The Difference in Ir-Catalyzed C(sp2)-H and C(sp3)-H Bond Activation Assisted by a Directing Group: Cyclometalation via Cis- or Trans-Chelation?

Iridium-catalyzed C-H borylation of aromatic and aliphatic hydrocarbons assisted by a directing group was theoretically investigated. Density functional theory (DFT) calculations revealed both Ir-catalyzed C(sp2)-H and C(sp3)-H borylations via an IrIII/IrV catalytic cycle, where the tetra-coordinated (C, N)IrIII(Bpin)2 complex with two vacant sites is an active species. Dramatically, the orientation of cyclometalation for C(sp2)-H bond activation assisted by a directing group is different from the C(sp3)-H one. The activation energy (ΔG°‡ = 28.5 kcal mol-1) of the C(sp2)-H bond via trans-chelation to form cyclometalation is lower than that (41.4 kcal mol-1) via cis-chelation. In contrast, the ΔG°‡ (26.6 kcal mol-1) of the C(sp3)-H bond via cis-chelation to form cyclometalation is lower than that (34.3 kcal mol-1) via trans-chelation. In addition, the rate-determining step of Ir-catalyzed C(sp2)-H borylation is oxidative addition of the C(sp2)-H bond, while that of C(sp3)-H analogues is hydride migration. Such differences arise from not only the differences in the steric hindrance of the C(sp2) and secondary C(sp3) atoms but also the differences in the trans effect and steric effect of the two vacant sites of active species. These findings were expected to facilitate further studies on the design and synthesis of innovative ligands for Ir-catalyzed C-H borylation.

An Iridium/Aluminum Cooperative Strategy for the ß-C(sp3 )-H Borylation of Saturated Cyclic Amines.

Despite the widespread success in the functionalization of C(sp2 )-H bonds, the deliberate functionalization of C(sp3 )-H bonds in a highly site- and stereoselective manner remains a longstanding challenge. Herein, we report an iridium/aluminum cooperative catalytic system that enables the ß-selective C-H borylation of saturated cyclic amines and lactams. Furthermore, we have accomplished an enantioselective variant using binaphthol-derived chiral aluminum catalysts to forge C-B bonds with high levels of stereocontrol. Computational studies suggest that the formation of a Lewis pair with the substrates is crucial to lower the energy of the transition state for the rate-determining reductive elimination step.

Atropisomeric Phosphine Ligands Bearing C-N Axial Chirality: Applications in Enantioselective Suzuki-Miyaura Cross-Coupling Towards the Assembly of Tetra-ortho-Substituted Biaryls.

Biaryl phosphines bearing C(Ar)-C(Ar) axial chirality are commonly known and have been successfully applied in many asymmetric catalyses. Nevertheless, the development of a chiral ligand having an axially chiral C(Ar)-N backbone remains elusive due to its undesirable less restricted rotational barrier. In fact, it is highly attractive to overcome this challenge in ligand development as the incorporation of an N-donor component at the chiral axis is more favorable toward the transient metal coordination, and thus, a better outcome of stereocommunication is anticipated to the approaching substrates. Herein, we present a rational design of a new collection of chiral phosphines featuring a C-N axial chirality and their applications in enantioselective Suzuki-Miyaura cross-coupling for accessing highly steric hindered tetra-ortho-substituted biaryls (26 examples up to 98:2 er). It is worth noting that the embodied carbazolyl framework is crucial to succeed the reaction, by the fruitful steric relief of bulky substrate coordination and transmetalation via a fleeting Pd-N jumping to Pd-π fashion. DFT calculation reveals an interesting Pd-arene-walking characteristic across the carbazolyl plane for attaining a lower energy-preferred route in a catalytic cycle. The theoretical study successfully predicts the stereooutcome and matches the enantioselectivity with the experimental results.

Theoretical Insight into the Multiple Roles of LiHMDS in Pd-Catalyzed Borylation of Fluorobenzene.

Pd-catalyzed borylation of fluorobenzene was theoretically studied. DFT calculations revealed that the reaction occurs through an unprecedented 3 + 6-membered ring transition state, in which one LiHMDS (HMDS = hexamethyldisilazane) acts as a ligand and another LiHMDS is essential to provide Li···N and Li···F interactions, overcoming the large destabilization of the strong phenyl-F bond distortion. The characteristic feature of LiHMDS was elucidated by comparing it with HMDS and NaHMDS analogues.

Efficient Electron Transfer from Electron-Sponge Polyoxometalate to Single-Metal Site Metal-Organic Frameworks for Highly Selective Electroreduction of Carbon Dioxide.

