Molecular titaniumhydroxamate complexes as models for TiO2 surface binding

Hydroxamates, as well as carboxylates and catecholates, chelate Fe(III) in natural siderophores (Greek: iron carriers). These iron-chelating compounds are secreted by microbes, adsorb strongly to mineral surfaces, and gather iron from their environment. These attributes make hydroxamic acid derivatives (Fig. 1) particularly attractive for functionalization of semiconductive oxide surfaces. They bind strongly and they are simple to synthesize from widelyavailable carboxylate precursors. Hydroxamic acids are relatively new anchoring groups in the context of surface functionalization, with significant advantages over the more common carboxylic acids and phosphonic acids for photoelectrochemical devices. Most notable are the greater resistance to hydrolysis compared to carboxylic acids and greater electronic coupling over carboxylic and phosphonic aids that ensures efficient electron transport. Hydroxamic acids of the form RCONHOH (R = H, Fig. 1) have two potentially labile protons (O–H, N–H). Therefore, it has been unclear whether the surface-bound form is mono(m) or dideprotonated (d) and how the protonation state affects the binding mode. Recent literature has focused on TiO2 as the most widely used metal oxide for photoelectrochemical devices, and has suggested many possible modes for hydroxamate binding. The most likely possibilities are shown in Fig. 2 where NH hydroxamic acids can chelate a single Ti (mc, dc) or bridge two Ti ions (mb and db). The mc binding mode is analogous to that of a previously synthesized N-alkyl titanium hydroxamate complex, and in the natural product ferrichrome in which an N-alkyl hydroxamic acid chelates iron. They could also bind as monodentate species, likely as the monodeprotonated monodentate species mm. Thus, they could bind in a monoanionic form via deprotonation at O (mc, mb, and mm), or dianionic form via deprotonation at O and N (db, dc). Herein, we show evidence for the monodeprotonated binding mode mc to TiO2. We analyzed hydroxamate binding by combining synthesis, IR spectroscopy and density functional theory (DFT) calculations of structure and IR spectra. Synthesis of a series of Ti-hydroxamate complexes from phenylhydroxamic acid precursors allowed for spectral comparisons to functionalized TiO2 surfaces. This series included m-oxo bridged Ti hydroxamate complexes prepared from NHand N-methyl-phenylhydroxamic acids (Fig. 3), the latter being incapable of double deprotonation. We assigned the IR spectra and the composition of the functionalized TiO2 surfaces by using DFT as described in the ESI.† Fig. 1 Hydroxamic acid functional group, where R and R are H, alkyl, or aryl.

Hydroxamates, as well as carboxylates and catecholates, chelate Fe(III) in natural siderophores (Greek: iron carriers). 1-3 These iron-chelating compounds are secreted by microbes, adsorb strongly to mineral surfaces, and gather iron from their environment. These attributes make hydroxamic acid derivatives ( Fig. 1) particularly attractive for functionalization of semiconductive oxide surfaces. [4][5][6][7][8] They bind strongly 9 and they are simple to synthesize from widelyavailable carboxylate precursors. 2,7 Hydroxamic acids are relatively new anchoring groups in the context of surface functionalization, with significant advantages over the more common carboxylic acids and phosphonic acids for photoelectrochemical devices. Most notable are the greater resistance to hydrolysis compared to carboxylic acids and greater electronic coupling over carboxylic and phosphonic aids that ensures efficient electron transport. [4][5][6][7][8] Hydroxamic acids of the form RCONHOH (R 2 = H, Fig. 1) have two potentially labile protons (O-H, N-H). Therefore, it has been unclear whether the surface-bound form is mono-(m) or dideprotonated (d) and how the protonation state affects the binding mode. Recent literature has focused on TiO 2 as the most widely used metal oxide for photoelectrochemical devices, and has suggested many possible modes for hydroxamate binding. 3,5,10,11 The most likely possibilities are shown in Fig. 2 where NH hydroxamic acids can chelate a single Ti (mc, dc) or bridge two Ti ions (mb and db). 5,10 The mc binding mode is analogous to that of a previously synthesized N-alkyl titanium hydroxamate complex, and in the natural product ferrichrome in which an N-alkyl hydroxamic acid chelates iron. 12,13 They could also bind as monodentate species, likely as the monodeprotonated monodentate species mm. Thus, they could bind in a monoanionic form via deprotonation at O (mc, mb, and mm), or dianionic form via deprotonation at O and N (db, dc). Herein, we show evidence for the monodeprotonated binding mode mc to TiO 2 .
