Characterisation and substitution kinetics of different cobalt(III) tripod complexes
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Date
2006-03
Authors
Molosioa, Phillip Sechaba
Journal Title
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Volume Title
Publisher
University of the Free State
Abstract
English: The synthesis and reactions of Co(III) complexes with tripod-type ligands such as l-leucine-N,N-diacetic acid (lda) and l-phenylalanine-N,N-diacetic acid (pda) have a widespread interest, mainly because of the fact that these complexes can be used as biological model complexes and because lda as well as pda labilises usually inert metal centres. Uehara and co-workers (1971:1552) were the first to prepare Co(III)-lda andpda complexes. Unfortunately metal complexes of cobalt(III) containing lda and pda as possible multidentate or tripod ligand are rarely mentioned in the literature and little information on their structure and chemistry is available. No kinetic studies on cobalt(III)-lda and-pda complexes have been published. The question regarding the identity of the different Co(III)-L (L = lda, pda) species in solution at different pH levels has been accounted for in this study (see Scheme 1). Scheme in PDF full text. l-Leucine-N,N-diacetic acid (lda) and l-phenylalanine-N,N-diacetic acid (pda) were synthesised according to the procedure developed by Bocarsly et al. (1990:4898). The synthesis of lda and pda were confirmed by means of IR and 1H NMR spectrometry. Both [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- were prepared similar to the method described by Uehara and co-workers (1971:1552). The synthesis of [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- were confirmed by means of IR, UV/VIS and 1H NMR spectrometry. The IR stretching frequencies obtained for above-mentioned complexes are indicative of COO- groups coordinated to a metal centre such as Co(III). The 1H NMR spectra for both [Co(lda)(µ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- also indicated that all the protons of lda and pda are non-equivalent. [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- undergo bridge-cleavage upon acidification with H+ ions to form [Co(lda)(H2O)2] and [Co(pda)(H2O)2], respectively. Further acidification of [Co(lda)(H2O)2] and [Co(pda)(H2O)2] leads to the stepwise dissociation of lda and pda, respectively. The formation of an ion association species between [Co(L)(H2O)2] (L = lda, pda) and H+ ions upon addition of acid is postulated. This ion associated species dissociates in the rate determining step to form the tridentate L (L = lda, pda) complex, [Co(µ3-L)(H2O)3]+. The values of k1 were determined as 0.128(8) s-1 at 25.9 µC for lda complex and as 0.115(7) s-1 at 25.7 µC for pda complex. Another acid-base equilibrium is observed when the pH of both [Co(lda)(H2O)2] and [Co(pda)(H2O)2] solutions are increased. It was concluded that the newly formed species are not the dimer, but rather [Co(lda)(H2O)(OH)]- and [Co(pda)(H2O)(OH)]- which reverts back to the dimer at pH 6 - 7 after several days. The acid dissociation constants of [Co(lda)(H2O)2] and [Co(pda)(H2O)2] were spectrophotometrically determined as 6.11(2) and 6.74(1), respectively. The substitution reactions between [Co(L)(H2O)2]/[Co(L)(H2O)(OH)]- (L = lda, pda) and NCS- ions 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(lda)(H2O)2] and [Co(pda)(H2O)2] (k1 = 2.43(3) x 10-3 M-1 s-1 at 24.5 °C) and (k1 = 2.3(4) x 10-2 M-1 s-1 at 25.0 °C), respectively, at low pH is about 125 times faster than the rate of substitution of the second aqua ligand (k3 = 1.52(6) x 10-4 M-1 s-1 at 25.0 °C) and (k3 = 1.36(2) x 10-4 M-1 s-1 at 25.0 °C), respectively. The [Co(lda)(H2O)(OH)]- and [Co(pda)(H2O)(OH)]-complexes reacts about 70 times faster at 25.0 °C with NCS-, respectively, than the [Co(lda)(H2O)2] and [Co(pda)(H2O)2] complexes with NCS- (k2 = 1.34(9) M-1 s-1 vs. 2.43(3) x 10-2 M-1 s-1 for k1 at 24.5 °C) and (k2 = 1.44(8) M-1 s-1 vs. 2.3(4) x 10-2 M-1 s-1 for k1 at 25.0 °C), respectively. 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 Co(III)-lda and-pda 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.
