5-Benzyl-5H-pyrido[3,2-b]indole

Letessier, J.; Schollmeyer, D.; Detert, H.

doi:10.1107/S1600536811032107

organic compounds

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ISSN: 2056-9890

Volume 67| Part 9| September 2011| Page o2341

doi:10.1107/S1600536811032107

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5-Benzyl-5H-pyrido[3,2-b]indole

Julien Letessier,^a Dieter Schollmeyer ^a and Heiner Detert ^a ^*

^aUniversity Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
^*Correspondence e-mail: detert@uni-mainz.de

(Received 8 August 2011; accepted 8 August 2011; online 17 August 2011)

The title compound, C₁₈H₁₄N₂, was prepared by twofold Pd-catalyzed arylamination of a cyclic pyrido–benzo–iodolium salt. In the crystal, two molecules of 9-benzyl-δ-carboline form centrosymmetrical dimers with distances of 3.651 (2) Å between the centroids of the pyridine rings and 3.961 (2) Å between the centroids of the pyrrole and pyridine rings. The phenyl rings point to the other molecule in the dimer and the carboline core is essentially planar [maximum deviation of 0.027 (2) Å].

Related literature

For δ-Carboline, see: Subbaraju et al. (2004 ); Paulo et al. (2000 ); Chernyshev et al. (2001 ); Namjoshi et al. (2011 ); Qu et al. (2009 ); Masterova et al. (2008 ). For synthetic strategies to carbolines, see: Späth & Eiter (1940 ); Sakamoto et al. (1999 ); Franck et al. (2008 ). For the transition-metal-catalyzed synthesis of carbazoles, see: Letessier (2011 ); Nemkovich et al. (2009 ). For the transition-metal-catalyzed synthesis of carbolines, see: Nissen et al. (2011 ), Dassonneville et al. (2010 ). For β-carboline, see: Torreiles et al. (1985 ); Love (2006 ); Dassonneville et al. (2011 ); Nissen & Detert (2011 ). For the synthesis of the title compound, see: Letessier & Detert (2011 ).

Experimental

Crystal data

C₁₈H₁₄N₂
M_r = 258.1
Monoclinic, P 2₁ /n
a = 11.295 (4) Å
b = 10.482 (4) Å
c = 11.961 (4) Å
β = 110.387 (11)°
V = 1327.4 (8) Å³
Z = 4
Mo Kα radiation
μ = 0.08 mm⁻¹
T = 173 K
0.51 × 0.25 × 0.02 mm

Data collection

Bruker SMART CCD diffractometer
15877 measured reflections
3149 independent reflections
1444 reflections with I > 2σ(I)
R_int = 0.128

Refinement

R[F² > 2σ(F²)] = 0.055
wR(F²) = 0.149
S = 0.98
3149 reflections
182 parameters
H-atom parameters constrained
Δρ_max = 0.22 e Å⁻³
Δρ_min = −0.21 e Å⁻³

Data collection: SMART (Bruker, 2006 ); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: PLATON (Spek, 2009 ); software used to prepare material for publication: PLATON.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536811032107/bt5607sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811032107/bt5607Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536811032107/bt5607Isup3.cml

checkCIF report

Comment top

The title compound was prepared as a part of a project focused on the transition metal catalyzed synthesis of carbazoles, see: Letessier (2011) and Nemkovich et al. (2009), carbolines, see: Nissen, Schollmeyer & Detert (2011) and related indolo-annulated heterocycles, see: Dassonneville et al. (2010). Whereas the β-carboline is the core of a large group of alkaloids (see: Torreiles et al. (1985); Love (2006)), only a few natural δ-carbolines are known. With Rh- or Ru-catalyzed [2 + 2+2] cycloadditions of alkynyl-ynamides we recently reported a new access to β- and γ-carbolines (Dassonneville et al., 2011), Nissen & Detert (2011), but this approach is not suitable for the synthesis of the δ-isomers. These can now be prepared in a twofold Pd-catalyzed arylation of primary amines with cyclic pyrido-benzo iodolium salts. This unique 9-substituted δ-carboline crystallizes in form of centrosymmetrical dimers. The phenyl group, pointing in the direction of the second molecule, is nearly orthogonal to the essentially planar carboline core (maximal deviations of 0.027 (2) Å from the least square plane). Short distances of the centroid of a pyridine ring of one molecule to the centroid of the pyridine of its counterpart of 3.65 Å and to the pyrrole centroid of 3.96 Å indicate a π-π interaction between the heterocycles.

