High-valent imido complexes of manganese and chromium corroies

Article abstract of DOI:10.1021/ic0484506

The oxidation reaction of M(tpfc) [M = Mn or Cr and tpfc = tris(pentafluorophenyl)corrole] with aryl azides under photolytic or thermal conditions gives the first examples of mononuclear imido complexes of manganese(V) and chromium(V). These complexes have been characterized by NMR, mass spectrometry, UV-vis, EPR, elemental analysis, and cyclic voltammetry. Two X-ray structures have been obtained for Mn(tpfc)(NMes) and Cr(tpfc)(NMes) [Mes = 2,4,6-(CH₃)₃C₆H₂]. Short metal-imido bonds (1.610 and 1.635 A) as well as nearly linear M-N-C angles are consistent with triple M=NR bond formation. The kinetics of nitrene [NR] group transfer from manganese(V) corroies to various organic phosphines have been defined. Reduction of the manganese(V) corrolato complex affords phosphine imine and Mn^III with reaction rates that are sensitive to steric and electronic elements of the phosphine substrate. An analogous manganese complex with a variant corroie ligand containing bromine atoms in the β-pyrrole positions, Mn(Br₈tpfc)(NAr), has been prepared and studied. Its reaction with PEt₃ is 250x faster than that of the parent tpfc complex, and its Mn^V/IV couple is shifted by 370 mV to a more positive potential. The EPR spectra of chromium(V) imido corroies reveal a rich signal at ambient temperature consistent with Cr^V=NR (d1, S = 1/2) containing a localized spin density in the d_xy orbital, and an anisotropic signal at liquid nitrogen temperature. Our results demonstrate the synthetic utility of organic aryl azides in the preparation of mononuclear metal imido complexes previously considered elusive, and suggest strong σ-donation as the underlying factor in stabilizing high-valent metals by corroie ligands.

Full text of DOI:10.1021/ic0484506

Inorg. Chem. 2005, 44, 3700−3708
High-Valent Imido Complexes of Manganese and Chromium Corroles
Nicola Y. Edwards,† Rebecca A. Eikey,† Megan I. Loring,† and Mahdi M. Abu-Omar*,‡
Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, California 90095, and Brown Laboratory, Department of Chemistry,
Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907
Received November 4, 2004
The oxidation reaction of M(tpfc) [M
photolytic or thermal conditions gives the first examples of mononuclear imido complexes of manganese(V) and
chromium(V). These complexes have been characterized by NMR, mass spectrometry, UV vis, EPR, elemental
analysis, and cyclic voltammetry. Two X-ray structures have been obtained for Mn(tpfc)(NMes) and Cr(tpfc)(NMes)
[Mes 2,4,6-(CH3)3C6H2]. Short metal imido bonds (1.610 and 1.635 Å) as well as nearly linear M C angles
are consistent with triple M NR bond formation. The kinetics of nitrene [NR] group transfer from manganese(V)
) Mn or Cr and tpfc ) tris(pentafluorophenyl)corrole] with aryl azides under
−
)
−
−N−
t
corroles to various organic phosphines have been defined. Reduction of the manganese(V) corrolato complex
affords phosphine imine and MnIII with reaction rates that are sensitive to steric and electronic elements of the
phosphine substrate. An analogous manganese complex with a variant corrole ligand containing bromine atoms in
the â-pyrrole positions, Mn(Br8tpfc)(NAr), has been prepared and studied. Its reaction with PEt3 is 250× faster than
/
IV
that of the parent tpfc complex, and its MnV couple is shifted by 370 mV to a more positive potential. The EPR
spectra of chromium(V) imido corroles reveal a rich signal at ambient temperature consistent with CrVtNR (d1, S
)
1/2) containing a localized spin density in the dxy orbital, and an anisotropic signal at liquid nitrogen temperature.
Our results demonstrate the synthetic utility of organic aryl azides in the preparation of mononuclear metal imido
complexes previously considered elusive, and suggest strong
high-valent metals by corrole ligands.
σ-donation as the underlying factor in stabilizing
Introduction
lated the development of several catalytic4 and stoichiomet-
ric5 methods for metal-mediated [NR] group transfers. The
active intermediate in these reactions is postulated to be a
terminal metal imido species analogous to the isoelectronic
metal oxo complexes prevalent in oxygen atom transfer
(OAT) reactions and cytochrome P450 chemistry.6 Hence,
there is a continuing interest in characterizing metal imido
complexes, particularly those of the first-row transition
elements, and testing their reactivity.7
Metal-mediated nitrogen atom transfer to olefins is an
efficient method for preparing nitrogen-containing organic
compounds such as aziridines and amines. This area of
research has received considerable attention in recent years
due to its promise in synthetic organic chemistry and in the
preparation of natural products.1,2 Groves and co-workers3
reported the first activation of a (nitrido)manganese(V)
porphyrin to effect the aziridination of cyclooctene in the
presence of trifluoroacetic anhydride. This discovery stimu-
(4) See, for example: (a) Evans, D. A.; Faul, M. M.; Bilodeau, M. T. J.
Am. Chem. Soc. 1994, 116, 2742. (b) Li, W.; Quan, R. W.; Jacobsen,
E. N. J. Am. Chem. Soc. 1995, 117, 5889. (c) Halfen, J. A.; Hallman,
J. K.; Schultz, J. A.; Emerson, J. P. Organometallics 1999, 18, 5435.
(5) (a) See, for example: DuBois, J.; Hong, J.; Carreira, E. M.; Day, M.
W. J. Am. Chem. Soc. 1996, 118, 915. (b) Minakata, S.; Ando, T.;
Nishimura, M.; Ryu, I.; Komatsu, M. Angew. Chem., Int. Ed. 1998,
37, 3392. (c) Minakata, S.; Ando T.; Nishimura, M.; Ryu I.; Komatsu,
M. Angew. Chem., Int. Ed. 1998, 110, 3596 (d) DuBois, J.; Tomooka,
C. S.; Hong, J.; Carreira, E. M. J. Am. Chem. Soc. 1997, 119, 3179.
(6) (a) Meunier, B. In Metalloporphyrin Catalyzed Oxidations; Montanary,
F., Casella, L., Eds.; Kluwer Academic: Dordrecht, The Netherlands,
1994; Chapter 1. (b) Ortiz de Montellano, P. R. Cyctochrome P450:
Structure, Mechanism and Biochemistry, 2nd ed.; Plenum Press: New
York, 1995.
* Author to whom correspondence should be addressed. Fax: (765) 494-
0238. E-mail: mabuomar@purdue.edu.
† University of California.
‡ Purdue University.
(1) (a) Park, T. K.; Kim, I. J.; Hu, S.; Bilodeau, M. T.; Randolph, J. T.;
Kwon, O.; Danishefsky, S. J. J. Am. Chem. Soc. 1996, 118, 11488.
(b) Noda, K.; Hosoya, N.; Irie, R.; Ito, Y.; Katsuki, T. Synlett 1993,
469.
