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Highly Reduced Porphyrins.

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of 1 and a dimer of 3 having an Sn-Sn double bond; since
such dimers of 3 are not knownr7]whereas a stannirene has
already been
we believe that the pathway via 6 is
more likely. Both pathways to the 1,2-digermacyclobutene
are possible in the case of the dialkylgermylenes.
Phosphadistannacyclobutenes, however, are probably formed by
12 + 21 cycloaddition.‘91
Experimental Procedure
4, 5: Compound 1 (0.78 g. 4.68 mmol) was added dropwise under argon to 2
( I .28 g, 4.68 mmol) or 3 (1.5 g, 4.68 mmol) in 5 mL of dry benzene at
10 ’C
over 5 min. resulting in the formation of yellow crystals. Aftcr purification (4.
recrystallization from benzene; 5, sublimation at 40-50”C/10-3 torr), 1.43 g
of 4 (86%) or 1.05 g of 5 (56%) was obtained.
+
Received: October 21. 1988 [Z 3020 IE]
German version: Angew. Chem. 101 (1989) 640
CAS Registry numbers: 1,26825-18-3; 2, 84806-15-5; 3,54724-62-8; 4,11978785-8: 5. 119793-90-7.
Fig. 1. Molecular structures o f 4 (top) and 5 (bottom). Important bond lengths
in the dimetallacyclobutenes: 4: Gel-Ge2 2.459 (1). G e l C22 2.030 (7). Ge2-C21 2.028 (6), C21-C22 = 1.34 (1); C22-Gel-Ge2 73.3 (l),
Ge2-C21-C22 105.0(1).5:Sn-Sn’2.803 (1). Sn-C11 2.27(I),Cll-C11’1.31 (2);
C11-Sn-Sn’70.7 (2), Sn-Cl1-CII‘ 109.3 (3) (51.
[A] and angles f“]
away from orthogonal positions with respect to the slightly
puckered digermacyclobutene. This twisting is due to the
sterically demanding tert-butyl groups on the nitrogen atoms
of the two peripheral rings. Compound 5 is not twisted because the Sn-Sn distance is 0.344 A longer, thereby reducing
this steric interaction.
Whereas the bond lengths and angles of the central ring of
4 are in the expected range, the C=C bond in 5 is quite short
(1.31 8, versus the expected value of 1.34 A) and the Sn-C
bonds are very long (2.27A versus the expected value of
2.1 7 A). [61 For comparison, in the stannirene synthesized by
Sita et al.,[31 the bond lengths are d(C=C) = 1.34A and
d(Sn-C) = 2.14 A. The Sn-C bond is thus very weak, which
would also explain the dissociation to the starting compounds observed in solution.
Compound 5 could have formed either via the stannirene
6 and subsequent insertion of 3 o r by a [2 + 21 cycloaddition
[I] A. Krebs. J. Berndt. Tetruhedron Len. 24 (1983) 4083; M. P. Egorov. S. P.
Kolesnikov, Yu. T. Struchkov, M. Yu. Antipin, S. V. Sereda, 0. M. Nefedov, J. Orgonomet. Chem. 290 (1985) C 27.
[2] A. Krebs, J. Wilke. Top. Curr. Chem. 109 (1983) 189.
[3] L. R. Sita. R . D. Bickerstaff. J; A m Chem. Sot. 110 (1988) 5208.
[4] M. Veith. M. Grosser, Z . Nuturforsch. B37 (1982) 1375: M. Veith. Angex..
Chem. 87 (1975) 287; Angew. Chem. I n f . Ed. Engl. 14 (1975) 263.
[5] 4: C,,H,,N,SSi,Ge,,
space group P2,!n, u = 10.761 (9).h = 31.85 (3).
c=12.54 (1)A. /1=114.61 (7),’. V = 3 9 0 8 A 3 , Z = 4 ,
=
1.197 gcm-3. p(MoK.) = 16.25 c m - ’ . 4634 unique reflections, 1047classified as not observed ( F s 3 a(Fl). Reflectioniparameter ratio = 15.
