Organometallic complexes for nonlinear optics. 15. Molecular quadratic hyperpolarizabilities of trans-bis{bis(diphenylphosphino)methane}ruthenium σ-aryl- and σ-pyridyl-acetylides: X-ray crystal structure of trans-[Ru(2-CCC 5H 3N-5-NO 2)Cl(dppm) 2

Abstract

The complexes trans-[Ru(2-CCC 5H 3N-5-R)Cl(dppm) 2] [R=H ( 6), NO 2 ( 7)] have been prepared and 7 has been structurally characterized; comparison with the structural study of trans-[Ru(4-CCC 6H 4NO 2)Cl(dppm) 2] ( 3) reveals a decrease in Ru–C(1) distance and increase in Ru–Cl(1) distance, consistent with stronger σ-bonding by the nitropyridylalkynyl ligand in 7 compared to the nitrophenylalkynyl ligand in 3. Electrochemical data for 3, 6, 7, precursor dichloro complexes [RuCl 2(dppm) 2] [ cis ( 1a), trans ( 1b)], and related alkynyl complexes trans-[Ru(4-CCC 6H 4R)Cl(dppm) 2] [R=H ( 2), 4-C 6H 4NO 2 ( 4), ( E)-4-CHCHC 6H 4NO 2 ( 5)] have been determined by cyclic voltammetry. Introduction of nitro substituent in progressing from 2 to 3 or 6 to 7 leads to a substantial increase in E o Ru II/III, most of which is lost on chain-lengthening the alkynyl ligand in progressing from 3 to 4 or 5. Replacement of phenyl by pyridyl in progressing from 2 to 6 or 3 to 7 results in a 0.1 V increase in E o Ru II/III. The optical spectra of representative complexes have been examined. Introduction of a nitro substituent onto the phenylalkynyl ligand, in progressing from 2 to 3, results in a substantial (ca. 11 000 cm −1) red-shift in the intense MLCT band of the Ru II–CC–Ar–NO 2 moiety. Complexes 3, 4 and 5 exhibit sizable solvatochromic shifts, suggestive of significant optical nonlinearities. Quadratic hyperpolarizabilities for 2– 7 were determined by hyper-Rayleigh scattering (HRS) at 1064 nm; measurements are consistent with an increase in β HRS upon incorporation of nitro substituent (progressing from 2 to 3 or 6 to 7), chain-lengthening the alkynyl ligand (progressing from 3 to 4 and then to 5) and replacing phenyl by pyridyl in progressing from 2 to 6, general trends that are maintained with the two-level-corrected data, and which parallel shifts in λ max to low energy. The observed and two-level-corrected β HRS values for 7 are lower than expected; it is perhaps significant that λ max for 7 is close to the second-harmonic.