Quantum chemical study of ethylene addition to group-7 oxo complexes MO2(CH3)(CH2) (M = Mn, Tc, Re)

Quantum chemical calculations using DFT at the B3LYP level have been carried out for the reaction of ethylene with the group-7 compounds ReO₂(CH₃)(CH₂) (Re1), TcO₂(CH₃)(CH₂) (Tc1) and MnO₂(CH₃)(CH₂) (Mn1). The calculations suggest rather complex scenarios with numerous pathways, where the initial compounds Re1-Mn1 may either engage in cycloaddition reactions or numerous addition reactions with concomitant hydrogen migration. There are also energetically low-lying rearrangements of the starting compounds to isomers which may react with ethylene yielding further products. The [2 + 2]_Re,C cycloaddition reaction of the starting molecule Re1 is kinetically and thermodynamically favored over the [3 + 2]_C,O and [3 + 2]_O,O cycloadditions. However, the reaction which leads to the most stable product takes place with initial rearrangement to the dioxohydridometallacyclopropane isomer Re1a that adds ethylene with concomitant hydrogen migration yielding Re1a-1. The latter reaction has a slightly higher barrier than the [2 + 2]_Re,C cycloaddition reaction. The direct [3 + 2]_C,O cycloaddition becomes more favorable than the [2 + 2]_M,C reaction for the starting compounds Tc1 and Mn1 of the lighter metals technetium and manganese but the calculations predict that other reactions are kinetically and thermodynamically more favorable than the cycloadditions. The reactions with the lowest activation barriers lead after rearrangement to the ethyl substituted dioxometallacyclopropanes Tc1a-1 and Mn1a-1. The manganese compound exhibits an even more complex reaction scenario than the technetium compounds. The thermodynamically most stable final product of ethylene addition to Mn1 is the ethoxy substituted metallacyclopropane Mn1a-2 which has, however, a high activation barrier.     

Comparative theoretical study of [3+2] and [2+2] cycloadditions of ethylene and WXYMe2; X, Y = (O), (NH), (CH2)

Quantum chemical calculations employing density functional theory (B3LYP) were carried out to compare the preference of [3+2] versus [2+2] cycloadditions of ethylene to WO₂(CH₃)₂ (W2), WONH(CH₃)₂ (W3), WNHCH₂(CH₃)₂ (W4), W(CH₂)₂(CH₃)₂ (W5), and W(NH)₂(CH₃)₂ (W6). The results are compared to previously published data on the related tungsten complex WOCH₂(CH₃)₂ (W1). In agreement with MoOCH₂(CH₃)₂ and ReO₂CH₃CH₂, all six tungsten complexes prefer a [2+2] pathway rather than a [3+2] cycloaddition which is the reverted preference compared to the analogous compounds TcO₂CH₃CH₂, MnO₂CH₃CH₂, RuO₃CH₂, OsO₃CH₂ and OsO₂(NH)₂, and MO₂CH₃CH₂ (M = Ir, Rh, Co).     

Ethylene addition to OsO3(CH2) - A theoretical study

Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO₃(CH₂). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]_O, C addition which has a barrier of only 2.3 kcal mol⁻¹. The [3+2]_O, O addition has a slightly higher barrier of 6.5 kcal mol⁻¹. Four other reactions of OsO₃(CH₂) with C₂H₄ have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol⁻¹. Even higher barriers are calculated for the [2+2] addition to the OsO bond (32.6 kcal mol⁻¹) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol⁻¹). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol⁻¹) and for the addition reactions of the latter with C₂H₄ are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the OsO bond which has an activation barrier of 46.6 kcal mol⁻¹ and the [3+2]_O, O addition which is an endothermic process with an activation barrier of 44.3 kcal mol⁻¹.     

