Electron spin-spin exchange coupling mediated by the porphyrin π system

Article abstract of DOI:10.1021/ic060209z

The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1-3) substituted with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin 3 is monoclinic, space group P2₁/n, with a = 12.239(2) A, b = 17.819(3) A, c = 34.445(7) A, α = 90°, β = 97.466(3)°, γ = 90°, and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with |J| ? |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3 exhibits frozen-solution EPR spectra with zero-field splitting and a Δm_s = 2 transition characteristic of a triplet state. The intensity of the Δm_s = 2 transition of 3 was measured as a function of temperature, and the data fit according to a singlet-triplet model to yield J(3,solution) = -75 cm^-1 (H = - 2JS₁·S₂). Polycrystalline samples of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were fit using a modified Bleaney-Bowers equation to give J(3,solid) = -29 cm^-1 (H = - 2JS₁·S ₂). The antiferromagnetic J values are consistent with the π topology of the porphyrin ring.

Full text of DOI:10.1021/ic060209z

Inorg. Chem. 2006, 45, 5752−5759
Electron Spin
System
−Spin Exchange Coupling Mediated by the Porphyrin π
David A. Shultz,*,† Christopher P. Mussari,†,‡ Krishna Kumar Ramanathan,†,§ and Jeff W. Kampf|
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
and Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
Received February 6, 2006
The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1−3) substituted
with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been
studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin
3 is monoclinic, space group P21/n, with a
97.466(3) γ ) 90 , and Z 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent
with . No evidence was observed for conformational modulation of J by rotation about single bonds as
| . |
shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3
exhibits frozen-solution EPR spectra with zero-field splitting and a ms 2 transition characteristic of a triplet
) 12.239(2) Å, b ) 17.819(3) Å, c ) 34.445(7) Å, R ) 90°, â )
°,
°
)
|
J
a|
∆
)
state. The intensity of the
according to a singlet
∆
ms
)
2 transition of 3 was measured as a function of temperature, and the data fit
1
−
triplet model to yield J(3,solution) ) −75 cm- (H ) − 2JSˆ1‚S2
ˆ
). Polycrystalline samples
of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were
fit using a modified Bleaney ). The antiferromagnetic
Bowers equation to give J(3,solid) ) −29 cm-1 (H ) − 2JSˆ1‚S2
J values are consistent with the topology of the porphyrin ring.
−
ˆ
π
Introduction
frameworks for the construction of novel high-spin mol-
ecules. Along these lines, Iwamura and Koga prepared free-
Open-shell molecular assemblies featuring metallopor-
phyrins are promising components of molecule-based mag-
netic materials.1-25 In addition, porphyrins offer versatile
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* To whom correspondence should be addressed. Tel: (919) 515-6972.
Fax: (919) 515-8920. E-mail: david_shultz@ncsu.edu. Web: http://
www.ncsu.edu/chemistry/das.html
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† North Carolina State University.
‡ Current address: Bristol-Myers Squibb, Pharmaceutical Research
Institute, Princeton, NJ.
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Mater. Chem. 2000, 10, 2507-2514.
§ Current address: Senior Research Scientist, CiVenti Chem, Pittsboro,
NC.
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| University of Michigan.
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5752 Inorganic Chemistry, Vol. 45, No. 15, 2006
10.1021/ic060209z CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/17/2006

Electron Spin-Spin Exchange Coupling
Scheme 1
Scheme 2
base and metalloporphyrins with one to four phenylcarbene
groups that coupled with each other and with copper(II)
bound in the porphyrin macrocycle.26,27 Kamachi reported
verdazyl-substituted porphyrins attached to polymer back-
bones.28 Iwamura and others have also studied the magnetic
behavior of pyridine nitroxides coordinated to chromium-
(III) tetraphenylporphyrin.29,30 Recently, several doubly
oxidized porphyrin dimers have been prepared that exhibit
exchange coupling.31-35
couplings to give 1(TBS)2 and 3(MOM)4 or to a modified
Sonogashira coupling31,40 to give 2(TBS)2 (TBS ) tert-
butyldimethylsilyl; MOM ) methoxymethyl). The syntheses
of the boronic esters and alkyne used in the couplings have
been reported previously.38,41 Deprotection of the TBS groups
of 1(TBS)2 and 2(TBS)2 was accomplished using tetra-n-butyl-
ammonium fluoride, and the corresponding hydroxylamines
were immediately oxidized to the bis(nitroxide) biradicals
using PbO2 as described previously.38
Our efforts have focused on preparing new porphyrins as
building blocks for coordination polymers and complexes
with interesting magnetic or magneto-optic properties.31,32,36-38
Herein, we describe the syntheses and electron paramagnetic
resonance (EPR) spectral characterizations of porphyrins 1-3
(Scheme 1), which have stable nitroxide and stable semi-
quinone (SQ) complexes covalently attached to trans-meso
positions of the porphyrin ring. One of these compounds, 3,
has been structurally characterized.
