A New Mo(VI) Complex of (E)-2-(((5-hydroxypentyl)imino)methyl)-6-methoxyphenol: Synthesis, Characterization, and X-Ray Crystal Structure

A tridentate ONO Schiff base ligand, (E)-2-(((5-hydroxypentyl)imino)methyl)-6-methoxyphenol [H2L], and its Mo(VI) complex [MoL], have been synthesized and structurally characterized by elemental analysis, FT-IR, NMR, and UV–Vis spectroscopy. The structure of the Mo(VI) complex has been determined by single-crystal X-ray diffraction. This complex has octahedral geometry around central atom. Yellow crystals crystallize in orthorhombic, space group Pbca with a = 17.8718(10) Å, b = 17.4516(10) Å, c, 18.1872(10) Å and α = β = γ = 90°.


Introduction
Schiff base compounds have absorbed a large amount of attention in recent years due to their facile synthesis, and wide applications in different areas. [1,2] Their functionality plays an important role in the mechanism of many enzymes such as aldose, and biochemical processes such as aminotransferases, using pyridoxal phosphate as a cofactor. [3,4] They are able to be coordinated with many metals and form related complexes in various oxidation states. [5] Furthermore molybdenum has got a great attention in coordination chemistry. [6,7] It is widely used in catalytic processes, [8][9][10] although the useful applications of molybdenum are not restricted to industrial type of catalytic reactions. Research shows that they exist in a number of redox enzymes such as sulfite oxidase, xanthine oxidase, aldehyde oxidase, and others. [11,12] In this work, we have prepared tridentate ONO Schiff base compound, (E)-2-(((5-hydroxypentyl)imino)methyl)-6-methoxyphenol [H 2 L] and its Mo(VI) complex as two new compounds. These compounds were fully characterized by elemental analysis, FT-IR, NMR, electronic spectra. The X-ray crystal structure of the Mo(VI) complex was also investigated.

Materials and Instrumentation
All chemicals and solvents used were of analytical reagent grade and used as received. Micro analyses were determined on an Elementar Vario MACRO analyzer. The Infrared spectra were recorded on a FT-IR TENSOR 27 spectrophotometer (400-4000 cm ¡1 ). NMR spectra were recorded in CDCl 3 and DMSO-d 6 solutions on Bruker Advance DPX 250 MHz AVANCE spectrometer. The electronic spectra of the solutions were recorded on a Perkin Elmer Lambda 45 spectrophotometer at concentrations of 1 £ 10 ¡3 and 2 £ 10 ¡5 M. Melting point of compounds was determined on a Electrothermal 9100 melting point apparatus. Conductance measurement was made by means of a Metrohm 712 Conductometer in DMSO at concentration of 1 £ 10 ¡3 M. Diffraction data were measured using a Bruker APEX II CCD area detector.

Synthesis of Tridentate ONO Schiff Base Ligand [H 2 L]
A methanolic solution (5 mL) of 5-amino-1-pantanol (0.103 g, 1 mmol) was added drop wisely to 5 mL methanolic solution of o-vaniline (0.152 g, 1 mmol). The mixture was refluxed for 6 h. After completion of the reaction, as indicated by TLC, the solvent was evaporated by rotary evaporator and the dense yellow liquid thus obtained was used as the ligand.

Synthesis of Mo(VI) complex [MoL]
To 8 mL methanolic solution of [H 2 L] (4 mmol, 0.948 g) was added 5 mL methanolic solution of Mo(acac) 2 (2 mmol, 0.654 g) and the mixture was refluxed in an oil bath for 8 h. After cooling, the resulting solid was filtered off, washed with cold absolute methanol, and dried in a desiccator over

Crystal Structure Determination
The data were collected at room temperature with a Bruker APEX II CCD area-detector diffractometer using Mo-Ka radiation (λ D 0.71073 A ). The data collection, cell refinement, data reduction and absorption correction were performed using multiscan methods with Bruker software. The non-hydrogen atoms were refined anisotropically by fullmatrix least-squares on F 2 using SHELXL. [13] All hydrogen atoms were placed at calculated positions and constrained to ride on their parent atoms. Details concerning collection and analysis are reported in Table 1.

