Platinum Complexes of

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

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Heterobimetallic Uranium−Nickel/Palladium/Platinum Complexes of Phosphinoaryl Oxide Ligands: A Theoretical Probe for Metal− Metal Bonding and Electronic Spectroscopy Yan-Ting Bi,† Li Li,† Yuan-Ru Guo,‡ and Qing-Jiang Pan*,† †

Inorg. Chem. Downloaded from pubs.acs.org by STOCKHOLM UNIV on 01/06/19. For personal use only.

Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education), School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China ‡ Key Laboratory of Bio-based Material Science & Technology (Ministry of Education), College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China S Supporting Information *

ABSTRACT: Heterobimetallic uranium−transition metal (U−TM) complexes have abundant active centers (two metals and several ancillary ligands with various donor atoms) and possible metal−metal bonding interaction, leading to diversified electronic structures and rather complicated electronic transition types. In this regard, a comprehensive and systematic theoretical study is highly desired although challenging. In the work, density functional theory (DFT) was utilized to examine a series of uranium−group 10 metal complexes supported by bidentate phosphinoaryl oxide ligands (labeled as L). TM (Ni, Pd, and Pt), uranium oxidation state (IV and III) and axial donor (I, Br, Cl, F, Me3SiO, and vacant) were varied. Calculations demonstrate an intrinsic TM → U dative bond. The order of bond strength of U−Ni > U−Pt > U−Pd is suggested by the formal shortness ratios, quantum theory of atoms in molecule (QTAIM) data, interaction energy (Eint), and bond orders calculated at various levels of theory. It is further evidenced by relativistic effects of heavy metal, natural orbital population and electronic spectroscopy. Regarding U−Ni complexes with different axial donors, metal−metal distances are found to be linearly correlated with QTAIM data/Eint/bond orders. Experimental UV−vis−NIR spectra were well reproduced by time-dependent DFT calculations. Complicated visible-light absorption bands, whose understanding remains unclear for many experimentally known heterobimetallic complexes, were rationalized in the work, along with NIR bands assigned as 5f → 5f transitions.

1. INTRODUCTION 1

With the assistance of 2-tert-butyl-4-methyl-6(diphenylphosphino)phenolate (labeled as LE in Chart 1), complete group 10 metals were successfully incorporated by Arnold and co-workers into a uranium(IV) precursor to yield (IUIV−TM0)(LE)3 (TM = Ni, Pd, and Pt).20 Subsequent

2

Since thorium−transition metal (nickel, platinum, and ruthenium)3 complexes were synthesized three decades ago, actinide−metal complexes have flourished owing to advances in synthetic techniques and massive efforts of experimentalists.4−31 Low-valent uranium, including stable UIV and reducing UIII, has been paid the most attention. Ligated uranium was found to weakly couple with a lot of main group metal (MM) such as group 13 (aluminum4,5 and gallium),5,6 group 14 (germanium7 and tin),8,9 and group 15 (antimony and bismuth).10 The key in the synthetic route is to construct a positively charged ligated U fragment and negatively charged ligated MM moiety,11−15 so the reaction of forming U−MM complex is driven by electrostatic attraction. The resultant complex is further stabilized by the created metal−metal bonding. Promoted by salt/alkane/amine elimination,16,17 chelation/self-assembly18 and photochemical19 routes, plenty of uranium−transition metal (U−TM) complexes were synthesized such as Ni,15,20,21 Pd,20 Pt,20 Cu,21,22 Ag,23 Co,15,18,19,24 Rh,25,26 Fe,27−32 Ru,32,33 and Re.16,17,34 © XXXX American Chemical Society

Chart 1. Experimentally Synthesized Heterobimetallic Uranium−Group 10 Metal Complexes20

Received: September 30, 2018

A

DOI: 10.1021/acs.inorgchem.8b02787 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

