- Email: [email protected]
Solid-state structure and solution behavior of two titanium oxo-alkoxide complexes with phenylphosphonate ligands
Journal Pre-proofs Solid-state structure and solution behavior of two titanium oxo-alkoxide complexes with phenylphosphonate ligands Fredric G. Svensson, Vadim G. Kessler PII: DOI: Reference:
S0277-5387(19)30721-1 https://doi.org/10.1016/j.poly.2019.114276 POLY 114276
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
16 November 2019 2 December 2019 3 December 2019
Please cite this article as: F.G. Svensson, V.G. Kessler, Solid-state structure and solution behavior of two titanium oxo-alkoxide complexes with phenylphosphonate ligands, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly. 2019.114276
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier Ltd. All rights reserved.
Solid-state structure and solution behavior of two titanium oxoalkoxide complexes with phenylphosphonate ligands Fredric G. Svensson* and Vadim G. Kessler Department of Molecular Sciences, Swedish University of Agricultural Sciences, Box 7015, 750 07 Uppsala, Sweden *Corresponding author. [email protected]
Abstract A novel titanium phenylphosphonate alkoxide complex, [Ti6(µ3-O)2(µ-OEt)6(µ2-OEt)6(µ3PPA)4]•0.5C7H8 (1) was synthesized by solvothermal processing and its structure was determined by single crystal X-ray diffraction. The solution stability was characterized in solution by 1H NMR, 13C NMR and 31P NMR and (1H) DOSY NMR and compared with a previously reported titanium phenylphosphonate alkoxide complex [(Ti4O(OEt)12PPA] of similar size. According to the NMR analyses, the solid-state structure is predominantly retained in solution for both 1 and 2. 31P NMR and 1H DOSY NMR of the mother liquor from the synthesis of 2 indicated presence of traces of at least two other complexes which could not be isolated in pure form.
Introduction Titanium coordination compounds are of interest for both materials science as precursor to organic-inorganic hybrid materials [1, 2] and to direct formation of oxide phases with complex structures on hydrolysis [3] as well as catalysts [4, 5]. The solid-state structure as determined by single crystal X-ray crystallography may not be retained in solution due to solvent interactions [6, 7]. Thus, when the structure of a metal oxo-complex is essential, it is important to characterize its stability in solution. Several techniques for this exists, for example small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) spectrometry [8]. While SAXS enables determination of, for example, size and shape of particles in a sample, as well as fingerprinting, it provides less structural chemical information and has size limitations [9]. NMR spectrometry enables to elucidate the chemical connectivity of a species although it can be difficult to distinguish a mixture of complexes in a 1D spectrum. Diffusion ordered NMR spectrometry (DOSY) can be used to construct a 2D NMR spectrum with chemical shift on the horizontal axis and diffusion constants on the vertical axis. Different compounds will move at different rates in solution depending on their size (and solvent viscosity) and hence have different diffusion constants. Overlap of chemical shifts may then be resolved by different diffusion constants on the vertical axis, enabling separation of multiple species in a complex mixture [8, 10]. Herein, we synthesized a novel titanium oxo-alkoxide complex modified with phenylphosphonic acid (PPA) ligands, [Ti6O2(OEt)12(PPA)4], 1, by reacting titanium(IV) ethoxide with two equivalents of PPA. When decreasing the PPA amount to a half equivalent, another complex [(Ti4O(OEt)12PPA], 2, previously reported by Coppens’ group [11] was obtained. Compounds 1 and 2 were used as models for characterization of titanium alkoxide
complexes by different NMR techniques in solution. It was found that both 1 and 2 predominantly retained their structures upon dissolution of crystals in chloroform. In addition, the mother solution of 2 was found to contain additional species which could not be isolated by crystallization.
Results and Discussion Compound 1 was obtained from hydrothermal processing titanium(IV) ethoxide with two equivalents of phenylphosphonic acid in a mixture of anhydrous toluene: ethanol (for detailed synthetic description, please see Materials and Methods). An analogue of 1 with isopropoxide ligands instead of ethoxide ligands was reported recently [12]. Detailed crystallographic data is provided in Supplementary Table T1. Compound 1 is an example of a structure formed by self-assembly with manifestation of Goldschmidt’s principles of dense packing of cations and anions [13]. The molecule of 1 possesses a barrel-type structure with a phosphonate body and two capping Ti3O-units, resembling the principal building block in the structure of the anatase phase. This is relatively unusual for the phosphonate cores, which commonly display a Ti4O core as found in 2. The Ti3O-units have the trimolybdate arrangement and 1 is structurally more related to larger polyoxometalates than to the tetrameric structure of titanium(IV) ethoxide in solution. Compound 1 crystallized in the monoclinic space group P21/n. Its two Ti3O subunits are linked by four triply bridging phenylphosphonic acid ligands. All titanium atoms have an octahedral coordination environment. Ti1, Ti2 and Ti3 polyhedrons all share edges (via O1, O4, O6, O8, and O22) and are further connected via a µ3-oxygen (O1). Ti1, Ti2 and Ti3 are also