Uranocenium: synthesis, structure and chemical bonding Article (Accepted Version) Layfield, Richard, Guo, Fu-Sheng, Tong, Ming-Liang, Chen, Yan-Cong and Mansikkamaki, Akseli (2019) Uranocenium: synthesis, structure and chemical bonding. Angewandte Chemie International Edition. ISSN 1433-7851 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/83665/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. Copyright and reuse: Sussex Research Online is a digital repository of the research output of the University. Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available. Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk AngewandteA Journal of the Gesellschaft Deutscher Chemiker International Edition Chemie www.angewandte.org Accepted Article Title: Uranocenium: Synthesis, Structure and Chemical Bonding Authors: Richard Layfield, Fu-Sheng Guo, Akseli Mansikkamaki, Ming- Liang Tong, and Yan-Cong Chen This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903681 Angew. Chem. 10.1002/ange.201903681 Link to VoR: http://dx.doi.org/10.1002/anie.201903681 http://dx.doi.org/10.1002/ange.201903681 Angewandte Chemie International Edition 10.1002/anie.201903681 COMMUNICATION Uranocenium: Synthesis, Structure and Chemical Bonding Fu-Sheng Guo,[a] Yan-Cong Chen,[b] Ming-Liang Tong,*[b] Akseli Mansikkamäki,*[c] and Richard A. Layfield*[a] In memory of Professor Paul O’Brien CBE FRS FREng 5 i Abstract: Abstraction of iodide from [( -C5 Pr5)2UI] (1) produces the unknown. Previous work on f-element metallocenes has led to the 5 i + 5 cationic uranium(III) metallocene [( -C5 Pr5)2U] (2) as a salt of suggestion that the {( -Cp)2M} structural motif is invariably bent – [11] [B(C6F5)4] . The structure of 2 consists of unsymmetrically bonded regardless of the metal and its oxidation state, although no cyclopentadienyl ligands and a bending angle of 167.82° at uranium. base-free examples are known for the actinides. The isolation of Analysis of the bonding in 2 shows that the uranium 5f orbitals are a homoleptic uranium metallocene would therefore furnish new strongly split and mixed with the ligand orbitals, leading to non- insight into the nature of metal-ligand bonding in actinide negligible covalent contributions to the bonding. Studying the dynamic compounds. Furthermore, such a metallocene should feature magnetic properties of 2 reveals that the 5f covalency leads to partially strong magnetic axiality and quantum tunneling properties, which quenched anisotropy and fast magnetic relaxation in zero applied could either assist with the design of high-performance single- magnetic field. Application of a magnetic field leads to dominant molecule magnets or qubits for quantum computing, an area relaxation via a Raman process. where lanthanides are prominent.[12] We therefore aimed to synthesize a cationic uranium(III) metallocene with the general R + R formula [(Cp )2U] , in which Cp is bulky cyclopentadienyl ligand Sandwich compounds containing cyclopentadienyl (Cp) and capable of stabilizing a pseudo-two-coordinate geometry. cyclo-octatetraenyl (COT) ligands play a pivotal role in The target compound was synthesized by abstracting iodide understanding the chemical bonding in f-block compounds and its from [(5-C iPr ) UI] (1) using the super-electrophile [(Et Si) (- [1] 5 5 2 3 2 implications for reactivity, spectroscopy and magnetism. [13] 5 H)][B(C6F5)4], resulting in the formation of [( - Organometallic sandwich compounds have yielded detailed i C5 Pr5)2U][B(C6F5)4] ([2][B(C6F5)4]), which was shown by X-ray insight into the balance of ionic and covalent factors in f-element diffraction to contain a uranocenium cation (Scheme 1).