Unusual Changes of C−H Bond Lengths in Chiral Zinc Complexes Induced by Noncovalent Interactions

In order to better understand the effect of non-covalent weak interactions on molecules, we have explored a variety of weak interactions, such as improper H-bonding (HB), tetrel bonds (TBs) and halogen bonds, in fluorinated chiral zinc complexes. High resolution neutron diffraction studies revealed a methylene carbon-hydrogen bond elongation and shortening due to TB and improper HB interactions, respectively. To show the accumulative effects of multiple weak interactions on the C−H bond, three types of tetrel bonds have been carefully examined. We have also shown how C−H bond elongation can be easily offset by forming an improper HB with the H atom from this C−H bond. Non-covalent interaction and electrostatic potential analysis investigations have been used to affirm the nature of the interactions based on density functional theory (DFT) and other related calculations.     

Rare-Earth Catalyzed C−H Bond Alumination of Terminal Alkynes

Organoaluminum reagents’ application in catalytic C−H bond functionalization is limited by competitive side reactions, such as carboalumination and hydroalumination. Herein, rare-earth tetramethylaluminate complexes are shown to catalyze the exclusive C−H bond metalation of terminal alkynes with the commodity reagents trimethyl-, triethyl-, and triisobutylaluminum. Kinetic experiments probing alkyl-group exchange between rare-earth aluminates and trialkylaluminum, C−H bond metalation of alkynes, and catalytic conversions reveal distinct pathways of catalytic aluminations with triethylaluminum versus trimethylaluminum. Most significantly, kinetic data point to reversible formation of a unique [Ln](AlR₄)₂⋅AlR₃ adduct, followed by turnover-limiting alkyne metalation. That is, C−H bond activation occurs from a more associated organometallic species, rather than the expected coordinatively unsaturated species. These mechanistic conclusions allude to a new general strategy for catalytic C−H bond alumination that make use of highly electrophilic metal catalysts.     

Influence of the Flexibility of Nickel PCP-Pincer Complexes on C−H and P−C Bond Activation and Ethylene Reactivity: A Combined Experimental and Theoretical Investigation

Using a pincer platform based on a bridgehead NHC donor with functional side arms, the combined effect of increased flexibility in six-membered pyrimidine-type heterocycles compared to the more often studied five-membered imidazole, and rigidity of phosphane side arms was examined. The unique features observed include: 1) the reaction of the azolium C_sp2−H bond with [Ni(cod)₂] affording a carbanionic ligand in [NiCl(PC_sp3^HP)] ( 8 ) rather than a carbene; 2) its transformation into the NHC, hydrido complex [NiH(PC^NHCP)]PF₆ ( 9 ) upon halide abstraction; 3) ethylene insertion into the Ni−H bond of the latter and ethyl migration to the N−C−N carbon atom of the heterocycle in [Ni(PC^EtP)]PF₆ ( 10 ); and 4) an unprecedented C−P bond activation transforming the P−C^NHC−P pincer ligand of 8 in a C−C^NHC−P pincer and a terminal phosphanido ligand in [Ni(PPh₂)(CC^NHCP)] ( 15 ). The data are supported by nine crystal structure determinations and theoretical calculations provided insights into the mechanisms of these transformations, which are relevant to stoichiometric and catalytic steps of general interest.     

Theoretical Study on Intermolecular Interactions and Thermodynamic Properties of Dimethylnitroamine Clusters

Ab initio SCF and Mφller-Plesset correlation correction methods in combination with counterpose procedure for BSSE correction have been applied to the theroetical studying of dimethylnitroamine and its dimers and trimers.Three optimized stable dimers and two trimers have been obtained.The corrected binding energies of the most stable dimer and trimer were predicted to be -24.68kJ/mol and -47.27kJ/mol,respectively at the MP2/6-31G^*//HF/6-31G^* level.The proportion of correlated interation energies to their total interaction energies for all clusters was at least 29.3 percent,and the BSSE of ΔE(MP2) was at least 10.0kJ/mol.Dispersion and/or electrostatic force were dominant in all clusters.There exist cooperative effects in both the chain and the cyclic trimers.The vibrational frequencies associated with N-O stretches or wags exhibit slight red shifts,but the modes associated with the motion of hydrogen atoms of the methyl group show somewhat blue shifts with respect to those of monomer.Thermodynamic properties of dimethylnitroamine and its clusters at different temperatures have been calculated on the basis of vibrational analyses.The changes of the Gibbs free energies for the aggregation from monomer to the most stable dimer and trimer were predicted to be 14.37kJ/mol and 30.40kJ/mol,respectively,at 1 atm and 298.15K.     

