Heteroleptic Copper Dipyrromethene Complexes: Synthesis, Structure, and Coordination Polymers

Article abstract of DOI:10.1021/ic0352295

The synthesis of neutral [Cu(dpm)₂] and [Cu(dpm)(acac)] (dpm = dipyrromethene, acac = acetylacetonato) complexes is presented. The formation of the asymmetric metal complexes was monitored by electronic absorption and infrared spectroscopy. Two of the complexes investigated, containing pyrdpm ligands (pyrdpm = pyridyldipyrromethene), form 1-dimensional coordination polymers. The coordination polymers formed by these complexes have been characterized by single-crystal X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The complexes possess square pyramidal coordination geometries with the apical position occupied by the mesopyridyl donor of a neighboring complex in the crystal lattice. The features of these coordination complexes that facilitate formation of extended solids have been probed. Symmetric [Cu(pyrdpm)₂] complexes are unable to form coordination solids due to steric hindrance at the metal center. Use of cyano donors in complexes such as [Cu(cydpm)(acac)] (cydpm = cyanodipyrromethene) in lieu of pyridyl donors also fail to form network solids. Through these systematic studies, both the basic coordination chemistry of these complexes and the fundamental design requirements for synthesizing this novel class of coordination polymers have been defined.

Full text of DOI:10.1021/ic0352295

Inorg. Chem. 2004, 43, 1242−1249
Heteroleptic Copper Dipyrromethene Complexes: Synthesis, Structure,
and Coordination Polymers
,†
Sara R. Halper,† Mitchell R. Malachowski,‡ Heather M. Delaney,† and Seth M. Cohen*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093-0358, and Department of Chemistry, UniVersity of San Diego,
San Diego, California 92110-2492
Received October 24, 2003
The synthesis of neutral [Cu(dpm)2] and [Cu(dpm)(acac)] (dpm ) dipyrromethene, acac ) acetylacetonato) complexes
is presented. The formation of the asymmetric metal complexes was monitored by electronic absorption and infrared
spectroscopy. Two of the complexes investigated, containing pyrdpm ligands (pyrdpm ) pyridyldipyrromethene),
form 1-dimensional coordination polymers. The coordination polymers formed by these complexes have been
characterized by single-crystal X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis.
The complexes possess square pyramidal coordination geometries with the apical position occupied by the meso-
pyridyl donor of a neighboring complex in the crystal lattice. The features of these coordination complexes that
facilitate formation of extended solids have been probed. Symmetric [Cu(pyrdpm)2] complexes are unable to form
coordination solids due to steric hindrance at the metal center. Use of cyano donors in complexes such as
[Cu(cydpm)(acac)] (cydpm ) cyanodipyrromethene) in lieu of pyridyl donors also fail to form network solids. Through
these systematic studies, both the basic coordination chemistry of these complexes and the fundamental design
requirements for synthesizing this novel class of coordination polymers have been defined.
Introduction
examples exist that use multidentate, asymmetric ligand
systems.11-14 Among the many systems studied, a number
of coordination solids containing copper ions have been
prepared, frequently with the use of rigid, multidentate poly-
(pyridyl) ligands as the organic spacer group. Among these
copper-containing compounds, several utilize [Cu(hfacac)2]
or [Cu(tfacac)2] (hfacac ) hexafluoroacetylacetonato, tfacac
) trifluoroacetylacetonato) as a building block.15-18 In these
Coordination polymers have rapidly developed into an
active area of chemical research as a route to materials with
useful electronic, magnetic, and zeolitic properties. To this
end, a wide variety of coordination solids that form 1-, 2-,
and 3-dimensional networks have been prepared and
studied.1-5 Attempts to control the dimensionality and
topology of these materials have focused on judicious
selection of multidentate ligands with metal ions of the
appropriate coordination geometry. Typically these structures
consist of rigid, symmetric ligands complexed to isolated
metal centers or metal clusters.6-10 Significantly fewer
(7) Hamilton, B. H.; Kelly, K. A.; Wagler, T. A.; Espe, M. P.; Ziegler,
C. J. Inorg. Chem. 2002, 41, 4984-4986.
(8) Na¨ther, C.; Greve, J.; Jeâ, I. Chem. Mater. 2002, 14, 4536-4542.
(9) Bernhard, S.; Goldsmith, J. I.; Takada, K.; Abrun˜a, H. D. Inorg. Chem.
2003, 42, 4389-4393.
(10) Shin, D. M.; Lee, I. S.; Chung, Y. K.; Lah, M. S. Inorg. Chem. 2003,
42, 5459-5461.
* To whom correspondence should be addressed. E-mail: scohen@
ucsd.edu. Telephone: (858) 822-5596. Fax: (858) 822-5598.
† University of California.
(11) Saalfrank, R. W.; Maid, H.; Hampel, F.; Peters, K. Eur. J. Inorg. Chem.
1999, 1859-1867.
‡ University of San Diego.
(12) Turner, S. S.; Collison, D.; Mabbs, F. E.; Halliwell, M. J. Chem. Soc.,
Dalton Trans. 1997, 1117-1118.
(1) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.;
O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330.
(2) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629-1658.
(3) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc.
Chem. Res. 1998, 31, 474-484.
(4) Tulsky, E. G.; Long, J. R. Chem. Mater. 2001, 13, 1149-1166.
(5) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511-522.
(6) Hamilton, B. H.; Kelly, K. A.; Malasi, W.; Ziegler, C. J. Inorg. Chem.
2003, 42, 3067-3073.
(13) Chen, B.; Fronczek, F. R.; Maverick, A. W. Chem. Commun. 2003,
2166-2167.
