| Abstract
| - Reaction of the amido complex(η5-C5H5)Re(NO)(PPh3)(N̈H2)(2) and hexafluoroacetone gives themethyleneamidocomplex(η5-C5H5)Re(NO)(PPh3)(N̈C(CF3)2)(3, 58%). Addition of TfOH to 3 yields theσ-imine complex[(η5-C5H5)Re(NO)(PPh3)(η1-N(H)C(CF3)2)]+TfO-(4, 96%). Similar reactions of 2 withtrifluoroacetaldehydeand then TfOH give the σ-imine complex[(η5-C5H5)Re(NO)(PPh3)(η1-N(H)C(CF3)H)]+TfO-(5, 78%) andsometimes small amounts of the corresponding π-trifluoroacetaldehydecomplex. Reaction of 5 andt-BuO-K+gives the methyleneamido complex(η5-C5H5)Re(NO)(PPh3)(N̈C(CF3)H)(6, 82%). The IR and NMR propertiesof 3−6 are studied in detail. The13C NMR spectra show CN signals (157−142 ppm)diagnostic of σ-bindingmodes. No evidence is observed for π isomers of 4 or5. Analogous OC(CF3)X complexesgive exclusivelyπ isomers, and rationales are discussed. Reactions of 3or 6 with MeOTf and heteroatom electrophiles arealsodescribed.
- The σ/π isomerization of imine ligands has beenproposed in catalytic reactions but never directly observed. Onlyσ-imine complexes of the rhenium Lewis acid[(η5-C5H5)Re(NO)(PPh3)]+have been detected previously, but it was thought that the moreπ-acidic ligands in the title compounds (X = CF3, H)would give greater amounts of π isomersas demonstrated for relatedketone and aldehyde complexes. However, only σ isomers areobserved within IR/NMR detection limits, and implications areanalyzed.
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