Nickel(II) Complex of a Hexadentate Ligand with Two o

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Nickel(II) Complex of a Hexadentate Ligand with Two o‑Iminosemiquinonato(1−) π‑Radical Units and Its Monocation and Dication Akram Ali,† Debanjan Dhar,† Suman K. Barman,†,‡ Francesc Lloret,§ and Rabindranath Mukherjee*,†,‡ †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India § Departament de Quımíca, Inorgànica/Instituto de Ciencia Molecular (ICMOL), Universitat de Valeńcia, Polígono de la Coma, s/n, 46980-Paterna, València, Spain ‡

S Supporting Information *

ABSTRACT: Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H4L3 = N,N′-bis(2-hydroxy-3,5-ditert-butylphenyl)-2,2′-diamino(diphenyldithio)-ethane, in CH3OH with NiII(O2CCH3)2·4H2O and Et3N afforded isolation of a reddish-brown crystalline solid [Ni(L3)] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [FeIII(η5-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of one-electron oxidized species [1]1+ as purple [1][PF6]·CH2Cl2 and two-electron oxidized species [1]2+ as dark purple [1][BF4]2·CH2Cl2, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [NiII{(LISQO,N)•−}{(LISQO,N)•−}{(LS,S)0}] 1, [NiII{(LIBQO,N)0}{(LISQO,N)•−}{(LS,S)0}][PF6]·CH2Cl2, and [NiII{(LIBQO,N)0}{(LIBQO,N)0}{(LS,S)0}][BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate(1−) π-radical (Srad = 1/2) (LISQ)•− and neutral o-iminoquinone (LIBQ)0 redox-levels. Complexes 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2−300 K) magnetic behavior of 1 and [1][PF6]·CH2Cl2, and a μeff value of [1][BF4]2·CH2Cl2 at 300 K. Both 1 and [1][PF6]·CH2Cl2 exhibit ferromagnetic exchangecoupling between the two electrons of NiII and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1]1+, and [1]2+. The observed UV−vis−NIR absorptions for 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 have been assigned, based on time-dependent (TD)-DFT calculations.

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INTRODUCTION Transition-metal complexes supported by redox-active ligands have recently garnered much attention,1−3 as these ligands offer an intriguing way to approach multielectron reactivity at a welldefined metal complex and play a role in catalysis.4,5 Because of the redox-active character, these ligands profoundly influence the electronic structural properties of the resulting complexes.6−16 o-Aminophenolates2,3,17−29 are prototypical redoxactive ligands whose ability to span oxidation-levels range from dianionic o-amidophenolate (L AP ) 2− to π-radical oiminosemiquinonate(1−) monoanion (LISQ)•− to neutral oiminoquinone (LIBQ)0 forms, allowing them to store redox equivalents. Using an o-aminophenol-based tridentate ligand in its deprotonated form L1(2−) (Figure 1), we reported30a three low-spin iron(III) complexes: [FeIII{(LO,N,N)2}•−] (S = 0), its monocation [FeIII{(LISQ)•−}2][BF4] (S = 1/2), and its monoanion [CoIII(η5-C10H15)2][FeIII{(LAP)2−}2] (S = 1/2). The first two complexes were structurally characterized, and the © XXXX American Chemical Society

complexes provided examples of a delocalized metal−ligand interaction. We demonstrated that valence-tautomerism is operative for the monocation and the monoanion. We then synthesized an o-aminophenol-based electronically localized tetradentate ligand (H2L2, Figure 1).30b Using L2(2−), we recently reported the synthesis, structural characterization, and electronic structure of [Pd II {(L AP O,N,S,N ) 2− }], [Pd II {(LISQO,N,S,N)•−}][PF6]·CH2Cl2, [PdII{(LIBQO,N,S,N)0}]-[BF4]2· 2CH2Cl2, and [PdII{(LISQO,N,N)•−}(PPh3)][PF6]·CH2Cl2. The PPh3-bound complexes [PdII{(LISQO,N,N)•−}(PPh3)][PF6]· CH2Cl230b and [PdII{(LAPO,N,S)2−}(PPh3)]30c were prepared to provide proof-of-concept of hemilability in complexes with redox-active ligands. Density functional theory (DFT) calculations at the B3LYP-level of theory allowed us to assign not only the correct oxidation-state of the metal ion but also the Received: November 19, 2015

A

DOI: 10.1021/acs.inorgchem.5b02688 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

mmol) was added in portions, under dinitrogen atmosphere. The resulting mixture was then refluxed under N2 atmosphere. After 5 h, the reaction mixture was cooled to room-temperature and kept in the refrigerator for 6 h. The resulting solid that formed was collected by filtration. The filtrate was kept for slow evaporation at 298 K, and the solid that formed was collected by filtration and mixed with a previously obtained residue. The combined solid was then thoroughly washed with CH3OH, until the washings were colorless. A white solid of the desired ligand was obtained. Yield: 1.2 g, ∼90%. Anal. Calcd for C42H56N2O2S2 (H4L3): C 73.68, H 8.19, N 4.09. Found: C 73.24, H 8.39, N 4.05. 1H NMR (CDCl3, 500 MHz): δ (ppm) 7.43 (d, J = 7.95 Hz, 2H; Ph-H2), 7.22 (s, 2H; Ph-H7), 7.11 (t, J = 7.62 Hz, 2H; PhH4), 6.95 (s, 2H; Ph-H6), 6.75 (t, J = 7.62 Hz, 2H; Ph-H3), 6.43 (d, J = 7.95 Hz, 2H; Ph-H5), 6.37 (s, 2H; Ph-H8), 6.18 (s, 2H; Ph-H9), 3.01 (s, 4H; −CH2−), 1.46 (s, 18H; tert-butyl), 1.24 (s, 18H; tert-butyl). IR (cm−1): 3378 (ν(NH)). ESI-MS of C42H56N2O2S2 (Mr = 684): 685.39 (M+ + 1 ion peak, 100%). Note: The yield was found to be critically dependent on the purity of the starting amine and may go down to as low as 60%, if the amine was not pure. Synthesis of Complexes. [Ni(L3)] (1). To a suspension of H4L3 (0.200 g, 0.29 mmol) in CH3OH (18 mL) was added Et3N (0.118 g, 1.17 mmol) and stirred for 5 min. To the resulting solution, solid NiII(O2CCH3)2·4H2O (0.073 g, 0.29 mmol) was added, and the solution was refluxed for 6 h. The reaction mixture was then cooled to room-temperature and filtered. The microcrystalline solid that formed was washed with ice-cold CH3OH and then diethyl ether to afford a reddish-brown solid. Yield: 0.175 g, ∼80%. Slow evaporation in air of a solution of the complex in a CH2Cl2−CH3OH mixture (1:1, v/v) yielded crystals of composition [Ni(L3)], suitable for X-ray diffraction studies. Anal. Calcd for C42H52N2O2S2Ni (1): C 68.23, H 7.04, N 3.79. Found: C 67.81, H 7.13, N 3.82. μeff (solid, 300 K) = 4.67 μB; μeff (CH2Cl2, 300 K) = 4.62 μB. [1][PF6]·CH2Cl2. Complex 1 (0.080 g, 0.11 mmol) was dissolved in CH2Cl2 (20 mL), and solid [FeIII(η5-C5H5)2][PF6] (0.035 g, 0.12 mmol) was added to it. The mixture was stirred in air at 298 K for 1 h. The solution was then concentrated to ∼2 mL, diethyl ether (6 mL) added, and kept in the refrigerator for 1 h. The solid that separated out was filtered and washed thoroughly with diethyl ether, until the washings were colorless. A purple microcrystalline solid was isolated. Yield: 0.080 g, ∼76%. Vapor-diffusion of diethyl ether to a CH2Cl2 solution of the complex afforded single-crystals suitable for X-ray diffraction of composition [Ni(L3)][PF6]·CH2Cl2. Anal. Calcd for C43H54N2Cl2F6O2PS2Ni ([1][PF6]·CH2Cl2): C 53.27, H 5.58, N 2.89. Found C 53.40, H 5.80, N 2.84. IR (KBr, cm−1, selected peak): 841 ν(PF6−). μeff (solid, 300 K) = 3.95 μB; μeff (CH2Cl2, 300 K) = 3.90 μB. [1][BF4]2·CH2Cl2. To a solution of 1 (0.050 g, 0.07 mmol) in CH2Cl2 (10 mL), solid AgBF4 (0.027 g, 0.14 mmol) was added. The mixture was stirred in air at 298 K for 1 h and then filtered (removal of metallic Ag). The solvent was removed by rotary evaporation under reduced pressure, and the solid that formed was recrystallized from a CH2Cl2−diethyl ether mixture (1:2, v/v). Dark purple crystals of composition [Ni(L3)][BF4]2·CH2Cl2 that formed were found to be suitable for X-ray structural study. Yield: 0.040 g, ∼59%. Anal. Calcd for C43H54B2Cl2F8N2O2S2Ni ([1][BF4]2·CH2Cl2): C 51.74, H 5.42, N 2.81. Found C 51.90, H 5.80, N 2.85. IR (KBr, cm−1, selected peak): 1052 ν(BF4−). μeff (solid, 300 K) = 2.86 μB; μeff (CH2Cl2, 300 K) = 2.78 μB. Physical Measurements. Elemental analyses were obtained using Thermo Quest EA1110 CHNS-O, Italy. IR spectra (KBr, 4000−600 cm−1) were recorded on a Bruker Vector 22 spectrophotometer. UV− vis spectra were recorded at 298 K using an Agilent 8453 diode-array spectrophotometer; NIR absorption spectra were recorded at 298 K using a JACSO V-670 (Japan) spectrophotometer. 1H NMR spectra (CDCl3) were obtained on a JEOL JNM LA 500 (500 MHz) spectrometer. Chemical shifts are reported in ppm referenced to TMS. ESI-MS spectra were recorded on a Waters-HAB213 spectrometer. Cyclic voltammetric (CV) experiments were performed at 298 K by using CH instruments, Electrochemical Analyzer/Workstation model 600B series. The cell contained a Beckman M-39273 platinum-inlay

