A Lewis Basicity Scale in Dichloromethane for Amines and Common Nonprotogenic Solvents Using a Zinc(II) Schiff-Base Complex as Reference Lewis Acid

A consistent, reliable scale of Lewis basicity in dichloromethane for 26 bases, involving amines and nonprotogenic solvents, is presented. A Lewis acidic Zn(II) Schiff-base complex, involving formation of stable 1:1 adducts is used as reference acid. Evaluation of binding constants is achieved from spectrophotometric titrations, by the least-squares nonlinear regression of multiwavelength spectrophotometric data. This Lewis basicity scale represents a unique set of data reflecting the actual Lewis basicity with respect this "real world" Lewis acidic species. The comparison of present Lewis basicity scale with data reported in the literature indicates that while for the involved solvents their relative basicity is scarcely affected by the reference Lewis acid, in contrast for sterically encumbered amines the Lewis basicity seems to be dependent from the reference species. Thus, Lewis basicity is governed by the steric hindrance at the donor atom and involves very different relative basicities than those predicted considering typical reference Lewis acids. This is expected to have a major involvement in the organic synthesis and catalysis, given the sterically encumbered nature of commonly involved Lewis acidic organometallic complexes.