Metalation and Methyl Group Migration in 21-, 22-, and 23-Methylcarbaporphyrins: Synthesis and Characterization of Palladium(II), Rhodium(I), and Rhodium(III) Derivatives

Article abstract of DOI:10.1021/acs.organomet.8b00863

A rational synthesis of 23-methylcarbaporphyrin has been developed. 3,4-Diethyl-1-methylpyrrole reacted with acetoxymethylpyrrole under acidic conditions to give N-methyltripyrrane, and following cleavage of the ester protective groups, the tripyrrolic in

Full text of DOI:10.1021/acs.organomet.8b00863

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Metalation and Methyl Group Migration in 21‑, 22‑, and 23-
Methylcarbaporphyrins: Synthesis and Characterization of
Palladium(II), Rhodium(I), and Rhodium(III) Derivatives
Alissa N. Latham, Gregory M. Ferrence, and Timothy D. Lash*
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, United States
S
* Supporting Information
ABSTRACT: A rational synthesis of 23-methylcarbaporphyrin has been
developed. 3,4-Diethyl-1-methylpyrrole reacted with acetoxymethylpyrrole
under acidic conditions to give N-methyltripyrrane, and following cleavage of
the ester protective groups, the tripyrrolic intermediate condensed with an
indene dialdehyde in the presence of trifluoroacetic acid to afford the required
N-methylcarbaporphyrin. Reaction with palladium(II) acetate in refluxing
acetonitrile for short time periods gave a 23-methyl palladium(II) complex
but prolonged reaction times afforded a rearranged 21-methyl product. The 23-
methyl complex can be isolated but gradually converts into the 21-methyl
derivative even in the solid state. 21-Methyl- and 22-methylcarbaporphyrins
reacted with [Rh(CO)2Cl]2 to give rhodium(I) complexes. However, when the
23-methylcarbaporphyrin was reacted under the same conditions, a rhodium-
(III) derivative was isolated. This complex incorporates a bridging methylene
unit between the Rh(III) and the 21-carbon and must therefore be formed by a methyl group migration−cyclization process.
INTRODUCTION
■
The porphyrins are among the most versatile of ligands and
form complexes with virtually every metal or metalloid in the
periodic table.1 This versatility is due in part to their NNNN
coordination core which can facilitate square planar complex-
ation, but they also retain sufficient flexibility to allow for other
coordination motifs.1 Core-modified porphyrins, where one or
more of the internal nitrogens have been replaced by other
elements such as O, S, Se, Te, or P, also exhibit interesting
coordination chemistry,2 although these ligands are far less
versatile. Carbaporphyrins (e.g., 1 and 2) and related systems,3
such as N-confused porphyrins 3,4 azuliporphyrins 4,5 and
benziporphyrins 5,6 have carbons in place of one or more of
the core nitrogen atoms (Figure 1). Although these macro-
cycles can only coordinate with a smaller number of metal
cations compared to porphyrins, they readily form stable
organometallic derivatives with a number of late transition
metal ions.7 Azuliporphyrins generally act as dianionic ligands,
forming organometallic complexes with Ni(II),8 Pd(II),8
Pt(II),8 and Ru(II).9 Rh(III) and Ir(III) azuliporphyrins
have also been reported,10,11 but these possess additional
axial carbon ligands. Benziporphyrins are also dianionic
ligands,12 whereas N-confused porphyrins can act as both
dianionic or trianionic ligands.13 True carbaporphyrins such as
1 and 2 are trianionic ligands and have been shown to form
Ag(III),14−16 Au(III),15 Rh(III),17 and Ir(III)17 derivatives.
However, the coordination chemistry can be altered by
replacing one of the remaining nitrogens with oxygen, sulfur,
or selenium. For instance, 23-oxa-, 23-thia- and 23-selena-21-
carbaporphyrins 6a−c reacted with palladium(II) acetate in
Figure 1. Selected examples of carbaporphyrinoid systems.
refluxing acetonitrile to give palladium(II) complexes 7a−c
(Scheme 1).18 Nickel(II) acetate also reacted with 6a or 6b to
afford the related nickel(II) derivatives 8a,b, but when the
reaction was carried out in the presence of air, oxy-
heterocarbaporphyrins 9a,b were isolated instead.18 Reports
of related organometallic complexes such as 1019 and 1120
have also appeared (Figure 2).
In an attempt to alter the coordination chemistry of regular
carbaporphyrins, 2 was reacted with methyl or ethyl iodide and
Received: November 28, 2018
© XXXX American Chemical Society
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initially gave Pd(II) complexes 14a or 14b, but these rapidly
rearranged to give the C-alkyl complexes 15. When the
reaction was carried out for 5 min, N-alkyl derivatives 14a,b
were the major products, but after 30 min, virtually all of the
material had been converted into 15a,b.21 Attempts to purify
14a or 14b by column chromatography were unsuccessful as
partial conversion to 15a,b occurred even at room temper-
ature. The mechanism for this rearrangement is not currently
known. It was suggested that a [3,3]sigmatropic rearrangement
could be involved or that a transient palladium−alkyl complex
was responsible for the transference.21 Similar rearrangements
were subsequently observed for carbaporphyrins without fused
benzo units22 and with naphthocarba[2,3-b]porphyrins.23
Palladium(II)-catalyzed rearrangements of p-benziporphyrins
to give related palladium(II) carbaporphyrins have been
reported,24 and palladium-mediated rearrangements of vacata-
porphyrin25 and some expanded porphyrin systems have been
noted.26 Therefore, reactions of this type appear to have
considerable significance and warrant further investigation. In
order to further probe this chemistry, the synthesis of a 23-
methylcarbaporphyrin 16 was carried out, and the formation of
palladium and rhodium derivatives was investigated.
Scheme 1. Metalation and Oxidation of 23-
Heterocarbaporphyrins18
RESULTS AND DISCUSSION
■
Methyl Group Migration in Palladium(II) Carbapor-
phyrins. Alkylation of carbaporphyrin 2 gave 22-alkyl (major)
and 21-alkyl (minor) products 12 and 13, respectively, but 23-
alkyl derivatives were not observed.21 Investigations into the
metalation of 23-methylcarbaporphyrin 16 were of great
potential interest because this system could not undergo a
simple [3,3]sigmatropic rearrangement to give a C-methyl
palladium complex. Therefore, metalation studies on 16 would
provide insights into the alkyl group migrations previously
observed for 22-alkylcarbaporphyrins. A rational synthesis of
16 was developed using the “3 + 1” version of the MacDonald
condensation (Scheme 3).27 Ethyl 3,4-diethylpyrrole-2-carbox-
ylate (17a) was prepared by reacting 3-acetoxy-4-nitrohexane
with ethyl isocyanoacetate and 2 equiv of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in THF−isopropyl
alcohol (Barton-Zard condensation). The product was isolated
Figure 2. Examples of metalated heterocarbaporphyrins.
potassium carbonate in refluxing acetone.21 The major
products resulting from these reactions were 22-alkylcarbapor-
phyrins 12, although C-alkylated byproducts 13 were also
observed (Scheme 2). Having replaced one of the inner
hydrogens with an alkyl group in 12a,b, it was anticipated that
the porphyrin analogues would now act as dianionic ligands.
Reaction of 12a or 12b with Pd(OAc)2 in refluxing acetonitrile
Scheme 3. MacDonald-Type “3 + 1” Synthesis of a 23-
Methylcarbaporphyrin
Scheme 2. Alkylation and Metalation of a Carbaporphyrin21
B
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in 70% yield, but the NMR spectrum showed that 17a was
contaminated with approximately 3% of the corresponding
isopropyl ester. As the ester group was saponified at a later
stage in the synthesis, 17a was used in this form. Alkylation of
17a with methyl iodide and KOH in DMSO afforded the
corresponding N-methyl derivative 17b (Scheme 4), and
meso-protons as two downfield singlets at 9.67 and 10.02 ppm.
The internal methyl group is strongly shifted upfield to give a
3H singlet at −4.21 ppm, whereas the internal CH afforded a
resonance at −4.86 ppm, and the NH appears as a broad peak
between 0 and −1 ppm. Carbaporphyrin 16 gave a UV−vis
spectrum (Figure 3) that was typical of an aromatic
Scheme 4. Synthesis of a N-Methyltripyrrane
subsequent saponification and decarboxylation with sodium
hydroxide in ethylene glycol at >180 °C generated 3,4-diethyl-
1-methylpyrrole (18). Pyrrole 18 had previously been prepared
from 1,2-dimethylhydrazine using a variation on the Fischer
indole synthesis.28 Tripyrranes can be prepared by condensing
α,α′-diunsubstituted pyrroles with 2 equiv of an acetoxyme-
thylpyrrole (e.g., 19) in the presence of an acid catalyst.29
However, attempts to react 18 with 19 in the presence of
Montmorillonite clay30 gave poor results, and reactions with
acetic acid in ethanol or other alcohol solvents31 also gave low
yields of the desired tripyrrolic product 20a. Superior results
were obtained by refluxing the reactants with acetic acid in
acetonitrile under nitrogen. Unlike N-unsubstituted tripyr-
ranes, tripyrrane 20a failed to precipitate from solution and
had to be purified by column chromatography. Some
tripyrranes decompose during chromatography, but this
problem did not arise in this case, and 20a could be isolated
as an oil that solidified in the freezer. Hydrogenolysis of the
benzyl ester protective groups over 10% palladium charcoal
gave the related dicarboxylic acid 20b.
