Arene -Complexes

Fairly little is known about self-assembly of transition metal complexes via Tr-arene interactions. A prominent example, [AgC104(C6H6)] , 94, was structurally characterized at a time when X-ray crystallography was still far from being a routine method [87]. Crystals of 94 can be obtained by recrystallizing silver(I) perchlorate 

Apart from the many derivatives of [Cr(CO)3( j-C6H6)] (see below) and the sandwich complexes described in Section X, very few arene compounds have been studied electrochemically. The synthesis of [V(CO)3(f/6-C6Ph6)] (386) is somewhat surprising in view of the two-electron oxidation of [V(CO)3-(f/6-mesitylene)] (E1/2 = -0.92 V in MeCN) (387). 

A mechanism, recently modified and discussed further in Section II,1C,6, has been proposed to account for the observation of both CO substitution and one-electron oxidation in the reactions between [Cr(CO)2L(f/-C6Me6)] and [NO]+. Rather than depending simply on the relative ° values, the products arise by the decomposition of the intermediate [Cr(NO)(CO)2-L(f/-C6Me6)]+, in which the NO group functions as a one-electron donor carbonyl displacement gives the cationic nitrosyl but loss of NO yields the radical cation (95). 

The two-electron reduction of [Cr(CO)3(r/-arene)j occurs at very negative potentials (e.g., arene = C6H5Me, 1/2 = -2.25 V in MeCN) (84,394,395). The process is irreversible for substituted benzene derivatives but stable dianions, in which the arene is bonded as an r/4-diene, can be generated by electrolytic reduction of the naphthalene analogs (395). 

The most familiar 6-electron ligands are benzene and substituted benzenes. Ligands of this type are called arene . The general methods of preparation of their complexes have already been described (p 169). Quite a wide range 

The neutral bis-w-arene complexes form well defined crystals, moderately soluble in common organic solvents. They may be sublimed in vacuum at about 100°C. Thermally they are reasonably stable, frequently up to 300°C. Some mean bond dissociation energies of the metal-ring bonds are listed in Table XVI together with some equivalent data for bis-ir-cyclopentadienyl complexes. 

These data suggest that the metal-ring bond in ferrocene is almost twice as strong as that in bis-w-benzene chromium, the corresponding 18-electron complex of the bis-ir-arene series. 

Most of the neutral bis-ir-arene metal complexes are readily oxidized to bis-arene cations. Indeed, bis-ir-benzene chromium acts as an electron donor, for example it forms 1 1 complexes with acceptor molecules such as tetracyanoethylene, which are probably best formulated as salts 

Electron-difiraction studies on bis-ir-benzene chromiiun in the vapour state show it to have the sandwich structure (D i, symmetry) in which all the C—C bond distances are equal (1423 0-002 A), and the arene rings are planar and parallel to each other. 

More sophisticated MO calculations, especially including relativistic corrections, suggest that whilst uranium 6d orbitals interact with the ligand orbitals, the 5f orbitals are relatively unperturbed. Photoelectron spectra do suggest that 5f interaction increases as more alkyl groups are introduced into the cyclooctatetraene rings. 

Half-sandwich compounds have been made, in the case of thorium by redistribution  

Uranium analogues can be made, not always by identical routes  

Certain other compounds of this type are known in the (-b3) state, such as [(j -C6H3Me3)U(BH4)3] also dimeric [(C6Me6)2U2Cl7]+ (AlCU), which is [(jj -CeMee) Cl2U(/x-Cl)3UCl2(j -C6Me6)]+ (AlCU) . These are considerably more reactive than uranocene. 

However, in contrast to the lanthanides, no arene complexes in a formal zero oxidation state have been isolated. 

The bis(benzenc)technetium(l) cation [Tc(C6H6)2] was synthesized by heating TCCI4, aluminum powder, AICI3, and benzene in a scaled tube at 135 °C for two days  

After hydrolysis of the anion of Tc(C6Hf,)2j[AlCl4], the cation was precipitated as hexafluorophosphate. The yellow-green, diamagnetic complex salt is stable in air, acids and bases [617]. Prior to the preparation of ponderable amounts, the cation was produced by irradiating h/A(benzene)molybdenum with thermal neutrons  

Taking into account the benzene n electrons, [Tc(C6H(,)2] attains the electron configuration of xenon. ITic cation is decomposed in melting Na202 [618, 619]. 

