Homoleptic, four-coordinate azadipyrromethene complexes of d10 zinc and mercury

Article abstract of DOI:10.1021/ic701190g

Tetraarylazadipyrromethenes, and especially their boron chelates, are a growing class of chromophores that are photoactive toward red light. The coordination chemistry of these ligands remains to be explored. Reported here are four-coordinate zinc(II) and mercury(II) complexes of tetraarylazadipyrromethene ligands. The new complexes contain two azadipyrromethenes bound per d¹⁰ metal center and are characterized by ¹H NMR, optical absorption spectroscopy, X-ray diffraction crystallography, and elemental analysis. Solid-state structures show that these bischelate complexes distort significantly from idealized D_2d symmetry. AM1 geometry optimizations indicate relaxation energies in the range of 6.8-15.2 kcal mol^-1; interligand π-stacking provides an added energetic impetus for distortion. The absorption spectra show a marked increase in the absorption intensity in the red region and, in the case of the zinc(II) complexes, the development of a second distinct absorption band in this region, which is red-shifted by ca. 40-50 nm relative to the free ligand. Semiempirical INDO/S computations indicate that these low-energy optical absorptions derive from allowed excitations among ligand-based orbitals that derive from the highest occupied molecular orbital and lowest unoccupied molecular orbital of the free azadipyrromethene.

Full text of DOI:10.1021/ic701190g

Inorg. Chem. 2008, 47, 2338-2346
Homoleptic, Four-Coordinate Azadipyrromethene Complexes of d10 Zinc
and Mercury
Thomas S. Teets, David V. Partyka, James B. Updegraff III, and Thomas G. Gray*
Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106
Received June 18, 2007
Tetraarylazadipyrromethenes, and especially their boron chelates, are a growing class of chromophores that are
photoactive toward red light. The coordination chemistry of these ligands remains to be explored. Reported here
are four-coordinate zinc(II) and mercury(II) complexes of tetraarylazadipyrromethene ligands. The new complexes
contain two azadipyrromethenes bound per d10 metal center and are characterized by 1H NMR, optical absorption
spectroscopy, X-ray diffraction crystallography, and elemental analysis. Solid-state structures show that these bis-
chelate complexes distort significantly from idealized D2d symmetry. AM1 geometry optimizations indicate relaxation
energies in the range of 6.8–15.2 kcal mol-1; interligand π-stacking provides an added energetic impetus for
distortion. The absorption spectra show a marked increase in the absorption intensity in the red region and, in the
case of the zinc(II) complexes, the development of a second distinct absorption band in this region, which is
red-shifted by ca. 40–50 nm relative to the free ligand. Semiempirical INDO/S computations indicate that these
low-energy optical absorptions derive from allowed excitations among ligand-based orbitals that derive from the
highest occupied molecular orbital and lowest unoccupied molecular orbital of the free azadipyrromethene.
Introduction
Azadipyrromethenes are an emerging class of bidentate
organic ligands that have occasioned scrutiny because of their
+
absorption of red and near-infrared light. In particular, BF2
chelates of aryl-substituted azadipyrromethenes possess
Figure 1. Structures of (a) the BF2+ chelate of a tetraarylazadipyrromethene
favorable properties for a variety of bio-optical applica-
and (b) unsubstituted dipyrrin.
tions.1–6 Figure 1a depicts the general structure of sym-
metrical BF2-azadipyrromethenes.
to be luminescent at room temperature; the tetraphenyl
compound (Ar1 ) Ar2 ) Ph) emits at 672 nm with a
fluorescence quantum yield ΦF ) 0.34.1 The absorption and
emission profiles of these chromophores are readily tuned
by altering the substitution of the aryl groups; electron-
donating groups2,3 such as -OMe and -NR2 as well as
conformationally restricted substituents4 red-shift both the
Boron azadipyrromethenes generally show intense absorp-
tion in the red region of the spectrum (λmax > 650 nm), with
ꢀ ∼ 75 000-85 000 M-1 cm-1. Several have also been shown
* To whom correspondence should be addressed. E-mail: tgray@
case.edu.
(1) Hall, M. J.; Allen, L. T.; O’Shea, D. F. Org. Biomol. Chem. 2006, 4,
776–780.
+
absorption and emission profiles. BF2 chelates of dimeth-
(2) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.;
O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619–10631.
(3) McDonnell, S. O.; O’Shea, D. F. Org. Lett. 2006, 8, 3493–3496.
(4) Zhao, W.; Carreira, E. M. Angew. Chem., Int. Ed. 2005, 44, 1677–
1679Chem.sEur. J. 2006, 12, 7254–7263.
ylamino-substituted tetraarylazadipyrromethenes,3 where Ar1
) p-N,N-dimethylaniline in Figure 1a, have been shown to
be efficient pH sensors. Terminal substitution with diethy-
lamine or morpholine functionalities produces photoinduced
electron-transfer-modulated sensors that respond to changes
in pH and microenvironmental polarity.1 Efficient mer-
cury(II) sensing was achieved through incorporation of
o-pyridyl donors onto the ligand scaffold,5 at the “Ar1”
position of Figure 1a. In addition, boron azadipyrromethenes,
(5) Coskun, A.; Yilmaz, M. D.; Akkaya, E. U. Org. Lett. 2007, 9, 607–
609.
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Gorman, A.; Killoran, J.; O’Shea, D. F. Br. J. Cancer 2005, 92, 1702–
1710. (b) McDonnell, S. O.; Hall, M. J.; Allen, L. T.; Byrne, A.;
Gallagher, W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2005, 127, 16360–
16361. (c) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.;
O’Shea, D. F. Chem. Commun. 2002, 1862–1863.
