Molecular squares structure

Several prominent examples of polynuclear poly-NHC metal complexes with original architectures were produced by the group of Hahn. To name but a few, in 2011, the group reported on the preparation of highly unusual di-NHC-based molecular squares (structure 58, Fig. 12) tetra-nuclear Ir(III) complexes with bridging Janus-type dicarbene Hgands were obtained from a tetranuclear complex precursor upon cycHzation of coordinated diisocyanides. More recently, the same group presented... 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

In the field of porous supramolecular metal complexes, both molecular and extended-solid materials have been extensively studied in recent years. A particularly well-studied class of compounds is the metal-containing molecular squares, that is, square-shaped porous tetrameric structures (30,108). These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis-coordination sites (109-113) (Fig. 13). However, the resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. 

Figure 1. Structure of a cyanide-bridged Fe(II)-M(II) molecular square (L = bpy M = Fe,Co,Cu).

Fujita, M., Sasaki, 0., Mitsuhashi, T., et al., On the structure of transition-metal-linked molecular squares. Chem. Commun. 1996, 1535-1536. 

FIGURE 5.19 (a) Chiral molecular square of the [M4L4] type, (b) ligand structure, and... 

Shiu K-B, Lee H-C, Lee G-H, Wang Y (2002) Synthesis, structures, and solvent-occlusion properties of a molecular loop and a molecular square using tetracarbonyl- and diphosphine-ligated diruthenium(I) as building blocks and dicarboxylates as linkers. Organometallics 21 4013 1016... 

Square structures are prepared from metal complexes containing the required 90° angle. Shown in Scheme 4, neutral-charged molecular square 37 is prepared by spontaneous self-assembly of complex 30 and 36 <1997JA2524>. Synthesis of precursor complex 30 is accomplished by coupling of Pt(ll) chloride 34 and ethynylpyridine anion 35. 

Reactions that involve cis-protected Mo2(DAniF)2 units have led to a variety of square structures with dicarboxylate anions. The linkers that have been used are oxalate (121), fumarate (122), ferrocenedicarboxylate (123) and 4,4 -biphenyldicarboxylate (124). The midpoints of the M02 units constitute a square, and their structures have been confirmed by H NMR spectroscopy and X-ray studies. Similar molecular squares containing Rh2 + units have been prepared with linkers such as oxalates (120), bicyclo[l.l.l]pentane-1,3-dicarboxylate (125), tetrafluoroterephthalate (126), 1,4-cubanedicarbboxylate (127), terephthalate (128), fumarate (129) and trans 1,4-cyclohexanedicarboxylate (130). The structures of these molecules have been established by spectroscopic and single-crystal diffraction studies. Detailed electrochemical investigations of these complexes have also been undertaken. ... 

Kou, H., Gao, S., Li, C. etal. (2002) Characterization of a soluble molecular magnet unusual magnetic behavior of cyano-bridged Gd(lll)-Cr(lll) complexes with one-dimensional and nanoscaled square structures. Inorganic Chemistry, 41, 4756-4762. 

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