Synthesis and spectroscopic/DFT structural characterization of coordination compounds of Nb(V) and Ti(IV) with bioactive carboxylic acids
Graphical abstract
The reactions of niobium pentachloride with aspirin, diclofenac and ethacrynic acid afforded dinuclear coordination compounds, with aspirin undergoing selective deacetylation. A mixed Ti(IV) isopropoxide-ethacrynate was also obtained.
Introduction
Amongst the three important classes of transition metal derivatives containing a M–O–C bond (i.e. metal alkoxides, β-diketonates and carboxylates), metal carboxylates have been known for the longest time [1]. The reactions of transition metal chlorides with acetic acid and related halo-α-substituted compounds usually take place via the release of hydrogen chloride and may represent an entry into the rich chemistry of carboxylato complexes [1], [2]. This reactivity has been ascertained, inter alia, for oxophilic halides of metals belonging to the groups 4 [3] and 5 [4]. More precisely, the reactions of several carboxylic acids or the corresponding alkali salts with titanium(IV) halides are known to yield derivatives of general formula TiX4-n(O2CR)n (n = 1–4, X = Cl, Br) [3]. The latter have been used as catalytic precursors for olefin polymerization [5] and for the synthesis of TiO2-based nano-crystalline materials [6], compounds with antiwear properties [7] or exhibiting solvent absorption and desorption properties [8]. Moreover, recently, titanium carboxylates have aroused interest for their possible biological activity [9] and, thus, for biomedical purposes [10]. Niobium and tantalum pentahalides remained for a long time in the shadow of metal complexes of group 4, probably in the light of the extraordinary applications of the latter in alkene chemistry [11]. However, in the last fifteen years, a significant progress has been traced concerning the chemistry of niobium and tantalum pentahalides [11], [12], encouraged by their easy availability, the relative nontoxicity of the metal elements [13], and the unusual reactivity patterns [12](e), [12](h), [14]. As far as metal carboxylates are concerned, adducts of the type MCl4(OOCR) (M = Nb, Ta, R = alkyl or haloalkyl substituents) [4], [15], beside a variety of oxo-chloride species of general formula MOCl(OOCR)2 [15a,15b], MOCl2(OOCR) [15a] and MO2(OOCR) [15a], were obtained from the reactions of MCl5 with RCOOH or (RCO)2O. On the other hand, the reactions of aryl carboxylic acids (ArCO2H) with MCl5 generally result in the formation of dinuclear carboxylato-bridged species, M2Cl8(OOCAr)2 (M = Nb, Ta, Ar = aryl) [16].
In this context, the exploration of the chemistry of early transition, high valent metal compounds with carboxylic acids containing additional functional groups still remains in its infancy. This is presumably a consequence of the variety of complicated reaction pathways that may be working when such oxophilic metal species are allowed to contact with oxygen functions [12](h), [17].
In view of this, and of the great interest aroused by the interaction of metal ions with bioactive molecules, finding extensive application for pharmacological issues [18], herein we describe a work on the reactivity of NbX5 (X = Cl, Br), TiCl4 and Ti(OiPr)4 with some carboxylic acids with a known biological role (Fig. 1) [19]. The formation of coordination compounds, their structural characterization by means of spectroscopic and computational studies, and the NbCl5-induced deacetylation of aspirin will be discussed.
Section snippets
Results and discussion
The reactions of NbX5 (X = Cl, Br) with acetylsalicylic acid led to the isolation of dark red solid compounds of formula NbX4(C7H5O3) (1a, X = Cl; 1b, X = Br), according to elemental analysis. Both the reactions proceeded with the release of the corresponding acetyl halide, as evidenced by NMR (see Experimental). After hydrolysis of the reaction mixtures [20], [21], salicylic acid was cleanly recovered as unique organic species, and detected by IR and NMR spectroscopy.
The deacetylation of aryl
Conclusions
Despite the chemistry of middle to late transition metal halides with carboxylic acids has been largely investigated, relatively little is known about the analogous reactions of high valent, early transition metal species. Herein, we have described the study of the reactivity of NbX5 (X = Cl, Br) and Ti(OiPr)4 with a selection of carboxylic acids exhibiting a known biological role. The reactions proceed to afford metal compounds containing bidentate carboxylates, whose structures have been
Experimental
All manipulations of air and/or moisture-sensitive compounds were performed under an atmosphere of pre-purified nitrogen using standard Schlenk techniques. The reaction vessels were oven dried at 150 °C prior to use, evacuated (10−2 mmHg) and then filled with argon.
TiCl4 (99%, Strem), NbCl5 (99+%, Strem), Ti(OiPr4) (98%, Strem), PCl5 (>98%, Sigma Aldrich), Ag2O (99+%, Sigma Aldrich) and organic reactants (highest purity available, from TCI Europe or Sigma Aldrich) were commercial products
Computational details
The computational geometry optimizations were carried out without symmetry constrains, using the range-separated DFT functional ωB97X [37], in combination with the split-valence polarized basis set of Ahlrichs and Weigend, with ECP for the niobium centre [38]. The C-PCM implicit solvation model was added to ωB97X calculations, considering chloroform as continuous medium [39]. The “restricted” formalism was always applied. The stationary points were characterized by IR simulations (harmonic
Acknowledgments
The authors wish to thank the University of Pisa for financial support.
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Current address: Department of Biosciences and Department of Chemistry, Durham University, Durham DH13LE, United Kingdom.