Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal

Mar 15, 2017 - The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which ind...
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Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals Lucie Nurdin,† Denis M. Spasyuk,†,‡ Warren E. Piers,*,† and Laurent Maron*,§ †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 LPCNO, Université de Toulouse, INSA, UPS, LPCNO, 135 Avenue de Rangueil, and CNRS, LPCNO, F-31077 Toulouse, France ‡ Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 2V3 §

S Supporting Information *

ABSTRACT: Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand B2Pz4Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C−C coupling of the azide aryl group and internal transfer of H• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π−π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co(III) imido radical intermediates that are the kinetic products of the reactions of 1 or 1-THF with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electronrich nature of the dianionic B2Pz4Py ligand framework render the imido ligand formed highly reactive.

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INTRODUCTION The engineering of ligand structure to enforce specific geometries and engender unique reactivity patterns in transition metal complexes is a strategy employed naturally by evolution and artificially by chemists. Manipulation of steric and electronic environments through ligand modifications is therefore key to uncovering new modes of reactivity or enhancing activity and selectivity in metal-mediated processes. The use of multidentate, chelating ligand frameworks1−3 is a particularly effective way of biasing compounds toward specific geometries because of the inherent rigidity of such ligands. A case in point is the class of tetrapodal pentadentate ligands4 exemplified by the polypyridyl donor set “PY5” introduced some years ago simultaneously by Stack5 and Feringa.6 These neutral L5 ligand systems form a square pyramidal platform upon which to conduct chemistry using the remaining vacant (or labilely coordinated) site.7 Here, the chelating nature of the ligand is augmented by strong bonds within an oxidatively stable scaffold, making for a robust stage upon which reactions as diverse as proton reduction,8−11 water oxidation,12−14 and hydrogen atom transfer15 can be conducted. There is therefore considerable interest in these types of ligands. While the PY5 system16,17 and its derivatives have © 2017 American Chemical Society

collectively received the most attention, other configurations are also known, with different donor combinations and variable symmetries.18−20 The majority of these ligands bear an overall neutral charge, rendering most of their metal ion complexes cationic. As the oxidation state of the metal rises, the dicationic nature of the complex contributes to heightened electrophilicity of these higher oxidation state species. Recently, efforts to include X-type donors within the pentadentate array have emerged,21−25 in order to lower the overall cationic charge of the molecules and perhaps stabilize the higher oxidation state transition metal compounds often proposed as intermediates in catalytic reactions.26 In this context, we have recently developed the diborate ligands B2Pz4Py27 (Chart 1). Topologically, these ligands are similar to the neutral Pz4Py ligand introduced by Gardinier et al.,28−30 but because of the transposition of boron for carbon in the chelate anchoring points, the ligand bears a 2− charge from the two scorpionate3 ligand-like arms of the pentadentate framework. As a result, complexes of M(II) ions are neutral rather than cationic, altering the solubility properties and potentially providing a ligand environment more amenable Received: January 19, 2017 Published: March 15, 2017 4157

DOI: 10.1021/acs.inorgchem.7b00174 Inorg. Chem. 2017, 56, 4157−4168

Article

Inorganic Chemistry

During this time the pink solid slowly turned brown. After this time, the glass vessel was sealed and immediately brought to the glovebox, and the solid was stored at −35 °C. Yield: 250 mg (94%). The product is paramagnetic, 1H and 13C NMR spectroscopy silent, and highly airsensitive. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 1 in benzene. 11B NMR (161 MHz, C6D6): δ 80.0 (br s). Anal. Calcd for C29H25B2CoN9: C, 60.04; H, 4.34; N, 21.73. Found: C, 59.78; H, 4.59; N, 21.63. UV−vis (C6H6, 1.72 × 10−3 M) [λmax, nm (ε, L mol−1 cm−1)]: 410(580), 680(261), 875(290) nm. HRMS (APCI): m/z calcd 580.1746 (M+), m/z found 580.1761 (M+). Evans method: sample mass 5 mg; solvent C6D6; standard = SiMe4 (6%); μeff = 3.82; s = 3/2, n = 3. Synthesis of [CoB2Pz4Py](NHPhtBu) (2-tBu). To a pressure tube equipped with a Teflon tap containing a suspension of 1-THF (150

Chart 1

to supporting neutral complexes with metals in higher oxidation states. Our initial report detailed some of the iron coordination chemistry of the B2Pz4Py ligand and the reactivity of the Fe(II) complexes toward organic azides.27 It was found that the products of these reactions were best described as Fe(III) imido radicals31 rather than Fe(IV) imido18,19 derivatives. As a result, these compounds were prone to scavenging hydrogen atoms from various sources to produce Fe(III) amido compounds as the eventual products. Here, we describe a companion study on the analogous cobalt(II) chemistry, which also suggests that the putative six-coordinate Co imido complexes that result from reactions with organic azides behave as nitrogen-based radicals.32−36

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mg, 0.230 mmol) in 5 mL of benzene was added all at once cyclohexadiene (10.878 μL, 0.115 mmol), and then a solution of 1azido-4-tert-butylbenzene (42 mg, 0.240 mmol in a solution of benzene stored over molecular sieves) was slowly added. Vigorous evolution of nitrogen gas was observed. The resulting forest green reaction mixture was stirred for 2 h. Pentane (14 mL) was layered on top of the obtained solution, and the resulting mixture left to crystallize at ambient temperature for 3 days. After this time, the green crystalline material was separated by filtration and dried in vacuo to give 85 mg (50%) of a green solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 2-tBu in benzene. 1H NMR (600 MHz, C6D6): δ 8.07 (4H, s, H8), 7.96 (4H, m, H5), 7.68 (4H, d, 3JHH = 2.4 Hz, H10), 7.40 (6H, m, H6 and H7), 7.28 (2H, t, 3JHH = 7.6 Hz, H2), 6.94 (2H, m, H13), 6.74 (1H, t, 3JHH = 7.6 Hz, H3), 6.10 (2H, m, H14), 5.84 (4H, t, 3JHH = 2.4 Hz, H9), 3.21 (1H, s, NH11), 1.30 (9H, s, H17). 13C{1H} NMR (151 MHz, C6D6): δ 172.7 (s, C1), 157.4 (s, C4), 143.4 (s, C8), 138.2 (s, C5), 137.4 (s, C10), 135.4 (s, C12), 134.3 (s, C3), 128.6 (s, C6), 128.4 (s, C13), 127.8 (s, C14), 127.6 (s, C7), 126.5 (s, C2), 121.2 (s, C15), 106.3 (s, C9), 34.3 (s, C16), 31.6 (s, C17). 11B NMR (161 MHz, C6D6): δ −1.3 (s). FT-IR (neat, cm−1): 3364.1 (w, NH), Anal. Calcd for C39H39B2CoN10: C, 64.31; H, 5.40; N, 19.23. Found : C, 64.35; H, 5.05; N, 19.09. UV−vis (C6H6, 1.37 × 10−3 M) [λmax, nm (ε, L mol−1 cm−1)]: 460(910), 630(728) nm. HRMS (APCI): m/z calcd 728.2872 (M+), m/z found 728.2874 (M+). Synthesis of [CoB2Pz4Py](NHPhtBu) (3-tBu). To a pressure tube equipped with a Teflon tap containing a suspension of 1 (50 mg, 0.086