The methoxymethyl-protected (MOM-protected) porphyrin
3(MOM)4 was reacted with catalytic concentrated aqueous
HCl in methanol to yield the corresponding bis(catechol)
porphyrin and remetalated using zinc acetate. The metalated
bis(catechol) porphyrin was reacted with 2 equiv of TpCum,Me
-
ZnOH {TpCum,Me ) hydro-tris[3(5)-cumenyl-5(3)-methylpyra-
zolyl]borate} to yield the corresponding bis(semiquinone)
complex.42-45 The formation of SQs is supported by EPR
spectroscopy and by X-ray crystallography. Attempts to
prepare X-ray quality crystals of both 1 and 2 were
unsuccessful.
Results and Discussion
Synthesis. The syntheses of porphyrins 1-3 are shown
in Scheme 2. Dibromoporphyrin 439 was subjected to Suzuki
Molecular Structure of 3. An ORTEP of 3 is shown in
Figure 1. Bond lengths for the dioxolene units are consistent
with the SQ oxidation state as opposed to catecholate or
protonated catecholate.46,47 The SQ torsion angles with
respect to the plane of the porphyrin ring are 58°, allowing
moderate delocalization of the SQ spin density into the π
system of the porphyrin. All other structural features are
listed in the Supporting Information.
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87.
Inorganic Chemistry, Vol. 45, No. 15, 2006 5753

Shultz et al.
Figure 1. ORTEP of bis(SQ) porphyrin, 3; monoclinic P21/n, a ) 12.239(2) Å, b ) 17.819(3) Å, c ) 34.445(7) Å, R ) 90°, â ) 97.466(3)°, γ ) 90°,
and Z ) 2. Data collected at 158 K. Solvent molecules, hydrogens, and cumenyl groups have been omitted for clarity.
Table 1. Bond Lengths (Å) for SQ Rings of 3
Zn(1)-O(2)
Zn(1)-O(1)
O(1)-C(1)
O(2)-C(6)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(4)-C(11)
C(5)-C(6)
1.952(10)
2.134(10)
1.243(17)
1.260(19)
1.48(2)
Figure 2. Mononitroxide ZnII porphyrins studied previously.
1.50(2)
1.356(18)
1.456(19)
1.41(2)
1.45(2)
1.38(2)
We reasoned that exchange coupling could be quite weak
in biradicals 1 and 2, and J might be strongly modulated
(and modulated differentially in 1 and 2) by Cmeso-C bond
torsion (Cmeso is the porphyrin meso-carbon; C is the carbon
of the nitroxide functionality: a phenyl carbon in 1 and an
ethyne carbon in 2), and the alternating line-width effect
might be observed.50-52 However, as shown in Figures 3 and
4, the EPR spectral shapes of both 1 and 2, respectively, are
temperature-invariant between ca. 250 and 365 K, suggesting
that (1) |J| . |a| at all torsion angles and (2) bond rotation
is extremely fast at all of the temperatures studied. The
second point is most likely for 2 because phenyl torsions in
phenylalkynes have extremely small torsional barriers of ca.
0.6 kcal/mol.53,54 For 1, the barrier is expected to be larger
than that for 2,55 such that J modulation might be more
reasonable for 1. However, the invariance of the fluid-
solution EPR spectral shape of 1 with temperature is
consistent with the fact that |J| . |a| regardless of the
phenylnitroxide torsion angles. This is reasonable because
|J| need only be greater than ca. 10-3 cm-1, the value of the
N-hyperfine interaction.
