DOI: 10.1002/chem.201501545
Communication
& f-Block Elements
f-Block Ansa Complexes in the Solid State: [3]Thoro- and [3]Uranocenophanes Holger Braunschweig,* Mehmet Ali Celik, Klaus Dìck, Florian Hupp, and Ivo Krummenacher[a] Abstract: The preparation of [3]thoro- and [3]uranocenophanes, the first structurally authenticated ansa-bridged complexes of actinocenes, is reported. Following a flytrap route, 1,2-bis(cyclooctatetraenyldimethylsilyl)methane was synthesized, reduced to its tetraanion, and subsequently converted into bridged uranocene and thorocene complexes by salt metathesis with the corresponding actinide tetrachlorides. In addition, their electronic structures have been investigated by experimental (UV/Vis spectroscopy, cyclic voltammetry) and theoretical (DFT) methods. Figure 1. Sandwich and ansa sandwich compounds of actinides.
Sandwich compounds of the d-block elements are of great interest to chemists due to their broad and varied reactivity and wide range of physical properties.[1] It is thus not surprising that molecules such as ferrocene have found use in numerous technological applications.[1] In contrast, sandwich compounds containing f-block metals have only been scarcely investigated, although such complexes possess promising magnetic, electronic, photophysical, and catalytic properties.[2] In the development of the organometallic chemisty of the f-block elements, the cyclooctatetraene dianion (COT) has played a similarly crucial role to that played by the cyclopentadienyl anion (Cp) in corresponding transition metal organometallic chemistry.[3] In particular, the synthesis and study of the electronic properties of uranocene, [U(h8-C8H8)2] (1 b), was a major breakthrough in this field as it showed that the f orbitals of the central metal atom participate in metal–ligand bonding.[3] In the following years, bis(COT) complexes of other actinides, such as thorium, protactinium, plutonium, and neptunium, were also reported (Figure 1).[4] Although these complexes exhibit diverse reactivities and properties, such as Lewis acidity or redox activity, little interest has been directed to preparing ansa derivatives, potential precursors for ring-opening polymerization.[1a, 5] Streitwieser et al. were the first and, to our knowledge, only research group involved in the development of bridged “met[a] Prof. Dr. H. Braunschweig, Dr. M. A. Celik, Dr. K. Dìck, Dr. F. Hupp, Dr. I. Krummenacher Institut fìr Anorganische Chemie Julius-Maximilians-Universitt Wìrzburg Am Hubland, 97074 Wìrzburg (Germany) Fax: (+ 49) 931-31-84623 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501545. Chem. Eur. J. 2015, 21, 9339 – 9342
allocenes” of the actinides.[6] This group reported the synthesis of the first ansa uranocene (2) by reaction of K4[(C8H7SiMe2)2C2H4], a tetraanionic ligand system, with uranium tetrachloride (Figure 1). It should be noted that the constitution of 2 was only postulated based on 1H NMR spectroscopy and mass spectrometry. We have now succeeded in the synthesis and full characterization, including structural analysis by X-ray crystallography, of an unprecedented bridged thorocene, which we report herein together with its uranium analog. The compounds represent the first unambiguouslycharacterized examples of ansa complexes of the actinide metals. The ligand precursor for the so-called “flytrap” reaction, (C8H7SiMe2)2CH2 (3), was prepared by treatment of (ClSiMe2)2CH2 with two equivalents of cyclooctatetraenyl lithium and obtained as a yellow oil in 68 % yield (see the Supporting Information, Schem S1). To test the potential of 3 as a ligand precursor for actinocenophane chemistry, we investigated its reduction behavior electrochemically.[7] The cyclic voltammogram of 3 in THF solution shows two overlapping reduction processes centered around ¢2.50 V vs. Fc/Fc + , consistent with stepwise reduction to the corresponding tetraanion (see the Supporting Information, Figure S5).[8] Chemical reduction to the corresponding tetraanion K4[(COTSiMe2)2CH2] (K4[3]) was achieved by treatment of 3 with potassium in THF. Subsequent conversion of the salt with [ThCl4(dme)2] (dme = dimethoxyethane) or UCl4 afforded [Th(h8-C8H7SiMe2)2CH2] (4 a) and [U(h8C8H7SiMe2)2CH2] (4 b), respectively, in moderate yields as yellow (4 a, 30 %) and green (4 b, 14 %) solids (Scheme 1). The actinocenophanes are moderately air stable but under inert conditions they can be stored in solution or as solids without any visible decomposition.
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Scheme 1. Synthesis of the actinocenophanes 4 a and 4 b.
