The Homoleptic Square-Antiprismatic Chelate ... - ACS Publications

Zr is coordinated by four chelating ligands, forming a square-antiprismatic coordination polyhedron. The structure is compared to similar complexes, a...
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CRYSTAL GROWTH & DESIGN

The Homoleptic Square-Antiprismatic Chelate Tetrakis(3-acetyl-2,4-pentanedionato)zirconium(IV): A Promising Coordination Motif for Tetrahedral Metal-Organic Frameworks

2006 VOL. 6, NO. 7 1720-1725

Daniel M. To¨bbens,*,† Reinhard Kaindl,§ Volker Kahlenberg,† Herwig Schottenberger,‡ and Michael Hummel‡ Institute of Mineralogy and Petrography, Institute of General, Inorganic and Theoretical Chemistry, and Christian-Doppler-Laboratory for AdVanced Hard Coatings at the Institute of Mineralogy and Petrography, UniVersity of Innsbruck, Innrain 52, A-6020, Austria ReceiVed February 20, 2006; ReVised Manuscript ReceiVed April 25, 2006

ABSTRACT: The novel analogue of the parent Zr(acac)4 complex, tetrakis(3-acetyl-2,4-pentanedionato)zirconium(IV), Zr[C7O3H9]4, has been synthesized straightforwardly by a salt-free methodology and was characterized by a number of complementary methods (1H NMR, 13C NMR, IR, and bulk density). From polycrystalline material, X-ray powder diffractograms and micro-Raman spectra were obtained and are discussed in detail. The crystal structure was determined from laboratory X-ray powder diffraction data by simulated annealing and subsequently refined with the Rietveld technique. The compound is monoclinic with space group P2/c. Zr, residing on a crystallographic 2-fold rotation axis, is coordinated by the four chelating ligands forming a square-antiprismatic coordination polyhedron. Differences and similarities to zirconium(IV)acetylacetonate, Zr[C5O2H7]4, and other similar complexes are discussed, addressing the conformational rigidity of this symmetrically substituted homoleptic acac complex. Introduction In the field of materials chemistry, the class of homoleptic β-diketonate complexes, especially of group 14 metals, has attracted ongoing interest due to their high potential as versatile oxide sources for nanopowders,1 reinforced,2 or other taskspecific ceramic composites,3,4 and MOCVD precursors for dielectric oxide films.5-9 In particular, symmetrically functionalized β-diketonate complexes, bearing linkable terminal substituents such as the tetrakis(3-acetyl-2,4-pentanedionato) complex presented herein, hold the potential for the synthesis of new coordination polymers, namely, metal-organic frameworks (MOFs). This class of porous materials is attracting continuous attention due to their large pore sizes, high apparent surface areas, and selective uptake of small molecules.10-14 Thus, the exceptional stability of β-diketonates in combination with tetravalent metals represents an inviting challenge to investigate new MOF motifs with novel tetrahedral primary building units.15,16 Therefore, the basic understanding of their structural peculiarities is of principal interest. Experimental Section Materials. Zirconium(IV)isopropoxide-2-propanol complex [1471756-7] and 3-(1-hydroxyethylidene)-2,4-pentanedione (synonyms: 3-acetyl2,4-pentanedione, triacetylmethane) [815-68-9] were purchased from Aldrich and used as received. Synthesis. Preparation of tetrakis(3-acetyl-2,4-pentanedionato-O2,O3)zirconium(IV), Zr[C7O3H9]4, [174472-72-1], called Zr[acacac]4 for short in the following, was performed in analogy to the reported synthesis of zirconium(IV)acetylacetonate, [17501-44-9]17 (see reaction Scheme 1). Zirconium(IV)isopropoxide-2-propanol complex (430 mg, 1.1 mM) was placed in a Schlenk-tube under argon and dissolved in 10 mL of freshly distilled anhydrous toluene. Subsequently, triacetylmethane (0.82 mL, 6.1 mM) was added at once via syringe, and the * To whom correspondence should be addressed. Phone: +43 (0)512 507 5532. Fax: +43 (0)512 507 2926. E-mail: [email protected]. † Institute of Mineralogy and Petrography. ‡ Institute of General, Inorganic and Theoretical Chemistry. § Christian-Doppler-Laboratory for Advanced Hard Coatings at the Institute of Mineralogy and Petrography.

