Oxidovanadium(v) complexes with l-proline-based amino acid phenolates

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Oxidovanadium(v) complexes with l-proline-based amino acid phenolates © 2019 Elsevier B.V. Accepted version (Final draft) Salonen, Pasi; Peuronen, Anssi; Sinkkonen, Jari; Lehtonen, Ari Salonen, P., Peuronen, A., Sinkkonen, J., & Lehtonen, A. (2019). Oxidovanadium(v) complexes with l-proline-based amino acid phenolates. Inorganica Chimica Acta, 489, 108-114. https://doi.org/10.1016/j.ica.2019.02.011 2019 1 Oxidovanadium(V) complexes with L-proline-based amino acid phenolates Pasi Salonena, Anssi Peuronena,b, Jari Sinkkonenc, Ari Lehtonena* a) Group of Inorganic Materials Chemistry, Department of Chemistry, University of Turku, FI-20014, Turku, Finland b) Laboratory of Inorganic Chemistry, Department of Chemistry, University of Jyväskylä, FI-40014, Jyväskylä, Finland c) Instrument Center, University of Turku, FI-20014, Turku, Finland *Ari Lehtonen Department of Chemistry, Group of Inorganic Materials Chemistry University of Turku FI-20014, Turku, Finland E-mail: [email protected] L-proline was used to prepare chiral, tridentate amino acid phenol proligands H2L1—4. These proligands react with vanadium precursors VO(acac)2, VOSO4 ∙ 5 H2O and VO(OPr)3 in methanol to form the corresponding oxidoalkoxidovanadium(V) complexes 1—4. The complexes crystallize from methanol, and are octahedrally coordinated with a general formula [VO(L1—4)(OMe)(MeOH)]. In solution, however, they adopt several different conformations or isomeric structures depending on the solvent. Highlights • Tridentate amino acid phenol proligands were made of L-proline. • Proligands react with various vanadium precursors to form oxidoalkoxidovanadium(V) complexes. • In solution, the complexes adopt different conformations or isomeric structures. • The solid-state structures resemble the active site found in vanadium bromoperoxidase. 2 1. Introduction The coordination chemistry of vanadium is generally studied due to the biological relevance and catalytic properties of different vanadium-based systems.[1–3] Especially, bioinorganic chemistry of vanadium, i.e. the central role of vanadium in certain haloperoxidases (VHPOs) and nitrogenase enzymes as well as the insulin-like effects of vanadium compounds have motivated synthetic chemists to prepare model compounds for structural studies and reactivity tests.[4] For example, the active sites of VHPOs have been modelled by vanadium phenoxides and catecholates. There are also some examples on the catechol oxidase mimicking activity of molecular vanadium(IV) and vanadium(V) complexes.[5–9] Moreover, artificial oxidovanadium(V) complexes can be used as catalysts in a number of organic oxidation reactions, e.g. sulfoxidation, epoxidation and oxidative bromination.[10–13] To date, some of the most active reported oxidovanadium(V) complexes for epoxidation, sulfoxidation and oxidative halogenation – that model vanadium haloperoxidases structurally and functionally – are based on tetradentate atrane-like aminotrisphenolate ligands.