In this work, by combining the superiority of polyoxometalates (POMs) and catalytic single-metal site Co of metalloporphyrin, a series of mixed-valence POM-based metal-organic frameworks (MOFs) composites is synthesized by a post-modification method. The electron-transfer property of POM@PCN-222(Co) composite is significantly enhanced owing to the directional electron-transfer from POM to single-metal site Co in PCN-222(Co). In particular, H-POM@PCN-222(Co) gives a high Faradaic efficiency of 96.2% for electroreduction of CO2 into CO and good stability over 10 h. DFT calculations confirm that the directional electron transfer, which accelerates the multi-electron transfer from the electrode to active single-metal site Co, enriches the electron density of the Co center, and ultimately reduces the energy of the rate-determining step, thus increasing the catalytic activity of CO2 reduction reaction (CO2 RR). This work therefore suggests some new insight for the design of efficient electrocatalysts for CO2 RR.

Methane Borylation Catalyzed by Ru, Rh, and Ir Complexes in Comparison with Cyclohexane Borylation: Theoretical Understanding and Prediction.

Methane borylation catalyzed by Cp*M(Bpin)n (M = Ru or Rh; HBpin = pinacolborane; n = 2 or 3) and (TMPhen)Ir(Bpin)3 (TMPhen = 3,4,7,8-tetramethyl-1,10-phenanthroline) was investigated by DFT in comparison with cyclohexane borylation. Because Ru-catalyzed borylation has not been theoretically investigated yet, its reaction mechanism was first elucidated; Cp*Ru(Bpin)3 1-Ru is an active species, and Cp*Ru(Bpin)3(H)(CH3) 4-Ru is a key intermediate. In 4-Ru, the Ru is understood to have an ambiguous oxidation state between +IV and +VI because it has a H··Bpin bonding interaction with a bond order of about 0.5. Methane borylation occurs through oxidative addition of methane C-H bond followed by reductive elimination of borylmethane in all of these catalysts. The catalytic activity for methane borylation increases following the order Cp*Ru(Bpin)3 < (TMPhen)Ir(Bpin)3 < Cp*Rh(Bpin)2. Cyclohexane borylation occurs in the same mechanism except for the presence of isomerization of a key intermediate. Chemoselectivity of methane over cyclohexane increases following the order Ir < Ru < Rh. In all of these catalysts, the rate-determining step (RDS) of cyclohexane borylation needs a larger ΔG°‡ than the RDS of methane borylation because the more bulky cyclohexyl group induces larger steric repulsion with the ligand than methyl. One reason for the worse chemoselectivity of the Ir catalyst is its less congested transition state of the reductive elimination of borylcyclohexane. Herein, use of a strongly electron-donating ligand consisting of pyridine and N-heterocyclic carbene with bulky substituents is computationally proposed as a good ligand for the Ir catalyst; actually, the Ir complex of this ligand is calculated to be more active and more chemoselective than Cp*Rh(Bpin)2 for methane borylation.

sp3 C-H Borylation Catalyzed by Iridium(III) Triboryl Complex: Comprehensive Theoretical Study of Reactivity, Regioselectivity, and Prediction of Excellent Ligand.

Iridium-catalyzed C-H borylation of THF was theoretically investigated as example of sp3 C-H functionalization. DFT computations show that ß-regioselective borylation occurs more easily than does α-regioselective, as reported experimentally, through oxidative addition of C-H bond to iridium(III) species and reductive elimination of B-C bond. The reductive elimination is both a rate-determining step and a regioselectivity-determining step. The lower energy transition state (TS) of the reductive elimination of ß-boryloxolane arises from the Ir···(ß-oxolanyl) interaction at TS being stronger than the Ir···(α-oxolanyl) one. The Ir···(ß-oxolanyl) interaction being stronger than the Ir···(α-oxolanyl) one is a result of the valence orbital energy of the α-oxolanyl group being higher than that of the ß-oxolanyl group due to antibonding overlap of the valence orbital with O 2p orbital, where SOMO of oxolanyl radical is taken as valence orbital hereinafter. Reactivity of substrate decreases following the order primary (ß) C-H of ethyl ether > primary C-H of n-pentane ∼ secondary (ß) C-H of THF > secondary C-H of cyclopentane > secondary (α) C-H of THF ∼ secondary C-H of n-pentane > secondary (α) C-H of ethyl ether. The primary C-H bond is more reactive than the secondary one because of its smaller steric repulsion and lower energy valence orbital of the primary alkyl group. The ß-C-H bond of THF is more reactive than the secondary C-H bond of cyclopentane because of valence orbital energy of the ß-oxolanyl group being lower than that of the cyclopentyl group. Both steric and electronic factors are important for determining reactivity of substrate. Bidentate ligand consisting of pyridine and N-heterocyclic carbene is predicted to be better than 3,4,7,8-tetramethyl-1,10-phenanthroline used experimentally.

