Porphyrins stoppers

The synthesis of [3]- (figuratively shown as 7) and a [5]rotaxane (8) with one central and two terminal porphyrins in the open configuration has been reported <96AG(E)906> also a rotaxane with two Ru(terpy>2 stoppers has appeared <96CC1915>. A pseudorotaxane comprised of a macroring of 2,9-diphenyl-1,10-phenanthroline unit and a molecular string... 

The axial coordination of metalloporphyrins to a pyridyl ligand was successfully exploited by two groups to produce porphyrin-stoppered rotaxanes. Sanders (48) assembled a rotaxane by simply mixing the constituent parts. Zn(II), Ru(II)CO, and Rh(II)Cl porphyrins were used as stoppers. Branda (49) reported the stoppering of a pseudorotaxane by adding two equivalents of a Ru(II)CO porphyrin that coordinated to... 

Molecular systems in which two porphyrins are held apart by a covalently linked spacer function give rise to bis-porphyrins with flexible or constrained geometries. Rotaxane architectures incorporating similar or dissimilar porphyrins as stoppers have been widely developed in our group, with the goal of building such systems capable of allowing modulation of electron transfer. 

Figure 7 illustrates the principle of the construction of this [2]-rotaxane, which bears two different porphyrin stoppers. 

This copper rotaxane could be demetallated, producing the free rotaxane 3 (Figure 6). As observed in some other cases, the demetallation reaction was accompanied by a pirouetting of the macrocycle, moving its dpp chelate outside the cleft formed by the porphyrin stoppers.173,74- In fact, free rotaxane 3 and bis-porphyrin thread 1 differ mainly in the fact that, in 3, ligand 1 is mechanically incarcerated in macrocycle 4. 

Fig. 7 Principle oftransition metal-templated construction of a [2]-rotaxane with two different porphyrin stoppers. The white diamond is a zinc porphyrin, and the hatched diamond is a gold porphyrin. The black disk is Cu(i).

Assemble the two-necked, round-bottomed 250 mL flask, the heating jacket and the stirrer. Dissolve the porphyrin (0.31 g) in 100 mL of acetic acid. Add potassium tetrachloroaurate (0.236 g) and sodium acetate (0.079 g). Introduce the stirring bar, assemble the condenser and stopper the remaining opening of the flask with a septum. 

Weigh 0.157 g of porphyrin. Put it into a 10 mL Schlenk flask and stopper it with a septum. Connect the flask to the vacuum line. Subject the apparatus to three vacuum/argon cycles. 

Figure 2.30. Copper(I)-templated synthesis of Cu(I)-complexed [3]-rotaxane 86 and compartmental [5]-rotaxane 87, bearing free-base porphyrins as stoppers.

Because porphyrins bind metal cations avidly, we found it necessary to protect the porphyrin stoppers by complexation prior to template removal studies. The sequence of reactions are shown schematically in Figure 2.31.61... 

Figure 2.31. Schematic representation of some coordination chemistry studies performed on bis-porphyrin-stoppered [3]-rotaxane 86. The black disk is Cu(I) black and hatched diamonds represent Au(III) and Zn(II) porphyrins, respectively. The thick lines represent chelate (phenan-throline) fragments.

The Cu(I)-complexed [2]-rotaxane 102 of Figure 2.36, containing Zn(II) and Au(III) porphyrins as stoppers, was designed and synthesized with the purpose of mimicking the ET processes occurring within the SP/BCh/BPh trichromophoric fragment described above. 

The precursors and the synthetic route leading to rotaxane 102 are represented in Figure 2.37.57b First macrocycle 58 was threaded onto the presynthesized Au(III) porphyrin-substituted phenanthroline 103, a semidumbbell molecule, in the presence of Cu(I), to afford prerotaxane 104 quantitatively. The second stopper (and functional end cap) was installed by the meso-porphyrin construction method. Thus reaction of 104 with 4,4/-dimethyl-3,3/-diethyl-2,2/-dipyrrylmethane 105 and 3,5-di-/m-bulyI benzaldehyde 84, followed by oxidation of the intermediate porphyrinogen with chloranil 85, gave the free-base Cu(I)-complexed [2]-rotaxane 106 in 25% yield. After metal-lation with Zn(0Ac)2 2H20 and exhaustive purification, Cu(I) complex 102 was obtained. It was subsequently demetallated with KCN, to afford the free [2]-rotaxane 107 in quantitative yield. 

An important question in biological electron transfer is related to through-space and through-bond processes.74 Whereas through-bond processes were studied with [2]-rotaxanes 102 and 107, [2]-rotaxane 111 of Figure 2.38 was synthesized with the purpose of addressing the through-space question in a novel approach.75,76 In such a rotaxane the donor (Zn porphyrin stoppers) and the acceptor (Au porphyrin appended to the macrocycle) components are maintained in the same molecule by mechanical bonds only. It is therefore... 

Figure 2.37. Copper(I)-templated synthesis of a Zn(II)/Au(III) bis-porphyrin-stoppered [2]-rotaxane as its Cu(I) complex (102) and as free [2]-rotaxane (107).

Porphyrins, owing to their outstanding photophysical and redox properties [74], are extensively used for the construction of photochemical molecular devices to achieve charge separation over nanometric distance [75]. Porphyrin units have been successfully incorporated into rotaxane structures [76, 77], first playing the role of stoppers. In CHjCN solution at room temperature, [2]rotaxane 17 (Fig. 15) [78, 79] undergoes a very fast (ca. 2 ps) electron transfer process from the Zn(II) to the Au(III) porphyrin upon excitation of the Zn(II) porphyrin moiety (process 1 in Fig. 15). Although direct back electron transfer from the Au(II) to the Zn(III) porphyrin indeed occurs (530 ps), the initial state is restored mainly via oxidation/reduction of the central copper unit, that is, through an electron transfer from... 

Figure 43 Principle of transition metal-templated construction of a [2]-rotaxane with two different porphyrinic stoppers (same conventions as in Figure 42). (i) Macrocycle (A) is threaded onto chelate (B), end-blocked by a porphyrin at one extremity and fiaictionalized with a reactive group X, which is a precursor to the second porphyrin stopper, affording prerotaxane (C) (ii) construction of the second porphyrin, leading to metal-complexed [2]-rotaxane (D).

Figure 49 One-pot construction of the two porphyrinic stoppers of copper(I)-complexed [2]-rotaxane (142). Also represented are the silver(l)- and lithium-complexed [2]-rotaxanes (144) and (145) [160].

In conclusion, the zinc(I0 and goId(lII) porphyrin-stoppered [2]-rotaxanes presented in this section allowed the study of two fundamental aspects of electron transfer the through-bond and the through-space processes. 

The precursors used and the synthetic route followed to prepare a bis-copper(l)-complexed [3]-rotaxane fitted with porphyrin stoppers are shown in Figures 52 and... 

Figure 51 Principle of transition metal-templated synthesis of a [3]-rotaxane, from two chelating macrocycles (B) and a bis-chelate-containing molecular thread (A) functionalized with reactive end groups X (same conventions as in Figure 43). (ii) Threading step, affording prerotaxane (C) construction of the porphyrin stoppers providing copper(I)-complexed [3]-rotaxane (D).

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