[14,15] In synthetic coordination chemistry, the environment of the metal centre can be modified by using diverse ligands. The different electronic and steric properties of ligands may have a remarkable effect on the reactivity of the metal species. In this regard, aminophenolates, and aminobisphenolates in particular, are a large family of well-known multidentate ligands that have found extensive use in the coordination chemistry of most transition metals.[16] This is because they are relatively easy to synthesize and as such they are easily modified to offer different steric and electronic properties. Although many complexes based on these ligands exist, aminophenols with amino acid side arm coupled to phenols, or amino acid phenols, are scarcely reported. These kinds of amino acid-based ligands can better mimic natural biomolecules, which comprise most enzyme active sites, including vanadium haloperoxidases (figure 1). 3 Figure 1. The oxidovanadium(v) centre of vanadium-depended bromoperoxidase as found in brown seaweed Ascophyllum nodosum (left).[17][18] General structure of the oxidoalkoxidovanadium(V) complexes prepared in this study (right). To this end, in the present work, we have used the amino acid phenol derivatives of L-proline as ligands to introduce a biomimetic environment around the oxidovanadium(V) centre. Herein we describe the preparation and characterization of four oxidoalkoxidovanadium(V) complexes derived from disubstituted (2-hydroxyphenol)-L-prolines. 2. Experimental All syntheses and manipulations were performed under ambient conditions. All solvents were of analytical grade unless stated otherwise and were used as received. All chemicals used in syntheses or catalysis were from commercial sources, of reagent grade unless stated otherwise, and used as received. IR spectra were recorded using a Bruker VERTEX 71 FTIR spectrometer equipped with an RTDLaDTGS detector. 32 scans were performed in ATR mode for each individual measurement with the Harrick VideoMVP accessory, in which the sample is compressed against a diamond. UV-Vis and CD spectra were recorded in MeOH, MeCN or CHCl3 using a ∅ 1 cm quartz cuvette with an Agilent CARY60 or a Chirascan qCD spectrometer. High-resolution mass spectra were recorded with a Bruker Daltonics MicrOTOF-Q II electrospray ionization time-of-flight mass spectrometer in both negative and positive ionization mode. For a complete list of IR, UV-Vis, CD and mass spectra and data, refer to the electronic supplementary information. 2.1. Syntheses 4 (3,5-di-tert-butyl-2-hydroxybenzyl)-L-proline, (H2L1). This proligand was synthesized following a modified published procedure.