Corroles cannot ruffle

Article abstract of DOI:10.1021/ic1017032

X-ray structures of Co^III[(CF₃)₃Cor] (PPh₃) [(CF₃)₃Cor = meso-tris(trifluoromethyl) corrolato] and Cu[(CF₃)₄Por] [(CF₃) ₄Por = meso-tetrakis(trif

Full text of DOI:10.1021/ic1017032

ARTICLE
pubs.acs.org/IC
Corroles Cannot Ruffle
Kolle E. Thomas,† Jeanet Conradie,†,‡ Lars Kristian Hansen,† and Abhik Ghosh*,†
†Department of Chemistry and Center for Theoretical and Computational Chemistry, University of Tromsø, 9037 Tromsø, Norway
‡Department of Chemistry, University of the Free State, 9300 Bloemfontein, Republic of South Africa
S Supporting Information
b
ABSTRACT: X-ray structures of CoIII[(CF3)3Cor](PPh3) [(CF3)3Cor =
meso-tris(trifluoromethyl)corrolato] and Cu[(CF3)4Por] [(CF3)4Por =
meso-tetrakis(trifluoromethyl)porphyrinato] revealed planar and highly
ruffled macrocycle conformations, respectively, in line with analogous
observations for a handful of other meso-perfluoroalkylated porphyrins
and corroles reported in the literature. To gain insights into the difference in
conformational behavior, we evaluated DFT (BP86-D/TZP) ruffling
potentials for a variety of corrole complexes, as well as their porphyrin
analogues. The calculations led us to conclude that corrole derivatives, in
essence, cannot ruffle.
’ INTRODUCTION
Intrigued by this apparent difference between porphyrins and
corroles, we set out to find an example of a ruffled corrole
derivative. The most common factor that leads to ruffling in
porphyrins is a small coordinated metal ion, most notably low-
spin Ni(II), but also low-spin Fe(III).7 This, however, is not a
productive approach to ruffled corroles, because nearly all
metallocorroles already have short MꢀN distances (∼1.9 Å).
Another strategy that leads to ruffling in porphyrins involves
bulky meso- substituents such as t-butyl, trifluoromethyl, and
other perfluoroalkyl groups. This is the approach we have chosen
here for corroles, arguing that meso-tris(trifluoromethyl)corrole
derivatives should be among the most likely to exhibit a ruffled
macrocycle. Toward this end, we solved single-crystal X-ray
structures of CoIII[(CF3)3Cor](PPh3) [(CF3)3Cor = meso-tris-
(trifluoromethyl)corrolato] and Cu[(CF3)4Por] [(CF3)4Por =
meso-tetrakis(trifluoromethyl)porphyrinato]. Although we failed
to find any evidence of a ruffled corrole, the X-ray structures
afford a number of insights into the structural chemistry of
corroles. The experimental studies were also supplemented by
extensive DFT calculations of porphyrin and corrole potential
energy surfaces. Together, the experimental and computational
results indicate that strong ruffling is virtually impossible for
corrole derivatives.
Since corroles became available via one-pot syntheses some 15
years ago,1 their chemistry has grown explosively and in certain
respects has begun to rival that of their better known congeners,
the porphyrins.2 In our laboratory, we have focused on funda-
mental aspects of metallocorroles, particularly on the question of
ligand noninnocence3,4 as well on the substituent-sensitive
electronic absorption spectra of certain metallocorroles.5 Re-
cently, we have sought to better understand the structural
chemistry of corroles, using a combined experimental5 (X-ray
crystallography) and theoretical6 (mostly DFT, but also ab
initio6d) approach. One of our more interesting findings is that
copper corroles, even sterically unhindered ones, are inherently
saddled; i.e., the pyrrole rings are alternately tilted relative to the
mean N4 plane.5 In this study, we sought to find and characterize
a ruffled corrole.
