Article pubs.acs.org/IC
Synthesis, Characterization, and Density Functional Theory Analysis of Uranium and Thorium Complexes Containing Nitrogen-Rich 5‑Methyltetrazolate Ligands Kevin P. Browne, Katie A. Maerzke, Nicholas E. Travia, David E. Morris, Brian L. Scott, Neil J. Henson,* Ping Yang,* Jaqueline L. Kiplinger,* and Jacqueline M. Veauthier* Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
ABSTRACT: Two nitrogen-rich, isostructural complexes of uranium and thorium, (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8), containing 5methyltetrazolate, have been synthesized and structurally characterized by single-crystal X-ray diffraction, electrochemical methods, UV−visible−near-IR spectroscopy, and variable-temperature 1H NMR spectroscopy. Density functional theory (DFT) calculations yield favorable free energies of formation (approximately −375 kJ/mol) and optimized structures in good agreement with the experimental crystal structures. Additionally, calculated NMR chemical shifts of 7 and 8 are in good agreement with the variable-temperature 1H NMR experiments. Time-dependent DFT calculations of both complexes yield UV−visible spectroscopic features that are consistent with experiment and provide assignments of the corresponding electronic transitions. The electronic transitions in the UV−visible spectroscopic region are attributed to C5Me5 ligand-to-metal charge transfer. The low-lying molecular orbitals of the tetrazolate ligands (∼2 eV below the HOMO) do not contribute appreciably to experimentally observed electronic transitions. The combined experimental and theoretical analysis of these new nitrogen-rich uranium and thorium complexes indicates the tetrazolate ligand behaves primarily as a σ-donor.
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INTRODUCTION Metal coordination compounds containing nitrogen-rich ligands represent a class of complexes that have long been of interest to inorganic and organometallic chemists. Transition metal complexes feature prominently in this group, with unique chemical and physical properties, including explosive compounds,1−3 nonlinear optical materials,4,5 coordination polymers,6−9 metal−organic frameworks,10,11 luminescent materials,12−15 and precursors to nanoporous foams.16,17 Nitrogenrich complexes of the lanthanides18,19 have also been investigated as emission sensitizers20−22 and as potential highrelaxivity agents for use in magnetic resonance imaging.23,24 In stark contrast, the chemistry of nitrogen-rich complexes of the actinides is comparatively unexplored, despite the wealth of scientific advances and enabling technologies that may result from focused research in this area. For example, uranium complexes of nitrogen-rich ligands have been proposed as potential precursors to uranium nitride for nuclear fuel applications;25−29 however, representative complexes have primarily featured azide ligands (e.g., Figure 1, complexes 1 and 2).25,30−39 Two recent examples of nonazido actinide complexes include the azobis(tetrazolate) uranyl complex by the Steinhauser research group (Figure 1, complex 3)27 and the pyrimidyl tetrazolate uranyl complex by Colacio and coworkers (Figure 1, complex 4).40 © XXXX American Chemical Society
We now introduce new members to this class of nitrogenrich actinide complexes and provide a robust theoretical explanation of the metal−ligand bonding and electronic structure in U(IV) and Th(IV) tetrazolate complexes. The goal of this study is twofold: (1) to develop a reliable synthetic protocol for preparing stable, nitrogen-rich complexes of the actinides and (2) to develop an electronic structure-based understanding of the chemical bonding in these complexes to predict favorable chemical reactions for unexplored nitrogenrich ligand motifs and actinide oxidation states. To this end, we report here the synthesis and characterization of the first uranium and thorium complexes of 5-methyltetrazolate, (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7), and (C5Me5)2Th[η2(N,N′)-tetrazolate]2 (8) (C5Me5 = pentamethylcyclopentadienyl).
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RESULTS AND DISCUSSION Synthesis. The addition of two equivalents of 5-methyl-1Htetrazole to a solution of (C5Me5)2U(CH3)2 (5) in either toluene or tetrahydrofuran (THF) resulted in immediate gas evolution and formation of a red-orange solution. Removal of the volatiles, followed by trituration of the solid with hexane, Received: February 26, 2016
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Figure 2. X-ray crystal structure of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) (50% probability ellipsoids). Selected bond distances (Å) and angles (deg): U(1)−N(1) 2.456(2); U(1)−N(2) 2.457(2); U(1)− N(5) 2.427(2); U(1)−N(6) 2.492(3); Ccent−U(1) 2.447(3); Ccent′− U(1) 2.444(3); N(1)−U(1)−N(2) 31.86(7); N(5)−U(1)−N(6) 31.87(8); Ccent−U(1)−Ccent′ 140.2(1).
Figure 1. Select examples of actinide complexes that feature nitrogenrich ligands.
afforded (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) as a redorange analytically pure powder in 91% isolated yield (Scheme 1).41 The thorium derivative was prepared in a similar fashion, and the reaction proceeded cleanly in toluene to yield (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) as an off-white powder in 79% isolated yield (Scheme 1).41 Unlike 7, complex 8 is unstable in THF. Scheme 1. Synthesis of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8)
Figure 3. X-ray crystal structure of (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) (50% probability ellipsoids). Selected bond distances (Å) and angles (deg): Th(1)−N(1) 2.5244(19); Th(1)−N(2) 2.5008(18); Th(1)−N(5) 2.4912(18); Th(1)−N(6) 2.5402(18); Ccent−Th(1) 2.511(2); Ccent′−Th(1) 2.514(2); N(1)−Th(1)−N(2) 31.33(6); N(5)−Th(1)−N(6) 31.13(6); C cent −Th(1)−C cent′ 136.65(7).
