Synthesis, Structure and Magnetic Properties of Regular Alternating μ-bpm/di-μ-X Copper(II) Chains (bpm = 2,2'-bipyrimidine; X = OH, F)

The preparation and X-ray crystal structure of four 2,2’-bipyrimidine (bpm)-containing copper(II) complexes of formula {[Cu2(μ-bpm)(H2O)4(μ-OH)2][Mn(H2O)6](SO4)2}n (1), {[Cu2(μ-bpm)(H2O)4(μ-OH)2]SiF6}n (2), {Cu2(μ-bpm)(H2O)2(μ-F)2F2}n (3) and [Cu(bpm)(H2O)2F(NO3)][Cu(bpm)(H2O)3F]NO3•2H2O (4) are reported. The structures of 1-3 consist of chains of copper(II) ions with regular alternation of bis-bidentate bpm and di-μ-hydroxo (1 and 2) or di-μ-fluoro (3) groups, the electroneutrality being achieved by either hexaaqua manganese(II) cations plus uncoordinated sulphate anions (1), uncoordinated hexafluorosilicate anions (2) or terminally bound fluoride ligands (3). Each copper(II) ion in 1-4 is six-coordinated in elongated octahedral surroundings. 1 and 2 show identical, linear chain motifs with two bpm-nitrogen atoms and two hydroxo groups building the equatorial plane at each copper(II) ion and the axial position being filled by water molecules. In the case of 3, the axial sites at the copper atom are occupied by a bpm-nitrogen atom and a bidentate fluoride ion, producing a ‘step-like’ chain motif. The values of the angle at the hydroxo and fluoro bridges are 94.11(6) (1), 94.75(4) (2) and 101.43(4)° (3). In each case, the copper-copper separation through the bis-bidentate bpm [5.428(1) (1), 5.449(1) (2) and 5.9250(4) Å (3)] is considerably longer than that through the di-μ-hydroxo [2.8320(4) (1) and 2.824(1) Å (2)] or di-μ-fluoro [3.3027(4) Å (3)] bridges. Compound 4 is a mononuclear species whose structure is made up of neutral [Cu(bpm)(H2O)2F(NO3)] units, [Cu(bpm)(H2O)3F]+ cations, uncoordinated nitrate anions and crystallization water molecules, giving rise to a pseudo-helical, 1D supramolecular motif. The magnetic properties of 1-3 have been investigated in the temperature range 1.9-300 K. Relatively large, alternating antiferro- [J = -149 (1) and -141 cm-1 (2) across bis-bidentate bpm] and ferromagnetic [αJ = +194 (1) and +176 cm-1 (2) through the di-hydroxo bridges] interactions occur in 1 and 2 [the Hamiltonian being defined as H = -J(S2i•S2i-1 - αS2i•S2i+1)]. These values compare well with those previously reported for parent examples. Two weak intrachain antiferromagnetic interactions [J = -0.30 and αJ = -8.1 cm-1 across bpm and the di-μ-fluoro bridges, respectively] whose values were substantiated by DFT-type calculations occur in 3.