Lanthanide/Platinum metallo-ligands: design strategies to heterometallic complexes and applications

The growing interest in d-f heterometallic complexes arises not only for the opportunity to combine and enhance the distinct properties of d-based and f-based metal centres within a single molecule, but also from the potential emergence of entirely new properties. Among the various reported strategies, the use of metallo-ligands as preformed molecular building blocks has proven to be an effective approach for assembling discrete and predictable heterometallic architectures. In the present work, the development of novel d-f heterometallic complexes was pursued through the design and synthesis of a series of metallo-ligands based on heteroditopic ligands bearing multiple donor sites. In the first section, three N-oxide ligands were explored for the preparation of d transition metal-based metallo-ligands. The reactivity of pyrzMO was investigated via bridge-splitting reactions of dinuclear precursors of general formula [M(μ-Cl)(ppy)x]2 (M = Pd, x = 1; M = Ir, Rh, x = 2). The presence of equilibria was incompatible with the use of any mononuclear metal complex as a metallo-ligand. The ligand N-(quinoxaline-4-oxide-methylene)-1-phenylmethanamine (G) was synthesised and reacted with [PtCl2(DMSO)2], affording [PtCl2(G)] as a single product. However, despite the presence of a free N-oxide donor potentially available for lanthanide coordination, this complex showed poor solubility and did not react with unsaturated lanthanide precursors. A series of d-based metallo-ligands was synthesised using pyrzcaMO as a divergent ligand. In these complexes, pyrzcaMO acts as a chelating agent toward transition metals (M = Pt, Ir, Rh) through its pyridine and carboxylate donor groups, while retaining the N-oxide moiety for potential coordination to unsaturated lanthanide fragments. Subsequent reactions with “[Y(tta)3]” yielded heterometallic [Y2M2] arrays, each forming a single product in solution as confirmed by NMR spectroscopy. The heterometallic nature of these arrays was further validated by SC-XRD studies on [Y(tta)3(µ-pyrzcaMO)(PtCl2(AsPh3))]2. In the second section, the complex reactivity of lanthanide centres was examined through the reactions of [PtCl(ppy)(pyrzMO)] with different unsaturated lanthanide fragments. The reaction with “[Ln(tta)3]” afforded heterometallic Ln2Pt2 arrays, while the use of [Ln(hfac)₃] led to the formation of supramolecular mesocate structures. These assemblies consist of two enantiomeric subunits (Λ and Δ), each featuring a nine-coordinate lanthanide centre bound to three terminal [PtCl(ppy)(pyrzMO)] ligands. The two [LnPt3] units are connected via uncommon CH···Cl-Pt weak interactions. To investigate potential structural changes arising from the substitution of the halogen atom involved in the CH···X-Pt interactions, the reaction was repeated with [PtBr(ppy)(pyrzMO)], affording analogous supramolecular architectures stabilized by CH···Br-Pt interactions. In the third section, the reactivity of DTPA bis-anhydride (DTPA-BA) with various transition metal-containing nucleophiles (Pt, Fe) was explored to synthesise new metallo-ligands. A series of Pt-based DTPA bis-ester derivatives was obtained, but hydrolysis prevented isolation of any heterometallic product. To address this limitation, more stable DTPA bis-amide derivatives were synthesised using ferrocene- and platinum-based nucleophiles. Their reactions with rare-earth ions afforded d–f heterometallic complexes as single products. The heterometallic nature of [YFe2] and [YPt2] was confirmed through ESI-MS experiments and further confirmed by SC-XRD studies for [YFe2].