Terminal Uâ¡E (E = N, P, As, Sb, and Bi) Bonds in Uranium

Article pubs.acs.org/JPCA

Terminal UE (E = N, P, As, Sb, and Bi) Bonds in Uranium Complexes: A Theoretical Perspective Qun-Yan Wu,† Jian-Hui Lan,† Cong-Zhi Wang,† Yu-Liang Zhao,† Zhi-Fang Chai,†,‡ and Wei-Qun Shi*,† †

Key Laboratory of Nuclear Radiation and Nuclear Energy Technology and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Radiological and Interdisciplinary Sciences (RAD-X), and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: The compound L−U−N [L = [N(CH2CH2NSiPri3)3]3−, Pri = CH(CH3)2] containing a terminal U−N triple bond has been synthesized and isolated successfully in experiments. To investigate the trend in the bonding nature of its pnictogen analogues, we have studied the L−U−E (E = N, P, As, Sb, and Bi) complexes using the scalar relativistic density functional theory. The terminal U−E multiple bond length increases in the order of U−N ≪ U− P < U−As < U−Sb < U−Bi, which can be supported by the hard and soft acids and bases (HSAB) theory. The U−E bond length, molecular orbital (MO), and natural bond orbital (NBO) reveal that the terminal U−E bonds should be genuine triple bonds containing one σ- and two π-bonding orbitals. Quantum theory of atoms in molecules (QTAIM) topological analysis and the electron localization function (ELF) suggest that the terminal U−E bond possesses covalent character and the covalency of U−E bonds decrease sharply when the terminal atom becomes heavier. This work presents a comparison about the bonding characteristic between the terminal UN bond and its heavier pnictogen (P, As, Sb, and Bi) analogues. It is expected that this work would shed light on the evaluation of the amount of 5f orbital participation in multiple bonds and further facilitate our deeper understanding of f-block elements. was the sign of a new breakthrough in this field. Despite increasing attention about the multiple U−N bonds,2,3 it is still extremely challenging for the synthesis of complexes containing terminal multiple bonds between the uranium atom and a pnictogen atom (P, As, Sb, and Bi). So far, the terminal phosphide molecule PUF3 has only been observed using IR spectroscopy by the reactions of uranium atoms with PF3 from matrix isolation experiments.29 Ab initio studies revealed that the PUF3 complex contained a UP triple bond with the effective bond order of 2.39.29 The molecule with a uranium− arsenic bond was also subsequently identified by the analogous experiment, which was described to be a weaker triple UAs bond with an effective bond order of 2.21.30 As it is well-known, various complexes with triple bonds between a transition metal and a pnictogen atom have been reported for several decades.31−33 Normally, a number of complexes concerning transition metal−pnictogen triple bonds were synthesized with a general formula of [LnME], where L represents a certain ligand, M = W, Mo, E = N, P, As, even Sb and Bi.31−43 For example, representative complexes containing Mo−P and W−P triple bonds were independently synthesized by the groups of Cummins and Schrock,31,32 respectively.

1. INTRODUCTION Uranium-ligand multiple bonding has become a topic of significant interest,1−3 and in particular multiple bonds concerning U−N,3−17 U−O,13,18,19 and U−C20−28 have been investigated with impressive progress. These studies represent benchmarks for f-block multiple bonds and enrich our understanding of f-block elements. Recently, the first L−U− N complex [L = [N(CH2CH2NSiPri3)3]3−, Pri = CH(CH3)2] (Scheme 1) containing a terminal triple U−N bond was synthesized and isolated by Liddle and his co-workers,4,5 which Scheme 1. Structures of the Compound L−U−N and Ligand [L = [N(CH2CH2NSiPri3)3]3− and Pri = CH(CH3)2]

Received: January 13, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/jp512950j J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 1. Selected Bond Distances (Å), Bond Angles (deg.), and Harmonic Vibrational Frequencies (cm−1) in Compounds L− U−E (E = N, P, As, Sb, and Bi) at the BP86 Level of Theory L−U−E L−U−N L−U−P L−U−As L−U−Sb L−U−Bi

U−E 1.773 1.780a 2.396 2.583 2.828 2.910

U−N2

U−N3

U−N4

U−N5

N5UE

ν(U−E)

b

2.291

2.295

2.300

2.643

177.35

938

2.401b 2.544b

2.269 2.268 2.258 2.256

2.252 2.252 2.244 2.243

2.255 2.253 2.243 2.246

2.683 2.673 2.687 2.685

174.38 175.08 174.34 174.80

381 183 173 132

1.759

a

Bond lengths of the U−N bond in L−U−N at BP86/TZP level of theory from ref 5. bBond lengths of the U−E bond in EUF3 with CASPT2 from refs 29 and 30.

