Hydride Mobility in Trinuclear Sulfido Clusters with the Core [Rh3(μ-H)(μ3-S)2]: Molecular Models for Hydrogen Migration on Metal Sulfide Hydrotreating Catalysts

Catalysis and structure of nitrogenases

In providing bioavailable nitrogen as building blocks for all classes of biomacromolecules, biological nitrogen fixation is an essential process for all organismic life. Only a single enzyme, nitrogenase, performs this task at ambient conditions and with ATP as an energy source. The assembly of the complex iron-sulfur enzyme nitrogenase and its catalytic mechanism remains a matter of intense study. Recent progress in the structural analysis of the three known isoforms of nitrogenase-differentiated primarily by the heterometal in their active site cofactor-has revealed a degree of structural plasticity of these clusters that suggest two distinct binding sites for substrates and reaction intermediates. A mechanistic proposal based on this finding integrates most of the available experimental data. Furthermore, the first applications of high-resolution cryo-electron microscopy have highlighted further dynamic conformational changes. Structures obtained under turnover conditions support the proposed alternating half-site reactivity in the C2-symmetric nitrogenase complex.

The Mechanism of the Intramolecular Hydrocarbyl Metathesis within a Planar Triruthenium Cluster: Combining Core Flexibility with Hydride Mobility

The transition metal catalysed formation and cleavage of C-C bonds is of utmost importance in synthetic chemistry. While most of the existing homogeneous catalysts are mononuclear, knowledge of the behaviour of polynuclear species is much more limited. By using computational methods, here we shed light into the mechanistic details of the thermally-induced isomerization of Cp*₃ Ru₃ (μ-H)₂ (μ₃ -η² -pentyne)(μ₃ -pentylidyne) (2) into Cp*₃ Ru₃ (μ-H)₂ (μ₃ -η² -octyne)(μ₃ -ethylidyne) (3), a process that involves the migration of a C₃ fragment between the hydrocarbyl ligands and across the plane formed by the three Ru centres. Our results show this to be a complex transformation that comprises of five individual rearrangements in an A→B→A→B→A order. Each so-called rearrangement A consists of the CH migration from the μ₃ -η² -alkyne into the μ₃ -alkylidine ligand in the other side of the Ru₃ plane. This process is facilitated by the cluster's ability to adopt open-core structures in which one Ru-Ru bond is broken and a new C-C bond is formed. In contrast, rearrangements B do not involve the formation or cleavage of C-C bonds, nor do they require the opening of the cluster core. Instead, they consist of the isomerization of the μ₃ -η² -alkyne and μ₃ -alkylidyne ligands on each side of the triruthenium plane into μ₃ -alkylidyne and μ₃ -η² -alkyne, respectively. Such transformation implies the migration of three H atoms within the hydrocarbyl ligands, and in this case, it is aided by the cluster's ability to behave as a H reservoir. All in all, this study highlights the plasticity of these Ru₃ clusters, whereby Ru-Ru, Ru-C, Ru-H, C-C, and C-H bonds are formed and broken with surprising ease.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Synthesis and dynamic behaviour of zwitterionic [M(η(6)-C6H5-BPh3)(coe)2] (M = Rh, Ir) cyclooctene complexes

The synthesis and structural characterization of zwitterionic [(η(6)-C6H5-BPh3)M(coe)2] (M = Rh, Ir) cyclooctene complexes is described. Both complexes exhibit an unusual exo-endo conformation of both cyclooctene ligands in the solid state. However, an equilibrium between the endo-endo and exo-endo rotational isomers arising from the hindered rotation about the metal-cyclooctene bond is observed in solution. Rotational barriers of around 65 kJ mol(-1) (Rh) and 84 kJ mol(-1) (Ir) have been determined by 2D EXSY NMR spectroscopy. The rotation process has also been studied by DFT calculations that showed that the dynamic behaviour is a consequence of the oscillation of the cyclooctene ligands about the metal-olefin bond instead of completing a full rotation.