The structural chemistry of titanium alkoxide derivatives with OH-substituted bidentate ligands

The organically modified titanium alkoxides Ti2(Oi-Pr)4(OOCCMe2O)2(i-PrOH)2 and Ti4(Oi-Pr)4(SA)6 were obtained from the reaction of Ti(Oi-Pr)4 with 2-hydroxy-isobutyric acid and salicyladoxime (SA-H2), respectively. Reaction of 1,3-dibenzoyl acetone (DBA-H) did not result in a substituted titanium alkoxide derivative, but instead in the oxo cluster Ti4O2(Oi-Pr)8(DBA)2 after allowing moisture to diffuse into the reaction mixture. The three titanium compounds show common structural features which are different to derivatives void of ligand OH groups. The latter play a decisive role in coordinating the ligands to the titanium centers.


Introduction
Metal alkoxides, M(OR) n , are usually not employed as such for sol-gel processing, but instead modified by organic ligands. Reasons for doing so are manifold (control of reactivity, creation of porosity, functionalization, etc.) and have been discussed elsewhere [1]. Bidentate monoanionic groups (X ∩ X or X ∩ Y) are preferred, because they are more strongly bonded than monodentate ligands. The organically modified derivatives are prepared by reacting M(OR) n with the corresponding protonated compounds.
The best-investigated (structural) chemistry of organically modified metal alkoxides is that of titanium. Substitution of titanium alkoxides, Ti(OR) 4 , with X ∩ X or X ∩ Y in a 1:1 ratio commonly results in derivatives of the composition Ti 2 (OR) 6 (X ∩ X/Y) 2 . In the compounds obtained by reaction of β-diketones, β-ketoesters, oximes, or aminoalcohols each titanium atom is chelated by one X ∩ X or X ∩ Y ligand (structure type A, Scheme 1) [2]. Two bridging bidentate ligands (structure type B, Scheme 1) were observed in Ti 2 (OMe) 6 [(CH 2 ) 2 PMe 2 ] 2 [3] and, with a slight variation (one carboxylate ligand bridging and the second hydrogen-bonded to a coordinated ROH), in the rare derivatives Ti 2 (OR) 6 (OOCRʹ) 2 (ROH) [4,5]. Reactions with carboxylic acids normally give oxo clusters of the type Ti a O b (OR) c (OOCRʹ) d , with bridging carboxylate ligands, because water is generated by reaction of the eliminated ROH with the carboxylic acid [2]. The dimeric nature of A and B is due to the preference of the Ti atoms for octahedral coordination, which is achieved by a pair of bridging OR groups.
When Ti(OEt) 4 was reacted with the amino acid glycine, a compound with the composition Ti 2 (OEt) 6 (glycinate) 2 (1, Scheme 2) was obtained (and not an oxo cluster), but the structure was different to B [6]. Instead of bridging carboxylate groups, five-membered chelate rings were formed with the α-amino group and only one COO oxygen coordinated to Ti. Thus, the structure of Ti 2 (OEt) 6 (glycinate) 2 is of the A type, with X = O and Y = N.
The structure of 1 shows that a substituent in the chelating/bridging ligand capable of also coordinating to the metal center (as the amino group in 1) might change the coordination of the ligand in the modified titanium alkoxides compared to the corresponding unsubstituted ligands. We now tested whether and how a hydroxy substituent would influence the structures. Derivatives Ti 2 (OR) 6 (X ∩ X/Y) 2 with such ligands were previously unknown; they were now obtained by the reaction of Ti(Oi-Pr) 4 with an OHsubstituted carboxylic acid and an OH-substituted oxime as well as a 1,3,5-triketone, which can formally be considered an OH-substituted β-diketone. We concentrate on the solidstate structures of the derivatives, and elaborate on some common structural features. The solution chemistry of the compounds is disregarded, because it is well known that IR and NMR spectra of metal alkoxide derivatives in most cases do not provide meaningful structural information; the structures are often dynamic so that the NMR spectra just show averaged signals.

