Bis(iminoxolene)iridium complexes over four oxidation states: from sodium–iridium bonding to ligand-centered radicals

Abstract

The novel iminoquinone N-(2,6-dibromophenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone (Briq), with electron-withdrawing and only mildly bulky 2,6-dibromophenyl substituents, is metalated by the iridium(I) complex {(coe)₂IrCl}₂ to give an inseparable mixture of (Briq)₂Ir, (Briq)₂IrCl, and (Briq)₂IrCl₂. This mixture can be funneled into a single pure compound either by exhaustive oxidation with iodobenzene dichloride to give (Briq)₂IrCl₂, or by exhaustive reduction by sodium naphthalenide to give (Briq)₂IrNa(THF)₂. The latter compound can be oxidized by one electron to give four-coordinate (Briq)₂Ir, or undergo two-electron oxidative addition reactions with iodine or iodomethane to yield (Briq)₂IrI or (Briq)₂IrCH₃. Structural data in the four different oxidation states indicate that each one-electron redox step is delocalized across the metal and iminoxolene ligand, with the redox changes becoming progressively more ligand-centered as the overall oxidation state increases. In the solid state, (Briq)₂IrNa(THF)₂ has a covalent bond between sodium and iridium (d = 2.9754(8) Å). The iridium–sodium bond partially heterolyzes in solution (ΔG° = 1.3 kcal mol⁻¹ in THF-d₈), and the four-coordinate anion has been characterized in the solid as the cobaltocenium salt. The iridium–sodium bond dissociation free energy is 74.3 kcal mol⁻¹.

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Introduction

Iminoxolene ligands have attracted interest as ancillary ligands because of the ready accessibility of multiple oxidation states (amidophenoxide/iminosemiquinone/iminoquinone).^1,2 The oxidation states differ in the occupancy of the molecular orbital dubbed the redox-active orbital (RAO) (Fig. 1): In the amidophenoxide, it is doubly occupied and constitutes the HOMO; in the iminosemiquinone, it is the SOMO; and in the iminoquinone, it is the LUMO. All three redox states are accessible because the RAO is moderate in energy. The RAO is thus close in energy to transition metal d orbitals, particularly those of elements in the middle of the d block. The close energy match and good spatial overlap of the RAO with d orbitals fosters highly covalent metal–ligand π bonding,^3,4 which can result in unusual patterns of reactivity.⁵

Fig. 1 Redox-active orbital of N-phenyl-2-imino-o-benzoquinone (Kohn–Sham LUMO).

We recently reported the preparation of a number of bis(iminoxolene)iridium complexes of the sterically hindered iminoxolene ligand Diso, N-(2,6-diisopropylphenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone.⁴ The complexes prepared spanned four oxidation levels, from high-valent (Diso)₂IrCl₂ to low-valent anionic Na[(Diso)₂Ir].⁶ We sought to assess how the steric bulk of the 2,6-diisopropylphenyl groups affected the synthesis and properties of the compounds by replacing the isopropyl substituents on the N-aryl group with smaller (and more oxidation-resistant) bromo substituents. Here we report that this novel iminoxolene ligand N-(2,6-dibromophenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone, abbreviated Briq, can support the same range of oxidation states in bis(iminoxolene)iridium compounds as its bulkier analogue Diso. The Briq ligand does significantly alter the synthetic chemistry, forming multiple products on metalation with iridium. It also fosters unusual covalent sodium–iridium bonding in the lowest-valent bis(iminoxolene)iridium complex.

Experimental

General procedures

Except as noted, all procedures were carried out in a nitrogen-filled glovebox to exclude air and moisture. Deuterated solvents were obtained from Cambridge Isotope Laboratories. PhICl₂ was prepared as described in the literature.⁷ Sodium naphthalenide was prepared as a THF solution by stirring naphthalene with 1.3 equiv. Na under N₂ for 3 h, then filtering through Celite. All other reagents were commercially available and used without further purification. NMR spectra were measured on a Bruker Avance DPX-400 or -500 spectrometer. Chemical shifts for ¹H and ¹³C are reported in ppm downfield of TMS, with spectra referenced using the chemical shifts of the solvent residuals. Infrared spectra were recorded on a Nicolet 380 FT-IR spectrometer or a Bruker Alpha II FT-IR spectrometer. UV-visible spectra were measured in 1 cm cells on an Agilent 8453 diode array spectrophotometer. Temperatures below ambient were controlled using a Unisoku CoolSpek cryostat. Near-IR spectra were acquired on a JASCO V-770 spectrophotometer. Cyclic voltammograms were performed using an Autolab potentiostat (PGSTAT 128N), with glassy carbon working and counter electrodes and a silver/silver chloride reference electrode. The electrodes were connected to the potentiostat through electrical conduits in the drybox wall. Samples were run with 0.1 M Bu₄NPF₆ as the electrolyte. Potentials were referenced to ferrocene/ferrocenium at 0 V (ref. 8) with the reference potential established by spiking the test solution with a small amount of ferrocene or decamethylferrocene (E° = –0.565 V vs. Cp₂Fe⁺/Cp₂Fe in CH₂Cl₂ ⁹). EPR spectra were measured on 1 : 1 toluene : dichloromethane glasses at 13 K using a Bruker EMX X-band spectrometer at 9.365 GHz. Elemental analyses were performed by Robertson Microlit (Ledgwood, NJ, USA) or Midwest Microlab (Indianapolis, IN, USA).

