The elusive terminal imido of manganese(v)

Article abstract of DOI:10.1002/1521-3773(20021004)41:19<3591::aid-anie3591>3.0.co;2-z

At last, terminal imido complexes of manganese(V), the postulated intermediates in aziridination and amination reactions, have been prepared. The structurally well-defined (imido)manganese(V) corroles, 2 and 3, are easily synthesized from manganese(III) c

Full text of DOI:10.1002/1521-3773(20021004)41:19<3591::aid-anie3591>3.0.co;2-z

COMMUNICATIONS
Angew. Chem. Int. Ed. 2002, 41, No. 19
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/02/4119-3591 $ 20.00+.50/0
3591

COMMUNICATIONS
The Elusive Terminal Imido of Manganese(v)**
Rebecca A. Eikey, Saeed I. Khan, and
Mahdi M. Abu-Omar*
The discovery that a (nitrido)manganese(v)porphyrin [1] can
be activated for the aziridination of cyclooctene with tri-
fluoroacetic anhydride (tfaa)has stimulated the development
of several catalytic[2 4] and stoichiometric methods[5] for metal-
mediated nitrene [NR] group transfer. The catalytic tosyla-
midation of alkanes by PhI ¼ NTs in the presence of
[6]
manganese(iii)pophyrins has also been reported. A major
goal of research on the use of catalytic manganese(iii)and
stoichiometric high-valent manganese(v)species is the iden-
tification of reaction intermediates. (Imido)manganese(v)
compounds have been proposed as the active species respon-
sible for the delivery of the nitrene group to double bonds,[1,7]
similar to what has been postulated for the related metal-
mediated oxygen-atom-transfer reaction.[8] Thus, it is not
surprising that while many structures of (nitrido)mangane-
se(v)complexes are known, [9] neither structures nor definitive
spectroscopic characterizations of (imido)manganese(v)com-
plexes are available.[7,10] Herein, we describe the preparation,
molecular structure, and preliminary reactivity studies of
novel (imido)manganese(v)corrole complexes. To our knowl-
edge, this report constitutes the first structure of a terminal
imido complex of manganese(v).[7,11]
Corroles, one-carbon atom short analogues of porphyrins,
have been shown to stabilize high oxidation states of various
transition metals.[12] The ambiguity of assigning oxidation
states that is characteristic of octaalkylcorroles[13] has been
eradicated by the introduction of the electron-poor 5,10,15-
tris(pentafluorophenyl)corrole (H3(tpfc), tpfc ¼ the trianion)
by Gross and co-workers.[14] Recently, Gross and Gray and
their co-workers extended the chemistry of manganese
corroles beyond the trivalent state by preparing (halo)man-
ganese(iv),[12c] (oxo)manganese(v),[12b] and (nitrido)mangane-
se(v)[12c] corroles. The manganese(iv)corroles were structur-
ally characterized, but the manganese(v)corroles were
identified spectroscopically.
Scheme 1. Synthesis of 2, 3, and 4 from [MnIII(tpfc)].
manganese(v)imido complex 2 under photolytic as well as
under thermal conditions. However, the reaction of 1 with
2,4,6-trichlorophenyl azide affords the desired product 3 only
under photolysis. Interestingly, when trimethylsilyl azide
(tmsa)was employed as the nitrene [NR] source, the
(nitrido)manganese(v)corrole 4, was isolated upon the
addition of a salt, [nBu4N][PF6]. The nitrido complex has
been prepared previously by the action of NaN3 followed by
photolysis.[12c] Complexes 2 and 3 are easily purified by
chromatography to afford red microcrystalline solids in
excellent and consistent overall yields of 80 70%.
The (imido)manganese complex 2 is sensitive to water and
air (the latter is most likely to be as a result of the moisture in
air since compound 2 persists for days under ambient
atmosphere in dry southern California weather). In solution
and in the solid state, 2 is stable for months under dry inert
atmosphere. On the other hand, solutions of 3 revert to green
MnIII species in a couple of days even under inert atmosphere.
A complex mixture of organic products is formed alongside
the identifiable azo compound.
Following the literature precedent of using NaN3 and
irradiation in the preparation of high-valent nitrido complex-
es,[9c] we have prepared (imido)manganese(v)corroles from
the one-pot reaction of organic azides[15 17] with (tpfc)manga-
nese(iii)complex 1 under photolysis (Scheme 1). The reaction
of 1 with 2,4,6-trimethylphenyl azide yields the corresponding
1
The H and 19F NMR spectra of both complexes 2 and 3
exhibit sharp resonance signals in the normal range for
chemical shifts, consistent with diamagnetic low-spin d2
manganese (Figure 1)centers. The phenyl proton resonances
of the imido ligands are shifted upfield (d ¼ 5.67 and 6.31 ppm
for 2 and 3, respectively)as a result of the ring current of the
corrole macrocycle. Noteworthy is that the green intermedi-
[*] Prof. Dr. M. M. Abu-Omar, R. A. Eikey, Dr. S. I. Khan
Department of Chemistry and Biochemistry
University of California
Los Angeles, CA 90095-1569
607 Charles E. Young Drive, East (USA)
Fax : (þ 1)310-206-4038
E-mail: mao@chem.ucla.edu
¼
[**] This research was supported by the US National Science Foundation
and the Arnold and Mabel Beckman Foundation (BYI to MMA).
Funds for the purchase of a diffractometer were made possible by an
NSF equipment grant (CHE-9871332). We are grateful to Dr. G. Erich
Wohlhieter for his help with some of the NMR experiments.
ate [(TMP)Mn( NCOCF3)(OCOCF3) ] (TMP¼ (5,10,15,20-
tetramethylporphyrinato)dianion) proposed by Groves and
Takahashi to be responsible for [NR] transfer is paramagnet-
ic.[1,18] This high-spin configuration of the MnV center could be
a result of the weaker p-donation ability of the acylimido
ligand or to the porphyrin ligand. Recent DFT calculations on
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
3592
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4119-3592 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 19

COMMUNICATIONS
Figure 1. 1H NMR spectra showing the b-pyrrole region of complexes a) 2
and b) 3 in CD3CN.
Figure 3. ORTEP diagram (H atoms are omitted for clarity)of
2
(oxo)manganese(v)corrole and porphyrin complexes showed
that diamagnetism is expected for corrole and paramagnetism
for porphyrin complexes.[19] However, a diamagnetic (oxo)-
MnV(hydroxo)porphyrin intermediate has been recently
characterized by 1H NMR spectroscopy.[10] The identification
of 2 and 3 as the imido complexes was confirmed by mass
spectrometry. The electronic spectra of these complexes
exhibit a single Soret band (410 nm)and a Q band at
528 nm. Figure 2 contrasts the UV/Vis spectrum of 2 with
that of 1 and 4. The most relevant changes are the loss of the
split Soret band and the band at 470 nm upon formation of
manganese(v).
(ellipsoids at 50% probability level). Selected bond lengths [ä] and
angles [8]: Mn1-N5 1.613(4), Mn1-Npyrrole 1.891(4)-1.947(4), N5-C38
1.374(6); C38-N5-Mn1 170.4(4), N5-Mn1-N pyrrole 103.30(19)-106.37(19),
and N5-C38-C43 118.9(5).
bonds in [MnVII(NtBu)3Cl] (the average Mn NR bond is
ꢁ
1.66 ä).[23] Hence, the Mn NR bond length and the Mn-N-CAr
ꢁ
ꢀ
bond angle in 2 are well within the range for a Mn NR triple
bond formulation. The average Mn N bond lengths for the
pyrrole nitrogen atoms in 2 (1.916 ä)is the same as those in
[MnIII(tpfc)(OPPh3)] (1.916 ä). [24] This situation is in contrast
ꢁ
ꢁ
to the lengthening of the Mn Npyrrole distances observed in
[MnIV(tpfc)Br] (1.925 ä) and [MnIV(tpfc)Cl] (1.932 ä)[12c] and
ꢁ
to the lengthening of the Cr Npyrrole distances found in
[(tpfc)CrV(O)] (1.927 1.943 ä). [25] The cause of this length-
ꢁ
ꢁ
ening in the Mn Npyrrole and Cr Npyrrole bonds was attributed
to the ™pronounced doming of the corrole framework∫ in the
manganese(iv)and chromium( v)structures. In our structure,
the pyrrole nitrogen atoms are essentially in the same plane as
the 19-membered carbon macrocycle. In both
2 and
[(tpfc)CrV(O)], the metals are displaced out of the plane
defined by the pyrrole nitrogen atoms. However, the displace-
ment is larger in [(tpfc)CrV(O)] (0.6 ä) resulting in a strong
interaction of the bonding orbitals of the pyrrole nitrogen
atoms with the p orbitals of the CrO moiety, which results in
Figure 2. Electronic spectra of a)[Mn III(tpfc)] (1; 1.5 î 10ꢁ5 m; ––),
V
ꢁ
b)[Mn V(tpfc)(N-mesityl)] (2; 1.6 î 10ꢁ5 m;
(4; 1.0 î 10ꢁ5 m; - - - -)in acetonitrile.
), and c) [Mn (tpfc)(N)]
* * * *
ꢀ
lengthening of the Cr O bond (1.57 ä)and doming of the
ligand.[25]
The molecular structure of 2 was confirmed by single-
crystal X-ray analysis (Figure 3).[20] The highlight features of
the structure are 1)a five-coordinate manganese( v)center
that is located 0.513 ä out of the plane defined by pyrrole
Preliminary investigations of the reactivity of (imido)man-
ganese(v)corroles suggested that olefins are not reactive
enough to accept an [NR] group from 2 or 3. This finding is
not surprising, given that MnIII salen complexes, which are
excellent epoxidation catalysts,[26] did not fair well in aziridi-
nation reactions.[2b,c] Interestingly, the analogous compound
[(tpfc)MnV(O)] was shown to be unreactive towards oxygen-
atom transfer to alkenes, which suggests that a higher
oxidation state of manganese is responsible for catalysis.[12b]
However, in the presence of pyridine (py), the terminal
mesitylimido group of 2 transfers to organic phosphanes
yielding the iminophosphane adduct of manganese(iii)cor-
role [Eq. (1)]. The UV/Vis spectrum of 5 (lmax in nm ¼ 401
ꢀ
nitrogen atoms, 2)a Mn NR bond length of 1.613(4)ä, and
ꢀ
3)a Mn-N-C Ar bond angle of 170.4(4)8. The Mn NR distance
is slightly longer than what is observed for (nitrido)manga-
ꢀ
nese(v)complexes (the average Mn N bond for porphyrin
and salen complexes is 1.52 ä)[9] and for (oxo)manganese(v)
[21]
ꢀ
ꢁ
complexes (Mn O bond of ~ 1.56 ä). The Mn NR bond in
ꢁ
our compound, however, is significantly shorter than the Mn O
bond observed for a terminal oxo complex of low-valent
manganese(iii)(1.80 ä)species, [22] and shorter than the imido
Angew. Chem. Int. Ed. 2002, 41, No. 19
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4119-3593 $ 20.00+.50/0
3593

COMMUNICATIONS
2: Method A. A solution of mesityl azide (36 mg, 230 mmol)and 1 (18 mg,
21 mmol)in toluene (10 mL)was refluxed under argon for 30 min or until
the disappearance of [MnIII(tpfc)] as monitored by TLC (12:1 mixture of
pentane:diethyl ether). Evaporation of the solvent under reduced pressure
followed by column chromatography (silica gel, 60 ä, 12:1 mixture of
pentane:diethyl ether)resulted in isolation of 2. Yield: 16 mg (75%). MS
(negative ion FAB): m/z (%)982 (100)[ Mꢁ], 848 (33)[ MꢁNmesꢁ]; UV/
1
Vis (CH3CN): l [nm] (log e) ¼ 354 (4.53), 410 (4.53), 528 (4.14); H NMR
(400 MHz, CD3CN, 258C): d ¼ 9.39 (d, J ¼ 4.5 Hz, 2H), 9.03 (d, J ¼ 4.1 Hz,
2H), 8.93 (d, J ¼ 4.7 Hz, 2H), 8.82 (d, J ¼ 4.9 Hz, 2H), 5.67 (s, 2H),
1.53 ppm (s, 3H), ꢁ0.61 (s, 6H); 19F NMR (400 MHz, CD3CN, 258C): d ¼
ꢁ140.82 (d, J ¼ 20.0 Hz, 2F), ꢁ141.00 (d, J ¼ 20.0 Hz, 2F), ꢁ141.27 (d, J ¼
24.0 Hz, 1F), ꢁ141.41 (d, J ¼ 24.0 Hz, 1F), ꢁ156.79 to ꢁ156.93 (m, J ¼
16.0 Hz, 3F), ꢁ164.55 (m, J ¼ 16.0 Hz, 3F), ꢁ164.79 ppm (m, J ¼ 20.0 Hz,
3F).
and 421 (split Soret), 493, 607 (Q band)) is similar to that of
[(tpfc)MnIII(OPPh3)].[12b] (The spectra of 5 and 6 are dis-
played in the Supporting Information.)Even though the
reactions of 2 with phosphanes are sluggish, complex 3 with
the electron-withdrawing trichlorophenyl substituent is much
more reactive and is more sensitive to the nucleophlicity of
the R3P group [Eq. (2)]. This finding is in agreement with
electrophilic [NR] group transfer from the manganese(v)
Method B. A solution of mesityl azide (64 mg, 397 mmol)and 1 (28 mg,
33 mmol)in acetonitrile (5 mL)was photolyzed under argon for 4 5 days.
Evaporation of the solvent under reduced pressure and purification of the
residue as in Method A resulted in isolation of 2. Yield: 12 mg (37%).
center. While [NR] transfer from activated (nitrido)manga-
3: A solution of 2,4,6-trichlorophenyl azide (36 mg, 162 mmol)and
(10 mg, 12 mmol)in acetonitrile (2.5 mL)was photolyzed under argon for
1
[5]
nese(v)salen complexes
and from complex 2 require
pyridine as an additive, reactions of complex 3 [Eq. (2)] do
not require pyridine, demonstrating the importance of the
R substituent on the imido ligand.
4 5 days. Evaporation of the solvent under reduced pressure and purifi-
cation of the residue as in Method A resulted in isolation of 3. Yield: 11 mg
(81%). MS (negative ion FAB): m/z (%)1043 (9)[ Mꢁ], 864 (100)
[MNꢁArꢁ], 848 (11)[ MꢁNArꢁ]; UV/Vis (Toluene): l [nm] (log e) ¼ 396
1
(4.22), 488 (3.63), 540 (3.75); H NMR (400 MHz, CD3CN, 258C): d ¼ 9.31
(d, J ¼ 4.5 Hz, 2H), 8.99 (d, J ¼ 4.3 Hz, 2H), 8.83 (d, J ¼ 4.9 Hz, 2H), 8.77
(d, J ¼ 4.9 Hz, 2H), 6.31 ppm (s, 2H); 19F NMR (400 MHz, CD3CN, 258C):
d ¼ ꢁ140.31 (d, J ¼ 20.0 Hz, 2F), ꢁ140.94 (t, J ¼ 20.0 Hz, 3F), ꢁ141.47 (d,
J ¼ 24.0 Hz, 1F), ꢁ156.65 (m, J ¼ 24.0 Hz, 3F), ꢁ164.31 to ꢁ164.61 ppm
(m, J ¼ 8.0 Hz, 6F).
Received: April 11, 2002 [Z19075]
Both of our complexes 2 and 3 are less reactive than their
proposed porphyrin and salen counterparts if the latter
complexes are indeed the species responsible for nitrene
[NR] transfer, which remains to be shown. This difference in
reactivity could be a result of the ability of the corrole ligand
to stabilize high oxidation state manganese(v)centers, or to
the difference in the electron-withdrawing ability of the
R substituent on the imido ligand. The latter is an appealing
explanation given that complex 3 is more reactive than 2.
In conclusion, we have described a facile one-step synthesis
of novel (imido)manganese(v)complexes containing the
electron-poor corrole tpfc as the ancillary ligand. The
molecular structure of [(tpfc)MnV(N-mesityl)] is consistent
[1] J. T. Groves, T. Takahashi, J. Am. Chem. Soc. 1983, 105, 2073.
¼
[2] For Mn-catalyzed aziridination with PhI NTs (Ts ¼ tosyl), see: a) D.
Mansuy, J.-P. Mahy, A. Dureault, G. Bedi, P. Battioni, J. Chem. Soc.
Chem. Commun. 1984, 1161; b)K. Noda, N. Hosoya, R. Irie, Y. Ho, T.
Katsuki, Synlett 1993, 469; c)K. J. O©Connor, S.-J. Wey, C. J. Burrows,
Tetrahedron Lett. 1992, 33, 1001; d)J.-P. Simonato, J. Pÿcaut, W. R.
Scheidt, J. C. Marchon, Chem. Commun. 1999, 989.
[3] For Fe-catalyzed aziridination with Chloramine T, see: L. Simkhovich,
Z. Gross, Tetrahedron Lett. 2001, 42, 8089.
¼
[4] For Cu-catalyzed aziridination with PhI NTs, see: a)D. A. Evans,
M. M. Faul, M. T. Bilodeau, J. Am. Chem. Soc. 1994, 116, 2742; b)Z.
Li, R. W. Quan, E. N. Jacobsen, J. Am. Chem. Soc. 1995, 117, 5889;
c)J. A. Halfen, J. K. Hallman, J. A. Schultz, J. P. Emerson, Organo-
metallics 1999, 18, 5435.
[5] For the activation and use of (nitrido)managanese(v)salen complexes,
see: a)J. DuBois, J. Hong, E. M. Carreira, M. W. Day, J. Am. Chem.
Soc. 1996, 118, 915; b)J. DuBois, C. S. Tomooka, J. Hong, E. M.
Carreira, J. Am. Chem. Soc. 1997, 119, 3179; c)S. Minakata, T. Ando,
M. Nishimura, I. Ryu, M. Komatsu, Angew. Chem. 1998, 110, 3596;
Angew. Chem. Int. Ed. 1998, 37, 3392.
[6] a)R. Breslow, S. H. Gellma, J. Chem. Soc. Chem. Commun. 1982,
1400; b)R. Breslow, S. H. Gellman, J. Am. Chem. Soc. 1983, 105, 6728;
c)X.-Q. Yu, J.-S. Huang, X.-G. Zhou, C.-M. Che, Org. Lett. 2000, 2,
2233; d)J. Yang, R. Weinberg, R. Breslow, Chem. Commun. 2000, 531.
[7] a)For a comprehensive review on imido complexes of the transition
metals, see: D. E. Wigley, Prog. Inorg. Chem. 1994, 42, 239. The
section on manganese is pp. 392 396, and references therein; b)For a
recent account of (imido)manganese complexes, see: A. A. Dano-
poulos, J. C. Green, M. B. Hursthouse, J. Organomet. Chem. 1999, 591,
36.
2
ꢁ
with a low-spin d configuration and a Mn NR triple bond.
Preliminary reactivity studies proved that these new and
structurally well-defined (imido)manganese(v)complexes are
suitable for [NR]-group transfer only to electron-rich sub-
strates, such as organic phosphanes. The results reported
herein set the stage for making and exploring other MnV and
new MnVI imido species, for designing an efficient catalyst for
[NR]-group transfer from organic azides,[27] and for develop-
ing the parallel alkylidene/carbene [CR2] organometallic
chemistry.
Experimental Section
[8] a)J. T. Groves, T. E. Nemo, R. S. Myers, J. Am. Chem. Soc. 1979, 101,
1032; b)J. T. Groves, W. J. Kruper, Jr., J. Am. Chem. Soc. 1979, 101,
7613; c)C. L. Hill, B. C. Schardt, J. Am. Chem. Soc. 1980, 102, 6374.
[9] a)C. L. Hill, F. J. Hollander, J. Am. Chem. Soc. 1982, 104, 7318;
b)J. W. Buchler, C. Dreher, K.-L. Lay, Y. J. A. Lee, W. R. Scheidt,
Inorg. Chem. 1983, 22, 888; c)J. DuBois, C. S. Tomooka, J. Hong,
Toluene was dried by distillation over CaH2. Acetonitrile was dried by
distillation over CaH2, deoxygenated, and stored under argon prior to use.
For recrystallization purposes, pentane was purified by sulfuric acid before
distillation over CaH2. Photolysis experiments were carried out with a
Hanovia Model 73A36 550-W medium pressure mercury lamp at 258C.
3594
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4119-3594 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 19

COMMUNICATIONS
E. M. Carreira, Acc. Chem. Res. 1997, 30, 364; d)C. J. Chang, W. B.
Connick, D. W. Low, M. W. Day, H. B. Gray, Inorg. Chem. 1998, 37,
3107; e)N. Svenstrup, A. Bogevig, R. G. Hazell, K. A. Jorgensen, J.
Chem. Soc. Perkin Trans. 1 1999, 1559.
[23] A. A. Danopoulos, G. Wilkinson, T. Sweet, M. B. Hursthouse, J.
¼
Chem. Soc. Chem. Commun. 1993, 495. The Mn N-C angles occur in
the range of 138.5(3) 141.8(3) 8. [MnVII( NtBu)3Cl] is isolated in only
¼
20% yield and is remarkably stable towards air, water, and acid.
[24] J. Bendix, H. B. Gray, G. Golubkov, Z. Gross, Chem. Commun. 2000,
1957.
1
[10] For a recent H NMR spectrum of an (oxo)manganese(v)porphyrin,
see: N. Jin, J. T. Groves, J. Am. Chem. Soc. 1999, 121, 2923.
[11] For
a
preliminary structure of an (imido)manganese porphyrin
[25] A. E. Meier-Callahan, H. B. Gray, Z. Gross, Inorg. Chem. 2000, 39,
3605.
[26] a)E. N. Jacobsen in Catalytic Asymmetric Synthesis (Ed.: I. Ojima),
VCH, New York, 1993, p. 159; b)Y. N. Ito, T. Katsuki, Bull. Chem.
Soc. Jpn. 1999, 72, 603.
[27] a)For the first report on the use of a transition-metal catalyst,
copper(0), for azirdination with an organic azide, see: H. Kwart, A. A.
Kahn, J. Am. Chem. Soc. 1967, 89, 1951. Four products were formed,
and the yield of aziridine was only 15%; b)For a recent report on the
use of RuII and CoII porphyrin catalysts for aziridination with
organic azides, see: S. Cenini, S. Tollari, A. Penoni, C. Cereda, J. Mol.
Catal. A 1999, 137, 135. The aziridine yields are low, ꢂ 30% at best.
complex containing an OI(Ph)Cl ligand, [MnV(TPP)(NtBu)-
(OI(Ph)Cl)], where TPP ¼ 5,10,15,20-tetraphenylporphyrinato)dian-
ꢀ
ion, see: R. Blanco, Ph.D. thesis, Tufts University, 1996. The Mn
N(tBu)bond length is 1.594(17)ä and the Mn NtBu bond angle is
175.8(15)8.
ꢀ
[12] a)S. Will, J. Lex, E. Vogel, H. Schmickler, J. P. Gisselbrecht, C.
Haubtmann, M. Bernard, M. Gross, Angew. Chem. 1997, 109, 367;
Angew. Chem. Int. Ed. Engl. 1997, 36, 357; b)Z. Gross, G. Golubkov,
L. Simkhovich, Angew. Chem. 2000, 112, 4211; Angew. Chem. Int. Ed.
2000, 39, 4045; c)G. Golubkov, J. Bendix, H. B. Gray, A. Mahammed,
I. Goldberg, A. J. DiBillio, Z. Gross, Angew. Chem. 2001, 113, 2190;
Angew. Chem. Int. Ed. 2001, 40, 2132; d)A. E. Meier-Callahan, A. J.
DiBillio, L. Simkhovich, A. Mahammed, I. Goldberg, H. B. Gray, Z.
Gross, Inorg. Chem. 2001, 40, 6788.
[13] a)S. Licoccia, E. Morgante, R. Paolesse, F. Polizio, M. O. Senge, E.
Tondello, T. Boschi, Inorg. Chem. 1997, 36, 1564; b)K. M. Kadish,
V. A. Adamian, E. Van Caemelbecke, E. Gueletii, S. Will, C. Erben,
E. Vogel, J. Am. Chem. Soc. 1998, 120, 11986; c)S. Cai, S. Licoccia,
F. A. Walker, Inorg. Chem. 2001, 40, 5795.
[14] a)Z. Gross, N. Galili, I. Saltsman, Angew. Chem. 1999, 111, 1530;
Angew. Chem. Int. Ed. 1999, 38, 1427; b)Z. Gross, N. Galili, L.
Simkhovich, I. Saltsman, M. Botoshansky, R. Blaser, R. Boese, I.
Goldberg, Org. Lett. 1999, 1, 599.
[15] Organic azides are easily prepared from aniline derivatives in a one-
pot reaction: a)S. Murata, H. Tomioka, J. Org. Chem. 1997, 62, 3055;
b)G. Smolinsky, J. Org. Chem. 1961, 26, 4108.
[16] Electron-deficient nitrene species are produced by the action of light
on organic azides: a)L. Horner, A. Christmann, Angew. Chem. 1963,
75, 707; Angew. Chem. Int. Ed. Engl. 1963, 2, 599; b)W. Lwowski,
Angew. Chem. 1967, 79, 922; Angew. Chem. Int. Ed. Engl. 1967, 6, 897;
c)A. Reiser, H. M. Wagner in The Chemistryof the Azido Group
(Ed.: S. Patai), Wiley, New York, 1971, pp. 441 501; d)referen-
ce [14a].
Light-Harvesting Dendrimers: Efficient Intra-
and Intermolecular Energy-Transfer Processes
in a Species Containing 65 Chromophoric
Groups of Four Different Types**
Uwe Hahn, Marius Gorka, Fritz Vˆgtle,*
Veronica Vicinelli, Paola Ceroni, Mauro Maestri,*
and Vincenzo Balzani*
Dedicated to J. Fraser Stoddart
on the occasion of his 60th birthday
An antenna for light harvesting is an organized system in
which several chromophoric molecular species absorb the
incident light and channel the excitation energy to a common
acceptor component.[1] Light-harvesting antennas are essen-
tial devices for natural photosynthetic processes.[2] In the last
decade, dendrimers[3] have been extensively used to construct
artificial antenna systems as suitable chromophoric groups
may be incorporated into their regular branched structures.[4,5]
Another interesting aspect of dendrimer chemistry is the
presence of internal cavities where ions or neutral molecules
can be hosted.[6,7] Energy-transfer processes between the
[17] Coordination of nitrenes to high d-electron count transition metals
has been hypothesized and proposed: a)R. Gleiter, R. Hoffman,
Tetrahedron 1968, 24, 5899; b)L. A. P. Kane-Maguire, F. Basolo, R. G.
Pearson, J. Am. Chem. Soc. 1969, 91, 4609; c)J. L. Reed, F. Wang, F.
Basolo, J. Am. Chem. Soc. 1972, 94, 7173.
ꢀ
[18] For detailed kinetics studies of the reaction of (porphyrin)Mn( N)
with tfaa, see: L. A. Bottomley, F. L. Neely, J. Am. Chem. Soc. 1988,
110, 6748.
[19] S. P. de Visser, F. Ogliaro, Z. Gross, S. Shaik, Chem. Eur. J. 2001, 7,
4954.
[20] Crystal data for 2¥(C7H8): red crystals were obtained by slow
evaporation of a toluene/pentane solution of 2 at room temperature;
Mr ¼ 1073.74; space group P21/c; unit cell dimensions a ¼ 14.935(5),
b ¼ 12.609(4), c ¼ 23.938(8)ä; b ¼ 99.525(6)8; V¼ 4446(2)ä 3; Z ¼ 4;
T¼ 100(2)K; 1calcd ¼ 1.604 gcmꢁ3; l (MoKa) ¼ 0.71073 ä. A total of
17429 reflections were collected and merged to 5963 independent
reflections (R indices [I > 2s(I)]: R1 ¼ 0.0568 and wR2 ¼ 0.0720; R1 ¼
0.1372 and wR2 ¼ 0.0845 for all data); largest difference peak and hole
are 0.463 and ꢁ0.308 eäꢁ3. CCDC-191398 (2)contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB21EZ, UK; fax: (þ 44)1223-336-033; or deposit
@ccdc.cam.ac.uk).
[21] a)T. J. Collins, R. D. Powell, C. Slebodnick, E. S. Uffelman, J. Am.
Chem. Soc. 1990, 112, 899; b)F. M. MacDonnell, N. L. P. Fackler, C.
Stern, T. V. O©Halloran, J. Am. Chem. Soc. 1994, 116, 7431; c)C. G.
Miller, S. W. Gordon-Wylie, C. P. Horwitz, S. A. Strazisar, D. K.
Peraino, G. R. Clark, S. T. Weintraub, T. J. Collins, J. Am. Chem. Soc.
1998, 120, 11540.
[*] Prof. F. Vˆgtle, U. Hahn, M. Gorka
Kekulÿ-Institut f¸r Organische Chemie
und Biochemie der Universit‰t Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax : (þ 49)228-735662
E-mail: voegtle@uni-bonn.de
Prof. M. Maestri, Prof. V. Balzani, V. Vicinelli, Dr. P. Ceroni
Dipartimento di Chimica ™G. Ciamician∫
Universit‡ di Bologna
via Selmi 2, 40126 Bologna (Italy)
Fax : (þ 39)051-2099456
E-mail: mmaestri@ciam.ubino.it
vbalzani@ciam.unibo.it
[**] This work has been supported in Italy by MIUR (Supramolecular
Devices Project), University of Bologna (Young Researcher grant to
V.V.)and the EC (HPRN-CT-2000-00029.) We are also grateful to
Deutscher Akademischer Austausch Dienst (DAAD)for financial
support.
[22] Z. Shirin, B. S. Hammes, V. G. Young, Jr., A. S. Borovik, J. Am. Chem.
Soc. 2000, 122, 1836.
Angew. Chem. Int. Ed. 2002, 41, No. 19
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4119-3595 $ 20.00+.50/0
3595