Complexation of a four-strand tetraalcohol with labile metal ions probed by electrospray mass spectrometry

The saturated, stereodefined tetraalcohol 2,3,5,6-endo,endo,endo,endo-tetrakis(hydroxymethyl)bicyclo[2.2.1]heptane (tetol, L-1) and the simple alcohol butane-1,3-diol (L-2) form complexes with alkali metal ions (lithium, sodium, potassium, rubidium and caesium), alkali earth cations (magnesium, calcium, strontium and barium) and Ga(Ill) and Ce(IV) in aqueous solution, characterised by electrospray ionisation mass spectrometry (ESMS). Metal ion exchange between the Li+ complex of L-1 and the other metal ions is rapid, with a range of M(L-1)(n)(m+) species detected, in addition to solvated species. With the alkal metal ions, M(L-1)(+) and M(L-1)(2)(+) are dominant, although speciation varies with metal ion size. For the alkaline earth ions, a range of complex ions up to n = 8 are observed, although n = 1-3 dominate. A preference for M(L-1)(2)(2+) with Mg2+ versus M(L-1)(3)(2+) with Ca2+ may again relate to a larger ion size. For the higher-charged Ga(III) and Ce(IV), hydroxo species M(OH)(L-1)(n)((m-1)+) are dominant reflecting bulk solution behaviour, which the ESMS studies appear to map generally. (C) 2002 Elsevier Science B.V. All rights reserved.