Helical coordination polymers and cyclic dimers formed from heteroleptic thioether-dipyrrinato copper(II) complexes

Article abstract of DOI:10.1039/b411991f

Heteroleptic copper complexes containing an acetylacetonato ligand and a thioether derivatized dipyrrinato ligand are shown to form oligomers and polymers in the solid state.

Full text of DOI:10.1039/b411991f

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Helical coordination polymers and cyclic dimers formed from
heteroleptic thioether-dipyrrinato copper(II) complexes{
Loi Do, Sara R. Halper and Seth M. Cohen*
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California,
92093-0358, USA. E-mail: scohen@ucsd.edu; Tel: 11 858-822-5596; Fax: 11 858-822-5598
Received (in Columbia, MO, USA) 4th August 2004, Accepted 27th August 2004
First published as an Advance Article on the web 11th October 2004
Heteroleptic copper complexes containing an acetylacetonato
ligand and a thioether derivatized dipyrrinato ligand are
shown to form oligomers and polymers in the solid state.
Coordination polymers represent a growing area of interest for the
discovery of novel magnetic, electronic, and functionalized
materials.1–3 Recently, dipyrromethene (dipyrrin) complexes of
copper(II) were found to form a variety of coordination polymers
with several interesting morphologies.4,5 These polymers are
constructed from self-complementary, heteroleptic copper(II) com-
plexes containing a heteroditopic dipyrrinato ligand and an
acetylacetonato (acac) ligand. Some of the compounds identified
were found to trap volatile solvents,4 while others formed extremely
unusual structures containing both discrete and extended chain
components.5 One limitation of the systems described in these
earlier studies is that only a single functional group, a meso-pyridyl
substituent, was found to mediate formation of polymeric
structures. Attempts to use other ligands, such as nitrile donors,
failed to form any oligomeric structures.4 Herein, two new
heterotopic dipyrrin ligands are described that form a self-
complementary helical coordination polymer and a cyclic dimer
with copper(II).
Scheme 1 General synthesis of (dipyrrinato)copper(II) complexes.
The heteroleptic dipyrrin complexes described to date result
from the formation of a square planar coordination sphere
established by the dipyrrin and acac chelates around a copper(II)
center. The polymeric compounds are then connected by axial
attachment of a peripheral donor atom found at the meso position
of the dipyrrinato ligand from a neighboring metal complex; this
generates a square pyramidal coordination environment around
the copper(II) ion.4,5 Thioether groups are not particularly strong
ligands; however, the coordination of this moiety to transition
metals is well documented.6 In order to determine whether
thioether substituted dipyrrins could form coordination polymers,
the ligand 5-(4-methylthiophenyl)dipyrromethene (4-mtdpm) was
synthesized according to literature procedures.4,7 Complexation of
4-mtdpm with Cu(acac)2 resulted in formation of the desired
heteroleptic complex [Cu(4-mtdpm)(acac)], but this complex
proved unstable (Scheme 1) and decomposed to the homoleptic
[Cu(4-mtdpm)2]. Fig. 1 shows the X-ray structure of [Cu(4-
mtdpm)2], which does not form a coordination polymer, consistent
with earlier studies on related homoleptic complexes.4{
Fig. 1 Structural diagram of [Cu(4-mtdpm)2] with partial atom numbering
schemes (ORTEP, 50% probability ellipsoids). Hydrogen atoms have been
omitted for clarity.
coordination sphere is occupied by the sulfur atom from the
thioether moiety of a neighboring complex. The Cu–S bond length
˚
is 2.78 A, which is long compared to most thioether–copper(II)
interactions found in the Cambridge Crystallographic Data Centre
˚
(y2.3–2.5 A). Nevertheless, several examples of long Cu–S
interactions with thioether ligands have been described when
coordinated axially.8–10
Like earlier described pyridyl-containing dipyrrins, [Cu(4-
mtdpm)(hfacac)] generates a 1-dimensional coordination polymer
in the solid state. The polymer chains orient along the crystal-
lographic c-axis and display a helical topology. The hfacac groups
are directed out to the periphery of the polymer backbone, where
they are displayed toward other hfacac groups from neighboring
polymer chains. This packing arrangement, where a layered
structure is formed from the localization and interdigitation of
hfacac groups (Fig. S1{), suggests that self-segregation of these
groups has a substantial influence on the packing of the polymer
chains in the crystal lattice. This tendency of fluoroalkanes to phase
separate has been observed in other self-assembling structures both
in solution and the solid-state.5,11 Thermal gravimetric analysis
(TGA) showed no weight loss up to y175 uC, suggesting that the
coordination polymer is thermally stable to this temperature.
In order to further probe this new polymer motif, the
compound [Cu(3-mtdpm)(hfacac)] was prepared (3-mtdpm ~
5-(3-methylthiophenyl)dipyrromethene).7,12,13 The complex was
In order to obtain a stable, heteroleptic complex, Cu(hfacac)2
(hfacac ~ hexafluoroacetylacetonato) was used to prepare the
