Molecular and Electronic Structure, and Hydrolytic Reactivity of a

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex Frankie D. White,† Cristian Celis-Barros,† Jillian Rankin,† Eduardo Solís-Ceś pedes,‡ David Dan,† Alyssa N. Gaiser,† Yan Zhou,† Jasmine Colangelo,† Dayań Paé z-Hernań dez,‡ Ramiro Arratia-Peŕ ez,‡ and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Relativistic Molecular Physics Group, Universidad Andres Bello, Replubica 275, Santiago, Chile

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‡

S Supporting Information *

ABSTRACT: The reaction of SmI2 with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu4N][BPh4], allows for the isolation of [SmII(DB30C10)][BPh4]2 as bright-red crystals in good yield. Exposure of [Sm(DB30C10)]2+ to solvents containing trace water results in the conversion to the dinuclear SmIII complex, Sm2(DB30C10)(OH)2I4. Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with SmII to a twisted conformation with SmIII. The optical properties of [SmII(DB30C10)][BPh4]2 exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Examination of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) shows that the ground state of SmII in [SmII(DB30C10)]2+ is a 4f6 state with low-lying 4f55d1 states, where the latter states have been lowered in energy by ∼12 000 cm−1 with respect to the free ion. The decacoordination of the SmII cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of SmII.

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differences in energies between the configurations are small.13 Thus, the particular configuration adopted is currently challenging to predict either experimentally or theoretically. The consequences of the placement of the additional electron profoundly affects the resultant physical and chemical properties of the complexes. Hence, an expansion of this chemistry beyond the rare examples that are currently known may provide the key to unlocking our understanding of what dictates configurational preferences in the f-block. Despite the unusual and potentially useful properties of encapsulated divalent lanthanide complexes, structurally characterized examples of cryptand and crown-ether complexes other than those containing EuII remain scarce owing to their substantially increased reactivity with water and oxygen.14 Oddly, SmII compounds have been known since the early 20th

INTRODUCTION Cryptands and crown ethers possess the ability to strongly bind lanthanide cations concomitant with substantial changes in the relative stabilities of the III+ versus II+ oxidation states.1−5 The resultant complexes can be used as sensors through selective and reversible quenching of the photoluminescence from several lanthanides, most notably EuIII and TbIII.6−10 Moreover, these shifts in the reduction potentials can be utilized to stabilize and perhaps isolate rare examples of divalent lanthanide complexes that can be used to understand coordination chemistry,11,12 electronic structure,1 and reactivity.2 The electronic structure of classical LnII compounds that are typically thought of as adopting 4f n+1 configurations is readily explained through the stabilization imparted by selfexchange that maximizes in half- and fully filled orbitals as exemplified by EuII (4f 7) and YbII (4f14). In contrast, other divalent lanthanides such as NdII and DyII have been observed in both 4f n+1 and 4f n5d1 configurations where calculated © XXXX American Chemical Society

Received: December 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b03566 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

of CH3CN. The DB30C10 solution was carefully added dropwise to avoid rapid precipitation, and the solution turned from dark green to bright red. After standing in a glovebox freezer at −34 °C for 12 h, bright red crystals suitable for single crystal crystallography formed in 78% yield (Figure S1). Anal. Calcd CHN for SmO10N2B2H43C80 (%): C, 68.27; H, 6.16; N, 1.99. Found (%). C, 68.12; H, 6.27; N, 1.31. High H and low N content due to loss of solvent and slight oxidation while under vacuum. Calculated CHN is more consistent with a single acetonitrile molecule being retained. 1H NMR of [Sm(DB30C10)]I2 (600 MHz, d-DMF): δ 8.2 (s, 8H); 7.13 (m, 30H), 6.98 (m, 10H); 4.33 (t, 8H); 4.00 (t, 8H); 3.89 (t, 8H); 43.82 (t, 8H); 2.33 (s, acetonitrile). Sm2(DB30C10)(OH)2I4. A 5 mL Schlenk flask was charged with SmI2 (10.2 mg, 0.025 mmol) and DB30C10 (14.7 mg, 0.027 mmol). CH3CN (3 mL) was added to the Schlenk flask without further drying and stoppered. After approximately 3−5 days, colorless crystals suitable for single crystal X-ray diffraction formed (Figure 2). Rapid

