4-[(1E,3E,5E)-6-(4-Pyrid­yl)hexa-1,3,5-trien­yl]pyridine

Bader, M.M.

doi:10.1107/S1600536809029092

organic compounds

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Volume 65| Part 8| August 2009| Page o2006

doi:10.1107/S1600536809029092

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4-[(1E,3E,5E)-6-(4-Pyridyl)hexa-1,3,5-trienyl]pyridine

Mamoun M. Bader ^a ^*

^aDepartment of Chemistry, Pennsylvania State University at Hazleton, 76 University Drive, Hazleton, PA 18202, USA
^*Correspondence e-mail: mmb11@psu.edu

(Received 19 July 2009; accepted 22 July 2009; online 29 July 2009)

The two independent molecules in the asymmetric unit of the title compound, C₁₆H₁₄N₂, are planar [dihedral angle between the terminal pyridine rings = 1.76 (2)°] and each display an all-trans configuration of C=C double bonds. One of the two molecules lies about a center of inversion. The dihedral angle between the two pyridine rings in the molecule lying on a general position is 1.65 (2)°.

Related literature

For acceptor-terminated polyenes, see: Gao et al. (2004 ). For the synthesis, see: Woitellier et al. (1989 ). For a related structure, see: Pham (2009 ).

Experimental

Crystal data

C₁₆H₁₄N₂
M_r = 234.29
Monoclinic, P 2₁ /n
a = 5.837 (1) Å
b = 17.171 (4) Å
c = 19.227 (4) Å
β = 97.685 (4)°
V = 1909.8 (7) Å³
Z = 6
Mo Kα radiation
μ = 0.07 mm⁻¹
T = 173 K
0.44 × 0.24 × 0.22 mm

Data collection

Bruker SMART Platform CCD diffractometer
Absorption correction: none
18771 measured reflections
3366 independent reflections
2460 reflections with I > 2σ(I)
R_int = 0.028

Refinement

R[F² > 2σ(F²)] = 0.032
wR(F²) = 0.089
S = 1.01
3366 reflections
328 parameters
All H-atom parameters refined
Δρ_max = 0.09 e Å⁻³
Δρ_min = −0.15 e Å⁻³

Data collection: SMART (Bruker, 2001 ); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SIR2002 (Burla et al., 2003 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809029092/ng2615sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809029092/ng2615Isup2.hkl

checkCIF report

Comment top

Pyridyl-terminated polyenes have been investigated for studying electron reactions with applications in biology, inorganic reaction mechanisms and molecular electronics. They have also been used in the synthesis of coordination polymers as well as template for solid state reactions.

For related systems of acceptor terminated polyenes, see: Gao et al. (2004); Pham (2009). For literature related to the synthesis, see: Woitellier et al. (1989).

Related literature top

For acceptor-terminated polyenes, see: Gao et al. (2004). For the synthesis, see: Woitellier et al. (1989). For a related structure, see: Pham (2009).

Experimental top

Synthesis was carried out following literature procedures (Woitellier et al., 1989) as follows: a solution of potassium tert-butoxide (1 2. l g in 200 ml of glyme) was added dropwise to a solution of tetraethyl ((E)-2-butene-1,4-diyl)diphosphonate (16.4 g, 0.05 mol) and pyridine-4-carboxaldehyde (10.70 g, 0.10 mol) in 100 ml of glyme at room temperature. After the addition of the base was complete, the resulting mixture was stirred at room temperature for 16 h. Cold water (ca 300–400 ml) was added and the product isolated by vacuum filtration (3.5 g, 30% crude yield) and recrystallized from acetone (2.3 g, 19%): mp 196–197. Crystals were grown from DMF.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SIR2002 (Burla et al., 2003); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top

	Fig. 1. Ellipsoid plot
	Fig. 2. 4-((1E,3E,5E)-6-(pyridin-4-yl)hexa-1,3,5-trienyl)pyridine.
	Fig. 3. Crystal packing viewed along the a axis.

