Turning over on sticky balls: preparation and catalytic studies of surface-functionalized TiO2 nanoparticles

We have investigated the reactivity of rhodium(iii) complex-functionalized TiO2 nanoparticles and demonstrate a proof-of-principle study of their catalytic activity in an alcohol oxidation carried out under aqueous conditions water in air. TiO2 nanoparticles (NPs) have been treated with (4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)phosphonic acid, 1, to give the functionalized NPs (1)@TiO2. Reaction between (1)@TiO2 NPs and either RhCl3·3H2O or [Rh2(μ-OAc)4(H2O)2] produced the rhodium(iii) complex-functionalized NPs Rh(1)2@TiO2. The functionalized NPs were characterized using thermogravimetric analysis (TGA), matrix-assisted laser desorption ionization (MALDI) mass spectrometry, 1H NMR and FT-IR spectroscopies; the single crystal structures of [Rh(1)2][NO3]3·1.25[H3O][NO3]·2.75H2O and of a phosphonate ester derivative were determined. 1H NMR spectroscopy was used to follow the reaction kinetics and to assess the recyclability of the NP-supported catalyst. The catalytic activity of the Rh(1)2@TiO2 NPs was compared to that of a homogeneous system containing [Rh(1)2]3+, confirming that no catalytic activity was lost upon surface-binding. Rh(1)2@TiO2 NPs were able to withstand reaction temperatures of up to 100 °C for 24 days without degradation.


Introduction
In general, catalytic processes are categorized as homogeneous or heterogeneous. Homogeneous catalysts have the advantage that all of the catalytic centres are potentially active, whereas in heterogeneous catalysts only the surface catalytic sites of solid phases are active as interior sites are inactive. 1 With solid phase heterogeneous catalysts, reaction only takes place at the interface rather than in the bulk of the reactant volume. On the other hand, it is oen easier to separate the products of heterogeneous catalysis from the spent catalyst than in the case of homogeneous catalysis. 2,3 Heterogeneous catalysts also have signicant benets in terms of catalyst recovery, a feature gaining increasing environmental and economic importance. Nanoparticle (NP) immobilized catalysts have the potential of bridging the gap between these two extreme types of catalysis. Firstly, NPs exhibit greater surface-to-volume ratios than bulk heterogeneous catalysts and the catalyst loading capacity, catalytic activity and turnover will be enhanced, in particular the catalytic site-to-volume ratio will be high. 4 Secondly, the ability to disperse NPs in solution gives some of the benets of homogeneous catalysts. NP-supported catalysts have attracted great interest and offer already outstanding diversity and range in the chemical and pharmaceutical eld. [5][6][7] The advantages of NP supported catalysts have been demonstrated for a number of key reactions. For the Suzuki-Miyaura coupling reaction, palladium-decorated benzene-1,2diamine-functionalized Fe 3 O 4 /SiO 2 magnetic NPs were utilized and showed excellent yields within short reaction times. 1 Gold NP-supported ruthenium catalysts have been used for ringopening metathesis polymerization of bicyclo[2.2.1]hept-2-ene (norbornene) and show a higher activity than the unsupported counterparts. 8 Nickel NP-based catalysts were active in the steam-reformation reaction of methane showing excellent conversion, H 2 selectivity and thermal stability. 9 These reactions oen make use of valuable and rare elements that cannot be efficiently recovered in the case of homogeneous catalysis. Recyclability of a catalyst is essential and conventional heterogeneous catalysts have the benet that they can be easily recovered compared to homogeneous catalysts. Dispersed NPs are more challenging to separate than conventional heterogeneous catalysts but the recoverability is still greater than with most homogenous catalysts. Furthermore, NPs can be modied to boost recoverability by introducing additional characteristics such as magnetic properties. The benets of NPs are well-established in the oxidation of alcohols, a key step in the synthesis of many organic compounds. 7 The transformation of alcohols to aldehydes, ketones or carboxylic acids generally needs stoichiometric quantities of hazardous, environmentally damaging and toxic oxidants such as chromium trioxide, dichromate, permanganate and chromic acid. 10 We have recently demonstrated 11 the functionalization of TiO 2 NPs with ligands 1, 2, 3 and 4 (see Scheme 1) which bind to the metal oxide surface through their phosphonate or phosphonic acid groups and we were interested in investigating the activity of complexes incorporating these surface-bound ligands. We report here the assembly of surface-bound rhodium complexes of ligand 1 related to [Rh 2 Cl 2 (m-OAc)(tpy) 2 ] + reported by Wang et al. 12 We compare the catalytic behaviour of functionalized TiO 2 NPs to the homogeneous catalytic performance of [Rh(1) 2 ]Cl 3 and [Rh(5) 2 ] [PF 6 ] 3 . We have also investigated the kinetics of these reactions, the recyclability of the NP catalyst over 5 cycles, and the inuence of the concentration of base on the catalytic activity of the functionalized nanoparticles.

