Racemic tricarbon­yl(η6-7-meth­oxy­flavan)chromium(0)

Johannes H van Tonder; Barend C B Bezuidenhoudt; J Marthinus Janse van Rensburg

doi:10.1107/S1600536810024992

. 2010 Jul 10;66(Pt 8):m907–m908. doi: 10.1107/S1600536810024992

Racemic tricarbonyl(η⁶-7-methoxyflavan)chromium(0)

Johannes H van Tonder ^a, Barend C B Bezuidenhoudt ^a,^*, J Marthinus Janse van Rensburg ^b

PMCID: PMC3007425 PMID: 21588147

Abstract

In the title compound [systematic name: tricarbonyl(η⁶-7-methoxy-2-phenyl-3,4-dihydro-2H-1-benzopyran)chromium(0)], [Cr(C₁₆H₁₆O₂)(CO)₃], the Cr(CO)₃ unit is coordinated by the phenylene ring of the flavan ligand, exhibiting a three-legged piano-stool conformation, with a point to plane distance of 1.750 (1) Å. The phenyl ring is twisted away from the fused ring system by 36.49 (5)° (r.m.s. deviation = 0.027 Å; fitted atoms are the C₆ ring and the attached fused-ring C and O atoms). The dihydropyran ring displays a distorted envelope configuration by displacement of the phenyl-bearing and the adjacent ring C atoms from the fused-ring system plane by 0.356 (2) and 0.402 (2) Å, respectively.

Related literature

7-Methoxyflavan was synthesized via hydrogenation from 7-methoxyflavanone, as described by Sato et al. (2006 ▶). For coordination of 7-methoxyflavan to chromium, see: Müller et al. (1999 ▶). For the importance of flavonoids in biological investigations, see: Rice-Evans & Packer (2003 ▶). For Cr(CO)₃ coordination to the phenylene ring of a flavanone compound, see: Dominique et al. (1999 ▶). For comparison bond distances, see: Allen et al. (1987 ▶). For related structures, see: van Tonder et al. (2009a ▶,b ▶). For the use of tricarbonyl(arene)chromium complexes in regioselective organic synthesis, see: Muschalek et al. (2007 ▶).

Experimental

Crystal data

[Cr(C₁₆H₁₆O₂)(CO)₃]
M _r = 376.32
Monoclinic,
a = 9.8422 (2) Å
b = 12.3850 (3) Å
c = 15.0146 (3) Å
β = 115.171 (1)°
V = 1656.42 (6) Å³
Z = 4
Mo Kα radiation
μ = 0.72 mm⁻¹
T = 173 K
0.41 × 0.34 × 0.24 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2004 ▶) T _min = 0.757, T _max = 0.847
12982 measured reflections
3985 independent reflections
3224 reflections with I > 2σ(I)
R _int = 0.030

Refinement

R[F ² > 2σ(F ²)] = 0.030
wR(F ²) = 0.085
S = 1.07
3985 reflections
227 parameters
H-atom parameters constrained
Δρ_max = 0.25 e Å⁻³
Δρ_min = −0.40 e Å⁻³

Data collection: APEX2 (Bruker, 2005 ▶); cell refinement: SAINT-Plus (Bruker, 2004 ▶); data reduction: SAINT-Plus and XPREP (Bruker, 2004 ▶); program(s) used to solve structure: SIR97 (Altomare et al., 1999 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg & Putz, 2005 ▶); software used to prepare material for publication: WinGX (Farrugia, 1999 ▶).

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536810024992/zb2005sup1.cif

e-66-0m907-sup1.cif^{(23.9KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810024992/zb2005Isup2.hkl

e-66-0m907-Isup2.hkl^{(191.4KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

Financial assistance from the University of the Free State and SASOL to JHvanT is gratefully acknowledged. Opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of SASOL. We would like to express our gratitude to the School of Chemistry at the University of the Witwatersrand for the use of the diffractometer. Special thanks are due to Dr M. A. Fernandes.

supplementary crystallographic information

Comment

The title compound, (I), [Cr(C₁₆H₁₆O₂)(CO)₃], where (C₁₆H₁₆O₂) = 7-methoxyflavan, has been examined due to the general biological activity of flavanoids (Rice-Evans & Packer, 2003) and the use of tricarbonyl(arene)chromium complexes in regioselective organic synthesis (Muschalek et al., 2007).

As with the tricarbonyl(η⁶-flavanone)chromium(0) complex reported by Dominique et al. (1999), the Cr(CO)₃ unit of the title compound is coordinated by the phenylene ring of the flavanoid backbone (Fig.1). The chromium metal centre is displaced by 1.750 (1) Å from the A-η⁶-coordinated arene ring centre. The dihydropyran ring displays a distorted envelope configuration by displacement of atoms C2 and C3 from the fused ring system plane, with distances of 0.356 (2) and 0.402 (2) Å respectively (r.m.s. of fitted atoms C4, C10, C5, C6, C7, C8, C9 and O5 = 0.027 °). Further molecular disorder is displayed by the phenyl ring twist away from the fused ring system plane, by 36.49 (5)°.

