Salan Complexes: Insight into the Pathway to the ... - ACS Publications

Oct 18, 2016 - In solution, as demonstrated by 1H and 51V NMR investigations, the structures of complexes 1–3 are consistent with their solid state...
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Oxidation of 1‑Methyl-1-phenylhydrazine with Oxidovanadium(V)− Salan Complexes: Insight into the Pathway to the Formation of Hydrazine by Vanadium Nitrogenase Ewa Kober, Zofia Janas,* and Julia Jezierska Faculty of Chemistry, University of Wrocław, 14, F. Joliot-Curie, 50-383 Wrocław, Poland S Supporting Information *

ABSTRACT: A series of oxidovanadium(V) complexes [VO(L-κ4O,N,N,O)(OR)] (1a, R = Et, L = L1; 1b, R = Me, L = L1; 2, R = Me, L = L2; 3, R = Me, L = L3) were synthesized by the σ-bond metathesis reaction between [VO(OR)3] and the linear diaminebis(phenol) derivatives H2L (salans) containing different para-substituents on the phenoxo group [CMe3CH2CMe2, L1; Me, L2; Cl, L3]. As shown by X-ray crystallography complexes 1a, 1b, and 2 exhibit cis-α geometry, do have a stereogenic vanadium center, and exist as a racemic mixture of the Δ cis-α and Λ cis-α enantiomers. In solution, as demonstrated by 1H and 51V NMR investigations, the structures of complexes 1−3 are consistent with their solid state. The reactions of 1b, 2, and 3 with NH2NMePh in n-hexane afforded the oxidovanadium(IV) [VO(L-κ4O,N,N,O)] (4, L1; 5, L2; 6, L3) and 1,4-dimethyl-1,4-diphenyl-2-tetrazene (Me2Ph2N4) (7) as the main products together with a small amount of hydrazido(2-) vanadium(V) [V(L3-κ4O,N,N,O)(NNMePh)(OMe)] (8). Proposed reaction course for the oxidation of NH2NMePh by 1b−3 is discussed. Compounds 4−8 were characterized by chemical and physical techniques including the X-ray crystallography for 4, 7, and 8. The solid-state (powder) electron paramagnetic resonance spectra and magnetic features strongly indicate that complexes 4−6 are formed as a mixture of a mononuclear (S = 1/2) and a dinuclear (S = 1) species.

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C6H2O)2]2− R = iPr},7b {L = [MeNCH2CH2NMe(CH2-4,6Me2-C6H2O)2]2−, R = Me},7a {L = [MeNCH2CH2NMe(CH24,6- t Bu 2 -C 6 H 2 O) 2 ] 2− , R = Me}, 7a and [V(O) 2 (HLκ4O,N,N,O)]·3H2O {L = [N,N′-ethylenebis(pyridoxylaminato)]2−},7g as well as for dinuclear [{VO(Lκ4O,N,N,O)}2(μ-O)] {L = [MeNCH2CH2NMe(CH2-4,6-Me2C6H2O)2]2−}7c and L = [PhNCH2CH2NPh(CH2C6H4O)2]2−},7d revealed an octahedral geometry for the central vanadium atom. With the exception of only one complex all complexes adopt cis-α geometry in the solid state, in contrast to the salen counterparts, which due to the planar nature usually form trans conformation.8 In the case of [{VO(L-κ 4 O,N,N,O)} 2 (μ-O)] {L = [PhNCH 2 CH 2 NPh(CH2C6H4O)2]2−} the phenolate oxygen atoms of the L ligand are cis-oriented to each other generating cis-β-type structure.7c The oxidovanadium(IV) complexes [VO(L-κ4O,N,N,O)] {L = [NHCH2CH2NH(CH2−C6H4O)2]2−},7f {L = [N,N′-ethylenebis (pyridoxylaminato)]2−},7g and those containing salan ligands derived from salicylaldehyde and chiral diamines (diaminecyclohexane and diphenylethylenediamine),7c were also synthesized and characterized. The formation of linear structures with ···VO···VO··· interactions in the solid state is postulated pursuant to low μeff and ν(VO) values as well as the EPR spectra of those complexes.7c,f,g Up to now, the application aspect of vanadium−salan complexes has been mainly focused on the oxidation of various organic substrates as

INTRODUCTION Vanadium complexes have received considerable attention from the point of view of their different applications in catalysis and biologically active systems.1−3 A new trend in vanadium chemistry concerns the incorporation of ligands that could create vanadium systems useful either for catalytic processes1,3 or for model compounds mimicking coordination sphere of the active site in vanadium-dependent enzymes.2 Among them, tripodal tetradentate diaminebis(aryloxido) ligands and their linear isomers, associated with vanadium and other metal complexes, have been explored and demonstrated an interesting reactivity.3 The advantage of these ligands is the great modification possibilities of their steric and electronic properties induced by variation of the amine side chain and phenol substituents. The tripodal diaminebis(aryloxido) ligands have proven to form efficient vanadium catalysts for ethene-α-olefin copolymerization4 and to mimic perfectly vanadium-dependent haloperoxidase functionalities in selective oxidation of thioanisole and benzyl alcohol.5,6 The linear diaminebis(aryloxido) ligands, so-called salans as a salen-reduced variant, have increased flexibility, stronger nitrogen donors, and greater resistance to hydrolysis than their Schiff-base analogues. These features, together with the stabilizing character of the hard phenolate oxygen, allowed isolation of a few oxidovanadium(V) and (IV) complexes.7 In most cases, the N-methylated ligands with different substituents at the ortho and para positions of the aromatic rings were employed to create oxidovanadium(V) complexes. The crystal structure data for monomeric [VO(Lκ4O,N,N,O)(OR)] {L = [MeNCH2CH2NMe(CH2-4,6-Me2© XXXX American Chemical Society

