Optical and Magnetic Properties of the Complex Bis

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Optical and Magnetic Properties of the Complex Bis(dicyclooctatetraenyl)diuranium. A Theoretical View Dayán Páez-Hernández,* Juliana Andrea Murillo-López, and Ramiro Arratia-Pérez Facultad de Ciencias Exactas, Universidad Andrés Bello, República 275, Santiago, Chile ABSTRACT: A systematic study on bis(dicyclooctatetraenyl)diuranium (U2COT4) has been performed using relativistic density functional theory. The molecule was calculated in four different electronic configurations and the two symmetries C2 and D2h. First we considered the high-spin quintuplet (5A) state, and from this state we built the broken-symmetry configuration following Noodleman's approach. Also, the triplet state was considered as a formal interaction of both uranocene fragments in triplet and singlet states simultaneously, and finally the low-spin singlet configuration was calculated. For both symmetries the ground state of the complex was the quintuplet, and on the basis of the broken-symmetry approach a significant ferromagnetic coupling between both metals was found. Time-dependent density functional theory (TDDFT) was used to calculate the excitation energies with the GGA SAOP functional. The obtained electronic transitions are in good correlation with reported experimental values, which are found between 600 and 720 nm: 633, 658, 664, 673, 685, and 717 nm.

1. INTRODUCTION In 1963, Fischer at the University of Hamburg published a theoretical paper in which he considered the metal−ring bonding in sandwich complexes.1 In the final section of this paper he discussed the C8H8 ligand. In addition to transitionmetal complexes containing a planar C8H8 ligand2,3 such as (C8H8)2Ti, he suggested the possibility of a stable uranium complex, (C8H8)2U, noting that there might be a possible additional gain in energy due to the participation of the f orbitals of heavy central atoms.3−7 As expected, the structure of uranocene showed it to be a π-bonded sandwich complex with D8h molecular symmetry.3,5 A qualitative discussion of the electronic structure of these compounds was given in the original report, in which the electronic structure was supposed to be similar to that of the bis(cyclopentadienyl)iron series, except that the orbitals of the rings and of the metal involved in bonding had an additional node in going around the main symmetry axis of the molecule.3,8−10 The preparation of uranocene was followed by extensive synthetic studies, whose goal was the further development of this interesting new area of actinide organometallic chemistry, for the most part carried out at Berkeley in Streitwieser's laboratories but also by chemists in other laboratories in the USA and Europe.8−13 Many novel compounds have appeared since, and their electronic structures and spectroscopic properties were reported.13 In particular, the synthesis of dinuclear actinocenes attracted the attention of many groups, with the objective to extend the well-known dinuclear sandwich in transition metals to lanthanide and actinide compounds.14−17 In 1981 Miller and Streitwieser reported the synthesis of a novel biuranocene (bis(dicyclooctatetraenyl)diuranium, U2COT4) obtained as a byproduct in the synthesis of 1,1′dicyclooctatetraenyluranocene.18,19 Curiously, this compound © 2012 American Chemical Society

has received little attention and many of its properties have not yet been reported, perhaps because of the experimental difficulties inherent in the study of actinocene compounds, which generally are extremely air sensitive and flammable and also are rapidly decomposed by aqueous bases and strong acids.3,18,19 For this reason these molecules are special candidates for a theoretical quantum chemistry investigation, particularly in a relativistic scheme, because the scalar and spin−orbit coupling effects are especially important in the correct descriptions of their electronic and spectroscopic properties.20−23 The present study provides relevant information for establishing the nature of electronic states and spectroscopic properties in the uranocene UCOT2 and its adduct U2COT4 on the basis of relativistic density functional theory calculations. Our aim was to perform a systematic study on the electronic structure and absorption spectra (visible and near-IR) in these two compounds, in order to understand the principal similarities and differences in their bonding nature and stabilization.