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.     

Catalytic C−C/C−H Bond Activation Relay for Synthesis of Fluorescent Naphthoquinolizinium Salts

Herein, we report the design and synthesis of a series of novel cationic nitrogen-embedded polyaromatic hydrocarbons with a planar geometry. The synthetic pathway is based on catalytic C−C/C−H bond activation relay that enabled preparation of regioselectively 5,6,10,11-tetrasubstituted naphtho[2,1,8-ija]quinolizinium salts bearing various types of substituents. Single-crystal X-ray analyses of selected compounds confirmed planarity of the quinolizinium core. Most of the prepared compounds exhibited strong fluorescence (Φ_s up to >99 %) ranging from 420–600 nm depending on the substitution pattern. According to DFT calculations LUMO is always distributed over the quinolizinium framework regardless of the attached substituents, whereas delocalization of HOMO is related to the substitution pattern. Electrochemical measurements show irreversible reduction of all compounds, which is supported by the calculated location of LUMO orbitals.     

Multivalent C−H⋅⋅⋅Cl/Br−C Interactions Directing the Resolution of Dynamic and Twisted Capsules

In this work, we report a mechanism by which stereoisomeric and twisted capsules P/M- 1 direct their dynamic chirality in the presence of haloalkane guests. The capsule comprises a static, but twisted, cage that is linked to a dynamic tris(2-pyridylmethyl)amine (TPA) lid at its top. From the results of experimental (NMR spectroscopy and X-ray crystallography) and computational (DFT) studies, the TPA lid was shown to assume clockwise (+) and counterclockwise (−) folds with diastereomeric (but racemic) capsules M- 1 (+) and M- 1 (−) interconverting at a rapid rate (ΔG^≠_189K=9.1 kcal mol⁻¹). The relative stability of the capsules was found to be a function of guest(s) residing in their interior (243/262 ų) with small CH₂Cl₂ (61 ų) yielding roughly equal population of diastereomeric inclusion complexes. Larger guests, such as CCl₄ (89 ų) and CBr₄ (108 ų), however, formed M- 1 (−)⊂CX₄ at the expense of M- 1 (+)⊂CX₄ in circa 3:1 ratio. To account for the observation, theory (DFT:M06-2X/6–31+G*) and experiments (¹H NMR spectroscopy) were used to deduce that CX₄ guests become localized inside the twisted cage of the capsule by forming a C−X⋅⋅⋅π halogen bond [N_c=d/(r_H+r_X)=0.91–0.92] with the benzene “floor” while encountering electrostatic repulsions with closer naphthalimide boundaries. At last, the TPA lid used its central methylene hydrogens to establish, within the M- 1 (−)⊂CX₄, three stabilizing C−H⋅⋅⋅X−C interactions with the guest. The same C−H⋅⋅⋅X−C interactions, however, became weaker (or possibly vanished) after the conformational reorganization of the lid and the formation of less stable M- 1 (+)⊂CX₄ complex. On individual basis, C−H⋅⋅⋅X−C intermolecular contacts are weak and hardly detectable in the solution phase. In the case of capsule P/M- 1 , however, these contacts were multivalent and altogether strong enough to direct the host's dynamic chirality.     

Coordination-Induced N−H Bond Splitting of Ammonia and Primary Amine of CuI−MOFs

We report a porous three-dimensional anionic tetrazolium based Cu^I−MOF 1 , which is capable of cleaving the N−H bond of ammonia and primary amine, as well as the O−H bond of H₂O along with spontaneous H₂ evolution. In the gas-solid phase reaction of 1 with ammonia and water vapor, Cu^I−MOF 1 was gradually oxidized to NH₂−Cu^II−MOF and OH−Cu^II−MOF, through single-crystal-to-single-crystal (SCSC) structural transformations, which was confirmed by XPS, PXRD and X-ray single-crystal diffraction. Density functional theory (DFT) demonstrated that Cu^I−MOF could lower N−H bond dissociation free energy of ammonia through coordination-induced bond weakening and promote H₂ evolution by the reduction potential of 1 . To our knowledge, this is the first example of MOFs that activate ammonia and amine in gas-solid manner.     

