One-pot conditional self-assembly of multicopper metallacycles

Article abstract of DOI:10.1021/ic1008957

The self-assembly of supramolecular metallacycles via the coordination-driven directional bonding approach can be modified to produce some unexpected structural variations. The combination of a flexible ligand-capped dinuclear transition-metal acceptor like [Cu₂(dppm)₂ (NCMe)₂]X₂ (1X₂; dppm = Ph₂PCH ₂PPh₂; X^- = BF₄^-, PF ₆^-, or BPh₄^-) with monodentate-bidentate donors like 2-, 3-, and 4-pyridylcarboxylates produced oligomeric compounds [{Cu₂(dppm)₂}(μ-(2-PyCO ₂))] ₂X₂ (2X₂), [{Cu ₂(dppm)₂} (μ-(3-PyCO₂))]₂ X ₂ (3X₂), and [{Cu₂(dppm)₂}(μ-(4- PyCO₂))] ₄X₄ (4X₄), respectively, as the thermodynamically stable products in one-pot reactions. However, the modified self-assembly is still subject to steric hindrance. The reaction of complex 1(BF₄)₂ with 6-Me-3-PyCO₂H did not produce a polygonal dimeric metallacycle but a simple dinuclear complex, [Cu₂(dppm)₂ (6-Me-3-PyCO₂)](BF₄) (5(BF₄)). The crystal structures of complexes 2(PF₆) ₂, 3(PF₆)₂, 4(BF₄)₄, and 5(BF₄) were determined using X-ray diffraction.

Full text of DOI:10.1021/ic1008957

9902 Inorg. Chem. 2010, 49, 9902–9908
DOI: 10.1021/ic1008957
One-Pot Conditional Self-Assembly of Multicopper Metallacycles
Kom-Bei Shiu,*,† Shih-An Liu,† and Gene-Hsiang Lee‡
†Department of Chemistry, National Cheng Kung University, Tainan, Taiwan 701, and
‡Instrument Center, National Taiwan University, Taipei, Taiwan 106
Received May 10, 2010
The self-assembly of supramolecular metallacycles via the coordination-driven directional bonding approach can be
modified to produce some unexpected structural variations. The combination of a flexible ligand-capped dinuclear
transition-metal acceptor like [Cu2(dppm)2(NCMe)2]X2 (1X2; dppm = Ph2PCH2PPh2; X- = BF4-, PF6-, or BPh4
)
-
with monodentate-bidentate donors like 2-, 3-, and 4-pyridylcarboxylates produced oligomeric compounds [{Cu2-
(dppm)2}(μ-(2-PyCO2))]2X2 (2X2), [{Cu2(dppm)2}(μ-(3-PyCO2))]2X2 (3X2), and [{Cu2(dppm)2}(μ-(4-PyCO2))]4-
X4 (4X4), respectively, as the thermodynamically stable products in one-pot reactions. However, the modified self-
assembly is still subject to steric hindrance. The reaction of complex 1(BF4)2 with 6-Me-3-PyCO2H did not produce a
polygonal dimeric metallacycle but a simple dinuclear complex, [Cu2(dppm)2(6-Me-3-PyCO2)](BF4) (5(BF4)). The
crystal structures of complexes 2(PF6)2, 3(PF6)2, 4(BF4)4, and 5(BF4) were determined using X-ray diffraction.
Introduction
monodentate or multibranched chelate donors with relative
ease in a one-pot reaction.5 A series of reports contributed by
Hor et al.6 demonstrated that the approach is even applicable
in cases with mixed-type donors such as pyridylcarboxylate,
which contains both the monodentate (i.e., the pyridyl nitro-
gen atom) and the bidentate (i.e., the carboxyl group) moie-
ties. The combination of rigid ligand-capped transition-metal
Supramolecular metallacycles have recently attracted a lot
of attention due to their potential application in areas as
diverse as selective guest encapsulation for chemical sensing,1
reaction catalysis,2 photoinitiated energy transfer,3 and inter-
molecular electron transfer.4 These metallacycles can be
rationally designed and synthesized via the coordination-
driven directional bonding approach that combines rigid
ligand-capped transition-metal acceptors and multibranched
complexes such as [Pt(PPh3)2(MeCN)2](OTf)2 (OTf-
=
CF3SO3-) with pyridylcarboxylate ligands afforded predic-
tive compounds based on the shape of the ligands. For
ligands, the turning angles of 60°, 120°, and 180° between
the pyridyl donor and the carboxylate donor sites for 2-, 3-,
and 4-pyridylcarboxylates, respectively, affect the formation
of either mononuclear complex or polygonal products, inclu-
ding [Pt(2-PyCO2)(PPh3)2](OTf), [Pt(3-PyCO2)((PPh3)2)]3-
(OTf)3, and [Pt(4-PyCO2)((PPh3)2)]4(OTf)4.6d However, the
present study shows that the employment of a flexible ligand-
capped transition-metal acceptor such as [Cu2(dppm)2-
(NCMe)2]2þ (dppm = Ph2PCH2PPh2)7 can convert various
pyridylcarboxylate donors into all polygonal oligomeric
compounds.
