The 1:1 charge-transfer complex dibenzo­tetra­thia­fulvalene–pyromellitic dianhydride (DBTTF–PMDA)

Payne, M.E.; Goetz, K.P.; Day, C.S.; Jurchescu, O.D.

doi:10.1107/S1600536814013324

organic compounds

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ISSN: 2056-9890

Volume 70| Part 8| August 2014| Pages o844-o845

doi:10.1107/S1600536814013324

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The 1:1 charge-transfer complex dibenzotetrathiafulvalene–pyromellitic dianhydride (DBTTF–PMDA)

Margaret E. Payne,^a Katelyn P. Goetz,^a Cynthia S. Day ^b ^* and Oana D. Jurchescu ^a ^*

^aDepartment of Physics, Wake Forest University, Winston-Salem, NC 27109, USA, and ^bDepartment of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA
^*Correspondence e-mail: daycs@wfu.edu, jurchescu@wfu.edu

(Received 21 May 2014; accepted 7 June 2014; online 2 July 2014)

The title charge-transfer (CT) complex, C₁₀H₂O₆·C₁₄H₈S₄, composed of donor dibenzotetrathiafulvalene (DBTTF) and acceptor pyromellitic dianhydride (PMDA), forms a mixed stacking pattern along the [-110] direction. The constituent molecules occupy crystallographic inversion centers. They are nearly parallel and lie ca.3.41 Å from each other. The crystals exhibit a high degree of donor/acceptor overlap [88.20 (4)%] in the long direction of the DBTTF and PMDA molecules as compared with 51.27 (5)% in the shortest direction of the molecules.

CCDC reference: 1007162

Related literature

General properties and potential applications of charge-transfer complexes in electronic devices are outlined by Goetz et al. (2014 ); Horiuchi et al. (2006 ); Tsutsumi et al. (2012 ); Kobayashi et al. (2012 ); Kagawa et al. (2010 ); Herbstein (2005 ); Ferraris et al. (1973 ); Kistenmacher et al. (1981 ); Takahashi et al. (2006 ); Wu et al. (2013 ). Related CT structures, containing the acceptor pyromellitic dianhydride (PMDA) include anthracene–PMDA (Robertson & Stezowski, 1978 ), phenanthrene–PMDA (Evans & Robinson, 1977 ), pyrene–PMDA (Herbstein & Snyman, 1969 ) and two polymorphs of biphenylene–PMDA (Stezowski et al., 1986 ). Structure–property relationships in molecular crystals have been described theoretically by Coropceanu et al. (2007 ) and experimentally by Mei et al. (2013 ), among others.

Experimental

Crystal data

C₁₀H₂O₆·C₁₄H₈S₄
M_r = 522.56
Triclinic, $[P \overline 1]$
a = 7.2292 (4) Å
b = 8.9572 (5) Å
c = 9.5224 (5) Å
α = 70.051 (1)°
β = 68.712 (1)°
γ = 70.136 (1)°
V = 523.39 (5) Å³
Z = 1
Mo Kα radiation
μ = 0.50 mm⁻¹
T = 213 K
0.20 × 0.20 × 0.02 mm

Data collection

Bruker APEX CCD diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 2012 ) T_min = 0.703, T_max = 0.746
10004 measured reflections
3023 independent reflections
2668 reflections with I > 2σ(I)
R_int = 0.021

Refinement

R[F² > 2σ(F²)] = 0.032
wR(F²) = 0.091
S = 1.07
3023 reflections
154 parameters
H-atom parameters constrained
Δρ_max = 0.30 e Å⁻³
Δρ_min = −0.33 e Å⁻³

Data collection: SMART (Bruker, 2002 ); cell refinement: SAINT (Bruker, 2011 ); data reduction: SAINT; program(s) used to solve structure: SHELXLS2013 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

