Rings cyclopentadienyl

Ferrocene Figure 2-47) provides a prime ex.ample of multi-haptic bonds, i.e, a situation where the electrons that coordinate the cyclopentadienyl rings with the iron atom arc contained in molecular orbitals delocalised over the iron atom and the 10 carbon atoms of the cyclopentadienyl rings [82. ... 

Representation of such a system by a connection tabic having bonds between the iron atom and the five carbon atoms of either one of the two cyclopentadienyl rings is totally inadequate. A few other examples of structures that can no longer be adequately described by a standard connection table are given in the Section 2.G.2. 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Fig. 8. Molecular stmcture of [Tj -C (CH2)5]2Zr(N2) 2(N2) (after Ref. 189). Each pentagon represents a pentamethyl cyclopentadienyl ring system.

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

Photolysis of Cp2TiAr2 in benzene solution yields titanocene and a variety of aryl products derived both intra- and intermolecularly (293—297). Dimethyl titan ocene photolyzed in hydrocarbons yields methane, but the hydrogen is derived from the other methyl group and from the cyclopentadienyl rings, as demonstrated by deuteration. Photolysis in the presence of diphenylacetylene yields the dimeric titanocycle (28) and a titanomethylation product [65090-11-1]. 

Examine the molecular model of ferrocene on Learning By Modeling. Does ferrocene have a dipole moment Would you expect the cyclopentadienyl rings of ferrocene to be more reactive toward nucleophiles or electrophiles Where is the region of highest electrostatic potential ... 

Tlie importance of bis(cyclopeniadienyl)irou (Fe(jj -C5H3)2( in the developmenl of organo-metallic chemistry has already been alluded to (p. 924). Tile compound, which forms orange crystals, mpl74°, has extraordinary thermal stability (>500°) and a remarkable structure which was unique when first established. It also has an extensive aromatic-lype reaction chernisiry which is reflected in its common name ferrocene The molecular structure of ferrocene in the ciysialline slac features two parallel cyclopentadienyl rings at one lime Ihese... 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

The cyclopentadienyl rings in ferrocene display some of the same chemical behavior as benzene. For example, ferrocene undergoes Friedel-Crafts mono and di acylation reactions, but at different rates. 

Interaction of the iron metal atoms with thiophenes (thiophene, 2-methyl-, and 2,5-dimethylthiophene) in the vapor phase at 77 K with subsequent heating in a carbon monoxide atmosphere also leads to the formation of ferrole 83 [76JOM(l 18)37, 77CJC3509]. The iron cyclopentadienyl ring is planar and all the bonds have multiple character. 

Introduction of substituents to the cyclopentadienyl ring of 7, or the use of complexes derived from other monodentate chiral alcohols, cause decreased enantioselectivity11,1 la,35a 36. 

In the context of pyridin synthesis from alkynes and nitriles homogeneously catalyzed by (cyclopentadienyl)cobalt complexes (Eq. 13), it was found that electron-withdrawing groups on the cyclopentadienyl ring significantly increase the activity [63]. During the screening of various cobalt half-sandwich complexes... 

Cp derivatives (Scheme 185) to form -oxo-bridged dimeric complexes. The stabilities of the Af,0-bound carbamate intermediates depend mainly on the cyclopentadienyl ring substituents, but there is also a small dependence on the nature of the length of the (dimethylamino)alkyl chain for the C5H4Me systems. ... 

Fig, 43. Reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms. Open circles, BH units shaded circles, C-CHj units and dark circles, C-H units. One cyclopentadienyl ring has been omitted on compound III, for clarity (J73). 

In an attempt to change the electronics of the chromium atom, we are replacing the carbon based cyclopentadienyl ring with ligands containing harder donor atoms. For example, we have employed the tris(pyrazolyl)borate moiety, an isoclectronic replacement for Cp featuring tridentate N-coordination.[9] Figure 2 shows the molecular structure of Tp SU Cr-Ph, a representative Cr° alkyl. It will be noted, that this complex is mononuclear, due to the steric protection of the extremely bulky tris(pyrazolyl)borate. 

The crystal and molecular structure of the 44, 45, and C5-vinylferro-cenyl-thymidine show that the substituted cyclopentadienyl ring is essentially co-planar with the nucleobase (164). DFT calculations indicate that, irrespective of the extent of saturation in the bridging C2-unit in ethynyl-, vinyl- or ethyl-ferrocenyl-C5-thymidine, a similar amount of spin density is transferred to the nucleobase (Fig. 48). The reduction potentials for these compounds are shifted little compared to the parent ferrocenyl derivatives. 

Finally, a weak jt-type halogen bonding involving a cyclopentadienyl ring and the iodine atom of an iodofluorocarbon [89] is a rare example of a Tt-bonded io do carbon derivative, in contrast to numerous examples of halogen bonding of the latter with n-type electron donors [2,20]. 

In 3, the amino functional group is two methylene units removed from the ferrocene nucleus. It appears from the instantaneous and quantitative formation of h from 3 that this feature minimizes steric effects and also enables 3 to undergo the Schotten-Baumann reaction readily without the classical a-metallocenylcarbenium ion effects providing any constraints. The IR spectrum of showed the characteristic N-H stretch at 3320 cm" (s), the amide 1 (carbonyl) stretch at 1625 an - -(s), the amide II (N—H) stretch at 1540 cm (s), and the amide III band at 1310 cm 1(m). In addition, characteristic absorptions of the ferrocenyl group were evident at 1100 and 1000 cm l (indicating an unsubstituted cyclopentadienyl ring) and at 800 cm"l. 

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