A Redox-Active Cascade Precursor: Isolation of a Zwitterionic

Article pubs.acs.org/IC

A Redox-Active Cascade Precursor: Isolation of a Zwitterionic Triphenylphosphonio−Hydrazyl Radical and an Indazolo−Indazole Derivative Sandip Mondal, Suvendu Maity, and Prasanta Ghosh* Department of Chemistry, R. K. Mission Residential College, Narendrapur, Kolkata 103, West Bengal, India S Supporting Information *

ABSTRACT: A redox-active [ML] unit (M = CoII and MnII; LH 2 = N′-(1,4-dioxo-1,4-dihydronap hthalen-2-yl)benzohydrazide) defined as a cascade precursor that undergoes a multicomponent redox reaction comprising of a C−N bond formation, tautomerization, oxidation, C−C coupling, demetalation, and affording 6,14-dibenzoylbenzo[f ]benzo[5,6]indazolo[3a,3-c]indazole-5,8,13,16-tetraone (IndL2) is reported. Conversion of LH2 → IndL2 in air is overall a (6H++6e) oxidation reaction, and it opens a route for the syntheses of bioactive diarylindazolo[3a,3-c]indazole derivatives. The reaction occurs via a radical coupling reaction, and the radical intermediate was isolated as a triphenylphosphonio adduct. In presence of PPh3 the [ML] unit promotes a reaction that involves a C−P bond formation, tautomerization, and oxidation to yield a stable zwitterionic triphenylphosphonio-hydrazyl radical (PPh3L±•). Conversion of LH2 → PPh3L±• is a (3H++3e) oxidation reaction. To authenticate the [ML] unit, in addition to the Ind L2, a zinc(II) complex, [(L3)ZnII(H2O)Cl]·2MeOH (1·2MeOH), was successfully isolated (L3H = a pyridazine derivative of 1,4 naphthoquinone) from a reaction of LH2 with hydrated ZnCl2. Conversion of 3LH2 → 1 is also a multicomponent (6H++6e) oxidation reaction promoted by zinc(II) ion via a radical intermediate. Facile oxidation of [L2−] to [L•−] that was considered as an intermediate of these conversions was confirmed by isolating a 1,4 naphthoquinone-benzhydrazyl radical (LH•) complex, [(LH•)ZnII(H2O)Cl2] (2H•). The intermediates of LH2 → IndL2, LH2 → PPh3L±•, and 3LH2 → 1 conversions were analyzed by electrospray ionization mass spectroscopy. The molecular and electronic structures of PPh3L±•, IndL2, 1·2MeOH, and 2H• were confirmed by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, and density functional theory calculations.

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INTRODUCTION Cascade reactions that are composed of C−C, C−N, or C−X (X = O, P, S) bond formations having reduced chemical waste and furnish functional molecules are worthy in chemical science.1 The nature follows cascade path for syntheses with higher atom economy.2 Isolation or detection of a chemical species that undergoes multicomponent cascade reaction generating bioactive products not isolable from a schematic path has a significant effect on pharmacy. In last two decades several cascade reactions were established in organic syntheses.3 These are classified as nucleophilic/electrophilic, radical, pericyclic, or transition-metal-catalyzed cascades.4 However, in the vast coordination chemistry, no such reaction was exploited effectively so far, and the same was explored in this project. In this report, a coordination entity that promotes cascade reactions defined as a cascade precursor is disclosed. The study reveals that the N′-(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide (LH 2 ) 5 coordinated to cobalt(II) and manganese(II) ions, [ML], is a cascade precursor and undergoes a multicomponent redox reaction in air producing diarylindazolo[3a,3-c]indazole derivative (IndL2). In presence of © 2017 American Chemical Society

triphenyl phosphine the redox reactions of [ML] unit yields a stable zwitterionic hydrazyl radical of the type 2-benzoyl-1-ide-1(1,4-dioxo-3-(triphenylphosphonio)naphthaalene-2-yl)hydrazyl (PPh3L±•) as shown in Chart 1. Both IndL2 and PPh3L±• are significant molecules in chemical science and were not successfully isolated by other routes. Notably, isolation of indazoles, which exhibit diverse biological activities6 and were considered as drugs in many cases,7 is significant in synthetic research. Various routes of the syntheses of indazole derivatives were reported,8 but the indazolo− indazole derivatives are rare, and no dihydroindazolo[3a,3c]indazole variety was documented so far. In this context the isolation of IndL2 by a cascade reaction is noteworthy. Stabilization of organic radicals, which are multifunctional, is a challenge in modern chemistry.9 Activity of organic radicals as antioxidants10 and in spin memory devices,11 rechargeable batteries,12 catalysis,13 and biology14 has been intriguing and a subject of topical investigation. Numerous organic radicals Received: March 29, 2017 Published: July 11, 2017 8878

DOI: 10.1021/acs.inorgchem.7b00818 Inorg. Chem. 2017, 56, 8878−8888

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Inorganic Chemistry Chart 1

2MeOH were confirmed by single-crystal X-ray crystallography, EPR spectroscopy, and density functional theory (DFT) calculations. The primary significance is that the conversions of LH2 to IndL2, PPh3L±•, and 1 by schematic methods will be unwieldy and were not achieved so far.

