Influence of arene dissociation and phosphine coordination on the catalytic activity of [RuCl(κ2-triphos)(p-cymene)]PF6
Graphical abstract
Ligand coordination/dissociation dynamics in a ruthenium(II)-arene compound containing a tripodal phosphine ligand play an important role in its catalytic activity.
Highlights
► The isolation of rare examples of partially coordinated triphos complexes. ► The low activity of [RuCl(ê2-triphos)(p-cymene)]PF6 in hydrogenation reactions. ► The determination of arene binding strengths by thermolysis reactions. ► Insights into the mechanisms of arene displacement in DMSO. ► Rationalisation of the low catalytic activity of [RuCl(κ2-triphos)(p-cymene)]PF6.
Introduction
Tripodal triphosphine ligands continue to find widespread application in coordination chemistry and catalysis [1], [1](a), [1](b), [1](c), [2], [2](a), [2](b), [2](c). Among these ligands, the C3 symmetric phosphine 1,1,1-tris(diphenylphosphinomethyl)-ethane (triphos) and it’s derivatives are the most extensively investigated, forming a large variety of transition metal complexes, many of which have been evaluated as catalysts [1], [1](a), [1](b), [1](c), [3], [3](a), [3](b), [3](c), [3](d), [3](e), [3](f), [3](g), [3](h). While predominately forming complexes adopting a κ3-coordinaton mode [2], [2](a), [2](b), [2](c), triphos is also known to partially bind to metal centres in a κ2-manner [4], [4](a), [4](b), [4](c), [4](d), [4](e), [4](f), [4](g). The interconversion between these two coordination modes, by ‘arm-off, arm-on’ dissociation/association of one of the phosphine centres, has significant consequences for the reactivity of the complex [5], [5](a), [5](b), [5](c), [5](d), [5](e). Examples of dynamic coordination include reversible arm-off triphos dissociation and concomitant addition of CO to [Rh(CO)H(κ3-triphos)], under hydroformylation conditions [6], and equilibration between κ3 and μ:κ2,κ1- coordination modes in [Rh(COD)(κ3-triphos)]PF6 on addition of [RuCl2(p-cymene)]2 [7].
Ruthenium(II)-arene complexes have been used extensively as catalyst precursors in many different reactions. Complexes of this type bearing chiral amino-amido ligands are especially notable as catalysts for the asymmetric transfer hydrogenation of carbonyl compounds [8], [8](a), [8](b), [8](c), [8](d). Other transformations mediated by ruthenium(II)-arene pre-catalysts include alkyne oxidation [9], C–C bond formation [10], olefin metathesis [11](c), [11](d), [11](e), [11](f), [11], [11](a), [11](b), Diels–Alder reactions [12], and free-radical polymerization [13]. As part of our on-going investigations on the catalytic activity of ruthenium(II)-arene complexes [14](c), [14](d), [14](e), [14](f), [14], [14](a), [14](b), [15](b), [15], [15](a), we report here further on a complex containing a κ2-coordinated triphos ligand, [RuCl(κ2-triphos)(p-cymene)]PF6 1. Isolation and characterisation of two isomers of this complex together with a study of their catalytic activity is described. The role of arene and phosphine coordination on the catalytic activity has been probed by comparison to structural analogues and thermolysis reactions.
Section snippets
Results and discussion
The κ2-triphos complex [RuCl(κ2-triphos)(p-cymene)]PF6 1 is readily prepared by reaction of triphos with the activated ruthenium arene precursor, [Ru2(μ-Cl)3(p-cymene)2]PF6 and [NH4]PF6 in refluxing methanol [16]. Two isomers of 1, differing by the orientation of the pendant arm of the triphos ligand with respect to the metallocyclic ring, are formed in a 1:1 ratio. Separation was achieved by selective precipitation of the less soluble anti-1 from methanol and subsequent recrystallisation,
Experimental section
All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques. CH2Cl2, diethyl ether, benzene, toluene and pentane, were dried under nitrogen using a solvent purification system (Innovative Technology Inc). Octane and CH2ClCH2Cl were distilled from CaH2 and P2O5, respectively, and stored over molecular sieves under nitrogen. All other solvents were p.a. quality and saturated with nitrogen prior to use, with the exception of DMSO which was purchased extra dry
Acknowledgement
This work was supported by the EPFL and the Swiss National Science Foundation. We thank the New Zealand Foundation for Research, Science and Technology for a Top Achiever Doctoral Fellowship (A. B. C.) and Dr R. Scopelliti and Dr E. Solari for technical support.
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