Vanadium(V) - ACS Publications - American Chemical Society

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Vanadium(V) Complexes with Substituted 1,5-bis(2hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane Diana Dragancea,† Natalia Talmaci,† Sergiu Shova,§ Ghenadie Novitchi,⊥ Denisa Darvasiová,# Peter Rapta,*,# Martin Breza,# Markus Galanski,‡ Jozef Kožıš́ ek,# Nuno M. R. Martins,∥ Luísa M. D. R. S. Martins,*,∥,∇ Armando J. L. Pombeiro,*,∥ and Vladimir B. Arion*,‡ †

Institute of Chemistry, Academy of Sciences of Moldova, Academiei Str. 3, MD-2028 Chisinau, Moldova Institute of Inorganic Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria § “Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487 Iasi, Romania ⊥ Laboratoire National des Champs Magnetiques Intenses-CNRS, Universite Joseph Fourier, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France # Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology, Radlinského 9, 81237 Bratislava, Slovakia ∥ Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal ∇ Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal ‡

S Supporting Information *

ABSTRACT: Six dinuclear vanadium(V) complexes have been synthesized: NH4[(VO2)2(HLH)] (NH4[1]), NH4[(VO2)2(t‑BuLH)] (NH4[2]), NH4[(VO2)2(ClLH)] (NH4[3]), [(VO2)(VO)(HLH)(CH3O)] (4), [(VO2)(VO)(t‑BuLH)(C2H5O)] (5), and [(VO2)(VO)(ClLH)(CH3O)(CH3OH/H2O)] (6) (where HLH4 = 1,5-bis(2-hydroxybenzaldehyde)carbohydrazone, t‑BuLH4 = 1,5-bis(3,5-di-tert-butyl-2hydroxybenzaldehyde)carbohydrazone, and ClLH4 = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)carbohydrazone). The structures of NH4[1] and 4−6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit −N−N−. In two pockets of NH4[1], two identical VO2+ entities are present, whereas, in those of 4−6, two different VO2+ and VO3+ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO3+ in 4−6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH4[1], NH4[2], and 4−6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.

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INTRODUCTION

alcohols, epoxidation of alkenes and allylic alcohols, oxidative bromination, sulfoxidation and oxidative Strecker reactions,10−12 and the development of a new class of multifunctional materials known as polyoxovanadates13−15 are of particular note. A significant contribution to the understanding

The catalytic activity of vanadium compounds and their role in biological systems (e.g., haloperoxydases1 and vanadium nitrogenase2), as well as their use as therapeutic agents, have led to an increasing interest in the coordination chemistry of this transition metal. The potential pharmacological effects such as action against diabetes mellitus and cancer,3−9 the use of vanadium compounds in catalytic oxidation of alkanes and © XXXX American Chemical Society

Received: April 22, 2016

A

DOI: 10.1021/acs.inorgchem.6b01011 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Ligand Precursors and Complexes Obtained in This Worka

a

Underlined numbers indicate complexes characterized by X-ray diffraction (XRD).

knowledge10,11 that vanadium complexes can act as catalysts or catalyst precursors for the oxidative functionalization of cyclohexane, under mild conditions, prompted us to search for new efficient and selective catalytic V-based systems. Herein, we describe the synthesis, full characterization, solution dynamics, spectroelectrochemistry, DFT calculations, and catalytic activity in the oxidation of cyclohexane of six dinuclear vanadium(V) complexes, namely, NH 4 [(VO 2 ) 2 ( H LH)] (NH 4 [1]), NH 4 [(VO 2 ) 2 ( t‑Bu LH)] (NH4[2]), NH4[(VO2)2(ClLH)] (NH4[3]), [(VO2)(VO)(HLH)(CH3O)] (4), [(VO2)(VO)(t‑BuLH)(C2H5O)] (5), and [(VO 2 )(VO)( Cl LH)(CH 3 O)(CH 3 OH/H 2 O)] (6), where H LH 4 = 1,5-bis(2-hydroxybenzaldehyde)carbohydrazone, t‑Bu LH4 = 1,5-bis(3,5-di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone and Cl LH 4 = 1,5-bis(3,5-dichloro-2hydroxybenzaldehyde)carbohydrazone (Scheme 1). The first type of complex is anionic, with NH4+ as the counterion. Both binding sites accommodate vanadium(V) as the VO2+ entity. The complexes are diamagnetic, with easily assignable resonances in the 1H NMR spectrum. The second type is neutral and consists of two different vanadium coresVO2+ and VO3+accommodated in ONN and ONO pockets, respectively. The half-widths of the 1H resonances in the NMR spectra are relatively broad and the origin of line broadening is rationalized. It should be also noted that complexes NH4[1] and 4−6 studied by X-ray crystallography contain co-crystallized solvent, which is easily lost or replaced by water upon standing in air at room temperature. Therefore, we do not include the type and amount of solvent in their formulas, which can be found in the experimental part.

and further development of vanadium coordination chemistry could provide the design and synthesis of suitable multidentate ligands. Acid hydrazides R−CO−NH−NH2 and their corresponding aroylhydrazones, R−CO−NH−NCH−R′, have received marked attention, because of their versatile coordination properties, due to the presence of several coordination sites and the ability to stabilize vanadium in its highest oxidation state.16 Bis(aryl)hydrazones, having well-defined and separated binding sites, are suitable for the synthesis of complexes with a predefined nuclearity. Recently, some of them, containing two binding moieties (provided by oxalic, malonic, adipic, or terephthalic central fragments) have been used for the synthesis of dinuclear17−19 and polymeric20 vanadium(V) species with high catalytic activity in oxidation of alkanes. Ditopic ligands based on the carbohydrazide fragment linked with pyridine, carboxylate, and oxime functional groups display coordination versatility and lead to various structural motifs.21−29 The polydentate Schiff base ligand 1,5-bis(salicylidene)carbohydrazide (H4L) that resulted from a condensation reaction between 2-hydroxybenzaldehyde and carbohydrazide provides two different binding sites, ONN and ONO, to accommodate metal ions. Thus, dioxidomolybdenum(VI), diorganotin(IV), nickel(II), and copper(II) complexes have been reported.30−33 In all of these compounds, the ligand acts as a trianion, in contrast to its thioanalogue, which also can be stabilized in metal complexes as a tetraanion.34 The production of large-scale commodities from abundant and inexpensive organic substrates in a sustainable way is a matter of great interest and a relevant example is the selective oxidation of cyclohexane.35 Although industrially and economically very important, in view of the significance of the products (cyclohexanol and cyclohexanone) for the manufacturing of adipic acid and caprolactam (precursors to polyamides widely used in several industries), the current industrial process requires high temperatures and leads to very low conversions (3−8%) to ensure a reasonable selectivity (ca. 85%). The

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EXPERIMENTAL SECTION

Materials. All reagents were used as received from Sigma−Aldrich. S y n t h e s i s of L i g a n d Pr e c u r s o r s . T he 1, 5 - b i s (2 hydroxybenzaldehyde)carbohydrazone (HLH4) and the other two ligand precursors (t‑BuLH4 and ClLH4) were prepared by a slight modification of the procedure reported previously.36 B


Article

Inorganic Chemistry

Table 1. Crystal Data, Data Collection Parameters, and Structure Refinement Details for NH4[1] and [4]2·3CH3OH, [5]2· 6C2H5OH, and [6]2·3.6CH3OH·0.75H2O empirical formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dcalc (mg/mm3) μ (mm−1) crystal size (mm3) GOFc R1a(I >2σ(I)) wR2b(all data) largest diff. peak/hole (e Å−3)

NH4[1]·CH3OH

[4]2·3CH3OH

[5]2·6C2H5OH

[6]2·3.6CH3OH·0.75H2O

C16H19N5O8V2 511.24 173(2) triclinic P1̅ 9.1328(7) 10.9497(8) 11.1790(7) 65.847(7) 84.709(6) 74.717(7) 983.82(13) 2 1.726 1.007 0.10 × 0.05 × 0.02 1.022 0.0519 0.0963 0.40/−0.43

C35H40N8O17V4 1048.51 173(2) monoclinic P21/c 12.2830(4) 28.0101(8) 13.3793(4) 90.00 111.917(2) 90.00 4270.4(2) 4 1.631 0.931 0.19 × 0.07 × 0.01 0.807 0.0484 0.0673 0.47/−0.46

C78H132N8O20V4 1705.67 100(2) triclinic P1̅ 9.2721(2) 14.5422(2) 18.1279(3) 66.678(2) 88.415(2) 87.740(2) 2242.65(8) 1 1.263 3.949 0.15 × 0.10 × 0.02 1.096 0.0547 0.1579 1.23/−1.37

C36.3H39.2Cl8N8O19.3V4 1383.72 120(2) triclinic P1̅ 12.4787(15) 14.6106(15) 15.592(2) 104.222(4) 91.701(3) 103.923(4) 2662.7(5) 2 1.726 1.160 0.22 × 0.07 × 0.02 1.023 0.0608 0.15998 1.29/−1.04

R1 = ∑| |F0| − |Fc| |/∑|F0|. bwR2 = {∑[w (F02 − Fc2)2]/∑[w(F02)2 ]}1/2. cGOF = {∑[w(F02 − Fc2)2]/(n − p)}1/2, where n is the number of reflections and p is the total number of parameters refined.

