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Electrochemistry and Ab Initio Study of the Dimetallofullerene La2@C80.

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Keywords: acetylene complexes . bridging ligands
compounds . porphyrinogens
titanium
[I] H. W. Kroto, A. W. Allaf. S. P. Balm, Chem. Rev. 1991, 91, 1213-1235; B. C.
Guo. K. P. Kerns, A. W. Castelman, Jr., Science 1992,255. 1411-1413; B. C .
Guo. S. Wei, J. Purnell, S. Buzad. A. W. Castleman, Jr., ibid. 1992, 256. 515516; S. Wei, B. C. Guo. J. Purnell. S. Buzza. A. W. Castleman, Jr.. ibid. 1992.
256, 818-820; S. F. Cartier, Z. Y. Chen. G. J. Walder, C. R. Sleppy, A. W.
Castleman, Jr., ihid. 1993, 260, 195-196.
[2] a) H. Lang, Angels. Chrn?. 1994, 106. 569-572: Angeii.. Chem. h i . Ed. Engi.
1994,33. 547-550, and references therein; b) J. A. Ramsden, W. Weng, A. M.
Arif. J. A. Gladysz. J. A m . Chem. Soc. 1992.114,5890-5891; c) W. Weng. J. A.
Ramsden, A. M. Arif, J. A. Gladysz, ibid. 1993,115, 3824-3825; d) Y. Zhou.
J. W. Seyler. W. Weng, A. M. Arif, J. A. Gladysz, ibid. 1993, 115. 8509-8510:
e) W. Weng, T. Bartik, J. A. Gladysz, Angew. Chein. 1994, 106. 2269-2272:
Angrw. Chem. hi. Ed. Engi. 1994. 33. 2199-2202; f ) K. Sonogashira. S.
Kataoka. S. Takahashi, N. Hagihara, J. Orgunon?eta/.Chem. 1978, 160. 319327; A. Wong. P. C. W. Kang, C. D. Tagge, D. R. Leon, Orgononieiaffics 1990.
9, 1992-1994; H. B. Fyfe. M. Mlekuz, D. Zargarian, N. J. Taylor. T. B.
Marder. J. Chem. Soc. Chein. Cominun. 1991,188-190; P. J. Stang. R.Tykwinski, J. Am. Chew. Soc. 1992. 114, 4411-4412; R. Crescenzi, C. Lo Sterzo.
OrgonoinrraNics 1992, 11.4301 -4305; T. Rappert, 0. Niirnberg, H . Werner,
ibid 1993, 12, 1359-1364; N. Le Narvor, C. Lapinte. J. Chem. Soc.
Chem. Commun. 1993. 357-359.
[3] M. Appel, J. Heidrich, W. Beck, Chem. Ber. 1987. 120, 1087-1089; J. Heidrich,
M. Steimann, W. Beck, J. R. Phillips. W. C. Trogler. Organometullics 1990. 9.
1296-1300; M. Akita. M. Terada. S. Oyama, Y Moro-oka. ibid. 1990. 9.
816-825: K. G. Frank, J. P. Selegue, J. Am. Chem. Soe. 1990,112.6414-6416:
H . Ogawa, K . Onitsuka. T. Joh, S. Takahashi, Y. Yamamoto. H. Yamazaki,
Orgunometulfics 1988, 7. 2257-2260; J. A. Davies, M. El-Ghanam, A. A.
Pinkerton. D . A. Smith, J Orgunomel. Chem. 1991, 409. 367-376.
[4] W. Beck. B. Niemer. M. Wieser. Angew. Chern. 1993, 105, 969-995;
Angen. Chem. Int. Ed. Engf. 1993, 32. 923-949, and references therein; G . A.
Koutsantonis. J. P. Selegue. J. Am. Chem. Sot. 1991. 113. 2316-2317; F. R.
Lemke, D. J. Szalda, R. M. Bullock, ibid. 1991, 113, 8466-8477.