Related literature top

For δ-Carboline, see: Subbaraju et al. (2004); Paulo et al. (2000); Chernyshev et al. (2001) ; Namjoshi et al. (2011); Qu et al. (2009); Masterova et al. (2008). For synthetic strategies to carbolines, see: Späth & Eiter (1940); Sakamoto et al. (1999); Franck et al. (2008). For the transition-metal-catalyzed synthesis of carbazoles, see: Letessier (2011); Nemkovich et al. (2009). For the transition-metal-catalyzed synthesis of carbolines, see: Nissen et al. (2011), Dassonneville et al. (2010). For β-carboline, see: Torreiles et al. (1985); Love (2006); Dassonneville et al. (2011); Nissen & Detert (2011). For the synthesis of the title compound, see: Letessier & Detert (2011).

Experimental top

A solution of 400 mg (0.93 mmol) of benzo[4,5]iodolo[3,2-b]pyridin-5-ium trifluoromethanesulfonate (Letessier & Detert, 2011) in dry toluene (10 ml) was deaerated in a Schlenk flask. Under argon, Pd₂(dba)₃ (34 mg, 0.04 mmol), Xantphos (64 mg, 0.11 mmol), and Cs₂CO₃ (850 mg, 2.61 mmol) were added. The mixture was stirred for 5 min at 300 K before benzyl amine (120 mg, 1.12 mmol) was added. After stirring for 15 h at 383 K, the mixture was cooled to ambient temparature, filtered through celite and concentrated. Purification by column chromatography (petroleum ether / ethyl acetate = 4 / 1) gave 156 mg (65%) of the title compound as colorless crystals with m. p. > 415 K.

Computing details top

Data collection: SMART (Bruker, 2006); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT (Bruker, 2006); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).

Figures top

Fig. 1. View of compound I. Displacement ellipsoids are drawn at the 50% probability level.

5-Benzyl-5H-pyrido[3,2-b]indole top

Crystal data top

C₁₈H₁₄N₂	F(000) = 544
M_r = 258.1	D_x = 1.293 Mg m⁻³
Monoclinic, P2₁/n	Melting point: 415 K
Hall symbol: -P 2yn	Mo Kα radiation, λ = 0.71073 Å
a = 11.295 (4) Å	Cell parameters from 1161 reflections
b = 10.482 (4) Å	θ = 2.6–22.3°
c = 11.961 (4) Å	µ = 0.08 mm⁻¹
β = 110.387 (11)°	T = 173 K
V = 1327.4 (8) Å³	Plate, colourless
Z = 4	0.51 × 0.25 × 0.02 mm

Data collection top

Bruker SMART CCD diffractometer	1444 reflections with I > 2σ(I)
Radiation source: sealed Tube	R_int = 0.128
Graphite monochromator	θ_max = 27.9°, θ_min = 2.1°
CCD scan	h = −14→14
15877 measured reflections	k = −13→13
3149 independent reflections	l = −15→15

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.055	H-atom parameters constrained
wR(F²) = 0.149	w = 1/[σ²(F_o²) + (0.0558P)² + 0.2129P] where P = (F_o² + 2F_c²)/3
S = 0.98	(Δ/σ)_max < 0.001
3149 reflections	Δρ_max = 0.22 e Å⁻³
182 parameters	Δρ_min = −0.21 e Å⁻³
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.016 (2)

Crystal data top

C₁₈H₁₄N₂	V = 1327.4 (8) Å³
M_r = 258.1	Z = 4
Monoclinic, P2₁/n	Mo Kα radiation
a = 11.295 (4) Å	µ = 0.08 mm⁻¹
b = 10.482 (4) Å	T = 173 K
c = 11.961 (4) Å	0.51 × 0.25 × 0.02 mm
β = 110.387 (11)°

Data collection top

Bruker SMART CCD diffractometer	1444 reflections with I > 2σ(I)
15877 measured reflections	R_int = 0.128
3149 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.055	0 restraints
wR(F²) = 0.149	H-atom parameters constrained
S = 0.98	Δρ_max = 0.22 e Å⁻³
3149 reflections	Δρ_min = −0.21 e Å⁻³
182 parameters