[19] 20 mmol 2,4-di-tert-butylphenol (4.13 g), 20 mmol paraformaldehyde (0.60 g) and 20 mmol L-proline (2.30 g) were suspended in 30 ml technical methanol in a 100 ml round-bottomed flask equipped with a magnetic stir-bar and a reflux condenser. The reaction mixture was stirred, heated to boil and refluxed for 16 hours. The target compound was obtained as a slightly purple powder after purification via flash chromatography using CH2Cl2: MeOH (ca. 95:5 V:V) eluent mixture. Proligands H2L2—4 were synthesized analogously. See electronic supplementary information for more details. Yield: 5.31 g (80 %). M.p. ca. 160 °C (decomposes). 1H NMR (CDCl3, 400 MHz, TMS) δ 9.01 (s, 2H), 7.31 (d, 1H, J = 2.2 Hz), 6.92 (d, 1H, J = 2.2 Hz), 4.62 (d, 1H, J = 13 Hz), 3.90 (d, 1H, J = 13 Hz), 3.79 (t, 1H, J = 7.0 Hz), 3.41 (m, 1H), 2.79 (q, 1H, J = 10 Hz), 2.30 (m, 2H), 1.95 (m, 2H), 1.41 (s, 9H), 1.27 (s, 9H). 13C NMR (CDCl3, 400 MHz, TMS) δ 173.70, 153.38, 141.86, 138.09, 125.26, 124.81, 120.12, 67.62, 56.84, 53.06, 35.03, 34.16, 31.59, 29.86, 28.73, 23.01. IR (KBr, cm–1): 2952s, 2905m, 2866m, 1709s, 1611s, 1481s, 1445s, 1361s, 1302s, 1230vs, 1201vs, 876m, 725m, 689m, 649m. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 283 (2660). (3-tert-butyl-2-hydroxy-5-methylbenzyl)-L-proline, (H2L2). Yield: 4.07 g (70 %). M.p. ca. 135 °C (decomposes). 1H NMR (CDCl3, 400 MHz, TMS) δ 7.97 (s, 2H), 7.06 (d, 1H, J = 1.5 Hz), 6.72 (d, 1H, J = 1.5 Hz), 4.53 (d, 1H, J = 13 Hz), 3.82 (d, 1H, J = 13 Hz), 3.74 (t, 1H, J = 7.3 Hz) 3.38 (m, 1H), 2.76 (m, 2H), 2.22 (s, 3H), 1.93 (m, 2H), 1.38 (s, 9H). 13C NMR (CDCl3, 400 MHz, TMS) δ 173.73, 153.51, 138.67, 128.91, 128.54, 128.45, 120.74, 67.52, 56.55, 53.15, 34.71, 29.77, 28.75, 23.05, 20.73. IR (KBr, cm–1) 2949s, 2915m, 2867m, 1712m, 1623s, 1481s, 1444s, 1390s, 1359s, 1298s, 1241s, 1203s, 862s, 762s, 688s, 588m, 528m. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 283 (2060). (5-tert-butyl-2-hydroxy-3-methylbenzyl)-L-proline, (H2L3). Yield: 3.50 g (60 %). M.p. ca. 145– 160 °C (decomposes). 1H NMR (CDCl3, 400 MHz, TMS) δ 7.97 (s, 2H), 7.06 (d, 1H, J = 1.5 Hz), 6.72 (d, 1H, J = 1.5 Hz), 4.53 (d, 1H, J = 13 Hz), 3.82 (d, 1H, J = 13 Hz), 3.74 (t, 1H, J = 7.3 Hz), 3.38 (m, 1H), 2.76 (m, 2H), 2.22 (s, 3H), 1.93 (m, 2H), 1.38 (s, 9H). 13C NMR (CDCl3, 400 MHz, TMS) δ 173.73, 153.51, 138.67, 128.91, 128.54, 128.45, 120.74, 67.52, 56.55, 53.15, 34.71, 29.77, 28.75, 23.05, 20.73. IR (KBr, cm–1) 2949s, 2915m, 2867m, 1712m, 1623s, 1481s, 1444s, 5 1390s, 1359s, 1298s, 1241s, 1203s, 862s, 762s, 688s, 588m, 528m. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 283 (2060). (2-hydroxy-3,5-dimethylbenzyl)-L-proline, (H2L4). Yield: 4.71 g (94 %). M.p. ca. 145°C (decomposes). 1H NMR (CDCl3, 500 MHz, TMS) δ 8.77 (s, 2H), 6.91 (d, 1H, J = 1.2 Hz), 6.70 (d, 1H, J = 1.2 Hz), 4.41 (d, 1H, J = 13 Hz), 3.92 (d, 1H, J = 13 Hz), 3.86 (t, 1H, J = 6.7 Hz), 3.45 (m, 1H), 2.82 (q, 1H, J = 8.8 Hz), 2.29 (m, 2H), 2.23 (s, 3H), 2.18 (s, 3H), 1.92 (m, 2H). 13C NMR (CDCl3, 500 MHz, TMS) δ 172.64, 152.81, 132.84, 128.65, 128.55, 126.39, 118.38, 68.05, 56.27, 53.38, 28.90, 23.31, 20.32, 16.46. IR (KBr, cm–1) 2952m, 2917m, 2869m,1709m, 1629s, 1490s, 1387s, 1356s, 1304s, 1221vs, 1157s, 1040m, 1007m, 861s, 744m, 682m, 567m. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 285 (2500). Complexes 1—4 were prepared following identical procedures, although X-ray quality crystals were obtained only for 1 and 4. The syntheses were repeated several times, and the reported values represent the average yield for each complex. Different vanadium precursors, i.e. VOSO4 ∙ 5 H2O, VO(acac)2 or VO(OPr)3 gave practically similar results. If VOSO4 ∙ 5 H2O was used, then two equivalents of triethylamine was added as well. [VO(L1)(OMe)(MeOH)], 1. 1 mmol of vanadium precursor and 1 mmol H2L1 were dissolved in 15 ml methanol, heated to ca. 60 °C and stirred for two hours in an open 50 ml roundbottomed flask. Brown crystals suitable for single crystal x-ray diffraction were obtained in two to three days at –25 °C in methanol. Yield: 0.15 g (33 %) 1H NMR (MeOH-d4, 500 MHz, TMS) δ 7.44 (d, 1H, J = 2.3 Hz), 7.17 (d, 1H, J = 2.3 Hz), 4,05 (dd, 1H, J1 = 8.5 Hz, J2 = 2.3 Hz), 3.81 (d, 1H, J = 12 Hz), 3.53 (d, 1H, J = 12.0 Hz), 3.37 (m, 1H), 3.21 (m, 1H), 2.39 (m, 2H), 2.06 (m, 1H), 1.89 (m, 1H), 1.50 (s, 9H), 1.34 (s, 9H). 13C NMR (MeOH-d4, 500 MHz, TMS) δ 181.27, 163.84, 143.20, 134.72, 123.90, 123.87, 123.79, 72.34, 57.47, 56.18, 48.47, 34.80, 33.89, 30.75, 29.61, 26.14, 21.32. 51V NMR (MeOH-d4, 500 MHz, VOCl3) δ –553.73, –493.35, –466.36. IR (KBr, cm–1) ~3400wb, 2954s, 1668s, 1617m, 1473m, 1441m, 1360m, 1236s, 1125s, 1050vs, 976s (V=O), 845s, 754s, 581vs, 539s, 484s. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 257 (8750), 313 (6010), 353 (6440), 637 (1440). ESI-MS+: [VO(L1)(OMe) + Na]+ m/zcalcd. = 452.161, m/zfound. = 452.168. ESI-MS–: [V(O)2(L1)]– m/zcalcd. = 414.149, m/zfound. = 414.133. Black single crystals suitable for X-ray diffraction of the dimeric 1’ were obtained from moist acetonitrile at –25 °C. 6 [VO(L2)(OMe)(MeOH)], 2. Yield: 0.20 g (52 %). 1H NMR (MeOH-d4, 400 MHz, TMS) δ 7.20 (d, 1H, J = 1.9 Hz), 6.96 (d, 1H, J = 1.9 Hz), 4.05 (dd, 1H, J1 = 8.5 Hz, J2 = 2.3 Hz), 3.78 (d, 1H, J = 12 Hz), 3.47 (d, 1H, J = 12 Hz), 3.38 (m, 1H), 3.19 (m, 1H), 2.37 (m, 2H), 2.32 (s, 3H), 2.05 (m, 1H), 1.88 (m, 1H), 1.48 (s, 9H). 13C NMR (MeOH-d4, 400 MHz, TMS) δ 181.26, 164.01, 135.29, 129.79, 127.61, 127.48, 124.17, 72.30, 57.37, 55.70, 48.46, 34.44, 29.56, 28.99, 26.08, 21.26, 19.60. 51V NMR (MeOH-d4, 400 MHz, VOCl3) δ –493.95, –465.53, IR (KBr, cm–1) 2956w, 2899w, 2871w, 2800w, 1640m, 1592s, 1465w, 1442w, 1389m, 1357w, 1301w, 1238s, 1052s, 972s (V=O), 835s, 619vs, 602s. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 258 (9060), 318 (6040), 352 (6710), 640 (1630). ESI-MS+: [VO(L2)(OMe) + Na]+ m/zcalcd. = 410.114, m/zfound. = 410.120. ESIMS–: [V(O)2(L2)]– m/zcalcd. = 372.102, m/zfound. = 372.100. [VO(L3)(OMe)(MeOH)], 3. Yield: 0.32 g (83 %). 