Rhodium Complexes of a New-Generation Sapphyrin: Unique Structures, Axial Chirality, and Catalysis

Article abstract of DOI:10.1002/chem.201804138

Rhodium insertion into the new 5,10,15,20-tetrakis(trifluoromethyl)sapphyrin was found to be much more facile than for other analogues, owing to NH???F hydrogen-bonding interactions that stabilise the pyrrole-inverted structure characteristic of the metallated product. The thus-obtained rhodium(I) complexes have axial chirality, and the enantiomers were resolved. The latter were found to interconvert quite rapidly in a process that involves a tautomerisation-like movement of the metal fragment between the five N atoms. The rhodium sapphyrins were investigated as catalysts for organic synthesis, by studying their carbene-transfer activity in the cyclopropanation of styrene with ethyl diazoacetate and comparing it to that of rhodium corroles.

Full text of DOI:10.1002/chem.201804138

A Journal of
Accepted Article
Title: Rh complexes of a new generation sapphyrin: unique structures,
axial chirality, and catalysis thereby
Authors: Zeev Gross, Qiu-Cheng Chen, Irena Saltsman, Natalia
Fridman, Zi-ye Xiao, Xuan Zhan, and Alexander Kaushansky
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To be cited as: Chem. Eur. J. 10.1002/chem.201804138
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Rh complexes of a new generation sapphyrin: unique structures,
axial chirality, and catalysis thereby
Qiu-Cheng Chen, Irena Saltsman, Alexander Kaushansky, Zi-ye Xiao, Natalia Fridman, Xuan Zhan,
and Zeev Gross*
Abstract: Rhodium insertion into the new 5,10,15,20-tetra(trifluoro-
methyl)sapphyrin was found to be much more facile than for other
analogues, attributable to unique NH-F hydrogen bonding
interactions that stabilize the pyrrole-inverted structure characteristic
of the metallated product. The thus obtained rhodium(I) complexes
are of axial chirality and the enantiomers were resolved. The latter
were found to interconvert quite fast in a process that involves a
tautomerization-like movement of the metal fragment between the
five N atoms. Also reported is the examination of the rhodium
sapphyrins as catalysts for organic synthesis, performed by looking
into their carbene-transfer activity using cyclopropanation of styrene
by ethyl diazoacetate (EDA) and comparing it to rhodium corroles.
Introduction
Scheme 1. The basic structures of porphyrin, corrole and sapphyrin, with the
bipyrrole moieties of corrole and sapphyrin highlighted in red and the latter
drawn in the normal and inverted conformations that are common for the β-
Metal complexes of porphyrins and related macrocycles play
indispensable roles in numerous catalytic biochemical
processes,[1] which continues to be the motivation for research
on expanded, contracted and modified porphyrins.[2] The first
isolated free-base porphyrin appears to be hematoporphyrin
(named cruentine by Thudichum who reported it in 1867),[3]
while the first expanded and contracted porphyrin analogs were
reported around 100 years later: sapphyrins by Woodward[4] and
corroles by Johnson.[5] Common to all three macrocycles is an
aromatic conjugation pathway and fast tautomerism that makes
pyrrole and meso-C-substituted sapphyrins, respectively. One particular
tautomer is drawn in all cases.
Sapphyrin chemistry is by large focused on the free-base
derivatives with impressive achievements in their utilization as
anion and cation receptors, PDT sensitizers and nonlinear
optical materials.[10] Meanwhile, their coordination chemistry
remained limited,[10b,11] and only rhodium complexes were
reported for meso-aryl-substituted sapphyrins.[11,12] The two
reasons for the restricted progress are: a) the N5 core of
sapphyrins is a much less suitable coordination sphere than the-
all inner
N
atoms equivalent.[6] The features shared by
sapphyrins and corroles are: the principle rotation axis being C2
rather than C4 in porphyrins; the presence of two directly linked
pyrrole rings; and three NHs rather than two in porphyrins
(Scheme 1). The last mentioned property remains the main
driving force for the extensive investigation of the very rich and
often unique coordination chemistry of metal ions chelated by
corroles.[7] Metalation of corroles by transition/post-transition
metal ions is not only facile, but in fact often stabilizes the ligand
and induces diverse functions.[8] These factors and the
revolutionary improvements of free-base corrole synthesis (such
as H3(tpfc), 5,10,15-tris-(pentafluorophenyl)-corrole) during the
last two decades led to the introduction of metallocorroles as
catalysts for organic, inorganic, and bioinorganic processes
relevant to the worldwide efforts devoted to cleaner energy
resources and disease fighting.[9]
Scheme 2. Synthesis of Rh(H2tfs)(CO)2 and Rh(H2tfs)(CO)(PPh3) and
drawing of the rhodium complexes of the other expanded or contracted
porphyrin mentioned in the text (py = pyridine). Note that the location of the
inner NH protons in the free-base sapphyrin is tentative only.