Figure 1. Ligands of pertinence to this work.

redox-level of the coordinated ligand in each complex. Hoping that a hexadentate ligand with two redox-active o-aminophenol components connected through an −S(CH2)2S− spacer may stop electronic delocalization between the two halves and hence may impart localization of ligand redox-level, we synthesized a ligand N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino(diphenyldithio)-ethane (H4L3, Figure 1). This work concerns complexes of nickel and forms part of our general program on o-aminophenolate-based ligand complexes of transition metal ions. We report here on the synthesis, structural characterization, and electronic structure of a new complex [NiII{(LISQO,N)•−}{(LISQO,N)•−}{(LS,S)0}] (1), through absorption spectroscopic, temperature-dependent magnetism, and cyclic voltammetric results. We also describe the synthesis and characterization of the monocation [1]1+ and the dication [1]2+ in the successful synthesis and magnetism of [NiII{(LIBQO,N)0}{(LISQO,N)•−}{(LS,S)0}][PF6]·CH2Cl2 (temperature-dependent studies) and [NiII{(LIBQO,N)0}{(LIBQO,N)0}{(LS,S)0}][BF4]2·CH2Cl2. We report here also the structural properties of the monocation and the dication. DFT calculation at the CAM-B3LYP-level of theory has been utilized to assign not only the correct oxidation-state of the nickel ion but also the redox-level of the coordinated ligand in each complex. Results of TD-DFT calculations are also presented here to assign the absorption spectral transitions of the complexes.

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EXPERIMENTAL SECTION

General Considerations. All reagents were obtained from commercial sources and used as received. Solvents were dried/purified as reported previously.30 1,2-Bis(2-aminophenylthio)ethane was synthesized following a reported procedure.31 [FeIII(η5-C5H5)2][PF6] was prepared, as reported in the literature.32 Tetra-n-butylammonium perchlorate (TBAP) was prepared and purified as before.30 Synthesis of H4L3. To a stirring suspension of 3,5-di-tertbutylcatechol (0.963 g, 4.34 mmol) and triethylamine (0.2 mL) in n-heptane (10 mL), 1,2-bis(2-aminophenylthio)ethane (0.6 g, 2.17 B


Article

Inorganic Chemistry Scheme 1a

Only one-half of L3(4−) is shown. L3(4−), {(LAP)2−}{(LAP)2−}; L3(3−), {(LISQ)•−}{(LAP)2−}; L3(2−), {(LISQ)•−}{(LISQ)•−}; L3(1−), {(LIBQ)0}{(LISQ)•−}; L3(0), {(LIBQ)0}{(LIBQ)0}. a

exchange coupling value 2J. The approach proposed by Noodleman is eq 2.41,42

working electrode, a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE), as reference electrode. Details of the cell configuration are as described before.32 The solutions were ∼1.0 mM in complex and 0.1 M in supporting electrolyte, TBAP. Under our experimental conditions, the E1/2 and peak-to-peak separation (ΔEp) values in CH2Cl2 for [FeIII(η5-C5H5)2]+/[FeII(η5-C5H5)2] (Fc+/Fc) couple were 0.49 V vs SCE and 120 mV, respectively.30 Room-temperature magnetic susceptibility measurements were made on polycrystalline samples (powder form) of [Ni(L3)] 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 by the Faraday method, using a locally built magnetometer.33 Temperature-dependent measurements on 1 and [1][PF6]·CH2Cl2 were carried out using a Quantum Design SQUID magnetometer (València) at 0.01 T for T < 50 K to avoid saturation effects and 0.1 T for T > 50 K. The effective magnetic moment was calculated from μeff = 2.828[χMT]1/2, where χM is the corrected molar susceptibility. The diamagnetic corrections were applied to the susceptibility data.34 Solution-state magnetic susceptibilities were obtained by the NMR technique of Evans35 in CH2Cl2 with a JEOL JNM LA 500 (500 MHz) spectrometer, and we made use of the paramagnetic shift of the methylene protons of CH2Cl2 as the measured NMR parameter. Crystal Structure Determination. Single-crystals of suitable dimensions were used for data collection. Diffraction intensities were collected on a Bruker SMART APEX CCD diffractometer, with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 100(2) K. The data were corrected for absorption. The structures were solved by SIR-97, expanded by Fourier-difference syntheses, and refined with SHELXL-97 incorporated in WinGX 1.64 crystallographic package.36 The positions of the hydrogen atoms were calculated by assuming ideal geometries but not refined. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares procedures on F2. Pertinent crystallographic parameters are summarized in Table S1, Supporting Information. CCDC-1424158 (1), 1424159 ([1][PF6]· CH2Cl2), and 1424160 ([1][BF4]2·CH2Cl2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. All DFT calculations were performed using the Gaussian 09 program37 with a hybrid exchange−correlation functional Coulomb-Attenuating Method-B3LYP (CAM-B3LYP).38 Triple-ζ quality basis set (TZVP)39 was used for nickel, nitrogen, oxygen, and sulfur, and the SVP basis set40 was used for carbon and hydrogen. For the spin-polarized symmetry-broken solution, the broken-symmetry formalism as introduced by Noodleman was used.41 For the two-center electron system, the magnetic-exchange coupling was modeled with the well-known Heisenberg−Dirac−van Vleck Hamiltonian:

ĤHDVV = − 2J12 . S1̂ . S2̂

J = − (E HS − E BS)/S2 max

(2)

where EHS and EBS are the energies of the high-spin (HS; MS = S1 + S2) and BS determinants (MS = |S1 − S2|). The second-one proposed by Bencini et al. is eq 3:43

J = − (EHS − EBS)/[Smax(Smax + 1)]

(3)

The third-one is proposed by Yamaguchi et al. (eq 4):

J = − (EHS − EBS)/( S2

HS

− S2

BS

)

44

(4)

The Yamaguchi approach uses explicitly the expectation values of the square of the spin (⟨S2⟩), provided by the output of the unrestricted DFT calculations. It has been well explained by Neese42a,b stating that the Yamaguchi approach is valid for a whole range of coupling strengths (from weak- to strong-exchange interactions) and that it reduces to the Noodleman equation at the weak-coupling limit. TD-DFT calculations were done employing the CAM-B3LYP functional and the polarizable continuum model, CPCM (CH2Cl2 as solvent).45 TD-DFT-derived electronic spectra were plotted using GaussSum.46 Corresponding orbitals and spin-density plots were made using the Chemcraft47 Visualization program.

