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

[摘要] 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 thefact that these complexes can be used as biological model complexes and because apdalabilises usually inert metal centres. The first Co(III)-apda complex was prepared byTsuchiya and co-workers (1969:1886), this complex was later conclusively characterisedby Gladkikh and co-workers (1997:1346) as [Co(apda)(H2O)2] by means of X-raycrystallography. Since then very few metal complexes containing apda as ligand arecited 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 atdifferent 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 methodobtained from Niclós Gutiérrez (University of Granada). The synthesis of apda wasconfirmed 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 asCo(III). The 1H NMR spectrum also indicated that apda acts as a tetradentate ligand withthe 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 isincreased. 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 wasspectrophotometrically determined as 6.23(2).The substitution reactions between [Co(apda)(H2O)2]/[Co(apda)(H2O)(OH)]- and NCSionshave been investigated. At pH = 2.00 NCS- ions substitute the aqua ligands in astepwise 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 ofsubstitution 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- thanthe [Co(apda)(H2O)2] complex with NCS- (k2 = 0.986(8) M-1 s-1 vs. 14(1) x 10-3 M-1 s-1for k1 at 25.0 °C). This clearly indicates that the hydroxo ligand labilises the cis-aquabond so that an increase in rate is observed. Hydroxide is not substituted by NCS- ions athigher pH so that only one reaction is observed spectrophotometrically.The synthesis and characterisation of a Co(III)-apda complex with apda acting as atridentate ligand were also undertaken. This complex was characterised by means of IRspectroscopy and X-ray crystallography as [Co(H2O)6][Co(Hapda)2]2Hּ2O. This complexcrystallises 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 strainexperienced by the glycinato rings of the apda ligand. This is the first Co(III)-apdacomplex with two tridentate apda ligands bonded to the same cobalt centre. Thiscomplex was also synthesised in the absence of competing ligands.The strain experienced by the glycinato rings of the coordinated apda decreases in theorder G > R for all the complexes studied. The R rings in all the complexes are almostperfectly planar in all cases, whilst the G rings are non-planar.The synthesis and characterisation of Na[Co(Hapda)2]xּH2O were also undertaken. Thiscomplex 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 isdecreased. It was concluded that the species present at pH 5.5 is [Co(apda)2]3- and thatthe [Co(Hapda)(apda)]2- and [Co(Hapda)2]- complexes form upon the addition of H+ tothis solution. The two acid dissociation constants, pKa1' and pKa2', were spectrophotometricallydetermined as 2.6(1) and 2.8(1), respectively.The various Co(III)-apda complexes that were isolated and characterised can successfullybe used as biological model complexes in future studies. These complexes could forexample be used to simulate the bonding of metal ion to functional groups of wool fibreor might have uses as models in pharmacology.

[发布机构] University of the Free State