Reversible oxidation state change in germanium(tetraphenylporphyrin) induced by a dative ligand: Aromatic GeII(TPP) and antiaromatic GeIV(TPP)(pyridine)2

Article abstract of DOI:10.1021/ja070794i

Treatment of GeCl₂(dioxane) with Li₂(TPP)(OEt ₂)₂ (TPP = tetraphenylporphyrin) in THF yields Ge(TPP), the first free Ge(II) porphyrin complex. In pyridine Ge(TPP) is converted to Ge(TPP)(py)₂, an antiaromatic Ge(IV) complex, whereas in benzene the reaction is reversed, and pyridine dissociates from Ge(TPP)(py)₂ to form Ge(TPP). That reversible reaction represents an unusual, if not unique, example of an oxidation-state change in a metal induced by coordination of a dative ligand. UV-vis and ¹H NMR spectroscopy show that Ge(TPP) is an aromatic Ge(II) porphyrin complex, while the ¹H NMR spectrum of Ge(TPP)(py)₂ clearly indicates the presence of a strong paratropic ring current, characteristic of an antiaromatic compound. Both Ge(TPP) and Ge(TPP)(py)₂ have been crystallographically characterized, and the antiaromaticity of Ge(TPP)(py)₂ leads to alternating short and long C-C bonds along the 20-carbon periphery of its porphine ring system. Coordination of pyridine to Ge(TPP) greatly increases its reducing ability: the Ge(TPP)^0/2+ redox potential is about +0.2 V, while the Ge(TPP)(py)₂^0/+ redox potential is -1.24 V (both vs. ferrocene). The equilibrium constant of the reaction Ge(TPP) + 2 py = Ge(TPP)(py)₂ in C₆D₆ is 22 M^-2. The germanium complex of the more electron-withdrawing tetrakis[3,5- bis(trifluoromethyl)-phenyl]porphyrin, Ge(TAr^FP), and its pyridine adduct Ge(TAr^FP)(py)₂ were synthesized. The equilibrium constant of the reaction Ge(TAr^FP) + 2 py = Ge(TAr ^FP)(py)₂ in C₆F₆/C₆D ₆ is 2.3 × 10⁴ M^-2. Density functional theory calculations are consistent with the experimental observation that M(TPP)(py)₂ formation from M(TPP) and pyridine is most favorable for M = Si, borderline for Ge, and unfavorable for Sn.

Full text of DOI:10.1021/ja070794i

Published on Web 06/06/2007
Reversible Oxidation State Change in
Germanium(tetraphenylporphyrin) Induced by a Dative
Ligand: Aromatic GeII(TPP) and Antiaromatic
GeIV(TPP)(pyridine)2
Julie A. Cissell,‡ Thomas P. Vaid,*,‡ and Glenn P. A. Yap§
Contribution from the Department of Chemistry and Center for Materials InnoVation,
Washington UniVersity, St. Louis, Missouri 63130, and Department of Chemistry and
Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received February 3, 2007; E-mail: vaid@wustl.edu
Abstract: Treatment of GeCl2(dioxane) with Li2(TPP)(OEt2)2 (TPP ) tetraphenylporphyrin) in THF yields
Ge(TPP), the first free Ge(II) porphyrin complex. In pyridine Ge(TPP) is converted to Ge(TPP)(py)2, an
antiaromatic Ge(IV) complex, whereas in benzene the reaction is reversed, and pyridine dissociates from
Ge(TPP)(py)2 to form Ge(TPP). That reversible reaction represents an unusual, if not unique, example of
an oxidation-state change in a metal induced by coordination of a dative ligand. UV-vis and 1H NMR
spectroscopy show that Ge(TPP) is an aromatic Ge(II) porphyrin complex, while the 1H NMR spectrum of
Ge(TPP)(py)2 clearly indicates the presence of a strong paratropic ring current, characteristic of an
antiaromatic compound. Both Ge(TPP) and Ge(TPP)(py)2 have been crystallographically characterized,
and the antiaromaticity of Ge(TPP)(py)2 leads to alternating short and long C-C bonds along the 20-
carbon periphery of its porphine ring system. Coordination of pyridine to Ge(TPP) greatly increases its
reducing ability: the Ge(TPP)0/2+ redox potential is about +0.2 V, while the Ge(TPP)(py)20/+ redox potential
is -1.24 V (both vs. ferrocene). The equilibrium constant of the reaction Ge(TPP) + 2 py ) Ge(TPP)(py)2
in C6D6 is 22 M-2. The germanium complex of the more electron-withdrawing tetrakis[3,5-bis(trifluoromethyl)-
phenyl]porphyrin, Ge(TArFP), and its pyridine adduct Ge(TArFP)(py)2 were synthesized. The equilibrium
constant of the reaction Ge(TArFP) + 2 py ) Ge(TArFP)(py)2 in C6F6/C6D6 is 2.3 × 104 M-2. Density functional
theory calculations are consistent with the experimental observation that M(TPP)(py)2 formation from M(TPP)
and pyridine is most favorable for M ) Si, borderline for Ge, and unfavorable for Sn.
