Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer

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

Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure Robert G. Surbella III,† Lucas C. Ducati,‡ Jochen Autschbach,§ Nicholas P. Deifel,∥ and Christopher L. Cahill*,† †

Department of Chemistry, The George Washington University, 800 22nd Street NW, Washington, D.C. 20052, United States Department of Fundamental Chemistry Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes 748, São Paulo, SP 05508-000, Brazil § Department of Chemistry, University at Buffalo, State University of New York, 312 Natural Sciences Complex, Buffalo, New York 14260, United States ∥ Department of Chemistry, Hampden-Sydney College, Hampden-Sydney, Virginia 23943, United States ‡

S Supporting Information *

ABSTRACT: The synthesis and structural characterization of seven new [UO2(NCS)5]3−- and [UO2(NCS)4Cl]3−-containing materials charge balanced by 4-phenylpyridinium or 4,4′bipyridinium cations are reported. Assembly of these materials occurs via a diverse set of noncovalent interactions, with the most prevalent involving the terminal sulfur atoms, which can both accept hydrogen bonds and/or form S···S and S···Oyl interactions. The electrostatic potential of the [UO 2 (NCS) 5 ] 3− and [UO2(NCS)4Cl]3− anions was calculated and mapped on the 0.001 au isodensity surface to rationalize the observed assembly modes and to provide an electrostatic basis to elucidate the role of the S atoms as both donors and acceptors of noncovalent interactions. Compounds 1−7 display a range of colors (red to yellow) as well as pronounced thermochromism. A computational treatment (time-dependent density functional theory, TDDFT) of the absorbance properties supports the temperature dependence on the ratio of inter- to intramolecular ligand to metal charge transfer (LMCT) bands as obtained from UV−vis diffuse reflectance analysis. Finally, the luminescence profiles of these materials feature additional peaks atypical for most uranyl-containing materials, and a combined spectroscopic (Raman, IR, and fluorescence) and computational (harmonic frequency calculations) effort assigns these as originating from vibronic coupling between the ν1 UO symmetric stretch and bending modes of the isothiocyanate ligands.

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INTRODUCTION Actinide isothiocyanates are of general interest for studies of soft-donor contributions to separation technologies.1,2 Several studies have explored the role of SCN− species for enhancing actinide ion selectivity over lanthanides in solvent extraction, explanations for which have included an increased covalent character within (for example) An(III) versus Ln(III) ions and varied coordination preferences for 4f vs 5f metals as a consequence.3−6 Our interests in [UO2]2+ isothiocyanates specifically, however, are a bit more broad in that systematic syntheses of families of materials provide a platform for exploring structural chemistry and advancing supramolecular assembly efforts. Whereas such knowledge may inform separation efforts going forward, our motivations are admittedly less applied and instead aim to probe the nature of solution speciation with respect to observed solid-state assembly motifs within uranyl hybrid materials. As such, we have recently prepared a family of [UO2(NCS)4H2O]2− compounds wherein assembly of hydrated, uranyl isothiocyanate anions with organic © XXXX American Chemical Society

cations gave rise to a range of structure types displaying various hydrogen- or halogen-bonding motifs.7 We are interested in defining assembly criteria in systems containing such noncovalent interactions (NCIs), and uranyl isothiocyanates provide a fertile combination of reproducible coordination geometries along with a variety of potential assembly motifs including electrostatic contributions, as well as NCIs at terminal S sites and the uranyl oxo atoms. We report here the synthesis, structural characterization, and optical properties of seven new compounds based on homoleptic [UO 2 (NCS) 5 ] 3 − and chloro-substituted [UO2(NCS)4Cl]3− anions charge balanced with pyridinium cations (Scheme 1). An analysis of assembly motifs and crystallographically observed synthons is presented, yet perhaps of greater interest is the deep variation in color and the pronounced thermochromism exhibited by these materials. Received: October 24, 2017

A

DOI: 10.1021/acs.inorgchem.7b02702 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

stirred as 6 molar equiv (based on U) of potassium thiocyanate was added. The solution became bright yellow and was covered and stirred overnight. The bright yellow solution was decanted from a white precipitate (KNO3), placed in a 20 mL scintillation vial, stirred, and gently heated. The volume of the solution was reduced until an orange to red solid remained. The solids were dissolved in DI H2O and stirred while 2 molar equiv of either 4-PP or 4,4′-Bipy was added. The pH was slowly adjusted to ∼1.0 using 3 M HCl, and the solution was heated (if needed) to dissolve the organic cation. The solution was cooled slowly and left to evaporate under ambient conditions in a fume hood. Single crystals of 1−7 formed over the course of several days to weeks. Crystals of 1−7 were harvested from their mother liquors for single-crystal X-ray diffraction analysis. Selected crystallographic information is provided in Table 1, whereas experimental and refinement details are given in the Supporting Information. Due to the tendency of these phases to coform, quantitative yield data were difficult to obtain and are therefore not included in this study. Ultraviolet−Visible Diffuse Reflectance Spectroscopy (UVvis-DR). Solid-state samples were analyzed at 298(2) K using a Jasco V-570 UV-vis-NIR spectrophotometer equipped with an ISN-470 integrating sphere. Crystals of 1−7 were crushed and dispersed in a barium sulfate matrix for analysis. Fourier Transform Infrared Spectroscopy (FTIR). Infrared spectra of single crystals of 1−7 were collected from 400 to 4000 cm−1 using a Bruker Tensor 27 FT-IR microspectrometer. Crystals were placed on glass microscope slides and crushed using a diamond attenuated total reflectance (ATR) microscope objective. Raman Spectroscopy. Raman spectra of single crystals of 1−7 were collected using a Bruker Sentinel system linked via fiber optics to a video-assisted Raman probe equipped with a 785 nm 400 mW source and a high-sensitivity TE-cooled, 1024 × 255 CCD array. Each spectrum was collected for 15 s with four signal accumulations over the range 80−3200 cm−1. Fluorescence Spectroscopy. Data were collected on single crystals of 1−7 using a Horiba Jobin Yvon spectrophotometer and processed using Horiba FluorEssence software. The crystals were crushed and placed in a quartz NMR tube, which was capped and submerged in a quartz Dewar filled with liquid nitrogen. An excitation wavelength of 410 nm was used for analysis, whereas emission slit widths and other experimental parameters were optimized for each sample. Computational Details. All of the calculations (below) were performed using the Gaussian 09 (rev. D. 01) program.23 Graphical material based on the calculations was created with GaussView (V5.O).24 Electrostatic Potentials. The electrostatic potentials of the [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− tectons from 3 and 7, respectively, were calculated with KS density functional theory using the M06-2X25 functional with the following basis sets: U, 60MWBSEG + ECP-60MWB;26−28 Cl, def2-TZVP; S, C, N, and O, def2TZVP; H, def2-SVP.29 The atomic coordinates were taken directly from the crystallographic data. The ESP, V(r), of a molecule at some point, r, is given by eq 1 in atomic units, where ZA is the charge of nucleus A, located at RA, and ρ(r) is the electron density of the molecule.

