Characterisation and substitution kinetics of cobalt(III) - N-(2-carboxyethyl)iminodiacetato complexes

English: The synthesis and reactions of Co(III) complexes with tripod-type ligands such as N-(2- carboxyethyl)iminodiacetic acid (apda) have a widespread interest, mainly because of the fact that these complexes can be used as biological model complexes and because apda labilises usually inert metal centres. The first Co(III)-apda complex was prepared by Tsuchiya and co-workers (1969:1886), this complex was later conclusively characterised by Gladkikh and co-workers (1997:1346) as [Co(apda)(H2O)2] by means of X-ray crystallography. Since then very few metal complexes containing apda as ligand are cited and little or no kinetic studies have been published on these types of complexes. The question regarding the identity of the different Co(III)-apda species in solution at different pH levels has been accounted for in this study (refer to Scheme 1). Scheme in PDF full text. N-(2-carboxyethyl)iminodiacetic acid (apda) was synthesised according to a method obtained from Niclós Gutiérrez (University of Granada). The synthesis of apda was confirmed by means of IR and 1H NMR spectrometry [Co(apda)(H2O)2] was prepared similar to the method described by Tsuchiya and coworkers (1969:1886). The synthesis of [Co(apda)(H2O)2] was confirmed by means of IR, UV/VIS and 1H NMR spectrometry. The IR stretching frequencies obtained for [Co(apda)(H2O)2] are indicative of COO- groups coordinated to a metal centre such as Co(III). The 1H NMR spectrum also indicated that apda acts as a tetradentate ligand with the longer propionato ring in the G (out-of-plane) position. An acid base equilibrium is observed when the pH of a [Co(apda)(H2O)2] solution is increased. It was concluded that the newly formed species is [Co(apda)(H2O)(OH)]- which is unstable at pH > 7, possibly due to dimer formation. The pKa was spectrophotometrically determined as 6.23(2). The substitution reactions between [Co(apda)(H2O)2]/[Co(apda)(H2O)(OH)]- and NCSions have been investigated. At pH = 2.00 NCS- ions substitute the aqua ligands in a stepwise fashion. The substitution of the first aqua ligand of [Co(apda)(H2O)2] (k1 = 14(1) x 10-3 M-1 s-1 at 25.0 °C) at low pH is about 125 times faster than the rate of substitution of the second aqua ligand (k3 = 1.2(6) x 10-4 M-1 s-1 at 25.0 °C). The [Co(apda)(H2O)(OH)]- complex reacts about 70 times faster at 25.0 °C with NCS- than the [Co(apda)(H2O)2] complex with NCS- (k2 = 0.986(8) M-1 s-1 vs. 14(1) x 10-3 M-1 s-1 for k1 at 25.0 °C). This clearly indicates that the hydroxo ligand labilises the cis-aqua bond so that an increase in rate is observed. Hydroxide is not substituted by NCS- ions at higher pH so that only one reaction is observed spectrophotometrically. The synthesis and characterisation of a Co(III)-apda complex with apda acting as a tridentate ligand were also undertaken. This complex was characterised by means of IR spectroscopy and X-ray crystallography as [Co(H2O)6][Co(Hapda)2]2Hּ2O. This complex crystallises in the triclinic space group P ī (R = 0.0228). The two anionic units, [Co(III)(Hapda)2]-, differ in terms of bond lengths and angles as well as strain experienced by the glycinato rings of the apda ligand. This is the first Co(III)-apda complex with two tridentate apda ligands bonded to the same cobalt centre. This complex was also synthesised in the absence of competing ligands. The strain experienced by the glycinato rings of the coordinated apda decreases in the order G > R for all the complexes studied. The R rings in all the complexes are almost perfectly planar in all cases, whilst the G rings are non-planar. The synthesis and characterisation of Na[Co(Hapda)2]xּH2O were also undertaken. This complex was characterised by means of IR, UV/VIS and 1H NMR spectrometry. Two acid base equilibria are observed when the pH of a Na[Co(Hapda)2]xּH2O solution is decreased. It was concluded that the species present at pH 5.5 is [Co(apda)2]3- and that the [Co(Hapda)(apda)]2- and [Co(Hapda)2]- complexes form upon the addition of H+ to this solution. The two acid dissociation constants, pKa1’ and pKa2’, were spectrophotometrically determined as 2.6(1) and 2.8(1), respectively. The various Co(III)-apda complexes that were isolated and characterised can successfully be used as biological model complexes in future studies. These complexes could for example be used to simulate the bonding of metal ion to functional groups of wool fibre or might have uses as models in pharmacology.