At present, the Cambridge Structural Database contains over
200 crystal structures of corrole derivatives, a rather small
number compared to the number of structurally characterized
porphyrins. Certain trends are nevertheless starting to emerge,
which suggest, interestingly, that the structural chemistry of
metallocorroles is rather different from that of metalloporphyr-
ins. Thus, in contrast to porphyrins, for which significant non-
planar distortions are common,7 the majority of metallocorroles
exhibit essentially planar macrocycle frameworks (although the
metal may reside somewhat outside the mean corrole plane).8
Several five-coordinate corrole complexes exhibit distinct
doming,2,9 but strong saddling is rare, being largely limited to
copper and silver corroles,5,10 and strong ruffling, where the
pyrrole rings are alternately twisted about the MꢀN bonds, is
unknown.
’ RESULTS
The X-ray structure of CoIII[(CF3)3Cor](PPh3), shown in
Figure 1a, reveals an essentially planar macrocycle. Except for an
Received: August 21, 2010
Published: March 02, 2011
r
2011 American Chemical Society
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Figure 1. ORTEPs (20% thermal ellipsoids). (a) CoIII[(CF3)3Cor](PPh3). Distances (Å): CoꢀN1, 1.875(2); CoꢀN2, 1.8847(17); CoꢀN3,
1.886(2); CoꢀN4, 1.873(2); and CoꢀP, 2.2034(6). (b) Cu[(CF3)4Por] (“top” and “side” views). Distances (Å): CuꢀN1, 1.988(9); CuꢀN2,
1.931(14); CuꢀN3, 1.931(12); and CuꢀN4, 1.977(10). Dihedrals (deg): C4ꢀN1ꢀN3ꢀC11, ꢀ44.6(16); C1ꢀN1ꢀN3ꢀC14, ꢀ39.6(15);
C6ꢀN2ꢀN4ꢀC19, 45.4(14); and C9ꢀN2ꢀN4ꢀC16, 37.6(15).
interesting corroleꢀcorrole stacking interaction (not shown),
the structure is otherwise unremarkable. The X-ray structures of
Re[(CF3)3Cor](O)11 and Ga[(n-C3F7)3Cor](py)12 (py =
pyridine) also exhibit planar corrole ligands, although the metal
ions in these cases exhibit somewhat larger out-of-plane displace-
ments. In constrast, the meso-perfluoroalkylated metalloporphy-
rin Zn[(n-C3F7)4Por](py),13 as well as the β-octabrominated
complex, Ni[Br8(n-C3F7)4Por],14 features a strongly ruffled
macrocycle, and the same is found here for Cu[(CF3)4Por]
(Figure 1b). Thus, seen from a porphyrin perspective, the
absence of ruffling in (CF3)3Cor complexes11,12 is striking.
These results suggest that ruffling is highly unfavorable for
corroles, a conclusion that is fully supported by DFT calculations
(see Experimental Section for details). Potential energy curves
were obtained for ruffling and saddling by a series of constrained
optimizations, where the ruffling or saddling dihedral (defined in
Figure 2a) was constrained to specific values, while all other
geometry parameters were fully optimized. Figure 2b presents
DFT ruffling potentials for CoIII[Cor](PH3), CoIII[(CF3)3Cor]-
(PH3), Cu(Por), and Cu[(CF3)4Por]. The results confirm what
is observed crystallographically: viz., whereas meso-CF3 groups
lead to a strongly ruffled minimum for a porphyrin, they fail to
ruffle a metallocorrole.
Very small central ions such as SiIV, PV, and AsV lead to strong
ruffling in porphyrins.15 For corroles, although a few PV com-
plexes have been synthesized,16 only one has been structurally
characterized, viz., [P(EMC)(OH)]Cl, where EMC = 8,12-
diethyl-2,3,7,13,17,18-hexamethylcorrole.16b We therefore eval-
uated DFT ruffling potentials for P(Cor)F2, [P(Cor)(OH)]þ,
As(Cor)F2, [As(Cor)(OH)]þ, and [P(Por)F2]þ (Figure 2c),
arguing that the corroles in this series should be among the most
likely to exhibit ruffled conformations. A planar minimum,
however, was found for each of the corrole derivatives, in
agreement with the crystal structure of [P(EMC)(OH)]Cl, in
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ligands falling off the iron, and the remaining FeIII(Cor)(PhNC)
complex had a relatively planar corrole ligand.
