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–Nbim = 2.1546(15) and 2.1489(15) Å] and two trans-positioned dca ligands [Co–Ndca = 2.1575(18) Å] 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 μ1,5-dca coordination mode [Co–Ndca = 2.190(3)–2.220(3) (2) and 2.127(3)–2.153(3) Å (3)]. Two trans-coordinated bim (2)/bmim (3) molecules achieve the six-coordination around each cobalt(II) ion [Co–Nbim = 2.128(3)–2.134(4) Å (2) and Co–Nbmim = 2.156(3)–2.163(39) Å (3)]. The values of the cobalt–cobalt separation through the single dca bridges are 8.927(2) and 8.968(2) Å in 2 and 8.7110(5) and 8.7158(5) Å 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 (Ea) 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) (Ea = 5.1 cm−1 under an applied static field of 1000 G), which was the subject of a previous report.