Terminal versus Bridging Boryl Coordination in N-Heterocyclic

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Terminal versus Bridging Boryl Coordination in N‑Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior Wiebke Drescher and Christian Kleeberg* Institut für Anorganische und Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, 38106 Braunschweig, Germany

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ABSTRACT: The B−B bond activation of the diborane(4) derivatives B2cat2 with the copper(I) alkoxido complex [(SIDipp)Cu−OtBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)Cu−Bcat] from PhMe or the μ-boryl complex [((SIDipp)Cu)2Bcat][cat2B] from THF. The relevant conversion of the linear boryl complex to the μ-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB−OtBu, concomitantly formed during the B−B activation. With Lewis acids such as BPh3 or [CPh3][BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)Cu−Bcat] or [(SIDipp)Cu−Bdmab] occurs and results in the μ-boryl complexes [((SIDipp)Cu)2Bcat/dmab][Ph3B−Bcat/dmab] and [((SIDipp)Cu)2Bcat][BArF]. The formation of [((SIDipp)Cu)2Bcat][cat2B] is generally accompanied by the concomitant formation of the μ-hydrido complex [((SIDipp)Cu)2H][cat2B]. The spiroborate [cat2B]− is formed from the initially formed Lewis acid/base adduct [catB−B(OtBu)cat]− presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the μ-boryl complexes in solution.

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INTRODUCTION Copper(I) boryl complexes of, in particular, dialkoxy boryl ligands of the type B(OR)2 (Bpin, Bcat; pin = (OCMe2)2, cat = 1,2-O2C6H4) have been established as intermediates in a plethora of copper catalyzed borylation reactions. Those complexes are, despite the presence of a three-coordinate, Lewis acidic, coordinatively unsaturated boron atom, versatile boron nucleophiles.1−3 The use of such complexes as the catalytically central intermediate in borylation reactions has been widely explored since the first independent reports by Miyaura et al. and Hosomi et al. in 2000.2,3 Since those, a plethora of such borylation reactions has been reported on substrates such as CO2, carbonyls, α,β-unsaturated carbonyls, dienes, allenes, alkynes, and aryl and alkyl halides.1a,b,c,2,4,5 Generally, those reactions follow a common pathway: a copper(I) boryl complex is generated by a σ-bond metathesis reaction of a suitable copper(I) precursor with a tetraalkoxy or -aryloxy diborane(4) derivative, and this complex reacts then with the (organic) substrate under nucleophilic boryl transfer.1,5 While the relevance of copper(I) boryl complexes for those reactions is apparent, comparably little is known on the coordination chemical properties of copper(I) boryl complexes. In their seminal work, Sadighi and coworkers reported on the isolation and characterization of the first copper(I) © XXXX American Chemical Society

dialkoxyboryl complex in 2005 as well as conducted detailed reactivity studies.1a,b The linear complex (IDipp)Cu−Bpin (2a) was obtained by reaction of a copper(I) alkoxido precursor with B2pin2 (1a) (Figure 1), in analogy to the proposed reaction pathway in catalysis (IDipp = 1,3-bis(2,6diisopropylphenyl)-imidazole-2-ylidene). This protocol was subsequently successfully applied to the isolation of a number of related sterically protected N-heterocyclic carbene (NHC)Cu(I) boryl complexes (2b−g, Scheme 1).1c,6a However, only recently, we reported on the first isolated and characterized

Figure 1. Overview of the diborane(4) derivatives used in this work. Received: April 11, 2019

A

DOI: 10.1021/acs.inorgchem.9b01041 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

cationic dinuclear mono-μ-boryl complex are observed, depending on the copper(I) precursor complex used. However, as the complex 3(BF4) is the only Bcat copper(I) boryl complex reported so far it may be considered that the coordination motif in 3+ is unique to Bcat. Considering the crucial role of copper(I) boryl complexes in copper catalyzed borylation reactions with diboranes(4) as the boron source, it is obvious that a detailed understanding of the coordination properties of copper(I) boryl complexes is important. In this work, we address some fundamental aspects regarding the coordination chemistry of copper(I) boryl complexes, in particular coordination properties of boryl ligands and herewith the change between terminal and bridging coordination modes.

Scheme 1. Synthesis and Structures of Selected NHC Copper(I) Boryl Complexes1a,6a

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EXPERIMENTAL SECTION

General Considerations. [(SIDipp)Cu−OtBu], 8a,b pinB− Bdmab (dmab = 1,2-(NMe)2C6H4),6a catB−Bdmab,8c [K(18Crown-6)OtBu],8d and catB−Cl8e were prepared according to literature procedures. B2cat2 (1b) was recrystallized from toluene prior to use. All other compounds were commercially available and used as received; their purity and identity were checked by appropriate methods. Unless otherwise noted, 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 nitrogen-filled glovebox (MBraun). NMR spectra were recorded on Bruker Avance II 300, Avance III HD 300, Avance III 400, or Avance III HD 500 spectrometers. For air sensitive samples, NMR tubes equipped with screw caps (WILMAD) were used, and the solvents were dried over potassium/benzophenone and degassed. Chemical shifts (δ) are given in ppm using the (residual) resonance signal of the solvents for calibration (C6D6: 1H NMR: 7.16 ppm, 13C NMR: 128.06 ppm; THF-d8: 1H NMR: 1.72 ppm, 13C NMR: 25.31 ppm).9 11B and 19F NMR chemical shifts are reported relative to pseudo external BF3· Et2O and CFCl3, respectively. 13C, 11B, and 19F NMR spectra were recorded by employing composite pulse 1H decoupling unless noted otherwise. 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 1H−13C HMBC). 11B NMR spectra were processed by applying a back linear prediction to suppress the broad background signal due to the borosilicate glass in the NMR tube and instrument and a Lorentz-type window function (LB = 10 Hz); the spectra were carefully evaluated to ensure that no genuinely broad signals of the sample were suppressed. Melting points were determined in flame-sealed capillaries under nitrogen using a Büchi 535 apparatus and are not corrected. Elemental analyses were performed at the Institut für Anorganische und Analytische Chemie of the Technische Universität Carolo-Wilhelmina zu Braunschweig using an Elementar vario MICRO cube instrument. X-ray Structure Determinations. The single crystals were transferred into inert perfluoroether oil inside a nitrogen-filled glovebox and, outside the glovebox, rapidly mounted on top of a human hair or MiTeGen loop and placed in the cold nitrogen gas stream on the diffractometer.10a The data were collected on an Oxford Diffraction Xcalibur E instrument using graphite monochromated Mo Kα radiation, an Oxford Diffraction Nova A, or a Rigaku Oxford Diffraction Synergy-S instrument using mirror-focused Cu Kα radiation or a Rigaku Oxford Diffraction Synergy-S instrument using mirror-focused Mo Kα radiation. The reflections were indexed and integrated, and appropriate absorption corrections were applied as implemented in the CrysAlisPro software package.10b The structures were solved by employing the program SHELXT and refined anisotropically for all nonhydrogen atoms by full-matrix leastsquares on all F2 using SHELXL software.10c,d Generally, hydrogen atoms were refined by employing a riding model; methyl groups were treated as rigid bodies and were allowed to rotate about the E−CH3

copper(I) boryl complexes with sterically less demanding NHC (2h−i and (2j−l)2, Scheme 1) and phosphine ligands.6b,c,7 In contrast to the sterically encumbered copper(I) boryl complexes that were found to adapt a linear coordination mode with terminal boryl ligands in the solid state (2a, b, f−i), sterically less demanding NHC (or phosphine) ligands together with little demanding boryl ligands exhibit dimeric, dinuclear structures with two bridging μ-boryl ligands ((2j−l)2, Scheme 1).6b,c A similar bridging μ-boryl coordination motif has also been reported by Sadighi and coworkers in 2016 in the cationic dinuclear mono-μ-boryl complex 3a+ in 3a(BF4), the only catB copper(I) complex reported so far (Scheme 2). The latter was Scheme 2. Synthesis and Structure of the μ-Boryl Copper(I) Complex 3a(BF4)1e

obtained upon reaction of the μ-silanolate copper(I) complex [((SIDipp)Cu)2(OTMS)][BF4] (SIDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazolidine-2-ylidene) with B2cat2 (1b) (Figure 1) via an analogous B−B activation as for the neutral complexes described above (Scheme 2).1e So far, it may be stated that for sterically less encumbered NHC ligands, neutral dimeric copper(I) bis-μ-boryl complexes are observed, whereas for sterically more demanding NHC ligands, mononuclear dimeric species are observed. For the sterically demanding dipp (2,6-diisopropylphenyl) derived NHC ligands either mononuclear linear complexes or a B

DOI: 10.1021/acs.inorgchem.9b01041 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

