Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl

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Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl Complexes: A Systematic Study Christian Kleeberg* and Corinna Borner Institut für Anorganische und Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, 38106 Braunschweig, Germany

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ABSTRACT: Five novel NHC copper(I) boryl complexes were synthesized by B−B activation via σ-bond metathesis of symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino diborane(4) derivatives. Despite their low stability, the NHC copper boryl complexes were thoroughly characterized spectroscopically and structurally. Variation of the NHC ligand (ItBu or Me2IiPr) as well as of the boryl ligand (Bpin, Bdmab, or BiPrEn) allowed, for the first time systematically, a study in such complexes of the dependence on steric encumbrance. For sterically more demanding ligand combinations, mononuclear linear complexes were obtained, while with less demanding ligand combinations, dimeric dinuclear complexes with two bridging μ-boryl ligands were obtained, exhibiting extremely short Cu···Cu distances (80 °C dec. Anal. Calcd for [C17H32BCuN2O2]2: C, 55.06; H, 8.70; N, 7.55. Found: C, 54.73; H, 8.74; N, 7.62. [(ItBu)Cu−Bdmab] (4b). In a nitrogen-filled glovebox, pinB− Bdmab (2b) (36.0 mg, 132.3 μmol, 1.05 equiv) and [(ItBu)CuOtBu] (40.0 mg, 126.5 μmol, 1.0 equiv) were dissolved separately in dry PhMe (0.85 mL) and cooled to −40 °C for 12 h. The solutions were combined in the cold and after 60 min at −40 °C layered with cold npentane (∼4 mL). After 48 h at −40 °C, X-ray quality crystals had separated. The supernatant solution was decanted from dark bronze crystals that were subsequently washed with cold n-pentane (2 × 2 mL, −40 °C) and briefly dried in vacuo at ambient temperature (37.0 mg, 95.3 μmol, 75%): 1H NMR (400 MHz, THF-d8, −40 °C) δ 7.29 (s, 2 H, CHN), 6.72−6.65 (m, 4 H, HCAr), 3.48 (s, 6 H, NCH3), 1.87 [s, 18 H, NC(CH3)3]; 13C{1H} NMR (100 MHz, THF-d8, −40 °C) δ 182.1 (CCarbene), 141.4 (CAr), 116.7 (HCAr), 116.6 (CHN), 106.2 (HCAr), 58.4 [NC(CH3)3], 33.9 (NCH3), 32.6 [NC(CH3)3]; 11 1 B{ H} NMR (128 MHz, THF-d8, −40 °C) δ 44.9 (s, Δw1/2 = 1000 Hz); mp >70 °C dec. Anal. Calcd for C19H30BCuN4: C, 58.69; H, 7.78; N, 14.41. Found: C, 56.40; H, 7.85; N, 14.60. All attempts to perform more accurate elemental analysis failed, presumably because of the presence of impurities that were inevitable for this highly unstable compound (vide supra). [(ItBu)Cu−B(iPrEn)] (4c). In a nitrogen-filled glovebox, pinB− B(iPrEn) (2c) (44.2 mg, 158.0 μmol, 1.0 equiv) was dissolved in npentane (0.15 mL) and [(ItBu)CuOtBu] (50.0 mg, 158 μmol, 1.0 equiv) was dissolved separately in a mixture of PhMe (0.65 mL) and n-pentane (0.15 mL). The solutions were combined to give a redpinkish solution that turned purple within 20 min. The solution was cooled to −40 °C after 30 min at ambient temperature (occasional shaking). After 24−72 h at −40 °C, cuboid colorless crystals that were suitable for X-ray diffraction had separated. The supernatant solution was decanted, and the crystals were washed with cold n-pentane (2 × 1 mL, −40 °C) and briefly dried in vacuo at ambient temperature (25.0 mg, 63.0 μmol, 40%): 1H NMR (400 MHz, PhMe-d8, −47 °C) δ 6.26 (s, 2 H, CHN), 4.52 [sept, J = 6.7 Hz, 2 H, NCH(CH3)2], 3.54 (s, 4 H, NCH2), 1.54 [d, J = 6.7 Hz, 12 H, NCH(CH3)2], 1.52 [s, 18

