Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex

Supramolecular assemblies of tetravalent terbium complex units: syntheses, structure, and materials properties

There is a growing interest in lanthanide complexes exhibiting unconventional oxidation states, primarily due to their unique electronic structures and accompanying physicochemical properties. Herein, likely the first examples of supramolecular assemblies of non-Ce(iv) tetravalent lanthanide complexes, with the general formula [Tb(OSiPh3)4Lx] n [1 (n = 2, L1 = 1,2-bis(4-pyridyl)ethane); 2 (L2 = 4,4'-bipyridine), 3 (L3 = 1,2-bis(4-pyridyl)acetylene), 4 (L4 = 1,2-bis(4-pyridyl)ethylene), and 5 (L5 = 1,4-bis(4-pyridyl)benzene)], are reported. Cyclic voltammetry studies show two successive redox events, indicating electronic interactions between the two Tb(iv) centers in the dimeric metallomacrocycle 1. Compounds 2-5 are zig-zag structured coordination polymers featuring complex units of Tb(OSiPh3)4 bridged by their respective pyridyl-based ditopic ligands. These tetravalent lanthanide species display impressive stability in air, which is believed to result from the stabilization effect of ligand Lx and the extensive multifarious interactions involving the aromatic rings of the anionic (Ph3SiO-) and bridging ligands. UV-vis absorption spectroscopic studies show that 2-5 are semiconducting, each with a narrow bandgap of ca. 1.7 eV. Magnetic property studies yielded magnetic entropy changes of ca. 8.0 J (kg K)-1 at 2.5 K and 7T, which is reasonable for a complex with high-molecular-weight ligands, suggesting the potential development of Tb(iv) complexes as molecular refrigerants due to their f7 electronic configuration.

Coordination Chemistry and Photoluminescence of Sm(II) Dibenzo-24-crown-8 Complexes

Three Sm(II) dibenzo-24-crown-8 (db24c8) complexes were synthesized in anhydrous, air-free conditions via the reaction of SmI2 with db24c8 and tetrabutylammonium tetraphenylborate ([TBA][BPh4]; where needed) in acetonitrile (CH3CN), dimethoxyethane (DME), and tetrahydrofuran (THF) to yield [Sm(db24c8)(CH3CN)2][BPh4][I]·CH3CN, [Sm(db24c8)(DME)]I2, and [Sm(db24c8)(THF)2]I2, respectively. In each case, a 10-coordinate, staggered dodecahedral (2:6:2) environment is formed around the Sm2+ center that is completed by either two solvent molecules (CH3CN or THF) or one bidentate solvent molecule (DME). Inner-sphere solvent molecules can be excluded by reacting SmI2 with db24c8 in 1:3 THF:toluene to yield Sm(db24c8)I2. This molecule features a distorted, eight-coordinate, hexagonal pyramidal Sm2+ metal center, where the coordinated db24c8 molecule shows a torsion angle unexpectedly close to the 180° antiperiplanar arrangement and two uncoordinated db24c8 oxygen atoms. Solution UV-vis-NIR measurements demonstrate that Sm2+ is a good size match for the cavity of various db24c8 conformations and that Eu2+ and Yb2+ exhibit competition between acetonitrile solvation and the Eu2+ and Yb2+/db24c8 complexes in solution. During excitation by 546 nm light, both [Sm(db24c8)(DME)]I2 and [Sm(db24c8)(THF)2]I2 exhibit mixed 5d → 4f and 4f → 4f emission at 20 °C and exclusively 4f → 4f at -180 °C, whereas Sm(db24c8)I2 only shows 5d → 4f emission regardless of temperature. Photoluminescence from [Sm(db24c8)(CH3CN)2][BPh4][I]·CH3CN is quenched.

Isolation of Inner-Sphere Aquo Complexes of Samarium(II)

