Chemistry of Monomeric and Dinuclear Non-Oxido Vanadium (IV) and

Article pubs.acs.org/IC

Chemistry of Monomeric and Dinuclear Non-Oxido Vanadium(IV) and Oxidovanadium(V) Aroylazine Complexes: Exploring Solution Behavior Subhashree P. Dash,† Sudarshana Majumder,† Atanu Banerjee,† M. Fernanda N. N. Carvalho,‡ Pedro Adaõ ,‡ Joaõ Costa Pessoa,*,‡ Krzysztof Brzezinski,§ Eugenio Garribba,*,∥ Hans Reuter,⊥ and Rupam Dinda*,† †

Department of Chemistry, National Institute of Technology, Rourkela 769008, Odisha, India Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal § Institute of Chemistry, University of Bialystok, Hurtowa 1, 5-399 Bialystok, Poland ∥ Dipartimento di Chimica e Farmacia, Università di Sassari, Via Vienna 2, I-07100 Sassari, Italy ⊥ Institute of Chemistry of New Materials, University of Osnabrück, Barbarastrasse 7, 49067 Osnabrück, Germany ‡

S Supporting Information *

ABSTRACT: A series of mononuclear non-oxido vanadium(IV) [VIV(L1−4)2] (1−4), oxidoethoxido vanadium(V) [VVO(L1−4)(OEt)] (5−8), and dinuclear μoxidodioxidodivanadium(V) [VV2O3(L1)2] (9) complexes with tridentate aroylazine ligands are reported [H2L1 = 2furoylazine of 2-hydroxy-1-acetonaphthone, H2L2 = 2thiophenoylazine of 2-hydroxy-1-acetonaphthone, H2L3 = 1naphthoylazine of 2-hydroxy-1-acetonaphthone, H2L4 = 3hydroxy-2-naphthoylazine of 2-hydroxy-1-acetonaphthone]. The complexes are characterized by elemental analysis, by various spectroscopic techniques, and by single-crystal X-ray diffraction (for 2, 3, 5, 6, 8, and 9). The non-oxido VIV complexes (1−4) are quite stable in open air as well as in solution, and DFT calculations allow predicting EPR and UV−vis spectra and the electronic structure. The solution behavior of the [VVO(L1−4)(OEt)] compounds (5−8) is studied confirming the formation of at least two different types of VV species in solution, monomeric corresponding to 5−8, and μ-oxidodioxidodivanadium [VV2O3(L1−4)2] compounds. The μoxidodioxidodivanadium compound [VV2O3(L1)2] (9), generated from the corresponding mononuclear complex [VVO(L1)(OEt)] (5), is characterized in solution and in the solid state. The single-crystal X-ray diffraction analyses of the non-oxido vanadium(IV) compounds (2 and 3) show a N2O4 binding set and a trigonal prismatic geometry, and those of the VVO complexes 5, 6, and 8 and the μ-oxidodioxidodivanadium(V) (9) reveal that the metal center is in a distorted square pyramidal geometry with O4N binding sets. For the μ-oxidodioxidodivanadium species in equilibrium with 5−8 in CH2Cl2, no mixedvalence complexes are detected by chronocoulometric and EPR studies. However, upon progressive transfer of two electrons, two distinct monomeric VIVO species are detected and characterized by EPR spectroscopy and DFT calculations.

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the so-called “bare” complexes, have been isolated and structurally characterized.7,8 This type of non-oxido VIV center was found in amavadin,9 a naturally occurring VIV compound isolated from the mushroom Amanita muscaria. On the other hand, it is well-known that, for a particular system, the solid state chemistry may be different from that in solution. In fact, in solution the complexes may undergo ligand exchange, in some cases also redox reactions and/or formation of oligomers; thus, the main species present in solution may differ from that of the parent solid.10

INTRODUCTION Hydrazones, −NH−NCRR′ (R and R′ = H, alkyl, aryl), as well as azines (RR′CNNCRR′), are versatile compounds, and the study of their complexes is of unabated interest because of their relevance to several distinct fields such as medicinal chemistry1 and catalysis.2 The phenolic hydrazone (or azine) O atoms are often good π-donor atoms and may stabilize high oxidation states with highly covalent M−O(phenolate) bonds. On the other hand, the diversified roles of vanadium in biological systems3−6 have been stimulating increased interest in vanadium coordination chemistry. In contrast to the well-known oxidovanadium(IV)/(V) complexes, relatively few non-oxido VIV and VV complexes, © XXXX American Chemical Society

Received: October 12, 2015

A

DOI: 10.1021/acs.inorgchem.5b02346 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Outline of the Pathways for the Synthesis of the Mononuclear Non-Oxido Vanadium(IV) [VIV(L)2] (1−4), Oxidovanadium(V) [VVO(L)(OEt)] (5−8), and Binuclear μ-Oxidovanadium(V) [VV2O3(L)2] (9) Compounds

monomeric as well as the corresponding μ-oxido dinuclear species. One of these species, 9, was now isolated in the solid state and characterized by spectroscopic techniques and its crystal and molecular structure determined by single-crystal Xray diffraction (XRD). To the best of our knowledge this is the first report where the intermediate species, being generated in solution, is isolated in the solid state and structurally characterized.

Vanadium complexes with organic ligands are often found to have improved therapeutic properties when compared with inorganic vanadium compounds, due to changes in their aqueous solubility and lipophilicity. This may lead to their use as potent orally administrable antidiabetic drugs.3h−k,11 Literature reveals that five-coordinated monooxido−vanadium(IV) complexes12,13 are common; five-coordinate monooxido− vanadium(V) complexes12b,13,14 are also known, and the vanadium(V) alkoxide [VVO(L)(OEt)] compounds correspond to one of the most common and stable types under aerobic conditions,15 which possess the unique ability to form11d several species in solution.10 Despite the reported involvement of such species to provide the alternative route for the generation of monooxido-bridged binuclear vanadium(V,V) species of the type (VVOL)2O, which hold the possibility of acting as precursors of mixed-valence V(IV,V) species,16 study of solution properties of these VVO complexes have so far received rather limited attention.16−18 We have been exploring the feasibility of using tridentate aroylhydrazone and azine ligands, which comprise the three donor atoms [ONO], for generating novel vanadium complexes, and reported several oxidovanadium, non-oxido vanadium, and μ-oxidodivanadium complexes of formulas [VVO(L)(OEt)], [VIV(L)2], and [VV2O3(L)2] with tridentate dinegative aroylazine ligands.19 Here, we extend our studies to a potentially tridentate azine ligand with the aim of scrutinizing the effect of the sterically hindered/bulky naphthoyl derivative and the bioactive heteroaromatic thiophenoyl and furoyl derivative arms on these types of ligands, which are found to yield homoleptic bis(tridentate) non-oxido vanadium(IV) complexes and monoligand, oxidoalkoxidovanadium(V) (i.e., VVO(L)(OEt)) compounds, whose relevance to solution chemistry has been described.13c,16,17,20 In particular, the binding of some new dibasic tridentate ligands (Scheme 1), abbreviated as H2L, was explored. Mononuclear non-oxido vanadium(IV) (1−4) and VVO species (5−8) were isolated, structurally characterized, and studied in solution. DFT methods were used to predict EPR and UV−vis spectra, as well as the electronic structure of 1−4. IR and NMR data for the VVO complexes (5−8) indicate that two species may simultaneously exist in solution, the

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EXPERIMENTAL SECTION

Materials and Methods. [VIVO(acac)2] was prepared as described in the literature.21 Reagent grade solvents were dried and distilled prior to use. Elemental analyses were carried out on a Vario ELcube CHNS elemental analyzer. IR spectra were recorded on a PerkinElmer Spectrum RXI spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker Ultrashield 400 MHz spectrometer using SiMe4 as an internal standard to obtain the corresponding chemical shifts δH and δC. The 51V NMR spectra were obtained on either Bruker Avance+ 400 or 300 MHz spectrometers. The 51V chemical shifts (δV) are expressed in ppm relative to neat VVOCl3, used as an external reference sample. Electronic spectra were recorded either on a Lambda 25 or on a Lambda 35 PerkinElmer spectrophotometer. Cyclic voltammograms (CV) of compounds 1−9 were obtained in TEAP (tetraethylammonium perchlorate)/CH2Cl2 (0.1 M) electrolyte solutions, using a CH1120A potentiostat, and a Pt disk or glassy carbon as working electrodes, Pt wire as counter electrode, and SCE as reference electrode. Controlled potential electrolysis (CPE) of compounds 5 and 9 was carried out in Bu4NBF4/CH2Cl2 (0.1 M) electrolyte solutions using a three compartment cell provided with a Pt gauze as working electrode and Pt and Ag wires as auxiliary and reference electrodes, respectively, using a VoltaLab PST050 equipment. The EPR spectra were recorded on a Bruker ESP 300E X-band spectrometer, in frozen samples at 77 K using the DPPH (2,2diphenyl-1-picrylhydrazyl) radical as reference. The measured spectra (first derivative X-band EPR) were simulated with the EPR simulation software developed by Rockenbauer and Korecz.22 Synthesis of the Azine Compounds (H2L1−4). Schiff bases, 2furoylazine of 2-hydroxy-1-acetonaphthone (H2L1), 2-thiophenoylazine of 2-hydroxy-1-acetonaphthone (H2L2), 1-naphthoylazine of 2hydroxy-1-acetonaphthone (H2L3), and 3-hydroxy-2-naphthoylazine of 2-hydroxy-1-acetonaphthone (H2L4) were synthesized by the condensation of 2-hydroxy-1-acetonapthone and the respective acid hydrazide in equimolar ratio in ethanol (EtOH) by adapting a B