complex [Cu(4-mtdpm)(hfacac)]. Use of the fluorinated hfacac
ligand was found to stabilize the heteroleptic [Cu(4-mtdpm)-
(hfacac)] complex, which was isolated without difficulty by silica
column chromatography as a red solid.
Slow evaporation of a solution of [Cu(4-mtdpm)(hfacac)] from
CH2Cl2 generated red crystals. X-Ray diffraction structural
characterization shows the complex has a square pyramidal
coordination geometry (Fig. 2).{ The square plane of the
˚
coordination sphere is made up of the dipyrrin (Cu–N 1.94 A)
˚
and hfacac (Cu–O 1.99 A) chelates. The apical position of the
{ Electronic supplementary information (ESI) available: synthesis, char-
acterization, and crystallographic details for all compounds. See http://
www.rsc.org/suppdata/cc/b4/b411991f/
2 6 6 2
C h e m . C o m m u n . , 2 0 0 4 , 2 6 6 2 – 2 6 6 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Fig. 3 Structural diagram of [Cu(3-mtdpm)(hfacac)] with partial atom
numbering schemes (ORTEP, 50% probability ellipsoids). Hydrogen
atoms, solvent molecules, and axial thioether ligation have been omitted for
clarity (top). Stick representation (bottom) shows the head-to-tail dimer
formed by the complex (viewed down the crystallographic b-axis).
prove useful in the preparation of related complexes and
supramolecular structures. The compounds presented here show
that small perturbations in ligand structure can lead not only to the
formation of mononuclear and polymeric structures, as previously
reported,4 but also discrete oligomeric structures. The stabilization
of these heteroleptic complexes by electron-withdrawing spectator
ligands has also been established. We anticipate that these findings
will aid in the discovery of interesting solid-state topologies based
on these dipyrrin motifs.
We thank Prof. A. L. Rheingold and Dr L. Zakharov for
assistance with X-ray structural determinations, Dr Y. Su for mass
spectrometry analysis, and Prof. E. A. Theodorakis for use of his
FT-IR. This work was supported by the University of California,
San Diego, a Chris and Warren Hellman Faculty Scholar award,
the donors of the American Chemical Society Petroleum Research
Fund, and a National Science Foundation Graduate Research
Fellowship (S. R. H.). S. M. C. is a Cottrell Scholar of the
Research Corporation.
Fig. 2 Structural diagram of [Cu(4-mtdpm)(hfacac)] with partial atom
numbering schemes (ORTEP, 50% probability ellipsoids). Hydrogen
atoms, solvent molecules, and axial thioether ligation have been omitted for
clarity (top). Stick representation (bottom) shows the 1-dimensional
coordination polymer formed by the complex.
crystallized and the solid-state structure of [Cu(3-mtdpm)(hfacac)]
shows the desired heteroleptic complex; however, to our surprise,
the compound does not form a coordination polymer.{ Instead,
this compound forms a head-to-tail cyclic dimer, connected
through axial thioether ligands (Fig. 3). The coordination
environment around the metal center is similar to [Cu(4-
˚
mtdpm)(hfacac)] with Cu–N distances of 1.94 A and Cu–O
˚
distances of 1.97 A. The dimer is held together by weak
interactions, with the thioether moiety binding in the axial position
of the coordination sphere, as found in [Cu(4-mtdpm)(hfacac)].
˚
The Cu–S distance in [Cu(3-mtdpm)(hfacac)] is 2.86 A, which is
slightly longer than that found in the coordination polymer [Cu(4-
mtdpm)(hfacac)]. A recently reported complex with 2,8-diethyl-
1,3,7,9-tetramethyl-5-(2-pyridyl) dipyrromethene and zinc(II) ions
forms a related cyclic binuclear structure.14
Notes and references
As mentioned above, the heteroleptic complex [Cu(4-mtdpm)-
(acac)] was found to rearrange to [Cu(4-mtdpm)2] during isolation.
We have found that the stability of these heteroleptic complexes
depends strongly on the nature of the acac ligand, the peripheral
meso donor on the dipyrrin ligand, and the presence of absorbents
such as alumina.4,15 Freshly prepared CH2Cl2–benzene (1 : 1)
solutions of 4-mtdpm were combined with one equivalent of
different copper(II) sources to generate [Cu(4-mtdpm)(acac)],
[Cu(4-mtdpm)(tfacac)] (tfacac ~ trifluoroacetylacetonato), and
[Cu(4-mtdpm)(hfacac)]. The reaction mixtures were stirred at room
temperature and examined by UV–visible spectroscopy to evaluate
the stability of each complex. Immediately after addition of the
copper(II) sources, the expected heteroleptic complexes were
formed as evidenced by a characteristic charge-transfer band
centered at y492 nm.4 After stirring for y1 h, the solution
containing [Cu(4-mtdpm)(acac)] had started to form [Cu(4-
mtdpm)2], which was identified by a new transition at y467 nm.
By y17 h a stable equilibrium mixture of both the homo- and
heteroleptic complexes was achieved. Complexes of [Cu(4-
mtdpm)(tfacac)] and [Cu(4-mtdpm)(hfacac)] showed no change
in electronic spectra over the same time period. These data indicate
that the more electron-withdrawing fluorinated acac ligands
stabilize these heteroleptic complexes, an observation that may
{ CCDC 246586–246588. See http://www.rsc.org/suppdata/cc/b4/b411991f/
for crystallographic data in .cif or other electronic format.
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C h e m . C o m m u n . , 2 0 0 4 , 2 6 6 2 – 2 6 6 3
2 6 6 3