2. THEORETICAL MODELS AND COMPUTATIONAL DETAILS The aim of the present work is to describe the electronic and spectroscopic properties of the product of the transmetalation reaction between two uranocene monomers which gives U2(C8H7)4 (U2COT4) as a product. For this reason we first describe briefly the principal electronic properties of the monomer UCOT2 in its principal electronic configurations (triplet and singlet), in order to understand what happens when the adduct of the reaction is formed. The idea is to obtain information about the metal−metal interaction, electronic Received: June 19, 2012 Published: August 28, 2012 6297

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properties, and possible modifications in the absorption spectrum produced by the transmetalation reaction. In a previous work we described, using DFT, the structure of the triplet state of the uranocene and here we calculate the singlet state to complete the study of this molecule and also to find the principal differences between the monomer and adduct product. All these calculations allow us to establish the nature of bonding, the contribution of the d and f orbitals, and the origin of stabilization in this kind of molecules. With this purpose a quantum chemical investigation was carried out using density functional theory (DFT). All structural and spectroscopic properties were obtained using Amsterdam density functional (ADF) code,24 where the relativistic scalar and spin−orbit effects were incorporated by the zeroth-order regular approximation (ZORA Hamiltonian). All molecular structures were fully optimized by an analytical energy gradient method as implemented by Verluis and Ziegler, using the local density approximation (LDA) within the Vosko−Wilk−Nusair parametrization for local exchange correlation.25−27 The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation exchange correlation functional was also used.28 Geometry optimizations were carried out by an all-electron scheme with an standard Slater-type orbital (STO) basis set with triple-ξ quality double plus polarization functions (TZ2P) for all atoms. The information reported previously indicates that the ground states of uranocene have a contribution of two states with different multiplicities (triplet and singlet).20−23,29 In our previous work we reported the calculations of the uranocene triplet state,21 and here we consider the singlet state, in which the two electrons of U4+ are in a formal f2 electronic structure which are considered antiferromagnetically coupled.20−23 In fact, this state has a rather minor contribution to the ground state of uranocene and its consideration here is only with the objective to characterize all possible ways in which the adduct may be formed. The adduct complex was calculated in four different electronic configurations. First we considered the high-spin quintuplet state, and from this state we built the broken-symmetry configuration following the Noodleman approach. This quintuplet (S = 2) configuration is the result of a formal interaction between both uranocene fragments in the triplet state and in the present case results in the ground state of the molecule. Also two other states were calculated: first the triplet state was considered as a formal interaction of both uranocene fragments in triplet and singlet states simultaneously, and finally the low-spin configuration was considered by the interaction of two uranocenes in their singlet state. The models were constructed with restrictions in symmetry in order to facilitate the analysis of properties. In the case of uranocene we worked with D8h symmetry. The adduct molecule was modeled under two different symmetries: first in a C2 point group related by the twist geometry reported by Streitwieser et al.,18 and we proposed an additional idealized structure with D2h symmetry in order to understand the effect of symmetry on the splitting of atomic orbitals and optical properties. To describe the contributions of different fragments (metal and COT ring) to the electronic structure of the systems, a density of states analysis was carried out using a projected density of states (PDOS) to obtain the contributions of the different metal atomic orbitals to the molecular orbitals of the two systems. Also we used the electron localization function (ELF) criteria to describequalitativelythe possible electron delocalization in the adduct complex and relate these results to the system stabilization.30,31 Finally the absorption spectra were calculated using time-dependent density functional theory (TDDFT).32,33 The GGA SAOP functional (statistical average of the orbital exchange correlation potentials) was used because it is specially designed for the calculation of optical properties.34,35 We have an special interest on the transitions in the near-infrared region (near-IR) because in general they involves electron transfers between metal orbitals and therefore allow a better description of the valence electronic structure of the system. The model used for adduct complexes is presented in Figure 1.

Figure 1. Molecular models for (η 8 -C 8 H 7 ) 2 U−U(η 8 -C 8 H 7 ) 2 (U2COT4) with C2 (A) and D2h (B) symmetry.