2338 Inorganic Chemistry, Vol. 47, No. 7, 2008
10.1021/ic701190g CCC: $40.75  2008 American Chemical Society
Published on Web 02/27/2008

Azadipyrromethene Complexes of d10 Zinc and Mercury
Scheme 1. Syntheses of 2a-c and 3
where pyrrolic protons are substituted with bromine, are
efficient sensitizers of oxygen,2,6 making them a target of
interest for a new class of nonporphyrin photodynamic
therapy mediators.
before boronation, so the opportunity to incorporate metals
into the ligand framework is readily apparent. Heavy-atom
binding to the azadipyrromethene core is one means of
attuning the ligand photophysics and of accessing photore-
activity from its triplet excited states.
Four-coordinate bis-chelates of the related dipyrromethene
ligand, containing a variety of 2+ metal ions, are abundant.7
The structure of the dipyrromethene ligand, often referred to
as “dipyrrin”, is shown in Figure 1b. The pyrrolic carbons and
the bridging meso carbon can be functionalized with a variety
of substituents, leading to numerous possible architectures.
Dipyrrinato bis-chelates of zinc(II) have been obtained as
side products during templated syntheses of zinc(II) por-
phyrins.8 In recent years, numerous homoleptic bis(dipyrri-
nato) complexes have been prepared and studied, employing
a variety of metal ions and encompassing several geometries.
Recent efforts have produced homoleptic bis-chelate com-
plexes of zinc(II)9 and nickel(II)10 and also tris-chelates of
group 13 metals,11 iron(III) and cobalt(III).12 Dipyrromethene
complexes of zinc(II),13 copper(II)14 and cobalt(II)15 have
been examined in the context of supramolecular chemistry,
as synthons for coordination polymers having significant
electronic, optical, and magnetic properties. Another recent
trend is the development of porphyrin-dipyrromethene
oligomers,16 which are targeted for use in efficient light-
harvesting arrays. The coordination chemistry of dipyr-
romethenes, which are structurally similar to azadipyr-
romethenes, has been pursued extensively.
Reported here is a series of four-coordinate zinc(II) and
mercury(II) bis-chelate complexes of tetraarylazadipyr-
romethenes. The new compounds were characterized by 1H
NMR, optical absorption spectroscopy, and elemental analy-
sis; all but one were structurally characterized by single-
crystal X-ray diffraction. Absorption in the red region greatly
intensifies in the complexes relative to the free azadipyr-
romethene, with a second distinct absorption transition
evident in this region for the zinc(II) compounds. The
complexes also demonstrate intramolecular π-stacking in-
teractions both in the solid state and in solution. We believe
these interactions stabilize the observed structures, which are
significantly distorted from D2d symmetry.
Results and Discussion
Synthesis. To prepare the zinc(II) complexes 2a-c,
zinc(II) acetate dihydrate was reacted with 2 equiv of
tetraarylazadipyrromethenes 1a-c in tetrahydrofuran (THF)
to yield the appropriate homoleptic complexes as crystalline
solids in good to excellent isolated yields, ranging from 54%
to 90% (Scheme 1).
The metallocomplex chemistry of azadipyrromethenes has
been slower to develop. Free azadipyrromethenes are isolated
Upon mixing, an instantaneous color change occurs to a
brighter, more greenish-blue tint relative to the free ligand.
Quantitative conversion was observed with 12 h reaction
times. In the case of 2c, heating at 40 °C was required to
ensure complete reaction; at room temperature, a small
amount of free ligand was observed in the 1H NMR spectra
of the product. The products were recrystallized by vapor
diffusion of pentane into THF solutions, which afforded the
complexes in good purity as judged by 1H NMR and
elemental analysis. The mercury(II) complex 3 was prepared
in an analogous manner by direct reaction of 1a and
mercury(II) acetate in THF. However, a slight excess of the
mercury salt, 0.75 equiv rather than the stoichiometric 0.5
equiv, and longer reaction time, 24 h rather than 12 h, were
required for the reaction to reach completion. Product 3 was
(7) Wood, T. E.; Thompson, A. Chem. ReV. 2007, 107, 1831–1861.
(8) Marchon, J. C.; Ramasseul, R.; Ulrich, J. J. Heterocycl. Chem. 1987,
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(b) Brückner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can.
J. Chem. 1996, 74, 2182–2193. (c) Halper, S. R.; Stork, J. R.; Cohen,
S. M. Dalton Trans. 2007, 1067–1074.
(10) Clarke, E. T.; Squattrito, P. J.; Rudolf, P. R.; Motekaitis, R. J.; Martell,
A. E.; Clearfield, A. Inorg. Chim. Acta 1989, 166, 221–231.
(11) Thoi, V. S.; Stork, J. R.; Magde, D.; Cohen, S. M. Inorg. Chem. 2006,
45, 10688–10697.
(12) Halper, S. R.; Cohen, S. M. Inorg. Chem. 2005, 44, 486–488.
(13) (a) Maeda, H.; Hasegawa, M.; Hashimoto, T.; Kakimoto, T.; Nishio,
S.; Nakanishi, T. J. Am. Chem. Soc. 2006, 128, 10024–10025. (b)
Wood, T. E.; Ross, A. C.; Dalgleish, N. D.; Power, E. D.; Thompson,
A.; Chen, X.; Okamoto, Y. J. Org. Chem. 2005, 70, 9967–9974. (c)
Thompson, A.; Dolphin, D. J. Org. Chem. 2000, 65, 7870–7877.
(14) (a) Do, L.; Halper, S. R.; Cohen, S. M. Chem. Commun. 2004, 23,
2662–2663. (b) Halper, S. R.; Cohen, S. M. Angew. Chem., Int. Ed.
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45, 2695–2703.
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M.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2004, 126, 7898–7907. (b)
Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
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Inorganic Chemistry, Vol. 47, No. 7, 2008 2339

Teets et al.