coordinated to transition-metal ions were isolated to define their spectroscopic and X-ray structural parameters.15 Practical uses of these organic radicals in many cases are limited owing to the metal ions. Thus, the isolation of free organic radicals is an important task in exploring functional materials.1,16 Three significant stable radicals are triphenylmethyl,17a 2,2,6,6tetramethylpiperidine-N-oxyl (TEMPO),17b and N,N-diphenylN-picrylhydrazyl (DPPH),17c which were discovered approximately six to nine decades back and represent three classes of organic radicals. DPPH is widely used in electron paramagnetic resonance (EPR) spectroscopy as a standard and standardization of spin and anti oxidant activity. No other hydrazyl radical was successfully isolated since the discovery of DPPH in 1922. However, the chemistry of π-delocalized verdazyl and triazolinyl radicals developed significantly.9 Thus, the isolation of PPh3L±• from a cascade reaction is an impetus to the growth of the chemistry of organic radicals. In this article, the intermolecular cascades developed by an [ML] unit are summarized in Scheme 1. The intermediates of the

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Reagents or analyticalgrade materials were obtained from the commercial suppliers and used without further purification. Spectroscopic-grade solvents were used for spectroscopic and electrochemical measurements. The C, H, and N contents of the compounds were obtained from a PerkinElmer 2400 Series II elemental analyzer. Infrared spectra of the samples were measured from 4000 to 400 cm−1 with KBr pellets at room temperature on a PerkinElmer Spectrum RX 1 FT-IR spectrophotometer. 1H and 13C NMR spectra in CDCl3 solvent were recorded at 296 K on a Bruker Avance 500 MHz spectrometer. ESI mass spectra were recorded on a Shimadzu LCMS 2020 mass spectrometers equipped with ESI ion source. Electronic absorption spectra in solution were obtained on a PerkinElmer Lambda 750 spectrophotometer in the range of 3300−175 nm. The X-band EPR spectra were measured on a Magnettech GmbH MiniScope MS400 spectrometer (equipped with temperature controller TC H03), where the microwave frequency was measured with an FC400 frequency counter. The EPR spectra were simulated using EasySpin software. The electroanalytical instrument BASi Epsilon-EC for cyclic voltammetric experiments in CH2Cl2 solutions containing 0.2 M tetrabutylammonium hexafluorophosphate as supporting electrolyte was used. The BASi platinum working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode were used for the measurements. The redox potential data are referenced versus ferrocenium/ferrocene, Fc+/Fc, couple. BASi SEC-C thin layer quartz glass spectro-electrochemical cell kit (light path length of 1 mm) with platinum gauze working electrode and SEC-C platinum counter electrode were used for spectro-electrochemistry measurements.

Scheme 1

reactions were detected by electrospray ionization (ESI) mass spectroscopy. To analyze the role of metal ions, the relatively stable [ZnL] unit was isolated as in [(L3)ZnII(H2O)Cl]·2MeOH (1·2MeOH) from a reaction of LH2 with zinc(II) chloride salt, which also affords IndL2 as a minor product. L3 is a modified ligand containing a pyridazine ring as depicted in Chart 1. In addition, a 1,4-naphthoquinone-benzhydrazyl radical (LH·) complex of zinc(II) of the type [(LH•)ZnII(H2O)Cl2] (2H•), which was characterized by different spectroscopy, was isolated. The molecular and electronic structures of IndL2, PPh3L±•, and 1·

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SYNTHESES N′-(1,4-Dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide (LH2). LH2 was synthesized by a reported procedure.5 6,14-dibenzoylbenzo[f]benzo[5,6]indazolo[3a,3-c]indazole-5,8,13,16-tetraone (IndL2). To LH2 (73 mg, 0.25 8879


