Multinuclear metal-binding ability of a carotene

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.

C arotenoids are naturally abundant pigments containing extended p-conjugated C ¼ C double-bond arrays. The fascinating physical and chemical properties of carotenoids, such as light-harvesting, photo-protective, antioxidative and conductive properties have been explored not only for better understanding of their roles in biological processes but also for the use in artificial chemical systems [1][2][3][4] . An attractive, although yet undeveloped function of carotenoids is their metal-binding ability. While several early reports showed that carotenoids bind mononuclear metal moieties through conventional 1,3-butadienetype tetrahapto-p-coordination [5][6][7][8][9] , the extended p-conjugated polyene moieties in their backbones may impart the ability to bind a large metal-metal (M-M)-bonded array through continuous multiple p-coordination bonds (Fig. 1). Such potential multidentate bridging p-coordination ability of carotenoids is highly attractive because of its usefulness in inorganic synthesis. Indeed, chemists have sought to use polynucleating multidentate ligands as the scaffolds or templates to control the metal assembly that provides methodology to synthesize molecularly well-defined metal clusters for catalysis and materials science [10][11][12][13][14] . However, the scaffold strategy has been applied mostly to the construction of metal clusters with small size owing to the difficulty to design practically useful large scaffold ligands that can assemble and then hold many metal atoms. In fact, only few synthetic scaffold ligands that can bind 10 or more metal atoms through multidentate bridging coordination have been developed. Recently, Peng and colleagues 15 developed synthetic multidentate N-donor ligands that lead to isolation of Ni 11 chain complexes. Shionoya and co-workers developed artificial metallo-DNA motifs that brought a method to construct a heterometal array of 10 metal ions, although direct M-M bonds are absent in the metal array 16 . While these previous studies used the s-type scaffolds bearing multiple hetero-atom sbinding sites, p-type extended p-conjugated unsaturated hydrocarbon scaffolds are also promising in light of their preferable rows of C ¼ C p-binding sites at regular intervals comparable to M-M bond lengths [17][18][19][20] . Furthermore, relatively weak C ¼ C p-coordination bond may cause dynamic metal binding at each C ¼ C site, which would be a key for assembling many metal atoms in a convergent manner. Carotenes are the rational choice as the extended p-conjugated scaffold for many metal atoms, because carotenes are one of the most readily available extended unsaturated hydrocarbons containing more than ten p-conjugated C ¼ C double bonds.
Herein, we report remarkable multinuclear metal-binding ability of b-carotene through synthesis and characterization of bis-(b-carotene) decanuclear metal chain complexes. The metallo-carotene framework shows interesting multinuclear metalation-demetalation reactivity, allowing us to construct heterobimetallic decanuclear chain. The bis-(b-carotene) decanuclear metal chain complexes are stable rod-like sandwich molecules, exhibiting parallel p-p stacking self-assembly in the crystalline state.

