Base Hydrolysis of Pentaaminecobalt(III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 2. Structure and reactivity of the pentacoordinate intermediate
Abstract
Abstract 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…
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