The constitution of the actinocenophanes 4 a and 4 b was proven by elemental analysis, NMR spectroscopy and X-ray diffraction analysis. Whereas compound 4 a shows the expected diamagnetic chemical shifts for its 1H, 13C and 29Si NMR resonances, the 1H NMR spectrum of compound 4 b shows distinct paramagnetic shifts, typical for uranocene and its derivatives.[3a, 6] For instance, the 1H NMR signals of the C8H7 ring protons appear in the range d = ¢32.14 to ¢39.82 ppm and are thus considerably shifted to lower frequencies in comparison to the starting material. The resonances for the carbon atoms of the bridge are also strongly shifted to lower frequencies in the 13C{1H} NMR spectrum [d = 1.65 ppm (Si(CH3)2), d = ¢30.3 ppm (CH2)], whereas the ring carbons of the ligand could not be detected. In addition, a weak signal for the silicon nuclei in the 29Si{1H} NMR spectrum could be observed at d(29Si) = ¢125.2 ppm. It should be noted that, due to the integer-spin 5f2 electronic configuration of the uranium ion, no EPR signal at the X-band (9.4 GHz) was observed for a solution of 4 b in toluene/THF over the temperature range of 10–300 K, consistent with the observation for uranocene and its substituted analogues.[9] We were able to obtain crystals suitable for X-ray analysis of both complexes (Figure 2). The structural parameters of 4 a indicate only a slightly distorted structure with a tilt angle of the COT rings (a) of 5.0(2)8 (XCOT-Th-XCOT: d = 176.88; XCOT = centroid of the C8H7 ring) and an exocyclic Cipso¢Si bond that lies approximately in the plane of the COT rings (b = 58; see the Sup-
Figure 2. Molecular structure of [Th(h8-C8H7SiMe2)2CH2] (4 a, left) and [U(h8C8H7SiMe2)2CH2] (4 b, right). Hydrogen atoms and thermal ellipsoids of the substituents on the silicon atoms are omitted for clarity. Thermal ellipsoids are displayed at the 50 % probability level. Selected bond lengths [pm] and angles [8]: 4 a (left): Th¢CCOT 268.2(3)–272.4(3), Th¢XCOT 198.3 and 199.3; C1Si1-C3 111.2(2), C2-Si2-C3 111.2(2), Si1-C3-Si2 123.1(2); a = 5.0(2), b = 4.4 and 5.9, g = 3.4 (Cipso-XCOT-X’COT-C’ipso), d = 176.8; 4 b (right): U¢CCOT 262.8(7)– 267.7(7), U¢XCOT 190.7 and 191.7; C1-Si1-C3 111.8(3), C2-Si2-C3 111.4(3), Si1C3-Si2 121.7(3); a = 3.3(3), b = 0.04 and 0.06, g = 4.1, d = 178.2 (XCOT = centroid of the C8H7 ring). Chem. Eur. J. 2015, 21, 9339 – 9342
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porting Information, Figure S1 for description of parameters). The eight-membered rings are close to an eclipsed conformation (twisted by ca. 38). Compared to the unbridged derivative [Th(h8-C8H8)2] (1 a), the Th¢C and Th¢XCOT distances are essentially unchanged.[4b] Furthermore, the C¢C bond lengths in the planar carbocyclic rings are almost equal (ca. 141 pm), consistent with the view that the 10 p-electron system of COT acts as an h8-ligand to thorium. Similarly, the effect of the ansa bridge on the molecular structure of the ansa uranocene 4 b is minor (Figure 2). The distortion induced by the bridge can be quantified by the angles a = 3.3(3)8, d = 178.28, and b = 0.18. Taken together with the uranium–carbon bond lengths, which remain virtually unchanged compared to uranocene 1 b,[4b] the crystallographic data indicate very little ring strain within the ansa complex 4 b. All C¢C bond lengths in 4 b are approximately 141 pm and are thus consistent with an aromatic 10 p-electron h8-coordinating ligand. To probe the electronic structure of the f-block compounds 4 a and 4 b, we carried out UV/Vis spectroscopy in the range of l = 230–700 nm (see the Supporting Information, Figure S2). The spectrum of the yellow thorocenophane 4 a shows absorption bands at l = 278, 304, and 320 nm with three shoulders around 270, 295 and 340 nm, tailing off to higher wavelengths. In comparison, the unbridged thorocene 1 a shows a broad lowest-energy absorption band at l = 450 nm with a low extinction coefficient (e 64 L mol¢1 cm¢1).[10] Despite having similar colors, a corresponding transition in the visible spectrum could not be detected for compound 4 a.[10a] The uranocenophane 4 b (Figure S2) gives rise to two absorption bands at l = 295 nm and 308 nm, as well as to a shoulder at approximately 390 nm. Furthermore, three intense peaks at l = 605, 632 and 652 nm were detected in the absorption spectrum, which are comparable to those of the parent compound [U(h8-C8H8)2] (1 b: lmax = 615, 641, and 680 nm).[11] To assign these characteristic UV/Vis absorption bands, which are responsible for the green color of 4 b, to specific electronic transitions, we carried out a TD-DFT (time-dependent density functional theory) calculation at the COSMOBP86/TZVP level (see the Supporting Information, Figure S3 and Table S3). The overall agreement between the calculated and the experimental spectra for 4 b is good. The calculated absorption band at l = 642.9 nm results from an excitation from the SOMO to LUMO + 10 (Table S3), which is mainly a U(5f)!s* transition. The calculated peak at l = 633.3 nm results from four excitations from the HOMO to LUMO + 2, the HOMO to LUMO, and the HOMO¢1 to LUMO, which all correspond to U(5f)!p* transitions, whereas the excitation from the HOMO¢1 to LUMO + 1 is attributable to a U(5f)!s* transition (Table S3). The calculated peak at l = 629.5 nm results from five excitations from the HOMO¢1 to LUMO, the HOMO¢1 to LUMO + 2, and the HOMO to LUMO + 2, which are U(5f)!p* transitions, whereas the other two excitations, from the HOMO to LUMO + 1 and the HOMO¢1 to LUMO + 1, are both U(5f)!s* transitions (Tables S3 and S4). The strong absorption bands of uranocene (1 a) in the visible region, responsible for the green color, have previously been assigned
