Macrotricycles

A relative of the latter class of compounds are the macrotricyclic quaternary ammonium salts which have been reported by Schmidtchen. The bridges may contain either methylenes or ethyleneoxy units and the nitrogens are quaternarized. The underlying principle is to provide a cavity suitable for solvating or at least trapping anions. Schmidtchen presents evidence which suggests the formation of halide inclusion complexes. The synthesis of these molecules is accomplished along more or less traditional lines Such a species is illustrated above as compound 19. 

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

Chapter 8 Macrobicyclic, macrotricyclic, or polycyclic compounds including crypt-ands, spherands, etc. 

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

Macropolycyclic ligands, 2,942 classification, 2,917 metal complexes binding sites, 2, 922 cavity size, 2,924 chirality, 2, 924 conformation, 2,923 dimensionality, 2, 924 electronic effects, 2, 922 shaping groups, 2,923 structural effects, 2,922 molecular cation complexes, 2,947 molecular neutral complexes, 2,952 multidentate, 2,915-953 nomenclature, 2,920 Macro tetrolide actins metal complexes, 2,973 Macrotricycles anionic complexes, 2,951 cylindrical... 

An impressive achievement of this strategy has been the construction of three-dimensional structures. Utilizing branched phenylacetylene sequences, double cyclization yielded macrobicyclic arrays 54 and 55 [43]. The zenith of Moore s approach is macrotricycle 56, a freely hinged system with a sizable 36xl2xl2A molecular cavity [44]. 

The expression bimacrocycle (OT macrocycle or po/ymacrocycle) is used instead of macrobicycle (macrotricycle, etc.) because polymacrocycles can contain further, non-macrocyclic ring systems (e.g. aromatic rings). 

Smaller macrobicycles and macrotricycles bearing less that six N-donors are also known. These ligands comprise tertiary or secondary amines and are derived from insertion of one or more straps between pairs of potentially bridgehead N-donors contained within a small ring macrocycle, and in this sense they may be considered analogs of the well-known azacryptands. The inability to offer six N-donors, and the presence of tertiary amines in these ligands leads to the preference for divalent over trivalent Co. 

For the macrotricyclic complexes (573)-(576), which retain the same 14-membered macrocyclic framework, the size of the subring moieties slightly affects the spectroscopic properties of the complexes.1465,1467 There is a qualitative relationship between Ni—N bond distances and Amax values. As the Ni—N bond distance increases, the Amax value increases.91... 

Ni1 complexes (944)-(950) with various different saturated polyaza macrotricyclic ligands having noncoordinated nitrogen atoms in the ligand framework have been prepared by reduction of the Ni11 complexes with Na/Hg in acetonitrile. The crystal and molecular structures of (948), (946), and (945) were determined.1582,2333 Both complexes were found to have essentially square... 

Construction of discrete and well-defined molecular architectures using organic ligands and metal ions through the self-assembly route has received much attention. A dinuclear Pd11 cage,636 includes a hexafluorophosphate ion inside the cavity. Self-assembly of a novel macrotricyclic cage... 

The macrotricyclic ligands (240) and (241) may be synthesized by multistep high-dilution procedures (Graf Lehn, 1975). They contain spherical cavities which are able to accommodate suitable guests whether they be cationic, neutral, or anionic. 

A system exhibiting chiral recognition. The chiral macrotricyclic tet-raamide (250) (Lehn, Simon Moradpour, 1978) has been used for the complexation, extraction and transport of primary ammonium salts. The tetraamide was used rather than the corresponding tetraamine because of the lower basicity of the nitrogens in the former ligand. This avoids the possibility of proton transfer occurring from the primary ammonium substrates R-NH3+ used as guests. In a typical experiment, a solution of a primary ammonium salt, such as naphthylethyl ammonium or phenylalanine methylester hydrochloride in hydrochloric acid was... 