Results and Discussion
Reactions of the tridentate ONO Schiff base ligand and Mo (acac) 2 in 2:1 molar ratio resulted the Mo(VI) complex in good yield (Scheme 1). Both ligand and its Mo(VI) complex are stable in air and soluble in most solvents. Molar conductivity value of [MoL] complex equal to 21.6 ohm ¡1 cm 2 mol ¡1 in DMSO, indicating its nonelectrolyte behavior.

X-Ray Crystal Structure
For determining the coordination spheres, a single crystal Xray diffraction study of [MoL] was made. Single-crystal Xray diffraction analysis revealed that this complex crystallized in orthorhombic system with space group Pbac. The ORTEP view with the atomic numbering scheme of complex is presented in Figure 1. As can be seen, the asymmetric unit of [MoL] consists of two Mo ions and two ONO tridentate ligands. Selected bond distance and interatomic angles are listed in Table 2. Structural analysis reveals that Mo(VI) ( Figure 1) has a distorted octahedral geometry in which the Schiff base [H 2 L] behaves as a monoanionic tridentate ligand. In this structure Mo(VI) has a six-coordination sphere in which central atom is surrounded by donor atoms of     (14) Symmetry transformations used to generate equivalent atoms: #1 x C 1/2, y, ¡z C 1/2 #2 x ¡ 1/2, y, ¡z C 1/2.  is shown in Figure 4 and hydrogen bond data is given in Table 3.

Vibrational Analysis
Assignments of selected prominent IR bands in the 400-4000 cm ¡1 region for HL and its Mo(VI) complex are listed in the Experimental section ( Figures S1 and S2). Comparison of the spectra of the [MoL] with the [H 2 L] provide some evidences for the coordination process. In the FT-IR spectrum of the ligand, a characteristic sharp azomethine band with high intensity at 1633 cm ¡1 is shifted to lower wavenumber at complex spectra and appeared at 1632 cm ¡1 . [14,15] This indicates the coordination of the azomethine group nitrogen atom to metal ion. The Shift of CÀ ÀO bond energy of [H 2 L] (1343 cm ¡1 ) to the lower wavenumber at 1315 cm ¡1 , is another reason for coordination of the ligand to metal through the phenolic oxygen atom. [16,17] Dioxidomolybdenum(VI) complex with a cis-MoO 2 moiety exhibits two strong bands at 897 and 933 cm ¡1 assignable to asymmetrical and symmetrical v(ODMoDO) vibrations. [18] In  Figure S5). The 13 C NMR spectrum of [H 2 L] shows a peak at 164.7 ppm, that can be assigned to the azomethine carbon (-CHHN-). The signals of C9 and C12 disclosed at 153 and 148.7 ppm respectively. Other aromatic carbons lie between in the range of 113.7-122.8 ppm and signals of aliphatic carbons appeared in the range of 23.3-67.7 ppm.   Sheikhshoaie et al.

UV-Vis Spectrum
The UV-Vis spectra of [H 2 L] and [MoL] were recorded in DMSO solution (Figures S6 and S7). In the electronic spectrum of ligand, two bands at 337 nm and 433 nm are assigned to p!p* and n!p* transitions, respectively. In Mo(VI) complex, The band which is appeared at 439 nm may be due to LMCT (O(p) !Mo(d)) transition. According to d 0 electronic configuration (C6 formal oxidation state) of molybdenum, the Mo-complex showed no d-d transition. [20] Conclusions The tridentate ONO Schiff base ligand (E)-2-(((5-hydroxypentyl)imino)methyl)-6-methoxyphenol [H 2 L] and its Mo (VI) complex were synthesized and characterized by prevalent spectroscopic methods such as FT-IR, NMR, and UV-Vis. Also X-ray diffraction was performed on Mo(VI) complex. It was concluded from spectral characterization that the metal complex has octahedral geometry.

Funding
Authors gratefully acknowledge the financial support provided for this work by the Shahid Bahonar University of Kerman.

Supplementary Material
Supplemental data for this article can be accessed at the publisher's website. CCDC 8080278 contains the supplementary crystallographic data for [MoL]. A copy of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, Cb2 1EZ, UK (fax: C44 1223 336 033); web page: http://www.ccdc.cam.ac.uk/cgi-bin/ catreq.cgi