Electropositive Substituents Research Articles

The nature of the NO bond in amine oxides

AbstractDespite the ubiquitous presence of amine oxides in chemistry, there is no consensus about the nature of the NO bond in these compounds. In this work, we have used electron density analysis to investigate the nature of this bond in substituted amine oxides, R3NO, and have compared it with the nature of the NO bond in hydroxylamines, R2NOR, and model molecules that have well‐established chemical bond character. The results showed that the NO bond length and relative stability are proportional to the inductive effect of the substituents. Quantum chemical topology, natural bond orbitals (NBO), and natural resonance theory (NRT) analyses indicated that the NO bond is polar covalent in all the studied amine oxides, but the ionic contribution is different. NBO and NRT analyses revealed that molecules with more electronegative substituents have strongly delocalized NO and NR bonds, whereas molecules with electropositive substituents have localized bonds.

Synthesis, Characterization and Calculations of Linear and Nonlinear Optical Properties of Acrylonitrile Substituted Spirocyclotriphosphazene Derivatives

There methoxy-substituted acrylonitrile compounds (2a, 2b and 2c) were synthesized using method described in the literature. Compounds 3a, 3b and 3c were obtained by using the microwave demethylation method for the first time, then they were reacted with 2,2-dichloro-4,4,6,6-bis[spiro(2′,2″-dioxy-1′,1″-biphenylyl)]-cyclotriphosphazene (DPP) to reach final products 4a, 4b and 4c. The structures of the substituted compounds were determined by elemental analysis, MALDI-TOF, 1H, [Formula: see text]C APT, [Formula: see text]P NMR and FT-IR spectroscopy. In order to investigate the usability potential of the synthesized products 4a, 4b and 4c in optoelectronic devices, we experimentally measured the band gaps and calculated their nonlinear optical properties by Density Functional Theory (DFT) method. Obtained results showed that the introduction of an electronegative substituent (F) or an electropositive substituent (CH[Formula: see text] to the parent compound 4a has minimal impact on the absorption spectra. However, the fluorine-substituted compound 4b exhibits considerably higher values in terms of dipole moment, averaged polarizability, and first-hyper polarizability when compared to compounds 4a and 4c. Calculated values of dipole moment, averaged polarizability, first-hyper polarizability, electronegativity and global hardness of the fluorine-substituted compound 4b were found to be 6.281 (debye), −393.592 (1.4818 × 10[Formula: see text] cm[Formula: see text], 899.857 (8.641 × 10[Formula: see text] cm5/e.s.u), 4.492 (eV) and 1.922 (eV), respectively. These values were the highest in magnitude in comparison to the associated values of 4a and 4c. Overall results indicated that the influence of introducing an electropositive substituent (CH[Formula: see text] to the parent compound 4a, resulting in decreased stability and increased reactivity. Conversely, the introduction of an electronegative substituent (F) to 4a leads to opposite effects.

Phosphole aromaticity enhancement by electron pumping through Schleyer hyperconjugative aromaticity: A comprehensive DFT study

Combination of aromaticity and hyperconjugation as two fundamental concepts in organic chemistry results hyperconjugative aromaticity. Due to trigonal pyramidal P (III) atom in phosphole ring, it is only slightly more aromatic than cyclopentadiene. In the present work, the hyperconjugative aromaticity of phosphole derivatives, bearing an electron donating group, have been studied at the DFT-B3LYP/6–311++G(d, p) level of theory using various aromaticity indices such as geometric, magnetic, electronic and energetic criteria. Our DFT calculations reveal that an electropositive substituent can enhance phosphole aromaticity through Schleyer hyperconjugative aromaticity. This hyperconjugative electron donation by the substituents results in a partially anionic phosphole ring. Among the electropositive groups, silyl and germanium substituents have the best performance. The excellent correlations are observed between all aromaticity indices. Our findings could help chemists to design novel hyperconjugative aromatic phospholes.

