Π-electron Donation Research Articles

Bicyclic-Ammonium-Incorporated Ylidic Nitrogen Groups for Strong π-Electron Donation in Push-Pull Benzene π-Conjugated Systems.

Ylidic nitrogen-based π-electron-donating groups, (quinuclidinio)amidyl (QA) and (1-azanorbornanio)amidyl (ANA) groups, were developed to shift the absorption of push-pull benzenes toward longer wavelengths. Changing the pyrrolidinyl group to the QA or ANA group achieved a bathochromic shift of 45-100 nm in the maximum absorption wavelength, depending on the push-pull π-conjugated system investigated (p-nitrobenzene, 1,8-naphthalimide, and an azo dye).

π-d Interaction to promote Zn ion de-solvation and transference toward high-performance Zn-MnO2 battery

Aqueous zinc ion batteries (AZIBs) are one class of high-safety and low-cost electrochemical energy storage devices supplementary to the current lithium ion technologies. However, their cycling performance has been severely tethered by the high de-solvation barrier of hydrated Zn ions and uncontrolled dendrite growth and side reactions on the Zn anodes. Herein, an artificial zincophilic passivation layer is constructed on the Zn anode surface by impregnating the porous Zn-based zeolitic imidazole frameworks (ZIF-8) with polypyrrole of rich π-electron donation. This not only lowers the de-solvation and nucleation barriers through π-d interaction, but also offers spatial room for accommodating 3D matrix Zn storage, lending smooth Zn plating/stripping with greatly improved kinetics in addition to inhibited dendrite formation and side reactions. Consequently, at 5 mA cm−2 the as-modified Zn anodes demonstrate a high CE of 99.6 % for more than 1500 cycles in half-cells and a stable operation for more than 2500 h in symmetrical cells. The aqueous Zn-MnO2 full cells further present a superb cycling performance with a capacity retention of 86.3 % after 1200 cycles at 1 A g−1. This work underscores the application of zincophilic and porous passivation layer to mediate smooth Zn plating/stripping by modulating the de-solvation and nucleation energy barriers.

Tapping the Potential of High-Valent Mo and W Metal Centers for Dynamic Electronic Structures in Multimetallic FeVO(OH)/Ni(OH)2 for Ultrastable and Efficient Overall Water Splitting.

Rationally designing a noble metal-free electrocatalyst for OER and HER is pivotal for large-scale energy generation via water splitting. A multimetallic electrocatalyst FeVO(OH)/Ni0.86Mo0.07W0.07(OH)2, aimed at tuning the electronic structure, is fabricated and shows considerable improvement in the water-splitting reaction kinetics, aided by low Tafel slope values of 24 mV/dec for OER and 67 mV/dec for HER, respectively. By taking advantage of (e̅-e̅) repulsions at the t2g level, we introduced high-valency Mo and W to provide a viable path for π-electron donation from oxygen 2p orbitals to vacant Mo and W orbitals for a dynamic electronic structure and an interfacial synergistic effect, which optimized the bond lengths for reaction intermediates to facilitate water splitting. The hybrid catalyst FeVO(OH)/NiMoW(OH)2 shows intrinsic activity and durability toward OER and HER tested for 48 h at a current density of 20 mA/cm2 and a cell voltage of 1.65 V @ 20 mA/cm2.

Polycyclic Arene-Fused Selenophenes via Site Selective Selenocyclization of Arylethynyl Substituted Polycyclic Arenes.

Arene-fused selenophenes were synthesized by a redox neutral process from arylethynyl substituted polycyclic arenes using selenium powder in refluxing N-methyl-2-pyrrolidone (NMP) with the assistance of the residual water in NMP as a catalytic proton source. The site-selective nature of this selenocyclization produces trans-alkenes as a competitive product, which is dependent on the π-electron donation ability of polycyclic arenes and the kind of arylethynyl group attached to it. DFT calculations were performed to understand the site selectivity in the selenophene formation reaction. The HOMO coefficient on the carbon adjacent to carbon having arylalkyne substituent of the polycyclic arene correlates with the selenocyclization tendency of the substrate. The wavelength of absorption and emission and quantum yield of emission increase with increasing the number of fused benzene rings in the polycyclic unit (from naphthalene to pyrene).

Isolation and Study of Ruthenium-Cobalt Oxo Cubanes Bearing a High-Valent, Terminal RuV-Oxo with Significant Oxyl Radical Character.

