Ligand as spacer

Transition-metal ions are employed as nodes and bifunctional ligands as spacers. Commonly used spacer ligands are pseudohalides such as cyanide, thiocyanate, and azide, and N-donor ligands such as pyrazine, 4,4 -bipyridine, and 2,2/-bipyrimidine. Besides discrete supermolecules, some one-, two-, and three-dimensional architectural motifs generated from this strategy are shown in Fig. 20.3.9. 

Recent investigations have shown that molybdenum and tungsten silanols with the r -cyclopentadienyl ligand as spacer between the metal and the silanol unit are stable with respect to self-condensation [1, 4, 5]. Extension of this work is now focused on compounds of this type containing, in addition, a metal-bound silanol function. 

The earliest examples of coordination polymers constructed through the node-and-spacer approach utilized metal ions as nodes and neutral N-donor ligands as spacers. Examples of neutral N-donor ligands utilized in the construction of coordination polymers include pyrazine, 4,4 -bipyridine, ° pyrimidine, triazine, and hexamethylenetetramine (HMTA) (Figures 4-6). Examples of permanently porous MOFs based on neutral N-donor ligands are scarce, mainly due to the relatively flexible nature of the N-M bonds around the metal center.In addition, framework interpenetration is commonly observed in this class of coordination polymers, limiting the maximal attainable pore size in isoreticular compounds constructed through expansion of the linker. ... 

When the metalloporphyrin bears a donor group on its periphery, it can behave as a self-complementary ditopic unit capable of metal-ligand induced dimerization. Many systems have been synthesized using different metals, ligands, and spacers. The length and geometry of the spacer groups determine the stoichiometry of the assembly process. 

The structure of material 6 is a 2D coordination network of twofold interpenetration, in which cubane-like Cu4I4 clusters act as connecting nodes and the dps ligands as the spacer. The intramolecular Cu—Cu distances range from 2.563(1) to 2.734(2) A. Like in the case of [ Cu(p,-I)2Cu (dps)2]ra 5, ligation... 

The combination of an efficient control over the environment of the active sites in a multi-functional catalyst and its immobilization within an insoluble macromolecular support was pioneered by Seebach et al. In their approach, the chiral ligand to be immobilized was placed in the core of a polymerizable dendrimer, followed by copolymerization of the latter with styrene as shown in Scheme 9 [58]. In this way, no further cross-finking agent was necessary, since the dendrimer itself acted as cross-linker. The dendritic branches are thought to act as spacer units, keeping the obstructing polystyrene backbone... 

Standard peptide coupling chemistry is performed (Box 25). The acid component is transformed to an activated ester derivative and then treated with the amine [17-19]. This procedure is repeated twice. In the final step the ligands la-le-H4 are deprotected by aryl ether cleavage with BBr3. By use of this simple reaction sequence derivatives with glycine, alanine, phenylalanine, valine, leucine, and other amino acids as spacers were obtained [16]. 

The microporous structure in 5 is produced based on Cu(II) hydroxide structural members joined together by the btec ligands (Fig. 4). The complex can be viewed as being made up from assemblies of tubes with rectangular sections in which the metal hydroxide chains are the walls of the tube and the btec " ligands act as spacers. The Cu(ll) atoms are all four coordinated (two carboxyl O atoms and two aqua O atoms) with square-plane configuration with two types. In one type, two carboxyl O atoms are in para position, while in the other type they are in trans place. 

Oligophenylene rods [92, 112] have attracted much interest as spacers [113], because of their thermal and photochemical stability, and their interesting tunable photophysical properties. Until recently their use as bridging ligands was limited to species containing only one or two phenylene units (see compounds 23-26, Figure 8) because of synthetic difficulties related to solubility problems [24, 33, 74, 97, 114-117]. 

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