Article pubs.acs.org/IC
Thorium and Uranium Hydride Phosphorus and Arsenic Bearing Molecules with Single and Double Actinide-Pnictogen and Bridged Agostic Hydrogen Bonds Lester Andrews,† Han-Gook Cho,†,‡ K. Sahan Thanthiriwatte,§ and David A. Dixon*,§ †
Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 406-772, South Korea § Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0366, United States ‡
S Supporting Information *
ABSTRACT: Thorium atoms from laser ablation react with phosphine during condensation in excess argon to produce two new infrared absorptions at 1467.2 and 1436.6 cm−1 near weak bands for ThH and ThH2, which increase on annealing to 25 and 30 K, indicating spontaneous reactions. Analogous experiments with uranium produced two similar bands at 1473.4 and 1456.7 cm−1 above UH at 1423.8 cm−1 and another absorption at 1388.2 cm−1. Electronic structure calculations at the coupled cluster CCSD(T) for Th and density functional theory calculations for U as well as their proximity to other actinide hydride absorptions support assignments of these bands to the simplest molecules HPThH2, HPUH2, and PH2−UH. Arsine gave the analogous products HAsThH2, HAsUH2, and AsH2−UH. The HEAnH2 molecules (E = P, As; An = Th, U) have strong agostic An−H(E) interactions with H−E−An angles in the range of 60−64°. The calculated agostic bond distances are 9% to 12% longer than terminal single An−H bonds, which suggests that these strong agostic bonds can be considered as bridge bonds since similar relationships are found for the dibridged M2H6 molecules (M = Al, Ga, In). The NBO analysis and the molecular orbitals show the presence of a σ and a π bond for HEAnH2 molecules that are heavily polarized with most of the density on the P or As.
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INTRODUCTION Actinide−ligand multiple bonding continues to attract the attention of synthetic as well as computational and spectroscopic chemists.1 However, there are fewer investigations as one goes down a column in the periodic table for the central atom in the ligand bound to the actinide. Although ligand supported phosphinidene complexes have been investigated previously,2 the first terminal parent phosphide and phospinindene complexes stabilized by U(IV) were reported in 2014.3 In 2015, a seminal work appeared on U(IV)−arsenic complexes with single, double, and triple bonding interactions.4 Liddle and co-workers have extended their uranium work to thorium and very recently reported complexes of thorium(IV)−phosphanide (Th−PH 2 , thorium(IV)−phosphinidene (ThPH), dithorium(IV)−phosphinidiide (Th−P(H)−Th), and phosphide (ThPTh) using the N(CH2CH2NSiPr3i)3 ligand.5 Molecules containing actinides with soft chalcogen ligands have recently been reviewed.6 We have contributed to this effort to better understand the properties of molecules containing actinides with chalcogen ligands and recently reported that laser ablated Th and U atoms react with H2S to give the double bonded SThH2 and SUH2 molecules.7 About a decade ago, we reported the preparation of the simplest actinide methylidene and imine bearing molecules CH2AnH2 and HNAnH2 using the reactions of laser © XXXX American Chemical Society
ablated Th and U atoms with CH4 and NH3 and characterized these new species with matrix infrared spectroscopy and density functional theory (DFT) calculations.8−12 Analogous experiments with the fluorine analogs NF3, PF3, and AsF3 produced the stable U(VI) products NUF3, PUF3, and AsUF3, and DFT was used to further characterize these simple terminal nitrides, phosphides, and arsenides.13,14 Recently, a bridging thorium phosphinidene complex was prepared.15 We recently reported an investigation of Ti, Zr, and Hf reactions with phosphine and arsine, which form strongly agostic HEMH2 molecules (E = P, As) with evidence for hydrogen bridge bonded E−H−M subunits.16 In the current work, we report results of the reactions of PH3 and AsH3 with Th and U atoms where we found new absorptions in the intense An−H stretching region of the infrared spectrum. Our combined experimental and computational results show that we have observed new An(IV) species containing An−E double bonds.