High-valent RuV-oxo intermediates have long been proposed in catalytic oxidation chemistry, but investigations into their electronic and chemical properties have been limited due to their reactive nature and rarity. The incorporation of Ru into the [Co3O4] subcluster via the single-step assembly reaction of CoII(OAc)2(H2O)4 (OAc = acetate), perruthenate (RuO4-), and pyridine (py) yielded an unprecedented Ru(O)Co3(μ3-O)4(OAc)4(py)3 cubane featuring an isolable, yet reactive, RuV-oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the RuV-oxo unit is experimentally demonstrated by radical coupling reactions between the oxo cubane and both 2,4,6-tri-tert-butylphenoxyl and trityl radicals. The oxo cubane oxidizes organic substrates and, notably, reacts with water to form an isolable μ-oxo bis-cubane complex [(py)3(OAc)4Co3(μ3-O)4Ru]-O-[RuCo3(μ3-O)4(OAc)4(py)3]. Redox activity of the RuV-oxo fragment is easily tuned by the electron-donating ability of the distal pyridyl ligand set at the Co sites demonstrating strong electronic communication throughout the entire cubane cluster. Natural bond orbital calculations reveal cooperative orbital interactions of the [Co3O4] unit in supporting the RuV-oxo moiety via a strong π-electron donation.

A Heterogeneous Metal-Free Catalyst for Hydrogenation: Lewis Acid-Base Pairs Integrated into a Carbon Lattice.

Designing heterogeneous metal-free catalysts for hydrogenation is a long-standing challenge in catalysis. Nanodiamond-based carbon materials were prepared that are surface-doped with electron-rich nitrogen and electron-deficient boron. The two heteroatoms are directly bonded to each other to form unquenched Lewis pairs with infinite π-electron donation from the surrounding graphitic structure. Remarkably, these Lewis pairs can split H2 to form H+ /H- pairs, which subsequently serve as the active species for hydrogenation of different substrates. This unprecedented finding sheds light on the uptake of H2 across carbon-based materials and suggests that dual Lewis acidity-basicity on the carbon surface may be used to heterogeneously activate a variety of small molecules.

A Fairly Stable Crystalline Silanone.

Silanones 2 substituted by bulky amino- and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron-donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2 =0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2 Si=O owing to the strong π-electron donation from the phosphonium ylide substituent.

Mechanisms for the formation of five-membered rings in ethene addition reactions with azomethine ylide and allyl anion

The mechanisms of pyrrolidine and cyclopentyl anion formation in the ethene addition reactions to allyl anion and azomethine ylide, as well as to their derivatives with π-electron-donating and π-electron-withdrawing substituents was studied for a wide range of systems by ab initio calculations using the MP2/6-311++G(d,p) basis set. The calculations showed that during the interaction of allyl anion with ethene the five-center intermediate species with CS symmetry was stabilized by direct π-electron donation effect, while in the case of azomethine ylide it was simultaneously affected by direct and reverse π-donation mechanisms. The introduction of π-electronwithdrawing substituents at the central position of allyl anion enabled a concerted synchronous cycloaddition process, while the reaction proceeded by single-step mechanism in the case of azomethine ylide having π-electron-donating or π-electron-withdrawing substituents at the nitrogen atom, except with sterically hindered reactants.

The half-sandwich 18- and 16-electron arene ruthenium iminophosphonamide complexes.

Novel half-sandwich 18ē and 16ē arene ruthenium iminophosphonamide complexes [(η6-C6Me6)RuCl{(R'N)2PR2}] (3a-c) and [(η6-C6Me6)Ru{(R'N)2PR2}]+(X-) (4a-c) (a, R = Ph, R' = p-Tol; b, R = Et, R' = p-Tol; c, R = Ph, R' = Me. X = BF4, PF6 or BArF4) were synthesized. The elongated Ru-Cl bond in the 18ē complexes is shown to dissociate even in apolar solvents to form the corresponding 16ē cations, which can be readily isolated as salts with non-coordinating anions. The coordinatively unsaturated 16ē complexes are stable species due to efficient π-electron donation from the nitrogen atoms of the zwitterionic NPN-ligand. The ruthenium iminophosphonamides are moderately active in the ROMP polymerization of norbornene; the 16ē complexes 4a,b yield high molecular weight polymers (Mn∼ 300 × 103) with a narrow distribution Mw/Mn∼ 1.6, while the 18ē complexes 3a,b give polymers of lower molecular weight (Mn < 50 × 103) with a wider polydispersity index Mw/Mn∼ 2.5.