Scheme 1. 4-Phenylpyridinium (Left) and 4,4′-Bipyridinium (Right)

Indeed, thermochromism has been observed previously in uranyl isothiocyanate7,8 and related systems,9−11 but is uncommon in 5f element chemistry, as the majority of thermochromic materials contain either transition or mainblock metals12−15 and (to a lesser extent) rare earths.16−18 Specific to uranyl systems, several thermochromic ionic liquids were reported by Aoyagi et al. that featured [UO2(NCS)5]3− complexes charge-balanced by methylimidazolium (mim) cations.8 The crystallographically characterized material, [1ethyl-3-mim]3[UO2(NCS)5], in that communication also exhibited temperature-dependent behavior as the crystals were yellow at room temperature and brown at 393 K.8 The origin of the thermochromic behavior of the liquids and solid were attributed to possible changes in the equatorial coordination number or alteration of the local coordination geometry due to anion−cation interactions. We focus here on exploring thermochromism in crystalline uranyl isothiocyanates wherein the ligand environment is not labile, and as such, the influence of temperature and second-sphere NCIs on the resulting optical properties can be delineated. We offer a thorough crystallographic characterization of these uranyl isothiocyanate materials at both 296(2) and 100(2) K in order to identify any possible temperature-dependent structural changes that may contribute to the observed thermochromism. Electrostatic potential (ESP) maps of the [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− anions (hereafter “tectons”, or molecular building units) were calculated using Kohn−Sham (KS) density functional theory in order to complement the structural analysis and to rigorously characterize the NCI acceptor−donor sites on the uranyl tectons. Further, crystallographic analysis is also used to support the diffuse reflectance data and a subsequent computational study (via TDDFT) aimed at identifying the molecular orbital pairs involved in the transitions that give rise to bright crystal colors. Moreover, the TDDFT calculations are used to provide a molecular-level rationale for the thermochromic properties. The luminescence spectra of these materials display features atypical of most uranyl materials19−22 yet are seemingly the norm within uranyl isothiocyanate containing systems. We have used a combined experimental, i.e. fluorescence, Raman, and infrared spectroscopy, and computational, i.e. harmonic frequency calculations, approach to assign these vibronic features as originating from the equatorial SCN ligands.

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EXPERIMENTAL SECTION

V (r) =

Synthesis and X-ray Diffraction. The reagents used in this study were purchased and used without further purification: potassium thiocyanate (Fisher Scientific, ≥98.5%), 4-phenylpyridine (4-PP; Sigma-Aldrich, 97%), and 4,4′-bipyridine (4,4′-Bipy; Sigma-Aldrich, 98%). The protonated (and doubly protonated) forms of the cations will be written as 4-PPH, 4,4′-BipyH, and 4,4′-BipyH2. Caution! These compounds contain depleted U, and as such, standard precautions and protective measures should be taken when handing this radioactive and toxic heavy metal. Synthesis of 1−7. Thorough details of our synthetic efforts are included in the Supporting Information; however, a brief procedure is included here. Uranyl nitrate hexahydrate was dissolved in acetone and

∑ A

ZA − |RA − r|

∫ |rρ′ (−r′)r| dV ′

(1)

The ESP refers to a fictitious positive probe charge of 1 atomic unit (au) at position r. A negative potential reflects an electron-rich region, whereas an electron-poor region is indicated by a positive potential. Neither the ESP nor its visual representation contains information regarding the response of the molecule to the presence of a probe charge. However, chemically useful information can be deduced when the ESP is mapped onto an isodensity surface. The 0.001 au isodensity (electrons bohr−3) surface was chosen, as it encompasses most of the integrated electron density and is considered to be the most relevant for evaluating noncovalent interactions.30,31 B


C

formula formula mass cryst size (mm3) cryst color cryst syst space group Z radiation type temp (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg)

4

1

5

2

6 (C10H10N2)2[UO2(NCS)5]·NO3 938.84 0.159 × 0.091 × 0.042 light yellow orthorhombic Pnma 4 Mo Kα 100(2) 14.0849(13) 23.637(2) 9.6739(9) 90.00 90.00 90.00

(C11H10N)3[UO2(NCS)5] 1029.03 0.277 × 0.217 × 0.191 yellow monoclinic C2/c 4 Mo Kα 100(2) 10.1231(12) 23.714(3) 16.959(2) 90.00 104.0890(10) 90.00 3948.7(8) 1.731 4.420 22011 5637 0.0342 0.0171 0.0429 0.0177 0.0431 1.075 1567362

(C10H10N2)2[UO2(NCS)4(NCS)0.75(Cl)0.25]·SCN 929.03 0.285 × 0.196 × 0.183 dark orange orthorhombic Pnma 4 Mo Kα 100(2) 14.5806(5) 23.2930(8) 9.6149(3) 90.00 90.00 90.00

(C11H10N)3[UO2(NCS)5]·3H2O 1083.07 0.122 × 0.095 × 0.042 orange triclinic P1̅ 2 Mo Kα 100(2) 9.5993(7) 14.3438(11) 16.2724(12) 94.7590(10) 101.8390(10) 97.8980(10) 2157.8(3) 1.667 4.054 32458 12071 0.0476 0.0348 0.0586 0.0507 0.0631 1.007 1567361

(C10H10N2)1.5[UO2(NCS)5]·2H2O 831.74 0.572 × 0.396 × 0.301 orange monoclinic C2/c 8 Mo Kα 100(2) 23.0483(14) 12.5667(8) 21.1340(13) 90.00 113.4900(10) 90.00

formula formula mass cryst size (mm3) cryst color cryst syst space group Z radiation type temp (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) unit cell volume (Å3) Dcalc (Mg m−3) abs coeff, μ (mm−1) no. of mead rflns no. of indep rflns Rint final R1 values (I > 2σ(I)) final wR2(F2) values (I > 2σ(I)) final R1 values (all data) final wR2(F2) values (all data) goodness of fit on F2 CCDC no.

Table 1. Selected Crystallographic Information of Compounds 1−7 3

(C10H10N2)2[UO2(NCS)4Cl]Cl·2H2O 923.66 0.195 × 0.133 × 0.075 dark yellow monoclinic P21/c 4 Mo Kα 100(2) 23.241(2) 9.7934(10) 14.0819(15) 90.00 94.275(2) 90.00

(C10H9N2)3[UO2(NCS)5] 1032.0 0.255 × 0.231 × 0.093 red orthorhombic Cmca 8 Mo Kα 100(2) 17.1831(12) 19.5670(14) 22.7608(16) 90.00 90.00 90.00 7652.7(9) 1.791 4.564 42256 5785 0.0631 0.0248 0.0532 0.0319 0.0558 1.018 1567363 7

Inorganic Chemistry Article


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Inorganic Chemistry

3220.7(5) 1.936 5.417 27495 3687 0.0503 0.0366 0.0814 0.0403 0.0826 1.184 1567366

6

3196.3(6) 1.919 5.551 45311 8427 0.0560 0.0371 0.1065 0.0537 0.1129 1.075 1567367

7

Quantum Theory of Atoms in Molecules (QTAIM). Noncovalent interactions involving the terminal S atoms were subjected to a QTAIM analysis to identify bond critical points and to characterize the electron density topology thereof to assess the interaction nature. Geometric parameters and intermolecular distances between the molecular fragments were taken directly from the crystallographic data and were not optimized. The analysis was performed using AIMALL.32 Calculated Absorbance Spectra. The absorption spectrum of a “chain” of three [UO2(NCS)5]3− tectons was calculated with timedependent density functional theory (TDDFT) with the same functional and basis set combination used for the ESP calculations. The atomic coordinates were taken directly from the crystallographic data of 3, yet the distances between the [UO2(NCS)5]3− units (measured via S···S distances) were manually varied within Avogadro V1.1.1,24,33 to probe the influence of distance upon the resulting absorbance spectrum. A total of four spectra (A−D) were calculated wherein the S···S distances were as follows: A, 4.0 Å; B, 3.448 Å; C, 3.351 Å; D, 2.9 Å. We note that the distances used in B and C were taken directly from the crystallographic data of 3 collected at 296(2) and 100(2) K, respectively. In each case, the 100 lowest-energy singlet electronic excitations were calculated, pushing the spectral cutoffs at high energy far enough such that even higher excitations are not expected to affect the Gaussian-broadened simulated spectra in the visible and near-UV spectral ranges. The simulated spectra were obtained as the sums of Gaussian functions centered at the vertical singlet excitation energies with σ = 0.4 eV to broaden the bands. Harmonic Frequency Calculations. The molecular geometries of single [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− anions (from 3 and 5) were optimized, and the infrared- and Raman-active vibration modes were calculated at the same level of theory as the ESP data. The anharmonic term (a negative contribution to the frequency calculation) was not included, and an error of ∼100 cm−1 is therefore expected.

