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Abstract
The binding of Zn(II) cations to a pentafoil (51) knotted ligand allows the synthesis of otherwise inaccessible metalated molecular pentafoil knots via transmetalation, affording the corresponding “first-sphere” coordination Co(II), Ni(II), and Cu(II) pentanuclear knots in good yields (≥85%). Each of the knot complexes was characterized by mass spectrometry, the diamagnetic (zinc) knot complex was characterized by 1H and 13C NMR spectroscopy, and the zinc, cobalt, and nickel pentafoil knots afforded single crystals whose structures were determined by X-ray crystallography. Lehn-type circular helicates generally only form with tris-bipy ligand strands and Fe(II) (and, in some cases, Ni(II) and Zn(II)) salts, so such architectures become accessible for other metal cations only through the use of knotted ligands. The different metalated knots all exhibit “second-sphere” coordination of a single chloride ion within the central cavity of the knot through CH···Cl– hydrogen bonding and electrostatic interactions. The chloride binding affinities were determined in MeCN by isothermal titration calorimetry, and the strength of binding was shown to vary over 3 orders of magnitude for the different metal-ion–knotted-ligand second-sphere coordination complexes.
Highlights
Molecular knots and entanglements occur in DNA,[1] some proteins,[2] and form spontaneously in polymer chains[3] of sufficient length and flexibility.[4]
We recently reported that a pentafoil (51) knot could be prepared by ring-closing olefin metathesis (RCM) of a pentameric circular Fe(II)5-helicate.[10]
The coordination chemistry of pentafoil knot 1 was investigated by first probing the rate of metalation of 1 with different zinc(II) salts (Scheme 1, steps a,b)
Summary
Molecular knots and entanglements occur in DNA,[1] some proteins,[2] and form spontaneously in polymer chains[3] of sufficient length and flexibility.[4]. Zn(II), direct metalation of 1 with some other first row transition metal salts (Fe(II), Co(II), Ni(II), or Cu(II)) failed to generate isolable quantities of fully metalated knots, even over extended reaction times (up to 60 h) at elevated temperatures (up to 80 °C).[14] Rather than start from metalfree ligand 1, we reasoned that as the Zn(II) cations in [Zn51· Cl](BF4)[9] organize the binding sites of 1 in the correct arrangement, the labile Zn(II) ions of that complex might be exchanged for a less labile metal (introduced in large excess) in a stepwise process. The chloride anion is located at different distances from the plane of the metal ions in the different knot complexes, illustrating how the size, electrostatics, and shape of the knotted ligand cavity change with coordination to the different metal cations (Figure 2d−f). Cu(II) ions distorts the folded geometry of the pentafoil knotted ligand to such an extent that the cavity is no longer of appropriate size or shape (e.g., no longer directs all of the polarized H1 protons toward a single point in space) to bind Cl−
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