Bonding and Reactivity of a Dicopper(I) μ-Boryl Cation

Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States. Organometallics , 2016, 35 (2), p...
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Bonding and Reactivity of a Dicopper(I) μ‑Boryl Cation Chelsea M. Wyss,† Jamie Bitting,† John Bacsa,‡ Thomas G. Gray,*,§ and Joseph P. Sadighi*,† †

School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States § Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ‡

S Supporting Information *

ABSTRACT: A siloxide-bridged dicopper(I) cation reacts with bis(catecholato)diboron to form the boryl-bridged dicopper cation {[(SIDipp)Cu] 2(μ-Bcat)} + (SIDipp = 1,3-bis(2,6diisopropylphenyl)imidazolin-2-ylidene; cat = 1,2-C6H4O2). The solid-state structure shows an acute angle about the boryl, with a short intermetallic distance. Density functional theory calculations indicate a small but significant copper−copper bond order. The boryl-bridged cation deprotonates phenylacetylene and reacts with methanol to form a hydride-bridged dicopper cation.

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ransition-metal boryl complexes1 are key intermediates in the hydroboration and diboration of unsaturated organic substrates2 and in the selective borylation of carbon−hydrogen bonds.3 Copper boryl complexes serve as catalytic intermediates in the activation of CO2,4,5 in hydroboration and diboration reactions,6,7 and in the borylation of carbon− heteroatom bonds.8,9 Theoretical studies have offered crucial insights into the reactivity of the copper−boron bond.10 Hydride- and vinyl-bridged dicopper(I) cations display reactivity distinct from that of their neutral, mononuclear analogues11 and are relevant to recently developed alkyne hydroalkylation catalysis.12 Although boryl-bridged complexes are known13 and include Mn/coinage metal clusters,14 boryls bridging only copper centers are limited to neutral clusters containing (diaminoboryl)copper(I) fragments.15,16 Herein we report the synthesis and structure of a singly boryl bridged dicopper cation supported by an N-heterocyclic carbene. The cation adopts a bent arrangement about the boryl with a short intermetallic distance. The boryl ligand displays Brønsted basic reactivity. Density functional theory (DFT) calculations reflect three-center bonding in the [(LCu)2B]+ core. The siloxide-bridged {[(SIDipp)Cu]2(μ-OSiMe3)}+BF4− (SIDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) reacts with bis(catecholato)diboron (catB−Bcat) to form {[(SIDipp)Cu]2(μ-Bcat)}+BF4− (1) and the hydrocarbonsoluble byproduct Me3SiOBcat (Scheme 1). The 1H NMR spectrum in CD2Cl2 displays a single set of resonances arising from the SIDipp ligands. The resonances assigned to the bridging Bcat moiety appear well upfield of those observed for catB−Bcat. The 11B NMR spectrum shows a single peak for the BF4− counterion at δ −1.47 ppm; the bridging boryl does not give rise to a discernible resonance. We speculate that coupling between boron and two quadrupolar copper nuclei, 63Cu or 65 Cu, broadens this resonance into the spectral baseline. © 2016 American Chemical Society

Scheme 1. Synthesis of Boryl-Bridged Dicopper Complex 1

Complex 1 is stable as a solid at −32 °C, but solutions in CD2Cl2 slowly deposit metallic copper at room temperature. Unlike the terminal (IDipp)Cu(Bpin),4b 1 does not react with CO2 at ambient temperature to form CO and the corresponding borate. We next examined the reactivity of the boryl-bridged cation toward alkynes, with which terminal boryl complexes have shown rich reactivity. Upon addition of phenylacetylene, a solution of 1 in CD2Cl2 turned bright yellow. After 8 h at −35 °C, the solution became colorless. The 11 B NMR spectrum indicated the formation of H−Bcat, and the 1 H NMR spectrum was consistent with the formation of {[(SIDipp)Cu]2(μ-CCC6H5)}+BF4− (2; Scheme 2).17 The ligands are equivalent on the NMR time scale, but this spectrum may represent the average of rapidly interconverting Scheme 2. Reaction of 1 with Phenylacetylene