Bis(nitroxide) Porphyrins 1 and 2. Several years ago,
we reported the preparation and EPR spectral characterization
of three nitroxide-substituted porphyrins shown in Figure 2
(Mes ) mesityl, 2,4,6-trimethylphenyl).38 One of our goals
is to maximize spin delocalization into the porphyrin to
achieve the maximum exchange coupling with bound
paramagnetic metal ions. We recognized that direct attach-
ment of the phenylnitroxide to the porphyrin would result
in substantial phenyl torsion48 that would attenuate delocal-
ization.45,49 The addition of a spacer group (e.g., -CHdCH-
or -CtC-) would dilute the spin density but would allow
for greater coplanarity of phenylnitroxide and porphyrin
rings, hopefully increasing the spin delocalization despite
spin dilution. Thus, assessing spin-delocalization-limiting
torsion versus spin-delocalization-promoting coplanarity is
critical for evaluating design principles for high-spin mol-
ecules based on porphyrins.45,49
Previously, we could not easily distinguish differences in
spin delocalization as a function of nitroxide functionality,38
and we hoped that the EPR spectral characterization of 1
and 2 could answer the question of which functionality
resulted in the greatest spin delocalization into the porphyrin
π system.
The frozen-solution EPR spectra of 1 and 2 are typical
for dinitroxides having small D values and N-hyperfine
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15426-15432.
5754 Inorganic Chemistry, Vol. 45, No. 15, 2006

Electron Spin-Spin Exchange Coupling
Figure 5. X-band frozen solution (10 mM solutions in toluene) EPR
spectra of biradical 3 (top, experimental; bottom, simulation). Inset: ∆ms
) 2 transition.
distinguish exchange coupling in 1 from exchange coupling
in 2 using EPR spectroscopy.
Figure 3. X-band fluid solution (1 mM in toluene) EPR spectra and
simulations of 1 at different temperatures.
Bis(semiquinone) Porphyrin 3. Pierpont et al. reported
the reaction of TpCum,MeZnOH with (tetrahydroxyphthalo-
cyaninato)nickel(II), a reaction that did not result in semi-
quinone formation.62 Instead, monodentate and weakly
coordinated bidentate complexes of TpCum,MeZn+ with the
catechol-like OH groups of (tetrahydroxyphthalocyaninato)-
nickel(II) were formed. The formation of semiquinone
complexes in the present cases is consistent with the weaker
electron withdrawing character of the porphyrin ring system
compared to that of a phthalocyanine ring system. The greater
electron withdrawing capacity of a phthalocyanine, therefore,
should require an oxidizing agent stronger than oxygen to
complete the formation of the semiquinone complex. The
lack of IR stretching near 1660 cm-1 (quinone) and 1250
cm-1 (catecholate) for 3 suggests the presence of semi-
quinone groups.
Positive evidence for the presence of SQ groups comes
from EPR spectroscopy. The fluid-solution EPR spectrum
consists of single broad lines lacking hyperfine structure.
However, as shown in Figure 5, the frozen-solution EPR
spectrum recorded at 77 K exhibits fine structure character-
istic of a triplet state.63-65 Also observable near half-field is
a ∆ms ) 2 transition (Figure 5, Inset). Both of these
observations are consistent with an S ) 1 bis(SQ) biradical.
Simulation66 of the ∆ms ) 1 region of the EPR spectrum of
3 gives zero-field-splitting parameters |D/hc| ) 0.0033 cm-1
Figure 4. X-band fluid solution (1 mM in toluene) EPR spectra and
simulations of 2 at different temperatures.
structure in the ∆ms ) 1 region.56-60 These spectral features,
unfortunately, preclude our simulating the spectra to extract
pertinent spin Hamiltonian parameters. However, both 1 and
2 exhibit very weak ∆ms ) 2 features near half-field,
indicating that dipole-dipole interactions are quite weak
because the intensity of this feature is proportional to the
zero-field-splitting parameter, D.61 Therefore, we cannot
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Inorganic Chemistry, Vol. 45, No. 15, 2006 5755

Shultz et al.
Figure 6. EPR Curie plot for biradical 3.
