K E N - I C H I S U G I U R A and Y O S H I T E R U SAKATA*
The Institute of Scientific and Industrial Research, Osaka University, Mihoga-oka, Ibaraki, Osaka 567, Japan
Abstract. 1,2-Bis(octaethylporphyrinyl)ethane and its metallocomplexes with various combinations of Zn, Cu, and Ni were prepared. On the basis of IH NMR spectra, it was concluded that interactions between the two porphyrins become attractive when at least one of the two rings is converted to zinc porphyrin. This phenomenon was explained by dipole-dipole interaction arising from large polarization in Zn complexes.
Keywgrds. Porphyrin rings; dimer; electronic spectra; monomers.
1. Introduction
Aggregation phenomena, which originate from intermolecular forces such as van der Waals and/or ionic interactions (Silinsh 1980L have been known for a long time and are seen in various n-systems. In general, aggregation or stacking brings about peculiar physical properties. F o r instance, a sharp intense absorption band, called the J-band in cyanine dyes (Sturmer 1977), hypochromism in nucleic acid bases (Seyama et al 1988), and exciton splitting in absorption spectra (Osuka and M a r u y a m a 1988) and so on. Porphyrins have highly extended n-electronic systems and they aggregate in the solid as well as in solution under neutral states (Scheer and Kats 1975) and ionic states (Scheidt and Lee 1987). Moreover, porphyrins can be coordinated in their central cavity by nearly all of the metal ions to give new n-electronic systems. The degrees of interaction between a series of metalloporphyrins are expected to be changed in combination with two differently perturbed chromophores and have been reported briefly (Abraham et al 1976). Katz observed for the first time the aggregation of some metallochlorophylls in solution by using N M R and IR techniques (Boucher and Katz 1967). They and, later, Abraham and Smith (Abraham et al 1991) identified two types of interaction. One involves coordination of side-chain carbonyl to central metal atom in another molecule and the other is n - n interaction independent of a metal to side-chain chromophore. A typical example of the former interaction is seen in the special dimer of photosynthetic reaction centre (Deisenhofer et al 1984). The nature of the aggregation due to the n - n interaction was studied theoretically and is explained by attractive interactions between the positively charged a-framework and negatively charged n-electrons of the other molecule (Hunter and Sanders 1990).
So far such attractive interactions have been investigated between intermolecularly or intramolecularly located porphyrins. However, systematic studies are still lacking especially on the interaction between differently coordinated porphyrin rings. F o r such a study, a new type of dimer porphyrin is required. We chose Arnold's porphyrin dimer 1 (Arnold et al 1977), in which two porphyrin rings are linked by a flexible
* For correspondence
735
736 Ken-ichi Suoiura and Yoshiteru Sakata
] (M 1 = M 2 = 2H) .~ (M 1 = 2H, M 2 = Ni) 3_ (M 1 = 2H, M 2 = Cu) t ( M1 = 2H, M 2 = Zn) 5 ( a I = N i , M 2 = N i )
6 (M I = N i , M 2 = C u ) 7 (M 1 = Ni, M2.= Zn) 8 (M ~ = Cu, M 2 = Cu)
(M 1 = Cu, M 2 = Zn) 10 (M 1 = Zn, M 2 = Zn)
1.!, (M = 2H) 12 (M = Ni) 1_3 (M = Cu) 14 (M = Zn)
Figure 1. Syn- and anti-forms of .1 and its metal complexes.
s HjSO,.
l
1 7.n(AeO~2H20~s I
9 i Ou(AcO:~.~40
cHcl~ ~lux 7
1.5 Cu(Ar Zn(AcO:~-21H~O
3 9
-... -
c.c~ zs "c 4_
Figure 2. Synthetic routes of 2-10.
2. Results and discussion
Free base ! was prepared by the methods of Arnold and/or Ponomarev (Arnold et a/
1977; Ponomarev and Shul'ga 1985). For the synthesis of homo-metal complexes _8 and 10, the free base compound _1 was treated with the corresponding metal acetate in boiling chloroform. Mono-metalated complexes _2, _3 and _4 were prepared under controlled conditions in moderate yields. Mixed-metal complexes _6 and 7 were derived from the mono-nfetalated complex _2 and the corresponding metal acetates. On the other hand, _9 was prepared from _3 and zinc acetate dihydrate.