Syntheses

N-(2,6-Dibromophenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone (Briq). Into a screw-cap vial are weighed 0.9066 g 2,6-dibromoaniline (3.61 mmol) and 0.8731 g 3,5-di-tert-butyl-1,2-benzoquinone (3.96 mmol, 1.1 equiv.). After adding 4 mL glacial acetic acid, the vial is securely capped and shaken until the solids dissolve. After standing at room temperature for 6 days, the precipitated solid is suction filtered, washed with 2 × 2 mL methanol, and air-dried 30 min to afford a first crop of the iminoquinone. A second crop is obtained from the filtrate in the same manner after an additional 7 d at room temperature. After collecting the second crop, the solvent is removed from the filtrate on a rotary evaporator and the residue taken up in 4 mL CH₃OH. After standing for a day, the precipitated solid is isolated as described above. The total yield of the three combined crops is 0.9138 g (56%). The compound is a 53 : 47 mixture of isomers in CDCl₃. ¹H NMR (CDCl₃): major isomer: δ 1.12, 1.33 (s, 9H each, ^tBu), 5.83, 6.98 (d, 2 Hz, 1H each, iminoquinone 3,5-H), 6.86 (t, 8 Hz, 1H, Ar 4-H), 7.55 (d, 8 Hz, 2H, Ar 3,5-H). Minor isomer: 1.18, 1.27 (s, 9H each, ^tBu), 6.65, 6.96 (d, 2 Hz, 1H each, iminoquinone 3,5-H), 6.78 (t, 8 Hz, 1H, Ar 4-H), 7.51 (d, 8 Hz, 2H, Ar 3,5-H). ¹³C{¹H} NMR (CDCl₃, both isomers): δ 28.44, 28.62, 29.42, 29.61 (C[CH₃]₃), 35.33, 35.57, 35.70, 35.78 (C[CH₃]₃), 110.17, 113.05, 115.13, 124.10, 124.53, 126.29, 132.13, 132.16, 133.80, 134.33, 147.43, 147.91, 149.14, 150.34, 155.45, 155.49, 158.26, 158.62, 179.41 (C=O), 183.81 (C=O). IR (ATR, cm⁻¹): 2968 (m), 1664 (s, ν_C=O), 1626 (m), 1578 (w), 1560 (m), 1547 (w), 1479 (w), 1419 (s), 1394 (w), 1377 (m), 1284 (w), 1267 (w), 1250 (m), 1215 (m), 1196 (w), 1146 (w), 1109 (w), 1088 (w), 1072 (w), 1024 (w), 968 (w), 931 (w), 897 (s), 862 (m), 833 (w), 798 (m), 789 (w), 764 (vs), 748 (m), 739 (w), 729 (s), 669 (m). Anal. calcd for C₂₀H₂₃Br₂NO: C, 53.00; H, 5.12; N, 3.09. Found: C, 53.32; H, 5.31; N, 3.09.

trans-(Briq)₂IrCl₂. A 20 mL scintillation vial is charged with 0.6094 g {(coe)₂IrCl}₂ (0.680 mmol), 1.2832 g Briq (2.83 mmol, 4.16 equiv.) and 6 mL dry C₆H₆. The vial is securely capped, shaken vigorously to dissolve the compounds and left to stand at room temperature for 1 week. The precipitate is then collected by vacuum filtration. The solids are washed with 3 × 5 mL CH₃OH before drying in vacuo overnight to afford 1.1405 g of a metalation mixture of (Briq)₂IrCl_x (x = 0, 1, 2, avg. x ≈ 1.09; 74%).

100.6 mg of this metalation mixture is treated with 13.1 mg (0.0477 mmol) of iodobenzene dichloride in 5 mL CH₂Cl₂ in a 20 mL vial. The vial is shaken until all solids are dissolved, and then removed from the drybox. After 3 h at room temperature, the vial is opened to the air and the solvent is removed on a rotary evaporator. The residue is slurried with 2 mL CH₃CN, and the solids are collected by vacuum filtration, washed with 2 × 5 mL CH₃CN, and air-dried 15 min to give 65.3 mg of (Briq)₂IrCl₂ (47% based on {(coe)₂IrCl}₂). ¹H NMR (CD₂Cl₂): δ 26.8 (v br), 13.49 (br, 18H, ^tBu), 1.84 (br, 18H, ^tBu), −4.19 (v br). IR (ATR, cm⁻¹): 3058 (w), 2959 (m), 2903 (w), 2866 (w), 2662 (w), 1590 (w), 1549 (w), 1519 (m), 1475 (m), 1427 (m), 1415 (m), 1395 (w), 1348 (s), 1283 (s), 1253 (s), 1230 (s), 1199 (s), 1181 (s), 1097 (s), 1072 (m), 1022 (s), 990 (s), 933 (m), 911 (s), 886 (s), 852 (s), 827 (w), 775 (s), 759 (m), 733 (s), 724 (s), 700 (m), 658 (m), 648 (m), 629 (w), 613 (s), 540 (s), 509 (s), 496 (s), 443 (m), 416 (s). UV-Vis-NIR (CCl₄): λ_max = 2060 nm (ε = 8700 L mol⁻¹ cm⁻¹), 1693 (5250), 1406 (7300), 1155 (1670), 788 (8000), 716 (sh, 6500), 605 (13 200), 410 (8040), 318 (12 000). CV (CH₂Cl₂): 0.37, −0.38 V. Anal. calcd for C₄₀H₄₆Br₄Cl₂IrN₂O₂: C, 41.08; H, 3.96; N, 2.40. Found: C, 39.26; H, 3.71; N, 2.07.

(Briq)₂IrNa(THF)₂. The metalation mixture of (Briq)₂IrCl_x prepared as described above (0.5158 g) is dissolved in 10 mL dry THF in a 20 mL vial. To this blue solution is added 1.7 mL of a 1.00 M solution of sodium naphthalenide in THF. The vial is capped and shaken vigorously. The reaction mixture is left to stand at room temperature for 30 min before the mixture is filtered through a plug of sand to remove NaCl. The solvent is then removed from the filtrate on the vacuum line. The solid residue is slurried in 5 mL hexanes, and the dark blue suspension filtered through a glass frit. The collected solids are washed with 2 × 5 mL hexanes before drying in vacuo for 30 min to afford 0.3405 g (Briq)₂IrNa(THF)₂ as dark blue solids (44% yield based on {(coe)₂IrCl}₂). ¹H NMR (CD₃CN): δ 8.05 (d, 8.1 Hz, 4H, NAr 3- and 5-H), 7.28 (t, 7.9 Hz, 2H, NAr 4-H), 6.67 (d, 2.2 Hz, 2H, iminoxolene 3- or 5-H), 5.47 (d, 2.2 Hz, 2H, iminoxolene 3- or 5-H), 1.86 (s, 18H, ^tBu), 1.15 (s, 18H, ^tBu). ¹³C{¹H} NMR (CD₃CN): δ 170.11 (iminoxolene CO), 150.97, 140.64, 133.67, 133.65, 133.59, 131.26, 128.25, 115.85, 111.08, 66.35 (THF α-C), 36.02 (C(CH₃)₃), 34.24 (C(CH₃)₃), 32.73 (C(CH₃)₃), 30.79 (C(CH₃)₃), 26.31 (THF β-C). IR (ATR, cm⁻¹): 2950 (m), 2900 (m), 2865 (m), 1550 (m), 1457 (m), 1427 (s), 1361 (m), 1301 (s), 1256 (s), 1234 (m), 1201 (s), 1174 (s), 1110 (w), 1039 (m), 1024 (m), 996 (m), 926 (m), 903 (m), 876 (w), 856 (m), 827 (w), 766 (s), 723 (s), 673 (w), 651 (w), 632 (m), 614 (w), 546 (m), 507 (m), 460 (w), 411 (m). UV-Vis (THF, 25 °C): λ_max = 810 nm (ε = 4650 L mol⁻¹ cm⁻¹), 664 nm (15 300 L mol⁻¹ cm⁻¹). Anal. calcd for C₄₈H₆₂Br₄IrN₂NaO₄: C, 45.54; H, 4.94; N, 2.21. Found: C, 45.53; H, 4.59; N, 2.25.