R = 0.057.-5: C,,H,,N,SSi,Sn,,
space group C2, u = 17.135 (Y), h =
10.223 (5). c = 14.233 (7) A. [f = 125.63 (4)‘, V = 2026 A3, Z = 2,
= 1.305 gcm-’, p(Mo,, = 12.52cm-’. 1410 unique reflections. 14
classified as not observed ( F 5 3 cr(F)). Reflectioniparameter ratio = 8.2.
R = 0.044.-Further details of the crystal structure investigations may be
obtained from the Fachinformationszentrum Energre, Physik. Mathematik
GmbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-53 454. the names of the authors. and the journal citation.
[6] M. Veith, 0. Recktenwald. Top. Curr. Chem. 104 (1982) 1 .
171 M. Veith, Angew. Chem. 99 (1987) I ; Angew. Chem. Int. Ed. Engl. 26 (1987)
1.
[XI 0.M. Nefedov, M. P. Egorov. A. M. Gal’minas, S. P. Kolesnikov,
A. Krebs. J. Berndt, 1 Orgunomel- Chem. 301 (1986) C 21
[9]A. H.Cowley, S . W. Hail, C. M. Nunn, J. M. Power, Angert. Chem. 100
(1988) X74: Angex. Chem. Znt. Ecf. Engl. 27 (1988) 838.
Highly Reduced Porphyrins **
By Robert Cosmo, Christian Kautz, Klaus Meerholz,
Jiirgen Heinze, * und Klaus Miillen *
Can an organic a system be used to store charge by successive addition of electrons and is the storage capacity increased on going to higher analogues containing several redox centers? We have examined this question for the first
time for porphyrins“] and have found that the zinc complex
1 of meso-tetratolylporphyrin 2 can reversibly accept six electrons. The NMR spectra of the intermediate di- and tetra-
tEu
I
1’1
I
tEu
[**I
6
604
0 VCH
firlu~sgesellsrliu~l
mhH. 0-6940 Weinheim, 1989
Prof. Dr. K . Miillen, Dr. R. Cosmo, Dipl.-Chem. C . Kautz
Institut fur Organische Chemie der Universitit
J.-J.-Becher-Weg 18- 22, D-6500 Mainz 1 (FRG)
Prof. Dr. J. Heinze, DipLChem. K. Meerholz
Institut fur Physikalische Chemie der Universitdt
Albert-Strasse 21, D-7800 Freiburg (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. R.C. thanks the Alexander von
Humholdt Foundation for a fellowship.
0 5 711-0833/~~9:0505-0604
$02.S0/0
Angctr. Chem. 6 1 1 . Ed. Engl. 28 (1989) No.
5
anions demonstrate their annulene-like bonding. We "doubled" the zinc porphyrin complex 1 to form the stilbenebridged diporphyrinyl system 3 and examined its redox
properties in analogy to those of 1. Complex 3 forms a dodecaanion !
the corresponding dianion (tetraanion) has a paratropic
(diatropic) 20 n (22 n) perimeter. We were able to carry out
corresponding redox experiments involving dianion and tetraanion formation with an [18]annulene. ['I The paratropism of a doubly charged porphyrin perimeter is especially revealed in the resonance of the meso proton (6, = - 3.9)
in the dianion (lithium salt) of the octaethyl zinc porphyrin
4.I81
b
CH3
1
'
M=Zn. R=CH,
2 ' M=ZH. R-CH,
5 ' M-2H. R-Br
6 M=2H. R=CHO
'
7 . M-Zn. R-CHO
78°C with active
Reduction of 1 31 in [DJTHF at
lithium or potassium results in several color changes ;however, the characterization of a diamagnetic primary product
(green-red) or a secondary product (black) by high-resolution NMR spectroscopy was only possible when the reaction
was followed spectroscopically and the concentration of the
intermediate radical anions was minimized. The number and
the charge-induced shifts of the 'H and '3C NMR signals of
the diamagnetic species (Table I), as well as trapping experiments with dimethyl sulfate141and mass-spectrometric characterization of the adducts, show that a dianion and a tetraanion with intact porphyrin frameworks are formed.
