Three unique coordination geometries involving 1,2-dimethoxy-4,5-bis(2-pyridylethynyl)benzene

Article abstract of DOI:10.1039/b109849g

Reaction of the new ligand 1,2-dimethoxy-4,5-bis(2-pyridylethynyl)benzene with different metal centers under similar reaction conditions led to three distinct structure formation processes: Molecular ring closure, dimerization, and polymer formation.

Full text of DOI:10.1039/b109849g

Three unique coordination geometries involving
1,2-dimethoxy-4,5-bis(2-pyridylethynyl)benzene
Jeffrey E. Fiscus,a Sandra Shotwell,a Ralph C. Layland,b Mark D. Smith,a Hans-Conrad zur
Loye*a and Uwe H. F. Bunz*a
a Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South
Carolina 29208, USA. E-mail: zurloye@sc.edu; bunz@mail.chem.sc.edu
b Division of Biological and Physical Sciences, Lander University, Greenwood, South Carolina, USA
Received (in Columbia, MO, USA) 14th September 2001, Accepted 30th October 2001
First published as an Advance Article on the web 3rd December 2001
Reaction of the new ligand 1,2-dimethoxy-4,5-bis(2-py-
ridylethynyl)benzene with different metal centers under
similar reaction conditions led to three distinct structure
formation processes: molecular ring closure, dimerization,
and polymer formation.
In synthetic, covalent organic chemistry the notion of con-
formers, oligomers and polymers are distinctly separated and
irreconcilable. In supramolecular synthesis on the other hand
this conception is less stringent. The utilized building blocks
and the variety of binding forces that hold supramolecular
assemblies together by arranging organic modules in the solid
state makes the border of the very notions of conformers, cycles,
and polymers more permeable. Here we wish to demonstrate
that one organic module 1,2-dimethoxy-4,5-bis(2-pyridylethy-
nyl)benzene 11 can form a supramolecular cycle, a dimer and
Fig. 1 A single molecule of 2. The hydrogen atoms and methanol group
have been omitted for clarity.
polymer utilizing different inorganic connectors Cu(OAc)2,
CoCl2 and [Rh(OAc)2]2.
Single crystals suitable for X-ray diffraction† of Cu(1)-
(OAc)2·CH3OH 2 were obtained by layering a methanol
solution (1 mL) of Cu(OAc)2·H2O (2.0 mg, 0.01 mmol) over a
dichloromethane solution (1 mL) of 1 (6.7 mg, 0.02 mmol), with
a layer of pure methanol separating them (20% yield). Crystals
of [Co(1)Cl2]2
3 were prepared similarly, substituting
CoCl2·6H2O for Cu(OAc)2·H2O and ethanol for methanol (68%
yield).‡ Crystals of catena-poly{[Rh(OAc)2]21·CH2Cl2} 4
were prepared similarly to 2 except that [Rh(OAc)2]2 was
substituted for Cu(OAc)2·H2O (maintaining the ligand to metal
ratio) and ethanol was substituted for methanol (40% yield).
These three systems each form a distinctly different structure
and between them, demonstrate the importance of the free
rotation of the pyridyl rings around the carbon–carbon bonds for
facilitating the formation of the three structure types.
Compound 2 demonstrates the preference of Cu2+ for square
planar coordination with copper positioned snugly between the
two pyridyl rings. The resulting N–Cu–N (Cu–N 2.01 Å, N–
Cu–N 172.98°) bonds close an eleven membered, triangular
ring (Fig. 1), which is nearly identical to the structures reported
by Bosch and Barnes for 1,2-bis(2-pyridylethynyl)benzene.2
The square planar coordination of copper is completed by two
trans oxygens from separate acetate groups.
Compound 3 contains Co2+, which is found in typical
tetrahedral coordination.3 This tetrahedral preference, appar-
ently, cannot be conveniently satisfied through a ring closure
that would require a rotation of the pyridyl rings away from the
180° angle found in the copper complex. The result of such a
rotation would be an elongated Co–N bond. Instead, the pyridyl
rings rotate away from each other by 128° and bind to separate
cobalt atoms with bond lengths of 2.03 Å, which is typical for
Co–N bonds. The tetrahedral coordination in each case is
completed by two chlorine atoms. The overall structure (Fig. 2)
consists of two molecules of 1 bridging two cobalt atoms. An
inversion center is located in the middle of the dimer.
Fig. 2 Hydrogen atoms have been omitted for clarity. (a) a view of 3 from
above showing the tetrahedral coordination of cobalt; (b) a view of 3 from
the side showing the opposing orientations of 1 and the rotation of the
pyridyl rings.
The polymeric structure of 4 (Fig. 3) results from the linear
coordination preference of [Rh(OAc)2]2. The structure consists
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Chem. Commun., 2001, 2674–2675
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b109849g

