Introduction

Metal oxo clusters of the general composition MaOb(OH/OR)c(OOCR′)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. Pb2Ti2O(OiPr)10, the structure of which is based on a Pb2Ti2(μ4-O) tetrahedron [4].

The (chain-like) structures of the carboxylate-substituted oxo clusters with a Pb:Ti ratio of 2:2–4, Pb2Ti2O(OiPr)8(OAc)2 (Pb2Ti2) [5], Pb2Ti3O2(OiPr)12(OOCC7H15)2 (Pb2Ti3) [6], and Pb2Ti4O2(OEt)14(OAc)2 (Pb2Ti4) [7], are based on Ti2(OR)x(μ-OOCR)y building blocks. The polynuclear compounds Pb2Ti4(OR)16(OOCR′)4 [6, 8] (without oxo groups, with different R and R′) have similar structures to that of Pb2Ti4.

The structures of the clusters Pb2Ti8O8(OBu)2X2(OMc)16(BuOH)2 (Pb2Ti8, X = OAc or OMc (OMc = methacrylate)) and Pb2Ti6O5(OiPr)3X(OMc)14 (X = OiPr or OMc) (Pb2Ti6) [9], with a higher Ti proportion, however, are derived from cyclic Ti8O8(OOCR)16. In the latter compound, each Ti atom is connected to both neighbouring Ti atoms through one μ 2-oxygen and two bridging OMc ligands each. The Ti8O8 ring has crown ether-like properties. In Pb2Ti8, two Pb(II) ions occupy the central cavity. Coordination of the oxygen atoms of the Ti8O8 ring to Pb is supported by bridging carboxylate ligands. In Pb2Ti6, the central Pb2 unit is coordinated by a semi-circular Ti6 fragment of the Ti8O8(OMc)16 metallacycle.

Common to the known carboxylate-substituted Pb/Ti oxo clusters is the metal ratio of Pb2Tix (x = 2–4, 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. Pb6Ti6O9(OAc)(OMc)17 (Pb6Ti6) and Pb4Ti8O10(OiPr)18(OAc)2 (Pb4Ti8).

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 structure of Pb6Ti6 (Fig. 1; Table 1) represents an interesting variation of the “Ti8O8 ring” motif, as it is based on a Ti6O6 ring as the central structural feature. While a great variety of Ti8O8(OOCR)16 (= [TiO(OOCR)2]8) structures is known (with different carboxylate ligands), Ti6O6 rings are not known, although a [TiO(OOCR)2]6 ring system appears to be stereochemically possible. The central unit of Pb6Ti6 comes close to such a structure (Fig. 1, right). The six titanium atoms are alternatively bridged by one or two oxygen atoms (whereas there is only one bridging oxygen between neighbouring Ti atoms in Pb2Ti8). The μ 2-oxygen atoms O(7)–O(9) are located within the Ti6 plane.

Fig. 1
figure 1

Left Molecular structure of Pb6Ti6O9(OAc)(OMc)17. Hydrogen atoms are omitted for clarity. Right Cluster core

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Table 1 Selected bond lengths/Å and angles/° for Pb6Ti6
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The six Pb atoms are arranged in two layers of three Pb atoms each above and below the Ti6 plane (Fig. 1, right). The three metal planes are almost parallel to each other. Each Pb atom is coordinated to one oxygen atom of the Ti2O2 units. Hence, these oxygen [O(1)–O(6)] atoms are μ 3, connecting two Ti and one Pb atom. The Pb–O axes, however, have different orientations. While four [Pb(1), Pb(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 Ti6 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 μ 3-O, only the outer Pb atoms are additionally connected to the Ti layer by bridging ligands (see below). The Pb-μ 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)–O(5) 2.519(2) Å] than that of the outer Pb atoms [2.244(3)–2.284(2) Å].

Pb2+ 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 Ti6 ring and towards each other. The Pb(2)···Pb(5) distance of 4.5535(7) Å is relatively short for a Pb···Pb distance of non-bridged lead atoms. This placement of two lead atoms above and below the centre of the Ti6 ring at a short distance is apparently very favourable. The sum of bond angles around the μ 3-oxygen atoms support this assumption: while O(2) and O(5) are clearly pyramidal [Σ M–O(2)–M 327.4(3)° and M–O(5)–M 330.6(3)°], indicating some pulling of the lead atoms towards the ring centre, the other μ 3-oxygen atoms are planar [350.6(4)–356.9(3)].