Article

Inorganic Chemistry previously reported procedure.19d The resulting white compounds were filtered, washed with ethanol, dried over fused CaCl2, and characterized by elemental analysis, NMR (1H and 13C), and IR data. 2-Furoylazine of 2-Hydroxy-1-acetonaphthone, H2L1. Yield: 74%. Anal. Calcd for C17H14N2O3: C, 69.38; H, 4.79; N, 9.52. Found: C, 69.35; H, 4.78; N, 9.50. IR (KBr pellet, cm−1): 3331 ν(O−H); 3102 ν(N−H); 1652 ν(CO); 1622 ν(CN). 1H NMR (400 MHz, DMSO-d6, ppm): δH 10.47 (s, 1H, OH), 9.06 (s, 1H, NH), 7.95−6.52 (m, 9H, aromatic), 2.32 (s, 3H, CH3).13C NMR (100 MHz, DMSOd6, ppm): δC 153.91, 153.55, 152.34, 146.48, 146.10, 131.87, 130.17, 129.13, 128.32, 128.18, 123.92, 122.85, 118.83, 115.97, 112.90, 112.65, 24.19. 2-Thiophenoylazine of 2-Hydroxy-1-acetonaphthone, H2L2. Yield: 71%. Anal. Calcd for C17H14N2O2S: C, 65.79; H, 4.55; N, 9.03. Found: C, 65.77; H, 4.56; N, 9.07. IR (KBr pellet, cm−1): 3353 ν(O−H); 3108 ν(N−H); 1658 ν(CO); 1621 ν(CN). 1H NMR (400 MHz, DMSO-d6, ppm): δH 10.31 (s, 1H, OH), 9.56 (s, 1H, NH), 8.08−7.20 (m, 9H, aromatic), 2.38 (s, 3H, CH3).13C NMR (100 MHz, DMSO-d6, ppm): δC 161.48, 152.83, 149.48, 135.52, 135.18, 134.23, 131.45, 130.42, 129.05, 128.44, 127.85, 127.08, 123.59, 123.00, 119.03, 113.62, 24.03. 1-Naphthoylazine of 2-Hydroxy-1-acetonaphthone, H2L3. Yield: 69%. Anal. Calcd for C23H18N2O2: C, 77.95; H, 5.12; N, 7.90. Found: C, 77.91; H, 5.14; N, 7.88. IR (KBr pellet, cm−1): 3448 ν(O−H); 3113 ν(N−H); 1661 ν(CO); 1611 ν(CN). 1H NMR (400 MHz, DMSO-d6, ppm): δH 10.32 (s, 1H, OH), 10.08 (s, 1H, NH), 8.02− 7.30 (m, 13H, aromatic), 2.40 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6, ppm): δC 165.05, 154.35, 153.00, 152.73, 133.77, 133.37, 131.17, 130.40, 130.36, 130.17, 128.95, 128.60, 128.32, 127.70, 125.86, 125.41, 125.24, 123.53, 123.36, 118.98, 114.47, 24.43. 3-Hydroxy-2-naphthoylazine of 2-Hydroxy-1-acetonaphthone, H2L4. Yield: 64%. Anal. Calcd for C23H18N2O3: C, 74.58; H, 4.90; N, 7.56. Found: C, 74.57; H, 4.88; N, 7.58. IR (KBr pellet, cm−1): 3648 ν(O−H); 3188 ν(N−H); 1652 ν(CO); 1622 ν(CN). 1H NMR (400 MHz, DMSO-d6, ppm): δH 11.87 (s, 1H, OH), 11.67 (s, 1H, naphthoyl hydrazide−OH), 10.15 (s, 1H, NH), 8.68−7.24 (m, 12H, aromatic), 2.38 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6, ppm): δC 166.82, 154.89, 153.33, 153.01, 136.33, 132.81, 130.22, 129.43, 128.79, 128.20, 127.68, 127.12, 126.24, 124.41, 124.35, 124.17, 123.25, 121.09, 119.91, 118.74, 111.19, 111.04, 19.33. Synthesis of Non-Oxido Vanadium(IV) Compounds [VIV(L1−4)2] (1−4). [VIVO(acac)2] (0.50 mmol) was added to a hot solution of the appropriate ligand H2L1−4 (1.0 mmol) in CH3CN (20 mL); the color immediately changed to greenish black. After 3 h under reflux black crystals were obtained from the reaction mixture, which were filtered off, washed thoroughly with ethanol, and dried. Some crystals were of good quality and were used directly for single-crystal XRD structure determination. [VIV(L1)2] (1). Yield: 68%. Anal. Calcd for C34H24N4O6V: C, 64.26; H, 3.81; N, 8.82. Found: C, 64.24; H, 3.82; N, 8.79. IR (KBr pellet, cm−1): 1608 ν(CN); 1243 ν(C−O)enolic; 1043 ν(N−N). UV−vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 620 (5430), 499 sh (5100), 365 sh (23770), 319 (38610), 266 (39270). [VIV(L2)2] (2). Yield: 60%. Anal. Calcd for C34H24N4O4S2V: C, 61.16; H, 3.62; N, 8.39. Found: C, 61.18; H, 3.61; N, 8.43. IR (KBr pellet, cm−1): 1611 ν(CN); 1242 ν(C−O)enolic; 1044 ν(N−N). UV−vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 623 (8820), 510 (8100), 370 (31430), 324 (49400), 260 (49190). [VIV(L3)2] (3). Yield: 61%. Anal. Calcd for C47.63H34.63Cl0.74N4.63O4V: C, 70.30; H, 4.25; N, 7.97. Found: C, 70.33; H, 4.27; N, 7.95. IR (KBr pellet, cm−1): 1591 ν(CN); 1244 ν(C−O)enolic; 1045 ν(N−N). UV−vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 603 (8590), 516 (8990), 368 (32050), 330 (47610). [VIV(L4)2] (4). Yield: 66%. Anal. Calcd for C46H32N4O6V: C, 70.14; H, 4.09; N, 7.11. Found: C, 70.15; H, 4.08; N, 7.13. IR (KBr pellet, cm−1): 3362 ν(O−H); 1609 ν(CN); 1239 ν(C−O)enolic; 1022 ν(N−N). UV−vis (CH2Cl2) [λmax, nm (ε, M−1 cm−1)]: 623 (4790), 517 (4170), 366 (25160), 321 (39510), 266 (46690). Synthesis of Oxidoethoxidovanadium(V) Compounds [VVO(L1−4)(OEt)] (5−8). [VIVO(acac)2] (1.0 mmol) was added to a hot

solution of the appropriate H2L1−4 (1.0 mmol) in EtOH (20 mL); the color immediately changed to brown. After 3 h under reflux the reaction mixture was filtered off. After 3 to 4 days crystals of good quality were obtained, which were used for single-crystal XRD structure determination. [VVO(L1)(OEt)] (5). Yield: 66%. Anal. Calcd for C19H17N2O5V: C, 56.45; H, 4.24; N, 6.93. Found: C, 56.46; H, 4.22; N, 6.92. IR (KBr pellet, cm−1): 1604 ν(CN); 1241 ν(C−O)enolic; 1034 ν(N−N); 994 ν(VO). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 421 (24636), 293 (63363), 229 (70909). IR (CHCl3, cm−1): 1009, 966 ν(VO); 820 ν(V−O−V). 1H NMR (400 MHz, DMSO-d6, ppm): δH 8.20− 6.63 (m, 18H, aromatic), 5.51−5.46 (q, 2H, CH2 (OEt)), 3.47−3.42 (q, 4H, CH2 (EtOH)), 2.92 (s, 3H, CH3), 2.86 (s, 3H, CH3), 1.49− 1.46 (t, 3H, CH3 (OEt)), 1.08−1.04 (t, 6H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6, ppm): δC 166.53, 166.03, 163.73, 163.53, 159.72, 159.49, 146.81, 146.47, 146.25, 145.45, 134.53, 133.81, 131.95, 131.74, 129.92, 129.49, 129.33, 128.82, 127.93, 127.55, 126.43, 126.13, 124.83, 124.33, 123.93, 119.42, 118.69, 118.24, 118.11, 116.30, 115.51, 112.62, 80.80, 56.52, 31.12, 23.89, 19.00, 18.63. 1H NMR (400 MHz, DMSO-d6 + EtOH, ppm): δH 8.09−6.67 (m, 9H, aromatic), 5.52− 5.46 (q, 2H, CH2 (OEt)), 3.48−3.43 (q, 27H, CH2 (EtOH)), 2.86 (s, 3H, CH3), 1.49−1.46 (t, 3H, CH3 (OEt)), 1.08−1.04 (t, 38H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6 + EtOH, ppm): δC 166.01, 163.73, 159.48, 146.38, 146.27, 133.74, 131.95, 129.56, 129.28, 127.49, 126.09, 124.27, 119.38, 118.24, 115.43, 112.53, 80.80, 56.52, 23.81, 18.88, 18.55. 51V NMR (DMSO-d6, ppm): δV −543, −522. 51V NMR (DMSO-d6 + EtOH, ppm): δV −522. [VVO(L2)(OEt)] (6). Yield: 66%. Anal. Calcd for C19H17N2O4SV: C, 54.29; H, 4.08; N, 6.66. Found: C, 54.27; H, 4.04; N, 6.67. IR (KBr pellet, cm−1): 1612 ν(CN); 1238 ν(C−O)enolic; 1037 ν(N−N); 999 ν(VO). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 420 (12871), 293 (29109), 230 (35842). IR (CHCl3, cm−1): 1005, 965 ν(VO); 817 ν(V−O−V). 1H NMR (400 MHz, DMSO-d6, ppm): δH 8.20− 7.08 (m, 18H, aromatic), 5.53−5.49 (q, 2H, CH2 (OEt)), 3.48−3.43 (q, 4H, CH2 (EtOH)), 2.92 (s, 3H, CH3), 2.86 (s, 3H, CH3), 1.50− 1.48 (t, 3H, CH3 (OEt)), 1.08−1.05 (t, 6H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6, ppm): δC 167.33, 166.50, 166.03, 158.91, 134.83, 133.77, 133.60, 131.97, 131.75, 131.54, 130.95, 129.96, 129.59, 129.50, 129.32, 128.55, 127.95, 127.55, 126.43, 126.15, 124.82, 124.32, 119.38, 118.18, 80.67, 56.53, 23.71, 19.00, 18.59. 1H NMR (400 MHz, DMSO-d6 + EtOH, ppm): δH 8.08−7.17 (m, 9H, aromatic), 5.52− 5.45 (q, 2H, CH2 (OEt)), 3.48−3.43 (q, 62H, CH2 (EtOH)), 2.85 (s, 3H, CH3), 1.51−1.48 (t, 3H, CH3 (OEt)), 1.08−1.04 (t, 95H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6 + EtOH, ppm): δC 167.33, 165.97, 158.89, 134.83, 133.59, 131.94, 131.22, 130.76, 129.58, 129.18, 128.28, 127.38, 126.03, 124.16, 119.26, 118.16, 80.68, 56.55, 23.48, 18.65, 18.27. 51V NMR (DMSO-d6, ppm): δV −538, −519. 51V NMR (DMSO-d6 + EtOH, ppm): δV −520. [VVO(L3)(OEt)] (7). Yield: 66%. Anal. Calcd for C25H21N2O4V: C, 64.66; H, 4.56; N, 6.03. Found: C, 64.63; H, 4.54; N, 6.06. IR (KBr pellet, cm−1): 1593 ν(CN); 1241 ν(C−O)enolic; 1037 ν(N−N); 998 ν(VO). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 418 (15049), 320 (29405), 229 (63564). IR (CHCl3, cm−1): 1004, 965 ν(VO); 818 ν(V−O−V). 1H NMR (400 MHz, DMSO-d6, ppm): δH 9.30− 7.25 (m, 26H, aromatic), 5.55−5.41 (m, 2H, CH2 (OEt)), 3.47−3.42 (q, 4H, CH2 (EtOH)), 3.03 (s, 3H, CH3), 2.95 (s, 3H, CH3), 1.49− 1.45 (t, 3H, CH3 (OEt)), 1.07−1.04 (t, 6H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6, ppm): δC 172.46, 171.75, 171.35, 166.78, 166.66, 166.32, 160.65, 160.14, 134.91, 134.01, 132.63, 132.20, 132.06, 131.85, 131.22, 130.97, 130.89, 130.27, 129.95, 129.57, 129.37, 129.21, 129.16, 129.02, 128.96, 128.01, 127.90, 127.61, 127.43, 127.34, 127.30, 126.57, 126.21, 125.59, 125.31, 124.91, 124.36, 119.58, 118.85, 118.60, 118.17, 118.02, 117.87, 80.05, 56.50, 24.40, 24.12, 19.02, 18.75. 1H NMR (400 MHz, DMSO-d6 + EtOH, ppm): δH 9.16−7.24 (m, 13H, aromatic), 5.56−5.42 (m, 2H, CH2 (OEt)), 3.48−3.43 (q, 92H, CH2 (EtOH)), 2.95 (s, 3H, CH3), 1.50−1.47 (t, 3H, CH3 (OEt)), 1.08− 1.04 (t, 136H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6 + EtOH, ppm): δC 172.45, 166.29, 160.15, 134.02, 133.92, 132.05, 131.79, 131.24, 129.59, 129.27, 129.15, 128.85, 127.49, 127.29, 127.21, 126.43, 126.13, 125.45, 124.25, 119.50, 118.16, 80.03, 56.51, 23.97, C