3. RESULTS AND DISCUSSION Geometry and Electronic Structure. The geometry optimization using both functionals is in agreement with the experimental results reported for uranocene (see Table 1). The difference in d(U−COT) is only 0.02 Å with respect to the experimental values for both functionals; particularly, with LDA the value is less than this experimental value and with PBE it is over it. In addition we compare our results with reported multiconfiguration CASPT2 calculations of uranocene by Kerridge et al.29 and find perfect agreement between these two results. The general geometry for the adduct does not change considerably with respect to the monomer structure; in general, the d(U−COT) value is slightly reduced in both calculations. The d(U−U) value is very large using both functionals; for this reason we do not consider that a direct strong interaction with charge transfer between the two metals in this system really exists. In the same way the d(U−COT) and d(U−C(COT) values are very similar to those of the monomer in the two symmetries considered for the adduct molecule. The principal differences in these symmetries are in d(U−U) and d(COT− COT) when we compare the D2h symmetry with respect to C2; in the first symmetry the distance between two uranium atoms is 5.376 Å and that for C2 is 5.265 Å at the same level of calculation with the PBE functional. These differences in the D2h symmetry are due to the four hydrogens of COT rings in the region of C−C bonding which are very close together, producing a strong distortion in the system due to the repulsion between them. Because of this, in a structure under C2 symmetry the distances between COT rings are slightly smaller than in D2h and the two uranium atoms may be slightly closer. In both structures the large distance between uranium atoms, rather than its covalent radii 196 pm,36 presumably does not allow a direct charge transfer among them but communication is possible throughout the carbons of the COT ring. The geometric parameters selected in Table 1 show also that the 6298

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Table 1. Structural Parameter Optimization of UCOT2 and U2COT4a state

d(U−COT)b

d(U−C(COT))

S Td S T T+(S)

1.902 1.945 1.941 1.957 1.944 1.924

2.652 2.694 2.693 2.683 2.645

1.894 1.881 1.954 1.957 1.956 1.948 1.936 1.956 1.946 1.945

molecule UCOT2 LDA PBE CASPT229 exptl10 U2COT4 LDA C2 D2h PBE C2

5

A B2g

5

1

A A ABSf 5 A 1 Ag B2gBS 3 B3u 5 B2g 3

D2h

d(U−U)

d(COT−COT)c

dihedrale

2.628 2.635

5.213 5.289

1.506 1.525

−38.04 0.00

2.675 2.693 2.692 2.685 2.670 2.687 2.675 2.685

5.265 5.265 5.264 5.265 5.376 5.378 5.367 5.377

1.528 1.529 1.529 1.531 1.547 1.559 1.552 1.550

−41.51 −40.41 −41.36 −39.84 0.00 0.00 0.00 0.00

a All distances are in Å. S and T refer to singlet and triplet, respectively. For the dimer the different states appear ordered from higher to lower energies. bDistance between metal and ring centroid. cThis value refers to the distance d(C−C) between two COT rings. dGeometric parameter for triplet state based on previous work.21 eRefers to the dihedral angle between COT rings; values given in deg. fBS stands for broken symmetry.

geometry of the adduct does not have an important variation with respect to the different electronic states considered here. Mainly this is because the four unpaired electrons occupy nonbonding f orbitals in the valence region of the molecule and for this reason the different electronic configurations considered do not affect directly the more internal bonding orbitals. The bonding interaction as shown in Figure 2 is the result of mixing between molecular orbitals from monomers throughout the rings. For both symmetries these orbitals have an important contribution of carbon p atomic orbitals with π symmetry, and the contribution due to uranium is mainly due to the fδ (fxyz) and fφ (fx and fy) with little contribution from fπ

and fσ. The rest of the orbitals near the valence region retain more or less the same identity as the monomers. The distances between two carbon atoms that link both monomers are 1.525 (1.506) Å and 1.550 (1.531) Å for LDA and PBE, respectively, in D2h (C2) symmetry. This possible electron delocalization between both structures is the principal electron transfer interaction that stabilizes the adduct molecule. In order to corroborate this idea, the first step was to analyze the topology of the density throughout the ELF function. For clarity in the selection of the cut planes we present here a qualitative discussion about the results of ELF calculations for the D2h symmetry of the adduct. As can be seen in Figure 3, the ELF representation shows a valence basin between the two COT rings, which communicates between the two fragments and introduces the idea of a bonding between them. Figure 3A shows the ELF in the plane of the rings where all valence basins are clearly defined, including the basin described above that appears between the two rings. If we move 1.00 Å in the direction of the metals, it is possible to see that all basins in the region of bonding between the carbon atoms appear fused (Figure 3B), indicating an important electron mobility even at ELF values of over 0.7. Also in this position it is possible to see a complete differentiation between the basins of rings and the metal; in particular, the unpaired electrons are located in fz3 orbitals. Figure 3C presents a vertical cut of ELF in which it is possible to see the last conclusion. In C2 symmetry the torsion between both monomer fragments produces a break of the coplanar positions of both COT rings and it is not possible to select planes for the ELF similar to the D2h case; however, the basin structures are similar. To continue with the bonding analysis in the adduct molecule, we pursued an energy decomposition based on the Morokuma−Ziegler scheme to understand the principal forces involved in the stabilization of this structure. In Table 2 we present the results of this calculation, where EPauli, EElstat, and E Orb are respectively the Pauli repulsion, electrostatic

