Formation of unprecedented ' triple bonds in methylidyne

Jonathan T. Lyon†, Han-Shi Hu‡, Lester Andrews†§, and Jun Li‡§

†Department of , University of Virginia, Charlottesville, VA 22904; and ‡Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

Edited by Malcolm H. Chisholm, Ohio State University, Columbus, OH, and approved October 11, 2007 (received for review July 27, 2007)

Chemistry of the actinide elements represents a challenging yet vital scientific frontier. Development of actinide chemistry requires 09.0 )i( fundamental understanding of the relative roles of actinide va- )h( lence-region orbitals and the nature of their chemical bonding. We (g) report here an experimental and theoretical investigation of the P ,F, Cl, Br), F2ClU'CH ؍ uranium methylidyne molecules X3U'CH (X and F3U'CF formed through reactions of laser-ablated uranium )f( and or carbon tetrafluoride in excess ar- )e( gon. By using matrix infrared and relativistic quan- ( )d bsorbance

tum chemistry calculations, we have shown that these actinide A complexes possess relatively strong U'C triple bonds between the P

U 6d-5f hybrid orbitals and carbon 2s-2p orbitals. Electron-with- UF5 FU 3 drawing ligands are critical in stabilizing the U(VI) oxidation state ( )c and sustaining the formation of uranium multiple bonds. These b( ) 00.0 )a( unique U'C-bearing molecules are examples of the long-sought 580 1- 005 actinide-alkylidynes. This discovery opens the door to the rational W va une bm ers ( cm ) synthesis of triple-bonded actinide–. Fig. 1. Infrared spectra in the 590–490 cmϪ1 region for laser-ablated U atoms codeposited with fluoromethanes in excess argon at 8 K. U and 1% CHF3 ͉ ͉ ͉ actinide multiple bond heavy element laser ablation matrix in argon codeposited for 1 h (a), after Ͼ290 nm irradiation (b), and after Ͼ220 ͉ isolation relativistic nm irradiation (c). U and 1% CDF3 in argon codeposited for 1 h (d), after Ͼ290 nm irradiation (e), and after Ͼ220 nm irradiation (f). U and 1% CF4 in argon Ͼ Ͼ hemical bonding and bond order are among the most codeposited for 1 h (g), after 290 nm irradiation (h), and after 220 nm irradiation (i). Precursor absorptions are labeled P. Cimportant fundamental concepts in modern chemistry since the birth of the theory of Lewis (1). Main-group and CHEMISTRY transition- compounds with multiple chemical bonds have L An'CR type of carbyne compounds is not expected to be always been fascinating to because of their pivotal role n highly stable. Well designed ligands that can stabilize the actin- in organic, inorganic, and , character- istic chemical and physical properties, and versatile applications ide center at their stable oxidation states are needed to accom- in biological and material science (2–6). Whereas numerous modate the actinide– carbon multiple bonds. organic and inorganic compounds with multiple bonds are High-oxidation state transition metal alkylidene and alkyli- known (7–17), f elements ( and ) with dyne complexes have received increasing attention over the past multiple bonds are relatively rare, except for early actinides. three decades owing to their importance as catalysts in a variety Such bonding has aroused great interest recently in the search for of synthetic organometallic processes (36). Recently, we have actinide complexes with multiple bonds between two actinide prepared simple methylidene and methylidyne molecules (18–21) and between actinide (An) and main-group through the reaction of laser-ablated early transition metal ligands (L) (22–28). atoms with or methyl halides (37). These studies were Among the actinide complexes with An–L multiple bonds, the extended to the accessible actinide metal atoms Th and U for the importance of first-row elements to bond to actinide metal preparation of the first actinide methylidene species centers has been highlighted by Burns (22), and molecular HXAnϭCH2 (X ϭ H, F, Cl, Br) (38–41). Although Mo and W complexes containing metal-nitride units have been prepared reactions also formed the analogous H2XM'CH methylidynes recently (24–26). Uranium as the leading example forms a (42–46), the H2XU'CH counterparts were energetically too plethora of UϭO bonds and a handful of UϭNR and UϭCR2 high to be produced in these experiments (40). However, very (R ϭ organic groups) bonds. Considerable interest has been recent investigations with the heavy metals Zr, Hf, and Re in developed in recent years in actinide complexes with An–L reactions have demonstrated that the highly double bonds, and most of these investigations have centered on exothermic driving force for transfer from carbon to organometallic systems. Examples include the compounds above heavy metal fosters the formation of the low-energy, very stable ϭ with N–U–N linkages (26) and organoimido (An NR) and trihalo metal carbynes (47, 48). phosphinidene (AnϭPR) groups (29, 30). The matrix isolation technique has revealed several inorganic uranium compounds ϩ with covalent triple bonds, including NUN, CUO, and NUO Author contributions: L.A. and J.L., contributed equally to this work; J.T.L. and H.-S.H. cation (31–33), which are isoelectronic with the ubiquitous performed research; L.A. and J.L. analyzed data; and L.A. and J.L. wrote the paper. uranyl dication. However, An–L multiple bonds are usually The authors declare no conflict of interest. formed between hard-acidic, high-valent actinides and hard This article is a PNAS Direct Submission. Ϫ 2Ϫ 2Ϫ Lewis bases, particularly F ,O , and NR (34, 35), and no §To whom correspondence may be addressed: E-mail: [email protected] or junli@ actinide alkylidyne complexes with An'CR triple bonds are tsinghua.edu.cn. known so far. Because of the high orbital energies of carbon, © 2007 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0707035104 PNAS ͉ November 27, 2007 ͉ vol. 104 ͉ no. 48 ͉ 18919–18924 Downloaded by guest on September 25, 2021 of or reagents with uranium are 0.1 )l( shown in Fig. 1, and the results from reactions with )k( and are detailed in Fig. 2. j( ) The geometries, vibrational frequencies, and electronic struc- )i( tures of the potential uranium product complexes were calcu- rBHC 3 )h( lated by using relativistic density functional theory (DFT) with )g( the generalized gradient PW91 approach (51) as implemented in ADF 2006.1 (52). Inasmuch as the 6s and 6p semicore orbitals