for the uranium atom. 5 6 − 5 8 The pseudopotentials ECP46MWB59 and ECP78MWB60 as well as their corresponding valence basis sets were used for Sb and Bi atoms, respectively. For the other atoms (H, C, N, Si, P, and As), the 6-31G(d) basis set was used. The quasi-relativistic smallcore pseudopotential replaces 60 core electrons for the uranium atom, whereas the remaining 32 valence electrons were represented by the associated valence basis set.56−58 The oxidation state of uranium in compounds L−U−E (E = N, P, As, Sb, and Bi) is hexavalent. The molecules studied here are considered to be closed-shell, hence the singlet electronic state for each compound is the ground state. Spin−orbit effects were not taken into consideration in this work. Harmonic vibrational frequencies obtained analytically at the optimized structures are all positive values. Natural population analysis (NPA) was carried out using NBO method with NBO 5.0 program61 as implemented in the Amsterdam density functional (ADF 2012.02) package.62,63 In NBO calculations, the BP86 method and the Slater type orbital (STO) basis set with the quality of triple-ζ plus polarization (TZP) basis set were used,64 without frozen core. The scalar relativistic (SR) effects were taken into account using the zero-order regular approximation (ZORA) approach.65 The topological analysis of the electron densities for the terminal multiple U−E (E = N, P, As, Sb, and Bi) bond was performed by employing quantum theory of atoms in molecules (QTAIM) with the ADF program and Multiwfn code.66 In addition, to further confirm the bonding properties, the electron localization function (ELF) was also provided using Multiwfn code.

Scheer et al. also synthesized the stable phosphide complex [(tBuO)3WP → W(CO)5] in 1995.33 Subsequently, Schrock and his co-workers isolated the first transition−arsenido complexes [(N3N)MAs] (M = Mo and W).34 In 2005, Scheer and his co-workers succeeded in the synthesis and spectroscopic and structural characterization of the first isolable stibido complex [(N3N)WSb].43 In the meantime, the bonding interactions between the transition metal and the pnictogen atom were also investigated theoretically,44−46 which revealed that the metal−pnictogen bonds are typical triple bonds containing a σ- and a pair of degenerate π-bonding orbitals. In addition, complexes with U=X (X = S, Se, and Te) bond have also been synthesized.47−49 Very recently, Liddle et al. reported the first complex containing the terminal UP bond with the distance of 2.613(2) Å.50 The corresponding DFT calculations also revealed a double bonding character with the UP Mayer bond order of 1.92. However, up to now, to the best of our knowledge, no complexes containing terminal triple uranium−pnictogen bonds (U−P, U−As, U−Sb, and U− Bi) have been obtained experimentally. In our previous work, we have studied a series of actinide (AnPa−Pu) complexes of L−An−N theoretically and showed that the terminal An−N bond contained one σ bond and two π bonds.51 It is revealed that the uranium 6d and 5f orbitals have significant contribution to the terminal An−N bond and the contributions of the 6d orbital of actinides to the covalency are larger in magnitude than those of the 5f orbital.51 In this work, to further compare the bonding difference between the light atom and heavy atom of group 15, we have subsequently studied the L−U−N compound5 (Scheme 1) as well as its pnictogen analogues L−U−E (E = P, As, Sb, and Bi). The bonding nature and covalent character of the terminal multiple U−E bonds (E = N, P, As, Sb, and Bi) in compounds L−U−E have been studied. On the one hand, a comparison on the multiple bonding character can be performed between the uranium- lighter (N) and heavier (P, As, Sb, and Bi) elements of group 15. On the other hand, these results can provide detailed information on the bonding principles of uranium and further facilitate our in-depth understanding of f-block elements.

3. RESULTS AND DISCUSSION 3.1. Optimized Geometries and Harmonic Vibrational Frequencies. We have optimized compounds L−U−E (E = N, P, As, Sb, and Bi) using the BP86 method which has proved to be reliable for compound L−U−N in our previous work.51 The selected bond distances, bond angles, and harmonic vibrational frequencies as well as infrared intensities are provided in Table 1. Previous works have extensively investigated the electronic structure of L−U−N complex and revealed that the terminal U−N bond has an obvious triple bonding.5,51 The terminal U−P bond distance (2.396 Å) in L− U−P complex is much longer than the corresponding U−N bond, whereas it is shorter than that of anionic [L-UPH]− complex with double UP bond of 2.613(2) Å.50 The predicted U−As bond length in complex L−U−As is 2.583 Å. These predicted U−E bond lengths in compounds L−U−E are comparable to those with triple U−E bonds character in E− UF3 molecules. For example, the U−E bond distance using CASPT2 methods is predicted to be 1.795, 2.401, and 2.544 Å in complexes N−UF3, P−UF3, and As−UF3, respectively.29,30