century, and are in fact historic enough to have once been denoted by samarous (or samaric 3+), and yet few structurally characterized examples exist outside of binary halides.13 Among the few well-characterized examples with SmII are [Sm(2.2.2-cryptand)]2+ that was recently obtained via an unanticipated displacement of K+ from [K(2.2.2-cryptand)]+,2 Sm(18-crown-6)(ClO4)2,15 where SmII is bound by both the crown ether and chelating perchlorate anions, and the sandwich complex, [Sm(15-crown-5)2][ClO4]2, where the perchlorate anions are outer sphere.15 The cavity size of these sequestering agents can also be expanded to accommodate multiple metal sites within a single ligand as is made possible with dibenzo-30-crown-10. However, examples of such complexes are largely restricted to alkali metals where they have been utilized in the selective extraction of radioactive 135,137Cs+ from nuclear waste using calixarene-crowns.16 The purpose of this paper is to provide the synthesis, structure, and electronic properties of a SmII dibenzo-30-crown-10 complex, the product of its reaction with water, and a comparison of the electronic structure of both complexes.

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EXPERIMENTAL SECTION

Syntheses. All manipulations were performed under Schlenk-type or glovebox conditions with exclusion of air. In the case of [Sm(DB30C10)][BPh4]2, tetrahydrofuran (THF) and acetonitrile (MeCN) were obtained from pressurized silica columns and further dried (NaK for THF, 3 Å molecular sieves for MeCN) before use in reactions. Caution! NaK is a sodium-potassium alloy that is extremely reactive with air and water. Care should be strongly taken in handling this material. For Sm2(DB30C10)(OH)2I4, acetonitrile (Sigma) was used as obtained from the manufacturer without further purification or drying. SmI2 (99.9%, Sigma), dibenzo-30-crown-10 (98%, Sigma), and tetrabutylammonium tetraphenylborate (99%, Sigma) were used as received without further purification. [Sm(DB30C10)][BPh4]2. The synthesis of [Sm(DB30C10)][BPh4]2 (Figure 1) was obtained by mixing SmI2 (10.1 mg, 0.025 mmol) with [Bu4N][BPh4] (27.7 mg, 0.049 mmol) in 0.5 mL of THF and 2.0 mL of CH3CN until all solids were completely dissolved. In a separate vial, DB30C10 (13.9 mg, 0.026 mmol) was dissolved in 2 mL

Figure 2. An illustration of the structure of the dinuclear, SmIII dibenzo-30-crown-10 complex, Sm2(DB30C10)I4(OH)2. The SmIII cations are bridging by bridging-OH anions that form hydrogen bonds with the crown ether. Two trans iodide anions complete the coordination sphere. 50% probability ellipsoids are depicted.

degradation of the crystals takes place shortly after forming, and the crystals change color and decompose into a pale-yellow powder. 1H NMR (600 MHz, CD3CN): δ 6.9 (s, 8H); 4.0 (s, 8H); 3.7 (s, 16H); 3.3 (s, 8H). The CHECKCIF program detected additional symmetry elements that were examined and proved to be noncrystallographic in orgin via the ADDSYM sub-routine of the PLATON program package. The structures were solved in OLEX2 equipped with the SHELXTL program suite and can be found in the CSD under deposition numbers 1843892 and 1843893.17,18 Computational Details. Geometry optimization and electronic properties were obtained by using the Amsterdam Density Functional (ADF) code, where the scalar relativistic and spin−orbit effects were incorporated using the zero-order regular approximation (ZORA).19−21 The structure was fully optimized via the GGA (Generalized Gradient Approximation) BP86 functional including the standard Slater-type-orbital (STO) basis sets with triple-ζ quality plus double polarization functions (TZ2P) for all atoms.22,23 The same level of theory was used to analyze the interactions between the two SmIII centers by means of the broken-symmetry approach (BS) developed by Noodleman and co-workers.24−29 In this approach, the real multideterminant electronic state with unpaired α and β electrons involving both metal centers is described by a single determinant Kohn−Sham wave function with electrons partially localized at different metal sites. Even though the energies and spin densities of the broken-symmetry state differ from those of the multideterminant antiferromagnetic state, the interaction parameter can be obtained from “mapping procedures” using spin-Hamiltonians and spin projection methods.