complexes have, to the best of our knowledge, not yet been considered.1−6

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EXPERIMENTAL SECTION

General Considerations. [(ItBu)CuOtBu], Me2IiPr, CuOtBu, pinB−Bdmab (2b), pinB−BiPrEn (2c), and pinB−BtBuEn (2d) were prepared according to literature procedures.7,10,18 All other compounds were commercially available and were used as received; their purity and identity were checked by appropriate methods. All solvents were dried using MBraun solvent purification systems, deoxygenated using the freeze−pump−thaw method, and stored under purified nitrogen. All manipulations were performed using standard Schlenk techniques under an atmosphere of purified nitrogen or in a nitrogenfilled glovebox (MBraun). Reactions in the glovebox were performed in scintillation vials (ø 18 mm × 58 mm). To handle temperature sensitive compounds inside the glovebox at low temperatures, a brass block with holes fitting to the scintillation vials precooled to −40 °C was used. NMR spectra were recorded on a Bruker Avance II 300, an Avance III HD 300, or an Avance III 400 spectrometer. For air sensitive samples, NMR tubes equipped with screw caps (WILMAD) were used and the solvents were dried over potassium/benzophenone and degassed. Low-temperature NMR spectra of temperature sensitive compounds were obtained as follows. Inside a nitrogenfilled glovebox, a precooled NMR tube was placed in a brass block with a suitable hole (ø 5.1 mm × 6 cm) cooled for >10 h to −40 °C. Inside the NMR tube, the respective compound was placed and the precooled solvent was added using a precooled pipet. The NMR tube was brought, still inside the brass bock, outside of the glovebox and immediately immersed in an appropriate cooling bath. The NMR tube was removed from the cooling bath, wiped, and inserted into the precooled magnet of the NMR instrument. Despite the handling time at room temperature being kept to a minimum, a brownish coloration of the sample was frequently observed during handling. Chemical shifts (δ) are given in parts per million, using the (residual) resonance signal of the solvents (99.5% deuteration, Eurisotop) for calibration (C6D6, 1H NMR 7.16 ppm, 13C NMR 128.06 ppm; THF-d8, 1H NMR 1.72, 3.58 ppm, 13C NMR 25.31, 67.21 ppm; PhMe-d8, 1H NMR 2.08 ppm, 13C NMR 20.43 ppm).19 11B and 31P NMR chemical shifts are reported relative to external BF3·Et2O and 85% aqueous H3PO4, respectively. 13C, 11B, and 31P NMR spectra were recorded by employing composite pulse 1H decoupling. If necessary, 2D NMR techniques were employed to assign the individual signals [1H−1H NOESY (1 s mixing time), 1H−1H COSY, 1H−13C HSQC, and 1 H−13C HMBC]. Melting points were determined in flame-sealed capillaries under nitrogen using a Büchi 535 apparatus and are not corrected. Elemental analysis (EA) was used to establish the purity of new compounds; however, for compounds 4b, 8, and 9, no satisfactory elemental analysis can be provided. Copper clusters 8 and 9 were obtained in only minute yields and could be characterized only by single-crystal X-ray diffraction. The highly unstable 4b, despite several independent attempts, gave reproducibly overly low carbon values, possibly because of the presence of decomposition products. Elemental analyses were performed at the Institut für Anorganische and Analytische Chemie of the Technische Universität Carolo-Wilhelmina zu Braunschweig using an Elementar vario MICRO cube instrument. It should be emphasized that the synthesis of all boryl complexes is highly reproducible, though the purity of the starting materials and the adherence to the experimental procedures, in particular with respect to the temperature and the amount of solvent(s), proved to be essential. X-ray Diffraction Studies. The crystals were, inside a nitrogenfilled glovebox, transferred into inert perfluoroether oil and, outside of the glovebox, rapidly mounted on top of a human hair and placed in the cold nitrogen gas stream on the diffractometer.20a The data were either collected on an Oxford Diffraction Xcalibur E instrument using graphite monochromated Mo Kα radiation (conventional sealed Xray tube) or an Oxford Diffraction Nova A instrument using mirrorfocused Cu Kα radiation (micro focus source). The reflections were H

