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

Abstract

Ti₆O₆(OiPr)₂(OOCR′)₁₀ (R′ = C₄H₇, Et) and Ti₆O₃(OiPr)₁₄(OOC–CH=CH–COO)₂ represent new structure types of carboxylate-substituted clusters with a Ti₆O_x 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.

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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.^1,2 The degree of substitution (d_s = RCOO/Ti ratio) and the degree of condensation (d_c = O/Ti ratio) of the obtained clusters reflect the proportion of both reactions; they are key parameters for which structures are formed. In the case of titanium, μ₃-O count for full for the calculation of d_c, μ₂-O for 2/3 and μ₄-O for 4/3, because in rutile and anatase (d_c = 2) every oxygen is μ₃-O.

Common to all Ti oxo clusters is that the Ti atoms are six-coordinate (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 Ti₆ clusters are schematically shown in Scheme 1. The Ti₆ cluster type with the highest d_c (and a wide variety of R/R′ combinations) is Ti₆O₆(OR)₆(OOCR′)₆,^3–18 where all oxygen atoms are μ₃-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 Ti₆O₆(OR)₆(OOCR′)₆ structure type is also demonstrated by the fact that the structures of many mixed-metal clusters are derived from this lead structure.¹⁹

Scheme 1 Schematic structures of the cluster cores of known carboxylate-substituted Ti₆O_x clusters. Terminal OR ligands and (always bridging) carboxylate ligands were omitted. The conformation of the cluster cores are drawn to emphasize structural similarities between them.

The four cluster types with a Ti₆O₄ core vary by different d_s and d_c. Ti₆O₄(OR)₈(OOCR′)₈^20–27 is the second most common Ti₆ cluster type, with a higher d_s and a lower d_c than Ti₆O₆(OR)₆(OOCR′)₆. The two types of Ti₆O₄(OR)₁₂(OOCR′)₄ clusters are only represented by a few examples.^28–31 The different proportion of terminal, μ₂ and μ₃ ligands (μ₄-O in Ti₆O₄(OEt)₁₄(OOCPh)₂³² is a very rare exception) in the Ti₆O₄ clusters results in different structures, i.e. in a different linkage of the [TiO₆] octahedra. There is only one example for a cluster with a Ti₆O₅ core, Ti₆O₅(OiBu)₆(OOCtBu)₈,¹⁶ which is structurally derived from the Ti₆O₄(OR)₈(OOCR′)₈ structure by the (formal) replacement of two terminal OR groups by a μ₂-O. The Ti₆O₄ and Ti₆O₅ clusters can be considered structural intermediates on the way to Ti₆O₆ clusters, with pre-formed elements of the Ti₆O₆ structure. The conversion into the corresponding Ti₆O₆(OR)₆(OOCR′)₆ cluster was proven experimentally in two cases, namely for Ti₆O₄(OEt)₈(OOCMe)₈³ and Ti₆O₅(OiBu)₆(OOCtBu)₈.¹⁶ The occurrence of Ti₃(μ₃-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 Ti₆ 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. Both contain Ti₃(μ₃-O) units as building blocks. As we have pointed out earlier (in an article on Ti₃O(OR)₈(OOCR′)₂ clusters³³) that the Ti₃O unit can be flat or pyramidal, without obvious correlation to other structural parameters. This possibly indicates that flat/pyramidal conversion is easy. To facilitate the comparison between the different cluster structures we have drawn the Ti₃O units in Scheme 2, and all other Schemes in this article, in pyramidal conformations.

Scheme 2 Schematic core structures of the new cluster reported in this article. Terminal OR ligands and carboxylate ligands were omitted. The conformation of the cluster cores are drawn to emphasize structural similarities to the clusters in Scheme 1.