while the methyl group of the other tetrazolate points away from the center (Figure 4, orientation B). The average Ccent−An distances of both 7 and 8 (2.445(3) and 2.513(2) Å, respectively) compare well to the value of 2.482 Å reported for (C5Me5)2U[η2-(N,N′)-pyrazolate]2 (9),42 and we note that the larger distance of 8 with respect to 7 is consistent with the larger ionic radius of Th(IV) compared to U(IV).43 The Ccent−An−Ccent′ angles of 7 and 8 (140.2(1)° and 136.65(7)°, respectively) are close to the value of 137.2° for complex 9 (see below). For analyzing bond lengths of the tetrazolate ligands themselves, there are no known organometallic uranium or thorium tetrazolate complexes, and so we focus here on comparing the metrical parameters of 7 and 8 to related η2(N,N′)-tetrazolate metallocene complexes of transition metals and lanthanides. While there is only a single dinuclear transition
X-ray Crystallography. The single-crystal X-ray structures of 7 and 8 are shown in Figures 2 and 3, respectively, and confirm that the complexes are mononuclear and of approximately tetrahedral geometry, with two C5Me5 ligands η5-bound to the metal centers and two 5-methyltetrazolate ligands coordinated in η2 fashion. For the uranium complex 7, the methyl groups of the tetrazolate ligands point inward toward each other along the plane in the metallocene wedge (Figure 4, orientation A). For the thorium complex 8, the methyl group of one of the tetrazolate ligands points inward, B
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Figure 4. Representations of different orientations of the tetrazolate methyl groups in complexes 7 and 8, in which An is η2-bound to N atoms adjacent to the methyl group. The C5Me5 ligands have been removed for clarity.
metal complex in this category ([(C5H5)2Y(μ−η1:η2-tetrazolate)2]2 (10)),44 Huang and co-workers prepared four dinuclear lanthanide complexes containing tetrazolate: [{(C5H4Me)(C5H5)Ln(μ-η1:η2-tetrazolate)}2][{(C5H4Me)2Ln(μ-η1:η2-tetrazolate)}2] (Ln = Gd (11), Dy (12), Ho (13)) and [(C5H4Me)2Yb(μ-η1:η2-tetrazolate)]2 (14).45 Evans and coworkers synthesized the monometallic lanthanide complex (C5Me5)SmI(C6H10N4) (15),46 which also contains the η2(N,N′)-tetrazole motif, although in this case tetrazole is a neutral ligand (see Figure 5 for 9−15).
Figure 6. Overlay of the calculated X-ray crystal structures (gray) and experimental structures of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (blue, 7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (red, 8). Note that the methyl groups of the tetrazolate ligands adopt orientation A in 7 and orientation B in 8. The hydrogen atoms have been omitted for clarity and tet = η2-(N,N′)-tetrazolate.
lengths are reproduced to within 3% of experiment and bond angles to within 7%, which gives confidence in the methods used. A detailed comparison of calculated and experimental selected structural parameters for the (C5Me5)2An[η2-(N,N′)tetrazolate]2 (An = U (7), Th (8)) complexes is provided in the Supporting Information. The free energies were calculated for the formation of 7 and 8 (Scheme 1) and were found to be highly favorable for each reaction (ΔG = −374.5 and −382.6 kJ/mol, respectively). Additionally, we used DFT to compare the ground-state free energies of the three possible (A−C) orientations (represented in Figure 4) of the two η2-(N,N′)-bound 5-methyltetrazolate ligands in 7 and 8 to understand why the X-ray structure of 7 indicates a preference for orientation A, while 8 prefers orientation B. Our calculations show that for 7 orientation A has the lowest free energy, with B and C being 7.1 and 22.0 kJ/ mol higher in free energy, respectively. For 8, B is favored by 6.5 kJ/mol more than A and 19.0 kJ/mol more than C. We note that orientation C, which has the highest calculated free energy, is not found in either crystal structure. One possible consequence of the predicted small free energy differences between structural orientations A and B for 7 and 8 is that, at room temperature, the tetrazolate rings could undergo rapid interconversion between A and B, especially when the energy barrier is sufficiently low. To test this hypothesis experimentally, we performed variable-temperature (VT) 1H NMR studies to determine whether we could observe these different orientations over a range of temperatures. VT 1H NMR Spectroscopy. In order to eliminate any possible confounding influence of chemical shifts for paramagnetic complexes that can vary with temperature (even in the absence of dynamic processes), we chose to focus on (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8), as it does not possess unpaired 5f electrons. Figure 7 is a plot of the chemical shift of the methyl protons of the tetrazolate ligands in 8 as a function of temperature in toluene-d8 solvent. We note that across the entire temperature range there is only a single peak for both methyl groups, and this peak position is plotted along the y-axis. This implies that if there is conversion between orientations, it takes place on a timescale faster than the characteristic 1H NMR timescale of ∼10−1−10−4 s.48 That is, in our study, it would be operating within the fastexchange regime. Second, the sigmoidal shape of the plot illustrates that at extremes in temperature (either low or high)
Figure 5. Examples of a transition metal complex (10) and lanthanide complexes (11−15) that contain tetrazolate ligands. While complexes 7 and 8 prepared in this work are the first examples of organometallic actinide tetrazolates, complex 9 illustrates the same η2-(N,N′)heterocyclic actinide-ligand bonding motif seen in 7 and 8.
For complex 10, the N(1)−N(2) distance is 1.333(6) Å, while the average corresponding distance for lanthanide complexes 11−14 is 1.33(4) Å, and for the actinides 7 and 8 in this work, the distance is 1.352(3) Å. The bond distances of the tetrazolate ligands are all close among the transition metal, lanthanide, and actinide complexes. Specifically, the average N(1)−C(5) distances are 1.334(6), 1.34(4), and 1.336(3) Å; the average N(2)−N(3) distances are 1.309(6), 1.31(6), and 1.310(3) Å; the average N(4)−C(5) distances are 1.328(6), 1.34(2), and 1.337(4) Å; and the average N(3)−N(4) distances are 1.330(6), 1.34(4), and 1.347(4) Å for complexes 10, 11− 14, and 7 and 8, respectively. DFT Structural Calculations. Computational studies were performed to understand the tetrazolate−metal binding and relative stability of ligand orientations A, B, and C illustrated in Figure 4 (C is not observed experimentally).47 Geometry optimizations were accomplished using density functional theory (DFT) for the singlet and triplet states of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and for the singlet state of (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8). For complex 7, corresponding to a U(IV) f2 configuration, the triplet spin state was calculated to be 118 kJ/mol more stable than the singlet state; thus, all future discussion of complex 7 refers to the triplet state. As shown by the overlay of experimental and calculated structures in Figure 6, the calculated structure is in good agreement with experiment. Notably, metal−ligand bond C
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(C5Me5)2U(CH2Ph)2, 7.62 ppm;52 (C5Me5)2U(SO3CF3)(CH3), 7.2 ppm53). At the other extreme, U(IV) complexes containing relatively strong π-donor ligand(s) have more electron density at the metal center (which is consequently donated back to the C5Me5 rings), resulting in chemical shifts further upfield (for example, (C5Me5)2U[η2-(N,N′)-PhCH2-NNCPh2]2, 2.5 ppm;54 (C5Me5)2U(N-2,4,6-t-Bu3C6H2), −3.9 ppm;55 (C5Me5)2U(−NCPh2)2, −4.7 ppm56). Cyclic Voltammetry. Cyclic and square-wave voltammetric data, shown in Figure 8, were collected for both (C5Me5)2U[η2-
Figure 7. Plot of chemical shift (ppm) of the tetrazolate methyl group protons of (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) as a function of temperature (K) in toluene-d8 solvent. Black squares are experimental data points, and the solid black line is from a fit of the experimental data to eq 2.
