2-Propanol vs Glycerol as Hydrogen Source in Catalytic Activation of Transfer Hydrogenation with (畏6-Benzene)ruthenium(II) Complexes of Unsymmetrical Bidentate Chalcogen Ligands

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• 作者：Alpesh K. Sharma ; Hemant Joshi ; Kamal Nayan Sharma ; Pancham L. Gupta ; Ajai K. Singh
• 刊名：Organometallics
• 出版年：2014
• 出版时间：July 14, 2014
• 年：2014
• 卷：33
• 期：13
• 页码：3629-3639
• 全文大小：548K
• ISSN：1520-6041

文摘

1H-Benzoimidazole on subjection to a sequence of reactions with benzyl bromide, PhECH₂Cl (E = S, Se), and elemental S or Se results in 1-benzyl-3-phenylchalcogenylmethyl-1,3-dihydrobenzoimidazole-2-chalcogenones (L1鈥?b>L4), which are unsymmetrical bidentate chalcogen ligands having a unique combination of chalcogenoether and chalcogenone donor sites. Half sandwich complexes, [(畏⁶-C₆H₆)Ru(L)Cl][PF₆] (1鈥?b>4), have been synthesized by reactions of [(畏⁶-C₆H₆)RuCl(渭-Cl)]₂ with the appropriate L at room temperature followed by treatment with NH₄PF₆. L1鈥?b>L4 and their complexes 1鈥?b>4 have been authenticated with HR-MS and ¹H, ¹³C{¹H}, and ⁷⁷Se{¹H} NMR spectra. The single-crystal structures of 1鈥?b>4 have been determined by X-ray crystallography. Each L acts as an unsymmetric (E,E) or (E,E鈥? bidentate ligand. The Ru atom in 1鈥?b>4 has pseudo-octahedral half-sandwich 鈥減iano-stool鈥?geometry. The Ru鈥揝 and Ru鈥揝e bond distances (脜) respectively are 2.358(3)/2.3563(18) and 2.4606(11)/2.4737(10) (thio- and selenoether), and 2.4534(17)/2.435(3) and 2.5434(9)/2.5431(10) (thione and selone). Catalytic activation with complexes 1鈥?b>4 has been explored for the transfer hydrogenation (TH) of aldehydes and ketones using various sources of hydrogen. 2-Propanol and glycerol have been compared and found most suitable among the sources screened. The catalytic efficiency of other sources explored, viz. formic, citric, and ascorbic acid, is dependent on the pH of reaction medium and is not promising. A comparative study of 2-propanol and glycerol as hydrogen sources for catalytic activation of TH with 1鈥?b>4 has revealed that with glycerol (for comparable conversion in the same time) more amount of catalyst is needed in comparison to that of 2-propanol. The catalytic process is more efficient with 3 (where Ru is bonded with selone), followed by 1 鈮?4, and 2 showing the least activity among all four complexes. The transfer hydrogenation involves an intermediate containing a Ru鈥揌 bond and follows a conventional alkoxide intermediate based mechanism. The results of DFT calculations appear to be generally consistent with experimental catalytic efficiencies and bond lengths/angles.