1H NMR (MeOH-d4, 400 MHz, TMS) δ 7.29 (d, 1H, J = 2.1 Hz), 7.14 (d, 1H, J = 2.1 Hz), 4.06 (dd, 1H, J1 = 9.1 Hz, J2 = 2.8 Hz), 3.80 (d, 1H, J = 12 Hz), 3.50 (d, 1H, J = 12 Hz), 3.25 (m, 1H), 3.17 (m, 1H), 2.39 (m, 2H), 2.36 (s, 3H), 2.05 (m, 1H), 1.88 (m, 1H), 1.33 (s, 9H). 13C NMR (MeOH-d4, 400 MHz, TMS) δ 181.34, 163.78, 143.76, 127.76, 123.27, 123.07, 122.52, 72.21, 57.02, 55.50, 48.45, 33.58, 30.69, 30.69, 26.01, 21.03, 14,83. 51V NMR (MeOH-d4, 400 MHz, VOCl3) δ –554.19, –494.90, –456.16. IR (KBr, cm–1) 3168wb, 2950m, ~2900w, 1650ms, 1474m, 1345m, 1252m, 1214m, 1050s, 1031s, 973s (V=O), 835m, 777m, 600vs, 552s. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 260 (8870), 316 (6340), 348 (6180), 635 (1150). ESI-MS+: [H2L3 + Na]+ m/zcalcd. = 314.173, m/zfound. = 314.189. [VO(L3)(OMe) + Na]+ m/zcalcd. = 410.114, m/zfound. = 410.118. ESI-MS–: [V(O)2(L3)]– m/zcalcd. = 372.102, m/zfound. = 372.098. [VO(L4)(OMe)(MeOH)], 4. Yield: 0.28 g (75 %). 1H NMR (MeOH-d4, 400 MHz, TMS) δ 7.29 (d, 1H, J = 2.1 Hz), 7.14 (d, 1H, J = 2.1 Hz), 4.06 (dd, 1H, J1 = 9.1 Hz, J2 = 2.8 Hz), 3.80 (d, 1H, J = 12 Hz), 3.50 (d, 1H, J = 12 Hz), 3.25 (m, 1H), 3.17 (m, 1H), 2.39 (m, 2H), 2.36 (s, 3H), 2.05 (m, 1H), 1.88 (m, 1H), 1.33 (s, 9H). 13C NMR (MeOH-d4, 400 MHz, TMS) δ 181.34, 163.78, 143.76, 127.76, 123.27, 123.07, 122.52, 72.21, 57.02, 55.50, 48.45, 33.58, 30.69, 30.69, 26.01, 21.03, 14.83. 51V NMR (MeOH-d4, 400 MHz, VOCl3) δ –554.19, –494.90, –456.16. IR (KBr, cm–1) 3168wb, 2950m, ~2900w, 1650ms, 1474m, 1345m, 1252m, 1214m, 1050s, 1031s, 973s (V=O), 835m, 777m, 600vs, 552s. UV-Vis (MeCN; λmax / nm (ε / M–1 cm–1)) 260 (8870), 316 (6340), 348 (6180), 635 (1150). ESI-MS+: [H2L4 + Na]+ m/zcalcd. = 272.126, m/zfound. = 272.140. 7 [VO(L4)(OMe) + Na]+ m/zcalcd. = 368.067, m/zfound. = 368.070. ESI-MS–: [V(O)2(L4)]– m/zcalcd. = 330.055, m/zfound. = 330.076. 2.2. NMR spectroscopy The 1H, 13C and 51V NMR spectra were recorded with Bruker Avance 400 (1H: 399.75 MHz, 13C: 100.52 MHz, 51V: 105.15 MHz) and Bruker Avance 500 (1H: 500.13 MHz, 13C: 125.76 MHz, 51V: 131.55 MHz) NMR spectrometers in CHCl3-d, MeOH-d4, MeCN-d3, DMSO-d6 and PhMe-d8 solutions at 25 °C (298 K). The spectrometers were equipped with a broad-band observe probe (Bruker BBO-5 mm-Zgrad). The proton and carbon chemical shifts were referenced to internal TMS (tetramethylsilane; δ TMS = 0.00 ppm). The 0 ppm vanadium reference frequency was calculated from the TMS 1H frequency using the unified chemical shift scale by IUPAC (Ξ(51V, VOCl3) = 26.302948).[20] The 1D 1H and 51V NMR spectra were measured with a single-pulse acquire sequence (flip angles 30° and 90°). The 1D 13C NMR spectra (flip angle 45°) were measured with broad-band 1H decoupling (waltz16). The gradient-selected 1H{1H} COSY spectra were recorded in doublequantum filtered mode. 1H{1H} NOE spectra were acquired with the noesygpph pulse sequence using a mixing time of 0.3 s. The 1H{13C} HSQC (with multiplicity editing) and HMBC experiments were optimized for 1JCH(short-range) = 145 Hz and JCH(long-range) = 10 Hz. Complete chemical shift assignment for all compounds can be found in the electronic supplementary material. 