Metal-Ligand Polyhedra: Difference between revisions

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Metal ions with square planar coordination, when mixed with bent bidentate ligands, can self-assemble into polyhedra of various sizes. Geometrical constraints limit the number of metal ions (vertices) to 6, 12, 24, 30, or 60 for entropically favored regular or semiregular polyhedra<ref>Coxeter, H. S. M., ''Regular Polytopes'', Dover Publications, New York, 3rd ed., 1973.</ref>. Shown at right (<scene name='Metal-Ligand_Polyhedra/Polyhedron_main_chains/4'>restore initial scene</scene>) is the "main chain" of a crystallographic model for the largest such structure reported as of May, 2010<ref name="sun-fujita-2010" >PMID: 20430973</ref>. <scene name='Metal-Ligand_Polyhedra/Polyhedron_main_chains/5'>24 palladium ions</scene> form the vertices of a 26-face polyhedron<ref>M24L48 forms a 26-faced ''rhombicubooctahedron'' with 18 square faces and 8 triangular faces. In this instance, the rectangular faces are very close to squares 13.35 &Aring;ngstroms on a side.</ref>. Three square faces and one triangular face meet at each vertex.

Each palladium ion is coordinated by <scene name='Metal-Ligand_Polyhedra/Polyhedron_main_chains/6'>four nitrogens</scene>. The nitrogens are bridged by a <scene name='Metal-Ligand_Polyhedra/Single_main_chain_ligand/1'>dipyridylthiophene</scene> ("ligand"). There are two ligand molecules (L) per metal ion (M); hence, this structure is called '''M24L48'''.

The models shown thus far are simplified, including only the "main chain". The actual M24L48 complex analyzed crystallographically contained a substituent of -OCH₂CH₂O- on each thiophene ring, PF₆^- counterions bound to the surface of the polyhedron, and hydrogen atoms. Here is the <scene name='Metal-Ligand_Polyhedra/M2l1_with_och2ch2o_h_pf6/1'>chemically complete M2L1 subunit</scene> (but lacking the three additional nitrogens coordinating each palladium, and water, which was not resolved crystallographically). Here is the <scene name='Metal-Ligand_Polyhedra/Full_polyhedron/2'>complete M24L48 polyhedron</scene> (but lacking PF₆ and water).

The dipyridylthiphene ligand pictured above, in the M24L48 polyhedron, has a bend angle of 149^o<ref name="sun-fujita-2010" />. A ligand with a sharper bend of 129^o (dipyridylfuran<ref>Dipyridylfuran differs from dipyridylthiophene in that oxygen replaces the sulfur.</ref>) forms a smaller polyhedron, M12L24<ref name="tominaga-fujita-2004">PMID: 15455450</ref>. A ligand with an even sharper bend of 90^o forms M6L12<ref name="suzuki-fujita-2009">PMID: 19294246</ref>.

Metal-ligand polyhedra could serve as nanoreactors containing a chemically defined nano-environment. Similar polyhedra have been constructed from ligands with covalent adducts facing the interior. Specifically, 24 perfluoroalkyl chains have been caged in an M12L24 polyhedron, forming a fluorous phase potentially useful for separation, purification, or reaction control in organic syntheses<ref>PMID: 16946067</ref>. In addition, the surfaces of such polyhedra have been decorated with attached groups. Photoresponsive nanoparticles have been demonstrated<ref>PMID: 20508119</ref>.

More generally, self-assembly of metal-ligand polyhedra demonstrates emergent behavior, in which microscopic differences lead to macroscopic differences.  Such self-assembly is reminiscent of the assembly of virus capsids and other biological structures.