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COT ligands (Figure 3 and Table S2 in the Supporting Information). Having a triplet spin state, molecule 4 b possesses two singly-occupied molecular orbitals (SOMO: ¢3.753 eV; SOMO + 1: ¢3.650 eV) and two near-isoenergetic lowest unoccupied molecular orbitals (LUMO: ¢2.917 eV; LUMO + 1: ¢2.915 eV) that are all mainly of metal character. In addition, the character of the frontier orbitals of the ansa actinocenes 4 a and 4 b was probed by cyclic voltammetry measurements in THF solution (see the Supporting Information, Figures S6 and S7). Besides two irreversible oxidation processes starting at around ¢0.30 V vs. Fc/Fc + , the uranium complex exhibits an irreversible reduction at Epc = ¢2.57 V vs. Fc/Fc + , corresponding to reduction to UIII.[14] In contrast, the thorium derivative 4 a does not show any oxidation events in the same potential window, but instead shows two consecutive irreversible reduction waves at Epc = ¢2.54 V and ¢3.00 V vs. Fc/Fc + . It is interesting to note that the electrochemical reduction processes leading to the dianion of 4 a, a species that would be isoelectronic with 4 b, are not reversible and thus likely involve structural reorganization of the original sandwich complex. In conclusion, we have described the synthesis and full characterization of ansa complexes derived from thoro- and uranocene, representing the first unambiguously characterized ansabridged actinocenes. The geometrical changes that result from incorporation of the three-atom bridge are only minor for both ansa sandwich complexes, as illustrated by the near-perfect parallel arrangement of the COT ligands and the small deviation of the XCOT-M-XCOT (M = U, Th) angle from 1808. Analysis of the Kohn–Sham molecular orbitals shows that the HOMO and HOMO¢1 correspond to bonding orbitals between the metal f orbitals and p orbitals of the COT ligand. The LUMO, LUMO + 1, and LUMO + 2 of the ansa thorocene (4 a) are a combination of metal f orbitals and ligand p orbitals, whereas the SOMO, SOMO + 1, LUMO, and LUMO + 1 for the ansa uranocene (4 b) are mainly of metal character. Moreover, with the help of TDDFT calculations, we could assign the intense visible absorption bands in the region of l = 600–700 nm for the uranium sandwich complex to both charge-transfer transitions from the uranium f orbitals to p* orbitals of the COT ligand, as well as to metal f–d transitions. The accessibility of ansa complexes of the f-block elements, potential precursors for ring-opening polymerization, opens up new avenues for the preparation of polymers incorporating lanthanide or actinide metals in the main chain. Our future efforts will thus be directed towards the transformation of more strained ansa compounds to produce materials with useful photophysical properties.
Experimental Section
Figure 3. Frontier molecular orbitals of [Th(h8-C8H7SiMe2)2CH2] (4 a, top) and [U(h8-C8H7SiMe2)2CH2] (4 b, bottom). Chem. Eur. J. 2015, 21, 9339 – 9342
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Synthesis of [Th(h8-C8H7SiMe2)2CH2] (4 a): [(COTSiMe2)2CH2] (3; 319 mg, 0.95 mmol) was added to a suspension of freshly cut potassium (390 mg, 9.48 mmol) in THF (15 mL) at room temperature and stirred over a time period of 30 h at this temperature. After filtration, a solution of [ThCl4(dme)2] (530 mg, 0.96 mmol) in THF (20 mL) was added dropwise to the brown solution at 0 8C during
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Communication 30 min. The yellow reaction mixture was slowly warmed to room temperature and stirred for an additional 48 h and then the solvent was completely removed. The resultant solid was extracted with THF (5 Õ 20 mL), the solvent removed by evaporation, and the remaining residue extracted with pentane (3 Õ 10 mL). After removal of the solvent, the solid residue was cooled to ¢78 8C and washed with cold pentane (3 Õ 10 mL) to yield 4 a as yellow solid (127 mg, 0.22 mmol, 30 %). 1H NMR (CD2Cl2, 500.1 MHz): d = 0.59 (s, 12 H, Si(CH3)2), 0.62 (s, 2 H, CH2), 6.53–6.55 (several m, 6 H, C8H7), 6.78–6.84 (several m, 4 H, C8H7), 6.88 (s, 2 H, C8H7), 6.90 ppm (s, 2 H, C8H7); 13C{1H} NMR (CD2Cl2, 125.8 MHz): d = 1.79 (Si(CH3)2), 5.37 (CH2), 107.23 (C8H7), 107.50 (C8H7), 110.28 (C8H7), 112.94 (C8H7), 116.60 ppm (Cipso8H7); 29Si{1H} NMR (CD2Cl2, 99.4 MHz): d = 4.82 ppm; UV/Vis (CH2Cl2): lmax (e) = 270 (sh), 278 (max, 3148), 295 (sh), 304 (max, 2332), 320 (max, 1585 L mol¢1 cm¢1), 340 nm (sh); elemental analysis calcd (%) for C21H28Si2Th (568.66): C 44.36, H 4.96; found: C 43.58, H 4.89.