[*]
Prof. Z. Gross, Dr. I. Saltsman, Dr. Alexander Kaushansky, Dr. N.
Fridman, Q. C. Chen, Z. Y. Xiao, and X. Zhan
Schulich Faculty of Chemistry
Technion-Israel Institution of Technology
Haifa 32000 (Israel)
E-mail: chr10zg@technion.ac.il
-N4 core present in porphyrins and corroles and b) pyrrole ring A
rotation (Scheme 1) dramatically alters the chelating properties
of the sapphyrin core.[10a,13] Neutral meso-aryl-substituted
sapphyrins have only recently been fully characterized,
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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disclosing structures both with and without an inverted pyrrole
ring A.[14,11] Truly surprising is that there is just a single case that
metal complexes of sapphyrins were claimed to be useful for
catalysis, which was one motivation for the current research.[15]
The other one was that we recently gained access to the first
meso-alkyl-substituted sapphyrin,[16] which displayed several
novel features, and it was hence of interest to see if and how
they may come into effect in practical applications. We now
report the results obtained with five rhodium complexes,
chelated by sapphyrins and corroles with either C6F5 or CF3
groups on their respective meso-C atoms (Scheme 2).
relative to H3(tpfs). In the former but not latter case, the
conformation with inverted pyrrole is absolutely preferred even
before metallation and there is hence no extra penalty in moving
from free-base to the metal complex. We note that the same
kind of explanation likely holds for the case where both the free-
base sapphyrin and the metal complex do not prefer an inverted
pyrrole: for the sapphyrin that has both meso-C6F5 and β-
pyrrole-alkyl substitutents, even the insertion of two rhodium
ions has been reported to proceed very smoothly.[11]
Results and Discussion
Insertion of rhodium into the new 5,10,15,20-tetra(trifluoro-
methyl)sapphyrin, H3(tfs), appeared to be much more facile (1 h,
RT, 90% yield) than for 5,10,15,20-tetra(pentafluorophenyl)-
sapphyrin H3(tpfs),[12] which is reminiscent of the mild reaction
condition for metallation of heterosapphyrin.[17] The decrease in
symmetry from C2v to C1 upon metalation came into effect by
four instead of two CF3-attributed signals in the 19F NMR
spectrum (Figure S1), as well as in the doubling of the number
of 1H NMR resonances due to the normal and inverted β-pyrrole-
H atoms (Figure 1a).[10b] The molecular structure of the product
was deduced to be Rh(H2tfs)(CO)2, composed of a d8 low-spin
rhodium(I) metal center coordinated by two N atoms of the
mono-anionic macrocycle and two carbonyls. This is consistent
with its high-resolution mass spectrum (Figure S10), the sharp
1H NMR spectrum and the appearance of two NH protons
therein (Figure 1a). The about 15 ppm difference in the chemical
shift of these two protons clearly indicates that pyrrole ring A
(Scheme 1) remains inverted after metalation (and even when
treated with up to 40 equivalents of trifluoroacetic acid). This is
reminiscent of the previously reported meso-C6F5 analogue
Rh(H2tpfs)(CO)2,[12] but different from the Rh(I) and Ir(I)
complexes of β-pyrrole-alkylated sapphyrins wherein all pyrroles
are not inverted[15] and also from the Rh(I) complexes of meso-
aryl heterosapphyrin wherein the metal ion is coordinated by the
N atoms of the bipyrrole sub-unit.[17a]
Confirmation of the above conclusions was obtained by the X-
ray crystal structures of both Rh(H2tfs)(CO)2 (Figure 1b-d) and
Rh(H2tpfs)(CO)2 (Figure 2), which also disclosed the following
information. The dihedral angles of the most deviating pyrrole
ring A were found to be roughly 35o (Figures 1d and S17b) and
the mean plane deviations (MPD) of Rh(H2tfs)(CO)2 and
Rh(H2tpfs)(CO)2 are 0.268 Å and 0.176 Å respectively. The
latter values are surprisingly close to those reported for the Rh(I)
complex of a sapphyrin that does not have an inverted pyrrole
ring[11], due to the presence of both meso-C6F5 and β-pyrrole-
alkyl substituents. DMF appeared to be necessary for obtaining
high quality X-ray crystals of Rh(H2tpfs)(CO)2, likely attributable
Figure 1. The 1H NMR spectrum of Rh(H2tfs)(CO)2 with assignment of the NH
and CH protons of pyrrole ring A (a) and three different views of it molecular
structure: (b) front view, with thermal ellipsoids of 50% and highlight of F-HN
distances; (c) side view (meso-CF3 and Rh-carbonyl substituents omitted),
with emphasis on pyrrole ring A (yellow colored), enantiomers arrangement
and the distance between them; (d) side view with emphasis on the square
planar coordination sphere of the rhodium(I) center.
