Synthesis and electrochemistry of undeca-substituted metallo- benzoylbiliverdins

Article abstract of DOI:10.1021/ic050841c

The high-yield preparation of metallo-benzoylbiliverdins 9, 10, and 11 from either oxidation of dodeca-substituted porphyrin 6 in the presence of NaNO ₂/TFA and air followed by metalation or by reaction of the Ni(II) or Cu(II) complexes of 6 wi

Full text of DOI:10.1021/ic050841c

Inorg. Chem. 2006, 45, 1463−1470
Synthesis and Electrochemistry of Undeca-substituted
Metallo-benzoylbiliverdins
Owendi Ongayi,† M. Grac
¸
a H. Vicente,*,† Zhongping Ou,‡ Karl M. Kadish,*,‡ M. Ravi Kumar,†
Frank R. Fronczek,† and Kevin M. Smith*,†
Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803, and
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204
Received May 24, 2005
The high-yield preparation of metallo-benzoylbiliverdins 9, 10, and 11 from either oxidation of dodeca-substituted
porphyrin 6 in the presence of NaNO2/TFA and air followed by metalation or by reaction of the Ni(II) or Cu(II)
complexes of 6 with m-chloroperoxybenzoic acid in pyridine under air is reported. The X-ray structures of complexes
9 and 10 and the electrochemistry and spectroelectrochemistry of metallo-benzoylbiliverdins 9−11 are presented
and discussed.
Introduction
heme oxygenase, occurs selectively at the meso R carbon to
form verdoheme 2 via a meso-hydroxyheme, which reacts
with a second molecule of oxygen to produce biliverdin IXR
(3), after loss of the meso R carbon as CO and demetalation
(Scheme 1).10 The regioselectivity of heme cleavage by heme
oxygenase is probably controlled by both electronic and steric
effects exerted by the protein.11 However, the in vitro
chemical oxidation of hemin, normally accomplished using
O2-ascorbic acid or O2-hydrazine in aqueous pyridine (the
so-called coupled oxidation)12-14 produces all four isomeric
biliverdins. This process has been widely used as a model
of the heme-oxygenase-catalyzed reactions.15 A variety of
other reaction conditions have also been used for the photo
and chemical oxidations of porphyrins, including H2O2 in
pyridine,16 thallium(III) and cerium(IV) salts,17 and O2-
NaOMe in methanol/THF.18 These reactions typically pro-
duce formylbiliverdins (e.g., 4, from magnesium octaethyl-
Metalloporphyrins are subject to oxidation at both the
metal center and the porphyrin ligand to form either metal-
or porphyrin-centered oxidation products.1 A number of
factors contribute to the preferential site of oxidation
including the electronegativity and oxidation state of the
central metal,2 the nature of peripheral substituents on the
porphyrin macrocycle,3 the presence and nature of metal axial
ligand(s),4 the medium,5 and the temperature.6 The oxidative
cleavage of porphyrin macrocycles is an important reaction
that mimics the natural processes of heme catabolism,7
chlorophyll degradation,8 and formation of algae biliproteins.9
The in vivo cleavage of heme 1, catalyzed by the enzyme
* To whom correspondence should be addressed.
† Louisiana State University.
‡ University of Houston.
(1) Fuhrhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174-
4181.
(2) Buchler, J. W.; Kadish, W.; Smith, P. D. Struct. Bonding (Berlin)
1978, 34, 79-134.
(10) Wilks, A.; Ortiz de Montellano, P. R. J. Biol. Chem. 1993, 268,
22357-22362.
(3) Kadish, K. M.; Morrison, M. M. Bioinorg. Chem. 1977, 7, 107-115.
(4) Whitten, D. G.; Meyer, T. J. J. Am. Chem. Soc. 1973, 95, 5939-
5942.
(11) Hernandez, G.; Wilks, A.; Paolesse, R.; Smith, K. M.; Ortiz de
Montellano, P. R.; La Mar, G. N. Biochemistry 1994, 33, 6631-6641.
(12) Bonnett, R.; McDonagh, A. F. J. Chem. Soc., Chem. Commun. 1970,
237-238.
(5) Kadish, K. M.; Bottomley, L. A. Inorg. Chem. 1981, 20, 1348-1357.
(6) Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1975, 97, 5288-5290.
(7) Ortiz de Montellano, P. R.; Auclair, K. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press:
Boston, 2003; Vol. 12, pp 183-210.
(8) Kra¨utler, B, A. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: Boston, 2003; Vol. 13, pp
183-209.
(9) (a) Frankenberg, N.; Lagarias, J. C. In The Porphyrin Handbook;
Kadish, K. M.; Smith, K. M.; Guilard, R., Eds.; Academic Press:
Boston, 2003; Vol. 13, pp 211-235. (b) Gossauer, A. In The Porphyrin
Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R., Eds.; Academic
Press: Boston, 2003; Vol. 13, pp 237-274.
(13) Bonnett, R.; McDonagh, A. F. J. Chem. Soc., Perkin Trans. 1 1973,
881-888.
(14) Crusats, J.; Suzuki, A.; Mizutani, T.; Ogoshi, H. J. Org. Chem. 1998,
63, 602-607.
(15) St. Claire, T. N.; Balch, A. L. Inorg. Chem. 1999, 38, 684.
(16) Bonnett, R.; Dimsdale, M. J. J. Chem. Soc., Perkin Trans. 1 1972,
2540-2548.
(17) Evans, B.; Smith, K. M.; Cavaleiro, J. A. S. J. Chem. Soc., Perkin
Trans. 1 1978, 768-773.