Received: November 19, 2015 Published: January 7, 2016 71

DOI: 10.1021/acs.organomet.5b00961 Organometallics 2016, 35, 71−74

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Organometallics σ,π-bridged structures. This reaction represents an unusual example of a boryl ligand reacting as a Brønsted base.15b,18 We next examined whether a nonacidic alkyne would undergo insertion to form a β-borovinyl complex. Upon addition of 3-hexyne to a solution of 1 in CD2Cl2 an intense yellow color, similar to that seen on addition of phenylacetylene, appeared. This color change, combined with subtle changes in the 1H NMR spectrum, led us to believe that the alkyne might be acting as a weak ligand rather than inserting. We attempted to crystallize an alkyne-bound complex from a solution of 1 in CH2Cl2, using 3-hexyne as cosolvent. The resulting colorless crystals unfortunately consisted of recovered 1 but proved well suited for analysis by X-ray diffraction (Figure 1).

Scheme 3. Proposed Sequence for Methanolysis of 1

The bonding of 1 was interrogated with density functional theory calculations. The geometry of 1 was fully optimized, and the converged structure is a minimum of the potential energy hypersurface; full details appear in the Supporting Information. Calculated copper−carbon distances are shorter than crystallographic values by up to 0.03 Å, and copper−boron distances are shorter still, by up to 0.07 Å. This overbinding recurs in the calculated copper−copper distance, which at 2.345 Å is shorter than the experimental value of 2.4082(8) Å. The Wiberg bond order in the Löwdin basis19 is 0.37 between the two copper atoms, very close to that of 0.38 found for a hydride-bridged dicopper cation.11 For comparison, the copper−carbon bond orders in 1 average 0.69 and the copper−boron bond orders average 0.66. Figure 2 depicts a partial Kohn−Sham orbital energy diagram of 1. The highest occupied Kohn−Sham orbital (HOMO) is an

Figure 1. Thermal ellipsoid depiction of 1 (50% probability). The BF4− anion, hydrogen atoms, and cocrystallized solvent are omitted; unlabeled atoms are carbon. Selected bond lengths (Å) and angles (deg): Cu1−Cu2 2.4082(2), C1−Cu1 1.941(5), Cu1−B2 2.051(6), B2−Cu2 2.041(6), Cu2−C28 1.923(5); C1−Cu1−B2 143.7(2), B2− Cu2−C28 142.7(2), Cu1−B2−Cu2 72.1(2).

The complex adopts a bent arrangement about the boryl, with a Cu−B−Cu angle of 72.1(2)° and an intermetallic Cu− Cu distance of 2.4082(2) Å. This distance is shorter than the Cu−Cu separation of 2.541(2) Å found for the hydride-bridged analogue {[(SIDipp)Cu]2(μ-H)}+OTf−,12 although crystalpacking effects may contribute to the difference between boryl and hydride. Still shorter Cu−Cu distances were measured for the neutral diaminoboryl (DAB) clusters [Cu 5 (μ 2 -O-t-Bu)(μ 2 -DAB) 4 ] 15a and [Cu 4 (μ 2 -Br) 2 (μ 2 DAB)2].15b In light of the deprotonation of phenylacetylene by 1, we reasoned that protonolysis of the boryl by an alcohol would form an alkoxide-bridged dicopper cation plus catecholborane. Subsequent metathesis of hydride and alkoxide should then generate the hydrido-bridged dicopper complex plus alkoxy(catechol)borate. Complex 1 reacts readily with CH3OH in THF-d8 at −32 °C, forming {[(SIDipp)Cu]2(μ-H)}+BF4− (3) and catBOCH3, as judged by 1H NMR spectroscopy (Scheme 3). The 1H NMR spectrum of the major SIDipp-containing product was identical with that of 3 prepared independently. This sequence, alcoholysis followed by hydride transfer, complements the identified pathway for copper-catalyzed hydroboration using a diboron reagent plus methanol.10a

