Article pubs.acs.org/Organometallics
Highly Diverse Bonding between Two U3+ Ions When Ligated by a Flexible Polypyrrolic Macrocycle Dong-Mei Su,† Xiu-Jun Zheng,† Georg Schreckenbach,*,‡ and Qing-Jiang Pan*,† †
Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China 150080 ‡ Department of Chemistry, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2 S Supporting Information *
ABSTRACT: A Schiff-base polypyrrolic ligand (H4L) can accommodate two U3+ ions and form a Pacman-like complex [U2(L)]2+ according to relativistic density functional theory. Sixteen species, featuring four structural models in four electronic states, are energetically stable. Ligand flexibility, lack of axial restriction, and suitable U−N interactions allow the two U3+ ions to stretch freely over a wide range, in contrast to U2@Cn (n = 60, 74, 80) studied previously. Diverse U3+− U3+ interactions are found. The quintet state of the Out−In model, which is calculated to be the global ground state both including and excluding the spin−orbit coupling energy, likely shows a weak single U2 bond. In both vertical and tilt In−In species, a triple bond is found. It is composed of two two-electron−two-center bonds and two one-electron−two-center bonds; moreover, the tilt conformer is almost isoenergetic with Out−In. The Out−Out species shows no U···U bonding. Comparison with explicitly THF-solvated diuranium complexes is also addressed.
1. INTRODUCTION The uranium−uranium multiple bond has been an interesting and exciting topic1−15 for a long time, essentially since a metal− metal quadruple bond was recognized in the complex K2[Re2Cl8].16−18 In 2005, Roos and Gagliardi successfully elucidated the U−U interaction in U2 as a quintuple bond, applying accurate complete active space wave function theory (WFT) in their study.1 Multiple bonds between two uranium atoms in various oxidation states were also investigated at the WFT,2−7 density functional theory (DFT),8−13 and X α scattered wave molecular orbital14 levels. Building on theoretical calculations, the complex PhUUPh, more likely in the form of the analogous MeUUMe, was proposed as a target for synthesis in a solid argon matrix,3 just as U2H2 and U2H4 had been experimentally isolated in solid argon.15 Computational studies were also performed for diuranium carboxylates5 and hydroxide.19 Regarding a relatively large molecule, the dimetalloendofullerene U2@C60 was first detected in a Fourier transform ion cyclotron resonance mass spectrometry experiment.20 Lu and Wu, via their DFT study, suggested a (U3+)2@C606− valence state and triply bonded U2.9 A year later, Gagliardi et al. argued that the constraints due to the small size of C60 artificially generate the U−U multiple bond, because this bond is greatly weakened and even vanishes upon increasing the fullerene size to C70 and C84.8 In these studies, one can note the pronounced charge transfer from uranium to carbon and the strong U−C bonds (within 2.4−2.5 Å), regardless of what size fullerene is © XXXX American Chemical Society
used. Thus, from a structural point of view, the fullerene is essentially not a good candidate to explore its endo-U2 bonding. The rigidity of the carbon cage definitely restricts the U−U free stretch; that is, the stretch range completely depends on the cage size; on the other hand, the large U → C charge transfer greatly enhances the U−C bonding, and the resulting energetic gain sufficiently compensates for any loss due to the dissociating U−U bond. Therefore, it is still an open question whether two U3+ ions are bonded or not in an experimentally ligated environment. Experimentally, it is required to carefully select ligands containing specific structural features, suitable donating ability, and excellent steric/electronic properties. Apart from donor atoms such as C, O or halogens,20−26 N-type donors such as oligomeric pyrroles are prospective candidates for complexating uranium ions.27−43 The U−N interaction has been shown to have some covalency, allowing for potential applications of this type of ligand in the separation science of uranium.25 A versatile polypyrrolic macrocycle (H4L in Figure 1) has successfully accommodated two metal atoms (actinide, lanthanide, and transition metal)31−42 to form “Pacman-like” complexes.44−46 In these types of complexes, the bite angles between the two N4-donor compartments of the macrocycle range from 0° to 90°, showing excellent flexibility.32−36,47−49 Even more importantly, there is no spatial restriction along the metal− Received: July 29, 2015
