Isocyanide

Article pubs.acs.org/IC

Theoretical Insights into Halogenated Uranium Cyanide/Isocyanide Compounds Zhengguo Huang,* Le Sun, Yuan Yuan, Yuying Li, and Xiaohong Wang Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic−Organic Hybrid Functional Materials Chemistry (Tianjin Normal University), Ministry of Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, People’s Republic of China S Supporting Information *

ABSTRACT: Two kinds of halogenated uranium cyanide/ isocyanide compounds, XUCN and XUNC (X = halogen) formed by the insertion of uranium atom into X−C(N) bonds of XCN (or XNC), were investigated by DFT and ab initio methods. Although XNC is less stable thermodynamically than XCN, XUNC is more stable than XUCN and is expected to be prepared and characterized in matrix isolation experiments. The C−N stretching vibration mode (νC−N) is the primary fingerprint for the identification of these isomers due to its redshift character with respect to the relevant precursor. Atoms-inmolecule (AIM) analysis illustrates that both X−U and U−C(N) bonds in XUCN and XUNC show closed-shell interaction character, although partial covalent character contributes to them, and can be denoted as X−U2+(CN)− and X−U2+(NC)−, respectively. Charge decomposition analysis (CDA) further reveals that the isocyanide exhibits better donation performance than the cyanide, which should be the root cause of the difference between XUCN and XUNC. spectrum and theoretical calculations as well.12 The reaction of U and NF3 produces the very stable NUF3,13 but a reaction with NH3 yields mostly HNUH2.12,14 Of the most interest is the reaction of uranium with species containing a CN group, since the CN radical can bond at either C or N, leading to cyanides and isocyanides, respectively. Previous theoretical research of uranyl−CN/NC bonding illustrated that the U−NC and U−CN bonds have comparable energies, which depend intimately on the local environment.21 Sonnenberg and co-workers reported that the pentacoordinated ion [UO2(CN)5]3− is more stable than the NC isomer, and the reverse holds true for the tetracoordinated analogues.22 Pyykkö et al. reported that UF4(NC)2 is slightly more exothermic than UF4(CN)2 and that the stretching frequencies are about 200 cm−1 lower for the isocyanide ligand than for the cyanide ligand.23 The dominant product in the reaction of uranium with CH3CN in solid argon is the isocyanide CH3UNC, which further isomerizes into CH2U(H)NC under irradiation.16 Reactions of laser-ablated uranium atoms with HCN produce HUNC, which is more stable than the cyanide isomer.19 Recently, reactions of laser-ablated uranium atoms with (CN)2 have produced UNC, U(NC)2, and U(NC)4 as the major products.24 This research spurred our interest in halogenated cyanides/isocyanides (XCN and XNC, X = halogen), since they are species containing a CN group as well. The halogenated xenon (and Kr) cyanide/isocyanide compounds (FKrCN,

1. INTRODUCTION Uranium is the most well-known actinide metal because it is a source of nuclear power, and the uranium chemistry has drawn much attention from the nuclear industry, earth science, and many other fields. The laser ablation technique combined with matrix isolation spectroscopy provides a straightforward way to study new reactive uranium molecular species. The reactions of uranium with various small molecules have been investigated in low-temperature noble-gas matrix isolation experiments, which have led primarily to products bearing single uranium atoms.1−20 For example, laser-ablated uranium atoms directly insert into the O−H bond of water to form H2UO in lowtemperature noble-gas matrices.1 The study of analogous hydrogen sulfide species reveals that H2US is the major product,2 which exhibits characteristics of both hydrides and chalcogenides. Reactions of laser-ablated uranium atoms with HF in argon and neon matrices produce HUF as the major product and UH and UF as minor products.3,4 Matrix isolation infrared spectroscopy has shown that the activation of methane by neutral uranium atom forms the methylidene complexes CH2UH2, which is distorted due to agostic interactions.5 Two sulfide oxides, SUO and SUO2, have also been identified in matrix isolation studies from reactions of the laser-ablated uranium atom with elemental sulfur vapors.6,7 The major product in the reactions of uranium with N2 in a solid argon matrix is the insertion dinitride NUN, with the diatomic uranium nitride UN as a minor product.8−11 Moreover, the NUNH molecule containing both triple and double uranium−nitrogen bonds was characterized by its infrared © XXXX American Chemical Society