Figure 2. Partial Kohn−Sham orbital energy diagram of 1. Plots of selected orbitals, with percentage compositions in terms of fragment electron density, appear at the right. Implicit THF solvation is included. Gray blocks depict closely spaced metal-based orbitals; for more detail see Figure S6 in the Supporting Information.

antiphase interaction between a catecholboryl π orbital (78%) and d orbitals on copper. The LUMO is more than 5 eV away and consists mainly of interactions between vacant copper p functions and s and p orbitals on the ligands. The two copper centers and boron have a three-center bond order20 of 0.21, which is the largest such value in 1. This three-center bond derives from constructive overlap of basis functions on copper with the boron-centered HOMO of Bcat−. Depictions of this orbital, and of filled orbitals of 1 deriving from it, appear as Figures S7 and S8 in the Supporting Information. The natural atomic charge21 calculated for boron is the highest in 1, at 0.50; charges on copper are 0.37 and 0.40. The large positive charge on boron suggests electron depletion from this formally anionic ligand. Consistent with this interpretation, a Dapprich− Frenking charge transfer analysis22 shows a net transfer of 1.28 electrons from the boryl to the dicopper(I) fragment. 72

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Organometallics

A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890−931. (4) (a) Zhang, L.; Cheng, B.; Carry, Z.; Hou, Z. J. Am. Chem. Soc. 2012, 134, 14314−14317. (b) Laitar, D. S.; Müller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127, 17196−17197. (5) Boron-mediated CO2 reduction: Bontemps, S. Coord. Chem. Rev. 2016, 308, 117−130. (6) (a) For recent reviews, see: Semba, K.; Fujihara, T.; Terao, J.; Tsuji, Y. Tetrahedron 2015, 71, 2183−2197. (b) Fujihara, T.; Semba, K.; Terao, J.; Tsuji, Y. Catal. Sci. Technol. 2014, 4, 1699−1709. (7) Selected examples: (a) Logan, K. M.; Smith, K. B.; Brown, M. K. Angew. Chem., Int. Ed. 2015, 54, 5228−5231. (b) Zhou, Y.; You, W.; Smith, K. B.; Brown, M. K. Angew. Chem., Int. Ed. 2014, 53, 3475− 3479. (c) Semba, K.; Fujihara, T.; Terao, J.; Tsuji, Y. Angew. Chem., Int. Ed. 2013, 52, 12400−12402. (d) Sasaki, Y.; Zhong, C.; Sawamura, M.; Ito, H. J. Am. Chem. Soc. 2010, 132, 1226−1227. (e) Lee, Y.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 3160−3161. (f) Beenen, M. A.; An, C.; Ellman, J. A. J. Am. Chem. Soc. 2008, 130, 6910−6911. (g) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. J. Am. Chem. Soc. 2006, 128, 11036− 11037 See also ref 4a. (8) Selected examples: (a) Schmid, S. C.; Van Hoveln, R.; Rigoli, J. W.; Schomaker, J. M. Organometallics 2015, 34, 4164−4173. (b) Van Hoveln, R.; Hudson, B. M.; Wedler, H. B.; Bates, D. M.; Le Gros, G.; Tantillo, D. J.; Schomaker, J. M. J. Am. Chem. Soc. 2015, 137, 5346− 5354. (c) Iwamoto, H.; Kubota, K.; Yamamoto, E.; Ito, H. Chem. Commun. 2015, 51, 9655−9658. (d) Yang, C.-T.; Zhang, Z.-Q; Tajuddin, H.; Wu, C.-C.; Liang, J.; Liu, J.-H.; Fu, Y.; Czyzewska, M.; Steel, P. G.; Marder, T. B.; Liu, L. Angew. Chem., Int. Ed. 2012, 51, 528−532. (e) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. Angew. Chem., Int. Ed. 2009, 48, 5350−5354. (f) Ito, H.; Sasaki, Y.; Sawamura, M. J. Am. Chem. Soc. 2008, 130, 15774−15775. (9) For a complementary strategy, see: Jayasundara, C. R. K.; Unold, J. M.; Oppenheimer, J.; Smith, M. R., III; Maleczka, R. E., Jr. Org. Lett. 2014, 16, 6072−6075. (10) (a) Moon, J. H.; Jung, H.