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3265.47(19) 1.890 5.401 24484 3462 0.0341 0.0585 0.1344 0.0616 0.1357 1.165 1567365

5

RESULTS Local Structure. The [UO2(NCS)5]3− tecton is common to 1−6 and features a central uranyl cation (UO 1.786 Å) that is coordinated equatorially by five approximately linear (Ncoordinated) isothiocyanate ligands (Figure 1 and Table 2).

The average U−N bond length is 2.430 Å, whereas the NC and CS bond lengths (of the isothiocyanate ligands) are 1.168 and 1.625 Å, respectively. We note that the bond lengths and the linearity of the isothiocyanate ligands of the [UO2(NCS)5]3− tectons do not vary appreciably from compound to compound, yet subtle variations in the N−U− N and OU−N bond angles are captured in Table S8. [UO2(NCS)4Cl]3− Tecton. ? is observed in 7 (and 5) and consists of a central uranyl cation that is coordinated equatorially by four nitrogen bound isothiocyanate ligands and one chloro ligand (Figure 1). The average U−N bond length is 2.441 Å, whereas the U−Cl bond length is 2.797(1) Å.

unit cell volume (Å3) Dcalc (Mg m−3) abs coeff, μ (mm−1) no. of mead rflns no. of indep rflns Rint final R1 values (I > 2σ(I)) final wR2(F2) values (I > 2σ(I)) final R1 values (all data) final wR2(F2) values (all data) goodness of fit on F2 CCDC no.

Table 1. continued

5614.0(6) 1.968 6.197 41187 8028 0.0323 0.0187 0.0419 0.0220 0.0428 1.047 1567364

4

Figure 1. (left) Ball and stick representation of the pentaisothiocyanate uranyl tecton. A central uranium(VI) atom (yellow sphere) is coordinated axially by two oxygen atoms (red spheres) and equatorially by five isothiocyanate ligands (N, blue spheres; C, black spheres; S, orange spheres). (center) View down the axial “yl” bond of the pentaisothiocyanate tecton. (right) The [UO2(NCS)4Cl]3− tecton (Cl, green sphere).

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(Figure 2 and Table 3). The 4-PPH cations form a second, cationic layer (also in the (100) plane), as the organic tectons form parallel columns via offset π interactions (Figure S10 and Table S9). One out of every three 4-PPH cations (of each column) deviates from planarity to form an N−H···Oyl hydrogen bond to the uranyl oxo (O2) of the [UO2(NCS)5]3− tectons. The N8−H8···Oyl hydrogen bond distance is 2.999(4) Å, and the ∠N8−H8···O2 angle is 138°. The offset π interactions here (and in 3) were not examined computationally yet feature interaction distances and angles that are within the crystallographic norm for pyridine-based aromatic molecules.34,35 Pseudochannels propagate through each layer along [100] as the voids between each pair of [UO2(NCS)5]3− anions align with those in adjacent layers. Noncoordinated (lattice) water molecules occupy the channels, between the [UO2(NCS)5] layers, and form hydrogen-bonded hexagonal arrays, observed elsewhere by Forbes et al.36 (Figure 2 and Table S9). Two of the three 4-PPH cations form N−H···Ow hydrogen bonds with two of the water molecules (Ow1 and Ow1′) of the hexamer. The relevant N−H···Ow hydrogen bond distances and angles are N6−Hn6···Ow1 2.761(4) Å, ∠N6−Hn6···Ow1 167° and N8− H8···Ow1 2.932(4) Å, ∠N8−H8···Ow1 133°. Two water molecules (Ow2 and Ow2′) of each hexamer form additional Ow−H···S hydrogen bonds (at 3.303(3) and 3.430(3) Å) with terminal sulfur atoms (S4 and S5) of the uranyl isothiocyanate tectons. Compound 2, (C10H10N)3[UO2(NCS)5]. Compound 2 crystallizes in the space group C2/c and consists of [UO2(NCS5)]3− anions charge-balanced by 4-PPH cations

Table 2. Selected Bond Lengths and Angles of the [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− Tectons from 3 and 7, Respectively [UO2(NCS)5]3−

bond or angle

[UO2(NCS)4Cl]3−

length (Å) or angle (deg)

U(1)−O(1) av U−N av N−C av C−S

1.786(2) 2.430 1.168 1.625

O(1)−U(1)−O(1′) N(1)−U(1)−N(2) N(2)−U(1)−N(3) N(3)−U(1)−N(4) N(4)−U(1)−N(5) N(5)−U(1)−N(1) av O(1)−U(1)−N av N−C−S

179.7(1) 72.5(1) 72.0(1) 71.7(1) 73.6(1) 70.2(1) 90.0 179.1

bond or angle

length (Å) or angle (deg)

U(1)−O(1) U(1)−O(2) U(1)−Cl(1) av U(1)−N av N−C av C−S

1.761(4) 1.771(4) 2.797(1) 2.441 1.171 1.627

O(1)−U(1)−O(1) N(1)−U(1)−Cl(1) N(1)−U(1)−N(2) N(2)−U(1)−N(3) N(3)−U(1)−N(4) N(4)−U(1)−Cl(1) av N−C−S

177.9(2) 71.8(1) 71.5(2) 71.6(2) 70.8(2) 74.3(1) 178.8

Select bond lengths and angles describing the [UO2(NCS)4Cl]3− are provided in Table 2. Structural Descriptions. Compound 1, (C10H10N)3[UO2(NCS)5]·3H2O. Compound 1 crystallizes in the space group P1̅ and is comprised of [UO2(NCS)5]3− anions charge-balanced by three crystallographically unique 4-PPH cations. The [UO2(NCS)5]3− anions form discrete pairs via dual S···S interactions to form anionic layers in the (100) plane

Figure 2. (top) Pairs of [UO2(NCS)5]3− tectons in 1 forming layers in (100) the plane. (bottom) Water molecules occupying pseudochannels along [100] and forming hydrogen-bonded hexagonal arrays. The 4-PPH cations are omitted here for clarity, whereas the hydrogen atoms are omitted throughout this paper. E


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Inorganic Chemistry Table 3. Selected Sulfur-Based Noncovalent Interaction Distances and Angles from 1−7 compound

interaction

distance (Å)

1 2 3

2 × S4···S5 S2···Oyl S3···S4 S5···C4(π)S4 S4···S4′ S2···S5 S5···S2 4 × S1···S2 4 × S1···S3 4 × S1···S2

3.539(13) 3.190(1) 3.3510(10) 3.385 3.3123(9) 3.3163(6) 3.3163(6) 3.319(3) 3.3414(19) 3.236(2)

4

5 6 7

angle (deg) C4−S4···S5 C2−S2···Oyl C3−S3···S4 C5−S5···C4(π)S4 C2−S2···S4′ C2−S2···S5 C5−S5···S2 C1−S1···S2 C1−S1···S2 C1−S1···S2

150.17(12) 148.64(6) 149.29(13) 168.8 164.7(1) 164.65(8) 127.95(7) 178.4(3) 179.15(16) 173.82(18)

Figure 3. (top) Global view of 2 along the [001] direction. (bottom left) [UO2(NCS)5]3− tectons forming chains along the [001] direction via S···O interactions. (bottom right) linear and bifurcated N−H···S hydrogen bonds anchoring the 4-PPH and [UO2(NCS)5]3− tectons together.

