Synthesis and solid-state molecular structures of bis- and mono-nitrosobenzene complexes of ruthenium porphyrins

Article abstract of DOI:10.1139/v02-155

Bis-nitrosobenzene complexes of the form (por)Ru(PhNO)₂ (por = TPP, TTP; TPP = tetraphenylporphyrinato) dianion, TTP = tetratolylporphyrinato dianion) have been prepared in good yields from the reaction of the (por)Ru(CO) precursor with excess PhNO in dichloromethane. The IR spectra of the complexes (as KBr pellets) displayed new bands at ～31348 cm^-1, due to ν_NO. The solid-state molecular structure of (TPP)Ru(PhNO)₂ (was determined by single-) crystal X-ray diffraction, and revealed that the PhNO ligands are bound to the Ru center via the N-binding mode. Reactions of the (por)Ru(PhNO)₂ (complexes with excess 1-methylimidazole gave the mono-nitrosobenzene complexes) (por)Ru(PhNO)(1-MeIm). The IR spectra revealed a lowering of ν_NO in these mononitrosobenzene derivatives by ～ 27 cm^-1, a feature consistent with the replacement of one π-acid PhNO ligand with the more basic 1-MeIm ligand. The solid-state molecular structure of (TPP)Ru(PhNO)(1-MeIm) reveals, in addition to the N-binding of the PhNO ligand, an essentially parallel arrangement of the C-N-O (of PhNO) and imidazole planes; this is in contrast with the (TPP)Ru(PhNO)₂ complex, in which the C-N-O planes (of PhNO) are essentially perpendicular.

Full text of DOI:10.1139/v02-155

1252
Synthesis and solid-state molecular structures of
bis- and mono-nitrosobenzene complexes of
ruthenium porphyrins
Jonghyuk Lee, Brendan Twamley, and George B. Richter-Addo
Abstract: Bis-nitrosobenzene complexes of the form (por)Ru(PhNO)2 (por = TPP, TTP; TPP = tetraphenylporphyrinato
dianion, TTP = tetratolylporphyrinato dianion) have been prepared in good yields from the reaction of the (por)Ru(CO)
precursor with excess PhNO in dichloromethane. The IR spectra of the complexes (as KBr pellets) displayed new
bands at -1348 cm–1, due to ꢀNO. The solid-state molecular structure of (TPP)Ru(PhNO)2 was determined by single-
crystal X-ray diffraction, and revealed that the PhNO ligands are bound to the Ru center via the N-binding mode. Re-
actions of the (por)Ru(PhNO)2 complexes with excess 1-methylimidazole gave the mono-nitrosobenzene complexes
(por)Ru(PhNO)(1-MeIm). The IR spectra revealed a lowering of ꢀNO in these mononitrosobenzene derivatives by
-27 cm–1, a feature consistent with the replacement of one ꢁ -acid PhNO ligand with the more basic 1-MeIm ligand.
The solid-state molecular structure of (TPP)Ru(PhNO)(1-MeIm) reveals, in addition to the N-binding of the PhNO
ligand, an essentially parallel arrangement of the C-N-O (of PhNO) and imidazole planes; this is in contrast with the
(TPP)Ru(PhNO)2 complex, in which the C-N-O planes (of PhNO) are essentially perpendicular.
Key words: nitroso, X-ray, ruthenium, porphyrin, imidazole.
Résumé : La réaction du précurseur (por)Ru(CO) avec un excès de PhNO, dans le dichlorométhane, permet de
préparer les complexes du bis-nitrosobenzène, de la forme (por)Ru(PhNO)2 (por = TPP, TTP; TPP = dianion
