Dynamic Formation of Coordination Polymers versus Tetragonal Prisms and Unexpected Magnetic Superexchange Coupling Mediated by Encapsulated Anions in the Cobalt(II) 1,3-Bis(pyrid-4-ylthio)propan-2-one Series

The reactions of cobalt(II) halides and flexible ligand L [L = 1,3-bis(pyrid-4-ylthio)propan-2-one] under different
conditions generated a series of complexes with various structural motifs ranging from tetragonal-prismatic cages
to 1−3D coordination polymers. The layer diffusion of cobalt(II) chloride and L in methanol/acetone at 25 °C gave
rise to a 3D polymer, [Co(L)2Cl2]·Me2CO (1). At 30 °C, the slow diffusion of diethyl ether into the blue
dimethylformamide (DMF) solution of complex 1 afforded a 1D polymer, Co(L)Cl2(DMF)2 (2). However, at 10 °C,
the diffusion of diethyl ether into the DMF solution of complex 1 produced a tetragonal-prismatic cage, [Co2(L)4Cl2]Cl2·Et2O·DMF·2MeOH·4H2O (3). The reaction of cobalt(II) bromide and L in DMF at 10 °C yielded a dimer,
[Co2(L)4Br2]Br2·6DMF·2H2O (4), with a cage structure similar to 3. The preparation of the series of compounds
indicates the subtle relationship between structures and tunable reaction conditions. It is also found that the structural
motifs vary according to the ligand conformations and that the formation of tetragonal-prismatic cages 3 and 4 may
be templated by anionic guests. Magnetic studies on complexes 1−4 in a temperature range 4−300 K disclose that
L is unfavorable for a long-range magnetic interaction; however, intramolecular spin-coupling constants of −19.6
and −21.5 cm-1 for 3 and 4 indicate rather strong magnetic superexchanges arising from the overlap of the dz2
orbitals of the cobalt(II) and pz orbitals of the encapsulated halide anions. Electron paramagnetic resonance (EPR)
spectra of complexes 3 and 4 in solution and solid give information that both complexes are high-spin cobalt(II)
compounds with a rhombic distortion of the axial zero-field splitting. Interestingly, the intramolecular magnetic-exchange coupling in 3 and 4 mediated by the encapsulated anion Cl- or Br- is also reflected by the EPR spectra.