Received: June 2, 2016

A

DOI: 10.1021/acs.inorgchem.6b01345 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 1. Structural Parameters (Bond Lengths in Å; Bond Angles and Dihedral Angles in deg) and the Expectation Values of the Total Spins (S2) of XUCN and XUNC (X = Halogen) as well as the Relevant Precursors Calculated at the B2PLYP/(C, N, X: def2-TZVPP; U: SARC) Level X in XUCN F RX−U RU−C(N) RC−N AX−U−C(N) AU−C(N)−N(C) DX−U−C(N)−N(C) δRX−U δRU−C(N) S2B2PLYP S2CCSD(T)

RX−C(N) RC−N

Cl

2.046 2.444 1.167 105.9 174.9 180.5 −0.294 −0.006 6.004 6.007

2.526 2.435 1.168 112.8 173.8 180.0 −0.164 −0.015 6.005 6.009 X in XCN

X in XUNC Br

I

F

2.690 2.435 1.168 118.4 175.5 180.1 −0.150 −0.015 6.005 6.010

2.907 2.429 1.168 117.0 173.4 180.0 −0.123 −0.021 6.006 6.012

2.050 2.293 1.182 106.9 171.2 179.8 −0.290 −0.117 6.004 6.007

Cl 2.531 2.284 1.183 113.3 169.3 180.1 −0.159 −0.126 6.005 6.008 X in XNC

Br

I

2.694 2.281 1.183 115.0 168.7 180.1 −0.146 −0.129 6.005 6.008

2.914 2.278 1.183 117.3 166.5 180.0 −0.116 −0.132 6.006 6.009

F

Cl

Br

I

F

Cl

Br

I

1.266 1.157

1.629 1.160

1.788 1.160

1.986 1.161

1.303 1.176

1.622 1.177

1.782 1.178

1.967 1.177

optimized by the B2PLYP method, followed by vibrational frequency calculations using analytical second derivatives to ensure that the optimized structure is the minimum and evaluate zero-point vibrational energies (ZPVE). On the basis of the UHF reference determinant, the CCSD(T) single-point energy over the optimized geometry was performed using the all-electron scalar relativistic basis sets to obtain more accurate energies. Moreover, to ensure that no serious spin contaminations were involved, the expectation values of the total spin (S2) for B2PLYP and CCSD(T) calculations were checked. On the basis of the wave functions generated using the recontracted scalar relativistic all-electron def2-TZVPP basis sets (and the SARC basis sets removing the g functions for U atoms), fuzzy bond orders (FBOs),35,36 charge decomposition analysis (CDA), 37,38 and Bader’s atoms-in-molecules (AIM)39,40 analyses were performed by the Multiwfn software41 to understand the nature of the bonding interactions in XUCN and XUNC molecules.

FXeCN, FXeNC, ClXeCN, BrXeCN, and BrXeNC) were identified by low-temperature matrix isolation techniques combined with quantum chemical calculations,25,26 which indicate that it is reasonable for NC/CN to bind to the metal centers. Therefore, we have systematically investigated the possible products of the reaction of XCN/XNC with uranium atoms: XUCN and XUNC (X = halogen) molecules. The focuses of our attention are whether XUCN and XUNC are stable enough to be prepared and identified experimentally, whether XUNC is more stable than its analogous XUCN, and what the fundamental causes are of such differences. Furthermore, we hope that this research will be helpful for the preparation of XUCN and XUNC molecules in cryogenic noble-gas matrices. This article is organized as follows. Section 2 provides an overview of the theories used in the calculations. The results are presented in section 3. A short summary is given in section 4.

2. THEORETICAL CALCULATIONS The structural, energetic, and spectroscopic properties of both XUCN and XUNC (X = F, Cl, Br, I) molecules were studied by density functional theory and ab initio methods using the ORCA program.27 To consider relativistic effects, scalar relativistic all-electron calculations can be performed with the zeroth-order regular approximation (ZORA),28,29 and the spin−orbit coupling (SOC) effect was considered simultaneously.30,31 The recontracted scalar relativistic all-electron basis sets def2-TZVPP32,33 were used for all atoms. It should be noted that the def2-TZVPP basis sets for the U atom are automatically replaced by the segmented all-electron relativistically contracted (SARC) basis sets32 in ORCA, which have been especially developed for scalar relativistic calculations and are individually adapted to the ZORA Hamiltonian. On the basis of the UKS reference determinant, the B2PLYP double-hybrid density functional method34 was used to optimize the structures of the molecules under investigation. In order to generate one valid initial wave function, the stability of the Kohn−Sham wave function for each molecule was tested prior to the structural optimization calculations, and the wave function was reoptimized if it had internal instabilities. On the basis of the optimized wave function, each molecule was first

