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Acta Crystallogr C Struct Chem
Acta Crystallogr C Struct Chem
Acta Cryst. C
Acta Crystallographica Section C: Structural Chemistry
2053-2296
International Union of Crystallography

39136285
son3002
10.1107/S2053229624006843
ACSCGG
S2053229624006843
Research Papers
Concerning the structures of Lewis base adducts of titanium(IV) hexa­fluoro­iso­pro­pox­ide
Lewis base adducts of titanium(IV) hexa­fluoro­iso­pro­pox­ide
Van Der Sluys William G. a*
a https://ror.org/04p491231 Department of Chemistry The Pennsylvania State University Commonwealth College at Altoona Altoona Pennsylvania 16601 USA
Correspondence e-mail: wgv102@psu.edu
01 9 2024
13 8 2024
13 8 2024
80 Pt 9 c240900 562566
05 6 2024
12 7 2024
© William G. Van Der Sluys 2024
2024
https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
A full version of this article is available from Crystallography Journals Online.
The reaction of titanium(IV) chloride with sodium hexa­fluoro­iso­pro­pox­ide, carried out in hexa­fluoro­iso­propanol, produces titanium(IV) hexa­fluoro­iso­pro­pox­ide, which is a liquid at room temperature. Recrystallization from coordinating solvents, such as aceto­nitrile or tetra­hydro­furan, results in the formation of bis-solvate com­plexes. These com­pounds are of inter­est as possible Ziegler–Natta polymerization catalysts.

The reaction of titanium(IV) chloride with sodium hexa­fluoro­iso­pro­pox­ide, carried out in hexa­fluoro­iso­propanol, produces titanium(IV) hexa­fluoro­iso­pro­pox­ide, which is a liquid at room temperature. Recrystallization from coordinating solvents, such as aceto­nitrile or tetra­hydro­furan, results in the formation of bis-solvate com­plexes. These com­pounds are of inter­est as possible Ziegler–Natta polymerization catalysts. The aceto­nitrile com­plex had been structurally characterized previously and adopts a distorted octahedral structure in which the nitrile ligands adopt a cis configuration, with nitro­gen lone pairs coordinated to the metal. The low-melting tetra­hydro­furan com­plex has not provided crystals suitable for single-crystal X-ray analysis. However, the structure of chlorido­tris­(hexa­fluoro­isopropoxido-κO)bis­(tetra­hydro­furan-κO)titanium(IV), [Ti(C3HF6O)3Cl(C4H8O)2], has been obtained and adopts a distorted octa­hedral coordination geometry, with a facial arrangement of the alkoxide ligands and adjacent tetra­hydro­furan ligands, coordinated by way of metal–oxygen polar coordinate inter­actions.

crystal structure
catalysis
polymerization
fluoro­alkoxide
ionization isomer
coordination chemistry
coordinating solvent
Ziegler-Natta polymerization catalyst
National Science Foundation CHE-0130835 This work was funded by National Science Foundation grant CHE-0130835 .
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pmcIntroduction

