Crystallographic Insights into the Behavior of Highly Acidic Metal Cations in Ionic Liquids from Reactions of Titanium Tetrachloride with [1-Butyl-3-Methylimidazolium][X] Ionic Liquids (X = Chloride, Bromide, Tetrafluoroborate)

Article abstract of DOI:10.1021/acs.inorgchem.8b00524

Highly charged metal ions are difficult to investigate in weakly coordinating ionic liquids (ILs) because of the insolubility of their solid forms, but the molecular liquid TiCl₄ offers a way to react tetravalent metal ions in an IL. Reactions of TiCl₄ with 1-butyl-3-methylimidazolium ([C₄mim]⁺)-based ILs containing chloride or bromide lead to mixtures of highly metastable amorphous solids and small amounts of crystalline chlorotitanate salts including [C₄mim]₂[TiCl₆] and two polymorphs of [C₄mim]₂[Ti₂Cl₁₀] in a manner not well correlated with stoichiometry or anion identity. The reaction of TiCl₄ with [C₄mim][BF₄] yields crystals of the mixed fluoro-chloro complex [C₄mim]₂[Ti₄F₆Cl₁₂], indicating spontaneous reaction of the IL ions to generate HF in situ. These unusual behaviors are explained in terms of the exceptionally high acidity of Ti⁴⁺ and the unusual behavior of TiCl₄ among metal halides as a nonpolar molecular compound.

Full text of DOI:10.1021/acs.inorgchem.8b00524

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Crystallographic Insights into the Behavior of Highly Acidic Metal
Cations in Ionic Liquids from Reactions of Titanium Tetrachloride
with [1-Butyl-3-Methylimidazolium][X] Ionic Liquids (X = Chloride,
Bromide, Tetrafluoroborate)
†
†,∇
†,○
,†,‡
Thomas P. Vaid,†,§
Manish Kumar Mishra, Steven P. Kelley,
Meghna Dilip,
David B. Cordes,† Scott T. Griffin, and Robin D. Rogers*
,#
†
†
Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
25 Solutions, Inc., P.O. Box 2206, Tuscaloosa, Alabama 35403, United States
S Supporting Information
‡5
*
ABSTRACT: Highly charged metal ions are difficult to
investigate in weakly coordinating ionic liquids (ILs) because
of the insolubility of their solid forms, but the molecular liquid
TiCl offers a way to react tetravalent metal ions in an IL.
4
Reactions of TiCl4 with 1-butyl-3-methylimidazolium
+
(
[C mim] )-based ILs containing chloride or bromide lead
4
to mixtures of highly metastable amorphous solids and small
amounts of crystalline chlorotitanate salts including [C mim] -
4
2
[
TiCl ] and two polymorphs of [C mim] [Ti Cl ] in a
6 4 2 2 10
manner not well correlated with stoichiometry or anion
identity. The reaction of TiCl with [C mim][BF ] yields
4
4
4
crystals of the mixed fluoro-chloro complex [C mim] -
4
2
[
Ti F Cl ], indicating spontaneous reaction of the IL ions
4 6 12
4
+
to generate HF in situ. These unusual behaviors are explained in terms of the exceptionally high acidity of Ti and the unusual
behavior of TiCl among metal halides as a nonpolar molecular compound.
4
1
. INTRODUCTION
halometallate ILs. Many such structures are reported for
8
9−12
11,13,14
11,15
divalent metal ions including Cr, Cu,
Co,
Pt, −Sn,
Fe,
There is a practically unlimited array of near-ambient
temperature ionic liquids (ILs) that can be made by combining
16
17
11
11,13
18
9,19−21
11
11,22
Ge, Hg, Mn, Ni,
Pb, Pd,
and
1
1,23
24
1
2
Zn.
In addition to the well-studied [AlCl4] systems,
different pairs or groups of ions. This great diversity contrasts
with molecular compounds, where the structure of the
molecule cannot be changed without affecting both its
chemical and physical properties, and has led to ILs being
viewed as “designer” solvents. One of the best examples of the
tunability of ILs are those which include ionic coordination
complexes such as halometallates, where a wide range of
speciation-diverse systems can be made by simply combining
an organic salt and a metal salt at different ratios. Adding
crystal structures are known for chlorometallates of trivalent
2
5,26
27
28,29
30
30,31
30,32
33−35
32
Au,
La,
Bi, Ce,
Dy, 29Er,
Eu,
Fe,
Gd,
36,37
32
32
38
32
,
29
30
Nd, Pr, Sb, Sc, Sm, Y, and Yb. However,
1
6
25
for tetravalent metal ions, the list is restricted to just Ge, Ir,
25
22
39−41
42
43
11,39,44−46
Pt, Sn, Ti,
U, V, and Zr,
for pentavalent
47
48,49
50
3
ions only Np, and Nb,
and Mo.
and for hexavalent ions only U
5
1,52
This may reflect how the weakly coordinating
ions found in many commonly used ILs cannot dissolve salts of
highly charged ions. However, these same ILs are also the most
interesting for exploring new, nonaqueous catalytic chemistry
of highly charged metal ions because their high Lewis acidity
will not be weakened by strong interactions with the solvent.