Strong Pancake 2e/12c Bond in π-Stacking Phenalenyl Derivatives Avoiding Bond Conversion.

The nature of the 2e/12c bond and its conversion to a carbon-carbon single bond in phenalenyl dimers have prompted a great deal of interests recently. In this work, we theoretically investigated a series of π-stacking phenalenyl derivatives with 2e/12c bonding character by density functional theory (DFT) calculations to elucidate origin of this unusual bond conversion. Results show that bond-conversion of the phenalenyl dimer easily occurs at room-temperature both dynamically and thermodynamically. However, bond-conversion of hetero π-stacking adducts, in which the two center carbon atoms were substituted by boron and nitrogen atoms, respectively, is much more difficult, because the 2e/12c bond is stabilized by its charge transfer character. Consequently, the bond-conversion is an endothermic process, albeit with a low conversion barrier. Interestingly, Lewis acid-base interactions would be induced by substitution of the center nitrogen atom to phosphorus atom. The 2e/12c bond is further stabilized by 5.9 kcal mol-1 and its conversion is also thermodynamically unfavorable.

Constructing Stable π-Dimers: Two Parallel Pancake π-π Bonds.

Stable phenalenyl (PLY) radical π-dimers still play an important role for new multifunctional materials owing to their intriguing molecular structure and unusual pancake π-π bonding mode. Herein, we design a new biphenalenyl biradicaloid (1) consisting of two PLYs and one benzene ring linkage tethered by single bonds, which presents an open-shell character. Further, three π-dimers (a1, b1, and c1) combined with 1 and conventional biphenalenyl biradicaloid (a, b, and c), which are capable of forming two staggered PLY dimers, exhibiting a short interlayer distance between the monomers. Interestingly, the analysis of the frontier molecular orbital reveals that two bonding orbitals, namely, the two highest occupied molecular orbitals (HOMO and HOMO-1) are doubly occupied. The results reveal that three π-dimers display two parallel pancake bonds. Moreover, they have small diradical and tetraradical characters, large interaction energies, and strong aromaticity, which indicate the stability of these π-dimers. The present work creates a new class of strong pancake bonding and might be utilized in devising an array of materials.

The Suzuki-Miyaura Coupling of Nitroarenes.

Synthesis of biaryls via the Suzuki-Miyaura coupling (SMC) reaction using nitroarenes as an electrophilic coupling partners is described. Mechanistic studies have revealed that the catalytic cycle of this reaction is initiated by the cleavage of the aryl-nitro (Ar-NO2) bond by palladium, which represents an unprecedented elemental reaction.

Connecting effect on the first hyperpolarizability of armchair carbon-boron-nitride heteronanotubes: pattern versus proportion.

Carbon-boron-nitride heteronanotubes (BNCNT) have attracted a lot of attention because of their adjustable properties and potential applications in many fields. In this work, a series of CA, PA and HA armchair BNCNT models were designed to explore their nonlinear optical (NLO) properties and provide physical insight into the structure-property relationships; CA, PA and HA represent the models that are obtained by doping the carbon segment into pristine boron nitride nanotube (BNNT) fragments circularly around the tube axis, parallel to the tube axis and helically to the tube axis, respectively. Results show that the first hyperpolarizability (ß0) of an armchair BNCNT model is dramatically dependent on the connecting patterns of carbon with the boron nitride fragment. Significantly, the ß0 value of PA-6 is 2.00 × 10(4) au, which is almost two orders of magnitude larger than those (6.07 × 10(2) and 1.55 × 10(2) au) of HA-6 and CA-6. In addition, the ß0 values of PA and CA models increase with the increase in carbon proportion, whereas those of HA models show a different tendency. Further investigations on transition properties show that the curved charge transfer from N-connecting carbon atoms to B-connecting carbon atoms of PA models is essentially the origin of the big difference among these models. This new knowledge about armchair BNCNTs may provide important information for the design and preparation of advanced NLO nano-materials.

Two-electron/24-center (2e/24c) bonding in novel diradical π-dimers.

A series of diradical π-dimers 2 with interesting pancake-shaped 2e/24c π-π bonding character were designed and investigated based on the famous phenalenyl (PLY) π-dimer with 2e/12c π-π bonding character. The position of stronger interaction between two layers of radicals was found by the Wiberg bond index (WBI) maximum component. Further, the different contributions of the interaction energy were analyzed quantitatively by energy decomposition analysis (EDA). Among these new diradical π-dimers, 2180 has the smallest layer distance and the largest interaction between two layers of radicals. The unusual PLY analogues can provide new insights into the unique features of two-electron/multicenter (2e/mc) π-π bonding.

The polar 2e/12c bond in phenalenyl-azaphenalenyl hetero-dimers: Stronger stacking interaction and fascinating interlayer charge transfer.

An increasing number of chemists have focused on the two-electron/multicenter bond (2e/mc) that was first introduced to interpret the bonding mechanism of radical dimers. Herein, we report the polar two-electron/twelve center (2e/12c) bonding character in a series of phenalenyl-azaphenalenyl radical hetero-dimers. Interestingly, the bonding energy of weaker polar hetero-dimer (P-TAP) is dominated by the overlap of the two different singly occupied molecular orbital of radicals, while that of stronger polar hetero-dimer (P-HAP) is dominated by the electrostatic attraction. Results show that the difference between the electronegativity of the monomers plays a prominent role in the essential attribution of the polar 2e/12c bond. Correspondingly, a stronger stacking interaction in the hetero-dimer could be effectively achieved by increasing the difference of nitrogen atoms number between the monomers. It is worthy of note that an interesting interlayer charge transfer character is induced in the polar hetero-dimers, which is dependent on the difference between the electronegativity of the monomers. It is our expectation that the new knowledge about the bonding nature of radical hetero-dimers might provide important information for designing radical based functional materials with various applications.

Evidence of Amine-CO2 Interactions in Two Pillared-Layer MOFs Probed by X-ray Crystallography.

Two pillared-layer metal-organic frameworks (MOFs; PMOF-55 and NH2 -PMOF-55) based on 1,2,4-triazole and terephthalic acid (bdc)/NH2 -bdc ligands were assembled and display framework stabilities, to a certain degree, in both acid/alkaline solutions and toward water. They exhibit high CO2 uptakes and selective CO2 /N2 adsorption capacities, with CO2 /N2 selectivity in the range of 24-27, as calculated by the ideal adsorbed solution theory method. More remarkably, the site and interactions between the host network and the CO2 molecules were investigated by single-crystal X-ray diffraction, which showed that the main interaction between the CO2 molecules and PMOF-55 is due to multipoint supramolecular interactions of C-H⋅⋅⋅O, C⋅⋅⋅O, and O⋅⋅⋅O. Amino functional groups were shown to enhance the CO2 adsorption and identified as strong adsorption sites for CO2 by X-ray crystallography.

Dynamic Control of Asymmetric Charge Distribution for Electrocatalytic Urea Synthesis.

Constructing dual catalytic sites with charge density differences is an efficient way to promote urea electrosynthesis from parallel NO 3 - ${\mathrm{NO}}_3^ - $ and CO2 reduction yet still challenging in static system. Herein, a dynamic system is constructed by precisely controlling the asymmetric charge density distribution in an Au-doped coplanar Cu7 clusters-based 3D framework catalyst (Au@cpCu7CF). In Au@cpCu7CF, the redistributed charge between Au and Cu atoms changed periodically with the application of pulse potentials switching between -0.2 and -0.6 V and greatly facilitated the electrosynthesis of urea. Compared with the static condition of pristine cpCu7CF (FEurea = 5.10%), the FEurea of Au@cpCu7CF under pulsed potentials is up to 55.53%. Theoretical calculations demonstrated that the high potential of -0.6 V improved the adsorption of *HNO2 and *NH2 on Au atoms and inhibited the reaction pathways of by-products. While at the low potential of -0.2 V, the charge distribution between Au and Cu atomic sites facilitated the thermodynamic C-N coupling step. This work demonstrated the important role of asymmetric charge distribution under dynamic regulation for urea electrosynthesis, providing a new inspiration for precise control of electrocatalysis.

After the electronic field: structure, bonding, and the first hyperpolarizability of HArF.