Finally, to obtain a “feel” for the energetics of ruffling versus
saddling, we evaluated ruffling and saddling potentials for two
different metallocorroles, Cu(Cor) and CoIII[Cor](PH3). As
shown in Figure 2d, for each molecule, the ruffling potential is
much steeper than the saddling potential. Stated differently,
corroles are much more deformable with respect to the saddling
dihedral χ than with respect to the ruffling dihedral ψ (see
Figure 2a).
’ DISCUSSION
The above results strongly suggest that corroles cannot ruffle,
at least to any appreciable degree. To what extent is this
conclusion supported by the more than 200 corrole crystal
structures reported to date? A search of the Cambridge Structural
Database did not reveal a single instance of a significantly ruffled
corrole. In no case did the ruffling dihedrals exceed values of
about 10° or so, compared with values of 30ꢀ50° that are
routinely found for porphyrins. Nor did we find a case where the
mean displacement of the corrole meso carbons above the N4
plane exceeded 0.20ꢀ0.25 Å; indeed in an overwhelming
majority of reported corrole crystal structures, the meso carbon
displacements are far smaller. It is worth pointing out, however,
that the term ruffling has occasionally been used simply to refer
to any form of nonplanar distortion. Thus, although certain
copper corroles have been described as ruffled, they are in fact
better described as saddled. This is consistent with our proposal,
now strongly supported by X-ray crystallography and DFT
calculations, that copper corroles are inherently saddled.5 In other
words, copper corroles are significantly saddled (χ = 30ꢀ45°)
even in the absence of sterically hindered substituents, although the
latter do accentuate the saddling.
A final question worth considering is whether the very idea
that strong ruffling is essentially impossible for corroles is a trivial
one, a fait accompli. Given that ruffling involves pyramidalization
of the C1ꢀC19 bipyrrole linkage (see Figure 1a for the atom
numbering), it is not surprising, one might argue, that it is highly
unfavorable. Indeed, mild pyramidalization of the bipyrrolic
double bond is observed for domed corroles; such pyramidaliza-
tion, however, never translates to ruffling, i.e., alternate twisting of
the pyrrole groups about the metalꢀnitrogen bonds. A similar
argument might also be made for saddling, which entails twisting
about the C1ꢀC19 linkage, and yet medium to strong saddling is
well-documented for copper corroles.5,16 Therefore, the finding
that the energetic cost of saddling is considerably lower than that
of ruffling (as shown in Figure 2d) is not trivial and, most likely,
could not have been a priori predicted.
Figure 2. (a) Definition of ruffling (ψ) and saddling (χ) dihedrals.
BP86-D/STO-TZP ruffling potentials for (b) Co and Cu porphyrins/
corroles and for (c) P and As porphyrins/corroles. (d) Comparison of
ruffling (solid lines) and saddling potentials (dotted lines).
