Metallo-Supramolecular Self-Assembly: the Case of Triangle-Square Equilibria

For the efficient self-assembly of metallo-supramolecular complexes, not only reversibility is required but also two other properties have to be controlled as well: (i) The right binding sites need to be programmed into the building blocks at the appropriate positions. (ii) The building blocks must be rigid enough to support the geometrical arrangement and to avoid the unfavorable entropy effects connected with the conformational fixation of flexible molecules. A series of different bis-pyridyl ligands is reported which self-assemble with (dppp)M(OTf)2 complexes (dppp = 1,3-bis-(diphenylphosphino)propane; M = PdII, PtII) to yield squares and/or triangles as the products. Enthalpic contributions (higher strain in the triangle) and entropic contributions (higher number of triangles from the same building blocks) determine the equilibrium. The effects of concentration, temperature, and solvent properties on the equilibrium have been studied. To characterize the complexes under study, a combination of 1H, 31P, and diffusion-ordered NMR spectroscopy, electrospray-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry, and X-ray crystallography is needed. Variable-temperature NMR spectroscopy provides evidence for fast ligand-exchange processes occurring for the Pd complexes, while the Pt complexes exchange ligands much more slowly

Metallo-Supramolecular Self-Assembly: the Case of Triangle-Square Equilibria

For the efficient self-assembly of metallo-supramolecular complexes, not only reversibility is required but also two other properties have to be controlled as well: (i) The right binding sites need to be programmed into the building blocks at the appropriate positions. (ii) The building blocks must be rigid enough to support the geometrical arrangement and to avoid the unfavorable entropy effects connected with the conformational fixation of flexible molecules. A series of different bis-pyridyl ligands is reported which self-assemble with (dppp)M(OTf)2 complexes (dppp = 1,3-bis-(diphenylphosphino)propane; M = PdII, PtII) to yield squares and/or triangles as the products. Enthalpic contributions (higher strain in the triangle) and entropic contributions (higher number of triangles from the same building blocks) determine the equilibrium. The effects of concentration, temperature, and solvent properties on the equilibrium have been studied. To characterize the complexes under study, a combination of 1H, 31P, and diffusion-ordered NMR spectroscopy, electrospray-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry, and X-ray crystallography is needed. Variable-temperature NMR spectroscopy provides evidence for fast ligand-exchange processes occurring for the Pd complexes, while the Pt complexes exchange ligands much more slowly

Mass Spectrometric Detection and Fragmentation Patterns of Synthetically Useful Chromium and Tungsten Carbene Complexes

A novel method for the detection of synthetically useful pentacarbonyl chromium and tungsten carbene complexes by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry is presented which utilizes the addition of CN- anions to the electrophilic carbene carbon. This method is nondestructive, provided the ionization proceeds under soft conditions. The scope and limitations of this method are examined by utilizing different types of carbene complexes. Depending on the α-CH acidity of the carbene complexes, [M − H]- anions can be observed either in competition with the CN- adducts or as the only ions, even in the presence of cyanide. Secondary aminocarbene complexes exclusively form [M − H]- anions by deprotonation of the NH moiety. The ions under study have been mass-selected and subjected to collision-induced decay (CID) experiments in order to examine their fragmentation reactions. CO loss is not the only observable process. Most prominently, 1,2-eliminations and a retro-aldol reaction within a sugar carbene complex compete with decarbonylation and thus have rather low activation barriers. Some reactions can be attributed to coordinatively unsaturated chromium atoms formed through preceding CO losses