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Different Isomers Of [RuII(NO+)(hedta)(H2O)] Prepared From Ru(NO)Cl3 Via Chelation By Hedta3− Than By NO2 − Addition To [Ru(H2O)(hedta)]−
Published 2003 · Chemistry
Abstract NO 2 − substitution on [Ru II (H 2 O)(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 2 O) 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 2 O 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 2 O 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 1 H and 13 C 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.