4-[(1E,3E,5E)-6-(4-Pyridyl)hexa-1,3,5-trienyl]pyridine top

Crystal data top

C₁₆H₁₄N₂	F(000) = 744
M_r = 234.29	D_x = 1.222 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 867 reflections
a = 5.837 (1) Å	θ = 2.5–27.5°
b = 17.171 (4) Å	µ = 0.07 mm⁻¹
c = 19.227 (4) Å	T = 173 K
β = 97.685 (4)°	Block, yellow
V = 1909.8 (7) Å³	0.44 × 0.24 × 0.22 mm
Z = 6

Data collection top

Bruker SMART Platform CCD diffractometer	2460 reflections with I > 2σ(I)
Radiation source: normal-focus sealed tube	R_int = 0.028
Graphite monochromator	θ_max = 25.0°, θ_min = 1.6°
ω scans	h = −6→6
18771 measured reflections	k = −20→20
3366 independent reflections	l = −22→22

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.089	All H-atom parameters refined
S = 1.01	w = 1/[σ²(F_o²) + (0.048P)² + 0.1535P] where P = (F_o² + 2F_c²)/3
3366 reflections	(Δ/σ)_max = 0.001
328 parameters	Δρ_max = 0.09 e Å⁻³
0 restraints	Δρ_min = −0.15 e Å⁻³

Crystal data top

C₁₆H₁₄N₂	V = 1909.8 (7) Å³
M_r = 234.29	Z = 6
Monoclinic, P2₁/n	Mo Kα radiation
a = 5.837 (1) Å	µ = 0.07 mm⁻¹
b = 17.171 (4) Å	T = 173 K
c = 19.227 (4) Å	0.44 × 0.24 × 0.22 mm
β = 97.685 (4)°

Data collection top

Bruker SMART Platform CCD diffractometer	2460 reflections with I > 2σ(I)
18771 measured reflections	R_int = 0.028
3366 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.032	0 restraints
wR(F²) = 0.089	All H-atom parameters refined
S = 1.01	Δρ_max = 0.09 e Å⁻³
3366 reflections	Δρ_min = −0.15 e Å⁻³
328 parameters