Introduction
germanium(IV) porphyrin complexes of the type Ge(porph)X2
are known.8-15 The one known type of germanium porphyrin
Many porphyrin complexes of group 14 elements have been
prepared, and the oxidation states of the group 14 atoms within
the known compounds follow a trend that is consistent with
the fact that the inert-pair effect1-3 becomes more important
lower in the periodic table: silicon and germanium are always
present as Si(IV) and Ge(IV) (with one type of exception for
germanium), tin is found as both Sn(IV) and Sn(II), and almost
all known lead porphyrins contain Pb(II). A normal porphyrin
ring system (excluding the central atom) has an oxidation state
of 2-, so the oxidation state of silicon is Si(IV) in a complex
of the type Si(porph)X2, where “porph” is a porphyrin ring
system and X is a substituent covalently bound to silicon.
Examples of X in known Si(porph)X2 complexes include
chloride, fluoride, triflate, and alkyl groups.4-7 Similarly, many
in which the germanium is best considered Ge(II) is (porph)-
GeFe(CO)4, wherein the germanium acts as a carbene-like two-
electron donor to the iron center.16,17 Some complexes of the
type Sn(porph), containing Sn(II), have been characterized,18,19
and a large number of Sn(IV) complexes, Sn(porph)X2, are
known.20 Several Pb(porph) complexes have been structurally
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R. J. Am. Chem. Soc. 1987, 109, 7705-7714.
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Q. Y.; Kadish, K. M. J. Chem. Soc., Dalton Trans. 1988, 1109-1113.
(13) Guilard, R.; Barbe, J. M.; Boukhris, M.; Lecomte, C. J. Chem. Soc., Dalton
Trans. 1988, 1921-1925.
‡ Washington University in St. Louis.
§ University of Delaware.
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3886-3891.
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9
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J. AM. CHEM. SOC. 2007, 129, 7841-7847
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A R T I C L E S
Cissell et al.
characterized,21,22 while Pb(OEP)Cl2 (OEP ) octaethylpor-
phyrin) is the only Pb(IV) porphyrin complex that has been
isolated and a few other Pb(IV) porphyrins have been studied
in solution.24,25
23
Ge(TPP). Li2(TPP)(Et2O)2 (410 mg, 0.529 mmol) and GeCl2-
(dioxane) (124 mg, 0.535 mmol) were stirred in 25 mL of THF at 22
°C for 12 h. Soon after warming to 22 °C the solution turned from the
vibrant dark blue-green color of Li2TPP(Et2O)2 to olive green. Eventu-
ally, a green microcrystalline precipitate formed. The solution was
reduced under vacuum to about 8 mL and then filtered, and the product
was held under vacuum for 1 h. Yield of green Ge(TPP): 235 mg,
65%. 1H NMR (C6D6): δ 9.00 (s, 8 H), 8.08 (dd, 8H), 7.44 (m, 12H).
UV-vis (toluene, nm): 395, 442, 495, 702. IR (Nujol, cm-1): 1826
(w), 1595, 1529 (w), 1506 (w), 1330 (m), 1200 (m), 1173 (s), 1069
(m), 1010 (w), 1000 (m), 982 (s), 914 (w), 876 (w), 802 (s), 749 (s),
717 (w), 702 (m), 661 (w). Anal. Calcd (found) for C44H28N4Ge: C,
77.11 (77.25); H, 4.12 (4.06); N, 8.18 (8.08).
We recently reported that the reduction of silicon tetraphe-
nylporphyrin dichloride, Si(TPP)Cl2, with two equivalents of
Na/Hg in THF yields the complex Si(TPP)(THF)2, which can
be converted to Si(TPP)(pyridine)2.26 In the complexes Si(TPP)-
(L)2, where L is a dative ligand, silicon does not adopt the Si-
(II) oxidation state, but instead remains Si(IV), and the porphyrin
ring system therefore has an oxidation state of 4-. The
aromaticity of a normal-valent porphyrin is generally attributed
to its 18 π-electron system; as a result, in the doubly reduced
ring system of Si(TPP)(L)2 one would expect an antiaromatic,
20 π-electron system. Exactly such a 20 π-electron circuit of
alternating double and single carbon-carbon bonds is present
along the periphery of the porphine ring of Si(TPP)(THF)2, as
observed in the crystal structure. The antiaromaticity of the Si-
(TPP)(L)2 compounds is also manifested in their NMR spectra,
which have highly shifted resonances due to a strong paratropic
ring current.27 These Si(TPP)(L)2 complexes were the first
antiaromatic porphyrins to be isolated.
Ge(TPP)(pyridine)2‚Pyridine. A suspension of Ge(TPP) (204 mg,
0.298 mmol) in 15 mL of pyridine was stirred at 22 °C for 48 h. The
solid slowly dissolved over the course of 24-36 h to form a bright-
red solution. The pyridine was almost completely removed, and a
microcrystalline material began to precipitate. Hexane was added to
precipitate more of the product. The product was isolated by filtration
and held under vacuum for 1 h. Yield of red-purple Ge(TPP)(pyridine)2‚
pyridine: 192 mg, 70%. UV-vis (pyridine, nm): 426, 482, 522, 563,
1
very broad 770. H NMR (pyridine-d5): δ 6.58 (m, 12 H), 5.86 (dd,
8H), 0.59 (br s, 8H). IR (Nujol, cm-1): 1610 (m), 1596 (m), 1573 (w),
1519 (w), 1358 (w), 1318 (w), 1241 (m), 1210 (m), 1192 (m), 1156
(m), 1067 (s), 1044 (s), 1015 (s), 988 (s), 917 (w), 761 (m), 746 (s),
690 (s), 661 (w), 644 (m).