Results and discussion
No oxo cluster was formed when Ti(Oi-Pr) 4 was reacted with an equimolar amount of 2-hydroxy-isobutyric acid, as in the reaction with α-amino acids, but instead a centrosymmetric dimer 2 with again only one COO oxygen and the α-hydroxy group of the hydroxycarboxylate ligands coordinated to titanium (Fig. 1). The hydroxycarboxylate ligands, however, were chelating-bridging contrary to the chelating amino carboxylate ligands in 1. The central Ti 2 O 2 unit and the two chelate rings of 2 are nearly coplanar.
The structure of 2 can be generalized to C, where O a and O b are the oxygen atoms of the chelating-bridging ligand and L a neutral ligand. A combination of the structural motifs A and C (Scheme 3) was found in Ti 4 (Oi-Pr) 4 (SA) 6 (3), which was obtained from the reaction of Ti(Oi-Pr) 4 with variable proportions of salicylaldoxime (H 2 SA) (molar ratio 1:1-1:5). The chelating-bridging entity in 3, however, is not a simple ligand, but instead a building block of the composition Ti(SA) 2  The solid-state structure of 3 contains two independent centrosymmetric molecules with only minor structural differences; only one is reproduced in Fig. 2 and Table 2. While the coordination octahedra of Ti3 and Ti3* are condensed with each other through the bridging oxygen atoms O23 and O23* and form the central unit D, Ti2 and Ti2* are part of the "chelating unit" and are connected to the central unit through bridging oximate groups. They are coordinated by a terminal Oi-Pr group and by two chelating SA ligands, which interact with the metal atom though the phenolate   4 (SA) 6 (3) An asterisk denotes inversion-related atoms. Only data of one of the independent molecules in the asymmetric unit are given Ti2-O6 187.9 (5) Ti3-O9 189.4(5) Ti2-O3 190.7 (5) Ti3-O13 188.6(5) Ti2-O11 176.9 (5) Ti3-O23 205.2(5) Ti2-O17 194.2 (5) Ti3-O23* 207.8(5) Ti2-N11 218.5 (6) Ti3-N10 223.0(6) Ti2-N9 223.5 (7) O9-N9 138.3(7) Ti3-O2 174.8 (5) O17-N10 137.5 (7) O23-N11 140.6(7) C1-N11-Ti2 129.2(5) O9-Ti3-O23* 155.1 (2) O23-N11-Ti2 119.3(4) O9-Ti3-N10 82.7 (2) Ti3-O23-Ti3* 112.2 (2) oxygen and the nitrogen atom (the two nitrogen and oxygen atoms being cis to each other). The coordination octahedron is completed by an oxygen atom from the SA ligand of the central unit D (see below). Ti3 of the central unit is coordinated by one chelating SA ligand, one Oi-Pr group and three oximate oxygens from the "chelating unit". Each SA ligand in 3 coordinates through the phenolic oxygen and the oximate nitrogen, but there are three different SA ligands with regard to the metal coordination (Scheme 4; the numbers in refer to the Harris notation of multidentate ligands [11]): the two SA ligands chelated to Ti2 differ in a way that the oximate oxygen of one of them (with O6 and N9) is coordinated to only one Ti3 atom (2.111, left in Scheme 4), while that of the second (with O3 and N11) is coordinated to both Ti3 and Ti3* (3.211). The third type of SA ligand (2.111, right in Scheme 4) chelates to Ti3 and additionally coordinates to Ti2 through the oximate oxygen. The 3.211 coordination mode was also found in another SA-substituted titanium alkoxide, viz. Ti 3 (Oi-Pr) 8 (SA) 2 [12] and the SA-substituted oxo derivative Ti 4 (OMe) 6 (SA) 4 , both being structurally unrelated to 3 [13]. This coordination of SA is in contrast to titanium alkoxide derivatives with aliphatic or aromatic aldoximate ligands without the OH group in ß-position, where the oximate NO group is always side-on coordinated to the same titanium atom. Representative examples with aromatic oximate ligands are Ti 2 (Oi-Pr) 4 (benzaldoximate) 4 and Ti 2 (Oi-Pr) 4 (anisaldoximate) 4 [14].
