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The reaction of cis-blocked, square-planar M II complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M 2n L n . 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 Pt 8 L 4 cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of M II 2n L n complexes are not well understood. Herein, we provide a geometric analysis of M 2n L n 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 Pt II complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt 8 L 4 , which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt 10 L 5 complex and a Pt 16 L 8 complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt 16 L 8 complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M 2n L n complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt 10 L 5 barrels, which are formed in nearly quantitative yields.

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe(II)8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe(II)4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd(II)-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd(II) cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N2O adducts depends on steric as well as electronic effects. The coordination of N2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes-N2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.

Chemically induced bond cleavage of nitrous oxide typically proceeds by rupture of the N-O bond with concomitant O-atom transfer and liberation of dinitrogen. On a few occasions, N-N bond scission has been observed instead. We report a reaction sequence involving an N-heterocyclic carbene and a vanadium complex that results in cleavage of both the N-O bond and the N-N bond., (© 2012 American Chemical Society)