Our original motivation for the synthesis of the highly reduced
Si(TPP)(L)2 molecules was to utilize them as n-dopants for other
porphyrin molecular semiconductors. The highly ruffled struc-
ture of Si(TPP)(THF)2 (and presumably of other Si(TPP)(L)2)
made it unsuitable for that purpose. The ruffling of Si(TPP)-
(THF)2 is caused, at least in part, by the small covalent radius
of silicon and the short Si-N bonds in the porphyrin complex.
We therefore decided to investigate Ge(TPP)(L)2, where we
anticipated that the larger covalent radius of germanium would
lead to a more nearly planar porphyrin ring. The synthesis and
characterization of Ge(TPP) and Ge(TPP)(py)2 are described
below, in addition to the synthesis of the germanium complex
of the more electron-withdrawing tetrakis[3,5-bis(trifluoro-
methyl)phenyl]porphyrin, Ge(TArFP), and its pyridine adduct
Ge(TArFP)(py)2.
H2(TArFP) was synthesized by the Lindsey method31 under the
specific conditions used previously for the synthesis of tetrakis-
(pentafluorophenyl)porphyrin.32 Pyrrole (0.90 mL, 12.7 mmol), 3,5-
bis(trifluoromethyl)benzaldehyde (2.1 mL, 12.7 mmol), and 750 mL
of CH2Cl2 were combined and degassed. BF3(OEt2) (0.45 mL, 3.5
mmol) was added under N2 counterflow. The solution was stirred under
N2 at 22 °C for 20 h. 2,3-Dichloro-5,6-dicyano-p-benzoquinone, DDQ,
(2.89 g, 12.7 mmol) was added, and the mixture was heated to reflux
for 2 h. Volatiles were evaporated, and the product was purified by
chromatography on neutral alumina with 7:3 chloroform/hexanes, and
recrystallized from CH2Cl2/CH3OH. Yield of pale brownish-pink H2-
1
(TArFP): 1.24 g, 34%. H NMR (CDCl3): δ 8.82 (s, 8 H), 8.70 (s,
8H), 8.39 (s, 4H), -2.87 (s, 2H). UV-vis (CH2Cl2, nm): 417, 512,
545, 587. IR (cm-1): 3316 (w), 1830 (w), 1619 (w), 1561(w), 1464
(m), 1316 (w), 1276 (s), 1253 (m), 1199 (s), 1164 (s), 1133 (s), 1107
(m), 1078 (w), 1054 (w), 1028 (w), 1014 (w), 979 (w), 920 (w), 904
(m), 847 (m), 807 (m), 794 (m), 726 (m), 708 (w), 692 (m), 680 (m).
Ge(TArFP). Li2(ArFP)(OEt2)2 was synthesized by deprotonation of
H2(TArFP) with LiN(SiMe3)2 in ether.28 The synthesis of Ge(TArFP)
was analogous to that of Ge(TPP), except that the product was
precipitated from THF using heptane. Yield of dark green-brown Ge-
Experimental Section
General Procedures and Materials. All manipulations were carried
out using Schlenk line or glove box techniques unless otherwise noted.
Reagents were purchased from commercial suppliers and used as
received unless their purification is noted as follows. Ethereal solvents
were distilled from a purple sodium benzophenone solution, and
hydrocarbon solvents were distilled from purple sodium benzophenone
solutions with added tetraglyme. Pyridine was distilled from potassium
hydroxide and stored over activated 3 Å molecular sieves. C6F6 was
dried over activated 3 Å molecular sieves. Li2(TPP)(Et2O)228 and [NEt4]-
[B(C6F5)4]29,30 were prepared by published procedures.
1
(TArFP): 218 mg, 65%. H NMR (C6F6 with small amt of C6D6): δ
9.20 (s, 8 H), 9.00 (s, 8H), 8.58 (s, 4H). UV-vis (R,R,R-trifluorotolu-
ene, nm): 394, 442, 497, 720. IR (Nujol, cm-1): 1617 (w), 1462 (s),
1278 (vs), 1184 (vs), 1132 (vs), 1078 (m), 1037 (m), 941 (w), 923
(w), 902 (s), 848 (m), 832 (w), 787 (m), 709 (w), 694 (s), 681 (s).
Ge(TArFP)(pyridine)2‚Pyridine. Ge(TArFP) (101 mg, 0.0822 mmol)
was dissolved in 5 mL of pyridine. The red-brown solution was allowed
to stir for 15 min, the pyridine was almost completely removed, and
hexanes was added to precipitate the product. The product was isolated
by filtration and held under vacuum for 1 h. Yield of red-gray Ge-
(TArFP)(pyridine)2‚pyridine: 102 mg, 85%. 1H NMR (C6F6 with small
amt of C6D6): δ 21.7 (pyridine, d, 4 H, J ) 5.5 Hz), 11.71 (pyridine,
t, 4H, J ) 7.1 Hz), 11.19 (pyridine, t, 2H, J ) 7.1 Hz), 7.25 (s, 4H),
(20) Arnold, D. P.; Blok, J. Coord. Chem. ReV. 2004, 248, 299-319.
(21) Barkigia, K. M.; Fajer, J.; Adler, A. D.; Williams, G. J. B. Inorg. Chem.
1980, 19, 2057-2061.
