Solid-State Changes in Ligand-to-Metal Charge-Transfer Spectra of (NH3)5RuIII(2,4-dihydroxybenzoate) and (NH3)5RuIII(xanthine) Chromophores

• Published in: Inorganic chemistry Vol. 47; no. 21; pp. 9813 - 9827
• Main Authors: Krogh-Jespersen, Karsten, Stibrany, Robert T, John, Elizabeth, Westbrook, John D, Emge, Thomas J, Clarke, Michael J, Potenza, Joseph A, Schugar, Harvey J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 03-11-2008
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic800511g

Five distorted-octahedral complexes containing (NH3)5RuIIIL ions, where L = 2,4-dihydroxybenzoate or a xanthine, have been studied using a combination of X-ray crystallography, solution and polarized single-crystal electronic absorption spectroscopy, and first principles electronic structure computational techniques. Both yellow (2) and red (3) forms of the complex (NH3)5RuIIIL, where L = 2,4-dihydroxybenzoate, as well as three xanthine complexes in which L = hypoxanthine-κN 7(4), 7-methylhypoxanthine-κN 9(5), and 1,3,9-trimethylxanthine-κN 7(6) were examined. In the solid state, some of these complexes exhibit split low-energy ligand-to-metal charge-transfer (LMCT) bands. Traditional solid-state effects, such as ligand π−π overlap or hydrogen bonding that might lead to splitting of electronic absorption bands, were probed in an attempt to identify the origins of these unusual observations. For comparison, companion studies were carried out for spectroscopically normal reference complexes of the same ligands. Time-dependent density-functional theory (TD-DFT) calculations, employing modified B3LYP-type functionals with increased contributions of exact exchange, attribute the color change in the crystalline complexes 2 and 3 to π(ligand) → Ru[d(π)] LMCT bands which, in the red form (3), arise from ligand donor π-orbitals split by strongly overlapping phenyl moieties in centrosymmetric (NH3)5RuIII(2,4-dihydroxybenzoate) dimers. Complex 5 does not show split visible absorptions, whereas both the polarizations and energies of the split visible absorptions shown by 4 and 6 also suggest assignment as LMCT. No support is found for relating the split absorptions of 4 and 6 to the details of π−π xanthine overlap in the solid state; indeed, complex 4 enjoys considerably less π-stacking overlap than does 5. We feel compelled to attribute the split absorptions in crystalline 4 and 6 to the emergence of a LMCT transition originating in the carbonyl lone pair, potentially deriving intensity from the significant intramolecular N−H···O hydrogen bonding present in both 4 and 6 (but not in 5). The electronic structure calculations suggest an O(n) → Ru[d(σ*)] LMCT transition; however, this novel assignment must be considered tentative.