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.