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

Abstract

Analysis of 1:1 solutions of V­(V) and the macrocyclic dihydroxamic acid siderophore putrebactin (pbH₂) in 1:1 H₂O/CH₃OH using triple quadrupole liquid chromatography–mass spectrometry (LC-MS-QQQ) (pH ≈ 4) showed two well-resolved peaks (<i>t</i>_R(1) 10.85 min; <i>t</i>_R(2) 14.27 min) using simultaneous detection modes (absorbance, 450 nm; selective ion monitoring, <i>m</i>/<i>z</i> 437) characteristic of the previously identified oxidoV­(V) complex [V^VO­(pb)]⁺ ([M]⁺, <i>m</i>/<i>z</i>_calc 437.1). Peak 1 gave mass spectrometry (MS) signals consistent with [V^VO­(pb)]⁺, together with [V^VO­(pb)­(OH)] and the dinuclear complexes [(V^VO­(pb))₂(μ-OH)]⁺ and [(V^VO­(pb))₂(μ-OH)₂]. Peak 2 gave MS signals consistent with [V^VO­(pb)]⁺, together with [V^VO­(pb)­(OCH₃)] and the dinuclear complexes [(V^VO­(pb))₂(μ-OCH₃)]⁺ and [(V^VO­(pb))₂(μ-OCH₃)₂]. This analysis showed that two groups of V­(V)/pbH₂ complexes with water- or methanol-derived ancillary ligands were resolved by liquid chromatography (LC). The detection of [V^VO­(pb)]⁺ in both peaks could be accounted for by its production from dissociation (peak 1: [(V^VO­(pb))₂(μ-OH)]⁺ → [V^VO­(pb)]⁺ + [V^VO­(pb)­(OH)]; peak 2: [(V^VO­(pb))₂(μ-OCH₃)]⁺ → [V^VO­(pb)]⁺ + [V^VO­(pb)­(OCH₃)]). The assignment of the signal at <i>m</i>/<i>z</i>_obs 959.2 (100%) as the dinuclear complex [(V^VO­(pb))₂(μ-OCH₃)₂] ([M + Na⁺]⁺, <i>m</i>/<i>z</i>_calc 959.3) and not an ion cluster of mononuclear [V^VO­(pb)­(OCH₃)] ({2­[M] + Na⁺}⁺, <i>m</i>/<i>z</i>_calc 959.3) was made unequivocal by the use of ⁵⁰V-enriched V₂O₅, which gave a signal with an isotope pattern comprising the sum of the patterns of the three constituent ⁵¹V–⁵¹V, ⁵¹V–⁵⁰V, and ⁵⁰V–⁵⁰V species. Coordination of methoxide was confirmed upon the replacement of CH₃OH with CD₃OD, which generated [(V^VO­(pb))₂(μ-OCD₃)₂] ([M + Na⁺]⁺, <i>m</i>/<i>z</i>_calc 965.3, <i>m</i>/<i>z</i>_obs 965.3). Analysis of 1:1 solutions of Mo­(VI) and pbH₂ showed a single peak in the LC (<i>t</i>_R 16.04 min), which gave MS signals that were characterized as mononuclear [Mo^VI(O)₂(pb)] ([M + Na⁺]⁺, <i>m</i>/<i>z</i>_calc 523.1, <i>m</i>/<i>z</i>_obs 523.1) and dinuclear [(Mo^VIO­(pb))₂(μ-O)₂] ([M + Na⁺]⁺, <i>m</i>/<i>z</i>_calc 1019.1, <i>m</i>/<i>z</i>_obs 1019.2). The steric and electronic effects of the <i>cis</i>-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 pbH₂ and other marine siderophores in V­(V)- and Mo­(VI)-rich surface ocean waters