Figure 8. Kekule´ resonance structures demonstrating antiferromagnetic
SQ-SQ exchange coupling for 3.
temperature. Modeling the temperature-dependent øpara of S
) 1 molecules can be achieved by fitting to a field-
independent van Vleck expression (in this case, a spin dimer
model), eq 2,69,70
2Ng2â2
kT[3 + e-2J/kT
øpara
)
(2)
]
where N is Avogadro’s number, g is the isotropic Lande´
factor (g ) 2.0023), â is the Bohr magneton, T is the
temperature in Kelvin, k is Boltzmann’s constant, and J is
the intramolecular SQ-SQ exchange coupling parameter.
The relative singlet- and triplet-state energies were derived
using the Hamiltonian, H ) -2JSˆ1‚Sˆ2, where Sˆ1 and Sˆ2 are
the spin operators for the SQs.
The curve fit result gives J ) -29 cm-1 and is consistent
with an antiferromagnetically coupled dimer. The inclusion
of a weak intermolecular term (zJ′) did not significantly
improve either the quality of the fit or the fit parameters.
The øpara versus T plot displays a signature maximum with
Tmax ) 1.285J/k.70 The J value from the fit (-29 cm-1)
compares well with the value of J determined from Tmax (J
) -27 cm-1).
Figure 7. øpara vs T plot for biradical porphyrin 3.
and |E/hc| ) 0.000 23 cm-1. From a point dipole model, an
interelectronic separation of ca. 9 Å is estimated,67 slightly
greater than the 6.94 Å separation between meso-carbons
but less than the 12.7 Å distance between SQ ring centers
(as determined from the crystal structure).
A Curie plot for the doubly integrated ∆ms ) 2 signal of
3 is shown in Figure 6. The biradical gives a nonlinear
response, consistent with J < 0 (antiferromagnetic cou-
pling).68 The data were fit to eq 1:68,69
Spin-Spin Coupling in 1-3. The study of spin-spin
coupling through nonalternate π systems is an unexplored
topic compared to studies of other (alternate) coupler groups.
In the present case, the moderately strong antiferromagnetic
coupling for 3 is easily explained by the resonance forms
shown in Figure 8.Indeed, similar, though less-important,
resonance forms predict antiferromagnetic coupling in 1 and
2 as well. The utility of such antiferromagnetically coupled
structures lies in part in the possibility of coupling each of
the radical spins with a porphyrin-bound paramagnetic metal
ion. In this way, ground-state S * 0 molecules can be
prepared, regardless of the sign of the metal-radical
coupling, if metal-radical coupling exceeds radical-radical
coupling, that is, |JM-rad| > |Jrad-rad|.
-2J
RT
3 exp
(
)
C
T
IEPR
)
(1)
-2J
RT
[
]
1 + 3 exp
(
)
where C is a constant and J is the exchange parameter (2J
is the singlet-triplet gap). Best fit results give J ) -75 (
3 cm-1 for 3.
Solid-State Magnetic Susceptibility Measurements. The
magnetic susceptibility data for 3 are shown in Figure 7 as
the molar paramagnetic susceptibility (øpara) versus the
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2810.
(68) Berson, J. A. The Chemistry of the Quinonoid Compounds; Patai, S.
R. Z., Ed.; John Wiley & Sons: New York, 1988; Vol. II, p 482.
(69) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London, Ser. A 1952, 214,
451-465.
Iwamura and co-workers have reported radical-substituted
porphyrins topologically similar to 1-3 in that zinc was the
(70) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
5756 Inorganic Chemistry, Vol. 45, No. 15, 2006

Electron Spin-Spin Exchange Coupling
2 do not cause |J| to approach the value of |a|. Frozen-
solution EPR spectra of 1 and 2 do not include a ∆ms ) 2
transition, consistent with weak zero-field splitting due to
limited interaction of the nitroxide moieties. Thus, exchange
coupling in the bis(nitroxide) porphyrins is weak and
apparently not differentially modulated by CdC and Ct
Csat least according to EPR spectroscopic measurements.
