ORGANOTRANSITION-METAL METALLACARBONES .18. ETA-6, ETA-5-BENZYLTETRAMETHYLCYCLOPENTADIENIDE(1-) AS A BRIDGING LIGAND IN MULTILEVEL IRON COBALT SANDWICH COMPLEXES

Benzyltetramethylcyclopentadiene (Me4C5CH2Ph) was prepared in high yield from commercial reagents and deprotonated with butyllithium to give the red title anion, which was converted to the orange sandwich complex (Et2C2B4H4)Co(Me4C5CH2Ph) (1) via reaction with CoCl2 and Na+Et2C2B4H5- in THF. Decapitation (apex BH removal) of 1 with wet TMEDA gave yellow (Et2C2B3H5)Co(Me4C5CH2Ph) (2), which in turn was bridge-deprotonated and reacted with CoCl2 and the title anion to give a burgundy-colored triple-decker complex, (Et2C2B3H3)Co2(Me4C5CH2Ph)2 (3). The thermal cyclooctatriene-displacement reaction of 1 with (Et2C2B4H4)Fe(eta-6-C8H10) produced an orange bimetallic species, (Et2C2B4H4)Co(Me4C5CH2Ph)Fe(Et2C2B4H4) (4). Similar treatment of 3 with the same cyclooctatriene-ferracarborane reagent gave the iron-dicobalt complex (Et2C2B3H3)Co2(Me4C5CH2Ph)2Fe(Et2C2B4H4) (5) and the diiron-dicobalt species (Et2C2B3H3)Co2(Me4C5CH2Ph)2Fe2(Et2C2B4H4)2 (6), both isolated as red oils. Complex 2 was benzylated at the B(5) location on the open face by bridge deprotonation to give the anion 2-, followed by reaction with benzyl bromide to form the yellow solid derivative (5-PhCH2Et2C2B3H4)Co(Me4C5CH2Ph) (7). Mono and bis(ferracarboranyl) derivatives of 7 were prepared by thermal displacement of cyclooctatriene from (Et2C2B4H4)Fe(eta-6-C8H10) by 7 to give primarily (Et2C2B4H4)Fe(5-PhCH2Et2C2B3H4)Co(Me4C5CH2Ph) (8) and a small amount of (Et2C2B4H4)Fe(5-PhCH2Et2C2B3H4)Co(Me4C5CH2Ph)Fe(Et2C2B4H4) (9). The monoiron species 8 consisted almost entirely of one isomer (8a), having the iron coordinated to the B-benzyl unit; the alternative isomer (8b), in which the iron is bonded to the cyclopentadienyl benzyl group, was detected in only trace amounts in NMR spectra. The much higher reactivity in 7 of the B-benzyl vs the cyclopentadienyl benzyl group toward iron coordination suggests deactivation of the latter by the adjacent cobalt atom.