contrast to [P(Por)F2]þ, for which a highly ruffled geometry was
obtained.15
’ CONCLUSION
Combined use of X-ray structural studies and DFT calcula-
tions has proved to be a valuable approach in mapping out the
structural chemistry of corroles,5 still a rather young field, relative
to similar studies on porphyrins. In this work, based on the crystal
structure of CoIII[(CF3)3Cor](PPh3) and a large set of DFT
calculations inspired thereby, we have shown that strong ruffling
is essentially impossible for corrole complexes. This is a negative
but nontrivial conclusion; it is a major difference from porphyrin
chemistry, where ruffling occurs widely and strongly modulates
physical and chemical properties as well as biological function.7
To further exclude the possibility of ruffling in a metallocor-
role, we also examined the bis-isocyanide complex FeIII(Cor)-
(PhNC)2. In analogous cationic iron(III) porphyrins, the axial
isocyanide ligands strongly stabilize the two dπ orbitals, leading
1
1
to a relatively unusual dxy ground state.17 Such dxy iron(III)
porphyrins are generally strongly ruffled, because ruffling allows
an otherwise symmetry-forbidden overlap between the iron dxy
orbital and the porphyrin a2u HOMO. For FeIII(Cor)(PhNC)2,
however, geometry optimization led to one of the isocyanide
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With the above results in place, we are now in a position to
comment on a large part of the conformational space of corrole
complexes. The overall picture may be summarized as follows:
(a) By and large, corrole complexes are relatively planar.2 (b)
Mild to moderate doming is also common for five-coordinate
complexes.2 (c) With a coordinated BHB group, even a kind of
waved conformation has been observed,18 although this must be
viewed as somewhat of a curiosity. (d) Saddling is uncommon,
although copper corroles, which are inherently saddled, are an
important exception. (e) Finally, as shown in this work, ruffling is
impossible for corroles, except to a purely nominal degree.
mixture heated to 80 °C over the course of an hour. Stirring was
continued at this temperature for an additional 4 h. The brown liquid
thus obtained was cooled to room temperature and dissolved in 10 mL of
CH2Cl2. DDQ (454 mg, 2 mmol, dissolved in 10 mL THF) was added,
and the suspension was stirred for 20 min. Workup and purification were
carried out as in part a to yield 6.0 mg (0.60%) of the free-base corrole as
tiny purple needles.
Synthesis of [5,10,15-Tris(trifluoromethyl)corrolato](tri-
phenylphosphine)cobalt(III). Free-base 5,10,15-tris(trifluorometh-
yl)corrole (12.0 mg, 0.024 mmol) and anhydrous sodium acetate (44.7
mg, 0.526 mmol, 22 equiv) were dissolved in absolute ethanol
(12.0 mL). After stirring for 5 min, Co(OAc)2 4H2O (44.7 mg,
3
0.179 mmol, 7.5 equiv) and triphenylphosphine (75.2 mg, 0.286 mmol,
12 equiv) were added in that order to the purple reaction mixture.
Stirring was continued for an additional 20 min, when TLC indicated
complete consumption of the free base. The reddish-brown reaction
mixture was evaporated, and the residue was chromatographed on a silica
gel column (31 cm ꢁ3 cm) with 95:5 n-hexane/CH2Cl2 as eluent
(700 mL) to yield the product as the first reddish-brown band. Yield: 14
mg (71.4%). Slow diffusion of a saturated benzene solution of the cobalt
corrole into n-hexane gave brown X-ray quality crystals of CoIII[(CF3)3-
Cor](PPh3). UVꢀvis (CHCl3), λmax (nm), [(log ε (Mꢀ1cmꢀ1)]: 369
(4.65) and 407 (4.75), 508 (3.71), 540 (3.64), 585 (3.97). 1H NMR: δ
9.22ꢀ9.17 (m, 2H); 9.16ꢀ9.11 (m, 2H); 9.05 (d, 2H); 8.91ꢀ8.85 (m,
2H); 7.03ꢀ6.97 (qt, 3H, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, para-H of PPh3);
’ EXPERIMENTAL SECTION
Materials. All reagents and solvents were used as purchased, except
pyrrole, which was predried and distilled from CaH2 at reduced pressure.
Silica gel 60 (0.04ꢀ0.063 mm particle size, 230ꢀ400 mesh; Merck) was
used for flash chromatography. Silica gel 60 preparative thin-layer
chromatographic plates (20 cm ꢁ 20 cm, 0.5 mm-thick, Merck) were
used for final purification of the copper(II) tetrakis(trifluoromethyl)-
porphyrin.
