Examination of the Pyridine Binding to the Bifunctional Lewis Acid B,B′-Diphenyldiboradiferrocene

The binding behavior of the bifunctional Lewis acid (η5-C5H5Fe)2[μ-(C5H3(BPh))2] (3) toward different pyridine derivatives as the Lewis base has been investigated. The 1:1 and 1:2 adducts, 3·D and 3·D2 (donor, D = 4-tert-butylpyridine (tBupy) and 4-dimethylaminopyridine (DMAP)), were obtained by low-temperature crystallization or slow solvent evaporation techniques. The complexes were fully characterized by multinuclear NMR, IR, and elemental analysis. Single-crystal X-ray diffraction data were collected for the 1:1 adduct with DMAP and for both 1:2 adducts, and the marked structural changes upon pyridine binding are discussed in detail. In solution, the 1:2 adducts were found to readily undergo dissociation with formation of the 1:1 adducts as the dominant species at room temperature. Variable-temperature NMR and UV−visible titration studies demonstrate that the first binding process is more than 5 orders of magnitude more favorable than the second binding process.