[5] a) D. Jacoby, C. Floriani, A. Chiesi-Villa, C. Rizzoli, J. Chern. Soc. Chem.
Commun. 1991, 220-222. 790-792: b) J. Am. Cheni. Soc. 1993, 115. 35953602, 7025-7026; c) S. DeAngelis, E. Solari. C. Floriani. A. Chiesi-Villa. C.
Rizzoli. J. Am. Chem. Soc 1994. 116, 5691-5701. 5702-5713: d) E. Solari, F.
Musso, C. Floriani, A. Chiesi-Villa. C. Rizzoli, J. Chem. Soc. Dafton Trans.
1994, 2015-2017.
M , = 885.1, mon[6] a) Crystal structure analysis of 1: C,,H,,N,O,Ti.l/2C,H,,
oclinic. space group 12/12. a = 17.330(2), h = 12.820(2). c = 22.732(2) A.
j = l O l . 4 7 ( 1 ) , V = 4 9 4 9 . 5 ( 3 1 ) ~ ' , Z = 4 , p,,,,,=1.188gcm~3, F(000) =
1912. Cu,, radiation (~i1 . 5 4 1 7 A.
8 ~(CU,,) =17.87 cm-l : crystal dimensions 0.23 x 0.35 x 0.45 mm. The structure was solved by the heavy-atom
method and anisotropically refined for all non-hydrogen atoms. The hydrogen
atoms were found from a difference Fourier map and introduced as fixed
contributors in the last stage of refinement (U,,, = 0.08 A'). 2775 Unique
observed reflections [I > 2u(I)] were collected at T = 295 K on a Rigaku
AFC6S diffractometer (6 < 28 < 140') and corrected for absorption;
R = 0.051 ( x R 2 = 0.153 for 4625 total unique reflections). All calculations
were carried out on an IBM PS2/80 personal computer and on an Encore 91
computer. b) Further details of the crystal structure investigation may be
obtained from the Director of the Cambridge Crystallographic Data Centre,
12 Union Road. GB-Cambridge CB2 I E Z (UK), on quoting the full journal
citation.
[7] S. DeAngelis, E. Solari, C. Floriani. A. Chiesi-Villa, C. Rizzoli, J. Chem. Sot.
Dalion Trims. 1994, 2467 - 2469.
M , = 1798.0,
[XI Crystal structure analysis of 2: C,,H,,Li2N8Ti2~2(C,,H,,Li0,).
triclinic, space group PT, a =16.648(2). b = 22.144(3), c = 15.948(2) A, a =
97.97(1), P =108.95(1). Y =79.02(1)", V = 5440.2(13) A'. Z = 2, pcalcd
=
1.098 g ~ m - ~F(OO0)
,
=1944. Cu,, radiation (2. =IS4178 A, ~(CU,,)=
16.51 cm-' : crystal dimensions 0.22 x 0.26 x 0.38 mm. The structure was
solved by the heavy-atom method (Patterson and Fourier synthesis) and anisotropically refined for all non-hydrogen atoms. Some ethyl chains and six thf
molecules of the two independent Li(thf), cations were disordered. The disorder was solved by splitting the atoms in two positions and refining isotropically
with the site occupation factors given in the supplementary material. All the
hydrogen atoms of the porphyrinogens except for those attached to the disordered atoms were located in difference Fourier maps and introduced as fixed
contributors in the last stage of refinement (Ulyo= 0.12 A2). The hydrogen
atoms of the thf molecules were ignored. 5277 Unique observed reflections
[ I > 2u(l)] were collected a t room temperature (6 < 28 < 140") and corrected
for absorption: R = 0.092 (wR2 = 0.244). All calculations were carried out
with the SHELX-76 and SHELXL-92 programs on an Encore 91 computer
WI.
[9] Lil-TilB 3.304(21), TilB-Li2 3.229(21), Li2-TilA 3.328(21), Ti1 A-Lil
3.206(21) A; Til-Lil-TilB 96.116. Lil-Ti2B-Li2 83.5(6). TilB-Li2-TilA
96.8(6). Li2-Ti1 A-Lil 83.7(6)".
