η1 versus η5 bonding modes in Cp*Al(I) adducts of 9-borafluorenes

The reactivity of highly Lewis acidic perfluorinated borafluorenes C12F8BR (R = C6F5, 1a; CH3, 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al](4) was investigated. The reaction of 1 with [Cp*Al](4) leads to the formation of thermally robust eta(1) Lewis acid-base adducts 3a,b as the. thermodynamically favored products. Use of the less Lewis acidic 2 does not alter the mode of reactivity, with the eta(1) Lewis acid-base 4 formed preferentially. Reduction of 2 to the 9-boratafluorene 2.Li-2(THF)(n) is readily accomplished in THF solution. However, reaction of 2.Li-2(THF)(n) with [Cp*AlCl2](2) or Cp*AlCl2(THF), 5, affords aluminum metal, 2.THF, and Cp*H as the main identifiable products. Compounds 3a, 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT computational study was conducted to probe the reason for the strong preference for eta(1) bonding, which essentially stems from the localization of aromaticity in the flanking phenyl rings in the 9-borafluorene ring system.