Shape‐Persistent [4+4] Imine Cages with a Truncated Tetrahedral Geometry

Abstract

The synthesis of shape‐persistent organic cage compounds is often based on the usage of multiple dynamic covalent bond formation (such as imines) of readily available precursors. By careful choice of the precursors geometry, the geometry and size of the resulting cage can be accurately designed and indeed a number of different geometries and sizes have been realized to date. Despite of this fact, little is known about the precursors conformational rigidity and steric preorganization of reacting functional groups on the outcome of the reaction. Herein, the influence of conformational rigidity in the precursors on the formation of a [4+4] imine cage with truncated tetrahedral geometry is discussed.

Host‐Guest Chemistry of Truncated Tetrahedral Imine Cages with Ammonium Ions

“…By extended NMR studies, thermodynamic and kinetic data have been generated suggesting that the uptake of ammonium ions is most likely be favored by a squeezing mechanism rather than by a gate‐opening mechanism. This is also in line with the previous observation that the [4+4] cages are not thermodynamically but rather kinetically controlled products . Guest uptake mechanisms play a pivotal role for the usage of shape‐persistent organic cages as confined molecular reaction vessels and therefore more studies will be pursued to finally pin down the mechanism and use the [4+4] cages as vessels, e. g. for catalytic reactions with cationic transition states …”

“…The compound crystallizes in the orthorhombic space group A ma2 ( Z =4) forming channels between the cage molecules with diameters of 9 Å ×11 Å, respectively (Figure c). The outer diameter of cage 3‐Me is with 1.5 nm nearly the same as found for cages 3‐H (1.6 nm) and 3‐Et (1.6 nm) . It is worth mentioning that in contrast to the structures of cages 3‐H and 3‐Et the imine bonds are found to exist in various conformations (Figure a).…”

“…The truncated [4+4] imine cages were synthesized by reacting the conformationally fixed triethyltriamine 1 with the corresponding trialdehydes 2 a – c in a 1 : 1 stoichiometry in acetonitrile at room temperature (Scheme ) . Here the missing link, cage 3‐Me was synthesized and isolated in 37 % yield, which is in between the prior reported yields of 27 % ( 3‐H ) and 46 % ( 3‐Et ) …”

“…In CDCl 3 , both NEt 4 + and NPr 4 + were bound with significantly larger association constants ( K a >1 ⋅ 10 5 M −1 ) than in DCM. From previous work we know that cage 3‐H is not stable in CHCl 3 and decomposes by time, most likely due to traces of hydrochloric acid . So it is in the case of NMe 4 + and NBu 4 + and decomposition is faster than complexation, making any assumption of association constants impossible.…”

A Constrained and “Inverted” [3+3] Salphen Macrocycle with an ortho‐Phenylethynyl Substitution Pattern

“…Mononuclear Zn II Schiff-base complexes are known to form dimeric structuresh aving at ypical Zn 2 O 2 central unit, mediated by strong intermolecularZ n ÀOc oordinative interactions (through m 2 -phenoxob ridging). [33][34][35][36][37][38] Oneo ft he main synthetic challenges is to obtain selectively one type of macrocycle of ag iven size in good yield. [7][8][9][10][19][20][21] In particular,t he aggregation behavior of multinuclear Zn II salphen complexes is strongly enhanced owing to their unusual oligomeric (ZnÀO) n coordination motifs, ap eculiar property that has been exploited for the construction of self-assembled nanostructures of very high stability.…”