Stephan Schenk

First-Principles Investigation of the Schrock Mechanism of Dinitrogen Reduction Employing the Full HIPTN3N Ligand

In this work, we investigate with density functional methods mechanistic details of catalytic dinitrogen reduction mediated by Schrocks molybdenum complex under ambient conditions. We explicitly take into account the full HIPTN 3N ligand without approximating it by model systems. Our data show that replacement of the bulky HIPT substituent by smaller groups leads to deviations in energy of up to 100 kJ mol (-1). Alternatives to the Chatt-like mechanism are also investigated. It turns out that for the generation of the first molecule of ammonia, protonation of the ligand plays a crucial role. With an increasing number of hydrogens on the terminal nitrogen atom, the reduction becomes more difficult. The energetically most feasible step is the generation of the first molecule of ammonia, while the preceding transfer of the second electron and proton is the most difficult one. Reaction energies are not only reported for decamethyl chromocene as in previous studies but also for a series of other metallocenes. Furthermore, results are provided in a way to allow for a convenient estimation of the thermochemical boundary conditions of catalysis with an arbitrary combination of acid and reductant. We demonstrate that the [Mo](NNH 3) (+) complex easily loses ammonia even in the absence of a reductant. For some complexes, spin states with higher multiplicity are the ground state instead of those with lower spin multiplicity.

Ligands for dinitrogen fixation at Schrock-type catalysts.

Catalytic dinitrogen reduction with the Schrock complex is still hampered by low turn-over numbers that are likely to result from a degradation of the chelate ligand. In this work, we investigate modifications of the original HIPTN(3)N ligand applied by Schrock and co-workers in catalytic reduction of dinitrogen with density functional methods. We focus on ligands that are substituted in the para position of the central phenyl ring of the terphenyl moieties and on a ligand where the bridging nitrogen is exchanged by phosphorus. In addition, results for tris(pyrrolyl-alpha-methyl)amine, tris(pyrrolyl-alpha-ethyl)amine, and tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine are reported. For this study, we take into account the full ligands without approximating them by model systems. Reaction energies for the various derivatives of HIPTN(3)N are found to be similar to those of the unchanged parent system. However, the most promising results for catalysis are obtained for the [{tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine}Mo](N(2)) complex. Feasibility of the exchange of NH(3) by N(2) is found to be the pivotal question whether a complex can become a potential catalyst or not. A structure-reactivity relationship is derived which allows for the convenient estimation of the reaction energy for the NH(3)/N(2) exchange reaction solely from the wavenumber of the N[triple bond]N stretching vibration. This relationship may guide experiments as soon as a dinitrogen Mo complex is formed.

Formation of a unique zinc carbamate by CO2 fixation: implications for the reactivity of tetra-azamacrocycle ligated Zn(II) complexes.

The macrocyclic ligand [13]aneN 4 ( L1, 1,4,7,10-tetra-azacyclotridecane) was reacted with Zn(II) perchlorate and CO 2 in an alkaline methanol solution. It was found that, by means of subtle changes in reaction conditions, two types of complexes can be obtained: (a) the mu 3 carbonate complex 1, {[Zn( L1)] 3(mu 3-CO 3)}(ClO 4) 4, rhombohedral crystals, space group R3 c, with pentacoordinate zinc in a trigonal bipyramidal enviroment, and (b) an unprecedenced dimeric Zn(II) carbamate structure, 2, [Zn( L2)] 2(ClO 4) 2, monoclinic crystals, space group P2 1/ n. The ligand L2 (4-carboxyl-1,4,7,10-tetra-azacyclotridecane) is a carbamate derivative of L1, obtained by transformation of a hydrogen atom of one of the NH moieties into carbamate by means of CO 2 uptake. In compound 2, the distorted tetrahedral Zn(II) coordinates to the carbamate moiety in a monodentate manner. Most notably, carbamate formation can occur upon reaction of CO 2 with the [Zn L1] (2+) complex, which implicates that a Zn-N linkage is cleaved upon attack of CO 2. Since complexes of tetra-azamacrocycles and Zn(II) are routinely applied for enzyme model studies, this finding implies that the Zn-azamacrocycle moiety generally should no longer be considered to play always only an innocent role in reactions. Rather, its reactivity has to be taken into account in respective investigations. In the presence of water, 2 is transformed readily into carbonate 1. Both compounds have been additionally characterized by solid-state NMR and infrared spectroscopy. A thorough comparison of 1 with related azamacrocycle ligated zinc(II) carbonates as well as a discussion of plausible reaction paths for the formation of 2 are given. Furthermore, the infrared absorptions of the carbamate moiety have been assigned by calculating the vibrational modes of the carbamate complex using DFT methods and the vibrational spectroscopy calculation program package SNF.

