Single-ion magnet behaviour in mononuclear and two-dimensional dicyanamide-containing cobalt(II) complexes

Three cobalt(II) complexes of formulae [Co(dca)(2)(bim)(4)] (1), [Co(dca)(2)(bim)(2)](n) (2) and [Co(dca)(2)(bmim)(2)](n) (3) [dca = dicyanamide, bim = 1-benzylimidazole and bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. Compound 1 is a mononuclear species where the cobalt(II) ion is six-coordinate with four bim molecules in the equatorial positions [Co-N-birn = 2.1546(15) and 2.1489(15) angstrom] and two trans-positioned dca ligands [Co-N-dca = 2.1575(18) angstrom] in the axial sites of a somewhat distorted octahedral surrounding. The structures of 2 and 3 consist of two-dimensional grids of cobalt(II) ions where each metal atom is linked to the other four metal centres by single dca bridges exhibiting the mu(1,5)-dca coordination mode [Co-N-dca = 2.190(3)-2.220(3) (2) and 2.127(3)-2.153(3) angstrom (3)]. Two trans-coordinated bim (2)/bmim (3) molecules achieve the six-coordination around each cobalt(II) ion [Co-N-bim = 2.128(3)-2.134(4) angstrom (2) and Co-N-bmim = 2.156(3)-2.163(39) angstrom (3)]. The values of the cobalt-cobalt separation through the single dca bridges are 8.927(2) and 8.968(2) angstrom in 2 and 8.7110(5) and 8.7158(5) angstrom in 3. Magnetic susceptibility measurements for 1-3 in the temperature range of 2.0-300 K reveal that these compounds behave as magnetically isolated high-spin cobalt(II) ions with a significant orbital contribution to the magnetic moment. Alternating current (ac) magnetic susceptibility measurements for 1-3 show a frequency dependence of out-of-phase susceptibility under static applied fields in the range of 500 2500 G, a feature which is characteristic of the single-ion magnet behaviour (SIM) of the Co(II) ion in them. The values of the energy barrier for the magnetic relaxation (E-a) are 5.45-7.74 (1), 4.53-9.24 (2) and 11.48-15.44 cm(-1) (3). They compare well with those previously reported for the analogous dca-bridged 2D compound [Co(dca)(2)(atz)(2)](n) (4) (E-a = 5.1 cm(-1) under an applied static field of 1000 G), which was the subject of a previous report.