Absorbance are important for actinide bonding, they are included explicitly ( )f e( ) in the variational space along with the 5f, 6d, 7s, and 7p valence )d( orbitals, whereas frozen-core approximation was applied to the ( )c U[1s2-5d10] atomic core. Slater basis sets with the quality of )b( triple-zeta plus two polarization functions (TZ2P) were used. 0.0 )a( The zero-order regular approximation was used to account for 45 0 -1 24 0 W uneva bm sre ( c )m the relativistic effects (53). We also performed ab initio calcu-

Ϫ1 lations at the level of coupled-cluster with single, double, and Fig. 2. Infrared spectra in the 550–410 cm region for laser-ablated U ϭ atoms codeposited with chloroform in excess argon at 8 K. U and 0.5% CHCl3 perturbative triple excitations [CCSD(T)] (54) on X3UCH (X in argon codeposited for 1 h (a), after ␭ Ͼ 290 nm irradiation (b), after ␭ Ͼ 220 H, F) with use of the Stuttgart quasi-relativistic pseudopotential 13 nm irradiation (c), and after annealing to 30 K (d). U and 0.5% CHCl3 in argon and valence basis set for U and 6-31ϩG* basis sets for C, F, and codeposited for 1 h (e), and after ␭ Ͼ 220 nm irradiation (f). U and 0.5% CDCl3 H (55, 56). The optimized CCSD(T) U'C distances (1.926 Å in in argon codeposited for 1 h (g), and after ␭ Ͼ 220 nm irradiation (h). U and H3UCH) lie in the same range as those from DFT calculations, ␭ Ͼ 2% CHBr3 in argon codeposited for 1 h (i), after 290 nm irradiation (j), after indicating that the later are applicable for evaluating these ␭ Ͼ 220 nm irradiation (k), and after annealing to 30 K (l). close-shell triple-bond actinide systems. In the fluoroform spectra stable binary uranium give Ϫ1 We report here an integrated experimental and theoretical rise to very weak absorptions at 496 and 584 cm for UF3 and UF5, respectively (57), which shows that uranium abstracts study of the actinide-methylidyne species, namely F3U'CH, Cl U'CH, Br U'CH, F ClU'CH, and F U'CF, which ren- from the precursor . Three new bands marked 3 3 2 3 with arrows in Fig. 1a are observed at 576.2, 540.2, and 527.5 der the long-sought U'C triple bonds in methylidyne com- cmϪ1 in the infrared spectrum recorded after the initial reaction pounds. Detailed bonding analysis based on a variety of relativ- ' of U and CHF3. These bands increase by 30% on UV irradiation istic quantum chemistry calculations indicates that the U C ␭ Ͼ ␭ Ͼ ␴ ␲ ( 290 nm) and another 20% on further UV irradiation ( triple bonds are composed of one (df-sp) bond and two (df-p) 13 220 nm). A more dilute CHF3 sample gave only the most bonds. Interestingly the U'C bond length and bond strength are Ϫ intense absorption shifted to 539.2 cm 1, which demonstrates tunable by changing the of the neighboring clearly that carbon is involved in the product species. The atoms attached to the U'C bond, making these complexes ' reaction with CDF3 shown in Fig. 1d gives the same upper band, attractive reactive intermediates. This discovery of the An C with the strong band shifting to 535.9 cmϪ1 and the lower band triple-bonded complexes provides important insights and is Ϫ1 shifting to the low 400 cm poor-signal region. Because CF4 is expected to assist in the rational synthesis of these compounds less