A Stable Six‐Coordinate Intermediate in Ammonia–Dinitrogen Exchange at Schrock's Molybdenum Catalyst

In this work, we investigate the mechanism of the ammonia-dinitrogen exchange reaction, which is the decisive step to close the catalytic cycle of Schrocks dinitrogen reduction sequence under ambient conditions. We identify several viable pathways for the approach of dinitrogen to the five-coordinate molybdenum center of the ammonia complex by means of first-principles molecular-dynamics simulations. These exploratory simulations are then complemented by rigorous quantum-chemical structure optimizations. Our calculations have been performed for the full Schrock catalyst without simplifying the large chelate ligand, and are hence not affected by model assumptions. We show that the reaction obeys an addition-elimination mechanism via a stable six-coordinate intermediate. This intermediate has been fully characterized by stationary quantum-chemical methods. The predicted infrared spectrum of this species features an N[triple bond]N stretching vibration, which is well separated in frequency from all other N[triple bond]N stretching vibrations of N(2)-binding complexes involved in the Schrock cycle. Depending on the life time of this intermediate in the reaction liquor, the production of this intermediate might even be monitored by the absorption of the N[triple bond]N stretching vibration.

MOVIPAC: Vibrational spectroscopy with a robust meta‐program for massively parallel standard and inverse calculations

We present the software package MOVIPAC for calculations of vibrational spectra, namely infrared, Raman, and Raman Optical Activity (ROA) spectra, in a massively parallelized fashion. MOVIPAC unites the latest versions of the programs SNF and AKIRA alongside with a range of helpful add‐ons to analyze and interpret the data obtained in the calculations. With its efficient parallelization and meta‐program design, MOVIPAC focuses in particular on the calculation of vibrational spectra of very large molecules containing on the order of a hundred atoms. For this purpose, it also offers different subsystem approaches such as Mode‐ and Intensity‐Tracking to selectively calculate specific features of the full spectrum. Furthermore, an approximation to the entire spectrum can be obtained using the Cartesian Tensor Transfer Method. We illustrate these capabilities using the example of a large π‐helix consisting of 20 (S)‐alanine residues. In particular, we investigate the ROA spectrum of this structure and compare it to the spectra of α‐ and 310‐helical analogs.

The missing link in COS metabolism: a model study on the reactivation of carbonic anhydrase from its hydrosulfide analogue.

Carbonic anhydrase (CA) is known to react with carbonyl sulfide, an atmospheric trace gas, whereby H2S is formed. It has been shown that, in the course of this reaction, the active catalyst, the His3ZnOH structural motif, is converted to its hydrosulfide form: His3ZnOH+COS→His3ZnSH+CO2. In this study, we elucidate the mechanism of reactivation of carbonic anhydrase (CA) from its hydrosulfide analogue by using density functional calculations, a model reaction and in vivo experimental investigation. The desulfuration occurs according to the overall equation His3ZnSH+H2O ⇌ His3ZnOH+H2S. The initial step is a protonation equilibrium at the zinc‐bound hydrosulfide. The hydrogen sulfide ligand thus formed is then replaced by a water molecule, which is subsequently deprotonated to yield the reactivated catalytic centre of CA. Such a mechanism is thought to enable a plant cell to expel H2S or rapidly metabolise it to cysteine via the cysteine synthase complex. The proposed mechanism of desulfuration of the hydrosulfide analogue of CA can thus be regarded as the missing link between COS consumption of plants and their sulfur metabolism.

Carbon dioxide and related heterocumulenes at zinc and lithium cations: bioinspired reactions and principles

This Perspective starts with the discussion of the properties of an interesting metalloenzyme (carbonic anhydrase, CA) that performs extremely successfully the activation of carbon dioxide. Conclusions from that are important for many synthetic procedures and include experimental and theoretical investigation (DFT calculations) of such metal mediated processes in the condensed and in the gas phase in which the zinc cation plays a dominant role. This is extended to the bio-analogue activation of further heterocumulenes such as COS, an important atmospheric trace gas, and CS(2). Novel metal complexes which serve as useful catalysts for the reactions (copolymerisations and cyclisation) of CO(2) and oxiranes are discussed subject to the inclusion of recently published DFT calculations. We continue with the discussion of the very general aspect of the insertion of CO(2) into metal-nitrogen bonds (formation of carbamates). This again is closely related to many biological or bio-analogue processes. We describe the synthesis and mechanistic aspects of characteristic metal carbamates of a wide variety of metals and include a discussion of the mechanistic aspects, especially for the formation of Mg(2+) and Li(+) carbamates and the formation of related cyclic products after addition of the heterocumulenes CO(2), Ph-NCO or CS(2) to novel ligands, the 4H-pyridin-1-ides which finally result in the formation of e.g. 1,3-thiazole-5(2H)-thiones.

1,2-Diazetines as Useful Tools for Ring Transformation Reactions with Isothiocyanates--A New Entry to 1,3,4-Thiadiazines

1,2-Diazetines (1) can be acylated with isocyanates (2) to give semicyclic urea derivatives (3). In contrast, isothiocyanates (4) react with 1 under mild conditions to furnish new derivatives of 1,3,4-thiadiazine (5). DFTcalculations show that two different mechanistic pathways for this ring transformation are possible. N-Acylation is preferred at lower temperatures; whereas an electrocyclic ring opening/cycloaddition process is possible at higher temperatures.