In this work, we add different strength of external electric field (E(ext)) along molecule axis (Z-axis) to investigate the electric field induced effect on HArF structure. The H-Ar bond is the shortest at E(ext) = -189 × 10(-4) and the Ar-F bond show shortest value at E(ext) = 185 × 10(-4) au. Furthermore, the wiberg bond index analyses show that with the variation of HArF structure, the covalent bond H-Ar shows downtrend (ranging from 0.79 to 0.69) and ionic bond Ar-F shows uptrend (ranging from 0.04 to 0.17). Interestingly, the natural bond orbital analyses show that the charges of F atom range from -0.961 to -0.771 and the charges of H atoms range from 0.402 to 0.246. Due to weakened charge transfer, the first hyperpolarizability (ß(tot)) can be modulated from 4078 to 1087 au. On the other hand, make our results more useful to experimentalists, the frequency-dependent first hyperpolarizabilities were investigated by the coupled perturbed Hartree-Fork method. We hope that this work may offer a new idea for application of noble-gas hydrides.

Theoretical investigation on the 2e/12c bond and second hyperpolarizability of azaphenalenyl radical dimers: strength and effect of dimerization.

An increasing number of chemists have focused on the investigations of two-electron/multicenter bond (2e/mc) that was first introduced to describe the structure of radical dimers. In this work, the dimerization of two isoelectronic radicals, triazaphenalenyl (TAP) and hexaazaphenalenyl (HAP) has been investigated in theory. Results show TAP2 is a stable dimer with stronger 2e/12c bond and larger interaction energy, while HAP2 is a less stable dimer with larger diradical character. Interestingly, the ultraviolet-visible absorption spectra suggest that the dimerization induces a longer wavelength absorption in visible area, which is dependent on the strength of dimerization. Significantly, the amplitude of second hyperpolarizability (γ(yyyy)) of HAP2 is 1.36 × 10(6) a.u. that is larger than 7.79 × 10(4) a.u. of TAP2 because of the larger diradical character of HAP2. Therefore, the results indicate that the strength of radical dimerization can be effectively detected by comparing the magnitude of third order non-linear optical response, which is beneficial for further theoretical and experimental studies on the properties of complexes formed by radical dimerization.

The excess electron in a boron nitride nanotube: pyramidal NBO charge distribution and remarkable first hyperpolarizability.

The unusual properties of species with excess electrons have attracted a lot of interest in recent years due to their wide applications in many promising fields. In this work, we find that the excess electron could be effectively bound by the B atoms of boron nitride nanotube (BNNT), which is inverted pyramidally distributed from B-rich edge to N-rich edge. Further, Li@B-BNNT and Li@N-BNNT are designed by doping the Li atom to the two edges of BNNT, respectively. Because of the interaction between the Li atom and BNNT, the 2s valence electron of Li becomes a loosely bound excess electron. Interestingly, the distribution of the excess electron in Li@N-BNNT is more diffuse and pyramidal from B-rich edge to N-rich edge, which is fascinating compared with Li@B-BNNT. Correspondingly, the transition energy of Li@N-BNNT is 0.99 eV, which is obviously smaller than 2.65 eV of Li@B-BNNT. As a result, the first hyperpolarizability (3.40×10(4) a.u.) of Li@N-BNNT is dramatically larger (25 times) than 1.35×10(3) a.u. of Li@B-BNNT. Significantly, we find that the pyramidal distribution of the excess electron is the key factor to determine the first hyperpolarizability, which reveals useful information for scientists to develop new electro-optic applications of BNNTs.

Spiral intramolecular charge transfer and large first hyperpolarizability in Möbius cyclacenes: new insight into the localized π electrons.

Much effort has been devoted to investigating the unusual properties of the π electrons in Möbius cyclacenes, which are localized in a special region. However, the localized π electrons are a disadvantage for applications in optoelectronics, because intramolecular charge transfer is limited. This raises the question of how the intramolecular charge transfer of a Möbius cyclacene with clearly localized π electrons can be enhanced. To this end, [8]Möbius cyclacene ([8]MC) is used as a conjugated bridge in a donor-π-conjugated bridge-acceptor (D-π-A) system, and NH(2)-6-[8]MC-10-NO(2) exhibits a fascinating spiral charge-transfer transition character that results in a significant difference in dipole moments Δµ between the ground state and the crucial excited state. The Δµ value of 6.832 D for NH(2)-6-[8]MC-10-NO(2) is clearly larger than that of 0.209 D for [8]MC. Correspondingly, the first hyperpolarizability of NH(2)-6-[8]MC-10-NO(2) of 12,467 a.u. is dramatically larger than that of 261 a.u. for [8]MC. Thus, constructing a D-π-A framework is an effective strategy to induce greater spiral intramolecular charge transfer in MC although the π electrons are localized in a special region. This new insight into the properties of π electrons in Möbius cyclacenes may provide valuable information for their applications in optoelectronics.