(18) Cavaleiro, J. A. S.; Hewlins, M. J. E.; Jackson, A. H.; Neves, M. G.
P. M. S. J. Chem. Soc., Chem. Commun. 1986, 142-144.
10.1021/ic050841c CCC: $33.50
Published on Web 01/13/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 4, 2006 1463

Ongayi et al.
Scheme 1
porphyrin),19-21 biliverdins12-14 (e.g., 3, via extrusion of CO)
from meso-unsubstituted porphyrins, and aroyl-substituted
bilenes from meso-tetraarylporphyrins, such as meso-tet-
raphenylporphyrin (TPP, 5). The yields obtained in these
reactions are usually low to moderate and depend on the
reaction conditions and the presence and nature of the
coordinated metal ion.
Scheme 2
Herein, we report the high-yield preparation of novel
metallo-benzoylbiliverdins (9, 10, and 11) from the chemical
oxidation of dodeca-substituted porphyrin 6 or its Ni(II) and
Cu(II) metal complexes 7 and 8 and a study of the
electrochemical behavior of the new compounds. Biliverdins
have been suggested to have both antioxidant22 and antiviral
activities.23,24 The study of the chemical and redox properties
of model metallo-benzoylbiliverdins, such as 9-11 may lead
to a better understanding of their possible physiological roles
and uncover new applications.25,26
The chemical oxidation of the zinc(II) complex of
TPP was first reported by Smith and co-workers27 using
thallium(III) salts (trifluoroacetate and nitrate). Subsequently,
the Zn(II), Cd(II), bis-Tl(I), and Mg(II) complexes of TPP
were ring-opened by photo-oxygenation.28-30 Under nitrating
conditions, using either a mixture of sulfuric and nitric acids31
or N2O4 in organic solvents,32 TPP (5) and its Mg(II) and
Zn(II) complexes also produced the corresponding benzoyl-
biliverdins as a side product. We now demonstrate the
preparation of metallo-benzoylbiliverdins 9-11 using two
different high-yielding routes (Scheme 2): (i) from the
reaction of porphyrin 6 with sodium nitrite in TFA in the
presence of oxygen followed by metalation33 and, for the
first time, (ii) from the metal complexes 7 and 8 by reaction
with m-chloroperoxybenzoic acid (mCPBA) in pyridine and
in the presence of oxygen. The oxidation of heme and other
iron porphyrins using mCPBA is known to generate iron-
oxo species, which catalyze the oxidation of various sub-
strates, such as alkenes and alkanes.34 However, to the best
of our knowledge, mCPBA has never been reported to open
the porphyrin macrocycle leading to metallo-biliverdins. The
electrochemistry of this class of compounds has not been
previously studied and is now described in the present
manuscript.
(19) Fuhrhop, J.-H.; Mauzerall, D. Photochem. Photobiol. 1971, 13, 453-
458.
(20) Wasser, P. K. W.; Fuhrhop, J.-H. Ann. NY Acad. Sci. 1973, 206, 533-
547.
(21) Fuhrhop, J.-H.; Wasser, P. K. W.; Subramanian, J.; Schrader, U.
Liebigs Ann. Chem. 1974, 1450-1466.
(22) Stocker, R.; Yamamoto, Y.; McDonagh, A. F.; Glazer, A. N. Ames,
B. N. Science 1987, 235, 1043-1046.
(23) Nakagami, T.; Taji, S.; Takahashi, M.; Yamanishi, K. Microbiol.
Immunol. 1992, 36, 381-390.
(24) Mori, H.; Otake, T.; Morimoto, M.; Ueba, N.; Kunita, N.; Nakagami,
T.; Yamasaki, N.; Taji, S. Jpn. J. Cancer Res. 1991, 51, 755-757.
(25) Lord, P. A.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 2000, 39,
1128-1134.
(26) Attar, S.; Balch, A. L.; Van Calcar, P. M.; Winkler, K. J. Am. Chem.
Soc. 1997, 119, 3317-3323.
Experimental Section
(27) Evans, B.; Smith, K. M.; Cavaleiro, J. A. S. Tetrahedron Lett. 1976,
52, 4863-4866.
Alumina (Brockmann EM Separations Grade III) was used for
column chromatography. Analytical thin-layer chromatography
(TLC) was performed using Merck Sorbent Technologies 60 F254
silica gel (precoated sheets, 0.2 mm thick). The syntheses were
(28) Smith, K. M.; Brown, S. B.; Troxler, R. F.; Lai, J.-J. Tetrahedron
Lett. 1980, 21, 2763-2766.
(29) Smith, K. M.; Brown, S. B.; Troxler, R. F.; Lai, J.-J. Photochem.
Photobiol. 1982, 36, 147-152.
(30) Matsuura, T.; Inoue, K.; Ranade, A. C.; Saito, I. Photochem. Photobiol.
1980, 31, 23-26.
(33) Ongayi, O.; Fronczek, F. R.; Vicente, M. G. H. Chem. Commun. 2003,
(31) Johnson, A. W.; Winter, M. Chem. Ind. 1975, 8, 351.
(32) Catalano, M. M.; Crossley, M. J.; Harding, M. M.; King, L. G. J.
Chem. Soc., Chem. Commun. 1984, 1535-1536.
2298-2299.
(34) Nam, W.; Lim, M. H.; Lee, H. J.; Kim, C. J. Am. Chem. Soc. 2000,
122, 6641-6647.