2. COMPUTATIONAL APPROACHES AND SCOPE OF CURRENT STUDY Structures of a series of U−TM complexes were fully optimized in the gas phase with Gaussian (version 2009).36 Relativistic Stuttgart small-core effective core potentials (RSCECPs) 37−39 were used for U and Ni together with corresponding basis sets. Associated with Hay−Wadt pseudopotentials, LANL08d was used for I, and 6-31G** for P, O, C, and H. The GGA-PBE functional was applied. Additionally, electron-spin density and charges of natural bond orbital (NBO) and Mulliken were obtained. On the basis of the PBE calculations, quantum theory of atoms in molecule (QTAIM)40,41 analyses at bond critical points (BCP) were performed using the Multiwfn 3.3.3 code.42 Electron density ρ(r), Laplacian electron density ∇2 ρ(r), energy density H(r), ellipticity ε, and delocalization index δ(U, TM/X) were obtained. H(r) was further discomposed into kinetic G(r) and potential V(r) terms. Among these, the unit of density parameters is “a.u.”. In addition, interaction energy Eint (in kcal/mol) of U−TM/X bonds was computed according to an empirical equation Eint = 0.5 × V(r).43−45 To explore electronic spectroscopy, 40 dipole-allowed singlet excited states were calculated using time-dependent DFT (TD-DFT)46−48 in the Gaussian program. One U−Ni complex, for instance, was even calculated for two hundred singlet excited states. The polarizable continuum model (PCM)49−52 was applied to simulate environmental effects of toluene. The dielectric constant of 2.37 was used. For comparison, Priroda (version 6)53−56 and ADF (version 2014)57−59 programs were also used to calculate molecular properties. A scalar relativistic all-electron (AE)60 approach was applied in Priroda, together with the PBE functional and all-electron correlation-consistent basis sets. We used an integration parameter of 6.0 in ADF calculations. Scalar relativistic ZORA61−63 Hamiltonian, PBE functional, and Slater-type TZP basis sets were employed. Thus, various bond orders were calculated at the above levels of theory. In this work, we first tested the performance of various theoretical approaches, ligand models and electron-spin states. Please see discussion and related results in Tables S1−S6, Chart S1, and Figure S1. Unless otherwise noted, the approach (Gau: PBE/B−I/ECPs/gas), the 2-methyl-6(dimethylphosphino)phenolate (labeled as L), and the highest-spin state of each complex were exploited throughout the work. Nine U−TM complexes were intensely computed. Their formulas and abbreviations are presented in Table 1, along with experimental ones.20 The TM center was changed across group 10 from Ni to Pd and Pt. Variation of the uranium oxidation state was made from IV to III, which is expected to provide some implication for experimental electrochemical reduction;20 however, the formed complex [(IUIII−Ni0)(L)3]− (UIII−Ni) may be regarded as the lower limit of metal−metal bond strength among various U−Ni-type complexes (see the detailed discussion in the text; “Results and Discussion” section). More systematic changes were carried out on donor atoms in the U−TM axial direction, including I, Br, Cl, F, and Me3SiO. Comparison was further made with (UIV−Ni0)(L)3 (Vac.U−Ni), which leaves the axial donor site vacant and is supposed to be a upper limit of U−Ni bond strength. Notably, analogues of U−TM (Ni, Pd, and Pt) and XU−Ni (X = F and OSiMe3) were experimentally synthesized and characterized,20

iodide substitution in the metal−metal axial direction afforded (XUIV−Ni0)(LE)3 (X = F and Me3SiO).20 5f2nd10 electronic configurations were attributed; the metal−metal bonding was recognized by various characterizations even though bridging LE ligands evidently had some constraints. U−Ni and U−Pd complexes were investigated by density functional theory (DFT) for electronic structures and U−TM bonding properties. Of great importance is that the order of metal−metal bond strength was concluded as (Me3SiO)U−Ni > FU−Ni > IU−Ni > IU−Pd > IU−Pt from electrochemical measurements. Regarding the above systems20 and previously reported actinide−transition metal (An−TM) complexes,15−34 seeking the regularity of electronic spectroscopy remains greatly challenging. First, these spectra involve electronic transitions between ground state and excited states, and the latter is difficultly traced by experimental techniques and simulated by theoretical computation. Second, the special structural feature of An−TM complexes makes the study rather complicated. Heterobimetallic U group 10 TM systems, for instance, contain many active centers like two metals, one axial donor, and three bidentate ligands; f-block U and d-block TM prefer entirely different coordinations of oxygen and phosphorus atoms. The bridging aryl group in LE can actively accept or release electron(s); moreover, U−TM bonding interaction is most likely present. Together, these result in too many possibilities in electronic transitions, which leaves unclear interpretation for UV−vis−NIR spectra in most experimental studies. In contrast, the U−TM bonding nature still requires in-depth understanding. Particularly, their relative bond strength is open to debate for systematic and regular complexes are extremely limited by synthetic difficulties and product stability. In this respect, it is desirable to systematically study U−TM complexes in theory, especially offering a comprehensive interpretation and clear regularity of electronic spectroscopy. DFT and wave function theory [single-reference methods: perturbation theory, couple cluster, and configuration interaction, as well as multireference ones: complete active space self-consistent field (CASSCF) and CASPT2 are prevalent in studying molecular properties. To date, DFT and CASSCF/ CASPT2 have been applied to investigate structural, electronic, and metal−metal bonding properties of An−TM complexes.17−20,23−30,33−35 Studies of theoretically rationalizing electronic spectroscopy (UV−vis−NIR), however, remain rare. Regarding theoretical approaches, DFT proved reliable and accurate for not only giving comparable results to multireference calculations27,35 but also greatly saving computational cost that makes it feasible to study excited states of relatively large-size molecular systems. In this work, we utilized relativistic DFT to systematically examine a series of uranium−group 10 metal complexes. Aiming to provide a deep insight into U−TM bonding nature, explore the regularity of metal−metal bond strength and complement previous study,20 TM sort (Ni, Pd, and Pt), uranium oxidation state (IV and III), and axial donor (I, Br, Cl, F, Me3SiO, and vacant) were varied. Excited-state calculations were performed to rationalize experimental UV−vis−NIR spectra, particularly focusing on the complicated visible-light absorptions. B


