Zn(II), Ni(II), Cu(II), and Fe(III) complexes of potentially bimetalating tris(pyridine- and imidazole-appended) picket-fence naphthylporphyrins with benzyl ether spacers: Implications for cytochrome c oxidase active-site modeling

Article abstract of DOI:10.1021/ic001274q

Two new unsymmetrical picket-fence naphthylporphyrin ligands, 1 and 2, and several of their metalated porphyrinato complexes have been synthesized as precursor model compounds for the binuclear (Fe/Cu) cytochrome c oxidase (CcO) active site. 1 and 2 have a naphthylporphyrin superstructure that has been specifically incorporated to confer long-term configurational stability to the atropisomeric products. The two picket-fence porphyrin ligands also bear covalently linked, axially offset tris(heterocycle) coordination sites for a copper ion, much like that found in the native enzyme. Monometallic porphyrin complexes [M = Zn(II), Ni(II), Cu(II), and Fe(III)] of the pyridine-appended ligand 1 have been prepared and spectroscopically and magnetically characterized. An unusual monomeric iron(III) hydroxo porphyrin complex was isolated upon workup of the compound formed under ferrous sulfate/acetic acid reflux conditions. There is general difficulty in forming binuclear complexes of 1, which is attributed to the conformational flexibility of the benzyl ether type picket spacers. The potential of ligands such as 1 and 2 for future CcO active-site modeling studies is considered.

Full text of DOI:10.1021/ic001274q

4556
Inorg. Chem. 2001, 40, 4556-4562
Zn(II), Ni(II), Cu(II), and Fe(III) Complexes of Potentially Bimetalating Tris(pyridine- and
imidazole-appended) Picket-Fence Naphthylporphyrins with Benzyl Ether Spacers:
Implications for Cytochrome c Oxidase Active-Site Modeling
Thomas P. Thrash and Lon J. Wilson*
Department of Chemistry and the Center for Nanoscale Science and Technology, M.S. 60,
Rice University, P.O. Box 1892, Houston, Texas 77251-1892
ReceiVed September 28, 2000
Two new unsymmetrical picket-fence naphthylporphyrin ligands, 1 and 2, and several of their metalated
porphyrinato complexes have been synthesized as precursor model compounds for the binuclear (Fe/Cu) cytochrome
c oxidase (CcO) active site. 1 and 2 have a naphthylporphyrin superstructure that has been specifically incorporated
to confer long-term configurational stability to the atropisomeric products. The two picket-fence porphyrin ligands
also bear covalently linked, axially offset tris(heterocycle) coordination sites for a copper ion, much like that
found in the native enzyme. Monometallic porphyrin complexes [M ) Zn(II), Ni(II), Cu(II), and Fe(III)] of the
pyridine-appended ligand 1 have been prepared and spectroscopically and magnetically characterized. An unusual
monomeric iron(III) hydroxo porphyrin complex was isolated upon workup of the compound formed under ferrous
sulfate/acetic acid reflux conditions. There is general difficulty in forming binuclear complexes of 1, which is
attributed to the conformational flexibility of the benzyl ether type picket spacers. The potential of ligands such
as 1 and 2 for future CcO active-site modeling studies is considered.
Introduction
The CcO X-ray structures revealed at least seven metal atoms
in the resting (oxidized) enzyme: a binuclear, mixed-valence
copper(I/II) site [CuA], an isolated low-spin iron(III) porphyrin
[heme a], a high-spin iron(III) porphyrin [heme a3] with a closely
positioned, monomeric copper(II) center [CuB], and, finally,
redox-inactive magnesium(II) and zinc(II) sites.4,5,7,8 The struc-
tures of the fully reduced and azide- and CO-bound enzyme
have also recently been reported.6,9 The likely electron-transfer
Cytochrome c oxidase (CcO) is the terminal enzyme in the
biological electron-transport cycle, catalyzing the four-electron
reduction of dioxygen into two molecules of water.1-3 The
energy liberated by this exergonic reaction is utilized by ATP
synthase to drive the conversion of ADP and inorganic
phosphate into ATP. Efforts toward understanding the enzymatic
mechanism have recently advanced significantly with reports
of the whole enzyme X-ray structures from mammalian4-6 and
bacterial7-9 sources. These 2.8 Å or better resolution X-ray
structures confirmed several structural features that had long
been proposed on the basis of spectroscopic evidence, particu-
larly EXAFS10-15 and EPR/ENDOR.16,17
route is cytochrome c f CuA f heme a f heme a3/CuB
f
H2O. The oxidized active-site structure (Figure 1) consists of
an Fe(III) heme having an axially bound histidine imidazole
ligand. The nearby CuB is a Cu(II) center coordinated by three
histidine imidazole nitrogens. The CuB atom is located ∼4.5 Å
from the heme a3 iron and is displaced ∼1 Å from the heme
perpendicular microsymmetry axis toward the histidines. None
of the X-ray structural studies definitively located a bridging
ligand between the active-site metal centers. This finding is
surprising, since strong antiferromagnetic coupling exists in the
native enzyme to form an overall S ) 2, EPR-silent state.1-3,18,19
EXAFS studies have suggested bridging atoms such as S or
Cl,10-15 and other ligands have been postulated as well (oxo,
hydroxo, histidyl).20
* To whom correspondence should be addressed.
