Mechanistic aspects of the chemistry of mononuclear Cr~(III)complexes withpendant-arm macrocyclic ligands and formation of discrete Cr~(III)/Fe~(II) andCr~(III)/Fe~(II)/Co~(III) cyano-bridged mixed valence compounds

摘要

The kinetics and mechanism of the redox reaction between [Fe~(II)(CN)_6]~(4-) and the macrocyclic ligandcomplex [CrClL_(15)]~(2+)(L_(15)= 6-methyl-1,4,8,12-tetraazacyclopentadecane-6-amine) has been studied atdifferent pH values. In acidic solution, the expected redox process occurs with no formation of any ofthe possible Cr~(III) /Fe~(II) mixed valence complexes, as those seen for the Co~(III) species of the same family,due to the enhanced lability of the Cr~(II) species formed on Fe~(II) to Fe~(III) oxidation. In alkaline conditions,the formation of the complex [Cr(L_(15))(OH)_2]~+ takes place as an initial step that precedes a simplesubstitution process producing the expected cyano-bridged Cr~(III) /Fe~(II) complex. In this species thepotentially pentadentate ligand, L_(15), has a tetradentate coordination mode with a protonated exocyclicprimary amine group and the redox potential is shifted to more negative values, thus disfavouring aredox driven reaction; the equivalent complex [CrCl(HL_(14))(H_2O)]~(3+) (L_(14) = 6-methyl-1,4,8,11-tetraaza-cyclotetradecane-6-amine) has been prepared by the same method and characterized by X-raycrystallography. The final [Fe~(II)(CN)_6]~(4-)substituted complex, [{(HL_(15))(OH)Cr~(III)NC}Fe~(II)(CN)_5]~- showspK_avalues of 3.8 and 7.4, as expected for the aqua and amino ligands, respectively. Its characterizationindicated its Class II mixed valence character with a very intense MMCT band at 350 nm showing amuch larger extinction coefficient than that observed for the Co~(III) complexes of the same family. Thisfact is in good agreement with the much larger Cr~(III)-Fe~(II) (t_(2g)-t_(2g)) coupling through cyanide bridgingligands expected for these complexes. The fully mixed metal/valence/ligand trimetallic complex[{(HL_(15))(OH)Cr~(III)NC}{L_(13)Co~(III)NC}Fe~(II)(CN)_4]~(2+)has been prepared following the same procedures andthe results are comparable. The final complex has the same Class II mixed valence character and itselectronic spectrum shows the characteristics of both the Fe~(II)-to-Cr~(III) and Fe~(II)-to-Co~(III) CT bands. Thestudy allows the application of the redox and/or substitutionally driven methodology, alreadydescribed and kinetically designed, for the preparation of discrete mixed valence complexes of inertcentres, so avoiding the unpredictable nature of other approaches seen in the past.