Scheme 1.

Reaction Scheme

reaction mixture was stirred for 19 h at room temperature. The resulting white suspension was centrifuged, and the supernatant toluene was decanted. Afterward, the remainder was suspended twice by ultrasonication in dry hexane (5 mL) and centrifuged again. The remainder was dried in a vacuum desiccator overnight yielding 440 mg of a white microcrystalline powder in analytical purity (yield 61% of theory). Another crop of crude material may be obtained by stripping off the toluene mother liquor. The product is sparingly soluble in toluene and freely soluble in tetrahydrofuran, dichloromethane, and acetonitrile. In contrast to a statistical ligand metathesis reaction,18 analytically pure Zr[acacac]4 is formed in this synthesis. Density. The density of the material was determined at 26.4 °C using a He-gas ultrapyknometer 1000. The resulting density from three independent measurements is 1.433(5) g/cm3. Infrared Spectroscopy. IR spectra of the polycrystalline material were recorded on a Nicolet 5700 FT-IR instrument equipped with a diamond-ATR-sensor. Observed peaks are listed in Table 1. NMR Spectroscopy. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance DPX 300 using CDCl3 as solvent (see also Table 1). Confocal Raman spectra. Confocal Raman spectra were obtained with a HORIBA JOBIN YVON LabRam-HR 800 Raman microspectrometer. Samples were excited at room temperature with the 488 nm line of a 30 mW Ar+-laser through an Olympus 100× objective. The laser spot on the surface had a diameter of approximately 1 µm and a power of 5 mW. Light was dispersed by a holographic grating with 1800 grooves/mm. Spectral resolution of about 1.8 cm-1 was experimentally determined by measuring the Rayleigh line. The dispersed light was collected by a 1024 × 256 open electrode CCD detector. Confocal pinhole was set to 1000 µm. Spectra were recorded unpolarized. All spectra were baseline-corrected by subtracting line segments and fitted to Gauss-Lorentz functions. Wavenumber calibration was done by regular measuring of the Rayleigh line. Wavenumber accuracy achieved by this method was better than 0.5 cm-1. The detection range was 100-4000 cm-1.

10.1021/cg060092p CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006

Tetrakis(3-acetyl-2,4-pentanedionato)zirconium(IV)

Crystal Growth & Design, Vol. 6, No. 7, 2006 1721

Table 1. IR and NMR Spectroscopic Data and Assignments IR 1H NMR 13C NMR

ATR, neat [cm-1] δ [ppm] (CDCl3) δ [ppm] (CDCl3)

3010, 1687, 1674, 1564, 1557, 1454, 1377, 1353, 1243, 1056, 682, 429 2,04 (s, 18 H), 2,17(s, 6 H), 2,36 (s, 12 H) 27,5 (CH3-acac), 33,9 (CH3-acetyl), 123 (C-3-acac), 189 (CdO-acac), 204 (CdO-acetyl)

Table 2. Parameters of X-ray Powder Diffraction and Rietveld Analysis instrument geometry radiation type, source generator settings discriminator detector effective µ‚t data collection temperature range in 2θ PSD step size internal integration step size space group chemical composition no. of contributing reflections no. of structural parameters no. of other parameters no. of restraints preferred orientation Rwp, Rexp, RBragg Be´rar-factor27