(2) (a) Tanner, D.; Harden, A.; Johansson, F.; Wyatt, P.; Andersson, P.
G. Acta Chem. Scand. 1996, 50, 361. (b) Tanner, D. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 599.
(3) Groves, J. T.; Takahasi, T. J. Am. Chem. Soc. 1983, 105, 2073.
3700 Inorganic Chemistry, Vol. 44, No. 10, 2005
10.1021/ic0484506 CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/07/2005

Imido Complexes of Manganese and Chromium Corroles
Scheme 1
further purification. The syntheses of H3(tpfc), Mn(tpfc), Mn(Br8-
tpfc), and Cr(tpfc)(py2) followed previously described methods in
the literature.11 All other solvents and reagents were of reagent grade
and used as received. Syntheses of Mn(tpfc)(NMes), Mes )2,4,6-
(CH3)3C6H2 (1), and Mn(tpfc)(NAr), Ar ) 2,4,6-Cl3C6H2 (2), have
been described previously.8 All azides were prepared using the
methodology of the Sandmeyer reaction or slight variations
thereof.12
Caution! Aryl azides haVe been reported to detonate if heated
to temperatures aboVe 100 °C. While organic azides should be
treated as potentially liable to detonate, particularly in the presence
of heavy metals or acids, no problems were encountered in this
study given the low concentrations used.
1
Physical Measurements. H, 19F, and 31P NMR spectra were
recorded on a Bruker ARX 400 spectrometer, MALDI mass spectra
on a Voyager-DE STR (matrix, R-cyano-4-hydroxycinnamic acid;
typical detailed instrument conditions are given in the Supporting
Information alongside one of the spectra), and electronic absorption
spectra on a Shimadzu UV-2501 spectrophotometer. Chemical shifts
In 2002 we reported the synthesis and characterization of
terminal imido manganese(V) complexes.8 The ancillary
ligand of choice was the tris(pentafluorophenyl)corrole (tpfc),
which was employed by Gray and Gross to stabilize high-
oxidation-state oxo and nitrido complexes.9 The chemistry
of high-oxidation-state manganese(V) oxo of the analogous
corrolazine ligand has been investigated by Goldberg and
co-workers.10 Our manganese imido complexes were derived
from the reaction of MnIII(tpfc) and an aryl nitrene, which
was generated by thermal or photochemical decomposition
of the corresponding azide in a one-pot reaction. Although
the imido ligand on manganese(V) transferred to phosphines,
resulting in the formation of the phosphine imine adduct, it
was not reactive toward olefins including electron-rich silyl
enol ethers.
1
for H were referenced relative to the solvent residual peak, and
31P chemical shifts were referenced to the peak for 1% P(OMe)3.
Magnetic susceptibility was measured using Evan’s method in
CDCl3 at 300 K.13 Photolysis experiments were carried out using
a Hanovia model 673A-0360 550 W medium-pressure mercury arc
lamp. Elemental analyses were done by Desert Analytics Laboratory
in Arizona. EPR spectra of Cr(tpfc)(NMes) (6) and Cr(tpfc)(NAr)
(7) solutions ranging in concentration from 0.20 to 0.50 mM were
taken in toluene at 4, 93, and 293 K on a Bruker EMX
Spectrometer.
X-ray Crystal Determination of Cr(tpfc)(NMes)‚C7H8 (6‚
C7H8). Crystals of 6‚C7H8 were obtained by the slow evaporation
of a toluene solution at room temperature. A suitable crystal of
dimensions 0.20 × 0.20 × 0.15 mm3 was coated with Paratone N
oil and subsequently mounted on a glass fiber that was placed under
a nitrogen cold stream. The single-crystal diffraction data were
collected at 100 K using a Bruker SMART 1000 CCD diffracto-
meter employing graphite-monochromatized Mo KR radiation (λ
) 0.71073 Å). The SMART software was used for data acquisition,
SAINT software for data extraction and reduction, and XPREP
v6.12 software for the empirical absorption correction. Initial atomic
positions were found by direct methods using XS followed by
subsequent difference Fourier syntheses. The structure was solved
and refined using the Bruker SHELTXTL (version 6.1) software
package, using the monoclinic space group P2(1)/c with Z ) 4 for
the formula unit C53H27CrF15N5. The final anisotropic full-matrix
least-squares refinement on F2 converged at R1 ) 4.37%, wR2 )
9.1%, and a goodness-of-fit of 1.022. The largest peak on the final
difference map was 0.389 e-/Å3. The calculated density for 6‚C7H8
(C53H27CrF15N5) is 1.604 Mg/m3, and F(000) is 2156 e-. Crystal-
lographic tables are given in the Supporting Information.
In this paper we provide a full account of the synthetic
utility of aryl azides to generate imido corrole complexes of
manganese and chromium (Scheme 1), electrochemical and
spectroscopic characterizations of these novel complexes, and
rate measurements on the reaction of manganese imido
complexes with various organic phosphines.
Experimental Section
Materials. Preparation and handling of air-sensitive materials
were carried out using standard vacuum line and glovebox
techniques. Toluene and acetonitrile were predried using molecular
sieves and distilled over CaH2 prior to use. When required, solvents
were deoxygenated via freeze-pump-thaw cycles. Deuterated
solvents were purchased from Cambridge Isotopes and used without
(7) For a comprehensive review of metal nitrido and imido complexes,
see: Eikey, R. A.; Abu-Omar, M. M. Coord. Chem. ReV. 2003, 243,
83. For examples of recently reported terminal imido complexes of
the first-row transition metals, see: (a) Brown, S. D.; Betley, T. A.;
Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322. (b) Jenkins D. M.;
Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. (c)
Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003, 125, 13350.
(d) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123,
4623.
(8) Eikey, R. A.; Khan, S. I.; Abu-Omar, M. M. Angew. Chem., Int. Ed.
2002, 41, 3592.
(9) (a) Meier-Callahan, A. E.; Gray, H. B.; Gross, Z. Inorg. Chem. 2000,
39, 3605. (b) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem.,
Int. Ed. 2000, 39, 4045. (c) Golubkov, G.; Bendix, J.; Gray, H. B.;
Mahammed, A.; Goldberg, I.; DiBillio, A. J.; Gross, Z. Angew. Chem.,
Int. Ed. 2001, 40, 2132.
Electrochemical Studies. Cyclic voltammetry employed a
Princeton Applied Research model 273A potentiostat with a glassy
(11) tpfc: (a) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed.
1999, 38, 1427. (b) Gross, Z.; Galili, N.; Simkhovich, L.; Saltsman,
I.; Botoshansky, M.; Blaser, D.; Boese, R.; Goldberg, I. Org. Lett.
1999, 1, 599. Mn(tpfc): (c) Gross, Z.; Golubkov, G.; Simkhovich, L.
