Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR3)2(Schiff base)]+ complexes

Complexes of the type trans-[Re(PR3)(2)(Schiff base)](+) (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu(4)N)[ReOCl4] with H(2)sal(2)en or H(2)sal(2)ibn followed by addition of a tertiary phosphine. The trans-[Re(PR3)(2)(sal(2)en)](+) complexes 2-4 were stable in solution, whereas the trans-[Re(PR3)(2)(sal(2)ibn)](+) complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR3)(sal(2)ibn)](+) products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR3)(2)(sal(2)ibn)](+) complexes is likely driven by steric interactions between the bulky backbone gem-dimethyl groups of the sal(2)ibn ligand and the phosphine ligands. These complexes were isolated and characterized by H-1 and C-13 NMR, FT-IR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. The results reported herein provide insight into the factors that drive trans-[Re(PR3)(2)(Schiff base)](+) complex formation. This will aid in the development of novel Re-186/188 therapeutic agents and the design of novel bifunctional N2O2 Schiff base ligands.