Effect of the Formation of Highly Ordered Platinum(II) Octaethylporphyrin Adlayer on the Surface Reconstruction of Gold and Supramolecular Assembly of Fullerenes

The structures of 2D molecular assemblies of platinum(II) octaethylporphyrin (PtOEP) on Au(111) and Au(100) surfaces were examined using in situ scanning tunneling microscopy (STM) in 0.05 M HClO4 solutions. The in situ STM observations revealed that the reconstructed rows of Au(111) and Au(100) were stabilized through the formation of highly ordered arrays by adsorption of PtOEP in a wide potential range of the electric double-layer region and that the PtOEP overlayer expanded the stable potential range for the reconstructed phase. Furthermore, the supramolecular assembly of C60 and C84 on a highly ordered PtOEP adlayer formed on Au(111) surfaces was investigated. STM images revealed site-selective supramolecular assembly of fullerenes, especially C84, which exhibited a low coverage on the PtOEP adlayer. C60 molecules exhibited a full coverage and formed highly ordered adlayers on PtOEP, whereas C84 exhibited disordered arrays. The results of this study showed that the strong π-electron donation by the PtOEP molecules stabilized the reconstructed rows of single-crystal Au planes and that the PtOEP adlayer acted as a strong electron-donating layer, aiding the formation of supramolecularly assembled C60 molecules.

Properties of Complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2: P···P Pnicogen Bonding at σ-Holes and π-Holes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A-P···P-A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A-P···P linear, but the P···P═C orientation of H2C═PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves π-electron donation and acceptance by H2C═PX. The order of binding energies of the three conformations of H2C═(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C═(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C═PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding σ* orbital on the other. However, in conformation C, charge transfer occurs from the π orbital of H2C═PX to the σ*P-A orbital of PXH2, and from the lone pair on P of PXH2 through the π-hole to the π*P═C orbital of H2C═PX. Changes in charges on P upon complexation do not correlate with changes in (31)P chemical shieldings. Computed EOM-CCSD spin-spin coupling constants correlate with P-P distances. At each distance, the ordering of (1p)J(P-P) is A > B > C. Binding energies and spin-spin coupling constants of conformation A complexes of (PH2X)2, H2C═(X)P:PXH2, and (H2C═PX)2 with A-P···P-A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)2 > H2C═(X)P:PXH2 > (H2C═PX)2, while the order is reversed for complexes formed from the more electropositive substituents. A plot of ΔE(PH2X)2/ΔE(H2C═PX)2 versus ΔE[H2C═(X)P:PXH2]/ΔE(H2C═PX)2 indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)2 tend to have larger spin-spin coupling constants and shorter P-P distances than H2C═(X)P:PXH2, which in turn have larger coupling constants and shorter P-P distances than (H2C═PX)2, although there is some overlap. Complexes having similar P-P distances have similar values of (1p)J(P-P).

On substituent effect on the benzodiazepinone system

Tautomerism, aromaticity, and electron density at ring critical points of substituted benzodiazepinones, composed of condensed 6- and 7-membered rings (7-substituted 1,3-dihydro-benzo[e][1,4]diazepin-2-ones), were studied at the B3LYP/6-31G∗∗ level. We found that in the gas phase and in water, the N1H tautomers are more stable than the N4H ones by at least 8kcal/mol. We have demonstrated that the greater the π-electron donation of the substituent, the more stable the N1H tautomer. Analysis of the HOMA index of the two rings shows that in the N1H tautomers, the benzo ring is aromatic and the diazepinone ring is antiaromatic. In the N4H tautomers the former ring loses aromaticity and the latter one gains a more antiaromatic character. This is the main reason for the relative instability of the N4H tautomers. The AIM analysis combined with the substituent effect analysis by use of the sEDA and pEDA descriptors reveal that the π-electron properties of the substituents attached to the 6-membered benzo ring linearly correlate with the electron density and Laplacian at ring critical point of the 7-membered diazepinone one. Surprisingly, the analogous correlation for the substituted at the 6-membered ring is far less significant, if it is significant at all. This strongly suggests that modification in pharmaceutical activity of benzodiazepinones can be partially controlled just by attaching a substituent of the proper π-electron donor–acceptor properties in the C7-position of the benzo ring.

Theoretical study of phthalane oxidation and effect of substituents by using Hyperchem program

The effect of substituents on the relative stabilization energy (ERS) of phthalane-X carbocations has been studied with PM3 semi-empirical molecular orbital theory. The results show that the amino group gives ERS less than zero for very big amount and that will increase stability of carbocation by π-electron donation, whereas the nitro group gives height amount of ERS more than zero and that will lead to difficulties of an intermediate carbocation formation because of its strong σ- and π-accepting nature. ERS decreases with increase in the phenyl rings attached to cation. When electron withdraws substituents attached, it will destabilize the cation, because they act as π-electron donors. Key words: Relative stabilization energy (ERS), PM3 semi-empirical, isodesmic reactions.

