Geometric and Electronic Structures of Nickel(II) Complexes of Redox

Article pubs.acs.org/IC

Geometric and Electronic Structures of Nickel(II) Complexes of Redox Noninnocent Tetradentate Phenylenediamine Ligands Jérémie Ciccione,† Nicolas Leconte,*,† Dominique Luneau,‡ Christian Philouze,† and Fabrice Thomas*,† †

Chimie Inorganique Redox, Département de Chimie Moléculaire (UMR CNRS 5250), Université Grenoble Alpes, UMR-5250, 38041 Grenoble Cedex 9, France ‡ Laboratoire des Multimatériaux et Interfaces (UMR CNRS 5615), Université Claude Bernard Lyon 1, 69622 Villeurbanne cedex, France S Supporting Information *

ABSTRACT: Five tetradentate ligands based on the N,N′-bis(2-amino-3,5-di-tertbutylphenyl)-o-phenylenediamine backbone were prepared, with different substituents at positions 4 and 5 (CH3 (3a), p-CH3O-C6H4 (3b), H (3c), Cl (3d), F (3e)). Their reaction with a nickel(II) salt in air affords the neutral species 4(a-e), which were isolated as single crystals. 4(a-e) feature two antiferromagnetically exchange-coupled diiminosemiquinonate moieties, both located on peripheral rings, and a diamidobenzene bridging unit. Oxidation of 4(a-e) with 1 equiv of AgSbF6 yields the cations 4(a-e)+, which harbor a single diiminosemiquinonate radical. Significant structural differences were observed within the series. 4b+ is mononuclear and contains a localized diiminosemiquinonate moiety. In contrast, 4c+ is a dimer wherein the diiminosemiquinonate radical is rather delocalized over both peripheral rings. 4d+ represents an intermediate case where the complex is mononuclear, but the radical is fully delocalized. Oxidation of 4(a-e) with 2 equiv of AgSbF6 produces the corresponding mononuclear dications. X-ray diffraction data on 4(bd)2+ reveals that the bridging ring retains its diamidobenzene character, whereas both peripheral rings have been oxidized into diiminobenzoquinone moieties. All the complexes were characterized by electrochemistry, EPR, and UV−vis−NIR spectroscopy. Remarkably, the electronic structures of the complexes differ from those reported by Wieghardt et al. for copper and zinc complexes of a related ligand involving a mixed N2O2 donor set (J. Am. Chem. Soc. 1999, 121, 9599). The easier oxidation of phenylenediamine moieties in comparison to aminophenols is proposed to account for the difference. involving tridentate5 and tetradentate6,7 amidophenolates are comparatively much scarcer. This is rather surprising since polydendate ligands however offer a decisive advantage in catalysis over bidentate ones. In particular, deactivation of the catalyst by metal release during turnovers can be prevented due to their high chelating ability. In addition, polydentate ligands allow for a better definition of the metal ion coordination sphere, essential for the design of new catalysts. Wieghardt et al. reported in the early 2000s the coordination chemistry of zinc and copper complexes of the tetradentate ligand H4LAP2 (Figure 1).7 They nicely showed that the ligand undergoes four successive and reversible electron-transfers and that the radical complexes were efficient catalysts for the aerobic oxidation of alcohols into aldehydes. Phenylenediamines (H2LPDA in Scheme 1; PDA = ophenylenediamine) are isoelectronic to aminophenols, but have received much less attention.8−10 Their metal complexes form an electron-transfer series where the ligand can adopt three relatively stable oxidation levels, which are symbolized as LPDA (initial dianionic ligand), LISQ (one-electron-oxidized

1. INTRODUCTION The coordination chemistry of redox noninnocent ligands with transition elements has attracted a renewed interest during the past few years.1 The redox noninnocence of a ligand manifests when the oxidation state of the complex (determined by spectroscopic tools) differs from that of the metal ion. This ambiguous situation is encountered when the redox-active orbitals of a ligand are energetically close to the metal ones and therefore accessible for intramolecular electron-transfer. Redox noninnocent ligands provide an elegant strategy to access oxidation states for the complexes that are inaccessible by considering the metal alone. In addition, the ligand can act as an electron reservoir, thereby assisting the metal ion for electron-transfer in catalysis.2 In this sense, coordination to a redox noninnocent ligand is a powerful approach to overcome the inability of some abundant transition metal ions to accommodate multielectron changes often encountered during turnovers. Sterically hindered bidendate aminophenols (H2LAP) are prototypical redox noninnocent ligands3 (Scheme 1), whose coordination chemistry has been reviewed recently.4 Although a number of complexes from simple bidentate3,4 amidophenolate ligands have been reported in the literature, metal complexes © XXXX American Chemical Society

Received: August 24, 2015

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DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Oxidation Levels of the Deprotonated Aminophenol and Phenylenediamine Ligands and Related Nomenclature

Figure 1. Complexes of interest in this study. phenyl)-o-phenylenediamine Derivatives 2(b-e). In a flame-dried resealable tube under argon atmosphere were added compound 1 (2.4 equiv), the desired o-phenylenediamine (0.1 M, 1.0 equiv), Cs2CO3 (4.0 equiv), and freshly distilled toluene. After the solution was degassed, Pd2dba3 (10 mol %) and rac-Binap (15 mol %) were added. The tube was immediately sealed and heated to 120 °C over 24 h. After cooling to rt (room temperature) the mixture was filtrated through a pad of Celite and abundantly washed with CH2Cl2 until a colorless filtrate. The organic phase was concentrated under reduced pressure, and the remaining crude product was purified by column chromatography on silica gel (CH2Cl2/pentane). N,N′-Bis(3,5-di-tert-butyl-2-nitrophenyl)-4,5-bis(4-methoxyphenyl)-o-phenylenediamine (2b). Orange solid, yield: 45%. Mp 196−198 °C (MeOH). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.15 (s, 2H), 7.12−7.14 (m, 4H), 6.99−7.04 (m, 4H), 6.70−6.75 (m, 4H), 5.94 (br s, 2H), 3.77 (s, 6H), 1.40 (s, 18H), 1.29 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 158.4, 153.5, 142.4, 141.1, 135.72, 135.68, 133.6, 133.0, 130.9, 123.1, 118.3, 114.6, 113.5, 55.3, 36.4, 35.4, 31.3, 31.2. MS (ESI): m/z = 685 [M − H]−. IR: ν (cm−1) 3370, 2959, 1578, 1492, 1416, 1363, 1242. Anal. Calcd for C48H58N4O6: C, 73.26; H, 7.43; N, 7.12. Found: C, 73.45; H, 7.39; N, 7.12. N,N′-Bis(3,5-di-tert-butyl-2-nitrophenyl)-o-phenylenediamine (2c). Orange solid. yield: 81%. Mp 193−195 °C (EtOH). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.09−7.13 (m, 4H), 6.82−7.03 (m, 2H), 6.90 (d, J = 2.0 Hz, 2H), 5.88 (br s, 2H), 1.40 (s, 18H), 1.22 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 153.5, 142.3, 141.1, 135.9, 134.3, 124.1, 121.7, 118.3, 114.3, 36.3, 35.3, 31.17, 31.16. MS (ESI): m/z = 575 [M + H]+, 597 [M + Na]+. IR: ν (cm−1) 3370, 2958, 2908, 2870, 1575, 1534, 1359, 1274. Anal. Calcd for C34H46N4O4, 0.5MeOH: C, 70.14; H, 8.19; N, 9.48. Found: C, 70.17; H, 8.19; N, 9.43. N,N′-Bis(3,5-di-tert-butyl-2-nitrophenyl)-4,5-dichloro-o-phenylenediamine (2d). Light green solid, yield: 33%. Mp 166−168 °C (MeOH). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.20 (d, J = 1.8 Hz, 2H), 7.15 (s, 2H), 6.87 (d, J = 1.8 Hz, 2H), 5.78 (s, 2H), 1.39 (s, 18H), 1.26 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 153.8, 134.4, 133.8, 126.6, 122.1, 119.5, 115.0, 49.7, 36.2, 35.3, 31.02, 30.00. MS (ESI): m/z = 641 [M − H; 35Cl,35Cl]−, 643 [M − H; 35Cl,37Cl]−, 645 [M − H; 37Cl,37Cl]−. IR: ν (cm−1) 3380, 2962, 2870, 1491, 810. Anal. Calcd for C34H44Cl2N4O4, 0.5MeOH: C, 62.82; H, 7.03; N, 8.49. Found: C, 63.01; H, 7.27; N, 8.36. N,N′-Bis(3,5-di-tert-butyl-2-nitrophenyl)-4,5-difluoro-o-phenylenediamine (2e). Yellow solid, yield: 36%. Mp 198−200 °C (MeOH). 1 H NMR (400 MHz, CDCl3): δ (ppm) = 7.17 (d, J = 2.0 Hz, 2H), 6.90 (t, J = 9.6 Hz, 2H), 6.84 (d, J = 2.0 Hz, 2H), 5.75 (sl, 2H), 1.39 (s, 18H), 1.25 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 153.7, 146.4 (dd, J = 245 Hz, J = 15.5 Hz), 142.3, 141.5, 135.1, 130.6− 130.5 (m), 119.2, 114.6, 110.1 (J = 13.6 Hz, J = 7.9 Hz), 36.2, 35.2,

diiminosemiquinonate radical form; ISQ = o-diiminosemiquinonate), and LIBQ (dioxidized diiminobenzoquinone form; IBQ = o-diiminobenzoquinone) in Scheme 1. We recently reported an exclusively nitrogenated counterpart of H4LAP2, namely, H4MeLPDA2 (Figure 1).11 To the best of our knowledge, a similar tetradentate N4 coordination pattern in a phenylenediamine-based ligand has been only recently reported for a rearranged product resulting from the reaction of Fe(II) complexes with dioxygen.12 This new ligand system represents an intriguing case where three redox noninnocent moieties of similar nature (PDA) are connected together and may compete for electron-transfers. We report in this Article the synthesis of five ligands based on the H4LPDA2 scaffold (Figure 1) and their corresponding nickel(II) complexes. By spectroelectrochemistry, X-ray diffraction, and UV−vis−NIR and EPR spectroscopies as well as DFT calculations we establish that the ligand alone supports the oxidative redox-activity. Remarkably, the electronic structure of some members of the electrontransferred series differs from that of phenolate derivatives described by Wieghardt et al. (Figure 1), due to the easier oxidation of phenylenediamine moieties.