General
Instrumentation details are given in the ESI. † Ligands 1 and 5 (Scheme 1), 13 were prepared according to the literature and their spectroscopic data matched those previously reported. TiO 2 NP activation and functionalization with ligand 1 were carried out according to our previously published procedure. 11 MALDI mass spectra were recorded using a-cyano-4-hydroxycinnamic acid (CHCA) as the matrix.
TiO 2 NPs (AEROXIDE TiO 2 P25) were purchased from Evonik Industries. The spherical NPs have an average radius of 10.5 nm (ref. 14) and an average surface area-to-volume ratio of 28%. The number of equivalents of NPs is dened as 0.28Â the total number of TiO 2 formula equivalents in the mass given, i.e. the effective surface concentration of TiO 2 . 11 Similarly, when clarifying equivalents or mmol of functionalized NPs, it refers to the estimated amount of ligand or complex bound to the surface.

Activation of commercial P25 TiO 2 NPs
The commercial NPs were activated as previously described 11 but the procedure was scaled up as follows. Commercial P25 TiO 2 NPs (2.00 g) were dispersed by sonication for 15 min in dilute aqueous HNO 3 (30 mL, 3 M). The mixture was then stirred for 30 min. The NPs were separated from the acid by centrifugation (10 min, 9000 rpm) and washed once with milliQ water (30 mL). The NPs were again dispersed in milliQ water (20 mL) through sonication for 10 min and the suspension was then stirred overnight. The NPs were separated by centrifugation (10 min, 9000 rpm) and washed with milliQ water (2 Â 20 mL). The activated NPs (1.85 g) were stored in a sealed vial under N 2 aer drying over high vacuum.

Recyclability of Rh(1) 2 @TiO 2 NPs
Rh(1) 2 @TiO 2 NPs (72.7 mg, 0.5 mol%) and milliQ water (1.67 mL) were added to a microwave vial. The suspension was sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) was added. The pH of the mixture was determined to be 7.8. rac-(1R)-1-Phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) was added to the suspension aer which the vial was sealed and the reaction mixture heated to 100 C for 24 h. Aer cooling, the reaction mixture was washed with Et 2 O (6 Â 3 mL). A small amount of the collected Et 2 O fractions (ca. 50 mL) was removed by syringe and the solvent was evaporated. The residue was dissolved in 500 mL D 2 O and added to an NMR tube. The 1 H NMR spectrum was measured to determine product : reactant ratios. The pH of the reaction mixture was measured using a pH electrode and adjusted to the initial value of 7.8 by adding aqueous NaOH (25 mM). The vial was resealed and the reaction mixture was heated again to 100 C for 24 h. The reaction was performed with this procedure 5 times in total. The product conversions were as follows: 20.3, 20.1, 18.3, 20.9 and 19.9%.

Kinetics of oxidation
Rh(1) 2 @TiO 2 NPs (145.4 mg, 0.5 mol%) and D 2 O (3.33 mL) were added into a microwave vial. The contents were sonicated (1 min) and then NaOH (25 mM in D 2 O, 0.667 mL) and rac-(1R)-1phenylethanol (0.2 mL, 1.65 mmol, 1 eq.) were added to the suspension. The vial was sealed and the reaction mixture was heated to 100 C for two weeks (ca. 300 h). At varying time intervals, a small amount of the reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The 1 H NMR spectrum was recorded to determine the product : reactant ratios at any point. Overall, data for 27 points were recorded over 12 days.

