Synthesis, Structure, and Vibrational Properties of [Ph4P]2NpO2Cl4

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

Synthesis, Structure, and Vibrational Properties of [Ph4P]2NpO2Cl4 and [Ph4P]2PuO2Cl4 Complexes David D. Schnaars and Richard E. Wilson* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The synthesis, structure, and vibrational properties are presented for an isostructural series of Np(VI) and Pu(VI) complexes of the form [Ph4P]2AnO2Cl4, where An = Np(VI) or Pu(VI). The reported complexes are readily synthesized in ambient laboratory conditions, and their molecular structures were determined using single crystal X-ray diffraction. Their vibrational spectra were studied using a combination of Raman and FT-IR vibrational spectroscopies. Analysis of the vibrational spectra and force constants highlight the periodic properties associated with the actinide contraction and filling of the 5f electronic shells. Additionally, we have assessed the utility of these complexes as conveniently synthesized starting materials for non-aqueous synthesis of transuranium molecules and materials.

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INTRODUCTION Within the early actinide elements, Th−Am, the changing relative energies of the 5f and 6d orbitals give rise to a multitude of oxidation states, ion geometries, and electronic properties.1 At uranium, the hexavalent oxidation state becomes accessible and is known to be stable for U, Np, Pu, and Am under appropriate conditions. These hexavalent ions exist as the molecular, linear dioxo-cations of the form AnO22+, known as the actinyls. Together the series of uranyl, neptunyl, and plutonyl ions form a convenient periodic series for studying the properties of these actinyl ions with respect to their structures and chemical properties, particularly with respect to the 5f electron population in these complexes. In comparison to the chemistry of the uranyl ion, which has been extensively studied, we have far fewer chemical models to study for the transuranium elements, particularly as isostructural series. This is unfortunate since recent studies have highlighted the utility of demonstrating clear periodic trends in the chemistry of the actinides through systematic studies across the series.2−7 Our prior work focused on investigating the changes in the vibrational properties of the actinyl ions as a periodic series and demonstrated differences in the structural and vibrational properties of hexavalent uranium and plutonium complexes that we could attribute to periodic effects in the chemistry of the early actinides.8,9 Here we complement this work with presentation of the plutonium and neptunium salts, characterized by single crystal X-ray diffraction and vibrational spectroscopy. These newly reported transuranium salts are isostructural with the uranyl salts we reported previously, making them potentially excellent model systems for spectroscopic characterization of their electronic properties.9 In addition to structural and spectroscopic characterizations, we also comment on the potential utility of these compounds as starting materials for non-aqueous transuranium chemistry. © XXXX American Chemical Society

MATERIALS AND METHODS

Caution! 237Np and 242Pu are alpha-emitting isotopes. All experiments described were performed in specially designed laboratories with negative pressure fume hoods and gloveboxes, using strict radiological controls. The following reactions were performed under ambient laboratory conditions, and all materials, with the exception of 237Np and 242Pu, were obtained from commercial sources and used as received. 237Np and 242Pu were purified using standard ion-exchange methods and oxidized to the hexavalent state by bubbling solutions of Np and Pu in concentrated HCl with ozone. Oxidation state purity of the Pu and Np was determined by optical spectrometry. KBr was ground and dried for a minimum of 48 h at 120 °C before use. [PPh4]2NpO2Cl4·2CH2Cl2. A 2 mL shell vial was charged with 500 μL of a dichloromethane solution containing [PPh4]Cl (9.5 mg, 0.025 mmol). On top of this colorless solution was layered 250 μL of an orange/green solution consisting of 0.049 M 237Np(VI)O22+ (3 mg of 237 Np, 0.013 mmol) in concentrated HCl. The solution was allowed to evaporate for a couple of hours in a fume hood, resulting in the deposition of [PPh4]2NpO2Cl4·2CH2Cl2 as green crystals. Despite the near stoichiometric combination of reagents, [PPh4]2NpO2Cl4· 2CH2Cl2 was isolated by decanting off the remaining mother liquor and rinsing the crystals with a minimal amount of water to remove any remaining HCl prior to complete evaporation of the solvents. The yield of the reaction was determined radiometrically using liquid scintillation counting of 237Np in the remaining mother liquors and rinse solutions and was determined to be approximately 80% based on the Np mass balance. It is worth noting that [PPh4]2NpO2Cl4· 2CH2Cl2 appears to be moderately soluble in H2O, resulting in a fraction of the desired product being dissolved in the aqueous rinse. IR (KBr pellet, cm−1): 524(vs), 615(w), 623(w), 690(s), 723(vs), 753(sh, m), 756(m), 762(sh, m), 853(w), 857(w), 914(s) (ν3), 985(sh, w), 995(m), 1026(w), 1073(sh, m), 1084(sh, m), 1108(vs), 1165(w), 1188(w), 1191(sh, w), 1278(sh, w), 1295(w), 1314(m), 1338(w), Received: September 15, 2017

A

DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. X-ray Crystallographic Data for All Complexes

empirical formula crystal habit, color crystal size (mm) crystal system space group volume (Å3) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z formula weight (g/mol) density (calculated) (Mg/m3) absorption coefficient (mm−1) F000 total no. reflections unique reflections final R indices [I > 2σ(I)] largest diff. peak and hole (e− Å−3) GOF

