Organometallic complexes for second-order non-linear optics: synthesis and molecular quadratic hyperpolarizabilities of η5-monocyclopentadienyliron(II) nitrile derivatives with different phosphines. X-ray crystal structure of [FeCp(DPPE)(p-NCC6H4NO2)][PF6

New complexes of the type [FeCp(P_P)(p-NCR)][PF6] (P_P=(S)-PROLOPHOS, DPPE and (R)-PROPHOS ((S)-PROLOPHOS=(S)-N-(diphenylphosphino)-2-diphenylphosphinooxymethylpyrrolidine; DPPE=1,2-bis(diphenylphosphino)ethane; (R)-PROPHOS=(R)-(+)-1,2-bis(diphenylphosphino)propane); R=C6H4N(CH3)2, C6H4NO2, (E)-C(H)C(H)C6H4NO2, C6H4C6H4NO2) have been synthesized and characterized. Spectroscopic data were evaluated in order to correlate the tuning of the electronic density at the metal center with the first molecular hyperpolarizabilities (β) determined by hyper-Rayleigh scattering at 1064 nm. Experimental β values increase with chain length and with the enhancement of the donor ability of the bidentate phosphine. [FeCp(DPPE)((E)-p-NCC(H)C(H)C6H4NO2)][PF6] is found to have the highest β value (570×10−30 esu) in the series. Molecular orbital calculations were carried out to understand the geometric and electronic factors involved and also to compute the static hyperpolarizabilities, β0. These results are consistent with the trend observed in the β0 values derived from experiment on the basis of the two-level model. The complex [FeCp(DPPE)(p-NCC6H4NO2)][PF6] was crystallographically characterized. The compound crystallizes in the centrosymmetric space group Pbcn. Bond distances within the nitrile ligand were discussed in order to evaluate the extension of π-backdonation suggested by spectroscopic data.