Received: May 27, 2016

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DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

radiation (λ = 0.710 73 Å) at 100 K.11 The experimental details and crystal data are given in Table S1 in Supporting Information. The structures were solved by direct methods and refined by the full-matrix least-squares techniques on all F2 data, using the SHELXTL software.12 All non-hydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were placed in geometrically calculated positions and refined using a riding model with Uiso set at 1.2Ueq(C) for aromatic and formic H atoms, and 1.5Ueq(C) for methyl H atoms. In 4·EtOH, the C atoms of methyl groups in the C(CH3)2CH2C(CH3)3 substituent are disordered, and they were refined in two positions with an 0.8:0.2 occupancy. General Procedure for the Synthesis of the Ligand Precursors H2L1−H2L3. A mixture of appropriate para-substituted phenol [4-(1,1,3,3-tetramethylbutyl)phenol for H2L1; 4-methylphenol for H2L2; 4-chlorophenol for H2L3], formaldehyde, and N,N′dimethylethylenediamine at 2:2:1 molar ratio, respectively, in methanol was stirred for 3 d at room temperature. A resulting white precipitate was filtered off, washed with ice-cold methanol, and dried in vacuo to yield a fine white powder. Compounds were purified by recrystallization from a mixture of MeOH/CH2Cl2. H2L1: Yield: 77.0%. Anal. Calcd for C34H56N2O2: C 77.81, H 10.75, N 5.34. Found: C 77.75, H 10.69, N 5.31%. 1H NMR (500 MHz, CDCl3, 298 K): δ 10.27 (br s, 2H, ArOH), 7.15 (dd, 3J = 8.0 Hz, 4J = 1.8 Hz, 2H, C6H3), 7.12 (d, 4J = 1.4 Hz, 2H, C6H3), 6.96 (d, 3J = 8.1 Hz, 2H, C6H3), 3.26 (s, 4H, ArCH2N), 2.18 (s, 4H, NCH2CH2N), 1.83 (s, 6H NCH3), 1.71 [s, 4H, C(CH3)2CH2C(CH3)3], 1.37 [s, 12H, C(CH3)2CH2C(CH3)3], 0.83 [s, 18H, C(CH3)2CH2C(CH3)3]. 13 C NMR (125 MHz, CDCl3, 298 K): δ 156.1, 139.9, 126.8, 126.3, 121.1, 115.9 (C6H3), 61.8 (ArCH2N), 57.1 [C(CH3)2CH2C(CH3)3], 53.7 (NCH2CH2N), 41.1 (NCH3), 37.7 [C(CH3)2CH2C(CH3)3], 37.6 [C(CH3)2CH2C(CH3)3], 32.3 [C(CH3)2CH2C(CH3)3], 31.7 [C(CH3)2CH2C(CH3)3]. IR (mineral oil mulls, cm−1): 2367 (w), 2038 (vw), 1879 (vw), 1781 (vw), 1759 (vw), 1683 (w), 1615 (m), 1596 (m), 1502 (s), 1417 (m), 1378 (s), 1364 (s), 1347 (m), 1312 (m), 1263 (s), 1212 (m), 1185 (m), 1162 (m), 1147 (m), 1130 (s), 1114 (s), 1079 (m), 1016 (m), 980 (m), 958 (m), 931 (m), 906 (w), 882 (m), 821 (s), 775 (m), 737 (m), 723 (m), 692 (w), 652 (w), 605 (m), 549 (vw), 488 (vw), 476 (w), 451 (w), 432 (w). ESI-MS: m/z: [H2L1 + H]+ = 525.4. H2L2: Yield: 87.0%. Anal. Calcd for C20H28N2O2: C 73.12, H 8.60, N 8.53. Found: C 73.08, H 8.58, N 8.51%. 1H NMR (500 MHz, CDCl3, 298 K): δ 10.28 (br s, 2H, ArOH), 6.97 (dd, 3J = 8.1 Hz, 4J = 1.8 Hz, 2H, C6H3), 6.76 (d, 4J = 1.4 Hz, 2H, C6H3), 6.74 (d, 3J = 8.2 Hz, 2H, C6H3), 3.65 (s, 4H, ArCH2N), 2.66 (s, 4H, NCH2CH2N), 2.28 (s, 6H, NCH3), 2.24 (s, 6H, ArCH3). 13C NMR (125 MHz, CDCl3, 298 K): δ 155.5, 129.6, 129.2, 128.3, 121.4, 116.0 (C6H3), 61.9 (NCH2CH2N), 54.3 (ArCH2N), 41.8 (NCH3), 20.6 (ArCH3). IR (mineral oil mulls, cm−1): 3318 (m), 3267 (m), 2935 (s), 2701 (m), 1880 (vw), 1802 (vw), 1752 (vw), 1652 (vw), 1615 (w), 1600 (w), 1546 (vw), 1495 (s), 1415 (w), 1357 (m), 1301 (w), 1273 (m), 1251 (s), 1221 (s), 1194 (m), 1143 (w), 1130 (m), 1115 (w), 1098 (m), 1054 (m), 1020 (m), 996 (w), 968 (w), 944 (w), 889 (w), 848 (w), 819 (s), 766 (m), 752 (w), 720 (vw), 651 (vw), 587 (w), 560 (m), 512 (vw), 455 (w), 431 (vw). ESI-MS: m/z: [H2L2 + H]+ = 329.2. H2L3: Yield: 79.0%. Anal. Calcd for C18H22N2O2Cl2: C 58.54, H 6.00, N 7.59, Cl 19.20. Found: C 58.48, H 5.96, N 7.53, Cl 19.18%. 1H NMR (500 MHz, CDCl3, 298 K): δ 10.45 (br s, 2H, ArOH), 7.12 (dd, 3 J = 8.6 Hz, 4J = 2.5 Hz, 2H, C6H3), 6.93 (d, 4J = 2.5 Hz, 2H, C6H3), 6.76 (d, 3J = 8.6 Hz, 2H, C6H3), 3.65 (s, 4H, ArCH2N), 2.64 (s, 4H, NCH2CH2N), 2.28 [s, 6H, NCH3]. 13C NMR (125 MHz, CDCl3, 298 K): δ 156.7, 129.1, 128.6, 124.1, 123.3, 117.9 (C6H3), 61.6 (NCH2CH2N), 54.2 (ArCH2N), 42.0 (NCH3). IR (mineral oil mulls, cm−1): 3002 (m), 2941 (vs), 2615 (w), 1877 (vw), 1877 (vw), 1798 (vw), 1752 (vw), 1644 (vw), 1605 (w), 1581 (w), 1497 (m), 1481 (vs), 1470 (vs), 1414 (w), 1389 (m), 1354 (w), 1295 (w), 1273 (s), 1264 (s), 1258 (s), 1197 (w), 1182 (w), 1133 (w), 1100 (m), 1086 (w), 1020 (m), 974 (m), 939 (vw), 919 (w), 878 (m), 847 (w), 827 (vs), 789 (vw), 762 (m), 721 (vw), 668 (m), 640 (w), 556 (w), 512 (vw), 476 (w), 450 (w), 432 (vw). ESI-MS: m/z: [H2L3 + H]+ = 370.2.

models of the vanadium-dependent haloperoxidases functionalities. They have been shown to have higher activity than the parent vanadium−salen complexes.7a,c,d Furthermore, the cytotoxic activity studies of vanadium(V) complexes based on the mono- and disubstituted salan ligands toward particular cells showed higher levels than those attained by cisplatin.7b To our knowledge there are no reports on the chemistry of vanadium−salan complexes either in activation of dinitrogen or in binding of its reduced intermediates. So far, model chemistry applied to mimic the structure and function of vanadium nitrogenase has been extensively studied.2c−g In effect, a lot of mononuclear and dinuclear vanadium dinitrogen complexes based on a wide range of ligands are known.2c,d,f,g Nevertheless, no genuine structural and functional nitrogenase mimic has yet been prepared. For many years our interest has laid strongly in the vanadium chemistry with aryloxido- and thiolato-supporting ligand environments in the context of the chemical models of the active center in vanadium nitrogenase.2e,9 It has been demonstrated that S,O- and O-ligated vanadium binds N2H4, NNR2 (R = Me, Ph), and NR (R = SiMe3), intermediate ligand species that might be involved in the reduction process of N2 to NH3. By this means we decided to use the salan ligands with different substituents at the para positions of the aromatic rings to create the vanadium−salan sites for the reduced dinitrogen intermediates. In this work, we report on synthesis, spectroscopic, and structural characterization of a series of oxidovanadium(V)−salan complexes. To gain the knowledge of their potential for coordination of the reduced dinitrogen intermediate species, 1-methyl-1-phenylhydrazine was employed.