Angew. Chem., Int. Ed. 2000, 39, 4045. Mn(Br8tpfc): (d) Golubkov,
G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.; DiBilio,
A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132. Cr(tpfc)-
(py)2: (e) Meirer-Callahan, A. E.; Di Bilio, A. J.; Simkhovich, L.;
Mahammed, A.; Goldberg, I.; Gray, H. B.; Gross, Z. Inorg. Chem.
2001, 40, 6788.
(10) (a) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H. L.;
Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. J. Am. Chem.
Soc. 2002, 124, 15170. (b) Wang, S. H. L.; Mandimutsira, B. S.; Todd,
R.; Ramdhanie, B.; Fox, J. P.; Goldberg, D. P. J. Am. Chem. Soc.
2004, 126, 18.
(12) (a)Murata, S.; Abe, S.; Tomioka, H. J. Org. Chem. 1997, 62, 3055.
(b) Leyva, E.; Munoz, D.; Platz, M. S., J. Org. Chem. 1989, 54, 5938.
(13) Evans, D. F. J. Chem. Soc. 1959, 2003.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3701

Edwards et al.
carbon working electrode, a platinum wire counter electrode, and
a Ag/AgCl reference electrode with 0.1 M (n-Bu4N)(PF6) in CH2-
Cl2 as the supporting electrolyte. Potentials measured are with
respect to the reference electrode but are corrected using the
ferrocenium/ferrocene couple as measured under identical condi-
tions. Solutions of the desired complexes 1, 4, 6, and 7 (concentra-
tions of approximately 1 mM) were prepared in dry, deoxygenated
methylene chloride. Samples were prepared in the glovebox and
loaded into the electrochemical cell using a gastight syringe.
Solutions were purged with argon for an additional 5 min prior to
data collection.
Kinetics of Imido Group Transfer from Mn(tpfc)(NAr) (2)
and Mn(Br8tpfc)(NAr) (5) to Various Phosphines. Solutions of
2 or 5 and various phosphines were prepared using dry, deoxy-
genated toluene in the glovebox. The concentrations of 2 and 5
were determined spectrophotometrically (5, log ꢀ558 ) 4.10 M-1
cm-1; 2, log ꢀ540 ) 3.75 M-1 cm-1). Due to the instability of the
aforementioned manganese complexes, solutions were freshly
prepared prior to the start of each kinetic experiment. Rate
measurements were carried out using either conventional UV-vis
or a stopped flow reaction analyzer (Applied Photophysics SX.18MV)
at 20.5 ( 0.5 °C.
The rates of reaction of 2 and 5 with various phosphines were
measured by monitoring the formation of the phosphine imine
adduct of manganese(III) at approximately 475 nm (varies from
470 to 485 nm depending on the phosphine) under pseudo-first-
order conditions with a phosphine concentration at least 10-fold in
excess of the manganese complex. Plots of the pseudo-first-order
rate constants (kψ) versus phosphine concentration were linear, and
the slopes afforded second-order rate constants (k1).
Typical Reaction Conditions for the Preparation of Phosphine
Imine Adducts Mn(tpfc)(NArPPh3) and Mn(tpfc)(NArPCy3). A
solution of 2 (25 mg, 24 µmol) and a stoichiometric amount of
PR3 in 5 mL of toluene was stirred at room temperature under an
argon atmosphere. The solution turned green in a matter of minutes.
The progress of reaction was followed by UV-vis until complete
conversion to the phosphine imine adduct. The following are data
for Mn(tpfc)(NArPPh3). MALDI MS: m/z ) 1303 (M - 2H)+ with
correct isotope pattern (see the Supporting Information). UV-vis
(toluene): λmax/nm ) 396, 416, 475, 609. The following are data
for Mn(tpfc)(NArPCy3). MALDI MS: m/z ) 1322 (M - H)+ with
correct isotope pattern (see the Supporting Information). UV-vis
(toluene): λmax/nm ) 389, 414, 470, 611. Attempts to obtain single
crystals from these solutions over prolonged periods of time were
not successful. Subjecting the reaction mixture to TLC showed a
spot corresponding to phosphine imine NArPR3 by comparison to
an authentic sample. Preparative TLC yielded starting (tpfc)MnIII
and phosphine imine NArPR3 in essentially quantitative yield
(g90%).
(toluene): λmax/nm (rel abs) ) 376 (2.8), 541 (1.0). MALDI MS
(rel intens) [assignment]: m/z 1007 (19) [M]+, 864 (7) [M - Ar′]+,
848 (100) [M - NAr′]+.
Mn(Br8tpfc)(NMes) (4). Method A. Mesityl azide (51.5 mg,
0.317 mmol) was added to a stirring solution of Mn(Br8tpfc) (21.4
mg, 14.5 µmol) in 8 mL of dry toluene under argon. The reaction
mixture was heated to 85 °C until TLC analysis showed complete
consumption of starting material (approximately 5 h) and evaporated
to dryness under vacuum. Chromatography as described for 3
followed by anaerobic recrystallization from hexanes and methylene
chloride afforded 4 (9 mg, 38%) as a burgundy microcrystalline
solid.
Method B. Mesityl azide (9.4 mg, 58.3 µmol) was added to a
stirring solution of Mn(Br8tpfc) (4 mg, 2.7 µmol) in acetonitrile
(dry, deoxygenated, 2 mL) under argon. The reaction mixture was
photolyzed until TLC analysis showed complete conversion of
starting material (approximately 3-4 days) and evaporated to
dryness under vacuum. Chromatography afforded 4 (1.5 mg, 38%
yield) as a burgundy crystalline solid. 1H NMR (CDCl3, 400.1 MHz,
300 K): δ 5.72 (s, 2 H), 1.69 (s, 3 H), -0.39 (s, 6 H). 19F NMR
(CDCl3, 400.1 MHz, 300 K): δ -136.75 (m, 2 F), -136.92 (m, 1
F), -138.68 (m, 3F), -152.08 (m, 3 F), -163.0 to -163.43 (m, 6
F). UV-vis (toluene): λmax/nm (log ꢀ) ) 426 (4.65), 549 (4.10),
791(2.55). UV-vis (CH2Cl2): λmax/nm (log ꢀ) ) 424 (4.82), 546
(4.27), 760 (3.04). MALDI MS (rel intens) [assignment]: m/z 1612
(40) [M]+, 1479 (100) [M - NMes]+. Anal. Calcd for MnF15N5C60-
H27Br8 (4‚2C7H8): C, 40.08; H, 1.50; N, 3.90. Found: C, 40.00;
H, 1.42; N, 3.94.
