Structural and Spectroscopic Analysis of Ln(II) 18-crown-6 and Benzo-18-crown-6 Complexes (Ln = Sm, Eu, Yb)

Abstract

Three Ln2+ 18-crown-6 complexes of the formula Ln­(18-crown-6)­I2 (Ln = Sm, Eu, Yb) were isolated from the explicit synthesis of the corresponding LnI2 salts with 18-crown-6 and tetrabutylammonium tetraphenylborate in organic media under air-free conditions. Each metal complex forms a distorted hexagonal bipyramidal geometry and crystallizes in the monoclinic space group P21/n. Comparatively, crystallization of Ln­(benzo-18-crown-6)­I2 (Ln = Sm, Eu, Yb) from the reaction of LnI2 with tetrabutylammonium tetraphenylborate and benzo-18-crown-6 in THF/ethanol under similarly air-free conditions yields two polymorphs. The first form, α, crystallizes in the monoclinic space group P21/c (or the nonstandard setting P21/n for α-Yb); whereas the second polymorph, β, crystallizes in P . While the geometries of the molecules only vary slightly, the molecular packing and intramolecular contacts are quite different. In the structure of β, π–π interactions between the benzo- moieties of adjacent molecules are observed, whereas these interactions are absent in α. Despite the similarities in these classically 4f n+1 lanthanide systems, the complexes display distinct spectroscopic features in their respective absorption and photoluminescence spectra. Broadband 5d → 4f photoluminescence was observed for the Sm and Eu compounds in the NIR region and UV–visible region, respectively. None of the three Yb compounds exhibit photoluminescence UV–visible-NIR region; however, a unique photooxidation event was observed resulting in characteristic Yb­(III) 4f → 4f transitions in the NIR region of the absorption spectra of these compounds. These findings are discussed along with structural comparisons of the 18-crown-6 and benzo-18-crown-6 compounds as well as other reported Ln­(II) crown complexes in the literature.

Isolation of a Californium(II) Crown-Ether Complex

“…Compared to the photoluminescence observed from the isomorphous Sm(18-crown-6)I2 under the same conditions, the emission exhibited by Cf(18-crown-6)I2 is far less intense. 22 The solution-phase absorption spectrum for this system corroborates the presence of Cf 2+ prior to crystallization. As shown in Figure 4D, the absorption spectrum of [Cf (18-crown-6)x] 2+ in acetonitrile exhibits several broad features between 32,500 and 15,000 cm −1 .…”

“…The Cf-I bond distance is 3.1563(4) Å which is notably shorter than that of Eu II -I, but statistically longer than that observed for Dy II -I. 22 The shortening of this bond is ascribed to the mixing of iodide 4pz orbitals with Cf II 6𝑑 z 2 orbitals (Figure 3A). Seeing as this is the first condensed-matter Cf II molecule that has been isolated and characterized, these bond metrics help bracket the upper and lower limits of the ionic radius of Cf II , which is currently not established.…”

“…[NBu4][BPh4] to the reaction mixture to inhibit ion pairing as was done in other Sm II reactions. 22 It is important to note that while the CfI3•nH2O (n ≤ 6) is obviously hydrated that this material has been dried in vacuo, and this process is known to convert hexahydrates to monohydrates. few minutes to a peach-brown color as shown in Figure 1C.…”

“…Single crystal X-ray diffraction studies reveal that Cf(18-crown-6)I2 (Figure 2B) adopts a structure-type first established with Ln II cations (Ln II = Eu, Sm, Yb, Dy, Tm) that has been the subject of recent investigations for single molecule magnetism with Tm(18-crown-6)I2. 22,24,25 The Cf(18-crown-6)I2 molecule consists of a Cf II cation bound within the cavity of the crown ether by the six etheric oxygen atoms and by two trans iodide anions. This yields a slightly distorted hexagonal bipyramidal environment where the Cf II cation is located on an inversion center.…”

Solid-state and solution-phase characterization of SmII-aza[2.2.2]cryptate and its methylated analogue

“…Complexes of divalent lanthanide (Ln II ) ions have attracted attention in recent years owing to their interesting luminescence, magnetic, and electrochemical properties. 1–25 One such group of Ln II complexes that have been the subject of extensive investigation with regard to their structural, photophysical, and electrochemical properties is the Ln II -cryptates. 2,26–42 In many studies, cryptands such as 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (222) and 5,6-benzo-4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacos-5-ene are used to encapsulate Ln II ions, 26–32,37–42 and reported Ln II -cryptates have luminescence and electrochemical properties that are useful in applications including light-emitting diodes, imaging, and catalysis.…”