Cyclopentadienyltins

The chemistry of cyclopentadienyltin compounds is reviewed in references (267) and (26S). The ready disproportionation of (CsH5)4Sn with SnCU (269), and the sensitivity of the cyclopentadienyl-tin bond to acidolysis and to photolysis (270) suggests that these compounds may find application in synthesis. 

New sources of RaSn- radicals that have been developed include the reversible thermal dissociation of bis(trialkylstannyl)pinacols (290-292), the )3-scission of /3-stannylalkyl radicals (293), and the photolysis of cyclopentadienyltin compounds (294). 

Another type of divalent organotin n complex has been reported by Noltes et al. (228,229). The cyclopentadienyltin halides LXIXa,b precipitate upon mixing concentrated THF solutions of stannocene with the tin dihalides [Eq. (38)]. The complex LXIXa can also be obtained by the reaction of LXVIIIa with hydrogen chloride in THF [Eq. (39)] (228). Similarly, the pentamethylcyclopentadienyltin compounds LXIXc,d have been prepared from decamethylstannocene LXVIIIc and the appropriate carboxylic acid [Eq. (40)] (230). 

Cyclopentadienyltin cations, the representatives of a third class of n-cyclopentadienyltin compounds, were first observed in the mass spectra of cyclopentadienyl-substituted organotin(II) and organotin(IV) species. The first such species to be isolated was the Me5C5Sn+ cation, in the form of the tetrafluoroborate salt (LXXa) (Scheme 6) (192,231). Subsequently, other electrophiles have been employed leading to the compounds LXXb-d (Scheme 6) (202,250). 

Scheme 6. Synthesis of compounds containing cyclopentadienyltin cations.

Stannocene, bis-Tj5-cyclopentadienyltin(II), and plumbocene, bis-Tj5-cy-clopentadienyllead(II), were first reported by Fischer and Gruber in 1956 (12). They observed the compounds to be monomeric in benzene and to have nonzero dipole moments and proposed the a-bonded structure shown in Fig. 1A. Wilkenson and co-workers reported the similarity between the IR spectrum of stannocene, plumbocene, and ferrocene and suggested the presently accepted angular sandwich structure for stannocene and plumbocene shown in Fig. IB (13). 

Cyclopentadienyltin(II) chloride and bromide can be made by the exchange reaction shown in Eq. (2) (79). This is apparently an equilibrium system, and the product is determined by the relative solubility of the reagents present. It is not possible to synthesize the iodide this way because it is too soluble in THF (31). 

Cyclopentadienyltin compounds undergo the same reaction, but much more readily, to generate the cyclopentadienyl and stannyl radicals.3 These reactions have been used for studying substitutent effects in cyclopentadienyl ([5]annulene) radicals (see Sections 9.3.3 and 20.1.1). 

The acidity of the cyclopentadienes makes the cyclopentadienyltin compounds readily accessible97 either through the reaction of other cyclopentadienylmetal derivatives with tin(IV) halides (equation 9-44),99 or by acidolysis of an aminostannane with a cyclopentadiene (equation 9-45).100 101 Ferrocenylstannanes can also be obtained from ferrocenylmercury compounds and organotin sulfides (equation 9-54).102... 

Some examples of cyclopentadienyltin(IV) compounds are shown in Table 9-1. 

The cyclopentadienyltin(IV) compounds are usually yellow oils or crystals, which are sensitive to air, light, and moisture (though some preparations have been worked up using a hydrolytic procedure), and tetrakis(cyclopentadienyl)tin has been reported to inflame in the air.99 On standing, they darken in colour, which is probably due, at least in part, to Diels-Alder polymerisation. 

The effect of the electronic configuration in conferring special characteristics on the cyclopentadienyltin(IV) compounds is discussed in Section 3.1.2.3. 

A number of properties of cyclopentadienyltin compounds emphasise that the electronegativity of the cyclopentadienyl ligand is greater than that of an alkyl ligand. The Mossbauer spectra of the compounds R3SnCp and R2SnCp2 show a substantial... 

The cyclopentadienyltin(IV) compounds undergo Diels-Alder cycloadditions with reactive dienophiles such as maleic anhydride, diethyl maleate, and diethyl acetylenedi-carboxylate," and an endoperoxide has been identified from the reaction with singlet oxygen.123... 

Cyclopentadienyltin(IV) compounds can act as cyclopentadienylating agents in the Stille reaction (Section 22.2), and Me3SnC5H5 reacts with C5I5Mn(CO)3 to give C5(C5H5)5Mn(CO)3 and thence the pentaanion (equation 9-52).124... 

The cyclopentadienyltin compounds (and cyclopentadienyl derivatives of other metals such as mercury and lead) are very sensitive to photolysis, and on irradiation with UV light in an ESR spectrometer they show the spectrum of the cyclopentadienyl radical the stannyl radical is difficult to observe directly under these conditions, but its presence is established by the reactions which it shows with substrates such as alkyl halides. This provides a useful route to a variety of stannyl radicals (see Section 20.1.1), and a method of generating cyclopentadienyl radicals for study by ESR spectroscopy.125 It would be interesting to extend these ESR studies to the more complex compounds 9-6-9-9, which are known to be photosensitive. 

Stannocene can also be prepared, via the cyclopentadienyltin(II) amine (21-61), by acidolysis of the diaminotin(II) compound 21-60 with cyclopentadiene98,99... 

Cyclopentadienyltin(lV) compounds can be prepared by treating tin tetrachloride, or an organotin chloride, with CpNa. These compounds are fluxionally a-bonded. They are photosensitive, and the stannanes CpSnRs, on irradiation with UV light, undergo homolysis to give Cp and RsSn radicals. 

In the tin sandwich compound shown in Figure 12, both 1,2-azaborolyl rings are more or less bonded as the Sn—N and Sn—B distances are found to be beyond binding interaction <85AG(E)602>. As in comparable cyclopentadienyltin sandwich complexes, the molecule has a bent structure as a result of the lone pair at the Sn atom. 

Sn----Cl intermolecular contacts are present for cyclopentadienyltin chloride in the solid. ... 

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