the chemical shifts reach separate plateaus and tend to converge asymptotically upon constant values. Taken together, these two observations are evidence that there is a dynamic process observable over the experimental temperature range that shifts the equilibrium population between the two most favored conformational states of complex 8 (A and B). For such a process,49 the observed chemical shift, δ, can be related to the free energy constants of the equilibrium according to eq 2,
( TΔSRT− ΔH )δB + δA T ΔS − ΔH 1 + exp( RT )
Figure 8. Cyclic voltammetric data for (C5Me5)2U[η2-(N,N′)tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) in CF3C6H5 solvent, referenced to ferrocenium/ferrocene. Black arrow indicates the rest potential for both complexes.
(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) in trifluorotoluene solvent (TFT), with ∼0.1 M [NBu4][B(3,5-(CF3)2-C6H3)4)] as the supporting electrolyte and referenced to ferrocenium/ferrocene. Additionally, the same electrochemical experiments were performed in THF solvent for 7. The cyclic voltammogram of complex 7 shows a reversible redox couple at −1.88 V in TFT (−1.76 V in THF). This redox couple is attributed to the U(IV)/U(III) reduction, and its value is comparable to those reported for other uranium metallocene complexes possessing primarily σ-donor ligands, such as (C5Me5)2UCl2 (−1.85 V in THF), (C5Me5)2UMe2 (−2.41 V in THF), (C5Me5)2U(CH2Ph)2 (−1.95 V in THF), and (C5Me5)2U(SO3CF3)(CH3) (−1.83 V in THF).57 DFT calculations for the U(IV)/U(III) redox couple in THF continuum solvent yielded a redox potential of −2.16 V relative to the ferrocenium/ferrocene couple with a spin density of 3.04 on the uranium center, in good agreement with the experimental data. We note that the calculated Mulliken charges change by −0.46e for the C5Me5 ligands and only −0.13e for the tetrazolate ligands. Similar to other uranium metallocene complexes possessing primarily σ-donors, (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) does not show a reversible U(IV)/U(V) oxidation process. This behavior is attributed to a lack of sufficient electron density around the metal center required to stabilize the U(V) oxidation state. Instead, there is only an irreversible peak in the anodic region, which is thought to be due to oxidation processes localized on the C5Me5 ligands, as this behavior is encountered with various C5Me5 derivatives both of actinides and with lanthanides and transition metals.57 DFT calculations on the putative species {[C5Me5)2U[η2-(N,N′)-tetrazolate]2}+ confirm that the C5Me5
exp δ=
(2)
where δA and δB correspond to the limiting chemical shifts at the low and high temperature extremes, at which the equilibrium is completely shifted to either “pure” orientation A or “pure” orientation B, respectively; ΔS and ΔH are the change in entropy and enthalpy, respectively, of the conversion between orientations A and B; R is the ideal gas constant; and T is temperature. Fitting the experimental data to this equation using nonlinear least-squares regression yields ΔH = 15.7 kJ/ mol, ΔS = 61.7 J/mol·K, δA = 2.45 ppm, and δB = 2.29 ppm. On the basis of the error associated with the solution, the 95% confidence interval for ΔG is found to be between −6.98 and 1.63 kJ/mol at 298 K. This value is consistent with ΔG = −6.5 kJ/mol calculated by DFT for the energy difference between orientations A and B for complex 8. In addition, the 0.16 ppm difference in isolated chemical shifts determined for A and B from the fitted data is in good agreement with the calculated difference in NMR shifts for configurations A and B (0.12 ppm). The 1H NMR spectrum of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) provides an important diagnostic for determining the amount of electron density at the metal center, as the chemical shift of the methyl protons attached to the cyclopentadienyl rings is an indicator of relative electron density on the metal center provided by additional ligands.50,51 The chemical shift of 10.3 ppm places complex 7 among U(IV) complexes that have weak σ-donor ligands and donate little electron density to the metal center (for example, (C5Me5)2UCl2, 13.5 ppm;52 D
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Figure S3), we do not consider π back-bonding to contribute significantly to the overall bonding picture. This observation is consistent with the experimental 1H NMR spectra, cyclic voltammetry, and UV−visible spectroscopic data discussed above. Below the frontier orbitals, and mixed in with the tetrazolate orbitals, are the [C5Me5]− ligand orbitals. The lowest energy virtual orbitals for the uranium complex (7) are primarily 5f orbitals on uranium, beginning at 3.06 eV above the HOMO. At higher energies, starting 5.33 eV above the HOMO, we observe orbitals with significant uranium−ligand overlap (both [C5Me5]− and tetrazolate). The virtual orbitals for (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) do not have a clear separation between purely thorium orbitals and orbitals with thorium− ligand overlap. Instead, they are interdispersed, beginning at an energy 4.52 eV above the HOMO. Experimental and Theoretical Electronic Spectroscopy. Figure 11 presents the UV−visible−near-IR spectroscopic data for complexes 7 and 8. In the context of prior experimental work, the UV−visible region for the electronic spectra of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) is consistent with a complex that has ligands of a mostly σ-donor nature, based upon the relatively low extinction coefficients of ∼2000−3000 M−1 cm−1.57 Features in this region can generally be ascribed to electronic transitions between metal-based 5f orbitals and higher energy 6d metal-based orbitals, as well as ligand-to-metal charge transfer (LMCT) bands from the [C5Me5]− and tetrazolate ligands, although there are potentially many electronic states in this spectroscopic window. Time-dependent density functional theory (TD-DFT) calculations revealed