A study, involving kinetic measurements on the stopped-flow and conventional UV/Vis timescales, ESI-MS, NMR spectroscopy and DFT calculations, has been carried out to understand the mechanism of the reaction of [Mo3 S4 (acac)3 (py)3 ][PF6 ] ([1]PF6 ; acac=acetylacetonate, py=pyridine) with two RCCR alkynes (R=CH2 OH (btd), COOH (adc)) in CH3 CN. Both reactions show polyphasic kinetics, but experimental and computational data indicate that alkyne activation occurs in a single kinetic step through a concerted mechanism similar to that of organic [3+2] cycloaddition reactions, in this case through the interaction with one Mo(μ-S)2 moiety of [1](+) . The rate of this step is three orders of magnitude faster for adc than that for btd, and the products initially formed evolve in subsequent steps into compounds that result from substitution of py ligands or from reorganization to give species with different structures. Activation strain analysis of the [3+2] cycloaddition step reveals that the deformation of the two reactants has a small contribution to the difference in the computed activation barriers, which is mainly associated with the change in the extent of their interaction at the transition-state structures. Subsequent frontier molecular orbital analysis shows that the carboxylic acid substituents on adc stabilize its HOMO and LUMO orbitals with respect to those on btd due to better electron-withdrawing properties. As a result, the frontier molecular orbitals of the cluster and alkyne become closer in energy; this allows a stronger interaction.

We report a trinuclear Mo 3 S 4 diamino cluster that promotes the semihydrogenation of alkynes. Based on experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the cluster act as the active site for this transformation. In the first step, two of these μ-S ligands react with the alkyne to form a dithiolene adduct; this process is formally analogous to the olefin adsorption on MoS 2 surfaces. Then, H 2 activation occurs in an unprecedented way that involves the third μ-S center, in cooperation with one of the dithiolene carbon atoms. Notably, this step does not imply any direct interaction between H 2 and the metal centers, and directly results in the formation of an intermediate featuring one (μ-S)−H and one C−H bond. Finally, such half-hydrogenated intermediate can either undergo a reductive elimination step that results in the Z-alkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates the E-alkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of Z-and E-stilbene. The results herein could have significant implications on the understanding of the catalytic properties of MoS 2 -based materials.

The [Mo 3 S 4 Cl 3 (dhprpe) 3 ] + (1 +) cluster cation has been prepared by reaction between Mo 3 S 4 Cl 4 (PPh 3) 3 (solvent) 2 and the watersoluble 1,2-bis(bis(hydroxypropyl)phosphino)ethane (dhprpe, L) ligand. The crystal structure of [1] 2 [Mo 6 Cl 14 ] has been determined by X-ray diffraction methods and shows the typical incomplete cuboidal structure with a capping and three bridging sulfides. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of the diphosphine ligand. Depending on the pH, the hydroxo group of the functionalized diphosphine can substitute the chloride ligands and coordinate to the cluster core to give new clusters with tridentate deprotonated dhprpe ligands of formula [Mo 3 S 4 (dhprpe-H) 3 ] + (2 +). A detailed study based on stopped-flow, 31 P{ 1 H} NMR, and electrospray ionization mass spectrometry techniques has been carried out to understand the behavior of acid−base equilibria and the kinetics of interconversion between the 1 + and the 2 + forms. Both conversion of 1 + to 2 + and its reverse process occur in a single kinetic step, so that reactions proceed at the three metal centers with statistically controlled kinetics. The values of the rate constants under different conditions are used to discuss on the mechanisms of opening and closing of the chelate rings with coordination or dissociation of chloride.

The aminophosphine ligand (2-aminoethyl)diphenylphosphine (edpp) has been coordinated to the W 3 (µ 3 -S)(µ-S) 3 the actual mechanism of substitutions in these clusters is strongly dependent on the nature of the leaving and entering anions. The interaction between an entering F -and the amino group coordinated to the adjacent metal have been also found to be especially relevant in the kinetics of these reactions.

An alternative reactivity mode of electron-rich allenylidene complexes via alkenylcarbyne species is reported for the first time in ruthenium complexes bearing monodentate phosphine ligands (triethylphosphine). Allenylidene complexes have been prepared and characterized from [Cp*RuCl(PEt 3 ) 2 ] and a series of propargyl alcohols bearing different substituents (HCtCCRR′(OH), where R, R′ ) Ph, Ph; R ) H, R′ ) p-MeO-C 6 H 4 , C 6 H 5 , p-F-C 6 H 4 ). Alkenylcarbyne complexes are prepared by protonation of the allenylidene precursors. The X-ray structures of one secondary allenylidene (R, R′ ) H, Ph) and one alkenylcarbyne complex (R, R′ ) Ph, Ph) constitute the first examples for ruthenium complexes bearing monodentate phosphine ligands. Neutral alkynyl, cationic γ-substituted vinylidene, and bicyclic carbene complexes have been isolated and characterized as a result of the allenylidene/alkenylcarbyne reactivity against anionic and neutral (protic and aprotic) nucleophiles. The cycloaddition products obtained by double addition of 1,3-cyclohexanedione and resorcinol (benzene-1,3-diol) to both allenylidene double bonds and the analysis of the effects of including electron-donor or -withdrawing groups in the allenylidene/ alkenylcarbyne reactivity are reported.