[2]
[3]
[4]
Synthesis of [U(h8-C8H7SiMe2)2CH2] (4 b): Freshly cut potassium (540 mg, 16.81 mmol) in THF (15 mL) was treated with [(COTSiMe2)2CH2] (3; 465 mg, 1.38 mmol) and stirred for 30 h. The suspension was filtered to remove excess potassium and then a solution of UCl4 (560 mg, 1.47 mmol) in THF (20 mL) was added dropwise over a period of 30 min at 0 8C. The reaction mixture was warmed to room temperature and stirred for another 48 h. The solvent was evaporated and the precipitate extracted with pentane (5 Õ 20 mL). The solvent was removed under vacuum and the green residue washed with cold pentane at ¢70 8C (3 Õ 10 mL). After removal of the solvent, the solid was dried in vacuum to give 4 b as green solid (114 mg, 0.20 mmol, 14 %). 1H NMR (C6D6, 500.1 MHz): d = ¢5.34 (s, 12 H, CH2), ¢25.88 (s, 2 H, Si(CH3)2), ¢32.14 (broad s, 2 H, C8H7), ¢34.85 (broad s, 4 H, C8H7), ¢37.08 (broad s, 4 H, C8H7), ¢39.82 ppm (broad s, 4 H, C8H7); 13C{1H} NMR (C6D6, 125.8 MHz): d = 1.65 (Si(CH3)2), ¢30.26 ppm (CH2), no other signals were found due to the paramagnetic character; 29Si{1H} NMR (C6D6, 99.4 MHz): d = ¢125.22 ppm; UV/Vis (THF): lmax (e) = 291 (max, 6280), 304 (max, 6170), 615(max, 1850), 641(max, 890), 660 (max, 600), 680 (max, 350), 689 (sh), 761 nm (max, 220 L mol¢1 cm¢1); elemental analysis calcd (%) for C21H28Si2U (574.65): C 43.89, H 4.91; found: C 43.94, H 5.30.
[5]
[6] [7]
[8]
[9]
[10]
Acknowledgements Support by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
[11]
[12]
Keywords: density functional calculations · metallocenes · solid-state structures · thorium · uranium [1] For general information on metallocenophanes, metalloarenophanes, and polymers derived thereof, see: a) D. E. Herbert, U. F. J. Mayer, I. Manners, Angew. Chem. Int. Ed. 2007, 46, 5060 – 5081; Angew. Chem. 2007, 119, 5152 – 5173; b) H. Braunschweig, T. Kupfer, Acc. Chem. Res. 2010, 43, 455 – 465; c) V. Bellas, M. Rehahn, Angew. Chem. Int. Ed. 2007, 46, 5082 – 5104; Angew. Chem. 2007, 119, 5174 – 5197; for properties of metallocene-based polymers, see: d) K. H. Pannell, V. I. Imshennik, Y. V. Maksimov, M. N. Il’ina, H. K. Sharma, V. S. Papkov, I. P. Suzdalev, Chem. Mater. 2005, 17, 1844 – 1850; e) C. J. Miller, D. O’Hare, J. Mater. Chem. 2005, 15, 5070 – 5080; f) S. Zou, M. A. Hempenius, H. Schçnherr, G. J. Vancso, Macromol. Rapid Commun. 2006, 27, 103 – 108; g) L. I. Espada,
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M. Shadaram, J. Robillard, K. H. Pannell, J. Inorg. Organomet. Polym. 2000, 10, 169 – 176; h) M. J. Maclachlan, M. Ginzburg, N. Coombs, T. W. Coyle, N. P. Raju, J. E. Greedan, G. A. Ozin, I. Manners, Science 2000, 287, 1460 – 1463; i) Z. Chen, M. D. Foster, W. Zhou, H. Fong, D. H. Reneker, Macromolecules 2001, 34, 6156 – 6158; j) P. W. Cyr, E. J. D. Klem, E. H. Sargent, I. Manners, Chem. Mater. 2005, 17, 5770 – 5773; for uraniumbridged ansa ferrocene, see: k) A. Bucaille, T. Le Borgne, M. Ephritikhine, J.-C. Daran, Organometallics 2000, 19, 4912 – 4914. a) J. D. Jamerson, A. P. Masino, J. Takats, J. Organomet. Chem. 1974, 65, C33 – C36; b) A. Greco, S. Cesca, G. Bertolini, J. Organomet. Chem. 1976, 113, 321 – 330; c) S. D. Jiang, S. S. Liu, L. N. Zhou, B. W. Wang, Z. M. Wang, S. Gao, Inorg. Chem. 2012, 51, 3079 – 3087; for lanthanocenes, see: G. A. Molander, J. A. C. Romero, Chem. Rev. 2002, 102, 2161 – 2185. a) A. Streitwieser Jr., U. Mìller-Westerhoff, J. Am. Chem. Soc. 1968, 90, 7364; b) A. Zalkin, K. N. Raymond, J. Am. Chem. Soc. 1969, 91, 5667 – 5668; c) D. Seyferth, Organometallics 2004, 23, 3562 – 3583. a) J. Goffart, J. Fuger, B. Gilbert, B. Kanellakopulos, Inorg. Nucl. Chem. Letters 1972, 8, 403 – 412; b) A. Avdeef, K. N. Raymond, K. O. Hodgson, A. Zalkin, Inorg. Chem. 1972, 11, 1083 – 1088; c) D. G. Karraker, Inorg. Chem. 1973, 12, 1105 – 1108; d) D. F. Starks, J. Am. Chem. Soc. 1973, 95, 3423 – 3424; e) D. F. Starks, T. C. Parsons, A. Streitwieser Jr., N. Edelstein, Inorg. Chem. 1974, 13, 1307 – 1308. a) J.