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EXPERIMENTAL SECTION

Caution! 1-Methyl-1-phenylhydrazine is acute toxic material. It can cause burns to the skin and eyes and is highly irritating to the mucous membrane. All operations should be conducted in a well-ventilated f ume hood and behind a safety shield. Caution! Tetrazenes are energetic compounds with sensitivity toward heat and impact. Although we had no problems in synthesis, proper protective measures are recommended when undertaking work involving tetrazenes on larger scales. General Information. All operations were performed under a dry dinitrogen atmosphere, using standard Schlenk techniques. All the solvents were distilled under dinitrogen from the appropriate drying agents prior to use. Reagents were purchased from the Aldrich Chemical Co. and used without further purification unless stated otherwise. Ligand precursors H2L2 and H2L3 were prepared by a modified synthetic procedure.10 NMR spectra were performed on Bruker Avance 500 (1H and 13C) and Bruker AMX 300 (51V) spectrometers. Chemical shifts are reported in parts per million and referenced to residual protons in deutered solvents. Infrared spectra were recorded on a PerkinElmer 180 spectro-photometer in mineral oil mulls (Nujol). Electronic paramagnetic resonance (EPR) spectra were measured using a Bruker Elexys E 500 Spectrometer equipped with NMR teslametr and frequency counter at X-band. The experimental spectra were simulated using the computer program VODimer (S = 1), written by Dr. Andrew Ozarowski from NHMFL, University of Florida. The magnetic susceptibility measurements were made on a Quantum Design MPMS-5 SQUID magnetometer (inhouse). A diamagnetic correction factor was estimated using Pascal’s constants. The electrospray mass spectra (ESI-MS) were recorded on a Bruker MicrOTOF-Q mass spectrograph. Microanalyses were conducted on a Vario EL III CHNS Elemental Analyzer (in-house). X-ray diffraction data for 1a, 1b, 4·EtOH, and 7 were collected on an Xcalibur PX diffractometer with CCD Onyx and for 2 and 8 on a KM4 diffractometer with a CCD sapphire camera and Mo Kα B

DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry General Procedure for the Synthesis of Oxidovanadium(V) Complexes [VO(L-κ4O,N,N,O)(OR)] (1−3). Complex [VO(L1κ4O,N,N,O)(OEt)] (1a) was prepared by the reaction of ligand precursor H2L1 with [VO(OEt)3] (1:1 ratio) in ethanol at room temperature. Complexes [VO(L1-κ4O,N,N,O)(OMe)](1b−3) were synthesized in the reaction of H2L1−3 with generated in situ [VO(OMe)3] from [VO(OEt)3] in methanol. The desired product was precipitated from the reaction mixture as a dark violet solid and collected by filtration, washed with cold ethanol or methanol, and dried under vacuum at room temperature. Single crystals of 1a, 1b, and 2 suitable for X-ray studies were grown from the filtrate at room temperature after few weeks. The elemental analysis and 1H NMR spectra for the crude and a crystalline product confirmed them to be the same compound. 1a: Yield 68.5%. Anal. Calcd for C36H59N2O4V: C 68.10, H 9.37, N 4.41. Found: C 68.06, H 9.34, N 4.39%. 1H NMR (500 MHz, CDCl3, 298 K): δ 7.27 (m, 4H, C6H3), 7.06 (m, 2H, C6H3), 5.59 (m, 2H, OCH2CH3), 4.86 (d, J = 14.4 Hz, 1H, ArCH2N), 4.74 (d, J = 13.4 Hz, 1H, ArCH2N), 3.19 (d, J = 14.5 Hz, 1H, ArCH2N), 2.90 (d, J = 13.6 Hz, 1H, ArCH2N), 2.70 (m, 2H, NCH2CH2N), 2.41, 2.30 (s, 3H, NCH3), 1.84, 1.79 [s, 2H, C(CH3)2CH2C(CH3)3], 1.52, 1.46 [s, 6H, C(CH3)2CH2C(CH3)3], 1.49 [m, 3H, OCH2CH3 overlap with 12 H, C(CH3)2CH2C(CH3)3], 1.30 (m, 2H, NCH2CH2N), 0.95, 0.91 [s, 9H, C(CH3)2CH2C(CH3)3]. 13C NMR (125 MHz, CDCl3, 298 K): δ 170.2, 163.3, 146.8, 141.4, 127.2, 127.0, 126.7, 125.4, 124.8, 117.6, 115.2, 113.6 (C6H3), 68.7 (OCH2CH3), 63.4, 58.6 (ArCH2N), 57.5, 57.3 [C(CH 3 ) 2 CH 2 C(CH 3 ) 3 ], 51.3 (NCH 2 CH 2 N), 49.8 (NCH2CH2N), 47.2, 45.3 (NCH3), 38.9, 38.2 [C(CH3)2CH2C(CH3)3], 32.6, 32.0 [C(CH3)2CH2C(CH3)3], 18.6 (OCH2CH3). 51V NMR (75 MHz, CDCl3, 298 K): δ −485. IR (mineral oil mulls, cm−1): ν(VO), 959 (s, shr). ESI-MS: m/z: [1a + H]+ = 635.4. 1b: Yield 57.6%. Anal. Calcd for C35H57N2O4V: C 67.75, H 9.19, N 4.52. Found: C 67.20, H 9.10, N 4.33%. 1H NMR (500 MHz, CDCl3, 298 K): δ 7.19 (dd, J = 8.41, 1.91 Hz, 2H, C6H3); 6.90, 6.88 (d, J = 2.29 Hz, 2H, C6H3), 6.68, 6.63 (d, J = 8.41, 8.81 Hz, 2H, C6H3), 4.99 (s, 3H, OCH3), 4.88 (d, J = 14.3 Hz, 1H, ArCH2N), 4.60 (d, J = 13.4 Hz, 1H, ArCH2N), 3.62 (d, J = 14.5 Hz, 1H, ArCH2N), 3.17 (d, J = 13.5 Hz, 1H, ArCH2N), 3.00 (dt, J = 13.00, 2.68 Hz, 1H, NCH2CH2N), 2.85 (dt, J = 13.00, 2.68 Hz, 1H, NCH2CH2N), 2.61, 2.43 (s, 3H, NCH3), 1.95 (m, 2H, NCH2CH2N), 1.69, 1.67 [s, 2H, C(CH3)2CH2C(CH3)3], 1.36, 1.32 [s, 6H, C(CH3)2CH2C(CH3)3], 0.79, 0.73 [s, 9H, C(CH3)2CH2C(CH3)3]. 13C NMR (125 MHz, CDCl3, 298 K): δ 161.7, 142.8, 140.7, 127.9, 126.5, 126.3, 126.1, 125.7, 124.8, 120.2, 116.7, 115.3 (C6H3), 66.9, 63.4 (ArCH2N), 53.7, 50.6 (NCH2CH2N), 48.5, 47.1 (NCH3), 57.3, 57.5 [C(CH3)2CH2C(CH3) 3], 38.2, 38.0 [C(CH3) 2CH2C(CH3) 3], 32.0, 31.8 [C(CH3)2CH2C(CH3)3]. 51V NMR (75 MHz, CDCl3, 298 K): δ −456. IR (mineral oil mulls, cm−1): ν(VO), 948 (s, shr). ESI-MS: m/z: [1b + H]+ = 621.4. 2: Yield 71.4%. Anal. Calcd for C21H29N2O4V: C 59.44, H 6.84, N 6.60. Found: C 59.40, H 6.80, N 6.60%. 1H NMR (500 MHz, CDCl3, 298 K): δ 7.01 (m, 2H, C6H3), 6.83 (d, J = 8.2 Hz, 1H, C6H3), 6,72 (m, 2H, C6H3), 5.00 (s, 3H, OCH3), 4.86 (d, 2J = 14.6 Hz, 1H, ArCH2N), 4.57 (d, 2J = 13.6 Hz, 1H, ArCH2N), 3.59 (d, 2J = 14.7 Hz, 1H, ArCH2N), 3.14 (d, 2J = 13.7 Hz, 1H, ArCH2N), 3.01, 2.86 (m, 1H, NCH2CH2N), 2.61, 2.41 (s, 3H, NCH3), 2.29, 2.28 (s, 3H, ArCH3), 1.94 (m, 2H, NCH2CH2N). 13C NMR (125 MHz, CDCl3, 298 K) δ: 161.9, 161.8, 130.8, 129.1, 129.3, 129.2, 129.1, 128.3, 125.5, 120.8, 117.4, 116.1 (C6H3), 72.6 (OCH3), 66.6, 63.0 (ArCH2N), 54.0 (NCH2CH2N), 50.9 (NCH2CH2N), 48.7, 47.3 (NCH3), 20.8, 20.6 (ArCH3). 51V NMR (75 MHz, CDCl3, 298 K): δ −475. IR (mineral oil mulls, cm−1): ν(VO), 960 (s, shr). ESI-MS: m/z: [2 + Na]+ = 447.1. 3: Yield 86.0%. Anal. Calcd for C19H23N2Cl2O4V: C 49.13, H 5.00, N 6.04. Found: C 49.11, H 4.99, N 6.03%. 1H NMR (500 MHz, CDCl3, 298 K): δ 7.15 (m, 2H, C6H3), 7.00 (dd, J = 4.6, 2.5 Hz, 2H, C6H3), 6.75 (m, 2H, C6H3), 5.08 (s, 3H, OCH3), 4.80 (d, J = 14.9 Hz, 1H, ArCH2N), 4.53 (d, J = 13.8 Hz, 1H, ArCH2N), 3.60 (d, J = 14.9 Hz, 1H, ArCH2N), 3.15 (d, J = 13.9 Hz, 1H, ArCH2N), 2.88 (m, 2H, NCH2CH2N), 2.61, 2.39 (s, 3H, NCH3), 2.00 (m, 2H, NCH2CH2N). 13 C NMR (125 MHz, CDCl3, 298 K): δ 162.4, 151.5, 129.8, 128.9,

128.5, 128.4, 126.7, 124.9, 123.5, 122.5, 119.0, 117.9 (C6H3), 73.8 (OCH 3 ), 65.9, 62.5 (ArCH 2 N), 54.0 (NCH 2 CH 2 N), 50.9 (NCH2CH2N), 48.8, 47.2 (NCH3). 51V NMR (75 MHz, CDCl3, 298 K): δ −472. ESI-MS: m/z: [3 + Na]+ = 487.0. IR (mineral oil mulls, cm−1): ν(VO), 960 (s, shr). General Procedure for Reactions of 1b−3 with NH2NMePh. To suspensions of 1b−3 in n-hexane was added NH2NMePh (2:1 ratio), and the mixture was stirred at room temperature overnight. The resulting light blue and light brown solids of 4, 5, and 6, respectively, were filtered off, washed with n-hexane, and dried under vacuum. Solids of 4−6 were characterized as follows: 4: Yield 83.8%. Anal. Calcd for C34H54N2O3V: C 69.23, H 9.24, N 4.75. Found: C 69.21, H 9.20, N 4.70%. ESI-MS: m/z: [4 + H]+ = 590.3. IR (mineral oil mulls, cm−1): ν(VO), 960 (s, shr). μeff = 1.12 μB for monomer at 293 K. Crystallization of the crude 4 from a diluted ethanol solution at ∼277 K yielded red crystals of 4·EtOH suitable for a single-crystal Xray diffraction study. 5: Yield 91.9%. Anal. Calcd for C20H26N2O3V: C 61.05, H 6.67, N 7.12. Found: C 61.00, H 6.61, N 7.13%. ESI-MS: m/z: [5 + H]+ = 394.1. IR (mineral oil mulls, cm−1): ν(VO), 947 (s, shr). μeff = 1.05 μB for monomer at 293 K. 6: Yield 57.6%. Anal. Calcd for C18H20N2O3Cl2V: C 49.79, H 4.64, N 6.45. Found: C 49.72, H 4.60, N 6.39%. ESI-MS: m/z: [6 + H]+ = 435.0. IR (mineral oil mulls, cm−1): ν(VO), 960. μeff = 0.98 μB for monomer at 293 K. Isolation of N4Me2Ph2 (7) and [V(L3-κ4O,N,N,O)(NNMePh)(OMe)] (8). Dark brown filtrate obtained during isolation of compound 6 was evaporated to dryness under reduced pressure. The resulting sticky residue was then dissolved in methanol and left for crystallization at 277 K. After a few weeks a mixture of colorless and light brown crystals of 7 and 8, respectively, were obtained. The crystals were separated manually for the X-ray diffraction studies. 7: The isolation of a sufficient amount of chemically pure form of compound 7 failed. The crystals have melted at room temperature; therefore, further characterization was unavailable. 8: Compound 8 was isolated by filtration at room temperature, washed with cold n-hexane, and dried under vacuum. Yield: 21.7% (for the crystalline form). Anal. Calcd for C26H31N4O3Cl2V: C 54.84, H 5.49, N 9.84. Found: C 54.78, H 5.47, N 9.80%. 1H NMR (500 MHz, CDCl3, 298 K): δ 7.13 (m, 2H, C6H3), 7.01 (m, 2H, C6H3), 6.78 (m, 2H, C6H3), 6.68 (m, 1H, C6H5), 6.49 (m, 2H, C6H5), 6.15 (m, 2H, C6H5), 5.31 (d, J = 13.4 Hz, 1H, ArCH2N), 5.09 (s, 3H, OCH3), 4.99 (d, J = 14.6 Hz, 1H, ArCH2N), 3.62 (d, J = 13.3 Hz, 1H, ArCH2N), 3.49 (s, 3H, NNCH3), 3.19 (d, J = 14.5 Hz, 1H, ArCH2N), 2.82 (m, 2H, NCH2CH2N), 2.65, 2.55 (s, 3H, NCH3), 1.97 (m, 2H, NCH2CH2N). 13C NMR (125 MHz, CDCl3, 298 K): δ 162.5, 151.7, 143.5, 131.9, 130.2, 129.6, 128.9, 127.4, 125.9, 123.4, 122.2, 121.3, 120.8, 119.3, 117.8, 116.7 (Ar), 74.8 (NNCH3), 73.7 (OCH3), 65.8, 62.4 (ArCH2N), 54.0, 50.9 (NCH2CH2N), 48.8, 47.2 (NCH3). 51 V NMR (75 MHz, CDCl3, 298 K): δ −531. ESI-MS: m/z: [8 + H]+ = 570.3.

