Specificity in template syntheses of hexaaza-macrobicyclic cages: [Pt(Me5-tricosatrieneN6)]4+ and [Pt(Me5-tricosaneN6)]4+

Abstract

The racemic C₃ hexadentate cage complex, [Pt(Me₅-tricosatrieneN₆)]Cl₄ (1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosa-3,14,18-triene)platinum(IV) tetrachloride), was synthesised stereospecifically and regiospecifically from a reaction of the bis-triamine template [Pt(tame)₂]Cl₄ (bis[1,1,1-tris(aminomethyl)ethane]platinum(IV) tetrachloride) with formaldehyde and then propanal, in acetonitrile under basic conditions. Largely, one racemic diastereoisomer was obtained in a surprisingly high yield (∼ 50%), even though the molecule has seven chiral centres. The origins of the stereoselective synthesis are addressed. The crystal structure of [Pt(Me₅-tricosatrieneN₆)](ZnCl₄)_1.5Cl·2H₂O showed that all three imines were attached to one tame fragment with a chiral amine site (ΛSSS, ΔRRR) and a chiral methine carbon site (ΛRRR, ΔSSS) on each ligand strand. The PtN₆⁴⁺ moiety had a slightly distorted octahedral configuration with the two types of Pt–N bonds related to the imine and the amine donors, 2.050(7) and 2.072(6) Å, respectively. Treatment with sodium borohydride (15 s, 20 °C) at pH ∼ 12.5 reduced the imine groups, but not the Pt(IV) ion, producing a C₃ saturated ligand complex [Pt(Me₅-tricosaneN₆)]Cl₄ ((1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane)platinum(IV)tetrachloride). X-ray crystallographic analysis showed that the average Pt–N bond distance in the cation increased upon imine reduction to 2.10 (av) Å. The cyclic voltammograms of the two cage complexes displayed irreversible two-electron reduction waves in aqueous media and a ∼ 0.3 V shift to more positive potentials compared to that of the smaller cavity sar (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) analogue. After reduction, net dissociation of one strand of the cage was also evident, to give unstable square planar Pt(II) macrocyclic products.