Short communication
Synthesis of Organotitanium(IV) Fluoride Phosphates
and the Crystal Structure of [(C5Me4Et)TiF(^-F){^-O2P(OSiMe3)2}]2
Andrej Pevec,1* Alojz Demšar,1 Jiri Pinkas2 and Marek Necas2
1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia 2 Department of Chemistry and CEITEC MU, Masaryk University, Kotlarska 2, CZ-61137 Brno, Czech Republic * Corresponding author: E-mail: andrej.pevec@fkkt.uni-lj.si
Received: 21-09-2011
Abstract
The complexes [(C5Me4R)TiF(|i-F){|i-O2P(OSiMe3)2}]2 [R = Me (1), Et (2)] were prepared from [(C5Me4R)TiF3]2, (R = Me, Et) and OP(OSiMe3)3. The molecular structure of 2 has been determined by single-crystal X-ray diffraction analysis. An eight-membered Ti2O4P2 metallacycle bridged by two fluorine ligands between two titanium centers is observed.
Keywords: Titanium, fluoride, phosphate, X-ray structure.
1. Introduction
In the recent years, a number of the Group 4 organo-metallic fluorides and organotitanium mixed-metal complexes have been prepared and structurally chracterized.1 This work was made possible by the discovery of Me3SnF as a mild and very effective fluorinating agent in comparison with other fluorinating reagents.2 This tool opened the chemistry of organotitanium fluoride complexes and number of them have been well characterized by X-ray crystallography.
In many of these cases, the driving force in the reactions is the formation of trimethylsilyl fluoride as a byproduct. Thus, the reaction of trimethylsilyl esters of sulfo-nic, phosphonic and carboxylic acids in combination with (C5Me5)TiF3 can lead to the formation of organotitanium sulphonates, phosphinates and carboxylates.34 The phosp-hinate complex which is derived from the reaction (1) is covalent dinuclear complex with the titanium atoms bridged by two fluorine atoms as well as by two phosphinate groups.
2(Me5C5)TiF3 + 2(Me3SiO)P(O)Ph2 ^ [(C5Me5)TiF(n-F)(|>O2PPh2)]2 + 2Me3SiF
(1)
Titanium fluoride complexes were found as very efficient and enantioselective bifunctional asymetric ca-
talysts.5 The high electronegativity of fluorine makes the titanium atom a strongly Lewis acidic centre capable of binding a carbonyl oxygen atom while fluorine interacts with silicon or aluminium of the nucleophilic reagents.6-8 We are interested in the preparation of new organotitanium mixed fluoro-oxo comlexes as new potential catalysts. For this purpose, we explored the reactions of organotitanium fluorides [(Me5C4R)TiF3]2, (R = Me, Et) with trimethylsilyl ester of phosphoric acid. We are not aware of any other report of organotitanium(IV) fluoride containing phosphate ligand.9 The present article discusses the synthesis, characterization and structural properties of the first organotitanium fluoride phosphates
[(C5Me4R)TiF(|>F){|>O2P(OSiMe3)2}]2 [R = Me (1), Et (2)]. 2. Experimental
2. 1. General
All manipulations were performed under a dry nitrogen atmosphere by Schlenk techniques or in a M. Braun Unilab dry box. Solvents were dried over and distilled from Na/benzophenone under nitrogen. Compounds [(C5Me5)TiF3]2, [(C5Me4Et)TiF3]2 were prepared according to the previously published procedures.2 OP(OSi-Me3)3 was purchased from Aldrich. NMR spectra (:H, 19F and 31P) were acquired on Avance DRX 300 MHz spectro-
meter at 30 °C. Chemical shifts were referenced to the residual resonances of solvent (7.26 ppm for XH in CDCl3). IR spectra (4000-400 cm-1) were collected on an EQUINOX 55/S/NIR FTIR spectrometer. The samples were prepared as KBr pellets.
2. 2. Typical Procedure for the Reactions
Neat OP(OSiMe3)3 (0.131 g, 0.417 mmol) was added dropwise by a syringe to the stirred solution of [(C5Me5)TiF3]2 (0.100 g, 0.417 mmol) in dry deoxygena-ted THF (20 mL). The reaction mixture was then stirred for 24 h, and all the volatiles were removed under vacuum.
2. 3. Spectral Data of New Compounds
[(C5Me5)TiF(^-F){^-O2P(OSiMe3)2}]2 (1). The residue was dissolved in hexane (6 mL) and cooled to -24 °C. Orange crystals of 1 were obtained in three days. 1H NMR (300 MHz, CDCl3): S 2.19 and 2.16, 1.10 (three s, 30H, C5Me5), 0.38 (s, 36H, SiCH3) ppm. 19F NMR (282 MHz, CDCl3): S6.50 and 6.20 (two d, JFF = 61 Hz), -19.05 (two overlapping t, JFF = 96 Hz), -35.17 and -35.53 (two t, JFF = 37 Hz), -53.14 (t, JFF = 29 Hz) ppm. 31P NMR (122 MHz, CDCl3): S-17.80 (d), -20.60 (s) , -24.54 (s) ppm. IR (KBr pellet, cm-1): 3436 bs, 2960 s, 2912 s, 1256 s, 1110 vw, 1032 vw, 851 s, 758 s, 656 s, 588 bs, 494 bs.
