Arene ligand

Arenes such as benzene and toluene can act as 6tt-electron donors as illustrated in equations 24.116 and 24.122. A wide range of arene complexes exist, and sandwich complexes can be made by co-condensation of metal and arene vapours (equation 24.123) or by reaction 24.124. 

The reaction of Cr(CO)6 or Cr(CO)3(NCMe)3 with benzene gives the half-sandwich complex (r -C6H5)Cr(CO)3 (24.73), and related complexes can be made similarly. The 

Cr(CO)3 unit in (r -arene)Cr(CO)3 complexes withdraws electrons from the arene ligand making it less susceptible to electrophilic attack than the free arene, but more susceptible to attack by nucleophiles (reaction 24.125). 

As in (r -C5Hg)2Cr, the benzene ligand in (ri -C6H6)Cr(CO)3 can be lithiated (equation 24.126) and then derivatized (scheme 24.127). The reactivity of halfsandwich complexes is not confined to sites within the tt-bonded ligand equation 24.128 illustrates substitution of a CO ligand for PPh3. 

Cyclobutadiene complexes can also be formed by the cycloaddition of alkynes as in reaction 24.134. 

3 and Section 26.7) has, since the 1980s, included the use of zirconocene derivatives. In the presence of methylalu-minoxane [MeAl( j,-0)] as a co-catalyst, compounds A, B and C (shown below) are active catalysts for propene polymerization. Compounds A and B are chiral because of the relative orientations of the two halves of the organic ligand. A racemic mixture of A facilitates the formation of iso tactic polypropene, while use of catalyst C results in syndiotactic polypropene (see footnote in Section 26.7 for 

Zirconocene derivatives are used to catalyse a range of organic hydrogenation and C—C bond-forming reactions. In the presence of methylaluminoxane, chiral complex A catalyses asymmetric hydrogenations (see Section 26.4), with the active species being a cationic zirconium hydrido complex. 

Surprisingly, the brown Cr complex is easily oxidized by I2 to the 17-electron, air-stable yellow [(q -C6H5)2Cr]. The ease of oxidation precludes (q -CgH6)2Cr from undergoing electrophilic substitution reactions. Electrophiles oxidize (q -C6H6)2Cr to [(q -C6H6)2Cr] which does not react further. The lithiated derivative (q -C6H5Li)2Cr can be 

The reaction of Cr(CO)6 or Cr(CO)3(NCMe)3 with benzene gives the half-sandwich complex (r] -C6H6)Cr(CO)3 (24.74), and related complexes can be made similarly. The Cr(CO)3 unit in (ri -arene)Cr(CO)3 complexes withdraws electrons from the arene ligand making it less susceptible to electrophilic attack than the free arene, but more susceptible to attack by nucleophiles (reaction 24.125). 

Cycloheptatriene (24.75) can act as a 67r-electron donor, and in its reaction with Mo(CO)g, it forms (ri -C7Hg)Mo(CO)3. The solid state structure of this complex (Fig. 24.27a) confirms that the Ugand coordinates as a triene, the ring 

In the complex (ri -C7Hg)Fe(CO)3, cycloheptatriene acts as a diene, giving the Fe(0) centre its required 18 electrons. Equation 24.130 shows that deprotonation generates a coordinated [ 7117] ligand which bonds in an t] manner, allowing the metal to retain 18 electrons. At room temperature, the [ 7117] ligand is fluxional, and on the NMR time-scale, the Fe(CO)3 unit effectively visits every carbon atom. 

Soluble 7t omplexed aromatic polysulfides 4.38 have also been prepared with (j/ -C5Me5)Ru moieties (Eq. 4.13) [81]. A variety of polymers with Fe(i/ -C5H5) groups attached to arene groups in the polymer main chain have been reported. 

These indude polyethers, polythioethers, and polyamines [82-85]. In addition to linear materials, star-shaped polymers such as 4.39 have been reported [86]. These materials are redox-active as a result of the presence of the cationic ArFe(i/ -CsHs) moieties, which undergo reversible reduction at -40 °C. The materials are photo- 

The complexation of Fe(i/ -C5H5) groups to ca. 60% of the main-chain arene moieties in low molecular weight samples (Afn=1700) of poly(n-hexylphenylene) to yield 4.41 has also been reported [88]. Interchain cross-linking appears to be induced by electrochemical reduction, resulting in the formation of an insoluble polymer film on the electrode. Attachment of the FeCp group also leads to a modification of the photoluminescence intensity and emission wavelength. 

Similar polymers with around 20% of the main-chain arene groups complexed by Mo(CO)3 (4.42) have also been reported [89]. The quenching of photoluminescence detected for 4.41 and 4.42 results from the presence of the metal-based LUMO between the valence and conduction bands of the conjugated polymer chain. This leads to a pathway for non-radiative decay after photoexcitation that involves electron transfer to the 3d LUMO on the metal, which occurs prior to recombination with holes [88, 90]. 

An analogous polymer with diethynyldiphenylene spacers, 4.43, which possesses complexation of arene groups to Cr(CO)3 moieties, has been prepared [91]. Its solubility was low, even though molecular weights M ) of ca. 7800 were estimated. In this case, the synthesis involved a cross-coupling reaction of the p-di-chloroarene complex with the bis(trimethylstannyl)dialkyne [91]. Similar materials with diethynylthiophene spacers have also been described [92]. 