reactive, on reagent codeposition (Fig. 1g) weaker bands are in larger quantities, which might be stabilized by electron- detected at 578.7 and 536.4 cmϪ1, which increase slightly during ' withdrawing substituents and steric protection of the U C the first irradiation (␭ Ͼ 290 nm), but undergo major growth in bonds with bulky ligands. the second irradiation (␭ Ͼ220 nm) (Fig. 1i). These bands are stable on annealing the sample to 30 K, and they increase slightly Results and Discussion on further UV irradiation. Reactions of laser-ablated U atoms were done with fluoroform, The new absorptions in the U–F stretching region are due to chloroform, bromoform, carbon tetrafluoride, and several iso- a reaction product trapped in solid argon, which is a uranium 13 ϭ topic modifications (CHX3, CHX3, CDX3 [X F, Cl, Br], and species other than binary UFx (x ϭ 1–6) (57). After our previous CF4) as precursors in excess argon (0.5%, 1%, or 2% concen- work on the titanium reaction with CF4, which produced the trations) during condensation onto an8Kcesium iodide window, electron-deficient methylidyne complex F3TiCF (58), and anal- as described (49, 50). Depleted high-purity uranium metal ogous work with heavy metal atoms (47, 48), which produced the targets (ORNL) were filed to remove surface contaminants. F3MCH molecules, we consider these absorptions for assignment Common bands in uranium experiments with different precur- to the F3UCH and F3UCF uranium-bearing methylidyne com- sors are limited to and nitrides (38–41, 49). Infrared pounds. Relativistic quantum chemistry calculations have been spectra were recorded on a Nicolet 550 spectrometer after performed on various structural isomers, isotopomers, and sample deposition, after annealing, and after irradiation by using different electronic states of these molecules to determine the a 175 W mercury arc street lamp. Infrared spectra from reactions thermodynamic stability and to validate the experimental as-

a b c d

149.1 .1 019 509.1 2 700.

Fig. 3. Optimized molecular structures of F3U'CH (a), Cl3U'CH (b), Br3U'CH (c), and F3U'CF (d) by using PW91. The U'C bond lengths are in angstroms.

18920 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0707035104 Lyon et al. Downloaded by guest on September 25, 2021 *H, D, F) molecules ؍ Table 1. Observed and calculated fundamental vibrational frequencies for the C3v F3UϵCX (X

13 F3UϵCH F3Uϵ CH F3UϵCD F3UϵCF

Mode description Obs. Calc. Obs. Calc. Obs. Calc. Obs. Calc.

‡ ‡ C-X str, a1 2,979 (2) 2,969 (2) — 2,200 (1) — 1,268 (312) ‡ †‡ ‡ § UϵCX str, a1 — 747 (46) — 721 (42) — 717 (41) — 441 (34) † U–F sym str, a1 576.2 585 (122) — 585 (123) 576.2 586 (123) 578.7 589 (118) U–F antisym str, e 540.2 561 (284) 539.2 559 (280) 535.9 541 (207) 536.4 544 (177) UϵC-X bend, e 527.5 508 (34) —† 506 (24) —§ 412 (49) — 311 (28)

*Vibrational frequencies (cmϪ1) and intensities (km/mol, in parentheses) are calculated by using PW91/TZ2P (see text). Three real lower-frequency bending modes (a1, e, e) are not listed. Absorptions are observed in argon matrix. †Sample too dilute to observe weaker bands. ‡Absorption masked by very intense precursor band. §Region noisy because of low detector response at the instrumental limit.

