Base Hydrolysis of Pentaaminecobalt (III) Complexes: The [CoX(dien) (dapo)]ⁿ⁺ system. Part 3. The internal conjugate base mechanism

Abstract

Azide anation and racemiration of optically pure mer-ao(H)-and n~er-endo(H)-[Co(OH)(dien)(dapo)]~+ (Aand B (X = OH), resp.; dien = N-(2-aminoethyl)ethane-l Jdiamine; dapo = I ,3-diaminopropan-2-oI) involve the same symmetrical pentacoordinate intermediate as the base hydrolyses of the corresponding mer-exo(Hj-and m~-endo(H)-[CoX(dien)(dapo)]~+ species A and B, respectively, where X = C1, Br, or N,. The kinetic parameters of the anation process are fully compatible with the independently measured competition ratio. The rate data reveal that substitution of OH-is unexpectedly fast, uiz. it is not consistent with the usual sequenceThis behavior is interpreted on the basis of an internal conjugate base mechanism which involves an amino-hydroxo/aminato-aqua tautomerism, tiz. the reaction is actually an OH--catalyzed substitution of [CoH20(dien)(dapo)13+ where deprotondtion occurs effectively at the secondary-amine site NH of dien.Introduction. -Base hydrolysis of pentaaminecobalt(II1) is an extensively studied example of an induced substitution process [ 11. The widely accepted S,, 1 CB mechanism incorporates the formation of a conjugate base in a fast pre-equilibrium and rate-determining loss of the leaving group'). We designed and investigated a pentaaminecobalt(II1) system, mer-exo (H)-and mer-endo (H)-[CoX(dien)(dapo)]" (A and B, resp.; X = C1-, A mer-exo(H) B mer-endo( H ) ' j ' )Deceased on September 20th, 1986. A recent study of kinetics of and competition during base hydrolysis of pentaaminecobak(II1) complexes was interpreted in terms of an IDCR mechanism [2] [3]. This is no1 fully coinpatible with our results, uiz. an I&B mechanism is only possible if the intermediate is allowed to relax stcrcochemically (see below and [MI).

Ligand Substitution Reactions at Inorganic Centers

“…Nordmeyer 161 first showed that the amide group when formed was most effective when cis to the leaving group and this was also corroborated in the case of active mer -NH. Much of the work since has largely corroborated these conclusions with some particularly reactive examples being mer -[Co(dien)(tn)Cl] 2+ 162 and mer -[Co(dien)(dapo)Cl] 2+ 13 (46). , Since then even more reactive pentamine complexes have been found, most notably with pyridine substituents, some of which show remarkable differences in the value of k OH even for very subtle changes in structure such as 14 compared to 15 . In 15 only a single fac -NH center is present wherein there is evidence of a competing pseudo-aminate mechanism involving deprotonation rather of the α-CH 2 group of the 2-pyridylmethylene arm confirmed by similar reactivity shown by the N−CH 3 derivative of 15 …”

17O NMR Study of Solvent Exchange in Some Aqueous [Co(tren)(X)(OH2/OH)]n+, [Co(cyclen)(X)(OH2/OH)]n+, and [Co(N-Mecyclen)(X)(OH2/OH)]n+ Systems (X = NH3, OH2/OH; n = 3, 2, 1)1

“…Hunt and Taube 25 made the interesting suggestion that ligand−OH - exchange could result from an indirect process involving amine deprotonation and loss of H 2 O, viz., eq 5, that is via an S N 1(CB) process on the aqua complex, and this allows for the rate of OH - exchange to exceed that of H 2 O exchange depending on the values of K and k ‘ OH . The unusually rapid substitution reactions of the [Co(dien)(dapo)OH] 2+ ion have similarly been interpreted as involving deprotonation at the dien sec -N H site in the conjugate acid, followed by water exchange in the resulting aqua−amido intermediate . Also, in the p -[Co(tren)(NH 3 )OH/H] 2+/3+ system coordinated OH - exchanges faster than coordinated OH 2 (15-fold; cf.…”

Base Hydrolysis of Pentaaminecobalt(III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 1. Syntheses, structures, configuration, and spectroscopy

“…The symmetries of the mer-, unsymzfuc-, and symzfuc-isomers of [CoXL3L2]"+ (L3 = dien, dpt; L' = en, tn, dapo) and [Co(dien)(dap~-N,N',O)]~+/~+ are reflected in their I3C-NMR spectra. The correctness of the assigned configurations is also supported by the base-hydrolysis reactivity of the various isomers and by the strereochemical changes observed for their base-catalyzed substitution reactions [7] [8]. However, the exo(H)us.…”

“…[7] and of the mer-exo(H)-and mer-endo(H)hydroxo and -aqua complexes A and B (X = OH and H,O, resp. ), respectively, [8] is given in the following papers.…”

“…The product was recrystallized from a minimum volume ofH,O using the respective precipitants. Yield of'D(ZnC1,) (X = Cl),8.6 g (85 Yo). Anal.…”