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Inorganic Chemistry mmol) in a round-bottom flask, methanol (25 mL) was added, and the mixture was heated to make a clear solution. To this solution, anhydrous CoCl2 (32 mg, 0.25 mmol) was added carefully. The solution immediately turned purple, which slowly turned green and then red in air. After 4 to 5 d, red crystals of Ind L2 separated out, which were collected upon filtration and dried in air. Yield: 45 mg (62% with respect to LH2). The similar reactions were performed using Mn(OAc)2 generating IndL2. Notably, the yield is higher (75%) when Mn(OAc)2 was used to promote the reaction. Anal. Calcd (%) for C34H18N4O6 (C, 70.59; H, 3.14; N, 9.68; Found: C, 70.52; H, 3.06; N, 9.55). Mass spectrum (ESI): m/z, 578.52 for [IndL2]+. 1H NMR (500 MHz, CDCl3): δ (ppm) = 8.41 (d, J = 7.5 Hz, 2H), 8.11 (d, J = 7.5 Hz, 2H), 7.93 (m, J = 7.0 and 7.5 Hz, 6H), 7.84 (t, J = 7.5 Hz, 2H), 7.53 (t, J = 7.5 Hz, 2H), 7.40 (t, J = 7.5 Hz, 4H). 13C NMR (500 MHz, CDCl3): δ (ppm) = 187.4, 178.5, 167.2, 146.7, 136.2, 136.0, 134.8, 134.5, 133.4, 130.9, 130.0, 128.3, 128.2, 127.7. IR/ cm−1 (KBr): ν = 1688 (s), 1595 (s), 1500 (s), 1461 (m) 1437 (s), 1264 (m), 1199 (s), 1146 (m), 1050 (m), 1010 (s), 770 (m). 2-Benzoyl-1-ide-1-(1,4-dioxo-3-(triphenylphosphonio)naphthalen-2-yl)hydrazyl (PPh3L±•). To N′(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide (LH2) (73 mg, 0.25 mmol) in methanol (25 mL) in a round-bottom flask, anhydrous CoCl2 (32 mg, 0.25 mmol) and triphenylphosphine (66 mg, 0.25 mmol) were added successively, and the mixture was refluxed gently for further 30 min. The solution turned violet. The hot solution was filtered, and the filtrate was allowed to evaporate slowly in air. Within 2 d, dark needles of PPh3 ±• L separated out, which were collected upon filtration. The violet filtrate was evaporated, and the dark mass obtained was purified on an alumina column using n-hexane−diethyl ether (for the removal of unreacted LH2 and PPh3) and diethyl ether− dichloromethane (for collecting PPh3L±•) mixtures. Total yield = 60 mg (43% with respect to (wrt) LH2). The similar reactions were performed using Mn(OAc)2 and Co(NO3)2 having 40− 43% yields. Anal. Calcd (%) for C35H24N2O3P (C, 76.22; H, 4.39; N, 5.08; Found: C, 76.01; H, 4.35; N, 4.99). Mass spectrum (ESI): m/z, 552.9 for [PPh3L±•]+. IR/cm−1 (KBr): ν = 1676 (s), 1634 (m), 1579 (s), 1539 (s), 1465 (m) 1436 (s), 1344 (m), 1270 (s), 1188 (m), 1120 (s), 998 (s), 720 (s), 695 (s) 520 (s). [(L 3 )Zn II (H 2 O)Cl]·2MeOH (1.2MeOH) and [(LH·)ZnII(H2O)Cl2] (2H·). To LH2 (73 mg, 0.25 mmol) in methanol (50 mL) in a round-bottom flask, ZnCl2·H2O (0.25 mmol) was added, and the reaction mixture was stirred for 2 h in air. The solution turned violet, and a black precipitate of 2H• separated out, which was filtered. The residue was washed with cold methanol and dried suitably in air for further analyses. Yield: 16 mg (15% wrt LH2). Anal. Calcd (%) for C17H14Cl2N2O4Zn (C, 45.72; H, 3.16; N, 6.27; Found: C, 45.42; H, 3.13; N, 6.16). Mass spectrum (ESI): m/z, 443 for [2H•]. IR/cm−1 (KBr): ν = 3430 (s, H2O), 3205(m, −NH−), 1647 (m, CO), 1300 (m) 1235 (m), 1095 (m), 701 (s). The violet filtrate obtained from the above reaction was allowed to evaporate slowly in air. The solution turned red, and dark red needles of 1·2MeOH separated out. The crystals were collected upon filtration, and the single crystals for X-ray diffraction study were selected from this crop. Yield: 25 mg (28% wrt LH2). The elemental analyses were performed after evaporating the solvents of the crystals in vacuum. Anal. Calcd (%) for C44H25ClN4O9Zn (C, 61.84; H, 2.95; N, 6.56; Found: C, 61.54; H, 2.94; N, 6.28). Mass spectrum (ESI): m/z, 857 for [1]. IR/cm−1 (KBr): ν = 3435(s, H2O), 1755(s, −COOPh), 1686 (s, CO), 1569 (s), 1545 (s), 1350 (m), 1260 (m), 1090 (m), 970

(m), 710 (s), 695 (m). 1H NMR (500 MHz, CDCl3): δ (ppm) = 8.44 (t, 4H), 8.05 (d, 4H), 7.84−4.60 (m, 8 H), 7.07−6.90 (m, 6H), 3.73 (s, 1H), 1.59 (s, 2H). The filtrate obtained after collecting the crystals of 1·2MeOH was evaporated to dryness and purified by column chromatography using basic alumina. After it was washed with n-hexane− diethyl ether (for the removal of unreacted LH2), the column was charged with diethyl ether−dichloromethane mixture, and the eluent was collected and evaporated. A small amount of IndL2 was obtained. Single Crystal X-ray Structure Determinations of the Complexes. Single crystals of PPh3L±•, IndL2, and 1·2MeOH were picked up with nylon loops and were mounted on a Bruker AXS D8 QUEST ECO diffractometer equipped with a Mo-target rotating-anode X-ray source and a graphite monochromator (Mo Kα, λ = 0.710 73 Å). Final cell constants were obtained from least-squares fits of all measured reflections. Intensity data were corrected for absorption using intensities of redundant reflections. The structures were readily solved by direct methods and subsequent difference Fourier techniques. The crystallographic data were listed in Table 1. The SHELXS-97 (Sheldrick 2008) software package was used for solution and SHELXL2014/6 (Sheldrick, 2014) was used for the refinement.18 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms Table 1. X-ray Crystallographic Data of PPh3L±•, IndL2, and 1· 2MeOH PPh3 ±•