4
,5
6
neutral ligands or combining different metal ions generates
even more chemical diversity from ILs based on ionic
coordination complexes.
Halometallate ILs were one of the first IL systems to be
Titanium tetrachloride (TiCl ) is especially interesting in
studied in detail, beginning with alkylpyridinium chloroalumi-
nate melts in 1979, but it is likely that the incredible structural
4
7
this regard, as it is an extremely strong Lewis acid and a
nonpolar liquid whose interactions have already been studied
variability of these systems will never be fully explored. There
remain certain areas of halometallate IL structural chemistry
that are comparatively under-investigated, particularly metal
ions with high oxidation states. This is especially clear if one
looks at the published crystal structures of 1,3-dialkylimidazo-
lium chlorometallates, an especially well-investigated subset of
5
3
with other liquids including nonpolar aromatic solvents,
5
4
55
polar organic solvents,
and supercritical CO2.
Its
Received: February 28, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00524
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Inorganic Chemistry
Article
5
6
interactions with inorganic salts are also well investigated and
All samples used had 98% minimum purity, as determined by NMR
in an appropriate solvent. Water contents were measured using
volumetric Karl Fischer titration (EM Star, Lawrence, KS) and
determined to be <1%.
of great importance in the preparation of TiCl , which is itself
the most important industrial precursor for Ti metal and
pigment-quality TiO2. Being a liquid itself, there is
potentially little limit to the range of ILs with which TiCl
could be reacted to exploit the extreme acidity of the Ti
cation in unexplored solvent environments.
A number of investigations on the combination of TiCl4
with ILs have been published, and they present an interesting,
apparently contradictory body of observations. One of the
earliest reports, from 1990, observed that the reaction of TiCl
and 1-ethyl-3-methylimidazolium ([C mim] ) chloride did not
give a liquid salt, in contrast to AlCl and many other high
valent metals. In 2009, it was reported that TiCl could be
reacted directly with 1-butyl-3-methylimidazolium ([C mim] )
chloride at 100−120 °C to give room-temperature ILs at 1:1
and 1:2 ratios of [C mim]Cl to TiCl . Raman spectroscopic
4
5
7
2
.2. Reactions of TiCl4 with ILs. CAUTION: TiCl4 is an
4
extremely corrosive and pyrophoric material and thus should be
handled with suitable personal protective equipment (PPE). Also, the
reactions reported herein are extremely exothermic.
The ILs, 1-butyl-3-methylimidazolium chloride (solid), bromide
(solid), or tetrafluoroborate (liquid) were dried overnight under a
4
+
high vacuum, and were purged with N for 1 h before being melted at
2
7
0 °C. All reactions were carried out in a N -filled drybox. TiCl was
2
4
4
+
transferred using a syringe under N . For 1:1 reactions, the ILs were
2
2
mixed with TiCl in 1:1 stochiometric ratios by adding 0.01 mol of
4
3
molten IL with 0.01 mol (ca. 1 mL) of TiCl . For 2:1 reactions, the
4
4
ILs were mixed with TiCl in 2:1 stochiometric ratios by adding 0.02
+
4
4
mol of molten IL to 0.01 mol (ca. 1 mL) of TiCl . For 1:3 and 3:1,
4
IL/TiCl conditions, 0.03 mol of the appropriate reactant was used.
4
58
4
4
All above-mentioned stoichiometric and nonstoichiometric reactions
4+
studies of these liquids indicated that Ti in these ILs was in
were conducted by adding the IL to TiCl and also in the reverse
4
equilibrium between the species [TiCl6]2 and [Ti Cl ] .