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RESULTS AND DISCUSSION Synthesis of Ligand Precursors H2L1−H2L3. The paramonosubstituted salans were prepared by a modified synthetic procedure previously described for the ortho/para-disubstituted ones.10 A Mannich condensation reaction between N,N′dimethylethylenediamine, formaldehyde, and appropriate substituted phenol at 1:2:2 molar ratio, respectively, in methanol at room temperature gave colorless solids H2L1−H2L3 in high yield (>70%). Compounds H2L1−H2L3 were recrystallized from a mixture of MeOH/CH2Cl2 prior to use for the preparation of vanadium complexes. Their purity was proven by IR and NMR spectra as well as mass spectrometry (ESI-MS). The IR and 1H NMR spectra of H2L1−H2L3 with broad absorptions at ∼3300 cm−1 and broad singlets at ∼10.00 ppm, respectively, suggested the presence of intramolecular hydrogen C

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simulated ESI-MS spectra for the [3 + Na]+ molecular ion is shown in Figure S6 in Supporting Information. Compounds 1−3 bearing the linear tetradentate L1−3 ligands can adopt three general octahedral coordination topologies: trans, cis-α, and cis-β.17 The trans coordination mode is achiral, while the cis-α and cis-β diastereomers exist as a pair of enantiomers. The presence of two different monodentate coligands (O and OR) can generate additional isomers. Moreover, the two internal nitrogens, after coordination to the vanadium atom, can adopt either the same or opposite configurations, increasing the number of possible diastereomers. For 1a, 1b, and 2, X-ray single-crystal diffraction studies were essential to unveil the type of isomer in the solid state. The molecular structure of 2 is shown in Figure 1a (for 1a and 1b,

bonds OH···N in solution. Reported previously an X-ray crystallographic analysis of H2L2 and H2L3 revealed an irrefutable proof for intramolecular hydrogen bonding between the tertiary nitrogen atom and the hydrogen atom of the phenoxo group leading to the formation of a six-membered ring.13,14 This indicates that H2L1 has similar structure to that for H2L2 and H2L3 either in the solid state or in solution. Synthesis and Characterization of the Oxidovanadium(V)−Salan Complexes. The hydrazido(2-) species bound to the vanadium center is believed to be involved in catalytic conversion of N2 to NH3 by the vanadium nitrogenase and model compounds thereof.2e,9,15 Generally, to generate hydrazido(2-) vanadium(V) complexes, condensation reaction of oxidovanadium(V) complexes with substituted hydrazines involving H2O elimination are mostly utilized.9,16 To examine the ability of salan ligands to create vanadium center capable of binding the hydrazido(2-) ligand, a series of oxidovanadium(V) complexes was synthesized. Key reactions based on the σ-bond metathesis reaction between the linear diaminebis(phenol) derivative and an appropriate metal precursor are shown in Scheme 1. The straightforward reaction Scheme 1. Synthetic Strategy for Complexes 1−3

Figure 1. (a) The molecular structure of complex 2 with crystallographic numbering of the donor atoms. Hydrogen atoms are omitted for clarity. (b) The Δ cis-α and Λ cis-α enantiomers in the crystal of 2.

Figures S1 and S2, respectively, in Supporting Information). Selected bond lengths and angles for 1a, 1b, and 2 are given in Table S2 in Supporting Information. All complexes are monomeric and exhibit cis-α geometry with the oxido and alkoxido groups occupying mutually cis coordination sites, and the amine nitrogen atoms are also mutually cis. The two aryloxido oxygen donors coordinate in a trans fashion. This geometry is not surprising, since the most structurally characterized salan−metal complexes are cis-α.7a−d,g,10b,17a,18 For achiral L1−3 ligands the Δ and Λ enantiomers are expected to be formed in a 1:1 ratio,17d and in fact complexes 1a, 1b, and 2 crystallize as rac-[VO(L-κ4O,N,N,O)(OR)] showing Δ and Λ helicity (Figure 1b). The V−Oaryloxido, V−Oalkoxido, and V−N bond lengths in 1a, 1b, and 2 are very similar and within statistical agreement with the corresponding distances in [VO(L-κ4O,N,N,O)(OiPr)] {L = [MeNCH2CH2NMe(CH2-4,6-Me2-C6H2O)2]2−}, [VO(Lκ4O,N,N,O) (OMe] {L = [MeNCH2CH2NMe(CH2-4,6-Me2C 6 H 2 O) 2 ] 2 − , and [MeNCH 2 CH 2 NMe(CH 2 -4,6- t Bu 2 C6H2O)2]2−}.7a−c As expected, the V−N1 bond is significantly longer than the V−N2 as a result of stronger trans interaction of nitrogen with oxido group than with alkoxido one.

