Chemistry in aldol complexes of metal dications: dehydration of the bisligand species

Abstract

The advent of electrospray has enabled the generation of microsolvated multiply charged metal ions. For dications, these had been produced for most common organic ligand classes including simple alcohols and ketones, but not aldols. Solution-phase aldol chemistry is reknown for dehydration and retro-aldol reactions. One of the simplest aldols is the acetone dimer, known as diacetone alcohol (DAA). It has recently been shown to coordinate gas-phase metal trications—the first precedent thereof for any protic solvent. Here we report on the formation and collisional fragmentation of DAA complexes for dications of divalent metals (M=Mg, Ca, Ba, Mn, Ni, Co, Fe, and Cu). Both retro-aldol and dehydration processes were observed in these species. Most notable is the extreme size-specificity of dehydration. Sequential loss of two waters dominates the dissociation of M 2+(DAA) 2 for all metals studied, yielding extraordinary M 2+(mesityl oxide) 2 peaks. However, M 2+(DAA) 3 essentially do not dehydrate. This suggests a geometry of complexes with two DAA in the first solvation shell, perhaps in a bidentate arrangement involving both carbonyl and hydroxyl. Similarly to their simple alcohol analogs, charge-reduction of M 2+(DAA) n proceeds via proton transfer, except in the case of Cu where proton and electron transfers compete. A comparison of the critical and minimum sizes for M 2+(DAA) n and M 3+(DAA) n reveals a major intrinsic gap between the stabilities of metal di- and trications in protic solvent complexes.