1094
<D VCH Verfugsgesellschafim h H . 0-69451 Weinheim, 1995
[lo] The LI-C distances [A] for the A [B] unit are: L i l - C l l 2.688(24) [2.735(22)].
Lil . . C12 2.402(26) [2.433(24)]. Lil . - C 1 3 2.565(24) [2.567(29)], L i 2 . . . C I
2.731(23) [2.664(22)], Li2-C2 2.398(24) [2.454(26)]. Li2ZC3 2.605(23)
[2.635(28)].
[ l l ] G. L. Wood. C . B. Knobler. M . E Hawthorne, Inorg. Chem. 1989. 28. 382384.
[I21 P. Binger. P. Miiller, P. Philipps. B. Gabor, R. Mynott, A. T. Herrman, F.
Langhauser. C. Kriiger. Chem. Err. 1992, 125, 2209-2211.
[I31 a) D. Mansuy, M. Lange, J.-C. Chottard, P. Guerin, P. Morliere. D. Brault,
M. Rougke. J. Chem. Sot. Chem. Commun. 1977, 648-649: D . Mansuy, JLP.
Lecomte. J.-C. Chottard. J.-E Bartoli, h u g . Chem. 1981. 20. 3119-3121 :
b) G. Rossi, V. L. Goedken. C. Ercolani, J. Chem. Soc. Chem. Comwzun. 1988,
46-47; C. Ercolani. M. Gardini. V. L. Goedken, G. Pennesi. G. Rossi, U .
Russo. P. Zanonato, Inorg. Chein. 1989, 28. 3097-3099.
Electrochemistry and Ab Initio Study of the
Dimetallofullerene La,@& **
Toshiyasu Suzuki,* Yusei Maruyama, Tatsuhisa Kato,
Koichi Kikuchi, Yasuhiko Nakao, Yohji Achiba,
Kaoru Kobayashi, and Shigeru Nagase*
Metallofullerenes are fullerene derivatives that contain some
metals inside their cages."] Although their structures, including
the cage structures and the positions of the metals, have not yet
been determined, the recent development of methods for purifying metallofullerenes allows more advanced studies of these new
molecules.[2- 'O1 Theoretical calculations suggest that their electronic structures are significantly changed by electron transfer
from highly electropositive metals to the electronegative carbon
cages.[", 12] Very recently, we reported that La@C,, and
Y @C,, showed almost identical electrochemical properties,'6, 'I
in accord with the ab initio calculations.['2e1It is interesting to
see how redox properties of the metallofullerene change when
two lanthanide atoms are present in the cage. Encapsulating two
lanthanide atoms is expected to result in an increase in total
charge transfer from the two metals to the cage and in the
interaction between the two metals. Since the first detection of
La,@C,, by mass spectrometry in 1991,['31there has been little
information about the electronic structure. Unlike La@C,, ,
La,@C,, is EPR-silent. We report here the electrochemistry of
La,@C,,, which significantly differs from those of empty fullerenes and monometallofullerenes, and the ab initio study of
La,@C,, at the HF level.
La,(iC,, was obtained as a by-product in the synthesis of
LaRC,, and isolated in pure form by HPLC.[141The cyclic
voltammogram (CV) of La,@C,, in 1,2-dichlorobenzene shows
two reversible oxidations and two reversible reductions (Fig. 1
and Table 1). The differential pulse voltammogram (DPV) displays four well-defined peaks with the same current intensities
as well as an additional ill-defined peak at -2.13 V (Fig. 1).
[*I Dr. T. Suzuki
Fundamental Research Laboratories, NEC Corporation
34, Miyukigaoka. Tsukuba 305 (Japan)
Telefax: Int. code + (298)56-6136
Prof. S. Nagase, K . Kobayashi
Department of Chemistry, Faculty of Education,
Yokohama National University
Yokohama 240 (Japan)
Prof. Y Maruyama, Prof. T. Kato
Institute for Molecular Science, Myodaiji, Okazaki 444 (Japan)
Prof. K . Kikuchi, Y. Nakao, Prof. Y. Achiba
Tokyo Metropolitan University (Japan)
[**I This work was supported by the Ministry of Education, Science and Culture of
Japan (No. 06224230).