1 Titanium, Zirconium, Hafnium and Scandium Solutions of the complex 

Ferrocene, ( 7 -cp2)Fe is an efficient quencher of triplet excited states. Quenching occurs by energy transfer with the lowest triplet level of ferrocene lying around 179 kJ mol The rate constants for energy transfer in benzene solution at 22 °C for a series of organic triplets are shown in Table 7.1. Donors that have triplet energies significantly above Aat of ferrocene are quenched by it at a diffusion 

The photochemistry of ruthenocene parallels that of ferrocene. The triplet energy of the excited state is approximately 237 kJ mol In cyclohexane solution, (rj -cp)2Ru is photochemically inert at 254 nm, but photolysis in the CTTS band in chlorocarbon solvents leads to oxidation to 

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Ti-arene complexes Complexes in which an aromatic system is bonded to a metal through its r-electrons. Generally only applied to complexes of uncharged aromatic systems, e.g. [(CeHe)2Cr] but formally applied to any complex of an aromatic system, e.g. [(CjH5)2Fe] as a complex of (C5H3)". 

The I9e electron-reservoir complexes Fe Cp(arene) can give an electron to a large number of substrates and several such cases have been used for activation. After ET, the [FenCp(arene)]+ cation left has 18 valence electrons and thus cannot react in a radical-type way in the cage as was the case for 20e Fe°(arene)2 species. Thus the 19e Fe Cp(arene) complexes react with the organic halide RX to give the coupled product and the [FeCp(arene)]+ cation. Only half of the starting complex is used e.g., the theoretical yield is limited to 50% [48] (Scheme VI) contrary to the reaction with Fe°(arene)2 above. 

The nucleophilic substitution of the nitro group in nitro-arene complexes works almost as well as that of Cl" and such substitutions were achieved by Chowdhurry et al. with O, S, and N nucleophiles and with stabilized carbanions [97,98] Eq. (28) and Table 8. 

Cl substituent. Upon hydrolysis, the monosubstituted iron-arene complex is thus isolated. 

Recently, it was shown that the attack of CN on [FeCp(C6H5Cl)]+ PFortho-position. In the intermediate cyclohexadienyl complex, the CN group migrates to the ipso-carbon, whereas Cl is displaced. The monosubstituted benzonitrile complex is subjected to a second ortho-CN- attack but hydride is not removed spontaneously to give back an arene complex (Scheme XIX). Removal of the hydride is achieved by oxidation using DDQ (2,3-dichloro-... 

As was suggested in the preceding discussion, most of the arene complexes isolated by metal-atom techniques are benzene derivatives. However, heterocyclic ligands are also known to act as 5- or 6-electron donors in transition-metal 7r-complexes (79), and it has proved possible to isolate heterocyclic complexes via the metal-atom route. Bis(2,6-di-methylpyridine)Cr(O) was prepared by cocondensation of Cr atoms with the ligand at 77 K (79). The red-brown product was isolated in only 2% yield the stoichiometry was confirmed by mass spectrometry, and the structure determined by X-ray crystal-structure analysis, which supported a sandwich formulation. 

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

Kiindig EP (2004) Synthesis of Transition Metal r/ -Arene Complexes. 7 3-20 Kiindig EP, Pape A (2004) Dearomatization via Complexes. 7 71-94... 

Further investigations revealed that this hydrogenation is accelerated in pentane solution. These results are shown in brackets in Table 3 [31]. Under optimized reaction conditions high catalyst TOF up to 5,300 were achieved when 10 was used. In the absence of both hydrogen and nitrogen, 10 was converted into the q -arene complexes such as the bis(imino)pyridine iron q -phenyl complex, 10-Phenyl, and the corresponding q -2,6-diisopropylphenyl complex, 10-Aryl, in the 85 15 ratio in... 

CeDs solution (Scheme 7). Both t -arene complexes were also determined by the X-ray diffraction and showed no reaction to hydrogen and olefins. Therefore, it was considered that the formation of the t -arene complexes was a deactivation pathway in the catalytic hydrogenation. 

Rigby J, Kondratenkov M (2004) Arene Complexes as Catalysts. 7 181-204 Risse T, Freund H-J (2005) Spectroscopic Characterization of Organometallic Centers on Insulator Single Crystal Surfaces From Metal Carbonyls to Ziegler-Natta Catalysts. 16 117-149... 

Several other versions of these catalysts have been developed. Arene complexes of monotosyl-l,2-diphenylethylenediamine ruthenium chloride give good results with a,(3-ynones.55 The active catalysts are generated by KOH. These catalysts also function by hydrogen transfer, with isopropanol serving as the hydrogen source. Entries 6 to 8 in Scheme 5.3 are examples. 

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

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). 

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