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RESULTS AND DISCUSSION Synthesis and Properties of [Ni(L3)] 1. The ligand H4L3 was synthesized via the condensation of 1,2-bis(2aminophenylthio)ethane with 2 equiv of 3,5-di-tert-butylcatechol in n-heptane in the presence of Et3N, under a dinitrogen atmosphere. It was characterized by elemental analysis, 1H NMR (Figure S1, Supporting Information), and ESI-MS (Figure S2, Supporting Information) spectra. Preparation of a reddish-brown complex [Ni(L3)] 1 was achieved in a straightforward manner in CH3OH by the reaction of NiII(O2CCH3)2·4H2O, H4L3, and Et3N as a base in the presence of air at refluxing temperature. Recrystallization of the microcrystalline solid from CH2Cl2−CH3OH resulted in X-ray quality single-crystals. The complex was correctly analyzed by elemental analysis. Positive ESI-MS of 1 in CH2Cl2 showed a peak at m/z = 738.28 corresponding to the species [M]+, based on the simulated mass and isotopic distribution pattern (Figure S3, Supporting Information). The formation of H2O2 during the synthesis of 1 in air29 was detected by following the development of the characteristic band for I3− spectrophotometrically (λmax = 353 nm; ε = 26 000 M−1 cm−1), upon reaction with I−.48 Since atmospheric dioxygen can oxidize I−, a blank experiment was also performed (Figure S4, Supporting Information).

(1)

On the basis of the above Hamiltonian (eq 1), three well-known approaches are reported in the literature42 to calculate the magneticC


Article

Inorganic Chemistry Redox Properties of 1. o-Amidophenolates(2−) are redoxactive and can exist in three redox-levels.2,3,4a,5a Scheme 1 depicts the geometric and electronic features of L3(4−). To explore the possibility of observing ligand-centered redox, CV experiments on 1 were carried out in CH2Cl2. Two oxidative responses at E1/2 values of 0.09 V (ΔEp = 140 mV) and 0.53 V (ΔEp = 150 mV) vs SCE were observed (Figure 2). Both the

Figure 2. Cyclic voltammogram (100 mV/s) of a 1.0 mM solution of [Ni(L3)] 1 exhibiting oxidative responses in CH2Cl2 (0.1 M in TBAP) at a platinum working electrode. Indicated peak potentials are in V vs SCE.

oxidation processes are chemically reversible (ratio of cathodic and anodic peak current, ipc/ipa ≈ 1) and electrochemically quasireversible30 electron-transfer reactions. Synthesis and Properties of [1][PF6]·CH2Cl2 and [1][BF4]2·CH2Cl2. The one-electron oxidized counterpart of 1, [1]1+, was isolated as purple crystals [1][PF6]·CH2Cl2 by stoichiometric chemical oxidation of 1 in air in CH2Cl2 by [FeIII(η5-C5H5)2][PF6] (E1/2 = 0.40 V vs SCE)49 at roomtemperature. Isolation of the two-electron oxidized product of 1, [1]2+, was achieved as dark purple crystals [1][BF4]2·CH2Cl2 by chemical oxidation of 1 in air in CH2Cl2 by AgBF4 (E1/2 = 1.05 V vs SCE)49 at room-temperature. Positive ESI-MS of [1][PF6]·CH2Cl2 in CH2Cl2 showed a peak at m/z = 738.27 corresponding to the species [M]+, based on the simulated mass and isotopic distribution pattern (Figure S5, Supporting Information). Positive ESI-MS of [1][BF4]2·CH2Cl2 in CH2Cl2 showed a peak at m/z = 738.28 corresponding to the species [M]+ (Figure S6, Supporting Information). CV results of [1][PF6]·CH2Cl2 and [1][BF4]2·CH2Cl2 are as expected. Description of the Structures of [Ni(L3)] 1, [1][PF6]· CH2Cl2, and [1][BF4]2·CH2Cl2. Crystal structure analysis confirms that the Ni atoms in [Ni(L3)] 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 are six-coordinate with NiO2N2S2 coordination (Figure 3 and Table S2, Supporting Information). Thus, the deprotonated form of the potentially hexadentate ligand L3(4−) provides coordination by two o-iminophenolate units utilizing N2O2 donor atoms and two thioether S atoms. The oxidized counterparts of 1 contain a molecule of CH2Cl2, as the solvent of crystallization. The metal coordination environments are identified by Ni−O1/O2(phenolate), Ni− N1/N2(iminate), and Ni−S1/S2(thioether) distances, respectively: 2.037(3)/2.022(3), 1.989(4)/1.995(4), and 2.3935(13)/ 2.4066(12) Å for 1; 2.019(2)/2.049(2), 1.966(3)/2.025(3),

Figure 3. Perspective views of metal coordination environment in (a) [Ni(L)] 1, (b) [1][PF6]·CH2Cl2, and (c) [1][BF4]2·CH2Cl2. Relevant metric parameters of the ligand backbone: C1−O1 1.298(5), C1−C2 1.431(6), C2−C7 1.359(6), C7−C8 1.433(6), C8−C13 1.362(6), C13−C14 1.414(6), C1−C14 1.462(6), N1−C14 1.360(5), C42−O2 1.281(5), C29−C42 1.460(6), C29−C30 1.412(6), C30−C31 1.378(6), C31−C36 1.427(6), C36−C37 1.366(6), C37−C42 1.430(6), and N2−C29 1.352(5) (1). C1−O1 1.297(4), C1−C2 1.430(5), C2−C7 1.376(5), C7−C8 1.427(5), C8−C13 1.370(5), C13−C14 1.411(5), C1−C14 1.460(5), N1−C14 1.362(5), C42−O2 D