U57U-oR33/95/1010-/0943 10.00+ .2Sj0
Angew. Chem. Int. Ed. Engf. 1995, 34, N o . 10
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I
----
0-
0.3 p A
I
T
I
,
1.5
1
,
,
0.5
.
.
.
.
-2-
.
0 -0.5 - 1 -1.5 - 2 -2.5
+-
EIV
Fig. 1. CV (top) and DPV (bottom) of La# C, at 20 mVs-' in 1.2-dichlorobenzene containing 0.1 M nBu,NPF,.
When the CV scan is continued beyond this potential, the second reduction also becomes irreversible. The first reduction potential of La,(~C,, is anodically shifted relative to those of
La@C8,-A['I and -Bl8] (major and minor isomers, respectively), suggesting that the dimetallofullerene is a stronger electron
acceptor than the monometallofullerenes (Table 1). The first
oxidation potential of La,@C,, is much more anodic than
those of La(trC,,-A and -B, implying that La,@C,, is a weaker
electron donor than the La(@C,,
Also, La,@C,,
has a remarkably narrow HOMO-LUMO gap compared to the
empty fullerenes such as C60[161and C,6[171(Eoxl-Eredl:
La,@ C,,, 0.87; C,,, 2.33; C,,, 1.75 V).
Table 1. Redox potentials of metallofullerenes and empty fullerenes[a].
Compound
EUx2
E,,1
Eredl
E d
Ere,3
La,w C " ,
LakrC,,-A
La(ri'C,,-B
f0.95
+1.07[b]
+1.08[b]
+0.56
+0.07
-0.07
+1.21[b]
+ 0.81
-0.31
-0.42
-0.47
-1.12
-0.94
-1.71
-1.37
-1.40[~]
-1.50
-1.26
-2.13[b]
-1.53
c,,,
+ 1.25
C,
-
-1.95
-1.72
Emd4
~
-2.26
-2.01
-2.41
-2.13
-
[a] Half-cell potentials unless otherwise stated. Values are in volts relative to ferrocene!ferrocenium couple. [b] Irreversible. Values were obtained by DPV. [c] Twoelectron process
To understand the redox behavior of La,@C,,, we carried
out ab initio theoretical calculations.['*l Among the seven C,,
isomers that obey the isolated-pentagon rule,["] we chose the
most symmetrical cage structure, I,-C,,, which was first proposed by Whetten et al.['51As shown in Figure 2, the La, dimer
is placed inside the Z,,-C,, cage along the C, axis that passes
through the centers of two six-membered rings. This endohedral
structure has D,, symmetry. The Hiickel molecular
orbital calculations suggest
that Z,,-Ca0 has fourfold degenerate HOMOs occupied
by two electrons and fourfold degenerate LUMOs.i201
In Figure 3, two electrons
are added to the a, orbital
for convenience. In the ab
initio calculations, the energy level of the a, orbital becomes lower than those of
the other three orbitals (b18,
Fig. 2 . View o l LA,((I~C,,with the fhb,,, and b3g).1211
Upon enC, cage
A n ~ r u. C h i w I n ! . Ed. EnRI. 1995, 34. No. 10
-R
1 '
'h%
Fig. 3. The orbital energy levels [eV] of I&,
0
and La,(n'C,, at the H F level..
dohedral doping, the HOMOs are fully occupied by six electrons from La, and stabilized at a La-La distance of 3.45 A.