The 13CNMR spectrum of 14"/4 Lie provides information on the charge distribution. Whereas all carbon centers
of the porphyrin ring experience an appreciable shielding
upon tetraanion formation (see Tdbk I), the centers of the
phenyl rings are deshielded. This finding indicates that, owing to twisting, the phenyl groups are not significantly involved in the charge delocalization. l9]
The cyclovoltammetric measurement of the reduction of 1
in absolutely dry dimethylamine[' with tetrabutylammonium bromide as conducting electrolyte at temperatures below
- 20 "C (Fig. 1 and Table 2) confirms that a stable tetraanion is formed. Furthermore, it shows that 1 can be reversibly
reduced all the way to the hexaanion. The reduction of 1 to
the dianion occurs in one-electron steps having separated
potentials of E o ( l / l o ) = - I .44 and E o (l/l'@) = - 1.98 V,
whereas both the tetraanion and the hexaanion are formed in
formal "two-electron'' reactions at E o (120/140) = - 2.78
Table 1. 'H and I3C NMR chemical shifts ([D,THF]) of the porphyrins 1 and
4 and of the corresponding amines.
8"
1
H-2.3
H-T.6
CH 3
8.95
8.10
7.55
2.70
H-5 (rneso-H)
-
H-3',5'
12e/2Ke
- 0.9
4.95
6.05
1.50
-
C-5 (rneso-C)
C-l,4
C - 2.3
CHs
121.1
150.4
131.9
21.5
140.0
134.4
127.2
137.0
C - 7.6
c-3',5'
C-4
6 70
7.85
6.59
2.14
-
CH,
c-I'
148/4Li0 4
-
42"/2 Lie
-
-
-
-
-
1.94
10.13
4.17
92.8
121.8
110.0 [a]
23.4
148.8
127.4 [a]
130.2 [a]
142.4
- 0.60
- 3.9
-
1.53
+ 28.3
+ 28.6
+ 21.9
1.9
- 8.8
- 7.0
- 3.0
- 5.4
1
-10
T
-15
-20
-25
-30
-35
E i v s Ag/AgCIIIVl
-
Fig. 1. Cyclic voltammogram of a) 1, b) 3; each
NBu,Br.
M in Me,NH/O.i M
[a] Signal assignment by selective decoupling
The salient features of the 'H NMR spectra are the resonances of the p protons of the pyrrole units at high field
(6, = - 0.9) in the dianion and at low field (6, = 6.70)-despite charge-induced shielding-in the tetraanion. If the
bonding in the neutral compound 1 is related to that in a
diatropic diaza[l8]annulene, [5*61 it follows necessarily that
A n p w . Chum. Int. Ed. EngI. 28 (1989) No. 5
and E o (I4@/l6@)= - 3.20 V. Simulations revealed that the
normal-potential separation between the 1 2e/13e(140/150)
and the 13@/140(150/160)redox pair is only 40 (50) mV.
Independent of the NMR data, cyclovoltammetric measurements on the Zn-free porphyrin system 2 showed that
only the charge state of the porphyrin framework and not
that of the Zn2@ion is changed upon reduction of 1. Here,
too, six redox steps are observed (Table 2). However, the
Q VCH Verlagsgrsell.whaft mhH. 0-6940 Weinhrrm. 1989
0S70-0833IH9iOSOS-060SB 02.5010
605
Table 2. Redox potentials E" [V] of the compounds 1-4 [a]
1
R/Re
Re/Rle
R2Q/R3e
R3Q/R4e
1
-
3
2
1.44
1.98
- 2.78 (44) [b]
1.19
-
- 1.54
::;;1
- 1.42 (10)
4
[b]
1;$
1
[a] All potentials were determined by cyclic voltammetry with
M solutions
of R in Me,NH/O.l M NBu,Br with respect to the Ag/AgCI electrode (calibration with Cp,Co@/Cp,Co); T = - 65 "C.[b] "Two-electron" redox steps (oneelectron potential separation [mV] in parentheses). [c] T = - 85°C.
anions of 2 are appreciably less stable than those of I, so
that, even for moderately fast scan rates, secondary reactions
can occur, despite the low temperatures employed.
Previous cyclovoltammetric investigations on the formation of highly charged porphyrins had always shown" l . "1
that protonated secondary products are formed beyond the
trianion stage.['21 This is also true for the octaethyl zinc
porphyrin 4, which we were able, for the first time, to reduce
reversibly to the tetraanion. The extreme potential values of
- 3.08 and - 3.25 V, respectively, for the 420/430and 43Q/
440 redox pairs confirm, at the same time, that these stages
are only accessible with difficulty by chemical reduction.
