Metal-Ligand Polyhedra

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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[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]. A 2022 extensive review[4] cites potential applications in sensing, catalysis, and drug delivery[5][6].

Contents

M24L48 Polyhedron (26 Faces)

Shown at right (restore initial scene) is a crystallographic model for the largest metal-ligand polyhedron reported as of May, 2010[2]. It has an interior cavity about 32 Å in diameter. 24 palladium ions form the vertices of a 26-face polyhedron[7]. Three square faces and one triangular face meet at each vertex.

C, N,  S , Pd

Each palladium ion is coordinated by four nitrogens. The nitrogens are bridged by a dipyridylthiophene ("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 chemically complete M2L1 subunit (but lacking the three additional nitrogens coordinating each palladium, and water, which was not resolved crystallographically). Here is the complete M24L48 polyhedron (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

  • 149o: M24L48. The dipyridylthiphene ligand described above, in the M24L48 polyhedron, has a bend angle of 149o[2].
  • 127o: M12L24. A ligand with a sharper bend of 127o (dipyridylfuran[8]) forms a smaller polyhedron, M12L24[9].

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].

  • 90o: M6L12. A ligand with an even sharper bend of 90o forms M6L12[10].

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[11]. 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.

Drag the structure with the mouse to rotate

References and Notes

  1. Coxeter, H. S. M., Regular Polytopes, Dover Publications, New York, 3rd ed., 1973.
  2. 2.0 2.1 2.2 2.3 Sun QF, Iwasa J, Ogawa D, Ishido Y, Sato S, Ozeki T, Sei Y, Yamaguchi K, Fujita M. Self-assembled M24L48 polyhedra and their sharp structural switch upon subtle ligand variation. Science. 2010 May 28;328(5982):1144-7. Epub 2010 Apr 29. PMID:20430973 doi:10.1126/science.1188605
  3. 3.0 3.1 Stefankiewicz AR, Sanders JK. Chemistry. Harmony of the self-assembled spheres. Science. 2010 May 28;328(5982):1115-6. PMID:20508119 doi:328/5982/1115
  4. McTernan, Charlie T., Jack A. Davies, and Jonathan R. Nitschke, Beyond Platonic: How to Build Metal–Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes, Chem. Rev. 2022, 122, 10393−10437.
  5. Ahmad N, Younus HA, Chughtai AH, Verpoort F. Metal-organic molecular cages: applications of biochemical implications. Chem Soc Rev. 2015 Jan 7;44(1):9-25. PMID:25319756 doi:10.1039/c4cs00222a
  6. Samanta SK, Isaacs L. Biomedical Applications of Metal Organic Polygons and Polyhedra (MOPs). Coord Chem Rev. 2020 May 15;410:213181. PMID:32255835 doi:10.1016/j.ccr.2020.213181
  7. 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 Ångstroms on a side.
  8. Dipyridylfuran differs from dipyridylthiophene in that oxygen replaces the sulfur.
  9. Tominaga M, Suzuki K, Kawano M, Kusukawa T, Ozeki T, Sakamoto S, Yamaguchi K, Fujita M. Finite, spherical coordination networks that self-organize from 36 small components. Angew Chem Int Ed Engl. 2004 Oct 25;43(42):5621-5. PMID:15455450 doi:10.1002/anie.200461422
  10. Suzuki K, Tominaga M, Kawano M, Fujita M. Self-assembly of an M6L12 coordination cube. Chem Commun (Camb). 2009 Apr 7;(13):1638-40. Epub 2009 Feb 17. PMID:19294246 doi:10.1039/b822311d
  11. Sato S, Iida J, Suzuki K, Kawano M, Ozeki T, Fujita M. Fluorous nanodroplets structurally confined in an organopalladium sphere. Science. 2006 Sep 1;313(5791):1273-6. PMID:16946067 doi:313/5791/1273

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