Mn(Br8tpfc)(NAr) (5). 2,4,6-Trichlorophenyl azide (40.5 mg,
0.182 mmol) was added to a stirring solution of Mn(Br8tpfc) (14
mg, 9.46 µmol) in acetonitrile (dry, deoxygenated, 5 mL) under
argon. The reaction mixture was photolyzed for 6 days. The
resulting brown solution was evaporated to dryness under vacuum,
washed with acetonitrile, dissolved in CH2Cl2, and chromatographed
(basic alumina activity IV, CH2Cl2) to yield 5 (13.5 mg, 85%) as
1
a burgundy microcrystalline solid. H NMR (CDCl3, 400.1 MHz,
300 K): δ 6.27 (s, 2 H). 19F NMR (CDCl3, 400.1 MHz, 300 K):
δ -136.91 (m, 2 F), -137.14 (d, 1 F), -138.17 (m, 3 F), -151.75
(m, 3 F), -162.75 to -163.16 (m, 6 F). UV-vis (toluene): λmax
/
nm (rel abs) ) 419 (3.2), 558 (1.0). MALDI MS (rel intens)
[assignment]: m/z 1479 (100) [M - NAr]+.
Cr(tpfc)(NMes) (6). Method A. Mesityl azide (18.3 mg, 0.114
mmol) was added to a stirring solution of Cr(tpfc)(py)2 (11.6 mg,
11.5 µmol) in 8 mL of dry toluene under argon. The reaction
mixture was refluxed until TLC analysis showed complete con-
sumption of starting material (approximately 3.5 h) and evaporated
to dryness under vacuum. Chromatography as described for 3
followed by aerobic recrystallization from pentane and toluene
afforded 6 (7.2 mg, 62%) as a burgundy semicrystalline solid.
Method B. Mesityl azide (77 mg, 0.478 mmol) was added to a
stirring solution of Cr(tpfc)(py)2 (12 mg, 12 µmol) in acetonitrile
(dry, deoxygenated, 4 mL) under argon. The reaction mixture was
photolyzed for approximately 9-10 days and evaporated to dryness
under vacuum. Chromatography as described for 3 followed by
aerobic recrystallization from pentane and toluene afforded 6 (2
mg, 17%) as a burgundy semicrystalline solid. UV-vis (CH3CN):
Mn(tpfc)(NAr′), Ar′ ) 2,6-Cl2C6H3 (3). 2,6-Dichlorophenyl
azide (40 mg, 0.21 mmol) was added to a stirring solution of Mn-
(tpfc) (13 mg, 15.3 µmol) in acetonitrile (dry, deoxygenated, 6 mL)
under argon. The solution was photolyzed until TLC analysis
showed complete consumption of starting material (approximately
3 days). The resulting burgundy solution was evaporated to dryness
under vacuum and chromatographed on silica gel (60 Å, 12:1
pentane/ether). 3 was isolated as a fragile burgundy microcrystalline
solid (7 mg, 7 µmol, 45%). 1H NMR (CDCl3, 400.1 MHz, 300 K):
δ 9.12 (d, J ) 4.3 Hz, 2 H), 8.83 (d, J ) 3.6 Hz, 2 H), 8.66 (d, J
) 3.7 Hz, 2 H), 8.58 (d, J ) 3.9 Hz, 2 H), 6.46 (t, J ) 7.4 Hz, 1
H), 6.10 (d, J ) 8.0 Hz, 2 H). 19F NMR (CDCl3, 400.1 MHz, 300
K): δ -137.29 (m, 2 F), -137.41 (m, 2 F), -137.75 (m, 2 F),
-153.26 (m, 3 F), -162.04 to -162.24 (m, 6 F). UV-vis
λ
max/nm (log ꢀ) ) 530 (2.97), 408 (3.07). MALDI MS (rel intens)
[assignment]: m/z 979 (100) [M]+, 845 (100) [M - NMes]+. µeff
(CDCl3, 300 K) ) 1.90 µB. Anal. Calcd for CrF15N5C52H29 (6‚1/
2C5H12‚1/2C7H8): C, 58.88; H, 2.76; N, 6.60. Found: C, 59.06; H,
3.26; N, 5.86.
Cr(tpfc)(NAr) (7). Method A. 2,4,6-Trichlorophenyl azide (47
mg, 0.211 mmol) was added to a stirring solution of Cr(tpfc)(py)2
3702 Inorganic Chemistry, Vol. 44, No. 10, 2005

Imido Complexes of Manganese and Chromium Corroles
(18 mg, 18 µmol) in 8 mL of dry toluene and refluxed under argon
for 24 h. The solvent was removed under vacuum and chromato-
graphed as described for 3. This was followed by aerobic recrys-
tallization from pentane and toluene to yield 7 (7.1 mg, 38%) as a
burgundy semicrystalline solid.
Method B. 2,4,6-Trichlorophenyl azide (38 mg, 0.17 mmol) was
added to a stirring solution of Cr(tpfc)(py)2 (10 mg, 10 µmol) in
acetonitrile (dry, deoxygenated, 5 mL) under argon. The reaction
mixture was photolyzed for approximately 4-5 days. Chromatog-
raphy as described for 3 followed by aerobic recrystallization from
pentane and toluene afforded 7 as a burgundy semicrystalline solid.
UV-vis (CH3CN): λmax/nm (log ꢀ) ) 536 (4.05), 391 (4.58).
MALDI MS (rel intens) [assignment]: m/z 1040 (95) [M]+, 845
(100) [M - NAr]+. µeff (CDCl3, 300 K) )1.93 µB. Anal. Calcd for
CrCl3F15N5C49H20 (7‚1/2C5H12‚1/2C7H8): C, 52.45; H, 1.80; N, 6.24.
Found: C, 52.22; H, 2.28; N, 5.58.
Results and Discussion
Synthesis, Characterization and Reactivity of Manga-
nese(V) Imido Corrole Complexes. The facile synthetic
route to the first well-characterized terminal imido complex
of manganese(V) has been communicated.8 This complex
was most likely derived from the reaction of Mn(tpfc) and
mesityl nitrene generated from thermal or photochemical
decomposition of the corresponding azide.14 This synthetic
methodology has been successfully employed in the synthesis
of manganese(V) imido complexes derived from other aryl
azides, namely, 2,4,6-trichlorophenyl and 2,6-dichlorophenyl
azides. Furthermore, the brominated corrole ligand Br8tpfc
can be used instead of tpfc to enhance the activity of the
metal center. The imido complexes were purified by flash
chromatography and characterized by NMR, mass spectrom-
etry, and elemental analyses. Yields are moderate, and
reactions were generally done thermally whenever possible
due to reduced time of reaction, higher product yields, and
amenability to larger scales.
Reactions with phosphines as well as lack of reaction with
organic acids such as trifluoroacetic acid (TFA) and methyl
iodide are a testimony to the electrophilicity of the NR
(imido) group in these complexes. In the case of Mn(tpfc)-
(NMes) (1), which is stable indefinitely under argon,
formation of the phosphine imine was complete after a day
at 85 °C.8 This is in startling contrast to the Mn(tpfc)(NAr)
(2), which must be stored in the freezer in the glovebox to
retard its decomposition, reacting with phosphines on the
time scale of minutes at room temperature. However, the
imido species are not reactive toward olefins of any kind
including the electron-rich silyl enol ethers.