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10.1021/ic001274q CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/03/2001

Zn, Ni, Cu, and Fe Complexes of Naphthylporphyrins
Inorganic Chemistry, Vol. 40, No. 18, 2001 4557
Figure 1. Schematic representation of the resting CcO active-site
structure as viewed parallel to the heme a3 plane. Peripheral side chains
of the protoporphyrin IX ligand have been omitted for clarity. The
dashed line from Tyr 244 represents a hydrogen bond to a heme a3
hydroxyfarnesylethyl group (not shown).
The present state of studying metalloenzyme systems with
model compounds has been the subject of a recent review.21
The CcO active site, in particular, has been the focus of
numerous small molecule modeling efforts,20,22-45 most of which
have utilized picket-fence porphyrin methodology.46 Some
Figure 2. The new tris(picket-fence) porphyrin CcO model compound
ligands: H2T(NAPOPY)3P (1) and H2T(NAPOIM)3P (2).
especially successful approaches toward modeling the CcO
active site have recently been disclosed. Collman has reported
several CcO model compounds bearing axial imidazole ligands
that efficiently perform four-electron dioxygen reduction.38,39,41,44
Karlin and Holm have prepared CcO model compounds in
which the Cu-binding site is linked to the porphyrin through
an unsupported µ-oxo bridge, producing antiferromagnetic
coupling between the Fe(III) and Cu(II) centers.20,45 A covalently
supported version of one of these compounds has also been
synthesized.47 Finally, it is only recently that the first CcO model
compounds bearing imidazoles at the CuB-binding site have been
reported.42-44
(21) Collman, J. P. Inorg. Chem. 1997, 36, 5145.
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The present work takes a somewhat different approach to
CcO active-site modeling with the synthesis of a new class of
picket-fence porphyrin compounds. The new ligands 1 and 2
(Figure 2) incorporate several structural features that were
predicted to give improved ease of handling, provide for
structural flexibility, and accurately reproduce the CcO active-
site architecture. First, naphthyl groups have replaced the more
typically used phenyl groups in the tetraarylporphyrin super-
structure. These sterically encumbered naphthyl moieties block
atropisomer interconversion, allowing the desired porphyrin
stereoisomer to be isolated and indefinitely retained. Second,
an unsymmetrical R,R,R-tris(2-hydroxy-1-naphthyl)porphyrin
precursor has been employed to prepare ligands with only three
heterocyclic groups above one porphyrin face.34-39,41-44 This
arrangement also forces the “upstairs” binding site to be located
off-axis as in the enzyme. Finally, a benzyl ether type OCH2
spacer has been utilized for the first time to promote a
conformationally flexible upstairs metal site with a probable
short metal (porphyrin)-metal (heterocycle) distance (<5 Å),
in contrast to the rigidity introduced by the more typically used
amide spacers which may promote longer M-M′ distances.
Preparation and characterization of picket-fence porphyrins 1
and 2 and several monometallic [Zn(II), Ni(II), Cu(II), and Fe-
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Chem. Soc., Dalton Trans. 1994, 3203.
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T. A.; Fu, L. J. Am. Chem. Soc. 1994, 116, 9783.
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Chem. Soc., Chem. Commun. 1999, 137.
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4558 Inorganic Chemistry, Vol. 40, No. 18, 2001
Thrash and Wilson
slurry-packed silica gel column. As soon as the product was thoroughly
adsorbed, the eluent was changed to acetone/hexanes (3:7), and the
adsorbed material eluted as a very broad, apparently unresolved, dark
red-purple band. Fractions were collected periodically and analyzed
by TLC [acetone/hexanes (3:7)] and NMR for product content. Pure
R,R,R-H2T(NAPOH)3P was obtained as the next-to-last eluted com-
pound (see the Results and Discussion). The desired product eluted
between R,R,R,â- and R,R,R,R-5,10,15,20-tetrakis(2-hydroxy-1-naph-
thyl)porphyrin, H2THNP. Final purification was achieved by recrys-
tallization from CHCl3/hexanes. Yield: 0.48 g (0.23% from pyrrole,
(III)] porphyrin complexes of 1 are described in this work as
an initial investigation toward the development of picket-fence
naphthylporphyrins as CcO active-site model compounds.48
Experimental Section
All compounds were reagent grade or better. Pyrrole (Acros) was
distilled from CaH2 under vacuum and stored over 4 Å molecular sieves
at -30 °C. The following reagents were used as received: 2-methoxy-
1-naphthaldehyde (Aldrich), p-tolualdehyde (Aldrich), neat BBr3 (Al-
drich), 3-picolyl chloride hydrochloride (Aldrich), Zn(CF3SO3)2‚H2O
(Alfa), Cu(CF3SO3)2 (Aldrich), Ni(CH3CO2)2‚4H2O (Fisher), and
FeSO4‚7H2O (Mallinckrodt). Solvents were used as received, with one
exception; for procedures requiring dry CH2Cl2, the solvent was refluxed
with CaH2 and then distilled under Ar. Chromatographic separations
were accomplished as appropriate on either neutral alumina (Aldrich,
Brockmann I, 150 mesh, 58 Å, surface area 155 m2 g-1) or silica gel
(Aldrich, grade 62, 60-200 mesh, 150 Å). NMR solvents CDCl3
(Cambridge Isotope Laboratories, 99.8% atom D) and pyridine-d5
(Aldrich, 99% atom D) were used as received.
NMR spectra were obtained on a Bruker Avance 400 MHz or Bruker
AF-250 MHz spectrometer. UV-vis characterization was accomplished
with a GBC model 918 UV-vis spectrophotometer. Mass spectral
analyses were performed on a Finnigan Mat 95 mass spectrometer.