L

formula fw cryst color cryst system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å) Z T (K) refl. collected (2θ) ρ calcd (g cm−3) unique ref/ref (I > 2σ) μ (mm−1)/λ (Å) F(000) R1b/goodness of fitc wR2d [I > 2σ(I)] no. of params/ restr resid density (eÅ−3)

Ind

L2

1·2MeOH

C35H24N2O3P 551.54 black monoclinic P21/c 13.9592(6) 13.9215(6) 15.7829(7) 90 116.067(2) 90 2755.2(2) 4 293(2) 51.928

C34H18N4O6 578.52 red monoclinic P21/c 12.7311(3) 14.7060(4) 14.7628(4) 90 108.8970(10) 90 2614.97(12) 4 293(2) 50.484

C46H34N4O11ClZn 919.59 red triclinic P1̅ 12.4726(4) 12.9389(4) 13.6110(4) 91.1650(10) 114.1480(10) 101.0340(10) 1955.33(11) 2 293(2) 51.54

1.332 5360/4031

1.469 4793/2934

1.562 6862/5491

0.140/0.710 73

0.103/0.710 73

0.769/0.710 73

1152 0.0565/1.087

1192 0.0529/1.074

940 0.0462/1.045

0.1464

0.1234

0.1250

370/0

398/0

575/0

0.497/−0.411

0.304/−0.230

0.576/−0.555

Observation criterion: I > 2σ(I). bR1 = ∑∥Fo| − |Fc∥/∑|Fo|. cGOF = {∑[w(Fo2 − Fc2)2]/(n − p)}1/2. dwR2 = {∑[w(Fo2 − Fc2)2]/ ∑[w(Fo2)2]}1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP], P = (Fo2 + 2Fc2)/3. a

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final product, IndL2 (calcd mass = 578.52; found m/z peak 579) as shown in Figure S4. It infers that the conversion of A → IndL2 occurs via the intermediate B that further undergoes oxidation and demetalation affording IndL2. In this regard, a recent report on the formation of C−C bond activating a C−H bond of 1,4naphthoquinone is noteworthy.24 The intermolecular 1,4 addition reaction and the generation of a bis(1,4-naphthosemiquinonate) as an intermediate was further authenticated by isolating PPh3L±•. PPh3 ±• L is stable in solid and solution, and was successfully purified by chromatography using a basic alumina column. The conversion of LH2 to PPh3L±•, which exhibits several canonical forms by delocalizing the spin/charge throughout the π system as shown in Scheme 3, undergoes via multiple steps as depicted in Scheme 4. The major steps are 1,4 addition of PPh3 to A, followed by tautomerization and deprotonation affording naphthalene-1,4-diol as an intermediate (C). PPh3 being a stronger nucleophile than an amine ceases 1,4 addition reaction between A and LH2. The one-electron oxidation of C generates a phosphonium-1,4-naphthosemiquinonate that further undergoes 2e oxidation in air to make a stronger − NN− bond and affords PPh3L±•. After 1 h of the reaction no m/z peak due to the phosphonium intermediate C (calcd mass = 682.41) was observed. However, a stronger m/z peak at 683 due to C in addition to the m/z peak at 553 due to PPh3L±• (calcd mass = 552.9), as illustrated in Figure S5, appeared after 6 h of the reaction, from which we infer that the C is a key intermediate of this redox reaction. Similarly, the alike intermediates, A [LMn(OAc)2] (calcd mass = 463.03, found m/z peak, 464) and C [(PPh3L)Mn(OAc)2] (calcd mass = 725.12, found m/z peak, 727) were detected by mass spectrometry (see Figure S6), when Mn(OAc)2 salt was used to promote the reaction in methanol, supporting the proposal of Scheme 4. Successful isolation of PPh3L±• strongly suggests that the conversion of LH2 → IndL2 undergoes having bis(1,4-naphthosemiquinonate) as an intermediate. The coordination of metal ion to LH2 in promoting the cascade reactions was established by isolating zinc(II) complexes, 1·2MeOH, and 2H·. Reaction of LH2 with hydrated ZnCl2 in MeOH affords a [ZnL] unit that promotes a nucleophilic addition to one of the keto functions of another LH2 molecule making a C−C bond giving an anionic intermediate D as shown in Scheme 5. Adduct D further reacts with another LH2 molecule and leads to the formations of C−O, C−N, and C−C bonds via a radical path as depicted in Scheme S1 (see, Supporting Information). Finally an aromatic pyridazine ring was achieved by eliminating the protonated PhCONH-NH- function. Elimination of one of the Cl− ions from the coordination sphere of the anion yields 1. The LH2 → 1 conversion as given in eq 1 is overall a (6e+6H+) oxidation reaction. The existence of intermediate [D-H2O]− in solution was authenticated by a m/z peak at 715 in ESI negative mass spectrum as illustrated in Figure S7.