Because the LUMO levels of C,, are not significantly affected
by endohedral doping, the low-lying empty 6s0, orbital of La,
becomes the LUMO of La,@C,, . Interestingly, this is not the
case for La(yCH2.in which the energy levels of the empty 5d and
6s atomic orbitals are higher than those of the unoccupied orbitals derived from the C,,
Upon electroreduction of
La,@C,,, the first electron goes to the LUMO (a,) which is
mostly localized on La,. This process is favored, and the a,
orbital is lowered from -2.6 to -4.4 eV. The first reduction
potential observed is very positive because the energy level of the
LUMO is relatively low. The second electron goes to the same
orbital (a,), and not to b,, or b3". This process costs much
energy because of the unfavorable electrostatic repulsion between the two electrons on the LUM0.[221Unlike electroreduction, the oxidation processes would take place on the C,, cage.
The electron-electron repulsion is expected to be larger on the
relatively small La, orbitals than on the delocalized fullerene
orbitals. This may explain why the potential difference between
the two reductions is much larger than that between the two
oxidations (1.40 and 0.39 V, respectively).
In summary, in the first electrochemical study of the pure
dimetallofullerene we have shown that La,(d:C,, is a stronger
electron acceptor than monometallofullerenes such as La@C,,A and -B and has a remarkably narrow HOMO-LUMO gap
compared to empty fullerenes. The theoretical calculations predict that upon electroreduction, at least the first two electrons go
to the lanthanum atoms, not to the C,, cage. Because only small
amounts of the materials are available, electrochemistry is a
quite useful method for characterizing metallofullerenes, especially, the EPR-silent ones.
Experimental Procedure
Cyclic (CV) and differential pulse voltammograms (DPV) were recorded on a BASBjW electrochemical analyzer. A three-electrode configuration was used throughout. All measurements were performed at ambient temperature under an argon
atmosphere in 0.1 M 1,2-dichlorobenaene solution of nBu,NPF,. The concentration
. compensation was employed throughout. A platinum
of La,(dC,, was 1 0 - 4 ~IR
working electrode (1 mm diameter) was polished with 0.05 pm alumina before
measurements. A platinum wire was used as the counter electrode. The reference
electrode was a Agi0.01 M AgNO, electrode filled with 0.1 M nBu,NCiO, In CH,CN.
All potentials are referenced to the ferrocene/ferrocenium couple (FcjFc') as the
$3 VCH Verlu~sgewl/.scku/rtnhH, 0-69451 Weinheim.1995
0570-0833195/1010-1095 X 10.00+ .25/0
1095
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internal standard. CV: scan rate, 2OmVs-'. DPV: pulse amplitude, 50 mV; pulse
width, 50 ms; pulse period, 200 ms; scan rate, 20 mVs-'.
Received: November 21. 1994 [Z7487IE]
German version: Angew. Chem. 199.5, 107, 1228
Keywords: ab initio calculations . electrochemistry . lanthanide
compounds
[l] For a recent review on metallofullerenes. see: D . S. Bethune. R. D. Johnson,
J. R. Salem, M. S. de Vries, C. S. Yannoni, Nature 1993, 366, 123.
[2] K. Kikuchi, S. Suzuki. Y Nakao, N. Nakahara, T. Wakabayashi. H. Shiromark!. K. %to. I. Ikemoto, Y. Achiba, Chem. P h w Lett. 1993, 216, 67.
[3] K . Yamamoto, H. Funasaka. T. Takahashi. T. Akasaka. J. Phys. Chem. 1994,
98. 2008.
[4] a) H. Shinohara, H. Yamaguchi, Y Hayashi. H . Sato, M. Ohkochi. Y Ando,
Y Saito, 1 Phys. Chem. 1993, 97,4259; b) H . Shinohara, N. Hayashi, H. Sato,
Y Saito. X.-D. Wang, T. Hashizume, T. Sakurai, ibid. 1993, 97, 13438.
151 R. Beyers. C.-H. Kiang, R. D. Johnson, J. R. Salem, M. S . de Vries, C. S.
Yannoni, I).S. Bethune. H . C. Dorn. P. Burbank, K . Harich. S. Stevenson,
Nature 1994, 370, 196.
