Metal-Ligand Polyhedra

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^[1]. In 2010 was reported self-assembly of a "giant" polyhedron with 24 metal ions, and a hollow spherical interior 36 Å in diameter^[2]. The self-assembly process demonstrates emergent behavior, and is reminiscent of the self-assembly of large biological structures, such as virus capsids. Such nano-spheres can also be functionalized to create, among other possibilities, synthetic receptors and nanoreactors^[3].

M24L48 Polyhedron (26 Faces)

Shown at right () is the "main chain" of a crystallographic model for the largest metal-ligand polyhedron reported as of May, 2010^[2]. form the vertices of a 26-face polyhedron^[4]. Three square faces and one triangular face meet at each vertex.

Each palladium ion is coordinated by . The nitrogens are bridged by a ("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 (but lacking the three additional nitrogens coordinating each palladium, and water, which was not resolved crystallographically). Here is the (but lacking PF₆^- and water).

The diameter of this M24L48 polyhedron is 40 Å (Pd to farthest Pd). A sphere of 50 Å circumscribes the molecular shell, and a sphere of 36 Å can be inscribed in the interior. In comparison, the diameter of the M12L24 polyhedron (see below) is 26 Å (Pd to farthest Pd).

Ligand Angle vs. Polyhedron Size

• 149^o: M24L48. The dipyridylthiphene ligand pictured above, in the M24L48 polyhedron, has a bend angle of 149^o[2].
• 127^o: M12L24 (). A ligand with a sharper bend of 127^o (dipyridylfuran^[5]) forms a smaller polyhedron, M12L24^[6].

Interestingly, mixtures of the two ligands (149 and 127 degrees) form only one size of polygon: a 3:7 mixture respectively (and up to 10:0) forms only M24L48, while a 2:8 mixture (and down to 0:10) forms only M12L24^[2].

• 90^o: M6L12. A ligand with an even sharper bend of 90^o forms M6L12^[7].

Significance

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: "endohedral functionalization". In one case, 24 perfluoroalkyl chains were caged in an M12L24 polyhedron, forming a fluorous phase potentially useful for separation, purification, or reaction control in organic syntheses^[8]. In addition, the surfaces of such polyhedra have been decorated with attached groups. Photoresponsive nanoparticles and other functionalizations have been demonstrated^[3].

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

Models

Models shown in this article were kindly provided by Makoto Fujita, who gave permission for their display here. In FirstGlance in Jmol, click Vines to display the molecule as sticks.

• M24L48 main chain atoms only: Image:M24L48 main chains.pdb
  • View in FirstGlance in Jmol
• M24L48 all atoms: Image:M24l48-single-polyhedron.pdb (This is actually in the XYZ atomic coordinate file format.)
  • View in FirstGlance in Jmol
• M12L24 all atoms: Image:M12L24-single-polyhedron.pdb
  • View in FirstGlance in Jmol