connected via a µ3-oxygen (O2) and share edges via O2, O17, O10, O8 and O9. The average bond length in the Ti-P(O) bonds are 1.964 Å (1.936 Å – 1.982 Å). There are one terminal ethoxide ligand for each titanium atom with an average Ti-O bond length of 1.772 Å (1.757 Å – 1.784 Å). Every titanium atom also have two bridging ethoxide ligands with average bond distance 2.009 Å, where the bond distances between two titanium atoms tend to be slightly asymmetric. The phenyl rings of the opposite P2 and P5 are turned in the same direction while the phenyl rings at the opposite P1 and P3 are oppositely turned. The elemental ratio of Ti:P was determined on a dried crystal of 1 by energy dispersive X-ray spectroscopy (EDS, Supplementary Figure S7) and was found to be higher than expected from the structure. Even modified titanium alkoxide complexes are not stable towards hydrolysis [3, 14, 15] and they will eventually react even with atmospheric water and form titania nanoparticles. The structure of 1 contains solvating toluene molecules which stabilize the crystals by increasing their hydrophobicity. The crystals were dried under a desktop lamp prior to SEM-EDS analysis which may have caused partial evaporation of solvating toluene, thus decreasing the stability and facilitating hydrolysis of 1 by atmospheric water. As compound 1 has a much more open structure than 2, more rearrangement of atoms would be needed during formation of nanoparticles than for 1, forcing the phosphonate ligands to the nanoparticle surfaces as the Ti3O units condense. When using the same protocol as for 1, but changing the Ti:PPA ratio to 2:1, another complex, [Ti4O(OEt)12PPA] previously reported by Coppens group [11], was obtained. Compound 2 consists of a Ti4O-core, with one single tridentatly coordinating PPA ligand. It was more sensitive than 1 and degraded quickly at ambient conditions if not protected by paraffin oil. Their protocol was somewhat different, suggesting 2 is a favored product under different
conditions. Hence, it was interesting to investigate its solution stability by different NMR methods, which was not performed in the original report [11]. EDS analysis of a crystal of 2 (Supplementary Figure S8) showed a Ti:P ratio well in agreement with the observed structure.
Figure 1. (a) Molecular structure of compound 1 and connectivity of its metal oxo-core (c, d). Molecular structure of 2 (b) and the connectivity of its metal oxo-core (e). Green it titanium, red is oxygen, magenta is phosphorous and grey is carbon. Hydrogen atoms and the solvating toluene molecule in 1 have been omitted for clarity.
NMR characterization For compound 1, 1H, 13C (Figure S4), 31P and (1H) DOSY NMR spectra were recorded; for compound 2, 1H, 31P and (1H) DOSY NMR spectra were recorded and for the mother solution of 2 (2M) 1H, 31P and (1H) DOSY NMR spectra were recorded. Anhydrous deuterated chloroform was used as solvent for all measurements and spectra acquisition were performed at 25oC. Results from the proton and carbon NMR are listed in under synthesis of compounds
(and spectra S4 to S6 in Supplementary Information) and the phosphorus and DOSY NMR will be discussed separately below.
Phosphorous NMR 31P
NMR spectra were recorded for compound 1, 2 and the mother solution of 2 (2M). Phosphorous NMR has previously been used to characterize titanium alkoxide phosphonates in solution and their stability apparently varies [16, 17]. Hayami and co-workers [17] recorded 31P NMR spectra of the complex [Ti O(OiPr) (PPA) ] over the course of ten days and could 4 8 3 observe an initial stability followed by degradation of the complex over time. This means that the life time of the complex in solution might have to be taken into account, depending on the application. An upshift in signals from 1 and 2 was observed compared to that of tertbutylphosphonic acid complexes [15] which is due to the electron-donating phenyl ring. The 31P NMR spectrum of 1 shows one dominating signal (7.83 ppm) assigned to the four chemically equivalent phosphorous atoms and one minor at 10.14 ppm (~2.8 area%). The phosphorous spectrum of compound 2 also shows one dominating signal, located at 14.32 ppm and is accompanied by two smaller signals at 14.85 ppm and 15.70 ppm (together comprising roughly 5 area%). A phosphorous NMR spectrum of 2M was recorded (Supplementary Figure S3). One major signal at 14.54 ppm (about 50 % of the total integrated area) was observed as well as about ten additional signals indicating the presence of more compounds than 2 in solution. One weak signal found at 14.25 ppm is likely from 2 with a small change in chemical shift caused by the addition of ethanol and toluene to the chloroform solvent. This suggests that compounds 1 and 2 predominantly retain their solid-state structures in solution when using a non-coordinating solvent as chloroform. The phosphorous spectrum of the reaction mixture indicates about a dozen chemically different phosphorous atoms. The presence of several other phosphorus resonances may indicate that the structure of 2 does not represent a deep energy minimum but complexes of similar stability form, possibly existing in equilibrium with 2 in the reaction mixture. In an attempt to elucidate the number of chemical species diffusion ordered spectroscopy (DOSY) was employed.
Figure 2. 31P NMR spectra of (a) compound 1 and (b) compound 2. Both spectra were recorded in anhydrous CDCl3 at 298 K. Full spectra are shown in Supplementary Figures S1 and S2.
