The Intricate Structural Chemistry of MII2nLn-Type Assemblies.

The reaction of cis-blocked, square-planar MII complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M2nLn. These assemblies typically adopt barrel-like structures, with the ligands paneling the sides of the barrels. However, alternative structures are possible, as demonstrated by the recent discovery of a Pt8L4 cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of MII2nLn complexes are not well understood. Herein, we provide a geometric analysis of M2nLn complexes, and we discuss how size and geometry of the ligand is expected to influence the self-assembly process. The theoretical analysis is complemented by experimental studies using different cis-blocked PtII complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt8L4, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt10L5 complex and a Pt16L8 complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt16L8 complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M2nLn complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt10L5 barrels, which are formed in nearly quantitative yields.

Carboxylic Acid Functionalized Clathrochelate Complexes: Large, Robust, and Easy-to-Access Metalloligands.

Polycarboxylate ligands are among the most important building blocks for the synthesis of metal-organic frameworks (MOFs). The ability to access these ligands in an efficient way is of key importance for future applications of MOFs. Here, we demonstrate that mono- and dinuclear clathrochelate complexes are versatile scaffolds for the preparation of polytopic carboxylate ligands. The largely inert clathrochelate complexes have a trigonal-bipyramidal shape. The synthesis of functionalized clathrochelates with two, three, four, or five carboxylic acid groups in the ligand periphery can be achieved in a few steps from simple starting materials. Apart from being easily accessible, the metalloligands display interesting characteristics for applications in metallasupramolecular chemistry and materials science: they are rigid, large (up to 2.2 nm), and robust and they can show additional functions (e.g., fluorescence or extra charge) depending on the metal ion that is present in the clathrochelate core. The utility of these new metalloligands in MOF chemistry is demonstrated by the synthesis of zinc- and zirconium-based coordination polymers. The combination of Zn(NO3)2 with clathrochelates having two or three carboxylic acid groups gives MOFs in which the clathrochelate ligands are connected by Zn4O clusters or zinc paddlewheel links. The structures of the resulting two- and three-dimensional networks could be established by single-crystal X-ray crystallography. The reaction of carboxylic acid functionalized clathrochelates with ZrCl4 gives amorphous powders that display permanent porosity after solvent removal.

Anionic bipyridyl ligands for applications in metallasupramolecular chemistry.

The facile synthesis of anionic bipyridyl ligands with dinuclear clathrochelate cores is described. These metalloligands can be obtained in high yields by the reactions of M(ClO4 )2 (H2 O)6 (M: Zn, Mn, or Co) with 4-pyridylboronic acid and 2,6-diformyl-4-methylphenol oxime or 2,6-diformyl-4-tert-butylphenol oxime, followed by deprotonation. The ligands are interesting building blocks for metallasupramolecular chemistry, as evidenced by the formation of a Pt-based molecular square and four coordination polymers with 2D or 3D network structures. Competition experiments reveal that the utilization of anionic bipyridyl ligands can result in significantly more stable assemblies.

Surface functionalization of dinuclear clathrochelates via Pd-catalyzed cross-coupling reactions: facile synthesis of polypyridyl metalloligands.

Dinuclear clathrochelate complexes are easily accessible by reaction of zinc(ii) triflate or cobalt(ii) nitrate with arylboronic acids and phenoldioximes. The utilization of brominated arylboronic acids and/or brominated phenoldioximes allows preparing clathrochelates with two, three, five or seven bromine atoms on the outside. These clathrochelates can undergo Pd-catalyzed cross-coupling reactions with 3- and 4-pyridylboronic acid to give new metalloligands featuring up to seven pyridyl groups. The pyridyl-capped clathrochelates display characteristics which make them interesting building blocks for structural supramolecular chemistry: they are rigid, large (up to 2.7 nm), luminescent (for M = Zn), and anionic. The pentatopic pyridyl ligands display an unusual trigonal bipyramidal geometry.

Dinuclear clathrochelate complexes with pendent cyano groups as metalloligands.

Dinuclear clathrochelate complexes with two, three, four, or five cyano groups in the ligand periphery were prepared following two distinct synthetic strategies: (a) zinc(ii)- or cobalt(ii)-templated polycondensation reactions of CN-functionalized arylboronic acids and phenoldioximes, or (b) postsynthetic cross-coupling reactions of polybrominated zinc(ii) clathrochelates with 4-cyanophenylboronic acid. The new clathrochelate complexes were used as metalloligands for the construction of heterometallic Zn2+/Ag+ and Co2+/Ag+ coordination polymers (CPs), which were characterized by single crystal X-ray diffraction and FT-IR. A one-dimensional CP was observed for ditopic clathrochelates, whereas two- and three-dimensional CPs were generated from tetra- and pentatopic metalloligands. The three-dimensional network is unique as it displays an unprecedented network topology with the point symbol (8·102)(82·104)(82·10)(83·103). Furthermore, it is a self-catenated net with an extremely high topological density.