Slow Magnetic Relaxation in Uranium(III) and Neodymium(III

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Slow Magnetic Relaxation in Uranium(III) and Neodymium(III) Cyclooctatetraenyl Complexes Jennifer J. Le Roy, Serge I. Gorelsky, Ilia Korobkov, and Muralee Murugesu* Chemistry Department and the Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5, Canada S Supporting Information *

ABSTRACT: The synthesis, structure, and magnetic properties of a uranium(III) sandwich complex, [Li(DME)3][UIII(COT″)2] (COT″ = bis(trimethylsilyl)cyclooctatetraenyl dianion), and its coordinatively analogous tetravalent equivalent, [UIV(COT″)2], were investigated. Additionally, a full structural and magnetic comparison to the isostructural and isoelectronic lanthanide complex, [Li(DME)3][NdIII(COT″)2], is provided. DFT calculations reveal that the UIII complex leads to weaker ligand-to-metal donation as compared with the tetravalent equivalent complex. Alternating current magnetic susceptibility results reveal slow magnetic relaxation in both UIII and NdIII complexes. The enhanced magnetic performance of the UIII congener further encourages the use of actinides in the design of single-molecule magnets. important to explore how new ligand fields effect the magnetic properties of actinides. With this in mind we have turned our attention toward uranocene-type sandwich complexes. The influence of a sandwich-type ligand field is unknown on the SMM properties of uranium. Moreover, strong ligand donation has been well established in uranocene [UVI(COT)2], which is a desirable property in a magnetic building block.7 However, magnetically UIII is preferable to UIV, where UIII has been recognized to have the required components such as anisotropy for attaining SMM behavior.5g Here, we report the use of 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion (COT″) as the ligand to isolate uranocene-type molecules with unsymmetrical coordination geometries and multiple oxidation states. We describe the synthesis, structure, and magnetic properties of [Li(DME)3][UIII(COT″)2], 1, and its coordinatively analogous tetravalent equivalent, [UIV(COT″)2], 2. The magnetic properties of several lanthanide-COT″ complexes have been established, with the ErIII analogue exhibiting a high blocking temperature of 8 K.2a,6a,8 Isostructural and isoelectronic complexes in the lanthanide and actinide series provide a unique opportunity to elucidate subtle differences such as how atomic mass (z) influences magnetic performance. Therefore, in addition to the computational study on the metal−ligand covalency of complexes 1 and 2, the magnetic properties of 1 are compared to structural analogue [NdIII(COT″)2][Li(DME)3], 3. Uranocene is unreactive to lithiation,9 and all reported trivalent uranocene-type complexes have been prepared via reduction of [UIV(COT-R)2] (R = CH3,10 1,4-(tBuMe2Si)211) by

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remendous effort has been put forth toward improving lanthanide single-molecule magnets (SMMs) for their use in magnetic materials.1 Many of the best candidates thus far have been single-ion lanthanide complexes, which display remarkable magnetic properties due to unquenched orbital angular momentum.2 However, the operating temperature of even the best lanthanide SMMs is well below what is required for practical applications. One of the main challenges toward higher temperature lanthanide SMMs is synthesizing magnetic complexes with strong superexchange-type metal−metal interactions, where metals are strongly coupled through diamagnetic bridging ligands. This requires strong metal−ligand covalency, which is challenging due to the poor radial extension of 4f orbitals. Uranium complexes have tremendous potential as SMMs, as they possess the key physical properties of large intrinsic total ground state spin (S) and more importantly uniaxial magnetic anisotropy (D) required for magnet-like behavior of slow relaxation of the magnetization.3 Akin to lanthanide ions, the heavy-element nature of uranium can additionally result in significant spin−orbit coupling constants.4 However, unlike lanthanides, actinides have enhanced 5f radial extension, which leads to substantial metal−ligand covalency, allowing for strong exchange coupling (J) between metals in multimetallic complexes. Despite the many obvious advantages, only a handful of actinide SMMs have been reported, none of which have shown enhance magnetic properties over the best performing lanthanide SMMs.5 It is well established that even subtle variations in the ligand field can drastically change the magnetic properties of lanthanide SMMs.6 Therefore, in order to isolate new uranium SMMs with large anisotropic barriers, it is © XXXX American Chemical Society