CCDC reference: 1007162

Crystal structure: contains datablock I. DOI: 10.1107/S1600536814013324/pk2526sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536814013324/pk2526Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536814013324/pk2526Isup3.cml

checkCIF report

Comment top

Organic charge-transfer (CT) complexes are combinations of electron donor (D) and electron acceptor (A) materials. They have been studied for decades, but have attracted significant interest recently due to their intriguing properties such as photoconductivity, tunable semiconductivity, metallicity, ferroelectricity, etc., which make them viable candidates for versatile electronic devices (Goetz et al., 2014; Horiuchi et al., 2006; Tsutsumi et al., 2012; Kobayashi et al., 2012; Kagawa et al., 2010). In the 1:1 D:A stoichiometry, they can exhibit either mixed stacking, where the repeating motif in the π-stacking direction is ···D—A—D—A···, or segregated stacking, where the donor and acceptor π-stack separately, as ···A—A—A—A··· and ···D—D—D—D··· (Herbstein, 2005). CT complexes of the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) have been widely explored. Examples include the organic metal with donor tetrathiafulvalene (TTF) (Ferraris et al., 1973) or the ambipolar semiconductor with dibenzotetrathiafulvalene (DBTTF) (Kistenmacher et al., 1981; Takahashi et al., 2006; Wu et al., 2013). With the exception of a few reports, CT complexes containing pyromellitic dianhydride (PMDA) as an acceptor have received little attention. Examples of CT complexes of PMDA include anthracene-PMDA (Robertson & Stezowski, 1978), phenanthrene-PMDA (Evans & Robinson, 1977), pyrene-PMDA (Herbstein & Snyman, 1969), and two polymorphs of biphenylene-PMDA (Stezowski et al., 1986). Here we report for the first time on the growth and crystal structure of the 1:1 CT complex containing the donor DBTTF and acceptor PMDA.

Single crystals of DBTTF-PMDA are triclinic, space group P1, with Z=1, where the DBTTF and PMDA molecules occupy crystallographic inversion centers at (0,0,0) and (1/2,1/2,0), respectively. The crystals are platelets, with their largest face corresponding to the (001) plane. The molecular structure is shown with thermal ellipsoids in Figure 1, where only the contents of the asymmetric unit are labeled. The DBTTF and PMDA molecules pack in a mixed-stack pattern, previously observed in other PMDA-based CT complexes. As shown in Figure 2, the DBTTF molecules lie on the corners of the unit cell, while the PMDA molecules lie in the center of the ab crystal faces. The mixed DA stacks build along the [-1 1 0] direction and are tilted by 45.43 (6)° (DBTTF) and 46.40 (6)° (PMDA) with respect to ab face. This tilt leads to a molecular overlap between the donor and acceptor wherein the fused 5-and 6-membered rings of each half DBTTF overlap ("straddle") the 3 fused rings of the PMDA molecules in the stack (Figure 3). The centroid of the central PMDA 6-membered ring to centroid of the 5- and 6-membered rings of the adjacent DBTTF molecules in the stack are 3.648 (1)Å and 3.585 (1)Å, respectively. The shortest centroid-centroid contact involving 5-membered rings of the DBTTF and PMDA molecules is 3.611 (1)Å while the shortest contacts involving the DBTTF 6-membered ring centroid are 3.527 (1)Å and 3.538 (1)Å to the centroids of the 5-membered PMDA and 6-membered DBTTF rings, respectively. The planes of the D/A molecules are nearly parallel with an interplanar angle of 1.31 (5)° and the long axes of the DBTTF and PMDA are also nearly parallel [1.8 (1)°]. The PMDA and DBTTF are symmetrically-spaced within each stack with an intermolecular separation of 3.408 (1)Å. This differs from anthracene-PMDA, where the DA spacing alternates between 3.32Å and 3.4Å (Robertson & Stezowski, 1978). There is a high degree of overlap between donor and acceptor molecules: the DBTTF molecule overlaps with 88.20 (4)% of the PMDA molecule in the longest direction of the molecule, and with 51.27 (5)% of the PMDA molecule in the shortest direction of the molecule. Measurements are in progress to evaluate the degree of charge transfer between the two moieties; however, this high degree of overlap suggests that a high value is to be expected. The large molecular overlap is a signature of good electrical properties, as suggested by theoretical (Coropceanu et al., 2007) and experimental studies (Mei et al., 2013).

