Pyridine–Quinoline and Biquinoline-Based Ruthenium <i>p</i>-Cymene Complexes as Efficient Catalysts for Transfer Hydrogenation Studies: Synthesis and Structural Characterization
Searching for new and efficient transfer hydrogenation catalysts, a series of new organometallic ruthenium(II)-arene complexes of the formulae [Ru(η<sup>6</sup>-<i>p</i>-cymene)(L)Cl][PF<sub>6</sub>] (<b>1</b>–<b>8</b>) and [Ru(η<sup>...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-07-01
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| Series: | Molecules |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1420-3049/30/14/2945 |
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| Summary: | Searching for new and efficient transfer hydrogenation catalysts, a series of new organometallic ruthenium(II)-arene complexes of the formulae [Ru(η<sup>6</sup>-<i>p</i>-cymene)(L)Cl][PF<sub>6</sub>] (<b>1</b>–<b>8</b>) and [Ru(η<sup>6</sup>-<i>p</i>-cymene)(L)Cl][Ru(η<sup>6</sup>-<i>p</i>-cymene)Cl<sub>3</sub>] (<b>9</b>–<b>11</b>) were synthesized and fully characterized. These were prepared from the reaction of pyridine–quinoline and biquinoline-based ligands (L) with [Ru(η<sup>6</sup>-<i>p</i>-cymene)(μ-Cl)Cl]<sub>2</sub>, in 1:2 and 1:1, metal (M) to ligand (L) molar ratios. Characterization includes a combination of spectroscopic methods (FT-IR, UV-Vis, multi nuclear NMR), elemental analysis and single-crystal X-ray crystallography. The pyridine–quinoline organic entities encountered, were prepared in high yield either via the thermal decarboxylation of the carboxylic acid congeners, namely 2,2′-pyridyl-quinoline-4-carboxylic acid (<b>pqca</b>), 8-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (<b>8-Mepqca</b>), 6′-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (<b>6′-Mepqca</b>) and 8,6′-dimethyl-2,2′-pyridyl-quinoline-4-carboxylic acid (<b>8,6′-Me<sub>2</sub>pqca</b>), affording the desired ligands <b>pq</b>, <b>8-Mepq, 6′-Mepq</b> and <b>8,6′-Me<sub>2</sub>pq</b>, or by the classical Friedländer condensation, to yield 4,6′-dimethyl-2,2′-pyridyl-quinoline (<b>4,6′-Me<sub>2</sub>pq</b>) and 4-methyl-2,2′-pyridyl-quinoline (<b>4-Mepq</b>), respectively. The solid-state structures of complexes <b>1</b>–<b>4</b>, <b>6, 8</b> and <b>9</b> were determined showing a distorted octahedral coordination geometry. The unit cell of <b>3</b> contains two independent molecules (Ru-<b>3</b>), (Ru′-<b>3</b>) in a 1:1 ratio, due to a slight rotation of the arene ring. All complexes catalyze the transfer hydrogenation of acetophenone, using 2-propanol as a hydrogen donor in the presence of KO<i>i</i>Pr. Among them, complexes <b>1</b> and <b>5</b> bearing methyl groups at the 8 and 4 position of the quinoline moiety, convert acetophenone to 1-phenylethanol quantitatively, within approximately 10 min with final TOFs of 1600 h<sup>−1</sup>. The catalytic performance of complexes <b>1</b>–<b>11</b>, towards the transfer hydrogenation of <i>p</i>-substituted acetophenone derivatives and benzophenone, ranges from moderate to excellent. An inner-sphere mechanism has been suggested based on the detection of ruthenium(II) hydride species. |
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| ISSN: | 1420-3049 |