−
−
order, i.e., adding TiCl to the IL. Suitable crystals for SCXRD formed
2
9
4
However, the reaction of TiCl with [C mim][AlCl ] was later
reported to give crystalline [C mim] [Ti Cl ], while the
reaction of [C mim]Cl with TiCl in dichloromethane yielded
C mim] [TiCl ] as a crystalline compound. Thus, the
upon cooling to room temperature for the reactions with [C
4
mim]Cl
4
4
4
40
and [C mim]Br. Crystals for the reaction with [C mim][BF ] were
4
4
4
4
2
2
10
obtained by further cooling to 0 °C in a refrigerator. All crystalline
products were extremely hygroscopic, but could be isolated for X-ray
diffraction studies using heavy hydrocarbon oil (Paratone-N,
Hampton Chemicals, CA).
4
4
41
[
4 2 6
+
physical state and even speciation in which [C mim] salts of
4
chlorotitanates are isolated appears to be especially sensitive,
not only to the ratio of Ti4 to halide but also to the method of
preparation.
Grinding experiments were also attempted at room temperature
+
using hand grinding under inert atmosphere. In general, TiCl (0.25
4
mmol) and each IL (0.25 mmol [C mim]Cl, [C mim]Br, or
4
4
In this study, we sought to expand upon these investigations
[C mim][BF ]) in a 1:1 ratio were ground in an agate mortar with
a pestle for 5−10 min each at room temperature under inert
atmosphere, leading to the formation of solids or liquids.
4 4
−
of TiCl speciation, which were limited to Cl containing
4
media, by exploring and comparing the reactions of TiCl with
4
+
−
−
2.3. X-ray Diffraction. Single crystal X-ray diffraction (SCXRD)
data were collected on a Bruker CCD area detector-equipped
diffractometer (Madison, WI) with graphite-monochromated Mo−
Kα radiation (λ = 0.71073 Å). The crystals were cooled to −100 °C
under a cold stream of nitrogen using an LT-2 cryostat (Bruker AXS).
[
[
C mim] )-based ILs with various anions (Cl Br , and
BF ] ). Surprisingly, these systems yielded crystalline Ti
4
,
−
4+
4
complexes only as minor products, even when the bulk
stoichiometries exactly matched that of the crystalline
compound. Observations from these reactions and structural
characterization of the isolated crystals give insights into the
60
Absorption corrections were performed using SADABS. Structure
61
solutions were carried out by direct methods using SHELXTL, and
2
unusual crystallization mechanism, unexpected crystal poly-
the structures were refined using full-matrix least-squares on F
+
62
morphism, and participation of the [C mim] cation in the
(SHELXL 2014). Non-hydrogen atoms were refined anisotropically,
4
+
formation of [C mim] chlorotitanates.
and hydrogen atoms were refined using a riding (CH, CH ) or riding-
2
4
rotating (CH ) model. Some of the butyl chains in all the structures
3
were found to be disordered. Additional partially occupied sites atoms
were located, and all atoms involved in the disorder were refined
anisotropically. Similarity restraints were required for modeling the
2
. EXPERIMENTAL SECTION
2
.1. Materials Used. 1-Chlorobutane, 1-methylimidazole, sodium
tetrafluoroborate (NaBF ), anhydrous dichloromethane (CH Cl ),
and silver nitrate (AgNO ) were purchased from Sigma-Aldrich (St.
Louis, MO, USA). TiCl was used as obtained from Fluka (Seelze,
Germany).
disordered butyl chains in [C mim] [TiCl ]. All three structures were
4 2 6
4
2
2
affected by twinning and were refined with incorporation of the
3
relevant twin law.
4
The crystals of [C
from the reactions of [C
[C mim]Br (1:1 ratio only) with TiCl
4
mim]
mim]Cl (3:1, 2:1, and 1:1 ratios) and
have identical unit cell
[TiCl ] from the sticky solid obtained
4
2 6
1
-Butyl-3-methylimidazolium chloride ([C mim]Cl) was synthe-
4
4
sized by reacting 5% molar excess of 1-chlorobutane with 1-
4
methylimidazole under reflux for 1 week, until no measurable amount
parameters. The structures of both complexes obtained from the
bromide IL were solved, and partial refinements of the data were used
to analyze if partial Cl/Br occupancy was present. Thermal
1
59
of 1-methylimidazole was detected using H NMR. To remove any
excess water and unreacted 1-chlorobutane, the product was dried
first on a rotary evaporator followed by a high vacuum line for at least
parameters, R-factors, and goodness of fit of the [C
and [C mim] [Ti Cl10] Form I structures obtained from the reaction
with [C mim]Br strongly suggested that no Br was present. A similar
analysis was carried out for the structure from the [C mim][BF
mim] [TiCl ]
4
2 6
1
2 h. 1-Butyl-3-methylimidazolium bromide ([C mim]Br) was
4
2
2
4
−
synthesized using the same procedure as [C mim]Cl, but with an
4
4
excess of 1-bromobutane in place of 1-chlorobutane.