1H
DOSY NMR
DOSY NMR spectra were recorded for 1, 2 and 2M (Figure 3a, b and c, respectively). In agreement with the phosphorous NMR, one major species is dominating each spectra for compound 1 and 2. For the mother solution of 2 (i.e. sample 2M), where a multitude of phosphorous signals were detected, two major compounds could be seen by DOSY. The obtained diffusion constants are listed in Table 1. The diffusion constant of 1 was determined to 2.32•10-9 m2/s, compared to 1.79•10-9 m2/s for compound 2. The diffusion constant is in part dependent on molecular size, the smaller molecule the larger diffusion constant. Here, however, compound 1 is slightly larger than 2. Also solvent interactions affect the diffusion constant, better solvent-solute interactions lead to higher values. The more, and evenly spaced, phenyl rings in 1 compared to 2 may contribute to better solvent interaction, increasing the diffusion constant of 1 compared to 2. The 1H DOSY spectrum of compound 1 indicates six major signals that belong to the dominant species. Minor additional signals can be ascribed to chloroform and toluene. The triplet at 0.83 ppm is assigned to the terminal CH3 of the ethoxide ligands and the quartet at 4.07 as CH2 in the terminal ethoxide ligands. Overlapping signals centered at 4.49 ppm and 1.33 ppm are assigned to CH2 and CH3 hydrogens of bridging ethoxide ligands, respectively. The appearance of double sets of signals are caused by asymmetry in the bridging ethoxide ligands. The multiplet centered at 7.35 ppm is assigned to the phenyl group. The 1H DOSY spectrum of compound 2 displays a similar pattern, although the splitting by the terminal and bridging CH2 and CH3 groups are much smaller, resulting in overlap of signals between the bridging and terminal ligands. The structural stability in solution of 1 and 2 proposed by 31P NMR is supported by the 1H DOSY measurements. Based on the spectra for 1 and 2, the two major species in 2M are presumably titanium (oxo-) alkoxy compounds.
Figure 3. DOSY NMR spectra of (a) compound 1, (b) compound 2 and (c) the mother solution (2M) of 2. Spectra a and b contain one major species each, i.e. compounds 1 and 2, respectively, while two major, unidentified, species dominate spectrum c (2M).
Table 1. Obtained diffusion constants for 1, 2 and 2M. Compound 1
Diffusion constant (D) 2.32•10-9 m2/s
2
1.79•10-9 m2/s
2M_a
5.62•10-9 m2/s
2M_b
7.50•10-9 m2/s
Conclusions In this work one novel titanium oxo-alkoxide phenylphosphonate has been synthesized and its structure was determined by single crystal X-ray diffraction. Its solution stability was investigated by employing different NMR techniques. Phosphorous NMR clearly suggest one dominating species upon dissolution in chloroform, and 1H DOSY NMR indicate preservation of in solution of both compound 1 and 2. The remnants in the mother solution of 2 was found to contain a large set of phosphorous signals. By employing DOSY NMR, two major species could be distinguished. In conclusion, phosphorous NMR is a good technique to characterize the solution stability of phosphorous containing titanium complexes in solution. 1H DOSY NMR is a useful compliment to phosphorous NMR to separate different species and facilitate structural identification of compounds in a complex mixture.
Materials and Methods Chemicals Titanium(IV) ethoxide (Aldrich, technical grade), phenylphosphonic acid (Aldrich, 98%) were used as received. Toluene (Merck, 99%) and ethanol (99.7%, Solveco), were refluxed and distilled over lithium aluminum hydride and calcium metal, respectively, prior to use. All work with titanium(IV) ethoxide was performed under nitrogen atmosphere. Syntheses of compounds
Compound 1, [Ti6(µ3-O)2(µ-OEt)6(µ2-OEt)6(µ3-PPA)4]•C7H8. 1.0 mmol of PPA was dissolved in a mixture of 0.2 mL toluene and 0.8 mL ethanol in a Teflon lined steel autoclave. Then, 0.50 mmol of titanium(IV) ethoxide was added. The RM was heated to 85oC (±2oC) at 4oC min-1 and held for 48 hours and then slowly cooled to room temperature. A few transparent crystals of several millimeters in size suitable for single crystal X-ray diffraction were obtained. The mother solution had turned rather turbid and white precipitates was observed alongside the crystals, hence the mother solution was not analyzed by NMR. Crystals for X-ray data collection was covered in paraffin oil and placed and sealed in a glass capillary. The crystals were washed several times with anhydrous toluene prior to spectrometric analyses. 31P NMR δ ppm (interpretation): 7.83. 13C NMR δ ppm: 127.44, 127.54, 129.35 131.64, 131.70 (CHarom); 135.79, (Carom-P); 18.59, 18.37, 18.04 (CH3). 1H NMR δ ppm: 7.99 (doublet of doublets); 7.35 (multiplet, Ar-H, phenyl ring) 4.49, (area of overlapping signals, CH2, bridging ethoxide); 4.07 (quartet, CH2, J=6.94 Hz, terminal ethoxide); 2.34 (singlet, Ar-CH3, toluene); 1.33 (area of overlapping signals, CH2, bridging ethoxide); 0.82 (triplet, J= 6.95 Hz, CH3, terminal ethoxide). Molar Ti:P ratio was according to EDS 0.86, observed was 1.5. Compound 2 [(Ti4O(OEt)12PPA]. 0.25 mmol of PPA was dissolved in 0.2 mL toluene and 0.8 mL ethanol in Teflon lined steel autoclave. Under nitrogen atmosphere 0.50 mmol of titanium(IV) ethoxide was added. The reaction mixture was heated to 85oC (±2oC) at 4oC min-1 and held for 48 hours, then slowly cooled to room temperature. When at room temperature the RM was transferred under nitrogen atmosphere to a glass vessel and stored at -18oC for crystallization. A few, millimeter sized, transparent crystals suitable for single crystal X-diffraction were obtained overnight (clear, transparent mother solution). Crystals for X-ray data collection was covered in paraffin oil and placed and sealed in a glass capillary (Orthorhombic, Pna2(1), a= 21.952(13) Å, b= 12.207(7) Å, c= 16.690(8) Å, α=β=γ= 90.000o, R1=0.073, wR2=0.2159). The crystals were washed several times with anhydrous toluene prior to spectrometric analyses. 31P NMR δ ppm: 14.32 (major) 14.86 (minor). 1H NMR δ ppm: 7.81 (doublet of doublets); 7.35 (overlapping signals, Ar-H, phenyl ring); overlapping signals between 4.69 to 4.30 ppm (CH2, ethoxide); overlapping signals between 1.34 to 1.18 ppm (CH3, ethoxide). Molar Ti:P ratio was according to EDS 4.5, observed was 4.