1464 Inorganic Chemistry, Vol. 45, No. 4, 2006

Undeca-substituted Metallo-benzoylbiliWerdins
monitored by TLC and spectrophotometry. The electronic absorp-
tion spectra were measured in dichloromethane solution using a
Perkin-Elmer Lambda 35 UV-vis spectrophotometer. Melting
points were measured on a MELT-TEMP apparatus. 1H NMR
spectra were obtained in CDCl3 or CD2Cl2, using a Bruker 250 or
400 MHz spectrometer; chemical shifts are expressed in ppm
relative to TMS (0 ppm) and residual CHCl3 (7.26 ppm). Low-
resolution mass spectra were obtained at the Mass Spectrometry
Facility at LSU and HRMS at the Ohio State University. Benzal-
dehyde, BF3‚OEt2, dichlorodicyanobenzoquinone (DDQ), mCPBA,
and the metal acetates were purchased (Aldrich) and used without
further purification. Benzonitrile (PhCN) was obtained from Fluka
or Aldrich and distilled over P2O5 under vacuum prior to use. Tetra-
n-butylammonium perchlorate (TBAP) was purchased from Sigma
or Fluka, recrystallized from ethyl alcohol, and dried under vacuum
at 40 °C for at least 1 week prior to use. All solvents were dried
and purified according to literature procedures.
Cyclic voltammetry was carried out using an EG&G Princeton
Applied Research (PAR) 173 potentiostat/galvanostat. A homemade
three-electrode cell was used for cyclic voltammetric measurements
and consisted of a platinum button or glassy carbon working
electrode, a platinum counter electrode, and a homemade saturated
calomel reference electrode (SCE). The SCE was separated from
the bulk of the solution by a fritted glass bridge of low porosity,
which contained the solvent/supporting electrolyte mixture. Thin-
layer UV-vis spectroelectrochemical experiments were performed
with a home-built thin-layer cell. Potentials were applied and
monitored with an EG&G PAR Model 173 potentiostat. Time-
resolved UV-vis spectra were recorded with a Hewlett-Packard
Model 8453 diode array spectrophotometer.
5,10,15,20-Tetraphenyl-2:3,7:8,12:13,17:18-butanoporphy-
rin (6). 6 was prepared as described in the literature.35,36 Freshly
distilled dry dichloromethane (106 mL) was added to a round-
bottom flask, fitted with a reflux condenser, under argon. 3:4-
Butanopyrrole (0.13 g, 1.06 mmol) and benzaldehyde (0.11 mL,
1.06 mmol) were added, and the solution was stirred at room
temperature under a slow, steady stream of argon for 15 min. The
flask was then shielded from ambient light, and BF3‚OEt2 (0.02
mL, 0.106 mmol) was added. This mixture was then stirred for 1
h at room temperature. DDQ (1.15 g, 5.10 mmol) was added to
the reaction flask, and the final solution turned dark pink instantly.
This solution was refluxed under argon for 1 h to give a dark green
solution. The solvent was reduced to dryness under vacuum, and
the resulting residue purified by alumina column chromatography
using 2% methanol in dichloromethane for elution. Recrystallization
from hot methanol gave purple crystals of the title porphyrin (0.17
g, 77% yield). 1H NMR (CDCl3, with 1 drop of d-TFA) 8.42 (s, 8
H, o-ArH) 7.91 (s, 12 H, p- and m-ArH) 2.52, 2.04, 1.79, 1.71
(broad s, 8 H each, CH2) -0.51 (s, 4H, NH). UV-vis (dication,
CH2Cl2) λmax 459 (214 200), 612 (13 000), 670 (25 000) nm. MS
(MALDI) m/z 831.75 (M + H)+.
removed under vacuum, and the residue was purified by column
chromatography on alumina, using a gradient elution starting with
pure dichloromethane. With 2% methanol in dichloromethane, the
metal-free benzoylbiliverdin was collected in an ∼3:1 ratio of the
hydrated forms. For the main conformer (violet), mp ) 194-196
°C, UV-vis (CH2Cl2) λmax 341 nm (19,200), 574 (41,100), 1H NMR
(CD2Cl2) 11.62, 11.27 (each s, 2 H, NH), 9.00 (s, 1 H, NH), 7.62-
7.57 (d, 2 H, o-ArH), 7.51-7.35 (m, 18 H, ArH), 6.01 (s, 1 H,
OH), 3.8-3.6 (m, 2 H, CH2), 2.49-1.20 (m, 30 H, CH2), HRMS
m/z 903.4224 (M + Na); for the pink conformer, mp) 170-173
°C, UV-vis (CH2Cl2) λmax 341 (21 700), 508 (13 800) nm, 1H NMR
(CD2Cl2) 12.57, 9.46 (each s, 2 H, NH), 7.56-7.18 (m, 20 H, ArH),
4.13 (s, 1 H, OH), 3.7 (s, 1 H, NH), 2.41-1.32 (m, 32 H, CH2),
HRMS m/z 903.4249 (M + Na). The hydrated benzoylbiliverdins
were dissolved in toluene (20 mL), and an excess of nickel(II)
acetate (24 mg, 0.1 mmol) was added to the solution with stirring.
The final mixture was refluxed for 24 h then cooled to room
temperature, and the solvent was removed under vacuum. Purifica-
tion was performed on an alumina plug to remove excess metal
acetate, followed by alumina column chromatography using 3:1
dichloromethane/petroleum ether for elution. The product was
collected and recrystallized from dichloromethane/methanol to give
the title biliverdin (84 mg) in 76% yield.