Instrumentation. Ultravioletꢀvisible spectra were recorded on an
HP 8453 spectrophotometer with dichloromethane as the solvent.
NMR spectra were recorded on a Mercury Plus Varian spectrometer
(400 MHz for 1H and 376 MHz for 19F) at room temperature in
chloroform-d. Proton chemical shifts (δ) in parts per million were
referenced to residual chloroform (δ = 7.2 ppm). Fluorine-19 chemical
shifts (δ) in parts per million were referenced to 2,2,2-trifluoroethanol-
d3 (δ = ꢀ77.8 ppm). MALDI-TOF mass spectra were recorded on a
Waters Micromass MALDI micro MX Mass Spectrometer using R-
cyano-4-hydroxycinnamic acid (CHCA) as matrix.
3
4
6.63ꢀ6.55 (dt, 6H, JHH = 7.8 Hz, JHH = 3.2 Hz, meta-H of PPh3);
4.28ꢀ4.18 (ddd, 6H, 3JHH = 11.6 Hz, 4JHH = 7.8 Hz, 5JHH = 1.2 Hz, ortho-
H of PPh3). 19F NMR: δ ꢀ43.89 (t, 3F, 5JFH = 3.0 Hz); ꢀ46.33 (t, 6F,
5JFH = 2.6 Hz). MS (MALDI-TOF, major isotopomer): Mþ = 819.21
(expt), 820.08 (calcd). Elemental analysis: 58.78% C (58.55% calcd),
2.75% H (calcd 2.83%), 6.81% N (calcd 6.83%).
Synthesis of [5,10,15,20-Tetrakis(trifluoromethyl)porph-
yrinato]copper(II). Trifluoroacetaldehyde hydrate (495 μL, 6 mmol),
pyrrole (420 μL 6 mmol), and trifluoromethanesulfonic acid (17 μL,
0.2 mmol, hissing upon addition) were introduced sequentially into a
25-mL round-bottomed flask. The mixture was heated to 75 °C over the
course of an hour and allowed to stir for 4.5 h at this temperature. The
orange-redgelthus obtainedwasallowedtocooltoroom temperature and
Synthesis of Free-Base 5,10,15-Tris(trifluoromethyl)-
corrole. a. From Trifluoroacetaldehyde Hydrate. Trifluoroacetalde-
hyde hydrate (ca. 75% in water, 467 μL, 6 mmol, d = 1.49 g cmꢀ3; Alfa
Aesar, tech grade) and trifluoromethanesulfonic acid (17 μL, 0.2 mmol;
Fluka, >99%) were introduced into a 25 mL round-bottomed flask in
that order. Pyrrole (420 μL, 6 mmol; Merck) was added, upon which the
mixture changed from colorless to pale yellow. The mixture was then
heated to 80 °C within 1 h and stirred at this temperature for an
additional 4 h. During this period, the mixture became orange and finally
brown. The brown liquid was allowed to cool to room temperature and
dissolved in 10 mL of CH2Cl2. DDQ (681 mg, 3 mmol, Fluka; dissolved
in 10 mL THF) was added and the suspension stirred for 15 min. The
reaction mixture was diluted with 20 mL of n-hexane and filtered
through silica gel on a B€uchner filter (3.5 ꢁ 5.5 cm) and further down
with small volumes of 1:1 n-hexane/CH2Cl2. The brown filtrate
obtained was evaporated, and the residue was subjected to column
chromatography on silica gel (20 cm in length) with 9:1 n-hexane/
CH2Cl2 as eluent (1100 mL). After 2ꢀ3 bands of green impurities, the
free base corrole was collected as the third purple band. The purple
eluate was evaporated, and the residue was crystallized from 1:1 hexane/
CH2Cl2 to yield 6.6 mg (0.65%) of the free-base corrole as tiny needles.
