An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A

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An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A Combined Experimental and Computational Study Prasenjit Sarkar,† Manas Kumar Mondal,† Amrit Sarmah,‡ Suvendu Maity,§ and Chandan Mukherjee*,† †

Department of Chemistry, Indian Institute of Technology Guwahati (IITG), Guwahati 781039, Assam, India Department of Molecular Modelling, Institute of Organic Chemistry and Biochemistry ASCR, Flemingovo nám. 2, CZ-16610 Prague 6, Czech Republic § Department of Chemistry, R. K. Mission Residential College, Narendrapur, Kolkata 700103, India ‡

S Supporting Information *

ABSTRACT: Ligand H4Sar(AP/AP) contained two terminal amidophenolate units that were connected by a disulfane bridge. The ligand reacted with VOSO4·5H2O in the presence of Et3N under air and provided a mononuclear octahedral oxidovanadium complex (1). X-ray crystal structure analysis of complex 1 revealed that the oxidation state of the V ion was V and the VO3+ unit was coordinated to an iminosemiquinone radical anion. An isotopic signal at g = 1.998 in the X-band electron paramagnetic resonance (EPR) spectrum and the solution magnetic moment μeff = 1.98 μB at 298 K also supported the composition. The formation of complex 1 preceded through the initial generation of a diamagnetic VO2+− iminoisemiquinone species, as established by time-dependent UV−vis− near-IR (NIR), X-band EPR, and density functional theory studies. The UV−vis−NIR spectrum of complex 1 consisted of four ligand-to-metal charge-transfer transitions in the visible region, while an intervalence ligandto-ligand charge transfer appeared at 1162 nm. The cyclic voltammogram of the complex showed four oxidation waves and one reduction wave. Spectroelectrochemical studies at fixed potentials revealed that the oxidation and reduction processes were ligand-based.

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centers. Hence, radical-coordinated VO3+ complexes are yet to be reported in the literature. Herein, we have taken the initiative for the plausible synthesis of a radical-coordinated VO3+ complex. In this endeavor, a noninnocent ligand, which will be denoted here as H4Sar(AP/AP),5 has been designed. Ligand H4Sar(AP/AP) can exist in various oxidation states in coordination complexes (Scheme 1). In the ligand backbone, a disulfane unit has been incorporated. We envisaged that the S atom in the disulfane, which is not a strong π-donor or acceptor, would undergo weak coordination with the V center and would stabilize the V atom in the IV oxidation state via the weak metal-to-ligand (dπ−dπ) and ligand-to-metal (pπ− dπ) interactions. The interactions would initially facilitate the ligand-centered oxidation, and, hence, the formation of a radicalcoordinated VO2+ species was expected. Further oxidation of the species would preferably promote the formation of VO3+ species from VO2+ species because the IV oxidation state for the V atom is not greatly stabilized by the S atom. Thus, radical-coordinated VO3+ species could be synthesized and characterized geometrically as well as spectroscopically. In this regard, ligand H4Sar(AP/AP) was reacted with a stoichiometric amount of VOSO4·5H2O under air, thus

INTRODUCTION Over the last 2 decades, significant attention has been given to the synthesis and spectroscopic understanding of first-row transition-metal-radical complexes because of the ubiquity of first-row transition-metal-radical species in metalloenzymes’ active sites and enzymatic activities.1 In this context, noninnocent ligands have been employed for the successful syntheses of radical-coordinated transition-metal complexes under the solo use of molecular oxygen as the oxidant.2 Although radical-containing copper, nickel, cobalt, iron, and manganese complexes are common, radical-containing vanadium complexes are indeed very rare.3 The plausible reason is that the amount of energy required to oxidize closed-shell, fully reduced noninnocent ligands to the corresponding radical forms is higher compared to the energy required for oxidation of the V ion.4 In radical-containing VO2+ complexes,3a−e apart from noninnocent ligands, π-interacting ligands are also present in the coordination environment. Stabilization of the V atom in the IV oxidation state, due to metal-to-ligand π interaction, energetically favors oxidation of the closed-shell, noninnocent ligands to their corresponding radical states. Thus, radical-containing VO2+ complexes are successfully synthesized. Further oxidation of the radical-containing VO2+ complexes does not provide radicalcoordinated VO3+ complexes because oxidation of the radical center is favored compared to the π-interaction-stabilized VO2+ © 2017 American Chemical Society

Received: March 27, 2017 Published: June 28, 2017 8068

DOI: 10.1021/acs.inorgchem.7b00789 Inorg. Chem. 2017, 56, 8068−8077

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Inorganic Chemistry Scheme 1. Various Plausible Oxidation States of Ligand H4Sar(AP/AP)

Scheme 2. (I) Schematic Representation for the Syntheses of H4Sar(AP/AP) and Complex 1 and (II) Two Possible Electronic Structures (X1 and X2) of Intermediate X

solution became blue and the complex started to precipitate out slowly. A detailed procedure is given in the Experimental Section. The time-dependent and simultaneous UV−vis−NIR and EPR spectra were recorded for a better understanding of the formation of complex 1 via the initial generation of the green intermediate X. In this context, the ligand, the metal salt, and Et3N were added in MeOH. The UV−vis−NIR and X-band EPR spectra were recorded instantaneously. No appreciable peak in the absorption spectrum in the 400−1600 nm region was observed, while the X-band spectrum exhibited an eight-line spectrum (Figure 1B), which was typical for VIV 3d1 species.6 The X-band EPR spectrum appeared because of the presence of VOSO4·5H2O salt in the solution. The EPR signal vanished within 10 min, and in the absorption spectrum, a new peak at around 615 nm appeared. It is worth noting that, in the absence of the metal salt, no change in the absorption spectrum was realized. This result further concluded that the presence of