Analyses X-ray data collection was performed in a crystal covered in paraffin oil protected in a glass capillary at room temperature. Single-crystal X-ray diffraction data were recorded with a Bruker D8 SMART APEX II CCD diffractometer (graphite monochromator) using λ(Mo-Kα) = 0.71073 Å radiation. The structure was solved by direct methods. Metal atom coordinates were obtained from the initial solutions and other non-hydrogen atoms by Fourier synthesis. Details of data collection and refinement is summarized in Supplementary Table S1. For elemental analyses a Hitachi TM-1000 scanning electron microscope, equipped with an Oxford Instruments EDS system, were used. A piece of a crystal was placed on carbon tape and was analyzed at different positions with EDS (spot analysis) and an average elemental composition was calculated. For NMR analysis a Bruker Avance III 600 MHz Smartprobe with Topspin version 3.5 was used. The spectra were processed and analyzed using Topspin version 3.6. All spectra were recorded at 298 K. The 1H and 13C spectra were calibrated against the chloroform solvent and for the 31P spectrum the calibration was made against an external solution of H3PO4.
Appendix A. Supplementary data CCDC 1966124 contains the supplementary crystallographic data for compound 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Additional NMR spectra and EDS spectra are presented in the Supplementary Information file.
Acknowledgements The support from the Swedish Research Council (Vetenskapsrådet) (grant 2014-3938) is gratefully acknowledged. We thank Dr. Peter Agback and Prof. Corine Sandström for assistance with the DOSY NMR.
References [1] L. Rozes, C. Sanchez. Chemical Society Reviews 40 (2011) 1006–1030. [2] M. Carraro, S. Gross. Materials 7 (2014) 3956-3989. [3] GA. Seisenbaeva, MP. Moloney, R. Tekoriute, A. Hardy-Dessources, JM. Nedelec, YK. Gun´ko, VG. Kessler. Langmuir 26 (2010) 9809–9817. [4] J. Bielefeld, E. Kurochkina, M. Schmidtmann, S. Doye. European Journal of Inorganic Chemistry (2019) 3713–3718. [5] P. Ghana, FD. van Krüchten, TP. Spaniol, J. van Leusen, P. Kögerler, J. Okuda. Chemical Communications 55 (2019) 3231. [6] P. Werndrup, VG. Kessler. Journal of the Chemical Society, Dalton Transactions (2001) 574–579. [7] T. Pawlak, D. Niedzielska, J. Vícha, R. Marek, L. Pazderski. Journal of Organometallic Chemistry 759 (2014) 58-66. [8] MN. Jackson Jr, MK. Kamunde-Devonish, BA. Hammann, LA. Wills, LB. Fullmer, SE. Hayes, PYH. Cheong, WH, Casey, M. Nyman, DW. Johnson. Dalton Transactions 44 (2015) 16982. [9] M. Nyman. Coordination Chemistry Reviews 352 (2017) 461–472. [10] H. Barjat, GA. Morris, S. Smart, AG. Swanson, SCR. Williams. Journal of Magnetic Resonance Series B 108 (1995) 170-172. [11] Y. Chen, E. Trzop, JD. Sokolow, P. Coppens. Chemistry – A European Journal 19 (2013) 16651 – 16655. [12] L Jin-Xiu, G. Mei-Yan, F. Wei-Hui. L. Zhang, J. Zhang. Angewandte Chemie International Edition 55 (2016) 5160-5165. [13] VG. Kessler. Chemical Communications (2003) 1213-1222. [14] FG. Svensson, GA. Seisenbaeva, VG. Kessler. European Journal of Inorganic Chemistry 2017, 4117-4122. [15] FG. Svensson, G. Daniel, CW. Tai, GA. Seisenbaeva, VG. Kessler. In revision. [16] RJ. Errington, J. Ridland, KJ. Willet, W. Clegg, RA. Coxall, SL. Heath. Journal of Organometallic Chemistry 550 (1998) 473-476. [17] R. Hayami, T. Sagawa, S. Tsukada, K. Yamamoto, T. Gunji. Polyhedron 147 (2018) 1–8.
Graphical Abstract
The formation of two structurally different complexes, 1 and 2, depends on the ratio between titanium(IV) ethoxide and phenylphosphonic acid ligand, using otherwise the same reaction conditions. The NMR spectra illustrate how DOSY NMR can be used to separate major species from a complex 1D 31P NMR spectrum.