a

C15H6Cl4N4O7V2: 298.90); m/z 612.82 [(VO)(VO2)(ClL)(CH3O)]− (calcd for C32H45N4O7V2: 699.22). [(VO2)(VO)(HLH)(CH3O)] (4). A mixture of 1,5-bis(2-hydroxybenzaldehyde) carbohydrazone (HLH4) (0.037 g (0.125 mmol) and VO(acac)2 (0.066 g, 0.25 mmol) in MeOH (15 mL) was stirred at room temperature for 1 h until a clear brown solution was obtained. This was filtered and placed in a parafilm sealed vial. After 1 week, light-brown rectangular crystals were filtered off, washed with ethanol, and dried in air. Yield: 0.035 g (54%). Anal. Calcd for C16H14N4O7V2 (Mr 476.19): C, 40.36, H, 2.96, N, 11.77. Found (%): C, 40.03, H, 2.90, N, 11.36. ESI-MS in methanol (negative): m/z 474.99 [(VO2)(VO)(HL)(CH3O)]− (calcd for C16H13N4O7V2: 474.97); m/z 460.98 [(VO2)2(HLH)]− (calcd for C15H11N4O7V2: 460.95); m/z 229.95 [(VO2)2(HL)]2− (calcd for C15H10N4O7V2: 229.97). [(VO2)(VO)(t‑BuLH)(C2H5O)]·H2O (5·H2O). A mixture of 1,5-bis(3,5di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone (t‑BuLH4) (0.065 g (0.125 mmol) and VO(acac)2 (0.066 g, 0.25 mmol) in EtOH (20 mL) was stirred at room temperature for 1 h until a brown solution was obtained. This was filtered and placed in a parafilm sealed vial. After 72 h, light-brown rectangular crystals were filtered off, washed with ethanol, and dried in air. Yield: 0.028 g (30.6%). Anal. Calcd for C33H50N4O8V2 (Mr 732.66): C, 54.10, H, 6.88, N, 7.65. Found (%): C, 54.26, H, 7.16, N, 7.25. ESI-MS in methanol (negative): m/z 699.25 [(VO2)(VO)(t‑BuL)(CH3O)]− (calcd for C32H45N4O7V2: 699.22); m/ z 685.22 [(VO2)2(t‑BuL)]2− (calcd for C31H43N4O7V2: 685.20); m/z 342.09 [(VO2)2(t‑BuLH)]− (calcd for C31H42N4O7V2: 342.10). IR (cm−1): 2953, 1606, 1513, 1037, 909, 860, 652. [(VO2)(VO)(ClLH)(CH3O)(H2O)]·1.5H2O (6·1.5H2O). To a solution of 0.132 g VO(acac)2 (0.5 mmol) in methanol (30 mL) 1,5-bis(3,5dichloro-2-hydroxybenzaldehyde)carbohydrazone 0.11 g (0.25 mmol) was added and the mixture was stirred for 5 min until it became clear. After filtering, the solution was left to stand at room temperature for 96 h. The brown precipitate was collected, washed with methanol, and recrystallized from methanol. Yield: 0.145 g (44%). Anal. Calcd for C16H15Cl4N4O9V2 (Mr 651.01): C, 29.52, H, 2.32, N, 8.61. Found (%): C, 29.54, H, 2.17, N, 8.62. IR (cm−1): 1599, 1506, 1433, 1215, 847, 771, 634. 1H NMR (DMSO-d6, δ): 3.18 (s, 3 H, CH3O), 7.70 (s, 4 H, CH arom.), 8.51 (s, 1 H, CHN), 9.58 (s, 1 H, CHN), 13.00 (br. s, 1H, NH). ESI-MS in methanol (negative): m/z 612.83 [(VO2)(VO)(ClL)(CH3O)]− (calcd for C16H9Cl4N4O7V2: 612.82).

Synthesis of Complexes. NH4[(VO2)2(HLH)]·0.5H2O (NH4[1]· 0.5H2O). A mixture of 1,5-bis(2-hydroxybenzaldehyde) carbohydrazone (HLH4) (0.074 g, 0.25 mmol) and ammonium metavanadate (0.059 g, 0.5 mmol) in methanol (20 mL) was stirred at room temperature for 2 h. The brown suspension was filtered and the filtrate was allowed to stand at room temperature for crystallization. The next day, yellow rectangular crystals were filtered off, washed with methanol, and dried in air. Yield: 0.034 g (28%). Anal. Calcd for C15H16N5O7.5V2 (Mr 488.20): C, 36.90, H, 3.30, N, 14.35. Found (%): C, 36.97, H, 2.86, N, 13.98. ESI-MS in methanol (negative): m/z 460.95 [(VO2)2(HLH)]− (calcd for C15H11N4O7V2: 460.95); m/z 229.94 [(VO2)2(HL)]2− (calcd for C15H10N4O7V2: 229.97); m/z 474.98 [(VO)(VO2)(HL)(CH3O)]− (calcd for C16H13N4O7V2: 474.97). NH4[(VO2)2(t‑BuLH)]·2H2O (NH4[2]·2H2O). A mixture of 1,5-bis(3,5di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone (t‑BuLH4) (0.130 g, 0.25 mmol) and ammonium metavanadate (0.059 g, 0.5 mmol) in methanol (20 mL) was stirred at room temperature for 2 h. The brown suspension was filtered and the filtrate was concentrated under reduced pressure to ∼1/2 and allowed to stand at room temperature for crystallization. The solvent was removed, and then the precipitate was washed with cold methanol (2 mL) and dried in air. Yield: 0.06 g (32%). Anal. Calcd for C31H51N5O7V2 (Mr 739.65): C, 50.34, H, 6.95, N, 9.47. Found (%): C, 50.36, H, 7.12, N, 9.19. ESI-MS in methanol (negative): m/z 685.22 [(VO2)2(t‑BuLH)]− (calcd for C31H43N4O7V2: 685.20); m/z 342.04 [(VO2)2(t‑BuLH)]2− (calcd for C31H42N4O7V2: 342.10); m/z 699.24 [(VO)(VO2)(t‑BuL)(CH3O)]− (calcd for C32H45N4O7V2: 699.22). IR (cm−1): 2956, 1607, 1515, 1431, 886, 856, 674. NH4[(VO2)2(ClLH)]·H2O (NH4[3]·H2O). A mixture of 1,5-bis(3,5dichloro-2-hydroxy-benzaldehyde)carbohydrazone (ClLH4) (0.11 g, 0.25 mmol) and ammonium metavanadate (0.098 g, 0.84 mmol) in methanol (40 mL) was stirred at room temperature for 1.5 h. The brown suspension was filtered and the filtrate was allowed to stand at room temperature for crystallization. After 6 days, a yellow-green precipitate was filtered off, washed with methanol, and dried in air. Yield: 0.06 g (37.8%). Anal. Calcd for C15H13Cl4N5O7V2 (Mr 634.99): C, 28.37, H, 2.06, N, 11.03. Found (%): C, 28.13, H, 2.12, 10.70. ESIMS in methanol (negative): m/z 598.79 [(VO2)2(ClLH)]− (calcd for C15H7Cl4N4O7V2: 598.80); m/z 298.89 [(VO2)2(ClL)]2− (calcd for C


Article

Inorganic Chemistry ESI-MS in CD3OD (negative): m/z 615.85 [(VO2)(VO)(ClL)(CD3O)]− (calcd for C16H6D3Cl4N4O7V2: 615.84); ESI-MS in ethanol: m/z 626.84 [(VO2)(VO)(ClL)(C2H5O)]− (calcd for C17H11Cl4N4O7V2: 626.83). Physical Measurements. Infrared spectra were recorded on an FT-IR spectrometer (Bruker, Model Vertex 70) by means of the attenuated total reflection technique. ESI mass spectra were measured on a mass spectrometer (Bruker, Model Esquire 3000), using methanol as a solvent. The m/z values are quoted for the most abundant isotope. Elemental analyses were performed at the Microanalytical Service of the Faculty of Chemistry of the University of Vienna. X-ray Crystallography. Single-crystal X-ray diffraction (XRD) measurements for NH4[1]·CH3OH were carried out with a CCD diffractometer (Oxford Diffraction, Model XCALIBUR E) that was equipped with graphite-monochromated Mo Kα radiation at 173 K. The crystal was placed at 40 mm from the CCD detector. The unit-cell determination and data integration were carried out using the CrysAlis package of Oxford Diffraction.37 Intensity data for [4]2·3CH3OH and [6]2·3.6CH3OH·0.75H2O were measured on a Bruker X8 APEXII CCD system. The data were processed using SAINT software.38 Data collection for [5]2·6C2H5OH was performed on STOE STADI VARI diffractometer, by microsource Cu Kα radiation (λ = 1.54186 Å) at 100(1) K. The data were processed using STOE X-Red32.39 All the structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares techniques on F2 with SHELXL.40 Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed at calculated positions and refined as riding atoms in the subsequent least-squares model refinements. The positional parameters of OH and NH hydrogen atoms were found from difference Fourier syntheses and verified by the geometric parameters of the corresponding hydrogen bonds. Crystal data, data collection parameters, and structure refinement details for all four crystal structures are summarized in Table 1. Charge Density Measurements. The data collection for [4]2· 3CH 3 OH was done on a STOE STADI VARI four-circle diffractometer that was equipped with a PILATUS 300 K detector, using the microsource Ag Kα radiation at 100(1) K. Data reduction was performed by STOE X-Red3239 and an average redundancy of 4.5 gives Rint = 0.0796. The quality of the crystal was not very good. However, since electron density around the heavy atom is a very robust property, the main features also could be determined from these data. Strategy for the multipole refinement was modified in order to eliminate the disadvantage of poor high-order diffraction. Highorder refinement gives the correct position of all non-hydrogen atoms and consecutive kappa refinement, restrictive multipole refinement (dipole, quadrupole, octapole, and hexadecapole for vanadium and oxygen atoms) and unrestrictive multipole refinement was done with diffractions up to 0.7 Å−1 sin(θ/λ). Distances to hydrogen atoms were put to the neutron values. High-order refinement with the entire dataset with the monopole population and kappa value from the unrestrictive multipole refinement was used to get better positions of the non-hydrogen atoms. Multipole refinement procedure was then repeated. Spectroelectrochemistry. The solutions were prepared in ethanol (max. 6% H2O, MikroChem, Slovakia) and dimethyl sulfoxide (SeccoSolv, max. 0.025% H2O, Merck). The septum-closed bottles were stored in desiccators. The analytical grade ferrocene (Fc) and lithium perchlorate (LiClO4) purchased from Sigma−Aldrich were used as received. Tetrabutylammonium hexafluorophosphate (TBAPF6) of puriss quality (Fluka), dried under reduced pressure at 70 °C for 24 h, was applied as the supporting electrolyte in dimethyl sulfoxide (DMSO). Cyclic voltammetric experiments were performed under argon atmosphere using a three-electrode arrangement with a platinum wire or glassy carbon rod as the working electrodes, a platinum wire as the counter electrode, and a Ag wire pseudo-reference electrode. The cyclic voltammetric studies were performed in a homemade miniature standard electrochemical cell (for 2 mL of solution) using a miniature platinum wire working electrode (platinum wire sealed in a glass

