Molecules with Large Cavities in Supramolecular Chemistry

Supramolecular chemistry is a new area of research that has rapidly developed from pure synthetic chemistry, and its novelty has led to interdisciplinary cooperation between organic and inorganic chemistry, biochemistry, physical and theoretical chemistry, and physics. Whereas molecular chemistry essentially deals with the covalent bonding of atoms, Supramolecular chemistry is predominantly involved in the study of the weaker intermolecular interactions resulting in the association and self-organization of several components to form larger aggregates (supramolecules). The first crown ether discovered by the subsequent Nobel prizewinner Pedersen was more the fortuitous reaction product of an impurity, but nowadays, some twenty-five years later, chemists are able to tailor host molecules to special requirements. Host compounds having a cyclophane skeleton make an important contribution, since their aromatic structural units ensure the necessary rigidity of the molecular structures and thereby improve the preorganization of the coordination sites for the cooperative binding of the guests. During the course of the rapid development of Supramolecular chemistry such a large number of synthetic hosts has been developed and their interaction with guests studied in such depth that we must restrict ourselves here to a discussion of a particular group of host compounds, namely cavity-supporting macrobicyclic and macrooligocyclic phanesu, which bear a similar relation to open-chain and monocyclic hosts as the metal-complexing cryptands to the podands and crown ethers. The molecular architecture of these three-dimensionally bridged macrooligocycles is a challenge for synthetic chemistry. (Not only the size and shape of the intramolecular cavity, but also the provision of the latter with suitable coordination centers have to be included in the synthesis strategy.) The capacity for the envelopment of guests from all sides and the expedient endo functionalization often also produce a particularly strong binding of host and guest, outstanding selectivities with regards to molecular recognition, and special properties of the Supramolecular complexes.     

Bildung eines Amin‐stabilisierten sechsgliedrigen Co2S4‐Heterocyclus: Synthese und Struktur eines ungewöhnlichen zweifach μ‐η1 : η1‐Disulfid‐verbrückten binuklearen Übergangsmetallkomplexes

The Formation of an Amine Stabilized Co₂S₄ Heterocycle: Structure and Synthesis of a Rare Example of a Double μ‐η¹ : η¹ Disulfido Bridged Binuclear Metal Complex Treatment of Co(BPh₄)₂ · 6 MeCN and Co(ClO₄)₂ · 6 MeCN with S₂^2– or S₄^2– in the presence of the macrocyclic tetraaza ligand 1,4,8,12‐tetraazacyclopentadecane ([15]aneN₄) afforded the new binuclear cobalt sulfur compounds [Co(μ‐S₂)([15]aneN₄)]₂[BPh₄]₂ ( 1 ) and [Co(μ‐S₂)([15]aneN₄)]₂[ClO₄]₂ ( 2 ), respectively, in which two cobalt atoms are bridged by two μ‐η¹ : η¹ disulfido groups forming six‐membered Co₂S₄ heterocycles.     

Synthesis of N,O-heterocyclic carbene and coordination to rhodium(I) and copper(I)

5,7-Di-tert-butyl-3-phenyl benzoxazolium tetrafluoroborate 1 could be prepared by simple reaction of the corresponding aminophenol with triethyl orthoformate under acidic conditions. Both rhodium(I) and copper(I) complexes with benzoxazol-2-ylidene ligand were then efficiently synthesised in a straightforward and smooth manner involving the reaction of benzoxazolium salt 1 with metal precursor and an external base. The complexes have been fully characterised and used in metal-catalysed hydrosilylation of ketones, where they showed poor catalytic activity, presumably due to low stability of the complexes under those conditions.     

Structural and mechanistic studies on ion insertion into the molecular box {[CpCo(CN)3]4[Cp*Ru]4}

The molecular box [CpCo(CN)₃]₄[Cp*Ru]₄ (Co₄Ru₄) reacts readily with a variety of monocations to form M⊂Co₄Ru₄⁺ (M=K⁺, Cs⁺, Rb⁺). Ion competition experiments, monitored by ESI-MS, show that the molecular box binds the smaller K⁺ more rapidly than Cs⁺, but that thermodynamically Co₄Ru₄ prefers the larger ion. The rates of ion-insertion for K⁺ and Cs⁺ into Co₄Ru₄ were found to qualitatively follow second order kinetics with K⁺, 300 M⁻¹ s⁻¹ and Cs⁺, 36 M⁻¹ s⁻¹. The ratio k_K/k_Cs qualitatively matched the ESI-MS results from ion competition experiments. The rates of ion-insertion into Co₄Ru₄ were found to depend on the counter anions. In particular, RbBF₄ reacted with Co₄Ru₄ more slowly than did RbOTf. The slower rates allowed us to establish second order kinetics. ¹H NMR studies reveal that the Cp signal for Co₄Ru₄ is very sensitive to the presence of entering ions, e.g., Rb⁺, whereas the corresponding Cp signal for Rb⊂Co₄Ru₄⁺ was insensitive to the presence of Rb⁺. The molecular structures of [Co₄Ru₄] · 6MeCN, [K⊂Co₄Ru₄]BF₄ · 7MeCN, [Cs⊂Co₄Ru₄]BF₄ · 6MeCN and [Tl⊂Co₄Ru₄]BF₄ · 6MeCN, determined by X-ray diffraction, showed that although the compounds crystallized in the same space group I23, a correlation exists between the Ru-N/Co-C bond distances and the size of the interstitial ion.     