*To whom the correspondence should be addressed. E-mail:
kbshiu@mail.ncku.edu.tw.
(1) (a) Mines, G. A.; Tzeng, B.-C.; Stevenson, K. J.; Li, J.; Hupp, J. T.
Angew. Chem., Int. Ed. 2002, 41, 154. (b) Kumazawa, K.; Biradha, K.;
Kusukawa, T.; Okano, T.; Fujita, M. Angew. Chem., Int. Ed. 2003, 42, 3909.
(c) Kumar, A.; Sun, S.-S.; Lees, A. J. Coord. Chem. Rev. 2008, 252, 922.
(2) (a) Ito, H.; Kusukawa, T.; Fujita, M. Chem. Lett. 2000, 598. (b) Merlau,
M. L.; Mejia, S. D. P.; Nguyen, S. T.; Hupp, J. T. Angew. Chem., Int. Ed. 2001,
40, 4239. (c) Fiedler, D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc.
Chem. Res. 2005, 38, 349.
(3) Splan, K. E.; Massari, A. M.; Morris, G. A.; Sun, S.-S.; Reina, E.;
Nguyen, S. T.; Hupp, J. T. Eur. J. Inorg. Chem. 2003, 2348.
(4) (a) Lau, V. C.; Berben, L. A.; Long, J. R. J. Am. Chem. Soc. 2002, 124,
9042. (b) Dinolfo, P. H.; Williams, M. E.; Stern, C. L.; Hupp, J. T. J. Am. Chem.
Soc. 2004, 126, 12989. (c) Dinolfo, P. H.; Hupp, J. T. J. Am. Chem. Soc. 2004,
126, 16814.
(5) (a) Olenyuk, B.; Stang, P. J. Acc. Chem. Res. 1997, 30, 502. (b)
Leninger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853. (c) Swiegers,
G. F.; Malefetse, T. J. Chem. Rev. 2000, 100, 3483. (d) Fujita, M.; Umemoto, K.;
Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001,
509. (e) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (f)
Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759. (g) Seidel,
S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (h) Ruben, M.; Rojo, J.;
Romero-Salgurero, F. J.; Uppadine, L. H.; Lehn, J.-M. Angew. Chem., Int. Ed.
2004, 43, 3644. (i) Steel, P. J. Acc. Chem. Res. 2005, 38, 243. (j) Fujita, M.;
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Gianneschi, N. C.; Masar, M. S.; Mirkin, C. A. Acc. Chem. Res. 2005, 38, 825.
(l) Stang, P. J. J. Org. Chem. 2009, 74, 2.
The cation, [Cu2(dppm)2(NCMe)2]2þ, contains two cop-
per atoms doubly bridged by two dppm ligands, probably
˚
at a d(Cu Cu) of around 3.4 A with no appreciable
3 3 3
cuprophilic interaction.8 Since this cation has good bonding
(6) (a) Teo, P.; Koh, L. L.; Hor, T. S. A. Inorg. Chem. 2003, 42, 7290.
(b) Teo, P.; Foo, D. M. J.; Koh, L. L.; Hor, T. S. A. Dalton Trans. 2004, 20, 3389.
(c) Teo, P.; Koh, L. L.; Hor, T. S. A. Inorg. Chim. Acta, 2006, 359, 3435. (d) Teo,
P.; Koh, L. L.; Hor, T. S. A. Chem. Commun. 2007, 2225. (e) Teo, P.; Koh, L. L.;
Hor, T. S. A. Inorg. Chem. 2008, 47, 6464. (f) Teo, P.; Koh, L. L.; Hor, T. S. A.
Inorg. Chem. 2008, 47, 9561.
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J. Chem. Soc., Dalton Trans. 1987, 1275.
r
pubs.acs.org/IC
Published on Web 10/07/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9903
Scheme 1
spectra were recorded on a Bruker TENSOR 27 instrument.
The 1H and 31P NMR spectra were recorded on a Bruker
AVANCE 300 spectrometer (1H, 300 MHz; 31P, 122 MHz) or
a Bruker AVANCE 500 spectrometer (1H, 500 MHz; 31P, 202
MHz) calibrated against internal deuterated solvents (1H) or
external 85% H3PO4 (31P). Microanalyses were carried out by
the staff of the Microanalytical Service of the Department of
Chemistry, National Cheng Kung University. The dicopper(I)
precursor complex, 1X2, was prepared according to a previously
reported procedure.7
Synthesis of [Cu2(dppm)2(2-PyCO2)]2(BF4)2 (2(BF4)2).
a. Method 1. Three drops of Et3N (ca. 0.0333 mL, 0.0462
mmol) were added to a suspension of 1(BF4)2 (0.1498 g, 0.130
mmol) and picolinic acid (0.0160 g, 0.130 mmol) in CH2Cl2 (25
mL), immediately forming a clear solution. The solution was
stirred at ambient temperature for 10 min. All of the solvent
and the excess Et3N were removed by a vacuum pump to
produce a pale yellow precipitate. The precipitate was washed
with deaerated water (ca. 15 mL) and pumped dry. Recrystal-
lization from CH2Cl2-hexane produced 2(BF4)2. Yield 0.139
g, 97%. IR (CH2Cl2): νa(CO2-), 1603; νs(CO2-), 1404;
ν(BF4-), 1098, 1060, 1038 cm-1. 1H NMR (CDCl3, 296 K): δ
3.30 (s, 8 H, P-CH2), 7.10-7.17 (m, 80 H, C6H5), 8.15 (m, 2 H),
8.56 (m, 2 H), 8.90 (m, 2 H), 9.03 (m, 2 H) for PyCO2
-
.