From the Ni(II) Complex 7.33 To a stirred solution of complex
7 (20 mg, 0.023 mmol) in pyridine (3 mL) was added mCPBA (30
mg, 0.17 mmol). After 30 min, the solution was evaporated under
reduced pressure and the residue was dissolved in dichloromethane
and washed with saturated aqueous NaHCO3. Purification as
described above yielded 9 (15 mg) in 82% yield. mp ) 245-247
°C. UV-vis (CH2Cl2) λmax 318 (28 900), 439 (23 500), 802 (9800)
1
nm. H NMR (CDCl3, with 1 drop of d-TFA) 8.04-7.21 (m, 20
H, ArH), 2.39-1.10 (m, 32 H, CH2). 13C NMR (CDCl3) 189.3,
181.8, 162.5, 159.1, 149.8, 149.3, 146.4, 145.1, 142.9, 141.9, 140.3,
140.0, 139.5, 138.3, 134.6, 134.5, 133.2, 132.5, 132.3, 131.4, 130.2,
130.0, 129.9, 129.8, 129.6, 129.5, 129.2, 128.3, 32.5, 31.2, 27.5,
27.3, 27.0, 26.5, 26.1, 25.9, 25.8, 24.8, 24.7, 24.5, 24.3, 24.2, 22.6,
15.6. MS (ESI) m/z 918.9 (M+). HRMS (ESI) for C60H52N4NiO2
+ Na: calcd 941.3341, found 941.3353. Anal. Calcd for C60H52N4-
NiO2: C 72.70, H 6.11, N 5.66. Found: C 73.19, H 6.14, N 5.25.
Copper(II) Benzoylbiliverdin (10). From Porphyrin 6. The
metal-free benzoylbiliverdin prepared as described above (100 mg,
0.12 mmol) was dissolved in toluene (20 mL), and copper(II) acetate
(239 mg, 1.2 mmol) was added to the solution with stirring. The
final mixture was refluxed for 8 h, cooled to room temperature,
and the solvent was then removed under vacuum. Purification was
first performed on an alumina plug to remove excess metal acetate
followed by alumina column chromatography using 3:1 dichlo-
romethane/petroleum ether for elution. The main band was collected
and recrystallized from dichloromethane/methanol to give the title
biliverdin (80 mg) in 75% yield.
From the Cu(II) Complex 8. Compound 8 was prepared by
insertion of Cu(II) into porphyrin 6, using copper(II) acetate in
methanol/chloroform. To a stirred solution of 8 (20 mg, 0.022
mmol) in pyridine (3 mL) was added mCPBA (30 mg, 0.17 mmol).
After 30 min, the solvent was evaporated under reduced pressure
and the residue was dissolved in dichloromethane and washed with
saturated aqueous NaHCO3. Purification as described above yielded
10 (16 mg) in 87% yield. mp ) 262-265 °C. UV-vis (CH2Cl2)
λmax 326 (28 200), 420 (24 200), 481 (28 400), 806 (10 400) nm.
MS (ESI) m/z 924.63 (M+). HRMS (ESI) for C60H52CuN4O2 +
Na: calcd 946.3284, found 946.3284. Anal. Calcd for C60H52-
CuN4O2: C 73.67, H 5.98, N 5.73. Found: C 73.86, H 5.81, N
5.28
Nickel(II) Benzoylbiliverdin (9). From Porphyrin 6. Porphyrin
6 (100 mg, 0.12 mmol) was dissolved in 3 mL of TFA, and NaNO2
(33 mg, 0.48 mmol) was added to the solution with stirring under
air at room temperature for 3 min. The reaction was quenched by
being poured into 50 mL of water followed by extraction with
dichloromethane (6 × 25 mL). The organic layers were washed
with saturated aqueous NaHCO3 (2 × 100 mL), then with water
(100 mL), and dried over anhydrous Na2SO4. The solvent was
(35) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt, J. A.
Tetrahedron Lett. 1990, 31, 3719-3722.
(36) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.;
Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627-3635.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1465

Ongayi et al.
Table 1. Crystal Data, Data Collection, and Refinement Parameters
compound
9
10
formula
color/shape
fw
cryst syst
space group
temp, K
cell constants
a, Å
2(C60H52N4NiO2)0.3(CHCl3)‚MeOH
C60H52CuN4O2‚CH2Cl2
brown/fragment
2229.7
triclinic
P1h
dark brown/parallelepiped
1009.5
triclinic
P1h
105
105
14.932(2)
17.009(2)
22.907(3)
70.580(5)
73.315(5)
88.440(6)
5240.9(12)
2
12.989(3)
14.384(3)
14.553(3)
86.078(8)
82.261(7)
64.438(9)
2430.4(9)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
formula units/unit cell
D
µ
calcd, g cm-3
1.413
6.51
1.380
6.10
calc, cm-1
transm. coeff
0.823-0.937
Nonius KappaCCD/ω
Mo KR (λ ) 0.71073 Å)
0.10 × 0.15 × 0.22
101 183
0.890-0.941
Nonius KappaCCD/ω
Mo KR (λ ) 0.71073 Å)
0.10 × 0.20 × 0.25
49 846
diffractometer/scan
radiation, graphite monochr.
