Complexation behaviour of hexadentate ligands possessing N₂O₄ and N₂O₂S₂ cores: differential reactivity towards Co(II), Ni(II) and Zn(II) salts and structures of the products

Abstract

Reactions of divalent metal salts of Co, Ni and Zn with 1,2-di(salicylaldimino-O-phenylthio)ethane (H₂L¹) and 1,2-di(naphthaldimino-o-phenylthio)ethane (H₂L²), having N₂O₂S₂ cores, and 1,2-di(O-salicylaldimino-o-hydroxyphenyl)ethane (H₂L³), having a N₂O₄ core, have been explored. Out of the three ligands and the nine products obtained from the corresponding reactions, two ligands and seven products were crystallographically characterized. However, all the ligands and the products were characterized by analytical and spectral methods. Reaction of H₂L¹ and H₂L² with Co(II) salts results in oxidative cleavage of the C-S bond to produce a Co(III) product bound to two dissimilar tridentate ligands formed as a result of the cleavage. The reaction of H₂L¹ and H₂L² with Co(III)acac in the presence of methanolic NaOH also shows cleavage of the C-S bond to give compounds similar to those obtained by reacting H₂L¹ and H₂L² with Co(II) salts in the absence of additional external base. However, the reaction of Co(II) salt with H₂L³ did not lead to any C-O bond cleavage; rather it produced an octahedral Co(II) complex of the hexadentate H₂L³. Reactions of all these three ligands with Ni(II) salt resulted in octahedral complexes of the corresponding hexadentate ligands. In the case of Zn(II), while H₂L³ with a N₂O₄ core resulted in an octahedral complex, H₂L¹ and H₂L², both with a N₂O₂S₂ core, produced pseudo-octahedral complexes whose Zn-S bond lengths are rather long. The conformations of both the 5-membered and 6-membered chelate rings formed in the products were evaluated. The extent of distortion exhibited in the geometry of these octahedral complexes was computed and appropriately compared.