A molecular orbital rationalization of ligand effects in N2 activation

Molecular orbital theory has been used to study a series of [(micro-N2){ML3}2] complexes as models for dinitrogen activation, with M=Mo, Ta, W, Re and L=NH2, PH2, AsH2, SbH2 and N(BH2)2. The main aims of this study have been to provide a thorough electronic analysis of the complexes and to extend previous work involving molecular orbital analyses. Molecular orbital diagrams have been used to rationalize why for L=NH2 ligand rotation is important for the singlet state but not the triplet, to confirm the effect of ligand pi donation, and to rationalize the importance of the metal d-electron configuration. The outcomes of this study will assist with a more in-depth understanding of the electronic basis for N2 activation and allow clearer predictions to be made about the structure and multiplicity of systems involved in transition-metal catalysis.     

Isatin‐Schiff base copper(II) complexes—A DFT study of the metal‐ligand bonding situation

Herein, we report results of calculations based on density functional theory (BP86/TZVP) of a set of isatin‐Schiff base copper(II) and related complexes, 1‐12, that have shown significant pro‐apoptotic activity toward diverse tumor cells. The interaction of the copper(II) cation with different ligands has been investigated at the same level of theory. The strength and character of the Cu(II)‐L bonding was characterized by metal‐ligand bond lengths, vibrational frequencies, binding energies, ligand deformation energies, and natural population analysis. The metal‐ligand bonding situation was also characterized by using two complementary topological approaches, the quantum theory of atoms‐in‐molecules (QTAIM) and the electron localization function (ELF). The calculated electronic g‐tensor and hyperfine coupling constants present significant agreement with the EPR experimental data. The calculated parameters pointed to complex 10 as the most stable among the isatin‐Schiff base copper(II) species, in good agreement with experimental data that indicate this complex as the most reactive in the series. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Carbon‐iodide bond activation by cyclometalated Pt (II) complexes bearing tricyclohexylphosphine ligand: A comparative kinetic study and theoretical elucidation

Cyclometalated Pt (II) complexes [PtMe(C^N)(L)], in which C^N = deprotonated 2,2′‐bipyridine N‐oxide (Obpy), 1 , deprotonated 2‐phenylpyridine (ppy), 2 , deprotonated benzo [h] quinolone (bzq), 3 , and L = tricyclohexylphosphine (PCy₃) were prepared and fully characterized. By treatment of 1–3 with excess MeI, the thermodynamically favored Pt (IV) complexes cis‐[PtMe₂I(C^N)(PCy₃)] (C^N = Obpy, 1a ; ppy, 2a ; and bzq, 3a ) were obtained as the major products in which the incoming methyl and iodine groups adopted cis positions relative to each other. All the complexes were characterized by means of NMR spectroscopy while the absolute configuration of 1a was further determined by X‐ray crystal structure analysis. The reaction of methyl iodide with 1–3 were kinetically explored using UV–vis spectroscopy. On the basis of the kinetic data together with the time‐resolved NMR investigation, it was established that the oxidative addition reaction occurred through the classical S_N2 attack of Pt (II) center on the MeI reagent. Moreover, comparative kinetic studies demonstrated that the electronic and steric nature of either the cyclometalating ligands or the phosphine ligand influence the rate of reaction. Surprisingly, by extending the oxidative addition reaction time, very stable iodine‐bridged Pt (IV)‐Pt (IV) complexes [Pt₂Me₄(C^N)₂(μ‐I)₂] (C^N = Obpy, 1b ; ppy, 2b ; and bzq, 3b ) were obtained and isolated. In order to find a reasonable explanation for the observation, a DFT (density functional theory) computational analysis was undertaken and it was found that the results were consistent with the experimental findings.     