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EXPERIMENTAL AND COMPUTATIONAL METHODS
Laser ablated Th and U atoms were reacted with phosphine (Matheson, after condensation at 77 K to remove volatile impurities) Received: December 19, 2016
A
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry diluted in argon or neon during condensation at 4 K using a closedcycle refrigerator (Sumitomo).8−14,16−18 (Natural Th and U are very long lifetime alpha emitters with 238U having a half-life of 4.5 × 109 years and 232Th having a half-life of 14 × 109, so they are safe to handle with minimal contact.) Arsine (Matheson, 10% in hydrogen, condensed at 77 K to remove H2) was used similarly. Deuterium exchanged phosphine was prepared by condensing PH3 on D2O several times in a D2O exchanged stainless steel vacuum line, which resulted in stronger PD3, PHD2, and PH2D absorptions as compared to residual PH3. These samples attained approximately 80% deuterium enrichments.16 Reagent gas mixtures ranged from 0.5% to 2% in argon or neon (Spectra Gases). [These reagents are highly toxic; our system pump was vented through a garden hose into a laboratory f ume hood exhausted through the roof.] The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto a rotating Th or U metal target (Oak Ridge National Laboratory) using 5−10 mJ/pulse. Infrared spectra were recorded at 0.5 cm−1 resolution using a Nicolet 750 spectrometer with a liquid nitrogen cooled Hg−Cd−Te range B detector after sample deposition. Next, samples were annealed using resistance heat, and selected samples were irradiated for 20 min periods by a mercury arc street lamp (175 W, range 220−580 nm) with the globe removed using a combination of optical filters.16,17 Support for assignments of the new experimental frequencies is provided by electronic structure calculations. For the Th(IV) compounds, coupled cluster CCSD(T) calculations19−22 as well as DFT calculations with the B3LYP23,24 and PW9125,26 exchangecorrelation functionals were performed. The calculations for the open shell U(IV) compounds were done at the DFT level with the same functionals. The aug-cc-pVTZ basis sets for H and N,27 aug-cc-p(V +d)TZ basis for P, 28 and aug-cc-pVTZ-pp basis sets with pseudopotentials for As29 and Th and U were used.30 Geometries were fully optimized for different spin-spatial symmetries as appropriate, and second derivatives were calculated to obtain vibrational frequencies. An ultrafine grid was used for the DFT calculations with grid specifications of (150, 974) for the H, N, P, and As and (225, 974) for Th and U. DFT provides reasonable predictions of harmonic vibrational frequencies, so the calculated values will be higher than the experimental values which include an anharmonic component.31,32 In addition, the matrix values in Ar can have an additional shift due to the presence of a matrix shift further lowering the experimental values. The calculations are performed without spin orbit corrections in the scalar relativistic approximation, so we can describe the spin states as singlets, triplets, et cetera. The same is true for the CCSD(T) calculations. The DFT calculations were done with Gaussian09,33 and the CCSD(T) calculations were done with MOLPRO.34,35
Figure 1. Infrared spectra of laser ablated thorium and phosphine reaction products: (a) after deposition of Th with 1% PH3 in argon at 4 K, (b) after annealing to 25 K, (c) after annealing to 33 K, and (d) after annealing to 39 K. mp = metal phosphinidene.
Several experiments were done with Th and AsH3 in argon, and two weak product bands at 1453.9 and 1427.6 cm−1 decreased on annealing to 22 and 30 K, Figure S1, but they were considerably weaker than the phosphine product absorptions in Figure 1. We suggest that this product may be photosensitive to the laser ablation plume. Uranium IR Spectra. Atomic uranium reacts readily with PH3 as the bottom set of argon matrix spectra in Figure 2 illustrates. The sharp band at 1423.8 cm−1 is due to the UH
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RESULTS AND DISCUSSION Thorium IR Spectra. Figure 1 shows the Th−H stretching region for new absorptions in the thorium atom reaction with phosphine. Sample deposition at 4 K produces new broad absorptions at 1467 and 1437 cm−1, which increase on annealing to 25 and 33 K and sharpen to 1467.2 and 1436.6 cm−1 (labeled mp for metal phosphinidene) together with weak bands at 1485.2 and 1455.8 cm−1 which are due to ThH and ThH2 as produced first from the Th reaction with H2 and then with H2O.36,37 Sample irradiation with the full light of a medium pressure mercury arc (λ > 220 nm) in another experiment reduced the mp bands by half without changing the ThH and ThH2 bands. Two neon matrix experiments produced broad absorptions at 1490.2 and 1458.1 cm−1, which decreased on annealing to 11 K. These 23.0 and 21.5 cm−1 blue shifts from the above solid argon values are comparable to the 17 and 19 cm−1 blue shifts observed for the two Th−H stretching modes of SThH26 and are characteristic of changing from the more polarizable argon to the less polarizable neon matrix.38
Figure 2. Infrared spectra of laser ablated uranium and phosphine or arsine reaction products: (a) after deposition (U + 1% PH3 in Ar), (b) after annealing to 20 K, (c) after annealing to 26 K, (d) after photolysis (λ > 220 nm), (e) after annealing to 20 K, (f) after annealing to 26 K, (g) after deposition (U + 1% AsH3 in Ar) and annealing to 25 K, (h) after annealing to 31 K, (i) after photolysis (λ > 220 nm), and (j) after annealing to 33 K. mp = metal phosphinidene. ma = metal arsinidene. i = insertion product. B
