Mixed-Metal Oxo Clusters Structurally Derived from Ti6O4(OR)8(OOCR′)8

The mixed-metal oxo clusters FeTi5O4(OiPr)4(OMc)10 (OMc = methacrylate), Zn2Ti4O4(OiPr)2(OMc)10, Cd4Ti2O2(OAc)2(OMc)10(HOiPr)2, [Ca2Ti4O4(OAc)2(OMc)10]n, and [Sr2Ti4O4(OMc)12(HOMc)2]n were obtained from the reaction of titanium alkoxides with the corresponding metal acetates and methacrylic acid. Their structures are derived from Ti clusters with the composition Ti6O4(OR)8(OOCR′)8. The Ca and Sr derivatives consist of chains of condensed clusters.


Introduction
Oxo clusters of the general composition Ti a O b (OR) c -(OOCRЈ) d are obtained when titanium alkoxides, Ti(OR) 4 , are treated with more than one molar equivalent of a carboxylic acid. [1] The carboxylic acid not only provides carboxylate ligands but also acts as an in situ water source through esterification with the eliminated alcohol. The outcome of the reaction depends, among others, on the groups R and RЈ as well as the Ti(OR) 4 /RЈCOOH ratio. Many oxo clusters have been isolated with different degrees of condensation (a:b ratio), different degrees of substitution (a:d ratio), different proportions of residual OR groups (a:c ratio), and, as a consequence, different structures. One of the more prominent structure types is Ti 6 O 4 (OR) 8 (OOCRЈ) 8 , which was obtained for several R/RЈ combinations. [2,3] An example, with R = iPr and OOCR=OMc (Ti6) (HOMc = methacrylic acid), is shown in Figure 1.
In this article, we describe the structures of mixed-metal oxo clusters that are derived from that of Ti 6 O 4 (OR) 8 -(OOCRЈ) 8 . The variability of this structural motif shows that this is a robust structure that not only tolerates variations of R and RЈ, but also that of the metal polyhedra of which it is composed.