Article

Inorganic Chemistry

in two different redox-levels.19b,20 There is a very recent report in which a pentadentate o-aminophenolate-based ligand21c,29 is coordinated to a mononuclear CuII in two different oxidationlevels of the N,O-coordinated halves.21c From this perspective and given the recent report of a NiII complex coordinated by two o-aminophenolate-based ligand providing O,N,S-coordination,6 the structural characterization of [1][PF6]·CH2Cl2, supported by a hexadentate ligand, is noteworthy. Complexes 1, the monocation [1][PF6]·CH2Cl2, and the dication [1][BF4]2·CH2Cl2 are ideally suited to distinctly distinguish the geometrical and spectroscopic features of the (O,N,S)2-coordinated, open-shell o-iminosemiquinonate(1−) π-radical (LISQ)•− (Srad = 1/2) and the closed-shell oiminoquinone (LIBQ)0, of the o-aminophenolate ligand (see Electronic Structural Aspects section). Comparison of Coordination Behavior between Bis(tridentate) and Hexadentate Ligand Systems. It is worth comparing the coordination behavior of the bis(tridentate) thioether-appended o-aminophenol ligand of Kaim and coworkers19b and the present hexadentate ligand. The special features of the present ligand are the following. (i) For the bis(tridentate) ligand, the complex “[NiII(L)2]” (L = 4,6-di-tertbutyl-N-(2-methylthiomethylphenyl)-o-iminobenzosemiquinonate(1−) π-radical) is approximately squareplanar, as the thioethers remain noncoordinated. The complex [NiII(L3)] 1 is six-coordinate, utilizing all donor sites. In both the complexes, however, the o-aminophenolate part of the ligand(s) is/are present in (LISQO,N)•− form. (ii) For the bis(tridentate) system [NiII(L)2], magnetic interactions are possible only between the two ligand radicals. However, in 1 additional interaction between the radical electrons and the NiII unpaired electrons is a possibility. (iii) For both ligands, oneelectron and two-electron oxidized counterparts are sixcoordinate, utilizing all potential donor sites, and the redoxlevels of the o-aminophenolate parts of the coordinated ligand(s) is/are identical: the one-electron oxidized form has one of the o-aminophenolates in the (LISQO,N)•− form and the other in the (LIBQO,N)0 form; the two-electron oxidized form has both o-aminophenolates in the (LIBQO,N)0 form. (iv) The bis(tridentate) system exhibits hemilability (the potentially O,N,S tridentate ligand coordinates only in a bidentate fashion with trans-positioned O and N donors),19b but the present hexadentate ligand does not show hemilability in either of the complexes 1, [1][PF6]·CH2Cl2, or [1][BF4]2·CH2Cl2. (v) For the bis(tridentate) ligand system in the one-electron and twoelectron oxidized complexes, the ligand coordinates with changed configuration (cis-positioned O donors in contrast to trans-positioned O,N donors in the parent complex). For 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2, the two O and two S donors are cis-positioned, and two N donors are transpositioned. This arrangement of donor atoms results from the geometrical constraint of the hexadentate ligand. Metrical Oxidation States (MOSs) of Ligands in 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2. Following Brown25 one can determine the empirical oxidation state of redox-active ligands, based on least-squares fitting of relevant C−O, C−N, and C−C bond lengths.30b The MOS of the ligand in 1 is calculated as −0.86 ± 0.09 (one side) and −0.83 ± 0.08 (the other side), which is consistent with the (LISQ)•− (Srad = 1/2) form of the ligand at both ends. Similarly, for [1][PF6]·CH2Cl2 the MOS values are calculated as −0.95 ± 0.07 (one side) and −0.04 ± 0.05 (the other side), which is consistent with the (LISQ)•− (Srad = 1/2) form of the ligand at one end and the

Figure 3. continued 1.242(4), C29−C42 1.518(5), C29−C30 1.433(5), C30−C31 1.352(5), C31−C36 1.457(5), C36−C37 1.354(5), C37−C42 1.456(5), and N2−C29 1.308(5) [1][PF 6 ]·CH 2 Cl 2 . C1−O1 1.240(5), C1−C2 1.461(6), C2−C7 1.345(6), C7−C8 1.473(6), C8−C13 1.350(6), C13−C14 1.420(6), C1−C14 1.500(6), N1−C14 1.308(6), C42−O2 1.242(5), C29−C42 1.516(6), C29−C30 1.422(6), C30−C31 1.354(6), C31−C36 1.463(6), C36−C37 1.341(6), C37−C42 1.458(6), and N2−C29 1.307(5) [1][BF4]2· CH2Cl2.

and 2.3788(10)/2.3626(10) Å for [1][PF6]·CH2Cl2; 2.069(3)/ 2.064(3), 2.001(4)/2.009(4), and 2.3660(13)/2.3371(14) Å for [1][BF4]2·CH2Cl2. In going from 1 to [1][PF6]·CH2Cl2, while one Ni−O distance remains almost constant [2.022(3) and 2.019(2) Å] the other distance increases [2.037(3) and 2.049(2) Å], and in going from [1][PF6]·CH2Cl2 to [1][BF4]2· CH2Cl2, the Ni−O distances further increase. For Ni−N distances, the observed result does not follow a definitive trend. The Ni−S distance in 1 is the longest. In going from [1][PF6]· CH2Cl2 to [1][BF4]2·CH2Cl2, while one Ni−S distance remains almost constant [2.3626(10) and 2.3660(13) Å] the other distance decreases [2.3788(10) and 2.3371(14) Å]. Clearly, the complexes are distorted octahedral (Table S2, Supporting Information). The nickel in 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 is present in its bivalent state, as evidenced by closely similar distances in NiII complexes of N,Ocoordinated o-iminophenolates in (L ISQ ) •− and (L IBQ ) 0 oxidation-levels.19 To note, the angles between the two oiminosemiquinonate planes in 1, between the o-iminosemiquinonate and o-iminoquinone planes in [1][PF6]·CH2Cl2, and between the two o-iminoquinone planes in [1][BF4]2·CH2Cl2 are ∼51°, ∼ 51°, and ∼52°, respectively. Understandably, the −S-CH2−CH2−S− spacer of the ligand L3(4−) imparts distortion of the coordination sphere. To the best of our knowledge, 1 provides the first example of an air-stable sixcoordinate Ni II complex of two O,N-coordinated oiminosemiquinonate(1−) π-radicals, supported by a hexadentate ligand. The only other example is the NiII complex of bis(tridentate) ligand 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol.19c Metric parameters for the O,N-coordinated o-aminophenolate part of the ligand (Scheme 1), however, are dramatically different in the three structures (Figure 3; Table S2, Supporting Information).17−30 A closer look at the C−O, C−N, and C−C bond distances within the quinonoid ring of 1 reveals that the two o-aminophenolate parts are in the o-iminosemiquinonate(1−) π-radical redox-level (LISQ)•−. Inspection of similar parameters of [1][PF6]·CH2Cl2 and [1][BF4]2·CH2Cl2 shows that the two o-aminophenolate parts of the monocation [1]1+ are in two different redox-levels, one in the oiminosemiquinonate(1−) π-radical (LISQO,N)•− and the other in the neutral o-iminoquinone (LIBQO,N)0, and the two oaminophenolate parts of the dication [1]2+ are in the neutral oiminoquinone redox-level (LIBQO,N)0. Thus, the X-ray crystallographic data strongly support the ligand redox-level formulation of 1 and the monocation [1][PF6]·CH2Cl2 and the dication [1][BF4]2·CH2Cl2 as [NiII{(LISQO,N)•−}{(LISQO,N)•−}{(LS,S)0}], [Ni II {(L IBQ ) 0 }{(L ISQ ) •− }{(L S,S ) 0 }] 1+ , and [Ni II {(L IBQ ) o } {(LIBQ)0}{(LS,S)0}]2+, respectively. The monocation [1]1+ joins a rare family of six-coordinate complexes in which two o-aminophenolate ligands are present E


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Inorganic Chemistry (LIBQ)0 form of the ligand at other end. For [1][BF4]2·CH2Cl2, the MOS values are calculated as −0.07 ± 0.06 (one side) and −0.03 ± 0.06 (other side), which is consistent with the (LIBQ)0 form of the ligand at both ends. Table 1 describes the comparison of MOS-derived calculated C−O, C−N, and C−C bond lengths with those of the X-ray determined values. Table 1. Comparison of X-ray Determined Experimental Bond Lengths with the Calculated Bond Lengths from Metrical Oxidation State; MOS (in Parentheses) of 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 [NiII(L3)] 1 C1−C14 C13−C14 C8−C13 C7−C8 C2−C7 C1−C2 C1−O1 C14−N1 C29−C42 C29−C30 C30−C31 C31−C36 C36−C37 C37−C42 C42−O2 C29−N2 C29−N2

1.462(6) 1.414(6) 1.362(6) 1.433(6) 1.359(6) 1.431(6) 1.298(5) 1.360(6) 1.460(6) 1.412(6) 1.378(6) 1.427(6) 1.366(6) 1.430(6) 1.281(5) 1.352(5) 1.352(5)

