Dihydroxamic acid complexes of iron(III): Ligand pK(a), 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 degrees C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (pK(a)) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The pK(a1) and pK(a2) values are: n = 2, R = CH3 (8.72, 9.37); n = 4, R = CH3 (8.79, 9.37); n = 6, R = CH3 (8.86, 9.42); 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 ferric 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 K-1 = -6.36 and log K-2 = -9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K-1-4, 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 K-2 values for the dimers are unexpectedly high, higher in fact than log K-1, 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 mu-hydroxo and mu-oxo bridges.