(22) Plater, M. J.; Aiken, S.; Gelbrich, T.; Hursthouse, M. B.; Bourhill, G.
Polyhedron 2001, 20, 3219-3224.
(23) Gouterman, M.; Schwarz, F. P.; Smith, P. D.; Dolphin, D. J. Chem. Phys.
1973, 59, 676-690.
1
6.33 (s, 8H), 0.22 (s, 8H). H NMR (pyridine-d5): δ 7.34 (s, 4 H),
(24) Ferguson, J. A.; Meyer, T. J.; Whitten, D. G. Inorg. Chem. 1972, 11, 2767-
2772.
(25) Richoux, M. C.; Neta, P.; Harriman, A. J. Chem. Soc., Faraday Trans. 2
1986, 82, 201-207.
(29) Stehling, U. M.; Stein, K. M.; Kesti, M. R.; Waymouth, R. M. Macro-
molecules 1998, 31, 2019-2027.
(26) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127,
12212-12213.
(30) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 39, 248-250.
(31) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz,
A. M. J. Org. Chem. 1987, 52, 827-836.
(27) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811-4815.
(28) Brand, H.; Capriotti, J. A.; Arnold, J. Inorg. Chem. 1994, 33, 4334-4337.
(32) Volz, H.; Schneckenburger, S. J. Prakt. Chem. 1993, 335, 283-284.
9
7842 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

Aromatic Ge(TPP) and Antiaromatic Ge(TPP)(py)2
A R T I C L E S
Figure 1. Solid-state structure of Ge(TPP). Important independent bond
lengths (see Figure 4 for atom-type labels): Ge-N, 2.194(3) Å; N-CR,
1.373(5), 1.376(5) Å; CR-Câ, 1.435(6), 1.433(6) Å; Câ-Câ 1.366(6) Å;
CR-Cmeso 1.394(6), 1.401(6) Å.
Figure 2. UV-vis spectrum of Ge(TPP) in toluene with maxima at 395,
442, 495, and 702 nm. The small absorption at 420 nm is due to a small
amount of oxidized species.
6.52 (s, 8H), 0.58 (bs, 8H). UV-vis (pyridine, partly unresolved): 427
nm. IR (Nujol, cm-1): 3081 (vw), 3059 (vw), 3046 (vw), 1613 (m),
1596 (m), 1582 (w), 1572 (w), 1529 (w), 1488 (m), 1452 (s), 1340 (s),
1280(vs), 1234 (s), 1177 (vs), 1131 (vs), 1069 (s), 1038 (m), 1006
(m), 927 (m), 899 (s), 847 (m), 837 (m), 768 (s), 753 (s), 702 (s), 693
(s), 680 (s), 644 (w), 626 (m), 604 (m).
hundreds of crystal structures of normal-valent, aromatic por-
phyrins34 and do not correspond to the averaged bond lengths
of an antiaromatic germanium porphyrin, GeIV(TPP)(pyridine)2
(see below). Additional evidence that Ge(TPP) contains Ge(II)
comes from the large deviation of the germanium atom from
the porphyrin plane. The covalent radius of germanium, 1.22
Å,35 is not too large to be accommodated in the porphyrin plane;
thus, the size of the germanium atom is not the cause of its
deviation from the plane. Rather, it is the presence of a
stereochemically active lone pair on Ge(II). In the computed
structure of Ge(porphine) (B3LYP/6-311G**), the germanium
atom is 0.868 Å above the N4 plane, and the computed HOMO
has a large component where the germanium lone pair would
be expected (see Supporting Information for a picture of the
HOMO). This view of the electronic structure of Ge(TPP) is
consistent with the known (porphyrin)GeFe(CO)4 complexes,
wherein Ge(II) acts as a carbene-like donor to iron.16,17
Results and Discussion
Synthesis and Characterization of Ge(TPP). Reduction of
Ge(TPP)Cl2 with 2 equiv of Na/Hg in THF (the conditions used
to produce Si(TPP)(THF)2) did not give a tractable product.
However, treatment of GeCl2(dioxane) with Li2(TPP)(OEt2)2 in
THF at 22 °C for 12 h gave green, microcrystalline, air-sensitive
Ge(TPP) in 65% yield. The behavior of the product in THF
contrasts with that of the analogous silicon porphyrin, which
forms Si(TPP)(THF)2; we were unable to isolate free Si(TPP).26
Single crystals of Ge(TPP) suitable for X-ray diffraction were
grown by slowly cooling a hot solution in THF. The crystal
structure is shown in Figure 1. In the refined structure the
porphine core of the ring system (i.e., the ring system excluding
the phenyl groups) appears planar, while the germanium atom
is 0.872 Å above the porphine plane. However, in the crystals
the Ge(TPP) molecules are disordered, with half of the
germanium atoms “above” the porphyrin plane and half “below”.
If the molecules are domed, with the nitrogens all slightly out
of the mean plane in the direction of the germanium atom and
the rest of the ring system domed appropriately, then in the
refined structure the porphine core would appear exactly planar.
An examination of the atomic displacement ellipsoids in the
refined structure reveals that the outermost carbons of the phenyl
rings have a large component of their ellipsoids along the
vertical direction, as would be expected for domed Ge(TPP)
with 1:1 “up” and “down” disorder. In addition, the calculated
structure of Ge(porphine) (Gaussian03,33 B3LYP/6-311G**) is
slightly domed, with an angle between the least-squares planes
of opposite pyrrole rings of about 5°. Overall it seems most
likely that Ge(TPP) is at least slightly domed.