2. EXPERIMENTAL SECTION 2.1. General. All experiments were performed under anaerobic conditions under a pure argon atmosphere using standard Schlenk techniques or in a drybox (O2 < 1 ppm). Anhydrous toluene and triethylamine were distilled over CaH2 under an argon atmosphere prior to use. Anhydrous dichloromethane, acetonitrile, and methanol were purchased from Acros. High-pressure reactions were carried out using a 0.6 L Parr Instrument stainless steel vessel. 3,5-Di-tert-butyl-2nitrobromobenzene 1,13 4,5-difluorophenylene-1,2-diamine,14 4,5-bis(4-methoxyphenyl)phenylene-1,2-diamine,15 and the complex [Ni(MeL)]0 4a11 were prepared according to reported procedures. All other chemicals were purchased from Acros, Alfa-Aesar, Sigma-Aldrich, or TCI and were used as received. NMR spectra were recorded on a Brüker Avance 400 (1H at 400 MHz, 13C at 100 MHz). Chemical shifts are given relative to solvent residual peak. Mass spectra were recorded on a Brüker Esquire 3000 (ESI/Ion Trap) instrument. Melting points were measured with a Büchi B-545 apparatus and were not corrected. Microanalyses were performed by the Service Central d’Analyse du CNRS (Lyon, France). Infrared spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer equipped with an ATR sampling accessory. 2.2. Synthesis of the Ligands and Dinitro Precursors. General Procedure for the Preparation of N,N′-Bis(3,5-di-tert-butyl-2-nitroB

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 31.03, 30.99. MS (ESI): m/z = 611 [M + H]+, 623 [M + Na]+. IR: ν (cm−1) 3386, 2963, 2872, 1582, 1364. Anal. Calcd for C34H44F2N4O4, 0.5MeOH: C, 66.11; H, 7.40; N, 8.94. Found: C, 66.24; H, 7.47; N, 8.90. General Procedure for the Preparation of N,N′-Bis(3,5-di-tertbutyl-2-aminophenyl)-o-phenylenediamine derivatives 3(b-e). In a Paar pressure vessel, to a solution of the dinitro compound 2(b-e) (0.03 M) in CH2Cl2 was added Pd/C (20 mol %). The resulting suspension was stirred at rt under H2 (25 bar) during 16−24 h. After complete consumption of the material (TLC monitoring), the mixture was filtrated through Celite and abundantly washed with CH2Cl2. The combined organic phase was dried over Na2SO4 and concentrated under reduced pressure. The remaining residue was purified either by recrystallization from MeOH or by column chromatography on silica gel (pentane/CH2Cl2 then CH2Cl2, then CH2Cl2/EtOH). N,N′-Bis(3,5-di-tert-butyl-2-aminophenyl)-4,5-bis(4-methoxyphenyl)-o-phenylenediamine (3b, H4ArL). White solid, yield: 74%. Mp 133−135 °C (MeOH). 1H NMR (400 MHz, CD3OD): δ (ppm) = 7.08 (d, J = 2.2 Hz, 2H), 6.99 (d, J = 2.2 Hz, 2H), 6.93 (d, J = 8.8 Hz, 4H), 6.67 (d, J = 8.8 Hz, 4H), 6.63 (s, 2H), 3.70 (s, 6H), 1.45 (s, 18H), 1.25 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 158.0, 141.1, 135.9, 134.5, 133.7, 133.0, 131.1, 130.1, 119.5, 119.3, 117.7, 113.3, 55.3, 34.9, 34.5, 31.8, 30.1. MS (ESI): m/z = 727 [M + H]+. IR: ν (cm−1) 3484, 3352, 2952, 2904, 1501. Anal. Calcd for C48H62N4O2: C, 79.30; H, 8.60; N, 7.71. Found: C, 79.52; H, 8.52; N, 7.40. N,N′-Bis(3,5-di-tert-butyl-2-aminophenyl)-o-phenylenediamine (3c, H4HL). White solid, yield: 86%. Pure analytical sample was obtained by crystallization from EtOH. Mp 159−160 °C. 1H NMR (400 MHz, CD3OD): δ (ppm) = 7.07 (d, J = 2.3 Hz, 2H), 6.84 (d, J = 2.3 Hz, 2H), 6.76 (dt, J = 7.3, 3.6 Hz, 2H), 6.65 (dt, J = 7.3, 3.6 Hz, 2H), 1.44 (s, 18H), 1.22 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 140.9, 136.0, 135.0, 134.2, 130.4, 121.6, 119.1, 117.9, 117.5, 34.8, 34.3, 31.8, 30.1. MS (ESI): m/z = 515 [M + H]+. IR: ν (cm−1) 3474, 3360, 3335, 2952, 2908, 2873, 1581, 1508, 1420, 1363, 1312, 1283, 1242. Anal. Calcd for C34H50N4, 0.5EtOH: C, 78.16; H, 9.93; N, 10.42. Found: C, 78.13; H, 9.88; N, 10.25. N,N′-Bis(3,5-di-tert-butyl-2-aminophenyl)-4,5-dichloro-o-phenylenediamine (3d, H4ClL). Pink solid, yield: 93%. Mp >250 °C (EtOH). 1 H NMR (400 MHz, CD3OD): δ (ppm) = 7.17 (d, J = 2.3 Hz, 2H), 6.90 (d, J = 2.3 Hz, 2H), 6.49 (s, 2H), 1.45 (s, 18H), 1.27 (s, 18H). 13 C NMR (100 MHz, CDCl3): δ (ppm) = 141.7, 135.8, 135.0, 134.8, 129.3, 124.2, 120.0, 118.8, 117.8, 34.9, 34.5, 31.7, 30.1. MS (ESI): m/z = 583 [M + H; 35Cl, 35Cl]+, 585 [M + H; 35Cl, 37Cl]+, 587 [M + H; 37 Cl, 37Cl]+. IR: ν (cm−1) 3478, 3348, 2955, 2867, 1578, 1473, 1359, 862. Anal. Calcd for C34H48Cl2N4: C, 69.96; H, 8.29; N, 9.60. Found: C, 69.99; H, 8.15; N, 9.51. N,N′-Bis(3,5-di-tert-butyl-2-aminophenyl)-4,5-difluoro-o-phenylenediamine (3e, H4FL). White solid, yield: 67%. Mp 223−225 °C (MeOH). 1H NMR (400 MHz, CD3OD): δ (ppm) = 7.13 (d, J = 2.2 Hz, 2H), 6.85 (d, J = 2.2 Hz, 2H), 6.34 (t, J = 10.2 Hz, 2H), 1.45 (s, 18H), 1.25 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 145.1 (dd, J = 240.0 Hz, J = 16.0 Hz), 141.3, 135.7, 134.6, 131.2 (t, J = 5.0 Hz), 129.8, 119.6, 117.3, 106.7 (dd, J = 12.0 Hz, J = 10.0 Hz), 34.7, 34.3, 31.6, 29.9. MS (ESI): m/z = 551 [M + H]+. IR: ν (cm−1) 3488, 3344, 2954, 2869, 1503, 1406. Anal. Calcd for C34H48F2N4: C, 74.14; H, 8.78; N, 10.17. Found: C, 74.07; H, 9.03; N, 9.89. 2.3. Preparation of the Complexes 4(b-e). General Procedure. Under argon, Et3N (4.0 equiv) was added to a degassed solution of the desired ligand RLH4 3(b-e) (5 mM) and Ni(OAc)2·4H2O (1.0 equiv) in dry CH3CN or MeOH. The solution was refluxed for 1 h. After cooling to rt, the solution was exposed to air and stirred for 2 h. A precipitate formed, which was filtrated and dried under vacuum. [Ni(ArL)]0 (4b). A black precipitate was recovered after reaction in CH3CN, yield 61%. Suitable crystals for X-ray analysis were obtained by diffusion of CH3CN in a benzene solution, mp >250 °C. MS (ESI): m/z = 727 [M + H]+. IR: ν (cm−1) 3402, 3386, 2952, 2901, 2867, 1585, 1480, 1182, 1157, 1100. Anal. Calcd for C48H58N4O2Ni, 0.5CH3CN: C, 73.36; H, 7.48; N, 7.86. Found: C, 73.30; H, 7.63; N, 8.09.

[Ni(HL)]0 (4c). A black precipitate was recovered after reaction in MeOH, yield 73%. Suitable crystals for X-ray analysis were obtained by slow diffusion at rt of CH3CN in a benzene solution, mp >250 °C. MS (ESI): m/z = 569 [M + H]+. IR: ν (cm−1) 3386, 2952, 2905, 2863, 1578, 1473, 1464, 1277, 1258, 745. Anal. Calcd for C34H46N4Ni, CH3CN, 0.5C6H6: C, 72.11; H, 8.07; N, 10.78. Found: C, 72.29; H, 8.43; N, 10.54. [Ni(ClL)]0 (4d). A dark blue precipitate was recovered after reaction in MeOH, yield 80%. Suitable crystals for X-ray analysis were obtained by slow evaporation at rt of a CH2Cl2/MeOH solution, mp >250 °C. MS (ESI): m/z = 637 [M + H; 35Cl, 35Cl]+, 639 [M + H; 35Cl, 37Cl]+, 641 [M + H; 37Cl, 37Cl]+. IR: ν (cm−1) 3623, 3373, 2952, 2905, 2863, 1576, 1483, 1359, 1274, 1261, 761, 751. Anal. Calcd for C34H44Cl2N4N, 0.5MeOH: C, 63.32; H, 7.09; N, 8.56. Found: C, 63.55; H, 7.35; N, 8.32. [Ni(FL)]0 (4e). A black precipitate was recovered after reaction in MeOH, yield 75%. Suitable crystals for X-ray analysis were obtained by slow evaporation at rt of an Et2O/MeOH solution, mp >250 °C. MS (ESI): m/z = 727 [M + H]+. IR: ν (cm−1) 3383, 3345, 2952, 2901, 2863, 1584, 1518, 1454, 1274, 1258, 767, 748. Anal. Calcd for C34H44F2N4Ni: C, 67.46; H, 7.33; N, 9.25. Found: C, 67.75; H, 7.76; N, 9.76. 2.4. Oxidation of Complexes 4(a-e). General Procedure. Under an argon atmosphere, AgSF6 (1 or 2 equiv) was added at rt to a 10−2 M solution of complex 4(a-e) in CH2Cl2. A color change was immediately observed, and the solution was stirred for 30 min. Filtration through a frit and subsequent addition of pentane to the filtrate led to the formation of a precipitate which was isolated, abundantly washed with pentane, and dried. [Ni(MeL)](SbF6) (4a+·SbF6−). Brownish black solid, yield 51%. Mp >250 °C. MS (ESI): m/z = 596 [M − (SbF6)]+. IR: ν (cm−1) 3360, 2962, 2870, 1606, 1464, 1397, 1337, 1242. Anal. Calcd for C36H50N4Ni·SbF6, 3(CH2Cl2): C, 43.05; H, 5.19; N, 5.15. Found: C, 42.90; H, 5.06; N, 4.79. [Ni(ArL)](SbF6) (4b+·SbF6−). Black solid, yield 91%. Mp >250 °C. MS (ESI): m/z = 780 [M − SbF6]+. IR: ν (cm−1) 3360, 2949, 2927, 2870, 1603, 1508, 1458, 1252, 1170. Anal. Calcd for C48H58N4NiO2· SbF6, C2H4Cl2: C, 53.79; H, 5.60; N, 5.02. Found: C, 54.13; H, 5.42; N, 4.91. [Ni(HL)](SbF6) (4c+·SbF6−). Black solid, yield 58%. Mp >250 °C. MS (ESI): m/z = 496 [M − (t-Bu) − (SbF6)]+, 568 [M − (SbF6)]+. IR: ν (cm−1) 3359, 2955, 2905, 2867, 1591, 1464, 1359, 1261. Anal. Calcd for C34H46N4Ni·.SbF6, 0.3(C5H12): C, 51.66; H, 6.08; N, 6.76. Found: C, 51.42; H, 6.08; N, 6.88. [Ni(ClL)](SbF6) (4d+·SbF6−). Greenish black solid, yield 95%. Suitable crystals for X-ray analysis were obtained by vapor diffusion of hexanes in a CH2Cl2 solution, mp >250 °C. MS (ESI): m/z = 636 [M − (SbF6); 35Cl35Cl]+, 638 [M − (SbF6); 37Cl35Cl, 35Cl37Cl]+, 640 [M − (SbF6); 37Cl37Cl]+. IR: ν (cm−1) 3345, 2958, 2908, 2867, 1584, 1464, 1359, 1242. Anal. Calcd for C34H44Cl2N4Ni·SbF6, 1.5(CH2Cl2): C, 42.57; H, 4.73; N, 5.59. Found: C, 42.54; H, 4.43; N, 5.63. [Ni(FL)](SbF6) (4e+·SbF6−). Greenish black solid, yield 72%. Mp >250 °C. MS (ESI): m/z = 604 [M − SbF6]+. IR: ν (cm−1) 3357, 2962, 2908, 2870, 1587, 1467, 1363, 1264, 1233. Anal. Calcd for C34H44F2N4Ni·SbF6, C2H4Cl2: C, 45.99; H, 5.15; N, 5.96. Found: C, 46.21; H, 5.14; N, 6.06. [Ni(MeL)](SbF6)2 (4a2+·2xSbF6−). Brownish black solid, yield 53%. Mp >250 °C. MS (ESI): m/z = 298 [M − 2(SbF6)]2+. IR: ν (cm−1) 3360, 2965, 2911, 2876, 1606, 1464, 1397, 1274, 1261. Anal. Calcd for C36H50N4Ni·2(SbF6): C, 40.45; H, 4.72; N, 5.25. Found: C, 40.64; H, 4.77; N, 5.10. [Ni(ArL)](SbF6)2 (4b2+·2xSbF6−). Purplish black solid, yield 99%. Mp >250 °C. MS (ESI): m/z = 390 [M − 2(SbF6)]2+. IR: ν (cm−1) 3354, 2962, 2908, 2873, 1600, 1508, 1461, 1249, 1173. Anal. Calcd for C48H58N4NiO2·2(SbF6), 3(C2H4Cl2): C, 41.84; H, 4.55; N, 3.61. Found: C, 42.02; H, 4.34; N, 3.71. [Ni(HL)](SbF6)2 (4c2+·2xSbF6−). Brownish black solid, yield 78%. An analytical sample was obtained by vapor diffusion of pentane in a DCE solution. Suitable crystals for X-ray analysis were obtained by vapor diffusion of pentane in a CH2Cl2 solution, mp >250 °C. MS (ESI): m/ C