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11250.
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Chem. 1999, 38, 5027-5033.
(16) Lee, B.-W.; Twamley, B.; Shreeve, J. M. J. Fluorine Chem. 2001,
108, 111-116.
1242 Inorganic Chemistry, Vol. 43, No. 4, 2004
10.1021/ic0352295 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/21/2004

Heteroleptic Copper Dipyrromethene Complexes
Chart 1. Multidentate meso-Substituted Dipyrromethene Ligands Used
in This Study
extended solids, the acac-derived ligand makes up the square
plane of the copper(II) coordination environment and mul-
tidentate nitrogen-based donor ligands link the complexes
by binding to the open axial sites on the copper center. An
unusual example of an asymmetric ligand system in these
types of structures is the use of combined acetylacetonate-
pyridine ligands that result in self-complementary complexes
that form square-grid and 3-dimensional interpenetrated
topologies.12,13
Figure 1. Structure of a dipyrrin ligand (pyrdpm ) pyridyldipyrromethene)
and some plausible schemes for the formation of homo- and heterometallic
coordination solids. Heterometallic (top) and homometallic 1-dimensional
(bottom) coordination polymers may both be accessible.
In the devising of building blocks for coordination
polymers that utilize copper or other transition metal ions,
meso-substituted dipyrromethene ligands have a rigid, easily
derivatized structure that make them attractive targets for
use in the synthesis of extended solids (Chart 1). Dipyr-
romethenes (dipyrrins) are monoanionic ligands that form
stable complexes with a variety of transition metal ions.19-21
Several discrete, neutral supramolecular structures with R-
or â-linked dipyrrins have been described, which were found
to be easily purified by conventional flash chromatogra-
phy.22,23 Synthetically, a wide range of functionalized ary-
laldehydes can be used to prepare meso-substituted dipyrrin
ligands with two distinct binding sites and a strictly defined
binding geometry.19,20 The spacing of these functional groups
can be systemically varied by using rigid aromatic or
phenylalkyl extensions, as has been previously described.24
Such compounds should allow for the synthesis of metal-
containing building blocks with 2-fold or 3-fold symmetry25,26
for further elaboration as homo- or heterometallic coordina-
tion solids.2,27-29 Furthermore, asymmetrically substituted
complexes can also potentially form coordination solids of
lower dimensionality (Figure 1).
As part of an effort to prepare heterometallic coordination
polymers based on multidentate dipyrrin ligands, several
meso-pyridyldipyrromethene (pyrdpm) ligands have been
synthesized and characterized. In the process of using
[Cu(acac)2] (acac ) acetylacetonato) as the metal source to
make copper(II) complexes of these ligands, it was found
that both the [Cu(pyrdpm)2] and asymmetric [Cu(pyrdpm)-
(acac)] complexes were stable compounds. In addition, two
of the [Cu(pyrdpm)(acac)] complexes were found to form
1-dimensional coordination polymers with the metal centers
connected by distal meso-pyridyl nitrogen atom ligation.
Several other dipyrrin compounds were synthesized to
explore the coordination chemistry of these ligands and to
determine what features of the dipyrromethene ligand
promote the formation of extended solids. All of the
complexes have been examined by electronic absorption and
infrared spectroscopy, and the solid-state structures of each
compound have been determined. Two novel coordination
polymers have been synthesized, and a number of other
complexes reveal the properties of the ligand and metal
geometry that facilitate formation of these unusual polymeric
structures.
(17) Bunn, A. G.; Carroll, P. J.; Wayland, B. B. Inorg. Chem. 1992, 31,
1297-1299.
Experimental Section
(18) Sano, Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.; Rabu,
P.; Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246-8252.
(19) Bru¨ckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim. Acta
1997, 263, 279-286.
General Methods. Unless otherwise noted, starting materials
were obtained from commercial suppliers and used without further
purification. The dipyrromethane precursors for the ligands 4-pyrdpm,
3-pyrdpm, 2-pyrdpm, and 4-cydpm were synthesized according to
literature procedures.30,31 Elemental analysis data were obtained
through NuMega Resonance Labs, Inc. (San Diego, CA). Infrared
spectra were collected on a Nicolet AVATAR 320 FT-IR instrument
at the Department of Chemistry and Biochemistry, University of
California at San Diego. UV-visible spectra were recorded in
CH2Cl2 using a Hewlett-Packard 4582A spectrophotometer under
PC control using the ChemStation software suite.
(20) Bru¨ckner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can. J. Chem.
1996, 74, 2182-2193.
(21) Cohen, S. M.; Halper, S. R. Inorg. Chim. Acta 2002, 341, 12-16.
(22) Zhang, Y.; Thompson, A.; Rettig, S. J.; Dolphin, D. J. Am. Chem.
Soc. 1998, 120, 13537-13538.
(23) Thompson, A.; Rettig, S. J.; Dolphin, D. Chem. Commun. 1999, 631-
632.
(24) Halper, S. R.; Cohen, S. M. Chem.sEur. J. 2003, 9, 4661-4669.
(25) Choe, W.; Kiang, Y.-H.; Xu, Z.; Lee, S. Chem. Mater. 1999, 11, 1776-
1783.
(26) Kiang, Y.-H.; Gardner, G. B.; Lee, S.; Xu, Z.; Lobkovsky, E. B. J.
Am. Chem. Soc. 1999, 121, 8204-8215.
[Cu(4-pyrdpm)2]. 5-(4-Pyridyl)dipyrromethane31 (0.30 g, 1.34
mmol) was dissolved in 150 mL of CHCl3, and the solution was
(27) Dong, Y.-B.; Smith, M. D.; zur Loye, H.-C. Inorg. Chem. 2000, 39,
1943-1949.