Received: November 29, 2014

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DOI: 10.1021/om501214c Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

(nearest C−H distance 2.76 Å (1); 2.74 Å (3)) undoubtedly contributes to the difference in tilt angle (Figure S5). Also of interest, 2 is only the second example of a uranocene with a bent structure where the (ring centroid)−U−(ring centroid) angle deviates from perfect linearity (180°) by 6.5°.14 Lorenz et al. recently reported a uranocene with bulky [C8H6(SiPh3)2]2− ligands leading to a large 11.3° (ring centroid)−U−(ring centroid) linear deviation.14 The less sterically bulky TMS groups account for the reduced bend in 2 in comparison to SiPh3 groups. Interestingly complex 1 has a (ring centroid)−U−(ring centroid) angle of 172.8°; we can therefore conclude that the oxidation has little effect of the bend angle. To fully understand the consequence of oxidation state on uranium−COT″ bonding, the electronic structure of 1 and 2 was probed using DFT calculations conducted at the spinunrestricted B3LYP/TZVP level of theory (the SDD basis set and effective core potential for the U atom). Single-point calculations were conducted using Gaussian 09 with the crystal structure geometries in which COT″ and methyl C−H bond distances were adjusted from the X-ray model values to 1.07 and 1.08 Å, respectively. The optimized wave functions for the ground states (with S = 1 for UIV and S = 3/2 for UIII) were used to evaluate bonding contributions. Figure S6 illustrates the spin density distribution of the electronic ground states. In complex 1, the UIII atom carries a spin density of 3.026 au due to three singly occupied 5f orbitals of the UIII ion (α-spin HOMO−2, HOMO−1, and the HOMO of the complex, Figure S7), while the COT″ ligands demonstrate weak spin polarization (a spin density of −0.013 au per ligand). Overall, each dianionic COT″ ligand donates ∼0.92 e− to UIII, resulting in the +1.16 au charge for the U atom. The Mayer bond order between UIII and each COT″ ligand is 1.73; α- and β-spin occupied orbitals contribute 0.98 and 0.75, respectively, to the total metal−ligand bond order. The analysis of the wave function in terms of contributions from fragment orbitals (the metal cation being one fragment and the two anionic ligands being the other two fragments) indicate that only charge donation from the COT″ ligands to the UIII contribute to the covalent bonding in complex 1. Five occupied π orbitals of the COT″2− ligands (Figure S8), namely, the highest occupied fragment orbital (HOFO), HOFO−1, HOFO−2, HOFO−3, and HOFO−7, participate significantly (change in orbital population is greater than 3%) in covalent bonding with the UIII ion. The changes in the populations of these fragment molecular orbitals are 11.3%, 11.5%, 3.3%, 4.1%, and 6.8% for α-spin manifold and 8.0%, 8.2%, 3.0%, 3.7%, and 6.2% for β-spin manifold, respectively. The analysis of populations of UIII atomic orbitals (Table S2) indicates that metal s, p, d, and f unoccupied orbitals participate in ligand-to-metal donation, with the 6d orbitals receiving almost half of the total electron density (1.84 e−) transferred from the two ligands. In 2, the UIV atom carries a spin density of 2.306 au due to two singly occupied 5f orbitals of the UIV (α-spin HOMO−3 and the HOMO of the complex, Figure S7), while the COT″ ligands demonstrate a more significant spin polarization (a spin density of only −0.153 au per ligand). This is due to a difference in charge donation from the dianionic COT″ ligands to UIV through α- and β-spin orbitals (see below). Overall, each dianionic COT″ ligand donates ∼1.5 e− to UIV, resulting in the +1.01 au charge for the UIV atom. The Mayer bond order between UIV and each COT″ ligand is 2.42, showing a significant increase in the metal−ligand covalency as compared to the corresponding UIII complex. The