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Inorganic Chemistry

experimentally unknown and theoretically designed herein, display close metal−metal distances. We also designed two extra U−Ni-type complexes in silico, Vac.U−Ni and UIII−Ni. Notably, these two complexes are anticipated to represent two extremes of uranium−nickel bond strength. Due to no axial donor coordination to uranium in Vac.U−Ni, valence electrons supposedly localized between uranium and donor atom would be to the largest extent shared between uranium and nickel; thus, Vac.U−Ni may have the strongest metal−metal bond strength among all U−Ni-type complexes studied in the work. UIII−Ni would show less driving force to form metal−metal bonding interaction because of having richer electrons around uranium. The reducing electron is assumed to enter a nonbonding 5f orbital of uranium and forms an antibonding orbital with respect to nickel. This would afford the weakest U−Ni bond. Above assumptions are evidenced by optimized U−Ni distances (Table 2), being the shortest 2.42 and longest 2.68 Å bond lengths for Vac.U−Ni and UIII−Ni, respectively. Notably, a 0.17 Å metal−metal elongation is found in UIII− Ni with respect to U−Ni (2.51 Å), which mainly originates from the difference of ionic radii between UIII and UIV due to the former being 0.15 Å longer. The antibonding interaction between two metal centers of UIII−Ni caused by the reducing electron has only a slight contribution (about 0.02 Å). This also results from the manner that the reducing electron is mainly localized on the uranium center (see section 3.1.2 below). Regarding complexes U−Pd and U−Pt, the computed metal−metal distances are 2.73 and 2.76 Å, a little longer than corresponding experimental values. Associated with U−Ni, the calculated bimetallic bond lengths show exactly the same trend as experimentally reported ones, i.e., U−Ni < U−Pd < U−Pt. It is known that bond length negatively correlates with bond strength in the most cases, so the order of above U−TM bond strengths will go a reverse way. To cancel the impact of different atom radius while comparing U−TM distances (also bond strength), the formal shortness ratio (FSRU−TM)64 was used (see its definition in footnotes of Table 2, where covalent atomic radii calculated by Pyykkö65 for uranium and transition metals were applied). Consequently, 0.90, 0.94, and 0.94 FSRU−TM values were given for optimized U−TM (TM = Ni, Pd, and Pt) distances, respectively. Applying this concept to experimentally measured distances gives FSRU−TM of 0.91, 0.93, and 0.92. We notice that U−Pd and U−Pt have the same FSRU−TM for our calculated distances, but the value of the former is larger regarding experimental results. As far as the U−TM bond lengths (more exactly the FSRU−TM) are concerned, U−Ni is undoubtedly the strongest bond. However, to distinguish the bonding strength between U−Pd and U−Pt, more evidence like bond orders, QTAIM data, and NBO analyses will give clear explanation below. In addition, when varying donor atom (X) in U−Ni-type complexes, optimized metal−metal distances change accordingly, which would give an order of U−Ni bond strength. 3.1.2. Electron-Spin Density and Charge. In Table 3, we list calculated electron-spin densities (S). SU ranges from 2.14 to 2.22 with the exception of that of UIII−Ni. Two paralleled 5f single electrons (α-spin) are denoted to reside in the uranium center, unraveling that uranium is tetravalent in nature. Accordingly, computed STM values are approximately equal to zero. This agrees with electronic configuration of zero-valent

Table 1. Formulas and Abbreviations of Complexes Together with Experimentally Known Analogues formulas

abbreviationsa

(IU −TM )(L)3 [(IUIII−Ni0)(L)3]− (XUIV−Ni0)(L)3 [(Me3SiO)UIV−Ni0](L)3 (UIV−Ni0)(L)3 (IUIV−TM0)(LE)3 (XUIV−Ni0)(LE)3

U−TM (TM = Ni, Pd, and Pt) UIII−Ni XU−Ni (X = Br, Cl, and F) SiOU−Ni Vac.U−Ni (U−TM)LE (TM = Ni, Pd, and Pt)b (XU−Ni)LE (X = F and Me3SiO)b

IV

0

a

For simplicity, the L symbol is omitted in abbreviations, while LE is kept. The same manner is for the oxidation state (actinide + IV and transition metal zero) and for the X ligand (iodine). bExperimentally synthesized complexes in ref 20.

which provide good support for our calculations and predictions.

3. RESULTS AND DISCUSSION 3.1. Structural Properties. 3.1.1. Geometry Parameters. Representative structures of U−TM complexes were illustrated in Figure 1. The uranium center was optimized to show a

Figure 1. Structures of (XUm−TM0)(L)3 (labeled as XUm−TM; TM = Ni, Pd, and Pt; X = I, Br, Cl, and F; and m = IV and III) (a), and [(Me3SiO)UIV−Ni](L)3 (marked as SiOU−Ni) (b). Noting that (UIV−Ni0)(L)3 (Vac.U−Ni) without the axial donor was not shown.

distorted trigonal bipyramidal configuration, which features one X donor and one TM in the axial direction and three oxo atoms in the equatorial plane. As seen in Table 2, X−U−TM (α1) angles were calculated to be approximately linear (>171°), which are comparable to reported values of experimentally known analogues;20 equatorial oxygen coordination is reflected by O−U−TM (α2) angles ranging from 89 to 96°. Angles of P−TM−U (α3) between 92 and 97° were calculated to be larger than a right angle. Twisted dihedral angles of O−U−TM−P (β) were calculated. Their deviation from the planar structure would facilitate heterobimetallic centers to approach each other and possibly produce metal− metal bonding. All optimized distances of U−X, U−O, and TM−P fall well within the range of experimental values. Short metal−metal bond lengths were calculated for these heterobimetallic complexes. U−Ni distances are 2.51, 2.49, and 2.50 Å for U−Ni, FU−Ni, and SiOU−Ni, respectively. They agree well with experimentally determined values of 2.53, 2.52, and 2.56 Å. Complexes BrU−Ni and ClU−Ni, which are C


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Inorganic Chemistry

Table 2. Optimized Geometry Parameters of Heterobimetallic Uranium−Transitional Metal Complexes Compared with Available Experimental Valuesa U−TMb

complexes calcd exp calcd exp calcd exp calcd calcd exp calcd calcd calcd exp calcd