of ligand precursor H2L1 with [VO(OEt)3] in EtOH and H2L1−3 with [VO(OMe)3] generated in situ from [VO(OEt)3] in MeOH (Experimental Section) yielded diamagnetic microcrystalline, dark violet solids of [VO(L-κ4O,N,N,O)(OR)] (1a, R = Et, L = L1; 1b, R = Me, L = L1; 2; R = Me, L = L2; 3, R = Me, L = L3). To obtain complexes [VO(L2,3-κ4O,N,N,O)(OEt)], the reactions of H2L2,3 with [VO(OEt)3] were performed in EtOH or n-hexane. Unfortunately, products are well-soluble in organic solvents, and their isolation in a chemically pure form failed. Complexes 1−3 are air-sensitive either in the solid state or in solution, and they are well-soluble in common organic solvents. Their IR spectra exhibit strong bands in the range of 948−960 cm−1 typical for the VO stretches in mononuclear vanadates. The ESI-MS spectrometry was recorded as representative for characterization. The molecular ion peaks appeared at m/z: 635.4 [1a + H]+, 621.4 [1b + H]+, 447.1 [2 + Na]+, 488.0 [3 + Na]+. Experimental and D

DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Reactions of 1b−3 with NH2NMePh

Scheme 3. Proposed Reaction Course for the Oxidation of NH2NMePh by 1b−3

The 1H NMR spectra of compounds 1−3 (see Experimental Section) are similar, reveal only one set resonances of protons due to the symmetry of ligands, and consist of an AB system for either the Ar−CH2-N or NCH2CH2N methylene units of the L1−3 ligands (for 2, Figures S3 and S4 in Supporting Information). Also 51V NMR measurements were performed to probe the effect of coligands (OR) and substituents in the L1−3 ligands on the vanadium center in 1−3. The 51V NMR spectra contain a single resonance at −485 ppm for 1a, −456 ppm for 1b, −475 ppm for 2, and −472 ppm for 3, in the region expected for six-coordinated vanadium complexes with a mixed N,O donor set (for 2, Figure S5 in Supporting Information).2a,d,4,5a,7a,d,19 There is a relatively small spread of resonances (456−485 ppm) within the series of 1−3; either the electron-donating methyl substituent in 2 or the electronwithdrawing chloride substituent in 3 show similar 51V chemical shifts. A small increase in the electron-donating capability of the (CH3)3CH2C(CH3)2 substituent in 1b in comparison with

methyl substituent in 2 was reflected by the chemical shift at the lower fields. Evidently, variation of the electronic nature of substituents at the phenyl ring in para position has only a minor effect on the shielding at vanadium center in 1−3. Similar trend has been observed for the octahedral oxidovanadium(V) complexes based on the tetra- and pentadentate amine/ diaminetris(aryloxido) ligands containing the phenolate substituents in the ortho and para positions.20a,b This contrasts with larger effect of either ortho or para substituents on the 51V resonances of the five-coordinate arylimido series [VCl3(NAr)], [VCl2(OEt) (NAr)], [V(OiPr)3(NAr)], and [VL(NAr)] (H3L = triethanolamine), which span the ranges of 258−429, 4−146, 549−648, and 224−340 ppm, respectively.20c−f Presumably the higher coordination number in 1−3 series and the buffering effect of the tetradentate ligands L1−3 contribute to the relatively small range of resonances. However, the coligands (OR) cause higher effect on the 51V nuclear shielding. It is wellseen in the case of the pair complexes 1a and 1b containing E

DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

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(V−NNH) intermediate produces hydrazine via additional electron transfer and protonation step (Scheme 4, pathway

ethoxido and methoxido coligands, respectively, the oxygen atom of the ethoxido group becomes less electronegative, decreases the 51V nuclear shielding, and causes the lower-field shift (1a: −485 ppm; 1b: −456 ppm). Moreover, the variabletemperature 1H and 51V NMR analysis of 1−3 do not show any significant differences (in CDCl3 from 233 to 313 K) confirming that their geometry is essentially retained in solution within the temperature range studied. Reactions of Oxidovanadium(V)−Salan Complexes 1b−3 with 1-Methyl-1-phenylhydrazine (NH2NMePh). The reactions of 1b−3 with NH2NMePh (2:1 molar ratio) in nhexane at room temperature gave light blue or light brown solids of oxidovanadium(IV) complexes [V(O)(Lκ4O,N,N,O)] (L1, 4; L2, 5, L3, 6; 58−92%) and 1,4-dimethyl1,4-diphenyl-2-tetrazene, Me2Ph2N4 (7) as the main products (Scheme 2). Besides, the formation of a small amount of vanadium(V) hydrazido(2-) derivatives was also observed (Scheme 2). Attempts to isolate them in a chemically pure form succeeded only in the case of the reaction between 3 and NH2NMePh. The light brown crystals of [V(L3-κ4O,N,N,O)(NNMePh)(OMe)] (8) together with the colorless crystals of 7 were isolated from the filtrate (see Experimental Section). These results indicate that the redox process involving the oxidation of the NH2NMePh to the 2-tetrazene (7) and the reduction of 1b−3 to the oxidovanadium(IV) complexes 4−6 predominate over the condensation reaction. Scheme 3 depicts proposed reaction course for the oxidation of NH2NMePh by 1b−3 in nhexane. It is quite likely that the first step is deprotonation of hydrazine to the hydrazido(2-) group with the simultaneous protonation of the RO group, liberated as ROH from the vanadium coordination sphere. This could enable the hydrazido(2-) group to coordinate to the vanadium centers in a bridging μ-NNMePh manner to form dinuclear intermediate species A. Although the species A was not isolated, the suggestion of its existence in the reaction mixture appears to be reasonable in the light of results achieved by Paul R. Sharp et. al for the hydrazido(2-) gold systems.21 The mechanism of transformation of well-defined gold(I) μ3-hydrazido(2-) complexes [Au3(μ3-NNR2)(L)3]+ (R = Ph, Me; L = PPh3) to the bitetrahedral gold(I,0) [Au6(PPh3)6] cluster and the 2-tetrazene (R4N4) has been well-documented. Additionally, the bridging coordination of the hydrazido(2-) ligand to the vanadium centers has been confirmed in the vanadium complex [NH2Me2][VCl3)2(μ-NNMe2)3]9e,16c The second step of the reactions in Scheme 3 could be the reductive elimination of the N-nitrenes (B) to form oxidovanadium(IV) complexes 4−6, which exist as mononuclear and dinuclear species with different contribution according to their EPR spectra in the solid state. Finally, the N-nitrenes (B) undergo dimerization to form the 2tetrazene (7) in accordance with the “normal” oxidation of disubstituted hydrazines; the so-called “abnormal” oxidation reaction would lead to the liberation of N2 including the formation of the appropriate hydrocarbons.22a−d The fact that 2-tetrazene (7) is generated via the oxidation of NH2NMePh by the vanadium(V) complexes 1b−3 can provide valuable insights into the mechanism of N2 reduction by the vanadium enzyme. In contrast to the molybdenum nitrogenase, the vanadium one can convert N2 not only to bioavailable ammonia but also to hydrazine as a byproduct.2a,c,h,15d According to a plausible mechanism for the reduction of dinitrogen by Schrock’s vanadium systems, branching at the diazenido(1-)

