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Formation of unprecedented actinide'carbon triple bonds in uranium methylidyne molecules Jonathan T. Lyon†, Han-Shi Hu‡, Lester Andrews†§, and Jun Li‡§ †Department of Chemistry, 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( atoms and trihalomethanes or carbon tetrafluoride in excess ar- )e( gon. By using matrix infrared spectroscopy 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–carbon compounds. 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 quantum chemistry 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 valence theory of Lewis (1). Main-group and CHEMISTRY transition-metal compounds with multiple chemical bonds have L An'CR type of carbyne compounds is not expected to be always been fascinating to chemists because of their pivotal role n highly stable. Well designed ligands that can stabilize the actin- in organic, inorganic, and organometallic chemistry, 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 (lanthanides and actinides) 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 metals (18–21) and between actinide (An) and main-group through the reaction of laser-ablated early transition metal ligands (L) (22–28). atoms with methane 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 trihalomethane reactions have demonstrated that the highly double bonds, and most of these investigations have centered on exothermic driving force for halogen 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 fluoroform or carbon tetrafluoride reagents with uranium are 0.1 )l( shown in Fig. 1, and the results from reactions with chloroform )k( and bromoform 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 fluorides 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- fluorine from the precursor molecule. 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 electronegativity of the neighboring clearly that carbon is involved in the product species.

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