Synthesis of a new phenolate-1-methylimidazole hexadentate ligand and the crystal structure of its first vanadyl complex

Article

Synthesis of a New Phenolate-1-methylimidazole Hexadentate Ligand and the Crystal Structure of its First Vanadyl Complex

Ademir Neves*, Melânia Tamanini, Valdir Rosa Correia, and Ivo Vencato

Laboratório de Bioinorgânica e Cristalografia-LABIN, Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis - SC, Brazil

Received: September 25, 1996; September 15, 1997

Reportamos neste trabalho, a síntese e a estrutura cristalina de OV^IV(Hbbimen)PF₆.2H₂O H₂bbimen = N,N-bis(2-hidroxibenzil)-N,N-bis (1-metilimidazol-2-il-metil) etilenodiamina como um raro exemplo de complexo V^IVO²⁺ o qual contém grupos 1-metilimidazol e fenolato coordenados simultaneamente ao centro metálico.

The synthesis and X-ray crystal structure of OV^IV(Hbbimen)PF₆.2H₂O H₂bbimen = N,N-bis(2-hydroxybenzyl)-N,N-bis (1-methylimidazole-2-yl-methyl) ethylenediamine are reported as a rare V^IVO²⁺ complex containing phenolate and 1-methylimidazole coordinated simultaneously to the metal center.

Keywords: imidazole-phenolate, VO²⁺ complex, crystal structure

Introduction

It is well established that vanadium plays an important role in many biological processes¹. In view of the recent reported binding of vanadium to tyrosinate residues in vanadium-modified transferrin², vanadium-phenolate chemistry is receiving a great deal of attention from the inorganic chemist^3-5. Despite the bindidng of iron(III)-transferrin has been established through crystallographic studies (NO₃ coordination and CO₃2- as synergistic anion)⁶, the coordination environment around the vanadium centre in vanadium-transferrins is not yet entirely known. It is well known that in the +3, +4, and +5 oxidation states, vanadium binds tighly to transferrin⁷.In the +4 oxidation state, a vanadyl complex structure with octahedral geometry having an NO₃ equatorial coordination (one tyrosinate, one aspartate, one histidine, and one monodentate carbonate) has been proposed recently for vanadium human lactoferrin, on the basis of computer simulations by using the atomic coordinates of the iron(III)- and copper(II)-lactoferrin complexes⁸.The coordination of histidine trans to the V=O bond has also been proposed⁸.

As a continuation of our work on the coordination chemistry of vanadium with biologically relevant ligands^4,5, we describe herein, the preparation and structural characterization of a VO²⁺ complex, with a new N₄O₂-donor ligand which contains the ethylenediamine backbone and two phenolate-type and two 1-methyl-imidazole-2-yl-methyl pendant arms. Interestingly, despite the numerous VO²⁺ complexes with nitrogen and oxygen donor atoms described in the literature,¹ there are only very few examples of structurally characterized vanadyl(IV) complexes which contain phenolate and imidazole coordinated simultaneously to the metal center⁹.

Experimental

Syntheses

The ligand N,N-bis(2-hydroxybenzyl)-N,N-bis(1-methylimidazole-2-yl-methyl)ethylenediamine (H₂bbimen) was prepared in good yield by the condensation reaction of 1-methyl-2-imidazolecarboxaldehyde¹⁰ and ethylenediamine and reduction with NaBH₄, followed by alkylation of the diamine (bimen) with 2-bromo-methylphenyl acetate¹¹, by the route in Scheme 1. The title complex was prepared as follows. A solution of VCl₃ in dry acetonitrile was refluxed for 15 min under an argon atmosphere and equimolar quantities of H₂bbimen and Et₃N in tetrahydrofuran (thf) were added to the hot solution, followed by 30 min of reflux. After the addition of NaPF₆ to the solution and cooling to room temperature under argon, a green precipitate of V^III(bbimen)PF₆1 was formed. This material is stable in the solid state, even in the presence of air, but oxidizes in acetonitrile solution under aerobic conditions. The lilac complex 2, OV^IV(Hbbimen)PF₆. 2H₂O, was obtained after leaving the green solution of 1 at room temperature over night, in the presence of air. Single crystals of 2 suitable for X-ray crystallography were obtained by recrystallization from a propan-2-ol-acetone-acetonitrile (1:1:1) solution of 2.