We analyzed hydroxamate binding by combining synthesis, IR spectroscopy and density functional theory (DFT) calculations of structure and IR spectra. Synthesis of a series of Ti-hydroxamate complexes from phenylhydroxamic acid precursors allowed for spectral comparisons to functionalized TiO 2 surfaces. This series included m-oxo bridged Ti hydroxamate complexes prepared from NH-and N-methyl-phenylhydroxamic acids (Fig. 3), the latter being incapable of double deprotonation. We assigned the IR spectra and the composition of the functionalized TiO 2 surfaces by using DFT as described in the ESI. †  Treatment of Ti(IV) isopropoxide with phenylhydroxamic acid (NH-PHA) forms the cis-bis-phenylhydroxamate-bis-isopropoxide Ti(IV) complex 1. We obtained the crystal structure of the monomeric titanium complex 2 (Fig. 4) by slow crystallization from 1 in ethylene glycol solvent, where labile isopropoxide ligands were replaced by ethylene glycol, which did not chelate in the system, but instead bound to Ti and formed a hydrogen bond with the NH group.
In moist DMSO solvent, the monomeric Ti complex 1 reacted with adventitious water to form the m-oxo bridged trimer 3 (Fig. 3), having the crystal structure shown in Fig. 4. For comparison, Nmethylphenylhydroxamic acid (NMe-PHA) 14 gave the N-methyl analogue, which crystallized as the bis-m-oxo dimer 4. These m-oxo structures are the closest extant small molecule models for hydroxamate binding to TiO 2 and show substantial similarities to the possible coordination sphere around titanium when NH-PHA and NMe-PHA bind to TiO 2 . We have, therefore, used them as the best available models for the surface-bound species.
The similarities between the NH-PHA and NMe-PHA structures follow from the comparison of the experimental and theoretical C-O, C-N, and N-O bond lengths, shown in Table S1 (ESI †). Doubly deprotonated coordination in the NH-PHA complexes would result in lengthened C-O bonds and shortened C-N bonds. Instead, the carbonyl bond lengths were 1.28, 1.26, and 1.28 Å for 2, 3, and 4, respectively, which are consistent with true CQO bonds as found in the prior literature. 15 The C-N bond lengths are also similar for all the complexes, with bond-lengths of 1.31, 1.30, and 1.31 Å in good agreement with known metalorganic coordination compounds containing monodeprotonated hydroxamate ligands (Table S2, ESI †). 15 We calculated the lowest energy structures for the functionalized material modeled as NH-PHA, anchored to a slab of anatase TiO 2 . We also computed the corresponding IR spectra of a {101} facet cluster model, obtained from a TiO 2 slab optimized with hydroxamate bound in various binding geometries. Fig. 5 compares the experimental and calculated spectra of NH-PHA bound to anatase TiO 2 and m-oxo bridged model complex 3 (See Fig. S2 and S5 for complex 2 and ligand spectra, ESI †). A scaling factor of 0.975 was determined by matching the theoretical and experimental model complex spectra. Correlation of vibrational frequencies to specific vibrations is complicated by strong coupling of the skeletal vibrations, as previously discussed. 3,10,16 However, assignments can still be made using the theoretical spectra, especially for important bands involving carbonyl stretches (see Table S4 for a summary, ESI †). Significant contributions from the carbonyl stretches (n CQO ) for NH-PHA on TiO 2 (vide infra) and for model complexes 2 and 3 occur at 1565/1604, 1566/1602,  and 1558/1594 cm À1 , respectively. Similarly, the theoretical peak for n CQO for NH-PHA on TiO 2 and 2 and 3 occur at 1514/1562/ 1598, 1552/1593, and 1556/1594 cm À1 , respectively. These are shifted to lower frequency from the free NH-PHA, where the carbonyl in the solid-state hydrogen-bonded form appears at 1645 cm À1 . The n CQO for monodeprotonated PHA appears at 1605 cm À1 . 17 Signals for the NH vibration are observed for NH-PHA bound to TiO 2 and in the spectra of 2 and 3 at 3193, 3189, and 3180 cm À1 , respectively (see Fig. S2, S3, S7, and S8, ESI †). The IR data for 3 show striking overlap with the IR spectrum of the surface-bound NH-PHA (Fig. 5). Corresponding vibrational bands are within 20 cm À1 of each other and have similar intensity ratios. This observation suggests that NH-PHA binds to TiO 2 in a monodeprotonated form as in the model complexes. Indeed, the presence of an NH stretch conclusively eliminates the dideprotonated forms as possibilities, leaving the only remaining possibilities as mc on a pristine surface or through an oxygen vacancy, as well as mm and mb. The similarity between the spectra of NH-PHA bound to TiO 2 and that of 3 provides good evidence for the chelating mode as a significant percentage of the surface-bound species. Bridging modes would be expected to cause significant changes in bond lengths and angles for the surface-bound species compared to the chelating modes, and