4
]
4
For the synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate
reaction, looking for partial F/Cl occupancy, and it was determined
that the thermal parameters, R-factors, and goodness of fit of the
structure strongly suggested the correct formula of the complex to be
[C mim] [Ti F Cl ].
(
[C mim][BF ]), [C mim]Cl (30.00 g, 172 mmol) was reacted with
4 4 4
59
NaBF (20.00 g, 182 mmol) in water with stirring. The product,
4
[
C mim][BF ], was extracted into CH Cl through repeated
4 4 2 2
4
2
4
6
12
extractions until the organic phase containing the IL showed no
Powder X-ray diffraction (PXRD) data of the bulk materials (the
detectable halide contamination using an aqueous AgNO solution.
mixtures of sticky solids and crystals obtained from the reactions of
3
Thereafter, the CH Cl phase was separated and dried first on the
the ILs and TiCl ) were recorded on a Bruker D2 PHASER
2
2
4
rotary evaporator followed by high vacuum for at least 12 h. The pure
C mim][BF ] is a colorless to pale yellow, viscous liquid at room
instrument with a Linxeye linear position-sensitive detector
(BrukerAXS, Madison, WI) using Ni-filtered Cu−Kα radiation.
Sample preparation for the PXRD experiment was done in a glovebag
[
4
4
temperature.
B
DOI: 10.1021/acs.inorgchem.8b00524
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Inorganic Chemistry
Article
Table 1. Reactions and Crystallographically Characterized Products of the ILs with TiCl4
under inert atmosphere. The bulk material was sticky and was placed
as a thin layer on a silicon low background sample holder. A dome
exceptional stability, although prior reports of the isolation of
40,41
crystalline complexes with these compositions
suggest they
(
BrukerAXS, Madison, WI) was placed over the sample holder and
should be metastable with respect to the crystalline complexes.
sealed against the O-ring. The diffraction data were measured across a
+
The outcomes of reactions of TiCl with [C mim] halides
4
4
2
θ range of 5−40° using a continuous fast scanning mode with a rate
appear to be rather poorly correlated with the identity of the IL
and the ratio of TiCl to halide, both within our study and
of 0.5 s/0.02°. A background measurement was also done for the
airtight dome sealed holder with the same parameters. The data are in
Supporting Information Figures S1−S4.
4
across the published literature. Instead, the systems appear to
be particularly sensitive to how they were prepared. We
hypothesize that this behavior has a kinetic-molecular
explanation: Within the heterogeneous reaction mixture,
3
. RESULTS
.1. Observations on the Reactivity of TiCl . Reactions
3
4
4
+
−
collisions between Ti containing species and Cl are the
of [C mim]Cl, [C mim]Br, and [C mim]BF with TiCl were
4
4
4
4
4
−
conducted by directly combining the ILs and TiCl , heating
least likely because the volume of Cl is so much smaller
4
the components to 100 °C to achieve a homogeneous melt,
and allowing them to cool. Both addition of the IL to TiCl4
relative to the other species. When anionic chlorotitanates do
form, they are more likely to collide with TiCl and result in
4
and addition of TiCl to the IL were examined (Table 1). In all
cases, the reactions proceeded vigorously with HCl gas
evolution and a color change from colorless or pale yellow
the formation of multinuclear anions with low charge. It has
been shown that the viscosity of chlorotitanate ILs is well-
4
63
accounted for by the hole theory of liquids, so the formation
of multinuclear halotitantes provides a mechanism for
vitrification. This hypothesis also explains the crystallization
of species such as [C mim] [TiCl ] from systems that have
to dark yellow. Reactions with [C mim]Cl and TiCl yielded
4
4
sticky solids upon cooling at 1:1, 2:1, and 3:1 ratios of
C mim]Cl to TiCl and yielded liquids with small amounts of
[
4
4
2
2
6
crystalline solid at a 1:3 ratio of [C mim]Cl to TiCl . [C mim]
4+
4
4
4
excess Ti ; the competing reaction of TiCl with anionic
chlorotitanates depletes the system of TiCl faster than Cl .
4
Br and [C mim][BF ] were reacted only at 1:1 ratios to give,
−
4
4
4
respectively, solid and liquid reaction mixtures. PXRD on all
solid reaction mixtures indicated that they were primarily
amorphous in all cases (see Figures S1 and S2); however all
reactions also yielded single crystals as minor components that
could be isolated by hand.
