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. 2021 Jan 29;11(10):5537-5547.
doi: 10.1039/d0ra09319j. eCollection 2021 Jan 28.

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

Sven A Freimann  1 , Alessandro Prescimone  1 , Catherine E Housecroft  1 , Edwin C Constable  1
Affiliations

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

Sven A Freimann et al. RSC Adv. .

Abstract

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.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Scheme 1
Scheme 1. Structures of compounds 1–5.
Fig. 1
Fig. 1. (a) The structure of the [Rh(1)2]3+ cation in the complex [Rh(1)2][NO3]3·1.25[H3O][NO3]·2.75H2O; H atoms except those in the phosphonic acid groups are omitted for clarity. Selected bond distances: Rh1–N1 = 2.047(3), Rh1–N2 = 1.962(3), Rh1–N3 = 2.055(3), Rh1–N4 = 2.055(3), Rh1–N5 = 1.964(3), Rh1–N6 = 2.043(3), P1–O1 = 1.501(3), P1–O2 = 1.544(3), P1–O3 = 1.552(3), P1–C28 = 1.786(4), P2–O4 = 1.550(3), P2–O5 = 1.546(4), P2–O6 = 1.489(3), P2–C40 = 1.800(4) Å. (b) Part of one hydrogen-bonded chain of [Rh(1)2]3+ cations. (c) The structure of the [Rh(5)2]3+ cation in [Rh(5)2][PF6]3·MeCN; H atoms are omitted for clarity. Selected bond parameters: Rh1–N1 = 2.040(6), Rh1–N2 = 1.964(6), Rh1–N3 = 2.046(7), Rh1–N4 = 2.047(7), Rh1–N5 = 1.984(6), Rh1–N6 = 2.050(7), P1–O1 = 1.471(10), P1–O2 = 1.540(15), P1–O3 – 1.605(14), P1–C37 = 1.803(11), P2–O4 = 1.453(8), P2–O5 = 1.707(15), P2–O6 = 1.634(13), P2–C19 = 1.787(9) Å.
Scheme 2
Scheme 2. Assembly of [Rh(1)2]3+ on TiO2 NPs starting from NPs functionalized with 1. Conditions: (i) RhCl3, EtOH : H2O, 95 °C, 6 h or [Rh2(μ-OAc)4(H2O)2], NaCl, EtOH, 80 °C, 18 h. NP surfaces are rough and could potentially provide two binding sites for the rhodium metal complex or two NPs could come together to form the rhodium metal complex.
Fig. 2
Fig. 2. Solid-state FT-IR spectra of activated NPs (black), (1)@TiO2 NPs (blue), [Rh(1)2]Cl3 (orange), and Rh(1)2@TiO2 NPs (red).
Fig. 3
Fig. 3. 1H NMR spectra (500 MHz, D2O, 298 K) of 1 in D2O and NaOH (a), (1)@TiO2 in D2O and NaOH (b), [Rh(1)2]Cl3 made using [Rh2(μ-OAc)4(H2O)2] in D2O and NaOH (c), Rh(1)2@TiO2 NPs made using [Rh2(μ-OAc)4(H2O)2] in D2O (d), Rh(1)2@TiO2 NPs made using [Rh2(μ-OAc)4(H2O)2] in D2O and NaOH (e), [Rh(1)2]Cl3 made using RhCl3·3H2O in D2O and NaOH (f), Rh(1)2@TiO2 NPs made using RhCl3·3H2O in D2O (g), Rh(1)2@TiO2 NPs made using RhCl3·3H2O in D2O and NaOH (h). Chemical shifts in δ/ppm.
Fig. 4
Fig. 4. TGA curves for activated NPs (black), (1)@TiO2 NPs (red), Rh(1)2@TiO2 NPs made using [Rh2(μ-OAc)4(H2O)2] (blue) and for Rh(1)2@TiO2 NPs made using RhCl3·3H2O (green).
Scheme 3
Scheme 3. Secondary alcohol oxidation of rac-(1R)-1-phenylethanol to acetophenone.
Fig. 5
Fig. 5. 1H NMR spectrum (500 MHz, D2O, 298 K) of the reaction solution after alcohol oxidation (38 h) using Rh(1)2@TiO2 NPs as catalyst (see Experimental).
Fig. 6
Fig. 6. Product concentration in percent against time in minutes during alcohol oxidation with varing NaOH base concentration, NaOH equivalents compared to starting material: 0.01 (black), 0.02 (red), 0.1 (blue) and 1 (green).
Fig. 7
Fig. 7. 1st order reaction graph, natural logarithm of starting material concentration during kinetic measurements of rac-(1R)-1-phenylethanol oxidation (see Experimental) against time in minutes (black), linear trendline through datapoints (red), insert of first 400 minutes illustrates incubation time.
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