DOI: 10.1021/acs.organomet.8b00672 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics H, NC(CH3)3]; 13C{1H} NMR (100 MHz, PhMe-d8, −47 °C) δ 183.1 (CCarbene), 114.7 (CHN), 57.4 [NC(CH3)3], 49.7 [NCH(CH3)2], 43.8 (NCH2), 31.8 [NC(CH3)3], 23.6 [NCH(CH3)2]; 11 1 B{ H} NMR (128 MHz, PhMe-d8, −47 °C) δ 49 (s, Δw1/2 = 1500 Hz); 1H NMR (300 MHz, PhMe-d8, room temperature) δ 6.36 (s, 2 H, CHN), 4.33 [sept, J = 6.7 Hz, 2 H, NCH(CH3)2], 3.42 (s, 4 H, NCH2), 1.56 [s, 18 H, NC(CH3)3], 1.43 [d, J = 6.7 Hz, 12 H, NCH(CH3)2]; 13C{1H} NMR (75 MHz, PhMe-d8, room temperature) δ 114.8 (CHN), 57.6 [NC(CH3)3], 49.8 [NCH(CH3)2], 44.4 (NCH2), 32.1 [NC(CH3)3], 23.8 [NCH(CH3)2] (carbene carbon atom not detected); 11B{1H} NMR (96 MHz, PhMe-d8, room temperature) δ 44.6 (s, Δw1/2 = 400 Hz); mp >90 °C dec. Anal. Calcd for C19H38BCuN4: C, 57.50; H, 9.65; N, 14.12. Found: C, 57.67; H, 9.76; N, 14.09. [(Me2IiPr)Cu−Bpin]2 [(5a)2]. In a nitrogen-filled glovebox, B2pin2 (2a) (70.0 mg, 275.4 μmol, 1.0 equiv) was dissolved in PhMe (0.5 mL) and cooled to −40 °C (16 h). Me2IiPr (49.6 mg, 275.3 μmol, 1.0 equiv) and CuOtBu (37.4 mg, 275.0 μmol, 1.0 equiv) were combined in PhMe (1.0 mL) and, after complete dissolution (3−6 h), cooled to −40 °C for 5−16 h. The solutions were combined in the cold to give a reddish solution that was kept at −40 °C for 30 h before it was layered with cold n-pentane (∼3 mL). After 24−60 h at −40 °C, welldeveloped light yellow prisms (suitable for X-ray diffraction) had separated (in some instances, the crystals were embedded in a brownish oil that was readily removed by washing). The supernatant solution was decanted, and the residue washed with cold n-pentane (3 × 1 mL, −40 °C) and briefly dried in vacuo at ambient temperature to give the product as a brown-yellowish solid (26.3 mg, 35.5 μmol, 26%). Alternatively, the residue after decanting may be dissolved in cold THF (0.5 mL) and recrystallized by being layered with cold npentane (3 mL) at −40 °C to give the THF solvate (5a)2(THF) (15.9 mg, 21.5 μmol, 16%): 1H NMR (400 MHz, THF-d8, −60 °C) δ 4.23 [br sept, J = 7 Hz, 4 H, NCH(CH3)2], 2.07 [s, 12 H, C(CH3)N], 1.66 [d, J = 6.6 Hz, 24 H, NCH(CH3)2], 0.88 [s, 24 H, OC(CH3)2]; 13 C{1H} NMR (100 MHz, THF-d8, −60 °C) δ 183.2 (CCarbene), 122.3 [C(CH3)N], 79.3 [OC(CH3)2], 49.9 [NCH(CH3)2], 26.3 [OC(CH3)2], 23.7 [NCH(CH3)2], 9.2 [NC(CH3)]; 11B{1H} NMR (128 MHz, THF-d8, −60 °C) δ no signal detected; mp >90 °C dec. Anal. Calcd for [C17H32BCuN2O2]2: C, 55.06; H, 8.70; N, 7.55. Found: C, 54.96; H, 8.81; N, 7.22. [((Me2IiPr)Cu)((Me3P)2Cu)(Bpin)2] (6). (5a)2 was prepared as described above (starting from 550 μmol of Me2IiPr). The mother liquor was decanted, and the residue was washed with cold n-pentane. After addition of precooled PMe3 (0.5 mL, −40 °C), THF (room temperature) was added in a dropwise manner to the slurry at room temperature until it was clear (∼0.3 mL); the solution was cooled to −40 °C. After 48−72 h, colorless crystals suitable for X-ray diffraction had separated. The mother liquor was decanted, and the crystals were washed with cold n-pentane (2 × 1 mL) and dried briefly in vacuo (36.2 mg, 50.7 μmol, 18%): 1H NMR (400 MHz, PhMe-d8, −47 °C) δ 3.95 [br sept, J = 6.7 Hz, 2 H, NCH(CH3)2], 1.89 [d, J = 6.7 Hz, 12 H, NCH(CH3)2], 1.51 (br s, 18 H, PMe3), 1.40 [s, 6 H, C(CH3)N], 1.22 [s, 24 H, OC(CH3)2]; 13C{1H} NMR (100 MHz, PhMe-d8, −47 °C) δ 178.4 (CCarbene), 120.7 [C(CH3)N], 79.1 [OC(CH3)2], 49.4 [NCH(CH3)2], 26.6 [OC(CH3)2], 24.3 [NCH(CH3)2], 19.3 (t, J = 8.1 Hz, PMe3), 8.8 [NC(CH3)]; 11B{1H} NMR (128 MHz, PhMe-d8, −47 °C) δ no signal detected; 31P{1H} NMR (162 MHz, PhMe-d8, −47 °C) δ −32.0 (s, Δw1/2 = 80 Hz); mp >60 °C dec. Anal. Calcd for C29H62B2Cu2N2O4P2: C, 48.82; H, 8.76; N, 3.93. Found: C, 48.63; H, 8.71; N, 4.03. [(Me2IiPr)Cu−Bdmab]2 [(5b)2]. In a nitrogen-filled glovebox, pinB−Bdmab (2b) (68.0 mg, 249.8 μmol, 1.0 equiv) was dissolved in PhMe (1.0 mL) and cooled to −40 °C (2−4 h). Me2IiPr (45.0 mg, 249.8 μmol, 1.0 equiv) and CuOtBu (34.0 mg, 250.0 μmol, 1.0 equiv) were combined in a mixture of PhMe (1.0 mL) and THF (0.5 mL) and, after complete dissolution, cooled to −40 °C for 2−4 h. The solutions were combined in the cold to give a reddish turbid solution that was kept at −40 °C. After 16−30 h at −40 °C, well-developed orange prisms (suitable for X-ray diffraction) had separated. The supernatant solution was decanted, and the residue washed with cold