potassium metal. Our synthetic strategy is a direct method where reaction of [UIIII3(1,4-dioxane)1.5]12 with [Li4(COT″)2(THF)4]8a in DME (dimethoxyethane) yields 1, the first [UIII(COT”)2]− lithium salt. Single crystals of 1 suitable for X-ray diffraction were grown from a 50:50 mixture of DME/ hexanes. Complex 2 was produced in a two-step synthesis where [Li4(COT″)2(THF)4]8a and [UIIII3(1,4-dioxane)1.5]12 were first combined in THF to produce [UIII(COT″)2][Li(THF)4], following which the metal was oxidized to UIV using FeCl2. Upon concentrating the solution, large green block crystals of 2 were isolated, suitable for single-crystal X-ray diffraction. [NdIII(COT″)2][Li(THF)4], 3, was synthesized with minor modifications to a previously published method.13 Single crystals of 3 were grown in a concentrated solution of a 50:50 mixture of DME/hexanes. Complex 1 crystallizes in the triclinic P1̅ space group where the UIII ion is bound to two COT″ ligands in a η8-fashion to form a distorted sandwich complex. The asymmetric η 8-COT″ coordination is reflected by UIII−CCOT″ bond distances ranging from 2.726(0) to 2.755(1) Å (Figure 1). The lithium counterion

Figure 1. Molecular structures of 1−3. Thermal ellipsoids are drawn at 50%. Hydrogen atoms are omitted for clarity. Color code: Si = green, UIII = pink, UIV = purple, NdIII = blue, C = gray, O = red, Li = light blue.

in the crystal lattice adopts an octahedral coordination environment filled by three DME molecules. Complex 2 crystallizes in a monoclinic P2/c space group, and the UIV ion is also asymmetrically coordinated by two η8-COT″ ligands with UIV−CCOT″ bond distances ranging from 2.646(3) to 2.669(3) Å. It is noteworthy, upon oxidation of the UIII ion to UIV, that metal−COT″ distances become shorter by an average of 0.07 Å as a consequence of the smaller atomic radius and possibly an enhanced covalent interaction between the COT″ and the higher oxidation state UIV ion. Complex 3 is isostructural to 1, crystallizing in a triclinic P1̅ space group. As expected, complexes 1 and 3 have very similar average metal−carbon distances of 2.74 and 2.73 Å, respectively. X-ray structures and a table containing the structural details of 1−3 are provided in the Supporting Information (Figures S1−3, Table S1). When viewing all three complexes from above, the CCOT″ atoms in each layer are staggered with CCOT″ atoms in other layers, most likely in order to sterically accommodate the bulky TMS (trimethylsilyl) groups (Figure S4).14 Complexes 1, 2, and 3 also have distinctly different tilt angles of 4.4°, 6.9°, and 3.8°, respectively. This difference in tilt angle can be due to several factors such as ionic radii, oxidation state15 of the metal ion, and intermolecular interactions. For complexes 1 and 3, the close contact between the sandwich molecule and the DME attached to the Li counterion through an asymmetrical steric interaction B