28.8 (CH(CH3′)), 29.2 (CH(CH3)2), 33.1 (NCH3), 53.3 (NCH2), 106.6 (3,6-CHdmab), 116.9 (4,5-CHdmab), 124.4 (3,5-CHdipp), 129.6 (4-CHdipp), 135.5 (ipso-Cdipp), 141.3 (1,2-Cdmab), 147.2 (2,6-Cdipp), 208.1 (CCarbene). 11B{1H} NMR (160.5 MHz, C6D6, rt): 44.4 (Δw1/2 = 860 Hz). 1H NMR (300.1 MHz, THF-d8, rt): δ 1.35 (12 H, d, J = 7.1 Hz, CH(CH3)2), 1.44 (12 H, d, J = 7.1 Hz, CH(CH3)2), 2.77 (6 H, s, NCH3), 3.26 (4 H, sept, J = 7.1 Hz, CH(CH3)2), 4.04 (4 H, s, NCH2), 6.41−6.48 (2 H, m, CHdmab), 6.49−6.56 (2 H, m, CHdmab), 7.30 (4 H, app. dd, J = 7.6, 1.6 Hz, 3,5-CHdipp), 7.39 (2 H, app. dd, J = 8.5, 6.5 Hz, 4-CHdipp). 11B{1H} NMR (96.3 MHz, THF-d8, rt): 43.2 (Δw1/2 = 860 Hz). Mp: decomp. >124 °C. Anal. Calcd for C35H48BCuN4 C, 70.16; H, 8.08; N, 9.35. Found: C, 70.11; H, 8.03; N, 9.78. Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. [((SIDipp)Cu)2Bcat][cat2B] (3a(cat2B)) and [((SIDipp)Cu)2H][cat2B] (4(cat2B)). For the synthesis of 3b(cat2B) with a minimal contamination by the hydrido complex 4(cat2B), rigorously inert conditions and clean starting materials are paramount. For the best results, the THF was stored over potassium prior to use, and the reaction vessels were treated with THF/potassium for 16 h prior to use. Generally, the reaction gave variable ratios of 3b(cat2B) and 4(cat2B). Adhering to the above protocol leads to a product containing >80% of 3b(cat2B).12 However, not doing so may lead exclusively to 4(cat2B). From B2cat2 (1b). In a nitrogen-filled glovebox, [(SIDipp)Cu− OtBu] (50.0 mg, 95.0 μmol, 1.0 equiv) and 1b (22.6 mg, 94.9 μmol, 1.0 equiv) were combined in a screw-cap vial in THF (2.5 mL). After 1 h at room temperature, the reaction mixture was layered with npentane (4 mL). After 20 h at −40 °C, the product had separated as colorless crystals. The solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give the product (20 mg, 16 μmol, 34% (based on Cu)). From 2m and catB−OtBu. In a nitrogen-filled glovebox, 2m (50.0 mg, 87.4 μmol, 1.0 equiv) and catB−OtBu (16.8 mg, 87.4 μmol, 1.0 equiv) were combined in a screw-cap vial in THF (2 mL). After 1 h at room temperature, the reaction mixture was layered with n-pentane (4 mL). After 20 h at −40 °C, the product had separated as colorless crystals. The solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give virtually pure product containing only negligible amounts of 4+ (13 mg, 10 μmol, 23% (based on Cu)). In Situ NMR Experiment [(SIDipp)Cu−OtBu] + B2cat2 in THF-d8. In a nitrogen-filled glovebox, [(SIDipp)Cu−OtBu] (15.0 mg, 28.5 μmol, 1.0 equiv) and 1b (6.8 mg, 28.6 μmol, 1.0 equiv) were combined in THF-d8 (0.7 mL) in a screw-cap NMR tube. NMR spectra were recorded in the given intervals (Figure S2.4).11 During the reaction, a colorless precipitate formed from the pale pinkish solution. In Situ NMR Experiment 2m + catB−OtBu (or pinB−OiPr). In a nitrogen-filled glovebox, 2m (15.0 mg, 26.2 μmol, 1.0 equiv) and catB−OtBu (5.0 mg, 26.2 μmol, 1.0 equiv) were combined in THF-d8 (0.7 mL) in a screw-cap NMR tube. NMR spectra were recorded in the given intervals.11 After 20 h, crystals of (3ax41−x)(cat2B)(THF)3.5−x suitable for X-ray structure determination had formed. The reactions in C6D6 and with pinB−OiPr (4.9 mg, 26.2 μmol, 1.0 equiv) were performed analogously. [((SIDipp)Cu)2 Bcat][cat2B] (3a(cat2 B)) (dissociation into [(SIDipp)Cu−cat2B] (5) and [(SIDipp)Cu−Bcat](2m) in THF-d8, only NMR data of 5 are given here): 1H NMR (500.3 MHz, THF-d8, rt): δ 1.20 (12 H, br. d, J = 7 Hz, CH(CH3)), 1.30 (12 H, d, J = 6.9 Hz, CH(CH3′)), 3.08 (4 H, sept, J = 6.9 Hz, CH(CH3)2), 4.08 (4 H, s, NCH2), 6.04−6.09 (4 H, m, CHcat2B), 6.31−6.35 (4 H, m, CHcat2B), 7.24 (4 H, d, J = 8 Hz, 3,5-CHdipp (overlapping)), 7.32−7.44 (2 H, 4CHdipp (overlapping)). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 23.9 (CH(CH3′)), 25.8 (CH(CH3)), 29.6 (CH(CH3)2), 54.8 (NCH2), 109.1 (CHcat2B), 118.4 (CHcat2B), 125.5 (3,5-CHdipp), 130.8 (br., 4CHdipp), 135.5 (ipso-Cdipp), 147.6 (2,6-Cdipp), 152.0 (1,2-Ccat2B), 202 (CCarbene, HMQC). 11B{1H} NMR (160.5 MHz, THF-d8, rt): 14.4

bond. During refinement and analysis of the crystallographic data, the programs WinGX, OLEX2, PLATON, DSR, Mercury, and Diamond were used.10e−k Unless noted otherwise, the shown ellipsoids represent the 50% probability level, and hydrogen atoms are omitted for clarity. Adapted numbering schemes may be used to improve the readability. [(SIDipp)Cu−Bcat] (2m). In PhMe. In a nitrogen-filled glovebox, [(SIDipp)Cu−OtBu] (50.0 mg, 95.0 μmol, 1.0 equiv) was suspended in PhMe (1.5 mL) in a screw-cap scintillation vial, and 1b (22.6 mg, 94.9 μmol, 1.0 equiv) was added together with PhMe (0.5 mL). The yellow solution was shaken until complete dissolution occurred and cooled to −40 °C. After 1 h at −40 °C, the solution was layered with n-pentane. After 16 h at −40 °C, a colorless crystalline solid (suitable for single crystal X-ray diffraction) had separated. The supernatant solution was decanted. The residue was washed with cold n-pentane (−40 °C) and dried thoroughly in vacuo to give 2m (40 mg, 70 μmol, 74%). In THF. In a nitrogen-filled glovebox, [(SIDipp)Cu−OtBu] (50.0 mg, 95.0 μmol, 1.0 equiv) and 1b (22.6 mg, 94.9 μmol, 1.0 equiv) were dissolved in THF (2.0 mL) in a silylated screw-cap scintillation vial (silylation was conducted by heating the glassware in an atmosphere of TMSCl to 100 °C at a slightly reduced pressure for 6 h followed by thorough drying at 100 °C in vacuo). The yellow solution was layered with n-pentane. After 16 h at −40 °C, a colorless crystalline solid (suitable for single crystal X-ray diffraction) had separated from which the supernatant solution was decanted. The residue was washed with cold n-pentane (−40 °C) and dried thoroughly in vacuo to give 2m (15 mg, 26 μmol, 27%). In Situ NMR Experiment ([(SIDipp)Cu−OtBu] + B2cat2 in C6D6). In a nitrogen-filled glovebox, [(SIDipp)Cu−OtBu] (15.0 mg, 28.5 μmol, 1.0 equiv) and 1b (6.8 mg, 28.6 μmol, 1.0 equiv) were combined in C6D6 (0.7 mL) in a screw-cap NMR tube. NMR spectra were recorded in the given intervals (Figure S2.3).11 During the reaction, a colorless precipitate formed. Crystallization of the reaction mixture yielded 2m. 1 H NMR (500.3 MHz, THF-d8, rt): δ 1.35 (12 H, d, J = 6.9 Hz, CH(CH3)), 1.46 (12 H, d, J = 6.9 Hz, CH(CH3′)), 3.23 (4 H, sept, J = 6.9 Hz, CH(CH3)2), 4.01 (4 H, s, NCH2), 6.65−6.69 (2 H, m, 4,5− CHcat), 6.77−6.82 (2 H, m, 3,6-CHcat), 7.26 (4 H, app. d, J = 7.9 Hz, 3,5-CHdipp), 7.34 (2 H, app. dd, J = 8.3, 7.0 Hz, 4-CHdipp). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 24.0 (CH(CH3)), 26.1 (CH(CH3′)), 29.7 (CH(CH3)2), 54.6 (NCH2), 111.3 (3,6-CHcat), 120.7 (4,5-CHcat), 124.9 (3,5-CHdipp), 130.0 (4-CHdipp), 136.0 (ipso-Cdipp), 147.8 (2,6-Cdipp), 149.9 (1,2-Ccat), 207.2 (CCarbene). 11B{1H} NMR (160.5 MHz, THF-d8, rt): 44.8 (Δw1/2 = 930 Hz). 1H NMR (300.1 MHz, C6D6, rt): δ 1.20 (12 H, d, J = 7.0 Hz, CH(CH3)2), 1.56 (12 H, d, J = 7.0 Hz, CH(CH3)2), 3.03 (4 H, sept, J = 7.0 Hz, CH(CH3)2), 3.14 (4 H, s, NCH2), 6.64−6.71 (2 H, m, CHcat), 6.99−7.04 (2 H, m, CHcat), 7.05 (4 H, app. d, J = 7.7 Hz, 3,5-CHdipp), 7.11−7.20 (4CHdipp, overlapping with solvent signal). 11B{1H} NMR (96.3 MHz, C6D6, rt): 45 (Δw1/2 = 1700 Hz). Mp: decomp. >98 °C. Anal. Calcd for C33H42BCuN2O2 C, 69.16; H, 7.39; N, 4.89. Found: C, 69.34; H, 7.40; N, 5.14. [(SIDipp)Cu−Bdmab] (2n). In a nitrogen-filled glovebox, [(SIDipp)Cu−OtBu] (50.0 mg, 95.0 μmol, 1.0 equiv) was suspended in PhMe (1.5 mL) in a screw-cap scintillation vial, and 1c (25.8 mg, 94.8 μmol, 1.0 equiv) was added as PhMe solution (0.5 mL). The yellow mixture was shaken until complete dissolution occurred and cooled to −40 °C. After 1 h at −40 °C, the solution was layered with n-pentane. After 16 h at −40 °C, a colorless crystalline solid (suitable for single crystal X-ray diffraction) had separated from which the supernatant solution was decanted. The residue was washed with cold n-pentane (−40 °C) and dried thoroughly in vacuo to give 2n (29 mg, 48 μmol, 51%). 1 H NMR (500.3 MHz, C6D6, rt): δ 1.24 (12 H, d, J = 6.8 Hz, CH(CH3)), 1.51 (12 H, d, J = 6.8 Hz, CH(CH3′)), 3.06 (4 H, sept, J = 6.9 Hz, CH(CH3)2), 3.08 (6 H, s, NCH3), 3.19 (4 H, s, NCH2), 6.88−6.93 (2 H, m, 3,6-CHdmab), 7.07−7.10 (2 H, m, 4,5-CHdmab), 7.12 (4 H, d, J = 7.8 Hz, 3,5-CHdipp), 7.23 (2 H, t, J = 7.8 Hz, 4CHdipp). 13C{1H} NMR (125.8 MHz, C6D6, rt): 23.8 (CH(CH3)), C

