TY - JOUR
T1 - A unique mechanism for base catalyzed hydrolysis of pentaaminecobalt(III) complexes containing picolyl residues
AU - Dickie, Alistair J.
AU - Hockless, David C.R.
AU - Willis, Anthony C.
AU - McKeon, Josephine A.
AU - Jackson, W. Gregory
PY - 2003/6/16
Y1 - 2003/6/16
N2 - A novel [Co(pentaamine)CI]2+ complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2′-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl]2+ species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl42- salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH]2+ is base catalyzed (kOH 0.70 M-1 s-1, 25 °C, l = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH2- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl]2+ and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD]2+ in OD-show that each act of deprotonation at the acidic methylene leads to loss of Cl-. This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (kH/kD = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.
AB - A novel [Co(pentaamine)CI]2+ complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2′-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl]2+ species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl42- salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH]2+ is base catalyzed (kOH 0.70 M-1 s-1, 25 °C, l = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH2- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl]2+ and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD]2+ in OD-show that each act of deprotonation at the acidic methylene leads to loss of Cl-. This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (kH/kD = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.
UR - http://www.scopus.com/inward/record.url?scp=0037607295&partnerID=8YFLogxK
U2 - 10.1021/ic0206995
DO - 10.1021/ic0206995
M3 - Article
SN - 0020-1669
VL - 42
SP - 3822
EP - 3834
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 12
ER -