[PPh4]2NpO2Cl4· CH2Cl2

[PPh4]2NpO2Cl4

Cl8H44C50O2P2Np irregular, light green 0.08 × 0.05 × 0.05 triclinic P1̅ 1265.22(12) 10.0491(6) 10.9720(6) 12.6173(7) 69.2270(8) 76.8990(8) 82.7040(8) 1 1259.39 1.653 2.578 619 22060 8632 R1 = 0.0345, wR2 = 0.0613 1.588 and −1.364

Cl4H40C48O2P2Np block, yellow-green 0.08 × 0.005 × 0.05 triclinic P1̅ 1091.5(3) 10.0363(14) 10.0886(14) 12.0963(14) 99.6340(19) 93.6480(18) 113.9780(17) 1 1089.54 1.658 2.737 535 16572 6881 R1 = 0.0330, wR2 = 0.0607 1.005 and −1.866

Cl4H40C48O2P2Pu irregular, light orange 0.12 × 0.12 × 0.06 triclinic P1̅ 1081.93(13) 9.9894(7) 10.0397(7) 12.0944(8) 99.6760(10) 93.8940(10) 113.7580(10) 1 1094.54 1.680 1.883 536 17875 7388 R1 = 0.0288, wR2 = 0.0529 1.233 and −0.744

Cl8H44C50O2P2Pu irregular, orange 0.10 × 0.06 × 0.06 triclinic P1̅ 1263.11(8) 9.7031(3) 11.7022(4) 12.8754(5) 63.1270(4) 83.6780(5) 75.5990(5) 1 1264.39 1.662 1.830 620 18091 6937 R1 = 0.0175, wR2 = 0.0373 0.552 and −0.391

Cl4H44C48O4As2Pu irregular rod, orange 0.20 × 0.08 × 0.07 monoclinic P21/n 2281.2(4) 10.9017(12) 15.8795(17) 13.1778(14) 90 90.3530(15) 90 2 1218.47 1.774 3.162 1184 37132 7926 R1 = 0.0396, wR2 = 0.0799 1.335 and −2.171

1.103

1.028

1.027

1.032

1.016

[PPh4]2PuO2Cl4

[PPh4]2PuO2Cl4· CH2Cl2

[AsPh4]2PuO2Cl4· (H2O)2

[PPh4]2PuO2Cl4. IR (KBr pellet, cm−1): 409(w), 431(w), 434(w), 447(w), 470(w), 472(w), 524(vs), 528(vs), 535(vs), 614(w), 680(sh, w), 688(s), 690(s), 693(sh, s), 723(vs), 748(m), 756(m), 763(m), 843(w), 862(w), 917(s) (ν3), 929(sh, w), 937(w), 943(w), 979(w), 986(sh, w), 996(s), 1025(w), 1072(w), 1098(sh, m), 1107(vs), 1160(sh, w), 1166(w), 1182(w), 1186(w), 1193(sh, w), 1271(w), 1313(m), 1332(w), 1338(m), 1385(m), 1405(m), 1435(vs), 1437(sh, s), 1441(s), 1483(s), 1570(w), 1579(w), 1584(m), 3012(s), 3024(s), 3034(s), 3044(sh, s), 3051(s), 3057(s), 3062(sh, s), 3066(sh, s), 3078(s), 3371(s). Raman (cm−1): 105, 122, 137, 202, 217, 248, 256, 290, 448, 617, 678, 800 (ν1), 928, 985, 1000, 1003, 1026, 1028(sh), 1098, 1108, 1160, 1168, 1191, 1577, 1584, 3059, 3063. CCDC: 1574551. [PPh 4 ] 2 PuO 2 Cl 4 ·2CH 2 Cl 2 . On one occasion, crystals of [PPh4]2PuO2Cl4·CH2Cl2 were isolated as the only tractable material from a reaction between [PPh4]2PuO2Cl4 and 1,10-phenanthroline in dichloromethane in an attempt to use [PPh4]2PuO2Cl4 as a starting material to produce the 1,10-phenanthroline complex. Raman (cm−1): 115(sh), 124, 200, 220, 253, 266, 295, 527, 561, 615, 680, 724, 751, 800 (ν1), 855, 934, 986, 1001, 1011(sh), 1020(sh), 1028, 1054, 1074, 1098, 1109, 1144, 1163, 1169, 1188, 1196, 1573, 1586, 3052(sh), 3060, 3071, 3146(br), 3167(br). CCDC: 1574552. [AsPh4]2PuO2Cl4·2H2O. IR (KBr pellet, cm−1): 416(w), 449(m), 469(sh, vs), 470(vs), 478(s), 613(w), 673(sh, w), 680(sh, m), 689(vs), 695(sh, m), 742(vs), 746(vs), 752(sh, s), 854(w), 904(sh, w), 920(vs) (ν3), 975(w), 986(w), 997(s), 1023(w), 1070(sh, w), 1079(s), 1081(sh, s), 1086(m), 1167(w), 1190(m), 1311(m), 1320(m), 1337(w), 1385(s), 1438(sh, vs), 1442(vs), 1477(sh, m), 1485(m), 1579(m), 1616(sh, m), 1630(m), 2991(s), 3003(s), 3019(s), 3038(s), 3051(s), 3077(s), 3095(s), 3100(s), 3154(s), 3438(s), 3478(sh, s), 3578(s), 3657(s). Raman (cm−1): 102, 122, 181(sh), 191, 201(sh), 214, 236(sh), 244(sh), 250, 342, 351, 478, 614, 671, 682, 753, 802 (ν1), 855, 939, 986, 1002, 1024, 1081, 1087, 1168, 1190, 1579, 3058, 3067, 3157. CCDC: 1574553. Vibrational Spectroscopy. Infrared samples were diluted (∼1−5 wt %) with dry KBr and pressed into a pellet before being collected on a Nicolet Nexus 870 FTIR system. Data were collected using 16 scans over 4000−400 cm−1 with a resolution of 1 or 2 cm−1. Raman data were collected on randomly oriented single crystals using a Renishaw

