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Trimetallaborides As Starting Points for the Syntheses of Large Metal­Rich Molecular Borides and Clusters

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Trimetallaborides As Starting Points for the Syntheses of Large Metal­Rich Molecular Borides and Clusters Trimetallaborides as starting points for the syntheses of large metal-rich molecular borides and clusters Article (Published Version) Braunschweig, Holger, Ewing, William C, Ghosh, Sundargopal, Kramer, Thomas, Mattock, James D, Östreicher, Sebastian, Vargas, Alfredo and Werner, Christine (2016) Trimetallaborides as starting points for the syntheses of large metal-rich molecular borides and clusters. Chemical Science, 7. pp. 109-116. ISSN 2041-6539 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/56777/ 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 Chemical Science View Article Online EDGE ARTICLE View Journal Trimetallaborides as starting points for the syntheses of large metal-rich molecular borides Cite this: DOI: 10.1039/c5sc03206g and clusters† Holger Braunschweig,*a William C. Ewing,a Sundargopal Ghosh,b Thomas Kramer,a James D. Mattock,c Sebastian Ostreicher,¨ a Alfredo Vargasc and Christine Wernera À Treatment of an anionic dimanganaborylene complex ([{Cp(CO)2Mn}2B] ) with coinage metal cations stabilized by a very weakly coordinating Lewis base (SMe2) led to the coordination of the incoming metal and subsequent displacement of dimethylsulfide in the formation of hexametalladiborides featuring planar four-membered M2B2 cores (M ¼ Cu, Au) comparable to transition metal clusters constructed around four-membered rings composed solely of coinage metals. The analogies between compounds consisting of B2M2 units and M4 (M ¼ Cu, Au) units speak to the often overlooked metalloid nature of boron. Treatment of one of these compounds (M ¼ Cu) with a Lewis-basic metal fragment (Pt(PCy3)2) Received 27th August 2015 Creative Commons Attribution 3.0 Unported Licence. led to the formation of a tetrametallaboride featuring two manganese, one copper and one platinum Accepted 19th October 2015 atom, all bound to boron in a geometry not yet seen for this kind of compound. Computational DOI: 10.1039/c5sc03206g examination suggests that this geometry is the result of d10–d10 dispersion interactions between the www.rsc.org/chemicalscience copper and platinum fragments. Introduction The relationship between molecular clusters and solid state materials has drawn interest and engendered discussions.2a Dating from the early work of Lipscomb and fellow main group From early on in the study of transition metal clusters, it has This article is licensed under a pioneers, the bonding arrangements behind the clustering of been postulated that these groupings of metal atoms linked by boron atoms into three dimensions has been methodically metal–metal bonding arrangements were capable of simulating explored.1 Alongside these advances, the chemistry of related the surfaces of bulk materials, thereby providing an opportunity to study chemistry at material interfaces.9 Bulk transition metal Open Access Article. Published on 20 October 2015. Downloaded 02/11/2015 12:28:14. metallaboranes has briskly proceeded in the shadow of organ- 10 ometallic chemistry.2 Several reviews and articles have demon- borides have found uses as super-hard materials, supercon- 11 12 strated that the progress made in the study of metal-rich ductors, and magnetic materials, and the doping of bulk metallaboranes is comparable to that coming from work on materials with boron is a commonly utilized strategy to 13 boron-rich, metal-containing cage clusters.3–5 To date, there has augment electronic properties. It follows that the construction been no need to evaluate these systems in a comparative sense, of polymetallic molecular structures featuring one or more as almost all known metallaboranes containing late transition boron atoms might provide a strategy for studying both the metal fragments follow the same structural paradigm as poly- electronics and surface chemistries of metal borides, as well hedral boranes.2a,3d,3e This fact is a clear measure of the success as effects induced by boron doping. and utility of cluster-electron counting rules and the isolobal For some time our group has been interested in developing analogy linking boranes, metallaboranes, and metal clusters in methods for the systematic construction of such metal-rich a simple, conceptually pleasing fashion.6–8 molecular