The mechanism of transition metal catalyzed carbonylation of allyl halides: A theoretical investigation

The mechanism of the carbonylation reaction of allyl halides catalyzed by nickel (Ni(CO)₄) and palladium (Cl₂Pd(PPh₃)₂) complexes is theoretically investigated at the DFT level using the hybrid B3LYP functional. The favored reaction path to carbonylation corresponds, for both catalysts, to a direct attack of the halogen on the metal. This affords η¹ intermediates that can undergo the final carbonylation step. It is also possible to obtain the acyl product (β,γ-unsaturated acyl halides) from η² and/or η³ intermediates. However, in this case, the barrier of the rate-determining step to carbonylation is much higher. Since a channel on the potential surface connects rather easily the η² or η³ intermediates to the η¹ intermediates, an alternative and competitive path leading to the acyl products can originate from the η² or η³ intermediates, follow the η²/η³ → η¹ transformation, then undergo the final carbonylation step.     

Vibrational center-ligand couplings in transition metal complexes

The mode-tracking principle [J. Chem. Phys. 2003, 118, 1634] for the direct quantum chemical calculation of preselected, characteristic molecular vibrations makes vibrational analyses of very large molecules feasible. This is demonstrated here for the [(Ph(3)PAu)(6)C](2+) complex, in which 18 phenyl groups in the ligand sphere are explicitly taken into account. We are aiming at the motion of the endohedral carbon atom, which is in an extraordinary bonding situation because it is surrounded by an octahedral core of gold atoms in this cluster. Secondary effects of the full ligand sphere on the vibrations of the [Au(6)C] core embedded in [(R(3)PAu)(6)C](2+) clusters are investigated. For this purpose, local vibrations of the octahedral core are generated, and their long-range couplings with the phosphine ligand sphere become visible in the mode-tracking iterations. The exact normal modes of these characteristic vibrations of the cluster are then obtained after convergence of the mode-tracking refinement. This protocol allows us to assess the coupling of the outer ligand sphere with the inner core of the cluster in terms of changes of the vibrational frequencies and of the collective motions of the atomic nuclei. The vibrational frequencies of the octahedral [Au(6)C] core split due to symmetry breaking in the C(1)-symmetric [(Ph(3)PAu)(6)C](2+) cluster. Our study demonstrates how effects of the periphery of a large molecule on local vibrations can be quantified. Furthermore, we predict the first set of characteristic vibrational frequencies obtained with first-principles methods for this gold cluster, whose vibrational spectra have not yet been recorded experimentally.     

Rigid ferrocenophane and its metal complexes with transition and alkaline-earth metal ions

The rigid [6]ferrocenophane, L¹, was synthesised by condensation of 1,1′-ferrocene dicarbaldehyde with trans-1,2-diaminocyclohexane in high dilution at r.t. followed by reduction. When other experimental conditions were employed, the [6,6,6]ferrocenephane (L²) was also obtained. Both compounds were characterised by single crystal X-ray crystallography. The protonation of L¹ and its metal complexation were evaluated by the effect on the electron-transfer process of the ferrocene (fc) unit of L¹ using cyclic voltammetry (CV) and square wave voltammetry (SWV) in anhydrous CH₃CN solution and in 0.1 M ⁿBu₄NPF₆ as the supporting electrolyte. The electrochemical process of L¹ between −300 and 900 mV is complicated by amine oxidation. On the other hand, an anodic shift from the fc/fc⁺ wave of L¹ of 249, 225, 81 and 61 mV was observed by formation of Zn²⁺, Ni²⁺, Pd²⁺ and Cu²⁺ complexes, respectively. Whereas Mg²⁺ and Ca²⁺ only have with L¹ weak interactions and they promote the acid-base equilibrium of L¹. This reveals that L¹ is an interesting molecular redox sensor for detection of Zn²⁺ and Ni²⁺, although the kinetics of the Zn²⁺ complex formation is much faster than that of the Ni²⁺ one. The X-ray crystal structure of [PdL¹Cl₂] was determined and showed a square–planar environment with Pd(II) and Fe(II) centres separated by 3.781(1) Å. The experimental anodic shifts were elucidated by DFT calculations on the [ML¹Cl₂] series and they are related to the nature of the HOMO of these complexes and a four-electron, two-orbital interaction.     

