heterobimetallic complex. In each case, two reactions took place, namely the conversion of the tris(3,5‐dimethylpyrazolyl)methanide ligand (Tpmd*) into the ‐methane derivative (Tpm*) and the reaction of the acidic hydride M?H bond with the M′?N(SiMe₃)₂ moiety. The latter produces HN(SiMe₃)₂ as a byproduct. The Group 2 representatives [Mg(Tpm*){MCp(CO)₃}₂(thf)] ( 3 a / b ) form isocarbonyl bridges between the magnesium and chromium/molybdenum centres, whereas direct metal–metal bonds are formed in the case of the ions [Zn(Tpm*){MCp(CO)₃}]⁺ ( 4 a / b ; [MCp(CO)₃]^? as the counteranion) and [Cd(Tpm*){MCp(CO)₃}(thf)]⁺ ( 5 a / b ; [Cd{MCp(CO)₃}₃]^? as the counteranion). Complexes 4 a and 5 a / b are the first complexes that contain Zn?Cr, Cd?Cr, and Cd?Mo bonds (bond lengths 251.6, 269.8, and 278.9 pm, respectively). Quantum chemical calculations on 4 a / b* (and also on 5 a / b* ) provide evidence for an interaction between the metal atoms.     

SF and SC Activation of SF6 and SF5 Derivatives at Rhodium: Conversion of SF6 into H2S

The degradation of SF₆ and SF₅ organyls by S? F and S? C bond‐activation reactions at [{Rh(μ‐H)(dippp)}₂] under mild conditions is reported. Fluorido and thiolato species were identified as products or intermediates, and were characterized by X‐ray diffraction analysis and multinuclear NMR spectroscopy. An unprecedented cyclic process for the conversion of the potent greenhouse gas SF₆ into H₂S was developed.     

Amending the Anisotropy Barrier and Luminescence Behavior of Heterometallic Trinuclear Linear [MIILnIIIMII] (LnIII=Gd,Tb, Dy; MII=Mg/Zn) Complexes by Change from Divalent Paramagnetic to Diamagnetic Metal Ions

The sequential reaction of a multisite coordinating compartmental ligand [2‐(2‐hydroxy‐3‐(hydroxymethyl)‐5‐methylbenzylideneamino)‐2‐methylpropane‐1,3‐diol] (LH₄) with appropriate lanthanide salts followed by the addition of [Mg(NO₃)₂] ? 6 H₂O or [Zn(NO₃)₂] ? 6 H₂O in a 4:1:2 stoichiometric ratio in the presence of triethylamine affords a series of isostructural heterometallic trinuclear complexes containing [Mg₂Ln]³⁺ (Ln=Dy, Gd, and Tb) and [Zn₂Ln]³⁺ (Ln=Dy, Gd, and Tb) cores. The formation of these complexes is demonstrated by X‐ray crystallography as well as ESI‐MS spectra. All complexes are isostructural possessing a linear trimetallic core with a central lanthanide ion. The comprehensive studies discussed involve the synthesis, structure, magnetism, and photophysical properties on this family of trinuclear [Mg₂Ln]³⁺ and [Zn₂Ln]³⁺ heterometallic complexes. [Mg₂Dy]³⁺ and [Zn₂Dy]³⁺ show slow relaxation of the magnetization below 12 K under zero applied direct current (dc) field, but without reaching a neat maximum, which is due to the overlapping with a faster quantum tunneling relaxation mediated through dipole–dipole and hyperfine interactions. Under a small applied dc field of 1000 Oe, the quantum tunneling is almost suppressed and temperature and frequency dependent peaks are observed, thus confirming the single‐molecule magnet behavior of complexes [Mg₂Dy]³⁺ and [Zn₂Dy]³⁺.     

