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13 Submitted Version Half-Sandwich Complexes of an Extremely Electron-Donating, Re- dox-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. No- ria 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 6 [(MeCN)3M(CO)3] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η -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 6 shows that ligand 1 is a remarkably strong electron donor. In particular, [(η -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 reversi- ble 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 well- Of the boron derivatives,12 neutral borabenzenes,13-14 in known of all organometallic compounds. Since the isolation of which a CH fragment of benzene is replaced by B, require sta- 6 [(η -C6H6)Cr(CO)3] by Fischer and Öfele in 1957 (Figure 1), bilisation from a Lewis base for their isolation. These are dis- the discovery that coordination to the Cr(CO)3 fragment makes tinguished from boratabenzenes, which contain an exocyclic, arenes susceptible to nucleophilic substitution has been instru- covalently bonded substituent at boron and possess an overall mental in organic synthesis.1-2 Although less well studied than negative charge. The introduction of electropositive boron into their all-carbon analogues, a variety of heteroarene complexes arenes causes an increase in energy of the occupied s orbitals.13 of the Group 6 M(CO)3 series are also known. Pyridines, typi- The expected strong donor ability of borabenzenes as ligands cally η1 donor ligands, can be coerced into η6 coordination by for transition metals was first observed experimentally by use of bulky substituents in the 2 and 6 positions,3-4 while half- Schmid and co-workers with the preparation of Group 6 6 5-7 8-9 sandwich complexes of η phosphabenzene, arsabenzene, M(CO)3 complexes of pyridine-borabenzene and pyridine-2- and stibabenzene9 ligands have also been reported. Silabenze- boranaphthalene, in which the CO stretching frequencies are nes10 and germabenzenes,11 kinetically stabilised by large sub- markedly lower than in arene analogues.15 The group of Fu (and stituents, have also been coordinated to group 6 tricarbonyl later others) developed a general synthetic protocol for such bo- fragments. rabenzene-base adducts,14, 16-17 and showed that their coordina- tion to Cr(CO)3 dramatically increases the rate of nucleophilic aromatic substitution at boron.18 Scheme 1. Synthesis of diborabenzene 1. It follows that incorporation of further boron atoms into the arene ring (with associated Lewis base donors) should produce Figure 1. Selected Group 6 arene and heteroarene tricarbonyl com- an even more strongly electron-donating ligand. Our reactivity plexes. studies on boron-boron multiply-bonded species recently pro- vided a route to the first neutral, 6π-aromatic diborabenzene Table 1. Selected crystallographically determined bond lengths (Å) and angles (°) in compounds 2-4 2 3 4 B1-C5 1.581(2) 1.582(4) 1.584(3) B2-C9 1.587(2) 1.585(3) 1.581(3) B1-C1 1.527(2) 1.530(3) 1.531(3) C1-C2 1.397(2) 1.397(3) 1.403(3) Scheme 2. Synthesis of compounds 2-4. C2-B2 1.530(2) 1.528(3) 1.530(4) compound, 1 (Scheme 1).19 The species was accessed via reac- tion of diboracumulene A with excess acetylene. The apparent B2-C3 1.526(2) 1.521(3) 1.528(4) aromaticity of 1, as well as the presence of benzene-like molec- C3-C4 1.418(2) 1.424(3) 1.430(3) ular orbitals according to DFT calculations, led us to explore its C4-B1 1.524(2) 1.527(3) 1.517(3) potential as an η6 ligand for transition metals. M-C13-15 1.827 1.947 1.952 (avg.) RESULTS AND DISCUSSION M-ring cen- 1.725 1.904 1.891 Synthesis of 2-4. Reaction of the 1,4-bis(CAAC)- troid diborabenzene 1 with [(MeCN)3M(CO)3] (M = Cr, Mo, W) in Σ (C-B1-C) 359.9 360.0 359.7 refluxing hexane afforded half-sandwich complexes [(h6- 1)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 con- firmed the coordination of the heteroarene in each case. Com- pounds 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 dis- play 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 ex- tended by roughly 0.025 Å upon complexation, presumably re- flecting 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 [(h6- 21-23 Figure 2. Crystal structure of compound 2 with atomic displace- C6H6)M(CO)3] derivatives and show the expected trend of Cr<W<Mo. ment ellipsoids shown at the 50% probability level. Hydrogen at- oms, except H1-H4, have been removed for clarity. The 11B NMR signals of the new compounds (6.0–7.0 ppm) are shifted significantly upfield from that of the free dibo- rabenzene (1: 24.8 ppm). Analogously to the benzene deriva- 24 Table 2. Carbonyl stretching frequencies of complexes 2-4 tives, the ring protons of the diborabenzene exhibit a signifi- 1 measured by infrared spectroscopy, alongside literature bench- cant shift in the H NMR spectrum upon complexation, from –1 13 marks, in cm . 7.31 ppm to 4.74 (2), 4.97 (3) and 4.78 ppm (4). The C NMR 2 spectrum also reveals the expected large shifts from 150.5 ppm 2 (Cr) 3 (Mo) 4 (W) [(C6H6)Cr(CO)3] [(py- 7 in 1 to 112.0 (2), 113.9 (3) and 109.3 ppm (4) for the ring carbon C5BH5)Cr(C 15 atoms, reflecting a reduction in aromaticity. The observation of O)3] a single resonance for the carbon and hydrogen atoms in the ν(CO)/ 1880, 1888, 1884, 1987, 1918 1925, 1850, ring indicates free rotation about the B-CCAAC bond in solution. cm-1 1788 1792 1792 1805 The signals for the carbonyl groups appear at 243.8 (2), 232.6 (3) and 222.7 ppm (4), each shifted to significantly lower field 6 IR and UV-vis Spectroscopy. In order to quantify the elec- than related [(η -arene)M(CO)3] complexes (between 227.0 and 238.5 ppm for Cr complexes) and indicative of exceptionally tron-donating properties of ligand 1, we measured IR absorp- strong backbonding.25-26 tion spectra of compounds 2-4 to obtain the stretching frequen- 2 cies of the carbonyl ligands. The results are summarised in Ta- aromaticity trend. As expected, it decreases in the order [Cr]: – 28 ble 2 alongside those of [(C6H6)Cr(CO)3] and Schmid’s pyri- 10.2, [Mo]: –9.5, [W]: –9.2, indicating lower aromaticity for the dine-borabenzene complex.15 It is immediately clear that the di- more strongly bound species. borabenzene complexes exhibit a large shift of the bands to lower energy. The A1 and E carbonyl stretching frequencies of Cr complex 2 are 107 cm–1 and 130 cm–1 lower, respectively, than those of the parent benzene complex. To the best of our knowledge, these values represent the lowest CO frequencies 6 yet observed for [(η -arene)M(CO)3] complexes, implying re- markably strong backdonation from the (diborabenzene)M fragment into the π* orbitals of the CO ligands. The UV-vis absorption spectra of 2-4 all display maxima around 400 nm (2: 424 nm, 3: 412 nm, 4: 400 nm), assigned to π-π* ligand transitions (see supporting information)). This rep- resents a significant blue shift compared to 1 (633 nm). The bands shift to shorter wavelength as one moves down Group 6, consistent with stronger binding to the metal and increased sta- bilisation of the HOMO.
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