Related literature top

General properties and potential applications of charge-transfer complexes in electronic devices are outlined by Goetz et al. (2014); Horiuchi et al. (2006); Tsutsumi et al. (2012); Kobayashi et al. (2012); Kagawa et al. (2010); Herbstein (2005); Ferraris et al. (1973); Kistenmacher et al. (1981); Takahashi et al. (2006); Wu et al. (2013). Related CT structures, containing the acceptor pyromellitic dianhydride (PMDA) include anthracene–PMDA (Robertson & Stezowski, 1978), phenanthrene–PMDA (Evans & Robinson, 1977), pyrene–PMDA (Herbstein & Snyman, 1969) and two polymorphs of biphenylene–PMDA (Stezowski et al., 1986). Structure–property relationships in molecular crystals have been described theoretically by Coropceanu et al. (2007) and experimentally by Mei et al. (2013), among others.

Experimental top

Dibenzotetrathiafulvalene (DBTTF) and pyromellitic dianhydride (PMDA), both obtained from Sigma Aldrich, were separately dissolved in xylenes and acetonitrile, respectively. The solid weights of the compounds were measured in the molar ratio 1:1. The solution concentrations were saturated, such that all of parent compound dissolved in as little solvent as possible. The solutions were mixed, and the complex was then crystallized by slow evaporation under ambient conditions. After about two days of evaporation, crystals were obtained as green-gold plates with approximate dimensions of 0.20 mm x 0.20 mm x 0.02 mm.

Computing details top

Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXLS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. The atom numbering scheme for DBTTF-PMDA with non-hydrogen atoms represented by 50% probability ellipsoids. Each molecule occupies an inversion center in the unit cell and only contents of the asymmetric unit are labeled.
	Fig. 2. Molecular packing viewed down the a axis (a) and the view across a stack of donor and acceptor molecules (b) with the six-membered rings vertically aligned.
	Fig. 3. Projection perpendicular to plane of DBTTF with PMDA (above - open bonds) (below - dashed open bonds) showing overlap of rings.

Dibenzotetrathiafulvalene–pyromellitic dianhydride (1/1) top

Crystal data top

C₁₀H₂O₆·C₁₄H₈S₄	Z = 1
M_r = 522.56	F(000) = 266
Triclinic, P1	D_x = 1.658 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.2292 (4) Å	Cell parameters from 5268 reflections
b = 8.9572 (5) Å	θ = 3.5–31.3°
c = 9.5224 (5) Å	µ = 0.50 mm⁻¹
α = 70.051 (1)°	T = 213 K
β = 68.712 (1)°	Plate, green-gold
γ = 70.136 (1)°	0.20 × 0.20 × 0.02 mm
V = 523.39 (5) Å³

Data collection top

Bruker APEX CCD diffractometer	3023 independent reflections
Radiation source: sealed x-ray tube	2668 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.021
ϕ and ω scans	θ_max = 30.0°, θ_min = 3.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 2012)	h = −10→10
T_min = 0.703, T_max = 0.746	k = −12→12
10004 measured reflections	l = −13→13

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.032	H-atom parameters constrained
wR(F²) = 0.091	w = 1/[σ²(F_o²) + (0.0529P)² + 0.115P] where P = (F_o² + 2F_c²)/3
S = 1.07	(Δ/σ)_max < 0.001
3023 reflections	Δρ_max = 0.30 e Å⁻³
154 parameters	Δρ_min = −0.33 e Å⁻³
0 restraints

