Cyclobutadiene complexes stability

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). 

All of the ethynylated cyclobutadienes are completely stable and can be easily manipulated under ambient conditions, as long as the alkyne arms carry substituents other than H. For the deprotected alkynylated cyclobutadiene complexes, obtainable by treatment of the silylated precursors with potassium carbonate in methanol or tetrabutylammonium fluoride in THF, the stability is strongly dependent upon the number of alkyne substitutents on the cyclobutadiene core and the nature of the stabilizing fragment. In the tricarbonyUron series, 27b, 27c, 29 b, and 28b are isolable at ambient temperature and can be purified by sublimation or distillation under reduced pressure. The corresponding tetraethynylated complex 63 e, however, is not stable under ambient conditions as a pure substance but can be stored as a dilute solution in dichloro-methane. It can be isolated at 0°C and kept for short periods of time with only... 

The CpCo-stabilized ethynylated cyclobutadienes are considerably more robust, and the parent 76 can be isolated as a yellow crystalline material, stable at ambient temperature for several hours. At 0°C 76 decomposes in the course of several days, which is indicated by darkening of the formerly brillant-yellow needles. The stability of 76 made in X-ray analysis feasible and the bond angles/distances obtained are in good agreement with reported values for ethynylated cyclobutadiene complexes already described [35,36]. 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

The validity of this interpretation is strengthened by an analysis of TMMFe(CO)3. In this compound, the best way to increase stabilization is to draw the CH2 groups closer to the Fe(CO)3by pyramidalizing the TMM. This is indeed observed. It is interesting to note that (a) this effect increases steric interactions between the TMM and Fe(CO)3fragments and (b) it is the reverse of the distortion observed in the cyclobutadiene complex. 

Similarly, in the tricarbonyliron cyclobutadiene complex 285 (78AJC1607), the metal carbonyl groups stabilize the positive charge in the a-positions to the complexed diene system (H-2 and H-6 8 8.29 ppm). 

Like its cyclobutadiene analog 39, the 7j4-l,3-diphosphacyclobutadiene complex 38 has high thermal stability, is stable to air, and generally has low reactivity. Reagents that normally release cyclobutadiene from transition metal-cyclobutadiene complexes by oxidative demetallation [i.e., HCN, (NH4)2Ce(N01)6, and alkali metals] do not react with compound 38. 

The main reaction modes and products that are formed from reaction of CpCoL2 and an alkyne are depicted in Scheme 27. If CpCo(PPh3)2 is treated with polar alkynes under ambient conditions, a monosubstitution product (27) can be isolated. Another isolable product is a cobaltacyclopentadiene (28) and, in certain instances, the dinuclear cobaltole complex (29) is formed, which has many analogs in the chemistry of Fe(CO)3 and other 14-electron fragments. The cyclobutadiene complex (30) is the direct product from collapse of (28) and is of high kinetic and thermodynamic stability. In catalytic cycles, (30) is inactive. If the ligand L in (28) is a third molecule of alkyne, insertion of the latter forms the arene (31). 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

Perhaps the most interesting preparative methods are those in which substituted cyclobutadienes are stabilized and isolated as metal 7r-complexes by the cyclization of diphenylacetylene (2-16)-(2-18). 

Actually the basic orbital pattern for any 18-electron polyene-ML3 complex will be very similar to that found in CpMn(CO)3. Figure 20.7 illustrates the situation for cyclobutadiene-Fe(CO)3. The set on cyclobutadiene is stabilized by 2e on Fe(CO)3. Likewise, the 02u orbital is stabilized by the loi and 2oi levels on Fe (CO)3. Three metal-centered orbitals are left nonbonding. Notice that the two fragments have been partitioned to be neutral. The set on cyclobutadiene and 2e set on Fe(CO)3 are each half-filled. It is reasonable to assume that lies at a lower energy than 2e (recall that the 2e set is carbonyl [Pg.580]

Stabilization of Unstable Intermediates. Transition metals can stabilize normally unstable or transient organic intermediates. Cyclobutadiene has never been isolated as a free molecule, but it has been isolated and fully characterized as an iron tricarbonyl complex (138) ... 

With the successful chemistry of the cymantrenes and the (cyclobuta-diene)tricarbonyl iron, the quest for tetraethynylated cyclobutadienes based on CpCo-stabilized complexes arose. Why would they be interesting Whereas all derivatives of 63 and 68 exhibit reasonable stability when their alkynyl substituents are protected by either an alkyl or a trimethylsilyl group, the desilylated parents are isolated only with difficulty and are much more sensitive. 

Disubstituted (cyclobutadiene)Fe(CO)3 complexes in which the two substituents are different may exist as enantiomers. Racemic cyclobutadiene carboxylic acids or cyclobutadiene amine complexes of this type have been separated by classical resolution methodology234. These optically active (cyclobutadiene)Fe(CO)3 complexes are stable with respect to racemization at 120°C for 24 h. This stability contrasts with acyclic... 

The prediction that cyclobutadiene would be stabilized by complex formation with transition metals (145) has been verified by the preparation of dichlorotctramethylcyclobutadienenickel(II) by the following reaction... 

The possibility of the stabilization of cyclobutadiene by complex formation was first suggested on theoretical grounds by Longuet-Higgins and... 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

---