Vernier template synthesis of molecular knots

Abstract

Molecular knots are often prepared using metal helicates to cross the strands. We found that coordinatively mismatching oligodentate ligands and metal ions provides a more effective way to synthesize larger knots using Vernier templating. Strands composed of different numbers of tridentate 2,6-pyridinedicarboxamide groups fold around nine-coordinate lanthanide (III) ions to generate strand-entangled complexes with the lowest common multiple of coordination sites for the ligand strands and metal ions. Ring-closing olefin metathesis then completes the knots. A 3:2 (ditopic strand:metal) Vernier assembly produces +3 1 #+3 1 and −3 1 #−3 1 granny knots. Vernier complexes of 3:4 (tetratopic strand:metal) stoichiometry selectively form a 378-atom-long trefoil-of-trefoils triskelion knot with 12 alternating strand crossings or, by using opposing stereochemistry at the terminus of the strand, an inverted-core triskelion knot with six alternating and six nonalternating strand crossings.

Knotting matters: orderly molecular entanglements

“…19). 27 The Vernier template synthesis approach combines aspects of the circular helicate and folding-and-entwining strategies. It facilitates the hierarchical construction of knots and the rapid assembly of large, complex entangled structures (with molecular weights exceeding 8 kDa) from relatively simple building blocks.…”

“…21 Long dreamt of 22 extended arrays of well-defined tangles have finally become realistic targets for chemical synthesis through molecular weaving [23][24][25][26] and Vernier template synthesis. 27 With such advances molecular knotting is ready to answer questions regarding nanoscale topology relevant to chemistry, biology and physics. Entangling molecular strands with robust backbones has two important consequences: (i) strand crossing regions cannot pass through each other, blocking pathways to particular conformations and altering molecular dynamics; (ii) the structure becomes non-trivial in topological terms (each crossing can be over or under with respect to others), imparting additional stereochemical complexity.…”

“…21 Long dreamt of 22 extended arrays of well-defined tangles have finally become realistic targets for chemical synthesis through molecular weaving 23–26 and Vernier template synthesis. 27…”

“…Fig.17 (a)The Vernier template approach to increase the complexity of a system by targeted coordinative mismatch.83 (b) The Vernier template approach as applied to linear, circular, and knotted systems in a 2 : 3 ratio. 84 (c) Synthesis of granny knot (L,L)-Lu III 2 50 by Vernier template synthesis 27. …”

“…Fig. 19 Synthesis of 'inverted core' triskelion knot (L 3 ,D)-Lu III 4 52 by Vernier template synthesis 27. …”