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DOI: 10.1021/acs.organomet.5b00649 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
criterion of 10−6 au for the SCF. Analytical frequency calculations were performed to confirm the local minima nature of the stationary points on the potential energy surface. Thermodynamic data were also obtained simultaneously. Priroda applies a scalar relativistic fourcomponent all-electron (AE) approach53,55 that arises from the full Dirac equation but with spin−orbit projected out56 and neglected. The GGA-PBE functional57 was used in these calculations. All-electron correlation-consistent Gaussian basis sets of double-ς polarized quality for the large component and corresponding kinetically balanced basis sets for the small component51 (labeled as B-I) were employed. Population-based (Mayer)58 bond orders were also calculated based on these PBE calculations. To provide further insight into metal−metal bonding, the electronic structures of the diuranium(III) complexes have been calculated using the ADF 2010.02 code.59−61 The default convergence criterion of 10−6 au was used, and an integration parameter of 6.0 was applied. No further structural optimization was carried out on these Prirodaoptimized geometries. In previous work of ours62−65 and others,66−68 it has been shown that reoptimization has only a very slight effect on the structural parameters and molecular properties. The implicit tetrahydrofuran (THF) solvation was incorporated using the conductor-like screening model, COSMO.69 Klamt radii were used for the uranium atom (1.70 Å)64,70−73 and the main group atoms (O = 1.72 Å, N = 1.83 Å, C = 2.00 Å, and H = 1.30 Å).74 The scalar relativistic ZORA approach75−78 and Slater-type ZORA-TZP basis sets (denoted as B-II) were used in these calculations. The core orbitals (1s−4f for U and 1s for C, N, and O) were frozen, and 32 valence electrons (5s25p65d106s26p65f36d17s2) were considered for the U atom (small-core basis sets). Scalar-level relativity has been commonly accepted for optimizing structures of actinide-containing complexes.1−3,8−10,72,73,79 On the other hand, studies of bimetallic complexes, especially those containing two U3+ ions with several 5f single electrons, require the inclusion of the spin−orbit coupling (SOC) effects. Thus, the lowest energy state of each model and the states that have total energies close to the global ground state (within 15 kcal/mol or less) were selected and calculated at the SOC level.
Figure 1. Optimized structures of the diuranium(III) complexes [U2(L)]2+ for the In−In (vertical/tilt), Out−Out, and Out−In models, where In and Out denote the position of the U atom inside and outside its N4-donor compartmental plane, respectively. (i) Structure of the lowest energy electronic state of each model (see the text), (ii) side-on view (right) and face-on view (left) of each model, and (iii) the flexible polypyrrolic macrocycle (H4L).
metal axial direction in the formed Pacman-like complex. This provides enough space for bimetallic stretches over a large range of distances. In this work, diuranium(III) complexes of the octadentate Schiff-base calixpyrrole (H4L) have been designed in silico and examined using a relativistic DFT approach. Four stable structural models of [U2(L)]2+ have been obtained in four possible electronic states, showing U−U distances over a wide range of 2.43−5.56 Å. Highly diversified U−U bonds have been found in these species, including multiple bonding, single bonding, and absence of any bonding.