I H - N M R spectra of all the monomers and dimers except Cu complexes _3, 6, _8, 9_
and 13 were measured in CDCI3. These spectra can be classified into two groups by considering whether at least one of the two porphyrin rings contains a zinc ion in the cavity or not. A typical example of zinc complexes is shown in figure 3 for 10.
In figure 3 there are two characteristic features. First, upfield shifts for meso-protons (meso-~t) were observed, compared with 14. Second, methylene protons of ethyl substituents were observed as non-equivalent signals. These characteristics are the same as seen in the cis isomer of (OEP)-CH = CH-(OEP) (Ponomarev et al 1993).
l__
'~ 6 ~ a, ;3 ~' 1
Chemical Shifts / ppm
Figure 3. IH-NMR spectra of _1 and 10 in CDCi 3.
738 Ken-ichi Suoiura and Yoshiteru Sakata
o ~
Z
I
Z
0
0
D I
I I I
6 o 6
+ + +
+ + +
6 6 6 6 6 6 + + + 1 + +
6 6 ~ 6 ~
E + + + + + +
In the case of O E P and the corresponding metal complexes, the chemical shifts of meso protons are found at around 10 ppm. Compounds 1, _2 and _5 show their peaks in the similar region. However, the corresponding protons of _4, _7 andlO show upfield shifts by more than 1 ppm. Since the introduction of 2-porphyrinylethyl substituents into a porphyrin nucleus at meso positions will make a small electronic perturbation to the porphyrin n-electronic system, the difference of the chemical shifts between meso protons of the complexes 1 ~ 10 with that of the corresponding OEP's can be ascribed to the ring current effect of the closely located porphyrin ring. Therefore, the values A . . . ~, - ~', - 13 and - fl' (differences in chemical shifts of meso-protons between complexes l ~ 10 and the corresponding metal complexes of OEP) can be used as an indication of their molecular structures. The values for 1, _2 and _5 are quite small, - 0.08 to + 0.45 ppm as shown in figure 4 and these compounds are considered to be anti conformation in solution. On the other hand, 4_, 7 and 10 show large values in the range of 0.71 to 2-00 ppm (figure 4). Therefore, we can conclude that these compounds take the syn conformation. Ring current effects also affect the chemical shifts for inner N-H protons of the Zn-2H complex _4. The broad peak was observed at - 4 . 3 4 p p m . Whereas 1 and _2 show the peaks at 2.83 and - 3 . 0 6 p p m and at
A 1 -- m - ~ - m-o~
= m - l ~ ' - m - ( x '
A > 0.5 ppm ~" I Syn'F~ I A < 0.5 ppm
;, IAnti-Form I
10 7 5 4 2 1
-0.50
Figure 4.
A1
A2
0.00
9 . , . . . . , . . . . , -
0.50 1.00 1.50
Chemical Shifts / ppm
Chemical shift differences (A 1 and A1) of 1-_5, _7, and 10.
2.00
740 Ken-ichi Sugiura and Yoshiteru Sakata
- 2.78 and - 2"90 ppm, respectively. These upfield shifts are also strong evidence for the syn-form.
As described before, methylene signals of complexes _4, _7 and !0 containing at least one zinc porphyrin ring are very different from the others and resemble the cis-porphyrinylethene 15 (Ponomarev et al 1993). All the methylene protons are split into eight sextets. The assignment of the signals was carried out using I H - 1 H correlation spectroscopy (1H-1H COSY) and the nuclear Overhauser effect (NOE).