Docking-Based 3D-QSAR Studies for 1,3,4-oxadiazol-2-one Derivatives as FAAH Inhibitors

This work aimed to construct 3D-QSAR CoMFA and CoMSIA models for a series of 31 FAAH inhibitors, containing the 1,3,4-oxadiazol-2-one moiety. The obtained models were characterized by good statistical parameters: CoMFA Q2 = 0.61, R2 = 0.98; CoMSIA Q2 = 0.64, R2 = 0.93. The CoMFA model field contributions were 54.1% and 45.9% for steric and electrostatic fields, respectively. In the CoMSIA model, electrostatic, steric, hydrogen bond donor, and hydrogen acceptor properties were equal to 34.6%, 23.9%, 23.4%, and 18.0%, respectively. These models were validated by applying the leave-one-out technique, the seven-element test set (CoMFA r2test-set = 0.91; CoMSIA r2test-set = 0.91), a progressive scrambling test, and external validation criteria developed by Golbraikh and Tropsha (CoMFA r20 = 0.98, k = 0.95; CoMSIA r20 = 0.98, k = 0.89). As the statistical significance of the obtained model was confirmed, the results of the CoMFA and CoMSIA field calculation were mapped onto the enzyme binding site. It gave us the opportunity to discuss the structure–activity relationship based on the ligand–enzyme interactions. In particular, examination of the electrostatic properties of the established CoMFA model revealed fields that correspond to the regions where electropositive substituents are not desired, e.g., in the neighborhood of the 1,3,4-oxadiazol-2-one moiety. This highlights the importance of heterocycle, a highly electronegative moiety in this area of each ligand. Examination of hydrogen bond donor and acceptor properties contour maps revealed several spots where the implementation of another hydrogen-bond-donating moiety will positively impact molecules’ binding affinity, e.g., in the neighborhood of the 1,3,4-oxadiazol-2-one ring. On the other hand, there is a large isopleth that refers to the favorable H-bond properties close to the terminal phenoxy group of a ligand, which means that, generally speaking, H-bond acceptors are desired in this area.

Molecular Modeling Study of c-KIT/PDGFRα Dual Inhibitors for the Treatment of Gastrointestinal Stromal Tumors.

Gastrointestinal stromal tumors (GISTs) are the most common Mesenchymal Neoplasm of the gastrointestinal tract. The tumorigenesis of GISTs has been associated with the gain-of-function mutation and abnormal activation of the stem cell factor receptor (c-KIT) and platelet-derived growth factor receptor alpha (PDGFRα) kinases. Hence, inhibitors that target c-KIT and PDGFRα could be a therapeutic option for the treatment of GISTs. The available approved c-KIT/PDGFRα inhibitors possessed low efficacy with off-target effects, which necessitated the development of potent inhibitors. We performed computational studies of 48 pyrazolopyridine derivatives that showed inhibitory activity against c-KIT and PDGFRα to study the structural properties important for inhibition of both the kinases. The derivative of phenylurea, which has high activities for both c-KIT (pIC50 = 8.6) and PDGFRα (pIC50 = 8.1), was used as the representative compound for the dataset. Molecular docking and molecular dynamics simulation (100 ns) of compound 14 was performed. Compound 14 showed the formation of hydrogen bonding with Cys673, Glu640, and Asp810 in c-KIT, and Cys677, Glu644, and Asp836 in PDGFRα. The results also suggested that Thr670/T674 substitution in c-KIT/PDGFRα induced conformational changes at the binding site of the receptors. Three-dimensional quantitative structure–activity relationship (3D-QSAR) models were developed based on the inhibitors. Contour map analysis showed that electropositive and bulky substituents at the para-position and the meta-position of the benzyl ring of compound 14 was favorable and may increase the inhibitory activity against both c-KIT and PDGFRα. Analysis of the results suggested that having bulky and hydrophobic substituents that extend into the hydrophobic pocket of the binding site increases the activity for both c-KIT and PDGFRα. Based on the contour map analysis, 50 compounds were designed, and the activities were predicted. An evaluation of binding free energy showed that eight of the designed compounds have potential binding affinity with c-KIT/PDGFRα. Absorption, distribution, metabolism, excretion and toxicity (ADMET) and synthetic feasibility tests showed that the designed compounds have reasonable pharmaceutical properties and synthetic feasibility. Further experimental study of the designed compounds is recommended. The structural information from this study could provide useful insight into the future development of c-KIT and PDGFRα inhibitors.