Further evidence that Ge(TPP) contains Ge(II) comes from
1
its spectroscopy. In the H NMR spectrum, the resonance for
the protons bound to Câ comes at 9.0 ppm, a typical value for
an aromatic porphyrin. In contrast, the resonance for the Câ
protons of SiIV(TPP)(pyridine)2 are shifted far upfield to 1.3
ppm. The UV-vis spectrum of Ge(TPP) in benzene, shown in
Figure 2, is very similar to the reported spectrum of Sn(TPP).19
There is a Soret band at 495 nm and a broad Q band at 702
nm. The absorption at 395 nm is due to a charge-transfer
transition from the Ge 4p lone pair to the porphyrin eg (π*)
orbital.36
Preparation and Characterization of Ge(TPP)(pyridine)2.
When green Ge(TPP) is placed in pyridine, it slowly dissolves
over the course of 1-2 days, forming a bright-red solution of
Ge(TPP)(py)2. Single crystals of Ge(TPP)(py)2‚py were grown
by slow diffusion of hexane into a concentrated solution of Ge-
(TPP)(py)2 in pyridine. The crystal structure is shown in Figure
3. As in Si(TPP)(THF)2, the structure is highly ruffled, with
the four nitrogen atoms of the porphyrin ring coplanar and nearly
perfect octahedral coordination around germanium. The meso
carbons, Cmeso, are above and below the porphyrin N4 plane by
an average of 0.825 Å, nearly as large as the deviation of 1.00
Å observed in Si(TPP)(THF)2. We had expected that the larger
There is crystallographic four-fold rotational symmetry for
each molecule of Ge(TPP). There is no evidence of the type of
bond-length alternation that is present in antiaromatic Si(TPP)-
(THF)2,26 but even if the bond-length alternation were present
in Ge(TPP), it would be averaged out by the crystallographic
four-fold rotational symmetry. Nevertheless, the absolute mag-
nitudes of the bond lengths provide evidence that the compound
is aromatic GeII(TPP) and not antiaromatic GeIV(TPP), in that
they are similar to the average bond lengths computed from
(34) Cyranski, M. K.; Krygowski, T. M.; Wisiorowski, M.; Van Eikema
Hommes, N. J. R.; Schleyer, P. v. R. Angew. Chem., Int. Ed. 1998, 37,
177-180.
(35) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, NY, 1960.
(36) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New
York, 1978; Vol. 3, pp 1-165.
(33) Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.
9
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A R T I C L E S
Cissell et al.
Figure 5. UV-vis spectrum of Ge(TPP)(py)2 in pyridine with maxima at
426, 482, 522, 563, and 770 nm.
Figure 3. Solid-state structure of Ge(TPP)(pyridine)2 in crystals of Ge-
(TPP)(pyridine)2‚pyridine.
system. The aromaticity of a normal-valent porphyrin is
generally ascribed to its system of 18 π-electrons, therefore the
π-system of Ge(TPP)(py)2 should contain 20 π-electrons and
be antiaromatic. The 20-carbon periphery of the porphine core
of Ge(TPP)(py)2 (highlighted in red in Figure 4) is the 20-π-
electron system. It is antiaromatic and is therefore not in
resonance with the structure with the double and single C-C
bonds interconverted, but instead has the alternating-bond
structure as its ground state. In Ge(TPP)(py)2 the average
difference in bond length between the formal single and double
bonds for Câ-Câ is 0.028 Å, CR-Câ is 0.027 Å, and Cmeso
-
CR is 0.036 Å. Those distortions are about half the size of those
26
observed in Si(TPP)(THF)2 and also about half the size of
those calculated (B3LYP/6-311G**) for Ge(porphine)(py)2
(calculated bond lengths are shown in blue in Figure 4). It is
not clear why the bond-length alternation is lower in Ge(TPP)-
(py)2. Spectroscopy, detailed below, clearly shows that the
complex exists in one of two distinct electronic states, either
GeII(TPP) or GeIV(TPP)(py)2, and not a hybrid of the two. It is
possible that the Ge(TPP)(py)2 molecules in crystals of Ge-
(TPP)(py)2‚py are not perfectly ordered with respect to the
formal single and double bonds in the structure and are instead
statically or dynamically disordered with a 3:1 population of
the two orientations. That situation would give an apparent bond-
length alternation of half of the actual magnitude in a given
molecule.
Figure 4. Valence-bond structure of Ge(TPP)(pyridine)2 with selected bond
distances (Å) from the crystal structure of Ge(TPP)(py)2‚py (black) and
calculated (B3LYP/6-311G**) for Ge(porphine)(py)2 (blue). C-C distances
are outside the ring, and C-N and Ge-N distances are inside the ring.
The Ge-N (pyridine) distances are 2.089(2) and 2.122(2) Å. The 20
π-electron circuit is highlighted in red.
covalent radius of germanium would lead to a more nearly
planar structure, and in fact, in the reported crystal structures
of Ge(IV) porphyrin complexes of the type Ge(porph)X2, the
structures range from moderately ruffled12,13 to nearly planar.14,15
One difference between Ge(TPP)(py)2 and those other Ge(IV)
porphyrins is that Ge(TPP)(py)2 has four covalent Ge-N(pyr-
role) bonds (see Figure 4), whereas the Ge(porph)X2 complexes
have the usual two covalent and two dative Ge-N bonds. That
leads to shorter Ge-N bonds in Ge(TPP)(py)2 and a more
ruffled structure. It is also possible that the antiaromaticity of
Ge(TPP)(py)2 is actively contributing to the ruffling of the ring
system.