capillary). Ferrocene served as the internal potential standard in DMSO and ethanol and all potentials are referred vs ferrocenium/ ferrocene (Fc+/Fc) redox couple. The concentration of investigated samples in DMSO and ethanol was 0.5 mM both in cyclic voltammetry and spectroelectrochemical experiments. As supporting electrolytes, 0.2 M n-Bu4N[PF6] in DMSO and 0.2 M LiClO4 in ethanol were used. A Heka PG310USB (Lambrecht, Germany) potentiostat with a PotMaster 2.73 software package served for the potential control in voltammetric and spectroelectrochemical studies. EPR measurements were performed on X-band Bruker EMX spectrometer. In situ ultraviolet−visible−near-infrared (UV-vis-NIR) measurements were performed on a spectrometer (Avantes, Model AvaSpec-2048x14-USB2, with a CCD detector, or Model AvaSpecNIR256−2.2, with an InGaAs detector. Halogen and deuterium lamps were used as light sources (Avantes, Model AvaLight-DH-S-BAL). The in situ spectroelectrochemical EPR/UV-vis-NIR cyclovoltammetric experiments were carried out under an argon atmosphere in a special flat spectroelectrochemical cell, suitable for the optical transmission EPR resonator (Model ER 4104 OR-C 9609) of the EMX EPR spectrometer (Bruker, Germany) with 100 kHz field modulation. The working electrode was a laminated platinum mesh with a small hole in the foil coincident with the light beam, which limited the active surface area of the electrode. A platinum wire counter electrode and a silver wire pseudo-reference electrode were used. The optical EPR resonator cavity was connected to the diode-array UV-vis-NIR spectrometer by optical fibers. UV-vis-NIR spectra were processed using the AvaSoft 7.7 software package. NMR Measurements. 1H, 13C, and 51V NMR spectra were recorded on Bruker Avance III spectrometers at 500.32 or 500.10 (1H) MHz and 125.82 or 125.76 (13C) MHz, respectively. The 1H and 13C chemical shifts were referred to the residual signals from the solvent and the 51V chemical shift to external VOCl3 (0 ppm) as a reference. Deuterated solvents CD3OD, and DMSO-d6 were obtained from Euriso-top, while dry DMSO-d6 from Sigma−Aldrich. The integrals of NMR signals were obtained by fitting Lorentzian/Gaussian functions to the experimental spectra using the MestReNova 9.0.1 program. Theoretical Calculations. The geometries of the complexes [(VO2)2HLH]− ([1]−) and [(VO)(VO2)HLH(OCH3)] (4), as well as those of their one- and two-electron reduced species in the singlet, doublet, and/or triplet spin states were optimized at the B3LYP41,42 level of theory under vacuum (starting from experimental XRD structures) and subsequently reoptimized in a DMSO environment without any symmetry restrictions using the Gaussian09 program package.43 Solvent effects were accounted within Self Consistent Reaction Field (SCRF) treatment using Integral Equation Formalism Polarizable Continuum Model (IEF PCM).44 Singlet spin states of the compounds under study have been treated using an unrestricted formalism (“broken symmetry” treatment). Standard 6-311G* basis sets45 were used for all atoms. The stability of the obtained structures has been tested by vibrational analysis (no imaginary vibrations). Atomic charges of relevant atoms and d electron populations of vanadium atoms (d×) were evaluated using natural bond orbital (NBO) analysis.46 Relative energies of various charge and spin states of the same complex have been corrected using restricted open-shell single-point calculations (replacing the electron energy in unrestricted energy data),47 except “broken symmetry” (BS) singlet state, where the energy difference between singlet (ES) and triplet (ET) states is evaluated as

ES − E T =

E BS − EuT 1 − 0.5⟨S2⟩BS

(1)

where EuT is an open-shell energy of the triplet state, EBS the energy value of the BS singlet state, and ⟨S2⟩BS the spin expectation value of the BS singlet state.48 Based on the optimized B3LYP geometries, the vertical transition energies and corresponding oscillator strengths f for electronic absorption spectra were computed by the TD-DFT method (100 excited states). The Molekel package49 has been used for the visualization of molecular orbitals and spin densities. D


Article

Inorganic Chemistry

Figure 1. View of the asymmetric part in the crystal structure of NH4[1]·CH3OH with thermal ellipsoids drawn at the 50% probability level.

Figure 2. Tetranuclear [(VO2)2(HLH)]22− cluster in the crystal structure of NH4[1]·CH3OH. Symmetry code: (i) 2 − x, 1 − y, 1 − z. ming mode, using a SGE BPX5 column (30 m × 0.25 mm × 0.25 μm). Solution samples were analyzed by GC after addition of nitromethane (as a standard compound). Subsequently, an excess of solid triphenylphosphine was added (to reduce the cyclohexyl hydroperoxide (primary product) to the corresponding alcohol, and hydrogen peroxide to water) and the mixture was analyzed again to estimate the amount of cyclohexyl hydroperoxide, following a method developed by Shul’pin.35b,50 For precise determination of the product concentrations, only data obtained after the reduction of the reaction sample with triphenylphosphine were typically used, taking into account that the original reaction mixture contained cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Reaction products were identified by comparison of their retention times with known reference compounds, and by comparing their mass spectra to fragmentation patterns obtained from the NIST spectral library stored in the computer software of the mass spectrometer. Blank experiments, in the absence of any catalyst, were performed under the studied reaction conditions and no significant conversion was observed.

Catalytic Studies. The catalytic oxidations of cyclohexane were carried out in sealed cylindrical Pyrex tubes (5 mL capacity with an internal diameter of 10 mm), under focused microwave irradiation (MW) in an Anton Paar Monowave 300 reactor fitted with a rotational system and an IR temperature detector. Typical reaction conditions are as follows: 1−10 μmol of the catalyst was added to 2.50 mmol of cyclohexane, whereafter 5.00 mmol of 70% aqueous tert-butyl hydroperoxide (TBHP) were introduced in the tube. This was then placed in the microwave reactor and the system was stirred (800 rpm) and irradiated (7 W) for 0.5−3 h at 60 or 100 °C. After the reaction, the mixture was allowed to cool to room temperature. For the assays in the presence of a radical trap, NHPh2, in a stoichiometric amount, relative to the oxidant, was added to the reaction mixture. For reactions performed under conventional heating, round-bottomed flasks (25 mL) that were equipped with reflux condensers in conventional oil baths, in air, were used. The reagents (see above) were introduced in the flask and vigorously stirred at the desired temperature (60−100 °C) during the desired reaction time (until 24 h). Gas chromatography(GC) measurements were carried out using a FISONS Instruments GC 8000 series gas chromatograph with a FID detector and a capillary column (DB-WAX, column length = 30 m, internal diameter = 0.32 mm) and the Jasco-Borwin v.1.50 software. The temperature of injection was 240 °C. The initial temperature was maintained at 100 °C for 1 min, then increased at a rate of 10 °C/min to 180 °C and held at this temperature for 1 min. Helium was used as the carrier gas. Gas chromatography−mass spectroscopy (GC-MS) analyses were performed using a PerkinElmer Clarus 600 C instrument (with helium as the carrier gas). The ionization voltage was 70 eV. Gas chromatography (GC) was performed in the temperature-program-

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RESULTS AND DISCUSSION Complexes NH4[1], NH4[2], and NH4[3] were prepared via the reaction of NH4VO3 with HLH4, t‑BuLH4, and ClLH4, respectively, in a 2:1 molar ratio in methanol. The reaction of VO(acac)2 with HLH4, t‑BuLH4, and ClLH4 in 2:1 molar ratio in methanol or ethanol afforded complexes 4, 5, and 6, respectively, in moderate yields. The ESI mass spectra of NH4[1]−NH4[3] in methanol measured in the negative ion E


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Figure 3. Molecular structure of [(VO2)(VO)(HLH)(CH3O)]2 ([4]2). Thermal ellipsoids are drawn at the 50% probability level. Co-crystallized solvent molecules have been omitted for the sake of clarity.