2. COMPUTATIONAL DETAILS The geometry optimizations were performed with the Gaussian 09 program.52 The pure gradient corrected BP86 functional was employed,53 which is a widely used functional for actinides.54,55 Our previous work has also confirmed that the results of compound L−U−N at the BP86 level show a surprisingly good agreement with available experimental data.51 Quasi-relativistic small-core pseudopotential ECP60MWB along with the corresponding ECP60MWB-SEG valence basis set was applied B


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The Journal of Physical Chemistry A These results indicate that the terminal U−E (E = N, P, and As) bonds in compounds L−U−E also possess triple bond characteristics. As for the heavier U−Sb and U−Bi bonds, the corresponding bond lengths are 2.828 and 2.910 Å, respectively. It is worth mentioning that the distance of the terminal U−E bond increases in the order U−N ≪ U−P < U− As < U−Sb < U−Bi. It can be concluded that the uranium atom interacts more easily with the contracted nitrogen atom than the diffusive heavy pnictogen atoms, which can be expected from the hard and soft acids and bases (HSAB) theory.67,68 The uranium(VI) is a kind of hard acid, and in contrast, the base character of group 15 elements gets softer when the atom becomes heavier. The three U−Namido bonds between the uranium atom and nitrogen atoms (N2, N3, and N4) of the amido group for each compound have similar bond distances. The largest difference of the average U−Namido bond length is only 0.05 Å between compound L−U−N and L−U−Bi. As for the U−N5 bond distance, it is the longest among all U−N bond distances for each compound. In addition, the N5UE bond angles are close to 180°, and the biggest deviation is only about 5.6° for compound L−U−Sb. The harmonic vibrational frequencies of the terminal U−E bond are also listed in Table 1. The predicted harmonic vibrational frequency of the terminal U−N bond in compound L−U−N is 938 cm−1, which is comparable with the corresponding observed experimental value for L−U−N (914 cm−1)5 and NUF3 molecule (938 cm−1),29 respectively. The corresponding frequencies of the terminal U−E bond gradually decrease from 381 cm−1 in compound L−U−P to 132 cm−1 in compound L−U−Bi. The trend of the harmonic vibrational frequencies for the terminal U−E (E = N, P, and As) bond is consistent with the data for WE bond in [N3N]WE complexes.34,43 The predicted harmonic frequencies can provide valuable information for experimental specialists. 3.2. Molecular Orbital. In order to investigate the interaction between the uranium atom and the terminal atom E in compounds L−U−E (E = N, P, As, Sb, and Bi), the models of the canonical valence molecular orbitals (MOs) that contribute to the terminal U−E bond are displayed in Figure 1

Figure 2. Visualizations and the energies (eV) of MOs between the uranium atom and terminal atom in compounds L−U−E (E = N, P, As, Sb, and Bi). Orbital plots are generated with a contour value of 0.03 au.

orbital, which reveals that the participation of the uranium 5f orbital is a predominant factor for forming the U−E triple bond. As shown in Figure 1, the σ MO of the U−E is principally composed of the uranium 5fσ orbital and the E npz orbital. Two π MOs of the U−E bond consist of the contribution of two uranium 5fπ orbitals and E npx and npy orbitals, respectively. The energies of the three MOs between uranium and terminal atom (E = N, P, As, Sb, and Bi) in compounds L−U− E are also provided in Figure 2. It is pointed out that the trend of energies for each type of MOs follows the order of U−N ≪ U−P < U−As < U−Sb < U−Bi, which reveals that there are significant differences in forming multiple bonds between light nitrogen atoms and heavy pnictogen atoms (P, As, Sb, and Bi). Taking σ MOs in compounds L−U−E as an example, the energies of the σ MOs are −5.254, −3.897, −3.788, −3.693, and −3.617 eV when the terminal atom changes from N to Bi. It is well-known that the energy of the MO involved in a certain bond is related to the bond strength in similar bonding situation.69 According to the energies of MOs in the terminal U−E bond, the order of the stability for the U−E bond is U−N ≫ U−P > U−As > U−Sb > U−Bi. These results are in accordance with the bond energies of U−E bonds (E = N, P, and As) in compounds E−UF3.29,30 The energies of two π MOs for each compound are almost similar, and the biggest deviation between two π MOs is only 0.03 eV for compound L−U−Bi. This result indicates that the two π MOs are quasi-degenerate in each compound. In addition, it can be clearly seen that for each complex the energy of the σ MO is higher compared to those of π MOs. For instance, in compound L−U−P, the energy of the σ MO is −3.897 eV, and the corresponding energies of the two π MOs are −4.046 and −4.073 eV, respectively, which suggests that the π MOs make a significant contribution to the stability of the U−E triple bond. 3.3. Topological Analysis. To obtain further insights about the bonding nature of the terminal U−E bond in compounds L−U−E (E = N, P, As, Sb, and Bi), topological analyses of the electron density of the terminal U−E bonds have been performed in the framework of QTAIM. The topological analysis of electron density can give valuable