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry diatomic molecule first observed with U and the H2 reagents and later using H2O, NH3, and HF.12,37,39,40 New absorptions were observed on sample deposition at 1473.7 and 1456.5 cm−1 and at 1388.2 and 1377.5 cm−1 (labeled mp and i, respectively; i = insertion product). Annealing to 20 and 26 K increased the 1388.2 cm−1 band. UV irradiation (λ > 220 nm) almost destroyed the i bands and increased the mp shoulder absorptions at 1486 and 1468 cm−1. Final annealing again to 20 and 26 K slightly increased the i peaks and the sharp mp bands. A weak band at 1050.9 cm−1, which increased on UV irradiation, is due to NUN from the reaction of atomic U with trace nitrogen impurity in the matrix sample.18 Spectra from the better of the two experiments with deuterium enriched PH3 in a stainless steel manifold passivated with D2O are illustrated in Figure 3. These spectra reveal sharp
reaction of Th with PH3 exhibits two strong absorptions at 1467.2 and 1436.6 cm−1, which increase together on early 25 and 33 K annealing cycles and decrease together on later 39 K annealing and on UV photolysis. The 1431 cm−1 shoulder is likely due to a matrix splitting of the main band. The higher frequency band remains one-third of the intensity of the lower band throughout this treatment, and these bands are appropriate for the symmetric and antisiymmetric stretching modes of a ThH2 subunit. In fact, the new bands are just 13 and 19 cm−1 lower than the two stretching modes for molecular ThH2, observed previously at 1480.1 and 1455.6 cm−1 with 1:6 relative intensities.36 Table 1 collects the absorptions in the Th−H stretching region of the spectrum for a number of small molecules41,42 Table 1. Stretching Frequencies, cm−1 (sym, antisym), for ThH2 and UH2 Subunits in Small Molecules Measured in Solid Argon molecules SeThH2 (1A′) HAsThH2 HPThH2 ThH2 (1A1) SThH2 (1A′) CH2ThH2 (1A) HNThH2 (1A′) OThH2 (1A′)
Figure 3. Infrared spectra of laser ablated uranium and deuterated phosphine reaction products: (a) after deposition of U with 2% PD0,1,2,3H3,2,1,0 in argon at 4 K, (b) after annealing to 25 K, (c) after annealing to 30 K, and (d) after annealing to 36 K. The c denotes a band common to these experiments. mp = metal phosphinidene. The deuterium enrichment is approximately 80% in these experiments.
frequencies 1477.5, 1445.8 1453.9, 1427.6 1467.2, 1436.6a 1480.1, 1455.6 1468.6, 1435.3 1435.7, 1397.1 1398.7, 1357.4 1397.2, 1352.4
molecules
frequencies
SeUH2 (3A″) HAsUH2
1493.7, 1470.0 1471.8, 1459.5 1473.7, 1456.5b
HPUH2
SUH2 (3A″) CH2UH2 (3A) HNUH2 (1A′) OUH2 (3A″) UH2 (5A2)
refs 42 this work this work 36
1488.9, 1462.4 1461.2, 1425.4 1436.3, 1403.0 1416,1377
7
1406.1, 1370.7
39
8, 9, 11 10, 12 37, 41
a Measured in solid neon at 1490.2 and 1458.1 cm−1, bMeasured in solid neon at 1500.9 and 1480.4 cm−1.
peaks for UH and UD at 1423.8 and 1016.5 cm−1 (H/D frequency ratio 1.4008).39 New peaks were observed in the U− H stretching region at 1473−1456 cm−1, close to those observed in the reaction with PH3 plus a stronger intermediate peak at 1465.7 cm−1, and 3-fold stronger bands at 1051−1041 cm−1 in the U−D stretching region. These bands increased and sharpened into triplet absorptions at 1473.4, 1465.7, and 1456.7 cm−1 and at 1051.2, 1046.8, and 1041.0 cm−1 on annealing to 25 and 30 K then decreased on annealing to 36 K. Corresponding experiments were done in excess neon, and the mp products were observed at 1500.9, 1480.4 cm−1 with D counterparts at 1070.9, 1057.4 cm−1 (H/D frequency ratios 1.4015, 1.4000). The 27.2 and 23.9 cm−1 shifts for the H counterparts are reasonable for small uranium bearing molecules: 22 and 20 cm−1 blue shifts were observed for S UH2.7 The i counterpart was observed in neon at 1408.9 cm−1, a 20.7 cm−1 blue shift from the argon matrix value. Atomic U also reacts with AsH3 as shown by the top spectra in Figure 2. The UH absorption is observed again, and two sharp new peaks were observed at 1471.8, 1459.5 cm−1 (labeled ma for metal arsinidene) following deposition and 25 K annealing. The UH absorption increased on annealing to 31 K, and a weak new band appeared at 1380.2 cm−1. UV−vis irradiation (λ > 220 nm) increased the bands at 1405.5, 1370.0 cm−1 due to UH2 and blue satellites on the ma bands. Thorium Dihydride Phosphinidine, HPThH2, and Arsenidene, HAsThH2. The major new product in the
with a ThH2 fragment and shows that our new product absorptions are only 1 cm−1 away from the Th−H modes for the third row bearing a SThH2 molecule and substantially higher (31 and 21 cm−1) than observed for the imine HN ThH2.7,10 These comparisons point to HPThH2 as the new reaction product. Frequency calculations at the CCSD(T) and DFT levels confirm that HPThH2 absorbs in this region, and that it is the most stable reaction product isomer. The CCSD(T) frequencies computed at 1523 and 1487 cm−1 are 3.8 and 3.5% higher, respectively, than the Ar matrix values with large respective 256 and 449 km/mol intensities at the PW91 level; these intensities are consistent with the observed 1:3 relative intensities. Other frequencies for HPThH2 are predicted to have much lower intensities and are too weak to be observed here. Table 2 collects the calculated frequencies for HPThH2. Our calculations predict that the lowest energy structure is a singlet HPThH2 isomer with C1 symmetry and an acute H−Th−U angle. The insertion product, H2 P−ThH, is potentially an intermediate in the reaction to form HPThH2, and this molecule is predicted to have frequencies in the region observed for mp, but it is not observed, probably because it is much higher in energy (ΔH298 = 16 kcal/mol at the CCSD(T) level). The same holds for the terminal phosphide C
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 2. Calculated An−H Vibrational Frequencies (cm−1) and Infrared Intensities (km/mol) for the Lowest State of HE-AnH2: HPThH2, HAsThH2, HPUH2, and HAsUH2 property B3LYP sym B3LYP asym PW91 sym PW91 asym CCSD(T) sym CCSD(T) asym Expt (Ar) sym Expt(Ar) asym Expt (Ne) sym Expt(ne) asym a5
1
H2ThPH (C1) 1521.0 1482.0 1509.3 1475.6 1522.9 1487.0 1467.2 1436.6 1490.2 1458.1
(321) (545) (256) (449)
1
H2ThAsH (C1) 1526.3 1486.5 1512.1 1478.8 1497.8 1465.1 1453.9 1427.6
H2P-UH (C1): 1438.0 (392) (B3LYP) and 1425.1 (289) (PW91).