Results
Synthesis and structure of bis-(b-carotene) decanuclear Pd complexes. 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 bonds 21,22 . At first, we investigated the full metalation of the bis-carotene p-framework. . It is noted that use of Pd 2 (dba) 3 Á CHCl 3 instead of Pd 2 (dba) 3 Á C 6 H 6 gave a complicated mixture from which 1 was not obtained. The major isomer (1-meso) is poorly soluble in CH 3 CN, whereas another isomer (1-rac) is p-framework can accommodate 10 Pd atoms array through remarkable multidentate bridging p-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 interactions 25,26 . The extended (pcarbon framework)-(metal clusters) contact found in 1-meso and 1-rac may be related with the interface structure of the sp 2carbon material and metal clusters that is of current interests in materials science and catalysis 27,28 .
Synthesis and structure of bis-(b-carotene) decanuclear PdPt complexes. To further explore the metal-binding ability of bcarotene, we next examined the binding of bimetallic chains by bis-b-carotene p-framework. It was difficult to obtain a single bimetallic chain product simply by using Pt 2 (dba) 3 together with Pd 2 (dba) 3 in the synthetic reaction. We then thought a stepwise synthesis, that is, if metal-deficient bis-b-carotene complexes [Pd n (b-carotene) 2 ] 2 þ (nr9) can be selectively constructed with Pd, subsequent incorporation of Pt may give bimetallic chains. The metal-deficient dications [Pd n (b-carotene) 2 ] 2 þ (5rnr9) were indeed observed when the formation of 1 (Fig. 2a) was monitored by electrospray ionization mass spectroscopy (ESI-MS; Fig. 2b). Upon mixing the starting materials at ambient temperature, Pd 6 and Pd 7 complexes of b-carotene were detected as the major MS-detectable species after 3 h. Relatively small MS signals for Pd 5 , Pd 8 and Pd 9 complexes were also detected. Further incorporation of Pd into the bis-b-carotene framework proceeded gradually but was incomplete at ambient temperature after 2 days, resulting in that Pd 7 and Pd 8 complexes were the major MS-detectable species. Heating at 60°C resulted in shift of the distribution of products to higher nuclearity species, eventually affording the Pd 10 chain complexes as the major MSdetectable product. However, it has been difficult to isolate and characterize each of metal-deficient products from the reaction mixtures of the build-up reaction. We confirmed the existence of regioisomers for a short chain model, that is, We then found that the metal-deficient complexes of b-carotene can be obtained as a single product by demetalation from the Pd 10 complex 1-meso with CO. Thus, the reaction of 1 with CO (1 atmosphere (atm)) at 5°C for 1 day afforded [Pd 5 (b-carotene) 2 ][B(Ar F ) 4 ] 2 (2-meso) in 74% yield, together with a significant amount of Pd black (Fig. 4a). 13 C{ 1 H} NMR analysis as well as the single-crystal X-ray structure analysis of 2-meso showed that the Pd 5 chain occupied the half-part of the bis-b-carotene framework (Fig. 5a). The ESI-MS monitoring experiments on the demetalation reaction with CO (1 atm) at 0°C showed that the starting Pd 10 complex 1-meso and the half-filled Pd 5 complex 2-meso were present as the major MS-detectable species during the reaction (Fig. 4b). The prolonged reactions for 1 week resulted in gradual increase of the MS signal for the Pd 4 complex. The Pd 7 complex [Pd 7 (b-carotene) 2 ][B(Ar F ) 4 ] 2 (3meso) was also obtained by exposing 1-meso to CO (1 atm) at 30°C for 3 h in 20% yield (Fig. 4a). During this reaction, the major ESI-MS-detectable species were the starting Pd 10 complex and the Pd 7 complex ( Supplementary Fig. 2), while the prolonged reactions resulted in further demetalation. The 13 C{ 1 H} NMR analysis as well as the single-crystal X-ray structure analysis (Fig. 5b) showed that the Pd 7 chain was located in the bis-bcarotene framework. The demetalation of 3-meso with CO (1 atm) at 0°C occurred rapidly to afford 2-meso with complete consumption of 3-meso within 15 min. Thus, the pseudo-superposed b-carotene stacking structure was preserved during the demetalation under a CO atmosphere, giving a single regioisomer of metal-deficient sandwich. The results of the ESI-MS monitoring experiments suggested that the loss of a Pd 0 atom from the Pd 10 complex and from the Pd 5 or Pd 7 complex are relatively slow. The loss of Pd 0 likely occurs from one end of the Pd chain, while it is not easy to explain the reason why the demetalation almost stopped at the Pd 5 species or the Pd 7 species in each reaction condition. There may be several factors that affect aggregation and dissociation of metals and organic ligands (for example, M-CO affinity 30 , M-carotene bond dissociation and M-M bond dissociation). In the case of associative ligand exchange, the relatively slow release of Pd 0 from the filled Pd 10 complex is probably due to the lower accessibility of the terminal Pd atoms, which are sterically hindered by the bulky terminal b-groups of the b-carotene ligands, to CO. Consistently, demetalation from the Pd 7 complex, where one end of the Pd 7 chain is less hindered by ligands, occurred much faster at 0°C than that from the Pd 10 complex.