providing mononuclear complex 1. X-ray crystallographic analysis of complex 1 showed that the central VO3+ unit contained an iminosemiquinone radical in its coordination sphere. The Evan’s method room temperature magnetic moment measurement (μeff = 1.98 μB) and ligand-centered g = 1.998 Xband electron paramagnetic resonance (EPR) signal also supported the formation of the unprecedented radicalcoordinated VO3+ complex. Time-dependent UV−vis−near-IR (NIR) and X-band EPR studies and density functional theory (TDDFT)-based calculation indicated that complex 1 was formed via the initial generation of a green diamagnetic VO2+− iminosemiquinone species (intermediate X). Furthermore, TDDFT calculations were performed for a better understanding of the UV−vis−NIR spectra of the intermediate and complex 1. Herein, we put forward the synthesis, characterization, and theoretical studies of H4Sar(AP/AP), intermediate X, and complex 1.

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RESULTS AND DISCUSSION A schematic diagram for the syntheses of ligand H4Sar(AP/AP) and complex 1 is presented in Scheme 2. The organic 1,2diphenyldisulfane-bridged ligand H4Sar(AP/AP) was synthesized in 73% yield by the condensation of 1:3 2,2′-disulfanediyldianiline and 3,5-di-tert-butylcatechol in hexane in the presence of triethylamine (Et3N). The synthesis of complex 1 was carried out by reacting the ligand with an equivalent amount of VOSO4· 5H2O in the presence of Et3N in methanol (MeOH) under air. Interestingly, a time-dependent change in the color of the reaction solution was observed. Initially, the color of the solution was green, which persisted for about 1 h. After that, the green

Figure 1. Time-dependent UV−vis−NIR (A) and simultaneous X-band EPR (B) spectral changes that occur during the formation of complex 1. X-band EPR spectra were measured at frequency = 9.45 GHz, power = 0.995 mW, modulation frequency = 100 kHz, amplitude = 10 G, and temperature = 298 K. 8069


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Inorganic Chemistry VOSO4·5H2O was essential for the formation of intermediate X (Figure S7). The absorption peak intensity increased with time, while no appreciable EPR signal was observed until 50 min. After that, a ligand-centered EPR signal with g ∼ 2.00 and a new peak at around 1130 nm in the UV−vis−NIR spectrum (Figure 1A) started to appear simultaneously. This feature consolidated the formation of complex 1 (vide infra) from the green intermediate X. Further time-dependent investigations showed that the peak at around 615 nm increased until 110 min and, after that, depleted with time, while the peak at around 1130 nm increased continuously with the progression of time. The X-band EPR spectrum recorded after 5 h of reaction (Figure 1B) concluded the formation of complex 1. Several attempts for the isolation of intermediate X by carrying out the reaction for 1 h and recrystallization of the isolated crude product under anaerobic conditions (N2 atmosphere) employing various solvent mixtures were unsuccessful. However, the disappearance of the VIV-centered X-band EPR spectrum with the formation of intermediate X clarified the generation of a diamagnetic species. Thus, the intermediate could be either a closed-shell ligand-coordinated VV 3d0 (S = 0) species (X1) or an antiferromagnetically coupled iminosemiquinone ligand (S = 1 /2)-coordinated VIV 3d1 (S = 1/2) species (X2) [Scheme 2]. DFT-based calculations indicated that X2 was 2.21 kcal/mol more stable (vide infra) than X1. Thus, we proposed that species X2 was the green intermediate X. TDDFT calculations indicated that the peak at around 615 nm appeared because of interligand π-to-π* charge transfer (LLCT; vide infra). It is worth noting that this type of LLCT band has also previously been found for iminosemiquinone-coordinated VO2+ species.3b,d In the IR spectrum of H4Sar(AP/AP), ν(O−H), ν(N−H), and ν(C−H) [tBu] stretching bands appeared at 3441, 3366, 2957, 2908, and 2868 cm−1. Upon complex formation, the ν(O−H) and ν(N−H) stretching bands vanished, indicating deprotonation of the groups. The presence of 2952, 2904, and 2866 cm−1 bands confirmed the existence of a 3,5-di-tert-butylcatecholate moiety in complex 1. The strong bands at 940 cm−1 appeared because of the presence of a vanadyl (VO) unit.3l,m The ν(V− O) stretching bands appeared in the 640−497 cm−1 region.3l,m,7 The electrospray ionization mass spectrometry (ESI-MS) spectrum in CH3CN for ligand H4Sar(AP/AP) provided a 100% peak at m/z 657.35, corresponding to [M + H]+ (M = molecular mass), and confirmed the composition C40H52N2O2S2 for the ligand (Figure S3). In the positive-mode ESI-MS spectrum of complex 1 in MeOH, a 100% peak at m/z 719.26 (M+) arose. Simulated isotope pattern distributions indicated a C40H48N2O3S2V1 composition for the observed peak (Figure S5). The geometry of complex 1 was determined by single-crystal X-ray diffraction measurement at 293(2) K. The complex crystallized in the triclinic space group P1̅. The molecular structure with the selected atom-numbering scheme is shown in Figure 2. The selected bond distances and bond angles are tabulated in Table 1. The complex was mononuclear and neutral in charge. In the complex, the central V atom (V1) was six-coordinate and surrounded by two NO (N1−O1 and N2−O2) donor sets from two amidophenolate units, one S atom (S1), and a terminal O atom (O3). The V1−O1 = 1.9328(16) Å bond was shorter compared to the V1−O2 = 1.9468(16) Å bond and indicated two different oxidation states for the two coordinated 2-amidophenolate-type units A and B (Figure 3, bottom). The V1−N2 = 2.198(2) Å bond was longer compared to the V1−N1 =

Figure 2. ORTEP diagram of complex 1. Thermal ellipsoids were drawn at the 50% probability level. H atoms are omitted for the sake of clarity.

Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for 1 V1−O3 V1−O2 V1−N2 S1−C20 S2−C21 O1−C2 N1−C15 N2−C26 O3−V1−O1 O1−V1−O2 O1−V1−N1 O3−V1−N2 O2−V1−N2 O3−V1−S1 O2−V1−S1 N2−V1−S1 C20−S1−V1 C21−S2−S1 C2−O1−V1 C15−N1−V1

1.6147(17) 1.9468(16) 2.198(2) 1.772(3) 1.777(3) 1.324(3) 1.405(3) 1.413(3) 112.67(9) 86.69(7) 79.21(7) 158.01(9) 75.22(7) 83.94(7) 111.06(5) 80.94(5) 94.41(9) 104.74(9) 115.66(14) 121.33(16)

V1−O1 V1−N1 V1−S1 S1−S2 O2−C32 N1−C1 N2−C27 C32−C31 O3−V1−O2 O3−V1−N1 O2−V1−N1 O1−V1−N2 N1−V1−N2 O1−V1−S1 N1−V1−S1 C20−S1−S2 S2−S1−V1 C32−O2−V1 C1−N1−V1 C27−N2−V1

1.9328(16) 1.9952(19) 2.5124(7) 2.0601(11) 1.299(3) 1.375(3) 1.340(3) 1.427(3) 95.66(8) 101.94(8) 160.66(7) 87.12(7) 90.78(8) 154.97(5) 79.04(6) 102.68(10) 114.16(4) 120.12(14) 114.12(15) 112.27(15)

Figure 3. Two possible electronic configurations of complex 1.

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of the formal oxidation state of noninnocent ligands in their corresponding coordination complexes. In complex 1, two amidophenolate units (A and B, Figure 3, bottom) coordinated to the central vanadyl unit. The C1−C2 = 1.424(3) Å, C2−C3 = 1.413(3) Å, C3−C4 = 1.377(4) Å, C4−C5 = 1.417(4) Å, C5−C6 = 1.367(4) Å, and C1−C6 = 1.399(3) Å bond distances along with a fairly single bond characterizing C1− N2 = 1.375(3) Å and C2−O1 = 1.324(3) Å bond distances supported the fully reduced, closed-shell amidophenolate [(AP)2−] form of the A unit. The slight quinoid-type distortion, i.e., elongation of the C1−C2 bond and contraction of the C3− C4 and C5−C6 bonds, in the phenyl ring in the A unit and shorter C1−O1 bond compared to pure phenolate C−O = 1.35 Å bond (Chart 1) emphasized the amidophenolate highest occupied molecular orbital (HOMO) to V d (ligand-to-metal) π donation.9 A much more prominent quinoidal distortion [C27− C28 = 1.418(3) Å, C28−C29 = 1.358(3) Å, C29−C30 = 1.431(4) Å, C30−C31 = 1.372(3) Å, C31−C32 = 1.427(3) Å, C27−C32 = 1.443(3) Å] and C27−N2 = 1.340(3) Å and C32− O2 = 1.299(3) Å bond distances closely resembled the previously reported one-electron-oxidized amidophenolate form2a−f and, thus, invoked the iminosemiquinone [{ISQ}−] form (Chart 1) of the B unit. Therefore, the coordinated ligand existed in the complex as [Sar(ISQ/AP)]3− (Scheme 1), and complex 1 can thus be formulated as [{VO}3+{Sar(ISQ/AP)}3−]0. However, it is worth noting that the oxidation states are descriptive conventions (not physically observable). The magnetic moment of complex 1 was examined at room temperature (25 °C) in CDCl3 employing Evan’s method (Figure S8). An effective magnetic moment (μeff) of about 1.98 μB was obtained. This indicated that the complex possessed a single unpaired electron (S = 1/2), which might be an organic radical in nature. To reconsolidate the radical nature of the unpaired electron, the X-band EPR spectrum of complex 1 was recorded in a CH2Cl2 solution as well as in the solid state at room temperature (25 °C). In both cases, an isotopic signal at g = 1.998 appeared (Figure 4A). The value was very close to free electron g = 2.00

1.9952(19) Å bond. This 0.20 Å bond elongation was because of the trans effect exerted by the vanadyl unit [V1−O3 = 1.6147(17) Å]. A weak coordination between the S1 and V1 atoms [V1−S1 = 2.5124(7) Å] was observed. The V atom was in vanadyl (VOn+) form. Thus, the oxidation state of the V atom can be either IV or V. The effect of the oxidation state of the V atom on the terminal VO bond distance is known as minimal. Thus, the formal oxidation state of the central V atom in the complex is to be established mainly by acute determination of the formal oxidation state of the coordinated organic ligand backbone. Because the complex is neutral in charge, the composition of complex 1 would be either [{VO}2+{Sar(ISQ/ISQ) }2−]0 or [{VO}3+{Sar(ISQ/AP)}3−]0 (Figure 3). In a metal-coordination complex, the oxidation state of the 3,5di-tert-butyl-group-containing 2-amidophenolate units can be determined by analyzing CPh−CPh, CPh−NPh, and CPh−OPh bond distances [CPh = phenyl ring belonging to a C atom, NPh = N atom attached to a phenyl ring (i.e., an aniline N atom), and OPh = O atom attached to a phenyl ring (i.e., phenol O atom)]. In the dianionic, fully reduced (closed-shell) 2-amidophenolate [(AP)2−] form, the C6 ring is aromatic and the CPh−CPh bond distances are intermediate between the C−C single and C−C double bonds. CPh−NPh and CPh−OPh are single bonds (Chart 1).8 In both one-electron-oxidized iminosemiquinone {(ISQ)−} Chart 18

