Cyclobutadiene, stabilization coordination

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination lo a metal and, indeed, were predicted theoretically on this basis by H. C. Longuei-Higgins and L, E, Orgel (1956) some 3y before the first examples were synthesized, In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy t /i and there would be an unpaired electron m each of the 2 degenerate oibilals 2, Coordination to a metal provides funhei interactions and avoids this unstable configuration, See also the discussion on ferra-boranes (p. 174). 

Cyclobutadiene (26) is antiaromatic and its isolation is not possible. Flowever, it can be stabilized by -coordination of Fp+ to one of the double bonds to give 27, and the uncomplexed double bond in 27 undergoes Diels-Alder reaction with cyclopentadiene to give 28 [4]. As described in Section 9.2, cyclobutadiene (26) can be stabilized as a diene by the )/4-coordination of Fe(CO)3. 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

Fig. 5. An extended carbon network stabilized by Fe(CO)3 groups coordinated to cyclobutadiene fragments.

Existence of cyclobutadiene as the simplest cyclic hydrocarbon with conjugated double bonds was predicted by Hiickel in the 1930s. Until recently, however, all attempts to prepare this compound ended in failure. In 1956 Longuet-Higgins deduced 166) that coordination of cyclobutadiene with a transition metal should stabilize its unstable triplet state. 

Also closely related to this theme are certain reactions involving the metal ion-promoted synthesis of (otherwise unstable) coordinated ligands such as cyclobutadiene, which are stabilized through coordination —e.g., the reaction described by Emerson, Watts, and Pettit (30),... 

Cyclobutadiene, C4H4, is anti-aromatic (i.e. it does not have 4n + 2 TT-electrons) and readily polymerizes. However, it can be stabilized by coordination to a low oxidation state metal centre. Yellow crystalline (ri -C4H4)Fe(CO)3 is made by reaction 23.115 and its solid state structure (Figure 23.25a) shows that (in contrast to the free ligand in which the double bonds are localized) the C—C bonds in coordinated C4H4 are of equal length. 

Alkylidene and carbene complexes contain the grouping [M]=CRR. Free carbenes CRR are very reactive and short lived species (p. 61). The isolation of complexes of these ligands is a good illustration of the stabilization of labile species by coordination to a transition element (cf. cyclobutadiene, p. 269 and trimethy-lenemethane p. 270). 

The reactive species, trimethylenemethane, has been observed only by photolysis of an adduct of allene and diazomethane in a matrix of hexafluorobenzene at — 185°C. The e.s.r. spectrum shows that it has two unpaired electrons (a triplet ground state) which is consistent with a planar structure with 3-fold symmetry (Djh). This species can, like cyclobutadiene, be stabilized by coordination to tricarbonyliron. The parent compound, a diamagnetic pale yellow solid, m.p. 28°C can be obtained by the two routes shown. 

While CH is stabilized by coordination to cyclobutadiene (see Exercise 42), the analogous structure in which a CH" is complexed to benzene, (CH)7% is not stabilized. Explain why this would be so. 

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