Isolation of Elusive Tetranuclear and Pentanuclear M(II)–Hydroximate Intermediates in the Assembly of Lanthanide [15-Metallacrown-5] Complexes

Two intermediates in the assembly of lanthanide metallacrowns (MCs) of divalent transition metals and ligands in the picoline hydroxamic acid (picHA)/α-amino hydroxamic acid family were synthesized and crystallographically characterized. Structures of the elusive M^II[12-MC_M^II,L-4]²⁺ were obtained with M = Ni, Zn and L = picHA, quinaldic hydroxamic acid. Consistent with previous calculations, the complex is highly concave, particularly with Zn­(II). ESI-MS and ¹H NMR reveal that the complexes retain their structure in solution. The Zn­(II) analogue reacts with Ln­(III) ions to form Ln^III[15-MC_Zn(II),picHA-5]³⁺ in pyridine. The greater stability of Zn^II[12-MC_Zn(II),picHA-4]²⁺ relative to the Cu­(II) and Ni­(II) analogues is inferred and attributed to the square-pyramidal Zn­(II) ions being complementary with the concave MC topology. A Zn₄(picHA)₂(OAc)₄(DMF)₂ species bearing a tetranuclear [6-MC_Zn^II,picHA-2] motif was also isolated. A mechanism for M^II[12-MC_M^II,L-4]²⁺ formation is proposed on the basis of structural analysis of tetranuclear [6-MC_M^II,L-2] complexes. These results contribute to the goal of controlling the reactivity of intermediates in the assembly of lanthanide MCs, and coordination driven macrocycles in general, to prepare complexes with greater stability or enhanced physical properties