Influence of Solvent and Counter Ion on Copper Complexes with N-Alkyl-Pyridine-2-Carboxamides as Studied by Electrospray Ionization Mass Spectrometry

Abstract

The copper complexes with the ligands containing pyridine-2-carboxamide functionality have attracted much attention in the last few years because of their interesting physico-chemical properties. 1-11 Some of the complexes were found to be effective HIV-1 protease inhibitors. 12-14 Electrospray ionization mass spectrometry (ESI-MS) is one of the most popular techniques used for the study of metal-ligand interactions. 15-18 However, ESI-MS has only been used occasionally for the study of the copper complexes with pyridine-2-carboxamide moieties. 1,11,12 This work concerns the interactions of copper(II) with Nalkyl-pyridine-2-carboxamides (1-3, Scheme 1) studied by ESI-MS. To establish the influence of the solvent, the solutions used for ESI mass spectra recording were prepared in methanol, acetonitrile or water (most popular solvents used for ESI-MS). To evaluate the influence of the counter ion, two copper salts, namely CuCl 2 and Cu(ClO 4) 2 , were used (most popular copper salts used for copper complex synthesis).

Self-Assembly of Quaterpyridine Ligands and Cu+ Cations into Helical Complexes of 2:2 Stoichiometry under Electrospray Ionisation Conditions

“…21,22 In acetonitrile, the equilibrium 2cu + = cu + cu 2+ is strongly shifted almost totally to form cu + . 23 therefore, under ESI conditions, copper(II) is easily reduced to copper(I) when acetonitrile is used as a solvent (much more easily than in methanol, in spite the higher ionisation energy of acetonitrlie than that of methanol), [17][18][19] but not always. 11 figure 3 shows the ESI mass spectra of acetonitrile and water solutions containing 1 and cucl 2 .…”

“…It is well known that upon electrospray ionisation (ESI) conditions copper(II)-containing complexes are easily reduced to copper(I)containing complexes. [11][12][13][14][15][16][17][18][19] therefore, it can be expected that, during ESI, the self-assembly process leading to the helical 2 : 2 complex formation will take place. In this communication we demonstrate that, indeed, such a process occurs.…”