that the broad shoulder at ∼280−330 nm (4.4 eV; 35 700 cm−1; Figure 11) seen in the UV−vis−NIR spectra for both complexes 7 and 8 is due to [C5Me5]− and tetrazolate ligand-based transitions. As shown in Figure 12, the NIR region for 7 features weak peaks that are Laporte-forbidden transitions (ε ≈ 10−80 M−1 cm−1), which is consistent with the presence of a uranium(IV) center.57 TD-DFT calculations showed that the lower energy transitions (greater than 290 nm; less than 4.3 eV; less than 35 000 cm−1) for (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) are primarily ligand-to-metal charge transfer (Figure 10). The transitions are from the [C5Me5]− frontier orbitals, which have up to 5% f character from thorium, and go to primarily thorium metal orbitals, with minor contributions from transitions to Th−tetrazolate orbitals. Between 230 and 285 nm (4.4−5.3 eV; 35 000−43 000 cm−1), transitions from the [C5Me5]− frontier orbitals to primarily Th-tetrazolate bonding orbitals are observed. The spectrum is more complicated for f2 uranium(IV) complex 7 (Figure 11). In particular, there are many more transitions at lower energies (greater than 320 nm; less than 3.8 eV; less than 31 000 cm−1). These low-energy transitions are from the uranium/[C5Me5]− frontier orbitals to the 5f orbitals on uranium. As previously discussed, the frontier orbitals in (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) include a significant contribution, up to 74%, from the unpaired 5f electrons. Below 300 nm (above 4.1 eV; above 33 000 cm−1) contributions from transitions originating in the tetrazolate orbitals are observed. The transitions from tetrazolate orbitals are to an admixture of uranium 5f orbitals and orbitals with contributions from uranium/tetrazolate/[C5Me5]−, where the metal contribution to the uranium−ligand orbital comes from a mix of 5f and 6d electrons.
ligands are oxidized, with a change in calculated charge of +0.48e on each C5Me5 ligand. Additionally, uranium has a spin density of 1.67, rather than the value of 1.00 that would indicate U(V). Figure 8 also shows the cyclic voltammetric data for (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) in TFT and, as expected for a Th(IV) species, does not display any reversible metal-based redox events.57 A small irreversible peak in the cathodic region may be due to irreversible reduction of the tetrazolate ligand coordinated to thorium, which is also observed in the voltammetric data collected for the uranium complex 7. As a final point, we note that among related uranium metallocene complexes, other researchers have found a linear correlation between the reduction potential and the 1H NMR resonance of the methyl group protons of the C5Me5 ligands.50 The values of −1.76 V and 10.3 ppm for complex 7 are in agreement with this previously observed linear correlation. Molecular Orbitals. The molecular orbitals of 7 and 8 were calculated by DFT methods, and the results are given in Figures 9 and 10. Analysis of the molecular orbitals for (C5Me5)2U[η2-
Figure 9. Molecular orbitals illustrating σ tetrazolate−An interactions in (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2(N,N′)-tetrazolate]2 (8), both in and out of the plane of the tetrazolate rings. The orbital energies are relative to the HOMO.
(N,N′)-tetrazolate]2 (7) indicates that the frontier orbitals are composed of metal 5f orbitals and [C5Me5]− ligand orbitals, whereas the frontier orbitals for (C5Me5)2Th[η2-(N,N′)tetrazolate]2 (8) are primarily [C5Me5]− ligand orbitals with minor metal 6d and 5f contributions (5−12%). For complex 7, the calculated Mulliken spin density of 2.19 indicates the presence of two 5f unpaired electrons on the uranium metal center, which are distributed with one electron lying principally within the HOMO−4 orbital with 74% 5f character and the other spread between HOMO−5 with 56% 5f character and HOMO−1 with 35% 5f character.58 (HOMO = highest occupied molecular orbital.) As shown in Figure 9, the tetrazolate ligand orbitals occur at significantly lower energies (1.77−2.98 eV) below the HOMO for both complexes 7 and 8, respectively. We do not see a large amount of An−N overlap in any of these orbitals; however, some tetrazolate orbitals do contain a minor contribution (4−6%) from the An 5f or 6d orbitals. Given the small degree of orbital mixing between U 5f and tetrazolate π* orbitals (HOMO−1 to HOMO−5; see SI E
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Figure 10. Molecular orbital diagram for (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8). Note that for simplicity we are only showing the α manifold for complex 7. All energies are relative to the HOMO, which is set to zero. The percentage contribution from the An 5f and 6d orbitals is included, as well as representative images for the different types of orbitals.
Figure 11. UV−visible spectra for (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) (left panels) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8) (right panels). In both the left and the right panels, the top spectra in each case are experimental and the bottom spectra have been calculated by TD-DFT. For the experimental spectra, the dotted lines correspond to spectral fitting for the solid experimental curve. For the theory spectra (bottom panels), the black lines correspond to the electronic transitions calculated by TD-DFT, and the height of the lines is proportional to the oscillator strengths of the corresponding transitions. The solid blue and red lines correspond to the electronic transition spectra for 7 and 8, respectively, which have been broadened with Gaussian peaks.
F
DOI: 10.1021/acs.inorgchem.6b00492 Inorg. Chem. XXXX, XXX, XXX−XXX
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materials with unique properties including new high-purity pathways to nuclear fuel materials such as uranium-oxides, -carbides, and -nitrides with properties distinctive of those prepared through conventional routes.