8-Aminoquinoline-Assisted Synthesis and Crystal Structure Studies of Ferrocenyl Aryl Sulfones.

A copper-catalyzed 8-aminoquinoline-directed oxidative cross-coupling of the C-H bond of ferrocene with sodium arylsulfinates has been achieved. The robust copper catalyst tolerates a range of methyl, tert-butyl, bromo, chloro, iodo and nitro functional groups in the phenyl ring, and set the stage for the synthesis of substituted ferrocene sulfones. Furthermore, X-ray crystal structure study on several ferrocenyl sulfones reveals the tetrahedral geometry around sulfur; interestingly, the O-S-O angle is larger than the electropositive substituent C-S-C angle which could be explained by Bent's rule. Further, unusual intramolecular O(S)⋅⋅⋅N(amide) short contacts (2.925-3) and O(S)⋅⋅⋅C=O were also noticed in ferrocenyl sulfones.

Computational study of paroxetine-like inhibitors reveals new molecular insight to inhibit GRK2 with selectivity over ROCK1

The G-protein coupled receptor kinase 2 (GRK2) regulates the desensitization of beta-adrenergic receptors (β-AR), and its overexpression has been implicated in heart failure. Hence, the inhibition of GRK2 is considered to be an important drug target for the treatment of heart failure. Due to the high sequence similarity of GRK2 with the A, G, and C family (AGC family) of kinases, the inhibition of GRK2 also leads to the inhibition of AGC kinases such as Rho-associated coiled-coil kinase 1 (ROCK1). Therefore, unraveling the mechanisms to selectively inhibit GRK2 poses an important challenge. We have performed molecular docking, three dimensional quantitative structure activity relationship (3D-QSAR), molecular dynamics (MD) simulation, and free energy calculations techniques on a series of 53 paroxetine-like compounds to understand the structural properties desirable for enhancing the inhibitory activity for GRK2 with selectivity over ROCK1. The formation of stable hydrogen bond interactions with the residues Phe202 and Lys220 of GRK2 seems to be important for selective inhibition of GRK2. Electropositive substituents at the piperidine ring and electronegative substituents near the amide linker between the benzene ring and pyrazole ring showed a higher inhibitory preference for GRK2 over ROCK1. This study may be used in designing more potent and selective GRK2 inhibitors for therapeutic intervention of heart failure.

The mechanism of electrophilic addition of singlet oxygen to pyrrolic ring

Pyrrolic compounds assume an important role in the chemistry of living organisms, coal surrogates and novel drugs. However, literature reports a few studies on their reactivity towards prominent oxidising agents. This contribution presents a comprehensive mechanistic study of the oxidation of unsubstituted pyrrole with singlet oxygen (O2 1∆g) by deploying a quantum chemical framework leading to the production of succinimide, as the major products, through a Diels–Alder addition of O2 1∆g to the aromatic ring. Other products such as maleimide, hydroperoxide, formamide and epoxide adducts appear to form via insignificant channels. The primary Diels–Alder channel encompasses a barrier of 41 kJ/mol with a fitted rate constant of k(T) = 1.87 × 10−13 exp(− 48,000/RT) cm3 mol−1 s−1. Furthermore, a kinetic study has been undertaken to investigate the influence of substituents on reaction rate of the Diels–Alder addition of singlet oxygen to a pyrrolic ring. The results clarify that electropositive substituents such as BeH and BH2 operate as π-acceptors and thus deactivate the ring towards electrophilic attack of singlet oxygen. However, substituents comprising of strong π-donors, e.g., NH2 and OH, destabilise the ring structure, increasing its oxidation reactivity.

Unconventional Aromaticity in Organometallics: The Power of Transition Metals.

Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Möbius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Hückel-type and Möbius-type. In comparison with numerous Hückel aromatics containing 4 n+2 π-electrons, Möbius aromatics with 4 n π-electrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Hückel antiaromaticity in the parent pentalyne into Craig-type Möbius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Möbius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Möbius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the π-conjugation in fully unsaturated rings; whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. We found that the unsaturated three-membered ring in cyclopropaosmapentalene is σ-aromatic. Very recently, we extended σ-aromaticity into in a fully unsaturated ring. The concepts and examples presented here show the importance of interplay and union between experiment and theory in developing novel aromatic systems and, especially, the indispensable role of computational study in rationalization of unconventional aromaticity. All these findings highlight the strong power of transition metals originating from participation of d orbitals in aromaticity, opening an avenue to the design of unique metalla-aromatics.

Pd(4-R3Si-IPr)(allyl)Cl], a Family of Silyl-Substituted Pd–NHC Complexes: Catalytic Systems for the Buchwald–Hartwig Amination

A family of Pd–IPr complexes (1Pd–6Pd) featuring electropositive and bulky R3Si substituents at the 4-position of the NHC ring was prepared and fully characterized. The catalytic performance of 1Pd (R3Si = SiMe3) and 3Pd (R3Si = Si(allyl)Me2) in the Buchwald–Hartwig amination was excellent, notably outperforming those of other previously reported catalytic systems by requiring shorter reaction times, lower catalyst loadings, and milder reaction conditions. Furthermore, a systematic evaluation of both the electronic and steric influences of the 4-R3Si-IPr ligand on the overall catalytic performance of 1Pd–6Pd revealed that electronic rather than steric factors play a dominant role.

Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

AbstractHyperconjugation and aromaticity are two of the most important concepts in chemistry. Mulliken and co‐workers combined both terms to explain the stability of cyclopentadiene. Here, we carried out DFT calculations on a series of mono‐ and disubstituted cyclopentadiene derivatives to investigate their hyperconjugative aromaticity. Our results revealed that one electropositive substituent can induce aromaticity, whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. When an electronegative substituent and a transition metal as an electropositive substituent were considered simultaneously, a slightly aromatic cyclopentadiene ring could be achieved, whereas an electropositive substituent of the main group in combination with an electronegative one will produce a nonaromatic cyclopentadiene ring.

Quantitative Structure–Activity Relationship and Combinatorial Design of 1,3,4-Oxadiazole-Based Thymidine Phosphorylase Inhibitors as Potential Anti-Cancer Agents

The 3D quantitative structure–activity relationship model representing r 2 = 0.8605, q 2 = 0.8193 and pred_r 2 = 0.6847 respectively, was generated for thymidine phosphorylase (TP) inhibitory activity of some 1,3,4-oxadiazole derivatives. Electronegative substituents at R 1 and less steric bulk with electropositive substituents at R 2 were found to be favourable for TP inhibition. The activity prediction of a combinatorial library of 1629 compounds resulted in 50 molecules whose predicted activity was comparable to the most active compound in the dataset and within the model’s applicability domain. Among them six molecules showed favourable interactions with the active site of TP proposing potential anticancer activity of the title compounds.

Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors.

Reactions between 1,8‐dichloroanthracenes with substituents in position 10 and ortho‐chloroaryne afford mixtures of 1,8,13‐ (syn) and 1,8,16‐trichlorotriptycenes (anti). The syn/anti ratio is dependent on these substituents. Electropositive substituents like SiMe3 and GeMe3 lead to preferred formation of the syn‐isomer, whereas CMe3 groups exclusively afford the anti‐isomer. Different quantum chemical calculations including location of transition states give conflicting results, but indicate the importance of dispersion forces for an at least qualitative prediction of results. The syn‐trichlorotriptycenes with SiMe3 and GeMe3 substituents were characterized by using NMR spectroscopy, mass spectrometry, and X‐ray diffraction experiments.

