Germanium phthalocyanine, GePc, and the reduced complexes SiPc(pyridine)2 and GePc(pyridine)2 containing antiaromatic π-electron circuits

Article abstract of DOI:10.1021/ic701248u

The reaction of GeCl₂(dioxane) with K₂Pc(DMF) ₄ yields germanium phthalocyanine, GePc. GePc dissolves in pyridine to form GePc(py)₂. The ¹H NMR spectrum of GePc(py) ₂ and nucleus-independent chemical shift (NICS) calculations on GePc(NMe₃)₂ both show the presence of a strong paratropic ring current. That ring current, along with the bond-length alternation in the crystal structure of GePc(^tBuPy)₂, indicates the presence of an antiaromatic π-electron circuit in GePc(py)₂. SiPc(py) ₂ was synthesized, and its electronic structure is similar to that of GePc(py)₂.

Full text of DOI:10.1021/ic701248u

Inorg. Chem. 2007, 46, 7713−7715
Germanium Phthalocyanine, GePc, and the Reduced Complexes
SiPc(pyridine)2 and GePc(pyridine)2 Containing Antiaromatic
Circuits
π-Electron
Julie A. Cissell,† Thomas P. Vaid,*,† Antonio G. DiPasquale,‡ and Arnold L. Rheingold‡
Department of Chemistry and Center for Materials InnoVation, Washington UniVersity, St. Louis,
Missouri 63130, and Department of Chemistry and Biochemistry, UniVersity of California,
San Diego, La Jolla, California 92093
Received June 25, 2007
The reaction of GeCl2(dioxane) with K2Pc(DMF)4 yields germanium
phthalocyanine, GePc. GePc dissolves in pyridine to form GePc-
(py)2. The 1H NMR spectrum of GePc(py)2 and nucleus-independent
chemical shift (NICS) calculations on GePc(NMe3)2 both show the
presence of a strong paratropic ring current. That ring current,
along with the bond-length alternation in the crystal structure of
porphyrin ring system has occurred to yield a Ge(IV)
complex with an antiaromatic porphyrin ring system.
There is one previous report of the synthesis of germanium
phthalocyanine, GePc, accomplished by the reduction of
GePcCl2 with NaBH4,4 but that compound was later shown
to be instead germanium triazatetrabenzocorrole hydroxide,
Ge(TBC)OH.5 We found that it is possible to synthesize blue-
purple GePc by the reaction of GeCl2(dioxane) with K2Pc-
(DMF)4 in THF for 12 h at 22 °C. The composition of the
product isolated by filtration was GePc‚0.2THF‚2KCl.
Because of the very low solubility of GePc in all non-
coordinating solvents, it was not possible to separate it from
the KCl byproduct. As with Ge(TPP),3 it appears that THF
is not a strong enough ligand to coordinate to GePc to form
GePc(THF)2, and the THF in the product is merely occluded
solvent that was not removed under vacuum.
GePc(tBuPy)2, indicates the presence of an antiaromatic
π-electron
circuit in GePc(py)2. SiPc(py)2 was synthesized, and its electronic
structure is similar to that of GePc(py)2.
We have reported the synthesis and characterization of
Si(TPP)(THF)2 (TPP ) tetraphenylporphyrin), a complex in
which the oxidation state of silicon is Si(IV) and the
porphyrin ring system has an oxidation state of 4-.1 In
normal, aromatic porphyrin complexes, the ring system has
an oxidation state of 2- and the aromaticity is generally
considered to be due to an 18 π-electron circuit in the ring
system. The doubly reduced Si(TPP)(THF)2 has a 20
π-electron circuit in its ring system and is antiaromatic, as
indicated by C-C bond-length alternation along the 20-
carbon periphery of the porphine ring system and NMR
chemical shifts that indicate a strong paratropic2 ring current.
Si(TPP)(THF)2 was the first antiaromatic porphyrin complex
to be isolated.