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EXPERIMENTAL SECTION
General Considerations. Reactions and manipulations were performed at 20 °C in a recirculating Vacuum Atmospheres NEXUS model inert atmosphere (N2) drybox equipped with a 40CFM dual purifier NI-Train. Glassware was dried overnight at 150 °C before use. All NMR spectra were obtained using a Bruker Avance 400 MHz spectrometer. Chemical shifts for 1H and 13C NMR spectra were referenced to solvent impurities. Materials. Unless otherwise noted, reagents were purchased from commercial suppliers and used without further purification. Celite (Aldrich), alumina (activated, neutral, Brockmann I, Aldrich), and 3 Å molecular sieves (Aldrich) were dried under dynamic vacuum at 250 °C for 48 h prior to use. All solvents (Aldrich) were purchased anhydrous and were dried over KH for 24 h, passed through a column of activated alumina, and stored over activated 3 Å molecular sieves prior to use. Benzene-d6 (Aldrich), toluene-d8 (Aldrich), and tetrahydrofuran-d8 (Cambridge Isotope Laboratories) were purified by storage over activated 3 Å molecular sieves prior to use. (C5Me5)2UMe2 (5) and (C5Me5)2ThMe2 (6) were prepared using established methods.59 [Bu4N][B(3,5-(CF3)2-C6H3)4] was prepared using an established method.50 Caution! Depleted uranium (primary isotope 238U) and natural thorium (primary isotope 232Th) are both weak α-emitters (4.197 and 4.012 MeV, respectively) with half-lives of 4.47 × 109 and 1.41 × 1010 years, respectively; manipulations and reactions should be carried out in monitored fume hoods or an inert-atmosphere drybox in a radiation laboratory equipped with α- and β-counting equipment. While none of the compounds reported here were not found to be mechanically unstable, tetrazolate ligands are nitrogen-rich and have the potential to form unstable compounds. Synthesis of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7). A 20 mL scintillation vial equipped with a magnetic stir bar was charged with 5methyl-1H-tetrazole (0.031 g, 0.37 mmol) and toluene (1.5 mL). To this white suspension was added a toluene solution (1.5 mL) of (C5Me5)2UMe2 (5) (0.100 g, 0.186 mmol) with stirring, resulting in a rapid evolution of gas. Next, the reaction mixture was stirred at room temperature for 2 h, during which time the solution became dark redorange in color. The volatiles were removed under reduced pressure, and the residual solid was triturated with hexane (4 mL). The product was dried under reduced pressure to afford analytically pure (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) as a dark red-orange solid (0.114 g, 0.169 mmol, 91%). 1H NMR (benzene-d6, 296 K): δ 10.31 (s, 30H, C5Me5), −18.72 (s, 6H, tetrazole-CH3). UV−vis−NIR (toluene) λ (nm) (ε, M−1 cm−1): 335 (4580), 344 (4290), 388 (2620), 447 (1970), 540 (1080), 683 (38), 701 (38), 829 (34), 916 (33), 977 (39), 1110 (77), 1344 (48), 1406 (57), 1478 (52), 1633 (62). IR (solid, cm−1): ν 2910 (s), 1479 (s), 1434 (m), 1380 (m), 1351 (m), 1213 (m), 1131 (w), 1098 (m), 1016 (m), 688 (m). U(IV)/U(III) E1/2 = −1.88 V (vs [(C5H5)2Fe]0/+ in CF3−C6H5 solvent). Mp: 268−269 °C. Anal. Calcd for C24H36N8U (674.64 g/ mol): C, 42.73; H, 5.38; N, 16.61. Found: C, 42.53; H, 5.74; N, 16.11. Synthesis of (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8). A 20 mL scintillation vial equipped with a magnetic stir bar was charged with 5methyl-1H-tetrazole (0.047 g, 0.56 mmol) and toluene (1.5 mL). To this white suspension was added a toluene solution (1.5 mL) of (C5Me5)2ThMe2 (6) (0.150 g, 0.282 mmol) with stirring, resulting in a rapid evolution of gas. Next, the reaction mixture was stirred at room temperature for 2 h, during which time the solution became turbid offwhite in color. The volatiles were removed under reduced pressure, and the residual gelatinous material was triturated with hexane (4 mL). The product was dried under reduced pressure and isolated from the reaction vessel to afford analytically pure (C5Me5)2Th[η2-(N,N′)tetrazolate]2 (8) as an off-white solid (0.150 g, 0.224 mmol, 79%). 1H NMR (benzene-d6, 297 K): δ 2.41 (s, 6H, tetrazole-CH3), 1.67 (s,
Figure 12. Near-IR absorption region for (C5Me5)2U[η2-(N,N′)tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)-tetrazolate]2 (8).
