The new type of [Zr6(μ3-O)4(μ3-OH)4] cluster core: Crystal structure and spectral characterization of [Zr6O4(OH)4(OOCR)12] (R = But, C(CH3)2Et)
Graphical abstract
Hexanuclear Zr(IV) oxo carboxylate derivatives of the general formula [Zr6O4(OH)4(OOCR′)12] (R′ = But (1), C(CH3)2Et (2)) were synthesized from Zr(OR)4 (R = Pri, C(CH3)2Et) and appropriate organic acids (HOOCR′, R′ = But, C(CH3)2Et) in a molar ratio 1:2. The structure of these compounds consists of hexanuclear Zr(IV) clusters formed by six zirconium atoms, linked by 4:4 system of μ3-oxo/hydoxy bridges and stabilized by syn–syn carboxylate groups. Results of crystallographic and spectroscopic data suggest that in [Zr6(μ3-O4)(μ3-OH)4] clusters four protons migrate between eight oxide bridges. Thermal stability studies of 1 and 2 using TGA/DTA and VT IR methods have been carried out.
Introduction
Synthesis and properties of zirconium oxo clusters have been studied extensively in search for the novel type inorganic–organic hybrid polymers [1], [2], and precursors of solid-state materials used mainly in sol–gel, but also in chemical vapor deposition methods (CVD) [2], [3], [4], [5]. The reaction of Zr(OR)4 with organic acids (HOOCR′) yields the multinuclear oxo complexes regardless of the molar ratio employed [6], [7], [8], [9], [10]. Kickelbick et al. reported that the reaction of Zr(IV) alkoxides with corresponding organic acids, using the molar ratio 1:4, proceeded with formation of carboxylate derivatives of general formulas [Zr6(OH)4O4(OMc)12] and [Zr4O2(OMc)12] [7], [8]. In the structure of [Zr6(OH)4O4(OMc)12], zirconium atoms (coordination number 8) form hexanuclear clusters linked by μ3-oxo and μ3-hydroxo bridges, which are stabilized by 12 methacrylate ligands. Three of them chelate the Zr(IV) atoms, while the remaining nine carboxylate groups participate in syn–syn bridges. The hydrogen atoms of the bridging groups have not been located, however, analysis of Zr–O bond distances and the charge balance indicates the presence of four μ3-OH groups [6]. A similar type of clusters has also been detected in structures of [Zr6(OH)2O6(OOCHPh2)10(phen)2] and also [Ti6O8(η5-C5H5)6] (in this case, the formation of [Ti(IV)4Ti(III)2O8] clusters was postulated) [11], [12]. Crystals of other complexes: [Zr4O2(OMc)12], [Zr6O2(OPri)16(OOC–C10H6O)2], [Zr6O2(OMe)4(OBun)2(OMc)14], [Zr6O2(OBun)10(OMc)10], have been obtained with a 6–15 folds excess of the organic acid (HOOCR′, R′ = C10H6OH, CMeCH2) [6], [7], [8], [13]. When Zr/carboxylate ligand ratio was increased from 1:2 to 1:3, as for [Zr4O2(OMc)12], the structure of cluster opened up and a distorted butterfly arrangement was formed [6]. The application of sterically varied carboxylic acids (HOOCR′; R′ = H, CH3, Pri, But, CH2But) in the reaction with zirconium(IV) alkoxides (molar ratio 1:1) leads to the formation of the following products: [Zr4O2(O2CH)2(OPri)10], [Zr3O(O2CCH3)5(OPri)5], [Zr2(OPri)6(O2CPri)2]2, [Zr2O(O2CBut)(OPri)7](HOPri)2, [Zr2O(O2CH2But)2(OPri)6](HOPri) [9].
A series of multinuclear Ti(IV) oxo carboxylate derivatives with oligomeric ring-shape molecules were synthesized through the reaction of branched acids (HOOCR′, R′ = But, C(CH3)2Et) with titanium(IV) alkoxides in a molar ratio 1:2 [14]. Therefore, the aim of the reported research was the synthesis and structural characterization of the novel multinuclear Zr(IV) oxo carboxylate derivatives, obtained in a reaction between Zr(OR)4 (R = Pri, C(CH3)2Et) and HOOCR′ (R′ = But and C(CH3)2Et). Thermal behavior of synthesized compounds was studied using thermogravimetric analysis and variable temperature (VT IR) spectroscopy.
Section snippets
Results and discussion
Hexanuclear Zr(IV) oxo carboxylate derivatives were prepared in the reaction of the respective zirconium alkoxide (Zr(OR)4, R = Pri, C(CH3)2Et) with the twofold excess of the organic acid (HOOCR′ (R′ = But, or C(CH3)2Et)) at room temperature under Ar atmosphere. Considering the literature data [7], the above reaction can be illustrated by the overall equation (1).where R′ = Pri, C(CH3)2Et, R = But, C(CH3)2Et.
Initially the reaction mixture
X-ray crystallographic structures
Single crystals X-ray analysis revealed that 1 and 2 crystallize in the centrosymmetric (no. 148) space group for the series of crystals, for which diffraction data were collected at 293 K (Table 1). The structures of these compounds consist of hexanuclear [Zr6(μ3-O)8]12+ clusters where six equivalent metal ions are arranged in the corners of an octahedron (Fig. 1). Consequently, in all the investigated structures the [Zr6(μ3-O)8]12+ cluster symmetry was C3i. Since two oxygen atoms (O1) were
Conclusions
Two multinuclear zirconium(IV) oxo carboxylates, [Zr6O4(OH)4(OOCR′)12] (R′ = But (1), C(CH3)2Et (2)) were prepared in a direct reaction of Zr(IV) alkoxides with the suitable branched organic acids in the molar ratio 1:2, and characterized by elemental analysis, IR, 13C NMR and single crystal X-ray diffraction techniques.
X-ray studies revealed that independently from the kind of Zr(IV) alkoxides (Zr(OPri)4 or Zr(OC(CH3)2Et) and organic acid used in the synthesis, a similar type of structure is
Materials and methods
The studied compounds were synthesized at room temperature under argon atmosphere, using standard Schlenk line and glovebox techniques. Zr(OR)4 (R = Pri, C(CH3)2Et) and ButCOOH, Et(CH3)2CCOOH (Aldrich) were used as received. The reactions were carried in the mixture of n-hexane and PriOH used as a solvent. Both solvents were distilled and stored over the molecular sieves.
Infrared (IR) spectra were recorded on Perkin–Elmer Spectrum 2000 spectrometer within range 400–4000 cm−1 with a medium slit
Acknowledgement
The authors wish to acknowledge the Polish Committee for Science Research for financial support Grant No. 4T09A 152 23.
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