EXPERIMENTAL SECTION

For general procedures and methods, see the Supporting Information. Synthesis of [CoB2Pz4Py]THF (1-THF). To a solution of ligand precursor (529 mg, 1.00 mmol) in THF (3 mL) was added all at once

LiOiPr (79.2 mg, 1.20 mmol) at room temperature. The resulting dilithium salt was left to stir for 15 min, and then a solution of CoBr2 (210 mg, 0.959 mmol) in 3 mL of THF was added in one portion. The resulting peach-colored solution was left to stir for 1 h. Then 1 mL of pentane was added. The resulting mixture was filtered, and to the filtrate was added another 5 mL of pentane. The mixture was placed in a freezer at −21 °C for 5 h. A crystalline solid was isolated by filtration, washed with a THF/pentane (1:2) mixture, and dried in vacuo for 1 h to give 417 mg (64%) of 1-THF. The product is paramagnetic and highly air-sensitive. Crystals suitable for X-ray crystallography were obtained by slow diffusion of pentane into a concentrated solution of the complex in THF. 1H NMR (500 MHz, [D8]THF): δ 66.47 (4H, s, H8), 55.99 (2H, s, H2), 51.06 (4H, s, H10), 26.34 (4H, s, H5), 15.03 (4H, d, 3JHH = 7.1 Hz, H6), 13.33 (2H, t, 3JHH = 7.1 Hz, H7), 3.61 (4H, s, H11), 1.70 (4H, s, H12), 1.12 (1H, s, H3), −41.06 (4H, s, H9). 13C{1H} NMR (126 MHz, [D8]THF): δ 773.4 (s, C8), 725.5 (s, C10), 658.9 (s, C9), 406.4 (s, C1), 160.6 (s, C5), 141.6 (s, C2), 137.7 (s, C6), 136.4 (s, C7), 103.7 (s, C3), 67.2 (s, C11), 25.3 (s, C12), C4 not observed. 11B NMR (161 MHz, [D8]THF): δ 99.0 (br s). Anal. Calcd for C33H33B2CoN9O: C, 60.77; H, 5.10; N, 19.33. Found: C, 60.59; H, 5.45; N, 19.61. UV−vis (C6H6, 1.53 × 10−3 M) [λmax, nm (ε, L mol−1 cm−1)]: 415(913), 645(294), 875(326). MS (ESI) for C29H25B2CoN9: m/z calcd 580.18 (100%), m/z found 580.1774 (100%). Synthesis of [CoB2Pz4Py] (1). A solid sample of 1-THF (300 mg, 0.460 mmol) in a thick-walled glass vessel equipped with a Kontes Teflon tap was placed in an oil bath at 25 °C. The vessel was evacuated under full dynamic vacuum, and the temperature in the oil bath was gradually increased to 150 °C and kept at this temperature for 2 h.

mmol) in 3 mL of benzene was added a solution of 1-azido-4-tertbutylbenzene (17 mg, 0.095 mmol in a solution of benzene stored over molecular sieves). Vigorous evolution of nitrogen gas was observed. The resulting forest green reaction mixture was left to stir for 20 min, 4158

DOI: 10.1021/acs.inorgchem.7b00174 Inorg. Chem. 2017, 56, 4157−4168

Article

Inorganic Chemistry and then pentane (10 mL) was layered on top of the resulting solution. The resulting mixture was left to crystallize at ambient temperature for 2 days. After this time the green crystalline material was separated by filtration and dried in vacuo to give 38 mg (61%) of a green solid. 1H NMR (500 MHz, C6D6): δ 8.60 (2H, d, 3JHH = 2.1 Hz, H8), 8.45 (2H, d, 3JHH = 2.1 Hz, H8′), 8.19 (2H, d, 3JHH = 2.6 Hz, H8″), 8.01 (4H, m, H5), 7.91 (2H, d, 3JHH = 2.6 Hz, H8‴), 7.82 (4H, m, H5′), 7.70 (2H, d, 3JHH = 2.6 Hz, H10″), 7.62 (4H, m, H10 and H10′), 7.56 (2H, d, 4JHH = 2.5 Hz, H14), 7.53 (2H, d, 3JHH = 2.6 Hz, H10‴), 7.45 (2H, d, 3JHH = 7.6 Hz, H2), 7.35 (8H, m, H6 and H6′), 7.14 (4H, m, H7 and H7′), 6.72 (2H, t, 3JHH = 7.6 Hz, H3 and H3′), 6.50 (2H, dd, 3JHH = 8.8, 4JHH = 2.5 Hz, H14′), 5.94 (2H, t, 3JHH = 2.3 Hz, H9), 5.86 (4H, t, 3JHH = 2.3 Hz, H9″ and H9‴), 5.82 (2H, t, 3JHH = 2.3 Hz, H9′), 4.86 (2H, d, 3JHH = 8.8 Hz, H13′), 4.37 (2H, s, NH11), 1.07 (18H, s, H17). 13C{1H} NMR (126 MHz, C6D6): δ 173.0 (s, C1 and C1′), 155.4 (s, C13), 145.2 (s, C8), 144.3 (s, C8′), 143.9 (s, C8″), 143.0 (s, C8‴), 137.5 (s, C10 and C10′), 136.3 (s, C12), 135.7 (s, C5), 135.2 (s, C5′), 134.3 (s, C10″ and C10‴), 132.4 (s, C15), 128.6 (s, C6 and C6′), 127.7 (s, C2 and C2′), 127.5 (s, C7 and C7′), 125.4 (s, C14 and C14′), 119.8 (s, C13′), 107.2 (s, C9), 106.7 (s, C9′), 106.4 (s, C9″and C9‴), 106.0 (s, C3 and C3′), 34.1 (s, C16), 31.6 (s, C17). C4 not seen. 11B NMR (128 MHz, C6D6): δ −0.6. FT-IR (neat, cm−1): 3305.8 (w, NH). Anal. Calcd for C78H76B4Co2N20: C, 64.40; H, 5.27; N, 19.26. Found: C, 64.48; H, 5.38; N, 18.91. UV−vis (C6H6, 6.87 × 10−4 M) [λmax, nm (ε, L mol−1 cm−1)]: 470(2909), 680(3637) nm. HRMS (APCI): m/z calcd 1454.5593 (M+), m/z found 1454.5568 (M+). Synthesis of [CoB2Pz4Py](NHPh(CF3)2) (2-CF3). To a pressure tube equipped with a Teflon tap containing a suspension of 1-THF