The connection of two porphyrin moieties through a stilbene group enticed us because stilbene is also a good electron
acceptor ( E o = - 2.26 vs. Ag/AgCI) and because twisting
around the formal single bond would allow a "decoupling"
of the two moieties as well as a minimization of the Coulombic repulsion in the anions. The synthesis of the diporphyrinyl system 4,4-bis[5-(10,15,20-tris(p-tolyl)porphyriny1)lstilbene (as its zinc complex 3) starts from the monobromo compound 5, ['I which was metalated with butyllithium
(9.2 equiv., 0 "C, diethyl ether, 2 h) and then formylated by
reaction with dimethylformamide (6.8 equiv., 0 "C, 30 min)
and hydrolysis with 2.5 M HCl to give the aldehyde 6[13'
(24% yield after chromatography on silica and recrystallization from CHCl,/CH,OH l / l ) . Compound 6 was converted
into the zinc complex 7 (98 YOafter recrystallization from
CHCI,/CH,OH) by reaction with zinc acetate dihydrater3]
and then subjected to McMurry coupling['41 (TiCI,/Zn,
THF, 0 "C). Chromatography on silica and recrystallization
(CHCI,/CH,OH) afforded the wine-red diporphyrinyl derivative 3 in 32% yield; 3 was sufficiently soluble in chloroform (1 mg in 0.14 mL CHC13).['51
'3
3 : M-Zn
606
8 VCH
VerlagsxeseflschaflmhH, 0-6940 Weinheim, 1989
The cyclovoltammetric experiment in dimethylamine now
shows (see Fig. 1 and Table 2) that 3 is reduced to the tetraanion in two formal "two-electron'' redox steps. Two potentially separated, reversible one-electron transfers are observed for the next redox reactions; the first already occurs
at - 2.32 V owing to the enlarged conjugation of the entire
system-the stilbene bridge has to be coplanar with the two
porphyrin frameworks in this charge state. The shift of the
next electron transfer by 350 mV [ E o(350/360)= - 2.67 V]
is explained by the fact that the accumulation of charge to
form the second trianionic electrophore is energetically disfavored by the Coulombic interaction through the coplanar
stilbene bridge. After this critical charge state is attained, the
sixfold charged dimer reversibly accepts another six (!) electrons to form the dodecaanion: however, the last stage already lies in the region of current rise due to electrolyte
reduction. Since the electron transfers again occur in formal
"two-electron'' redox steps, the further accumulation of
charge must take place locally in the two porphyrin units,
without larger Coulombic effects acting through the stilbene
bridge.
Thus, a tetraaryl porphyrin is a highly efficient electron
acceptor. A doubling of the storage capacity per molecule
can be achieved by suitable linking of the two porphyrin
units. Current work is aimed at the development of
analogous charge storage systems containing more than two
porphyrin redox centers.
Received: August 4, 1988;
revised: January 12, 1989 [Z 2908 IE]
German version: Angew. Chem. 101 (1989) 638
CAS Registry numbers:
1, 19414-67-6; IZe/2Ke, 119770-75-1; I4O/4Lie, 119770-76-2; lbe, 11978746-1: 2,14521-51-6; 3,119787-45-0; 4,17632-18-7; 42Q/2Lio, 119770-77-3;44e,
119770-79-5; 5, 119770-80-8;6, 119770-81-9;7, 119770-78-4.
[I] For comparable investigations on phthalocyanine complexes, see
R. Taube, Z . Chem. 6 (1966) 8 .
[2] A. D. Adler, F. R. Longo. I. D. Finarelli, J. Goldmdcher. J. Assour,
L. Korsakoff, J. Org. Chem. 32 (1967) 476.
131 A. D. Adler. F. R. Longo. F. Kampas. J. Kim, J. Inorg. Nucl. Chem. 32
( I 970) 2443.
[4] A. Botulinski, J. W. Buchler. N. E. Abbes, W. R. Scheidt, Liebigs Ann.