Afrikaans: Die sintese en reaksies van Co(III) komplekse met driepoot-tipe ligande soos l-leusien- N,N-diasynsuur (lda) en l-fenielalanien-N,N-diasynsuur (pda) lok wye belangstelling omdat hierdie komplekse as biologiese modelkomplekse gebruik kan word en omdat die lda en pda ligande metaalsentra wat gewoonlik inert is, labiliseer. Die eerste bereiding van Co(III)-lda en-pda komplekse is gedoen deur Uehara en medewerkers (1971:1552). Ongelukkig word metaalkomplekse van cobalt(III) wat lda en pda as moonklike multidentate of driepootligand bevat, selde in die literatuur vermeld. Daar is dus min beskikbare inligting oor die struktuur en chemie van hierdie komplekse. Daar is nog geen kinetiese studie van cobalt(III)-lda en-pda komplekse gepubliseer nie. Die vraag rondom die indentiteit van die verskillende Co(III)-L (L = lda, pda) spesies in oplossing (sien Skema 1) by verskillende pH’s is grootliks in hierdie studie beantwoord. Skema in PDF volteks. l-Leusien-N,N-diasynsuur (lda) en l-fenielalanien-N,N-diasynsuur (pda) was gesintetiseer volgens die prosedure van Bocarsly et al. (1990:4898). Die sintese van lda en pda was bevestig met behulp van IR en 1H KMR spektrometrie. Beide [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- was gesintetiseer volgens die metode wat beskryf is deur Uehara and medewerkers (1971:1552). Die sintese van [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- was bevestig met behulp van IR, UV/VIS en 1H KMR spektrometrie. Die IR strekkingsfrekwensies van die bogenoemde komplekse dui op COO- groepe wat gekoördineerd is aan metale soos Co(III). Die 1H KMR spektrum van beide [Co(lda)(µ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- het ook bewys dat al die protone van lda en pda nie-ekwivalent is. Hierdie studie het bevind dat die hidrokso-brûe van [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- gesplyt word met byvoeging van H+ ione om [Co(lda)(H2O)2] en [Co(pda)(H2O)2], respektiewelik te vorm. Verdere byvoeging van suur lei tot die stapsgewyse dissosiasie van die lda en pda ligande. Dit word gepostuleer dat ‘n ioongeassosieerde spesie tydens die reaksie tussen [Co(L)(H2O)2] (L = lda, pda) en H+ ione met byvoeging van suur vorm. Hierdie ioongeassosieerde spesie dissosier in die tempobepalende stap om ‘n tridentate L (L = lda, pda) kompleks, [Co(µ3-L)(H2O)3]+, te vorm. Die waarde van k1 by 25.9 µC is as 0.128(8) s-1 van die lda kompleks en is as 0.115(7) s-1 by 25.7 µC van die pda kompleks bepaal. Nog ‘n suur-basis ewewig is waargeneem toe die pH van beide [Co(lda)(H2O)2] en [Co(pda)(H2O)2] oplossing verhoog is. Die gevolgtrekking is dat die nuutgevormde spesies nie die dimeer is nie, maar eerder [Co(lda)(H2O)(OH)]- en [Co(pda)(H2O)(OH)]- wat terugkeer na die dimeer by pH 6 - 7 na verloop van ‘n paar dae. Die pKa van [Co(lda)(H2O)2] en [Co(pda)(H2O)2] was spektrofotometries as 6.11(2) en 6.74(1), respektiewelik bepaal. Die substitusiereaksies tussen [Co(L)(H2O)2]/[Co(L)(H2O)(OH)]- (L = lda, pda) en NCSione is ook ondersoek. NCS- substitueer