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. 2015 Apr 10:6:6742.
doi: 10.1038/ncomms7742.

Multinuclear metal-binding ability of a carotene

Shinnosuke Horiuchi  1 , Yuki Tachibana  2 , Mitsuki Yamashita  3 , Koji Yamamoto  3 , Kohei Masai  3 , Kohei Takase  4 , Teruo Matsutani  4 , Shiori Kawamata  1 , Yuki Kurashige  5 , Takeshi Yanai  5 , Tetsuro Murahashi  6
Affiliations

Multinuclear metal-binding ability of a carotene

Shinnosuke Horiuchi et al. Nat Commun. .

Abstract

Carotenes are naturally abundant unsaturated hydrocarbon pigments, and their fascinating physical and chemical properties have been studied intensively not only for better understanding of the roles in biological processes but also for the use in artificial chemical systems. However, their metal-binding ability has been virtually unexplored. Here we report that β-carotene has the ability to assemble and align ten metal atoms to afford decanuclear homo- and heterometal chain complexes. The metallo-carotenoid framework shows reversible metalation-demetalation reactivity with multiple metals, which allows us to control the size of metal chains as well as the heterobimetallic composition and arrangement of the carotene-supported metal chains.

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Figures

Figure 1
Figure 1. Schematic representation of multinuclear metal binding by β-carotene.
Many metal atoms are assembled on the π-conjugated plane of β-carotene.
Figure 2
Figure 2. Synthesis of bis-(β-carotene) Pd10 chain complexes.
(a) Synthesis of [Pd10(μ10-β-carotene)2][B(ArF)4]2 (1), (b) ESI-MS monitoring of the formation of 1 at 60 °C showing the formation of metal-deficient intermediates [Pdn(β-carotene)2]2+ (n=5, 6, 7, 8 and 9).
Figure 3
Figure 3. Structures of bis-(β-carotene) Pd10 chain complexes.
(a) Thermal ellipsoid (50%) drawing of meso-[Pd10(μ10-β-carotene)2][B(ArF)4]2 (1-meso). (b) Ball-stick drawing of 1-meso. (c) Thermal ellipsoid (30%) drawing of rac-[Pd10(μ10-β-carotene)2][B(ArF)4]2 (1-rac). (d) Ball-stick drawing of 1-rac. (e) C–C bond lengths in 1-meso and free β-carotene (CCDC-253816), determined by X-ray structural analyses. (f) A view of a part of an intermolecular backbone π–π stacking column of 1-meso in the crystalline state. (g) A side view of a part of the π–π stacking column of 1-meso. For a–d,f,g, B(ArF)4 anions and non-coordinating solvent molecules were omitted for clarity.
Figure 4
Figure 4. Synthesis of bimetallic PdPt chain sandwich complexes of β-carotene.
(a) Demetalation and metalation of bis-β-carotene framework. Demetalation of 1-meso with CO afforded the metal-deficient complex [Pd5(μ5-β-carotene)2][B(ArF)4]2 (2-meso) or [Pd7(μ7-β-carotene)2][B(ArF)4]2 (3-meso). Subsequent metalation of 2-meso with Pt0 and then with Pd0 gave a mixed metal complex [Pd5Pt3(μ8-β-carotene)2][B(ArF)4]2 (4-meso) and [Pd5Pt3Pd2(μ10-β-carotene)2][B(ArF)4]2 (5-meso), respectively. (b) ESI-MS monitoring of the demetalation from 1-meso at 0 °C under CO (1 atm) atmosphere.
Figure 5
Figure 5. Structures of metal-deficient sandwich complexes of β-carotene.
(a) Thermal ellipsoid (30%) drawing of [Pd5(μ5-β-carotene)2][B(ArF)4]2 (2-meso). (b) Thermal ellipsoid (30%) drawing of [Pd7(μ7-β-carotene)2][B(ArF)4]2 (3-meso). (c) Thermal ellipsoid (30%) drawing of [Pd5Pt3(μ8-β-carotene)2][B(ArF)4]2 (4-meso). For a–c, B(ArF)4 anions and non-coordinating solvent molecules were omitted for clarity. The coordination modes of two β-carotene ligands in the crystalline 3-meso or 4-meso are slightly different. Furthermore, in the crystal structures of the metal-deficient complexes, 2-meso, 3-meso and 4-meso, the uncoordinated cyclohexenyl group are not superposed due to the rotation at C6–C7 bond. The NMR spectra of 2-meso, 3-meso or 4-meso showed only a single set of β-carotene signals.
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