Figure 4. Uranyl tectons in 3 forming S···S interactions (circled) and multiple S···π short contacts (red dashed lines) to form sheets in the (100) plane.

(Figure 3). The [UO2(NCS5)]3− tectons are linked via S···Oyl interactions to form chains along the [001] direction (Table 3). The chains are flanked by 4-PPH cations that in turn form

linear and symmetric bifurcated N−H···S hydrogen bonds to the [UO2(NCS)5]3− tectons. The interaction distance and angle of the linear hydrogen bonds are 3.2009(16) Å and F


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Figure 5. Hydrogen bonds and S···S interactions in 4 linking the 4,4′-BipyH2 cations and [UO2(NCS)5]3− anions together to form “double-wide” chains along the [101] direction.

Figure 6. [UO2(NCS)5]3− tectons in 5 (6 and 7) linked via S···S interactions to form sheets in the (100) plane. The 4,4′-BipyH2 cations and SCN anions have been omitted for clarity.

∠N6−H6a···S2 162°, whereas the bifurcated interactions are 3.3849(16) Å and ∠N7−H7a···S3 130°. Compound 3, (C10H9N2)3[UO2(NCS)5]. Compound 3 crystallizes in the space group Cmca and consists of two distinct layers, the first of which is comprised of [UO2(NCS)5]3− anions, whereas the second contains monoprotonated 4,4′BipyH cations (Figure S5). The uranyl tectons form corrugated chains along [010] via S···S interactions, whereas multiple S···πNCS interactions link the neighboring chains into sheets (Figure 4 and Table 3). The 4,4′-BipyH cations form chains via end-to-end N−H···N hydrogen bonds and arrange in a parallel fashion to form sheets via offset π interactions (Figure S6 and Table S10). The hydrogen bond distances and angles are N8− H8···N8′ 2.770(3) Å, ∠N8−H8···N8 174(6)° and N7−H7···N6 2.778(3) Å, ∠N7−H7···N6 171(6)°, whereas relevant offset π-

interaction parameters can be located in the Supporting Information. Compound 4, (C10H10N2)1.5[UO2(NCS)5]·2H2O. Compound 4 crystallizes in the space group C2/c and forms sheets comprised of [UO2(NCS)5]3− anions and doubly protonated 4,4′-BipyH2 cations (Figure 5). The [UO2(NCS)5]3− tectons form double-wide chains along [101] as three of the five isothiocyanate ligands (on each tecton) form S···S interactions to link each tecton to a total of three others (Table 3). The double-wide chains feature defined cavities that are occupied by the 4,4′-BipyH2 cations. The cations form two (end to end) bridging bifurcated N−H···S hydrogen bonds with the two sulfur atoms (S2 and S5) on separate and neighboring [UO2(NCS)5]3− tectons (Figure 5). The relevant hydrogen bond distances and angles are N8−H8···S2 3.340(2) Å, ∠N8− H8···S2 133° and N8−H8···S5 3.340(2) Å, N8−H8···S5 147°, G


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Inorganic Chemistry

Figure 7. Doubly protonated 4,4′-Bipy cations in 7 forming hydrogen bonds with free chloride anions and water molecules to form cationic chains. The uranyl tectons decorate the chain and form intermolecular sulfur−sulfur interactions.

respectively. One out of every three [UO2(NCS)5]3− tectons (of each chain) is linked via an elaborate hydrogen-bonding network (described in the Supporting Information) such that the chains ultimately form sheets in the (010) plane (Figure S7 and Table S11). Compound 5, (C10H10N2)2[UO2(NCS)4(NCS)0.75(Cl)0.25]·SCN. The uranyl isothiocyanate anions in 5 contain an equatorial site that is occupied by an isothiocyanate (∼75%) and chloro (∼25%) ligand. There is no apparent structural influence of the substitution, and as such, the following structural description is provided with preference to the major ligand component. Compound 5 crystallizes in the space group Pnma and consists of [UO2(NCS)5]3− tectons and noncoordinated thiocyanate anions that are charge-balanced by doubly protonated 4,4′-BipyH2 cations. Each [UO2(NCS)5]3− is linked to four others via S···S interactions to form anionic sheets in the (011) plane (Figure 6 and Table 3). The 4,4′-BipyH2 cations and SCN− anions form charge-balancing cationic sheets in (100) via a N−H···N and N−H···S hydrogen-bonding network (Figure S8). The thiocyanate anions align normal to the (100) plane and are oriented in an alternating fashion such that both the nitrogen and sulfur moieties function as hydrogen bond acceptors. The relevant hydrogen bond interaction distances and angles are N5−Hn5···S4: 3.299(8) Å, ∠N5−Hn5···S4 139° and N6−Hn6···N4 2.896(10) Å, ∠N6−Hn6···N4 152°. The 4,4′BipyH2 cations and SCN− anions also form hydrogen bonds and short contacts, respectively, with the “yl” oxo ligands of the [UO 2 (NCS) 5 ]3− tectons. The relevant hydrogen bond interaction parameters are N5−H5a···Oyl: 3.039(10) Å, ∠ N5− H5a···Oyl 115° and NCS···Oyl 2.952(11) Å, ∠C4S4···O2 174.0(7)°. Compound 6, (C10H10N2)2[UO2(NCS)5]·NO3. Compound 6 is isomorphous with 5, despite the incorporation of a nitrate anion in lieu of SCN−. The [UO2(NCS)5]3− tectons form anionic sheets via S···S interactions that are nearly indistinguishable from those in 5 (Figure 6 and Table 3). The incorporation of the larger nitrate anion in 6 causes a slight elongation in the unit cell parameters (along [010]), yet the nitrate still functions as a hydrogen bond acceptor and preserves the cationic sheet motif as described in 5 (Figure S9). The relevant hydrogen bond distances and angles are N5− Hn5···O3 2.701(5) Å, ∠N−H···O 163° and N6−Hn6···O4 2.788(6) Å, ∠N−H···O 112°. We note a N−H···S hydrogen bond (N6−Hn6···S2 3.307(4) Å, ∠N−H···S 147°) between the