tétraphénylporphyrinato, TTP = dianion tétratolylporphyrinato), avec de bons rendements. Les spectres IR des com-
plexes (sous la forme de pastilles de KBr) présentent de nouvelles bandes aux environ de 1348 cm–1 dues à ꢀNO
.
Faisant appel à la diffraction des rayons X sur un cristal unique, on a déterminé la structure moléculaire à l’état solide
du (TPP)Ru(PhNO)2; elle révèle que les ligands PhNO sont liés au centre Ru par le biais d’un mode de fixation par
l’azote. Les réactions des complexes (por)Ru(PhNO)2 avec un excès de 1-méthylimidazole conduisent à la formation de
complexes mononitrosobenzène (por)Ru(PhNO)(1-MeIm). Le spectre IR de ces dérivés mononitrosobenzène met en
évidence un abaissement d’environ 27 cm–1 de la ꢀNO, une caractéristique en accord avec le remplacement d’un ligand
PhNO ꢁ -acide par le ligand plus basique 1-MeIm. La structure moléculaire à l’état solide du (TPP)Ru(PhNO)(1-MeIm)
met en évidence la fixation par l’azote du ligand PhNO ainsi qu’un arrangement essentiellement parallèle des plans
C-N-O (du PhNO) et de l’imidazole; cette observation est en opposition à ce qui est observé avec le complexe
(TPP)Ru(PhNO)2 dans lequel les plans C-N-O (du PhNO) sont essentiellement perpendiculaires.
Mots clés : nitroso, diffraction des rayons X, ruthénium, porphyrine, imidazole.
[Traduit par la Rédaction] Lee et al. 1258
Introduction
prehensive discussion of this subject area has been presented
in a recent review (1). For example, various amine-
containing drugs can be metabolized by cytochrome P450,
resulting in RNO formation and binding to the heme, and
the tight binding of the RNO group can inhibit the enzy-
matic activity.
Much of our understanding of RNO binding to the heme
site in heme-containing biomolecules still comes from stud-
ies of RNO binding to synthetic metalloporphyrins. Mansuy
et al. (2) reported, in 1983, that i-PrNO binds to ferrous por-
phyrins via the N-binding mode. Several years ago, we re-
ported the first X-ray structural study of the binding of
nitrosoarenes to ferrous and ferric porphyrins, and showed
that the nature of binding (N-bound vs. O-bound) depended
on the formal oxidation state of the metal (3). Since then,
other structural reports have also been published on RNO
binding to metalloporphyrins of Fe (4), Mn (5), Co (6), Ru
(6), and Os (7).
The binding of nitrosobenzene and related RNO com-
pounds (R = alkyl, aryl) to the metal centers in heme
biomolecules is a naturally occurring phenomenon, and evi-
dence for RNO binding to the heme site has been established
for hemoglobin, myoglobin, cytochrome P450, guanylyl
cyclase, NO synthase, and other heme biomolecules; a com-
Received 1 June 2002. Published on the NRC Research Press
Web site at http://canjchem.nrc.ca on 13 September 2002.
J. Lee and G.B. Richter-Addo.1 Department of Chemistry
and Biochemistry, University of Oklahoma, 620 Parrington
Oval, Norman, OK 73019, U.S.A.
B. Twamley. X-ray Structural Laboratory, University
Research Office, 109 Morrill Hall, University of Idaho,
Moscow, ID 83844–3010, U.S.A.
1Corresponding author (e-mail: grichteraddo@ou.edu).
Can. J. Chem. 80: 1252–1258 (2002)
DOI: 10.1139/V02-155
© 2002 NRC Canada