31P{1H}
NMR (CDCl3, 296 K): δ -7.66 (s). Elem anal. calcd for
C112H96B2Cu4F8N2O4P8 (%): C, 60.88; H, 4.38; N, 1.27.
Found: C, 60.54; H, 4.39; N, 1.26.
b. Method 2. A suspension of 1(BF4)2 (0.1753 g, 0.152 mmol)
and picolinic acid (0.0187 g, 0.152 mmol) in CH2Cl2 (25 mL) was
stirred at ambient temperature for 30 min. The suspension
formed a clear solution. The solution was then stirred for
another 10 min. All of the solvent was removed by a vacuum
pump to produce a pale yellow precipitate, which was then
washed with Et2O (ca. 25 mL) and collected. (If necessary,
further recrystallization from CH2Cl2-hexane can be carried
out.) Compound 2(BF4)2 was produced. Yield: 0.163 g, 97%.
Synthesis of [Cu2(dppm)2(3-PyCO2)]2(BF4)2 (3(BF4)2). Com-
pound 3(BF4)2 was prepared in yields of 97% and 96% using
procedures similar to those used in methods 1 and 2, respec-
tively, for compound 2(BF4)2. IR (CH2Cl2): νa(CO2-), 1603;
νs(CO2-), 1402 cm-1; ν(BF4-), 1098, 1060, 1038 cm-1. 1H NMR
(CDCl3, 296 K): δ 3.42 (s, 8 H, P-CH2), 6.92-7.20 (m, 80 H,
interactions with various oxyanions,10 it is not surprising that
the bonded nitrate anions of [Cu2(dppm)2(NO3)2] impede the
formation of polygonal oligomeric metallacycles; the reac-
tion of the complex with 2-PyCO2H thus produces [Cu2-
(dppm)2(η2-(2-PyCO2H)2](NO3)2.11 In the present study,
the cation with noncoordinating anions, [Cu2(dppm)2-
(MeCN)2]X2 (1X2; X- = BF4-, PF6-, or BPh4-), was found
to readily self-assemble pyridylcarboxylates, 2-, 3-, and 4-
PyCO2-, into distinctive polygonal oligomeric metallacycles,
including dimeric, dimeric, and tetrameric structures, respec-
tively (Scheme 1), as the only isolated, thermodynamically
controlled, products in one-pot reactions. The results for the
flexible ligand-coordinated cation 12þ hence contrast sharply
with the mononuclear, trigonal, and square structures, ob-
tained by Hor et al.,6d for the rigid ligand-capped cation,
[Pt(PPh3)2(MeCN)2]2þ. Importantly, the results indicate that
a flexible ligand-capped dinuclear (and other multinuclear)
transition-metal acceptor like 12þ can promote oligomeriza-
C6H5), 8.31 (m, 2 H), 8.91 (m, 2 H), 9.21 (m, 2 H), 9.47 (s, 2 H)
-
for PyCO2
.
31P{1H} NMR (CDCl3, 296 K): δ -8.01 (s). Elem
anal. calcd for C112H96B2Cu4F8N2O4P8 (%): C, 60.88; H, 4.38;
N, 1.27. Found: C, 61.13; H, 4.41; N, 1.25.
Synthesis of [Cu2(dppm)2(4-PyCO2)]4(BF4)4 (4(BF4)4). Com-
pound 4(BF4)4 was prepared in a yield of 97% using a procedure
similar to that used in method 2 for compound 2(BF4)2. IR
(CH2Cl2): νa(CO2-), 1604; νs(CO2-), 1403 cm-1; ν(BF4-), 1098,
-
tion for the 2-PyCO2 donor and diminishes the shape dif-
ference between 2- and 3-PyCO2- donors. The employment
of a flexible ligand-capped transition-metal acceptor to re-
place a rigid analogue can bring unexpected structural vari-
ation in the products obtained by conventional directional
bonding approach. However, the new oligomerization is still
subject to steric or electronic hindrance, as demonstrated below.
1
1060, 1038 cm-1. H NMR (CD2Cl2, 296 K): δ 3.23 (s, 16 H,
P-CH2), 7.03-7.24 (m, 160 H, C6H5), 8.55 (d, 8 H, J = 5.7), 9.00
(d, 8 H, J = 5.7), for PyCO2
-
.
31P{1H} NMR (CDCl3, 296 K): δ
-7.94 (s). Elem anal. calcd for C224H192B4Cu8F16N4O8P16 (%):
C, 60.88; H, 4.38; N, 1.27. Found: C, 61.01; H, 4.08; N, 1.19.