Figure 2. Crystal structures of 2a, 2c, and 3 with partial labeling schemes and 50% probability ellipsoids. Hydrogen atoms are omitted for clarity, and
carbon atoms are not labeled. Data were collected at 100 ( 2 K.
isolated in 61% yield. Attempts at the synthesis of cadmium
analogues of 2 and 3 produced intractable mixtures of the
free ligand and complex.
to those found in structurally characterized zinc(II) dipyrrin
complexes.9c,16b The ligand bite angles, defined as ∠N-M-N,
are 94.56(8)° for 2a and 96.44(13)° for 2c; again these values
are comparable to those for zinc dipyrrin complexes. The
bite angles in 3 are smaller at 87.11(7)° and 86.51(7)°. This
decrease in the bite angle within increasing radius of the
metal atom mirrors the trend observed for previously reported
coinage metal complexes of azadipyrromethenes.17 Backbone
C-Nmeso-C bond angles, where Nmeso is the bridging
nitrogen atom not involved in metal ligation, are larger than
those reported for two BF2-azadipyrromethenes.2,6c With
the smaller boron atoms in the ligand pocket, the C-Nmeso-C
bond angles are 119.7(3)° and 119.5(3)°, while in the
metal(II) bis-chelates reported here, the angle has widened
to 126.3(2)° for 2a, 127.93(14)° for 2c, and 128.5(2)° and
129.5(3)° for 3. These larger angles indicate sufficient
flexibility in the ligand backbone to allow distortion from
ideal sp2 hybridization at nitrogen upon metal-ion binding.
As with the other structural parameters, these angles in 2a
and 2c are comparable to analogous C-Cmeso-C angles in
zinc dipyrrinato complexes. The most notable structural
difference in 3 compared to 2a and 2c is an asymmetry in
Hg-N bond lengths for each chelated azadipyrromethene;
the two lengths are nearly equal in the zinc(II) complexes;
see Table 1. The two six-membered chelate rings in 3 each
consist of one long Hg-N bond and one short Hg-N bond,
which differ by more than 0.2 Å. This asymmetry likely
reflects a mismatch between the size of the mercury(II) cation
and the ligand cavity.
1H NMR readily establishes the formation of bis(aza-
dipyrromethene) complexes. When measured in CDCl3, the
singlet associated with the pyrrolic protons shifts upfield by
0.4–0.5 ppm in the zinc(II) complexes relative to the free
ligand. One of the aromatic doublets associated with the
proximal azadipyrromethene phenyl rings (i.e., the phenyl
substituents deriving from acetophenone)2 exhibits an upfield
shift of the same magnitude in the complexes. Furthermore,
in 2b, the methoxy singlet shifts upfield by 0.55 ppm. The
remaining ligand peaks are generally unresponsive to com-
plex formation, shifting by <0.1 ppm upon the formation
1
of zinc(II) complexes. The H NMR spectrum of 3 is very
similar to that of 2a, but the upfield shift of the pyrrolic
proton resonance is smaller, only shifting ∼0.2 ppm in 3
relative to 1a.
Crystal Structures. Irregular, block-shaped crystals of 2a,
2c, and 3 were grown from concentrated THF solutions by
vapor diffusion of pentane at room temperature; attempts to
grow X-ray-quality crystals of 2b resulted in fibrous needles
that were too thin to mount on a goniometer. Figure 2 depicts
crystal structures of 2a, 2c, and 3. Table 1 collects crystal-
lographic data.
Compound 2a crystallized in the orthorhombic space group
Fdd2; 2c crystallized in the monoclinic space group C2/c,
and 3 crystallized in the orthorhombic space group P212121.
Molecules of 2a and 2c reside on crystallographically
imposed C2 axes such that the asymmetric unit consists of
the zinc atom and one azadipyrromethene ligand. Table 2
summarizes relevant interatomic distances and angles for 2a,
2c, and 3.
The coordination geometries of 2a, 2c, and 3 distort
significantly from D2d symmetry, as indicated by the angle
between best-fit planes of the two six-membered chelate
rings. This angle is 63.50° in 2a, 68.67° in 2c, and 67.91°
The Zn-N bond lengths, which range between 1.984(2)
and 1.9905(13) Å, are unremarkable and, in general, similar
(17) Teets, T. S.; Partyka, D. V.; Esswein, A. J.; Updegraff, J. B.; Zeller,
M.; Hunder, A. D.; Gray, T. G. Inorg. Chem. 2007, 46, 6218–6220.
2340 Inorganic Chemistry, Vol. 47, No. 7, 2008

Azadipyrromethene Complexes of d10 Zinc and Mercury
Table 1. Crystallographic Data for 2a, 2c, and 3
2a
2c
3
formula
fw
C64H44N6Zn
962.42
C68H52N6O4Zn
1082.53
C68H52HgN6O
1169.79
cryst syst
space group
orthorhombic
Fdd2
monoclinic
C2/c
orthorhombic
P212121
a, Å
b, Å
c, Å
19.8576(11)
44.419(2)
10.8128(5)
21.149(6)
10.888(3)
22.737(5)
12.2174(9)
15.6645(12)
27.527(2)
R, deg
ꢀ, deg
94.700(5)
γ, deg
cell volume, Å3
Z
9537.6(8)
8
1.340
100 ( 2
0.565
5218(2)
4
1.378
100 ( 2
0.531
5268.2(7)
4
1.475
100 ( 2
2.974
Dcalcd, Mg, m3
T, K
µ, mm-1
F(000)
4000
2256
2360
cryst size, mm
θmin-θmax, deg
no. of reflns collected
no. of indep reflns
no. of refined param
GOFa on F2
final R indicesb [I > 2σ(I)]
R1
0.41 × 0.29 × 0.16
1.83-27.34
58 263
5402
321
0.971
0.28 × 0.16 × 0.14
1.80-27.50
29 971
5987
359
1.040
0.42 × 0.39 × 0.22
1.82-27.50
60 315
12057
685
1.000
0.0388
0.1065
0.0339
0.0800
0.0187
0.0420
wR2
R indices (all data)
R1
wR2
0.0498
0.1192
0.0443
0.0854
0.0225
0.0430
a GOF ) [∑w(Fo - Fc )2/(n - p)]1/2; n ) number of reflections, p ) number of parameters refined. b R1 ) ∑(|Fo - Fc|)/∑|Fo|; wR2 ) [∑w(Fo
-
2
2
2
2
4
Fc )2/∑wFo ]1/2
.