When crystals form, it is ultimately in a system where free
[
C mim]Cl is in excess relative to free TiCl . The variable
4
4
identities of the crystals thus reflect the fact that when they
grow, the composition of the liquid has already changed due to
vitrefaction.
Our observation that TiCl and [C mim]Cl gave a sticky
4
4
In contrast to the reactions with halide ILs, when reacted at
solid reaction mixture conflicted with prior reports that this
58
a 1:1 ratio, [C mim][BF ] and TiCl yielded a liquid which
4
4
4
reaction should give a room temperature liquid or that pure
39
only gave crystals identified as [C mim] [Ti F Cl ] upon
4 2 4 6 12
storage at 0 °C for several days. These crystals were
reproducibly isolated in separate experiments irrespective of
[
C mim] [Ti Cl ] was a crystalline solid. In order to avoid
4 2 2 10
supercooling, we also attempted 1:1 molar reactions of TiCl at
4
room temperature using hand grinding in an agate mortar and
pestle to drive the reaction, which led to homogeneous yet
the order of addition of IL and TiCl . The source of the
4
−
fluoride anions must be the [BF4] anion, as the starting
amorphous solid powders (see Figure S3) for [C mim]Cl and
4
[
C mim][BF ] was analyzed for free fluoride anion content by
[
C mim]Br and to a dark orange liquid for [C mim][BF ]. We
4 4
4
4
4
ion chromatography and found to contain less than 1 ppm fre−e
attempted to induce a solid-to-solid phase transition of the
amorphous material by grinding seed crystals of [C mim] -
fluoride. There is the possibility that small amounts of F
4
2
6
4
[
Ti Cl ] into them, but this resulted in no observable changes
formed by hydrolysis of [C
mim][BF
4
4
]; however, because
2
10
4
+
3+
in the PXRD pattern of the material (see Figure S4). Overall,
the amorphous solids obtained in this study appear to show
Ti like B , is a hard Lewis acid, halide anion transfers
between BX and TiX of the type shown in eq 1 are known:
65
n
m
C
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Inorganic Chemistry
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Figure 1. 50% Probability ellipsoid plot of the formula unit of [C mim] [TiCl ] (a). Packing diagrams showing short (less than the sum of the van
4
2
6
der Waals radii) contacts to each unique cation (b) and anion (c). The dashed green lines indicate short contacts. Packing diagram showing 2 × 2
×
2 unit cells viewed down the crystallographic a axis (d). The disorder has been omitted for clarity.
−
−
F B‐F + TiCl → F B + F‐TiCl
(1)
and contains one unique formula unit per asymmetric unit
Figure 1). Both cations have butyl groups disordered over two
3
4
3
4
(
It has also been shown that even without the presence of
metal ions, 1,3-dialkylimidazolium [BF ] ILs can thermally
decompose into HF and the ylidene-BF adduct. Further
halide exchanges allow equilibration across various boron and
titanium polyhalide molecules and polyhalide anions, leading
mostly overlapping positions which are oriented nearly
perpendicular to the imidazolium ring plane. The crystal
packing of [C mim] [TiCl ] is virtually identical to [C mim] -
−
4
6
6
3
4
2
6
4
2
11
[ZrCl6], which was solved in the lower symmetry space
group P2 ; however, we believe the higher symmetry space
1
2−
+
eventually to the thermodynamically favored [Ti F Cl ] .
group P2 2 2 is correct, even with disorder in the [C mim]
4
6
12
1
1
1
4
Interestingly while the deprotonation of dialkylimidazolium
cations.
67
cations in metal-containing IL systems is known, it has only
been described as a source of carbene complexes and not yet
reported to explain the speciation of metal halides in an IL.
In this structure, each of the two unique cations is
surrounded by four anions which make short contacts (defined
as two atoms within a distance of less than the sum of the van
der Waals radii) to one or two hydrogen atoms each to the
imidazolium ring and butyl chain, roughly in the plane of the
ring (Figure 1b). In dialkylimidazolium salts, Coulombic
interactions tend to occur between anions and carbon atoms
3
.2. Observations on the Crystal Structures. A
summary of the crystallographic results is presented in Table
. We will first summarize the structures obtained and then
discuss the implications of these results on the Lewis acidity of
1
4
+
68
Ti and the basicity of the resulting halotitanate species.
and are perpendicular to the plane of the ring, while in-plane
3
.3. [C mim] [TiCl ]. The crystal structure of [C mim] -
6 1 1 1
interactions to ring hydrogen atoms are typically assigned to
hydrogen bonding, even for weakly Lewis basic halometallates
4
2
6
4
2
[
TiCl ] was solved in the orthorhombic space group P2 2 2
D
DOI: 10.1021/acs.inorgchem.8b00524
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Figure 2. 50% probability ellipsoid plot of the formula unit of [C mim] [Ti Cl ] (Form II) (a). Nonlabeled atoms are symmetry equivalents of
4
2
2
10
labeled atoms. Packing diagrams showing short contact environments around the cation (top) and anion (bottom) in Form II (b). Dashed green
lines indicate short contacts. The disorder has been omitted for clarity.