b5
(327) (519) (254) (430)
H2UPH (C1)a
3
1503.9 1480.7 1482.0 1463.2
1473.7 1456.5 1500.9 1480.4
(421) (507) (311) (426)
H2UAsH (C1)b
3
1461.9 1454.9 1474.4 1457.7
(616) (585) (339) (416)
1471.8 1459.5
H2As-UH (C1): 1437.6 (377) (B3LYP) and 1423.6 (274) (PW91).
stretching modes. The observed numbers are a median U−H stretch at 1465.1 cm−1 with an intermediate band at 1465.7 cm−1 and a median U−D stretch at 1046.0 cm−1 with an intermediate band = 1046.8 cm−1. We were not able to observe the predicted small difference between Dcis and Dtrans on the U−H and U−D stretching modes. However, the intermediate bands that we do observe in both the U−H and U−D stretching regions confirm this observation of a uranium dihydride containing product, and its position in Table 1 verifies this identification of the HPUH2 molecule. Our calculations predict that the two lowest energy structures at the PW91 level are a 3HPUH2 isomer with C1 symmetry and an acute H−P−U angle and a 5HUPH2 C1 structure; they have the same energy to within 0.1 kcal/mol. The Cs conformer of 3HPUH2 with an obtuse H−P−U angle is 6.5 kcal/mol higher in energy at the PW91 level. The calculated frequencies at the PW91 level for the acute C1 isomer are 8.6 and 6.5 cm−1 higher than the observed values, in excellent agreement and in the expected direction, but the calculated frequencies for the obtuse Cs conformer are 24.2 and 28.0 cm−1 lower than the observed values, which rules out observation of the obtuse conformer. The observed doublet at 1471.8 and 1459.5 cm−1 from the U + AsH3 reaction behaves like the above HPUH2 product, and the shift from PH3 to AsH3 amounts to 2−3 cm−1. Our PW91 calculation predicts that the triplet HAsUH2 product with C1 symmetry and an acute H−As−U angle has strong U−H stretching modes within 5 to 8 cm−1 of the HPUH2 values and within 2 cm−1 of the observed values, which is in agreement with the experiment within the accuracy of our computed frequencies. The higher energy Cs obtuse conformer (ΔH298 = 8.2 kcal/mol) has computed frequencies 8 and 29 cm−1 lower than the observed values, so the obtuse conformer can be ruled out on the basis of both calculated frequencies and the relative energy. Uranium Hydride Phosphide, H2PUH, and Arsenide, H2AsUH. The single sharp i band at 1388.2 cm−1 (Figure 2) with a matrix site splitting at 1377.5 cm−1 behaves differently from HPUH2 as the 1388.2 cm−1 band increases more on annealing, and it is almost destroyed on photolysis. This single band, which is less than the value for diatomic UH at 1423.8 cm−1, invites consideration for a single U−H bond stretching mode. The 5H2P−UH insertion isomer with C1 symmetry is predicted to be isoenergetic with the lowest energy 3HPUH2 isomer at the PW91 level. The 5H2P−UH isomer is predicted to have a stong U−H absorption 38 cm−1 below the lower frequency U−H stretch for 3HPUH2 consistent with the 68.3
triplet PThH3, which is computed to be 31 kcal/mol higher in energy than HPThH2 at the same computational level, and this much higher energy product is surely not formed here. The two weak product absorptions with thorium and arsine in argon are 13.3 and 9.0 cm−1 lower than the corresponding bands for HPThH2. The electronic structure calculations all predict a small increase for the symmetric and asymmetric stretches of HAsThH2 compared to HPThH2, but the differences with respect to the experiment are still small. The bands at 1453.9 and 1427.6 cm−1 are assigned tentatively to HAsThH2. Uranium Dihydride Phosphinidine, HPUH2, and Arsinidine, HAsUH2. The two bands at 1473.7 and 1456.5 cm−1 with U and PH3 track together on annealing and photolysis, and they appear in the U−H stretching region just above UH itself39,39 (Figure 2). The weak shoulder bands are most likely due to a different matrix packing arrangement around the product responsible for the stronger above absorptions. Similar experiments with D enriched phosphine resulted in broader features at 1473.4, 1465.7, and 1456.7 cm−1, slightly shifted from the two strongest bands using PH3 and including a stronger median component. Sharper, stronger bands were observed in the U−D stretching region: a doublet at 1051.0 and 1041.0 cm−1 with a weaker intermediate component at 1046.8 cm−1 These absorptions exhibit H/D frequency ratios of 1.4022 and 1.3991 for the doublet and 1.4002 for the intermediate band. These high isotopic ratios are appropriate for an H- or D-heavy metal stretching mode, and the appearance of two such bands again suggests the symmetric and antisymmetric modes of a dihydride species (Table 1). The intermediate bands are expected for the analogous single H and single D stretching