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for 2a,
2c, and 3
2a
2c
3
M-N1
1.984(2)
1.987(2)
N/A
1.9882(14)
1.9905(13)
N/A
2.3642(19)
2.0971(17)
2.1086(18)
2.3297(18)
M-N3
M-N4
M-N6
N/A
N/A
N-M-N (bite angle)
C-Nmeso-C
Θa
94.56(8)
126.3(2)
63.50
96.44(5)
127.93(14)
68.67
87.11(7), 86.51(7)
129.5(3), 128.5(2)
67.91
a Dihedral angle between six-membered chelate rings.
in 3, leading to a distorted tetrahedral arrangement of donor
nitrogen atoms at the metal center. The structures of 2a and
2c approximate D2 symmetry. Figure 3a shows a “top” view
of the structure of 2a, looking down the Nmeso-Zn-Nmeso
axis, clearly showing the deviation from D2d symmetry in
the complex. The planes shown in Figure 3a represent best-
fit planes of the two chelate rings.
We propose that intramolecular π-stacking is an essential
contributor to the crystallographically observed geometry.
In the solid state, the pyrrole ring of one of the azadipyr-
romethenes aligns parallel to and within π-stacking distance
of a proximal phenyl ring on the opposite azadipyrromethene
ligand. In 2a, a minimum distance of 3.629 Å is observed
between the centroids of adjacent rings; in 3, an even shorter
contact of 3.602 Å is observed, while in 2c, the smallest
such distance is 3.942 Å, which indicates more limited
interaction, if any. Figure 3b shows another view of 2a,
where the stacking interaction is apparent.
Figure 3. (a) Crystal structure of 2a illustrating the dihedral angle between
best-fit planes of the two six-membered chelate rings. (b) View of the crystal
structure of 2a showing π-stacking between the proximal phenyl group of
one ligand and a pyrrole moiety of the opposite ligand.
acetophenone),2 which are observed to be involved in π
stacking in the solid state, undergo a significant upfield shift
in the complexes. This upfield shift is consistent with the
existence of the stacking interactions in solution.18 While in
the solid state some rings are more closely stacked than
NMR spectra of the complexes indicate that π-stacking
interactions persist in solution. As previously mentioned, the
1H NMR resonances of the pyrrolic protons and the proximal
phenyl rings (i.e., the phenyl substituents deriving from
(18) Martin, C. B.; Mulla, H. R.; Willis, P. G.; Cammers-Goodwin, A. J.
Org. Chem. 1999, 64, 7802–7806.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2341

Teets et al.
Table 3. Summary of UV/Vis Absorption Wavelengths (nm) and Molar
Absorptivities (M-1 cm-1) for Ligands and Complexes
λmax, ꢀ ) near-UV
λ
max, ꢀ ) red
λ, ꢀ ) shoulder peak
1a
1b
1c
2a
2b
2c
3
302, 42 000
318, 32 000
301, 34 000
304, 64 000
321, 58 000
301, 60 000
303, 53 000
596, 46 000
620, 43 000
607, 42 000
591, 85 000
612, 86 000
598, 85 000
601, 80 000
640, 58 000
670, 55 000
645, 62 000
second distinct transition resolved as is in 2a. Careful
examination of the low-energy band in 3 shows that it is
unsymmetrical and tails out into the red region, possibly
indicating a second, unresolved transition at redder wave-
lengths. The spectra of both 2a and 3 show low-intensity
bands near 374 and 475 nm. The wavelength of the other,
near-UV absorption peak (ca. 300–320 nm) is largely
unshifted, changing by only 1–2 nm in the complexes relative
to free ligands, with the molar absorptivity also increasing.
Table 3 summarizes optical absorption data for all compounds.
The spectra of 2b and 2c are analogous to that of 2a.
Methoxy substitution on either the proximal or remote phenyl
rings bathochromically shifts the low-energy absorptions
relative to the parent tetraphenyl ligand and complex. When
the methoxy substituent is distal to the chelating nitrogens,
as in 1c and 2c, the corresponding absorptions shift by only
7–9 nm relative to 1a and 2a. However, in 1b and 2b, with
the methoxy group proximal to the chelating nitrogens, the
change is greater and the correlated absorption peaks red-
shift by 20–30 nm relative to 1a and 2a.
Figure 4. Absorption spectra of 1a, 2a, and 3, recorded in THF at 298 (
2 K. For comparison, the absorption spectrum of difluoroboron azadi-
pyrromethane 1a·BF2 is illustrated.
others, the (room temperature) NMR spectrum is perfectly
symmetric, indicating that the structure is fluxional on the
NMR time scale, giving an “average” structure in which all
rings undergo the same degree of π stacking.
Optical Absorption Spectra. One of the most attractive
features of azadipyrromethenes is their intense absorption
in the red region of the spectrum, making them ideally suited
for in vivo applications.1–6 Free azadipyrromethenes 1a-c
show two distinct absorption bands of nearly equal intensity,
one at ca. 600 nm and the other at ca. 300 nm. UV/vis
absorption spectra for the complexes were measured and
compared to the free azadipyrromethene ligands. Figure 4
overlays UV/vis absorption spectra for 2a, 3, and their parent
ligand, 1a, measured in THF at micromolar concentrations.