The cis-anti-cis and cis-syn-cis isomers of [Sm(dicyclohexano-18-crown-6)(H2O)2]I2 exhibiting trans water molecules bound to the Sm2+ ion have been isolated and characterized. Sm2+ possesses an electrochemical potential sufficient for water reduction, and thus these complexes add to the recent body of evidence that the oxidation of Sm2+ by water can operate by a mechanism that is not straightforward. These complexes are obtained by the direct addition of stoichiometric amounts of water to solutions of the respective Sm(dicyclohexano-18-crown-6)I2 isomers under an inert atmosphere. The parent complex, Sm(dicyclohexano-18-crown-6)I2, lacking coordinating water molecules can be obtained through rigorous exclusion of water. It was determined that the bulky cyclohexano-substituents deter intramolecular interactions between [Sm(dicyclohexano-18-crown-6)(H2O)2]I2 complexes and slow the oxidization of the metal centers. The extent of the stability of these complexes to the presence of water has been further probed through cyclic voltammetry, where it was found that the redox potential of both isomers of [Sm(dicyclohexano-18-crown-6)(H2O)2]I2 maintains quasi-reversible behavior with a 50,000-fold excess of water to Sm2+ in solution with the cis-syn-cis complex being quasi-reversible at even higher concentrations of water. Solution-phase spectroscopy of these complexes in acetonitrile shows a corresponding hypsochromic shift of the Sm2+ 4f → 5d transition typically observed in the visible region from Sm2+ complexes. The crystalline compounds obtained in this study support solid-state spectroscopic trends observed from other Sm2+ crown-ether complexes containing iodide counterions, wherein the proximity of the iodide ions to the metal center determines whether the complex can exhibit 4f → 4f photoluminescence.

Crown-Ether Coordination Compounds of Europium and 24-Crown-8

Crown-ether coordination compounds of europium(II/III) and the crown ether (C2H4O)8 (24-crown-8, 24c8) are prepared, aiming at novel compounds, structures, and coordination modes as well as potential luminescence properties. By reacting EuCl2, EuI2, or EuCl3 with 24c8 or its derivatives in ionic liquids, the novel compounds [Bu3MeN]2[Eu(II)(NTf2)4] (1), [BMIm]6[Eu3I12] (2), [EuCl2(dibenzo-18c6)] (3), [EuI2(dibenzo-24c8)] (4), [(Eu(III)Cl3)2(C14H30O8)](24c8) (5), and [Eu(III)Cl(24c8)]I2 (6) are obtained (BMIm: 1-butyl-3-methylimidazolium; EMIm: 1-ethyl-3-methylimidazolium). Based on different reaction conditions, different coordinative modes including the absence of the crown ether in the product (1, 2), splitting of the crown ether (5), and coordination of 24c8 via six of eight oxygen atoms (4) and, finally, via all oxygen atoms (6) are observed. Crystallization of the title compounds is generally difficult, which can be attributed to the flexibility of the crown-ether molecule that can be rotated around all 24 tetrahedral (C) and pseudo-tetrahedral (O) centers. Besides structural characterization via single-crystal structure analysis and X-ray powder diffraction with Rietveld refinement, compounds 1-6 are examined by infrared spectroscopy and thermal analysis. The title compounds show blue to red emission, and the influence of structure and coordinative mode on the luminescent properties is analyzed.

Structure and bonding of a radium coordination compound in the solid state

The structure and bonding of radium (Ra) is poorly understood because of challenges arising from its scarcity and radioactivity. Here we report the synthesis of a molecular Ra2+ complex using 226Ra and the organic ligand dibenzo-30-crown-10, and its characterization in the solid state by single-crystal X-ray diffraction. The crystal structure of the Ra2+ complex shows an 11-coordinate arrangement comprising the 10 donor O atoms of dibenzo-30-crown-10 and that of a bound water molecule. Under identical crystallization conditions, barium (Ba2+) yielded a 10-coordinate 'Pac-Man'-shaped structure lacking water. Furthermore, the bond distance between the Ra centre and the O atom of the coordinated water is substantially longer than would be predicted from the ionic radius of Ra2+ and by analogy with Ba2+, supporting greater water lability in Ra2+ complexes than in their Ba2+ counterparts. Barium often serves as a non-radioactive surrogate for radium, but our findings show that Ra2+ chemistry cannot always be predicted using Ba2+.

Free Energy Calculations and Conformational Analysis of Dibenzo-30-crown-10 with Sm2+, Eu2+, and Three Halide Salts in THF Using the AMOEBA Force Field