Control experiment 1: catalyst inuence on reaction rate
Activated NPs (72.7 mg) and milliQ water (1.67 mL) were added to a microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) and rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspension. The vial was sealed and the reaction mixture was heated to 100 C for 24 h. A small amount of the reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The 1 H NMR spectrum was recorded and revealed that no acetophenone had formed. Further control experiments were conducted using either unactivated commercial NPs, activated NPs, (1)@TiO 2 NPs or [Rh 2 (m-OAc) 4 (H 2 O) 2 ]. The control experiment using (1)@TiO 2 NPs and [Rh 2 (m-OAc) 4 (H 2 O) 2 ] showed 7% product conversion while all other experiments showed less than 1% product conversion aer 24 hours (Table S1 †).

Control experiment 2: temperature inuence on reaction rate
Rh(1) 2 @TiO 2 NPs (72.7 mg, 0.5 mol%) and milliQ water (1.67 mL) were added to a microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) and rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspension. The vial was sealed and the reaction mixture was le stirring at room temperature for 72 h. A small amount of the reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The 1 H NMR spectrum was recorded which revealed no acetophenone had formed (Table S1 †).

Control experiment 3: NaOH inuence on functionalization stability
Rh(1) 2 @TiO 2 NPs (72.7 mg, 0.5 mol%) and milliQ water (1.67 mL) were added to a microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) was added to the suspension. The vial was sealed and the reaction mixture was heated to 100 C for 72 h. The supernatant solution was separated from the NPs by centrifugation (30 min, 17 500 rpm). The supernatant solution was ltered and added into a separate microwave vial and rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) was added to the solution. The vial was sealed and the reaction mixture was heated to 100 C for 24 hours. A small amount of the reaction mixture (ca. 500 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The 1 H NMR spectrum was recorded and revealed that no acetophenone had formed. The separated NPs were added together with milliQ water (1.67 mL) into a separate microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspension. The vial was sealed and the reaction mixture was heated to 100 C for 24 hours. A small amount of the reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The 1 H NMR spectrum was recorded and showed acetophenone had been formed. This experiment was repeated without any base and yielded the same result. The reaction using the separated supernatant solution did not show acetophenone formation (<1%) while the reaction containing the separated NPs showed normal product conversion (19.9%).

Control experiment 4: NaOH concentration inuence on reaction rate
Rh(1) 2 @TiO 2 NPs (72.7 mg, 0.5 mol%) and milliQ water (1.67 mL) were added to a microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspension. Simultaneously three other vials were prepared under the same conditions with different NaOH (0.331 mL, 50 mM, 0.25 M, 2.5 M) concentrations. The vials were sealed and the reaction mixtures were heated to 100 C for 3 weeks. A small amount of each reaction mixture (ca. 50 mL) was removed from each vial by syringe and dispersed in 500 mL D 2 O in separate NMR tubes. The 1 H NMR spectrum of each sample was recorded to determine the product : reactant ratios at any point. Overall, 11 data points were recorded over 24 days.

Control experiment 5: oxidation using free rhodium complexes
[Rh(1) 2 ]Cl 3 (3.79 mg, ca. 0.5 mol%) and milliQ water (1.67 mL) were added to a microwave vial. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) and rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspension. The vial was sealed and the reaction mixture was le stirring at 100 C for 24 h. A small amount of the reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in an NMR tube. The NMR data revealed that aer 24 h, the ratio of product to reactant was 1.0 : 5.3 (15.8% product, Table S1 †). The procedure was repeated using [Rh(5) 2 ][PF 6 ] 3 and showed a ratio of product to reactant of 1.0 : 4.2 (19.2% product, Table S1 †).