A good example for such a situation is a recent report on the synthesis of the macrotricyclic core 63 of roseophilin [40,41]- RCM was able to form the rather strained ansa chain of this target molecule only after the cyclization had been biased by a conformational control element X which helps to bring the unsaturated chains closer together and lowers the enthalpic barrier during ring formation (Scheme 18). 

The nucleophilic displacement of the halogen from 2,4-dinitrohalobenzenes by azide ion is catalysed by macrotricyclic ammonium salts [69], Kinetic studies indicate that the azide ion is entrapped and transported within the macrocyclic cage. The highly explosive tetra-azido-p-benzoquinone is obtained when the tetrachloro-quinone is reacted with an excess of sodium azide under phase-transfer catalytic conditions [70]. When only a twofold excess of the azide is used, the 2,5-diazido-3,6-dichloro compound is obtained. 

F. Fages, J.-P. Desvergne, H. Bouas-Laurent, J.-M. Lehn, J. P. Konopelski, P. Marsau, and Y. Barrans, Synthesis and fluorescence emission properties of a bis-anthracenyl macrotricyclic ditopic receptor. Crystal structure ofits dinuclear rubidium cryptate, J. Chem. Soc., Chem. Commun. 655(1990). 

A recent example, with references to previous literature, is in nickel(II) template synthesis of macrotricyclic complexes from formaldehyde and triamines, M. P. Suh, W. Shin, S.-G. Kang, M. S. Lah and T.-M. Chung, Inorg. Chem. 28, 1602 (1989). 

Fig. 4. Synthetic strategies towards macrotricyclic ligands of type J...

Table 6. Approximate cavity diameters in macrocyclic, macrobicyclic and macrotricyclic ligands...

Various evidence shows that the macrotricyclic ligand 45 forms 1 1 inclusion complexes, [3]-cryptates, with AC s and AEC s (96). NMR spectroscopic data indicate that the cation is non-symmetrically located in the cavity (95). 1 2 complexes with two Ag+ or T1+ cations have also been observed (Pd). Complex formation of ligand 46 with K+, Na+, Li+ and Ca++ has been reported (92). 

Figure 6. X-ray crystal structure of the iodide cryptate of the macrotricyclic quaternary ammonium receptor 16." ...

A theoretical (AMI molecular orbital) treatment of boron analogues of the katapinand series as well as macrotricyclic compounds related to 16 has also recently appeared. As with the ammonium-based host molecules the macrotricyclic hosts containing four boron atoms exhibited a greater degree of anion specificity as a consequence of the rigidity of their binding sites. In all cases, size selective complexation of halide anions was observed with accompanying decreases in B - B distances and partial sp -> sp rehybridization. ... 

The propagation reaction of ethylene oxide anionic polymerization has been studied in THF at 20 C with " , " + [222], Cs+ and Cs + [TC] as counterions (10,11). [TC] is a spheroidal macrotricyclic tetramine hexaether (25) which forms a very stable complex with Cs+. 

Thus the spherical macrotricyclic molecule represented below gives a tetraprotonated species that has very unusual binding properties. It binds very strongly the spherical chloride anion, holding it in an ideal tetrahedral arrangement of N+—H Cl- hydrogen bonds. This complex is much more stable and the Cl /Br selectivity is much higher than any other known to date. These properties may be attributed to the presence... 

In cases where nitroethane is used instead of methylamine as shown in Eq. (4), the Ni(II) complex of hexaaza macrotricyclic ligand, L2, can be prepared (19). 

Properties of Square-Planar Nickel(II) Complexes of AT-Alkylated Macrotricyclic Ligands... 

Since Pedersen s disclosures, and the subsequent extension into the third dimension through Lehn and his macrobicyclic and macrotricyclic cryptands,29 there have been a plethora of papers concerning many aspects of M"+ complexation, the majority being centred on crown ether and related species. Their literature has been extensively reviewed and attention is drawn to articles by Poonia and Bajaj30 and by Vogtle and Weber31 which are comprehensive and provide excellent bibliographies of the area. 

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