1390(w), 1409(w), 1436(vs), 1441(sh, s), 1472(sh, m), 1483(m), 1523(w), 1576(sh, w), 1585(m), 1610(w), 1684(w), 1779(w), 1819(w), 1905(w), 1984(w), 2202(w), 2577(w), 2679(w), 2852(w), 2904(sh, m), 2921(m), 2955(m), 3002(m), 3013(sh, m), 3022(m), 3051(sh, m), 3057(m), 3080(m), 3091(sh, m), 3142(m), 3167(m). Raman (cm−1): 105, 122, 197, 252(sh), 257, 289, 617, 680, 696 (dichloromethane CCl2 symmetric stretch), 725, 797 (ν1), 908(br), 986, 1001, 1028, 1098, 1111, 1164, 1173, 1189, 1575, 1586, 2963, 2989, 3008, 3059, 3145(br), 3169(br), 3267(br), 3474(br), 3615(br), 3783(br). CCDC: 1574549. [PPh4 ] 2 NpO 2 Cl 4 . On one occasion, after the crystals of [PPh4]2NpO2Cl4·2CH2Cl2 were rinsed with H2O, the aqueous rinse solution was allowed to evaporate, resulting in the formation of the unsolvated neptunyl species, [PPh4]2NpO2Cl4. Raman (cm−1): 104, 125, 132(sh), 190, 205, 219(sh), 248, 256, 274(sh), 290, 391, 448, 524, 537, 616, 678, 726, 747, 765, 801 (ν1), 832, 888, 928, 974, 985, 1000, 1007(sh), 1026, 1077, 1098, 1108, 1161, 1169, 1191, 1577, 1585, 3062, 3373(sh, br), 3489(sh, br), 3550(br), 3654(br), 3822(br). CCDC: 1574550. Synthesis of [PPh4]2PuO2Cl4 and [AsPh4]2PuO2Cl4·2H2O. A 2 mL shell vial was charged with 200 μL of 0.062 M 242Pu(VI)O22+ (3 mg of 242Pu, 0.012 mmol) in concentrated HCl. To this orange solution was added 500 μL of 0.050 M [EPh4]Cl (E = P, As) (0.025 mmol) in CH2Cl2, resulting in the formation of an aqueous/organic layered sample. The orange color quickly transferred from the aqueous phase to the organic phase, and evaporation of the solution within a fume hood (∼2 h) resulted in the deposition of the desired product [PPh4]2PuO2Cl4, [AsPh4]2PuO2Cl4·2H2O, as irregular orange crystals. The plutonyl salts were isolated by decanting off the remaining mother liquor and rinsing the crystals with a minimal amount of water to remove any remaining HCl. Similarly to the reactions reported with Np(VI), the reported Pu(VI) complexes appear to be somewhat soluble in H2O, resulting in a fraction of the desired product being dissolved in the rinse. Radiometric measurements using liquid scintillation counting of the mother liquor and rinse solutions provided reaction yields of approximately 72% and 65% for [PPh4]2PuO2Cl4 and [AsPh4]2PuO2Cl4·2H2O, respectively, based on the Pu mass balance. B

DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry inVia Raman Microscope with a circularly polarized excitation line of 532 or 785 nm as indicated. X-ray Crystallography. The solid state molecular structures for all complexes were determined similarly with exceptions noted (Table 1). Crystals were mounted on a glass fiber using a quick drying epoxy as a fixative. Data (0.5° frame widths) were collected using a Bruker SMART or QUAZAR diffractometer equipped with an APEXII detector using Mo Kα radiation. All data were collected at 100 K using an Oxford Cryosystems cryostat. The data were integrated and corrected for absorption using the APEX2 suite of crystallographic software, while structure solutions and refinements were completed using XShell.10