modes of a mixed UHD product species, where the U−H band is double the intensity of the U−D band. Our calculations at the PW91 density functional theory level reveal precisely this pattern (where X = H or D) for the XP UH2, XPUHD, and XPUD2 U−H or U−D stretching modes. Since the U−P−H bonding is not linear and the molecule is nonplanar, the two hydride atoms are not equivalent, and we find eight different isotopic product molecules in the mix bracketed by HPUH2 and DPUD2, including DPUH2 and HPUD2, and the sets DPUHDcis, DPUHDtrans, HPUHDcis, and HPUHDtrans. Frequency calculations (SI) predict that the PH or PD affects the U−H stretch by a few 0.1 cm−1, and the four different XUHD structural isomers have values close to the median of the computed HPUH2 and DPUD2 frequencies except for the one case where the P−D stretch interacts with the U−H D
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry cm−1 shift to the 1388.2 cm−1 i band. Unfortunately, the deuterium counterpart is probably masked by PH3 in the mixed isotopic precursor sample. The Ne matrix counterpart for 5H2PUH is observed at 1408.9 cm−1, a 20.7 cm−1 blue shift. The Ne matrix value is 16 cm−1 below our calculated gas phase harmonic PW91 value. We note that the 1388.2 cm−1 band is within one wavenumber of the strongest absorption of HO2, but another prominent HO2 band at 1103 cm−1 is not observed here, and HO2 grows on UV irradiation,43 whereas the present 1388.2 cm−1 band is almost destroyed. Therefore, the present 1388.2 cm−1 band is not due to HO2. Finally, the terminal uranium phosphide 1PUH3 is computed to be 30 kcal/mol higher energy than 3HPUH2 or 5H2P−UH, and this much higher energy product is not formed here in contrast to the PF3 reaction where PUF3 is the major product.13 The AsH3 experiment reveals a different band at 1380.2 cm−1, which also increases on annealing and decreases on UV photolysis. This band is tentatively assigned to H2As−UH. Our PW91 calculations predict that the 5H2As−UH isomer is only 2 kcal/mol above the 3HAsUH2 isomer, so both could be observed. The predicted U−H stretching frequency shift from H2P−UH to H2As−UH of 2 cm−1 is consistent with the small shift of 8 cm−1 that is observed. The terminal uranium arsenide 1 AsUH3 is computed to be 28 kcal/mol higher in energy than 3HAsUH2; in contrast, AsUF3 is is the major product in the reaction with AsF3.13,14 Predicted Molecular Structures. The calculated structures for the An and EH3 reaction products are illustrated in Figure 4 where the analogous ammonia reaction products are included for comparison with those for phosphine and arsine. The lowest energy isomers for H2AnEH and HAnEH2 are shown. An interesting observation is the marked change in the H−EAn structure from the nearly linear H−NU angle (175°) with Cs symmetry for the imines to the acute H−PAn and H−AsAn angles (63−65° and 59−60°) for the phosphinidenes and arsinidenes. The latter acute angle isomers have an obtuse angle conformer higher in energy by 6 kcal/mol for P and 8 kcal/mol for Th and U, respectively. Part of this structural change for the lowest isomer can be attributed to the high p character of the P−H and As−H bonds,44 and part is due to the formation of a strong terminal E−H to an agostic bonding interaction. The higher energy obtuse angle conformers without agostic interactions are predicted to have shorter AnE bond lengths (Δr ∼ 0.06 Å for ThP, Δr ∼ 0.02 Å for UP, Δr ∼ 0.08 Å for ThAs, and Δr ∼ 0.08 Å for UAs) and shorter E−H bond lengths (Δr ∼ 0.06 Å for (Th)P−H, Δr ∼ 0.06 Å for (U)P−H, Δr ∼ 0.08 Å for (Th)As−H, and Δr ∼ 0.09 Å for (U)As−H) than those in the agostic form. Thus, the stabilizing agostic interaction comes at the expense of both the AnE bond and E−H bonds as both lengthen in the agostic form. The structures for HAnEH2 show a similar variation as for the H2AnEH with the HAnNH2 showing a planar structure with a planar nitrogen. In contrast, the P and As derivatives for HAnEH2 show very nonplanar geometries at the E with strong agostic interactions from the two E−H bonds back to the An. Although the HAnEH2 structures for AnTh and EP and As are not likely to be energetically accessible, those for AnU are and are observed. The two P−H agostic interactions with U in 5HUPH2 are 0.25 Å longer than in 3H2UPH, and the U−P bond is also 0.22 Å longer. Surprisingly, the As−H agostic interaction is apparently stronger in 5HUAsH2 than in the P derivative with an increase of only 0.18 Å from the 3H2UAsH
Figure 4. Optimized structures of the lowest energy isomers of H2AnEH and HAnEH2 for AnTh and U and E = N, P, and As at the PW91 level. Relative PW91 energies ΔH(298) in kcal/mol.