Figure 3. Packing diagrams of [C mim] [Ti Cl ] Form II (a) showing organization of anions and cations into infinite chains along the
4
2
2
10
crystallographic b axis through interactions with C2 (dashed green lines indicate short contacts). Comparison of the two most closely associated
ions in Form I vs Form II (b). Packing diagram of Form II in 2 × 2 × 2 unit cells viewed down the b axis (c) and Form I viewed down the bc plane
(
d) showing hydrogen bonded sheets. Dashed green lines in Form I indicate short contacts. Disorder omitted for clarity.
−
69
such as [AlCl ] . The environments around the two cations
imates a CsCl lattice with the two cations acting as a pseudo-
dication (Figure S5).
4
are very similar. The anion makes short contacts to eight
cations which form two sheets above and below the anion
Figure 1c). The arrangement nearly has 4-fold symmetry,
consistent with the nearly tetragonal lattice constants, but the
symmetry-inequivalent cations have their butyl groups on the
wrong sides of the ring relative to each other and thus cannot
be related by 4-fold rotation. Overall the structure approx-
2−
The [TiCl6] anion is distorted octahedral with axial Ti−Cl
bonds (Ti−Cl5 = 2.353(1) Å; Ti−Cl3 = 2.353(1) Å) longer
than the equatorial Ti−Cl bonds (Ti−Cl1 = 2.334(1) Å; Ti−
Cl2 = 2.343(1) Å; Ti−Cl4 = 2.336(1) Å, Ti−Cl6 = 2.339(1)
Å). This same axial lengthening of two Ti−Cl bonds is
(
41
observed in the crystal structure of [C mim] [TiCl ]·CH Cl
2
4
2
6
2
and [C mim] [ZrCl ], although to a lesser degree. The fact
4
2
6
E
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Figure 4. 50% probability ellipsoid plot of the formula unit of [C mim] [Ti F Cl ] (a). Packing diagrams showing short contacts to each unique
4
2
4
6
12
cation (b) and the anion (c). Dashed green lines indicate short contacts. Packing diagram showing 2 × 2 × 2 unit cells viewed down
crystallographic a axis (d). The disorder has been omitted for clarity.
that this small distortion is conserved across multiple
structures argues that it is the result of a significantly stabilizing
packing effect. The reported crystal structures of
Each cation makes short contacts to four cations and four
anions (Figure 2b, top), while each anion makes short contacts
to eight cations and no anions (Figure 2b, bottom). The
cation−anion pairs are connected through C−H···Cl hydrogen
bonds donated by hydrogen atoms on the imidazolium ring,
the methyl and methylene groups α to the nitrogen atoms, and
the methylene group β to the nitrogen atom. The short
cation−cation contacts involve the two disorder carbon atoms
at the end of the butyl chain possibly indicating repulsive
interactions.
[
C mim] [TiCl ]·CH Cl and [C mim] [ZrCl ] argue that
4 2 6 2 2 4 2 6
the stabilization of this type of structure is substantial.
.4. [C mim] [Ti Cl ]. This compound was isolated as
3
4
2
2
10
39
both the known form of [C mim] [Ti Cl ] (here Form I)
4
2
2
10
and a new polymorph (Form II). The reported Form I
crystallizes in the monoclinic space group P2 /n with one
1
+
2−
[
C mim] cation and half of a binuclear [Ti Cl ] anion in
4 2 10
70
the asymmetric unit connected through C−H···Cl interactions.
Form I and II are packing polymorphs. In Form II, the
cations are oriented so that they lie in between two anions,
with the imidazolium ring plane roughly parallel to the Ti−Cl
bond and the C2−H bond pointing toward the equatorial
Form II also crystallized in P2 /n with one cation and one-half
1
of the anion in the asymmetric unit (Figure 2a). The binuclear
anion is slightly skewed about its center. The butyl chain of the
cation is an all-anti configuration with the two disordered
carbon atoms.
