Influence of the metal size in the structure of the complexes derived from a pentadentate [N3O2] hydrazone

Abstract

The influence of the metal size in the nuclearity of the complexes derived from the hydrazone ligand 2,6-bis(1-salicyloylhydrazonoethyl)pyridine [H₄daps] has been investigated. We have synthesised a series of new complexes [M(H_xdaps)]·yH₂O, (x = 2,3; y = 0–3) with M = Ag (1), Cd (2), Al (3), Sn (4) and Pb (6), using an electrochemical procedure. The crystal and molecular structures have been determined for the mononuclear complexes [Sn(H₂daps)(H₂O)₂]·4H₂O (5) and [Pb(H₂daps)(CN)][Et₄N] (7). Complex 5 is the first neutral Sn(II) complex derived from a pentadentate hydrazone Schiff base ligand. Complex 7 shows the lead coordinated to the hydrazone donor set and a cyanide ligand, being the first reported complex with the lead atom coordinated to a monodentate cyanide group. Additionally, we have synthesised the lead complex using chemical conditions, in the presence of sodium cyanide which allowed us to isolate the neutral complex [Pb(H₂daps)] (8). Evaporation of these mother liquors led the novel compound [Pb(Hdaphs)(CH₃COO)] (9). Complex 9 shows the initial ligand hydrolysed in one of the imine bonds giving rise to a new tetradentate ligand [H₂daphs] coordinated to the lead atom and a bidentate acetate group. Moreover, the solution behaviour of the complexes has been investigated by ¹H, ¹¹³Cd, ¹¹⁷Sn and ²⁰⁷Pb NMR techniques. In particular multinuclear NMR has provided new useful data to correlate factors such as oxidation state, coordination number and nature of the kernel donor atoms due to the new coordination found in complexes 5 and 7. The comparative study of the structures of the complexes derived from this pentadentate [N₃O₂] hydrazone ligand let us to conclude that the metal size is a key factor to control the nuclearity of the complexes derived from the ligand [H₄daps].