Systematische Studie zu den Koordinationseigenschaften des Guanidin‐Liganden Bis(tetramethylguanidino)propan mit den Metallen Mangan,Cobalt, Nickel,Zink, Cadmium,Quecksilber und Silber

The transition metal complexes with the ligand 1,3‐bis(N,N,N′,N′‐tetramethylguanidino)propane (btmgp), [Mn(btmgp)Br₂] ( 1 ), [Co(btmgp)Cl₂] ( 2 ), [Ni(btmgp)I₂] ( 3 ), [Zn(btmgp)Cl₂] ( 4 ), [Zn(btmgp)(O₂CCH₃)₂] ( 5 ), [Cd(btmgp)Cl₂] ( 6 ), [Hg(btmgp)Cl₂] ( 7 ) and [Ag₂(btmgp)₂][ClO₄]₂·2MeCN ( 8 ), were prepared and characterised for the first time. The stoichiometric reaction of the corresponding water‐free metal salts with the ligand btmgp in dry MeCN or THF resulted in the straightforward formation of the mononuclear complexes 1 – 7 and the binuclear complex 8 . In complexes with M^II the metal ion shows a distorted tetrahedral coordination whereas in 8 , the coordination of the M^I ion is almost linear. The coordination behavior of btmgp and resulting structural parameters of the corresponding complexes were discussed in an comparative approach together with already described complexes of btmgp and the bisguanidine ligand N¹,N²‐bis(1,3‐dimethylimidazolidin‐2‐ylidene)‐ethane‐1,2‐diamine (DMEG₂e), respectively.     

Complexes of 4- and 5-bromo derivatives of 2-(hydroxymethyl)pyridine with copper(II) and cobalt(II) salts. Synthesis and X-ray crystal structures

Four copper(II) complexes (1–4) and a cobalt(II) complex (5) derived from 4-bromo-2-(hydroxymethyl)pyridine (L¹) or 5-bromo-2-hydroxymethyl)pyridine (L²) with Cu(NO₃)₂·3H₂O, CuCl₂·2H₂O and CoCl₂·6H₂O have been synthesized and their respective crystal structures studied. They show specific influences owing to the different kind of metal cations and counter anions, the hydration as well as the different position of the bromine substitution on both the coordination of the complex unit and the network structure of the crystal lattice. The Cu(II) complexes of L¹ are five-coordinate [Cu(L¹)₂NO₃]NO₃·H₂O (1) and [Cu(L¹)₂Cl]Cl·H₂O (2) species with distorted quadratic pyramidal and trigonal bipyramidal coordination geometries of the N₂O₃ and N₂O₂Cl donor atoms around the Cu(II), respectively. The Cu(II) complexes of L² are six-coordinate [Cu(L²)₂(NO₃)₂] (3) and [Cu(L²)₂Cl(H₂O)]Cl·H₂O (4) species with distorted octahedral coordination geometries of the N₄O₂ and N₂O₃Cl donor atoms. A distorted octahedral coordination geometry of the N₂O₂Cl₂ donor atoms is also found in the complex unit [Co(L²)₂Cl₂] of the Co(II) complex 5 but showing the oxygen atoms of the chelating ligand as well as the chloride ions in a cis-position. Depending on the complex, water molecules and chloride anions are shown to act as stabilizing components of the crystal structure. The comparative structural investigation includes also known structures of the bromine-free ligand analogue 2-(hydroxymethyl)pyridine, illustrating the basic implication of the bromine substitution, mostly perceptible in the different modes of crystal packing.     

Stanna‐closo‐dodecaborate: A New Ligand in Coordination Chemistry

Most of the divalent compounds of tin have a lone pair and hence can act as donors. In tin‐transition metal chemistry neutral molecules as well as anions have been studied as ligands. This research report summarizes recent research on coordination compounds with a closo‐heteroborate cage compound stanna‐closo‐dodecaborate [SnB₁₁H₁₁]^2?. The syntheses of the first coordination compounds and studies on the ligand abilities of this tin borate are discussed in this article.     

A self-assembling metallosupramolecular cage based on cavitand-terpyridine subunits

Metal-directed self-assembly of a terpyridyl-functionalized cavitand yields a large hexameric coordination cage.