Early transition-metal coordination chemistry involving Group IV metals (Ti, Hf, and Zr) has been of inter­est for many years, with applications as Ziegler–Natta (ZN)-type polymerization catalysts (Ziegler et al., 1955 ▸). It is generally accepted that effective ZN catalysts involve alkyl­ated and coordinatively unsaturated species that com­plex an olefin monomer, and then allow for insertion into the metal–carbon bond by way of the so-called Cossee–Arlman mechanism (Hartwig, 2010 ▸). Theoretical studies predict that the electron densities of the titanium ion play a major role in the effectiveness of the catalyst (Piovano et al., 2021 ▸). Previously, the author and co-workers published a description of a series of titanium fluoro­alkoxide com­plexes, obtained primarily by alcohol-exchange reactions. The nature of the products appeared to depend upon the acidity and steric requirements of the fluoro­alcohol and the alkyl groups of the titanium alkoxide starting materials (Campbell et al., 1994 ▸; Fisher et al., 1993 ▸). In several cases, the products adopted dimeric structures with bridging alkoxide ligands and/or coordinated alcohol ligands to satisfy the coordinative unsaturation of the titanium(IV) ions. In at least one case, a titanium coordination site was filled by the inter­action with an F atom, that wrapped around and formed a weak inter­action. The primary impetus of this work was to use the electron-withdrawing fluoro­alkoxide ligands as pseudohalides to provide significantly more Lewis acidity to the metal ion, yet retain the steric control associated with the alkoxide ligands. Sawamoto & Kamigaito (1996 ▸) have described using TiCl2(OR)2 com­pounds, where OR represents both alkyl alkoxides and phenoxides, as living polymerization catalysts that are potentially stereospecific. As part of prior work on the related titanium(IV) fluoro­alkoxides, the author and co-workers described the preparation and characterization of two hexa­fluoro­iso­pro­pox­ide com­plexes with the general formula Ti(ORf)4L2, where L represents the coordinating solvents aceto­nitrile (in com­plex 1) and tetra­hydro­furan (THF) (in 2). A single-crystal X-ray structure determination of the aceto­nitrile com­plex indicated a monomeric structure in which the coordination geometry of the titanium was essentially octa­hedral, with cis-nitrile ligands acting as Lewis bases (Fig. 1 ▸), coordinating by way of nitro­gen lone pairs. This result was somewhat inter­esting, but not terribly surprising, and consistent with the structures of other similar com­plexes that had been characterized previously (Bradley, 1959 ▸; Bradley et al., 1978 ▸).

On the other hand, we did not report the crystal structure of the tetra­hydro­furan com­plex 2. In fact, we attempted to obtain a single-crystal X-ray structure of this com­pound on several occasions, but were unable to obtain crystals of suitable quality to publish the results. However, the diffraction data we did obtain appeared to be most consistent with an autoionization isomer, in which an octa­hedrally coordinated [Ti(ORf)2L4]2+ cation and a [Ti(ORf)6]2− anion had formed. We have structurally characterized one other example of an octa­hedrally coordinated TiIV dianionic hexa­kis­fluoro­phenoxide, Na2Ti(OC6F5)6(THF)2, so at least the formation of the dianion is plausible. [We have recently described the X-ray structures of the tetra­kis­(penta­fluoro­phenoxide)bis­(tetra­hydro­furan)­titanium(IV) com­plex and the hexa­kis­(penta­fluoro­phenoxide)titanium(IV) com­plex anion (Van Der Sluys et al., 2018 ▸).]

There are a limited number of examples of cationic titanium(IV) alkoxides which also include cyclo­pentadienyl ligands (Fandos et al., 2007 ▸). Perhaps the most relevant example that we are aware of is a cyclo­penta­dienyl titanium(IV) trication, in which there are coordinated aceto­nitrile ligands and three SbCl6− anions that act as counter-ions (Willey et al., 1994 ▸). Our spectroscopic evidence for autoionization, based on NMR and IR spectroscopic and conductivity measurements, was inconclusive at best and we were unsure if the ionization isomer represented a minor com­ponent or was representative of the bulk of the material. While ionization isomers for metal com­plexes are known (Barbier et al., 1972 ▸), we were unwilling to publish our speculative results. More recent results concerning the autoionization of metal coordination com­pounds (Tebbe & Muetterties, 1967 ▸; Kamata et al., 2012 ▸; Giesbrecht et al., 2004 ▸; Xie et al., 1996 ▸; Niemeyer, 2001 ▸) has encouraged a revisit of this work. Described herein are the most recent efforts, including the X-ray crystal structure of a mixed chloride/fluoro­alkoxide com­plex, which adopts a structure similar to that of the neutral aceto­nitrile com­plex.