A quantum chemical study on the mechanism of S-coordinated tetrazole-thiolato formation by the reaction of organic isothiocyanates with metal azido complexes of Pt(II), Pd(II), and Sn

The mechanism of the reaction of isothiocyanates with metal-azido complexes of Pt(II), Pd(II), and Sn as well as hydrazoic acid is studied using the density functional theory method. The relative stability between two possible product isomers (S-coordinated tetrazole-thiolato and N-coordinated tetrazolato complexes) does not directly relate to the experimentally synthesized product. The overall reaction proceeds via three steps. The first step is the approach of the S-atom of the organic isothiocyanate to the central metal atom followed by the nucleophilic attack of the coordinated N-atom of the azido group to the C-atom of the isothiocyanate. The activation barrier of this step is 22-24 kcal mol⁻¹, and the resulting intermediate has the imidoyl azide form. In the second reaction step, electrophilic attack of the terminal N-atom of the azido group to the N-atom of the isothiocyanate transforms the intermediate to the S-coordinated tetrazole-thiolato product with a barrier of about 11 kcal mol⁻¹. The N-coordinated tetrazole could be made from the S-coordinated tetrazole-thiolato complex only after the third step, in which the metal coordination migrates from the S- to the N-atom.     

Tricarbonylchromium complexes of [5]- and [6]metacyclophane: an experimental and theoretical study

Tricarbonylchromium complexes of [5]- and [6]metacyclophane were prepared and the interaction between the Cr(CO)₃ tripod and the cyclophane fragment was evaluated by both an experimental and a theoretical study. The tricarbonylchromium complex of [5]metacyclophane could only be obtained in solution and was characterized by its ¹H NMR spectrum. The tricarbonylchromium complex of [6]metacyclophane was isolated and an X-ray crystal structure was obtained, which reveals that no significant geometric changes occur upon coordination of the severely distorted aromatic ring. Computations on the tricarbonylchromium complexes of m-xylene, [5]- and [6]metacyclophane furthermore demonstrate that the corresponding complexation energy is remarkably unaffected by the degree of distortion of the aromatic ring. Theoretical analyses of the above model systems as well as complexes of planar and artificially deformed benzene with Cr(CO)₃ show that this is primarily the result of two counteracting effects: (i) a stabilization due to an increased back-donation from the metal center to the benzene and (ii) a destabilization due to the increasing strain in the aromatic ring.     

Quinoxaline-based Schiff base transition metal complexes: review

The literature survey highlights spectra and biological activity of transition metal complexes derived from Schiff bases of quinoxaline. The extensive studies of synthesis, spectral, structural characterization, and biological activities of the metal complexes with heterocyclic Schiff bases of quinoxaline are reviewed.     

Stable trans isomer as the kinetic and theromodynamic product for the oxidative addition of MeI to cycloplatinated(II) complexes comprising isocyanide ligands

The present investigation introduces a new series of cycloplatinated(II) complexes, with the general formula Pt(O‐bpy)(Me)(CN‐R)] (R = benzyl, 2‐naphtyl and tert‐butyl), which are able to generate the stable trans‐Pt(IV) product in the solution after the reaction with iodomethane. In fact, the trans product is both the kinetic and thermodynamic product of the reaction; this observation was supported by DFT calculations. These Pt(II) complexes are supported by 2,2'‐bipyridine N‐oxide (O‐bpy) and one of several isocyanides as the cyclometalated and ancillary ligands, respectively. These new Pt(II) complexes undergo oxidative addition with MeI to give the corresponding trans‐Pt(IV) complexes. All the complexes were identified employing the multi‐nuclear NMR spectroscopy and single crystal X‐ray crystallography. The kinetic investigations were also performed for the oxidative addition reactions in order to measure the reaction rates; the reaction was followed by UV‐Vis spectroscopy. The rates obtained follow the trend CN‐^tBu > CN‐Bz > CN‐2 Np for the CN‐R ligands in the Pt(II) complexes. The order can be related to the degree of electron‐donation of the R group (tert‐butyl > benzyl > 2‐naphtyl).     

