Lewis acidity of tris(pentafluorophenyl)borane: crystal and molecular structure of B(C6F5)3·OPEt3

The synthesis and characterisation (IR, multielement NMR spectroscopy) of B(C6F5)3·L {L=CH3CO·OEt (1), Et3PO (2)} are described together with a single-crystal X-ray diffraction study of 2. The latter reveals an angular B–O–P linkage {161.04(16)°} with BO and PO bondlengths of 1.533(3) and 1.4973(17) Å, respectively. Lappert’s Δν(CO) and Gutmann’s acceptor number (AN) methods have been applied to 1 and 2 to assess the relative Lewis acidity of B(C6F5)3 which is confirmed as weaker than BF3. A strong linear correlation has been obtained for Gutmann’s AN and Childs’ 1H NMR Lewis acidity scales.