From Organometallic Cations to Carbenes: An NMR, Structural and Reactivity Study.

Abstract

<p>The electronic requirements of the short-lived 4n π anti-aromatic fluorenyl indenyl and cyclopentadienyl cations have been studied by investigating the dynamic processes exhibited by their dicobalt hexacarbonyl alkynyl clusters in solution. It is suggested that the 12π fluorenyl complex requires the least amount of electronic assistance from a neighbouring Co(CO)₃ moiety (and consequently the cyclopentadienyl ligand requires the most). That is, the experimental evidence indicates the following cationic stability trend: fluorenyl (12π electrons) > indenyl (8π electrons) > cyclopentadienyl (4π electrons). However, these cationic clusters did not yield single crystals suitable for an X-ray crystallographic study and efforts were focused on synthesizing neutral models for these complexes. Isobal replacement of a formal Co(CO)₃⁺ vertex with a Fe(CO)₃ moiety, in the fluorenyl and indenyl systems yields neutral iron-cobalt clusters, (Me₃Si-C=C=C₉H₈)FeCo(CO)₆ and (1-trimethylsilylethynyl-2,3-diphenylindenyl)FeCo(CO)₆, the structures of which were determined by X-ray diffraction. This work provided insight into the mechanism for the formation of these iron-cobalt complexes. Conclusions about the electron requirements that these 4n π anti-aromatic cations impose on the neighboring metal center have been drawn. In contrast to anti-aromatic species, aromatic ligands such as C₇H₇⁺ do not require electronic assistance from neighboring organometallic moieties. However, if these species bulky substituents, then a compromise between π delocalization and steric strain occurs. The attempt to monitor the electronic requirements of a C₇Cl₆R⁺ moiety led to an investigation of the chemistry of the polychlorinated cyclic system, C₇Cl₈. Treatment of C₇Cl₈ with various organometallic reagents yielded syn-C₁₄Cl₁₂, presumably via coupling of intermediate C₇Cl₆carbenes. The reactions of dimethoxycarbene [(MeO)₂C:)], generated by the thermal decomposition of 2,2-dimethoxy-5,5-dimethyl-∆³1,3,4-oxadiazoline with polychlorinated olefins and ketones were investigated. Treatment of [(MeO)₂C:] with octachlorocycloheptatriene or octachlorobicyclo[3.2.0]hepta-3,6-diene yielded dimethoxy(hexachloro)heptafulvene and dodecachloroheptafulvene. It is proposed that an initial displacement of a chloride ion (Sɴ2') by dimethoxycarbene occurs resulting in cationic intermediates which can then undergo further reaction to yield the observed products. Reaction of hexachlorotropone and dimethoxycarbene was also investigated and methyl-2-pentachlorophenyl-2-oxo-ethanoate was observed. Mechanisms to account for the observed products were proposed, and the structures of some of these products were determined using X-ray crystallography.</p>