Figure 4. ORTEP view of the dimeric associate of dinuclear vanadium(V) complexes 5 in [5]2·6C2H5OH. Thermal ellipsoids are drawn at the 30% probability level.

mode are very similar, showing peaks with m/z 460.95, 229.94, and 474.98 (NH4[1]), 685.22, 342.04, and 699.24 (NH4[2]), and 598.79, 298.89, and 612.82 (NH4[3]), which were assigned to [(VO2)2(RLH)]−, [(VO2)2(RL)]2−, and [(VO2)(VO)(RL)(CH3O)]− with R = H, t-Bu, and Cl, respectively, fitting the theoretical isotopic distribution patterns well. Methanolic solutions of 4 and 5 exhibited signals with m/z 474.99, 460.98, and 229.95 (4) and 699.25, 685.22, and 342.04 (5), which are attributable to [(VO 2 )(VO)( R L)(CH 3 O)] − , [(VO2)2(RLH)]−, and [(VO2)2(RL)]2−. The mass spectrum of 6 in CH3OH, CD3OD, and C2H5OH showed peaks at 612.83, 615.85, and 626.84 attributed to [(VO2)(VO)(ClL)(CH3O)]−, [(VO 2 )(VO)( C l L)(CD 3 O)] − and [(VO 2 )(VO)( C l L)(C2H5O)]−, respectively, providing evidence for a lability of alkoxido ligand in 4−6. Crystal Structures. NH4[(VO2)2(HLH)]·CH3OH (NH4[1]· CH3OH). The result of XRD study of 1 is shown in Figure 1. The asymmetric part of the crystal structure consists of a dinuclear complex anion, a NH4+ cation, and one molecule of CH3OH in 1:1:1 ratio. In the complex anion, the triply deprotonated hexadentate ligand HLH3− accommodates two

dioxidovanadium(V) ions in ONN and ONO binding sites with a V···V separation of 4.994(2) Å. The V1 and V2 atoms are five-coordinate and use cis-oxido ligands to complete their first coordination spheres. The bond lengths within the carbohydrazide fragment (Table S1 in the Supporting Information) indicate that the ligand adopts the enolate tautomeric form. The bond lengths of vanadium to donor atoms are in the range found for other reported complexes containing the dioxidovanadium(V) entity.36,51−54 Two dinuclear complex anions are associated in a tetranuclear centrosymmetric cluster via short contacts of the type V2···O3i(2 − x, 1 − y, 1 − z) at 2.744(2) Å, as depicted in Figure 2. As a result, the coordination geometry of V2 atom can be described as distorted octahedral, while the V1 atom exhibits merely a distorted square-pyramidal coordination geometry (τ = 0.43).55 In the crystal, the dianionic tetranuclear cluster, NH4+ cations, and co-crystallized molecules of CH3OH are involved in N−H···O and O−H···O intermolecular hydrogen bonding, generating a three-dimensional (3D) supramolecular architecture, as shown in Figure S1 in the Supporting Information. F


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Figure 5. ORTEP view of [6]2.

[(VO 2)(VO)( HLH)(CH 3 O)] 2 ·3CH 3 OH ([4] 2 ·3CH 3 OH) and [(VO2)(VO)(t-BuLH)(C2H5O)]2·6C2H5OH ([5]2·6C2H5OH). The single-crystal XRD study of [4]2·3CH3OH revealed the presence of a tetranuclear molecular associate in the asymmetric part of the unit cell, which comprises two VO3+ and two VO2+ ions. A view of tetranuclear cluster with two pseudo-inversion symmetry-related molecules [(VO2)2(VO)2(HLH)2(CH3O)2] is shown in Figure 3, while the bond distances are listed in Table S1. As for [1]−, the trianionic ligand HLH3−, which forms two five-membered chelate rings and two six-membered chelate rings, adopts an almost-planar configuration with the dihedral angle between two phenyl rings of 1.620° and 8.914° for the ligand coordinated to V1 and V2 and to V3 and V4 atoms, respectively. Note that the bond lengths V2−O8 (2.348(3) Å) and V4−O2 (2.307(3) Å) in [4]2 are significantly shorter, compared to V2···O3i contact of 2.744(2) Å found in [1]−. The coordination geometry of vanadium atoms in [4]2 is similar to that found in [1]−. Indeed, the τ values of 0.39 and 0.44 for V1 and V3 atoms, respectively, indicate a distorted (O3N2) squarepyramidal coordination geometry for these atoms. The O5N environment of V2 and V4 atoms can be characterized as a strongly distorted octahedral, with the displacement of V2 at 0.413(2) Å and V4 at 0.397(2) Å toward their oxido ligands O4 and O11, respectively. The bond lengths for this coordination site are in agreement with those reported recently for the VO2+ unit in a closely related species.36 In addition, the analysis of V−O and V−N bond distances, along with the presence of one methoxido ligand at V2 and one at V4, indicate that all four crystallographically independent V ions in [4]2 are in the 5+ oxidation state and the charge balance corresponds to the neutral species [(VO2)(VO)(HLH)(CH3O)]2. The interaction of the tetranuclear cluster with co-crystallized solvent molecules (CH3OH and H2O) in the crystal occurs via numerous N−H···O and O−H···O hydrogen bonds, which results into the formation of two-dimensional (2D) supramolecular layers arranged parallel to the (011) plane. The structure of one such layer is shown in Figure S2 in the Supporting Information.

A very similar tetranuclear molecule (Figure 4, Table S1), consisting of two pairs of VO3+ cations and two VO2+ cations joined via two centrosymmetry-related linkages V1−O3−V2 (V1−O3 = 1.656(2) Å, O3−V2i(−x, − y, 1 − z)) was found in the crystal structure of 5. Two pairs of vanadium(V) atoms, one in a distorted square-pyramidal environment (τ = 0.43 for V1) and the other in a distorted-octahedral environment are bridged by the N−N diazine unit of the hexadentate triply deprotonated (t‑BuHL3−) ligands with a V1···V2 separation of 5.023(2) Å. The coordination environment of the VO3+ ions is completed by an ethoxido ligand to form a molecular species [(VO2)(VO)(t‑BuLH)(C2H5O)]2. [(VO2)(VO)(ClLH)(CH3O)(CH3OH/H2O)] 3.6CH3OH·0.75H2O ([6]2·3.6CH3OH·0.75H2O). The result of XRD study of [6]2· 3.6CH3OH·0.75H2O is shown in Figure 5. The asymmetric part includes a similar tetranuclear unit; however, in contrast to structures NH4[1] and [4]2, this molecular aggregate is consolidated by a single V1−O8−V3 linkage. The bond distances in the latter are 1.670(4) Å for V2−O8 and 2.239(4) Å for V3−O8 (Table S1). The V2 and V3 atoms have a distorted-octahedral coordination formed by NO5 and N2O4 donor atom sets, respectively. The environment of the other two vanadium atoms is different: V1 and V4 adopt a distorted square-pyramidal coordination with τ values of 0.46 and 0.40, respectively. The coordination polyhedron of V2 is completed by one molecule of CH3OH trans to the oxido ligand O4, and one methoxido ligand trans to the N1 atom, while that of V4 is completed by a methoxido ligand trans to O14. Note that the position of CH3OH ligand is shared with a coordinated water molecule in a 3:1 ratio, and their hydrogen atoms are involved in the formation of hydrogen bonding to oxido ligand O13 (Figure 5). Another special feature of [6]2· 3.6CH3OH·0.75H2O is the formation of centrosymmetric octanuclear molecular cluster through the second V3−O12− V4i(−x, − y, 1 − z) linkage with V3−O12 (bond distance = 1.637(4) Å) and V4−O12 (bond distance = 2.231(4) Å). The structure of the octanuclear associate is shown in Figure S3 in the Supporting Information. In addition, the octanuclear cluster is stabilized by intramolecular π−π stacking interactions with centroid-to-centroid distances in the range of 3.811−3.884 Å. G


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The spectra of NH4[2] are similar (see Figure 6). The presence of two inequivalent aldimine protons (−CNH) with similar chemical shifts (9.76 and 8.56 ppm), as well as the presence of the triple split signal of NH4+, indicate similar composition and mode of binding of t‑BuLH3− to vanadium(V) in [2]−. The NMR spectra of 4 and 5 in CD3OD are shown in Figures S9b and S10b in the Supporting Information. As for NH4[1] and NH4[2], two inequivalent aldimine protons are present. The aromatic regions appear slightly different, because of the use of a solvent with another polarity. Comparative analysis of spectra of 4 and 5 with those of NH4[1] and NH4[2] suggests the same dinuclear structure of the complexes in solutions. Dissociation of the tetranuclear cluster of [4]2 in solution is also corroborated by mass spectra showing the presence of the peak with m/z 460.95 attributed to [(VO2)2(HLH)]− for NH4[1] and m/z 474.99 assigned to [(VO2)(VO)(HL)(CH3O)]− for 4. The proton NH signal is not seen in the 1H NMR spectra of 4 and 5, because of the H/ D exchange in methanol-d4 and residual molecules of water present in the solvent. The absence of resonances of NH4+ and the presence of signals typical for C2H5OH in 5 confirm the similarity of the structure and the composition with those of compound 4 (Figure S10 in the Supporting Information). Assignment of 1H and 13C resonances for NH4[2], 4 and 5 has been performed by the same 2D NMR techniques as in the case of NH4[1] (see the Supporting Information). 51 V NMR chemical shifts of vanadium(V) complexes with redox inactive ligands, also referred to as “innocent”’, are observed in the range from −300 ppm to −700 ppm, both in solution and in solid state, when using VOCl3 (δ = 0.0 ppm) as a reference.56−59 This is because a linear correlation between isotropic chemical shifts in the solid state and in solution was observed.60 The 51V chemical shifts can provide information on (i) the number of nonequivalent vanadium sites, (ii) coordination number, (iii) the identity of donor atoms bound to vanadium, (iv) distortion of the coordination sphere, and (v) the association of vanadium−oxygen polyhedra.56 The 51V NMR spectra of NH4[2] and 5 in CD3OD show predominantly or only two signals (see Figure 7, as well as Table S2 in the Supporting Information), in agreement with coordination of two vanadium(V) ions in two distinct pockets of the

