Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR 3 ) 2 (Schiff base)] + complexes.

Complexes of the type trans-[Re(PR ₃ ) ₂ (Schiff base)] ⁺ (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu ₄ N)[ReOCl ₄ ] with H ₂ sal ₂ en or H ₂ sal ₂ ibn followed by addition of a tertiary phosphine. The trans-[Re(PR ₃ ) ₂ (sal ₂ en)] ⁺ complexes 2-4 were stable in solution, whereas the trans-[Re(PR ₃ ) ₂ (sal ₂ ibn)] ⁺ complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR ₃ )(sal ₂ ibn)] ⁺ products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR ₃ ) 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 ¹ H and ¹³ C 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(PR ₃ ) ₂ (Schiff base)] ⁺ complex formation. This will aid in the development of novel ^186/188 Re therapeutic agents and the design of novel bifunctional N ₂ O ₂ Schiff base ligands.