Synthesis, Characterisation and Molecular Hyperpolarisabilities of Pseudo-Octahedral Hydrido(nitrile)iron(II) Complexes for Nonlinear Optics: X-ray Structure of [Fe(H)(dppe)2(4-NCC6H4NO2)][PF6]·CH2Cl2

A series of ionic pseudo-octahedral trans-hydrido(nitrile)iron(II) complexes with the general formula [Fe(H)(dppe)2(4-NCR)][PF6] [dppe = 1,2-bis(diphenylphosphanyl)ethane; R = acceptor-substituted conjugated ligand] have been synthesised by chloride abstraction from the starting compound trans-[FeHCl(dppe)2] and fully characterised. First hyperpolarisabilities (β) have been determined by hyper-Rayleigh scattering (HRS) at the fundamental wavelength of 1072 nm and the high near-resonant values obtained (up to 1130 × 10–30 esu) are interpreted in terms of the two-level model (TLM) and are correlated with IR and NMR spectroscopic data. Wavelength-dependent HRS has been performed in the 1072–1580 nm range for two of the compounds, namely [Fe(H)(dppe)2{4-NC(CH)(CH)C6H4NO2}][PF6] and [Fe(H)(dppe)2{4-NCC6H4(CH)(CH)C6H4NO2}][PF6]. These results clearly show the two-photon resonance but also the shortcomings of the TLM when it comes to deriving reliable static β values. A structural study of the compound [Fe(H)(dppe)2(4-NCC6H4NO2)][PF6] by X-ray diffraction shows that it crystallises in the centrosymmetric monoclinic space groupP21/n, with four molecules in the unit cell and a pairwise antiparallel alignment of the dipoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)