The influence of the ligand structure on activation of hafnocene polymerization catalysts: A theoretical study

The influence of ligand structure of hafnocenes on activation of the polymerization catalysts has been studied by quantum chemical methods. Altogether 54 hafnocenes were included in the analysis, supplemented by four zirconocenes for comparison. The trends in structural and electronic parameters relevant in the catalyst activation step were studied for the dichloride, dimethyl and cationic monomethyl forms of the catalysts. The effects of ligand modifications were analyzed as functions of the metal, ancillary cyclopentadienyl-based ligand, ligand substituent and the ligand bridge, making comparisons to experimental data. Generally, large aromatic ligands together with electron donating ligand substituents stabilize the catalytically active species, thus facilitating the catalyst activation process. The obtained trends are expected to aid in the development of new high-performance polymerization catalysts.     

Solvent effects on the excited state properties of 2-aminopurine—a theoretical study by the ONIOM and Supramolecular method

In this paper, the micro-solvated effects on the lowest-energy vertical transition state and adiabatic excited states of 2-aminopurine (2Ap) were studied by Supramolecular method (B3LYP/6-31++G(d)) and ONIOM (B3LYP/6-31++G(d):PM3) method. The results are as follows: (1) In 2Ap molecule surrounded by six water molecules the pyramidalization of amino group in 2Ap almost disappears, and the hex-atomic ring is obviously buckled. The adiabatic lowest-energy valence excitation of gaseous 2Ap also causes the disappearance of amino pyramidalization. (2) The energy for lowest-energy singlet π→π^* vertical transition in water is predicted as 3.99 and 4.29 eV by Supramolecular and ONIOM method, respectively. Both values are in good agreement with the reported experimental result, 4.11 eV. The energy for the second lowest-energy n→π^* transition, 4.72 eV, by the Supramolecular method is obviously deviated from the reported experimental value 4.46 eV. The corresponding value given by the two-layer ONIOM method, 4.43 eV, is in good agreement with the experimental value. (3) The optimized energy of the fluorescent emission state (S₁ state) are 3.61 and 3.87 eV by Supramolecular and ONIOM methods, respectively. The calculated oscillator strengths, in both gas and water clusters, were compared with reported experimental and theoretical results. These results indicated that both Supramolecular and ONIOM methods, combined with TD DFT B3LYP/6311++G(d), can provide good results of calculating excited state and spectra properties of 2Ap in condensed phase. This fact encouraged us to extend our study to 2Ap-T base pair and its solvated model so as to obtain the spectra properties of 2Ap in real DNA environment.     

The influence of hydrophobic amino acid side groups on the acidity of the aromatic imidazole ring of histidine: A theoretical study

In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine‐Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔG_S values are located between ?69.38 and ?18.82 kcal mol^?1 which are related to His⁺–Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas‐phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self‐consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His⁺–ILe and His⁺–Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical study on the electronic structures and optical properties of blue‐green and blue phosphorescent iridium(III) complexes with tetraphenylimidodiphosphinate ligand

The electronic structures and photophysical properties of five iridium(III) complexes Ir(tfmppy)₂(tpip) (1), Ir(dfppy)₂(tpip) (2), Ir(afCNppy)₂(tpip) (3), Ir(CNpyN₃)₂(tpip) (4), and Ir(2fphpta)₂(tpip) (5) [where tfmppy = 4‐trifluoromethylphenylpyridine; dfppy =4,6‐difluorophenylpyridine; afCNppy = 6‐fluoro‐4‐octyloxy‐5‐cyano‐phenylpyridine; CNpyN₃ = 2‐(4‐cyano‐phenyl)‐[1,2,3]‐triazole; 2fphpta=2‐(2,6‐difluoro‐phenyl‐[1,2,4]‐triazol‐3‐yl)‐pyridine; tpip=tetraphenylimido‐diphosphinate] have been investigated by using density functional theory (DFT) methods and time‐dependent DFT ones, aiming at elucidating the influences of different substituents and cyclometalated ligands on the emission properties and quantum yield. The calculated results revealed that the different substituents in 1 ‐ 3 have a great influence on the energy levels, in particular highest occupied molecular orbital. Meanwhile, we have also get a further insight into the reason for different phosphorescence quantum yields of the studied complexes. The higher quantum yield (Φ) reported for 1 was found to be closely related to both its smaller S₁–T₁ splitting energy ( ) and larger transition electric dipole moment ( ) upon the S₀ → S₁ transition. Complex 5 is expected to be a potential candidate for blue‐emitting material with good organic light‐emitting diodes performances. We propose that the optical properties of this class of materials can be tuned by the modifications of the cyclometalated ligands. © 2013 Wiley Periodicals, Inc.     