Scheme 4. Alternative Pathways for the Formation of NH2NH2 by Vanadium Nitrogenase

a).15a Taking into account our results and the fact that the parent tetrazene N4H4 decomposes into N2 and hydrazine,22f the reductive elimination of the N-nitrenes (N−NH2 ↔ N NH2) ought to be considered as an alternative step, responsible for the production of hydrazine by vanadium nitrogenase (Scheme 4, pathway b). This step does not require additional electron and proton transfer. Despite the fact that the oxidovanadium complexes 1b−3 do not meet requirements to mimic the vanadium center in the FeV cofactor, which lacks the oxido group, their participation in the formation of 7 is noteworthy in both the synthetic and biomimetic contexts. Characterization of Oxidovanadium(IV)-, Hydrazido(2-), and Vanadium(V)−Salan Complexes and 2-Tetrazene Compound. Compounds 4−6 were fully characterized by analytical and spectroscopic methods, including the magnetic susceptibility studies. The IR spectra of 4−6 present ν(VO) at ca. 947−960 cm−1, these high stretching frequencies indicate that the polymeric ···VIVO···VIVO··· linear interaction in the solid state does not occur.7c,f,g,23 The ESI-MS spectrometry was recorded as representative for characterization in solution. The molecular ion peaks appeared at m/z 590.4 [4 + H]+, 394.1 [5 + H]+, and 435.0 [6 + H]+, and the observed isotopic distributions are in agreement with these assigned formulations. Experimental and simulated ESIMS spectra for the [5 + H]+ and [6 + H]+ molecular ions are shown in Figures S7 and S8, respectively, in Supporting Information. The μeff values of 1.12 μB for 4, 1.05 μB for 5, and 0.98 μB for 6 are substantially lower than expected for monomeric oxidovanadium(IV) compounds {d1, μeff (spin only) = 1.73 μB; 1.970 μB for gav = 1.964 for vanadyl [VO]2+ complexes}. The presence of ferromagnetic interaction between two oxidovanadium(IV) cores would lead to the increase of magnetic moments. The ferromagnetic dinuclear oxidovanadium(IV) were observed when their VO groups are oriented antiparallel or twisted in respect to the coordination planes.24 It is noteworthy that the oxidovanadium(IV) dimer [{VO(μ-L-κ4O,N,N,O)}2] {L = Me2NCH2CH2N(CH2-4-R-C6H3O)2, R = C(CH3)2CH2C(CH3)3} bearing the tripodal isomer of the salan ligand L1 was identified crystalographically and that its VO groups are oriented antiortogonally.25 The antiorthogonal or syn-orthogonal orientation of the VO groups results in anti-ferromagentic interaction due to direct overlap between magnetic dxy orbitals, decreasing the magnetic moment in accordance with our observation. The EPR spectra provide unambiguous proof that the dinuclear and monomeric species with S = 1/2 and S = 1 spin states, respectively, are formed for the studied complexes 4−6. It suggests that the anti-ferromagnetic interactions in a dimeric form of 4− 6 lead to the observed decrease of the magnetic moments and that their geometry is similar as compared to that for [{VO(μ-L-κ 4O,N,N,O)}2] {L = F

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disproportionation of 4 analogously as in the case of [VO(salen)] complexes in polar solvents.27 The X-ray study of 4·EtOH monocrystal revealed its mononuclear structure, which indicates that the crystallization in ethanol destroyed its dimeric form. The molecular structure of 4·EtOH is shown in Figure 3, and selected bond lengths and angles are collected in

Me2NCH2CH 2N(CH2-4-R-C6H3O)2, R = C(CH3)2CH2C(CH3)3}.25 The solid-state (powder) EPR spectra of complexes 4−6 are presented in Figure 2.

Figure 3. Molecular structure of complex 4·EtOH with crystallographic numbering of the donor atoms. Hydrogen atoms, except H4 engaged in a hydrogen-bond interaction, are omitted for clarity.

Figure 2. EPR spectra of the powdered compounds 4−6 together with the theoretical spectrum of 6 (sim 6) simulated with spin Hamiltonian for S = 1 and parameters given in the text.

Table S2 in Supporting Information. The coordination environment around the vanadium atom is a distorted square pyramid with the trigonality index τ = 0.10. The distortion is best illustrated by the variation of the N1−V−O3 and N2−V− O1 angles of 144.64(7) and 150.70(7), respectively. The two aryloxido oxygens (O1, O3) and two tertiary amine nitrogens (N1, N2) of the L1 ligand bounded to vanadium(IV) form the basal plane of the square pyramid, and the oxido (O2) atom occupies the apical site. The vanadium atom sits 0.568 Å out of the plane defined by N1−N2−O1−O3. The V−O2 oxido distance of 1.5916(16) Å is typical for five-coordinate vanadyl species.28,29 The observed V−Oaryloxido bond lengths are unequal [V−O1, 1.9286(16) Å; V−O3, 1.9408(16) Å] due to a different engagement in an intermolecular hydrogen-bond interaction with the H atom of EtOH molecule implicated by the O1···H4−O4 and O3···H4−O4 distances of 3.290(3) and 2.823(3) Å with the angles of 130° and 162°, respectively. This also finds reflection in the V−Namine distances; the V−N2 bond length [2.183(2) Å] is longer than the V−N1 bond distance [2.149(2) Å] as a result of stronger trans interaction of the O1 and N2 atoms than the O3 and N1 one. Obviously, the V−N distances are distinctly longer than the V(IV)−Nimine bond distances in many vanadium(IV) Schiff-base complexes, which are expected of sp3 and sp2 hybridization, respectively, for the tertiary amine and imine nitrogen atoms.8 A search of the current Cambridge Crystallographic Database does not provide any examples of the salan-based oxidovanadium(IV) complexes structurally characterized. Thus, to our knowledge complex 4 is the first crystallographically characterized example of a vanadyl compound bearing the salan ligand. It is worthy to underline that the 2-tetrazene (7) was isolated for the first time in a crystalline form allowing to determine its molecular structure by the X-ray diffraction. Previously, 7 has been obtained as a yellow-orange oil or colorless plates in the oxidation reaction of MePhNNH2 by Me3SiCl, Me3SiSiMe3, and MePh2SiSiPh2Me in the presence of potassium hydride or mercuric oxide and by action of diazo salts on aromatic sulphonamides.30 The molecular structure of 7 is shown in Figure 4, and selected bond distances and angles are presented in Table S2 in Supporting Information. Compound 7 is composed of a four-