(28) Shorrock, C. J.; Jong, H.; Batchelor, R. J.; Leznoff, D. B. Inorg. Chem.
2003, 42, 3917-3924.
(30) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323-7344.
(31) Gryko, D.; Lindsey, J. S. J. Org. Chem. 2000, 65, 2249-2252.
(29) Leznoff, D. B.; Xu, B.-Y.; Batchelor, R. J.; Einstein, F. W. B.; Patrick,
B. O. Inorg. Chem. 2001, 40, 6026-6034.
Inorganic Chemistry, Vol. 43, No. 4, 2004 1243

Halper et al.
stirred in an ice bath. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(0.31 g, 1.34 mmol) was dissolved in 100 mL of benzene, and the
solution was added slowly dropwise. After addition, the solvent
was evaporated to half the volume and [CuCl2‚2H2O] (0.092 g,
0.54 mmol) dissolved in 50 mL of MeOH was added. The mixture
was stirred for 10 min to form the copper complex. The solution
was evaporated to dryness, and the product was purified by column
chromatography (SiO2; CHCl3 with 1.0% MeOH) to afford a
dichroic red/green film. Yield: 25%. FAB-MS: m/z 504.5, [M +
H]+. Anal. Calcd for C28H20N6Cu: C, 66.72; H, 4.00; N, 16.67.
Found: C, 66.35; H, 3.70; N, 16.45. λmax ) 230, 302, 378, 470,
502 nm. IR (film from CH2Cl2): ν 995, 1022, 1037, 1244, 1335,
X-ray Crystallographic Analysis. Single crystals of each
compound suitable for X-ray diffraction structural determination
were mounted on quartz capillaries with Paratone oil and were
cooled in a nitrogen stream on the diffractometer. Data were
collected on either a Bruker AXS or a Bruker P4 diffractometer
each equipped with area detectors. Peak integrations were performed
with the Siemens SAINT software package. Absorption corrections
were applied using the program SADABS. Space group determina-
tions were performed by the program XPREP. The structures were
solved by either Patterson or direct methods and refined with the
SHELXTL software package.32 All hydrogen atoms were fixed at
calculated positions with isotropic thermal parameters, and all non-
hydrogen atoms were refined anisotropically unless otherwise noted.
Differential Scanning Calorimetry and Thermogravimetric
Analysis. Differential scanning calorimetry (DSC) experiments were
performed on a Shimadzu DSC-50 instrument in a helium atmo-
sphere with a flow rate of 30 cm3/min. Crystalline samples were
heated from ∼25 to 500 °C at a rate of 10 °C/min. Thermogravi-
metric analysis (TGA) experiments were performed on a Shimadzu
TGA-50 with a flow rate of 40 cm3/min of nitrogen. Crystalline
samples were heated from ∼25 to 800 °C at a rate of 4 °C/min.
1376, 1404, 1536, 2924, 3094 cm-1
.
[Cu(3-pyrdpm)2]. The same procedure was used as in the
synthesis of [Cu(4-pyrdpm)2] starting from 5-(3-pyridyl)dipyr-
romethane.31 Yield: 16%. FAB-MS: m/z 504.5, [M + H]+. Anal.
Calcd for C28H20N6Cu 0.5H2O: C, 64.42; H, 4.25; N, 16.10.
Found: C, 64.50; H, 4.11; N, 15.90. λmax ) 232, 318, 372, 468,
502 nm. IR (film from CH2Cl2): ν 995, 1023, 1037, 1244, 1335,
1376, 1405, 1547, 2925, 3094 cm-1
.
[Cu(4-cydpm)2]. The same procedure was used as in the
synthesis of [Cu(4-pyrdpm)2] starting from 5-(4-cyanophenyl)-
dipyrromethane.30 Yield: 51%. MALDI-TOF-MS: m/z 551.2,
[M]+. Anal. Calcd for C32H20N6Cu‚1.5H2O: C, 66.37; H, 4.00; N,
14.51. Found: C, 66.66; H, 4.01; N, 14.69. λmax ) 236, 306, 472,
498 nm. IR (film from CH2Cl2): ν 994, 1028, 1247, 1337, 1383,
Results
The syntheses of the dipyrromethane precursors (1 and 2;
Scheme 1) were performed according to literature proce-
dures.30,31 The dipyrromethanes were oxidized to the corre-
sponding dipyrrins by reaction with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) in a chloroform/benzene solution.
The dipyrrin ligands were not isolated but were used in situ
to generate the desired metal complexes.19-21,24 To form
heteroleptic [Cu(dpm)(acac)] complexes, the oxidation reac-
tion mixtures containing the dipyrrin ligand were combined
with ∼1 molar equiv (based on the quantity of dipyr-
romethane precursor) of [Cu(acac)2]. The desired complexes
were formed at room temperature within 30 min, after which
the products were purified by silica column chromatography
as bright orange-red compounds that generally appeared
lustrous green upon removal of solvent. The solids were
readily dissolved in a variety of organic solvents such as
benzene, chloroform, methylene chloride, and acetone. The
heteroleptic [Cu(dpm)(acac)] complexes were found to be
stable in solution and in the presence of silica gel. However,
when exposed to basic alumina, the complexes were found
to rapidly decompose to the homoleptic [Cu(dpm)2] com-
plexes. To prepare the homoleptic [Cu(dpm)2] complexes,
the same general procedure was used, but only ∼0.4 equiv
(based on dipyrromethane precursor) of [Cu(acac)2] or
[CuCl2‚2H2O] was combined with the oxidized dipyrrin
mixture. Again, column chromatography was used to isolate
the complexes in modest yields. All of the metal complexes
were characterized by mass spectrometry, elemental analysis,
infrared spectroscopy, and absorption spectroscopy.