were placed at the calculated positions and refined as riding atoms with isotropic displacement parameters. Density Functional Theory Calculations. All calculations reported in this article were done with the Gaussian 03W19 program package supported by Gauss View 4.1. The DFT20 calculations were performed at the level of Becke threeparameter hybrid functional with the nonlocal correlation functional of Lee−Yang−Parr (B3LYP).21 Gas-phase geometries of PPh3L±• and 2H· with doublet spin state and those of [PPh3L±]− and IndL2 with singlet spin state were optimized using Pulay’s Direct Inversion6 in the Iterative Subspace (DIIS), “tight” convergent self-consistent field (SCF) procedure22 ignoring symmetry. Valence double-ζ with diffuse and polarization functions, 6-31+G* as basis set,23 was employed for all the atoms for all the calculations. The closed-shell singlet (CSS) solution of [PPh3L±]− is stable.

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RESULTS AND DISCUSSION Cascade Reactions. Details of the syntheses are outlined in the Experimental Section. The 1H and 13C NMR spectra of IndL2 recorded in CDCl3 are illustrated in Figures S1 and S2. The NMR shielding tensors and the spin−spin coupling constants were summarized in Experimental Section. The reaction of LH2 with cobalt(II) and manganese(II) salts in MeOH in air at room temperature (RT) affords IndL2′, whereas the same reaction in the presence of PPh3 in boiling MeOH furnishes PPh3L±• in higher yields. The change of UV−vis absorption during LH2 → IndL2 conversion using CoCl2 recorded in methanol exhibits several isosbestic points as shown in Figure 1. Notably, in both cases no

Figure 1. Change of UV−vis absorption spectra during LH2 → conversion in MeOH in presence CoCl2 at RT.

Ind

L2

reaction was observed in absence of metal ions. Coordination and deprotonation of LH2 affording [ML] were authenticated by ESI mass spectroscopy (vide infra). The activity of the [ML] unit does not depend on the coligand of the cobalt ion. The reactions were performed with CoCl2·6H2O, anhydrous CoCl2, Co(NO3)2, and Mn(OAc)2 have similar products. The cascade path of the LH2 → IndL2 conversion promoted by cobalt(II) ions is depicted in Scheme 2. The intermediate A [LCoCl2] (calcd mass = 418.94, found m/ z peak, 419) was detected in the ESI mass spectrum as shown in Figure S3. It is overall a (6H++6e) oxidation reaction involving two LH2. The reaction progresses with 1,4-addition of LH2 to A followed by tautomerization and deprotonation generating bis(naphthalene-1,4-diol) that undergoes facile oxidation to a bis(1,4-naphtho-semiquinonate) adduct promoting a C−C radical coupling reaction affording B. The ESI mass spectrum of the reaction mixture recorded just after 1 h displays the m/z signal at 419, due to intermediate A only. However, after 2 h the mixture exhibits m/z signals due to the intermediates A and B (calcd mass = 708.13, found m/z peak, 709) in addition to the

3LH 2 + ZnCl 2·H 2O → [(L3)Zn II(H 2O)Cl](2) + Cl− + 5H+ + PhCONHNH 2

(1)

In air [ZnL] unit as in 22− of Scheme 6 undergoes oxidation to an anion radical, 2·−. Monoprotonation of 2•− affords the paramagnetic 2H•, which was successfully isolated as a black solid. The existence of neutral LH• in the powder was 8881


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Inorganic Chemistry Scheme 2. Proposed Cascade of the Formation of IndL2

Scheme 3. Canonical Forms of PPh3L±•

isolating 1·2MeOH and 2H•. The [ZnL] unit is relatively stable, and the reaction of zinc(II) salt with LH2 affords IndL2 as a minor product and promotes another (6H++6e) cascade yielding 1. The radical path of this cascade was established by isolating 2H•. X-ray Crystallography. The molecular geometries of PPh3 ±• Ind L , L2, and 1·2MeOH were confirmed by single-crystal X-ray crystallography. The crystallographic data are summarized in Table 1. Both PPh3L±• and IndL2 crystallize in P21/c space group. The molecular structures in crystals and the atom labeling scheme of IndL2 and PPh3L±• are illustrated in Figure 2a,b respectively. The selected bond parameters are summarized in Table 2. The N−N, N−C, and the C−OQ lengths of IndL2 and PPh3 ±• L are significantly different. The N(1)−N(2) and N(3)− N(4) lengths of IndL2 are 1.376(3) and 1.381(3) Å, while the C(2)−N(1) and C(20)−N(4) lengths are 1.287(3) and 1.287(3) Å, which are consistent with the o-iminoquinone