Article

Inorganic Chemistry Table 1. Crystal Data and Refinement Details for 2, 3, 5, 6, 8, and 9 2 empirical formula

C34H24N4O4S2V

formula wt temp, K cryst syst space group unit cell dimens

vol, Å3 Z density (calcd), g cm−3 abs coeff F(000) cryst size, mm3 θ range for data collection, deg indep reflns final R indices [I > 2σ(I)]

3

5

6

8

9

C19H17N2O5V

C19H17N2O4SV

C25H21N2O5V

C34.50H25ClN4O9V2

667.63 100(2) monoclinic P21/c a = 13.03105(9) Å b = 23.24945(14) Å c = 10.37682(9) Å α = 90° β = 104.2945(8)° γ = 90° 3046.48 (4) 4 1.456

C47.63H34.63Cl0.74 N4.63O4V 812.98 293(2) monoclinic P21/c a = 11.8182(1) Å b = 19.0878(2) Å c = 16.5463(2) Å α = 90° β = 97.829(1)° γ = 90° 3697.78(7) 4 1.460

404.29 100(2) triclinic P−1 a = 8.3450(6) Å b = 10.1037(7) Å c = 10.5931(7) Å α = 79.973(3)° β = 81.273(2)° γ = 78.461(2)° 855.39(10) 2 1.570

420.35 100(2) monoclinic P21/n a = 9.75757(9) Å b = 15.62094(14) Å c = 11.92282(11) Å α = 90° β = 91.8964(8)° γ = 90° 1816.31(3) 4 1.537

480.38 100(2) triclinic P−1 a = 6.06519 (7) Å b = 17.9476 (3) Å c = 21.00558(19) Å α = 69.4104(11)° β = 89.5791(8)° γ = 88.6613(10)° 2139.93(4) 4 1.491

776.92 100(2) triclinic P−1 a = 10.39154 (19)Å b = 12.21149 (18) Å c = 13.8273(2) Å α = 99.9444(13)° β = 108.6135(16)° γ = 103.0069(14)° 1562.34(4) 2 1.651

4.39 mm−1 1372 0.21 × 0.19 × 0.02 3.5−67.2

0.378 mm−1 1682 0.20 × 0.12 × 0.12 1.6−28.0

0.615 mm−1 416 0.32 × 0.16 × 0.12 2.6−28.0

0.69 mm−1 864 0.23 × 0.14 × 0.07 3.0−37.0

4.23 mm−1 992 0.53 × 0.11 × 0.06 2.6−76.4

6.37 mm−1 790 0.23 × 0.18 × 0.15 3.5−72.9

5304 R = 0.034 Rw = 0.095

8929 R = 0.0363 Rw = 0.0863

4113 R = 0.0302 Rw = 0.0791

9244 R = 0.031 Rw = 0.091

8945 R = 0.065 Rw = 0.185

6187 R = 0.045 Rw = 0.119

18.78, 18.59. 51V NMR (DMSO-d6, ppm): δV −540, −522. 51V NMR (DMSO-d6 + EtOH, ppm): δV −523. [VVO(L4)(OEt)] (8). Yield: 66%. Anal. Calcd for C25H21N2O5V: C, 62.51; H, 4.41; N, 5.83. Found: C, 62.48; H, 4.40; N, 5.82. IR (KBr pellet, cm−1): 3156 ν(O−H); 1571 ν(CN); 1239 ν(C−O)enolic; 1035 ν(N−N); 1001 ν(VO).UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 423 (15462), 314 (35028), 234 (74421). IR (CHCl3, cm−1): 1009, 970 ν(VO); 822 ν(V−O−V). 1H NMR (400 MHz, DMSOd6, ppm): δH 11.78 (s, 1H, OH), 11.42 (s, 1H, OH), 8.54−7.10 (m, 24H, aromatic), 5.72−5.67 (q, 2H, CH2 (OEt)), 3.47−3.42 (q, 4H, CH2 (EtOH)), 3.01 (s, 3H, CH3), 2.91 (s, 3H, CH3), 1.61−1.58 (t, 3H, CH3 (OEt)), 1.07−1.04 (t, 6H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6, ppm): δC 170.65, 166.19, 159.74, 155.19, 136.79, 134.39, 131.93, 131.06, 129.97, 129.63, 129.40, 129.34, 129.06, 128.83, 128.32, 128.14, 127.79, 127.36, 127.24, 126.42, 126.17, 125.96, 124.91, 124.54, 124.09, 123.70, 122.91, 119.43, 118.79, 117.86, 117.31, 111.01, 110.74, 82.36, 56.51, 23.56, 19.00, 18.84. 1H NMR (400 MHz, DMSO-d6 + EtOH, ppm): δH 11.77 (s, 1H, OH), 8.53−7.22 (m, 12H, aromatic), 5.73−5.67 (q, 2H, CH2 (OEt)), 3.47−3.42 (q, 47H, CH2 (EtOH)), 2.91 (s, 3H, CH3), 1.61−1.58 (t, 3H, CH3 (OEt)), 1.07− 1.04 (t, 72H, CH3 (EtOH)). 13C NMR (100 MHz, DMSO-d6 + EtOH, ppm): δC 170.66, 166.17, 159.69, 155.22, 136.79, 134.32, 131.92, 131.02, 129.63, 129.35, 129.27, 128.74, 127.72, 127.35, 126.37, 126.12, 124.47, 124.02, 119.38, 117.85, 117.30, 110.97, 82.36, 56.51, 23.48, 18.88. 51V NMR (DMSO-d6, ppm): δV −565, −529. 51V NMR (DMSO-d6 + EtOH, ppm): δV −533. Synthesis of μ-Oxidodioxidodivanadium(V) Compound [VV2O3(L1)2] (9). Complex 5 was dissolved in CH2Cl2 and layered with CH3CN. The above mixture was kept for recrystallization. After 7 days single-crystal XRD quality crystals were obtained, filtered off, washed thoroughly with CH3CN, and dried. Some of the crystals were selected for single-crystal XRD structure determination. Yield: 66%. Anal. Calcd for C34.50H25ClN4O9V2: C, 53.29; H, 3.22; N, 7.21. Found: C, 53.31; H, 3.21; N, 7.24. IR (KBr pellet, cm−1): 1608 ν(CN); 1237 ν(C−O)enolic; 1040 ν(N−N); 999, 963 ν(VO); 823 ν(V−O− V). UV−vis (CHCl3) [λmax, nm (ε, M−1 cm−1)]: 427 (26040), 292 (70594), 230 (80990). 1H NMR (400 MHz, DMSO-d6, ppm): δH 8.20−6.63 (m, 9H, aromatic), 2.92 (s, 3H, CH3).13C NMR (100 MHz, DMSO-d6, ppm): δC 166.52, 163.51, 163.17, 159.97, 159.70, 146.82, 146.13, 145.53, 145.41, 134.52, 131.73, 130.18, 129.92, 129.50, 129.13,

