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Half-Sandwich Complexes of an Extremely ElectronDonating, Redox-Active #6-Diborabenzene Ligand Julian Böhnke, Holger Braunschweig, J. Oscar C. Jiménez-Halla, Ivo Krummenacher, and Tom E. Stennett J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12394 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017
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Journal of the American Chemical Society
Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η6-Diborabenzene Ligand Julian Böhnke,†,‡ Holger Braunschweig,*,†,‡ J. Oscar C. Jiménez-Halla,§ Ivo Krummenacher†,‡ and Tom E. Stennett†,‡ †
Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Institute for Sustainable Chemistry & Catalysis with Boron, Am Hubland, 97074 Würzburg, Germany § Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta S/N, Col. Noria Alta, Guanajuato, C.P. 36050, Gto., Mexico ‡
ABSTRACT: The heteroarene 1,4-bis(CAAC)-1,4-diborabenzene (1; CAAC = cyclic (alkyl)(amino)carbene) reacts with [(MeCN)3M(CO)3] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η6-diborabenzene)M(CO)3] (M = Cr (2), Mo (3), W (4)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand 1 is a remarkably strong electron donor. In particular, [(η6-arene)M(CO)3] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes 2-4 revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η4 binding mode. Treatment of 4 with lithium metal in THF led to identification of the paramagnetic complex [(1)W(CO)3]Li·2THF (5). Compound 1 can also be reduced in the absence of a transition metal to its dianion 12–, which possesses a quinoid-type structure.
Half-sandwich complexes of arenes are among the most wellknown of all organometallic compounds. Since the isolation of [(η6-C6H6)Cr(CO)3] by Fischer and Öfele in 1957 (Figure 1), the discovery that coordination to the Cr(CO)3 fragment makes arenes susceptible to nucleophilic substitution has been instrumental in organic synthesis.1-2 Although less well studied than their all-carbon analogues, a variety of heteroarene complexes of the Group 6 M(CO)3 series are also known. Pyridines, typically η1 donor ligands, can be coerced into η6 coordination by use of bulky substituents in the 2 and 6 positions,3-4 while halfsandwich complexes of η6 phosphabenzene,5-7 arsabenzene,8-9 and stibabenzene9 ligands have also been reported. Silabenzenes10 and germabenzenes,11 kinetically stabilised by large substituents, have also been coordinated to group 6 tricarbonyl fragments.
Of the boron derivatives,12 neutral borabenzenes,13-14 in which a CH fragment of benzene is replaced by B, require stabilisation from a Lewis base for their isolation. These are distinguished from boratabenzenes, which contain an exocyclic, covalently bonded substituent at boron and possess an overall negative charge. The introduction of electropositive boron into arenes causes an increase in energy of the occupied s orbitals.13 The expected strong donor ability of borabenzenes as ligands for transition metals was first observed experimentally by Schmid and co-workers with the preparation of Group 6 M(CO)3 complexes of pyridine-borabenzene and pyridine-2boranaphthalene, in which the CO stretching frequencies are markedly lower than in arene analogues.15 The group of Fu (and later others) developed a general synthetic protocol for such borabenzene-base adducts,14, 16-17 and showed that their coordination to Cr(CO)3 dramatically increases the rate of nucleophilic aromatic substitution at boron.18
Scheme 1. Synthesis of diborabenzene 1.
Figure 1. Selected Group 6 arene and heteroarene tricarbonyl complexes.
It follows that incorporation of further boron atoms into the arene ring (with associated Lewis base donors) should produce an even more strongly electron-donating ligand. Our reactivity studies on boron-boron multiply-bonded species recently provided a route to the first neutral, 6π-aromatic diborabenzene
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Table 1. Selected crystallographically determined bond lengths (Å) and angles (°) in compounds 2-4
Scheme 2. Synthesis of compounds 2-4.
compound, 1 (Scheme 1).19 The species was accessed via reaction of diboracumulene A with excess acetylene. The apparent aromaticity of 1, as well as the presence of benzene-like molecular orbitals according to DFT calculations, led us to explore its potential as an η6 ligand for transition metals. RESULTS AND DISCUSSION Synthesis of 2-4. Reaction of the 1,4-bis(CAAC)diborabenzene 1 with [(MeCN)3M(CO)3] (M = Cr, Mo, W) in refluxing hexane afforded half-sandwich complexes [(h61)M(CO)3] as black, crystalline solids in good yields (2 (Cr): 55%; 3 (Mo): 62%; 4 (W): 66%) after recrystallisation from benzene (Scheme 2). Single-crystal X-ray diffraction confirmed the coordination of the heteroarene in each case. Compounds 2-4 are essentially isostructural in the solid state (Figure 2, Table 1). The diborabenzene ligand coordinates in an η6 fashion to each of the metals, retaining its planarity. The flanking cyclic (alkyl)(amino)carbene (CAAC)20 ligands display a cis conformation, as in the free ligand, and are somewhat twisted out of the diborabenzene plane. The bond distances in the ligand are identical within error over the three complexes, but significant differences are observed compared to the free diborabenzene. The B-C bonds to the CAAC substituent are extended by roughly 0.025 Å upon complexation, presumably reflecting a reduction in the π-donation from the aromatic ring to the CAAC moiety due to competition from the metal, while the ring C-C bonds are also slightly elongated. In each case, the metal sits slightly off-centre, with the bond distances to the ring carbon atoms adjacent to the bulky Dipp groups roughly 0.05 Å longer than those opposite. The distances from the ring centroid to the metal are essentially identical to those in the [(h6C6H6)M(CO)3] derivatives21-23 and show the expected trend of Cr