[Cp₂Co][(Briq)₂Ir]. To a 20 mL vial, 307.7 mg of the metalation mixture of (Briq)₂IrCl_x prepared as described above, 128.2 mg cobaltocene (0.678 mmol, 2.5 equiv. per Ir) and 10 mL benzene are added. The vial is capped and shaken vigorously, then left to stand at room temperature for 3 d. The precipitated solids are collected by suction filtration, washed with 2 × 5 mL C₆H₆ followed by 3 × 4 mL CH₃OH, and dried in vacuo overnight to afford 306.2 mg [CoCp₂][(Briq)₂Ir] as a fine turquoise powder (65% based on {(coe)₂IrCl}₂). ¹H NMR (CD₃CN): δ 8.04 (d, 8.0 Hz, 4H, NAr 3- and 5-H), 7.27 (t, 8.1 Hz, 2H, NAr 4-H), 6.67 (d, 2.2 Hz, 2H, iminoxolene 3- or 5-H), 5.66 (s, 10H, Cp₂Co), 5.47 (d, 2.2 Hz, 2H, iminoxolene 3- or 5-H), 1.86 (s, 18H, ^tBu), 1.14 (s, 18H, ^tBu). IR (ATR, cm⁻¹): 3097 (w), 2950 (m), 2902 (w), 2861 (w), 1548 (w), 1457 (w), 1422 (s), 1390 (w), 1359 (w), 1301 (s), 1277 (m), 1260 (s), 1203 (m), 1174 (w), 1141 (w), 1117 (w), 1076 (w), 1026 (w), 998 (m), 926 (m), 903 (m), 864 (m), 852 (m), 827 (w), 766 (s), 723 (s), 706 (w), 675 (w), 649 (w), 612 (w), 564 (w), 544 (m), 517 (w), 507 (w), 454 (s), 411 (w). UV-vis (THF): λ_max = 800 nm (ε = 8580 L mol⁻¹ cm⁻¹), 680 (10 670). Anal. calcd for C₅₀H₄₆Br₄CoIrN₂O₂: C, 46.63; H, 4.38; N, 2.18. Found: C, 46.46; H, 4.22; N, 2.20.

(Briq)₂Ir. A 20 mL scintillation vial is charged with 87.2 mg (0.0689 mmol) (Briq)₂IrNa(THF)₂ and 24.1 mg (0.0938 mmol, 1.36 equiv.) silver trifluoromethanesulfonate in 6 mL benzene. The reaction mixture is then shaken vigorously and sealed before the vial is removed from the drybox and placed in a 60 °C oil bath overnight. The vial is brought back into the drybox and 5 mL CH₂Cl₂ is added. The resulting mixture is filtered through Celite and the solvent is removed from the filtrate on the vacuum line. The residue is suspended in 5 mL CH₃OH for 1.5 h before the mixture is filtered through a fritted glass funnel. The collected solids are washed with 2 × 5 mL CH₃OH before drying in vacuo overnight to afford 42.7 mg of (Briq)₂Ir (56%) as a dark blue powder. ¹H NMR (C₆D₆): 10.50 (br, 18H, ^tBu), 0.75 (br, 2H, Ar 4-H), −1.57 (br, 4H, Ar 3,5-H), −2.40 (v br, 18H, ^tBu). IR (ATR): 2954 (s), 2904 (m), 2865 (m), 1548 (m), 1519 (w), 1459 (w), 1441 (m), 1422 (s), 1383 (m), 1359 (s), 1320 (m), 1303 (m), 1258 (s), 1232 (m), 1199 (s), 1178 (m), 1160 (s), 1108 (m), 1076 (w), 1026 (m), 996 (m), 918 (m), 903 (w), 852 (m), 825 (w), 766 (s), 720 (s), 659 (w), 649 (m), 630 (m), 542 (m), 509 (m), 413 (w). UV-Vis (CH₂Cl₂): λ_max = 807 nm (ε = 15 400 L mol⁻¹ cm⁻¹), 734 (12 100), 648 (23 300). Anal. calcd for C₄₀H₄₆Br₄IrN₂O₂: C, 43.73; H, 4.22; N, 2.55. Found: C, 42.10; H, 3.94; N, 2.38.

