Synthesis of Cyclopentadienyl Iron Alkoxycarbene Complexes via Vinylidene Intermediates: X-ray Structures of [Fe{C(OMe)Me}(dppe)Cp][I] and [Fe{C(CH2)3O}(dppe)Cp][PF6]

The reaction of [FeI(dppe)Cp], 1 (dppePh2PCH2CH2PPh2) with HC≡CSiMe3 in refluxing methanol results in formation of a mixture of the vinylidene complex [Fe(CCH2)(dppe)Cp]+, 2 and the heteroatom stabilised alkoxycarbene [Fe{C(OMe)Me}(dppe)Cp]+, 3. The carbene 3 is formed by addition of the alcohol solvent to the reaction intermediate vinylidene 2, and prolonged reflux in methanol (4 days) resulted in complete conversion of 2 to 3. The methoxycarbene, 3 was isolated as a stable solid and fully characterised by spectroscopic and structural methods. Vinylidene 2 also reacts with ethanol to give the ethoxycarbene [Fe{C(OEt)Me}(dppe)Cp]+, 4 but no reaction was observed in refluxing isopropanol, thus providing a specific synthesis of vinylidene 2 from 1 and HC≡CSiMe3. An example of an alcohol based, intramolecular nucleophilic addition to a mono-substituted vinylidene [Fe(CCHR)(dppe)Cp]+ was uncovered in the reaction of 1 with 3-butyn-1-ol which leads to the synthesis of the cyclic oxacarbene complex [Fe{C(CH2)3O}(dppe)Cp][PF6], 5. X-ray crystallographic investigations on 3[I] and 5[PF6] determine Fe-Cα bonds lengths of 1.851(5) and 1.834(2) Å respectively, intermediate between previously reported Fe-Cα distances for the amino carbene [Fe{C(NH2)Me}(dppe)Cp][PF6] and the alkylidene complex [Fe{C(H)Me}(dppe)Cp*][PF6].