Organometallic Catalysis in Aqueous Solution. The Hydrolytic Activity of a Water-Soluble ansa-Molybdocene Catalyst

A bioinspired bifunctional catalyst: an amphiphilic organometallic catalyst for ring-closing metathesis forming liquid droplets in aqueous media

Inspired by phase-separated biopolymers with enzymatic activity, we developed an amphiphilic catalyst consisting of alternating hydrophilic oligo(ethylene glycol) and hydrophobic aromatic units bearing a Hoveyda-Grubbs catalyst center (MAHGII). MAHGII served as both a droplet-forming scaffold and a catalyst for ring-closing metathesis reactions, providing a new biomimetic system that promotes organic reactions in an aqueous environment.

Mechanistic Insights into Promoted Hydrolysis of Phosphoester Bonds by MoO2Cl2(DMF)2

A phosphoester bond is a crucial structural block in biological systems, whose occurrence is regulated by phosphatases. Molybdenum compounds have been reported to be active in phosphate ester hydrolysis of model phosphates. Specifically, MoO₂Cl₂(DMF)₂ is active in the hydrolysis of para-nitrophenyl phosphate (pNPP), leading to heteropolyoxometalate structures. We use density functional theory (DFT) to clarify the mechanism by which these species promote the hydrolysis of the phosphoester bond. The present calculations give insight into several key aspects of this reaction: (i) the speciation of this complex prior to interaction with the phosphate (DMF release, Mo-Cl hydrolysis, and pH influence on the speciation), (ii) the competition between phosphate addition and the molybdate nucleation process, (iii) and the mechanisms by which some plausible active species promote this hydrolysis in different conditions. We described thoroughly two different pathways depending on the nucleation possibilities of the molybdenum complex: one mononuclear mechanism, which is preferred in conditions in which very low complex concentrations are used, and another dinuclear mechanism, which is preferred at higher concentrations.

Reactions in Water - A Greener Approach Using Ruthenium Catalysts

Reactions using transition metals as catalysts have emerged as an efficient method in the recent times. However, the selection of solvent plays a crucial role in this regard. Several solvents used traditionally suffer majorly with problems of toxicity; high boiling point etc. leading to drastic reaction conditions. Water being a non-toxic, non-inflammable and environmentally benign can replace the hazardous organic solvents in laboratory as well as industry. Maintaining a minimum catalyst loading percentage we can advantageously avail high levels of selectivity. Water was found to be a good solvent medium for several metal catalysed reactions. An intramolecular deprotonation mechanism is followed by the ruthenium (II) catalysts in water; thereby, facilitating the catalytic action of the metal. These studies can help the industrial chemists to utilize water as a solvent for their reactions towards improvement of their waste management procedure. This review mainly focuses on the several recent developments in the above direction.

Understanding the hydrolysis mechanism of ethyl acetate catalyzed by an aqueous molybdocene: a computational chemistry investigation

A thoroughly mechanistic investigation on the [Cp2Mo(OH)(OH2)](+)-catalyzed hydrolysis of ethyl acetate has been performed using density functional theory methodology together with continuum and discrete-continuum solvation models. The use of explicit water molecules in the PCM-B3LYP/aug-cc-pVTZ (aug-cc-pVTZ-PP for Mo)//PCM-B3LYP/aug-cc-pVDZ (aug-cc-pVDZ-PP for Mo) computations is crucial to show that the intramolecular hydroxo ligand attack is the preferred mechanism in agreement with experimental suggestions. Besides, the most stable intermediate located along this mechanism is analogous to that experimentally reported for the norbornenyl acetate hydrolysis catalyzed by molybdocenes. The three most relevant steps are the formation and cleavage of the tetrahedral intermediate immediately formed after the hydroxo ligand attack and the acetic acid formation, with the second one being the rate-determining step with a Gibbs energy barrier of 36.7 kcal/mol. Among several functionals checked, B3LYP-D3 and M06 give the best agreement with experiment as the rate-determining Gibbs energy barrier obtained only differs 0.2 and 0.7 kcal/mol, respectively, from that derived from the experimental kinetic constant measured at 296.15 K. In both cases, the acetic acid elimination becomes now the rate-determining step of the overall process as it is 0.4 kcal/mol less stable than the tetrahedral intermediate cleavage. Apart from clarifying the identity of the cyclic intermediate and discarding the tetrahedral intermediate formation as the rate-determining step for the mechanism of the acetyl acetate hydrolysis catalyzed by molybdocenes, the small difference in the Gibbs energy barrier found between the acetic acid formation and the tetrahedral intermediate cleavage also uncovers that the rate-determining step could change when studying the reactivity of carboxylic esters other than ethyl acetate substrate specific toward molybdocenes or other transition metal complexes. Therefore, in general, the information reported here could be of interest in designing new catalysts and understanding the reaction mechanism of these and other metal-catalyzed hydrolysis reactions.

Direct coupling of nitriles and aniline to form the triazapentadiene species [upper bond 1 start]Rh(III){NH=C(R)N(Ph)C(R=N[upper bond 1 end]H}

The facile and high-yield reaction between aniline and cis-positioned nitriles of [((t)bpy)(2)Rh(NCR)(2)][OTf](3) [R = Me (2), Ph (3); (t)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine; OTf = CF(3)SO(3)(-)] produces the Rh(III)-triazapentadiene species [((t)bpy)(2)[upper bond 1 start]Rh{NH=C(R)N(Ph)C(R=N[upper bond 1 end]H}][OTf](3) [R = Me (4), Ph (5)]. It represents the first example of direct coupling between two RCN ligands and a primary amine to form this type of metallacycle. X-ray structures of 2 and 4 are reported.

Di-μ(3)-oxido-di-μ(2)-oxido-tetra-oxido-bis-(1,1,2,2-tetra-methyl-ethylenedicyclo-penta-dien-yl)-dimolyb-denum(IV)-dimolybdenum(VI) hexa-hydrate

The title compound, [Mo(4)(C(16)H(20))(2)O(8)]·6H(2)O, is a centrosymmetric ansa-molybdocene complex in which two dinuclear [C(2)Me(4)(η(5)-C(5)H(4))(2)]Mo(μ(2)-O)(2)MoO(2) units dimerize by forming two μ(3)-O bridges between three Mo atoms. The ansa-molybdocene [C(2)Me(4)(η(5)-C(5)H(4))(2)]Mo unit has a typical bent-sandwich metallocene structure with an inter-ring angle of 127.98 (8)°. The Mo atom in the bridging (μ(2)-O)(μ(3)-O)(2)MoO(2) group has a distorted trigonal-bipyramidal coordination. The Mo-(μ(3)-O) and Mo-(μ(2)-O) bond distances inside the units [2.0869 (14) and 2.1014 (15) Å, respectively] are slightly longer than the Mo(-x + 1, -y + 1, -z)-(μ(3)-O) bond distance between the units [1.9986 (14) Å]. The solvent water mol-ecules together with complex O atoms form a network of O-H⋯O hydrogen bonds.