Infrared spectra were obtained as KBr disks using a Nicolet Magna-IR
760 spectrometer. Paramagnetic compounds were characterized with a
Varian E-Line EPR spectrometer (X-band) and by SQUID magnetom-
etry (Quantum Design, MPMS-5S). Bulk magnetic measurements were
corrected by subtracting the experimentally measured diamagnetic
contributions from the quartz sample cell and the free ligand. Elemental
analyses were obtained commercially from Galbraith Laboratories
(Knoxville, TN). Metal analyses were performed either in-house by
flame atomic absorption spectroscopy (GBC model 908) or com-
mercially by Galbraith Laboratories. Qualitative elemental composition
studies were accomplished by EDS electron microprobe analysis
(Cameca SX-50).
r,r,r-5,10,15-Tris(2-hydroxy-1-naphthyl)-20-p-tolylporphyrin,
H2T(NAPOH)3P. The desired unsymmetrical porphyrin atropisomer
was synthesized as a mixture of porphyrin products from a mixed-
aldehyde condensation with pyrrole.49 In a typical reaction, 14.01 g of
2-methoxy-1-naphthaldehyde (75 mmol), 2.9 mL of p-tolualdehyde (25
mmol), and 6.9 mL of pyrrole (100 mmol) were combined in 250 mL
of propionic acid and refluxed for 1 h. The propionic acid solvent was
then removed under reduced pressure, and the product was chromato-
graphed twice on alumina with CH2Cl2 eluent. Separation of the desired
atropisomeric product was not possible at this point due to similar
polarities of the compounds, and the product mixture (13 porphyrin
atropisomers) was used without further purification for the step
immediately below. Yield (of mixture): 2.46 g (∼10% porphyrins).
The methoxy groups of the product mixture were removed under
mild conditions using BBr3.50 In a typical reaction, 6.20 g of the
porphyrin mixture was dissolved in 140 mL of dry CH2Cl2, and the
solution was chilled to -60 °C. A 4.4 mL sample of neat BBr3 (46.5
mmol, excess) was added dropwise to the cold solution via syringe,
during which time the reaction mixture turned green. The solution was
maintained at -60 °C for 1 h and then gradually warmed to room
temperature overnight. Excess BBr3 was quenched at that time by slowly
adding 50 mL of deionized water to the reaction mixture and vigorously
stirring for 15 min. The CH2Cl2 solution was diluted with 500 mL of
EtOAc and neutralized with three 10% NaHCO3 washes. The EtOAc
extract was then washed twice with deionized water, dried over
anhydrous Na2SO4, and gravity filtered. The solution was taken to
dryness under reduced pressure, and R,R,R-H2T(NAPOH)3P was
isolated chromatographically as follows. The crude reaction product
was dissolved in a minimum amount of CHCl3 and loaded onto a CHCl3
1
two steps). H NMR (CDCl3): δ ) 8.88 (d, 2H), 8.69 (d, 2H), 8.60
(m, 4H), 8.24 (t, 3H), 8.05 (m, 5H), 7.60 (m, 5H), 7.37 (m, 3H), 7.05
(t, 3H), 6.90 (t, 3H), 5.10 (br s, 3H), 2.69 (s, 3H), -2.40 (br s, 2H).
UV-vis (CH2Cl2): 424 (Soret), 516, 548, 589, 644 nm. MS (CI+):
M + 1 ) 827, C57H38N4O3 + H.
r,r,r-5,10,15-Tris(2-(3-pyridyl)methyleneoxy-1-naphthyl)-20-p-
tolylporphyrin, H2T(NAPOPY)3P (1). In a typical reaction, 250 mg
of H2T(NAPOH)3P (0.3 mmol) and 250 mg of powdered KOH (4.5
mmol, excess) were dissolved in 16 mL of DMSO, and the solution
was stirred under Ar for 30 min. A 300 mg sample of 3-picolyl chloride
hydrochloride (1.8 mmol) was dissolved in 8 mL of DMSO and added
to the reaction mixture. The reaction was stirred at room temperature
for 8 h and then poured into 75 mL of deionized water. The porphyrin
was extracted into EtOAc, and the extract was then sequentially washed
with 1 M NaOH, deionized water, 1 M NaOH, deionized water, and
finally saturated aqueous NaCl twice. The solution was dried over
anhydrous MgSO4, gravity filtered, and reduced to dryness. The crude
reaction product was dissolved in CHCl3 and chromatographed on a
slurry-packed silica gel column with MeOH/CHCl3 (1:19). Two narrow
red bands moved just behind the solvent front and were followed by a
much broader, slower-moving, dark red-purple product band. The
product eluent was purified further by recrystallization from CHCl3/
hexanes. Yield: 160 mg (48%). 1H NMR (CDCl3): δ ) 8.78 (d, 2H),
8.54 (d, 2H), 8.44 (m, 4H), 8.24 (t, 3H), 8.07 (m, 10H), 7.89 (d, 1H),
7.66 (t, 3H), 7.51 (t, 2H), 7.33 (t, 3H), 7.00 (m, 4H), 6.91 (d, 2H),
6.54 (ddt, 2H), 6.38 (m, 3H), 6.18 (dd, 1H), 5.03 (s, 4H), 4.99 (s, 2H),
2.67 (s, 3H), -2.24 (br s, 2H). 13C NMR (CDCl3, selected peaks): δ
) 156.13/156.08 (naphthyl, C-2), 148.63/148.55 and 147.91/147.79
(pyridyl, C-2 and C-6), 68.93/68.89 (OCH2), 21.50 (Ar-CH3). UV-
vis (CH2Cl2): 425 (Soret), 516, 550, 591, 646 nm. MS (CI+): M + 1
) 1100, C75H53N7O3 + H. Anal. Calcd for C75H53N7O3‚H2O: C, 80.55;
H, 4.96; N, 8.77). Found: C, 80.44; H, 4.97; N, 8.65.