Figure 1. Representations of the three canonical molecular orbitals containing the terminal U−E bond (n = 2−6 for N, P, As, Sb, and Bi, respectively). U−E bond orientates along the z axis.

and the specific MOs for each complex are also provided in Figure 2. It is worthwhile to note that one σ and two π MOs are involved for all compounds, which indicates that the terminal U−E bonds have genuine triple bond characteristics. The atomic orbital compositions of MOs were obtained using the natural atomic orbital approach and the specific atomic orbital contributions to σ and π MOs for the three compounds are listed in Table S1. It is worth mentioning that the np (n = 2−6 for N, P, As, Sb, and Bi, respectively) orbital of the terminal atom and the 5f and 6d orbitals of the uranium atom have significant contribution to all MOs concerned above. Furthermore, the contribution of the uranium 5f orbital to the UE triple bond is larger than that of the uranium 6d C


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ρ(r) and H(r) between the terminal U−N and uranium− pnictogen (P, As, S,b and Bi) bonds indicate the contribution of group 15 elements to the covalency of U−E bond is distinguishable. We also performed the ELF analysis to further confirm the properties of the bonding interactions. Ordinarily, ELF is within the range of 0 to 1, ELF = 1 represents the perfect localization of electron, while ELF = 0 suggests a completely delocalized situation.78,79 It has been found that the ELF analysis can provide useful information on chemical bonds for a wide variety of systems.80−82 A large ELF value indicates a more covalent character of a chemical bond. The twodimensional colored ELF images of molecular fragments containing N5−U−E planes and one-dimensional ELF curves for the terminal U−E bonds are provided in Figure 3a−f. It can be seen that the values of the ELF for the five terminal U−E bonds are relatively larger, revealing that the U−E bonds have covalent character. Moreover, Figure 3f suggests intuitively that the ELF value between the uranium and the terminal atom E decreases when the terminal atom E changes from N to Bi. The ELF analysis also reveals that the covalency decreases from the U−N to the U−Bi bond, which is in excellent agreement with the electron density analysis. 3.4. NBO Analysis. Figure 2 clearly shows that the canonical MOs are delocalized over the complexes. To obtain more details on the terminal multiple U−E bonding and analyze how the 5f and 6d orbitals of uranium contribute to the covalency, we have also performed the localized natural bond orbital (NBO) analysis which can provide chemically a more intuitive description of the bonding nature.83,84 The natural electronic configurations and natural charges on uranium and the terminal atoms are provided. Bond multiplicities including Mayer bond order and Nalewajski−Mrozek (NM) valence indices, the compositions, as well as the contribution of each atomic orbital for NBOs of U−E bonds are also fully discussed. The natural electronic configurations of uranium and the terminal atom E (E = N, P, As, Sb, and Bi) are listed in Table 3. It is interesting to note that, for each compound, most of the valence electrons of the uranium atom reside in the 5f shell, and the remaining valence electrons occupy the 6d and 7s shells. Moreover, significant differences about the 5f population occupancies between complex L−U−N and its pnictogen analogues have been found. The corresponding values are 2.77, 3.14, 3.18, 3.19, and 3.19, respectively, when the terminal atom changes from N to Bi, which is in excellent agreement with the MO analysis. As for the number of electrons in the 7s and 6d shells, it changes a little with the increase of the atomic number of group 15. In addition, the valence electrons mainly reside on the ns and np (n = 2−6 for N, P, As, Sb, and Bi, respectively) shells of the terminal atoms. It is clearly observed that compared to the natural electronic configuration of the isolated terminal atom (ns2np3), the number of the electrons decreases for the ns shell, whereas it increases for the np shell. Therefore, the redundant electrons of the np shell of the terminal atom mostly come from the ns shell of the terminal atom and uranium 5f shell. Moreover, similar to the uranium 5f population occupancies, there are also significant differences on the np population occupancies between light nitrogen (4.05) and heavy pnictogen atoms (P, As, Sb, and Bi is about 3.6). The natural charges on the uranium and the terminal atoms are presented in the SI, Table S5. It can be clearly seen that the natural charges on the terminal atoms decreases from N to Bi,

