Titanium oxo-clusters derivatized from the Ti 10 O 12 (cat) 8 ( py) 8 complex: structural investigation and spectroscopic studies of light absorption †

A series of deep-red colored nano-sized titanium oxo-clusters bearing catecholato ligands is reported. These architectures are produced via post-synthetic modi ﬁ cation of the Ti 10 O 12 (cat) 8 (py) 8 (cat = catecholato, py = pyridine) complex through quantitative substitution of labile pyridine ligands by three substituted pyridines (pico, 4-Phpy and pyrald). The crystal structure analysis reveals a common Ti 10 O 12 (cat) 8 backbone for the three isolated molecular architectures. Partial charge analysis indicates two types of titanium atoms within these complexes with one resembling titanium( IV ) found in TiO 2 . These complexes strongly absorb visible light in solution ( λ max = 411 nm, ε = 10 800 for Ti 10 O 12 (cat) 8 (py) 8 in CHCl 3 ) and in the solid-state. The band gaps estimated from the re ﬂ ectance spectra are between 1.85 eV and 1.97 eV. The present work also details the HOMO and LUMO representations obtained via DFT calculation for Ti 10 O 12 (-cat) 8 (py) 8 and a virtual Ti 10 O 12 (cat) 8 complex as well as the DOS (density of states) plots calculated for those structures. This computational study highlights an impact of the pyridine ligand on the DOS plots.


Introduction
Titanium dioxide (TiO 2 ) is a cheap non-toxic semiconductor, which elicits a major interest for applications in photocatalytic water splitting, 1 photocatalytic degradation of pollutants 2 or solar energy conversion. 3 However, TiO 2 does not absorb the visible light of solar radiation due to its high band gap (3.2 eV). Therefore, to develop more practical TiO 2 -based photo-active materials, research studies are actually focused on materials that can absorb light in the visible domain.
Titanium oxo-clusters 4 are a fascinating class of compounds that are intensively investigated as models of bulk TiO 2 . In particular, in the context of visible-light-active TiO 2based materials, the synthesis and study of the optical properties of polyoxotitanium complexes absorbing light in the visible region represent an intensive field of research. Mainly, two classes of polyoxotitanate architectures are developed. The broad family of metal-doped polyoxotitanium complexes are viewed as ligand-stabilized molecular fragments of metaldoped TiO 2 , and several metal ions, including Fe(II), 5 Ga(III), 6 Co(II), 7 Mn(II), 8 lanthanide ions, 9 and alkali metal ions, 10 have been incorporated in these structures. The band gap of these compounds has shown to be reduced in comparison with classical oxo-titanium clusters. Another class of titanium oxo-clusters concerns nanoclusters decorated with sensitizers. 11 High nuclearity clusters bearing catecholato 10,12 or 2,2′-biphenolato 11,13 ligands have been reported as the Ti-O ( phenolato) bond is known to absorb in the visible domain, due to the charge transfer between the lone pair of oxygen atoms and the empty 3d-orbitals of the Ti(IV) ions. 14 Very recently, we have reported the structural characterization of a high nuclearity catecholato-based Ti(IV) architecture bearing pyridine ligands. This complex formulated as Ti 10 O 12 -(cat) 8 ( py) 8 (cat = catecholato, py = pyridine) displays a S 4 symmetry as shown in the crystal structure in Fig. 1. 15 Importantly, the unusual behaviour of this complex in solution was studied by various NMR techniques highlighting the labile behaviour of the pyridine ligands.
In order to compare these oxo-clusters to TiO 2 phases, the partial charge distribution was determined for these species. In parallel, we present DFT calculations to determine the HOMO-LUMO gaps for these complexes. These data are correlated with the experimental spectroscopic data recorded for this series of red-colored polyoxotitanium complexes.