128.21, 127.93, 127.04, 126.43, 124.83, 122.87, 118.69, 118.55, 118.11, 117.98, 116.30, 116.13, 112.69, 24.22, 24.10. 51V NMR (DMSO-d6, ppm): δV −544. X-ray Crystallography. Crystals suitable for X-ray diffraction studies were obtained at room temperature for several of the complexes prepared. A summary of crystal data and refinement details for 2, 3, 5, 6, 8, and 9 is provided in Table 1. For 3 data were collected on a Bruker smart CCD area diffractometer at 293(2) K, using Mo Kα radiation (0.71073 Å). Data were corrected for absorption effects using the numerical method (SADABS). For 5 data were collected on a Bruker Kappa APEX II CCD-based 4-circle X-ray diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Integrated intensities were obtained with the Bruker SAINT software package using a narrow-frame algorithm performing spatial corrections of frames, background subtractions, Lorentz and polarization corrections, profile fittings, and error analyses. The structures of 3 and 5 were solved by direct methods and subsequent difference Fourier syntheses of the program SHELXS and refined by full-matrix least-squares techniques on F2 with SHELXL. For other compounds, the X-ray diffraction data were collected at 100(2) K on a SuperNova diffractometer (Agilent) with a CCD detector and Mo Kα (6) or Cu Kα (2, 8, and 9) radiation. The crystal data were processed with CrysAlisPro (data collection, cell refinement, and data reduction).23 The crystal structures were solved using direct methods with SHELXD and refined with SHELXL.24 All hydrogen atoms were initially located in electron-density difference maps and were constrained to idealized positions, with C−H = 0.95−0.99 Å and with Uiso(H) = 1.5Ueq(C) for methyl hydrogen atoms and Uiso(H) = 1.2Ueq(C) for others. The non-hydrogen atoms were refined anisotropically, and the PLATON software25 was used to validate the crystallographic data. Cyclic Voltammetry and Controlled Potential Electrolysis. The redox properties of complexes 1−9 were studied by cyclic voltammetry using a three compartment cell provided with Pt disk or glassy carbon working electrodes. The potentials were quoted in volts (±10 mV) and referred to SCE. Controlled potential electrolysis (CPE) experiments were carried out in Bu4NBF4/CH2Cl2 (0.1 M) using a platinum gauze as working electrode. All the experiments were done under nitrogen atmosphere. Typically the CPE experiments were followed until transfer of the number of coulombs calculated for one D


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Inorganic Chemistry

Figure 1. ORTEP (50%) diagrams of (a) [VIV(L2)2] (2), (b) [VIV(L3)2] (3), (c) [VVO(L1)(OEt)] (5), (d) [VVO(L2)(OEt)] (6), and (e) [VVO(L4)(OEt)] (8). exchange-correlation functional B3P86, which ensures a good degree of accuracy in the prediction of the structures of first-row transition metal complexes.27 The basis set was 6-311g for 1, 3, 4; for 2 polarization and diffuse functions were added on S atom according to the data in the literature.28 The g and 51V A tensors were calculated using the functional PBE0 and VTZ basis set with ORCA software,29 according to the procedures published.30 The theory background was described in detail previously.31 It must be taken into account that for a VIV species the hyperfine coupling constant (A) values can be negative, but in the

electron per mole of compound. Then, the process was interrupted, a cyclic voltammogram obtained, and a sample of the electrolyzed solution collected and immediately transferred to an EPR tube (previously purged with nitrogen gas) and the solution frozen by immersion of the tube in liquid nitrogen. The CPE process was then continued until constant current (end), a new cyclic voltammogram recorded, and another sample of the solution collected for EPR measurements and frozen. DFT Calculations. The geometry of 1−4 was optimized in the gas phase with Gaussian 09 (revision C.01)26 software, using the hybrid E


Article

Inorganic Chemistry Table 2. Selected Bond Distances (Å) and Angles (deg) for Non-Oxido VIV Complexes 2 and 3 distances (Å) V(1)−O(1) V(1)−O(2) V(1)−O(3) V(1)−O(4) V(1)−N(1) V(1)−N(3) N(1)−N(2) N(3)−N(4)

angles (deg)

2

3

1.905(1) 1.909(1) 1.900(1) 1.912(1) 2.076(2) 2.069(1) 1.400(2) 1.389(2)

1.910(1) 1.923(1) 1.902(1) 1.891(1) 2.070(1) 2.087(2) 1.389(2) 1.394(2)

O(1)−V(1)−N(1) O(1)−V(1)−O(2) O(2)−V(1)−N(1) O(3)−V(1)−N(3) O(4)−V(1)−N(3) O(2)−V(1)−O(3) O(1)−V(1)−O(4) O(3)−V(1)−O(4) O(2)−V(1)−O(4) O(1)−V(1)−O(3)

literature their absolute value is usually reported. This simplification was also adopted in this work in a number of points. Time-dependent density functional theory (TD-DFT) calculations32 were used to predict the excited states of compounds 1 and 2 and obtain the expected electronic absorption spectrum. The calculations were carried out on the geometry optimized in the gas phase using CAM-B3LYP and BHandHLYP functionals, available on Gaussian 09, and 6-31+g(d) basis set.33

3

82.33(5) 127.04(5) 74.37(5) 82.54(6) 74.43(5) 86.05(5) 86.49(5) 127.51(5) 137.26(5) 84.23(5)

81.41(6) 131.77(6) 74.50(6) 81.81(6) 73.98(6) 88.36(6) 86.43(6) 131.42(6) 131.69(6) 81.80(5)

Table 3. Selected Bond Distances (Å) and Angles (deg) for Oxido VV Complexes 5, 6, and 8 V(1)−O(1) V(1)−O(2) V(1)−O(3) V(1)−O(4) V(1)−N(1) N(1)−N(2) O(1)−V(1)−N(1) O(1)−V(1)−O(3) O(1)−V(1)−O(2) O(2)−V(1)−O(4) O(2)−V(1)−O(3) O(2)−V(1)−N(1) O(3)−V(1)−O(4) O(3)−V(1)−N(1) O(4)−V(1)−N(1)

■

RESULTS AND DISCUSSION Complexes [VIV(L1−4)2] (1−4) and [VVO(L1−4)(OEt)] (5− 8). Synthesis. Scheme 1 shows the pathways through which the non-oxido vanadium(IV), VVO, and V2VO3 complexes were obtained. The monomeric [VIV(L1−4)2] compounds (1−4) were synthesized by the reactions of aroylazines (H2L1−4) with [VIVO(acac)2] in CH3CN. The similar reaction in ethanol led to the formation of the monomeric [VVO(L1−4)(OEt)] (5−8). The reactions are clean, affording pure crystalline products in good yield (∼70%). The stability of these non-oxido vanadium(IV) complexes may be due to the simultaneous complete charge neutralization and the formation of six strong covalent bonds at the VIV center with the N and O donor atoms of the sterically hindered tridentate ligands L1−4, which possibly preclude the approach of reagent species within the coordination sphere of the well-protected VIV center in these compounds. These factors allow, in CH3CN, the removal of the O-oxido as water (eq 1):7c,n,o,9c,19d 2H 2L + VIV O(acac)2 → VIV (L)2 + 2Hacac + H 2O

2

5

6

8

1.885(1) 1.926(1) 1.587(1) 1.771(1) 2.126(1) 1.392(2) 80.87(5) 102.78(5) 143.72(4) 95.26(5) 105.67(5) 75.03(4) 102.80(5) 94.28(5) 162.19(5)

1.8385(7) 1.9123(7) 1.592(7) 1.7826(7) 2.0994(8) 1.391(1) 81.28(3) 110.63(3) 132.38(3) 90.48(3) 112.21(3) 74.96(3) 104.72(3) 95.48(3) 158.53(3)

1.833(2) 1.931(3) 1.591(3) 1.755(2) 2.098(3) 1.394(4) 82.4(1) 104.8(1) 138.9(1) 86.4(1) 111.0(1) 74.9(1) 106.4(1) 95.3(1) 155.3(1)

[VIV(L2−3)2] (2 and 3). The molecular structures of [VIV(L2)2] (2) and [VIV(L3)2] (3) are similar and are depicted in Figure 1a,b, and selected geometric parameters are collected in Table 2. The remarkable features of the structures are the trigonal prismatic coordination around the VIV atom: the VIV centers are coordinated by two azine ligands, each corresponding to a NO2 binding set, forming trigonal prismatic structures.7a All azine ligands form one five-membered CN2OV and one sixmembered C3NOV chelate ring. The four V−O bonds around the central vanadium atoms are in the range 1.923(1) to 1.891(1) Å. The other distances and angles are in the expected range as previously reported for several other compounds containing the [VIV(ONO)2] moiety.7a,i,l,x,19d [VVO(L1,2,4)(OEt)] (5, 6, and 8). In complexes [VOL1(OEt)] (5), [VOL2(OEt)] (6), and [VOL4(OEt)] (8), the binding around each VV center corresponds to an O4N donor set (Figure 1c−e). The τ values being 0.31 (5), 0.44 (6), and 0.27 (8), respectively,16,19c,34,35 the geometries may be considered to be closer to a square pyramidal than to a trigonal bipyramidal geometry. Some selected geometric parameters are collected in Table 3. The basal plane is made up by the O-phenolate, O-enolate, N-imine donor atoms from the ligand L and the O atom from deprotonated alkoxide, forming six-membered and fivemembered chelate rings. The apical positions are occupied by the O-oxido atom O(3). The short V(1)−O(3) distances, in the range of 1.592(7)−1.587(1) Å, are within those commonly found in VIVO and VVO complexes.3638 The V−O bond lengths

(1)

The oxidoalkoxidovanadium(V) (i.e., VVO(L)(OEt)) compounds were obtained by refluxing stoichiometric amounts (1:1 mol ratio) of [VIVO(acac)2] and the ligand H2L, in ethanol in open air. During this reaction, VIV is oxidized by oxygen of air and the isolation of alkoxidovanadium(V) complexes highlights the ability of the alkoxido group to stabilize high oxidation states.16 The compounds are quite soluble in CH2Cl2, DMF, and DMSO and sparingly soluble in MeOH, EtOH, and CH3CN. Single-Crystal X-ray Diffraction Analysis of Complexes 2, 3, 5, 6, and 8. The observed elemental (C, H, N) analytical data of all [VIV(L)2] and [VVO(L)(OEt)] are consistent with their expected composition. The coordination mode of the azines in these compounds was confirmed by the determination, by single-crystal X-ray diffraction, of the molecular structure of compounds 2, 3, 5, 6, and 8. These are shown in Figure 1 together with the atom numbering scheme; selected bond distances and angles are collected in Tables 2 and 3. F


