Chemical Science View Article Online EDGE ARTICLE View Journal New evidence for 5f covalency in actinocenes determined from carbon K-edge XAS and electronic Cite this: DOI: 10.1039/c3sc52030g structure theory† Stefan G. Minasian,ab Jason M. Keith,‡b Enrique R. Batista,*b Kevin S. Boland,b David L. Clark,b Stosh A. Kozimor,*b Richard L. Martin,*b David K. Shuh*a and Tolek Tyliszczaka Evidence for metal–carbon orbital mixing in thorocene and uranocene was determined from DFT calculations and carbon K-edge X-ray absorption spectra (XAS) collected with a scanning transmission X-ray microscope (STXM). Both the experimental and computational results showed that the 5f orbitals 2À engaged in significant d-type mixing with the C8H8 ligands, which increased as the 5f orbitals dropped in energy on moving from Th4+ to U4+. The first experimental evidence for extensive f-orbital Received 19th July 2013 interactions has been provided by the C K-edge XAS analysis of thorocene; however, f-type covalency in Accepted 30th September 2013 uranocene was negligible. The results highlighted two contrasting trends in orbital mixing from one DOI: 10.1039/c3sc52030g pair of highly symmetric molecules, and showed that covalency does not increase uniformly for different www.rsc.org/chemicalscience molecular orbital interactions with later actinides. Introduction intensities are proportional to coefficients of orbital mixing between metals and ligands.22 Applications of ligand K-edge –“ ” ¼ Actinide sandwich complexes actinocenes ,(C8H8)2An (An XAS to quantify orbital mixing in M–Cl and M–S bonds is well- Th, Pa, U, Np, Pu) – have played a central role in the develop- established for transition metals22 and actinides,23,24 and recent ment of electronic structure models in organoactinide chem- developments in C and O K-edge XAS has enabled quantitative 1–4 istry. Prior experimental and theoretical studies of (C8H8)2An examinations of a variety of metal interactions with light atoms Published on 05 November 2013. Downloaded 19/11/2013 15:25:14. have provided considerable evidence of covalent mixing in molecular systems.25–29 – between the ligands and metal-based 5f and 6d orbitals.5 18 In this study, C K-edge XAS and time-dependent density Much of our current understanding of actinocene bonding and functional theory (TDDFT) was used to evaluate the electronic orbital mixing is based on earlier gas phase photoelectron structures of thorocene, (C8H8)2Th, and uranocene, (C8H8)2U. spectroscopy (PES) studies, which probed the occupied molec- These molecules were ideal for evaluating the relative role of 5f – ular orbitals.9,19 21 Covalency in the empty or partially occupied versus 6d covalency in actinide bonding because of their variable 5f and 6d orbitals has not been studied extensively, despite the electron count and because hybridization of the 5f and 6d orbitals foundational role of actinocenes in the development of ligand is symmetry-forbidden in the D8h point group. The results were eld theory for organoactinide complexes. consistent with earlier synthetic,8,30,31 spectroscopic, and theo- Recent advances in so X-ray synchrotron radiation have retical analyses of actinocenes, and when taken together provide shown that studies of empty or partially occupied 5f and 6d a comprehensive bonding picture for both the occupied and orbitals are within reach using ligand K-edge X-ray absorption unoccupied actinide orbitals.5,8,9,12–17 Experimental evidence for f spectroscopy (XAS). The technique measures ligand centered 1s an unusual -bonding interaction in (C8H8)2Th is also discussed. / np(n ¼ principle quantum number) transitions, whose Results and discussion aLawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. E-mail: dkshuh@ Ground state electronic structure and molecular orbital lbl.gov description bLos Alamos National Laboratory, Los Alamos, NM 87545, LA-UR-13-25346, USA. E-mail: [email protected]; [email protected]; [email protected] Before discussing the C K-edge XAS spectra in detail it is † Electronic supplementary information (ESI) available: Background subtracted instructive to provide a framework for evaluating the molecular ¼ and normalized C K-edge XAS data for (C8H8)2An (An Th, U), as well as orbital interactions in each complex. Because the electronic Kohn–Sham orbital pictures and tables of spectral and computational data. See DOI: 