−
plane of Cl ligands. This orientation appears to maximize the
out-of-plane C2···Cl interactions, which is generally the
F
DOI: 10.1021/acs.inorgchem.8b00524
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
4. DISCUSSION OF LEWIS ACIDITY OF Ti4+ AND
strongest electrostatic interaction in 1,3-dialkylimidazolium
7
1
salts, while also allowing hydrogen bonding to the C2−H
hydrogen atom, which is the most acidic. This bridges the
cations and anions into infinite chains along the crystallo-
graphic b axis (Figure 3a). By contrast, in [C mim] [Ti Cl ]
BASICITY OF THE HALOTITANATE SPECIES
It is interesting to note here two additional crystalline phases
were obtained from the reactions of TiCl with [C mim]Cl
4
4
4
2
2
10
and the 1:1 reaction of TiCl with [C mim]Br, respectively
4
4
Form I, the cation is oriented so that it overlaps with the
bridging, rather than the axial, chlorides. This leads to
maximized hydrogen bonding with no short contacts related
to Coulombic stacking in Form I as compared to Form II
(
see Table 1). These two additional crystalline phases were
found to be polymorphs of the compound [C mim] [Ti Cl ].
Polymorphism is prevalent in [C mim] salts, but is usually
4
2
2
10
+
4
7
6
correlated with conformational flexibility of the cation.
Interestingly, in both forms of [C mim] [Ti Cl ] the
(
Figure 3b).
4
2
2
10
The infinite chains in Form II (Figure 3a) arrange into
conformations of all ions are similar, and it is the relative
orientations of the ions which differ between the two.
The bond distance differences with the four monocoordi-
infinite sheets perpendicular to the ac diagonal, which stack by
interacting through weaker hydrogen bonds involving the butyl
chain (Figure 3c). On the other hand, the cation−anion
hydrogen bonds in Form I lead to the formation of hydrogen
bonded sheets that stack along the ac diagonal (Figure 3d).
The sheets interact with each other through weaker hydrogen
bonds involving the methyl groups α to the nitrogen atoms.
The Ti(IV) cations in Form I adopt a distorted octahedral
geometry with adjacent Cl−Ti−Cl angles ranging from
2
−
nated chlorides of the Form II [Ti Cl ] anion range from
2
10
2
.231(3) to 2.306(3) Å, while the bridging chlorides show
longer bond lengths of 2.472(3) and 2.512(3) Å. These
structural features have all been noted in other crystalline salts
2−
77
containing [Ti Cl ] . This may account for the higher
2
10
2−
−
Lewis acidity of the [Ti Cl ] anion than the [TiCl ] anion
2
10
6
as the elongated bridging Ti−Cl bonds will leave the Ti center
7
1
8.64(7)° for the angle across the bridging chlorides, to
01.36(9)° for the angle trans to this. In Form II, the unique
coordinatively partially unsatisfied.
2−
4
+
The formation of [Ti F Cl ] from [C mim][BF ] and
4 6 12 4 4
Ti center has a slightly less distorted octahedral geometry as
compared to Form I, with adjacent Cl−Ti−Cl angles ranging
from 79.05(8)°, for the angle between the bridging chlorides,
to 100.71(11)° for the angle trans to this.
TiCl has implications for other reactions that utilize this
4
mixture such as the reported hydrolysis of a solution of TiCl4
in [C mim][BF ] to yield TiO nanoparticles with the anatase
4
4
2
78
structure. In contrast, a similar hydrolysis of TiCl4 in
3
.5. [C mim] [Ti F Cl ]. This structure is triclinic P1
̅
with
4
2
4
6
12
+
[C mim]Cl under similar conditions leads to a mixture of
an asymmetric unit consisting of two [C mim] cations and
the cage anion [Ti F Cl ] (Figure 4a). Butyl groups of both
4
4
2
−
anatase and rutile TiO , which eventually converts to pure
2
4
6
12
79
2−
rutile TiO2. The presence of [Ti F Cl ] or similar mixed
cations are oriented nearly perpendicular to the imidazolium
ring plane, although the last two carbon atoms of one cation
are disordered over two slightly offset positions. The cage
4
6
12
fluoride-chloride species in [C mim][BF ] may slow the
4
4
hydrolysis and lead to anatase TiO exclusively.
2
4+
The possibility of disproportionation or substitution
between the metal halide has not received much consideration
in determining the speciation of Lewis acidic halometallate ILs
as it is not very intuitive. The high vapor pressure and low
structure comprises a tetrahedral arrangement of Ti cations,
−
with all six edges of the tetrahedron formed by F ligands each
4
+
−
bridging two Ti cations. Three terminal Cl ligands cap each
4
+
Ti such that each titanium ion has a distorted octahedral
conductivity of pure TiCl would suggest that it does not
coordination geometry. The Ti···Ti distances range from
4
autoionize to any significant extent, and the phase diagrams of
halide ILs with the more well-studied Lewis acid metal chloride
3
.838(6) to 3.848(7) Å.