[(C5Me4Et)TiF(^-F){^-O2P(OSiMe3)2}]2 (2). The solid residue was dissolved in hexane (6 mL). The compound 2 crystallized overnight in the form of orange crystals by a slow evaporation of the solvent at a reduced pressure. 1H NMR (300 MHz, CDCl3): S 2.69 and 2.57 (two q, 4H, CH2CH3), 2.18 and 2.07 (two m, 24H, C5Me4), 0.96 and 0.8(5 (two t, 6H, CH2CH3), 0.34 (m, 36H, SiCH3) ppm. 19F NMR (282 MHz, CDCl3): S7.40 and 7.12 (two d, JFF = 63 Hz), -12.53 (t, JFF = 94 Hz), -35.18 and -35.55 (two t, JFF = 38 Hz), -53.25 (t, JFF = 28 Hz) ppm. 31P NMR (122 MHz, CDCl3): S -16.29 (s), -19.44 (s), -24.55 (s) ppm. IR (KBr pellet, cm-1): 3433 bs, 2963 s, 2912 s, 2872 s, 1254 s, 1133 vw, 1108 vw, 1031 vw, 851 s, 760 s, 655 s, 596 s, 500 s, 415 bs.
2. 4. X-ray Structure Determination
Diffraction data were collected on a KUMA KM-4 K-axis diffractometer with graphite-monochromated Mo Ka radiation (A = 0.71073 A) and a CCD camera at 120 K. The intensity data were corrected for Lorentz and polarization effects. The structures were solved by direct methods using SIR-9210 and refined with a full-matrix least-squares procedure based on F2 using SHELXL-97.11 All of the non-hydrogen atoms were refined anisotropically while the hydrogen atoms were positioned geometrically and refined as riding.
Crystal structure analysis for 2: C34H70F4O8P2Si4Ti2, M = 953.00 g mol-1, triclinic system, space group P-1, a = 10.046(2), b = 11.109(2), c = 12.111(2) A, a = 66.49(2), p = 72.18(2), y = 79.16(1)°, V = 1176.5(3) A3, Z = 1, Dx = 1.345 g cm-3, ^ = 0.568 mm-1, 16304 reflections measured, 5359 were independent of symmetry, of which 3474 were observed [I>2o(I)], R1 = 0.0378, wR2 (all data) = 0.0824, 255 parameters.
CCDC-844363 contains the supplementary crystal-lographic data for 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3. Results and Discussion
The reaction of [(C5Me5)TiF3]2 and [(C5Me4Et)Ti-F3]2 with OP(OSiMe3)3 in a 1:1 molar ratio in THF resulted in the formation of [(^Me^TiFju-FKiu-O^OSi-Me3)2}]2 (1) and [(^Me^OTiF^-FKiu-O^OSiMe^}^ (2), respectively. The structures of 1 and 2 have been determined by a single-crystal X-ray diffraction analysis at a low temperature. Compounds 1 and 2 crystallize in the monoclinic P21/c and triclinic P-1 space group, respectively. The molecular structures of both compounds are similar with the exception of organic ligand but the quality of the crystals of 1 was not sufficient for a refinement of the structure.12 The molecular structure of 2 is shown in Fig. 1 and the selected bond distances and angles are given in Table 1.
Figure 1. ORTEP plot of 2. Ellipsoids are drawn at 50% probability level.
The asymmetric unit of 2 consists of a half of the complex [(^Me^OTiF^-FKi-O^OSiMe^}^ with the center of symmetry in the middle of the molecule. Two titanium atoms are connected by two bridging fluorine
Table 1. Selected bond lengths (Â) and angles (°) for 2.
