Oxidatively Induced Isomerization of Square-Planar [Ni(1,4,8,11-tetraazacyclotetradecane)](ClO₄)₂

Abstract

Previous proton NMR and electronic spectroscopy studies have demonstrated that the square-planar complex of Ni(II) and 1,4,8,11-tetraazacyclotetradecane (cyclam) exists in two stable isomeric conformations, namely the R,S,R,S form or trans-I isomer and the R,R,S,S form or trans-III isomer. Electrochemical analysis of the trans-I isomer of Ni(II)-cyclam has demonstrated rapid conversion to the trans-III cyclam conformation following oxidation to Ni(III). The mechanism and kinetics for this oxidatively-induced isomerization were studied by the technique of cyclic voltammetry (CV) with simulation of CV traces by finite-difference computations. The most crucial mechanistic indicator was found to be the transient trans-I-Ni(III) species. This intermediate was detected by CV over a pH range 2-4 and was found to have an apparent half-life of ca. 400 ms at room temperature. Remarkably, this life-time was approximately a billionfold shorter than the corresponding trans-I-Ni(II) species. Measurements made at varied solution temperature and pH demonstrated that the oxidatively induced isomerization followed an apparent square-scheme, where the trans-I/III isomerization process of Ni(III) was independent of pH. This finding precluded a base-catalyzed isomerization process that has been previously identified for the Ni(II) system. Arrhenius plots of the forward isomerization rate constant allowed the extraction of activation parameters for the Ni(III) process. These parameters are discussed with respect to possible rate-determining steps.

Configurations of Nickel–Cyclam Antiviral Complexes and Protein Recognition

“…However, in solution, trans-I, trans-III and cis-V isomers of [NiA C H T U N G T R E N N U N G (cyclam)] 2 + complexes have been observed. [9,[17][18][19] The work reported here shows that nickel-cyclam complexes can rapidly convert from paramagnetic trans-III to diamagnetic trans-I configurations in aqueous solution. However, if the cyclam backbone initially adopts the cis-V configuration, this too can convert to a trans-I configuration, but much more slowly.…”

“…It can be seen that the Ni À N1 bond (the bond to the N bearing the benzyl substituent) is longer in the Ni II -benzylcyclam complexes (2.1899 (17) in 5 and 2.134(3)…”

Inclusion compounds of the copper(ii) and zinc(ii) complexes with cyclam in cucurbit[8]uril: synthesis and structure

“…not included in the cavitand, complexes with cyclam contain the ligand configuration trans III, which is found in the crystalline state, along with other cyclam isomers of cyclam. 14,23, 24 In the present paper, we report the synthesis of the inclusion compounds in CB [8] [8] (1800-400 cm -1 ) and water and also narrow medium intensity bands at 2926 and 2855 cm -1 , which can be attributed to stretching vi brations of the N-H bond in coordinated cyclam.…”

“…This is, for instance, the zinc(II) complex with the phe nylazomethine cyclam derivatives 30 or the nickel(II) com plex [Ni(cyclam)](ClO 4 ) 2 in which, according to the 1 Н NMR data at room temperature, the isomer trans I is about 15%. 23, 24 The configuration of cyclam trans II is rather rare, it is known for the copper(II) complex with tetramethyl substituted cyclam, 31 and for unsubstituted cyclam its formation was observed for the first time in inclusion compound 3 (see Ref. 21).…”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 18, 2007) and the discipline

“…So has oxamide itself [423][424][425] with disconcertingly contentious results, i.e., a 60 kJ mol -1 spread is found among the three cited values for the enthalpy of formation of the solid. Multiple isomers of the tetramine metal complex may be drawn with varying amine hydrogen locations-although they have not been differentiated for the copper complex, such studies have been made for Ni(II) and Ni(III) [426].…”