Paramagnetic ruthenium-biimidazole derivatives [(acac)2RuIII(LHn)]m, n/m = 2/+, 1/0, 0/−. Synthesis, structures, solution properties and anion receptor features in solution state

Abstract

The paramagnetic ruthenium-biimidazole complexes [(acac)₂Ru^III(LH⁻)] (1 = red-brown), [(acac)₂Ru^III(LH₂)](ClO₄) (2 = pink) and Bu₄N[(acac)₂Ru^III(L²⁻)] (3 = greenish yellow) comprising of monodeprotonated, neutral and bideprotonated states of the coordinated biimidazole ligand (LH_n, n = 1, 2, 0), respectively, have been isolated (acac⁻ = acetylacetonate). Single-crystal X-ray diffraction of 1 reveals that the asymmetric unit consists of three independent molecules: A–C, where molecule A corresponds to complex 1 and the other two molecules B and C co-exist as a hydrogen bonded dimeric unit perhaps between the cationic 2⁺ and anionic 3⁻. The packing diagram further reveals that the molecule A in the crystal of 1 also forms a hydrogen bonded dimer with the neighbouring another unit of molecule A. The formation of [(acac)₂Ru^III(LH₂)](ClO₄) (2) has also been authenticated independently by its single-crystal X-ray structure. The packing diagram of 2 shows multiple hydrogen bonds between the N–H protons of coordinated LH₂ and the counter ClO₄⁻. Paramagnetic complexes show ¹H NMR spectra over a wide range of chemical shift, δ (ppm), +10 to −35 in CDCl₃. One-electron paramagnetic 1–3 (μ/B.M. ∼1.9) exhibit distinct rhombic-EPR spectra with relatively large g anisotropic factors: <g> 2.136–2.156 and Δg 0.65–0.77, typical for distorted octahedral ruthenium(III) complexes. The complexes 1–3 are inter-convertible as a function of pH. The pK_a1 and pK_a2 of 6.8 and 11, respectively, for 2 are estimated by monitoring the pH dependent spectral changes. The Ru(III)–Ru(IV) couple near 1.25 V vs. SCE remains almost invariant in 1–3 whereas the corresponding Ru(III)–Ru(II) couple varies appreciably in the range of −0.52 to −0.85 V vs. SCE based on the protonated-deprotonated states of the coordinated biimidazole ligand. Compounds 1–3 exhibit one weak ligand to metal charge transfer (LMCT) transition near 500 nm and intense intraligand transitions in the higher energy UV region. The spectrophotometric titrations of 2 with the TBA (TBA = tetrabutylammonium) salts of a wide variety of anions, F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, OAc⁻, H₂PO₄⁻ in CH₃CN reveal that the possible hydrogen bonds between the N–H protons of LH₂ in 2 and Cl⁻ or Br⁻ or I⁻ or HSO₄⁻ or H₂PO₄⁻ anion are rather weak or negligible. However, in presence of excess H₂PO₄⁻ anion, the molar ratio of 2 to H₂PO₄⁻ being 1 : 4, simple liberation of one N–H proton of the coordinated LH₂ in 2 has been taken place which in effect yields 1 and H₃PO₄. On the contrary, the spectrophotometric titrations of 1 : 1 molar solution of 2 and OAc⁻ or F⁻ anion suggest the initial formation of hydrogen bonds between the N–H protons of LH₂ in 2 and the anion with the calculated log K value of 5.92 or 4.7, respectively, which eventually leads to the transfer of one of the N–H protons of LH₂ in 2 to the anion, resulting in 1 and HOAc or HF. On addition of excess OAc⁻ to the above solution of 1 (molar ratio of OAc⁻ to 1, 4 : 1), further hydrogen bonding between the N–H proton of LH⁻ in 1 and OAc⁻ occurs but without the abstraction of the N–H proton of LH⁻. However, excess F⁻ anion concentration (molar ratio of anion to 1, 5 : 1) facilitates the removal of the remaining N–H proton of LH⁻ in 1 which in turn yields 3 incorporating the bideprotonated form L²⁻.