Ti 10 O 12 (cat) 8 ( py) 8 in solution: NMR investigation Before studying the substitution of pyridine ligands linked to the Ti 10 O 12 (cat) 8 core of the complex, full characterization of the complex in solution is mandatory. At room temperature, the DOSY experiment run in CDCl 3 clearly suggests the decoordination of the pyridine ligands. 14 Furthermore, three large signals at δ = 8.40, 7.53 and 7.10 ppm attributed to pyridine highlight an exchange process as shown by the Ti 10 O 12 -(cat) 8 (py) 8 spectrum at room temperature ( Fig. 3c). At low temperature (see Fig. 3b), the 1 H NMR spectrum changes drastically since sharp signals are predominant. However, due to the complexity of the spectrum, the distinction between signals belonging to the catecholato and pyridine ligands is rather difficult. Thus, the spectrum at low temperature of a new complex bearing deuterated pyridine ligands was recorded. This complex formulated as Ti 10 O 12 (cat) 8 ( py-d 5 ) 8 is prepared like Ti 10 O 12 (cat) 8 ( py) 8 . At low temperature, the 1 H NMR spectrum of Ti 10 O 12 (cat) 8 ( py-d 5 ) 8 appears to be much simpler than the nondeuterated complex since only signals attributed to the catecholato ligands are observed (Fig. 3a). Now, the assignment of Fig. 1 View of the molecular structure of Ti 10 O 12 (cat) 8 ( py) 8 evidencing the S 4 -symmetry of the complex. The two inequivalent catecholato ligands are colored differently (in orange and light purple). The pyridine ligands are displayed in two colors depending on their localizations in the complex. The pyridine ligands colored in light green (named as py (int.)) correspond to ligands sandwiched between a catecholato ligand (in purple) and a second pyridine ligand colored in dark green (named as py(ext.)). The titanium atoms and the oxygen atoms are in blue and red respectively. Hydrogen atoms are omitted for clarity.  the pyridine ligand signals in spectrum 1b becomes straightforward. The presence of two sets of pyridine proton resonances (δ = 7.32 ppm, 6.77 ppm and δ = 7.05 ppm, 6.27 ppm) for protons H 3 and H 4 (Fig. 4) is in full accord with the two types of pyridine ligands observed in the crystal structure (named py(int.) and py(ext.) in Fig. 1 legend). As in the structure, four py(int.) ligands are sandwiched between catecholato and py(ext.) ligands, the assignment of the upfield shifted pyridine proton resonances to py(int.) protons is proposed (δ = 7.05 ppm and 6.27 ppm). Consequently, the signals at δ = 7.32 ppm and 6.77 ppm correspond to the NMR signature of H 4 and H 3 of py(ext.) respectively. The COSY spectrum permits also the partial assignment of the signals of the catecholato ligands. The signal at δ = 6.03 ppm is abnormally upfield shifted in comparison with the other catecholato proton signals. A close examination of the Ti 10 O 12 (cat) 8 ( py) 8 crystal structure provides evidence to the environment of the H d protons (the labels of the catecholato protons are given in Fig. 4). These protons, belonging to the bridging catecholato ligands are localized in the shielding cone of a second bridging catecholato ligand. The distance measured between these protons and the nearest catecholato ring centroids is estimated to be 3.7 Å. This nicely explains the chemical shift observed for this signal. Next, according to the correlation observed by COSY, the assignment of the resonances of all protons belonging to the bridging catecholato ligands is made. Finally, the rest of the catecholato proton resonances (from δ = 6.7 to 6.4 ppm) are attributed to protons of the singly bridging catecholato ligands.

Complex synthesis and crystal structure description
After the Ti 10 O 12 (cat) 8 ( py) 8 low-temperature 1 H NMR spectrum assignment, the pyridine ligand substitution study was undertaken as proposed in eqn (1). The post-synthetic modification of titanium-oxo clusters represents a very challenging task since a reorganization of the titanium oxo backbone after the substitution reaction is generally observed. 16 Only a few examples of a post-synthetic cluster modification without any alteration of the inorganic core of the initial Ti(IV) oxo-cluster are reported in the literature. 17 Many different substituted pyridine compounds have been tested with very limited success concerning the isolation of the desired products as crystalline solids. The tested pyridine derivatives include lutidine derivatives, pyridine rings functionalized with electron donating or withdrawing groups or pyridine rings substituted with sterically hindered groups. However, when the reactions were conducted with the pico, 4-Phpy and pyrald compounds, after the diffusion of diethylether, crystals of the targeted compounds were isolated in quantitative yields. It is worth noticing that the pico and 4-Phpy compounds are commercially available products whereas the pyrald derivative is a key intermediate for the preparation of several molecules. 18 These new complexes were characterized by NMR spectroscopy, infrared spectroscopy and elemental analysis. The S 4 symmetry of these titanium oxo-clusters in solution was confirmed by 13 C NMR since four resonances attributed to C̲ -O (catecholato) are observed in the spectra of the complexes dissolved in CDCl 3 .