(Briq)₂IrI. 141.7 mg of (Briq)₂IrNa(THF)₂ (0.1119 mmol) is dissolved in 15 mL dry THF in a 100 mL round-bottom flask. A stir bar is placed in the flask and the flask is capped with a rubber septum and removed from the drybox. Into the rapidly stirring solution is injected a solution of 32.3 mg I₂ (0.127 mmol, 1.13 equiv.) in 10 mL dry THF. The deep purple solution is stirred for 5 min at room temperature before the flask is opened to air and the solvent removed on a rotary evaporator. The solid residue is dissolved in 15 mL CH₂Cl₂. The solution is washed with 10 mL 0.1 M aqueous thiosulfate and 10 mL water, then dried over anhydrous MgSO₄. After removing the dichloromethane on the rotary evaporator, the solid residue is collected to afford 102.7 mg of (Briq)₂IrI (75%). ¹H NMR (CD₂Cl₂): δ 8.00 (dd, 8.2, 1.3 Hz, 2H, Ar 3- or 5-H), 7.66 (dd, 8.1, 1.2 Hz, 2H, Ar 3- or 5-H), 7.25 (t, 8.1 Hz, 2H, Ar 4-H), 7.12 (d, 2.0 Hz, 2H, iminoxolene 3- or 5-H), 6.62 (d, 2.0 Hz, 2H, iminoxolene 3- or 5-H), 1.38 (s, 18H, ^tBu), 1.27 (s, 18H, ^tBu). ¹³C{¹H} NMR (CD₂Cl₂): δ 180.04 (CO), 152.29, 147.46, 141.75, 138.53, 133.32, 132.77, 129.87, 124.84, 124.52, 120.79, 113.74, 35.59 (C(CH₃)₃), 35.14 (C(CH₃)₃), 30.97 (C(CH₃)₃), 29.84 (C(CH₃)₃). IR (ATR, cm⁻¹): 2956 (m), 2904 (w), 2865 (w), 1544 (m), 1457 (m), 1443 (m), 1425 (s), 1394 (m), 1383 (m), 1359 (s), 1320 (m), 1283 (m), 1250 (s), 1230 (s), 1197 (s), 1172 (s), 1108 (s), 1074 (w), 1026 (m), 1002 (m), 918 (m), 854 (m), 825 (w), 772 (s), 727 (s), 667 (s), 649 (w), 632 (w), 620 (w), 544 (m), 511 (m), 413 (m). UV-Vis (CH₂Cl₂): λ_max = 756 nm (ε = 24 300 L mol⁻¹ cm⁻¹), 535 (12 100). Anal. calcd for the acetone solvate C₄₃H₅₂Br₄IIrN₂O₃: C, 40.24; H, 4.08; N, 2.18. Found: C, 40.14; H, 4.20; N, 2.00.

(Briq)₂IrCH₃. In a 20 mL scintillation vial, 97.1 mg of the metalation mixture (Briq)₂IrCl_x, 51.1 mg Cp₂Co (0.270 mmol, 3.2 equiv.) and 8 mL THF are combined. The vial is capped and shaken to give a dark blue solution, to which 8.5 μL (0.137 mmol, 1.60 equiv. per Ir) of CH₃I is added. The reaction is stirred at room temperature overnight before the vial is removed from the drybox. The reaction mixture is opened to the air and filtered through a plug of silica, eluting with THF. The dark purple eluent is collected and evaporated to dryness. The residue is slurried with 5 mL of methanol and the solids collected by suction filtration, washed with an additional 5 mL CH₃OH and air-dried for 30 min to yield 69.0 mg of (Briq)₂IrCH₃ as dark purple solids (72%). ¹H NMR (CD₂Cl₂): δ 7.91 (dd, 8.2, 1.2 Hz, 2H, NAr 3- or 5-H), 7.76 (dd, 8.1 Hz, 1.3 Hz, 2H, NAr 3- or 5-H), 7.21 (t, 8.1 Hz, 2H, NAr 4-H), 6.72 (d, 2.1 Hz, 2H, iminoxolene 3- or 5-H), 6.47 (d, 2.1 Hz, 2H, iminoxolene 3- or 5-H), 2.99 (s, 3H, IrCH₃), 1.31 (s, 18H, ^tBu), 1.27 (s, 18H, ^tBu). ¹³C{¹H} NMR (CD₂Cl₂): δ 178.83 (iminoxolene CO), 149.76, 146.19, 140.38, 136.51, 133.06, 132.82, 129.26, 127.89, 123.27, 122.65, 112.53, 35.55 (C(CH₃)₃), 34.75 (C(CH₃)₃), 31.54 (C(CH₃)₃), 29.63 (C(CH₃)₃), −17.40 (IrCH₃). IR (ATR, cm⁻¹): 3074 (w), 2953 (m), 2903 (m), 2864 (m), 1763 (w), 1590 (w), 1544 (m), 1530 (m), 1477 (w), 1459 (m), 1441 (m), 1423 (s), 1382 (m), 1360 (m), 1308 (m), 1279 (w), 1258 (m) 1246 (m), 1226 (s), 1194 (s), 1173 (s), 1146 (m), 1108 (s), 1075 (m), 1022 (s), 1001 (m), 924 (m), 904 (m), 897 (m), 884 (m), 853 (s), 847 (m), 825 (m), 767 (s), 770 (s), 732 (m), 724 (s), 670 (m), 650 (m), 633 (m), 616 (w), 568 (w), 546 (m), 539 (m), 516 (m), 508 (m), 443 (w), 414 (m). UV-Vis (CH₂Cl₂): λ_max = 800 nm (ε = 52 200 L mol⁻¹ cm⁻¹), 543 (2800), 474 (2100), 400 (1600). CV (CH₂Cl₂): 0.80, 0.21, −1.28 V. Anal. calcd for C₄₁H₄₉Br₄IrN₂O₂: C, 44.22; H, 4.43; N, 2.52. Found: C, 43.90; H, 4.85; N, 2.39.

Analysis of thermodynamic parameters for Ir–Na heterolysis by variable-temperature NMR spectroscopy

A solution of (Briq)₂IrNa(THF)₂ was prepared in THF-d₈ in the drybox and sealed in an NMR tube fitted with a Teflon-lined screw cap. ¹H NMR spectra were acquired in the range of 183–333 K. The data were fit to a model which included a linear temperature dependence of the chemical shifts of the two iminoxolene protons of the two species (eqn (1)). The actual chemical shifts were then calculated using the mole fractions of the bonded form (b) and the ion-paired form (ip) based on the equilibrium constant calculated from the modeled ΔH° and ΔS° of reaction (eqn (2)–(3)).

