pH-Dependent Selective Transfer Hydrogenation of α,β-Unsaturated Carbonyls in Aqueous Media Utilizing Half-Sandwich Ruthenium(II) Complexes

Half-sandwich ruthenium(II) PTA complexes bearing the 1,2-dihydropentalenyl (C8H9-, Dp) and indenyl
(C9H7-, Ind) ancillary ligands have been synthesized and characterized using multinuclear NMR
spectroscopy and X-ray crystallography. The complexes DpRu(PTA)(PPh3)Cl, DpRu(PTA)2Cl, IndRu(PTA)(PPh3)Cl, and [IndRu(PTA)2(PPh3)]Cl were obtained in good to excellent yields. The solid-state
structures of these compounds exhibit piano stool geometries with η5-coordination of the indenyl and
dihydropentalenyl moieties. DpRu(PTA)2Cl is water-soluble (S25°C = 43 mg/mL), while the mixed
phosphine compounds are slightly soluble in acidic solutions. The Ru−H complexes, Cp‘Ru(PTA)(PPh3)H
(Cp‘ = Ind, Cp), have been synthesized in good yield and spectroscopically and structurally characterized.
The ruthenium hydrides undergo an H/D exchange reaction with CD3OD with relative rates CpRu(PTA)(PPh3)H ≫ IndRu(PTA)(PPh3)H > CpRu(PTA)2H. The air-stable Cp‘Ru(PTA)(PR3)Cl complexes (Cp‘
= Cp, Dp, Ind; PR3 = PPh3 or PTA) exhibit activity in the regioselective transfer hydrogenation of
α,β-unsaturated carbonyls in aqueous media with HCOONa, HCOOH, or isopropanol/Na2CO3 serving
as the hydrogen source. They were found to be effective in the selective reduction of the carbonyl
functionality of cinnamaldehyde and the CC bond of benzylidene acetone and chalcone. IndRu(PTA)(PPh3)Cl was less active than Cp‘Ru(PTA)(PPh3)Cl (Cp‘ = Cp or Dp). Results of the transfer hydrogenation
of unsaturated substrates using CpRu(PTA)(PPh3)H are also reported.

本研究合成了带有1,2-二氢戊搭烯基（1,2-dihydropentalenyl，缩写为Dp，化学式C8H9-）与茚基（indenyl，缩写为Ind，化学式C9H7-）辅助配体的半夹心型钌(II) 1,3,5-三氮杂-7-磷杂金刚烷（PTA）配合物，并通过多核核磁共振波谱法与X射线晶体衍射法对其进行了表征。目标配合物DpRu(PTA)(PPh3)Cl、DpRu(PTA)2Cl、IndRu(PTA)(PPh3)Cl以及[IndRu(PTA)2(PPh3)]Cl均以良好至优异的产率获得。上述化合物的固态结构均呈现钢琴凳构型，其中茚基与二氢戊搭烯基片段以η5配位模式结合。DpRu(PTA)2Cl具有水溶性（25℃下溶解度为43 mg/mL），而混合膦配体配合物仅在酸性溶液中微溶。钌氢配合物Cp'Ru(PTA)(PPh3)H（Cp'=Ind、Cp，其中Cp为环戊二烯基）以良好产率合成，并经光谱及结构表征。该类钌氢化物与氘代甲醇（CD3OD）发生氢氘交换反应，相对反应速率顺序为：CpRu(PTA)(PPh3)H ≫ IndRu(PTA)(PPh3)H > CpRu(PTA)2H。空气稳定的Cp'Ru(PTA)(PR3)Cl型配合物（Cp'=Cp、Dp、Ind；PR3=三苯基膦（PPh3）或PTA）可在水相介质中催化α,β-不饱和羰基化合物的区域选择性转移氢化反应，反应以甲酸钠（HCOONa）、甲酸（HCOOH）或异丙醇/碳酸钠（Na2CO3）作为氢源。该类配合物可选择性还原肉桂醛的羰基官能团，以及亚苄基丙酮与查尔酮的碳碳双键。IndRu(PTA)(PPh3)Cl的催化活性低于Cp'Ru(PTA)(PPh3)Cl（Cp'=Cp或Dp）。本研究同时报道了以CpRu(PTA)(PPh3)H为催化剂时，不饱和底物的转移氢化反应结果。