information about the nature of different chemical bonds.70,71 AIM analysis shows that there is one bond critical point (BCP) between two atoms bonded to each other. The bonding interactions can be characterized and classified according to the properties of the electron density [(ρ(r)] and its Laplacian [∇2ρ(r)] as well as energy density [H(r)] (the sum of the kinetic and potential energy densities) at these BCPs. In general, the electron density at BCP ρ(r) > 0.20 au and ∇2ρ(r) < 0 describes a covalent bond, while ρ(r) < 0.10 au and ∇2ρ(r) > 0 is for an ionic bond. H(r) at the BCP is negative for interactions with significant sharing of electrons, and its magnitude can reflect the covalence of the bonding interaction. QTAIM has been used for assessing the bonding properties of actinide compounds and could provide valuable information about chemical bonds generally.4,5,50,72−75 It should be mentioned that QTAIM calculations may be sensitive to the method and basis set in the case of multiple and more polarized bonds.76,77 In this work, to study the reliability of QTAIM calculations, we chose compound L−U−N as an example to compute ρ(r), ∇2ρ(r), and the eigenvalues λi (i = 1−3) of the Hessian matrix at the terminal U−N BCP with respect to six methods (BP86, PBE, BLYP, TPSS, B3LP, and TPSSH) and five basis sets (DZ, TZ, DZP, TZP, and TZ2P), respectively. The ADF program was used in these examinations. As shown in the SI, Tables S2−S3, the ρ(r) values are almost similar (about 0.32) with respect to various methods and basis sets, while the corresponding ∇2ρ(r) changes are in the range of 0.04−0.12 with different methods and basis sets. Liddle et al. have also carried out the QTAIM analysis of compound L−U− N at the BP86/TZP level of theory and found that the ρ(r) at triple U−N BCP is 0.39.5 Herein, the calculated ρ(r) and H(r) at the U−E BCP have been obtained using ADF and Multiwfn programs, as shown in Table 2. The ρ(r) of the terminal U−E Table 2. Electron Density [ρ(r), au] and the Total Electronic Energy Density [H(r), au] at the Terminal U−E Bond Critical Point in Compounds L−U−E (E = N, P, As, Sb, and Bi) L−U−E L−U−N L−U−P L−U−As L−U−Sb L−U−Bi

ρ(r)a 0.325 0.123 0.092 0.069 0.062

ρ(r)b 0.332 0.123 0.092 0.066 0.058

H(r)b −0.329 −0.066 −0.040 −0.027 −0.023

a The values obtained with the ADF program. bThe values obtained with the Multiwfn program.

BCP decreases sharply from 0.325 au at U−N BCP to 0.062 au at U−Bi BCP, suggesting that the uranium−pnictogen (P, As, Sb, and Bi) bonds have weaker covalent character compared to the U−N bond. It should be pointed out that all ∇2ρ(r) at U− E BCP are positive with small values, as shown in the SI, Table S4. Moreover, the magnitude of H(r) can reflect the degree of ionic and covalent interactions between the uranium and the terminal E atoms. The negative energy density H(r) at the terminal U−E bonds also reveals that the terminal U−E bonds exhibit covalent character. Similar to the electron density at the terminal U−E BCP, the energy density becomes less negative with the increasing of atomic number of group 15, which also means that the covalent interaction between the uranium atom and the terminal atom decreases when group 15 elements become heavier. In addition, the significant differences of the D


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Figure 3. Two dimensional ELF contours for the N5−U−E planes containing the terminal U−E bonds (a) L−U−N, (b) L−U−P, (c) L−U−As, (d) L−U−Sb, (e) L−U−Bi, as well as (f) one-dimensional ELF curves along the terminal U−E bond direction; the origin of the x axis is the position of uranium atom.

Bond order is a straightforward measurement of bond multiplicities; its value is important for evaluating the bonding nature. Mayer bond order has been used as a useful tool for bonding analysis.85,86 The NM valence indices comprise covalent and ionic contributions, 87,88 which can more accurately describe structural properties from experiments.46,89,90 Herein, the obtained NM and Mayer bond indices of the terminal U−E bond and other U−N bonds at the BP86/TZP level of theory are listed in Table 4. It should be mentioned that the NM bond indices are higher than the corresponding Mayer bond orders, although they show a similar trend. Liddle et al. reported that the calculated Mayer bond order of terminal U−N bond in complex L−U−N is 2.92,5 which is comparable with our theoretical finding (2.966). In this work, we used the Mayer bond order to analyze the trend of the terminal U−E bond with increasing atomic number. The Mayer bond orders are about over 2.72 for the terminal U−E bonds, which reveals that the terminal U−E bonds are prominent triple bonds. This result is in excellent agreement with the results of bond lengths and MO analyses. Moreover, the Mayer bond order of the terminal U−E bond decreases when the terminal atoms become heavier except for the U−As bond with a little deviation, and this trend is similar to that of the effective bond orders of the U−E bonds in E UF3 molecules, which were obtained by CASPT2 methods29,30 and also displayed in Table 4 for comparison. This result also suggests that the strength of the triple U−N bond in complex

Table 3. Natural Electronic Configurations of Uranium and the Terminal Atoms (E = N, P, As, Sb, and Bi) in the Compounds L−U−E L−U−E