[14] chemistry and the important roles played by d- and f-orbitals,[2] leading to, for example, advances in catalysis[3] and small- [4] i i molecule activation, and in the stabilization of unusual oxidation Pr Pr iPr iPr [(Et3Si)2(-H)] [5] [6] i Manuscript states and bonding motifs. Pr iPr iPr iPr i Pr [B(C6F5)4] iPr Few f-element sandwich compounds have had greater U I U i i [B(C6F5)4] 8 Pr -Et3SiH Pr i impact than uranocene, the iconic D8h-symmetric species [U( - iPr i Pr iPr Pr 2 -Et3SiI i C8H8)2] in which the uranium(IV) centre has a 5f ground-state i i Pr iPr Pr Pr configuration.[7] Extensive investigations into uranocene and its derivatives have established that the uranium-carbon bonds Scheme 1. Synthesis of [2][B(C6F5)4]. possess an appreciable degree of covalent character, which derives mainly from overlap of 6d orbitals with the ligand orbitals, [8] but with a significant contribution from the 5f orbitals. Whilst Significant changes to structure of the metallocene unit occur [9] other bis(COT) actinide compounds are known, homoleptic upon formation of 2 from 1 (Figure 1, Table S1). In 1, the two n [10] bis( -ligand) sandwich compounds are otherwise rare, and cyclopentadienyl ligands interact in a similar way with the uranium 5 homoleptic bis( -cyclopentadienyl)actinide complexes are centre, as shown by the U–C distances, i.e. 2.767(4)-2.862(4) Å and 2.786(3)-2.854(4) Å, respectively, with associated U–Cpcent distances of 2.5323(15) Å and 2.5408(15) Å (cent = centroid). The [a] Dr F.-S. Guo, Prof. Dr R. A. Layfield resulting Cp-U-Cp angle is 152.63(6)° and the U–I bond length is Department of Chemistry University of Sussex 3.0721(3) Å. In the cation 2, one cyclopentadienyl ligand Falmer, Brighton, BN1 9QR (U.K.) coordinates unsymmetrically to uranium, with U–C distances of E-mail: [email protected] 2.702(5)-2.804(6) Å and a U–Cp distance of 2.472(3) Å. The cent Accepted [b] Dr Y.-C. Chen, Prof. Dr M.-L. Tong other cyclopentadienyl ligand in 2 bonds more symmetrically, with Key Laboratory of Bioinorganic and Synthetic Chemistry of the Ministry of Education, School of Chemistry U–C distances of 2.744(6)-2.802(6) Å and U–Cpcent = 2.496(3) Å. Sun Yat-Sen University The Cp-U-Cp angle in 2 is much wider at 167.82(8)°. E-mail: [email protected] Since 2 is the first base-free actinide bis(cyclopentadienyl) Guangzhou 510275 (P.R. China) sandwich complex, an understanding of the metal-ligand bonding [c] Dr A. Mansikkamäki Department of Chemistry, Nanoscience Center in uranocenium is of importance from a fundamental perspective, University of Jyväskylä whilst also providing insight into properties such as magnetism P.O. Box 35, Jyväskylä, FI-40014 (Finland) and chemical reactivity. The uranium-carbon interactions in 2 E-mail: [email protected] were studied by means of DFT calculations as implemented in the Supporting information for this article is given via a link at the end of Amsterdam Density Functional code.[15] A schematic molecular the document. orbital diagram for the frontier orbitals in 2 (Figure 2) shows that This article is protected by copyright. All rights reserved. Angewandte Chemie International Edition 10.1002/anie.201903681 COMMUNICATION the temperature dependence of MT, where M is the molar magnetic susceptibility, was measured in an applied field of 1 kOe 3 for both compounds. The values of MT at 300 K are 1.38 cm K –1 3 –1 mol and 1.71 cm K mol for 1 and [2][B(C6F5)4], respectively, both of which are in the expected range for monometallic uranium(III) compounds.[16] On decreasing the temperature, only a slight decrease in MT was observed for 1, with a value of 1.28 cm3 K mol–1 being reached at 2 K (Figure S7). The decrease in MT with temperature is more pronounced for [2][B(C6F5)4], with a sharp drop observed below about 20 K and a value of 0.67 cm3 K mol–1 at 2 K (Figure S9). The field (H) dependence of the magnetization (M) was measured at 2, 3 and 5 K for both compounds, with the same general features being observed. A relatively rapid increase in the magnetization occurs as the field Figure 1. Molecular structure of 1 (left) and 2 (right). Thermal ellipsoids at 50% increases to approximately 20 kOe, and at higher fields the probability and hydrogen atoms omitted for clarity.