General Procedure for Metalation with Zn(II), Cu(II), and Ni-
(II): [MIIT(NAPOPY)3P]. Metalation with Zn(II), Cu(II), and Ni(II)
was accomplished by refluxing 5-10 equiv of the metal salt (either
the triflate or acetate) in an appropriate solvent (either CHCl3/MeOH
or DMF) until UV-vis spectroscopy indicated completion of the
reaction. At this point, the solvent was removed under reduced pressure,
and the residue was redissolved in CHCl3 and washed with deionized
water several times to remove excess metal salt. The CHCl3 solution
was reduced to dryness, and the metalated porphyrin product was
recrystallized from CHCl3/hexanes. Full experimental details and data
can be found in the Supporting Information.
r,r,r-5,10,15-Tris(2-(3-pyridyl)methyleneoxy-1-naphthyl)-20-p-
tolylporphyrinatoiron(III) Hydroxide, [FeIIIT(NAPOPY)3P(OH)].
Metalation with iron was accomplished by a modification of the ferrous
sulfate/acetic acid method.51 A 110 mg sample of 1 (0.10 mmol) was
dissolved in 55 mL of glacial acetic acid, and 280 mg of FeSO4‚7H2O
(1.0 mmol) was added. The solution was refluxed in air for 1 h, cooled,
and then reduced to dryness under vacuum. The green-brown residue
was partitioned between CHCl3 and deionized water with the aid of
brief sonication, and the organic phase was then washed five times
with deionized water. The CHCl3 solution was gravity filtered after
extraction, and some residual brown-black particulates were discarded.
The filtered solution was reduced to dryness, dried further under
vacuum, and recrystallized from CHCl3/hexanes. The product was
homogeneous by TLC [MeOH/CHCl3 (1:19)]. Yield: 95 mg (78%).
(48) Harmjanz, M.; Scott, M. J. J. Chem. Soc., Chem. Commun. 2000, 397.
(49) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(50) Sta¨ubli, B.; Fretz, H.; Piantini, U.; Woggon, W.-D. HelV. Chim. Acta
(51) Fuhrhop, J.-H.; Smith, K. H. In Porphyrins and Metalloporphyrins;
1987, 70, 1173.
Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 803.

Zn, Ni, Cu, and Fe Complexes of Naphthylporphyrins
Inorganic Chemistry, Vol. 40, No. 18, 2001 4559
Figure 3. 1H NMR spectrum of H2T(NAPOPY)3P at 400 MHz. The inset is an expansion of the 9.0-6.0 ppm region.
UV-vis (CH2Cl2): 334, 421 (Soret), 507 (sh), 576, 630 (sh) nm. EPR
(CH2Cl2, 11 K): g ) 5.87, g| ) 2.03. Magnetism (1.7-300 K, 1 T):
on 1 for this initial investigation. The synthesis and character-
ization of 2 can be found in the Supporting Information.
1
µeff(300 K) ) 5.11 µB. Paramagnetic H NMR (CDCl3): δ ) 80 (br,
The unsymmetrical meso substitution pattern of 1 is displayed
by its 1H NMR H-â splitting pattern (Figure 3). The â protons
adjacent to the tolyl group appear as a resolved downfield
doublet at 8.78 ppm (H-â1), whereas those whose nearest
neighbor is a naphthyl group and second-nearest neighbor is
the tolyl group occur as a doublet at 8.54 ppm (H-â2). The â
protons whose both nearest and second-nearest neighbors are
naphthyl groups occur as overlapping doublets at 8.44 ppm (H-
H-â). MS (CI+): M + 1 ) 1153, C75H51N7O3Fe + H. MS (FAB+,
3-nitrobenzyl alcohol matrix): M + 1 ) 1170, C75H52N7O4Fe + H.
Anal. Calcd for C75H52N7O4Fe‚3H2O: C, 73.53; H, 4.77; N, 8.00; Fe,
4.56. Found: C, 73.02; H, 4.41; N, 7.87; Fe, 4.40. Acetate by ion
chromatography: found only 0.21.
Results and Discussion
Ligand Synthesis. Picket-fence naphthylporphyrin ligands
1 and 2 were synthesized in three steps from commercially
available starting materials. The desired atropisomeric porphyrin
scaffold was obtained as an inseparable mixture of porphyrin
products and atropisomers from a mixed-aldehyde condensation
with pyrrole. However, the R,R,R-configuration triphenolic
porphyrin H2T(NAPOH)3P was easily isolated after demethy-
lation. The product configuration was established on the basis
of the compound’s polarity and relative elution position between
two highly polar H2TNHP atropisomers whose configurations
â3,4). Thus, H-â1 most resembles the â protons in 5,10,15,20-
tetrakis(p-tolyl)porphyrin (8.85 ppm, singlet), and H-â3,4 are
similar in H-â resonance position to a H2TMNP atropisomeric
mixture (8.42 ppm, multiplet).52 The upfield shifts of the â
protons near the naphthyl groups are consistent with increased
disruption of the porphyrin ring current by the bulky naphthyl
rings, compared to that produced by the relatively unencumbered
phenyl substituent.55
The R,R,R-configuration of 1 is fully consistent with the 1H
and selected 13C NMR data. Neither 1H nor 13C NMR
spectroscopy can conclusively distinguish between the R,R,R-
and R,â,R-configurations, regardless of the field strength, since
these two atropisomers are differentiated only by the configu-
ration of the unique naphthyl picket on the symmetry plane.