DOI: 10.1021/acs.inorgchem.9b01041 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (Δw1/2 = 28 Hz). Mp: decomp. >180 °C.12 Anal. Calcd for C72H88B2Cu2N4O6 ((3a(cat2B)) C, 68.95; H, 7.07; N, 4.47. Anal. Calcd for (3a0.7740.23)(cat2B) C, 69.09; H, 7.15; N, 4.55. Found: C, 69.09; H, 7.56; N, 5.10.12 [((SIDipp)Cu)2H][cat2B] (4(cat2B)): 1H NMR (500.3 MHz, THF-d8, rt): δ −4.38 (1 H, s, Cu2H), 1.00 (24 H, d, J = 6.9 Hz, CH(CH3)), 1.24 (24 H, d, J = 6.9 Hz, CH(CH3′)), 2.97 (8 H, sept, J = 6.9 Hz, CH(CH3)2), 3.96 (8 H, s, NCH2), 6.22−6.27 (4 H, m, CHcat2B), 6.27−6.32 (4 H, m, CHcat2B), 7.19 (8 H, d, J = 7.7 Hz, 3,5CHdipp), 7.34 (4 H, dd, J = 7.7 Hz, 4-CHdipp). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 23.7 (CH(CH3′)), 26.2 (CH(CH3)), 29.4 (CH(CH3)2), 54.9 (NCH2), 108.1 (CHcat2B), 117.2 (CHcat2B), 125.3 (3,5-CHdipp), 130.6 (4-CHdipp), 135.2 (ipso-Cdipp), 147.4 (2,6Cdipp), 153.7 (Ccat2B), 202.0 (CCarbene). 11B{1H} NMR (160.5 MHz, THF-d8, rt): 14.9 (Δw1/2 = 27 Hz). Mp: decomp. >210 °C. Anal. Calcd for C66H85BCu2N4O4 C, 69.76; H, 7.54; N, 4.93. Found: C, 69.66; H, 7.43; N, 5.26. [((SIDipp)Cu)2Bdmab][cat2B] (3b(cat2B)). In a nitrogen-filled glovebox, a scintillation vial was charged with [(SIDipp)Cu−OtBu] (50.0 mg, 95.0 μmol, 1.0 equiv) and 1d (25.1 mg, 95.0 μmol, 1.0 equiv) dissolved in THF (2.5 mL). After 20 h at −40 °C, the mixture was layered with n-pentane (4 mL) and stored at −40 °C. After 9 days at −40 °C, a colorless crystalline solid (suitable for single crystal X-ray diffraction) had separated from which the supernatant solution was decanted. The residue was washed with cold n-pentane (−40 °C, 2 × 2 mL) and dried thoroughly in vacuo to give 2n (4 mg, 3 μmol, 6% (based on Cu)). The yields of the reaction proved to be quite variable; the typical yield was significantly lower (∼1%). A more complete characterization was not possible due to the very low yields obtained. According to NMR spectroscopy, the isolated material contained about 10% of the hydrido complex 4(cat2B) and 5% of the linear complex 2n (Figure S2.2).11 See discussion below. 1 H NMR (500.3 MHz, THF-d8, rt): δ 0.85 (24 H, d, J = 6.8 Hz, CH(CH3)), 1.15 (24 H, d, J = 6.8 Hz, CH(CH3′)), 2.89 (8 H, sept, J = 6.8 Hz, CH(CH3)2), 2.25 (6 H, s, NCH3), 3.88 (8 H, s, NCH2), 6.24−6.30 (8 H, m, CHcat2B), 6.57−6.60 (2 H, m, 3,6-CHdmab), 6.80− 6.84 (2 H, m, 4,5-CHdmab), 7.13 (8 H, d, J = 7.7 Hz, 3,5-CHdipp), 7.34 (4 H, t, J = 7.7 Hz, 4-CHdipp). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 24.2 (CH(CH3′)), 26.1 (CH(CH3)), 29.4 (CH(CH3)2), 33.2 (NCH3), 54.9 (NCH2), 108.0 (CHcat2B), 108.8 (3,6-CHdmab), 117.1 (CHcat2B), 119.4 (4,5-CHdmab), 125.4 (3,5-CHdipp), 130.4 (4-CHdipp), 135.7 (ipso-Cdipp), 139.8 (1,2-Cdmab), 147.5 (2,6-Cdipp), 153.9 (Ccat2B), 201.8 (CCarbene). 11B{1H} NMR (160.5 MHz, THF-d8, rt): 14.9 (Δw1/2 = 33 Hz). [K(18-Crown-6)][cat2B]. In a nitrogen-filled glovebox, solutions of [K(18-Crown-6)OtBu] (20.0 mg, 53 μmol, 1.0 equiv) and 1b (12.5 mg, 53 μmol, 1.0 equiv), each in PhMe/Et2O (1:1, 0.5 mL), were combined. After 5 min, the solution was filtered over a plug of glass wool and stored at rt. Within 12−36 h, the product was separated as colorless crystals (16 mg, 30 μmol, 57%). 1 H NMR (300.3 MHz, THF-d8, rt): δ 3.51 (24 H, s, OCH2), 6.30− 6.46 (8 H, m, CHcat2B). 13C{1H} NMR (75.5 MHz, THF-d8, rt): 71.0 (OCH2), 108.1 (CHcat2B), 117.2 (CHcat2B), 153.9 (Ccat2B). 11B{1H} NMR (96.3 MHz, THF-d8, rt): 15.0 (Δw1/2 = 17 Hz). Mp: 164−169 °C. Anal. Calcd for C24H32BK2O10 C, 54.35; H, 6.08. Found: C, 54.35; H, 5.89. [nBu4N][cat2B]. Synthesized following the procedure given by Duggan, Humphrey and coworkers.13 1 H NMR (300.3 MHz, THF-d8, rt): δ 0.92 (12 H, t, J = 7.6 Hz, CH3), 1.30 (8 H, app. sext., J = 7.6 Hz, CH2), 1.46−1.97 (8 H, m, CH2), 3.10−3.20 (8 H, m, NCH2), 6.33−6.49 (8 H, m, CHcat2B). 13 C{1H} NMR (75.5 MHz, THF-d8, rt): 13.9 (CH3), 20.4 (CH2), 24.5 (CH2), 58.7 (NCH2), 108.3 (CHcat2B), 117.6 (CHcat2B), 153.7 (Ccat2B). 11B{1H} NMR (96.3 MHz, THF-d8, rt): 15.0 (Δw1/2 = 16 Hz). catB−OtBu. catB−Cl (200 mg, 1.30 mmol, 1.0 equiv) and KOtBu (145 mg, 1.29 mmol, 1.0 equiv) were combined in THF (6 mL). After 1 h at room temperature, the mixture was filtered through a plug of Celite, and the solvent was removed in vacuo to give catB−OtBu as a colorless oil (120 mg, 0.63 mmol, 49%). 1H NMR (300.3 MHz,

THF-d8, rt): δ 1.48 (9 H, s, C(CH3)3), 6.92−6.98 (2 H, m, CHcat), 7.02−7.09 (2 H, m, CHcat). 11B{1H} NMR (96.3 MHz, THF-d8, rt): 22.3 (Δw1/2 = 88 Hz). 1H NMR (300.3 MHz, C6D6, rt): δ 1.28 (9 H, s, C(CH3)3), 6.70−6.77 (2 H, m, CHcat), 6.90−6.98 (2 H, m, CHcat). 11 1 B{ H} NMR (96.3 MHz, C6D6, rt): 22.7 (Δw1/2 = 91 Hz). [((SIDipp)Cu)2Bcat][Ph3B−Bcat] (3a(Ph3B−Bcat)). In a nitrogenfilled glovebox, 2m (30.0 mg, 52.4 μmol, 1.0 equiv) and BPh3 (5.7 mg, 23.6 μmol, 0.45 equiv) were combined in a screw-cap vial in PhMe (2 mL). The turbid solution was cooled to −40 °C. After 20 h at −40 °C, the product had separated as a colorless solid (in this, a few solid crystals suitable for single crystal X-ray diffraction were found, though of low quality).11 The solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give the product as PhMe solvate 3a(Ph3B−Bcat)(PhMe)n with varying amounts of PhMe present (18 mg, 12 μmol, 51% (based on BPh3)). Slightly better quality single crystals suitable for X-ray diffraction studies were obtained by recrystallization of the isolated product from THF/n-pentane at −40 °C. The NMR spectra suggest the presence of two major species 2m and 3a(Ph3B−Bcat); only the data of the latter are given here. For a detailed discussion of the NMR data, see below. 1 H NMR (500.3 MHz, THF-d8, rt): δ 0.99 (24 H, d, J = 6.7 Hz, CH(CH3)), 1.18 (24 H, d, J = 6.7 Hz, CH(CH3′)), 2.85 (8 H, sept., J = 6.7 Hz, CH(CH3)2), 3.80 (8 H, s, NCH2), 6.63 (3 H, tt, J = 7.3, 1.4 Hz, 4-CHPh), 6.72−6.76 (2 H, m (CHcat)Ph3B−Bcat, (overlapping)), 6.76 (6 H, app. t, J = 7.4 Hz, 3,5-CHPh, (overlapping)), 6.73−6.78 (2 H, br. overlapping, (CHcat)boryl), 6.88−6.92 (2 H, m (CHcat)Ph3B−Bcat), 6.94−6.98 (2 H, br. overlapping, (CHcat)boryl), 6.98 (8 H, d, J = 7.6 Hz, 3,5-CHdipp), 7.07−7.14 (4 H, 4-CHdipp (overlapping)), 7.17 (6 H, d, J = 7.6 Hz, 2,6-CHPh). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 23.8 (CH(CH3′)), 26.0 (CH(CH3)), 29.4 (CH(CH3)2), 54.9 (NCH 2 ), 111.7 (CH cat,Ph3B−Bcat), 113.2 ((CHcat )boryl ), 120.7 ((CHcat)Ph3B−Bcat), 122.1 (4-CHPh), 122.4 ((CHcat)boryl), 125.2 (3,5CHdipp), 126.2 (3,5-CHPh), 130.6 (4-CHdipp), 134.4 (ipso-Cdipp), 136.3 (2,6-CHPh), 147.0 (2,6-Cdipp), 149 ((Ccat)boryl, HMBC), 151.1 ((CHcat)Ph3B−Bcat), 163 (br. s, B−CPh), 201 (CCarbene, HMBC). 11 1 B{ H} NMR (160.5 MHz, THF-d8, rt): −12.8 (Δw1/2 = 46 Hz). Mp: decomp. >146 °C. Anal. Calcd for C 84 H 99 B 3 Cu 2 N 4 O 4 ((3a(Ph3B−Bcat)) C, 72.67; H, 7.19; N, 4.04. Anal. Calcd for 3a(Ph3B−Bcat)(C7H8)2 C, 74.85; H, 7.37; N, 3.56. Found: C, 74.98; H, 7.31; N, 3.20. [(SIDipp)Cu(Ph3B−Bcat)]. In a nitrogen-filled glovebox, 2m (43.0 mg, 75.2 μmol, 1.0 equiv) and BPh3 (18.2 mg, 75.2 μmol, 1.0 equiv) were combined in a screw-cap vial in PhMe (2 mL). The turbid solution was cooled to −40 °C, and the product was separated within 20 h as colorless crystals suitable for single crystal X-ray diffraction. The solution was decanted, and the residue was washed with cold npentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give the product (43 mg, 53 μmol, 70%) as toluene solvate. Recrystallization from Et2O at −40 °C gave the ether solvate in Xray quality crystals. The different species present are reported separately; for a detailed discussion, see below. SIDipp Species 1 (Red in Figure 9). 1H NMR (400.4 MHz, THFd8, −10 °C): δ 0.97 (12 H, br. d, J = 6.8 Hz, CH(CH3)), 1.17 (12 H, d, J = 6.8 Hz, CH(CH3′)), 2.84 (4 H, sept, J = 6.8 Hz, CH(CH3)2), 3.80 (4 H, s, NCH2), 6.97 (4 H, d, J = 7.5 Hz, 3,5-CHdipp (overlapping)), 7.10 (2 H, app. t, J = 7.5 Hz, 4-CHdipp (overlapping)). 13 C{1H} NMR (100.7 MHz, THF-d8, −10 °C): 23.6 (CH(CH3′)), 26.1 (CH(CH3)), 29.4 (CH(CH3)2), 54.7 (NCH2), 125.2 (3,5CHdipp), 134.3 (ipso-Cdipp), 146.9 (2,6-Cdipp), 200.2 (CCarbene); the 4CHdipp signal was not unambiguously identified. SIDipp Species 2 (Green in Figure 9). 1H NMR (400.4 MHz, THF-d8, −10 °C): δ 1.18 (12 H, br. d, J = 7.0 Hz, CH(CH3)), 1.22 (12 H, d, J = 7.0 Hz, CH(CH3′)), 3.02 (4 H, sept, J = 7.0 Hz, CH(CH3)2), 3.94 (4 H, s, NCH2), 7.26 (4 H, br. d, J = 7.7 Hz, 3,5CHdipp), 7.31−7.41 (2 H, br. m, 4-CHdipp). 13C{1H} NMR (100.7 MHz, THF-d8, −10 °C): 24.8 (CH(CH3′)), 25.1 (CH(CH3)), 29.4 (CH(CH3)2), 54.8 (NCH2), 125.5 (3,5-CHdipp), 136.2 (ipso-Cdipp), 147.4 (2,6-Cdipp), 203.2 (CCarbene); the 4-CHdipp signal was not unambiguously identified. D