Stoe symmetric transmission X-ray, Cu KR1 40 kV, 40 mA primary beam, curved Ge(111) monochromator linear PSD, 6° width 0.14 28.0(5) °C 3-115° 0.1°, 240 s/step 0.01° P12/c1 Zr[C7O3H9]4, Z ) 2, Z′ ) 1 2125 total, 569 effective 61 + 54 for hydrogen positions 21 44 + 54 for hydrogen positions March-Dollase, || [1, 0, -0.55] G1 ) 0.69(1), fraction ) 68(1) % 3.60, 2.90, 4.03% 2.1

X-ray Powder Diffraction. Data for structure determination and refinement were collected on a Stoe STADI-MP diffractometer in flatplate transmission geometry using a round sample of 3 mm diameter prepared between two thin foils. The diffractometer was equipped with an asymmetric primary beam Ge(111) monochromator (yielding a strictly monochromatic Cu KR1 radiation) and a linear PSD with 6° detector range. The effective value of 0.14 for the product µt (µ ) linear absorption coefficient; t ) sample thickness) used for the absorption correction was determined experimentally from the intensity ratios at θ ) 0° with and without the sample. Details of the data collection and the refinement are given in Table 2. Structure Solution. The crystal structure of the material was solved from X-ray powder diffraction data. Peak positions for unit cell determination were determined by profile fitting using WinXPow.19 Indexing of the reflections was done with the Crysfire suite20 in combination with Chekcell.21 The symmetry was found to be monoclinic with lattice parameters of a ) 16.477 Å, b ) 12.187 Å, c ) 7.859 Å, β ) 102.33°. No systematic extinctions were found by Chekcell, indicating extinction symbol 2/mP1_1. A LeBail-refinement, performed with the program FullProf,22 provided starting values for the unit cell metric as well as background and peak width/shape parameters. These values were directly used for the subsequent structure solution. Unit cell volume of 1541 Å3 and experimentally determined density of F ) 1.433(5) g/cm3 indicated that the number of formula units per unit cell is Z ) 2. The corresponding X-ray density Fxcalc ) 1.41 g/cm3 is in good agreement with the experimentally determined value. Patterson analysis with GFourier23 gave strong maxima for the interatomic vector [0.5, 0.313, 0.5] (and its inverse). With a total of two heavy scatterers () Zr-atoms) in the unit cell, this not compatible with the existence of an my symmetry. Therefore, P2 remains as the only possible space group compatible with the observed extinctions with two symmetrically independent Zr-atoms on special positions (1a): (0, y, 0) and (1d): (0.5, y′, 0.5) with y′ ) y + 0.313. Determination of the position of the chelating ligands was performed by parallel simulated annealing using the program FOX.24 The 3-acetyl2,4-pentanedionato groups were modeled using soft constraints for bond lengths and angles; these restrictions were kept in the final structure refinement. The respective values of the applied constraints are given in Table 4 together with the observed values from the final structure. Additional constraints for the torsion angles (to keep the main part of the 3-acetyl-2,4-pentanedionato groups (C1‚‚‚C6,O1,O2) flat) were applied during the structure determination but were removed in the Rietveld refinement. Very soft restrictions on Zr-O-distances and Zrinvolving torsion angles were also applied during the structure determination to restrict the chelate ring folding to reasonable low values. Hydrogen atoms were not included in the structure determination

Table 3. Structural Parameters of Zr[acacac]4 in Space Group P2/ca atom

Wyk

x

y

z

Biso

Zr C1a* C2a C3a C4a C5a* C6a C7a* O1a O2a O3a C1b* C2b C3b C4b C5b* C6b C7b* O1b O2b O3b

2e 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g 4g

0(-) -0.0835(3) 0.0301(4) 0.2130(3) 0.3141(3) 0.5121(3) 0.3068(10) 0.4154(3) -0.0501(7) 0.2578(7) 0.2627(7) -0.3844(3) -0.2083(7) -0.0961(6) 0.0776(6) 0.1953(3) -0.1572(11) -0.1229(3) -0.1876(8) 0.1236(8) -0.2382(7)