DOI: 10.1021/om501214c Organometallics XXXX, XXX, XXX−XXX

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Organometallics α- and β-spin occupied orbitals contribute 1.34 and 1.08, respectively, to the total metal−ligand bond order. Similar to the UIII complex, only charge donation from the COT″ ligands to UIV contribute to the covalent bonding in the UIV complex, and five occupied π orbitals of the COT″2− ligands (Figure S8) participate in covalent bonding with the UIV ion. The changes in the populations of the HOFO, HOFO−1, HOFO−2, HOFO−3, and HOFO−7 of the COT2− ligand are 22.7%, 22.5%, 6.0%, 5.3%, and 7.2% for α-spin manifold and 14.8%, 14.8%, 5.1%, 4.4%, and 7.6% for β-spin manifold, respectively. When going from UIII to UIV, the energies of the metal orbitals were lowered due to a higher ionic charge and a new 5f orbital became empty and thus opened up for donation. Thus, ligand-to-metal donation became stronger (Table S2), with the 5f and 6d orbitals receiving most of the total electron density (2.99 e−) transferred from the two ligands. As can be seen from the electronic structure descriptors such as bond orders, atomic charges, and changes in populations of fragment orbitals, the U−C bonds in the UIV complex are more covalent than the U−C bonds in the UIII complex. The observation of shorter UIV−C bonds (average value of 2.66 Å) in the X-ray structure of 2 relative to the UIII−C bonds (average value of 2.74 Å) in the X-ray structure of 1 is consistent with a stronger, more covalent metal−ligand interaction in 2. Although UIII−COT″ covalency in 1 is less than that of 2, the Mayer bond order of 1 is still far greater than the 1.2 calculated for structurally analogous Dy III complex [Li(DME) 3 ][DyIII(COT″)2].8a This confirms the radial extension of actinide ions is greater than that of the lanthanide ions. This increase signifies 1 has potential as a desirable SMM building block unit for creating larger UIII-SMMs with U ions coupled via a superexchange mechanism. One additional advantage of utilizing actinides as SMMs is the theoretical increase in spin−orbit coupling constant with atomic mass.4 However, to understand the effect a higher z has on the magnetic properties of a SMMs, isoelectronic complexes such as 1 and 3 need to be carefully examined. Only one direct comparison between isostructural and isoelectronic 4f/5f molecular magnets has been presented, where UIII and NdIII trispyrazolylborate (Tp) complexes exhibited slow magnetic relaxation with Ueff = 2.84(7) and 3.81(8) cm−1, respectively, under a 100 Oe applied dc field.5h Reported UTp3 exhibited only slightly enhanced magnetic properties over the NdTp3, where temperature dependence of the magnetization in both complexes was very small due to the distorted triangular dodecahedron geometry. A major challenge in the rational design of UIII SMMs is the limited predictability of how different ligand field environments would affect SMM properties. Baldovi et al. recently suggested complexes with ligand electron density along the symmetry axis as well as a trigonal prismatic geometry may provide ideal conditions to harness UIII SMM behavior.16 The sandwich architecture of complexes 1−3 provides the strictly axial coordination environment that is magnetically favorable for elements with a prolate f-electron distribution like erbium.2a,17 The oblate nature of the f-electron distribution of NdIII and UIII likely means that the magnetic properties of 1 and 3 will behave similarly to the recently published DyIII analogue, which exhibited a spin reveal barrier of Ueff = 25 K under zero applied direct current (dc) field.6a Direct current magnetic susceptibility measurements of 1−3 were performed in the temperature range 1.8−300 K under a 1000 Oe applied dc field (Figure S9). The room-temperature χT values of 1.13 cm3·K·mol−1 (1) and 1.50 cm3·K·mol−1 (2) are within the range typically reported for UIII and UIV monomers,