-C. Berthet, P. Thu¦ry, M. Ephritikhine, Organometallics 2008, 27, 1664 – 1666; b) D. C. Eisenberg, A. Streitwieser Jr., W. K. Kot, Inorg. Chem. 1990, 29, 10 – 14; c) J. A. Butcher Jr., R. M. Pagni, J. Q. Chambers, J. Organomet. Chem. 1980, 199, 223 – 227; d) A. Herv¦, N. Garin, P. Thu¦ry, M. Ephritikhine, J.-C. Berthet, Chem. Commun. 2013, 49, 6304 – 6306; e) J.-C. Berthet, P. Thu¦ry, N. Garin, J.-P. Dognon, T. Cantat, M. Ephritikhine, J. Am. Chem. Soc. 2013, 135, 10003 – 10006; f) A. Herv¦, P. Thu¦ry, M. Ephritikhine, J.-C. Berthet, Organometallics 2014, 33, 2088 – 2098; g) J.-C. Berthet, P. Thu¦ry, M. Ephritikhine, Comptes Rendus Chimie 2014, 17, 526 – 533. A. Streitwieser, Jr., M. T. Barros, H.-K. Wang, T. R. Boussie, Organometallics 1993, 12, 5023 – 5024. a) A. C. Cope, F. A. Hochstein, J. Am. Chem. Soc. 1950, 72, 2515 – 2520; b) N. Hu, L. Gong, Z. Jin, W. Chen, J. Organomet. Chem. 1988, 352, 61 – 66; c) H. P. Fritz, H. Keller, Chem. Ber. 1961, 94, 158 – 173. a) T. J. Katz, W. H. Reinmuth, D. E. Smith, J. Am. Chem. Soc. 1962, 84, 802 – 809; b) H. Lehmkuhl, S. Kintopf, E. Janssen, J. Organomet. Chem. 1973, 56, 41 – 52; c) R. D. Allendoerfer, J. Am. Chem. Soc. 1975, 97, 218 – 219. a) E. Souli¦, G. Folcher, B. Kanellakopulos, Can. J. Chem. 1980, 58, 2377 – 2380; b) N. Edelstein, A. Streitwieser, D. G. Morrell, R. Walker, Inorg. Chem. 1976, 15, 1397 – 1398. a) C. Levanda, A. Streitwieser Jr, Inorg. Chem. 1981, 20, 656 – 659; for calculated UV/Vis spectra, see: b) F. Ferraro, C. A. Barboza, R. Arratia-P¦rez, J. Phys. Chem. A 2012, 116, 4170 – 4175; c) D. Pez-Hernndez, J. A. Murillo-Lûpez, R. Arratia-P¦rez, J. Phys. Chem. A 2011, 115, 8997 – 9003. A. Streitwieser Jr., U. Mìller-Westerhoff, G. Sonnichsen, F. Mares, D. G. Morrell, K. O. Hodgson, C. A. Harmon, J. Am. Chem. Soc. 1973, 95, 8644 – 8649. R. F. Dallinger, P. Stein, T. G. Spiro, J. Am. Chem. Soc. 1978, 100, 7865 – 7871. For comparable analyses of molecular orbitals in uranocene and thorocene, see: a) M. Pepper, B. E. Bursten, Chem. Rev. 1991, 91, 719 – 741; b) J. Li, B. E. Bursten, J. Am. Chem. Soc. 1998, 120, 11456 – 11466; c) A. Kerridge, R. Coates, N. Kaltsoyannis, J. Phys. Chem. A 2009, 113, 2896 – 2905; d) A. Kerridge, Dalton Trans. 2013, 42, 16428 – 16436; e) S. G. Minasian, J. M. Keith, E. R. Batista, K. S. Boland, D. L. Clark, S. A. Kozimor, R. L. Martin, D. K. Shuh, T. Tyliszczaka, Chem. Sci. 2014, 5, 351 – 359. J. A. Butcher Jr., J. Q. Chambers, R. M. Pagni, J. Am. Chem. Soc. 1978, 100, 1012 – 1013.
Received: April 21, 2015 Published online on May 26, 2015
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& f-Block Elements
f-Block Ansa Complexes in the Solid State: [3]Thoro- and [3]Uranocenophanes Holger Braunschweig,* Mehmet Ali Celik, Klaus Dìck, Florian Hupp, and Ivo Krummenacher[a] Abstract: The preparation of [3]thoro- and [3]uranocenophanes, the first structurally authenticated ansa-bridged complexes of actinocenes, is reported. Following a flytrap route, 1,2-bis(cyclooctatetraenyldimethylsilyl)methane was synthesized, reduced to its tetraanion, and subsequently converted into bridged uranocene and thorocene complexes by salt metathesis with the corresponding actinide tetrachlorides. In addition, their electronic structures have been investigated by experimental (UV/Vis spectroscopy, cyclic voltammetry) and theoretical (DFT) methods. Figure 1. Sandwich and ansa sandwich compounds of actinides.