0.052 mmol) in 2 mL of benzene was added a solution of 3,5bis(trifluoromethyl)phenyl azide (13 mg, 0.052 mmol in solution of benzene). Vigorous evolution of nitrogen gas was observed. The resulting forest green reaction mixture was left to stir for 30 min, and then pentane (10 mL) was layered on top of the resulting solution. The resulting mixture was left to crystallize at ambient temperature for 2 days. After this time the green crystalline material was separated by filtration and dried in vacuo to give 28 mg (67%) of a green solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 3-CF3 in benzene. 1H NMR (500 MHz, C6D6): δ 8.49 (2H, s, H8), 8.37 (2H, s, H8″), 8.35 (2H, s, H8‴), 8.08 (4H, d, 3JHH = 7.4 Hz, H5), 7.98 (2H, s, H8′), 7.89 (2H, d, 3 JHH = 2.6 Hz, H10), 7.80 (2H, d, 3JHH = 2.6 Hz, H10″), 7.73 (4H, m, H6), 7.65 (2H, d, 3JHH = 2.6 Hz, H10′), 7.58 (2H, d, 3JHH = 2.6 Hz, H10‴), 7.49 (4H, t, 3JHH = 7.4 Hz, H6′), 7.42 (2H, t, 3JHH = 7.4 Hz, H7), 7.38 (8H, m, H5′, H7′ and H2′), 7.00 (2H, s, H15), 6.77 (2H, d, 3 JHH = 7.5 Hz, H2), 6.59 (2H, t, 3JHH = 7.5 Hz, H3 and H3′), 6.02 (2H, s, H9), 5.92 (2H, s, H9″), 5.91 (2H, s, H9‴), 5.81 (2H, s, H9′), 4.48 (2H, s, H13′), 2.51 (2H, s, NH11). 13C{1H} NMR (126 MHz, C6D6): δ 183.0 (s, C1 and C1′) 157.3 (s, C13), 144.8 (s, C8), 143.9 (s, C8″), 143.7 (s, C8‴), 143.1 (s, C8′), 138.5 (s, C10), 138.3 (s, C10″), 138.1 (s, C10‴), 137.6 (s, C10′), 135.3 (s, C5), 135.0 (s, C3 and C3′), 134.7 (s, C13′), 130.5 (s, C5′), 130.3 (s, C14), 128.7 (s, C6), 128.60 (s, C2 and C2′), 128.56 (s, C7), 128.50 (s, C7′), 125.8 (s, C6′ and C14′), 125.1 (s, C12), 114.3 (s, C15), 108.2 (s, C9), 107.63 (s, C9″), 107.60 (s, C9‴), 107.4 (s, C9′). C4, C4′, C16, and C16′ are not seen. 19F NMR (471 MHz, C6D6): δ −57.3 (6F, s, C16F), −63.9 (6F, s, C16′F). 11B NMR (128 MHz, C6D6): δ −0.4. FT-IR (neat, cm−1): 3364.0 (w, NH). Anal. Calcd for C74H56B4Co2F12N20: C, 55.05; H, 3.50; N, 17.35. Found: C, 54.69; H, 3.52; N, 16.98. UV−vis (C6H6, 6.19 × 10−4 M) [λmax, nm (ε, L mol−1 cm−1)]: 365(1776), 450(1292), 645(1009) nm. HRMS (APCI): m/z calcd 1614.3841 (M), 1614.3836 (M+), m/z found 1614.3856 (M), 1614.3856 (M+). Synthesis of [CoB2Pz4Py](NHSO2PhtBu) (2-SO2Ar). To a pressure tube equipped with a Teflon tap containing a suspension of

(30 mg, 0.046 mmol) in 2 mL of benzene was added cyclohexadiene (2.176 μL, 0.023 mmol), and then a solution of 3,5-bis(trifluoromethyl)phenyl azide (12 mg, 0.046 mmol in a solution of benzene stored over molecular sieves) was slowly added. Vigorous evolution of nitrogen gas was observed. The resulting forest green reaction mixture was stirred for 2 h. Pentane (10 mL) was layered on top of the resulting solution, and the mixture was left to crystallize at ambient temperature for 1 day. After this time, a green crystalline material was separated by filtration and dried in vacuo to give 29 mg (78%) of a green solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 2-CF3 in benzene. 1H NMR (500 MHz, C6D6): δ 7.95 (4H, m, H5), 7.81 (4H, d, 3JHH = 2.6 Hz, H8), 7.50 (4H, m, H10), 7.41 (6H, m, H6 and H7), 7.32 (1H, d, 4JHH = 1.5 Hz, H13), 7.06 (1H, s, H15), 7.00 (1H, d, 4JHH = 1.5 Hz, H13′), 6.84 (2H, d, 3JHH = 7.6 Hz, H2), 6.58 (1H, t, 3JHH = 7.6 Hz, H3), 5.84 (4H, m, H9), 2.14 (1H, s, NH11). 13C{1H} NMR (126 MHz, C6D6): δ 172.0 (s, C1), 158.1 (s, C4), 147.9 (s, C8), 144.7 (s, C10), 142.6 (s, C12), 137.7 (s, C3), 134.9 (s, C5), 128.60 (s, C2), 128.56 (s, C13), 128.4 (s, C13′), 128.2 (s, C14 and C14′), 128.0 (s, C6), 127.5 (s, C7), 107.3 (s, C9), 104.3 (s, C15). C16 and C16′ not observed. 19F NMR (471 MHz, C6D6): δ −63.2 (3F, s, C16F), −64.2 (3F, s, C16′F). 11B NMR (128 MHz, C6D6): δ −1.2. FT-IR (neat, cm−1): 3370.0 (w, NH). Anal. Calcd for C37H29B2CoF6N10: C, 54.98; H, 3.62; N, 17.33. Found: C, 54.82; H, 3.62; N, 17.19. UV−vis (C6H6, 1.24 × 10−3 M) [λmax, nm (ε, L mol−1 cm−1)]: 350(1920), 435(808), 605(707) nm. HRMS (APCI): m/z calcd 808.1999 (M), 808.1993 (M+), m/z found 808.2030 (M), 808.2030 (M+). Synthesis of [CoB2Pz4Py](NHPh(CF3)2) (3-CF3). To a pressure tube equipped with a Teflon tap containing a suspension of 1 (30 mg,