Fig. 3 A view of 4 showing the connectivity of 1 and the rhodium dimer. Hydrogens have been omitted for clarity.
of two acetate-bridged rhodium atoms that connect to the
provided by the NSF Instrumentation for Materials Research
pyridyl rings of two separate molecules of 1 and mimics a
conjugated organic polymer. The rhodium is six-coordinate
with four equatorial oxygens from the acetate groups, one axial
rhodium from the other half of the dimer and one nitrogen
belonging to the pyridyl group on the ligand.
Program through Grant DMR:9975623.
Notes and references
† Synthesis of the ligand 1: under nitrogen, 1,2-dimethoxy-4,5-diiodo-
benzene (2.00 g, 5.28 mmol),12 2-ethynylpyridine (1.09 g, 10.6 mmol),13
(Ph3P)2PdCl2 (50 mg, 71 mmol), CuI (50 mg, 263 mmol) and piperidine (15
mL) are placed in a Schlenk flask and stirred for 24 h at ambient
temperature. Aqueous workup followed by chromatography with EtOAc–
hexanes (1:9) furnishes the ligand in 22% yield (395 mg) as a colorless
powder (mp 154 °C). Spectroscopic data: IR, n/cm21 2200 (m), 1590 (w),
1570 (s), 1550 (w), 1510 (s), 1460 (m), 1450 (m), 1435 (w), 1425 (w) 1415
(w), 1360 (s) . 1H NMR (CD3CN), d 3.90 (s, 6H, OCH3), 7.24 (s, 2H, aryl-
H), 7.38 (ddd, 2H, pyridyl-H), 7.77 (dt, 2H, pyridyl-H), 7.82 (td, 2H,
pyridyl-H), 8.65 (d, 2H, pyridyl-H). 13C NMR (CDCl3), d 150.28, 149.812,
143.83, 136.38, 127.54, 122.92, 118.65, 114.67, 91.94, 88. 33, 56.34. MS
EI, m/z 340 (100%, M+), 341 (25%, M+), 342 (5%, M+), 325 (M+ 2 CH3,
15%,) 309 (M+ 2 OCH3, 4%), 263 (M+ 2 pyridine, 97%).
Tetrakis(carboxylato)rhodium compounds were first dis-
covered in 1960,4 but it was not until 1981 that the first
polymeric species containing such a rhodium dimer was
synthesized.5 A survey of the CSD indicates that 1 is the largest
ligand yet used in such a polymeric species. The polymeric
structure is charge balanced, eliminating the need for counter
ions competing for binding sites. This leads to higher site
symmetry and makes such rhodium dimers attractive building
blocks for coordination polymers.6 Because of its length, the
dirhodium moiety cannot fit between the pyridyl rings and
achieve ring closure, as in 2, despite the favorable linear
arrangement of the binding sites. Consequently, the pyridyl
rings rotate outward by 180° to form a polymer chain with the
rhodium dimer bridging adjacent ligands in a zig-zag fashion.
This polymer is a supramolecular analogue of the hitherto
unknown ortho-PPE7 and as such is a fascinating structure. The
Rh–N distance within the polymer is 2.25 Å, typical for Rh–N
bonds in such systems.5,8–10
One interesting aspect of these structures are the carbon–
carbon separations between the alkyne groups on the ligand 1
which, if close enough, could potentially be crosslinked in a
Bergman reaction.11 In 2, the closed ring conformation results
in a C6…C17 separation of 4.089 Å. This is almost the same as
the C6…C6 distance of 4.100 Å in 4, which should represent an
unstrained system. By comparison, in 3, the C6…C17 distance
within the same molecule of 1 is 3.886 Å, while the alkyl groups
on separate ligands are only 3.548 Å apart (within the range for