δ_species = δ_species,0C + α_species(T − 273.15 K) (1)

K = e^{(−ΔH°+TΔS°)/RT} (3)

The ten parameters (δ_0C and α for each resonance and each species, and ΔH° and ΔS°) were optimized using unweighted least-squares fitting as implemented in the Solver routine of Microsoft Excel;¹⁰ the spreadsheet used is included in the ESI.† Uncertainties in the fitted parameters were estimated as described in the literature.¹¹

X-ray crystallography

Crystals were placed in inert oil before transferring to the N₂ cold stream of a Bruker Apex II CCD diffractometer. Data were reduced, correcting for absorption, using the program SADABS. Calculations used SHELXTL (Bruker AXS),¹² with scattering factors and anomalous dispersion terms taken from the literature.¹³

In (Briq)₂IrCH₃, there was a small amount of a component where the iridium was located about 0.4 Å away from the main Ir site on the vector away from the iridium–methyl bond. Only the Ir center of this component was modeled; it refined to 2.8(2)% occupancy. The lattice methanol in (Briq)₂Ir·CH₃OH was disordered about an inversion center and was refined isotropically. All hydrogen atoms were found on difference Fourier maps and refined isotropically, with the exception of those in (Briq)₂IrI·C₃H₆O, the IrCH₃ group in (Briq)₂IrCH₃, and the methanol in (Briq)₂IrCl₂·CH₃OH. These hydrogens were placed in calculated positions and refined with their isotropic thermal parameters tied to the atom to which they are attached (1.5× for CH₃ groups, 1.2× for all others). Further details about the structures are in Tables 1–2.

Table 1 Summary of crystal data

	Briq	trans-(Briq)2IrCl2·CH3OH	(Briq)2IrNa(THF)2	[Cp2Co][(Briq)2Ir]·2 C6H6	(Briq)2Ir·2 CHCl3	(Briq)2IrI·(CH3)2CO	(Briq)2IrCH3
Formula	C20H23Br2NO	C41H50Br4Cl2IrN2O3	C48H62Br4IrN2NaO4	C62H68Br4CoIrN2O2	C42H48Br4Cl6IrN2O2	C43H52Br4IIrN2O3	C41H49Br4IrN2O2
Formula mass	453.21	1201.57	1265.82	1443.95	1337.36	1283.60	1113.66
T/K	120(2)	120(2)	120(2)	120(2)	120(2)	120(2)	120(2)
Crystal system	Monoclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic	Orthorhombic	Monoclinic
Space group	P21/c	P1̄	C2/c	P21/c	P21/c	P212121	P21/c
λ/Å	0.71073 (Mo Kα)	0.71073 (Mo Kα)	1.54178 (Cu Kα)	0.71073 (Mo Kα)	0.71073 (Mo Kα)	0.71073 (Mo Kα)	0.71073 (Mo Kα)
Total data collected	36 428	21 734	119 244	205 171	32 230	113 473	107 470
No. of indep reflns.	4811	5631	9632	14 275	5950	12 542	12 812
R int	0.0211	0.0148	0.0427	0.0847	0.0308	0.0666	0.0476
Obsd refls [I > 2σ(I)]	4350	5410	9432	11 355	5245	11 573	10 938
a/Å	12.1021(17)	10.2904(14)	29.8280(13)	13.7924(5)	15.6386(4)	10.202(2)	14.4389(12)
b/Å	11.6583(16)	11.1541(15)	18.3608(13)	19.3444(6)	10.6069(3)	15.082(3)	10.0550(8)
c/Å	13.935(2)	11.6974(16)	19.4665(10)	22.2366(8)	14.7633(4)	29.489(6)	29.257(2)
α/°	90	65.495(2)	90	90	90	90	90
β/°	99.375(2)	67.090(2)	114.354(2)	104.2700(10)	101.7892(9)	90	101.060(2)
γ/°	90	80.666(2)	90	90	90	90	90
V/Å^3	1939.8(5)	1125.3(3)	9712.5(10)	5749.8(3)	2397.24(11)	4537.4(16)	4168.8(6)
Z	4	1	8	4	2	4	4
μ mm^−−1	4.185	6.673	9.609	5.424	6.490	7.184	7.072
Crystal size/mm	0.22 × 0.32 × 0.41	0.11 × 0.28 × 0.29	0.05 × 0.11 × 0.12	0.08 × 0.11 × 0.22	0.13 × 0.14 × 0.18	0.12 × 0.14 × 0.20	0.08 × 0.16 × 0.23
No. refined params	309	334	789	873	355	487	640
R1, wR2 [I > 2σ(I)]	0.0210, 0.0524	0.0167, 0.0407	0.0169, 0.0422	0.0317, 0.0575	0.0214, 0.0439	0.0375, 0.0971	0.0288, 0.0587
R1, wR2 [all data]	0.0251, 0.0542	0.0180, 0.0414	0.0176, 0.0424	0.0501, 0.0631	0.0279, 0.0459	0.0423, 0.0991	0.0398, 0.0617
Goodness of fit	1.041	1.037	1.119	1.013	1.054	1.058	1.095

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Table 2 Selected distances, angles, and metrical oxidation states of iminoxolene ligands of Briq, (Briq)₂IrNa(THF)₂, [Cp₂Co][(Briq)₂Ir], (Briq)₂Ir, (Briq)₂IrCH₃, (Briq)₂IrI, and trans-(Briq)₂IrCl₂. Values given are the average of chemically equivalent measurements, with esd's reflecting both the variance in the measured values and the statistical uncertainty of the crystallographic model

	Briq	(Briq)2IrNa(THF)2	[Cp2Co][(Briq)2Ir]	(Briq)2Ir	(Briq)2IrCH3	(Briq)2IrI	trans-(Briq)2IrCl2
Distances/Å
Ir–O1		1.993(2)	2.001(13)	1.9548(16)	1.992(3)	1.997(10)	2.0022(14)
Ir–N1		1.933(3)	1.935(6)	1.936(2)	1.954(3)	1.952(10)	2.0090(15)
Ir–X		2.9754(8)			2.105(4)	2.6322(9)	2.3371(5)
O1−C11	1.2134(18)	1.330(6)	1.330(4)	1.343(3)	1.320(4)	1.312(12)	1.297(2)
N1−C12	1.2863(18)	1.391(6)	1.389(4)	1.386(3)	1.375(7)	1.358(10)	1.338(2)
C11−C12	1.531(2)	1.418(4)	1.417(7)	1.416(3)	1.418(4)	1.417(13)	1.450(3)
C12−C13	1.4469(19)	1.401(3)	1.393(4)	1.396(3)	1.405(4)	1.416(19)	1.419(3)
C13−C14	1.3456(19)	1.390(4)	1.389(6)	1.391(3)	1.375(4)	1.377(16)	1.364(3)
C14−C15	1.4686(19)	1.418(3)	1.417(5)	1.405(4)	1.420(4)	1.422(12)	1.441(3)
C15−C16	1.3474(19)	1.388(3)	1.389(5)	1.395(4)	1.383(5)	1.380(13)	1.374(3)
C11−C16	1.4824(19)	1.414(4)	1.414(4)	1.406(3)	1.421(5)	1.423(12)	1.427(3)
Angles/°
O1−Ir–N1		79.96(14)	80.08(16)	81.19(8)	80.11(9)	79.5(6)	79.08(6)
O1−Ir–O2		179.23(5)	178.92(9)	180	178.85(10)	174.5(3)	180
N1−Ir–N2		175.77(6)	178.04(10)	180	168.21(11)	157.9(3)	180
Metrical oxidation state (MOS) ^14	+0.28(7)	–1.52(8)	–1.54(7)	–1.67(2)	–1.33(6)	–1.21(7)	–0.83(4)

