Chromium(V) and chromium(VI) nitrido complexes of tris(pentafluorophenyl)corrole

Article abstract of DOI:10.1002/anie.200351793

Nitrogen-atom transfer from a (nitrido)-manganese(v) complex to a Cr ^III complex of the 5,10,15-tris (pentafluorophenyl)corrole trianion gave the stable Cr^V nitrido complex 1 (Ar=C₆F ₅). In contrast, treatment of the same Cr^III starting material with ammonia/sodium hypochlorite gave the Cr^VI analogue of 1.

Full text of DOI:10.1002/anie.200351793

Angewandte
Chemie
High-Oxidation-State Cr Complexes
Chromium(v) and Chromium(vi) Nitrido
Complexes of Tris(pentafluorophenyl)corrole**
Galina Golubkov and Zeev Gross*
A unique feature of corroles relative to porphyrins and other
related ligands is their ability to stabilize transition-metal ions
in high oxidation states.[1,2] This phenomenon can not be
related to the traditional means of obtaining high-valent
metal complexes, which relies on p-donating and nonoxidiz-
able ligands such as FÀ, O2À, and N3À, since the corrole
macrocycle in non-transition-metal complexes is easily oxi-
dized.[3] Instead, recent theoretical and experimental evi-
dence suggests that s donation by corrole trianions raises the
energy of metal d orbitals to a large extent. This leads to
destabilization of low-valent oxidation states and is also
manifested in the unique activation of molecular oxygen by
chromium(iii) and iron(iii) corroles.[4,5] Taken together, the
cooperation of both p and s effects in oxo- and nitrido-
metallocorroles could be an attractive methodology for
obtaining novel complexes. The validity of this hypothesis is
indicated by the stability of corrole complexes with penta-
valent metal ions: [(tpfc)CrV(O)] (1),[4b] [(tpfc)MnV(O)] (2),[6]
[(tpfc)MnV(N)]À (3),[7] and [(tpfc)MnV(NAr)] (4),[8] where
tpfc stands for the trianion of 5,10,15-tris(pentafluorophe-
nyl)corrole (Scheme 1).[9] However, complexes of the highly
oxidizing CrVI ion were not reported for corroles or other
related ligands.[10] A relevant result that explains why such
complexes could not be obtained is that the one-electron
oxidation of 1 is corrole- rather than metal-centered and leads
to [(tpfcC)CrV(O)]+ rather than [(tpfc)CrVI(O)]+.[11] Since a
comparison of nitrido and oxo complexes with identical
ligands and metals reveals that the former are invariably more
stable, we decided to explore the possibility of obtaining
stable (nitrido)chromium corroles. These attempts were
successful: both [(tpfc)CrV(N)]À (5a) and [(tpfc)CrVI(N)]
(5b) were isolated and characterized by a combination of
spectroscopic methods and electrochemistry.
The first approach was to treat [(tpfc)CrIII(py)2] (6) with
an excess of NH3 in the presence of a mild oxidant, under the
assumption that ligand substitution and subsequent oxidation
of the coordinated ammonia would lead to an imido or nitrido
complex. Using Br2 or N-bromosuccinimide (NBS), the
reaction proceeded to the CrV(N) state, but partial bromina-
À
tion (up to four) of the b-pyrrolic C H bonds of the corrole
took place concurrently. On the other hand, the oxo complex
[*] Prof. Dr. Z. Gross, G. Golubkov
Department of Chemistry and
Institute of Catalysis Science and Technology
Technion—Israel Institute of Technology
Haifa 32000 (Israel)
Fax: (+972)4-823-3735
E-mail: chr10zg@tx.technion.ac.il
[**] This research (No. 368/00) was supported by the Israel Science
Foundation, the Petroleum Research Fund (ACS), and the Fund for
the Promotion of Research at the Technion.
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DOI: 10.1002/anie.200351793
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Figure 1. Spectral changes upon transformation of [(tpfc)CrIII(pyr)2] (6)
into [(tpfc)CrV(N)]À (5a) by addition of [(salop)Mn(N)] (7).