1406, 1561, 1592, 2225, 2916 cm-1
.
[Cu(4-pyrdpm)(acac)]. 5-(4-Pyridyl)dipyrromethane31 (0.50 g,
2.24 mmol) was dissolved in 150 mL of CHCl3 and the solution
was stirred in an ice bath. 2,3-Dichloro-5,6-dicyano-1,4-benzo-
quinone (0.51 g, 2.24 mmol) was dissolved in 100 mL of benzene,
and the solution was added slowly dropwise. [Cu(acac)2] (0.59 g,
2.24 mmol) was dissolved in 20 mL of CHCl3 and the solution
was added to the reaction mixture, which was stirred for 10 min to
form the copper complex. The solution was evaporated to dryness,
and the product was purified by column chromatography (SiO2;
CHCl3 with 0.5% MeOH) to afford brown/red crystals. Yield: 40%.
FAB-MS: m/z 383.4, [M + H]+, 283.2, [M - acac]+. Anal. Calcd
for C19H17N3O2Cu: C, 59.60; H, 4.48; N, 10.97. Found: C, 59.68;
H, 4.68; N, 11.04. λmax ) 230, 300, 364, 496 nm. IR (film from
CH2Cl2): ν 993, 1025, 1245, 1338, 1387, 1520, 1557, 1592, 2920,
3045, 3093 cm-1
.
[Cu(3-pyrdpm)(acac)]. The same procedure was used as in the
synthesis of [Cu(4-pyrdpm)(acac)] starting from 5-(3-pyridyl)-
dipyrromethane.31 Yield: 58%. FAB-MS: m/z 383.4, [M + H]+.
Anal. Calcd for C19H17N3O2Cu‚0.5CH2Cl2: C, 55.06; H, 4.27; N,
9.88. Found: C, 55.01; H, 4.67; N, 9.94. λmax ) 232, 302, 362,
496 nm. IR (film from CH2Cl2): ν 993, 1026, 1244, 1337, 1378,
1520, 1549, 1591, 2919, 3094 cm-1
.
[Cu(2-pyrdpm)(acac)]. The same procedure was used as in the
synthesis of [Cu(4-pyrdpm)(acac)] starting from 5-(2-pyridyl)-
dipyrromethane.31 Yield: 17%. ESI-MS: m/z 383.1, [M + H]+.
Anal. Calcd for C19H17N3O2Cu‚0.5CH2Cl2: C, 55.06; H, 4.27; N,
9.88. Found: C, 55.32; H, 4.34; N, 9.95. λmax ) 230, 300, 366,
498 nm. IR (film from CH2Cl2): ν 996, 1028, 1246, 1337, 1384,
1520, 1555, 1586, 2854 cm-1
.
Formation of the desired hetero- or homoleptic complex
was confirmed by examination of the absorption and infrared
spectra of the compounds. Indeed, the UV-visible spectra
of the crude oxidation/complexation mixtures could be used
to follow the progress of the reaction. Figure 2 shows the
[Cu(4-cydpm)(acac)]. The same procedure was used as in the
synthesis of [Cu(4-pyrdpm)(acac)] starting from 5-(4-cyanophenyl)-
dipyrromethane.30 Yield: 41%. FAB-MS: m/z 406.3, [M + H]+.
Anal. Calcd for C21H17N3O2Cu: C, 61.98; H, 4.21; N, 10.33.
Found: C, 62.34; H, 4.51; N, 10.73. λmax ) 236, 306, 496, 506
nm. IR (film from CH2Cl2): ν 996, 1028, 1245, 1336, 1403, 1555,
1588, 2229, 2340, 2360.
(32) Sheldrick, G. M. In SHELXTL Vers. 5.1 Software Reference Manual;
Brucker AXS: Madison, WI, 1997.
1244 Inorganic Chemistry, Vol. 43, No. 4, 2004

Heteroleptic Copper Dipyrromethene Complexes
Scheme 1. General Approach for Preparing dpm Ligands, [Cu(dpm)(acac)] Complexes, and [Cu(dpm)2] Complexes
cm-1). The distinct spectroscopic features of the symmetric
and asymmetric complexes allow for facile evaluation of the
reaction products.
The structures of [Cu(4-pyrdpm)2], [Cu(4-cydpm)2], [Cu-
(4-pyrdpm)(acac)], [Cu(3-pyrdpm)(acac)], [Cu(2-pyrdpm)-
(acac)], and [Cu(4-cydpm)(acac)] were determined by single-
crystal X-ray diffraction studies. The complexes [Cu(4-
pyrdpm)2] and [Cu(4-cydpm)2] show 4-coordinate, distorted
square planar metal environments with average Cu-N bond
distances of ∼1.95 Å (Figure 3). The twist angle between
Figure 2. Electronic absorption spectra for [Cu(4-pyrdpm)(acac)] (solid,
s), [Cu(4-pyrdpm)2] (dotted, ‚‚‚), [Cu(3-pyrdpm)(acac)] (dashed dotted,
-‚-), and [Cu(3-pyrdpm)2] (dashed, - -).
