Meridional anchorage of coordinate occupancy by a planar tridentate ligand and its effect on ligand substitution reactions of octahedral ruthenium(II) complexes

Abstract

A kinetic study of the substitution behavior of octahedral [Ru(terpy)(bipy)(OH2)]2+ and [Ru(terpy)(tmen)(OH2)]2+ {terpy = 2,2:6′,2″-terpyridine, bipy = 2,2′-bipyridine and tmen = N,N,N′,N′-tetramethylethylenediamine} with thiourea, 1,3′-dimethyl-2-thiourea, and acetonitrile nucleophiles (Nu) as a function of concentration in pH of 4.0 aqueous media using UV-Vis spectroscopy has been made. The reactions are first order in both the concentration of the Nu and the ruthenium complex in accordance to the two-term rate law k obs = k 2[Nu] + k− 2. The ligand effect of the cis-coordinated bidentates (NN) on the substitutional lability of the aqua leaving group in the [Ru(terpy)(NN)(OH2)]2+ complexes increases in the order: NN = dppro < dopro < phen ≈ bipy < tmen < diox < Me2phen. This order reflects the steric as well as the electronic properties of the bidentate ligand where the meridionally coordinated terpy enacts stereoelectronic rigidity on the bidentate ligand in addition to providing an efficient drainage of electron density at the metal centers. In the tmen complex, the retardation of the incoming groups caused by a dominant cis σ-effect from the tmen toward the metal center controls the rate of the reaction, as a result of the induced weakening of the scorpionatic effect of the steric tmen ligand due to the strong π-repulsive backbone of the meridionally coordinated terpy.