0.3442(2) 0.0251(2) 0.1161(4) 0.1061(3) 0.1960(4) 0.1955(2) 0.0039(4) -0.0567(2) 0.2030(4) 0.2884(3) -0.0460(6) 0.5055(2) 0.4875(4) 0.5781(3) 0.5698(4) 0.6696(2) 0.6853(4) 0.7136(2) 0.3961(4) 0.4818(4) 0.7576(5)

0.25(-) 0.3704(2) 0.3515(7) 0.3637(6) 0.3519(6) 0.3869(2) 0.3968(4) 0.3487(2) 0.3221(5) 0.3210(4) 0.4510(4) 0.3672(2) 0.3487(7) 0.3591(7) 0.3535(7) 0.3710(2) 0.3819(2) 0.4726(2) 0.3163(5) 0.3244(5) 0.3389(4)

1.87(5) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8) 3.38(8)

a Atoms marked with an asterisk have attached three hydrogen atoms each, which were controlled during the refinement by restraints on bond distances and angles (1.0 Å and 109.3°, respectively). All esds should be multiplied by 2.1 for realistic uncertainties. a ) 7.85889(2) Å, b ) 12.18734(8) Å, c ) 16.67247(7) Å, b ) 105.0934(4)°.

Table 4. Bond Distances and Bond Anglesa atoms

distance [Å]

restraint [Å]

C1a-C2a C4a-C5a C6a-C7a C2a-C3a C3a-C4a C3a-C6a C2a-O1a C4a-O2a C6a-O3a

1.507(6) 1.513(3) 1.508(8) 1.404(4) 1.400(7) 1.479(7) 1.264(7) 1.269(7) 1.213(10)

1.51 1.51 1.51 1.40 1.40 1.48 1.27 1.27 1.21

C1b-C2b C4b-C5b C6b-C7b C2b-C3b C3b-C4b C3b-C6b C2b-O1b C4b-O2b C6b-O3b

1.511(8) 1.510(5) 1.505(4) 1.395(6) 1.396(9) 1.476(8) 1.267(9) 1.267(9) 1.208(8)

1.51 1.51 1.51 1.40 1.40 1.48 1.27 1.27 1.21

a

atoms

angle [deg]

restraint [deg]

O1a-C2a-C1a O1a-C2a-C3a C1a-C2a-C3a C2a-C3a-C4a C2a-C3a-C6a C4a-C3a-C6a O2a-C4a-C3a O2a-C4a-C5a C3a-C4a-C5a C3a-C6a-O3a C3a-C6a-C7a O3a-C6a-C7a O1b-C2b-C1b O1b-C2b-C3b C1b-C2b-C3b C2b-C3b-C4b C2b-C3b-C6b C4b-C3b-C6b O2b-C4b-C3b O2b-C4b-C5b C3b-C4b-C5b C3b-C6b-O3b C3b-C6b-C7b O3b-C6b-C7b

115.8(6) 121.2(6) 123.0(4) 120.6(5) 121.4(6) 117.8(6) 127.0(6) 112.0(5) 120.5(4) 119.3(9) 120.7(5) 117.5(8) 116.1(7) 127.0(7) 116.3(6) 122.1(7) 119.7(6) 118.1(8) 117.9(9) 122.3(6) 119.3(5) 130.5(7) 118.5(7) 111.0(6)

120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

All esds should be multiplied by 2.1 for realistic uncertainties.

but were added during the structure refinement; their positions are controlled by restraints on bond distances and angles (1.0 Å and 109.3°, respectively). The high symmetry of the 3-acetyl-2,4-pentanedionato group would allow its positioning on a special crystallographic position of 2-fold rotational symmetry with only very limited degree of disorder. In order not to exclude this possibility from the structure model, an initial annealing run in space group P1 with four independent ligands was performed. The central Zr atoms were placed initially on the positions known from Patterson analysis. After one million trials, the structure in P1 showed clearly that the 3-acetyl-2,4-pentanedionato groups were