Special details top

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	U_iso*/U_eq
N1	0.5720 (2)	0.58755 (7)	−0.07734 (6)	0.0569 (3)
N2	−0.45576 (19)	1.10512 (6)	0.36976 (5)	0.0481 (3)
N3	−0.00550 (19)	1.24674 (6)	0.23495 (6)	0.0497 (3)
C1	0.6991 (3)	0.65228 (9)	−0.06772 (8)	0.0543 (4)
H1	0.838 (3)	0.6547 (8)	−0.0931 (7)	0.062 (4)*
C2	0.6494 (2)	0.71361 (8)	−0.02588 (7)	0.0462 (3)
H2	0.751 (2)	0.7581 (8)	−0.0194 (7)	0.052 (4)*
C3	0.4553 (2)	0.71081 (7)	0.00886 (6)	0.0388 (3)
C4	0.3222 (2)	0.64283 (7)	−0.00011 (7)	0.0431 (3)
H4	0.185 (2)	0.6363 (7)	0.0227 (6)	0.044 (3)*
C5	0.3870 (3)	0.58440 (8)	−0.04271 (7)	0.0511 (4)
H5	0.293 (2)	0.5370 (8)	−0.0495 (6)	0.056 (4)*
C6	−0.5830 (2)	1.04076 (8)	0.35549 (7)	0.0479 (3)
H6	−0.722 (3)	1.0359 (8)	0.3800 (7)	0.060 (4)*
C7	−0.5305 (2)	0.98292 (8)	0.31014 (7)	0.0445 (3)
H7	−0.628 (2)	0.9374 (8)	0.3018 (6)	0.049 (4)*
C8	−0.3348 (2)	0.98987 (7)	0.27589 (6)	0.0385 (3)
C9	−0.2039 (2)	1.05746 (7)	0.29001 (6)	0.0411 (3)
H9	−0.064 (2)	1.0657 (7)	0.2688 (6)	0.041 (3)*
C10	−0.2692 (2)	1.11169 (8)	0.33646 (7)	0.0449 (3)
H10	−0.176 (2)	1.1575 (7)	0.3472 (6)	0.044 (3)*
C11	−0.1535 (2)	1.23391 (8)	0.17687 (7)	0.0487 (3)
H11	−0.298 (2)	1.2632 (7)	0.1723 (7)	0.055 (4)*
C12	−0.1104 (2)	1.18348 (8)	0.12416 (7)	0.0453 (3)
H12	−0.223 (2)	1.1766 (8)	0.0831 (7)	0.057 (4)*
C13	0.0985 (2)	1.14333 (7)	0.12916 (6)	0.0384 (3)
C14	0.2518 (2)	1.15541 (7)	0.19037 (7)	0.0415 (3)
H14	0.401 (2)	1.1294 (7)	0.1986 (6)	0.049 (4)*
C15	0.1931 (2)	1.20645 (8)	0.24030 (7)	0.0473 (3)
H15	0.300 (2)	1.2158 (8)	0.2839 (7)	0.059 (4)*
C16	0.3959 (2)	0.77741 (7)	0.05039 (6)	0.0414 (3)
H16	0.504 (2)	0.8205 (7)	0.0528 (6)	0.040 (3)*
C17	0.2082 (2)	0.78441 (7)	0.08307 (6)	0.0404 (3)
H17	0.100 (2)	0.7412 (7)	0.0822 (6)	0.043 (3)*
C18	0.1516 (2)	0.85257 (7)	0.12091 (6)	0.0432 (3)
H18	0.260 (2)	0.8965 (7)	0.1235 (6)	0.049 (4)*
C19	−0.0351 (2)	0.85901 (7)	0.15427 (7)	0.0431 (3)
H19	−0.140 (2)	0.8147 (7)	0.1538 (6)	0.047 (4)*
C20	−0.0919 (2)	0.92526 (7)	0.19440 (6)	0.0414 (3)
H20	0.012 (2)	0.9683 (8)	0.1966 (6)	0.044 (3)*
C21	−0.2736 (2)	0.92738 (7)	0.23017 (6)	0.0420 (3)
H21	−0.376 (2)	0.8825 (8)	0.2281 (6)	0.049 (4)*
C22	0.1515 (2)	1.09529 (7)	0.07030 (7)	0.0420 (3)
H22	0.030 (2)	1.0935 (7)	0.0295 (7)	0.050 (4)*
C23	0.3498 (2)	1.05781 (7)	0.06546 (7)	0.0417 (3)
H23	0.473 (2)	1.0581 (7)	0.1052 (6)	0.044 (3)*
C24	0.3997 (2)	1.01778 (7)	0.00320 (7)	0.0439 (3)
H24	0.275 (2)	1.0192 (7)	−0.0377 (7)	0.052 (4)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0517 (7)	0.0549 (8)	0.0644 (8)	0.0045 (6)	0.0091 (6)	−0.0115 (6)
N2	0.0471 (7)	0.0512 (7)	0.0468 (6)	0.0063 (5)	0.0095 (5)	0.0009 (5)
N3	0.0557 (7)	0.0448 (6)	0.0486 (7)	0.0072 (5)	0.0073 (6)	−0.0015 (5)
C1	0.0454 (9)	0.0607 (9)	0.0581 (9)	0.0038 (7)	0.0118 (7)	−0.0031 (7)
C2	0.0441 (8)	0.0453 (8)	0.0494 (8)	−0.0044 (6)	0.0065 (6)	0.0033 (6)
C3	0.0428 (7)	0.0382 (7)	0.0340 (6)	0.0018 (5)	0.0002 (5)	0.0060 (5)
C4	0.0428 (8)	0.0420 (7)	0.0441 (7)	−0.0017 (6)	0.0049 (6)	0.0013 (6)
C5	0.0499 (9)	0.0437 (8)	0.0587 (9)	−0.0027 (6)	0.0034 (7)	−0.0047 (7)
C6	0.0413 (8)	0.0560 (9)	0.0477 (8)	0.0050 (6)	0.0107 (6)	0.0079 (7)
C7	0.0410 (8)	0.0444 (8)	0.0479 (8)	−0.0026 (6)	0.0052 (6)	0.0071 (6)
C8	0.0396 (7)	0.0377 (7)	0.0371 (7)	0.0021 (5)	0.0012 (5)	0.0065 (5)
C9	0.0384 (7)	0.0425 (7)	0.0431 (7)	−0.0015 (6)	0.0076 (6)	0.0014 (6)
C10	0.0451 (8)	0.0422 (8)	0.0473 (8)	−0.0008 (6)	0.0060 (6)	−0.0024 (6)
C11	0.0467 (8)	0.0480 (8)	0.0518 (8)	0.0099 (6)	0.0080 (7)	0.0049 (6)
C12	0.0437 (8)	0.0490 (8)	0.0417 (8)	0.0016 (6)	−0.0003 (6)	0.0050 (6)
C13	0.0437 (7)	0.0339 (6)	0.0377 (7)	−0.0030 (5)	0.0060 (6)	0.0042 (5)
C14	0.0411 (8)	0.0376 (7)	0.0450 (7)	0.0022 (6)	0.0027 (6)	−0.0003 (6)
C15	0.0531 (9)	0.0423 (7)	0.0445 (8)	0.0033 (6)	−0.0009 (6)	−0.0040 (6)
C16	0.0497 (8)	0.0359 (7)	0.0379 (7)	−0.0048 (6)	0.0029 (6)	0.0035 (5)
C17	0.0504 (8)	0.0361 (7)	0.0340 (7)	−0.0020 (6)	0.0027 (6)	0.0031 (5)
C18	0.0553 (9)	0.0364 (7)	0.0370 (7)	−0.0019 (6)	0.0026 (6)	0.0023 (6)
C19	0.0517 (8)	0.0369 (7)	0.0394 (7)	−0.0006 (6)	0.0011 (6)	0.0013 (6)
C20	0.0476 (8)	0.0364 (7)	0.0391 (7)	−0.0025 (6)	0.0022 (6)	0.0018 (5)
C21	0.0463 (8)	0.0356 (7)	0.0431 (7)	−0.0027 (6)	0.0022 (6)	0.0030 (6)
C22	0.0480 (8)	0.0395 (7)	0.0377 (7)	−0.0033 (6)	0.0028 (6)	0.0017 (5)
C23	0.0503 (8)	0.0351 (7)	0.0394 (7)	−0.0046 (6)	0.0049 (6)	0.0004 (5)
C24	0.0541 (8)	0.0363 (7)	0.0415 (7)	−0.0046 (6)	0.0073 (7)	0.0006 (6)