Author Contribution Statement FGS: investigation, writing – original draft. VGK: writing – review & editing, supervision.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0277-5387(19)30721-1 https://doi.org/10.1016/j.poly.2019.114276 POLY 114276
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
16 November 2019 2 December 2019 3 December 2019
Please cite this article as: F.G. Svensson, V.G. Kessler, Solid-state structure and solution behavior of two titanium oxo-alkoxide complexes with phenylphosphonate ligands, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly. 2019.114276
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier Ltd. All rights reserved.
Solid-state structure and solution behavior of two titanium oxoalkoxide complexes with phenylphosphonate ligands Fredric G. Svensson* and Vadim G. Kessler Department of Molecular Sciences, Swedish University of Agricultural Sciences, Box 7015, 750 07 Uppsala, Sweden *Corresponding author. [email protected]
Abstract A novel titanium phenylphosphonate alkoxide complex, [Ti6(µ3-O)2(µ-OEt)6(µ2-OEt)6(µ3PPA)4]•0.5C7H8 (1) was synthesized by solvothermal processing and its structure was determined by single crystal X-ray diffraction. The solution stability was characterized in solution by 1H NMR, 13C NMR and 31P NMR and (1H) DOSY NMR and compared with a previously reported titanium phenylphosphonate alkoxide complex [(Ti4O(OEt)12PPA] of similar size. According to the NMR analyses, the solid-state structure is predominantly retained in solution for both 1 and 2. 31P NMR and 1H DOSY NMR of the mother liquor from the synthesis of 2 indicated presence of traces of at least two other complexes which could not be isolated in pure form.
Introduction Titanium coordination compounds are of interest for both materials science as precursor to organic-inorganic hybrid materials [1, 2] and to direct formation of oxide phases with complex structures on hydrolysis [3] as well as catalysts [4, 5]. The solid-state structure as determined by single crystal X-ray crystallography may not be retained in solution due to solvent interactions [6, 7]. Thus, when the structure of a metal oxo-complex is essential, it is important to characterize its stability in solution. Several techniques for this exists, for example small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) spectrometry [8]. While SAXS enables determination of, for example, size and shape of particles in a sample, as well as fingerprinting, it provides less structural chemical information and has size limitations [9]. NMR spectrometry enables to elucidate the chemical connectivity of a species although it can be difficult to distinguish a mixture of complexes in a 1D spectrum. Diffusion ordered NMR spectrometry (DOSY) can be used to construct a 2D NMR spectrum with chemical shift on the horizontal axis and diffusion constants on the vertical axis. Different compounds will move at different rates in solution depending on their size (and solvent viscosity) and hence have different diffusion constants. Overlap of chemical shifts may then be resolved by different diffusion constants on the vertical axis, enabling separation of multiple species in a complex mixture [8, 10]. Herein, we synthesized a novel titanium oxo-alkoxide complex modified with phenylphosphonic acid (PPA) ligands, [Ti6O2(OEt)12(PPA)4], 1, by reacting titanium(IV) ethoxide with two equivalents of PPA. When decreasing the PPA amount to a half equivalent, another complex [(Ti4O(OEt)12PPA], 2, previously reported by Coppens’ group [11] was obtained. Compounds 1 and 2 were used as models for characterization of titanium alkoxide
complexes by different NMR techniques in solution. It was found that both 1 and 2 predominantly retained their structures upon dissolution of crystals in chloroform. In addition, the mother solution of 2 was found to contain additional species which could not be isolated by crystallization.
Results and Discussion Compound 1 was obtained from hydrothermal processing titanium(IV) ethoxide with two equivalents of phenylphosphonic acid in a mixture of anhydrous toluene: ethanol (for detailed synthetic description, please see Materials and Methods). An analogue of 1 with isopropoxide ligands instead of ethoxide ligands was reported recently [12]. Detailed crystallographic data is provided in Supplementary Table T1. Compound 1 is an example of a structure formed by self-assembly with manifestation of Goldschmidt’s principles of dense packing of cations and anions [13]. The molecule of 1 possesses a barrel-type structure with a phosphonate body and two capping Ti3O-units, resembling the principal building block in the structure of the anatase phase. This is relatively unusual for the phosphonate cores, which commonly display a Ti4O core as found in 2. The Ti3O-units have the trimolybdate arrangement and 1 is structurally more related to larger polyoxometalates than to the tetrameric structure of titanium(IV) ethoxide in solution. Compound 1 crystallized in the monoclinic space group P21/n. Its two Ti3O subunits are linked by four triply bridging phenylphosphonic acid ligands. All titanium atoms have an octahedral coordination environment. Ti1, Ti2 and Ti3 polyhedrons all share edges (via O1, O4, O6, O8, and O22) and are further connected via a µ3-oxygen (O1). Ti1, Ti2 and Ti3 are also connected via a µ3-oxygen (O2) and share edges via O2, O17, O10, O8 and O9. The