Figure 1. UV-vis spectra in toluene of (A) 40 µM 2 (red) and its reaction
product with 400 µM PPh3, MnIII(tpfc)(NArPPh3) (green), and (B) 40 µM
MnIII(tpfc) (orange) and the same compound in the presence of 400 µM
PPh3 (purple) compared to the product from the reaction of 2 with PPh3,
MnIII(tpfc)(NArPPh3) (green).
phine), corresponding to the formation of the phosphine
imine adduct. The UV-vis spectra of 2 and its reaction
product with PPh3, MnIII(tpfc)(NArPPh3), are shown in
Figure 1A. In addition to the pronounced LMCT band at
475 nm, other key features are the splitting of the Soret band
and the shift of the Q-band upon product formation. These
are indicative of a change in oxidation state and manifested
by a change in color from burgundy to green. It is worth
noting that the UV-vis spectrum of the resulting phosphine
imine adduct, MnIII(tpfc)(NArPPh3), is distinguishable from
the UV-vis spectra of MnIII(tpfc) and MnIII(tpfc) with excess
PPh3 (Figure 1B). The addition of a large excess of PPh3
had very subtle effects on the UV-vis spectrum of Mn(tpfc).
In comparison, the spectra of the reaction product of 2 and
PR3 yielded distinct spectra with variable maxima between
470 and 485 nm depending on the identity of the organic
phosphine. Furthermore, a peak corresponding to the mass
of the manganese phosphine imine adduct with the correct
isotope pattern was detected by MALDI-TOF MS in reaction
mixtures of all phosphines. Three representative mass spectra
are shown in the Supporting Information for (tpfc)Mn-
(NArPPh3), (tpfc)Mn(NArP(4-FC6H4)3), and (tpfc)Mn(NAr-
PCy3). Therefore, on the basis of UV-vis and mass
spectrometry results, we conclude that the initial product
from the reaction of (imido)manganese(V) corrole complexes
Kinetic Studies of NR Group Transfer from Mn(tpfc)-
(NAr) to Various Organic Phosphines. Rates of reduction
of 2 by a series of phosphines that vary widely in steric and
electronic properties have been measured. The progress of
reaction was monitored at approximately 470 nm (which
varies in the range 470-485 nm depending on the phos-
(14) For examples of aryl azides as nitrene sources in the formation of
metal imido complexes, see: (a) Hillhouse, G. L.; Bercaw, J. E.
Organometallics 1982, 1, 1025. (b) Antonelli, D. M.; Schaefer, W.
P.; Parkin, G.; Bercaw, J. E. J. Organomet. Chem. 1993, 462, 213.
(c) La Monica, G.; Cenini, S. J. Chem. Soc., Dalton Trans. 1980,
1145.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3703

Edwards et al.
Table 1. Second-order Rate Constants (k1) for the Reaction of Organic
Phosphines with 2 along with Tolman’s Steric (Cone Angle/deg) and
Electronic (ø/cm-1) Parameters
cone
angle/deg
a
phosphine
k1 /M-1 s-1
ø/cm-1
P(p-FC6H4)3
PPh3
PPh2Et
P(p-Tol)3
PEt3
0.062(6)
0.71(5)
1.09(2)
0.78(6)
12.1(2)
145
145
140
145
132
170
15.7
13.25
11.3
10.5
6.3
PCy3
0.0480(2)
1.4
a In toluene at 20.5 ( 0.5 °C.
Figure 2. A plot of the observed pseudo-first-order rate constant (kψ)
versus [PEt3] for the reaction of PEt3 with 2 at 20.5 ( 0.5 °C.
with phosphines is the manganese(III) phosphine imine
adduct.
The reaction of manganese(V) corrole complexes with
phosphines to afford manganese(III) phosphine imine (MnIII
r N(Ar)dPR3) is taken to represent complete imido (NAr)
group transfer from the metal to phosphorus. The starting
manganese(V) imido is diamagnetic, and the product is
paramagnetic. The UV-vis spectral features of the reaction
(see above) are consistent with the assigned oxidation states.
Furthermore, phosphine imine does not react with manga-
nese(III) corrole to give manganese(V) imido.
Product formation was authenticated for the reaction with
PPh3. Addition of a slight excess of PPh3 to 2 resulted in
the formation of a green reaction mixture within minutes.
Purification via preparative TLC subsequently led to the
isolation of Mn(tpfc) and the free phosphine imine in
essentially quantitative yields. The latter was identified by
mass spectrometry and 1H and 31P NMR via comparison to
an authentic sample prepared independently from the reaction
of phosphine and aryl azide.15
It should be noted that the use of (imido)manganese(V)
corrole complexes in the preparation of phosphine imines
would have no practical value whatsoever, nor does it
constitute by any means the purpose for this investigation.
Phosphine imines can be synthesized conveniently from the
reaction of phosphine and azide (the Staudinger reaction).15
Nevertheless, the point of this kinetic study is to gain insight
into the reactivity and reaction mechanisms of novel terminal
imido complexes of manganese.
Reduction of 2 by trialkyl- and alkylarylphosphines
displayed single-exponential time profiles under pseudo-first-
order conditions with phosphine concentrations at 10-fold
or more excess. The observed rate constant, kψ, varied
linearly with phosphine concentration in accordance with an
overall second-order rate law: -d[2]/dt ) k1[2][PR3]. A
representative plot for the reaction with PEt3 is shown in
Figure 2. Second-order rate constants (k1) determined from
the slopes of kψ versus [PR3] plots are listed in Table 1.
In contrast to trialkyl- or arylalkylphosphines, reduction
of 2 by triarylphosphines displayed complex kinetics. Bi-
phasic time profiles were obtained under pseudo-first-order
Figure 3. Dependence of the fast (kf) and slow (ks) stages on PPh3
concentration for its reaction with 2.
conditions. The data were fit to a nonlinear least-squares
biexponential equation. The observed rate constants, kf (fast)
and ks (slow), varied linearly with the phosphine concentra-
tion and featured discernible nonzero y intercepts. Typical
plots of kf and ks are illustrated in Figure 3 for the reaction
with PPh3. The y intercepts cannot be due to complex
decomposition since, over the time scale of these reactions,
the manganese complex is stable. Furthermore, a study of
reaction kinetics based on a series of phosphines with
different substituents in the para position of the aryl ring
(15) (a) Staudinger, H.; Meyer, J. HelV. Chim. Acta 1919, 2, 635. (b)
