Wheel structures

The use of dirhodium(II) catalysts for catalytic reactions with diazo compounds was initiated by Ph. Teyssie [14] in the 1970s and rapidly spread to other laboratories [1]. The first uses were with dirhodium(II) tetraacetate and the more soluble tetraoctanoate, Rh2(oct)4 [15]. Rhodium acetate, revealed to have the paddle wheel structure and exist with a Rh-Rh single bond [16], was conve-... 

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

Fig. 20 SubPc (a) and C o (b) form at 1 1 ratio on the Ag(lll) surface a six-molecule pin-wheel structure [51]. The STM image (c, 13.6nm x 13.6 nm) reveals locally only clusters of equal handedness. All subPc molecules (triangles) of a single pinwheel are rotated into the same direction. Reprinted with permission from Wiley...

Fig. 4.57. Ferris-wheel structure formed with lanthanum chloride interacting with p-sulfonatocalix[4]arene and 18-crown-6. Redrawn from A. Drljaca et al., Chem. Commun. 1135, 1999.

Potentially bidentate ligands, such as the formamidinates [RNCR NR], the monoanion derived from 2,6-bis(phenylamino)pyridine and 7-methyl-l,8-naphthyridin-2-one, and others, formed complexes with a paddle wheel structural motif. Figure 5(d), and some of these complexes could be oxidized by ferrocenium cations to give mixed-valent species. Generally, such oxidation led to a slight lengthening of the metal-metal bond. 

In situ STM studies of the oxidation of a Pd film in the SMSI state at elevated temperature show a thickening of the encapsulating film (Fig. 8.7a-c). The film prior to oxidation had a hexagonal pin-wheel structure on the raised triangular island on the Pd film. After oxidation (Fig. 8.7c), the island was decorated heavily with a thickened, rough layer of Ti implying the formation of an oxide film of higher stoichiometry (possibly TP+), or mass transport of Ti to the Pd surface from the Ti surface. 

The structures labeled (w, and w )-TiO on Pt(l 11) correspond to three hexagonal wagon-wheel structures found by LEED and STM. The w phase grows under oxidizing conditions, the w phase is produced by exposing the w phase to reducing conditions, and the w. appears as an intermediate phase between these conditions which always coexists with at least one of the other w phases. 

Fig. 8.10 The original proposal for the pin-wheel structure with a Pd-O-Ti stacking sequence. The O layer sits in threefold Pd sites, the hexagonal Ti layer has a lattice constant of 3.25 A and is rotated 3° with respect to the Pd lattice. Substrate and overlayer unit cells are outlined. Reprinted with permission from Bennett et al. [20], copyright (2002) American Chemical Society...

The synthesis of [Rh2(OAc)4(H20)2] from RhCl3-3H20, HOAc and NaOAc first gives [Rh2Cl2(OAc)4]2, which has now been crystallised and found to have the expected paddle-wheel structure the cations of the hydrated sodium salt are co-ordinated to one Cl and one OAc oxygen on adjacent dimers and to two water molecules.152 [Rh2(02CCF3)4(thf)] exists as two isomers, one being polymeric with... 

The Er + ions are linked by hydroxo and 0x0 bridges to give two types of small cluster cores cubic [Er4(/t3-0)(/i3-0H)3] + (Er4) and dimeric [Er2(/i3-OH)2] + (Er2) cores (Figure 104, middle). Different from the familiar cubane [R4(/i3-OH)4] + core which contains four /is-OH groups, the present tetranuclear core consists in three /la-OH groups and one atom, which is unprecedented. The (Er4) cores link alternately the (Er2) units to form a nanosized (Er3g) wheel structure. 

Schmidbaur. H. Hartmann, C. Reber. G. Muller, G. Isovalent and mixed-valent ylide complexes of gold The synthesis of trinuclear compounds having double paddle-wheel structure. Angew. Chem.. Int. Ed. Engl. 1987. 26. 1146. 

Introducing a second metal also opens the way to bimetallic redox synergy, which has been observed in paddlewheel binuclear rhodiumfll) catalysts. These chiral binuclear complexes are able to promote a variety of catalytic reactions like cyclopropanation [50], cyclopropenation [51], and C-H insertion [52-54] reactions achieving very high enantioselectivities (Scheme 12) (for reviews, see [55-57]). In addition to contributing to the paddle wheel structure of the complex, the role of the second rhodium is to stabilize its rhodium(II) neighbor that accounts for the high catalytic activity. 

In order to examine the possibility of this speculation, an approach using synthetic peptides was made [10]. Three kinds of peptides (H, S, and R) with 16 amino acid residues were synthesized, and their secondary structure and surface properties were investigated to clarify the effects of conformational amphiphilicity. The amino acid compositions of the three peptides were the same (8 Leu and 8 Glu residues), but their sequences were different. The helical wheel structure of peptide H is shown in Fig. 2e [5], As shown in this structure, peptide H was designed to form an amphiphilic a-helix, whereas the other peptides were designed not to show such amphiphilicity, even when... 

FIG. 2 Helical wheel structures of (a) Os,-casein residues 12-23 (b) bovine serum albumin residues 383-396 (c) bovine serum albumin residues 541-555 (d) P-lacto-globubn residues 125-143 and (e) synthetic peptide H. Hydrophobic amino acids are shown by closed circles, hydrophUic charged amino acids by open circles, and hydro-phibc uncharged amino acids by shaded circles. (From Ref. 5.)... 

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