Synthesis, insecticidal activities and structure–activity relationship study of dual chiral sulfilimines

To investigate the "methyl" impact on bioactivity of sulfiliminyl dicarboxamides, a total of 16 novel N-cyano and N-trifluoroacetyl sulfiliminyl dicarboxamides containing m-heptafluoroisopropylated aromatic amino moiety were studied. Two series of sulfiliminyl substituents were designed, synthesized and evaluated against oriental armyworm (Pseudaletia separata Walker) for their insecticidal activities. Their chemical structures were established by corresponding [Formula: see text] NMR, HRMS and optical polarimetry. Bioassay results revealed that some of the title compounds showed potent insecticidal activities against oriental armyworm. Notably, compounds IIa, IIIa, IVa exhibited 100% activity at [Formula: see text], in particular, IIa showed a comparable control efficacy to that of the commercial product flubendiamide. The SAR of these N-cyano sulfiliminyl isomers can be summarized as follows (Sc, Ss) [Formula: see text] (Sc, Rs), while the N-trifluoroacetyl sulfiliminyl isomers is (Sc, Rs) [Formula: see text] (Sc, Ss). Comparative molecular field analysis indicated that an electropositive substituent, [Formula: see text] group in the benzene ring was very important for the improvement in biological activity. These results could hold promise for novel chiral sulfiliminyl RyR regulators.

Exceptionally Strong Electron-Donating Ability of Bora-Ylide Substituent vis-à-vis Silylene and Silylium Ion.

Electropositive boron-based substituent (phosphonium bora-ylide) with an exceptionally strong π- and σ-electron donating character dramatically increases the stability of a new type of N-heterocyclic silylene 2 featuring amino- and bora-ylide-substituents. Moreover, the related silylium ion 4 and transition-metal-silylene complexes, with trigonal-planar geometries around the silicon center, are also well stabilized. Therefore, the N,B-heterocyclic silylene 2 can be used as a strongly electron-donating innocent ligand in coordination chemistry similarly to N-heterocyclic carbenes.

Assessment of long-range transport potential of polychlorinated Naphthalenes based on three-dimensional QSAR models.

Experimentally determined octanol-air partition coefficients (K OA) for 43 polychlorinated naphthalene (PCN) congeners and experimentally determined subcooled liquid vapor pressures (P L) for 17 PCN congeners were used with comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) to generate three-dimensional quantitative structure-activity relationship (3D-QSAR) models. The data were used to predict K OA values for the other 32 congeners and P L values for the other 58 congeners. The CoMFA and CoMSIA model contour maps showed that the electrostatic fields of the PCN molecules are the most important factors affecting the K OA and P L values. The long-range transport potentials of several PCN homologs were assessed using the following grading system: high mobility (MoCNs), relatively high mobility (DiCNs to TeCNs), relatively low mobility (PeCNs to HeCNs) and low mobility (HeCNs and OCN). The PCN-2 molecule was modified using the contour maps of the two models, and the results showed that introducing an electronegative R1 substituent increased the K OA value but introducing an electropositive R6 substituent decreased the P L value. PCN-2 was in the high mobility class, but introducing these substituents moved the long-range transport potentials of the modified molecules to the relatively high mobility class.

Carbenes as Electron-Pair Donors for P⋅⋅⋅C Pnicogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH2 )2 , C(OH)2 , and cyclic C(OCH)2 [OHC] with H2 XP molecules, with X=F, Cl, NC, OH, CH3 , CN, CCH, and H. The H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes have Cs symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes arise only when X is one of the more electronegative substituents. Coplanar H2 XP:C(NH2 )2 complexes form when X is one of the more electropositive substituents, whereas coplanar H2 XP:C(OH)2 complexes exist for all X. H2 XP:C(NH2 )2 and H2 XP:C(OH)2 are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H2 (CH3 )P:C(OH)2 and H3 P:C(OH)2 are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H2 XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H2 (CH3 )P:OHC and H3 P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H2 XP:C(NH2 )2 , H2 XP:C(OH)2 , and H2 XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin-spin coupling constants.