Scheme 1. Formulas of recently reported oxo-, imido-, and nitrido-
metallocorroles and of the new chromium complexes.
spectroscopy. The spectrum of 5a (Figure 2b) is similar to that
of the (oxo)chromium(v) complex 1 (Figure 2a): both display
hyperfine coupling to 53Cr (A53Cr) and superhyperfine coupling
to nitrogen atoms (A14N) at room temperature. The presence
of five 14N atoms in 5a as opposed to four in 1 is clearly
evident from the 11 rather than nine lines, best seen in the
inset of Figure 2b. In addition, the A53Cr value of 2.67 mTin 5a
is within the range of 2.49–2.84 mTobtained for other
(nitrido)chromium(v) complexes and, as expected, much
larger than in 1 (1.96 in CH3CN and 1.64 mTin CH 2Cl2).
1 was formed when iodosylbenzene was utilized as oxidant.
Because of these limitations, a very different route was
adopted. It consists of preparing the stable (nitrido)manga-
nese(v) complex of salophene,[12] [(saloph)Mn(N)] (7), and
transferring its nitrogen atom to 6 (Scheme 2).[13]
Scheme 2. Synthesis of [(tpfc)CrV(N)]À (5a) by nitrogen-atom transfer
from [(salop)Mn(N)] (7) to [(tpfc)CrIII(pyr)2] (6).
Pronounced changes occur when the two reagents are
mixed: Compund 6 is green and displays a very rich electronic
spectrum, while 5a is purple-green and shows a sharp Soret
band at 427 nm and a characteristic Q band at 585 nm
(Figure 1). Similar to the previously reported (nitrido)man-
ganese(v) complex 3, 5a is particularly stable in acetonitrile,
and accordingly the spectroscopic measurements were per-
formed in this solvent. The strongest support for the assign-
ment of 5a as a (nitrido)chromium(v) complex resulting from
a two-electron atom transfer from 7 is provided by EPR
Figure 2. EPR spectra (in CH3CN at RT) of a) [(tpfc)CrV(O)] (1),
b) [(tpfc)CrV(N)]À (5a), c) [(15N4-tpfc)CrV(N)]À (15N4-5a), and
d) [(tpfc)CrV(15N)]À (5a-15N). Insets: experimental (exp) and simulated
(sim) spectra of the low-field region.
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Examination of Figure 2b reveals one A14N value of about
0.27 mTfor all five nitrogen atoms. This is reminiscent of the
recurring puzzle in the EPR spectra of all (nitrido)chro-
mium(v) complexes with equatorial N donors (for example,
salen, cyclam, and porphyrins),[14] in which the isotropic
hyperfine couplings to the axial nitrido ligand and the other
nitrogen atoms were found to be practically identical, with
and + 1.04 V are supportive of porphyrin- rather than
chromium-centered processes.
Since the electrochemistry of 5a suggested that the
(nitrido)chromium(vi) complex 5b should be quite stable,
the chromium(iii) complex 6 was treated with sodium
hypochlorite.[18] In contrast to porphyrin and salen complexes,
for which this is the method of choice for preparation of the
corresponding (nitrido)chromium(v) complexes,[12,14] the sole
reaction product in the case of 6 was 5b and not 5a. This was
proven by spectroscopy: 5b is EPR-silent and displays well-
resolved 19F and 1H NMR spectra (Figure 4). This is perfectly
in line with expectations for a genuine chromium(vi) complex
(d0).[19]
A
14N = 0.23–0.28 mT. What is more, A14N(nitrido) = 0.29 mTin
the pentacyano(nitrido)chromium(v) complex prepared by
Jesper et al.,[15] the only example to date for which the
superhyperfine coupling tensor to a nitrido ligand could be
measured independently of couplings to other nitrogen atoms.
In an attempt to solve the puzzle, analogues of 5a in which
either the four nitrogen atoms of the corrole or the unique
nitrido ligand are 15N-labeled ([(15N4-tpfc)CrV(N)]À (15N4-5a)
and [(tpfc)CrV(15N)]À (5a-15N), respectively) were pre-
pared.[16] The spectra shown in Figure 2c and d reveal
significantly different superhyperfine coupling constants,
and the best simulations were achieved with A14N(nitrido) =
0.26 mTand A14N(corrole) = 0.36 mTfor 15N4-5a and A14N(ni-
trido) = 0.36 mTand A14N(corrole) = 0.27 mTfor 5a-15N. Since
the isotropic hyperfine coupling constant of 15N is 40% larger
than that of 14N,[17] these measurements prove that the
hyperfine couplings to the axial nitrido ligand and the other
nitrogen atoms in complex 5a are indeed identical.
