Mass spectrometry catenanes

Singly and doubly interlocked [2]catenanes can exist as topological stereoisomers (see p. 144 for a discussion of diastereomers). Catenanes 35 and 36 are such stereoisomers and would be expected to have identical mass spectra. Analysis showed that 35 is more constrained and cannot readily accommodate an excess of energy during the mass spectrometry ionization process and, hence, breaks more easily. 

Two-dimensional NMR studies and mass spectrometry identified the isomers as being the cyclic tetramer 6 and the [2]catenane 7 consisting of two interlocked cyclic dimers. Later X-ray crystal structures confirmed these results [16, 17],... 

A second experiment should prove that macromonocycles are actually the intermediate supramolecular templates in the course of catenane formation. Therefore macromonocycle 17 was reacted with 5 and 3, and the first [2]catenane 18 of the amide type consisting of two different macromonocycles was isolated (Figure 8). Unsymmetric catenanes like 18 can be identified unambiguously by mass spectrometry, because the corresponding tetrameric macromonocycle can not be formed in this reaction sequence. This confirms the presumption that catenation here proceeds via a macrocycle rather than via intertwining open chain units. 

Figure 7-42. Mass spectrometry allows us to distinguish between isomeric [2+2] macrocycles and catenanes of mass 2M. The mass spectrum of a catenane should show no peaks between m/z 2M and M.

The existence of interlocked molecules such as 10.95 can be established by a variety of spectroscopic techniques. Most importantly, mass spectrometry provides very characteristic patterns for catenanes. The mass spectra for catenated species are very different from those of covalently linked precursors (such as 10.94) but are more than the sum of their two individual components. Catenane mass spectra are characterised by the appearance of peaks at high mlz corresponding to the parent species as well as fragments corresponding to the transfer of hydrogen atoms from one macrocycle to the functional... 

Dietrich-Buchecker, C., Leize, E., Nierengarten, J. F., Sauvage, J. P, Vandorsselaer, A., Singly and doubly interlocked [2]-catenanes - influence of the degree of entanglement on chemical-stability as estimated by fast-atom-bombardment (Fab) and electrospray-ionization (Esi) mass spectrometries (Ms). J. Chem. Soc., Chem. Commun. 1994, 2257-2258. 

We have seen how elegantly transition metals can template the formation of knots, but what about Nature s favourite templating interaction, the hydrogen bond A remarkably efficient molecular trefoil knot synthesis based on this interaction was reported by Vogtle and co-workers, who made a knotane in 20% yield [39]. This amazing route (Fig. 11) was uncovered serendipitously during the synthesis of catenanes. The crystal structure of the compound was the definitive proof for the structure, because neither NMR nor mass spectrometry could tell it apart conclusively from the macrocycles that are also formed. 

The Stoddart group has used the principles employed in their catenane studies to synthesise both a molecular trifoil knot and its isomeric large ring in very low yield. These isomers were separated by HPLC and characterised by liquid secondary ion mass spectrometry. The synthetic procedure, which is illustrated schematically in Figure 5.26, was based on the formation of a double-stranded supramolecular complex between an acyclic 7C-electron-rich, 1,5-dioxynaphthalene-based polyether and an acyclic 7i-electron-deficient bipyridinium-based tetracation. The extremely low efficiency of the synthesis in this case appears to reflect an unfavourable solution geometry of the intermediate host-guest complex - the latter... 

In the case of L2" + it was not possible to synthesize [2]catenanes because the cavity of this cyclophane is too large to give stable complexes with aromatic crown ethers in the templated synthetic approach. Starting from Li" +, however, it was possible to prepare the [2]catenane ligands and L4 + which were then used to prepare several mononuclear [2]catenane complexes (Figure 23). These compounds were characterized by NMR spectroscopy, mass spectrometry and, in some cases, X-ray crystallography. 

