Synthesis and rearrangements of the pentaamminecobalt(III) linkage isomers of some amidic acids

Abstract

The unidentate pentaamminecobalt(III) linkage isomers of phthalamic and malonamic acids bonded through the deprotonated amide nitrogen or the carboxylate group have been synthesized and characterized. The carboxylate-bonded complexes were synthesized directly from their amidic acids; they, as well as carboxylate-bonded succinamic acid, solvolyzed very slowly in aqueous acid ( 1 M, t 1 2 days) and at pH 10 they decomposed very slowly; the amide group was not hydrolyzed while the amidic acid ligand was bonded to the metal. The malonamato- N complex was prepared by base hydrolysis at pH 7 of the corresponding cyanoacetate- N complex. Pentaammine (phthalamato- N)cobalt(III) was synthesized by base catalyzed hydration of the corresponding phthalimido- N complex; the latter was prepared directly from free ligand and pentaammine (dimethyl sulfoxide) cobalt(III). The rate law for the base catalyzed hydration (10 −7 M ≤ [OH −] ≤ 0.05 M) of the phthalimido- N complex is: k ohs = k OH [OH −] where K OH = 0.40 M −1 s 1 (25°C, I = 1.0 M (NaCl). In acid the amido- N linkage isomers are protonated on both the amide oxygen and on the carboxylate group. In water they undergo the usual solvolysis and amide- N to O rearrangement, but, concurrently, amide- N to carboxylate-bonded rearrangement. This first-formed amide- O complexes aquate relatively rapidly so that the observed products were the pentaammineaquacobalt(III) complex and the carboxylate-bonded complexes of malonamic acid (73%), phthalamic acid (36%) and succinamic acid (24%), respectively. In Me 2SO-d 6 amide- N to O and amide- N to carboxylate-bonded rearrangements were observed directly, with the ultimate products being pentaammine (dimethyl sulfoxide) cobalt(III) and the carboxylate-bonded complexes of malonamic acid (10%), phthalamic (33%) and succinamic acid (5%), respectively. The effects of a one-versus two-C length organic backbone, flexible (−CH 2CH 2−) versus rigid ( o-phenylene) geometry, and solvent (H 2O versus Me 2SO) in influencing the competitive rearrangements are considered.