4,4′-BipyH2 cations and [UO2(NCS)5]3− anions that was not observed in 5 (Figure S9). Compound 7, (C10H10N2)2[UO2(NCS)4Cl]·Cl·2H2O. Compound 7 crystallizes in the space group P21/c and is the lone example featuring a fully (chloro) substituted [UO2(NCS)4Cl]3− tecton (Figure 1). The structure of 7 is very similar to those of 5 and 6, despite the difference in space group. The [UO2(NCS)4Cl]3− once again form sheets in 7 via S···S interactions, whereas the cationic sheet motif (common to both 5 and 6) is also preserved (Figure S10 and Table 3). The noncoordinated Cl− anions and (lattice) water molecules and facilitate the formation of the hydrogen-bonding network in roles analogous to the SCN− and NO3− anions in 5 and 6, respectively (Figure 7). There are two crystallographically unique 4,4′-BipyH2 cations in 7 that (each) form two hydrogen bonds, the first with the Cl− anion (Cl2 and Cl2′), whereas the second is with a noncoordinated water molecule (Ow1 or Ow2) (Table S11). The lattice water molecules also form hydrogen bonds between themselves at 3.004(6) Å and a second hydrogen bond with a chloro ligand of the [UO2(NCS)4Cl]3− tecton. Phase Transformation of 1 to 2. Single crystals of 1, (C11H10N3)3[UO2(NCS)5]·3H2O, are unstable outside of the mother liquor, as removal will induce a phase transformation to yield compound 2. This transformation can be monitored visibly, as the orange crystals of 1 will turn opaque and yellow over the course of several minutes. The transformation was studied via optical microscopy and powder and single crystal Xray diffraction, the details of which are located in the Supporting Information. In short, the phase transformation occurs via dehydration of 1 followed by a structural rearrangement to yield 2. We were able to structurally characterize an intermediate phase (1M, (C10H11N)3[UO2(NCS)5]) between 1 and 2 that contains structural motifs common to both 1 and 2 (Figure S2). Details pertaining to the X-ray diffraction data collection and the structural refinement of 1M are also located in the Supporting Information.

■

DISCUSSION Coulombic attraction between the anionic and cationic tectons is of course the primary driver for assembly in 1−7 as the strengths of such interactions are large in comparison to a typical hydrogen or halogen bond. We are interested, however, in the more subtle influence of the NCIs upon the molecular H


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Inorganic Chemistry

Figure 8. ESP of the [UO2(NCS)5]3− tecton mapped at the 0.001 au isodensity surface. The color scheme represents the magnitude of the ESP as areas of highest and lowest potential are denoted in red and blue, respectively. The ball and stick representation depicts the orientation of the [UO2(NCS)5]3− tecton as in the ESP map.

Table 4. Topological Parameters of Electron Densitya compound

atoms

ρ

∇2 ρ

ε

H(ρ)

V(ρ)

G(ρ)

−G(ρ)/V(ρ)

1 2 3 3b 3 4 4 4b 6c

S4···S5 S2···Oyl S3···S4 S3···S2 S5···C4(π)S4 S5···S2 S4···S4′ S4···S3 S1···S3

7.70 7.70 9.90 2.30 6.30 10.68 9.07 0.02 8.61

+24.33 +30.11 +32.31 +6.29 +20.83 +34.64 +33.14 +0.12 +31.30

0.08382 0.08695 0.18380 0.04520 0.84548 0.13697 0.00146 0.07960 0.07134

1.156 1.570 1.375 0.382 1.090 1.346 1.547 0.011 1.503

−3.769 −4.387 −5.326 −0.808 −3.029 −5.969 −5.190 −0.008 −4.817

+4.925 +5.958 +6.701 +1.190 +4.119 +7.315 +6.737 +0.019 +6.321

1.3067 1.3581 1.2582 1.4728 1.3599 1.2255 1.2981 2.3750 1.3122

a The electron density (ρ), its Laplacian (∇2ρ), local kinetic energy density (G), potential energy density (V), and electronic energy density (H) parameters are in 103 au, whereas the ellipticity (ε) is dimensionless. bThe electron density parameters of 3 and 4 are small, and accordingly the interaction distances associated with S3···S2 and S4···S3 are well outside of the van der Waals radii for S at 4.323 and 6.771 Å, respectively. This highlights the ability to discern between meaningful interactions and those that are seemingly inconsequential. cThe S···S interactions in 5 and 7 were not explored, as they are analogous to those in 6.

anisotropic, as the “sides” feature a higher potential of −779 kJ mol−1 relative to the region coaxial to the CS bond at −643 kJ mol−1. This is expected, as the electrons associated with the S 3p orbitals reside on the sides of the S atoms (the acceptor sites), and in fact, this is consistent with crystallographic trends, as this region commonly accepts hydrogen and halogen bonds.37,42−48 The anisotropic distribution of ESP about the S atoms allows for the formation of the S···S (in 1 and 3−7) and S···Oyl (in 2) interactions. The electrostatically poor regions of the “donor” S atoms are oriented toward (and overlaps with) the electron-rich region of the acceptor S. The CS···S interaction angles are all ∼160° with S···S/S···Oyl distances that are all within the sum of the vdW radii for S (and O). In a related manner, multiple C

level disposition of the tectons and, as such, have characterized the acceptor and donor properties of the [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− anions. The S atoms accept and donate NCIs, whereas the “yl” oxo atoms function solely as acceptors. This behavior is explored using ESP calculations, whereas the formation of the S-based NCIs themselves are quantified using QTAIM analysis and rationalized electrostatically. We note consistencies between the observed acceptor−donor geometries in this and related systems.37−41 Acceptor−Donor Properties of the SCN and “yl” Oxo Ligands. There are two distinct acceptor regions on the SCN ligands. The first is located between the CS bond, where the ESP is −837 kJ mol−1, and the second is localized on the S atoms (Figure 8 and Table 4). The ESP on the sulfur atoms is I


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Inorganic Chemistry Table 5. Selected ESP Values in kJ mol−1 of the [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− Tectons [UO2(NCS)5]3−

region

O1: −828 O2: −827 S1: −656 S2: −658 S3: −625 S4: −636 S5: −641 red: −840 yellow: −798 green: −745 teal: −683 blue: −643

oxo ligands coaxial with UO bond sulfur atoms coaxial with the CS bond

perpendicular extension of the SCN ligand

[UO2(NCS)4Cl]3− O1: −859 O2: −844 S1: −683 S2: −679 S3: −680 S4: −678 red: −941 yellow: −877 green: −833 teal: −761 blue: −761

Figure 9. ESP of the [UO2(NCS)4Cl]3− tectons mapped at the 0.001 au isodensity surface. The ball and stick representation depicts the orientation of the [UO2(NCS)4Cl]3− tecton as in the ESP map.

S···π interactions (average ∠CS···π 168.8°) are observed in 3, as the electron-deficient region of the S donors are oriented toward the electron-rich region between the CS bonds (Figure 4). It is important to note that the S-based interactions form between [UO2(NCS)5]3− anions, which may suggest that the interactions are unattractive or insignificant. The presence of bond critical points, however (per the QTAIM analysis), between the interacting atoms of each S···S, S···Oyl, and S···π interaction confirms that their presence is not merely a consequence of crystal packing (Table 4). Analysis of the electron density topology at the BCPs indicates that these interactions are indeed attractive, yet are classified as weak, noncovalent, or electrostatic in origin49 and likely result from polarization. This conclusion is consistent with the ESP analysis here and with studies elsewhere50−54 aimed at exploring this rather common NCI.2,7,55 Of note is that the magnitude of the ESP on the “yl” oxo atoms, in the region coaxial to the UO bond, is −828 kJ mol−1 and is therefore one of the best NCI acceptor sites on the [UO2(NCS)5]3− tectons (Figure 8 and Table 5). As such, the “yl” oxo atoms participate in a range of NCIs, yet a single reoccurring interaction motif across 1−7 is not observed. The [UO2(NCS)4Cl]3− Tecton. The coordination of a chloro ligand has two observable effects upon the ESP of the [UO 2 (NCS) 4 Cl] 3− tecton in comparison that of the [UO2(NCS)5]3−. First, the magnitude of the ESP on the two “yl” oxo ligands is larger at −844 and −859 kJ mol−1, respectively. Second, the magnitudes of the ESPs both at the sulfur atoms (coaxial with the CS bond) and on the sides of the SCN ligands (⊥ to the U-NCS bond) are also larger: i.e.,