Crystal data top

C₁₀H₂O₆·C₁₄H₈S₄	γ = 70.136 (1)°
M_r = 522.56	V = 523.39 (5) Å³
Triclinic, P1	Z = 1
a = 7.2292 (4) Å	Mo Kα radiation
b = 8.9572 (5) Å	µ = 0.50 mm⁻¹
c = 9.5224 (5) Å	T = 213 K
α = 70.051 (1)°	0.20 × 0.20 × 0.02 mm
β = 68.712 (1)°

Data collection top

Bruker APEX CCD diffractometer	3023 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 2012)	2668 reflections with I > 2σ(I)
T_min = 0.703, T_max = 0.746	R_int = 0.021
10004 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.032	0 restraints
wR(F²) = 0.091	H-atom parameters constrained
S = 1.07	Δρ_max = 0.30 e Å⁻³
3023 reflections	Δρ_min = −0.33 e Å⁻³
154 parameters

Special details top

Experimental. Absorption correction: data were corrected for scaling and absorption effects using the multi-scan technique [SADABS (Sheldrick, 2012)]. The ratio of minimum to maximum apparent transmission was 0.942.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
S1	−0.06704 (4)	0.15387 (4)	0.15691 (4)	0.02936 (10)
S2	−0.31329 (5)	−0.02880 (4)	0.11851 (4)	0.03025 (10)
C1	−0.08003 (17)	0.02644 (15)	0.05769 (14)	0.0250 (2)
C2	−0.32321 (18)	0.17849 (14)	0.27098 (14)	0.0245 (2)
C3	−0.43823 (18)	0.09185 (14)	0.25298 (14)	0.0246 (2)
C4	−0.64247 (19)	0.10259 (16)	0.34069 (16)	0.0291 (3)
H4	−0.7200	0.0443	0.3290	0.035*
C5	−0.7294 (2)	0.20070 (17)	0.44543 (16)	0.0333 (3)
H5	−0.8672	0.2096	0.5042	0.040*
C6	−0.6146 (2)	0.28587 (17)	0.46427 (15)	0.0332 (3)
H6	−0.6754	0.3511	0.5362	0.040*
C7	−0.4108 (2)	0.27550 (16)	0.37773 (15)	0.0292 (3)
H7	−0.3334	0.3329	0.3909	0.035*
O1	0.08852 (14)	0.63648 (12)	0.27821 (12)	0.0338 (2)
O2	0.30802 (18)	0.75698 (14)	0.29107 (13)	0.0441 (3)
O3	−0.05194 (14)	0.48790 (13)	0.21480 (13)	0.0395 (2)
C8	0.2784 (2)	0.67350 (16)	0.23163 (15)	0.0301 (3)
C9	0.41355 (18)	0.59058 (14)	0.10654 (14)	0.0237 (2)
C10	0.30252 (17)	0.50435 (14)	0.08434 (14)	0.0237 (2)
C11	0.09415 (18)	0.53456 (16)	0.19353 (15)	0.0282 (2)
C12	0.61565 (17)	0.58997 (14)	0.02281 (14)	0.0252 (2)
H12	0.6905	0.6484	0.0375	0.030*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.02281 (15)	0.03348 (17)	0.03676 (18)	−0.00701 (12)	−0.00525 (12)	−0.01796 (14)
S2	0.02316 (15)	0.03528 (18)	0.03745 (19)	−0.00885 (12)	−0.00331 (12)	−0.01913 (14)
C1	0.0214 (5)	0.0260 (5)	0.0281 (6)	−0.0048 (4)	−0.0051 (4)	−0.0102 (5)
C2	0.0249 (5)	0.0227 (5)	0.0237 (5)	−0.0048 (4)	−0.0055 (4)	−0.0057 (4)
C3	0.0234 (5)	0.0239 (5)	0.0246 (5)	−0.0043 (4)	−0.0059 (4)	−0.0062 (4)
C4	0.0249 (5)	0.0291 (6)	0.0303 (6)	−0.0072 (5)	−0.0050 (5)	−0.0060 (5)
C5	0.0282 (6)	0.0341 (7)	0.0274 (6)	−0.0061 (5)	0.0008 (5)	−0.0063 (5)
C6	0.0378 (7)	0.0298 (6)	0.0255 (6)	−0.0067 (5)	−0.0007 (5)	−0.0094 (5)
C7	0.0347 (6)	0.0266 (6)	0.0261 (6)	−0.0084 (5)	−0.0059 (5)	−0.0083 (5)
O1	0.0265 (4)	0.0378 (5)	0.0343 (5)	−0.0065 (4)	0.0015 (4)	−0.0179 (4)
O2	0.0466 (6)	0.0508 (6)	0.0436 (6)	−0.0154 (5)	−0.0036 (5)	−0.0288 (5)
O3	0.0246 (5)	0.0465 (6)	0.0462 (6)	−0.0133 (4)	−0.0009 (4)	−0.0153 (5)
C8	0.0295 (6)	0.0318 (6)	0.0280 (6)	−0.0075 (5)	−0.0028 (5)	−0.0119 (5)
C9	0.0242 (5)	0.0249 (5)	0.0228 (5)	−0.0061 (4)	−0.0051 (4)	−0.0082 (4)
C10	0.0204 (5)	0.0253 (5)	0.0239 (5)	−0.0065 (4)	−0.0048 (4)	−0.0050 (4)
C11	0.0234 (5)	0.0288 (6)	0.0287 (6)	−0.0054 (4)	−0.0031 (4)	−0.0084 (5)
C12	0.0239 (5)	0.0270 (6)	0.0274 (6)	−0.0082 (4)	−0.0065 (4)	−0.0086 (5)