1-THF (150 mg, 0.230 mmol) in 5 mL of benzene was added 4-tertbutylbenzenesulfonyl azide (61 mg, 0.253 mmol). Vigorous evolution of nitrogen gas was observed. The obtained pink reaction mixture was stirred for 2 h at room temperature. Pentane (15 mL) was layered on top of the resulting solution. The resulting mixture was left to crystallize at ambient temperature for 1 day. After this time the pink crystalline material was separated by filtration and dried in vacuo to give 125 mg (69%) of a pink solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 2-SO2Ar in benzene. 1H NMR (500 MHz, CD2Cl2): δ 8.39 4159

DOI: 10.1021/acs.inorgchem.7b00174 Inorg. Chem. 2017, 56, 4157−4168

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Inorganic Chemistry (4H, m, H8), 7.95 (4H, m, H5), 7.92 (4H, dd, 3JHH = 2.6 Hz, 0.7 Hz, H10), 7.55 (4H, m, H6), 7.53 (2H, d, 3JHH = 8.5 Hz, H14), 7.49 (2H, t, 3JHH = 8.4 Hz, H7), 7.42 (2H, d, 3JHH = 8.5 Hz, H13), 7.30 (1H, t, 3 JHH = 7.5 Hz, H3) 7.12 (2H, d, 3JHH = 7.5 Hz, H2), 6.33 (4H, t, 3JHH = 2.4 Hz, H9), 1.55 (1H, s, NH11), 1.35 (9H, s, H17). 13C NMR (126 MHz, CD2Cl2): δ 172.9 (s, C1), 154.1 (s, C4), 147.1 (s, C12), 145.2 (s, C15), 144.8 (s, C8), 138.1 (s, C10), 135.5 (s, C3), 135.1 (s, C5), 128.7 (s, C13), 128.5 (s, C6), 127.9 (s, C7), 125.8 (s, C14), 125.5 (s, C2), 107.1 (s, C9), 35.2 (s, C16), 31.4 (s, C17). 11B NMR (161 MHz, CD2Cl2): δ −4.4 (s). FT-IR (neat, cm−1): 3135.7 (w, NH). Anal. Calcd for C39H39B2CoN10O2S: C 59.11; H 4.96; N 17.68. Found: C 58.75; H 5.06; N 17.31. UV−vis (C6H6, 1.26 × 10−3 M) [λmax, nm (ε, L mol−1 cm−1)]: 515(871), 665(159), 875(120) nm. HRMS (APCI): m/z calcd 792.2491 (M+), m/z found 792.2514 (M+). Synthesis of [CoB2Pz4Py]μ2(NSO2PhtBu) (4). To a pressure tube equipped with a Teflon tap containing a suspension of 1 (40 mg, 0.069

with LiOiPr in THF prior to treatment with a solution of CoBr2 in THF (Scheme 1). Over the course of an hour of stirring, a Scheme 1

blue to light orange color change was observed, and upon workup a crop of peach-colored crystals of an air- and moisturesensitive, paramagnetic compound were obtained; its structure was confirmed via X-ray crystallography (Figure S1) to be the (B2Pz4Py)Co(II)THF complex 1-THF. In 1-THF, the B2Pz4Py ligand coordinates in a square pyramidal array, and the THF ligand occupies the position trans to the pyridyl group to form an overall pseudo-octahedral geometry. An Evans method measurement37 yields a solution magnetic moment of μeff = 3.79 μB, consistent with a high spin (HS) electron configuration with three unpaired electrons (S = 3/2). The 1H NMR spectrum of 1-THF taken in d8-THF (Figure S2) shows all eight of the expected ligand resonances, with appropriate integrations in the spectral range of +70 to −42 ppm; the resonance for the singular para-pyridyl proton was found at 1.12 ppm. The compound is readily oxidized in air, and cyclic voltammetry (Figure S3) indicates that the Co(II)/Co(III) redox couple lies at +0.22 V vs SHE, compared to that of +0.88 V vs SHE found under similar conditions for the dicationic [(PY5)Co(L)]2+ complex.10 This illustrates the ability of the B2Pz4Py ligand to stabilize higher oxidation state complexes, although no further oxidations are observed when scanning up to +2.3 V. This is curious in light of the fact that the free ligand [B2Pz4LiPyH]2 undergoes two irreversible oxidations at +1.71 and +1.99 V vs SHE (Figure S3), presumably due to oxidation of the borate moieties. Thus, the coordination of the ligand to the metal evidently renders it more oxidatively robust. It was anticipated that the THF ligand in the 19-electron complex 1-THF would be labile and subject to removal to form a five-coordinate square pyramidal Co(II) derivative similar to what was observed in the analogous 18-electron (B2Pz4Py)Fe(II)THF complex.27 Indeed, the parent ion for 1-THF in the electrospray ionization (ESI) mass spectrum appeared at m/z = 580.18, corresponding to the THF-free (B2Pz4Py)Co fragment. Exposure of solid 1-THF to high vacuum with gradual heating to 150 °C for 2 h was sufficient to drive off the THF ligand, leaving the five-coordinate derivative 1 behind as a brown solid in essentially quantitative yield. Unlike the THF adduct, 1 was completely 1H NMR silent in nondonor solvents, but a characteristic resonance for the compound was observed at 80

