Functionalization of Titanium Oxide Cluster Ti₁₇O₂₄(OⁱC₃H₇)₂₀ with Catechols: Structures and Ligand‐Exchange Reactivities

Abstract

The functionalization of Ti17O24(OiC3H7)20 (Ti17) with substituted catechols is studied by using crystallography, Raman spectroscopy, and stopped‐flow kinetics. The knowledges on the number of accessible functionalities, their exact location correlated with their Raman assignment, and the kinetic parameters are acquired. A catecholate ligand binds to a five‐coordinated surface Ti of Ti17 (denoted as Tia site) adopting the mono‐protonated, chelate‐bidentate binding mode, whereas it binds to a six‐coordinated surface‐Ti (denoted as Tib site) adopting the mono‐dentate mode. With low numbers of equivalents of added catechols the Tia sites show higher reactivity than the Tib sites toward functionalization. Two binding modes may co‐exist and equilibrate in solution. Our results also imply that at most eight of the twenty OiC3H7 ligands of Ti17 are exchangeable without damage of the core structure. The kinetic studies point out that the ligand‐exchange reaction is second order and occurs very fast. The current findings are helpful for the controlled functionalization of Ti17 and other Ti oxide clusters, and the further application of them as building blocks in supramolecular chemistry for the assembly of well‐defined organic–inorganic hybrid materials.

Supramolecular Chemistry of Titanium Oxide Clusters

“…Whereas Zhang et al. showed that phenolate with a charge of −1 acts as μ 2 ‐bridging (Scheme 1, mode e) or terminal ligands (Scheme 1, mode f) of TOCs, [36] we observed that hydroquinone bound to the surface of TOC as a neutral ligand (Scheme 1, mode g) [35] …”

“…The TOCs functionalized with catecholate or salicylate usually exhibit intense EPR peaks at g ≈2, for example, the Ti 4 O(O i Pr) 6 (DTBC) 4 molecule shown in Figure 14 a [39] . The signals are assigned as phenoxyl radicals, [35, 40] similar to that of the same adsorbent on a TiO 2 nanoparticle [41] . Interestingly, under visible‐light irradiation, the EPR signal intensity of a solution of TOC increased significantly.…”

“…Four binding modes of catecholate on the surfaces of TOCs, that is, modes a–d, as shown in Scheme 1, have been revealed. Among them, catecholate in modes a and b is monoprotonated, [35] whereas that in mode c is fully deprotonated [27, 38] . Mode d was only found in Ti 10 Ct 2 [38] .…”

“…Recently, we systematically investigated the postfunctionalization of Ti 17 with substituted catechols (SCt) [35] . Whereas X‐ray diffraction was used to characterize the single crystals of the intermediates/products, spectroscopic methods, such as UV/Vis and Raman, were applied to monitor the reaction progress.…”

“…The inset highlights the structure of Ti 17 Ct from ref. [35]. c) Preparation of the Ti 17 film substrate [40] …”