3. RESULTS AND DISCUSSION 3.1. Geometries for Four Structural Models. While optimizing the binuclear complex [U2(L)]2+, two trivalent uranium ions were initially placed into the ligand cavity. Stable configurations corresponding to four structural models (In−In (vertical), In−In (tilt), Out−Out, and Out−In) were obtained, Figure 1. For each model, minimum-energy structures in all possible electronic states (septet, quintet, triplet, and singlet) were fully optimized and further confirmed by frequency calculations. It is of interest that these species feature U−U distances that cover a wide range (bonding and nonbonding). In the various optimized structures, the uranium atoms are situated relatively far away from their respective N4-donor planes of the macrocyclic ligand. In the In−In models, the two uranium ions are attractive and approaching each other. As a result, each uranium atom is situated 0.60−1.29 Å out of its N4donor plane in the direction of the other uranium, Table 1. In contrast, the two uranium atoms are departing away from each other in the Out−Out species, being situated −0.90 to −0.98 Å out of their N4-donor planes. The above positive and negative signs correspond to uranium atoms located inside and outside the mouth of the “Pacman” ligand, respectively. Finally, geometries with one uranium outside (−1.17 to −1.88 Å) and one inside the cavity (1.19−1.35 Å) are found for the Out−In species. The bite angles (α) between the two N4-donor planes of the macrocyclic ligand were calculated to be largest for the In−In (vertical) model. A lateral twist of the macrocycle has been
2. SCOPE AND COMPUTATIONAL DETAILS Scope of the Current Study. Binuclear trivalent uranium complexes [U2(L)]2+ have been created and optimized. The formed complexes exhibit Pacman-like structures, where the uranium atoms could be situated either outside or inside the mouth of the polypyrrolic ligand (see Figure 1). Thus, we designed different structural models including In−In, Out−Out, and Out−In. The Out−In model, for instance, is a structure with one uranium atom outside the ligand mouth and the other inside. Second, several possible subarrangements of the two uranium atoms were investigated for the In−In model. As seen in Figure 1 (top), the U2 bond can be pointing in the x-, y-, or zaxis directions, corresponding to In−In (horizontal-x), In−In (horizontal-y), and In−In (vertical), respectively. Unfortunately, the horizontal complexes could not be obtained, despite several attempts. Instead, optimizations resulted in an extra structure In−In (tilt), in which the U2 line displays an angle with respect to the x−y plane. In addition, each U3+ ion contains three 5f single electrons, and the resulting diuranium(III) complex in each model is capable of adopting electronic states of septet, quintet, triplet, and singlet. In the sections discussing geometries and relative stability, all 16 species (four models and four electronic states) in the gas phase are discussed in detail. In the subsequent section on electronic structures and uranium−uranium bonding, special attention has been paid to the lowest energy electronic state of each model. Implicit and explicit THF solvation was also taken into account. Computational Details. All structures for each model mentioned above in the various electronic states were fully optimized in the gas phase without any symmetry constraints, using the Priroda code (version 6).50−54 The gradients in the geometry optimizations were converged to 10−5 au (tight tolerance), together with the gradient B
DOI: 10.1021/acs.organomet.5b00649 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Regarding the In−In (vertical) species, the U−U bond lengths were calculated to be within 2.43 and 2.62 Å, Table 2. Their bond orders are in the range 3.08−3.18, indicative of UU triple-bond character. Vibrational frequencies calculated at 188−220 cm−1 were assigned to the U−U stretching vibration. Short U−U distances ranging from 2.43 to 2.82 Å were also calculated for the In−In (tilt) species, while relatively long U−U bond lengths (3.18−3.76 Å) were found for Out− In. The calculated bond orders indicate diverse U−U bonds, including triple, double, and single bonds (both strong and weak). Our calculated U−U distances are comparable to literature values for U2@C60 (2.72 Å calculated at the PBE level)8,9 and diuranium chlorides/carboxylates (2.29−3.42 Å calculated by WFT).1−5 Besides the above structures, the flexible Schiff-base polypyrrolic ligand also supports Out−Out species. Calculated U−U distances range from 5.30 to 