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 planarlocked quinoline-based PNN ligands, which cannot form key inner-sphere transition states and intermediates, were synthesized: 2-((ditertbutylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNP tBu ), 2-((diisopropylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNP iPr ), and 2-((diphenylphosphaneyl)methyl)-N,N-diethylquinolin-8-amine (QNP Ph ). 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.
The formation of TEMPOH from a mixture of [Mn(CO)3(μ3-OH)]4 (1) and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is shown to occur through a light-initiated CO photolysis from 1 (illumination at 300–375 nm). One hypothesis is that the loss of carbon monoxide (CO) causes significant O–H bond weakening to render proton-coupled electron transfer (PCET) to TEMPO favorable. For instance, the ground-state O–H bond dissociation free energy (BDFEO–H) of 1 (computed with density functional theory and estimated using effective BDFE reagents) is too high to transfer an H-atom to TEMPO. We also demonstrate that TEMPO and 1 interact in the dark through a hydrogen-bonded “precomplex” (1···TEMPO). We suggest that the PCET reaction that forms TEMPOH is the result of a H-atom-transfer reaction that occurs immediately after photolysis of a CO ligand(s).
Metal-ligand cooperativity (MLC) involving reversible aromatization/dearomatization of pyridine-derived pincer ligands is considered important in acceptorless (de)hydrogenation for dihydrogen generation and storage. Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties. Thus, we were surprised to find an aromatized kappa-3-NCP binding mode in [{LutP`}Ru(CO)(H)(PPh3)] (2) upon dehydrohalogenation of the lutidine-derived PN/P complex [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1) with KOtBu. The reaction of H2 with 2 results in formation of a cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photochemistry that we leveraged to demonstrate a unique strategy for unsensitized single-component photocatalytic H2 production via acceptorless alcohol dehydrogenation. Although labeling studies implicate MLC processes during the photocatalytic reaction, they may be off-path intermediates, emphasizing that aromatization/dearomatization may not be necessary for acceptorless transformations.
Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties, such as in Milstein’s PNN-Ru(CO)(Cl)(H) (1Py) catalyst. Thus, we were surprised to find an aromatized κ3-N,C,P binding mode in the lutidine-derived bidentate analogue [{LutP′}Ru(CO)(H)(PPh3)] (2), instead of a dearomatized compound, upon dehydrohalogenation of [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1). The reaction of 2 with H2 results in the formation of the cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3), and labeling studies confirm cooperative metal–ligand activation. 3 exhibits reversible photoisomerization, forming another cis-dihydride isomer (4) upon irradiation. The lability of 4 toward ligand substitution was leveraged to demonstrate photoenhanced H2 production via acceptorless alcohol dehydrogenation. Labeling studies implicate metal–ligand cooperative (MLC) processes during the photocatalytic reaction, but they appear to be off-path processes on the basis of our mechanistic study of the system. The latter emphasizes that aromatization/dearomatization may not be necessary for acceptorless transformations, which is generally consistent with several contemporary studies on analogous Ru catalysts.
Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often lead to dearomatized moieties, such as in Milstein's PNN-Ru(CO)(Cl)(H) (1Py) catalyst. Thus, we were surprised to find an aromatized k 3 -N,C,P binding mode in the lutidine-derived bidentate analog [{LutP`}Ru(CO)(H)(PPh3)] (2), instead of a dearomatized compound, upon dehydrohalogenation of [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1). The reaction of 2 with H2 results in formation of a cis-dihydride [{LutP}Ru(CO)(H)2( PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photochemistry, forming another cis-dihydride isomer (4). The lability of 4 toward ligand substitution was leveraged to demonstrate a unique example of photoswitchable H2 production via acceptorless alcohol dehydrogenation. Labeling studies implicate metal-ligand cooperative (MLC) processes during the photocatalytic reaction, but they appear to be off-path processes based on our mechanistic study of the system. The latter emphasizes that aromatization/dearomatization may not be necessary for acceptorless transformations, which is generally consistent with several contemporary studies on analogous Ru catalysts.
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