3. RESULTS AND DISCUSSION 3.1. Structures. The selected structural parameters of XUCN and XUNC (X = halogen) calculated at the B2PLYP/ (C, N, X: def2-TZVPP; U: SARC) level are given in Table 1, and the expectation values of the total spin (S2) for B2PLYP and CCSD(T) calculations are also presented in Table 1. For comparison, all molecules were also optimized at the MP2(full)/(C, N, F, Cl, Br: aug-cc-pVTZ; I, U: SDD) level, and the results are presented in Table S1 in the Supporting Information. It was found that the optimized geometries obtained using the B2PLYP method are essentially consistent with the MP2(full) method. For each XUCN/XUNC (X = halogen) molecule, all geometries with possible electronic spin multiplicities have been optimized, and the quintet was finally confirmed to be the ground state. The expectation values of the total spin (S2) for B2PLYP and CCSD(T) calculations given in Table 1 show that the spin contamination is small and negligible. All molecules are planar structures with C s symmetry. As shown in Table 1, the C−N bond in XUCN (or XUNC) is elongated in comparison with that in the XCN (or XNC) precursor, which indicates that the insertion of the uranium atom favors activation of the C−N bond. Moreover, B


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Inorganic Chemistry

Table 2. Fragmentation Energies (kcal mol−1) of XUCN and XUNC (X = Halogen) along Different Pathways using B2PLYP and CCSD(T) Methods with All-Electron Relativistic Basis Sets B2PLYPa FUCN ClUCN BrUCN IUCN FUNC ClUNC BrUNC IUNC

CCSD(T)b

ΔE1

ΔE2

ΔE3

ΔE4

139.0 121.6 120.3 109.5 214.2 170.2 162.1 144.6

261.3 223.9 211.4 195.8 265.7 228.7 216.2 200.6

158.2 160.8 159.2 160.4 162.6 165.6 164.0 165.2

234.6 187.0 174.3 167.9 230.3 183.1 170.3 164.0

F Cl Br I

ΔE2

ΔE3

ΔE4

113.8 235.7 102.3 203.9 101.2 191.5 86.6 177.6 189.5 240.8 151.6 209.5 144.3 197.4 123.7 184.0 ΔE(XUCN−XUNC)c

ΔE1

149.3 144.0 142.6 139.2 154.4 149.6 148.5 145.6

226.6 183.7 170.6 172.0 225.9 183.6 170.8 172.7

B2PLYPa

CCSD(T)b

4.4 4.7 4.7 4.8

5.1 5.6 5.9 6.4

ZPE corrections were made for the B2PLYP energies. bNo ZPE corrections were considered in the CCSD(T) energies. cΔE = E(XUCN) − E(XUNC). a

consuming. To further investigate the thermodynamic stabilities of these molecules, the fragmentation energies associated with four possible fragmentation pathways were taken into account:

the C−N bond in XUCN is shorter than that in its analogue XUNC, which indicates that the C−N bond in XUCN may be stronger than that in XUNC. Likewise, the X−U bond in XUCN should be slightly stronger than that in its analogue XUNC due to the shorter bond length. However, although the RX−U value in XUCN (or XUNC) is elongated remarkably as X varies from F to I, the strengths of X−U bonds involving different halogen atoms are not directly comparable by the RX−U value due to the different radii of the halogen atoms. Similarly, the strengths of U−C bonds in XUCN are not directly comparable with those of U−N bonds in XUNC as well. Although the RU−C value of about 2.4 Å in XUCN (X = halogen) is longer than the RU−N value (about 2.3 Å) in XUNC, this does not mean that the U−C bonds in XUCN must be weaker than the corresponding U−N bonds in XUNC because the radius of the carbon atom is larger than that of the nitrogen atom. In order to evaluate A−B bonds involving different atoms, the structural parameter δRA−B is defined to unify interactions to estimate their strengths even if different pairs of atoms are used:42,43 δRA − B = RA − B − RA − RB

ΔE1 = E(U) + E(XCN) − E(XUCN) or ΔE1 = E(U) + E(XNC) − E(XUNC)

(2)

ΔE2 = E(X) + E(U) + E(CN) − E(XUCN) or ΔE2 = E(X) + E(U) + E(NC) − E(XUNC) (3)

ΔE3 = E(XU +) + E(CN−) − E(XUCN) or ΔE3 = E(XU +) + E(NC−) − E(XUNC)

(4)

ΔE4 = E(X−) + E(UCN+) − E(XUCN) or ΔE4 = E(X−) + E(UNC+) − E(XUNC)

(5)