A similar cage structure has been reported for the
2
−
72−74
AlCl indicate that there is simply a ratio-dependent addition
tetranuclear [Ti F ] anion.
The Ti−F distances for
3
4
18
−
7
of Cl to AlCl . However, it may offer insight into the reason
the bridging fluoride anions in these reported structures are
longer (ranging from 1.959(4) to 2.019(4) Å) than the
terminal Ti−F distances (ranging from 1.752(4) to 1.790(4)
3
why electroplating Ti(0) from ILs has been a particular
80
challenge.
2
−
The disproportionation is also important to consider in
mixed-halide systems. In the literature, a random statistical
Å). The bridging Ti−F bond distances in [Ti F Cl ]
4
6
12
(
1.967(5)−1.997(5) Å) fall into the same range for bridging
−
distribution of Br anions in the metal chloride complexes of
fluoride anions. The Ti−Cl range from 2.206(3) to 2.239(3)
3
+
3+
−
Al or Bi is known on simple addition of Br to salts of these
Å, which is somewhat shorter than the terminal Ti−Cl
8
1,82
2−
distances previously observed in the [Ti Cl ] anion (2.227−
metals.
However, in the case of TiCl here, the Lewis
4
2
10
3
9,40
4+
2
.317 Å).
This is likely due to the high electronegativity of
acidity of Ti drives it to maximize the number of the
fluorine, which leads to a higher fractional positive charge on
strongest interactions possible per metal ion, i.e., Ti−Cl, even
2−
−
titanium in [Ti F Cl ] , which in turn leads to a stronger
in the presence of excess Br as in the reactions with
4
6
12
interaction with the bound chlorides.
[C
The contrast between the polymorphic behavior of
[C mim] [Ti Cl10] with the crystallization of [C mim]
[TiCl ] as part of an isomorphous series, highlights the effect
of the size-to-charge ratio on the basicity of halometallates.
mim]Br.
4
One cation makes short contacts to four anions, while the
other makes short contacts to five anions (Figure 4b). The
anion makes short contacts to nine cations (Figure 4c), and
-
2
4
2
2
4
6
75
there are no short contacts between like ions. The packing
essentially consists of charge-ordered columns of cations or
anions along the crystallographic a-axis which are bridged by
these interactions along b- and c-axes into a 3D network
2−
2−
The increase in size from [TiCl
]
6
to [Ti
Cl10
]
with no
2
corresponding increase in charge is correlated with the onset of
crystal polymorphism caused entirely by differences in the
packing of molecules, not their conformation or molecular
structure. Polymorphism has been directly implicated as a
(
Figure 4d). It also appears that the butyl groups of one of the
unique cations form narrow, continuous lipophilic channels
along the crystallographic b-axis, while the butyl groups of the
other cation do not appear to aggregate.
71
melting point lowering mechanism in ILs. Halometallate
4
+
complexes with highly acidic ions like Ti may be useful in
G
DOI: 10.1021/acs.inorgchem.8b00524
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
exploring this phenomenon, as the geometry of the molecules
themselves tends to be highly conserved with other structures
due to the strength of the metal-halide bonds.
§(T.P.V.) Department of Chemistry, University of Michigan,
Ann Arbor, MI 48109, USA.
(D.B.C.) School of Chemistry, University of St Andrews, St
#
Andrews, KY16 9ST, UK.
5
. CONCLUSIONS
Notes
The results of this study indicate a number of deviations from
typical halometallate chemistry in ILs which are apparently
driven by the high acidity of the Ti4 ion and the strength of
the resulting Ti-halide bonds. The reaction of TiCl with
halide or [BF ] ILs led to the crystallization of [TiCl ] ,
The authors declare no competing financial interest.
+
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AUTHOR INFORMATION
Corresponding Author
E-mail: rdrogers@ua.edu.
ORCID
Manish Kumar Mishra: 0000-0002-8193-3499
Steven P. Kelley: 0000-0001-6755-4495
Meghna Dilip: 0000-0003-3773-5396
Thomas P. Vaid: 0000-0003-4597-0847
Robin D. Rogers: 0000-0001-9843-7494
Present Addresses
S.P.K.) Department of Chemistry, University of Missouri,
Columbia, MO 65211, USA.
M.D.) Department of Chemistry, Worcester State Uni-
versity, Worcester, MA 01602, USA.
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