DOI: 10.1021/acs.inorgchem.9b01041 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(br. s), −164.2 (t, J = 20 Hz), −167.6 (br. t, J = 18 Hz). 11B{1H} NMR (160.5 MHz, THF-d8, rt): −16.1 (Δw1/2 = 21 Hz). Mp: decomp. >181 °C. Anal. Calcd for C84H80BCu2F20N4O2 (3a(BArF)) C, 59.13; H, 4.73; N, 3.28. Anal. Calcd for 3a(BArF)(C5H12)1/2 C, 59.63; H, 4.98; N, 3.22. Found: C, 59.29; H, 4.75; N, 3.38. The content of approximately 0.5 equiv of n-pentane is in agreement with the X-ray structure and the 1H NMR data. [((SIDipp)Cu)2Bcat][BF4] 3a(BF4). In a nitrogen-filled glovebox, 2m (30.0 mg, 52.4 μmol, 1.0 equiv) and [CPh3][BF4] (8.6 mg, 26.2 μmol, 0.5 equiv) were combined in a screw-cap vial in THF (3 mL). After 1 h at −40 °C, the solution was layered with n-pentane (ca. 4 mL) and cooled to −40 °C. After 36 h at −40 °C, a few colorless crystals (also of low quality suitable for single crystal X-ray diffraction) had separated; the solution was decanted, and the solid residue washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give 3a(BF4) (3 mg, 3 μmol, 12%). The NMR properties and cell parameters match those reported by Sadighi.1e [((SIDipp)Cu)(OEt2)][BArF]. In a nitrogen-filled glovebox, 2m (30.0 mg, 52.4 μmol, 1.0 equiv) and [CPh3][B(C6F5)4] (48.8 mg, 52.4 μmol, 1.0 equiv) were combined in a screw-cap vial in Et2O (1.5 mL). After 16 h at −40 °C, the mixture was layered with n-pentane (ca. 4 mL). After 20 h at −40 °C, the product had separated as a microcrystalline solid; the solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo (42 mg, 35 μmol, 67%). Note that it was found that the NHC:Et2O ratio varied according to 1H NMR spectroscopy around unity dependent on the time of vacuum drying, possibly due to solvent adhering to the solid as well as partial loss of the ligand Et2O (see also the analytical data). Single crystals suitable for X-ray diffraction studies were obtained by recrystallization of the isolated product from Et2O/n-pentane at −40 °C. 1 H NMR (500.3 MHz, THF-d8, rt): δ 1.11 (6 H, t, J = 6.9 Hz, OCH2CH3), 1.33 (12 H, d, J = 6.8 Hz, CH(CH3)), 1.35 (12 H, d, J = 6.8 Hz, CH(CH3′)), 3.17 (4 H, sept, J = 6.8 Hz, CH(CH3)2), 3.38 (4 H, q, J = 6.9 Hz, OCH2CH3), 4.14 (4 H, s, NCH2), 7.35 (4 H, app. d, J = 7.7 Hz, 3,5-CHdipp), 7.43 (2 H, dd, J = 7.0, 8.4 Hz, 4-CHdipp). 13 C{1H} NMR (125.8 MHz, THF-d8, rt): 15.7 (OCH2CH3), 23.9 (CH(CH3′)), 25.7 (CH(CH3)), 29.6 (CH(CH3)2), 54.6 (NCH2), 66.3 (OCH2CH3), 125.5 (3,5-CHdipp), 130.6 (4-CHdipp), 125 (br, B− CBArF), 136.6 (ipso-Cdipp), 137.1 (br. m, m-CF, J = 248 Hz), 139.1 (br. d, p-CF, J = 253 Hz), 148.0 (2,6-Cdipp), 149.1 (br. d, o-CF, J = 243 Hz), 203.6 (CCarbene). 19F{1H} NMR (470.7 MHz, THF-d8, rt): −131.8 (br. s), −164.1 (t, J = 21 Hz), −167.6 (br. t, J = 19 Hz). 11 1 B{ H} NMR (160.5 MHz, THF-d8, rt): −16.1 (Δw1/2 = 19 Hz). Mp: 166−173 °C. Anal. Calcd for C55H48BCuF20N2O ([((SIDipp)Cu)(OEt2)][BArF]) C, 54.72; H, 4.01; N, 2.32. Anal. Calcd for C51H38BCuF20N2 ([(SIDipp)Cu][BArF]) C, 54.06; H, 3.38; N, 2.47. Anal. Calcd for C58H56BCuF20N2O2 ([((SIDipp)Cu)(OEt2)][BArF])(C4H10O) C, 54.96; H, 4.45; N, 2.21. Found: C, 55.27− 53.96; H, 4.77−4.11; N, 2.45−1.48. Repeated attempts to obtain better elemental analysis failed, possibly due to the presence of additional solvent in the solid and partial removal of the ligand Et2O (vide supra).

Ph3B−Bcat. 1H NMR (400.4 MHz, THF-d8, −10 °C): δ 6.63 (3 H, tt, J = 7.2, 1.5 Hz, 4-CHPh), 6.74 (6 H, app. t, J = 7.4 Hz, 3,5-CHPh), 6.80−6.85 (2 H, m CHcat), 6.94−6.99 (2 H, m CHcat), 7.04 (6 H, app. d, J = 7.7 Hz, 2,6-CHPh).13C{1H} NMR (100.7 MHz, THF-d8, −10 °C): 111.9 (CHcat), 121.0 (CHcat), 122.9 (4-CHPh), 126.7 (3,5CHPh), 135.4 (2,6-CHPh), 150.9 (Ccat), 160 (br. s, B−CPh). 11B{1H} NMR (128.5 MHz, THF-d8, −10 °C): −13.5 (Δw1/2 = 65 Hz), 29.3 (Δw1/2 = 1350 Hz). Mp: decomp. >111 °C. Anal. Calcd for C51H57B2CuN2O2 ([(SIDipp)Cu(Ph3B−Bcat)]) C, 75.14; H, 7.05; N, 3.44. Anal. Calcd for C54.5H61B2CuN2O2 ([(SIDipp)Cu(Ph3B− Bcat)](PhMe)1/2) C, 76.00; H, 7.14; N, 3.25. Found: C, 75.98; H, 7.31; N, 3.20. The presence of PhMe is in agreement with the composition of ([(SIDipp)Cu(Ph3B−Bcat)](PhMe)0.63) according to the 1H NMR data. [((SIDipp)Cu)2Bdmab][Ph3B−Bdmab] (3b(Ph3B−Bdmab)). In a nitrogen-filled glovebox, 2n (25.0 mg, 41.8 μmol, 1.0 equiv) and BPh3 (5.1 mg, 20.9 μmol, 0.5 equiv) were combined in a screw-cap vial in PhMe (2 mL). After 16 h at −40 °C, the product had separated as a colorless solid. The solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give 3b(Ph3B−Bdmab) (21 mg, 15 μmol, 70% (based on BPh3)). Single crystals suitable for X-ray diffraction studies were obtained by recrystallization of the isolated product from THF/ n-pentane at −40 °C. The NMR spectra suggest the presence of two major species 2n and presumably 3b(Ph3B−Bdmab); only the data of the latter are given here. For a detailed discussion of the NMR data, see below. 1 H NMR (500.3 MHz, THF-d8, rt): δ 0.85 (24 H, d, J = 7.0 Hz, CH(CH3)), 1.17 (24 H, d, J = 7.0 Hz, CH(CH3′)), 2.25 (6 H, s, (NCH3)boryl), 2.81 (6 H, s, (NCH3)Ph3B−Bdmab), 2.87 (8 H, sept, J = 7.0 Hz, CH(CH3)2), 3.81 (8 H, s, NCH2), 6.54−6.58 (4 H, m, (CHdmab)Ph3B−Bdmab), 6.58−6.60 (2 H, m, (CHdmab)boryl), 6.61 (3 H, tt, J = 7.1, 1.3 Hz, 4-CHPh), 6.80 (6 H, app. t, J = 7.3 Hz, 3,5-CHPh), 6.82−6.86 (2 H, m, (CHdmab)boryl), 7.14 (8 H, d, J = 7.7 Hz, 3,5CHdipp), 7.37 (4 H, d, J = 7.7 Hz, 4-CHdipp), 7.41 (6 H, br. d, J = 7.8 Hz, 2,6-CHPh). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 24.1 (CH(CH 3 ′)), 25.7 (CH(CH 3 )), 29.5 (CH(CH 3 ) 2 ), 31.8 ((NCH3)Ph3B−Bdmab), 33.2 ((NCH3)boryl), 54.7 (NCH2), 105.6 ((CHdmab)Ph3B−Bdmab), 108.8 ((CHdmab)boryl), 116.3 ((CHdmab)Ph3B−Bdmab), 119.6 ((CHdmab)boryl), 121.0 (4-CHPh), 125.5 (3,5-CHdipp), 126.0 (3,5-CHPh), 130.6 (4-CHdipp), 135.6 (ipso-Cdipp), 137.0 (2,6-CHPh), 139.7 ((Cdmab)boryl), 142.1 ((Cdmab)Ph3B−Bdmab), 147.4 (2,6-Cdipp), 166 (br. s, B−CPh, HMBC), 202.3 (CCarbene). 11 1 B{ H} NMR (160.5 MHz, THF-d8, rt): −11.5 (Δw1/2 = 55 Hz), 38.6 (Δw1/2 = 610 Hz). Mp: decomp. >130 °C. Anal. Calcd for C88H111B3Cu2N8 ((3b(Ph3B−Bcat)) C, 73.38; H, 7.77; N, 7.78. Found: C, 73.44; H, 7.78; N, 8.17 (recryst. from THF). [((SIDipp)Cu)2Bcat][B(C6F5)4] (3a(BArF)). In a nitrogen-filled glovebox, 2m (30.0 mg, 52.4 μmol, 1.0 equiv) and [CPh3][B(C6F5)4] (24.4 mg, 26.2 μmol, 0.5 equiv) were combined in a screw-cap vial in PhMe (1.5 mL). After a few minutes at rt, the solution was layered with npentane (ca. 4 mL) and cooled to −40 °C. After 20 h at −40 °C, a viscous layer had separated that solidified to an off-white to yellowish solid within 3 days at −40 °C. The solution was decanted, and the residue was washed with cold n-pentane (2 × 2 mL, −40 °C) and thoroughly dried in vacuo to give 3a(BArF)(C5H12)1/2 (16 mg, 9 μmol, 36%). Single crystals suitable for X-ray diffraction studies were obtained by recrystallization of the isolated product from Et2O/npentane at −40 °C. 1 H NMR (500.3 MHz, THF-d8, rt): δ 1.02 (24 H, d, J = 6.6 Hz, CH(CH3)), 1.20 (24 H, d, J = 6.6 Hz, CH(CH3′)), 2.90 (8 H, br. sept, J = 6.6 Hz, CH(CH3)2), 3.95 (8 H, s, NCH2), 6.75−6.80 (2 H, m, CHcat), 6.93−6.98 (2 H, br. m, CHcat), 7.00 (8 H, d, J = 7.7 Hz, 3,5-CHdipp), 7.11 (4 H, app. t, J = 7.7 Hz, 4-CHdipp). 13C{1H} NMR (125.8 MHz, THF-d8, rt): 23.7 (CH(CH3′)), 26.0 (CH(CH3)), 29.5 (CH(CH3)2), 54.7 (NCH2), 113.2 (CHcat), 122.4 (CHcat), 125.4 (3,5-CHdipp), 130.6 (4-CHdipp), 125 (br, B−CBArF), 134.4 (ipso-Cdipp), 137.2 (br. d, m-CF, J = 244 Hz), 139.1 (br. d, p-CF, J = 250 Hz), 147.0 (2,6-Cdipp), 148.9 (1,2-Ccat), 149.0 (br. d, o-CF, J = 243 Hz), 200.8 (CCarbene). 19F{1H} NMR (470.7 MHz, THF-d8, rt): −131.8