absorption spectra for [Cu(4-pyrdpm)(acac)], [Cu(3-pyrdpm)-
(acac)], [Cu(4-pyrdpm)2], and [Cu(3-pyrdpm)2]. The hetero-
leptic complexes [Cu(4-pyrdpm)(acac)] and [Cu(3-pyrdpm)-
(acac)] show characteristic absorption maxima around 301
and 495 nm, while the homoleptic complexes show major
transitions at 302/318 (4-pyrdpm/3-pyrdpm), 378/372, and
469 nm with a shoulder feature around 503 nm.24 The intense
low-energy bands (495/469 nm) have been previously
assigned to charge-transfer processes associated with the
dipyrrin ligand.19,20,24 In the infrared spectra (Figure S1),
characteristic features between 800 and 1800 cm-1 could be
used to distinguish the two complexes. The bis(dipyrrin)
complexes have several features at approximately 995, 1022,
1244, 1335, 1375, and 1545 cm-1 characteristic of the
dipyrrin ligand. The [Cu(dpm)(acac)] complexes retain these
features but also display two additional bands at ∼1520 and
∼1590 cm-1 that are found in [Cu(acac)2] (1517 and 1580
Figure 3. Structural diagram of [Cu(4-pyrdpm)2] (top) and [Cu(4-cydpm)2]
(bottom) with partial atom numbering schemes (ORTEP, 50% probability
ellipsoids). Hydrogen atoms have been omitted for clarity.
the two chelators in each structure is ∼44°. As expected,
the meso-pyridyl/phenyl substituents are twisted out of plane
with the dipyrrin π-system by ∼65°. No significant inter-
molecular interactions, particularly those associated with the
pyridine or cyano nitrogen atoms, are found in the structures.
The [Cu(4-pyrdpm)2] and [Cu(4-cydpm)2] structures are quite
Inorganic Chemistry, Vol. 43, No. 4, 2004 1245

Halper et al.
Table 1. X-ray Data for [Cu(4-pyrdpm)2] and [Cu(4-cydpm)2]
tion geometry (Figure 4). The base of the square pyramid
geometry is provided by the acac and dipyrrin ligands with
Cu-O and Cu-N bond lengths of 1.96 and 1.98 Å,
respectively. Unlike the symmetric complex [Cu(4-pyrdpm)2],
the square plane in [Cu(4-pyrdpm)(acac)] is not distorted
and is essentially an idealized geometry. The copper atom
is only slightly displaced ∼0.05 Å out of the square plane
toward the apical ligand. The apical position of the square
pyramid is occupied by a pyridine nitrogen donor atom from
an adjacent molecule of [Cu(4-pyrdpm)(acac)] with a
substantially longer Cu-N bond distance of 2.32 Å. The
pyridyl ring of the 4-pyrdpm ligand is nearly orthogonal to
the plane of the dipyrrin chelator with a dihedral angle of
∼88°. Binding of the pyridyl nitrogen atom results in the
formation of an infinite 1-dimensional zigzag coordination
polymer.
[Cu(4-pyrdpm)2]
[Cu(4-cydpm)2]
empirical formula
fw
C28H20N6Cu
504.04
C32H20N6Cu
552.09
space group
unit cell dimens (Å, deg)
Aba2 (No. 41)
a ) 8.9282(7)
R ) 90
P21/n (No. 14)
a ) 11.9165(12)
R ) 90
b ) 16.0218(17)
â ) 91.223(2)
c ) 12.9956(14)
γ ) 90
2480.6(5), 4
100(2)
0.710 73
1.478
0.916
R1 ) 0.0565
wR2 ) 0.1111
R1 ) 0.0798
wR2 ) 0.1181
b ) 8.9150(7)
â ) 90
c ) 28.113(2)
γ ) 90
2237.7(3), 4
100(2)
0.710 73
1.496
V (Å3), Z
temp (K)
λ ( Å)
D
calcd (Mg m-3
)
µ (mm-1
)
1.007
final R indices (I > 2σ(I))a
R1 ) 0.0266
wR2 ) 0.0728
R1 ) 0.0268
wR2 ) 0.0730
R indices (all data)a
The structure of [Cu(3-pyrdpm)(acac)] also forms a
1-dimensional coordination polymer like [Cu(4-pyrdpm)-
(acac)] (Table 2), but the structure of the complex and the
resulting polymer is noticeably different (Figure 4). The [Cu-
(3-pyrdpm)(acac)] complex has a square pyramidal coordina-
tion sphere about the copper(II) ion. The bond lengths at
the base of the square pyramid are 1.96 Å for the Cu-O
bonds (acac) and 1.98 Å for the Cu-N bonds (dipyrrin),
nearly identical with those found in [Cu(4-pyrdpm)(acac)].
Again, the copper(II) ion resides slightly above the square
plane (∼0.06 Å) toward the apical site, which is occupied
by the pyridyl nitrogen atom from an adjacent molecule in
the polymeric chain (Cu-N bond length 2.29 Å, Table 3).
The pyridyl ring is twisted perpendicular relative to the
dipyrrin plane with a dihedral angle of ∼73°. A notable
feature of [Cu(3-pyrdpm)(acac)] is the apparent curvature
in the overall profile of the complex, which appears to have
a concave shape away from the face of the molecule where
the pyridyl nitrogen is bound.
a R1 ) ∑|Fo| - |Fc|/∑|Fo|; R2 ) {∑[w(Fo - Fc )2]/∑[wFo ]}1/2
.
2
2
4
Unlike [Cu(4-pyrdpm)(acac)] and [Cu(3-pyrdpm)(acac)],
the structure of [Cu(2-pyrdpm)(acac)] shows that this
complex is a simple, discrete mononuclear metal complex
(Table 2). The copper(II) ion possesses a square planar
coordination geometry as observed in the other heteroleptic
complexes, but there is no apparent axial coordination by
an additional ligand (Figure 4). The Cu-O and Cu-N bond
lengths are slightly shorter than the other complexes de-
scribed above (Table 3). The pyridyl ring is less twisted
relative to the dipyrrin group with a dihedral angle of ∼52°.