Consistent (although often ignored) with the C1 symmetry, the
crystal structures of Rh(H2tfs)(CO)2 and Rh(H2tpfs)(CO)2
disclose the presence of two enantiomers in a 1:1 ratio (Figure
1c). Separation of the racemic mixture by HPLC with a chiral
column was not only quite difficult, but appeared to be non-
practical for obtaining pure enantiomers. Injection of the
separated fractions revealed the re-formation of a mixture
(Figure S20), clearly indicative of a very facile racemization
pathway for these complexes.
to fixation of the most flexible pyrrole ring
A due to
intermolecular DMFO---HNpyrrole hydrogen bonding interactions
(Figure 2). The structure of Rh(H2tfs)(CO)2 uncovers
intramolecular N-H—F bonding interactions (Figure 1b), which
provides a hint for much more facile Rh insertion into H3(tfs)
Figure 2. Hydrogen bonding between Rh(H2tpfs)(CO)2 and DMF shown by a
dnorm Hirshfeld surface.
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Similar to Rh(H2tfs)(CO)2, the X-ray crystal structure of
Rh(H2tfs)(CO)(PPh3) disclosed pairs of associated enantiomers
with MPD values of 0.263 Å and pyrrole ring A angles of 32.3o
(Figure 4a & Figure 3b). In this case, an almost perfect
separation of the racemic mixture was achieved by chiral HPLC
(Figure 4b-c) and the circular dichroism (CD) spectra of the two
enantiomers displayed strong signals in both the UV and the
visible parts (Figure 4d). The enantiomers were however still
found to be stable only when hexane was used as both eluent
and solvent, while fast racemization took place when
dichloromethane was used as solvent, reminiscent to what
happened with Rh(H2tfs)(CO)2 (Figure S20). This might suggest
that the process responsible for racemization involves
movement of pyrrole ring A, as the intramolecular CF3…H-N
interactions (Figure 1b) that fix its position and prevent the flip
through the core movement of the metal with its ligands are
weakened by a polar solvent.
Figure 3. The X-ray crystal structure of Rh(H2tfs)(CO)(PPh3): (a) front view,
with thermal ellipsoids of 50% (phenyl groups omitted); (b) side view with
emphasis on dihedral angles between pyrrole ring A and the least-square
plane.
Considering that one plausible possibility is that the Rh(CO)2
moiety is able to flip through the core, Rh(H2tfs)(CO)2 was
treated with triphenylphosphine (PPh3) for increasing the steric
hindrance and blocking that racemization route. 1H NMR
analysis of the reaction product, isolated in quite high yield
(Scheme 2), easily revealed that only one carbonyl was replaced
by PPh3 (Figure S7) and that pyrrole ring A remained inverted in
Rh(H2tfs)(CO)(PPh3). The other analytical data such as high
resolution MS (Figure S14) and IR (one rather than two kinds of
carbonyl stretches) were also consistent with the
Rh(H2tfs)(CO)(PPh3) structure, while the position of PPh3
therein was clarified by X-ray crystallography (Figure 3a) to be in
the less sterically crowded part of the macrocycle’s core. Indeed,
attempted replacement of both carbonyls by using either excess
of PPh3 or the bidentate ethylenebis(diphenylphosphine)
(DIPHOS) induced decomposition rather than formation of any
desired product.
Figure 5. Two proposed racemization mechanism of Rh(H2tfs)(CO)2: “Flip-
through” (red labeled) and “Walking-tour” (blue labeled) pathways.
