Enhanced nucleophilic substitution with coordinated 4,4‘-dichloro-2,2‘-bipyridine: X-ray structures of 4,4‘-dichloro-2,2‘-bipyridine (Bipy-Cl2), cis-dichlorobis(4,4‘-dichloro-2,2‘-bipyridine)rhodium(III) hexafluorophosphate [Rh](PF6), and tris(4,4‘-dichloro-2,2‘-bipyridine)ruthenium(II) hexafluorophosphate [Ru](PF6)2

Abstract

The chemical reactivity of 4,4‘-dichloro-2,2‘-bipyridine (bipy-Cl2) changes profoundly upon coordination to a [Ru]2+ center. When not coordinated to [Ru]2+, bipy-Cl2 is relatively unreactive toward nucleophiles; but when coordinated to [Ru]2+, the chlorine atoms become susceptible to nucleophilic displacement.The X-ray structures of bipy-Cl2, cis-dichlorobis (4,4‘-dichloro-2,2‘-bipyridine)rhodium (III) hexafluorophosphate [Rh](PF6), and tris(4,4‘-dichloro-2,2‘-bipyridine)ruthenium (II) hexafluorophosphate [Ru](PF6)2 reveal that the carbon-chlorine bond lengths do not change substantially upon coordination to the rhodium or ruthenium centers – implying that the carbon-chlorine bond strengths also do not change substantially.B3LYP calculations reveal that the standard enthalpy of activation (ΔH°≠) for the nucleophilic substitution of the chlorine atom in [Ru (bipy)2{bipy-Cl}]2+(bipy = 2,2‘-bipyridine; bipy-Cl = 4-chloro-2,2‘-bipyridine) by OCH3− is 46.7 kJ mol−1, while the calculated ΔH°≠ value for the nucleophilic substitution of the chlorine atom in free bipy-Cl by OCH3− is 72.8 kJ mol−1. However, the B3LYP calculations of the ΔH°≠ values for the nucleophilic displacement of the chlorine atom in the cis and trans isomers of [Ru (bipy) (2,2‘-biphenyl){bipy-Cl}], which are neutral complexes, are 76.0 and 73.8 kJ mol−1 respectively – comparable to that for the reaction involving free bipy-Cl. Hence, the calculations suggest that the overall positive charge of the complex is primarily responsible for lowering the activation barrier to nucleophilic substitution in coordinated chloro-bipyridines.