more negative (Figure 9 and Table 5). The chloro ligand features two distinct areas of ESP. The first is in the region coaxial with the U−Cl bond (−831 kJ mol−1) whereas the second is localized along the perpendicular extension of the U− Cl bond (−880 kJ mol−1). We note that this anisotropic distribution about the Cl ligands is consistent with observations in related [AnO2Cl4]2−-containing systems wherein the chloro ligands readily accept NCIs.56 Despite this similarity, however, the chloro ligands in this system are minor contributors to the observed NCI motifs. Structural Diversity. The structures of 1−7, interestingly, do not share a reoccurring NCI motif (or synthon) involving both the [UO2(NCS)5]3− or [UO2(NCS)4Cl]3− anions and the 4-PPH or 4,4-BipyH/H2 cations. Whereas the “yl” oxo atoms (in 1 and 5) and the sides of the S atoms (in 2 and 4) accept N−H···Oyl and N−H···S hydrogen bonds, the interaction geometries differ and these motifs are absent in 3, 6, and 7. Perhaps the 4-PPH or 4,4′-BipyH/H2 cations are poorly suited (geometrically) to readily form NCIs with the uranyl tectons, or perhaps, the ESP distribution on the latter is not optimal to support the formation of hydrogen bonds. We note this here, as in [UO2(NCS)4H2O]2−-containing systems, the S atoms readily accept halogen and hydrogen bonds from smaller 4-Xpyridinium cations, X = Cl, Br, I, SCN, CH3, N(CH3)2.7 The absence of a robust hydrogen-bonding network may contribute to the sensitivity of product formation to pH and anion concentration and the susceptibility of 1 toward dehydration and phase transformation to yield 2. Material Properties: Color, Thermochromism and Luminescence. The optical properties of 1−7 were probed J


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Inorganic Chemistry

Figure 10. Experimental UV−vis diffuse reflectance spectra of 1−4.

using a combined experimental (UV-vis-DR and fluorescence spectroscopy) and computational (TDDFT) approach. We comment first on the origin of the crystal color and second on the structural and electronic factors that govern the thermochromic behavior. Also of note is that the crystals of 1−7 are luminescent at 77 K upon excitation at λex 420 nm and that the typical uranyl emission contains additional vibronic features that are seemingly characteristic of uranyl isothiocyanates. We have utilized Raman and infrared spectroscopy coupled with harmonic frequency calculations to delineate the origin of these additional vibronic features. Origin of Crystal Color: 1 and 2. The UV−vis diffuse reflectance spectrum of 1 contains two prominent bands. The first is broad and gradually sloping (∼550−360 nm), and as it extends into the green region of the spectrum it is responsible for the orange crystal color (Figure 10). Aoyagi et al. reported a similar CT band in the absorbance spectra of a uranyl isothiocyanate containing ionic liquid where the CT was assigned as ligand-to-metal charge transfer (LMCT),8 which was observed and assigned here (computationally) as being SCN → U(VI) in nature. The second band at 275 nm is strong and narrow and is attributed to the n → π* transition of the 4PP cations,57 yet it likely also contains a minor contribution from uranyl absorption.58,59 Crystals of 2 are bright yellow, and accordingly, the SCN → U(VI) LMCT band is blue-shifted (514−370 nm) relative to 1. The yellow crystal color and strong absorbance at 415 nm is more typical of UO22+-containing salts and suggests that the characteristic U(VI) → Oyl LMCT is the dominant contributor to material color.60 The strong band at 277 nm (n → π*) is slightly red shifted relative to 1, which is attributed to the participation of the 4-PP cations in N−H···S hydrogen bonds.57 Origin of Crystal Color: 3−5. The SCN → U(VI) LMCT band in 3 extends well into the blue-green region of the spectrum (∼595−480 nm, Figure 10), and as such, the crystals

are deep red (photo S3). The higher energy absorption feature at 261 nm in 3 is once again attributed to the n → π* transition of the cation, in this case the monoprotonated 4,4′-BipyH.61 The crystal colors of 4 and 5 are both shades of orange, and accordingly, each respective SCN → U(VI) LMCT band is blue-shifted (relative to 3), spanning 530−440 nm and 515− 391 nm, respectively (Figure 10 and Figure S19). The n → π* transition in 4 and 5 is also shifted (relative to 3) to 251 and 284 nm (in 4) and 264 nm in 5, due to the 4,4′-BipyH2 cations in 4 and 5 being doubly protonated and involved in different hydrogen-bonding and offset-π interaction motifs. Origin of Crystal Color: 6 and 7. Despite the presence of an SCN → U(VI) LMCT band in the spectra of 6 and 7 at 537−395 and 510−365 nm, respectively, the crystals of 6 and 7 are yellow and pale yellow (Figures S21 and S23). The reason for this is admittedly unclear and highlights the complex nature of the uranyl isothiocyanate system in general. One may speculate that the presence of a nitrate in 6, (C10H10N2)[UO2(NCS)5]·NO3, as opposed to the SCN anion in 5, or the coordination of an equatorial chloro ligand in 7, (C10H10N2)[UO2(NCS)4Cl]·Cl·2H2O, makes an unknown contribution to the deviation in the trend of crystal colors relative to 3−5. Thermochromism. When the crystals of 1 and 3−5 are cooled from 296(2) to 100(2) K, a color change occurs from either red or orange to light orange or yellow (photo S3). Given that a low-energy LMCT band is responsible for the crystal color (at 296(2) K), it is logical to expect this CT band to (blue) shift in response to a decrease in temperature, and in fact, this is observed experimentally in the low-temperature diffuse reflectance spectrum of 3 (Figure S25). To identify the occupied−unoccupied molecular orbital (MO) pair contributions that give rise to this transition, i.e. to confirm the assignment of the LMCT as SCN → U(VI) in nature, we calculated (using TTDFT) an approximated absorbance spectrum of 3 (Figure 11). K


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Inorganic Chemistry

Figure 11. (top) A truncated chain of three [UO2(NCS)5]3− tectons from 3 used as a model for the TDDFT calculations. (bottom) The SCN → UO22+ LMCT band features contributions from both intra- and intermolecular character.

the upper end of the calculated spectrum there are four major excitations at 292, 290, 289, and 279 nm, each of which are assigned as intramolecular in nature. Spectrum B: S···S Distances of 3.448 Å. The first five electronic excitations of the leading edge in B are located at 381, 373, 369, 364, and 344 nm (Figure S26 and Table S14). These excitations are primarily intramolecular SCN → U(VI) in nature, yet there is a clear contribution from intermolecular SCN → U(VI) transitions. We note that the intense excitations near the upper cutoff (tailing edge) of the spectrum are redshifted, yet to a lesser extent in comparison to the longwavelength (leading edge) section, and are primarily intramolecular SCN → U(VI) transitions. There is, however, a clear contribution from intermolecular SCN → U(VI) transitions in this spectral range, which is of note, as these where absent in spectrum A. Spectrum C: S···S Distances of 3.351 Å. The five primary transitions in the leading edge of C are located at 382, 374, 371, 365, and 436 nm (Figure S27 and Table S15). The first four transitions are primarily intramolecular SCN → U(VI) LMCT in nature, whereas the most intense excitation at 436 nm is now primarily an intermolecular SCN → U(VI) transition. The contributions of intra- and intermolecular SCN → U(VI) CT in the tailing edge is relatively unchanged with respect to B, and the entire edge is once again red-shifted, yet to a much lesser extent than the leading edge. We note that there is now evidence for UO22+-based Oyl → U(VI) LMCT at 296 nm. Spectrum D: S···S Distances of 3.0 Å. The transitions of the leading edge in D are now primarily intermolecular SCN → U(VI) in nature (Figure S27 and Table S16). It is of note that the most intense excitation in the leading edge (in D at 356 nm) has increased in oscillator strength from 0.1515 in A to 0.2439 in D and now features uranyl-based Oyl→ U(VI) LMCT