respectively.5,18 The room-temperature χT value of 1.63 cm3·K· mol−1 for 3 is in good agreement with the theoretical value of 1.64 cm3·K·mol−1 for a mononuclear NdIII complex. The temperature dependence of the χT product of 1 shows a slight decrease in magnetic susceptibility with decreasing temperature to reach a minimum value of 0.33 cm3·K·mol−1 at 1.8 K. The χT product of 2 decreases gradually with decreasing temperature; below 50 K there is a sharp decrease to a minimum value of 0.14 cm3·K·mol−1 at 1.8 K (Figure S9). The sharp decrease in the χT product is most likely due to the thermal depopulation of higher excited states upon decrease of temperature, as often seen for a UIV ion (ground term 3H4). The quenching of the magnetic moment could also result form the low symmetry and loss of Kramer degeneracy for UIV. The temperature dependence of the χT product of 3 decreases gradually with decreasing temperature, with a sharp decrease below 10 K to reach a minimum value of 0.33 cm3·K·mol−1 at 1.8 K. The final decrease at low temperature is most likely due to single-ion effects such as significant anisotropy as often encountered in LnIII compounds rather than intermolecular interactions (closest NdIII−NdIII distance is 10.53 Å). The magnetization as a function of field in 1 and 3 (Figure S10−S12) shows no saturation even at low temperature and high fields. The reduced magnetization plot shows nonsuperimposition of iso-temperature lines, indicating significant magnetic anisotropy in both complexes. In the case of 2, the vanishingly small values are most likely arising from the low-lying excited states, which can be easily populated even at 1.8 K (Figure S11). Although no hysteresis was observed for 1−3, to probe slow magnetization relaxation dynamics, alternating current (ac) susceptibility measurements were performed. Under zero dc field no ac signal was observed for 1−3. To compare the slow relaxation of complexes 1 and 3, all ac susceptibility measurements were performed under an identical dc field (1000 Oe). Frequency-dependent studies reveal a strong frequency-dependent in-phase (χ′) and out-of-phase magnetic susceptibility (χ″) in both 1 and 3 (Figure 2) indicative of field-induced SMM

Figure 2. Out-of-phase magnetic susceptibility of 1 (left) and 3 (right) under a 1000 Oe applied dc field between indicated temperatures.

behavior. The anisotropic energy barrier, Ueff, can be obtained from the high-temperature regime of the slow magnetic relaxation data where it is thermally induced (Arrhenius law τ = τ0 exp(Ueff/kBT)). Effective energy barriers of 27 and 21 K with pre-exponential factors (τ0) of 4.6 × 10−6 and 5.5 × 10−5 s for 1 and 3, respectively, were extracted from the ac data (Figure S13). This obtained value for 1 is consistent with other recently reported UIII single-ion magnets (SIMs) (Ueff = 4−29 K).5 The obtained Ueff of 21 K for 3 is significantly higher than the 3.9 K (2.8 cm−1) previously reported for a NdIII SMM.5h In the case of 2 no SIM behavior was observed, once more confirming the nonmagnetic ground state of UIV ions.18 C