Figure 1. A view of the structure of the [SmII(DB30C10)]2+ cation in [SmII(DB30C10)][BPh4]2·2CH3CN. The SmII cation is bound by all 10 etheric oxygen atoms from the dibenzo-30-crown-10 ligand creating a sphenocorona geometry around the metal center. 50% probability ellipsoids are depicted. B


Article

Inorganic Chemistry For both Sm2(DB30C10)I4(OH)2 and [Sm(DB30C10)]2+, multiconfigurational Complete Active Space Self Consistent Field (CASSCF) calculations were performed in MOLCAS 8.230 for a better understanding of the nature of the ground state and low-lying excited states. The dinuclear SmIII local properties were investigated making a diamagnetic substitution by replacing one SmIII site with LaIII. First, CASSCF calculation was performed employing an active space consistent in five electrons in seven 4f orbitals CAS(5,7). Spin− orbit coupling (SOC) was introduced in the second step by diagonalizing the SOC operator based on the optimized CASSCF wave functions by the RASSI (Restricted Active Space States Interaction) method. Scalar relativistic effects are considered by means of the Douglas−Kroll−Hess transformation, and the spin− orbit integrals are calculated using the AMFI (Atomic Mean Field Integrals) method. In each case, the all-electron ANO-RCC basis set with TZP quality was employed for every atom. The same method was also applied to [SmII(DB30C10)]2+. However, expanded active space was utilized. Divalent lanthanides are characterized by a 5d subshell closer to the 4f in energy, making it possible to have a crossover between the 4f n+1 and 4f n5d1 states. Therefore, the expanded active space consisted of 6 electrons in 12 orbitals CAS(6,12), although only 1 of the 5d orbitals was considered as truly active because the occupation numbers of the remaining orbitals is negligible. Using this active space allows reproducing the proper ground state and understand the nature of the low-lying states. Also, since dynamical correlation could play a relevant role in this system, second order perturbation theory corrections (CASPT2) were included in the final results. Additionally, time-dependent density functional theory (TD-DFT) was used as complement to the ab initio electronic structure by calculating the absorption spectrum of [Sm(DB30C10)]2+. The ORCA 4.0.1 package31 was used to perform these calculations. Two functions were tested for this aim, the parametrized CAMB3LYP and PBE0, which are both hybrid functions alongside with the TZVP basis set. The former was discarded since it was unable to properly reproduce the ground state even though it is the most common functional used for these purposes. The PBE0 ground state provided an agreement with the CASSCF results.

mated as a sphenocorona, which is the most isotropic geometry for a 10-coordinate metal ion and likely results from the conformational flexibility of this large crown ether. The Sm−O bond distances range from 2.645(4) to 2.791(3) Å, with the longest interactions occurring to the etheric oxygen atoms that are bound to the phenyl rings. These distances are quite similar to those measured in [Sm(15-crown-5)2][ClO4]2,15 but are much longer than found in SmIII crownether complexes where distances of 2.4−2.5 Å have been measured.32,33 Of further interest is that the crown ether is folded into a socalled “Pac-Man” conformation that fully encapsulates the SmII cation. This conformation is reinforced by π−π stacking of the two phenyl groups as depicted in Figure 1 with a centroid to centroid distance of 3.594(7) Å. The form adopted by the crown ether in [SmII(DB30C10)][BPh4]2·2CH3CN differs substantially from that observed with Na+ and K+ cations whose ionic radius SmII lies between.34,35 When the crown ether hosts alkali metal cations, the phenyl groups are staggered and not eclipsed, and a Pac-Man conformation is not adopted. Figure 3 shows a comparison of conformations of dibenzo-30-crown-10 as pure substance, with alkali metal cations, and with divalent samarium.

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RESULTS AND DISCUSSION Syntheses. The synthesis of [SmII(DB30C10)][BPh4]2· 2CH3CN requires rigorous exclusion of air and water from the reaction mixture. Although much like observed for [Sm(15crown-5)2][ClO4]2,15 crystals of [SmII(DB30C10)][BPh4]2· 2CH3CN are surprisingly stable and can be stored under immersion oil for days in moist air without decomposition. Unexpectedly, the crystals can be placed directly in deoxygenated water for several days without decomposition. In contrast to the low reactivity of the solid, it was observed that, if the solvents used at any point in this synthesis contained even trace amounts of water, the initial red color gradually faded, even in an argon-filled glovebox, and large, colorless crystals would form prior to the addition of [Bu4N][BPh4]. X-ray analysis of these crystals revealed that they consist of the dinuclear SmIII complex, Sm2(DB30C10)(OH)2I4. One could speculate due to the poor reproducibility that this product forms via the reduction of water by SmII. However, SmII might first be oxidized by O2, followed by hydrolysis to create the Sm2(OH)2 core (vide inf ra). Surprisingly, this compound decomposes much more rapidly than [SmII(DB30C10)][BPh4]2·2CH3CN upon exposure to air. Structural Features. The crystal structure of [SmII(DB30C10)][BPh4]2·2CH3CN contains a SmII cation bound by all 10 etheric oxygen atoms from the DB30C10 as shown in Figure 1. The geometry around the samarium center is best approxi-