The frozen-solution EPR spectrum of 3 exhibits zero-field
splitting and a ∆ms ) 2 transition characteristic of a triplet
state. The intensity of the ∆ms ) 2 transition of 3 was
measured as a function of temperature, and the data were fit
according to a singlet-triplet model to yield J ) -75
cm-1sa substantial exchange parameter considering the size
of the porphyrin π system through which coupling occurs.
Consistent with the π topology, the exchange parameter is
antiferromagnetic. This interaction is substantially greater
than that between phenylcarbene units through the porphyrin
π system measured by Iwamura’s group (J ) -3 cm-1).26,27
In the latter case, the carbene units were generated from a
bis(diazo) precursor in which porphyrin-phenyl torsion
minimized carbene conjugation with the porphyrin ring, and
the second phenyl group attached to the carbene delocalized
some of the π spin density. Both of these effects would serve
to attenuate the exchange interaction between the carbene
units. In the solid state, the exchange parameter (J ) -29
cm-1) is attenuated compared to the value obtained from
fitting the EPR spectral intensity data. Because we have
shown that the exchange coupling in π-type biradicals varies
with the cos2 of the torsion angles,45,49 the stronger frozen-
solution coupling suggests that the frozen-solution structure-
(s) have smaller SQ-porphyrin torsion angles that permit
greater SQ spin delocalization.
Figure 9. Bis(carbene) ZnII porphyrin prepared by Iwamura and co-
workers.
bound metal ion, and the spins were part of delocalized
biradicals.26,27 The spin carriers were diaryl carbenes gener-
ated by photolysis of the bis(diazo) precursor, as shown in
Figure 9. A J value of -3 cm-1 was reportedsa significantly
weaker antiferromagnetic coupling than that measured for
3. We propose two mechanisms for weaker coupling in
Iwamura’s carbene porphyrins. First, it is likely that the
meso-aryl carbene groups are unable to relax to a conforma-
tion that would maximize spin delocalization into the
porphyrin ring following photolysis, and second, the terminal
phenyl groups also delocalize spin, thus diluting porphyrin
spin density.
Milgrom et al. have shown that the oxidation and
subsequent strong antiferromagnetic coupling of trans-
phenoxyl radicals can give rise to a diamagnetic, quinoidal
porphyrin with NiII, whereas the larger PdII precludes the
requisite ruffled conformation and resists oxidation.71-74
Comparison of 1-3. Semiquinones have greater ring spin
densities (ca. 0.8) than phenyl-tert-butylnitroxides (ca. 0.1-
0.15).38,49,75 Because of the greater spin density near the
porphyrin ring, the average interelectronic distance in the
bis(SQ)porphyrins is less than that for the bis(nitroxide)
porphyrins, and this is manifested in both zero-field splitting
and exchange coupling in biradical 3. Clearly, 3 should be
superior to both 1 and 2 for the future construction of
magnetic chain coordination polymers.
The utility of antiferromagnetically coupled biradicals 1-3
lies in the possibility of ferromagnetically coupling each of
the radical spins with a bound paramagnetic metal ion. In
this way, overall ferromagnetic alignment might be achieved.
The present results represent the first examples of stable
radicals exchange-coupled through a porphyrin π system and,
for 3, the first structurally characterized example of such.
Porphyrins 1-3 also complement the studies of exchange-
coupled porphyrin dimers.31-35 Future efforts will focus on
coordination polymers of these biradical ligands.
Experimental Section
Conclusions
Chemicals were purchased from Aldrich Chemical Co. unless
noted otherwise. Solvent distillations, synthetic procedures, and
electrochemistry were carried out under an argon atmosphere.
Tetrahydrofuran (THF) was distilled from sodium benzophenone-
ketyl prior to use. Methylene chloride was distilled from calcium
hydride. X-band EPR spectra were recorded on an IBM-Bru¨ker
E200SRC spectrometer fitted with an Oxford model 900 cryostat.
Radical formation was accomplished as reported previously.40
Magnetic susceptibilities were measured on a Quantum Design
MPMS-XL7 SQUID magnetometer using an applied field of 1 T
for Curie plots. Data for crystalline samples were corrected for
molecular diamagnetism using Pascal’s constants. Microcrystalline
The bis(nitroxide) porphyrins 1 and 2 exhibit fluid-solution
EPR spectra consistent with |J| . |a|. No evidence was
observed for the modulation of J as shown by the temperature
invariance of the EPR spectra. Thus, bond rotations in 1 and
(71) Milgrom, L. R.; Yahioglu, G.; Jogiya, N. N. Free Radical Res. 1996,
24, 19-29.
