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).

本研究从以1:1:2摩尔比混合相应二价金属氯化物水合物、2,2′-联吡啶（2,2′-bipyridine，简称bipy）与焦磷酸钠的水溶液中，分离得到三个通式为{[M­(bipy)­(H₂O)­(H₂P₂O₇)]₂·2H₂O}的同结构化合物，其中M分别为Ni（化合物1）、Co（化合物2）和Mn（化合物3）。上述化合物的结构均通过单晶X射线衍射（single-crystal X-ray diffraction）解析确定。化合物1~3的结构由中性的水合(2,2′-联吡啶)二价金属双核单元构成，该单元通过两个以双齿/单齿配位模式结合的焦磷酸二氢根桥联。化合物1~3中的每个金属离子均采取六配位畸变八面体几何构型，金属离子处螯合bipy所张的角度减小[分别为79.6(1)°（1）、77.32(7)°（2）和72.9(1)°（3）]是导致该畸变的主要因素。分子内金属-金属间距分别为5.271(1) Å（1）、5.3065(8) Å（2）和5.371(1) Å（3）。在1.9~300 K的温度范围内对化合物1~3的多晶样品进行磁化率测试，结果表明其存在弱的分子内铁磁性耦合[J = +1.86(2) cm⁻¹（1）与+0.25(1) cm⁻¹（2）]及反铁磁性耦合[J = −0.48(1) cm⁻¹（3）]，对应的自旋哈密顿量（spin Hamiltonian）定义为H = −JSM₁·SM₁ₐ。这种较为罕见的焦磷酸二氢根配位模式，可使镍(II)或钴(II)配合物呈现铁磁性耦合相互作用。