A comparative DFT study of some N-based aromatic ligand metal complexes as anticancer agents and analysis of their mode of interaction with DNA base pair

Metal complexed anticancer agents interact with DNA nucleobase pairs (AT and GC) through different types of binding mode such as intercalation, groove binding, covalent binding, etc. Minor and major groove binding mechanism of DNA base pair is the key factor for all kinds of anticancer agent; as metal complexes have a great affinity to bind with DNA nucleobase either through minor or major groove. Ligands in metal complexes also play a vital role during the interaction with DNA base pairs; these ligands directly interact with DNA through different interacting modes. Generally, anticancer agents with less sterically hindered N-based aromatic and planar ligands are the key component for DNA binding; as the structure of such ligands are quite compatible for following intercalation and groove binding mechanism. Since, the experimental investigation for drug-DNA nucleobase complexes are extremely complicated, therefore; quantum mechanical calculations might be very helpful for computing the actual interactions in drug-DNA complexes. Quantum mechanical approaches such as density functional theory (DFT) might be a very important and useful tool to investigate the actual mode of interaction of metal complexed antitumor agents with DNA nucleobase. Herein, we have taken some metal complexes with N-based aromatic ligands as antitumor agents to investigate the proper mode of interaction between drug-DNA complexes.     

多核过渡金属配合物自旋阻挫的理论研究

区别于双核配合物，自旋阻挫是多核配合物重要的磁现象之一．在分子磁体系中，自旋阻挫引起体系基态的多变和简并以及可能的基态自旋中间值等特征．简要地介绍多核配合物磁耦合竞争自旋阻挫的理论研究进展．     

Vibrational corrections to geometries of transition metal complexes from density functional theory

Zero-point vibrational corrections are computed at the BP86/AE1 level for the set of 50 transition-metal/ligand bonds that have recently been proposed as testing ground for DFT methods, because of the availability of precise experimental gas-phase geometries (Bühl and Kabrede, J Chem Theory Comput 2006, 2, 1282). These corrections are indicated to be transferable to a large extent between various density-functional/basis-set combinations, so that they can be used to estimate zero-point averaged r0g distances from re values optimized at other theoretical levels. Applying this approach to a number of popular DFT levels does not, in general, improve their overall accuracy in terms of mean and standard deviations from experiment. The hybrid variant of the meta-functional TPSS is confirmed as promising choice for computing structures of transition-metal complexes.     

Catalytic and biological activity of transition metal complexes of salicylaldiminopropylphosphine

Primary phosphine complexes of transition metals have been synthesized from salicylaldiminopropylphosphine. The complexes were characterized by elemental analysis, infrared, electronic, ¹H NMR, ³¹P NMR spectra, magnetic susceptibility, and conductivity measurements. The studies indicate square planar geometry for copper, cobalt, and nickel complexes. The EPR spectra of copper complex in acetonitrile at 300 and 77 K were recorded. The biological activities of the ligand and metal complexes have been studied on microorganisms such as Salmonella typhi, Staphylococcus aureus, Escherichia coli, Aspergillus niger, and Aspergillus flavus by the well-diffusion method. The zone of inhibition values were measured at 37°C for 24 h. The electrochemical behavior of copper complexes was studied by cyclic voltammetry. The copper(II) complex oxidizes cinnamaldehyde using hydrogen peroxide as oxidant.     

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.     

Pathways of cycloaddition of carbodiimides to N-alkenylidenetriflamides: A theoretical study

Possible cyclization pathways for the reaction of (1E,2E)-N-(but-2-en-1-ylidene)triflamide with N,N'-dimethylcarbodiimide have been investigated by DFT and MP2 calculations. The [4+2] route is shown to be thermodynamically unfavorable due to a small content of the reactive s-cis conformer of the azadiene. The [2_CN+2_CN] route has ΔGº>0 and therefore is thermodynamically forbidden. The only allowed route is the [2_CN+2_CC] cycloaddition, which has ΔGº < 0 and leads to 2-methylimino-3-(2-trifliminomethyl)-1,4-dimethylazetidine with further isomerization to 3-(triflamidomethylidene)-2-methylimino-1,4-dimethylazetidine in full agreement with the experimental results. These results indicate the necessity of considering free energy changes rather than only enthalpy changes for accurate prediction of the course of cyclization reactions.     