1722 Crystal Growth & Design, Vol. 6, No. 7, 2006

To¨bbens et al. Final Rietveld refinement of the structure was performed with FullProf, using atomic structure factors and an overall isotropic DebyeWaller factor for all light atoms. Thompson-Cox-Hastings-PseudoVoigt functions were employed to describe the peak shape including an asymmetry correction following Finger et al.26 Background was modeled by linear interpolation between points with no or low Bragg intensity contribution. The experimentally determined absorption correction was applied to the data using the program-implemented function for the Stoe transmission geometry. A moderate degree of preferred orientation was best described using the March-Dollase model with a preferred orientation vector [1,0,-0.55], normal to c* and b*. The structure refinement converged to residuals of Rwp ) 3.7%, χ2 ) 1.57, RBragg ) 4.1% with an effective reflection/intensity-dependent parameter ratio of 4.8. The correction factor for estimated standard deviations of the refined parameters as proposed by Be´rar27 is 2.1. A graphical comparison between the observed and the calculated powder patterns is given in Figure 1; final structure parameters for the non-hydrogen atoms are given in Table 3.

Figure 1. Observed (circles) and calculated (solid line) step intensities and their difference (line at bottom of figure) of Zr[acacac]4. To obtain a more concise representation of the high-angle region, intensity in this area has been magnified by a factor of 10. Peak positions permitted by unit-cell metric are indicated by tick marks (middle portion). Residuals of the Rietveld refinement are Rwp ) 3.6%, χ2 ) 1.5. in general positions, with two symmetrically independent pairs of ligands at each Zr atom. After the symmetry was increased to P2, another one million trials were calculated. An inspection of the atomic coordinates of the model obtained from the structure determination using the MISSYM algorithm implemented in the program PLATON25 revealed the existence of an additional nyglide plane, relating the two molecules symmetrically independent in P2. At first glance, this result seems to contradict the outcome of the analysis of systematic absences obtained from Chekcell. However, the program searches not for individual symmetry elements but for absences conforming to potential space groups. For incomprehensible reasons, P2/n is missing in the internal list. Transforming the unit cell to the standard setting leads to the space group symbol P2/c with lattice parameters of a ) 7.85889(2) Å, b ) 12.18734(8) Å, c ) 16.67247(7) Å, β ) 105.0934(4) Å.

Discussion of Crystal Structure The 8-fold coordination of the Zr4+cation by the terminal ligand oxygens is square-antiprismatic, with a pseudo-symmetry very close to D2-222, even though the actual site symmetry is only C2-2 (Figure 2). Deviations of interatomic angles and distances from their respective average values (Tables 4 and 5) are very small for the inner coordination polyhedron, as well as for the entire molecule. The edges of the square sides of the oxygen coordination polyhedron are spanned by the 3-acetyl2,4-pentanedionato-chelate rings in such a way that two opposing edges of each square are built up by intra-ring O-O separations of the two symmetrically independent 3-acetyl-2,4pentanedionato groups. The inter-ring separations, which make up the remaining square edges, are slightly shorter than the intraring ones. Interatomic O-O distances constituting the square-connecting edges are in general slightly longer, with the notable exception of O1b-O2a. This region of the molecule also contains the strongest deviations from the expected bond

Figure 2. The molecule structure of Zr[acacac]4 (without H atoms, darker shades used as depth cue, prime denoting atoms generated by the 2-fold rotation axis) showing the square-antiprismatic coordination polyhedron (prepared using the program ATOMS28).

