Metallation effects on the thermal interconversion of atropisomers of di(orthomethylarene)-substituted porphyrins

Article abstract of DOI:10.1039/b312898a

A new series of meso-substituted diaryl free-base and metalloporphyrins have been prepared. Each arene has been substituted with both a methyl group in the ortho position and a formyl group in the meta position. Rotation of the arene units is prevented at

Full text of DOI:10.1039/b312898a

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Metallation effects on the thermal interconversion of atropisomers
of di(orthomethylarene)-substituted porphyrins
Paul G. Plieger,a Anthony K. Burrell,*a Geoffrey B. Jamesonb and David L. Officerb
a Actinide, Catalysis and Separations Chemistry, C-SIC, Los Alamos National Laboratories,
MS J514, Los Alamos, NM 87545, USA. E-mail: burrell@lanl.gov
b Institute of Fundamental Sciences, Massey University, Private Bag 11222, Palmerston North,
New Zealand
Received 15th October 2003, Accepted 20th November 2003
First published as an Advance Article on the web 12th December 2003
A new series of meso-substituted diaryl free-base and metalloporphyrins have been prepared. Each arene has been
substituted with both a methyl group in the ortho position and a formyl group in the meta position. Rotation of the
arene units is prevented at room temperature due to the steric restrictions imposed by the flanking methyl groups at
the porphyrin β-pyrrolic positions on the methyl groups at the ortho position on the meso-substituted arene unit.
This allowed the αα and αβ atropisomers of this porphyrin to be separated and characterised. X-Ray crystallographic
determination of the structure of the free-base porphyrin revealed a very flat porphyrin core. Metallation resulted
in the isolation and characterisation of the nickel, zinc and copper derivatives. The assignments of the αα and
αβ isomers are confirmed by X-ray crystallographic determination of the structures of the Cu() analogues.
The copper αα structure exhibits a very twisted porphyrin core, the copper αβ structure is also distorted, but to
a lesser degree. The activation energy for rotation has been calculated for each of the 2H, Ni and Zn derivatives.
The energy required to rotate the arene ring increases in the order Ni < Zn ∼ 2H. No significant difference in the
free energy of rotation was observed between experiments carried out with the αα and αβ isomers.
it was easier to rotate the phenyl rings of the dicationic form of
Introduction
the ortho-methoxy substituted arene porphyrin than those of
Atropisomerism arises when steric hindrance caused by
the corresponding free base, as determined by the average free
adjacent atoms prevents an otherwise unhindered single bond
from rotating. Atropisomerism is important in porphyrin chem-
istry, as the room temperature isomers (usually ortho-substi-
tuted tetraarenes) are locked in place, which allows a degree of
energy of rotation.8 A similar effect was found by Zimmer et al.
for their tetra-catechol substituted porphyrin.11 A theoretical
study of aryl ring rotation in arylporphyrins has been con-
ducted by Okuno and co-workers.18 They examined the
geometric control to be exerted on the porphyrin substituents.
atropisomerism of a mono-arene substituted porphyrin and
In the mid 1970’s, Collman and co-workers utilised the α4 atro-
pisomer of ortho-substituted meso-tetraphenylporphyrin as a
structural scaffold to create picket-fence porphyrins, which
concluded that a considerable deformation of the porphyrin
ring in the transition-state region was required for aryl ring
rotation.18
acted as biological mimics for dioxygen-binding hemoproteins.1
Almost all these studies of porpyrin atropisomerism have
More recently, others have utilised porphyrin atropisomers in
concentrated on tetraaryl-substituted porphyrins. The simpler
applications as diverse as catalysis and molecular recognition,2
diaryl systems have been largely overlooked.12 The removal of
as well as continuing to explore their uses as mimics for bio-
two adjacent aryl rings from the meso positions changes the
logical systems.3,4 The ability to control substituents orthogonal
rigidity of the porphyrin core and this is likely to reduce the
to the porphyrin plane has also been exploited in the area of
energy required to deform the porphyrin core and, hence, rotate
supramolecular chemistry.5
the substituted arene ring. To date, little information has been
Atropisomerism in porphyrins with meso-aryl substituents
reported on the energies associated with isomer interconversion
was first described by Gottwald and Ullman.6 Since this first
of the 5,15-di-meso substituted arene analogues.13,15,19
account, atropisomerism has been seen in tetra-,3,7–11 di-12–15
We have recently synthesised a new di-meso substituted
arylporphyrin building block for use in the synthesis of larger
porphyrin arrays. This has provided us with an opportunity to
investigate the atropisomerism of this class of porphyrin.
and mono-substituted porphyrin16 systems containing meso-
aryl and -alkyl substituents,16 both with hydrogen and larger
flanking β-pyrrolic substituents. For porphyrins that are meso-
arene substituted, atropisomerism usually arises due to ortho
substitution on the arene ring. However, in at least a few cases,
Results and discussion
suitably bulky substituents have led to atropisomerism of
meta-arene substituted porphyrins.17
Porphyrin synthesis
A handful of studies have been undertaken to identify the
factors contributing to the ease of arene rotation in porphyrin
atropisomers. In the first comprehensive study of atropisomer-
ism of tetraaryl-substituted porphyrins, Freitag and Whitten
examined both thermally induced and photoinduced atropi-
somerism of a series of picket-fence porphyrins and demon-
strated that a correlation exists between non-polar core
distortions and the atropisomerisation rates.7 For example,
introduction of a metal ion into the porphyrin core resulted in a
rigid porphyrin backbone, as seen by an increase in the relative
energy required for atropisomer interconversion. These results
have been mirrored in other systems.10,13,16 Gust et al. found that
The diaryl-substituted porphyrins were constructed via a five-
step synthetic strategy, starting from 4-methylbenzaldehyde
(Scheme 1). By using dichloromethane as the reaction medium
instead of the reported 1,2-dichloroethane,20 a substantial
increase in the yield of 1 was obtained (89 vs. 63%). Protection
of benzaldehyde 1 to give 2 was achieved quantitatively using
2,2-dimethyl-1,3-propanediol in the presence of p-toluene-
t
sulfonic acid (p-TsOH). Substitution of 2 using BuLi and
DMF at Ϫ78 ЊC and subsequent hydrolysis produced 3
as a light yellow peach-scented oil. This was purified via
column chromatography before the porphyrin condensation
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 1 9 – 3 2 6
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Scheme 1 Reagents and conditions: (i) AlCl3, CH2Cl2, Br2, argon; (ii) HOCH2C(CH3)2CH2OH, p-TsOH, C6H6, reflux, 18 h; (iii) tBuLi, THF, Ϫ78 ЊC,
DMF, argon, HCl(aq); (iv) 3,3Ј-di-n-butyl-4,4Ј-dimethyl-2,2Ј-dipyrrolylmethane, CH2Cl2, TFA, DBU, p-chloranil; (v) CHCl3–TFA–H2O, rt, 1 h,
argon; (vi) chromatography.
was attempted. The diaryl-substituted porphyrin 4 was formed
via an acid-catalysed condensation, using trifluoroacetic acid
(TFA). Initial attempts to separate the resulting atropisomers
failed, as the difference in polarity was insufficient to allow
clean separation. However, once the diacetal groups were
deprotected, the differences in the polarity of porphyrins 6-H2
and 7-H2 was sufficient as to allow careful separation via
chromatography. The overall yield for a multi-gram synthesis of
the isomeric mixture of porphyrins was 32%.
Separation and identification of the atropisomers
The diphenylporphyrins with ortho-methyl substituents exist as
atropisomeric mixtures, due to the restricted rotation of the
phenyl rings. Careful column chromatography on silica gel (1 : 1
CH2Cl2–hexane doped with 0–0.5% diethyl ether) resulted
in the isolation of the αβ isomer, 6-H2, followed by the αα
isomer, 7-H2 (1 : 1 CH2Cl2–hexane doped with 1% diethyl
ether). The relative stereochemistry of the two free-base iso-
mers was initially assigned on the basis of polarity, with the less
polar αβ isomer eluting first. The assignment was further con-
firmed by examining the 1H NMR signal corresponding to the
protons on the α-carbon of the n-butyl groups in the 2, 8, 12
and 18 positions of the porphyrin skeleton. For the symmetric-
ally equivalent αα isomer, it is expected the chemical shifts of
the signals corresponding to the protons on the α-carbon will
be equivalent, resulting in the observation of a simple 8H trip-
let. This was indeed observed in the 1H NMR of the more polar
isomer. The signals corresponding to the α-carbon protons in
the αβ isomer were more complex, with the observation of two
closely overlapping triplets due the diastereotopic nature of the
CH2 groups.
Fig. 1 Solid-state structure of 6-H2. The formyl group in the meta-aryl
Solid-state structures
position is 50% disordered over both sites (for clarity, only one is
shown). The H atoms and n-butyl disorder have been removed for
clarity. Atoms labelled with an A are related by symmetry (1/2 Ϫ x,
3/2 Ϫ y, Ϫz) to those with the same number.
Conclusive assignment of the geometries was completed by
single crystal X-ray examination of the porphyrins and their
metallated derivatives. The molecular structure of 6-H2 is
shown in Fig. 1, with structural parameters listed in Table 1.
The asymmetric unit contains half of the molecule, which lies
across a crystallographic inversion centre. This crystallographic
symmetry imposes an αβ symmetry on the molecule, which
allows us to assign this porphyrin as the αβ atropisomer. How-
ever, the presence of disorder in the structure with the methyl
groups and the aldehydes (50 : 50 occupation factors), makes
unequivocal assignment of the atropisomer impossible.
each aryl ring is almost perpendicular [86.0(4) and 89.3(4)Њ for
molecules I and II, respectively] with respect to the plane of
the porphyrin.
Preparation of both copper complexes enabled the conclu-
sive assignment of the relative atropisomers. Crystals of 6-Cu
and 7-Cu suitable for X-ray diffraction were grown by the slow
evaporation of concentrated CHCl3–MeOH solutions contain-
ing the appropriate atropisomer. Diffuse solvent equivalent to
three chloroform molecules (343 e per cell) per unit cell was
treated in the manner described by van der Sluis and Spek21 for
7-Cu. The molecular structure of 6-Cu is shown in Fig. 2, with
structural parameters listed in Table 1. Both of these structures
The 24 atoms of the porphyrin macrocycle all lie within
0.05 Å of their least-squares plane. The aryl groups are slightly
tilted away from the porphyrin core by angles of 173.30 and
176.95Њ (for molecules I and II, respectively). The plane of
D a l t o n T r a n s . , 2 0 0 4 , 3 1 9 – 3 2 6
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Table 1 X-Ray parameters for the porphyrins 6-H2, 6-Cu and 7-Cu
6-H2
6-Cu
7-Cu
Empirical formula
Formula wt.
T /K
C28H33N2Oa
827.11
150
C56H64N4O2Cu
888.65
150
C56H64N4O2Cu
888.65
150
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
Monoclinic
C2/c
32.2970(7)
11.8287(3)
13.4306(3)
90
111.321(1)
90
8
Monoclinic
P2(1)/c
27.2070(6)
10.8242(2)
15.8465(3)
90
93.121(1)
90
4
Triclinic
P1
¯
17.8253(5)
18.6667(5)
31.1012(7)
85.070(1)
77.896(1)
89.020(1)
8
Z
U/Å3
4779.7(2)
4201
4659.8(2)
9777
10081.1(5)
85186
Reflections observed
Independent reflections
Rint
R1
2781
6955
35333
0.0767
0.0866
0.2598
0.0454
0.0938
0.2685
0.0447
0.0493
0.1334
wR2
a The molecular formula for 6-H2 is C56H66N4O2.
respect to the plane of the porphyrin. There are four independ-
ent molecules of 7-Cu in the asymmetric unit, three of which
are considerably more distorted/ruffled than 6-Cu (Fig. 3). The
porphyrin core atoms of molecules A, C and D correlate
reasonably well with one another (Fig. 4), (<0.17 correlation
error for the 24 core porphyrin atoms), the porphyrin core
atoms of B have poor correlations with the other three mole-
cules (>0.40), but correlate reasonably with the porphyrin core
atoms of 6-Cu (∼0.19). The 24 core atoms of the three mole-
cules A, C and D protrude by as much as 0.468 Å from their
least-squares plane. However, the Cu2ϩ atoms associated with
these molecules still sit within this plane (Ϫ0.044–0.065 Å).
Each arene ring is less tilted above and below the porphyrin
core as compared with 6-Cu (average angle of 174.8Њ). The dis-
tortion in the porphyrin core is further evidenced by the large
Fig. 2 Solid-state structure of 6-Cu, The H atoms and n-butyl
disorder have been removed for clarity.
enable conclusive assignment of the atropisomers, which
matches our predictions from the NMR data and the relative
polarity of the atropisomers.
The 24 atoms of the porphyrin macrocycle in 6-Cu are more
ruffled compared to 6-H2, with deviations of up to 0.15 Å
(cf. 0.05 Å for the free base) from their least-squares plane and
the Cu2ϩ atom sitting essentially within this plane (0.01 Å). The
aryl groups are tilted above and below on either side from the
porphyrin core by angles of 172.2 and 170.5Њ, more obtuse than
for the free-base equivalent. The slight distortion in the por-
phyrin core is further evidenced in the β-pyrrolic methylene
groups, which are further above and below the porphyrin plane
(Ϫ0.14 to ϩ0.28 Å) than in 6-H2. The planes of each aryl ring
are again almost perpendicular [86.0(2) and 85.2(2)Њ] with
Fig. 3 Solid-state structure of one of four independent molecules of 7-
Cu in the asymmetric unit. The H atoms have been removed for clarity.
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Table
2
Activation parameters and atropisomerisation rates of
the porphyrins studied. Equation used as obtained from Laidler and
Meiser;22 parameters have their usual meaningsa
T /K
K/sϪ1
∆G‡ b/kJ molϪ1
6-H2
6-Zn
6-Ni
7-Ni
409
401
394
386
379
3.5171 × 10Ϫ5
1.1816 × 10Ϫ5
6.9339 × 10Ϫ6
2.6557 × 10Ϫ6
1.1917 × 10Ϫ6
136
137
136
137
137
Average ∆G‡ = 137 kJ molϪ1
∆H‡ c = 137 kJ molϪ1
∆S‡ d = 2 J molϪ1 KϪ1
409
401
394
386
379
5.6637 × 10Ϫ6
3.0588 × 10Ϫ6
2.1933 × 10Ϫ6
7.9347 × 10Ϫ7
4.5804 × 10Ϫ7
142
142
140
141
142
Average ∆G‡ = 141 kJ molϪ1
Fig. 4 An overlay of two of the four independent porphyrins of 7-Cu
(A and C) showing the close correlation between the 24 core atoms of
each porphyrin.
∆H‡ c = 105 kJ molϪ1
∆S‡ d = Ϫ90 J molϪ1 KϪ1
409
394
379
365
352
4.4629 × 10Ϫ4
1.0257 × 10Ϫ4
3.2982 × 10Ϫ5
8.2428 × 10Ϫ6
2.0204 × 10Ϫ7
128
127
126
125
125
displacements from the 24-atom porphyrin plane of the methyl-
ene groups attached at the β-pyrrolic positions of the porphyrin
(average displacement 0.55 Å, maximum displacement 0.92 Å).
The angles between the planes between the aryl groups and the
porphyrin cores are now less acute, resulting in further twisting
of the arene rings away from perpendicular [typical angles
range from 75.1(6) and 87.5(6)Њ] with respect to the plane of the
porphyrin. The overall visual effect for these three porphyrins
(A, C and D) is that it appears that the porphyrin core has been
twisted along the C5–C15 axis (Fig. 3). The core atoms of the
fourth independent molecule in the asymmetric unit (B) sit
flatter than those of the other three molecules, resulting in a
less twisted appearance, more akin to the core atoms of 6-Cu
(Fig. 2). The planes of the aryl groups are closer to perpen-
dicular [81.6(4) and 88.3(5)Њ] with respect to the plane of the
porphyrin. The methylene groups attached to the porphyrin
β-pyrrolic positions are less out of plane (0.42 Å) than their
more twisted counterparts, yet, surprisingly, the arene rings
are further above and below the porphyrin core plane then in
molecules A, C and D.
Average ∆G‡ = 126 kJ molϪ1
∆H‡ c = 108 kJ molϪ1
∆S‡ d = Ϫ49 J molϪ1 KϪ1
394
379
365
3.55213 × 10Ϫ4
1.17929 × 10Ϫ4
2.63703 × 10Ϫ5
122
122
123
Average ∆G‡ = 122 kJ molϪ1
∆H‡ c = 105 kJ molϪ1
∆S‡ d = Ϫ45 J molϪ1 KϪ1
a Uncertainty in measurement for compounds under study 2.5%.
b Calculated from ∆G‡ = RT[ln k Ϫ ln(κT/ប)]. c Calculated from an
Arrenhius plot. d Calculated from ∆S‡ = R[ln A Ϫ 1 Ϫ ln(κT/ប)].
whereas our system has methyl substituents. It has also been
postulated that diarylporphyrin skeletons are more flexible than
their tetraaryl analogues.15 It appears that in our work, both
factors effectively cancel each other out. The reasons for the
substantial increase in the free energy of activation for rotation
of 6-H2 as compared to the porphyrins of Young and Chang
are currently unclear. All of these porphyrins are diaryl with
flanking β-pyrrolic methyl substituents, yet the difference in ∆G
between the two systems is ∼20%. The relative sizes of the ortho
substituents will contribute to some of this difference, as will
inductive interactions caused by the addition of the meta-
substituted formyl group (in the present examples). It has been
shown that the addition of para substituents affects the free
energy of activation for rotation in free-base and metallated
Kinetic studies of the isomerisation
Atropisomerism was expected in these porphyrins due to the
steric hindrance imposed by the presence of ortho-methyl sub-
stituents on the aryl rings. Rates of atropisomerisation were
determined for 6-H2, 6-Zn and 6-Ni in toluene. The rate of
atropisomerisation for 7-Ni was also determined. The rate con-
stants at various temperatures and the activation parameters
for the atropisomerisation of 6-X (where X = H2, Ni and Zn) to
a mixture of 6-X and 7-X are shown in Table 2, and are com-
pared with those of selected literature examples in Table 3.
These results indicate that the nickel complexes have lower acti-
vation barriers to rotation than both the free-base and zinc
porphyrins. The free energy of activation for rotation of the
phenyl ring increases in the following order: 7-Ni ≈ 6-Ni < 6-Zn
≈ 6-H2. A similar trend has been observed for ortho-substituted
tetraphenylporphyrins.7 The ∆G‡ values for 6-Ni and 7-Ni lie
within experimental error, despite a three-fold difference in
their atropisomerisation rates. The free energy of activation for
rotation of the phenyl ring of the free-base porphyrin 6-H2 is
similar in magnitude to those for the related ortho-phenylene
substituted (picket fence) tetraphenylporphyrins of Freitag and
Whitten,7 yet is larger than the energies for the diarylporphyrins
of Young and Chang.15 The rates of interconversion of the
studied diarylporphyrins are also slower than those observed by
Young and Chang15 and by Redman and Sanders.13 The picket
fence tetraarylporphyrins have protons on the flanking pyrroles,
tetraarylporphyrins by as much as 9.6 kJ molϪ1 8,23
A similar
.
effect may also be present for meta-substituted arylporphyrins.
The 6-Zn porphyrin has a free energy of activation for
rotation similar in magnitude to that of 6-H2, but significantly
larger than for the 6-Ni or 7-Ni porphyrins. Previous authors7
have attributed this to the porphyrin core becoming rigid upon
coordination of zinc, which locks the porphyrin core into a flat
configuration, thus increasing the energy barrier to rotation.
Zinc porphyrins also have a propensity to acquire a fifth ligand
in an axial position, whether it be solvent or other coordination
ligands. It is possible that this ligation has a significant effect on
the flexibility of the porphyrin core. This effect has been seen
with other axially ligated metalloporphyrins.23 In our examples,
there is no external ligand, nor is there any evidence of co-
1
ordinated solvent in their H NMR spectra. Potentially, the
porphyrin complex could also dimerise, but again, no evidence
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(an upfield shift in the CHO NMR signal, for example) was
observed in our spectra. Both of these factors may contribute
to the observed differences in the rate of atropisomerism in zinc
porphyrins in other systems. A more plausible explanation of
the difference in the entropy of activation between the nickel
and zinc derivatives lies in the much smaller ionic radius of
Ni(), which leads to strongly ruffled porphyrin conformations
in order to shorten the Ni–N(porphyrin) separation. Zn(), on
the other hand, forces the central hole of a completely planar
porphyrin dianion to expand slightly (from 2.015 to 2.035 Å),
leaving a rather more flexible porphyrin. Relative to Zn() por-
phyrins, the Ni() analogue has lower entropy (more ordered)
and, hence, a smaller, less unfavourable entropy change to the
highly ordered transition state. For the nickel and zinc deriv-
atives, the enthalpy of activation is very similar, leading to the
marked difference observed in the free energy of activation
between them. The free-base porphyrin, on the other hand (see
Table 2), has a noticeably higher enthalpy of activation coupled
with a negligible entropy of activation. This is consistent with
an activated complex in which the internal pyrrolic N–H ؒ ؒ ؒ N
hydrogen bonds are weakened through distortion of the
porphyrin (contribution to ∆H‡), which acquires additional
flexibility (whence ∆S‡ ≈ 0). A relatively small ∆S‡ is also seen
for free-base tetraaryl-substituted porphyrins with non-bulky
ortho substituents.7 Further work is underway to elaborate the
factors that influence the rate of atropisomerism in metallo-
porphyrins.
Conclusions
A new series of meso-substituted diaryl free-base and metallo-
porphyrins have been prepared. Each arene has been substi-
tuted with both a methyl group in the ortho position and a
formyl group in the adjacent meta position. Rotation of the
arene units is prevented at room temperature by the steric
restrictions imposed by the presence of flanking methyl groups
at the β-pyrrolic positions on the methyl groups. This allowed
the αα and αβ atropisomers of this porphyrin to be separated
and characterised. X-Ray crystallographic determination of the
free-base porphyrin reveals a very flat porphyrin core. Metal-
lation of 6-H2 provided nickel, zinc and copper derivatives. The
assignments of αα and αβ atropisomers were confirmed by the
X-ray crystallographic determination of the Cu() analogues.
The copper αα structure exhibits a very twisted porphyrin
skeleton; the copper αβ structure is similar but less distorted.
The entropic and enthalpic contributions to the free energy of
rotation of the arene rings have been determined for each of the
H2, Ni() and Zn() derivatives. The free energy of activation
for rotation of the arene ring increased in the order Ni < Zn ≈
H2. The origin of the differences between the Ni and Zn deriv-
atives most likely lies in the entropy of activation; for the free
base, a significantly higher enthalpy of activation and much less
negative entropy of activation was observed compared to the
metallated derivatives. Further work is needed to assess the con-
tributing factors. A three-fold difference in atropisomerisation
rates between the Ni() αα and αβ isomers did not lead to a
significant difference in their free energy of rotation.
Experimental
Materials and methods
Melting points were recorded on a Kofler hot stage and are
uncorrected. Microanalyses were performed by the Campbell
Microanalytical Laboratory, University of Otago. 1H NMR
spectra were measured with a Bruker Avance A400 spectro-
meter operating at 400.13 MHz. Chemical shifts were deter-
mined with respect to the NMR solvent. Variable temperature
NMR measurements were performed with a VT BVT 3300
unit. Temperature calibration over the range 298 to 419 K was
D a l t o n T r a n s . , 2 0 0 4 , 3 1 9 – 3 2 6
323

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achieved with an ethylene glycol/DMSO sample yielding the
solution (10 ml) was added and the resulting suspension was
stirred (5 min). The aqueous layer was extracted twice with
CH2Cl2. The combined organic layers were washed with water,
dried (MgSO4) and reduced in vacuo to give the product as a
yellow oil. Purification was achieved via flash column chrom-
atography (9 × 5 cm, 1 : 1 CH2Cl2–hexane) to give 3 as a pale
yellow oil. (5.62 g, 68%). Found: C, 71.04; H, 7.48; C14H18O3
requires C, 71.77; H, 7.74%. δH (400 MHz, CDCl3): 0.78 (3H, s,
CH3), 1.26 (3H, s, CH3), 2.63 (3H, s, PhCH3), 3.64 (2H, d,
J 11.2, CH2), 3.75 (2H, d, J 11.2, CH2), 5.40 (1H, s, CH), 7.24
(1H, d, J 7.8 Ph), 7.59 (1H, dd, J 7.8 and 1.9 Ph), 7.91 (1H, d,
J 1.9 Ph), 10.23 (1H, s, CHO). EI-MS: m/z 234 (Mϩ, 45), 147
(Mϩ Ϫ 87, 100%).
Ϫ4
2
following equation Tactual = Ϫ7.2663 × 10
T
ϩ 1.5899Tset
set
Ϫ110.3. FAB-MS spectra were recorded using a Varian VG-
250S double focusing magnetic sector mass spectrometer. Sam-
ples were supported in a p-nitrobenzyl alcohol matrix. Electron
impact (EI) mass spectra were recorded on an AEI MS 902
spectrometer at 70 eV. MALDI MS spectra were recorded using
a Micromass MALDI (TOF) – Reflectron mass spectrometer
operating a nitrogen UV laser at 337nm wavelength, with a dual
micro-channel plate detector. Samples were supported in a
p-nitrobenzyl alcohol matrix. Column chromatography was
performed using Merck silica gel 60 Type 9835 (40–60 µm).
Analytical thin layer chromatography was run on Merck silica
gel 60 F254 pre-coated sheets (0.2 mm). Typical elutents for TLC
were dichloromethane, hexane and methanol. Where solvent
mixtures are used, proportions are given by volume. All
solvents were AR grade which, were either used as received
(methanol) or dried and distilled prior to use (dichloro-
methane).
Preparation of 4. To a degassed solution of 3,3Ј-di-n-butyl-
4,4Ј-dimethyl-2,2Ј-dipyrrolmethane (1.284 g, 4.482 mmol) and
3 (1.00 g, 4.268 mmol) in CH2Cl2 (430 ml) was added TFA
(0.329 ml, 4.268 mmol). The resulting deep red solution
was stirred for 45 min in the dark, after which time DBU
(0.638 ml, 4.268 mmol) was added, followed a few minutes later
by p-chloranil (2.623 g, 10.67 mmol). The mixture was then
refluxed for 3.5 h in darkness. Upon cooling, Et3N (2.4 ml)
was added and the reaction mixture was reduced in volume
(100 ml). An equal volume of MeOH was then added and the
reaction was left to sit at 0 ЊC overnight. The intensely coloured
product 4 was isolated via filtration as an inseparable mixture
of atropisomers (1.357 g, 63.6%). δH (400 MHz, CDCl3): Ϫ2.38
(2H, br s, NH), 0.77 (6H, s, CH3), 1.07 (12H, t, J 7.3, CH2CH3),
1.31 (6H, s, CH3), 1.72 (8H, m, J 7.3, CH2CH2CH3), 1.96 (5.4H,
s, αα-PhCH3), 2.03 (6.6H, s, αβ-PhCH3), 2.15 (8H, m, J 7.3,
CH2CH2CH2), 2.44 (12H, s, CH3), 3.66 (4H, d, J 10.8, CH2),
3.80 (4H, d, J 10.8, CH2), 3.96 (8H, t, J 7.3, CH2CH2CH2),
5.55 (1.1H, s, αβ-CH), 5.56 (0.9, s, αα-CH), 7.61 (0.9H, d,
J 7.8 αα-Ph), 7.63 (1.1H, d, J 8.1, αα-Ph), 7.87 (2H, d, J 8.1,
αα/αβ-Ph), 7.96 (1.1H, s, αβ-Ph), 8.04 (0.9H, s, αα-Ph), 10.19
(2H, s, meso).†
Synthesis
Preparation of 3-bromo-4-methylbenzaldehyde (1). The pro-
cedure was adapted from that of Eizember and Ammons.20 To a
suspension of AlCl3 (93.34 g, 0.700 mol) in refluxing dichloro-
methane (120 ml) under argon was added 4-methylbenzalde-
hyde (98%, 49.07 g, 0.400 mol) in a dropwise fashion. The reac-
tion mixture was stirred for 30 min at 40 ЊC. Bromine (22.67 ml,
0.440 mol) was then added dropwise over 1 h while maintaining
the reaction temperature between 30–40 ЊC. The resulting solu-
tion was stirred for a further 30 min, then gently poured over a
stirred ice slurry (400 ml). The organic layer was diluted with
CHCl3 (150 ml) and separated. The aqueous layer was washed
twice with CHCl3 (2 × 100 ml) and the organic layers were
combined and concentrated. The organic layer was washed
twice with water, dried (MgSO4) and reduced in vacuo to give a
pale yellow oil that, on standing overnight, yielded colourless
crystals of 1. These were filtered, crushed and dried (70.71 g,
89%). Mp 47 ЊC (from CHCl3; lit.20 48–49 ЊC). Found: C, 48.28;
H, 3.59; Br, 40.37; C8H7BrO requires: C, 48.27; H, 3.54; Br,
40.14%. δH (400 MHz, CDCl3): 2.45 (3H, s, PhCH3), 7.37 (1H,
d, J 7.8, Ph), 7.69 (1H, dd, J 7.8 and 1.1, Ph), 8.00 (1H, d, J 1.1,
Ph), 9.88 (1H, s, CHO). EI-MS: m/z 199 (Mϩ, 100%).
Preparation of 5. To a stirred solution of 4 (1.040 g, 1.04
mmol) in CHCl3 (60 ml) was added water (20 ml) and TFA (60
ml). The biphasic mixture was left to stir under argon for 1 h.
The aqueous and organic layers were diluted (an additional 100
ml in each) and the organic layer was separated. The aqueous
layer was washed with CHCl3 (2 × 100 ml). The organic layers
were combined and washed with aqueous NaHCO3 until clear
(3 × 150 ml). The solution was dried (MgSO4) and evaporated
in vacuo to give 5 as a mixture of atropisomers.
Preparation of 1-bromo-2-methyl-5-(4,4-dimethyl-2,6-dioxan-
1-yl)benzene (2). A solution of 1 (60.0 g, 0.301 mol), 2,2-di-
methylpropane-1,3-diol (34.5 g, 0.331 mol) and p-toluene-
sulfonic acid (5.7 g, 30.1 mmol) in dry benzene (900 ml) was
refluxed for 18 h using a Dean–Stark apparatus. Upon cooling,
triethylamine (4.2 ml, 30.1 mmol) was added and the solution
was reduced to dryness. The residue was purified by dissolving
in CH2Cl2 (500 ml) and washing with water (2 × 200 ml). The
organic layer was separated and dried (MgSO4). The solvent
was removed to give 2 as a white powder (82.05 g, 96%). Mp
59 ЊC (CH2Cl2). Found: C, 54.84; H, 5.93; Br, 27.11; C13H17-
BrO2 requires: C, 54.84; H, 6.01; Br, 28.02%. δH (400 MHz,
CDCl3): 0.78 (3H, s, CH3), 1.26 (3H, s, CH3), 2.36 (3H, s,
PhCH3), 3.61 (2H, d, J 11.0, CH2), 3.74 (2H, d, J 11.0, CH2),
5.31 (1H, s, CH), 7.20 (1H, d, J 7.9, Ph), 7.30 (1H, dd, J 7.9 and
1.5, Ph), 7.67 (1H, d, J 1.5, Ph). EI-MS: m/z 285 (Mϩ, 50), 199
(Mϩ Ϫ 86, 100%).
Separation of the atropisomers 6-H2 and 7-H2. The atropi-
somers were separated via column chromatography (initial
loading: 3.5 cm × 11 cm, Merck 60 silica gel, 1 : 1 CH2Cl2–
hexane). Isomer 6-H2 was separated first (0–0.5% diethyl ether,
1 : 1 CH2Cl2–hexane) followed by isomer 7 (CH2Cl2, 1% diethyl
ether). 6-H2: δH (400 MHz, CDCl3): Ϫ2.35 (2H, br s, NH), 1.09
(12H, t, J 7.6, CH2CH3), 1.24 (12H, s, CH3), 1.73 (8H, m, J 7.6,
CH2CH2CH3), 2.13 (6H, s, PhCH3), 2.16 (8H, m, J 7.6, CH2-
CH2CH2), 2.41 (12H, s, CH3), 3.98 (8H, t, J 7.6, CH2CH2CH2),
7.80 (2H, d, J 8.0, Ph), 8.25 (2H, d of d, 1J 8.0, 3J 1.6, Ph), 8.42
3
(2H, d, J 1.6, Ph), 10.20 (2H, s, meso), 10.24 (2H, s, CHO).
δH (400 MHz, CD3C6D5): Ϫ1.58 (2H, br s, NH), 1.11 (12H, t,
J 7.6, CH2CH3), 1.77 (8H, m, J 7.6, CH2CH2CH3), 1.87 (6H, s,
CH3), 2.23 (8H, m, J 7.6, CH2CH2CH2), 2.44 (12H, s, CH3),
3.98 (8H, t, J 7.6, CH2CH2CH2), 7.31 (2H, d, J 8.0, Ph), 8.07
(2H, d of d, 1J 8.0, 3J 1.2, Ph), 8.07 (2H, d, 3J 1.2, Ph), 9.88 (2H,
s, meso), 10.44 (2H, s, CHO). UV-vis (CHCl3) λmax/nm (ε/103
dm3 molϪ1 cmϪ1): 434 (445.4), 536 sh (3.6), 574 (23.0), 618 (7.7).
EI-MS: m/z 827.7 (Mϩ, 100%). MALDI-TOF MS: m/z 827.3
Preparation of 2-methyl-5-(4,4-dimethyl-2,6-dioxan-1-yl)-
benzaldehyde (3). A 1.5 M solution of tBuLi in pentane (52 ml,
77.5 mmol) was slowly (over 1 h) added to a degassed solution
of 2 (10.05 g, 35.2 mmol) in dry THF (200 ml) at Ϫ78 ЊC under
argon. After 20 min at Ϫ78 ЊC, the solution was allowed to
warm to 0 ЊC over 1 h, then cooled again to Ϫ78 ЊC. Dry DMF
(6 ml, 77.5 mmol) was then added and the resulting mixture
allowed to warm to 0 ЊC (over 50 min). An aqueous 1 M HCl
† NMR assignments for the αα and αβ isomers of the free base (H2)
diacetal porphyrin were achieved by working back from the purified
dialdehyde αα and αβ isomers.
D a l t o n T r a n s . , 2 0 0 4 , 3 1 9 – 3 2 6
324

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(Mϩ, 100%). 7-H2: δH (400 MHz, CDCl3): Ϫ2.29 (2H, br s, NH),
1.11 (12H, t, J 7.6, CH2CH3), 1.27 (12H, s, CH3), 1.76 (8H, m,
J 7.6, CH2CH2CH3), 2.09 (6H, s, PhCH3), 2.19 (8H, m, J 7.6,
CH2CH2CH2), 2.44 (12H, s, CH3), 4.01 (8H, m, J 7.6,
CH2CH2CH2), 7.80 (2H, d, J 8.0, Ph), 8.27 (2H, d of d, 1J 8.0,
(2H, d of d, 1J 8.0, 3J 1.6, Ph), 8.03 (2H, d, 3J 1.6, Ph), 9.62 (2H,
s, CHO), 9.90 (2H, s, meso). UV-vis (CHCl3) λmax/nm (ε/103 dm3
molϪ1 cmϪ1): 324 (18.4), 410 (186.5), 531 (11.4), 566 (16.8).
MALDI-TOF MS: m/z 882.3 (Mϩ, 100%).
3
3J 1.6, Ph), 8.50 (2H, d, J 1.6, Ph), 10.23 (2H, s, meso), 10.28
6-Cu and 7-Cu. Procedure as for nickel. 6-Cu (αβ isomer):
UV-vis (CHCl3) λmax/nm (ε/103 dm3 molϪ1 cmϪ1): 334 (22.1), 411
(304.5), 534 (15.1), 570 (14.0). MALDI-TOF MS: m/z 888.3
(Mϩ, 100%). 7-Cu (αα isomer): UV-vis (CHCl3) λmax/nm (ε/103
dm3 molϪ1 cmϪ1): 332 (4.9), 411 (277.1), 534 (12.6), 570 (11.9).
MALDI-TOF MS: m/z 888.3 (Mϩ, 100%).
(2H, s, CHO). δH (400 MHz, CD3C6D5): Ϫ1.59 (2H, br s, NH),
1.10 (12H, t, J 7.6, CH2CH3), 1.76 (8H, m, J 7.6, CH2CH2CH3)
1.91 (6H, s, CH3), 2.23 (8H, m, J 7.6, CH2CH2CH2), 2.44 (12H,
s, CH3), 3.98 (8H, t, J 7.6, CH2CH2CH2), 7.32 (2H, d, J 8.0, Ph),
1
8.07 (2H, d, J 8.0, Ph), 8.07 (2H, s, Ph), 9.89 (2H, s, meso),
10.42 (2H, s, CHO). UV-vis (CHCl3) λmax/nm (ε/103 dm3 molϪ1
cmϪ1): 434 (177.4), 538 sh (1.6), 574 (9.4), 618 (2.9). MALDI-
TOF MS: m/z 827.5 (Mϩ, 100%).
Kinetic studies
The porphyrin to be studied was dissolved into d8 toluene (typi-
cal concentration 5 mmol lϪ1) and placed into a pressure NMR
tube which was then sealed. The sample was heated at a fixed
temperature for a set period of time. Accurate proton inte-
grations could not be determined from the 1H spectrum of the
atropisomers at elevated temperatures due to the overlap of
signals of interest; therefore, the reaction was quenched by co-
oling the sample rapidly to room temperature and the 1H NMR
spectrum was recorded. Rates were determined from a plot of
change in concentration of reactant against time, which follows
first-order kinetics in the initial stages of the interconversion.
Rates were determined from extrapolation of several data
points. Typically, 10–15 data points were used for the majority
of temperatures (at the very highest temperature, the rate
of reaction was such that only 3 points could be obtained).
Enthalpy values were determined from an Arrenhius plot over
five different temperatures (three for 7-Ni).
6-Zn and 7-Zn. To 6-H2 or 7-H2 (0.030 g, 0.036 mmol)
dissolved in CH2Cl2 (20 ml) was added Zn(CH3COO)2ؒ2H2O
(0.048 g, 0.217 mmol) dissolved in MeOH (5 ml). The reaction
mixture was stirred for 1 h, then reduced to dryness. The residue
was dissolved in CH2Cl2 (10 ml) and then filtered through a
silica plug to give 6-Zn or 7-H2 (0.032 g, 100%). 6-Zn (αβ iso-
mer): δH (400 MHz, CDCl3): 1.10 (12H, t, J 7.6, CH2CH3), 1.75
(8H, m, J 7.6, CH2CH2CH3), 2.09 (6H, s, CH3), 2.18 (8H, m,
J 7.6, CH2CH2CH2), 2.43 (12H, s, CH3), 4.00 (8H, m, J 7.6,
CH2CH2CH2), 7.79 (2H, d, J 7.6, Ph), 8.27 (2H, d of d, 1J 7.6,
3
3J 1.6, Ph), 8.49 (2H, d, J 1.6, Ph), 10.22 (2H, s, meso) 10.27
(2H, s, CHO). δH (400 MHz, CD3C6D5): 1.14 (12H, t, J 7.6,
CH2CH3), 1.79 (8H, m, J 7.6, CH2CH2CH3), 2.01 (6H, s, CH3),
2.26 (8H, m, J 7.6, CH2CH2CH2), 2.47 (12H, s, CH3), 4.01 (8H,
t, J 7.6, CH2CH2CH2), 7.38 (2H, d, J 8.0, Ph), 8.07 (2H, d of d,
3
3
1J 8.0, J 1.6, Ph), 8.30 (2H, d, J 1.6, Ph), 9.89 (2H, s, meso),
10.36 (2H, s, CHO). UV-vis (CHCl3) λmax/nm (ε/103dm3 molϪ1
cmϪ1): 349 (20.6), 411 (288), 539 (22.5), 575 (13.9). MALDI-
TOF MS: m/z 888.2 (Mϩ, 100%). 7-Zn (αα isomer): δH (400
MHz, CD3C6D5): 1.14 (12H, t, J 7.6, CH2CH3), 1.79 (8H, m,
J 7.6, CH2CH2CH3) 2.06 (6H, s, CH3), 2.26 (8H, m, J 7.6,
CH2CH2CH2), 2.46 (12H, s, CH3), 4.02 (8H, t, J 7.6, CH2-
CH2CH2) 7.41 (2H, d, J 8.0, Ph), 8.11 (2H, d of d, 1J 8.0, 3J 1.6,
Acknowledgements
The authors would like to thank Pat Edwards for the temper-
ature calibration data and the Marsden fund for support for
P. G. P.
3
Ph), 8.24 (2H, d, J 1.6, Ph), 9.88 (2H, s, meso), 10.35 (2H, s,
References
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mmol) suspended in MeOH was added 6 or 7 (0.137 g, 0.166
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δH (400 MHz, CDCl3): 1.07 (12H, t, J 7.4, CH2CH3), 1.65 (8H,
m, J 7.4, CH2CH2CH3), 2.02 (8H, m, J 7.4, CH2CH2CH2), 2.09
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CH2CH2) 7.69 (2H, d, J 8.0, Ph), 8.16 (2H, d of d, 1J 8.0, 3J 1.6,
3
Ph) 8.25 (2H, d, J 1.6, Ph), 9.47 (2H, s, meso) 10.14 (2H, s,
CHO). δH (400 MHz, CD3C6D5): 1.03 (12H, t, J 7.6, CH2CH3),
1.63 (8H, m, J 7.6, CH2CH2CH3) 1.84 (6H, s, CH3), 2.05 (8H,
m, J 7.6, CH2CH2CH2), 2.18 (12H, s, CH3), 3.66 (8H, t, J 7.6,
CH2CH2CH2), 7.29 (2H, d, J 8.0, Ph), 7.93 (2H, d of d, 1J 8.0,
3J 1.6, Ph), 8.04 (2H, d, 3J 1.6, Ph), 9.61 (2H, s, CHO), 9.91 (2H,
s, meso). UV-vis (CHCl3) λmax/nm (ε/103 dm3 molϪ1 cmϪ1): 350
(13.7), 410 (275.3), 531 (19.5), 566 (26.5). MALDI-TOF MS:
m/z 882.2 (Mϩ, 100%). 7-Ni (αα isomer): δH (400 MHz, CDCl3):
1.05 (12H, t, J 7.2, CH2CH3), 1.63 (8H, m, J 7.2, CH2CH2CH3),
2.00 (6H, s, CH3), 2.00 (8H, m, J 7.2, CH2CH2CH2), 2.16 (12H,
s, CH3), 3.66 (8H, t, J 7.2, CH2CH2CH2), 7.66 (2H, d, J 7.6, Ph),
8.15 (2H, d, J 7.6, Ph), 8.31 (2H, s, Ph), 9.45 (2H, s, CHO),
10.14 (2H, s, meso). δH (400 MHz, CD3C6D5): 1.03 (12H, t,
J 7.6, CH2CH3), 1.63 (8H, m, J 7.6, CH2CH2CH3), 1.84 (6H, s,
CH3), 2.05 (8H, m, J 7.6, CH2CH2CH2), 2.18 (12H, s, CH3),
3.66 (8H, t, J 7.6, CH2CH2CH2), 7.29 (2H, d, J 8.0, Ph), 7.94
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D a l t o n T r a n s . , 2 0 0 4 , 3 1 9 – 3 2 6
326