The titanium alkoxide derivatives 2 and 3 demonstrate the great impact of OH substituents on the structures formed upon reaction of Ti(OR) 4 with carboxylic acids or oximes. In both cases, the C-OH group was deprotonated and bridged two titanium atoms. It thus had a structure-determining role and resulted in completely different structures compared with the compounds with unsubstituted carboxylate or oximate ligands. Along this line, we also tested 1,3,5-triketones, which can be considered OH substituted β-diketones. We were unable to isolate a substituted titanium alkoxide derivative when 1,5-diphenyl-pentane-1,3,5-trione (1,3-dibenzoyl acetone, DBA-H) was reacted with Ti(Oi-Pr) 4 in dichloromethane. However, when ambient moisture was allowed to diffuse slowly in the reaction solution, the crystalline titanium oxo cluster Ti 4 O 2 (Oi-Pr) 8 (DBA) 2 (4) (Fig. 3 and Table 3) was reproducibly obtained (Scheme 5).
In the centrosymmetric tetramer 4, each titanium atom is octahedrally coordinated; the four [TiO 6 ] polyhedra share edges (Fig. 4). The same cluster core structure was observed for Ti 4 O 4 (Oi-Pr) 4 (OCHPhCHMeNHMe) 4 [15]. The comparison of 4 with Ti 4 O 2 (Oi-Pr) 10 (acac) 2 (acac = acetylacetonate) [16], one of the few titanium oxo clusters with β-diketonate ligands, is interesting (Fig. 4), where-formally-the DBA is replaced by (acac + Oi-Pr). While all titanium atoms in 4 are octahedrally coordinated, two of the titanium atoms in Ti 4 O 2 (Oi-Pr) 10 (acac) 2 (the ones without acac ligands) are only 5-coordinate. Furthermore, the polyhedra are differently connected. The reason for this difference is that the central oxygen atoms of the (planar) DBA ligands bridge two Ti atoms. This supplementary coordination renders the inner titanium atoms also octahedrally coordinated. The highly condensed cluster core of 4 results from three different bridging groups: DBA linking Ti1 and Ti2, μ 3 -O20 connecting Ti2, Ti2* and Ti1, and a µ 2 -Oi-Pr bridge (O7) between Ti2 and Ti1* The Ti-O acac distances in Ti 2 (Oi-Pr) 6 (acac) 2 (with chelating acac ligands) are 207.3(4) and 203.0(3) pm [17]. They are typical Ti-O bond lengths of coordinated β-diketonates. In comparison, the Ti-O2 distance of the bridging DBA oxygen (O2) in 4 is much longer (Ti1-O2  (129.1-130.9 pm). As a matter of fact, the Ti-O2 distance in 4 is also much longer than the corresponding distances of the bridging oxygen atoms in 2 and 3. A possible interpretation could be that a mesomeric form of the coordinated DBA prevails, in which the oxygen atoms in the 1and 5-position of the DBA ligand carry a higher negative charge, while that in the 3-position is more ketone-like. This is supported by the bond distances trans to O2: the bridging Oi-Pr ligand O7 is very unsymmetrical (Ti2-O7 191.0(1) and Ti1-O7* 218.7(1) pm), the shorter distance being trans to O2. Likewise, the Ti1-O5 distance (177.2(1) pm), also being trans to O2, is shorter than typical distances of Ti-OR groups.

X-ray structure analyses
All measurements were performed using Mo Kα (λ = 71.073 pm) radiation. Data were collected on a Bruker AXS SMART APEX II four-circle diffractometer with κ-geometry. Data were collected with φ and ω-scans and 0.5° frame width. 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. Details of the X-ray investigations are given in Table 4. The structures were solved by the Patterson method (SHELXS97). 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, if not otherwise stated, and refined riding with the corresponding atom. CCDC-2017662 (for 2), -2017663 (for 3), and -2017664 (for 4) contain the supplementary crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam. ac.uk/struc tures /.
Funding Open access funding provided by TU Wien (TUW).
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