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(3) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, U.K., 2006. (4) (a) Khudyakov, I. V.; Serebrennikov, Y. A.; Turro, N. J. Chem. Rev. 1993, 93, 537. (b) Meihaus, K. R.; Long, J. R. Dalton Trans. 2015, 44, 2517. (5) (a) Rinehart, J. D.; Long, J. R. J. Am. Chem. Soc. 2009, 131, 12558. (b) Mougel, V.; Chatelain, L.; Pecaut, J.; Caciuffo, R.; Colineau, E.; Griveau, J.-C.; Mazzanti, M. Nat. Chem. 2012, 4, 1012. (c) Mills, D. P.; Moro, F.; McMaster, J.; van Slageren, J.; Lewis, W.; Blakel, A. J.; Liddle, S. T. Nat. Chem. 2011, 3, 454. (d) Coutinho, J. T.; Antunes, M. A.; Pereira, L. C. J.; Bolvin, H.; Marçalo, J.; Mazzantic, M.; Almeida, M. Dalton Trans. 2012, 41, 13568. (e) Antunes, M. A.; Pereira, L. C. J.; Santos, I. C.; Mazzanti, M.; Marçalo, J.; Almeida, M. Inorg. Chem. 2011, 50, 9915. (f) King, D. M.; Tuna, F.; McMaster, J.; Lewis, W.; Blake, A. J.; McInnes, E. J. L.; Liddle, S. T. Angew. Chem., Int. Ed. 2013, 52, 4921. (g) Moro, F.; Mills, D. P.; Liddle, S. T.; Slageren, J. V. Angew. Chem., Int. Ed. 2013, 125, 1. (h) Rinehart, J. D.; Long, J. R. Dalton Trans. 2012, 41, 13572. (i) Rinehart, J. D.; Harris, T. D.; Kozimor, S. A.; Bartlett, B. M.; Long, J. R. Inorg. Chem. 2009, 48, 3382. (j) Rinehart, J. D.; Meihaus, K. R.; Long, J. R. J. Am. Chem. Soc. 2010, 132, 7572. (k) Magnani, N.; Apostolidis, C.; Morgenstern, A.; Colineau, E.; Griveau, J.-C.; Bolvin, H.; Walter, O.; Caciuffo, R. Angew. Chem., Int. Ed. 2011, 50, 1696. (l) Coutinho, J. T.; Antunes, M. A.; Pereira, L. C. J.; Marçalo, J.; Almeida, M. Chem. Commun. 2014, 50, 10262. (m) Pereira, L. C. J.; Camp, C.; Coutinho, J. T.; Chatelain, L.; Maldivi, P.; Almeida, M.; Mazzanti, M. Inorg. Chem. 2014, 53 (22), 11809. (n) Meihaus, K. R.; Minasian, S. G.; Lukens, W. W., Jr.; Kozimor, S. A.; Shuh, D. K.; Tyliszczak, T.; Long, J. R. J. Am. Chem. Soc. 2014, 136 (16), 6056. (o) Antunes, M. A.; Santos, I. C.; Bolvin, H.; Pereira, L. C. J.; Mazzanti, M.; Marcalo, J.; Almeida, M. Dalton Trans. 2013, 42, 8861. (6) (a) Le Roy, J. J.; Jeletic, M.; Gorelsky, S. I.; Korobkov, I.; Ungur, L.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2013, 135, 3502. (b) Petit, S.; Pilet, G.; Luneau, D.; Chibotaru, L. F.; Ungur, L. Dalton Trans. 2007, 40, 4582. (c) Cucinotta, G.; Perfetti, M.; Luzon, J.; Etienne, M.; Car, P.-E.; Caneschi, A.; Calvez, G.; Bernot, K.; Sessoli, R. Angew. Chem., Int. Ed. 2012, 51, 1606. (7) (a) Dallinger, R. F.; Stein, P.; Spiro, T. G. J. Am. Chem. Soc. 1978, 100, 7865. (b) Chang, A. H. H.; Pitzer, R. M. J. Am. Chem. Soc. 1989, 111, 2500. (8) (a) Jeletic, M.; Lin, P.-H.; Le Roy, J. J.; Korobkov, I.; Gorelsky, S. I.; Murugesu, M. J. Am. Chem. Soc. 2011, 133, 19286. (b) Le Roy, J.; Korobkov, I.; Murugesu, M. Dalton Trans. 2014, 43, 2737. (9) Seyferth, D. Organometallics 2004, 23, 3562. (10) Boussie, T. R.; Eisenberg, D. C.; Bigsbee, J.; Streitwieser, A., Jr.; Zalkin, A. Organometallics 1991, 10, 1922. (11) Parry, S. J.; Cloke, F. G. N.; Coles, S. J.; Hursthouse, M. B. J. Am. Chem. Soc. 1999, 121, 6867. (12) Monreal, M. J.; Thomson, R. K.; Cantat, T.; Travia, N. E.; Scott, B. L.; Kiplinger, J. L. Organometallics 2011, 30, 2031. (13) Edelmann, A.; Lorenz, V.; Hrib, C. G.; Hilfert, L.; Blaurock, S.; Edelmann, F. T. Organometallics 2013, 32, 1435. (14) (a) Lorenz, V.; Schmiege, B. M.; Hrib, C. G.; Ziller, J. W.; Edelmann, A.; Blaurock, S.; Evans, W. J.; Edelmann, F. T. J. Am. Chem. Soc. 2011, 133, 1257. (b) Tsoureas, N.; Summerscales, O. T.; Cloke, F. G. N.; Roe, S. M. Organometallics 2013, 32, 1353. (15) Boussie, T. R.; Eisenberg, D. C.; Bigsbee, J.; Streitwieser, A., Jr.; Zalkin, A. Organometallics 1991, 10, 1922. (16) Baldovi, J. J.; Cardona-Serra, S.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Arino, A. Chem. Sci. 2013, 4, 938. (17) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078. (18) (a) Siladke, N. A.; Meihaus, K. R.; Ziller, J. W.; Fang, M.; Furche, F.; Long, J. R.; Evans, W. J. J. Am. Chem. Soc. 2012, 134, 1243. (b) Castro-Rodrí guez, I.; Meyer, K. Chem. Commun. 2006, 1353. (c) Seaman, L. A.; Pedrick, E. A.; Tsuchiya, T.; Wu, G.; Jakubikova, E.; Hayton, T. W. Angew. Chem., Int. Ed. 2013, 52, 10589. (d) Lewis, A. J.; Williams, U. J.; Carroll, P. J.; Schelter, E. J. Inorg. Chem. 2013, 52, 7326.

