Copper catenate

Figure 14.2 Electrochemically induced molecular rearrangements undergone by the copper catenate 12+/ +. In the text, the subscript 4 or 5 indicates the number of nitrogen atoms coordinated to the metal. This number is explicitly shown on the figure.

Figure 3. Cyclic voltammograms of bis-chelate copper complexes (a) [Cu(phen)2] and (b) [Cu(dmbp)2] in CH3CN- n-C4H9)4NC104 (0.1 M), and (c) of copper-catenate Cu.5 in CH3CN for oxidation, in DMF for reduction. Oxidation on Pt, reduction on the hanging mereury electrode.

In the case of the phen complex, reduction of the monovalent state leads to copper metal, the initial electron transfer occurring either in the Cu 4s orbital or via a phen n orbital. Fast dissociation follows this monoelectronic reduction step. The redox orbitals involved during the reduction process of the copper catenate are likely to be ligand-localized. This is also supported by the small difference between the redox potentials of the Cu+ / and Cu°/ couples (A i/2 200 mV). Electrolysis of Cu.5+ in the cavity of an EPR spectrometer confirms the radical anion nature of the formally copper(O) complex obtained by one-electron reduction of the catenate g = 2.000 + 0.002, Ai/ = 39 G. 

It is noteworthy that, due to the topologically different properties of the copper-catenate and the copper-entwined complex [Cu(dap)2]+, electrolysis of the second leads only to copper metal and free dap. 

The present examples are intended to demonstrate the potential of the copper catenates as mobile, electro- and photoactive components in the design of molecular devices which may be of use in the development of electron transfer-driven molecular machines. 

Figure 11 Electrochemically induced molecular rcarrangemcnls undergone by the copper catenate 2+1+...

The synthesis of the key catenate [Cu(I)N4]+ PF6- = 2(4) + (Fig. 14.8a) (one should notice that, as usual, the subscripts 4, 5, and 6 indicate the coordination number of the copper center) derives from the usual three-dimensional template strategy.23,24... 

When either the 2(6)2 + solution resulting from this process or a solution prepared from a sample of isolated solid 2(6)2 + (BF4 )2 were electrochemically reduced at — IV, the tetracoordinate catenate was quantitatively obtained. The cycle depicted in Fig. 14.3 was thus completed. The changeover process for the monovalent species is faster than the rearrangement of the Cu(II) complexes, as previously observed for the previously reported simpler catenate.16 In fact, the rate is comparable to the CV timescale, and three Cu species are detected when a CV of a CH3CN solution of 2(6)2 + (BF4 )2 is performed. The waves at + 0.63 V and —0.41V correspond, respectively, to the tetra- and hexacoordinate complexes mentioned above. By analogy with the value found for the previously reported copper-complexed catenane,16 the wave at —0.05 V is assigned to the pentacoordinate couple (Fig. 14.4b). 

The changeover reaction converting 23 2+ to the stable 5-coordinate species 23 2+ is quantitative. It is easily monitored by visible absorption spectroscopy, since the product of the rearrangement reaction is only slightly colored (pale olive green /.m lx = 640 nm s = 125). The geometry of the copper(n) catenate obtained through decomplexation and remetallation has been confirmed by comparison of its physi-... 

Cesario, M., Dietrich-Buchecker, C.O., Gui Them, J., Pascard, C. and Sauvage, J.P. 1985. Molecular structure of a catenand and its copper (I) catenate complete rearrangement of the interlocked macrocyclic ligands by complexation, J. Chem. Soc. Chem. Commun., 244-247. 

To our knowledge, topologically chiral molecules have not yet been resolved into enantiomers. However, we may anticipate that their energy barrier to racemization will be extremely high, compared to Euclidean chiral molecules. Therefore they are expected to be useful in enantioselective interactions or reactions. For example, it has been shown that tetrahedral copper(I) bis-2,9-diphenyl-l,10-phenanthroline complexes (which form the catenate subunits) are good reductants in the excited state [97] therefore the chiral Cu(I) catenates could be used for enantioselective electron-transfer reactions. Alternatively, the resolution of topologically chiral molecules would allow to answer fundamental questions, such as what are the chiroptical properties of molecular trefoil knots ... 

Copper(I) catenate 10+, made of two interlocked 30-membered rings, is obtained in 42% yield by slow addition of an equimolar solution of precatenate 9+ and link 7 in DMF to a vigorously stirred suspension of C CC in DMF under argon at 65 °C (high-dilution conditions) (Scheme 9.8). 

The free catenand 11 is easily obtained in quantitative yield by treating the copper(I) catenate with a large excess of potassium cyanide (Scheme 9.9). This is the most convenient decomplexing agent, notwithstanding its toxicity. 

Demetallation of copper(l) catenate Structure 10 (BF4 ) to afford the free catenand Stucture 11 (Scheme 9.9)... 

Figure 2.21. Copper(I)-templated synthesis of [2]-catenate 56 and its demetallation to the corresponding [2]-caten<2nd 57.

The coordination of catenated nitrogen ligands to transition metals also dates back to the early work of Griess 89, 90), which included references to copper and silver derivatives of 1,3-diphenyltriazene. Around the turn of the century Meldola and Streatfeild 146-148), Meunier 150-152), Niemen-towski and Roszkowski 159), Cuisa and Pestalozza 55,56), and others reported extensively on triazene complexes of copper, silver, and mercury, and in the late 1930s and early 1940s Dwyer and colleagues 69-74) extended this work to include derivatives of nickel and palladium. However, most work on the coordination chemistry of triazenes and other catenated... 

Electrochemical Properties of Metallocatenates and Knots 587 Table 1. redox potentials of various copper [2]catenates... 

In CH3CN or CH2CI2, all copper]I) [2]catenates are reversibly oxidized to the divalent complexes (Figure 3). The oxidation potentials are high ( 0.6 V versus SCE), making the Cu species relatively strong oxidants. 

As expected, the redox potential of the Cu +/Cu+ couple is slightly shifted towards anodic values in CuAg.l0 + as compared to Cu.S" ". This effect is even more pronounced in CuCo.l0 + the copper(II) state is significantly more difficult to electro-generate than for the mono-nuclear species Cu.5+. The positive shift of the Cu +/Cu+ redox potential in di-metallic species as compared to Cu.5+ shows that the two metal complex subunits of the [3]catenates do interact. This may reflect an... 

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