title: B-B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2and B2pin2.
abstract: In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B2pin2) and bis(neopentyl glycolato)diboron (B2neop2), as well as bis(ethylene glycolato)diboron (B2eg2) and tetrakis(dimethylamino)diboron (B2(NMe2)4), and their reactivity, along with that of bis(catecholato)diboron (B2cat2) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B2eg2, at room temperature for B2neop2and at higher temperature for B2cat2. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B2eg2with sterically less demanding NHCs such as Me2ImMeand iPr2Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L2B(OR)2]+and borenium [LB(OR)2]+cations have been observed from the reaction of the bis-borate B2eg3with the NHCs iPr2Im and Me2ImMe, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Martin
authors.initials: M
authors.lastname: Eck
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Würtemberger-Pietsch
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Antonius
authors.initials: A
authors.lastname: Eichhorn
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Johannes H J
authors.initials: JH
authors.lastname: Berthel
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Bertermann
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Ursula S D
authors.initials: US
authors.lastname: Paul
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Heidi
authors.initials: H
authors.lastname: Schneider
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Friedrich
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106 Braunschweig, Germany. ch.kleeberg@tu-braunschweig.de.
authors.forename: Christian
authors.initials: C
authors.lastname: Kleeberg
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Udo
authors.initials: U
authors.lastname: Radius
authors.affiliation: Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. u.radius@uni-wuerzburg.de todd.marder@uni-wuerzburg.de.
authors.forename: Todd B
authors.identifier: http://orcid.org/0000-0002-9990-0169
authors.initials: TB
authors.lastname: Marder