Multidentate ligands for the synthesis of multi-metallic complexes

Abstract

The reaction of the Schiff base species tris-((2-hydroxybenzylidene)aminoethyl)-amine (TrenSal) and tris-((2-hydroxy-5-bromobenzylidene)aminoethyl)amine (Tren5BrSal) with the acetates of nickel and zinc are reported. Two trimetallic complexes (M₃L₂) of Tren5BrSal with nickel and zinc have been crystallographically characterised. The attempted crystallisation of bis-(tris-((2-hydroxybenzylidene)aminoethyl)amine nickel) nickel from solutions containing TMEDA lead to the production of two novel complexes: namely a nickel adduct of the partially hydrolysed TrenSal ligand and an interesting nickel bromide–carbonate salt. [(TrenSal)₂Ni₃] is reacted with PbCl₂ to form a novel tetrametallic complex, [{(TrenSal)Ni}Pb(NC₅H₅)Cl]₂, where a Pb₂Cl₂ moiety replaces the nickel at the core of the complex. Extending the study to include the related hexadentate ligand, 1,1,1-tris-((2-hydroxybenzylidene)-aminomethyl)propane (TEtSal), we were able to isolate and characterise both [(TEtSal)₂Ni₃] and [{(TEtSal)Ni}₂Pb].

Introduction

Tetradentate Schiff base complexes of nickel [1], [2], [3], [4], [5], [6], [7], zinc [8], [9], [10], [11], [12], [13] and to a lesser extent copper [14] adopt a trimetallic motif when prepared from the relevant metal acetate and a symmetric ligand constructed from two rigid chelating salicylidene units separated by a flexible spacer (e.g. 1,3-propanediamine, 1,4-butanediamine). This motif creates two different metal binding environments (Fig. 1). Domain one has the metal (M1) chelated within the Schiff base ligand (N1, N2, O1, O2) with ancillary ligands (acetate, O3; solvent, N3) in the axial positions. This mode of binding creates a second domain namely the two chelating clefts within the phenoxide donor (O1, O2) into which the second metal (M2) binds. This arrangement taken in combination with two bridging acetates creates a regular O₆ octahedral motif for the second metal.

Replacing the diamine spacer by a tripodal triamine within the Schiff base motif allows the introduction of a third salicylidene unit (Fig. 2). This should maintain the octahedral binding motif while dispensing with the need for bridging acetates. Although many of these ligands have been known for many years [1], [15], [16], [17] the catalogue of complexes derived from them is still somewhat limited. As a tri-anionic motif it is not surprising that most of the well-defined complexes are derived from trivalent metals [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] and that this combination facilitates the encapsulation of the cation and the formation of simple discrete complexes (Fig. 3). Divalent metals make additional demands on these tri-anionic ligands and an inspection of the small number of complexes derived from these combinations suggest that the trimetallic motif (Fig. 1) will re-establish itself with metals such as nickel and zinc [1], [28], [29], [30].

As a result of our studies on ligand flexibility [6], [12], [30], [31] we have begun to study the series of multidentate Schiff base species derived from tris-(2-aminoethyl)amine (Fig. 2, Tren) and salicylaldehyde, which potentially offers seven donor atoms [17]. Significantly this motif allows flexibility around the aminoethyl regions in conjunction with the rigidity of the salicylidene units. In our formative study of this ligand system we have elected to revisit the chemistry of nickel and zinc in the expectation that we will expand the catalogue of trimetallic complexes. A comparison is made with the hexadentate ligand system, tris-((2-hydroxybenzylidene)-aminomethyl)propane (TEtSal) which is expected behave in an identical manner to the previously reported TMeSal system [29].