Theoretical Studies on the Stabilities and Hydrogen Bond Actions of （H2O）n Clusters

1 INTRODUCTION In the latest ten years, the structure and function of water clusters have captured the interest of chemists. One of the most important study objects in water cluster is to describe the behavior of water so- lution quantitatively at molecule level, which will pave the way for the solving of some environmental and other scientific problems, such as the formation of acid rain and nucleation mechanism of little water drop. Besides, weak interaction in water clusters could be al…     

Multicomponent Aromatic and Benzylic Mannich Reactions through C−H Bond Activation

Multicomponent Mannich reactions through C−H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo-benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition-metal-catalyzed sp² or sp³ C−H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to Rh^III-catalyzed sp² C−H bond functionalization and extended to Cu^II-catalyzed sp³ C−H bond functionalization.     

C−H and Si−H Activation Reactions at Ru/Ga Complexes: A Combined Experimental and Theoretical Case Study on the Ru−Ga Bond

Treatment of [Ru(COD)(MeAllyl)₂] and [Ru(COD)(COT)] with GaCp* under hydrogenolytic conditions leads to reactive intermediates which activate Si−H or C−H bonds, respectively. The product complexes [Ru(GaCp*)₃(SiEt₃)H₃] ( 1 ) and [Ru(GaCp*)₃(C₇H₇)H₃] ( 2 ) are formed with HSiEt₃ or with toluene as the solvent, respectively. While 1 was isolated and fully characterized by NMR, MS, IR and SC-XRD, 2 was too labile to be isolated and was observed and characterized in situ by using mass spectrometry, including labelling experiments for the unambiguous assignment of the elemental composition. The structural assignment was confirmed by DFT calculations. The relative energies of the four isomers possible upon toluene activation at the ortho-, meta-, para- and CH₃-positions have been determined and point to aromatic C−H activation. The Ru−Ga bond was analyzed by EDA and QTAIM and compared to the Ru−P bond in the analogue phosphine compound. Bonding analyses indicate that the Ru-GaCp* bond is weaker than the Ru-PR₃ bond.     

Not Carbon s–p Hybridization,but Coordination Number Determines C−H and C−C Bond Length

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp³ to sp² to sp.     

Trapping of a Polyketide Synthase Module after C−C Bond Formation Reveals Transient Acyl Carrier Domain Interactions

Modular polyketide synthases (PKSs) are giant assembly lines that produce an impressive range of biologically active compounds. However, our understanding of the structural dynamics of these megasynthases, specifically the delivery of acyl carrier protein (ACP)-bound building blocks to the catalytic site of the ketosynthase (KS) domain, remains severely limited. Using a multipronged structural approach, we report details of the inter-domain interactions after C−C bond formation in a chain-branching module of the rhizoxin PKS. Mechanism-based crosslinking of an engineered module was achieved using a synthetic substrate surrogate that serves as a Michael acceptor. The crosslinked protein allowed us to identify an asymmetric state of the dimeric protein complex upon C−C bond formation by cryo-electron microscopy (cryo-EM). The possible existence of two ACP binding sites, one of them a potential “parking position” for substrate loading, was also indicated by AlphaFold2 predictions. NMR spectroscopy showed that a transient complex is formed in solution, independent of the linker domains, and photochemical crosslinking/mass spectrometry of the standalone domains allowed us to pinpoint the interdomain interaction sites. The structural insights into a branching PKS module arrested after C−C bond formation allows a better understanding of domain dynamics and provides valuable information for the rational design of modular assembly lines.     

Photoredox-Catalyzed Isomerization of Highly Substituted Allylic Alcohols by C−H Bond Activation

Photoredox-catalyzed isomerization of γ-carbonyl-substituted allylic alcohols to their corresponding carbonyl compounds was achieved for the first time by C−H bond activation. This catalytic redox-neutral process resulted in the synthesis of 1,4-dicarbonyl compounds. Notably, allylic alcohols bearing tetrasubstituted olefins can also be transformed into their corresponding carbonyl compounds. Density functional theory calculations show that the carbonyl group at the γ-position of allylic alcohols are beneficial to the formation of their corresponding allylic alcohol radicals with high vertical electron affinity, which contributes to the completion of the photoredox catalytic cycle.     