Figure 4. (A) Experimental X-band EPR spectrum (9.45 GHz) of complex 1 measured at room temperature, power = 0.995 mW, modulation frequency = 100 kHz, and amplitude = 0.1 G. (B) UV−vis− NIR spectrum of complex 1.

and inferred a radical-centered unpaired electron with an S = 1/2 ground state for complex 1. Therefore, the X-band EPR spectral analysis further justified the [{VO}3+{Sar(ISQ/AP)}3]0 formulation for the complex. The absence of any 51V(I = 7/2) hyperfine coupling pattern in the spectrum indicated localization of the radical within the iminosemiquinone unit. The electronic absorption spectrum (UV−vis−NIR) for complex 1 in a CH2Cl2 solution is illustrated in Figure 4B. The spectrum was dominated by intense charge-transfer transitions.

and two-electron-oxidized iminoquinone {(IQ)0}, the C6 ring is in quinoid form, while CPh−NPh and CPh−OPh are double bonds in (IQ)0, and the bonds are intermediate between the single and double bonds in (ISQ)−. Thus, the systematic change in the CPh−CPh, CPh−NPh, and CPh−OPh bond distances (Chart 1) has been well explored and established for the successful assignment 8071


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Inorganic Chemistry Complex 1 was comprised of both (AP)2− and (ISQ)− units (vide supra). Therefore, (AP)2−-to-VO3+ as well as (ISQ)−-toVO3+ charge-transfer (LMCT) transitions are expected to appear in the spectrum. The peak at λmax = 690 nm (ε = 3600 M−1 cm−1) was attributed to (AP)2−-to-VO3+ charge transfer, while the peak at λmax = 487 nm (ε = 8000 M−1 cm−1) was ascribed to a (ISQ)−to-VO3+ transition. The 792 nm (ε = 3500 M−1 cm−1) peak possibly arose due to intraligand charge transfer in the (ISQ)− unit. The low-energy absorption manifold centered at 1162 nm (ε = 5250 M−1 cm−1) possibly appeared because of the ligandcentered (AP)2−-to-(ISQ)− intervalence ligand-to-ligand chargetransfer (IVLLCT) and metal-to-ligand charge-transfer (MLCT) transitions. The electrochemical behavior of complex 1 was investigated by cyclic voltametry. The complex at a scan rate of 50 mV s−1 showed one one-electron reversible reduction process [E1/2red1 = −0.681 V; peak difference (ΔE) = 85 mV] and two one-electron oxidation processes; among them, the oxidation occurring at lower potential (E1/2ox1 = 0.047; ΔE = 80 mV) was reversible, and the other (Epox2 = 0.954 V) was irreversible (Figure 5A).

Herein, we performed spectroelectrochemical studies for oneelectron reduction and one-electron oxidation processes. During the fixed potential coulometric one-electron reduction of complex 1, the intervalence charge-transfer (IVCT) peak at 1162 nm almost vanished, while the LMCT peaks at 792, 690, and 487 nm diminished in intensity (Figure 6B). The peak at 487

Figure 6. Changes in the UV−vis−NIR spectrum of complex 1 during a one-electron fixed potential coulometric: (A) oxidation; (B) reduction.

nm also shifted to 515 nm. The disappearance of the IVCT band clearly inferred reduction of the (ISQ) − unit to the corresponding (AP)2− unit. This led to formation of the expected [{VO}3+{Sar(AP/AP}4−]− species. A minimal change in the spectral feature of complex 1 was observed during the coulometric one-electron oxidation of the complex. The IVCT peak at 1162 nm decreased in intensity, and the peak shifted to 1175 nm (ε = 4100 M−1 cm−1). The presence of this low-energy peak supported the existence of the (ISQ)− unit in 1+ species and could be assigned as a LLCT transition. Thus, the formation of nonradical-containing species [{VO}3+{Sar(IQ/AP)}2−]+ as 1+ was being discarded, and the generation of [{VO}3+{Sar(ISQ/ISQ)}2−]+ species was postulated. Notably, no diradical-containing vanadium(V) species has been reported in the literature. Therefore, direct spectroscopic comparison was not possible. Computational Study. DFT-based calculations were performed on the monoanionic intermediates X1 and X2 (Scheme 2) and neutral complex 1. In the case of intermediate X2, both an antiferromagnetically coupled singlet state and a ferromagnetically coupled triplet state were considered. Computational details are given in the Experimental Section. The V−O/N/S bond distances, obtained from structural optimization for X1, X2, and complex 1, are given in Table S1. We checked the stability of the wave function for the optimized complex 1 as well as two anionic systems. It was found from the calculation that the closed-shell-singlet (CSS) → open-shellsinglet (OSS) perturbation induced instability to the CSS solution of X2. Interestingly, some singlet radical characteristics have been observed for the HOMO in the case of complex 1, whereas the HOMO in X2 exhibited a highly probable coupled electronic state, as depicted in Figure 7. Perhaps, implementation of an unrestricted broken-symmetry (BS) solution is the best possible tool to describe the electronic coupling in this particular situation. The BS solution for X2 was stable under the considered CSS → OSS perturbation. It is worth mentioning here that the BS solution for an antiferromagnetically coupled state was 2.21 kcal/mol lower in energy than that for X1. In a similar observation, the triplet state for intermediate X2 was found to be 3.52 kcal/mol higher in energy compared to the antiferromagnetically coupled state. The probability of an energetically favorable antiferromagnetic coupling can be pictorially visualized from the computed spin density maps obtained from the BS solution. Subsequently, the density of

Figure 5. Cyclic voltammograms of complex 1 measured at 50, 100, and 200 mV: (A) −1.00 to +1.20 V; (B) −0.90 to +0.30 V.