[1.450] [1.415] [1.367] [1.433] [1.374] [1.433] [1.293] [1.342] [1.452] [1.415] [1.367] [1.434] [1.373] [1.431] [1.291] [1.341] [1.341]

[1][PF6]·CH2Cl2

[1][BF4]2·CH2Cl2

1.460(5) 1.411(5) 1.370(5) 1.427(5) 1.376(5) 1.430(5) 1.297(4) 1.362(9) 1.518(5) 1.433(5) 1.352(5) 1.457(5) 1.354(5) 1.456(5) 1.242(4) 1.308(5) 1.308(5)

1.500(6) 1.420(6) 1.350(6) 1.473(7) 1.345(6) 1.461(6) 1.240(6) 1.308(7) 1.516(6) 1.422(6) 1.354(6) 1.463(6) 1.341(6) 1.458(6) 1.242(5) 1.307(5) 1.307(5)

[1.445] [1.413] [1.370] [1.430] [1.376] [1.427] [1.299] [1.347] [1.507] [1.432] [1.347] [1.465] [1.353] [1.453] [1.240] [1.299] [1.299]

[1.504] [1.432] [1.348] [1.464] [1.353] [1.452] [1.242] [1.301] [1.508] [1.432] [1.347] [1.466] [1.352] [1.454] [1.240] [1.299] [1.299]

Magnetism. Figure 4a displays the temperature-dependence of the χMT values of 1. The observed room-temperature effective magnetic moment data of 1 [μeff (300 K) = 4.67 μB] imply that the complex possesses an St = 2 ground-state. Since the complex is neutral, it renders six-coordinate NiII (d8) with two unpaired electrons and the two o-aminophenol-derived bidentate ligand part with each at the (LISQ)•− redox-level. This could be attained via strong intramolecular ferromagnetic coupling between the magnetic orbital of the NiII ion (SNi = 1) and the π-orbital of the two o-iminosemiquinonate(1−) πradicals (Srad = 1/2) from two arms of the hexadentate ligand. Thus, 1 should be described as [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] (see Description of the Structures section). As stated above, we consider a triple-spin system (two ligand radicals and a NiII center): SR1 = 1/2, SNi = 1, and SR2 = 1/2. The corresponding Hamiltonian (eq 5), taking into account radical-NiII (J) and radical−radical (j) magnetic interactions (in the DFT Results section it is expressed as 2J), and zero-field splitting (D) for NiII, is

Figure 4. χMT versus T plot for powdered samples of (a) [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] (1) and (b) magnetization plot for 1, and (c) χMT versus T plot for powdered samples of [NiII{(LIBQO,N)0}{(LISQO,N)•−}{(LS,S)0}][PF6]·CH2Cl2. The solid lines represent the best theoretical fit using the equations described in the text.

H = −JSNi(SR1 + SR 2) − j(SR1SR 2) + D(S2z ,Ni − 2/3) + βH(gNiSNi + gR1SR1 + gR 2SR 2)

(5)

where gR1 = gR2 = 2.0 (kept constant during the fitting process). Intermolecular interactions were also taken into account in the form of (T − θ). These antiferromagnetic intermolecular interactions are related to the π−π contacts between radicals from different complexes. Magnetic interactions are expressed in terms of J/j values. It is not possible to give an unambiguous value for the parameter jRR. The parameters jRR and θ are strongly correlated, and their influence on the fit is negligible (it can be positive or negative). In fact, if we neglect this value (jR‑R

= 0), we obtain gNi = 2.18(1), DNi = 18.3(3) cm−1, JNi‑R = +305(4) cm−1, gR = 2.0 (fixed), and θ = −26.1(3) K (zjinter = −9.0 cm−1, where z is the number of nearest neighbors, and jinter is their magnetic interactions). However, if we keep jR‑R at +10.61 (DFT-calculated value; see DFT section) constant in the fitting processes, we obtain basically the same values: gNi = 2.18(1), DNi = 18.1(3) cm−1, JNi‑R = +305(4) cm−1, gR = 2.0 F


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Inorganic Chemistry (fixed), and θ = −26.9(3) K (zj = −9.3 cm−1). It is clear that we can give an unambiguous J(NiR) value (see below), which is not dependent on the values of the other parameters (not correlated). It is important to mention here that the intermolecular antiferromagnetic interactions in 1 fully cancel its effective magnetic moment, yielding a diamagnetic material with lowering in temperature. In fact, the magnetic moment values tend to approach zero at low temperatures (Figure 4a). Moreover, we observed a maximum in the susceptibility curve at ca. 5.0 K, indicating a diamagnetic ground state (in the solid state). These features do not allow one to observe any EPR signal, as well as the use of magnetization vs applied magnetic field to characterize the S = 2 ground-state of the isolated molecule, NiII(radical)2. As observed in Figure 4b (magnetization curve of 1), the magnetization is a straight line whose slope shows very weak magnetic susceptibility at 2 K. The observed room-temperature effective magnetic moment data of [1][PF6]·CH2Cl2 [μeff (300 K) = 3.95 μB] implies that the complex [1][PF6]·CH2Cl2 possesses an St = 3/2 ground state (Figure 4c). Since the complex is monocationic and one part of the ligand is in the o-iminoquinone form, [1][PF6]· CH2Cl2 should be described as [NiII{(LIBQO,N)0}{(LISQO,N)•−}(LS,S)0][PF6]·CH2Cl2 (see Description of the Structures section). The S = 3/2 ground-state could be attained via strong intramolecular ferromagnetic coupling between a NiII ion (SNi = 1) and an (LISQO,N)•− π-radical (Srad = 1/2). Considering a double-spin system (a radical and a NiII) with SR = 1/2 and SNi = 1, taking into account radical-NiII (J), and zerofield splitting (D) for NiII, the corresponding Hamiltonian is eq 6.

values (as indicated in the Figures) and keeping constant the other parameters obtained in the best-fit: gNi = 2.20, DNi = 17.9 cm−1, gR = 2.0 (fixed), jR‑R = +10.61 cm−1, and θ = −25.4 K for 1 and gNi = 2.18, DNi = 18.3 cm−1, and gR = 2.0 (fixed) for [1][PF6]·CH2Cl2. Similar behavior has been reported for other octahedral NiII complexes containing coordinated organic π-radicals.19 The dx2_y2 and dz2 magnetic orbitals (σ-symmetry) of the NiII ion and the ligand π-orbital containing the unpaired radical electron are strictly orthogonal, and so, the intramolecular spin-exchange coupling is therefore ferromagnetic in nature for both compounds (1 and [1][PF6]·CH2Cl2). Electronic Structural Aspects: DFT Results. To correctly assign the electronic structure of complexes with redox-active ligands, DFT calculations offer valuable information not only to correctly assign the oxidation-state of the metal ion and the redox-level of the coordinated ligand but also help to explain the spectroscopic signature of the complexes. Assignment of correct oxidation-level of the coordinated ligand L3(4−) and the oxidation-state of Ni in 1, [1]1+, and [1]2+ has been done by DFT calculations at the CAM-B3LYP level of theory (see Computational Details in the Experimental Section). Geometry-optimization of 1 (Table S3, Supporting Information) was done using X-ray coordinates. Complex 1 can have three different possible electronic configurations with S = 2 (highspin, HS), Ms = 0 (low-spin1, LS1), and Ms = 1 (LS2) (Scheme 2). All three spin-states were optimized, and their respective Scheme 2a