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Inorganic Chemistry

wavelength range 423−418 nm are assignable to ligand-tometal charge transfer transitions, whereas the other bands in the higher energy region (320−229 nm) are likely mainly due to ligand centered transitions.16,19a To get more information on the nature and intensity of the transitions, the electronic absorption spectra of 1 and 2 were calculated by TD-DFT methods, according to procedures reported in the literature.41−43 It has been recently observed that a long-range corrected functional, such as CAM-B3LYP, and a half-and-half hybrid functional, such as BHandHLYP, allow a better prediction of a UV−vis spectrum than simple hybrid and stand-alone functionals.33 Even if a quantitative agreement with the experimental results cannot be reached, these types of calculations reproduce well the spectrum, in both the visible and UV regions; in particular, CAM-B3LYP allowed us to predict the experimental spectrum, mainly in the UV region. The comparison between the experimental and the calculated (with CAM-B3LYP and BHandHLYP functional) spectrum for 1 is given as an example in Figure 3; the results for compound 2

follow the order V−O(oxido) < V−O(alkoxido) < V− O(phenolate) < V−O(enolate). These data indicate the stronger binding of the O-alkoxido atom compared to those of phenolate and enolate O atoms.36−38 In complex 5 there is a weak vanadium oxygen interaction between two molecules which are related to each other by a center of symmetry expanding the coordination number of vanadium from five (square pyramid arrangement) to six (distorted octahedron arrangement). This kind of dimerization is shown in Figure S1. IR Spectroscopy. Selected IR data of all the ligands H2L1−4 and their corresponding metal complexes 1−8 are given in the Experimental Section. All ligands possess a band in the range 3648−3331 cm−1 assigned to the −OH of the ketone moiety, which is not found in the corresponding metal complexes due to the coordination. For complexes 4 and 8 the presence of a band in the range 3362−3156 cm−1 is assigned to the pending −OH group attached to the hydrazide moiety of H2L4, which does not take part in coordination. Disappearance of bands for −NH and −CO and appearance of new bands in the range 1244−1238 cm−1 indicate the enolization of these two groups forming a −NC−O− type moiety. The loss of the O-oxido ligand in compounds 1−4 is confirmed by the nonobservation of the strong characteristic ν(VO) stretching in the 900− 1000 cm−1 region, indicating a “bare” vanadium center.37,38 For 5−8 the presence of sharp bands in the range 1001−994 cm−1 is assigned to the VO stretching.19a,39 UV−Vis Spectroscopy. The electronic spectra of 1−8 were recorded in CH2Cl2. The spectral data are summarized in the Experimental Section. Complexes 1−4 possess similar characteristic spectral features and are depicted in Figure 2. There are

Figure 3. Experimental spectrum of 1. The transitions predicted by the functionals CAM-B3LYP (blue) and BHandHLYP (red) are shown by the full lines.

are similar. Differently from VIVO species, for which the bands observed in the visible range are mainly pure d−d transitions with very low oscillator strength (corresponding to molar absorption coefficients lower than 200 M−1 cm−1), for 1 the d− d bands fall in the near IR region (900−1150 nm), whereas the visible absorptions between 400 and 800 nm are mainly ligandto-metal charge transfer (LMCT) transitions, particularly those from the MOs formed by the π orbital of N−N bond perpendicular to the backbone of the ligandtoward the V orbitals dxz, dyz, and dxy. This is in agreement with what was proposed by Raymond and co-workers, i.e., that for non-oxido vanadium(IV) species all the transitions have to be considered LMCT in which the metal orbitals involved are dxy, dxz, dyz, and dx2−y2 (see Table 4).44 The oscillator strength for the excitations in the visible range was generally more than 3 orders of magnitude larger than that for the VIVO complexes (ε values are >10000 M−1 cm−1 at 400 nm). In the UV region, intraligand charge transfer (LLCT) appears, mainly between MOs with π character, which contribute significantly to the total absorption. In Table 4, the most important calculated electronic transitions, expressed by absorption wavelength and oscillator strength, are compared with the experimental results.

Figure 2. UV−vis spectra of compounds 1−4 in CH2Cl2; the solutions were 1 × 10−5 M.

two bands in the 623−499 nm range, among which the lowest energy transition bands around 623−603 nm are situated in the d−d transition region. The intensity (εmax, 8820−4790 M−1 cm−1) of these bands appears too large for pure d−d transitions, which are expected to be much weaker.38 The high intensity of these bands and the analogy with other “bare” vanadium(IV) complexes7a−c,e,h,j,o,p,x,40 suggest that these transitions should be mainly assigned to ligand-to-metal charge transfer bands.41 To the other bands at higher energy (370− 260 nm), ligand centered transitions also could contribute (see below).16 For the VVO complexes 5−8 some of the spectral features are similar. The strong absorptions observed in the G


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Inorganic Chemistry Table 4. Main Calculated and Experimental Electronic Transitions for 1 Down to 250 nm main transition

charactera,b

λc

f × 105d

H-4(α) → L(α) H-4(α) → L+2(α) H(α) → L(α) H(α) → L(α) H-1(α) → L(α) H(α) → L(α) H(β) → L(β) H-8(α) → L(α)/H-1(α) → L+1(α)/H-1(β) → L(β) H-1(α) → L-1(α)/H-1(β) →L(β) H(α) → L+3(α)/H-1(β) → L+1(β) H-2(α) → L+2(α) H-8(α) → L+1(α)/H-4(β) → L+1(α) H(α) → L+7(a)/H-8(β) → L(β) H-14(α) → L(α) H-14(α) → L(α) H-5(α) → L(α)/H-3(β) → L+2(β) H-8(α) → L+1(α)/H-2(β) → L+3(β)

dxy → dxz dxy → dyz π(N−N) → dxz π(N−N) → dxz π(N−N + phen) → dxz π(N−N) → dxz π(N−N) → dxy π(N−N) → dxz/π(N−N) → π(imine)/π(N−N) → dxy π(N−N + phen) → π(imine)/π(N−N) → dxy π(N−N) → π(imine)/π(N−N) → π(imine) π(phen) → dyz π(N−N)→ π(imine)/dxy → π(imine) π(N−N) → dz2/π(imine) → π(imine) O(phen) → dxz O(phen) → dxz π(fur) → dxz/π(phen) → dxz dxy → π(imine)/π(phen) → dyz

1113.7 919.7 605.0 548.1 474.2 455.6 401.4 363.3 360.8 320.4 303.0 289.1 283.1 275.6 269.5 267.6 256.6

10 30 140 280 380 5320 4610 6880 21700 25230 8810 7740 8250 7810 30620 20170 20590

λexptl/εexptlc,e,f

620/5430 499/5100 sh 428/10390 sh 365/23770 319/38610

266/39270

a

Even if MOs are formed by the combination of several atomic orbitals, we indicated for them the orbital which has the largest contribution in the specific MO. bImine indicates RN−NR fragment, phen indicates the ring with the phenolate-O, and fur indicates the furane ring. cλ values measured in nm. dOscillator strength. eε values measured in M−1 cm−1. fsh indicates a shoulder absorption.

detected at ca. −506 ppm, while the remaining species are observed upfield at ca. −512 ppm and −520 ppm. The minor species are tentatively assigned to conformational isomers present in solution,45 but partial decomposition cannot be ruled out. The obtained chemical shifts compare well with those reported and/or calculated for other VVO−hydrazonato complexes.2b,46,47 Electrochemical Properties of Compounds (1−8). The non-oxido VIV complexes (1−4) display, by cyclic voltammetry in TEAP/CH2Cl2 (0.1 M), one anodic and one cathodic quasireversible wave involving transfer of one electron per mole. As a representative example, the cyclic voltammogram of 1 is displayed in Figure 4.

NMR Spectroscopy. 1H and 13C NMR of compounds H2L1−4 were recorded in DMSO-d6, and the data are given in the Experimental Section. The spectra of these free ligands exhibit resonances in the ranges δH = 11.9−10.3 ppm due to −OH, 10.2−9.1 ppm due to −NH, and 2.4−2.3 ppm due to the −CH3 protons. In H2L4 an extra peak is observed at 11.7 ppm due to the −OH group attached to the hydrazide moiety. All aromatic protons of the ligands are clearly observed in the expected region: δH = 8.7−6.5 ppm.16 From the 13C NMR spectra signals for the (CO−N) and [NC(Me)] carbons are observed in the downfield region in the range δC = 166.8−153.9 ppm and 154.9−152.8 ppm, respectively, whereas the carbons due to the methyl groups (−CH3) are found in the upfield region (24.4−19.3 ppm). Furthermore, appearance of signals in the range 153.3−111.0 ppm due to aromatic carbons is in agreement with what might be expected for these compounds.2a The 51V NMR spectra of the VV compounds 5−8 were recorded in CD2Cl2, as well as in DMSO-d6 and DMSO-d6 + EtOH. Several of the spectra obtained for 5 to 8 are included in Figures S2 to S8. The observed 51V shifts (δV) are listed in Table 5. Table 5. 51V NMR Shifts δV (ppm) Obtained for Compounds 5 to 9 in Deuterated Solvents complex 5 6 7 8 9

CD2Cl2 −506 −500 −509 −523 −506

(major), (major), (major), (major), (major),

−512, −521 −509, −516 −513, −519 −530 −521

DMSO-d6 −543, −538, −540, −565, −544

−522 −519 −522 −529

Figure 4. Cyclic voltammogram of complex 1 in TEAP/CH2Cl2 (0.1 M); potential in volts vs SCE.

DMSO-d6 + EtOH −522 −520 −523 −533 −522

The quasi-reversibility character is coherent with the nonoxido VIV arrangement and supports that the “bare” structure is maintained during the redox processes.48 The anodic and cathodic processes are attributed to VIV → VV oxidation and VIV → VIII reduction processes, respectively (Table 6). The potentials of the anodic processes in complexes 1−4 are within the range of the values reported for the oxidation (VIV → V V ) of the related dithiocarbazate non-oxido V IV complexes.33 In contrast, the cathodic processes occur at considerably lower potentials (−0.40 to −0.53 V, Table 6) than those reported for the dithiocarbazate vanadium complexes

The 51V NMR spectra of complexes 5−8 in DMSO-d6 display two sets of bands with an approximate ratio of 1:1 (Table 5 and Figures S6a, S7a, and S8a). These are discussed below in the context of 1H NMR spectra in solution. In the obtained spectra (in CD2Cl2) the resonances are observed in similar chemical shift ranges: a major species is H


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Inorganic Chemistry Table 6. Cyclic Voltammetric Dataa for [VIV(L1−4)2] (1−4), [VVO(L1−4)(OEt)] (5−8), and [VV2O3(L1)2] (9) complex [VIV(L1)2] (1) [VIV(L2)2] (2) [VIV(L3)2] (3) [VIV(L4)2] (4)

E1/2 (VIV → VIII)

E1/2 (VIV → VV)

−0.53

1.13

−0.40

1.10

−0.50

1.05

−0.52

1.20

complex [VVO(L1)OEt] (5) [VVO(L2)OEt] (6) [VVO(L3)OEt] (7) [VVO(L4)OEt] (8) [VV2O3(L1)2] (9)