Experimental

Synthesis

All synthetic procedures were carried out using standard Schlenk techniques or in a purified nitro­gen atmosphere using a Braun UNIlab glove-box. Solvents were purified by distillation from a sodium benzo­phenone ketal solution and stored in glass containers fitted with solvent seal fittings. IR spectra were recorded as KBr pellets, by grinding small portions of the sample with dried potassium bromide using an agate mortar and pestle in the glove-box. The Fourier transform IR (FT–IR) spectra were recorded on a ThermoScientific Nicolet iS10 FT–IR spectrometer. A background spectrum was subtracted to produce the percent transmittance spectrum of the sample. Residual gaseous carbon dioxide asymmetric vibrations were sometimes ob­served in the 2400 cm−1 region, due to incom­plete background subtraction, and calibration was checked regularly using a film of polystyrene. NMR spectra were collected using a Bruker DPX 300 MHz spectrometer. Deuterated solvents, such as benzene-d6 (≥99.6 atom% D) and di­chloro­methane-d2 (≥99.9 atom% D) were purchased from Aldrich and degassed using a freeze–pump–thaw (FPT) method, and stored in the glove-box over mol­ecular sieves.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The selected crystal was twinned by non-merohedry. The twinning in intensity data was effectively removed by the integration and absorption correction programs. The positions of the H atoms were initially determined by geometry and were refined using a riding model. H-atom displacement parameters were set at 1.2 times the isotropic equivalent displacement parameters of the bonded atoms.

Results and discussion

Synthesis

Mazdiyasni et al. (1971 ▸) described the synthesis of a series of hexa­fluoro­iso­pro­pox­ides of the Group IV elements by way of a metathesis approach, in which the metal chlorides were reacted with four equivalents of sodium hexa­fluoro­iso­pro­pox­ide, with the corresponding alcohol as solvent (Scheme 1). The titanium(IV) com­pound is a colorless liquid that can be obtained in high yield by distillation. Mazdiyasni reported that these com­pounds could be recrystallized from a variety of solvents, including benzene, acetone, diethyl ether, and tetra­hydro­furan (THF), and indicated that the ethers reacted to form solvate com­plexes, but did not provide structural characterization data for these solvate com­plexes.

In our laboratory, we first prepared the sodium hexa­fluoro­iso­pro­pox­ide in situ by reacting sodium hydride with excess hexa­fluoro­iso­propanol and, after the evolution of hydrogen stopped, titanium(IV) chloride was added slowly while stirring. The product was recovered by first removing the excess hexa­fluoro­alcohol in vacuo and then gently heating the somewhat less volatile titanium(IV) hexa­fluoro­iso­pro­pox­ide and condensing the liquid product in a liquid-nitro­gen-cooled trap, which usually resulted in excellent yields. We typically accessed the purity of the product based on NMR spectroscopy, but we suspect that there is rapid ligand exchange on the NMR time scale at room temperature between various species that might be present in solution. Borden & Hammer (1970 ▸) also ob­served rapid ligand exchange at room temperature for a mixture of TiCl4 and TiF4 in THF solution. At −60 °C, exchange was slowed sufficiently to observe several species in solution. The 19F NMR data were most easily inter­preted as having resulted from a com­plex mixture of com­pounds in which cis-TiF4(THF)2 was present, but also that there were additional com­pounds having both chloride and fluoride coordinated to monomeric octa­hedrally coordinated titanium(IV) species with two THF mol­ecules occupying cis, but not trans, positions. While their inter­pretation of the data is very well reasoned, it is not totally clear that these data eliminate the possible presence of ionic species as well, such as TiF5(THF)−, TiF62−, TiF3(THF)3+, and TiF2(THF)42+, as additional com­ponents in solution.

When the titanium(IV) hexfluoro­iso­pro­pox­ide is recrystallized from coordinating solvents, such as aceto­nitrile or THF, the bis-solvate com­plexes, Ti(ORf)4L2, are formed based on combustion elemental analysis and spectroscopic techniques, such as NMR and IR (Campbell et al., 1994 ▸; Fisher et al., 1993 ▸). The aceto­nitrile com­plex is a very stable crystalline solid that can be sublimed intact to form large nearly cubic crystals. The THF com­plex is extremely soluble in THF and has a relatively low melting point, which is slightly above room temperature (Campbell et al., 1994 ▸). Unfortunately, it has not been possible to produce single crystals of com­pound 2 of suitable quality for a convincing single-crystal X-ray analysis. However, on at least one occasion, recrystallization from a mixture of hexane and THF at −20 °C, produced a small number of crystals which were suitable for X-ray analysis. It appears that this higher-melting crystalline material was a minor com­ponent, with the formula TiCl[OCH(CF3)2]3(THF)2 (3), resulting from incom­plete substitution of chlorides for fluoro­alkoxides in the metathesis reaction.

