Concomitant Twinning and Polymorphism of Ti(C5H4tBu)2Cl2

Mar 13, 2009 - Ilia A. Guzei*, Amitabha Mitra and Lara C. Spencer. Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, ...
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Concomitant Twinning and Polymorphism of Ti(C5H4tBu)2Cl2 Ilia A. Guzei,* Amitabha Mitra, and Lara C. Spencer Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2287–2292

ReceiVed September 25, 2008; ReVised Manuscript ReceiVed January 30, 2009

ABSTRACT: The title compound (Ti(C5H4tBu)2Cl2, 1) exists in three polymorphs and undergoes two enantiotropic phase transitions between them. The low-temperature, non-merohedrally twinned monoclinic phase III (space group P21) undergoes a first-order phase transition into an orthorhombic phase II (space group P212121) at 147(1) K. Subsequent heating of the crystal results in a gradual second-order k2 transformation of this phase into the high-temperature orthorhombic phase I (P21212). This phase transition is completed at ∼330 K. The phase transitions were monitored by single-crystal X-ray diffraction, differential scanning calorimetry, and powder diffraction; however, the II f I transition did not register on the differential scanning calorimetry curve or powder patterns. The molecular conformations and mutual arrangement of molecules in the crystal in the three phases are very similar. The location of the ancillary ligands relative to the Cl-Ti-Cl wedge in the solid-state structures of 1 and 32 related Ti(C5H4R)2Cl2 complexes seems to be principally determined by weak C-H · · · Cl intramolecular interactions between the R substituents and Cl ligands rather than by steric factors. An example of an advanced structural refinement technique using SHELXL to compute standard uncertainties on mathematically derived parameters is also given. Introduction Crystallographic characterization of non-merohedrally twinned crystals has become essentially routine with the help of such programs as CELL_NOW,1 TWINABS,2 PLATON,3 and a number of others (http://www.ccp14.ac.uk/solution/ twinning/index.html) as can be attested by numerous publications.4 There is also a large body of literature on solid-state phase transitions.5,6 However, only a handful of documented examples of solid-state phase transitions from a single crystal to a non-merohedrally twinned crystal has been reported for molecular crystals. The ionic complex [NO2][Ga(NO3)4] crystallizes in the tetragonal space group I4j at RT but reversibly changes into its non-merohedrally twinned monoclinic polymorph (space group I2) below ∼250 K.7 A colorless high-spin complex {Fe[HC(3,5-Me2pz)3]2}(BF4)2 (pz ) pyrazolyl ring) exists in the monoclinic space group C2/c above ∼204 K but converts into a purple non-merohedrally twinned triclinic (P1j ) 1:1 mixture of high and low spin complexes below 206 K.8,9 The high-temperature orthorhombic phase (space group Pnma) of the Cu(II) complex LiPrCu(OC6H4tBu) where LiPr ) 2,4-bis((2,6-diisopropylphenyl)imido)pentane undergoes a phase transition at 265(5) K into a 2:1 non-merohedrally twinned monoclinic (P21/n) phase accompanied by molecular conformational changes.10 Similarly, the orthorhombic (Pnma, RT) polymorph of barbituric acid dehydrate (C4H4N2O3 · 2H2O) undergoes a phase change into a non-merohedrally twinned monoclinic (P21/n) polymorph at 216(2) K.5 An R T β phase transformation in the giant magnetocaloric material Gd5(Si2Ge2) can be controlled by composition, temperature, and magnetic field.11 The orthorhombic (Pnma) R phase observed at 163 and 243 K becomes a non-merohedrally twinned monoclinic (P1121/a) β phase above ∼260 K. 1-Phenyl-2-methyl-4-nitro5-bromoimidazole undergoes two phase transitions between 100 and 295 K. The lower temperature phase transition in the range 115-118 K is manifested by a change in crystal symmetry from the monoclinic lattice (P21/c) into a twinned * To whom correspondence should be addressed. E-mail: iguzei@ chem.wisc.edu.