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CONCLUSIONS We have developed a robust framework for understanding the structure and bonding in uranium and thorium complexes of nitrogen-rich tetrazolate ligands. Experimentally, we have synthesized and characterized two isostructural organometallic complexes, (C5Me5)2An[η2-(N,N′)-tetrazolate]2 (An = U (7), Th (8)). Single-crystal X-ray diffraction confirms the identity of both complexes, and VT-NMR experiments, in conjunction with DFT calculations, prove the room-temperature rotation of the tetrazolate ligand on the metal center. Three lines of evidence independently suggest that, in these complexes, tetrazolate acts primarily as a σ-donor ligand. The first comes from electrochemical characterization of the uranium derivative (7): cyclic voltammetry experiments show a quasi-reversible redox couple at −1.88 V, attributable to U(IV)/U(III) reduction, and the absence of U(IV)/U(V) oxidation, which places it in the same family as other complexes whose X ligand is a σ-donor. Second, a relatively low molar absorptivity for complex 7 in both the UV−visible region of the electronic spectrum and the near-IR region positions this complex among other compounds in the (C5Me5)2UX2 family that feature σdonor ligands. The third piece of evidence comes from the downfield location of the resonance in the 1H NMR spectrum at 10.3 ppm for the methyl groups of the U analogue, which is indicative of X ligands that are σ-donors. Electronic structure calculations on the uranium and thorium complexes are consistent with experimentally determined bond lengths and UV−visible spectroscopy measurements and yield favorable free energies of formation for their synthetic routes. Most importantly, these calculations determine the molecular orbital picture that confirms the tetrazolate−metal interaction to be primarily a σ-bond, with no π-overlap. Furthermore, TDDFT calculations predict electronic transitions that agree well with those found experimentally in the UV−visible region, with the main contributions in this spectroscopic region assigned to C5Me5-ligand-to-metal charge-transfer transitions. The reliable synthetic methodology and electronic structure based understanding of chemical bonding developed in this work provides a firm basis to predict and further explore the chemistry of actinide complexes with nitrogen-rich ligands. As an emerging field of science, nitrogen-rich actinide chemistry has the potential to yield new methodologies and new actinide G
DOI: 10.1021/acs.inorgchem.6b00492 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
hemisphere of data was collected using ω scans, with 10 s frame exposures and 0.5° frame widths. Data collection and initial cell refinement were handled using APEX 2 software.71 Frame integration, including Lorentz−polarization corrections, and final cell parameter calculations were carried out using SAINT software.72 The data were corrected for absorption using redundant reflections and the SADABS program.73 Decay of reflection intensity was monitored through analysis of redundant frames. The structure was solved using direct methods and difference Fourier techniques. Unless otherwise noted, non-hydrogen atoms were refined anisotropically and hydrogen atoms were treated as idealized contributions. Structure solution, refinement, and creation of publication materials were performed using SHELXTL.74
30H, C5Me5). 13C NMR (benzene-d6, 304 K): 156.8 (CN4-Me), 127.9, (C5Me5), 10.6 (C5Me5), 10.4 (CN4-Me). UV−vis (toluene) λ (nm) (ε, M−1 cm−1): 296 (4620). IR (solid, cm−1): ν 2909 (s), 2861 (s), 1481 (s), 1435 (m), 1378 (m), 1353 (m), 1211 (m), 1165 (w), 1125 (w), 1103 (m), 1014 (m), 684 (m). Mp: 358 °C (dec). Anal. Calcd for C24H36N8Th (668.65 g/mol): C, 43.11; H, 5.43; N, 16.76. Found: C, 42.85; H, 5.34; N, 16.69. Computational Methods. The electronic structures of (C5Me5)2U[η2-(N,N′)-tetrazolate]2 (7) and (C5Me5)2Th[η2-(N,N′)tetrazolate]2 (8) were examined with DFT using the hybrid B3LYP exchange−correlation functional,60 the Stuttgart small-core relativistic effective core potential for the actinides,61,62 and the TZVP basis set for the N, C, and H atoms.63 Following the work of Dixon and coworkers, we replaced the most diffuse functions in the U basis set, those with an exponent of 0.005, with s, p, d, and f functions with exponents of 0.013, 0.059, 0.026, and 0.067, respectively.64 All calculations were performed using Gaussian 09.65 Calculation of the redox potentials was done using a thermodynamic cycle with a continuum solvent of THF following the method outlined by Batista and co-workers for transition metals.66 The solvation free energies (required for the thermodynamic cycle) were determined using the polarizable continuum model as implemented in Gaussian 09.67 The UV−vis spectra were calculated using TD-DFT.68−70 Instrumentation and Sample Protocols. Voltammetric data were obtained in the Vacuum Atmospheres drybox system described above. All data were collected using a PerkinElmer Princeton Applied Research Corporation (PARC) model 263 potentiostat under computer control with PARC model 270 software. All sample solutions were ∼1−2 mM in complex with 0.1 M [Bu4N][B(3,5(CF3)2-C6H3)4] supporting electrolyte50 in THF or trifluorotoluene solvent. All data were collected with the positive-feedback IR compensation feature of the software/potentiostat activated to ensure minimal contribution to the voltammetric waves from uncompensated solution resistance (typically ∼1 kΩ under the conditions employed). For experiments at ambient temperature, solutions were contained in PARC model K0264 microcells consisting of a ∼3 mm diameter Pt disk working electrode, a Pt wire counter electrode, and a Ag wire quasi-reference electrode. Scan rates from 50 to 5000 mV/s were employed in the cyclic voltammetry scans to assess the chemical and electrochemical reversibility of the observed redox transformations. Half-wave potentials were determined from the peak values in the square-wave voltammograms or from the average of the cathodic and anodic peak potentials in the reversible cyclic voltammograms. Potential calibrations were performed at the end of each data collection cycle using the ferrocenium/ferrocene couple as an internal standard. Electronic absorption and cyclic voltammetric data were analyzed using Wavemetrics IGOR Pro (version 6.31) software. Electronic absorption spectral data were obtained for toluene solutions of complexes over the wavelength range 270−2000 nm on a PerkinElmer model Lambda 1050 UV−visible−near-infrared spectrophotometer. All data were collected in 0.1 cm path length cuvettes loaded in the recirculating Vacuum Atmospheres drybox system discussed above. Samples were typically run at two dilutions, 0.5 and 20 mM, to optimize absorbance in the UV−visible and near-infrared, respectively. Spectral resolution was 1 nm in the visible region and in the nearinfrared. Sample spectra were obtained versus air and corrected for solvent absorption subsequent to data acquisition. IR spectra were obtained using a Thermo Scientific Nicolet iS5 FTIR spectrometer, using a Golden Gate Diamond ATR (ZnSe lenses) with a reaction anvil (neat solid samples). Elemental analyses were performed by Atlantic Microlab, Inc., in Norcross, GA. Melting points were measured with a Barnstead Thermolyne MEL-TEMP capillary melting point apparatus using capillary tubes flame-sealed under nitrogen; values are uncorrected. X-ray Crystallography. Data for 7 and 8 were collected on a Bruker D8 Quest diffractometer, with a CMOS detector in shutterless mode. The crystals were cooled to 100 K employing an Oxford Cryostream liquid nitrogen cryostat. The data collection employed graphite-monochromatized Mo Kα (λ = 0.710 73 Å) radiation. A
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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.6b00492. Crystallographic data for 7 and 8 have also been deposited with the Cambridge Crystallographic Data Center under depository numbers 1431790 and 1431791, respectively. Tables with bond distances and angles for experimental and computational structures of 7 and 8; spectral fitting results for the electronic spectral data for 7 and 8; molecular orbital pictures for 7 of MOs with a significant contribution from the unpaired U 5f electrons; molecular orbital energies for 7 and 8 and percentage contributions from An f and d orbitals; pictures of the calculated orbitals (including virtual orbitals) for the α-manifold for 7 (PDF) DFT structure of 7 (PDB) DFT structure of 8 (PDB) Crystallographic parameters for 7 (CIF) Crystallographic parameters for 8 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS For financial support of this work, we acknowledge the U.S. Department of Energy through the LANL LDRD Program (K.P.B., K.A.M., B.L.S., N.E.T., N.J.H., P.Y., J.L.K., J.M.V., materials and supplies), the LANL G.T. Seaborg Institute for Transactinium Science (PD Fellowships to K.P.B., N.E.T.), and the Office of Basic Energy Sciences, Heavy Element Chemistry Program (P.Y., J.L.K., B.L.S., materials and supplies). This research used resources provided by the Los Alamos National Laboratory Institutional Computing Program. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (contract DE-AC5206NA25396).
