Lewis Acid Properties of Tris(pentafluorophenyl)borane. Structure and Bonding in L−B(C₆F₅)₃ Complexes

Abstract

A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated by reaction of a Lewis base with an excess of B(C 6 F 5 ) 3 in pentane. In this way, nitrile complexes (C 6 F 5 ) 3 B·NCR (R = CH 3 1a, p -CH 3 −C 6 H 4 1b, p -NO 2 −C 6 H 4 1c ), isonitrile complexes (C 6 F 5 ) 3 B·CNR (R = C(CH 3 ) 3 3a, C(CH 3 ) 2 CH 2 C(CH 3 ) 3 3b, 2,6-(CH 3 ) 2 −C 6 H 3 3c ), and the phosphine adduct (C 6 F 5 ) 3 B·P(C 6 H 5 ) 3 ( 6 ) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses ( 1a, 1c, 3a, 3b, and 6 ). Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a substantial increase in the C⋮N bond strength. This is evident from a marked shift of the ν̃ C ⋮ N IR band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C⋮N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C 6 F 5 ) 3 B·L, L = CNCH 3, NCCH 3, PH 3, CO. A detailed analysis revealed that the bonding in (C 6 F 5 ) 3 B·L complexes is mainly dominated by electrostatic interaction, which in turn is responsible for the observed structural and spectroscopic changes. In the context of this work, the bonding of the neutral B(C 6 F 5 ) 3 Lewis acid is compared to the positively charged organometallic d 0 -Cp 3 M + system (M = Zr, Hf). It was found that electrostatic effects are more pronounced for B(C 6 F 5 ) 3 than for the transition metal fragments. The question as to the existence of a nonclassical main group carbonyl complex, (C 6 F 5 ) 3 B·CO, is addressed.