Use of the macrocyclic ligand cucurbit[6]uril for isolation of tetranuclear lanthanide aquahydroxo-carboxylate complexes from aqueous solutions

Abstract

0.5 }(NO 3 ) 9 •nH 2 O (Ln = Ho, Gd, or Er) were prepared by heating (130 °C) aqueous solutions of lanthanide nitrates, cucurbit[6]uril (C 36 H 36 N 24 O 12 ), and 4 cyanopyridine. The tetradentate coordination of the macrocyclic cucurbit[6]uril ligands through the portals leads to the formation of sandwich compounds, in which the tetranuclear hydroxo complex is located between two macrocyclic molecules. The polynuclear complexes are additionally stabilized by the chelating effect of the isonicotinate ligands generated by hydrolysis of 4 cyanopyridine. In the complexes, the aromatic moiety of the isonicotinate ion is encapsulated into the hydrophobic inner cavity of cucurbit[6]uril. In the absence of cucurbit[6]uril, the reaction with 4 cyanopyridine produces only the polymeric complexes [Nd(C 5 NH 4 COO) 3 (H 2 O) 2 ] and [Ln(C 5 NH 4 COO) 2 (H 2 O) 4 ]NO 3 (Ln = Pr, Sm, or Gd), whose structures were established by X ray diffraction. In water and aqueous solutions of nonionic and cationic surfactants, irreversible changes of the tetranuclear fragment of the complex (Ln = Gd) were observed after storage for two days, whereas the anionic surfactant stabilizes the complexes.

Tetranuclear Lanthanide Aqua Hydroxo Complexes with Macrocyclic Ligand Cucurbit[6]uril

“…The bond lengths in both types of lanthanide cores are presented in Table 1. The distances Ln-O agree well with the average bond lengths in Ln 4 hydroxo complexes [9][10][11][12][13][14][15][16][17]23] and decrease from 1 to 7 in accordance with the decrease of the lanthanide radius (1.13 Å for Pr 3+ and 1.01 Å for Yb 3+ [37] ). The sandwich cation is a common fragment for 1-7 but there are some differences in the structures and crystal packing of the compounds.…”

“…As we have shown earlier, slow hydrolysis of 4-cyanopyridine is necessary for the isolation of tetranuclear complexes from lanthanide nitrate aqueous solutions. [23] Slow cooling of the sealed reaction tubes and allowing them to stand at room temperature for several days, produces high yields of well-shaped pale colored or colorless crystals of complexes 1-7.…”

“…[18][19][20] Interest in gadolinium(III) nanoclusters with tunable electron relaxation behavior can be conceived for magnetic resonance imaging applications. [21][22][23] Great efforts have been focused on the europium(III) nanoclusters to construct luminescent nanodevices. [24,25] The sharpness of the intra-f-shell emission lines in lanthanide luminescence spectra makes them interesting candidates for phosphors, optical sensors, and probes in biochemistry.…”

“…Last year we managed to isolate and structurally characterize a tetranuclear lanthanide complex with CB [6] from aqueous solutions of holmium(III) nitrate for the first time. [23] Here we report the synthesis of a series of lanthanide(III) hydroxo complexes from aqueous solutions of CB [6], lanthanide chlorides, and 4-cyanopyridine. Two types of lanthanide complexes were obtained, in which metal atoms form a cubane-like fragment {Ln 4 (µ 3 -OH) 4 } 8+ and are coordinated additionally by two or four µ 2 -hydroxo bridges with formation of {Ln 4 (µ 3 -OH) 4 (µ 2 -OH) 2 …”

One-Dimensional Coordination Polymers of Lanthanide Cations to Cucurbit[7]uril Built Using a Range of Tetrachloride Transition-Metal Dianion Structure Inducers

“…Cucurbit[n]urils(Q[n]s) [9][10][11][12][13][14] consist of a rigid hydrophobic cavity of low polarity, accessible through two open polar portals rimmed with carbonyl groups, which can interact with various metal ions [15][16][17][18][19][20][21][22][23][24][25][26], or their complexes [27][28][29][30][31][32][33][34][35][36][37][38][39][40] or clusters [41][42][43][44][45][46][47][48][49][50] [53]. In recent years, there has been a trend towards introducing an organic or inorganic species into the metal-Q[n] systems as a "structure inducer" or trigger, since the properties, structural novelties, and functionalities of the resulting coordination polymers have often exceeded those observed in the absence of such agents.…”