Figure 2. Bonding molecular orbitals for the ground state (S = 2), where it is possible to see the δ interaction between atoms and the sixmembered ring formed by two uranium atoms and four carbons of the rings. 6299

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Figure 3. Cut-plane ELF at selected positions in the ground state (S = 2): (A) at the plane of the rings; (B) 1 Å near uranium atoms; (C) vertical position.

EOrb and EPauli. In the language of the Morokuma−Ziegler scheme, in this molecule the orbital interaction is higher than the steric interaction (repulsive) and for that reason the analysis of the two fragments considered here results in a bonding interaction. These results show the importance of the orbital interaction between both fragments, and the analysis of the density of states was done in order to corroborate the metal and ligand influence on the interaction described above. This type of calculation allows us to establish three different regions in the electronic structure of these molecules, including different levels of contributions of all fragments in the structure (U atoms and COT rings). As can be seen in Figure 4, the density of states for the monomer in the triplet state shows that the region under −6.0 eV has a predominantly COT p orbital character and the metal contribution is minimal here. The most

Table 2. Bonding Energy (eV) Decomposition Analysis Based on the Morokuma−Ziegler Scheme for All Spin States Treated Here BE = E Pauli + E Estat + EOrb symmetry C2

D2h

state

EPauli

EElstat

EOrb

BE

5

61.69 58.71 66.55 67.96 68.04 65.44

−16.84 −16.08 −18.83 −15.91 −15.93 −15.79

−60.52 −51.48 −56.97 −61.02 −60.87 −55.58

−17.08 −8.86 −9.27 −8.96 −8.75 −5.94

A 3 A 1 A 5 B2g 3 B3u 1 Ag

interaction, and orbital-mixing terms. A detailed description of the physical significance of these properties has been given by Bickelhaupt and Baerends.37 In general, the distortion in the more stable C2 symmetry introduces, on the basis of our calculations, an energy difference between C2 and D2h of 19.1 kcal/mol (at the PBE level). In this bonding energy analysis we consider the three principal electronic states for both symmetries described here. In both cases the molecule was divided into monomer fragments and the bonding energy between them was determined. For both symmetries the most stable interaction is produced in a quintuplet state and, as expected, the 5A (C2) state has a stronger interaction. In D2h symmetry 5B2g is the more stable state, but the bonding energy in this case decreases due to the hydrogen repulsion described above. In all cases the EOrb value has the principal role in the stabilization forces and this is sufficient to conclude that the interaction between both fragments have an important covalent character, around 70% of the stabilization forces; however, the contribution of the EElstat components is very important in the stabilization of the system. As can be seen in Table 2, the Pauli repulsion between both monomers is very strong in all states and is relatively higher in D2h symmetry, being even higher than EOrb in some states, for which this interaction has a determinant destabilizing role and the electrostatic additional stabilization is the force that compensates for the slight difference between

Figure 4. Density of states for the monomer UCOT2 in the triplet state. The Fermi level is represented by a vertical line. 6300