Figure 4. 1H (inset) and 19F NMR spectra of [(tpfc)CrVI(N)], 5b, in
CD3CN at RT.
Another major difference between 5a and 1 is found in
their cyclovoltammograms (Figure 3). As expected, nitrido
complex 5a is harder to reduce and easier to oxidize than oxo
This study shows that the combination of s donation by
corrole trianions and p donation by N3À is a fruitful approach
for the preparation of metal complexes with unusually high
oxidation states. The (nitrido)chromium(v) corrole was
prepared by nitrogen-atom transfer from (nitrido)mangane-
se(v) salophene, which also serves to demonstrate the large
thermodynamic stability of the former complex. The ammo-
nia/sodium hypochlorite method that is extensively used for
the preparation of (nitrido)chromium(v) complexes of por-
phyrins and salens leads to (nitrido)chromium(vi) when
applied to chromium(iii) corroles.
Received: May 2, 2003 [Z51793]
Keywords: chromium · cyclic voltammetry · EPR spectroscopy ·
.
N ligands · porphyrinoids
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Chem. 1999, 38, 2676.
Figure 3. Cyclovoltammograms (in CH3CN at RT) of a) [(tpfc)CrV(O)]
(1) and b) [(tpfc)CrV(N)]À (5a).
complex 1. Furthermore, the difference between the corre-
sponding E1/2 values in 5a is only 0.59 V, and both its
reduction and oxidation potentials (À0.03 and + 0.56 V,
respectively) are far from the values required for corrole-
centered redox processes (< À1 and > + 1 V, respectively).[3b]
This is remarkably different from the (nitrido)chromium(v)
porphyrin [(tpp)Cr(N)],[14a] whose redox potentials of À1.08
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[3] a) K. M. Kadish, Z. P. Ou, V. A. Adamian, R. Guilard, C. P.
Vis), sodium sulfate was added, and the solution was filtered and
evaporated to dryness to afford 5b (21.3 mg, 24.8 mmol). MSÀ:
m/z (%): 859 [MÀ] (10); UV/Vis (MeOH) lmax (relative e): 429
Gros, C. Erben, S. Will, E. Vogel, Inorg. Chem. 2000, 39, 5675;
b) L. Simkhovich, A. Mahammed, I. Goldberg, Z. Gross, Chem.
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1
(100), 581 nm (25); H NMR (200 MHz, CD3CN): d = 9.32 (d,
J = 3.99 Hz, 2H), 8.97 (d, J = 3.81 Hz, 2H), 8.93 (d, J = 4.54 Hz,
2H), 8.79 ppm (d, J = 4.34 Hz 2H); 19F NMR (188 MHz,
CD3CN): d = À138.8 (d, J = 23.9 Hz, 2F), À138.9 (d, J =
20.7 Hz, 1F), À139.5 (d, J = 24.05 Hz, 2F), À139.6 (d, J =
28.7 Hz, 1F), À156.6 (t, J = 20.4 Hz, 2F), À156.8 (t, J =
20.2 Hz, 1F), À164.0 ppm (m, 6F).
[4] a) S. P. de Visser, F. Ogliaro, Z. Gross, S. Shaik, Chem. Eur. J.
2001, 7, 4954; b) A. E. Meier-Callahan, H. B. Gray, Z. Gross,
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Am. Chem. Soc. 2003, 125, 1162; b) L. Simkhovich, I. Goldberg,
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4211; Angew. Chem. Int. Ed. 2000, 39, 4045.
[7] G. Golubkov, J. Bendix, H. B. Gray, A. Mahammed, I. Goldberg,
A. J. DiBilio, Z. Gross, Angew. Chem. 2001, 113, 2190; Angew.
Chem. Int. Ed. 2001, 40, 2132.
[19] For a recent diamagnetic d0 silver(iii) corrole, see C. Brꢁckner,
C. A. Barta, R. P. Brinas, J. A. K. Bauer, Inorg. Chem. 2003, 42,
1673.
[8] R. A. Eikey, S. I. Khan, M. M. Abu-Omar, Angew. Chem. 2002,
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Angew. Chem. Int. Ed. 1999, 38, 1427; b) Z. Gross, N. Galili, L.
Simkhovich, I. Saltsman, M. Botoshansky, D. Blꢀser, R. Boese, I.
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[10] A. L. Odom, C. C. Cummins, Polyhedron 1998, 17, 675.
[11] A. E. Meier-Callahan, A. J. Di Bilio, L. Simkhovich, A. Maham-
med, I. Goldberg, H. B. Gray, Z. Gross, Inorg. Chem. 2001, 40,
6788.
[12] J. Du Bois, C. S. Tomooka, J. Hong, E. M. Carreira, Acc. Chem.
Res. 1997, 30, 364.
[13] [(tpfc)CrV(N)]À (5a): Solid [(saloph)MnV(N)] (22 mg, 57.4 mmol)
was added to a solution of [(tpfc)CrIII(py)2] (25 mg, 29.6 mmol) in
acetonitrile (10 mL). After the color change from green to
purple was complete (1 min, also monitored by UV/Vis), 5a
(16 mg, 18.6 mmol) was isolated by column chromatography
(silica gel, CH3CN). MSÀ: m/z (%): 859 [MÀ] (100), 845 [MÀÀN],
80%; UV/Vis (CH3CN) lmax (relative e): 427 (100), 585 nm (15);
19F NMR (188 MHz, CD3CN): d = À139.0 (brs, 6F), À156.6 (brs,
3F), À163.6 ppm (brd, 6F).
[14] a) J. T. Groves, T. Tadashi, B. M. William, J. Am. Chem. Soc.
1983, 105, 844; b) K. Meyer, J. Bendix, T. Weyhermueller, B.
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Inorg. Chim. Acta 1997, 266, 29.
[15] a) J. Bendix, R. J. Deeth, T. Weyhermuller, E. Bill, K. Wieghardt,
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Weyhermueller, B. Eckhard, N. Metzler-Nolte, K. Wieghardt,
Inorg. Chem. 1998, 37, 1767.
[16] 15N4-5a: The labeled corrole was prepared by solvent-free
condensation of [15N]pyrrole (Aldrich) and pentafluorobenzal-
dehyde.[8] Insertion of chromium(iii) ions and atom transfer from
[(saloph)MnV(N)] (13.5 mg, 35.3 mmol) to [(15N4-tpfc)CrIII(py)2]
(15 mg, 17.7 mmol) in acetonitrile (5 mL) provided 15N4-5a
(14 mg, 16.2mmol). MSÀ: m/z (%): 863 [MÀ] (45), 849
14
[MÀÀ N]. 5a-15N: [(saloph)MnV(15N)] (27.7 mg, 72 mmol) was
prepared by addition of 15NH4OH (3n, 0.68 mL, 2 mmol) and
sodium hypochlorite (7 mL, 0.28 mmol) to [(saloph)MnIIICl]
(50 mg, 135 mmol) in methanol (5 mL). Atom transfer from
[(saloph)Mn(15N)] (8.2 mg, 21.3 mmol) to [(tpfc)CrIII(py)2]
(15 mg, 17.8 mmol) in acetonitrile (5 mL) yielded 5a-15N
(14 mg, 16.3 mmol). MSÀ: m/z (%): 860 [MÀ] (50), 845 [M-15N]
(100%).
[17] J. E. Wertz, J. R. Bolton, Electron Spin Resonance: Elementary
Theory and Practical Applications, McGraw-Hill, New York,
1972, Table C, inside cover.
[18] [(tpfc)CrVI(N)], 5b: Pyridine (1 drop), NH4OH (0.014 mL,
0.36 mmol), and sodium hypochlorite (7.5 mL, 0.14 mmol)
were added to a solution of (tpfc)CrIII(py)2 (25 mg, 24.9 mmol)
in acetonitrile (10 mL) under argon. After the color change from
green to purple was complete (1 min, also monitored by UV/
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