signments. The optimized geometries of X3U'CH (X ϭ F, Cl, The C–H stretching modes are too weak to be observed here, Br) and F3U'CF by using the PW91 approach for the singlet and the C–F stretching mode is masked by very intense CF4 ground-state molecules are depicted in Fig. 3, and their vibra- precursor absorption. Finally, the U'CH stretching mode is tional frequencies and IR intensities are listed in Tables 1 and 2. unfortunately in the region of very strong haloform precursor The upper bands in Fig. 1 are assigned to the symmetric U–F bands, and the U'CF stretching mode is predicted at the low stretching mode, which shows very little change between these end of our region of detection where the signal-to-noise is two F3U'CR (R ϭ H, F) molecules. The strongest absorption diminished. However, in the molecule of lower symmetry, is due to the very strong degenerate U–F stretching mode, which F2ClU'CH, prepared from the reaction with 22, the 13 Ϫ1 shifts slightly from F3UCH to F3U CH, to F3UCD and to U'CH stretching mode is found as a weak band at 671.3 cm , Ϫ1 Ϫ1 13 F3UCF. Finally, the weak 527.5 cm band is assigned to the which is shifted to 648.7 cm by using CHF2Cl, confirming the degenerate H–C–U bending mode, which is weak in this mole- vibrational assignment of U'CH stretching mode. Note that the cule probably, in part, because of intensity gained by the nearby large 22.6 cmϪ1 carbon-13 shift for the U'CH stretching mode degenerate U–F stretching mode. In the deuteriated species, this agrees well with theoretical calculations; our PW91 calculation Ϫ1 mode shifts into a region of low signal near our limit of detection. predicts the U'CH mode in F2ClU'CH at 682.4 cm with an This assignment is confirmed in chloroform and bromoform isotopic shift of 23.6 cmϪ1. experiments, where the H–C–U bending mode gains substantial Our calculations also indicate that the X3U'CH (X ϭ F, Cl, Ϫ1 infrared intensity and shifts slightly to 527.2 and to 527.6 cm , Br) and F3U'CF molecules prefer a singlet ground state, with respectively, and the U–X stretching frequencies are much lower the triplet and quintet states being much higher in energy. The because of the heavier halogen mass. The lack of a significant methylidynes are indeed more stable than their methylidene and CHEMISTRY difference between the chloroform and bromoform reaction methyl uranium halide counterparts, and the predicted C3v product absorptions demonstrates clearly that the halogen atoms molecular symmetry, vibrational frequencies, infrared absorp- in the product structure are farther separated from the hydrogen tion intensities, and isotopic frequencies are all in excellent than in the precursor haloform molecules. Notice in Fig. 2 agreement with the experimental observations, as shown in that the strong new band with chloroform at 527.2 cmϪ1 in- Tables 1 and 2. This agreement confirms our vibrational assign- Ϫ1 13 creases on UV irradiation, and shifts to 522.8 cm with CHCl3 ments and the identification of uranium methylidyne molecules. Ϫ1 and to 415.9 cm with the CDCl3 precursor. The PW91 Additional chemical support for this exothermic ␣-halogen calculation of frequencies (Table 2) for isotopic Cl3UCH mol- transfer reaction of the uranium systems is found in our subse- ecules reveals one strong absorption at 522 cmϪ1, which exhibits quent investigations with Re, Mo, and W atoms (48, 59). These almost exactly the same isotopic shifts as the observed new metals react with haloforms to give analogous X3M'CH mol- product absorption and defines the bending mode. We note that ecules with C3v symmetry for M ϭ Mo and W. In the case of Re, the frequency of the H–C–U bending mode increases as the Jahn–Teller distortion lowers the symmetry to Cs, which in- C'U bond becomes stronger and shorter in the X3U'CH creases the IR intensity of the C–H stretching absorption due to series, and the reaction of excited U atom with chloroform and polarization of the triple bond, and high C–H stretching fre- bromoform is even more favorable than the reaction with quencies characteristic of sp hybridization are also observed. fluoroform. Like U the heavy group 6 metals Mo and W also support

*Cl, Br) molecules ؍ Table 2. Observed and calculated fundamental vibrational frequencies for the C3v X3UϵCH (X

13 Cl3UϵCH Cl3Uϵ CH Cl3UϵCD Br3UϵCH

Mode description Obs.† Calc. Obs. Calc. Obs. Calc. Obs. Calc.