Crystal structure

The mol­ecular structue of 3 shown in Fig. 2 ▸ emphasizes the nearly octa­hedral coordination geometry of the Ti atom. The fluoro­alkoxide ligands form a facial arrangement, with the chloride and two THF ligands on opposing sides. Table 2 ▸ provides relevant bond length and angle data. Fractional coordinates and other crystallographic data can be found in the supporting information.

The coordination geometry of 3 is best described as distorted octa­hedral. The average fluoro­alkoxide Ti—O bond length [1.85 (1) Å] com­pares very well with the average fluoro­alkoxide Ti—O bond lengths reported previously for com­pound 1 [1.84 (1) Å]. Not surprisingly, the longest metal–ligand bond length is the bond between the titanium and chloride [2.3399 (9) Å], which is similar to the terminal Ti—Cl distances ob­served in other octa­hedrally coordinated titanium com­pounds (Sarsfield et al., 1999 ▸; McCarthy et al., 2020 ▸; Nielson et al., 2001 ▸; Wright & Williams, 1968 ▸). There appears to be a slight but significant trans influence (Burdett & Albright, 1979 ▸) in the Ti—ORf bond lengths, with the Ti—O3 bond length, which is trans to the chloride ligand, being slightly longer than the Ti—O1/O2 bond lengths, which are trans to the coordinated THF ligands. The Ti—O bond lengths for the coordinated THF ligands are significantly longer than the alkoxide bond lengths [average 2.13 (1) Å], consistent with the weaker polar coordinate inter­actions of these ligands. The trans influence is due to the weakly coordinating neutral THF ligands that allow the trans-fluoro­alkoxides to bond more strongly to titanium than the alkoxide that is trans to the formally anionic chloride ligand.

The large Ti—O—C angles of the fluoro­alkoxides are also com­parable to those ob­served in com­pound 1 and consistent with other structurally characterized TiIV alkoxide com­plexes (Schubert et al., 2020 ▸). These angles are best inter­preted as resulting from significant oxygen-to-titanium pπ→dπ bonding. Titanium(IV) has a 3d0 electronic configuration and therefore in an octa­hedral ligand field, the empty t2g set (dxy, dxz, and dyz) of d orbitals can accept electrons from π-donor ligands, such as alkoxides. Presumably this is tempered somewhat in fluoro­alkoxide ligands due to the electron-withdrawing nature of the –CF3 groups, as com­pared with alkyl­alkoxide ligands, making the Ti—ORf inter­actions more halide-like in nature. The presumed order of π-bonding is proposed to be ORf > Cl > THF, consistent with the trans-influence effects that we observe in 3 and consistent with the order of F > Cl > THF used by Borden & Hammer (1970 ▸) to justify their proposed stereochemical inter­pretation of the 19F solution NMR data for the TiF4/TiCl4/THF system. Perhaps even more straightforward, the trans influence in these com­pounds can be inter­preted based on hard–soft Lewis acid–base theory (Pearson, 1963 ▸; Jolly, 1984 ▸).

Conclusions

While the crystal structure of com­pound 3 reported here clearly indicates a monomeric mol­ecular coordination com­pound and not an ionization isomer, as speculated for com­pound 2, it is not totally clear if replacing the final chloride ligand would tip the scale in favor of such a structure. There may be a subtle balance between the steric factors of the coordinated ligands, the degree of covalency or ionic inter­actions associated with the metal cation and the ligands, as well as inter­molecular forces of attraction between the coordination com­plex and the solvent. The initial hypothesis suggested that an increase in the ionic nature of the alkoxide ligands would produce favorable properties that would facilitate polymerization catalysis. Coordinately unsaturated cationic transition-metal species are particularly promising in this regard. We note that the ionic radius of the titanium(IV) ion (0.745 Å), is smaller than the highly ionic lanthanide ions (1.00–1.17 Å) (Shannon, 1976 ▸), with the latter producing autoionization in certain cases (Fandos et al., 2007 ▸). It is also not clear if there are several structural possibilities that might exist in solution, all of which are rapidly exchanging, and that crystallization of a solid does not necessarily indicate what species are present in solution. It is hoped that the solvent mol­ecules are labile enough to provide open coordination sites and that alkyl­ation of these com­pounds will produce effective olefin polymerization catalysts. Further studies are currently in progress.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2053229624006843/son3002sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2053229624006843/son3002Isup2.hkl

