A heterobimetallic copper–titanium oxo cluster with a new structural motif

Abstract Mixed-metal oxo clusters Cu4Ti5O6(OOCR)16 (OOCR = methacrylate, propionate) were obtained by reaction of titanium alkoxides and copper carboxylates with propionic or methacrylic acid. Graphical abstract


Introduction
Metal oxo clusters can be obtained by controlled hydrolysis of metal alkoxides and regarded as intermediates or building blocks in the formation of sol-gel materials. They may also serve as model compounds for structural units in amorphous gels. Especially for bimetallic gels, nearly nothing is known about distribution and mutual arrangement of the metals. The structural characterization of heterobimetallic oxo clusters may therefore provide some evidence in this respect.
Schubert et al. have previously developed a convenient approach to synthesize heterobimetallic oxo clusters with early transition metals (Ti, Zr, Y) as one of the metals. To this end, carboxylic acids were initially reacted with metal alkoxide mixtures. The oxo groups of the cluster core are produced in situ through esterification of the carboxylic acid with alcohol which is liberated by substitution of alkoxo ligands against carboxylate ligands. A simplification of this method is to use only one metal alkoxide, and a metal acetate as the source for the second metal [1][2][3].
The structures of most titanium-containing heterobimetallic clusters are based on a few common motifs, despite variation of coordination number, ionic size, and charge of the second metal. Some clusters with Pb, Sr [1], La, and Ce [2] are structurally derived from cyclic Ti 8 O 8 (OOCR) 16 . Another series is based on partial replacements of the Ti atoms in Ti 6 O 4 (OR) 8 (OOCR') 8 by Fe, Zn, Cd, Ca, or Sr, with concomitant rearrangement of the ligand shell [3]. The structures of clusters with Zr [4], Hf [5], Y [6], Sm, Eu, Gd, or Ho [2] are zig-zag chains of condensed [TiO 6 ] and [MO x ] polyhedra. In this article, we describe a Cu/Ti oxo cluster the structure of which is not related to these series although the radius and coordination properties of Cu 2? ions are not too different to that of other divalent metals used before.

Results and discussion
When Ti(OiPr) 4 , Ti(OPr) 4 , or Ti(OBu) 4 were reacted with Cu(OAc) 2 and methacrylic acid (HOMc) in different molar ratios, the centrosymmetric heterobimetallic cluster Cu 4 Ti 5 (l 3 -O) 6 (l 2 -OMc) 16 (1, Fig. 1 [1,7] was supressed by a greater excess of methacrylic acid. The isostructural cluster Cu 4 Ti 5 O 6 (OProp) 16 (2) was obtained from Ti(OiPr) 4 , Cu(OAc) 2 , and propionic acid (HOProp). The Ti atoms in 1 and 2 are octahedrally coordinated by six oxygen atoms, the coordination geometry of the copper atoms is a square pyramid. The [TiO 6 ] and [CuO 5 ] polyhedra are connected through six l 3 -oxygen atoms, each of which being bonded to two Ti and one Cu atom. The five Ti atoms and the core oxygen atoms are coplanar; the copper atoms are located above and below the Ti 5 O 6 plane. While the [TiO 6 ] octahedra are corner-sharing among each other and form a face-centred rectangle, two [CuO 5 ] square pyramids (Cu2) share a corner with two [TiO 6 ] octahedra and the other two share an edge with that of central Ti1 (which is located on an inversion centre). The central Ti atom is thus coordinated to four l 3 -O, the other Ti atoms to two l 3 -O, and the Cu atoms to either one or two.
The  6 (l 2 -OMc) 16 was also obtained with cocrystallized CH 2 Cl 2 (1S). The molecular structure is the same as that of 1, but the crystal structure is different (Fig. 2).

Conclusions
The structures of Cu 4 Ti 5 O 6 (OOCR) 16 represent a new structural motive, which cannot be derived from known Ti oxo clusters in a straightforward manner. There is a faint resemblance to the structure of Zn 2 Ti 4 O 4 (OiPr) 2 (OMc) 10 [3], where two [ZnO 4 ] tetrahedra are connected to a ring of four corner-sharing [TiO 6 ] octahedra through two l 3 -O groups. The structure of 1 and 2 is obtained by cutting the Zn/Ti cluster in two halves, replacing the [ZnO 4 ] tetrahedra by [CuO 5 ] square pyramids and inserting the rod-shaped unit Cu1-Ti1-Cu1* unit between both halves.
Formation of oxo clusters by reaction of metal alkoxides and carboxylic acids usually depends to a considerable degree on the OR groups of the employed metal alkoxide as well as the kind of carboxylic acid. This is surprisingly not the case for the Ti/Cu cluster type reported in this communication. This cluster type therefore appears to be a thermodynamically favoured structural entity. Another unusual feature is that the cluster contains no residual OR ligands, as in most other titanium/metal oxo clusters isolated until present. Both features may be connected with each other.

Experimental
All experiments were carried out under Ar atmosphere using standard Schlenk techniques. Ti(OPr) 4 , Ti(OBu) 4 , and Cu(OAc) 2 were obtained from Aldrich, Ti(OiPr) 4 from ABCR. Water-free copper acetate was obtained by drying in vacuum at 130°C over night. The drying process was monitored by IR spectroscopy. Cu(OMc) 2 was synthesized according to literature [8].

X-ray crystallography
Crystallographic data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with j-geometry at 100 K using MoK a (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 checked with the program PLATON. The structures were solved by charge flipping (JANA2006). Refinement was performed by the full-matrix least-squares method based on F 2 (SHELXL97 [9]) 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 propionate ligands of 2 show positional disorder.