C–H str, a1 3,001 (2) 2,991 (2) 2,219 (7) 3,005 (3) ‡ ‡ ‡ ‡ UϵCX str, a1 — 770 (69) — 744 (65) — 738 (64) — 777 (70) UϵC-H bend, e 527.2 522 (224) 522.8 518 (218) 415.9 410 (216) 527.6 527 (178) § U–X sym str, a1 339 (29) 339 (29) 339 (29) 225 (84) U–X antisym str, e 329 (140) — 329 (140) — 326 (100) 216 (10)§

*Vibrational frequencies (cmϪ1) and intensities (km/mol, in parentheses) are calculated by using PW91/TZ2P (see text). Three real lower-frequency bending modes (a1, e, e) are not listed. †Absorptions are observed in argon matrix. ‡Absorption masked by very intense precursor band. §Mode symmetries reversed.

Lyon et al. PNAS ͉ November 27, 2007 ͉ vol. 104 ͉ no. 48 ͉ 18921 Downloaded by guest on September 25, 2021 Table 3. Natural charges (qN), natural localized molecular orbital (NLMO) compositions, natural hybrid orbitals, and bond U-C orders

qU qC NLMO BOW BOM BOGJ

0.05 0.05 1.03 1.06 H3UϵCH 2.45 Ϫ1.17 22% U(s p d f) ϩ 78% C(sp ) 2.51 2.47 3.06 39% U(d0.34f) ϩ 61% C(p) 0.10 0.05 0.96 0.97 F3UϵCH 2.87 Ϫ0.95 18% U(s p d f) ϩ 82% C(sp ) 2.48 2.38 2.88 47% U(d0.19f) ϩ 53% C(p) 0.09 0.03 1.05 0.96 Cl3UϵCH 2.42 Ϫ0.91 20% U(s p d f) ϩ 80% C(sp ) 2.53 2.40 2.90 48% U(d0.19f) ϩ 52% C(p) 0.09 0.03 1.07 0.97 Br3UϵCH 2.35 Ϫ0.93 21% U(s p d f) ϩ 79% C(sp ) 2.53 2.39 2.90 47% U(d0.21f) ϩ 53% C(p) 0.45 0.03 1.27 0.45 F3UϵCF 2.78 Ϫ0.33 11% U(s p d f) ϩ 89% C(sp ) 2.14 2.18 2.57 51% U(d0.19f) ϩ 49% C(p)

Only one of the two ␲-orbitals is listed as they are equivalent. BOW, natural Wiberg bond order; BOM, Mayer bond order; BOGJ, Gophinatan–Jug bond order.

hexavalency, leading to the formation of the symmetric trons and exhibits maxima at the most probable positions of X3M'CH molecules. In addition to the MX3 stretching and localized electron pairs (62, 63). Fig. 5 illustrates the ELF ' H–C–M deformation fundamentals, high C–H and M C isosurfaces of HCCH, F3UCH, Cl3UCH, Br3UCH, and F3UCF. stretching frequencies are observed to further characterize these The polarized ring attractors between U and C indicate that all methylidyne molecules (59). of these actinide molecules possess triple bonds as is the case for To understand the relative roles of the U 6p, 5f, and 6d orbitals acetylene. In addition, the calculated NLMOs (60) reveal the and their chemical bonding, a series of theoretical analyses have formation of the U'C triple bonds by one ␴-bond between been performed. Table 3 lists the natural charges, natural the U df␴ and C sp␴ hybrid orbitals and two ␲-bonds between localized molecular orbitals (NLMOs), natural hybrid orbitals, the U df␲ and C p␲ orbitals, thus providing unequivocal support ' ' and the bond orders calculated for X3U CH and F3U CF to the formation of triple bonds in these methylidyne molecules. ␴ ␲ molecules. The orbital contributions to the - and -orbitals in As shown by the NLMO results and the orbital contributions X3U'CH are listed in Table 4 for comparison with those of the listed in Tables 3 and 4, the threefold symmetry of the X3U'CR formal triple bonds in uranyl. The natural population analysis molecules allows significant hybridization or orbital mixing (60) reveals that U carries highly positive charges in these U(VI) between U 6d and 5f orbitals, which enhances the bonding complexes, which helps to form stable molecules by both cova- strengths of the U'C triple bonds. Table 4 also reveals that the lent and ionic interactions. The DFT calculations predict that the ␴-bond of F U'CH molecule involves nonnegligible U 6p U'C distances in H UCH (a model compound), Br UCH, 3 3 3 hybridization with 5f/6d orbitals, similar to the ‘‘pushing from Cl3UCH, F3UCH, and F3UCF are 1.901, 1.905, 1.910, 1.941, and 2.007 Å, respectively. These U'C distances are slightly longer below’’ 6p-5f mixing in uranyl (64). Our calculations on the series X3U'CR (X, R ϭ H, F, Cl, Br, than the sum of the U and C triple-bond radii (17), but correlate ' well with what is expected from the experimentally measured I, CH3, SiH3, CO) also reveal that the U C bond distances and distances for UϭC (2.29 Å) and U–C (2.54 Å) bonds (61). The bond strengths depend on the ligands and the coordination pyramidality around the U and C atoms. In particular, the strong U'C triple bonds in the X3U'CR complexes are further established by theoretical calculations from different method- inductive effect of electronegative fluorine supports a large ologies. As shown in Fig. 4, the canonical molecular orbitals from positive charge on the central U atom, which helps to retain the DFT calculations also reveal one ␴- and two ␲-orbitals with six most stable U(VI) oxidation state and to push the U 7s orbital electrons, consistent with orthodox triple-bond models (1, 2). The effective bond orders calculated by using different for- HC≅ HC F U≅ HC F U≅ FC malisms are also listed in Table 3 which agree well with 3 3 triple-bond covalency. A further support of the U'C triple bonds in these complexes are obtained from calculations of the characteristic topology of the electron localization function π1-MO (ELF), which measures the spatial distribution of paired elec-