Furthermore, no NOE experiment can distinguish between the
two configurations either. However, the high polarity of the H2T-
(NAPOH)3P precursor and its observed elution position in the
predicted location between R,R,R,R- and R,R,R,â-H2THNP offer
convincing evidence of the proposed configuration assignment
(polarity order: R,R,R,R > R,R,R > R,R,R,â > R,â,R).
Repeated attempts for more than one year to crystallize 1 and
its metalated products for single-crystal X-ray structural analysis
have been unsuccessful to date.
1
are readily established by H NMR spectroscopy.52 Further
evidence of the product configuration was provided by the peak
1
symmetry pattern in the H and 13C NMR spectra of 1 as
discussed in more detail below.
No evidence of atropisomer interconversion was observed
for the solid R,R,R- H2T(NAPOH)3P intermediate over one year
of storage at room temperature or for any derived reaction
product. The absence of atropisomer interconversion attests to
the usefulness of naphthyl picket-fence porphyrins for preparing
CcO model compound ligands. Both 1 and 2 were prepared in
good yields via nucleophilic displacement at a benzylic center.
Difficulty in handling the light-sensitive, imidazole-appended
porphyrin53,54 2, however, led to most studies being performed
The inequivalence of one picket arm is manifested in the
narrowly separated H NMR resonances of the benzylic and
(52) Abraham, R. J.; Plant, J.; Bedford, G. R. Org. Magn. Reson. 1982,
19, 204.
(53) Tsuchida, E.; Komatsu, T.; Arai, K.; Nishide, H. J. Chem. Soc., Dalton
Trans. 1993, 2465.
1
(54) Grimmett, M. R. In ComprehensiVe Heterocyclic Chemistry; Katritzky,
(55) Janson, T. R.; Katz, J. J. In The Porphyrins; Dolphin, D., Ed.;
A. R., Rees, C. W., Eds.; Pergamon: Oxford, 1984; Vol. 5, p 420.
Academic: New York, 1979; Vol. IV, p 28.

4560 Inorganic Chemistry, Vol. 40, No. 18, 2001
Thrash and Wilson
pyridyl protons, as well as for selected 13C signals (see the
Experimental Section). The benzyl methylene protons appear
as two resolved singlets (2:1 integral ratio) near 5 ppm,
demonstrating this inequivalence. Likewise, the corresponding
carbon signals occur as narrowly separated peaks in ap-
chloride), failed to produce any changes in the NMR spectrum.
1
In contrast to the broad H NMR spectrum in CDCl3, [ZnIIT-
(NAPOPY)3P] gave a spectrum with very sharp signals in
pyridine-d5. These results can be rationalized as follows.
Zinc(II) porphyrins are either four- or five-coordinate, with
a loosely held, neutral ligand sometimes filling the axial
coordination position.59,60 In pyridine-d5, the ligating bulk
solvent fully occupies the axial coordination site, and the
observed spectrum is that of the axial pyridine-d5 complex. In
CDCl3 this site is open, and intermolecular axial coordination
of the appended pyridines is apparently more favored than is
intramolecular chelation of a second metal at the tris(3-pyridyl)
site. Intermolecular ligation results in transient oligomer forma-
tion in solution, producing the broad proton resonances.
Intramolecular coordination of the appended pyridines is not
possible, since the OCH2 picket spacer is too short to reach the
metal site within the same porphyrin. The five-coordinate nature
of the Zn(II) complex is supported by its UV-vis spectrum in
CH2Cl2 (R:â intensity ratio 0.37), since ratios greater than 0.25
are typically indicative of axial coordination.59 To our knowl-
edge, intermolecular oligomerization has not been previously
noted in Zn(II)-metalated picket-fence porphyrin CcO model
compounds.
1
proximately a 2:1 intensity ratio. Although the benzylic H
resonances occur as singlets in 1, diastereotopic splitting of these
protons was seen in the imidazole-appended ligand 2 at room
temperature. Similar diastereotopic methylene splitting has been
previously observed in a picket-fence porphyrin, but only at
reduced temperatures.29
1
The pyridine H resonances display upfield and downfield
shifts relative to the unappended heterocycle, since these protons
lie in both the shielding and deshielding regions of the porphyrin
ring current. Further, slight chemical shift differences for one
set of resonances again indicate the presence of the unique
pyridine ring. The pyridine H-2 protons (Hpy-2) are manifested
as narrowly split doublets at 8.07 and 7.89 ppm. The downfield
signal is obscured by the naphthyl H-3/H-5/m-tolyl multiplet,
but integration places the two additional protons under this peak.
The higher field signal integrates to one proton, and the narrow
doublet is consistent with long-range coupling to H-4. The
overlapping doublets (2:1 integral ratio) at 7.66 ppm may be
attributed to the pyridine H-6 resonances (Hpy-6), leaving the
three far-upfield-shifted signals (6.54, 6.38, and 6.18 ppm)
assignable to the inequivalent sets of pyridine ring H-4 and H-5
protons (Hpy-4,5). The proton resonances for the pyridine on
the symmetry plane all lie slightly farther upfield than do those
of the symmetry-related rings. The narrowly separated 13C NMR
resonances of the pyridine C-2 and C-6 positions also display
peak height intensity ratios of approximately 2:1, again dem-
onstrating the inequivalence of one picket arm.
