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Osaka University 100 Papers : 10 Selected Papers

ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2006-2007

Engineering

Discrete Sandwich Compounds of Monolayer Palladium SheetsPaper in journals : this is the first page of a paper published in Science.[Science] 313, 1104-1107 (2006)

From Science, 313, Murahashi, T. et al., Discrete Sandwich Compounds of Monolayer Palladium Sheets, 1104-1107, 2006. Reprinted with permission from AAAS.

Discrete Sandwich Compounds of Monolayer Palladium SheetsMURAHASHI Tetsuro and KUROSAWA Hideo(Graduate School of Engineering)

22 ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2006-2007

salt [C7H7][BF4], which is the seven-membered aromatic cati-on, in the presence of PPh4Cl at ambient temperature (Fig. 2A). The molecular structure of the complex 1 was determined by X-ray crystallographic analysis (Fig. 2B). The seven-membered hydrocarbon rings flank the triangular array of three palladium atoms capped by Cl ligands. The Pd−Pd distances (2.7550(5) Å, 2.7446(5) Å, 2.7889(5) Å) are in the range of normal Pd−Pd bond distances (cf. the Pd−Pd length of bulk Pd metal is 2.76 Å). The complex 1 is air- and moisture stable, and no decom-position was observed even after heating in solution at 60 °C, while the mononuclear sandwich complexes of Pd such as Pd(h3-allyl)2 are usually unstable in solution. Substitution of Cl ligands with other ligands proceeded without decomposition of the bis-cycloheptatrienyl-Pd3 sandwich framework: [Pd3(μ3-C7H7)2(CH3CN)3][BF4]2 (1-CH3CN) was formed by treatment of 1 with AgBF4 in CH3CN, and [Pd3(m3-C7H7)2(PPh3)3][BF4]2 (1-PPh3) was formed by addition of excess PPh3 to 1-CH3CN.

The trinuclear sandwich structure of 1 was analyzed by theoretical studies. The structural optimization of [Pd3(m3-C7H7)2Cl3]− using density functional theory (DFT) calculations reproduced well the structure of 1. MO-based fragment overlap population analysis suggested that dσ−dσ antibonding orbit-als of [Pd3Cl3]3− fragment mainly participate in back-donation to the [C7H7]+ ligands, while dπ−dπ bonding orbitals are also involved in the back-donating interaction. Donating interaction from the π-orbitals of the [C7H7]+ ligands to the Pd3 orbitals

Introduction

Half a century ago, the sandwich structure of ferrocene Fe(C5H5)2 was elucidated by G. Wilkinson, R. B. Wood-

ward, and E. O. Fischer (Scheme 1).1 Since then, sandwich complexes have been synthesized for nearly all metallic ele-ments in the periodic table. The structural concept of the molec-ular sandwich revolutionized the chemistry of metal com-plexes, and brought us the principle of π-coordination of cyclic unsaturated hydrocarbons to a metal center. Typical examples of known sandwich complexes are shown in Scheme 1. Most of the known sandwich complexes are the mononuclear com-plexes in which cyclic unsaturated hydrocarbon ligands flank a single metal atom. Some dinuclear sandwich complexes are also known. In light of the fact that metal atoms tend to gather by themselves to form a metal cluster, it is of great interest to verify whether three or more metal atoms can be incorporated into the sandwich structure. Thus, we hypothesized the exist-ence of stable multinuclear metal sandwich complexes,2 and aimed at synthesizing the unprecedented metal sheet sandwich complexes, of which the illustrated images are shown in Fig. 1. In our paper, we described the successful isolation and struc-tural characterization of the first discrete sandwich complexes containing metal sheets.

SYNTHESIS AND STRUCTUREThe first stable sandwich complexes of triangular trimetal

core were synthesized by the reaction of a zerovalent Pd com-plex Pd2(dba)3 (dba = dibenzylideneacetone) and a tropylium

The following is a comment on the published paper shown on the preceding page.

Fig 1. Illustrated models of hypothetical metal monolayer sandwich com-pounds.

Scheme 1. Typical examples of known mono- and dinuclear sandwich com-plexes (From left, Fe(C5H5)2 (ferrocene), Cr(C6H6)2, and Pd2(C6H6)2(AlCl4)2).

Fig 2. (A) Synthesis of the cycloheptatrienyl tripalladium sandwich complex 1. (B) Thermal ellipsoid (50%) drawing of 1 (PPh4 cation and water were omitted for clarity).

References

1 (a) G. Wilkinson, et al. J. Am. Chem. Soc. 74, 2125 (1952). (b) E. O. Fischer, W. Pfab, Z. Naturforsch. B., 7, 377 (1952).

2 (a) T. Murahashi, et al. J. Am. Chem. Soc. 121, 10660 (1999). (b) T. Murahashi, et al. J. Am. Chem. Soc. 125, 8436 (2003).

3 For our publications after this paper: (a) T. Murahashi, et al. Angew. Chem. Int. Ed. 46, 3509 (2007). (b) T. Murahashi, et al. Angew. Chem. Int. Ed. 46, 5440 (2007).

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Osaka University 100 Papers : 10 Selected Papers

ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2006-2007

also contribute to the bonding interaction between the C7H7 ligands and the Pd3 center. The Wiberg bond indices (WBIs) indicate the presence of weak Pd−Pd bonding interaction in the triangular tripalladium core.

The palladium sheet sandwich complex was also obtained by using polycyclic aromatic hydrocarbon ligands. The pen-tapalladium sandwich sheet complex 4-toluene was formed by the reaction of [Pd2(CH3CN)6][BF4]2 (3), naphthacene, and NaBArf (BArf = B{3,5-(CF3)2C6H3}4) in refluxing 1,2-dichlo-roethane, followed by recrystallization from CH2Cl2/toluene solution. The toluene ligand dissociated during precipitation from CH2Cl2/n-hexane to give a sandwich complex 4 (Fig. 3). The structure of 4-toluene was determined by X-ray crystallo-graphic analysis (Fig. 4). The two naphthacene ligands flank the Pd5 sheet through μ5-h2:h2:h2:h3:h3-coordination mode. Pd−Pd distances are in the range of normal Pd−Pd bond distances, while Pd4−Pd5 length is somewhat long (Fig. 4C). 13C{1H} NMR chemical shifts indicate that the Pd5(naphthacene)2 sandwich structure in the crystalline state is maintained in solution, although it is not clear whether toluene ligand is bound to the apical Pd in the solution. Theoretical analyses of [Pd5(naphthacene)2]2+ suggested that the donation from naph-thacene ligands and back-donation from the [Pd5]2+ moiety contribute to the bonding between [Pd5]2+ fragment and naph-thacene ligands. In the Pd5 sheet, there are very weak Pd−Pd interactions.

Fig 3. Synthesis of compounds 4-toluene and 4.

CONCLUSION

This work showed for the first time that metal sheet sand-wich complexes exist as the stable, discrete molecules. We propose that sandwich compounds containing different sizes and shapes of metal sheet can be synthesized by employing dif-ferent extended pπ-conjugated carbon frameworks, with aid of their template effect.3 Also, other metal elements may be used in the metal sheet sandwich complexes.

Fig 4. Molecular structure of 4-toluene: (A) 50% probability ellipsoids diagram, (B) top view with ball-stick drawing, (C) bond distances (Å) and angles (deg) of the Pd5 sheet in 4-toluene. (D) 1H and 13C{1H} NMR chemical shifts (ppm) of the unsaturated hydrocarbon ligands of 4-toluene•3(C7H8) in CD2Cl2 at 25 °C.