61.8−62.7 × 10−4 cm−1) for 1−4 are typical of hexacoordinated VIV species.51 EPR spectra of compounds 1−4 at 77 K are characterized by the spin Hamiltonian parameters reported in Table 7; the spectra recorded on 1−4 dissolved in CH2Cl2 are shown in Figure 6. The spectra of hexacoordinated non-oxido VIV complexes can be divided into two groups: (1) those with gz ≪ gx ∼ gy < 2.0023 and Az ≫ Ax ∼ Ay, recently named type 1 spectra with the SOMO (singly occupied molecular orbital) based mainly on V-dxy atomic orbital and geometry close to the octahedral one, and (2) those with gx ∼ gy ≪ gz ∼ 2.0023 and Az ≪ Ax ∼ Ay, type 2 spectra, with the SOMO composed by a mixture of V-dxy, V-dxz, V-dyz, and V-dz2 atomic orbitals and geometry close to the trigonal prism.33 The spin Hamiltonian parameters were calculated by DFT methods and are compared with the experimental values in Table 7. As recently noticed,7s,51,52 the best agreement can be reached using ORCA software, which calculates the second-order spin−orbit effects, whose contribution to A is more important for a “bare” VIV than for VIVO species. It can be noted that DFT calculations correctly predict for 1−4 that Az ≫ Ax ∼ Ay. The percent deviations from the experimental values of Aiso and Az are from −8.8 to −10.6%, and from −4.4 to −7.0%, respectively. The results of the calculations also confirm the formation of a non-oxido structure for compounds 1−4. Recently, some of us noticed that the arrangement of the coordinating tridentate ligand with respect to V, mer or fac, has marked effects on the electronic structure and A values of nonoxido VIV complexes.7s,33,52 If Ω is the angle formed by the two external donors of the tridentate ligand with vanadium (it is 180° for a mer and 90° for a fac isomer), a correlation between the values of Ai (i = x or z, depending on the symmetry of the species) and Ω can be found. This is shown in Figure 7, where the data for 18 “bare” V complexes formed by cyclic ligands forming fac structures (OOO ligands derivatives of cisinositol7y,z,51), open-chain ligands which form intermediate structures (ONO and ONS ligands,7s,33 plus 1−4), and openchain ligands forming mer structures (rigid ONO ligands7x,52) were considered. The structure of the 18 ligands taken into account is reported in Scheme S1 and the list of the values for Ai and Ω in Table S1. It can be observed that the meridional VIV complexes are characterized by Ai (Az) larger than 140 × 10−4 cm−1, the complexes with intermediate geometry by Ai (Ax or Az) in the range 118−128 × 10−4 cm−1, and those with facial arrangement of the ligands by Ai (Ax) smaller than 110 × 10−4 cm−1. As expected for 1−4, which are characterized by Ω from 127.3 to 136.2° (experimental for 2 and 3, calculated for 1 and 4), intermediate values of Ai are predicted and observed. The relatively low values of Ai for non-oxido VIV species were related to the mixing of V-dxy with the excited V-dxz, V-dyz, and V-dz2 orbitals; for an exhaustive explanation the reader is referred to the recent paper by Kundu et al.33 In this work, the electronic structure and molecular orbital composition of compounds 1−4 were evaluated choosing a coordinate system in which the O(enolate)−V−O(enolate) direction is oriented along the z axis and the two N−V− O(phenolate) directions roughly occupy the x and y axes. The energy levels of the molecular orbitals (MOs) derived from V d orbitals are shown in Scheme 2. For clarity, the energy of the MOs is relative to the SOMO, set as reference at 0.0 eV. To interpret Scheme 2, it must be remembered that (i) only the MOs derived from V d atomic orbitals were considered; (ii) for each MO only the total contribution of V atomic orbitals was

E1/2 (VV → VIV)b 0.35 0.31 0.32 0.31 0.34b

In TEAP/CH2Cl2 (0.1 M). Values measured at 100 mV s−1 and quoted in V vs SCE reference electrode. bA ca. 1/2 intensity cathodic wave (Epred = −1.23 V) is observed. a

(−0.085 to −0.106 V).33 Such a trend is consistent with complexes 1−4 being more electron-rich than the dithiocarbazate complexes, a fact that is attributed to the net electron donor ability of the tridentate ligands, that essentially differ between a sulfur atom in dithiocarbazate ligand (ONS)33 and an oxygen atom in the tridentate aroylazine ligands (ONO) in complexes 1−4. In fact, the charge on each ONO ligand anion, calculated by DFT methods, is between −1.872 and −2.009 for 1−4, whereas it is between −1.459 and −1.516 for ONS dithiocarbazate ligands. The electron withdrawal effect of sulfur is also perceptible in complex 2, with a S atom in ligand (H2L2, Scheme 1), that displays a higher potential for the VIV → VIII cathodic process than 1, 3, or 4, although in this case the effect is less pronounced since the sulfur is not the coordinating atom (see below for the characteristics of molecular orbitals and energies involved in complexes 1−4). The VVO complexes 5−8 display one reversible cathodic process in a narrow range of potential (0.31 to 0.35 V, Table 6) which is attributed to the VV → VIV reduction. The cyclic voltammogram of 6 is depicted as a representative example (Figure 5). No reduction process that could be attributed to VIV → VIII was observed for 5−8.

Figure 5. Cyclic voltammogram (partial) of complex 6; potential in mV vs SCE.

EPR Spectra and Electronic Structure of Non-Oxido Vanadium(IV) Complexes 1−4. X-band EPR spectra of 1−4 were recorded in CH2Cl2 solution at 298 K. The data are listed in Table 7. A representative spectrum for 2 is shown in Figure S9, which reveals a well-resolved 8-line hyperfine structure, characteristic of a mononuclear VIV complex (51V, I = 7/2).49,50 The spin Hamiltonian parameters (giso, 1.969−1.972; Aiso, I


Article

Inorganic Chemistry Table 7. Experimental and Calculated Spin Hamiltionian Parameters for 1−4a exptl g values 1 2 3 4

exptl A values

% devb

calcd A values

giso

gx

gy

gz

Aiso

Ax

Ay

Az

Aiso

Ax

Ay

Az

Aiso

Az

1.972 1.971 1.970 1.969

1.982 1.953 1.981 1.984

1.986 1.993 1.992 1.976

1.955 1.955 1.956 1.957

−62.0 −61.8 −62.7 −62.4

c −2.1 −33.4 −34.2

−28.5 −13.4 −27.6 −19.2

−126.9 −126.9 −126.9 −124.6

−55.8 −55.9 −56.0 −56.9

−27.6 −28.2 −28.5 −28.7

−21.8 −21.5 −21.4 −22.9

−118.0 −118.0 −118.2 −119.1

−10.0 −9.5 −10.6 −8.8

−7.0 −7.0 −6.9 −4.4

a Values of A reported in 10−4 cm−1 units. bPercent deviation from the experimental values calculated as 100 × [(|Ai|calcd − |Ai|exptl)/|Ai|exptl], with i = iso or z. cValue not measurable.

been already reported for “bare” VIV structures.7u,v,53 The order of V d orbitals is as follows: dxy < dxz < dyz < dz2 < dx2−y2. The mixing of the V orbitals is particularly evident in Figure 8, where it is observable that the MOs formally derived from d orbitals do not occupy only one plane but have components on all of the three axes (see, for example, V-dxy, Figure 8a, which is not confined in the xy plane). In Figure 8 are shown all the MOs deriving from V d atomic orbitals for compound 1. It is interesting to observe that the contribution to SOMO of V-dxy for 1−4 (60.0−66.8%) is significantly higher than that observed for a recent series of three non-oxido VIV species formed by dithiocarbazate-based tridentate ONS ligands (29.0−30.0%).33 In that recent study, it was demonstrated that the mixing between V-dxy and the V d orbitals with components along the z axis results in a lowering of the largest value of 51V hyperfine coupling constant (as the absolute value).33 The values of A experimentally measured for 1−4 (124.6−126.9 × 10−4 cm−1) are in agreement with the higher V-dxy contributions to the SOMO than the VIV species formed by the dithiocarbazate ONS ligands (A in the range 119.6− 120.0 × 10−4 cm−1).33 Solution Behavior of Oxidoethoxidovanadium(V) Complexes (5−8). IR studies of the VVO compounds 5−8 were carried out in DMSO/CHCl3. The IR spectra in solution in the ranges 1009−1004 and 970−965 cm−1 depict two bands assigned to VO stretching, which suggests that two different VO moieties exist in solution.17 Other new bands probably corresponding to the ν(V−O−V)16,19a,46 were also observed in the range 822−817 cm−1,39a which were not observed in the spectra of 5−8 in the solid state. The IR spectra in solution suggest partial retention of their mononuclear nature, similar to that in the solid state, but with the simultaneous formation of the corresponding dinuclear complexes (with a structure similar to 9, see below), as a second distinct vanadium-containing species. These observations suggesting that two species may be formed in solution require adequate explanation. The 1H NMR spectra of 5−8 in DMSO-d6 yielded two close but separate sets of bands in an approximate 1:1 ratio. A representative spectrum of 7 is shown in Figure 9a. The set of sharp 1H NMR signals correspond to the monomeric form of species [VVO(L1−4)(OEt)] 5−8, which were isolated in the solid state and in a few cases structurally characterized. The somewhat less sharp signals were assigned to the corresponding second species. These NMR results also indicate that two VVcontaining species exist in DMSO solution. The 1H NMR spectra of 5−8 exhibit two sets of singlets corresponding to the −CH3 group in the range 3.0−2.9 ppm, and that of 8 exhibits two sets of OH resonances at 11.8 and 11.4 ppm. The aromatic protons of 5 and 6 (a total of 18), 7 (a total of 26), and 8 (a total of 24) appear in the range 9.3−6.6 ppm. In addition, all VV complexes exhibit separate bands corresponding to bound Oethoxido in the ranges 5.7−5.4 (−CH2) and 1.6−1.5 (−CH3)

Figure 6. First derivative of the X-band EPR spectra of complexes 1−4 in CH2Cl2 measured at 77 K.