Geometric parameters (Å, º) top

S1—C1	1.7543 (13)	C6—H6	0.9400
S1—C2	1.7560 (12)	C7—H7	0.9400
S2—C3	1.7505 (13)	O1—C11	1.3923 (16)
S2—C1	1.7567 (12)	O1—C8	1.3993 (16)
C1—C1ⁱ	1.353 (2)	O2—C8	1.1878 (17)
C2—C7	1.3967 (17)	O3—C11	1.1907 (16)
C2—C3	1.3985 (17)	C8—C9	1.4800 (17)
C3—C4	1.3963 (16)	C9—C12	1.3864 (16)
C4—C5	1.3892 (19)	C9—C10	1.3933 (16)
C4—H4	0.9400	C10—C12ⁱⁱ	1.3839 (16)
C5—C6	1.391 (2)	C10—C11	1.4836 (16)
C5—H5	0.9400	C12—C10ⁱⁱ	1.3838 (16)
C6—C7	1.3913 (19)	C12—H12	0.9400

C1—S1—C2	95.38 (6)	C6—C7—C2	118.93 (13)
C3—S2—C1	95.34 (6)	C6—C7—H7	120.5
C1ⁱ—C1—S1	122.00 (13)	C2—C7—H7	120.5
C1ⁱ—C1—S2	122.26 (13)	C11—O1—C8	110.17 (10)
S1—C1—S2	115.74 (6)	O2—C8—O1	121.25 (13)
C7—C2—C3	120.37 (11)	O2—C8—C9	131.52 (13)
C7—C2—S1	123.12 (10)	O1—C8—C9	107.23 (11)
C3—C2—S1	116.50 (9)	C12—C9—C10	122.97 (11)
C4—C3—C2	120.30 (12)	C12—C9—C8	129.32 (11)
C4—C3—S2	122.83 (10)	C10—C9—C8	107.70 (10)
C2—C3—S2	116.88 (9)	C12ⁱⁱ—C10—C9	123.01 (10)
C5—C4—C3	119.04 (12)	C12ⁱⁱ—C10—C11	129.51 (11)
C5—C4—H4	120.5	C9—C10—C11	107.48 (11)
C3—C4—H4	120.5	O3—C11—O1	121.29 (12)
C4—C5—C6	120.69 (12)	O3—C11—C10	131.31 (13)
C4—C5—H5	119.7	O1—C11—C10	107.40 (10)
C6—C5—H5	119.7	C10ⁱⁱ—C12—C9	114.03 (10)
C5—C6—C7	120.66 (13)	C10ⁱⁱ—C12—H12	123.0
C5—C6—H6	119.7	C9—C12—H12	123.0
C7—C6—H6	119.7

