Square Pyramidal Dialkoxo-Bound Monooxo-Vanadium(V) Complex and Its Behavior in Solution Mooshin Moon, ² Myoungho Pyo, ‡ Young Chan Myoung, § Chong Il Ahn, § and Myoung Soo Lah* ,² Department of Chemistry, College of Science, Hanyang University, 1271 Sa-1-dong, Ansan, Kyunggi-do 425-791, Korea, Department of Chemistry, College of Natural Science, Soonchun National University, Soonchun 540-742, Korea, and Department of Chemical Science and Technology, Agency for Technology and Standard, MOCIE, 2 Choongang-dong, Kwacheon, Kyunggi-do 427-010, Korea ReceiVed September 17, 1999 Introduction Interest in vanadium chemistry has increased since the recognition of the involvement of vanadium ions in many important biological systems. 1 The distinctive preference of the vanadium ion for N/O donor atom types has prompted the synthesis and characterization of many model complexes containing N/O donor ligands, the spectroscopic properties and solution behavior of which have been investigated. 2 A short vanadium-oxygen bond length (1.72 Å) for the coordination environment of the vanadium(V) center in vanadate-dependent bromoperoxidase, based on the EXAFS, 3 increased interest in alkoxo-bound vanadium(V) model complexes. In addition, crystallographic studies on chloroperoxidase (at 2.1-Å resolu- tion) showed a five-coordinate trigonal bipyramidal vanadium center as an active site, where three nonprotein oxygen atoms are situated at short distances from the vanadium(V) center (average 1.65 Å). 4 While five-coordinate monooxo-vanadium- (IV) model complexes 5,6 are common, only a few five-coordinate monooxo-vanadium(V) model complexes 5b,6,7 are known. In this study, we report the synthesis and structural characterization of a five-coordinate square pyramidal dialkoxo-bound mo- nooxo-vanadium(V) complex and its behavior in solution. Experimental Section Materials. The following were used as received with no further purification: salicylhydrazide (shz), vanadium(III) acetylacetonate, chloroform-d, and tetrabutylammonium tetrafluoroborate (TBABF4) from Aldrich, Inc.; methanol (MeOH) and ethanol (EtOH) from Carlo Erba. Instrumentation. C, H, N, and V determinations were performed by the Elemental Analysis Laboratory of the Korean Institute of Basic Science. Infrared spectra were recorded as KBr pellets in the range 4000-600 cm -1 on a Bio-Rad FT-IR spectrometer. The solution IR spectra were recorded between KBr plates. Absorption spectra were obtained using a Perkin-Elmer Lambda spectrometer. 1 H NMR spectra were obtained using a Varian-300 spectrometer and 51 V NMR spectra were obtained using a Bruker DMX 600 spectrometer. Positive-ion electron spray ionization (ESI) mass spectrum was obtained using a JEOL HX110A/HX110A tandem mass spectrometer in ethanol. Room- temperature magnetic susceptibilities of well-ground solid samples were measured by using an Evans balance. 8 All the electrochemical measure- ments described in this study were carried out at room temperature using the BAS CV-50W with a conventional three-electrode configu- ration. The working electrode was a glassy carbon disk (GC, electro- chemical area ) 0.064 cm 2 ) freshly polished with activated aluminum oxide (150 mesh, 58 Å, Aldrich) before use. The reference and counter electrodes were Ag/AgCl (3 M KCl) and Pt gauze, respectively. All the potentials mentioned in this paper were referenced to Ag/AgCl. Synthesis: V(V)O(Hacshz)(OEt), 1. (Hacshz 2- is a doubly depro- tonated dianionic acetylacetosalicylhydrazone). A 0.234-g (1.53-mmol) sample of shz was dissolved in 15 mL of ethanol. When a 0.531-g (1.52-mmol) sample of vanadium(III) acetylacetonate was added to the solution, the solution changed its color to dark brown. After 20 min of stirring, the solution was filtered. Slow evaporation of the filtrate solution over 3 days produced dark brown crystals (0.270 g, 51.4%). Anal. Calcd for VO(Hacshz)(OEt) (C14H17N2O5V) (fw ) 344.24): C, 48.85; H, 4.98; N, 8.14; V, 14.80. Found: C, 48.88; H, 4.92; N, 8.21; V, 14.6. IR (KBr, cm -1 ): ν 998 vs νVdO. 1 H NMR (300 MHz, CDCl3): A form. δ 11.00 (s), 1H for phenolic proton; 7.82 (d), 7.31 (t), 6.95 (d), 6.87 (t), 4H for phenyl protons; 5.66 (s), 1H for -CH-; 5.39 (m), 2H for V-OCH2CH3; 2.42 (s), 2.23 (s), 6H for two methyl groups; 1.57 (t), 3H for V-OCH2CH3. B form. δ 10.04 (s), 1H for CH3CH2OH; 7.22 (t), 7.01 (d), 6.87 (d), 6.61 (t), 4H for phenyl protons; 5.57 (s), 1H for -CH-; 3.65 (qr), 2H for HOCH2CH3; 2.42 (s), 1.82 (s), 6H for two methyl groups; 1.17 (t), 3H for free HOCH2CH3. Ratio A:B ) 4:1. 51 V NMR (157.7 MHz, CDCl3): A form. δ -519.98. B form. δ -514.21. Ratio A:B ) 4.7:1. 51 V NMR (157.7 MHz, MAS): δ -511.68. ESI mass spectrum: m/z of [VO(Hacshz)(OEt) + H] + , 345. UV-vis (CH3Cl) [λmax ()]: 240 (13 300 M -1 cm -1 ), 267 (18 800 M -1 cm -1 ), 339 (12 200 M -1 cm -1 ), 390 nm (shoulder, 6600 M -1 cm -1 ). X-ray Crystallography. A dark brown crystal of complex 1 was mounted on a glass fiber. Preliminary examination and data collection were performed with Mo KR radiation (λ ) 0.71069 Å) on a Siemens SMART CCD equipped with a graphite crystal, incident-beam mono- chromator. Data were collected at room temperature. Lp and absorption corrections were applied to the data. The structure was solved by direct methods and refined on F 2 by full-matrix least-squares techniques with * To whom correspondence should be addressed. ² Hanyang University. ‡ Soonchun National University. § Agency for Technology and Standard, MOCIE. (1) (a) Chasteen, N. D. Vanadium in Biological Systems; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990. (b) Butler, A.; Carrano, C. J. Coord. Chem. ReV. 1991, 109, 61-105. (c) Rehder, D. Angew. Chem., Int. Ed. Engl. 1991. 30, 148-167. (2) (a) Dutta, S. K.; Samanta, S.; Kumar, S. B.; Han, O. H.; Burckel, P.; Pinkerton, A. A.; Chaudhury, M. Inorg. Chem. 1999, 38, 1982-1988. (b) Kosugi, M.; Hikichi, S.; Akita, M.; Moro-oka, Y. Inorg. 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