Figure 3. Depictions of the conformations of dibenzo-30-crown-10 in the solid state. The top figure (a) shows the form adopted by the crown ether without a metal ion. The second conformation (b) is adopted with alkali metal cations such as Na+ and K+. The final PacMan conformation (c) is found with SmII in [Sm(DB30C10)][BPh4]2·2CH3CN.

The structure of the SmIII hydrolysis product, Sm2(DB30C10)(OH)2I4, is composed of a Sm2(OH)2 diamond core within a DB30C10 ligand with two, trans, iodide anions bound to each SmIII cation as illustrated in Figure 2. The core bears some similarities to those observed with other LnIII cations encapsulated within crown ethers where nitrate most often C


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Inorganic Chemistry

Figure 4. Absorption (left at 298 K), and excitation and photoluminescence (right at 77 K) spectra of [SmII(DB30C10)][BPh4]2·2CH3CN. The absorption spectrum displays broad-band peaks indicative of 4f → 5d transitions, whereas the excitation and photoluminescence spectra obtained at 77 K reveal characteristic fine 4f → 4f transitions. The photoluminescence spectrum shows the 5D0 → 7FJ (J = 0, 1, 2, 3) transitions.

[BPh4]2·2CH3CN, it is found at 14 577 cm−1 placing both complexes intermediate in the range of energies. Broken-Symmetry (DFT) Calculations. The first method used to understand the nature of the interaction between SmIII centers in Sm2(DB30C10)(OH)2I4 was the Broken Symmetry (BS) approach. The molecule was optimized using two different spin polarizations, Δρ = 10 and Δρ = 0, to represent both the high (HS) and low spin (LS) states, respectively. In both cases, the geometrical parameters have practically the same values with some small deviations in the distances of the peripheral ligands. The Sm−Sm and Sm−I distances were 3.673 and 3.125 Å, respectively, while the distance from the Sm to the bridging oxygens was ca. 2.256 Å. The HS state shows a strong localization of the unpaired electrons on each SmIII center (Δρ = 5.187) which is a common feature in lanthanide compounds due to the shielded nature of the 4f shell. The LS state is near the HS state, but lower in energy. The stabilization of this state could be explained at the DFT level using the broken-symmetry (BS) formalism to avoid the use of the extensive post-Hartree−Fock corrections. The BS wave function is formed by one Slater determinant constructed using molecular orbitals localized onto the two paramagnetic centers with opposite spins (Figure 5). This determinant is not

serves to complete the coordination sphere and balance charge.36 Each SmIII center is also ligated by three etheric oxygen atoms from the crown ether. Thus, the SmIII cations are bound by five oxygen atoms that are also found to be approximately coplanar. The SmIII−O etheric oxygen atom distances are significantly shorter than those observed in [Sm(DB30C10)]2+ and average 2.464(7) Å. The SmIII−O distance to the bridging hydroxyl group is 2.249(9) Å. The coordination sphere is completed by the two, trans iodide anions with expected long distances of 3.115(11) Å. These distances agree with those observed in other lanthanide compounds with peripheral iodide ligands.37,38 The accommodation of the [Sm2(OH)2] core within the crown ether causes it to unfold from that observed in [Sm(DB30C10)]2+ into a twisted conformation as shown in Figure 3. This conformation is buttressed by hydrogen bonds between the bridging hydroxyl groups and the two etheric oxygen atoms that are not bound to the SmIII centers on each side of the crown ether with hydrogen-bonding distances of 3.096(7) Å. One could speculate that the weak SmIII−I and SmIII−O (etheric) interactions, the low coordination number of samarium, and the rather open pentagonal faces create opportunities for degradation of this complex in moist air. Spectroscopic Properties. Divalent samarium displays rich photophysics that are quite distinct from that exhibited by the trivalent state. Excitation and photoluminescence spectra were obtained from crystals of [Sm(DB30C10)][BPh4]2· 2CH3CN at 77 K and are depicted in Figure 4. The excitation spectrum shows both broad and sharp features with the former being assigned to transitions between the 4f6 ground state (7F0) and a 4f55d1 configuration (e.g., an 4f → 5d transition), with the latter fine features owing their origin to intra-f transitions. The f → f transitions occurring between the 5D0 → 7 FJ (J = 0, 1, 2, 3) states produce the fine features that are characteristic of lanthanides. The luminescence from [Sm(DB30C10)][BPh4]2·2CH3CN agrees well with that obtained from [Sm(15-crown-5)2][ClO4]2.15 A detailed discussion and review of SmII doped into a variety of hosts was used in the evaluation of [Sm(15-crown-5)2][ClO4]2 where it was shown that the energy of the 5D0 → 7F0 transition in a variety of host lattices is typically found in the range of ∼14 475 to 14 700 cm−1.15 In [Sm(15-crown-5)2][ClO4]2, the energy of this transition is at 14 636 cm−1, whereas, in [Sm(DB30C10)]-