Synthesis of [Cu2(dppm)2(6-Me-3-PyCO2)](BF4) (5(BF4)).
Compound 5(BF4) was prepared in a yield of 96% using a
procedure similar to that used in method 2 for compound
Experimental Section
2(BF4)2. IR (CH2Cl2): νa(CO2-), 1600; νs(CO2-), 1404 cm-1
;
General Information. All reactions were performed under
prepurified nitrogen using freshly distilled solvents. The IR
ν(BF4-), 1103, 1061, 1038 cm-1. 1H NMR (CD2Cl2, 296 K): δ
2.87 (s, 3 H, CH3), 3.17 (m, 4H, P-CH2), 7.02-7.15 (m, 40 H,
C6H5), 7.95 (d, 1 H, J = 4.4), 8.95 (d, 1 H, J = 4.4), 9.24 (s, 1 H)
.
31P{1H} NMR (CDCl3, 296 K): δ -7.79 (s). Elem
-
˚
for PyCO2
(8) The distance, d(Cu Cu), of 3.426(3) A found in the structure of
3 3 3
7
[Cu2(dppm)2(NCMe)4](ClO4)2 is much larger than the sum of the van der
anal. calcd for C57H50BCu2F4NO2P4 (%): C, 61.19; H, 4.50; N,
1.25. Found: C, 61.15; H, 4.73; N, 1.25.
I
9
˚
Waals radii of two Cu centers of 2.80 A.
(9) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(10) Bera, J. K.; Nethaji, M.; Samuelson, A. G. Inorg. Chem. 1999, 38,
1725.
(11) Liu, Y.; Zhao, D.; Yang, R.; Zhu, J.; Sun, Y.; Zhang, C. Russ. J.
Inorg. Chem. 2004, 49, 1828.
X-Ray Crystallography. Single crystals were obtained by
layering hexane on top of a CH2Cl2 or CH2Cl2-MeOH solution
of the complexes. Data collection was performed on a Bruker
SMART-CCD diffractometer at 100(2) K for crystal 2(PF6)2,

9904 Inorganic Chemistry, Vol. 49, No. 21, 2010
Shiu et al.
Table 1. Crystal Data
crystal
2(PF6)2 2CH2Cl2
3
3(PF6)2 4CH2Cl2
3
4(BF4)4 CH2Cl2
3
5(BF4) 4CH2Cl2
3
formula
fw (g mol-1
C114H100Cl4Cu4F12N2O4P10 C116H104Cl8Cu4F12N2O4P10 C225H194B2Cl2Cu8F8N4O8P16 C61H59B2Cl8Cu2F8NO2P4
)
2495.62
0.20 ꢀ 0.15 ꢀ 0.15
monoclinic
P21/c
2665.47
4330.20
1546.27
1.00 ꢀ 0.80 ꢀ 0.60
monoclinic
P21/c
cryst size (mm3)
cryst syst
0.15 ꢀ 0.10 ꢀ 0.10
monoclinic
P21/n
1.00 ꢀ 0.80 ꢀ 0.20
triclinic
P1
space group
˚
a (A)
15.506(3)
22.758(4)
16.992(3)
90
12.0588(2)
25.3520(4)
19.3009(3)
90
14. 564(2)
22.880(3)
23.563(3)
61.091(2)
80.927(2)
81.605(2)
6765.3(16)
1
12.578(5)
20.407(8)
27.288(10)
90
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
113.779(3)
90
103.992(1)
90
102.702(6)
90
3
˚
V (A )
Z
5487.3(15)
2
1.510
5725.49(16)
2
1.546
6833(4)
4
1.503
D
calc (g cm-3
)
1.063
0.781
2224
μ, (mm-1
F(000)
)
1.082
2544
1.133
2712
1.093
3136
reflns collected, unique
R1/wR2 (I > 2σ(I))
GOF on F2
39430/13607
0.0670/0.1581
0.867
57689/10081
0.0510/0.1218
1.077
26542/26542
0.0720/0.1754
0.817
42559/14061
0.0686/0.1901
1.039
-3
˚
largest diff peak and hole (e A
)
2.348/-1.507
0.586/-0.812
1.412/-0.480
1.320/-1.497
on a NONIUS Kappa CCD diffractometer at 150(2) K for
crystal 3(PF6)2, and on a Bruker SMART Apex II diffracto-
meter at 296(2) K for a crystal of 4(BF4)4 and at 100(2) K for
crystal 5(BF4). The three instruments were equipped with
˚
graphite-monochromated Mo KR radiation (λ = 0.71073 A).
Empirical absorption corrections were made on the basis of an
azimuthal scan. The structures of crystals were determined using
the direct method and then refined using the full-matrix least-
squares method on F2, with all non-hydrogen atoms refined with
anisotropic temperature parameters using SHELXL-97
software.12 The PLATON-SQUEEZE program was applied to
crystal 4(BF4)4 for the position-disordered solvent molecules in
the structure refinement.13 Almost all hydrogen atoms were
either located from the difference maps or generated by geome-
˚
try with d(C-H) = 0.930 or 0.950 A. A summary of the crystal
data, data collection, and refinement parameters is provided in
Table 1.
