Chirality and thermal stability of four salen complexes: Crystallographic, theoretical and TGA studies

Abstract

Four complexes of the general formula [M(L)] where (M = Cu, Ni, Pd or Co) and H2L is 2,2′-[cis-1,2-diaminocyclohexanediylbis(nitrilo-methylidyne)]bis(5-diethyl-amino]phenol were synthesized. The four complexes were characterized using FTIR and high-resolution mass spectroscopy. The crystal structures of [Cu(L)], [Ni(L)] and [Pd(L)] were determined. The asymmetric unit of [Cu(L)] or[Ni(L)] complexe consists of two enantiomer independent molecules which are proved to be two different enantiomers. The two enantiomers arise from the chirality of cis-1,2-disubstitutedcyclohexane. The thermal stability of the four complexes was investigated using thermal gravimetric analysis (TGA). The thermal stability of the ligand in the four complexes and that of free ligand follow the order [Ni(L)] > H2L ∼ [Cu(L)] > [Co(L)] > [Pd(L)]. The thermal stability trend was rationalized using three factors; the ionic characters of the M−N and M−O bonds, the strength M−O and M−N bonds (taking bond order as criterion) and the back donation from the metal to the antibonding orbitals localized on the ligand. NBO and QTAIM analysis indicate that ionic characters of the bonds of Cu-O and Cu-N are higher than the ionic characters in the corresponding bonds in the other three complexes. Also, the back donation from Cu center to the ligand is minimal, this rationalizes the fact the free H2L and the ligand in the [Cu(L)] complex decompose at similar temperatures. For the other three complexes, the M−O and M−N bonds have lower ionic characters and similar bond orders. Thermal stability is rationalized using stabilization energy due to ligand to metal back donation. Plotting of the decomposition temperature against the stabilization energy due to back donation gave a straight line with correlation coefficient R2 = 0.9906.