authenticated by IR, ESI mass (see, Figure S8), and EPR spectroscopies. The existence of 22− and 2azo was identified by cyclic voltammetry (vide infra); however, 2H− and [ZnLH2] were not successfully detected. Isolation of 2H• and PPh3L±• strongly suggests the radical paths of the LH2 → IndL2 and LH2 → 1 conversions that promote C−C coupling reactions. The study infers that the coordination of the metal ions to LH2 leads to the deprotonation and that the [ML] unit becomes reactive promoting an intermolecular 1,4 addition reaction and generates a bis(naphthalene-1,4-diol) intermediate. In air the naphthalene-1,4-diol unit undergoes oxidation to a bis(1,4 naphthosemiquinonate anion radical). Coupling reaction between two radical units results in a C−C bond formation. It is one of the major steps of the redox cascade of the LH2 → IndL2 conversion. The radical intermediate was successfully isolated as a zwitterionic triphenylphosphonio-hydrazyl radical (PPh3L±•). The coordination of the metal ions to LH2 was confirmed by 8882


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Inorganic Chemistry Scheme 4. Proposed Path of Formation of PPh3L±•

Scheme 5. Proposed Path of Formation of 1·2MeOH

Scheme 6

DPPH (N−N, 1.352(7) Å).17c It is noteworthy that the N−N length in phenyl osazone coordinated to transition-metal ions spans a range of 1.37 ± 1 Å.25 Notably, the C(3)−N(1) and C−P lengths of PPh3L±• are relatively shorter. The features can be analyzed by the resonance structures of PPh3L±• as depicted in

state of IndL2. The C−C length in the bicycle is 1.544(3) Å. The average C−OQ lengths are 1.213(3) Å. The bond parameters of PPh3L±• are different from those of the DPPH, which is a neutral hydrazyl radical. The N−N bond length of PPh3L±• is 1.372(3) Å, which is longer than that of the 8883


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Figure 2. Molecular geometries of (a) IndL2 and (b) PPh3L±•. (C) 1·2MeOH in crystals (40% thermal ellipsoids, hydrogen atoms and MeOH solvent molecules are omitted for clarity).

between aromatic fragments. The Zn(1)−O(1) and Zn(1)− O(3) lengths are significantly different because of the neutral state of the quinone fragment and the monoanionic state of the hydrazide, −NC(O−)-Ph, function. The o-iminoquinone state of the coordinated fragment was predicted by the C−O (1.234(4)) and C−N (1.295(4) Å) bond lengths. The C−C bond, C(3)−C(18), 1.596(4) Å, between two chiral centers is relatively longer than C(26)−C(27) bond, 1.384(4) Å, that belongs to the aromatic pyridazine ring. The N−N distance of the pyridazine ring is 1.330(4) Å. EPR Spectroscopy and DFT Calculations. The X-band EPR signals of powder and CH2Cl2 solution of PPh3L±• at 298 K are relatively broader, and no hyperfine splitting due to 14N (I =

Scheme 3. Because of the resonance, the C(1)−OQ bond is relatively longer than C(4)−OQ (Q = quinone). The average C− OQ lengths, 1.240(3) Å, are similar to those of a zwitterionic pyridinio-semiquinone π-radicals reported recently.16d 1·2MeOH crystallizes in P1̅ space group. The molecular structure in crystals and the atom labeling scheme of 1·2MeOH are depicted in Figure 2c. The selected bond parameters are summarized in Table 3. The molecule exhibits a trigonal bipyramidal (tbp) geometry having H2O and Cl− ligands in the equatorial plane. A notable feature is that the chiral C(3) atom containing the coordinated fragment lies in a plane, while the chiral C(18) atom containing the pyridazine ring makes another plane. Two planes are parallel and display π−π interactions 8884


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Inorganic Chemistry Table 2. Selected Experimental and Calculated Bond Lengths of (Å) of PPh3L±• and IndL2 PPh3 ±•

Ind

L

C(1)−O(1) C(4)−O(2) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(2)−P(1) C(3)−N(1) N(1)−N(2) N(2)−C(11) C(11)−O(3) C(11)−C(12)

exptl

calcd

1.247(3) 1.234(3) 1.419(4) 1.447(3) 1.491(4) 1.762(3) 1.309(3) 1.372(3) 1.378(3) 1.221(3) 1.490(4)

1.246 1.227 1.450 1.423 1.500 1.850 1.350 1.290 1.415 1.230 1.493

O(1)−C(1) C(4)−O(2) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(2)−N(1) N(1)−N(2) N(2)−C(11) N(2)−C(28) C(28)−O(6) C(11)−C(12)

L2 1.215(3) 1.212(3) 1.487(4) 1.499(4) 1.534(4) 1.287(3) 1.376(3) 1.495(3) 1.382(3) 1.217(3) 1.535(4)

Table 3. Selected Experimental Bond Lengths of (Å) of 1· 2MeOH Zn(1)−Cl(1) Zn(1)−O(9) Zn(1)−N(1) Zn(1)−O(1) Zn(1)−O(3) C(1)−O(1) C(11)−O(3) C(4)−O(2) C(1)−C(2) C(2)−N(1) N(1)−N(2) N(2)−C(11) C(2)−C(3)

