Synthesis, characterization and molecular structures of six-coordinate manganese nitrosyl porphyrins

Article abstract of DOI:10.1039/b308143p

Manganese(II) porphyrins are isoelectronic with iron(III) porphyrins, and previously reported work suggests that manganese nitrosyl porphyrins are good structural models for their kinetically unstable and biologically relevant ferric-NO analogues. We have

Full text of DOI:10.1039/b308143p

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Synthesis, characterization and molecular structures of
six-coordinate manganese nitrosyl porphyrins†
Zaki N. Zahran, Jonghyuk Lee, Susan S. Alguindigue, Masood A. Khan and
George B. Richter-Addo*
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval,
Norman, OK 73019, USA. E-mail: grichteraddo@ou.edu
Received 16th July 2003, Accepted 23rd October 2003
First published as an Advance Article on the web 14th November 2003
Manganese() porphyrins are isoelectronic with iron() porphyrins, and previously reported work suggests that
manganese nitrosyl porphyrins are good structural models for their kinetically unstable and biologically relevant
ferric–NO analogues. We have prepared a new set of six-coordinate manganese nitrosyl porphyrins of the general
form (por)Mn(NO)(L) (por = TTP, T(p-OCH3)PP; L = piperidine, methanol, 1-methylimidazole) in moderate to
high yields. The (por)Mn(NO)(pip) complexes were prepared from the reductive nitrosylation of the (por)MnCl
compounds with NO in the presence of piperidine. The IR spectra of the (por)Mn(NO)(pip) compounds as KBr
pellets show new strong bands at 1746 cmϪ1 (for TTP) and 1748 cmϪ1 (for (T(p-OCH3)PP) due to the NO ligands.
Attempted crystallization of one of these compounds (por = TTP) from dichloromethane–methanol resulted in
the generation of the methanol complex (TTP)Mn(NO)(CH3OH). Reaction of the (por)Mn(NO)(pip) compounds
with excess 1-methylimidazole gave the (por)Mn(NO)(1-MeIm) derivatives in good yields. The IR spectra of these
compounds show νNO bands that are ∼12 cmϪ1 lower than those of the (por)Mn(NO)(pip) precursors, indicative of
greater Mn NO π-backdonation in the 1-MeIm derivatives. X-Ray crystal structures of three of these compounds,
namely (TTP)Mn(NO)(CH3OH), (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)PP)Mn(NO)(1-MeIm) were obtained,
and reveal that the NO ligands in these complexes are linear.
proteins containing NO continue to provide very valuable
Introduction
information to complement that obtained from their iso-
The biological role of nitric oxide (NO) is tied in with its inter-
electronic and biologically relevant FeCO analogues. Examples
actions with iron porphyrins in heme biomolecules.1 For
of such Mn-substituted nitrosyl complexes include those of
example, the receptor for NO is the heme-containing enzyme
hemoglobin (Hb), myoglobin (Mb), sGC, neuronal nitric oxide
soluble guanylyl cyclase (sGC), and this enzyme contains his-
synthase, cytochrome c and peroxidases.4,12–15 MnHbNO and
tidine as an axial ligand to iron. NO binds to the ferrous heme
MnMbNO have been prepared as spectroscopic models for
site in sGC to give a six-coordinate (por)Fe(NO)(His) complex
their EPR-silent Fe–CO analogues.16–18 The MnHbNO deriv-
that eventually converts to a five-coordinate (por)Fe(NO)
ative was shown to behave similarly to native HbCO in terms of
species.2–5 The binding of NO to the heme iron in sGC has been
ligand rebinding kinetics and cooperativity.19 In the case of Mn-
correlated with activation of this enzyme, resulting eventually
substituted sGC, binding of NO generated a six-coordinate
in vasodilation. Other histidine-liganded hemes also form
(porphyrinato)Mn(NO)(His) species that did not activate the
adducts with NO, and these include hemoglobin, myoglobin,
enzyme.4 Other manganese hemes have been prepared for
cytochrome oxidase, nitrophorins, FixL and nitrite reductase.1
NO-sensing20–22 and for peroxynitrite decomposition.23,24
In many cases, the binding of NO has been shown to be
biologically relevant.
