Macrocyclic Se₄N₂[7,7]ferrocenophane and Se₂N[10]ferrocenophane containing benzyl unit: Synthesis, complexation, crystal structures, electrochemical and optical properties

Two novel macrocyclic polyselena[n]ferrocenophanes containing a pendent benzyl unit, 20-membered Se₄N₂[7,7]ferrocenophane (L1) and 10-membered Se₂N[10]ferrocenophane (L2), were designed and synthesized. The reaction of L1 with two molar amounts of metal salts (M = Cu⁺, Cu²⁺, Pd²⁺ and Hg²⁺) led to six dimetallic complexes 1-6. A crystallographic study revealed that each metal center in 1-5 was tetracoordinated to two selenium atoms from different ferrocene units, one aliphatic nitrogen atom and one co-ligand. The structures of the complexes contain a two-fold axis perpendicular to the molecular plane with two pendant benzyl moieties in an anti-conformation. Macrocycle L1 gives significant electrochemical, linear and third-order nonlinear optical responses to Cu²⁺ and Hg²⁺. The DFT/B3LYP calculations of 4 demonstrated a small HOMO-LUMO energy gap and delocalization of the π-electron cloud in the frontier molecular orbitals, which led to the enhancement of molecular NLO properties after complexation. The results show that the oxidation state of the ferrocene unit is accompanied by significant differences in the corresponding absorption spectra and third-order NLO properties.