More recently, we reported the synthesis of Ge(TPP) by
the reaction of GeCl2(dioxane) and Li2(TPP)(ether)2.3 In Ge-
(TPP), the oxidation state of germanium is Ge(II) and the
complex is aromatic. Ge(TPP) undergoes a reversible reac-
tion with pyridine, forming GeIV(TPP)(py)2, wherein a formal
transfer of two electrons from the germanium center to the
Characterization of GePc was complicated by its very low
solubility in solvents that do not form complexes of the type
GePc(L)2, where L is the solvent acting as a dative ligand
to germanium. Efforts at single-crystal growth of GePc for
X-ray diffraction were thwarted by its low solubility, and
sublimation did not yield single crystals. NMR spectroscopy
of GePc was not possible. Attempts to obtain the UV-vis
spectrum of GePc yielded spectra identical to that of Ge-
(TBC)OH (see Supporting Information).5 It seems there are
traces of Ge(TBC)OH in our sample of GePc‚0.2THF‚2KCl.
GePc has extremely low solubility in organic solvents, while
the solubility of Ge(TBC)OH is higher,5 so when a sample
of our product is placed in a solvent such as THF, the small
amount of Ge(TBC)OH in the sample dissolves while the
GePc remains as a solid. Treatment of our GePc with benzoyl
peroxide leads to a compound with a UV-vis spectrum
consistent with the normal phthalocyanine GePc(OBz)2. In
contrast, Ge(TBC)OH does not react with oxidizing agents
* To whom correspondence should be addressed. E-mail: vaid@
wustl.edu.
† Washington University in St. Louis.
4
‡ University of California, San Diego.
such as Br2 and could not be converted to GePc(OBz)2 by
(1) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005,
127, 12212-12213.
(2) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811-4815.
(3) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. J. Am. Chem. Soc. 2007,
129, 7841-7847.
(4) Stover, R. L.; Thrall, C. L.; Joyner, R. D. Inorg. Chem. 1971, 10,
2335-2337.
(5) Fujiki, M.; Tabei, H.; Isa, K. J. Am. Chem. Soc. 1986, 108, 1532-
1536.
10.1021/ic701248u CCC: $37.00
Published on Web 08/18/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 19, 2007 7713

COMMUNICATION
Figure 2. X-ray crystal structure of GePc(tBuPy)2.
Figure 1. 1H NMR spectrum of GePc(py)2 in C6D6. Resonances labeled
“a” are due to coordinated pyridine, “b” are free pyridine, and “c” are
phthalocyanine ring resonances.
benzoyl peroxide, so it appears that the bulk of our product
is GePc. Further evidence for the identity of our product is
provided by its conversion to GePc(L)2 complexes and the
characterization of those compounds (see below). The small
amount of Ge(TBC)OH probably originates from traces of
water in the reaction of GeCl2(dioxane) and K2Pc(DMF)4.
GePc dissolved in pyridine over the course of several hours
to form a bright purple solution of GePc(pyridine)2. Precipi-
tation with hexane yielded brown-purple GePc(py)2‚2.7KCl
(the KCl remains from the GePc‚0.2THF‚2KCl starting
material, and it appears that the KCl was more completely
precipitated than GePc(py)2 was in this preparation). The
UV-vis spectrum of GePc(py)2 is given in the Supporting
Information.
1
GePc(py)2 has some solubility in benzene; its H NMR
spectrum in C6D6 is shown in Figure 1. The resonances of
the coordinated pyridine, at 21.7, 9.8, and 9.0 ppm for the
2-, 3-, and 4-hydrogens, respectively, indicate the presence
of a strong paratropic ring current, characteristic of an
antiaromatic compound.2 The Pc ring proton resonances
occur at 5.1 and 4.3 ppm, upfield from the typical resonances
for a normal phthalocyanine complex, which occur at about
9.6 and 8.3 ppm.6 (There are actually four types of chemically
inequivalent Pc protons in GePc(py)2, as explained below,
but they are interconverting quickly on the NMR time scale.)
The Pc ring resonances of GePc(py)2 are in the range
Figure 3. Valence-bond structure of GePc(tBuPy)2 with bond distances
(Å) from the crystal structure of GePc(tBuPy)2 (black) and calculated
(B3LYP/6-31G*(C,H,N),6-311G*(Ge)) for GePc(py)2 (blue). C-C and
C-Nmeso distances are given outside the ring, while Ge-N and C-Nisoindole
distances are given inside the ring. The germanium atom is on a
crystallographic inversion center, and symmetry-equivalent distances are
not shown.