To date, there are only two X-ray crystal structures of man-
ganese nitrosyl porphyrins, namely those of (TTP)Mn(NO) and
NO binds to both ferric and ferrous hemes, although NO
(TPP)Mn(NO)(4-Mepip).25 In order to better understand the
binding to ferric hemes is weaker than that for ferrous hemes.6
nature of binding of NO to manganese porphyrins and to
determine the nature of the trans effect of the bound NO ligand
in these complexes, we have prepared a new set of synthetic
The biological relevance of NO binding to ferric heme is best
exemplified by ferric nitrophorins from Rhodnius prolixus.7 The
nitrophorins are ferric heme proteins contained in the saliva of
manganese nitrosyl porphyrin complexes containing methanol
the organism, and they bind NO. When the insect bites the host,
and N-donor ligands. In this article, we report their syntheses
NO is released from the ferric nitrosyl nitrophorins, resulting
and structural characterization by spectroscopy and high-
in local vasodilation which ensures that the insect obtains a
resolution X-ray crystallography.
sufficient blood meal.
Efforts to study well-characterized six-coordinate ferric
Experimental
nitrosyl porphyrins have been hampered by the fact that they
are difficult to obtain pure, and only a few have been obtained
in sufficient quantities for detailed spectroscopic and crystallo-
graphic studies.8 Such ferric nitrosyl porphyrins belong to the
{FeNO}6 class as defined by Enemark and Feltham.9–11
We are interested in the study of manganese nitrosyl por-
phyrins belonging to the {MnNO}6 classification. These are
isoelectronic with their ferric nitrosyl {FeNO}6 and d6 ferrous
carbonyl FeCO counterparts. Manganese-substituted hemo-
All reactions were performed under an atmosphere of pre-
purified nitrogen using standard Schlenk glassware and/or in an
Innovative Technology Labmaster 100 Dry Box. Solutions for
spectral studies were also prepared under a nitrogen atmos-
phere. Solvents were distilled from appropriate drying agents
under nitrogen just prior to use: CH2Cl2 (CaH2).
Chemicals
(TTP)MnCl and (T(p-OCH3)PP)MnCl were prepared by liter-
ature methods.26,27 Piperidine (99.5ϩ%), 1-methylimidazole
(1-MeIm, 99ϩ%) and anhydrous CH3OH (99.8%) were
† Electronic supplementary information (ESI) available: Molecular
structure of (TTP)Mn(NO)(1-MeIm). See http://www.rsc.org/
suppdata/dt/b3/b308143p/
D a l t o n T r a n s . , 2 0 0 4 , 4 4 – 5 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
44

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purchased from Aldrich Chemical Co. and used as received.
Chloroform-d (99.8%) was obtained from Cambridge Isotope
Laboratories. Nitric oxide (98%, Matheson Gas) for the syn-
thesis work was passed through KOH pellets and two cold traps
(dry-ice/acetone, Ϫ78 ЊC) to remove higher nitrogen oxides.
(app t, J = 13, 1H of pip). The peaks of the piperidine ligand
were not observed in the 1H NMR spectrum of the complex at
room temperature. ESI mass spectrum: m/z 787 [(T(p-OCH3)-
PP)Mn]ϩ (100%).
Preparation of (por)Mn(NO)(1-MeIm) compounds (por ؍
TTP, T(p-OCH3)PP). To a stirred CH2Cl2 solution (8 mL) of
(TTP)Mn(NO)(pip)ؒ0.65CH2Cl2 (0.020 g, 0.022 mmol) was
added 1-MeIm (0.01 mL). The mixture was stirred for 5 h. The
solvent was removed in vacuo, and the residue was redissolved
in a CH2Cl2–CH3OH (1 : 1, 10 mL) mixture. To this solution
was added 1-MeIm (0.1 mL). Slow evaporation of this solution
at room temperature under inert atmosphere gave spectro-
scopically pure (TTP)Mn(NO)(1-MeIm) (0.012 g, 0.014 mmol,
Instrumentation
Infrared spectra were recorded on a Bio-Rad FT-155 FTIR
spectrometer. Proton NMR spectra were obtained on Varian
VXR-S 500 MHz or Varian Mercury VX 300 MHz spectro-
meters for low-temperature and room-temperature measure-
ments, respectively, and the signals referenced to the residual
signal of the solvent employed (CHCl3 at 7.24 ppm). All coup-
ling constants are in Hz. ESI mass spectra were obtained on a
Micromass Q-TOF mass spectrometer. Elemental analyses
were performed by Atlantic Microlab, Norcross, Georgia.