Results

Reaction of Ti(OiPr)₄ with 4 molar equivalents of cyclobutane carboxylic acid resulted in the formation of Ti₆O₆(OiPr)₂(OOCC₄H₇)₁₀ (1a, Fig. 1). An isostructural cluster, Ti₆O₆(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 Ti₃O units, with nearly trigonal-planar μ₃-O atoms (sum of angles 354.6°), are bridged by two μ₂-O and four carboxylate ligands. The Ti/O core structure is basically the same as that of the Ti₆O₄(OR)₈(OOCR′)₈ clusters (Scheme 1), but the two bridging OR groups in the Ti₆O₄ clusters are replaced by μ₂-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.

Fig. 1 Molecular structure of Ti₆O₆(OiPr)₂(OOCC₄H₇)₁₀ (1a). Hydrogen atoms were omitted for clarity. Selected bond distances [Å] and angles [°]: Ti1–O1 2.093(3), Ti2–O1 1.867(3), Ti3–O1 1.918(3), Ti1–O2 1.837(3), Ti2–O2 1.810(3), Ti2–O3 2.003(3), Ti3–O3 1.722(3). Ti2–O1–Ti3* 133.0(2), Ti2–O1–Ti1 91.9(1), Ti3–O1–Ti1 129.6(2), Ti2–O2–Ti1 102.8(2), Ti3–O3–Ti2 131.8(2).

For this reason, the arrangement of ligands decorating the Ti₆O₆ cluster core in 1 is slightly different compared to Ti₆O₄(OR)₈(OOCR′)₈. Only the outer Ti atoms (Ti1 and symmetry-related Ti1* in 1a) of the ellipse-shaped Ti₆ arrangement still carry a terminal OiPr group (these atoms are substituted by two terminal OR ligands in the Ti₆O₄ 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 Ti₆ oxo clusters. In passing, the highest possible d_s for Ti oxo clusters (with 6-coordinate Ti atoms) is 2 as in [TiO(OOCR′)₂]₈.³⁴

The Ti₃O 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 Ti₃O group is also observed in the Ti₆O₄(OR)₈(OOCR′)₈ clusters and possibly due to the terminal OR group being trans to Ti1–O1 (for example in Ti₆O₄(OEt)₈(OOC–CMe=CH₂)₈: Ti–O 1.895(4), 1.895(5), 2.093(3); Ti–O–Ti 129.4(2), 128.2(2), 101.9(2)²³).

The main difference in the core structures of 1 and Ti₆O₄(OR)₈(OOCR′)₈, resulting from the replacement of a μ₂-OR by a μ₂-O in the Ti₃O unit, is the position of the oxygen atoms bridging the two Ti₃O entities. This oxygen (O3 and O3* in 1a) is trans to another bridging oxygen (O2, which is essentially symmetrically located between Ti1 and Ti2) at Ti2 in 1a, but trans to a bridging OR group in Ti₆O₄(OR)₈(OOCR′)₈. While O3 in 1a is shifted away from Ti2 [Ti2–O3 2.003(3), Ti3–O3 1.722(3) Å], the oxygen bridging the two Ti₃O entities in Ti₆O₄(OR)₈(OOCR′)₈ is shifted towards the respective Ti atom (with a smaller difference between both Ti–O distances). This is apparently due to a different trans influence of μ₂-O compared with μ₂-OR. The pronounced asymmetry of the Ti2–O3–Ti3 arrangement in 1a also affects bonding of the carboxylate ligand bridging Ti1 and Ti3*, where Ti1–O7 (2.010(4) Å) is shorter and Ti3–O8 (2.152(3) Å) much longer than the other carboxylate Ti–O distances (2.010(4)–2.063(3) Å).

The situation in solution is hard to comprehend, because the number of signals of 1a in both the ¹H and ¹³C 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 ¹H 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 Ti₂(OiPr)₆(OOC–C₆H₄–COOⁱPr)₂(iPrOH).¹⁵ In the analogous reaction of Ti(OiPr)₄ with maleic anhydride we now isolated a small proportion of Ti₆O₃(OiPr)₁₄(OOC–CH=CH-COO)₂ (2) (denoted as Ti₆O₃(OR)₁₄(OOCR′)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 Ti₆ clusters.