mmol) in 2 mL of benzene was slowly added a solution of 4-tertbutylbenzenesulfonyl azide (8 mg, 0.034 mmol) in 1 mL of benzene. Vigorous evolution of nitrogen gas was observed during this time. The resulting dark brown reaction mixture was stirred for 1 h. Pentane (14 mL) was layered on top of the resulting solution, and the resulting mixture was left to crystallize at ambient temperature for 1 day. After this time, the dark brown, crystalline material was separated by filtration and dried in vacuo for 2 h to give 31 mg (66%) of a brown solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in a saturated solution of 4 in benzene. 1H NMR (500 MHz, C6D6): δ 9.22 (2H, d, 3JHH = 2.6 Hz, H8), 9.02 (2H, d, 3 JHH = 2.4 Hz, H8″), 8.97 (2H, d, 3JHH = 2.4 Hz, H8‴), 8.68 (2H, d, 3 JHH = 2.6 Hz, H8′), 7.96 (4H, d, 3JHH = 7.3 Hz, H5), 7.88 (4H, d, 3 JHH = 7.4 Hz, H5′), 7.83 (2H, d, 3JHH = 2.6 Hz, H10′), 7.76 (2H, d, 3 JHH = 2.6 Hz, H10), 7.71 (2H, d, 3JHH = 2.4 Hz, H10‴), 7.48 (2H, d, 3 JHH = 2.4 Hz, H10″), 7.43 (8H, m, H6′, H7 and H7′), 7.38 (4H, d, 3 JHH = 7.1 Hz, H6), 7.21 (2H, d, 3JHH = 7.5 Hz, H2), 7.05 (2H, d, 3JHH = 8.0 Hz, H13), 6.87 (2H, d, 3JHH = 8.0 Hz, H14), 6.77 (2H, d, 3JHH = 7.5 Hz, H2′), 6.66 (1H, t, 3JHH = 7.5 Hz, H3), 6.33 (1H, t, 3JHH = 7.5 Hz, H3′), 6.11 (2H, t, 3JHH = 2.6 Hz, H9), 5.96 (2H, t, 3JHH = 2.4 Hz, H9‴), 5.93 (2H, t, 3JHH = 2.6 Hz, H9′), 5.79 (2H, t, 3JHH = 2.4 Hz, H9″), 1.28 (9H, s, H17). 13C NMR (126 MHz, C6D6): δ 173.5 (s, C1′), 173.1 (s, C1), 150.7 (s, C12), 149.0 (s, C15), 146.9 (s, C8), 146.1 (s, C8″), 146.0 (s, C8‴), 145.6 (s, C8′), 145.0 (s, C4 and C4′), 137.4 (s, C10), 137.2 (s, C10′), 137.0 (s, C10‴), 136.5 (s, C10″), 135.5 (s, C5), 135.0 (s, C5′), 134.9 (s, C3′), 134.1 (s, C3), 128.44 (s, C6′), 128.35 (s, C7′ and C7), 128.0 (s, C2′), 127.4 (s, C6), 127.0 (s, C2), 126.0 (s, C13), 123.4 (s, C14), 107.0 (s, C9), 106.8 (s, C9′), 105.9 (s, C9″), 105.6 (s, C9‴), 34.5 (s, C16), 31.7 (s, C17). 11B NMR (128 MHz, C6D6): δ −1.2. Anal. Calcd for C68H63B4Co2N19O2S: C 59.55; H 4.63; N 19.40. Found: C 59.35; H 4.77; N 19.37. UV−vis (C6H6, 7.29 × 10−4 M) [λmax, nm (ε, L mol−1 cm−1)]: 495(1235), 745(206), 875(686) nm. HRMS (APCI): m/z calcd 1372.4242 (M + H)+, m/z found 1372.4204 (M + H)+.

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RESULTS AND DISCUSSION As described previously,27 the diborate ligand can be synthesized in gram quantities as its mono lithium pyridinium salt [B2Pz4LiPyH]2. This precursor can be deprotonated in situ 4160

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room temperature or with gentle heating to 60 °C. However, when 1-THF was reacted with 4-tert-butylphenyl azide, a rapid reaction was signaled by both effervescence and a pink to forest green color transition. By proton NMR spectroscopy, a mixture of two diamagnetic products, present in a 2:1 ratio, was observed. Subsequent investigations showed these products to be the monomeric Co(III) aryl amido complex 2-tBu (minor product) and the dimeric Co(III) amido species 3-tBu, formed via C−C coupling at an ortho position with migration of the ortho hydrogen of the azide aryl ring to the nitrogen atom (major product). This dimeric species is produced cleanly in good yield when the THF-free complex 1 is employed instead of the THF adduct (Scheme 2). The source of the amido

ppm in the 11B NMR spectrum taken in C6D6. The solution magnetic moment (μeff = 3.82 μB), determined via the Evans method, indicates that the THF-free species retains a HS, S = 3/2 configuration, and elemental analysis of the solid supports the absence of THF. Crystallization via slow diffusion of pentane into a saturated benzene solution of 1 yields X-ray quality single crystals, and a structure determination confirms this assignment (Figure 1).

Scheme 2

Figure 1. (A) Thermal ellipsoid diagram (50%) of 1, with hydrogen atoms removed for clarity. Selected bond distances (Å): Co1−N1, 2.090(7); Co1−N2, 1.947(8); Co1−N4, 1.990(6); Co1−N6, 1.929(7); Co1−N8, 1.968(8). Selected bond angles (deg): N2− Co1−N4, 87.4(3); N2−Co1−N6, 92.8(3). (B) Partial packing diagram illustrating the close contacts between the cobalt centers and pyrazolyl carbons on adjacent molecules.

The cobalt center in 1 is necessarily distorted from an ideal square pyramidal geometry due to the ca. 5−7° difference between the narrow bay N2−Co1−N4 angle of 87.4(3)° and the wide bay N2−Co1−N6 angle of 92.8(3)°, giving the molecule overall C2v symmetry. The Co1−N1 (pyridyl nitrogen) distance of 2.090(7)Å is typical for these compounds, and there is a noticeable tilt between the planes defined by the six atoms of the pyridine ring and that proscribed by the four pyrazolyl nitrogens bonded to Co1 (tilt angle = 74.4°). Although the sixth coordination site is vacant, as in the iron congener,27 the compound crystallizes in such a way that close contacts of ∼3.1 Å between the open coordination site on the Co center and one of the pyrazolyl carbon atoms are observed (see Figure 1B). Although we did not attempt to ascertain whether or not this interaction is present in solution, we presume that it is weak and unlikely to play a major role in the chemistry of 1. Density functional theory (DFT) optimizations of 1 and 1THF were performed using the B3PW91 functional, and in each case the HS quartet configuration was more stable than the low-spin (LS) doublet, but by narrower margins than the differences observed in the iron congeners. For 1, the HS configuration was only 1.0 kcal/mol more stable than the LS, while for 1-THF, the difference was slightly greater at 3.7 kcal/ mol. In the iron systems, the differences in energy between the HS and LS configurations were 14.4 and 10.2 kcal/mol for the THF-free and THF-ligated compounds, respectively, favoring the HS, S = 2 states.27 With these Co(II) starting materials in hand, we explored their reactivity with a variety of azide reagents. No reactivity was observed toward the sterically bulky adamantyl azide at

hydrogen atom in the formation of the monomeric species 2tBu was likely THF, a relatively weak hydrogen atom donor, and when the reaction between 1-THF and the azide was conducted in the presence of one equivalent of the more effective hydrogen atom donor 1,4-cyclohexadiene (CHD), monomeric 2-tBu was the sole cobalt-containing product observed; C6H6 was also produced in this reaction, as detected in NMR experiments conducted in C6D6. As can be seen in Scheme 2, similar chemistry was observed when 3,5-bis(trifluoromethylphenyl) azide was employed. The propensity of the cobalt imido products to accept hydrogen atoms33 bears some similarity to the chemistry observed in the iron system,27 but the C−C coupling chemistry was not observed for iron. As alluded to above, both the monomeric and the dimeric Co(III) amido products, like the Fe(III) amidos, are LS complexes but are diamagnetic as opposed to the S = 1/2 Fe(III) derivatives and so are readily characterized by the usual multinuclear NMR techniques. The para-pyridyl hydrogens give rise to diagnostic resonances that can be assigned on the basis of integration, and the monomers 2 can be distinguished from the dimers 3 via the pattern of aromatic proton resonances for the N-aryl groups. For the CF3substituted compounds, 19F NMR spectroscopy is also helpful, with the monomer 2-CF3 giving rise to two broad singlets at −62.3 and −63.3 ppm (C7D8), which coalesce at higher temperatures (315 K, ΔG⧧315 ≈ 14.2 kcal mol−1; see Figure S4). This indicates that there is restricted rotation about the 4161

