Formation of η<sup>2</sup>‑Coordinated
Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer
from Ruthenium(II) to Pyridinium Cations
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Abstract
Reactions
between various pyridinium cations with and without a
−CF<sub>3</sub> substituent at the 3-position and [Ru(tpy)(bpy)H]<sup>+</sup> (tpy = 2,2′:6′,2″-terpyridine and bpy
= 2,2′-bipyridine) were investigated in detail. The corresponding
1,4-dihydropyridines coordinating to a Ru(II) complex in η<sup>2</sup> mode through a CC bond were quantitatively formed
at the initial stage. The only exception observed was in the case
of the 1-benzylpyridinium cation, where a mixture of two adducts with
1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio
96:4. Cleavage of the Ru–(CC) bond proceeded at a slower
rate in all reactions, giving the corresponding dihydropyridine and
[Ru(tpy)(bpy)(NCCH<sub>3</sub>)]<sup>2+</sup> when acetonitrile was
used as a solvent. Kinetic activation parameters for the adduct formation
indicated that the 1,4-regioselectivities were induced by formation
of sterically constrained structures