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REFERENCES
(1) Geisberger, G.; Klapoetke, T. M.; Stierstorfer, J. Eur. J. Inorg. Chem. 2007, 2007, 4743−4750. H
DOI: 10.1021/acs.inorgchem.6b00492 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (2) Huynh, M. H. V.; Hiskey, M. A.; Meyer, T. J.; Wetzler, M. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 5409−5412. (3) Reitzner, B.; Manno, R. P. Nature 1963, 198, 991−991. (4) Liao, J.-Z.; Chen, D.-C.; Li, F.; Chen, Y.; Zhuang, N.-F.; Lin, M.J.; Huang, C.-C. CrystEngComm 2013, 15, 8180−8185. (5) Ye, Q.; Li, Y. H.; Song, Y. M.; Huang, X. F.; Xiong, R. G.; Xue, Z. L. Inorg. Chem. 2005, 44, 3618−3625. (6) Zhang, J. Y.; Cheng, A. L.; Sun, Q. A.; Yue, Q.; Gao, E. Q. Cryst. Growth Des. 2010, 10, 2908−2915. (7) Wang, X. S.; Tang, Y. Z.; Huang, X. F.; Qu, Z. R.; Che, C. M.; Chan, P. W. H.; Xiong, R. G. Inorg. Chem. 2005, 44, 5278−5285. (8) Tong, X. L.; Wang, D. Z.; Hu, T. L.; Song, W. C.; Tao, Y.; Bu, X. H. Cryst. Growth Des. 2009, 9, 2280−2286. (9) Pachfule, P.; Chen, Y. F.; Sahoo, S. C.; Jiang, J. W.; Banerjee, R. Chem. Mater. 2011, 23, 2908−2916. (10) Tseng, T.-W.; Luo, T.-T.; Chen, S.-Y.; Su, C.-C.; Chi, K.-M.; Lu, K.-L. Cryst. Growth Des. 2013, 13, 510−517. (11) Cui, P.; Chen, Z.; Gao, D. L.; Zhao, B.; Shi, W.; Cheng, P. Cryst. Growth Des. 2010, 10, 4370−4378. (12) Zhong, D. C.; Lu, W. G.; Jiang, L.; Feng, X. L.; Lu, T. B. Cryst. Growth Des. 2010, 10, 739−746. (13) Yang, G.-W.; Ma, Y.-S.; Li, Q.-Y.; Zhou, Y.; Gu, G.-Q.; Wu, Y.; Yuan, R.-X. J. Coord. Chem. 2009, 62, 1766−1774. (14) Wang, F.; Yu, R.; Zhang, Q.-S.; Zhao, Z.-G.; Wu, X.-Y.; Xie, Y.M.; Qin, L.; Chen, S.-C.; Lu, C.-Z. J. Solid State Chem. 2009, 182, 2555−2559. (15) Sun, L.; Li, G.-Z.; Xu, M.-H.; Li, X.-J.; Li, J.-R.; Deng, H. Z. Anorg. Allg. Chem. 2012, 638, 1200−1203. (16) Tappan, B. C.; Steiner, S. A., III; Luther, E. P. Angew. Chem., Int. Ed. 2010, 49, 4544−4565. (17) Tappan, B. C.; Huynh, M. H.; Hiskey, M. A.; Chavez, D. E.; Luther, E. P.; Mang, J. T.; Son, S. F. J. Am. Chem. Soc. 2006, 128, 6589−6594. (18) Steinhauser, G.; Giester, G.; Wagner, C.; Leopold, N.; Sterba, J. H.; Lendl, B.; Bichler, M. Helv. Chim. Acta 2009, 92, 1371−1384. (19) Steinhauser, G.; Giester, G.; Leopold, N.; Wagner, C.; Villa, M. Helv. Chim. Acta 2009, 92, 2038−2051. (20) Andreiadis, E. S.; Demadrille, R.; Imbert, D.; Pecaut, J.; Mazzanti, M. Chem. - Eur. J. 2009, 15, 9458−9476. (21) Shavaleev, N. M.; Eliseeva, S. V.; Scopelliti, R.; Bunzli, J. C. G. Inorg. Chem. 2014, 53, 5171−5178. (22) Giraud, M.; Andreiadis, E. S.; Fisyuk, A. S.; Demadrille, R.; Pecaut, J.; Imbert, D.; Mazzanti, M. Inorg. Chem. 2008, 47, 3952− 3954. (23) Facchetti, A.; Abbotto, A.; Beverina, L.; Bradamante, S.; Mariani, P.; Stern, C. L.; Marks, T. J.; Vacca, A.; Pagani, G. A. Chem. Commun. 2004, 1770−1771. (24) Aime, S.; Cravotto, G.; Crich, S. G.; Giovenzana, G. B.; Ferrari, M.; Palmisano, G.; Sisti, M. Tetrahedron Lett. 2002, 43, 783−786. (25) Thomson, R. K.; Cantat, T.; Scott, B. L.; Morris, D. E.; Batista, E. R.; Kiplinger, J. L. Nat. Chem. 2010, 2, 723−729. (26) Fortier, S.; Wu, G.; Hayton, T. W. Dalton Trans. 2010, 39, 352− 354. (27) Steinhauser, G.; Giester, G.; Wagner, C.; Weinberger, P.; Zachhuber, B.; Ramer, G.; Villa, M.; Lendl, B. Inorg. Chem. 2012, 51, 6739−6745. (28) Nocton, G.; Pecaut, J.; Mazzanti, M. Angew. Chem., Int. Ed. 2008, 47, 3040−3042. (29) King, D. M.; Liddle, S. T. Coord. Chem. Rev. 2014, 266, 2−15. (30) Thomson, R. K.; Graves, C. R.; Scott, B. L.; Kiplinger, J. L. Eur. J. Inorg. Chem. 2009, 2009, 1451−1455. (31) Zi, G. F.; Jia, L.; Werkema, E. L.; Walter, M. D.; Gottfriedsen, J. P.; Andersen, R. A. Organometallics 2005, 24, 4251−4264. (32) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835−1838. (33) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 4565−4571. (34) Berthet, J. C.; Lance, M.; Nierlich, M.; Vigner, J.; Ephritikhine, M. J. Organomet. Chem. 1991, 420, C9−C11.