311G*(Ge)) bond lengths for GePc(py)2. The valence-bond
structure of Figure 3, with four covalent bonds to Ge and
the single- and double-bond pattern throughout the rest of
the ring system that those four covalent Ge-N bonds require,
appears to be a reasonable representation of the electronic
structure of the molecule. If the aromaticity of a normal-
valent phthalocyanine ring system is considered to be due
to an 18 π-electron circuit (although the situation is really
more complex for this polycyclic system), then the doubly
reduced GePc(py)2 should contain an antiaromatic 20
π-electron circuit. That circuit is highlighted in purple in
Figure 3. Because the molecule is antiaromatic, the structure
with the double and single bonds of the purple 20-atom
circuit interconverted is not in resonance with the one shown,
but instead the alternating-bond structure is the ground state,
as in cyclobutadiene. The difference in bond length between
the formal single and double CR-Nmeso bonds is 0.069 Å,
CR-Câ is 0.057 Å, and Câ-Câ is 0.024 Å. The bond-length
differences for CR-Nmeso and CR-Câ are very similar to those
observed for Si(TPP)(THF)2, while the Câ-Câ bond-length
1
expected for nonaromatic alkene protons. Overall, the H
NMR data from GePc(py)2 indicate that there is an antiaro-
matic π-electron circuit in the central part of the phthalo-
cyanine ring system and the peripheral benzo groups have
lost some of their aromaticity relative to a normal phthalo-
cyanine complex. The electronic structure is discussed further
below.
Crystals of GePc(tBuPy)2 (tBuPy ) 4-tert-butylpyridine)
suitable for single-crystal X-ray diffraction were grown by
slowly cooling a solution of GePc in hot 4-tert-butylpyridine.
The structure is shown in Figure 2. In contrast to the
1
porphyrin complexes Si(TPP)(THF)2 and Ge(TPP)(py)2,3
there is no ruffling in GePc(tBuPy)2 and the ring system is
nearly planar. However, there is a distinct bond-length
alternation in GePc(tBuPy)2, as in Si(TPP)(THF)2 and Ge-
(TPP)(py)2. The bond lengths are given in Figure 3, along
with the calculated (Gaussian03,7 B3LYP/6-31G*(C,H,N),6-
(6) Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever,
A. B. P., Eds.; VCH Publishers, Inc.: New York, 1989; Vol. 1-4.
(7) Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT,
2004.
7714 Inorganic Chemistry, Vol. 46, No. 19, 2007

COMMUNICATION
represent a positive NICS(1) value and a paratropic ring
current. The area of each circle is proportional to the absolute
value of its calculated NICS(1) (all NICS(1) values are given
in the Supporting Information). As expected for MgPc, at
all locations there are negative NICS(1) and the entire system
is aromatic. For GePc(NMe3)2, there are large, positive NICS-
(1) in the inner ring system and clearly a strong paratropic
ring current. The largest value is +55.2, calculated 1 Å above
the midpoint of the two isoindole C-N bonds. The smaller
positive NICS(1) values at the centroids of the pyrrole rings
indicate that the paratropic ring current of the 20 π-electron
antiaromatic circuit (highlighted in purple) splits between
the inner and outer paths through the pyrrole rings. The small,
negative NICS(1) values for the benzo groups are consistent
with their decreased aromaticity. It is somewhat surprising
that the calculated NICS(1) are not significantly different
for the top/bottom and left/right benzo groups, while the
calculated geometry does correctly predict the bond-length
alternation in the top/bottom benzo groups and the lack of
bond-length alternation in the left/right benzo groups.
Treatment of silicon phthalocyanine dichloride, SiPcCl2,
with 2 equiv of Na/Hg in THF at 22 °C for 12 h gave dark
gray-purple SiPc(THF)2. Dissolution of SiPc(THF)2 in py-
ridine and precipitation of the product with hexanes yielded
dark purple SiPc(py)2. The 1H NMR spectrum of SiPc(py)2
in C6D6 has resonances for coordinated pyridine at 19.9, 9.5,
and 8.8 ppm for the 2-, 3-, and 4-hydrogens, respectively,
and Pc ring resonances at 5.3 and 4.6 ppm. Those chemical
shifts indicate a slightly smaller paratropic ring current in
SiPc(py)2 than in GePc(py)2, although the overall electronic
structures of SiPc(py)2 and GePc(py)2 appear to be quite
similar. The NICS(1) calculated for SiPc(NMe3)2 are similar
to those for GePc(NMe3)2. We made many attempts at
growing crystals of SiPc(L)2 (with L ) THF, pyridine, 4-tert-
butylpyridine, DMF, or anisole) suitable for single-crystal
X-ray diffraction, but none were successful. The calculated
structure of SiPc(py)2 (B3LYP/6-31G*) has bond-length
alternation similar to that in the calculated structure of GePc-
(py)2.