Formation of Phosphorylated 3H‐Pyrroles from NefIsocyanidePerkow Adducts and Tosylmethyl Isocyanide

Imidoyl chlorides, generated from isocyanides and acyl chlorides, react with trialkyl phosphites, in a Perkow‐type reaction, to afford 3‐(alkylimino)‐2‐[(dialkyloxyphosphoryl)oxy]acrylates, which undergo a smooth reaction with tosylmethyl isocyanide (TsMIC) to furnish 4‐(alkylamino)‐3‐[(dialkyloxyphosphoryl)oxy]‐5‐[(4‐methylphenyl)sulfonyl]‐3H‐pyrrole‐3‐carboxylates in moderate‐to‐good yields.     

From Diphosphino‐Functionalized 1,3‐Dialkylimidazolium Cations to Imidazolones through Dehydrogenative CN Coupling

Diphosphino‐functionalized 1,3‐dialkylimidazolium salts react with KOH affording amine/formamide open‐chain products, which fully revert to the imidazolium cation by treatment with a variety of acids or are converted to 2‐imidazolones by noncatalyzed intramolecular dehydrogenative C?N coupling, a process that is modulated by coordination of the phosphino functionalities to transition metals.     

Metal‐Free Oxidative CC Bond Formation through CH Bond Functionalization

The formation of C?C bonds embodies the core of organic chemistry because of its fundamental application in generation of molecular diversity and complexity. C?C bond‐forming reactions are well‐known challenges. To achieve this goal through direct functionalization of C?H bonds in both of the coupling partners represents the state‐of‐the‐art in organic synthesis. Oxidative C?C bond formation obviates the need for prefunctionalization of both substrates. This Minireview is dedicated to the field of C?C bond‐forming reactions through direct C?H bond functionalization under completely metal‐free oxidative conditions. Selected important developments in this area have been summarized with representative examples and discussions on their reaction mechanisms.     

Manganese‐Catalyzed Dehydrogenative [4+2] Annulation of NH Imines and Alkynes by CH/NH Activation

Described herein is a manganese‐catalyzed dehydrogenative [4+2] annulation of N? H imines and alkynes, a reaction providing highly atom‐economical access to diverse isoquinolines. This transformation represents the first example of manganese‐catalyzed C? H activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H₂ as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition‐metal‐catalyzed C? H activation. Mechanistic studies revealed the five‐membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.     

First Diastereoselective PasseriniSmiles and UgiSmiles Reactions Using Chiral Aldehydes

A diastereoselective O/N‐arylative Passerini/Ugi reaction of chiral aldehydes with commercially available isocyanides affording O‐aryl amides and N‐aryl amides, respectively, as products in moderate to good yields, together with de values, is reported.     

Rhodium‐Catalyzed CS and CN Functionalization of Arenes: Combination of CH Activation and Hypervalent Iodine Chemistry

Rhodium‐catalyzed sulfonylation, thioetherification, thiocyanation, and other heterofunctionalizations of arenes bearing a heterocyclic directing group have been realized. The reaction proceeds by initial Rh^III‐catalyzed C?H hyperiodination of arene at room temperature followed by uncatalyzed nucleophilic functionalization. A diaryliodonium salt is isolated as an intermediate, which represents umpolung of the arene substrate, in contrast to previous studies that suggested umpolung of the coupling partner.     

Mechanistic Study on Rh‐Catalyzed Stereoselective CC/CH Activation of tert‐Cyclobutanols

A mechanistic study was performed on the Rh‐catalyzed stereoselective C?C/C?H activation of tert‐cyclobutanols. The present study corroborated the previous proposal that the reaction occurs by metalation, β‐C elimination, 1,4‐Rh transfer, C?O insertion, and a final catalyst‐regeneration step. The rate‐determining step was found to be the 1,4‐Rh transfer step, whereas the stereoselectivity‐determining step did not correspond to any of the aforementioned steps. It was found that both the thermodynamic stability of the product of the β‐C elimination and the kinetic feasibility of the 1,4‐Rh transfer and C?O insertion steps made important contributions. In other words, three steps (i.e., β‐C elimination, 1,4‐Rh transfer, and C?O insertion) were found to be important in determining the configurations of the two quaternary stereocenters.