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important region appears between −6.0 and 6.0 eV, where the contributions of d and f uranium atomic orbitals to the valence molecular orbitals of the complex are important. Figure 4 also shows that in the frontier orbital region the molecule has a greater contribution of f orbitals than d orbitals and the region over the Fermi level shows two different bands with f and d character, indicating a greater stabilization of the f orbitals in the uranium atom. In the case of the adduct the same density of states distribution for both symmetries was found, as can be seen in Figure 5. The principal difference with the monomer is that in

the reduction in the symmetry point group of this molecule is important in the intensity of these transitions. The conclusions obtained in the analysis of the density of states were corroborated in the molecular orbital diagrams of both molecules presented in Figure 6. The four unpaired electrons in the more stable configuration 5A (5B2g) of the adduct are located in molecular orbitals with essentially pure 5f of the uranium atoms, located in nonbonding positions in the energy level diagram of Figure 6. The ground state found for the adduct molecule is a quintuplet state with four unpaired electrons, but in the present study we also found some other states which are very close in energy with the quintuplet state. To find these electronic configurations, we applied the broken-symmetry Noodleman approach; this approach makes use of an unrestricted or spinpolarized formalism and a broken-symmetry solution for the low-spin state. The energy difference between these states (high spin and broken symmetry) is used to obtain the magnetic coupling constant based on the Heisenberg Hamiltonian with the form Ĥ = −2JŜ1·S2̂ where J refers to the magnetic coupling constant and the convention sign is taken in such way that a positive J means ferromagnetic coupling and S1̂ ·Ŝ2 is the direct product between the total spin angular momentum of the interacting states. Figure 7 gives a diagram with all electronic states considered for the adduct molecule. For both symmetries the broken-symmetry (BS) triplet and singlet states were calculated and their formal electronic distribution is presented. For both symmetries the monomers interacted magnetically by a ferromagnetic coupling and with the calculated map of spin density it is possible to conclude that in the adduct ground state the unpaired electrons are strongly localized over the uranium centers; even the spin densities over the uraniums are slightly higher than 2 as a consequence of electron donation from the ring to the metal. In the triplet state the same occurs, and the spin density map shows that over the uranium atom an amount of negative density appears. In this case, as indicated by the expectation value of ⟨S2⟩, the spin contamination is very important, revealing the multireference character of the molecular wave function. Here it is appropriate to mention that the ⟨S2⟩ expectation values are well-defined for the unrestricted Hartree−Fock method, while in the context of unrestricted DFT the finding of these expectation values obtained from Kohn−Sham orbitals is not a well-defined procedure. In our case the spin states for the configuration of high spin and broken symmetry are respectively |S1 = 1, Ms1 = +1; S2 = 1, Ms2 = +1⟩ and |S1 = 1, Ms1 = +1; S2 = 1, Ms2 = −1⟩. The energy expectation values for these configurations are −2J and 4J, respectively, and in this way the coupling parameter is thus calculated by multiplying the difference between high-spin and broken-symmetry Kohn−Sham energies by 1/6, but the broken-symmetry Kohn−Sham wave functions are not orthogonal to each other (due to the overlapping spin densities for different metal centers), which may require additional corrections. For this reason even when the broken-symmetry wave functions are relatively easy to compute and interpret, the mapping of the eigenvalues and eigenstates of the “exact” Hamiltonian into the Heisenberg Hamiltonian requires the definition of a model space expressed from localized orthogonal orbitals or the equivalent symmetry-adapted molecular orbitals. In Table 3 we present the results of the calculation of the magnetic coupling constants (J) based on the mapping scheme proposed by Nishino et al. (eq 1)38 and the mapping proposed by Noodleman et al. (eq 2).39−41 The principal differences

Figure 5. Density of states of the adduct U2COT4 with C2 (A) and D2h (B) symmetry. The Fermi level is represented by a vertical line.

this case the densities of states due to f and d orbitals in the valence region of the molecule are higher. The bonding interaction orbitals in the molecule (Figure 2) are located in the region around −5.0 eV, where the p orbitals of the carbon and f and d of uranium have an important density of states and a mixture between them is possible. As a conclusion, it is possible to see that the adduct has a greater density of states than the monomer with an important concentration of f states in the frontier orbital region; for that reason the presence of transitions with f → f and f → d character are expected in addition to the charge transfer electronic transitions. Of course, 6301

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Figure 6. Qualitative energy level diagram for U2COT4. Red lines indicate contributions under 50%, and blue lines indicate contributions over 50%. The molecular orbitals where the four unpaired electrons in C2 symmetry are located are presented.