The observed relaxation barriers for both 1 and 3 are smaller than the one observed in our analogous DyIII compound; however it is important to emphasize that the inherent spin value for the DyIII ion is much higher than for the UIII and NdIII compounds.2a Thus, from the direct comparison of isostructural and isoelectronic complexes of 1 and 3, it is reasonable to conclude that in the same ligand field environment the inherent magnetic anisotropy of the UIII ion is larger. In conclusion, we have successively synthesized two structurally similar U(COT″)2 sandwich complexes. Through the employment of TMS-substituted COT″ ligands, structural characterization of trivalent and tetravalent U complexes was obtained. Contrary to previous understanding, the comparative structural study in our case reveals the oxidation state of the metal ion has little effect on the (ring centroid)−U−(ring centroid) bend angle. A detailed DFT study reveals ligand-tometal donation is stronger for the UIV ion compared to UIII due to the additional vacant metal orbital. The observed difference in low-temperature magnetic behavior is due to the presence of significant spin−orbit coupling in the UIII ion. The latter effect not only enhances the anisotropy of the system but also contributes toward the magnet-like behavior seen in 1. Although this first UIII sandwich SIM has a smaller Ueff barrier than the analogous DyIII complex, the covalent bonding of the UIII ion with the COT″ ligand suggests these mononuclear units can be ideal for the preparation of multinuclear SMMs with large energy barriers.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra and energies of the computed species. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Ottawa, NSERC (Discovery and RTI grants); ERA, CFI, and ORF.

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REFERENCES

(1) (a) Layfield, R. A. Organometallics 2014, 33, 1084. (b) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Chem. Rev. 2013, 113, 5110 and references therein. (c) Fahrendorf, S.; Atodiresei, N.; Besson, C.; Caciuc, V.; Matthes, F.; Blügel, S.; Kögerler, P.; Bürgler, D. E.; Schneider, C. M. Nat. Commun. 2013, 4, 2425. (d) Mannini, M.; Pineider, F.; Danieli, C.; Totti, F.; Sorace, L.; Sainctavit, P.; Arrio, M. A.; Otero, E.; Joly, L.; Cezar, J. C.; Cornia, A.; Sessoli, R. Nature 2010, 468, 417. (2) (a) Le Roy, J. J.; Korobkov, I.; Murugesu, M. Chem. Commun. 2014, 50, 1602. (b) Ungur, L.; Le Roy, J. J.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. Angew. Chem., Int. Ed. 2014, 53, 4413. (c) Jiang, S.-D.; Wang, B.-W.; Sun, H.-L.; Wang, Z.-M.; Gao, S. J. Am. Chem. Soc. 2011, 133, 4730. (d) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Am. Chem. Soc. 2003, 125, 8694. (e) Ganivet, C. R.; Ballesteros, B.; de la Torre, G.; Clemente-Juan, J. M.; Coronado, E.; Torres, T. Chem.Eur. J. 2013, 19, 1457. (f) Zhang, P.; Zhang, Li.; Wang, C.; Xue, S.; Lin, S.-Y.; Tang, J. J. Am. Chem. Soc. 2014, 136, 4484− 4487. D

DOI: 10.1021/om501214c Organometallics XXXX, XXX, XXX−XXX