Type II Ti₆O₄(OR)₁₂(OOCR′)₄ clusters consist of two Ti₃O(OR)₆(OOCR′)₂ units (with Ti₃O(OR)₂ cores) which are connected by two μ₃-O. The building blocks of cluster 2 are the same Ti₃O(OR)₂ units, but connected by one μ₂-O and two bridging dicarboxylate ligands. The structure of 2 (Fig. 2) can thus formally (!) be derived from the Ti₆O₄ cluster type by removing one μ₃-O between the Ti₃O(OR)₂ units and replacing the second μ₃-O by a μ₂-O (compare Schemes 1 and 2). Charges and coordination sites are compensated by additional carboxylate groups. The substitution pattern of the bystander ligands in the Ti₃O units of 2 is essentially the same as in Ti₃O(OiPr)₈(OOCPh)₂³⁶ and Ti₃O(OCH₂CMe₃)₈(OOCH)₂.⁵ Another way of looking at the structure of 2 therefore is that two such Ti₃O clusters are bridged by a μ₂-O (O3), where the μ₂-O replaces a terminal OR ligand in each Ti₃O cluster unit (see Discussion section).

Fig. 2 Molecular structure of Ti₆O₃(OiPr)₁₄(OOC–CH=CH–COO)₂ (2). Selected bond distances [Å] and angles [°]: Ti1–O1 1.979(2), Ti2–O1 1.981(2), Ti3–O1 1.860(2), Ti4–O2 1.981(2), Ti5–O2 2.005(3), Ti6–O2 1.844(3), Ti3–O3 1.780(2), Ti4–O3 1.849(2). Ti1–O1–Ti3 144.5(1), Ti2–O1–Ti3 107.4(1), Ti1–O1–Ti2 105.3(1), Ti4–O2–Ti6 144.1(1), Ti5–O2–Ti6 107.0(1), Ti4–O2–Ti5 104.3(1), Ti3–O3–Ti4 156.6(1).

One Ti atom in each Ti₃O unit (Ti3 and Ti6) is 5-coordinate, as in the reference Ti₃O structures. Interestingly, the 5-coordinate Ti atoms are in geometrically different positions: in one unit (Ti1–Ti3) this is Ti3, bonded to the bridging oxygen O3, and in the second (Ti4–Ti6) Ti6 which is only bridged to one of its neighbors by a μ₂-OR group. Correspondingly, the bridging O3 atom is closer to the 5-coordinate Ti atom (Ti3–O3 1.780(2) Å) than to Ti4 (Ti4–O3 1.849(2) Å), and the Ti–O distances between the μ₃-oxygens O1 and O2 and the 5-coordinate Ti atoms (Ti3–O1 1.860(2), Ti6–O2 1.844(3) Å) are shorter than to the 6-coordinate ones (1.979(2)–2.005(3) Å).

Because 2 has no molecular symmetry, fourteen different groups of signals of the OiPr groups are expected in the ¹H 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 CH₃ 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′ and the Ti(OR)₄/R′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.

However, comparative analysis of different cluster structures may give clues on how the structures develop during the reactions. We have discussed in a previous article³³ that stable Ti₃O(OR)₈(OOCR′)₂ clusters are apparently only obtained if the groups R or R′ are bulky. The occurrence of Ti₃O units in the majority of Ti oxo clusters with d_c ≤ 1 indicates that Ti₃O clusters are formed early in the reactions and serve – if not stabilized by bulky groups – as building blocks for the thermodynamically favored products Ti₆O₆(OR)₆(OOCR′)₆ (=[TiO(OR)(OOCR′)]₆) and Ti₈O₈(OOCR′)₁₆ (=[TiO(OOCR′)₂]₈). The other structurally characterized Ti oxo clusters are possibly snapshots for intermediate stages.

