Synthesis, structure, and kinetics and stereochemistry of base-catalyzed hydrolysis of meso - and rac -[Co 2 (tmpdtne)Cl 2 ] 4+ , bis(pentaamine) complexes devoid of deprotonatable NH centers

Abstract

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co 2(tmpdtne)Cl2]4+ which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co2(tmpdtne)(OH)2]4+ with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl- ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl- via a hydroxobridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The 1H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. δ -0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published 1H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH-], and deprotonation at an α-CH2 center (α to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of kOH for the second-order base-catalyzed reaction are ca. 4.0 M-1 s-1 for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.