Gololobov, Y. G.; Zhmurova, I. N.; Kasukhin, L. F. Tetrahedron 1981,
37, 437-472. (c) Wei, P.; Chan, K. T. K.; Stephan, D. W. Dalton
Trans. 2003, 3804. (d) Masuda, J. D.; Walsh, D. M.; Wei, P.; Stephan,
D. W. Organometallics 2004, 23, 1819.
3704 Inorganic Chemistry, Vol. 44, No. 10, 2005

Imido Complexes of Manganese and Chromium Corroles
Scheme 2. Ligand-Assisted Mechanism Which May Account for the
Observed Biphasic Kinetics with Triarylphosphines
was attempted. In the case of P(p-Tol)3, rate constants for
both fast and slow stages were comparable to those of
unsubstituted PPh3, and the reaction with P(p-FC6H4)3
displayed a significant y intercept. Thus, no Hammett
correlation can be drawn from these findings. Since the fast
stage accounts for the bulk of absorbance change and is
consistent with the reduction of manganese(V) to manganese-
(III), the rate constants from the fast stage (kf) were taken to
represent NR group transfer to triarylphosphines and are
presented in Table 1. Similar biphasic and complex kinetics
have been observed for imido transfer from a ruthenium
porphyrin to triarylphosphines.16 One possible assignment
of the slow stage is ligand dissociation of the resulting
phosphine imine on manganese(III) assisted or catalyzed by
phosphine substrate. However, the physical basis of a
substrate-independent pathway (i.e., y intercepts in Figure
3) cannot be accounted for by this proposal. Additionally, a
mechanism in which the arylphosphine complex replaces the
phosphine imine adduct is not supported by the observation
that the resulting spectrum of the product at the end of the
kinetic run is distinct from that of Mn(tpfc) and phosphine
(Figure 1B).
Figure 4. Electronic map (log k1 vs Tolman’s electronic parameter ø) for
the reaction of phosphines with 2.
Biphasic kinetics might be the result of a ligand-assisted
pathway in which the phosphine substrate coordinates trans
to the imido ligand prior to NR transfer (Scheme 2). To
explore the role of trans-coordinating ligands on the rate of
reaction, experiments were carried out with added pyridine,
which was varied in concentration between 5 and 40 equiv
with respect to 2. No difference in reaction rates was
detected, and hence, a ligand-assisted pathway is not fully
supported.
Steric and Electronic Effects of Phosphines. Table 1
contains rate constants for several phosphine substrates along
with Tolman’s electronic and steric parameters.17 The
electronic map (Figure 4) demonstrates that phosphine imine
formation is favored by more nucleophilic phosphines with
the exception of PCy3 in which activity is dictated by a steric
effect. Log k1 decreases in the order PEt3 (ø ) 6.3 cm-1),
PPh2Et (ø ) 11.3 cm-1), PPh3 (ø ) 11.3 cm-1), and P(p-
FC6H4)3 (ø ) 15.7 cm-1). Rate constants obtained span over
2 orders of magnitude. Phosphine imine formation is clearly
favored by less sterically hindered phosphines as measured
by their cone angles in degrees (Figure 5, steric map). Log
k1 increases in the order PCy3 (170°), PPh3 and P(p-Tol)3
(145°), PPh2Et (140°), and PEt3 (132°). P(p-FC6H4)3, isosteric
with PPh3 and P(p-Tol)3, is the mere exception to this trend.
Its relatively small value of k1 reflects its intrinsically low
nucleophilicity. Both factors together, the sterics and elec-
tronics of phosphines, govern the rate of phosphine addition
to the manganese(V) imido in 2 to afford the manganese-
(III) phosphine imine.
Figure 5. Steric map (log k1 vs Tolman’s cone angle) for the reaction of
phosphines with 2.
Tuning the Reactivity via Ligand Modification. This
work and previous studies done on both porphyrin and
corrole complexes9,18 gave rise to the speculation that more
electron-deficient imido corrole complexes are potential
stoichiometric nitrogen atom transfer agents to olefins.
Toward this end, analogues of the imido species based on
corrole ligands with Br atoms in the â-pyrrole positions of
the macrocycle were synthesized.
To directly measure the electrophilicity around the metal
center and hence its reactivity, electrochemical properties
of the imido complexes were investigated via cyclic volta-
mmetry (Table 2). The cyclic voltammograms of the imido
complexes were measured relative to the Ag/AgCl electrode
in 0.10 M (n-Bu4N)(PF6) in CH2Cl2 at ambient temperature.
They feature two one-electron reversible redox processes:
a ligand-based oxidation corresponding to the corrole/corrole
π-radical cation (tpfc/tpfc•+) couple and a metal-based
reduction corresponding to the metal(V/IV) couple. Peak
assignments were based on comparison to the (tpfc)Cr(V)O
complex.9 Reversibility was established by the near-unity
value of the cathodic and anodic peak current intensities and
(18) For brominated porphyrins and corroles, see, for example: (a)
Grinstaff, M. W.; Hill, M. G.; Birbaum, E. R.; Schaefer, W. P.;
Labinger, J. A.; Gray, H. B. Inorg. Chem. 1995, 34, 4896. (b)
Mahammed, A.; Meier-Callahan, A. E.; Gross, Z. J. Am. Chem. Soc.
2003, 125, 1162. For fluorinated corroles, see: Lui, H.-Y.; Lai, T.-
S.; Yeung, L.-L.; Chang, C. K. Org. Lett. 2003, 5, 617.
(16) Leung, W.-H.; Hun, T. S. M.; Hou, H.-W.; Wong, K.-Y. J. Chem.
Soc., Dalton Trans. 1997, 237.
(17) Tolman, C. A. Chem. ReV. 1977, 77, 313.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3705

Edwards et al.
Table 2. Redox Potentials (V) vs Ag/AgCl for Metal(V) Imido Corrole Complexesa
complex
E
1/2(tpfc/tpfc+‚
)
E1/2(M5+/4+
)
complex
E1/2(tpfc/tpfc+‚
)
E1/2(M5+/4+
)
Mn(tpfc)(NMes) (1)
Mn(Br8tpfc)(NMes) (4)
Cr(tpfc)(NMes) (6)
1.21
1.67
1.31
-0.36
0.01
-0.47
Cr(tpfc)(NAr) (7)
1.29
1.25
-0.13
0.11
Cr(tpfc)(O)b
a Redox potentials were obtained from E1/2 ) (Epa + Epc)/2 and were corrected by the potential of the ferrocenium/ferrocene couple. Cyclic voltammograms
were obtained in CH2Cl2 with 0.1 M (n-Bu4N)(PF6) as the supporting electrolyte at 298 K. The scan rate was 25 mV s-1 b Reference 9a.
.
stoichiometry by the comparability of the peak current to
that of the ferrocene/ferrocenium redox couple of the same
concentration under identical conditions.
Comparisons of the electrochemical measurements be-
tween the imido species based on the tpfc and Br8tpfc ligand
systems were done using those derived from mesityl azide
due to their stability. However, comparisons of rate measure-
ments were done using imido species derived from trichlo-
rophenyl azide due to their enhanced reactivity. In the cyclic
voltammogram of 1, the tpfc/tpfc•+ and the MnV/IV couples
occur at potentials of 1.21 and -0.36 V, respectively. In the
case of 4, the tpfc/tpfc•+ redox couple and the MnV/IV redox
couple occur at potentials of 1.67 and 0.01 V, respectively.