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RESULTS AND DISCUSSION Linear Complexes: [(SIDipp)Cu−Bcat] (2m) and [(SIDipp)Cu−Bdmab] (2n). The linear terminal boryl complex [(SIDipp)Cu−Bcat] (2m) was obtained upon reaction of the alkoxido complex [(SIDipp)Cu−OtBu] with the diborane(4) derivative B2cat2 (1b) in toluene in 74% yield (Scheme 3). The diaminoboryl complex [(SIDipp)Cu− Bdmab] (2n) was obtained similarly from pinB−Bdmab (1c) in 51% yield (dmab = 1,2-(NMe)2C6H4). Structurally and spectroscopically, both complexes 2m and n exhibit no unexpected behavior. The 11B NMR shifts are indicative of boryl complexes at 45 ppm (2m) and 43 ppm (2n), respectively, and fit well in the chemical shift ranges observed for related linear (NHC)Cu−boryl complexes of 41− E

DOI: 10.1021/acs.inorgchem.9b01041 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

PhMe, leading to 2m, to THF, the outcome of the reaction changes dramatically (Scheme 4). Instead of the terminal boryl

Scheme 3. Reaction of [(SIDipp)Cu−OtBu] with Diboranes(4) in PhMe and Syntheses of the Linear Copper(I) Boryl Complexes 2m and 2n

Scheme 4. Reaction of [(SIDipp)Cu−OtBu] with Diboranes(4) in THF and Formation of the μ-Boryl Copper(I) Complexes 3a(cat2B) and 3b(cat2B)

49 ppm.1a,6a,c,7c The carbene carbon atoms resonate at 207.2 and 208.1 ppm, respectively, in the range found for related (SIDipp)Cu complexes.1e Structurally, the complexes 2m and n were characterized by single crystal X-ray diffraction as the solvates 2m(PhMe) and 2n(PhMe)2, respectively, each with one independent molecule in the asymmetric unit in the space group types P21/n and Pbca, respectively (Figure 2).11 Note

complex 2m, now the dinuclear μ-boryl complex 3a(cat2B) is obtained in yields around 30% (based on Cu). Analogously, the reaction of [(SIDipp)Cu−OtBu] with catB−Bdmab (1d) in THF yields the dinuclear μ-boryl complex 3b(cat2B) in very low and varying yields of ≤6% (Scheme 4). Moreover, the isolated μ-boryl complexes 3a(cat2B) and 3b(cat2B) were generally contaminated with variable amounts of the μ-hydrido cation 4+ (Scheme 4). The μ-boryl complex 3a(cat2B) crystallizes as THF solvate 3a(cat2B)(THF)y in a monoclinic space group P21/c with two independent units of 3a(cat2B) in the asymmetric unit (Z = 8, Z′ = 2). The cocrystallized THF could not be properly refined, and its contribution to the diffraction was mathematically removed using the SQUEEZE algorithm (Figure 3). From the structurally related cocrystals of 3a(cat2B) and 4(cat2B), a composition of 3a(cat2B)(THF)2.5 may be estimated (vide infra). The structures of the two independent 3a+ cations are very similar with respect to bond length and angles; however, the overall conformation differs slightly as evidenced by the significantly different interplanar angles included between the [Cu2B] moiety and the NHC rings (Figure 3). It is similarly true in comparison to the 3a+ cation in the complex 3a(BF4) reported by Sadighi and coworkers (Scheme 2), albeit in 3a(BF4), also a slightly shorter Cu−Cu distance (2.4082(2) Å) is observed.1e Beside the discussed structure of 3a(cat2B), seven structures of 3a(cat2B) with cocrystallized 4(cat2B) were obtained as the THF solvates (3ax41−x)(cat2B)(THF)3.5−x (I−VII) (Figures S1.2−1.9, Table S1.2, S1.2).11 The amount of 3a+ varies in these structures from x = 0.92 (I) to 0.25 (VII). One of these seven structures, (3a0.2540.75)(cat2B)(THF)0.75+y (VII), is shown here exemplarily (Figure 4, top). The structures of (3ax41−x)(cat2B)(THF)3.5−x (I−VII) are isomorphs in the space group type P21/n with the asymmetric unit comprising one entity of (3ax41−x)(cat2B) and four sites of cocrystallized (disordered) THF molecules. However, in some

Figure 2. Molecular structures of 2m (right) from 2m(PhMe) and 2n (left) from 2n(PhMe)2. Selected bond lengths (Å) and angles (deg): 2m: C1−Cu1 1.930 (1), B1−Cu1 1.9842(15), C1−Cu1−B1 172.52(7), ∠ (N1,C1,N2Cu1)/(N3,B1,N4,Cu1) 71.9(2); 2n: C1−Cu1 1.9309(15), B1−Cu1 1.978(2), C1−Cu1−B1 174.22(7), ∠(N1,C1,N2Cu1)/(N3,B1,N4,Cu1) 7.1(6).11,14

that 2m was also obtained as the THF solvate 2m(THF)2 (vide inf ra); however, no major differences in the molecular structures were found for either solvate (see the Supporting Information).11 Structurally, the complexes 2m and n are linear complexes resembling the molecular structure of related (NHC)Cu boryl complexes such as 2a, b, f, and g and 2h and i but also [(IMes)Cu−B((Ndipp)2C2H4)] and [(IMes)Cu− B((Ndipp)2C2H2)] (Scheme 1). Those complexes exhibit CCarbene−Cu and B−Cu distances as well as CCarbene−Cu−B angles in ranges of 1.91−1.94 and 1.98−2.00 Å and 165.49− 179.43°, respectively.1a,c,6a,c,7c However, a direct comparison of 2n with the unsaturated congener 2f reveals subtle structural differences. While for 2n the [B1,N3,N4,Cu1] and the [C1,N1,N2,Cu1] mean planes are virtually coplanar, indicated by an interplanar angle of 7.1(6)°, for 2f, a nearly perpendicular arrangement with an angle of 88.3(5)° is found.6a The isolation of 2m indicates that linear terminal copper(I) boryl complexes of the Bcat ligand are accessible; together with 2n, it also indicates that the SIDipp ligand is able to support terminal as well as μ-boryl complexes. μ-Boryl Complexes: [((SIDipp)Cu) 2 Bcat][cat 2 B] ( 3 a ( c a t 2 B ) ) a n d [ ( ( S I D i p p ) C u ) 2 B d ma b ] [ c a t 2 B ] (3b(cat2B)). Synthesis and Structures. Switching the solvent of the reaction of [(SIDipp)Cu−OtBu] with B2cat2 (1b) from F

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Figure 3. View of the asymmetric unit of 3a(cat2B)(THF)y; disorder of the [cat2B]− anions omitted. Selected bond lengths (Å) and angles (deg): C1−Cu1 1.939(3), C28−Cu2 1.931(3), B1−Cu1 2.093(4), B1−Cu2 2.058(4), Cu1−Cu2 2.4192(5), Cu1−B1−Cu2 71.3(1), ∠(Cu1,Cu2,B1)/(Bcat) 85.1(2)°, ∠(Cu1,Cu2,B1)/(NHC(C1)) 28.5(1)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 10.4(1)°; C1′−Cu1′ 1.940(3), C28′−Cu2′ 1.933(3), B1′−Cu1′ 2.094(4), B1′− Cu2′ 2.057(4), Cu1′−Cu2′ 2.4244(5), Cu1−B1−Cu2 71.5(1), ∠ (Cu1′,Cu2′,B1’)/(Bcat) 83.5(1)°, ∠ (Cu1′,Cu2′,B1’)/(NHC(C1’)) 30.3(1)°, ∠(Cu1′,Cu2′,B1’)/(NHC(C28’)) 15.6(1)°.11,14

instances, the solvate THF could not be refined and was treated using the SQUEEZE algorithm.11 In this series of structures, the Bcat ligand is not fully occupied but disordered with a hydride ligand and one (partly occupied) THF molecule (Figure 4, top). The amount of 3a+ present is calculated on the basis of the freely refined site occupation factor of the Bcat site. Only for the hydride-rich structure of (3a0.2540.75)(cat2B)(THF)0.75+y was the hydride ligand (with all due caution) identified in the difference Fourier map and refined freely.11 Besides that, the pure μ-hydrido complex 4(cat2B) (Figure S1.10) was also obtained and found to crystallize isomorphous to the structures I−VII of (3ax41−x)(cat2B)(THF)3.5−x with x = 0 (an additional polymorph of 4(cat2B) was also identified, Figure S11).11,15 However, the structure of hydride free (3a) (cat2B)(THF)y (Figure 3) is, not surprisingly, also related to the structures I−VII. It represents a superstructure with doubled cell volume, Z and Z′ of the structures I−VII.11 Comparison of the cationic parts of the structures of (3a0.2540.75)(cat2B)(THF)0.75+y (VII), 3a(cat2B)(THF)y, and 4(cat2B)(THF)y reveals subtle but distinct differences: the C− Cu distances are smaller for 4+ than for 3a+ by >0.02 Å, while for (3a0.2540.75)+, a value closer to that of 4+ is found, in agreement with high 4+ content of this species. For the Cu−Cu distance, an inverse trend is observed: it elongates from 3a(cat2B)(THF)y over (3a0.2540.75)(cat2B)(THF)0.75+y (VII) to 4(cat2B)(THF)y. In fact, both trends progress monotonously, within error, for the structures (3ax41−x)(cat2B)(THF)3.5−x (I−VII) from x = 0.92 (I) to 0.25 (VII).11 This allows cautious conclusions on the coordination properties of the hydride and the boryl ligand. The longer Cu−CNHC distances for the strongly donating boryl ligand may be rationalized in analogy to the ubiquitous trans-influence, whereas the shorter Cu−Cu distance is reminiscent of the effective three-centerbinding in the μ-boryl complex.1e Besides the μ-Bcat cation 3a+, the μ-Bdmab species 3b+ also crystallized as 3b(cat2B)(THF)2 and was, though of only mediocre quality, subjected to a single crystal X-ray structure determination (Figure 4 (bottom), Table S1.2b, Figure