Table 4. PW91 MO percentages (%) of U valence and semicore H, F) molecules ؍ orbitals in uranyl and X3UCH (X π OM- U C/O 2

MO 6 s 6p 5f 6d 7 s 7p 2 s 2p 2ϩ ␴ UO2 u 857 2 33 ␴g 14 2 7 77 ␲u 134 64 σ OM- ␲g 19 79 H3UϵCH ␲ 43 7 38 ␴ 1 15 5 11 7 21

F3UϵCH ␲ 39 9 46 ␴ ␲ ' ␴ 6 12 13 2 12 26 Fig. 4. Comparison of the - and -molecular orbitals of ethyne HC CH and the uranium-methylidyne F3U'CH and F3U'CF complexes. (Isosurface *Orbital contribution Ͻ1% is not shown. value ϭ 0.05 atomic unit.)

18922 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0707035104 Lyon et al. Downloaded by guest on September 25, 2021 Fig. 5. Isosurfaces of the electron localization function (ELF) calculated at value 0.8 for HCCH, F3UCH, Cl3UCH, Br3UCH, and F3UCF (from left to right). The ring attractors in all of these molecules clearly show the triple bonds.

to high energy to facilitate strong bonding between U and CR. research field based on the need to dispose of environmentally For the halogen substitutions on U, the less electronegative hazardous , the most significant aspect of this dis- ligands decrease the U'C bond length by increasing the effec- covery is that X3U'CH (X ϭ F, Cl, Br), F2ClU'CH, and tive overlap between U 6d and C 2p orbitals. Hence, among the F3U'CF are examples of the long-sought actinide-alkylidyne molecules with halogen substituents, the triple bond in I3U'CH molecules that have been prepared and characterized. Although is the strongest and it has the shortest U'C distance (1.895 Å), the chemistry of these species prepared and isolated in a close to the calculated distance in (SiH ) U'CH (1.893 Å). The low-temperature noble-gas matrix is difficult to investigate ex- 3 3 ' calculations on F3U'CR (R ϭ F, Cl, Br, I, H, Me) also perimentally, these unique diamagnetic X3U CR molecules demonstrate that the U'C bonds are shortest for less electro- might be synthesized by coordination of U with highly electron- negative R such as H atom. withdrawing ligands and steric protection of the U'C bonds The experimental results suggest that laser-energy-excited with bulky ligands, possibly with pseudo-threefold local symme- atomic uranium is a vigorous reducing agent. The reactions try. The ability of fluorine-substituted or sterically encumbering occur as U cleaves the strong X–C (X ϭ F, Cl, Br) bond and then ligands (25) to generate new types of metal–ligand multiple two successive ␣-halogen transfers to uranium occur, forming bonds might be extended to the still-elusive actinide–carbon multiple-bond alkylidyne and arylidyne systems. The discovery the U(VI) products identified in this work. Consistent with this ' notion, theoretical calculations show that halogen transfer from of these actinide complexes with An C triple bonds is poten- C to U atoms is strongly favored thermodynamically, with each tially important for organosynthesis, , interstellar chem- step being strongly exothermic because of the difference in X–U istry, , and actinide chemistry under extreme conditions. and X–C bond energies. The overall reaction (1) requires Extending the well known methylidyne chemistry of the initiation by using electronically excited uranium (noted U*) transition metals (36) to actinide metals has proven to be a from laser ablation or UV irradiation, and the reaction with difficult challenge. Although this may simply be because actinide CHEMISTRY fluoroform is calculated to be exothermic by 183 kcal/mol with 6d and 5f valence orbitals do not behave like transition-metal nd approximate