Different isomers of [Ru II(NO +)(hedta)(H 2O)] prepared from Ru(NO)Cl 3 via chelation by hedta 3− than by NO 2 − addition to [Ru(H 2O)(hedta)] −

Abstract

NO 2 − substitution on [Ru II(H 2O)(hedta)] −, followed by acidification, forms almost exclusively the cis-equatorial [Ru II(NO +)(hedta)] isomer ( 1a) ( υ NO=1846 cm −1). However, the reaction of a pre-formed ruthenium nitrosyl complex, Ru(NO)Cl 3(H 2O) 2, with hedta 3− (pH∼3.0, 1.5 h, ∼80 °C) produced two different products ( 2a, 2b). Complexes 2a and 2b, [Ru(NO)(hedta)X], have structures that are derived from the energetically much less favorable cis-polar [Ru II(NO +)(hedta)] isomer by a displacement of one glycinato donor by X=H 2O or Cl −, either trans ( 2b) or cis ( 2a) to NO +. Both isomer products ( 2a, 2b) have υ NO=1882 cm −1, indicative of rather similar oxygen donors trans to NO +. Complex 2a (66%) has the NO + group cis to in-plane X=H 2O or Cl − and the two N donors of hedta 3−. Complex 2a also has one axial glycinato chelate from the N-hydroxyethyl nitrogen ( R), and one in-plane glycinato chelate from the second N donor ( R). The ( R, R) placement of the pendant ethyl and glycinato units are in an ‘up,down’ arrangement for 2a, minimizing repulsions for the pendant groups. Complex 2b (33%) has two in-plane glycinato chelate rings and the two in-plane en-type N donors, as for cis-polar [Ru(NO)(hedta)], but with the axial glycinato group of 2a moved to the in-plane location, and retaining the ( R, R) ‘up,down’ chiralities. The coordination assignments for the glycinato rings from 1H and 13C NMR data are supported by MMFF94 calculations wherein 2a ( R, R) (‘up,down’) is more stable than ( R, S) (‘up,up’) by approximately 36 kcal mol −1. The calculations are sensitive to the central angles with the Ru center, which were constrained to 90° angles in the calculations. For lesser isomer 2b, it was determined that the stability of the placement of pendant groups follows the order ( R, R) (‘up,down’)>( S, R) (‘down,down’)∼( S, S) (‘down,up’) >( R, S) (‘up,up’). The Ru II(NO +)/Ru II(NO −) reduction potential of 2a and 2b is −0.12 V versus NHE, similar to −0.10 V for 1a. Since the [Ru(NO)(hedta)] coordination is most favored for a cis-equatorial complex than complexes derived from cis-polar, the addition of the hedta 3− ligand forms 2a and 2b under kinetic control wherein substitution of the first and second N donors occur cis to the NO + group.