AN ELECTROSPRAY MASS-SPECTROMETRY STUDY OF SOME METAL-ION CAGE COMPLEXES

A series of cationic metal complexes of the bicyclic hexaamine cage co mpound 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 t he corresponding fast atom bombardment (FAB) mass spectra. The most ab undant ion in the ESI mass spectra of divalent metal-ion cage complexe s is the doubly charged molecular ion [M(cage)](2+). For trivalent met al-ion complexes spectra obtained using a low-resolution quadrupole ma ss spectrometer suggested that the most abundant ion is of the type [M (cage)(3+) - H+](2+). However, when the spectra of several of these ca ge complexes were obtained using a high-resolution sector instrument i t can be shown that the most intense peaks are due to mixtures of thes e and other ions, Fi(cage)(2+), formed by reduction of the metal ion i h the ion source. The ESI mass spectra of both di- and tri-valent meta l-ion complexes also show the presence of ion pairs [M(cage)(x+) + ani on(-)]((x-1)+). In general ions arising from the free cage are not obs erved which makes the ESI technique suited for characterising the comp lex cations. However, varying the cone and skimmer potentials can alte r the relative abundances of ions, and the degree to which reduction o f the central ion occurs, so these parameters must be carefully contro lled. The ESI mass spectra of analogous cobalt(III) complexes containi ng ammonia or ethane-1,2-diamine displayed more extensive fragmentatio n compared to those of the cobalt(III) cage complexes. This study demo nstrates the potential of ESI mass spectrometry for the characterisati on of metal cage complexes as a powerful adjunct to NMR spectroscopy a nd microanalysis.