[6] Suzuki, T.; Maruyama, Y; Kato, T.; Kikuchi. K.; Achiba. Y. J. A m . Chem. Soc.
1993, 115, 11006.
[7] K. Kikuchi, Y. Nakao. S . Suzuki. Y. Achiba, T. Suzuki. Y. Maruyama. J. A m .
ChiJm.Soc. 1994, 116, 9367.
[8] K. Yamamoto, H. Funasaka. T. Takahashi. T. Akasaka, T. Suzuki, Y. Maruyama, J. Phys. Chem. 1994. 98, 12833.
[9] S. Hino, H . Takahashi. K. Iwasaki, K. Matsumoto, T. Miyazaki, S. Hasegawa,
K. Kikuchi, Y Achiba, Phys. Rev. Leu. 1993, 71, 4261.
[lo] D. M. Poirier, M. Knupfer. J. H . Weaver. W. Andreoni, K . Laasonen, M.
Parrinello. D. S. Bethune, K . Kikuchi, Y. Achiba, Phys. Rev. B 1994.49, 17403.
[11] K . Laasonen, W. Andreoni, M. Parrinello, Science 1992, 258, 1916.
[12] a) S. Nagase. K . Kobayashi, T. Kato. Y. Achiba, Chem. Phys Lett. 1993, 201.
475; b) S. NagaSe. K. Kobayashi. ibid. 1993, 214. 57; c) S. Nagase, K.
Kobayashi, ibid. 1994. 228, 106; d ) S. Nagase, K. Kobayashi, ibid. 1994, 231,
319; e) S. NagaSe. K. Kobayashi, J. Chem. Soc.. Chern. Commun. 1994, 1837.
[13] M. M. Alvarez. E. G . Gillan, K. Holczer, R. B. Kaner, K. S. Min, R. L.
Whetten. J. Phys. Chem. 1991, 95, 10561.
[14] For experimental details, see ref. [2]. Retention times for La,(aC,,: 490 min
(polystyrene column, JAIGEL Co., CS, eluant) and 9.4 min (Buckyclucher I
column, Regis Co., toluene eluant).
[15] On the basis of the mass spectrometric studies. Whetten et al. suggested that:
1) La,@ C,, has an ionization potential higher than La(u;C,,, and 2) La,@ C,,
has an electron affinity higher than known fullerenes. These observations are
consistent with our electrochemical results. C. Yeretzian. K. Hansen, M. M.
Alvarez. K. S. Min. E. G . Gillan. K. Holczer. R. B. Kaner, R. L. Whetten,
Chem. Phys. Let,. 1992, 196. 337.
[16] D . Duhois, K. M. Kadish, S. Flanagan, L. J. Wilson. J. Am. Chem. Soc. 1991,
113,1773.
[17] Q. Li. F. Wudl, C. Thilgen, R. L. Whetten. F. Diederich, F. J. Am. Chem. Sac.
1992, 114, 3994.
[18] Computational details: Calculations were carried out using Hartree-Fock
(HF) molecular orbital method and the Gaussian 92/DFT program. The effective core potential and basis set developed by Hay and Walt were used on La
but the outermost core electrons in the 5s25p6 configuration were explicitly
treated as valence electrons [lsa]. The basis set of La was (5s5p3d)/[4s4p3d] in
standard notation. The split-valence 3-21G hasis set was used for C [18 b].
These give 782 contracted Gaussian functions for La,(dC,,. The geometry
optimized with the AM1 method was used for the C,, cage [lSc]. a) P. J. Hay,
W. R. Wadt. J . Chem. Phys. 198.5, 82. 299; b) J. S. BinkIey, J. A. Pople, W J.
Hehre, J . Am. Chem. Sac. 1980, 102, 939; c) M. J. S. Dewar, E. G . Zoebisch,
E. F. Healy. J. J. P. Stewart, ibid. 1985, 107, 3902.
[19] The a b initio calculations by Nakao et al. suggest that /h-C8, has the largest
bond energy among the seven C,, isomers. K. Nakao. N. Kurita. M. Fujita,
Phys. Rev. B 1994, 49. 11415.
