SYNTHESIS OF BIS(METHOXYCARBENE) AND BIS(ALKYLIDENE) LIGANDS BRIDGING2 IRON CENTERS IN THE CP-ASTERISK-FE(L1)(L2) SERIES - X-RAY CRYSTAL-STRUCTURE OF THE IRON ALKYLIDENE [CP-ASTERISK-FE(DPPE)(=C(H)ME)][PF6]

The carbene [CpFe(dppe)(=C(OMe)Me)][CF3SO3] [2b, Cp* = eta(5)-C(5)Me( 5), dppe = ethylenebis(diphenylphosphine)] was prepared in 97% yield f rom [CpFe(CO)(2)(=C(OMe)Me)][CF3-SO3] upon photochemical displacement of the carbonyl ligands, and the iron methoxycarbene [CpFe(CO)(PMe(3 ))(=C(OMe)Me)][CF3SO3] (2c) was obtained (80%) upon alkylation of the acyl derivative CpFe(PMe(3))(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 hexafluorophospha te acid. The ethylidene 6 is very stable in solution as in the solid s tate, and no decomposition reaction was observed by NMR spectroscopy a t 50 degrees C. The X-ray crystal structure of 6 was solved and refine d. The Fe-C(37) bond distance (1.787 Angstrom) reveals the double-bond character of the metal-carbon bond. Reaction of the iron carbene comp lexes 2a-c and 6 with 2 equiv of potassium tert-butoxide in THF gave t he corresponding vinyl derivatives 7a-d in high yield (80-90%). CV ana lyses of the vinyl complexes 7a-d display at 20 degrees 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 CH2Cl2 at -80 degrees C. Warming to 20 degrees C of the 17-electron iron(III) complexes [7a-d](.+)[PF6-] i n solid state allows the vinyl-vinyl coupling providing the new binucl ear bis(carbene) complexes 8a-d in 60-95% yield. The complex 8a was is olated 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 diastereoiso mers were separated by CH2Cl2 extraction which only solubilizes the dl pair.