Structures and Magnetism of a Series Mn(II) Coordination Polymers Containing Pyrazine-Dioxide Derivatives and Different Anions

The reaction of MnCl2·4H2O and 2,5-dimethylpyrazine-dioxide (2,5-dmpdo) with various anions in
H2O affords different complexes, the solid structures of which are controlled by the choice of counteranions. Reaction
of MnCl2·4H2O and 2,5-dmpdo with NH4NCS yields [Mn(NCS)2(2,5-dmpdo)1.5(H2O)2]·0.5(2,5-dmpdo)·H2O (1), which
contains Mn(II) dimers bridged by 2,5-dmpdo. However, with the replacement of NH4NCS with NaN3 or sodium
dicyanamide (Na(dca)) in the above reaction, Mn(N3)2(2,5-dmpdo)(H2O)2 (2) or Mn(dca)2(2,5-dmpdo) (3) was isolated.
Compound 2 consists of a one-dimensional zigzag chain constructed by 2,5-dmpdo bridging ligands. Compound 3 is
comprised of two-dimensional layers constructed by bridging 2,5-dmpdo and dca ligands. These results unequivocally
indicate that the nature of the counteranions, which play different roles in each complex, is the key factor governing
the structural topologies of them. While using 2,3-dimethylpyrazine-dioxide (2,3-dmpdo) instead of 2,5-dmpdo,
[Mn(dca)2(2,3-dmpdo)]·H2O (4) was obtained, which contains a CdSO4-like three-dimensional open framework
constructed by 2,3-dmpdo and dca, indicating a minor change of the bridging ligand can have great effect on the
topology of the crystal structure. Magnetic studies reveal that the four complexes exhibit spin-flop transitions below
2.12 K for 1, 6.14 K for 2, 6.23 K for 3, and 5.37 K for 4, respectively.