64% isolated yield) as purple crystals. IR (KBr, cmϪ1): νNO
=
1738s, 1732s; also 3128vw, 3019vw, 2952vw, 2918vw, 2866vw,
1532w, 1506m, 1457vw, 1347m, 1304vw, 1284vw, 1233w, 1209w,
1181m, 1108m, 1081w, 1071m, 1001s, 943vw, 908vw, 846vw,
796s, 731w, 719m, 670vw, 660vw, 628vw, 615vw, 523m. 1H
NMR (CDCl3, Ϫ40 ЊC): δ 8.69 (s, 8H, pyrrole-H of TTP), 8.04
(d, J = 7, 4H, o-H of TTP), 7.91 (d, J = 7, 4H, oЈ-H of TTP),
7.50 (d, J = 7, 4H, m-H of TTP), 7.45 (d, J = 7, 4H, mЈ-H of
TTP), 4.60 (s, 1H of 1-MeIm), 2.66 (s, 12H, CH3 of TTP), 2.06
(s, 3H, CH3 of 1-MeIm), 1.17 (s, 1H of 1-MeIm), 0.73 (s, 1H of
1-MeIm). The peaks of the 1-MeIm ligand were not observed in
Preparation of (por)Mn(NO)(pip) compounds (por ؍ TTP,
T(p-OCH3)PP). A Schlenk flask was charged with (TTP)MnCl
(0.035 g, 0.046 mmol), CH2Cl2 (8 mL) and piperidine (0.7 mL).
The mixture was stirred to generate a green solution, and NO
gas was bubbled through the solution for 20 min. [Note: during
this time, an uncharacterized white gaseous product was gener-
ated from solution.] The color of the reaction mixture turned
bright red. Nitrogen was bubbled through the solution for 5 min
to remove any unreacted NO and other gaseous products.
Anhydrous methanol (15 mL) was added to the solution, and
the volume of the solution was reduced in vacuo until a solid
precipitated. The supernatant solution was discarded, and the
red–purple solid was dried in vacuo to give (TTP)Mn(NO)(pip)ؒ
0.65CH2Cl2 (0.026 g, 0.029 mmol, 63% isolated yield). Anal.
Calc. for C53H47N6OMnؒ0.65CH2Cl2: C, 72.07; H, 5.44; N, 9.40;
Cl, 5.15. Found: C, 71.78; H, 5.66; N, 9.53; Cl, 4.99%. IR (KBr,
cmϪ1): νNO = 1746s; also 3022w, 2935w, 2920w, 2859w, 1533m,
1503m, 1450m, 1404vw, 1346m, 1305w, 1268vw, 1209w, 1181m,
1108w, 1070m, 1027vw, 1015w, 1001s, 871w, 847vw, 796s, 718m,
1
the H NMR spectrum of the complex at room temperature.
ESI mass spectrum: m/z 805 [(TTP)Mn(1-MeIm)]ϩ (57%), 723
[(TTP)Mn]ϩ (100%).
The purple (T(p-OCH3)PP)Mn(NO)(1-MeIm) compound
was obtained in 77% isolated yield after recrystallization from
CH2Cl2–CH3OH (2 : 1) in the presence of excess 1-MeIm at
room temperature under inert atmosphere. IR (KBr, cmϪ1): νNO
= 1736s; also 3128vw, 3033vw, 2998vw, 2958vw, 2934vw,
2906vw, 2833vw, 1606m, 1533w, 1501s, 1466w, 1460w, 1439w,
1346m, 1303vw, 1285m, 1247s, 1205vw, 1177s, 1108w, 1089vw,
1073vw, 1037vw, 1024w, 1001s, 849vw, 808m, 799m, 735w,
1
719w, 670vw, 662vw, 607w, 582vw, 540w. H NMR (CDCl3,
1
631w, 552vw, 522m. H NMR (CDCl3, Ϫ50 ЊC): δ 8.74 (s, 8H,
Ϫ40 ЊC): δ 8.70 (s, 8H, pyrrole-H of T(p-OCH3)PP), 8.07 (dd,
J = 8/2, 4H, o-H of T(p-OCH3)PP), 7.93 (dd, J = 8/2, 4H, oЈ-H
of T(p-OCH3)PP), 7.22 (dd (overlapping with CHCl3 peak),
4H, m-H of T(p-OCH3)PP), 7.18 (dd, J = 8/2, 4H, mЈ-H of
T(p-OCH3)PP), 4.60 (s, 1H of 1-MeIm), 4.06 (s, 12H, OCH3 of
T(p-OCH3)PP), 2.07 (s, 3H, CH3 of 1-MeIm), 1.15 (s, 1H of
1-MeIm), 0.72 (s, 1H of 1-MeIm). The peaks of the 1-MeIm
ligand were not observed in the 1H NMR spectrum of the
complex at room temperature. ESI mass spectrum: m/z 787
[(T(p-OCH3)PP)Mn]ϩ (100%).
