Manipulating clusters by regulating N,O chelating ligands: structures and multistep assembly mechanisms

Abstract

We obtained a binuclear Gd(III) complex [Gd₂(L¹)₂(NO₃)₄] (1, HL¹ = (E)-2-(2-(pyridin-2-ylmethyleneamino)ethoxy)ethanol) by reacting N₂O₂-tetradentate chelating ligand HL¹ with Gd(NO₃)₃·6H₂O at solvothermal temperature of 100 °C. Time-dependent high-resolution electrospray ionization mass spectrometry (HRESI-MS) followed the formation of 1 and revealed it's the assembly mechanism: HL¹ → Gd(L¹) → Gd₂(L¹)₂. When we used the NO₂-tridentate chelating ligand HL² (HL² = 2-(2-aminoethoxy)ethanol) under the same reaction conditions, we obtained an “hourglass-like” nonanuclear Gd(III) cluster [Gd₉(L²)₈(μ₃-OH)₈(μ₄-O)₂(NO₃)₈]·2CH₃OH·H₂O (2). Their assembly mechanism was proposed: HL² → Gd(L²) → Gd₂(L²)₂ → Gd₃(L²)₃ → Gd₄(L²)₄ → Gd₅(L²)₄ → Gd₉(L²)₈. Most notably, when we used the NO-bidentate chelate ligand HL³ (HL³ = picolinic acid), we obtained a dodecanuclear Gd(III) cluster [Gd₁₂(L³)₈(OH)₁₆(NO₃)₈(OH)₄(H₂O)₄]·22CH₃OH·25CH₃CN (3) under the same reaction conditions. We found that the formation process of cluster 3 involved two different and mutually interfering self-assembly processes, namely, multitemplate-induced assembly (HL³ → Gd(L³) → Gd₂(L³)₂ → Gd₁₂(L³)₈) and stepwise assembly (HL³ → Gd(L³) → Gd₂(L³)₂ → Gd₃(L³)₂ → Gd₄(L³)₂ → Gd₅(L³)₂ → Gd₆(L³)₂ → Gd₁₂(L³)₈).