With the realization that bimetallic complex formation was
apparently being impeded by competitive oligomerization in the
five-coordinate zinc(II) porphyrins, efforts refocused on the Ni-
(II) and Cu(II) complexes, which are usually only four-
coordinate porphyrin species.60 It was hoped that, by freeing
the heterocyclic pickets from intermolecular coordination, they
would readily chelate and form intramolecular bimetallic
complexes. As expected, [NiIIT(NAPOPY)3P] was easily formed,
1
and its H NMR spectrum in CDCl3 was sharp and readily
One interesting NMR spectroscopic feature of 1 that deserves
special mention is the splitting pattern of its o-tolyl protons.
The m-tolyl signals are hidden beneath the multiplet at 8.07
ppm, but the o-tolyl protons (Hortho) are clearly observed at 7.51
ppm. The ortho protons are assigned as the upfield signal, since
they are more shielded by the porphyrin ring current. Instead
of the expected doublet, however, an apparent triplet is observed.
The “triplet” is actually, upon careful observation, a set of
overlapping doublets. Slow tolyl group rotation evidently
distinguishes the protons above the R and â porphyrin faces,
resulting in the overlapping doublets. Very slight peak broaden-
ing is also consistent with this assessment. Similar slow rotation
of unhindered porphyrin aryl groups (such as p-tolyl) has
recently been noted.56
Monometallic Zn(II), Cu(II), and Ni(II) Complexes. Before
preparation of any Fe(III) compound, the homobimetallic Zn-
(II), Ni(II), and Cu(II) complexes of 1 were initially pursued as
simpler reference compounds. In addition, it was hoped that
the solution-state Cu(II)-Cu(II) distance could be indirectly
determined in the dicopper complex through use of the EPR
dipolar coupling constant.40,57,58
assigned to a monomeric metalloporphyrin structure. Interest-
ingly, the o-tolyl proton resonance occurs as the expected
doublet here, instead of the “triplet” observed for the free ligand,
indicating an increased rate of phenyl ring rotation in the
metalated compound.
Various mass spectrometry techniques gave parent-ion masses
for only the monometallic Zn(II), Ni(II), and Cu(II) complexes
of 1 (see the Experimental Section). Bimetallic metalloporphyrin
compounds have sometimes failed to display bimetallic parent-
ion mass peaks;23,28,31,37,61 however, elemental analyses clearly
demonstrated that the Ni(II) and Cu(II) compounds were only
monometallic. Further treatment of the mononuclear Ni(II)
complex with various salts of Ni(II), Cu(II), Zn(II), and Co(II)
under mild conditions (stoichiometric to excess metal cation,
CHCl3/MeOH, room temperature, aqueous workup) failed to
produce any changes in the 1H NMR spectrum, such as
paramagnetic broadening or conformationally induced shifts.
Also, EDS electron microprobe analysis did not detect even
traces of the new metals in the retreated compounds.
All spectral properties of the Cu(II) complex of 1 were also
consistent with only a mononuclear formulation. The EPR
spectrum of [CuIIT(NAPOPY)3P] showed no evidence for a
triplet state, which is indicative of a bimetallic Cu(II) com-
pound.57,58 The EPR spectrum of [CuIIT(NAPOPY)3P] in a
frozen CH2Cl2 solution at 11 K was characteristic of copper(II)
porphyrins and displayed typical anisotropic g values (g| ) 2.23,
Zn(II) metalation of 1 proceeded readily as determined by
UV-vis spectroscopy. However, upon workup of the reaction
mixture, a very broad 1H NMR spectrum was obtained in CDCl3
at room temperature. Metalation under various conditions,
including use of counteranions other than triflate (acetate or
g
≈ 2.05).62 The bulk magnetic properties of the Cu(II)
(56) Koerner, R.; Wright, J. L.; Ding, X. D.; Nesset, M. J. M.; Aubrecht,
K.; Watson, R. A.; Barber, R. A.; Mink, L. M.; Tipton, A. R.; Norvell,
C. J.; Skidmore, K.; Simonis, U.; Walker, F. A. Inorg. Chem. 1998,
37, 733.
(57) Smith, T. D.; Pilbrow, J. R. Coord. Chem. ReV. 1974, 13, 173.
(58) Eaton, S. S.; More, K. M.; Sawant, B. M.; Eaton, G. R. J. Am. Chem.
Soc. 1983, 105, 6560.
(59) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100, 5075.
(60) Buchler, J. W. In The Porphyrins; Dolphin, D., Ed.; Academic: New
York, 1979; Vol. I, p 425.
(61) Woo, L. K.; Maurya, M. R.; Jacobson, R. A.; Yang, S.; Ringrose, S.
L. Inorg. Chem. 1992, 31, 913.

Zn, Ni, Cu, and Fe Complexes of Naphthylporphyrins
Inorganic Chemistry, Vol. 40, No. 18, 2001 4561
Figure 5. Positive-ion FAB mass spectrum of [FeIIIT(NAPOPY)3P-
(OH)].
Figure 4. Comparative infrared spectra of H2T(NAPOPY)3P (a), [NiIIT-
(NAPOPY)3P] (b), and [FeIIIT(NAPOPY)3P(OH)] (c).
complex were also consistent with only a mononuclear com-
pound having simple Curie-like behavior from 1.7 to 150 K
(µeff ) 1.69 µB at 150 K; see Table S-1 in the Supporting
Information).