Paths 2 and 3 are the two- and three-body dissociation pathways, respectively, while paths 4 and 5are the ionization dissociation pathways. Fragmentation energies and the energy differences between XUCN and XUNC are given in Table 2. As shown in Table 2, for the fragmentation energies along with different fragmentation pathways, the B2PLYP results give trends similar to those of CCSD(T); especially for the case of ΔE4, the B2PLYP results for most of the molecules are very close to those of CCSD(T). The differences of other fragmentation energies between B2PLYP and CCSD(T) are less than about 30 kcal mol−1, which is attributed to the overestimation of B2PLYP. The CCSD(T) method is usually regarded as the “golden critical”, and previous studies have shown that the CCSD(T) method gives the most accurate values for the HCN → H + CN dissociation energy in comparison to the B3LYP and MP2 methods.45 Therefore, the following discussions are based on the CCSD(T) results. Although XCN is more stable than XNC, XUNC is more stable than the analogous XUCN, similar to the cases of U + HCN/CH3CN, among which the primary products of interest were the isocyanides HUNC and CH3UNC, respectively.16,19

(1)

where RA−B is the A−B bond length and RA and RB are the single-bond covalent radii of A and B atoms,44 respectively. The smaller δRA−B is, the stronger the interaction is, and vice versa. As shown in Table 1, for both XUCN and XUNC molecules, the X−U bond is weakened as X varies F to I due to the increasing δRX−U value. The δRU−N value of XUNC is smaller than the δRU−C values of XUCN, which indicates that the U−N bond in XUNC should be stronger than the U−C bond in XUCN. Since the covalent radii in ref 44 are not designed specifically for M−X bonds between transition metals M and halides X, it should be noted that δR just provides a rough estimation of the strengths of bonds (X−U, U−C, and U−N); analyses of the nature of these bonds will be given later. 3.2. Energies. On the basis of the optimized structures, the single-point energy of each molecule was calculated using the B2PLYP and CCSD(T) methods with the same all-electron scalar relativistic basis sets, in which ZPVE correction was not considered by the CCSD(T) method, since it is very timeC


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Inorganic Chemistry

Table 3. Selected Vibrational Frequencies (Intensities) of XUCN, XUNC (X = Halogen), and the Relevant Precursors Calculated at the B2PLYP/(C, N, X: def2-TZVPP; U: SARC) Levela FUCN ClUCN BrUCN IUCN FUNC ClUNC BrUNC IUNC FCN ClCN BrCN ICN FNC ClNC BrNC INC

νC−N

ΔνC−Nb

2143.6 (25) 2140.1 (31) 2134.3 (35) 2137.3 (35) 2052.3 (446) 2045.6 (487) 2043.5 (503) 2041.6 (522) 2349.1 (137) 2237.6 (52) 2219.2(28) 2206.7(13) 2175.4(29) 2114.6(142) 2104.8(196) 2103.9(244)

−205.5 −97.5 −84.9 −69.4 −123.1 −69.0 −61.3 −62.3

νX−U 564.6 321.1 215.8 172.9 560.4 318.4 209.7 166.3

(133) (93) (29) (17) (132) (68) (31) (22)

νU−C 321.7 331.5 319.1 328.0

νU−N

(63) (30) (66) (73) 369.2 377.6 377.6 387.9

(96) (92) (104) (110)

a Frequencies are in cm−1, and intensities (in parentheses) are in km mol−1. bΔνC−N is the difference in the νC−N values between XUCN (or XUNC) and XCN (or XNC).

3.3. Frequencies. IR spectroscopy is a powerful tool to characterize active species in matrix isolation experiments because the vibrational fingerprints can provide important information for molecular identification and structural analysis. The calculated vibrational frequencies of XUCN, XUNC (X = halogens), and the relevant precursors are presented in Table 3. As shown in Table 3, the C−N bonds in both XCN and XNC (X = halogen) are typical triple bonds, and the stretching vibrational frequencies (νC−N) are about 2100−2350 cm−1; moreover, the νC−N values of XCN are higher than those of XNC. Likewise, the calculated νC−N values of XUCN are within the range 2137−2144 cm−1, which is obviously higher than that of XUNC. The calculated νC−N values of XUNC are within the range 2041−2053 cm−1 and are very close to the νC−N values of HUNC (2047.1 cm−1) and CH3UNC (2050.1 cm−1), which were calculated at the same level (see Table S2 in the Supporting Information for the harmonic vibrational frequencies of HUNC and CH3UNC, and their optimized structures are also given in Figure S1 in the Supporting Information), and previous works showed that the νC−N values of HUNC and CH3UNC are 2028 and 2029.7 cm−1 in solid argon matrices, respectively;16,19 therefore, the νC−N values of XUNC are expected to be nearby. The differences in νC−N values between XUCN and XUNC can be used to identify these molecules in matrix isolation infrared spectroscopy experiments. As X varies from F to I, the νC−N values of XUCN (or XUNC) change slightly. Moreover, the νC−N values of XUCN (or XUNC) are smaller than that of XCN (or XNC) and exhibit red-shift character, and such a red shift corresponds to the elongation and the weakening of the C−N bond, which is consistent with the structural results mentioned above. The largest red shifts of νC−N in XUCN (or XUNC) are for FUCN and FUNC, respectively; moreover, the degree of the red shift decreases as X varies from F to I. The calculated νX−U values of XUCN are close to those of XUNC, which demonstrates that the X−U bonds in both XUCN and XUNC seem to have similar strengths. Furthermore, the νX−U values of XUCN (or XUNC) decrease as X varies from F to I, which is the comprehensive result of the increasing reduced mass and weakening of the X−U bond. The