2.3. X-ray crystallography Single crystal data collection of 1’ and 4 was carried out with an Agilent SuperNova microfocus dual source (Cu/Mo) diffractometer using CuKα radiation (λ = 1.54184 Å). A suitable crystal was placed in a MiTeGen MicroMount™ using Fomblin oil and mounted on the diffractometer where it was kept under a stream of liquid nitrogen at 123 K. CrysAlisPro (v. 1.171.37.35 or 1.171.38.41) program[20] was used for data collection and reduction as well as to apply analytical numeric absorption correction based on a multifaceted crystal model. For 1, the crystallographic data was recorded at 170 K with a Nonius-Kappa diffractometer equipped with APEXII CCD area-detector using MoKα radiation (λ = 0.71073 Å). Data collection and processing were carried out using COLLECT[21] and DENZO-SMN[22], respectively, 8 whereas SADABS[23] was used to apply absorption correction. The structures were solved with either ShelXS by direct methods and refined with ShelXL[24] using Least Squares minimization within the Olex2[25] solution program. All non-hydrogen atoms were refined anisotropically. C—H hydrogen atoms were calculated to their ideal positions and refined using a riding model with Uiso values 1.2—1.5 times to respective host atoms. O–H hydrogen atoms were located from the difference map and refined isotropically. CCDC 18551491855152 contain the supplementary crystallographic data for 1, 1’, 4 and H2L1∙2,4-di-tertBuPhOH, respectively. Table 4. Summary of crystallographic data for 1, 1’ and 4. 1 1' 4 Empirical formula C 22 H 36 NO 6 V C 42.37 H 64.37 N 3.19 O 10.41 V 2 C 17 H 28 NO 7 V Formula weight 461.46 886.77 409.34 Temperature/K 170 123 123 Crystal system Monoclinic Monoclinic Orthorhombic Space group C2 P2 1 P2 1 2 1 2 a/Å 45.4595(7) 16.7775(2) 7.2700(3) b/Å 8.9963(2) 16.60171(17) 28.6337(8) c/Å 12.2069(2) 18.2292(2) 9.3128(2) α/° 90 90 β/° 101.9720(10) 115.5289(16) 90 γ/° 90 90 90 Volume/Å3 4883.64(16) 4581.77(11) 1938.61(10) Z 8 4 4 Z' 2 2 1 ρ calc g/cm3 1.255 1.286 1.403 μ/mm-1 0.441 3.891 4.611 F(000) 1968 1881 864 Crystal size/mm3 0.38 × 0.30 × 0.30 0.221 × 0.132 × 0.105 0.211 × 0.033 × 0.029 Radiation MoKα (λ = 0.71073) CuKα (λ = 1.54184) CuKα (λ = 1.54184) 2Θ range for data collection/° 3.41 to 51.996 5.992 to 137.966 9.496 to 139.958 Index ranges -56 ≤ h ≤ 55, -11 ≤ k ≤ 10, -14 ≤ l ≤ 15 -20 ≤ h ≤ 18, -20 ≤ k ≤ 15, -19 ≤ l ≤ 22 -7 ≤ h ≤ 8, -20 ≤ k ≤ 34, -11 ≤ l ≤ 10 15 Omethanol bond lengths of 1.77—1.80 Å and 2.25—2.32 Å. The neutral nitrogen donors are coordinated to vanadium ions with the bond lengths of 2.23—2.25 Å. A visual comparison of the crystal structures of 1 and 4 is shown in figure S45, which further demonstrates the general geometrical similarity of the complexes. The most apparent geometrical differences arise from the orientation of the methoxide and methanol ligands that have significant rotational freedom with respect to the V—O bond axis. In addition to 1 we were able to analyse the related dinuclear complex 1’ by means of single crystal X-ray diffraction. 