-Y.; Lee, Y. J.; Lee, S. W.; Yun, J.; Lee, J. Y. Organometallics 2015, 34, 2151−2159. (b) Jang, H.; Zhugralin, A. R.; Lee, Y.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 7859−7871. (c) Zhao, H.; Dang, L.; Marder, T. B.; Lin, Z. J. Am. Chem. Soc. 2008, 130, 5586−5594. (d) Dang, L.; Zhao, H.; Lin, Z.; Marder, T. B. Organometallics 2007, 26, 2824−2832. (e) Zhao, H.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128, 15637−15643. (11) Wyss, C. M.; Tate, B. K.; Bacsa, J.; Gray, T. G.; Sadighi, J. P. Angew. Chem., Int. Ed. 2013, 52, 12920−12923. (12) Suess, A. M.; Uehling, M. R.; Kaminsky, W.; Lalic, G. J. Am. Chem. Soc. 2015, 137, 7747−7753. (13) See for example: (a) Anju, R. S.; Roy, D. K.; Geetharani, K.; Mondal, B.; Varghese, B.; Ghosh, S. Dalton Trans. 2013, 42, 12828− 12831. (b) Braunschweig, H.; Radacki, K.; Shang, R. Chem. Commun. 2013, 49, 9905−9907. (c) Murphy, J. M.; Lawrence, J. D.; Kawamura, K.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 13684− 13685. (d) Braunschweig, H.; Radacki, K.; Rais, D.; Whittell, G. R. Angew. Chem., Int. Ed. 2005, 44, 1192−1194. (e) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Harlow, R. L.; Calabrese, J. C.; Lam, K. C.; Lin, Z. Polyhedron 2004, 23, 2665−2677. (f) Korostylev, A.; Gridnev, I.; Brown, J. M. J. Organomet. Chem. 2003, 680, 329−334. (g) Curtis, D.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.; Starbuck, J. J. Chem. Soc., Dalton Trans. 1999, 1687−1694. (14) Braunschweig, H.; Radacki, K.; Shang, R. Chem. Sci. 2015, 6, 2989−2996. (15) (a) Borner, C.; Kleeberg, C. Eur. J. Inorg. Chem. 2014, 2014, 2486−2489. (b) Kajiwara, T.; Terabayashi, T.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2008, 47, 6606−6610. (16) Isolated terminal boryls of copper: (a) Semba, K.; Shinomiya, M.; Fujihara, T.; Terao, J.; Tsuji, Y. Chem. - Eur. J. 2013, 19, 7125− 7132. (b) Okuno, Y.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2011, 50, 920−923. (c) Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2007, 46, 6710−6713 See also refs 4b and 15. (17) (μ2-alkynyl)Cu2 complexes: (a) Jin, L.; Tolentino, D. R.; Melaimi, M.; Bertrand, G. Sci. Adv. 2015, 1, e1500304. (b) Siu, S. K.-

Calculations on the terminal boryl (SIDipp)CuBcat found the HOMO to be the Cu−B σ-bonding molecular orbital, consistent with studies 10c of (IDipp)CuBpin (Bpin = pinacolboryl). In boryl-bridged 1, the highest occupied orbital drawing significant density from the Bcat− HOMO is the HOMO-12, which lies 1.0 eV below the HOMO (4% boron density). The HOMO-37 (3.1 eV below the HOMO) and HOMO-75 (4.8 eV below the HOMO) also have substantial contributions from the Bcat− HOMO (8% and 3% boron density, respectively). Theoretical studies10 suggest that, in 1,2insertion reactions of a copper boryl, electrons flow from the Cu−B σ bond to coordinated substrate, a path consistent with experimental findings. Stabilization of the Cu−B bond through bridging may account for the lack of reactivity between 1 and CO2 or 3-hexyne. In summary, an N-heterocyclic carbene ligand supports a bent [Cu2Bcat]+ complex. The boryl-bridged dicopper cation deprotonates phenylacetylene and reacts readily with methanol to form a hydride-bridged dicopper cation. Density functional theory (DFT) calculations elucidated the three-center bonding between boron and the dicopper core.

