Examination of the pyridine binding to the bifunctional Lewis acid B,B′-diphenyidiboradiferrocene

The binding behavior of the bifunctional Lewis acid (775-C5H5Fe)(2)[mu-(C5H3(BPh))(2)] (3) toward different pyridine derivatives as the Lewis base has been investigated. The 1: 1 and 1:2 adducts, 3 center dot D and 3 center dot 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.