Kinetics and Mechanism of Ligand Substitution in Some Chromium(III) Tetraaza Macrocyclic Complexes in Acidic Media: Differences in Reactivity Between cis and trans Isomers

Abstract

Thiocyanate anation of the macrocyclic complexes cis‐[Cr(cycb)(OH2)2]3+ and trans‐[Cr(cyca)(OH2)2]3+ (cycb and cyca are rac‐ and meso‐5,5,7,12,12,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane, respectively), and mercury(II)‐induced aquation of the product thiocyanato‐N complexes, has been studied in acidic solution. The rate retardation with an increase of the acid concentration for both types of reactions follows the rate expression kobs = (k1 + k2Ka/[H+(aq)])/(1 + Ka/[H+(aq)]), corresponding to parallel reaction paths through aqua and hydroxo complexes. Some general trends in the differences in reactivity between the two geometrical isomers have been observed and are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Kinetics of the heterobimetallic complexes formation in the reactions between chromium(III) macrocyclic species and [Fe(CN)6]4− ions

“…It is noticeable that this value is very close to the one obtained previously [28] for anation and aquation of these macrocyclic chromium(III) complexes.…”

“…The acidic properties of the studied macrocyclic chromium(III) species have been reported in literature. The pK a1 and pK a2 values at 298 K are equal 2.44 and 6.93 for the trans isomer and 3.49 and 7.11 for the cis one [28]. In the studied pH range (5.0-8.4) presence of the diaqua form of the trans isomer can be ignored due to its high acidity.…”

“…Validity of the proposed reaction mechanism can be tested by comparison of the kinetic parameter K a2 (Equation 7) to the value of the second acidity constant reported in the literature [28]. The pK a2 value for the trans isomer calculated from Equation (8) is 6.3 and the literature one, 6.9 are satisfactory convergent.…”

“…This process is not interfered by aquation and/or isomerisation of the reactants due to the well known inertness of the chromium(III) [27,28] and iron(II) complexes. Special attention has been paid to the explanation of the role played by protolytic reactions in the process of heterobimetallic complex formation.…”

“…6 ] AE 3H 2 O, KOH, KBr, 40% HBr (POCh, Gliwice, Poland) were of pro analysi grade quality. Both chromium(III) complexes were prepared according to a literature method [28]. MES and TRIS (Aldrich) buffers were applied to adjust the suitable pH of the solutions.…”