Effect of H-bonding on the ambivalence of SCN− towards copper(II)

Condensations of 2-(2-aminoethyl)pyridine with 4-methylimidazole-5-carboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde generate the tridentate N donor ligands L and L′ respectively. Reactions of Cu(NCS)2 with L and L′ yield respectively CuL(SCN)(NCS) (1) containing a CuN4S core and CuL′(NCS)2 (2) having a CuN5 core. Both the cores are square pyramidal with SCN− bound in 1 at the axial position through the S end. This differential behaviour of SCN− in the two complexes despite the ligands being very similar, is investigated by DFT calculations at the B3LYP/TZV level. It is found that DFT calculations predict isolation of the Cu(ligand)(NCS)2 species for both the ligands L and L′. Presence of an offsetting intermolecular H-bonding between the N atom of the thiocyanate and the N–H proton of the ligand L of an adjacent molecule makes the binding of SCN− via the S end feasible in 1 resulting in the H-bonded dimer Cu2L2(SCN)2(NCS)2. The strength of the H-bond is estimated as 27.1 kJ mol−1 from the DFT calculations. The question of such H-bonding does not arise with L′ as it lacks in a similar H atom. Dimeric 1 represents a case of two non-interacting spins.

Binding of thiocyanate to Cu(II) through the S end as found here is uncommon. DFT calculations reveal that this has become possible because of a compensatory H-bond between the N atom of thiocyanate and an N–H proton of the ligand of an adjacent molecule. When the ligand lacks in this N–H proton, Cu(ligand)(NCS)2 is obtained. (Colour code: orange, Cu; gray, C; blue, N; yellow, S; white, H.)Figure optionsDownload as PowerPoint slide