The somewhat less facile racemization of Rh(H2tfs)(CO)(PPh3)
relative to Rh(H2tfs)(CO)2 (Figure S21) triggered our interest in
understanding the racemization pathway. The initial sought was
that the Rh(CO)2 sub-unit is able to move through the
coordination core (Figure 5, “flip-through”) and that this is less
facile for the more bulky Rh(CO)(PPh3) moiety present in
Rh(H2tfs)(CO)(PPh3). This “flip-through” hypothesis was
immediately ruled out by DFT calculations, which confirmed that
the strong repulsive interactions between two CO ligands and
sapphyrin skeleton prevent the racemization via this mechanism.
Potential energy surfaces of alternative “flip-through” pathways,
such as of less bulky Rh(CO) and Rh moieties formed in concert
with temporary dissociation of CO ligand(s), also passed through
intermediates and transition states that are much too high in
energy (Figure S25). The same holds for a variety of other
investigated mechanisms (Figures S19-21), except of the one
that involves migration of both the Rh(CO)2 moiety and protons
among the inner core’s N atoms (Figures 5-7).
Figure 4. The arrangement of two enantiomers in the X-ray structure of
Rh(H2tfs)(CO)(PPh3) (meso-CF3, carbonyl and PPh3 groups omitted), with
emphasis on pyrrole ring A (yellow colored) (a); HPLC profiles (n-hexane as
eluent, OD-H column, UV-detector 488 nm, 0.9 mL/min flow rate) of the
racemic mixture at 9o (purple trace), 25o (grey trace) and 40o C (black trace)
(b); and of the separated fractions at 40o C (c); circular dichroism spectra of
the first (blue trace) and the second (red trace) fractions, at room temperature
in n-hexane (d).
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Figure 7. Free-energy profile for racemization mechanism in DCM at PBE0-
D3(BJ)/BS2//PBE0-D3(BJ)/BS1 level of theory and the proposed structure of
TS1.
Figure 6. Steps of “walking” pathway basing on DFT study. The movement of
pyrrole ring A and the NH proton(s) were highlighted in red.
-using only two N atoms for coordination to the metal centre
(Figure 8b). The effect of CF3 vs. C6F5 on the redox potential for
the sapphyrin was larger than for corroles, with positive shifts of
340 and 190 mV, respectively (Figure 9). Also noteworthy is that
the redox processes of the corroles were much more reversible
than those of the sapphyrins, a phenomenon that will be
addressed only in future studies as it requires multiple
approaches for studying these potentially non-innocent
ligands.[19]
One key feature of the “walking tour” pathway is the 29.2
kcal/mol barrier transformation of Sap1 to Sap2 (step a in
Figures 6 and 7), which is substitution of NC by ND via the 5-
coordinate transition state TS1 (NA, NB, NC, ND, and NE are the N
atoms of the respective pyrrole rings). Further walking of the
Rh(CO)2 moiety would produce Sap1’, the enantiomer of Sap1,
but that does not happen because of a formidably high energy
barrier for direct proton transfer between NB and NC. Inversion of
pyrrole A as to produce Sap3 does however proceed through a
relatively low-lying transition state (step b) and that may now be
followed by much more facile proton transfer from NA to NC (step
c). The thus formed non-chiral intermediate Sap4 may return to
either Sap1 or Sap1’ by reversal of the described steps, thus
completing the racemization process. Transition state TS3 is of
the highest energy (31.8 kcal/mol) in that mechanism and it
includes the inversion of ring A. This might be the rational for the
slower racemization rate in hexane, wherein the loss of CF3…H-
N interactions upon moving from Sap3 to Sap4 could be most
significant.
Figure 8. (a) Crystal structure of Rh(tfc)(py)2; (b) Mean-plane deviation
diagram calculated from the core atoms for Rh(tfc)(py)2 (green),
Rh(H2tfs)(CO)2 (blue) and Rh(H2tfs)(CO)(PPh3) (red).
