Two new hexanuclear titanium oxo cluster types and their structural connection to known clusters

Ti6O6(OiPr)2(OOCR′)10 (R′ = C4H7, Et) and Ti6O3(OiPr)14(OOC–CHCH–COO)2 represent new structure types of carboxylate-substituted clusters with a Ti6Ox core (x = 3–6). They complement the already known six structure types and thus allow conclusions on how the structures of such clusters evolve during substitution and condensation processes.


Introduction
Reaction of Ti(OR) 4 with carboxylic acids results in the formation of carboxylate-substituted oxo clusters with a broad range of nuclearities. In such reactions, partial or complete substitution of the OR groups by carboxylate ligands and generation of oxo groups through ester formation between the carboxylic acid and the eliminated alcohol compete with each other. 1 Common to all Ti oxo clusters is that the Ti atoms are sixcoordinate (rarely five-coordinate, see below) and the carboxylate ligands almost always bridge two Ti atoms. Stable clusters are obtained if the metal charges are balanced by the ligands and all available coordination sites are occupied. A large percentage of (the uncharged) carboxylate-substituted titanium oxo clusters are hexanuclear where the six octahedrally coordinated Ti atoms sum up to 24 positive charges and 36 coordination sites in total. The cluster cores of the known Ti 6 clusters are schematically shown in Scheme 1. The Ti 6 cluster type with the highest d c (and a wide variety of R/R 0 combinations) is Ti 6 O 6 (OR) 6 (OOCR 0 ) 6 , [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] where all oxygen atoms are m 3 -O. This leaves 12 positive charges unbalanced and 18 coordination sites unoccupied which are compensated by 6 terminal OR (one per Ti) and 6 bridging carboxylate ligands.
The robustness of the Ti 6 O 6 (OR) 6 (OOCR 0 ) 6 structure type is also demonstrated by the fact that the structures of many mixed-metal clusters are derived from this lead structure. 19 The four cluster types with a Ti 6 O 4 core vary by different d s and d c . Ti 6 O 4 (OR) 8 (OOCR 0 ) 8 20-27 is the second most common Ti 6 cluster type, with a higher d s and a lower d c than Ti 6 O 6 (OR) 6 (OOCR 0 ) 6 8 . 16 The occurrence of Ti 3 (m 3 -O) units in nearly all Ti oxo clusters, also in clusters with low d c , is a strong indication that this unit is formed in an early stage of the condensation process.
In this article, we report two new structure types of Ti 6 oxo clusters (Scheme 2), which demonstrate the structural variability of this cluster class, depending on d s and d c , but nevertheless have some features in common and thus complement the series of clusters shown in Scheme 1.

