Synthesis and structural characterisation of lead(II) isothiocyanate complexes with receptors derived from 1,10-diaza-15-crown-5

Abstract

Compounds of formula Pb(L 1)(SCN) 2·CH 3CN ( 1) and Pb(L 3)(SCN) 2·0.5EtOH ( 2) (where L 1 is the lariat crown ether N, N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and L 3 is a Schiff-base macrobicyclic receptor derived from the condensation of L 1 with 2,6-diformylpyridine) were isolated and structurally characterised both in the solid state and in acetonitrile solution. The X-ray crystal structure of 1 shows the metal ion coordinated to the seven donor atoms of the receptor and to the nitrogen atom of an isothiocyanate group. This structure is different to that found for the previously described perchlorate analogous [Pb(L 1)](ClO 4) 2, evidencing that the nature of the anionic group present has an important effect in the conformation and fold of L 1 when coordinated to lead(II). In 2, the metal ion is nearly centred inside the macrobicyclic cavity of L 3 clearly bound to the three oxygen atoms, to one pivotal nitrogen atom, to one imine nitrogen atom and to the pyridine nitrogen donor of L 3 , while the remaining donor atoms of L 3 are only weakly coordinated to the lead(II) ion. The coordination sphere of lead(II) is completed by the nitrogen atom of an isothiocyanate group, and the coordination polyhedron may be described as a distorted tricapped trigonal prism. A comparison between this structure and that previously described for [Pb(L 3)(ClO 4)] + demonstrates that for the L 3 complexes the different nature of the anion present (thiocyanate or perchlorate) hardly has an effect on the fold and conformation that the macrobicycle adopts when acting as a receptor for lead(II). So, the effect that the anion present has on the structure of the lead(II) complexes of L 1 and L 3 appears to be determined by the degree of flexibility of the organic receptor.