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Inorganic Chemistry N−Cipso bond of the amido ligand. For the dimer 3-CF3, the CF3 groups appear in the 19F NMR spectrum as two noncoalescing signals of equal intensity at −57.3 and −63.9 ppm. X-ray structural analysis confirmed the structures of the monomeric Co(III) amides 2-CF3 (Figure 2) and 2-tBu

Figure 3. Thermal ellipsoid diagram (50%) of 3-CF3, with all hydrogens except the amido hydrogen omitted for clarity. Selected bond distances (Å): Co1−N1, 1.969(3); Co1−N10, 1.972(3); Co2− N11, 2.000(3); Co1−N20, 1.952(4); C66−C67, 1.500(6). Selected bond angles (deg): N1−Co1−N10, 178.74(15); N1−Co2−N20, 175.62(15).

Figure 2. (A) Thermal ellipsoid diagram (50%) of 2-CF3, with all hydrogens except the amido hydrogen omitted for clarity. Selected bond distances (Å): Co1−N1, 1.955(3); Co1−N2, 1.949(2); Co1− N5, 1.954(2); Co1−N6, 1.930(3); N6−C20, 1.360(4). Selected bond angles (deg): N1−Co1−N6, 179.46(12); N2−Co1−N2′, 89.94(12); N5−Co1−N5′, 85.46(12); N2−Co1−N5, 92.30(8); Co1−N6−C20, 135.0(3). (B) Top view showing the alignment of the amido ligand with the narrow bay of the B2Pz4Py ligand bisecting the N2−Co1−N2′ and N5−Co1−N5′ angles.

this product was a dinuclear complex in which a NS(Ar)O2bridged Co(III)−Co(III) species is formed, as was observed in the iron chemistry previously.27 Indeed, reaction of 1 with 0.5 equiv of the sulfonyl azide reagent cleanly produced compound 4 as show in Scheme 3. Both 2-SO2Ar and 4 were fully characterized spectroscopically and by elemental analysis and X-ray crystallography, and the molecular structures of 2-SO2Ar and 4 are given in Figures 4 and 5, respectively. In addition to its observed diamagnetism, the metrical parameters associated with the bridging sulfonylimide ligand in 4 are consistent with the formulation of the compound as having a Co(III)−Co(III) ground state as opposed to a Co(IV)−Co(II) configuration. For example, the Co1−N10 and Co2−O2 distances of 1.939(2) and 1.935(2) Å, respectively, are consistent with M−E single bonds, while the S1−O2 distance of 1.516(2) Å is significantly elongated in comparison to that of S1−O1, at 1.458(2) Å. These observations suggest that the presumed imido product of reaction of 1 with azide N3SO2-4-tBuC6H4 is highly reactive toward a second equivalent of 1, and even inverse addition strategies (i.e., adding a solution of 1 to an excess of the azide) could not prevent exclusive formation of 4. Complexes 2-R and 3-R are rare examples of six-coordinate, octahedral Co(III) amido derivatives. In fact, cobalt(III) amido complexes of any coordination number are relatively scarce and, excluding cobalt(III) porphyrin derivatives, whose pyrrole rings are a special type of amido group, structurally characterized Co(III) amido complexes are few in number. Examples from the literature are compiled in Table S1. Power’s three-coordinate Co[N(SiMe3)2]338 and its one-electron reduced anion39 constitute archetypical examples, while others include NR2 donors as part of multidentate pincer ligand arrays in four40- or five41-coordinate geometries. The examples most relevant to compounds 2-R and 3-R are the four-, five-, and sixcoordinate derivatives I,42 II,43 and III,44 shown in Chart 2. Compounds I and II were formed via reaction of low-valent

(Figure S5, connectivity only). In addition, the structure of the dimer 3-CF3 was determined. Carbon−carbon coupling through the ortho position on the aryl ring is apparent (C66−C67, Figure 3), and notably for 3-CF3 no evidence of coupling through the para position was observed.31 In all of these structures, the amido group aryl substituent aligns with the narrow N−Co−N bays of the B2Pz4Py ligand framework. In other words, in 2-CF3, the plane of the amido ligand bisects the N2−Co−N2′ angle, rather than the N2− Co−N5 angle. Likely this is sterically determined, as the wider N2−Co−N5 bays are partially blocked by the C−H bonds of the alpha pyrazolyl carbons and there is actually more steric space within the narrower N2−Co−N2′ and N5−Co−N5′ bays. The restricted rotation of the 3,5-(CF3)2-C6H3 group within the narrow bay about the N6−C20 bond on the NMR time scale at room temperature is also likely due to steric effects. Thus, despite the appearance of a sterically open hemisphere in the B2Pz4Py ligand environment, the pyrazolyl groups do have an influence on the orientation and dynamics of axial ligands of moderate steric bulk. In light of the tendency of the product of reaction of 1 with ArN3 reagents to dimerize, we next explored the reactivity of compounds 1-THF and 1 with the sulfonyl azide N3SO24-tBuC6H4 (Scheme 3), postulating that the C−C coupling would be prevented in this instance. Like before, in the reaction with 1-THF, rapid evolution of N2 was observed at room temperature, but here the sole product was the monomeric Co(III) amido complex 2-SO2Ar arising from the scavenging of a hydrogen atom, likely from the THF. The reaction involving THF-free 1 with one equivalent of sulfonyl azide was also rapid at room temperature, affording a new diamagnetic product along with 0.5 equiv of unreacted azide. We hypothesized that 4162

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Figure 4. Thermal ellipsoid diagram (50%) of one of the two molecules of 2-SO2Ar in the unit cell; the molecules are not significantly different in terms of their metrical parameters. All hydrogen atoms except the amido hydrogen are omitted for clarity. Selected bond distances (Å): Co1−N1, 2.012(3); Co1−N2, 1.957(3); Co1−N4, 1.946(3); Co1−N6, 1.976(3); Co1−N8, 1.933(3); Co1− N10, 1.965(3). Selected bond angles (deg): N1−Co1−N10, 179.35(12); N2−Co1−N4, 84.86(11); N6−Co1−N8, 91.69(11); N2−Co1−N6, 91.83(12); N4−Co1−N8, 91.75(11); Co1−N10−S1, 133.29(18).