triclinic phase (space group P1j ).12 Orthorhombic crystals of triferrocenylboroxine [Fe3(C5H5)3(C15H12B3O3)] transform from orthorhombic (Cmc21) into twinned monoclinic (P21) upon cooling below 283(2) K.13 All the above phase transitions are reversible. The current project originated serendipitously in the course of a routine structural investigation of a titanium-containing red, crystalline unknown. The routine unit cell determination at 100 K revealed that the selected crystal was nonmerohedrally twinned, as indicated by CELL_NOW.1 An effort was made to find a single crystal in the available batch, but when six different crystals proved to be twinned, a data collection was conducted on the best diffracting twin. The monoclinic unit cell obtained did not match any unit cell either in the Cambridge Structural Database4 or in the inhouse Reciprocal Net14 database. The subsequent structural solution in space group P21 established the unknown compound to be Ti(C5H4tBu)2Cl2 (1) (rather than the desired compound), a complex previously characterized in the orthorhombic space group P21212 at room temperature.15 A differential scanning calorimetry analysis of 1 in the range 108-305 K showed one phase transition at ∼147 K. It was subsequently established that this temperature corresponds to the transformation of the twinned monoclinic phase observed at 100 K into an orthorhombic phase, with the space group P212121 rather than P21212. This suggested a second phase transition between 147 K and RT, which, however, was not seen in a repeated DSC experiment. The study described herein reveals that the 1ow-temperature orthorhombic phase (P212121) gradually transforms into the hightemperature, higher symmetry orthorhombic phase (P21212) with the complete conversion around 330 K. The nature of this phase transition is such that it registers neither on the DSC curve nor in the powder diffraction patterns. Thus, the space group of the previously reported structure of 1 at RT should be corrected to P212121. Remarkably, in the temperature range 296-395 K the structure of 1 can be satisfactorily refined both in P21212 and in P212121; the latter space group is correct at 296 K and incorrect above 330 K. Here we

10.1021/cg8010772 CCC: $40.75  2009 American Chemical Society Published on Web 03/13/2009

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Guzei et al.

Scheme 1

present a detailed description of the two phase transitions and the three phases of 1. Discussion Phase Transitions. Three phases of 1 (Scheme 1, Figure 1) have been identified with single-crystal X-ray diffraction: nonmerohedrally twinned monoclinic (P21) below 147 K (phase III), orthorhombic P212121 from 147 to 330 K (phase II), and orthorhombic P21212 above 330 (phase I). The solid-state transitions I T II and II T III are reproducible, nondestructive, and enantiotropic. Phase transition I-II is presumed to be second order and is assigned type k2. During the transition of 1-I to 1-II the symmetry changes from space group P21212 into its k minimal nonisomorphic subgroup (type IIb) P212121 (c′ ) 2c) of 1-II. The index 2 was computed according to eq (1)17 where ik is the number of antiphase domains, it is the number of twin domains, Z is the number of formula units in the primitive unit cell, and P is the order of the corresponding point group.

i ) ik × it )

Z(P212121) Z(H) |P(G)| × ) × Z(G) |P(H)| Z(P21212) P(P21212) 4 4 ) × )2 P(P212121) 2 4

(1)

The phase transition II-III is first order. During the transition the orthorhombic crystal with space group P212121 of 1-II is transformed into a non-merohedrally twinned monoclinic crystal of 1-III with space group P21. The β angle changes from 90 to 92.456(2) and the c axis is halved, Figure 2. The transformation cannot be classified with a group-subgroup relation index due to the twinning. The structural relationship among the three polymorphs is illustrated in Figure 2 which depicts several unit cells for each polymorph. The high-temperature phase (Figure 2a) contains the Ti complex residing on a 2-fold axis; only one-half of complex 1-I is symmetry independent. The second orthorhombic phase, 1-II, Figure 2b, stable in the range of temperatures 147-330 K, does not possess a crystallographic 2-fold axis. Each Ti complex occupies a general position, and the c axis is twice the length of the c axis of the high-temperature polymorph 1-I. The twinned low-temperature phase 1-III (Figure 1c) is produced by lowering the symmetry from orthorhombic to monoclinic. The β angle of 92.456(2)° becomes the unique angle and the c axis is halved when the temperature is lowered below 147 K. The c axis can be thought of as the twin axis of the polymorph. A 180° rotation about the [001] vector superimposes one twin component onto the other. Thus, the twin component ratio should be 1:1. Indeed, the experimental refinement bears this out; the resulting twin component ratio is 50.8(1):49.2(1). This phase transition exemplifies concomitant twinning and polymorphism. Two differential scanning calorimetry experiments were conducted with two separate batches of crystals of 1: one in the 108-305 K and the other in the 193-403 K region, both