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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.5b00961. Experimental procedures, spectral and crystallographic data (PDF) Crystallographic data for 1 (CIF) Computational output (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.G.G.: [email protected]. *E-mail for J.P.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. National Science Foundation for support of the synthetic work under Award CHE-1300659 to J.P.S. Prof. Jake D. Soper kindly allowed us the use of his group’s FTIR spectrometer. Work at Case Western Reserve University (DFT calculations and analysis) was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under Award DE-FG0213ER46977 to T.G.G.

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REFERENCES

(1) For an extensive review of metal boryl architectures, see: Kays, D. L.; Aldridge, S. Struct. Bonding (Berlin) 2008, 130, 29−122. (2) For reviews of these and related reactions, see for example: (a) Geier, S. J.; Westcott, S. A. Rev. Inorg. Chem. 2015, 35, 69−79. (b) Oestreich, M.; Hartmann, E.; Mewald, M. Chem. Rev. 2013, 113, 402−441. (c) Cid, J.; Gulyás, H.; Carbó, J. J.; Fernández, E. Chem. Soc. Rev. 2012, 41, 3558−3570. (d) Takaya, J.; Iwasawa, N. ACS Catal. 2012, 2, 1993−2006. (e) Dang, L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987−3995. (3) Selected reviews: (a) Mankad, N. P. Synlett 2014, 25, 1197− 1201. (b) Wencel-Delord, J.; Glorius, F. Nat. Chem. 2013, 5, 369−375. (c) Hartwig, J. F. Acc. Chem. Res. 2012, 45, 864−873. (d) Mkhalid, I. 73

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Organometallics L.; Ko, C.-C.; Au, V. K.-M.; Yam, V. W.-W. J. Cluster Sci. 2014, 25, 287−300. (c) Hattori, G.; Sakata, K.; Matsuzawa, H.; Tanabe, Y.; Miyake, Y.; Nishibayashi, Y. J. Am. Chem. Soc. 2010, 132, 10592− 10608. (d) Bruce, M. I.; Zaitseva, N. N.; Skelton, B. W.; Somers, N.; White, A. H. Inorg. Chim. Acta 2007, 360, 681−685. (e) Díez, J.; Gamasa, M. P.; Gimeno, J.; Aguirre, A.; García-Granda, S.; Holubova, J.; Falvello, L. R. Organometallics 1999, 18, 662−669. (f) Kuang, S.-M.; Zhang, Z.-Z.; Wang, Q.-G.; Mak, T. C. W. J. Chem. Soc., Dalton Trans. 1998, 1115−1119. (g) Edwards, A. J.; Paver, M. A.; Raithby, P. R.; Rennie, M.-A.; Russell, C. A.; Wright, D. S. Organometallics 1994, 13, 4967−4972. (h) Olbrich, F.; Behrens, U.; Weiss, E. J. Organomet. Chem. 1994, 472, 365−370. (18) (a) Kays, D. L.; Rossin, A.; Day, J. K.; Ooi, L.-L.; Aldridge, S. Dalton Trans. 2006, 399−410. (b) Yasue, T.; Kawano, Y.; Shimoi, M. Angew. Chem., Int. Ed. 2003, 42, 1727−1730. (19) Natiello, M. A.; Medrano, J. A. Chem. Phys. Lett. 1984, 105, 180−182. (20) (a) Sannigrahi, A. B.; Kar, T. Chem. Phys. Lett. 1990, 173, 569− 572. (b) Kar, T.; Marcos, E. S. Chem. Phys. Lett. 1992, 192, 14−20. (c) Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. J. Am. Chem. Soc. 2006, 128, 278−290. (21) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735−746. (22) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352−9362.

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