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_bim = 2.1546(15) and 2.1489(15) Å] and two trans-positioned dca ligands [Co–N_dca = 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 μ_1,5-dca coordination mode [Co–N_dca = 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–N_bim = 2.128(3)–2.134(4) Å (2) and Co–N_bmim = 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 (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.

Several 2,2′:6′,2′′-terpyridines substituted in the 4′-position were synthesized and their photophysical properties were investigated by absorption and photoluminescence spectroscopy in dilute solutions and solid state. The studies confirmed that the absorption and emission wavelengths, fluorescence quantum yields and lifetimes of 1-R^1–16 are strongly structure-related, demonstrating a decisive role of the nature of the substituent in determining the photophysical properties of 4′-functionalized terpyridines. Additionally, the density functional theory (DFT) calculations were performed for 1-R^1–16 to get insight into their electronic structure and spectroscopic properties.

Photophysics of [ReCl(CO)₃(4′-R-terpy-κ²N)].

The preparation, X-ray crystal structure, spectroscopic and variable-temperature dc and ac magnetic properties of two six-coordinate cobalt(ii) complexes of formula [Co(bim)₄(tcm)₂] (1) and [Co(bmim)₄(tcm)₂] (2) are reported.