die akwaligande stapsgewys by pH = 2.00. Die substitusie van die eerste akwa ligand van [Co(lda)(H2O)2] en [Co(pda)(H2O)2] (k1 = 2.43(3) x 10-3 M-1 s-1 by 24.5 °C) en (k1 = 2.3(4) x 10-2 M-1 s-1 by 25.0 °C), respektiewelik by lae pH is ongeveer 125 keer vinniger as die tempo van die tweede akwa-substitusie (k3 = 1.52(6) x 10-4 M-1 s-1 by 25.0 °C) en (k3 = 1.36(2) x 10-4 M-1 s-1 by 25.0 °C), respektiewelik. Die [Co(lda)(H2O)(OH)]- en [Co(pda)(H2O)(OH)]- komplekse reageer ongeveer 70 keer vinniger by 25.0 °C met NCS-, respektiewelik as die [Co(lda)(H2O)2] en [Co(pda)(H2O)2] komplekse met NCS- (k2 = 1.34(9) M-1 s-1 vs. 2.43(3) x 10-2 M-1 s-1 vir k1 by 24.5 °C) en (k2 = 1.44(8) M-1 s-1 vs. 2.3(4) x 10-2 M-1 s-1 vir k1 by 25.0 °C), respektiewelik. Hierdie resultate toon duidelik dat die hidroksoligand die cis-akwa binding sodanig labiliseer dat ‘n toename in tempo waargeneem word. Verder toon dit aan dat die hidroksied ligande nie deur NCS- ione by hoër pH gesubstitueer word nie, aangesien slegs een reaksie by hierdie pH waargeneem word. Die Co(III)-lda en-pda komplekse wat geïsoleer en gekarakteriseer is in hierdie studie kan suksesvol as biologiese modelkomplekse gebruik word in toekomstige studies. Hierdie komplekse kan byvoorbeeld gebruik word om die binding van ‘n metaal aan ‘n funksionele groep van wol na te boots of kan gebruik word as modelkomplekse in farmakologie.
Afrikaans: Die sintese en reaksies van Co(III) komplekse met driepoot-tipe ligande soos l-leusien- N,N-diasynsuur (lda) en l-fenielalanien-N,N-diasynsuur (pda) lok wye belangstelling omdat hierdie komplekse as biologiese modelkomplekse gebruik kan word en omdat die lda en pda ligande metaalsentra wat gewoonlik inert is, labiliseer. Die eerste bereiding van Co(III)-lda en-pda komplekse is gedoen deur Uehara en medewerkers (1971:1552). Ongelukkig word metaalkomplekse van cobalt(III) wat lda en pda as moonklike multidentate of driepootligand bevat, selde in die literatuur vermeld. Daar is dus min beskikbare inligting oor die struktuur en chemie van hierdie komplekse. Daar is nog geen kinetiese studie van cobalt(III)-lda en-pda komplekse gepubliseer nie. Die vraag rondom die indentiteit van die verskillende Co(III)-L (L = lda, pda) spesies in oplossing (sien Skema 1) by verskillende pH’s is grootliks in hierdie studie beantwoord. Skema in PDF volteks. l-Leusien-N,N-diasynsuur (lda) en l-fenielalanien-N,N-diasynsuur (pda) was gesintetiseer volgens die prosedure van Bocarsly et al. (1990:4898). Die sintese van lda en pda was bevestig met behulp van IR en 1H KMR spektrometrie. Beide [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- was gesintetiseer volgens die metode wat beskryf is deur Uehara and medewerkers (1971:1552). Die sintese van [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- was bevestig met behulp van IR, UV/VIS en 1H KMR spektrometrie. Die IR strekkingsfrekwensies van die bogenoemde komplekse dui op COO- groepe wat gekoördineerd is aan metale soos Co(III). Die 1H KMR spektrum van beide [Co(lda)(µ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- het ook bewys dat al die protone van lda en pda nie-ekwivalent is. Hierdie studie het bevind dat die hidrokso-brûe van [Co(lda)(μ-OH)]2 2- en [Co(pda)(μ-OH)]2 2- gesplyt word met byvoeging van H+ ione om [Co(lda)(H2O)2] en [Co(pda)(H2O)2], respektiewelik te vorm. Verdere byvoeging van suur lei tot die stapsgewyse dissosiasie van die lda en pda ligande. Dit word gepostuleer dat ‘n ioongeassosieerde spesie tydens die reaksie tussen [Co(L)(H2O)2] (L = lda, pda) en H+ ione met byvoeging van suur vorm. Hierdie ioongeassosieerde spesie dissosier in die tempobepalende stap om ‘n tridentate L (L = lda, pda) kompleks, [Co(µ3-L)(H2O)3]+, te vorm. Die waarde van k1 by 25.9 µC is as 0.128(8) s-1 van die lda kompleks en is as 0.115(7) s-1 by 25.7 µC van die pda kompleks bepaal. Nog ‘n suur-basis ewewig is waargeneem toe die pH van beide [Co(lda)(H2O)2] en [Co(pda)(H2O)2] oplossing verhoog is. Die gevolgtrekking is dat die nuutgevormde spesies nie die dimeer is nie, maar eerder [Co(lda)(H2O)(OH)]- en [Co(pda)(H2O)(OH)]- wat terugkeer na die dimeer by pH 6 - 7 na verloop van ‘n paar dae. Die pKa van [Co(lda)(H2O)2] en [Co(pda)(H2O)2] was spektrofotometries as 6.11(2) en 6.74(1), respektiewelik bepaal. Die substitusiereaksies tussen [Co(L)(H2O)2]/[Co(L)(H2O)(OH)]- (L = lda, pda) en NCSione is ook ondersoek. NCS- substitueer die akwaligande stapsgewys by pH = 2.00. Die substitusie van die eerste akwa ligand van [Co(lda)(H2O)2] en [Co(pda)(H2O)2] (k1 = 2.43(3) x 10-3 M-1 s-1 by 24.5 °C) en (k1 = 2.3(4) x 10-2 M-1 s-1 by 25.0 °C), respektiewelik by lae pH is ongeveer 125 keer vinniger as die tempo van die tweede akwa-substitusie (k3 = 1.52(6) x 10-4 M-1 s-1 by 25.0 °C) en (k3 = 1.36(2) x 10-4 M-1 s-1 by 25.0 °C), respektiewelik. Die [Co(lda)(H2O)(OH)]- en [Co(pda)(H2O)(OH)]- komplekse reageer ongeveer 70 keer vinniger by 25.0 °C met NCS-, respektiewelik as die [Co(lda)(H2O)2] en [Co(pda)(H2O)2] komplekse met NCS- (k2 = 1.34(9) M-1 s-1 vs. 2.43(3) x 10-2 M-1 s-1 vir k1 by 24.5 °C) en (k2 = 1.44(8) M-1 s-1 vs. 2.3(4) x 10-2 M-1 s-1 vir k1 by 25.0 °C), respektiewelik. Hierdie resultate toon duidelik dat die hidroksoligand die cis-akwa binding sodanig labiliseer dat ‘n toename in tempo waargeneem word. Verder toon dit aan dat die hidroksied ligande nie deur NCS- ione by hoër pH gesubstitueer word nie, aangesien slegs een reaksie by hierdie pH waargeneem word. Die Co(III)-lda en-pda komplekse wat geïsoleer en gekarakteriseer is in hierdie studie kan suksesvol as biologiese modelkomplekse gebruik word in toekomstige studies. Hierdie komplekse kan byvoorbeeld gebruik word om die binding van ‘n metaal aan ‘n funksionele groep van wol na te boots of kan gebruik word as modelkomplekse in farmakologie.
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Keywords
Cobalt(III), l-leucine-N,N-diacetic acid (lda) and l-phenylalanine-N,N-diacetic acid (pda), Ionisation constants, Substitution reactions, Dissertation (M.Sc. (Chemistry))--University of the Free State, 2006