The Al(OR^F)₃/H₂O/Phosphane [R^F = C(CF₃)₃] System – Protonation of Phosphanes and Absolute Brønsted Acidity

Abstract

Abstract The synthesis of the classical, neutral donor–acceptor adducts Ph 2 MeP–/Ph 3 P–/Ph 3 As–Al(OR F ) 3 and H 2 O–Al(OR F ) 3 [ 1 , 2 , 3 , 4 , OR F = OC(CF 3 ) 3 ] is reported. The intermediate H 2 O–Al(OR F ) 3 ( 4 ) was generated by substitution of PhF in PhF–Al(OR F ) 3 with H 2 O and was analyzed in a long‐term NMR study over 22 days. This Brønsted acidic system was used in orienting experiments to protonate phosphanes such as PMePh 2 , PPh 3 , PCy 3 , P( t Bu) 3 , and PCy 2 [2,4,6‐( i Pr) 3 C 6 H 2 ]. Depending on the use of one or two equivalents of PhF–Al(OR F ) 3 , the new weakly coordinating anions [(R F O) 3 Al(μ‐OH)Al(OR F ) 3 ] – or [HOAl(OR F ) 3 ] – were obtained. However, in dependence of the steric bulk of the phosphanes, stable and unreactive R 3 P–Al(OR F ) 3 adducts were also observed in the NMR experiments. The absolute acidity of the key H 2 O–Al(OR F ) 3 adduct was evaluated by the relaxed COSMO cluster‐continuum (rCCC, COSMO = conductor‐like screening model) model in fluorobenzene solution. For a 0.001 M solution of H 2 O–Al(OR F ) 3 , the medium acidity resulted as –986 kJ mol –1 or a pH abs value of 173. Long‐term hydrolysis of H 2 O–Al(OR F ) 3 ( 4 ), probably to give HOR F and HOAl(OR F ) 2 followed by trimerization, gave [HOAl(OR F ) 2 ] 3 ( 10 ), which was identified by X‐ray diffraction.