Single crystals of the complexes were obtained by dissolving the Ti 10 O 12 (cat) 8 ( py) 8 complex in chloroform with a slight excess of the substituted pyridines. Dark red single crystals suitable for X-ray diffraction analysis were obtained for the pico, 4-Phpy and pyrald compounds by slow diffusion of diethylether in a few days period. The crystal structure resolution reveals that compounds obtained with 4-Phpy and pyrald crystallize in the I4 1 /a space group whereas the third compound crystallizes in the P1 space group. In each crystalline network, the complex co-crystallizes with chloroform molecules. The molecular structures of these complexes are shown in Fig. 5(a-c).
The three complexes are nanosized objects with molecular volumes estimated to be 1370 Å 3 , 2110 Å 3 and 2464 Å 3 for Ti 10 (Fig. 1)). The pyridine protons are labelled as H 3 , H 4 and H 5 ext. or int. (ext. and int. referring to the localization of these ligands in the complex). H(int.) concerns the protons of the pyridine ring py(int.) (colored in light green in the structure (Fig. 1)). H(ext.) refers to the protons of the pyridine rings, py(ext.) that decorate the external part of the complex and colored in dark green in the structure (Fig. 1). Titanium ions in black, red and pink refers to TiO 6 , TiO 5 N (bridging) and TiO 5 N respectively.
For the new complexes no residual pyridine ligand is found and eight substituted pyridine ligands ( py′) are linked to the inorganic core of the complexes leading to clusters displaying the respective Ti 10 O 12 (cat) 8 ( pico) 8 , Ti 10 O 12 (cat) 8 (4-Phpy) 8 and Ti 10 O 12 (cat) 8 ( pyrald) 8 formulae. In these structures, as for the Ti 10 O 12 (cat) 8 ( py) 8 complex, two types of rarely observed coordination modes for the catecholato ligands are noticed. The purple colored catecholato ligands in Fig. 5 represent the four ligands adopting a singly bridging chelate μ 2 -(O,O′,O′) mode, whereas the singly bridging μ 2 -(O,O′) mode involves the four catecholato ligands colored in orange. The inorganic core of the complexes is formed by four μ 2 -oxo and eight μ 3 -oxo bridging ligands with two types of inequivalent TiO 5 N subunits and one type of TiO 6 polyhedron. It is worth noticing that Concerning the metrical description of the complexes, the Ti⋯N bond distances are similar to those found in Ti 10 O 12 -(cat) 8 ( py) 8 (the Ti-N bond distances are given in Table SI †). Clearly, the lengths of the Ti-N bonds are not affected by the nature of the substituents decorating the pyridine rings.

Partial charge analysis
The partial charge calculation for the four complexes was performed with the PACHA software. 19 The charges found in these complexes for the oxygen atoms are unexceptional. These charges are listed in Table 1. The charges of the oxygen atoms belonging to the catecholato ligands range from  8 . In these structures, the two inequivalent catecholato ligands are differently colored (in orange and light purple). The pyridine ligands are colored in green. The titanium atoms and the oxygen atoms are in blue and red respectively. This inorganic core is composed of ten titanium atoms, eight μ 3 -oxo bridges (two inequivalent μ 3 -oxo bridges are present), and four μ 2 -oxo bridges; the other oxygen atoms belong to the catecholato ligands.  Table 2. The charges for the titanium ions surrounded by 6 oxygens are around q = +2.06 ± 0.02, whereas the titanium ions having a TiO 5 N coordination sphere and linked with the catecholato ligand adopting a bridging μ 2 -(O,O′) mode, display more positive charges between q = +2.42 and q = +2.45. These values contrast strongly with the partial charges computed for the titanium atoms inscribed in the second TiO 5 N subunit (these titanium atoms are coordinated by the catecholato ligands adopting the singly bridging chelate μ 2 -(O,O′,O′) mode). These titanium atoms have less positive values around q = +1.85 ± 0.02. Interestingly, charges of titanium inferior to q = 2 are rather uncommon for neutral catecholato-based Titanium(IV) neutral complexes. According to the 13 structures of neutral Ti(IV) complexes incorporating the catecholato ligand reported in the V1.1.1 webCSD release of the Cambridge Crystallographic Database, the charge of titanium ions are found superior to q = +2 with a mean value of q = +2.50 ± 0.34. A notable exception concerns a recently reported high nuclearity complex incorporating 17 titanium(IV) atoms. Here, the q(Ti) values range from q = +1.00 to +2.10. 10 According to the charges calculated from these four structures, two categories of titanium could be distinguished. For the titanium atoms with q(Ti) > 2, these atoms have charges compatible with titanium centers found in low nuclearity molecular complexes. The second category of titanium atoms (q = +1.85 ± 0.02) resemble more to a titanium atom found in TiO 2 (anatase or rutile) q(Ti) = 1.63.