The shortest distance between the 2-pyridyl nitrogen of one
complex and its nearest neighbor is >6.0 Å, which is too
long for a Cu-N bond. In addition, the molecules are not
positioned in any particular orientation in the solid state that
would suggest ordering in a coordinate-covalent, polymeric
fashion.
The structure of [Cu(4-cydpm)(acac)] shows that this
compound is a square planar copper(II) complex (Figure 5).
The bond lengths are comparable to those found in the other
asymmetric complexes described (Table 3). Unlike the [Cu-
(4-pyrdpm)(acac)] complex, [Cu(4-cydpm)(acac)] does not
form a coordination polymer but is rather a simple, discrete
complex similar to [Cu(2-pyrdpm)(acac)] (Figure 5). The
Figure 4. Structural diagrams of [Cu(4-pyrdpm)(acac)] (top), [Cu(3-
pyrdpm)(acac)] (middle), and [Cu(2-pyrdpm)(acac)] (bottom) with partial
atom numbering schemes (ORTEP, 50% probability ellipsoids). Hydrogen
atoms and solvent molecules have been omitted for clarity. Coordination
from the adjacent meso-pyridyl donor and to the neighboring copper(II)
center are shown for [Cu(4-pyrdpm)(acac)] and [Cu(3-pyrdpm)(acac)] to
illustrate the polymeric nature of these structures.
typical of other 4-coordinate R-unsubstituted dipyrrin com-
plexes (Table 1) of copper(II) and nickel(II).20,24
The structure of [Cu(4-pyrdpm)(acac)] is quite distinct
from the homoleptic complex [Cu(4-pyrdpm)2] (Table 2).
[Cu(4-pyrdpm)(acac)] displays a square pyramidal coordina-
1246 Inorganic Chemistry, Vol. 43, No. 4, 2004

Heteroleptic Copper Dipyrromethene Complexes
Table 2. X-ray Data for [Cu(4-pyrdpm)(acac)], [Cu(3-pyrdpm)(acac)], [Cu(2-pyrdpm)(acac)], and [Cu(4-cydpm)(acac)]
[Cu(4-pyrdpm)(acac)]
[Cu(3-pyrdpm)(acac)]
19.5H18N3O2ClCu
[Cu(2-pyrdpm)(acac)]
[Cu(4-cydpm)(acac)]
empirical formula
fw
C25.33H22.67N4O2.67Cu1.33
C
C19H17N3O2Cu
382.90
C21H17N3O2Cu
406.92
510.35
425.36
space group
unit cell dimens (Å, deg)
Pnma (No. 62)
a ) 14.0337(10)
R ) 90
P1h (No. 2)
P212121 (No. 19)
a ) 9.4788(6)
R ) 90
C2/c (No. 15)
a ) 11.9543(10)
R ) 90
b ) 20.6386(18)
â ) 103.165(2)
c ) 7.6606(7)
γ ) 90
1840.3(3), 4
218(2)
0.710 73
1.469
1.208
R1 ) 0.0372
wR2 ) 0.0995
R1 ) 0.0392
wR2 ) 0.1010
a ) 8.3811(5)
R ) 117.520(1)
b ) 15.5837(10)
â ) 91.737(1)
c ) 16.5921(11)
γ ) 98.950(1)
1714.7(2), 4
100(2)
b ) 11.9862(9)
â ) 90
b ) 9.9354(6)
â ) 90
c ) 10.4162(7)
γ ) 90
1752.1(2), 3
100(2)
0.710 73
1.452
c ) 35.677(2)
γ ) 90
3359.9(4), 8
100(2)
0.710 73
1.514
V (Å3), Z
temp (K)
λ (Å)
0.710 73
1.498
1.319
R1 ) 0.0545
wR2 ) 0.1473
R1 ) 0.0647
wR2 ) 0.1544
D
calcd (Mg m-3
)
µ (mm-1
)
1.263
1.317
Final R indices (I > 2σ(I))a
R1 ) 0.0268
wR2 ) 0.0778
R1 ) 0.0293
wR2 ) 0.0787
R1 ) 0.0330
wR2 ) 0.0726
R1 ) 0.0359
wR2 ) 0.0738
R indices (all data)a
a R1 ) ∑|Fo| - |Fc|/∑|Fo|; R2 ) {∑[w(Fo - Fc )2]/∑[wFo ]}1/2
.
2
2
4
the [Cu(3-pyrdpm)(acac)] polymer does not readily release
the cocrystallized solvent and can effectively trap this volatile
compound. Furthermore, TGA experiments revealed that the
endotherm observed at ∼225 °C was associated with a sharp
weight loss of 18.1%, followed by another gradual weight
loss of 4.8% up to 470 °C (data not shown). The exact nature
of this weight loss is unclear, but it may be associated with
liberation of the acac ligand (predicted weight loss 23.3%).
A similar weight loss (20.2%) was also found for the [Cu-
(4-pyrdpm)(acac)] polymer (Figure 6). Above 470 °C an
additional break point was observed in both compounds
followed by a gradual weight loss with increasing temper-
ature consistent with decomposition of the samples. In
contrast, DSC measurements on [Cu(2-pyrdpm)(acac)],
which does not form an extended solid, showed a single
endothermic event at substantially lower temperature (∼120
°C) than found for the polymeric compounds (data not
shown).
Figure 5. Structural diagram of [Cu(4-cydpm)(acac)] with partial atom
numbering schemes (ORTEP, 50% probability ellipsoids). Hydrogen atoms
have been omitted for clarity.
Table 3. Selected Bond Lengths for Each Copper(II) Complex
compd
Cu-Ndipyrrin (Å) Cu-Oacac (Å) Cu-Npyridyl (Å)
[Cu(4-pyrdpm)2]
[Cu(4-cydpm)2]
[Cu(4-pyrdpm)(acac)]
[Cu(3-pyrdpm)(acac)]
[Cu(2-pyrdpm)(acac)]
[Cu(4-cydpm)(acac)]
1.95
1.96
1.98
1.98
1.96
1.95
n/a
n/a
1.96
1.96
1.94
1.92
n/a
n/a
2.32
2.29
n/a
n/a
closest distance between a copper(II) ion and a cyano
nitrogen atom from a neighboring molecule is ∼6.0 Å clearly
indicating that no intermolecular bonding interactions are
present.
Discussion
The initial goal of this study was to design multidentate
dipyrromethene ligands for preparing extended solids25,26,33
via the formation of bis(dipyrrin)copper complexes.20,24
During the course of these investigations, it became apparent
that heteroleptic [Cu(dpm)(acac)] complexes were stable,
isolatable species. These complexes could be readily distin-
guished from the [Cu(dpm)2] complexes by examination of
their electronic absorption and infrared spectroscopy. Fur-
thermore, solid-state structural characterization revealed that
some of these asymmetric complexes, namely [Cu(4-pyrdp-
m)(acac)] and [Cu(3-pyrdpm)(acac)], were “self-comple-
mentary” and spontaneously formed 1-dimensional coordi-
nation polymers (Figure 4). The ability of these complexes
to form polymers in the solid state and the nature of the
resulting polymers was found to be strongly influenced by
the position of the nitrogen atom in the pyridine heterocycle.
Placement of the coordinating nitrogen atom in the 4- or
The polymeric materials, [Cu(4-pyrdpm)(acac)] and [Cu-
(3-pyrdpm)(acac)], were further examined by differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA). DSC experiments on dried samples [Cu(4-pyrdpm)-
(acac)] and [Cu(3-pyrdpm)(acac)] gave similar results with
sharp, endothermic thermal decompositions at around 225
°C. The DSC profile for [Cu(3-pyrdpm)(acac)] shows an
additional broader, endothermic feature centered at ∼94 °C
(Figure 6). On the basis of the crystallographic and elemental
analysis data (vide supra), this observation was tentatively
assigned to the loss of CH2Cl2 from the coordination
polymer. TGA analysis showed that this DSC event cor-
responded to an 8.3% weight loss from the material. The
stoichiometry of CH2Cl2 was 1 molecule/2 [Cu(3-pyrdpm)-
(acac)] complexes (on the basis of both the X-ray structure
and elemental analysis), which would give a predicted weight
loss of 9.98%, in satisfactory agreement with the experi-
mental data. Even after drying at ∼50 °C under vacuum,
(33) Kiang, Y.-H.; Lee, S.; Xu, Z.; Choe, W.; Gardner, G. B. AdV. Mater.
2000, 12, 767-770.
Inorganic Chemistry, Vol. 43, No. 4, 2004 1247

Halper et al.
Figure 6. DSC (left) and TGA (right) analysis of the coordination polymers [Cu(4-pyrdpm)(acac)] (solid, s) and [Cu(3-pyrdpm)(acac)] (dotted, ‚‚‚). The
experiments show the generally similar behavior of the two compounds, except for the loss of trapped CH2Cl2 from [Cu(3-pyrdpm)(acac)].
ion is somewhat puckered out of the square plane geometry
toward the axial pyridine donor, and the pyridine ring system
is directed nearly perpendicular to the dipyrrin π-system in
both polymeric complexes. In contrast to [Cu(4-pyrdpm)-
(acac)] and [Cu(3-pyrdpm)(acac)], the 2-pyridyl derivative,
[Cu(2-pyrdpm)(acac)], did not form a coordination polymer
although the complex maintains a similar square planar
coordination geometry with respect to the dipyrrin and acac
ligands. The lack of intermolecular interactions is supported
by the long Cu-N distances between neighboring molecules
Figure 7. Space-filling representation of the coordination polymers formed
and the “relaxed” conformation of the pyridine heterocycle
by [Cu(4-pyrdpm)(acac)] (top left) and [Cu(3-pyrdpm)(acac)] (top right).
relative to the plane of the dipyrrin moiety. Inspection of
A stick representation of [Cu(3-pyrdpm)(acac)] (bottom) is viewed down
space-filling models of the [Cu(2-pyrdpm)(acac)] complex
suggests that the 2-pyridyl ligand is situated such that the
nitrogen donor is sterically prevented from establishing a
proximate relationship with a neighboring copper(II) center.
In an attempt to extend the general motif of these
complexes toward coordination polymers with different
donor ligands, the [Cu(4-cydpm)(acac)] complex, which
possesses a cyano functionality extending from the meso-
phenyl group of the dipyrrin ligand, was synthesized. The
cyano group was chosen on the basis of a select number of
earlier examples of copper-containing coordination polymers
linked by this functional group.11,17 Structural characterization
of [Cu(4-cydpm)(acac)] showed that a 4-coordinate square
planar metal complex rather than a coordination polymer was
isolated. Apparently the cyano group is not a sufficiently
strong donor to drive polymer formation. This finding is not
completely surprising because although copper-containing
coordination polymers based on cyano bridges are known,
the failure of this moiety to generate such interactions has
been documented in several attempts to synthesize related
structures.15,16,36
the crystallographic c-axis, which shows the interdigitation of the acac
ligands between paired polymer chains. Solvent molecules (top right,
bottom) and hydrogen atoms (bottom only) have been omitted for clarity.
