SYNTHESIS AND REACTIVITY OF THE PENTAAMMINECOBALT(III) LINKAGE ISOMERS OF SUCCINIMIDE

The linkage isomers of (succinimido)pentaamminecobalt(III) have been selectively synthesized and characterized by H-1 and C-13 NMR, IR, and UV-visible spectroscopies. The deprotonated imide ligand bonds to the metal through oxygen or nitrogen. The oxygen-bonded isomer is the less stable form. In water (k(s)ON = 1.7 x 10(-4) s-1, 25-degrees-C) and Me2SO (k(s)ON = 5.1 x 10(-5) s-1) it spontaneously isomerizes to the nitrogen-bonded form; in aqueous acid (pK(a) = 2.7, I = 0.1 M, LiClO4) and acidified Me2SO it protonates and rapidly solvolyzes; the protonated species in water has reactivity comparable (k(H) = 2.3 x 10(-2) s-1, I = 0.1 M, LiClO4, 25-degrees-C) to the most reactive isolable [(NH3)5CoX]n+ species known. In aqueous base three competing reactions have been detected, namely solvolysis (40%, 25-degrees-C), base-catalyzed O- to N-bonded linkage isomerization (30%), and nucleophilic attack on the coordinated carbonyl group by hydroxide ion leading to the formation of the carboxylate-bonded isomer of (succinamato)-pentaamminecobalt(III) (30%) (k(OH)(obsd) = 9.0 x 10(-2) M-1 s-1, I = 0.1 M, KF, 25-degrees-C). The individual rates and rate laws for all these reactions have been determined. In acid and base the nitrogen-bonded imido complex is less reactive than the O-bonded form. It is base hydrolyzed relatively slowly, and a term second order with respect to hydroxide ion is dominant in the rate law (k(N) = 6.1 x 10(-3) M-2 s-1, I = 1.0 M, NaClO4, 25-degrees-C); O-18 studies establish the reversible addition of OH- in the first step. The product is the nitrogen-bonded succinamato complex, which has been characterized through crystallization in its basic and acidic forms (pK(a) = 1.8 (amide) and 3.55 (carboxylic acid), I = 1.0 M, NaCl, 25-degrees-C). The succinimido-N complex is protonated in water and Me2SO only in very strong acid. The protonated species has been crystallized and characterized; it is a strong acid (pK(a) < 1), and in acidic solution it solvolyzes very slowly (t1/2 = days, 25-degrees-C). A N- to O-bonded isomerization reaction has not been detected. The structure and reactivity of these imide complexes are compared with those of the related amide and urea complexes.