Figure 5. Spin density representation for the High-Spin (HS) and Broken-Symmetry (BS) states. D


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Inorganic Chemistry an S2 eigenstate but corresponds to a total Ms = 0 state that occasionally can be related to the real Ms = 0 state in the socalled strong delocalization limit. However, in this case, a more proper approach would be to use the strong localization limit owing to the small overlap between the orbitals containing unpaired electrons.39 Using this approach, the calculated coupling constant between both states is 16.2 cm−1, which is consistent with antiferromagnetic coupling between both SmIII ions. However, considering the inherent overestimation of GGA functionals of these values, it is possible to conclude that, in this molecule, the metal centers are essentially uncorrelated. The energy of the BS state contains the most important contributions to the HS−LS separation, including the ligandbridge effects often called ligand-spin polarization observed in Figure 5. Ab Initio Calculations. The electronic structures of both SmIII and SmII compounds were investigated by means of the CASSCF multiconfigurational approach. In the dinuclear compound, the minimal active space containing the seven 4f orbitals for each metal center was sufficient to properly reproduce the electronic structure of the ground and low-lying excited states. In the case of SmII, the 5d subshell was necessary to be included in the active space because its energy is modifiable by the local environment, allowing for potential changes in the nature of the ground state. Sm2(DB30C10)(OH)2I4. As expected, in the dinuclear SmIII compound, each SmIII site shows an electronic structure very close to the free ion that is typical for trivalent lanthanides (Table 1). As shown in Table 1, the first five multiplets have pure contribution from the 6H spin-free ground term and are ordered in increasing energy. Additionally, the energy difference between multiplets is sufficient to confirm that all of them are well isolated as it generally occurs in trivalent lanthanide compounds. The analysis of the crystal-field parameters shows that both metal centers experience almost the same effects from the surrounded ligands (Table 2). It is noticeable that the B02 parameter is negative and corresponds to the largest value, indicating some axial character of the crystal field. This corresponds, according to the form of the related Stevens operator O02 = 3J2z − J(J + 1), to a stabilization of the MJ = 1/2 state as ground state. At the same time, the nonaxial parameters (mainly B−1 2 ) are also large, which reduces the axial character and produces a more isotropic crystal field. This can be explained in terms of the charge distribution of the free lanthanide ion at the lowest J state, where SmIII is associated with a prolate distribution. This means that strong-field ligands in axial positions introduce high local anisotropy in the electron density. However, in our case, the local anisotropy is reduced because of the weak-field iodide anions in axial positions in combination with the equatorial oxo ligands. This combination produces a more intense deformation of the charge distribution that can be diminished by replacing the axial weak-field ligands with strong-field ligands such as chlorine and fluorine anions. In that case, anisotropy is sought. [SmII(DB30C10)][BPh4]2. As previously mentioned, the electronic structure of LnII ions is richer than LnIII ions because of the well-known stabilization of the 5d subshell in the LnII ion that allows for crossover between 4f n+1 and 4f n5d1 configurations (Figure 6) in the ground state.40,41 Thus, the ligand field plays a key role as it affects the energy of the unoccupied 5d orbitals more strongly than the 4f core orbitals. Consequently, the spectroscopic properties, particularly the 4f

Table 1. Calculated Excitation Spectrum for Both Sm(III) Centers in Sm2(DB30C10)(OH)2I4 at SO-CASPT2(5,7) Level of Theory term

center I

center II

J

E (cm−1)