p–p interactions). This suggests that the copper cation fits
between the pyridyl rings without inducing any strain, while in
3, the dimer formation strains the ligand, bending the pyridyl
ligands towards one another, which effects a shorter C6…C17
distance.
¯
‡ Crystal data for 2: C27H26CuN2O7, M = 554.04, triclinic, space group P1,
a = 8.2357(4), b = 12.5088(6), c = 13.3128(6) Å, a = 78.3000(10)°, b =
72.1170(10)°, g = 76.6810(10)°, U = 1257.12(10) Å3, T = 293(2) K, Z =
2, l = 0.71073 Å, 11656 reflections measured, 5144 unique (Rint = 0.0199)
which were used in all calculations. R1 = 0.0374 and wR2 = 0.0922. For
3: C44H32Cl4Co2N4O4, M = 940.40, monoclinic, space group P21/n, a =
8.5272(6), b = 18.3653(13), c = 13.3493(9) Å, b = 103.574(2)°, U =
2032.2(2) Å3, T = 190(2) K, Z = 2, l = 0.71073 Å, 13541 reflections
measured, 4156 unique (Rint = 0.0347) which were used in all calculations.
R1 = 0.0504 and wR2 = 0.1107. For 4: C31H30Cl2N2O10Rh2, M = 867.29,
monoclinic, space group C2/c, a = 21.5985(13), b = 20.2480(12), c =
8.0207(5) Å, b = 103.2650(10)°, U = 3414.1(4) Å3, T = 190(2) K, Z =
4, l = 0.71073 Å, 15208 reflections measured, 3504 unique (Rint = 0.0545)
which were used in all calculations. R1 = 0.0613 and wR2 = 0.1222.
CCDC reference numbers 173558–173560. See http://www.rsc.org/supp-
data/cc/b1/b109849g/ for crystallographic data in CIF or other electronic
format.
1 S. Shotwell and U. H. F. Bunz, manuscript in preparation.
2 E. Bosch and C. L. Barnes, Inorg. Chem., 2001, 40, 3097.
3 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Reed
Educational and Professional Publishing, Oxford, 2nd edn., p. 1129.
4 I. I. Chernyaev, E. V. Shenderetskaya and A. A. Karyaguba, Russ. J.
Inorg. Chem., 1960, 5, 559.
These three structures demonstrate the diversity, which can
be achieved using 1 that is made possible by the ability of the
ligand to distort itself to the preferred coordination environment
of the metal center. While one may expect a slight bending of
the pyridylethynyl legs either towards or away from each other,
these three structures show that rotation of the pyridyl ring
around the ethynyl linkage seems more favorable.
Financial support was provided in part by the National
Science Foundation through Grants DMR:9873570 and
CHE:9814118 and in part by the South Carolina Commission
on Higher Education through Grant CHE:R00-U25. The Bruker
CCD Single Crystal Diffractometer was purchased using funds
5 F. A. Cotton and T. R. Felthouse, Inorg. Chem., 1981, 20, 600.
6 F. A. Cotton, C. Lin and C. A. Murillo, Acc. Chem. Res., 2001, in
press.
7 R. H. Grubbs and D. Kratz, Chem. Ber., 1993, 126, 149.
8 T. Niu, J. Lu, G. Crisci and A. J. Jacobson, Polyhedron, 1998, 17,
4079.
9 H. Kitamura, T. Ozawa, K. Jitsukawa, H. Masuda, Y. Aoyama and H.
Einaga, Inorg. Chem., 2000, 39, 3294.
10 F. A. Cotton and T. R. Felthouse, Inorg. Chem., 1981, 20, 584.
11 R. G. Bergman, Acc. Chem. Res., 1973, 6, 25; T. Chandra, J. C. Huffman
and J. M. Zaleski, Inorg. Chem. Commun., 2001, 4, 434.
12 J. D. Kinder and W. J. Youngs, Organometallics, 1996, 15, 460.
13 D. E. Ames, D. Bull and C. Takundwa, Synthesis, 1981, 364.
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