Formation of η²-Coordinated Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium Cations

Abstract

Reactions between various pyridinium cations with and without a −CF3 substituent at the 3-position and [Ru(tpy)(bpy)H]+ (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 η2 mode through a C═C 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–(C═C) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru(tpy)(bpy)(NCCH3)]2+ 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.