Lead-rich carboxylate-substituted titanium–lead oxo clusters

Abstract The carboxylate-substituted mixed-metal oxo clusters Pb6Ti6O9(acetate)(methacrylate)17 and Pb4Ti8O10(OiPr)18(acetate)2 contain a higher number of lead atoms in the cluster core than previously reported compounds. The metal atoms in both clusters are arranged in three layers of different composition, which are connected through oxygen, propionate and/or carboxylate bridges. Graphical abstract


Introduction
Metal oxo clusters of the general composition M a O b (OH/ OR) c (OOCR 0 ) d are obtained when early transition metal alkoxides, M(OR) n , are reacted with more than one molar equivalent of carboxylic acid [1]. The carboxylic acid not only provides carboxylate ligands but also the oxo groups through esterification with the alcohol eliminated during the substitution reaction. This protocol can be extended to mixed-metal oxo clusters by employing mixtures of metal alkoxides or, alternatively, a metal alkoxide and a metal salt.
A variety of titanium/metal oxo clusters has been obtained by this route. The structures of some of them are based on common structural motives, as has been discussed elsewhere in detail [2,3]. Carboxylate-substituted Pb/Ti oxo clusters are in a sense unique as a variety of compounds with different Pb:Ti proportions are known. This allows gaining insight in how the structural features depend on the Ti/metal ratio. Carboxylate-substituted mixed-metal clusters generally have a richer structural chemistry than polynuclear compounds with a similar composition but without such ligands, because the bridging ligands provide more possibilities for connecting metals. Thus, contrary to the many examples of carboxylate-substituted Pb/Ti oxo clusters, only one unsubstituted cluster is known, viz. Pb 2 Ti 2 O(OiPr) 10 , the structure of which is based on a Pb 2 Ti 2 (l 4 -O) tetrahedron [4].
In the latter compound, each Ti atom is connected to both neighbouring Ti atoms through one l 2 -oxygen and two bridging OMc ligands each. The Ti 8  Common to the known carboxylate-substituted Pb/Ti oxo clusters is the metal ratio of Pb [2][3][4]6,8), notwithstanding the different structures, the different metal:oxygen ratios and the different ligand shell composition. In this article we report two new Pb/Ti oxo clusters with a greater number of lead atoms, viz. Pb 6

