Tethered Arene Complexes of Ruthenium

Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 lEW, UK 

had been a Research Assistant at the University of Cambridge, working on organoarsenic chemistry with Sir William Jackson Pope.  

Potentially chelating ligands containing both alkene and tertiary phosphine or arsine donor centres were first synthesised in Nyholm s group in 1961 with the 

The methyl o-toluate complex [RuCl2(/i -l,2-MeCgH4C02Me)]2 (1) employed in earlier studies on planar chiral areneruthenium complexes proved to be a suitably labile precursor to the tethered arene complexes. It reacts with the donors mentioned above in a 1 2 mol ratio in dichloromethane at room temperature to give quantitatively the corresponding P-bonded adducts [RuCl2( / - 

Smith and Wright have reported that complex 3 is formed in 50% yield by heating the p-cymene complex [RuCl2( / -l,4-MeC6H4CHMe2)- 

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Tethered arene complexes of ruthenium(II) are also formed readily when the conformational freedom of the tether is restricted, for which purpose the biphenyl unit has proved ideal. Thus, (2-dicyclohexylphosphino)biphenyl and its derivatives react with [RuCl2(r 6-C6H6)]2 in DMF at 100 °C to give directly the tethered complexes 67-69 in yields of 31-96% without detection or isolation of the presumed P-coordinated intermediates.65... 

An example of a tethered arene complex of ruthenium(II) in which the auxiliary ligand is a a-bonded carbon atom is complex 80, which is formed by the action of AgBF4 and P(OMe)3 on the r 4-tetraphenylcyclobutadiene r 3-allyl complex 79 [Eq. (13)]. In the proposed mechanism, the allyl group migrates to the four-membered ring, which opens to generate an intermediate cation 81, the pendant arene of which coordinates to the metal.74... 

It is clear from the preceding section that the field of tethered arene-metal complexes is dominated by ruthenium and by arene-phosphines as ligands. In part, this situation has arisen because of the current surge of interest in the catalytic properties of ruthenium complexes in organic synthesis.85,86 Moreover, the tethered arene complexes are usually air-stable, crystalline solids with a well-defined, half-sandwich molecular geometry that, in principle, can lock the configuration at the metal centre. These compounds should, therefore, be ideal both for the study of the stereospecificity of reactions at the metal centre and for stereospecific catalysis. 

The complexed arene rings in tethered complexes of ruthenium(II) are close to planar, though the ipso-carbon atom is often pulled slightly towards the metal atom. In the phosphine complexes, the Ru-C(arene) distances trans to the P-donor (2.22-2.29 A) are significantly greater than those trans to the Ru-Cl bonds (2.15-2.25 A). This feature is also evident in non-tethered complexes of the type [RuCl2(r 6-arene)(PR3)] and can be attributed to the higher trans-influence of PR3 relative to that of Cl-.88... 

Redox Potentials (vs FcH/FcH+) of Selected Tethered Arene-Phosphine Complexes of Ruthenium (II)... 

Nazarov, A. A. et al. Anthracene-Tethered Ruthenium(II) Arene Complexes as Tools To Visualize the Cellular Localization of Putative Organometallic Anticancer Compounds. Inorg. Chem. 51, 3633-3639 (2012). 

Fig. 12. (a) General structure of the half-sandwich, piano-stool ruthenium—arene complexes (b) X and Y are commonly occupied by a bidentate ligand L giving a monofunctional complex (c) tethering of a monodentate ligand to the arene results in a bifunctional complex. 

The bifunctional amine-tethered ruthenium(II) arene complexes [Ru(r6 ti1-C6H5CH2(CH2)i1NH2)C12] (n = 1,2) (13a,b) show two consecutive hydrolysis steps to yield the mono- and bis-aqua complexes (64). At extracellular chloride concentrations, the majority of the complexes could be expected to be present as the mono-aqua adduct. Equilibrium constants were determined for both steps (for 13b, Ki = 145 mM K2 = 5.4 mM) and found to be considerably higher than those of cisplatin, which also has two reactive sites available. 

As expected, the presence of the tether does hinder the release of the arene from the coordination sphere at the ruthenium(ii) level. Thus, whereas benzene is displaced completely from the unstrapped complex Ph2P(CH2)3Ph ] in refluxing acetonitrile over 48 hours, displacement of the tethered arene in complex 3 does not proceed to completion, even after 11 days in both cases, the product is a mixture of cis- and trans-isomers of [RuCl -Ph2P(CH2)3Ph (NCMe)JCl. 

In a reaction that similarly relies on the acidity of benzylic protons in arene-ruthenium(II) complexes, the r 6-mesitylene complex 92 containing bidentate (C6F5)2PCH2CH2P(C6F5)2 loses two molecules of HF on treatment with proton sponge to give the di-strapped salt 93 in which each tether contains three carbon atoms [Eq. (18)]).83... 

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