Figure 4. Structures of (3a0.2540.75)+ (top) from (3a0.2540.75)(cat2B)(THF)0.75+y (VII) (disordered Bcat and THF/H moieties as spheres with arbitrary radii) and of 3b+ (bottom) from 3b(cat2B)(THF)2. Selected bond lengths (Å) and angles (deg): (3a0.2540.75)+: C1−Cu1 1.921(2), C28−Cu2 1.915(2), B1−Cu1 2.19(2), B1−Cu2 2.05(1), Cu1−Cu2 2.5259(4), Cu1−B1−Cu2 73.1(5), ∠(Cu1,Cu2,B1)/(Bcat) 81.9(3)°, ∠(Cu1,Cu2,B1)/(NHC(C1)) 25.7(3)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 11.0(3)°, Cu1−H100−Cu2 112(2), ∠(Cu1,Cu2,H100)/(NHC(C1)) 46(2)°, ∠(Cu1,Cu2,H100)/(NHC(C28)) 32(1)°; ∠(Cu1,Cu2,H100)/(Cu1,Cu2,B1) 21(2)°; 3b+: C1−Cu1 1.934(4), C28−Cu2 1.957(4), B1−Cu1 2.028(5), B1−Cu2 2.082(5), Cu1−Cu2 2.4921(8), Cu1−B1−Cu2 74.6(2), ∠(Cu1,Cu2,B1)/(Bdmab) 83.90(7)°, ∠ (Cu1,Cu2,B1)/(NHC(C1)) 78.0(3)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 28.6(2)°.11,14

G

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Figure 5. 1H NMR spectrum of a sample of (3ax41−x)(cat2B) and proposed dissociation of 3a(cat2B) into 5 and 2m in THF solution; nonoverlapping signals of 2m and 4+ are assigned (500.3 MHz, rt, THF-d8, *: residual solvent signal).11

S1.12).11 3b(cat2B)(THF)2 crystallizes in a triclinic space group P1̅ with one formula unit in the asymmetric unit. The geometrical parameters in 3b(cat2B)(THF)2 are in the range also found for 3a+ with a few distinct differences. The Cu−Cu and CNHC−Cu distances in 3b+ are slightly longer than those in 3a+, which is possibly an effect of the stronger σ-donor properties of the diaminoboryl ligand.16 The most pronounced feature, however, is the relative orientation of the NHC rings, while in 3a+, the NHC rings included only two small angles around 30° and 12° with the [Cu2B] plane, are in 3b+ the angles significantly larger (Figure 4, bottom), presumably an effect of the increased steric demand of the Bdmab moiety.11 It should be emphasized that the solid state structure of 3b(cat2B) does not suggest the presence of the hydrido complex 4+, an effect of the nonisomorphous crystal structures of 3b(cat2B)(THF)2 and 4(Bcat2)(THF)y. NMR Spectroscopy. The complexes 3a(cat 2 B) and 3b(cat2B) were also characterized in solution by multinuclear NMR spectroscopy. The 1H NMR of a sample of 3a(cat2B) is surprisingly complex (Figure 5).12 Immediately, three major SIDipp species were identified by the three major signals of the imidazolidine-2-ylidene backbone around 4 ppm (and also by the iso-propyl methylene proton signals around 3.1 ppm). Inspection of this spectrum reveals by comparison with spectra of authentic samples that the linear complex 2m as well as the hydrido complex 4+ are two of these species. Integration of the well-separated signals (imidazolidine-2-ylidene backbone, isopropyl methylene proton, and iso-propyl methyl proton) reveals a ratio of 10:10:6 for 5:2m:4+ with 5 being the so far unidentified species. It has to be emphasized that the spectrum shown is typical for the material obtained by the reaction of [(SIDipp)Cu−OtBu] with 1b under the discussed conditions. Inspection of the 1H−1H NOESY NMR spectrum reveals a contact between one of the methyl group signals (1.21 ppm) of 5 with one of the catechol signals (6.06 ppm), suggesting their spatial proximity. The two clearly separated catechol signals around 6.06 and 6.33 ppm are assigned to the spiroborate [cat2B]+, also identified by it characteristic narrow 11B{1H}

NMR signal at 14.5 ppm (Δw1/2 = 32 Hz). However, the 1H NMR chemical shift of these signals differs significantly from those of the spiroborate in 4(cat2B), 3b(cat2B), [K(18-Crown6)][cat2B], and [nBu4N][cat2B], where two only slightly separated multiplets in the range 6.20−6.49 ppm are observed. This also agrees with an aggregation of the spiroborate in (3ax41−x)(cat2B) in THF solution. It may be concluded that 5 is the complex [(SIDipp)Cu−cat2B]. This would allow the conclusion that 3a(cat2B) dissociates in THF solution quantitatively to give the linear complex 2m and the cation [(SIDipp)Cu]+ that is coordinated by [cat2B]− to give 5. The 1:1 ratio of 5:2m (by 1H NMR spectroscopy) as well as the integral ratio of the backbone methylene signal of 5 (or alternatively the iPr-group methin signal) and the spiroborate signals of 4:11 (expected 4:8) agree with this interpretation, taking an impurity of 23 mol % 4(cat2B) (vide supra) with respect to 3a(cat2B) into account. Hence, an averaged signal of the spiroborate was observed. Moreover, 1H−1H NOESY NMR spectrum (Figure S2.1b) of the mixture of 5/2m/ 4(cat2B) exhibits exchange signals between all equivalent groups of the (SIDipp)Cu fragment in 5, 2m, and 4(cat2B), suggesting appreciable exchange between these species. Further, it should be noted that the 11B NMR signal of the linear complex 2m was not observed in the mixture 5/2m/ 4(cat2B) possibly due to both broadening by chemical exchange as well as the quadrupolar nature of the 11B nucleus. Further, it should be noted that for the diaminoboryl complex 3b(cat2B), no (or only marginal) dissociation into 5 and 2n is observed (Figure S2.2). Additionally, it should be emphasized that neither for 3b(cat2B) nor the other μ-boryl complexes (vide infra) discussed here were 11B NMR signals observed, which is in line with Sadighi’s report on 3a(BF4).1e So far, it may be summarized that the reaction of [(SIDipp)Cu−OtBu] with 1b leads, depending on the solvent toluene or THF, either to the linear terminal boryl complex 2m or to the μ-boryl complexes 3a(cat2B). Whereby, the formation of the latter is accompanied by the formation of the μ-hydrido complex 4(cat2B) to a variable extent (Scheme 4). H

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Figure 6. In situ 1H NMR and 11B{1H} spectra of the reaction of [(SIDipp)Cu−OtBu] with 1b (bottom) and 2m with catB−OtBu (middle) and pinB−iPr (top), respectively, in THF-d8 (300.3/96.3 MHz, rt, after 3 h at rt, *: residual solvent signal).11

Figure 7. In situ 1H NMR and 11B{1H} spectra of the reaction of [(SIDipp)Cu−OtBu] with 1d (bottom and middle) and with 1c (top) in THF-d8 (300.3/96.3 MHz, rt, *: residual solvent signal).11

additional (broad) signals around 1.3, 3, and 4 ppm assignable to iso-propyl groups and the methylene backbone. The 11 1 B{ H} NMR spectrum exhibits a broad signal indicative of 2m around 45 ppm, a narrow signal at 22.3 ppm assigned to catB−OtBu, and three additional narrow signals indicative of four-coordinate boron. One of the latter, at 14.8 ppm (Δw1/2 = 40 Hz), is assigned to the spiroborate [cat2B]− by comparison with authentic samples. On the other hand, performing the same reaction in C6D6, 2m and catB−OtBu are formed in a clean reaction without appreciable side product formation (Figure S2.3).11 Hence, it may be concluded that the solvent, THF vs PhMe/C6D6, does change the course of the reaction of [(SIDipp)Cu−OtBu] with 1b, as evidenced both preparatively as well as by in situ NMR spectroscopy. The above findings suggest the primary formation of the terminal boryl complex 2m upon reaction of [(SIDipp)Cu−

This, to the best of our knowledge unparalleled behavior, gives rise to three distinct questions: (1) How and why are the cationic μ-boryl complexes formed? (2) How is the counterion, the spiroborate [cat2B]−, formed? (3) How is the formation of the hydride complex 4+ connected to this? However, before these questions are addressed, it must be emphasized that glass surface appears to play a crucial role, as performing the reaction of [(SIDipp)Cu−OtBu] with 1b in THF in silylated glassware leads to the exclusive isolation of the terminal boryl complex 2m as THF solvate 2m(THF)2 (27% yield). 1. How and Why Are the Cationic μ-Boryl Complexes Formed? The reaction of [(SIDipp)Cu−OtBu] with 1b monitored by in situ NMR spectroscopy in THF-d8 (Figure 6, bottom) shows that 2m along with 4+ and the expected boric acid ester catB−OtBu are formed.11 However, the formation of additional SIDipp containing species is evident by I

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Inorganic Chemistry OtBu] with 1b, the dinuclear complexes 3a+ and 4+ are then produced in secondary reactions from 2m. To verify this, the reaction of 2m with catB−OtBu in THF-d8 was studied by in situ NMR spectroscopy, and indeed, the outcome of this reaction is virtually identical to that of the reaction of [(SIDipp)Cu−OtBu] with 1b in THF-d8 (Figure 6, middle).11 Moreover, in the analogous experiment with 2m/pinB−OiPr in THF-d8 (Figure 6, top) and 2m/catB−OtBu in C6D6 (Figure S2.6), no reaction is observed.11 So, the following conclusions may be drawn: (i) In less polar aromatic solvents, 2m is formed as sole product and does not react with catB−OtBu, the byproduct of its formation by B−B bond cleavage of 1b. (ii) In the more polar THF as solvent, the formation of ion pairs is facilitated, and ionic species such as 3a+ (as evidenced by isolation) and 4+ (as evidenced by isolation as well as by in situ NMR experiments) are produced by reaction of 2m with catB−OtBu (the question of the hydride source may be addressed later). (iii) The less Lewis acidic pinB−OtBu does, under identical conditions, not react with 2m. In short, besides a polar solvent, the presence of a suitable Lewis acid is necessary for the formation of 3a+ from 2m. To confirm this, further isolated 2m was reacted with catB−OtBu in THF on a preparative scale, and 3a(cat2B) in 23% yield virtually free of 4(cat2B) was obtained. Following this rationing, one would expect the exclusive formation of 2n from the reaction of [(SIDipp)Cu−OtBu] with pinB−Bdmab (1c) in THF; whereas catB−Bdmab (1d), leading also to catB−OtBu as byproduct, should deliver 2n as primary product, but ultimately, 3b+ should be obtained. Exactly this reactivity pattern is observed as evidenced by in situ NMR spectroscopy (Figure 7, Figure S2.7).11 The reaction of [(SIDipp)Cu−OtBu] with catB−Bdmab (1d) in THF gives 3b+ together with so far unidentified side products (Figure 7 bottom and middle). It should be noted that, in contrast to the reaction with 1b (Figure 6), no formation of the hydrido complex 4+ is observed.11 The observation of 3b+ is explained by the stability of 3b+ in THF solution in contrast to the dissociation of 3a+ into 2m and 5, as discussed above. The 11B{1H} NMR spectra show clearly the initial formation of catB−OtBu and pinB−OtBu, respectively, during these reactions (Figure 7, bottom and top). In the further course of the reaction with 1d, the initially formed catB−OtBu is consumed, and the spiroborate [cat2B]− is formed (Figure 7, middle). However, the 11B{1H} NMR spectra exhibit additional so far unidentified boron species. Given the results so far, the formation to the μ-boryl complexes 3a+ and 3b+ are proposed to proceed from initially formed 2m and 2n by reaction with one equivalent of the concomitantly formed catB−OtBu as Lewis acid (Scheme 5). The abstraction of a boryl ligand from 2m and n under formation of the diborane(4) adduct [catB(OtBu)−Bcat]− and [catB(OtBu)−Bdmab]− is followed by association of the formed formal [(SIDipp)Cu]+ cation with one equivalent of unreacted 2m to give 3a+ (or 3b+, respectively). The formed diborane(4) adduct [catB(OtBu)−Bcat]− and [catB(OtBu)− Bdmab]− reacts by a so far unknown reaction pathway to the spiroborate [cat2B]− (vide infra). Reactivity Toward Lewis Acids: BPh3. Under the reasoning that the μ-boryl complex formation and the formation of the spiroborate are related but distinct processes, Lewis acids other than catB−OtBu should also be able to abstract a boryl ligand from a terminal boryl complex, leading to cationic μ-boryl complexes, but not give [cat2B]− as counteranions. To support