(35) Prasad, L.; Gabe, E. J.; Glavincevski, B.; Brownstein, S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, 39, 181−184. (36) Benaud, O.; Berthet, J.-C.; Thuery, P.; Ephritikhine, M. Chem. Commun. 2011, 47, 9057−9059. (37) Lam, O. P.; Franke, S. M.; Nakai, H.; Heinemann, F. W.; Hieringer, W.; Meyer, K. Inorg. Chem. 2012, 51, 6190−6199. (38) Herve, A.; Garin, N.; Thuery, P.; Ephritikhine, M.; Berthet, J.-C. Chem. Commun. 2013, 49, 6304−6306. (39) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. Organometallics 2013, 32, 3279−3285. (40) Rodriguez-Dieguez, A.; Mota, A. J.; Seco, J. M.; Palacios, M. A.; Romerosa, A.; Colacio, E. Dalton Trans. 2009, 9578−9586. (41) Reaction completion was determined by monitoring the disappearance of the starting material peaks and the appearance of 7 and 8 in deuterated solvent by 1H NMR spectroscopy. No additional products were observed. (42) Eigenbrot, C. W.; Raymond, K. N. Inorg. Chem. 1982, 21, 2653−2660. (43) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (44) Zhou, X. G.; Zhang, L. X.; Huang, Z. E.; Cai, R. F.; Huang, X. Y. Synth. React. Inorg. Met.-Org. Chem. 2000, 30, 965−978. (45) Zhou, X. G.; Huang, Z. E.; Cai, R. F.; Zhang, L. X.; Hou, X. F.; Feng, X. J.; Huang, X. Y. J. Organomet. Chem. 1998, 563, 101−112. (46) Evans, W. J.; Drummond, D. K.; Hughes, L. A.; Zhang, H. M.; Atwood, J. L. Polyhedron 1988, 7, 1693−1703. (47) We were unable to locate any known crystal structures in which tetrazolate binds to a transition metal, lanthanide, or actinide through other N atoms (e.g., through N(2)/N(3)) or in which tetrazolate adopts other binding modes (e.g., in a facial π manner). Tetrazolate bonding in η1 fashion to U and Th was attempted computationally, but did not result in any stable η1-bound structures. (48) Bryant, R. G. J. Chem. Educ. 1983, 60, 933−935. (49) Limbach, H.-H. In Deuterium Shift Calculation; Diehl, P.; Fluck, E.; Günther, H.; Kosfeld, R.; Seelig, J., Eds.; NMR Basic Principles and Progress 23; Springer: Berlin, Germany, 1991; pp 63−164. (50) Thomson, R. K.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. C. R. Chim. 2010, 13, 790−802. (51) Lukens, W. W.; Beshouri, S. M.; Blosch, L. L.; Stuart, A. L.; Andersen, R. A. Organometallics 1999, 18, 1235−1246. (52) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650−6667. (53) Kiplinger, J. L.; John, K. D.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organometallics 2002, 21, 4306−4308. (54) Cantat, T.; Graves, C. R.; Jantunen, K. C.; Burns, C. J.; Scott, B. L.; Schelter, E. J.; Morris, D. E.; Hay, P. J.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 17537−17551. (55) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448− 9460. (56) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organometallics 2002, 21, 3073−3075. (57) Morris, D. E.; Da Re, R. E.; Jantunen, K. C.; Castro-Rodriguez, I.; Kiplinger, J. L. Organometallics 2004, 23, 5142−5153. (58) See Figure S3 for MO pictures illustrating the orbitals that contain significant contribution from the unpaired 5f electrons. (59) Pagano, J. K.; Dorhout, J. M.; Waterman, R.; Czerwinski, K. R.; Kiplinger, J. L. Chem. Commun. 2015, 51, 17379−17381. (60) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (61) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535−7542. (62) Cao, X. Y.; Dolg, M.; Stoll, H. J. Chem. Phys. 2003, 118, 487− 496. (63) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560−571. (64) Jackson, V. E.; Gutowski, K. E.; Dixon, D. A. J. Phys. Chem. A 2013, 117, 8939−8957. (65) Frisch, M. J., et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. I
DOI: 10.1021/acs.inorgchem.6b00492 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (66) Roy, L. E.; Jakubikova, E.; Guthrie, M. G.; Batista, E. R. J. Phys. Chem. A 2009, 113, 6745−6750. (67) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43−54. (68) Casida, M. E. J. Mol. Struct.: THEOCHEM 2009, 914, 3−18. (69) Casida, M. E.; Huix-Rotllant, M. In Annu. Rev. Phys. Chem. Johnson, M. A.; Martinez, T. J., Eds.; 2012, 63, 287−32310.1146/ annurev-physchem-032511-143803. (70) Dreuw, A.; Head-Gordon, M. Chem. Rev. 2005, 105, 4009− 4037. (71) APEX2, v. 2014.7-1; Bruker AXS: Madison, WI, 2014. (72) SAINT, v. 8.34A; Bruker AXS: Madison, WI, 2013. (73) SADABS, v. 2014/3; Bruker AXS: Madison, WI, 2013. (74) SHELXTL, v. 6.14; Bruker AXS: Madison, WI, 2000.
J
DOI: 10.1021/acs.inorgchem.6b00492 Inorg. Chem. XXXX, XXX, XXX−XXX