isomer. The increase in the U−As bond is also smaller at 0.17 Å. Thus, the As−H agostic interaction distance of 2.40 Å in 5 HUAsH2 is shorter by 0.07 Å than in 5HUPH2. Actinide−Pnictinide Bonding. The actinide−pnictinide bonds have been explored using a natural bond order (NBO) analysis (see Table 3).45−48 Complete details are given in the Supporting Information. This analysis is performed for wave functions calculated in the scalar relativistic approximation without consideration of spin orbit effects. For H2ThPH, a Th− P σ bond is generated from the lone pair on P with 1.32 e donating into an empty orbital on Th with 0.68 e on the Th. There is a TH−P π bond with 1.41 e on P and 0.69 e on Th. The bonding at Th is dominated by d orbitals with some additional s character in some bonds. There is always less than 15% f character on the Th in these bonds. The P−H bond has 0.79 e on P, 1.05 e on H, and the 0.18 e agostic interaction on the Th; again the bonding component on the Th is dominated E
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
to 1.63 e on the P. This is consistent with the shorter Th−P bond in this structure and the lack of the agostic P−H interaction. The agostic interaction clearly changes the qualitative picture of the bonding by removing the ability of the Th to interact with the P by occupying vacant orbitals and leading to a longer bond in the lowest energy form. The sum of the double bond radii49 for Th and P is 2.45 Å, and our values for the PTh bond for the Cs structure are consistent with this value and not with the triple bond value50 of 2.30 Å. The bonding for the most stable C1 structure of H2ThAsH is very similar to that of H2ThPH for the two Th−As bonds. The As−H agostic interacting bond has 0.72 e on As, 1.09 e on H, and 0.20 e on Th (see Figure 5 for the orbitals). The Cs structure for H2ThAsH is similar to that of the Cs structure of H2ThPH with three highly polarized As−Th bonds. The sum of the double bond radii for Th and As is 2.57 Å, and our values for the AsTh bond for the Cs structure are consistent with this value and not with the triple bond value50 of 2.42 Å. The bonding in H2ThNH has three polarized Th−N bonds with one derived from the lone pair on N (1.63 e) interacting with an empty orbital on Th (∼0.37 e). In addition, there are two other very polarized Th−N bonds, one with 0.32 e on Th and 1.68 e on N and one with 0.40 e on Th and 1.60 e on N. Both of these bond have dominant d character on the Th. The Th−N bond distance in H2ThNH (Cs) is predicted to be 1.95 Å at the PW91 level and 1.96 Å at the CCSD(T) level. The sum of the triple bond covalent radii50 for Th and N is 1.90 Å, and the sum of double bond covalent radii49 gives 2.03 Å. Our calculated bond distance is intermediate between these two values and is consistent with the three predicted highly polarized Th−N bonds. As an additional benchmark, the imine Th−N stretching frequency in H2ThNH is 793.4 cm−1 in solid argon10 in comparison with our calculated PW91 value of 820 cm−1 and CCSD(T) value of 806 cm−1. The ThE diatomic molecules (2∑) are predicted to have bond distances of 1.82 Å (CCSD(T)) and 1.81 Å (PW91) for ThN, 2.39 Å (CCSD(T)) and 2.36 Å (PW91) for ThP, and 2.54 Å (CCSD(T)) and 2.48 Å (PW91) for ThAs. For ThN, the calculated frequencies are 946 cm−1 (CCSD(T) and 975 cm−1 (PW91) in comparison with the argon matrix value of 934.6 cm−1.10 The ThN bond length is shorter than the sum of the triple bond covalent radii, and the ThP and ThAs bond distances are longer than the sum of the triple bond radii, which suggests that these diatomics may not be good models for this type of bond or that the radii need to be further improved. The NBO analysis shows the presence of two unpaired electrons on the U in 3H2UPH. The bonding for 3H2UPH is similar to that in 1H2ThPH except that there is more U and more f