max cryst dimens, mm3
reflns measured
Rint
0.058
0.052
independent reflns
2θ range, deg
range of h,k,l
reflns observed
criterion for observed
data/params
21 712
12 459
5.0< 2θ < 53.0
(18, (21, (28
14 056
I > 2σ(I)
21 712/1334
0.068
5.6< 2θ < 57.4
(17, (19, (19
8366
I > 2σ(I)
12 459/632
0.057
R (obs)
R (all data)
0.119
0.164
1.017
1.50, -1.74
0.103
0.142
1.037
0.84, -1.20
Rw, F2(all data)
GOF
max resid. peaks (e‚Å-3
)
Zinc(II) Benzoylbiliverdin (11). The metal-free benzoylbiliver-
din prepared as described above (100 mg, 0.12 mmol) was dissolved
in CHCl3 (20 mL), and a saturated solution of zinc(II) acetate in
methanol (5 mL) was added to the solution with stirring. The final
mixture was refluxed for 8 h, cooled to room temperature, and the
solvent was removed under vacuum. The residue was redissolved
in dichloromethane (20 mL), and this solution was washed once
with saturated aqueous NaHCO3 (100 mL) and once with water
(100 mL) before being dried over anhydrous NaSO4. Recrystalli-
zation from dichloromethane/methanol gave the title biliverdin (66
mg) in 60% yield. mp ) 174-177 °C. UV-vis (CH2Cl2) λmax 329
Results and Discussion
Porphyrin 6 was synthesized in 77% yield by condensation
of 4,5,6,7-tetrahydroisoindole with commercially available
benzaldehyde, as previously reported.35 The precursor 4,5,6,7-
tetrahydroisoindole was obtained via Barton-Zard condensa-
tion using 1-nitrocyclohexene and ethyl isocyanoacetate in
80-92% yield followed by basic hydrolysis and decarboxyl-
ation in 70-85% yield. Because of the core-deformation
from planarity in dodeca-substituted porphyrins, these com-
pounds generally have lower oxidation potentials compared
with their related octa- and tetra-substituted porphyrins.31
Thus, the chemical oxidation of porphyrin 6 was ac-
complished under mild conditions, by adding 6 equiv of
NaNO2 to a concentrated solution of 6 in TFA under air, at
room temperature.33 Under similar conditions TPP is mainly
nitrated at the para positions of the meso phenyl groups.38
The green color of the acidic reaction mixture immediately
changed to brown. After 10 min, the reaction was quenched
with water and neutralized with aqueous sodium bicarbonate
solution. The major purple product was isolated in 77% yield,
along with 6% recovered starting material and trace amounts
of polynitrated products. When this procedure was performed
under an inert atmosphere of argon, only starting material 6
was recovered, indicating that air oxygen is essential and is
probably added to one of the porphyrin meso positions. In
addition, when the reaction was performed using dichlo-
1
(21 900), 446 (17 200), 752 (6300) nm. H NMR (CDCl3) 7.90-
7.25 (m, 20 H, ArH), 2.40-1.30 (m, 32 H, CH2). HRMS for
C60H52N4O2Zn: calcd 925.3460, found 925.3473.
X-ray Data Collection and Structure Determination. Intensity
data were collected for solvates of compounds 9 and 10 using
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) on a
Nonius KappaCCD diffractometer fitted with an Oxford Cryostream
cooler. Data reduction included absorption corrections by the
multiscan method. Crystal data and experimental details are given
in Table 1. Structures were solved by direct methods and refined
by full-matrix least squares, using SHELXL97.37 C-H hydrogen
atoms were treated as riding in idealized positions. Compound 9
has two independent Ni complexes in the asymmetric unit, one of
which has a disorder of one of its cyclohexene rings. In that ring,
the two C atoms opposite the double bond take on two conforma-
tions with approximately 2:1 population. All residual electron
density peaks greater than 1 e‚Å-3 were located near heavy-atom
positions.
(37) Sheldrick, G. M. SHELXL97: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(38) Luguya, R.; Jaquinod, L.; Fronczek, F. R.; Smith, K. M.; Vicente, M.
G. H. Tetrahedron 2004, 60, 2757-2763.
1466 Inorganic Chemistry, Vol. 45, No. 4, 2006

Undeca-substituted Metallo-benzoylbiliWerdins
Figure 1. Molecular structures of metallo-benzoylbiliverdins 9 (left) and 10 (right). (A) Top view (hydrogen atoms were omitted for clarity); (B) side view
showing square planar distortion (hydrogen and butano carbon atoms were removed for clarity).
romethane as a solvent and a catalytic amount of TFA, or in
the absence of TFA, only starting material 6 was recovered.
When a radical scavenger (2,6-di-tert-butyl-4-methylphenol)
was added to the reaction mixture, the ring-opening reaction
was completely inhibited. These results suggest that the
porphyrin macrocycle is initially oxidized to the correspond-
ing π-cation radical by NO+, and upon reaction with air
oxygen, ring opening occurs, relieving steric strain. The
metallo-benzoylbiliverdins 9-11 were prepared by treating
the conformationally flexible metal-free, ring-opened product
with the appropriate metal salts. Both nickel(II) and copper-
(II) were inserted in refluxing toluene using the correspond-
ing metal acetates. On the other hand, zinc(II) was inserted
using chloroform as the solvent and a saturated solution of
zinc(II) acetate in methanol. During the metalation reaction
with copper(II) and nickel(II) acetate, a deep blue intermedi-
ate complex with a strong absorption at λmax 650 nm was
formed. The 1H NMR spectrum of this intermediate showed
one broad singlet N-H peak at 8.95 ppm, along with
multiple peaks in the aromatic region, and the MALDI-MS
showed a molecular ion at 936.3. These results suggest that
the metal ion initially coordinates to only three nitrogens
and the oxygen of the benzoyl group, in agreement with
observations of Callot and co-workers39 for a nickel and
copper complex of a TPP derivative. Further heating in the
presence of the metal salt leads to complete disappearance
of the blue intermediate and formation of the metallo-
benzoylbiliverdins in nearly quantitative yield from the metal-
free biliverdin.
porphyrins 7 and 8 to the corresponding porphyrin-cation
radicals, which then react with air oxygen to produce metallo-
benzoylbiliverdins 9 and 10 in high yield. Since the Zn(II)
complex of porphyrin 6 is easily demetalated, biliverdin 11
was prepared in higher yield by zinc(II) insertion into the
metal-free benzoylbiliverdin rather than by the mCPBA
route. Complex 11 was found to be readily demetalated under
acidic conditions, in contrast with the nickel(II) and copper-
(II) derivatives 9 and 10.
