On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M₃S₄ Clusters with Alkynes: Insights from Experiment and Theory

Abstract

Whereas the cluster [Mo3 S4 (acac)3 (py)3 ](+) ([1](+) , acac=acetylacetonate, py=pyridine) reacts with a variety of alkynes, the cluster [W3 S4 (acac)3 (py)3 ](+) ([2](+) ) remains unaffected under the same conditions. The reactions of cluster [1](+) show polyphasic kinetics, and in all cases clusters bearing a bridging dithiolene moiety are formed in the first step through the concerted [3+2] cycloaddition between the C≡C atoms of the alkyne and a Mo(μ-S)2 moiety of the cluster. A computational study has been conducted to analyze the effect of the metal on these concerted [3+2] cycloaddition reactions. The calculations suggest that the reactions of cluster [2](+) with alkynes feature ΔG(≠) values only slightly larger than its molybdenum analogue, however, the differences in the reaction free energies between both metal clusters and the same alkyne reach up to approximately 10 kcal mol(-1) , therefore indicating that the differences in the reactivity are essentially thermodynamic. The activation strain model (ASM) has been used to get more insights into the critical effect of the metal center in these cycloadditions, and the results reveal that the change in reactivity is entirely explained on the basis of the differences in the interaction energies Eint between the cluster and the alkyne. Further decomposition of the Eint values through the localized molecular orbital-energy decomposition analysis (LMO-EDA) indicates that substitution of the Mo atoms in cluster [1](+) by W induces changes in the electronic structure of the cluster that result in weaker intra- and inter-fragment orbital interactions.

Cycloaddition of alkynes to diimino Mo3S4 cubane-type clusters: a combined experimental and theoretical approach

“…9 Given that [1] + and [Mo 3 S 4 (acac) 3 (py) 3 ] + only differ in the ligands bound to the molybdenum centres, we believe that the results shown herein confirm our previously postulated hypothesis that the polyphasic kinetics of the reactions between [Mo 3 S 4 (acac) 3 (py) 3 ] + and alkynes involve participation of the pyridine ligands. 9,10…”

“…They appear at 135.1 and 134.0 ppm, values that compare well with those reported previously (133.1 and 136.7 ppm). 9,10 The remaining quaternary carbon atoms were identified by their HMBC long C-H correlation with neighboring protons. The carboxylate methyl groups in dmad adduct show a three-bond correlation with the carboxylic carbon atom, which appear at 164.1 and 164.9 ppm.…”

“…The mechanism of formation of type I products between the clusters [1] + and [2] + and alkynes has been computationally analysed (see Computational details). The objective of the study is threefold: (a) to understand why [2] + does not react with alkynes whereas [1] + does; (b) to confirm that those reactions lead to type I products via concerted [3+2] cycloaddition processes, as has been described for other cuboidal clusters, [8][9][10] and (c), to analyse the effect of the substituents at the alkyne on the kinetics and thermodynamics of these cycloadditions. To do so, the reactions of the clusters [1] + and [2] + with all the alkynes employed in the experimental section were studied.…”

“…For instance, while the Mo 3 S 4 4+ aqua ion reacts with acetylene and other alkynes in a single kinetic step to produce the [Mo 3 (m 3 -S)(m 2 -S)(m 3 -SCHQCHS)(H 2 O) 9 ] 4+ adduct or its analogues, 6,8 the [Mo 3 S 4 (acac) 3 (py) 3 ] + (acac = acetylacetonate, py = pyridine) cluster shows a polyphasic kinetics which involves substitution of the pyridine ligands and further ligand reorganization. [9][10] Interestingly, no reaction is observed upon replacement of molybdenum by tungsten in the case of the acac-py cluster. 10 In contrast, the isothiocyanate tungsten complex [W 3 S 4 (SCN) 9 ] 5À reacts with acetylene to afford a final product containing two acetylene groups with different coordination modes [W 3 (m 3 -S)(m 3 -SCHQCHS)(m-SCHQCH 2 )(NCS) 9 ] 5À .…”

“…[9][10] Interestingly, no reaction is observed upon replacement of molybdenum by tungsten in the case of the acac-py cluster. 10 In contrast, the isothiocyanate tungsten complex [W 3 S 4 (SCN) 9 ] 5À reacts with acetylene to afford a final product containing two acetylene groups with different coordination modes [W 3 (m 3 -S)(m 3 -SCHQCHS)(m-SCHQCH 2 )(NCS) 9 ] 5À . 5 In order to shed more light on the mechanism of these processes, we decided to investigate the reaction of the trinuclear [M 3 S 4 Cl 3 (dbbpy) 3 ] + complexes (M = Mo ([1] + ) and W ([2] + )) with various alkynes.…”