Guanine-containing copper(ii) complexes: synthesis, X-ray structures and magnetic properties

Abstract

Three new compounds of formula {[Cu(gua)(H₂O)₃](BF₄)(SiF₆)_1/2}_n (1), {[Cu(gua)(H₂O)₃](CF₃SO₃)₂·H₂O}_n (2) and [Cu(gua)₂(H₂O)(HCOO)]ClO₄·H₂O·1/2HCOOH] (3) [gua = 2-amino-1H-purin-6(9H)-one] showing the unprecedented coordination of neutral guanine, have been synthesised and structurally characterized. The structures of the compounds 1 and 2 contain uniform copper(II) chains of formula [Cu(gua)(H₂O)₃]_n²ⁿ⁺, where the copper atoms are bridged by guanine ligands coordinated viaN(3) and N(7). The electroneutrality is achieved by uncoordinated tetrafluoroborate and hexafluorosilicate (1) and triflate (2). Each copper atom in 1 and 2 is five-coordinated in a distorted square pyramidal environment: two water molecules in trans positions and the N(3) and N(7a) nitrogen atoms of two guanine ligands build the basal plane whereas a water molecule fills the axial position. The values of the copper–copper separation across the bridging guanine ligand are 7.183(1) (1) and 7.123(1) Å (2). 3 is an ionic salt whose structure is made up of mononuclear [Cu(gua)₂(H₂O)(HCOO)]⁺ cations and perchlorate anions plus water and formic acid as crystallization molecules. The two guanine ligands in the cation are coordinated to the copper centre through the N(9) atom. The copper atom in 3 is four-coordinated with two monodentate guanine molecules in the trans position, a water molecule and a monodenate formate ligand building a quasi square planar surrounding. Magnetic susceptibility measurements for 1 and 2 in the temperature range 1.9–300 K show the occurrence of significant intrachain antiferromagnetic interactions between the copper(II) ions across the guanine bridge [J = –9.6(1) (1) and –10.3(1) cm^–1 (2) with Ĥ = –J∑_iŜ_i·Ŝ_i+1].