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 2. Synthesis of the Ligands

z = 284 [M − 2(SbF6)]2+. IR: ν (cm−1) 3357, 2968, 2870, 1610, 1397, 1274, 1258, 748. Anal. Calcd for C34H46N4Ni·2(SbF6), 3(C2H4Cl2): C, 35.91; H, 4.37; N, 4.19. Found: C, 35.83; H, 4.18; N, 4.55. [Ni(ClL)](SbF6)2 (4d2+·2xSbF6−). Brownish black solid, yield 98%. Suitable crystals for X-ray analysis were obtained by vapor diffusion of hexanes in a CH2Cl2 solution, mp >250 °C. MS (ESI): m/z = 318 [M − 2(SbF6); 35Cl35Cl]2+, 319 [M − 2(SbF6); 37Cl35Cl, 35Cl37Cl]2+, 320 [M − 2(SbF6); 37Cl37Cl]2+. IR: ν (cm−1) 3351, 2965, 2873, 1610, 1451, 1331, 1138. Anal. Calcd for C34H44Cl2N4Ni·2(SbF6), 3(CH2Cl2): C, 32.57; H, 3.69; N, 4.11. Found: C, 32.49; H, 3.58; N, 4.49. [Ni(FL)](SbF6)2 (4e2+·2xSbF6−). Brownish black solid, yield 87%. Mp >250 °C. MS (ESI): m/z = 302 [M − 2(SbF6)]2+. IR: ν (cm−1) 3354, 2971, 2914, 2876, 1603, 1578, 1492, 1397, 1337, 1302. Anal. Calcd for C34H44F2N4Ni·2(SbF6), 0.5(C5H12): C, 39.39; H, 4.53; N, 5.03. Found: C, 39.08; H, 4.29; N, 4.80. 2.5. Physical Methods. X-band EPR spectra were recorded on a Bruker EMX Plus spectrometer controlled with the Xenon software and equipped with a Bruker teslameter. A Bruker nitrogen flow cryostat connected to a high-sensitivity resonant cavity was used for 100 K measurements. An Oxford Instrument helium flow cryostat connected to a dual-mode resonant cavity was used to run experiments at 10 K. Spectra were simulated using the SIMFONIA software (Bruker). UV−vis−NIR spectra at 298 K were recorded on a PerkinElmer Lambda 1050 spectrophotometer or a Varian Cary 50 spectrophotometer. The quartz cell path length was 1 mm. Cyclic voltammetry curves of complexes 4a, 4c, and 4e were recorded on a CH Instruments 620 potentiostat in a standard three-electrode cell under argon atmosphere. For 4b and 4d, a BioLogic SP 300 potentiostat connected to an O2- and H2O-free glovebox under argon atmosphere was used. A AgNO3/Ag (0.01 M) reference electrode was used. All potentials given in the text are referenced against the Fc+/Fc couple. A vitrous carbon disk electrode (5 mm diameter) polished with 1 μm diamond paste was used as working electrode. Electrochemical oxidations were performed on an EG&G PAR 273A potentiostat, under an argon atmosphere at 298 K, using a carbon felt working electrode. Magnetic susceptibilities were measured using a Quantum Design MPMS-XL5 SQUID magnetometer equipped with an EVER Cool system at University Claude Bernard Lyon 1 on polycrystalline samples in the temperature range from 2 to 300 K, with an applied field of 0.1 T. 2.6. Crystal Structure Analysis. Collected reflections were corrected for Lorentz and polarization effects but not for absorption in the cases [Ni(HL)]0 4c and [Ni(HL)]2+ 4c2+. For the other structures SADABS-2004/1 was used for absorption correction. Structures were solved by direct methods and refined using OLEX2 software.16 All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were generated in idealized positions, riding on the carrier atoms, with isotropic thermal parameters except the hydroxyl ones, which were localized on the Fourier map and fixed. 2.7. DFT Calculations. Geometry optimizations were carried out with the GAUSSIAN 09 software (revision D.01),17 using the B3LYP

functional18 in combination with the TZVP19 basis set for all atoms. They were performed in the gas-phase, by using tight SCF convergence criteria. For all the electron-transferred series a frequency calculation was systematically carried out in order to ensure that the optimized structure was located at a minimum of the potential energy surface. We additionally checked the stability of the wave function by the stable option of the program. The optical properties were investigated by time-dependent DFT (TD-DFT)20 by using the same functional and basis set. Solvent effects were included through a polarized continuum model (PCM),21 and 25 excited states were calculated in each case. The ORCA software (release 3.0.1)22 was used to calculate the EPR parameters (g-factors) on the above optimized structures. For this purpose the B3LYP functional was used, in combination with the RIJCOSX approximation23 and an increased grid (X5), with a radial grid of 7 for the Ni atom. The Def2-TZVP basis set24 was used for the atoms of the ligand, while a partially contracted core-property basis set CP(PPP) based on the TurboMole DZ basis developed by Ahlrichs and co-workers and obtained from the basis set library under ftp.chemie.unikarlsruhe.de/pub/basen was used for the metal ion. Relativistic effects were treated by using the ZORA Hamiltonian.25

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Ligands and Complexes. In a recent article, we described the synthesis of the ligand N,N′bis(3,5-di-tert-butyl-2-aminophenyl)-4,5-dimethyl-o-phenylenediamine (H4MeL), following an efficient two-step sequence from 3,5-di-tert-butyl-2-nitrobromobenzene 1.11 The same strategy was applied for the preparation of the ligands H4RL 3(b-e) (Scheme 2). The very reliable methodology reported by the Beifuss’ group26 allows for the Pd-catalyzed N-arylation of the selected 4,5-disubstituted o-phenylenediamine with an excess of 1. The related N,N′-bis(3,5-di-tert-butyl-2-nitrophenyl)-4,5disubstituted-o-phenylenediamines 2(b-e) were isolated in moderate to good yields depending on the nature of the substituents. Subsequent reduction of the nitro groups by catalytic hydrogenation cleanly led to the targeted ligands 3(be) in good yields. The compounds were found to be unstable in nonprotic solvents in the presence of air. Thus, except for methanol or ethanol, colorless solutions turned rapidly deep purple within a few minutes. In the solid state, the ligands were stable and kept out from light without any other precaution. A general methodology was used to synthesize the Ni(II) compounds [Ni(RL)]0 4(b-e) (Scheme 3). Et3N (4.0 equiv) was added dropwise to a solution of the ligand H4RL and the metal salt (Ni(OAc)2·4H2O) in equimolar amounts, under an argon atmosphere. The reaction took place in either methanol or acetonitrile. The resulting mixture was refluxed during 1 h, followed by a prolonged exposure to air at rt (2 h), affording a dark microcrystalline precipitate in yields ranging from 61% to D

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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/2) signal that accounts for less than 5% of the total sample concentration (identified as the monocation). The ESI mass spectra of the complexes display a [M + H]+ molecular ion peak consistent with a metal:ligand ratio of 1:1. The IR spectra of 4a, 4c, 4e, 4(a−e)+, and 4(a−e)2+ exhibit a unique ν(N−H) vibration band in the 3340−3390 cm−1 region. In the case of 4b two ν(N−H) stretching frequencies at 3402 and 3386 cm−1 are observed. For 4d and 4e a ν(O−H) stretching band is also visible in the same range, due to a cocrystallized methanol molecule. 3.2. X-ray Crystal Structure of the Neutral Complexes. Single crystals were successfully obtained for the neutral complexes 4(a-e). Selected bond distances are given in Table 1. The X-ray crystal structure of 4d is depicted in Figure 2, while the other ones are shown in SI. The metal ion lies in a square-planar geometry with a ligand:metal ratio of 1:1. The nickel ion is coordinated by the N1, N2, N3, and N4 atoms, with the former and latter corresponding to the terminal NH groups. Small geometrical differences are observed in the present series. For example, the ligand and the metal lie in a nearly perfect plane in complexes 4a and 4b, whereas the chlorinated bridge in complex 4d is twisted from 9° when compared to the coordination plane. In the last two structures, 4c and 4e, the central and lateral rings are tilted from the plane formed by the nitrogen atoms N1−