CCDC reference: 2370983

The author would like to thank the Pennsylvania State University, Altoona, Office of Research and Sponsored Programs for their support through a Research and Development (RDG) grant. The author thanks Doug Powell, Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma, USA (https://orcid.org/0000-0001-7133-468X) for his assitance in collecting the X-ray data with the support of the National Science Foundation (grant CHE-0130835) and the University of Oklahoma for funds to purchase the X-ray instrument and com­puters. The author also thanks Professor Nan Xu (PSU, Altoona) for allowing the use of his inert atmosphere glove-box. The author declares no com­peting financial inter­est.

Figure 1 The presumed octa­hedral structure of monomeric bis-Lewis base adducts of titanium(IV) alkoxides.

Figure 2 The mol­ecular structure of com­pound 3, showing the numbering scheme used. Displacement ellipsoids are drawn at the 50% probability level.

Table 1 Experimental details

Crystal data	
Chemical formula	[Ti(C3HF6O)3Cl(C4H8O)2]	
M r	728.67	
Crystal system, space group	Monoclinic, P21/c	
Temperature (K)	100	
a, b, c (Å)	14.050 (4), 10.701 (3), 18.692 (5)	
β (°)	108.450 (3)	
V (Å3)	2665.9 (13)	
Z	4	
Radiation type	Mo Kα	
μ (mm−1)	0.58	
Crystal size (mm)	0.26 × 0.18 × 0.18	
 	
Data collection	
Diffractometer	Bruker APEX CCD	
Absorption correction	Multi-scan [TWINABS (Sheldrick, 2015c ▸) and SADABS (Krause et al., 2015 ▸)]	
Tmin, Tmax	0.864, 0.904	
No. of measured, independent and ob­served [I > 2σ(I)] reflections	67346, 4968, 3934	
R int	0.058	
(sin θ/λ)max (Å−1)	0.610	
 	
Refinement	
R[F2 > 2σ(F2)], wR(F2), S	0.042, 0.123, 0.98	
No. of reflections	4968	
No. of parameters	379	
H-atom treatment	H-atom parameters constrained	
Δρmax, Δρmin (e Å−3)	0.56, −0.71	
Computer programs: APEX2 (Bruker, 2007 ▸), SAINT (Bruker, 2007 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2018 (Sheldrick, 2015b ▸), and Mercury (Macrae et al., 2020 ▸).

Table 2 Selected geometric parameters (Å, °)

Ti1—O1	1.8386 (17)	Ti1—O4	2.1043 (17)	
Ti1—O2	1.8426 (17)	Ti1—O5	2.1593 (17)	
Ti1—O3	1.8679 (17)	Ti1—Cl1	2.3399 (9)	
 	 	 	 	
O1—Ti1—O2	100.14 (8)	O2—Ti1—Cl1	90.52 (5)	
O1—Ti1—O3	94.69 (7)	O3—Ti1—Cl1	172.19 (6)	
O2—Ti1—O3	92.72 (7)	O4—Ti1—Cl1	90.02 (5)	
O1—Ti1—O4	88.84 (7)	O5—Ti1—Cl1	85.92 (5)	
O2—Ti1—O4	170.98 (7)	C2—O1—Ti1	140.75 (15)	
O3—Ti1—O4	85.68 (7)	C5—O2—Ti1	142.97 (15)	
O1—Ti1—O5	167.56 (7)	C8—O3—Ti1	130.80 (15)	
O2—Ti1—O5	92.10 (7)	C13—O4—Ti1	129.17 (14)	
O3—Ti1—O5	86.87 (7)	C10—O4—Ti1	120.92 (13)	
O4—Ti1—O5	78.95 (6)	C14—O5—Ti1	129.22 (14)	
O1—Ti1—Cl1	91.72 (6)	C17—O5—Ti1	121.54 (13)
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