thus significantly affect the spectra. In fact, in the theoretical geometries for mb and mc, we see a large difference in the O-N-C bond angle (1331 and 1151, respectively), the N-C-O bond angle (1261 and 1161, respectively), and the binding O-Ti bond length (B2.17 Å and o2.08 Å, respectively) leading to mb as a higher energy surface binding mode. However, free energy calculations for the binding modes show mm, mb, and mc (via an oxygen vacancy) to be the most stable surface anchoring possibilities in that order, with mc being the least stable of those three modes (Table S3, ESI †). No single calculated binding geometry was found to reproduce the surface-bound spectra. Thus, a heterogeneous binding mixture utilizing all three binding modes was used to calculate the theoretical spectrum of NH-PHA on TiO 2 in Fig. 5. Fig. 6 shows the IR analysis of 4 as compared to that of the ligand bound to TiO 2 (see Fig. S6 for ligand spectrum, ESI †). Much like for NH-PHA, the spectra of NMe-PHA bound to TiO 2 and model complex 4 have similarities. The n CQO are 1565/1595 cm À1 , Fig. 5 Comparison of experimental (solid) and calculated (dashed) IR spectra of NH-PHA on TiO 2 (black) and model complex 3 (red). Theoretical calculation of NH-PHA on TiO 2 incorporates an equal mixture of mc via an oxygen vacancy, mb, and mm binding modes, and includes inverted peaks due to loss of surface hydration upon binding. Fig. 6 Comparison of experimental (solid) and calculated (dashed) IR spectra of NMe-PHA on TiO 2 (black) and model complex 4 (red). Theoretical calculation of NMe-PHA on TiO 2 incorporates an equal mixture of mc via an oxygen vacancy, mb, and mm binding modes, and includes inverted peaks due to loss of surface hydration upon binding.
1564/1586 cm À1 , and 1597 cm À1 for NMe-PHA on TiO 2 , in 4, and for the ligand, respectively. The theoretical equivalents are 1424/1428/1584 cm À1 , 1551/1586 cm À1 , and 1646 cm À1 . The values for n CQO are similar to those of the NH-PHA analogues, described above. The trends in the free energy calculations for the binding modes mimic those of the NH-PHA analogues, and a similar mixture of mm, mb, and mc (via an oxygen vacancy) was used to model NMe-PHA bound to TiO 2 .
Overall, there are more significant differences between the spectra of 4 with its surface-bound form than with the NH-PHA analogues, likely due to differences in the octahedral geometries of 3 and 4. TiO 2 anatase has a Ti-O-Ti bond angle 18 of 156.21 compared to 140.81 for 3 and only 96.51 for 4, which is closer to octahedral. Thus, complex 3 could more closely match the chelating geometry of the NH-PHA ligand bound to the surface of anatase TiO 2 than the NMe-PHA analogues due to a greater degree of electronic similarity. UV-visible spectroscopy of the model complexes in solution and surface-bound to TiO 2 was performed ( Fig. S9-S11, ESI †), but these data do not provide additional information on the binding geometry due to overlap of the UV absorption from TiO 2 with all the absorption bands of the model compounds.
The similarity of NH-PHA to both 4 and NMe-PHA on TiO 2 provides substantial evidence for the presence of the mc binding mode, as also found for the binding of N-alkylhydroxamates to metal ions in siderophores. 12 Our calculations suggest other binding modes are more energetically favorable, but for the calculations it must be assumed that the TiO 2 cannot undergo reconstruction to form a lower energy surface. As an experimental test, we compared the IR spectra of NH-PHA and NMe-PHA bound to commercially available nanoparticulate anatase and rutile TiO 2 (see Fig. S20 and S21, ESI †). The results show that all observable peaks are in alignment for each ligand on each crystal form, suggesting that the coordination of the ligand to the surface could cause significant bond rearrangement, ending in a lower energy surface that is similar between the two crystal phases of the starting materials. This hypothesis is additionally supported by the IR spectroscopic features of model complex 4, which should be more electronically similar to rutile than anatase, yet are dissimilar to both.
Our study provides new evidence on the binding of hydroxamates to TiO 2 . The crystal structure data, supported by IR analysis and DFT calculations, show that the mc binding mode is present in the model complexes. For the binding of hydroxamates to TiO 2 , we propose that there is likely a mixture of binding geometries on the surface, possibly due to surface reconstruction upon ligand binding. Our IR data suggest that mc is the main binding mode on the TiO 2 surface. The 5-membered chelate formed between the hydroxamate and Ti is a stable structural motif, providing a rationale for the tight binding and excellent hydrolytic stability of hydroxamates adsorbed to TiO 2 surfaces. This material is based upon work supported as part of the