Interestingly, at higher scan rates (100 and 200 mV), two additional oxidations occurred between the two previously observed oxidation processes (Figure 5A). The appearance of additional oxidation waves at higher scan rate was presumably due to rearrangement processes of the ligand following the first oxidation, which were on a time scale that was in the range of the higher scan rates but faster than the slowest scan rate. X-ray crystallographic analysis (vide supra) implied that complex 1, [{VO}3+{Sar(ISQ/AP)}3−]0, acquired one iminosemiquinonate (ISQ)− unit and one amidophenolate (AP2−) unit in the coordination sites (Table 2). Thus, the π-radical center was the only site that could be reduced feasibly at the observed potential E1/2red1 = −0.678 V. Therefore, the reduction process could be assigned to reduction of the π-radical-containing iminosemiquinone unit to the corresponding closed-shell amidophenolate unit, and [{VO}3+{Sar(AP/AP)}4−]− would be generated. However, the first oxidation process for complex 1 could be either the oxidation of a π radical that would lead to the formation of an iminoquinone (IQ)0 unit from the iminosemiquinone (ISQ)− unit or the oxidation of the amidophenolate unit to an iminosemiquinone unit. An iminoquinone species is a strong oxidant, while an amidophenolate species is a strong reductant. Thus, the coexistence of both species in the same coordination sphere of a metal ion in a stable coordination complex is unexpected. Therefore, the formation of [{VO} 3 + {Sar ( I Q / A P ) } 2 − ] + was unlikely. Hence, the [{VO}3+{Sar(ISQ/ISQ)}2−]+ species would be generated by the one-electron oxidation of complex 1. 8072


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also correlated well with the predicted electronic environment of complex 1 and emphasized the presence of a single (unpaired) β electron at the highest occupied level. To understand the possible electronic excitations responsible for the observed peak maxima in the experimental UV−vis−NIR spectra of intermediate X2 and complex 1, we performed extensive TDDFT calculations. The optimized structures of both species were further subjected to TDDFT calculations at the same level of theory, i.e., M06-2x/def2tzvp. The implicit solvent environment for DCM has also been considered in TDDFT calculations. As many as 30 lowest triplet excitation energies for the optimized structure of X2 with triplet spin states and the same number of excitation energies for complex 1 have also been analyzed using the TDDFT formalism. It is worth mentioning here that the TDDFT-calculated electronic excitations can be directly correlated with the particular peak maxima of the experimental UV−vis−NIR spectra. The vertical excitation energies and corresponding oscillator strength values obtained from TDDFT-based computation for X2 and complex 1 are given in Table S2. The pictorial representations of the computer-simulated UV− vis−NIR spectrum based on TDDFT calculations for X2 and complex 1 are reported in Figure S12. It was encouraging to have some precise correlations between the simulated and experimental spectra. The peak appearing at 615 nm in the experimental UV−vis−NIR spectrum of X2 mainly originated from the HOMO-to-LUMO+1 (85%) transition, as it appeared from the computed configuration interaction orbital analysis. A closer look into the FMOs (Figure S11A) predicted the characteristic π-to-π* ligand-to-ligand (the A unit to the B unit) charge transfer (IVCT) for this particular electronic excitation. However, there were some significantly weak metalto-ligand charge-transfer (MLCT) contributions during this particular transition. On the other hand, two major excitations were prevalent for the experimental UV−vis−NIR spectrum of complex 1. The peak appearing at 1162 nm was basically a HOMO-to-LUMO (85%) electronic excitation (Figure S11B). FMO analysis revealed the existence of a LLCT characteristic associated with n-to-π* types of electronic excitations for the process. Another peak at 480 nm corresponded to a combination of HOMO-to-LUMO+1 (64%) and HOMO−3-to-LUMO+1 (13%) electronic excitations. A more sophisticated way to visually represent the proposed charge-transfer process during electronic excitation in the two systems is the density difference plots reported in Figure 8. Primarily, this was the outcome of the subtraction of the groundstate electronic charge density from the excited-state electronic

Figure 7. HOMOs of intermediate X2 and complex 1.

states (DOS) and projected density of states (PDOS) plots for the ferromagnetically coupled triplet and antiferromagnetically coupled singlet states (reported in Figure S9) were also supported by the above-mentioned observation. Theoretical findings suggested some significant variations in the distribution of electronic states between the triplet and singlet states of the system. It was also important to note that there was a large energy gap between the two β-spin unpaired electrons in the triplet state. Again the PDOS analysis of X2 in the triplet state revealed some interesting contradictions about the electron occupancy. It appeared that the β-HOMO was energetically higher than the corresponding α lowest unoccupied molecular orbital (αLUMO). Thus, the wave-function stability analysis implied a far-reaching impact of the unpaired electron-induced instability to the electronic wave function for that particular state. To be precise, this was not the preferable electronic configuration for X2. However, the stability of the wave function for the BS solution was found to be stable under the considered perturbation and correlated well with the PDOS plot (Figure S9C). Here, the two unpaired electrons appeared to be in an antiferromagnetically coupled closed-shell state. Accordingly, the HOMO was shifted to lower energy (Figure S9B) and the HOMO−LUMO gap increased. Considering the analogy from the aforementioned discussion regarding the best possible electronic environment in X2, it was essential to determine the preferable oxidation state of the central V metal ion in the intermediate. It was much more evident from Figure S9A that the majority of the unpaired α-spin density (62%) was localized on the dxy orbital of the V atom and corresponding the β-spin was located on the N2 (26%) and O5 (22%) atoms of the ligand. The significantly higher amount of localized spin density on the V atom authenticated the possibility of a IV oxidation state for the V atom of the oxidovanadium unit in X2. Additionally, the possibility of a strongly coupled electronic state for X2 was also evident from the frontier molecular orbital (FMO) analysis. Thus, it can be argued that the findings of theoretical calculations were in good agreement with the experimental findings. In a similar note, we investigated the stability of the electronic wave function for complex 1, and it was found to be stable under the considered perturbation. A doublet spin multiplicity associated with some singlet radical characteristic was evident from the theoretical calculations for the complex. A computed spin-density map clearly suggests delocalization of the excess unpaired electron density over the 2-amidophenolate-type ring B (Figure S10A). On the other hand, the absence of localized spin density on the metal atom implied a higher oxidation state for the V atom in complex 1. Henceforth, taking the analogy from the discussion on the anionic system (intermediate X), the central V atom was supposed to be in the V oxidation state; i.e., the 3d and 4s orbitals were empty. The calculated DOS plot (Figure S10B)