U

E

L−U−N L−U−P L−U−As L−U−Sb L−U−Bi

[Rn]7s0.175f2.776d0.507p0.028s0.01 [Rn]7s0.165f3.146d0.587p0.02 [Rn]7s0.155f3.186d0.537p0.02 [Rn]7s0.185f3.196d0.537p0.03 [Rn]7s0.185f3.196d0.527p0.03

[He]2s1.902p4.053d0.03 [Ne]3s1.903p3.673d0.01 [Ar]4s1.914p3.644d0.02 [Kr]5s1.915p3.59 [Xe]6s1.926p3.55

the reason for which could be that light nitrogen is more electronegative than heavy pnictogen (P, As, Sb, and Bi). Like the results of the terminal atoms, the natural charges on the uranium in five compounds follow a similar trend. This result suggests that the interaction between the uranium atom and nitrogen atom is stronger compared to its pnictogen analogues, which is also very in line with the results of bond lengths, infrared intensities as well as MO analyses. In addition, the natural charges on the nitrogen atoms (N2−N5) of the same topological position are similar when the terminal atom changes from N to Bi, which indicates that the terminal atom can only affect the natural charge distribution of the adjacent uranium atom. From this point of view, it can perfectly explain the fact that the terminal U−E bond length increases with the increasing atomic number of E, whereas the other U−N bond lengths almost keep constant.

Table 4. Mayer Bond Order and Nalewajski−Mrozek (NM) Valence Indices of U−E and U−N Bonds in Compounds L−U−E (E = N, P, As, Sb, and Bi)a L−U−E

U−E

L−U−N

2.966/3.566 2.92b 2.916/3.351 2.719/3.264 2.771/3.205 2.751/3.179

L−U−P L−U−As L−U−Sb L−U−Bi

U−N2

U−N3

U−N4

U−N5

2.78c

0.894/1.286

0.874/1.274

0.880/1.266

0.383/0.453

2.39c 2.21c

0.946/1.304 0.945/1.306 0.972/1.320 0.965/1.319

0.925/1.337 0.925/1.338 0.952/1.348 0.944/1.345

0.934/1.326 0.934/1.329 0.972/1.344 0.970/1.339

0.342/0.456 0.353/0.463 0.322/0.454 0.324/0.459

a −/− indicate that Mayer bond order/NM valence indices. bMayer bond order of U−N bond in L−U−N from ref 5. cEffective bond order of U−E bond in EUF3 with CASPT2 from refs 29 and 30.

E


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Furthermore, the uranium percentage of σ orbital is lower than that of the π orbital for compounds L−U−P, L−U−As, L−U− Sb, and L−U−Bi, whereas it is a little bit higher in compound L−U−N. As discussed above in the section of MOs analysis, it is worthy to realize that the uranium 5f and 6d orbitals and the np orbital of atom E all have predominant contributions to both σ and π bonding orbitals. However, the percentage of uranium 5f and 6d orbitals participating in σ and π bonding orbital is significantly different, as shown in Figure 4b,c. In addition, there is a little contribution of uranium 7s and E ns orbitals to the σ bonding orbital. For example, in the σ bonding orbital of compound L−U−P, the component of the uranium 7s orbital and nitrogen 2s orbital is about 4.44% and 4.68%, respectively. In Figure 4b, the contribution of the uranium 5f orbital for σ bonding orbital gradually increases when the terminal atom gets heavier and the contribution of the uranium 6d orbital gradually decreases. For the π bonding orbital in Figure 4c, the contribution of the uranium 5f orbital also gradually increases except for the complex L−U−Sb when the atom E gets heavier. However, unlike the case of σ bonding orbital, in the case of the π bonding orbital, the difference between the component percentage of the uranium 5f and 6d orbitals is distinctly larger. Taking compound L−U−As as an example, the percentage of the uranium 5f orbital is about 92.5%, while the percentage of the uranium 6d orbital is only about 7.5%. Based on above results, the following conclusions can be reached (i) the contribution of the uranium 5f orbital is larger than that of the 6d orbital to the σ and π bonding orbitals; (ii) the contribution of the uranium 5f orbital to π bonding orbital is significantly larger compared to that of the σ bonding orbital. This reveals that the uranium 5f orbital is the predominant contributor to the terminal U−E triple bond. Previous works showed that the contributions of the actinide 6d orbital to the covalency are larger in magnitude than that of the 5f orbital in a series of actinide compounds.51,91 In this work, according to NBO, QTAIM, and ELF analyses, the contribution of uranium 5f orbital to the terminal U−E (E = N, P, As, Sb, and Bi) bonds is larger than that of the 6d orbital, while the covalency of the terminal U−E bonds seems to be dominated by the contribution of 6d orbital. Moreover, the covalency of the terminal U−E bond involving uranium 5fσ and 5fπ orbitals decreases with increasing the atomic number of group 15.