treatment of the spin-orbit-coupling effects. The orbitals (65), we indeed see glimpses of transition-metal-like solid argon matrix soaks up this excess energy and stabilizes the behavior in the early actinides. This and our earlier research final methylidyne product. work on Th and U methylidenes (38–41) provide clues to the existence and nature of actinide–carbon, multiple-bonded U* ϩ CHX 3 ͓XU–CHX ͔* 3 3 2 species. ͓ ϭ ͔ 3 ϵ X2U CHX * X3U CH [1] The calculations were performed by using a HP Itanium2 cluster at Through a combined experimental and theoretical effort, we Tsinghua National Laboratory for Information Science and Technology. have hereby shown that laser-ablated uranium atoms are quite We thank two anonymous reviewers for very constructive and helpful suggestions. This research was supported by National Science Founda- reactive, and even the unreactive CF4 molecule falls prey and tion Grant CHE 03-52487 and by the Chinese National Key Basic yields fluorine atoms to the active uranium metal center, leading Research Science Foundation (NKBRSF) Grants 2006CB932305 and to the formation of a relatively strong U'C triple bond. 2007CB815200 and the National Natural Science Foundation of China Although the activation of C-halogen bonds itself is an important (NNSFC) Grant 20525104.

1. Lewis GN (1916) J Am Chem Soc 38:762–785. 16. Cummins CC (2006) Angew Chem Int Ed 45:862–870. 2. Pauling L (1964) Nature 203:182–183. 17. Pyykko¨P, Riedel S, Patzschke M (2005) Chem Eur J 11:3511–3520. 3. Cotton FA, Curtis NF, Harris CB, Johnson BFG, Lippard SJ, Mague JT, 18. Gagliardi L, Roos BO (2005) Nature 433:848–851. Robinson WR, Wood JS (1964) Science 145:1305–1307. 19. Straka M, Pyykko¨P (2005) J Am Chem Soc 127:13090–13091. 4. Fischer EO (1976) Adv Organometallic Chem 14:1–32. 20. Cavigliasso G, Kaltsoyannis N (2006) Inorg Chem 45:6828–6839. 5. Schrock RR (1983) Science 219:13–18. 21. Frenking G, Tonner R (2007) Nature 446:276–277. 6. Lee M, Lenman M, Banas A, Bafor M, Singh S, Schweizer M, Nilsson R, 22. Burns CJ (2005) Science 309:1823–1824. Liljenberg C, Dahlqvist A, Gummeson PO, et al. (1998) Science 280:915–918. 23. Arney DSJ, Burns CJ, Smith DC (1992) J Am Chem Soc 114:10068–10069. 7. Sekiguchi A, Kinjo R, Ichinohe M (2004) Science 305:1755–1757. 24. Hayton TW, Boncella JM, Scott BL, Palmer PD, Batista ER, Hay PJ (2005) 8. Cotton FA, Nocera DG (2000) Acc Chem Res 33:483–490. Science 310:1941–1943. 9. Suzuki N, Nishiura M, Wakatsuki Y (2002) Science 295:660–663. 25. Castro-Rodriguez I, Nakai H, Zakharov LN, Rheingold AL, Meyer K (2004) 10. Frenking G (2005) Science 310:796–797. Science 305:1757–1759. 11. Nguyen T, Sutton AD, Brynda M, Fettiger JC, Long GJ, Power PP (2005) 26. Evans WJ, Kozimor SA, Ziller JW (2005) Science 309:1835–1838. Science 310:844–847. 27. Gagliardi L, Pyykko¨P (2004) Angew Chem Int Ed 43:1573–1576. 12. Piro NA, Figueroa JS, McKellar JT, Cummins CC (2006) Science 313:1276– 28. Ephritikhine M (2006) Dalton Trans 2501–2516. 1279. 29. Brennan JG, Andersen RA (1985) J Am Chem Soc 107:514–516. 13. Radius U, Breher F (2006) Angew Chem Int Ed 45:3006–3010. 30. Arney DS, Burns CJ, Schnabel RC (1996) J Am Chem Soc 118:6780–6781. 14. Berry JF, Bill E, Bothe E, Geoge SD, Mienert B, Neese F, Wieghardt K (2006) 31. Green DW, Reedy GT (1976) J Chem Phys 65:2921–2922. Science 312:1937–1941. 32. Li J, Bursten BE, Liang B, Andrews L (2002) Science 295:2242–2245. 15. Mindiola DJ (2006) Acc Chem Res 39:813–821. 33. Zhou M, Andrews L (1999) J Chem Phys 111:11044–11049.