Also inlaid is the absorption spectrum of the boron adduct
1a · BF2. Spectra of difluoroboron azadipyrromethenes have
been extensively discussed.2 Upon binding of the Lewis
acidic fragment BF2+, the absorption maximum of the lowest-
energy transition red-shifts some 57 nm and its molar
absorptivity increases by 31%.
Complexes 2a and 3 show marked changes in their
absorption spectra relative to the free ligand 1a, notably in
the red region. In 1a, the absorption maximum occurs at 596
nm, with ꢀ ) 46 000 M-1 cm-1 (extinction coefficients are
normalized to molar concentrations of compounds, not to
the number of ligands). In 2a, λmax blue-shifts slightly to
591 nm, but the absorption intensity nearly doubles to 85 000
M-1 cm-1. Also readily apparent in the spectrum of 2a is
the development of a second absorption transition, which
appears as a shoulder at λ ) 640 nm, ꢀ ) 58 000 M-1 cm-1.
Thus, even though λmax is slightly blue-shifted in 2a-c
relative to 1a-c, the zinc(II) complexes absorb a much wider
range of red wavelengths because of this second absorption
transition. Gaussian deconvolution of the 500–800 nm region
of the spectra of 1a and 2a indicates maxima at 563.9 and
602.4 nm (1a) and 590.1 and 642.7 nm (2a) (Figures S3
and S4 in the Supporting Information). The spectrum of 3
likewise shows an intensified red-region absorption with λmax
) 601 nm and ꢀ ) 80 000 M-1 cm-1. However, there is no
The bis(azadipyrromethene)zinc(II) and -mercury(II) com-
plexes are nonemissive. Free ligands 1a-c are weak emitters,
with quantum yields of emission of 0.014 (1b) and smaller.
BF2 chelates of 1a-c are highly emissive, with Φf ) 0.34,
2
0.36, and 0.23, respectively.
Monoazadipyrromethene
coinage metal complexes also emit, albeit weakly.17 How-
ever, the zinc(II) and mercury(II) complexes show no
emission when excited anywhere within the absorption
profile. Rotation of the pendant phenyl groups may induce
nonradiative decay, and luminescence from group 12 com-
plexes of rigidified azadipyrromethenes may be realizable.4
Calculations. Semiempirical molecular orbital calculations
have been undertaken to rationalize the canted structure of
bis(azadipyrromethene) chelates, where the conjugated ligand
backbones stray from perpendicularity. Geometries of 2a-c
were optimized within the AM1 Hamiltonian19,20 beginning
from crystal structures (2a and 2c) or from an idealized D2d
geometry (2b). Symmetry was imposed during all geometry
optimization. All canted, D2-symmetric structures are po-
tential energy minima by harmonic frequency calculations.
The idealized D2d-symmetric structures have 8 (2a and 2b)
or 12 (2c) imaginary frequencies and are nonstationary
geometries. Two views of optimized 2a appear in Figure
5a. Optimization of the zinc(II) complex of the unsubstituted
azadipyrromethene produced an energy minimum having
perfect D2d symmetry (i.e., the planes of the ligand backbones
(19) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Steward, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902–3909.
(20) Dewar, M. J. S.; Merz, K. M., Jr. Organometallics 1988, 7, 522–524.
2342 Inorganic Chemistry, Vol. 47, No. 7, 2008

Azadipyrromethene Complexes of d10 Zinc and Mercury
Figure 5. (a) Optimized (AM1) structures of D2d and D2 structures of 2a. (b) Cartoon diagrams describing ligand canting in bis(azadipyrromethene)zinc(II)
complexes. Canting in mercury complex 3 is similar.
are orthogonal). This result indicates that the observed ligand
canting is not metal-imposed, as is expected for d10 metal
centers.
Recent investigations of azadipyrromethene complexes1–6
have emphasized the ligands’ optical absorption properties.
The INDO/S configuration interaction singles protocol27–34
was applied to bis(azadipyrromethene)zinc(II) complexes.
Calculations on the experimental geometries35 of 2a and 2c,
as opposed to AM1-optimized structures, afforded better
fidelity to the experimental absorption spectra. Implicit THF
solvation was included using a self-consistent reaction field
method.36
The calculations indicate that the intense absorption
features between ca. 500 and 800 nm (Figure 4) represent
several overlapping optically allowed transitions. A sizable
energy gap, ca. 7800 cm-1, separates these low-lying states
from the next higher-energy singlets. Agreement with the
observed absorption spectra is fair. The shoulder observed
The optimized geometry as depicted in Figure 5a is
congested and difficult to apprehend. Figure 5b depicts the
ligand canting in cartoon form. The proximal benzene rings
(i.e., the phenyl substituents nearer to the metal center) rotate
by the same angle and in the same direction. The opposing
azadipyrromethene ligand also tilts in the same direction, as
if in response to the initial leaning of the proximal phenyls.
The depiction in Figure 5b is not intended as a mechanism
for canting but as an aid in visualizing the geometries of the
new complexes. Figure 5b indicates that the energy of the
D2d f D2 distortion, as calculated by AM1, is 6.8 kcal mol-1
for 2a; corresponding figures are 14.9 kcal mol-1 (2b) and
15.2 kcal mol-1 (2c). A third process might be added to the
schematic of Figure 5b: formation of a π-stacking interaction
between the pyrrolyl ring of one ligand and a proximal
phenyl of the opposite ligand. The semiempirical calculations
herein fail to reproduce this interaction, presumably for lack
of an adequate treatment of dispersion.21 Present indications
are that azadipyrromethene complexes are much given to
π-stacking.17 Such interactions are abundantly precedented
outside of nucleic acid chemistry; they result primarily from
favorable electrostatics.22–26
(25) Newcomb, L. F.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116, 4993–
1994.