X-ray Crystallography

Crystal data for C₂₆H₃₁N₆O₃V.PF₆.2H₂O, M = 707.51, triclinic, P-1, (No. 2), Z = 2, a = 10.703(2), 13.857(3), c = 14.166(3) Å, a = 118.70(3), b = 103.77(3), g = 102.78(3)º, V = 1645.2(6) ų, D_c = 1.428 g cm^-3, F(000) = 730, m = 0.428 mm^-1 for Mo-Ka radiation (l = 0.71073 Å, crystal size 0.13 x 0.13 x 0.46 mm. Nonius CAD4 diffractometer, T = 293 K. Measured reflections: 6021, with 4049 unique reflections of which 2071 had I > 2s(I). The structure was solved by direct methods, full-matrix refinement on F², anisotropic refinement for all non-H atoms of the complex molecule, isotropic refinement for water molecules and disordered PF₆ group. The number of refined parameters was 390, R = 0.127 , wR = 0.273.

The quality of the refinement was limited by the intrinsic quality of the data and by the high thermal motion of the disordered PF₆ group. All the H atoms were placed at geometrically calculated positions, except those of the water molecules that were not found. Crystallographic calculations were performed using the MOLEN, SHELXS86, SHELXL93 and ZORTEP computer program packages¹².

Results and Discussion

The structure and atomic numbering scheme of 2 are shown in Fig. 1. The vanadium (IV) ion is in a pseudo-octahedral environment, in which one phenolate oxygen, one amine, and two 1-methylimidazole nitrogen atoms in trans positions, form the equatorial plane. The other amine nitrogen atom of the ethylenediamine backbone, in the trans position with respect to the terminal oxo group, completes the coordination sphere. It is important to note that, despite the similarities between the hexadentate H₂bbimen and H₂bbpen⁵ ligands, H₂bbpen = N,N-bis(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)ethylenediamine, there are significant structural differences in their corresponding VO²⁺ complexes. While in OV^IV(bbpen⁵ one pyridyl group of the ligand H₂bbpen is pendant, in 2 one phenol group of H₂bbimen is not coordinated to the vanadium (IV) center. This most probably arises due to distinct basicities of the coordinating pyridyl and 1-methylimidazole nitrogen atoms. As expected, the V-N_(amine) of 2.36(1) Å trans to the V=O bond in 2, is 0.13 Å longer than the V-N_(amine) bond in the equatorial plane and is a consequence of the strong trans influence of the oxo group. A similar trend in these bond lenghts has been observed in OV^IV(bbpen), but with longer V-N_(amine) bonds (0.05 and 0.03 Å for trans and cis to the oxo group, respectively) in complex 2. This fact can be associated with the higher rigidity of 2, due to the presence of three five- and one six-membered rings compared to OV^IV(bbpen), which possesses two five- and two six-membered rings. In addition, the displacement of the V atom from the N₃O plane (0.331 Å) for the present complex is smaller compared with that (0.345 Å) in OV^IV(bbpen). The V-O_(phenolate) distance in 2 (1.905(8) Å) is similar to those found in the OV^IV(bbpen) complex⁵ (av. 1.923 Å) but is shorter than in the isostructural VO²⁺- (imidazole)(phenolate) complexes (1.981-1.973 Å) which contain the OV^IV(salimH) fragment {HsalimH = 4-(2-(salicylideneamino)ethyl)imidazole} and variable bidentate ligands⁹. The V- N_{(1-methylimidazole)} distances (av. 2.08 Å) cis to the V=O bond, are similar to the V-N_(imidazole) bond of 2.099 Å observed in the OV^IV(salimH)(acac) complex⁹.

Finally, further preparative, structural and physicochemical (UV-Vis, EPR and electrochemical) studies on both V^III and V^IV complexes are in progress in our laboratory in order to compare with the corresponding properties of V^III- and V^IV- transferrins. These results will be reported later in a full paper.

Acknowledgments

This work was supported by Grants from CNPq, PADCT and FINEP (Ministério da Ciência e Tecnologia of Brazil).

References

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Publication Dates

• Publication in this collection
  10 Sept 2010
• Date of issue
  1997

History

• Received
  25 Sept 1996
• Accepted
  15 Sept 1997