U−Ni U−Pd U−Pt UIII−Ni FU−Ni ClU−Ni BrU−Ni SiOU−Ni Vac.U−Ni

2.514 2.534 2.732 2.690 2.762 2.708 2.679 2.488 2.520 2.506 2.497 2.497 2.556 2.421

(0.90) (0.91) (0.94) (0.93) (0.94) (0.92) (0.91)c (0.89) (0.90) (0.90) (0.89) (0.89) (0.91) (0.86)

U−Xb 3.020 3.010 3.020 3.001 3.027 3.011 3.123 2.051 2.091 2.603 2.786 2.077 2.093

(U−O)av

(TM−P)av

α1d

α2avd

α3avd

βavd

2.154 2.147 2.145 2.130 2.144 2.138 2.200 2.193 2.177 2.164 2.163 2.174 2.186 2.123

2.208 2.231 2.371 2.365 2.351 2.325 2.173 2.210 2.217 2.208 2.209 2.209 2.214 2.226

170.5 179.3 175.7 179.1 172.8 179.0 172.1 175.9 178.8 170.9 170.6 174.1 178.6

92.8 89.7 93.6 92.0 94.2 92.6 95.7 92.0 90.1 92.9 93.0 92.7 90.8

96.0 93.0 91.9 91.5 92.6 92.3 96.2 95.2 93.2 95.7 96.0 96.2 93.5 93.9

31.5 29.1 30.9 27.8 29.0 27.0 31.2 34.0 27.1 32.9 32.8 32.4 28.0 31.3

(1.00) (0.99) (1.00) (0.99) (1.00) (0.99) (0.98)c (0.88) (0.89) (0.97) (0.98) (0.89) (0.90)

a

Distances in angstrom and angles in degrees. Experimental values from ref 20. bThe formal shortness ratio (FSRU−A) listed in parentheses is defined as FSRU−A = DU−A/(RU + RA),64 where DU−A is the U−A bond length and the RU and RA values are the atomic radii of uranium and A (TM and X) from Pyykkö’s values65 (U = 1.70 Å, Ni = 1.10, Pd = 1.20, Pt = 1.23, O = 0.63, F = 0.64, Cl = 0.99, Br = 1.14, and I = 1.33). cBecause UIII is 0.15 Å longer in the ionic radius than UIV, a corrected one (1.85 Å) is used for UIII. dα1= X-U−TM, α2 = X−U−O, α3 = P−TM-U and β = O− U−TM-P

Table 3. Calculated Electron-Spin Density (S) and NBO Charge (Q) of Uranium−Transitional Metal Complexes Together with Natural Orbital Population of Metal Centers complexes

SU

STM

SX

S3L

QU

QTM

QX

Q3L

U

TM


2.223 2.196 2.191 2.819 2.154 2.187 2.194 2.139 2.223

−0.084 −0.037 −0.037 0.177 −0.072 −0.074 −0.075 −0.051 −0.093

−0.042 −0.043 −0.044 −0.033 −0.030 −0.034 −0.038 −0.039

−0.098 −0.115 −0.110 0.037 −0.051 −0.078 −0.082 −0.045 −0.130

1.179 1.261 1.241 1.283 1.442 1.245 1.196 1.456 1.616

−0.635 −0.597 −0.745 −0.896 −0.613 −0.617 −0.628 −0.622 −0.635

−0.262 −0.277 −0.279 −0.454 −0.417 −0.334 −0.287 −1.012a

−0.282 −0.387 −0.217 −0.932 −0.412 −0.293 −0.281 −0.822 0.019

5f3.036d1.387s0.227p0.20 5f2.996d1.347s0.227p0.21 5f3.006d1.337s0.227p0.22 5f3.156d1.157s0.247p0.17 5f3.046d1.187s0.167p0.19 5f3.066d1.317s0.197p0.21 5f3.056d1.357s0.207p0.21 5f3.046d1.207s0.167p0.18 5f2.966d1.177s0.187p0.13

3d9.384s0.504p0.78 4d9.465s0.495p0.68 5d9.326s0.656p0.80 3d9.464s0.504p0.95 3d9.364s0.504p0.79 3d9.374s0.504p0.77 3d9.384s0.504p0.78 3d9.364s0.494p0.81 3d9.354s0.524p0.78

a

X is the oxygen atom for SiOU−Ni.

Table 4. QTAIM Parameters at U−TM Bond Critical Points (BCPs) for Uranium−Transitional Metal Complexes Associated with Interaction Energy Eint (kcal/mol) complexes

ρ(r)

∇2 ρ(r)

H(r)

V(r)

G(r)

ε

δ(U, TM)

Eint


0.0670 0.0605 0.0656 0.0549 0.0703 0.0679 0.0690 0.0690 0.0815

0.1627 0.1195 0.1265 0.0935 0.1830 0.1699 0.1750 0.1783 0.2313

−0.0168 −0.0161 −0.0178 −0.0159 −0.0167 −0.0167 −0.0170 −0.0163 −0.0206

−0.0743 −0.0620 −0.0672 −0.0551 −0.0792 −0.0760 −0.0777 −0.0772 −0.0990

0.0575 0.0459 0.0494 0.0393 0.0625 0.0592 0.0607 0.0609 0.0784

0.0328 0.0343 0.0360 0.0502 0.0151 0.0296 0.0295 0.0221 0.0174

0.9565 0.7554 0.8191 0.7176 0.9970 0.9707 0.9797 1.0093 1.0952

−23.31 −19.45 −21.08 −17.29 −24.85 −23.85 −24.38 −24.22 −31.07

nd10 transition metal. Electron-spin densities of other fragments in these complexes (SX and SL) are close to zero in most cases. In brief, no pronounced electron transfer is found between two metal centers. Notably, UIII−Ni has SU = 2.82 and SNi = 0.18, which is approximately consistent with electronic configurations of U(III) and Ni(0). However, the results reflect that the reducing electron mainly enters uranium-based unfilled orbital of the U−Ni precursor and partially disperses into the nickeltype orbital. This will be reflected in electronic structures of UIII−Ni and U−Ni discussed below.