Interestingly, the only observable structural change in 3 (or across 1−7 for that matter) between 296(2) and 100(2) K is a systematic decrease in the NCI distances. We also note (as one may expect) a subsequent decrease in the unit cell lengths across 1−7, and as a consequence, the distance between each uranyl center is likewise decreased. For this reason, a total of four spectra were calculated wherein the S···S distances were fixed at the values spectrum A 4.000 Å, spectrum B 3.448 Å, spectrum C 3.351 Å, and spectrum D 2.900 Å, in order to probe the influence of intermolecular tecton distance (the only observable structural change as a function of temperature) on the nature of the transitions comprising the SCN → U(VI) LMCT band. The S···S distances in spectra B and C are analogous to those observed crystallographically in 3 at 296 and 100 K, respectively (Table S7). Spectrum A: S···S Distances of 4.0 Å. Spectrum A (and B− D) contains two important regions, the “leading” and “tailing” edges, at long wavelength/low energy and near the shortwavelength/high-energy cutoff, respectively, with the former corresponding to the SCN → U(VI) LMCT band (Figure S26). The leading edge contains a cluster of four tightly grouped transitions at 366, 361, 349, and 340 nm, each of which feature contributions from several occupied and unoccupied MOs (Figure S26 and Table 13). These transitions are localized on a single [UO2(NCS)5]3− tecton and are primarily SCN and U(VI) based; accordingly, they are designated intramolecular SCN → U(VI) LMCT transitions (Figure 11). Of note is that the transitions at 349 and 340 nm also have a minor contribution from intermolecular SCN → U(VI) LMCT, meaning that the SCN ligand and U(VI) center are located on two distinct [UO2(NCS)5]3− tectons. The most intense transition of the leading edge is at 328 nm and is primarily intramolecular SCN → U(VI) LMCT in nature. At L


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Inorganic Chemistry

Figure 12. (left) Luminescence spectrum of 3 (λexc 420 nm). (right) Expanded view of the additional vibronic features that correspond to the vibronic coupling with the SCB bending modes.

Table 6. Wavelengths (nm) of the Vibronic Peaks in the Emission Spectra of 1−7a peak maxima (nm) compound 1 2 3 4 5 6 7 a

499 497 496 496 496 496

497 (511) (510) (508) (499) (508) (500)

519 521 518 518 518 518 518

(534) (532) (531) (523) (530.5) (522)

545 542 541 542 541 541

541 (560) (557) (555) (546) (556) (547)

571 568 (584) 567 568 (574) 567 (583) 567 (573)

601 596 595 596 595.5 595 (602)

Vibronic peaks originating from equatorial ligand vibrational modes are given in parentheses.

speculation that “interactions among the anion complexes are significant”8 is confirmed here, as intermolecular SCN → U(VI) CT is indeed substantiative. The experimental and calculated spectra blue and red shift, respectively, as the distance between the [UO2(NCS)5]3− tectons decreases. The opposite directions of the shifts are expected and are perhaps best rationalized by the changes in band gap energy across A−D (Table S17). The HOMO is delocalized about the SCN ligands, whereas the LUMO is localized on the U(VI) metal center, an observation consistent with computational results of Baker et al.,55 wherein DFT and QTAIM analysis was used to probe the nature of UO22+/SCN bonding. Decreasing the S···S distances between the three (non-charge-balanced) anionic [UO2(NCS)5]3− tectons across A−D increases the total energy of the system, but in a disproportionate fashion. The energy of the MOs localized on the SCN ligands increases to a greater extent in comparison to those of the U(VI) metal center due to greater orbital overlap in the former. The net result is a red shift or decrease in energy to the band gap as the energy difference between the HOMO and LUMO decreases (Table S17). An analogous effect is not observed experimentally, as the crystal structure is chargebalanced and the S atoms can offset (somewhat) decreases to interatomic distances via polarization and dispersion. More likely, however, is that the increase in the SCN orbital energy weakens the U(VI)/SCN coupling, leading to a larger energy difference between the orbital pairs and ultimately a blue shift in the resulting LMCT band. This argument is consistent with the luminescence behavior in this system as uranyl emission is quenched at 296 K and is observed at 77 K. Moreover, at short S···S distances the orbital pair contributions in the leading edges of the calculated spectra begin to exhibit Oyl → U(VI) based transitions, which supports a decoupling of the U(VI)/ SCN interaction. Absorbance properties that are more typical of uranyl-containing salts seemingly prevail at short intertecton

character. It is also of note that many of the excitations in D do not originate from discrete SCN atomic orbitals, but rather are from orbitals that are delocalized over several neighboring SCN ligands, both on the same tecton and across adjacent tectons. Once again the entire spectrum is red-shifted, yet the effect is greatest in the tailing edge. Thermochromism: Discussion. The calculated spectra A−D are admittedly qualitative approximations of the actual absorbance properties of compound 3. Nevertheless, the leading edges of the calculated spectra are comparable to and a good approximation of the low-energy LMCT band in the diffuse reflectance spectrum of 3 (Figure 10). The most notable orbital transitions in the leading edges are SCN to U(VI) based, which is consistent with our assessment that the experimentally observed, low-energy LMCT band in 3 arises from thiocyanate to uranium charge transfer. The oscillator strengths of the orbital transitions in the leading edges of the calculated spectra are sensitive to S···S distance and, as such, provide insight into the thermochromic behavior. The SCN → U(VI) CT transitions are predominately intramolecular in nature when S···S distances are long and intermolecular in nature at short S···S distances. The most intense transition in the leading edge also increases in oscillator strength as a function of decreasing S···S distance. Experimentally, the S···S (and intertecton) distances are longer at 296 K, where a red crystal color is observed, whereas this distance decreases from 3.448(3) to 3.351(1) Å upon cooling to 100 K, where the crystal color is orange. We can therefore conclude that the interplay between the intra- and intermolecular CT transitions are a prerequisite for thermochromic behavior in uranyl isothiocyanate systems. This is consistent with observations by Aoyagi et al., as it was noted that upon dilution of a thermochromic, uranyl isothiocyanate containing ionic liquid the solution color changed from orange to yellow and no longer exhibited thermochromic behavior.8 Thus, their M


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Inorganic Chemistry

Figure 13. (left) Luminescence spectrum of 7 (λexc 420 nm). Of note are the shoulders that correspond to the vibronic coupling with the equatorial chloro ligand. (right) Expanded view of the additional vibronic features that correspond to the vibronic coupling with the NCS bending modes.

