Stereoselective non-equivalent bis-diimine coordination to Co(II) ion: Structure, luminescence and density functional theory calculations

Stereoselective coordination affording a luminescent tcc-isomer of dihalobis(diimine)Co(II) family in which two photoactive diimine ligands (L_NO2^?) are unsymmetrical and non-equivalent is reported (L_NO2^? = (E)-3-nitro-N-(pyridine-2-ylmethylene)aniline; ? = dihedral angle between the diimine unit including pyridine ring and the phenyl ring planes; tcc refers trans-cis-cis positions with respect to pyridine nitrogen-imine nitrogen-chloride donors). The single step reaction of 2-pyridinecarboxaldehyde, 3-nitro aniline and anhydrous cobalt dichloride affords only the tcc-(L_NO2^?1)(L_NO2^?2)Co^IICl₂ as the product which is characterized by IR, Mass, UV-Vis spectra and magnetic susceptibility measurements. Single crystal X-ray structure determination has established the orientation of the two ligands in the crystal as (L_NO2^33.7)(L_NO2^79.7)Co^IICl₂·0.25CH₃OH (1). The isomer is luminescent in frozen methanol glass at 77 K with the excitation and emission maxima respectively at 370 and 525 nm. The average luminescence life time is 1.86 ns measured on exciting at 356 nm. Density functional theory (DFT) calculation shows that the non-equivalence of the two diimine ligands prevails in the gas phase also and the geometry is (L_NO2^41.0)(L_NO2^60.5)Co^IICl₂. The calculation has identified four types of closely spaced localized π^∗ orbitals as unoccupied photoactive molecular orbitals (UPMOs). TD-DFT calculations have assigned the Co(II) to π^∗ charge transfer (MLCT) and π to π^∗ of charge transfer (LLCT) as the primary origin of absorption above 340 nm in methanol. Pendant nitro groups of the ligands and their intermolecular interactions have been the driving force of the stereoselectivity of the product. The binuclear L₄Co₂Cl₄ unit of 1 has constructed the helical 2D assembly and cis-CoCl₂ mediated strong inter-helices H-bonding interactions result the channeled 3D framework. Geometry optimization of other possible ttt, ctc, ccc and cct isomers has established that in the gas phase, the tcc-isomer is more stable at least by 20-42 kJ mol⁻¹ compared to other isomers where the two diimine ligands are equivalent.