5.56 Å. No bonding is found between the two uranium atoms. One may note the calculated U−U bond order of 0.79 in the singlet state. Careful inspection demonstrates that this bond order arises from perturbation of the uranium and pyrrole interaction and is therefore an artifact of the calculations and not an indication of actual bonding. This is further evidenced by the lateral twist angle of 20°, being the largest among the four states of Out−Out. The electron-spin density around the uranium atoms was calculated. It correlates well with the electronic state of each species (Table 2). In the In−In (vertical), for instance, the electron spin of each uranium atom is 3.00, 2.05, 1.07, and 0.00. This denotes that 6, 4, 2, and 0 5f single electrons are present in the molecular species, respectively, and that these electrons are localized on the metals. Therefore, it corresponds to the electronic states of septet, quintet, triplet, and singlet. Some amount of charge transfer is found in the other model structures. For example, 2.85 electrons (less than 3.0) in the septet state of Out−Out were calculated to reside in each uranium, but 1.45 electrons (more than 1.0) are localized on each metal center in the triplet state. Besides the charge transfer between uranium and ligand, a pronounced charge transfer is also found between the two uranium atoms in the quintet state. Its two uranium atoms have electron-spin densities of 1.50 and 2.78, although they are supposed to be 2.0 for each metal. Thus, the system is likely to form a U(IV)−U(II) complex in the quintet state rather than having a pure U(III)−U(III) configuration. 3.2. Relative Stability. Unless otherwise noted, we will discuss relative energies excluding SOC, as SOC does not change the energetic ordering of the species (the last column of Table 2). Table 2 and Figure 2a show the energetic ordering to be In−In (tilt) < In−In (vertical) < Out−Out (total energy, ΔE). Large energetic changes have been found for the series of the Out−In complexes (Figure 2a). The quintet state of the Out−In model was calculated to be the lowest in energy among all 16 species. Its energy was used as reference accordingly. Among the tilt species, the triplet state is the lowest in energy. It is approximately energetically degenerate with the global ground state, being only 0.4 kcal/mol above. The energy difference is slightly larger (0.8 kcal/mol) when considering the SOC energy. As reflected by the flat line in Figure 2a, the four states of the tilt model are close in energy. The largest difference is 5.3 kcal/mol, suggesting that these states can be readily transformed into each other at room temperature. A similar case is obtained for the vertical species. They are 9−14 kcal/mol higher in energy compared with the tilt ones. The
Table 1. Optimized Geometry Parameters of Diuranium(III) Complex [U2(L)]2+ in Various Electronic States (ES) of Different Models model
ES
r1a
r2a
In−In (vertical)
septet quintet triplet singlet septet quintet triplet singlet septet quintet triplet singlet septet quintet triplet singlet
1.033 1.014 1.023 1.002 0.602 0.770 0.859 0.902 −0.927 −0.955 −0.983 −0.902 −1.173 −1.877 −1.882 −1.491
1.033 1.014 1.023 1.002 1.149 1.231 1.277 1.285 −0.927 −0.954 −0.872 −0.901 1.186 1.349 1.328 1.317
In−In (tilt)
Out−Out
Out−In
r3a
αb
β1c
2.165 1.678 1.684 1.668
78.0 74.0 72.7 71.3 45.7 43.9 44.3 46.7 64.7 58.3 57.0 57.2 37.4 28.8 30.4 30.9
4.1 4.0 4.5 4.8 32.0 29.6 28.1 29.0 16.0 15.7 18.3 20.2 20.8 18.0 13.4 22.0
a
The distance (r in Å) denotes the deviation of the uranium center from its N4-donor plane, i.e., the normal distance from the uranium atom to the N4-donor plane. Positive and negative values correspond to inside and outside of N4-donor planes, respectively. For Out−In, the r3 is the normal distance from the inner uranium atom to the other N4-donor plane. bBite angle (α in deg) between two N4-donor planes (see Chart 1). The optimized free H4L ligand shows a bite angle of 69.2°. cTwist angle (β in deg), i.e., right angle (90°) minus the angle (mean) between the N4-donor plane and the aryl hinge plane (see Chart 1).
found in the other model species. This is directly observable from their face-on structures in Figure 1, and it is reflected by the calculated twist angles (β) of 13−32°. The angles α and β are defined in Chart 1. The In−In (vertical) species have very Chart 1. Bite Angle (α in deg) between the Two N4-Donor Planes and Twist Angle (β in deg), i.e., Right Angle (90°) Minus the Angle (Mean) between the N4-Donor Plane and the Aryl Hinge Plane
small β angles (
