Dihydroxamic acid complexes of iron (III): ligand pKa and coordinated water hydrolysis constants
A series of dihydroxamic acid ligands of the formula [RN(OH)C(O)]2(CH2)n, (n = 2, 4, 6, 7, 8; R = CH3, H) has been studied in 2.0 M aqueous sodium perchlorate at 25.0 °C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (pKa) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The pKa1 and pKa2 values are: n = 2, R = CH3 (8.72, 9.37); n = 4, R = CH3 (8.79, 9.37); n = 6, R = CH3; n = 7, R = CH3 (8.95, 9.47); n = 8, R = CH3 (8.93, 9.45); n = 8, R = H (9.05, 9.58). Equilibrium constants for the hydrolysis of coordinated water (log K) have been estimated for the 1:1 feeric complexes of the ligands n = 2, 4, 8; R = CH3. The N = 8 ligand forms a monomeric complex with Fe(III) while the n = 2 and 4 ligands form dimeric complexes. For hydrolysis of the n = 8 monomeric complex, log K1 = -6.36 and log K2 = -9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K1-4, for the dimeric complexes are as follows: n = 2 (-6.37, -5.77, -10.73, -11.8); n = 4 (-5.54, -5.07, -11.57, -10.17). The log K2 values for the dimers are unexpectedly high, higher in fact than log K1, inconsistent with the formation of simple ternary hydroxo complexes. A scheme is proposed for the hydrolysis of the ferric dihydroxamate dimers, which includes the possible formation of μ-hydroxo and μ-oxo bridges. © 1995.
Caudle, MT; Caldwell, CD; Crumbliss, AL
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