Figure 8. LLCT in (A) X2 and (B) complex 1. Electron density differences between the T1 (triplet excited state) and T0 (triplet ground state) (left) for X2 and between the S1 (excited state) and S0 (ground state) (right) for complex 1. Blue corresponds to negative values (higher electron density in the ground state), while purple corresponds to positive ones (higher electron density in the excited state). 8073


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understand the relative stability, electronic environment, and photophysical behavior of the complexes. The Gaussian09 package13 was used throughout the calculations. We have tested various DFT functionals (both hybrid and meta-GGA), and on the basis of its performance and previous reports, M06-2x (global hybrid functional containing 56% Hartree−Fock exchange)14 was considered to be the most effective method. Perhaps, M06-2x was found to be a good performer for maingroup thermochemistry, kinetics, noncovalent interactions, etc.15 The initial geometries of the complex are optimized at the M06-2x/ def2tzvp16 level without any imposed constraint. The solvent effect is taken care through the self-consistent-reaction-field17 approach using the “Integral Equation Formalism-Polarizable Continuum Model” (IEFPCM)18 formalism for DCM, as implemented in Gaussian09. Subsequently, frequency calculations were performed to ensure global energy minima of the optimized structures.

charge density. These computer-aided higher-level ab initio simulations, in combination with the experimental results, strongly suggested that the lowest-energy excitation (636 nm in X2 and 1197 nm in 1) in the complexes are mainly ligandcentered along with some minor contributions from the MLCT states.

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CONCLUSION In conclusion, we presented the successful synthesis of a sixcoordinate mononuclear neutral oxidovanadium(V) complex (1). X-ray structural analysis of complex 1 revealed the presence of an iminosemiquinone radical anion and an aminophenolate dianion in the coordination sphere. To the best of our knowledge, the complex is the first isolated iminosemiquinonecoordinated mononuclear VO3+ species. The paramagnetic nature of the complex with an S = 1/2 spin state, due to the presence of the solo paramagnetic iminosemiquinone radical, was consolidated by the solution magnetic moment (Evan’s method, μeff = 1.98 μB) and X-band EPR spectrum (isotopic signal at g = 1.998) measurements. Investigation on complex 1 formation by simultaneously measuring the time-dependent UV−vis−NIR and X-band EPR spectra of the reaction solution indicated that complex 1 was generated via the initial formation of a diamagnetic species. DFTbased calculations established that the diamagnetic species was the iminosemiquinone-coordinated oxidovanadium(IV) species. The species was 2.21 kcal/mol more stable than its corresponding resonance form, the amidophenolate-coordinated oxidovanadium(V) species, and an antiferromagnetic interaction between the SR = 1/2 and SVIV = 1/2 prevailed. Thus, herein, metal center oxidation of an iminosemiquinone-coordinated VO2+ species was successfully pursued in the presence of a disulfane ligand backbone.

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Table 2. Crystallographic Parameters and Refinement Data for 1 empirical formula fw CCDC number, cryst habit, color cryst size (mm3) temperature T wavelength λ (Å) cryst syst space group unit cell dimens a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume V (Å3) Z calcd density, Mg m−3 abs coeff μ (mm−1) F(000) θ range for data collection limiting indices reflns collected/unique completeness to θ max and min transmission refinement method data/restraints/parameters GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

EXPERIMENTAL SECTION

Materials. All of the chemicals and solvents were obtained from commercial sources and used as supplied, unless noted otherwise. 3,5Di-tert-butylcatechol and 2-aminothiophenol were purchased from Sigma-Aldrich. 2,2′-Disulfanediyldianiline was synthesized according to a previous report.10 Solvents were obtained from Merck (India). Mass spectra were measured in a HPLC-grade either acetonitrile or methanol solution. Physical Methods. X-ray crystallographic data were collected using Supernova, single source at offset, Eos diffractometer. Data refinement and cell reduction were carried out by CrysAlisPro.11 Structures were solved by direct methods using SHELXS-97 and refined by a full-matrix least-squares method using SHELXL-97.12 All of the non-H atoms were refined anisotropically. SQUEEZE was used to deal with a diffuse contribution of water to the overall electron density. IR spectra were recorded on a PerkinElmer instrument at normal temperature with KBr pellets by grinding the sample with KBr (IR-grade). 1H and 13C NMR spectra of the ligand were recorded in a Bruker 600 MHz NMR spectrometer. UV−vis−NIR spectra and spectroelectrochemical data were recorded on a PerkinElmer Lamda 750 UV−vis−NIR spectrometer by preparing a known concentration of the samples in HPLC-grade CH2Cl2 at room temperature (25 °C) using a cuvette of 1 cm width. The cyclic voltammograms of the complex were recorded in CH2Cl2 solutions containing 0.10 M [(nBu)4N]ClO4 as the supporting electrolyte at a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The experiments were performed at different scan rates. Ferrocene was used as an internal standard, and potentials were referenced versus the ferrocenium/ ferrocene couple. Computational Details. An extensive theoretical simulation using a sophisticated DFT-based method (augmented with long-range correction) with significantly large basis sets was performed to