L−U−N is stronger than those of the uranium−pnictogen counterparts, which is also in accordance with the results of MOs as discussed above. The Mayer bond orders of the three U−Namido bonds are in the range of 0.87−0.97, indicating that the three U−Namido bonds are single bonds. Unlike the terminal U−E bond order, the trend of U−Namido bond order gradually increases with increasing the atomic number. In addition, the Mayer bond order of U−N5 bond is lower (about 0.32−0.38) than those of other uranium atom involved bonds, suggesting that the electrostatic interactions play dominant roles for the U−N5 bonding. To obtain more bonding details and evaluate the relative role of 5f versus 6d on the covalency of the terminal U−E bonds, the calculated compositions and the contribution of each atomic orbital for U−E bonds in the compounds L−U−E are presented in Table S6. The natural localized molecular orbitals of U−E are distinct σ and π bonds, as displayed in Figure S1. In order to clearly compare the compositions of σ and π bonds and the trend of the percentage of uranium 5f and 6d orbital involved in bonding, we provided the composition and the percentage of each bonding orbital with increasing the atomic number of group 15 in Figure 4a−c. It is interesting to note that in Figure 4a the uranium percentage of σ orbital in the five L−U−E compounds is almost the same (about 30%), while the corresponding value of the π orbital gradually increases from 30% in compound L−U−N to 44% in compound L−U−Bi.

4. CONCLUSIONS In summary, complexes L−U−E containing a terminal U−E triple bond have been studied using the scalar relativistic DFT method. We found that the terminal U−E bonds in L−U−E complexes (E = N, P, As, Sb, and Bi) have genuine triple bond character by the geometrical and electronic analyses. The triple U−E bond lengths increase in the order of U−N ≪ U−P < U− As < U−Sb < U−Bi, which can be supported by the theory of HSAB. MOs show that the terminal U−E bond contains one σ and two quasi-degenerate π bonding orbitals, which also suggest that the terminal U−E bonds have obvious triple bonding character. Moreover, the MOs energies of the compounds L−U−E increase sharply when the terminal atom E gets heavier, which indicates that there are significant differences in forming uranium multiple bonding between lighter nitrogen atom and heavier pnictogen atoms (P, As, Sb, and Bi). QTAIM topological analysis shows that the terminal U−E bonds possess covalent characteristic and the covalent interaction of the terminal U−E bonds decreases with the

Figure 4. Percentage of uranium atom for σ and π bonding orbitals (a), and the contribution of uranium 5f and 6d atomic orbitals for σ (b) and π (c) bonding orbitals. F


