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

A series of amphiphilic salen complexes, [(LM)-M-1a,b] and [(LM)-M-2a,b], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic pi-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [(LM)-M-1b] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [(LCo)-Co-1b(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, [(LNi)-Ni-1b] and [(LNi)-Ni-2b], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [(LNi)-Ni-1a] and [(LNi)-Ni-2a]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [(LNi)-Ni-1b] and [(LNi)-Ni-2b], are more flexible than the corresponding bis-TEG analogues, [(LNi)-Ni-1a] and [(LNi)-Ni-2a], and that the phenanthrene derivatives [(LNi)-Ni-1a,b] were more rigid than the corresponding naphthalene analogues, [(LNi)-Ni-2a,b]. The Langmuir-Blodgett (LB) films of one of the complexes, [(LNi)-Ni-1b], 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.