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Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand.

ABSTRACT: Thioethers are good ligands for photoactivatable ruthenium(II) polypyridyl complexes, as they form thermally stable complexes that are prone to ligand photosubstitution. Here, we introduce a novel symmetric chelating bis(thioether) ligand scaffold, based on 1,3-bis(methylthio)-2-propanol (4) and report the synthesis and stereochemical characterization of the series of novel ruthenium(II) polypyridyl complexes [Ru(bpy)₂(L)](PF₆)₂ ([1]-[3](PF₆)₂), where L is ligand 4, its methyl ether, 1,3-bis(methylthio)-2-methoxypropane (5), or its carboxymethyl ether, 1,3-bis(methylthio)-2-(carboxymethoxy)propane (6). Coordination of ligands 4-6 to the bis(bipyridine)ruthenium center gives rise to 16 possible isomers, consisting of 8 possible ? diastereoisomers and their ? enantiomers. We found that the synthesis of [1]-[3](PF₆)₂ is diastereoselective, yielding a racemic mixture of the ?-(S)-eq-(S)-ax-OH_eq-[Ru]²⁺ and ?-(R)-ax-(R)-eq-OH_eq-[Ru]²⁺ isomers. Upon irradiation with blue light in water, [1]-[3](PF₆)₂ selectively substitute their bis(thioether) ligands for water molecules in a two-step photoreaction, ultimately producing [Ru(bpy)₂(H₂O)₂]²⁺ as the photoproduct. The relatively stable photochemical intermediate was identified as cis-[Ru(bpy)₂(?¹-L)(H₂O)]²⁺ by mass spectrometry. Global fitting of the time evolution of the UV-vis absorption spectra of [1]-[3](PF₆)₂ was employed to derive the photosubstitution quantum yields (?₄₄₃) for each of the two photochemical reaction steps separately, revealing very high quantum yields of 0.16-0.25 for the first step and lower values (0.0055-0.0093) for the second step of the photoreaction. The selective and efficient photochemical reaction makes the photocleavable bis(thioether) ligand scaffold reported here a promising candidate for use in e.g. ruthenium-based photo-activated chemotherapy.

SUBMITTER: Meijer MS

PROVIDER: S-EPMC6724527 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand.

Meijer Michael S MS Bonnet Sylvestre S

Inorganic chemistry 20190821 17

Thioethers are good ligands for photoactivatable ruthenium(II) polypyridyl complexes, as they form thermally stable complexes that are prone to ligand photosubstitution. Here, we introduce a novel symmetric chelating bis(thioether) ligand scaffold, based on 1,3-bis(methylthio)-2-propanol (4) and report the synthesis and stereochemical characterization of the series of novel ruthenium(II) polypyridyl complexes [Ru(bpy)2(L)](PF6)2 ([1]-[3](PF<s ...[more]

PMID: 31433170

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Project description:Metal complexes that release ligands upon photoexcitation are important tools for biological research and show great potential as highly specific therapeutics. Upon excitation with visible light, [Ru(TQA)(MeCN)2](2+) [TQA = tris(2-quinolinylmethyl)amine] exchanges one of the two acetonitriles (MeCNs), whereas [Ru(DPAbpy)MeCN](2+) [DPAbpy = N-(2,2'-bipyridin-6-yl)-N,N-bis(pyridin-2-ylmethyl)amine] does not release MeCN. Furthermore, [Ru(TQA)(MeCN)2](2+) is highly selective for release of the MeCN that is perpendicular to the plane of the two axial quinolines. Density functional theory calculations provide a clear explanation for the photodissociation behavior of these two complexes. Excitation by visible light and intersystem crossing leads to a six-coordinate (3)MLCT state. Dissociation of acetonitrile can occur after internal conversion to a dissociative (3)MC state, which has an occupied d?* orbital that interacts in an antibonding fashion with acetonitrile. For [Ru(TQA)(MeCN)2](2+), the dissociative (3)MC state is lower than the (3)MLCT state. In contrast, the (3)MC state of [Ru(DPAbpy)MeCN](2+) that releases acetonitrile has an energy higher than that of the (3)MLCT state, indicating dissociation is unfavorable. These results are consistent with the experimental observations that efficient photodissociation of acetonitrile occurs for [Ru(TQA)(MeCN)2](2+) but not for [Ru(DPAbpy)MeCN](2+). For the release of the MeCN ligand in [Ru(TQA)(MeCN)2](2+) that is perpendicular to the axial quinoline rings, the (3)MLCT state has an occupied quinoline ?* orbital that can interact with a d?* Ru-NCCH3 antibonding orbital as the Ru-NCCH3 bond is stretched and the quinolines bend toward the departing acetonitrile. This reduces the barrier for the formation of the dissociative (3)MC state, leading to the selective photodissociation of this acetonitrile. By contrast, when the acetonitrile is in the plane of the quinolines or bpy, no interaction occurs between the ligand ?* orbital and the d?* Ru-NCCH3 orbital, resulting in high barriers for conversion to the corresponding (3)MC structures and no release of acetonitrile.

Dataset Information

Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand.

Publications

Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand.

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