Although the methylene protons of both the syn and the anti-forms are prochiral, only the syn-form shows complex signals by strong magnetic anisotropic effects of the other porphyrin ring. In variable temperature N M R s:tudies of 10, no change in their shapes was observed even up to 110~ in chlorobenzene-ds. This means that the syn conformation is stable with the activation energy of more than 3.18 kJ mol-
Electronic spectra of ! ~ 10 were measured in CHC13. Typical spectra are shown in figure 5 and data are summarized in table 2. From figure 5 one can see that the spectra of dimers are different from those of monomers. However, there exists no distinct difference between syn- and anti-conformers as seen in ~H NMR spectra.
Therefore, electronic spectra cannot be used for the assignment of the two isomers.
Our present results clearly show that the attractive interactions between two porphyrins overcome the steric repulsion of ethyl substituents when at least one of the two rings is complexed with zinc ion. This conclusion is given for the first time by this experiment, althottgh attractive interactions between two zinc porphyrin rings have been reported (Abraham et al 1976; Leighton et al 1988; Uemori et al 1992).
When neither of two porphyrin rings is complexed with zinc ions, the interaction between them becomes repulsive. Attractive interaction can be explained as follows.
Based on the electronic structure of several metalloporphyrins the net electron population of metals was determined to be + 0.40 for Zn, + 0.28 for Cu, and + 0.30 for Ni (Zerner and Gouterman 1966). X-ray photoelectron spectroscopy studies for the Is orbitals of porphyrin nitrogen atoms also confirmed this tendency (Karweik and Winograd 1976). Considering these values the intramolecular polarization of zinc
0
0 0 2 . 0 .
. Q
,~ 1.0"
0 0.0 15
600 500 400 300
4 . 0 | ' i , I , I , , ... 11 3.0
20 25 30 35
W a v e n u m b e r s / X 10 3 cm "1
(nm) (nm)
400
I
6OO 5OO
4.0 I i ' ' i ,
i
3.0 1
2.0
1.0 ~ ,
0.0 . . .
15 20
, I
- - 10
. . . 1 4
: I
. . . . i . . . .
25 30 35
Wavenumbers / X 10 3 cm "1 Figure 5. Electronic spectra of monomer and dimer porphyrins in CHC13.
23810 19420, 18380, 17210, 15700 24390 19400, 18280, 17060, 15650 24810 19460, 18180, 17010, 15630 23890 18550, 17420
23750, 24180 18480, 17440 7 23640, 24210 18280, 17380 25250 18330, 17320 25280 18180, 17240, 16950
10 25220 18080, 17180, 16890
11 25160 18760, 17670, 1685~16140
12 25540 19400, 18130
13 25190 19120, 17870
14 25030 18850, 17640
1 in CDCI 3
complex is the largest among those of other metalloporphyrins such as nickel and copper. Qualitatively, this polarization is governed by the electronegativities for metal ions: Zn, 1.6; Cu, 1.9; Ni, 1.8. The strong dipole-dipole interaction arises from this polarization in the case of zinc complexes, when the two rings locate with offset stacking. For other metal complexes and free bases, this attractive force is weak and steric repulsion forces dominate, and anti conformation is favoured. Attractive geometry of the two zinc porphyrins with parallel and offset stacking was supported by X-ray analysis of o-his(zinc porphyrinyl)benzene (Osuka et al 1991). In the solid state molecular assembly, this specific intermolecular dipole-dipole interaction induced by zinc ion was also observed. The phase transition enthalpy and entropy of the 5, 10, 15, 20-tetrakis (4-n-dodecylphenyl)porphyrinato zinc are considerably greater than those of other metal complexes, i.e., Co, Ni, Cu and Pd (Shimizu et al 1993).
3. Conclusion
On the basis of 1H N M R studies on metallocomplexes of flexible dimer porphyrins, it is concluded that attractive interactions between porphyrins are dominant when zinc is incorporated into one or both of the two porphyrin rings. This attractive interaction is explained by dipole-dipole interaction generated from polarized n-electronic structure which is induced by inserted metal ions. The results in this study may be general for the interaction of porphyrin macrocycles and strong attractive interactions seem to be one of the most dominant factors for the aggregation of porphyrins, including chlorophyll.
742 Ken-ichi Su#iura and Yoshiteru Sakata References
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