Armed with the acquired knowledge on the Rh(I) complexes of
the two sapphyrins, their potential catalytic activity was
addressed, in comparison with the rhodium(III) complexes of
corroles with the same meso-C substitutents. Rh(tpfc)(py)2 was
already reported as good catalysts for carbene-transfer
reactions;[18] and we have now prepared the much smaller
1
Rh(tfc)(py)2. The 19F and H NMR spectra (Figure S4) allowed
for full characterization of this new corrole, including the
identification of two pyridine molecules that are strongly bound
to the metal. These conclusions were fully confirmed by analysis
of the X-ray structure (Figure 8a), which disclosed Rh-Npyridine
bond lengths of 2.065(5) Å, in the range of other Rh(III)
corroles.[18] With a MPD of 0.05, Rh(tfc)(py)2 may be considered
as practically planar, in contrast with its sapphyrin analogs
Rh(H2tfs)(CO)2 and Rh(H2tfs)(CO)(PPh3) that have a flexible
pyrrolic unit and a rhodium(I) centre of square planar geometry-
Figure 9. Cyclic voltammetry (RT, CH2Cl2, 0.5 mM substrate, 0.1 M TBAP,
glassy carbon working electrode, 50 mVs-1 scan rate, vs. Fc+/0 in 25 mV) of (a)
Rh(H2tfs)(CO)2 (black trace) and Rh(H2tpfs)(CO)2 (red trace); and (b)
Rh(tfc)(py)2 (black trace) and Rh(tpfc)(py)2 (red trace). The corresponding
square wave spectra of the sapphyrin rhodium complexes are provided in
Figure S22.
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All examined complexes were found to be good catalysts for the
model reaction:[20] cyclopropanation of styrene by ethyl
diazoacetate (EDA, Scheme 3 and Table 1). The complexes
with meso-C6F5 groups, Rh(tpfc)(py)2 and Rh(H2tpfs)(CO)2,
were more selective regarding cyclopropanation vs. EDA
dimerization and anti vs. syn products, than Rh(tfc)(py)2 and
Rh(H2tfs)(CO)2 with their meso-CF3 substituents. Considering
that the rate limiting step in these reaction is commonly the
formation of the metal-carbene intermediate and not the pre-
association of EDA to the metal complex,[21] the above deduced
stronger electron withdrawing effect of CF3 relative to C6F5 is
apparently disadvantageous for this kind of reaction. Meanwhile,
the significant yields of the less stable syn-isomer are
lead to the introduction of these very interesting aromatic[23] but
yet quite flexible molecules[24] as metal-supporting ligands in
many applications much beyond the one we have introduced
here.
Experimental Section
Computational details
All DFT calculations reported in this work were carried out using
the G09 quantum chemical software.[25] Single-point energy
calculations were carried out at the PBE0-D3(BJ)/BS2//PBE0-
D3(BJ)/BS1[26] level of theory with solvation effects modeled by
IEFPC[27] in dichloromethane, BS1 denoting a mixed basis set of
the quasi-relativistic Stuttgart–Dresden–Cologne (SDD) effective
core potential and basis set (SDD) for Rh[28], and 6-31+g for all
other atoms. While BS2 denoting a mixed basis set of effective
core-potential and basis SDD for Rh, and def2-tzvp for all other
atoms. Transition-state structures were confirmed to connect
corresponding reactants and products by intrinsic reaction
coordinate (IRC) calculations.[29]
reminiscent of other corrole and porphyrin-base catalysts (Entry
22]
1, 2, 5&6).[18,
An attempt of further increasing the relative
amounts of that isomer by increasing the steric hindrance
around the metal center by CO/PPh3 substitution was successful
(the last two entries of Table 1), but that came with a price. The
total yield was much lower, probably due to the earlier noticed
diminished stability of the catalysts.
Instrumentations
All routine chemical reagents and solvents were purchased from
commercial sources and were purified by standard procedures
before use. Column chromatography was performed on silica gel
(Kieselgel 60, 230-400 mesh). 1H and 19F NMR spectra were
recorded on Bruker Avance III 400 spectrometer (400 MHz for
1H and 377 MHz for 19F). Chemical shifts are reported in ppm
relative to the residual hydrogen atoms in the deuterated solvent
Scheme 3. Metallosapphyrin/metallocorrole-catalyzed cyclopropanation of
styrene by ethyl diazoacetate (EDA).
CDCl3 (δ
= 7.26). High-resolution mass spectra for the
compounds were performed on a Bruker MaXis Impact mass
spectrometer. Absorption spectra of the samples were
measured on HP 8453 diode array spectrometer.