UVꢀvis (CHCl3), λmax (nm), [(log ε (Mꢀ1 cmꢀ1)]: 397 (4.89) and 403
1.30ꢀ1.15 (m, 4H, β-H); 0.87ꢀ0.78 (m, 4H, β-H). 19F NMR:
δ ꢀ38.60 to ꢀ39.20 (broad). MS (MALDI-TOF, major isotopomer):
[M þ H]þ = 503.08 (expt), 503.09 (calcd). Elemental analysis:
52.58% C (calcd 52.60%), 2.09% H (calcd 2.21%), 11.15% N (calcd
11.15%).
b. From Trifluoroacetaldehyde Methyl Hemiacetal. Trifluoroacetal-
dehyde methyl hemiacetal (574 μL, 6 mmol, Alfa Aesar) and trifluor-
omethanesulfonic acid (17 μL, 0.2 mmol, Fluka, >99.0%) were
introduced into a 50 mL round-bottomed flask in that order. After
stirring for 5 min, pyrrole (420 μL, 6 mmol, Merck) was added and the
then was dissolved in 5 mL of pyridine. Cu(OAc)2 H2O (600 mg,
3
3 mmol) was added to the red solution, and the mixture was stirred at
75 °C for 4 h. The resulting black viscousreactionmixture was evaporated.
After the addition of 5 mL of CH2Cl2 (5 mL), the residue, now a slurry,
was slowly vacuum-filtered through silica gel (3.5 cm thickness, 5.5 cm in
diameter) on a B€uchner funnel and washed down with 1:1 n-hexane/
CH2Cl2. The purple and orange fractions of the filtrate were evaporated,
and the residue thus obtained was chromatographedon a silica gel column
(20 cm ꢁ 3 cm) with 5:1 n-hexane/CH2Cl2 as eluent, giving the copper
porphyrin as the first purple eluate (15 mg). The last orange eluate after
two additional purple impurity bands was the corresponding copper
corrole (which is rather unstable and is not described here in depth).
Further purification of the purple solid by preparative TLC with 4:1
n-hexane/CH2Cl2 as eluent yielded 13 mg (1.3%) of the pure copper
porphyrin. Purple rectangular X-ray quality crystals were obtained by slow
evaporation of a saturated CHCl3 solution of the complex within 2ꢀ3
weeks. UVꢀvis (CHCl3), λmax (nm), [(log ε (Mꢀ1cmꢀ1)]: 406 (5.28),
1
(4.88), 497 (3.47), 537 (3.80), 550 (3.85), 598 (3.61). H NMR: δ
544 (3.84), 584 (4.27). MS (MALDI-TOF, major isotopomer): M þ
=
642.86 (expt), 642.98 (calcd). Elemental analysis: 44.88% C (44.77%
calcd), 1.21% H (calcd 1.25%), 8.62% N (calcd 8.70%).
Computational Methods. All calculations were carried out with
the BP8619 exchange-correlation functional, with Grimme’s dispersion
corrections,20 all as implemented in ADF 2009.21 An STO-TZP basis set
was used throughout, as well as fine meshes for numerical integration of
matrix elements and tight criteria for geometry optimization. Ruffling
and saddling potentials were obtained via constrained optimizations,
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: abhik@chem.uit.no.
’ ACKNOWLEDGMENT
This work was supported by the Research Council of Norway
and the National Research Foundation of the Republic of South
Africa.
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(9) For an exceedingly strongly domed corrole acting as a binucleat-
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(10) (a) Br€uckner, C.; Brinas, R. P.; Bauer, J. A. K. Inorg. Chem. 2003,
42, 4495–4497. (b) Luobeznova, I.; Simkhovich, L.; Goldberg, I.; Gross,
Z. Eur. J. Inorg. Chem. 2004, 1724–1732. (c) Br€oring, M; Bregier, F.;
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(11) Tse, M. K.; Zhang, Z.; Chan, K. S. Chem. Commun. 1998,
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