Sandwich compounds of the d-block elements are of great interest to chemists due to their broad and varied reactivity and wide range of physical properties.[1] It is thus not surprising that molecules such as ferrocene have found use in numerous technological applications.[1] In contrast, sandwich compounds containing f-block metals have only been scarcely investigated, although such complexes possess promising magnetic, electronic, photophysical, and catalytic properties.[2] In the development of the organometallic chemisty of the f-block elements, the cyclooctatetraene dianion (COT) has played a similarly crucial role to that played by the cyclopentadienyl anion (Cp) in corresponding transition metal organometallic chemistry.[3] In particular, the synthesis and study of the electronic properties of uranocene, [U(h8-C8H8)2] (1 b), was a major breakthrough in this field as it showed that the f orbitals of the central metal atom participate in metal–ligand bonding.[3] In the following years, bis(COT) complexes of other actinides, such as thorium, protactinium, plutonium, and neptunium, were also reported (Figure 1).[4] Although these complexes exhibit diverse reactivities and properties, such as Lewis acidity or redox activity, little interest has been directed to preparing ansa derivatives, potential precursors for ring-opening polymerization.[1a, 5] Streitwieser et al. were the first and, to our knowledge, only research group involved in the development of bridged “met[a] Prof. Dr. H. Braunschweig, Dr. M. A. Celik, Dr. K. Dìck, Dr. F. Hupp, Dr. I. Krummenacher Institut fìr Anorganische Chemie Julius-Maximilians-Universitt Wìrzburg Am Hubland, 97074 Wìrzburg (Germany) Fax: (+ 49) 931-31-84623 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501545. Chem. Eur. J. 2015, 21, 9339 – 9342
allocenes” of the actinides.[6] This group reported the synthesis of the first ansa uranocene (2) by reaction of K4[(C8H7SiMe2)2C2H4], a tetraanionic ligand system, with uranium tetrachloride (Figure 1). It should be noted that the constitution of 2 was only postulated based on 1H NMR spectroscopy and mass spectrometry. We have now succeeded in the synthesis and full characterization, including structural analysis by X-ray crystallography, of an unprecedented bridged thorocene, which we report herein together with its uranium analog. The compounds represent the first unambiguouslycharacterized examples of ansa complexes of the actinide metals. The ligand precursor for the so-called “flytrap” reaction, (C8H7SiMe2)2CH2 (3), was prepared by treatment of (ClSiMe2)2CH2 with two equivalents of cyclooctatetraenyl lithium and obtained as a yellow oil in 68 % yield (see the Supporting Information, Schem S1). To test the potential of 3 as a ligand precursor for actinocenophane chemistry, we investigated its reduction behavior electrochemically.[7] The cyclic voltammogram of 3 in THF solution shows two overlapping reduction processes centered around ¢2.50 V vs. Fc/Fc + , consistent with stepwise reduction to the corresponding tetraanion (see the Supporting Information, Figure S5).[8] Chemical reduction to the corresponding tetraanion K4[(COTSiMe2)2CH2] (K4[3]) was achieved by treatment of 3 with potassium in THF. Subsequent conversion of the salt with [ThCl4(dme)2] (dme = dimethoxyethane) or UCl4 afforded [Th(h8-C8H7SiMe2)2CH2] (4 a) and [U(h8C8H7SiMe2)2CH2] (4 b), respectively, in moderate yields as yellow (4 a, 30 %) and green (4 b, 14 %) solids (Scheme 1). The actinocenophanes are moderately air stable but under inert conditions they can be stored in solution or as solids without any visible decomposition.
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Scheme 1. Synthesis of the actinocenophanes 4 a and 4 b.
The constitution of the actinocenophanes 4 a and 4 b was proven by elemental analysis, NMR spectroscopy and X-ray diffraction analysis. Whereas compound 4 a shows the expected diamagnetic chemical shifts for its 1H, 13C and 29Si NMR resonances, the 1H NMR spectrum of compound 4 b shows distinct paramagnetic shifts, typical for uranocene and its derivatives.[3a, 6] For instance, the 1H NMR signals of the C8H7 ring protons appear in the range d = ¢32.14 to ¢39.82 ppm and are thus considerably shifted to lower frequencies in comparison to the starting material. The resonances for the carbon atoms of the bridge are also strongly shifted to lower frequencies in the 13C{1H} NMR spectrum [d = 1.65 ppm (Si(CH3)2), d = ¢30.3 ppm (CH2)], whereas the ring carbons of the ligand could not be detected. In addition, a weak signal for the silicon nuclei in the 29Si{1H} NMR spectrum could be observed at d(29Si) = ¢125.2 ppm. It should be noted that, due to the integer-spin 5f2 electronic configuration of the uranium ion, no EPR signal at the X-band (9.4 GHz) was observed for a solution of 4 b in toluene/THF over the temperature range of 10–300 K, consistent with the observation for uranocene and its substituted analogues.[9] We were able to obtain crystals suitable for X-ray analysis of both complexes (Figure 2). The structural parameters of 4 a indicate only a slightly distorted structure with a tilt angle of the COT rings (a) of 5.0(2)8 (XCOT-Th-XCOT: d = 176.88; XCOT = centroid of the C8H7 ring) and an exocyclic Cipso¢Si bond that lies approximately in the plane of the COT rings (b = 58; see the Sup-
Figure 2. Molecular structure of [Th(h8-C8H7SiMe2)2CH2] (4 a, left) and [U(h8C8H7SiMe2)2CH2] (4 b, right). Hydrogen atoms and thermal ellipsoids of the substituents on the silicon atoms are omitted for clarity. Thermal ellipsoids are displayed at the 50 % probability level. Selected bond lengths [pm] and angles [8]: 4 a (left): Th¢CCOT 268.2(3)–272.4(3), Th¢XCOT 198.3 and 199.3; C1Si1-C3 111.2(2), C2-Si2-C3 111.2(2), Si1-C3-Si2 123.1(2); a = 5.0(2), b = 4.4 and 5.9, g = 3.4 (Cipso-XCOT-X’COT-C’ipso), d = 176.8; 4 b (right): U¢CCOT 262.8(7)– 267.7(7), U¢XCOT 190.7 and 191.7; C1-Si1-C3 111.8(3), C2-Si2-C3 111.4(3), Si1C3-Si2 121.7(3); a = 3.3(3), b = 0.04 and 0.06, g = 4.1, d = 178.2 (XCOT = centroid of the C8H7 ring). Chem. Eur. J. 2015, 21, 9339 – 9342
