Synthesis and nonlinear optical properties of η⁵-monocyclopentadienyliron(II) acetylide derivatives. X-ray crystal structures

A series of new acetylide complexes of the type [Fe(η⁵-C₅H₅)(P down curve sign P)(p-C≡CC₆H₄R)] (P down curve sign P = DPPE (=l,2-bis(diphenylphosphino)ethane), (P)-PROPHOS (=R)-(+)-l,2-bis(diphenylphosphino)propane), R = NO₂, C₆H₄NO₂, (Z)-C(H)=C(H)C₆H₄NO₂, (E)-C(H)=C(H)C₆H₄NO₂) have been synthesized by halide abstraction from the precursors [Fe(η⁵-C₅H₅)(P down curve sign P)(I)] and fully characterized. Quadratic hyperpolarizabilities (β) for the complexes have been determined by hyper-Rayleigh scattering at 1064 nm. The influence on the nonlinear response of acetylide chain length in proceeding from 4-nitrophenylethynyl to 4-nitrobiphenylethynyl and 4-nitro-(E)-stibenylethynyl has been studied, revealing values of the first hyperpolarizabilities among the highest reported for organometallic molecular materials. Comparisons on the nonlinear efficiencies are drawn with the related well-known families of compounds [Ru(η⁵-C₅H₅)-(PR₃)₂(p -C≡C-(aryl)-NO₂)] and [Fe(η⁵-C₅H₅)(P down curve sign P)(p-N≡C-(aryl)-NO₂)]⁺. Cubic hyperpolarizabilities (γ) determined by Z-scan at 800 nm are consistent with an increase in γupon replacing Ru by the more easily oxidizable Fe and upon chain-lengthening the delocalizable π-bridging unit (proceeding from 4-C₆H₄ to (E)-4,4′-C₆H₄CH=CHC₆H₄). X-ray crystallographic structures of complexes [Fe(η⁵-C₅H₅(DPPE)(p-C≡CC₆H ₄NO₂)] and [Fe(η⁵C₅H₅)(DPPE)((E)-p-C≡CC₆ H₄C(H)=C(H)C₆H₄NO₂)] were studied in order to investigate the existence of π-back-donation suggested by spectroscopic and electrochemical data. Crystal packing was analyzed with the aim of assessing the alignment of the molecules in the lattice and hence suggesting the magnitude of NLO properties at the macroscopic level.