In the 1
H NMR spectrum of Ge(TPP)(pyridine)2 in pyridine-
d5, the chemical shift of the Câ protons is 0.6 ppm, far upfield
of the usual chemical shift of about 9 ppm for the Câ protons
of an aromatic porphyrin, and slightly upfield of the chemical
shift of 1.3 ppm for the Câ protons of Si(TPP)(pyridine)2. Those
upfield chemical shifts are due to a strong paratropic ring current
in Ge(TPP)(pyridine)2 and Si(TPP)(pyridine)2, which circulates
in the opposite direction as the diatropic ring current of aromatic
compounds and is characteristic of an antiaromatic compound.27
A 1H NMR spectrum of Ge(TPP)(pyridine)2 can be obtained in
C6D6, although pyridine slowly dissociates from the complex.
The chemical shift of the ortho protons on pyridine of Ge(TPP)-
(pyridine)2 in C6D6 is 21.6 ppm, and that large downfield shift
for protons near the center of the porphyrin ring is also indicative
of a paratropic ring current in the porphyrin ring system.
The UV-vis spectrum of Ge(TPP)(pyridine)2 in pyridine is
shown in Figure 5. It is similar to the spectrum of Si(TPP)-
(pyridine)2, which has been analyzed in detail.38
The most unusual structural feature in Ge(TPP)(py)2 is the
alternating lengthened and shortened C-C bonds around the
periphery of the porphine core (see Figure 4 for bond lengths).
This type of structural distortion has been observed previously
26
in only two porphyrin complexes, Si(TPP)(THF)2 and Al-
(TPP)(THF)2,37 both reduced porphyrins like Ge(TPP)(py)2. The
valence-bond structure given in Figure 4, different from valence-
bond structure of a normal-valent porphyrin, accounts for the
structural distortion. The oxidation state of 4+ for germanium
means that the oxidation state of the porphyrin ring system is
4-, rather than the usual 2- of an aromatic porphyrin ring
(37) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. Inorg. Chem. 2006, 45, 2367-
(38) Song, H.-e.; Cissell, J. A.; Vaid, T. P.; Holten, D. J. Phys. Chem. B 2007,
111, 2138-2142.
2369.
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7844 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

Aromatic Ge(TPP) and Antiaromatic Ge(TPP)(py)2
A R T I C L E S
2+/+
+/0
Table 1. Selected Redox Potentials of Ge(TPP), Ge(TPP)(py)2,
Sn(TTP), and Sn(TTP)(py)2
direction show the Ge(TPP)(py)2
and Ge(TPP)(py)2
waves. The relative instability of Sn(TTP)(py)2 is in agreement
with DFT calculations described at the end of the Results and
Discussion section.
potential (V vs Fc)
0/−
−/2−
The irreversible (on the electrochemical time scale) oxidation
of Sn(TTP) occurs at about +0.1 V vs ferrocene0/+, and,
similarly, the irreversible oxidation of Ge(TPP) in CH3CN/
benzene occurs at about +0.2 V (we were unable to observe
the oxidation in THF). Thus, Ge(TPP)(py)2 is much more easily
oxidized than Ge(TPP) is, at -1.24 V instead of +0.2 V. When
Ge(TPP) is oxidized, two electrons are removed from Ge,
converting it from Ge(II) to Ge(IV). In contrast, in Ge(TPP)-
(py)2 the germanium is already present as Ge(IV), and the two
extra electrons are in the porphyrin ring system, which has an
oxidation state best described as 4-. An electron is easily
removed from that reduced, antiaromatic porphyrin ring system,
and the potential for oxidation of Ge(TPP)(py)2 is therefore quite
negative. In other words, Ge(TPP)(py)2 is a relatively good
reducing agent, which was our original purpose in its synthesis.
Ge(TArFP) and Ge(TArFP)(pyridine)2. If the equilibrium
GeII(porph) + 2 pyridine ) GeIV(porph)(py)2 is considered to
be a transfer of two electrons from Ge(II) to the porphyrin ring
system, then one would expect a complex with a more electron-
withdrawing porphyrin to have an equilibrium that lies further
toward the product. We synthesized tetrakis[3,5-bis(trifluorom-
ethyl)phenyl]porphyrin, H2(TArFP), by the Lindsey method.
Cyclic voltammetry in THF with 0.10 M [NBu4][PF6] showed
that H2(TArFP) was reversibly reduced at -1.34 and -1.72 V
(vs ferrocene+/0) and H2(TPP) was reduced at -1.65 and -2.04
V under the same conditions. Both reductions of H2(TArFP)
occur at potentials about 0.32 V positive of H2(TPP), a value
close to that predicted by a published correlation between the
redox potentials of the free-base form of several tetra(4-X-
phenyl)porphyrins and the Hammett parameters of the X
substituents,42 using a Hammett parameter of 0.46 for a meta
CF3 group.43 Treatment of H2(TArFP) with 2 equiv of LiN-
(SiMe3)2 in ether gave Li2(TArFP)(ether)2, and the reaction of
Li2(TArFP)(ether)2 and GeCl2(dioxane) in THF for 12 h at 22
°C gave Ge(TArFP). Green-brown Ge(TArFP) dissolves quickly
in pyridine to form red Ge(TArFP)(pyridine)2.
