Cyclooctatetraene Dianion Complexes

Just as C5H5 and its analogues are aromatic 6-electron ligands, the cyclooctatetraene dianion, CgHg , is an aromatic 10-electron ligand. Several types of complex have been obtained. The product of the reaction between LnCb and K2CgH8 in THF depends upon stoichiometry  

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Quan and co-workers (93) have synthesized the aminoborollide complex 27, bearing yet another dianionic Cp analogue. The cyclooctatetraene dianion complex 28 has shown fair polymerization activity with MAO (94). 

Uranium complexes analogous to these compounds were also described, but with the cot (cyclooctatetraene dianion) ligand rather than the Cp or Cp ones. Both the dianionic U(IV) [(cot)U(dddt)2]2 [47] and monoanionic U(V) [(cot)U(dddt)2] [48] complexes were isolated and structurally characterized (Fig. 5). Spectacular distortions of the US2C2 metallacycles were rationalized on the basis of DFT calculations, which reproduced the spectacular folding of the endo US2C2 metalla-cycle when the dianionic species undergoes an oxidation. 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

The diamagnetic complex is not reduced further by the cyclooctatetraene dianion. This prevents the conversion of the azoxybenzene anion-radicals into azodianions. Potassium cation plays an important role in this limitation of the reduction process, which, generally, proceeds readily (the... 

The structure of a potassium salt of a cyclooctatetraene dianion has been determined by Raymond and co-workers (269). In this bright yellow complex (XlXb in Fig. 3) a diglyme molecule is coordinated to each of the potassium atoms through all three oxygen atoms. Two potassium-diglyme units lie on either side of the planar carbocyclic ring equidistant from the ring center. 

Reaction of the cyclooctatetraene dianion with [Cp RhCl2]2 or [Cp IrCl2]2 at low temperature (<—10°C) gave the complexes 146 or 147, respectively (Sche-... 

In analogy to the cyclooctatetraene dianion, 71 also forms a very stable sandwich complex 75 upon reaction with UC14 in THF (Scheme 18) [69]. The H NMR spectrum of this complex 75 revealed a very characteristic singlet at 6 = -21 ppm ( ) which is in good agreement with the chemical shift of urano-cene,the bis(cyclooctatetraenediyl)uranium complex [70]. 

In analogy to the cyclooctatetraene dianion, 71 also forms a very stable sandwich complex 75 upon reaction with UCI4 in THF (Scheme 18) [69]. The... 

Controversy exists in naming sandwich organometallic actinide and lanthanide compounds of cyclooctatetraene dianion [8]annulene dianion and [8]annulene-metal(X) complex are chosen here because these names properly describe the delocalization of charge in the ligand and also emphasize the formal oxidation state of the metal in the complex. 

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

The complexation with the dianion of cyclooctatetraene leads to the formation of the mixed or half-sandwich complexes see Half-sandwich Complexes) as shown in Scheme The coordination of the planar... 

Cyclooctatetraene could form sandwich or, more likely, halfsandwich complexes in which the 10 v electrons of the planar dianion CsHg - are involved in bonding to a transition metal (Fig. If). 

The bis([8]annulene)lanthanate(III) anions which are complexed by an [8]annulenelanthanate(III) cation are the only other class of complexes which contain two [8]annulene dianions in a sandwich arrangement about a lanthanide(III) ion. This class was first reported for the cerium(III) ion, from the reduction of a cerium(IV) alkoxide with triethylaluminum in the presence of cyclooctatetraene.3 ... 

In order to study a bis[8]annulene compound with a divalent central metal atom we have synthesized and characterized the divalent ytterbium complex K2[Yb(CgHg)2]F K2[Yb(CgHg)2]r and its calcium analogue, K2[Ca(CgHg)2].51 These compounds were prepared by a reaction which utilizes the solubility of ytterbium and calcium metals in liquid ammonia. Reaction of cyclooctatetraene, potassium and either ytterbium or calcium, in liquid ammonia solution, gives the desired potassium salt of the complex dianion. 

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