with 10°/min heating regimes. The lower temperature run indicated a phase transition at approximately 147(1) K. The rate of cooling and crystal quality are likely to play a role. Mnyukh noted that the purer the compound and slower the cooling rate the more likely sample overcooling will occur.18 The higher temperature DSC curve did not show a signal attributable to a phase transition. This observation is consistent with our singlecrystal variable temperature unit cell measurements and the nature of the phase transition, Vide infra. We believe the hightemperature orthorhombic-orthorhombic k2 phase transition to be gradual in nature and to span the temperature range 147330 K. The phase transitions were also monitored by single-crystal X-ray diffraction. A selected crystal was cooled to 100 K and then slowly heated up stepwise to 395 K with periodic (4-30 K) unit cell determinations. The 16 e j titanium(IV) complex 1 is stable in the entire temperature range; however, the crystal quality deteriorated somewhat upon prolonged (20 h) exposure to elevated temperatures (385 K). Figure 3 displays the number of reflections acquired during the extended unit cell determination routine as a function of temperature. In the range 100-147 K a total of 2298 reflections was harvested with the reflections nearly evenly split between the two twin domains. The discontinuity of the graph (red and blue lines in Figure 2) between 147 and 160 K is due to the first-order III-II phase transition. Once the III-II phase transition has occurred at 147 K, the number of reflections increased to 2808 at 160 K, due to the doubling of the unit cell. The entire set of harvested reflections could be indexed for the 1-II polymorph. The reflection count is a convenient means to monitor the II-I phase transformation in the 147-395 K range. It may be argued that the k2 phase transition between polymorphs II and I commences immediately upon completion of the III-II transition with concomitant appearance of 1-I nucleation sites.18 Figure 3 traces the total number of reflections, reflections hkl, l ) even and hkl, l ) odd. As the temperature rises the total number of reflections decreases due to the disappearance of reflections hkl, l ) odd. The number of reflections hkl, l ) even remains essentially invariant with only a slight decrease due to the decline in reflection intensities corresponding to the increased atomic thermal motion observed at higher temperatures. The low slope of the hkl, l ) odd curve below 200 K becomes progressively steeper as the crystal is heated. The curve reaches an inflection point at ∼302 K and merges with the abscissa at ∼330 K. The graph corresponding to the total number of reflections (top line in Figure 3) parallels this behavior. Note that at 296 K (RT) reflections with hkl, l ) odd are definitely present, substantiating the correct assignment of the space group P212121 at this temperature. The gradual nature of the II-I transformation apparently precludes its detection by the DSC method. In all three phases the molecular packing is very similar, and thus the II-I phase transition is not clearly detected by powder diffraction. Previously, Guzei et al.19 described a similar k2 enantiotropic transition between two orthorhombic phases P212121/P21212 of an ionic 222-cryptand complex with LiClO4, but in that case the transition registered on the DSC curve and was of type order-disorder. The relative intensities of observed reflections in the variabletemperature unit cell experiment are plotted in Figure 4. The average intensity of hkl, l ) even decreases with increasing temperature, as expected. During the high temperature transition reflections hkl, l ) odd never entirely disappear but the graph for these reflections levels off at ∼330 K, when the percentage of these reflections comprises 3.0%. The persisting reflections

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Figure 1. A molecular drawing of 1-II shown with 50% probability ellipsoids. All H atoms are omitted.

Figure 2. The molecular arrangement in the unit cells of the three polymorphs of 1 viewed along the b axis: (a) phase 1-I: orthorhombic polymorph (space group P21212) with the Ti complexes residing on 2-fold axes (shown), Z′ ) 0.5, T > 330 K; (b) phase 1-II: the other orthorhombic polymorph (space group P212121), Z′ ) 1, T ) 175 K; note the doubling of the c axis, V(1-II) ≈ 2V(1-I); (c) phase 1-III: non-merohedrally twinned monoclinic P21 polymorph, the twinned components are related by a 180° rotation about [001], Z′ ) 1, T ) 100 K, V(1-III) ≈ V(1-I).