Crown ether complexes have been tailored for use in industrial separations of lanthanides (Ln) as a part of rare earth mining and refining. Dibenzo-30-crown-10 (DB30C10) is one of the most efficient complexants for the separation of rare earth mixtures based on the cation size. To understand the origin of this complexation, molecular dynamics (MD) simulations of DB30C10 have been performed using different combinations of divalent Sm and Eu and three halide salts Cl^-, Br^-, and I^- in tetrahydrofuran (THF) solvent. DB30C10 was parameterized here for the polarizable atomic multipole optimized energetics for biomolecular simulation (AMOEBA) force field, and the existing parameters of THF, Sm²⁺, and Eu²⁺ were employed from our previous efforts. The large conformational fluctuations present in the DB30C10 systems were found to be dependent both on the identity of the lanthanide and halide complexes. For Cl^- and Br^- systems, there were no observed conformational changes at 200 ns, while in I^- systems, there were two conformational changes with Sm²⁺ and one with Eu²⁺ within that same timeframe. In SmI₂-DB30C10, there were three stages of conformational changes. In the first stage, the molecule is unfolded, in the second stage, the molecule is partly folded, and finally, in the third stage, the molecule is completely folded. Lastly, the Gibbs binding free energies of DB30C10 with SmBr₂ and EuBr₂ have been computed, which resulted in nearly identical ΔG_comp values for each lanthanide with Sm²⁺ being slightly more favorable. Considering the folding mechanism of the SmI₂ system with DB30C10, the Gibbs binding free energies of DB30C10 and dicyclohexano-18-crown-6 (DCH18C6) with SmI₂ were calculated separately and compared to probe their complexation affinities, in which the former was found to be more favorable.

Isolation of a californium(II) crown-ether complex

The actinides, from californium to nobelium (Z = 98-102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing Cf^II materials, but investigations are hampered by the fact that they have remained difficult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce Cf^III to Cf°. Here we show that a Cf^II crown-ether complex, Cf(18-crown-6)I₂, can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf^III can be quantitatively reduced to Cf^II, and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf^II and Cf^III complexes without the Al/Hg amalgam. Quantum-chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.

Molecular Dynamics and Free Energy Calculations of Dicyclohexano-18-crown-6 Diastereoisomers with Sm2+, Eu2+, Dy2+, Yb2+, Cf2+, and Three Halide Salts in Tetrahydrofuran and Acetonitrile Using the AMOEBA Force Field

With the continual development of lanthanides (Ln) in current technological devices, an efficient separation process is needed that can recover greater amounts of these rare elements. Dicyclohexano-18-crown-6 (DCH18C6) is a crown ether that may be a promising candidate for Ln separation, but additional research is required. As such, molecular dynamics (MD) simulations have been performed on four divalent lanthanide halide salts (Sm2+, Eu2+, Dy2+, and Yb2+) and one divalent actinide halide salt (Cf2+) bound to three diastereoisomers of DCH18C6. Dy2+, Yb2+, Cf2+, DCH18C6, and tetrahydrofuran (THF) solvent were parameterized for the AMOEBA polarizable force field for the first time, whereas existing parameters for Sm2+ and Eu2+ were utilized from our previous efforts. A coordination number (CN) of six for Ln2+/An2+-O solvated in THF indicated that the cations interacted almost entirely with the oxygens of the polyether ring. A CN of one for Ln2+/An2+-N solvated in acetonitrile for systems containing iodide suggested that the N atom of acetonitrile was competitive with I- for cation interactions. Fluctuation between five and six CNs for Dy2+ and Yb2+ suggested that although the cations remained in the polyether ring, the size of the ring may not be an ideal fit as these cations possess comparatively smaller ionic radii. Gibbs binding free energies of Sm2+ in all DCH18C6 diastereoisomers solvated in THF were calculated. The binding free energy of the cis-syn-cis diastereoisomer was the most favorable, followed by cis-anti-cis, and then trans-anti-trans. Finally, two major types of conformation were observed for each diastereoisomer that were related to the electrostatic interactions and charge density of the cations.

Hydration of divalent lanthanides, Sm2+ and Eu2+ : A molecular dynamics study with polarizable AMOEBA force field

The chemistry of divalent lanthanides, Ln2+ , is a growing sub-field of heavy element chemistry owing to new synthetic approaches. However, some theoretical aspects of these unusual cations are currently underdeveloped, especially as they relate to their dynamic properties in solution. In this work, we address the hydration of two of the classical Ln2+ cations, Sm2+ and Eu2+ , using atomic multipole optimized energetic for biomolecular applications (AMOEBA) force fields. These cations have not been parameterized to date with AMOEBA, and few studies are available because of their instability with respect to oxidation in aqueous media. Coordination numbers (CN's) of 8.2 and 8.1 respectively for Sm2+ and Eu2+ , and 8.8 for both Sm3+ and Eu3+ have been obtained and are in good agreement with the few available AIMD and X-ray absorption fine structures studies. The decreased CN of Ln2+ compared with Ln3+ arises from progressive water exchange events that indicates the gradual stabilization of 8-coordinate structures with respect to 9-coordinate geometries. Moreover, the effects of the chloride counter anions on the coordination of Ln2+ cations have been studied at different chloride concentrations in this work. Lastly, water exchange times of Ln2+ cations have been calculated to provide a comprehensive understanding of the behavior of Eu2+ and Sm2+ in aqueous chloride media.