Results and Discussion
Although several clusters of the composition Ti 6 O 4 (OR) 8 -(OOCRЈ) 8 are known with various R/RЈ combinations, we prepared the derivative with R = iPr and OOCR=OMc (Ti6) (Figure 1) for better comparison of the structural parameters with that of the mixed-metal clusters reported in this article. The centrosymmetric Ti 6 O 4 core is formed by two Ti 3 (μ 3 -O) units, which are connected through two μ 2oxygen atoms. An alternative description of the structure is that of a Ti 4 O 4 ring of four corner-sharing octahedra to which two Ti octahedra (called "outer Ti" in the following) are condensed through shared edges. Balancing of charges and coordination numbers is achieved by two bridging OiPr ligands (connecting the two edge-sharing octahedra), eight bridging carboxylate ligands and six terminal OiPr ligands.
Treatment of Fe(OAc) 2 and Ti(OiPr) 4 (2 equiv.) with methacrylic acid (17 equiv.) resulted in reddish-brown crystals of FeTi 5 O 4 (OiPr) 4 (OMc) 10 (FeTi5) (Figure 2). The cluster core of FeTi5 is isostructural to that of Ti6. Although attachment of the "outer" Ti octahedra is the same as in Ti6, the four Ti atoms of the Ti 4 O 4 ring are partly replaced by Fe atoms owing to the nearly identical ionic radii of Ti 4+ (0.605 Å [8] ) and low-spin Fe 2+ in an octahedral coordination (0.61 Å). Distinction between these two elements in the crystal structure is not straightforward. The Ti/Fe1 site (corresponding to Ti2 in Ti6) was refined with an occupancy for Fe of 34 % and that of Ti/Fe2 (corresponding to Ti1 in Ti6) with 16 %. To prove incorporation of both metals, the crystals were washed with dry n-heptane and their metal content was checked with energy-dispersive X-ray spectroscopy (EDX), through which both Fe and Ti were found in the crystals.  FeTi5 needs two negative ligands less than Ti6 because of the lower charge of Fe 2+ , but the total number of coordination sites to be occupied by the ligands is the same because all metal atoms in both Ti6 and FeTi5 are octahedrally coordinated. Thus, the two terminal OiPr groups O12 (on Ti2) and O14 (on Ti3) in Ti6, which are nearly parallel to each other are replaced by one bridging OMc ligand in FeTi5 (O9 and O10) (compare Figures 1 and 2). As a consequence of this substitution, the coordination octahedra in FeTi5 are slightly tilted relative to Ti6. This results, among other things, in slightly different distances between the metal centers (3.4215,3.4917,and 3.0466 Å in FeTi5 compared with 3.3783,3.5862,and 3.1062 Å in Ti6 for analogous distances).
in Ti6), and the OiPr ligands bridging Ti1 and Ti3 in Ti6 are replaced by a bridging OMc ligand in Zn2Ti4.
In addition to the μ 3 -oxygen atom (O1), the Zn atom is connected to both neighboring Ti atoms through bridging OMc ligands, two to Ti1 and one to Ti2. The oxygen atoms of these three OMc ligands, together with the μ 3 -oxygen atom, form a tetrahedron around Zn. The Zn-O distances of the OMc ligands vary only slightly between 1.940(2) and 1.957(2) Å and are not much shorter than the Zn-O1 distance of 1.975(1) Å. The O-Zn-O angles vary between 103.24(6) and 116.79(6)°.
Reaction of Cd(OAc) 2 and an equimolar proportion of Ti(OiPr) 4 with a tenfold excess amount of methacrylic acid resulted in centrosymmetric Cd 4 Ti 2 O 2 (OAc) 2 (OMc) 10 -(HOiPr) 2 (Cd4Ti2, Figure 4). The structure of this cluster can again be related to that of Ti6, but there are more profound changes. (i) In contrast to the structures discussed before, the four Ti atoms in the center are replaced by Cd atoms, (ii) acetate groups [originating from the Cd(OAc) 2 precursor] were incorporated in the structure, (iii) two Cd atoms are bridged across the Cd 4 O 4 ring, and (iv) the structure contains no μ 2 -oxygen atoms, only the μ 3 -O units are retained.
The very short Ti-O1 bond length of 1.698 (2) Å is due to the coordinated ROH in the trans position.
All the metal atoms in both Ti6 and Cd4Ti2 are octahedrally coordinated. The total positive charge of the metals, however, is +24 in Ti6 but only +16 in Cd4Ti2. This means that a smaller number of (monoanionic) ligands must satisfy the coordination requirements of the metals. In addition to the ligands discussed before, coordination of the Cd 4 core must be completed by two OMc and two OAc ligands. This can only be achieved if each of the carboxylate ligands is tridentate. Thus, one oxygen atom (O2) of the remaining OMc ligands bridges Cd1 and Cd1Ј, and the second (O3) is coordinated to Cd2. The acetate ligands are bridging-chelating [O4 bridges Cd1 and Cd2; O4 and O5 chelate Cd2]. The coordination octahedron of Cd2 is much more distorted than that of Cd1 due to the chelating carboxylate. Whereas the cis O-Cd1-O angles of Cd1 are between 75.0(1) and 97.9(1)°, those of Cd2 are between 55.50 (8) and 109.49(9)°.
The μ 3 -OMc and the chelating/bridging acetate cause a shorter distance between the symmetry-related Cd1 atoms than the corresponding Ti atoms in Ti4 or Zn2Ti4. The distance between Cd1 and Cd1Ј [3.6618 (8)  Another carboxylate-substituted Cd/Ti oxo cluster reported in the literature, Cd 4 Ti 4 O 6 (OCHCH 2 NMe 2 ) 4 -(OCCF 3 ) 4 (OAc) 4 , [9] is also based on four interconnected Cd polyhedra. In this case, however, the Cd 4 unit is capped by two condensed Ti octahedra on both sides.
Reaction of an equimolar mixture of Ca(OAc) 2 and Ti(OiPr) 4 or Ti(OBu) 4 with methacrylic acid (4.5 equiv.) resulted in crystals of [Ca 2 Ti 4 O 4 (OAc) 2 (OMc) 10 ] n (Ca2Ti4) besides much of a colorless precipitate of unknown composition. In Ca2Ti4 the outer two Ti octahedra of the Ti6 structure are replaced by Ca distorted pentagonal bipyramids, but the structure of the Ti 4 O 4 core is retained, similar to that in Zn2Ti4. Owing to the lower charge of Ca 2+ and its higher coordination number, the clusters are condensed to endless parallel chains of Ca 2 Ti 4 units ( Figure 5), contrary to the molecular clusters discussed before. The Ca 2 Ti 4 repeating units are connected through a chelating-bridging acetate ligand. The acetate ligand is chelating Ca1, while one of its oxygen atoms (O3) is bridging Ca1 and Ti1, and the other (O4) Ca1 and Ca1Ј. The Ca1-O4 bond length [2.660(2) Å] is much longer than that of Ca1Ј-O4 [2.292(2) Å].  (8), Ca1-Ti1 3.5606 (7), Ca1-Ti2Ј 3.6737 (7), Ca1-O1 2.500(2), Ti1-O1 1.931 (2), Ti1-O2 1.745 (2), Ti2-O1 1.774 (2), Ti2-O2 1.875 (2) (8) 4 , and methacrylic acid. [5] A compound with a different structure was obtained when Sr(OAc) 2 was treated with Ti(OBu) 4 . The arrangement of the metal polyhedra of polymeric [Sr 2 Ti 4 O 4 (OMc) 12 (HOMc) 2 ] n (Sr2Ti4, Figure 7) is the same as that of Ca2Ti4 and Ca2Ti4a, but there are differences in the ligand sphere, especially in the coordination of Sr. In addition to μ 3 -oxygen atoms, Sr1 is connected to Ti2 and Sr2 to Ti4 through two bridging OMc ligands each, similar to Ca1 and Ti1 in Ca2Ti4. The connection of Sr1 and Sr2 to Ti3 and Ti1 by two OMc ligands each is slightly different. In one pair of OMc ligands, one oxygen atom of the COO group bridges Sr1 and Ti1, whereas the other oxygen atom coordinates to Sr2 (correspondingly, one oxygen atom bridges Sr2 and Ti3, and the other coordinates to Sr1). In the second pair of OMc li-gands, one oxygen atom of the COO group bridges the two Sr atoms and the other oxygen atom binds to Ti1 (or Ti3, respectively).