The theoretical study on anionic polymerization mechanism of maleimide: Chain propagation by p‐π conjugation process

A complete research on the mechanism of the anionic polymerization of maleimide was performed, not only including the chain initiation, but the propagation as well. The density functional theory method is employed to investigate the reaction pathway using 6‐311+G* basis set, and the Onsager model is also applied to imitate the effect of solvent on the structures and thermodynamic functions of the key steps. It is found that the initiation starts with a nucleophilic reaction, in which the key transition state shows a π‐complex structure. In contrast, the calculated chain propagation (both dimer and trimer process) employs a p‐π conjugation chain propagation mechanism (p‐π CCPM), characterized by the formation of p‐π conjugation orbital between the chain terminal C atom and monomer C?C double bond. This mechanism is in good agreement with the frontier molecular theory and the principle of conservation of molecular orbital symmetry. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

The role of aromaticity on the building of nanohybrid materials functionalized with metalated (Au(III), Ag(III), Cu(III)) extended porphyrins and single‐walled carbon nanohorns: A theoretical study

An ab initio, systematic study on the aromaticity involving the group of metalated extended porphyrins, termed meso‐hexakis(pentafluorophenyl)‐substituted[26]hexaphyrin(1.1.1.1.1.1) (HP), was performed for the first time. The aromatic behavior of the system shifted to antiaromatic in the [28]HP analogue, due to the presence of hydrogen atoms that break the orbital symmetry. The absorption bands observed in the experiment were assigned to an intraligand charge transfer, where the intrametallic character is also important. The excited states reveal the absorption of visible light and the possibility of electronic transfer to different systems. We propose a system such as single‐walled carbon nanohorns (SWCNHs), due to their special electronic properties, and predict a novel nanohybrid material. The evidence of electronic communication between both species is presented in this work. The HP aromaticity and the spatial configuration of the interaction with SWCNHs are also related to the strength of electronic transfer among the systems, making the HP metalated antiaromatic species and their corresponding nanohybrids potential candidates to be used as building blocks in photovoltaic cell materials. © 2012 Wiley Periodicals, Inc.     

The negative differential resistance mechanism of a molecular device based on double‐cage fluorinated fullerene C20F18(NH)2C20F18: A theoretical study

The electronic transport properties of the molecular device based on double‐cage fluorinated fullerene C₂₀F₁₈(NH)₂C₂₀F₁₈ were studied theoretically. The results show that the device exhibits two negative differential resistance (NDR) peaks in its I‐V curve. The NDR peak under low bias voltage originates from the bias‐induced alignment of the molecular orbitals, and the conduction channel being suppressed at a certain bias voltage is the main reason for the NDR peak under a relatively high bias voltage.     

A comparative study on aldol‐type condensation reactions of cyclic,acyclic and substituted cyclic ketones by the W(CO)6/CCl4/UV system

Aldol‐type condensation reactions of a number of cyclic, acyclic and substituted cyclic ketones were investigated using the W(CO)₆/CCl₄/UV system. The progress of the reactions was followed by IR and GC‐MS techniques. The cyclic ketone derivatives with β‐ and γ‐substituent gave the expected condensation products. However, the α‐substituted cyclic, acyclic and unsubstituted cyclic ketones with rings larger than six did not. Formation of [W]–ketone complexes with all of the ketones used was observed by FTIR. With respect to our studies, a mechanism involving an intermediate seven‐coordinate tungsten complex has been proposed. Copyright © 2005 John Wiley & Sons, Ltd.