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Results and discussion

Synthesis and metalation of Briq

N-Aryl-3,5-di-tert-butyl-2-imino-o-benzoquinones with 2,6-disubstituted aryl groups have typically been prepared by the carboxylic acid-catalyzed condensation of the aniline with commercially available 3,5-di-tert-butyl-o-benzoquinone.¹⁵ This method is successful with 2,6-dibromoaniline (eqn (4)), but the reaction is quite slow, requiring weeks at room temperature in neat acetic acid. The slow rate is attributed to the electron-withdrawing character of the bromo substituents, as the sterically similar 2,6-dimethylaniline is reported to react in hours at −20 °C in methanol with traces of formic acid as catalyst.¹⁵ Slow condensation was also observed with 2,6-bis(triisopropylsilylethynyl)aniline and was attributed to the electron-poor nature of the sp-hybridized carbon substituents.¹⁶

In the solid state, Briq is observed as the E isomer (Fig. 2), though it exists as a nearly equimolar mixture of E and Z isomers in solution. The most noteworthy feature of its structure is the appreciable twisting of the central six-membered ring out of planarity, as indicated for example by the N1–C12–C11–O1 dihedral angle of 15.7°. The other three structurally characterized N-aryl-o-benzoquinoneimines are similar, with ϕ = 10.8° for Ar = 2,6-ⁱPr₂C₆H₃,¹⁷ 15.9° for Ar = 4-CF₃C₆H₄,¹⁸ and 17.2° for Ar = 2,6-(3,5-(CF₃)₂C₆H₃)₂C₆H₃.¹⁹ The twisting and bond localization (Table 2) are reasonable given the formally antiaromatic 4π electron character of the central ring. The N-aryl substituent is significantly out of the mean plane of the iminoquinone ring, with a C12–N1–C21–C26 dihedral angle of 76.7°. This is larger than the corresponding angle for the 4-trifluoromethylphenyl group (51.1°)¹⁸ but smaller than the value for the 2,6-diisopropylphenyl group (86.4°),¹⁷ consistent with the steric profile of the bromo substituent being intermediate between hydrogen and isopropyl.

Fig. 2 Thermal ellipsoid plot of Briq (50% ellipsoids), with hydrogen atoms omitted for clarity.

Treatment of benzene solutions of the iminoquinone with the iridium(I) precursor {(coe)₂IrCl}₂ (coe = η²-cyclooctene) results in ligation of Briq to iridium and complete displacement of cyclooctene. However, in contrast to the analogous reaction of Diso, which gives diamagnetic (Diso)₂IrCl in high yield, the reaction of Briq gives a mixture of diamagnetic (Briq)₂IrCl and paramagnetic (Briq)₂Ir and (Briq)₂IrCl₂ (Scheme 1). A typical mixture has the three compounds in a ratio of 1.9 : 1 : 1.4 (average composition (Briq)₂IrCl_1.09), though this ratio varies considerably from sample to sample. The distribution of compounds appears to be kinetically controlled, as it is unchanged on heating the isolated metalation mixture.

Scheme 1 Metalation of Briq and subsequent normalization of oxidation states of the metalation mixture.

We have not been able to separate the components of this mixture. To obtain pure compounds, we adopted a strategy of normalizing the oxidation states by exhaustive oxidation. Thus, trans-(Briq)₂IrCl₂ is isolated in 47% yield based on {(coe)₂IrCl}₂ by treatment of the metalation mixture with excess PhICl₂ (Scheme 1). The broadness and paucity of peaks in the ¹H NMR spectrum of this paramagnetic substance makes it useless for determining stereochemistry, but the similarity of the compound's UV-Vis-NIR spectrum (Fig. S20†) to that of trans-(Diso)₂IrCl₂ and its solid-state structure (Fig. 3) confirm the trans geometry.

Fig. 3 Thermal ellipsoid plot (50% ellipsoids) of trans-(Briq)₂IrCl₂·CH₃OH, with lattice solvent and hydrogen atoms omitted for clarity.

Because the electron density on the iminoxolene ligand in the redox-active orbital affects the intraligand bond lengths, analysis of these bond lengths can be used to determine an apparent or metrical oxidation state (MOS) of the iminoxolene.¹⁴ Such an analysis on (Briq)₂IrCl₂ gives a MOS of −0.83(4) for each iminoxolene group, corresponding to a nominal oxidation state of iridium of close to +4. Nevertheless, the unpaired electron appears to be in a ligand-centered orbital, judging from the EPR spectrum (Fig. 4a), which shows a rhombic signal with little anisotropy (g = 2.0533, 1.9963, 1.9672; g_iso = 2.0059) and no resolvable hyperfine coupling. This is consistent with the SOMO being the A_u-symmetry combination of ligand RAOs, which corresponds to the HOMO of octahedral compounds such as trans-(Diso)₂IrCl(py).²⁰

Fig. 4 X-band EPR spectrum of (a) (Briq)₂IrCl₂ and (b) (Briq)₂Ir (13 K, CH₂Cl₂/toluene glass).

Preparation of (Briq)₂IrNa(THF)₂ and the nature of the sodium–iridium bonding

The oxidation states of the metalation mixture can also be normalized by exhaustive reduction. Thus, treatment of the metalation mixture with excess sodium naphthalenide results in formation of (Briq)₂IrNa(THF)₂ (Scheme 1).