(72) Golder, A. J.; Milgrom, L. R.; Nolan, K. B.; Povey, D. C. J. Chem.
Soc., Chem. Commun. 1989, 1751-1753.
(73) Golder, A. J.; Milgrom, L. R.; Nolan, K. B.; Povey, D. C. J. Chem.
Soc., Chem. Commun. 1987, 1788-1790.
(74) Milgrom, L. R. Tetrahedron 1983, 39, 3895-3898.
(75) Forrester, A. R.; Hay, J. M.; Thompson, R. H. Organic Chemistry of
Stable Free Radicals; Academic: London, 1968.
Inorganic Chemistry, Vol. 45, No. 15, 2006 5757

Shultz et al.
samples were loaded into the sample space of a Delrin sample
holder and mounted directly to the sample rod. The sample holder
was subtracted out using an empty Delrin sample holder as the
background.
5’,15’-Bis-[3,4-di-(methoxymethoxy)-5-tert-butyl]phenyl-10’,20’-
di-phenylporphyrin 3(MOM)4. A 50 mL Schlenk flask containing
3,4-[di-methoxymethoxy]-5-tert-butyl-phenyl-1-boronic pinacol es-
ter (0.268 g, 0.805 mmol), 5′,15′-di-bromo-10′,20′-diphenylpor-
phyrin (0.178 g, 0.287 mmol), Pd(PPh3)4 (0.033 g, 0.029 mmol),
Na2CO3 (2M, 0.7 mL) in ethanol (0.5 mL), and THF (8 mL) was
pumped and purged under N2 seven times. The reaction mixture
was refluxed for 2 days. Once cool, the solvent was removed under
reduced pressure. The crude material was diluted with ether and
filtered through a pad of Celite to remove the inorganic salts. The
remaining crude mixture was purified by radial chromatagraphy
using 4-10% THF/petroleum ether. The collected fraction was a
Porphyrin 1(TBS)2. A 100 mL flask containing 4-[N-tert-butyl-
N-(tert-butyldimethylsilyloxy)amino]phenyl-1-boronic acid pinacol
ester (1.07 g, 2.99 mmol), 4 (0.765 g, 1.19 mmol), Pd(PPh3)4
(0.0828 g, 0.072 mmol), and Na2CO3 (2M, 3 mL) in THF (50 mL)
was pumped and purged under N2 five times. The reaction mixture
was refluxed for 2 days. Once cool, the solvent was removed under
reduced pressure, and ether was added, and then, the reaction
mixture was filtered to remove the inorganic salt. The remaining
crude mixture was purified by an alumina column with 10-30%
CH2Cl2/petroleum ether and chromatotran with 30% CH2Cl2/
petroleum ether to give the first band, free-base 1(TBS)2 (0.449 g,
37%), and the second band, 1(TBS)2 (0.515 g, 40%). Free-base
1(TBS)2. 1H NMR (CDCl3, δ): 8.86 (d, 8H, J ) 4.4 Hz), 8.23 (d,
4H, J ) 7.6 Hz), 8.08 (d, 4H, J ) 8.4 Hz), 7.76 (d, 6H, J ) 5.6
Hz), 7.64 (d, 4H, J ) 7.2 Hz), 1.37 (s, 18H), 1.05 (s, 18H), 0.15
(s, 12H), -2.74 (s, 2H). 13C NMR (CDCl3, δ): 151.1, 142.4, 138.6,
134.8, 133.8, 131.3, 131.2 (broad), 127.9, 126.8, 123.8, 120.3,
120.2, 61.3, 26.6, 26.5, 18.4, -4.3. IR (film from CDCl3): 3306.4,
2920.4, 2849.8, 1555.7, 1463.8, 1351.9, 1246.3, 1193.0, 935.3,
855.1, 832.2, 793.3, 697.1, 665.3 (cm-1). MS-FAB C64H76O2N6-
Si calcd exact mass: 1016.5568. Observed: 1016.5594. 1(TBS)2.