[20] a) P. W. Fowler, Chem. P h w Leu. 1986, 131, 444. b) P. W. Fowler, D . E.
Manolopoulos. Nature 1992, 355. 428.
[21] Because we keep the Ih symmetry of the C,, cage during our calculations. this
is not due to the Jahn-Teller distortion.
I221 I n this case. the us orbital is raised from -4.4 to -0.1 eV.
Assembly and Crystal Structure of a Photoactive
Array of Five Porphyrins""
Sally Anderson,* Harry L. Anderson, Alan Bashall,
Mary McPartlin,* and Jeremy K. M. Sanders*
The investigation of conformationally homogeneous porphyrin assemblies is essential for the better understanding of
energy and electron transfer in natural photosynthetic reaction
centers.['] This area of research has been stimulated by the characterization of natural photosynthetic reaction centers by
Deisenhofer, Huber, and Michel,['] and by the theoretical work
of Marcus.131 Structures containing many porphyrin chromophores are of particular interest as models for light-harvesting antennae; only a few antennae models containing more than
three porphyrin units have been studied,[,] and all of these structures are rather flexible, which hampers understanding of their
photophysical properties. The structure of the five-porphyrin
complex presented in this communication is exceptionally well
defined; its conformation in solution, deduced by NMR, is essentially identical to that in the solid state, as determined by
single-crystal X-ray diffraction. It should be straightforward to
vary the structure of this complex systematically, because the
central porphyrin is inserted in the last step with strong but
reversible noncovalent binding interactions.
Corey-Pauling-Koltun (CPK) models of the cyclic zinc porphyrin tetramerI5I Zn,-1 indicate that it is rather flexible, and
NMR spectroscopy confirms that the molecule exists in several conformations, which interchange slowly on the chemical shift time scale.r5".6l
One possible conformer is shown schematically in Figure 1.
When
meso-tetra(4pyridy1)porphyrin H,Py,P is added to a solution of cyclic tetramer
Zn,-1, a strong 1 : I
4
complex forms, which
is in slow exchange
with excess of either
component on the
Zn4-l
'H NMR
chemical
shift time scale and runs on a silica column as a single band. The
H,-Py,P ligand locks the tetramer in a single symmetrical conformation: the signals of the ester methyl (Me,) and ring methyl
(Me,) groups each give rise to two singlets of equal intensity
separated by Ah = 0.28 and 0.05, respectively, which rules out
the D,, conformation shown schematically in Figure 1. Similar
splittings are also observed in the 13CNMR spectrum. Molecular mechanics calculations and CPK models indicate that the
[*I
['I
[**I
1096
Q VCH Verlugsgesellschuft mbH, 0-69451 Weinheim, 1995
Dr. S. Anderson.''] Dr. H. L. Anderson."' Dr. J. K. M. Sanders
Cambridge Centre for Molecular Recognition
University Chemical Laboratory
Lensfield Road, GB-Cambridge CB2 1EW (UK)
Telefax: Int. code + (1223)336913
Prof. M. McPartlin, A. Bashall
School of Chemistry, University of North London
Holloway Road, GB-London N7 8DB (UK)
Telefax: Int. code + (171)753-5402
Current address: Dyson Perrins Laboratory
South Parks Road. GB-Oxford OX1 3QY (UK)
Telefax: lnt. code + (18651275674
This work was supported by the Science and Engineering Research Council
(UK), RhGne Poulenc Rorer, Magdalene College Cambridge, and Trinity College Cambridge. We are grateful to Prof. F. Diederich (ETH Zurich, Switzerland) for generously providing facilities for fluorescence measurements.
i10.00+ .2510
0570-0833~9SjlOlO-I096!
Angew. Chem. Int. Ed. Engl. 1995, 34, No. 10
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electrochemistry, stud, initio, la2, c80, dimetallofullerenen
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