transition metal borides featuring three or more metals directly bonded to boron.14 In hopes of thoroughly understanding what we envision as an entry point into more aInstitut fur¨ Anorganische Chemie, Julius-Maximilians-Universitat¨ Wurzburg,¨ Am metal-rich materials, we recently investigated the structural ¨ Hubland, 97074 Wurzburg, Germany. E-mail: [email protected]. ff de e ects involved in changing both metals and ligands in a set bDepartment of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, of trimetallaboride complexes formed of the interactions of India base-stabilized Lewis acidic coinage metal cations with an c À 15 Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 anionic dimanganaborylene ([{Cp(CO)2Mn}2B] , 1). In these 9QJ, Sussex, UK compounds, the coinage metal is found complexed to the † Electronic supplementary information (ESI) available: All synthetic, linear [Mn]B]Mn] unit of the borylene either in a position computational and crystallographic details. CCDC 1420470–1420473. For ESI and crystallographic data in CIF or other electronic format see DOI: equidistant from the two Mn centers, in an arrangement held 10.1039/c5sc03206g together by intermetallic bonds between the incoming metal and This journal is © The Royal Society of Chemistry 2015 Chem. Sci. View Article Online Chemical Science Edge Article Mn, or spanning one of the B]Mn bonds, using the p-system as a side-on ligand with interactions roughly described by the Dewar–Chatt–Duncanson bonding model (Fig. 1).15d Which of these bonding geometries a complex took, was found to depend on the combination of the metal's identity (Cu, Ag, or Au) and its stabilizing base. The magnitudes of both the s-basicity and p-acidity of the three ligands studied, tricyclo- hexylphosphine (PCy3), 1,3-bis(4-tolyl)imidazol-2-ylidene (ITol), and 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2- ylidene (CAAC), had a direct inuence on the bonding prefer- ences of their accompanying coinage metals. Despite the observed differences, each of the three ligands explored in ref. 15d may be considered a strong s-donor, limiting the overall range of the s-acidity of the metal fragments studied. To explore the bonding of highly s-acidic metal fragments, a weaker Scheme 1 Reaction of 1 to 2 and 3 and the calculated energetics of stabilizing ligand was required. the two postulated constitutive steps. For 2, the values of DG were calculated in the gas phase at 298.15 K at the B3LYP/6-311 + G(d,p) level for all atoms. For 3, the values of DG were calculated in the gas Results and discussion phase at 298.15 K and the B3LYP/6-311 + G(d,p) level for all small atoms and with the LANL2DZ pseudopotential for Mn and Au. To assess the effects of a highly s-acidic metal stabilized by a weak s-donor, we synthesized AuCl and CuCl stabilized by dimethyl sulde (DMS) and reacted each with 1 (Scheme 1). of a class of transition metal cluster complexes constructed around Creative Commons Attribution 3.0 Unported Licence. In both cases, the reaction solution turned red concomitant homometallic four-membered planar cores of naked coinage with the formation of a white precipitate (presumably LiCl). metal ions, with each M–M bond similarly bridged by a transition The conversion of 1 was monitored by the disappearance of its 16 metal fragment. 2 and 3 can be directly compared to two such 11B NMR resonance at 199 ppm, and the growth of a new peak compounds, the Cu4[(CO)4Co]4 and Au4[Cp(CO)2Mo]4 clusters re- at 208 ppm (in both reactions). As this chemical shi fell 16b,e ported by Klufers¨ and Braunstein, respectively (Fig. 3). The near the range established as normal for trimetallaborides size difference between boron and either gold or copper can be (209–216 ppm),15 it was initially assumed that the reactions expected to induce changes in the core, in this case distorting proceeded to yield similar compounds with the DMS ligand the central squares into diamond-shaped geometries with elon- intact. However, X-ray analyses of painstakingly-grown single This article is licensed under a gated B/B distances and short M/M distances. In Braunstein's crystalline material did not show the expected trimetallaboride, Au compound, the cross-center Au/Au distances are 3.9391(11) but instead dimerization of two sulde-free trimetallaborides in and 3.9041(10) A.˚ The 2.8021(4) A˚ between the Au atoms of 3 is the formation of hexametalladiborides containing two coinage Open Access Article.
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