Results and Discussion
Synthesis, Solution Behavior, and Structures of Multi-
copper Metallacycles. As illustrated in Scheme 1, the
synthesis of the multicopper metallacycles (2X2-4X4)
was readily achieved by the direct treatment of the
dicopper complex, 1X2, with 2-, 3-, and 4-PyCO2H in
the presence or absence of Et3N in CH2Cl2 at ambient
temperature. The production yields for metallacycles
2X2, 3X2, and 4X4 were almost quantitative. However,
the time-dependent 31P{1H} NMR spectra (Figure 1), ob-
tained by mixing an equal amount of [Cu2(dppm)2-
(MeCN)2](BF4)2 (1(BF4)2) and PyCO2H in CD2Cl2 in
an NMR tube, indicate that metallacycles 2(BF4)2 and
4(BF4)4 formed almost completely, but that metallacycle
3(BF4)2 and one unidentified product, 3u(BF4), which
resonate in 31P{1H} NMR experiments at δ -8.01 and
-9.93, respectively, were in an equilibrium ratio of ca.
1:0.43, favoring metallacycle 3(BF4)2, after 7 h. Following
the removal of MeCN using a vacuum pump (see Experi-
mental Section), metallacycle 3(BF4)2 was obtained as the
only product. The intensity of the signal at δ -8.01
increases upon the addition of pure metallacycle 3(BF4)2,
Figure 1. Time-dependent 31P{1H} NMR spectra for reactions (a)
between complex 1X2 and 2-PyCO2H with signals for impurity marked
by ꢀ, (b) between complex 1X2 and 3-PyCO2H, (c) between complex 1X2
and 4-PyCO2H, and (d) between complex 1X2 and 6-Me-3-PyCO2H in
CD2Cl2 at ambient temperature.
whereas the intensity of the signal at δ -9.93 increases
immediately with a complete disappearance of the signal
at δ -8.01 after the addition of one drop of MeCN (ca.
0.031 mL) into the NMR tube. Noteworthily, the 1H
NOESY spectra obtained14 support that metallacycles 2-
(BF4)2, 3(BF4)2, and 4(BF4)4 can react with the added
MeCN molecules to form MeCN adducts. All the evidence
(14) A typical NOESY experiment was performed using a solution of
metallacycle (ca. 10-3M) and MeCN (1 drop, ca. 0.03 mL) in CD2Cl2
(0.5 mL). In the NOESY spectrum, the methyl group of the added MeCN
molecules shows a cross peak to the phenyl group of dppm in a metallacycle,
€
€
(12) Sheldrick, G. M. SHELXTL-97; University of Gottingen: Gottingen,
Germany.
(13) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
22þ, 32þ, or 44þ
.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9905
Scheme 2
indicate that the adducts are probably [Cu2(dppm)2( μ-
L)(MeCN)](BF4) (L = 2-PyCO2, 2u(BF4); 3-PyCO2, 3u-
(BF4); and 4-PyCO2, 4u(BF4)),15 which are also in equi-
librium with respective metallacycles (Scheme 2). Fur-
ther, IR spectra of the CD2Cl2 solution used to measure
the NOESY spectra contain two ν(CN) absorption
bands,17 one at 2291 cm-1 assigned to free MeCN mole-
cules and the other at 2276, 2277, and 2272 cm-1 assigned
to the coordinated MeCN molelcules in compounds 2u-
(BF4), 3u(BF4), and 4u(BF4), respectively. It is apparent
that a metallacycle is a thermodynamically stable product
from the reactions of complex 1X2 with 2-, 3-, and
4-PyCO2H in CH2Cl2.
Three crystal structures, [{Cu2(dppm)2}(μ-(2-PyCO2))]2-
(PF6)2 (2(PF6)2), [{Cu2(dppm)2}(μ-(3-PyCO2))]2 (PF6)2
(3(PF6)2), and [{Cu2(dppm)2}(μ-(4-PyCO2))]4(BF4)4 (4-
(BF4)4), were determined using X-ray diffraction. OR-
TEP plots for cations 22þ, 32þ, and 44þ are presented in
Figures 2, 3, and 4, respectively.
From the figures, it is evident that a dimerization
occurs for 2-PyCO2- and 3-PyCO2-, forming 8- and 12-
membered metallacycles, respectively, whereas a tetra-
merization occurs for 4-PyCO2-, forming a 28-membered
metallacycle. Two dimeric structures with diagonal dis-
Figure 2. (a) Cation fragment of [Cu2(dppm)2(2-PyCO2)]2(PF6)2
(2(PF6)2) with phenyl rings on phosphorus and hydrogen atoms removed
for clarity. (b) Secondary interactions with hydrogen bonds (shown as
dashed lines) and π-π stacking between two nearby phenyl planes, {N1,
C50, C52, C53, C54, C55} and {C25, C26, C27, C28, C29, C30}. (c) 1D
columnar packing of complex 22þ, viewed along the c axis with hydrogen
atoms omitted for clarity.
atom, Cu(2), forming a five-membered ring. O(1) also
serves as a one-atom bridge, coordinating with Cu(1).
