Unexpected Formation of Complexes between a Protonated Organic Ligand and a Neutral Salt Molecule Studied by Electrospray Ionization Mass Spectrometry

Protonated organic ligands containing pyridine moieties form complexes with undissociated salt molecules present in solution, especially with ZnCl₂, as proved by electrospray ionization mass spectrometry. In the gas phase, the ions observed, for example [L1 H+ZnCl₂]⁺, lose an HCl molecule producing ions [L1 +ZnCl]⁺ (L1= 1,1,3,3-tetramethyl-1,3-bis(3-(3-(2-pyridyl)propoxy)propyl)disiloxane). The use of other zinc salts (for example, ZnI₂), also yielded the complexes between protonated ligands and neutral salt molecules (for example, [L1H+ZnI₂]⁺), but losses of respective acid molecules (for example, HI), proceeded with higher efficiencies than the loss of HCl molecule. An increase in solvent polarity, namely the use of water instead of methanol, led to the substantial lowering of [L1H+ZnCl₂]⁺ ion abundances, since fewer undissociated species (for example, ZnCl₂) were present in the solution. LigandL2(1,3,5,7-tetramethyl-1,3,5,7-tetrakis(3-(3-(2-pyridyl)propoxy)propyl)cyclotetrasiloxane, larger thanL1), forms doubly charged ions containing one and two ZnCl₂ molecules, for example [L1H₂+ZnCl₂]²⁺.