Crystal structure of tetrakis(1,1,1,5,5,5-hexafluoroacetylacetonato)hafnium(IV)

The crystal structure of the square-antiprismatic complex tetrakis(1,1,1,5,5,5-hexafluoroacetylacetonato)hafnium(IV) is reported.

The crystal structure of the title compound, [Hf(C 5 HF 6 O 2 ) 4 ], has been determined. The asymmetric unit contains two Hf(hfac) 4 molecules (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate); both are located on general positions and have identical structures apart from the disorder involving three CF 3 groups in one of the two molecules. The molecules of Hf(hfac) 4 are arranged in layers that are parallel to the ab plane, and the coordination geometry of each hafnium(IV) center is a distorted square antiprism. An interesting aspect of the structure is that the hfac ligands are arranged so that the Hf(hfac) 4 molecules have idealized 2 point symmetry, in which two of the hfac groups bridge between the two squares. Although all other M(-diketonate) 4 compounds of Hf (and Zr) also have square-antiprismatic geometries; in almost all of them the ligands are arranged so that the molecules have 222 point symmetry (in which none of the hfac ligands bridges between the two squares). The factors that favor one structure over another are not clear.
Thin films of HfO 2 are widely used as the gate oxide in integrated circuits because of its high dielectric constant. Although most CVD precursors for HfO 2 , such as the dialkylamide Hf(NMe 2 ) 4 , have molecular weights less than 500, it has recently been discovered that higher molecular weight ISSN 2056-9890 precursors can enable superconformal growth in high aspect ratio features (i.e., faster growth deeper in the feature), which is an important goal in the microelectronics industry (Wang et al., 2014). The molecular weight of Hf(hfac) 4 is quite high (1006.8), but it nevertheless is highly volatile owing to the fluorine substituents, which reduce the strength of intermolecular interactions (Jones et al., 2009). Here we report on the crystal structure of Hf(hfac) 4 .

Structural commentary
There are two crystallographically inequivalent Hf(hfac) 4 molecules in the asymmetric unit (Fig. 1). The two molecules are structurally identical (r.m.s. deviation = 0.004 Å ), except that three of the CF 3 substituents in the Hf1 molecule are disordered over two sites (Fig. 2). Each hafnium atom is bound to four bidentate hfac ligands; the eight oxygen atoms define a square antiprism in which two of the hfac ligands bridge between the squares, giving an idealized point symmetry of 2. The Hf-O bond lengths range from 2.134 (2) to 2.210 (3) Å , whereas the C-O bond lengths range from 1.247 (5) to 1.275 (5) Å . All of the distances are as expected except that the C-F bond distances vary over a larger range than usual owing to the disorder.

Supramolecular features
The molecules are well separated and the shortest HfÁ Á ÁHf distance is 8.3610 (1) Å . The molecules form layers parallel to the ab plane (Figs. 3 and 4). Of the intermolecular C-HÁ Á ÁF contacts (ignoring the minor site F atoms; Table 1), only three are comparable to the 2.60 Å sum of the van der Waals radii (1.2 Å for H, and 1.40 Å for F attached to a primary alkyl (Bondi, 1964): H8Á Á ÁF5 (2.50 Å ), H13Á Á ÁF35 (2.65 Å ), and H3Á Á ÁF24 (2.68 Å ). Only one intermolecular FÁ Á ÁF interaction (again ignoring the minor site F atoms) is shorter than 2.8 Å , the sum of van der Waals radii for two F atoms attached to a primary alkyl, that involving F2Á Á ÁF22 (2.76 Å ). All these intermolecular contacts must be weak, given that the The asymmetric unit of Hf(hfac) 4 with displacement ellipsoids drawn at the 50% probability level. F10A-F12A, F16A-F18A and F22A-F24A are the minor components of the F atoms of the disordered CF 3 groups in the Hf1 molecule. Color coding: C -grey, O -red, H -white, F-green, Hf -blue.

Figure 2
Overlay of the crystallographically inequivalent Hf(hfac) 4 molecules with molecule 1 in orange, molecule 2 in blue, and the disordered CF 3 groups in green; r.m.s. deviation = 0.004 Å . compound sublimes in moderate vacuum only slightly above room temperature.