Scheme 3. Synthesis of the Complexes

80%. Aiming to get single crystals, various combination of solvents and techniques were successfully used to recrystallize the materials: whatever the method (vapor diffusion or slow evaporation), methanol and acetonitrile were identified as the best antisolvents associated with diethyl ether, dichloromethane, or benzene. The NMR spectra of 4(a-e) were surprisingly broad and barely interpretable at 25 °C, whatever the deuterated solvent and the concentration used. This behavior likely results from residual paramagnetism in the sample. It could be due to either a thermally accessible paramagnetic excited state or a paramagnetic contaminant in the tube. Support for the second hypothesis comes from EPR, which shows a very weak (S = Table 1. Selected Bond Distances (Å) [Ni(MeL)]0 4a Ni−N1 Ni−N4 Ni−N2 Ni−N3 C1−N1 C7−N4 C2−N2 C8−N3 C13−N2 C14−N3

Ni−N1 Ni−N4 Ni−N2 Ni−N3 C1−N1 C7−N4 C2−N2 C8−N3 C13−N2 C14−N3 Ni−N1 Ni−N4 Ni−N2 Ni−N3 C1−N1 C7−N4 C2−N2 C8−N3 C13−N2 C14−N3

1.809(3) 1.807(3) 1.807(2) 1.816(3) 1.343(4) 1.344(4) 1.366(4) 1.364(4) 1.423(4) 1.416(4) [Ni(ArL)]+ 4b+ 1.814(2) 1.844(2) 1.835(2) 1.830(2) 1.344(3) 1.315(3) 1.365(3) 1.333(3) 1.397(3) 1.403(3) [Ni(ArL)]2+ 4b2+ 1.846(2) 1.851(2) 1.844(2) 1.840(2) 1.306(4) 1.303(4) 1.322(3) 1.324(3) 1.399(3) 1.389(3)

[Ni(ArL)]0 4b

[Ni(HL)]0 4c

[Ni(ClL)]0 4d

1.825(2) 1.815(2) 1.807(2) 1.816(2) 1.349(3) 1.345(3) 1.357(3) 1.361(3) 1.413(3) 1.421(3)

1.823(2)

1.820(2) 1.810(2) 1.810(2) 1.816(2) 1.343(3) 1.348(3) 1.359(3) 1.364(3) 1.413(3) 1.417(3)

1.809(2) 1.342(2) 1.348(2) 1.415(2)

1.824(5) 1.820(5) 1.811(4) 1.820(4) 1.355(6) 1.347(6) 1.343(6) 1.344(6) 1.426(6) 1.417(6) [Ni(ClL)]+ 4d+

[Ni(HL)]+ 4c+ Ni1A-Ni1B Ni1A-N1A/Ni1B−N1B Ni1A−N4A/Ni1B−N4B Ni1A−N2A/Ni1B−N2B Ni1A−N3A/Ni1B−N3B C1A−N1A/C1B−N1B C7A−N4A/C7B−N4B C2A−N2A/C2B−N2B C8A−N3A/C8B−N3B C13A−N2A/C13B−N2B C14A−N3A/C14B−N3B [Ni(HL)]2+ 4c2+ Ni1−N1/Ni2−N8 Ni1−N4/Ni2−N5 Ni1−N2/Ni2−N7 Ni1−N3/Ni2−N6 C1−N1/C35−N8 C7−N4/C41−N5 C2−N2/C36−N7 C8−N3/C42−N6 C13−N2/C47−N7 C14−N3/C48−N6

2.784(10) 1.844(4)/1.843(3) 1.832(3)/1.835(3) 1.833(3)/1.844(3) 1.834(4)/1.825(3) 1.330(5)/1.308(5) 1.326(5)/1.325(5) 1.336(5)/1.330(5) 1.337(5)/1.349(5) 1.418(6)/1.422(5) 1.410(5)/1.404(5)

1.856(5)/1.857(5) 1.855(5)/1.855(5) 1.840(5)/1.842(5) 1.843(4)/1.834(5) 1.293(7)/1.279(7) 1.281(7)/1.297(7) 1.307(7)/1.327(7) 1.318(7)/1.323(7) 1.409(7)/1.420(7) 1.402(7)/1.405(7) E

[Ni(FL)]0 4e

Ni−N1 Ni−N4 Ni−N2 Ni−N3 C1−N1 C7−N4 C2−N2 C8−N3 C13−N2 C14−N3 Ni−N1 Ni−N4 Ni−N2 Ni−N3 C1−N1 C7−N4 C2−N2 C8−N3 C13−N2 C14−N3

1.826(3) 1.820(3) 1.831(3) 1.827(3) 1.328(5) 1.322(5) 1.345(5) 1.345(4) 1.401(4) 1.399(4) [Ni(ClL)]2+ 4d2+ 1.864(5) 1.846(5) 1.834(5) 1.856(5) 1.307(7) 1.301(8) 1.323(7) 1.315(7) 1.419(8) 1.400(7)

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peripheral rings are assigned to diiminosemiquinonate radicals.9 It is worth noticing that the electronics of the substituents (Me, p-MeOC6H4, H, Cl, F) have only a marginal effect on the electronic structure of the complexes, as reflected by the homogeneity in the C−C and C−N bond lengths found in the series. It is instructive now to compare the electronic structure of 4(a−e) with those reported by Wieghardt et al. for the copper(II) and zinc(II) complexes of H4LAP2, which represents the oxygenated derivative of 4a (Figure 3a).6 Under experimental conditions closely related to ours they isolated two neutral complexes wherein the ligand adopts a closed-shell N,N′-bis(phenolato)benzoquinonediimine structure. This electronic structure was unambiguously established by X-ray diffraction, especially through the quinoidal distribution of bond lengths in the central aromatic ring. The difference between 4(a−e) and Wieghardt’s complexes is assigned to the lower oxidation potential of anilines27 (or phenylenediamines)8 in comparison to phenols28 (or aminophenols).3,4 The peripheral rings are easier to oxidize in the precursor 4(a− e)2‑ than in [MII(LAP2)]2−. Each undergoes a one-electron oxidation in the presence of air, affording the bis(iminosemiquinonate) radical species 4(a−e). In the case of [MII(LAP2)]2− this is the central ring that is two-electronoxidized, with the peripheral ring keeping the amidophenolato character (Figure 3c). In this sense, the electronic structure of 4(a−e) is closer to that reported for homoleptic bis(odiiminosemiquinonato)nickel(II) complexes (Figure 3b).9 3.3. Electrochemistry. The electrochemical behavior of each complex was studied by cyclic voltammetry (CV) in CH2Cl2 solutions containing 0.1 M NBu4ClO4 (TBAP). The redox potentials are referenced to ferrocenium/ferrocene couple (Fc+/Fc). The CV of [Ni(ArL)]0 (4b) is shown in Figure 4, while those of the other complexes are provided in SI. The overall shape of the CVs is rather homogeneous within the series. They display four reversible or quasireversible monoelectronic redox processes within the following ranges: −1.80 to −1.63 V for

Figure 2. X-ray structure of [Ni(ClL)]0 4d at the 30% ellipsoid probability. H atoms are omitted for clarity, except the anilido ones.

N2−N3−N4. Both lateral rings adopt an umbrella shape with a tilt angle of 25° and 23° with respect to the bridge, respectively. In the case of complex 4b (see SI) the dihedral angles between the p-methoxyphenyl moieties and the central ring are 46° and 51°. The steric clash between the p-methoxyphenyl moieties is likely responsible for these large dihedral angles, which prevents electronic delocalization. For complexes 4(a−e) the coordination bond lengths range between 1.807 and 1.825 Å, consistent with a low-spin nickel(II) electronic configuration of the metal ion. Within the central ring the C−N bonds lengths are almost equivalent at 1.42 ± 0.01 Å, while the C−C bonds fall in the range 1.38− 1.42 Å. The absence of quinoidal distribution of bond lengths is consistent with a diamido character of this ring. Regarding the peripheral rings, the C−N bond lengths are much shorter: The mean C1−N1 and C7−N4 (terminal nitrogens) are similar, at 1.350 ± 0.005 Å, while the C2−N2 and C8−N3 (connecting nitrogens) are 1.355 ± 0.010 Å. Further, an examination of the C−C bond distances within both peripheral rings evidence a quinoidal distribution of bond lengths, with average C1−C2, C2−C3, C3−C4, C5−C6, and C1−C6 bond lengths at 1.443, 1.404, 1.371, 1.417, 1.377, and 1.437 Å, respectively. Thus, both

Figure 3. Bond distances and electronic structures (from X-ray diffraction) of complexes: (a) [M(LAP2)] (where M = Cu(II), Zn(II)),6 (b) [Ni(LISQ)],9 and (c) 4(a−e). F

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(Figure 3b),9 which basically displays a single two-electron oxidation wave. Thus, the aromatic connection between the peripheral rings dramatically strengthens their electrochemical communication, stabilizing the mono-oxidized species against comproportionation. On the other hand, the electronics of the substituents does not influence significantly ΔE1/22−1 and ΔE1/24−2. Therefore, this is the nature of the connecting group (diamidobenzene) that grossly tunes the communication between the redox-active peripheral rings. An interesting comparison can be made between 4(a−e) and [MII(LAP2)].6 First, the latter displays four redox couples, similar to 4(a−e), but all are anodically shifted by ca. 0.4 V. This trend follows that reported when salen28 complexes are compared to anilinosalen27 complexes and reflects the easier oxidation of phenyl moieties harboring a nitrogen instead of an oxygen exocyclic group. Second, two redox waves assigned to the oxidation of the diamidobenzene central ring were reported in the case of [MII(LAP2)] at ca. −1.3 V (first oxidation) and −0.6 V (second oxidation, producing the diiminobenzoquinone). We did not observe any oxidation of the diamidobenzene central ring in a potential region extended to +0.4 V for 4(a−e). The lower electron density on the peripheral radical rings of 4(a−e) most probably contributes to the increase in oxidation potential of the central bridge by conjugation. Thus, the first oxidative event is crucial in the sense that it dictates the full oxidation sequence. 3.4. X-ray Crystal Structure of the Monocations. The one-electron-oxidized species 4(a−e)+ were generated by reaction with a silver salt. Single crystals of 4(b−d)+ could be isolated and were subjected to an X-ray diffraction analysis. Unlike the parent complexes 4(a−e), which display strong analogies in their geometric and electronic structures, striking differences are observed between the mono-oxidized complexes 4(b−d)+. The structure of 4b+ (Figure 5) displays a mononuclear nickel(II) complex, where the metal ion lies in an essentially square-planar geometry. The nickel ion is coordinated by the N1, N2, N3, and N4 atoms, with N1 and N4 belonging to the terminal NH groups. The Ni−N1 bond length is shorter than the opposite Ni−N4 one (1.814(2) vs 1.844(2) Å), whereas the Ni−N2 and Ni−N3 ones are almost equivalent (1.835(2) vs 1.830(2) Å). Close inspection of the interatomic distances within the ligand framework of 4b+ (Table 1) reveals that the C1−N1 and C2−N2 bond lengths (1.344(3) and 1.365(3) Å, respectively) are roughly similar to those observed in the crystal structure of 4b, showing that this moiety has a diiminosemiquinonate radical character. In contrast, the C7−N4 and C8− N3 bonds (opposite ring) are shorter (1.315(3) and 1.333(3) Å, respectively) and more in agreement with diiminobenzoquinones. Thus, 4b+ features a diiminosemiquinonate radical on one peripheral ring, and a diiminobenzoquinone on the

Figure 4. CV curve of a 0.5 mM CH2Cl2 solution (+ 0.1 M TBAP) of 4b. Scan rate: 100 mV s−1, T = 298 K.