One can observe that variations of transition metals and donor atoms have only a slight effect on SU values of complexes. Differently, relatively large impacts are found for calculated NBO charges, Table 3. For example, QU ranges from 1.18 to 1.62, and QNi falls between −0.61 and −0.90. NBO yields smaller QU and QTM (absolute values) than those from Mulliken (Table S7). In these complexes, NBO shows quite similar natural orbital population for uranium centers (Table 3); however, the difference of relativistic effect of group 10 metal result in platinum having more (n + 1)s/p and less nd electrons than palladium and nickel. D


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Inorganic Chemistry Table 5. U−TM Bond Orders Calculated at Different Levels of Theorya


Gaussian

Priroda

δ(U, TM)

Mayer

Mayer

G−J

N−M(1)

ADF N−M(2)

N−M(3)

0.957 0.755 0.819 0.718 0.997 0.971 0.980 1.009 1.095

0.92 0.71 0.86 0.78 1.04 0.96 0.96 1.02 1.11

1.17 0.83 0.88 0.95 1.25 1.21 1.21 1.27 1.32

0.90 0.62 0.82 0.60 0.95 0.91 0.92 0.95 1.05

1.12 0.61 1.02 0.81 1.20 1.14 1.15 1.18 1.34

0.98 0.25 0.94 0.70 0.98 0.98 0.99 0.98 1.10

1.04 0.45 0.97 0.76 1.10 1.06 1.06 1.08 1.23

a

Bond orders include types of Mayer, Gophinatan-Jug (G-J) and Nalewajski-Mrozek (N-M) bond orders, where three-level N-M ones were calculated according to different two-electron valence indices.

Figure 2. Plots of U−Ni distances (Dis.) versus QTAIM parameters and interaction energy Eint for heterobimetallic uranium−nickel complexes.

3.2. Uranium−Transition Metal/Donor Ligand Bonding Properties. 3.2.1. Topological Analyses of U−TM Bonds. QTAIM data based on electron density are able to classify chemical bond types, characterize bond nature, and distinguish bond strength somehow. Previous studies66−74 proposed a general creation for actinide−metal/ligand bonds. In the case of ρ(r) > 0.1, ∇2 ρ(r) < 0, and H(r) < 0 (a.u.), a typical covalent (shared shell) bond is suggested. An ionic (closed shell) interaction is given, while ρ(r) > 0, ∇2 ρ(r) > 0 and H(r) > 0. Regarding 0 < ρ(r) < 0.1, ∇2 ρ(r) > 0, and H(r) < 0, a dative or electron-transfer bond is assigned. With respect to U−TM complexes, calculated QTAIM parameters at the metal−metal BCP are listed in Table 4. We note that small ρ(r) values were obtained for all U−TM complexes, ranging from 0.055 to 0.082. Small and negative H(r) values were calculated, along with positive ∇2 ρ(r). According to above criterion, the U−TM interaction is attributed to a dative or an electron-transfer bond. After

checking calculated electron-spin density in Table 3, the electron-transfer bond is ruled out. Thus, U−TM is the dative bond per se, showing the coordination of transition metal toward uranium. As a measure of bond order, delocalization index δ(U, TM) is a good indicator to denote bond strength. Calculated δ(U, TM) values between 0.76 and 1.10 suggest single-bond character, which is further supported by ellipticity ε values that are close to zero. Notably, one must carefully use QTAIM data because the heavy element uranium has different matching extents with Ni, Pd, and Pt while forming a metal− metal bond. Furthermore, the interaction energy (Eint) of U−TM bond was computed to help characterize bond strength. As seen in Table 4, Eint values were calculated between −17.3 and −31.1 kcal/mol. Herein, more negative values denote stronger bond. In a previous study, Marks and co-workers experimentally determined uranium−metal bond disruption enthalpies of Cp3UIV−MPh3 (Cp = C5H5; M = Si, Ge, and Sn) and E


