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Multinuclear metal-binding ability of a carotene.

Horiuchi S, Tachibana Y, Yamashita M, Yamamoto K, Masai K, Takase K, Matsutani T, Kawamata S, Kurashige Y, Yanai T, Murahashi T - Nat Commun (2015)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8787, Japan.

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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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.
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f3: 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.

Mentions: We examined the homoleptic carotene–metal systems in which all auxiliary ligands contained in the starting metal complexes are replaced by carotene to afford a sandwich-type multimetal-binding motif. Pd was used because of its feasibility to undergo convergent metal assembly with the aids of relatively weak metal–ligand and metal–metal bonds2122. At first, we investigated the full metalation of the bis-carotene π-framework. The redox–condensation reaction of [Pd2(CH3CN)6][BF4]2 (ref. 23) and excess Pd2(dba)3·C6H6 (ref. 24) in the presence of β-carotene at 60 °C gave the decanuclear palladium complex [Pd10(μ10-β-carotene)2][B(ArF)4]2 (1, B(ArF)4=B(3,5-(CF3)2C6H3)4) in 33% yield as a mixture of two isomers (Fig. 2a, 1-meso:1-rac=7:3). It is noted that use of Pd2(dba)3·CHCl3 instead of Pd2(dba)3·C6H6 gave a complicated mixture from which 1 was not obtained. The major isomer (1-meso) is poorly soluble in CH3CN, whereas another isomer (1-rac) is soluble, enabling us to separate the two isomers. The molecular structures of thus obtained yellow complexes 1-meso and 1-rac were determined by X-ray crystallographic analysis (Fig. 3). Remarkably, two β-carotene molecules flank an array of 10 Pd atoms through unprecedentedly large μ10-bridging π-coordination. The two β-carotene ligands are stacking in an eclipsed form in 1-meso and in a staggered one in 1-rac. The nine Pd–Pd bond lengths (2.5827(7)–2.7172(6) Å in 1-meso; 2.6010(11)–2.7111(11) Å in 1-rac) are shorter than that of bulk Pd (2.76 Å), indicating that 10 Pd atoms in 1-meso or 1-rac are connected through Pd–Pd bonds (Supplementary Table 1). The calculated indices of Mayer bond order for each Pd–Pd are ranged from 0.23 to 0.14 for 1-meso; cf., for a typical Pd–Pd-bonded complex, Pd2Cl2(PH3)4, the index of Meyer bond order for Pd–Pd was calculated to be 0.60. The β-carotene ligands in 1-meso showed reduced and inversed C=C/C–C bond length alternation, that is, the long/short alternation (1.46/1.41 Å) was found for the inner nonaene substructure in the β-carotene ligands in 1-meso, being in between the C–C bond lengths in ethane (1.54 Å) and ethylene (1.34 Å; cf. 1.33/1.47 Å for the bond length alternation of free β-carotene; Fig. 3e). It is noted that the bis-β-carotene Pd10 chain dication of 1-meso formed infinite intermolecular π–π stacking columns (the shortest intermolecular C···C distance is 3.51 Å) in the crystalline state (Fig. 3f,g). Such intermolecular backbone π–π stacking represents the typical property of the planar π-conjugated system. The dication of 1-rac formed the π–π stacking dimer instead of the infinite column in the crystalline state. The 1H and 13C{1H} nuclear magnetic resonance (NMR) spectra of 1-meso and 1-rac in CD2Cl2 showed that all olefinic proton and carbon resonances of the β-carotene ligands appeared at the high-field region (olefinic moieties appeared at δ=3.5–2.6 ppm for 1H; δ=111–69 ppm for 13C), being consistent with the solid state structures determined by X-ray crystallography. The decanuclear sandwich complexes 1-meso and 1-rac are stable in solution even in the aerobic condition. Thus, it has been proven that bis-β-carotene π-framework can accommodate 10 Pd atoms array through remarkable multidentate bridging π-coordination. The decanuclear complexes 1-meso and 1-rac are the soluble and isolable organometallic clusters having a long metal chain. Existence of long inorganic palladium wires in solution was recently reported, where di- or tetranuclear palladium units are self-assembled through Pd–Pd interactions2526. The extended (π-carbon framework)–(metal clusters) contact found in 1-meso and 1-rac may be related with the interface structure of the sp2-carbon material and metal clusters that is of current interests in materials science and catalysis2728.