Figure 3. UV−vis spectra of 23-methylcarbaporphyrin 16 in 1%
triethylamine−dichloromethane (free base, red line) and 1% TFA−
dichloromethane (monocation 16H+, purple line).
porphyrinoid, showing a strong Soret absorption at 439 nm
and Q bands at 538, 567, 624, and 683 nm. Addition of
trifluoroacetic acid (TFA) afforded a monoprotonated species
16H+ that showed a diminished Soret band at 444 nm (Figure
3). Crystals of 16 that were suitable for X-ray crystallographic
analysis were obtained, and the results showed that the pyrrole
ring bearing the internal methyl group was tilted by 24.34(3)°
from the macrocyclic plane, defined by atoms C1, C2, C3, C4,
C5, C6, C7, C8, C9, C10, C11, C14, C15, C16, C17, C18,
C19, C20, and C21 (Figure 4). The remainder of the
N-Alkyl tripyrranes had not previously been applied to the
preparation of porphyrinoid products. However, “2 + 2”
MacDonald condensation of a N-methyldipyrrylmethane with
a dipyrrylmethane dialdehyde had been used to synthesize a N-
methylporphyrin,32 and related porphyrins were prepared from
a,c-biladiene intermediates.33 The presence of an internal
substituent might sterically inhibit cyclization, and given the
instability of tripyrranes under the acidic conditions used to
carry out these reactions, there was a possibility that
decomposition would ensue. Fortunately, when 20b was
reacted with indene dialdehyde 2134 in the presence of TFA,
followed by oxidation with DDQ, the targeted 23-methyl-
carbaporphyrin 16 was generated in 32% yield. Fairly
concentrated conditions were used (<20 mL of dichloro-
methane for every 100 mg of tripyrrane), but no improvement
in yield was observed under more dilute conditions. The
carbaporphyrin was purified by column chromatography,
followed by recrystallization from chloroform−methanol. The
N-substituted carbaporphyrin retains fully aromatic character-
istics, and the proton NMR spectrum in CDCl3 showed the
Figure 4. Color POV-Ray rendered ORTEP III drawing (50%
probability level, hydrogen atoms rendered arbitrarily small for
clarity) of 23-methylcarbaporphyrin 16.
macrocyclic framework is rather flat, with the two remaining
pyrrole rings and the indene subunit only tilted 0.46(3),
3.04(3), and 2.96(2)°, respectively, from the macrocyclic
plane. In addition, the N-methyl bond is significantly bent
relative to the pyrrole ring plane, as evidenced by the
139.2(1)° C12−C11−N23−C25 and 137.8(1)° C13−C14−
N23−C25 torsion angles. These results differ from the
structure obtained for a benzocarbaporphyrin without the N-
alkyl substituent as this showed that the indene moiety was
tilted by 15.5° relative to the mean macrocyclic plane, whereas
C
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the pyrrole to mean macrocyclic plane tilts were 2.5, 4.4, and
4.9°.35 With the exception of the C1−C2 bond length, a
Mogul geometry check validated all bond distances, angles, and
torsions to be within typical ranges.36 The long 1.4822(19) Å
C1−C2 and 1.4753(19) Å C3−C4 bond lengths are consistent
with more single-bond-like character, which suggests the
phenyl π-system of the indene subunit is isolated from the
main macrocyclic π-system.
23-Methylcarbaporphyrin 16 was reacted with palladium(II)
acetate in refluxing acetonitrile. When the reaction was carried
out for 1 h, a mixture of metalated products was obtained.
However, when the reaction was terminated after 5 min and
the crude products were purified by column chromatography
and recrystallized from chloroform−methanol, a N-methyl
palladium complex 22 was isolated in 67% yield (Scheme 5).
Scheme 5. Preparation of Palladium and Rhodium
Organometallic Derivatives from 23-Methylcarbaporphyrin
16
Figure 5. UV−vis spectra of palladium(II) carbaporphyrin complexes
22 (23-methyl, red line) and 15a (21-methyl, blue line).
10.27 ppm, whereas the inner methyl group appeared upfield
at −3.21 ppm. Again, the proton and carbon-13 NMR spectra
were consistent with 15a possessing a plane of symmetry. The
structure of 15a was confirmed by X-ray crystallography
(Figure 6). The metrics of the framework bond distances are
Figure 6. Color POV-Ray rendered ORTEP III drawing (50%
probability level, hydrogen atoms rendered arbitrarily small for
clarity) of palladium(II) 21-methylcarbaporphyrin 15a.
consistent with an overall aromatic carbaporphyrin with a sp3-
hybridized C21. The palladium(II) center resides in an
approximately square planar coordination environment with
2.058(2) Å Pd−C21, 2.037(2) Å Pd−N22, 2.066(2) Å Pd−
N23, and 2.028(1) Å Pd−N24 bond lengths. In fact, the metal
coordination environment is indistinguishable from that of the
closely related complex (8,12,13,17-tetraethyl-2,3-bis-
(methoxycarbonyl)-7,18,21-trimethyl-21-carbaporphyrinato)-
palladium(II).16 Complex 15a has the palladium(II) ion
coordinating η1 to C21, and as the carbon is alkylated, this
leads to the distended 1.491(2) and 1.498(2) Å sp3→sp2
C21−C1 and C21−C4 bond distances, and C21 is clearly
separated from the macrocycle’s π-system. This is supported by
the nonplanar 7.8(2) and 8.9(2)° torsion angles observed,
respectively, for C3−C2−C1−C21 and C2−C3−C4−C21,
along with the C21 atom being located 0.360(2) Å out of the
PdN3 plane and 0.196(2) Å out of the plane defined by the
other indene subunit atoms. The plane of the other indene
subunit atoms is tilted 14.82(4)° from the PdN3 plane. The
C25 methyl group lies 1.935(2) Å out of the PdN3 plane and
1.707(2) Å out of the plane defined by the other indene
subunit atoms. The sp3-hybridized C25 methyl group in 15a is
not available to interact with the palladium(II) metal center, as
illustrated by the over 2.76 Å Pd···C(methyl) separation. The
C25−C21−Pd bond angle is 98.1(1)°, and the C25−C21−C1
This species retained a strong aromatic ring current, showing
the meso-protons downfield at 9.85 and 10.00 ppm and the
internal methyl unit as an upfield singlet at −2.86 ppm. The
symmetry of the complex was evident from both the proton
and carbon-13 NMR spectra. The benzo unit gave rise to two
2H multiplets at 7.47 and 8.50 ppm, demonstrating that the
complex has a similar conjugation pathway to 16. The UV−vis
spectrum of 22 was complex, giving several strong absorptions
between 390 and 520 nm and additional broad peaks at higher
wavelengths (Figure 5). When the reaction of 16 with
palladium(II) acetate in refluxing acetonitrile was carried out
for 16 h, the initially formed complex was completely
converted to the C-methyl derivative 15a. Hence, alkyl group
migration still occurs for 23-methylcarbaporphyrin 16 but at a
much slower rate. The 21-methyl complex was identical to the
previously isolated product from the reaction of 22-
methylcarbaporphyrin 12a with Pd(OAc)2.21 The UV−vis
spectrum for 15a is quite different from 22, showing a
weakened Soret-like band at 421 nm and a fairly strong
absorption at 697 nm (Figure 5). Although 15a is aromatic, the
conjugation pathway is redirected through the benzo unit. This
causes the benzo-proton resonances to shift much further
downfield to give multiplets at 8.24 and 9.41 ppm. The meso-
protons for 15a were identified as two 2H singlets at 9.56 and
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and C25−C21−C4 bond angles are 107.2(1) and 107.8(1)°,
respectively. The C1−C2 and C3−C4 bond lengths for 15a
are ca. 0.03 Å shorter than those found in carbaporphyrin 16,
which is consistent with the aromatic delocalization pathway
being relocated through the indene unit in the metalated
derivative.
Scheme 6. Proposed Mechanism for Methyl Group
Migration in Palladium(II) Carbaporphyrins
A time course experiment was conducted in order to gain
further insights into the transformation of 22 into 15a. The
relative proportions of the two metallocarbaporphyrins were
assessed by proton NMR spectroscopy by comparing the
integrations for the meso-, benzo-, and internal methyl protons.
Apart from 15a and 22, no other porphyrinoid species could
be identified in the NMR spectra for these mixtures. The
formation of 22 is evidently very rapid, but its conversion to
15a was comparatively slow and was only 50% complete after
refluxing for 2.5 h in acetonitrile (Figure S16). Although it is
possible that 14a could be an intermediate in the formation of
15a, the presence of this species was not detected in any of the
proton NMR spectra. As a slight excess of Pd(OAc)2 is used to
prepare these palladium complexes, there was a possibility that
this promoted the rearrangement. However, the presence or
absence of Pd(OAc)2 had no measurable effect on the rate of
conversion of 22 to 15a. A second time course experiment was
performed where 22 was dissolved in acetonitrile-d3 at 70 °C
(Figure 7). The data showed an inexorable conversion of 22 to
Rhodium Complexes of Internally Alkylated Carba-
porphyrins. The synthesis, reactivity, and catalytic activity of
rhodium porphyrins has been subjected to extensive
investigations over the last 50 years.37 Rhodium porphyrins
have been used to selectively activate C−H and C−C bonds38
and can catalyze enantioselective cyclopropanation reactions.39
Rhodium(I) and rhodium(III) complexes of carbaporphyrins
were only reported recently,17 although examples of rhodium
N-confused porphyrins40 and azuliporphyrins10 had been
described earlier. Reaction of carbaporphyrin 2 with di-μ-
chlorotetracarbonyldirhodium(I) in refluxing dichloromethane
afforded rhodium(I) complex 24 in up to 90% yield (Scheme
7).17 When 24 was heated under reflux in pyridine,
Scheme 7. Synthesis of Rhodium(I), Rhodium(III), and
Iridium(III) Complexes of Carbaporphyrin 217
Figure 7. Time course experiment showing the conversion of 22
(black line) into the 21-methylcarbaporphyrin 15a (red line). The
experiment was carried out at 70 °C in acetonitrile-d3 and was
monitored by proton NMR spectroscopy.
rhodium(III) complex 25a was generated. A related iridium-
(III) derivative 25b was also reported.17 In order to investigate
how the presence of internal alkyl substituents would influence
this chemistry, 21-, 22-, and 23-methylcarbaporphyrins 13a,
12a, and 16 were reacted with [Rh(CO)2Cl]2 in refluxing
dichloromethane. 22-Methylcarbaporphyrin 12a reacted under
these conditions to give rhodium(I) complex 26 in 50% yield
(Figure 8). The complex retained fully aromatic properties,
and the proton NMR spectrum for 26 showed the meso-
protons downfield as four 1H singlets between 9.64 and 10.16
ppm (Figure 9). The external methyl substituents were
significantly deshielded, producing two 3H singlets at 3.36
and 3.69 ppm, whereas the internal methyl and indene
resonances appeared strongly upfield at −4.50 and −4.68 ppm,
respectively. The carbon-13 NMR spectrum showed the
carbonyl resonances as doublets at 178.8 and 179.4 ppm
15a that was 50% complete after ca. 380 min. The reaction still
occurs, albeit much more slowly, at room temperature in
solution or in the solid state. A crystalline sample of 22 that
had been stored for approximately 1 year was completely
converted into the 21-methyl derivative 15a. The new results
provide some insights into these rearrangements, but the
mechanism remains ambiguous. It is possible that the 22-
methyl palladium(II) complex is formed as an intermediate as
the conversion of 14a to 15a is comparatively fast compared to
the rearrangement of 22. However, the intermediacy of a
palladium−methyl complex is more plausible. We propose that
an oxidative addition occurs to afford the palladium(IV)
complex 23, and a subsequent reductive transfer of the methyl
unit to C21 then produces the observed product (Scheme 6).
E
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Figure 8. Rhodium(I) complexes of 22- and 21-methylcarbaporphyr-
ins.
Figure 10. UV−vis spectra of rhodium(I) complex 26 (red line) and
methylene-bridged rhodium(III) derivative 28 (purple line) in
dichloromethane.
convert 26 or 27 into rhodium(III) complexes were
unsuccessful.
The placement of the internal methyl substituent in 16
prevents the formation of rhodium(I) complexes of the type
described above. Nevertheless, when 16 was reacted with
[Rh(CO)2Cl]2 in refluxing toluene, a rhodium complex was
generated in 31% yield (Scheme 5). The proton NMR
spectrum for the new derivative showed a broad 2H doublet at
−3.22 ppm (2JRh−H = 1 Hz) rather than an internal methyl
substituent (Figure 11). The meso-protons were identified as
Figure 9. Proton NMR spectrum at 500 MHz of rhodium(I) 22-
methylcarbaporphyrin complex 26.
(1JRh−C = ca. 69 Hz) due to coupling to rhodium-103 (I = 1/2).
The meso-carbons gave rise to four peaks at 97.1, 102.5, 104.6,
and 106.6 ppm, and the internal indene CH appeared at 121.2
ppm. The presence of the carbonyl ligands was also evident
from the IR spectrum due to the presence of two strong peaks
at 1985 and 2057 cm−1. The UV−vis spectrum for 26 afforded
a Soret-like band at 481 nm and minor absorptions at 561 and
637 nm (Figure 10). 21-Methylcarbaporphyrin 13a reacted
similarly to give rhodium(I) complex 27 (Figure 8). The UV−
vis spectrum of 27 afforded a similar UV−vis spectrum with a
Soret-like band at 476 nm and weaker absorptions at 562, 627,
and 604 nm. The proton NMR spectrum again confirmed that
27 is a highly diatropic species that no longer possesses a plane
of symmetry. The meso-protons gave rise to four 1H singlets at
9.52, 9.56, 9.69, and 9.82 ppm, and the internal CH3 and NH
protons appeared upfield at −5.66 and −2.83 ppm,
respectively. The benzo unit gave rise to multiplets between
7.3 and 8.2 ppm, indicating that the major aromatic π-
delocalization pathway does not involve the arene moiety. The
carbon-13 NMR spectrum again shows two doublets for the
carbonyl ligands between 177 and 179 ppm, whereas the meso-
carbons appeared at 93.0, 99.9, 103.5, and 108.6 ppm. In
addition, the IR spectrum of 27 gave two peaks at 1996 and
2063 cm−1 corresponding to the carbonyl units. Attempts to
Figure 11. Proton NMR spectrum at 500 MHz of rhodium(III)
complex 28. The 2H resonance at −3.22 ppm due to the bridge CH2
unit is coupled to 103Rh, and this gives rise to the observed doublet.
two 2H singlets at 9.51 and 10.13 ppm, confirming that the
macrocycle retains strongly diatropic characteristics, and both
the proton and carbon-13 NMR spectra show that the
porphyrinoid has a plane of symmetry. Furthermore, the
benzo-protons give rise to two 2H multiplets that are
substantially shifted downfield to 8.18 and 9.39 ppm, which
implies that the aromatic conjugation pathway passes through
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this unit. These data are consistent with methylene-bridged
rhodium(III) complex 28. However, the electrospray ioniza-
tion mass spectrum for this compound showed no sign of the
proposed molecular ion and instead gave a base peak that
corresponded to rhodium complex 29 (calcd for C35H34N3Rh
m/z 599.1808; found 599.1805), together with additional
peaks for unidentified species. The original analysis was carried
out in the presence of 0.5% formic acid, but the same result
was obtained when the formic acid was omitted. It is proposed
that protonation and elimination of a methyl radical results in
the formation of 29 (Scheme 8). Subsequent analysis of 28 by
The structure of 28 was confirmed by X-ray crystallography
(Figure 13). The porphyrinoid framework conformation is
Scheme 8. Fragmentation of Rhodium Complex 28 during
Analysis by ESI MS
Figure 13. Color POV-Ray rendered ORTEP III drawing (50%
probability level, hydrogen atoms rendered arbitrarily small for
clarity) of methylene-bridged rhodium(III) carbaporphyrin 28.
remarkably similar to that of palladium complex 15a, the most
obvious difference being the presence of the three-membered
rhodacycle. The metrics of the framework bond distances are
consistent with an overall aromatic carbaporphyrin with a sp3-
hybridized C21, and the C1−C2 and C3−C4 bond lengths of
ca. 1.447 Å are consistent with the delocalization pathway
passing through the indene unit. The C1−C21−C4 bond angle
is 107.3(1)°, and the C25−C21−C1 and C25−C21−C4 bond
angles are both 120.5(1)°. The C1−C21−Rh and C4−C21−
Rh bond angles are both 119.8(1)°, and the C25−C21−Rh
bond angle is 64.09(6)°. The rhodium(III) center resides in a
strongly distorted square pyramidal coordination environment
with the equatorial plane consisting of C21 and the three
pyrrole nitrogen atoms, with 2.1329(11) Å Rh−C21,
2.0369(9) Å Rh−N22, 2.0431(9) Å Rh−N23, and 2.0410(9)
Å Rh−N24 bond lengths. The metal coordination environ-
ment is very similar to that of a closely related tetraphenyl
rhodium(III) carbaporphyrin complex,41 although there are
some notable differences. Like this complex, 28 has the
rhodium(III) ion coordinating η2 to C21 and C25. Attachment
of the methylene group to C21 leads to the distended 1.463(2)
and 1.462(2) Å sp3→sp2 C21−C1 and C21−C4 bond
distances, which separates C21 from the macrocycle’s π-
system. This is supported by the nonplanar 3.1(1) and 3.8(1)°
torsion angles observed, respectively, for C3−C2−C1−C21
and C2−C3−C4−C21, along with the C21 atom being located
0.412(1) Å out of the RhN3 plane and 0.035(1) Å out of the
plane defined by the other indene subunit atoms. The plane of
the other indene subunit atoms is tilted 18.21(3)° from the
RhN3 plane. Methylene unit C25 lies 1.585(1) Å out of the
RhN3 plane and 0.872(1) Å out of the plane defined by the
other indene subunit atoms. The sp3-hybridized C25
methylene group in 28 is clearly interacting with the
rhodium(III) metal center, as illustrated by the 1.9932(11)
Å Rh···C25(methylene) separation. This is on the shorter end
of the 2.09(4) Å distribution of 297 known Rh−CH2 bond
distances found in the Cambridge Structural Database (v5.39,
Aug 2018 updates) and notably 0.04 Å shorter than the
rhodium(III) tetraphenylcarbaporphyrin complex.41
electron impact mass spectrometry gave a strong molecular ion
at m/z 613 that matched the molecular formula for the
methylene-bridged rhodium(III) complex. The formation of
28 from 23-methylcarbaporphyrin 16 was unexpected but
presumably involves a rearrangement similar to the one
observed for palladium(II) complex 22 where the N-methyl
group effectively somersaults over the rhodium center. Related
bridged rhodium(III) complexes were previously obtained by
the ring contraction of benziporphyrin complexes.41,42 The
carbon-13 NMR spectrum of the rhodium(III) complex gave a
doublet for the bridging CH2 unit at 46.3 ppm (1JRh−C = 21.4
Hz), whereas the quaternary carbon at position 21 showed up
as a doublet at 54.5 ppm (1JRh−C = 14.2 Hz). The meso-carbons
afforded two resonances at 103.7 and 104.2 ppm. The UV−vis
spectrum of 28 (Figure 10) was very different from
rhodium(I) complexes 26 or 27, showing a peak at 333 nm,
followed by a series of broad absorptions culminating in a band
that was centered on 682 nm. Although 18 π-electron
delocalization pathways have been emphasized for palladium
complexes 15 and the rhodium(III) derivative 28, 22 π-
electron circuits can also potentially contribute to these
structures (see structures 15′ and 28′ in Figure 12). Indeed,
these pathways help to explain the pronounced downfield
shifts observed for the benzo-protons in the proton NMR
spectra for these metalated derivatives, as well as the observed
bathochromic shifts in the UV−vis spectra resulting from the
macrocycle’s extended chromophore.
Figure 12. Alternative 22π-electron delocalization pathways for
metallocarbaporphyrinoids 15 and 28.
G
DOI: 10.1021/acs.organomet.8b00863
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dichloromethane layers were washed with water (5 × 100 mL), dried
over magnesium sulfate, and the solvent removed under reduced
pressure. The residue was vacuum distilled to yield N-methylpyrrole
ester 17b (3.20 g, 15.3 mmol, 79%) as a pale yellow oil: bp 98−100
°C at 1 Torr; 1H NMR (500 MHz, CDCl3) δ 1.12 (3H, t, 3JHH = 7.6
Hz, 3-CH2CH3), 1.14 (3H, t, 3JHH = 7.6 Hz, 4-CH2CH3), 1.30 (3H, t,
3JHH = 7.2 Hz, OCH2CH3), 2.37 (2H, q, 3JHH = 7.6 Hz, 4−CH2CH3),
CONCLUSIONS
■
Metalation of a 23-methylcarbaporphyrin with palladium(II)
acetate in refluxing acetonitrile has been shown to initially give
the corresponding N-methyl palladium(II) complex, but this
species gradually rearranges to generate a C-methyl palladium-
(II) derivative. The N-methyl complex rearranges in the
absence of palladium(II) acetate, and the process slowly
proceeds at room temperature even in the solid state. A
mechanism for this transformation is proposed that involves a
transient palladium(IV) species. The 21- and 22-Methylcarba-
porphyrins reacted with [Rh(CO)2Cl]2 to give rhodium(I)
complexes, but the 23-methylcarbaporphyrin afforded a
methylene-bridged rhodium(III) complex that had undergone
a similar transformation to the one observed for the palladium
series. These observations demonstrate that carbaporphyrin
ligands facilitate unexpected reactivity that can lead to diverse
organometallic structures.
3
2.72 (2H, q, JHH = 7.6 Hz, 3−CH2CH3), 3.75 (3H, s, N-Me), 4.24
3
(2H, q, JHH = 7.2 Hz, OCH2), 6.43 (1H, s, 5-H); 13C NMR (125
MHz, CDCl3) δ 13.8 (OCH2CH3), 14.5 (4-CH2CH3), 15.2 (3-
CH2CH3), 17.4 (4-CH2), 18.5 (3-CH2), 36.5 (N-Me), 58.7 (OCH2),
118.6, 123.5, 125.8 (5-CH), 133.4, 161.2 (CO); HR-MS (EI) m/z
calcd for C12H19NO2 209.1416; found 209.1419.
3,4-Diethyl-1-methylpyrrole (18). Pyrrole ester 17b (2.82 g,
13.5 mmol) was added to sodium hydroxide (1.07 g) in ethylene
glycol (11 mL), and the mixture was heated for 2.5 h at 200 °C. After
being cooled to room temperature, the mixture was partitioned
between water (50 mL) and hexanes (50 mL). The aqueous layer was
drawn off and further extracted with hexanes (3 × 50 mL). The
combined hexanes layers were dried over magnesium sulfate, and the
solvent was removed under reduced pressure to yield 3,4-diethyl-1-
methylpyrrole (1.55 g, 11.3 mmol, 84%) as a pale yellow oil: bp 78−
EXPERIMENTAL SECTION
■
80 °C at 18 Torr; 1H NMR (500 MHz, CDCl3) δ 1.46 (6H, t, 3JHH
=
Melting points are uncorrected. NMR spectra were recorded using a
400 or 500 MHz NMR spectrometer and were run at 302 K unless
otherwise indicated. 1H NMR values are reported as chemical shifts δ,
relative integral, multiplicity (s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet; br, broad peak), and coupling constant (J).
Chemical shifts are reported in parts per million (ppm) relative to
CDCl3 (1H residual CHCl3 singlet δ 7.26, 13C CDCl3 triplet δ 77.23)
or DMSO-d6 (1H residual DMSO-d5 pentet δ 2.49, 13C DMSO-d6
septet δ 39.7), and coupling constants were taken directly from the
spectra. NMR assignments were made with the aid of 1H−1H COSY,
HSQC, DEPT-135, and NOE difference proton NMR spectroscopy.
The 2D experiments were performed using standard software. High-
resolution mass spectra (HR-MS) were carried out using a double
focusing magnetic sector instrument. 1H and 13C NMR spectra for all
new compounds are reported in Supporting Information.
Ethyl 3,4-Diethyl-1-methylpyrrole-2-carboxylate (17b). 3-
Acetoxy-4-nitrohexane (52.5 g, 0.278 mol) and ethyl isocyanoacetate
(26.0 g, 0.230 mol) were mixed with THF (165 mL) and 2-propanol
(67 mL) in a three-neck round-bottom flask fitted with a
thermometer, addition funnel, and drying tube. DBU (86.0 g, 0.565
mol) was added dropwise, maintaining the temperature between 20
and 30 °C with the aid of a salt-ice bath. After the addition was
complete, the orange solution was stirred at room temperature
overnight. The solvent was removed under reduced pressure and the
residue poured into water (225 mL). Diethyl ether (225 mL) was
added and the aqueous layer drawn off and extracted twice with 225
mL of diethyl ether. The combined ether layers were washed with
10% aqueous HCl (2 × 300 mL), dried over magnesium sulfate, and
the solvent removed on a rotary evaporator. The residue was vacuum
distilled to yield ethyl 3,4-diethylpyrrole-2-carboxylate (17a)43 (31.6
g, 0.162 mol, 70%) as a pale yellow oil: bp 102−105 °C at 0.6 Torr;
1H NMR (400 MHz, CDCl3) δ 1.16 (3H, t, 3JHH = 7.5 Hz), 1.20 (3H,
3
7.6 Hz, 2 × CH2CH3), 2.71 (4H, q, JHH = 7.6 Hz, 2 × CH2CH3),
3.80 (3H, s, N-Me), 6.58 (2H, s, 2,5-H); 13C NMR (125 MHz,
CDCl3) δ 14.6 (2 × CH2CH3), 18.3 (2 × CH2CH3), 35.1 (N-Me),
118.2 (2,5-CH), 124.1 (3,4-C); HR-MS (EI) m/z calcd for C9H15N
137.1204; found 137.1204.
2,5-Bis(5-benzyloxycarbonyl-3-ethyl-4-methyl-2-pyrrolyl-
methyl)-3,4-diethyl-1-methylpyrrole (20a). Benzyl 5-acetoxy-
methyl-4-ethyl-3-methylpyrrole-2-carboxylate (1.28 g, 4.10 mmol)
and 3,4-diethyl-1-methylpyrrole (0.279 g, 2.04 mmol) were dissolved
in acetonitrile (15 mL) and acetic acid (1 mL). The mixture was
refluxed overnight under nitrogen. The solution was then cooled to
room temperature, diluted with dichloromethane (50 mL), and then
washed with water (100 mL) and aqueous 10% sodium bicarbonate
solution (100 mL). The dichloromethane layer was dried over sodium
sulfate and the solvent removed under reduced pressure. The residue
was purified by column chromatography on silica gel, eluting with
80:20:1 dichloromethane/hexanes/triethylamine, and afforded the N-
methyl tripyrrane (0.624 g, 0.964 mmol, 48%) as an orange oil that
1
slowly solidified when stored in the freezer: H NMR (500 MHz,
3
CDCl3) δ 1.05 (6H, t, JHH = 7.6 Hz, 3′,3″-CH2CH3), 1.09 (6H, t,
3JHH = 7.6 Hz, 3,4-CH2CH3), 2.27 (6H, s, 4′,4″-CH3), 2.39−2.45
(8H, m, 4 × CH2CH3), 2.98 (3H, s, N-Me), 3.81 (4H, s, 2 × bridge-
CH2), 5.24 (4H, s, 2 × OCH2), 7.28−7.39 (10H, m, 2 × Ph), 8.23
(2H, br s, 2 × NH); 13C NMR (125 MHz, CDCl3) δ 10.7 (4′,4″-Me),
15.4 (3′,3″-CH2CH3), 17.24 (3,4-CH2CH3), 17.28, 18.0 (4 ×
CH2CH3), 21.9 (2 × bridge-CH2), 30.4 (N-Me), 65.6 (2 ×
OCH2), 117.1, 122.2, 122.9, 123.4, 128.1 (Ph), 128.7 (Ph), 131.1,
136.9, 161.6 (2 × CO). HR-MS (ESI) m/z [M + H]+ calcd for
C41H50N3O4 648.3801; found 648.3792.
N-Methyl Tripyrrane Dicarboxylic Acid 20b. N-Methyl
tripyrrane 20a (0.703 g, 1.09 mmol) was placed in a hydrogenation
vessel and dissolved in acetone (100 mL), methanol (34 mL), and
triethylamine (14 drops). The mixture was purged with nitrogen; 10%
palladium charcoal (67 mg) was added and the solution shaken under
an atmosphere of 40 psi hydrogen at room temperature overnight.
The catalyst was removed by suction filtration and the solvent
removed on a rotary evaporatory while maintaining the bath
temperature below 30 °C. The residue was dissolved in 5% aqueous
ammonium and diluted to a final volume of 50 mL with water. The
resulting solution was cooled to 0−5 °C and then acidified by adding
acetic acid dropwise while maintaining the temperature below 5 °C.
The mixture was allowed to sit in a salt-ice bath for 30 min, and the
precipitate was then collected by suction filtration and washed
multiple times with water to remove all traces of acid. The solid was
dried overnight in a vacuum desiccator to yield the dicarboxylic acid
(0.419 g, 0.897 mmol, 83%) as an unstable pale purple powder that
3
3
t, JHH = 7.5 Hz) (3,4-CH2CH3), 1.36 (3H, t, JHH = 7.2 Hz,
3
OCH2CH3), 2.46 (2H, q, JHH = 7.5 Hz, 4−CH2CH3), 2.77 (2H, q,
3JHH = 7.5 Hz, 3−CH2CH3), 4.33 (2H, q, 3JHH = 7.2 Hz, OCH2), 6.68
(1H, d, 3JHH = 2.9 Hz, 5-H), 9.00 (1H, br s, NH). The proton NMR
spectrum shows the presence of ca. 3% of the corresponding isopropyl
ester due to transesterification with 2-propanol.
Potassium hydroxide pellets (3.84 g) were heated with
dimethylsulfoxide (15 mL) until a yellow solution was obtained.
The solution was cooled to room temperature, and the foregoing
pyrrole ester 17a (3.80 g, 19.5 mmol) was added. After the mixture
had been stirred for 1 h, methyl iodide (5.2 g) was added dropwise,
keeping the temperature below 30 °C with the aid of a salt-ice bath.
After the addition was completed, the solution was stirred for an
additional 1 h, keeping the temperature below 30 °C. The solution
was poured in ice/water (200 mL), and the aqueous layer was
extracted with dichloromethane (2 × 200 mL). The combined
1
was used without further purification; H NMR (500 MHz, DMSO-
H
DOI: 10.1021/acs.organomet.8b00863
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
3
d6) δ 0.73 (6H, t, JHH = 7.5 Hz), 0.97 (6H, t, JHH = 7.5 Hz) (4 ×
CH2CH3), 2.12 (6H, s, 4′,4″-CH3), 2.11 (4H, q, 3JHH = 7.5 Hz), 2.35
MHz, CDCl3) δ 11.7 (7,18-Me), 16.5 (12,13-CH2CH3), 17.6 (8,17-
CH2CH3), 20.16, 20.21 (4 × CH2CH3), 45.3 (N-Me), 98.9 (10,15-
CH), 109.6 (5,20-CH), 118.8 (21,31-CH), 126.0 (22,32-CH), 133.8,
135.1, 136.3, 138.9, 140.7, 142.7, 145.6, 149.8; HR-MS (ESI) m/z
calcd for C36H37N3Pd 617.2022; found 617.2044.
3
(4H, q, JHH = 7.5 Hz) (4 × CH2CH3), 2.99 (3H, s, N-Me), 3.81
(4H, s, 2 × bridge-CH2), 10.14 (2H, br s, 2 × NH), 11.53 (2H, v br, 2
× CO2H); 13C NMR (125 MHz, CDCl3) δ 9.9 (4′,4′′-Me), 14.8,
16.3, 16.5, 17.2, 21.6 (2 × bridge-CH2), 30.1 (N-Me), 116.5, 119.6,
122.6, 123.1, 125.5, 130.4, 162.1 (2 × CO); HR-MS (ESI) m/z
calcd for C27H37N3O4 490.2682; found 490.2672.
[8,12,13,17-Tetraethyl-7,18,21-trimethylbenzo[b]-
carbaporphyrinato]palladium(II) (15a). Palladium(II) acetate (10
mg, 0.0450 mmol) was added to a solution of 23-methylbenzocarba-
porphyrin 16 (10 mg, 0.0195 mmol) in acetonitrile (10 mL) and the
mixture refluxed overnight. The solvent was removed under reduced
pressure and the residue chromatographed on a silica gel column
eluting with dichloromethane/hexanes (70:30). A brown band was
collected and the solvent removed under reduced pressure to give 15a
(10.2 mg, 0.0165 mmol, 85%) as a dark brown solid: mp >300 °C; 1H
NMR (500 MHz, CDCl3) δ −3.21 (3H, s, 21-CH3), 1.74 (6H, t, 3JHH
= 7.7 Hz), 1.75 (6H, t, 3JHH = 7.7 Hz) (4 × CH2CH3), 3.33 (6H, s),
3.71−3.78 (8H, m, 4 × CH2CH3), 8.22−8.25 (2H, m, 22,32-H),
9.39−9.43 (2H, m, 21,31-H), 9.56 (2H, s, 10,15-H), 10.27 (2H, s,
5,20-H). The sample had identical spectroscopic properties to a
previously prepared sample obtained by reacting 12a with Pd-
(OAc)2.21
8,12,13,17-Tetraethyl-7,18,23-trimethylbenzo[b]-
carbaporphyrin (16). N-Methyl tripyrrane dicarboxylic acid 20b
(0.144 g, 0.308 mmol) was dissolved in TFA (1.5 mL) and stirred
under nitrogen for 2 min. Dichloromethane (27 mL) was added,
followed immediately by indene dialdehyde (54.0 mg, 0.314 mmol),
and the solution was stirred under nitrogen for 2 h. The mixture was
neutralized by the dropwise addition of triethylamine; DDQ (72.0
mg, 0.317 mmol) was added, and the mixture was stirred for a further
1 h. The solution was washed with water (100 mL) and then sodium
bicarbonate (100 mL), and the solvent was removed under reduced
pressure. The residue was chromatographed twice on grade 3 neutral
alumina eluting with dichloromethane and then chloroform, and a
dark brown band was collected. The product was recrystallized from
chloroform−methanol to yield the 23-methylbenzocarbaporphyrin
(52.5 mg, 0.102 mmol, 32%) as dark purple-blue crystals: mp >300
°C; UV−vis (1% Et3N−CH2Cl2) λmax/nm (log ε) 306 (4.42), 379
(4.56), 439 (5.10), 538 (4.15), 567 (3.91), 624 (3.86), 683 (3.36);
UV−vis (1% TFA−CH2Cl2) λmax/nm (log ε) 310 (4.42), 403 (sh,
4.77), 444 (4.97), 481 (4.47), 568 (4.01), 608 (4.08), 677 (3.26); 1H
NMR (500 MHz, CDCl3) δ −4.95 (1H, s, 21-H), −4.25 (3H, s, N-
[8,12,13,17-Tetraethyl-7,18,22-trimethylbenzo[b]-
carbaporphyrinato](dicarbonyl)rhodium(I) (26). 22-Methylben-
zocarbaporphyrin 12a (10.4 mg, 0.0203 mmol) was dissolved in
dichloromethane (22 mL) under nitrogen. Anhydrous sodium acetate
(16.3 mg) was added, followed by [Rh(CO)2Cl]2 (7.8 mg, 0.0201
mmol), and the mixture was refluxed overnight under nitrogen. The
solvent was removed under reduced pressure, and the residue was
purified by column chromatography on silica gel eluting with
dichloromethane/hexanes (50:50). The rhodium complex was
collected as a brown band. Recrystallization from chloroform/hexanes
afforded 26 (6.8 mg, 0.010 mmol, 50%) as dark crystals: mp >300 °C;
UV−vis (CH2Cl2) λmax/nm (log ε) 319 (sh, 4.39), 360 (4.57), 407
(sh, 4.44), 481 (4.85), 561 (4.07), 586 (sh, 3.83), 637 (3.68); IR
(ZnSe) νCO 1985, 2057 cm−1; 1H NMR (500 MHz, CDCl3) δ −4.68
(1H, s, 21-H), −4.50 (3H, s, N-Me), 1.56 (3H, t, 3JHH = 7.7 Hz), 1.68
3
Me), −0.63 (1H, v br, NH), 1.47 (6H, t, JHH = 7.7 Hz, 12,13-
CH2CH3), 1.83 (6H, t, JHH = 7.7 Hz, 8,17-CH2CH3), 3.51 (6H, s,
3
7,18-Me), 3.67−3.75 (2H, m), 3.76−3.84 (2H, m) (12,13-CH2),
3.88−4.02 (4H, m, 8,17-CH2), 7.67−7.71 (2H, m, 22,32-H), 8.79−
8.83 (2H, m, 21,31-H), 9.67 (2H, s, 10,15-H), 10.03 (2H, s, 5,20-H);
1H NMR (500 MHz, TFA-CDCl3) δ −6.53 (1H, br s, 21-H), −4.79
3
(2H, s, 2 × NH), −4.51 (3H, s, N-Me), 1.50 (6H, t, JHH = 7.7 Hz,
3
12,13-CH2CH3), 1.77 (6H, t, JHH = 7.7 Hz, 8,17-CH2CH3), 3.60
(3H, t, 3JHH = 7.7 Hz), 1.76 (3H, t, 3JHH = 7.7 Hz), 1.79 (3H, t, 3JHH
=
(6H, s, 7,18-Me), 3.69−3.80 (4H, m, 12,13-CH2), 4.05 (4H, q, 3JHH
=
7.7 Hz, 8,17-CH2), 7.70−7.80 (2H, br, 22,32-H), 8.64−8.72 (2H, m,
21,31-H), 10.00 (2H, s, 10,15-H), 10.26 (2H, s, 5,20-H); 13C NMR
(125 MHz, CDCl3) δ 11.5 (7,18-Me), 16.9 (12,13-CH2CH3), 17.4
(8,17-CH2CH3), 20.0 (8,17-CH2), 20.1 (12,13-CH2), 31.4 (N-Me),
96.4 (10,15-CH), 103.4 (5,20-CH), 118.8 (21-CH), 120.4 (21,31-
CH), 126.5 (22,32-CH), 134.2, 135.2, 136.3, 140.7, 144.0, 147.6; 13C
NMR (125 MHz, TFA-CDCl3) δ 11.7 (7,18-Me), 16.5 (12,13-
CH2CH3), 16.9 (8,17-CH2CH3), 20.13, 20.15 (4 × CH2CH3), 30.7
(N-Me), 95.8 (10,15-CH), 103.6 (5,20-CH), 116.5 (21-CH), 121.9
(21,31-CH), 128.7 (22,32-CH), 136.2, 136.8, 138.8, 138.9, 140.6,
141.5, 141.7, 150.6; HR-MS (ESI) m/z [M + H]+ calcd for C36H40N3
514.3222; found 514.3198.
7.7 Hz) (4 × CH2CH3), 3.36 (3H, s), 3.52 (3H, s) (7,18-Me), 3.56−
3.69 (2H, m), 3.76−3.99 (6H, m) (4 × CH2CH3), 7.66−7.71 (2H, m,
22,32-H), 8.74−8.78 (2H, m, 21,31-H), 9.640 (1H, s), 9.643 (1H, s)
(10,15-H), 9.97 (1H, s), 10.16 (1H, s) (5,20-H); 13C NMR (125
MHz, CDCl3) δ 12.0, 12.3, 15.8, 17.6, 18.3, 18.4, 19.9, 20.0, 20.1,
20.6, 97.1, 102.5, 104.6, 106.6, 120.32, 120.35, 121.2 (br), 126.6,
130.1, 132.3, 132.9, 134.8, 136.5, 139.2, 139.4, 142.2, 143.7, 144.4,
1
147.8, 148.4, 149.1, 150.0, 152.5, 155.0, 178.8 (d, JRhC = 68.5 Hz,
1
CO), 179.4 (d, JRhC = 69.4 Hz, CO); HR-MS (ESI) m/z [M + H]+
calcd for C38H39N3O2Rh 672.2097; found 672.2104.
[8,12,13,17-Tetraethyl-7,18,21-trimethylbenzo[b]-
carbaporphyrinato](dicarbonyl)rhodium(I) (27). Anhydrous so-
dium acetate (11.4 mg) was added, followed by [Rh(CO)2Cl]2 (5.5
mg, 0.0142 mmol), to a solution of 21-methylbenzocarbaporphyrin
13a (7.0 mg, 0.0136 mmol) in dichloromethane (15 mL) under
nitrogen. The resulting mixture was heated under reflux overnight.
The solvent was removed under reduced pressure and the residue
purified by column chromatography on silica gel eluting with
dichloromethane/hexanes (50:50). The complex eluted as a green
band. Recrystallization from chloroform/hexanes gave the rhodium(I)
complex (6.2 mg, 0.00924 mmol, 67%) as as dark crystals: mp >300
°C; UV−vis (CH2Cl2) λmax/nm (log ε) 327 (4.18), 368 (4.34), 476
(4.56), 562 (3.71), 604 (3.71), 627 (3.64); IR (ZnSe) νCO 1996,
[8,12,13,17-Tetraethyl-7,18,23-trimethylbenzo[b]-
carbaporphyrinato]palladium(II) (22). Palladium(II) acetate
(10.0 mg, 0.0450 mmol) was added to a solution of 23-
methylbenzocarbaporphyrin 16 (10.0 mg, 0.0195 mmol) in
acetonitrile (10 mL) and heated under reflux in a preheated oil
bath for 5 min. The solution was diluted with chloroform (30 mL)
and washed with water (50 mL). The aqueous solution was back
extracted with chloroform, and the combined organic layers were
evaporated under reduced pressure. The residue was purified by
column chromatography on silica gel eluting with dichloromethane/
hexanes (70:30), and a dark green band was collected. The solvent
was removed on a rotary evaporator and the residue recrystallized
from chloroform−methanol to give the palladium complex (8.0 mg,
0.013 mmol, 67%) as dark green crystals: mp >300 °C; UV−vis
(CH2Cl2) λmax/nm (log ε) 309 (4.50), 397 (4.61), 404 (sh, 4.60), 470
(sh, 4.52), 496 (4.76), 518 (4.36), 561 (3.63), 613 (3.93), 635 (sh,
1
2063 cm−1; H NMR (500 MHz, CDCl3) δ −5.66 (3H, s, N-Me),
−2.83 (1H, br s, NH), 1.69 (3H, t, 3JHH = 7.7 Hz), 1.78−1.83 (9H, 3
overlapping triplets) (4 × CH2CH3), 3.35 (3H, s), 3.52 (3H, s) (7,18-
Me), 3.67−3.85 (3H, m), 3.88−4.01 (5H, m) (4 × CH2CH3), 7.35
4
3
4
(1H, dt, JHH = 0.9 Hz, JHH = 7.3 Hz), 7.41 (1H, dt, JHH = 0.9 Hz,
3JHH = 7.4 Hz) (22,32-H), 7.97 (1H, d, JHH = 7.2 Hz), 8.20 (1H, d,
3
1
3.90), 667 (3.84); H NMR (500 MHz, CDCl3) δ −2.86 (3H, s, N-
3JHH = 7.3 Hz) (21,31-H), 9.52 (1H, s), 9.56 (1H, s, 10,15-H), 9.69
(1H, s), 9.82 (1H, s) (5,20-H); 13C NMR (125 MHz, CDCl3) δ 11.5,
11.8 (21-Me), 12.3, 17.4, 17.6, 18.0, 18.3, 19.75, 19.84, 19.91, 20.5,
93.0, 99.9, 103.5, 108.6, 120.0, 120.2, 126.1, 126.2, 127.8, 131.5,
Me), 1.67 (6H, t, 3JHH = 7.7 Hz, 12,13-CH2CH3), 1.81 (6H, t, 3JHH
=
7.7 Hz, 8,17-CH2CH3), 3.48 (6H, s, 7,18-Me), 3.84−4.00 (8H, m, 4
× CH2CH3), 7.45−7.49 (2H, m, 22,32-H), 8.48−8.52 (2H, m, 21,31-
H), 9.85 (2H, s, 10,15-H), 10.00 (2H, s, 5,20-H); 13C NMR (125
I
DOI: 10.1021/acs.organomet.8b00863
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Organometallics
Article
132.8, 132.89, 132.94, 134.7, 136.1, 137.5, 139.1, 139.9, 140.1, 141.6,
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific
Publishing, 2012; Vol. 18, pp 303−413.
(2) (a) Gupta, I.; Ravikanth, M. Recent Developments in
Heteroporphyrins and their Analogues. Coord. Chem. Rev. 2006,
1
143.2, 145.8, 147.7, 153.1, 154.8, 177.6 (d, JRhC = 67.0 Hz, CO),
1
179.0 (d, JRhC = 66.2 Hz, CO); HR-MS (ESI) m/z [M + H]+ calcd
for C38H39N3O2Rh 672.2097; found 672.2088.
[8,12,13,17-Tetraethyl-7,18-dimethyl-21-methylenebenzo-
[b]carbaporphyrinato]rhodium(III) (28). 23-Methylbenzocarba-
porphyrin 16 (20.0 mg, 0.0390 mmol) was dissolved in toluene (40
mL) under nitrogen. Anhydrous sodium acetate (33.3 mg) was added,
followed by [Rh(CO)2Cl]2 (14.3 mg, 0.0368 mmol), and the mixture
refluxed overnight. The solvent was removed under reduced pressure,
and the residue was purified twice by column chromatography on
silica gel eluting with dichloromethane. The product eluted as a dark
band. Recrystallization from chloroform−methanol gave the rhodium-
(III) complex (7.4 mg, 0.0121 mmol, 31%) as dark crystals: mp >300
°C; UV−vis (CH2Cl2) λmax/nm (log ε) 333 (4.68), 386 (sh, 4.45),
424 (sh, 4.36), 486 (4.14), 682 (4.03); 1H NMR (400 MHz, CDCl3)
́
250, 468−518. (b) Chmielewski, P. J.; Latos- Grazynski, L. Core
̇
Modified Porphyrins − a Macrocyclic Platform for Organometallic
Chemistry. Coord. Chem. Rev. 2005, 249, 2510−2533.
(3) (a) Lash, T. D. Carbaporphyrins and Related Systems. Synthesis,
Characterization, Reactivity and Insights into the Nature of
Porphyrinoid Aromaticity. In Handbook of Porphyrin Science - With
Applications to Chemistry, Physics, Material Science, Engineering, Biology
and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World
Scientific Publishing, 2012; Vol. 16, pp 1−329. (b) Lash, T. D.
Carbaporphyrinoid Systems. Chem. Rev. 2017, 117, 2313−2446.
(4) (a) Srinivasan, A.; Furuta, H. Confusion Approach to
Porphyrinoid Chemistry. Acc. Chem. Res. 2005, 38, 10−20.
(b) Toganoh, M.; Furuta, H. Blooming of Confused Porphyrinoids
− Fusion, Expansion, Contraction, and More Confusion. Chem.
Commun. 2012, 48, 937−954.
(5) Lash, T. D. Out of the Blue! Azuliporphyrins and Related
Carbaporphyrinoid Systems. Acc. Chem. Res. 2016, 49, 471−482.
(6) Lash, T. D. Benziporphyrins, a Unique Platform for Exploring
the Aromatic Characteristics of Porphyrinoid Systems. Org. Biomol.
Chem. 2015, 13, 7846−7878.
(7) Lash, T. D. Metal Complexes of Carbaporphyrinoid Systems.
Chem. - Asian J. 2014, 9, 682−705.
2
3
δ −3.22 (2H, br d, JRhH = 1.0 Hz, Rh-CH2), 1.74 (6H, t, JHH = 7.7
Hz, 8,17-CH2CH3), 1.78 (6H, t, 3JHH = 7.7 Hz, 12,13-CH2CH3), 3.34
(6H, s, 7,18-Me), 3.62−3.70 (4H, m, 8,17-CH2), 3.72−3.83 (4H, m,
12,13-CH2), 8.16−8.20 (2H, m, 22,32-H), 9.37−9.41 (2H, m, 21,31-
H), 9.51 (2H, s, 10,15-H), 10.12 (2H, s, 5,20-H); 13C NMR (125
MHz, CDCl3) δ 11.6 (7,18-Me), 17.4 (8,17-CH2CH3), 18.5 (12,13-
1
CH2CH3), 19.6 (12,13-CH2), 19.9 (8,17-CH2), 46.3 (d, JRhC = 21.4
1
Hz, Rh-CH2), 54.5 (d, JRhC = 14.2 Hz, 21-C), 103.7 (10,15-CH),
104.2 (5,20-CH), 121.3 (21,31-CH), 127.3 (22,32-CH), 135.8, 140.2,
141.7, 142.4, 144.3, 145.4, 149.6, 154.9; HR-MS (EI) m/z calcd for
C36H36N3Rh 613.1964; found 613.1659.
(8) (a) Graham, S. R.; Ferrence, G. M.; Lash, T. D. Organometallic
Chemistry of Carbaporphyrinoids: Synthesis and Characterization of
Nickel(II) and Palladium(II) Azuliporphyrins. Chem. Commun. 2002,
38, 894−895. (b) Lash, T. D.; Colby, D. A.; Graham, S. R.; Ferrence,
G. M.; Szczepura, L. F. Organometallic Chemistry of Azuliporphyrins:
Synthesis, Spectroscopy, Electrochemistry and Structural Character-
ization of Nickel(II), Palladium(II) and Platinum(II) Complexes of
Azuliporphyrins. Inorg. Chem. 2003, 42, 7326−7338.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00863.
Experimental details for the X-ray crystallography and
́
(9) Bialek, M. J.; Latos- Grazynski, L. Merging of Inner and Outer
̇
1
1
selected UV−vis, IR, H NMR, H−1H COSY, HSQC,
Ruthenium Organometallic Coordination Motifs within an Azulipor-
phyrin Framework. Chem. Commun. 2014, 50, 9270−9272.
(10) Stateman, L. M.; Ferrence, G. M.; Lash, T. D. Rhodium(III)
Azuliporphyrins. Organometallics 2015, 34, 3842−3848.
(11) Lash, T. D.; Pokharel, K.; Zeller, M.; Ferrence, G. M.
Iridium(III) Azuliporphyrins. Chem. Commun. 2012, 48, 11793−
11795.
DEPT-135, 13C NMR, and mass spectra (PDF)
Accession Codes
CCDC 1877450−1877452 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
́
(12) Stepien, M.; Latos-Grazynski, L. Benziporphyrins: Exploring
̇
Arene Chemistry in a Macrocyclic Environment. Acc. Chem. Res.
2005, 38, 88−98.
(13) (a) Toganoh, M.; Furuta, H. In Handbook of Porphyrin Science
− With Applications to Chemistry, Physics, Material Science, Engineering,
Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
World Scientific Publishing: Singapore, 2010; Vol. 2, pp 295−367.
(b) Harvey, J. D.; Ziegler, C. J. Developments in the Metal Chemistry
of N-Confused Porphyrin. Coord. Chem. Rev. 2003, 247, 1−19.
(14) Muckey, M. A.; Szczepura, L. F.; Ferrence, G. M.; Lash, T. D.
Silver(III) Carbaporphyrins: The First Organometallic Complexes of
True Carbaporphyrins. Inorg. Chem. 2002, 41, 4840−4842.
(15) Lash, T. D.; Colby, D. A.; Szczepura, L. F. New Riches in
Carbaporphyrin Chemistry: Silver and Gold Organometallic Com-
plexes of Benzocarbaporphyrins. Inorg. Chem. 2004, 43, 5258−5267.
(16) Sahota, N.; Ferrence, G. M.; Lash, T. D. Synthesis and
Properties of Carbaporphyrin and Carbachlorin Dimethyl Esters
Derived from Cyclopentanedialdehydes. J. Org. Chem. 2017, 82,
9715−9730.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tdlash@ilstu.edu.
ORCID
Timothy D. Lash: 0000-0002-0050-0385
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
under Grant No. CHE-1465049 and the Petroleum Research
Fund, administered by the American Chemical Society. The
authors also thank NSF-CHE (Grant No. 1039689) for
funding the X-ray diffractometer.
(17) Adiraju, V. A. K.; Ferrence, G. M.; Lash, T. D. Rhodium(I),
Rhodium(III) and Iridium(III) Carbaporphyrins. Dalton Trans 2016,
45, 13691−13694.
(18) Lash, T. D.; Ferrence, G. M. Metalation and Selective
Oxidation of Diphenyl-23-oxa-, thia-, and Selena-21-carbaporphyrins.
Inorg. Chem. 2017, 56, 11426−11434.
(19) (a) Liu, D.; Lash, T. D. Synthesis, Spectroscopy and
Metallation of Mixed Carbaporphyrinoid Systems. Chem. Commun.
REFERENCES
■
(1) Mironov, A. Transition Metal Complexes of Porphyrins and
Porphyrinoids. In Handbook of Porphyrin Science - With Applications to
Chemistry, Physics, Material Science, Engineering, Biology and Medicine;
J
DOI: 10.1021/acs.organomet.8b00863
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2002, 2426−2427. (b) Liu, D.; Ferrence, G. M.; Lash, T. D.
Oxybenziporphyrins, Oxypyriporphyrins, Benzocarbaporphyrins, and
Their 23-Oxa- and 23-Thia-Analogues: Synthesis, Spectroscopic
Characterization, Metalation, and Structural Characterization of a
Palladium(II) Organometallic Derivative. J. Org. Chem. 2004, 69,
6079−6093.
Res. 1986, 19, 209−215. (b) Wayland, B. B.; Sherry, A. E.; Coffin, V.
L. Homogeneous Transition Metal Catalyzed Reactions; American
Chemical Society: Washington, DC, 2009, Vol. 230, pp 249−259.
(c) Cui, W.; Wayland, B. B. Hydrocarbon C-H Bond Activation by
Rhodium Porphyrins. J. Porphyrins Phthalocyanines 2004, 8, 103−110.
(d) de Bruin, B.; Hetterscheid, D. G. H. Paramagnetic (Alkene)Rh
and (Alkene)Ir Complexes: Metal or Ligand Radicals? Eur. J. Inorg.
Chem. 2007, 2007, 211−230.
(39) (a) Maxwell, J. L.; O’Malley, S.; Brown, K. C.; Kodadek, T.
Shape-selective and Asymmetric Cyclopropanation of Alkenes
Catalyzed by Rhodium Porphyrins. Organometallics 1992, 11, 645−
652. (b) O’Malley, S.; Kodadek, T. Asymmetric Cyclopropanation of
Alkenes Catalyzed by a Rhodium Chiral Fortress Porphyrin.
Organometallics 1992, 11, 2299−2302. (c) Maxwell, J. L.; Brown, K.
C.; Bartley, D. W.; Kodadek, T. Mechanism of the Rhodium
Porphyrin-Catalyzed Cyclopropanation of Alkenes. Science 1992, 256,
1544−1547.
́
(20) Berlicka, A.; Dutka, P.; Szterenberg, L.; Latos-Grazynski, L.
̇
Towards True Carbaporphyrinoids: Synthesis of 21-Carba-23-
thiaporphyrin. Angew. Chem., Int. Ed. 2014, 53, 4885−4889.
(21) Lash, T. D. Unexpected Alkyl Group Migration in Palladium-
(II) Benzocarbaporphyrins. Org. Lett. 2011, 13, 4632−4635.
(22) Li, D.; Lash, T. D. Synthesis and Reactivity of Carbachlorins
and Carbaporphyrins. J. Org. Chem. 2014, 79, 7112−7121.
(23) Grabowski, E. Y.; AbuSalim, D. I.; Lash, T. D. Naphtho[2,3-
b]carbaporphyrins. J. Org. Chem. 2018, 83, 11825−11838.
́
(24) Szyszko, B.; Latos-Grazynski, L.; Szterenberg, L. A Facile
̇
Palladium-Mediated Contraction of Benzene to Cyclopentadiene:
Transformations of Palladium(II) p-Benziporphyrin. Angew. Chem.,
Int. Ed. 2011, 50, 6587−6591.
(40) (a) Srinivasan, A.; Toganoh, M.; Niino, T.; Osuka, A.; Furuta,
H. Synthesis of N-Confused Tetraphenylporphyrin Rhodium
Complexes Having Versatile Metal Oxidation States. Inorg. Chem.
2008, 47, 11305−11313. (b) Niino, T.; Toganoh, M.; Andrioletti, B.;
Furuta, H. Rhodium N-Confused Porphyrin-Catalyzed Alkene
Cyclopropanation. Chem. Commun. 2006, 42, 4335−4337.
(25) Pacholska-Dudziak, E.; Skonieczny, J.; Pawlicki, M.;
́
̇
nski, L. Palladium
Szterenberg, L.; Ciunik, Z.; Latos- Grazy
Vacataporphyrin Reveals Conformational Rearrangements Involving
Huckel and Mobius Macrocyclic Topologies. J. Am. Chem. Soc. 2008,
̈
̈
́
(41) Idec, A.; Szterenberg, L.; Latos-Grazynski, L. From para-
̇
130, 6182−6195.
Benziporphyrin to Rhodium(III) 21-Carbaporphyrins: Imprinting
Rh···η2-CC, Rh···η2-CO, and Rh···η2-CH Coordination Motifs. Chem.
- Eur. J. 2015, 21, 12481−12487.
(26) Yoneda, T.; Saito, S.; Yorimitsu, H.; Osuka, A. Palladium(II)-
Triggered Rearrangement of Heptaphyrins to N-Confused Porphyr-
ins. Angew. Chem., Int. Ed. 2011, 50, 3475−3478.
́
(42) Hurej, H.; Pawlicki, M.; Latos-Grazynski, L. Rhodium-Induced
̇
(27) (a) Lash, T. D. Porphyrin Synthesis by the “3 + 1” Approach:
New Applications for an Old Methodology. Chem. - Eur. J. 1996, 2,
1197−1200. (b) Lash, T. D. What’s in a Name? The MacDonald
Condensation. J. Porphyrins Phthalocyanines 2016, 20, 855−888.
(28) Chapelle, J. P.; Elguero, J.; Jacquier, R.; Tarrago, G.
Enehydrazines [(alk-1-en-1-yl)hydrazines]. III. Reaction of 1,2-
Dimethylhydrazine with Carbonyl Compounds; Formation of N-
Methylpyrroles and 3-Pyrazolines. Bull. Soc. Chim. Fr. 1970, 3147−
3155.
Reversible C-C Bond Cleavage: Transformations of Rhodium(III) 22-
Alkyl-m-benziporphyrins. Chem. - Eur. J. 2018, 24, 115−126.
(43) (a) Sessler, J. L.; Mozaffari, A.; Johnson, M. R. 3,4-
Diethylpyrrole and 2,3,7,8,12,13,17,18-Octaethylporphyrin. Organic
Syntheses; John Wiley & Sons, Inc.: Hoboken, NJ, 1992; Vol. 70, p 68.
(b) Roth, S. D.; Shkindel, T.; Lightner, D. A. Intermolecularly
Hydrogen-bonded Dimeric Helices: Tripyrrindiones. Tetrahedron
2007, 63, 11030−11039.
(29) Sessler, J. L.; Johnson, M. R.; Lynch, V. Synthesis and Crystal
Structure of a Novel Tripyrrane-containing Porphyrinogen-like
Macrocycle. J. Org. Chem. 1987, 52, 4394−4397.
(30) Boudif, A.; Momenteau, M. A New Convergent Method for
Porphyrin Synthesis Based on a ‘3 + 1’ condensation. J. Chem. Soc.,
Perkin Trans. 1 1996, 1235−1242.
(31) Lash, T. D. Porphyrins with Exocyclic Rings. Part 9. Porphyrin
Synthesis by the “3 + 1” Approach. J. Porphyrins Phthalocyanines 1997,
1, 29−44.
(32) Jackson, A. H.; Pandey, R. K.; Roberts, E. Total Synthesis of an
N-Methylporphyrin. J. Chem. Soc., Chem. Commun. 1985, 470−471.
(33) Pandey, R. K.; Jackson, A. H.; Smith, K. M. Ring Syntheses of
the Four N-Methylprotoporphyrin Isomers and of Related N-Methyl
and N,N’-Dimethylporphyrins. J. Chem. Soc., Perkin Trans. 1 1991,
1211−1220.
(34) Arnold, Z. Synthetic Reactions of Dimethylformamide. XXII.
Formation and Preparation of Formyl Derivatives of Indene. Collect.
Czech. Chem. Commun. 1965, 30, 2783−2792.
(35) Lash, T. D.; Hayes, M. J.; Spence, J. D.; Muckey, M. A.;
Ferrence, G. M.; Szczepura, L. F. Conjugated Macrocycles Related to
the Porphyrins. 21. Synthesis, Spectroscopy, Electrochemistry, and
Structural Characterization of Carbaporphyrins. J. Org. Chem. 2002,
67, 4860−4874.
(36) Bruno, I. J.; Cole, J. C.; Kessler, M.; Luo, J.; Motherwell, W. D.
S.; Purkis, L. H.; Smith, B. R.; Taylor, R.; Cooper, R. I.; Harris, S. E.;
Orpen, A. G. Retrieval of Crystallographically-derived Molecular
Geometry Information. J. Chem. Inf. Comput. Sci. 2004, 44, 2133−
2144.
(37) Thompson, S. J.; Brennan, M. R.; Lee, S. Y.; Dong, G. Synthesis
and Applications of Rhodium Porphyrin Complexes. Chem. Soc. Rev.
2018, 47, 929−989.
(38) (a) Brothers, P. J.; Collman, J. P. The Organometallic
Chemistry of Transition-Metal Porphyrin Complexes. Acc. Chem.
K
DOI: 10.1021/acs.organomet.8b00863
Organometallics XXXX, XXX, XXX−XXX