Special details top

Experimental. ¹H-NMR (400 MHz, CDCl₃): δ = 8.59 (dd, J = 4.7 Hz, J = 1.2 Hz, 1H, 2-H), 8.43 (d, J = 7.8 Hz, 1H, 9-H), 7.64 (dd, J = 8.2 Hz, J = 1.2 Hz, 1H, 6-H), 7.53 (td, J = 8.3 Hz, J = 1.2 Hz, 1H, 7-H), 7.42 (d, J = 8.2 Hz, 1H, 6-H), 7.26-7.34 (m, 5H, CH), 7.12 (m, 2H, CH), 5.52 (s, 2H, CH₂). ¹³C-NMR (75 MHz, CDCl₃): δ = 141.9 (d, C-2), 141.8 (s, C-9b), 141.4 (s, C-5a), 136.5 (s, C-1), 134.0 (s, C-4a), 128.9 (d, C-2), 127.9 (d, C-7), 127.7 (d, C-4), 126.3 (d, C-3), 122.2 (s, C-9a), 120.9 (d, C-3), 120.1 (d, C-9), 120.0 (d, C-8), 115.8 (d, C-4), 109.2 (d, C-6), 46.5 (t, CH₂). IR (neat, ATR): ν = 1621 (w), 1588 (w), 1482 (m), 1451 (m), 1412 (s), 1334 (m), 1318 (s), 1242 (w), 1193 (m), 1115 (w), 1012 (w), 913 (w), 845 (m), 781 (s), 742 (vs), 730 (vs), 721 (vs), 695 (s)cm^-1. FD-MS: m/z = 258.1 [C₁₈H₁₄N₂]⁺.

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	0.74211 (17)	0.47469 (18)	0.17200 (17)	0.0360 (5)
C2	0.7372 (2)	0.4852 (2)	0.2855 (2)	0.0334 (6)
C3	0.8208 (2)	0.5471 (2)	0.3853 (2)	0.0410 (6)
H3	0.8945	0.5885	0.3825	0.049*
C4	0.7916 (3)	0.5456 (2)	0.4876 (2)	0.0468 (7)
H4	0.8463	0.5873	0.5568	0.056*
C5	0.6848 (3)	0.4849 (2)	0.4923 (2)	0.0501 (7)
H5	0.6678	0.4861	0.5647	0.060*
C6	0.6034 (2)	0.4233 (2)	0.3955 (2)	0.0424 (7)
H6	0.5304	0.3817	0.3998	0.051*
C7	0.6295 (2)	0.4231 (2)	0.2914 (2)	0.0356 (6)
C8	0.5662 (2)	0.3720 (2)	0.1737 (2)	0.0365 (6)
N9	0.45662 (19)	0.3047 (2)	0.1336 (2)	0.0488 (6)
C10	0.4201 (2)	0.2725 (3)	0.0174 (3)	0.0514 (8)
H10	0.3436	0.2259	−0.0154	0.062*
C11	0.4853 (3)	0.3021 (2)	−0.0574 (2)	0.0522 (8)
H11	0.4528	0.2755	−0.1384	0.063*
C12	0.5981 (2)	0.3703 (2)	−0.0164 (2)	0.0452 (7)
H12	0.6447	0.3914	−0.0664	0.054*
C13	0.6378 (2)	0.4055 (2)	0.1036 (2)	0.0370 (6)
C14	0.8358 (2)	0.5335 (2)	0.1306 (2)	0.0409 (6)
H14A	0.8610	0.6162	0.1719	0.049*
H14B	0.7963	0.5513	0.0442	0.049*
C15	0.9524 (2)	0.4548 (2)	0.1502 (2)	0.0360 (6)
C16	1.0688 (2)	0.5125 (3)	0.1743 (2)	0.0475 (7)