4. Conclusions
Ti1-F1	1.971(1)	P1-O2	1.501(2)
Ti1-F2	1.828(1)	P1-O3	1.564(2)
Ti1-F1a	2.155(1)	P1-O4	1.559(2)
Ti1-O1	2.038(2)	Si1-O3	1.675(2)
Ti1a-O2	2.047(2)	Si2-O4	1.678(2)
P1-O1	1.504(2)		
F1-Ti1-F2	150.20(6)	O1-P1-O2	116.95(9)
F1-Ti1-F1a	70.41(6)	O1-P1-O3	109.2(1)
Ti1-O1-P1	132.48(9)	O1-P1-O4a	107.73(9)
Ti1-F1-Ti1a	109.59(6)	P1-O3-Si1	134.6(1)
atoms and by two phosphate groups. The coordination geometry around Ti atoms is best described as a distorted octahedron, a geometry which is commonly observed for other TiIV complexes.1 The Ti-F distances for the two bridging F atoms are different (difference 0.18 Á). This is also the case in organotitanium fluoro-phosphinate [(C5Me5)TiF(u-F)(^-O2PPh2)]23 and fluoro-carboxylate complexes [(C5Me5)TiF(u-F)(u-O2CR)]2 (R = CF3, C6F5)4 but not in the fluoro-sulphonate [(C5Me5)TiF(u-F) (u-O2S(O)-p-C6H4CH3)]23 complex with more symmetrical central Ti2F2 rhomboid. The Ti-F distance for the terminal F atoms in 2 (1.828 Á) is also typical for other orga-notitanium fluoride complexes with pentamethylcyclo-pentadienyl ligans.13 These cyclopentadienyl ligands in complexes 1 and 2 adopt a trans mutual arrangement. This is also the arrangement found in dimeric [(C5Me5)TiF3]2 but not in tetrameric [(C5Me5)TiF3]4 complexes.14 The same trans arrangement was also found in the previously reported organotitanium fluoro-phosphinate and -carboxy-late structures but not in the sulfonate complex in which a cis mutual arrangement of pentamethylcyclopentadienyl ligands is more favourable in the crystal structure.3,4 There are no significant intermolecular contacts in the crystal lattice of compound 2.
The 1H, 19F and 31P NMR spectra of 1 and 2 suggest the presence of isomers in the solution. The spectra of 1 have three 1H NMR Cp* resonances, three 31P NMR resonances and four 19F NMR resonances of bridging fluorine atoms.15 Two of the bridging-fluorine resonances (approx. -19 and -35 ppm) could be simulated with the same coupling constants and belong presumably to the same isomer with two nonequivalent bridging fluorines. The other two bridging-fluorine resonances (approx. 6.5 and -53 ppm) originate from additional two isomers, each with two equivalent bridging fluorines, resembling the structure found in the solid state of 2. The presence of three isomers in the solution of 1 is also in accordance with the 1H and 31P NMR spectra. The 1H, 19F and 31P NMR spectra of 2 are similar to the spectra of 1. The isomers were previously observed in solution of sulfonates, phosphinates and carboxylates of organotitanium fluorides3,4 and in solutions of organotitanium fluorides.14
Molecular and macromolecular titanium phosphates have found variety of applications, including as catalysts (see ref. 16 and citations therein). Since the fluoride li-gand bonded to titanium increases its Lewis acidity,5 the reported compounds could be seen as a model for Lewis-acid titanium fluoride phosphates catalysts with increased activity.
5. Acknowledgement
This work was supported by the MSM0021622410, CZ.1.05/1.1.00/02.0068, and KONTAKT MEB-091018. A. P. thanks MU for a postdoctoral fellowship.
6. References
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3.	S. A. A. Shah, H. Dorn, J. Gindl, M. Noltemeyer, H.-G. Schmidt, H. W. Roesky, J. Organomet. Chem. 1998, 550, 1-6.
4.	J. Gindl, F.-Q. Liu, M. Noltemeyer, H.-G. Schmidt, H. W. Roesky, Inorg. Chem. 1995, 34, 5711-5714.
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9.	Cambridge Structural Database (version 5.32, update August 2011). Search for titanium complexes with C-Ti-O-P connections.
10.	A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi, J. Appl. Cryst. 1994, 27, 435-435.
11.	G. M. Sheldrick, Acta Cryst. Sect. A 2008, 64, 112-122.
12.	Crystal data for 1: C32H66F4O8P2Si4Ti2, M = 924.95, space group monoclinic P21/c, a = 15.012(3)À , b = 10.781(2)À , c = 16.381(3)À , P= 99.93(3)°, V = 2611.5(9) À3, Z = 2.
13.	F. Perdih, A. Demšar, A. Pevec, S. Petricek, I. Leban, G. Gie-ster, J. Sieler, H. W. Roesky, Polyhedron 2001, 20, 1967- 1971.
14.	F. Perdih, A. Pevec, S. Petricek, A. Petric, N. Lah, K. Kogej, A. Demšar, Inorg. Chem. 2006, 45, 7915-7921.
15.	We considered only the bridging fluorine resonances in the 19F NMR spectra. The frequency range of the recorded spectra was too narrow for the observation of terminal fluorine resonances. Those resonances usually occur above 150 ppm, see ref. 14.
16.	R. Murugavel, S. Kuppuswamy, Inorg. Chem. 2008, 47, 7686-7694.
Povzetek
Izolirali smo dve spojini s formulo [(C5Me4R)TiF(|i-F){|i-O2P(OSiMe3)2}]2 [R = Me (1), Et (2)] pri reakciji med [(C5Me4R)TiF3]2, (R = Me, Et) in OP(OSiMe3)3. Kristalna struktura spojine 2 je bila določena s pomočjo rentgenske strukturne analize. V strukturi je opažen osemčlenski obroč Ti2O4P2, povezan z dvema mostovnima fluorovima ligando-ma med obema titanovima atomoma.