orbital participation in the U−P bond for the α spin electrons than found in the Th analog. The Kohn−Sham orbitals for the α electrons are consistent with this picture as shown in Figure 6. The analysis for the β spin electrons show that they are more similar to the Th analogs. The NBO analysis shows the presence of four unpaired electrons on the U in 5HUEH2 and a single U−E bond for E P and As with ∼0.5 e on U with 65% d character. The En−H bond distorts away from the U−En axis due to the very acute H−En−U angle. The lone pair on the P or As can slightly delocalize onto the U from the EnH2 side of the molecule that is bent toward the U. The Kohn−Sham orbitals are shown in Figure 7 and are consistent with this picture. The single bonds are ∼0.15 Å as compared to the sum of the covalent single bond radii.49
Table 3. (NBO6) Natural Electron Configuration and Atomic Charges on Th (B3LYP) and U (PW91) compounds molecule 1
H2ThNH HThNH2 3 H3ThN 1 H2ThPH 1 H2ThPH 1 HThPH2 3 H3ThP 1 H2ThAsH 1 H2ThAsH 1 HThAsH2 3 H3ThAs 3 H2UNH 3 HUNH2 1 H3UN 5 HUPH2 3 HUPH2 3 H2UPH 3 H2UPH 1 H3UP 5 HUAsH2 3 HUAsH2 3 H2UAsH 3 H2UAsH 1 H3UAs 1
symmetry Cs Cs C3v C1 Cs Cs(trans) C3v C1 Cs Cs(trans) C3v Cs Cs C3v C1 C1 C1 Cs C3v Cs Cs C1 Cs C3v
q (M) 1.93 1.10 0.97 1.23 1.48 0.92 1.74 1.35 1.43 0.89 1.71 1.78 1.15 1.50 0.84 1.01 1.20 1.32 1.02 0.79 0.97 1.26 1.45 1.04
M occ 0.39
7s 6d1.465f0.33 7s1.686d1.015f0.21 7s0.406d1.445f0.24 7s0.526d1.975f0.34 7s0.446d1.865f0.30 7s1.526d1.335f0.25 7s0.486d1.65f0.21 7s0.506d1.935f0.27 7s0.446d1.895f0.29 7s1.476d1.415f0.21 7s0.506d1.935f0.27 7s0.376d1.385f2.56 7s1.086d0.775f2.97 7s0.506d1.665f2.54 7s0.906d1.085f3.18 7s1.016d0.925f3.05 7s0.506d1.715f2.62 7s0.396d1.445f2.89 7s0.466d1.945f2.65 7s0.896d1.155f3.17 7s1.006d0.965f3.05 7s0.516d1.505f2.76 7s0.406d1.375f2.82 7s0.476d2.675f1.87
by the d orbitals. There is another lone pair on P which has 80% s character. As shown in Figure 5, the Kohn−Sham orbitals are consistent with this pattern. The higher energy 1H2ThPH (Cs) structure has three highly polarized Th−P bonds with 0.37 to 0.54 e on the Th and 1.46
Figure 5. Kohn−Sham orbitals for H2ThPH and H2ThAsH for the key Th−E interactions. F
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 6. Kohn−Sham α-orbitals for H2UPH and H2UAsH for the key Uh−E interactions.
of 1.94 Å, but not nearly as short as the 1.72 Å sum of triple bond covalent radii, and this frequency is observed as 819.9 cm−1 in solid argon. However, the ground state UN bond length is computed to be 1.759 Å, and the UN frequency is observed as 995.6 cm−1 in solid argon,18 which are in accord with triple bond status for the UN diatomic molecule. Finally, if we take the An−H bond length from the obtuse structure in H2AnEH as a measure of a single bond length to compare with the agostic bond, we find that the agostic bond is slightly longer than this single bond. For HPThH2, the ratio of agostic (Th−H(P) single covalent Th−H (from the Cs structure) bond length is 2.31/2.061.12, so the agostic bond is 12% longer than a single Th−H bond. For HAsThH2, the corresponding agostic interaction is 11% longer than the corresponding Th−H bond. For HPUH2, the agostic interaction is 9% longer than the corresponding U−H bond and for HAsUH2, the agostic interaction is also a 9% increase. These agostic H−An bonds are only 9 to 12% longer than a single An−H bond, and we can conclude that the agostic hydrogen bond has bridged bond character. In support of this result, dibridged group 13 M2H6 molecules (M = Al, Ga, In) have bridged to terminal M−H bond length ratios of 1.10, 1.13, and 1.13, respectively.51−54 In fact, the agostic bridged HP ThH2 and HPUH2 molecules can be viewed as intermediates to the much higher energy PThH3 and PUH3 molecules, which were not observed here.
Figure 7. Kohn−Sham α-orbitals for H2PUH and H2AsUH for the key U−E interactions.