Figure 5. Thermal ellipsoid diagram (50%) of 4; all hydrogen atoms are omitted for clarity. Selected bond distances (Å): Co1−N1, 1.985(3); Co1−N10, 1.939(2); Co2−O2, 1.935(2); Co2−N11, 1.964(3); S1−N10, 1.499(2); S1−O1, 1.458(2); S1−O2, 1.516(2). Selected bond angles (deg): N1−Co1−N10, 176.83(10); N19−Co2− O1, 176.23(10); Co1−N10−S1, 134.15(16); Co2−O2−S1, 134.42(14); N10−S1−O1, 117.51(13); N10−S1−O2, 106.02(14); O1−S1−O2, 114.03(13).

Chart 2 Co(I) precursors with azides; subsequent hydrogen atom abstraction from a C−H bond attached to the supporting ligand (I) or the aryl group of the azide (II) and Co−carbon bond formation via coupling of the resulting alkyl radical and the Co(II) center leads to the observed Co(III) amido products. The six-coordinate complex III44 is to our knowledge the only other structurally characterized octahedral Co(III) amido complex and was prepared for the purpose of acting as a precursor to anilino radical complexes via reversible oneelectron oxidation. The Co−Namido bond distances are consistent with single bonds and rise with increasing coordination number. In the present examples 2-R and 3-R, the range of Co−Namido distances is 1.930(3) Å in 2-CF3 to 1.982(10) Å in 2-tBu, somewhat longer than in III, where the amido group is part of a chelating array. These long distances are consistent with the expectation that π−π repulsions will be significant in these d6 Co(III) derivatives, and indeed,

pyramidalization at the amido nitrogen is evident in both the experimental and the computed structures of these species. This pyramidalization at N is minimized in 2-CF3, where the nitrogen lone pair is stabilized by the electron-withdrawing aryl ring (note the relatively short N6−C20 bond distance of 1.360(4) Å, Figure 2), but for 2-tBu and 2-SO2Ar, the 4163

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Figure 6. Experimental and simulated isotropic X-band EPR spectrum of the imido radical intermediate 5-tBu in toluene solution at 220 K (frequency = 9.385 005 GHz; mod. amp. = 10 G; microwave power = 2.0 mW). Simulation of the EPR spectrum was performed using the Winsim2002 EPR simulation program to >99% correlation. giso value: 2.007; hyperfine coupling constants: 57Co, expt, 13.101 (simulated, 13.090); 14 N, expt n/a (simulated 0.742); correlation, 99.407%.

pyramidalization is pronounced (Figures S5 and 4, respectively). The Co−Namido distances in the computed structures of 2-tBu (1.95 Å) and 2-CF3 (1.93 Å) are close to the experimental value for 2-CF3, and the pyramidalization at the amido nitrogen is also evident in these computed structures (see Figure 7A below for the structure of the DFT-optimized structure of 2-tBu). Co(III) amido complexes are thought to be important intermediates in cobalt porphyrin-catalyzed C−H amination32,33 and olefin aziridination32,34,45 reactions using organoazides, and so well-defined examples of such compounds provide models for these catalytic intermediates. In the Co(II) porphyrin and related systems, the azide reacts with the metal complex to form a highly reactive species formulated as a d6 Co(III) imido radical, as opposed to a d5 Co(IV) imido complex. In the former, the unpaired electron is largely associated with the imido nitrogen and has very little d character in the singly occupied molecular orbital.32 This radical abstracts a hydrogen atom from the substrate in C−H amination or reacts with an olefin in aziridination; the resulting Co(III) amido species in each case is trapped in a “rebound” step with the coproduced alkyl radical to form a C−N bond and return the cobalt to its Co(II) oxidation state. We hypothesized that a similar scenario was applicable here, with dimerization or H atom trapping of the imido radical in lieu of the C−N bond-forming step, which we do not observe here (or in the related iron system27). The amido products observed in the reactions of 1 and 1-THF with azides therefore likely arise from an initially formed Co(III) imido radical complex (5-R, Scheme 4); the alternative formulation as a Co(IV) imido (V, Scheme 4) is not consistent with the observed reactivity. The facility by which 5-R are generated is dependent on the steric properties of the substituent on the azide, as this affects its ability to assume the necessary coordination through the alpha nitrogen (IV, Scheme 4) prior to N2 loss. Coordination of the azide in this fashion creates steric interaction between ortho aryl groups and the

Figure 7. Density functional theory (B3PW91/SDD(Co)/6-31G(d,p) other atoms) optimized geometry of (A) 2-tBu; (B) 5-tBu. (C) Depiction of the SOMO of 5-tBu. For full details on the methods used, see the Supporting Information.

nitrogens of the azide and two of the pyrazolyl groups on the ligand, as depicted in Scheme 4. This model accounts for the failed attempts to stabilize (and perhaps isolate) any examples of the imido radical using sterically encumbered azides such as adamantyl azide or 2,4,6-trimethylphenylazide. These reagents 4164