The energy differences between XUCN and XUNC calculated at the CCSD(T) level fall in the range of 5.1−6.4 kcal mol−1, which is close to that (6.3 kcal mol−1) between HUCN and HUNC calculated at the B3LYP level. Considering that HUCN and CH3UCN have not been observed experimentally, XUNC is expected to be the primary product in the reaction of uranium with XCN and has a greater chance of being prepared and identified experimentally. As shown in Table 2, the fragmentation of XUCN (or XUNC, X = halogen) along path 2 is highly endothermic; therefore, XUCN (or XUNC) is thermodynamically stable with respect to the two-body dissociation (U + XCN/XNC). Similarly, both XUCN and XUNC are also thermodynamically stable with respect to the three-body dissociation (U + X + CN) due to the positive ΔE2 value. Moreover, as X varies from F to I, the thermodynamic stability of XUCN (or XUNC) decreases due to the decreasing ΔE1 as well as ΔE2; therefore, FUNC is expected to be the most stable thermodynamically, while IUCN has the least thermodynamic stability. The ΔE1 value of XUNC is significantly larger than that of XUCN, which is the result of the comprehensive effects of two factors: the greater stability of XCN in comparison with that of XNC and the lesser stability of XUCN in comparison with that of XUNC. In addition, for both XUCN and XUNC, ΔE2 is remarkably larger than ΔE1 because the dissociation of XCN (or XNC) into X + CN requires substantial energy. As shown in Table 2, the fragmentations of XUCN (or XUNC, X = halogen) along ionization paths 4 and 5 are endothermic as well. Moreover, the bond energies of U−C (or U−N) and X−U bonds can be estimated approximately by ΔE3 and ΔE4. The strength of the X−U bond in XUCN (or XUNC) is weakened significantly as X varies from F to I due to decreasing ΔE4, which is consistent with the structural results mentioned above, whereas the small change of ΔE3 of XUCN (or XUNC) indicates that the strength of the U−C (or U−N) bond in XUCN (or XUNC) is hardly influenced by the halogen atom substituents. Another noteworthy case is that the ΔE4 value of XUCN (or XUNC) is noticeably larger than the ΔE3 value, which demonstrates that the cleavage of the U−C (or U−N) bond is easier than that of the X−U bond. D


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Inorganic Chemistry νF−U values of both FUCN and FUNC are the two largest vibrational frequencies among all species studied here, whereas the rest of the νX−U values of other XUCN and XUNC molecules are less than about 320 cm−1, which are beyond the limit of the infrared spectrum. Similar things also happened in the νU−C values of XUCN and νU−N values of XUNC; the calculated νU−C values of XUCN are about 319−332 cm−1 and are smaller than the νU−N values (ca. 369−388 cm−1) of XUNC. Both νU−C and νU−N values are less than 400 cm−1 and can hardly be detected by an infrared spectrum. Therefore, other technologies are required to identify these molecules. 3.4. AIM Analyses. AIM analyses have been carried out to explore the nature of bonds in XUCN and XUNC (X = halogens), and the results are presented in Table 4; Figure 1

