Institute of Organic Chemistry, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany
Redox active crown ethers, consisting of quinones (redox reaction) and crown ethers (complexing reaction) are well suited for studies of electrochemically driven ion transport mechanisms [3]. This combination of redox and complexing reactions is also provided by ortho quinones due to the close lying oxygens [4].
If redox active crown ethers or ortho quinones are covalently linked to porphyrin, the photoinduced electron transfer (ET) and coupling reactions can be studied in the same system. Loading of the ortho quinones (e.g. radicals 1, 2 and 3) or the crown ethers (e.g. radicals 4, 5cis and 5trans) with alkali metal cations are known to decrease the reducing potential of the quinones, thereby changing the driving force G of the ET reaction [2].
These systems can be studied not only by optical spectroscopy or by cyclic voltammetry, but also by EPR and ENDOR spectroscopy.
In Figures 1 - 6 we present the ENDOR spectra of chemically generated (alkaline reduction) semiquinone anion radicals from porphyrin-quinones in 2-propanol.
Three different, directly covalently linked porphyrin-1,2-benzosemiquinone anion radicals (Fig. 1, 2, 3) were examined as chelate complexes with alkali counter ions. Tetra-n-butylammonium hydroxide (n-C4H9)4N+OH- (40% solution in water) was used as reference. As shown in Table 1, in all cases of metal chelatation the hfc constant a_4 (position of high spin density) increases and those for positions of small spin density (a_3, a_6) decrease. For Li+- and Na+-chelates 7Li- and 23Na-ENDOR hfc constants were obtained. In the case of the tetra-n-butylammonium counter ion small hfc constants for gamma- and delta-porphyrin protons were found.
Three differently bridged porphyrin-quinone crown ethers (1,4-butylene-, cis-1,4-cyclohexylene- and trans-1,4-cyclo-hexylene) were used for studies of counter ion effects (Fig. 4, 5, 6). For the butylene bridged anion radical 4 relatively large and positive hfc constants for 7Li+ and 23Na+ ions were determined. Surprisingly, the hfc constant for 23Na+ is more than 1.1 MHz larger than a_Li+ (gamma_7Li+/gamma_23Na+ = 1.4). The different counter ions create hfc shifts (in opposite direction) for the proton couplings, indicating a different mechanism of complexation.
At first sight, the ENDOR spectra of the Na+-complex of 5cis and 5trans might be interpreted assuming large and different cyclohexyl beta- and crown ether beta-proton couplings (Fig. 5 and 6). However, this interpretation is not in accordance with the EPR spectra (total splitting and hyperfine pattern). In comparison with our previously published results on 2-cyclohexyl-3-methyl-1,4-benzosemiquinone derivatives [5], ENDOR signals I and II must be due to different species. Clearly, sterical hindrance between the neighboring cyclohexyl and crown ether is responsible for this effect. Moreover, the cyclohexyl beta-proton coupling decreases significantly (< 0.5 MHz). Obviously this beta-proton is forced into the plane of the semiquinone ring. Thus, the two species give different sets of hfc constants (Table 2).
Signs of the hfc constants were determined by general TRIPLE resonance.
Table 1. Hyperfine coupling constants [MHz] of complexed 1,2-benzosemiquinone radical anions in 2-propanol
subst. T [K] a3 a4 a5 a6 aM+
1R4N+ 280 -2.89 -0.50 a, +0.20 b -8.94 -3.70 ---
1Li+ 290 -2.42 -0.69 -10.33 -2.42 -1.22
1Na+ 290 -2.32 -0.56 a, +0.20 b -9.40 -3.22 -0.62
2R4N+ 250 +1.32 c -0.51 -8.86 +1.32 c ---
2Li+ 280 +0.62 c --- -9.52 +0.62 c -1.32
2Na+ 270 +0.59 c --- -9.27 +0.79 c -0.26
2K+ 270 +0.64 c --- -9.16 +0.83 c ---
a1,8 a2,7 a4,5 a6 aM+
3R4N+ 270 -4.18, -4.05 +0.78 +1.23, +1.08 -4.55 ---
3Li+ 270 -4.53 +0.95 +1.15 -4.90 -1.32
3Na+ 260 -4.41 +0.98 +1.12 -4.83 -0.62
3K+ 290 -4.33 +0.86 +1.10 -4.81 ---
a gamma-protons (2 H), b delta-protons (2 H), c beta-methylprotons (3 H).
Table 2. Hyperfine coupling constants [MHz] of complexed 1,4-benzosemiquinone crown ether radical anions in 2-propanol
subst. T [K] a2(beta,gamma) a3(beta) a5(beta) a6 aM+
4R4N+ 250 +3.34 a, 0.34 b +2.68, +2.35 +4.52 -6.45 ---
4Li+ 260 +2.82 c, 0.34 b +2.82 c +4.30 -6.57 +1.84
4Na+ 250 +3.51 a, 0.30 b +2.75, +1.94 +5.04 -6.22 +2.96
4K+ 310 +3.53 a, 0.30 b +3.00, +2.60 +3.86 -7.03 ---
5(cis)R4N+ 260 0.35 d +2.59 +3.82 -6.48 ---
5(cis)Na+ 260 0.32 d +3.31 (II) +4.29 (II) -7.27 (I) +3.43
+2.51 (I) e +3.76 (I) e -6.52 (II) e
5(trans)R4N+ 260 0.51 d +2.80 +3.73 -6.34 ---
5(trans)Na+ 280 0.40 d +2.86 (II) +3.98 (II) -7.96 (I) +3.04
+2.17 (I) e +3.42 (I) e -7.41 (II) e
a beta-protons (2 H), b gamma-protons (2 H), c broadened signals (4 H), d only gamma-protons (4 H) detectable, e 2 species (I and II) from sterical hindrance between crown ether and cyclohexane substituent (2 conformers).
Radical anions prepared by alkaline reduction in 2-propanol are not free ions, because they are often in close contact with residual unreduced molecules, with other ions, especially with their counter ions and with solvent molecules in their immediate environment. Lubitz et al. [6] and Stegmann et al. [7] investigated the effects of different cations and solvents on 1,2-benzosemiquinone or related anion radicals. In accordance to these results, a particular model for contact ion pair binding can be discussed. As shown above, there are two different mechanisms of complexation possible, to build a contact ion pair in pi-plane (left side) or out-of-plane (right side). The physical and chemical properties of radical anion, cation and solvent can be characterized by an equilibrium between these limiting cases. For Li+, which is the smallest alkali cation, we assume an in-plane complexation in polar solvents. The ENDOR results for Li+-chelates, negative hfc constants (positive spin density), independent of substitution of 1,2-semiquinone, are in line with this hypothesis. For Na+-chelates the interaction in the ion pair is generally small, e. g. for the Na+-complex of 3,6-dimethyl-1,2-benzosemiquinone anion radical 2 we found a small hfc constant for Na+ due to sterical hindrance by the two methyl groups.
We obtained opposing results for benzosemiquinone crown ethers. The geometry of the molecule implies two planes (semiquinone and crown ether `plane'). The average angle between these two planes is about 30 deg., thus, a complexed cation in the crown ether plane lies above or below the semiquinone ring. Since an interaction between cation and radical anion can be observed, we assume a pi-overlap complexation. The ENDOR results for 4 support this hypothesis. The positive hfc constant (negative spin density) for Na+ is larger than for Li+; an explanation may be the different location of these cations (more pi-overlap interaction for Na+ in the crown ether plane).
In summary it can be stated that ENDOR experiments can contribute to a better understanding of the different geometrical arrangements in ion pair complexes.
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