1H NMR (CDCl3, δ): 8.97 (s, 8H), 8.24 (s, 4H), 8.08 (s, 4H), 7.76
(s, 6H), 7.65 (s, 4H), 1.38 (s, 18H), 1.05 (s, 18H), 0.16 (s, 12H).
13C NMR (CDCl3, δ): 150.9, 150.6, 150.4, 143.1, 139.2, 134.8,
134.6, 133.6, 132.1, 127.7, 126.7, 123.7, 121.3, 61.3, 26.6, 26.5,
18.4, -4.3. IR (film from CDCl3): 2956.2, 2855.2, 1646.0, 1487.8,
1337.8, 1249.1, 1202.0, 1070.2, 998.4, 942.5, 909.0, 858.0, 833.2,
781.2, 706.1, 666.6 (cm-1). MS-FAB C64H74O2N6SiZn2 calcd exact
mass: 1078.4703. Observed: 1078.4681.
1
dark purple solid, 3(MOM)4 (0.150 g, 54%). H NMR (CDCl3,
δ): 8.93 (d, 4H, J ) 4.3 Hz), 8.85 (d, 4H, J ) 4.3 Hz), 8.22 (d,
4H, J ) 6.2 Hz), 7.91 (d, 4H, J ) 11.6 Hz), 7.76 (d, 6H, J ) 6.1
Hz), 5.55 (s, 4H), 5.35 (s, 4H), 3.83 (s, 6H), 3.56 (s, 6H), 1.60 (s,
18H), -2.78 (s, 2H). MS-FAB C66H62N4O8 calcd exact mass:
966.4568. Observed: 966.4581.
Porphyrin 3. To a 25 mL round-bottom flask was added 3-
(MOM)4 (0.078 g, 0.08 mmol), two drops of concentrated HCl,
and MeOH (10 mL). The reaction mixture was purged several times
with N2 and set to reflux for 12 h. After cooling, the MeOH was
removed under reduced pressure. The crude reaction mixture was
diluted with CH2Cl2 and washed with NH4Cl (1 × 15 mL) and
brine (3 × 15 mL), dried over Na2SO4, and filtered, and the solvent
was removed in vacuo. The purple solid was purified by a pentane
wash. This gave 0.062 g (97% yield) of the free-base bis(catechol)-
1
porphyrin as a purple crystalline solid. H NMR (300 MHz, d6-
acetone, δ): 9.05 (d, 4H, J ) 4.0 Hz), 8.88 (d, 4H, J ) 3.9 Hz),
8.28 (s, 4H), 7.87 (d, 6H, J ) 3.4 Hz), 7.73 (d, 4H, J ) 7.3 Hz),
7.64 (s, 2H), 1.66 (s, 18H), -2.46 (s, 2H). 13C NMR (d6-acetone,
δ): 148.4, 145.9, 142.4, 141.4, 136.9, 134.5, 131.1, 128.1, 127.7,
126.7, 121.7, 120.2, 120.1, 22.9.
Porphyrin 1. The TBS-protected porphyrin was deprotected and
oxidized as described previously.38 Anal. Calcd for C52H44N6O2:
C, 79.56; H, 5.64. Found: C, 79.22; H, 5.44.
The product was placed in a 50 mL round-bottom flask, and
Zn(OAc)2‚H2O (0.188 g, 0.857 mmol), EtOH (1 mL), and distilled
THF (25 mL) were added. The reaction was stirred under N2 and
reflux conditions for 3 h. After cooling, the solvent was removed
under reduced pressure. The crude reaction mixture was diluted
with CH2Cl2 and washed with brine (2 × 20 mL) and distilled H2O
(1 × 15 mL), dried over Na2SO4, and filtered, and the solvent was
removed in vacuo. The crude product was washed with pentane.
