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

Kundu, Tanaya ; Mobin, Shaikh M. ; Lahiri, Goutam Kumar (2010) 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 Dalton Transactions, 39 (17). pp. 4232-4242. ISSN 0300-9246

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Official URL: http://pubs.rsc.org/en/content/articlelanding/2010...

Related URL: http://dx.doi.org/10.1039/B919036H

Abstract

The paramagnetic ruthenium-biimidazole complexes [(acac)2RuIII(LH-)] (1 = red-brown), [(acac)2RuIII(LH2)](ClO4) (2 = pink) and Bu4N[(acac)2RuIII(L2-)] (3 = greenish yellow) comprising of monodeprotonated, neutral and bideprotonated states of the coordinated biimidazole ligand (LHn, 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)2RuIII(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 Δ g 0.65-0.77, typical for distorted octahedral ruthenium(III) complexes. The complexes 1-3 are inter-convertible as a function of pH. The pKa1 and pKa2 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-.

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Deposited On:29 Mar 2012 04:45
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