Chem. 1987,305; A. Botulinski, J. W. Buchler, K.-L. Lay, H. Stoppa, 2nd.
1984, 1259.
[5] M. Gouterman 111 D. Dolphin (Ed.). The Porphyrins, Val. 3, Academic
Press. New York 1978, p. 1.
(61 E. Vogel. W Haas. B. Knipp, J. Lex, H. Schmickler, ilngew. Chem. 100
(1988) 445; Angew. Chem. In[.Ed. Engl. 27(1988) 406, and references cited
therein.
[7] K. Miillen, W Huber, M. Nakagawa, M. Iyoda, J. Am. Chem. SOC.104
(1982) 5403.
[S] G. N. Sinyakov, A. M. Shul'ga, G. P. Gurinovich, Zh. Prikl. Spekrrosk.
28 ( 3 ) (1978) 504; C/7em.Absrr. 89 (1978) 3375211.
191 From the charge-induced shift of all I3C NMR signals of the porphyrin
ring (without tolyl substituents) in I4O/4 Lie and the proportionality constant K = 160 ppm per unit charge, one obtains an estimate of 3.2 e0 for
the excess charge in the porphyrin framework. If the same charge density
is assumed for the four nitrogen centers as for the ring carbon atoms, one
obtains an estimate of 3.9 ee for the excess charge.
[lo] K . Meerholz, J. Heinze. J. Am. Chemi. Sac. 111 (1989) 2325. The purification is accomplished by condensing gaseous dimethylamine (99 % purity)
011 actlvdted alumina. After heating at reflux for 1 h. the dimethylamine is
condensed at reduced pressure into an electrochemical cell suitable for
low-temperature measurement; see also R. P. VanDuyne, C. N. Reilley,
Anal. Chem. 44 (1972) 142.
Ill] D. W. Clark, N. S. Hush, J. Am. Chem. Soc. 87 (1965) 4238.
I121 G . Peychal-Heiling, G. S. Wilson. Anal. Chem. 43 (1971) 545, 551
0570-0833]89~0505-0606$02.50/0
Angen. Chem. In[. Ed. Engl. 28 (1989) No. 5
[13] H. Ericsson, 0. Wennerstrom. unpublished. We thank Prof. 0. Wmner!irijm for providing us with the results.
[I41 1. E. McMurry. Arc. Chem. Res. 16 (1983) 405.
=,
[lSl Spectroscopic data for 3: m.p. > 300‘C; UVjVIS (CH,CL,): i,,
228 nm (lgi: = 5.96). 304 (5.23), 402 sh (5.46). 424 (6.36), 550 (5.23). 591
(4.88); ‘ H NMR (400 MHz, CDCI,): 6 = 9.06, 9.05 (AB,, JA,” = 4.5 Hz,
XH.2xH-2”,3”,7”,8”),8.96(s,8H,ZxH-12”,13”,
17”.18”),8.29(d.4H,
J = 7.9 Hz, 2 x H-3.3, 8.1 (m, 12H. 2 x (4 H-b, 2 H-d)), 8.05 (d, 4H.
J = 7 9 Hz, 2 x H-2.6). 7.76 (s, 2H. 2 x H-a), 1.56 (m, 12H, 2 x (4 H-c.
2 x H-e)), 2.71 (s, 18H. 2 x (3 aryl-CH,)); MS (zinc isotope pattern): mi-.
( M e t 1469.41 (20.9%). 1468.5 (45.2). 1467.44 (58.3), 1466.47 (75.7)
1465.5 (79.1), 1464.44 (loo), 1463.41 (64.4). 1462.44 (83.3,1461.44 (51.3).
734.2 (65.8). 732.2 (100).
1460.41 (33.0); mi; ( M Z e J
variant in the mechanism of the metal-ion-induced activation of C-H and C-C bonds and, further, provide insight
into several fundamental aspects of elementary reactions of
organometallic chemistry.
Electron-impact ionization (100 eV) of a mixture of
Fe(CO), and C,H,NH, (1 : 5) in a mass spectrometer affords
a complex of composition C,H,NFee, which, in a tandem
gives the prodmass spectrometry (MSMS) e~periment,’~]
ucts shown in Equation (a). The identity of the neutral
molecules, based on the mass differences (Am), is firmly established for energetic reasons.[’I Their origin can be determined by investigation of the Fe@complexes of the deuterated propylamines 6a-d (Table 1).
