Organometallic complexes for nonlinear optics. Part 27. Syntheses and optical properties of some iron, ruthenium and osmium alkynyl complexes

The syntheses of the alkynyl complexes M(4-C≡CC₆H₄NO₂)(dppe)(η-C ₅H₅) [M=Fe (1), Ru (2), Os (3)], Os(4-C≡CC₆H₄NO₂)(PPh₃) ₂(η-C₅H₅) (4) and Ru(4-C≡CC₆H₄NO₂)(CO)₂ (η-C₅H₅) (5) are reported. Structural studies reveal a decrease in Ru-C(1) distance on proceeding from 5 to 2, consistent with greater back-donation of electron density to the alkynyl ligand from the more electron-rich metal center in 2. Electrochemical data show that the M^II/III couple for the dicarbonyl complex 5 is at a significantly more positive potential than that of the related diphosphine complex 2, consistent with ligand variation modifying the electron richness and hence donor strength of the metal center. Time-dependent density functional calculations on model complexes M(4-C≡CC₆H₄NO₂)(PH₃) ₂(η-C₅H₅) (M=Fe, Ru, Os) have been employed to assign the intense low-energy optical transition in these complexes as MLCT in character, the higher energy band being phenyl-phenyl* in nature. Molecular quadratic optical nonlinearities have been measured using the hyper-Rayleigh scattering procedure at 1064 nm. β values vary as Fe≤Ru≤Os for metal variation and CO<phosphines for co-ligand variation, the latter consistent with the variation in donor strength of the metal center inferred from electrochemical and crystallographic data. The observed trend in β on metal variation follows the trend in backbonding energies calculated by DFT.