Figure 7. Ai (as the absolute values) for non-oxido VIV complexes as a function of the angle Ω. The pink squares indicate complexes 1−4. Ai (i = x or z) are the largest values of the 51V hyperfine coupling tensor A. The dotted gray line represents the best linear fitting of the 18 points. For the structure of the ligands and the exact values of Ai and Ω the readers are referred to Scheme S1 and Table S1.

taken into account; (iii) the percent amount reported refers to the weight of the specific V d orbital with respect to the total V d contribution. For example, in the MO LUMO+6 of 1 the percent contribution of V d orbitals is 52.2% and, of this amount, 44.4% belongs to dz2 (i.e., the relative contribution of V-dz2 orbital with respect to the total V contribution is 85.1%). Therefore, at a first approximation it can be considered that LUMO+6 derives from V-dz2. The other MOs derived from V have contributions from other d orbitals. In particular, SOMO has the main contribution from V-dxy but also from V-dxz, V-dyz, and V-dz2. The mixing of V-dxy and V-dz2 in the SOMO has J


Article

Inorganic Chemistry Scheme 2. Relative Energy Levels of V d Orbitals for Compounds 1−4a

a

In bold the V d atomic orbital that mostly contributes to the MO is shown. The total percent contribution of the V d orbital with respect to the total V contribution in the specific MO is indicated within parentheses.

ethanol, the alcoholic proton is not detected, probably because of the lower concentration or overlap of the signals. Upon addition of a few drops of ethanol to the DMSO-d6 solution of complexes 5−8, the second set of bands disappeared and a clear spectrum resulted (Figure 9b). The spectrum exhibits only one OH resonance at 11.8 ppm for 8, one set of −CH3 proton signals in the range 3.0−2.9 ppm for 5−8, and signals for a total of nine aromatic protons for 5 and 6, 13 for 7, and 12 for 8 in the range 9.2−6.7 ppm. In addition, the coordinated ethoxido bands in the ranges 5.7−5.4 ppm (−CH2) and 1.6−1.5 ppm (−CH3)11a,13a,16,17 broadened, and the intensity of free ethanol bands in the ranges 3.5−3.4 ppm (due to −CH2) and 1.1−1.0 ppm (due to −CH3) increased. In agreement with the 1H NMR data, the 51V NMR spectra of complexes 5−8 in DMSO-d6 display two sets of bands with an approximate ratio of 1:1 in the ranges (i) −565 to −538 ppm and (ii) −529 to −519 ppm (Table 5 and, e.g., Figure S7a), and the second set of bands disappeared upon addition of small amounts of ethanol (Figure S7b). Equivalent observations were also obtained from 13C NMR data (Figure S10). With compounds 5−8 in CD2Cl2 the major 51V NMR bands are recorded at −523 to −500 ppm (Figures S2 to S5). So, the NMR results of compounds 5−8 in DMSO-d6 support the existence of a second species in solution. Most probably this second VV species is the corresponding dinuclear complex as it is evidenced by the IR as well as 51V NMR spectrum of 9, which is described in a later section. In CD2Cl2 one major VV species is detected corresponding to the mononuclear complexes. The slightly different δV values observed for this species in different solvents can be explained by solvation effects.54 For example, with oxidized solutions of [VIV(sal(chan))] (H2sal(chan) = N,N′-salicyl-R,R-cyclohexane-

Figure 8. Molecular orbitals for 1: (a) SOMO (mixing of V-dxy, V-dz2, V-dxz, and V-dyz); (b) LUMO (mixing of V-dxz, V-dyz, and V-dxy); (c) LUMO+2 (mixing of V-dyz, V-dxz, and V-dx2−y2); (d) LUMO+6 (mixing of V-dz2 and V-dxy); (e) LUMO+9 (mixing of V-dx2−y2, V-dyz, and V-dxz). The hydrogen atoms were omitted for clarity. The three Cartesian axes are also indicated.

ppm11a,16,17 and the sharp peaks observed in the range 3.5−3.4 (−CH2) and 1.1−1.0 (−CH3) ppm were assigned to free, nonligated ethanol that is most likely formed in situ in solution. Although the chemical shifts of the methylene and methyl protons of the ethanol are very close to those of the free K


Article

Inorganic Chemistry

Figure 9. 1H NMR spectrum of complex 7 in DMSO-d6: (a) before addition of ethanol; (b) after addition of ethanol.

μ-oxido dimer) in solution. The confirmation of the second species as μ-oxidodioxidodivanadium(V) was further evidenced by the isolation of the complex 9 in the solid state. Further details of synthesis and characterization of this dinuclear complex are described below. Complex [V V 2 O 3 (L 1 ) 2 ] (9). Synthesis. The μoxidodioxidodivanadium(V) compound [VV2O3(L1)2] (9) was obtained (Scheme 1) from [VVO(L1)(OEt)] (5) in CH2Cl2/ CH3CN mixture. The reaction afforded pure crystalline products in good yield (∼70%). Compound 9 is highly soluble in CH2Cl2, DMF, and DMSO and sparingly soluble in MeOH, EtOH, and CH3CN. Single-Crystal X-ray Diffraction Analysis of Complex 9. Compound 9 yielded suitable crystals as the solvate VV2O3L2· 0.5CH2Cl2. A view of the binuclear complex is shown in Figure 10, and selected bond parameters are listed in Table 8. The geometry around the V atom is similar to that in 5, 6, and 8, but here the pyramidal base is constituted by ONO of L2− and the bridging atom O(4). Both metal atoms have distorted square pyramidal geometry13b,19a,56 with a O4N donor set, and the basal positions are occupied by the donor atoms O(1)/O(5), N(1)/N(3), and O(2)/O(6) from the tridentate ligand L2− and the bridging atom O(4), the two coordination pyramids being metrically distinct. The VO distances 1.586(2) Å [V(1)− O(3)] and 1.585(1) Å [V(2)−O(7)] are typical of V−oxido bonds.13b,19a,38,56 The angles at the vanadium centers subtended by the terminal and bridging oxido atoms, that is, O(3)−V(1)−O(4) [106.52(7)°] and O(7)−V(2)−O(4) [105.97(7)°], are in the range expected for a nearly ideal square pyramidal geometry around V(1) and V(2), respectively (τ = 0.03 (V1) and τ = 0.05 (V2)). Most of distances and angles (Table 8) are in the expected range as reported for compounds containing the [V V 2 O 3 (ONO) 2 ] moiety.13b,19a,46,56,57

diaminium), differences of δV values of ca. 11 ppm were measured between the δV(CH2Cl2) and δV(DMSO), and a linear solvation energy relationship (LSER) analysis with the Kamlet−Taft55 parameters and the chemical shift of the VV complex in five solvents allowed it to be concluded that the dipolarity/polarizability of the solvent was the most important factor for the differences observed.54 In the present case, for complexes 5−8 the corresponding differences measured for the δV(CH2Cl2) and δV(DMSO) are of ca. 11 to 16 ppm. The solution behavior of 5−8 could be ascribed to the equilibrium shown in Scheme 3, established in DMSO/CHCl3 Scheme 3. Schematic Diagram for the Interconversion Involving 5 (Monomeric) and the Corresponding Dinuclear Species 9 in Solution

containing a small amount of water (it is assumed that the possible water source was either from NMR solvents or from water molecules in the air that were probably physisorbed on the surface of the complexes, and/or in the glass containers where experiments were carried out), which also explains the simultaneous presence of the two species, M (monomer, isolated in the solid state and characterized by X-ray crystallography) and D (second species, generated in situ as L


Article

Inorganic Chemistry

Figure 10. ORTEP (50%) diagram of [VV2O3(L1)2] (9), showing the atom labeling scheme.

DMSO-d6 indicates the presence of 9 in solution obtained upon dissolving 5. When a drop of ethanol was added, all peaks corresponding to complex 9 disappeared and a clean spectrum was observed corresponding to the mononuclear VVO complex, [VVO(L1)(OEt)] (5), the one isolated in the solid state. The 51 V NMR of 9 was recorded in CD2Cl2, as well as in DMSO-d6. As an example the spectrum in CD2Cl2 is depicted in Figure S12. Electrochemical Properties of μ-Oxidodioxidovanadium(V) Complex [VV2O3(L1)2] (9). The cyclic voltammogram of 9 (Figure S13) displays one reversible cathodic process. The potential (Table 6) of the reversible cathodic process (0.34 V, vs. SCE) is well in the range of the potentials reported for the V V,V → V IV,V reduction processes in the related μoxidodioxidodivanadium(V) with ONO tridentate ligands.13b,58 Chronocoulometry of 5 and 9 and Subsequent EPR Studies. Controlled potential electrolysis (chronocoulometry) of complexes 5 and 9 was carried out at suitable potentials to allow VV → VIV reduction in Bu4NBF4/CH2Cl2 (0.1 M) solutions. Chronocoulometry of 5 and 9 was followed until the number of coulombs transferred achieved the expected value for one electron per mole of compound. Then, the cyclic voltammogram of each solution was recorded (using a Pt wire electrode), a sample of the electrolyzed solution was collected for EPR measurements, and the chronocoulometry was allowed to proceed until constant current. At this point, a new cyclic voltammogram was recorded and another sample of the electrolyzed solution collected. Figure 11 shows the recorded cyclic voltammograms (a) and chronocoulometry data (b) obtained on 9.