Database survey
A search of the Cambridge Structural Database (CSD) returned 21 structures of the form Hf(RCOCHCOR 0 ) 4 and 21 of the form Zr(RCOCHCOR 0 ) 4 (Groom et al., 2016). The R and R 0 groups, which could either be the same or different, included Me, CF 3 , i Pr, CH 2 t Bu, t Bu, thiofuranyl, CMe 2 (OMe), OSiMe 3 , OMe, OEt, O t Bu, Ph, and CH 2 COO t Bu. In all cases, the eight oxygen atoms describe a square antiprism about the metal center. This is the geometry expected for M(bidentate) 4 molecules in which the bidentate ligand has a large bite angle (Kepert, 1982).
Interestingly, of the 42 structures in the CSD, three different arrangements of the ligands have been seen, corresponding to the three idealized molecular point symmetries possible for a square-antiprismatic coordination geometry with four bidentate ligands (Fig. 5) (Marchi et al., 1943;Hoard & Silverton, 1963;Muetterties & Wright, 1967). The majority of them describe molecules with idealized 222 symmetry (in which none of the ligands bridge between the two squares), two describe molecules with idealized point symmetries of 2 (in which two of the ligands bridge between the two squares), and one describes a molecule with idealized 422 symmetry (in which all four ligands bridge between the two squares; in all cases, these point symmetries describe the arrangement of the ligands, and neglect differences between the R and R 0 groups, if any).
The current molecule Hf(hfac) 4 adds to the small number of group 4 M(RCOCHCOR 0 ) 4 complexes that adopt the structure with an idealized point symmetry of 2; interestingly, one of the others is the zirconium analog Zr(hfac) 4 (Calderazzo et al., 1998). There is no obvious reason why Hf(hfac) 4 and Zr(hfac) 4 adopt this geometry rather than one of the other two. Irrespective of the structure adopted, the Hf-O and Zr-O bond distances in all Zr and Hf -diketonates are all near 2.2 Å .
The NMR data for Hf(hfac) 4 show that all four C-H groups and all eight CF 3 groups are chemically equivalent on the NMR time scale at room temperature, so that there must be a dynamic process that interconverts the different hfac environments.

Figure 3
A single layer of Hf(hfac) 4 molecules as viewed along the c axis. The layered structure of Hf(hfac) 4 as viewed along the a axis.

Figure 5
The three different isomers of square antiprismatic tetrakis(bidentate) metal complexes. Point symmetries: (a) 2, (b) 222, (c) 422. and allowed to stir overnight. The solution was filtered, and the filtrate was taken to dryness under vacuum. The colorless product Hf(hfac) 4 was sublimed out of the brown residue at 15 mTorr and 303 K onto a water-cooled cold finger. Yield: 0.44 g (28%). 1 H NMR (400 MHz, C 6 D 6 ): 6.12 (s). 19 F NMR (400 MHz, C 6 D 6 ): À77.01 (s). The NMR spectra are similar to those previously reported for this compound in CCl 4 ( 1 H NMR: 6.54; 19 F NMR: À74.7); note that this previous work used the opposite chemical shift sign convention and a different 19 F NMR shift reference (Chattoraj et al., 1968).
X-ray quality crystals were grown by allowing Hf(hfac) 4 (0.1 g) to sublime inside an evacuated 50 mL Schlenk tube placed on top of a warm oven. After 12 h, crystals had formed on the cooler parts of the tube.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H-atom positions were positioned geometrically and refined as riding: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). The F10-F12, F16-F18, and F22-F24 atoms are disordered over two sites; their occupancies refine to 0.644 (18):0.356 (18), 0.507 (6):0.493 (6) and 0.61 (2): 0.39 (2), respectively. Within each disordered CF 3 group, the C-F distances were restrained to 1.35AE0.01 Å , and the F-C-F and C-C-F bond angles were limited to near-tetra-hedral values by restraining the FÁ Á ÁF and -CÁ Á ÁF distances to 2.15 (1) and 2.3 (5) Å , respectively. The displacement parameters for all F atoms were restrained to be approximately isotropic (ISOR 0.005). The (111), (021), (011), (110), (113), (122), (111), (220), and (121) reflections were obscured by the beam stop and were omitted from the final refinement. The largest electron density peak in the difference map (4.36 e Å À3 ) is located 0.85 Å from Hf2 and is certainly a Fourier truncation ripple.   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.