Revision and Modification of the Traditional
Mechanism of C-H/C-C Activation by “Naked”
Transition-Metal Ions **
By Sigurd KarraJ, Karsten Eller, Christian Schulze,
and Helmut Schwarz *
Many reactions of “naked” (i.e., ligand- and counterionfree) transition-metal ions M@with substituted alkanes 1 in
the gas phase can be satisfactorily explained in terms of a
mechanism formulated for the first time by Allison and
Ridge”] (Scheme 1). Oxidative addition of the C-X bond to
M @(1 .+ 2) is followed by a a-hydrogen transfer; the resulting olefin complex 3 undergoes competitive loss of HX (reductive elimination) and RCH = CH,.
1
= OH.
“27’”I
HC’H
I J
I
/
4
lr
5
Scheme 2.
We report here on experiments that reveal a significant
[‘I
[**I
[YO]
H2
NH,
C,H,
C3H6
C,H,NH,/FeQ
6/Fe0
22
4
55
19
(a)
Table 1. Neutral isotopomers, generated from the Fee complexes of 6a-6d
[a1.
CH,CH,CH,ND, 6 a CH,CH,CD,NH, 6 b
CH,CD,CH,NH, 6c CD,CH,CH,NH, 6d
Neutral
isotopomers
6a/Fee
6b/Fee
6c/Fee
6d/Fee
halogen, NH,
For longer-chain CN-substituted n-alkanes (cf. 4,
X = CN). we recently found a new type of reaction!’] In this
case, the dominant process is due to an interaction of remote
C-H bonds with the transition-metal ion Me “anchored” to
the functional group (“remote functionalization”[31). Depending on Me(e.g., Fee, Co@,Ni@),different segments of
the alkyl chain are activated in an oxidative addition
(Scheme 2).
€
Neutral
molecule
3
2
Scheme 1 . X
Am
Prof. Dr. H. Schwarz, DipLChem. S. KarraO, DipLChem. K . Eller.
Dip!.-Chem. C. Schulze
lnstitut fur Organische Chemie der Technischen Universiti!
StraBe des 17. Juni 135, D-1000 Berlin 12
This work was supported by the Fonds der Chemischen Industrie, the
Stiftung Volkswagenwerk, and the Gesellschaf! von Freunden der Technischen Universitat Berlin.
AngeTc”‘.t h e m . In!. Ed. Engl. 28 (1989) N o . 5
f;
[a] The product distribution is normalized with respect to the corresponding
neutral compound (Zi,.,o,omei= 100%).
It was found that molecular hydrogen originates exclusively from the P and y positions of the alkyl chain (1,2-elimination); hydrogen shifts do not occur. Scheme 2 (X = NH,;
pathway a) provides a satisfactory description of the reaction mechanism. Oxidative addition of an N-H bond, followed by P-H transfer and reductive elimination of H, to
give an Fe@-complexedimine, plays no role.f61
The elimination of ethylene from the complex C,H,NH,/
Fee is not explainable in terms of Scheme 1, since only
propene should be formed from propylamine. The reaction
mechanism shown in Scheme 2 is also wrong, since ethylene
originates specifically from the a-and P-methylene groups of
the amine (Table 1). The a- and P-methylene groups are also
involved in the formation of NH, and C,H,-in
fact, to the
same extent (+ 2%). According to Scheme 1, NH, should
contain a hydrogen atom only from the 0 position of the
alkyl chain, and this hydrogen atom should be absent in the
propene formed (R = CH,). The data in Table 1 show that,
before the elimination of ammonia or propene, the a- and
P-methylene groups are equilibrated. Hydrogen shifts (e.g.,
via dyotropic rearrangement[”) are ruled out here (no loss of
a hydrogen atom from the a-CH, group). Instead a (formal)
interchange of the two intact methylene groups takes place,
possibly via the olefin complex 8 (Scheme 3 ) . Complex 8
V C H VerlugsgesellschafrmbH, 0-6940 Weinheim,1989
0570-0833i89jOSOS-0507dO2.SOjO
607
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