Mechanisms of formation of pseudocalixarene Schiff base macrocycles investigated by ESI-MS

• Main Author: Kulmaczewski, Rafal
• Published: Loughborough University 2011
• Subjects: 546
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.634766

Starting from 4-substituted phenols, three dialdehydes were synthesised as Schiff base pseudocalixarene macrocyclic precursors. Two of the dialdehydes, 2,2'methylene-bis- [(6-hydroxymethyl)-4-methylphenol] and 2,2'-methylene-bis-[(6-hydroxymethyl)-4- phenylphenol] were structurally characterised. For the phenyl substituted compound condensation with 1,3-diaminopropan-2-ol, with transition metal ions as template, was investigated and a series of dinuclear complexes was synthesised and characterised by IR, FAB-MS and elemental analysis. The analytical data implied that the complexes have the same saddle shape conformation controlled by hydrogen bonds resulting from mono-deprotonation of the methylendiphenol units as was observed in previous work. A range of TM2M trinuclear complexes [TM = Cu(II), Ni(II) and M = Li(I), Na(I), Mg(II), Ca(II)] of (2+2) macrocycles was synthesised and characterised by IR, MS (FAB, ESI) and elemental analysis. Additionally [Cu2Ca(2+2)(NO3)2](MeOH)2 was characterised by X-ray crystallography. An ESI-MS was used to follow condensation reactions between 2,2'-methylene-bis-[(6- hydroxymethyl)-4-tert-butylphenol] and 1,3-diaminopropan-2-ol in solution with various templates. It was found that, when a transition metal is used alone, the reaction produces only the (2+2) macrocycle. Cu(II) produced equilibrium mixtures containing dicopper(II) and tricopper(II) species but Ni(II) and Zn(II) yielded only dinuclear complexes. When transition metal ions were used in combination with group 1 or group 2 metal ions, the size of the macrocycle and nuclearity of the complex depended on the synthetic route and nature of alkali or alkaline earth metal. Among the products identified in the ESI-MS spectra were trinuclear complexes of the (2+2) macrocycle, pentanuclear sandwich complexes of two (2+2) macrocycles, tetranuclear (3+3) complexes, pentanuclear (4+4) and hexanuclear (6+6) species. One of the routes resulted in formation of [BaCu4(4+4)]2+ ion via a [BaCu4(dialdehyde)4]2+ cluster which was established to be a double template process where both metals are necessary for formation of the macrocycle. The central Ba(II) ion holds the dialdehydes together and the Cu(II) ion orients the carbonyl groups for Schiff base condensation.