˚
˚
tances of 4.711(10) A (see Figure 2a) and of 7.035(5) A
(see Figure 3a) between Cu(1) and Cu(10) are caused by
two bonding modes adopted by pyridylcarboxylate do-
nors, respectively. 2-PyCO2- use one oxygen atom, O(1),
and one nitrogen atom, N(1), to coordinate with a copper
-
This atom also links with the O(20) of another 2-PyCO2
.
Two carboxylate oxygen atoms, rather than the nitrogen
atom, of one 2-PyCO2- are hence involved in the skeleton
of the eight-membered metallacycle (Figure 2a). How-
ever, 3-PyCO2- uses two oxygen atoms, O(1) and O(2), to
(15) The crystal structures of compounds 2uþ, 3uþ, and 4uþ are believed
to be similar to that observed for the cation of [Cu2(dppm)2(μ-OAc)-
(MeCN)](PF6).16
(16) Shiu, K.-B.; Liu, S.-A.; Lee, G.-H. Unpublished Results.
(17) The detection of two ν(CN) absorption bands is reasonable given
that the time scale for IR is much shorter than that for NMR spectroscopy.18
coordinate with copper atoms, Cu(1) and Cu(2), respec-
tively. Cu(1) also links with N(10) of another 3-PyCO2
One of two carboxylate oxygen atoms and the nitrogen
atom of one 3-PyCO2- are thus involved in the skeleton of
the 12-membered metallacycle (Figure 3a). The bonding
-
.

9906 Inorganic Chemistry, Vol. 49, No. 21, 2010
Shiu et al.
Figure 3. (a) Cation fragment of [Cu2(dppm)2(3-PyCO2)]2(PF6)2
(3(PF6)2) with phenyl rings on phosphorus and hydrogen atoms removed
for clarity. (b) Secondary interactions with hydrogen bonds (shown as
dashed lines) and C-H π interaction between H(6A) and one nearby
3 3 3
phenyl plane, {C90, C100, C110, C120, C130, C140}.
modes for 2- and 3-PyCO2- were previously described in
12
the structures of [Cu2(dppm)2(η2-(2-PyCO2H)2](NO3)2
and [Cu2(dppm)2(μ-OAc)](BF4).19 Like the bonding
mode of 3-PyCO2-, 4-PyCO2 also uses two oxygen
-
atoms, O(1) and O(2), to coordinate with copper atoms,
Cu(1) and Cu(2), respectively. Cu(1) also links with the
N(2) of another 4-PyCO2-. One of two carboxylate
-
oxygen atoms and the nitrogen atom of one 4-PyCO2
are involved in the skeleton of the 28-membered metalla-
˚
cycle with diagonal distances of 13.119(9) A between
0
˚
Cu(1) and Cu(1 ) and 12.641(11) A between Cu(3) and
Cu(30) (Figure 4a). All of the copper atoms in structure
22þ are four-coordinate, whereas in structures 32þ and
44þ, one-half of the copper atoms are three-coordinate
and the other half are four-coordinate. The geometry
adopted by a four-coordinate copper(I) atom is close to
being tetrahedral, and that adopted by a three-coordinate
copper(I) atom is close to being trigonal planar. Both
geometries conform to simple structural models such as
the VSEPR theory, because a CuI atom has a spherical
(d10) electron core.20
Figure 4. (a) Cation fragment of [Cu2(dppm)2(4-PyCO2)]4(BF4)4
(4(BF4)4) with phenyl rings on phosphorus and hydrogen atoms
removed for clarity. (b) Secondary interactions with hydrogen bonds
(shown as dashed lines) π-π stacking between two nearby phenyl
planes, {N1, C5, C4, C2, C12, C7} and {C13, C20, C34, C33, C40,
The flexible {Cu2(dppm)2} unit of the starting complex,
[Cu2(dppm)2(NCMe)2]X2 (1X2), is believed to have a
8
˚
distance of around 3.4 A between two copper atoms.
C24}, and C-H π interaction between H(68A) and one nearby
3 3 3
phenyl plane, {C3, C22, C42, C27, C31, C94}. (c) 1D columnar packing
for complex 44þ, viewed along the a axis with hydrogen atoms omitted
for clarity.
(18) Muetterties, E. L. Inorg. Chem. 1965, 4, 769.
(19) Harvey, P. D.; Drouin, M.; Zhang, T. Inorg. Chem. 1997, 36, 4998.