1
The Zn(II) and Ni(II) benzoylbiliverdins had similar H
NMR spectra, showing two distinct sets of doublets, down-
field shifted between 8.04 and 7.72 ppm, corresponding to
the ortho aromatic protons of the benzoyl group. The
remaining aromatic protons appeared upfield, clustered
between 7.64 and 7.21 ppm. A sharp singlet corresponding
to two CH2 protons of the cyclohexenyl ring adjacent to the
benzoyl group appeared downfield at about 2.4 ppm, and
the remaining CH2 cyclohexenyl protons appeared as a
complex set of peaks between 2.2 and 1.1 ppm. The 13C
spectra of metallobenzoylbiliverdins 9 and 11 showed, as
expected, two carbonyl resonances at about 189 and 182
ppm, the aromatic carbons appeared between 162 and 128
ppm, and the CH2 between 32 and 16 ppm. Suitable crystals
for X-ray analysis of complexes 9 and 10 were grown from
slow diffusion of chloroform into methanol, and the molec-
ular structures of these compounds are shown in Figure 1.
Compound 9 has two independent molecules in the asym-
metric unit, and only one of the molecules is shown in the
figure. We have recently published the structure of benzoyl-
biliverdin 933 as a mixed CH2Cl2/MeOH/H2O solvate. That
structure had only one independent benzoylbiliverdin mol-
ecule but suffered from considerable solvent disorder prob-
lems, and the precision of the determination was somewhat
lower than the CHCl3/MeOH solvate reported here. In both
complexes 9 and 10, the metal atoms have tetrahedrally
distorted square planar geometries enforced by the oxidized
nonmacrocyclic nature of the ligand. In the Ni(II) complex,
The Ni(II) and Cu(II) benzoylbiliverdins were also ob-
tained in a single step, by treating the metal complexes 7
and 8 with mCPBA in pyridine, in the presence of air oxygen.
When the reaction was performed under argon, again, no
metallobiliverdins were detected by either TLC or spectro-
photometry. We believe that mCPBA initially oxidizes
(39) Jeandon, C.; Krattinger, B.; Ruppert, R.; Callot, H. J. Inorg. Chem.
2001, 40, 3149-3153.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1467

Ongayi et al.
Table 2. Half-wave Potentials (V vs SCE) of Complexes 7-11 in
PhCN, 0.1 M TBAP
oxidation
reduction
a
compound
third second first
first second
third ∆E1/2
(BV)Ni 9
(BV)Cu 10
(BV)Zn 11
(TPBP)Ni 7 1.64
(TPBP)Cu 8
-
1.30b 0.80 -0.79 -1.30b
1.80b 1.25 0.81b -0.65 -1.08 -1.53
-
1.59
1.46
1.26b 1.10b 0.72b -0.49 -0.63 -1.16b 1.21
0.93
1.01
0.73 -1.49 -1.83
-
-
2.22
2.05
-
0.55 -1.50 -1.90b
a ∆E1/2 ) the potential difference between the first oxidation and first
reduction. b Peak potential at a scan rate of 0.1 V/s.
the N atoms at the oxygenated ends of the benzoylbiliverdin
ligand lie out of the plane defined by the Ni and the two
central N atoms by 0.635(4) and -0.716(4) Å (0.677(4) and
-0.693(4) Å for the second molecule). The distortion is
slightly larger for the Cu complex, with out-of-plane devia-
tions of 0.759(3) and -0.884(3) Å for those N atoms. The
Ni-N bond lengths fall within the range 1.845(3)-1.868(3)
Å for the Ni(II) complex and 1.924(2)-1.932(2) Å for the
Cu(II) complex. The benzoyl O atom also forms a longer
intramolecular interaction with the metal, having M‚‚‚O
distances of 2.973(4) and 3.076(4) Å for MdNi and 2.933-
(3) Å for MdCu. Although to date we have not obtained
crystals of the Zn(II) complex 11 suitable for X-ray deter-
mination of its structure, we believe that this complex is
significantly more distorted than the Ni(II) and Cu(II)
derivatives. This conclusion is based on the following
observations: (1) Unlike 9 and 10, the Zn(II) complex 11
readily undergoes demetalation under mildly acidic condi-
tions, such as during purification using alumina or silica gel
chromatography. (2) The Zn(II) complex 11 is significantly
less stable than 9 or 10, decomposing upon storage under
air in a few days. In contrast, the Ni(II) and Cu(II) complexes
are very stable under the same conditions even after several
months. (3) The UV-vis spectrum of the Zn(II) complex
11 in CH2Cl2 shows a blue-shifted long-wavelength band at
752 nm compared with the corresponding absorptions for
the Ni (802 nm) and the Cu (806 nm) complexes. (4) The
electrochemistry of the Zn(II) complex is distinctly different
from that of the Ni(II) and Cu(II) derivatives (vide infra).
These observations strongly suggest a different conformation
for the Zn(II) derivative from those shown in Figure 1 for
the Ni(II) and Cu(II) complexes; the ring-opened ligand is
able to distort a lot more than a porphyrin macrocycle in
order to accommodate the larger and possibly penta- or hexa-
coordinated Zn ion.
Figure 2. Cyclic voltammograms of Ni(II) benzoylbiliverdin 9 and Ni-
(II) porphyrin 7 in PhCN, 0.1 M TBAP.