Control experiment 6: inert atmosphere and light inuence on reaction rate
Four microwave vials were prepared to all of them Rh(1) 2 @TiO 2 NPs (72.7 mg, 0.5 mol%) and milliQ water (1.67 mL) was added. The contents were sonicated (1 min) and then aqueous NaOH (25 mM, 0.331 mL) and rac-(1R)-1-phenylethanol (0.1 mL, 0.827 mmol, 1 eq.) were added to the suspensions. The suspensions in two vials were bubbled with Argon for 15 minutes. The vials were sealed and one under inert atmosphere and one under air were covered from light with alumina foil. Leading to vials having inert atmosphere, no light, inert atmosphere and no light, and a control with air not covered from light. The four vials were le stirring at 100 C for 72 h. A small amount of each reaction mixture (ca. 50 mL) was removed by syringe and dispersed in 500 mL D 2 O in separate NMR tubes. The NMR data revealed that aer 72 h, the ratio of product to reactant for the control vial was 1.0 : 2.4 (29.3% product, Table S1 †) while the ratio of product to reactant when using inert gas was 1.0 : 5.0 (16.5% product, Table S1 †). Performing the reaction without light had no effects on the product formation.

Results and discussion
Choice of anchoring ligand and substrate 2,2 0 :6 0 ,2 00 -Terpyridines (tpy) are well-established chelating ligands which undergo a conformational change from the equilibrium transoid-arrangement upon coordination to metal centres. 21 The tpy metal-binding domain acts as a good s-donor and a strong p-acceptor, making tpy ligands excellent candidates for the stabilization of low oxidation state metal centres. This leads to their applications in a wide range of homogeneous catalytic reactions involving metals such as Ni, Cu, Ru, Pd, Rh, Fe, Mg and Co. 22 The functionalization of metal chalcogenide NPs with carboxylic and phosphonic acids is well-established. [23][24][25][26][27][28][29][30][31][32] We have extended surface-modication strategies developed for dye-sensitized solar cells to nanoparticles and have illustrated that TiO 2 NPs can be functionalized with bpy or tpy ligands bearing carboxylic or phosphonic acid anchoring units. 11 We further demonstrated the preferential binding of phosphonic acids over carboxylic acids, and the ability of ligandfunctionalized NPs to complex metal ions such as copper(I) and iron(II) to form robust coordination-complex functionalized NPs. 11 TiO 2 NPs have benets beyond being able to strongly bind anchoring ligands (carbocylic or phosphonic acids): they comprise earth abundant elements, are relatively cheap, nontoxic, thermodynamically stable and temperature resistant. TiO 2 NPs can also be specically prepared in a wide variety of sizes and shapes. This makes them a desirable choice for further investigation as a substrate for catalysis.
One potential problem with TiO 2 NPs is that they can exhibit large amounts of surface-adsorbed water creating problems for catalytic processes in which it is crucial to avoid exposure to water and redox-related species such as dioxygen. 11,22 It is therefore of signicant interest to investigate catalytic systems that can tolerate both water and air. 12 These "green" conditions are in any case desirable. The rhodium(III) complexes selected for the present investigation are tolerant of both water and air. No peaks assigned to acetato species were observed in the mass spectrum and the 1 H NMR spectrum (Fig. S2 †) showed no signals arising from an acetate group.
A comparison of the 1 H NMR spectrum of 1 with that of the black product suggested the formation of a homoleptic [Rh(1) 2 ] n+ complex. The shi to lower frequency for the signal assigned to proton A6 (see Scheme 1) is consistent with this proton lying over the ring of a second tpy domain and the spectrum (which shows only one set of tpy signals) indicates the formation of a homoleptic bis(tpy) complex. We therefore concluded that the product was [Rh(1) 2 ]Cl 3 . In order to conrm this proposal, we adapted the protocol described by Thomas and coworkers 15 for the preparation of [Rh(4 0 -Phtpy) 2 ][PF 6 ] 3 to prepare [Rh(1) 2 ]Cl 3 from RhCl 3 $3H 2 O and 1. The presence of chloride counter ion in the product was established by dissolving the compound in concentrated HNO 3 and adding a drop of silver nitrate which lead to the precipitation of white silver chloride (Fig. S3 †). The MALDI mass spectrum of [Rh(1) 2 ]Cl 3 (Fig. S4 †) was similar to that described above (Fig. S2 †) (Fig. S5 and S6 †) were assigned using 2D methods (Fig. S7-S9 †) and were identical to those of the product from the attempted synthesis of [Rh 2 (m-OAc)(1) 2 Cl 2 ]Cl (Fig. S10-S13 †). We speculate that the acidic phosphonic acid substituents on ligand 1 labilize the acetato ligands and prevent the isolation of [Rh 2 Cl 2 (m-OAc)(1) 2 ] + .
The compound [Rh(1) 2 ]Cl 3 formed during the synthesis was insoluble in most solvents and could only be dissolved in concentrated HNO 3 or in water under very basic conditions. This pH dependent solubility suggested the formation of a zwitterionic species in basic conditions.  (Fig. 1a). An ORTEP Fig. 1 (a)   representation of the cation is displayed in Fig. S21a. † The octahedral coordination environment with two chelating tpy domains is unexceptional with the [Rh(tpy) 2 } core closely resembling that observed in other [Rh(Xtpy) 2 ] 3+ cations (X ¼ 4 0phenyl, 4 0 -(pyridin-4-yl), 4-ferrocenyl (DAHDAS, 15 DAHDIA, 15 DAHDEW, 15 XIFTIS 34 ). The Rh-N bond lengths are given in the caption to Fig. 1a, and the chelate N-Rh-N bond angles are in the range 79.84(13)-80.60 (13) . Each P atom is tetrahedrally sited and P-O and P-C bond lengths are given in the caption to Fig. 1a; the bond angles centred on P1 and P2 lie in the range 104.20(17)-114.96 (18) . The most dominant packing interaction involving the [Rh(1) 2 ] 3+ cations is hydrogen bonding between PO(OH) 2 units leading to the assembly of 1D-chains (Fig. 1b). For the centrosymmetric hydrogen-bonded motifs, pertinent (1) 2 ] 3+ cations pack in the lattice with head-to-tail pairings of ligands on adjacent complexes, there are no signicant pstacking interactions between phenyl and tpy domains.