The series of clusters Ti₆O₄(OR)₈(OOCR′)₈, Ti₆O₅(OiBu)₆(OOCtBu)₈¹⁶ and Ti₆O₆(OR)₂(OOCR′)₁₀ (1) shows how condensation can proceed (increasing d_c) structurally without greatly affecting the cluster core (Scheme 3). In going from the Ti₆O₄ to the Ti₆O₅ cluster, two terminal OR groups are replaced by a μ₂-O. This corresponds to a condensation process. Since this replacement does neither affect the charge balance nor the number of occupied coordination sites, no modification of the ligand sphere is necessary. Only the core of the Ti₆O₅ cluster is twisted compared with that of Ti₆O₄ due to the additional μ₂-O bridge connecting the two Ti₃O units. In going from the Ti₆O₄ to the Ti₆O₆ cluster, two μ₂-OR groups are replaced by two μ₂-O. This implicates some modifications of the ligand sphere, as discussed above. Since additional carboxylate ligands are required, d_s also increases. Note that this discussion only refers to the development of the structure which not necessarily mirrors a reaction pathway from one cluster to another.

Scheme 3 Interrelation of the cluster core structures of Ti₆O₅ and Ti₆O₆ with that of Ti₆O₄. The new groups/bonds are drawn in bold. The conformation of the cluster cores are drawn to emphasize structural similarities to the clusters in Schemes 1 and 2.

The structures of Ti₃O(OR)₈(OOCR′)₂, Ti₆O₃(OR)₁₄(OOCR′)₄ and Ti₆O₄(OR)₁₂(OOCR′)₄ (type II) can similarly be interrelated (Scheme 4). Ti₆O₃(OR)₁₄(OOCR)₄ is structurally derived from a formal condensation of two Ti₃O clusters. The structure of Ti₆O₃(OR)₁₄(OOCR)₄ can similarly be converted into that of Ti₆O₄(OR)₁₂(OOCR′)₄ by intramolecular condensation of two OR groups with concomitant conversion of μ₂-O into μ₃-O, by which all Ti atoms become 6-coordinate. Note again that this only a structural discussion, and formation of the clusters may proceed differently, especially in the case of 2, where a dicarboxylate group is involved, rather than two monocarboxylate ligands.

Scheme 4 Interrelation of the cluster core structures of Ti₃O, Ti₆O₃ and Ti₆O₄ (type II). The new bonds/groups are drawn in bold. The conformation of the cluster cores are drawn to emphasize structural similarities to the clusters in Schemes 1 and 2.

Experimental

General

All experiments were carried out under Ar atmosphere using standard Schlenk techniques. Ti(OiPr)₄ was obtained from ABCR. All solvents used for NMR spectroscopy (Eurisotop) were degassed prior to use and stored over molecular sieve. ¹H and ¹³C solution NMR spectra were recorded on a Bruker AVANCE 250 (250.13 MHz [¹H], 62.86 MHz [¹³C]) equipped with a 5 mm inverse-broadband probe head and a z-gradient unit.

Synthesis of Ti₆O₆(OiPr)₂(OOCC₄H₇)₁₀, 1a

Ti(OiPr)₄ (0.31 ml, 1 mmol) was slowly added to 0.38 ml (4 mmol) of cyclobutane carboxylic acid under stirring. A clear solution was obtained of which crystals of 1a were obtained after 3 weeks. Yield 80 mg (32%). ¹H NMR (CDCl₃, 250 MHz) δ (ppm) 1.22 (d, J = 6.28 Hz, 6H, CHCH₃), 1.31 (d, J = 6.12 Hz, 6H, CHCH₃), 1.73–2.05 (m, 20H, CH₂CH₂CH₂), 2.06–2.50 (m, 40H, CH₂CH), 2.95–3.11 (m, 5H, CHCH₂), 3.14–3.33 (m, 5H, CHCH₂), 4.82 (m, J = 6.12 Hz, 1H, CHCH₃), 5.00 (m, J = 6.28 Hz, 1H, CHCH₃). ¹³C{¹H} NMR (CDCl₃, 62.9 MHz) δ (ppm) 18.24, 18.35, 18.52, 18.72 (CH₂CH₂CH₂), 21.79, 24.38 (CHCH₃), 25.14, 25.24, 25.29, 25.40, 25.46, 25.57, 25.61, 25.70 (CH₂CH), 37.74, 38.37, 39.47, 40.00, 40.14, 40.24 (CHCOO), 67.26, 77.19, 81.47 (CHMe₂), 180.80, 182.88, 184.38, 185.18, 185.78, 186.80 (COO).