Results and discussion
Metal oxo clusters are very reproducibly obtained when all reaction parameters are meticulously kept, whereas seemingly minor variations may result in different clusters. For example, crystals of Pb2Ti8 were formed within three weeks when Pb(OAc) 2 , Ti(OBu) 4 , and methacrylic acid were reacted in a 1:1:4 ratio at room temperature [9]. In contrast, colourless crystals of Pb6Ti6 were obtained after four months when equimolar amounts of Pb(OAc) 2 and Ti(OiPr) 4 were first reacted at 70°C in allylic alcohol, and two equivalents of methacrylic acid were added after cooling. The same reaction at room temperature resulted in the same cluster as Pb2Ti8 with OAllyl instead of OBu ligands. Note that Pb6Ti6 contains no residual OR ligands as in the other PbTi clusters.
The six Pb atoms are arranged in two layers of three Pb atoms each above and below the Ti 6 plane ( Fig. 1 (3), Pb (4) and Pb (6)] point away from the ring centre, the other two Pb atoms [Pb(2) and Pb (5)] are located above and below the centre of the Ti 6 ring. Pb (3), Pb (4), and Pb (6) show positional disorder, but this does not affect the ligands. While each Pb atom is connected to two Ti atoms through a l 3 -O, only the outer Pb atoms are additionally connected to the Ti layer by bridging ligands (see below). The Pb-l 3 -O bonds of the central lead atoms Pb (2) and Pb(5) are significantly longer [Pb(2)-O(2) 2.509(2) Å , Pb (5) Pb 2? has a lone pair of electrons, which is often stereochemically active. This is indicated by truncated coordination polyhedra of the corresponding metals. In Pb6Ti6, the lone pairs of the outer Pb atoms [Pb(1), Pb (3), Pb (4), and Pb (6)] point away from the cluster centre. In contrast, the lone pairs of Pb (2) and Pb(5) point to the centre the Ti 6 ring and towards each other. The Pb (2)ÁÁÁPb (5) distance of 4.5535 (7) Å is relatively short for a PbÁÁÁPb distance of non-bridged lead atoms. This placement of two lead atoms above and below the centre of the Ti 6 ring at a short distance is apparently very favourable. The sum of bond angles around the l 3 -oxygen atoms support this assumption: while O(2) and O (5)  The positioning of Pb (2) and Pb (5) above and below the Ti 6 ring centre at a short PbÁÁÁPb distance is probably the reason for the non-centrosymmetric positions of the other lead atoms and the ligand shell around the Ti 6 ring (Fig. 2). Only Ti (1) and Ti(4) have OMc bridges to both neighbouring Ti atoms, but there is no methacrylate bridge between Ti (2) and Ti (3), or Ti (5) and Ti (6). The central Ti 6 ring system and the attached Pb atoms are approximately C 2 -symmetric, with the axis of rotation passing through O(3) and the centre of the Ti 2 O 2 ring, formed by O (6), O (7), Ti (5), and Ti (6). The pairs of Ti atoms which are not connected by a OMc bridge have instead OMc bridges to the adjacent Pb 3 layers. Ti (2) and Ti (3) are connected to one Pb 3 layer through a l 3 -OMc ligand each and to the other by a l 2 -OMc ligand. Thus, Ti (3) is connected to Pb(1) and Pb (2) of the top layer through a l 3 -OMc ligand and to Pb(4) of the bottom layer through a l 2 -OMc ligand. Conversely, Ti (2) is connected to Pb(4) and Pb (5) of the bottom layer and to Pb(1) of the top layer. On the other side of the Ti 6 ring, Ti (5) and Ti(6) are connected to one Pb atom of both adjacent Pb 3 layers (Pb (3) and Pb (6)) by two OMc bridges. The two Ti atoms with two OMc bridges to both neighbouring Ti atoms also connect to the Pb 3 layers by one OMc bridge each [Ti(1)ÁÁPb (1) and Ti (4)ÁÁPb (4)].
The remaining four carboxylate ligands connect the Pb atoms in the Pb 3 layers. One carboxylate group bridges all three Pb atoms. It is chelating the central Pb atom [Pb(2) or Pb (5)] and bridging to both other Pb atoms of the layer. In the top layer this carboxylate ligand is an acetate group, but a methacrylate ligand in the bottom layer. Each of the other two OMc ligands is again chelating the central Pb atom [Pb(2) or Pb (5)] and bridges only to one other Pb atom.
In an (unsuccessful) attempt to synthesize Pb2Ti2 from Pb(OAc) 2 and Ti(OiPr) 4 according to the literature [5] colourless crystals of Pb 4 Ti 8 O 10 (OiPr) 18 (OAc) 2 (Pb4Ti8) ( Fig. 3; Table 2) were obtained after one year. This is of course no viable synthesis method; the structure of Pb4Ti8 is nevertheless included in this paper to demonstrate the structural richness of Pb/Ti oxo clusters and to discuss some construction principles. Its cluster core can again formally be split in three layers of metals connected through oxygen atoms. The cluster core is approximately mirror-symmetric, with the mirror plane through Pb(1), Ti (3), Ti (6), and Pb (4) and perpendicular to the three metal layers. The overall symmetry of the cluster is lower than that of the cluster core due to the orientation of the peripheral isopropyl groups.
The bottom layer (Fig. 4) is formed by a Pb 3 O 3 ring (Pb (1) (4) Å ]. This is not only a consequence of the asymmetry of the bottom layer, but also of the unsymmetrical substitution of the Ti 3 O unit. A l 3 -O connects Ti(4), Ti(6), and Pb (2), as well as Ti (5), Ti(6) and Pb(3) [O (4) and O (5), respectively]. There is no equivalent group connecting Ti(4) and Ti(5). Instead, Ti(4) and Ti(5) are bonded to the bottom layer [to Ti(1) and Ti(2)] through the two acetate bridges and two bridging OiPr groups. Both have shorter Ti-O bond lengths to the Ti atoms of the central layer, Ti(4) and Ti(5). While Ti(4) and Ti(5) are octahedrally coordinated, Ti (3) has only a coordination number of 5 with a trigonal bipyramidal coordination geometry. This is quite unusual for Ti, as it prefers an octahedral coordination. The central Ti (6)  The top layer (Fig. 4) contains two Ti and one Pb atom, which are bridged by a l 3 -OiPr group [O(12)]. This OiPr group is perpendicular to the Ti 2 Pb plane and mirrors the l 3 -OiPr group of the bottom layer. Pb(4), Ti (7), and Ti (8) are additionally connected through l 4 -O (8), which also binds to Ti(6). The Ti-O bond lengths of Ti (7) and Ti(8) to O(8) are relatively long [Ti (7) (7) and Ti(8).

Conclusions
The structures of the previously reported Pb 2 Ti x (x = 2,4,6,8) oxo clusters are based on structural motifs typical of monometallic titanium oxo clusters (see ''Introduction''). This is no longer the case when the number of Pb 2? ions is increased. The high tendency of the [TiO 6 ] octahedra to connect with each other is also observed in Pb6Ti6 and Pb4Ti8. The same is true for the Pb/O polyhedra (with different coordination numbers), where the Pb 2? lone pair is stereochemically active. The preferred condensation of polyhedra of the same kind is probably due to the different ionic radii of Ti(IV) and Pb(II) and especially pronounced in Pb6Ti6. Six [TiO 6 ] octahedra in Pb6Ti6 form a planar sixmembered ring, which is capped by Pb 3 units from above  (2) 3.8287 (4) Ti (4) [5]. Dry Pb(OAc) 2 (1.306 g, 4 mmol) and 3.41 g of Ti(OiPr) 4 (12 mmol) were stirred in 20 cm 3 of dry n-hexane. After 3 days [because of the low solubility of Pb(OAc) 2 ] a clear solution was obtained. After two weeks the solution was concentrated to 15 cm 3 . Colourless crystals were obtained after 1 year, beside much white precipitate.

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 [10]) 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 3.
CCDC 1530015 (Pb6Ti6) and 1530016 (Pb4Ti8) contain supplementary crystallographic data. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_ request/cif.