Geometric parameters (Å, º) top

N1—C1	1.3355 (18)	C11—H11	0.975 (14)
N1—C5	1.3436 (18)	C12—C13	1.3931 (18)
N2—C6	1.3394 (17)	C12—H12	0.964 (14)
N2—C10	1.3396 (16)	C13—C14	1.3952 (17)
N3—C11	1.3354 (17)	C13—C22	1.4663 (17)
N3—C15	1.3422 (17)	C14—C15	1.3765 (18)
C1—C2	1.380 (2)	C14—H14	0.971 (13)
C1—H1	0.999 (15)	C15—H15	0.989 (13)
C2—C3	1.3910 (18)	C16—C17	1.3394 (18)
C2—H2	0.964 (13)	C16—H16	0.969 (12)
C3—C4	1.3999 (18)	C17—C18	1.4395 (18)
C3—C16	1.4627 (18)	C17—H17	0.973 (13)
C4—C5	1.3796 (19)	C18—C19	1.3408 (19)
C4—H4	0.970 (13)	C18—H18	0.982 (13)
C5—H5	0.980 (14)	C19—C20	1.4382 (18)
C6—C7	1.3830 (19)	C19—H19	0.976 (13)
C6—H6	0.997 (14)	C20—C21	1.3390 (18)
C7—C8	1.3974 (18)	C20—H20	0.954 (13)
C7—H7	0.969 (13)	C21—H21	0.973 (13)
C8—C9	1.3961 (17)	C22—C23	1.3380 (18)
C8—C21	1.4616 (17)	C22—H22	0.985 (13)
C9—C10	1.3788 (18)	C23—C24	1.4436 (18)
C9—H9	0.969 (12)	C23—H23	0.978 (12)
C10—H10	0.964 (13)	C24—C24ⁱ	1.341 (3)
C11—C12	1.3815 (18)	C24—H24	0.997 (13)

C1—N1—C5	115.69 (12)	C13—C12—H12	119.3 (8)
C6—N2—C10	115.81 (12)	C12—C13—C14	116.18 (12)
C11—N3—C15	115.75 (12)	C12—C13—C22	119.88 (11)
N1—C1—C2	124.04 (14)	C14—C13—C22	123.85 (12)
N1—C1—H1	116.0 (8)	C15—C14—C13	119.46 (12)
C2—C1—H1	120.0 (8)	C15—C14—H14	118.7 (7)
C1—C2—C3	120.14 (13)	C13—C14—H14	121.9 (7)
C1—C2—H2	120.4 (8)	N3—C15—C14	124.58 (13)
C3—C2—H2	119.5 (8)	N3—C15—H15	115.0 (8)
C2—C3—C4	116.35 (12)	C14—C15—H15	120.4 (8)
C2—C3—C16	120.18 (12)	C17—C16—C3	126.49 (12)
C4—C3—C16	123.45 (12)	C17—C16—H16	118.4 (7)
C5—C4—C3	119.21 (13)	C3—C16—H16	115.1 (7)
C5—C4—H4	119.6 (7)	C16—C17—C18	124.57 (13)
C3—C4—H4	121.2 (7)	C16—C17—H17	119.5 (7)
N1—C5—C4	124.55 (13)	C18—C17—H17	115.9 (7)
N1—C5—H5	116.1 (8)	C19—C18—C17	124.39 (13)
C4—C5—H5	119.3 (8)	C19—C18—H18	118.2 (7)
N2—C6—C7	123.92 (13)	C17—C18—H18	117.4 (7)
N2—C6—H6	116.0 (8)	C18—C19—C20	125.63 (13)
C7—C6—H6	120.1 (8)	C18—C19—H19	118.8 (7)
C6—C7—C8	119.99 (13)	C20—C19—H19	115.5 (8)
C6—C7—H7	120.5 (8)	C21—C20—C19	123.54 (13)
C8—C7—H7	119.5 (8)	C21—C20—H20	119.9 (7)
C9—C8—C7	116.10 (12)	C19—C20—H20	116.5 (7)
C9—C8—C21	123.76 (12)	C20—C21—C8	127.09 (12)
C7—C8—C21	120.12 (12)	C20—C21—H21	118.8 (7)
C10—C9—C8	119.73 (12)	C8—C21—H21	114.1 (8)
C10—C9—H9	119.7 (7)	C23—C22—C13	126.71 (12)
C8—C9—H9	120.5 (7)	C23—C22—H22	117.8 (7)
N2—C10—C9	124.44 (13)	C13—C22—H22	115.3 (7)
N2—C10—H10	115.9 (7)	C22—C23—C24	124.00 (13)
C9—C10—H10	119.6 (7)	C22—C23—H23	119.9 (7)
N3—C11—C12	123.79 (13)	C24—C23—H23	116.1 (7)
N3—C11—H11	116.4 (8)	C24ⁱ—C24—C23	124.67 (17)
C12—C11—H11	119.8 (8)	C24ⁱ—C24—H24	119.5 (8)
C11—C12—C13	120.21 (13)	C23—C24—H24	115.8 (8)
C11—C12—H12	120.5 (8)

