Cycloheptatrienyl Compounds

Treatment of Nd(BH4)3(THF)2 with KC7H9 gave the the first cycloheptatrienyl compound of a Af element, [Nd(BH4)2(THF)]2(/i-7]7 7]7-C7H7), as green microcrystals in 77% yield. The formation of the cycloheptatrienyl resulted from the disproportionation reaction 3C7HT — C7F173- + 2C7FE0. Its crystal structure has been determined.800 

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Cycloheptatrienes with chromium, 5, 337 in molybdenum carbonyls, 5, 480 tj7-Cycloheptatrienyl complexes, with tungsten carbonyls and isocyanides, 5, 693 Cycloheptatrienyl compounds actinide complexes, 4, 227 with actinides, 4, 226 lanthanide complexes, 4, 122... 

The kinetics of reactions of the cyclohexadienyl and cycloheptatrienyl compounds of iron and osmium (10) with acetylacetone or dimedone have been determined in acetonitrile solution. The rate law suggests that the... 

Manganese.—The pentahapto-cycloheptatrienyl compound (11) is fluxional. The activation energy for fluxional rearrangement is several kcalmol" higher than for the analogous, but charged, iron compound (12). The... 

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

The cycloheptatrienyl carbocation, also known as the tropylium cation, has six pi electrons. It is also aromatic and is quite stable. In fact, 7-bromo-l,3,5-cycloheptatriene actually exists as an ionic compound. 

Also in many of the organometallic compounds various important organic ligands can function as CT acceptors (eg ethines, carbenes, arenes, or cyclohexadi-enyl or cycloheptatrienyl cations). Accordingly, organometallic complexes with such ligands are frequently characterized by the MLCT absorptions and their CT excitation can lead to emission or photo-oxidation [35,36]. 

Compare the piQ of cyclopentadiene with that of cycloheptatriene. Whilst the anion of the former has 6 7t electrons (which makes it isoelectronic with benzene), the anion of the latter has 8 ti electrons. Remember that on p. 176 we saw how 4n n electrons made a compound anti-aromatic The cycloheptatrienyl anion does have 4 7t electrons but it is not anti-aromatic because it isn t planar. However, it certainly isn t aromatic either and its pKa of around 36 is about the same as that of propene. This contrasts with the cyclopropenyl anion, which must be planar since any three points define a plane. Now the compound is anli-aromatic and this is reflected in the very high pKit (about 62). Other compounds may become aromatic on losing a proton. We looked at fluorene a few pages back now you will see that fluorene is acidic because its anion is aromatic (14 n electrons). 

T. Arliguie, M. Lance, M. Nierlich, J. Vigner, and M. Ephritikhine, Synthesis and Crystal Structure of [K(C12H2406)][U(r -C7H7)2], the First Cycloheptatrienyl Sandwich Compound, J. Chem. Soc., Chem. Comm. 1995, 183-184. 

The mixed cyclopentadienyl-cycloheptatrienyl ( 5-7 ) titanium complex Cp( -C7H7)Ti has been prepared in 33 -40% yield by the reduction of CpTiCb with isopropylmag-nesium bromide or Mg in the presence of cycloheptatriene. Reduction of Cp TiCb in THF with Mg in the presence of cycloheptatriene gives Cp ( -C7H7)Ti in 68% yield. Titanium 5-7 complexes exhibit sandwich structures see Sandwich Compound) with the five-membered and seven-membered rings nearly parallel to each other. 

Figure 29.10. Aromatic compounds with 6 w electrons. Configuration of IT electrons in cyclopentadienyl anion, benzene, and cycloheptatrienyl cation.

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