Influence of Outer-Sphere Anions on the Photoluminescence from Samarium(II) Crown Complexes

Three samarium(II) crown ether complexes, [Sm(15-crown-5)₂]I₂ (1), [Sm(15-crown-5)₂]I₂·CH₃CN (2), and [Sm(benzo-15-crown-5)₂]I₂ (3), have been prepared via the reaction of SmI₂ with the corresponding crown ether in either THF or acetonitrile in good to moderate yields. The compounds have been characterized by single crystal X-ray diffraction and a variety of spectroscopic techniques. In all cases, the Sm(II) centers are sandwiched between two crown ether molecules and are bound by the five etheric oxygen atoms from each crown ether to yield 10-coordinate environments. Despite the higher symmetry crystal class of 1 (R3c), the samarium center resides on a general position, whereas in 2 and 3 (both in P2₁/c) the metal centers lie upon inversion centers. Moreover, the complexes in 2 and 3 are approximated well by D_5d symmetry. The molecule in 1, however, is distorted from idealized D_5d symmetry, and the crown ethers are more puckered than observed in 2 and 3. All three complexes luminesce in the NIR at low temperatures. However, the nature of the luminescence differs between the three compounds. 1 exhibits broadband photoluminescence at 20 °C but at low temperatures transitions to narrow peaks. 2 only exhibits nonradiative decay at 20 °C and at low temperatures retains a mixture of broadband and fine transitions. Finally, 3 displays broadband luminescence regardless of temperature. Spin-orbit (SO) CASSCF calculations reveal that the outer-sphere iodide anions influence whether broadband luminescence from 5d → 4f or fine 4f → 4f transitions occur through the alteration of symmetry around the metal centers and the nature of the excited states as a function of temperature.

Insights of the Structure and Luminescence of Mn2+/Sn2+-Containing Crown-Ether Coordination Compounds

Crown-ether coordination compounds with Mn²⁺ and Sn²⁺ as cations and 12-crown-4, 15-crown-5, and 18-crown-6 as ligands are synthesized. Their luminescence properties and quantum yields are compared and correlated with their structural features. Thus, MnI₂(15-crown-5) (1), MnCl₂(15-crown-5) (2), [Mn(12-crown-4)₂]₂[N(Tf)₂]₂(12-crown-4) (3), Sn₃I₆(15-crown-5)₂ (4), and SnI₂(18-crown-6) (5) are obtained by an ionic-liquid-based reaction of MX₂ (M: Mn, Sn; X: Cl, I) and the respective crown ether. Whereas 1, 2, and 5 exhibit a centric coordination of Mn²⁺/Sn²⁺ by the crown ether, 3 and 4 show a sandwich-like coordination of the cation with two crown-ether molecules. All title compounds show visible emission, whereof 1, 2, and 5 have good luminescence efficiencies with quantum yields of 47, 39, and 21%, respectively. These luminescence properties are compared with recently realized compounds such as Mn₃Cl₆(18-crown-6)₂, MnI₂(18-crown-6), Mn₃I₆(18-crown-6)₂, or Mn₂I₄(18-crown-6), which have significantly higher quantum yields of 98 and 100%. Based on a comparison of altogether nine crown-ether coordination compounds, the structural features can be correlated with the luminescence efficiency, which allows extraction of those conditions encouraging intense emission and high quantum yields.

Understanding the Stabilization and Tunability of Divalent Europium 2.2.2B Cryptates

Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl]⁺ complex is obtained from a methanol-THF solvent mixture, while a methanol-acetonitrile solvent mixture affords a [Eu(2.2.2B)(CH₃OH)]²⁺ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f⁶5d¹ to 4f⁷ transitions, computational results show that the substitution of a Cl^- anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. The synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

The role of the excited state dynamic of the antenna ligand in the lanthanide sensitization mechanism

A fragmentation scheme has been used to describe the photophysical phenomena associated with the antenna effect in organometallic lanthanide complexes. The theoretical protocol allows justifying the sensitization pathways.