Single crystals of Ge(TArFP)(pyridine)2‚pyridine were grown
by slowly cooling a hot solution in pyridine/hexane. The crystal
structure of Ge(TArFP)(pyridine)2 is shown in Figure 7, and its
valence-bond structure and bond lengths are shown in Figure
8. For this compound the bond-length difference between the
formal single and double bonds for Câ-Câ is 0.050 Å, for CR-
Câ is 0.052 Å, and for Cmeso-CR is 0.067 Å, close to the
calculated values for Ge(porphine)(py)2 of 0.054, 0.056, and
0.068 Å, respectively.
All the spectra of Ge(TArFP) and Ge(TArFP)(pyridine)2 are
consistent with aromatic GeII(TArFP) and antiaromatic GeIV-
(TArFP)(pyridine)2, respectively. A 1H NMR spectrum in C6F6/
C6D6 of a mixture containing both Ge(TArFP) and Ge(TArFP)-
(pyridine)2 is shown in Figure 9. The equilibrium constant of
the reaction Ge(TArFP) + 2 pyridine ) Ge(TArFP)(py)2 in C6F6/
C6D6 at 22 °C was found to be 2.3 × 104 M-2, favoring the
product more than in the case of Ge(TPP), as expected for the
more electron-withdrawing TArFP.
In THF
-1.64
-1.65
Sn(TTP)
Ge(TPP)
-2.06
-2.09
2+/
+
+/0
In Pyridine
-0.76
Sn(TTP)(py)2
Ge(TPP)(py)2
-1.20
-1.24
-0.77
A mixture of Ge(TPP) and pyridine in C6D6 was allowed to
come to equilibrium for 7 days at 22 °C. Integration of the H
1
NMR spectrum showed that the equilibrium constant of the
reaction Ge(TPP) + 2 pyridine ) Ge(TPP)(py)2 is 22 M-2. If
the standard state of a species in solution is taken to be 1 M
and because the temperature of the experiment was near the
standard temperature of 25 °C, the standard free energy of the
reaction is very close to zero.
Electrochemistry. In a THF solution of Ge(TPP), the only
porphyrin species that we expect to be present is GeII(TPP) with
no axial ligands. In contrast, the equilibrium constant given
above indicates that in pyridine solution, Ge(TPP) will be
present almost exclusively as GeIV(TPP)(pyridine)2. One would
expect the difference in electronic structures between those two
molecules to have a profound influence on their electrochemical
behavior, and we found that to be true, as discussed below. The
results are in very good agreement with a previous study of the
cyclic voltammetry of Sn(TTP) (TTP ) tetratolylporphyrin) in
THF and pyridine,39 with the only difference being the relative
stabilities of the M(porph) and M(porph)(pyridine)2 complexes
for the two metals.
The redox potentials determined by cyclic voltammetry are
given in Table 1, along with those from the previous study on
Sn(TTP). In THF, Ge(TPP) undergoes two single-electron
reductions to the monoanion and dianion at -1.65 and -2.09
V vs ferrocene0/+ (see Figure 6). Those values are close to those
reported for Sn(TTP) in THF of -1.64 and -2.06 V vs
ferrocene0/+ (with ferrocene0/+ in THF taken to be +0.56 V vs
SCE40), and they are at reasonable potentials for two single-
electron reductions of a porphyrin ring system.41 For Ge(TPP)-
(py)2 in pyridine, two new reversible waves were observed at
-0.77 and -1.24 V (see Figure 6), which are ascribed to the
Ge(TPP)(py)22+/+ and Ge(TPP)(py)2+/0 couples, respectively (the
irreversible wave at ca. -0.3 V is of unknown origin). Again,
the potentials correspond closely to those previously reported
2+/+
for
(py)2
Sn(TTP)(py)2
and
Sn(TTP)-
+/0
in pyridine at -0.76 and -1.20 V.39 For Sn(TTP),
however, those waves were observable only when the potential
was first swept in the positive direction, oxidizing Sn(TTP) to
Sn(TTP)2+, which formed Sn(TTP)(py)22+, and the potential was
then swept in the negative direction at the relatively fast rate of
10 V/s to form transiently stable Sn(TTP)(py)2+ and Sn(TTP)-
(py)2. In contrast, neutral Ge(TPP)(py)2 is present in pyridine
solution under normal conditions, and potential sweeps in either
(39) Kadish, K. M.; Dubois, D.; Barbe, J. M.; Guilard, R. Inorg. Chem. 1991,
30, 4498-4501.
(40) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(41) Kadish, K. M., Smith, K. M., Guilard, R., Eds. The Porphyrin Handbook;
Academic Press: San Diego, 2000; Vol. 9.
(42) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc. 1976, 98, 3326-3328.
(43) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry, 5th ed.;
John Wiley & Sons: New York, 2000.
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A R T I C L E S
Cissell et al.
Figure 8. Valence-bond structure of Ge(TArFP)(py)2 with selected bond
distances (Å) from the crystal structure of Ge(TArFP)(py)2‚py (black) and
calculated (B3LYP/6-311G**) for Ge(porphine)(py)2 (blue). C-C distances
are outside the ring, and C-N and Ge-N distances are inside the ring.