Lyon et al. PNAS ͉ November 27, 2007 ͉ vol. 104 ͉ no. 48 ͉ 18923 Downloaded by guest on September 25, 2021 34. Pepper M, Bursten BE (1991) Chem Rev 91:719–741. 52. te Velde G, Bickelhaupt FM, van Gisbergen SJA, Guerra CF, Baerends EJ, 35. Clark DL (2000) Los Alamos Sci 26:364–381. Snijders JG, Ziegler T (2001) J Comput Chem 22:931–967. 36. Schrock RR (2002) Chem Rev 102:145–179. 53. van Lenthe E, Baerends EJ, Snijders JG (1993) J Chem Phys 99:4597–4610. 37. Andrews L, Cho HG (2006) Organometallics 25:4040–4053. 54. Raghavachari K, Trucks GW, Pople JA, Head-Gordon M (1989) Chem Phys 38. Andrews L, Cho HG (2005) J Phys Chem A 109:6796–6798. Lett 157:479–483. 39. Lyon JT, Andrews L (2005) Inorg Chem 44:8610–8616. 55. Ku¨chle W, Dolg M, Stoll H, Preuss H (1994) J Chem Phys 100:7535–7542. 40. Lyon JT, Andrews L (2005) Inorg Chem 45:1847–1852. 56. Ditchfield R, Hehre WJ, Pople JA (1971) J Chem Phys 54:724–728. 41. Lyon JT, Andrews L, Malmqvist PÅ, Roos BO, Wang T, Bursten BE (2006) 57. Hunt RD, Thompson C, Hassanzadeh P, Andrews L (1994) Inorg Chem Inorg Chem 46:4917–4925. 33:388–391. 42. Cho HG, Andrews L (2005) Chem Eur J 11:5017–5023. 58. Lyon JT, Andrews L (2006) Inorg Chem 45:9858–9863. 43. Cho HG, Andrews L (2005) J Am Chem Soc 127:8226–8231. 59. Lyon JT, Cho HG, Andrews L (2007) Organometallics, in press (Cr, Mo and W ϩ 44. Cho HG, Andrews L (2005) Organometallics 24:5678–5685. CHX ,CX). 45. Cho HG, Andrews L, Marsden C (2005) Inorg Chem 44:7634–7643. 3 4 46. Cho HG, Andrews L (2006) J Phys Chem A 110:13151–13163. 60. Reed AE, Curtiss LA, Weinhold F (1988) Chem Rev 88:899–926. 47. Lyon JT, Andrews L (2007) Organometallics 26:2519–2527. 61. Cramer RE, Maynard RB, Paw JC, Gilje JW (1981) J Am Chem Soc 48. Lyon JT, Cho HG, Andrews L, Hu HS, Li J (2007) Inorg Chem 46:8728–8738. 103:3589–3590. 49. Souter PF, Kushto GP, Andrews L, Neurock M (1997) J Am Chem Soc 62. Becke AD, Edgecombe KE (1990) J Chem Phys 92:5397–5403. 119:1682–1687. 63. Silvi B, Savin A (1994) Nature 371:683–686. 50. Andrews L, Citra A (2002) Chem Rev 102:885–911. 64. Tatsumi K, Hoffmann R (1980) J Am Chem Soc 19:2656–2658. 51. Perdew JP, Wang Y (1992) Phys Rev B 45:13244–13249. 65. Kaltsoyannis N (2003) Chem Soc Rev 32:9–16.

18924 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0707035104 Lyon et al. Downloaded by guest on September 25, 2021