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 sulfate 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 bis-monodentate fluoride anion, 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, one-dimensional (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 dihydroxo
bridges] interactions occur in 1 and 2 [the
Hamiltonian being defined as H = −J∑i=1n/2 (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 density functional theory (DFT)-type
calculations occur in 3.