3-position allowed for the formation of two related, but
topologically distinct, coordination polymers (Figure 7). The
typical motifs for 1-dimensional coordination polymers are
zigzag chains and helicies,2 the latter being a rather rare
morphology. Both [Cu(4-pyrdpm)(acac)] and [Cu(3-
pyrdpm)(acac)] form structures quite typical of zigzag
chains.27,34,35 [Cu(3-pyrdpm)(acac)] shows a particularly
intriguing arrangement in the solid state: the packing
diagram of [Cu(3-pyrdpm)(acac)] (Figure 7, bottom) shows
that two coordination chains assemble in an antiparallel
fashion with interdigitation of the acac ligands in an
alternating manner. This results in a tight packing of the bulk
structure into paired polymeric chains.
Formation of these extended polymers has little effect on
the metal and ligand geometry. The Cu-O and Cu-N bond
distances for the acac and dipyrrin chelators are essentially
the same as the mononuclear, square planar complexes that
do not form polymeric structures (Table 3). The copper(II)
Structural characterization of [Cu(4-pyrdpm)2] and [Cu-
(3-pyrdpm)2] reveals that these compounds are mononuclear
(34) Mimura, M.; Matsuo, T.; Nakashima, T.; Matsumoto, N. Inorg. Chem.
1998, 37, 3553-3560.
(35) Stocker, F. B.; Troester, M. A.; Britton, D. Inorg. Chem. 1996, 35,
3145-3153.
(36) Zhang, W.; Jeitler, J. R.; Turnbull, M. M.; Landee, C. P.; Wei, M.;
Willett, R. D. Inorg. Chim. Acta 1997, 256, 183-198.
1248 Inorganic Chemistry, Vol. 43, No. 4, 2004

Heteroleptic Copper Dipyrromethene Complexes
pyridyl substituents were found to form 1-dimensional
coordination polymers of varying topologies while similar
complexes containing meso-4-cyanophenyl or geometrically
inaccessible meso-pyridyl substituents formed discrete com-
pounds. In addition, homoleptic compounds were found to
be unable to form coordination polymers with either cyano
or pyridyl donors due to steric constraints. Ongoing studies
are focused on varying both the dipyrrin and acac ligands to
better understand the formation of these compounds and to
discover new polymer structures. In addition, studies focused
on using homoleptic complexes as monomers for heterome-
tallic coordination polymers are underway and will be
reported in due course.
Figure 8. Chemical (left) and space-filling (right, from X-ray coordinates)
representations of [Cu(4-cydpm)(acac)] (top) and [Cu(4-pyrdpm)2] (bottom).
The structures clearly show that the steric interactions between the R-pyrrolic
protons play a major role in controlling the coordination geometry of these
complexes. Note that the axial positions in [Cu(4-pyrdpm)2] are largely
blocked by the twisting of the dipyrrin ligands relative to one another.
Acknowledgment. We thank Prof. Arnold L. Rheingold,
Dr. Lev N. Zakharov, and David T. Puerta (U.C. San Diego)
for help with the X-ray structure determinations, Prof.
Emmanuel A. Theodorakis (U.C. San Diego) for use of his
FT-IR, and Melissa A. Grunlan and Prof. William P. Weber
(U.S.C.) for instruction and use of their TGA and DSC
instruments. This work was supported by the University of
California, San Diego, a Chris and Warren Hellman Faculty
Scholar award, and a Hellman Fellows award. Acknowledg-
ment is made to the donors of the American Chemical
Society Petroleum Research Fund for support of this research.
H.M.D. was supported by the NSF-MASEM program
through the U.C. San Diego Summer Training Academy for
Research in the Sciences (STARS), and S.R.H. was supported
in part by GAANN Fellowship No. GM-602020-03. This
material is based upon work supported under a National
Science Foundation Graduate Research Fellowship (S.R.H.).
metal complexes. The solid-state structure of these complexes
show the metal centers possess a highly distorted square
planar geometry that is more typical of bis(dipyrrin)metal
complexes.20,24 The inability of these compounds to form
more idealized square planar geometries is attributed to a
steric clash between the R-pyrrole protons on opposing
ligands (Figure 8).20 The severely distorted square planar
geometry in these complexes obstructs coordination of a fifth
ligand such as a pyridyl nitrogen atom, although on the basis
of spectroscopic studies it has been suggested that such
coordination is possible.20 Although these compounds do not
form self-complementary coordination polymers, they hold
promise as building blocks for heterometallic coordination
polymers by subsequent reaction with salts of silver(I),
cadmium(II), or other metal ions that can form coordinate
bonds with nitrogenous ligands.2,13,25,26,33,37,38
Supporting Information Available: An additional figure,
experimental details, and crystallographic data in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org. X-ray crystallographic files in CIF format are available
free of charge via the Internet at http://www.ccdc.cam.ac.uk. Refer
to CCDC reference numbers 222290, 222291, 222292, 222293,
222294, and 222295.
In summary, a series of homo- and heteroleptic copper-
(II) dipyrromethene complexes were synthesized and char-
acterized. Certain heteroleptic complexes containing meso-
(37) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.;
Biradha, K. Chem. Commun. 2001, 509-518.
(38) Hirsch, K. A.; Wilson, S. R.; Moore, J. S. Chem. Commun. 1998,
13-14.
IC0352295
Inorganic Chemistry, Vol. 43, No. 4, 2004 1249