Oxorhenium complexes with the 8-quinolinolato ligand. X-ray structure and DFT calculations for [ReOX2(hqn)(AsPh3)] and [ReOX2(hqn)(PPh3)] complexes

The reactions of [ReOX₃(AsPh₃)₂] and [ReOX₃(PPh₃)₂] with 8-hydroxyquinoline (Hhqn) have been examined and the complexes [ReOX₂(hqn)(AsPh₃)] and [ReOX₂(hqn)(PPh₃)] (X = Cl, Br) have been obtained, respectively. The crystal and molecular structures of [ReOCl₂(hqn)(AsPh₃)] (1) and [ReOBr₂(hqn)(PPh₃)] (4) have been determined. The electronic structure of 1 has been calculated with the density functional theory (DFT) method. The spin-allowed electronic transitions of 1 have been calculated with the time-dependent DFT method, and the UV–Vis spectrum of [ReOCl₂(hqn)(AsPh₃)] has been discussed on this basis.     

Synthesis, characterisation of two hexa-iron clusters with {Fe2S2(CO)x} (x = 5 or 6) fragments and investigation into their inter-conversion

Reaction of a trithiol ligand, 2-(mercaptomethyl)-2-methylpropane-1,3-dithiol (H₃L), with tri-iron dodecacarbonyl in toluene produces two hexa-iron clusters (1 and 2). The two clusters are characterised crystallographically and spectroscopically. NMR spectroscopy reveals that the cluster 2 exists in two conformations in equilibrium 2_anti ⇔ 2_syn and the equilibrium constant K_eq = 0.55 under CO atmosphere. In the cluster 2, the central {Fe₂S₂(CO)₆} sub-unit is flanked by two identical {Fe₂S₂(CO)₆} satellite sub-units through thiolate linkages whereas one of the thiolate linkages can further form Fe-S bond with the proximal Fe atom in one of the two satellite sub-units to produce the cluster 1 by expelling one CO. This conversion can be entirely reversed by continuously purging CO through the solution of the cluster 1. As suggested by DFT calculations, the conversion features a key step, the rotation of the Fe_prox(CO)₃ to expose a vacant site for exogenic ligand binding (the S atom from the central sub-unit in this case) with concomitant switch for one of the three CO ligands in the unit of Fe_prox(CO)₃ from terminal to bridging orientation. The conversion from the clusters 1-2 involving one CO uptake is much faster than its reverse process since the latter is an endergonic process characterised by large reaction barriers, as revealed by the DFT calculations.     

Estimation of reliability of quantum-chemical calculations of electronic transitions in aqua complexes of transition metals

The correlation comparison of energy levels of aqua complexes of row IV transition metals was calculated using different methods on the basis of the WinGAMESS program with standard potentials of these systems and experimental values of the rate constants. The calculation showed that the use of DFT (density functional theory) considerably increases the reliability of calculations.     

New oxorhenium complexes with the 8-quinolinolato ligand: X-ray structure and DFT calculations for [ReOBr(hqn)2]

The reactions of [ReO(OEt)X₂(PPh₃)₂] (X = Cl or Br) with 8-hydroxyquinoline (Hhqn) have been examined and the [ReOX(hqn)₂] complexes have been obtained. The crystal and molecular structures of [ReOBr(hqn)₂] have been determined. The electronic structure of [ReOBr(hqn)₂] has been calculated with the density functional theory (DFT) method, and additional information about binding in the ReO³⁺ unit has been obtained by NBO analysis. The spin-allowed electronic transitions of [ReOBr(hqn)₂] have been calculated with the time-dependent DFT method, and the UV–Vis spectra of the [ReOX(hqn)₂] compounds have been discussed on this basis.