Remarkably, the complex in the solid state (Fig. 5) features a direct sodium–iridium bond (d = 2.9754(8) Å, Table 2). Alkali metal–iridium bonds are very rare. This appears to be the first example of a structurally characterized sodium–iridium bond, and the only alkali metal–iridium bond previously observed in the solid state is the potassium–iridium bond of 3.299(3) Å in K[(bpa–2H)Ir(cod)].²¹ The potassium compound is a coordination polymer, with the potassium bonded in an η⁵ fashion to the IrN₂C₂ rings of two doubly deprotonated dipicolylamine ligands; the K–N and K–C distances are all shorter than the K–Ir distance. In contrast, in (Briq)₂IrNa(THF)₂, the Na atom is canted toward one of the iminoxolene oxygens (O1), but the distance is very long (2.9842(16) Å), and the distances to the other atoms in the iminoxolene are all greater than 3.4 Å. Thus, the sodium is best considered bonded only to the iridium and not to the iminoxolenes. The coordination sphere of the sodium is completed by two bound THF molecules and two dative bonds from the o-Br substituents of the Briq ligand. The average Na–Br distance is similar to those observed in other species with sodium–bromoarene interactions (Table 3).²²

Fig. 5 Thermal ellipsoid plot (50% ellipsoids) of (Briq)₂IrNa(THF)₂, with hydrogen atoms omitted for clarity.

Table 3 Structural data regarding the sodium coordination in (Briq)₂IrNa(THF)₂

	Distance/Å		Angle/°
		Ir–Na–Br1	82.07(2)
Na–Ir	2.9754(8)	Ir–Na–Br4	83.11(2)
Na–Br1	3.1477(9)	Ir–Na–O5	131.21(5)
Na–Br4	3.0299(9)	Ir–Na–O6	107.84(5)
Na–O5	2.2930(16)	Br1−Na–Br4	155.32(3)
Na–O6	2.3149(19)	Br1−Na–O5	90.95(4)
		Br1−Na–O6	85.59(5)
		Br4−Na–O5	113.62(5)
		Br4−Na–O6	80.32(5)
		O5−Na–O6	119.73(6)

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Based on structural data, the sodium–iridium bond appears to have substantial covalency. The average unsupported metal–metal bond between neutral iridium(II) centers has a distance of 2.74(6) Å (9 examples).²³ Combined with the Na–Na distance in gas-phase Na₂ (3.0789 Å),²⁴ this would predict an Ir–Na covalent bond distance of 2.91(3) Å, within three standard deviations of the observed value. In contrast, the Na–Br distances in (Briq)₂IrNa(THF)₂ (3.09 Å avg) are much longer than the average between the homodiatomic molecules (2.68 Å), as expected for a dative bond.

If (Briq)₂IrCl_x is reduced in the absence of an alkali metal, for example by cobaltocene, then the ionic compound [Cp₂Co][(Briq)₂Ir], containing a square planar anion, is produced (Fig. 6). This compound is structurally analogous to that observed with the 2,6-diisopropylphenyl-substituted iminoxolene Diso, [Na(acetone)₆][(Diso)₂Ir].⁶ Presumably the greater bulk of the isopropyl group and the lack of a Lewis basic site near the iridium precludes the possibility of sodium–iridium bonding in the latter compound. The geometric parameters of the four-coordinate iridium center in [Cp₂Co][(Briq)₂Ir] are remarkably similar to those in (Briq)₂IrNa(THF)₂ (Table 2). In particular, the MOS of the Briq ligand in [Cp₂Co][(Briq)₂Ir], −1.54(7), is identical within experimental error to that in (Briq)₂IrNa(THF)₂.

Fig. 6 Thermal ellipsoid plot (50% ellipsoids) of [Cp₂Co][(Briq)₂Ir]·2 C₆H₆, with lattice solvents and hydrogen atoms omitted for clarity.

Consistent with the ease of formation of a four-coordinate anion, the sodium–iridium bond in (Briq)₂IrNa(THF)₂ appears to be susceptible to heterolysis in solution. In the nonpolar solvent toluene-d₈, ¹H NMR spectra of (Briq)₂IrNa(THF)₂ display C₂ symmetry up to 110 °C (Fig. S28†), consistent with retention of the Ir–Na bond in solution. In contrast, NMR spectra in THF-d₈ or CD₃CN show C_2h symmetry (Fig. S4†), consistent with either rapid Na⁺ exchange between faces of a bonded complex or net formation of a solvent-separated ion pair in solution. Variable-temperature UV-vis spectroscopy in THF (Fig. 7) indicates that there is a temperature-dependent equilibrium between the bonded and the ion-pair form (eqn (5)). The former is favored at higher temperatures and the latter at low temperatures, as judged by the similarity of the low-temperature spectra to Cp₂Co[(Briq)₂Ir] (Fig. S21†). ¹H NMR spectroscopy shows analogous behavior, with sigmoidal dependence of chemical shifts with temperature (Fig. 8), consistent with a temperature-dependent equilibrium. Quantitative analysis of the chemical shifts gives thermodynamic parameters for eqn (5) as ΔH° = –7.3(6) kcal mol⁻¹ and ΔS° = –29(2) cal mol⁻¹ K⁻¹. The decrease in entropy on ionization is expected, as the combined effect of creating ions (which orders the surrounding solvent more than that surrounding neutral solutes) and coordinating additional THF to the sodium ion (there are 5–6 molecules of THF in the first coordination sphere of sodium ion²⁵) outweighs the dissociation of the sodium from the iridium complex.

Fig. 7 Variable-temperature optical spectra of (Briq)₂IrNa(THF)₂ in THF.

Fig. 8 Chemical shifts of the iminoxolene ring protons of (Briq)₂IrNa(THF)₂ in THF-d₈. Solid lines are the best fit to eqn (1)–(3).

By combining the thermodynamics of heterolysis, the reduction potentials of (Briq)₂Ir and of Na⁺, and the energies of vaporization and solvation of Na, one can calculate the thermodynamic parameters for the homolysis of the Na–Ir bond in (Briq)₂IrNa(THF)₂ in THF solution (eqn (6); see ESI† for details). The free energy change of this reaction is the Na–Ir bond dissociation free energy (BDFE) in THF solution. The BDFE of 74.3 kcal mol⁻¹ is much larger than the free energy of heterolysis (1.3 kcal mol⁻¹), as expected given the difficulty of reducing Na⁺ to Na atoms.