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porting Information, Figure S1 for description of parameters). The eight-membered rings are close to an eclipsed conformation (twisted by ca. 38). Compared to the unbridged derivative [Th(h8-C8H8)2] (1 a), the Th¢C and Th¢XCOT distances are essentially unchanged.[4b] Furthermore, the C¢C bond lengths in the planar carbocyclic rings are almost equal (ca. 141 pm), consistent with the view that the 10 p-electron system of COT acts as an h8-ligand to thorium. Similarly, the effect of the ansa bridge on the molecular structure of the ansa uranocene 4 b is minor (Figure 2). The distortion induced by the bridge can be quantified by the angles a = 3.3(3)8, d = 178.28, and b = 0.18. Taken together with the uranium–carbon bond lengths, which remain virtually unchanged compared to uranocene 1 b,[4b] the crystallographic data indicate very little ring strain within the ansa complex 4 b. All C¢C bond lengths in 4 b are approximately 141 pm and are thus consistent with an aromatic 10 p-electron h8-coordinating ligand. To probe the electronic structure of the f-block compounds 4 a and 4 b, we carried out UV/Vis spectroscopy in the range of l = 230–700 nm (see the Supporting Information, Figure S2). The spectrum of the yellow thorocenophane 4 a shows absorption bands at l = 278, 304, and 320 nm with three shoulders around 270, 295 and 340 nm, tailing off to higher wavelengths. In comparison, the unbridged thorocene 1 a shows a broad lowest-energy absorption band at l = 450 nm with a low extinction coefficient (e 64 L mol¢1 cm¢1).[10] Despite having similar colors, a corresponding transition in the visible spectrum could not be detected for compound 4 a.[10a] The uranocenophane 4 b (Figure S2) gives rise to two absorption bands at l = 295 nm and 308 nm, as well as to a shoulder at approximately 390 nm. Furthermore, three intense peaks at l = 605, 632 and 652 nm were detected in the absorption spectrum, which are comparable to those of the parent compound [U(h8-C8H8)2] (1 b: lmax = 615, 641, and 680 nm).[11] To assign these characteristic UV/Vis absorption bands, which are responsible for the green color of 4 b, to specific electronic transitions, we carried out a TD-DFT (time-dependent density functional theory) calculation at the COSMOBP86/TZVP level (see the Supporting Information, Figure S3 and Table S3). The overall agreement between the calculated and the experimental spectra for 4 b is good. The calculated absorption band at l = 642.9 nm results from an excitation from the SOMO to LUMO + 10 (Table S3), which is mainly a U(5f)!s* transition. The calculated peak at l = 633.3 nm results from four excitations from the HOMO to LUMO + 2, the HOMO to LUMO, and the HOMO¢1 to LUMO, which all correspond to U(5f)!p* transitions, whereas the excitation from the HOMO¢1 to LUMO + 1 is attributable to a U(5f)!s* transition (Table S3). The calculated peak at l = 629.5 nm results from five excitations from the HOMO¢1 to LUMO, the HOMO¢1 to LUMO + 2, and the HOMO to LUMO + 2, which are U(5f)!p* transitions, whereas the other two excitations, from the HOMO to LUMO + 1 and the HOMO¢1 to LUMO + 1, are both U(5f)!s* transitions (Tables S3 and S4). The strong absorption bands of uranocene (1 a) in the visible region, responsible for the green color, have previously been assigned
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COT ligands (Figure 3 and Table S2 in the Supporting Information). Having a triplet spin state, molecule 4 b possesses two singly-occupied molecular orbitals (SOMO: ¢3.753 eV; SOMO + 1: ¢3.650 eV) and two near-isoenergetic lowest unoccupied molecular orbitals (LUMO: ¢2.917 eV; LUMO + 1: ¢2.915 eV) that are all mainly of metal character. In addition, the character of the frontier orbitals of the ansa actinocenes 4 a and 4 b was probed by cyclic voltammetry measurements in THF solution (see the Supporting Information, Figures S6 and S7). Besides two irreversible oxidation processes starting at around ¢0.30 V vs. Fc/Fc + , the uranium complex exhibits an irreversible reduction at Epc = ¢2.57 V vs. Fc/Fc + , corresponding to reduction to UIII.[14] In contrast, the thorium derivative 4 a does not show any oxidation events in the same potential window, but instead shows two consecutive irreversible reduction waves at Epc = ¢2.54 V and ¢3.00 V vs. Fc/Fc + . It is interesting to note that the electrochemical reduction processes leading to the dianion of 4 a, a species that would be isoelectronic with 4 b, are not reversible and thus likely involve structural reorganization of the original sandwich complex. In conclusion, we have described the synthesis and full characterization of ansa complexes derived from thoro- and uranocene, representing the first unambiguously characterized ansabridged actinocenes. The geometrical changes that result from incorporation of the three-atom bridge are only minor for both ansa sandwich complexes, as illustrated by the near-perfect parallel arrangement of the COT ligands and the small deviation of the XCOT-M-XCOT (M = U, Th) angle from 1808. Analysis of the Kohn–Sham molecular orbitals shows that the HOMO and HOMO¢1 correspond to bonding orbitals between the metal f orbitals and p orbitals of the COT ligand. The LUMO, LUMO + 1, and LUMO + 2 of the ansa thorocene (4 a) are a combination of metal f orbitals and ligand p orbitals, whereas the SOMO, SOMO + 1, LUMO, and LUMO + 1 for the ansa uranocene (4 b) are mainly of metal character. Moreover, with the help of TDDFT calculations, we could assign the intense visible absorption bands in the region of l = 600–700 nm for the uranium sandwich complex to both charge-transfer transitions from the uranium f orbitals to p* orbitals of the COT ligand, as well as to metal f–d transitions. The accessibility of ansa complexes of the f-block elements, potential precursors for ring-opening polymerization, opens up new avenues for the preparation of polymers incorporating lanthanide or actinide metals in the main chain. Our future efforts will thus be directed towards the transformation of more strained ansa compounds to produce materials with useful photophysical properties.