average bond length in the Ti-P(O) bonds are 1.964 Å (1.936 Å – 1.982 Å). There are one terminal ethoxide ligand for each titanium atom with an average Ti-O bond length of 1.772 Å (1.757 Å – 1.784 Å). Every titanium atom also have two bridging ethoxide ligands with average bond distance 2.009 Å, where the bond distances between two titanium atoms tend to be slightly asymmetric. The phenyl rings of the opposite P2 and P5 are turned in the same direction while the phenyl rings at the opposite P1 and P3 are oppositely turned. The elemental ratio of Ti:P was determined on a dried crystal of 1 by energy dispersive X-ray spectroscopy (EDS, Supplementary Figure S7) and was found to be higher than expected from the structure. Even modified titanium alkoxide complexes are not stable towards hydrolysis [3, 14, 15] and they will eventually react even with atmospheric water and form titania nanoparticles. The structure of 1 contains solvating toluene molecules which stabilize the crystals by increasing their hydrophobicity. The crystals were dried under a desktop lamp prior to SEM-EDS analysis which may have caused partial evaporation of solvating toluene, thus decreasing the stability and facilitating hydrolysis of 1 by atmospheric water. As compound 1 has a much more open structure than 2, more rearrangement of atoms would be needed during formation of nanoparticles than for 1, forcing the phosphonate ligands to the nanoparticle surfaces as the Ti3O units condense. When using the same protocol as for 1, but changing the Ti:PPA ratio to 2:1, another complex, [Ti4O(OEt)12PPA] previously reported by Coppens group [11], was obtained. Compound 2 consists of a Ti4O-core, with one single tridentatly coordinating PPA ligand. It was more sensitive than 1 and degraded quickly at ambient conditions if not protected by paraffin oil. Their protocol was somewhat different, suggesting 2 is a favored product under different
conditions. Hence, it was interesting to investigate its solution stability by different NMR methods, which was not performed in the original report [11]. EDS analysis of a crystal of 2 (Supplementary Figure S8) showed a Ti:P ratio well in agreement with the observed structure.
Figure 1. (a) Molecular structure of compound 1 and connectivity of its metal oxo-core (c, d). Molecular structure of 2 (b) and the connectivity of its metal oxo-core (e). Green it titanium, red is oxygen, magenta is phosphorous and grey is carbon. Hydrogen atoms and the solvating toluene molecule in 1 have been omitted for clarity.
NMR characterization For compound 1, 1H, 13C (Figure S4), 31P and (1H) DOSY NMR spectra were recorded; for compound 2, 1H, 31P and (1H) DOSY NMR spectra were recorded and for the mother solution of 2 (2M) 1H, 31P and (1H) DOSY NMR spectra were recorded. Anhydrous deuterated chloroform was used as solvent for all measurements and spectra acquisition were performed at 25oC. Results from the proton and carbon NMR are listed in under synthesis of compounds
(and spectra S4 to S6 in Supplementary Information) and the phosphorus and DOSY NMR will be discussed separately below.
Phosphorous NMR 31P
NMR spectra were recorded for compound 1, 2 and the mother solution of 2 (2M). Phosphorous NMR has previously been used to characterize titanium alkoxide phosphonates in solution and their stability apparently varies [16, 17]. Hayami and co-workers [17] recorded 31P NMR spectra of the complex [Ti O(OiPr) (PPA) ] over the course of ten days and could 4 8 3 observe an initial stability followed by degradation of the complex over time. This means that the life time of the complex in solution might have to be taken into account, depending on the application. An upshift in signals from 1 and 2 was observed compared to that of tertbutylphosphonic acid complexes [15] which is due to the electron-donating phenyl ring. The 31P NMR spectrum of 1 shows one dominating signal (7.83 ppm) assigned to the four chemically equivalent phosphorous atoms and one minor at 10.14 ppm (~2.8 area%). The phosphorous spectrum of compound 2 also shows one dominating signal, located at 14.32 ppm and is accompanied by two smaller signals at 14.85 ppm and 15.70 ppm (together comprising roughly 5 area%). A phosphorous NMR spectrum of 2M was recorded (Supplementary Figure S3). One major signal at 14.54 ppm (about 50 % of the total integrated area) was observed as well as about ten additional signals indicating the presence of more compounds than 2 in solution. One weak signal found at 14.25 ppm is likely from 2 with a small change in chemical shift caused by the addition of ethanol and toluene to the chloroform solvent. This suggests that compounds 1 and 2 predominantly retain their solid-state structures in solution when using a non-coordinating solvent as chloroform. The phosphorous spectrum of the reaction mixture indicates about a dozen chemically different phosphorous atoms. The presence of several other phosphorus resonances may indicate that the structure of 2 does not represent a deep energy minimum but complexes of similar stability form, possibly existing in equilibrium with 2 in the reaction mixture. In an attempt to elucidate the number of chemical species diffusion ordered spectroscopy (DOSY) was employed.
Figure 2. 31P NMR spectra of (a) compound 1 and (b) compound 2. Both spectra were recorded in anhydrous CDCl3 at 298 K. Full spectra are shown in Supplementary Figures S1 and S2.