between both approaches are that the first is valid for any bonding situation and the Noodleman approach is more effective in the case of the weak coupling region.42 In all cases J has a positive value, indicating in our sign convention a ferromagnetic coupling. These results support the fact that the paramagnetic properties of the adduct arise from the magnetic properties of the independent uranocene fragments,18 especially because of the existence of a strong spin localization over the uranium atoms. Electronic Transitions. The electronic spectra of actinide compounds usually involve a combination of f → f, which being Laporte forbidden tend to be weak and do not feature strongly in absorption spectra, or f → d transitions, and also charge transfer transitions, not only because of the large number of states derived from a single fn configuration, but also because of the splitting of these states by ligand field and spin−orbit coupling. In a previous work21 we discussed the principal transition in uranocene complex and in the present work we extend these calculations to the adduct molecule treated here. The experimental visible absorption spectrum of uranocene contains four well resolved bands between 600 and 700 nm.8 The most correct assignment of these bands is due to charge transfer transition from filled π orbitals on the COT rings to f orbitals at the uranium center. The calculated visible absorption spectrum for the adduct molecule has many important differences with respect to the monomer calculated by us in the previous work21 (see Figure 8). In the visible region this molecule has an important group of absorption bands between 400 and 600 nm with high oscillator strength; in addition, we find some absorption bands over 600 nm; particularly for the adduct in C2 symmetry an important band appears between 600 and 680 nm. For the molecule in D2h symmetry the absorption bands are narrower and it is possible to observe some fine structure. The positions of the bands are similar in both cases, and the differences are in the intensities of the bands. In Table 4 we present the assignment of the spectrum, the principal and more intense bands, the character of the molecular orbitals involved in the excitations, and the dipole moment of the transition. The electronic transitions in the first region of the spectrum mainly contains transitions with ligand to metal charge transfer (LMCT) character, where the molecular orbitals involved in

the excitations are similar to those of the individual monomers with δ and π symmetry. The more intense transitions for the complex in both symmetries appear at 420.9 nm (C2), 423.5 nm (D2h) and 484.7 nm (C2), 507 nm (D2h). To understand the difference in the intensities of these bands in both symmetries, it is necessary to know the atomic orbital composition of the molecular orbitals involved in these transitions. In Table 4 it is possible to see that the change in the dipole moment of the transition for molecule in C2 symmetry is approximately half of the value for D2h symmetry and because of the intensity for the first symmetry is reduced with respect to the D2h case. In D2h symmetry we found that the orbitals with δ symmetry involved in the transition at 423.5 nm has an 80% contribution due to p orbitals of the carbon atoms in the COT rings and 20% dxy in uranium. In the same way the contributions for the orbitals involved in the transition at 507.4 nm are 90% due to p of the carbon and 10% dx2−y2 of the uranium. The virtual orbitals have respectively 95% of fz2y and 60% fx, 20% fz2x of the uranium atoms. These different orbital contributions imply that the electron transition involves two regions of the molecule, leading to a change in the dipole moment greater than that in the C2 symmetry. The distortion produces a loss of the inversion center of the molecule, and the mixing between d and f orbitals is now possible; this produces a reduction of the dipole moment of the transition obtained in the calculations. In this calculation we found an important correlation between the calculated values and the transitions reported in an earlier paper of Streitwiesser et al.18 These authors reported that in the synthetic compounds there appear transitions between 600 and 720 nm at 633, 658, 664, 673, 685, and 717 nm. In Table 4 it is possible to see that our results are in very good agreement with the experimental values. The spectrum for D2h symmetry presented shows similar characteristics but has a slight blue shift of the absorption bands around 500 nm.

4. CONCLUSIONS The present study is the first theoretical systematic investigation on the structure of the bis(dicyclooctatetraenyl)diuranium complex (U2COT4) since its synthesis was reported in the late 1980s by Streitweiser et al. On the basis of on this 6302

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Figure 8. Simulated absorption spectra in the visible region for U2COT4 in C2 (black line) and D2h (red line) symmetries.

Table 4. Characterization of Electronic Transitions for U2COT4 in C2 and D2h Symmetriesa λ (nm)

Figure 7. Energy level diagram for the different spin configurations calculated for the complex U2COT4 in C2 (A) and D2h (B) symmetries. The figure shows the energy difference among the three states (left). In addition, the ⟨S2⟩ expectation values and spin density (Δρ = ρα − ρβ) over uranium atoms are presented (right).