The molecular packing displays two types of soft intermolecular contacts, this between O2···H8 [2.682 (1) Å] forming a O2···H8—C8 angle of 152.5 (1)° and O1···H6 [2.459 (1) Å] forming a O1···H6—C6 angle of 125.9 (1)° (Fig.2).

Experimental

7-Methoxyflavan was synthesised via H₂SO₄ catalyzed hydrogenation (5 bar) over 10% Pd/C from 7-methoxyflavanone, as described by Sato et al. (2006). 7-Methoxyflavan-4-one (1.00 g; 3.9 mmol), 10 % Pd/C (0.10 g), 3 M H₂SO₄ (aq.) (1 ml), EtOH (30 ml). Purification by means of flash column-chromatography yielded 7-methoxyflavan (0.67 g; 70.6%) as a colourless oil.

R_f 0.65 (H:DCM:EtOAc; 50:50:1); ¹H NMR (600 MHz, CDCl₃) δ ppm 7.44 – 7.41 (2H, m, H-2' and H-6'), 7.40 – 7.37 (2H, m, H-3' and H-5'), 7.34 – 7.31 (1H, m, H-4'), 6.99 – 6.97 (1H, m, H-5), 6.50 – 6.47 (2H, m, H-6 and H-8), 5.05 (1H, dd, J = 2.37, 10.19 Hz, H-2), 3.77 (3H, s, -OCH₃), 2.92 (1H, ddd, J = 6.02, 10.92, 16.08 Hz, H-4(a)), 2.74 (1H, ddd, J = 3.40, 5.12, 16.08 Hz, H-4(e)), 2.22 – 2.18 (1H, m, H-3), 2.11– 2.04 (1H, m, H-3); ¹³C NMR (600 MHz, CDCl3) δ ppm 24.47 (C-4), 30.19 (C-3), 55.38 (-OCH₃), 77.98 (C-2), 101.71 (C-6/8), 107.54 (C-6/8), 114.01, 126.11, 127.93, 128.61, 130.05, 141.79, 155.91, 155.91, 159.23

Preparation of the title compound, tricarbonyl(A-η⁶-7-methoxyflavane)chromium(0), was based on a method described by Müller et al. (1999). A solution of 7-Methoxyflavane (0.27 g, 1.1 mmol) and Cr(CO)₆ (0.25 g, 1.1 mmol: 1 eq.) in Bu₂O:THF (9:1; 10 ml per 100 mg Cr(CO)₆ was degassed with argon, using standard Schlenk techniques, and refluxed (48 h) under an oxygen free atmosphere. The reaction mixture was cooled to room temperature and the solvent evaporated in vacuo. Purification through flash column-chromatography yielded tricarbonyl(A-η⁶-7-methoxyflavane)chromium(0) (0.07 g; 16.6.0%) as a yellow solid. Recrystallization from diethyl ether yielded yellow cuboidal crystals.

R_f 0.23 (Hexane: Acetone; 8:2); Mp 148.4 °C; Note: A, B and C-ring labelling refers to the benzene, phenyl and dihydropyrane rings respectively. ¹H NMR (600 MHz, CDCl₃) δ ppm 7.49 (2H, d, J = 7.15 Hz, H-2' and H-6'), 7.41 (2H, dd, J = 7.15, 8.66 Hz, H-3' and H-5'), 7.39 – 7.35 (1H, m, H-4'), 5.65 (1H, d, J = 6.61 Hz, H-5), 5.15 (1H, s, H-8), 4.90 – 4.86 (2H, m, H-2 and H-6), 3.72 (3H, s, -OCH₃), 2.93 (1H, ddd, J = 4.89, 12.43, 15.65 Hz, H-4(a)), 2.54 (1H, dd, J = 4.14, 15.65 Hz, H-4(e)), 2.31 (1H, ddd, J = 4.14, 12.43, 13.68 Hz, H-3(a)), 2.13 (1H, dd, J = 4.89, 13.68 Hz, H-3(e)); ¹³C NMR (600 MHz, CDCl₃) δ ppm 25.52 (C-4), 29.52 (C-3), 55.86 (-OCH₃), 68.31 (C-8), 74.42 (C-2/6), 80.56 (C-2/6), 89.11, 94.55 (C-5), 126.61, 128.83, 128.89, 139.62, 140.30, 143.33, 234.44 (-Cr(CO)3); MS m/z 376 (M+, 13.0), 344 (0.2), 320 (0.1), 292 (70.7), 277 (0.2), 256 (0.1), 240 (5.8), 225 (0.5), 209 (0.3), 188 (100.0), 173 (0.4), 146 (10.0), 137 (2.1), 121 (2.0), 104 (5.1).