Phosphonate-substituted zirconium oxo clusters

Abstract The phosphonate-substituted zirconium oxo clusters Zr6O2(OBu)12(O3PPh)4 and Zr7O2(OiPr)12(O3PCH2CH2CH2Br)6, with octahedrally coordinated Zr atoms, were synthesized by reaction of zirconium alkoxides with phosphonic acid bis(trimethylsilyl) esters. The basic structural motif are Zr3O(µ2-OR)3(OR)3 units which are connected in different ways. Graphical abstract

Phosphonate-substituted metal compounds are commonly prepared from the corresponding phosphonic acids or their metal salts. We have recently shown that titanium oxo clusters can be easily obtained from the reaction of titanium alkoxides with phosphonic acid bis(trimethylsilyl) esters [14,15]. The esters have the advantage of being soluble in organic solvents. Their reaction with alcohol added to the reaction mixture liberates phosphonic acid which substitutes part of the OR groups of Ti(OR) 4 in a relatively fast reaction. Oxo groups are generated in situ either by water originating from esterification of (coordinated or non-coordinated) phosphonic acid or by nonhydrolytic processes.

Results and discussion
Crystals of Zr 6 O 2 (OBu) 12 (O 3 PPh) 4 (1, Fig. 1 [16]). The crystallographic symmetry of 1 is retained in solution since the 31 P NMR spectrum in CD 2 Cl 2 showed only one signal at 6.57 ppm. The expected number of signals with the expected shifts was observed in the 1 H NMR spectrum.
The most surprising feature of 1 is that all zirconium atoms are octahedrally coordinated. This is remarkable since higher coordination numbers (7-9) are mostly found in zirconium oxo clusters. The structure of 1 is different from that of oxo clusters obtained from reactions of Ti(OiPr) 4 with bis(trimethylsilyl) phosphonates although Ti is also six-coordinated there. M 3 O(l 2 -OR) 3 12 (O 3 PR) 6 (R=CH 2 CH 2 CH 2 Cl or benzyl) are connected through a central Ti atom [14]. In the case of titanium, structures Ti 4 (l 3 -O)(l 2 -OiPr) 3 (OiPr) 5 -(O 3 PR) 3 L (L = neutral ligand) and dimers thereof were also obtained, where the Ti 3 O unit is capped by a Ti(OiPr) 2 L group.
The symmetry of 2 is retained in solution as only one signal at 30.6 ppm was observed in the 31 P NMR spectrum in C 6 D 6 . The 1 H NMR spectrum shows only two doublets for the isopropoxo CH 3 groups as well as two multiplets of the CH groups. Therefore, all terminal as well as all bridging isopropoxo ligands are symmetry related in solution.

Conclusions
The coordination chemistry of titanium and zirconium, including that of metal oxo clusters, is usually quite different even if the same reaction conditions and stoichiometric ratios of the reactants are employed. This is due to the different coordination numbers. The surprising outcome of the work reported in this article is that oxo clusters were obtained in the reaction of M(OR) 4 (M = Ti, Zr) with bis(trimethyl)silylphosphonates where the coordination numbers and geometries of both Ti and Zr were the same. For this reason, the structures of the obtained Zr clusters were the same as those of Ti oxo clusters (for 2) or very closely related (for 1). A possible reason for this feature might be that the M 3 O(l 2 -OR) 3 (-OR) 3 moiety appears to be a very robust building block, as already postulated earlier [14].

Experimental
All operations were carried out in a moisture-and oxygenfree argon atmosphere using Schlenk techniques. 2-Propanol and 1-butanol were dried by distilling twice from sodium metal. The phosphonates were prepared as previously reported [14,15]. Zirconium isopropoxide and zirconium n-butoxide were obtained from Sigma-Aldrich and used without further purification.

X-Ray structure analyses
All measurements were performed using MoK a radiation (k = 71.073 pm). Data were collected on a Bruker AXS Smart Apex II four-circle diffractometer with j-geometry at 100 K with u and x-scans and 0.5°frame width ( Table 1). The data were corrected for polarization and Lorentz effects, and an empirical absorption correction (SADABS) was applied. 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 checked with the program PLATON. The structure was solved by the Patterson method (SHELXS97 [18]). Refinement was performed by the fullmatrix least-squares method based on F with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were inserted in calculated positions and refined riding with the corresponding atom. Four of the six crystallographic independent butoxo ligands in 1 were disordered and refined with about 50 % for each position. The same treatment was done for three of the 3-bromopropyl moieties and two isopropoxo ligands in 2. Furthermore, one 3-bromopropyl moiety was refined using three different positions with 42, 36, and 21 % occupancy.
CCDC-1402779 (for 1) and 1402780 (for 2) contain the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.