Influence of the Ti−O−C Angle on the Oxygen-to-Titanium π-Donation in [Cp2*Ti(III)OR] Complexes

The constrained geometry alkoxide [Ti(III)(η5-C5Me5){η5-C5Me4CH2CMe2O-κO}] (2) and titanocene alcoholates [Cp*2TiOR] for R = 1-adamantyl (3) and pentafluorophenyl (4) were prepared by reacting acetone and the respective alcohols with the singly tucked-in titanocene [Ti(III)(η5-C5Me5)(η5:η1-C5Me4CH2)] (1) in order to add the simplest member to constrained geometry alkoxides 5 and 6 and to extend the series of [Cp*2TiOR] (R = Me (8), tBu (7), Ph (9), and H (10)) compounds for one of the most bulky and the most electron attracting substituent. The titanocene alkoxides with the alkoxy carbon atom tethered to one of the cyclopentadienyl ligands showing the crystallographic Ti−O−C angle close to 133° are a suitable group of compounds to be compared with the [Cp*2TiOR] compounds, where the Ti−O−C angle is about 174° for 3 and 7 and 180° for 4. In spite of these Ti−O−C angle differences, the wavelength of the first electronic transition λexp(1a1 → b2), supposed to indicate the extent of the oxygen π-electron donation into the Ti−O bond (as found by Andersen et al. J. Am. Chem. Soc. 1996, 118, 1719−1728), was slightly higher for 2 (1390 nm) compared to 3 (1335 nm) or 7 (1300 nm). Since the Ti−O−C carbon atom has a similar environment in all these compounds (three methylenes for 3, three methyl groups for 7, and one tethered methylene and two methyl groups for 2), the batochromic shift of λexp(1a1 → b2) for 2 could be attributed to a smaller extent of the oxygen π-electron donation effect. These differences in λexp are, however, very small and hardly can be correlated with the crystallographic Ti−O bond lengths, whose differences of max. 0.02 Å can be attributed also to the unequal steric congestion in 2, 3, and 7. On the other hand, the electron-attracting C6F5 group in 4 decreased the oxygen π-electron donation considerably, as indicated by the large bathochromic shift of λexp(1a1 → b2) to 1950 nm and the discernible elongation of d(Ti−O)exp and shortening of d(O−C)exp distances. These observations are in full agreement with our previous experimental and theoretical study (Varga et al. Organometallics 2009, 28, 1748) concluding that an increasing polarity of the Ti−O bond decreases the oxygen π-donation. The time-dependent DFT calculations of λcalc(1a1 → b2) could follow the trend λexp(1a1 → b2) (4 ≫ 9 > 10 > 3 > 7 > 8), although with the theoretical values biased systematically toward higher energies by about 200 nm. This shift has been ascribed to the insufficient treatment of electron correlation by DFT, resulting in the overestimation of interelectron repulsion. Complete active space SCF studies for [Cp*2TiOMe] (8) involving both the ground and the first excited state gave an excellent agreement of λcalc(1a1 → b2) with λexp(1a1 → b2) and proved no involvement of other orbitals in this transition. The DFT study on [Cp*2TiOH] (10) covering the range of linear-to-perpendicular Ti−O−H angles established the energy minimum for the Ti−O−H angle at 118.4° and the OH group situated in the Cg,Ti,Cg plane, in excellent agreement with the single-crystal model. The difference in the overall delocalization energies Edeloc(O→Ti) for the bent and the linear arrangement was, however, small in comparison with variations of Edeloc(O→Ti) in [Cp*2TiOR] compounds induced by the nature of substituents R.

Meso-tetracinnamylporphyrin: Synthesis, characterization and the catalytic activity of its Mn(III) complex in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate

Meso-tetracinnamylporphyrin (H 2tcp) has been synthesized and characterized by UV–Vis, 1H NMR and IR spectroscopies. Protonation of H 2tcp with HCl is accompanied by a blue shift (77 cm −1) of the Q(0,0) band, probably indicating a decrease in the efficiency of the π-electron donation from the meso-substituents to the a 2u orbital in comparison with the corresponding dication of meso-tetraphenylporphyrin (H 2tpp). The unusual NH signal multiplicity observed in 1H NMR spectrum of H 2tcp shows evidence of a long distance coupling between the vinyl protons and NH ones. This long range coupling can occur between the remote centers using the a 2u orbital as a connector. Mntcp(OAc) shows higher catalytic activity (ca. 1.2-fold higher) than Mntpp(OAc) in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate (TBAO). The low ratio (∼1.5) of cis- to trans-stilbene oxide, in competitive oxidation of cis- and trans-stilbene with TBAO, strongly suggests the involvement a common high-valent manganese oxo complex as the reactive intermediate responsible for oxygen atom transfer.