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tBu was chosen as the subject of these investigations because analogous calculations on the iron congener were recently reported27 and comparisons between these two related species are of interest. The lowest energy structure of 5-tBu is a doublet, as would be expected in either the imido radical or the Co(IV) imido (V) formulation. Whereas the optimized structure of the Co(III) amido 2-tBu (Figure 7A) clearly shows the pyramidalization at the amide nitrogen, in the imido complex, the N-aryl group is aligned in such a way as to suggest sp2 hybridization at the imido nitrogen (Figure 7B). However, the Co−Nimido distance of 1.89 Å is only marginally shorter than the Co−Namido distance of 1.982(10) Å (calculated: 1.95 Å), arguing against significant multiple bond character in this linkage. Known Co−imido bonds all occur in lower coordinate two-,49 three-,42,50,51 or four52−57-coordinate species and exhibit much shorter bond distances in the range 1.609(3)−1.691(6) Å; almost invariably they possess essentially linear Co−Nimido− C geometries. A longer Co−Nimido in a higher coordinate species such as 5tBu would be expected, but the computed distance is more consistent with single-bond character, as is the bent geometry assumed by the imido ligand; the Co−Nimido−Cipso bond angle in the computed structure of 5-tBu is 130°. Since π−π repulsions are present in the amido complexes 2-R, the lack of π bonding in the Co−Nimido bond is not unexpected.58 Interestingly, the isostructural iron derivative27 has a shorter Fe−Nimido bond length (1.696 Å) and a wider M−N−Cipso angle (143.3°), indicating a higher bond order in this d5 Fe(III) species in comparison to the d6 Co(III). Analysis of the singly occupied molecular orbital (SOMO) confirms a lack of Co−N π bonding and shows that it is highly localized on the aryl imido group, mainly associated with the imido nitrogen, but also delocalized into the ortho and para positions of the aryl ring (Figure 7C). There is very little dcharacter to this orbital, and so a formulation of intermediate 5tBu as an imido radical32−34 is also supported by the computed structure. The delocalization of spin density onto the aryl ring, which is not present to nearly the same extent in the iron congener,27 also explains the tendency of these compounds to dimerize via C−C bond formation through the ortho positions. That dimerization is able to compete with C−H activation in the cobalt system but is not observed to be significant in the iron congener27 is also reflected in the computed barriers to transfer a H atom to the imido nitrogen for the iron and cobalt systems. For the iron imido radical, toluene was a suitable hydrogen atom donor, despite the relatively large 89 kcal/mol bond dissociation energy59 (BDE) of the benzylic C−H bond; the calculated barrier for hydrogen atom transfer to the iron imido radical was only 6.4 kcal/mol. For the cobalt system, however, transfer of a hydrogen atom from toluene to the imido radical was not observed at room temperature. Here, the barrier was computed to be 25.5 kcal mol−1 (Figure S8), much higher because the transition state is late and involves considerable C−H bond breakage in comparison to that in the iron compound, which has a more symmetrical transition state. When the transition state for transfer of a hydrogen atom from 1,4-cyclohexadiene to 5-tBu was examined, a much lower barrier of 2.1 kcal/mol was found, consistent with the significantly lower BDE of 77 kcal/mol60 for this hydrogen atom donor,61,62 and the observed rapid formation of 2-tBu when 5-tBu is generated in the presence of one equivalent of CHD.

Scheme 4

did not react with 1 under any conditions that we explored (heating to ∼80 °C in an oil bath or microwave reactor), possibly because, for these substrates, adduct IV cannot form. Thus, aryl substituents with no ortho groups are required for reactivity, but the imido radicals 5-R that result are not stable enough to isolate even in the absence of hydrogen atom donors due to the facile dimerization process leading to amides 3-R. While the imido radicals could not be isolated, 5-tBu could be detected spectroscopically when the reaction of 1 with 4-tertbutylphenyl azide was monitored via X-band EPR spectroscopy at low temperatures. At room temperature, the EPR spectrum of 1 in toluene (1 μM) is broad and poorly defined due to low signal-to-noise (Figure S6); this is not unusual for HS d7 complexes.46 As the temperature was cooled to 160 K, however, a spectral signature emerged that is consistent with what would be expected for compound 1 (Figure S6).47,48 This sample was frozen, and a solution containing an excess of 4-tert-butylphenyl azide (∼100 equiv) was added to the sample; it was then warmed to melt and shaken before returning to the EPR probe cooled to 160 K. The resulting spectrum indicated that 1 was consumed completely and its EPR signature replaced by a new pattern (Figure S7). Optimization of the temperature improved the resolution of the spectrum such that, at 220 K, a hyperfine splitting pattern emerged (Figures 6; S7) that is reminiscent of those recorded in the Co(III) imido radical porphyrin complexes reported recently by de Bruin et al. 32,33 Furthermore, simulation of the spectrum yields comparable EPR parameters. The spectrum is persistent at low temperatures, but as the sample is warmed to 260 K, in addition to loss of fine structure, the intensity of the signal diminishes and the sample becomes EPR silent (Figure S7) as dimerization to 3tBu takes place, the clean formation of which was confirmed by NMR spectroscopy. Together, these results provide convincing evidence for the intermediacy of the moderately stable Co(III) imido radical 5-tBu in the reaction of 1 with 4-tert-butylphenyl azide. In order to obtain more detailed structural information on these imido radicals, DFT computations were undertaken; 54165

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Inorganic Chemistry

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CONCLUSIONS Cobalt(II) complexes of the diborate tetrapodal pentadentate ligand B2Pz4Py, 1 and 1-THF, have been prepared and reacted with organoazides. When the azide substituent is of moderate steric bulk, rapid reaction (resulting in evolution of N2) signals the formation of Co imido derivatives, but the isolated products are Co(III) amidos that arise from hydrogen atom scavenging (monomeric amidos 2-R) or a C−C bond-forming dimerization through the ortho carbons of the aryl group on the azide (dimeric products 3-R). In the latter, C−C bond formation is accompanied by migration of the ortho hydrogen from the carbon to the imido nitrogen. Attempts to stymie this dimerization and isolate a Co imido derivative by using an arylsulfonyl azide were thwarted by the ready formation of the dinuclear derivative 4, in which the sulfonyl imido group rapidly picks up another equivalent of the Co(II) starting material 1. Despite our inability to isolate examples of the cobalt imido products of these reactions, they were probed by EPR spectroscopy and DFT methods and are best characterized as d6 Co(III) imido radical complexes 5-R. The experimental and computational data show that significant spin density is localized on the imido nitrogen and onto the imido aryl group, which is fully consistent with the observed reactivity of these compounds to give amido products. Their behavior is thus similar to their iron congeners, which were observed to be powerful hydrogen atom scavengers. Interestingly, in the iron case, dimerization through C−C coupling was not observed. We attribute this to the fact that it is only in the more electron rich cobalt derivatives that the SOMO is pushed out onto the aryl group of the imido ligand, triggering the C−C bond forming pathway observed in these systems. The potential of this unique reactivity for catalytic synthesis of biaryl amines will be the focus of future investigations.

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Foundation (W.E.P.and L.M.) and the Chinese Academy of Science (L.M.) are acknowledged for financial support. L.M. is member of the Institut Universitaire de France. L.M. and W.P. also acknowledge CNRS for a bilateral grant through the PICS Program. CalMip is acknowledged for a generous grant of computing time. D.M.S. thanks the University of Calgary for an Eyes High Postdoctoral Fellowship. L.N. thanks Alberta Innovates Technology Futures for Scholarship support. Dr. David W. Bi is acknowledged for collecting the X-ray data for compounds 1, 2-tBu, and 4.

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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.inorgchem.7b00174. General experimental procedures and spectroscopic, electrochemical, and X-ray crystallographic data for all new compounds and computational details; table of comparative literature data (PDF) CCDC 1521068−1521074 containing X-ray crystallographic data (CIF) Cartesian coordinates for calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Warren E. Piers: 0000-0003-2278-1269 Laurent Maron: 0000-0003-2653-8557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by NSERC of Canada (Discovery Grant) the Canada Research Chair secretariat (Tier I CRC 2013−2020) to W.E.P. The Alexander von Humboldt 4166

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Inorganic Chemistry

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