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-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.