concentration along the X−U bond line direction, while the charge of the U atom is depleted toward the X atom. Therefore, it is clear that both the X−U and U−C (or U−N) bonds have closed-shell interaction characters, and there is charge transfer (CT) from the U atom to the X atom and CN group. We note that the “closed-shell” (and “open-shell” below) term refers to a chemical bond rather than a molecule (the same below). In addition, the fact that the X atom in all studied molecules has a circular-shaped area of charge concentration suggests that additional electronic charge has been donated, yielding an X− anion. The analyses of the BCPs of the relevant bonds presented in Table 4 can provide more useful information on the nature of bonding in both XUCN and XUNC (X = halogen). Some descriptors at BCPs have been used widely to characterize the bonding interaction between the atoms, such as the electron density (ρ(r)), the Laplacian of electron density (∇2ρ(r)), the total energy density (H(r)), and the absolute ratio of potential and kinetic energy densities (|V(r)|/G(r)).46 It has been established that the values of both H(r) and ∇2ρ(r) at the BCP allow the interaction to be characterized:46−49 H(r) < 0 and ∇2ρ(r) < 0 indicate an accumulation of charge density at the BCP and therefore a covalent interaction between the interacting atoms; H(r) > 0 and ∇2ρ(r) > 0 indicate a depletion of charge density at BCP and therefore a closed-shell interaction between the interacting atoms; H(r) < 0 but ∇2ρ(r) > 0 is indicative of partially covalent interactions. The |V(r)|/ G(r) ratio was proposed such that this quantity at the BCP can also be used to discriminate interaction types: |V(r)|/G(r) < 1 corresponds to a pure closed-shell interaction and |V(r)|/G(r) > 2 corresponds to a pure covalent (open-shell) interaction, while 1 < |V(r)|/G(r) < 2 corresponds to an intermediate interaction.46 As shown in Table 4, both H(r) and ∇2ρ(r) at the C−N BCPs in both XUCN and XUNC (X = halogens) have negative values; therefore, the C−N bond is a typical covalent bond, which is further confirmed by the |V(r)|/G(r) ratio descriptor since the |V(r)|/G(r) ratio at the C−N BCPs is more than 2.0. For the BCPs of X−U, U−C, and U−N bonds, H(r) values are negative and ∇2ρ(r) values are positive; therefore, these bonds exhibit partially covalent characters; moreover, the |V(r)|/G(r) ratios at their BCPs are about 1.2−1.3, which further supports the statement. As shown in Table 4, the F−U bonds in both FUCN and FUNC are special cases; their ρ(r) and ∇2ρ(r) values are remarkably larger than those for other X−U (X = Cl, Br, I) bonds, and their |V(r)|/G(r) ratios are about 1.28 and are smaller than those of other X−U bonds, which illustrates that the F−U bond exhibits less covalent character than other X−U bonds. The reason for this is that the largest electronegativity of F atom among halogen atoms leads to the stronger polarity characteristics of the F−U bond. As X varies from F to I, the ρ(r) and ∇2ρ(r) values at the X−U BCPs decrease, while the | V(r)|/G(r) ratios increase; therefore, the covalent character (closed-shell interaction) of the X−U bonds increases along with the decrease in electronegativity of the X atoms. 3.5. Bond Order and CDA. To understand the bonding interactions in XUCN and XUNC (X = halogen), the fuzzy bond orders (FBOs)35,36 were calculated using the B2PLYP method with all-electron scalar relativistic basis sets. The calculated results are given in Table 5. As shown in Table 5, in comparison with the relevant XUN (or XNC) precursor, the C−N bond in XUCN (or XUNC) is weakened due to its smaller FBO. Moreover, the FBO of the C−N bond in XUCN

Table 4. AIM Results of XUCN and XUNC (X = F, Cl, Br, I) Carried Out using the B2PLYP Method with All-Electron Relativistic Basis Setsa molecule

bond

ρ(r)

∇2ρ(r)

|V(r)|/G(r)

H(r)

FUCN

F−U U−C C−N Cl−U U−C C−N Br−U U−C C−N I−U U−C C−N F−U U−N N−C Cl−U U−N N−C Br−U U−N N−C I−U U−N N−C

0.134 0.075 0.491 0.083 0.076 0.491 0.071 0.077 0.490 0.058 0.077 0.490 0.134 0.086 0.475 0.083 0.088 0.475 0.070 0.089 0.474 0.058 0.090 0.474

0.509 0.163 −0.200 0.187 0.163 −0.201 0.133 0.162 −0.203 0.084 0.164 −0.202 0.507 0.280 −0.639 0.184 0.282 −0.652 0.134 0.283 −0.655 0.081 0.285 −0.659

1.283 1.307 2.054 1.331 1.317 2.055 1.348 1.323 2.056 1.404 1.324 2.055 1.283 1.217 2.206 1.335 1.228 2.211 1.338 1.231 2.213 1.410 1.233 2.215