Table 8. Selected Bond Distances (Å) and Angles (deg) for 9 V(1)−O(1) V(1)−O(2) V(1)−O(3) V(1)−O(4) V(1)−N(1) N(1)−N(2) O(1)−V(1)−O(3) O(1)−V(1)−O(4) O(1)−V(1)−N(1) O(2)−V(1)−N(1) O(2)−V(1)−O(4) O(3)−V(1)−O(4) O(1)−V(1)−O(2) O(4)−V(1)−N(1) V(1)−O(4)−V(2)

1.833(2) 1.971(1) 1.586(2) 1.798(2) 2.082(2) 1.397(2) 102.11(7) 106.45(7) 82.88(7) 74.53(7) 83.38(7) 106.52(7) 147.41(7) 149.08(7) 109.81(8)

V(2)−O(5) V(2)−O(6) V(2)−O(7) V(2)−O(4) V(2)−N(3) N(3)−N(4) O(5)−V(2)−O(7) O(5)−V(2)−O(4) O(5)−V(2)−N(3) O(6)−V(2)−N(3) O(6)−V(2)−O(4) O(7)−V(2)−O(4) O(5)−V(2)−O(6) O(4)−V(2)−N(3) O(6)−V(2)−O(7)

1.811(2) 1.961(2) 1.585(1) 1.824(2) 2.090(2) 1.399(3) 104.21(7) 105.97(7) 82.97(7) 75.14(7) 82.67(7) 105.97(7) 146.55(7) 149.44(7) 104.18(7)

Spectroscopy. The spectral (IR, UV−vis, and NMR) data of 9 are summarized in the Experimental Section and Table 5. In the IR spectrum, a pair of sharp bands observed at 999 and 963 cm−1 are assigned to the ν(VO), and the moderately strong band at 823 cm−1 is assigned to the V−O−V asymmetric bridge vibration.13a,39a,46 The UV−vis spectral features of 9 resemble those of the mononuclear VVO(OEt) complexes 5−8. The strong absorption observed at 427 nm is assignable to LMCT transitions, whereas the other bands at higher energy (292−230 nm) are likely to be due to ligand centered transitions.16,19a The 1H NMR spectrum of 9 (Figure S11) shows a multiplet in the range 8.20−6.63 ppm for nine aromatic protons and a singlet at 2.92 ppm due to a −CH3 group attached in the ligand fragment. Comparison of the 1H NMR spectra of 5 and 9 in

Figure 11. (a) Cyclic voltammograms of 9: () before electrolysis; (---) upon ca. one electron per mole was transferred; (···) at the end of electrolysis; potential in volts vs SCE. (b) Charge (Q) and current variation during electrolysis. M


Article

Inorganic Chemistry

during the entire electrochemical reduction process (at least as far as might be concluded by EPR). The parameters obtained for this species are consistent with an (OOAr, Oazn, Nimine, Owater)eq donor atom set (Oazn = O-aroylazinato), which is comparable to what was reported for related VIV O− hydrazonato complexes.59,60 A water ligand should occupy the axial position. Conversely, two species were detected by EPR during the reduction of 9 (Figures 13 and S15). After the full reduction was achieved, the signals for both species presented not much different intensities (Figure 13). No indication of spin coupling of the two 51V nuclei was detected under the conditions used for the EPR measurements. The DFT calculations (Table 9) suggest the formation of a square pyramidal species A with a (OOAr, Nimine, Oazn, OH2O)eq binding set and an octahedral species B with a (OOAr, Nimine, Oazn, OH2O)eq(OH2O)ax binding set. The axial coordination of an axial water may indeed decrease significantly the value of Az.31b For these species the Az calculated by DFT methods is slightly underestimated, according to the previous results.31c In Scheme 4 a summary of

On the electrolyzed samples of complexes 5 and 9, evidence for VIVO species is clearly observed in the EPR spectra; the full EPR spectra of the reduced forms of complexes 5 and 9, obtained during the coulometric reduction, are included in Figures S14 and S15. The high-field regions of the spectra are depicted in Figures 12 and 13, and the spin Hamiltonian parameters obtained by simulation of the spectra are listed in Table 9.

Figure 12. High-field region of the first derivative of the EPR spectrum (at 77 K) of a solution of 5 upon electrochemical reduction in CH2Cl2 at room temperature: (a) after one electron per mole was transferred (calculated value) by the electrochemical reduction; (b) after the current completely stabilized; (c) a few minutes after the electrolysis was finished. The corresponding full spectra are depicted in Figure S14.

Scheme 4. Possible Pathways for the Coulometric Stepwise Reduction of 9 and the Formation of Mononuclear VIVO Species A (Az = 166.5 × 10−4 cm−1) and B (Az = 159.4 × 10−4 cm−1) in CH2Cl2 (0.1 M TEAP)a

Figure 13. High-field region of the first derivative of the EPR spectrum (at 77 K) of the solution of 9 upon electrochemical reduction in CH2Cl2 at room temperature: (a) after one electron per mole was transferred by the electrochemical reduction; (b) after two electrons per mole were transferred by the electrochemical reduction; (c) a few minutes after the electrolysis was finished. The corresponding full spectra are depicted in Figure S15.

a

Solv indicates a general molecule of solvent (water in the DFT calculations).

the processes taking place is depicted: as a result of the reduction and protonation, the μ-O bridge of 9 is broken and two mononuclear VIVO complexes A and B are formed.

The obtained spin Hamiltonian parameters were compared with those obtained from the DFT calculations. In the case of 5 (Figures 12 and S14), only one VIVO species was observed

Table 9. Experimental and Calculated Spin Hamiltonian Parameters Obtained for the Solutions of the Electrochemically Reduced Compounds 5 and 9 in CH2Cl2, at 77 K complex

gx, gy

Ax, Ay (×104 cm−1)

gz

Az (×104 cm−1)

Acalcd (×104 cm−1) z

binding seta

5 first species

1.977

54.9

1.949

−159.2

−158.9

(OOar, Nimine, Oazn, OH2O)eq(OH2O)ax

9 first species (A)

1.979

59.3

1.950

−159.4

−158.9

(OOar, Nimine, Oazn, OH2O)eq(OH2O)ax

1.949

−166.5

−163.8

(OOar, Nimine, Oazn, OH2O)eq

−173.3

(OOar, Oazn, OH2O, OH2O)eq(Nimine)ax

second species (B)

a

The binding set assumed in the DFT calculations. N


Article

Inorganic Chemistry

exhibit all the characteristic spectroscopic signals of a μ-oxido bridged VV2O3 species, which agree well with results reported earlier.16,57 Chronocoulometric and subsequent EPR studies of solutions of 5−8 in CH2Cl2 did not allow us to detect the formation of mixed-valence μ-oxido VIV,V complexes in equilibrium with the mononuclear species. Probably a dinuclear VIV species is formed by transfer of two electrons, which subsequently yields two distinct monomeric V IV complexes, detected and characterized by EPR spectroscopy and DFT calculations.

Finally, it is noteworthy that the voltammograms are less influenced than the EPR spectra by the presence of two species in solution, and this depends on the small differences in the structure of A and B and by the different resolution potential of the instrumental signals. Indeed, a reduction of the Az value of about 5% (from ca. 167 × 10−4 cm−1 of A to ca. 159 × 10−4 cm−1 of B) is easily detectable in an EPR spectrum, but the corresponding effect is not expected to be measurable in cyclic voltammograms.

■

■

CONCLUSIONS Four aroylazine ligands, possessing a rigid bulky 2-hydroxy-1acetonaphthone core and a selection of sterically hindered/ bulky naphthoyl derivatives, were synthesized and used in the synthesis of the corresponding mononuclear non-oxido vanadium [VIV(L)2] (1−4), oxidoethoxidovanadium [VVO(L)(OEt)] (5−8), and dinuclear μ-oxidodioxidodivanadium [VV2O3(L)2] (9) complexes. The “bare” monomeric [VIV(L)2] compounds 2 and 3, not containing an O-oxido atom, were structurally characterized. Besides L being dianionic ligands that bind strongly to vanadium, their bulkiness and aromatic nature impose some “preorganization” of the ligand, these factors probably being relevant for the stability of these non-oxido VIV compounds. Complexes 2 and 3 show the N2O4 donor sets defining a trigonal prismatic geometry. The EPR spectra and DFT calculations provided data indicating that the Az values (between −124.6 and −126.9 × 10−4 cm−1) are intermediate between those of meridional and facial arrangement of the ligands, in agreement with the angles Ω experimentally measured (for 2 and 3) and calculated (for 1 and 4). The spin Hamiltonian parameters as well as the MOs were obtained for 1−4, the calculated EPR and UV−vis spectra giving good agreement with those recorded for the corresponding compounds. All the non-oxido vanadium(IV) complexes 1−4 are quite stable both in the solid state and in solution. Cyclic voltammetric data suggest that the electrochemical oxidations and reductions are metal-centered. The [VVO(L1−4)(OEt)] (5−8) were also successfully synthesized and characterized through various spectroscopic techniques. The single-crystal XRD studies of 5, 6, and 8 revealed that the arrangement of ligands corresponds to distorted square pyramidal geometry (τ values of 0.31 (5), 0.44 (6), and 0.27 (8), respectively), where the O,N,O donor ligand and the O-ethoxido atom constitute satisfactory O3N basal planes. The solution behavior of 5−8 in CHCl3, CH2Cl2, and DMSO was also examined. Two species were observed, 5−8 plus a new dinuclear μ-oxido species (such as 9 also characterized by single-crystal XRD). Addition of a few drops of ethanol to these solutions yielded predominantly single species corresponding to the monomeric complexes [VVO(L1−4)(OEt)] isolated in the solid state. Thus, an equilibrium is established in solution (2 [VVO(L)(OEt)] + H2O ⇆ [(VVO(L))2O] + 2 EtOH), which shifts to the right when compounds 5−8 are dissolved in (not totally dry) CH2Cl2, CHCl3, or DMSO; when EtOH is added, it shifts to the left. From the successful isolation of the VV2O3 complex, 9 in the solid state and its characterization by single-crystal XRD, as well as from the solution study results (UV−vis, NMR, and CV) of mononuclear and the dinuclear VVO complexes, it is clear that the mononuclear VVO complexes 5−8 and the V2O3 complexes may both exist in solution. Additionally, the dinuclear species

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02346. Experimental data, spectra (51V, 13C, and 1H NMR; EPR), and CV (PDF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF) Crystallographic data for 5 (CIF) Crystallographic data for 6 (CIF) Crystallographic data for 8 (CIF) Crystallographic data for 9 (CIF) The crystallographic data for the structural analysis of 2, 3, 5, 6, 8, and 9 have been deposited with the Cambridge Crystallographic Data Centre. CCDC No. for 2 is 969041, for 3 is 1009872, 5 is 1419322, 6 is 969042, 8 is 969044, and 9 is 1004184. A copy of this information may be obtained free of charge from the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (tel, + 44 (0) 1223 762911; e-mail, [email protected]. uk).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the reviewers for their comments and suggestions, which were helpful in preparing the revised version of the manuscript. Funding for this research was provided by the Department of Science and Technology, India [Grants SR/ WOS-A/CS-145/2011 (S.P.D.) and SR/FT/CS-016/2008 (R.D.)]. R.D. thanks Ekkehard Sinn, M. Reichelt, and P. S. Mukherjee for assistance with X-ray diffraction analysis. J.C.P. thanks the Portuguese Foundation for Science and Technology (FCT), (UID/QUI/00100/2013), RECI/QEQ-QIN/0189/ 2012), and RECI/QEQ-MED/0330/2012. E.G. thanks Fondazione Banco di Sardegna for the financial support (Project Prot. U924.2014/AI.807.MGB; Prat. 2014.0443). The X-ray diffractometer was funded by EFRD as part of the Operational Programme Development of Eastern Poland 2007−2013 (project No. POPW.01.03.00-20-034/09-00).

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