Synthesis and characterization of [Rh(5) 2 ][PF 6 ] 3
Because of initial uncertainties regarding the protonation state of the homoleptic complex obtained with 1, we prepared an analogous compound with a phosphonate ester. Compound 5 (Scheme 1) was prepared according to the literature 15 and its reaction with RhCl 3 $3H 2 O followed by anion exchange using NH 4 PF 6 yielded [Rh(5) 2 ][PF 6 ] 3 as a pale pink solid in 82.7% yield. Mass spectrometric and 1 H, 13 C and 31 P NMR spectroscopic data (see the Experimental section, Fig. S14-S16 †) were consistent with the formation of the homoleptic complex. 1 H and 13 C NMR spectra were assigned using NOESY, HMQC and HMBC spectra (Fig. S17-S19 †). The solution absorption spectrum (in MeCN) of [Rh(5) 2 ][PF 6 ] 3 (Fig. S20 †) exhibits intense bands below 370 nm which are assigned to ligand-centred p* ) p transitions similar to that reported for [Rh(tpy) 2 ](PF 6 ) 3 . 33 The lack of absorptions in the visible region arising from metalto-ligand charge transfer is consistent with the low-spin d 6 conguration of the Rh(III) centre. X-ray quality crystals of [Rh(5) 2 ][PF 6 ] 3 $MeCN were grown by vapour diffusion of Et 2 O into an MeCN solution and the structure of the [Rh(5) 2 ] 3+ cation is shown in Fig. 1c; an ORTEP representation is displayed in Fig. S21b. † Important bond lengths and angles are given in the caption to Fig. 1c. As anticipated, the Rh(III) centre is octahedral, bound by two bis-chelating tpy domains. The structure is unexceptional but serves to conrm the formation of the homoleptic complex, and there are no noteworthy packing interactions.