Energetics of C–C Bond Scission in Ethane Hydrogenolysis: A Theoretical Study of Possible Intermediates and Reaction Pathways

C–C bond scission steps, which are often considered as rate-determining in ethane hydrogenolysis, are studied by the Unity Bond Index–Quadratic Exponential UBI–QEP method. The binding energies of atomic carbon with Group VIII and IB metal surfaces Ni(111), Pd(111), Pt(111), Rh(111), Ru(001), Ir(111), Fe(110), Cu(111), and Au(111) are estimated using experimental data on the adsorption of various species on these surfaces. These estimates are corrected using data from density functional theory (DFT) on the adsorption heats of the CH _x species. Metal surfaces are arranged in the following series according to the binding strength of a carbon atom: Cu(111) < Au(111) < Pd(111) < Ru(001) Pt(111) < Ni(111) Rh(111) < Ir(111) < Fe(110). The values of chemisorption heats range from 121 kcal/mol for Au(111) to 193 kcal/mol for Fe(110). The activity of these surfaces toward C–C bond scission increases in the same series. The results of this work suggest that the most probable C–C bond scission precursors are ethyl, ethylidyne, adsorbed acetylene, CH₂CH, CH₂C, and CHC. Theoretical data obtained by different methods are compared and found to agree well with each other. An overview of experimental data on ethane hydrogenolysis mechanisms is given.     

Rhoda-Electrocatalyzed C−H Methylation and Paired Electrocatalyzed C−H Ethylation and Propylation

The use of electricity over traditional stoichiometric oxidants is a promising strategy for sustainable molecular assembly. Herein, we describe the rhoda-electrocatalyzed C−H activation/alkylation of several N-heteroarenes. This catalytic approach has been successfully applied to several arenes, including biologically relevant purines, diazepam, and amino acids. The versatile C−H alkylation featured water as a co-solvent and user-friendly trifluoroborates as alkylating agents. Finally, the rhoda-electrocatalysis with unsaturated organotrifluoroborates proceeded by paired electrolysis.     

Infrared Spectrum of the Adamantane+–Water Cation: Hydration-Induced C−H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C−H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀H₁₆⁺–H₂O, Ad⁺–W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn–Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge–dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺–W potential.     

Synthesis of Conjugated Polymers via Transition Metal Catalysed C−H Bond Activation

Transition metal catalysed C−H bond activation chemistry has emerged as an exciting and promising approach in organic synthesis. This allows us to synthesize a wider range of functional molecules and conjugated polymers in a more convenient and more atom economical way. The formation of C−C bonds in the construction of pi-conjugated systems, particularly for conjugated polymers, has benefited much from the advances in C−H bond activation chemistry. Compared to conventional transition-metal catalysed cross-coupling polymerization such as Suzuki and Stille cross-coupling, pre-functionalization of aromatic monomers, such as halogenation, borylation and stannylation, is no longer required for direct arylation polymerization (DArP), which involve C−H/C−X cross-coupling, and oxidative direct arylation polymerization (Ox-DArP), which involves C−H/C−H cross-coupling protocols driven by the activation of monomers’ C(sp²)−H bonds. Furthermore, poly(annulation) via C−H bond activation chemistry leads to the formation of unique pi-conjugated moieties as part of the polymeric backbone. This review thus summarises advances to date in the synthesis of conjugated polymers utilizing transition metal catalysed C−H bond activation chemistry. A variety of conjugated polymers via DArP including poly(thiophene), thieno[3,4-c]pyrrole-4,6-dione)-containing, fluorenyl-containing, benzothiadiazole-containing and diketopyrrolopyrrole-containing copolymers, were summarized. Conjugated polymers obtained through Ox-DArP were outlined and compared. Furthermore, poly(annulation) using transition metal catalysed C−H bond activation chemistry was also reviewed. In the last part of this review, difficulties and perspective to make use of transition metal catalysed C−H activation polymerization to prepare conjugated polymers were discussed and commented.     

Late-Stage Aryl C−H Bond Cyclopropenylation with Cyclopropenium Cations

Herein, we disclose the first regio-, site- and chemoselective late-stage (hetero)aryl C−H bond cyclopropenylation with cyclopropenium cations (CPCs). The process is fast, operationally simple and shows an excellent functional group tolerance in densely-functionalized drug molecules, natural products, agrochemicals and fluorescent dyes. Moreover, we discovered that the installation of the cyclopropene ring in drug molecules could not only be used to shield against metabolic instability but also as a synthetic tool to reach medicinally-relevant sp³-rich scaffolds exploiting the highly-strained nature of the cyclopropene ring with known transformations.     

Pd-Catalyzed Directed Thiocyanation Reaction by C−H Bond Activation

The Pd-catalyzed directed thiocyanation reaction of arenes and heteroarenes by C−H bond activation was achieved. In the presence of an electrophilic SCN source, this original methodology offered an efficient tool to access a panel of functionalized thiocyanated compounds (21 examples, up to 78 % yield). Post-functionalization reactions further demonstrated the synthetic utility of the approach by converting the SCN-containing molecules into value-added scaffolds.