pyrrole-H of TTP), 8.03 (d, J = 7, 4H, o-H of TTP), 7.99 (d,
J = 7, 4H, oЈ-H of TTP), 7.52 (app t (overlapping d’s), 8H,
m/mЈ-H of TTP), 5.29 (s, CH2Cl2), 2.67 (s, 12H, CH3 of TTP),
0.15 (br d, 1H of pip), Ϫ0.40 (br d, 2H of pip), Ϫ0.78 (br d, 1H
of pip), Ϫ1.34 (br d, 2H of pip), Ϫ3.42 (app q, J = 13, 2H of
pip), Ϫ3.76 (br d, 2H of pip), Ϫ5.48 (app t, J = 13, 1H of pip).
The peaks of the piperidine ligand were not observed in the 1H
NMR spectrum of the complex at room temperature. ESI mass
spectrum: m/z 723 [(TTP)Mn]ϩ (100%).
Attempts to obtain suitable crystals of this sample for a
single-crystal X-ray structural determination have so far not
been successful. Unexpectedly, however, crystals grown by slow
solvent evaporation of a CH2Cl2–CH3OH (2 : 1) solution
mixture of (TTP)Mn(NO)(pip) under inert atmosphere were
identified as (TTP)Mn(NO)(CH3OH) by X-ray crystallography.
The IR spectrum of (TTP)Mn(NO)(CH3OH) (as a KBr pellet)
Structural determinations by X-ray crystallography
X-Ray data were collected on a Bruker Apex diffractometer
using Mo-Kα (λ = 0.71073 Å) radiation. The structures were
solved by the direct method using the SHELXTL system (Ver-
sion 6.12; 1997) and refined by full-matrix least squares on F 2
using all reflections. All the non-hydrogen atoms were refined
anisotropically. All the hydrogen atoms were included with
idealized parameters. Displacement ellipsoids in Figs. 1 and 2
are drawn at the 35% probability level. Details of the crystal
data and refinement are given in Table 1.
showed a strong band at 1743 cmϪ1 assigned to νNO
.
The (T(p-OCH3)PP)Mn(NO)(pip) compound was prepared
similarly from (T(p-OCH3)PP)MnCl and NO gas in the pres-
ence of piperidine. The red-purple product was obtained in
81% isolated yield. IR (KBr, cmϪ1): νNO = 1748s; also 3032vw,
2997vw, 2935w, 2834w, 1608m, 1574vw, 1533m, 1512s, 1502s,
1464m, 1439m, 1410vw, 1347m, 1303w, 1287m, 1248s, 1206w,
1175s, 1107w, 1071w, 1039w, 1026m, 1002s, 872vw, 848w, 803s,
719m, 637vw, 632vw, 607m, 575vw, 555w, 538m. 1H NMR
(CDCl3, Ϫ50 ЊC): δ 8.73 (s, 8H, pyrrole-H of T(p-OCH3)PP),
8.04 (d, J = 8, 4H, o-H of T(p-OCH3)PP), 8.01 (d, J = 8, 4H,
oЈ-H of T(p-OCH3)PP), 7.22 (br (overlapping with CHCl3
peak), 8H, m/mЈ-H of T(p-OCH3)PP), 5.29 (s, CH2Cl2), 4.07 (s,
12H, OCH3 of T(p-OCH3)PP), 0.12 (br d, 1H of pip), Ϫ0.43 (br
d, 2H of pip), Ϫ0.80 (br d, 1H of pip), Ϫ1.36 (br d, 2H of pip),
Ϫ3.46 (app q, J = 13, 2H of pip), Ϫ3.79 (br d, 2H of pip), Ϫ5.50
(i) (TTP)Mn(NO)(CH3OH). Crystals for X-ray crystallo-
graphy were grown during an attempt at crystallizing (TTP)-
Mn(NO)(pip) using CH2Cl2–CH3OH. X-Ray diffraction
intensity data, which approximately covered the full sphere of
the reciprocal space, were measured as a series of ω oscillation
frames each 0.3Њ for 21 s frameϪ1. The detector was operated in
512 × 512 mode and was positioned 6.00 cm from the crystal.