Article

Inorganic Chemistry Cp3UIV−TM(CO)2Cp (TM = Fe and Ru), affording values between −35.9 and −40.4 kcal/mol.75 The energies of UIV−Ru bonds in a series of tripodal triamido complexes fell within the same range.33 Differently, only −5.1 to −9.6 kcal/mol bond dissociation enthalpies were calculated for (CpSiMe3)3UIII− MMCp* (MM = Al and Ga; Cp* = C5Me5).5 Comparatively, our data are located between above data. 3.2.2. Metal−Metal Bond Strength among Complexes U− Ni/Pd/Pt. Recent studies have shown that QTAIM parameters and Eint are able to quantify bond strength for they have good correlation.42,71,76 Herein, we will use the conclusion to testify our results. Regarding three complexes U−TM, ρ(r) decreases going from TM = Ni to Pt to Pd; corresponding values of ∇2 ρ(r), V(r) (absolute value), G(r), δ(U, TM), and Eint (absolute value) decrease accordingly. All these indicate an order of bond strength: U−Ni > U−Pt > U−Pd. Notably, while canceling the impact of different atom radius, FSRU−TM values deduced the order of U−Ni > U−Pt ≥ U−Pd, implying the U−Pt bond may be stronger than the U−Pd one. Our topological analyses provide more evidence for U−Pt being stronger than U−Pd. The conclusion is also supported by the fact that the heavier platinum has stronger relativistic effect than the palladium. The relativity of heavy metal leads to the radial expansion and energetic destabilization of nd orbital and the radial contraction and energetic stabilization of (n + 1)s/p orbitals;77 as a net effect, more electrons disperse into (n + 1)s/p orbitals from nd for Pt than Pd. This has been reflected by our natural orbital population analyses for U−Pt and U−Pd, yielding 5d 9 . 3 2 6s 0 . 6 5 6p 0 . 8 0 for Pt and 4d9.465s0.495p0.68 for Pd. More electrons distributing over (n + 1)s/p orbitals evidently enhance the U−Pt bond over U−Pd. Moreover, subsequent analyses on electronic structures (see below) indicate the U−TM single bond is primarily of σcharacter. Pt with more (n + 1)s/p electrons and less nd ones would improve the U−TM σ-bonding interaction than Pd. Apart from δ(U, TM) calculated by the Gaussian program, a variety of bond orders of U−TM were further obtained using Priroda and ADF codes. As seen in Table 5, Mayer bond orders of U−Ni/Pd/Pt were calculated to be 0.92/0.71/0.86 and 1.17/0.83/0.88, respectively. In the ADF calculations, bond orders of G−J and N−M all give the same sequence, i.e., U−Ni bond being the strongest, U−Pt the next, and U−Pd the weakest. In brief, the order of bond strength, U−Ni > U−Pt > U−Pd, is suggested, which differs from the assumption deduced from metal−metal bond lengths and by a previous report.20 3.2.3. Metal−Metal Bond Strength among U−Ni-Type Complexes. Regarding XU−Ni series complexes as well as Vac.U−Ni and UIII−Ni, varying the axial donor atom and introducing one electron induce interesting regular changes of U−Ni bond strength. We have found that U−Ni distances linearly correlate with QTAIM parameters [ρ(r), ∇2 ρ(r), V(r), G(r), and δ(U, Ni)] and Eint in Figure 2. The leastsquares linear regression correlation coefficients (R2) range from 0.882 to 0.991. Thus, larger QTAIM/Eint (absolute value) corresponds to shorter U−Ni distance and stronger metal−metal bond. Previous studies68,69,76 also indicated the QTAIM metrics are good indicators for An−metal/ligand bond strengths. The current results provide further evidence. In Table 5, various bond orders of U−Ni calculated by different approaches help to give an order of bond strength. It is not surprising that a good anticorrelation is built with respect

to U−Ni distances as seen in Figure S2. The R2 values were fitted within 0.875 and 0.993. Among these U−Ni-type complexes, Vac.U−Ni and UIII− Ni have the shortest and longest metal−metal distances, respectively. They are attributed as two extremes, corresponding to the strongest and weakest U−Ni bonding strengths. One can envision that Vac.U−Ni without the axial donor atom would release more uranium valence electrons than any XU− Ni having a real donor. In return, these valence electrons would be partially shared with nickel, leading to the strongest U−Ni bond and being regarded as the upper limit. In contrast, electronic structures reveal that UIII−Ni possess the σ*(U− Ni)-character HOMO, being exactly the same as LUMO of U−Ni. These evidently indicate that UIII−Ni should have weaker metal−metal bond than any tetravalent uranium complexes. More importantly, the above deduction is evidently supported by results of QTAIM parameters, Eint, and bond orders at various levels of theory (Tables 4 and 5, and Figures 2 and S2). Additionally, our results indicate that U−Ni with the iodine donor atom has the second weakest U−Ni bond strength. Therefore, an order of U−Ni bonding strength is suggested in the sequence of Vac.U−Ni > SiOU−Ni ≈ FU−Ni > ClU− Ni ≈ BrU−Ni > U−Ni > UIII−Ni. This agrees with the recent study where experimental analogues of SiOU−Ni, FU−Ni, and U−Ni were found to show the same order. Through theoretically designing relevant complexes, current calculations provide more systematic predictions, being expected to help future experimental synthesis and property exploration. 3.2.4. Topological Analyses of U−X Bonds. Herein, we also carried out QTAIM calculations at the U−X BCP. As discussed, U−X bond strongly affects U−TM bond via competing valence electrons of uranium. Applying the criterion of defining chemical bond, U−X is attributed to a dative bond, while X = I, Br, and Cl. Their Eint values were calculated in the range from −8.5 to −19.9 kcal/mol in Table S8, being close to bond dissociation enthalpies (−9.8 to −17.2 kcal/mol) of U− O/N/C reported by Minasian et al.5 In contrast, a covalent bond with strong polarity is assigned for complexes SiOU−Ni and FU−Ni, because large values of ρ(r) (>0.10), ∇2 ρ(r), and H(r) (absolute) are afforded; moreover, calculated Eint is around −60 kcal/mol. Additionally, the calculated δ(U, X), ε, and other types of bond orders (Table S9) indicate a singlebond character for all the U−X bonds. 3.3. Electronic Structures and Spectroscopy. 3.3.1. Electronic Structures. Compositions of α-spin molecular orbitals (MOs) of U−TM (TM = Ni, Pd, and Pt) are presented in Tables S10−S12, together with orbital diagrams in Figures S3−S5. Energy levels of orbitals with metal-involved character are illustrated in Figure 3. Careful inspection finds some similarity and difference for these three complexes. First, U−TM all have seven U(5f)-dominated frontier MOs, including two high-lying occupied HOMO and H−1, and five low-lying unoccupied LUMO ∼ L+4. For example, HOMO/ H−1 of U−Ni are composed of 87%/93% U(5f) contribution, compared with 90%/93% for U−Pd and 91%/94% for U−Pt. Two paralleled 5f electrons fill these two orbitals (α-spin), giving a triplet ground state for U−TM. U(5f)-based character also forms LUMO to L+4 of U−TM. When considering the interplay of uranium with transition metal, the first four have σ*(U−TM) and π*(U−TM) character, and the last one (L +4) is nearly pure U(5f) due to almost no participation of transition metal. U−Ni has the uranium-dominated σ*(U−Ni) F