Scheme 5. Proposed Reaction Pathway for the Formation of the μ-Boryl Copper(I) Complex 3a(cat2B)

this assumption, the reaction of 2m with the Lewis acids BPh3 (and [CPh3]+, vide infra) was studied. Reacting 2m with BPh3 leads, depending on the stoichiometry, 1:1 or 2:1, to two products, either the μ-boryl complex 3a(Ph3B−Bcat), analogous to 3a(cat2B), or the complex [(SIDipp)Cu(Ph3B−Bcat)] in good yields (Scheme 6). Scheme 6. Reaction Pathways for the Reaction of 2m with BPh3

Hence, according to the employed ratio 2m:BPh3, either half an equivalent or all boryl ligands from 2m are abstracted. The first case leads after association of unreacted 2m to the μ-boryl cation 3a+ accompanied by the formal diborane(4) phenyl adduct [Ph3B−Bcat]− as the counteranion. Whereas in the second case, [(SIDipp)Cu(Ph3B−Bcat)] is obtained, which was thoroughly characterized in solution by multinuclear NMR spectroscopy as well as in the solid state by single crystal X-ray diffraction (as PhMe and Et2O solvate, Figures S1.1d and e).11 Two pseudopolymorphic PhMe solvates of 3a(Ph3B−Bcat) were crystallized. As both were, despite all efforts, of only mediocre quality, only the slightly better one is addressed here.11 The μ-boryl 3a+ cation in the PhMe solvate 3a(Ph3B− Bcat)(PhMe)n is structurally comparable with the structures of 3a+ in 3a(cat2B), 3a(BF4) and in the pseudopolymorph J

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Inorganic Chemistry 3a(Ph3B−Bcat)(PhMe)1/2 (Figure 8 (top), Figure S1.1c).1e The anion [Ph3B−Bcat]− may be understood as an sp2−sp3

diboron compound, as the Lewis acid/base adduct of the diborane(4) catB−BPh2 with a phenyl anion. As such, it is analogous to the proposed intermediate [catB(OtBu)−Bcat]− (Scheme 5) in the formation of [cat2B]−; however, in contrast to that, [Ph3B−Bcat]− does not react further. Use of 2n instead of 2m leads to the analogous μ-Bdmab complex [((SIDipp)Cu)2Bdmab][Ph3B−Bdmab] (3b(Ph3B− Bdmab)) (Figure 8 (bottom)).11 This complex exhibits again structurally similar features as the above-discussed derivative of 3b+. The NMR spectroscopic characterization of the three complexes 3a(Ph 3 B−Bcat), 3b(Ph 3 B−Bdmab), and [(SIDipp)Cu(Ph3B−Bcat)] is again quite complex as for all three complexes, multiple dynamically exchanging species are observed. However, the two μ-boryl complexes 3a(Ph3B− Bcat) and 3b(Ph3B−Bdmab) exhibit comparable features (Figure 9). For both complexes, essentially two SIDipp containing species are observed. The minor of the two species is identified for both complexes as the linear complex, 2m and 2n, respectively (blue in Figure 9). For 3a(Ph3B−Bcat), one major additional species is observed; the ratio of this species and 2m is estimated by integration of the 1H NMR spectrum to 5:1. In contrast, for 3b(Ph3B−Bdmab), a ratio of 12:1 is found. It may be proposed that 3a(Ph3B−Bcat) and 3b(Ph3B− Bdmab) dissociated in THF solution to a minor amount into the respective linear complexes; hence, the major species observed would be the actual μ-boryl complexes 3a(Ph3B− Bcat) and 3b(Ph3B−Bdmab), respectively. It should be emphasized that characteristic exchange signals between the major species, presumably 3a(Ph3B−Bcat) and 3b(Ph3B− Bdmab), respectively, and the linear complexes are observed by 1H−1H NOESY NMR spectroscopy (Figures 2, 10c, and 2.11c).11 Furthermore, it should be noted that the 11B NMR data for 3a(Ph3B−Bcat) and 3b(Ph3B−Bdmab) exhibit sharp signals at

Figure 8. Structures of 3a(Ph3B−Bcat) (top) and 3b(Ph3B−Bdmab) (bottom).11 Selected bond lengths (Å) and angles (deg): 3a(Ph3B− Bcat): C1−Cu1 1.936(4), C28−Cu2 1.928(3), B1−Cu1 2.078(5), B1−Cu2 2.073(4), Cu1−Cu2 2.3800(7), B2−B3 1.719(8), Cu1− B1−Cu2 69.96(14), ∠(Cu1,Cu2,B1)/(Bcat) 86.8(1)°, ∠(Cu1,Cu2,B1)/(NHC(C1)) 16.8(2)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 19.0(2)°; 3b(Ph3B−Bcat): C1−Cu1 1.961(2), C28−Cu2 1.948(2), B1−Cu1 2.078(2), B1−Cu2 2.073(2), Cu1−Cu2 2.4570(4), B2−B3 1.726(4), Cu1−B1−Cu2 72.57(8), ∠ (Cu1,Cu2,B1)/(Bdmab) 85.5(5)°, ∠ (Cu1,Cu2,B1)/(NHC(C1)) 43.2(2)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 46.2(2)°.11,14

Figure 9. 1H NMR spectra of 3a(Ph3B−Bcat) (bottom), 3b(Ph3B−Bcat) (middle), signals of 2m and n highlighted in blue, and [(SIDipp)Cu(Ph3B−Bcat)] (top), signals of the two distinct species in green/red (500.3/400.4 MHz, rt, THF-d8, *: residual solvent signal).11 K

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Inorganic Chemistry −12.8 and −11.5 ppm, respectively, indicative of the presence of a four-coordinate boron atom (Figures S2.10b and S2.11b).11 For 3b(Ph3B−Bdmab), an additional broad signal at 38.6 ppm is discernible (Figure S2.11b).11 These signals are assigned to the [Ph3B−Bcat]− and [Ph3B−Bdmab]− anion, respectively. For the first anion, however, the signal of the three-coordinate boron atom was not detected due to broadening because of the quadrupolar nature of the 11B nucleus as well as the presence of dynamic processes arising from the presence of formally free BPh3 upon dissociation of 3a(Ph3B−Bcat). Note that the line shape in the 1H NMR spectrum of 3a(Ph3B−Bcat) is also in agreement with the presence of more pronounced dynamic processes than in 3b(Ph3B−Bdmab) (Figure 9). This is also in line with the observance of dissociation of 3a(cat2B) into 2m and 5, while it was not observed (or to an only negligible extent) for the Bdmab analogue 3b(cat2B) (vide supra).11 It may be summarized that the NMR spectroscopic data of the complexes 3a(Ph3B−Bcat) and 3b(Ph3B−Bdmab) suggest that they dissociate dynamically in THF solution to a small, but significant, amount into the linear complexes 2m and 2n, respectively, and formally free BPh3. At last, the NMR properties of [(SIDipp)Cu(Ph3B−Bcat)] should be addressed (Figure 9, top). Here, two distinct sets of signals indicative of the SIDipp moieties are observed in a 1:1 ratio (green:red in Figure 9) at room temperature (at −10 °C, a ratio 10:7 is observed). One set of signals (red in Figure 9) may be assigned to the same species as present in 3a(Ph3B− Bcat) that was attributed there to the μ-boryl complex (Figure S2.9a).11 Nonetheless, the 1H−1H NOESY experiment indicated close proximity of the BPh3 entity to the SIDipp ligand for the other set of signals (green in Figure 9) (Figure S2.9c).11 This is in agreement with a structure similar to the one found in the solid state for [(SIDipp)Cu(Ph3B−Bcat)] with a catB−BPh3···Cu(SIDipp) coordination.11 The 11B{1H} NMR is suggestive for an sp2−sp3 hybridized diboron species, [Ph3B−Bcat]−, exhibiting one broadened and one narrow signal at 29.3 and −13.5 ppm, respectively (Figure S2.9b).11 Those data agree well with the 11B NMR chemical shifts of the diboron anion discussed above for 3a(Ph3B−Bcat) and 3b(Ph3B−Bdmab). These observations may suggest an equilibrium involving the dissociation of two equivalents of [(SIDipp)Cu(Ph3B−Bcat)] into 3a+, [Ph3B−Bcat]−, and BPh3. No evidence of the presence of free BPh3 is found; however, the signals assigned to the [Ph3B−Bcat]− anion appear slightly shifted compared to those in 3a(Ph3B−Bcat), a possible indication of the presence of dynamic processes. In conclusion, it can be stated that the complexes 3a(Ph3B− Bcat), 3b(Ph3B−Bcat), and [(SIDipp)Cu(Ph3B−Bcat)] exhibit a quite complicated and yet not fully understood solution state behavior. Nonetheless, it is clear that redistribution processes of the boryl ligand are central to this behavior; hence, copper(I) boryl complexes, in particular in the presence of Lewis acidic species, should be considered labile and facile sources of boryl anions. Reactivity Toward Lewis Acids: [CPh3]+. Replacing BPh3 as Lewis acid by the isosteric [CPh3]+ as [BArF]− ([B(C6F5)4]−) or [BF4]− salt results in a similar reaction (Scheme 7). Employing half an equivalent of the trityl salt leads to the expected μ-boryl complexes (Figure 10), whereas an equimolar amount leads in the BArF case in ethereal solvent by complete abstraction of the boryl ligands to the complex [(SIDipp)Cu− OEt2][BArF] (Figure S1.1f).11

Scheme 7. Reaction of 2m with [CPh3][BArF] and [CPh3][BF4], Respectively

Figure 10. Structure of 3a(BArF).11 Selected bond lengths (Å) and angles (deg): C1−Cu1 1.927(2), C28−Cu2 1.925(2), B1−Cu1 2.039(3), B1−Cu2 2.046(3), Cu1−Cu2 2.4030(4), ∠(Cu1,Cu2,B1)/(Bcat) 89.34(8)°, ∠(Cu1,Cu2,B1)/(NHC(C1)) 23.6(2)°, ∠(Cu1,Cu2,B1)/(NHC(C28)) 55.6(1)°.11,14