a linear OAnO bond angle. To maintain an overall neutral charge, the actinyl anion is charge balanced by two PPh4+ cations or two AsPh 4 + cations as in the case of [AsPh4]2PuO2Cl4·2H2O. Additionally, for [PPh4]2NpO2Cl4· CH2Cl2 and [PPh4]2PuO2Cl4·CH2Cl2, each contains two molecules of CH2 Cl 2 within the crystal lattice, while [AsPh4]2PuO2Cl4·2H2O contains two noncoordinated water molecules in the crystal lattice. For compounds [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2NpO2Cl4, the NpO/NpCl bond lengths are 1.764(2)/2.654(1) and 2.656(1) Å and 1.766(2)/2.665(1) and 2.683(1) Å, respectively (Table 2). These bond lengths are consistent with those of previously reported NpO2Cl42− anions, such as [Ph3PNH2]2NpO2Cl4 (NpO, 1.748(3) and 1.752(3) Å; NpCl, 2.647(1)−2.664(1) Å),11 Cs2NpO2Cl4 (NpO, 1.775(17) Å; NpCl, 2.653(3) Å),12 and [NBu4]NpO2Cl4 (NpO, 1.733(5) and 1.740(6) Å; NpCl, 2.637(2)− 2.676(2) Å).13 Similarly, the bond lengths of the anion in the reported plutonium compounds (PuO, 1.742(3) to 1.750(2) Å; PuCl, 2.657(1) to 2.685(1) Å (Table 2)) are also consistent with literature values for PuO2Cl42− compounds containing cations such Rb,8 Cs,8,14 Me4N,8 C10H10N2,15 C12H12N2,15 C13H16N2,15 C10H11N2,15 and Ph3PNH2,16 which range from 1.709(10) to 1.759(2) Å for the PuO bond and 2.634(2) to 2.676(1) Å for the PuCl bond. As was observed for the uranyl analogues, the compounds reported here contain cation−anion interactions with both the oxygen and chloride ligands. 8,9 For PPh 4 + containing compounds the shortest HPPh···Oyl and HPPh···Cl interactions range from 2.55(3) ([PPh4]2NpO2Cl4·CH2Cl2) to 2.80(3) Å ([PPh4]2PuO2Cl4) and from 2.72(3) ([PPh 4]2NpO2Cl4· CH2Cl2) to 2.78(3) Å ([PPh4]2PuO2Cl4), respectively (Table 2 and Figure 2). For [AsPh4]2PuO2Cl4·2H2O, where the central P atom of the cation is replaced with an As, the HPPh···Oyl interaction is slightly shorter (2.35 Å), while the distance of the shortest HPPh···Cl interaction (2.77 Å) falls within the range observed for the PPh4+ containing compounds. These interactions are consistent with the reported mean values for (PPh4)C−H···OM (2.700 Å)17 and C−H···ClM (2.876 Å)18 interactions (AM = A is terminally bound to a metal atom), but with the exception of [PPh4]2NpO2Cl4·CH2Cl2 and [AsPh4]2PuO2Cl4·2H2O, the distances between the cation and the actinyl oxo ligand are longer than those observed in UO2Cl42− compounds containing PPh4+ and AsPh4+ cations (2.34(3) to 2.64 Å).9 As was stated for the uranyl analogues, the HPPh···Oyl and HPPh···Cl distances in the compounds reported here suggest a relatively weak anion−cation interaction. In addition to cation−anion interactions, [PPh4]2NpO2Cl4· CH2Cl2, [PPh4]2PuO2Cl4·CH2Cl2, and [AsPh4]2PuO2Cl4· 2H2O also contain interactions between the lattice solvent and the chloride ligands of the anion. For [PPh4]2NpO2Cl4· CH2Cl2, the shortest Hsolvent···Cl and Csolvent···Cl distances are 2.60(3) and 3.534(3) Å, respectively, while the corresponding distances for [PPh4 ]2PuO2Cl4·CH 2Cl 2 are 2.68(2) and 3.582(2) Å, respectively. These values are consistent with the mean Hsolvent···Cl (2.53(3) Å) and Csolvent···Cl (3.57(3) Å) distances reported for hydrogen bonding interactions between dichloromethane and Cl− ions.19 In the crystallographic refinement of [AsPh4]2PuO2Cl4·2H2O we were unable to definitively refine the location of hydrogen atoms on the solvent water. Comparison of the Osolvent···Cl distance (3.274(7) Å) in [AsPh4]2PuO2Cl4·2H2O to the mean reported

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RESULTS AND DISCUSSION Synthesis and Structural Descriptions. Layering a concentrated HCl solution of Np6+ or Pu6+ on top of a dichloromethane solution containing 2 equiv of [PPh4]Cl or [AsPh4]Cl followed by evaporation in a fume hood results in the deposition of crystalline [PPh4 ] 2 NpO 2 Cl 4 ·CH 2 Cl 2 , [PPh4]2PuO2Cl4, or [AsPh4]2PuO2Cl4·2H2O. On one occasion, after crystals of [PPh4]2NpO2Cl4·CH2Cl2 has been rinsed with H2O, the rinse solution was allowed to evaporate, resulting in the formation of the unsolvated neptunyl species, [PPh4]2NpO2Cl4. Similarly, crystals of the CH2Cl2 solvate of [PPh4]2PuO2Cl4, [PPh4]2PuO2Cl4·CH2Cl2, were isolated as the only tractable material from a reaction between [PPh4]2PuO2Cl4 and 1,10-phenanthroline in dichloromethane. While this work will primarily focus on compounds [PPh 4 ] 2 NpO 2 Cl 4 ·CH 2 Cl 2 , [PPh 4 ] 2 PuO 2 Cl 4 , and [AsPh4]2PuO2Cl4·2H2O, we have included [PPh4]2NpO2Cl4 and [PPh4]2PuO2Cl4·CH2Cl2 in order to allow for structural comparisons to the newly reported complexes here and to previously reported uranyl analogues.9 All of the reported complexes crystallize in the triclinic space group P1̅, except [AsPh4]2PuO2Cl4·2H2O which crystallizes in the monoclinic space group P21/n. In all five of the compounds, the AnO2Cl42− anion adopts a distorted octahedral geometry where the two oxygen atoms are bound trans to one another, forming the actinyl moiety (AnO22+), and the four chloride ligands occupy the equatorial plane (Figure 1). Within the solid state molecular structure of each compound, the actinide atom rests in a special position with inversion symmetry, resulting in

Figure 1. Solid state structures of [PPh4]2NpO2Cl4·2CH2Cl2. Np (gray), chlorine (green), oxygen (red), phosphorus (magenta), carbon (black). H atoms on the PPh4+ cation have been omitted for clarity. C

DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bond Lengths (Å), Angles (deg), and Cation···Anion and Solvent···Anion Interactionsa AnO

An−Cl

HPPh···Oyl

[PPh4]2NpO2Cl4·CH2Cl2

1.764(1)

2.55(3)

137(2)

[PPh4]2NpO2Cl4

1.766(1)

