Subcomponent Self‐Assembly of a Cyclic Tetranuclear Fe^II Helicate in a Highly Diastereoselective Self‐Sorting Manner

Abstract

An enantiomerically pure diamine based on the 4,15‐difunctionalized [2.2]paracyclophane scaffold and 2‐formylpyridine self‐assemble into an optically pure cyclic metallosupramolecular Fe4L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self‐assembly process. The cyclic assembly results from steric strain that prevents the formation of a smaller linear dinuclear triple‐stranded helicate, and hence, leads to the larger strain‐free assembly that fulfils the maximum occupancy rule. Interestingly, use of the racemic diamine also leads to a racemic mixture of the homochiral cyclic helicates as the major product in a highly diastereoselective narcissistic chiral self‐sorting manner given the fact that the assembly contains ten stereogenic elements, which can in principle give rise to 149 different diastereomers. The metallosupramolecular aggregates could be characterized by NMR, UV/Vis and CD spectroscopy, mass spectrometry, and X‐ray crystallography.

Charge‐State‐Dependent Fragmentation of [2.2]Based Metallosupramolecular Cyclic Helicates in the Gas Phase

“…They are formed via the subcomponent self‐assembly approach [32,33] by mixing a [2.2]based amine, pyridine carbaldehyde, and iron(II) triflate in a ratio of 6 : 12 : 4. The formation is strikingly selective when enantiopure diamine is used [31] . The cyclic helical structure of this compound is not as symmetric as the ones we studied earlier – adjacent metal centers are alternately bridged by one and two ligands.…”

“…The formation is strikingly selective when enantiopure diamine is used. [31] The cyclic helical structure of this compound is not as symmetric as the ones we studied earlier -adjacent metal centers are alternately bridged by one and two ligands. One obvious question is whether this is reflected in the gas-phase fragmentation pattern.…”

“…Isotope patterns and accurate mass determinations fully support this interpretation. [31] According to our previous findings with metallo-supramolecular macrocycles, [25] rhombs, [30] and squares, [22,26] the fragmentation of the very same cationic core can significantly vary with the number of anions attached to it.…”

“…[22,25,26,27,28,29,30] Herein, we describe the gas-phase fragmentation behavior of chiral tetranuclear cyclic helicates depicted in (Scheme1). [31] Their synthesis and indepth characterization were recently reported including X-ray of the interesting peculiar structure. They are formed via the subcomponent self-assembly approach [32,33] by mixing a [2.2] based amine, pyridine carbaldehyde, and iron(II) triflate in a ratio of 6 : 12 : 4.…”

“…We have started to study the gas‐phase fragmentations of some metallo‐supramolecular aggregates in order to better understand the basic principles of their fragmentation patterns and the best conditions for more reliable structural information [22,25,26,27,28,29,30] . Herein, we describe the gas‐phase fragmentation behavior of chiral tetranuclear cyclic helicates depicted in (Scheme1) [31] . Their synthesis and in‐depth characterization were recently reported including X‐ray of the interesting peculiar structure.…”