H = −JSNiSR1 + D(S2 z , Ni − 2/3) + βH(gNiSNi + gR SR ) (6)

where gR = 2.0 (kept constant during the fitting process). The solid-line in Figure 4c represents a best-fit using the above spinHamiltonian with parameters J = +310(6) cm−1, gNi = 2.18(1) and a zero-field splitting DNi = 18.3(3) cm−1. Notably, the J(NiR) and DNi values of 1 are very close to those corresponding to[1][PF6]·CH2Cl2. Because of the large value of the magnetic-exchange coupling parameter, JNi‑R, for these complexes, the ground state (S = 3/2 or S = 2) is basically populated at room temperature, so a plateau in the χMT curve is observed at high-temperature (i.e., the values of χMT are nearly constant). In such a case, it is not possible to give an exact value for J. As is shown in Figure 4a and c, the values of JNi‑R are in the range of 250−350 cm−1 for 1 and 350−450 cm−1 for [1][PF6]·CH2Cl2, and they are correlated with the magnetic corrections (temperatureindependent magnetism such as diamagnetism of the sample holder and the ligand and temperature-independent-paramagnetism (TIP) of the NiII ions). For these reasons, we have carefully measured the values for the sample holder. Therefore, experimentally measured values were corrected for the sample holder, the diamagnetism of the ligands (−473 × 10−6 for 1 and −537 × 10−6 cm3 mol−1 for [1][PF6]·CH2Cl2, and the TIP of NiII (100 × 10−6 cm3 mol−1). Using these corrections, we obtain the curves shown in Figure 4a and c and the J values: ∼ +300 cm−1 (for 1) and ∼ +400 cm−1 (for [1][PF6]·CH2Cl2). In Figure 4a and c, we have corrected the experimental susceptibility values more carefully (as indicated above), and the theoretical curves have been calculated by varying the J

a For [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] 1, different spin-orientations can be expressed as shown above.

energies are collected in Table 2. Inspection of these energy values reveals that the HS state is lower in energy than other electronic configurations (the HS state is stabilized by −1.7153 kcal/mol, compared to the LS1 state; the HS state is stabilized by −0.8728 kcal/mol, compared to the LS2 state). This implies the existence of ferromagnetic coupling between the two NiII electrons and the two (LISQO,N)•− π-radical electrons on both sides of the ligand in [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] 1. To compute the two magnetic-exchange coupling constants, JNi‑R and JR‑R, three possible electronic configurations (HS, LS1, and LS2) (Scheme 2) were converged and combined with each other. The corresponding Heisenberg spin-Hamiltonian for the three-center spin-system (Scheme 2) can be expressed from eq 1 as Ĥ = −2JNi‑R(S1̂ ·S2̂ + Ŝ2·Ŝ3) − 2JR‑RŜ1·S3̂ . In this section, the magnetic interactions are expressed in terms of 2J. The energy of the HS state can be expressed as EHS = −2JNi − R (1/2 × 1) − 2JNi − R (1/2 × 1) − 2JR − R (1/2 × 1/2)

(7)

ELS1 = −2JNi − R ( −1/2 × 1) − 2JNi − R ( −1/2 × 1) − 2JR − R (1/2 × 1/2) G

(8) DOI: 10.1021/acs.inorgchem.5b02688 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Computed J values for [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] (1) Ni

S,O,N

C,H

functional

−EHS (a.u.) S = 2

−ELS1 (a.u.) Ms = 0

−ELS2 (a.u.) Ms = 1

2JNi‑R (cm−1)

2JR-R (cm−1)

TZVP

TZVP

SVP

CAM-B3LYP

4194.53981492

4194.53708149

4194.53842407

+299.95

+10.60

ELS2 = −2JNi − R (1/2 × 1) − 2JNi − R ( −1/2 × 1) − 2JR − R ( −1/2 × 1/2)

(9)

Combining eq 7 and eq 8, E HS − ELS1 = −4JNi − R

(10)

Combining eq 7 and eq 9, EHS − ELS2 = −2JNi − R − JR − R

(11)

From eq 10 and eq 11, we obtain 2JNi‑R = +299.95 cm−1 and 2JR‑R = +10.60 cm−1. The experimental range of 2JNi‑R value for 1 is 250−350 cm−1. The computed values are consistent with experimentally observed strong ferromagnetic interaction between nickel(II) and radical centers. It is appropriate to mention here that for strong ferromagnetic nickel(II)-radical systems, it is very difficult to quantify the exact coupling constant value experimentally.50 We should mention here that the DFT calculations carried out were not intended to reproduce the absolute value of the coupling constants, rather to demonstrate the nature, range, and extent (strong or weak) of magnetic-exchange coupling. Mulliken spin-population analysis on 1 reveals +1.62 spin on the metal (consistent with octahedral NiII), and each oaminophenolate part of the ligand holds +1.19 spin with a total of +2.38 spin over entire ligand. The spin-density plot for 1 is depicted in Figure 5a. This justifies the existence of osemiquinonate(1−) π-radical oxidation-level on both sides of the ligand and oxidation state of nickel as bivalent. Analysis of the magnetic-orbitals (Figure 5b) reveals that unpaired electrons reside mainly on the o-semiquinonate ring of the ligand and on the dx2-y2, dz2 orbitals of NiII. Geometry-optimization of [1]1+ (coordinates were taken from [1][PF6]·CH2Cl2; Table S4, Supporting Information) was performed with S = 3/2 and Ms = 1/2, and their respective energies are collected in Table 3. Analysis of respective optimized energy values reveals that high-spin state (S = 3/2) [NiII{(LIBQO,N)0}{(LISQO,N)•−}(LS,S)0]1+ electronic-configuration is lower in energy (the S = 3/2 state is stabilized by −1.126 kcal/mol, compared to the Ms = 1/2 state) compared to its doublet broken-symmetry state (Table 3). This implies the existence of ferromagnetic coupling between the two NiII electrons and one unpaired o-semiquinonate(1−) π-radical electron on the ligand in [NiII{(LIBQO,N)0}{(LISQO,N)•−}(LS,S)0]1+. To compute the J value for the two-center electronic system, it is a well-established fact that the use of the Yamaguchi approach is more appropriate as it is valid for a whole range of coupling (from weak- to strong-exchange interaction).42a,b Thus, following eq 4, 2JNi‑R is calculated as +268.36 cm−1. This computed value is consistent with experimentally observed strong ferromagnetic interaction between nickel(II) and the radical center. However, the computed value is “slightly” less than that of the experimental data-fitted range of 350−450 cm−1 (cf. the Magnetism section). It is worth mentioning here that for strong ferromagnetic nickel(II)-radical systems, it is very difficult to quantify the exact coupling constant value experimentally.50 For example,

Figure 5. (a) Spin-density plot and (b) magnetic-orbitals for [NiII{(LISQO,N)•−}{(LISQO,N)•−}(LS,S)0] (1) (high-spin state with S = 2).

for complex 2, 2JNi‑R = +310(6) cm−1 also provides good fitting (see the Magnetism section). The results of DFT calculation on 1 and 2 point toward the nature of the magnetic-interaction present, and the computed values are close to the range observed experimentally. Figure 6a displays the spin-density plot for [1][PF6]·CH2Cl2 where the +1.60 spin resides on the NiII ion and the +1.24 spin on one side of the ligand and +0.16 spin on other side of the ligand. This justifies that one side of the ligand exists in the osemiquinonate(1−) π-radical (LISQ)•− form, whereas the other H


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Inorganic Chemistry Table 3. Computed J Values for NiII{(LIBQO,N)0}{(LISQO,N)•−}(LS,S)0]1+ Ni

S,O,N

C,H

functional

−EHS (a.u.) S = 3/2

−ELS1 (a.u.) Ms = 1/2

2JNi‑R (cm−1)

TZVP

TZVP

SVP

CAM-B3LYP

4194.33043142

4194.32865840

+268.36

plot (Figure 7a) where the nickel center possesses +1.60 spinpopulation and the ligand contains a residual +0.40 spin-