Further interactions between octanuclear units through the weaker intermolecular π−π stacking interactions at 3.977 and 4.168 Å determine the formation of infinite supramolecular ribbons running along the crystallographic c-axis, as shown in Figure S4 in the Supporting Information. NMR Spectroscopy. Structure and solution dynamics of NH4[1], NH4[2], 4, and 5 were studied using NMR spectroscopy. The 1H NMR spectrum of NH4[1] is shown in Figure 6. It displays signals in the aromatic region (7.7−6.8

Figure 6. 1H NMR (500 MHz) spectra of (a) NH4[1] and (b) NH4[2] in DMSO-d6 at room temperature.

ppm) with well-defined splitting and a broad line between 7.3 ppm and 6.9 ppm. In the low-field region, there are three signals with an equal population at 12.63, 9.75, and 8.57 ppm. In accord with the XRD analysis, the broad signal at 12.63 ppm can be assigned to the hydrogen at the hydrazinic nitrogen atom, which is probably involved in proton exchange with residual molecules of water, methanol, and ammonium protons (NH4+). The other two signals at 9.75 and 8.57 ppm are aldimine protons (C9H, C7H), which are inequivalent, as a consequence of triple deprotonation of the Schiff base ligand and the formation of two different binding pockets with ONN and ONO donor sets (see Figure 6). The absence of cross signals in 1H,13C HMBC 2D NMR spectra (Figure S6 in the Supporting Information) between the hydrazinic proton and aldimine carbon makes the assignment of resonances difficult; therefore, arguments based on XRD data were used. In accordance with the latter, the imine group C7H should be more deshielded, in comparison with C9H, because of the presence of a deprotonated hydrazinic nitrogen (N2) and the two oxygens (O6 and O7) of the VO2+ ion (Figure 6). The two sets of aromatic signals with the ABCD spin system are shown in Figure S7 in the Supporting Information. The assignments have been made using the 2D NMR tools (1H, 1H COSY (Figure S8 in the Supporting Information) and 1H,13C HMBC (Figure S6)), as well as by simulation of NMR spectra (see details in the Supporting Information). The broad signal at δ = 7.3−6.9 ppm was attributed to the NH4+ ion split into a triplet due to the presence of coupling between the proton and 14N (nuclear spin I = 1, 1J14N1H = 50.4 Hz). The signal is broad because of the proton exchange reactions with methanol and residual water present in solution.

Figure 7. 51V NMR spectra of NH4[2] and 5. H


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Inorganic Chemistry carbohydrazone ligand. We applied a referencing scale that was proposed by Rehder et al.61 to estimate the 51V shielding in vanadium(V) complexes with coordination numbers from 4 to 6, which is based on a correlation of 51V chemical shifts with the sum of electronegativities of the coordinated atoms, Σχ to our complexes. The sum of electronegativities for two dioxidovanadium(V) atoms in the two different binding pockets ONN and ONO in NH4[2] are 17.05 and 17.63, according to the electronegativity data tabulated by Zhang.62 Applying these values to the referencing scale would predict δ values in the upfield region between −400 ppm and approximately −480 ppm. These values are notably lower, compared to the experimentally observed chemical shifts. Marked deviations from the predictions based on the electronegativity data were reported for vanadium(V) complexes with non-innocent ligands.59,60 The same complexes (NH4[2] and 5) in DMSO-d6 show two and three broad resonances, respectively, which probably indicate their involvement in exchange/substitution processes. The alkoxido ligand in 5 can be substituted by solvent molecules (water or DMSO). It has been found that the alkoxido group in related vanadium complexes [VOL(OR)] (where OR = OMe, OEt, or OPr, and H 2 L = 2hydroxybenzaldehyde 4-phenylthiosemicarbazone) can be easily and reversibly substituted when the complex is dissolved in an appropriate alcohol.63 In order to clarify the origin of a significant increase in the line width and appearance of supplementary lines in 51V NMR, a detailed NMR investigation of 5 was performed. The NMR spectrum of freshly prepared solution of 5 in DMSO is displayed in Figure 8.

Figure 9. Time dependence of 1H NMR spectra (selected regions) of 5 (500 MHz, concentration (C) = 3.5 mM in DMSO-d6 at room temperature).

decrease with initial resonances of 5 and the signals of outer sphere ethanol increase with the population at the same rate. All this indicates substitution of the ethoxido ligand by the solvent, and this process is slow on the time scale of NMR spectroscopy. The chemical shift of the HDO (the residual signal in DMSO-d6) is also modified, suggesting that water molecules are also involved in this process. The equation proposed for the hydrolysis reaction is as follows: [(VO2 )(VO)(t ‐BuLH)(C2H5O)] + H 2O form A

⇌ [(VO2 )(VO)(t ‐BuLH)(OH)] + C2H5OH form B

(2)

Additional information about this substitution reaction could be extracted from the analysis of the time-dependent 1H NMR spectra. The time evolution of population rate for NH and CHN proton signals of species A and B (eq 2) and CH3 protons of free EtOH and ethoxido ligand was extracted by fitting the NMR spectra. The CH′N signal, which is attributed to the moiety containing the VO2+ ion, is not sensitive to substitution and is not shifted over time. Therefore, this signal was chosen as reference for estimation of population rates. The signals of species A decrease proportionally with increasing the signals of species B (Figure 9) (see also Figure S13 in the Supporting Information). This evolution of population rates is fully synchronized with the evolution of signals belonging to ethoxido ligand and outersphere ethanol. The concentration of forms A and B, as a function of time, is shown in Figure 10. Close inspection of NMR spectra reveals that the signal of the CH3 group of ethoxido ligand has evident chemical shift dependence (Figure S12) over time, which is possible under protonation, according to eq 3:

Figure 8. Selected regions of 1H NMR spectra of 5 in DMSO-d6.

Two sets of signals can be seen for CHN and t-Bu protons as expected from the molecular structure. The signals for coordinated ethoxido group and outer sphere ethanol can be also detected (see Figure 8, as well as Figure S11a in the Supporting Information). Coordination of the ethoxido ligand is evidenced by a specific splitting of the O−CH2− group in 1H NMR, showing the inequivalence of the CH2 protons (Figure S11b). Over time, the spectra of 5 are modified and additional signals appear (Figure 9). This indicates the presence of two different vanadium(V)-containing species in solution. The new signals increase over time, while the initial signals decrease, suggesting the transformation of 5 into another species. Note that the signals of coordinated ethoxido ligand simultaneously

[(VO2 )(VO)(t ‐BuLH)(C2H5O−)] + H 2O form A

⇌ [(VO2 )(VO)(t ‐BuLH)(C2H5OH)]+ + OH− form A ′

(3)

The hydroxide ion formed is consumed further in the bimolecular reaction with A (eq 4): I


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in situ UV-vis-NIR, as well as EPR spectroelectrochemistry, in DMSO/n-Bu4N[PF6] (for NH4[1] and NH4[2]) and ethanol/ LiClO4 (for 5). The cyclic voltammograms (CV) of NH4[1] and NH4[2] measured at a scan rate of 100 mV s−1 in DMSO/ n-Bu4N[PF6] clearly indicate one reduction peak with the halfpeak potential Ep/2 at −1.46 V for NH4[1] and at −1.48 V for NH4[2]. Two oxidations of the species formed upon reduction, namely, one strongly shifted broad peak at −0.9 V and another small at 0.25 V (all vs Fc/Fc+) were observed during the reverse scan for both complexes (see Figure 11).

Figure 10. Evolution of relative population rates of CHN signals for A and B and their derivatives (solid lines), as a function of reaction time in DMSO-d6 for 5 (1H, 500 MHz, c = 3.5 mM in DMSO-d6) at room temperature (also see Figure S13).

[(VO2 )(VO)(t ‐BuLH)(C2H5O)] + OH− ⇌ [(VO2 )(VO)(t ‐BuLH)(OH)] + C2H5O−

(4)

Based on NMR investigation the probable pathway of hydrolysis of 5 is proposed as depicted in Scheme 2. Scheme 2. Proposed Substitution Pathway of EtO− in 5

Figure 11. Cyclic voltammograms of (a) NH4[1] and (b) NH4[2] in DMSO/n-Bu4N[PF6] at Pt working electrode at a scan rate of 100 mV s−1 (black lines represent the first scan, red lines represent the second scan).

