Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule

The
complexes trans-[RuIII(NH3)4(4-pic)­(H2O)]­(CF3SO3)3 (1) and [RuIII(NH3)5(4-pic)]­(CF3SO3)3 (2) were isolated and studied experimentally by electron paramagnetic
resonance (EPR) and UV–vis spectroscopies, cyclic voltammetry,
and X-ray crystallography and theoretically by ligand-field theory
(LFT) and density functional theory (DFT) calculations. Complex 1 is reported in two different crystal forms, 1a (100 K) and 1b (room temperature). EPR and UV–vis
spectroscopies suggest that aqua ligand interaction in this low-spin
ruthenium­(III) complex changes as a function of hydrogen bonding with
solvent molecules. This explicit water solvent effect was explained
theoretically by DFT calculations, which demonstrated the effect of
rotation of the aqua ligand about the Npic–Ru–Oaq axis. The UV–vis spectrum of 1 shows
in an aqueous acid solution a broad- and low-intensity absorption
band around 28 500 cm–1 (ε ≈
500 M–1 cm–1) that is assigned
mainly to a charge-transfer (CT) transition from the equatorial ligands
to the Ru β-4dxy orbital (β-LUMO)
using DFT calculations. The electronic reflectance spectrum of 1 shows a broad and intense absorption band around 25 500
cm–1 that is assigned to a CT transition from 4-picoline
to the Ru β-4dxz orbital (β-LUMO)
using DFT calculations. The t2g5 set of orbitals
had its energy splitting investigated by LFT. LFT analysis shows that
a rhombic component arises from C2v symmetry by a simple π-bonding ligand (H2O in our case) twisting about the trans (C2) axis. This twist was manifested in the EPR spectra, which were
recorded for 1 as a function of the solvent in comparison
with [Ru­(NH3)5(4-pic)]3+ and [Ru­(NH3)5(H2O)]3+. Only 1 shows an evident change in the g-tensor values, wherein
an increased rhombic component is correlated with a higher nucleophilicity
(donor) solvent feature, as parametrized by the Abraham system.