Conclusion
The structure of all the clusters described in this article can be derived from that of Ti6. The general structure is retained when part of the Ti atoms is replaced by twovalent atoms (Fe 2+ , Zn 2+ , Cd 2+ , Ca 2+ , Sr 2+ ), but the lower charge of the second metal renders modification of the ligand sphere necessary. Depending on the preferred coordination number of the two-valent atoms, this adaptation occurs differently. In addition to different coordination of the negatively charged ligands, completion of the coordination sphere is also possible by coordination of neutral ligands (ROH, McOH), as observed in Cd4Ti2, Ca2Ti4a, or Sr2Ti4.
Fe 2+ and Ti 4+ have similar bonding characteristics with oxygen and the ionic radii are almost equal (0.61 and 0.605 Å), hence the four inner Ti 4+ sites are partly replaced by Fe 2+ atoms (FeTi5). In contrast, the ionic radius of Cd 2+ is much bigger (0.95 Å). This results in a different arrangement of the coordination octahedra in Cd4Ti2, namely, re-placement of the inner Ti atoms by Cd. The lower charge is compensated by a different coordination behavior of the smaller number of ligands.
Although the size of Zn 2+ is the same as that of Fe 2+ and Ti 4+ (0.61 Å), it usually exhibits tetrahedral coordination. Hence a partial substitution of the Ti atoms is not possible. In Zn2Ti4, the two outer Ti octahedra are replaced by Zn tetrahedra, with corresponding adjustment of the connecting ligands.
Ca 2+ and Sr 2+ ions are much bigger, have higher coordination numbers (seven-coordinated Ca 2+ 2.00 Å, eight-coordinate Ca 2+ 2.17 Å, ten-coordinated Sr 2+ 2.33 Å), and the bonds are less directed. In the Ca and Sr compounds, the outer Ti octahedra are substituted with Ca or Sr polyhedra. Different to FeTi5, Zn2Ti4, and Cd4Ti2 where molecular clusters were obtained, Ca2Ti4, Ca2Ti4a, and Sr2Ti4 form chains of condensed clusters in the crystal lattice. This is enabled by the higher coordination of Ca 2+ and Sr 2+ .
The structures reported in this article impressively demonstrate the subtle balancing of charges and coordination behavior of both the metals and the ligands to reach a stable cluster. [6] In light of the reported results, it is to some extent surprising that such closely related structures were obtained with a given set of ligands, despite the considerably different ionic radii and coordination numbers of the metals.

Experimental Section
General: All experiments were carried out under an argon atmosphere using standard Schlenk techniques. Ti(OBu) 4 , Fe(OAc) 2 , and Sr(OAc) 2 were obtained from Aldrich, Ti(OiPr) 4 from ABCR, Ca(OAc) 2 ·H 2 O and Zn(OAc) 2 ·H 2 O from Fluka, and Cd(OAc) 2 ·H 2 O from Merck. Water-free metal acetates were obtained by drying under vacuum at 130°C overnight (verified by IR spectroscopy). All solvents used for NMR spectroscopy (Eurisotop) were degassed prior to use and stored over molecular sieves. Ti6 was prepared analogously to the OnPr derivative. [2] 1 H and 13 C solution-state NMR spectra were recorded on a Bruker Avance 250 (250.13 MHz [ 1 H], 62.86 MHz [ 13 C]) equipped with a 5 mm inverse-broadband probe head and a z-gradient unit.
General Preparative Procedure: Ti(OiPr) 4 , the corresponding waterfree metal acetate and an excess amount of methacrylic acid were mixed. No solvent was added unless otherwise stated. The mixture was left standing in a closed vessel until crystals were formed.
X-ray Structure Analyses: Crystallographic data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with κgeometry at 100 K using Mo-K α (λ = 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. [10] The structures were solved by charge flipping (JANA2006). Refinement was performed by the full-matrix least-squares method based on F 2 (SHELXL97) 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 Tables 1 and 2. CCDC-1005662 (for Ti6), -1005663 (for FeTi5), -1005664 (for Zn2Ti4), -1005665 (for Cd4Ti2), -1005666 (for Ca2Ti4), -1005667 (for Ca2Ti4a), and -1005668 (for Sr2Ti4) contain the supplementary crystallographic data for this paper. These data can be ob-