Synthesis of Ti₆O₃(OiPr)₁₄(OOC–CH=CH–COO)₂, 2

Ti(OiPr)₄ (8.8 ml, 30.3 mmol) was added to 2.96 g (30.1 mmol) of maleic anhydride in 9.3 ml of iPrOH. Crystals of 2 were obtained after 16 weeks at room temperature. Yield 200 mg (3%). ¹H NMR (C₆D₆) δ 1.34–1.54 (m, 60H, CH₃), 1.63 (d, J = 6.22 Hz, 6H, CH₃), 1.73 (d, J = 6.26 Hz, 6H, CH₃), 1.92 (d, J = 6.26 Hz, 6H, CH₃), 1.94 (d, J = 6.22 Hz, 6H, CH₃), 5.00–5.40 (m, 14H, CH), 6.06 (s, 4H, =CH). ¹³C{¹H} NMR (C₆D₆) δ 24.47, 24.84, 24.90, 25.10, 25.44, 25.60, 25.71, 25.80, 25.90, 26.04, 26.11 (CH₃), 76.58, 77.25, 77.86, 78.40, 78.81, 79.28, 79.35 (CHMe₂), 134.41 (=CH), 171.45, 173.21 (COO).

X-ray structure analyses

Crystallographic data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with κ-geometry using MoK_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.³⁸

The structures were solved by charge flipping (JANA2006). Refinement was performed by the full-matrix least-squares method based on F² (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.

Table 1 Crystal data, data collection parameters and refinement details

	1a	1b	2
a ω = 1/[σ^2(Fo^2) + (xP)^2 + yP], where P = (Fo^2 + 2Fc^2)/3.
Empirical formula	C56H82O28Ti6	C36H64O28Ti6	C50H102O25Ti6
M r	1490.6	1232.3	1390.7
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄
a (Å)	9.6136(4)	9.363(1)	12.5972(3)
b (Å)	12.9095(7)	11.467(2)	15.6563(3)
c (Å)	13.5706(7)	12.838(2)	19.9053(4)
α (°)	95.160(2)	73.427(8)	73.238(1)
β (°)	96.711(2)	89.342(7)	76.797(1)
γ (°)	105.180(2)	75.625(7)	83.588(1)
V (Å^3)	1601.5(1)	1277.0(4)	3655.3(1)
Z	1	1	2
D x (g cm^−3)	1.546	1.602	1.264
T (K)	100	100	295
μ (mm^−1)	0.797	0.981	0.690
Crystal size (mm)	0.2 × 0.1 × 0.05	0.33 × 0.31 × 0.29	0.30 × 0.27 × 0.20
No. measd, indep, obs. refl. (I > 2σ(I))	12 864, 5603, 3421	27 774, 6202, 3413	102 190, 12570, 9709
R int	0.057	0.131	0.037
θ max (°)	25.01	28.15	24.82
R[F^2 > 2σ(F)], ωR(F^2), S	0.057, 0.125, 1.004	0.053, 0.082, 0.952	0.047, 0.137, 1.074
No. of parameters	436	323	758
Weighting scheme^a	x = 0.06587, y = 0.7700	x = 0.0352, y = 0	x = 0.0886, y = 2.9622
δρmax, δρmin (e Å^−3)	0.912, −0.429	0.904, −0.914	0.969, −0.406

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Conflicts of interest

There are no conflicts to declare.

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Footnotes

† CCDC 1826634 (1a), 1826635 (1b) and 1826636 (2). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c8nj01077c

‡ This article is dedicated to Professor Dietmar Stalke on the occasion of his 60th birthday.

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