Low-Valent Rhodium and Iridium Assemblies Directed by Uracilate and Guaninate Linkers

Nucleobases combine rigid heterocyclic scaffolds with versatile N, O donor sets and intrinsic hydrogen-bonding capability, enabling access to diverse coordination architectures. Reactions of [{M(μ-OMe)(cod)}₂] (M = Rh, Ir) with uracil (H₂Ura) yield isostructural hexanuclear complexes [{M₂(cod)₂(μ₄-κ²N¹,N³:κ²O²,O⁴-Ura)}₃] (M = Rh (1), Ir (2)) featuring a double-decker metallacalix[3]arene motif sustained by dianionic μ₄ uracilate linkers. In solution, 1 establishes an equilibrium with an octanuclear uracilate-based species [{Rh₂(cod)₂(Ura)}₄] (3), as supported by DOSY NMR measurements. In contrast, reaction with guanine (H₂Gua) affords the octanuclear complex [{Rh₂(cod)₂(μ₄-κ⁴N¹,N⁷,N³,N⁹-Gua)}₄] (4), in which each dianionic guaninate bridges four Rh(I) centers through a μ₄ coordination mode. The resulting Rh₈ framework forms a closed metal-organic polyhedron with a gyrobifastigium topology reinforced by a continuous N-H···O hydrogen-bond belt. These results highlight how differences in nucleobase donor topology and hydrogen-bonding capability influence the assembly of low-valent Rh(I) and Ir(I) architectures.