New zirconium and zirconium–titanium oxo cluster types by expansion or metal substitution of the octahedral Zr6O8 structural motif

The cluster core structures of Zr10O8(OBu)16(OOC–C6H4–CH2Cl)8 and Zr9O6(OBu)18(OOCCCEt)6 are derived from that of known Zr6O4(OH)4(OOCR)12 clusters by expansion of the octahedral Zr6O8 core. The core structures of the heterobimetallic clusters Ti2Zr4O5(OH)2(OPr)(OOCCMe3)11 and Ti3Zr3O4(OH)3(OBu)3(OOCCMe3)10 can be regarded as hybrids between that of Zr6O4(OH)4(OOCR)12 and Ti6O6(OR)6(OOCR′)6.

In the clusters Zr 6 O 4 (OH) 4 (OOCR) 12 , the eight triangular faces of the M 6 octahedron are alternatively capped by l 3 -O or l 3 -OH groups. Each Zr atom is coordinated by eight oxygen atoms. According to quantum mechanical calculations, a structure where the 12 carboxylate ligands bridge the 12 Zr-Zr edges (i.e. an O hsymmetric cluster) are lowest in energy [13]. In most crystal structures, however, only 9 carboxylate ligands are bridging and three chelate the Zr atoms of one triangular face. Several variations of this structural motif were found, such as the dimeric structures [Zr 6 O 4 (OH) 4 (OOCR) 12 ] 2 [2,14] in which two Zr 6 units are bridged by carboxylate ligands or clusters Zr 6 O 4 (OH) 4 (OOCR) 12 (LH) where a bridging carboxylate ligand is converted to a g 1 -OOCR ligand with concomitant coordination of a water or alcohol molecule (LH) to the emptied coordination site. Coordination of LH is supported by a hydrogen bond to the g 1 -OOCR ligand [2,6]. A third possibility to modify the Zr 6   The octahedrally coordinated Zr atoms form a tetrahedron among each other, i.e. the structure of 1 can be described as an octahedral Zr 6 unit inscribed in a larger Zr 4 tetrahedron (Fig. 2).
In the parent structure, Zr 6 O 4 (OH) 4 (OOCR) 12 , the l 3 -OH groups are asymmetrically bonded, with two shorter and one longer Zr-O distance. Although the Zr-O distances of 1 are more uniform, some asymmetry is found in the Zr-O-Zr bond angles between the octahedral (outer) Zr atoms and that of the Zr 6 core: one angle each is in the range 136.0-140.8(2) Å, while the other two are 104.0-107.9(2) Å.
While even structural details of the Zr 6 O 8 cluster core are very well preserved upon coordination of the additional Zr atoms, the ligand sphere has undergone some changes. At the first glimpse it appears surprising that different to the symmetric cluster core, the substitution pattern of the Zr atoms is not uniform. The coordination of Zr1 and Zr6 is completed by the bridging ligands (two OBu groups and one carboxylate group) to the outer Zr atoms in addition to the l 3 -and l 4 -oxygen atoms. Zr3 has two bridging OBu groups to outer Zr atoms and two bridging carboxylate ligands to Zr2 and Zr4 of the Zr6 core. Two carboxylate ligands chelate Zr5. This results in a coordination number of seven for Zr1, Zr2, Zr4 and Zr6 and a coordination number of eight for Zr3 and Zr5 (in the parent cluster all Zr atoms are 8-coordinate). An explanation for this observation might be that this is a compromise between the preferential coordination requirements of the Zr atoms and the number of bidentate ligands available. If all Zr atoms were 8-coordinate, 24 mono-anionic ligands (in addition to the eight l 3 -and l 4 -O) would have to occupy 52 coordination sites; this is not possible with the available ligands (OR and OOCR).
Crystals of Zr 9 O 6 (OBu) 18 (OOCC"CEt) 6 (2, Fig. 3) were obtained when water was added to an equimolar mixture of Zr(OBu) 4 and 2pentynoic acid in BuOH. The core structure of 2 is derived from that of 1 by removing one Zr atom (Zr5 in 1) together with two (chelating) carboxylate ligands from the central Zr6 octahedron (Fig. 4).
This Zr atom in 1 is connected by two l 3 -and two l 4 -O atoms with the other Zr atoms. In 2, the two l 4 -oxygen atoms become l 3 and, for charge balancing reasons, the l 3 -O are replaced by l 2 -OBu groups. The remaining carboxylate and OBu ligands coordinate in the same way as in 1, also the Zr-O bond lengths are in the same range.
It was previously observed that clusters of different composition may be obtained if the precursor composition is slightly varied, such as metal alkoxides with different alkoxo groups [19] or the stoichiometric ratio of the precursors [20]. In the present case we obtained a second Ti/Zr mixed-metal cluster at different reaction conditions, namely Zr 3 Ti 3 (l 3 -O) 4 (l 3 -OH) 3 (OBu) 3 (OPiv) 10 Á HOPiv (Figs. 7 and 4). The structure of 4 is a hybrid of the Zr 6 O 8 and Ti 6 O 6 structure types; one face of the M 6 octahedron is    The Ti atoms have the same ligand environment as in the Ti 6 O 6 clusters (including a terminal OR ligand) and the Zr atoms show almost the same coordination as the Zr 6 O 8 clusters (Fig. 7). The only major exception is that only two Zr atoms (Zr2 and Zr3) have chelating ligands, while the third (Zr1) has two g 1 -OPiv ligands which show strong hydrogen bonds to neighboring l 3 -OH groups [O2Á Á ÁO28 2.556 (3) Fig. 8).

Conclusions
The core Zr 6 O 8 of the Zr 6 O 4 (OH) 4 (OOCR) 12 structures can be regarded as the smallest possible structural section of tetragonal zirconia which is stabilized by organic ligands. Along these lines, clusters 1 and 2 are expansions of this embryonic development of the zirconia structure (with one Zr atom missing in 2). Comparison of the Zr 6 O 8 structures with that of 1 also shows that the number and coordination of the organic ligands must be adjusted to the increased cluster size in order to provide sufficient stabilization.
The same line of reasoning applies to the Ti/Zr oxo clusters 3 and 4. The titania-zirconia phase diagram [21] contains several ordered and disordered solid solutions of (Zr,Ti) 2 O 4 the structures of which are not fully understood. We therefore speculate that the cluster cores of 3 and 4 might be structurally related to structural motif(s) in tetragonal (Zr,Ti) 2 O 4 in the same way as Zr 6 O 8 and 1 are related to tetragonal zirconia.

X-ray structure analyses
Crystallographic data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with j-geometry at 100 K using Mo Ka (k = 0.71073 Å) radiation. The data were corrected for polarization and Lorentz effects, and an empirical absorption correction (SADABS) was employed. The cell dimensions were refined with all unique reflections. SAINT PLUS software (Bruker Analytical X-ray Instruments, 2007) was used to integrate the frames. Symmetry was then checked with the program PLATON [22]. The structures were solved by charge flipping (JANA2006). Refinement was performed by the full-matrix least-squares method based on F 2 (SHELXL97 [23]) with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were inserted in calculated positions and refined riding with the corresponding atom. Crystal data, data collection parameters and refinement details are listed in Table 1.
Some organic groups in the structures showed disorder. The atom positions were split and their occupancies were refined. The large positive and negative residual electron density in 1 is located close to the CCl bond. Besides two molecules of pivalic acid and one molecule of butanol in 4, residual electron density remained in a void of 280 Å 3 , which could not be identified. Because the residual density was too far away from the cluster, SQUEEZE by PLATON was applied to remove this electron density.