The positioning of Pb(2) and Pb(5) above and below the Ti6 ring centre at a short Pb···Pb distance is probably the reason for the non-centrosymmetric positions of the other lead atoms and the ligand shell around the Ti6 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 Ti6 ring system and the attached Pb atoms are approximately C2-symmetric, with the axis of rotation passing through O(3) and the centre of the Ti2O2 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 Pb3 layers. Ti(2) and Ti(3) are connected to one Pb3 layer through a μ 3-OMc ligand each and to the other by a μ 2-OMc ligand. Thus, Ti(3) is connected to Pb(1) and Pb(2) of the top layer through a μ 3-OMc ligand and to Pb(4) of the bottom layer through a μ 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 Ti6 ring, Ti(5) and Ti(6) are connected to one Pb atom of both adjacent Pb3 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 Pb3 layers by one OMc bridge each [Ti(1)··Pb(1) and Ti(4)··Pb(4)].

Fig. 2
figure 2

Schematic drawing of the three metal layers of Pb6Ti6. Only the COO groups of the OMc ligands are drawn. Dashed lines indicate bonds to metals of adjacent layers

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The remaining four carboxylate ligands connect the Pb atoms in the Pb3 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.

Pb6Ti6 co-crystallizes with an allylic alcohol molecule which weakly interacts with Pb(6), with a Pb–O distance of 3.12(2) Å.

In an (unsuccessful) attempt to synthesize Pb2Ti2 from Pb(OAc)2 and Ti(OiPr)4 according to the literature [5] colourless crystals of Pb4Ti8O10(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.

Fig. 3
figure 3

Left Molecular structure of Pb4Ti8O10(OiPr)18(OAc)2 (Pb4Ti8). Hydrogen atoms are omitted for clarity. Right Cluster core

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Table 2 Selected bond lengths/Å and angles/° for Pb4Ti8
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The bottom layer (Fig. 4) is formed by a Pb3O3 ring (Pb(1)–Pb(3) and O(1)–O(3)) which is capped by a rare μ 3-OiPr group (O(11)). The Pb2+ lone pairs are trans to the O(11) and thus point away from the Pb3O3 ring centre. The bond distance of the μ 3-OiPr group to Pb(1) (the Pb atom on the imagined mirror plane) is much shorter than to Pb(2) and Pb(3) [Pb(1)–O(11) 2.295(4) Å, Pb(2)–O(11) 2.622(4) Å, Pb(3)–O(11) 2.620(4) Å]. The ring oxygen atoms O(1) and O(2) are also shifted towards Pb(1) [Pb(1)–O(1) 230.6(4)/Pb(2)–O(1) 239.8(4) Å and Pb(1)–O(2) 229.9(4)/Pb(3)–O(2) 239.6(4) Å]. The asymmetry of the Pb3O(11) unit and the uneven distribution of the Pb–O bond lengths in the Pb3O3 ring is a consequence of the composition of the top layer, as will be discussed later. Three Ti atoms [Ti(1)–Ti(3)] are attached to the Pb3O3 ring through the ring oxygen atoms (Fig. 4). The Ti atoms are slightly above the plane of the Pb atoms, shifted towards the central layer. All Ti atoms of the bottom layer are additionally bonded to both neighbouring Pb atoms through μ 2-OiPr groups with relatively long Pb–O bond lengths [2.634(4)–2.870(4) Å]. Three terminal OiPr groups (one on each Ti atom) complete the ligand sphere of Ti(1)–Ti(3).

Fig. 4
figure 4

Schematic drawing of the layers in Pb4Ti8. Dashed lines indicate bonds to metals of adjacent layers