2.1842(9) 2.015(3) 2.058(3) 2.279(2) 2.092(3) 1.234(4) 1.255(4) 1.207(4) 1.482(4) 1.295(4) 1.353(3) 1.361(4) 1.501(4)

C(3)−C(4) N(3)−N(4) N(3)−C(36) N(4)−C(37) C(25)−O(4) C(3)−C(18) C(26)−C(27) C(28)−O(5) C(35)−O(6) C(38)−O(8) C(37)−C(26) C(36)−C(27) C(18)−O(7)

1.529(4) 1.330(4) 1.334(4) 1.340(4) 1.204(4) 1.596(4) 1.384(4) 1.208(4) 1.213(4) 1.199(4) 1.395(4) 1.398(4) 1.365(4)

Figure 3. X-band EPR spectra of (a) PPh3L±• and (b) 2H· in CH2Cl2 at 298 K (experimental, black and simulated, red). (c) Atomic spin of PPh3 ±• L (O1, 0.14; O2, 0.07; C2, 0.20; C3, 0.20; N1, −0.06; N2, 0.37; O3, 0.08).

average C−OQ, 1.249 Å, lengths in comparison to those of L . Redox Activity and Electronic Spectra. PPh3L±• is redoxactive. The anodic wave due to PPh3L+/PPh3L±• redox couple at +0.52 V with respect to Fc+/Fc couple is irreversible, while the cathodic wave due to PPh3L±•/PPh3L− redox couple at −1.51 V is quite reversible as shown in Figure 4a. The CSS solution of [PPh3L±]− is stable, predicting an electronic state of the anion as given in Scheme 7. The cathodic peak of IndL2 at −1.45 V due to Q/SQ redox couple is irreversible as shown in Figure S11. The cyclic voltammogram of 2H• is different from that of PPh3L±•. Notably, the cyclic voltammogram of 2H•, which undergoes deprotonation in solution to 2•−, displays a reversible anodic wave at 0.25 V due to 2azo/2•− redox couple and a reversible cathodic wave at −0.53 V due to 2•−/22− redox couple as shown in Figure 5a. The existence of 2•−/22− redox couple at relatively lower potential is significant predicting the oxidation of the coordinated hydrazide fragment to a hydrazyl radical that was isolated as 2H• and was considered as an intermediate in the LH2 → IndL2 and LH2 → 1 transformations. The UV−vis absorption spectra of LH2, PPh3L±•, IndL2, 1, and 2H• were recorded in CH2Cl2 and are shown in Figures 4b and 5b. The spectral data are listed in Table 4. PPh3L±• exhibits an absorption band at 540 nm, which is absent in LH2. The calculated energy difference between β-HOMO and β-LUMO is 2.24 eV, which corresponds to 553 nm. Thus, the lower-energy absorption band of the zwitterion is assigned to hydrazyl → πQ* charge-transfer transition. The change of UV−vis absorption spectra during PPh3L±• → PPh3L− conversion was recorded by spectro-electrochemical measurement and is shown in Figure 4c. The spectral change during the irreversible PPh3L±• → PPh3L+ conversion was also recorded by the spectro-electrochemical measurement and was depicted in Figure 4d. It is established that upon oxidation or reduction the intensity of the band at 540 nm PPh3 ±•

1) nuclei resolved as depicted in Figure 3a and Figure S9. The g value, 2.001, obtained from the simulations of solution (lw, 1.4 mT) and powder (lw, 1.2 mT) spectra is consistent with those of DPPH and other organic radicals.17d Similarly, 2H· exhibits stronger EPR signals (see Figure 3b) at 1.997 both in solution and solid, due to the coordinated hydrazyl radical. The gas-phase geometries of PPh3L±• and 2H· were optimized with the doublet spin state at the B3LYP/DFT level of the theory. The calculated bond parameters are listed in Table 2, the calculated C−P and C(3)−N(1) lengths are relatively longer than the experimental lengths, while N−N length is shorter, may be due to the different weight-age of the resonance structures in crystal and gas phase. The frontier molecular orbitals are illustrated in Figure S10. The β-HOMO (HOMO = highest occupied molecular orbital) is localized on the hydrazyl fragment, while the singly occupied molecular orbital (SOMO), α-LUMO (LUMO = lowest unoccupied molecular orbital), and β-LUMO disperse on both 1,4 quinone and hydrazyl fragments. The atomic spin obtained from the Mulliken spin population analysis scatters over -NH-(CO)- and quinone fragments correlating the resonance structures of PPh3 ±• L (see Scheme 3) as depicted in Figure 3c. The calculation reveals that 37% of the spin is localized on the N(3) atom, and 14% is on the O(3) atom. The rest is delocalized over the 1,4naphthoquinone moiety, predicting PPh3L±• a resonance hybrid of reduced 1,4-naphthoquinone and hydrazyl radical as given in Scheme 7. PPh3L±• undergoes reduction to the anionic zwitterion [PPh3L±]−, which exhibits relatively longer N−N, 1.330 Å, and 8885


