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

Abstract

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 (1)H 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.

Highly Emitting Near-Infrared Lanthanide “Encapsulated Sandwich” Metallacrown Complexes with Excitation Shifted Toward Lower Energy

“…64 This synthesis proceeded in moderate yield (58%) but proved less tedious than direct esterification 65,66 or solid-phase methods. 67 The complete deprotonation of quinHA resulted in the formation of a tetradentate dianionic ligand whose charge complements Zn 2+ in MC assembly.…”

Crystal structure of N-hydroxyquinoline-2-carboxamide monohydrate

“…N-Hydroxyquinoline-2-carboxamide, also known as quinoline-2-hydroxamic acid (QuinHA), has been used for the preparation of various metallacrown complexes (Stemmler et al, 1999;Trivedi et al, 2014;Jankolovits et al, 2013). Presently, the Cambridge Structural Database (Groom et al, 2016) contains ten entries on coordination compounds based on N-hydroxyquinoline-2-carboxamide, nine of which have been reported within the past four years.…”

Penta and hexanuclear nickel tiara-like clusters with two different thiolate bridges

“…3,4,[7][8][9][10][11] To form metallacrowns, one can substitute heteroatoms with the methylene carbon atoms of the parent ether complexes. [15][16][17][18][19][20][21][22][23] As to S-bridged metallacrowns, nickel thiolates with tiaralike structures ijNiIJμ-SR 2 ) n ] have been a fundamental issue in the development of S-bridged metallocrowns, since the pioneering work of Woodward in the middle of the 1960s. [12][13][14][15][16][17] Up to now, there are plenty of examples for the ubiquitous O and/or N-bridged metallacrowns, but only a limited number of literature reports are available concerning S-bridged metallacrowns.…”