Figure 4. Temperature dependence of intensities of the hkl, l ) even and hkl, l ) odd reflections for 1-II. The total number of reflections considered at 160 K is 2808, at 395 K - 1377. The number of hkl, l ) odd reflections at 330, 340, 360, 380, and 395 K is 44 (3.0%), 30(2.1%), 16(1.1%), 9(0.6%), and 7(0.5%), respectively. Figure 3. The number of observed reflections harvested by an extended unit cell determination routine. The reflections for the monoclinic twinned crystal are between 100 and 147 K. The reflections in the 147-395 K region were indexed for the orthorhombic phase 1-II (the larger cell) in order to monitor the disappearance of reflections hkl, l ) odd associated with the k2 transition.

at 385 K are 01j1, 01j1j, 1j1j1, 1j1j1j, 1j1j3j, 15j3j, and 06j3j, of which the first five are rather strong. Determination of the origin of these reflections has proven to be difficult. The behavior of the axial dimensions of the phases of 1 as a function of temperature deserves a comment (Figure 5). All axes

of the monoclinic polymorph have small positive thermal expansion coefficients. Upon the III f II transition at ∼147 K the a axis becomes slightly shorter in the orthorhombic phase, but then slowly lengthens, with a positive thermal expansion coefficient. The length of the b axis slightly increases in the monoclinic phase III between 100 and 147 K, then remains constant within the experimental uncertainty throughout the “orthorhombic” 160-395 K temperature range. The c axis has a positive linear thermal expansion coefficient in all three phases. Nothing atypical was observed for the thermal behavior of the axial dimensions of 1-III. All axial lengths marginally

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Figure 5. Temperature dependence of the axial lengths of the three polymorphs of 1. 100-147 K - phase III, 160-330 K - phase II, above 330 K - phase I.

Figure 6. Temperature dependence of β angle of the monoclinic polymorph of 1-III.

expand with increasing temperature. The unit cell volume increases, while the unique β angle narrows from 92.456(2)° at 100 K to 90° at 160 K, Figure 6. As the unique angle approaches 90° the area of the ac face parallelogram increases, an observation consistent with the thermal expansion of the unit cell. Molecular Geometry. In this section we will follow the changes in the molecular symmetry of 1 during the solid-state transformations. The highest attainable molecular symmetry of 1 is C2V. This symmetry, however, has been observed neither for 1, nor for any of the 32 crystallographically characterized congener clino-sandwich Ti complexes of general formula Ti(C5H4R)Cl2.4 Most of the 32 complexes and 1 possess either the C2 molecular symmetry coinciding with crystallographic symmetry, or molecular symmetry close to C2. The 2-fold axis bisects the Cl-Ti-Cl angle. There are five complexes with CS symmetry. A facile way of determining how close a particular molecule is to a certain geometry is by calculating an appropriate continuous symmetry measure S (CSM20). The value S ) 0 corresponds to the exact symmetry. The higher the numerical value of S the more substantial the deviation of the molecule from the idealized symmetry. Complex 1 resides on a 2-fold crystallographic axis in 1-I, but occupies a general position in the crystal structures of 1-II and 1-III. The CSM S values for the Ti complexes 1-I (385 K), 1-II (296 K), 1-II (175 K), and 1-III (100 K) are 0, 0.002, 0.014, and 0.028. Thus, the lower the temperature, the more distorted the approximate C2 geometry of the Ti complex becomes; nevertheless, in all cases the deviation is very small. CSM data for the 32 related Ti complexes are compiled in the Supporting Information.

Guzei et al.