Coverage of unique data was 99.0% complete to 54Њ (2θ). Cell
parameters were determined from a non-linear least squares fit
of 7582 reflections in the range of 2.8 < θ < 25.9Њ. A total of
D a l t o n T r a n s . , 2 0 0 4 , 4 4 – 5 0
45

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37158 reflections were measured. The data were corrected for
absorption by multi-scan method from equivalent reflections
giving the minimum and maximum transmissions of 0.9193 and
0.9403. The asymmetric unit contains one and a half units of
the C49H40N5O2Mn complex (with the Mn(2) atom situated
very near the inversion center) and fractional amounts of di-
chloromethane (0.2) and methanol (1.15) solvent molecules.
SHELXTL restrains of DFIX and ISOR were applied to the
atoms belonging to the disordered axial group and the solvent
molecules to achieve convergence during least squares refine-
ment. The final R1 = 0.067 is based on 12836 “observed reflec-
tions” [I > 2σ(I )], and wR2 = 0.178 is based on all reflections
(13303 reflections).
(ii) (TTP)Mn(NO)(1-MeIm)ؒ0.28CH2Cl2. Suitable crystals
for X-ray crystallography were grown by slow evaporation of a
CH2Cl2–CH3OH (1 : 1) solution of the compound containing
1-MeIm at room temperature under inert atmosphere. Cover-
age of unique data was 99.1% complete to 53Њ (2θ). Cell param-
eters were determined from a non-linear least squares fit of
8729 reflections in the range of 3.6 < θ < 26.2Њ. A total of 13983
reflections were measured. The data were corrected for absorp-
tion by multi-scan method from equivalent reflections giving
the minimum and maximum transmissions of 0.8602 and
0.9036. The asymmetric unit contains one and a half units of
the C52H42N7OMn complex (with the Mn(2) atom situated very
near the inversion center) and fractional dichloromethane sol-
vent molecules. The unit of the C52H42N7OMn complex that lies
at the inversion center is disordered at the axial sites. The N(10)
atom from the NO group lies very close to the N(11) atom
of the 1-methylimidazole ligand. As a result, the refinement of
the positional and thermal parameters is compromised. The
Mn–N–O angles and N–O bond distances in the two molecules
are significantly different. However, in view of the axial dis-
order in the disordered molecule, it is not possible to ascertain
whether these differences in bond angles and bond lengths are
real or an artifact of the poor refinement due to the disorder.
SHELXTL restrains of DFIX, ISOR and EADP were applied
to achieve convergence during least squares refinement (the dis-
ordered NO group in the molecule #2 was allowed to refine
without any restraints). The final R1 = 0.078 is based on 13129
“observed reflections” [I > 2σ(I )], and wR2 = 0.222 is based on
all reflections (13983 reflections).
Fig. 1 (a) Molecular structure of (TTP)Mn(NO)(CH3OH). Only the
crystallographically ordered molecule is shown. (b) View of the
orientation of the methanol ligand relative to the porphyrin skeleton.
The porphyrin tolyl substituents have been omitted for clarity.
(iii) (T(p-OCH3)PP)Mn(NO)(1-MeIm)ؒCH2Cl2. Suitable
crystals for X-ray crystallography were grown by slow evapor-
ation of a CH2Cl2–CH3OH (2 : 1) solution of the compound
containing 1-MeIm at room temperature under inert atmos-
phere. Coverage of unique data was 98.4% complete to 50Њ (2θ).
Cell parameters were determined from a non-linear least
squares fit of 7823 reflections in the range of 3.2 < θ < 24.4Њ.
A total of 23420 reflections were measured. The data were
corrected for absorption by the multi-scan method from equiv-
alent reflections giving minimum and maximum transmissions
of 0.9223 and 0.9820.