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(5) King, D. M.; Tuna, F.; McInnes, E. J. L.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Isolation and Characterization of a Uranium(VI)-Nitride Triple Bond. Nat. Chem. 2013, 5, 482−488. (6) Green, D. W.; Reedy, G. T. The Identification of UN in Ar Matrices. J. Chem. Phys. 1976, 65, 2921−2922. (7) Hunt, R. D.; Yustein, J. T.; Andrews, L. Matrix Infrared Spectra of NUN Formed by the Insertion of Uranium Atoms into Molecular Nitrogen. J. Chem. Phys. 1993, 98, 6070−6074. (8) Andrews, L.; Wang, X.; Gong, Y.; Vlaisavljevich, B.; Gagliardi, L. Infrared Spectra and Electronic Structure Calculations for the NUN(NN)1−5 and NU(NN)1−6 Complexes in Solid Argon. Inorg. Chem. 2013, 52, 9989−9993. (9) Camp, C.; Pécaut, J.; Mazzanti, M. Tuning Uranium−Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands. J. Am. Chem. Soc. 2013, 135, 12101−12111. (10) Smiles, D. E.; Wu, G.; Hayton, T. W. Synthesis of Uranium− Ligand Multiple Bonds by Cleavage of a Trityl Protecting Group. J. Am. Chem. Soc. 2013, 136, 96−99. (11) Andersen, R. A. Tris((hexamethyldisilyl)amido)uranium(III): Preparation and Coordination Chemistry. Inorg. Chem. 1979, 18, 1507−1509. (12) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin, J. G.; Zwick, B. D. A Convenient Entry into Trivalent Actinide Chemistry: Synthesis and Characterization of AnI3(THF)4 and An[N(SiMe3)2]3 (AnU, Np, Pu). Inorg. Chem. 1994, 33, 2248− 2256. (13) Arney, D. S. J.; Burns, C. J. Synthesis and Properties of HighValent Organouranium Complexes Containing Terminal Organoimido and Oxo Functional Groups. A New Class of Organo-f-Element Complexes. J. Am. Chem. Soc. 1995, 117, 9448−9460. (14) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R. Exchange of an Imido Ligand in Bis(imido) Complexes of Uranium. J. Am. Chem. Soc. 2006, 128, 12622−12623. (15) Lam, O. P.; Bart, S. C.; Kameo, H.; Heinemann, F. W.; Meyer, K. Insights Into the Mechanism of Carbonate Formation Through Reductive Cleavage of Carbon Dioxide with Low-Valent Uranium Centers. Chem. Commun. 2010, 46, 3137−3139. (16) Lam, O. P.; Franke, S. M.; Nakai, H.; Heinemann, F. W.; Hieringer, W.; Meyer, K. Observation of the Inverse Trans Influence (ITI) in a Uranium(V) Imide Coordination Complex: An Experimental Study and Theoretical Evaluation. Inorg. Chem. 2012, 51, 6190−6199. (17) Matson, E. M.; Crestani, M. G.; Fanwick, P. E.; Bart, S. C. Synthesis of U(IV) Imidos From Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by Extrusion of Bibenzyl. Dalton Trans. 2012, 41, 7952−7958. (18) Kraft, S. J.; Walensky, J.; Fanwick, P. E.; Hall, M. B.; Bart, S. C. Crystallographic Evidence of a Base-Free Uranium(IV) Terminal Oxo Species. Inorg. Chem. 2010, 49, 7620−7622. (19) Barros, N.; Maynau, D.; Maron, L.; Eisenstein, O.; Zi, G.; Andersen, R. A. Single but Stronger UO, Double but Weaker UNMe Bonds: The Tale Told by Cp2UO and Cp2UNR. Organometallics 2007, 26, 5059−5065. (20) Cramer, R. E.; Maynard, R. B.; Paw, J. C.; Gilje, J. W. A Uranium-Carbon Multiple Bond. Crystal and Molecular Structure of (.eta.5-C5H5)3UCHP(CH3)2(C6H5). J. Am. Chem. Soc. 1981, 103, 3589−3590. (21) Cantat, T.; Arliguie, T.; Noël, A.; Thuéry, P.; Ephritikhine, M.; Floch, P. L.; Mézailles, N. The UC Double Bond: Synthesis and Study of Uranium Nucleophilic Carbene Complexes. J. Am. Chem. Soc. 2009, 131, 963−972. (22) Tourneux, J.-C.; Berthet, J.-C.; Thuery, P.; Mezailles, N.; Le Floch, P.; Ephritikhine, M. Easy Access to Uranium Nucleophilic Carbene Complexes. Dalton Trans. 2010, 39, 2494−2496. (23) Cooper, O. J.; Mills, D. P.; McMaster, J.; Moro, F.; Davies, E. S.; Lewis, W.; Blake, A. J.; Liddle, S. T. Uranium−Carbon Multiple Bonding: Facile Access to the Pentavalent Uranium Carbene [U{C(PPh2NSiMe3)2}(Cl)2(I)] and Comparison of UVC and UIVC Bonds. Angew. Chem., Int. Ed. 2011, 50, 2383−2386.

terminal atom getting heavier, which can be also supported by the ELF analysis. The Mayer bond order and NM valence indices also indicates that the terminal U−E bonds are prominently triple bonds and the corresponding bond orders decrease in the order of U−N > U−P > U−As > U−Sb > U− Bi. The localized NBOs of the U−E triple bonds have been evaluated that the contribution of the uranium 5f and 6d orbital to the σ bonding orbital is almost the same, whereas the corresponding contribution to the π bonding orbital is significantly larger. Moreover, the relative contribution of the uranium 5f orbital for forming the terminal U−E bonds is larger compared to the uranium 6d orbital. According to the analyses of the QTAIM, ELF, and NBO, the contribution of the uranium 6d orbital to the covalency of the U−E bonds is larger in magnitude than that of the 5f orbital, which is in accordance with the results of the series of compounds L−An−N in our previous work.51 This work presents a comparison of the characteristic of uranium multiple bonding between the lighter (N) and heavier elements (P, As, Sb, and Bi) of group 15 and predicts the terminal UP bond and the even heavier UBi linkages may also be accessible. It is expected that this work would shed light on the evaluation of the amount of 5f orbital participation in actinide multiple bonding and further facilitate our deeper understanding of the f-block elements.

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ASSOCIATED CONTENT

S Supporting Information *

Contribution (%) of the uranium and the terminal atom E to the U−E σ and π delocalized canonical MOs and localized NBOs for the three complexes L−U−E (E = N, P, As, Sb, and Bi), the electron density [ρ(r)], its Laplacian [∇2ρ(r)], and the eigenvalues λi (i = 1−3) of the Hessian matrix at terminal U−N BCP in compound L−U−N with respect to methods and basis sets, the natural charges on the uranium and terminal atoms, and the images of NBO are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-10-88233968. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 91126006, 91326202, 11205169, and 21477130) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA030104). The results described in this work were obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

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Terminal Uâ¡E (E = N, P, As, Sb, and Bi) Bonds in Uranium

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Terminal Uâ¡E (E = N, P, As, Sb, and Bi) Bonds in Uranium