The μ-boryl complex 3a[BF4] was earlier studied by Sadighi and coworkers (vide supra) and offered no additional insights. The solid state structure of 3a+ in the n-pentane solvate 3a(BArF)(C5H12)1/2 compares well with those discussed above; however, the angles included by the mean the NHC and (Cu1, Cu2, B1) planes cover a wider range than that found for 3a+ with other counteranions (Figure 10). The NMR spectroscopic characterization of 3a(BArF), however, exhibits some more interesting features (Figure S2.12). In contrast to the spectra of 3a+ discussed so far, no evidence for extensive dissociation is apparent. This may be rationalized with the very weak coordination properties of the BArF− anion, as evidenced by the formation of the ether complex [(SIDipp)Cu−OEt2][BArF] in contrast, e.g., to the complex [(SIDipp)Cu(Ph3B−Bcat)] discussed above. 2. How is the Counterion, The Spiroborate [cat2B]−, Formed? The formation of the spiroborate [cat2B]− during the reaction of [(SIDipp)Cu−OtBu] and 1b or 1d is not necessarily expected, although its undesirable formation from 1b has been addressed earlier.17 The data discussed above suggest strongly that the spiroborate [cat2B]− is formed from the sp2−sp3 diboron compound [catB−B(OtBu)cat]− formed by abstraction of a boryl anion by the Lewis acidic catB−OtBu (Scheme 5). To shed light on this the reaction of the diborane(4) derivative 1b with [(18-Crown-6)KOtBu], that L

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Inorganic Chemistry

Figure 11. In situ 11B{1H} NMR spectrum of the reaction of 1b with [(18-Crown-6)KOtBu] (300.3 MHz, rt, THF-d8).11

3. How is the Formation of the Hydrido Complex 4+ Connected to This? Last but not least, the formation of the hydrido complex 4+ deserves some consideration. It has to be emphasized that the hydrido complex is only observed in connection with the spiroborate counterion [cat2B]−; hence, upon conducting the reaction of 1b with [(SIDipp)Cu−OtBu] in THF, no evidence for the presence of a hydrido complex is observed for the other μ-boryl complexes (vide supra). While we found that purity of all starting materials and rigorous exclusion of moisture is essential for the outcome of this reaction (vide supra), we observed clearly that silylation of the glassware prevents the reaction to the μ-boryl complexes 3a[cat2B] and delivers the linear complex 2m (vide supra). Therefore, we suggest as a possible explanation19 that the glassware (or rather its surface) plays an intrinsic role in this reaction and that the formation of the hydrido complex and the spiroborate does not occur independently.

should lead to the formation of the same Lewis acid/base adduct [catB−B(OtBu)cat]−, was investigated. Monitoring the reaction of 1b with [(18-Crown-6)KOtBu] in THF-d8 at room temperature by 1H and 11B{1H} NMR spectroscopy gives a comprehensive picture: immediately upon combination of the starting materials, two signals in the 11 1 B{ H} NMR spectrum are found at shifts of 38.2 (Δw1/2 = 900 Hz) and 8.4 ppm (Δw1/2 = 300 Hz), indicative of the initial formation of a sp2−sp3 diboron compound, presumably [catB−B(OtBu)cat]− (Figure 11).18 Over time, a narrow signal at 15.0 ppm (Δw1/2 = 35 Hz) becomes predominant. This chemical shift is identical to the one observed for isolated [K(18-Crown-6)][cat2B] and [nBu4N][cat2B], respectively. Moreover, over time, an additional signal that may be assigned to free 1b is also apparent. This behavior is also reflected in the in situ 1H NMR spectra obtained; however, those spectra do not support the presence of 1b at the later stages of the reaction (Figure S2.13).11 Performing the reaction of 1d with [(18-Crown-6)KOtBu] in an analogous manner gives a more complicated picture; however, [cat2B]− formation is also suggested by its characteristic 11B NMR signal among other unidentified boron containing species (Figure S2.14). Performing the reaction of 1b with [(18-Crown-6)KOtBu] on a preparative scale in an Et2O/PhMe mixture leads to the exclusive isolation of the spiroborate [K(18-Crown-6)][cat2B] as a crystalline solid in 57% yield (Figure S1.1b).11 All attempts to isolate the postulated initial sp2−sp3 diborate [catB− B(OtBu)cat]− were unsuccessful. Neither variation of the solvent(s) (PhMe, THF, n-pentane, Et2O, and mixtures thereof) nor temperature nor the use of different auxiliary ligands (TMEDA, Crown ether) or silylated glassware led to any isolable product besides [K(18-Crown-6)][cat2B]. It is concluded that the formation of the spiroborate [cat2B]− is an intrinsic property of the sp2−sp3 diboron compound [catB−B(OtBu)cat]− and not related to the Cu boryl complexes present. However, the exact course of the reaction leading from [catB−B(OtBu)cat]− to [cat2B]−, in particular the remaining formal [tBuO−B] unit, remains the subject of further studies, but we suggest that the glass surface is involved in this process (vide infra). In particular, no formation of the μ-boryl complex 3a(cat2B) was observed in silylated glassware (vide supra). However, other pathways involving, e.g., THF activation are also feasible.19

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CONCLUSION The B−B activation reaction of the diborane(4) derivative B2cat2 (1b) with the copper(I) complex [(SIDipp)Cu−OtBu] delivers, depending on the solvent used, either the linear complex [(SIDipp)Cu−Bcat] (2m) (PhMe) or the dinuclear μ-boryl complex [((SIDipp)Cu)2Bcat][cat2B] (3a(cat2B)) (THF). However, the latter complex is always accompanied by the hydrido complex [((SIDipp)Cu)2H][cat2B] (4(cat2B)), resulting in a series of cocrystals of these complexes with the composition (3ax41−x)(cat2B)(THF)3.5−x (x = 0.92−0.25). Substitution of 1b by the unsymmetrical diborane catB− Bdmab (1d) leads to the structurally analogous Bdmab complex 3b(cat2B). On the other side, the analogous reaction of pinB−Bdmab (1c) results, independently of the reaction conditions, exclusively in the linear complex 2n. Hence, it is concluded that the presence of the higher Lewis acidic Bcat moiety, in the form of catB−OtBu, one of the side products of the B−B activation, is prerequisite for the formation of the μboryl complex. This is corroborated by the formation of the complexes 3a(Ph3B−Bcat), 3b(Ph3B−Bdmab), 3a(BF4), and 3a(BArF) upon reaction of the respective linear boryl complexes 2m and n with the respective Lewis acids BPh3, [CPh3][BF4], and [CPh3][BArF], respectively. In the solid state, structurally comparable complex cations 3a+ and 3b+ exhibit different solution state behavior. Generally, M

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Inorganic Chemistry for the Bcat complexes containing 3a+, dynamic dissociation into (among others) the linear complex 2m is frequently observed in THF solution, whereas the Bdmab-based complexes 3b+ exhibit no or much less dissociation. In short, it may be summarized that linear and μ-boryl bridged copper(I) complexes may be interconverted by action of Lewis acids and, moreover, that mono-μ-boryl bridged copper(I) boryl complexes may dynamically dissociate to linear boryl complexes. The latter is in contrast to bis-μ-boryl complexes where no evidence for dissociation was observed.6c These findings may trigger further development of the flourishing field of copper(I) catalyzed borylation reactions.1a,2,4,5 Copper(I) boryl complexes may not necessarily be linear mononuclear complexes, as frequently assumed, or dimeric μ-boryl complexes, but between both structural motifs, interconversion is facile either intrinsically in solution or triggered by a Lewis acid.

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(2) For recent reviews, see: (a) Neeve, E. C.; Geier, S. J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse. Chem. Rev. 2016, 116, 9091−9161. (b) Semba, K.; Fujihara, T.; Terao, J.; Tsuji, Y. Coppercatalyzed borylative transformations of non-polar carbon−carbon unsaturated compounds employing borylcopper as an active catalyst species. Tetrahedron 2015, 71, 2183−2197. (c) Fujihara, T.; Semba, K.; Terao, J.; Tsuji, Y. Regioselective transformation of alkynes catalyzed by a copper hydride or boryl copper species. Catal. Sci. Technol. 2014, 4, 1699−1709. (d) Dang, L.; Lin, Z.; Marder, T. B. Boryl ligands and their roles in metal-catalysed borylation reactions. Chem. Commun. 2009, 3987−3995. (3) (a) Takahashi, K.; Ishiyama, T.; Miyaura, N. Addition and Coupling Reactions of Bis(pinacolato)diboron Mediated by CuCl in the Presence of Potassium Acetate. Chem. Lett. 2000, 29, 982−983. (b) Ito, H.; Yamanaka, H.; Tateiwa, J. I.; Hosomi, A. Boration of an α,β-enone using a diboron promoted by a copper(I)−phosphine mixture catalyst. Tetrahedron Lett. 2000, 41, 6821−6825. (4) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. A facile route to aryl boronates: room-temperature, copper-catalyzed borylation of aryl halides with alkoxy diboron reagents. Angew. Chem., Int. Ed. 2009, 48, 5350−5354. (5) (a) Zhao, H.; Lin, Z.; Marder, T. B. Density Functional Theory Studies on the Mechanism of the Reduction of CO2 to CO Catalyzed by Copper(I) Boryl Complexes. J. Am. Chem. Soc. 2006, 128, 15637− 15643. (b) Dang, L.; Zhao, H.; Lin, Z.; Marder, T. B. DFT Studies of Alkene Insertions into Cu−B Bonds in Copper(I) Boryl Complexes. Organometallics 2007, 26, 2824−2832. (c) Zhao, H.; Dang, L.; Marder, T. B.; Lin, Z. DFT Studies on the Mechanism of the 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. (e) Cid, J.; Carbó, J. J.; Fernández, E. Disclosing the Structure/ Activity Correlation in Trivalent Boron-Containing Compounds: A Tendency Map. Chem. - Eur. J. 2012, 18, 12794−12802. (6) (a) 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. (b) Borner, C.; Anders, L.; Brandhorst, K.; Kleeberg, C. Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures, and Reactivity. Organometallics 2017, 36, 4687−4690. (c) Borner, C.; Kleeberg, C. Syntheses, Structures and Reactivity of NHC-Copper(I)-Boryl Complexes: A Systematic Study. Organometallics 2018, 37, 4136−4146. (7) It is emphasized that copper(I) boryl complexes derived from sterically demanding lithium boryl derivatives have also been studied in some detail: (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. (8) (a) Uehling, M. R.; Suess, A. M.; Lalic, G. Copper-Catalyzed Hydroalkylation of Terminal Alkynes. J. Am. Chem. Soc. 2015, 137, 1424−1427. (b) Landers, B.; Navarro, O. Microwave-Assisted Synthesis of (N-Heterocyclic carbene)MCl Complexes of Group 11 Metals. Eur. J. Inorg. Chem. 2012, 2012, 2980−2982. (c) Oschmann,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01041. Additional in situ NMR spectra and analytical and crystallographic data (PDF) Accession Codes

CCDC 1907080−1907081, 1907098−1907109, and 1907136−1907143 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_ [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

*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 C.K. and W.D. gratefully acknowledge support from a Research Grant (KL 2243/5-1) of the Deutsche Forschungsgemeinschaft (DFG). The authors thank AllyChem Co. Ltd. for a generous gift of diborane(4) reagents.

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REFERENCES

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