Electrochemical measurements were recorded with an
EmStat3+ electrochemical system. All the potentials are
referenced versus the Fc+/0 redox potential added as an internal
standard (E1/2 = 0.025 V vs. Ag/Ag+ in 0.1 M TBAP / 0.01 M
Table 1. Cyclopropanation of styrene by ethyl diazoacetate (EDA) catalysed
by the rhodium(I) and rhodium(III) complexes depicted in Scheme 2.[a]
Catalyst
Yield of
diethyl
maleate
2%
2%
3%
3%
1%
1%
Yield of
cyclopropyl
esters
85%
anti/syn
ratio
Rh(tpfc)(py)2
Rh(tfc)(py)2
2.3
2.0
3.0
2.3
1.8
1.7
36%
82%
68%
16%
Rh(H2tpfs)(CO)2
Rh(H2tfs)(CO)2
AgNO3 solution).
A conventional three electrode system
Rh(H2tpfs)(CO)(PPh3)[b]
Rh(H2tfs)(CO)(PPh3)
consisting of a glassy carbon working electrode, a platinum wire
as counter electrode and silver wire separated from the bulk
solution by a sample holder with a porous glass frit in 0.1 M
TBAP / 0.01 M AgNO3 solution. HPLC analysis are performed on
a combination of Elite LaChrom organizer, diode array detector
L-2455, auto-sampler L-2200, oven L-2300 and pump L-2130.
Flow rate: 0.3-1 ml/min; eluent: 100% n-hexane; column:
CHIRALCEL® OD-H, 250 x 4.6mm ID. A GC system consist of
Sion 4210 GC System and Astec® CHIRALDEX® B-PM
Capillary GC Column (L × I.D. 30 m × 0.25 mm, df 0.12 µm) was
used for calculating the syn/trans ration as well as the relative
amount of dimerization product. Hirshfeld surface calculations[30]
19%
[a] catalyst/EDA/styrene = 1/500/5000, addition of EDA in one portion, 0.28-
0.30 mM of catalyst, under Ar, in dichloromethane, at room temperature for 24
h. Yields were determined by GC, relative to an internal standard. [b] This
complex was not isolated, but prepared in situ via addition of 1.6 eq. of PPh3,
based on a titration experiment monitored by UV-vis spectroscopy (Figure
S16).
Conclusions
were performed by using the program CrystalExplorer 3.1[31]
.
Four novel rhodium complexes, three chelated by monoanioinic
sapphyrin and one by trianionic corrole, were synthesized and
fully characterized in this study. The emphasis was on deducing
their structural and electronic properties, including elements of
chirality, as the prerequisite for the application of
metallosapphyrins, which is a surprisingly unfertilized field. We
trust that the level of understanding provided in this study will
Hirshfeld surfaces with dnorm were mapped on Rh(H2tpfs)(CO)2.
dnorm is a normalised contact distance.
X-Ray Crystallographic Details
Single crystals immersed in Paratone-N oil were quickly fished
with a glass rod and mounted on a Kappa CCD diffractometer
under a cold stream of nitrogen at 200 K. Data collection was
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carried out with monochromated Mo Kα radiation using φ and ω
scans to cover the Ewald sphere.[32] Accurate cell parameters
were obtained with complete collections of intensities, and these
were corrected in the usual way.[33] The structure was solved by
SHELXS-97 direct methods[34] and refined by the SHELXL-97
program package. The atoms were refined anisotropically.
Hydrogen atoms were calculated using the riding model.
Synthesis of 5, 10, 15-tris(trifluoromethyl)corrolato
rhodium(III) (Rh(tfc)(py)2).
A mixture of H3(tfc) (20 mg), dry K2CO3 (10 mg), and di-µ-
chloro-bis[dicarbonylrhodium(I)] (70 mg) in pyridine (20 mL) was
refluxed for 2 h until the disappearance of the starting material
(TLC, DCM/n-hexane
=
1/9). Solvent evaporation and
subsequent silica gel chromatography (DCM/n-hexane/pyridine
= 5/95/0.01) afforded Rh(tfc)(py)2 as a deep-red solid material
(19 mg, 82% yield). X-ray quality crystals were obtained by
recrystallization from DCM/n-heptane in the presence of two
Synthesis
of
H3(tfs),
H3(tpfs),
Rh(tpfc)(py)2,
and
Rh(H2tpfs)(CO)2.