1H
DOSY NMR
DOSY NMR spectra were recorded for 1, 2 and 2M (Figure 3a, b and c, respectively). In agreement with the phosphorous NMR, one major species is dominating each spectra for compound 1 and 2. For the mother solution of 2 (i.e. sample 2M), where a multitude of phosphorous signals were detected, two major compounds could be seen by DOSY. The obtained diffusion constants are listed in Table 1. The diffusion constant of 1 was determined to 2.32•10-9 m2/s, compared to 1.79•10-9 m2/s for compound 2. The diffusion constant is in part dependent on molecular size, the smaller molecule the larger diffusion constant. Here, however, compound 1 is slightly larger than 2. Also solvent interactions affect the diffusion constant, better solvent-solute interactions lead to higher values. The more, and evenly spaced, phenyl rings in 1 compared to 2 may contribute to better solvent interaction, increasing the diffusion constant of 1 compared to 2. The 1H DOSY spectrum of compound 1 indicates six major signals that belong to the dominant species. Minor additional signals can be ascribed to chloroform and toluene. The triplet at 0.83 ppm is assigned to the terminal CH3 of the ethoxide ligands and the quartet at 4.07 as CH2 in the terminal ethoxide ligands. Overlapping signals centered at 4.49 ppm and 1.33 ppm are assigned to CH2 and CH3 hydrogens of bridging ethoxide ligands, respectively. The appearance of double sets of signals are caused by asymmetry in the bridging ethoxide ligands. The multiplet centered at 7.35 ppm is assigned to the phenyl group. The 1H DOSY spectrum of compound 2 displays a similar pattern, although the splitting by the terminal and bridging CH2 and CH3 groups are much smaller, resulting in overlap of signals between the bridging and terminal ligands. The structural stability in solution of 1 and 2 proposed by 31P NMR is supported by the 1H DOSY measurements. Based on the spectra for 1 and 2, the two major species in 2M are presumably titanium (oxo-) alkoxy compounds.
Figure 3. DOSY NMR spectra of (a) compound 1, (b) compound 2 and (c) the mother solution (2M) of 2. Spectra a and b contain one major species each, i.e. compounds 1 and 2, respectively, while two major, unidentified, species dominate spectrum c (2M).
Table 1. Obtained diffusion constants for 1, 2 and 2M. Compound 1
Diffusion constant (D) 2.32•10-9 m2/s
2
1.79•10-9 m2/s
2M_a
5.62•10-9 m2/s
2M_b
7.50•10-9 m2/s
Conclusions In this work one novel titanium oxo-alkoxide phenylphosphonate has been synthesized and its structure was determined by single crystal X-ray diffraction. Its solution stability was investigated by employing different NMR techniques. Phosphorous NMR clearly suggest one dominating species upon dissolution in chloroform, and 1H DOSY NMR indicate preservation of in solution of both compound 1 and 2. The remnants in the mother solution of 2 was found to contain a large set of phosphorous signals. By employing DOSY NMR, two major species could be distinguished. In conclusion, phosphorous NMR is a good technique to characterize the solution stability of phosphorous containing titanium complexes in solution. 1H DOSY NMR is a useful compliment to phosphorous NMR to separate different species and facilitate structural identification of compounds in a complex mixture.
Materials and Methods Chemicals Titanium(IV) ethoxide (Aldrich, technical grade), phenylphosphonic acid (Aldrich, 98%) were used as received. Toluene (Merck, 99%) and ethanol (99.7%, Solveco), were refluxed and distilled over lithium aluminum hydride and calcium metal, respectively, prior to use. All work with titanium(IV) ethoxide was performed under nitrogen atmosphere. Syntheses of compounds
Compound 1, [Ti6(µ3-O)2(µ-OEt)6(µ2-OEt)6(µ3-PPA)4]•C7H8. 1.0 mmol of PPA was dissolved in a mixture of 0.2 mL toluene and 0.8 mL ethanol in a Teflon lined steel autoclave. Then, 0.50 mmol of titanium(IV) ethoxide was added. The RM was heated to 85oC (±2oC) at 4oC min-1 and held for 48 hours and then slowly cooled to room temperature. A few transparent crystals of several millimeters in size suitable for single crystal X-ray diffraction were obtained. The mother solution had turned rather turbid and white precipitates was observed alongside the crystals, hence the mother solution was not analyzed by NMR. Crystals for X-ray data collection was covered in paraffin oil and placed and sealed in a glass capillary. The crystals were washed several times with anhydrous toluene prior to spectrometric analyses. 31P NMR δ ppm (interpretation): 7.83. 13C NMR δ ppm: 127.44, 127.54, 129.35 131.64, 131.70 (CHarom); 135.79, (Carom-P); 18.59, 18.37, 18.04 (CH3). 1H NMR δ ppm: 7.99 (doublet of doublets); 7.35 (multiplet, Ar-H, phenyl ring) 4.49, (area of overlapping signals, CH2, bridging ethoxide); 4.07 (quartet, CH2, J=6.94 Hz, terminal ethoxide); 2.34 (singlet, Ar-CH3, toluene); 1.33 (area of overlapping signals, CH2, bridging ethoxide); 0.82 (triplet, J= 6.95 Hz, CH3, terminal ethoxide). Molar Ti:P ratio was according to EDS 0.86, observed was 1.5. Compound 2 [(Ti4O(OEt)12PPA]. 0.25 mmol of PPA was dissolved in 0.2 mL toluene and 0.8 mL ethanol in Teflon lined steel autoclave. Under nitrogen atmosphere 0.50 mmol of titanium(IV) ethoxide was added. The reaction mixture was heated to 85oC (±2oC) at 4oC min-1 and held for 48 hours, then slowly cooled to room temperature. When at room temperature the RM was transferred under nitrogen atmosphere to a glass vessel and stored at -18oC for crystallization. A few, millimeter sized, transparent crystals suitable for single crystal X-diffraction were obtained overnight (clear, transparent mother solution). Crystals for X-ray data collection was covered in paraffin oil and placed and sealed in a glass capillary (Orthorhombic, Pna2(1), a= 21.952(13) Å, b= 12.207(7) Å, c= 16.690(8) Å, α=β=γ= 90.000o, R1=0.073, wR2=0.2159). The crystals were washed several times with anhydrous toluene prior to spectrometric analyses. 31P NMR δ ppm: 14.32 (major) 14.86 (minor). 1H NMR δ ppm: 7.81 (doublet of doublets); 7.35 (overlapping signals, Ar-H, phenyl ring); overlapping signals between 4.69 to 4.30 ppm (CH2, ethoxide); overlapping signals between 1.34 to 1.18 ppm (CH3, ethoxide). Molar Ti:P ratio was according to EDS 4.5, observed was 4.
