Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air–water interface

Abstract

A series of amphiphilic salen complexes, [L^1a,bM] and [L^2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L^1bM] (M = Ni, Cu, Zn), formed a monolayer at the air–water interface, while the monocationic [L^1bCo(MeNH₂)₂](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L^1bNi] and [L^2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L^1aNi] and [L^2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L^1bNi] and [L^2bNi], are more flexible than the corresponding bis-TEG analogues, [L^1aNi] and [L^2aNi], and that the phenanthrene derivatives [L^1a,bNi] were more rigid than the corresponding naphthalene analogues, [L^2a,bNi]. The Langmuir–Blodgett (LB) films of one of the complexes, [L^1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air–water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.