There were no significant changes in the shape and intensity of the reduction peak over multiple redox cycles. The large peak-to-peak separation at the scan rate of 100 mV s−1 at platinum working electrode indicates either slow electron transfer processes or molecular structural changes upon reduction of NH4[1] and NH4[2]. In situ UV-vis-NIR spectroelectrochemical experiments revealed that, by decreasing the scan rate to 10 mV s−1, the shape of the cyclic voltammogram becomes more reversible with the ratio of the cathodic to anodic current in the corresponding cyclic voltammogram being closer to 1, as representatively shown for NH4[2] in Figure 12a. The complementary UV-vis spectra generated in the second and third voltammetric scans are shown in Figure 12b. However, the peak-to-peak separation remains large, with the potential difference between the first reduction peak and the corresponding oxidation peak ΔE = 0.79 V. The large peak-topeak separation at low scan rates indicates significant structural differences between the initial and reduced states of NH4[2]. As shown in Figure 12b, by repeating the cyclic scan several times,

The time-dependent experiments using the anhydrous DMSO-d6 were also performed. Since water is present in the complex (along with some residual water in the solvent), this was consumed in the hydrolysis reaction and an equilibrium was reached over time. Further decreases in D2O shifts the equilibrium far to the right and the substitution is complete. The addition of D2O from the very beginning of the reaction also indicates that the rate of hydrolysis is strongly dependent on the concentration of water in the reaction mixture, suggesting a bimolecular mechanism of hydrolysis (SN2). Elucidation of the mechanism of substitution process in the series of dinuclear vanadium complexes by investigation of the temperature and pH dependence of 51V, 17O, 1H NMR spectra is underway in our laboratories and will be reported separately. Electrochemistry and Spectroelectrochemistry. Vanadium complexes with organic ligands show interesting redox behavior, because of the variety of easily accessible oxidation states both on metal and ligand(s).64 The redox behavior of NH4[1], NH4[2], and 5 was studied by cyclic voltammetry and J


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Figure 13. UV-vis spectra detected simultaneously during (a) the in situ reduction of NH4[2] in the region of the first cathodic peak (from −1.2 V to −2.0 V vs Fc/Fc+) and (b) during the back reoxidation in the region from −1.0 V to +0.4 V vs Fc/Fc+.

Figure 12. In situ UV-vis spectroelectrochemistry for NH4[2] in 0.2 M n-BuN4[PF6] in DMSO (scan rate of 10 mV s−1, Pt-microstructured honeycomb working electrode): (a) CVs with color-highlighted potential region, where spectra were taken during the second (solid lines) and immediately afterward, the third (dashed lines) voltammetric scan; (b) complementary UV-vis spectra.

The optical spectra of NH4[2] in 0.2 M n-BuN4[PF6]/DMSO measured before (green line in Figure S15 in the Supporting Information) and after bulk electrolysis at −1.8 V vs Fc/Fc+ (blue line in Figure S15) correspond well to the spectroelectrochemical results shown in Figure 13a. It was proved that the electrolyzed solution of NH4[2] at the first cathodic peak is EPR silent (see blue line in Figure S16 in the Supporting Information). The CV of 5 at a scan rate of 100 mV s−1 in EtOH/LiClO4 is shown in Figure S17 in the Supporting Information. Although broadened reduction peaks and two strongly shifted oxidation peaks were observed, the chemically reversible redox processes of 5 in EtOH/LiClO4 were confirmed by UV-vis spectroelectrochemistry (Figure S18 in the Supporting Information), as similarly observed for NH4[2]. A complete recovery of the initial optical bands observed during the voltammetric reverse scan at strongly anodically shifted potentials at the low scan rates confirms chemical reversibility of these redox processes (Figure S19 in the Supporting Information). Note that an EPR signal characteristic of vanadium(IV) (S = 1/2, I = 7/2) after bulk electrolysis of 5 in 0.2 M LiClO4/EtOH at −1.6 V vs Fc/ Fc+ was obtained (see Figure 14, as well as the purple line in Figure S16). Theoretical Calculations. In order to further explore the electronic structure of the vanadium species NH4[2] and 5 present in solution and those generated experimentally in spectroelectrochemical studies, time-dependent density functional theory (TD-DFT) transitions were computed for the smaller model compounds NH4[1] and 4, respectively (hydrogen atoms instead of the tert-butyl groups) as shown in Figure 15 for vacuum and in Figure S20 in the Supporting

reversible changes in the intensity of the characteristic UV-vis spectra of NH4[2] were observed. However, note that the reoxidation of reduced NH4[2] back to the initial compound occurs at strongly anodically shifted potentials. For clarity, the UV-vis spectroelectrochemical responses obtained from the first cycle for selected potential regions are shown in Figure 13 in a 2D plot. Here, in the region of the first reduction peak (from −1.2 V to −2.0 V vs Fc/Fc+), a new optical band at 440 nm and a decrease of the intensity of the band of the initial complex NH4[2] at 305 nm via an isosbestic point at 340 nm were observed (Figure 13a). In the reverse scan, the products that formed upon reduction are reoxidized when increasing the potential up to +0.4 V vs Fc/Fc+. Full recovery of the initial optical bands during the voltammetric reverse scan at strongly anodically shifted potentials confirms the chemical reversibility of the redox process (Figure 13b). In the simultaneous EPR spectroelectrochemical measurements for NH4[2] in 0.2 M n-BuN4[PF6] in DMSO, no EPR signal was detected during the reduction at the first cathodic peak, even if a large platinum-mesh working electrode was used. This indicates a two-electron transfer, leading to an EPR-silent reduction product. Controlled potential electrolysis performed at the first reduction peak confirmed the consumption of two electrons per molecule (n = 1.8). Two-electron reduction was also proved by comparison of the oxidation peak of ferrocene (Fc) and the reduction peak of NH4[2] taken in equivalent amounts as shown in Figure S14 in the Supporting Information. K


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[4]− (red lines in Figure 15b) and for the diamagnetic twoelectron reduced states 1[1]3− and 1[4]2− (blue lines in Figure 15). Energy data of the optimized structures both for vacuum (Table S3 in the Supporting Information) and DMSO (Table S4 in the Supporting Information) indicate that the initial singlet vanadium complex 1[1]− is more stable than the triplet one by ca. 2 eV. As expected, DMSO solvent significantly stabilizes all charged species formed upon reduction. The first computed optical transition for 1[1]− with significant oscillator strength (f) found at 446 nm ( f = 0.06) under vacuum and at 409 nm ( f = 0.25) in DMSO corresponds to the HOMO → LUMO transition (Figures 15a and Figure S20a) and confirms the predominantly ligand-to-ligand transition with increased vanadium LUMO contribution in DMSO (see Figures S21a and S21b and Figures S22a and S22b in the Supporting Information). We suppose that two electrons are added sequentially rather than concertedly upon reduction of samples NH4[1] and NH4[2] and the two sequential one-electron-transfer reactions produce a single voltammetric peak. 65 Therefore, the theoretical investigation of NH4[1] reduction is based on the analysis of two separate one-electron processes. The shape of LUMO for 1[1]− under vacuum indicates that the prevailing first reduction locus is on the ligand framework (Figure S21b); however, in DMSO, the contribution from both vanadium atoms is more evident (Figure S22b). Interestingly, the shape of the α-HOMO computed for the doublet state 2[1]2− (formed upon one-electron reduction), which actually represents the singly occupied molecular orbital (SOMO), is almost the same both under vacuum (Figure S21c) and DMSO (Figure S22c) and is very similar to the HOMO of 1[1]− under vacuum only. The calculated spin density of this paramagnetic species 2[1]2− is located predominantly on the ligand (Figures S21d and S22d). This confirms that the first reduction peak observed for the complexes NH4[1] and NH4[2] can be predominantly attributed to a ligand-based reduction, as already indicated in the electrochemical and spectroelectrochemical studies discussed above. For the two-electron reduced vanadium complex 1[1]3−, the computed transition under vacuum with the highest oscillator strength at 410 nm (see black arrow in Figure 15a) corresponds to the α-HOMO−1 → α-LUMO ligand-to-ligand transition from the ligand framework accommodating one vanadium atom to the ligand framework accommodating the second vanadium atom (Figures S21e and S21f), as observed analogously for the initial state 1[1]−. This intense transition computed corresponds well to the experimentally observed new intense absorption band at λmax = 440 nm upon cathodic reduction of complex NH4[2] at the first reduction peak (Figure 13a). Although the electron transitions calculated for 1[1]3− in DMSO are positioned in the similar range of wavelengths as that observed under vacuum, their agreement with the experiment is worse and the interpretation of the electronic absorption spectra is much less straightforward. Similar to 1[1]−, the singlet vanadium complex 1[4]0 is more stable than the triplet one both under vacuum and in DMSO. The highest occupied molecular orbital (HOMO) of 1[4]0 is localized, in both cases, mostly on the ligand framework accommodating one vanadium(V) atom with a small contribution from the second vanadium atom (see Figure 16a and Figure S23a in the Supporting Information). However, in contrast to 1[1]−, the LUMO of 1[4]0 is localized predominantly on the second vanadium atom and its 2

Figure 14. EPR spectrum measured at 100 K after bulk electrolysis of 5 in 0.2 M LiClO4/EtOH at −1.6 V vs Fc/Fc+.

Figure 15. (a) Calculated electronic transitions (under vacuum) for 1 [1]− (green lines) and 3[1]3− (blue lines). (b) Calculated electronic transitions (under vacuum) for 1[4]0 (green lines), 2[4]− (red lines), and 1[4]2− (blue lines).