Article

Inorganic Chemistry

TM-dominated α-spin orbitals, more than five, are found for each of complexes U−TM, as presented in Figure 3 and Tables S10−S12. This is largely due to the intrusion of bidentate ligands (L), uranium, and donor atom. For all three complexes, transition metal dx2−y2 and dxy compositions contribute to their respective H−2 and H−3. Notably, the nickel character accounts for 58 and 61% for U−Ni, together with over 24% L participation. For other two complexes, TM composition greatly reduces to about 38%, while L contribution increases accordingly. Strongly polarized π(U−Ni) character is attributed to H−8 to H−10 of U−Ni, for dxz/dyz electrons are highly localized around Ni with much less U participation ( U−Pt > U−Pd.

Figure 3. Diagrams of orbitals with the metal character for U−TM (TM = Ni, Pd, and Pt). (a) The α-spin orbital energy levels are used, and (b) orbitals with similar character were marked with same color lines.

LUMO, being the same as HOMO of UIII−Ni. The reduction evidently weakens the U−Ni bond. Moreover, this intriguing electronic feature agrees with calculated SNi of −0.08 for U−Ni and 0.18 for UIII−Ni. One can see that the reducing electron has some distribution over nickel atom. The second feature is about transition metal involved orbitals. According to nd10 electronic configuration of zerovalent transition metal (Ni, Pd, and Pt), five α-spin occupied orbitals with five β-spin equivalents are supposed to have primary TM(d) character. In the calculations, however, seven

Figure 4. Simulated absorption spectra of U−TM (TM = Ni, Pd, and Pt) in toluene from the TD-DFT calculations, along with the vertical bars from calculated transitions and oscillator strengths in panels b−d. All three spectra were normalized for convenient comparison (a). G


Article

Inorganic Chemistry

Table 6. Electronic Absorptions of U−TM (TM = Ni, Pd, and Pt) in Toluene from the TD-DFT Calculations along with the Assignment and Experimental Values bandsa

U−Ni

U−Pd

U−Pt

I II III IV V VI VI I II III IV V VI VI I II III IV V VI VI

816 744 683 640 585 550 (sh) 517 750 724 676 622 569 550 (sh) 528 765 737 673 622 578 542 520 (sh)

calcd transitionsb

assignment

816 744 683 628−645 585 550 497−526 750 722, 725 681, 671 622 569 539−556 506−538 759, 770 737 673 614, 625 558−578 529−548 511−515

5f → Ni + L Ni + L → 5f Ni + L → 5f 5f → L 5f → L L → 5f L → 5f Pd + L → 5f Pd + L → 5f 5f → Pd + L, Pd + L → 5f Pd + L → 5f 5f → L + Pd L → 5f L → 5f Pt + L → 5f, 5f → Pt + L Pt + L → 5f Pt + L → 5f Pt + L → 5f 5f → L + Pt L → 5f L → 5f

expc 814 (8) 709 (51) 666 (41)

511 (598) 732 (11) 686 (29), 661 (22)

576 (27) 527 (103) 728 685 668 578 533

(11) (38) (34) (36) (90)

a Theoretically simulated bands, as marked in Figure 4b,c. bCalculated electronic transitions, corresponding to the vertical bars in Figure 4b,c. See detailed information in Tables S13−S15. cExperimental values from ref 20 and molar absorptivity (M−1·cm−1) are listed in parentheses.

originate from three electronic transitions (645, 630, and 628 nm). A U(5f) → L character is assigned. Band V at 585 nm has the same charge transfer property. Differently, L → U(5f) charge transfer character is found for band VI. The strongest peak at 517 nm is comparable to experimental one at 511 nm with the largest absorbance coefficient among all visible-light absorptions. U−Pd and U−Pt both exhibit similar characteristic absorption bands within the range between 450 and 850 nm. However, there is some difference due to the participation extent of heavy Pd/Pt from light Ni. First, bands I−III of U− Pd and U−Pt are shorter in the wavelength, i.e., higher in absorption energy, than those of U−Ni. As summarized in Table 6, TM + L → U(5f) character is becoming more dominant in these peaks. Electronic structures associated with electronic transitions may explain the changes. One can note that TM + L corresponds to orbitals H−2 and below, while U(5f) to LUMO ∼ L+5 for all three complexes. Electronic transitions are closely correlated with their orbital energy difference (ΔE). For instance, ΔE values of H−2 and L+5 are 2.88, 3.09, and 2.90 eV for U−TM (TM = Ni, Pd, and Pt), respectively. It shows a trend of Ni < Pt < Pd, which indicates the trend of above absorption bands. Because these electronic transitions involve transition metal, the trend would also provide a subtle implication for metal−metal bond strength. Second, as for those of U−Ni, bands V and VI of U−Pd and U−Pt have the same charge transfer properties, i.e., between U(5f) and L. It is worth noting that band VI with L → 5f character was calculated at 528, 542, and 517 nm, respectively. They are comparable to experimental values of 527, 533, and 511 nm.20 Third, band VI of U−Pd/U−Pt shows TM + L → U(5f) charge transfer property, differing from U(5f) → L of U−Ni.