The Ge-N (pyridine) distances are 2.087(4) and 2.096(4) Å.
Figure 6. Cyclic voltammograms of Ge(TPP) in THF (top, 0.10 M [NEt4]-
[B(C6F5)4], decamethylferrocene standard, 100 mV/s) and Ge(TPP)(py)2
in pyridine (bottom, 0.10 M [NEt4][B(C6F5)4], ferrocene reference, 100 mV/
s).
Figure 9. 1H NMR spectrum of Ge(TArFP)(py)2 and Ge(TArFP) in C6F6/
C6D6. Resonances are marked as follows: a for pyridine in Ge(TArFP)-
(py)2, b for uncomplexed Ge(TArFP) resonances, and c for porphyrin
resonances of Ge(TArFP)(py)2.
For silicon, the antiaromatic complex SiIV(TPP)(L)2 is favored
over one containing Si(II), while for tin the antiaromatic
SnIV(TPP)(py)2 is only transiently stable. Germanium porphyrins
can be isolated in either state.
The energy of the reaction of eq 1 can be estimated by
calculating the energy of each of the three molecules individu-
ally.
Figure 7. Solid-state structure of Ge(TArFP)(pyridine)2 in crystals of Ge-
(TArFP)(pyridine)2‚pyridine. The -CF3 groups with spherical fluorines were
refined as disordered over two conformations; only one is shown.
MII(porphine) + 2 pyridine h MIV(porphine)(pyridine)2
(1)
MII(porph) + 2 py ) MIV(porph)(py)2 for M ) Si, Ge, or
Sn. There is a clear trend in the propensity of porphyrin
complexes of group 14 elements, MII(porph), to undergo a
reaction with a dative ligand L to form MIV(porph)(L)2. Si-
(TPP) has thus far been isolated only as a complex with donor
ligands, in Si(TPP)(THF)2 and Si(TPP)(py)2. Ge(TPP) does not
form an adduct with THF but in the presence of the stronger
ligand pyridine forms Ge(TPP)(py)2. Finally, no Sn(TPP)(L)2
complexes have been isolated. The trend is rooted in the
difference in oxidation state of the metal in MII(porph) versus
that in MIV(porph)(L)2. In MIV(porph)(L)2 the porphyrin ring
system is antiaromatic, which is energetically unfavorable,
whereas in MII(porph) the metal has an oxidation state of 2+,
which is unfavorable for elements near the top of group 14.
Calculations were performed for M ) Si, Ge, and Sn with the
B3LYP functional and 6-311G** basis set, except for the Sn
atom, which required an effective core potential, and cc-pVDZ-
PP was used. Using just the electronic energy, the calculated
energy of the reaction for M ) Si was -54.4 kcal/mol; that for
Ge was +2.5 kcal/mol, and that for Sn was +24.5 kcal/mol.
Similar values were obtained with B3LYP and a different basis
set, cc-pVDZ (and cc-pVDZ-PP for Sn): Si, -49.4 kcal/mol;
Ge, +3.8 kcal/mol; Sn, +23.9 kcal/mol. Single-point energy
calculations were done at the MP2/6-311G** level using the
B3LYP/6-311G** geometries, and the energy of the reaction
was calculated to be -93.0 kcal/mol for Si and -32.0 kcal/
mol for Ge. Therefore, it appears to be coincidental that the
9
7846 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

Aromatic Ge(TPP) and Antiaromatic Ge(TPP)(py)2
A R T I C L E S
B3LYP-calculated energy of reaction for M ) Ge is so close
to zero, but the trend in energies of the reaction moving down
the periodic table is correct for both the B3LYP and MP2
calculations. In addition, a full vibrational analysis was per-
formed (B3LYP/6-311G**) to calculate the actual free energy
of the reaction; the ∆Grxn for Si was -15.4 kcal/mol, and that
for Ge was +34.9 kcal/mol. There is probably inaccuracy in
those values because of the presence of very low-energy
torsional modes in M(porphine)(py)2 and ruffling modes in
M(porphine), but again the trend is correct, moving from Si to
Ge.
an oxidation-state change in a metal center, is, as far as we
know, without precedent. A similar molecule with a more
electron-withdrawing porphyrin ring, Ge(TArFP), behaves simi-
larly, but its reversible reaction with pyridine favors the
antiaromatic product, Ge(TArFP)(py)2. The propensity of group
14 porphyrin complexes to form their antiaromatic adducts with
pyridine follows the trend predicted by the inert pair effect.
Acknowledgment. We thank Brian Barnes and Lev Gelb for
assistance with the quantum calculations. Funding was provided
by NSF Grant CHE-0133068 and the computational chemistry
resource was supported by NSF Grant CHE-0443511.
Conclusions
The first free Ge(II) porphyrin complex, Ge(TPP), has been
isolated and characterized. The oxidation state of germanium
in Ge(TPP) is Ge(II), and the complex is aromatic. In the
presence of pyridine Ge(TPP) undergoes a remarkable trans-
formation, forming the antiaromatic Ge(IV) porphyrin complex
Ge(TPP)(py)2. That reaction, in which a dative ligand induces
Supporting Information Available: Crystal structure data in
CIF format, crystallographic details, picture of Ge(porphine)
HOMO, and the complete version of ref 33. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA070794I
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