n-pentane (2 × 1 mL, −40 °C) and dried in vacuo at −40 °C for 1 h to give (5b)2(PhMe)2 as a brownish solid (74.3 mg, 77.4 μmol, 62%): 1 H NMR (400 MHz, THF-d8, −46 °C) δ 6.6 (br s, 8 H, CHAr), 4.73 [br s, 4 H, NCH(CH3)2], 3.47 (s, 12 H, NCH3), 2.04 [br s, 12 H, NC(CH3)], 0.88 [br s, 24 H, NCH(CH3)2]; 13C{1H} NMR (100 MHz, THF-d8, −46 °C) δ 142.0 (CAr), 122.7 [C(CH3)N], 116.0 (HCAr), 104.6 (HCAr), 52.8 [NCH(CH3)2], 33.9 (NCH3), 22.5 [NCH(CH3)2], 9.5 [C(CH3)N] [carbene carbon atom not detected; in addition, resonances indicative for PhMe (1 equiv) were observed]; 11 1 B{ H} NMR (128 MHz, THF-d8, −46 °C) δ no signal detected; mp >60 °C dec. Anal. Calcd for C52H76B2Cu2N8 [(5b)2(PhMe)2]: C, 64.93; H, 7.96; N, 11.65. Found: C, 64.94; H, 8.12; N, 11.46. This composition agrees with the composition established by X-ray diffraction and by NMR spectroscopy. [(Me2IiPr)3Cu][pinB(OtBu)−Bdmab] (7). In a nitrogen-filled glovebox, pinB−Bdmab (2b) (22.0 mg, 80.8 μmol, 1.0 equiv) was dissolved in THF (0.5 mL) and cooled to −40 °C (16 h). Me2IiPr (45.0 mg, 249.8 μmol, 3.1 equiv) and CuOtBu (11.0 mg, 80.9 μmol, 1.0 equiv) were dissolved in THF (1.0 mL) and, after complete dissolution, cooled to −40 °C for 16 h. The solutions were combined in the cold to give a yellow solution, which was layered with n-pentane after 4−16 h at −40 °C. After 16−48 h, colorless prisms (suitable for X-ray diffraction) had separated. The supernatant solution was decanted, and the residue washed with cold n-pentane (3 × 1 mL, −40 °C) and dried in vacuo at room temperature (60.9 mg, 64.2 μmol, 79%): 1H NMR (400 MHz, THF-d8, −43 °C) δ 6.56 (br s, 4 H, CHAr), 4.88 [br sept, J = 6.8 Hz, 6 H, NCH(CH3)2], 3.46 (s, 6 H, NCH3), 2.18 [br s, 18 H, NC(CH3)], 1.35 [br d, J = 6.8 Hz, 36 H, NCH(CH3)2], 1.06 [s, 6 H, OC(CH3)2], 0.99 [s, 9 H, OC(CH3)3], 0.83 [s, 6 H, OC(CH3)2]; 13C{1H} NMR (100 MHz, THF-d8, −43 °C) δ 185.4 (CCarbene), 142.0 (CAr), 124.8 [C(CH3)N], 115.9 (HCAr), 105.4 (HCAr), 76.8 [OC(CH3)2], 66.7 [OC(CH3)3], 53.4 [NCH(CH3)2], 33.6 [OC(CH3)3], 31.2 (NCH3), 27.7 [OC(CH3)2], 27.5 [OC(CH3)2], 22.5 [NCH(CH3)2], 9.9 [C(CH3)N]; 11B{1H} NMR (128 MHz, THF-d8, −43 °C) δ 6.4 [s, Δw1/2 = 300 Hz, pinB(OtBu)− Bdmab] (only one signal detected); 1H NMR (300 MHz, THF-d8, room temperature) δ 6.60−5.52 (m, 4 H, CHAr), 4.88 [br s, 6 H, NCH(CH3)2, Δw1/2 = 45 Hz], 3.48 (s, 6 H, NCH3), 2.15 [br s, 18 H, NC(CH3)], 1.38 [d, J = 7.0 Hz, 36 H, NCH(CH3)2], 1.10 [s, 6 H, OC(CH3)2], 1.01 [s, 9 H, OC(CH3)3], 0.85 [s, 6 H, OC(CH3)2]; 11 1 B{ H} NMR (96 MHz, THF-d8, room temperature) δ 30.4 [s, Δw1/2 = 200 Hz, pinB(OtBu)−Bdmab], 5.7 [s, Δw1/2 = 7 Hz, pinB(OtBu)−Bdmab]; mp >80 °C dec. Anal. Calcd for C51H91B2CuN8O3: C, 64.51; H, 9.66; N, 11.80. Found: C, 64.28; H, 9.83; N, 11.45. [Cu5(Me2IiPr)3(Bdmab)3] (8). In a nitrogen-filled glovebox, (5b)2(PhMe)2 (30 mg, 31 μmol) was dissolved in dry THF (0.9 mL) and layered with n-pentane (3.0 mL) at room temperature. After 1−2 days at −40 °C, a small amount of optically uniform single crystals of 8(THF)2 may have separated. It must be emphasized that 8(THF)2 is obtained in only approximately two of 10 runs. [(Me2IiPr)10Cu23(PMe3)2] (9). In a nitrogen-filled glovebox, 6 (20 mg, 28 μmol) was dissolved in dry PhMe (0.7 mL). After ∼24 h at ambient temperature, a small amount of optically uniform single crystals of 9 was deposited on the glass surface. It must be emphasized that 9 is obtained in only approximately two of 10 runs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00672. Additional analytical (in situ NMR spectra) and crystallographic data (PDF) Accession Codes