The molecular parameters of 1-I, 1-II, and 1-III are very similar, Table 2, and agree well with the literature data. The complex possesses a pseudotetrahedral geometry about the Ti center; the tert-butyl substituents are located on the opposite sides of the centroid-Ti-centroid plane and outside the lateral Cl-Ti-Cl triangle, Figure 1. All Ti-C and Ti-Cl distances are normal. There are no significant conformational changes attributable to phase transitions or twinning. An examination of the crystal structures of 1 revealed that the differences in packing among the three are minor. The packing coefficients3 for 1-I (385 K), 1-II (296 K), 1-II (175 K), and 1-III (100 K) become progressively tighter at 66.2, 67.3, 69.4, and 70.5%, respectively. This is one of the reasons why the I-II k2 transformation does not distinctly manifest itself in the powder diffraction patterns. The metric molecular parameters of Ti(C5H4R)2Cl2 complexes can be characterized according to the nomenclature suggested by Grimmond et al.,21 Figure 7. We have tabulated a summary of the relevant parameters for the 32 related complexes reported to the Cambridge Structural Database.4 More details are available in the Supporting Information. The variations in the observed parameters reflect both the different nature of the compounds and their arrangement in the solid state as well as different experimental conditions of the single-crystal X-ray studies (e.g., temperature, different precision, and complex solvation). The Ti(C5H4R)2Cl2 complexes exemplify a relatively tight system with a delicate balance among the interligand interactions, Figure 7. Generally speaking, the larger the R ligand and the stronger its electron donor properties, the larger the ∆S parameter. The lateral ∆S shift is a characteristic of the η-ligation mode of the Ti center. Small values of ∆S correspond to the η5-coordination mode, whereas ∆S values between 0.69 and 0.79 Å correspond to the Cp ligand binding in the η3 fashion. In the 32 complexes at hand ∆S does not exceed 0.302 Å. The ∆R displacement is always positive (away from the metal center), but there is no direct correlation with the size of the ligand, because sterics is not the only factor influencing ∆R. Parameters D1 and D2 depend on the ring substitution and are tied to ∆S. A small change in the R angle frequently induces a change in the β angle and vice versa. It was observed22 that the X-M-X angle in complexes (η5-C5H5)2MX2 was dependent on the number of nonbonding electrons, and in the case of the metal d0 configuration was 94-97°. In that study the metals did not include titanium. However, our survey results concur; the Cl-Ti-Cl angles in the 32 Ti(C5H4R)2Cl2 complexes average 93.2(11)°. The θd angle varies dramatically and depends both on the R substituent and on packing effects in the crystal. Preferred Molecular Geometries of Complexes Ti(C5H4R)Cl2. The 16-electron disubstituted titanocene dichloride complexes exhibit pseudotetrahedral geometries, typical for clino-sandwich compounds. The mutual orientation of the ring R substituents can be characterized by their position relative to the Cl-Ti-Cl wedge and the centroid-Ti-centroid plane, Figure 8. All four types of geometries have been experimentally observed. We have analyzed the frequency of observation of these geometries in Ti(C5H4R)2Cl2 complexes where the R substituent is connected to the cyclopentadienyl ring via an sp3-hybridized carbon atom, Table 3. In the case of the primary carbon atom in complex Ti(C5H4Me)2Cl2 the geometry is “over-under”, Figure 8. When the substituent is connected to the Cp ring with a secondary carbon atom, any of the four geometries in the solid state may be observed; however, the over-under orientation is

Polymorphism of Ti(C5H4tBu)2Cl2

Crystal Growth & Design, Vol. 9, No. 5, 2009 2291 Table 1. Experimental Data Table for 1

Tmin, Tmax Rint R[F2 > 2σ(F2)], wR(F2), S no. of reflections no. of parameters H-atom treatment ∆Fmax, ∆Fmin (e Å-3) Flack parameter16 µ (mm-1) crystal size (mm) diffractometer absorption correction

0.715, 0.859 0.036 0.030, 0.083, 1.00 11624 197

0.732, 0.899 0.151, 0.168 0.171, 0.423 0.034 0.044 0.024 0.029, 0.069, 1.02 0.031, 0.086, 1.02 0.026, 0.074, 1.01 5111 3217 1782 197 197 100 H-atom parameters constrained 0.41, -0.24 0.34, -0.20 0.15, -0.19 0.20, -0.12 -0.009 (16) -0.032 (19) -0.003 (8) -0.002 (8) 0.78 0.77 6.6 6.5 0.46 × 0.20 × 0.20 0.43 × 0.21 × 0.14 0.46 × 0.44 × 0.42 0.42 × 0.40 × 0.16 Bruker CCD-1000 area detector Bruker SMART APEX2 area detector multiscan SADABS (Bruker-AXS, 2007) Table 2. Molecular Geometries of I and 32 Clino-Sandwich Ti(C5H4R)2Cl2 Compoundsa Ti-Cl, Å

range for 32 clino-sandwich Ti(C5H4R)2Cl2 compounds 1-III (100 K) 1-II (175 K) 1-II (296 K) 1-I (385 K) a

Ti-C, Å

∆S, Å

∆R, Å

R,°

β,°

2.330-2.399

2.308-2.528

0.000-0.302

0.001-0.296

129.68-133.47

90.82-95.94

2.3718(12) (avg.) 2.3714(6) (avg.) 2.3668(6) 2.3666(6)

2.3338-2.4837 2.331-2.4844 2.325-2.478 2.3269-2.486

0.074 (avg.) 0.123 (avg.) 0.150 0.137

0.2563 0.2563 0.263 0.269

131.02(3) 131.13(4) 131.06(9) 130.95(11)

92.262(16) 92.106(18) 92.61(3) 92.57(3)

The parameters are defined in Figure 7.