C2—S1—C1—C1ⁱ	176.20 (15)	C11—O1—C8—O2	−178.78 (13)
C2—S1—C1—S2	−4.14 (8)	C11—O1—C8—C9	0.82 (14)
C3—S2—C1—C1ⁱ	−176.37 (15)	O2—C8—C9—C12	−0.6 (2)
C3—S2—C1—S1	3.97 (8)	O1—C8—C9—C12	179.86 (12)
C1—S1—C2—C7	−178.74 (11)	O2—C8—C9—C10	178.37 (15)
C1—S1—C2—C3	2.71 (10)	O1—C8—C9—C10	−1.17 (14)
C7—C2—C3—C4	0.54 (18)	C12—C9—C10—C12ⁱⁱ	0.2 (2)
S1—C2—C3—C4	179.13 (9)	C8—C9—C10—C12ⁱⁱ	−178.82 (11)
C7—C2—C3—S2	−179.00 (9)	C12—C9—C10—C11	−179.90 (11)
S1—C2—C3—S2	−0.41 (13)	C8—C9—C10—C11	1.05 (13)
C1—S2—C3—C4	178.35 (11)	C8—O1—C11—O3	179.96 (12)
C1—S2—C3—C2	−2.12 (10)	C8—O1—C11—C10	−0.18 (14)
C2—C3—C4—C5	0.16 (18)	C12ⁱⁱ—C10—C11—O3	−0.9 (2)
S2—C3—C4—C5	179.68 (10)	C9—C10—C11—O3	179.27 (14)
C3—C4—C5—C6	−0.7 (2)	C12ⁱⁱ—C10—C11—O1	179.29 (12)
C4—C5—C6—C7	0.5 (2)	C9—C10—C11—O1	−0.57 (13)
C5—C6—C7—C2	0.2 (2)	C10—C9—C12—C10ⁱⁱ	−0.21 (19)
C3—C2—C7—C6	−0.73 (18)	C8—C9—C12—C10ⁱⁱ	178.62 (12)
S1—C2—C7—C6	−179.23 (10)

Symmetry codes: (i) −x, −y, −z; (ii) −x+1, −y+1, −z.

Experimental details

Crystal data
Chemical formula	C₁₀H₂O₆·C₁₄H₈S₄
M_r	522.56
Crystal system, space group	Triclinic, P1
Temperature (K)	213
a, b, c (Å)	7.2292 (4), 8.9572 (5), 9.5224 (5)
α, β, γ (°)	70.051 (1), 68.712 (1), 70.136 (1)
V (Å³)	523.39 (5)
Z	1
F(000)	266
Radiation type	Mo Kα
No. of reflections for cell measurement	5268
θ range (°) for cell measurement	3.5–31.3
µ (mm⁻¹)	0.50
Crystal size (mm)	0.20 × 0.20 × 0.02

Data collection
Diffractometer	Bruker APEX CCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 2012)
T_min, T_max	0.703, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	10004, 3023, 2668
R_int	0.021
(sin θ/λ)_max (Å⁻¹)	0.704

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.032, 0.091, 1.07
No. of reflections	3023
No. of parameters	154
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.30, −0.33

Computer programs: SMART (Bruker, 2002), SAINT (Bruker, 2011), SHELXLS2013 (Sheldrick, 2008), SHELXL2013 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

The WFU X-ray Facility thanks the National Science Foundation (grant CHE-0234489) for funds to purchase the X-ray instrument and computers. This work has been partly supported by the National Science Foundation grant DMR-1105147. KPG acknowledges the NSF Graduate Research Fellowship Program under grant DGE-0907738.

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