Synthesis, Structures, and Optical Properties of Ruthenium(II) Complexes of the Tris(1-pyrazolyl)methane Ligand

Four
new complex salts [RuIICl­(Tpm)­(LA)2]­[PF6]n [Tpm = tris­(1-pyrazolyl)­methane; n = 1, LA = pyridine (py) 1 or ethyl
isonicotinate (EIN) 2; n = 3, LA = N-methyl-4,4′-bipyridinium (MeQ+) 3 or N-phenyl-4,4′-bipyridinium
(PhQ+) 4] have been prepared and characterized.
Electronic absorption spectra show intense d → π* metal-to-ligand
charge-transfer (MLCT) absorption bands, while cyclic voltammetry
reveals a reversible RuIII/II wave, accompanied by quasireversible
or irreversible LA-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me2CO, 2, and 3•Me2CO. For 2–4, molecular first hyperpolarizabilities β have
been measured in acetonitrile solutions via the hyper-Rayleigh scattering
(HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic
studies on the MLCT bands in frozen butyronitrile allow the indirect
estimation of static first hyperpolarizabilities β0. The various physical data obtained for 3 and 4 are compared with those reported previously for
related cis-{RuII(NH3)4}2+ species [Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845]. TD-DFT calculations
on the complexes in 1–4 confirm that
their lowest energy absorption bands are primarily RuII → LA MLCT in character, while RuII →
Tpm MLCT transitions are predicted at higher energies. DFT agrees
with the Stark, but not the HRS measurements, in showing that β0 increases with the electron-accepting
strength of LA. The 2D nature of the chromophores is evidenced
by dominant βxxy tensor components.