UV-visible spectroscopy
The UV-visible light absorption properties of the titanium oxoclusters were studied in solution (CHCl 3 ) and in the solid- Table 1 Charges computed for the oxygen atoms in Ti 10 O 12 (cat) 8 ( py) 8 , Ti 10 O 12 (cat) 8 (4-Phpy) 8 , Ti 10 O 12 (cat) 8 ( pico) 8 and Ti 10 8 , and Ti 10 O 12 (cat) 8 ( pico) 8 (c = 10 −5 M) due to the insolubility of the Ti 10 O 12 (cat) 8 ( pyrald) 8 complex. These spectra are shown in Fig. 7. The spectra for the three complexes are rather similar. The complexes start to absorb light around 600 nm with a maximum in the visible region around 410 nm. In the UV domain, the complex bearing the 4-Phpy ligand absorbs strongly in comparison with its two analogues. Nevertheless, the nature of the pyridine ligands that substitute the oxo-clusters seems not to really affect the absorption properties of visible light. These solid-state spectra are viewed in Fig. 8.
In the solid state, all species incorporating the Ti 10 O 12 (cat) 8 backbone display similar optical properties with a broad range of absorption in the visible domain between 400 nm and 650 nm. The band gaps are estimated for the five complexes using direct extrapolation of the absorption edges to the energy axis (see the ESI †). The E g values found for the different titanium-based species are compiled in Table 3. The band gaps for the complexes are rather similar ranging from 1.85 eV to 1.97 eV. Again, the nature of the nitrogen-ligands  seems not to significantly affect the solid-state electronic properties of the complexes. It should be noted that these band gaps estimated for our series of Ti 10 O 12 (cat) 8 structures are much lower compared to those found for TiO 2 -based materials or nanoparticles or polyoxotitanate cages that ranges between 2.4 eV and 3.8 eV. 20

Theoretical calculations
Density functional theory (DFT) calculations on the Ti 10 O 12 (cat) 8 and Ti 10 O 12 (cat) 8 ( py) 8 have been performed with the Gaussian09 package, 21 using the hybrid exchange correlation functional B3LYP with a double-ζ basis function set with an ultrafine grid was used in all our calculations. This approach was chosen to be in line with previous theoretical studies reported on similar complexes. 22 For both complexes, geometry optimization started from the crystal structure geometry of Ti 10 O 12 (cat) 8 ( py) 8 . In the case of Ti 10 O 12 (cat) 8 , pyridine fragments were removed before calculation. The crystallographic symmetries were maintained in all of the calculations. GaussView was used for visualization of the computational results, 23 while DOS plots have been obtained with the GaussSum 2.2 software. 24 Even though the Kohn-Sham eigenstates should only be considered as a first approximation to one-particle energy levels and hence the obtained band gaps should therefore not be taken too literally, they are often informative in providing qualitative understanding of the electronic structures of materials and of the trend in band gaps. 6 The HOMO, HOMO−1, HOMO−2 and LUMO, LUMO+1, LUMO+2 representations determined for Ti 10 O 12 (cat) 8 and Ti 10 O 12 (cat) 8 ( py) 8 are given in Fig. 9. For both complexes, the HOMO, HOMO−1 and HOMO−2 orbitals are localized exclusively on the catecholato ligands. Concerning the unoccupied orbitals (LUMO, LUMO+1, LUMO+2), they are originated almost exclusively from titanium d orbitals. Therefore, low energy transitions observed for the Ti 10 O 12 (cat) 8 ( py) 8 complex arise undoubtedly from catecholato-to-titanium charge transfer. Interestingly, the presence of the pyridine influences drastically the energy of the HOMO-LUMO gap. Energies of ΔE = −2.94 eV and ΔE = −3.32 eV are respectively found for Ti 10 O 12 (cat) 8 and Ti 10 O 12 (cat) 8 ( py) 8 (see Table 4). Ti 10 O 12 (cat) 8 has the lowest HOMO-LUMO gap calculated for a neutral catecholato-based titanium complex to date. 22 The density of states (DOS) plots for Ti 10 O 12 (cat) 8 and Ti 10 O 12 (cat) 8 ( py) 8 are reproduced in Fig. 10. These DOS plots evidence discrete energy levels for both complexes. However, we notice the influence of the pyridine ligands onto the Ti 10 O 12 (cat) 8 ( py) 8 DOS plots as the occupied levels are less energetically isolated compared to the occupied levels in Ti 10 O 12 (cat) 8 .