−0.050 −0.018 −0.967 −0.023 −0.019 −0.967 −0.018 −0.019 −0.966 −0.014 −0.020 −0.966 −0.050 −0.019 −0.935 −0.023 −0.021 −0.933 −0.017 −0.021 −0.933 −0.014 −0.022 −0.932

CLUCN

BrUCN

IUCN

FUNC

ClUNC

BrUNC

IUNC

a

The recontracted scalar relativistic def2-TZVPP basis sets were used for all atoms. Note that, for the U atom, the def2-TZVPP basis sets are automatically replaced by the segmented all-electron relativistically contracted (SARC) basis sets.

gives the contour line diagrams of the Laplacian of electron density (∇2ρ(r)) for both XUCN and XUNC. According to the AIM theory, the existence of BCP is the first descriptor for an interatomic interaction. As shown in Figure 1, it is clear that BCPs are located at the crossing points of the bond paths and zero-flux surfaces. For both XUCN and XUNC, the C−N bond has an area of charge concentration and the BCP is at the boundary of the valence shell region of the carbon atom, which illustrates that the C−N bond is a typical polar covalent bond. The C atom in XUCN (or N atom in XUNC) has an area of charge concentration toward the U atom, while the U atom has an area of charge depletion along the U−C (or U−N) bond line direction. Similar things also happen in the X−U bond in all studied molecules; the X atom has an area of charge E


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Inorganic Chemistry

Figure 1. Contour line diagrams of ∇2ρ(r) for both XUCN and XUNC (X = halogen), obtained by the B2PLYP method with all-electron relativistic basis sets. Dashed lines indicate areas of charge concentration (∇2ρ(r) < 0), while solid lines show areas of charge depletion (∇2ρ(r) > 0). The bold brown solid lines connecting the atomic nuclei are the bond paths, and the solid blue lines separating the atomic nuclei indicate the zero-flux surfaces in the molecular plane. The crossing points of the bond paths and zero-flux surfaces are the bond critical points (BCP).

is expected to be weaker than the U−N bond in XUNC. In addition, the smaller FBO of the F−U bonds among X−U bonds indicates that the F−U bonds seem to be weaker than other X−U (X = Cl, Br, I) bonds, which is in contrast with the structural (and energy) results of δRX−U mentioned above, since a smaller δRF−U implies a stronger binding interaction, and one plausible explanation is that the FBO is not a perfect descriptor of such a high-polarity bond.50 The CDA is a useful tool for analyzing the interactions between molecular fragments on a quantitative basis. X−, U2+, and CN− (or NC−) were defined as three fragments, which is a reasonable choice because the X−U and U−C(N) bonds mainly exhibit ionic characters. The CDA algorithm is used to calculate three terms, the amount of electron donation (d), the amount of electron back-donation (b), and repulsive polarization (r). The term d − b can be viewed as the number of net transferred electrons from the electron donator to the acceptor. Therefore, the CDA indicates the direction as well as extent of electron flow within the studied molecules. The CDA results are presented in Table 6. As shown in Table 6, it is observed that there occurs an electron flow from the halogens and the cyanide (or isocyanide) to the uranium atom. The net electron

Table 5. Fuzzy Bond Orders (FBOs) of XUCN and XUNC (X = F, Cl, Br, I) Calculated using the B2PLYP Method with All-Electron Relativistic Basis Sets molecule

X−U

U−C(N)

C−N

precursor

X−C(N)

C−N

FUCN ClUCN BrUCN IUCN FUNC ClUNC BrUNC IUNC

1.866 1.960 2.000 2.008 1.857 1.947 1.989 1.989

1.155 1.163 1.165 1.168 1.372 1.389 1.395 1.401

2.760 2.752 2.752 2.747 2.467 2.451 2.446 2.439

FCN ClCN BrCN ICN FNC ClNC BrNC INC

1.450 1.434 1.398 1.345 1.458 1.509 1.449 1.401

2.843 2.778 2.791 2.789 2.592 2.499 2.519 2.522

is about 2.8 and is obviously larger than that (ca. 2.5) of the C− N bond in XUNC, which also reveals that the C−N bond is effectively activated when the nitrogen atom of the isocyanide is coordinated to the U atom. The FBO of the X−U bonds in XUCN is slightly larger than that of the X−U bonds in XUNC; therefore, the X−U bonds in XUCN are stronger than those in their XUNC analogues, which is in line with the results above. Moreover, the U−C bond in XUCN has a smaller FBO than the U−N bond in XUNC; therefore, the U−C bond in XUCN F


Article

Inorganic Chemistry

Table 6. Charge Decomposition Analysis (CDA) Results for XUCN and XUNC Obtained using the B2PLYP Method with AllElectron Relativistic Basis Setsa FUCN d b d−b r

