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Coordination Chemistry of a Molecular Pentafoil Knot Liang Zhang, Alexander J. Stephens, Jean-François Lemonnier, Lucian Pirvu, Inigo J. Vitorica-Yrezabal, Christopher J. Robinson, and David A. Leigh J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019
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Journal of the American Chemical Society
Coordination Chemistry of a Molecular Pentafoil Knot Liang Zhang,†‡ Alexander J. Stephens,‡ Jean-François Lemonnier,‡ Lucian Pirvu,‡ Iñigo J. VitoricaYrezabal,‡ Christopher J. Robinson§ and David A. Leigh†‡* † School
of Chemistry and Molecular Engineering, East China Normal University, 200062 Shanghai, China. of Chemistry, University of Manchester, Manchester M13 9PL, UK. § SYNBIOCHEM, Manchester Institute of Biotechnology, University of Manchester, Manchester M1 7DN, UK. ‡ School
ABSTRACT: The binding of Zn(II) cations to a pentafoil (51) knotted ligand allows the synthesis of otherwise inaccessible metallated molecular pentafoil knots via transmetallation, 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 metallated 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 shown to vary over three orders of magnitude for the different metal-ion−knotted-ligand second sphere coordination complexes.
Introduction Molecular knots and entanglements occur in DNA,1 some proteins,2 and form spontaneously in polymer chains3 of sufficient length and flexibility.4 Synthetic routes to several small-molecule knots have been developed,5,6 or serendipitously discovered,7 and physical and chemical consequences of knotting have been demonstrated,4 including anion binding,7h,8 asymmetric9 and allosteric regulation10 of catalysis, and the securing of a threaded structure by the increase in steric bulk that accompanies tying a knot in a molecular strand.11 However, despite metal template synthesis being a common route to molecular knots,4a,c,f,h,5a,c-k,7d,8 effects on metal-ion coordination induced by knotting of the ligand strand have rarely10,12 been described. 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 framework of this 51 knot contains only kinetically robust covalent bonds and so, unlike previous examples,5e,f the metal ions can be removed from a molecular pentafoil knot without destroying the knot topology. Direct remetallation of the knotted ligand proved only possible with Zn(II) cations, presumably because the coordination dynamics of other metal(II) cations is too slow to allow ‘mistakes’ regarding which bipyridine moieties coordinate to which metal ion to be corrected. However, complexation of the five Zn(II) cations holds the knotted ligand in the conformation necessary to bind to five metal ions. We reasoned that this might enable sequential substitution of other metal(II) cations12 for Zn(II) without permitting the knotted ligand to adopt conformations that can bind to incoming metal ions through ‘wrong’ combinations of bipyridine units.13 Here we show that this enables the efficient synthesis of the otherwise inaccessible Co(II), Ni(II) and Cu(II) metallated knots, enabling the exploration of the
coordination chemistry of the knotted ligand. The unknotted ligand monomer only forms circular helicates with Fe(II) and Zn(II)10 and so the transmetallation strategy, and resulting breadth of coordination chemistry, is only achievable with a knotted ligand. Results and Discussion Direct metallation of knotted ligand 1 The coordination chemistry of pentafoil knot 1 was investigated by first probing the rate of metallation of 1 with different zinc(II) salts (Scheme 1, steps a,b). The rate and efficiency of introduction of the zinc(II) ions proved to be highly dependent on the salt used (Table 1). Treatment of 1 with Zn(BF4)2 in MeOH/CH2Cl2 (most conveniently followed using deuterated solvents) generated [Zn51•Cl](BF4)9 in 98 % yield after 2 h at 40 °C followed by work up with one equivalent of Bu4NCl (Table 1, entry 1). Introducing the metal ions with Zn(CF3SO3)2 proved more sluggish, resulting in a 44 % yield of [Zn51•Cl](CF3SO3)9 after 16 h and work up (Table 1, entry 2). However, treatment of 1 with ZnCl2 afforded [Zn51•Cl](BF4)9 quantitatively in less than 5 minutes at room temperature, followed by exchange of the nine non-cavity bound Cl- anions for BF4- for solubility reasons (Table 1, entry 3). The X-ray crystal structure of [Zn51•Cl](PF6)9 features a chloride anion tightly bound in the central cavity as a consequence of electrostatic interactions and ten CH….Clhydrogen bonds (Figure 2c and 2f).10 Given the very different rates of metallation with the different zinc salts, it seems likely that halide binding in the central cavity plays a significant role in facilitating the metallation of the knotted ligand, encouraging rapid rearrangement of wrongly coordinated bipyridine residues. In the 1H NMR spectrum of the titration of
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ZnCl2 into knot 1, free ligand and [Zn51•Cl]9+ signals dominate. Asymmetric species, i.e. partially metallated pentafoil knots [Zn1-41•Cl]1-7+, are only present in very minor amounts (Figure S13). Coordination of the first zinc cations to the knotted ligand appears to preorganize the remaining empty coordination pockets expediting the subsequent coordination events. In contrast to the complexation of the knotted ligand with Zn(II), direct metallation 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 metallated knots, even over extended reaction times (up to 60 h) at elevated temperatures (up to 80 °C).14 Rather than start from metal-free 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. Such stepwise transmetallation could allow the knotted ligand to retain the relative positions of its bipyridine chelating groups as the metals are successively exchanged, and hence reduce the degree of ligand strand rearrangement required. Scheme 1. Metallation of knotted ligand 1 to generate the corresponding M(II)5-1 knot complexesa
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Reagents and conditions: (a) ZnCl2, dichloromethane/methanol (1:1), room temperature,