Analyses X-ray data collection was performed in a crystal covered in paraffin oil protected in a glass capillary at room temperature. Single-crystal X-ray diffraction data were recorded with a Bruker D8 SMART APEX II CCD diffractometer (graphite monochromator) using λ(Mo-Kα) = 0.71073 Å radiation. The structure was solved by direct methods. Metal atom coordinates were obtained from the initial solutions and other non-hydrogen atoms by Fourier synthesis. Details of data collection and refinement is summarized in Supplementary Table S1. For elemental analyses a Hitachi TM-1000 scanning electron microscope, equipped with an Oxford Instruments EDS system, were used. A piece of a crystal was placed on carbon tape and was analyzed at different positions with EDS (spot analysis) and an average elemental composition was calculated. For NMR analysis a Bruker Avance III 600 MHz Smartprobe with Topspin version 3.5 was used. The spectra were processed and analyzed using Topspin version 3.6. All spectra were recorded at 298 K. The 1H and 13C spectra were calibrated against the chloroform solvent and for the 31P spectrum the calibration was made against an external solution of H3PO4.
Appendix A. Supplementary data CCDC 1966124 contains the supplementary crystallographic data for compound 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Additional NMR spectra and EDS spectra are presented in the Supplementary Information file.
Acknowledgements The support from the Swedish Research Council (Vetenskapsrådet) (grant 2014-3938) is gratefully acknowledged. We thank Dr. Peter Agback and Prof. Corine Sandström for assistance with the DOSY NMR.
References [1] L. Rozes, C. Sanchez. Chemical Society Reviews 40 (2011) 1006–1030. [2] M. Carraro, S. Gross. Materials 7 (2014) 3956-3989. [3] GA. Seisenbaeva, MP. Moloney, R. Tekoriute, A. Hardy-Dessources, JM. Nedelec, YK. Gun´ko, VG. Kessler. Langmuir 26 (2010) 9809–9817. [4] J. Bielefeld, E. Kurochkina, M. Schmidtmann, S. Doye. European Journal of Inorganic Chemistry (2019) 3713–3718. [5] P. Ghana, FD. van Krüchten, TP. Spaniol, J. van Leusen, P. Kögerler, J. Okuda. Chemical Communications 55 (2019) 3231. [6] P. Werndrup, VG. Kessler. Journal of the Chemical Society, Dalton Transactions (2001) 574–579. [7] T. Pawlak, D. Niedzielska, J. Vícha, R. Marek, L. Pazderski. Journal of Organometallic Chemistry 759 (2014) 58-66. [8] MN. Jackson Jr, MK. Kamunde-Devonish, BA. Hammann, LA. Wills, LB. Fullmer, SE. Hayes, PYH. Cheong, WH, Casey, M. Nyman, DW. Johnson. Dalton Transactions 44 (2015) 16982. [9] M. Nyman. Coordination Chemistry Reviews 352 (2017) 461–472. [10] H. Barjat, GA. Morris, S. Smart, AG. Swanson, SCR. Williams. Journal of Magnetic Resonance Series B 108 (1995) 170-172. [11] Y. Chen, E. Trzop, JD. Sokolow, P. Coppens. Chemistry – A European Journal 19 (2013) 16651 – 16655. [12] L Jin-Xiu, G. Mei-Yan, F. Wei-Hui. L. Zhang, J. Zhang. Angewandte Chemie International Edition 55 (2016) 5160-5165. [13] VG. Kessler. Chemical Communications (2003) 1213-1222. [14] FG. Svensson, GA. Seisenbaeva, VG. Kessler. European Journal of Inorganic Chemistry 2017, 4117-4122. [15] FG. Svensson, G. Daniel, CW. Tai, GA. Seisenbaeva, VG. Kessler. In revision. [16] RJ. Errington, J. Ridland, KJ. Willet, W. Clegg, RA. Coxall, SL. Heath. Journal of Organometallic Chemistry 550 (1998) 473-476. [17] R. Hayami, T. Sagawa, S. Tsukada, K. Yamamoto, T. Gunji. Polyhedron 147 (2018) 1–8.
Graphical Abstract
The formation of two structurally different complexes, 1 and 2, depends on the ratio between titanium(IV) ethoxide and phenylphosphonic acid ligand, using otherwise the same reaction conditions. The NMR spectra illustrate how DOSY NMR can be used to separate major species from a complex 1D 31P NMR spectrum.
Author Contribution Statement FGS: investigation, writing – original draft. VGK: writing – review & editing, supervision.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: