Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins

Article abstract of DOI:10.1021/ic010958a

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (ΔG?_ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H²⁺) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) ¹H NMR spectroscopy are used to examine the relationship between the arylporphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H²⁺ and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase ΔG?_ROT, which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H²⁺. These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.

Full text of DOI:10.1021/ic010958a

Inorg. Chem. 2003, 42, 2227−2241
Unusual Aryl−Porphyrin Rotational Barriers in Peripherally Crowded
Porphyrins
Craig J. Medforth,*,†,‡ Raid E. Haddad,§ Cinzia M. Muzzi,† Neal R. Dooley,† Laurent Jaquinod,†
,†,|
David C. Shyr,† Daniel J. Nurco,† Marilyn M. Olmstead,† Kevin M. Smith,* Jian-Guo Ma,§ and
,‡,
John A. Shelnutt*
Department of Chemistry, UniVersity of California, DaVis, California 95616,
Biomolecular Materials and Interfaces Department, Sandia National Laboratories,
Albuquerque, New Mexico 87185, Department of Chemistry and Department of Chemical
and Nuclear Engineering, UniVersity of New Mexico, Albuquerque, New Mexico 87131,
Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803, and
Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received September 7, 2001
Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl−porphyrin rotation
‡
(∆G ROT) varies as a function of the core substituent M and is lower for a small metal (M ) Ni) compared to a
2+
large metal (M ) Zn) and for a dication (M ) 4H ) versus a free base porphyrin (M ) 2H). This has been
attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle
caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations,
1
and variable temperature (VT) H NMR spectroscopy are used to examine the relationship between the aryl−
porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylpor-
phyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs),
where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of
DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle
2+
conformations being observed for M ) Zn, 2H or M ) 4H and saddle and/or ruffle conformations for M ) Ni.
‡
VT NMR studies show that the effect of protonation in the TArOPPs is to increase ∆G ROT, which is the opposite
of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when
2+
M ) 4H . These and other findings suggest that the aryl−porphyrin rotational barriers in the DArPs are closely
linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being
rotated out of the porphyrin plane.
Introduction
porphyrins.2 Significant changes in the properties of the
porphyrin macrocycle (e.g., redox behavior and photophys-
ics) are now known to result from two types of nonplanar
deformation (ruffling or saddling).1,3,4 These findings have
lead to speculation about the role of the nonplanar deforma-
tions seen for tetrapyrroles in biological systems,3 and to
investigations of how peripheral crowding might be used to
Recent investigations of peripherally crowded porphyrins
(e.g., 1-5, Figure 1) have helped to delineate the relationship
between the substituents on the porphyrin ring and the
amount and type of nonplanar deformation induced by those
substituents.1 Figure 2 illustrates the four nonplanar deforma-
tions commonly observed in crystallographic studies of
* Authors to whom correspondence should be addressed. E-mail:
medforth@chem.ucdavis.edu (C.J.M.); kmsmith@lsu.edu (K.M.S.);
jasheln@unm.edu (J.A.S.).
(1) For a recent review of nonplanar porphyrins see: Senge, M. O. In
The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Eds.; Academic Press: Boston, 2000; Vol. 1; p 239.
(2) Jentzen, W.; Song, X.-Z.; Shelnutt, J. A. J. Phys. Chem. B 1997, 101,
1684.
(3) For a recent review of the possible biological significance of porphyrin
nonplanar distortions see: Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia,
S.-L.; Jentzen, W.; Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31.
† University of California.
‡ Sandia National Laboratories.
§ University of New Mexico.
| Louisiana State University.
University of Georgia.
10.1021/ic010958a CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/06/2003
Inorganic Chemistry, Vol. 42, No. 7, 2003 2227

Medforth et al.
Table 1. Activation Energies for Aryl-Porphyrin Rotation (∆G‡ROT; kJ
mol-1) in 5,10,15,20-Tetraarylporphyrins (TArPs)
porphyrin
M
∆G‡
ROT
619
ZnII
PdII
CuII
NiII
2H
131
130
124
108
122
110
108
96
4H2+
2H
718
817
4H2+
RuII(CO)(tBu-Py)
InIIICl
72-77a
63-71a
60-68a
TiIVdO
a Depending upon the para substituent X.
rotation are increased in cobalt(III) complexes of strongly
ruffled T(tBu)P (5) or strongly saddled OETPP (1).15,16 The
decreased activation energy for NH tautomerism in H2T-
(tBu)P has been rationalized14 in terms of a contraction of
the porphyrin core due to ruffling of the porphyrin macro-
cycle; this results in enhanced intramolecular hydrogen
bonding of the NH protons, a situation which is analogous
to the transition state for NH tautomerism. The higher barriers
for ligand rotation were explained15,16 by the ligands being
oriented in deep cavities formed by the nonplanar porphyrin
macrocycles and their substituents.
Figure 1. Structures of previously investigated peripherally crowded
porphyrins and 5,10,15,20-tetraarylporphyrins.
Other changes in the dynamic properties of peripherally
crowded nonplanar porphyrins versus regular porphyrins can
reasonably be anticipated, and in this regard, it is interesting
to note that differences between the activation energies for
aryl-porphyrin rotation (∆G‡ROT) in 5,10,15,20-tetraarylpor-
phyrins (TArPs) such as 6-8 (Figure 1, Table 1) have long
been thought to arise in significant part from nonplanar
distortions and changes in the deformability of the macro-
cycle induced by the core substituent M.17-19 For example,
it has been hypothesized that ruffling induced by small metal
ions (e.g., M ) NiII vs ZnII for porphyrin 6 or metals with
higher oxidation states for porphyrin 8) acts to move the
Figure 2. Representations of the lowest energy nonplanar distortion modes
of the porphyrin macrocycle.2
produce materials such as molecular receptors5-7 and mo-
lecular switches.8
Even though the effects of nonplanarity or peripheral
crowding on the dynamic properties of porphyrins and other
tetrapyroles might also be significant, to date this area has
received comparatively little attention.8-16 Optical studies
have indicated that interconversion between different non-
planar conformations is likely a key factor determining the
excited state dynamics of peripherally crowded porphyrins.8-13
meso aryl groups out-of-plane, thereby allowing them to
17,19
rotate more easily and lowering ∆G‡
.
Nonplanarity
ROT
has also been invoked to explain part of the observed
1
Variable temperature (VT) H NMR investigations have
decrease in ∆G‡
for dications (M ) 4H2+) compared to
ROT
shown that the activation energy for NH tautomerism is
decreased in the strongly ruffled porphyrin H2T(tBu)P (5,
M ) 2H),14 and that the activation energies for axial ligand
free base porphyrins (M ) 2H) for 6 and 7,18,19 as protonation
(9) Gentemann, S.; Medforth, C. J.; Ema, T.; Nelson, N. Y.; Smith, K.
M.; Fajer, J.; Holten, D. Chem. Phys. Lett. 1995, 245, 441.
(10) Drain, C. M.; Kirmaier, C.; Medforth, C. J.; Nurco, D. J.; Smith, K.
M.; Holten, D. J. Phys. Chem. 1996, 100, 11984.
(11) Gentemann, S.; Nelson, N. Y.; Jaquinod, L.; Nurco, D. J.; Leung, S.
H.; Medforth, C. J.; Smith, K. M.; Fajer, J.; Holten, D. J. Phys. Chem.
B 1997, 101, 1247.
(12) Retsek, J. L.; Gentemann, S.; Medforth, C. J.; Smith, K. M.; Chirvony,
V. S.; Fajer, J.; Holten, D. J. Phys. Chem. B 2000, 104, 6690.
(13) Retsek, J. L.; Medforth, C. J.; Nurco, D. J.; Gentemann, S.; Chirvony,
V. S.; Smith, K. M.; Holten, D. J. Phys. Chem. B 2001, 105, 6396.
(14) Somma, M. S.; Medforth, C. J.; Nelson, N. Y.; Olmstead, M. M.;
Khoury, R. G.; Smith, K. M. Chem. Commun. 1999, 1221.
(15) Medforth, C. J.; Muzzi, C. M.; Smith, K. M.; Abraham, R. J.; Hobbs,
J. D.; Shelnutt, J. A. J. Chem. Soc., Chem. Commun. 1994, 1843.
(16) Medforth, C. J.; Muzzi, C. M.; Shea, K. M.; Smith, K. M.; Abraham,
R. J.; Jia, S.; Shelnutt, J. A. J. Chem. Soc., Perkin Trans. 2 1997,
833.
(4) Recent investigations of the large red shifts seen in the optical spectra
of peripherally crowded nonplanar porphyrins have led some research
groups (Wertsching, A. K.; Koch, A. S.; DiMagno, S. G. J. Am. Chem.
Soc. 2001, 123, 3932. Ryeng, H.; Ghosh, A. J. Am. Chem. Soc. 2002,
124, 8099) to propose that the observed shifts (and by implication the
changes seen in other properties) are simply the result of substituent
effects. A recent paper by our group (ref 73) confirms that the large
red-shifts seen in peripherally crowded nonplanar porphyrins are indeed
caused by nonplanar deformation and shows that the attribution of
the red-shifts to substituent effects resulted from the use of inap-
propriate model structures in the earlier calculations.
(5) Mazzanti, M.; Marchon, J.-C.; Shang, M.; Scheidt, W. R.; Jia, S.;
Shelnutt, J. A. J. Am. Chem. Soc. 1997, 119, 12400.
(6) Furusho, Y.; Kimura, T.; Mizuno, Y.; Aida, T. J. Am. Chem. Soc.
1997, 119, 5267.
(7) Muzzi, C. M.; Medforth, C. J.; Smith, K. M.; Jia, S.-L.; Shelnutt, J.
A. Chem. Commun. 2000, 131.
(8) Drain, C. M.; Gentemann, S.; Roberts, J. A.; Nelson, N. Y.; Medforth,
C. J.; Jia, S. L.; Simpson, M. C.; Smith, K. M.; Fajer, J.; Shelnutt, J.
A.; Holten, D. J. Am. Chem. Soc. 1998, 120, 3781.
(17) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1977, 99, 6594.
(18) Dirks, J. W.; Underwood, G.; Matheson, J. C.; Gust, D. J. Org. Chem.
1979, 44, 2551.
(19) Freitag, R. A.; Whitten, D. G. J. Phys. Chem. 1983, 87, 3918.
2228 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
typically distorts the porphyrin into a very saddled structure1
to relieve crowding of the protons in the core. In the present
work, we describe an investigation of the structures and
aryl-porphyrin rotational barriers of some peripherally
crowded and very nonplanar porphyrins based on the novel
2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (DPP)
system 4.
DPP and related 2,3,5,7,8,10,12,13,15,17,18,20-dode-
caarylporphyrins (DArPs) have been widely studied during
the past decade because of their unusual structural and
spectroscopic properties.10,12,13,20-37 Given the earlier studies
of TArPs,17-19 DArPs such as 5,10,15,20-tetraryl-2,3,7,8,-
12,13,17,18-octaphenylporphyrins (TArOPPs; 10) seemed to
be ideal systems for further investigating the effect of
nonplanar deformation on the aryl rotational barriers in
porphyrins. The DArPs raise some interesting questions
regarding the impact of nonplanar deformations on aryl-
porphyrin rotational barriers. First, the dependence of the
rotational barriers in the DArPs as a function of the core
substituent M is not immediately obvious given that all of
the complexes will be very nonplanar.26,30,32 Second, although
the addition of more aryl groups to the porphyrin macrocycle
(e.g., eight phenyl groups to a TArP 9 to produce a TArOPP
10 or four phenyl groups to an OArP 11 to produce an
OArTPP 13) might be expected to significantly increase the
peripheral steric crowding, and thus the rotational barriers,
it is not clear to what extent the resulting nonplanar
deformations will also attenuate the increase in rotational
barrier. The aim of our study is to answer these and other
related questions, and to see if it is possible to rationalize
the aryl-porphyrin rotational barriers in DArPs using the
same out-of-plane deformation model previously applied to
TArPs.17-19
Figure 3. Structures of the investigated meso aryl-substituted porphyrins.
(20) Medforth, C. J.; Smith, K. M. Tetrahedron Lett. 1990, 31, 5583.
(21) Tsuchiya, S. Chem. Phys. Lett. 1990, 169, 608.
(22) Takeda, J.; Ohya, T.; Sato, M. Chem. Phys. Lett. 1991, 183, 384.
(23) Tsuchiya, S. J. Chem. Soc., Chem. Commun. 1991, 716.
(24) Takeda, J.; Sato, M. Inorg. Chem. 1992, 31, 2877.
(25) Tsuchiya, S. J. J. Chem. Soc., Chem. Commun. 1992, 1475.
(26) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt,
J. A. J. Am. Chem. Soc. 1992, 114, 9859.
(27) Charlesworth, P.; Truscott, T. G.; Kessel, D.; Medforth, C. J.; Smith,
K. M. J. Chem. Soc., Faraday Trans. 1994, 1073.
(28) Takeda, J.; Sato, M. Chem. Lett. 1995, 971.
Figure 4. Structures of the investigated â aryl-substituted porphyrins.
(29) Takeda, J.; Sato, M. Chem. Lett. 1995, 939.
(30) Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Olmstead, M. M.; Smith,
K. M. J. Am. Chem. Soc. 1996, 118, 10918.
(31) Clement, T. E.; Nguyen, L. T.; Khoury, R. G.; Nurco, D. J.; Smith,
K. M. Heterocycles 1997, 45, 651.
(32) Barkigia, K. M.; Nurco, D. J.; Renner, M. W.; Melamed, D.; Smith,
K. M.; Fajer, J. J. Phys. Chem. B 1998, 102, 322.
(33) Guilard, R.; Perie, K.; Barbe, J.-M.; Nurco, D.; Smith, K. M.; Van
Caemelbecke, E.; Kadish, K. M. Inorg. Chem. 1998, 37, 973.
(34) Kadish, K. M.; Van Caemelbecke, E.; D’Souza, F.; Lin, M.; Nurco,
D. J.; Medforth, C. J.; Forsyth, T. P.; Krattinger, B.; Smith, K. M.;
Fukuzumi, S.; Nakanishi, I.; Shelnutt, J. A. Inorg. Chem. 1999, 38,
2188.
(35) Muzzi, C. M.; Medforth, C. J.; Voss, L.; Cancilla, M.; Lebrilla, C.;
Ma, J.-G.; Shelnutt, J. A.; Smith, K. M. Tetrahedron Lett. 1999, 6159.
(36) Kadish, K. M.; Lin, M.; Van Caemelbecke, E.; De Stefano, G.;
Medforth, C. J.; Nurco, D. J.; Nelson, N. Y.; Krattinger, B.; Muzzi,
C. M.; Jaquinod, L.; Xu, Y.; Shyr, D. C.; Smith, K. M.; Shelnutt, J.
A. Inorg. Chem. 2002, 41, 6673.
The porphyrins investigated in our study can be divided
into two general groups. The first group consists of the TArPs
(9) and the highly substituted and very nonplanar 5,10,15,
20-tetraryl-2,3,7,8,12,13,17,18-octaphenylporphyrins
(TArOPPs; 10) (Figure 3). These porphyrins were used to
compare the barriers for rotation of meso aryl groups in
uncrowded and crowded porphyrin systems, which is the
primary focus of this study. The second group consists of
the 2,3,7,8,12,13,17,18-octaarylporphyrins (OArPs; 11) and
the 2,3,7,8,12,13,17,18-octaaryl-5,10,15,20-tetraphenylpor-
phyrins (OArTPPs; 13) (Figure 4). These compounds were
chosen to investigate the barriers for the rotation of â aryl
groups in uncrowded and highly crowded porphyrins. In
addition, the barriers for rotation of â aryl groups in an
intermediately crowded system were investigated using some
(37) Retsek, J. L.; Drain, C. M.; Kirmaier, C.; Nurco, D. J.; Medforth, C.
J.; Smith, K. M.; Chirvony, V. S.; Fajer, J.; Holten, D. Submitted for
publication.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2229

Medforth et al.
2,3-diaryl-5,10,15,20-tetraphenylporphyrins (DiArTPPs; 12).
The core substituent effects examined in our study were
chosen to match those previously studied for the TArPs.17-19
The effect of metal size was investigated using complexes
with a small metal (M ) Ni) or a large metal (M ) Zn).
The effect of protonation was examined using the free base
porphyrin (M ) 2H) and the porphyrin dication (M ) 4H2+).
As in the previous studies of TArPs,17,18,38-41 variable
temperature (VT) 1H NMR spectroscopy is used to determine
the aryl-porphyrin rotational barriers. In such experiments,
it is usually necessary to generate asymmetry in the porphyrin
complex, either by the use of asymmetrical aryl substitu-
ents18,38,39,41 or by asymmetrical ligation of a centrally
coordinated metal ion.17,39,40 The former approach was
employed in this study, and porphyrins 9b, 10b, 11b, 12b,
and 13b, where the aryl group is 3-methoxyphenyl, were
synthesized. The 3-methoxyphenyl group was chosen as the
standard substituent because it could be readily introduced
onto the porphyrin macrocycle using existing synthetic
methodologies.35,42 As some of the porphyrins used in our
studies are known to display additional dynamic processes
such as NH tautomerism and macrocyclic inversion,43 aryl
substituents with varying steric requirements, and thus
different rotational barriers, were used to unambiguously
assign the aryl-porphyrin rotation process. The 2-methox-
yphenyl group has been employed in earlier studies of 5,-
10,15,20-tetraarylporphyrins18 and was also used in this study
to increase the aryl-porphyrin rotational barrier (porphyrin
13d). The 3-thienyl substituent was successfully tested as a
way to lower the aryl-porphyrin rotational barrier (porphy-
rins 10c, 12c, and 13c).
TArOPP 10b. MM calculations were carried out because the
rotational barriers may be dependent on the ability of the
porphyrin to deform along a particular coordinate during the
rotation process, an effect that may not always be reflected
in the crystallographic data. The MM calculations allow us
to examine the conformational changes that occur for the
porphyrin macrocycle during the aryl-porphyrin rotation
process. Finally, in section III, the barriers for aryl-porphyrin
rotation are determined experimentally using variable tem-
perature (VT) 1H NMR spectroscopy. The NMR results are
then interpreted in terms of the available structural informa-
tion and the barriers compared to the results obtained from
the MM calculations.
Results and Discussion
I. X-ray Crystallographic Studies. In this section, X-ray
crystallography is used to investigate the relationship between
the peripheral and core substituents present for the TArPs,
OArPs, DArPs (TArOPPs/OArTPPs), or DiArTPPs and the
conformation of the porphyrin macrocycle. Particular em-
phasis is placed on determining the conformations of the
DArPs and comparing them to those seen for the TArPs.
Note that the compounds investigated in this section are not
necessarily identical to those discussed in sections II and
III. This is because the compounds used in the NMR studies
in section III have asymmetrical aryl substituents to allow
rotation to be detected, whereas most of the porphyrins
examined by X-ray crystallography have phenyl substituents
or symmetrical aryl substituents to avoid the problems
presented by the porphyrin crystallizing with multiple
orientations of the aryl substituents (atropisomers). The effect
on the structure of adding or changing a substituent (e.g.,
adding a methoxyl group at the meta position of the phenyl
ring) is expected to be small in the systems we are studying,
as confirmed by the MM calculations discussed in section
II, so it will not alter the conclusions reached in our work.
Full details of the synthesis, characterization, and crystal
structure determinations of the porphyrins investigated in our
study are provided in the Experimental Section. Table 2
provides crystallographic data for the new crystal structures
reported in this paper. Tables 3 and 4 summarize the
nonplanar deformations for the TArPs, OArPs, DArPs, and
DiArTPPs as determined using normal-coordinate structural
decomposition (NSD).2,46 NSD is a recently developed
procedure for quantitatively analyzing the out-of-plane (and
in-plane) deformations of porphyrins. The NSD method
characterizes the porphyrin conformation in terms of equiva-
lent displacements (normal deformations) along the normal
coordinates of the D4h-symmetric porphyrin macrocycle.
Nonbonded interactions at the porphyrin periphery obtained
from selected X-ray structures, which are used as an
In section I of the Results and Discussion section, X-ray
crystallographic studies of the TArPs, OArPs, DArPs
(TArOPPs/OArTPPs), and DiArTPPs are described. The
crystal structures are then analyzed using normal-coordinate
structural decomposition (NSD),2 a recently developed
technique for quantifying the out-of-plane deformations in
porphyrins. In this way, the effects of different peripheral
or core substituents on the conformation of the macrocycle
are delineated. Note that the majority of the compounds
discussed in this section contain symmetrical aryl substituents
(mainly phenyl rings) and are thus not identical to the
compounds used in the VT NMR studies or the molecular
mechanics (MM) calculations (e.g., the X-ray data is based
on structure 9a rather than on structure 9b). However,
molecular mechanics calculations indicate that the effect of
the methoxyl substituent on the macrocycle conformation is
small.
In section II, MM calculations44-46 are used to calculate
the barriers for aryl-porphyrin rotation in TArP 9b and
(38) Walker, F. A.; Avery, G. L. Tetrahedron Lett. 1971, 52, 4949.
(39) Eaton, S. S.; Eaton, G. R. J. Chem. Soc., Chem. Commun. 1974, 576.
(40) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660.
(41) Crossley, M. J.; Field, L. D.; Forster, A. J.; Harding, M. M.; Sternhell,
S. J. Am. Chem. Soc. 1987, 109, 341.
(42) Takeda, J.; Sato, M. Chem. Pharm. Bull. 1994, 42, 1005.
(43) For a recent review of NMR spectroscopy of diamagnetic porphyrins
see: Medforth, C. J. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: Boston, 2000; Vol.
5; p 1.
(44) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.; Smith,
K. M. J. Am. Chem. Soc. 1991, 113, 4077.
(45) Song, X.-Z.; Jaquinod, L.; Jentzen, W.; Nurco, D. J.; Jia, S.-L.; Khoury,
R.; Ma, J.-G.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. Inorg.
Chem. 1998, 37, 2009.
(46) For a recent review see: Shelnutt, J. A. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press:
Boston, 2000; Vol. 7; p 167.
2230 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
slightly ruffled structure has been reported), and H4TPP2+
is strongly saddled irrespective of the anion present (dsad 2.5-
3.3 Å). No structural data is available for OPP, possibly
because of the low solubility of this compound.48 However,
it is likely to be very similar to that of 2,3,7,8,12,13,17,18-
octaethylporphyrin (OEP) for which numerous crystal struc-
tures have been reported including ZnOEP, NiOEP, H2OEP,
and H4OEP2+ (Table 3). Overall, the structural trends seen
for OEP as a function of the core substituent M are similar
to those seen for TPP, although OEP seems to be more
flexible and more planar than TPP. This is evidenced by
NiOEP crystallizing in planar or ruffled conformations, and
by the amount of distortion seen for H4OEP2+ being strongly
dependent upon the anion.
The structural changes seen for OEP and TPP as a function
of M are also seen for other porphyrins.1,49 Generally, small
metals which require short metal-nitrogen distances favor
ruffling because this distortion mode produces the smallest
core.45 In contrast, protonation favors saddling because this
deformation reduces the steric crowding between the protons
in the core. The earlier interpretations17-19 of the rotational
barriers in TArPs were based in large part on the X-ray for
TPP and OEP. The ruffling apparent for the nickel complexes
but not for the zinc complexes was used to explain the lower
rotational barrier in 6 (M ) Ni) versus 6 (M ) Zn)19 (Table
1). It was suggested that the easier deformation and
concomitant out-of-plane movement of the aryl groups in
the nickel complexes of TArPs facilitated aryl-porphyrin
rotation by relieving interactions between the aryl group and
the porphyrin macrocycle.17,19 Nonplanar deformation (sad-
dling) was also invoked to explain the lower barriers in the
dications versus the free bases of porphyrins 6 and 7.18,19
In an earlier report,19 it was suggested that the nonplanar
distortions seen in the crystal structures of nickel porphyrins
might be viewed as the “limit of deformability”, with the
zinc atom enforcing a planar structure and restricting
deformation. Curiously, an analysis of the close contacts at
the porphyrin periphery for ruf NiTPP and planar ZnTPP
shows that the distance between the ipso carbons of the meso
phenyl rings and the â pyrrole carbons are similar (2.92 Å).
This makes it difficult to understand the decrease in rotational
barrier purely on the basis of the static distortions seen in
the crystal structures and suggests that the nickel complex
deforms more during the rotation process than is apparent
in the X-ray structure (i.e., the deformability of the macro-
cycle is important).
Table 2. Crystallographic Data for 12c (M ) Ni), 13c (M ) Ni), and
13e (M ) 4H2+
)
12c (M ) Ni)
13c (M ) Ni) 13e (M ) 4H2+
)
2+
chemical formula C52H26N4Ni1S2 C76H44N4NiS8 C92H56F8N4
‚2.6(CHCl3)
‚2(C6H2N3O7)-
‚4.5(CH2Cl2)
2207.79
P21/c (No. 14)
14.7084(5)
24.5060(8)
29.1288(10)
103.130(1)
10224.8(6)
4
fw
835.68
1638.80
I41/a (No. 88)
23.192(2)
23.192(2)
82.412(7)
90
space group
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
C2/c (No. 15)
24.2800(10)
15.3387(6)
10.7881(4)
105.808(1)
3865.8(3)
4
44328(5)
24
λ (Å)
0.71073
90(2)
0.655
1.434
0.0550
1.54178
130(2)
5.314
1.454
0.1256
0.71073
90(2)
0.344
1.452
T (K)
µ (mm-1
)
D
calcd (g cm-3
)
2
R1 (Fo )
0.1015
(>2σ(I))a
2
wR2 (Fo )
(all data)a
0.1572
0.3495
0.3421
2 2
2 2
a R1 ) ∑||Fo - Fc||/∑|Fo| and wR2 ) [∑[w(Fo2 - Fc ) ]/∑[w(Fo ) ]]1/2
,
2
2
2
w ) 1/[σ2(Fo ) + ((X)P)2 + (Y)P] where P ) (Fo + 2Fc )/3. For 12c (M
) Ni): X ) 0.0876000 and Y ) 5.91870. For 13c (M ) Ni): X ) 0.173300
and Y ) 1304.26. For 13e (M ) 4H2+): X ) 0.198600 and Y ) 17.7353.
approximate measure of the steric crowding of the aryl
substituents, are presented in Table 5.
We consider first the published data for the uncrowded
TArP parent system 5,10,15,20-tetraphenylporphyrin (TPP,
9a) and the uncrowded OArP parent system 2,3,7,8,12,13,-
17,18-octaphenylporphyrin (OPP, 11a). Table 3 lists the total
out-of-plane deformation (dobsd) calculated using all the out-
of-plane modes, as well as the contributions from the lowest
frequency mode of each symmetry type (e.g. dsad which
corresponds to the lowest energy B2u ‘saddling’ mode).47
These low frequency modes typically encompass most of
the nonplanar deformation.2 The saddling (sad), ruffling (ruf),
doming (dom), and waving (wav) modes discussed in Table
3 are those illustrated in Figure 2.
The ruffling (ruf) and saddling (sad) distortions constitute
the majority of the nonplanar deformation observed in each
structure (Table 3) with smaller contributions from the
doming (dom), waving (wav), and propellering (pro) modes.
This is not surprising as ruffling and saddling are the softest
out-of-plane modes and therefore produce the largest out-
of-plane distortions.2 The following discussion is restricted
to the energetically relevant ruf and sad deformation modes,
which conveniently correspond to out-of-plane displacements
along the axes of substitution for the porphyrins investigated
in our study. For example, ruffling will move the meso aryl
substituents out-of-plane whereas saddling will move the â
aryl substituents out-of-plane (see Figure 2).
The TArP dications are expected to be difficult to deform
by ruffling because the resulting strong contraction of the
core45 will significantly increase steric interactions between
the four inner hydrogens. A more probable mechanism for
lowering the rotational barriers in the TArP dications would
seem to be an increase in the conformational space available
to the meso aryl substituent due to the large out-of-plane
movement of the adjacent pyrrole rings upon saddling. In
TPP is the most extensively investigated of all the aryl-
substituted porphyrins, and crystal structures have been
reported for the zinc complex (ZnTPP), the nickel complex
(NiTPP), the free base porphyrin (H2TPP), and the dication
(H4TPP2+). The crystallographic studies show that ZnTPP
is essentially planar (dobsd 0.2-0.3 Å), NiTPP is slightly
ruffled (druf 1.3 Å), H2TPP is typically planar (although a
(48) Takeda, J.; Ohya, T.; Sato, M. Chem. Pharm. Bull. 1990, 38, 264.
(49) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581.
(47) Note that dobsd is calculated as the square root of the sum of the squares
for all the modes.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2231

Medforth et al.
Table 3. NSD Analysis of the Out-of-Plane Deformations for TPP and OEP Complexes
dsad
druf
(B1u)
ddom
(A2u)
dwav(x)
(Eg(x)
dwav(y)
(Eg(y)
dpro
(A1u)
a
porphyrin
CCDC refcode
dobsd
(B2u)b
)
)
ZnTPP (monoclinic)
ZnTPP (triclinic)
NiTPP
ZZZTAY03
ZZZTAY02
ZZZUUC01
TPHPOR01
JIVRAH
0.175
0.297
1.295
0.333
0.046
0.065
0.233
1.082
2.520
2.670
2.743
3.042
3.272
0.273
0.330
1.461
0.087
0.144
0.126
0.085
1.221
1.389
2.137
0.000
0.000
-0.256
0.000
0.000
0.000
0.000
0.183
2.434
2.586
2.662
2.964
3.114
0.082
-0.098
-0.116
0.000
0.000
0.000
0.000
1.113
1.276
2.056
0.000
0.000
-1.266
0.000
0.000
0.000
0.000
1.055
0.000
0.169
0.000
0.007
0.754
0.041
0.026
1.456
0.000
0.000
0.000
0.000
0.186
0.346
0.210
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.045
0.025
0.181
0.021
0.000
0.236
-0.278
0.000
0.000
0.000
0.000
0.000
0.082
0.036
0.101
-0.158
-0.194
0.000
0.114
0.009
0.003
0.057
0.000
0.000
0.000
0.053
0.069
0.000
0.040
0.137
0.000
0.072
-0.066
0.063
0.060
0.064
0.005
0.018
0.018
0.136
0.000
0.232
0.011
0.039
0.197
0.000
0.031
0.068
0.108
0.040
0.000
0.012
-0.009
0.000
-0.031
-0.117
0.090
0.006
0.097
0.123
0.109
0.000
0.000
-0.001
0.000
0.000
0.000
0.000
0.000
0.000
0.022
0.014
0.024
0.000
-0.010
0.005
0.000
0.000
0.000
0.000
0.000
0.044
0.060
0.006
H2TPP
H2TPP [benzyl alcohol]
H2TPP [m-xylene]
H2TPP [p-xylene]
H2TPP
SEMNIH
SEMNUT
TPHPOR10
LEXSIQ
CAXHAK
RUHQEQ
YEVJAN
H4TPP2+(HSO4)2
H4TPP2+(CF3CO2)4(UO2)
H4TPP2+(ClO4)2 [C6H6]
H4TPP2+(ClO4)2 [CH3OH]
H4TPP2+(Cl)(FeCl4)
ZnOEP(1-Me-Im)
ZnOEP(Py)
TPPFEC
GAKWEU
EPOPZN10
NOEPOR
NOEPOR11
NOEPOR02
OETPOR10
HOETPN
LEYFOK
RUHQAM
YEVKIT
NiOEP (tetragonal)
NiOEP (triclinic A)
NiOEP (triclinic B)
H2OEP
H4OEP2+Rh(CO)2Cl2
H4OEP2+(CH3SO4)2
H4OEP2+(ClO4)2
H4OEP2+(CF3CO2)2
a Total out-of-plane deformation (Å). b Deformation in the lowest frequency mode of each symmetry type (Å).
Table 4. NSD Analysis of the Out-of-Plane Deformations for DArP and DiArTPP Complexes
dsad
druf
(B1u)
ddom
(A2u)
dwav(x)
dwav(y)
dpro
(A1u)
a
porphyrin
H2DiEtTPP
ZnDiEtTPP (3-picoline)
ZnDiEtTPP (MeOH)
NiDiEtTPP
CCDC refcode
dobsd
(B2u)b
(Eg(x)
)
(Eg(y)
)
TATPOT01
RUTNEZ
RUTNID
RUTMUO
this work
ZAWRES
XAWSAP
TEZXIF
XAWROC
XAWRUI
XAWSET
this work
this work
LADGAY
XAWRIW
this work
0.619
1.066
0.361
2.648
0.131
2.896
3.074
2.441
3.550
2.717
2.522
3.282
2.826
3.032
3.922
3.858
-0.596
-0.896
0.328
-2.520
0.000
-2.882
3.059
-0.060
-3.098
0.917
0.827
-2.770
1.247
2.963
3.835
3.769
-0.055
-0.473
-0.035
0.782
-0.128
0.000
0.044
0.162
-0.085
0.036
0.000
-0.084
0.000
0.000
-0.156
0.051
0.058
0.002
-0.079
0.499
0.031
0.156
-0.086
0.147
0.000
0.000
0.000
0.000
0.000
-0.027
0.004
-0.004
0.000
-0.098
-0.184
-0.049
-0.137
0.187
0.042
0.000
0.005
0.000
0.000
0.000
0.000
0.009
-0.040
0.062
0.000
0.000
0.000
0.221
0.022
-0.010
-0.006
0.013
-0.001
0.000
0.000
0.000
-0.083
-0.028
-0.014
0.019
12c (M ) Ni)
ZnII DPP #1
ZnII DPP #2
0.000
NiII DPP #1
-2.439
-1.707
-2.555
-2.377
-1.751
-2.530
0.000
NiII DPP #2
NiII DPP #3
NiII DPP #4
13c (M ) Ni) molecule 1
13c (M ) Ni) molecule 2
H2DPP #1
0.078
0.000
0.000
0.031
H2DPP #2
0.000
0.420
-0.173
-0.016
13e (M ) 4H2+) (picrate)2
a Total out-of-plane deformation (Å). b Deformation in the lowest frequency mode of each symmetry type (Å).
Table 5. Nonbonded Interactions (Å) at the Periphery of the Aryl-Substituted Porphyrins
Cipso(meso),
C(â)
Cipso(meso),
ipso (â)
Cipso (â),
Cipso (â)′
Cipso (â),
C(meso)
porphyrin
CCDC refcode
conformation
C
ZnTPP (monoclinic)
NiTPP
ZZZTAY03
ZZZUUC01
SEMNIH
YEVJAN
ZAWRES
TEZXIF
planar
ruffled
planar
saddled
saddled
ruffled
saddled
saddled
2.92
2.92
2.91
3.04
3.01
3.01
3.03
3.10
N/A
N/A
N/A
N/A
3.02
3.00
3.04
3.12
N/A
N/A
N/A
N/A
3.06
2.95
3.13
3.16
N/A
N/A
N/A
N/A
3.25
3.26
3.21
3.20
H2TPP
H4TPP2+ (ClO4)2 [CH3OH]
ZnII DPP #1
NiII DPP #1
H2DPP #1
LADGAY
this work
13e (M ) 4H2+) (picrate)2
this regard, it is worth noting that saddling in the TArP
dications can result in a displacement out of the porphyrin
least-squares plane of greater than 1 Å for the pyrrole â
carbon atoms. An examination of the close contacts shows
that in the case of protonation there is indeed a significant
increase in the Cipso(meso)-Câ distance (from 2.91 to 3.04
Å). This may indicate a mechanistic dichotomy in how core
substituents lower the aryl-porphyrin rotational barriers in
TArPs. Small metals might favor out-of-plane rotation due
to their greater deformability via ruffling, whereas protona-
tion might favor in-plane rotation due to the increased
conformational space achieved by saddling the macrocycle.
We will return to this point in the molecular mechanics
calculations described in section II.
We now turn to the peripherally crowded and very
nonplanar dodecaarylporphyrins (DArPs). 2,3,5,7,8,10,12,-
13,15,17,18,20-Dodecaphenylporphyrin (DPP, 4) is the par-
ent DArP from which TArOPPs 10 and OArTPPs 13 are
2232 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
Figure 5. The two independent molecules seen in the crystal structure of
13c (M ) Ni). The aryl substituents have been omitted to better illustrate
the conformations of the porphyrin macrocycles.
derived. DPP has been the subject of several crystallographic
investigations,26,30,32 all of which have shown that the
porphyrin macrocycle is very distorted due to the need to
minimize steric repulsions between the peripheral phenyl
substituents. The two crystal structures reported for ZnDPP32
show quite similar amounts of nonplanar deformation (dobsd
approximately 3 Å) that is predominantly of the sad type
(Table 4). The two crystal structures reported for H2DPP26,32
also show primarily sad distortion of the porphyrin macro-
cycle, although a significant doming component is present
for the first structure (LADGAY). The second structure
(XAWRIW) is significantly more nonplanar than the first
(dobsd 3.9 vs 3.0 Å). No fewer than four X-ray structures
have been reported for NiDPP.30,32 These show that the
macrocycle can adopt structures with ruf and/or sad defor-
mations; i.e., the macrocycle is conformationally flexible with
this metal. One structure (CCDC refcode TEZXIF) is ruf,
two (XAWUI and XAWSET) are mainly ruf with some sad
deformation, and one (XAWROC) is sad with a significant
amount of ruf deformation. The increased preference of the
nickel complex versus the zinc complex for a ruffled
conformation has an obvious structural rationale given the
known tendency of nickel to favor short metal-nitrogen
distances and the greater core contraction that occurs with
ruf versus sad deformation.45
Figure 6. Crystal structure of 13e (M ) 4H2+) (pic)2. Hydrogen atoms,
except for those in the core, have been omitted for clarity. Hydrogen-bonding
interactions are indicated by dashed lines.
bond distances in 13c (M ) Ni) are short (1.896 Å in
conformation 1 and 1.878 Å in conformation 2) as described
for other very nonplanar porphyrins [e.g., 1.885-1.909 Å
in NiDPP and 1.906 Å in NiOETPP (1, M ) Ni)].50 Note
that this is not the first time that two independent molecules
have been found in the asymmetric cell of a peripherally
crowded porphyrin. A similar phenomenon has been reported
for NiOETNP (3, M ) Ni).51
Crystallographic data has not been reported for the dication
of dodecaphenylporphyrin (H4DPP2+) or any other DArP
(TArOPP or OArTPP), but such a structure was required to
complete our series of porphyrins with different core
substituents. Attempts to grow crystals of H4DPP2+ suitable
for X-ray crystallographic studies were unsuccessful using
a range of acids (trifluoroacetic, hydrochloric, and picric
acids) with several crystallization techniques. However, we
were able to grow crystals of OArTPP 13e (M ) 4H2+),
which has 4-fluorophenyl rather than phenyl groups at the
â position of the porphyrin (see Figure 4), using picric acid.
The presence of a fluoro group at the para positions of the
â phenyl rings is expected to have a minimal impact on the
observed structure.
The greater conformational flexibility of the nickel com-
plexes of the DArPs was also seen in the present study in
the crystal structure of OArTPP 13c (M ) Ni), one of the
porphyrins used in the NMR experiments in section III.
Porphyrin 13c (M ) Ni) is not an ideal candidate for a crystal
structure determination because of the many different isomers
(atropisomers) possible for this molecule. However, it did
yield crystals suitable for a structure determination, and the
resulting structure is shown in Figure 5. As expected, the
3-thienyl groups are rotationally disordered over two posi-
tions. More significantly, porphyrin 13c (M ) Ni) adopts
two different conformations in the asymmetric unit (Figure
5). Both conformations are very nonplanar (dobsd ) 3.3 and
2.8 Å) and contain mixtures of sad and ruf deformations.
Conformation 1 has principally sad deformation, and con-
formation 2 has mainly ruf deformation (Table 4). The Ni-N
As far as we are aware, this is the first time that the
structure of the picrate salt of a porphyrin dication has been
determined. The structure of porphyrin 13e (M ) 4H2+)
(pic)2 displays a very nonplanar conformation (Figure 6).
The nonplanar deformation (dobsd 3.9 Å) is predominantly
sad (3.8 Å) with a small amount of ruf (0.4 Å).47 The tilt
angle of a pyrrole ring with respect to the least-squares plane
(50) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.;
Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627.
(51) Senge, M. O. J. Chem. Soc., Dalton Trans. 1993, 3539.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2233

Medforth et al.
of the porphyrin macrocycle is as large as 41°. The
nonplanarity of the structure is greater than seen in the first
crystalline modification of H2DPP (dobsd 3.0 Å) and in the
dications of TPP (dobsd 2.5-3.3 Å) or OEP (dobsd 0.1-2.1
Å) but is comparable to that seen in the second crystalline
form of H2DPP (dobsd 3.9). Overall, the structure of 13e (M
) 4H2+) (pic)2 is consistent with the known tendency of both
peripheral crowding and protonation to increase nonplanarity
of the porphyrin macrocycle (i.e., it is more nonplanar than
the less crowded H4TPP2+ and than at least one form of
unprotonated H2DPP).
Hydrogen atoms were observed on each of the pyrrole
nitrogen atoms indicating that two protons were donated by
the picric acid molecules to the porphyrin macrocycle.
Compound 13e (M ) 4H2+) (pic)2 should therefore be
classified as a salt and not as a complex.52 The picrate anions
are oriented approximately parallel to the cavities formed
on either side of the macrocycle by the saddling of the
porphyrin ring and the 4-fluorophenyl substituents on the
pyrrole rings. A similar orientation effect has been observed
for the aromatic amine ligands in cobalt(III),15,16 nickel,53
and iron54 complexes of OETPP. A significant difference
between the picrate anions and the aromatic amine ligands
is the tilting of the aromatic plane away from the perpen-
dicular for the picrates (Figure 6).
Figure 7. Crystal structure of porphyrin 12c (M ) Ni). Hydrogen atoms
have been omitted for clarity, and only one orientation of the rotationally
disordered 3-thienyl rings is shown.
conformation is presumably the result of the ruf conformation
being better able to provide the short metal-nitrogen distance
favored by NiII. Overall, the structural changes seen for the
DArPs as a function of the core substituent M are similar to
those seen for the TArPs: switching to a smaller metal (Ni
vs Zn) increases the tendency of the macrocycle to ruffle,
whereas protonation increases the amount of saddle deforma-
tion (at least compared to one H2DPP structure).
The picrate anion on the upper face of the molecule is
disordered over both tilting positions, although only the right
tilting position is shown in Figure 6. The distances between
the porphyrin nitrogen atoms and the picrate phenolate
oxygen atom for the upper picrate are 2.90 Å (N3-O8) and
3.04 Å (N1-O8). These are longer than the corresponding
N-O(phenolate) distances in pyridinium picrate (2.62 Å)55
and imidazolium picrate (2.71 and 2.83 Å)56 but comparable
to the N-O distances in the perchlorate salts of TPP (CCDC
refcode RUHQEQ) and OEP (RUHQAM) (2.99 and 2.95
Å). The lower picrate anion is not disordered but is slipped
along the cavity (to the left in Figure 6). This results in
considerable asymmetry in the N-O(phenolate) distances
(2.74 Å for N2-O1 and 3.88 Å for N4-O). The slippage
may be possible because N4 can also interact with one of
the oxygen atoms (O7) from a nitro group (N-O distance
2.88 Å). A related interaction between the pyridinium
nitrogen and the oxygen atom of one of the picrate nitro
groups has been observed in pyridinium picrate (N-O
distance 2.92 Å).55
The close contacts at the periphery of the macrocycle for
the DArPs are given in Table 5. The trends in the close
contacts for the meso aryl substituents in the DArPs parallel
those seen for the TArPs but are about 0.1 Å longer in the
case of the DArPs. The Cipso(meso)-Câ distances are about
the same (3.02 ( 0.01 Å) for ruf NiDPP, sad ZnDPP, and
H2DPP (CCDC refcode LADGAY) but approximately 0.1
Å greater for 13e (M ) 4H2+) (pic)2. A similar pattern is
seen for the distance between Cipso(meso) and Cipso(â). Based
purely on these findings, it would seem reasonable to expect
the dication to have the lowest aryl-porphyrin rotational
barrier, although as we will show later this is not the case.
Finally, we examined DiArTPPs 12 which are of inter-
mediate peripheral crowding. By analogy with previous
studies of the effects of peripheral substitution,1 the addition
of two phenyl substituents to the â position of TPP (9a) to
produce the intermediately crowded DiArTPP 2,3,5,10,15,-
20-hexaphenylporphyrin (HPP, 12a) might be expected to
increase the amount of nonplanar deformation. Such an
increase in nonplanar deformation compared to TPP has been
observed in the related 2,3-diethyl-5,10,15,20-tetraphenylpor-
phyrins (DiEtTPPs) reported by Senge and Kalisch57 (Table
4). Interestingly, the crystal structure of DiArTPP 12c (M
) Ni) (Figure 7) shows that the macrocycle is not appreciably
distorted. The molecule is approximately planar (dobsd 0.13
Å, Table 4) and is actually less nonplanar than NiTPP (dobsd
1.30 Å, Table 3). The mean deviation of an atom from the
least-squares plane of the porphyrin ring for 12c (M ) Ni)
is only 0.02 Å, and the angles between the pyrrole planes
and the least-squares plane of the porphyrin ring are less
than 2°. The average Ni-N distance (1.963 Å) is also quite
In summary, the structural data for the DPPs (DArPs)
shows that a sad conformation is favored by the macrocycle
when M ) Zn, 2H or 4H2+, but both the sad and/or the ruf
conformations can be accessed when a small metal (NiII) is
present. The ability of the NiII complexes to access the ruf
(52) Herbstein, F. H.; Moshe, K. Acta Crystallogr. 1991, C47, 1131.
(53) Renner, M. W.; Barkigia, K. M.; Melamed, D.; Smith, K. M.; Fajer,
J. Inorg. Chem. 1996, 35, 5120.
(54) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, K.
M.; Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc.
2001, 123, 6564.
(55) Takayanagi, H.; Kawaoka, R.; Chin, K.; Goto, M.; Yamaguchi, S.-I.;
Ogura, H. Anal. Sci. 1990, 6, 321.
(56) Soriano-Garcia, M.; Schatz-Levine, M.; Toscano, R. A.; Villena Iribe,
R. Acta Crystallogr. 1990, C46, 1556.
(57) Senge, M. O.; Kalisch, W. W. Inorg. Chem. 1997, 36, 6103.
2234 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
long and consistent with those seen in other planar nickel
porphyrins (e.g., triclinic A NiOEP 1.958 Å).58 The macro-
cycle does show a slight elongation along the axis of the
substituted pyrrole ring leading to longer average Ni-N
distances in this direction (1.982 vs 1.945 Å). However, the
additional aryl rings at the pyrrole position of 12c (M ) Ni)
appear to do little to increase the amount of nonplanar
distortion.
DREIDING II force field has been fully described in the
literature,44,45 so details of the force field are included as
Supporting Information.
The force field was originally parametrized for nickel
porphyrins44 and subsequently extended to other transition
metals including zinc,49 so it could be readily applied to the
metal complexes (Ni and Zn) used in our study. The use of
the force field to calculate the structures of metal free
porphyrins (i.e., free base porphyrins and porphyrin dications)
has not been reported in the literature. To calculate the
structures of the free base porphyrins and porphyrin dications,
hydrogen bonding H atoms were used to represent the inner
hydrogen atoms. Counterions were not included in the
calculations, and the default charge equilibration scheme45
was employed. For consistency with earlier studies, the
calculations were performed with a dielectric constant of 2.64
(for CS2). This dielectric constant also reasonably ap-
proximates the nonpolar solvents used in the NMR studies
(e.g., CD2Cl2 or C6D5CD3).
The finding that 12c (M ) Ni) is more substituted but
also more planar than NiTPP underscores the need for
caution when interpreting the spectroscopic properties of
porphyrins using the static picture obtained from X-ray
crystallographic studies. The apparently anomalous behavior
of 12c (M ) Ni) is probably related to the fact that the energy
required to deform the macrocycle is small for small amounts
of deformation; i.e., there is a fairly shallow potential energy
surface.59 This is particularly true for porphyrins with small
metals such as nickel, where there is a tradeoff between the
structural requirements of the metal (short metal-nitrogen
distance, which favors ruffling) and the porphyrin ring
(maximizing π-overlap, which favors a planar macrocycle).
Given the fine balance between these two forces, the
crystallization conditions and crystal packing forces become
important in determining the observed conformation. For
example, it is known that NiOEP exists in both planar and
nonplanar conformations in the solid state (Table 3) and in
solution.60,61 It is probable that 12c (M ) Ni) exhibits similar
behavior.
II. Molecular Mechanics (MM) Calculations. Molecular
simulations are increasingly being used to investigate the
structural and dynamic properties of porphyrins.46 We
decided to determine if molecular mechanics (MM) calcula-
tions might provide additional insights into the structural
preferences of the TArPs and DArPs as well as the aryl-
porphyrin rotation processes in these systems. MM calcula-
tions were carried out using POLYGRAF software (Molec-
ular Simulations, Inc.) and a force field for metalloporphyrins
that has been used extensively to calculate the structures of
porphyrins7,8,26,44-46,49,62-73 and to investigate some dynamic
processes (e.g., axial ligand rotation).15,16 The modified
The molecular mechanics calculations were divided into
two parts. First, the global minimum energy structure was
calculated. Then, the aryl group was rotated using a standard
dihedral rotation procedure, with the porphyrin being allowed
to relax fully at each step of the drive, to obtain the aryl-
porphyrin rotational barrier.
Global minimum energy structures and rotational barriers
were initially calculated for TArPs 9a and 9b using a full
range of core substituents (e.g., M ) Ni, Zn, 2H or 4H2+).
Similar results were obtained for both porphyrins, so only
the data for compound 9b, which is used in the NMR studies
in section III, is discussed. The similarity of both the
structures and barriers for TArPs 9a and 9b supports the
earlier contention that the addition of a meta methoxyl
substituent results in only a minor structural perturbation.
Note that the relative orientation of the methoxyl groups (the
atropisomer) was also found to have a negligible effect on
the energy minimized structure and on the calculated
rotational barrier, so the energy minimized RâRâ atropisomer
(which has alternating up and down methoxyl groups) was
used as the starting structure for the dihedral rotation.
The global minimum energy structure of 9b (M ) Ni)
was found to be modestly ruffled, 9b (M ) Zn) was
approximately planar, 9b (M ) 2H) was slightly saddled,
and 9b (M ) 4H2+) was strongly saddled. These results are
in general agreement with the crystallographic data for TPP
derivatives (Table 3) as they show that nickel(II) favors
ruffling of the macrocycle and that protonation induces
(58) Cullen, D. L.; Meyer, E. F. J. Am. Chem. Soc. 1974, 96, 2095.
(59) This behavior is discussed in a recent paper from our group (ref 73).
(60) Brennan, T. D.; Scheidt, W. R.; Shelnutt, J. A. J. Am. Chem. Soc.
1988, 110, 3919.
(61) Alden, R. G.; Crawford, B. A.; Doolen, R.; Ondrias, M. R.; Shelnutt,
J. A. J. Am. Chem. Soc. 1989, 111, 2070.
(62) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt, J. A.
Tetrahedron Lett. 1990, 31, 3719.
(63) Shelnutt, J. A.; Majumder, S. A.; Sparks, L. D.; Hobbs, J. D.; Medforth,
C. J.; Senge, M. O.; Smith, K. M.; Miura, M.; Luo, L.; Quirke, J. M.
E. Raman Spectrosc. 1992, 23, 523.
(64) Hobbs, J. D.; Majumder, S. A.; Luo, L.; Sickelsmith, G. A.; Quirke,
J. M. E.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem.
Soc. 1994, 116, 3261.
(65) Medforth, C. J.; Senge, M. O.; Forsyth, T. P.; Hobbs, J. D.; Shelnutt,
J. A.; Smith, K. M. Inorg. Chem. 1994, 33, 3865.
(66) Sparks, L. D.; Anderson, K. K.; Medforth, C. J.; Smith, K. M.;
Shelnutt, J. A. Inorg. Chem. 1994, 33, 2297.
(67) Jentzen, W.; Hobbs, J. D.; Simpson, M. C.; Taylor, K. K.; Ema, T.;
Nelson, N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti,
M.; Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, I. W. A.;
Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085.
(69) Song, X.-Z.; Jentzen, W.; Jia, S.-L.; Jaquinod, L.; Nurco, D. J.;
Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1996,
118, 12975.
(70) Jentzen, W.; Unger, E.; Song, X. Z.; Jia, S. L.; Turowska-Tyrk, I.;
Schweitzer-Stenner, R.; Dreybrodt, W.; Scheidt, W. R.; Shelnutt, J.
A. J. Phys. Chem. A 1997, 101, 5789.
(71) Song, X.-Z.; Jentzen, W.; Jaquinod, L.; Khoury, R. G.; Medforth, C.
J.; Jia, S.-L.; Ma, J.-G.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem.
1998, 37, 2117.
(72) Nelson, N. Y.; Medforth, C. J.; Nurco, D. J.; Jia, S.-L.; Shelnutt, J.
A.; Smith, K. M. Chem. Commun. 1999, 2071.
(68) Medforth, C. J.; Hobbs, J. D.; Rodriguez, M. R.; Abraham, R. J.; Smith,
K. M.; Shelnutt, J. A. Inorg. Chem. 1995, 34, 1333.
(73) Haddad, R. E.; Gazeau, S.; Pe´caut, J.; Marchon, J.-C.; Medforth, C.
J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2235

Medforth et al.
optimized Câ-CR-Cm-Ci torsion angle of 50° was found
to produce only a slightly higher (62 kJ mol-1) barrier for
out-of-plane rotation than for in-plane rotation (51 kJ mol-1).
In the case of 9b (M ) Zn), rotating the aryl group in-plane
about the minimum energy structure produced a higher
barrier (88 kJ mol-1) than that obtained by constraining the
Câ-CR-Cm-Ci torsion angles to force the aryl substituent
to rotate out-of-plane (80 kJ mol-1). A similar effect was
observed for the free base porphyrin 9b (M ) 2H), although
in this case the difference between the in-plane (118 kJ
mol-1) and out-of-plane (59 kJ mol-1) pathways was much
larger. The optimized Câ-CR-Cm-Ci torsion angles for 9b
(M ) Zn) and 9b (M ) 2H) (50-55°) were found to be
similar to that seen for 9b (M ) 4H2+) (50°).
Figure 8. Diagrams showing (a) the torsion angles used in the molecular
mechanics calculations of the meso aryl rotational barriers and (b) the
definition of clockwise rotation of a meso aryl group in a saddle deformed
porphyrin.
Overall, the MM calculations correctly predict both the
structures and the trends in the experimental rotational
barriers for the TArPs.17-19 A lower barrier is seen for the
nickel complex versus the zinc complex of porphyrin 9b (50
vs 80 kJ mol-1) and also for the dication versus the free
base porphyrin (51 vs 59 kJ mol-1). The ordering of the
calculated rotational barriers (Ni < 4H2+ < 2H < Zn) is
also the same as that reported for porphyrin 619 (Table 1).
MM calculations were then performed for TArOPP 10b,
a porphyrin that is used in the NMR studies in section III.
Porphyrin 10b is related to porphyrin 9b by the addition of
phenyl rings to all of the unsubstituted pyrrole positions.
Calculations were carried out for porphyrin 10b with two
core substituents: M ) Ni, which based on the X-ray data
is expected to be the most conformationally flexible system
and to adopt sad and/or ruf structures, and M ) 4H2+, which
is expected to be the least conformationally flexible system
because the protons in the core will strongly favor a saddle
structure. The global minimum energy structure of 10b (M
) 4H2+) was calculated to be strongly saddle distorted in
agreement with the crystal structure of 13e (M ) 4H2+)
(Figure 6). Nickel complex 10b (M ) Ni) was also found
to have a sad conformation as the global minimum energy
structure, although the ruf conformation was also obtained
as a stable local minima only 15 kJ mol-1 higher in energy
than the sad conformation. A stable ruffle conformation could
not be obtained for 10b (M ) 4H2+). The finding of sad
and ruf conformations of similar energies for the nickel
complex of 10b is consistent with the crystallographic data
discussed in section I, which showed both sad and/or ruf
conformations for NiDPP (Table 4) and mixed sad/ruf
conformations for 13c (M ) Ni) (Figure 5, Table 4).30,32
The MM calculations predict a considerable difference in
Table 6. Barriers for meso 3-Methoxyphenyl Rotation (∆E‡ROT; kJ
mol-1) Obtained from Molecular Mechanics Calculations
porphyrin
∆E‡
ROT
9b (M ) Zn)
9b (M ) Ni)
9b (M ) 2H)
9b (M ) 4H2+
10b (M ) Ni)
80a
50b
59a
)
51c
58b
10b (M ) 4H2+
)
102b
a Calculated with the rotated meso aryl group constained out-of-plane
using two Câ-CR-Cm-Ci torsion angles. Rotating the meso aryl substituent
about the global minimum energy structure gives a higher barrier [88 kJ
mol-1 for 9b (M ) Zn) and 118 kJ mol-1 for 9b (M ) 2H)]. b Calculated
by rotating the meso aryl substituent about the global minimum energy
structure. c Calculated by rotating the meso aryl substituent about the global
minimum energy structure. A higher barrier (62 kJ mol-1) is obtained when
the rotated meso aryl group is constrained out-of-plane using two Câ-CR-
Cm-Ci torsion angles.
saddling of the macrocycle. Using the energy minimized
RâRâ structure as a starting point, the rotational barriers were
then calculated using a dihedral rotation. The CR-Cm-Ci-
Co torsion angle shown in Figure 8a was used to rotate the
aryl group, which was rotated clockwise or counterclockwise
in 10° increments with smaller changes around the energy
maximum. The rest of the porphyrin structure was allowed
to relax fully at each of the steps. The rotational sense of
the meso aryl group was only found to be important for the
saddle structures, and is defined in Figure 8b.
The rotational barriers (∆E‡ROT) determined from the MM
calculations are given in Table 6. For 9b (M ) Ni), the
lowest barrier (50 kJ mol-1) was obtained by rotating the
aryl group about the minimum energy ruf structure; i.e.,
rotation occurred when the aryl group was out of the least-
squares plane of the porphyrin macrocycle. In contrast, for
9b (M ) 4H2+), the lowest barrier (51 kJ mol-1) was
obtained by rotating the aryl group about the minimum
energy sad structure, which corresponds to rotation in the
plane of the porphyrin macrocycle. The out-of-plane rotation
pathway was also investigated for 9b (M ) 4H2+). The out-
of-plane pathway was approximated by constraining the aryl
substituent to move out-of-plane using two Câ-CR-Cm-Ci
torsion angles as shown in Figure 8a. The CR-Cm-Ci-Co
and Câ-CR-Cm-Ci torsion angles were then varied inde-
pendently to determine the lowest energy pathway. An
the effect on ∆E‡
of adding eight phenyl rings to
ROT
porphyrin 9b to produce porphyrin 10b for the two core
substituents investigated (M ) Ni or 4H2+). The lowest
barrier for rotation of the 3-methoxyphenyl group in 10b
(M ) Ni) was found when the macrocycle was ruffled and
the meso aryl group moved out of the porphyrin plane.
Surprisingly, the calculated barrier (58 kJ mol-1) was only
marginally higher than that obtained for 9b (M ) Ni) (50
kJ mol-1) despite the addition of eight phenyl rings to the
porphyrin periphery. The lowest barrier for rotation of the
3-methoxyphenyl group in 10b (M ) 4H2+) was much higher
2236 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
Table 7. Activation Energies (∆G‡ROT; kJ mol-1) and Coalescence
Temperatures for meso 3-Methoxyphenyl Rotation Obtained from
Variable Temperature H NMR Studies
(102 kJ mol-1) and was obtained when the aryl group was
rotated clockwise from the saddled minimum energy struc-
ture, see Figure 8b, which forced the meso aryl group to
move out-of-plane. Rotating the meso aryl group counter-
clockwise did not move the aryl group out-of-plane and gave
a significantly higher barrier (133 kJ mol-1). The barrier for
the dication of 10b (102 kJ mol-1) was much higher than
that calculated for the dication of 9b (51 kJ mol-1). The
barrier for the dication of 10b (102 kJ mol-1) was also
considerably higher than the barrier for the nickel complex
of 10b (58 kJ mol-1).
1
porphyrin
M ) Zn
M ) Ni
M ) 2H
45 ( 541
M ) 4H+
9b
62 ( 2
(273 K)
67 ( 3
(303 K)
49 ( 3
(243 K)
55 ( 2
(273 K)
47 ( 2
(218 K)
91 ( 3
(398 K)
10b
65 ( 3
(288 K)
Table 8. Activation Energies (∆G‡ROT; kJ mol-1) and Coalescence
Temperatures for â 3-Methoxyphenyl Rotation Obtained from Variable
1
Temperature H NMR Studies
porphyrin
M ) Zn
M ) Ni
M ) 2H
M ) 4H2+
As noted earlier, an analysis of crystallographic data for
the DArPs shows that the Cipso(meso)-C(â) and Cipso(meso)-
C
11b
12b
a
a
a
g31 ( 3b
45 ( 2
66 ( 3
(294 K)
52 ( 2
(263 K)
65 ( 3
(293 K)
54 ( 3
(251 K)
60 ( 4
ipso(â) distances are longer in 13e (M ) 4H2+) (pic)2 than
(273 K)
(233 K)
42 ( 2
13b
g46 ( 2c
in ruf NiDPP (Table 5), implying that the rotational barrier
should be lower in the dication. However, the results of the
MM calculations in Table 6, which are confirmed by the
VT NMR results in section III, indicate that the barrier is
actually much higher in the dication than in the nickel
complex of TArOPP 10b (102 versus 58 kJ mol-1). One
explanation for the MM results is that the meso aryl rotational
barrier in TArOPP 10b is strongly influenced by the
deformability along the ruffling coordinate. Ruffling defor-
mation effectively allows the aryl group being rotated to
move out of the porphyrin plane to minimize interactions
with the rest of the porphyrin (as previously suggested for
the metal complexes of TArPs).17,19 A core substituent which
favors or permits ruffling (e.g., nickel) will facilitate the meso
aryl group moving out-of-plane to minimize aryl-porphyrin
interactions and will decrease the rotational barrier. In
contrast, a core substituent which disfavors ruffling (such
as four protons, where severe steric crowding would occur
in the core) will inhibit the aryl group from moving out-of-
plane and will increase the rotational barrier.
It should be pointed out that the actual mechanism of aryl
rotation in the TArOPPs might be more complicated than
that suggested by the MM calculations. For example, rotation
of some or all of the aryl groups might be a concerted
process. However, the good agreement between the MM and
NMR barriers (see section III) plus the fact that we found
no evidence of such effects in the MM calculations make
this scenario less likely. It is also worth noting that the only
time the lowest energy pathway for aryl-porphyrin rotation
is not calculated to involve out-of-plane movement of the
meso aryl group is in the case of 9b (M ) 4H2+). This may
be because such a large amount of saddle deformation occurs
upon protonation of 9b (M ) 2H), allowing in-plane rotation
to compete with out-of-plane rotation in the dication. A
related in-plane rotation mechanism may not compete as
effectively for the TArOPPs because the dication is so highly
substituted.
(213 K)
a Proton NMR spectra showed broadening at low temperatures, but ∆G‡
could not be determined with any degree of accuracy. b The dynamic process
with ∆G‡ ) 31 kJ mol-1 might be inversion of the nonplanar porphyrin
macrocycle. As aryl-porphyrin rotation must be slow on the NMR time
scale rotation to allow inversion to be detected, 31 kJ mol-1 is the lower
limit for ∆G‡
.
c Atropisomers could be detected when macrocyclic
ROT
inversion (∆G‡ ) 46 kJ mol-1) was slow on the NMR time scale. As aryl-
porphyrin rotation must be slow on the NMR time scale rotation to allow
inversion to be detected, this is the lower limit for ∆G‡
.
ROT
determined using coalescence data from variable temperature
(VT) 1H NMR studies, are provided in Table 7. The
corresponding data for the â 3-methoxyphenyl groups in
porphyrins 11b, 12b, and 13b are given in Table 8. The
porphyrins used in our study are known to display several
dynamic processes including NH tautomerism (in the free
base porphyrins), substituent rotation (for both the meso and
â aryl groups), and inversion of the nonplanar macrocycle
(TArOPPs and OArTPPs).43 In an initial study of DPP, we
mistakenly assigned one of the dynamic processes for one
particular core substituent (M ) 4H2+) to macrocyclic
inversion instead of â aryl rotation.26 This error was rectified
when additional TArOPPs and OArTPPs were synthesized.35
In the present study, great care was taken to assign all of
the dynamic processes expected for each porphyrin. For
example, only one process is expected for 9b (M ) Ni) (meso
aryl rotation) whereas four processes are expected for 10b
(M ) 2H) (NH tautomerism, macrocyclic inversion, meso
aryl rotation, and â aryl rotation).
Definitive assignment of the aryl-porphyrin rotation
process was achieved by synthesizing porphyrins with aryl
groups that were expected to have higher or lower aryl
rotation barriers than the 3-methoxyphenyl group. In such a
case, the barriers for the other dynamic processes (e.g., NH
tautomerism or macrocyclic inversion) were essentially
unchanged allowing an unambiguous assignment of the aryl-
porphyrin rotation process. Aryl rotation barriers for the
porphyrins with the modified aryl groups are given in Table
9. The more bulky 2-methoxyphenyl substituent was found
to increase ∆G‡ROT by approximately 50 kJ mol-1 compared
to the 3-methoxyphenyl group. The 3-thienyl group was
investigated as a way of lowering the rotational barrier and
was found to decrease ∆G‡ROT by approximately 20 kJ mol-1
compared to the value for the 3-methoxyphenyl group.
III. Variable Temperature 1H NMR Spectroscopy. 1H
NMR spectra of TArP 9b, TArOPP 10b, OArP 11b, OArTPP
13b, and DiArTPP 12b were analyzed as a function of
temperature with a view to determining the rotational barriers
of the 3-methoxyphenyl substituents in these systems.
Activation energies (∆G‡ROT; kJ mol-1) for rotation of the
meso 3-methoxyphenyl groups in porphyrins 9b and 10b,
Inorganic Chemistry, Vol. 42, No. 7, 2003 2237

Medforth et al.
Table 9. Activation Energies (∆G‡ROT; kJ mol-1) for 3-Thienyl and
mol-1) is also seen for 10b (M ) Ni) versus 10b (M ) Zn)
and presumably has similar origins, i.e., the ability of the
nickel complex to more readily deform along the ruf
coordinate and move the meso substituent out-of-plane to
facilitate rotation (a finding supported by the X-ray data for
NiDPP and ZnDPP in section I). On the basis of previous
studies of TArPs,18,19 a lower activation energy for aryl
rotation was expected for 9b (M ) 4H2+) versus 9b (M )
2H), although the barriers were found to be experimentally
indistinguishable (Table 7). This may reflect the lower
barriers resulting from the less hindered aryl groups in
porphyrin 9b compared to porphyrins 6 and 7, and the
correspondingly greater significance of the experimental
error.
1
2-Methoxyphenyl Rotation Obtained from Variable Temperature H
NMR Studies
porphyrin
M ) Zn
M ) Ni
M ) 2H
M ) 4H2+
8
10818
9618
g84a
79 ( 3
b
13d
10c
12c
13c
g96a
44 ( 3
42 ( 3
38 ( 4
51 ( 4
b
c
47 ( 2
b
30 ( 2
e27a
a Estimated from line broadening using the standard equations.86 b Por-
phyrin could not be prepared (see Experimental Section). c Sample was not
1
sufficiently soluble at low temperatures for H NMR studies.
The activation energies for 3-methoxyphenyl rotation
(Tables 7 and 8) are surprisingly similar given the significant
structural differences present among the porphyrins inves-
tigated. Most of the barriers fall between 40 and 70 kJ mol-1,
with the exception of 10b (M ) 4H2+) where the barrier is
much higher as predicted by the MM calculations. Because
of the many atropisomers present in the slow exchange
spectra, and difficulties in accurately determining the coa-
lescence temperature (Tc) and the slow-exchange chemical
shift difference (δν), the errors in the NMR measurements
are fairly large (up to 5 kJ mol-1).74 Despite these limitations,
the NMR results for meso aryl-substituted porphyrins 9b and
10b (Table 7) agree well with the barriers obtained from
the MM calculations. Equally importantly, the NMR results
support an interpretation based on the deformability of the
macrocycle being a significant factor controlling the rota-
tional barriers. This is most clearly illustrated by the barriers
for porphyrins 10b (M ) Ni) and 10b (M ) 4H2+) discussed
The NMR data for the â aryl-substituted porphyrins 11b,
12b, and 13b is incomplete, primarily because of difficulties
in accurately measuring the rotational barriers in OArP 11b
(Table 8). However, the barriers which could be measured
can be readily interpreted in terms of the out-of-plane
deformability model discussed for the TArOPPs. Just as
ruffling lowers the barrier for meso aryl rotation by moving
the meso position out-of-plane, so saddling (which moves
the pyrrole â positions out-of-plane) appears to lower the
barrier for â aryl rotation. Thus, protonation of DiArTPP
12b (M ) 2H) to form a more saddled dication structure is
seen to decrease ∆G‡
for the â 3-methoxyphenyl group
ROT
by approximately 15 kJ mol-1. Note that this is the opposite
of the effect seen for the meso substituted TArOPP 10b,
where protonation markedly increased the aryl-porphyrin
rotational barrier.
in the following paragraphs, where the difference in ∆G‡
ROT
(36 kJ mol-1) is clearly too large to be explained by
In addition, ∆G‡ROT is found to be lower in OArTPP 13b
than in DiArTPP 12b irrespective of the core substituent M.
This is consistent with the more highly substituted OArTPP
being easier to deform along the sad coordinate because of
the larger number of peripheral substituents. Put another way,
while the immediate environment around the â 3-methox-
yphenyl groups is similar in both porphyrins (i.e., a phenyl
ring on one side of the ring being rotated and a 3-methox-
yphenyl ring on the other side), steric interactions arising
from the six additional 3-methoxyphenyl rings in 13b make
it easier to move the â aryl group being rotated out of the
porphyrin plane. Interestingly, there is no statistically
experimental errors or differences in ∆S‡.75
We consider first porphyrins 9b and 10b which were
investigated in detail in the MM studies in section II. The
NMR studies of these porphyrins yield rotational barriers
which agree fairly well with those obtained from the MM
calculations: 9b (M ) Ni) 49 ( 3 (calcd 50), 9b (M )
4H2+) 47 ( 2 (calcd 51), 10b (M ) Ni) 55 ( 2 (calcd 58),
10b (M ) 4H2+) 91 ( 3 (calcd 102). The NMR studies
verify both the small increase in the rotational barrier
predicted for 10b (M ) Ni) versus 9b (M ) Ni) (obsd 6;
calcd 8 kJ mol-1) and the much larger increase predicted
for 10b (M ) 4H2+) versus 9b (M ) 4H2+) (obsd 44; calcd
51 kJ mol-1). This provides additional support for the
argument made in section II that easier deformability along
the ruf coordinate lowers the meso aryl rotational barriers
in the TArOPPs. The rotational barriers for porphyrins 9b
and 10b with other core substituents are also consistent with
this model. A smaller metal (Ni vs Zn) is seen to decrease
significant difference in ∆G‡
for DiArTPP 12b or
ROT
OArTPP 13b when a smaller metal (Ni vs Zn) is present
(Table 8), although there is for TArP 9b and TArOPP 10b
(Table 7). This can be explained by the ruffling deformation
favored by the nickel atom not moving the â aryl groups
out-of-plane in 12b and 13b, whereas it does move the meso
aryl groups out-of-plane in TArP 9b and TArOPP 10b.
∆G‡
by 13 kJ mol-1 for 9b in agreement with earlier
ROT
In summary, the NMR results for the TArOPPs, OArTPPs,
and DiArTPPs support the idea that out-of-plane deform-
ability of the macrocycle is important in lowering the
activation energy for aryl-porphyrin rotation. Overall, they
suggest a straightforward relationship between the position
of the aryl substituent being rotated (meso or â) and the
symmetry of the deformation mode (ruffling or saddling)
required to lower the rotational barrier. Ruffling is seen to
studies of TArPs (Table 1).17-19 A related decrease (12 kJ
(74) Given the complicated nature of the NMR spectra, no attempt was
made to perform a detailed simulation and to extract ∆H‡ and ∆S‡.
(75) It seems unlikely that the differences in ∆G‡ could arise solely from
the entropic component ∆S‡, as the barriers obtained in our study were
generally measured over a small temperature range (see Tables 7 and
8) and previous studies of aryl rotation in TArPs have suggested that
∆S‡ is small (Freitag, R. A.; Whitten, D. G. J. Phys. Chem. 1983, 87,
3918.).
2238 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
ppm (CHCl2CDCl2), or 7.26 ppm (CHCl3). For the variable
temperature studies, CD2Cl2 or C6D5CD3 was typically used as a
low temperature solvent and CDCl2CDCl2 or C6D5CD3 as a high
temperature solvent. The temperature control unit was calibrated
using a published procedure.77 Unless noted, spectra reported in
the Experimental Section were obtained at ambient temperature
(293-298 K).
lower the barrier for meso aryl rotation whereas saddling
lowers the barrier for â aryl rotation.
Significantly, the peripherally crowded porphyrins inves-
tigated in our study reveal some unusual trends in the
rotational barriers which are difficult to explain without
invoking the deformability model: (1) adding eight phenyl
rings to TArP 9b (M ) Ni) to give TArOPP 10b (M ) Ni)
produces a very small increase (only 6 kJ mol-1) in the
rotational barrier of the meso 3-methoxyphenyl substituent
(explained using the deformability model by the highly
substituted nickel porphyrin being able to ruffle and move
the aryl substitutent out of the porphyrin plane to facilitate
rotation); (2) adding eight phenyl rings to TArP 9b (M )
4H2+) to give TArOPP 10b (M ) 4H2+) produces a very
large increase (44 kJ mol-1) in the rotational barrier of the
meso 3-methoxyphenyl substituent (because deformation
along the ruffling coordinate would result in severe crowding
of the four protons in the core, making ruffling distortion
highly unfavorable for 10b (M ) 4H2+)); (3) the rotational
barriers are uniformly lower in 13b than in 12b, even though
the former is more peripherally crowded (this can be
understood in terms of the easier deformability of 13b due
to the presence of more substituents).
Synthesis. The porphyrins used in the NMR studies were
typically prepared in their free base form (M ) 2H). Nickel
complexes were then prepared by adding Ni(OAc)2 in methanol to
the free base porphyrin in refluxing chloroform, or by refluxing a
solution of the free base porphyrin in toluene containing Ni(acac)2.
Zinc complexes were prepared by adding Zn(OAc)2 in methanol
to the free base porphyrin in refluxing chloroform. The metal
complexes were purified by recrystallization from dichloromethane
by addition of methanol. The dications were prepared in situ prior
to the NMR experiments by addition of 1 vol % of trifluoroacetic
acid to the free base porphyrins. Visible spectra absorption maxima
and absorption coefficients were recorded on either a Hewlett-
Packard 8450A diode array spectrophotometer or a Hewlett-Packard
8452A diode array spectrometer using CH2Cl2 as solvent for the
nickel or zinc complexes, 99% CH2Cl2/1% N(CH3CH2)3 for the
free base porphyrins, and 99% CH2Cl2/1% CF3CO2H for the
dications. Mass spectral analyses were generally performed using
matrix-assisted laser desorption/ionization Fourier transform mass
spectrometry (MALDI-FTMS) in the positive ion detection mode
as described previously.78 Some mass spectra were obtained using
LSIMS at the Mass Spectrometry Facility at the University of
California, San Francisco. Melting points (uncorrected) were
measured on a Thomas/Bristoline microscopic hot stage apparatus.
5,10,15,20-Tetrakis(3-methoxyphenyl)porphyrin [9b (M )
2H)]. The synthesis of this porphyrin has been reported previously.41
Finally, it is worth noting that the changes in the dynamic
properties of the substituents demonstrated in our study may
well be general in nature and independent of the mechanism
by which the nonplanar deformation is produced. If this is
the case, the nonplanar deformations present in heme
proteins46,76 may also affect the dynamic properties of the
porphyrin substituents, provided the deformations are of the
correct symmetry.
1
9b (M ) 2H). H NMR (CD2Cl2): 8.88 (s, 8H, Hâ), 7.79 (d, 4H,
H6), 7.76 (s, 4H, H2), 7.64 (t, 4H, H5), 7.33 (m, 4H, H4), 3.96 (s,
1
24H, OCH3), -2.9 (s, 2H, NH). 9b (M ) 4H2+). H NMR (CD2-
Experimental Section
Cl2 + 1% CF3CO2H): 8.77 (s, 8H, Hâ), 8.13 (s, 4H, H2), 8.11 (d,
4H, H6), 7.92 (t, 4H, H5), 7.57 (m, 4H, H4), 4.15 (s, 24H, OCH3),
-1.3 (s, 4H, NH). 9b (M ) Ni). H NMR (CD2Cl2/CS2 1:4 by
volume): 8.73 (s, 8H, Hâ), 3.94 (s, 24H, OCH3). The sample was
X-ray Crystallography. Crystals were grown via solvent
diffusion methods using CH2Cl2/MeOH for 12c (M ) Ni), CHCl3/
MeOH for 13c (M ) Ni), and CH2Cl2/cyclohexane for 13e (M )
4H2+). X-ray diffraction data were collected on a Bruker SMART
1000 diffractometer with a sealed tube source [λ(Mo KR) ) 0.71073
Å] for 12c (M ) Ni) and 13e (M ) 4H2+) and on a Siemens P4
diffractometer with a rotating anode source [λ(Cu KR) ) 1.54178
Å] for 13c (M ) Ni). The Bruker SHELXTL V. 5.03 software
package was used for structure solution and refinement; scattering
factors were used as supplied. Structures were refined based on F2
using all independent data by full-matrix least-squares methods.
Crystallographic data are given in Table 2. Full experimental details,
in CIF format, are available as Supporting Information to this article.
Molecular Mechanics Calculations and Normal-Coordinate
Structural Decomposition. Full details of the force field used in
the molecular mechanics calculations are given as Supporting
Information. The normal-coordinate structural decomposition pro-
gram used to analyze the nonplanar deformations present in the
crystal structures has been fully described in the literature,2,46 and
a browser based version is available for general use at http://
jasheln.unm.edu.
1
too dilute to reliably assign other signals from the 3-methoxyphenyl
1
group. 10b (M ) Zn). H NMR (CD2Cl2): 8.96 (s, 8H, Hâ), 7.78
(d, 4H, H6), 7.73 (s, 4H, H2), 7.63 (t, 4H, H5), 7.29 (m, 4H, H4),
3.92 (s, 24H, OCH3).
5,10,15,20-Tetrakis(3-methoxyphenyl)-2,3,7,8,12,13,17,18-oc-
taphenylporphyrin [10b (M ) 2H)]. This compound was obtained
as green crystals in 11% yield from the condensation of 3,4-
diphenylpyrrole with 3-methoxybenzaldehyde using a modified
Adler-Longo procedure.42 10b (M ) 2H). Mp: >300 °C. MALDI-
1
FTMS [M + H]: 1343.5, calculated 1343.5. H NMR (CD2Cl2):
7.22 (br, 4H, aryl-H6), 7.11 (br, 4H, aryl-H2), 6.73 (br, 44H,
phenyl-H and aryl-H5), 6.34 (m, 4H, aryl-H4), 3.67 (s, 12H, OCH3),
NH signal not observed. Visible spectrum (CH2Cl2 + 1% N(C2H5)3),
λ/nm (ꢀ/cm-1 mol-1 dm-3): 466 (234 000), 562 (25 800), 614
(24 200), 712 (16 400). 10b (M ) 4H2+). 1H NMR (CD2Cl2 + 1%
CF3CO2H): 7.51-7.70 (8H, aryl-H2 and H6), 7.04-7.10 (4H, meso-
H5), 6.65-6.85 (44H, phenyl-H and aryl-H4), 3.82-3.87 (24H,
OCH3), NH signal not observed. Visible spectrum (CH2Cl2 + 1%
CF3COOH), λ/nm (ꢀ/cm-1 mol-1 dm-3): 496 (196 000), 720
(43 600). 10b (M ) Ni). Mp: >300 °C. 1H NMR (CD2Cl2): 6.60-
NMR Spectroscopy. 1H NMR spectra were recorded at a
frequency of 300 MHz using sample concentrations of ap-
proximately 5 mM. The spectra were referenced to TMS or the
solvent signals at 2.09 ppm (C6D5CHD2), 5.30 ppm (CHDCl2), 5.94
(77) van Geet, A. L. Anal. Chem. 1970, 42, 679.
(78) Green, M. K.; Medforth, C. J.; Muzzi, C. M.; Nurco, D. J.; Shea, K.
M.; Smith, K. M.; Shelnutt, J. A.; Lebrilla, C. B. Eur. Mass Spectrom.
1997, 3, 439.
(76) Jentzen, W.; Ma, J. G.; Shelnutt, J. A. Biophys. J. 1998, 74, 753.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2239

Medforth et al.
6.74 (br, 52H, phenyl-H and aryl-H2, H5, and H6), 6.23 (m, 4H,
aryl-H4), 3.47 (br s, 12H, OCH3). Visible spectrum (CH2Cl2), λ/nm
(ꢀ/cm-1 mol-1 dm-3): 448 (100 000), 566 (7900), 610 (8800). 10b
(M ) Zn). H NMR (CD2Cl2): 6.7-7.2 (br, 52H, phenyl-H and
aryl-H2, H5, and H6), 6.28 (m, 4H, aryl-H4), 3.61 (s, 12H, OCH3).
NMR (CDCl3): 8.69 (s, 2H, H12,13), 8.57 (d, 2H, H8,17), 8.37 (d,
2H, H7,18), 7.98 (d, 4H, 10,15-Hortho), 7.66 (m, 6H, 10,15-Hmeta and
Hpara), 7.49 (d, 4H, 5,20-Hortho), 7.20 (t, 2H, 5,20-Hpara), 7.09 (t,
4H, 5,20-Hmeta), 6.77 (t, 2H, aryl-H5), 6.49 (m, 4H, aryl-H4 and
H6), 6.35 (s, 2H, aryl-H2), 3.54 (s, 6H, OCH3). 12b (M ) 2H). 1H
NMR (CDCl3): 8.80 (s, 2H, H12,13), 8.71 (d, 2H, H8,17), 8.57 (d,
2H, H7,18), 8.21 (d, 4H, 10,15-Hortho), 7.82 (m, 6H, 10,15-Hmeta and
1
2,3,7,8,12,13,17,18-Octaphenyl-5,10,15,20-tetrakis(3-thienyl)-
porphyrin [10c (M ) 2H)]. This compound was obtained as brown
crystals in 19% yield from the condensation of 3,4-diphenylpyrrole
with thiophene-3-carboxaldehyde using a modified Adler-Longo
reaction.42 10c (M ) 2H). Mp: >300 °C. LSIMS [M + H]: 1248.2,
calculated 1247.3. 1H NMR (CD2Cl2): 7.34 (br s, 4H, thienyl-H2),
7.12 (m, 4H, thienyl-H5), 6.81 (br, 40H, phenyl-H), 6.57 (m, 4H,
thienyl-H4), -1.00 (br s, 2H, NH). Visible spectrum (CH2Cl2 +
1% N(C2H5)3), λ/nm (ꢀ/cm-1 mol-1 dm-3): 470 (184 000), 572
Hpara), 7.28 (d, 4H, 5,20-Hortho), 7.24 (m, 6H, 5,20-Hmeta and Hpara),
6.79 (t, 2H, aryl-H5), 6.53 (m, 4H, aryl-H2 and H6), 6.46 (m, 2H,
aryl-H4), 3.61 (s, 6H, OCH3), -2.5 (s, 2H, NH). 12b (M ) 4H2+).
1H NMR (CDCl3 + 1% CF3CO2H): 8.68 (s, 2H, H12,13), 8.53 (m,
4H, 10,15-Hortho), 8.50 (d, 2H, H8,17), 8.38 (d, 2H, H7,18), 8.20 (m,
4H, 5,20-Hortho), 8.00 (m, 6H, 10,15-Hmeta and Hpara), 7.58 (m, 6H,
5,20-Hmeta and Hpara), 6.81 (t, 2H, aryl-H5), 6.53 (m, 2H, aryl-H4),
6.4 (m, 4H, aryl-H2 and H6), 3.55 (s, 6H, OCH3), -2.1 (s, 4H,
1
(10 800), 622 (13 300), 728 (7000). 10c (M ) 4H2+). H NMR
1
NH). 12b (M ) Zn). H NMR (CDCl3): 8.85 (d, 2H, H8,17), 8.65
(CD2Cl2 + 1% CF3CO2H): 7.87 (br, 4H, thienyl-H2), 7.56 (br, 4H,
thienyl-H5), 6.71-6.96 (m, 44H, phenyl-H and thienyl-H4). Visible
spectrum (CH2Cl2 + 1% CF3COOH), λ/nm (ꢀ/cm-1 mol-1 dm-3):
(s, 2H, H12,13), 8.65 (d, 2H, H7,18), 7.78 (d, 4H, 5,20-Hortho), 7.72
(d, 4H, 10,15-Hortho), 7.69 (m, 6H, 10,15-Hmeta and Hpara), 7.50 (m,
6H, 5,20-Hmeta and Hpara), 6.79 (t, 2H, aryl-H5), 6.60 (s, 2H, aryl-
H2), 6.55 (m, 2H, aryl-H6), 6.46 (m, 2H, aryl-H4), 3.62 (s, 6H,
OCH3).
5,10,15,20-Tetraphenyl-2,3-bis(3-thienyl)porphyrin [12c (M
) Ni)]. This compound was obtained as purple crystals in 33%
yield from the Suzuki coupling reaction82,83 of nickel(II)-2,3-
dibromotetraphenylporphyrin84 with thiophene-3-boronic acid. The
porphyrin decomposed when demetalation was attempted. 12c (M
) Ni). Visible spectrum (CH2Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3): 422
(226 000), 500 (6300), 537 (23 600). 1H NMR (CD2Cl2): 8.66 (s,
2H, H12,13), 8.55 (d, 2H, H8,17), 8.37 (d, 2H, H7,18), 7.96 (d, 4H,
10,15-Hortho), 7.66 (m, 6H, 10,15-Hmeta and Hpara), 7.52 (d, 4H, 5,-
20-Hortho), 7.22 (m, 6H, 5,20-Hpara, Hmeta), 6.79 (m, 2H, thienyl-
H4), 6.63 (m, 4H, thienyl-H2), 6.50 (m, 2H, thienyl-H5).
2,3,7,8,12,13,17,18-Octakis(3-methoxyphenyl)-5,10,15,20-tet-
raphenylporphyrin [13b (M ) 2H)]. This compound was obtained
as purple crystals in 43% yield using the Suzuki coupling
reaction82,83 of 2 (M ) 2H)85 with 3-methoxyphenylboronic acid.
13b (M ) 2H). Mp: 260-263 °C. MALDI-FTMS [M + H]:
1463.6, calculated 1463.6. 1H NMR (CDCl3): 7.63 (d, 8H, phenyl-
Hortho), 6.82 (m, 12H, phenyl-Hmeta and Hpara), 6.61 (t, 8H, aryl-
H5), 6.37 (br, 8H, aryl-H6), 6.29 (m, 8H, aryl-H4), 6.23 (br, 8H,
aryl-H2), 3.50 (s, 24H, OCH3), NH signal not observed. Visible
spectrum (CH2Cl2 + 1% N(C2H5)3), λ/nm (ꢀ/cm-1 mol-1 dm-3):
468 (210 000), 566 (13 900), 620 (13 900), 728 (8100). 13b (M )
4H2+). 1H NMR (CD2Cl2 + 1% CF3CO2H): 8.05 (m, 8H, phenyl-
1
496 (170 000), 740 (44 100). 10c (M ) Ni). Mp: >300 °C. H
NMR (CD2Cl2): 6.79 (m, 40H, phenyl-H), 6.68 (m, 4H, thienyl-
H2), 6.52 (m, 4H, thienyl-H5), 6.23 (m, 4H, thienyl-H4). Visible
spectrum (CH2Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3): 450 (200 000),
1
568 (17 300), 612 (16 200). 10c (M ) Zn). H NMR (CD2Cl2):
6.78 (m, 40H, phenyl-H), 7.15 (m, 4H, thienyl-H2), 7.00 (m, 4H,
thienyl-H5), 6.45 (m, 4H, thienyl-H4).
2,3,7,8,12,14,17,18-Octakis(3-methoxyphenyl)porphyrin [11b
(M ) 2H)]. This compound was isolated as a light brown powder
in 10% yield from the condensation of 3,4-bis(3-methoxyphenyl)-
pyrrole with formaldehyde using a modification of a published
procedure.48 3,4-Bis(3-methoxyphenyl)pyrrole was prepared from
3,3′-dimethoxybenzil and dimethyl-N-acetyliminodiacetate.79 3,3′-
Dimethoxybenzil was obtained by condensing 3-methoxybenzal-
dehyde to form the benzoin80 and then oxidizing the benzoin to
the corresponding benzil.81 11b (M ) 2H). MALDI-FTMS [M +
1
H]: 1159.2, calculated 1159.5. H NMR (CD2Cl2): 10.38 (s, 4H,
Hmeso), 7.60 (s, 8H, H6), 7.55 (d, 8H, H2), 7.54 (t, 8H, H5), 7.11
(m, 8H, H4), 3.82 (s, 24H, OCH3), -3.1 (s, 2H, NH). Visible
spectrum (CH2Cl2), λ/nm: 419, 512, 549, 578, 632. 7 (M ) 4H2+).
1
11b (M ) 4H2+). H NMR (CD2Cl2 + 1% CF3CO2H): 10.77 (s,
4H, Hmeso), 7.57 (t, 8H, H5), 7.44 (s, 8H, H2), 7.43 (d, 8H, H6),
7.19 (m, 8H, H4), 3.81 (s, 24H, OCH3), -2.5 (s, 4H, NH) (at 203
K). 11b (M ) Ni). 1H NMR (CD2Cl2): 10.04 (s, 4H, Hmeso), 7.47
(t, 8H, H5), 7.43 (d, 8H, H6), 7.38 (s, 8H, H2), 7.05 (d, 8H, H4),
1
3.78 (s, 24H, OCH3). 11b (M ) Zn). H NMR (CD2Cl2): 10.45
Hortho), 7.18 (m, 12H, phenyl-Hmeta and Hpara), 6.69 (t, 8H, aryl-
(s, 4H, Hmeso), 7.63 (d, 8H, H6), 7.55 (s, 8H, H2), 7.53 (t, 8H, H5),
7.08 (m, 8H, H4), 3.80 (s, 24H, OCH3).
H5), 6.39 (m, 8H, aryl-H4), 6.29 (br, 8H, aryl-H2), 6.25 (br, 8H,
aryl-H6), 3.50 (s, 24H, OCH3), NH signal not observed. 13b (M )
2,3-Bis(3-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin [12b
(M ) Ni)]. This compound was isolated as blue crystals in 38%
yield from the Suzuki coupling reaction82,83 of nickel(II)-2,3-
dibromotetraphenylporphyrin84 with 3-methoxyphenylboronic acid.
The porphyrin was demetalated by treatment with concentrated
sulfuric acid for 20 min. 12b (M ) Ni). Visible spectrum (CH2-
Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3): 422 (221 000), 538 (14 700) 1H
1
Ni). H NMR (CD2Cl2): 7.04 (br, 8H, phenyl-Hortho), 6.72 (t, 4H,
phenyl-Hpara), 6.61 (br, 16H, phenyl-Hmeta and aryl-H5), 6.29 (d,
8H, aryl-H4), 6.2-6.4 (br, 16H, aryl-H2 and H6), 3.45 (s, 24H,
OCH3). Visible spectrum (CH2Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3):
448 (203 000), 568 (16 500), 612 (17 800). 13b (M ) Zn). 1H NMR
(CD2Cl2): 7.53 (br, 8H, phenyl-Hortho), 6.78 (t, 4H, phenyl-Hpara),
6.69 (t, 8H, phenyl-Hmeta), 6.55 (t, 8H, aryl-H5), 6.23 (m, 24H, aryl-
H2, H4 and H6), 3.54 (s, 24H, OCH3).
5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(3-thienyl)-
porphyrin [13c (M ) 2H)]. This compound was obtained as
brown/green crystals in 20% yield from the Suzuki coupling
reaction82,83 of 2 (M ) 2H)85 with thiophene-3-boronic acid. 13c
(M ) 2H). Mp: >300 °C. MALDI-FTMS [M + H]: 1271.9,
(79) Friedman, M. J. Org. Chem. 1965, 30, 859.
(80) Williamson, K. L. Macroscale and Microscale Organic Experiments;
D. C. Heath and Co.: Lexington, MA, 1989; p 534.
(81) Wilcox, C. F. J. Experimental Organic Chemistry, A Small Scale
Approach; MacMillan Publishing Co.: New York, 1988; p 427.
(82) Chan, K. S.; Zhou, X.; Luo, B.-S.; Mak, T. C. W. J. Chem. Soc.,
Chem. Commun. 1994, 271.
(83) Zhou, X.; Zhou, Z.; Mak, T. C. W.; Chan, K. S. J. Chem. Soc., Perkin
Trans. 1 1994, 2519.
(84) Jaquinod, L.; Khoury, R. G.; Shea, K. M.; Smith, K. M. Tetrahedron
1999, 55, 13151.
(85) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239.
(86) Abraham, R. J.; Fisher, J.; Loftus, P. Introduction to NMR Spectros-
copy; Wiley: Chichester, U.K., 1988.
2240 Inorganic Chemistry, Vol. 42, No. 7, 2003

Aryl-Porphyrin Rotational Barriers
calculated 1271.2. 1H NMR (CD2Cl2): 7.81 (m, 8H, phenyl-Hortho),
7.03 (m, 12H, phenyl-Hmeta and -Hpara), 6.64 (m, 8H, thienyl-H4),
6.57 (br, 8H, thienyl-H2), 6.40 (br, 8H, thienyl-H5), -0.80 (br s,
2H, NH). Visible spectrum (CH2Cl2 + 1% N(C2H5)3), λ/nm (ꢀ/
cm-1 mol-1 dm-3): 478 (98 700), 570 (8100), 622 (8000), 730
Visible spectrum (CH2Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3): 442
(136 000), 562 (16 300), 602 (9600).
2,3,7,8,12,13,17,18-Octakis(4-fluorophenyl)-5,10,15,20-tet-
raphenylporphyrin [13e (M ) 2H)]. The synthesis of this
porphyrin has been reported previously.34 The picrate salt used in
the crystal structure determination of 13e (M ) 4H2+) was prepared
by washing a solution of 13e (M ) 2H) in CH2Cl2 with a saturated
aqueous solution of picric acid. The organic layer was then separated
and dried by filtration through anhydrous sodium sulfate and the
solvent removed under vacuum.
1
(3400). 13c (M ) 4H2+). H NMR (CD2Cl2 plus 1% CF3CO2H):
8.15 (m, 8H, phenyl-Hortho), 7.29 (m, 12H, phenyl-Hmeta and -Hpara),
6.72 (m, 8H, thienyl-H4), 6.63 (m, 8H, thienyl-H2), 6.41 (m, 8H,
thienyl-H5), NH signal not observed. Visible spectrum (CH2Cl2 +
1% CF3COOH), λ/nm (ꢀ/cm-1 mol-1 dm-3): 506 (76 200), 740
(22 500). 13c (M ) Ni). 1H NMR (CD2Cl2): 7.16 (m, 8H, phenyl-
H
ortho), 6.90 (m, 12H, phenyl-Hpara), 6.78 (m, 8H, phenyl-Hmeta),
Acknowledgment. This work was supported by grants
from the National Institutes of Health (HL 22252) and the
National Science Foundation (CHE-99-04076). This work
was partially supported by the Division of Materials Sciences
and Engineering, Office of Basic Sciences, U.S. Department
of Energy. Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed-Martin company, for the
United States Department of Energy under Contract DE-
ACO4-94AL85000. Mass spectrometric analyses were per-
formed by the UCSF Mass Spectrometry Facility (A. L.
Burlingame, Director) supported by the Biomedical Research
Technology Program of the National Center for Research
Resources, NIH NCRR BRTP 01614.
6.60 (m, 8H, thienyl-H4), 6.44 (m, 8H, thienyl-H2), 6.25 (m, 8H,
thienyl-H5). Visible spectrum (CH2Cl2), λ/nm (ꢀ/cm-1 mol-1 dm-3):
454 (182 000), 570 (14 400), 618 (18 100). 13c (M ) Zn). 1H NMR
(CD2Cl2): 7.68 (m, 8H, phenyl-Hortho), 6.93 (m, 12H, phenyl-Hpara
and Hmeta), 6.60 (m, 8H, thienyl-H4), 6.44 (m, 8H, thienyl-H2), 6.27
(m, 8H, thienyl-H5).
2,3,7,8,12,13,17,18-Octakis(2-methoxyphenyl)-5,10,15,20-tet-
raphenylporphyrin [13d (M ) 2H)]. This compound was obtained
as brown/green crystals in 14% yield from the Suzuki coupling
reaction82,83 of 2 (M ) 2H)85 with 2-methoxyphenylboronic acid.
13d (M ) 2H). Mp: >300 °C. MALDI-FTMS [M + H]: 1463.6,
1
calculated 1463.6. H NMR (CD2Cl2): 6.1-7.6 (52H, aromatic
protons), 2.8-3.7 (24H, OCH3), NH signal not observed. Visible
spectrum (CH2Cl2 + 1% N (C2H5)3), λ/nm (ꢀ/cm-1 mol-1 dm-3):
452 (187 000), 546 (1900), 596 (12 800), 625 (10 000), 692 (7100).
13d (M ) 4H2+). H NMR (CDCl3 + 1% CF3CO2H): 5.4-8.2
(52H, aromatic protons), 2.4-2.6 (pseudoaxial OCH3), 3.6-3.9
(pseudoequatorial OCH3), NH signal not observed. Visible spectrum
(CH2Cl2 + 1% CF3COOH), λ/nm (ꢀ/cm-1 mol-1 dm-3): 484
Supporting Information Available: X-ray crystallographic files
in CIF format for porphyrins 12c (M ) Ni), 13c (M ) Ni), and
13e (M ) 4H2+). Full details of the force field used in the molecular
mechanics calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
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(92 600), 712 (23 800). 13d (M ) Ni). Mp: >300 °C. H NMR
(CD2Cl2): 5.9-7.8 (52H, aromatic protons), 3.0-3.5 (24H, OCH3).
IC010958A
Inorganic Chemistry, Vol. 42, No. 7, 2003 2241