Figure 3. Cyclic voltammogram of Cu(II) benzoylbiliverdin 10 in PhCN,
0.1 M TBAP.
This result suggests that the second reduction of compound
9 is coupled with one or more subsequent chemical reactions.
The electrochemical behavior of (BV)Ni 9 is different from
that of the Ni(II) complex 7 [(TPBP)Ni]40 which shows
normal porphyrin electrochemical properties (see Figure 2b).
The Cu(II) complex 8 [(TPBP)Cu] also shows normal
porphyrin behavior. The compound is reduced at E1/2
)
-1.50 V and Epc ) -1.90 V for a scan rate of 0.1 V/s and
is oxidized at E1/2 ) 0.55 and 1.01 V vs SCE in PhCN, 0.1
M TBAP. In contrast, three reversible reductions are located
at E1/2 ) -0.65, -1.08, and -1.53 V are observed for the
copper(II) benzoylbiliverdin, [(BV)Cu] 10, in PhCN (see
Figure 3). The electrochemical behavior of the zinc(II)
benzoylbiliverdin, [(BV)Zn] 11, is similar to that of the
copper(II) complex 10. However, only the first two reduc-
tions of compound 11 are reversible (Table 3).
Up to three oxidations can be observed for complexes
9-11 in PhCN containing 0.2 M TBAP. The first oxidation
of the Ni(II) complex is located at E1/2 ) 0.80 V, and a small
rereduction wave at -0.36 V can be observed in addition to
the main anodic process when the potential scan is reversed
after the first oxidation (Figure 2a). The first oxidation of
the Cu(II) and Zn(II) complexes is irreversible and is located
at 0.81 and 0.72 V, respectively (Table 2). The difference
of potentials between the first reduction and the oxidation
(HOMO-LUMO gap) is 1.59 V for complex 9, 1.46 V for
10, and 1.21 V for 11 (Table 2). These values are much
Electrochemistry and Spectroelectrochemistry. Cyclic
voltammetry of metallo-benzoylbiliverdins 9-11 was carried
out in PhCN containing 0.1 M TBAP. The half-wave
potentials of these complexes are summarized in Table 2.
Each compound undergoes two or three reductions in PhCN,
0.1 M TBAP. The first reduction for all three compounds
9-11 is reversible and is located at E1/2 ) -0.79 V for Ni(II),
-0.65 V for Cu(II) and -0.49 V for Zn(II), respectively.
The second reduction of 9-11 ranges from -1.30 to -0.63
V (Table 2).
(40) Kadish, K. M.; Van Caemelbecke, E.; Boulas, P.; D’Souza, F.; Vogel,
E.; Kisters, M.; Medforth, C. J.; Smith, K. M. Inorg. Chem. 1993, 32,
4177-4178.
Nickel(II) benzoylbiliverdin [(BV)Ni] undergoes two
reductions, the second of which is not reversible (Figure 2a).
1468 Inorganic Chemistry, Vol. 45, No. 4, 2006

Undeca-substituted Metallo-benzoylbiliWerdins
Table 3. UV-vis Spectral Data of M(II) Benzoylbiliverdins 9-11 in
PhCN, 0.2 M TBAP
λ
max, nma
visible region
M(II)
Ni
rxn
Eapp (V)
Soret region
326 442
none
none
804
1st ox
2nd ox
1st red
+1.00 367 415
+1.50 322 442
-1.00 331 460
552 580 662 720
779
579
774 897
626 734
2nd red -1.50 330 402
none
1st ox
2nd ox
1st red
2nd red -1.40 315 398
3rd red -1.80 354
none
1st ox
2nd ox
1st red
437
484
Cu
Zn
none
344 422
762 814
628sh 680 793
680 824
+1.00 349 415
+1.50 352 412
-0.90 365 459
530
790 878
645 791
599 646
460
none
+0.90 329 455
+1.20 328 404sh 453sh
330 455
752
752
661 739
-0.55 339 437 482sh 580 646 752 824
600 647
2nd red -0.80 347 446
a sh ) shoulder peak.
Figure 4. UV-vis spectra (TPBP)Ni 7 (full line) and Ni(II) benzoyl-
biliverdin 9 (dash line) in PhCN containing 0.2 M TBAP.
smaller than the HOMO-LUMO gap (∼2.25 V) for generic
TPP or OEP complexes.41
The plots of the redox potentials as a function of the ratio
between the metal ion electronegativity and the radius (EN/
ri) for compounds 9-11 are given in the Supporting
Information (Figure S1). The potentials for the first and
second oxidations of the biliverdin complexes increase upon
going from Zn to Cu to Ni complex. The electrochemical
results indicate that the HOMO orbital energy of the Zn(II)
complex is higher than that of the Cu(II) and Ni(II)
complexes. The slope of the plot for the first oxidation equals
93 mV, while the slope for the second oxidations is 200 mV.
This result suggests that the coordinated metal ion might have
a stronger effect on the second oxidation of the biliverdin
complexes. When M(II) is varied from Zn to Ni, the first
reduction shifts negatively with a slope of -271 mV, while
the second reduction exhibits a much larger negative shift
of -645 mV. This confirms that the metal ion also has a
strong effect on the second reductions of complexes 9-11.
A comparison between the slope of the plots for reduction
and oxidation of complexes 9-11 indicates clearly that the
reductions exhibit significant metal ion dependence.