These values represent a positive shift of 460 mV for the
oxidation process and 370 mV for the reduction process
(Table 2). This substantial positive shift for the MnV/IV redox
couple was accompanied by enhanced reactivity as rate
measurements of imido group transfer from Mn(Br8tpfc)-
(NAr) (5) to PEt3 afforded a second-order rate constant of
3022 ( 61 M-1 s-1, over 2 orders of magnitude greater
(252×) than that measured for 2 (12.2 ( 0.1 M-1 s-1).
Despite this significant change in both redox potential and
phosphine reactivity, 4 and 5 remained unreactive toward
olefins.
The redox potentials for the analogous chromium imido
complexes are also given in Table 2 alongside the manganese
compounds for comparison. Since the chromium imido
complex with chloride substituents on the aryl imido ligand
is sufficiently stable, its cyclic voltammogram was measured.
While the chloro substituents on the aryl imide have no
bearing on the oxidation potential of the corrole ligand (tpfc/
tpfc•+), they cause a positive shift of 340 mV in the MnV/IV
potential. Hence, the combined effects of brominating the
corrole ligand and having chloride substituents on the aryl
imide provide a predicted value for the MnV/IV potential in
5 of 0.35 V. However, this could not be confirmed
experimentally due to a lack of stability of 5.
Synthesis, Characterization, and Reactivity Trends of
Chromium(V) Imido Corrole Complexes. Chromium cor-
role imido complexes were synthesized in a fashion similar
to that of the corresponding manganese complexes. Both
imido complexes derived from mesityl azide (6) and trichlo-
rophenyl azide (7) are quite stable in air and can be
synthesized using either thermal or photochemical activation
of substituted aryl azides. The resulting complexes are easily
purified by column chromatography followed by aerobic
crystallization from toluene/pentane solvent mixtures. The
CrV oxidation state was confirmed by their room temperature
EPR spectra and values of 1.90 µB and 1.93 µB (1 µB )
9.27 × 10-24 J T-1) obtained for the paramagnetic suscep-
Figure 6. Molecular structure of 6 showing 50% probability ellipsoids
and partial atom labeling schemes. Select bond distances and angles are
Cr-N1 ) 1.955(2) Å, Cr-N2 ) 1.941(2) Å, Cr-N3 ) 1.943(2) Å, Cr-
N4 ) 1.932(2) Å, Cr-N5 ) 1.635(2) Å, and Cr-N5-C38 ) 169.59-
(18)°.
tibility of 6 and 7, respectively. These values are slightly
greater than the spin-only value for one unpaired electron
but in excellent accord with values obtained for chromium-
(V) nitrido complexes in the literature.19 Like their manga-
nese counterparts, chromium imido complexes react with
triphenylphosphine, resulting in the formation of the phos-
phine imide adduct, a manifestation of the electrophilic nature
of the NR group. For 7 the reaction is complete in 48 h at
85 °C compared with less than 10 min at room temperature
for 2. The additional stability of the imido complexes as an
effect of changing the metal from Mn to Cr is best understood
in terms of the CrV/IV and MnV/IV redox potentials. The CrV/IV
redox couple for 6 is shifted negatively with respect to the
MnV/IV redox couple in the analogous complex 1. The CrV/IV
potentials in 6 (-0.47 V) and 7 (-0.13 V) are also shifted
negatively compared to that of Cr(tpfc)(O) (0.11 V), reflect-
ing the greater π-donating strength of the imido ligand.
Differences in the observed reactivities cannot be easily
correlated to structural elements of the complexes since the
two available structures of 1 and 6 feature similar structural
parameters. A discussion of the structure of complex 6 and
its comparison to other related structures including that of
its manganese analogue complex 1 is undertaken next.
X-ray Molecular Structure of Cr(tpfc)(NMes)‚C7H8 (6‚
C7H8). The molecular structure of Cr(tpfc)(NMes) (6) is
shown in Figure 6. The critical distances (Å) and angle (deg)
are Cr-N1 ) 1.955(2), Cr-N2 ) 1.941(2), Cr-N3 )
1.943(2), Cr-N4 ) 1.932(2), Cr-N5 ) 1.635(2), and Cr-
N5-C38 ) 169.59(18). The metal-nitrogen pyrrole dis-
(19) (a) Meyer, K.; Bendix, J.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 1998, 37, 5180. (b) Che, C. M.; Ma, J. X.; Wong, W.
T.; Lai, T. F.; Chung, K. P. Inorg. Chem. 1988, 27, 2547.
3706 Inorganic Chemistry, Vol. 44, No. 10, 2005

Imido Complexes of Manganese and Chromium Corroles
Table 3. Comparison of Structural Parameters of 6 to Those of Related
Structures
metal displacement
from the plane of M-Npyrrole
complex
the macrocyclea/Å
range/Å
M-Laxialb/Å
ref
Cr(tpfc)(NMes) (6)
Cr(tpfc)(py)2
Mn(tpfc)(NMes) (1)
Cr(tpfc)(O)c
domed
0.54
0.14
0.51
0.56
0.57
1.932-1.955 1.64
1.926-1.952 2.109, 2.129 11e
1.891-1.947 1.61
1.936-1.942 1.57
1.927-1.943 1.57
this work
8
9a
twisted
Cr(tpp)(O)
0.47
2.028-2037 1.572
21
a Plane of macrocycle is defined by the four pyrrolic nitrogen atoms.
b Laxial designates the axial ligand. c Two conformers exist for this complex
in the solid-state structure, one in which all the pyrroles bend downward
(Domed) and the other in which one of the pyrrole rings bends upward
(Twisted).
Figure 7. X-band EPR spectrum of Cr(tpfc)(NAr) (7) in toluene at 298
K. Experimental conditions: microwave frequency 9.770 GHz, microwave
power 2.0 mW, modulation amplitude 1.00 G.
tances are within the range of those of both its isoelectronic
counterpart Cr(tpfc)(O)9a and the related CrIII(tpfc)(py2),11e
while they are slightly longer than those of Mn(tpfc)(NMes).8
As there is no crystal structure of chromium imido porphy-
rin,20 we utilize the structure of a chromium(IV) oxo
porphyrin, Cr(tpp)(O), for comparison to 6.21 Metal-nitrogen
pyrrole distances for Cr(tpp)(O) are longer than those of our
corrole complex 6. This difference between the corrole and
the porphyrin complex is almost exclusively due to the
different sizes of the ligands. The imido linkage exhibits a
short Cr-N distance of 1.635 Å and is almost linear (Cr-
N5-C38 ) 169.60°), which is consistent with a formal triple
bond. The Cr atom is displaced out of the plane as defined
by the four pyrrole nitrogen atoms by 0.54 Å. This metal
out-of-plane displacement is almost identical to those of the
CrV(tpfc)O conformers (0.56 and 0.57 Å),9a slightly greater
than that of the Mn(tpfc)(NMes) complex (0.51 Å),8 and
significantly larger than that of the Cr(tpp)O complex (0.47
Å).21 Shorter metal-nitrogen pyrrole distances and larger
metal out-of-plane displacements along with a unique ability
to destabilize low-valent metal complexes (through raising
the energy of metal d orbitals) are the main factors that have
been attributed to the greater stability of corrole complexes
over those of porphyrin ligand systems.9a The main structural
parameters of 6 are compared to those of related structures
of porphyrin and corrole macrocycles in Table 3.
