Copper(I)- or Iron(II)-Templated Synthesis of Molecular Knots Containing Two Tetrahedral or Octahedral Coordination Sites

Abstract

Molecular trefoil knots have been prepared from metal-assembled precursors using the ring closing metathesis (RCM) cyclization methodology. The templating metal is either copper(I) or iron(II) and the coordinating fragments 1,10-phenanthroline or 2,2′:6′,2"-terpyridine, respectively. The RCM approach, newly applied to the field of catenanes and knots, represents a spectacular synthetic improvement in terms of yield and experimental conditions (no high dilution required). The dicopper(I) trefoil knot has been synthesized in a 74% yield. A similar approach also led to the first knot constructed around two iron(II) bis(terpyridine) moieties, demonstrating that iron(II) can also be used as a highly efficient template. Moreover, for both Cu(I) and Fe(II) knots, it has been possible to quantitatively reduce the cyclic olefins formed during the macrocyclization by catalytic (Pd/C) hydrogenation. An X-ray structure of the double helix of iron(II) bis-(1,2-bis(5-(5"-methyl-2,2′:6′,2"-terpyridinyl))ethane) is given which shows that the double-stranded helical precursor is well predisposed for the formation of a molecular knot.

General Approach for Template‐Directed Synthesis of Macroheterocycles by Ring‐Closing Metathesis (RCM)

“…Most of these procedures use monometallic complexes as the templates. [9] Also the application of metathesis in the synthesis of catenanes, [10] molecular knots [11] and molecular wires, insulated with a double helix of alkene chains have been reported, in which a metal ion was used as the template to ensure the desired geometry. [12] Recently, we reported a model study in which we applied monometallic ECE-pincer complexes (E ¼ N, S, metal ¼ palladium, platinum) coordinated to bisolefin functionalized pyridines in ring-closing metathesis (RCM) reactions.…”

A synthetic molecular pentafoil knot

“…Although other syntheses of trefoil knots have been reported [15][16][17][18][19][20][21][22] (as have composites of trefoil knots 23 and other molecular topologies such as catenanes [24][25][26][27][28] and Borromean links 29 ), higher-order molecular knots remain elusive. Here, we report on the synthesis of a molecular pentafoil knot that combines the use of metal helicates to create crossover points 30 , anion template assembly to form a cyclic array of the correct size [31][32][33] , and the joining of the metal complexes by reversible imine bond formation 34-37 aided by the gauche effect 38 to make the continuous 160-atom-long covalent backbone of the most complex non-DNA molecular knot prepared to date.…”

Molekulare Knoten

“…Das Te mplat konnte ebenfalls variiert werden;e sw urde oktaedrisch umgebenes Fe II mit zwei dreizähnigen Liganden verwendet. Der molekulare Kleeblattknoten 7 [66] wurde nach RCM in 20 %A usbeute erhalten.…”