Synthesis and coordination chemistry of 1,4,7,10,13-pentakis(2-hydroxyethyl)-1,4,7,10,13-pentaazacyclopentadecane: a five armed pendant donor macrocycle

Abstract

Synthesis of the five armed pendant donor macrocycle 1,4,7,10,13-pentakis(2-hydroxyethyl)-1,4,7,10,13-pentaazacyclopentadecane (phec15) has been achieved by reaction of 1,4,7,10,13-pentaazacyclopentadecane ([15]aneN₅) with ethylene oxide in the absence of light and oxygen over an eight day period. The first three pK_a values for phec15 have been determined by potentiometric titration (I = 0.1 mol dm⁻³ NEt₄ClO₄) to be 8.51 ± 0.01, 7.57 ± 0.01 and 3.74 ± 0.01, respectively, and the final two are < 2.3. The stability constants (log(K/dm³ mol⁻¹) for [M(phec15]²⁺ complexes in aqueous solution where M = Co(II), Cu(II), Zn(II), Cd(II) and Pb(II) are 7.45 ± 0.03, 14.69 ± 0.02, 8.47 ± 0.01, 12.57 ± 0.03, and 10.35 ± 0.03, respectively. Gas phase ab initio modelling predicts that phec15 adopts the trans-I configuration, by virtue of linked hydrogen bonding between the five pendant hydroxyl groups, and that [Ba(phec15)]²⁺ and [Sr(phec15)]²⁺ are ten coordinate with an approximately pentagonal prismatic structure in which the plane of the five oxygen atoms is rotated either clockwise or anticlockwise with respect to the nitrogen atom plane, giving rise to Λ and Δ enantiomers, respectively. Variable temperature ¹³C{¹H} NMR spectra support these structural predictions and in the case of [Ba(phec15)]²⁺ complete lineshape analysis gave the kinetic parameters k = (4.0 ± 0.3) × 10⁴ s⁻¹, ΔH^‡ = 45.4 ± 2.2 kJ mol⁻¹ and ΔS^‡ = −4.5 ± 8.5 J K⁻¹ mol⁻¹ for intramolecular exchange between the Λ and Δ forms. Gas phase ab initio modelling and ¹³C{¹H} NMR spectra indicate that [Cd(phec15)]²⁺ is most probably six coordinate with an approximately trigonal prismatic N₃O₃ donor atom set arising from coordination of non-adjacent nitrogen atoms and their associated pendant hydroxyl groups.