Dicopper helicates

The reaction of sulfanilic acid with 2,9-diformyl-l,10-phenanthroline, copper(I) oxide, and sodium bicarbonate gave a quantitative yield of the anionic double 

In addition to sulfanilic acid, numerous other primary amines could be used to construct helicates. The conditions under which different amines were incorporated into these helicates were investigated. Table 1.1 summarizes the selection rules discovered. 

Water was preferred to acetonitrile as the solvent, allowing moderately hindered and anionic amines to self-assemble. Acetonitrile is a much better ligand for copper(I) than water, making it more difficult for hindered ligands (such as the one formed from serinol, third entry in Table 1.1) to form complexes in competition with the solvent. More hindered amines as well as cationic amines were not incorporated in either solvent, which we attribute to steric and Coulombic repulsion, respectively. 

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Baxter, P. N. W. Lehn, J. M. Rissanen, K. Generation of an equilibrating collection of circular inorganic copper(I) architectures and solid-state stabilization of the dicopper helicate component. Chem. Commun. 1997,14, 1323-1324. 

Double-strand dicopper helicate complexes are interesting systems in that they may show hysteresis (as observed with ligand 16), thus giving rise to a rare example... 

Tricopper helicates could also be synthesized using a simple modification of the dicopper helicate preparation [28]. When three equivalents of copper(I) were employed and 8-aminoquinoline was used in place of an aniline, tricopper double-helicate 4 was formed as the unique product (Scheme 1.6). 

The reaction of dicopper helicate 25 (Scheme 1.24) with o-phenylenediamine produced the dimeric tetracopper helicate 26 in 51% yield. This reaction is the first step of a step polymerization reaction [48] during the course of the reaction, a substantial amount of insoluble brown material is also produced. The elemental analysis of this insoluble co-product is consistent with that of a higher oligomer... 

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

However, it has been recently demonstrated by Pallavicini et al. that the lifetime of the dicopper(II) double-strand helicate [ 2 (16)]4 + can be significantly increased by introducing hindering substituents on the framework of 16. In particular, this was shown to occur with the copper complexes of the bis-bidentate ligand 17.21... 

Besides the knot, the major cyclization product (24%) obtained in the latter reaction could be identified as a dicopper complex consisting of two 43-membered rings arranged around the metallic centers in an approximate face-to-face geometry [94]. This unknotted compound originates from a non-helical precursor which is in equilibrium with the expected double helix. Figure 18 describes in a schematic way the alternative cyclization reaction leading to the unknotted face-to-face complex and the equilibrium which interconverts the helical and the non-helical precursors. 

Analogous to the previously synthesized dicopper(I) knots [97], Cu2(K-84)p+ gives characteristic 1H NMR and FABMS spectroscopy data [102], In particular, some of the aromatic protons of the bis-chelates are strongly shielded, which indicates that Cu2(K-84)p is compact and that its helical core is geometrically very similar to that observed in 32. This was fully confirmed by subsequent X-ray structure determination. 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

This concept may also be extended to polynuclear helicates [38]. When 2-amino-quinoline and 4-chloroaniline were mixed with the phenanthroline dialdehyde shown in Scheme 1.10, a dynamic library of potential ligands was observed to form. The addition of copper(I) causes this library to collapse, generating only dicopper and tricopper helicates. As in the mononuclear case of Scheme 1.9, the driving force behind this selectivity appeared to be the formation of structures in which all ligand and metal valences are satisfied. The use of supramolecular (coordination) chemistry to drive the covalent reconfiguration of intraligand bonds thus... 

The preparation of structure 10, shown in Scheme 1.11, requires a different kind of selectivity in the choice of ligand subcomponents. Whereas during the simultaneous formation of dicopper and tricopper helicates (Scheme 1.10) all mixed ligands were eliminated from the dynamic library initially formed, in Scheme 1.11 the mixed ligand forms the unique structure selected during equilibration [39]. 

This differential selectivity results from the differing numbers of donor atoms offered by the two dialdehydes upon which these structures are based. Phenanth-roline dicarbaldehyde readily lends itself to the construction of a set of homo-ligands bearing a number of donor atoms divisible by 4, matching the coordination preference of copper(I), as seen in the dicopper and tricopper helicate structures discussed earlier. 

Scheme 1.10 Simultaneous preparation of dicopper and tricopper helicates from a dynamic library of ligands.

The dicopper double-helicate moiety [31] has exhibited rich and varied substitution chemistry, as discussed below [17, 28, 46], It is more rigid and structurally better-defined than the mononuclear complexes discussed above, which allows one to use it as a persistent, well-defined tecton [13],... 

Scheme 1.24 Dimerization reaction of dicopper hel icate 25 and o-phenylenediamine to give tetracopper helicate 26.

Reports of double-helical complexes have appeared in the literature since the sixties. Despite the early interest, it is only more recently that emphasis has been given to the use of metal template synthesis for obtaining a wide range of doubly-and triply-stranded systems. In part, this attention has had its origins in an early report by Lehn et al. in which the spontaneous assembly of a dicopper(I)-containing double helix was described. 

Anion-binding properties of [Cu2(L14)2], which are further explored via solid-state and solution chemistry to define the effects of the strength of anion binding in the helical metallomacrocyclic cavity, have shown that these zwitterionic dicopper hosts can bind anions with an approximate volume of 0.09 nm and smaller. The dicopper host [Cu2(L14)2] is capable of binding anions in... 

MOLECULAR KNOTS CONSTRUCTED ON DICOPPER(I) HELICAL COMPLEXES... 

Preparation of the double-stranded helical precursor with copper(l) using the 1,3-phenylene linker turned ont to be qnantitative. Reaction of this tetraphenolic donble helix with two eqnivalents of the di-iodo derivative of hexaethyleneglycol, in the presence of caesinm carbonate, afforded a single isolable copper(I) complex the dicopper(l) knot was isolated in 29% yield after chromatography. H-NMR spectroscopy data indicated that the knot contained a compact helical core. This was fully confirmed by its subsequent X-ray strnctnre determination. The spectacularly improved yield can be increased even further by optimizing the covalent-bond-forming reactions, whose conditions can harm the helical precnrsors. 

Therefore, combining the quantitative formation and high stability of the helical precursor composed of Cu(I) bis-phenanthroline units linked by 1,3-phenylene units and the highly efficient RCM methodology developed by Grubbs and coworkers, the dicopper(l) trefoil knot 7 + could be obtained in seven steps from commercially available 1,10-phenanthroline, with an overall yield of 35%. The cis or trans nature of the two cyclic olefins formed in the... 

Trefoil knots, and therefore the molecular knots discussed in this section, are chiral (Figure 4-29). The resolution of a dicopper(I) knot prepared from a helical precursor containing the 1,3-phenylene-linked bis-phenanthroline ligand described above (LI 198) has been achieved by crystallisation of the racemic cation with (5)-(+)-l,l -binaphthyl-2,2 -diyl phosphate [343]. As commented by these authors, the preparation of optically pure knot complexes is of great potential interest in relation both to interactions with biological molecules and, where the complexed metal has more than one accessible oxidation state, to enantioselective electron transfer [344]. 

Wallbank Al, Corrigan IF (2002) Triply bridged dicopper-bis(trimethylsilylchalcogenolates) synthesis and characterization of the series of helical complexes [(Me3SiE-Cu)2 (/i-PhjPCsCPPha-tc Pls] (E = S, Se, Te). Can J Chem 80 1592-1599... 

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