Synthesis and structural and magnetic characterization of {[(phen) ₂Ni]₂(μ-P₂O₇)}·27H ₂O and {[(phen)₂Mn]₂(μ-P₂O ₇)}·13H₂O: Rare examples of coordination complexes with the pyrophosphate ligand

The reaction in water of M(II) [M = Ni or Mn] with 1,10-phenanthroline (phen) and sodium pyrophosphate (Na₄P₂O₇) in a 2:4:1 stoichiometry resulted in the crystallization of dinuclear complexes featuring the heretofore rare bridging pyrophosphate. Single-crystal X-ray diffraction studies revealed the complexes to be {[(phen)₂Ni] ₂(μ-P₂O₇)}·27H₂O (1) and {[(phen)₂Mn]₂(μ-P₂O7)}·13H ₂O (2) where the asymmetric M(phen)₂ units are bridged by bis-bidentate pyrophosphate, each metal ion exhibiting a distorted octahedral geometry. The bridging pyrophosphate places adjacent metal centers at 5.031 Å in 1 and 4.700 Å in 2, and its conformation also gives rise to an intramolecular π-π interaction between two adjacent phen ligands. Intermolecular π-π interactions between phen ligands from adjacent dinuclear complexes create an ornate 3D network in 1, whereas a 2D sheet results in 2. The hydrophilic nature of the pyrophosphate ligand leads to heavy hydration with the potential solvent-accessible area for 1 and 2 accounting for 45.7% and 26.4% of their unit cell volumes, respectively. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 1 and 2 revealed net weak intramolecular antiferromagnetic coupling between metal centers in both compounds with J = -3.77 cm^-1 in 1 and J = -0.88 cm^-1 in 2, the Hamiltonian being defined as H = -JS _A·S_B. The ability of the bis-bidentate pyrophosphate to mediate magnetic interactions between divalent first row transition metal ions is discussed bearing in mind the number and nature of the interacting magnetic orbitals.