E (cm−1)

0.0 199.5 399.6 1098.9 1273.4 1353.7 1440.4 2385.9 2524.1 2604.4 2639.2 2735.9 3800.1 3921.1 4002.2 4053.7 4092.3 4196.2 5279.6 5391.6 5467.3 5526.8 5590.5 5661.4 5743.4 6731.8 6851.7 6904.8 6982.7 7081.8 7163.7 7281.3

0.0 202.8 401.6 1108.3 1278.9 1340.6 1446.2 2392.4 2528.3 2602.4 2633.0 2740.5 3804.8 3929.8 4001.1 4044.8 4093.3 4200.3 5279.4 5401.2 5473.9 5521.1 5579.9 5664.4 5752.1 6725.6 6855.7 6919.7 6978.8 7076.9 7167.9 7278.7

6

H

5/2

6

H

7/2

6

H

9/2

6

H

11/2

6

H

13/2

6

H, 6G, 6F

15/2

Table 2. Ab Initio Computed Crystal-Field Parameters That Show the Axial Character of the Dinuclear SmIII Complex, Sm2(DB30C10)(OH)2I4 center I

center II

k

Q

Bkq

Bkq

2

−2 −1 0 1 2 −4 −3 −2 −1 0 1 2 3 4

−0.62 10.68 −16.87 3.79 −4.47 −0.59 −2.23 1.79 −2.33 −0.35 −1.39 0.73 −1.78 −0.25

10.10 5.77 −16.97 2.20 −4.66 0.38 −3.63 −1.77 −2.30 −0.30 1.51 0.32 0.60 −8.79

4

→ 5d transitions, can be tuned by modifying the ligand field. On the basis of these criteria, our calculations aimed to determine the ground state configuration and the optical E


Article

Inorganic Chemistry

Table 3. Calculated Excitation Spectrum for [Sm(DB30C10)][BPh4]2 at SO-CASPT2(6,12) Level of Theory state 7

F0 F1

7

7

F2

7

F3

7

F4

Figure 6. Energy diagram showing the lanthanide states (black horizontal lines) and the interval of energy in which the f → d transitions appear (green square).

properties of SmII in a decacoordinated environment. The analysis of the multiconfigurational wave function reveals that the 4f6 configuration is the ground state, which corresponds to the 7F0 ground term, followed by the other 7FJ (J = 1−6) terms distributed by about 7500 cm−1 (Table 3). The excited states with configurations of 4f55d appear at ∼14 500 cm−1 and form a broad band that extends to 25 000 cm−1 (starts at 26 200 cm−1 in the free ion40). The arrangement of these states agrees with the experimental absorption spectrum determined for divalent samarium. However, the quintuplet states start at ∼15 000 cm−1 and extend beyond 25 000 cm−1 (Table 3). A more detailed analysis on the emissive 5DJ states shows that the 5 D0 state appears at 15 234 cm−1 which corresponds to an overestimation of only 657 cm−1 (less than 0.1 eV) with respect to the experiment (5D0 → 7F0 luminescent transition). Accordingly, a simplified photophysical mechanism underlying the observed emission might be based on the energy transfer from an excited septuplet with 4f55d configuration to a quintuplet 5DJ, followed by a vibrational relaxation and subsequent emission. This mechanism is also supported by the observed spectral overlap between the absorption and emission spectra (Figure 4). Time-Dependent DFT. TD-DFT calculations were carried out to understand in depth the nature of the transitions in the [Sm(DB30C10)]2+ ion. Figure 7 shows the calculated absorption spectrum that can be associated with the room temperature absorption spectrum (Figure 4). Table 4 summarizes the primary transitions involved in the experimental measured region. It is remarkable that all of the transitions are combinations of 4f → 5d and 4f → π* metal ligand charge transfer (MLCT) transitions in nature, differing only in the main contributor. The first three bands are dominated by the promotion of one electron from the 4f subshell to the 5d subshell, whereas the last three are dominated by CT transitions.