Figure 7. (a) Spin-density plot and (b) magnetic-orbitals for [NiII{(LIBQO,N)0}{(LIBQO,N)0}-{(LS,S)0}][BF4]2·CH2Cl2 (with S = 1). Figure 6. (a) Spin-density plot and (b) magnetic-orbitals for [NiII {(L IBQO,N )0}{(L ISQO,N)•−}{(L S,S)0 }][PF6 ]·CH2Cl2 (high-spin state with S = 3/2).

population on the coordinating atoms. Figure 7b describes the magnetic-orbitals for [1]2+, where two unpaired electrons are residing on the metal-centered orbitals (196α and 197α). This confirms the benzoquinone form of the coordinated ligand and the metal as octahedral NiII. The calculated metal−ligand bond distances agree well with the experimental values of 1, [1][PF6]·CH2Cl2, and [1][BF4]2· CH2Cl2. The bond distances associated with the ligandbackbone accurately reproduce the redox-level of the coordinated ligand (Table S2 and Table S6, Supporting Information). DFT calculations thus confirm that all of the redox processes (cf. Redox Properties section) are ligand-based, and thus the oxidation-state of the metal (NiII) remains invariant, as reflected by the similar spin-population value of ∼ +1.6, in each case. Absorption Spectra and TD-DFT Results. In order to assign the observed electronic spectral transitions of the parent complex [NiII{(LISQO,N)•−}{(LISQO,N)•−}{LS,S}0] 1, and of its mono- and dicationic forms [NiII{(LIBQN,O)}0}{(LISQN,O)}•−}{(LS,S)0}][PF6]·CH2Cl2, and [NiII{(LIBQN,O)}0}{(LIBQN,O)}0}{(LS,S)}0][BF4]2·CH2Cl2, TD-DFT calculations were performed for 1, [1]1+, and [1]2+ (Tables 4, 5, and 6, respectively).

side of the ligand exists as the o-iminoquinone (LIBQ)0 form. It is important to note that oxidation of 1 to [1]1+ does not change the spin-population on the NiII center (+1.62 for 1 and +1.60 for [1]1+), whereas the spin-population on one side of the ligand reduces from +1.19 to +0.16. This proves that oxidation has taken place on the ligand-center. Analysis of magnetic-orbitals for [1]1+ (Figure 6b) reveals that out of three unpaired electrons residing orbitals, two (196α and 197α) are metal-based (dx2-y2 and dz2) and that the other one (198α) is ligand-based. It is worth-mentioning here that for 1, two unpaired electrons are metal-based, and the other two are ligand-based. Thus, during oxidation of 1 to [1]1+, an electron was removed from the ligand part. Further, two-electron oxidation of 1 leads to [1][BF4]2· CH 2 Cl 2 , the electronic configuration of which is [NiII{(LIBQO,N)0}{(LIBQO,N)0}(LS,S)0]2+, where the ligand is fully oxidized (benzoquinone form) with no unpaired electron on the ligand. Geometry-optimization of [1]2+ (Table S5, Supporting Information) and the corresponding spin-density I


Article

Inorganic Chemistry Table 4. TD-DFT-Calculated Electronic Transitions of [NiII(L3)] 1 excitation energy (eV)

λ (nm)

1.7092

725

0.15

2.8922

429

0.0446

3.4507

359

0.1043

3.4699

357

0.4109

4.0064

f

310

transition

character

β-H [∼98% L] → β-L+1 [∼98% L] (46%) β-H-1 [∼99% L] → β-L [∼99% L] (47%) β-H-2[∼98% L] → β-L[∼99% L] (29%) β-H-3[∼85% L] → β-L+1[∼98% L] (10%) β-H-4[∼96% L] → β-L+1[∼98% L] (16%) α-H[∼98% L] → α-L+3 [∼99% L] (20%) α-H-1[∼99% L] → α-L [∼99% L] (10%) α-H-1[∼99% L] → α-L+2 [∼96% L] (18%) α-H [∼98% L] → α-L+2 [∼96% L] (18%) α-H-1[∼99%L]→ α-L+3 [∼99%L] (18%) β-H-4[∼96% L] → β-L [∼99% L] (15%) β-H-5[∼95% L] → β-L+1 [∼98% L] (11%) β-H-7[∼87% L] → β-L+1 [∼98% L] (13%)

0.0845

intraligand charge-transfer in phenyl-iminosemiquinonate ring intraligand charge-transfer in iminosemiquinonate ring charge-transfer in phenyl-iminosemiquinonate part thioether-phenyl-iminosemiquinonate ring → phenyl-iminosemiquinonate part along with small MLCT thioether-phenyl-iminosemiquinonate ring → phenyl-iminosemiquinonate part charge-transfer in phenyl-iminosemiquinonate part phenyl-iminosemiquinonate part → amino thioether ring charge-transfer in phenyl-iminosemiquinonate part charge-transfer in phenyl-iminosemiquinonate part charge-transfer in phenyl-iminosemiquinonate part thioether-phenyl-iminosemiquinonate ring → phenyl-iminosemiquinonate part amino thioether ring → phenyl-iminosemiquinonate part phenolate-amino thioether ring → iminosemiquinonate part and small MLCT

Table 5. TD-DFT-Calculated Electronic Transitions of [1]1+ excitation energy (eV)

λ (nm)

f

1.6637 2.4543

745 505

0.0937 0.1194

2.5772

481

0.0328

2.6048 2.8651

476 433

0.0215 0.044

3.5234

352

0.2095

3.7667

330

0.0572

transition

character

β-H [∼98% L] → β-L+1[∼98% L] (92%) charge-transfer in phenyl-iminosemiquinonate part α-H-2[∼84% L] → α-L[∼99% L] (21%) charge-transfer from imino nitrogen phenolate oxygeen-thioether to phenyliminoquinone, along with MLCT α-H-3[∼95% L] → α -L[∼99% L] (15%) charge-transfer in phenyl-iminoiquinone part β-H-1 [∼99% L] → β-L [∼99% L] (33%) charge-transfer in phenyl-iminoiquinone part α-H-3[∼95% L] → α-L[∼99% L] (27%) charge-transfer in phenyl-iminoiquinone part β-H-6 [∼98% L] → β-L[∼99% L] (21%) charge-transfer in phenyl-iminoiquinone part α-H-3[∼95% L] → α-L[∼99% L] (32%) charge-transfer in phenyl-iminoiquinone part β-H-2[∼98% L] → β-L+1 [∼98% L] charge-transfer in phenyl-iminosemiquinonate part (64%) charge-transfer from phenyl-iminosemiquinonate to amino-thioether part α-H[∼99% L] → α- L+3[∼99% L] (16%) α-H[∼99% L] → α- L+4[∼98% L] charge-transfer in phenyl-iminosemiquinonate part (26%) β-H-2[∼98% L] → β-L+1 [∼98% L] charge-transfer in phenyl-iminosemiquinonate part (11%) β-H-2[∼98% L]→β-L [∼99% L] (47%) charge-transfer from phenyl-iminosemiquinonate to phenyl-iminoquinone

Table 6. TD-DFT-Calculated Electronic Transitions of [1]2+ excitation energy (eV)

λ (nm)

f

transition

2.3541

527

0.0979

2.3838

520

0.198

3.3731

368

0.088

3.5256

352

0.2144

α-H-1[∼99% L] → α-L[∼99% L] (23%) α-H [∼99% L] → α-L+1[∼98% L] (23%) β-H-1 [∼99% L] → β-L[∼99% L] (17%) β-H [∼99% L] → β-L+1[∼98% L] (15%) α-H [∼99% L] → α-L[∼99% L] (23%) α-H-1[∼99% L] → α-L+1[∼98% L] (19%) β-H [∼99% L] → β-L[∼99% L] (14%) β-H-1 [∼99% L] → β-L+1[∼98% L] (14%) β-H-1 [∼97% L] → β-L+1[∼98% L] (11%) β-H [∼99% L] → β-L[∼99% L] (25%) α-H-7[∼90% L] → α-L+1[∼98% L] (10%)

character

α-H-1[∼99% L] → α-L+1[∼98% L] (10%) β-H-2 [∼99% L] → β-L [∼99% L] (14%)