Multinuclear metal-binding ability of a carotene.

Horiuchi S, Tachibana Y, Yamashita M, Yamamoto K, Masai K, Takase K, Matsutani T, Kawamata S, Kurashige Y, Yanai T, Murahashi T - Nat Commun (2015)

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.
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f3: 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.
Mentions: We examined the homoleptic carotene–metal systems in which all auxiliary ligands contained in the starting metal complexes are replaced by carotene to afford a sandwich-type multimetal-binding motif. Pd was used because of its feasibility to undergo convergent metal assembly with the aids of relatively weak metal–ligand and metal–metal bonds2122. At first, we investigated the full metalation of the bis-carotene π-framework. The redox–condensation reaction of [Pd2(CH3CN)6][BF4]2 (ref. 23) and excess Pd2(dba)3·C6H6 (ref. 24) in the presence of β-carotene at 60 °C gave the decanuclear palladium complex [Pd10(μ10-β-carotene)2][B(ArF)4]2 (1, B(ArF)4=B(3,5-(CF3)2C6H3)4) in 33% yield as a mixture of two isomers (Fig. 2a, 1-meso:1-rac=7:3). It is noted that use of Pd2(dba)3·CHCl3 instead of Pd2(dba)3·C6H6 gave a complicated mixture from which 1 was not obtained. The major isomer (1-meso) is poorly soluble in CH3CN, whereas another isomer (1-rac) is soluble, enabling us to separate the two isomers. The molecular structures of thus obtained yellow complexes 1-meso and 1-rac were determined by X-ray crystallographic analysis (Fig. 3). Remarkably, two β-carotene molecules flank an array of 10 Pd atoms through unprecedentedly large μ10-bridging π-coordination. The two β-carotene ligands are stacking in an eclipsed form in 1-meso and in a staggered one in 1-rac. The nine Pd–Pd bond lengths (2.5827(7)–2.7172(6) Å in 1-meso; 2.6010(11)–2.7111(11) Å in 1-rac) are shorter than that of bulk Pd (2.76 Å), indicating that 10 Pd atoms in 1-meso or 1-rac are connected through Pd–Pd bonds (Supplementary Table 1). The calculated indices of Mayer bond order for each Pd–Pd are ranged from 0.23 to 0.14 for 1-meso; cf., for a typical Pd–Pd-bonded complex, Pd2Cl2(PH3)4, the index of Meyer bond order for Pd–Pd was calculated to be 0.60. The β-carotene ligands in 1-meso showed reduced and inversed C=C/C–C bond length alternation, that is, the long/short alternation (1.46/1.41 Å) was found for the inner nonaene substructure in the β-carotene ligands in 1-meso, being in between the C–C bond lengths in ethane (1.54 Å) and ethylene (1.34 Å; cf. 1.33/1.47 Å for the bond length alternation of free β-carotene; Fig. 3e). It is noted that the bis-β-carotene Pd10 chain dication of 1-meso formed infinite intermolecular π–π stacking columns (the shortest intermolecular C···C distance is 3.51 Å) in the crystalline state (Fig. 3f,g). Such intermolecular backbone π–π stacking represents the typical property of the planar π-conjugated system. The dication of 1-rac formed the π–π stacking dimer instead of the infinite column in the crystalline state. The 1H and 13C{1H} nuclear magnetic resonance (NMR) spectra of 1-meso and 1-rac in CD2Cl2 showed that all olefinic proton and carbon resonances of the β-carotene ligands appeared at the high-field region (olefinic moieties appeared at δ=3.5–2.6 ppm for 1H; δ=111–69 ppm for 13C), being consistent with the solid state structures determined by X-ray crystallography. The decanuclear sandwich complexes 1-meso and 1-rac are stable in solution even in the aerobic condition. Thus, it has been proven that bis-β-carotene π-framework can accommodate 10 Pd atoms array through remarkable multidentate bridging π-coordination. The decanuclear complexes 1-meso and 1-rac are the soluble and isolable organometallic clusters having a long metal chain. Existence of long inorganic palladium wires in solution was recently reported, where di- or tetranuclear palladium units are self-assembled through Pd–Pd interactions2526. The extended (π-carbon framework)–(metal clusters) contact found in 1-meso and 1-rac may be related with the interface structure of the sp2-carbon material and metal clusters that is of current interests in materials science and catalysis2728.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8787, Japan.

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.

Show MeSH