Oxidations of (Briq)₂IrNa(THF)₂

Oxidation of the low-valent complex (Briq)₂IrNa(THF)₂ is a convenient way to make other bis(iminoxolene)iridium compounds in pure form. For example, treatment of (Briq)₂IrNa(THF)₂ with ferrocenium hexafluorophosphate or with silver trifluoromethanesulfonate affords the one-electron oxidation product, neutral (Briq)₂Ir (eqn (7)). In the reaction with silver(I) salts, a diamagnetic C₂-symmetric intermediate, tentatively assigned as (Briq)₂IrAg, can be observed by NMR at short reaction times at room temperature. Silver salts of weakly coordinating anions are known to form Ag–Ir bonds with Vaska's complex, where the square planar geometry of (Ph₃P)₂Ir(CO)Cl is only minimally perturbed by the binding of the Ir d_z² orbital to Ag.²⁶ A tris(isonitrile)iridium(−1) anion forms an unsupported Ir–Tl bond, whereas the potassium salt does not show metal–metal bonding.²⁷

Neutral (Briq)₂Ir is a four-coordinate, square planar monomer (Fig. 9). The metrical oxidation state of its iminoxolene ligands is similar to those in the sodium compound, or if anything slightly more reduced in the neutral compound (−1.67(2) vs. −1.52(8) in (Briq)₂IrNa(THF)₂). This suggests that oxidation takes place essentially only on the metal, consistent with an EPR spectrum that is consistent with a metal-centered radical (g = 3.756, 1.843, 1.636; Fig. 4b). These features, as well as its optical spectrum and paramagnetic NMR spectrum, are all similar to (Diso)₂Ir, which has been previously characterized as having its unpaired electron in a metal dπ orbital.⁴

Fig. 9 Thermal ellipsoid plot of (Briq)₂Ir·2 CHCl₃ (50% ellipsoids), with solvent molecules and hydrogen atoms omitted for clarity.

The sodium–iridium complex (Briq)₂IrNa(THF)₂ also undergoes net two-electron oxidative addition reactions. For example, it reacts with iodine to give square pyramidal (Briq)₂IrI (eqn (8)). This reaction likely proceeds via two one-electron steps, as neutral (Briq)₂Ir reacts with I₂ to give the five-coordinate iodide complex. The iridium-sodium complex, or more conveniently the cobaltocenium salt [Cp₂Co][(Briq)₂Ir], reacts with methyl iodide to afford the iridium methyl complex (Briq)₂IrCH₃ (eqn (9)). This likely occurs by a two-electron S_N2 pathway, as has been demonstrated for the analogous [(Diso)₂Ir]⁻.⁶

Structurally, both (Briq)₂IrI (Fig. 10) and (Briq)₂IrCH₃ (Fig. 11) are square pyramidal (Reedijk τ₅ values²⁸ of 0.28 and 0.18, respectively). The metrical oxidation states of the iminoxolene ligands (−1.21(7) and −1.33(6), respectively) are slightly different, suggestive of a more electron-rich iminoxolene in the alkyl complex. This pattern has previously been noted between bis(iminoxolene)iridium and -cobalt halide and alkyl complexes.⁶ The difference may be due to enhanced π donation from the iminoxolene orbitals to the Ir–X σ* orbital in the halides compared to the alkyls.

Fig. 10 Thermal ellipsoid plot of (Briq)₂IrI·(CH₃)₂CO (50% ellipsoids), with lattice solvent and hydrogen atoms omitted for clarity.

Fig. 11 Thermal ellipsoid plot of (Briq)₂IrCH₃ (50% ellipsoids), with hydrogen atoms omitted for clarity.

Overall, the iminoxolene ligands in the two (Briq)₂IrX compounds (average MOS = −1.27) are significantly oxidized compared to those in (Briq)₂Ir⁻ and (Briq)₂Ir (average MOS = −1.58). This is part of a consistent trend that as the overall oxidation state of the complex becomes more positive, the oxidations become increasingly ligand-centered. Thus, the one-electron oxidation from (Briq)₂Ir⁻ to (Briq)₂Ir is essentially localized on the metal; from (Briq)₂Ir to (Briq)₂IrX is about 60% ligand-centered; and from (Briq)₂IrX to (Briq)₂IrCl₂ is about 90% ligand-centered.

Conclusions

The 2,6-dibromophenyl-substituted iminoquinone Briq is metalated with an iridium(I) chloride precursor to give an inseparable mixture of (Briq)₂Ir, (Briq)₂IrCl, and (Briq)₂IrCl₂. This mixture can be transformed into a single oxidation state either by exhaustive oxidation (to give pure (Briq)₂IrCl₂) or exhaustive reduction (to give pure [(Briq)₂Ir]⁻). Compounds with intermediate oxidation states can be prepared from the anion by one-electron (to give (Briq)₂Ir) or two-electron (to give (Briq)₂IrI or (Briq)₂IrCH₃) redox reactions. In this way a suite of (Briq)₂Ir complexes in four oxidation states can be accessed. Structural data indicate that each successive oxidation becomes increasingly localized at the ligands, with the (Briq)₂Ir⁻/(Briq)₂Ir oxidation being essentially localized at the metal, while the (Briq)₂IrX/(Briq)₂IrCl₂ oxidation is about 90% ligand-centered. If exhaustive reduction is carried out using sodium naphthalenide, a compound containing a covalent sodium–iridium bond, (Briq)₂IrNa(THF)₂, is formed. The Na–Ir bond is retained in nonpolar organic solvents but dissociates readily in more polar solvents. A bond dissociation free energy of 74.3 kcal mol⁻¹ can be calculated for the sodium–iridium bond in THF solution.

Data availability

Crystallographic data have been deposited with the CCDC for Briq (accession number 2417993), (Briq)₂Ir·2CHCl₃ (2417994), (Briq)₂IrI·(CH₃)₂CO (2417995), (Briq)₂IrCH₃ (2417996), (Briq)₂IrNa(THF)₂ (2417997), trans-(Briq)₂IrCl₂·CH₃OH (2417998) and [Cp₂Co][(Briq)₂Ir]·2C₆H₆ (2417999).† Other data supporting this article are included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the US National Science Foundation (CHE-1955933 and CHE-2400019). We thank Dr Allen G. Oliver for his assistance with X-ray crystallography, and the NSF for support of the acquisition of diffractometers (MRI award CHE-2214606).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic, spectroscopic, and thermochemical details. CCDC 2417993–2417999. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00244c

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