Figure 4. NICS(1) values for MgPc and GePc(NMe3)2. Blue circles
represent negative NICS(1), and red circles represent positive NICS(1). The
area of each circle is proportional to the absolute value of the NICS(1) at
that point.
difference in GePc(tBuPy)2 is about half as large as that in
Si(TPP)(THF)2. That is presumably due to the fact that the
Câ-Câ bonds in GePc(tBuPy)2 are also part of the fused
benzo groups. The four benzo groups are not all equivalent
in GePc(tBuPy)2. The benzo groups to the left and right in
Figure 3 show only very little, if any, bond-length alternation,
and are not significantly different from the benzo groups in
a normal-valent phthalocyanine.8 That is consistent with the
valence-bond picture of Figure 3, where the left and right
benzo groups have a benzenoid aromatic sextet.9 The top
and bottom benzo groups, on the other hand, exhibit signi-
ficant bond-length alternation. The average difference be-
tween the Câ-Cγ and Cγ -Cδ bonds is 0.056 Å. That implies
that those benzo groups have lost some of their aromaticity,
consistent with the 1H NMR data and the nucleus-
independent chemical shift (NICS) calculations below.
NICS10 reveals the presence of diatropic or paratropic ring
currents. In a conjugated, monocyclic π-system, a diatropic
ring current is indicative of aromaticity and a paratropic ring
current is indicative of antiaromaticity.2 The question of
aromaticity and antiaromaticity becomes somewhat more
complicated for polycyclic systems such as phthalocyanines
and porphyrins, but NICS calculations at several points
within the molecule will give an overall picture of the ring
currents present. NICS(1) calculations, performed 1 Å out
of the plane of the ring system in question, have been shown
to be a better gauge of ring currents than NICS(0) calcula-
tions, which are performed in the plane of the ring system.11
Figure 4 shows the results of NICS(1) calculations on MgPc
and GePc(NMe3)2. Calculations were done at the B3LYP/
6-31G* level, with the 6-311G* basis set used for the
germanium atom. Trimethylamine ligands were used in place
of pyridine to avoid effects from the pyridine ring current.
In Figure 4, the blue circles represent a negative NICS(1)
value and a diatropic ring current while the red circles
In conclusion, we have synthesized, isolated, and charac-
terized the first phthalocyanine complexes containing anti-
aromatic π-electron circuits. The crystal structure of GePc-
(tBuPy)2 shows that the antiaromaticity results in an alternating
single- and double-bond structure that has not been previ-
ously observed in phthalocyanine complexes.
Acknowledgment. Funding was provided by NSF Grant
No. CHE-0133068, and the Washington University compu-
tational chemistry resource was supported by NSF Grant No.
CHE-0443511.
(8) Matsumoto, S.; Matsuhama, K.; Mizuguchi, J. Acta Crystallogr. C
1999, 55, 131-133.
Supporting Information Available: Synthetic details, spectra,
table of NICS(1), calculated structure of SiPc(NMe3)2, complete
ref 7, and crystallographic details, including a CIF file. This material
is available free of charge via the Internet at http://pubs.acs.org.
(9) Clar, E. The Aromatic Sextet; John Wiley and Sons: New York, 1972.
(10) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P.
v. R. Chem. ReV. 2005, 105, 3842-3888.
(11) Schleyer, P. v. R.; Manoharan, M.; Wang, Z.-X.; Kiran, B.; Jiao, H.;
Puchta, R.; van Eikema Hommes, N. J. R. Org. Lett. 2001, 3, 2465-
2468.
IC701248U
Inorganic Chemistry, Vol. 46, No. 19, 2007 7715