UV-vis spectra for the nickel porphyrin (TPBP)Ni 7 and
the Ni(II) biliverdin 9 are presented in Figure 4. A typical
porphyrin spectrum with a strong Soret band at 427 nm and
two weak visible bands at 545 and 581 nm can be seen for
(TPBP)Ni 7 in PhCN. However, Ni(II) biliverdin 9 exhibits
only two weak, broad, short-wavelength bands at 326 and
442 nm, and there is a single broad visible band at 804 nm
in the same solvent.
Thin-layer UV-vis spectroelectrochemistry was carried out
in PhCN containing 0.2 M TBAP. The spectral data obtained
upon controlled-potential reduction or oxidation of complexes
9-11 are summarized in Table 3 while, the spectral changes
obtained for 9-11 in PhCN, 0.2 M TBAP are shown in
Figures 5-7 and Figure S2 of the Supporting Information.
Upon the first reduction of the Ni(II) complex 9 at -1.00
V, the short-wavelength bands at 326 and 442 nm red-shift
Figure 5. Thin-layer UV-vis spectral changes of M(II) benzoylbiliverdins
9-11 obtained upon the controlled-potential reduction in PhCN, 0.2 M
TBAP.
to 331 and 460 nm with only slight changes in intensity.
The visible band at 804 nm for this complex almost
disappears, while a new band at 897 nm grows up during
the first controlled-potential reduction in PhCN, 0.2 M TBAP
(Figure 5a). Similar spectral changes are observed upon the
(41) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435-605.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1469

Ongayi et al.
Spectral changes upon the first oxidation of Cu(II) complex
10 are shown in Figure 7. As is also seen for the Ni(II)
complex 9, a new Soret band at 415 nm appears when the
oxidation potential is set at 1.00 V (see Figures 6a and 7).
A very strong visible band at 680 nm is also observed upon
the first oxidation. This result is different from that of the
Ni(II) complex 9, which has no strong visible bands and only
four weak bands in this spectral region.
For the Zn(II) complex 11, the spectral changes obtained
upon the oxidations are totally different from those of Ni(II)
and Cu(II) complexes, and this is shown in Figure S2 of the
Supporting Information. Upon the first oxidation at 0.90 V,
all absorption bands decrease only slightly in intensity and
no new bands can be discerned.
Conclusions
Two high-yielding synthetic methods for the preparation
of metallo-benzoylbiliverdins have been developed using the
chemical oxidation of a dodeca-substituted porphyrin and
its Ni(II) and Cu(II) complexes. Benzoylbiliverdins 9 and
10 were prepared in 75-87% overall yields either from
porphyrin 6 using NaNO2/TFA and air followed by metal-
ation or from complexes 7 and 8 by reaction with mCPBA/
pyridine in the presence of air. The zinc(II) complex 11 was
obtained by oxidation of porphyrin 6 followed by metalation
due to the instability of the Zn(II) porphyrin in the presence
of mCPBA; this route does not depend on the oxidation
potential or the stability of the metallo-porphyrin. The
molecular structures of complexes 9 and 10 were determined
and show significantly distorted square planar geometries
for the central metal ions. The electrochemistry and spec-
troelectrochemistry of metallo-benzoylbiliverdins 9-11 are
carried out in PhCN contaning 0.1 or 0.2 M TBAP. Up to
three reductions and three oxidations are observed. The
differences of potentials between the first reduction and the
first oxidations range from 1.21 to 1.59 V vs SCE, which is
much smaller than the value of 2.25 ( 0.15 V for generic
TPP or OEP complexes or 2.05-2.22 V for the two
porphyrin complexes 7 and 8 examined in the present study.
Figure 6. Thin-layer UV-vis spectral changes of Ni(II) benzoylbiliverdin
9 obtained upon the controlled-potential oxidations in PhCN, 0.2 M TBAP.
Figure 7. Thin-layer UV-vis spectral changes of Cu(II) benzoylbiliverdin
10 obtained upon the first controlled-potential oxidation in PhCN, 0.2 M
TBAP.
first reduction of the Cu(II) complex 10 under the same
experimental conditions (Figure 5b). However, the spectral
changes for the Zn(II) complex 11 are different from the
Ni(II) and Cu(II) derivatives. Here bands at 330 and 455
nm decrease in intensity upon the first reduction at -0.55
V (Figure 5c).
Acknowledgment. The research described was supported
by the National Science Foundation (Grant No. CHE-304833,
M.G.H.V.), the National Institutes of Health (Grant No. EB-
002064, K.M.S.), and the Robert A. Welch Foundation
(Grant No. E-680, K.M.K.).
During the first oxidation of the Ni(II) complex 9 at 1.00
V, the absorption bands at 442 and 804 nm disappear while
a strong Soret band at 415 nm and four visible bands at 552,
580, 662, and 720 nm appear (Figure 6a). The bands at 415,
552, and 580 nm for the final spectrum of the oxidized
species are similar to that of the nickel porphyrin, (TPBP)Ni
7, which has a Soret band at 427 nm and two visible bands
at 545 and 581 nm. Upon further oxidation of 9 at 1.50 V,
a spectrum with Soret bands at 322 and 442 nm and a broad
visible band at 779 nm was observed which is similar to the
neutral spectrum of 9 (Figure 6b). This result suggests that
an open-chain structure is re-formed under the experimental
conditions.
Supporting Information Available: X-ray data in CIF format
for 9 and 10, the redox potentials of complexes 9-11 in PhCN
containing 0.1 M TBAP (Figure S1), and the thin-layer UV-vis
spectral changes of Zn-biliverdin 11 obtained upon the first and
second oxidations in PhCN, 0.2 M TBAP (Figure S2). This material
is available free of charge via the Internet at http://pubs.acs.org.
IC050841C
1470 Inorganic Chemistry, Vol. 45, No. 4, 2006