Figure 7. Selected geometrical characteristics of the Ti(C5H4R)2Cl2 complexes.

Figure 8. Classification of mutual R substituent positions in clinosandwich complexes and their highest idealized symmetry. Table 3. Relative Frequencies of the Observed Geometries of Ti(C5H4R)2Cl2 Complexes in the Solid State R CH3 CH2R1 CHR1R2 CR1R2R3

outside-outsideoutside-outsidetrans over-outside over-under cis 2 3 7

1 1 1

1 11 5

1

the most likely. With a tertiary R carbon the over-under geometry is still the most likely, yet it is not as likely to occur as with a secondary carbon. In the case of a quaternary atom the outside-outside-trans geometry predominates. We originally proposed the reasons for these observations to be twofold. First of all, as the size of the substituents increases the space above and below the Cl-Ti-Cl wedge becomes more crowded. Consequently, larger ligands are preferentially positioned outside the wedge to relieve steric congestion. Second, the primary, secondary, and tertiary carbon atoms bear H atoms that form attractive intramolecular C-H · · · Cl interactions with the two Cl atoms, stabilizing the over-under geometry. The steric conflict among the ligands may not play a significant role, however. We have characterized all Ti(C5H4R)2Cl2 compounds from the point of view of steric saturation of the metal coordination sphere by two methods. The first method involves computations

of the ligand solid angles and the extent of the shielding of the central metal by the ligands. The ligand solid angles are more objective ligand steric requirement descriptors than the familiar cone angles. The shielding percentage is represented by the G-parameter,23 which averages for the complexes with primary, secondary, tertiary, and quaternary carbon atoms to 88.6(2), 89.4(9), 91.3(17), and 92.2(4)%, respectively. The overall trend is toward higher metal shielding with larger numbers of substituents on the R carbon, but the differences are not statistically significant. The second method is to utilize the “buried volume”.24 The buried volume is defined as the volume of a sphere of an arbitrary radius (usually 3.0 Å) about the central metal occupied by coordinated and uncoordinated atoms. The average “buried” percentages of the 3.0 Å sphere for the complexes with primary, secondary, tertiary, and quaternary alpha carbons comprised 75.5(4), 76.0(5), 76.2(7), and 76.0(4)%, respectively. There is no obvious trend among these numbers. Thus, the ligand steric behavior in complexes of this type is comparable and is unlikely to control the molecular conformation. If steric factors play a role in the determination of the overall complex geometry, it is likely due to steric interactions among neighboring molecules in the solid state. In order to compare the relative stability of the outsideoutside-trans and over-under geometries we conducted DFT computations at the B3LYP/6-311+G(d) level with Gaussian0325 on three Ti(C5H4R)2Cl2 complexes with R ) Me, tBu, and CCl3. For the Me the over-under geometry is more stable by 1.04 kcal/mol. The over-under geometry was also more stable for the tert-butyl, but only by 0.57 kcal/mol. Taking into consideration the conclusion from our computations of the ligand steric parameters above this result does not seem surprising. In the case of -CCl3 the absence of any attractive intramolecular C-H · · · Cl interactions resulted in the outside-outside-trans arrangement being more stable by 1.49 kcal/mol. This is another argument that interligand electronic interactions in Ti(C5H4R)2Cl2 complexes principally define the overall complex geometry. Conclusions We have uncovered three polymorphic phases of Ti(C5H4tBu)2Cl2 and two solid-state phase transitions between them. The transitions are reproducible, nondestructive, and