An initial data set was collected at 100(2) K and yielded poor
refinement. Better data were obtained at 178(2) K, showing
much sharper spots on the frames. There is a highly disordered
CH2Cl2 solvent molecule and some minor disorder involving
the nitrosyl O(1) atom, the imidazole N(7) atom and the
p-methoxy C(27), C(34), C(41) and C(48) atoms of the por-
phyrin aryl substituents. The non-solvent atoms were refined
without resolving into their disordered components due to the
very close proximity of these components. The overall geom-
etry of the molecule of interest is sound. SHELXTL restrains
of DFIX, ISOR and SADI were applied to the atoms belonging
to the disordered solvent molecule to achieve convergence dur-
ing least squares refinement. The final R1 = 0.075 is based on
4461 “observed reflections” [I > 2σ(I )], and wR2 = 0.1896 is
based on all reflections (8054 unique reflections). A reviewer
Fig. 2 (a) Molecular structure of (T(p-OCH3)PP)Mn(NO)(1-MeIm).
Hydrogen atoms have been omitted for clarity. (b) View of the
orientation of the 1-MeIm ligand relative to the porphyrin skeleton.
The porphyrin p-methoxyphenyl substituents have been omitted for
clarity.
D a l t o n T r a n s . , 2 0 0 4 , 4 4 – 5 0
46

Table 1 Crystal data and structure refinement
(TTP)Mn(NO)(CH3OH)ؒsolva
C49.90H43.33Cl0.27N5O2.77Mn
821.69
96(2)
(TTP)Mn(NO)(1-MeIm)ؒ0.28CH2Cl2
C52.28H42.56Cl0.56N7OMn
859.65
(T(p-OCH3)PP)Mn(NO)(1-MeIm)ؒCH2Cl2
C53H44Cl2N7O5Mn
984.79
Formula
Mr
T/K
120(2)
178(2)
Crystal system
Space group
Triclinic
P1
Triclinic
P1
Triclinic
P1
¯
¯
¯
a/Å
b/Å
11.3170(6)
12.3768(7)
14.9067(8)
15.0365(8)
12.1992(16)
12.7458(17)
c/Å
α/Њ
β/Њ
γ/Њ
23.0336(12)
97.9110(10)
15.8656(9)
104.7090(10)
90.8320(10)
15.151(2)
90.136(2)
90.287(2)
102.9260(10)
97.0580(10)
97.3400(10)
99.228(2)
V, Z
3074.9(3) Å3, 3
1.331
3407.4(3) Å3, 3
1.257
2325.3(5) Å3, 2
1.407
Dc/g cmϪ3
µ/mmϪ1
F(000)
Crystal size/mm
0.389
1288
0.22 × 0.18 × 0.16
1.68–27.00
0.370
1343
0.42 × 0.38 × 0.28
1.84–26.50
0.457
1020
0.18 × 0.04 × 0.04
1.62–25.00
θ Range for data collection/Њ
Index ranges, hkl
Reflections collected
Independent reflns (Rint
Max., min. transmission
Data/restraints/parameters
Goodness-of-fit on F 2
Ϫ14 to 14, Ϫ15 to 15, Ϫ29 to 29
37158
13303 (0.0162)
0.9403, 0.9193
13303/124/916
1.127
Ϫ18 to 18, Ϫ18 to 18, Ϫ19 to 19
36783
13983 (0.0198)
0.9036, 0.8602
13983/69/948
1.193
Ϫ14 to 14, Ϫ14 to 15, Ϫ17 to 17
23420
8054 (0.0715)
0.9820, 0.9223
8054/63/672
1.004
)
Final R indices [I > 2σ(I )]
R Indices (all data)
R1 = 0.0670, wR2 = 0.1769
R1 = 0.0685, wR2 = 0.1778
1.311, Ϫ0.776
R1 = 0.0780, wR2 = 0.2205
R1 = 0.0811, wR2 = 0.2220
1.257, Ϫ0.592
R1 = 0.0750, wR2 = 0.1721
R1 = 0.1437, wR2 = 0.1896
1.190, Ϫ0.458
Largest diff. peak, hole/e ÅϪ3
D
a Crystal contains both CH3OH and CH2Cl2 in fractional occupancy.

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suggested that we consider the possibility of a monoclinic crys-
tal system, since two angles are close to 90Њ. We re-examined
this possibility, and better fits of the data were obtained for a
triclinic cell (Rint = 0.07) than for a monoclinic cell (Rint = 0.57);
the correct Laue symmetry is 1 rather than 2/m. We thank the
reviewer for this suggestion.
CCDC reference numbers 215499–215501.
See http://www.rsc.org/suppdata/dt/b3/b308143p/ for crystal-
lographic data in CIF or other electronic format.
pure samples of the nitrosyl products can be obtained in size-
able quantities.
