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]
Citations Over TimeTop 18% of 1996 papers
Abstract
The carbene [Cp*Fe(dppe)(C(OMe)Me)][CF3SO3] [2b, Cp* = η5-C5Me5, dppe = ethylenebis(diphenylphosphine)] was prepared in 97% yield from [Cp*Fe(CO)2(C(OMe)Me)][CF3SO3] upon photochemical displacement of the carbonyl ligands, and the iron methoxycarbene [Cp*Fe(CO)(PMe3)(C(OMe)Me)][CF3SO3] (2c) was obtained (80%) upon alkylation of the acyl derivative Cp*Fe(PMe3)(CO)(COCH3) (4). The iron−methylidene [Cp*Fe(dppe)(C(H)Me)][PF6] (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 (ipa/ipc) current ratio less than unity. CV and ESR measurements established the stability of the 17-electron vinyl radical in CH2Cl2 at −80 °C. Warming to 20 °C of the 17-electron iron(III) complexes [7a−d]•+[PF6-] 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 CH2Cl2 extraction which only solubilizes the dl pair.
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