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reversible. The low-temperature (147 K) transition between the orthorhombic and non-merohedrally twinned monoclinic phases is first order, while the transformation between two orthorhombic phases is second order and occurs over a wide temperature range (150-330 K). The phase changes are accompanied by insignificant changes in molecular geometries and crystal packing. An analysis of related Ti(C5H4R)2Cl2 complexes indicates that their overall geometry is primarily defined by attractive intramolecular interligand interactions whereas steric factors play a lesser role. Experimental Procedures Synthesis. Compound 1 was purchased from Strem Chemicals and used without further purification. Crystals of compound 1 were grown by slow evaporation of a saturated toluene solution with a few drops of hexane under nitrogen at room temperature. The NMR spectrum was recorded on a Bruker AC-300 spectrometer and referenced to residual solvent peaks. 1H NMR (RT, C6D6, ppm): 1.30 (s, 18 H, C(CH3)2), 5.83 (t, 4 H, J ) 3.1, 2.3 Hz, CH), 6.15 (t, 4 H, J ) 2.8, 2.8 Hz, CH). Instrumentation. Two instruments were used for data acquisition: a Bruker-AXS APEXII diffractometer equipped with a sealed tube Cu KR radiation source and an Oxford Cryostream 700 cooling device and a Bruker-AXS SMART-1000 diffractometer with a sealed Mo tube and a Bruker Kryoflex cooling device. The Oxford Cryostream on the former was carefully calibrated in the range 100-400 K with a Si diode model DT-421-HR-4 L (DT-421 miniature silicon diode) and temperature monitor model 211, both from Lakeshore Cryotronics, Inc. The Bruker Kryoflex was calibrated in the 100-296 K range. X-ray Data Collection. Full-sphere data sets were collected on four different crystals at 100, 175, 296, and 385 K. The full data collection strategies employed 0.5° ω and φ scans on the APEXII instrument and 0.3° ω and φ scans on the SMART CCD-1000 system, Table 1. The unit cell determinations at 100, 110, 120, 130, 140, 150, 160, 200, 240, 280, 290, 295, 300, 305, 310, 315, 320, 325, 330, 340, 360, 380, and 395 K were based on three 80-degree (2θ) runs with 0.6° ω scans. A transparent red crystal was attached to the tip of a glass fiber with two-component epoxy glue. A fifth crystal was used for the 23 unit cell data determinations between 100 and 395 K. The unit cell data acquisitions were all conducted in one nonstop experiment in automated mode with Bruker APEXII software that allowed automatic temperature adjustments and equilibration delays to allow the crystal to acclimate at the new temperature for 20 min. Data Processing and Refinement. The data were integrated with SAINT,26 corrected for absorption either with SADABS (2008/1)27 or with TWINABS (2007/5)2 as appropriate, and solved and refined with SHELXTL.28 Molecular drawings were prepared with DIAMOND.29 The structures were refined by standard techniques, including the use of the TWIN/BASF instructions. The nonstandard axis setting was used in the case of the monoclinic twinned crystal in order to facilitate our discussion of unit cell transformations. In the case of the twinned data set no racemic twinning was detected in addition to the nonmerohedral twinning. All non-hydrogen atoms were refined with anisotropic displacement coefficients. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. In the Supporting Information we provide a detailed procedure describing how to compute to standard uncertainties for derivative metric parameters using SHELXL. Our procedure is a slight modification of the original procedure designed by James Fettinger.30

Acknowledgment. The authors gratefully acknowledge invaluable intellectual and experimental contributions of Prof.

Guzei et al.

Ha˚kon Hope (University of California-Davis) to this project. We thank Dr. L. Zhang and Y. Sun (University of WisconsinMadison) for conducting the DSC experiments, and Dr. H. Cordes (Bruker-AXS) for conducting a variable temperature powder diffraction experiment. We are grateful to Prof. Bruce M. Foxman (Brandeis University) and Dr. Victor G. Young, Jr. (University of Minnesota) for numerous fruitful discussions on twinning and phase transitions. Supporting Information Available: A detailed procedure for computation of standard uncertainties for derivative metric parameters using SHELXL. Table S1: CSD data mining results for geometries of Ti(C5H4R)2Cl2 complexes where the R substituent is connected to the cyclopentadienyl ring via an sp3-hybridized carbon atom; crystallographic information files for the three polymorphs of 1 characterized at 100, 175, 296, and 385 K. This information is available free of charge via the Internet at http://pubs.acs.org.

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