ClUCN

a

IUCN

F→U

CN → U

Cl → U

CN → U

Br → U

CN → U

I→U

CN → U

0.2017 −0.0362 0.2378 −0.2544

0.1137 −0.0168 0.1305 0.0565

0.2707 −0.0136 0.2844 −0.1717

0.0945 0.0081 0.0863 −0.0755

0.2538 −0.0060 0.2597 −0.1653

0.0963 0.0064 0.0899 −0.0763

0.3717 −0.0026 0.3744 0.0243

0.0981 0.0426 0.0556 −0.0891

F→U

NC → U

Cl → U

NC → U

Br → U

NC → U

I→U

NC → U

0.2062 −0.0427 0.2489 −0.2624

0.1538 −0.0121 0.1658 −0.2060

0.2661 −0.0162 0.2823 −0.1715

0.1533 −0.0062 0.1595 −0.2024

0.2484 −0.0099 0.2583 −0.1610

0.1516 −0.0040 0.1556 −0.1941

0.3854 0.0130 0.3724 −0.0356

0.1279 −0.0121 0.1400 −0.1862

FUNC d b d−b r

BrUCN

ClUNC

BrUNC

IUNC

Abbreviations: d, donation; b, back-donation; d − b, charge transfer; r, repulsion.

transfer term d − b of X → U is larger significantly than that of CN (or NC) → U, which indicates that the halogens play a key role in electron donation in the studied molecules. The d − b term of X → U in XUCN is close to that of XUNC, which indicates that the X−U bonds in both XUCN are XUNC are similar. However, the d term of NC → U in XUNC is obviously larger than that of CN → U in XUCN, while the absolute value of the b term of NC → U in XUNC is smaller than that of CN → U in XUCN; therefore, the comprehensive effect of the two factors lead to the net electron transfer term d − b of NC → U in XUNC being remarkably larger than that of CN → U in XUCN. Therefore, the isocyanide exhibits better donation performance than the cyanide, which results in the U−N bond in XUNC being stronger than the relevant U−C bond in XUCN. XUNC demonstrates more thermodynamic stability than its analogue XUCN, which is in line with previous works of Pyykkö and co-workers.23

(4) AIM analyses show that both X−U and U−C(N) bonds mainly exhibit closed-shell interaction (ionic bond) character, although partial covalent character contributes to them, and can be denoted as X−U2+(CN)− and X−U2+(NC)−, respectively. (5) The FBO calculations show that the U−N bond in XUNC is stronger than the corresponding U−C bond in XUCN. CDA further reveals that the better donation performance of the isocyanide over the cyanide results in a stronger U−N bond in XUNC, which seems to be the root cause of XUNC being more thermodynamically stable than its XUCN analogue.

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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.6b01345. Structures and frequencies of HUNC and CH3UC obtained at the B2PLYP/(C, N, H: def2-TZVPP; U: SARC) level, and the geometries of XUNC/XUCN (X = halogen) obtained at the MP2(full)/(C, N, F, Cl, Br: aug-cc-pVTZ; I, U: SDD) level (PDF)

4. CONCLUSIONS Both XUCN and XUNC (X = halogen) have been investigated by DFT and ab initio calculations. Equilibrium geometries, harmonic vibrational frequencies, and energies were calculated, and AIM and CDA calculations were also performed to understand the bonding interactions in the studied molecules. (1) The ground states of all molecules are quintets, and all molecules are planar structures with Cs symmetry. (2) Both XUCN and XUNC are stable enough thermodynamically with respect to three-body and two-body dissociations, and XUNC molecules are more stable than their XUCN analogues, which is attributed to the U−N bond in XUNC being shorter and stronger than the U−C bond in their XUCN analogues. FUNC is predicted to the most stable, while IUCN has the lowest thermodynamic stability. Not only the stabilities of the studied molecules but also the X−U bond dissociation energies are affected significantly by the halogen atom substituents. (3) The main vibrational fingerprint of XUCN/XUNC is the C−N stretching vibrational mode, which exhibits red-shift character in both XUCN and XUNC. Such behaviors of νC−N can be used to identify these molecules in matrix isolation infrared spectroscopy experiments. Except for the ν F−U stretching vibrational frequency, all νX−U, νU−C, and νU−N frequencies in the studied molecules are less than 400 cm−1 and can hardly be detected by an infrared spectrum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.H. Huang: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Tianjin (No. 12JCYBJC13400) and the Program for Innovative Research Team in University of Tianjin (TD12-5038).

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REFERENCES

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Isocyanide

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