E1/21, −1.09 to −0.97 V for E1/22, −0.48 to −0.29 V for E1/23, and −0.04 to 0.11 V for E1/24 (Table 2). All are assigned to ligand-centered redox events (vide supra). The electronics of the substituents on the central ring has a small but sizable effect on the potentials, giving rise to a linear dependence of E1/2 as a function of σHammett of the substituent (see SI). For instance, the methyl substituent (4a), which is the most electrondonating group, induces a cathodic shift of the redox waves by 0.05−0.07 V when compared to that of complex 4c (R = H). Conversely, the most electron-withdrawing chlorine groups induce an anodic shift of ca. 0.1 V with respect to 4c. The substituent induced shifts are within the range or smaller than those reported for transition metal complexes derived from Naryl-o-amidophenolato-based ligands (R = any aromatic substituent).29 In these latter cases the substituent exerts an inductive effect on the redox couple of the remote oamidophenolato ring. A similar situation is found for 4(a−e), where the electron-transfer occurs on the peripheral rings, rather than the central bis(substituted) one. Interestingly, the pmethoxyphenyl substituents (4b) do not exert a significant electron-donating effect, consistent with the large dihedral angles between the p-methoxyphenyl moieties and the central ring (see above). Although many factors could affect the separation of the waves,30 it has been suggested that the degree of electronic delocalization between two similar redox-active moieties could be estimated through the difference in their oxidation potentials or reduction potentials, respectively. In the present case ΔE1/22−1 and ΔE1/24−2 were calculated as the differences between the reduction and oxidation potentials, respectively, for 4(a−e). Both ΔE1/22−1 and ΔE1/24−2 are found to be relatively large (Table 2), consistent with significant electronic communication between the peripheral rings. It is noteworthy that the ΔE1/24−2 values are much larger than those reported for the bis(o-diiminosemiquinonato)nickel(II) complex [Ni(LISQ)] Table 2. Redox Potentials for 4(a−e)a compd Me

[Ni( L)] 4a [Ni(ArL)] 4b [Ni(HL)] 4c [Ni(ClL)] 4d [Ni(FL)] 4e

E1/21

E1/22

ΔE2−1b

E1/23

E1/24

ΔEox4−3b

−1.80 −1.73 −1.75 −1.63 −1.66

−1.09 −1.06 −1.04 −0.97 −0.98

0.71 0.67 0.71 0.66 0.68

−0.48 −0.42 −0.41 −0.29 −0.34

−0.04 0.01 0.02 0.11 0.07

0.44 0.43 0.43 0.40 0.41

a In V vs Fc+/Fc. Scan rate = 100 mV s−1. T = 298 K. Determined for 0.5 mM CH2Cl2 (+0.1 M TBAP) solutions of the complexes. bΔE2−1 = E1/22 − E1/21; ΔEox4−3 = E1/24 − E1/23.

G

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. X-ray structure of 4b+ (left) and 4d+ (right) at the 30% ellipsoid probability. H atoms are omitted for clarity, except the anilido ones.

Figure 6. X-ray structure of [4c]22+ at the 30% ellipsoid probability. Left: top view. Right: side view. On both views counterions and H atoms are omitted for clarity, except the anilido ones. For the same purpose, t-Bu groups were omitted on the side view.

remains in the same range (1.328 Å). These C−N bond distances are much smaller than those observed in the central diamidobenzene ring (1.41 ± 0.01 Å) and in-between diiminosemiquinone and diiminobenzoquinones. Thus, the diiminosemiquinonate radical is not localized on a single ring as in 4b+, but delocalized over both peripheral ones. Because the dimer is dicationic, it immediately follows that both nickel ions adopt a (+II) oxidation state in [4c]22+. The solid-state structure of complex 4d+ (Figure 5) is a compromise between 4b+ and [4c]22+, in the sense that it adopts a mononuclear structure, as does 4b+, but the metrical parameters in the organic framework better match those of 4c+. The C−N bonds in the lateral rings are identical within 0.013 Å, with a mean length of 1.335 Å. The C13−N2 and C14−N3 bonds (central ring) are significantly longer, at 1.401(3) and 1.399(3) Å. Thus, 4d+ is assigned to a mixed-valent radical complex,31,32 featuring a diamidobenzene bridging ring and an iminosemiquinonate π radical that is delocalized over both peripheral rings. This structural investigation underlines the crucial role of the substituents in modulating the nuclearity of the cations. 4c+, which has no substituent on the phenyl bridging ring, forms a dimer at the solid state, whereas 4b+ and 4d+ do not. Similar

opposite side in the solid state. The central ring then exhibits a diamidobenzene character, similarly to 4b. This idea is reinforced by the examination of the C13−N2 and C14−N3 bonds lengths, which are significantly larger with an average distance of 1.400 ± 0.003 Å. The structure of 4c+ is depicted in Figure 6. In contrast to 4b+, which is monomeric, this complex is assembled into a dimer of global formula [4c]22+. The dimer [4c]22+ exhibits a 90°-turned eclipsed conformation. The aromatic bridge of each ligand is stacked over a single peripheral ring of the second ligand moiety. The distances between the stacked rings in the pair range between 3.26 and 3.93 Å, consistent with stabilizing π−π interactions. The intermetallic Ni1A−Ni1B distance is 2.784 Å, with the metal ions Ni1A and Ni1B being, respectively, displaced 0.181 and 0.226 Å away from the mean N1−N2− N3−N4 coordination planes. Interestingly, each moiety of the dimer adopts an umbrella shape like the neutral precursor 4c, with a mean tilt angle of 19° between the aromatic central bridge and the peripheral rings. Examination of the bond lengths in the ligand framework reveals several important features. For instance, all the C−N bond distances are similar at 1.332 ± 0.005 Å in the peripheral rings of subunit A. They slightly diverge in subunit B, but the mean C−N bond distance H

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry dimerization processes have been reported for oxidized cisoriented Ni complexes derived from the 3,5-di-tert-butylphenylene-1,2-diamine.9 The intermetallic distance was very close to ours and the nickel oxidation state assigned to (+II). Nevertheless, there is major conformational difference between these dimers and [4c]22+: In the former case the complexes dimerize through π-stacking interaction between each ligand radical, while in [4c]22+ a 90°-turned eclipsed conformation is observed, presumably to minimize steric constraints. It is likely that dimerization of 4b+ and 4d+ is prevented by the sterically hindered p-methoxyphenyl and chlorine substituents, respectively. It is clear that the dimerization process observed for 3+ is not the consequence of a delocalization of the ligand radical SOMO as it would also have been observed for 4d+ but it was not. Further, the difference in ligand radical localization in 4(b−d)+ deserves a comment: Delocalization could have been influenced by the electronics of the substituents, as 4b+ harbors electron-donating groups on the aromatic bridge, whereas 4d+ harbors electron-withdrawing chlorine groups. However, vis− NIR measurements (see below) do not support this hypothesis. The most likely reason for the change in ligand radical delocalization in 4(b−d)+ is therefore packing effects at the solid state. Thus, 4b+ and 4d+ could be considered as benchmarks for localized and delocalized radical species in the solid state, respectively, allowing for a precise estimation of the changes in bond length associated with SOMO relocalization. 3.5. X-ray Crystal Structure of the Dications. The twoelectron-oxidized 4(a−e)2+ were chemically generated by addition of 2 equiv of AgSbF6 to the neutral precursors. 4(b−d)2+ were isolated as single crystals for structural determinations. All are mononuclear species, which display closely related structures. In 4b2+ the aromatic bridge is twisted by 11° with respect to the mean coordination plane. The twist is less pronounced in the other complexes, as a result of weaker steric demand of the substituents. Two crystallographically independent molecules were found in the crystal cell of 4c2+ (Figure 7). One dication

shows an umbrella shape with an average angle of 25° between the bridge and the peripheral rings. The metal ion lies 0.14 Å above the mean coordination plane, as a consequence of a weak interaction between the nickel and a counterion in the axial position (Ni1−F7 distance of 2.771 Å). In the second molecule the two peripheral diiminobenzoquinone moieties adopt an unusual chairlike conformation, while the metal ion is again subjected to interactions with the SbF6− counterion. In this case two SbF6− molecules are located at both apical positions, at the Ni2−F15 and Ni2−F21 distances of 2.936 and 3.089 Å, respectively. The same global feature is observed in the chlorinated 4d2+ with an average angle of 20° between the bridge and the peripheral rings. The electronic structure of the dication could be established by focusing on the bond lengths in the aromatic framework. Within the peripheral rings the average C1−N1 and C7−N4 bond distances (terminal nitrogens) are 1.290 ± 0.015 Å, while the C2−N2 and C8−N3 bond distances (connecting nitrogens) are 1.320 ± 0.015 and 1.320 ± 0.005 Å, respectively. These metrical parameters unambiguously show that both peripheral rings are diiminobenzoquinones. Thus, double oxidation of the neutral precursors occurred on both peripheral rings, affording a closed-shell ligand. Accordingly, the bond distances within the bridging aromatic ring are poorly affected by oxidation, as judged by the mean C13−N2 and C14−N3 bond distances at 1.41 ± 0.01 and 1.40 ± 0.01 Å, which compare fairly with those of the diamidobenzene ring in the neutral precursor. Finally, the diiminobenzoquinones are expected to be weaker ligands than diiminosemiquinonates, resulting in longer coordination bonds. This is experimentally verified for 4(b−d)2+, which show a global expansion of the coordination sphere by ca. 0.04 Å with respect to 4(b−d). 3.5. Electronic Spectra of the Neutral and Mono- and Dioxidized Species. The UV−vis−NIR spectra of the members of each electron-transfer series were measured in CH2Cl2 at 298 K. The neutral complexes 4(a−e) (Figure 8 shows data for 4b, 4d; see SI for results for 4a, 4c, 4e) exhibit two main absorption bands: an intense band is found in the region 950−980 nm (ε ∼ 1.0 to 1.5 × 104 L mol−1 cm−1), while another one is observed in the range 370−400 nm (ε ∼ 3.5 × 104 L mol−1 cm−1). The latter is assigned to a CT transition. The strong NIR band is a fingerprint of square-planar Ni(II) complexes featuring two diiminosemiquinonate ligands.9,10 On the basis of TDDFT calculations (see below) it is assigned to an LLCT (ligand-to-ligand-charge-transfer) transition involving the diiminosemiquinonate radical moieties. We recently reported that 3a could form complexes of stoichiometry 1:2 (M:L) with the nickel, where the metal ion lies in a squareplanar geometry coordinated to two diiminosemiquinonate moieties. The main NIR band was observed at 877 nm with a much larger intensity (55 160 M−1 cm−1). Further, this NIR band is detected at 790 nm with a similar high intensity (ε = 54 000 M−1 cm−1) in the homoleptic complex [NiII(LISQ)2].9 The lower intensity of the NIR band in 4(a−e) is attributed to a smaller diradical index,10 resulting from the connexion of the diiminosemiquinonate moieties by an aromatic ring. It is noticeable that the λmax is observed in a sharp spectral window for all the compounds (Table 3): 954 nm (12 050 L mol−1 cm−1) (4a), 966 nm (10 110 L mol−1 cm−1) (4b), 966 nm (15 240 L mol−1 cm−1) (4c), 974 nm (11 770 L mol−1 cm−1) (4d), and 970 nm (12 410 L mol−1 cm−1) (4e). This marginal substituent effect corroborates the fact that the electronic