H16	1.0757	0.6027	0.1820	0.057*
C17	1.1752 (2)	0.4406 (3)	0.1872 (2)	0.0548 (8)
H17	1.2544	0.4817	0.2035	0.066*
C18	1.1675 (3)	0.3100 (3)	0.1769 (2)	0.0583 (8)
H18	1.2407	0.2606	0.1857	0.070*
C19	1.0529 (3)	0.2523 (3)	0.1537 (3)	0.0606 (9)
H19	1.0468	0.1621	0.1471	0.073*
C20	0.9462 (2)	0.3233 (3)	0.1399 (2)	0.0487 (7)
H20	0.8673	0.2815	0.1232	0.058*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0301 (11)	0.0362 (12)	0.0403 (13)	−0.0008 (9)	0.0106 (9)	0.0009 (9)
C2	0.0340 (13)	0.0264 (13)	0.0380 (15)	0.0064 (11)	0.0102 (11)	0.0032 (11)
C3	0.0415 (14)	0.0330 (14)	0.0454 (17)	−0.0039 (12)	0.0113 (12)	0.0013 (12)
C4	0.0553 (17)	0.0366 (16)	0.0437 (17)	−0.0044 (14)	0.0113 (13)	−0.0011 (13)
C5	0.0628 (19)	0.0421 (16)	0.0487 (17)	0.0031 (15)	0.0234 (15)	0.0025 (14)
C6	0.0381 (15)	0.0372 (15)	0.0577 (19)	0.0002 (12)	0.0239 (14)	0.0030 (13)
C7	0.0313 (13)	0.0277 (13)	0.0459 (16)	0.0049 (11)	0.0112 (12)	0.0052 (11)
C8	0.0263 (12)	0.0303 (14)	0.0499 (16)	0.0017 (11)	0.0097 (11)	0.0029 (12)
N9	0.0354 (12)	0.0359 (13)	0.0652 (16)	0.0046 (11)	0.0050 (11)	0.0021 (12)
C10	0.0363 (15)	0.0358 (16)	0.066 (2)	0.0038 (12)	−0.0024 (15)	−0.0015 (14)
C11	0.0535 (18)	0.0396 (17)	0.0447 (17)	0.0077 (15)	−0.0066 (14)	−0.0034 (13)
C12	0.0494 (16)	0.0372 (15)	0.0442 (17)	0.0081 (13)	0.0102 (13)	0.0032 (12)
C13	0.0295 (13)	0.0314 (14)	0.0440 (16)	0.0058 (11)	0.0050 (12)	−0.0006 (12)
C14	0.0408 (14)	0.0394 (15)	0.0438 (16)	−0.0017 (12)	0.0161 (12)	0.0034 (12)
C15	0.0353 (14)	0.0397 (15)	0.0324 (14)	−0.0023 (12)	0.0112 (11)	−0.0013 (11)
C16	0.0444 (16)	0.0535 (17)	0.0422 (16)	−0.0130 (14)	0.0123 (13)	−0.0050 (13)
C17	0.0344 (15)	0.081 (2)	0.0485 (18)	−0.0076 (16)	0.0140 (13)	−0.0022 (16)
C18	0.0416 (17)	0.075 (2)	0.061 (2)	0.0133 (17)	0.0214 (14)	0.0001 (17)
C19	0.0534 (19)	0.0505 (19)	0.084 (2)	0.0045 (15)	0.0321 (17)	−0.0043 (16)
C20	0.0379 (15)	0.0436 (16)	0.0663 (19)	−0.0008 (13)	0.0202 (13)	−0.0050 (14)