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Inorganic Chemistry Scheme 7

Figure 4. (a) Cyclic voltammogram of PPh3L±• in CH2Cl2 at 298 K (conditions: 0.2 M [N(n-Bu)4]PF6 supporting electrolyte; scan rate, 100 mV s−1; platinum working electrode; potential vs Fc+/Fc redox couple), (b) UV−vis absorption spectra of LH2 (black), PPh3L±• (red), IndL2 (blue), in CH2Cl2 at 298 K. Spectro-electrochemical measurements of (c) PPh3L±• → PPh3L− and (d) PPh3L±• → PPh3L+ conversions in CH2Cl2 at 298 K.

Figure 5. (a) Cyclic voltammogram of 2H· in CH2Cl2 at 298 K (conditions: 0.2 M [N(n-Bu)4]PF6 supporting electrolyte; scan rate, mV s−1; platinum working electrode; potential vs Fc+/Fc redox couple), (b) UV−vis absorption spectra of 2H· (orange) and 1(black) in CH2Cl2 at 298 K. Spectro-electrochemical measurements of (c) 2•− → 2azo (d) 2•− → 22− conversions in CH2Cl2 at 298 K.

Table 4. UV−Vis Spectral Data of LH2, PPh3L±•, and IndL2 in CH2Cl2 at 298 K

Ind

decreases. No band is present in the visible spectrum of L2 that absorbs at 323 nm. The UV−vis absorption spectrum of 1 displays a band at 580 nm. 2H· exhibits a broader absorption band at 800 nm as shown Figure 5b. Such band that is absent in 1 is assigned to hydrazyl → πQ* charge-transfer transition. The changes of the absorption spectra of 2•−→ 2azo and 2•−→ 22− conversions were recorded by spectro-electrochemical measurements and are illustrated in Figure 5c,d. During oxidation, the intensity of the lower-energy absorption bands gradually decreases, while during reduction the change is reversed, from which we infer that the redox reaction occurs at the same center as predicted in Scheme 6.

compounds

λmax, nm (ε, 1 × 105 M−1 cm−1)

LH2 PPh3 ±•

395 (0.05), 328 (0.08), 290 (0.19) 540 (0.08), 308 (0.30), 272 (0.24) 323 (0.08), 280 (0.20), 261 (0.28) 580 (0.12), 430 (0.72) 800 (0.10), 555 (0.22), 502 (0.30), 420 (0.52), 330 (1.28)

L Ind L2 1 2H•

are making of C−N bonds, tautomerization, oxidation to anion radical intermediate, C−C bond formation via radical coupling, and demetalation. In presence of PPh3 the intermolecular cascade of the [ML] unit is different, and it is overall a (3H++3e) oxidation reaction that stabilizes the radical intermediate as PPh3 ±• L . In addition to the IndL2 as a minor product, the relatively more stable [ZnL] unit promotes another (6H++6e) redox cascade involving three LH2 ligands and yields a zinc(II) complex, [(L3)ZnII(H2O)Cl]·2MeOH (1·2MeOH) (L3 = a pyridazine derivative of 1,4-naphthoquinone). In air the oxidation of [ZnL] unit to [ZnL•−] that was considered as an intermediate of LH2 → IndL2 and LH2 → 1 conversions was established by isolating a hydrazyl radical complex of the type [(LH•)ZnII(H2O)Cl2] (2H•). IndL2, PPh3L±•, and 1 were not successfully isolated by other routes, making these coordinationpromoted cascades of LH2 worthy in chemical science.

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CONCLUSION The study disclosed three redox cascades of a 1,4-naphthoquinone-benzhydrazide derivative promoted by 3d metal ions. Conversions of N′-(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide (LH2) to a hitherto unknown diarylindazolo[3a,3-c]indazole (IndL2) derivative and a stable zwitterionic triphenylphosphonio-hydrazyl radical (PPh3L±•), which are significant in developing fundamental chemistry of organic radicals and in designing antioxidants and drugs in the presence of metal ions, are authenticated. The [ML] (M = CoII, MnII, and ZnII) unit is redox-active and promotes (6H++6e) and (3H++3e) redox reactions. The significant steps of LH2→ IndL2 conversion 8886


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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.7b00818. X-ray crystallographic CIF files for the PPh3L±• and IndL2; 1 H and 13C NMR spectra; ESI mass spectra of the intermediates; EPR spectrum and optimized coordinates (PDF) Accession Codes

CCDC 1533102−1533103 and 1554449 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-33-2428-7347. Fax: +91-33-2477-3597. ORCID

Prasanta Ghosh: 0000-0002-2925-1802 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 Financial support received from University Grants Commission (F. No. 43-214/2014(SR), SERB-DST, New Delhi (EMR/ 2016/005222) and WB-DST, India. S.M. gratefully acknowledges UGC, New Delhi, India, for his fellowship (No. F. 11-24/ 2013 (SA-1) dated 08.07.1014.

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