A Systematic Study of Structure and E-H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes.

A series of new germylene compounds has been synthesized offering systematic variation in the σ- and π-capabilities of the α-substituent and differing levels of reactivity towards E-H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ERn )] (1-6: ERn =NHDipp, CH(SiMe3 )2 , P(SiMe3 )2 , Si(SiMe3 )3 or B(NDippCH)2 ; terphenyl=C6 H3 Mes2 -2,6=Ar(Mes) or C6 H3 Dipp2 -2,6=Ar(Dipp) ; Dipp=C6 H3 iPr2 -2,6, Mes=C6 H2 Me3 -2,4,6), while the related complex [{(Me3 Si)2 N}Ge{B(NDippCH)2 }] (8) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X-ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ-donating boryl or silyl ancillary donors. HOMO-LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)-germylene system [Ar(Mes) Ge{B(NDippCH)2 }] (6-Mes), which features a wide C-Ge-B angle (110.4(1)°) and (albeit relatively weak) ancillary π-acceptor capabilities, has the smallest HOMO-LUMO gap (119 kJ mol(-1) ). These features result in 6-Mes being remarkably reactive, undergoing facile intramolecular C-H activation involving one of the mesityl ortho-methyl groups. The related aryl(silyl)-germylene system, [Ar(Mes) Ge{Si(SiMe3 )3 }] (5-Mes) has a marginally wider HOMO-LUMO gap (134 kJ mol(-1) ), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H2 and NH3 to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E-H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E-H bond activation.

H2XP:OH2 Complexes: Hydrogen vs. Pnicogen Bonds

A search of the Cambridge Structural Database (CSD) was carried out for phosphine-water and arsine-water complexes in which water is either the proton donor in hydrogen-bonded complexes, or the electron-pair donor in pnicogen-bonded complexes. The range of experimental P-O distances in the phosphine complexes is consistent with the results of ab initio MP2/aug’-cc-pVTZ calculations carried out on complexes H2XP:OH2, for X = NC, F, Cl, CN, OH, CCH, H, and CH3. Only hydrogen-bonded complexes are found on the H2(CH3)P:HOH and H3P:HOH potential surfaces, while only pnicogen-bonded complexes exist on H2(NC)P:OH2, H2FP:OH2, H2(CN)P:OH2, and H2(OH)P:OH2 surfaces. Both hydrogen-bonded and pnicogen-bonded complexes are found on the H2ClP:OH2 and H2(CCH)P:OH2 surfaces, with the pnicogen-bonded complexes more stable than the corresponding hydrogen-bonded complexes. The more electronegative substituents prefer to form pnicogen-bonded complexes, while the more electropositive substituents form hydrogen-bonded complexes. The H2XP:OH2 complexes are characterized in terms of their structures, binding energies, charge-transfer energies, and spin-spin coupling constants 2hJ(O-P), 1hJ(H-P), and 1J(O-H) across hydrogen bonds, and 1pJ(P-O) across pnicogen bonds.

How carbo-benzenes fit molecules in their inner core as do biologic ion carriers?

The present computational study complements experimental efforts to describe and characterize carbo-benzene derivatives as paradigms of aromatic carbo-mers. A long-lasting issue has been the possibility of the π-electron crown of the C18 carbo-benzene ring to fit metals or any chemical agents in its core. A systematic screening of candidate inclusion complexes was carried out by density functional theory calculations. Mayer bond order, aromaticity indices, and energy decomposition analyses complete the understanding of the strength of the host–guest interaction. The change in steric and electronic properties induced by the guest agent is investigated by means of steric maps. Substitution of H atoms at the carbo-benzene periphery by electron-withdrawing or electron-donating groups is shown to have a determining influence on the stability of the inclusion complex ions: while electronegative substituents enhance the recognition of cations, electropositive substituents do the same for anions. The results confirm the experimental failure hitherto to evidence a carbo-benzene complex. Nevertheless, the affinity of carbo-benzene for the potassium cation appears promising for the design of planar hydrocarbon analogues of biologic ion carriers.