Results
Reaction of Ti(OiPr) 4 with 4 molar equivalents of cyclobutane carboxylic acid resulted in the formation of Ti 6 O 6 (OiPr) 2 -(OOCC 4 H 7 ) 10 (1a, Fig. 1). An isostructural cluster, Ti 6 O 6 (OiPr) 2 -(OOCEt) 10 (1b) was obtained as a minor side-product in a different reaction, which shows that 1a is not an isolated case. There are no significant structural differences between 1a and 1b. The centrosymmetric structures consist of six roughly coplanar titanium atoms. Two Ti 3 O units, with nearly trigonalplanar m 3 -O atoms (sum of angles 354.61), are bridged by two m 2 -O and four carboxylate ligands. The Ti/O core structure is basically the same as that of the Ti 6 O 4 (OR) 8 (OOCR 0 ) 8 clusters (Scheme 1), but the two bridging OR groups in the Ti 6 O 4 clusters are replaced by m 2 -O atoms. This replacement, however, also affects the bystander ligands, because the Ti charges must be balanced and all coordination sites occupied. To keep the cluster uncharged, two additional singly charged ligands (OR) must be removed, which, however, leaves two coordination sites unoccupied. Therefore two further terminal (OR) groups must be replaced by two bridging (carboxylate) ligands.
For this reason, the arrangement of ligands decorating the Ti 6 O 6 cluster core in 1 is slightly different compared to Ti 6 O 4 (OR) 8 (OOCR 0 ) 8 . Only the outer Ti atoms (Ti1 and symmetryrelated Ti1* in 1a) of the ellipse-shaped Ti 6 arrangement still carry  a terminal OiPr group (these atoms are substituted by two terminal OR ligands in the Ti 6 O 4 clusters). All the other coordination sites are occupied by oxygen atoms of bridging carboxylate ligands. Thus, Ti1 and Ti2 are coordinated by three bridging carboxylate ligands, and Ti3 by four. This results in the highest d s among all the Ti 6 oxo clusters. In passing, the highest possible d s for Ti oxo clusters (with 6-coordinate Ti atoms) is 2 as in [TiO(OOCR 0 ) 2 ] 8 . 34 The Ti 3 O group in 1 is quite unsymmetrical, the Ti1-O1 distance being much longer than Ti2-O1 and Ti3-O1 and, correspondingly, Ti2-O1-Ti3* being much larger than Ti2-O1-Ti1 and Ti3-O1-Ti1. This distortion of the Ti 3 O group is also observed in the Ti 6 O 4 (OR) 8  The situation in solution is hard to comprehend, because the number of signals of 1a in both the 1 H and 13 C solution NMR spectra at ambient temperature corresponds neither to the solid-state structure nor to a fully dynamic situation. Most striking is that two sets of signals of equal intensity are observed in the 1 H NMR spectrum for the OiPr groups and the CH protons of the cyclobutyl groups. The inequivalence of the OiPr groups indicates that either the inversion symmetry is lifted in solution or free rotation of the OiPr groups is restricted. On the other hand, the appearance of (only) two sets of cyclobutyl signals points to intramolecular ligand exchange processes. Both are common phenomena for Ti oxo clusters in solution. 35 We have shown previously that reaction of Ti(OiPr) 4 with an equimolar amount of phthalic anhydride resulted in transfer of an OiPr group from the metal to one carbonyl group of the anhydride and coordination of the thus formed phthalic monoester to titanium to give Ti 2 (OiPr) 6 (OOC-C 6 H 4 -COO i Pr) 2 (iPrOH). 15 In the analogous reaction of Ti(OiPr) 4 with maleic anhydride we now isolated a small proportion of Ti 6 O 3 (OiPr) 14 (OOC-CHQCH-COO) 2 (2) (denoted as Ti 6 O 3 (OR) 14 (OOCR 0 ) 4 in Scheme 1 for comparison with monocarboxylate ligands). This compound was almost certainly formed by the unintentional introduction of moisture in the system. We nevertheless report the structure of 2 here, because it nicely complements the structural series of known Ti 6 clusters.
Type II Ti 6 O 4 (OR) 12  Because 2 has no molecular symmetry, fourteen different groups of signals of the OiPr groups are expected in the 1 H NMR spectrum if the structure is static in solution. Although only four groups can be clearly resolved and the other ten groups only give a broad range of signals in the CH 3 region of the spectrum this appears to be the case. The CH is also not resolved. An assignment of the signals to specific OiPr groups is therefore not possible.

Discussion
Various types of carboxylate-substituted titanium oxo clusters have been isolated from reactions of titanium alkoxides with carboxylic acids. Clusters of a particular composition and structure are reproducibly formed, if the precursors and reaction conditions stay meticulously the same. While the structure of a given cluster can be rationalized, as discussed in the Introduction, it is currently not possible to predict which cluster type will be formed in a particular reaction environment. This is due to the fact that substitution and condensation reactions compete with each other and the relative rates of both reactions are influenced by a number of parameters, among them the electronic and steric properties of the groups R and R 0 and the Ti(OR) 4 /R 0 COOH ratio. 37 Furthermore, the reactions cannot be monitored in situ, because the IR and NMR spectra are largely uninformative. The isolation of a specific cluster from a reaction mixture (possibly containing different cluster species) could therefore also be due to a higher crystallization tendency.

X-ray structure analyses
Crystallographic data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with k-geometry using MoK a (l = 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. 38 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 Table 1.

Conflicts of interest
There are no conflicts to declare.