Ligand-to-Metal Charge Transfer Resulting in Metalloaromaticity of [R,R-Cyclohexyl-1,2-bis(2-Oxidonaphthylideneiminato-N,N′,O,O′)]Cu(II): A Scrutinized Structural Investigation

Molecular and crystal structure analysis of a tetradentate ligand-to-metal charge transfer (LMCT) complex, [R,R-cyclohexyl-1,2-bis(2-oxidonaphthylideneiminato-N,N′,O,O′)]Cu(II), are described and characterized by X-ray crystallography and FTIR spectroscopy. The Cu(II) centers are coordinated by four atoms of the donor set [N2O2] in a distorted square-planar fashion, which can be attributed to an active electronic delocalization within metallacycles and polychelating property of the pro-ligand. Bond valences of copper centers in four crystallographically independent monomers are slightly distinguished from each other, which is associated with differences in charge delocalization levels of the pro-ligands. Total valence of the copper center is increased with increasing π-electron donation from naphthalene fragments to metallacycles. This charge transfer leads to π···π interactions between metallacycles including imine bridges. In addition, decreases in the centroid–centroid distances of π···π interactions are associated with increased aromatic character of metallacycles. Molecules of the complex are stacked as dimers in the crystal structure formed by π···π interactions and aggregations of these dimeric formations along a-axis are strengthened by C–H···O type H-bonds and C–H···π interactions.

Unsaturated SiC4H6Si-bridged ansa-permethyltitanocene(TiIII) acetylide and hydroxide

Paramagnetic complexes ansa-[Ti(CCCMe3){η5:η5-C5Me4SiMe2CHCHCH2CH2SiMe2C5Me4}] (1) and ansa-[Ti(OH){η5:η5-C5Me4SiMe2CH2CHCHCH2SiMe2C5Me4}] (2) were prepared by reactions of ansa-[Ti(III){η3:η5:η5-C5Me4SiMe2CHCHCHCH2SiMe2C5Me4}] (3) with t-butylethyne or water, respectively. The X-ray single crystal diffraction analysis of 1 and 2 showed that the cis-double bond is situated asymmetrically in the ansa-bridge of 1 whereas, in 2, the double bond is trans, and lies in the bridge centre. ESR and UV-near IR spectra indicated a weaker oxygen π-electron donation into the Ti–O bond of 2 compared to analogous methoxide ansa-[Ti(III)(OMe){η5:η5-C5Me4SiMe2CH2CHCHCH2SiMe2C5Me4}] (4) or decamethyltitanocene hydroxide (5).

Molecular Structures of Alumina Nanoballs and Nanotubes: A Theoretical Study.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reverse SiC Bond Polarization as a Means for Stabilization of Silabenzenes: A Computational Investigation

Quantum-chemical calculations at the B3LYP/6-31+G(d,p) hybrid density functional level reveal that reverse polarization, i.e., reduction of the partial positive charge at Si by π-electron donation from ortho and/or para substituents, is a viable tool to increase the thermodynamic stability of silabenzenes. With 2,4,6-triamino substitution the silabenzene is dominated by zwitterionic resonance structures that place negative charge on Si, and this silabenzene is nearly equally stable as a monomer as it is as a dimer. Ipso silyl substitution further increases the stability so that 1-silyl-2,4,6-triaminosilabenzene is more stable as a monomer than as a dimer. However, bond geometrical data and nucleus-independent chemical shifts indicate that aromaticity decreases with increasing importance of reverse polarization. To design novel silabenzenes that are stable at ambient temperatures, one can work with three parameters: the steric bulk of the substituents, the inductive stabilization obtained through ipso-silyl substitution, and the conjugative stabilization achieved through π-donor groups in ortho and/or para positions.

Density functional study of the adsorption of 1,4-cyclohexadiene on Pt(1 1 1): origin of the C–H stretch red shift

The adsorption of 1,4-cyclohexadiene (CHD14) on Pt(1 1 1) was analysed using first-principle density functional theoretical calculations. CHD14 adsorbs at the bridge site via quadra-σ-type bonding. The adsorption energy was calculated to be 146 kJ/mol. CHD14 can also adsorb at the hollow site via the formation of a di-σ- and a π-bond with an adsorption energy of 142 kJ/mol. Upon adsorption CHD14 adopts a boat conformation. Strong π-electron donation to the platinum surface in combination with a hyperconjugative interaction of these π-orbitals with the axial σCH orbital helps to explain the observed red shift of the axial C–H stretching frequency and the corresponding C–H bond lengthening.