2.68(3)

[PPh4]2PuO2Cl4

1.750(1)

[PPh4]2PuO2Cl4·CH2Cl2

1.743(1)

[AsPh4]2PuO2Cl4·2H2O

1.742(3)

2.654(1) 2.656(1) 2.665(1) 2.683(1) 2.657(1) 2.676(1) 2.659(1) 2.685(1) 2.664(1) 2.667(1)

C−HPPh···Oyl

HPPh···Cl/HSolvent···Cl

C−HPPh···Cl/C−HSolvent···Cl

151(3)

2.72(3) 2.60(3) 2.74(3)

130(2) 176(3) 147(2)

2.80(3)

146(2)

2.78(3)

144(2)

2.73(2)

141.8(17)

2.35*

153.2*

2.75(2) 2.68(2) 2.77* 3.274(7)**

165.0(19) 155.8(16) 156.3*

a * indicates no esds due to HFIX of hydrogen atoms within the solid state molecular structure, and ** indicates the distance is from the oxygen atom of the lattice water to the chloride ligand of the PuO2Cl42− moiety.

Figure 2. Ball and stick model showing the hydrogen bonding interactions in [PPh4]2PuO2Cl4. Dashed lines indicate cation−anion interactions (Pu = purple, Cl = orange, O = red, P = blue-green, C = black, and H = beige).

distance for hydrogen bonding between water and Cl− ions (3.190(3) Å)19 reveals a weak interaction between the anion and lattice solvent. Interestingly, while the lattice solvent in [PPh4]2NpO2Cl4·CH2Cl2, [PPh4]2PuO2Cl4·CH2Cl2, and [AsPh4]2PuO2Cl4·2H2O interacts with the chloride ligands of the anion, they do not interact with the oxo ligands of the actinyl moiety. This is similar to what we observed for the uranyl analogues where the CH2Cl2, MeCN, and MeOH lattice solvent interacted only with the anion through the chloride ligands.9

Vibrational Spectroscopy. In addition to obtaining the solid state molecular structures, we also investigated the vibrational properties of the five new transuranium compounds (Figure 3). All five of the compounds presented herein were examined by Raman spectroscopy, and compounds [PPh 4 ] 2 NpO 2 Cl 4 ·CH 2 Cl 2 , [PPh 4 ] 2 PuO 2 Cl 4 , and [AsPh4]2PuO2Cl4·2H2O were also probed using infrared spectroscopy. Since [PPh4]2NpO2Cl4 and [PPh4]2PuO2Cl4·CH2Cl2 were isolated only on one occasion and in small quantities, we were unable to obtain the infrared spectra for these species. The possible desolvation of [PPh4]2NpO2Cl4·CH2Cl2 and D

DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

O moiety. This linear geometry gives rise to three vibrational modes for the actinyl: the symmetric stretching mode (ν1, Raman active), a doubly degenerate bending mode (ν2, IR active), and the asymmetric stretching mode (ν3, IR active).20 For NpO22+, the ν1 symmetric stretch of the aqueous compound has been reported to appear in the Raman spectrum at 854/863 cm−1, while the ν3 asymmetric stretch of the same species is located at 969 cm−1 in the infrared spectrum.21−24 In the case of plutonium, the ν1 symmetric and ν3 asymmetric stretches of the analogous PuO22+ aqueous species are slightly red shifted from those of the neptunyl moiety, appearing at 833/835 and 962 cm−1, respectively.21−24 Due to the wavelength limitations of our infrared spectrometer, we are unable to investigate the actinyl ν2 bending mode which is located below 400 cm −1 (199−210 cm −1 for uranyl compounds23 and 267 cm−1 for Cs2NpO2Cl4).12,25 For all compounds, the ν1 symmetric stretching frequency is located within a 10 cm−1 region between 795 and 805 cm−1 (Table 3). While these stretches are significantly red-shifted Table 3. Symmetric (ν1, cm−1) and Asymmetric (ν3, cm−1) −yl Stretches for the Reported Complexes, Along with the Stretching Force Constant (k1, mdyn/Å) and the Interaction Force Constant (k12, mdyn/Å) for the AnO Bond [PPh4]2NpO2Cl4·CH2Cl2 [PPh4]2NpO2Cl4 [PPh4]2PuO2Cl4 [PPh4]2PuO2Cl4·CH2Cl2 [AsPh4]2PuO2Cl4·2H2O

ν1

ν3

k1

k12

797 801 800 800 802

914 914a 917

6.46 6.49 6.51

−0.47 −0.44 −0.48

920

6.55

−0.49

a

Value observed in the infrared spectrum obtained from compound [PPh4]2NpO2Cl4·CH2Cl2, which may have undergone elimination of the lattice solvent during preparation of the sample.

