Predicting Dinitrogen Activation via Transition-Metal-Involved [4+2] Cycloaddition Reaction

As the strongest triple bond in nature, the N≡N triple bond activation has always been a challenging project in chemistry. On the other hand, since the award of the Nobel Prize in Chemistry in 1950, the Diels-Alder reaction has served as a powerful and widely applied tool in the synthesis of natural products and new materials. However, the application of the Diels-Alder reaction to dinitrogen activation remains less developed. Here we first demonstrate that a transition-metal-involved [4+2] Diels-Alder cycloaddition reaction could be used to activate dinitrogen without an additional reductant by density functional theory calculations. Further study reveals that such a dinitrogen activation by 1-metalla-1,3-dienes screened out from a series of transition metal complexes (38 species) according to the effects of metal center, ligand, and substituents can become favorable both thermodynamically (with an exergonicity of 28.2 kcal mol⁻¹) and kinetically (with an activation energy as low as 13.8 kcal mol⁻¹). Our findings highlight an important application of the Diels-Alder reaction in dinitrogen activation, inviting experimental chemists’ verification.     

Aromatic hydrocarbons, carbocyclic ligands spanning several oxidation states in both Main Group and transition elements. Recent advances with early transition d elements

In this paper some synthetic procedures to obtain (η⁶-arene)metal derivatives are reviewed. The metal-atom-arene-vapor co-condensation technique is the most appropriate to generate complexes of polycyclic aromatic hydrocarbons or heterocycles. As far as the aluminium halide-mediated synthesis is concerned, two classes of reaction are observed. When AlX₃ is used with a metal halide in the presence of an aromatic hydrocarbon in the absence of any reducing agent, AlX₃ can function as a dehalogenating agent, to give ionic compounds of general formula [M(η⁶-arene)n](AlX₄)_m, or it can add across the M---X bond with formation of M(μ-X)_nAlX_4−n systems. In both cases the metal displays its typical oxidation state. However, the use of AlX₃ in combination with aluminium (the Fischer-Hafner reducing system) affords ionic or covalent low-oxidation-state metal(η⁶-arene) complexes. Attention is focused on our most recent results concerning the synthesis, properties and reactivity of η⁶-arene derivatives of Group 4 and 5 elements, showing, inter alia, the first example of a tetraarylborate anion behaving as a 12-electron donor to one metal atom and low-valent η⁶-arene compounds as useful reagents in the inorganic and coordination chemistry of the corresponding metal in nonaqueous systems.     

碳酸及碳酸酯热消除反应的量子化学研究

用AM1方法研究了碳酸和碳酸酯的热消除反应．发现迁移基团R和基团R’共同制约这类吸热反应的活化能。     

Theoretical study on interaction of different coordination modes of BH4 ligand with transition metal in [TM(BH4)(CO)4] (TM = Cr, Mo)

Density functional calculations were performed on bonding and structural features of [(ηⁿ-BH₄)TM(CO)₄]⁻ (n = 1, 2, 3; TM = Cr, Mo) complexes. Calculations show that the ground state is bidentate which is in good agreement with experimental results. It has been found that the bridge and terminal hydrogen atoms will interchange by two pathways: (i) twist of BH₄ about one of the bridge B-H and (ii) twist of BH₄ about one of the terminal B-H. The molecular orbital calculations and natural bond orbital methodologies for different isomers of these complexes have been evaluated. The final results indicate that case (i) is more preferable relative to another case.     

Solvothermal syntheses of Cu3P via reactions of amorphous red phosphorus with a variety of copper sources

Polycrystalline Cu₃P was successfully prepared under a wide variety of solvothermal conditions. The reaction of red phosphorus with several copper sources (copper metal, copper (I) iodide, copper (I) chloride and copper (II) chloride) at 150-200 °C for 1-2 days in water produced Cu₃P. Products were examined with powder X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Thermal analyses and optical spectroscopy were also performed. A detailed list of reaction conditions, products and impurity phases (where applicable) are reported.     

Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Geometry optimizations at the UHF/6-31G^* and UMP2/6-31G^* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS^+·₁ and TS^+·₂, were obtained which have C₁ and C₂ symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS^+·₁ is the true transition state connecting N^+· and Q^+·, while TS^+·₂ is a second order saddle point.     

Density functional calculations of NMR chemical shifts and ESR g-tensors

An overview is given on recent advances of density functional theory (DFT) as applied to the calculation of nuclear magnetic resonance (NMR) chemical shifts and electron spin resonance (ESR) g-tensors. This is a new research area that has seen tremendous progress and success recently; we try to present some of these developments. DFT accounts for correlation effects efficiently. Therefore, it is the only first-principle method that can handle NMR calculations on large systems like transition-metal complexes. Relativistic effects become important for heavier element compounds; here we show how they can be accounted for. The ESR g-tensor is related conceptually to the NMR shielding, and results of g-tensor calculations are presented. DFT has been very successful in its application to magnetic properties, for metal complexes in particular. However, there are still certain shortcomings and limitations, e.g., in the exchange-correlation functional, that are discussed as well. Received: 24 October 1997 / Accepted: 19 December 1997     

On the automatic restricted-step rational-function-optimization method

The rational function optimization algorithm is one of the widely used methods to search stationary points on surfaces. However, one of the drawbacks of this method is the step reduction procedure to deal with the overstepping problem. We present and comment on a method such that the step obtained from the solution of the rational function equations possesses the desired correct length. The analysis and discussion of the method is mainly centered on the location and optimization of transition states. Received: 18 June 1998 / Accepted: 17 September 1998 / Published online: 23 November 1998     

On the Fe-CO molecule

Using a perturbational configuration interaction approach, it is found that the Fe-CO molecule has a low-spin ground state (i.e. ^3–), at variance with similar compounds formed by the first transition elements of this series (e.g. Sc and Ti). Binding energies, interatomic distances and vibration frequencies have been calculated for the ^3– state as well as for the ^5– high-spin state.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday