Copper(I) complexes with a proximal aromatic ring: Novel copper-indole bonding

Article abstract of DOI:10.1002/(sici)1521-3773(19990816)38:16<2401::aid-anie2401>3.0.co;2-v

A unique bonding of indole to a metal ion has been established. Two new tridentate N ligands with a pendent indole ring gave Cu¹ complexes 1 and 2, the latter of which exhibits the new form of bonding between the Cu(I) ion in a distorted tetrahedral geometry and the indole C(2)-C(3) moiety. The bond is dependent upon the length of the side chain and therefore the accessibility of the ring to the metal center.

Full text of DOI:10.1002/(sici)1521-3773(19990816)38:16<2401::aid-anie2401>3.0.co;2-v

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aromatic side chain, which depend on the length of the
3-indolylalkyl chain. The unit cell of 1 consists of two
crystallographically independent CuI complex ions and two
PF6 ions. The molecular structure is shown in Figure 1, where
Copper(i) Complexes with a Proximal Aromatic
Ring: Novel Copper± Indole Bonding**
Yuichi Shimazaki, Hiroshi Yokoyama, and
Osamu Yamauchi*
Noncovalent interactions between metal ions and aromatic
rings have been implicated for copper(ii) complexes such as
[1]
[2]
[Cu(bpy)(l-Trp)(H2O)]ClO4 and [Cu(phen)(l-Trp)]ClO4
(bpy ˆ 2,2'-bipyridine, phen ˆ 1,10-phenanthroline, Trp ˆ
tryptophanate), where stacking between the side chain and
coordinated aromatic rings, and close contact between the
metal ion and the aromatic ring have been detected. Even in
the absence of a coordinated aromatic ring as a stacking
partner, the indole ring in CuII ± glycyltryptophanate[3] and the
phenol ring in [Cu(l-Tyr)2][4] (Tyrˆ tyrosinate) have been
found to be close to the CuII ion with a square-planar
geometry, which indicated that there could be a bonding
interaction between the CuII ion and the aromatic ring.
Recently a CuI ± macrocyclic NS2 ligand complex with a
naphthyl ring was structurally characterized to have a bond
between the CuI center and the naphthyl ring p bond.[5] Our
preliminary ab initio molecular orbital calculations with the
density functional method supported the formation of a
bonding orbital between the CuII ion and the five-membered
ring of the indole nucleus above the coordination plane in
‡
Figure 1. ORTEP view of the [Cu(Me2IMP)(CH3CN)] ion in 1 drawn
with the thermal ellipsoids at the 50% probability level and atomic labeling
scheme.
the coordination around the CuI ion has a tetrahedral
geometry formed by two pyridine nitrogen atoms, a tertiary
amine nitrogen atom, and an acetonitrile nitrogen atom. The
side-chain indole ring is uncoordinated, and no stacking or
Cu ± aromatic ring interaction was detected. Complex 2 has
the structure shown in Figure 2, with the CuI ion bound in a
‡
[Cu(bpy)(l-Trp)(H2O)] .[6] Herein we report that a tridentate
N-donor ligand with a pendent indole group forms a distorted
tetrahedral CuI complex that involves a unique coordination
of the indole ring through the indole C(2) C(3) moiety.
A tridentate ligand, N-(3-indolylmethyl)-N,N-bis(6-methyl-
2-pyridylmethyl)amine (Me2IMP) or N-(3-indolylethyl)-N,N-
bis(6-methyl-2-pyridylmethyl)amine (Me2IEP), was treated
‡
Figure 2. ORTEP view of the [Cu(Me2IEP)] ion in 2 drawn with the
thermal ellipsoids at the 50% probability level and atomic labeling scheme.
distorted tetrahedral geometry to three nitrogen atoms and
the C(2) C(3) moiety of the indole ring. The C(2) C(3) bond
length in 2 (1.379(7) Š) is only slightly longer than that of the
uncoordinated indole ring in 1 (1.365(9) Š), which is in line
with the observation that the C C bond length of coordinated
ethylene (1.329 ± 1.360 Š) in several CuI ± alkene complexes[8]
is virtually unchanged from that of free ethylene (1.337 Š).[9]
Whereas the CuI ± naphthyl bond in the CuI complex of the
macrocyclic ligand has one short (2.129(6) Š) and one long
(2.414(6) Š) Cu C bond,[5] the two Cu C bond lengths in 2
(2.228(5) and 2.270(5) Š for Cu C(2) and Cu C(3), respec-
tively) are nearly the same, but the two bonds are much longer
than those in CuI ± ethylene complexes (1.943 ± 2.028 Š).[8] We
see from the structures of 1 and 2 and the space-filling models
with [Cu(CH3CN)4]PF6 in methanol to give crystals of
[Cu(Me2IMP)(CH3CN)]PF6 (1) and [Cu(Me2IEP)]PF6 (2),
respectively. X-ray crystal structure analyses[7] have shown
that they have structures with different conformations of the
[*] Prof. Dr. O. Yamauchi, Y. Shimazaki, H. Yokoyama
Department of Chemistry, Graduate School of Science and
Research Center for Materials Science
Nagoya University, Nagoya 464-8602 (Japan)
Fax: (‡81)52-789-2953
E-mail: b42215a@nucc.cc.nagoya-u.ac.jp
[**] This work was supported by Grants-in-Aid for Scientific Research
(No. 09304062 and No. 10149219 (Priority Areas ªMetal-assembled
Complexesº) to O.Y.) from the Ministry of Education, Science, Sports,
and Culture of Japan.
Angew. Chem. Int. Ed. 1999, 38, No. 16
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was added in small portions to the resulting solution with stirring. The
reaction mixture was stirred for three days at room temperature, acidified
with conc. HCl, and then concentrated almost to dryness under a reduced
pressure. The residue was dissolved in a saturated aqueous solution of
NaHCO3 (50 mL) and extracted with CHCl3 (3 Â 50 mL). The brown oily
product obtained from the extract was purified by column chromatography
on silica gel with CHCl3/methanol as eluent to give Me2IEP (4.34 g,
70.2%). 1H NMR (300 MHz, CDCl3, 258C, TMS): d ˆ 2.53 (s, 6H, CH3py),
2.96 (m, 4H, CH2CH2), 3.92 (s, 4H, CH2py), 6.94 (d, 1H, indole-2-H), 6.98
(d, 2H, py-H), 7.03 ± 7.15 (m, 2H, indole-4,7-H), 7.32 (d, 2H, py-H), 7.32 (m,
1H, indole-5,6-H), 7.44 (m, 1H, indole-5,6-H), 7.46 (t, 2H, py-H), 8.0 (brs,
1H, indole-NH).
that the observed structural difference may be a consequence
of the steric requirements for the side-chain indole ring to
approach the CuI center. In connection to this, a closely
related tripodal ligand, tris(6-phenyl-2-pyridylmethyl)amine,
with an additional substituted pyridine ring in place of an
indole ring of Me2IEP is reported to form a CuI complex with
a trigonal pyramidal geometry with four nitrogen atoms
coordinated.[10]
The absorption spectrum of 2 in CH2Cl2 exhibited a
characteristic band centered at 308 nm (e ˆ 18000), which is
assigned to the charge transfer from CuI to the indole ring
(metal to ligand charge transfer (MLCT)). This band dis-
appeared when 2 was dissolved in CH3CN, and the resulting
spectrum was similar to that of 1, which has no CuI ± indole
bonding. This indicates that the indole ring in 2 is weakly
bound to CuI and is easily replaced by CH3CN. The 1H NMR
chemical shift differences between the indole protons of the
complexes and those of the free ligands were found to be
rather small, which is in contrast with the finding that CuI-
coordinated styrene exhibits upfield shifts of Dd ˆ 0.1 ± 0.5 as
a result of the p back donation.[11] On the other hand, whereas
the spectrum of Me2IEP in CD2Cl2 showed the ethylene
signals at d ˆ 2.96 as a multiplet, two separate triplets were
observed for 2 at d ˆ 3.03 and 3.25, which demonstrates that
the two methylene groups ( CH2CH2 ) are not equivalent as
a result of the coordination of the indole ring.
1 and 2: The complexes were prepared in methanol under a nitrogen
atmosphere from [Cu(CH3CN)4]PF6 and Me2IMP and Me2IEP, respective-
ly, as pale yellow crystals. 1: Yield, 59%; 1H NMR (300 MHz, CD2Cl2,
258C, TMS): d ˆ 2.69 (s, 6H, CH3py), 3.68 (d, 2H, CH2py), 4.06 (s, 2H,
CH2indole), 4.08 (d, 2H, CH2py), 7.06 (d, 2H, py-3,5-H), 7.07 (dd, 1H,
indole-5,6-H), 7.13 (td, 1H, indole-5,6-H), 7.22 (d, 2H, py-3,5-H), 7.36 (d,
1H, indole-2-H), 7.39 (m, 1H, indole-4,7-H), 7.42 (dt, 1H, indole-4,7-H),
7.63 (t, 2H, py-4-H), 9.36 (brs, 1H, indole-1-H); elemental analysis (%)
calcd for C25H27N5CuF6P: C 49.54, H 4.49, N 11.55; found: C 49.41, H 4.60,
N 11.24. 2: Yield, 64%; 1H NMR (300 MHz, CD2Cl2, 258C, TMS): d ˆ 2.64
(s, 6H, CH3py), 3.03 (t, 2H, CH2CH2), 3.25 (t, 2H, CH2CH2), 3.80 (d, 2H,
CH2py), 4.10 (d, 2H, CH2py), 6.98 (td, 1H, indole-5,6-H), 7.10 (d, 1H,
indole-2-H), 7.16 (m, 1H, indole-5,6-H), 7.16 (m, 2H, py-3,5-H), 7.24 (d,
2H, py-3,5-H), 7.31 (d, 1H, indole-4,7-H), 7.44 (d, 1H, indole-5,8-H), 7.69 (t,
2H, py-4-H), 8.96 (brs, 1H, indole-1-H); elemental analysis (%) calcd for
C24H26N4CuF6P: C 49.78, H 4.53, N 9.68; found: C 50.08, H 4.50; N 9.65.
Received: May 27, 1998
Revised version: March 12, 1999 [Z11904IE]
German version: Angew. Chem. 1999, 111, 2561 ± 2563
The cyclic voltammograms of 1 and 2 were recorded in
CH2Cl2 under a nitrogen atmosphere to give quasi-reversible
Keywords: coordination chemistry ´ copper ´ indole ligands
´ noncovalent interactions ´ redox chemistry
redox waves with E1/2
ˆ
0.06 V (DE ˆ 0.12 V) and 0.01 V
(DE ˆ 0.15 V) versus ferrocene/ferrocenium, respectively.
The finding that 2 has a higher redox potential than that of
1 may be interpreted as a result of the p back donation from
the CuI ion to the coordinated indole ring.
[1] H. Masuda, T. Sugimori, A. Odani, O. Yamauchi, Inorg. Chim. Acta
1991, 180, 73 ± 79.
[2] a) K. Aoki, H. Yamazaki, J. Chem. Soc. Dalton Trans. 1987, 2017 ±
2021; b) H. Masuda, O. Matsumoto, A. Odani, O. Yamauchi, Nippon
Kagaku Kaishi 1988, 783 ± 788.
[3] M. B. Hursthouse, S. A. A. Jayaweera, H. Milburn, A. Quick, J. Chem.
Soc. Dalton Trans. 1975, 2569 ± 2572.
[4] D. van der Helm, C. E. Tatsch, Acta Crystallogr. Sect. B 1972, 28,
2307 ± 2312.
In conclusion, the CuI complexes of two indole-containing
ligands have different structures that depend either on the
side-chain length or the mobility of the indole ring. Despite
the various types of direct metal ± indole bonds that have been
reported,[12±15] complex 2 presents a novel mode of metal
binding by the indole ring and suggests the reactivity of the
C(2) C(3) moiety and the versatility of the indole ring are
important in metal complex formation.
[5] W. S. Striejewske, R. R. Conry, Chem. Commun. 1998, 555 ± 556.
[6] H. Masuda, N. Fukushima, T. Sugimori, O. Yamauchi, J. Inorg.
Biochem. 1993, 51, 158.
[7] X-Ray crystal structure determination: The X-ray diffraction data
were collected at 295 K with a Rigaku AFC-5R four-circle diffrac-
tometer and graphite-monochromated CuKa radiation (l ˆ
1.54178 Š). Crystal data for 1: C25H27N5CuF6P, Mr ˆ 606.03, crystal
Experimental Section
Me2IMP: Indole (1.77 g, 10 mmol) and a small amount of acetic acid was
added to
a solution of bis(6-methyl-2-pyridylmethyl)amine (2.27 g,
Å
size: 0.6  0.3  0.2 mm, triclinic, space group P1, a ˆ 14.109(4), b ˆ
10 mmol) in methanol (100 mL). Aqueous formaldehyde (37%, 0.81 g)
was added in small portions to the resulting solution with stirring. The
reaction mixture was stirred for two days at room temperature and then
concentrated almost to dryness under a reduced pressure. The residue was
dissolved in a saturated aqueous solution of NaHCO3 (50 mL) and
extracted with CHCl3 (3 Â 50 mL). The brown oily product obtained from
the extract was purified by column chromatography on silica gel with
CHCl3/methanol as eluent to give Me2IMP (2.01 g, 56.3%). 1H NMR
(300 MHz, CDCl3, 258C, TMS): d ˆ 2.51 (s, 6H, CH3py), 3.81 (s, 4H,
CH2py), 3.87 (s, 2H, CH2indole), 6.98 (d, 2H, py-H), 7.10 ± 7.16 (m, 2H,
indole-4,7-H), 7.20 (d, 1H, indole-2-H), 7.33 (m, 1H, indole-5,6-H), 7.43 (d,
2H, py-H), 7.53 (t, 2H, py-H), 7.71 (m, 1H, indole-5,6-H), 8.1 (brs, 1H,
indole-NH).
16.601(5), c ˆ 12.392(7) Š, a ˆ 100.86(4), b ˆ 90.48(4), g ˆ 90.49(3)8,
Vˆ 2760(2) Š3, Z ˆ 2, 1calcd ˆ 1.458 gcm 3, m ˆ 22.42 cm 1, F(000) ˆ
1240.0, 9958 independent reflections, 6895 reflections used, 686
parameters, R ˆ 0.088, Rw ˆ 0.088. Crystal data for 2: C24H26N4CuF6P,
Å
Mr ˆ 579.01, crystal size: 0.6  0.2  0.1 mm, triclinic, space group P1,
a ˆ 11.573(1), b ˆ 14.151(1), c ˆ 8.0593(9) Š
,
a ˆ 104.091(8), b ˆ
95.778(8),
g ˆ 92.197(8)8,
Vˆ 1270.9(2) Š3,
Z ˆ 2,
1calcd
ˆ
1.513 gcm 3, m ˆ 23.93 cm 1, F(000) ˆ 592.0, 3749 independent reflec-
tions, 3280 reflections used, 326 parameters, R ˆ 0.065, Rw ˆ 0.066.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC-115148 and 115149 for 1 and 2, respectively. Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
Me2IEP: Tryptamine (2.70 g, 16.8 mmol) and a small amount acetic acid
was added to a solution of 6-methylpyridine-2-aldehyde (4.08 g, 33.7 mmol)
in methanol (100 mL). Sodium cyanotrihydroborate (2.14 g, 33.7 mmol)
2402
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[8] a) J. S. Thompson, R. L. Harlow, J. F. Whitney, J. Am. Chem. Soc.
1983, 105, 3522 ± 3527; b) J. S. Thompson, J. F. Whitney, Inorg. Chem.
1984, 23, 2813 ± 2819; c) H. Masuda, N. Yamamoto, T. Taga, K.
Machida, S. Kitagawa, M. Munakata, J. Organomet. Chem. 1987, 322,
121 ± 129.
CH3
N
CH3NHCH2CO2H
CH2O, ∆
[9] a) L. S. Bartell, E. A. Roth, C. D. Hollowell, K. Kuchitsu, J. E.
Young, Jr., J. Chem. Phys. 1963, 42, 2683 ± 2686; b) D. M. P. Mingos in
Comprehensive Organometallic Chemistry, Vol. 3 (Eds.: G. Wilkinson,
F. G. A. Stone, E. W. Abel), Pergamon, New York, 1982, pp. 1 ± 88.
[10] C. Chuang, K. Lim, Q. Chen, J. Zubieta, J. W. Canary, Inorg. Chem.
1995, 34, 2562 ± 2568.
40%
(1)
CH3
N
I
CH3
CH3I
[11] H. Masuda, K. Machida, M. Munakata, S. Kitagawa, H. Shimono, J.
Chem. Soc. Dalton Trans. 1988, 1907 ± 1910.
DMSO
100%
[12] a) Y. Omote, N. Fukada, N. Sugiyama, Nippon Kagaku Zasshi 1969,
90, 1283 ± 1285; b) M. G. Reinecke, J. F. Sebastian, H. W. Johnson, Jr.,
C. Pyun, J. Org. Chem. 1971, 21, 3091 ± 3095; c) M. G. Reinecke, J. F.
Sebastian, H. W. Johnson, Jr., C. Pyun, J. Org. Chem. 1972, 22, 3066 ±
3068, and references therein.
1
[13] O. Yamauchi, M. Takani, K. Toyoda, H. Masuda, Inorg. Chem. 1990,
29, 1856 ± 1860.
[14] M. Takani, H. Masuda, O. Yamauchi, Inorg. Chim. Acta 1995, 235,
367 ± 374.
benzene (Scheme 1). Two layers were obtained, and the
mixture was agitated by shaking until an emulsion formed.
The emulsion dissipated after 15 min, and the two layers
separated. The benzene layer contained a flocculent, hairlike
material (75% yield), which was inspected by transmission
electron microscopy (TEM). Rodlike structures were ob-
served with diameters of 14 ± 120 nm and lengths of over
70 mm (Figures 1a, b). The precise molecular orientation and
spacing of 1 in the supramolecular structure is not known;
however, that such extended fulleroid structures could self-
assemble is striking, especially since a long hydrophobic tail is
not appended onto the fullerene rings. To our knowledge,
there is no similar process for the rapid supramolecular
assembly of fullerenes or their derivatives in such high yields.
The scope of this process was investigated. Self-assembly of
1 into rodlike structures was not seen at concentrations less
than 2.5 mm. The precise order of solvent addition was also
critical: addition of benzene, then water, to a solution of 1 in
DMSO followed by agitation failed to provide nanorods. To
screen the effect of different solvents on nanorod formation,
benzene was replaced by hexane, cyclohexane, diethyl ether,
chloroform, and dichloromethane. The self-assembly experi-
ments in chloroform and dichloromethane were inconclusive,
resulting in thick milky white solutions. Using diethyl ether as
solvent resulted in two clear layers absent of solid. No self-
assembled structures were observed in these layers by TEM.
Use of cyclohexane and hexane led to much shorter nanorods
having lengths of 1 ± 2 mm and diameters similar to those
obtained with benzene.
We also explored changes to 1 that might affect assembly
into nanorods. Changing the counterion in a surfactant can
influence essential parameters necessary to form micellar
structures.[3] In the case of 1, exchanging the iodide for
bromide, chloride, and nitrate did influence the formation of
nanorods. Ion exchange of the iodide on 1 was performed by
passing a solution of 1 in DMSO through an ion-exchange
resin containing the desired anion. Anion exchange was
detected by energy dispersive X-ray analysis (EDAX). In the
case of nitrate, the absence of iodide by EDAX was proof of
counterion exchange since the lighter elements cannot be
detected with the system utilized. Under the same conditions
employed with the iodide salt, nanorods formed from both the
bromide and nitrate salts. However, no rods were detected
when chloride was the counterion. We speculate that with the
[15] R. Robson, Inorg. Chim. Acta 1982, 57, 71 ± 77.
Self-Assembling Supramolecular
Nanostructures from a C60 Derivative:
Nanorods and Vesicles**
Alan M. Cassell, C. Lee Asplund, and James M. Tour*
Extended fullerene structures such as those found in carbon
nanotubes have attracted considerable interest from both
structural and applications perspectives.[1] Similarly, nonex-
tended C60 derivatives have shown promise for numerous
applications.[1] In an effort to bridge the gap between self-
assembling, extended structures and modified fullerene
materials, we describe here the supramolecular assembly of
a derivatized C60. C60-N,N-dimethylpyrrolidinium iodide (1),[2]
prepared in two steps from C60 [Eq. (1)], can self-assemble
into either nanorods or vesicles depending on how the
solution is treated.
Nanorod structures were prepared from a two-phase
mixture by diluting a solution of 1 in dimethylsulfoxide
(DMSO) with one part water and then adding one part
[*] Prof. J. M. Tour,[‡] A. M. Cassell, C. L. Asplund
Department of Chemistry and Biochemistry
University of South Carolina
Columbia, SC 29208 (USA)
‡
[ ] Current address:
Rice University
Department of Chemistry and Center for Nanoscale Science and
Technology
MS-222, 6100 Main Street, Houston, TX 77005 (USA)
Fax: (‡1)713-737-6250
E-mail: tour@rice.edu
[**] The Office of Naval Research funded this work.
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