(20) (a) Kaupp, M. Angew. Chem., Int. Ed. 2001, 40, 3534. (b) Gillespie,
R. J.; Robinson, E. A. Chem. Soc. Rev. 2005, 34, 396.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9907
˚
Table 2. Hydrogen Bonding Geometries (A, deg) Existing in Cyclic Structures
Scheme 3
structure
D-H
A
D-H
H
A
D
A
D-H
3 3 3
A
3 3 3
3 3 3
3 3 3
22þ
C12A-H12A O2a 0.95
2.46
2.57
2.47
2.34
2.56
2.67
2.59
2.50
3.37
2.84
2.78
2.94
2.83
3.20
2.85
3.09
159.8
96.7
98.9
120.7
97.6
116.6
96.4
121.8
3 3 3
3 3 3
C54A-H54A O2
0.95
0.95
0.95
0.93
0.93
0.93
0.93
32þ
44þ
C2-H2A O1
3 3 3
C2-H2A O1b
3 3 3
C4-H4A O1
3 3 3
C5-H5A O4
3 3 3
C55c-H55Ac O4
3 3 3
3 3 3
C56-H56A O1
a Symmetry transformation used to generate the equivalent atoms:
1 - x, 1 - y, 2 - z. b Symmetry transformation: -x, -y, -z. c Symmetry
transformation: 1 - x, 1 - y, -z.
Table 3. Phenyl-Phenyl Interactions Existing in Cyclic Structures
interaction central separation shortest contact angle
(deg)
˚
(A)
˚
(A)
structure
type
22þ π-π stacking
3.85
3.78
3.56
β = 10.94
θ = 157
β = 9.55
θ = 151
32þ C-H
π
3.04a
3.61
3 3 3
44þ π-π stacking
2.92a
they may work cooperatively to help the observed cycli-
zation in Scheme 1. The dimeric cycles for complex 22þ
form 1D columns along the a axis (Figure 2c), but each
C-H
π
3 3 3
a Separation of an H atom and π ring centroid; θ = angle of
C-H ring centroid in C-H π interaction; β = angle between
2
3 3 3
3 3 3
˚
column has a small rectangular cavity of 3.4 ꢀ 3.2 A ,
the ring normal and centroid-centroid vector in π-π stacking.
˚
˚
where 3.4 A is d(Cu(1) C(56)) and 3.2 A is d(Cu(1)
3 3 3
˚
C(560)) (Figure 2a). Although a similar cavity of 3.2 ꢀ
The nonbonded Cu Cu distance shortens to 3.133(1) A
3 3 3
3 3 3
in cyclic structure 22þ, 3.126(1) A in structure 32þ, and
˚
2
5.5 A is also found with 3.2 A = d(Cu(1) C(1)) and 5.5
˚
˚
3 3 3
2.968(2) A and 2.833(4) A in structure 44þ. These values
˚
˚
A = d(Cu(1) C(10)) (Figure 3a), the dimeric cycles for
˚
3 3 3
˚
complex 32þ do not form 1D columns. This complex has
are within the reported range, 2.71-3.76 A, for some
typical complexes containing one or more {Cu2(dppm)2}-
units.19,21 However, among the four distances found in
structures 22þ, 32þ, and 44þ, it is surprising to have the
nearly eclipsed dppm ligands with an average torsion
angle of 8°, sterically impeding the 1D columnar packing.
The dppm ligands for complex 22þ adopt a staggered
conformation with an average torsion angle of 30°.
Interestingly, for complex 44þ, one-half of the dppm
ligands adopt an eclipsed conformation with an average
torsion angle of 10°, and the other half adopt a staggered
˚
longest Cu Cu distance of 3.133(1) A, rather than the
3 3 3
shortest value, for the only structure, 22þ, containing a
one-atom bridge, O(1), linking the two Cu atoms of each
Cu2(dppm)2 unit. A careful check of the structural details
reveals the presence of secondary interactions, H-bond-
ing (Table 2) and phenyl-phenyl interactions (Table 3),
at different locations in the three structures. In structure
conformation with an average torsion angle of 28°. The
2
˚
˚
large cavity of 9.13 ꢀ 9.09 A with 9.13 A = d(Cu(1)
Cu(3)) and 5.5 A = d(Cu(1) Cu(30)) (Figure 4a) still
˚
3 3 3
3 3 3
22þ, four H-bonding interactions and two π π (i.e.,
allows the tetrameric cycles for complex 44þ to form 1D
columns along the c axis (Figure 4c).
3 3 3
face-to-face) stacking interactions were found outside the
dimeric metallacycle (Figure 2b). In structure 32þ, four
H-bonding interactions were found inside the dimeric
The solid-state structures indicate that cationic com-
plexes 22þ, 32þ, and 44þ have point-group symmetries of
C2h, C2h, and C4h, respectively, in solution. Hence, the
probable 31P-spin systems assigned for the complexes are
A2A02B2B02 for complex 22þ, A2A02B2B02 for complex
metallacycle and two C-H π (i.e., edge-to-face stack-
3 3 3
ing) interactions were found outside the dimeric metalla-
cycle (Figure 3b). In structure 44þ, eight H-bonding
interactions were found inside the tetrameric metalla-
32þ, and A2A02A00 A0002B2B02B00 B000 for complex 44þ
.
2
2
2
However, unexpectedly, the 31P{1H} NMR spectra for
the complexes (CD2Cl2, 202 MHz, 303 K) are very simple.
