Six-Membered Metallacycles

Compared with five-membered metallacycles, relatively fewer reports are known on the preparative methods and reaction chemistry of six-membered metallacycles. Whitby and coworkers have systematically investigated insertion of carbenoids into five-membered zirconacycles and developed a number of interesting six-membered zirconacycles [17, 26]. Isonitriles, 1-halo-l-lithioalkenes, allenyl carbenoids, allyl carbenoids, propargy carbenoids, benzyl carbenoids, and 1-nitrile-1-lithio epoxides can all insert into zirconacyclopentanes and zirconacyclopentenes to afford various six-membered zirconacycles (Eqs. 23,24). 

Lithiated chloromethyltrimethylsilane was reported to insert into zircona-cyclopentadienes to give zirconacyclohexadiene derivatives (Eq. 25) [27]. This has been so far the only example of carbenoid insertion into 5-membered met-allacyclopentadienes. 

In a similar manner, reactions of zirconocene-benzyne complexes with bis(phenylalkynyl)phosphine afforded benzocyclozirconacyclohexadiene derivatives (Eq. 28) [28]. 

A dibenzotitanacyclohexadiene, the first example of this kind, was prepared in 1988 from the reaction of 2,2 -dilithiobiphenyl and Cp2TiCl2 and structurally characterized (Eq. 29) [29]. 

Obviously, titanocene analogues of six-membered metallacycles are very rare, due to lack of preparative methods. More preparative methods are desirable and interesting reaction chemistry can be expected of six-membered metallacycles. 

---

R=Me the connectivity is TiCp2(/U-S3)(/U-S4)NMe. This different behavior between the SyNH and SyNMe systems cannot be attributed to either recognizable steric or electronic reasons. By contrast, the reaction with RN(/t-S2)2NR (R=Me, n-Oct) in hexane at 20 °C gave not the expected seven-membered ring compound but a six-membered metallacycle, TiCpyl/f-SylNR [36] (Scheme 5). 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

The synthesis and stmctnral characterization of a series of spirocyclic organozincates containing two five- or six-membered metallacycles in which zinc is the central spiro atom, is shown in Scheme 1. Compound 8a was prepared via an elegant one-pot synthesis, starting from 1,5-dichloropentane, ZnCl2 and a lithium/sodinm alloy (1% sodinm) in diethyl ether as a solvent (eqnation 4 in Scheme 1). Snbseqnent treatment of a soln-tion of 8a with TMEDA afforded the corresponding TMEDA complex 8b of which the structnre in the solid state was unambiguously established by an X-ray crystal structnre determination. 

According to X-ray diffraction data, in the solid-state complex 87 adopts a six-membered metallacyclic structure, as the result of P-Pd-Cl-B bridging coordination. The interaction of the borane moiety with the chlorine atom is apparent from the short Cl-B distance (2.165(2) A) and the pyramidalization of the boron environment (EBa = 349.1°). The bridging coordination of the Pd-Cl bond by the PB 7b contrasts with the B-pendant coordination mode adopted by complex 72, deriving from the related ligand 7e featuring mesityl groups at boron (see Section IV.A). This illustrates the role of steric factors in the participation, or not, of the Lewis acid in the coordination assembly. 

The final neutral palladium(II) complex with the amide functionalised carbene hgand features two six membered metallacycles, just as the cationic not yet deprotonated one does. The amine nitrogen atom has four substituents (H, Pd and two C). Since the two metallacycles can adopt two distinct conformations that slowly flip into each other for steric reasons, the nitrogen atom appears chiral due to atropisomerism despite its overall C -symmetry. The deprotonated complex lacks this feature, clearly visible in the respective NMR spectra. 

Another example for a new titanium complex [Cp2Ti(As2Me4)]2[AsFg]4 contains a six-membered metallacycle. The temperature-dependent H NMR spectra indicate dynamic behaviour and a chair conformation (40). 

Although oxidative addition of C—O or C—N bonds to Ni, Pd, or Pt is not widespread, a number of such examples do exist. The Ni(0) complex [Ni(bipy)(COD)] reacts with phthalic anhydride by insertion of a Ni-containing moiety into one of the C—O bonds to generate a six-membered metallacycle ... 

The titanacycle derivative Cp 2Ti[C(=CH2)CH2CH2] reacts with isonitriles or diphenylketones at room temperature to give five- and six-membered metallacycle derivatives Cp 2Ti[C(=CH2)CH2CH2C(=NR)] and Cp 2Ti[C(=CH2)CH2CH2C(=CPh2)0], the structures of which have been determined by X-ray diffraction. The unsaturated molecules Ph2C=C=0, C02, or isocyanates are not inserted.1366... 

In addition to the (CANAC)-type ligands, several examples of (NACAN) pincer ligands have been used in cyclometallated compounds [53,54]. None of these complexes seem to have interesting photochemical or photophysical properties due to the fact that all of them contain either sp3 hybridized N-ligands, six-membered metallacycles, or other ligating atoms, like halides, phosphines, and so on. 

Exchange of one ligand in a metallacycle by another ligand has been observed in Pd(II) complexes dissolved in acidic solution [74]. Synthesis of a cyclometallic complex comprising six-membered metallacycles from species containing five-membered metallacycles has been achieved in this way. 

Azole substituents at the nitrogen atom of an exocyclic C=N bond are predominantly coordination-active. The site for coordination is the nitrogen atom of the p5uidine type. However, for the ring closure to stable five- or six-membered metallacycles, spatially favorable conditions are necessary. Illustrations include chelates of pyrazoles 225 (97JCS(D)2155) and 226 (X = O, S) (00ICA(310)183). Complexes of an imidazole azomethine involve chelates with different numbers of... 

---