J

charge-transfer charge-transfer charge-transfer charge-transfer charge-transfer charge-transfer charge-transfer charge-transfer

from from from from from from from from

phenylphenylphenylphenylphenylphenylphenylphenyl-

iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone

to to to to to to to to

iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone iminoquinone

ring ring ring ring ring ring ring ring

charge-transfer from phenyl- iminoquinone to iminoquinone ring charge-transfer charge-transfer MLCT charge-transfer charge-transfer

from phenyl- iminoquinone to iminoquinone ring from amino thioether to quinone ring along with small from phenyl- iminoquinone to iminoquinone ring from amino thioether to iminoquinone ring


Article

Inorganic Chemistry

plexes, [ Ni I I {(L I S Q O , N ) • − }{(L I S Q O , N ) • − }{L S , S } 0 ] 1 , [Ni II {(L IBQ N,O } 0 }{(L ISQ O,N ) •− }{L S,S } 0 ][PF 6 ]·CH 2 Cl 2 , and [Ni II{(LIBQ N,O) 0}{(LIBQ N,O ) 0}{LS,S} 0]-[BF 4]2 ·CH 2Cl2 , have been achieved. (ii) For a hexadentate ligand, the coordination by two bidentate o-aminopholate units in two different oxidation-levels, o-iminosemiquinonate(1−) π-radical and oiminoquinone, as in [1][PF6]·CH2Cl2 is unprecedented. It is demonstrated that the oxidation-level of the coordinated ligands and NiII oxidation-state can be deduced by X-ray crystallographic metric parameters of reasonable quality in conjunction with mass, UV−vis−NIR spectral, and cyclic voltammetric results. (iii) A well-defined three-membered electron-transfer series has been identified. (iv) Temperaturedependent magnetic studies reveal ferromagnetic coupling between the magnetic-orbitals of NiII (S = 1) and two (LISQO,N)•− radicals in 1 (S = 2) and an (LISQO,N)•− radical in [1][PF6]·CH2Cl2 (S = 3/2). Two-electron oxidized complex [1][BF4]2·CH2Cl2 has an S = 1 ground-state, as expected. The experimental results are corroborated by DFT calculations, which support the proposed electronic structural assignments and observed absorption spectral features. Exploration of the generality and versatility of the coordination behavior of L3(4−) toward other transition-metal ions is being carried out in our laboratories.

The UV−vis-NIR spectra of the three complexes are displayed in Figure 8. The spectral signatures of these six-

Figure 8. Electronic absorption spectra of (a) [NiII{(LISQO,N)•−}{(LISQO,N)•−}{(LS,S)}0] 1 (black), [NiII{(LIBQO,N)0}{(LISQO,N)•−}{(LS,S)}0][PF6]·CH2Cl2 2 (red), and [NiII{(LIBQO,N)0}{(LIBQO,N)0}{(LS,S)}0][BF4]2·CH2Cl2 3 (blue) in CH2Cl2.

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coordinate NiII complexes are dominated by relatively intense charge-transfer (CT) transitions, in which the coordinated ligands are present in its three accessible oxidation-levels. Spectral features as observed for 1 and 3 are reported by Wieghardt and co-workers for [NiII(tren){(LISQN,O)}•−}][ClO4] and [NiII(tren){(LIBQN,O)}0}][PF6]2 (tren = tris(2aminoethyl)amine).19 In CH2Cl2, 1 shows intense absorptions in the 300−1000 nm region: 310 sh (ε = 23 500 M−1 cm−1), 405 (31 200), 528 (6120), 750 sh (6220), 850 (9330), and 960 sh (7700). TDDFT calculation on 1 (Figures S7 and S8, Supporting Information; Table 4) reveals that all transitions are mainly due to CT in the phenyl-iminosemiquinonate part. In CH2Cl2, [1][PF6]·CH2Cl2 displays intense absorption in the range 300− 1000 nm: 304 sh (ε = 18 160 M−1 cm−1), 396 (23 800), 495 sh (10 800), 530 (11 200), 770 sh (2540), 870 (3330), and 990 sh (2470). TD-DFT calculation on [1]1+ (Figures S7 and S9, Supporting Information; Table 5) reveals that the low-energy transitions are associated with CT involving phenyl-iminosemiquinonate, whereas high-energy transitions are combinations of CT in phenyl-iminosemiquinonate and charge-transfer from phenyl-iminosemiquinonate to phenyl-iminoquinone. In going from 1 to [1]1+, TD-DFT calculations (Tables 4 and 5) show that the oscillator-strength of the lowest-energy band decreased from 0.15 (725 nm) to 0.0937 (745 nm). This results in the decrease in intensity of the corresponding lowest-energy band (Figure 8) upon oxidation of 1 to [1]1+, which is attributed to the removal of one of the (LISQ)•− moieties to (LIBQ)0, due to one-electron oxidation. In CH2Cl2, [1][BF4]2·CH2Cl2 displays absorptions at 420 nm (ε = 15 420 M−1 cm−1) and 530 (14 580). TD-DFT calculation (Figures S7 and S10, Supporting Information; Table 6) reveals that these transitions are due to iminobenzoquinone ring-based CT.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02688. 1 H NMR and ESI-MS spectra of H4L3 (Figures S1 and S2, respectively); positive ESI-MS spectra of [Ni(L3)] 1 [M]+, [Ni(L3)][PF6]·CH2Cl2 2 [Ni(L3)]+, and [Ni(L3)][BF4]2·CH2Cl2 3 [M]+ (Figures S3, S5−S6); electronic spectra of the formation of I3− ion in presence of H2O2 (Figure S4); TD-DFT-calculated electronic spectra of [NiII{(LISQN,O)•−}{(LIBQN,O)0}{(LS,S)0}] 1, [Ni I I {(L I B Q N , O ) 0 }{(L I S Q N , O ) • − }{(L S , S ) 0 }] 1 + , and [NiII{(LIBQN,O)0}{(LIBQN,O)0}{(LS,S)0}]2+ (Figure S7) and their representative molecular-orbitals (Figures S8−S10); crystal data collection and structure refinement and metrical parameters for [Ni(L3)] 1, [1][PF6]· CH2Cl2, and [1][BF4]2·CH2Cl2 (Tables S1 and S2, respectively), DFT-optimized Cartesian coordinates for 1 (Table S3) and for [1]1+ (Table S4), DFT-optimized Cartesian coordinates of [1] 2+ (Table S5), and comparison of X-ray structural bond lengths of 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2, and DFTcalculated bond lengths of 1, [1]1+, and [1]2+ (Table S6) (PDF) Crystallographic graphic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-512-2597437. Fax: +91-512-2597436. E-mail: [email protected].

■

Notes

The authors declare no competing financial interest.

SUMMARY AND CONCLUDING REMARKS The findings of this work can be summarized as follows. (i) Using a hexadentate redox-active ligand with two o-aminophenol arms joined by a −SCH2CH2S− spacer, isolation and structural characterization of three six-coordinate NiII com-

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ACKNOWLEDGMENTS This work is supported by the J.C. Bose fellowship grant by the Department of Science & Technology (DST), Government of K


Article

Inorganic Chemistry

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India. R.M. sincerely thanks DST for a J.C. Bose fellowship. A.A. gratefully acknowledges the award of an SRF by Council of Scientific & Industrial Research (CSIR), Government of India, and S.K.B. acknowledges the award of an SRF by CSIR, postdoctoral fellowships by IISER Kolkata and by Science and Engineering Research Board (SERB, DST). We thank the reviewers for their helpful comments at the revision stage.

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