Pentalene metal complex

Gold chloride forms an unstable complex with cyclo-octatetraene at low temperatures. This decomposes at — 20°C to l,2-dichlorocyclo-octa-3,5,7-triene, which gradually cyclizes to (562). In an attempt to form pentalene-metal complexes, cyclo-octatetraene was reduced to a mixture of trienes and bicyclo [4,2,0] octadiene, which was then treated with substituent ruthenium carbonyls, affording a variety of complexes, including (563). °... 

Metal complexes of pentalene have been prepared Knox Stone /tec. Chem. Res. 1974, 7, 321-328. 

Bicyclo[3.3.0]octa-l,3,5,7-tetraene (2), trivially called pentalene [26, 27], is the second member in the series of fully unsaturated oligoquinanes. Huckel MO theory predicts that this planar hydrocarbon with its 8 r-electron system should be an antiaromatic species [25]. 2-Methylpentalene (37) has been generated by a retro-Diels-Alder reaction and deposited as a film at —196 °C on a NaCl or quartz plate for its spectroscopic characterization. It rapidly dimerized upon warming the cold plates to temperatures above —140 °C [26]. Only two stable derivatives of pentalene not complexed to a metal [29], the hexaphenyl- (38) [30] and 2,4,7-tri-tert-butylpentalene (39) [31], have hitherto been reported (Figure 4). 

The first metal complex (50) of unsubstituted pentalene (2) was prepared in 1973 by reaction of the pentalene [2+2] dimer 43 with Fe2C09 (Figure 6) [42]. Several other dinuclear complexes of 2 have been prepared from the dilithium pentalenediide 22 -2Li+ [43]. In all... 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

In 1973 Hafner prepared the first metal complex of unsubstituted pentalene 46 (Fig. 4) by reaction of the [2 + 2] dimer of pentalene 2 with Fe2C09 [39]. Several other dinuclear complexes of 2 have been prepared from the pentalenediide 45 as reviewed previously by Paquette [1]. All these structures contain a planar pentalene ligand, in which each five-membered ring is complexed to a different metal atom as in bis(pentamethylcyclopentadienylnickel)pentalene 47 obtained by reaction of 45 with Cp Ni(acac) [40]. 

Attempts to prepare the antiaromatic compound pentalene have so far failed. Various transition metal complexes of pentalene are, however, easily prepared [for example, ruthenium carbonyl complexes, Eq. (167) (Brookes et at., 1973)] so that pentalene might be generated in situ from such complexes and trapped with various reagents (Knox and Stone, 1974). 

Pentalene (135) and acepentalene (137) are antiaromatic cross-conjugated compounds with two or three fulvene subunits. According to theoretical calculations [142-147], 135 and 137 are unstable species [147, 148] but can be ligands in transition metal complexes. The respective anions, the planar 136 [149, 150] and the slightly bowl-shaped 138 [151, 152], can be generated by deprotonation of a dihydropentalene or by treatment of4,7-bis(trimethylstannyl)-4,7-dihydroacepentalene with methyllithium at -60 °C (Scheme 10.48). 

It is interesting that some metal complexes of the pentalene system have been prepared [266]. 

Weidmuller and Hafner have observed that fission of pentalene dimers can be achieved by transition metals and, in particular, to yield complexes of the type 184 or 185 when treated with Fe2(CO)9 at 50 °C in methylcyclohexane under a carbon monoxide atmosphere.262 The complexes of pentalene arid 1,3-dimethyl-pentalene were given particularly close scrutiny. 

These ligands often display unusual chemical behaviour not found, or rarely found, in mononuclear metal compounds. For example in [Ru3(CO)8(C8H6)J the triruthenium cluster stabilises pentalene(l), an organic molecule having only a transient existence under normal conditions, while in the complex... 

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