C5—N1—C1—C2	−0.5 (2)	C11—C12—C13—C22	−174.59 (11)
N1—C1—C2—C3	−0.7 (2)	C12—C13—C14—C15	−1.72 (18)
C1—C2—C3—C4	1.43 (18)	C22—C13—C14—C15	174.85 (12)
C1—C2—C3—C16	−176.92 (12)	C11—N3—C15—C14	1.1 (2)
C2—C3—C4—C5	−1.03 (18)	C13—C14—C15—N3	0.1 (2)
C16—C3—C4—C5	177.26 (11)	C2—C3—C16—C17	175.93 (12)
C1—N1—C5—C4	0.9 (2)	C4—C3—C16—C17	−2.3 (2)
C3—C4—C5—N1	−0.1 (2)	C3—C16—C17—C18	−177.16 (11)
C10—N2—C6—C7	0.60 (19)	C16—C17—C18—C19	−179.16 (12)
N2—C6—C7—C8	−0.2 (2)	C17—C18—C19—C20	177.79 (11)
C6—C7—C8—C9	−0.64 (17)	C18—C19—C20—C21	−176.06 (13)
C6—C7—C8—C21	177.48 (11)	C19—C20—C21—C8	175.54 (11)
C7—C8—C9—C10	1.12 (17)	C9—C8—C21—C20	−1.5 (2)
C21—C8—C9—C10	−176.93 (11)	C7—C8—C21—C20	−179.51 (12)
C6—N2—C10—C9	−0.08 (19)	C12—C13—C22—C23	175.18 (13)
C8—C9—C10—N2	−0.8 (2)	C14—C13—C22—C23	−1.3 (2)
C15—N3—C11—C12	−0.6 (2)	C13—C22—C23—C24	−173.53 (11)
N3—C11—C12—C13	−1.0 (2)	C22—C23—C24—C24ⁱ	177.66 (15)
C11—C12—C13—C14	2.12 (18)

Symmetry code: (i) −x+1, −y+2, −z.

Experimental details

Crystal data
Chemical formula	C₁₆H₁₄N₂
M_r	234.29
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	173
a, b, c (Å)	5.837 (1), 17.171 (4), 19.227 (4)
β (°)	97.685 (4)
V (Å³)	1909.8 (7)
Z	6
Radiation type	Mo Kα
µ (mm⁻¹)	0.07
Crystal size (mm)	0.44 × 0.24 × 0.22

Data collection
Diffractometer	Bruker SMART Platform CCD diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	18771, 3366, 2460
R_int	0.028
(sin θ/λ)_max (Å⁻¹)	0.596

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.032, 0.089, 1.01
No. of reflections	3366
No. of parameters	328
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.09, −0.15

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SIR2002 (Burla et al., 2003), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

This work was supported in part by Research Development grants from the Pennsylvania State University and partially by the MRSEC Program of the National Science Foundation under award No. DMR-0819885. The author also acknowledges William W. Brennessel, Victor G. Young Jr and the X-ray Crystallographic Laboratory at the University of Minnesota.

References

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