Tetrakis(3-acetyl-2,4-pentanedionato)zirconium(IV)

Crystal Growth & Design, Vol. 6, No. 7, 2006 1723

Figure 3. Molecule arrangement in the structures of Zr[acacac]4 (left) and β-Zr[acac]4 (right30). Columns of molecules are highlighted by dotted borders; unit cells are indicated by solid lines.

angles within the 3-acetyl-2,4-pentanedionato groups. The distortion at the interatomic bond angles at the C2b and C4a atoms attached to O1b and O2a, respectively, can thus be attributed to competitive steric requirements of the ligand geometry and the Zr(IV) coordination. The Zr(IV) center does not lie in the main plane of the chelate rings, resulting in a ring folding of 20° along the O-O edge. This leads to an overall shape of the molecule with two “flat” sides, composed of the squares of the coordination polyhedron and the two adherent rings, as well as two laterally elongated “long” domains, where the ligands are located, and two “open” directions. The short O1b-O2a edges are the ones in the open sides of the molecule. As described above, Zr[3-acetyl-2,4-pentanedionato]4 follows closely the behavior observed in the related structure of the β-polymorph of tetrakis(acetylacetonato)zirconium(IV), [1750144-9], Zr[C5O2H7]4, termed β-Zr[acac]4 in the following.29,30 A notable difference lies in the orientation of the 2-fold symmetry axis, which, for Zr[acacac]4, lies parallel to the long direction of the molecule and for β-Zr[acac]4 parallel to the open direction. The most prominent breach of the D2-222 quasi-symmetry of the molecule is the position of the free acetyl groups, which are not part of the square-antiprism, relative to the main plane of the rings. The corresponding torsion angle is about -50° for the a-labeled group and -86° for the b-labeled one (Figure 2). This is the result of the different positionings of the two ligands with respect to neighboring molecules and the correspondingly different conformations of the acetyl group. The b-ligand is oriented so that its C7bH3 group points directly to the center of the a-ligand’s chelate ring, keeping a maximum distance from all the ring atoms. The acetyl group of the a-ligand, on the other hand, adopts an arrangement influenced by methyl groups of its own columns and the ones of neighboring columns as well. With the similarity between the molecular conformations of Zr[3-acetyl-2,4-pentanedionato]4 and β-Zr[acac]4, it is hardly surprising that there are similarities in the overall arrangement, too. In Zr[3-acetyl-2,4-pentanedionato]4, the molecules are arranged in staggered columns, with the “flat” sides of the molecules facing toward each other. The Zr centers of the molecules are shifted orthogonally to the column axis, alternately in opposite directions along the long domain of the molecule (Figure 3). This results in two of the ligands of consecutive molecules lying directly adverse but antiparallel to each other.

Table 5. Interatomic Distances and Angles in the Inner Coordination Sphere of Zra distance [Å]

atoms

angles [deg]

Zr-O1a Zr-O2a Zr-O1b Zr-O2b

atoms

2.192(7) 2.172(5) 2.155(8) 2.160(6)

Zr-O1a-C2a Zr-O2a-C4a Zr-O1b-C2b Zr-O2b-C4b

136.7(6) 132.5(5) 131.1(8) 138.2(7)

O1a-O2a O1b-O2b O2a-O2b O1a-O1b

2.638(8) 2.630(9) 2.589(7) 2.581(7)

O1a-Zr-O1b O2a-Zr-O1b O2a-Zr-O2b O1a-Zr-O2a O1b-Zr-O2b

72.8(4) 72.9(4) 73.4(3) 74.4(3) 75.1(4)

O1a-O1a′ O1a-O2a′ O2a-O1b O1b-O2b′ O2b-O2b′

2.717(12) 2.723(9) 2.571(10) 2.732(12) 2.724(10)

O1a-Zr-O1a′ O1a-Zr-O2a′ O1b-Zr-O2b′ O2b-Zr-O2b′

76.6(5) 77.2(4) 78.5(5) 78.2(4)

a A prime denotes an atom on a ligand generated by the 2-fold rotation axis through Zr. All esds should be multiplied by 2.1 for realistic uncertainties.