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The central layer (Fig. 4) consists of an open Ti3O triangle [Ti(4)–Ti(6) and O(6)] which is connected to the bottom layer through the three oxygen atoms of the Pb3O3 unit. These oxygen [O(1)–O(3)] atoms are thus μ 4, connecting two Ti and two Pb atoms. Ti(4)–O(1) and Ti(5)–O(2) are about 0.1 Å longer than Ti(6)–O(3) [Ti(4)–O(1) 2.050(4) Å, Ti(5)–O(2) 2.053(4) Å, Ti(6)–O(3) 1.950(4) Å] thus compensating the shorter Pb(1)–O(1) and Pb(1)–O(2) bond distances in the bottom layer. The central oxygen atom O(6) of the Ti3O unit is nearly planar [sum of bond angles 356.5(5)], but its coordination is best described as T-shaped [Ti(4)–O(6)–Ti(5) 169.5(2)°, Ti(4)–O(6)–Ti(6) and Ti(5)–O(6)–Ti(6) 95.0(2)°]. Concomitantly, Ti(4)–O(6) and Ti(5)–O(6) are about 0.1 Å shorter than Ti(6)–O(6) [Ti(4)–O(6) 1.921(4) Å, Ti(5)–O(6) 1.917(4) Å, Ti(6)–O(6) 2.027(4) Å]. This is not only a consequence of the asymmetry of the bottom layer, but also of the unsymmetrical substitution of the Ti3O unit. A μ 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) of the Ti3O unit is coordinated to μ 3-O atom which connects the central layer to Ti(3) of the bottom layer and Pb(4) of the top layer. There are no bridging OiPr or carboxylate ligands within the central layer, and between the central and the top layer.

The top layer (Fig. 4) contains two Ti and one Pb atom, which are bridged by a μ 3-OiPr group [O(12)]. This OiPr group is perpendicular to the Ti2Pb plane and mirrors the μ 3-OiPr group of the bottom layer. Pb(4), Ti(7), and Ti(8) are additionally connected through μ 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)–O(8) 2.118(4) Å, Ti(8)–O(8) 2.112(3) Å], whereas Ti(6)–O(8) is much shorter [1.955(4) Å]. In addition to O(7), which links all three layers, two μ 2-oxygen atoms bridge Ti atoms of the top layer and the central layer [Ti(4)–O(9)–Ti(7) and Ti(5)–O(10)–Ti(8)].

Every metal of the top layer is connected to both neighbouring metals through a μ 2-OiPr group. The Ti–O bonds of Pb–Ti bridging alkoxo groups are about 0.1 Å shorter than those bridging Ti–Ti [Ti(7)–O(25) 1.946(4) Å, Ti(8)–O(26) 1.944(4) Å, Ti(7)–O(27) 2.045(4) Å, Ti(8)–O(27) 2.037(4) Å]. Two terminal OiPr groups complete the octahedral coordination sphere of Ti(7) and Ti(8).

Conclusions

The structures of the previously reported Pb2Tix (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 Pb2+ ions is increased. The high tendency of the [TiO6] 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 Pb2+ 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 [TiO6] octahedra in Pb6Ti6 form a planar six-membered ring, which is capped by Pb3 units from above and below. In Pb4Ti8, five [TiO6] octahedra connect with each other in a three-dimensional arrangement (the center and top layer) to which one isolated Pb/O polyhedron is condensed. This PbTi5 unit is attached to a Pb3O3 ring (the bottom layer), to which two isolated [TiO6] octahedra and one [TiO5] trigonal bipyramid [Ti(3)] are bonded.

Experimental

All experiments were carried out under Ar atmosphere using standard Schlenk techniques. Pb(OAc)2·2 H2O was obtained from Merck and Ti(OiPr)4 from ABCR. Water-free lead acetate was obtained by drying in vacuum at 130 °C over night. The drying process was monitored by IR spectroscopy.

PbTi oxo clusters

Pb6Ti6O9(OAc)(OMc)17 (Pb6Ti6)

Pb(OAc)2 (651 mg, 2 mmol), 568 mg of Ti(OiPr)4 (2 mmol), and 278 mg of dry allylic alcohol (4 mmol) were heated for 2 h at 70 °C. The clear solution was allowed to cool to room temperature and then 1.62 g of methacrylic acid (18.9 mmol) was added. A white precipitate was formed, which disappeared after 1 h. Colourless crystals of Pb6Ti6O9(OAc)(OMc)17 were obtained after four months. Yield: 39 mg (34% rel. Ti).

Pb4Ti8O10(OiPr)18(OAc)2 (Pb4Ti8)

It was attempted to synthesize Pb2Ti2O(OiPr)8(OAc)2 according to the literature [5]. Dry Pb(OAc)2 (1.306 g, 4 mmol) and 3.41 g of Ti(OiPr)4 (12 mmol) were stirred in 20 cm3 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 cm3. 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 κ-geometry at 100 K using MoKα (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 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.

Table 3 Crystal data, data collection parameters and refinement details
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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.