The w-indenyl ligand in the molecule [Mo(w-C9H,)(CO)aI] (32) is regarded as occupying three co-ordination sites. The co-ordination geometry 

The two independent molecules in the crystal structure of [MofCyHg) -(Me)(NO)] are approximately the mirror images of each other. Their molecular geometry (34) can be derived from that of [Mo(CsHs)j(NO)] by 

Photolysis of the benzene complex rj -C6H6Cr(CO)3 in a Nujol mull at 77 K shows that for this complex the loss of CO occurs as a reversible process. This result, along with similar observations made in methane matrices at 12 K, provides further good support that loss of CO is a primary photoprocess in these complexes.  

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Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Borazine is isoelectronic and isostructural with benzene and may act as a six-electron donor in complex chemistry. In contrast to arene ligands of arene-transi-tion-metal complexes, coordinated borazines lose their planarity and are slightly puckered . Nevertheless, the B atoms show interactions with metal atoms. 

Another accessible organomercuiyd) compound is that with Hg(I) complexed to ir-arene ligands. A melt of equimol Hg2Cl2 and AICI3 reacts with benzene at 60°C to... 

The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst environment. Instead, the Fe center can influence a catalytic asymmetric process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. This interaction is often comparable to the stabilization of a-ferrocenylcarbocations 3 (see Sect. 1) making use of the electron-donating character of the Cp2Fe moiety, but can also be reversed by the formation of feirocenium systems thereby increasing the acidity of a directly attached Lewis acid. Alternative applications in asymmetric catalysis, for which the interaction of the Fe center and the catalytic center is less distinct, have recently been summarized in excellent extensive reviews and are outside the scope of this chapter [48, 49], Moreover, related complexes in which one Cp ring has been replaced with an ri -arene ligand, and which have, for example, been utilized as catalysts for nitrate or nitrite reduction in water [50], are not covered in this chapter. 

In a closely related study, Marecek et al. [46] used the pendant drop video-image method to investigate the adsorption and surface reactions of calix[4]arene ligands at the ideally polarized water-1,2-dichloroethane interface. The difference between the surface tensions in acidic and alkaline media was ascribed to a difference in the charge on the... 

D. Arosio, M. Fontanella, L. Baldini, L. Mauri, A. Bemardi, A. Casnati, F. Sansone, and R. Ungaro, A synthetic divalent Cholera Toxin glycocalix[4] arene ligand having higher affinity than natural GM1 oligosaccharide, J. Am. Chem. Soc., 127 (2005) 3660-3661. 

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... 

Calixarenes, when in their cone-conformation (54), represent versatile host systems for metalated container molecules and many examples have been reported in the literature (55-61). Reinaud and coworkers have carried out extensive work concerned with calix[6]arenes that are functionalized at the small rim by nitrogen arms (62), aiming to reproduce the hydro-phobic binding site of mononuclear zinc and copper metalloen-zymes. A recent example is the calix[6]arene ligand L1 (Fig. 3), in which a tris(2-methylpyridyl)amine unit covalently caps the calixarene small rim (63). The ligand forms copper complexes of... 

The smaller p -tert-butyl-calix [4] arenes have a rich coordination chemistry as well (65). Of these, however, only the upper-rim modified calixarenes seem to support metal complexes with confined binding sites (66), except in those cases where the lower-rim substituents form an appended cavity. Thus, Matt and coworkers have reported (67) a pocket-shaped calix[4]arene ligand L2 bearing two lower-rim [([Pg.410]

A new imidazole-functionalized calix[4]arene ligand, able to form a dinuclear Cu2+ complex, has been reported to hydrolyze HPNP and ethyl p-nitropheny lphosphate [70]. The dinuclear complex was found to be 22-and 330-fold more reactive than the corresponding monomer towards the above substrates, respectively. Dinuclear Cu2+ complexes of linked triazacyclononane ligands are reported to promote the hydrolysis of the monoribonucleotide GpppG, a model for the 5 -cap structure of mRNA [71]. The dinuclear complexes offer some 100-fold higher reactivity compared to the mononuclear Cu2+-triazacyclononane system. 

Based on the insight that a dissociative mechanism plays the major role along the metathesis pathway [11], these catalysts have been designed such that only one bulky phosphine, one chloride and one cumulenylidene ligand are attached to a Ru(II) center. Because arene ligands are known to be labile on such a metal fragment, they will easily liberate free coordination sites ( ) for the interaction with the alkene substrate. Although the precise mode of action of such allenyli-... 

If the arene ligand is kept the same and the metal changed, Table 15 shows that >(M-mesitylene) increases with increasing atomic number. In Fig. 7 the variation... 

Treatment of lead(n) chloride with aluminum trichloride and o-xylene furnishes bis(o-xylene)lead(ll) bis(tetrachloro-aluminate) 41. According to the results of an X-ray structure determination, 41 consists of a monomeric complex with two arene ligands in a distorted 77-mode of coordination and two bidentate AICI4- ions at the central Pbz+ ion. The interaction of the lead center with the tetrachloroaluminate ligands is presumably highly ionic, suggesting that 41 is a triad of arene-stabilized ions, rather than a molecule.7... 

Hz found for 11. These values correlate well with the X-ray determined Si-H distance of 1.61(4) A in 27, which is shorter than the range 1.75(4)-1.802(5) A determined in 11. The stronger Si-H interaction in 27 is apparently the result of a weaker donor ability of the arene ligand compared with the cyclopentadienyl ligand, which leads to a weaker backdonation from the chromium center. A similar correlation between the r(Si-H) and the /(Si-H) in complexes 12 has been discussed above. 

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