Porphyrin 2(TBS)2. A 100 mL flask containing 4-trimethylsi-
lanylethylnyl-1-[N-tert-butyl-N-(tert-butyldimethylsilyloxy)amino]-
benzene (0.535 g, 1.423 mmol), 5-Zn (0.332 g, 0.485 mmol),
K2CO3 (0.295 g, 2.130 mmol), piperidine (0.96 mL, 9.70 mmol),
Pd(PPh3)4 (56 mg, 0.049 mmol), CuI (5.0 mg, 0.024 mmol), THF
(30 mL), and distilled methanol (4 mL) was pumped and purged
10 times and then refluxed for 58.5 h. Once cool, saturated NH4Cl
was added. The organic layer was washed with ether twice
separated, and dried over Na2SO4, and the solvent was removed in
vacuo. The mixture was purified by flash chromatography (alumina,
20% CH2Cl2/petroleum ether for the first and second bands, 90%
CH2Cl2/petroleum ether for the third band, and 100% CH2Cl2),
giving 2(TBS)2 (0.2560 g, 47% yield).1H NMR (CDCl3, δ): 9.70
(d, 4H, J ) 4.7 Hz), 8.86 (d, 4H, J ) 4.7 Hz), 8.18 (d, 4H, J ) 7.1
Hz), 7.91 (d, 4H, J ) 8.4 Hz), 7.79 (d, 6H, J ) 5.8 Hz), 7.47 (d,
4H, J ) 7.9 Hz), 1.23 (s, 18H), 1.00 (s, 18H), 0.01 (s, 12H). 13C
NMR (CDCl3, δ): 152.2, 152.1, 150.2, 142.4, 134.6, 132.8, 131.2,
131.0, 127.8, 126.9, 125.6, 122.9, 120.3, 102.3, 97.4, 92.1, 61.6,
26.5, 26.4, 18.3, -4.3. IR (film from CH2Cl2): 2956.9, 2928.1,
2855.7, 1598.1, 1498.4, 1461.8, 1359.9, 1256.1, 1206.9, 1002.1,
943.5, 859.8, 834.0, 781.6, 701.4 (cm-1). MS-FAB C68H76O2N6-
Si2Zn2 calcd exact mass: 1126.4703. Observed: 1126.4718.
1
This yielded a purple solid (0.065 g, 97%). H NMR (300 MHz,
d6-acetone, δ): 9.08 (d, 4H, J ) 4.5 Hz), 8.90 (d, 4H, J ) 4.6
Hz), 8.2 (d, 4H, J ) 2.3 Hz), 7.83 (d, 6H, J ) 2.8 Hz), 7.72 (d,
4H, J ) 10.2 Hz), 1.66 (s, 18H).
The product was placed in a 25 mL round-bottom flask
containing TpCum,MeZnOH (0.112 g, 0.162 mmol) in a mixture of
MeOH (5 mL) and CH2Cl2 (5 mL) and was stirred overnight in
the air. The reaction mixture was concentrated by rotary evaporation
and washed with cold MeOH. The reddish-brown precipitate was
collected by filtration and then recrystallized from 1:1 CH2Cl2/
pentane. IR (film from CH2Cl2) ν (cm-1): 2924, 2546, 1574, 1551,
1519, 1441, 1362, 1262, 1186, 1066, 836, 794, 746, 701. UV-vis
λ
max (nm): 418, ꢀ (M-1cm-1) ) 96 074. MS-FAB C130H132N16O4B2-
Zn3 calcd exact mass: 2194.87. Observed: 2195.02.
Acknowledgment. This work was funded by the National
Science Foundation (CHE-0091247 and CHE-0345263).
Mass spectra were obtained at the Mass Spectrometry
Laboratory for Biotechnology. Partial funding for the facility
Porphyrin 2. The TBS-protected porphyrin was deprotected and
oxidized as described previously.38 Anal. Calcd for C56H44N6O2:
C, 80.74; H, 5.32. Found: C, 80.32; H, 5.29.
5758 Inorganic Chemistry, Vol. 45, No. 15, 2006

Electron Spin-Spin Exchange Coupling
Supporting Information Available: X-ray crystallographic files
for 3 in CIF format and relevant crystallographic data. This material
is available free of charge via the Internet at http://pubs.acs.org.
was obtained from the North Carolina Biotechnology Center
and the National Science Foundation. The MALDI-TOF
mass spectrometer was funded in part by the North Carolina
Biotechnology Center.
IC060209Z
Inorganic Chemistry, Vol. 45, No. 15, 2006 5759