Water exchange of the (o-phenylenediamine-N,N,N',N'-tetraacetato)ferrate(III) complex in aqueous solution as studied by variable-temperature, -pressure, and -frequency oxygen-17 NMR techniques

Abstract

The crystal structure of potassium aqua(o-phenylenediamine-N,N,"'-tetraacetato)ferrate(III) dihydrate, (K[Fe(OH2)-(phdta)].2H20), has been determined by X-ray crystallography. The central iron(II1) ion is seven-coordinate, with one water molecule being hydrated. Water-exchange rates of the iron(II1) complex with phdtaC in aqueous solution have been studied as a function of temperature and pressure by the oxygen-17 NMR line-broadening method. Activation parameters for water exchange have been determined as follows: k(25 'C) = (1.2 i 0.2) X IO' s-I, AH* = 26 i 3 kJ mol-', AS* = -22 * 9 J K-' mol-', and AI" = 4.6 f 0.2 cm3 mol-'. The positive activation volume indicates that the water exchange proceeds via a dissociative interchange mechanism. IntroductionValuable results have been accumulated from a number of studies on t h e solvent exchange a t solvated metal ions by the high-pressure NMR method. For example, the study of water exchange on divalent metal ions across the first-row transitionmetal series has demonstrated the changeover from associative interchange I, to dissociative interchange Id mechanisms along the series.Is2 This changeover has been rationalized in terms of changes in ionic radius and of filling of d orbitals. The solvent exchange of the d5 manganese(I1) ion is known to proceed via an associative interchange mechanism in water,' methan01,~ and a~etonitrile.~ Recently, however, positive volumes of activation have been reported for solvent exchange on t h e manganese( 11) ion in N,N-dimethylformamide (DMF)S and acetic acid.6 Such a positive activation volume has been interpreted in terms of the bulkiness of coordinated solvent molecules. Furthermore, the activation volume for the solvent exchange of manganese(I1) acetate in acetic acid has been determined t o be +6.7 f 0.6 cm3 which is more positive than that (+0.4 f 0.7 cm3 mol-') for the hexakis(solvent)manganese(II) ion in acetic acid. I t has been claimed that the coordinated solvent molecules are labilized by electron donation from bound ligands such as CH3COO-t o the metal ion and that the activation mode of the reaction becomes less associative.

Applications for Transition-Metal Chemistry in Contrast-Enhanced Magnetic Resonance Imaging

“…Magnetic relaxation in the presence of Fe 3+ has not been explored as thoroughly as that for Mn 2+ and Gd 3+ , but NMRD measurements performed on Fe-EDTA and relaxometric evaluation of free versus protein-bound Fe 3+ -EHPG indicate that r 1 is controlled predominantly by τ R . , Fe 3+ is a stronger Lewis acid compared to Gd 3+ or Mn 2+ , and care must be taken to ensure that the water coligand p K a is high enough to avoid the formation of Fe hydroxide or Fe oxo ligands, which may substantially reduce inner-sphere contributions to r 1 . The water coligand p K a values of four structurally related acyclic chelates are compared in Table and demonstrate how for Fe 3+ complexes the water coligand p K a is strongly influenced by changes in the ligand environment. ,, For example, swapping the trans -1,2-cyclohexylene backbone of Fe-CyDTA for a 1,2-phenylene backbone (Fe-PhDTA) can alter the acidity of the water coligand by >5 p K a units. Care must also be taken to ensure stabilization of high-spin Fe 3+ because the increased charge-to-radius ratio results in chelates with greater crystal-field splitting energy compared to the corresponding Mn 2+ chelates.…”

“…The water coligand pK a values of four structurally related acyclic chelates are compared in Table 5 and demonstrate how for Fe 3+ complexes the water coligand pK a is strongly influenced by changes in the ligand environment. 101,128,129 For example, swapping the trans-1,2-cyclohexylene backbone of Fe-CyDTA for a 1,2-phenylene backbone (Fe-PhDTA) can alter the acidity of the water coligand by >5 pK a units. Care must also be taken to ensure stabilization of high-spin Fe 3+ because the increased charge-to-radius ratio results in chelates with greater crystal-field splitting energy compared to the corresponding Mn 2+ chelates.…”

Crystal structure of the 3-D complex [(H2O)Cd(μ-3,4-TDTA)Cd(H2O)]. Potentiometric and 113Cd NMR studies in aqueous solution (3,4-TDTA = 3,4-toluenediamine-N,N,N ′,N ′-tetraacetate)

“…o-PhDTA, 3,4-TDTA and 4-Cl-o-PhDTA are usually found to act as hexadentate ligands, however these ligands are not flexible, which gives a considerable distortion from octahedral geometry twisting towards trigonal prismatic as has been observed for complexes of Co(), 14 Zn() 15 and Cu() 16 (with o-PhDTA), as well as for the cobalt() complex with 4-Cl-o-PhDTA 17 and zinc() complex with 3,4-TDTA. 18 Sevencoordination is found for complexes of Mg(), 19 Mn() 20 and Fe() 21 (with o-PhDTA), as well as Fe() with 3,4-TDTA 22 and 4-Cl-o-PhDTA. 12 In these complexes one water molecule occupies the seventh position.…”

17O NMR study of oxo metalloporphyrin complexes: Correlation with electronic structure of M O moiety

“…17 O NMR spectroscopy has been also utilized to detect the hydrogen bonding interaction in amides and peptides [54]. For inorganic compounds, the 17 [55][56][57][58][59][60][61][62][63][64]. Furthermore, 17 O NMR spectroscopy has been applied for polyoxometallates for studying their structure, electronic states, and reactivities [65][66][67][68][69][70][71][72][73][74][75][76][77][78][79].…”