The Fmoc-protected precursor of an unnatural polyoxometalate amino acid (POM-AA, X_paa), in which an Anderson–Evans (AE)-type POM cluster is carried on the side chain, has been synthesised and used directly as a building block in standard automated solid-phase peptide synthesis. A tris(hydroxymethyl)azidobenzamide ligand was first grafted onto an AE-POM, and its azide handle was then linked to Fmoc-protected propargylglycine by a click reaction. Because the POM cluster is confined to the side chain, the amino-acid backbone retains its native reactivity: chain assembly proceeds by routine in situ coupling, without the pre-formed Fmoc amino-acid anhydrides required in our previous main-chain POM-peptide synthesis. Four positional isomers of X_paaLVFF were assembled on a commercial automated synthesiser. This work provides a practical route to peptides bearing POM side chains and points towards a new class of artificial metalloproteins, the polyoxometalloproteins.

The incorporation of lanthanides into polyoxometalates (POMs) is a fascinating area of research due to their unique chemical and physical properties, as well as their potential applications. Polyoxopalladate (POP) clusters feature flexible central cavities and coordination geometries dictated by their central metal ion templates and external capping groups. Two primary structure types have been reported: {Pd₁₂} cubes and {Pd₁₅} stars, which can accommodate various central metal ions, ranging from common transition metals in the +2 oxidation state to less conventional lanthanides in the +3 oxidation state. This work explores POP clusters containing cerium ions, leading to the discovery of two new structure types: [CePd₁₃O₉(SeO₃)₉(H₂O)]⁶⁻ (1) and [Ce₃Pd₂₂O₁₆(SeO₃)₁₆]⁸⁻ (2). Cluster 1 adopts a fused “star-cube” structure, combining half {Pd₁₅} star and half {Pd₁₂} cube with Ce⁴⁺ as the central template. 2 exhibits a twisted dumbbell-shaped structure (D_2d symmetry), where two Ce⁴⁺-centred {CePd₁₁} cubes are bridged by a third Ce⁴⁺ ion at the cluster core. Parallel experiments with other lanthanides yielded two propeller-like chiral POPs [LnPd₁₂O₇(SeO₃)₈Cl(H₂O)₂]⁴⁻ (Ln = Eu³⁺ (3); Ln = Gd³⁺ (4)), both displaying C₃ symmetry. Notably, this represents one of the rare instances in POP chemistry where a capping ligand (SeO₃²⁻) directly coordinates to the central lanthanide ions (Eu³⁺ and Gd³⁺). The twisted coordination of this capping ligand induces a uniform tilt in a set of {PdO₄} planes, resulting in the clusters’ propeller-like chirality. In contrast, similar reaction conditions for Tb³⁺ produced [TbPd₁₂O₈(SeO₃)₈]⁵⁻ (5), a normal cubic structure.