Assembly of a homoleptic rhodium(III) complex on TiO 2 NPs
Established procedures for TiO 2 NP activation involve HNO 3 (ref. 35) treatment and sonication to optimize surface functionalization and particle dispersion respectively. 36 Commercial P25 TiO 2 NPs were activated using aqueous HNO 3 and then functionalized with ligand 1 (see the Experimental section). For the functionalization, an aqueous suspension of dispersed ligand 1 and activated NPs was heated to 130 C for 3 h under microwave conditions. The (1)@TiO 2 NPs were separated from the liquid phase by centrifugation. The functionalization procedure differs only from that previously described 11 in that it was scaled up and the ratio TiO 2 to 1 had to increased by 10% to ensure binding of all the ligand.
Thermogravimetric analysis (TGA) of activated NPs, (1) @TiO 2 and Rh(1) 2 @TiO 2 NPs was carried out, and the results are presented in Fig. 4. All samples show a weight loss of 1.5-2% in two steps (isotherm maxima <120 C and <330 C). The two steps can be attributed to the loss of physisorbed followed by chemisorbed water. The mass of the non-functionalized NPs undergoes no further signicant change (Fig. S30 †) upon being heated to 900 C for 30 minutes. The (1)@TiO 2 NPs and Rh(1) 2 @TiO 2 NPs exhibit additional 3% and 4-5% weight losses above ca. 400 C (Fig. S24 and S25 †) ascribed to decomposition of the ligand. Additionally, Rh(1) 2 @TiO 2 NPs show a weight increase occurring during the 30 minute 900 C isotherm.   (1)

NP supported homoleptic rhodium complex for alcohol oxidation
Wang and coworkers 12 reported the dehydrogenation and aerobic oxidation of alcohols under air to produce carboxylic acids or ketones with [Rh 2 Cl 2 (m-OAc)(tpy) 2 ]Cl as catalyst under aqueous conditions. We chose the reaction shown in Scheme 3 for a proof-of-principle investigation of the catalytic activity of Rh(1) 2 @TiO 2 NPs.
For the alcohol oxidation, Rh(1) 2 @TiO 2 NPs were dispersed by sonication in milliQ water. Aqueous NaOH and rac-(1R)-1phenylethanol were added to the suspension and the mixture was again dispersed by sonication. A control (Control experiment 3) was carried out to check that NaOH (at the concentrations used in the reactions) did not strip the catalyst from the surface. The reaction was performed under air at 100 C for 18 h, 24 h and 38 h. The product : reactant ratio was measured using 1 H NMR spectroscopy by removing a small amount of reaction solution and dispersing it in D 2 O (see Experimental section). Since the reaction shown in Scheme 3 involves one reactant forming one product, it was possible to reliably determine the reactant to product ratio by comparing the peak area of the aromatic protons and the methyl protons (Fig. 5).
Control experiments (see Experimental section) were performed to investigate several key factors during the reaction. Control experiment 1 compared Rh(1) 2 @TiO 2 NPs with pristine commercial NPs or activated NPs were added to the reaction vial. Aer the reaction, 1 H NMR spectroscopy revealed no acetophenone formation conrming that Rh(1) 2 @TiO 2 NPs are the active catalyst. Control experiment 2 investigate the inuence of temperature with the reaction being performed at room temperature (ca. 22 C) instead of 100 C. 1 H NMR spectroscopy revealed <1% conversion to acetophenone, even aer 72 h, at the lower temperature. Hence, elevated temperatures are important for product formation.
We also ensured that the reaction was catalysed by Rh(1) 2 @TiO 2 NPs as opposed to [Rh(1) 2 ] 3+ that had been removed from the surface. Control experiment 3 investigated if defunctionalization could occur under the basic reaction conditions. Firstly, Rh(1) 2 @TiO 2 NPs and NaOH were dispersed in milliQ water. The mixture was then heated to 100 C for 72 h to simulate the reaction conditions. Next, the NPs were separated from the solution by centrifugation, and the NPs and the supernatant solution separated into two sample vials. The substrate rac-(1R)-1-phenylethanol was added to each vial (see Experimental section) and the reaction mixtures were then heated to 100 C for 24 h. Aer the reaction, 1 H NMR spectroscopy revealed no acetophenone had formed in the vial containing the supernatant solution whereas it was found in the vial containing the NPs. The results of Control 3 indicated that under the basic conditions used in the reaction, little or no defunctionalization of the NPs occurred.
Control experiment 4 investigated the inuence of the concentration of the base concentration on the oxidation and allowed us to determine at what point defunctionalization occurred. Reactions were performed with NaOHrac-(1R)-1phenylethanol ratios of 0.01, 0.02, 0.1 and 1 (see Experimental section). Fig. 6 illustrates the reaction course of each experiment, and shows that the base concentration does not strongly inuence the activity of Rh(1) 2 @TiO 2 NPs with 0.01, 0.02 or 0.1 equivalents of NaOH. The catalytic activity of the Rh(1) 2 @TiO 2 NPs is only affected strongly basic conditions (Fig. 6, green line).
Control experiment 5 was performed to compare the differences in catalytic activities of to [Rh(1) 2 ]Cl 3 . We can make a number of general observations: (i) the attempt to prepare an immobilized dinuclear catalyst analogous to the established homogeneous species was unsuccessful (ii) the presence of the  phosphonate functionality hinders the formation of the homogeneous dinuclear species (iii) both solution and surface chemistry leads to mononuclear complexes with 1 ligands (iv) the catalytic activity of homogeneous species depends to some extent upon the synthetic route used and (v) the catalytic activity of Rh(1) 2 @TiO 2 NPs is generally similar to the homogeneous species.
Although [Rh(tpy) 2 ] n+ species do not appear to have been used as photocatalysts, [Rh(bpy) 3 ] n+ (n ¼ 2 or 3) are well-established in multicomponent systems for photocatalytic reduction. We do not speculate in detail upon the mechanism of the photo-oxidation but it seems likely that the observed photooxidation product arises from the alcohol acting as a sacricial reductant. We note that the yield of the oxidation product is somewhat reduced when the reaction is performed under argon (Table S1 †). We have not observed dihydrogen production. [40][41][42][43][44][45] Kinetics of Rh(1) 2 @TiO 2 NP catalysed alcohol oxidation The kinetics of the alcohol oxidation by Rh(1) 2 @TiO 2 NPs was investigated. The Rh(1) 2 @TiO 2 NPs were dispersed in D 2 O, rac-(1R)-1-phenylethanol and aqueous NaOH were added and the mixture again dispersed by sonication. The alcohol oxidation was performed under air at 100 C for two weeks. The productreactant ratio was determined by measuring a small amount of reaction solution dispersed in D 2 O at certain time intervals with 1 H NMR spectroscopy (see Experimental section).
Analysis of the data indicated rst order kinetics (Fig. 7). The data further suggested an incubation time during the rst 6 h in which the reaction rate was slower. However overall rac-(1R)-1phenylethanol was able to perform linearly over an extended period of time yielding over 71% product aer the reaction.

