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Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site

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Abstract

The reactions of the diruthenium tetrahydrido complex, Cp*Ru(μ-H)4RuCp* (1) (Cp* = η5-C5Me5), with pyridines were investigated in relation to the dehydrogenative coupling of 4-substituted pyridines. Complex 1 reacted with γ-picoline to yield the bis(μ-pyridyl) complex, {Cp*Ru(μ-H)(μ-4-MeC5H3)}2 (2a), with the elimination of dihydrogen. Complex 2a immediately reacted with the liberated dihydrogen to yield μ-η2-dihydrobipyridine (dhbpy) complex 4a via C–C bond formation between the two pyridyl groups, in which one of the pyridine rings underwent partial hydrogenation. The X-ray structure of 4a shows that the dhbpy moiety adopts a μ-η2 coordination mode at the Ru2 site. Complex 4a was reversibly converted to 5a via the elimination of dihydrogen in which the dhbpy moiety adopts a μ-η22 mode. Although 5a was coordinatively saturated, 5a readily reacted with tBuNC to yield 6a. This was owing to the ability of the dhbpy ligand changing its coordination mode between the μ-η22 and μ-η2 modes. This also causes the dehydrogenation from the dhbpy ligand to yield μ-η22-bipyridine complex 7a at 140 °C. However, 7a was not shown to be an intermediate of the catalysis. The reaction of 1 with 1,10-phenanthroline afforded μ-η2-phenanthroline complex 8 containing two hydrides, which can be a model compound for the bipyridine elimination from the Ru2 site. Dynamic NMR studies suggested that 8 was isomerised to an unsaturated μ-N-heterocyclic carbene (NHC) complex. The unsaturated nature of the μ-NHC complex is likely responsible for the uptake of the third pyridine molecule to turn over the catalytic cycle.

Graphical abstract: Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site

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Publication details

The article was received on 04 Apr 2019, accepted on 26 Apr 2019 and first published on 26 Apr 2019


Article type: Paper
DOI: 10.1039/C9FD00029A
Faraday Discuss., 2019, Advance Article

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    Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site

    T. Takao, T. Kawashima, R. Nagae, H. Kanda and W. Watanabe, Faraday Discuss., 2019, Advance Article , DOI: 10.1039/C9FD00029A

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