These products of eqn. (2) are purple and have similar solu-
bilities and stabilities as those of their piperidine precursors.
The IR spectra of the (por)Mn(NO)(1-MeIm) complexes (as
KBr pellets) reveal bands at 1738/1732 cmϪ1 (split band) and
1736 cmϪ1 for the TTP and T(p-OCH3)PP derivatives, respect-
ively, assignable to νNO. The split νNO band of (TTP)Mn(NO)-
(1-MeIm) is likely due to the presence of two orientations of the
1-MeIm ligand in the bulk solid. In any event, the νNO bands in
(por)Mn(NO)(1-MeIm) are ∼12 cmϪ1 lower in energy than the
νNO bands of the precursor (por)Mn(NO)(pip) complexes. This
feature is consistent with the replacement of piperidine ligand
with the π-interacting 1-MeIm ligand, which makes more
electron density available for MnII NO backdonation, result-
¯
Results and discussion
The synthesis and chemistry of manganese- and other metallo-
porphyrin nitrosyl complexes has been reviewed recently.1
Scheidt and coworkers showed, almost thirty years ago, that
reductive nitrosylation of manganese() porphyrins took place
in the presence of aliphatic amines to generate manganese
nitrosyl porphyrins.28,29 We have used similar methodology for
the preparation of (TTP)Mn(NO)(pip) and (T(p-OCH3)PP)-
Mn(NO)(pip) in 63 and 81% isolated yields, respectively
(eqn. (1); por = TTP or T(p-OCH3)PP; pip = piperidine).
ing in the lowering of νNO
.
The low-temperature 1H NMR spectra of the (por)Mn(NO)-
(1-MeIm) complexes show, in addition to the peaks due to the
porphyrin rings, the peaks of the bound 1-MeIm ligands. The
ESI mass spectrum of (TTP)Mn(NO)(1-MeIm) shows ion
fragments assigned to loss of the NO ligand or loss of
both axial ligands, whereas that of (T(p-OCH3)PP)Mn(NO)-
(1-MeIm) shows ion fragments from loss of both axial ligands.
(por)MnCl ϩ NO (excess) ϩ pip
(por)Mn(NO)(pip) (1)
X-Ray crystallographic characterization
These red–purple complexes are soluble in CH2Cl2, CHCl3
and acetone, but are only slightly soluble in hexane and meth-
anol. The complexes are moderately stable in air in the solid
state, showing no noticeable signs of decomposition over a one
month period as judged by IR and H NMR spectroscopy.
Their solutions, however, are air-sensitive.
We were successful in obtaining suitable crystals of (TTP)-
Mn(NO)(CH3OH), (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)-
PP)Mn(NO)(1-MeIm) for single-crystal X-ray crystallography.
The molecular structures of (TTP)Mn(NO)(CH3OH) and
(T(p-OCH3)PP)Mn(NO)(1-MeIm) are shown in Figs. 1 and 2.
The structure of (TTP)Mn(NO)(1-MeIm) is provided in the
ESI.†Selectedstructuralparametersforallthreecompoundsare
summarized in Fig. 3. As stated in the Experimental Section,
the asymmetric units in the crystals of (TTP)Mn(NO)(CH3OH)
and (TTP)Mn(NO)(1-MeIm) contain one crystallographically
ordered molecule and a second disordered molecule. Only the
more accurate data from the ordered components are discussed
here.
The Mn–N–O moieties in all three complexes are linear, and
this observed linearity is consistent with the classification of
these (por)Mn(NO)(L) compounds as {MnNO}6 species.9–11
The axial Mn–N(O) bond length in (TTP)Mn(NO)(CH3OH) is
1.680(2) Å while the N–O bond length is 1.165(2) Å. The
Mn–N(por) distances fall in the 2.008(2)–2.032(2) Å range
while the Mn atom is displaced by 0.12 Å from the 24-atom
mean porphyrin plane toward the π-acid NO ligand. The
methanol ligand in (TTP)Mn(NO)(CH3OH) is oriented essen-
tially between two porphyrin N-atoms (Fig. 1(b)), and the Mn–
O bond length is 2.086(2) Å while the Mn–O2–C49 angle is
124Њ. The Mn–O bond length is shorter than those observed in
other structurally characterized MnII and MnIII porphyrins
1
The IR spectra of the (por)Mn(NO)(pip) complexes (as KBr
pellets) show new strong bands at 1746 cmϪ1 (for TTP) and
1748 cmϪ1 (for (T(p-OCH3)PP), respectively, which are attri-
buted to the terminal NO ligands. These bands are similar to
that reported for the crystallographically characterized (TPP)-
Mn(NO)(4-Mepip) compound (νNO 1740 cmϪ1) that exhibits a
linear NO geometry.28,29 Although the (por)Mn(NO)(pip) com-
pounds are isoelectronic with their ferric–NO counterparts, the
νNO bands of the manganese compounds are significantly lower
than those of the isolable ferric [(por)Fe(NO)(N-base)]ϩ com-
plexes (νNO 1894–1921 cmϪ1) reported by Scheidt and Ellison.30
Parthasarathi and Spiro have suggested, based on resonance
Raman studies, that there is reduced metal NO backbonding
in FeIII–NO compared with MnII–NO due to the higher effect-
ive charge on iron in FeIII–NO.16 Clearly, such an effect is
responsible for the differences in the observed NO stretching
frequencies between the Mn and Fe compounds.