(26) Phillip, D.; Gramlich, V.; Seiler, P.; Diederich, F. J. Chem. Soc., Perkin
Trans. 2 1995, 875–886.
(27) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111–134.
(28) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T.
J. Am. Chem. Soc. 1980, 102, 589–599.
(29) Zerner, M. C. ReV. Comput. Chem. 1991, 2, 313–366.
(30) Martin, C. H.; Zerner, M. C. Inorganic Electronic Structure and
Spectroscopy; Wiley-Interscience: New York, 1999; Vol. 1, pp
555–660..
(31) Thompson, M. A.; Zerner, M. C. J. Am. Chem. Soc. 1991, 113, 8210–
8215.
(32) Thompson, M. A.; Glendening, E. D.; Feller, D. J. Phys. Chem. 1994,
98, 10465–10476.
(21) Cramer, C. J. Essentials of Computational Chemistry: Theories and
Models, 2nd ed.; Wiley: New York, 2004; p 149.
(33) Thompson, M. A.; Schenter, G. K. J. Phys. Chem. 1995, 99, 6374–
6386.
(22) Williams, V. E.; Lemieux, R. P.; Thatcher, G. R. J. J. Org. Chem.
1996, 61, 1927–1933.
(34) Thompson, M. A. J. Phys. Chem. 1996, 100, 14492–14507.
(35) Non-hydrogen atom positions are those determined crystallographi-
cally; bond lengths to hydrogen atoms were optimized with AM1,
with all other atoms being frozen.
(23) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1992, 114, 9701–9702.
(24) Cozzi, F.; Cinquini, M.; Annuziata, R.; Siegel, J. S. J. Am. Chem.
Soc. 1993, 115, 5330–5331.
(36) Karelson, M. M.; Zerner, M. C. J. Phys. Chem. 1992, 96, 6949–6957.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2343

Teets et al.
zinc(II) bis(azadipyrromethenes) are composed of four.
Table 4. One-Electron Components of Low-Energy, Optically
Allowed Transitions in Bis(azadipyrromethene)zinc Complexes As
Calculated within the INDO/S CIS Approximationa
At higher energies, between ca. 350 and 500 nm, numerous
transition energies occur, all of comparatively slight oscillator
strength. The INDO/S calculations find these to be ligand-
centered, as expected for complexes of zinc(II) and, by
extension, mercury(II). Configuration interaction in these
higher-energy states is extensive, and the calculated excita-
tions are combinations of one-electron promotions. An
intense feature occurs near 300 nm in all complexes. A
magnified view of this part of the spectrum appears in Figure
7. The absorption wavelengths of these features are insensi-
tive to metal-ion complexation (or its lack); they do vary
with ring substitution on the pendant phenyl groups. INDO/S
CI computations on 2a and 2c reproduce these near-UV
features and indicate them to be ligand-based. For 2a,
absorptions are calculated at 309.4 and 299.6 nm, respec-
tively; both are excitations to B1 states. In compound 2c,
where methoxy groups occupy the para positions of the
Compound 2a
energy oscillator
(nm) strengthb
principal orbital
contributions
interconfigurational
oscillator strengthb
685.8 0.4094 LUMO+1 (b3) r HOMO-1 (a)
LUMO (b2) r HOMO (b1)
0.2686
0.2464
1.2769
1.1936
0.2686
0.2464
1.2769
1.1936
595.3 0.1043 LUMO (b2) r HOMO-1 (a)
LUMO+1 (b3) r HOMO (b1)
581.9 0.0727 LUMO+1 (b3) r HOMO-1 (a)
LUMO (b2) r HOMO (b1)
535.4 2.3447 LUMO (b2) r HOMO-1 (a)
LUMO + 1 (b3) r HOMO (b1)
Compound 2c
681.8 0.4418 LUMO+1 (b3) r HOMO-1 (a)
LUMO (b2) r HOMO (b1)
0.3584
0.3036
1.2322
1.1481
0.3584
0.3036
1.2322
1.1481
585.9 0.0466 LUMO (b2) r HOMO-1 (a)
LUMO+1 (b3) r HOMO (b1)
564.8 0.1465 LUMO+1 (b3) r HOMO-1 (a)
LUMO (b2) r HOMO (b1)
545.3 2.3098 LUMO (b2) r HOMO-1 (a)
1
1
remote phenyl rings, the corresponding B1r A transition
is calculated at 300.0 nm. The dependence of the higher-
energy (near 300 nm) features on ring substitution indicates
phenyl group participation, and the INDO/S CI calculations
indeed suggest a contribution from pπ orbitals on these
substituents.
LUMO+1 (b3) r HOMO (b1)
a Calculations employed the crystallographic geometry with hydrogens
in AM1-optimized positions. All transitions are to 1B states. b Oscillator
strengths were calculated with use of the dipole length operator.31
at 643 nm (from Gaussian deconvolution) in the spectrum
of 2a is 39 nm blue-shifted from the calculated transition at
682 nm, whereas the predominant transition at 590 nm is
red-shifted by 45 nm from the calculated value. Some
assignments can be ventured.
Conclusions
Syntheses and characterization of d10 zinc(II) and mer-
cury(II) complexes of azadipyrromethene ligands are dem-
onstrated. The transition-metal coordination chemistry of
these interesting ligands, which are noted for their intense
absorption in the red region of the spectrum, has remained
under-explored. The complexes described herein adopt a
pseudotetrahedral geometry in the solid state, with a signifi-
cant distortion from D2d symmetry. Semiempirical molecular
orbital (AM1) calculations indicate that electronic relaxation
partly drives these distortions. They fail to account for
intramolecular ligand-ligand π-stacking, which reinforces
ligand canting (D2d f D2 symmetry). NMR spectra suggest
that these π interactions are maintained in solution. The
crystal structures also show that the meso nitrogen deviates
from perfect sp2 geometry upon metal binding. One of the
more prominent features of these complexes is their optical
absorption profiles. In the zinc(II) complexes, the molar
absorptivity for the peak absorbance nearly doubles relative
to the respective free ligand, and a second distinct absorption
transition is evident at longer wavelengths. The mercury(II)
complex also displays an intensified absorption in the red
region, but only one such band is observed. Transition-metal
chelation offers a novel means of tuning the absorption
properties of azadipyrromethenes, an emerging class of
organic fluorophores with several potential in vivo applica-
tions.
