Syntheses, crystal structures and electronic properties of [Cu(bipym)(C₅O₅)(H₂O₂]·H₂O and [Cu₂(bipym)(C₅O₅)₂(H₂O)₂·4H₂O (bipym = 2,2′-bipyrimidine)

Abstract

Two new complexes [Cu(bipym)(C5O5)(H2O)2·H2O 1 and [Cu2(bipym)(C5O5)2(H2O)2]·4H2O 2[bipym = 2,2′-bipyrimidine and C5O52–= dianion of croconic acid (4,5-dihydroxycyclopent-4-ene-1,2,3-trione)] have been synthesised and characterized by spectroscopic and X-ray diffraction methods. Crystals of 1 and 2 are triclinic, space group p, with a= 6.604(1), b= 9.491(1), c= 12.019(1)Åα= 88.37(1), β= 79.77(1), γ= 86.08(1)° and Z= 2 for 1, and a= 6.638(1), b= 8.765(2), c= 10.093(2)Å, α= 95.96(2), β= 93.11(2), γ= 91.73(2)° and Z= 1 for 2. Compound 1 consists of mononuclear [Cu(bipym)(C5O5)(H2O)2] units in which the copper atom shows a slightly distorted elongated octahedral co-ordination with two bipym nitrogens and two croconate oxygens forming the equatorial plane, and two water molecules in the axial positions. The structure of compound 2 is made up of centrosymmetric bipym-bridged dinuclear [Cu2(bipym)(C5O5)2(H2O)2] units in which the copper atom exhibits a slightly distorted square-pyramidal co-ordination, with two bipym nitrogens and two croconate oxygens defining the equatorial plane whereas a water molecule occupies the axial position. In addition, the second axial position is occupied by a water molecule at a distance of close to 3 Å. The intradimer metal-metal separation is 5.384 Å. The croconate group acts as a bidentate ligand in both complexes whereas bipym adopts the chelating and bis(chelating) co-ordination modes in 1 and 2, respectively. The temperature dependence of the magnetic susceptibility of 2 reveals the occurrence of a relatively strong intramolecular antiferromagnetic coupling, J(singlet-triplet energy gap)=–160 cm–1. This value is compared with those of related compounds on the basis of structural data and theoretical calculations.