In Zr[acacac]4, as well as in β-Zr[acac]4, the axis of the column is parallel to c, so that successive molecules are symmetryrelated by the cy-glide plane. Therefore, the key difference between the two structures is a result of the orientation of the molecules with respect to the column axis and the overall arrangement of the columns resulting from this. While in Zr[acacac]4 the molecules lie with their flat side normal to the column axis, in β-Zr[acac]4 their long axis is tilted by an angle of 26.7°, bringing all molecule centers exactly into a common bc-plane. This results in drastic changes of the molecule distances. In Zr[acacac]4, the shortest Zr-Zr-distance of 7.859 Å, corresponding to the translation along the a-axis, connects the open sides of the molecules. In β-Zr[acac]4, the corresponding distance is 8.360 Å, resulting from translation along the b-axis. However, the shortest Zr-Zr distance in β-Zr[acac]4 is along the column axis with 7.138 Å. In Zr[acacac]4 with 9.161 Å, this distance is considerably longer. This is a direct result of the sterically more demanding ligand moieties in Zr[acacac]4. The additional acetyl group increases the distance between the two adverse ligands of neighboring molecules as well as the staggering displacement along the long direction of the molecules. Furthermore, no staggering along the open direction of the molecules occurs in Zr[acacac]4, while in β-Zr[acac]4 it is effective to allow for close packing of the ligands of neighboring molecules. As a final consequence, an approximately round cross section of the columns in β-Zr[acac]4 leaves each column being surrounded by six neighboring columns in a nearly pseudohex-

1724 Crystal Growth & Design, Vol. 6, No. 7, 2006

To¨bbens et al.

cies in the 500-1600 cm-1 region are assigned to modes that predominantly involve primary motions of the ligand atoms.31,34 They are in very good agreement with other metal-acetylacetonate complexes (Table 6) and confirm that vibrational frequencies for this type of complexes vary only slightly with changing mass of the central metal. The chelating carbonyl stretching region between 1300 and 1600 cm-1 is very similar to that of bis(acac)oxovanadium complexes. Although very weak, a band at 1626 cm-1 may indicate ketonic carbonyl modes, as reported for the keto form of acetylacetone37 and γ-carbon-bonded acetylacetonates of platinum(II).38,39 The bands between 2900 and 3100 cm-1 are related to symmetric and asymmetric CH and CH3 stretching vibrations.33 Figure 4. Raman spectrum of polycrystalline Zr[acacac]4.

agonal arrangement. Columns in Zr[acacac]4, on the other hand, are of a flat cross-sectional shape and are packed in a primitive orthogonal arrangement. Solid-State Micro-Raman Spectroscopy The unpolarized Raman spectrum and Raman shifts of polycrystalline Zr[acacac]4 material are given in Figure 4 and Table 6. For comparison, Raman shifts of β-Zr[acac]4, of methyl and ethyl derivatives of bis(acetylacetonato)oxovanadium [VO(acac)2], [VO(3-methyl-acac)2], and [VO(3-ethyl-acac)2] are given.31-36 On the basis of these data, the observed bands of Zr[acacac]4 were assigned to vibrational modes of atomic groups in the structure. The region of 70-500 cm-1 is expected to involve dominantly metal-oxygen symmetric and asymmetric stretching modes.31 The very strong band at 445 cm-1 and the weaker one at 417 cm-1 have been identified as metal-oxygen symmetric stretching vibrations.31-33 Increased frequencies in the Zr-O modes of Zr[acacac]4 compared to β-Zr[acac]4 suggest increased Zr-ligand bond distances, accompanied by decreased Zr-O and increased C-O bond strength.31 Vibrational frequen-

