Ferromagnetic Coupling in “Double-Bridged” Dihydrogenpyrophosphate Complexes of Cobalt(II) and Nickel(II)

Three
isostructural compounds of the formula {[M­(bipy)­(H2O)­(H2P2O7)]2·2H2O} [bipy = 2,2′-bipyridine; M = Ni (1), Co (2), Mn (3)] have been isolated from aqueous solutions
containing the corresponding metal­(II) chloride hydrate with a bipy
and sodium pyrophosphate solution in a 1:1:2 molar ratio, and their
structures were determined by single-crystal X-ray diffraction. The
structures of 1–3 consist of neutral
aqua­(2,2′-bipyridine)­metal­(II) dinuclear units bridged by two
dihydrogenpyrophosphate groups adopting a bidentate/monodentate mode.
Each metal ion in 1–3 is six-coordinate
in a distorted octahedral geometry, with the reduced value of the
angle subtended by the chelating bipy at the metal ion [79.6(1)°
(1), 77.32(7)° (2), and 72.9(1)°
(3)] being the main source of this distortion. The values
of the intramolecular metal–metal separation are 5.271(1) Å
(1), 5.3065(8) Å (2), and 5.371(1)
Å (3). Magnetic susceptibility measurements on polycrystalline
samples of 1–3 in the temperature
range 1.9–300 K shows weak intramolecular ferromagnetic [J = +1.86(2) cm–1 (1) and
+0.25(1) cm–1 (2)] and antiferromagnetic
[J = −0.48(1) cm–1 (3)] coupling, with the spin Hamiltonian being defined as H = −JSM1·SM1a.
This rarely observed coordination mode for dihydrogenpyrophosphate
leads to ferromagnetic coupling in complexes of nickel­(II) or cobalt­(II).