Rational design of 1-D metal–organic frameworks based on the novel pyrimidine-4,6-dicarboxylate ligand. New insights into pyrimidine through magnetic interaction

Abstract

Single crystal X-ray analysis of compounds H₂pmdc·2H₂O (1), KHpmdc (2), and K₂pmdc (3) shows that the pyrimidine-4,6-dicarboxylate (pmdc) dianion presents an almost planar geometry which confers a potential capability to act as a bis-bidentate bridging ligand, and therefore, to construct 1-D metal complexes. Based on this assumption, we have designed the first six transition metal complexes based on this ligand of formula {[M(µ-pmdc)(H₂O)₂]·H₂O}_n [M(II) = Fe (4), Co (5), Ni (6), Zn (7), Cu (8)] and {[Cu(µ-pmdc)(dpa)]·4H₂O}_n (9) (dpa = 2,2′-dipyridylamine). The crystal structure of all of these complexes has been determined by single crystal X-ray measurements, except for compound 6 whose X-ray powder diffraction pattern reveals that it is isostructural to compounds 4–7. The bis-chelating pmdc ligand bridges sequentially octahedrally coordinated M(II) centres leading to polymeric chains. The hexacoordination of the metal centres is completed by two water molecules in compounds 4–8 and by the two endocyclic-N atoms of a terminal dpa ligand in compound 9. Cryomagnetic susceptibility measurements show the occurrence of antiferromagnetic intrachain interactions for compounds 4–6 and 8–9 (J = −2.5 (4), −5.2 (6), −32.7 (8), and −0.9 (9) cm⁻¹). Model calculations and analyses of the available experimental data have been used to examine the influence of several factors on the nature and magnitude of the magnetic coupling constants in pyrimidine bridged complexes, showing that metal deviation from the pyrimidine mean plane could lead to ferromagnetic behaviour.