While it is quite straightforward to distinguish these two species, the problem becomes more challenging when comparing the tetra- and octalactam macrocycles, catenanes and the trefoil knot shown in Fig. 5.11. For both, fragmentation of the octalactam macrocycle and of the catenane, covalent bonds need to be broken and one may ask whether mass spectrometry is still able to provide criteria for differentiating the two topologies. 

Aromatic templates, in conjunction with coordinative bonds, have been employed by Sanders et al. [42] to self-assemble a [2]catenane incorporating a chiral metallomacro-cycle. The 1,5-dioxynaphthalene-based macrocyclic polyether 60 threads onto the r-elec-tron-deficient compound 61 in MeCN. Thus, when both compounds and Zn(OS02CF3)2 are mixed in this solvent, threading of 60 onto 61 is followed by the [2 + 2] assembly of a helical metallomacrocycle as a result of the tetrahedral coordination of two Zn centers by the bipyridine ligands appended to the r-electron-deficient recognition sites. The resulting [2]catenane 62 was characterized by a combination of H-NMR spectroscopy and electrospray mass spectrometry. 

Fast Atom Bombardment-Fourier Transform-Mass Spectrometry (FAB-FT-MS) and low-energy collisional activation experiments proved that the oligomers of j3-ketolactone generally have a macrocyclic ring structure rather than a catenane ring. ... 

Wu, J., Chen, C., Kurth, M.J., and LebrUla, C.B., Mass Spectrometry Analyses of j8-Ketolactone Oligomers, Macrocyclic or Catenane Structures, Anal. Chem., 68, 38, 1996. 

NMR and Mass Spectrometries of Metal-free Catenane (1) and Copper [2)Catenane... 

The sahent features of the polycatenanes, as discussed above, are summarized in Table 17.1. Whilst many analytical tools, including NMR spectroscopy, mass spectrometry, GPC, and FTIR, have been used to characterize the polycatenanes, studies of their properties have been hampered by poor yields, even when using readily prepared poly[2]catenane systems. Gonsequendy, the development of more efficient synthetic methods, and/or of more readily-prepared systems, is critical to the research and development of these materials. 

By application of the recently developed Pt-templated macrocyclization protocol (see above) to the bis-phenanthroline-Cu(I) complex, an interlocked tris-metalated Pt(II)-Cu(I)-Pt(II)-catenate 4.61 was obtained from precursor 2.59. Elimination of the Pt comers with iodine under simultaneous C-C bond formation afforded Cu(I)-catenate 4.62, which represented the higher homologue of Cu(I)-catenate 4.58. In this case, due to the larger ring sizes (34-membered) decomplexation of the Cu(I) center was achieved by reaction with KCN and conjugated catenane 4.63 was isolated in pure form after chromatography (Scheme 1.56). Evidence for the interlocked stmcture of 4.63 came from NMR spectroscopy and tandem mass spectrometry [427]. 

Bauerle and coworkers have also prepared the catenate 3 (Chart 5.2) and characterized this species by electrospray ionization Fourier transform ion cyclotron resonance (ESI-F TlCR) mass spectrometry [32]. Reductive elimination of the platinum centers from this complex yielded a Cu+-linked conjugated catenate, which was also characterized by mass spectrometry, and the Cu could be removed with KCN to give the pure conjugated, metal-lfee catenane. The copper could not be removed from the terthiophene analogue of 3 which was prepared earlier by the same group [33]. 

Evidence for the formation of the [2]catenane was provided by mass spectrometry crucially no ions with... 

A wide range of instrumental techniques are needed to characterise products fully - X-ray crystallography, mass spectrometry (especially FAB-MS and electrospray MS), H and NMR, UV-Vis spectroscopy, and electrochemistry - in the solid state and in solution. As much information as possible is needed in order to establish both the exact nature and long-range structural features (superstructure) of rotaxanes, catenanes and knots. As noted at appropriate points in the text, there is considerable interest in applications for these classes of compounds, particularly in respect to molecular switching devices. 

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