from those of the aqueous actinyl compounds, they are consistent with previously observed NpO22+ and PuO22+ compounds, such as Cs2NpO2Cl4 (802 cm−1)12,25 and M2PuO2Cl4 (M = Rb, Cs, Me4N, C10H10N2, C12H12N2, C13H16N2, and C10H11N2; ν1 = 793−813 cm−1).8,15 Similarly, the asymmetric stretching frequency for [PPh4]2NpO2Cl4· CH2Cl2, [PPh4]2PuO2Cl4, and [AsPh4]2PuO2Cl4·2H2O undergoes a red shift in comparison to that of the aqueous species; the infrared spectra of these compounds exhibit a ν3 signal at 914, 917, and 920 cm−1, respectively. These values are consistent with the asymmetric stretching frequencies reported for Cs2NpO2Cl4 (919 cm−1)12,25 and M2PuO2Cl4 (M = Rb, Cs, Me4N; ν1 = 909−932 cm−1).8 We believe the Raman spectrum measured for [PPh4]2PuO2Cl4·CH2Cl2 more accurately represents the product resulting from desolvation of the lattice dichloromethane. One piece of evidence pointing toward the desolvation of [PPh 4 ]2 PuO 2 Cl 4 ·2CH 2 Cl 2 is that the ν 1 symmetric stretching frequency for [PPh4]2PuO2Cl4·CH2Cl2 is identical to the corresponding frequency in [PPh4]2PuO2Cl4. A similar observation was made during our previous investigation in which we followed the desolvation of [PPh4]2UO2Cl4·2CH2Cl2 and [AsPh4]2UO2Cl4·2CH2Cl2 over a period of 80 and 120 min, respectively. Similar to what we observe for [PPh4]2PuO2Cl4·CH2Cl2, desolvation of the uranium compounds resulted in a spectrum almost identical to that of the unsolvated analogue, [PPh4]2UO2Cl4.9 Additionally, the spectrum for [PPh4]2PuO2Cl4·CH2Cl2 lacks a signal

Figure 3. Infrared (blue) and Raman (red) spectra for [PPh4]2NpO2Cl4·2CH2Cl2 (upper), [PPh4]2PuO2Cl4 (middle), and [AsPh4]2PuO2Cl4·2H2O (lower).

[AsPh4]2PuO2Cl4·2H2O during preparation of the compounds for analysis by infrared spectroscopy, as well as the possible desolvation of [PPh4]2PuO2Cl4·CH2Cl2 prior to analysis by Raman spectroscopy, will be addressed below. The actinide ions in all of the compounds reside on special positions with Ci site symmetry, resulting in a linear OAn E

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Inorganic Chemistry attributable to the dichloromethane CCl2 symmetric stretch,20 which appears at ∼698 cm−1 in the Raman spectra for solvated species of [PPh4]2NpO2Cl4·CH2Cl2, [PPh4]2UO2Cl4·2CH2Cl2, and [AsPh4]2UO2Cl4·2CH2Cl2.9 This evidence leads us to believe that [PPh4]2PuO2Cl4·CH2Cl2 desolvated between the time when the crystals were isolated and when the sample was analyzed by Raman spectroscopy. Similar to the desolvation proposed for [PPh4]2PuO2Cl4· CH2Cl2, preparation of [PPh4]2NpO2Cl4·CH2Cl2 and [AsPh4]2PuO2Cl4·2H2O for infrared spectroscopy (grinding the sample and pressing a KBr pellet) may lead to elimination of the unbound solvent contained within the crystal lattice. This loss of lattice solvent in [PPh4]2NpO2Cl4·CH2Cl2 and [AsPh4]2PuO2Cl4·2H2O would result in the reported infrared spectra for these compounds more accurately representing the unsolvated species, [PPh4]2NpO2Cl4 and [AsPh4]2PuO2Cl4, respectively. Support for this idea can be seen when comparing the infrared spectrum of [PPh4]2NpO2Cl4·CH2Cl2 to the attenuated total reflectance (ATR) spectra of freshly isolated, unground [PPh4]2UO2Cl4·2CH2Cl2 and [AsPh4]2UO2Cl4· 2CH2Cl2 crystals.9 In both of the solvated uranyl compounds, the signal corresponding to the CH2 wagging mode of the dichloromethane is clearly visible at 1270 cm−1.9 The corresponding signal is noticeably absent in the infrared spectrum of [PPh4]2NpO2Cl4·CH2Cl2, leading us to conclude that the compound desolvated during preparation of the sample. Taking this into consideration, we have used the ν3 asymmetric stretching frequency observed in the infrared spectrum of [PPh4]2NpO2Cl4·CH2Cl2 to approximate the k1 stretching and k12 interaction force constants for the desolvated species [PPh4]2NpO2Cl4 which was isolated in a small yield on only one occasion (Table 3). Using the methods described in our previous work, we can use the symmetric (ν1) and asymmetric (ν3) stretching frequencies of each compound to calculate the stretching force constant (k1) and interaction force constant (k12) of the actinyl moiety.8 For a linear arrangement like NpO22+ and PuO22+, the stretching force relates to the AnO bond, while the interaction force constant describes the interaction between the two oxo ligands. Additionally, these calculations provide a quantitative value that can be used to compare the Np and Pu compounds to their uranyl analogues.9 The stretching force constant (k1) and interaction force constant (k12) for [PPh4]2NpO2Cl4·CH2Cl2 are 6.46 and −0.47 mdyn/Å, respectively, while the corresponding values for [PPh4]2NpO2Cl4 are 6.49 and −0.44 mdyn/Å, respectively (Table 3). These constants are lower and less negative than those for NaNpO2(Ac)3 (6.98 and −0.54 mdyn/Å, respectively)26 and significantly lower than the stretching force constant reported for the NpO22+ aquo species (7.82 mdyn/ Å).23 As discussed above, the force constants presented for [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2NpO2Cl4 are both calculated using the asymmetric stretching frequency (ν3) from the infrared spectrum of [PPh4]2NpO2Cl4·CH2Cl2, which appears to undergo elimination of the lattice solvent during preparation of the KBr pellet. For [PPh4]2PuO2Cl4 and [AsPh4]2PuO2Cl4·2H2O, the stretching force constants (6.51 and 6.55 mdyn/Å, respectively) and interaction force constants (−0.48 and −0.49 mdyn/Å, respectively) are similar to those observed for the PuO22+ species Rb2PuO2Cl4 (6.69 and −0.52 mdyn/Å), Cs2PuO2Cl4 (6.58 and −0.53 mdyn/Å), and (Me4N)2PuO2Cl4 (6.39 and −0.48 mdyn/Å).8

