Mechanism of [3+2] Cycloaddition of Alkynes to the [Mo₃S₄(acac)₃(py)₃][PF₆] Cluster

Abstract

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.

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

“…Both from ak inetic and thermodynamic point of view the alkynes can be divided in two groups,a dc and its methylated derivative dmad, and the remaining alkynes. It is noteworthy that for the second group of alkynes the DG r values correspond to slightly endergonic processes, in spite of the factthat all of these do take place experimentally.T his discrepancy,a lready observed in previous reports, [6,7] does not necessarily disagree with the formationo f type Ic ycloadditionp roducts,a si tc ould be explained by taking into account that both the excess of alkyne used in the experiments and the existence of subsequent exergonic transformationsw ill drive the reaction towards product formation. The observation in the present work that the reactions with some of these alkynes actually occur under conditions of rever-sible equilibria adds further support to this conclusion.…”

“…The reaction of clusters [ [7] and we were interested in checking the reactivity of the analogous tungsten cluster [2]PF 6 .N otably, [ …”

“…The major difference with respectt ot he other data in Ta ble2 is the existence of an intermediate kinetic step signaled by as hift in the band from l = 845 to 875 nm and with az ero order with respect to the alkyne.T he existence of three steps was previously observed for the reactionw ith adc in acetonitrile, and it was interpreted in terms of formationo fad ifferent type Ia dduct, probably by substitution of pyridine by ac oordinated alkyne. [7] In any case, the reaction of cluster [1] + with alkynes always leads initially to compounds showing an intense band in the near-infrared region. Shibaharaa nd co-workers have reported that this band is typical of type Ia dducts, [5] and recent time-dependent (TD) DFT calculations indicated that it can be assigned to charge-transfer transitions from the alkyne to the cluster core.…”

“…[5] By meanso fc ombined experimental and computational studies, we have recently confirmed the proposal of Shibahara and concluded that the formation of type I products can be described as ac oncerted [3+ +2] cycloaddition reactionb etween aM ( m-S) 2 moiety of the cluster and the CC bond of the alkyne (Scheme1), that is, both CÀSb onds are formed in as ingle step and no intermediates with one CÀSbond can be characterized. [6,7] Knowledge of the kinetico-mechanistic properties of these reactions is not only interesting in relation to the alkyne activation and CÀSbond formation, but also because these reactions could be used as novel click reactions for the incorporation of cuboidal clusters into more complex structures, with new reactivities and catalytic properties. In fact, Shibaharae tal.…”

“…[10] The experiments in acetonitrile solutions carried out in previous work have already revealed the dramatic effect of the Rg roup of the alkyne [RÀCCÀR; R= CH 2 OH (btd);C OOH (adc)] on the kinetics of the reaction with [Mo 3 S 4 (acac) 3 (py) 3 ] + ([1] + )( acac = acetylacetonate, py = pyridine), whichshowedarate constantthree orders of magnitude faster with adc than with btd. [7] This was thoroughly analyzed by combining an activation strain model (ASM) [11] analysis with frontierm olecular orbital (FMO) calculations, and the resultsi ndicatedt hat the major differencei nt he activationb arrierf or the reaction with both alkynes originates from their different degree of interaction with the clusteratthe transition-state geometries. In the framework of the frontier molecular orbital theory the effect was traced to the differences in the frontier orbitals of the alkynes, which in the case of adc becomec loser to those of the cluster and allow for as tronger orbitali nteraction.…”