Base Hydrolysis of Pentaaminecobalt(III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 2. Structure and reactivity of the pentacoordinate intermediate
Authors: Comba Peter , Jackson W. Gregory , Marty Werner , Zipper Luc 
Published: December 2004

Base hydrolysis of optically pure mer‐exo(H)‐ and mer‐endo(H)‐[CoCl(dien)(dapo)]2+ (A and B (X = Cl)), resp.; dien = N‐(2‐aminoethyl)ethane‐1,2‐damine; dapo = 1,3‐diaminopopan‐2‐ol, kOH = (1.13 ±0.09)·105 M−1s−1 (A (X = Cl), kOH = (1.18 ± 0.11)·105M−1s−1 (B (X = Cl)); I = 1.0M (NaClO4 or NaN3)1, T = 298 K) is accompanied by retention of the mer‐geometry and full racemization (99 ± 1%). It is shown this is not due to racemization of either reactants or products. This result, together with the fact that both A and B yield the same mer‐exo(H)‐product distribution, indicates the intermediacy of a pentacoordinate species II which is symmetrical (at least in the time average), viz. trigonal bipyramidal with a deprotonated (‘flat’) secondaryamine moiety. The H‐exchange rates of the coordinated amine groups are consistent with this interpretation and indicate that loss of Cl− is the rate‐determining step, in agreement with an SN1CB mechanism. The reactivity of the unsym‐fac‐exo(OH)‐ and unsy,‐fac‐endo(OH)‐isomers C and D, respectively, is in sharp contrast: base hydrolysis is 3 orders of magnitue slower, and the reaction is accompanied by some change of coordination geometry (C, 23%; D, 10%, some inversion of configuration (C, 15%; D, 19%)); much lower acceleration of hydrolysis in base (106 vs. 1010). Azide competition during base hydrolysis of the mer‐isomer A and B is quite large (R = [CoN3]∞/[CoOH]∞[N] = 1.4 ±0.2M−1, I= 1.0M, T = 298 K) and indicates that the coordinatively unsaturated intermediate II is highly selective. The ratios of exo(H)‐ and endo(H)‐azide competition products A and B (X = N3), respectively, immediately after the substitution reaction (kinetic control) are independent of the engaged epierm A or B: 31.7 ± 0.9% of B (X = N3) and 68.3 ± 0.9% of A (X = N3, determined after ca. 10.t½ of the base hydrolysis). This is agreement with the effective site of deprotonation at the secondayr(central)‐amine group of dien, cis to the leaving group X, and with a common set of intermediates. Epimerization of A and B (X = Cl, N3) is shown to proceed solely via the pentacoordinate (base hydrolysis) intermediate II, viz. the direct route involving a six‐coordinate deprotonated intermediate is immeasurably slow. For the hydroxo products A and B (X = OH), the direct rotue may compete with the H2O‐substitution(exchange) path which can occur by an internal conjugate‐base process. The kinetically controlled distribution of complexes A/B (X = N3) is different from the quasi‐thermodynamic one (19.1 ± 0.8% of B (X = N3) and 80.9 ± 0.8% of A (X = N3)). This is consistent with the differences in the base‐hydrolysis rates of the reactants (kOh (A (X = N3))= (1.59 ± 0.04)·102M−1s−1; kOH (B (X = N3)) = (2.89 ± 0.22).102M−1s−1). Various aspects of the investigated reactions are discussed on the basis of the widely studied reaction of base hydrolysis of pentaaminecobalt(III) complexes. Also, the structure and reactivity of the pentacoordinate intermediate II are discussed in relation to various current models.