Figure 7. X-ray structure of 4c2+ at the 30% ellipsoid probability. Counterions and H atoms are omitted for clarity, except the anilido ones. I

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. UV−vis spectra of 4b0/+/2+ and 4d0/+/2+ in CH2Cl2 solutions (5 × 10−4 M, 0.1 M TBAP, T = 298 K).

reminiscent of monoradical species is additionally observed at ca. 400 nm (ε ∼ 1.5 to 3.1 × 104 L mol−1 cm−1) and attributed to a CT transition. The shape of the vis−NIR spectrum evolves only marginally when CH2Cl2 is replaced by toluene. It was not possible to extend this study to THF and polar or protic solvents due to the poor stability of the complexes in these media. Since the global shape of the low-energy band is similar in the series, with some differences in intensities, it is interesting to compare the solution data in CH2Cl2 with the solid-state structures described above. X-ray diffraction showed that 4b+ and 4d+ are localized and delocalized radicals, respectively. The NIR band is however found to be more intense in 4b+ than in 4d+. This trend is opposite to what would be expected if one has considered pure IVCT character of the band and the fact that the complexes retain their solid-state structure in solution. Thus, it is likely that (i) the localized radical character of 4b+ in the solid state is a consequence of crystal packing effect and (ii) the bridge slightly contributes to the transition. The two-electron-oxidized 4(a−e)2+ exhibit only a featureless tail instead of a high-intensity band in the NIR region, consistent with a closed-shell electronic configuration of the complexes. The visible spectrum is dominated by a pair of lowenergy bands of medium intensity in the 600−800 nm and 460−510 nm (ε ∼ 1.7−2.8 × 104 L mol−1 cm−1) regions that are assigned to CT transitions involving mainly the diiminobenzoquinone moieties. The former band is red-shifted, while its intensity is enhanced in comparison to [M(LISQ)2]+9 due to the extended conjugation. We electrochemically generated only 4d−, which is the most accessible anion due to the strong electron-withdrawing effect of the chlorine substituents, for spectroscopic characterizations. Its electronic spectrum displays a strong band at 1420 nm (8920 M−1 cm−1), whose intensity exceeds that of the NIR band of 4d+. Both its low energy and high intensity are indicative of a LLCT transition, which predominantly involves the peripheral fragments. 3.6. EPR Spectroscopy and Magnetic Susceptibility. The neutral compounds 4(a−e) were found to be mainly EPR silent in the temperature range 10−150 K, although they are diradical species. This is in agreement with measurements of the magnetic susceptibility carried out on compounds 4b and 4d that show they are diamagnetic. This suggests the occurrence of a strong intramolecular antiferromagnetic coupling between the iminosemiquinonate moieties. The antiferromagnetic exchange between the two diiminosemiquinonate moieties in the unsubstituted derivative of [NiII(LISQ)2]

Table 3. Electronic Spectra of the Complexes compd [Ni(MeL)]0 (4a) [Ni(MeL)]+ (4a+)a [Ni(MeL)]2+ (4a2+)a [Ni(ArL)]0 (4b) [Ni(ArL)]+ (4b+)b [Ni(ArL)]2+ (4b2+)b [Ni(HL)]0 (4c) [Ni(HL)]+ (4c+)a [Ni(HL)]2+ (4c2+)a [Ni(ClL)]0 (4d) [Ni(ClL)]+ (4d+)b [Ni(ClL)]2+ (4d2+)b [Ni(ClL)]− (4d‑) [Ni(FL)]0 (4e) [Ni(FL)]+ (4e+)a [Ni(FL)]2+ (4e2+)a

λmax, nm (ε, L mol

−1

−1

cm )

376(35 870), 478(4860), 542(4090), 954(12 051) 392(31 575), 485(20 607), 524sh(17 375), 608(11 220), 813sh(5191), 899(11 325) 407sh(16 617), 466(26 377), 686(7020), 738(7514) 320sh(11 968), 393(39 535), 520sh(2647), 563(3945), 668(1098), 966(10 114) 331sh(11 771), 412(21 170), 573(16 279), 658sh(7968), 908(7136) 388sh(8569), 452sh(12 241), 510(18 863), 693(10 536), 767(10 821) 306(15 670), 370(34 192), 453sh(3076), 544(1988), 966(15 238) 298sh(17 393), 392(26 170), 461sh(16 032), 511sh(12 371), 617(12 825), 811sh(4013), 895(9227) 400sh(20 344), 460(28 646), 677sh(8710), 719(9059) 304sh(11 170), 379(31 874), 545(1772), 974(11 772) 300sh(10 885), 337(10 401), 397(16 109), 472sh(11 538), 532sh(7376), 622(6824), 900(5416) 288sh(14 549), 411sh(12 397), 463(17 125), 623sh(4115), 673(4584), 737(4821) 385(22 040), 550(2510), 650(2110), 890(1816), 1420(8920) 370(28 797), 542(1901), 970(12 409) 332(12 579), 387(17 542), 432(14 971), 464sh(13 357), 518sh(8735), 616(8671), 808sh(3273), 894(6939) 393sh(12 240), 464(19 740), 689sh(6148), 728(6265)

a Electrochemically oxidized species (5 × 10−4 M in CH2Cl2 containing 0.1 M TBAP). bIsolated oxidized species after reaction with AgSbF6 (5 × 10−4 M in CH2Cl2).

structure is homogeneous within the series and that the main contributors to the NIR band are the peripheral radical rings. The UV−vis spectra of the one-electron-oxidized complexes 4(a−e)+ differ markedly from those of their neutral precursors (Figure 8). The intensity of the NIR band is lower (ε ∼ 0.5 to 1.0 × 104 L mol−1 cm−1), while it is shifted to higher energies (only 4a+ has a similar intensity as its parent 4a): 899 nm (11 330 L mol−1 cm−1) (4a+), 908 nm (7140 L mol−1 cm−1) (4b+), 895 nm (9230 L mol−1 cm−1) (4c+), 900 nm (5420 L mol−1 cm−1) (4d+), and 894 nm (6940 L mol−1 cm−1) (4e+). On the basis of its high intensity it is attributed to an LLCT involving the peripheral radical rings. TDDFT calculations support this assignment and provide a detailed picture of the molecular orbitals involved (see below). A higher energy band J

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry has been modeled by quantum chemical calculations.10,33 By using ab initio methods the singlet−triplet gap was estimated at 3000−4000 cm−1. This value is indicative of a strong antiferromagnetic exchange between the two diiminosemiquinonate moieties, which is mediated by an out-of-plane filled t2g orbital and explains the diamagnetic behavior that was observed from magnetic susceptibility measurements. The electrochemically generated cations 4(a−e)+ exhibit a rhombic EPR signal centered at around g ∼ 2, which is typical for radical species having an S = 1/2 ground state. The g values measured for the complexes are of the same order (Table 4). As

as a dimer [4c]22+. Antiferromagnetic interactions between each (S = 1/2) radical unit lead to the diamagnetism experimentally observed. Thus, these EPR data establish that the dimeric structure of [4c]22+ is preserved in frozen CH2Cl2 solution (double integration of the EPR spectrum reveals that only 5% of the cation is EPR-active). The intensity of the EPR signal of 4d+ is similar to that of 4c+. Though a mononuclear π-radical cation was crystallized in the former case, EPR data show that a dimer is likely formed in solution similarly to [4c]22+. This analysis can be extended to 4e+, for which no crystal structure is available. The highest intensity in the EPR signal was measured for [Ni(ArL)]+ (4b+), i.e., the compound that harbors two sterically demanding para-methoxyphenyl substituents, which prevent dimerization. Double integration of the EPR spectrum of 4b+ and comparison with a standard (DPPH) recorded under similar conditions confirms that 4b+ is solely present under its mononuclear form in CH2Cl2. The EPR data are therefore fully consistent with the solid-state structure, which displays a mononuclear radical complex. The methylated derivative 4a+ also mainly exists under a mononuclear form in frozen solution, as judged by the intensity of its EPR spectrum. Clearly, the electronics of the substituents on the bridge is not a main determinant for the electronic structure of the radical complexes. In contrast, it directly affects the ability of the complex to dimerize into a diamagnetic species, as revealed by the direct correlation between the intensity of the EPR signal and the steric bulk provided by the substituents: bulkier para-methoxyphenyl and methyl substituents favor monomeric structures, while H, F, and even Cl substituents favor dimeric structures. These EPR data therefore support the existence of the following equilibrium in solution, which is controlled by the steric demand of the R substituent:

Table 4. EPR Parameters of [RLNi]+ 4(a−e)+ and 4d−a,b compd

g1

g2

g3

gaverage

Δg

4a+

2.020 2.023 2.020 2.018 2.027

2.004 2.002 2.003 2.004 1.999

1.994 1.977 1.993 1.992 1.979

0.026 0.046 0.027 0.026 0.048

2.015 2.025 2.099 2.108 2.016

2.003 1.999 2.003 2.009 2.004

1.991 1.980 1.947 1.944 1.992

2.006 2.000c 2.005 2.005 2.001c diamagneticd 2.003 2.001c 2.016 2.020c 2.004

4b+ 4c+

4d+ 4d− 4e+

0.024 0.045 0.152 0.164 0.024

Electrochemically generated (5 × 10−4 M in CH2Cl2 solutions containing 0.1 M TBAP), 100 K. bIn italic type: Computed g-values (B3LYP/Def2TZVP/NiCP(PPP)/ZORA). cg-tensors for the delocalized radical. dThe dimer has an (S = 0) ground spin state a

an example, the following values were obtained for complex 4b+: g1 = 2.020, g2 = 2.003, g3 = 1.993 (gav = 2.005). The ganisotropy is relatively small, while the gav value deviates only slightly from that expected for free imisosemiquinonate radicals. This leads to the conclusion that the SOMO is essentially ligand-centered, with only a marginal metal contribution. Although the shape of the EPR signal remains mostly similar in the series, there are large differences in intensities (Figure 9). The following trend in intensity was observed: [Ni(ArL)]+ (4b+) > [Ni(MeL)]+ (4a+) ≫ [Ni(FL)]+ (4e+) > [Ni(ClL)]+ (4d+) ≈ [Ni(HL)]+ (4c+). The origin of such difference could be found in the crystallographic data discussed above. The weakest EPR signal is observed for 4c+, which was crystallized