We have not been able to observe the parent ions of the
(por)Mn(NO)(pip) compounds in their ESI mass spectra; the
spectra show ion fragments due to loss of both axial ligands.
Attempts to grow crystals of the (TTP)Mn(NO)(pip) complex
from dichloromethane–methanol resulted in the replacement of
the piperidine ligand with methanol solvent to give (TTP)-
Mn(NO)(CH3OH) (νNO 1743 cmϪ1), as determined from the
X-ray crystal structure of the crystallization product (see later).
The (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)PP)Mn(NO)-
(1-MeIm) derivatives were prepared in 64 and 77% isolated
yields, respectively, from the reaction of their piperidine
precursors with an excess of 1-MeIm in CH2Cl2 (eqn. (2)).
containing
alcohol
ligands:
[(TPP)Mn(CH3OH)2]ClO4
(2.252(2) and 2.270(2) Å),32 [(TPP)Mn(CH3OH)2]SbCl6
(2.283(5) Å),33 (TPP)Mn(N3)(CH3OH) (2.329(7) Å),34 (P*)Mn-
(CH3OH)(OH) (2.251(7) Å; P* = D4 symmetrical chiral
porphyrin),35 (TCPP)Mn(CH3OH)(H2O) (2.246 Å; TCPP =
tetracarboxyphenylporphyrin),36 and [(OEP)Mn(EtOH)]ClO4
(2.145(2) Å).37
The axial Mn–N(O) bond lengths in the two (por)Mn(NO)-
(1-MeIm) complexes are in the 1.645(3)–1.650(2) Å range and
are similar to that observed previously in (TPP)Mn(NO)-
(4-Mepip) (1.644(5) Å),25,28 but are shorter than that deter-
mined for (TTP)Mn(NO)(CH3OH) (1.680(2) Å). This feature
is suggestive of greater Mn NO π-backdonation in these
(por)Mn(NO)(N-base) compounds. The lower νNO of the
1-methylimidazole complexes compared with that of the
methanol complex supports this view of a greater Mn NO
π-backdonation in (por)Mn(NO)(1-MeIm). Importantly, the
axial trans Mn–N(imidazole) bond lengths of 2.096(2) and
(por)Mn(NO)(pip) ϩ 1-MeIm
(por)Mn(NO)(1-MeIm) ϩ pip (2)
Prior to this work, the (por)Mn(NO)(imidazole) compounds
reported in the literature were prepared in situ from exposure of
NO to solutions of MnII-porphyrins containing an excess of the
imidazole ligand, and the success of this synthetic procedure
was very limited.16,31 We find that the preparative route
described by eqn. (2) provides a convenient method by which
D a l t o n T r a n s . , 2 0 0 4 , 4 4 – 5 0
48

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Fig.
3
Structural data for the ordered molecule of (TTP)Mn(NO)(CH3OH), the ordered molecule of (TTP)Mn(NO)(1-MeIm) and
(T(p-OCH3)PP)Mn(NO)(1-MeIm). Selected bond lengths and angles are shown in the top sketches. Perpendicular atom displacements from the
24-atom porphyrin planes (in 0.01 Å units) are shown in the bottom sketches.
2.097(3) Å for the (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)-
PP)Mn(NO)(1-MeIm) compounds, respectively, are shorter
References
1 L. Cheng and G. B. Richter-Addo, Binding and Activation of
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In summary, we have prepared several isolable six-coordinate
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