1’ crystallizes in the space group P21 and consists of two independent molecules in the asymmetric unit (see S44 in Supplementary Information). The structure has both acetonitrile and water solvent molecules included in the crystal lattice with respective amounts of 1.2 and 0.4 molecules per each complex formula unit. The molecular structure of the complex is formed of two different V-centred units. In one unit, the metal ion has an approximate square pyramidal coordination sphere in which the central atom is coordinated to the tridentate amino acid phenolate ligand as well as to one terminal and one bridging oxido ligands. The other unit in the dinuclear molecule has a hexacoordinated vanadium centre with a coordination sphere resembling the structure of 1 with the bridging oxido and a water molecule completing the coordination environment instead of the methoxide and methanol ligands found in 1. Table 1. Selected bond angles (°) and distances (Å) in complexes 1 and 4. 1A 1B 4 Distances V1—O1 1.838(2) 1.821(2) 1.847(3) V1—O2 1.963(3) 1.972(3) 1.964(3) V1—O2 1.963(3) 1.972(3) 1.964(3) V1—O2 1.963(3) 1.972(3) 1.964(3) V1—O4 1.587(3) 1.576(3) 1.597(3) V1—O5 1.775(3) 1.775(3) 1.800(3) V1—O5 2.282(3) 2.324(3) 2.253(3) V1—N8 2.232(3) 2.250(3) 2.250(3) Angles O1—V1—O2 154.67(13) 153.33(11) 151.30(13) O5—V1—N8 162.77(12) 161.36(13) 166.01(13) O4—V1—O6 176.99(15) 175.03(15) 176.48(14) O4—V1—N8 94.96(13) 93.39(14) 91.22(13) 16 4. Conclusions Chiral, tridentate amino acid phenol ligand precursors H2L1—4 can be easily prepared from Lproline, 2,4-disubstituted phenols and formaldehyde in a one-pot fashion. These tridentate proligands react with oxidovanadium precursors VO(acac)2, VOSO4 ∙ 5 H2O and VO(OPr)3 in methanol to form oxidomethoxidovanadium(V) complexes of the general formula of [VO(L1— 4)(OMe)(MeOH)]. The x-ray studies show that the complexes are octahedrally coordinated in the solid state, having structures resembling that found in the active site of VBrPO. However, in solution the NMR spectra indicate the presence of several different conformations or isomeric structures, depending on the solvent. This fact significantly hampers any further solution state studies, including catalytic experiments, as it would be difficult to draw any meaningful structure-reactivity conclusions. 5. Notes The authors declare no competing interests. Acknowledgements A.P. gratefully acknowledges the financial support from the Academy of Finland (project no. 315911). References [1] D.C. Crans, J.J. Smee, E. Gaidamauskas, L. Yang, The Chemistry and Biochemistry of Vanadium and the Biological Activities Exerted by Vanadium Compounds, Chem. Rev. 104 (2004) 849– 902. doi:10.1021/cr020607t. [2] C. Bolm, Vanadium-catalyzed asymmetric oxidations, Coord. Chem. 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