An electrospray mass spectrometry study of some metal-ion cage complexes

Abstract

A series of cationic metal complexes of the bicyclic hexaamine cage compound fac-1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane have been examined by electrospray ionisation (ESI) mass spectrometry, along with metal complexes of related smaller and larger hexaamine cages. The ESI mass spectra are considerably simpler than the corresponding fast atom bombardment (FAB) mass spectra. The most abundant ion in the ESI mass spectra of divalent metal-ion cage complexes is the doubly charged molecular ion [M(cage)]²⁺. For trivalent metal-ion complexes spectra obtained using a low-resolution quadrupole mass spectrometer suggested that the most abundant ion is of the type [M(cage)³⁺– H⁺]²⁺. However, when the spectra of several of these cage complexes were obtained using a high-resolution sector instrument it can be shown that the most intense peaks are due to mixtures of these and other ions, [M(cage)]²⁺, formed by reduction of the metal ion in the ion source. The ESI mass spectra of both di- and tri-valent metal-ion complexes also show the presence of ion pairs [M(cage)^x++ anion^–]^(x–1)+. In general ions arising from the free cage are not observed which makes the ESI technique suited for characterising the complex cations. However, varying the cone and skimmer potentials can alter the relative abundances of ions, and the degree to which reduction of the central ion occurs, so these parameters must be carefully controlled. The ESI mass spectra of analogous cobalt(III) complexes containing ammonia or ethane-1,2-diamine displayed more extensive fragmentation compared to those of the cobalt(III) cage complexes. This study demonstrates the potential of ESI mass spectrometry for the characterisation of metal cage complexes as a powerful adjunct to NMR spectroscopy and microanalysis.