New Chiral Cyclopentadienylrhenium Lewis Acids Featuring Fluorinated Triarylphosphanes and Enhanced Acceptor Abilities − An Unusual Carbon−Fluorine Bond Activation in a Metal Coordination Sphere

Abstract

The acetonitrile complex [(η5-C5H5)Re(NO)(NCCH3)(CO)]+ BF4− (1) and P(4-C6H4CF3)3 slowly react (2 d, room temperature) to give the substitution product [(η5-C5H5)Re(NO){P(4-C6H4CF3)3}(CO)]+BF4− (3, 85%). The reaction of 1 and less nucleophilic P(C6F5)3 must be conducted in a melt at 140 °C, but [(η5-C5H5)Re(NO){P(C6F5)3}(CO)]+BF4− (11) is isolated in 90% yield. Reduction of 3 by NaBH4 gives the methyl complex (η5-C5H5)Re(NO){P(4-C6H4CF3)3}(CH3), which is treated with TfOH and H3CN=C(H)C6H5 to give the imine complex [(η5-C5H5)Re(NO)(PPh3){N(CH3)=C(H)C6H5}]+TfO− (9). Complex 9 was, like the non-fluorinated analog, unreactive towards allyltin nucleophiles. Complex 11 should lead to a more reactive imine complex, but NaBH4 gave a methyl complex that was difficult to purify. However, NaOCH3 or NaSCH3/methanol gave easily purified (η5-C5H5)Re(NO){P(4-C6F4OCH3)3}(CO2CH3) (85%) or (η5-C5H5)Re(NO){P(4-C6F4SCH3)3}(CO2CH3) (51%), in which all para-fluorine atoms have undergone nucleophilic displacements, and the carbonyl ligands methoxide additions. This suggests that catalytic chemistry of the chiral Lewis acid [(η5-C5H5)Re(NO){P(C6F5)3}]+ (I-F15) will be complicated by nucleophilic degradation. Nonetheless, density functional calculations show that I-F15 is a much stronger σ acceptor and weaker π donor than less fluorinated analogs.