Moreover, we have ruled out the possibility of coupling that originates from the U−N stretching frequencies, as these are expected to be lower in energy.67−69 Luminescence of 1, 2, 4, and 6. The emission spectrum of 1 is weak due to self-quenching (by the SCN → U(VI) LMCT band) and features only three distinct peaks, given in Table 6. It is of note that excitation of 1 at the excitation maximum of the 4-PPH cations (280 nm) results in organicbased fluorescence (340−480 nm) as well as weak uranyl emission. In contrast, compound 2 exhibits strong uranyl emission with five major and three minor vibronic bands, the latter arising from the SCN ligands. The luminesence spectra of 4 and 6 are nearly identical with that of 2, and as such, the locations of the emissive bands are summarized in Table 6 whereas the spectra are provided in the Supporting Information. Luminescence of 5 and 7. The position of the major vibronic bands in 5 and 7 are nearly identical with those in 1−4 and 6, yet here the peaks are broad and feature pronounced shoulders (Figure 13). The shoulders are offset from the apex of the major peak by an average of 178 cm−1 and likely denote vibronic coupling to the U−Cl stretching or rocking mode.64,66,70 This is consistent with the fact that 5 and 7 are the only compounds to contain the [UO2(NCS)5−nCln]3− (n = 1, 0.25) tecton. Harmonic frequency calculations reveal several Raman-active U−Cl vibrational modes from 90 to 220 cm−1 that are mixed with analogous U−NCS modes. The calculations do not support a sole contribution from the U−Cl vibrational modes, and we note that a high-resolution Raman and fluorescence study is needed to provide definitive assignment. Moreover, the uranyl-based ν3 bending mode is calculated to be at 285 cm−1 and therefore may also be observed in the vibronic structure. The SCN bending modes are observed in the spectra of both 5 and 7, yet they are not well resolved.

distances and therefore provide a good explanation for the yellowing of crystal color as a function of cooling. Luminescence of 3. The SCN → U(VI) LMCT band of 3 extends into the green region of the spectrum (to 595 nm), which is where the uranyl typically emits, and as such the crystals are self-quenching. Consequently, large single crystals (0.830 × 0.720 × 0.630 mm) are not luminescent even at 2 K, yet smaller crystals (approximately 0.050 mm3) and samples that have been lightly crushed or ground are in fact emissive at 77 K. Powder X-ray diffraction data collected on lightly crushed or ground crystals of 3 indicate that there are no structural changes upon fracturing or grinding. It is likely that more emissive sites are created by the increase in surface area or modification thereof, which limits the effects of the selfquenching. Moreover, the smaller crystallite size may also decrease the optical density of the specimen (relative to a large crystal), which may abate the self-quenching. A thorough highresolution spectroscopic study is needed to rule out the possibility of emission stemming from defect sites, however. Excitation of 3 at 420 nm (at 77 K) results in typical UO22+ emission, as five vibronic peaks are observed that progress harmonically with an average energy spacing of 836 cm−1 (Figure 12 and Table 6). These vibronic bands arise from the coupling of the symmetric ν1 UO stretch (observed experimentally at 836 cm−1, Table S19) with the 3Πu electronic triplet excited state.62 Between each of the major peaks are less intense vibronic bands at 510 nm (19607.8 cm−1), 532 nm (18796.9 cm−1), 557 nm (17953.3 cm−1), and 584 nm (17123.3 cm−1). The average spacing between these weaker bands is 828.2 cm−1, which indicates that they progress harmonically with the ν1 stretch.63,64 The average spacing between the apex of the major and the “minor” peaks is ∼500 cm−1, indicating their origin is not uranyl based, as the ν2 and ν3 modes are typically observed at 920 and 200−250 cm−1,65,66 respectively, but rather originates from the equatorial ligand environment. The harmonic frequency calculations show several isothiocyanate bending modes that range from 495 to 512 cm−1, which is in agreement with the experimental Raman spectrum of 3, as two rather weak, yet distinct, peaks at 494 and 479.5 cm−1 are observed (Table S18). As metal-coordinated isothiocyanate ligands feature two nondegenerate Raman-active bending modes (∼470 cm−1),67 we are confident that the less intense vibronic structure originates from the coupling of the SCN bending modes with the ν1 UO stretch. We note that there are no other calculated vibrational modes (Raman or IR) in the 490 cm−1 region, as the two nearest are at 799 cm−1 (SCN stretches) and 285 cm−1 (UO bending) (Table S18).

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CONCLUSION The synthesis of seven new homoleptic [UO2(NCS)5]3− and chloro-substituted [UO2(NCS)4Cl]3−-bearing compounds charge-balanced with 4-phenylpyridinium and 4,4′-bipyridinium cations is reported, and structural characterization reveals assembly motifs via a wide range of NCIs, most notably of which are the sulfur-based S···S and S···Oyl interactions. The absence of a single, dominant (or robust) supramolecular synthon across 1−7 renders product formation highly sensitive to reaction conditions (e.g., pH and anion concentration), and the structure of 1 is unstable and prone to a phase transformation that yields 2. The calculated ESP maps of the N


Article

Inorganic Chemistry [UO2(NCS)5]3− and [UO2(NCS)4Cl]3− tectons reveal that the “yl” oxo species and the SCN ligands are potentially good acceptor sites of NCIs, yet hydrogen bonding between the organic cations and the uranyl tectons is largely absent. Crystals of 1−7 ranged in color from red to yellow, which resulted from an experimentally observable SCN → U(VI) LMCT charge transfer band, which TDDFT calculations revealed to be either intra- or intermolecular SCN → U(VI) in nature depending on S···S distances between the [UO2(NCS)5]3− tectons. The computational treatment indicated that intramolecular SCN → U(VI) LMCT character is favored at long S···S distances, whereas intermolecular SCN → U(VI) LMCT is favored at short distances. The interplay between these types of charge transfer bands (intra vs inter) is affected by temperature and is therefore the most likely explanation for the thermochromic behavior in this series of materials. Moreover, the additional weak vibronic bands present in the 77 K emission spectra of the uranyl isothiocyanates are attributed to the coupling of the SCN bending modes with that of the uranyl symmetric stretch. This combined experimental and computational treatment has provided much needed detail regarding structure−property relationships and is inspiring more targeted efforts of structurally driven property manipulation in our group.

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Heavy Elements Program, under grant DE-FG02-05ER15736 at GWU. The computations were carried out by L.C.D. and J.A. with support from the DOE Heavy Elements Program grant DE-SC0001136. L.C.D. is also grateful for fellowships from the São Paulo Research Foundation (FAPESP) (2014/21930-9) and the Ministry of Science, Technology, Innovation and Communications (CNPq) (202068/2015-3). The authors are grateful to Korey P. Carter for assistance with Raman and ATIR measurements, which were collected at the ND Energy Center, and to whom we are thankful for instrument time.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02702. Synthetic notes, details regarding the phase change from 1 to 2 and the single-crystal X-ray diffraction experiment of 1−7 and 1M, 298(2) K structural refinements of 1−7, bond lengths and angles, additional figures of 1−7, electrostatic potential maps of the 4-phenylpyridinium and 4,4′-bipyridinium cations, calculated UV−vis absorbance spectra, experimental Raman and IR spectra, experimental spectra (UV-vis-DR, ATIR, Raman, fluorescence) of 1−7, and thermal ellipsoidal representations of 1−7 (PDF) Accession Codes

CCDC 1567354−1567368 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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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*C.L.C.: tel, (202) 994-6959; e-mail, [email protected]. ORCID

Lucas C. Ducati: 0000-0002-6539-4325 Jochen Autschbach: 0000-0001-9392-877X Christopher L. Cahill: 0000-0002-2015-3595 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This primary source of support for this research was the U.S. Department of Energy, Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Sciences, Office of Science, O


Article

Inorganic Chemistry

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Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer

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