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Planar-Locked Ru-PNN Catalysts in 1‑Phenylethanol Dehydrogenation
Ru-PNN pincer catalysts of the general form [{PNN}Ru(H)(Cl)(CO)] can dehydrogenate alcohols through inner- and outer-sphere mechanisms, but determining the favored path is challenging. To address this challenge, the following planar-locked quinoline-based PNN ligands, which cannot form key inner...
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Published in: | Organometallics 2020-10, Vol.39 (20), p.3628-3644 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | Ru-PNN pincer catalysts of the general form [{PNN}Ru(H)(Cl)(CO)] can dehydrogenate alcohols through inner- and outer-sphere mechanisms, but determining the favored path is challenging. To address this challenge, the following planar-locked quinoline-based PNN ligands, which cannot form key inner-sphere transition states and intermediates, were synthesized: 2-((ditertbutylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNPtBu), 2-((diisopropylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNPiPr), and 2-((diphenylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNPPh). In addition to the quinoline-derived ligands, we also prepared the isoquinoline PNN ligand N-((1-((ditert-butylphosphaneyl)methyl)isoquinolin-3-yl)methyl)-N-ethylethanamine (IsoQNP) and two known picoline- and lutidine-derived ligands 2-((ditert-butylphosphaneyl)methyl)pyridine (PicP) and 2-((ditert-butylphosphaneyl)methyl)-6-methylpyridine (LutP). These six ligands were coordinated to Ru(II) ions to prepare six new complexes of the general formulation [{L}Ru(H)(Cl)(CO)] analogous to Milstein’s PNN catalyst precursor (1PyCl). The X-ray structural, NMR, UV–vis, and FTIR spectroscopic properties of the new complexes are similar to parent complex 1PyCl and were used in catalytic 1-phenylethanol acceptor-less and transfer dehydrogenation. The comparative results demonstrate that 1Py outperforms the other catalysts. DFT reaction profiles were computed for 1Py and the planar-locked catalysts. The results suggest that 1Py has access to a lower-energy inner-sphere path, whereas the planar-locked catalysts can only proceed through a high-energy outer-sphere mechanism and may even get trapped in unreactive alkoxide sinks. |
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ISSN: | 0276-7333 1520-6041 |
DOI: | 10.1021/acs.organomet.0c00327 |