Third, U−TM complexes have two types of L-involved orbitals. One is π(Ph) + π(U−O) character, related to H−4 to H−6 of U−TM, while π(Ph) + σ(TM−P) is for H−12 to H− 14 of U−Ni and H−10 to H−12 of U−Pd/U−Pt. One π(I)based orbital (H−11) was found for U−Ni. In contrast, U−Pd and U−Pt have two π(I) orbitals (H−7 and H−8) with over 80% composition. 3.3.2. Electronic Spectroscopy. TD-DFT was performed to calculate excited states of U−TM (TM = Ni, Pd, and Pt), affording electronic absorption transitions in solution. Accordingly, electronic spectra were simulated in Figure 4. All of them were summarized in Table 6, together with experimental results. More detailed information is given in Tables S13−S15. As shown in Figure 4a, three complexes display similar general patterns, i.e., six bands in the visible region. In the nearinfrared (NIR) region (over 1100 nm), our calculations obtained 5f → 5f transitions involving uranium center, which agree with experimentally measured weak tails in the UV−vis− NIR absorption spectra.20 For U−Ni, we calculated more spinallowed excited states (two hundred) and found an intensive peak ranging from 330 to 360 nm. It is related to the highenergy peak in experiment.20 In the following work, electronic absorption in the visible region will be addressed. In Figure 4b, U−Ni was observed to display one strong peak (with a shoulder one) and five relatively weak bands. The lowest-energy peak, band I, was calculated at 816 nm, which is contributed by HOMO → L+5 excitation configuration with a coefficient of 0.988. The peak has U(5f) → Ni + L charge transfer character and is related to experimentally determined 814 nm adsorption. Similarly, both bands II and III come from the contribution of a single electronic transition, being attributed to Ni + L → U(5f) property. In contrast, complicated transitions were calculated for relatively highenergy bands. Band IV was simulated at 640 nm, which H


Article

Inorganic Chemistry Notes

4. CONCLUSIONS A series of uranium−transition metal complexes of phosphinophenolate ligands were computationally investigated, and some important points were reached. QTAIM and electron-spin density calculations reveal a U− TM single bond and transition metal toward uranium dative bond per se. The order of bond strength of U−Ni > U−Pt > U−Pd is suggested by calculated FSR U−TM , QTAIM parameters, Eint, and various bond orders, revising the previous result where U−Pt is the weakest. Our conclusion is further supported by relativistic effects of heavy metal that heavier Pt has more (n + 1)s/p radial contraction and nd expansion than Pd does, as well as calculated natural orbital populations and electronic spectra. Regarding U−Ni-type complexes, a good linear correlation of U−Ni distances versus QTAIM data/Eint/ bond orders has been built, which gives an order of metal− metal bond strength of Vac.U−Ni > SiOU−Ni ≈ FU−Ni > ClU−Ni ≈ BrU−Ni > U−Ni > UIII−Ni. This agrees with the previous order for experimentally known analogues. Good agreement of TD-DFT excited-state calculations with experimental UV−vis−NIR spectra was achieved. For example, band VI of complexes U−TM (TM = Ni, Pd, and Pt) was calculated to be 517, 528, and 542 nm, respectively, comparable to experimental values of 511, 527, and 533 nm. Absorption bands in the visible-light region, which are intrinsically complicated for uranium−transition metal complexes and still remain open for clear interpretation, were divided into six bands in general in this work. Low-energy four bands are attributed to mixed characters of U(5f) → TM + L and TM + L → U(5f); moreover, the latter plays more significant role in U−Pd/U−Pt than in U−Ni. This is responsible for shifting of these bands. High-energy bands V and VI are assigned to U(5f) → L and L → U(5f), respectively. In brief, the in-depth understanding in the current study is expected to provide support for designing and synthesizing novel An−TM complexes. A probe of photo/electrocatalysis and inverse trans influence of heterobimetallic complexes is in progress.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (21671060 and 21273063). We are grateful to Dr. Dimitri Laikov for providing us with the Priroda code.

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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.8b02787.

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REFERENCES

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Figures of ligand models and corresponding complexes; plots of U−Ni distances versus bond orders; tables of geometry parameters and molecular properties of (U− Ni)Lx as well as of U−Ni calculated by various levels of theory; tables of bond orders and QTAIM data of U−X and Mulliken charges; diagrams of orbitals and tables of absorptions and orbital compositions (%) for U−TM (TM = Ni, Pd, and Pt); full references of Gaussian and ADF codes, and Cartesian coordinates of optimized complexes (PDF)

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Corresponding Author

*E-mail: [email protected]. ORCID

Yuan-Ru Guo: 0000-0002-9015-4577 Qing-Jiang Pan: 0000-0003-2763-6976 I


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