Study of doubly charged alkaline earth metal and 3‐azidopropionitrile complexes by electrospray ionization mass spectrometry

Abstract

The present work describes a study of the complexation of calcium and magnesium by 3-azidopropionitrile by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of calcium and magnesium salts of the type CaX2 and MgX2 (where X = Cl or NO3) in water and methanol/water. The complexes detected were mainly double positively charged, with various stoichiometries not depending on the solvent, since water and 3-azidopropionitrile were always the main ligands. Solvation with methanol was not observed unlike in a previous study of complexation of nickel and cobalt by 3-azidopropionitrile. The complex ions [M(II)Az4(H2O)](2+), [M(II)Az5](2+) (where M = Ca and Mg) are the most abundant for both metals, and both counter ions. Tandem mass spectrometric (MS/MS) analysis showed that, under collision-induced dissociation (CID) conditions, the most important processes occurring were loss of neutral ligands and the replacement of 3-azidopropionitrile by water. A complex species containing reduced alkaline earth metal was due to radical loss, resulting from homolytic cleavage in the azide ligand. Some terminal ions, in the fragmentation sequences, point to the nitrile group as the coordination site in the 3-azidopropionitrile. Density functional theory (DFT) calculations confirmed this coordination site and proved that 3-azidopropionitrile behaves as a monodentate ligand in the systems under study. Moreover, the theoretical study proved that the presence of water ligand introduces stability through a hydrogen bond established between the water molecule and one nitrogen atom of the azido group. In addition, the strong dipole moment of 3-azidopropionitrile (4.76 D), which is mainly related to presence of the nitrile group, favors the stabilization of the metal-ligand complexes through charge-dipole interactions and the coordination of the metal to the nitrile group.

Differentiation of Isomeric HydroxypyridineN-Oxides Using Metal Complexation and Electrospray Ionization Mass Spectrometry

“…In the case of alkaline earth metals, doubly charged adduct ions were predominant, while singly charged species with chloride attached were important for transition metals such as cobalt and zinc. This complexation behavior has been reported previously for metals such as calcium, magnesium, cobalt, and nickel, with a different monodentate ligand, namely 3-azidopropionitrile [23,24]. As an exception, singly charged ions resulting from reduction of Cu (II) to Cu (I) were detected for isomer 2 in the case of copper, which is known to be easily reduced during the ESI process [25].…”

An electrospray ionization mass spectrometry study of azidoacetic acid/transition metal complexes

“…In those studies, the possibility of coordination by either the Nα or Nγ (Scheme ) of the azido group was established but no specific trend was found. We have been applying mass spectrometry (MS) combined with theoretical studies as a tool to investigate the fragmentation behaviour of several organic azides and their coordination to transition metals as an alternative to previous X‐ray crystallography studies . Some α‐carbonyl azides, which have proved to be useful in enzymatic studies, have been characterized by electron ionization (EI)‐MS .…”

“…Some α‐carbonyl azides, which have proved to be useful in enzymatic studies, have been characterized by electron ionization (EI)‐MS . However, the interaction of some organic azides with metals has been characterized using a different MS route involving electrospray ionization (ESI) …”

“…ESI, a softer ionization technique, is characterized by its ability to transfer ions in solution into the gas phase with their structures intact . Therefore, this technique supplies an obvious advantage since metal complexes can be easily transferred from solution to the gas phase maintaining the oxidative state as seen in solution . Moreover, the study of metal‐containing ions in the gas phase may provide information on the intrinsic physical and chemical properties of these ions in the absence of complicating factors such as solvation and ion‐pairing effects .…”

“…In our previous studies, ESI‐MS allowed us to investigate the complexation of 3‐azidopropionitrile and azidoacetone with different transition metals (nickel and cobalt) which were selected because of their important roles in biological processes . In those studies, azido‐metal complexes with different stoichiometries were observed and metal insertion in C–C bonds was inferred by tandem mass spectrometry (MS/MS) .…”