Information for a DMSO environment. Theoretical spectra were computed for the initial compounds 1[1]− and 1[4]0 (green lines in Figure 15), for monoreduced paramagnetic state L


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Inorganic Chemistry

of the complex both under vacuum (Table 2) and in DMSO (Table S5). It indicates that the reduction of complex 1[1]− is Table 2. NBO Atomic Charges and d-Electron Populations (d×) of Vanadium Atoms in the Systems under Study (under Vacuum) (see Figures 1 and 3 for Atom Labeling) V1

Figure 16. (a) HOMO and (b) LUMO of 1[4]0 under vacuum at the 0.05 a.u. level; (c) spin density of 2[4]2− under vacuum at the 0.02 a.u. level; and (d) β-HOMO of 2[4]− under vacuum at the 0.05 a.u. level.

near neighborhood (Figure 16b and Figure S23b), clearly indicating that the first reduction event occurs on a single vanadium center in the complex. Spin density calculated for the monocharged doublet state 2[4]− is located mainly on a single central vanadium atom (on V2 under vacuum while on V1 in DMSO, as shown in Figure 16c and Figure S23c). This correlates well with our EPR spectroelectrochemical measurements for 5 in ethanol, where EPR signal characteristic of vanadium(IV) species was observed upon reduction both at room and low temperatures (see Figure 14 and Figure S16). As indicated in Figures 16a and 16b, the HOMO → LUMO transition for 1[4]0 under vacuum and DMSO is predominantly of ligand-to-metal character from the ligand framework accommodating one vanadium atom to the second vanadium atom and its nearby neighborhood. In the UV-vis spectroelectrochemistry of 5, a more complex spectroelectrochemical response was found in comparison to NH4[2]. More absorption bands were observed in the region from 350 nm to 500 nm (Figure S19), and we have not observed a clear isosbestic point, as was the case for NH4[2] upon reduction, going from the initial to the two-electron reduced state (see Figure 13). Going to more-negative potentials, the observed new absorption bands are slightly shifted to higher energies (Figure S19a), which correlates well with the theoretical calculations shown in Figure S21. The most intense computed transitions for monocharged 2[4]− under vacuum were found at 467 nm (f = 0.05), representing the β-HOMO → β-LUMO transition and at 376 nm (f = 0.09) representing the α-HOMO2 → α-LUMO, β-HOMO-2 → β-LUMO transitions. For the doubly charged singlet state 1[4]2− under vacuum, we found intense transitions at slightly higher energies at 413 nm (f = 0.14) corresponding to the β-HOMO-1 → β-LUMO transitions and at 359 nm (f = 0.16), which corresponds to the β-HOMO-2 → β-LUMO and α-HOMO-2 → α-LUMO transitions. The agreement of the calculated electron transitions in DMSO with the experiment is less satisfactory for NH4[1] and worse for 4, where a large number of theoretical transitions was found in the wavelength range from 250 nm to 500 nm (Figure S20b), which complicates the data interpretation. Electron density distribution between central vanadium atoms and ligands upon reduction can be very simply ascribed by the changes of vanadium atomic charges. In the studied systems, the vanadium atomic charges are positive and practically independent of the increasing negative total charge

q

Ms

−1 −1 −2 −3 −3

1 3 2 1 3

0 0 −1 −2 −2

1 3 2 1 3

charge

V2 d×

Compound [1]q 1.323 3.42 1.305 3.43 1.329 3.40 1.347 3.38 1.347 3.38 Compound [4]q 1.319 3.43 1.304 3.44 1.319 3.42 1.328 3.40 1.328 3.40

charge

d×

1.348 1.347 1.358 1.371 1.371

3.40 3.40 3.38 3.35 3.35

1.416 1.445 1.461 1.468 1.469

3.32 3.32 3.30 3.28 3.28

localized on the ligand with negligible electron density transfer to the central atom(s). On the other hand, for complex 1[4]0, its LUMO shape and the spin density calculated for 2[4]− indicate vanadium-centered reduction for the first electron addition (see Figures 16b and 16c and Figures S23b and S23c). Since the total atomic charge on this vanadium center remains constant, it implies the subsequent electron density transfer from the central vanadium atom to the ligand. This means that the α-electron addition to the vanadium center upon reduction is followed by both α- and β-electron density transfer to the ligand via the decrease of vanadium contribution (and simultaneous increase of ligand atoms contributions) in other occupied MOs below SOMO. The second electron (β-spin) addition is ligand-centered (see β-LUMO of 2[4]− in Figure S22) leading to the more stable “broken symmetry” singlet state 1[4]2− with the spatially separated spin densities of opposite signs at V2 and at ligand on the V1 side. Similarly, as in the case of atomic charges discussed above, the changes of d-electron populations on vanadium atoms with increasing negative total charges q of the species corresponding to the first and second reduction steps are negligible (differences of ca. 0.01 electron). Preliminary charge density studies show that all vanadium atoms in [4]2 have approximately the same charges, in the range from +1.56 to +1.64 (see Table S6 in the Supporting Information). Hexacoordinate vanadium atoms V2 and V4 have slightly more positive charges than the pentacoordinated ones (V1 and V3), in agreement with the results of quantumchemical calculations on 1[4]0 (Table 2), despite the use of different population analysis. Also notable is the difference in the VO bonds strengths for pentacoordinated and hexacoordinated vanadium centers, where electron density at the bond critical point (BCP) is the highest for double bonds V1O1 and V3O9, compared to V2−O4 and V4−O11 (see Table S7 and Figure S25 in the Supporting Information). Catalytic Studies. Compounds NH4[1], NH4[2], and 4−6, as well as their precursors, were tested as homogeneous catalysts for the microwave (MW)-assisted peroxidative (with aqueous tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane to cyclohexanol and cyclohexanone (Table S8 in the Supporting Information) via the formation of cyclohexyl M


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Inorganic Chemistry

catalytic activity of the corresponding compounds, when compared with the analogous (NH4[1] and 4, respectively) bearing RLH3− with unsubstituted phenyl groups (see Figure 17 and Table S8). In fact, very similar total yields of cyclohexanol and cyclohexanone (17.7% and 18.5%, respectively) are obtained with NH4[1] and NH4[2] under the assayed conditions, which are in accord with the very similar first reduction potentials (−1.46 V and −1.48 V vs Fc/Fc+, see above) exhibited by these compounds. The reduction potential is possibly an important factor for their catalytic behavior (see the mechanistic considerations noted below). The amount of catalyst, as well as the reaction temperature, play a significant role on the catalytic activity of the vanadium compounds, as depicted in Figure 18 for the most active

hydroperoxide (CyOOH) as the primary product. This further evolves in a mixture of cyclohexanol and cyclohexanone (final products, Scheme 3). The formation of CyOOH was proved by Scheme 3. Microwave (MW)-Assisted Neat Oxidation of Cyclohexane to Cyclohexyl Hydroperoxide, Cyclohexanol and Cyclohexanone with tert-Butylhydroperoxide Catalyzed by the Vanadium(V) Complexes NH4[1], NH4[2], and 4−6

using the method proposed by Shul’pin:50 the addition of PPh3 prior to the GC analysis of the products resulted in a marked increase of the amount of cyclohexanol (due to the reduction of CyOOH by PPh3, with the formation of phosphane oxide) and in a corresponding decrease of cyclohexanone (Scheme 3). Under the optimized conditions of these solvent- and additive-free systems, 100 °C and 1.5 h of low power (7 W) microwave irradiation, good yields (up to 19% for 4, ca. 4 times higher than those reported for the industrial aerobic process, to guarantee good selectivity)50,66 of the oxygenated products are obtained (see Figure 17) using 0.2% molar ratio of vanadium catalyst, relative to cyclohexane. Cyclohexanol and cyclohexanone were the only products detected by GC-MS analysis under these assayed conditions, thus revealing a very selective oxidation system. However, “overoxidation” products, such as 1,3- and 1,4-cyclohexanediol, are detectable by GC-MS for longer reaction times. Decomposition of H2O2 (30% or 50% aqueous solution) under the used reaction conditions (MW, 100 °C) hampered the use of this less-expensive and environmentally friendly oxidant (yields were reduced, e.g., for 4, from 19% to 2% when TBHP was replaced by H2O2 50% aqueous solution). As shown in Figure 17 and Table S8, the best results under the above eco-friendly conditions were obtained with the neutral complex [(VO2)(VO)(HLH)(CH3O)] (4). The presence of t-Bu substituents at the phenyl ring of RLH3− in NH4[2] and 5 does not appear to have a clear influence on the

Figure 18. Effect of the reaction time, temperature, and amount of catalyst 4 on the yield of cyclohexanol and cyclohexanone obtained by additive-free microwave (MW)-assisted neat oxidation of cyclohexane with THBP.

catalyst (4). The increase from 1 μmol to 5 μmol of 4 in the reaction medium leads to a yield increment and allows one to reach the maximum yield faster (in 1.5 h, instead of the 2 h of MW irradiation needed for 1 μmol of 4). In addition, MW irradiation at 100 °C was required to achieve good yields of oxygenated products in a shorter reaction time (Figure 18). Nevertheless, the low-power MW-assisted reaction used provides a much more efficient synthetic method than conventional heating under open atmosphere or nonpressur-

Figure 17. Total yield of cyclohexanol and cyclohexanone obtained by microwave (MW)-assisted neat oxidation of cyclohexane with THBP catalyzed by the vanadium(V) complexes NH4[1], NH4[2], and 4−6. N