We next confirmed that metal-refilling reaction from isolated metal-deficient complexes proceeds smoothly by using the Pd 5 complex 2-meso, that is, addition of Pd 2 (dba) 3 Á C 6 H 6 to 2-meso in C 2 D 4 Cl 2 at 60°C afforded the Pd 10 complex 1-meso (41% yield). We then tested whether metal refilling of 2-meso with Pt 0 is possible. Thus, the bimetallic Pd 5 Pt 3 chain complex [Pd 5 Pt 3 (bcarotene) 2 ][B(Ar F ) 4 ] 2 (4-meso) was formed by treatment of 2meso with Pt 2 (dba) 3  at 30°C for 1 day (59% yield; Fig. 4a). In the absence of ethylene, the metalation with Pt 2 (dba) 3 did not proceed at the present condition. Addition of ethylene generated Pt-ethylene complexes in situ, which might be more reactive and soluble than Pt 2 (dba) 3 (ref. 31). The Pd 5 À Pt 3 mixed metal arrangement in 4-meso was confirmed by X-ray crystallographic analysis (Fig. 5c)  bond (2.657(2) Å). The Pd 5 Pt 3 arrangement was also confirmed by 13 C NMR analyses in CD 2 Cl 2 where only one set of b-carotene signals was observed at 25°C, and 13 C signals for Pt-bound carbons appeared at relatively higher field compared with those for Pd-bound carbons. Further metalation of 4-meso with Pd 2 (dba) 3 Á C 6 H 6 at 70°C gave decanuclear bimetallic chain complex [Pd 5 Pt 3 Pd 2 (b-carotene) 2 ][B(Ar F ) 4 ] 2 (5-meso) (Fig. 4a). The alternative metal arrangement, Pd 5 -Pt 3 -Pd 2 was confirmed by assignment of the upfield shifted Pt-bound carbons of the b-carotene ligands in 13 C{ 1 H} NMR analyses in CD 2 Cl 2 , that is, the substantial upfield shifts of the Pt-bound carbons of the b-carotene ligands in 5-meso relative to those in 1-meso (Dd ¼ 8-13 ppm) were observed, while the chemical shifts of the Pd-bound carbons in 5-meso are similar to those in 1-meso (Dd ¼ 0-2 ppm). Thus, b-carotene has the ability to bind bimetallic decanuclear chain, where stepwise demetalationmetalation sequence is useful in controlling the bimetal arrangement. Such reversible accommodation/liberation of multinuclear metal atoms has rarely been attained in metal cluster chemistry 32 , representing a facile dynamic metal-binding feature derived from weakly coordinating olefin p-coordination 33,34 as well as M-M bonds in organometallic sandwich frameworks 35 .
Absorption spectra of bis-(b-carotene) Pd complexes. It is noted that [Pd n (b-carotene) 2 ][B(Ar F ) 4 ] 2 showed a nuclearity-dependent absorption profile, that is, the red-shift of maximum absorption bands was observed according to decrease of the number of Pd atoms (362 nm for 1-meso, 373 nm for 3-meso and 468 nm for 2meso) (Supplementary Fig. 3). The absorption spectra were well described by the time-dependent density functional theory calculations at the Coulomb-attenuated B3LYP level 36 , suggesting that (i) the absorption bands originate mainly from the ligand-to-metal charge transfer (Supplementary Fig. 4 and Supplementary Table 2

Discussion
In this report, it has been proven that b-carotene, a naturally abundant and readily available unsaturated hydrocarbon pigment, has the ability to bind decanuclear homo-and heterometal chains through unprecedentedly large m 10 -bridging p-coordination. The present results showed that natural extended pconjugated unsaturated hydrocarbons can be utilized as the multidentate p-scaffolds for the construction of giant metal clusters. Future studies will focus on the physical and chemical properties of the rod-like bis-carotene decametal chain sandwich complexes, such as self-assembling behaviour, multielectron redox behaviour and charge mobility.

Methods
Synthesis and characterization of compounds. All manipulations were conducted under a nitrogen atmosphere using standard Schlenk or drybox techniques. The b-carotene-metal complexes were characterized by elemental analyses, ESI-MS analyses and NMR. The assignment of each resonance in NMR analysis was made with aid of heteronuclear single-quantum correlation (HSQC) or heteronuclear multiple-quantum correlation (HMQC) and heteronuclear multiplebond correlation (HMBC) techniques. Furthermore, the five complexes (1-meso, 1rac, 2-meso, 3-meso and 4-meso) were structurally determined by X-ray crystallographic analyses (Supplementary Data 1-5).