Table 4 summarizes principal components of calculated
singlet–singlet excitations and the associated oscillator
strengths. Figure 6 depicts optically allowed transitions
among low-lying singlet states of 2a alongside plots of
several pertinent orbitals. The calculations find that those
transitions with the greatest oscillator strengths have heavy
contributions from the one-electron b2 LUMO r a
HOMO-1 and b3 LUMO+1 r b1 HOMO excitations (both
1
to B2 states in D2 symmetry). The one-electron b2 LUMO
r b1 HOMO transition has roughly 20% of the oscillator
strength of either of these other one-electron excitations. The
ligand orbitals appearing in Figure 6 are recognizable
descendents of the molecular orbitals of the C2V-symmetric
azadipyrromethene anion, as diagrammed in the Supporting
Information. The b1 HOMO and a HOMO-1 of 2a are both
related to the a2 HOMO of the free ligand. The HOMO and
HOMO-1 differ in that the orbital phase on the lower
azadipyrromethene (as shown in Figure 6, left) reverses sign.
The b2 LUMO and b3 LUMO+1 are composed of the
individual LUMOs of the free ligand. In Figure 6, right, the
phase of the upper ligand alternates sign between the LUMO
and LUMO+1. The figure also indicates the relatively greater
oscillator strength associated with the b2 LUMO r a
HOMO-1 and b3 LUMO+1 r b1 HOMO one-electron
transitions, compared to other single transitions among the
frontier orbitals. These promotions contribute significantly
to the calculated transitions, which themselves are linear
combinations of one-electron excitations. Whereas the low-
energy transitions of monoazadipyrromethene complexes are
predominated by two one-electron transitions,17 those of
Experimental Section
General Procedures. Zn(OAc)·2H2O (OAc ) acetate) and all
solvents were obtained commercially and used without purification.
Tetraarylazadipyrromethene ligands2 1a-c were prepared as previ-
1
ously described. H NMR spectra were recorded on a Varian AS-
2344 Inorganic Chemistry, Vol. 47, No. 7, 2008

Azadipyrromethene Complexes of d10 Zinc and Mercury
Figure 6. Calculated (INDO/S, CIS) singlet transitions of 2a in the crystallographically determined geometry. Hydrogens are in AM1-optimized positions.
Darker arrows indicate excitations of higher calculated oscillator strength. THF solvation is included implicitly in a self-consistent reaction field model. Left
and right: frontier orbitals as indicated; 0.03 au contour level. Interconfigurational transitions having substantial one-electron oscillator strength (i.e., LUMO
r HOMO-1 and LUMO+1 r HOMO) are indicated with blue arrows along the outside of the figure; calculated excitation transitions (black arrows,
center) are linear combinations of these interconfigurational, one-electron promotions.
(CDCl3): δ 7.86 (dd, 8H), 7.49 (dd, 8H), 7.36–7.42 (m, 12H),
7.06–7.13 (m, 12H), 6.71 (s, 4H) ppm. UV/vis (THF): λ (ꢀ) 304
(64 000), 591 (85 000), 640(sh) (58 000) nm. Anal. Calcd for
C64H44N6Zn: C, 79.87; H, 4.61; N, 8.73. Found: C, 79.74; H, 4.38,
N, 8.54.
{[5-(4-Methoxyphenyl)-3-phenyl-1H-pyrrol-2-yl][5-(4-meth-
oxyphenyl)-3-phenylpyrrol-2-ylidene]amine}zinc(II) (2b). To a
solution of Zn(OAc)2 ·2H2O (12 mg, 0.055 mmol) in 10 mL of
THF was added a solution of 1b (54 mg, 0.11 mmol) in 10 mL of
THF. The resulting solution was stirred at room temperature for
12 h. All volatiles were removed by rotary evaporation; the resulting
residue was dried in vacuo, then taken up in ca. 5 mL of CH2Cl2,
and filtered through Celite. The solvent was evaporated by rotary
evaporation to give a dark-colored solid, which was recrystallized
from THF by vapor diffusion of pentane at room temperature to
give the product as a fibrous, purple solid. Yield: 52 mg (90%). 1H
NMR (CDCl3): δ 7.90 (dd, 8H), 7.44 (dd, 8H), 7.35–7.44 (m, 12H),
6.71 (s, 4H), 6.61 (d, J ) 8.8 Hz, 8H), 3.37 (s, 12H) ppm. UV/vis
(THF): λ (ꢀ) 321 (58 000), 612 (86 000), 673(sh) (55 000) nm. Anal.
Figure 7. Absorption features near 300 nm of bis(azadipyrromethene)
complexes, in THF.
Calcd for C68H52N6O4Zn: C, 75.44; H, 4.84; N, 7.76. Found: C,
74.85; H, 4.28, N, 7.67.
400 spectrometer operating at 400 MHz. Chemical shifts are
referenced to residual solvent resonances. UV/vis absorption spectra
were recorded on a Cary 5G spectrophotometer in tetrahydrofuran
(THF) solutions at micromolar concentrations at which all new
compounds obeyed Beer’s law.
