Octafluorocyclooctatetraene transition-metal chemistry: 1,2-.eta. and 1,2,3,6-.eta. complexes of iron and platinum

Abstract

Cyclooctatetraene (COT)' has occupied a prominent place in the historical development of organometallic chemistry.2 It is a versatile ligand which has exhibited a flexible hapticity in its modes of bonding to transition metals, lanthanides, and actinides.'J Moreover, the elucidation of the nature of metallotropic rearrangements in COT complexes, particularly in Fe(COT)(C0)3 (l), occupied the attention of many leading research groups for a number of years.* Fluorocarbon organometallics have often proved to be more stable thermally and to exhibit different bonding modes from their hydrocarbon a n a l o g~e s .~ This communication reports examples of 1,2-7 complexes of octafluorocyclooctatetraene (OFCOT, 2)4 with iron(0) and platinum(0). These undergo intramolecular oxidative addition reactions, with formation of a metal-carbon u bond, to afford 1,2,3,6-7 bonded complexes.F F at equal areas per molecule for each of these comparisons, the inherent cratic entropy difference of R In Z5 between the racemic and enantiomeric forms should make a constant contribution to the differences in the final equilibrium states of the films." How this factor affects rates of mixing in these experiments is not clear to us. On the strength of the above experiments, the resolution of a racemic film was approached by seeding with pure crystals of either enantiomer. The racemic film was spread on 10 N H2S0, from a dilute hexane solution. After the usual 20 min for solvent evaporation was allowed, the barrier drive was started, and the film was compressed to 28 dyn/cm. The dashed curve in Figure 3 shows the surface pressure decrease caused by seeding the compressed film with pure racemic crystals. After 1.5 h the surface pressure had decreased nearly to the ESP of the racemic film.The experiment was now repeated by compressing a freshly spread racemic film, but this time crystals of pure R-(+) enantiomer were sprinkled on the surface. The solid curve of Figure 3 shows the rapid decrease of surface pressure to a point well below the ESP of the racemicfilm and approaching that of a pure enantiomeric film.These results imply the deposition of R-(+) molecules on the added crystals, leaving a partially resolved film which was composed of predominantly S-(-) molecules. Equivalent results were obtained by seeding with S-(-) crystals which would leave predominantly an R-(+) film. We have already presented evidence above that the free energy of enantiomer crystals is lower compared to liquid or film states than are racemic crystals.We have tried repeatedly to detect unseeded spontaneous resolution by leaving racemic films under compression for periods of 20 h, the present limit for maintaining reasonably constant conditions. In no case have we been able to detect a spontaneous decrease in surface pressure which could not be attributed to leaks or other artifacts.Resolution of racemates from bulk solution by seeding with enantiomers is only possible in cases, such as the present one, where the racemic crystals are more soluble than those of the en...

Fluorine as a ligand substituent in organometallic chemistry: A second chance and a second research career

“…Contributions to this area were reviewed 20 years ago [21], but a few examples are presented here for the purpose of perspective, and also to introduce some recent illustrative results from DFT calculations. For example, while COT, first reported by Willstä tter in 1911 [22], was shown to react with Fe(CO) 5 to give 1, in which the COT is bound as a 1,3-diene, and in which the Fe(CO) 3 unit undergoes a facile peripheral perambulation around the ring [23,24], OFCOT forms the previously mentioned complex 2 in which the coordination mode is quite different and the molecule is non-fluxional [25].…”

Fluorinated dienes in transition-metal chemistry – the rich chemistry of electron-poor ligands

“…Hughes, and complexes of both early and late transition-metals have been structurally characterised. [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] The great variability in coordination modes is unprecedented by any other fluorinated molecule. However, the scope of OFCOT chemistry exceeds this perspective article, therefore only a brief overview based on representative examples will be given; a comprehensive review has been published by Hughes.…”

“…The most simple bonding situation, a coordination of only one double bond is observed in thermolabile intermediates on the way to higher hapticities. The 1,2-h 2 -OFCOT ligand in the ruthenium species 49 is thermally converted to a 1,2-h 2 -5,6-h 2 -diene ligand in 50 after loss of ethene, 46 while the iron species 51 loses carbon monoxide on heating to give the 1,2,3-h 3 -6-h 1 -complex 52 (Scheme 15). 44 An h 1 -h 1 -coordination Scheme 14 Perfluorocyclooctatetraene (47) and its valence isomers perfluorosemibullvalene (48a) and -bicyclo[4.2.0]octatriene (48b).…”

Hexafluorobenzene Photochemistry: Wellspring of Fluorocarbon Structures

“…Cyclooctatetraene 6 has proven to be a versatile ligand in organotransition metal chemistry. Diiron nonacarbonyl in hexane at room temperature gave tetracarbonyl complex 66 , which at reflux was transformed into the oxidative addition product 67 . Cyclooctatetraene itself does not form an iron complex analogous to either.…”