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

Abstract

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 CuN 4S core and CuL′(NCS) 2 ( 2) having a CuN 5 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 Cu 2L 2(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.