Design and Study of Structural Linear and Nonlinear Optical Properties of Chiral [Fe(phen)₃]²⁺ Complexes

The dependence of nonlinear optical properties upon the spin state in molecular switches is still an unexplored area. Chiral [Fe­(phen)₃]²⁺ complexes are excellent candidates for those studies because they are expected to show nonlinear optical properties of interest and at the same time show photoconversion to a short-lived metastable high-Spin state by ultrafast optical pumping. Herein, we present the synthesis, crystallographic, and spectroscopic comparison of chiral [Fe­(phen)₃]²⁺ complexes obtained with chiral anions, a new lipophilic derivative of the D₂-symmetric (As₂(tartrate)₂)^2–, and D₃-symmetric tris­(catechol)­phosphate­(V) (TRISCAT), tris­(catechol)­arsenate­(V) (TRISCAS), and 3,4,5,6-tetrachlorocatechol phosphate­(V) (TRISPHAT). Complexes [Fe­(phen)₃]­(rac-TRISCAT)₂ (2) and [Fe­(phen)₃]­(X-TRISCAS)₂ (X = rac (3), Δ (4), Λ (5)) were found to be isomorphous in the R32 Sohncke space group with twinning by inversion correlated with the starting chiral anion optical purity. The structures show the [Fe­(phen)₃]²⁺ complex interacting strongly along its 3-fold axis with two anions. Only the structure of a [Fe­(phen)₃]­(rac-TRISPHAT)₂ solvate (6) could be obtained, which showed no particular anion/cation interaction contrary to what was observed previously in solution. The [Fe­(phen)₃]­(X-As₂(tartrate)₂) (X = Δ (7), Λ (8), and racemic mixture (9)) crystallizes in enantiomorphic space groups P3₁21/P3₂21 with the same solid-state packing. Dichroic electronic absorption studies evidenced racemization for all chiral complexes in solution due to ion pair dissociation, whereas the asymmetric induction is conserved in the solid state in KBr pellets. We evidenced on chiral complexes 4 and 5 strong nonlinear second harmonic generation, the intensity of which could be correlated with the complex electronic absorption.