Recyclability of Rh(1) 2 @TiO 2 NPs
The recyclability of the Rh(1) 2 @TiO 2 NPs was studied. In contrast to the typical procedure described earlier, the pH was monitored using a pH electrode and was returned to pH 7.8 aer each cycle. The conversion of each cycle was determined from the product and starting material extracted with Et 2 O from the reaction solution. This had the benets of not removing any Rh(1) 2 @TiO 2 NPs from the experiment with consequent distortion of the results from the next cycle. Five reaction cycles were measured under which the conversions stayed steady at a product to reactant ratio of 1.0 : 5 (20% product) aer 24 hours reaction time.

Conclusions
In this work we have built upon our previously established metal binding ligand functionalized TiO 2 NPs and use their properties to form a surface bound homoleptic rhodium complex (Rh(1) 2 @TiO 2 NPs). The functionalized NPs were investigated using TGA and 1 H NMR and FT-IR spectroscopies. We further demonstrate a proof-of-principle investigation of their catalytic activity in an alcohol oxidation in water and under air. By tracking the product conversion over time with 1 H NMR spectroscopy we were able to study the reaction kinetics and the recyclability of the catalyst. Rh(1) 2 @TiO 2 NPs were further compared to their non-bound counterpart concluding that the NP-bound catalyst performs similarly if not slightly better. Rh(1) 2 @TiO 2 NPs were able to withstand reactions at 100 C for at least 24 days without showing decomposition or degradation. We further studied the resistance of the functionalization to higher base concentrations. Several other control experiments were performed to exclude inuencing circumstances that could lead to unintentional product formation.

Conflicts of interest
There are no conicts to declare.