Table 4. EPR Parameters for a Series of Cr(V) Complexes
53
14
complex
giso
A
Cr/mT
A N/mT
Cr(tpfc)(NAr)a
Cr(tpfc)(NMes)a
Cr(tpfc)(O)b
Cr(tpfc)(N)c
Cr(salen)(N)d
Cr(bpb)(N)e
Cr(ttp)(N)f
1.987
1.985
1.986
2.18
2.18
1.64
2.67
2.83
2.73
2.85
2.83
0.31
0.31
0.30
0.27
0.25
0.23
0.28
0.28
1.978
1.982
1.985
1.982
Cr(oep)(N)f
a This work. b Reference 9a. c Reference 22. d Reference 23. e Reference
24. H2bpb ) 1,2-bis((2-pyridylcarboxy)amido)benzene. f Reference 25. oep
) octaethylporphinate(2-) and ttp ) tetratolylporphinate(2-).
on either side by two smaller satellites due to coupling to
53Cr (I ) 3/2 and abundance 9.4%). The isotropic hyperfine
53
coupling constants of 6 and 7 were determined to be A
2.18 mT and A ) 0.31 mT.
)
Cr
14N
Table 4 reveals that the EPR parameters determined for 6
and 7 fall within the range of those found for other Cr(V)
species based on corrole, porphyrin, and salen ancillary
ligands. A comparison of the hyperfine coupling constants
for the nitrido, imido, and oxo tpfc complexes is notewor-
53
14
thy: A Cr/A N constants (mT) are 2.67/0.27, 2.18/0.31, and
1.64/0.31 for the nitrido, imido, and oxo moieties, respec-
tively. In the case of the oxo complex, these values, a
manifestation of the relative spin densities on the metal (low)
and ligand (high), have been explained in terms of the strong
σ donation from the pyrrole nitrogen atoms to the metal and
the subsequent transfer of that electron density to the axial
ligand via a π-type donation.9a This argument is undoubtedly
also applicable to the nitrido, but the strong π-donating ability
of this ligand negates the σ-donating effect, resulting in
relative spin densities that are much higher on the metal and
lower on the ligand than in the oxo and imido counterparts.
In our imido complexes, there is interplay of the σ-donating
effect of the pyrrolic nitrogens and the π-donating ability of
the axial ligand, resulting in spin densities that remain high
on the ligand but greater on the metal than in the case of the
Electron Paramagnetic Resonance (EPR) Spectra. The
room temperature EPR spectrum of Cr(tpfc)(NAr) (7) in
toluene (Figure 7) displays a sharp isotropic signal at go )
1.987 reflecting the 3dxy1 ground-state electronic configura-
tion. It features an intense central resonance of eleven lines
attributable to coupling of the unpaired electron to five 14
N
nuclei, which indicates that all five nitrogen atoms (four
pyrrole nitrogens and the axial imido nitrogen) in 7 are
magnetically equivalent. This is a recurring phenomenon
observed with chromium(V) nitrido complexes and thought
to result from Fermi contact or through bond resonances
occurring to the same extent.22 The main signal is flanked
(20) For chromium(IV) imido porphyrin complexes, see: Moubaraki, B.;
Murray, K. S.; Nichols, P. J.; Thomson, S.; West, B. O. Polyhedron
1994, 13, 485.
(21) Groves, J. T.; Kruper, W. R., Jr.; Haushalter, R. C.; Butler, W. M.
Inorg. Chem. 1982, 21, 1363.
14
oxo moiety. The A coupling constant, being an effective
N
measure of the σ-donation of the pyrrolic nitrogen atoms to
the metal, is greatest for the oxo and imido corrole
complexes, leading to short metal-nitrogen pyrrole distances
(22) Golubkov, G.; Gross, Z. Angew. Chem., Int. Ed. 2003, 42, 4507.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3707

Edwards et al.
metalated corrole of aryl nitrene, generated via thermal or
photochemical decomposition of the corresponding organic
azide. Manganese(V) complexes containing electron-with-
drawing substituents on the aryl imide ligand were found to
be sufficiently reactive with various organic phosphines,
allowing detailed rate measurements. Reduction of manga-
nese(V) to manganese(III) accompanied by nitrene [NR]
transfer was sensitive to steric and electronic parameters of
the substrate. Modification of the corrole ligand by introduc-
ing bromine atoms in the â-pyrrole positions had a pro-
nounced effect on the complex activity as well as caused a
significant positive shift of the MnV/IV electrochemical
potential.
An X-ray single-crystal structural analysis of a chromium-
(V) corrole has been performed. Shorter metal-nitrogen
pyrrole distances and larger metal out-of-plane displacements
were observed for CrV(tpfc)(NMes) (6) in comparison to a
CrIVdO porphyrin, but within the ranges displayed by other
high-valent corrole complexes. The EPR spectra of chromi-
um(V) imido corroles are similar to those of their chromium
nitrido analogues and, as expected, are characteristic for a
d1 system with a strong spin localization in the dxy orbital.
Figure 8. EPR spectrum of Cr(tpfc)(NAr) (7) in toluene at 93 K.
and hence complexes that are more stable than those based
on salen and porphyrin ancillary ligands.
An anisotropic, in particular, axially symmetric signal was
observed for 6 and 7 at 93 K (Figure 8), as has been noted
for both chromium(V) nitrido and oxo complexes.26 It is
worth noting that, in going to imido, a weaker π-donating
ligand than nitrido, there is a shift to lesser anisotropic
behavior in the low-temperature EPR signal.
Acknowledgment. We thank Dr. Saeed I. Khan for
assistance with the crystal structure determination and Dr.
Clifton K. F. Shen for assistance with MALDI MS measure-
ments. This work was supported by the NSF.
Conclusions
The first examples of manganese(V) and chromium(V)
imido corroles have been prepared and fully characterized.
The synthetic methodology relied on the interception by the
Supporting Information Available: MALDI-TOF MS for the
phosphine imine adducts (tpfc)Mn(NArPPh3), (tpfc)Mn(NArP(4-
FC6H4)3), and (tpfc)Mn(NArPCy3) (with instrument parameters) and
NMR spectra of complexes 3 and 5 (PDF) and tables of crystal
data, structure solution and refinement details, atomic coordinates,
bond lengths and angles, and anisotropic thermal parameters for 6
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(23) Azuma, N.; Imori, Y.; Yoshida, H.; Tajima, K.; Li, Y.; Yamauchi, J.
Inorg. Chim. Acta 1997, 266, 29.
(24) Azuma, N.; Ozama, T.; Tsuboyama, S. J. Chem. Soc., Dalton Trans.
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