Conclusions
The post-synthetic modification of a catecholato polyoxotitanate nano-sized cluster has been demonstrated by taking advantage of the labile behavior of the pyridine ligands in Ti 10 O 12 (cat) 8 ( py) 8 . Three S 4 -symmetric complexes having an identical Ti 10 O 12 (cat) 8 backbone were isolated from Ti 10 O 12 -(cat) 8 ( py) 8 and structurally characterized. These Ti 10 O 12 (cat) 8based complexes incorporate two types of titanium atoms according the charge partial analysis: four Ti atoms in these structures display charges similar to those found in TiO 2 whereas the rest of the Ti atoms within these structures are related to Ti atoms found in molecular coordination compounds. These species strongly absorb light in the visible domain. According to DFT calculation, these strong absorbances are originated from HOMO-LUMO electronic transitions where the HOMOs are localized on the catecholato ligands and the LUMOs are centered on the d orbitals of titanium atoms. The DOS plots of Ti 10 O 12 (cat) 8 are far from displaying conduction and valence bands. Nevertheless, the pyridine ligands decorating the Ti 10 O 12 (cat) 8 backbone appear to impact the DOS plots towards the one expected for semiconductor nanoparticles. Overall, the Ti 10 O 12 (cat) 8 fragment appears to be an attractive building unit to generate Ti-O based hybrid materials absorbing visible light rather than ultraviolet light. 25

Experimental section
General All chemicals, solvents and reagents were of the best commercially available grade, purchased from Alfa Aesar, Sigma Aldrich and TCI Europe and used as received. 1 H and 13 C NMR spectra were recorded with a Bruker Avance 500 (500 MHz) spectrometer with the use of a deuterated solvent as the lock. The chemical shifts were expressed in parts per million ( ppm, δ) and referenced to residual solvent protons as internal standards ( 1 H NMR: CDCl 3 : δ = 7.26 ppm, CD 2 Cl 2 : δ = 5.32 ppm, DMSO: δ = 2.50 ppm; 13 C NMR: CDCl 3 : δ = 77.16 ppm, CD 2 Cl 2 : δ = 53.84 ppm, DMSO: δ = 39.52 ppm). The description of signals include: s singlet, and dd doublet of doublets. The signal abbreviations include: Phenpyr-H (aromatic proton of 4-phenylpyridine), Pico-H (aromatic proton of 4-picoline), and Cat-H (aromatic proton of catechol). Infrared (IR) spectra were recorded with a Bruker Alpha FT-IR spectrometer. UV-Vis spectra were recorded with a BioTek UVIKON XL spectrometer. Thermogravimetric analyses (TGA) were performed on a Perkin-Elmer Pyris 6 TGA thermogravimetric analyser. Differential Scanning Calorimetry (DSC) spectra were obtained with a Perkin-Elmer Jade DSC calorimeter. Microanalyses were per-  8 . The reaction was conducted under air. In a small glass vial, to a solution of Ti 10 O 12 (cat) 8 ( py) 8 (5 mg, 0.0023 mmol) dissolved in 0.750 mL of CHCl 3 was added a slight excess of 4-phenylpyridine (4-Phpy, 3 mg, 0.019 mmol) in 0.250 mL of CHCl 3 . The reaction mixture was agitated for few seconds and set aside to crystallize by slow vapor diffusion of diethyl ether. Small dark red crystals were recovered by filtration and dried under vacuum during one night to afford Ti 10 O 12 (cat) 8 (4-Phpy) 8 in quantitative yield (6 mg). 1