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ized refluxing, allowing the attainment of similar yields in much shorter times (e.g., 20% yield of cyclohexanol and cyclohexanone was obtained after 24 h of reaction in the presence of 4 under the same conditions but using an oil bath (see the Experimental Section)). Vanadium-containing catalytic systems using TBHP as the oxidant agent for the oxidation of cyclohexane are only scarcely reported.67 Moreover, the performance of such zeolite67a or phosphorus67b oxide heterogeneous catalysts to convert cyclohexane seems to be worse than that of our system, possibly also because of the MW effect used in our case. Addition of a radical trap (e.g., Ph2NH) to the reaction mixture results in the suppression of the catalytic activity. This behavior, along with the formation of cyclohexyl hydroperoxide (typical intermediate product in radical-type reactions) supports the hypothesis of a free-radical mechanism for the cyclohexane oxidation carried out in this study. Similarly to the reported for vanadium-catalyzed oxidation of cyclohexane by hydrogen peroxide,68,69 the vanadium-assisted decomposition of t-BuOOH leading to the oxygen-centered radicals t-BuOO• and t-BuO• (reactions 5 and 6) can be proposed, as illustrated below for complex 4. Cyclohexyl radical (Cy•) is then formed upon H-abstraction from cyclohexane (CyH) by t-BuO• (reaction 7). Reaction of Cy• with O2 leads to CyOO• (reaction 8), and CyOOH can then be formed e.g., upon Habstraction from t-BuOOH by CyOO• (reaction 9). Vanadium complex-assisted decomposition of CyOOH to CyO• and CyOO• (reactions 10 and 11) would then lead to cyclohexanol (CyOH) and cyclohexanone (Cy−H=O) products (reactions 12 and 13).70 [4]0 + t ‐BuOOH → t ‐BuOO• + H+ + [4]−

(5)

[4]− + t ‐BuOOH → t ‐BuO• + [4]0 + HO−

(6)

t ‐BuO• + CyH → t ‐BuOH + Cy •

(7)

Cy • + O2 → CyOO•

(8)

CyOO• + t ‐BuOOH → CyOOH + t ‐BuOO•

(9)

CyOOH + [4]− → CyO• + [4]0 + HO−

(10)

CyOOH + [4]0 → CyOO• + H+ + [4]−

(11)

CyO• + CyH → CyOH + Cy •

(12)

2CyOO• → CyOH + Cy−HO + O2

(13)

Article

CONCLUSIONS

By reactions of RLH4 with NH4NO3 and VO(acac)2 in 1:2 molar ratio in alcohol two types of complexes have been prepared, namely, NH 4 [(VO 2 ) 2 ( H LH)] (NH 4 [1]), NH 4 [(VO 2 ) 2 ( t‑Bu LH)] (NH 4 [2]), NH 4 [(VO 2 ) 2 ( Cl LH)] (NH4[3]), and [(VO2)(VO)(HLH)(CH3O)] (4), [(VO2)(VO)(t‑BuLH)(C2H5O)] (5), and [(VO2)(VO)(ClLH)(CH3O)(CH3OH/H2O)] (6). The ditopic ligands act as a trianion R LH3− and accommodate two homovalent vanadium(V) ions as two identical VO2+ entities in complexes NH4[1]−NH4[3] or two different oxido and dioxidovanadium cores (VO3+ and VO2+) in 4−6. The compounds characterized by single-crystal X-ray crystallography form dimeric associates (dimers of dimers) via two or one V−μ-O−V linkages in the crystal. Interestingly, in NH4[1], 4, and 5, two dinuclear complexes are associated in a tetranuclear associate in a mutually trans fashion, whereas in 6, the two dinuclear complexes are bound in a cis manner. The 3,5-substitution in the 2-hydroxybenzaldehyde moiety by electron-withdrawing or releasing groups has a marked effect on the coordination pattern of the two vanadium(V) ions and on the formation of associates in the solid state. The VO2+ entity accommodated in the NNO binding site of the dinucleating ligand provides an axial position for the vanadium(V) atom in the ONO binding pocket of the ligand in the second neighboring complex NH4[1]. A similar distribution of roles (functions) is observed in 4 and 5, although different entities (VO2+ and VO3+) are found in the ONN and ONO binding sites, respectively. In stark contrast, in 6, the fivecoordinate vanadium atom V1 as a part of VO2+ entity in the ONN-pocket provides an axial position for the V3 atom, which is also accommodated in the ONN binding site of the neighboring complex, while V2 and V4 atoms are six- and five-coordinate, respectively. The dimeric associates formed in the solid state are not preserved in solution dissociating in dinuclear vanadium(V) species, as confirmed by multinuclear NMR experiments and ESI mass spectrometry measurements. Although irreversible reduction peaks and two strongly shifted oxidation peaks were observed upon cathodic reduction of complexes NH4[1], NH4[2], and 5, the chemically reversible redox processes were confirmed by UV-vis spectroelectrochemistry. In contrast to 1[1]−, where the ligand-centered reduction was indicated by theoretical calculations, for complex 1[4]0, the first reduction is localized predominantly on a single vanadium atom and its nearby neighborhood. Spin density calculated for the most stable monocharged doublet state 2[4]− is located mainly on a single central vanadium atom, as proved by EPR spectroelectrochemical measurements, where the EPR signal characteristic of vanadium(IV) species was observed. The changes of atomic charges and d-electron populations at vanadium atoms during the first and second reduction steps are negligible. Compounds NH4[1], NH4[2], and 4−6 act as homogeneous catalysts for the MW-assisted and solvent-free peroxidative (with aqueous TBHP) oxidation of cyclohexane, via a radical mechanism, affording cyclohexanol and cyclohexanone in high selectivity and in a yield that is ca. 4 times higher than that of the industrial aerobic process for good selectivity, which also operates at higher temperature. Our vanadium(V) complexes are reducible, which is consistent with the involvement of reduced vanadium species. Moreover, the study indicates the key role played by the carbohydrazone ligand in the formation

The availability of reducible vanadium(V) species by the peroxide is shown by electrochemical studies and is crucial for the peroxidative oxidation. In fact, the formation of t-BuOO• and t-BuO• radicals is a key step for the occurrence of the C−H abstraction from the cyclohexane. The above theoretical calculations indicate a relevant role played by the ligand (RLH3−) in the redox process. In fact, the reduction of the metal complex can be centered at the ligand and not at the vanadium ion, which preserves its +5 oxidation state (which is the case of ionic [1]− complex) or can be centered at the metal but followed by e− transfer to the ligand (which is the case of of [4]). The recognition of such a ligandassisted catalyst redox behavior is unprecedented in this field. O


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Inorganic Chemistry

funded by the European Region Development Funds), for computing facilities. We also thank Dr. Luca Carrella for collection of X-ray data for compound 4 and Anatolie Dobrov for the preparation of single crystals of complex 6.

of the reduced form of the catalyst with preservation of the vanadium +5 oxidation state. This feature has not been recognized earlier in this field and deserves to be further explored. Other advantages of the present catalytic systems are the mild and environmental benign conditions such as (i) solventand additive-free protocol, (ii) use of environmentally acceptable oxidant (aqueous TBHP) and energy source (MW irradiation), and (iii) short reaction time. These are significant features toward the development of a sustainable chemical process for the cyclohexane oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre (via the Internet (www.ccdc.cam.ac.uk/conts/retrieving.html) or by contacting Cambridge Crystallographic Data Centre directly (Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; or [email protected]. uk)). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01011. CCDC-1459019 for NH4[1]·CH3OH (CIF) CCDC-1459020 for [4]2·3CH3OH (CIF) CCDC-1459013 for [5]2·6C2H5OH (CIF) CCDC-1459015 for [6]2·3.6CH3OH·0.75H2O (CIF) Additional X-ray data (Figures S1−S5, Table S1), NMR data (Figures S6−S11, Table S2), kinetics data (Figures S12 and S13), spectroelectrochemical data (Figures S14−S19), DFT results (Figures S20−S24, Tables S3− S5), charge density measurements data for complex 4 (Figure S25, Tables S6 and S7) and selected data on catalytic oxidation of cyclohexane (Table S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

REFERENCES

[email protected] (P. Rapta). lmartins@deq isel.ipl.pt (L. M. D. R. S. Martins). [email protected] (A. Pombeiro). [email protected] (V. B. Arion).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Fundaçaõ para a Ciência e a Tecnologia (FCT), Portugal, for the CATSUS fellowship SFRH/BD/52371/2013 to N.M.R.M. and the PTDC/QEQERQ/1648/2014, PTDC/QEQ-QIN/3967/2014 and UID/ QUI/00100/2013 projects, is gratefully acknowledged. S.S. is indebted to support by European Regional Development Fund, Sectoral Operational Programme “Increase of Economic Competitiveness”, Priority Axis 2 (SOP IEC-A2-O2.1.2-20092, ID 570, COD SMIS-CSNR: 12473, Contract 129/2010POLISILMET). The computational and spectroelectrochemical studies were supported by the Slovak Grant Agency VEGA (under Contract Nos. 1/0598/16 and 1/0307/14) and by the Slovak Research and Development Agency (under Contract Nos. APVV-15-0053 and APVV-15-0079). We thank the HPC center at the Slovak University of Technology in Bratislava, which is a part of the Slovak Infrastructure of High Performance Computing (SIVVP Project No. 26230120002, P


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Inorganic Chemistry

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