Geometric parameters (Å, º) top

N1—C2	1.382 (3)	C11—C12	1.393 (4)
N1—C13	1.383 (3)	C11—H11	0.9500
N1—C14	1.453 (3)	C12—C13	1.395 (3)
C2—C3	1.397 (3)	C12—H12	0.9500
C2—C7	1.403 (3)	C14—C15	1.502 (3)
C3—C4	1.373 (3)	C14—H14A	0.9900
C3—H3	0.9500	C14—H14B	0.9900
C4—C5	1.382 (4)	C15—C16	1.383 (3)
C4—H4	0.9500	C15—C20	1.384 (3)
C5—C6	1.365 (3)	C16—C17	1.380 (4)
C5—H5	0.9500	C16—H16	0.9500
C6—C7	1.376 (3)	C17—C18	1.374 (4)
C6—H6	0.9500	C17—H17	0.9500
C7—C8	1.442 (3)	C18—C19	1.366 (4)
C8—N9	1.358 (3)	C18—H18	0.9500
C8—C13	1.398 (3)	C19—C20	1.376 (4)
N9—C10	1.348 (3)	C19—H19	0.9500
C10—C11	1.378 (4)	C20—H20	0.9500
C10—H10	0.9500

C2—N1—C13	107.80 (19)	C11—C12—C13	115.1 (3)
C2—N1—C14	125.75 (19)	C11—C12—H12	122.4
C13—N1—C14	126.3 (2)	C13—C12—H12	122.4
N1—C2—C3	129.1 (2)	N1—C13—C12	130.5 (2)
N1—C2—C7	110.2 (2)	N1—C13—C8	109.2 (2)
C3—C2—C7	120.8 (2)	C12—C13—C8	120.3 (2)
C4—C3—C2	117.1 (2)	N1—C14—C15	114.7 (2)
C4—C3—H3	121.4	N1—C14—H14A	108.6
C2—C3—H3	121.4	C15—C14—H14A	108.6
C3—C4—C5	121.7 (2)	N1—C14—H14B	108.6
C3—C4—H4	119.1	C15—C14—H14B	108.6
C5—C4—H4	119.1	H14A—C14—H14B	107.6
C6—C5—C4	121.5 (3)	C16—C15—C20	118.0 (2)
C6—C5—H5	119.2	C16—C15—C14	120.7 (2)
C4—C5—H5	119.2	C20—C15—C14	121.2 (2)
C5—C6—C7	118.4 (2)	C17—C16—C15	120.7 (3)
C5—C6—H6	120.8	C17—C16—H16	119.6
C7—C6—H6	120.8	C15—C16—H16	119.6
C6—C7—C2	120.5 (2)	C18—C17—C16	120.6 (3)
C6—C7—C8	133.9 (2)	C18—C17—H17	119.7
C2—C7—C8	105.5 (2)	C16—C17—H17	119.7
N9—C8—C13	124.4 (2)	C19—C18—C17	119.0 (3)
N9—C8—C7	128.3 (2)	C19—C18—H18	120.5
C13—C8—C7	107.4 (2)	C17—C18—H18	120.5
C10—N9—C8	114.1 (2)	C18—C19—C20	120.9 (3)
N9—C10—C11	124.9 (3)	C18—C19—H19	119.6
N9—C10—H10	117.5	C20—C19—H19	119.6
C11—C10—H10	117.5	C19—C20—C15	120.8 (3)
C10—C11—C12	121.1 (3)	C19—C20—H20	119.6
C10—C11—H11	119.4	C15—C20—H20	119.6
C12—C11—H11	119.4

C13—N1—C2—C3	−179.4 (2)	C10—C11—C12—C13	−0.3 (4)
C14—N1—C2—C3	−3.2 (4)	C2—N1—C13—C12	178.9 (2)
C13—N1—C2—C7	−0.1 (2)	C14—N1—C13—C12	2.8 (4)
C14—N1—C2—C7	176.1 (2)	C2—N1—C13—C8	0.1 (2)
N1—C2—C3—C4	178.4 (2)	C14—N1—C13—C8	−176.1 (2)
C7—C2—C3—C4	−0.8 (3)	C11—C12—C13—N1	−178.6 (2)
C2—C3—C4—C5	0.4 (4)	C11—C12—C13—C8	0.2 (3)
C3—C4—C5—C6	0.2 (4)	N9—C8—C13—N1	179.3 (2)
C4—C5—C6—C7	−0.3 (4)	C7—C8—C13—N1	0.0 (3)
C5—C6—C7—C2	−0.2 (4)	N9—C8—C13—C12	0.3 (4)
C5—C6—C7—C8	−178.6 (2)	C7—C8—C13—C12	−179.0 (2)
N1—C2—C7—C6	−178.6 (2)	C2—N1—C14—C15	88.4 (3)
C3—C2—C7—C6	0.7 (4)	C13—N1—C14—C15	−96.1 (3)
N1—C2—C7—C8	0.1 (2)	N1—C14—C15—C16	−146.8 (2)
C3—C2—C7—C8	179.5 (2)	N1—C14—C15—C20	35.9 (3)
C6—C7—C8—N9	−0.8 (4)	C20—C15—C16—C17	0.2 (4)
C2—C7—C8—N9	−179.3 (2)	C14—C15—C16—C17	−177.2 (2)
C6—C7—C8—C13	178.4 (3)	C15—C16—C17—C18	−0.3 (4)
C2—C7—C8—C13	−0.1 (2)	C16—C17—C18—C19	−0.1 (4)
C13—C8—N9—C10	−0.6 (3)	C17—C18—C19—C20	0.5 (4)
C7—C8—N9—C10	178.5 (2)	C18—C19—C20—C15	−0.5 (4)
C8—N9—C10—C11	0.5 (4)	C16—C15—C20—C19	0.2 (4)
N9—C10—C11—C12	−0.1 (4)	C14—C15—C20—C19	177.6 (2)

Experimental details

Crystal data
Chemical formula	C₁₈H₁₄N₂
M_r	258.1
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	173
a, b, c (Å)	11.295 (4), 10.482 (4), 11.961 (4)
β (°)	110.387 (11)
V (Å³)	1327.4 (8)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.08
Crystal size (mm)	0.51 × 0.25 × 0.02

Data collection
Diffractometer	Bruker SMART CCD diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	15877, 3149, 1444
R_int	0.128
(sin θ/λ)_max (Å⁻¹)	0.659

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.055, 0.149, 0.98
No. of reflections	3149
No. of parameters	182
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.22, −0.21

Computer programs: SMART (Bruker, 2006), SAINT (Bruker, 2006), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009).

Acknowledgements

The authors are grateful to Heinz Kolshorn for invaluable discussions and the NMR spectra.

References

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