Experimental Section
Figure 3. Frontier molecular orbitals of [Th(h8-C8H7SiMe2)2CH2] (4 a, top) and [U(h8-C8H7SiMe2)2CH2] (4 b, bottom). Chem. Eur. J. 2015, 21, 9339 – 9342
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Synthesis of [Th(h8-C8H7SiMe2)2CH2] (4 a): [(COTSiMe2)2CH2] (3; 319 mg, 0.95 mmol) was added to a suspension of freshly cut potassium (390 mg, 9.48 mmol) in THF (15 mL) at room temperature and stirred over a time period of 30 h at this temperature. After filtration, a solution of [ThCl4(dme)2] (530 mg, 0.96 mmol) in THF (20 mL) was added dropwise to the brown solution at 0 8C during
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Communication 30 min. The yellow reaction mixture was slowly warmed to room temperature and stirred for an additional 48 h and then the solvent was completely removed. The resultant solid was extracted with THF (5 Õ 20 mL), the solvent removed by evaporation, and the remaining residue extracted with pentane (3 Õ 10 mL). After removal of the solvent, the solid residue was cooled to ¢78 8C and washed with cold pentane (3 Õ 10 mL) to yield 4 a as yellow solid (127 mg, 0.22 mmol, 30 %). 1H NMR (CD2Cl2, 500.1 MHz): d = 0.59 (s, 12 H, Si(CH3)2), 0.62 (s, 2 H, CH2), 6.53–6.55 (several m, 6 H, C8H7), 6.78–6.84 (several m, 4 H, C8H7), 6.88 (s, 2 H, C8H7), 6.90 ppm (s, 2 H, C8H7); 13C{1H} NMR (CD2Cl2, 125.8 MHz): d = 1.79 (Si(CH3)2), 5.37 (CH2), 107.23 (C8H7), 107.50 (C8H7), 110.28 (C8H7), 112.94 (C8H7), 116.60 ppm (Cipso8H7); 29Si{1H} NMR (CD2Cl2, 99.4 MHz): d = 4.82 ppm; UV/Vis (CH2Cl2): lmax (e) = 270 (sh), 278 (max, 3148), 295 (sh), 304 (max, 2332), 320 (max, 1585 L mol¢1 cm¢1), 340 nm (sh); elemental analysis calcd (%) for C21H28Si2Th (568.66): C 44.36, H 4.96; found: C 43.58, H 4.89.
[2]
[3]
[4]
Synthesis of [U(h8-C8H7SiMe2)2CH2] (4 b): Freshly cut potassium (540 mg, 16.81 mmol) in THF (15 mL) was treated with [(COTSiMe2)2CH2] (3; 465 mg, 1.38 mmol) and stirred for 30 h. The suspension was filtered to remove excess potassium and then a solution of UCl4 (560 mg, 1.47 mmol) in THF (20 mL) was added dropwise over a period of 30 min at 0 8C. The reaction mixture was warmed to room temperature and stirred for another 48 h. The solvent was evaporated and the precipitate extracted with pentane (5 Õ 20 mL). The solvent was removed under vacuum and the green residue washed with cold pentane at ¢70 8C (3 Õ 10 mL). After removal of the solvent, the solid was dried in vacuum to give 4 b as green solid (114 mg, 0.20 mmol, 14 %). 1H NMR (C6D6, 500.1 MHz): d = ¢5.34 (s, 12 H, CH2), ¢25.88 (s, 2 H, Si(CH3)2), ¢32.14 (broad s, 2 H, C8H7), ¢34.85 (broad s, 4 H, C8H7), ¢37.08 (broad s, 4 H, C8H7), ¢39.82 ppm (broad s, 4 H, C8H7); 13C{1H} NMR (C6D6, 125.8 MHz): d = 1.65 (Si(CH3)2), ¢30.26 ppm (CH2), no other signals were found due to the paramagnetic character; 29Si{1H} NMR (C6D6, 99.4 MHz): d = ¢125.22 ppm; UV/Vis (THF): lmax (e) = 291 (max, 6280), 304 (max, 6170), 615(max, 1850), 641(max, 890), 660 (max, 600), 680 (max, 350), 689 (sh), 761 nm (max, 220 L mol¢1 cm¢1); elemental analysis calcd (%) for C21H28Si2U (574.65): C 43.89, H 4.91; found: C 43.94, H 5.30.
[5]
[6] [7]
[8]
[9]
[10]
Acknowledgements Support by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
[11]
[12]
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Received: April 21, 2015 Published online on May 26, 2015
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