New chiral cyclopentadienylrhenium Lewis acids featuring fluorinated triarylphosphanes and enhanced acceptor abilities - An unusual carbon-fluorine bond activation in a metal coordination sphere

The acetonitrile complex [(eta (5)-C5H5)Re(NO)(NCCH3)(Co)](+) BF4- (1) and P(4-C6H4CF3)(3) slowly react (2 d, room temperature) to give the substituti on product [(eta (5)-C5H5)Re(NO){P(4-C6H4CF3)(3)}(CO)]+BF4- (3, 85%). The r eaction of 1 and less nucleophilic P(C6F5)(3) must be conducted in a melt a t 140 degreesC, but [(eta (5)-C5H5)Re(NO){P(C6F5)CO)]+BF4- (11) is isolated in 90% yield. Reduction of 3 by NaBH4 gives the methyl complex (eta (5)-C5 H5)Re(NO){P(4-C6H4CF3)(3)}(CH3), which is treated with TfOH and H3CN = C(H) C6H5 to give the imine complex [(eta (5)-C5H5)Re(NO)(PPh3){N(CH3)=C(H)C6H5} ]+TfO- (9). Complex 9 was, Like the non-fluorinated analog, unreactive towa rds allyltin nucleophiles. Complex 11 should lead to a more reactive imine complex, but NaBH4 gave a methyl complex that was difficult to purify. Howe ver, NaOCH3 or NaSCH3/methanol gave easily purified (eta (5)-C5H5)Re(NO)-{P (4-C6F4OCH3)(3)}{(CO2CH3) (85%) or (eta (5)-C5H5)Re(NO)-{P(4-C6F4SCH3)(3)}{ (CO2CH3) (51%), in which all para-fluorine atoms have undergone nucleophili c displacements, and the carbonyl ligands methoxide additions. This suggest s that catalytic chemistry of the chiral Lewis acid [(eta (5)-C5H5)Re(NO){P (C6F5)(3)}](+) (I-F-15) (I-F-15) will be complicated by nucleophilic degrad ation. Nonetheless, density functional calculations show that I-F,, is a mu ch stronger sigma acceptor and weaker re donor than less fluorinated analog s.