C40H48N2O3S2V 719.86 1537090, block, brown 0.26 × 0.19 × 0.17 293(2) 0.71073 triclinic P1̅ 10.4828(5) 12.8429(5) 16.1967(8) 84.433(4) 72.558(4) 83.908(3) 2063.66(15) 2 1.158 0.377 762 2.99−25.00° −12 ≤ h ≤ 12, −15 ≤ k ≤ 15, −19 ≤ l ≤ 15 14457/7284 [R(int) = 0.0224] 99.8% (θ = 25.00°) 0.938/0.918 SHELXL-97 (Sheldrick, 1997) 7284/0/445 1.017 R1 = 0.0473, wR2 = 0.1271 R1 = 0.0597, wR2 = 0.1363 +0.85 and −0.35 e Å−3

Synthesis of H4Sar(AP/AP), [C40H52N2O2S2]. To a solution of 2,2′disulfanediyldianiline (1.24 g, 5 mmol) and 3,5-di-tert-butylcatechol (3.33 g, 15 mmol) in hexane (40 mL) was added Et3N (0.1 mL). The solution was refluxed for 24 h. During the period, the color of the solution changed from green to reddish-brown. The resulting solution was further stirred at room temperature (25 °C) for 2 h. After that, the solution was evaporated in vacuum, and the residue was purified by column chromatography (1% EtOAc/hexane). A yellow amorphous product was isolated. Yield: 2.40 g, 73%. FTIR (KBr pellet, cm−1): δ 3441, 3366, 3069, 2957, 2906, 2868, 1586, 1570, 1476, 1448, 1420, 1391, 1362, 1310, 1265, 1220, 1200, 1157, 1116, 1033, 975, 881, 822, 808, 749, 649. 1H NMR (600 MHz, CDCl3): δ 7.31 (d, J = 7.8 Hz, 2H), 7.24 (s, 2H), 7.19 (t, J = 7.8 Hz, 2H), 6.94 (s, 2H), 6.71 (t, J = 7.2 Hz, 2H), 6.49 (d, J = 8.4 Hz, 2H), 6.20 (s, 2H), 6.15 (s, 2H), 1.44 (s, 18H), 1.26 (s, 18H). 13C NMR (151 MHz, CDCl3): δ 149.73, 148.77, 142.56, 8074