The synthetic details for the preparation of H3(tfs), H3(tpfs),
Rh(tpfc)(py)2, and Rh(H2tpfs)(CO)2 were provided in pervious
publication.[12, 16, 18, 35]
Synthesis of 5,10,15,20-tetra(trifluoromethyl)sapphyrinato
dicarbonylrhodium(I) (Rh(H2tfs)(CO)2).
1
drops of pyridine. H NMR (400 MHz, CDCl3) δ = 9.55 (dd, J =
5.1, 2.6, 2H), 9.43 (dd, J = 5.2, 2.8, 2H), 9.26 – 9.21 (m, 2H),
9.15 (d, J = 4.5, 2H), 6.00 (t, J = 7.6, 2H), 5.06 (t, J = 7.2, 4H),
1.23 (s, 4H). 19F NMR (377 MHz, CDCl3) δ = -36.93 (s, 3F), -
39.53 (s, 6F). HRMS- (ACPI, negative mode) for C22H8N4F9Rh:
m/z = 601.9655 (calculated), 601.9650 (observed). UV-vis
(pyridine): max (ε(M-1 cm-1)) = 409 nm (90000), 570 (28000).
H3(tfs) (50 mg) was dissolved in dichloromethane (50 mL).
Excess amount of anhydrous sodium acetate was added to the
solution followed by di-µ-chloro-bis[dicarbonylrhodium(I)] (20
mg), and the mixture was stirred for 1 hour monitored by TLC
(DCM/n-hexane = 1/9) to show a fully conversion. The solvent
was evaporated in vacuo, and the residue was
chromatographed a silica column (DCM/n-hexane = 1/99). The
golden brown fraction was collected (55 mg, 90 % yield), and
Acknowledgements
1
recrystallized in DCM/hexanes to give green crystals. H NMR
This research was supported by a grant from the Israel Science
Foundation. The funding of IS by the Center for Absorption in
Science, Ministry of Immigration (KAMEA Foundation), is also
acknowledged.
(400 MHz, CDCl3) δ = 13.20 (s, 1H), 10.42 (d, J = 4.5, 1H),
10.05 (d, J = 2.3, 1H), 10.01 (d, J = 4.9, 1H), 9.87 (dd, J = 4.9,
2.5, 1H), 9.78-9.71 (app. ddd, J = 10.5, 4.9, 2.5, 2H), 9.68 (d, J =
2.4, 1H), 9.58 (dd, J = 4.7, 2.0, 1H), -2.28 (s, 1H), -2.47 (dd, J =
4.8, 1.6, 1H), -3.42 (dd, J = 4.8, 1.6, 1H). 19F NMR (377 MHz,
CDCl3) δ = -36.62 (s, 3H), -39.00 (s, 3H), -46.34 (s, 3H), -48.61
(s, 3H).HRMS- (ACPI, negative mode) for C30H12F12N5O2Rh: m/z
= 804.9854 (calculated), 804.9860 (observed). UV-vis (toluene):
max (ε(M-1 cm-1)) = 357 nm (23500), 491 (116000), 642 (10600),
697 (12200), 764 (5400), 876 (6200). IR (neat, cm-1): 2067, 2002.
Synthesis of 5,10,15,20-tetra(trifluoromethyl)sapphyrinato
triphenylphosphine-carbonylrhodium(I)
Conflict of interest
The authors declare no conflict of interest.
Keywords: Sapphyrin • Racemization • Porphyrin • Chirality •
Cyclopropanation
(Rh(H2tfs)(CO)(PPh3)).
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5-10%
DCM
in
n-hexane)
afforded
Rh(H2tfs)(CO)(PPh3) (20 mg, 80% yield). X-ray quality crystals
were obtained by recrystallization from DCM and n-heptane. H
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NMR (400 MHz, CDCl3) δ = 12.99 (s, 1H), 9.75 (dd, J = 4.7, 2.5,
1H), 9.69 (dq, J = 5.4, 2.7, 1H), 9.59 (dd, J = 4.7, 2.2, 1H), 9.56
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C12H27ON5PF12Rh: m/z
= 1039.0811(calculated), 1039.0849
(observed). UV-vis (CHCl3): max (ε(M-1 cm-1)) = 496 nm (65800),
654 (11000), 703 (12000). IR (neat, cm-1): 1966.
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Larger affinity to rhodium, easy
resolution of racemic rhodium(I)
complexes, and increased selectivity
to the kinetic cyclypropanation product
are some of the unique features
displayed by the meso-CF3-subsituted
sapphyrin.
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