Table 3. Energy Difference (eV) between States and Magnetic Coupling Constant J (cm−1) Obtained Following Two Mapping Schemes E − E HS J = 2 BS S HS − S2 BS (1) J=

E BS − E HS S

2

C2 D2h

(2)

max

symmetry 5 5

state

ΔE

Δ⟨S2⟩

J1

J2

A−ABS B2g−B2gBS

0.38 0.65

4.04 4.06

758.2 1290.5

505.7 863.0

first report we calculated the geometrical parameters and spectroscopic properties of the complex in two different symmetries, determining its three low-lying electronic configurations, with the quintuplet configuration being the ground state for both symmetries. Because of the determination of the spin density and the important localization of the unpaired electrons over uranium atoms, we can conclude that the

f

assignment

420.9 424.6 431.3 440.0 459.6 461.8 472.0 484.7 503.9 510.3 536.1 548.0 581.3 588.2 621.6 636.0 641.5 650.2 671.3 674.0 676.0 713.5 795.7

3.0 2.0 4.3 2.7 2.6 2.1 1.6 7.8 1.3 2.9 3.5 2.9 1.7 1.5 1.1 2.0 5.2 1.3 2.1 8.7 1.4 1.6 1.2

× × × × × × × × × × × × × × × × × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−6 10−5

423.5 456.1 494.5 507.4 533.4 541.3 556.5 693.7

3.0 1.3 9.3 3.2 4.9 1.4 6.6 1.0

× × × × × × × ×

10−2 10−4 10−3 10−2 10−3 10−3 10−3 10−3

C2 δ → f (LMCT) δ → f (LMCT) π → f (LMCT) δ → f (LMCT) π → f (LMCT) δ → f (LMCT) δ → f (LMCT) f→d δ → f (LMCT) f→d f→d π → f (LMCT) f→f f→f f→f f→f f→f f→f f→f f→f f→f f→f f→f D2h δ → f (LMCT) δ → f (LMCT) f→d δ → f (LMCT) f→d f→f δ → f (LMCT) f→f

⟨μ⟩ (au) 2.1 1.7 2.5 2.0 2.0 1.8 1.6 3.5 1.5 2.2 2.5 2.3 5.7 5.5 4.8 6.4 1.0 5.3 6.8 4.4 1.8 1.5 1.8

× × × × × × × × × × × × × × × × × × × × × × ×

10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−2 10−2 10−2 10−2 10−1 10−2 10−2 10−2 10−2 10−2 10−2

6.5 4.4 3.8 7.3 2.9 1.6 3.5 1.5

× × × × × × × ×

10−1 10−2 10−1 10−1 10−1 10−1 10−1 10−1

By definition molecular orbitals with δ and π symmetries are the results of the following linear combinations: φδ = c1[fxyz(fz), dxz(dyz)](U) + c2π(COT) and φπ = c1[fxz2(fyz2)](U) + c2π(COT). The criterion to classify a transition as LMCT is because in these cases c2 2 ≫ c1 2 . a

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paramagnetic properties of the adduct molecule arise from the magnetic properties of the individual uranocene fragments. Similar to the case for the monomers in this molecule, the highspin configurations are more stable and the electronic states with a formal interaction of two monomers in the singlet configuration are quite destabilized with respect to the ground state. The calculation of the magnetic coupling constant between both uranium atoms using the Heisenberg Hamiltonian showed a strong ferromagnetic coupling, which is established between the 5f orbitals where the unpaired electrons are located. On the other hand, the absorption spectra calculated showed several electronic transitions mainly with ligand to metal charge transfer character (LMCT) and f → d character in the region between 400 and 550 nm; the near-IR shifted transitions have f → f character and match nicely with the experimental reported transitions. Finally, we think that relativistic DFT is a good approach for the calculation of the properties discussed here. At this theoretical level we found the ground state of the complex and obtained valuable information about the principal low-lying electronic configurations.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge funding from Grant Nos. FONDECYT 1110758, UNAB-DI-05-11/I, UNAB-DI-195-12/I, and UNAB-DI-17-11/R and an MECESUP fellowship.

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dx.doi.org/10.1021/om300560h | Organometallics 2012, 31, 6297−6304