We compute the UN bond in HNUH2 to be 1.906 Å in length, slightly shorter than the double bond covalent radii sum G
DOI: 10.1021/acs.inorgchem.6b03055 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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Uranium(IV)-Arsenic Complexes Containing One-, Two-, and Threefold U-As Bonindg Interactions. Nat. Chem. 2015, 7, 582−590. (5) Wildman, E. P.; Balázs, G.; Wooles, A. J.; Scheer, M.; Liddle, S. T. Thorium−Phosphorus Triamidoamine Complexes Containing Th−P Single- and Multiple-Bond Interactions. Nat. Commun. 2016, 7, 12884. (6) Ephritikhine, M. Molecular Actinide Compounds with Soft Chalcogen Ligands. Coord. Chem. Rev. 2016, 319, 35−62. (7) Wang, X.-F.; Andrews, L.; Thanthiriwatte, K. S.; Dixon, D. A. Infrared Spectra of H2ThS and H2US in Noble Gas Matrixes: Enhanced H-An-S Covalent Bonding. Inorg. Chem. 2013, 52, 10275− 10285. (8) Andrews, L.; Cho, H.-G. Infrared Spectrum and Structure of CH2=ThH2. J. Phys. Chem. A 2005, 109, 6796−6798. (9) Cho, H.-G.; Andrews, L. Methane Activation by Laser-Ablated Th Atoms: Matrix Infrared Spectra and Theoretical Investigations of CH3ThH and CH2=ThH2. J. Phys. Chem. A 2015, 119, 2289− 2297. (10) Wang, X. F.; Andrews, L.; Marsden, C. J. Infrared Spectrum and Structure of Thorimine (HNThH2). Chem. - Eur. J. 2007, 13, 5601− 5606. (11) Lyon, J. T.; Andrews, L.; Malmqvist, P.-A.; Roos, B. O.; Yang, T.; Bursten, B. E. Infrared Spectrum and Bonding in Uranium Methylidene Dihydride, CH2=UH2. Inorg. Chem. 2007, 46, 4917− 4925. (12) Wang, X. F.; Andrews, L.; Marsden, C. J. Reactions of Uranium Atoms with Ammonia: Infrared Spectra and Quasi-Relativistic Calculations of the U:NH3, H2NUH, and HNUH2 Complexes. Chem. - Eur. J. 2008, 14, 9192−9201. (13) Andrews, L.; Wang, X.; Lindh, R.; Roos, B. O.; Marsden, C. J. Simple NUF3 and PUF3 Molecules with Triple Bonds to Uranium. Angew. Chem., Int. Ed. 2008, 47, 5366−5370. (14) Andrews, L.; Wang, X.-F.; Roos, B. AsUF3 Molecule with a Weak Triple Bond to Uranium. Inorg. Chem. 2009, 48, 6594−6598. (15) Behrle, A. C.; Castro, L.; Maron, L.; Walensky, J. R. Formation of a Bridging Phosphinidene Thorium Complex. J. Am. Chem. Soc. 2015, 137, 14846−14849. (16) Andrews, L.; Cho, H.-G. Matrix Infrared Spectra and Quantum Chemical Calculations of Ti, Zr, and Hf Dihydride Phosphinidene and Arsinidene Molecules. Inorg. Chem. 2016, 55, 8786−8793. (17) Andrews, L.; Cho, H.-G. Matrix Preparation and Spectroscopic and Theoretical Investigations of Simple Methylidene and Methylidyne Complexes of Group 4−6 Transition Metals. Organometallics 2006, 25, 4040−4053. (18) Andrews, L.; Wang, X.- F.; Gong, Y.; Kushto, G. P.; Vlaisavljevich, B.; Gagliardi, L. Infrared Spectra and Electronic Structure Calculations for NN Complexes with U, UN, and NUN in Solid Argon, Neon, and Nitrogen. J. Phys. Chem. A 2014, 118, 5289− 5303. (19) Purvis, G. D., III; Bartlett, R. J. A Full Coupled Cluster Singles and Doubles Model: The Inclusion of Disconnected Triples. J. Chem. Phys. 1982, 76, 1910−1918. (20) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. A Fifth-Order Perturbation Comparison of Electron Correlation Theories. Chem. Phys. Lett. 1989, 157, 479−483. (21) Watts, J. D.; Gauss, J.; Bartlett, R. J. Coupled-Cluster Methods with Noniterative Triple Excitations for Restricted Open Shell Hartree-Fock and Other General Single Determinant Reference Functions. Energies and Analytical Gradients. J. Chem. Phys. 1993, 98, 8718−8733. (22) Bartlett, R. J.; Musial, M. Coupled Cluster Theory in Quantum Chemistry. Rev. Mod. Phys. 2007, 79, 291−352. (23) Becke, A. D. Density-functional thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (24) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (25) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron Gas Correlation Energy. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244−13249.
CONCLUSIONS Laser ablated Th and U atoms react with phosphine and arsine on condensation in excess argon and neon to form the simplest phosphinidene and arsinidene molecules HEAnH2 with acute agostic angles in the 60−64° range for these strong agostic bonding interactions. The NBO analysis shows that these double bonds consist of a σ bond and a π bond heavily polarized to the N. The E−Th bonding is dominated by d orbitals with