7

F5

7

F6

energy (cm−1) 0.0 116.5 162.4 561.4 571.4 1136.6 1254.6 1356.8 1443.6 1513.6 1752.0 1781.6 1963.9 2084.5 2334.5 2481.0 2781.8 2851.1 2895.8 3045.2 3057.2 3929.5 3980.5 4490.9 4496.2 4786.7 4787.7 4883.8 4934.6 5113.8 5256.3 5261.3 5264.6 5486.1 5497.6 5659.9 5676.7 5737.1 5741.1 6046.9 6117.1 6458.6 6475.2 6821.0 6822.2 7144.7 7144.7 7434.8 7434.8

state 5

D0 D1

5

5

D2

5

D3

energy (cm−1) 15234.4 17793.1 17794.9 17897.1 20484.2 20511.2 20550.9 20606.3 20884.7 24657.3 24668.5 24686.1 24686.9 24697.6 24699.1 24701.9

The nature of the orbitals involved in these transitions can be largely classified into pure 4f, 5d, and unoccupied ligand orbitals (π*) (Figure 8). It is interesting that the Pac-Man conformation (Figure 3c) is stabilized by π−π interactions that are also responsible for the broadening in the absorption spectrum of divalent samarium by MLCT transitions from 4f electrons to low-lying unoccupied orbitals with bonding character. This stabilizing interaction can be observed in the F


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Inorganic Chemistry

reduction potentials. While SmII borders a half-filled 4f shell, this does not explain the rather moderate reduction potential for reducing SmIII to SmII in a rigorous manner. Furthermore, the classical description of SmII as adopting a 6 4f ground state also appears to be overly simplistic. Our examination of the electronic structure of SmII in [Sm(DB30C10)][BPh4]2 and the electronic spectra of previously reported SmII complexes is consistent with a 4f6 with low-lying 4f55d1 states. The strong crystal-field effect produced by the 10coordinate crown ether allows for the stabilization of these 4f55d1 states to be lowered by approximately 12 000 cm−1 with respect to the free ion. The changes from broad features at room temperature to fine features at 77 K conform to what would be expected for a state with an isolated 4f6 configuration in the ground state. This description provides a much more satisfying explanation for the changes in the electronic absorption of SmII.

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Figure 7. Calculated TDDFT absorption spectrum of [SmII(DB30C10][BPh4]2.

Table 4. TD-DFT Absorption Spectrum Decomposition Where fosc Corresponds to the Oscillator Strengths of Each Transition in [Sm(DB30C10)][BPh4]2a E (cm−1)

fosc (× 10−3)

.transition

17614

1.92

21109

1.36

22665

2.30

23903

1.42

26280

1.36

28558

2.26

4f → 5d 4f → π* (MLCT) 4f → 5d 4f → π* (MLCT) 4f → 5d 4f → π* (MLCT) 4f → 5d 4f → π* (MLCT) 4f → 5d 4f → π* (MLCT) 4f → 5d 4f → π* (MLCT)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03566. Experimental details, computational analysis, crystallographic details for data collection, and structural determination of Sm2(DB30C10)(OH)2I4 and [SmII(DB30C10)][BPh4]2 (PDF) Accession Codes

CCDC 1843892 and 1843893 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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a

Dominant transitions are denoted in bold letters.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frankie D. White: 0000-0002-8644-1231 Cristian Celis-Barros: 0000-0002-4685-5229 Eduardo Solís-Céspedes: 0000-0002-7834-524X Yan Zhou: 0000-0002-7290-1401 Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311

Figure 8. Types of orbitals involved in excitation of [SmII(DB30C10)][BPh4]2.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Non-Covalent Interactions (NCI) surface shown in Figure S4 featuring a typical π−π stacking.

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Notes

The authors declare no competing financial interest.

CONCLUSION Samarium, europium, and ytterbium are considered the classical examples of lanthanide elements that are readily reduced to the divalent state, and examples of compounds containing these Ln2+ ions have been known for well over a century. All three ions have been described as adopting 4f n+1 configurations, i.e., 4f6, 4f 7, 4f14, for SmII, EuII, and YbII, respectively. The synthetic accessibility of the latter two examples is readily explained by the energetic stabilization imparted by self-exchange in spherically symmetric ground states, and is in turn reflected in the small, negative, standard

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ACKNOWLEDGMENTS This joint work was supported as part of the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0016568 and the Heavy Elements Chemistry Program (HEC) under Award Number DE-FG02-13ER16414. CAST provided the computational aspects of the research while (HEC) provided the experimental research. Part of the G


Article

Inorganic Chemistry

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computational calculations were provided by Fondecyt 1180017 and Fondecyt 1150629 grants.

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Molecular and Electronic Structure, and Hydrolytic Reactivity of a

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