10.1039/c3sc52030g structures for actinocenes are well established, this discussion ‡ Current address: Department of Chemistry, Colgate University, 13 Oak Dr., will focus on the metal-based unoccupied orbitals that are rele- Hamilton, NY 13346, USA. vant to the C K-edge XAS experiment. For the purposes of this This journal is ª The Royal Society of Chemistry 2013 Chem. Sci. View Article Online Chemical Science Edge Article study, we dene covalency in bonding as mixing between two orbitals, in accordance with recently published studies.17 The currently accepted actinocene bonding model5,8,9,12–18 is consis- tent with results of our DFT calculations on (C8H8)2Th and (C8H8)2U as shown in Fig. 1 and 2 and Table 1. In general, the DFT calculations showed that bonding in the (C8H8)2An compounds involved orbital mixing between the sixteen 4À (C8H8)2 orbitals that are C–C p-bonding (perpendicular to the individual ring planes) and the metal 5f and 6d orbitals. The actinide 6d valence orbitals interacted to form molecular anti- –s –p –d bonding orbitals of 2a1g (6d ), 2e1g (6d ), and 2e2g (6d ) symmetries, while the actinide 5f valence orbitals were split into the 2a2u (5f–s), 2e1u (5f–p), 2e2u (5f–d), and 1e3u +2e3u (5f–f) molecular orbitals. In accordance with previous theoretical studies,12,33,34 our DFT calculations conrmed a dominant role in bonding for the gerade and ungerade e2 (d)orbitalsandshowed Fig. 2 Representations of the 5f- and 6d-antibonding interactions in actino- – that the a (s)ande1 (p) orbitals were engaged in signicantly cenes. For a complete table of all the virtual Kohn Sham orbitals for (C8H8)2Th † less mixing. It has been proposed that the origin of this and (C8H8)2U, refer to the ESI. increased mixing is related to the directionality of the lobes of –d the 2e2u (5f ) orbitals, which pointed more towards the carbon 2À 5 atoms of the C8H8 rings. The DFT calculations also revealed addressed in the study of M–C bonding for Group 4 bent met- an unusual case of f-bonding in the bonding 1e3u and anti- 29 allocene dichlorides, (C5H5)2MCl2 (M ¼ Ti, Zr, Hf). Using that bonding 2e3u (5f–f) orbitals, which has been discussed in methodology, small droplets of (C8H8)2Th and (C8H8)2U were previous theoretical reports.35 C K-edge and N5,4-edge measurements a b Table 1 Calculated energies and atomic compositions of selected virtual Obstacles associated with measuring the C K-edge XAS spectra molecular orbitals for (C8H8)2Th and (C8H8)2U. The lowest energy molecular of nonconducting molecular compounds were recently orbitals given are the antibonding LUMO for (C8H8)2Th and the doubly occupied SOMOs for (C8H8)2U MO (DFT, %) Energy Orbital (eV) C 2p C 2s M 6d M 5f M 7s M 7p Published on 05 November 2013. Downloaded 19/11/2013 15:25:14. (C8H8)2Th 1b1u (An-C nb) 4.40 0.98 0 0 0 0 0 1b2g (An-C nb) 4.10 0.97 0 0 0 0 0 2e1g (6d–p) 2.18 0.10 À0.18 0.71 0 0 0 2e3u (5f–f) 1.62 0.49 0 0 0.47 0 0 –d À 2e2g (6d ) 1.57 0.23 0.02 0.75 0 0 0 À 1e3g (An-C nb) 1.17 0.96 0.01 0 0 0 0 2e2u (5f–d) 0.69 0.13 0 0 0.86 0 0 2e1u (5f–p) À0.05 0.01 0.01 0 0.91 0 0.05 –s À 2a2u (5f ) 0.29 0.03 0 0 0.88 0 0 1e3u (5f–f) À0.69 0.33 0 0 0.65 0 0 2a1g (6d–s) À1.13 0.03 0.02 0.73 0 0.14 0 (C8H8)2U 1b1u (An-C nb) 4.54 0.98 0 0 0 0 0 1b2g (An-C nb) 4.14 0.94 0 0 0 0 0 –p À 2e1g (6d ) 2.50 0.13 0.13 0.44 0 0 0 –d À 2e2g (6d ) 2.30 0.22 0.02 0.74 0 0 0 1e3g (An-C nb) 1.35 0.96 À0.01 0 0 0 0 2e3u (5f–f) 1.17 0.89 0 0 0.07 0 0 –s À 2a1g (6d ) 0.97 0.02 0 0.77 0 0.03 0 2e2u (5f–d) À1.14 0.28 0 0 0.71 0 0 2e1u (5f–p) À2.09 0.02 0 0 0.97 0 0 2a2u (5f–s) À2.37 0.03 0 0 0.96 0 0 1e (5f–f) À4.10 0.06 0 0 0.94 0 0 Fig. 1 Quantitative molecular orbital diagram showing density functional 3u theory (DFT) calculated energies for an idealized (C H ) 4À fragment in D a – b 8 8 2 4h Alpha spin orbital energies are reported for (C8H8)2U. The use of a symmetry (left) in comparison to those for (C8H8)2Th (middle) and (C8H8)2U non-orthogonal basis set can cause Mulliken analysis to have 92 (right). For (C8H8)2U, alpha spin–orbital energies are used. nonphysical results such as compositions >100% or <0. Chem. Sci. This journal is ª The Royal Society of Chemistry 2013 View Article Online Edge Article Chemical Science allowed to evaporate on Si3N4 windows, forming thin crystallites that enabled transmission XAS measurements that were within the linear regime of the Beer–Lambert law.