R

R

2[Ni II( LISQ/IBQ )]+ ⇌ {[Ni II( LISQ/IBQ )]2 }2 +

We finally investigated the anion 4d−, whose EPR spectrum is depicted in SI. It is typical of an S = 1/2 species having rhombic symmetry (g1 = 2.099, g2 = 2.003, g3 = 1.947). The ganisotropy (Δg = 0.152) is much larger than that observed for the cation 4d+ (Δg = 0.024) and indicative of an enhanced metal contribution to the SOMO in anion. This is further confirmed by the giso at 2.016 that deviates significantly from the free electron value. On the other hand, Ni(I) complexes exhibit much larger giso (in the range 2.15−2.20), without component lower than 2.34 Thus, it is clear that 4d− is not a genuine nickel(I) complex, but a radical anion with some delocalization of the SOMO onto the metal. The enhanced metal contribution in the anion in comparison with the cation can be easily rationalized by geometrical considerations (see the DFT details below). 3.7. Theoretical Investigation. DFT calculations were performed by using the hybrid functional B3LYP35 on the dimer [4c]22+ and two-electron-transferred series: 4a0/+/2+, which feature electron-donating methyl groups on the central ring, and 4d−/0/+/2+, which feature electron-withdrawing chlorine substituents on this ring. Similar conclusions can be reached for both electron-transferred series; therefore, we will limit our discussion to 4d−/0/+/2+ in the text. Full details about the electronic structures and calculated spectroscopic parameters are given as Supporting Information. Geometric and Electronic Structures. For the neutral complexes the optimization was conducted by considering

Figure 9. Superimposed X-band EPR spectra at 100 K of the electrochemically generated cations 4(a−e)+ (0.5 mM complex in CH2Cl2, 0.1 M TBAP) compared to DPPH (dashed blue line, 0.5 mM in CH2Cl2, 0.1 M TBAP). K

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. Spin density plots for (a) the open-shell singlet 4d and (b) the symmetrical doublet 4d+ and (c) 4d−, including the Mulliken spin population (larger than |0.06|).

negative spin population on the three remaining ones (Figure 10). Finally, the energetic analysis conducted on the diradical 4d provides evidence for a triplet−singlet gap of only 0.7 kcal/ mol, which is within the accuracy of the DFT method. Thus, calculations consistently point to a diradical character of the neutral complexes, in agreement with experimental data. The geometry optimization conducted on doublet 4d+ allowed for the localization of two minima in the potential energy surface (PES). Frequency calculations confirm that both correspond to real minima and not transition states. One is a symmetrical structure (C−NH bond distance at 1.325 Å), while the other is unsymmetrical, with a large C−NH bond on one side (1.344 Å) and a much shorter one on the other side (1.311 Å). Clearly, only the former structure matches the experimental one, with again an excellent agreement between calculated and experimental bond distances. This is especially the case in the peripheral rings (±0.012 Å of the experimental values), wherein one can clearly distinguish the increased quinoidal distortion of bond distances when compared to the neutral precursor (as an example the C−N bonds are shortened by 0.02 Å in comparison to 4d). It is also significant that calculations correctly predict the elongation by about 0.02 Å of the coordination sphere on going 4d to symmetrical 4d+. The symmetrical structure is therefore assigned to a mixed-valence compound, where the spin density is equally shared between the two halves of the molecule. Regarding the unsymmetrical structure, positive spin density is found only on one side of the complex. It is therefore assigned to a localized radical species. Although the fully delocalized structure is more stable, the localized one remains very close in energy, as reflected by the difference in energy of 0.7 kcal/mol. Therefore, both forms may be relevant, and external factors such as crystal packing effects may favor either of the structures. This behavior is in line with the solid-state data obtained in the present series, which evidence a delocalized radical character for 4d+ but a localized radical species in another compound, namely, 4b+. When delocalized 4d+ is compared to 4d the spin population analysis

both a triplet diradical and singlet spin states. The coordination bond distances are reproduced within ±0.02 Å of the experimental results in the case of the singlet (±0.04 Å for the triplet), which is in the acceptable discrepancy inherent to the B3LYP DFT method. Most importantly, the quinoidal distribution of bond lengths within the peripheral rings is nicely reproduced by calculations, with the computed bond distances being ±0.009 Å of the experimental values. A good agreement is also found between the experimental and calculated values in the central ring (±0.013 and 0.022 Å of the experimental values for the singlet and triplet, respectively). It is significant that the triplet diradical is 4.7 kcal/mol more stable than the closedshell singlet. As expected, a stability analysis of the wave function of the closed-shell singlet reveals an UKS/RKS instability.35 Reoptimization allowed for the localization of a lower-lying open-shell singlet, which is identified 4 kcal/mol below the closed-shell singlet. The molecular orbital diagram of the closed-shell singlet 4d confirms this trend of the neutral complex to exist under a diradical form since the HOMO− LUMO gap is only 0.98 eV (the HOMO and LUMO are depicted in SI). The same analysis applies for 4a, which exhibits a somewhat larger energetic gap between the closed-shell singlet and open-shell singlet (7.2 kcal/mol in favor of the open-shell form). Thus, it is evident that both 4a and 4d are diradical species. The spin density plot of the open-shell singlet 4d is depicted in Figure 10, while that of 4a is shown in SI. Large Mulliken spin populations are found on the N atoms of 4d (0.26 and −0.26 for the terminal nitrogens, 0.20 and −0.20 for those shared with the bridge). Consistent with experimental findings, closely related features are observed for 4a (Mulliken spin population of 0.23 and −0.24 for the terminal nitrogens, 0.24 and −0.22 for those share with the bridge). 4d features an antisymmetric combination of the SOMOs of two diiminosemiquinonate fragments, which extends over the aromatic bridge, especially on the resonant positions Cortho and Cpara (see SI). Consequently, positive Mulliken spin population is also found on three carbons of the central ring, and there is L

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 11. TD-DFT assignment of the intense low-energy band for (a) 4d−, (b) the open-shell singlet 4d, and (c) the symmetrical doublet 4d+ and (d) 4d2+.

Dimerization therefore promotes a slight relocalization of the spin density on the stacked rings. Regarding the closed-shell dication 4d2+, calculations consistently reproduced the exacerbated quinoidal distortion of each peripheral ring in comparison to 4d+ (the C−N bonds shorten by 0.02 Å) and the global elongation of the coordination sphere (by ca. 0.02 Å). It is noteworthy that the extent of quinoidal distortion on going from 4d+ to 4d2+ is rather similar to that going from 4d to symmetrical 4d+. The doublet monoanion 4d− could only be converged into a symmetrical structure, which exhibits the longest C−N bond distances within the series (+0.02 Å in comparison with 4d). The quinoidal distribution of bond distances cannot be clearly observed in any ring, with four out of six C−C bond distances of the peripheral rings being for example in the small range 1.392−1.402 Å. This suggests an enhanced participation of the metal in that of the SOMO. This idea is reinforced by the analysis of the Mulliken spin population, which reveals a population at the Ni atom of 0.22. Regarding the N atoms it is 0.18 and 0.06 for the two distinct populations of nitrogens (N1, N4 and N2, N3, respectively), with similar or smaller contribution of the endocyclic carbons. This dramatic increase of the Mulliken spin population on the metal can be easily explained by geometric considerations: There is an efficient mixing of the ligand radical SOMO with an out-of-plane Ni orbital for symmetry reasons, resulting in a larger metal character of the SOMO. Calculation of the Spectroscopic Properties. In order to validate our theoretical approach and assign the lowest-energy vis−NIR bands we performed TD-DFT calculations on the geometry-optimized 4a0/+/2+ and 4d−/0/+/2+ (Figure 11). The results are summarized in Table 5. The same level of theory as before (B3LYP/TZVP) was used, but the solvent was included in the calculations (PCM/CH2Cl2). We additionally computed the EPR parameters of 4a+, 4d+, and 4d− by using a softer basis set for the nickel atom (CP(PPP)). We will first comment on the TD-DFT results obtained for neutral 4d. When the triplet form is considered, NIR bands of moderate intensity are predicted at 7776 cm−1 ( fosc = 0.0340) and 9033 cm−1 (fosc = 0.0682). They mainly arise from the βHOMO → βLUMO and βHOMO − 1 → βLUMO excitations, respectively. Their energies and intensities are however far from the experimental ones, consistent with a

points to a decreased contribution of the terminal N atoms (less than 0.06 for N1 and N4, see the numbering in Figure 2 and picture in Figure 10b) at the expense of the N2, N3, C1, and C7 ones. A similar trend is observed for 4a+. This indicates that the radical has an enhanced anilinyl character in the monocations in comparison to the neutral precursor. Geometry optimization on the dimer [4c]22+ was carried out by considering both triplet and singlet ground states. For comparison the corresponding monomer 4c+ was also geometry-optimized. The dimeric structure is retained during optimization in the case of the singlet, whereas constraints must be applied in the case of the triplet to preserve it. We will therefore limit the present discussion to the singlet. The coordination sphere of the two nickel ions is slightly unsymmetrical, in agreement with experimental findings. The coordination bond distances in the geometry-optimized dimer are slightly overestimated (+0.02 Å) in comparison to the solidstate data, but remain largely acceptable. It is noteworthy that the central Ni−N bond distances (Ni−N2 and Ni−N3) are identical to those obtained for the geometry-optimized monomer 4c+, while the peripheral ones are 0.01−0.02 Å larger. Most importantly, the peculiarities of the dimer are preserved although the calculated Ni−Ni bond distance is larger than the experimental one (3.340 Å). For example the nickel is located 0.174 Å above the mean plane formed by the N1, N2, N3, and N4 atoms, instead of remaining in this plane as in the optimized monomer 4c+. In addition, the Ni−Ni axis is almost orthogonal to this plane, with N−Ni−Ni angles in the range 83−108°. Owing to the fact that a closed-shell configuration was considered in the calculation it is obvious that this pair comprises a bis(nickel(II)) core, with π stacking interactions between aromatic rings. The stability analysis reveals the presence of a lower-lying open-shell singlet, which is localized 1.2 kcal/mol below the closed-shell singlet. The openshell singlet displays large positive spin population on one subunit that constitutes the dimer and negative spin population on the other one. It is therefore clear that [4c]22+ is composed of two antiferromagnetically exchanged-coupled nickel(II)semiquinonate radical entities 4c+. The extent of ligand radical delocalization differs somewhat in comparison to that observed for mononuclear 4c+. The Mulliken spin densities on the N1, N2, N3, and N4 atoms are 0, 0.11, 0.13, 0.06 on each half of [4c]22+, respectively, and 0.03, 0.16, 0.16, 0.03 for 4c+. M