The lack of binuclear compound formation in the Zn(II), Ni-
(II), and Cu(II) complexes of 1 can likely be attributed to
conformational flexibility of the benzyl ether picket spacers.
Previous CcO model picket-fence porphyrin ligands have
utilized more rigid amide spacers or preformed cyclic chelating
sites to encourage a favorable ligand configuration for binding
of a second metal. The present model compounds are the first
to lack both of these features. Originally, it was hoped that the
OCH2 spacer flexibility would lead to a “conformationally
relaxed” heterocyclic binding site (with an especially short
metal-metal distance) without having detrimental effects on
its chelating ability. Instead, the entropy of these spacer groups
is apparently so great that a configuration favorable for chelation
is never attained. The absence of binuclear compounds is not
likely explained by an incorrect atropisomer assignment. As
discussed above, the R,R,R-configuration is strongly implicated
by elution characteristics, although the R,â,R-configuration
cannot be conclusively disproven by NMR spectroscopy. Even
in the event of an incorrect configuration assignment, the trans-
5,15-(3-pyridyl) pickets of the R,â,R-isomer could still coor-
dinate to a second metal atom if binuclear compound formation
were at all feasible. Similar trans-5,15-binding of Cu(I) has been
reported for an amide-linked picket-fence porphyrin model
compound.29
Fe(III) Complexes. The mononuclear Fe(III) complex of 1
was also readily prepared by the ferrous sulfate/acetic acid
method. Unlike the other mononuclear porphyrin compounds,
however, the Fe(III) complex displayed several unanticipated
characteristics.
The iron(III) porphyrin compound isolated upon workup of
the metalation reaction is apparently the axial hydroxide
complex instead of the anticipated acetate compound. This
assignment was established on the basis of several independent
techniques. First, the infrared spectrum of [FeIIIT(NAPOPY)3P-
(OH)] was nearly identical to those of ligand 1 and the analogous
Ni(II) complex (Figure 4). This fact leads to two important
conclusions: (1) The axial ligand cannot be acetate, since a
strong carbonyl stretch is not present near 1650 cm-1, as
displayed in other iron(III) acetato porphyrins.63 (2) The product
is also most likely not a µ-oxo dimer, as evidenced by the
Figure 6. UV-vis spectrum of [FeIIIT(NAPOPY)3P(OH)] in CH2Cl2.
absence of any new Fe-O-Fe antisymmetric stretching bands
in the 850-900 cm-1 region of the spectrum.64 Sulfur-based
axial ligands were similarly excluded on the basis of the infrared
spectrum, as well as by EDS electron microprobe analysis. This
process of elimination leaves hydroxide as the only remaining
axial ligand choice, but its existence can be only indirectly
inferred from the infrared spectrum, since its OH stretching
frequency is coincident with that of bulk water. The absence of
an acetate ligand has also been directly established through a
negligible ion-chromatography acetate analysis (see the Experi-
mental Section).
Like the other mononuclear porphyrins, [FeIIIT(NAPOPY)3P-
(OH)] readily gave a M + 1 ) 1153 parent ion by positive-ion
chemical ionization mass spectrometry, with the axial ligand
being lost during the ionization process. However, in contrast
to the chemical ionization mass spectrum, the positive-ion FAB
mass spectrum showed a weak M + 1 parent-ion peak at 1170
mass units for the axial hydroxide complex, as well as a much
more intense signal for the ligandless parent ion (Figure 5).
Detection of an intact axial hydroxide porphyrin complex by
mass spectrometry has not been reported for other known iron-
(III) hydroxo porphyrin compounds.
The electronic spectrum of [FeIIIT(NAPOPY)3P(OH)] is also
consistent with the axial ligand assignment (Figure 6). The
relatively featureless visible region of the spectrum is in good
qualitative agreement with those previously reported for iron-
(III) hydroxo porphyrins.26,28,65-73 Typically, the visible region
(64) White, W. I. In The Porphyrins; Dolphin, D., Ed.; Academic: New
York, 1979; Vol. V, p 318.
(65) Amundsen, A. R.; Vaska, L. Inorg. Chim. Acta 1975, 14, L49.
(66) Cense, J.-M.; Le Quan, R.-M. Tetrahedron Lett. 1979, 3725.
(62) Assour, J. M. J. Chem. Phys. 1965, 43, 2477.
(63) Oumous, H.; Lecomte, C.; Protas, J.; Cocolios, P.; Guilard, R.
Polyhedron 1984, 3, 651.

4562 Inorganic Chemistry, Vol. 40, No. 18, 2001
Thrash and Wilson
is somewhat better defined for the µ-oxo dimer in those systems
which can exist in either the µ-oxo or hydroxo form. The band
near 334 nm is also characteristic of only the previously reported
iron(III) hydroxo porphyrins.