Article

Inorganic Chemistry

the Cun-O2 structure and subsequent reactivity. JBIC, J. Biol. Inorg. Chem. 2004, 9, 669−683. (f) Kitajima, N.; Moro-oka, Y. Copper-Dioxygen Complexes. Inorganic and Bioinorganic Perspectives. Chem. Rev. 1994, 94, 737−757. (g) Kopf, M. A.; Karlin, K. D. In Biomimetic Oxidations; Muenier, B., Ed.; Imperial College Press: London, U.K., 2000; Chapter 7. (h) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Geometric and Electronic Structure/Function Correlations in Non-Heme Iron Enzymes. Chem. Rev. 2000, 100, 235−350. (i) Akita, M.; Hikichi, S. Inorganic Chemistry Based on Tp Ligands From Dioxygen Complexes to Organometallic Systems. Bull. Chem. Soc. Jpn. 2002, 75, 1657−1679. (j) Tshuva, E. Y.; Lippard, S. J. Synthetic Models for Non−Heme Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics. Chem. Rev. 2004, 104, 987−1012. (2) Some references are as follows: (a) Mukherjee, C.; Pieper, U.; Bothe, E.; Bachler, V.; Bill, E.; Weyhermüller, T.; Chaudhuri, P. LigandDerived Oxidase Activity. Catalytic Aerial Oxidation of Alcohols (Including Methanol) by Cu(II)Diradical Complexes. Inorg. Chem. 2008, 47, 8943−8956. (b) Chaudhuri, P.; Wieghardt, K.; Weyhermüller, T.; Paine, T. K.; Mukherjee, S.; Mukherjee, C. Biomimetic metalradical reactivity: aerial oxidation of alcohols, amines, aminophenols and catechols catalyzed by transition metal complexes. Biol. Chem. 2005, 386, 1023−1033. (c) Mondal, M. K.; Biswas, A. K.; Ganguly, B.; Mukherjee, C. Unprecedented iminobenzosemiquinone and iminobenzoquinone coordinated mononuclear Cu(II) complex formation under air. Dalton Trans. 2015, 44, 9375−9381. (d) Rakshit, R.; Ghorai, S.; Biswas, S.; Mukherjee, C. Effect of Ligand Substituent Coordination on the Geometry and the Electronic Structure of Cu(II)Diradical Complexes. Inorg. Chem. 2014, 53, 3333−3337. (e) Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Transition metal complexes with bulky 4,6ditertbutylNaryl(alkyl)oiminobenzoquinonato ligands: Structure, EPR and magnetism. Coord. Chem. Rev. 2009, 253, 291−324. (f) Blackmore, K. J.; Sly, M. B.; Haneline, M. R.; Ziller, J. W.; Heyduk, A. F. Group IV Imino-Semiquinone Complexes Obtained by Oxidative Addition of Halogens. Inorg. Chem. 2008, 47, 10522−10532. (g) Smith, A. L.; Hardcastle, K. I.; Soper, J. D. Redox-Active Ligand-Mediated Oxidative Addition and Reductive Elimination at Square Planar Cobalt(III): Multielectron Reactions for CrossCoupling. J. Am. Chem. Soc. 2010, 132, 14358−14360. (h) Thompson, J. S.; Calabrese, J. C. Synthesis, spectroscopy, and structures of copper(II)3,5ditertbutylosemiquinone complexes. Inorg. Chem. 1985, 24, 3167− 3171. (i) Mukherjee, C.; Weyhermüller, T.; Bothe, E.; Rentschler, E.; Chaudhuri, P. A Tetracopper(II)Tetraradical Cuboidal Core and Its Reactivity as a Functional Model of Phenoxazinone Synthase. Inorg. Chem. 2007, 46, 9895−9905. (j) Bag, S. S.; Ghorai, S.; Jana, S.; Mukherjee, C. Solvatochromic fluorescent cyanophenoxazine: design, synthesis, photophysical properties and fluorescence light−up sensing of ct-DNA. RSC Adv. 2013, 3, 5374−5377. (k) Mukherjee, C.; Weyhermüller, T.; Bothe, E.; Chaudhuri, P. Targeted Oxidase Reactivity with a New RedoxActive Ligand Incorporating N2O2 Donor Atoms. Complexes of Cu(II), Ni(II), Pd(II), Fe(III), and V(V). Inorg. Chem. 2008, 47, 11620−11632. (l) Hindson, K.; de Bruin, B. Cooperative & Redox NonInnocent Ligands in Directing Organometallic Reactivity. Eur. J. Inorg. Chem. 2012, 2012, 340−342. (m) Luca, O. R.; Crabtree, R. H. Redox-active ligands in catalysis. Chem. Soc. Rev. 2013, 42, 1440− 1459. (n) Das, D.; Scherer, T. M.; Das, A.; Mondal, T. K.; Mobin, S. M.; Fiedler, J.; Priego, J. L.; Jimenez-Aparicio, R.; Kaim, W.; Lahiri, G. K. The intricate paramagnetic state of [Os(Q)2(bpy)]+, Q = 4,6ditertbutyloiminobenzoquinone. Dalton Trans. 2012, 41, 11675− 11683. (o) Rajput, A.; Sharma, A. K.; Barman, S. K.; Koley, D.; Steinert, M.; Mukherjee, R. Neutral, Cationic, and Anionic LowSpin Iron(III) Complexes Stabilized by Amidophenolate and Iminobenzosemiquinonate Radical in N,N,O Ligands. Inorg. Chem. 2014, 53, 36−48. (p) Pierpont, C. G.; Lange, C. W. The Chemistry of Transition-Metal Complexes Containing Catechol and Semiquinone Ligands. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: Hoboken, NJ, 1994. (q) Rakshit, R.; Ghorai, S.; Sarmah, A.; Tiwari, A.; Roy, R. K.; Mukherjee, C. Inter−ligand azo (N = N) unit formation and stabilization of a Co(II)−diradical complex via metal−to−ligand dπ−

137.07, 135.75, 132.40, 126.93, 122.68, 122.20, 120.20, 119.56, 114.25, 35.24, 34.58, 31.79, 29.71. ESI-MS(+). Calcd for [C40H52N2O2S2 + H]+: m/z 657.3542. Found: m/z 657.3583. Synthesis of Complex 1, [C40H48N2O3S2V]. VOSO4·5H2O (0.053 g, 0.21 mmol) and Et3N (0.1 mL) were added sequentially to a stirred solution of H4Sar(AP/AP) (0.140 g, 0.21 mmol) in MeOH (5 mL). The solution color immediately changed to deep green. After 1 h of stirring at room temperature (25 °C), the color turned to blue. The solution was further stirred for 4 h. A blackish precipitate was formed. The precipitate was filtered and washed with MeOH (15 mL). Recrystallization of the solid from a Et2O/MeOH (1:1) solvent mixture provided a crystalline solid in 6 h. The obtained crystal was suitable for single-crystal X-ray diffraction measurement. Crystalline solid was also isolated from the filtrate part. Yield: 0.048 g, 32%. FTIR (KBr pellet, cm−1): 3479, 3052, 2952, 2904, 2866, 1583, 1556, 1532, 1456, 1413, 1387, 1361, 1330, 1310, 1258, 1226, 1198, 1175, 1111, 1061, 1027, 996, 940, 911, 763, 751, 640. ESI-MS(+). Calcd for [C40H48N2O3S2V]+: m/z 719.2545. Found: m/z 719.2616. Anal. Calcd for C40H48N2O3S2V·0.25H2O: C, 66.32; H, 6.75; N, 3.87. Found: C, 66.45; H, 6.23; N, 3.78.

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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.7b00789. IR, MS, and NMR spectra of the ligand and complex 1, TDDFT-calculated UV−vis−NIR spectra of the intermediate and complex 1, and coordinates for geometrically optimized structures of possible intermediates and complex 1 (PDF) Accession Codes

CCDC 1537090 contains 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 data_ [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]. ORCID

Chandan Mukherjee: 0000-0002-2771-2468 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by SERB [EMR/2015/002491], India. PS and MKM thank Indian Institute of Technology Guwahati (IITG) for their doctoral fellowship. The Department of Chemistry and CIF, IIT Guwahati are thankfully acknowledged for instrumental facility. The computational facilities from IOCB, Prague, and discussions with Prof. Pavel Hobza are thankfully acknowledged by AS.

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REFERENCES

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An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A

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