Conclusion The molecular structure, especially the chelate bonding situation of Zr[acacac]4, is very similar to the comparable parent complex Zr[acac]4.17,30 This is reflected by the nearly undistinguishable Raman frequencies of corresponding vibrations. The intermolecular arrangement, on the other hand, is markedly different. Since no hydrogen bonds are present, it depends on weak interactions only and thus is subject to small differences in the molecular structure. As a supplement to the widely employed, bridged dicarboxylic acid salts, the introduction of chelates with exceptionally stable acac-based secondary building units (SBU) as bidentate rigid ligand interlinks40 opens a plethora of further possibilities for applications in the field of MOFs. Zr(acetyl-acac)4, as a model for tetrahedrally connected network topologies, provides reliable support, so that it is expected that positions and orientation of the chelate ligands and the substituents in interlinkable derivatives are subject to change only in a very limited range. Therefore, a high diversity of linkable terminal substituents can be envisioned. Furthermore, the high symmetry of the complex minimizes the potential of mismatches during

Table 6. Raman Shifts and Probable Assignment of Zr[acacac]4 Bands (in cm-1) and Comparison with Literature Data of Other Acetylacetonato Metal Complexesa Zr[acacac]4

β-Zr[acac]4b

[VO(acac)2]c

123 vw 142 vw 202 vw 228 vw 241 vw 257 vw 280 vw, b 405 vw, sh 417 w 445 vs 536 vw 563, 569 vw, d 659 w 676 w 773 vw 950 w 1017, 1029 vw, d 1184 vw 1279 vw, 1292 m, d 1310 vw, sh 1359 w, sh 1372 s 1425, 1436 vw, d 1525 vw 1586 vw 1626 vw 2922 vs 2969 vw 2994 vw 3023 vw 3083, 3096 sh, w, d

x x x x x x ∼300 w, b 416 m, b 441 s, sp 535 w 563 m, sp 661 m, b 687 w 772 w ∼945 m ∼1027 m 1281 vs, sp 1366 s, sp 1528 w x x x x x x x

x x x x 240 m 261 m 290 w 424 m 562 m 660 vw 945 s 1027 w 1187 m 1289 s 1374 s 1440 w 2920 s 2980 w 3005 w 3095 vw

[VO(methyl-acac)2]c x x x x 243 w 257 m 300 vw 420 w 735 s 1305 s 1362 m 1435 w 2922 s 2965 w 3010 w x x

[VO(ethyl-acac)2]c x x x x x x x x x x x x x x 723 s 1300 m 1445 w 2932 s 3083 vw

assignment ν(M-O) ν(M-O) ν(M-O) ν(M-O) ν(M-O) ν(M-O) νas(M-O) M-O? νas(M-O) or δ(C-CH3) νs(M-O) ? π(ring) or ν(M-O) νs(M-O) + δ(C-CH3) or δ(ring) δ(ring) π(C-H) or ν(C-C) ν(C-CH3) F(CH3) δ(C-CH3) νs(C-C-C) ? ? CH3 sym. def. or ν(C-O) δas(CH3) νas(C-C-C) ? ν(C-O) νs(CH3) νas(CH3) νas(CH3) ? ν(C-H)

a ν , symmetric stretch, ν , asymmetric stretch; δ, in plane deformation; π, out of plane deformation; F, rocking; vs, very strong; s, strong; m, medium; s as w, weak; b, broad; sp, sharp; sh, shoulder; d, doublet; -, not observed; x, not reported. b Fay and Pinnavaia (1968).30 c Claro et al. (2005).33

Tetrakis(3-acetyl-2,4-pentanedionato)zirconium(IV)

cross-complexation. This holds the potential for the synthesis of task-specifically tailored three-dimensional coordination polymers. Related Zr(acac)4-based MOFs will be reported in due course. Acknowledgment. We are grateful to Elisabeth Gstrein, Faculty of Chemistry and Pharmacy, for the determination of the bulk density of tetrakis(acacac)zirconium(IV). M.H. acknowledges a diploma grant from Aktion D. SwarovskiLeopold-Franzens-Universita¨t Innsbruck, Fo¨rderungsfonds 2004. Supporting Information Available: Crystallographic information file Zr-Acacac4.cif. This material is available free of charge via the Internet at http://pubs.acs.org.

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