CCDC 1859613−1859622 contain the supplementary crystallographic data for this paper. These data can be obtained I

DOI: 10.1021/acs.organomet.8b00672 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Diboration of Aldehydes Catalyzed by Copper(I) Boryl Complexes. J. Am. Chem. Soc. 2008, 130, 5586−5594. (d) Moon, J. H.; Jung, H.-Y.; Lee, Y. J.; Lee, S. W.; Yun, J.; Lee, J. Y. Origin of Regioselectivity in the Copper-Catalyzed Borylation Reactions of Internal Aryl Alkynes with Bis(pinacolato)diboron. Organometallics 2015, 34, 2151−2159. (7) Borner, C.; Kleeberg, C. Selective Synthesis of Unsymmetrical Diboryl PtII and Diaminoboryl CuI Complexes by B−B Activation of Unsymmetrical Diboranes(4) {pinB−B[(NR)2C6H4]}. Eur. J. Inorg. Chem. 2014, 2014, 2486−2489. (8) Borner, C.; Anders, L.; Brandhorst, K.; Kleeberg, C. Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures, and Reactivity. Organometallics 2017, 36, 4687−4690. (9) (a) Kajiwara, T.; Terabayashi, T.; Yamashita, M.; Nozaki, K. Syntheses, Structures, and Reactivities of Borylcopper and -zinc Compounds: 1,4-Silaboration of an α,β-Unsaturated Ketone to Form a γ-Siloxyallylborane. Angew. Chem., Int. Ed. 2008, 47, 6606−6610. (b) Okuno, Y.; Yamashita, M.; Nozaki, K. One-Pot Carboboration of Alkynes Using Lithium Borylcyanocuprate and the Subsequent Suzuki−Miyaura Cross-Coupling of the Resulting Tetrasubstituted Alkenylborane. Eur. J. Org. Chem. 2011, 2011, 3951−3958. (c) Segawa, Y.; Yamashita, M.; Nozaki, K. Boryl Anion Attacks Transition-Metal Chlorides To Form Boryl Complexes: Syntheses, Spectroscopic, and Structural Studies on Group 11 Borylmetal Complexes. Angew. Chem., Int. Ed. 2007, 46, 6710−6713. (d) Okuno, Y.; Yamashita, M.; Nozaki, K. Borylcyanocuprate in a One-Pot Carboboration by a Sequential Reaction with an ElectronDeficient Alkyne and an Organic Carbon Electrophile. Angew. Chem., Int. Ed. 2011, 50, 920−923. (10) Oschmann, W.; Borner, C.; Kleeberg, C. Unsymmetrical diborane(4) derivatives by copper mediated B−B coupling. Dalton Trans. 2018, 47, 5318−5327. (11) See the Supporting Information for details. (12) While no product could be isolated from reactions involving Me2IiPr, CuOtBu, and 2c, the formation of a black precipitate, presumably elemental copper, is in agreement with the formation of a copper(I) boryl complex as a transient species. (13) Wiberg, N. Holleman-Wiberg, Lehrbuch der Anorganischen Chemie; Walter de Gruyter: Berlin, 2007; pp 2002−2006. (14) It is noted that also in the spectrum at −40 °C a small amount of decomposition products of 4b is observed (∼15%). However, this is attributed to decomposition during the preparation of the NMR sample; no appreciable decomposition was observed within 6 h at −40 °C. (15) The elemental copper formed during the decomposition of the boryl complexes is formed either as a black precipitate or as a copper mirror on the vessel used; concomitant formation of both has also been observed. In our hands, the formation of elemental copper in one form or the other was neither reproducible nor predictable. (16) (a) Kleeberg, C.; Crawford, A. G.; Batsanov, A.; Hodgkinson, P.; Apperley, D. C.; Cheung, M. S.; Lin, Z.; Marder, T. B. Spectroscopic and Structural Characterization of the CyNHC Adduct of B2pin2 in Solution and in the Solid State. J. Org. Chem. 2012, 77, 785−789. (b) Eck, M.; Würtemberger-Pietsch, S.; Eichhorn, A.; Berthel, J. H. J.; Bertermann, R.; Paul, U.; Schneider, H.; Friedrich, A.; Kleeberg, C.; Radius, U.; Marder, T. B. B−B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2 and B2pin2. Dalton Trans. 2017, 46, 3661−3680. (17) (a) Albert, C. F.; Healy, P. C.; Kildea, J. D.; Raston, C. L.; Skelton, B. W.; White, A. H. Lewis-base adducts of Group 11 metal(I) compounds. 49. Structural characterization of hexameric and pentameric (triphenylphosphine)copper(I) hydrides. Inorg. Chem. 1989, 28, 1300−1306. (b) Nguyen, T.-A. D.; Jones, Z. R.; Goldsmith, B. R.; Buratto, W. R.; Wu, G.; Scott, Z. L.; Hayton, T. W. A Cu25 Nanocluster with Partial Cu(0) Character. J. Am. Chem. Soc. 2015, 137, 13319−13324. (c) Nguyen, T.-A. D.; Jones, Z. R.; Leto, D. F.; Wu, G.; Scott, S. L.; Hayton, T. W. Ligand-Exchange-Induced Growth of an Atomically Precise Cu29 Nanocluster from a Smaller Cluster. Chem. Mater. 2016, 28, 8385−8390. (d) Protchenko, A. V.; Dange,

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Institut für Anorganische and Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106 Braunschweig, Germany. Fax: +49 (0) 531 391 5387. E-mail: [email protected]. ORCID

Christian Kleeberg: 0000-0002-6717-4086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from a Research Grant (KL 2243/5-1) of the Deutsche Forschungsgemeinschaft (DFG) and from the Fonds der Chemischen Industrie. The authors thank AllyChem Co. Ltd. for a generous gift of diboron(4) reagents.

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00672 Organometallics XXXX, XXX, XXX−XXX