Only one singlet was observed. When the measurement
temperature was reduced to 203 K, only the signal for
complex 32þ was broadened to have a shoulder in the
upfield position. The signals for complexes 22þ and 44þ
were simply broadened. When the number of scans was
fixed at 100, the full width at half-maximum (FWHM)
increased from 85.6 to 136.2 Hz and from 21.1 to 28.3 Hz
for complex 22þ and complex 44þ, respectively. With the
crystal structures of [Cu2(dppm)2(μ-OAc)](BF4)19 and
[Cu2(dppm)2(6-Me-3-PyCO2)](BF4) (5(BF4)), described
below, it is proposed that a quick equilibrium between a
probable intermediate, [Cu2(dppm)2(LCO2)]X (L= 2-,
3-, and 4-Py), and a metallacycle occurs in solution
at ambient temperature, producing the observed singlet
cycle, two π π stacking interactions were found along
3 3 3
the tetrameric metallacycle, and two C-H π interac-
3 3 3
tions were found outside the tetrameric metallacycle
(Figure 4b). In the three structures, the H-bonding dis-
˚
tances of 2.34-2.67 A and angles of 96-160° (Table 2) are
indicative of weak hydrogen bonding interactions.22 The
strength and location of the secondary interactions are
believed to affect the Cu Cu distances. Importantly,
3 3 3
(21) (a) Wu, M.-M.; Zhang, L.-Y.; Qin, Y.-H.; Chen, Z.-N. Acta Crystal-
logr., Sect. E 2003, 59, m195. (b) Jiang, K.; Zhao, D.; Guo, L.-B.; Zhang, C.-J.;
Yang, R.-N. Chin. J. Chem. 2004, 22, 1297. (c) Guo, L.-B.; Zhao, D.; Zheng, X.;
Zhang, X.; Yang, R. Russ. J. Inorg. Chem. 2005, 50, 1681. (d) Angaridis, P.;
Cotton, F. A.; Petrukhina, M. A. Inorg. Chim. Acta 2001, 324, 318. (e) Chen,
Y.-D.; Zhang, L.-Y.; Chen, Z.-N. Chin. J. Struct. Chem. 2004, 23, 395. (f) Yam,
V. W.-W.; Lam, C.-H.; Cheung, K.-K. Chem. Commun. 2001, 545.
(22) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley:
New York, 2000; p 23.

9908 Inorganic Chemistry, Vol. 49, No. 21, 2010
Shiu et al.
of 3-PyCO2 in structure 32þ (Figure 3b), a simple re-
placement of the hydrogen atom with a methyl group may
destroy the favorable interaction by introducing steric
-
-
hindrance. Indeed, when 6-Me-3-PyCO2 was used to
react with[Cu2(dppm)2(MeCN)2](BF4)2 (1), the expected
oligomerization was inhibited, and the reaction produced
a dinuclear product, [Cu2(dppm)2(6-Me-3-PyCO2)](BF4)
(5(BF4)) (Figure 1d). The crystal structure of this complex
was also determined by X-ray diffraction (Figure 5). The
˚
Cu-O distances of 2.029(4) and 2.015(4) A are similar to
˚
those of 2.029(18) and 1.993(15) A in [Cu2(dppm)2( μ-
OAC)](BF4).19 Likewise, the Cu Cu distance of
3 3 3
˚
2.869(5) A in 5(BF4) is also similar to that of 2.789(11)
˚
A in [Cu2(dppm)2(μ-OAC)](BF4).
Conclusion
The self-assemby of supramolecular metallacycles via the
coordination-driven directional bonding approach can be
modified to produce some unexpected structural variations
via the employment of a flexible ligand-capped di- or multi-
nuclear transition-metal acceptor like complex 1X2 and a
mixed-type donor like pyridylcarboxylate. However, the
modified self-assembly is still subject to steric hindrance.
The reaction of complex 1(BF4)2 with 6-Me-3-PyCO2H did
not produce a polygonal dimeric metallacycle but a dinuclear
complex, [Cu2(dppm)2(6-Me-3-PyCO2)](BF4) (5(BF4)).
Figure 5. Cation fragment of [Cu2(dppm)2(6-Me-3-PyCO2)](BF4)
(5(BF4)) with phenyl rings on phosphorus and hydrogen atoms removed
for clarity.
signals for the complexes in the 31P{1H} NMR spectra
(Scheme 3). With L = 2-Py, the intermediate may rear-
range to form another intermediate, containing a (N,O)-
linked geometry similar to that observed for [Cu2-
(dppm)2(η2-(2-PyCO2H)2](NO3)2.11 Thus, the cyclization
probably starts with the replacement of MeCN of com-
Acknowledgment. This work was financially supported
by the National Science Council of the Republic of China
(under grant NSC95-2113-M-006-012-MY3).
-
plex 1X2 with LCO2 to form one or two intermediates
depending on the identity of L, which then cyclize to form
metallacycles 22þ, 32þ, and 44þ
.
Synthesis, Solution Behavior, and Structure of Uncy-
clized Dicopper Complex 5(BF4). Since one of the favor-
able secondary interactions involves a hydrogen atom,
H(6A), at a ring position next to the nitrogen atom, N(1),
Supporting Information Available: X-ray crystallographic
data, in CIF format, and three 1H NOESY spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.