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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cm−1). The fact that the coefficients of the low-energy excitation (Table 5) are similar for the α- and β-spin electrons has been recently reported by Bachler et al.35 for the homoleptic biradical [Ni(ISQL)2] complex. In fact only the oscillator strength of the NIR band differs significantly between the open-shell and closed-shell configurations, but remains large in both cases ( fosc = 0.1656 and 0.2367, respectively), in agreement with experimental data. In addition to this prominent band, calculations on the open-shell form predict lower-energy excitations at 1127 nm ( fosc = 0.0212, mainly α/ βHOMO − 1 → α/βLUMO transition), 1593 and 1624 nm ( fosc = 0.0076 and 0.0177, mainly α/βHOMO → α/βLUMO transition). They perfectly match three broad features experimentally observed in the NIR spectrum of 4d (see SI). When the closed-shell singlet is considered a single excitation is predicted in the NIR region, whose energy is largely underestimated (λcalc = 2120 nm, fosc = 0.0525). Thus, this analysis of the NIR spectrum again supports an open-shell electronic configuration of the singlet. In addition to the singlet → singlet excitations, we explored the singlet → triplet excitations for closed-shell 4d, as suggested by Casida.36 Owing to the fact that they are spin forbidden the oscillator strength is naturally zero. Most importantly, an imaginary triplet excitation energy was found, consistent with the fact that a broken symmetry solution is energetically favored over the symmetry unbroken solution for the ground state. TD-DFT calculations were then carried out on doublet 4d+ under both the symmetrical and unsymmetrical mononuclear forms. When the symmetrical structure is considered an electronic excitation is calculated at 12 034 cm−1 (fosc = 0.1682), which arises principally from the βHOMO → βLUMO + 1 transition (percentage contribution C = 53%, Figure 11c). It is assigned to the band experimentally detected at 11 123 cm−1. When the unsymmetrical form is considered an intense band corresponding to the βHOMO → βLUMO + 1 transition (percentage contribution C = 46%) is predicted at 10 650 cm−1 ( fosc = 0.1212). The spin expectation value is however substantially larger at both the ground (⟨S⟩2 = 0.85 before annihilation) and excited states (1.25),37 pointing to significant spin contamination in this case. Moreover, the experimental trend in energy on going from the neutral to the cationic complex is only reproduced when the symmetrical form is considered, further supporting a delocalized structure for 4d+. For the other doublet radical, namely 4d−, an electronic excitation is calculated at 7364 cm−1 with a large oscillator strength ( fosc = 0.1861), which perfectly matches the experimental band detected at 7042 cm−1. The excitation primarily originates from a βHOMO − 1→ βLUMO transition (C = 76%, Figure 11a). It is worth noting that the HOMO − 1 in the anion basically corresponds to the HOMO of the cation, while the LUMO + 1 of the anion represents the LUMO of the cation. In each radical the donor MO is a delocalized π orbital, which primarily involves the peripheral moieties and, at a lower extend, the bridging ring. The acceptor MO is again a delocalized π orbital, but it is clearly more developed on the peripheral rings. The intense vis−NIR band is therefore assigned to an LLCT transition for both the anion and cation radicals. Regarding the dication 4d2+, the lowest-energy electronic excitation of significant oscillator strength arises from a HOMO → LUMO + 1 transition (Figure 11d). The MOs involved in this excitation are similar in shape to those involved in the NIR

Table 5. TD-DFT Assignment of the Low-Energy Absorption Bands of 4a and 4d under Various Oxidation States complex 4a (closedshell singlet) 4a (open-shell singlet)

Eexpa (ε)

Ecalcda

fosc

11 481

0.2416

5880br (2130)b

5271

0.0108

7250br (1870)b

6266

0.0096

8711

0.0224

11 429

0.1508

7855

0.0377

8460

0.0502

9066

0.0231

4717

0.0525

11 351

0.2367

5900 (1634)b

6161

0.0177

b

6277

0.0076

7752 (1185)b

8873

0.0212

10 267 (11 772)

11 468

0.1656

7776

0.0340

9033

0.0682

10 482 (12051) 4a (triplet)

4d (closedshell singlet)

4d (open-shell singlet)

6370 (1606)

4d (triplet)

4d+

7500br (950)b

8576e

0.0205 e

0.1682

11 111 (5416)

12 034

8000br (1630)

9141e

0.0211

11 123 (11 325)

12 210e

0.1888

4d2+

13 569 (4821)

12 837

0.1826

4a2+

13 550 (7514)

12 870

0.2093

4d‑

7042 (8918)

7364

0.1861

4a+

assignment (C in %)d HOMO − 1 → LUMO (81) αHOMO → αLUMO (82) βHOMO → βLUMO (84) βHOMO − 1 → βLUMO (61) βHOMO − 1 → βLUMO (57) βHOMO → βLUMO (94) βHOMO → βLUMO + 1 (57) βHOMO → βLUMO + 1 (22) HOMO → LUMO (81) HOMO − 1→ LUMO (78) HOMO → LUMO (90)c HOMO → LUMO (94)c HOMO − 1→ LUMO (94)c HOMO − 1→ LUMO (86)c βHOMO → βLUMO (100) βHOMO − 1→ βLUMO (89) βHOMO → βLUMO (82) βHOMO → βLUMO + 1 (53) βHOMO → βLUMO (79) βHOMO → βLUMO + 1 (52) HOMO → LUMO + 1 (97) HOMO → LUMO + 1 (100) βHOMO − 1 → βLUMO (76)

a Transitions are given in cm−1 bValues from NIR spectra. cThe βHOMO → βLUMO and αHOMO → αLUMO transitions have close coefficients.35 Only a global value corresponding to the sum is given. The same situation is observed for the βHOMO − 1 → βLUMO and αHOMO − 1 → αLUMO transitions. dThe percentage contribution C of an individual excitation is expressed as the ratio between the square of the coefficient for a given excitation divided by the sum of the square of the coefficients for all the excitations. eThe values correspond to the symmetrical complexes.

singlet ground spin state of the complex. TD-DFT calculations were then performed on both the open-shell and closed-shell singlets. The agreement between experience and theory is much better in these two cases. An intense band is indeed predicted at 11 468 cm−1 (open-shell form) and 11351 cm−1 (closed-shell form), which mainly corresponds to the α/ βHOMO − 1 → α/βLUMO transition (Figure 11b). This excitation, which is LLCT in nature, accounts for the intense NIR band experimentally detected at 10 352 cm−1 (Γ1/2 = 1820 N

DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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intensity varies little in the series and without evident connection with solid-state data. We therefore interpret the localization observed in the crystal structures as arising from packing effects. Conversely, we observed a clear correlation between the intensity of the EPR signal and the steric bulk provided by the substituents of the central aromatic ring: the larger the steric bulk, the higher the intensity of the signal. Thus, more than the electronics, it is the steric demand of the R substituents that dictates the geometric structure of the cations, which have the propensity to assemble as dimers when the R group is the least bulky. Finally, the cations (4a−e)2+ were shown to harbor two peripheral diiminobenzoquinone rings, based on the C−N bond distances that are the shortest in the series. Consistent with this electronic structure in solution, they display only a featureless NIR tail instead of a high-intensity NIR band.

band of the cation, again reflecting an LLCT character of the transition. Serendipitously, for closed-shell compounds the agreement between theory and experiment is excellent, with the transition being computed at 12 837 cm−1 ( fosc = 0.1826, C = 97%) for an experimental band observed at 13 569 cm−1. The EPR parameters were computed for the cations 4a+, 4c+, and 4d+ as well as the anion 4d−. As shown in Table 4, there is a fairly good agreement between theory and experience. It is gratifying that calculations correctly predict the increase in ganisotropy (Δg) on going from the cation to the anion. The computed Δg is indeed 0.045 for symmetrical 4d+ (Δgexp = 0.024), while it is 0.164 (Δgexp = 0.155) for 4d−. The increase in Δg is directly correlated to the enhanced metal contribution to the SOMO in the anion (see above). The ground state acquires a substantial orbital angular momentum, which results in a larger g-anisotropy. Calculations further allow for the assignment of the g-components. In the anion, which gives rise to the larger Δg, the two highest g-components are in the x,y plane, while the lowest one points orthogonal to the x,y plane, similarly to the homoleptic [Ni(LISQ)2]− complex. Finally, the calculated isotropic g-tensor is in the range 2.000−2.001 for the cations, highlighting their dominant radical character. It is higher for the anion (2.020), consistent with the increased metallic contribution to the SOMO. Altogether DFT calculations support the notion that the redox processes are ligand-centered in both 4a and 4d and confirm the biradical character of the neutral complexes. Furthermore, they show that the substituents on the bridge (electron-donating methyl or electron-withdrawing chloride) show little influence on the electronic structure of the compounds.

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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.5b01947. Crystallographic data (CIF) 1 H and 13C NMR spectra of compounds 2(b−e) and 3(b−e), crystal structures, UV−vis−NIR spectra, and CV curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +33 476 51 4373 Fax: +33 476 51 4836.

4. CONCLUSION In this Article, we present the syntheses of five tetradentate ligands based on the N,N′-bis(3,5-di-tert-butyl-2-aminophenyl)o-phenylenediamine backbone, where the central phenylenediamine ring is differently substituted at the positions 4 and 5 (R = CH3 (3a), PhOCH3 (3b), H (3c), Cl (3d), F (3e)). Their nickel complexes were prepared and shown to form an electron-transfer series 4(a−e)2+/+/0/1−/2−, where all redox processes are centered on the peripheral rings. The electronic structures of 4(a−e) are remarkably homogeneous. These neutral compounds harbor two diiminosemiquinonate radicals located on both peripheral rings. They are singlet diradicals as a result of strong antiferromagnetic interaction between the radical moieties. 4(a−e) can undergo four reversible or quasireversible electron-transfers, two in reduction and two in oxidation. The solid-state structures of three cations were determined, showing a great diversity of geometrical arrangements. In the mononuclear complex 4b+ one peripheral ring has a diiminosemiquinonate character, and the other one is a benzodiiminoquinone character. In contrast, 4d+ is a mixed-valence compound, wherein the diiminosemiquinonate radical is delocalized over both peripheral rings. Finally, 4c+ is a dimer (Ni−Ni bond distance at 2.784 Å) stabilized by π-stacking interactions between the least sterically hindered bridging ring of one ligand molecule and one peripheral ring of the other ligand molecule. Delocalization of the diiminosemiquinonate radical is again observed in each ligand. Vis−NIR and EPR spectroscopies were used in order to probe the radical delocalization and dimerization ability, respectively, of the compounds in CH2Cl2. All the cations exhibit an intense LLCT transition in the NIR region, whose

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to sincerely thank the theoretical chemistry team of the DCM for fruitful discussion, especially Prof. M. E. Casida regarding TD-DFT calculations and Dr. P. Girard for technical assistance in these calculations, as well as Dr. M. Orio (iSm2, Marseille, France) for preliminary DFT calculations. We also gratefully acknowledge the Centre de Calcul Intensif en Chimie de Grenoble (CECIC) for providing the computational resources. This work has been partially supported by the Labex ARCANE (ANR-11-LABX-0003-01).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01947 Inorg. Chem. XXXX, XXX, XXX−XXX