Reactivity of the Arene Ligand

Tj -Cyclohexadienyl ruthenium complexes have been obtained either by addition of nucleophiles to the arene ring of arene ruthenium(II) complexes or by protonation of ruthenium(O) complexes. The first complex prepared, the benzene cyclohexadienyl ruthenium cation 236a, has been obtained together with the zero-valent arene cyclohexadiene ruthenium(O) complex 196a, by reaction of 235a with lithium aluminum hydride (118) [Eq. (27)]. 

Mesitylene and hexamethylbenzene cyclohexadienyl ruthenium complexes 236 have also been prepared in good yield by reduction of their corresponding bisarene ruthenium dications 235 with sodium borohydride in water (142,143). Cyclohexadienyl derivative 236c can be easily formed by treatment of bisarene ruthenium(O) complex 197 (arene = hexamethylbenzene) with a solution of hydrochloric acid in acetone. The structure of 236c and the presence of the endo C—H bond have been clearly resolved by infrared and 1H-NMR spectra (144,145) [Eq. (28)]. 

Other nucleophiles such as phosphines and trialkylphosphites undergo nucleophilic addition to bisbenzene ruthenium and osmium dications 235a and 142 to yield the cyclohexadienyl phosphonium adducts 237 and 238. 

Kinetic studies have shown that electrophilicity in the iron triad is strongly metal dependent with Fe Ru, Os, and the nucleophilic reactivity order is PPh3 P(0-tBu)3. Adducts 237 (PR3 = phosphites) react with water to give the cyclohexadienyl phosphonate complexes 239. Complex 235 is a effective catalyst for the conversion of phosphites to HP(0)(0R)2 (99,146,147) [Eq. (29)]. In a similar fashion, benzene ruthenium dications 

240 react with nucleophiles such as hydride, hydroxide, and cyanide to give the corresponding stable cyclohexadienyl ruthenium complexes 241. No reaction with phosphines is observed (21,148) [Eq. (30)]. 

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The complexation of an arene to the tricarbonylchromium unit promotes the addition of nucleophiles to the arene ring due to the strong electron-withdrawing ability of the Cr(CO)3 group. Other effects of the coordination of the metal on the reactivity of the arene ligand have been well-documented in the literature [1] and concern (Scheme 1) (i) the stabilization... 

Such a charge transfer from the ligated arene can lead to (a) nucleophilic addition or substitution, (b) electron transfer, and (c) proton elimination/transfer, thus revealing the dose relationship between all of these processes. The reactivity of the arene ligands towards nudeophiles in (arene)ML complexes depends on the electrophilidty of the metal fragments [MLn], this increasing in the order [Cr(CO)3] < [Mo(CO)3] [FeCp]+ < [Mn(CO)3]+ [2]. For example, in (arene)FeCp+, which is widely used for synthetic purposes, a chloro or nitro substituent on the arene is readily substituted by such nudeophiles as amides, eno-lates, thiolates, alkoxides, and carbanions [45]. 

When a carbon monoxide ligand is replaced by Iriphenylphosphine (in LV), the electrophilic reactivity of the arene ligand is lower, but transient / -cyclohexadienyl ligands are again implicated in reactions with nucleophiles . ... 

Of the handful of transition metal systems that are known to form stable q complexes with aromatic molecules [8-14], only d octahedral metal complexes have been shown to enhance the reactivity of the aromatic ligand toward electrophiles to date [15]. For nearly a decade, despite our best efforts, this mode of arene activation was known only for the pentaammineosmium(II) system. However, in the past few years a new generation of dearomatization agents have been developed based on a careful matching of the d /d reduction potential of rhe-... 

Unactivated, unsaturated hydrocarbons are not routinely subject to attack by nucleophiles. Complexation of alkenes, alkynes, or arenes to a positively charged metal center may lead to activation of the hydrocarbyl group towards nucleophilic attack and, in particular, to addition of hydride ion. The effect of the [LnM] fragment on the reactivity of the unsaturated ligand is largely due to the transfer of electron density from the unsaturated hydrocarbyl to the metal center upon coordination, a process that is most effective when the metal center is positively charged. Figure 10.5 ... 

Loss of Coordinated Arene. We previously stated that the arene ligand in ruthenium(II)-arene complexes is relatively inert towards displacement under physiological conditions. While this is generally true, there are a few exceptions to this rule and this type of reactivity can be used to advantage. Weakly bound arenes, for instance, can be thermally displaced, a property convenient for the synthesis of ruthenium-arene complexes that are not readily available through more common synthetic routes. This way, the reaction of a precursor dimer, [RuCl2(etb)]2 (etb, ethylbenzoate) (68), with either 3-phenyl-1-propylamine or... 

The complexes are air stable indeed, a limitation is the need for powerful oxidizing agents, such as Jones reagent CrVI, to detach the arene ligand.38 They are highly reactive toward nucleophiles. This limits the number of compatible synthesis manipulations that can be carried on in the presence of the [(arene)Mn(CO)3] unit but broadens the scope of effective nucleophiles. 

Carbon nucleophiles of type (iii) add to the arene ligand and do not rearrange examples include the very reactive anions, such as 2-lithio-2-methyl-l,3-dithiane, and the less sterically encumbered anions, such as lithio acetonitrile and /-butyl lithioacetate. In these cases, the anion adds to an unsubstituted position (mainly ortho or meta to Cl, as in 22) and does not rearrange. Then iodine quenching, even after a long period at 25 °C, gives almost exclusively the products from formal substitution for hydrogen, as from (22) in Scheme 8. 

Consequently, organometallic ruthenium(II) and osmium(II) arene complexes have recently attracted interest as anticancer agents [51]. The presence of a 7i-bonded arene in Ru11 (and Os11) complexes can have a dramatic influence on their chemical reactivity. There is a delicate balance between electron donation from the arene into the empty Ru 4d orbitals and back-donation from the filled 4d6 orbitals into vacant arene orbitals. This is influenced by the donor-acceptor power of the arene (e.g. hexamethylbenzene as a strong donor, in contrast to biphenyl which may act as acceptor) and by the other ligands on Ru11 which can influence the... 

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