Specificity in template syntheses of hexaaza-macrobicyclic cages: [Pt(Me 5 -tricosatrieneN 6 )] 4+ and [Pt(Me 5 -tricosaneN 6 )] 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-t riene)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 times 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]platinu m(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 (3,6,10,16,19-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.