The theoretical (simulated) spectrum calculated using spin Hamiltonian for S = 1 spin state allows to discriminate the signals due to resonance transitions within S = 1 states (split in zero magnetic field) from that associated with monomeric oxidovanadium(IV) species. For 4, the almost symmetric line at ∼3500 G is similar to that usually observed for undiluted diamagnetically S = 1/2 compounds, for example, for VOSO4· H2O powder, and should be assigned to strongly dominant monomeric form; the forbidden |ΔMs| = 2 line at ∼1660 G becomes visible only as a result of ∼20 times amplification. For 5 the line due to S = 1/2 corresponding to the mononuclear form decreases over these associated with resonance transitions within the S = 1 states and disappears for 6. It is noteworthy that for 6 the resolution of hyperfine splitting of approximately one-half of the value for the mononuclear compound results from interaction with two V nuclei (∑I = 7/2 + 7/2) and is distinctly seen for |ΔMs| = 2 signal at ∼1600 G additionally providing the spin state S = 1. The best fitting of the experimental spectra of 6 was obtained using S = 1 spin Hamiltonian parameters gx = 1.985, gy = 1.974, gz = 1.945, D = 0.007 cm−1, E = 0.040 cm−1, Ax = 20 × 10−4 cm−1, Ay = 30 × 10−4 cm−1, and Az = 77 × 10−4 cm−1, the parameters close to those found for another oxidovanadium(IV) dinuclear complexes.26 Similar parameters without hyperfine coupling constants were used for simulation of 5 spectrum. The hyperfine splitting (due to two vanadium nuclei) in EPR spectrum of 6 indicates that the centers in S = 1 state are diluted by diamagnetic ones in S = 0 state in a much greater population. To unveil the solid-state nuclearity of 4−6 by X-ray singlecrystal diffraction studies, recrystallization from different organic solvent (toluene, benzene, CH2Cl2, CH3CN, EtOH) was performed. Red crystals of 4·EtOH were obtained from a diluted ethanol solution of 4, left at 253 K under anaerobic conditions for 1 h. However, when the crystallization was prolonged for a longer time, complete transformation of 4 to the crystalline form of 1a occurred. It means that compound 4 is not stable in EtOH. Excluding the oxidation of 4 by accidental incorporation of air into the crystallized sample the formation of 1a most probably occurs via the proton-induced G

DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

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CONCLUSIONS In this paper we presented a series of oxidovanadium(V) complexes 1−3 based on the tetradentate linear diaminebis(aryloxido) ligands (salans), comprising of different substituents at position para to the phenolic oxygen atom. Complexes 1−3 are monomeric, exhibit cis-α geometry, and do have a stereogenic vanadium center. No significant electronic and steric effects of substituents on the structural parameters of 1−3 in the solid state and in solution are observed. Complexes 1−3 showed unprecedented reactivity toward substituted hydrazine NH2NMePh. The oxidation of NH2NMePh to 2-tetrazene (7) and reduction of 1b−3 to oxidovanadium(IV) complexes 4−6 have been successfully proven. It is highly probable that the lack of substituents at ortho position in the L ligands plays a vital role in the reduction of vanadium center in 1b−3. Nevertheless, extension of investigation on the oxidovanadium(V) complexes containing ortho- and para-substituted salan ligands are needed to provide additional support. A small quantity of expected hydrazido(2-) vanadium(V) species, a result of the condensation reaction between NH2NMePh and the oxido group in 1b− 3, has been also isolated. However, the formation of bridging V2-μ-N-NMePh species via deprotonation of hydrazine by the coligand OMe in 1b−3 is postulated to lie on the plausible pathway to the 2-tetrazene, (7). The generation of 7 by vanadium complexes seems to have a crucial contribution to better understanding of the biological formation of hydrazine.

Figure 4. Molecular structure of 7 with crystallographic numbering.

nitrogen chain with a central double bond. The torsion angle N1−N2−N2′−N1′ of 180° shows that the molecular structure adopts an E configuration about the azo N2N2′ bond and is in good agreement with those previously found, for example, in 1,4-di-tert-butyl-1,4-bis(chlorophenyl)-2-tetrazene,22e 1,4-bis(1methyltetrazol-5-yl)-1,4-dimethyl-2-tetrazene, 22h 4,4′-bis(morpholine)-2-tetrazene,22j and 1,4-bis(hydroxyethyl)-1,4-diphenyl-2-tetrazene and its bis(trimethylsilyl)derivative.22h The geometry and the N−N and NN distances of 1.364(3) and 1.268(2) Å, respectively, in 7 closely resemble those of other tetrazenes.22e−j The molecular structure of compound 8 is depicted in Figure 5. Selected bond parameters associated with metal ion are listed

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01283. Additional crystallographic data for 1a, 1b, 2, 4, 7, 8, the 1 H, 13C, and 51V NMR spectra for 2, the ESI-MS spectra for 3, 5, and 6 (PDF) XRD data for 1a, 1b, 2, 4, 7, and 8 (PDF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF)

Figure 5. Molecular structure of complex 8 with crystallographic numbering. Hydrogen atoms are omitted for clarity.

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AUTHOR INFORMATION

Corresponding Author

in Table S2 in Supporting Information. Compound 8 has essential octahedral geometry at vanadium with the V− Oaryloxido, V−Oalkoxido, and V−N average distances of 1.905, 1.840, and 2.274 Å, respectively, very similar to those in 1 and 2. The NNMePh ligand is nearly linear [angle N3−N4−V 168.08(15)°] with the V−N distance of 1.684(3) Å very close to those in [V{N(CH2CH2S)3}(NNMe2)][1.681(3) Å],9e [V{O(CH2CH2S)2}X(NNMe2)] [X = OSiMe3, 1.678(2) Å; X = OC6H3iPr2, 1.675(3) Å],9c [VL(NNR2)] [L = tris(2phenolato)amine; R = Me, 1.689(2) Å; R = Ph, 1.683(4) Å]16a and longer than in tetrahedral [V(OAr)3(NNMe2)] [OAr = OC6H3iPr2; 1.654(3) Å].9d The N−N distance in the hydrazido(2-) ligand [1.331(2) Å] is in the range generally found in hydrazido(2-) complexes of the early transition metals including other structurally characterized vanadium complexes.9,16,20

*E-mail: zofi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Scientific Centre (Poland; Grant No. 2012/05/N/ST5/00697) for financial support of this work. We are grateful to Prof. T. Lis and Dr. A. Bieńko (Univ. of Wrocław) for their assistance with the X-ray crystallography and the magnetic measurements, respectively.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01283 Inorg. Chem. XXXX, XXX, XXX−XXX