We expected a clear trend among the vibrational frequencies and force constants. However, as shown in Table 3, the observed vibrational frequencies and calculated force constants for Np are lower than those of Pu. This should not be the case. From our prior work we have demonstrated that the vibrational frequencies of the actinyl ions are strongly influenced by solid state effects such as packing/symmetry and lattice solvent.9 Because of the difficulties with desolvation of the dichloromethane solvates, we hesitate to comment any further on this anomalous behavior. Structural Comparison across the Actinides. The compounds presented afford the opportunity to investigate periodic changes in the solid state molecular structures of isostructural compounds across the actinides. A comparison of [PPh4]2NpO2Cl4 and [PPh4]2PuO2Cl4 with their uranyl analogue reveals a slight contraction of the AnO bond when traversing the actinides from U (1.776(2) Å) to Np (1.7663(18) Å) to Pu (1.7498(16) Å).9 A similar trend is also observed when comparing [PPh4]2NpO2Cl4·CH2Cl2 with the isostructural dichloromethane solvate of uranyl, [PPh4]2UO2Cl4·2CH2Cl2, where the AnO bond length undergoes a 0.01 Å decrease upon replacing uranium with neptunium; [PPh4]2PuO2Cl4·CH2Cl2 is not included in this comparison due to the difference in the unit cell.9 For these two series of isostructural compounds, the shortening of the AnO bond across the actinides may be subtle, but the change is consistent with the difference in ionic radii of the metal ions (U6+ 6-coordinate, 0.73 Å; Np6+ 6-coordinate, 0.72 Å; Pu6+ 6coordinate, 0.71 Å).27 Exploration of the literature reveals a number of experimental and theoretical examples exhibiting a similar trend of contracting AnO and AnO interactions when traversing the actinides from left to right across the periodic table: AnO 2 (H 2 O) 5 2+ (An = U, Np, Pu), 22 AnO2(NO3)2(H2O)2 (An = U, Pu),28 M2AnO2Cl4 (M = Cs, Me4N; An = U, Pu),8 AnO2(SO4)(H2O)3 (An = U, Pu),28 K4AnO2(CO3)3 (An = U, Np),29,30 AnO2 (An = Th, Pa, U, Np, Pu, Am),31 An(Aracnac)4 (An = Th, U, Pu),32 [An(α2P2W17O61)2]16− (An = Th, U, Np, Pu, Am),33 and [C(NH2)3]4[An(C2O4)4]·2H2O (An = Th, U, Pu).34 As previously mentioned, the actinide atoms in [PPh4]2AnO2Cl4 and [PPh4]2AnO2Cl4·2CH2Cl2 reside on a special position with Ci site symmetry. This gives rise to two distinct sets of AnCl bonds, one of which is typically longer than the other. Examination of these AnCl interactions for isostructural compounds reveals a slight contraction of the bond length when traversing from left to right across the actinide series. For [PPh4]2AnO2Cl4·2CH2Cl2, longer and shorter AnCl bond lengths each decrease by 0.01 Å when traversing from uranium9 to neptunium ([PPh4]2NpO2Cl4· CH2Cl2). As with the changes observed for the AnO bond lengths, this contraction is consistent with the change in ionic radii of the metal ions (U6+ 6-coordinate, 0.73 Å; Np6+ 6coordinate, 0.72 Å).27 In the [PPh4]2AnO2Cl4 series of compounds, the change in the AnCl bond length across the series is smaller, with almost no change between U (2.669(1) and 2.686(1) Å) and Np (2.665(1) and 2.683(1) Å) and a contraction of only 0.01 Å from U to Pu (2.657(1) and 2.676(1) Å).9 Very few series of isostructural and isomorphic compounds across the actinides contain An−Cl bonds,35 making it difficult to judge the validity of the An−Cl contraction observed in [PPh4]2AnO2Cl4 (An = U, Np, Pu) and [PPh4]2AnO2Cl4· 2CH 2 Cl 2 (An = U, Np). Three of these series, F

