The Milstein Bipyridyl PNN Pincer Complex of Ruthenium Becomes a Noyori-Type Catalyst under Reducing Conditions

Hydrogenation of the dearomatized
PNN ligand of the Milstein bipyridyl
complex RuH­(CO)­[PNN] (2) gives a square-pyramidal Ru­(II)
product RuH­(CO)­[pPNN] (5). The central ring of the pPNN
ligand is a piperidine. A minor byproduct of the hydrogenation reaction
is complex 6 which has a dimeric structure made of two
Ru­(II) fragments each possessing a partly hydrogenated PNN ligand.
The structures of 5 and 6 have been elucidated
by NMR spectroscopy and X-ray crystallography. The PNN ligand of 2 is also hydrogenated under the conditions of the catalytic
dehydrogenative coupling of ethanol to ethyl acetate. No direct evidence
of the aromatized dihydride RuH2(CO)­[PNN] (4) was found in this study. However, treating RuHCl­(CO)­[PNN] with
Li­[HBEt3] or reacting 2 with H2 at low temperature resulted in a structurally characterized hydride-bridged
dimer (7) bearing intact aromatized bipyridyl ligands.
M06-L/def2-QZVP DFT calculations provided insights into the thermodynamics
of the stoichiometric reactions of this work and into the nature of
the intermediates of the catalytic ester hydrogenation facilitated
by RuH2(CO)­[pPN­(H)­N] (8) formed from 5 under H2.

米尔斯坦联吡啶配合物RuH(CO)[PNN]（2）的脱芳构化PNN配体的氢化反应，可得到四方锥构型的Ru(II)产物RuH(CO)[pPNN]（5）。pPNN配体的中心环为哌啶环。该氢化反应的次要副产物为配合物6，其具有双聚结构，由两个Ru(II)结构单元组成，每个单元均带有部分氢化的PNN配体。配合物5与6的结构已通过核磁共振（NMR）波谱学与X射线晶体学解析确定。在乙醇催化脱氢偶联生成乙酸乙酯的反应条件下，配合物2的PNN配体同样会发生氢化反应。本研究未发现芳构化二氢化物RuH₂(CO)[PNN]（4）的直接证据。然而，将RuHCl(CO)[PNN]与三乙基氢硼化锂（Li[HBEt₃]）进行反应，或在低温下使配合物2与氢气发生反应，均可得到经结构表征的氢桥联双聚配合物7，该配合物带有完整的芳构化联吡啶配体。采用M06-L/def2-QZVP水平的密度泛函理论（Density Functional Theory, DFT）计算，为本研究中计量反应的热力学过程，以及由5在氢气氛围下生成的RuH₂(CO)[pPN(H)N]（8）所催化的酯加氢反应的中间体本质，提供了理论见解。