only magnetic properties were seen (i.e., µeff ) 5.9 µB at 300
K, data not shown).74 The low magnetic moments of the Fe-
(III) complexes in the literature tetrakis(pyridine) system were
attributed to “weakly coupled interactions” in the polymeric
solid-state structure.80 The magnetic moments of previously
reported iron(III) hydroxo porphyrins have typically been in the
usual range for high-spin S ) 5/2 iron(III) porphyrins, except
for one compound with a room-temperature magnetic moment
of only 5.1 µB.65
The CH2Cl2 frozen solution EPR spectrum of [FeIIIT-
(NAPOPY)3P(OH)] at 11 K showed typical behavior for an
axially symmetric, high-spin iron(III) porphyrin compound, with
g and g| values of 5.87 and 2.03, respectively. Similarly, the
paramagnetic 1H NMR spectrum of [FeIIIT(NAPOPY)3P(OH)]
in CDCl3 was also consistent with an S ) 5/2 high-spin
assignment for Fe(III) as evidenced, for example, by its broad
H-â resonances centered near 80 ppm (see the Experimental
Section). In the solid state, however, the bulk magnetic
properties demonstrated considerable deviation from pure S )
5/2 high-spin Fe(III) character. At an applied field of 1 T, the
room-temperature effective magnetic moment of [FeIIIT-
(NAPOPY)3P(OH)] was only 5.11 µB, compared to typical
values of 5.7-5.9 µB for S ) 5/2 iron(III) porphyrins.74 The
magnetic moment versus temperature curve reached a maximum
near 80 K, before declining slightly to the room-temperature
value. Zero-field splitting of the Fe(III) center accounts for a
substantial drop in the magnetic moment below 20 K.75 The
full temperature data (1.7-300 K; see Table S-2 in the
Supporting Information) also exclude the possibility that an Fe-
(III) spin-equilibrium process gives rise to the low observed
room-temperature magnetic moment.76 Such a spin-equilibrium
situation is known to exist in ferric myoglobin hydroxide.77,78
Similarly, the proximity of the EPR g value to “6” eliminates
the possibility of an S ) 5/2, 3/2 quantum mechanically admixed
state leading to the low magnetic moment.79
Magnetic moments below the characteristic values for an S
) 5/2 spin state have been previously noted in the Fe(III)
complexes of a tetrakis(pyridine)-appended picket-fence por-
phyrin, and the measured moments are extremely sensitive to
the identity of the axial ligand and presence of admixed
crystallization solvents.80 For example, in the stated literature
case the room-temperature magnetic moments range from as
low as 5.17 µB for the axial bromide complex to a near spin-
only value of 5.88 µB for the axial hydroxide complex. A related
dependence on axial ligand identity was apparently seen in the
present study. As discussed previously, the axial iron(III)
hydroxo complex of 1 demonstrated a substantially depressed
magnetic moment. In comparison, when the axial iron(III) chloro
complex of 1 was prepared by standard methods,81 simple spin-
Aside from its notable spectroscopic and magnetic properties,
[FeIIIT(NAPOPY)3P(OH)] is also unusual for other reasons. The
fact that it exists in the hydroxide-ligated form and shows no
tendency to form a µ-oxo dimer is typical of other bifacially
encumbered iron(III) porphyrins.65-68,70,72,73 1 has both a
proximal face protected by the appended pyridines and a distal
face blocked by the naphthyl rings. These two structural features
likely explain the easy formation and stability of the iron(III)
hydroxo porphyrin, but the fact that the hydroxide form is
assumed “spontaneously” upon workup of the metalation
reaction is unprecedented. The axial acetate ligand is evidently
lost during aqueous workup and replaced by adventitious
hydroxide generated in the bulk solvent. Space-filling molecular
models indicate that the acetate anion may fit poorly into the
binding pocket, potentially explaining its easy loss. Admixed
water in the CHCl3 extraction solvent (∼1% H2O) and the
enhanced basicity of the alkyl-substituted pyridine moieties are
evidently sufficient to produce enough hydroxide to effect ligand
exchange for the weakly bound acetate. The significantly
different physical properties of the chloride and hydroxide
complexes in the present study suggest that chloride is firmly
held in the binding pocket and does not exchange for adventi-
tious hydroxide during workup. Further, only the Fe(III) axial
hydroxide complex has displayed any evidence to undergo
further reaction with Cu(II) to produce an as yet unidentified,
possibly heterobinuclear compound.82
In conclusion, two new configurationally stable CcO model
compound ligands and several of their monometallic porphyrin
complexes have been synthesized and characterized. There is
general difficulty in forming bimetallic compounds with the
present ligands, presumably due to inherent flexibility of the
picket spacers, but there is some evidence that heterobinuclear
compounds might be approached through an iron(III) hydroxo
porphyrin intermediate. We continue to explore new synthetic
strategies designed to expand the utility of such potentially
binucleating picket-fence porphyrin ligands, while preserving
the basic structural features that make them so appealing for
modeling the CcO active site.
(67) Miyamoto, T. K.; Tsuzuki, S.; Hasegawa, T.; Sasaki, Y. Chem. Lett.
1983, 1587.
(68) Cheng, R.-J.; Latos-Grazynski, L.; Balch, A. L. Inorg. Chem. 1982,
21, 2412.
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Acknowledgment. The Robert A. Welch Foundation (Grant
C-0627) is gratefully acknowledged for support of this research.
(74) Hambright, W. P.; Thorpe, A. N.; Alexander, C. C. J. Inorg. Nucl.
Chem. 1968, 30, 3139.
Supporting Information Available: Synthetic procedures and
experimental data for 2 and the Zn(II), Ni(II), and Cu(II) complexes
of 1 and molar magnetic susceptibility versus temperature data for
[CuIIT(NAPOPY)3P] and [FeIIIT(NAPOPY)3P(OH)]. This material is
available free of charge via the Internet at http://pubs.acs.org.
(75) Mitra, S. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.;
Addison-Wesley: Reading, MA, 1983; Vol. II, p 9.
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Soc. 1977, 99, 3886.
IC001274Q
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references therein.
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(81) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
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(82) Reaction of [FeT(NAPOPY)3P(OH)] with CuCl2‚2H2O in CHCl3/
MeOH produced a compound that gave a positive-ion FAB mass
spectrum consistent with Cu incorporation (M + 1 ) 1216).