Dinuclear [(VVO(putrebactin))2(μ-OCH3)2] Formed in Solution as Established from LC-MS Measurements Using 50V-Enriched V2O5

Analysis of 1:1 solutions of V(V) and the macrocyclic dihydroxamic acid siderophore putrebactin (pbH(2)) in 1:1 H2O/CH3OH using triple quadrupole liquid chromatography-mass spectrometry (LC-MS-QQQ) (pH approximate to 4) showed two well-resolved peaks (t(R)(1) 10.85 min; t(R)(2) 14.27 min) using simultaneous detection modes (absorbance, 450 nm; selective ion monitoring, m/z 437) characteristic of the previously identified oxidoV(V) complex [(VO)-O-V(pb)](+) ([M](+), m/z(calc) 437.1). Peak 1 gave mass spectrometry (MS) signals consistent with [(VO)-O-V(pb)](+), together with [(VO)-O-V(pb)-(OH)] and the dinuclear complexes [((VO)-O-V(pb))(2)(mu-OH)](+) and [((VO)-O-V(pb))(2)(mu-OH)(2)]. Peak 2 gave MS signals consistent with [(VO)-O-V(pb)](+), together with [(VO)-O-V(pb)(OCH3)] and the dinuclear complexes [((VO)-O-V(pb))2(mu-OCH3)](+) and [((VO)-O-V-(pb))(2)(mu-OCH3)(2)]. This analysis showed that two groups of V(V)/pbH(2) complexes with water- or methanol-derived ancillary ligands were resolved by liquid chromatography (LC). The detection of [(VO)-O-V(pb)](+) in both peaks could be accounted for by its production from dissociation (peak 1: [((VO)-O-V(pb))(2)(mu-OH)](+) -> [(VO)-O-V(pb)](+) + [(VO)-O-V(pb)(OH)]; peak 2: [((VO)-O-V(pb))(2)(mu-OCH3)](+) [(VO)-O-V(pb)](+) + [(VO)-O-V(pb)(OCH3)]). The assignment of the signal at m/z(obs) 959.2 (100%) as the dinuclear complex [((VO)-O-V(pb))(2)(mu-OCH3)(2)] ([M + Na+](+), m/z(calc) 959.3) and not an ion cluster of mononuclear [(VO)-O-V(pb)(OCH3)] ({2[M] + Na+}(+), m/z(calc) 959.3) was made unequivocal by the use of V-50-enriched V2O5, which gave a signal with an isotope pattern, comprising the sum of the patterns of the three constituent V-51-V-51, V-51-V-50, and V-50-V-50 species. Coordination of methoxide was confirmed upon the replacement of CH3OH with CD3OD, which generated [(VVO(pb))(2)(mu-OCD3)(2)] ([M + Na+](+), m/z(calc) 965.3, m/z(obs) 965.3). Analysis of 1:1 solutions of Mo(VI) and pbH(2) showed a single peak in the LC (t(R) 16.04 min), which gave MS signals that were characterized as mononuclear [Mo-VI(O)(2)(pb)] ([M + Na+](+), m/z(calc) 523.1, m/z(obs) 523.1) and dinuclear [((MoO)-O-VI(pb))(2)(mu-O)(2)] ([M + Na+](+), m/z(calc) 1019.1, m/z(obs) 1019.2). The steric and electronic effects of the cis-dioxido group(s) in [Mo-VI(O)(2)(pb)] mitigated coordination of solvent-derived ancillary ligands. The work highlights the value of using isotopically enriched metal ion sources and deuterated solvents to deconvolute metal/siderophore solution speciation. The results have relevance for an improved understanding of the coordination chemistry of pbH(2) and other marine siderophores in V(V)- and Mo(VI)-rich surface ocean waters.