Metathesis Reaction of Hydrocarbyl Ligands across the Triruthenium Plane

Abstract

Carbon-chain recombination, similar to that observed during the cracking of alkanes and in the Shell higher olefin process, is one of the most important reactions in the chemical industry. There has been a continuous demand for a process that can yield hydrocarbons of desired chain length. Recently, a method for converting lower alkanes into higher alkanes, which is referred to as alkane metathesis, has been reported. [1] Alkane metathesis has considerable scientific and industrial importance because it can convert gaseous hydrocarbons into liquid fuels. Such interconversion involves three catalytic reactions: activation of the alkane CÀH bond, olefin metathesis, and hydrogenation of the formed olefins. However, because of the cross-metathesis of the alkenes during the recombination process, the product will have a molecular weight distribution to some extent.Coupling reactions between two hydrocarbyl ligands placed on both faces of a trimetallic plane have often been seen in organometallic chemistry, [2] as has CÀC bond scission on a cluster.[3] These reactions have attracted considerable attention and are considered to be characteristic of cluster compounds. We have studied C À C bond formation on a triruthenium cluster by directing our attention to the roles of an MÀM bond.[4] Recently, we reported the synthesis of 2 using the coupling reaction of m 3 -benzyne and m 3 -vinylidene ligands separated by the Ru 3 plane of 1 (Scheme 1).[4c] In this reaction, the formation of the ruthenacyclopentadiene skeleton was accompanied by the rupture of a Ru À Ru bond. We also reported the scission of a CÀC bond in a ruthenacyclopentadiene skeleton in which a Ru 3 triangle was reconstructed.[4a] Such flexibility in a cluster skeleton would enable a novel recombination of hydrocarbons. Herein, we report an unprecedented selective interconversion of two C 5 fragments into C 8 and C 2 fragments on a triruthenium cluster.The thermolysis of 4, which was obtained by the sequential treatment of 3 with 1-pentene and 1-pentyne, [5] resulted in the quantitative formation of an equilibrated mixture of 5 a and 5 b (Scheme 2). At ambient temperature, the motion of the hydrido ligand was so rapid that the signals of 5 a and 5 b coalesced. For simplification, we denote this equilibrated mixture as 5.[6] The structures of 4 and 5 were determined by X-ray diffraction studies (Figure 1 and Figure 2, respectively).[7] These results clearly indicate that the m 3 -pentylidyne and m 3 -pentyne ligands in 4 were respectively transformed into the m 3 -ethylidyne and m 3 -octyne ligands in 5.This rearrangement required the migration of the C 3 fragment across the Ru 3 plane. In order to elucidate the mechanism, we monitored the reaction by means of NMR spectroscopy. Upon heating at 80 8C, formation of the m 3 -butylidyne-m 3 -hexyne complex 6 was observed (Path a in Scheme 3). For the formation of 6, a carbyne group must Scheme 1. CÀC bond formation in a trimetallic cluster accompanied by MÀM bond breaking. Cp* = pentamethylcyclopentadienyl. Sche...

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

“…Based on the behaviour of analogous Ru 3 hydride clusters it has been proposed that this transformation may involve intermediates featuring open-cluster structures in which aR u ÀRu bond is broken and aC ÀCb ond formed between the CH moiety of the terminal alkyne ligand and the m 3 -C centre of the alkylidyne. [9] Indeed, graduals hortening of the C5ÀC6 distance in 2 resulted in the formation of intermediate I1 (see Fig-ure 2a), in which the Ru2a nd Ru3 centresa re located 3.80 apart (cf. 2.76 in 2)w hile ab ond between C5 and C6 is now evident( d(C5ÀC6) = 1.49 ,c f. 2.90 in 2).…”

“…Notably,i nt he case of 2,l ow-temperature NMR experiments have shown that it exists as am ixture of two isomers in a 90:10 ratio that differ in the positiono ft he m-H ligands. [9] Note that there are only two m-H ligands but three possible bridging sites betweent he metal centres.I na greement with this data, a 70:30 ratio (DG = 0.45 kcal mol À1 )i sc omputed in favour of the major isomer,w hose structure is included in Figure 1( see Figure S1 in SupportingI nformation fort he minor isomer). Interestingly, at the selected computational level A is found to be only 0.1kcal mol À1 less stable than 2,a nd thus the process can be practically considered thermoneutral (see Scheme 3).…”

“…Surprisingly,t oo ur knowledge computational methods have so far never being appliedt ot his peculiar process and the only availablem echanistic information is based on NMRa nd X-ray data. [9] Such experimentali nformation is nonetheless quite informative, as by carefullyp laying with the reactiont emperature, the researchers were able to observe an umber of reactioni ntermediates, ultimately leading them to proposet he multistep mechanism shown in Scheme2.T hus, the overall reactioni sb elieved to involvet wo different types of rearrangement, labelledh ere as A and B, which take place in an alternative manner.T he so-called rearrangement A consists of the migration of aC Hu nit from a m 3alkylidyne into a m 3 -h 2 -alkyne ligand,a nd it has been proposed to involveo pen-core intermediates. [9] Once ar earrangement A has taken place, before the next similarr earrangement can proceedt he hydrocarbyl ligands within the resulting species have to undergo an additional isomerization for which no mechanistic proposal has even been formulated.…”

“…[9] Such experimentali nformation is nonetheless quite informative, as by carefullyp laying with the reactiont emperature, the researchers were able to observe an umber of reactioni ntermediates, ultimately leading them to proposet he multistep mechanism shown in Scheme2.T hus, the overall reactioni sb elieved to involvet wo different types of rearrangement, labelledh ere as A and B, which take place in an alternative manner.T he so-called rearrangement A consists of the migration of aC Hu nit from a m 3alkylidyne into a m 3 -h 2 -alkyne ligand,a nd it has been proposed to involveo pen-core intermediates. [9] Once ar earrangement A has taken place, before the next similarr earrangement can proceedt he hydrocarbyl ligands within the resulting species have to undergo an additional isomerization for which no mechanistic proposal has even been formulated. In other words, species A in Scheme2,f eaturing m 3 -butylidyne and m 3h 2 -hexyne ligands, must first be converted into m 3 -h 2 -butyne and m 3 -hexylidyne ligands, respectively.T his rearrangement, denoted here as B,d oes not modify the number of Ca toms within these hydrocarbyl ligands but only involves Hm igrations.…”

“…This is the case of the trinucleart itanium heptahydride complex [(C 5 Me 4 SiMe 3 )Ti] 3 (m 3 -H)(m-H) 6 ,w hich was shown recently to cleave the inert CÀCb ond of benzene, [8] the process being coupled to the cleavage of aT i ÀTi bond and the formation of new metal-ligandi nteractions.A ne ven more peculiar behaviour has been observed by Suzuki et al while studying the reaction of the Ru 3 polyhydrido cluster (1)w ith 1-pentene and 1pentyne (Scheme 1). [9] The resulting (m 3 -pentylidyne)(m 3 -h 2 -pentyne) species 2 is stable at room temperature, however,i ts thermolysis generatest he (m 3 -h 2 -octyne)(m 3 -ethylidyne) cluster 3 via the formal migration of aC 3 fragment across the triruthenium plane.…”