{[3-(4-Methoxyphenyl)-5-phenyl-1H-pyrrol-2-yl][3-(4-meth-
oxyphenyl)-5-phenylpyrrol-2-ylidene]amine}zinc(II) (2c). To a
solution of Zn(OAc)2 ·2H2O (11 mg, 0.050 mmol) in 10 mL of
THF was added a solution of 1c (51 mg, 0.10 mmol) in 10 mL of
THF. The resulting solution was stirred at 40 °C for 12 h. The
reaction mixture was allowed to cool to room temperature, and all
volatiles were removed by rotary evaporation. The resulting solid
was taken up in ca. 10 mL of CH2Cl2 and filtered through Celite.
The solvent was removed by rotary evaporation to give a black
solid, which was recrystallized from THF by vapor diffusion of
pentane at room temperature to give the product as a crystalline,
gold-colored solid. Yield: 29 mg (54%). 1H NMR (CDCl3): δ 7.83
(d, J ) 9.2 Hz, 8H), 7.47 (dd, 8H), 7.02–7.10 (m, 12H), 6.97 (d,
J ) 8.8 Hz, 8H), 6.62 (s, 4H), 3.92 (s, 12H) ppm. UV/vis (THF):
[(3,5-Diphenyl-1H-pyrrol-2-yl)(3,5-diphenylpyrrol-2-ylidene)am-
ine]zinc(II) (2a). To a solution of Zn(OAc)2 ·2H2O (12 mg, 0.055
mmol) in 10 mL of THF was added a solution of 1a (50 mg, 0.11
mmol) in 10 mL of THF. The resulting solution was stirred at room
temperature overnight. The solvent was removed by rotary evapora-
tion, and the resulting residue was taken up in ca. 5 mL of CH2Cl2
and filtered through Celite. The solvent was removed by rotary
evaporation to give the product as a metallic, dark-colored solid.
The product was recrystallized from THF by vapor diffusion of
pentane at room temperature. Yield: 45 mg (84%). 1H NMR
Inorganic Chemistry, Vol. 47, No. 7, 2008 2345

Teets et al.
λ (ꢀ) 301 (60 000), 598 (85 000), 645(sh) (62 000) nm. Anal. Calcd
for C68H52N6O4Zn: C, 75.44; H, 4.84; N, 7.76. Found: C, 74.55;
H, 4.29; N, 7.72.
program Gaussian03.37 All calculations were spin-restricted with
singlet spin multiplicity. Geometries were optimized using the Berny
algorithm within redundant internal coordinates. Symmetry (D2d
or D2) was imposed on all optimizations. Harmonic frequency
calculations verified converged structures of D2 symmetry to be
energy minima.
Configuration-interaction singles (CIS) calculations were per-
formed on the crystallographically determined geometries of 2a
and 2c (except that C-H bond lengths are AM1-optimized values)
using the intermediate neglect of the differential overlap (INDO/
S) model of Zerner and co-workers,27–34 as implemented in the
program ArgusLab 4.0.1.38 The active space consisted of the 10
highest-occupied and 10 lowest-unoccupied spin-restricted orbitals
using a minimal-valence STO-6G basis set. Two-electron Coulomb
integrals were calculated with the Weiss parameter set to 1.2.
Implicit THF solvation (ꢀ ) 7.565; nD ) 1.408, 20 °C) was
incorporated through a self-consistent reaction field model where
excitation is instantaneous and only the electronic polarization of
the solvent responds to the excited-state charge distribution.
[(3,5-Diphenyl-1H-pyrrol-2-yl)(3,5-diphenylpyrrol-2-ylidene)
amine]mercury(II) (3). To a suspension of Hg(OAc)2 (47 mg, 0.15
mmol) in ∼6 mL of THF was added solid 1a (88 mg, 0.20 mmol)
followed by an additional 2 mL of THF. The reaction was stirred
at room temperature for 24 h, yielding a bright-blue solution. The
solvent was removed via rotary evaporation, and the resulting
residue was extracted into toluene (∼40 mL) and filtered through
Celite. Removal of the solvent via rotary evaporation gave a shiny
red residue, which was triturated with pentane to afford a dark-red
solid. The solid was collected, washed with 2 × 10 mL of pentane,
and dried in vacuo to give the product in pure form as judged by
NMR. Yield: 66 mg (61%). Analytically pure product was obtained
by recrystallization of the product from a concentrated THF solution
via vapor diffusion of pentane. 1H NMR (CDCl3): δ 7.79 (dd, 8H),
7.52 (dd, 8H), 7.36–7.38 (m, 12H), 7.10–7.13 (m, 12H), 6.93 (s,
4H) ppm. UV/vis (THF): λ (ꢀ) 303 (53 000), 601 (80 000) nm.
Anal. Calcd for C64H44N6Hg: C, 70.03; H, 4.04; N, 7.66. Found:
C, 68.76; H, 4.22; N, 6.98.
Acknowledgment. The authors thank Case Western
Reserve University and the donors of the Petroleum Research
Fund, administered by the American Chemical Society
(42312-G3 to T.G.G.) for support. T.S.T. thanks the Case
Western Reserve University SOURCE program for funding.
The diffractometer was funded by NSF Grant CHE0541766.
We thank L. Gao and M. A. Peay for experimental assistance.
Calculations. Semiempirical molecular orbital calculations used
the AM1 Hamiltonian19,20 and parameters as codified within the
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M. A.; Cheeseman, J. R.; Montgomery, Jr. J. A.; Vreven, T.; Kudin,
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Austin A. J.; Cammi R.; Pomelli C.; Ochterski J. W.; Ayala P. Y.;
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Supporting Information Available: X-ray crystallographic data
in CIF format, optical absorption spectra for 1b, 2b, 1c, and 2c,
calculated transition energies, HOMO and LUMO, and AM1-
optimized geometries. This material is available free of charge via
the Internet at http://pubs.acs.org.
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2346 Inorganic Chemistry, Vol. 47, No. 7, 2008