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Inorganic Chemistry [Me4N]2AnO2Cl4 (An = U, Pu),8 [Ph3PNH2]2AnO2Cl4 (An = Np, Pu),11,16 and [PPh4]4AnCl6·4MeCN (An = Th, U, Pu),6 all exhibit a contraction of the An−Cl bond when traversing the actinides from left to right. In contrast, contraction of the metal−halide distance is not observed across the Cs2AnO2Cl4 series (An = U, Np, Pu),8,12,14 where the Np−Cl distance is reported to be shorter than the Pu−Cl bond length. The small sample size and contradicting trends suggest that more isostructural and isomorphic actinide compounds are required to verify a contraction of the An−Cl bond across the series. The isostructural series of [PPh4]2AnO2Cl4 (An = U, Np, Pu) compounds can be used to examine changes in the HPPh··· Oyl and HPPh···Cl interactions that occur upon exchanging the identity of the actinide metal ion. Examination of this set of compounds reveals that the HPPh···Oyl interaction length increases slightly from uranium (2.64 Å) to neptunium (2.68(3) Å) to plutonium (2.80(3) Å), while the HPPh···Cl interaction length is relatively consistent for all three compounds (U, 2.77 Å; Np, 2.74(3) Å; Pu, 2.78(3) Å).9 Interestingly, the increase in the HPPh···Oyl interaction length, 0.04 Å for U to Np and 0.12 Å for Np to Pu, does not correlate directly to the oxo ligand being bound closer to the actinide metal, as shown by the change in AnO bond length which contracts only 0.010 Å from uranium to neptunium and 0.016 Å from neptunium to plutonium. Surbella and co-workers recently studied a series of AnO2Cl42− hybrid materials and investigated the noncovalent interactions between the actinyl perhalo anion and the charge compensating cations. Their study demonstrated that the electrostatic potential in these actinyl anions decreased when moving from U to Pu, and they concluded that there was a reduced propensity for the −yl oxygens to form noncovalent interactions, a phenomenon that seems to be born out in the observation of longer interaction lengths moving from uranium to plutonium in this study.36 Potential Starting Materials for Non-Aqueous Transuranic Chemistry. A paucity of viable high-valent transuranic starting materials has restricted the ability of researchers to probe the non-aqueous chemistry of Np(VI) and Pu(VI). Typically, precursors for non-aqueous chemistry of the transuranium elements are limited to either the metallic forms of the elements, which are difficult to come by, or the tri- and tetrahalides produced directly from the metals.37−40 Furthermore, their increased redox activity and their ability to form actinyl bonds vitiate the applications of the elegantly simple synthesis of ThCl4 demonstrated by Kiplinger and Cantat to Np and Pu.41 Absent the availability of the metals or anhydrous halides, researchers have been primarily limited to using [NpO2Cl2(THF)]n as a starting material for non-aqueous NpO22+ chemistry.40,42 While this compound is soluble in THF, it appears to be somewhat unstable toward reduction to Np5+. Dissolution in THF results in the formation of a minor Np(V) component as observed by vis-nIR spectroscopy, and the mixed-valent species [(NpVIO2Cl2)(NpVO2Cl(THF)3)2] can be isolated from the solution after a few days.40,42 In the case of Pu(VI), while compounds such as M2PuO2Cl4 (M = Et4N, Me4N) were reported as early as 1965,43 the first (and to our knowledge only) starting material to be specifically synthesized for non-aqueous Pu(VI) chemistry, [PuO2Cl2(THF)2]2, was not reported until 2007.44 One issue that has been reported for both [NpO2Cl2(THF)]n and [PuO2Cl2(THF)2]2 is the potential for protonating agents such as HCl and TMSCl to

carry through from the synthesis of the starting material and effect further reactivity.11,16 We believe that compounds [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2PuO2Cl4 have the potential to be very useful as starting materials for non-aqueous transuranic chemistry. Both compounds appear to crystallize in a pure AnO22+ oxidation state and are soluble in organic solvents such as acetonitrile and dichloromethane. The easy synthesis and rapid crystallization of [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2PuO2Cl4 provide some advantages for these compounds over previous non-aqueous Np(VI) and Pu(VI) starting materials. Compounds [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2PuO2Cl4 can be synthesized directly from freshly ozonized HCl stock solutions of Np(VI) and Pu(VI), respectively. This one-step process alleviates the need to isolate intermediate materials such as NpO2(OH)2·xH2O for [NpO2Cl2(THF)]n42 or PuO2(CO3) in the case of [PuO2Cl2(THF)2]2,44 which reduces the number of synthetic steps and lowers the risk of actinide reduction. Additionally, the speed of crystallization for [PPh4]2NpO2Cl4· CH2Cl2 and [PPh4]2PuO2Cl4 also helps to avoid reduction to An(V). Crystalline material of these compounds can typically be isolated within a couple of hours after initiation of the reaction depending on how quickly the dichloromethane fraction evaporates. Finally, [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2PuO2Cl4 do not require the use of dry solvents and inert atmospheres to maintain their oxidation states. All manipulations of these compounds were performed under ambient conditions using the solvents as they were received from the vendor. In our hands, initial reactivity studies utilizing [PPh4]2NpO2Cl4·CH2Cl2 and [PPh4]2PuO2Cl4 as non-aqueous AnO22+ starting materials have proven fruitful, and we anticipate publishing our results in the future.

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CONCLUSION We have presented the synthesis and structure of five new transuranium hexavalent actinide complexes, determined their molecular structure, and studied their vibrational properties. Structurally, these complexes represent the completion of the periodic series of actinyl tetrachlorides that we had previously studied for uranium(VI). Our investigation of the vibrational spectra revealed a red shifting of the symmetric and asymmetric vibrational modes associated with the neptunyl and plutonyl ions with respect to the uranyl ion. However, comparison between the small shifts observed between neptunyl and plutonyl frequencies is complicated by the demonstrated sensitivity of these vibrational modes to crystal packing forces as well as the presence of solvent crystallization.9 Nevertheless, this series of complexes should be useful for further spectroscopic study of their electronic properties, particularly as it relates to our developing description of the actinide− ligand bond. Finally, the complexes and their preparation should find utility in further developing non-aqueous transuranium chemistry, the development of which is hindered by a lack of easily prepared and appropriate starting materials.

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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.7b02382. Thermal ellipsoid plots and complete vibrational spectra (PDF) G

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Inorganic Chemistry Accession Codes

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CCDC 1574549−1574553 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Richard E. Wilson: 0000-0001-8618-5680 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed at Argonne National Laboratory, operated by UChicago Argonne LLC for the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements program under Contract DE-AC0206CH11357.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02382 Inorg. Chem. XXXX, XXX, XXX−XXX