Synthesis of Bis(methoxycarbene) and Bis(alkylidene) Ligands Bridging Two Iron Centers in the Cp*Fe(L1)(L2) Series. X-ray Crystal Structure of the Iron Alkylidene [Cp*Fe(dppe)(C(H)Me)][PF₆]

The carbene [Cp*Fe(dppe)( C(OMe)Me)][CF 3 SO 3 ] [ 2b, Cp* = η 5 -C 5 Me 5, dppe = ethylenebis(diphenylphosphine)] was prepared in 97% yield from [Cp*Fe(CO) 2 ( C(OMe)Me)][CF 3 SO 3 ] upon photochemical displacement of the carbonyl ligands, and the iron methoxycarbene [Cp*Fe(CO)(PMe 3 )( C(OMe)Me)][CF 3 SO 3 ] ( 2c ) was obtained (80%) upon alkylation of the acyl derivative Cp*Fe(PMe 3 )(CO)(COCH 3 ) ( 4 ). The iron−methylidene [Cp*Fe(dppe)( C(H)Me)][PF 6 ] ( 6 ) was synthesized (95%) by treatment of Cp*Fe(dppe)(CH(OMe)Me) ( 5 ) with an aqueous solution of hexafluorophosphate acid. The ethylidene 6 is very stable in solution as in the solid state, and no decomposition reaction was observed by NMR spectroscopy at 50 °C. The X-ray crystal structure of 6 was solved and refined. The Fe−C(37) bond distance (1.787 Å) reveals the double-bond character of the metal−carbon bond. Reaction of the iron carbene complexes 2a − c and 6 with 2 equiv of potassium tert -butoxide in THF gave the corresponding vinyl derivatives 7a − d in high yield (80−90%). CV analyses of the vinyl complexes 7a − d display at 20 °C an oxidation wave at a platinum electrode with the ( i p a / i p c ) current ratio less than unity. CV and ESR measurements established the stability of the 17-electron vinyl radical in CH 2 Cl 2 at −80 °C. Warming to 20 °C of the 17-electron iron(III) complexes [ 7a − d ] • + [PF 6 - ] in solid state allows the vinyl−vinyl coupling providing the new binuclear bis(carbene) complexes 8a − d in 60−95% yield. The complex 8a was isolated as a pure diastereoisomer, whereas 8b was a mixture of the meso 8b( RS, SR ) and dl 8b( RR, SS ) isomers in the 2/1 ratio. The diastereoisomers were separated by CH 2 Cl 2 extraction which only solubilizes the dl pair.