Lee et al.
1253
We are particularly interested in RNO binding to Ru(II)
1157 (vw), 1072 (m), 1010 (vs), 859 (m), 833 (w), 794 (m),
754 (s), 737 (vw), 714 (w), 702 (s), 665 (w), 528 (vw). H
1
porphyrins, since the latter species serve as good models for
low-spin d6 systems similar to low-spin ferrous heme. In this
report, we present and describe the successful syntheses of
bis-nitrosobenzene complexes of Ru(II) tetraarylporphyrins
— namely (por)Ru(PhNO)2 — and their ready conversion by
reaction with 1-methylimidazole to the axially unsymmetri-
cal (por)Ru(PhNO)(1-MeIm) derivatives. In particular, we
examine the electronic and structural consequences of PhNO
binding, and discuss the effect of the axial imidazole ligand
on the orientation of the trans PhNO ligand.
NMR (CDCl3) ꢄ: 8.54 (s, 8H, pyrrole-H of TPP), 8.12 (m,
8H, o-H of TPP), 7.71 (m, 12H, m/p-H of TPP), 6.46 (br t,
2H, p-H of PhNO), 5.99 (br t, 4H, m-H of PhNO), 2.41 (br d,
J = 6.8 Hz, 4H, o-H of PhNO). LR-FAB-MS m/z (%): 851
([(TPP)Ru(NO)(PhNO)]+, 26), 821 ([(TPP)Ru(PhNO)]+, 19),
714 ([(TPP)Ru]+, 100).
The (TTP)Ru(PhNO)2 compound was prepared similarly
from (TTP)Ru(CO) and excess PhNO. The brown product
was obtained in 75% isolated yield. UV–vis (CH2Cl2) ꢃ
(nm): 310 (18), 413 (100), 525 (9). IR (KBr pellet) (cm–1):
ꢀNO 1346 (s) (overlapping with the porphyrin band); also
3022 (vw), 2921 (vw), 1586 (vw), 1529 (w), 1454 (w), 1305
(m), 1212 (w), 1181 (m), 1108 (w), 1072 (m), 1010 (vs),
858 (m), 798 (s), 766 (m), 716 (m), 691 (m), 669 (w), 643
Experimental section
All reactions were performed under an atmosphere of
prepurified nitrogen, using standard Schlenk glassware, and
(or) in an Innovative Technology Labmaster 100 dry box.
Solutions for spectral studies were also prepared under a ni-
trogen atmosphere. Solvents were distilled from the appro-
priate drying agents under nitrogen, just prior to use
(CH2Cl2 (CaH2), hexane (CaH2)).
1
(vw), 524 (m). H NMR (CDCl3) ꢄ: 8.54 (s, 8H, pyrrole-H
of TTP), 7.99 (d, J = 8 Hz, 8H, o-H of TTP), 7.50 (d, J =
8 Hz, 8H, m-H of TTP), 6.43 (br, 2H, p-H of PhNO), 5.95
(br, 4H, m-H of PhNO), 2.67 (s, 12H, CH3 of TTP), 2.38 (br,
4H, o-H of PhNO). LR-FAB-MS m/z (%): 907
([(TTP)Ru(NO)(PhNO)]+, 32), 877 ([(TTP)Ru(PhNO)]+, 15),
770 ([(TTP)Ru]+, 100).
Chemicals
(TPP)Ru(CO) and (TTP)Ru(CO) were prepared by pub-
lished procedures (8). The samples were converted, in moist
air, to (por)Ru(CO)(H2O)x (x ? 1) species (por = TPP, TTP)
(9). PhNO (97%) and 1-methylimidazole (1-MeIm, 99+%)
were purchased from Aldrich Chemical Co. and used as re-
ceived. Chloroform-d (99.8%) was obtained from Cam-
bridge Isotope Laboratories.
Preparation of (por)Ru(PhNO)(1-MeIm) compounds
(por = TPP, TTP)
To a CH2Cl2 (10 mL) solution of (TPP)Ru(PhNO)2
(0.020 g, 0.021 mmol) was added 2 equiv of 1-MeIm. This
mixture was stirred at room temperature for 30 min, during
which time it turned from brown to red-purple. The solvent
1
was removed in vacuo. An H NMR spectrum of the residue
Instrumentation
in CDCl3 showed the quantitative formation of
(TPP)Ru(PhNO)(1-MeIm), together with the presence of
Infrared spectra were recorded on a Bio-Rad FT-155 FT-
IR spectrometer. Proton NMR spectra were obtained on a
Varian 400 MHz spectrometer and the signals referenced to
the residual signal of the employed solvent. All coupling
constants are in Hz. FAB mass spectra were obtained on a
VG-ZAB-E mass spectrometer. UV–vis spectra were re-
corded on a Hewlett-Packard model 8453 diode array instru-
ment. Wavelengths are reported with ꢂ values, or as %
intensities for the samples whose yields were too small for
accurate concentration measurements.
some
unreacted
1-MeIm.
Spectroscopically
pure
(TPP)Ru(PhNO)(1-MeIm) was obtained in 80% yield by
recrystallization of the residue from a CH2Cl2–hexane solu-
tion (1:3) at –20°C. UV–vis (3.19 x 10–6 M in CH2Cl2) ꢃ
(nm) (ꢂ (mM–1, cm–1)): 307 (42), 413 (257), 533 (24). IR
(KBr pellet) (cm–1): ꢀNO 1322 (s); also 3127 (vw), 3052 (w),
3022 (w), 1596 (m), 1529 (m), 1482 (w), 1440 (m), 1348
(m), 1303 (m sh), 1237 (w), 1207 (w), 1177 (w), 1157 (w),
1109 (w), 1072 (m), 1008 (vs), 949 (vw), 924 (vw), 888
(vw), 833 (vw), 815 (vw), 793 (m), 752 (s), 701 (s), 665
1
(w), 615 (w), 527 (w). H NMR (CDCl3) ꢄ: 8.37 (s, 8H,
Preparation of (por)Ru(PhNO)2 compounds (por = TPP,
TTP)
pyrrole-H of TPP), 8.05 (m, 8H, o-H of TPP), 7.64 (m, 12H,
m/p-H of TPP), 6.39 (tt, J = 7.2, 1.2 Hz, 1H, p-H of PhNO),
5.96 (m, 2H, m-H of PhNO), 4.74 (dd (apparent t), J = 1.2,
1.2 Hz, 1H of 1-MeIm), 2.64 (m, 2H, o-H of PhNO), 2.18
(s, 3H, CH3 of 1-MeIm), 1.58 (br dd (apparent br t), 1H of
1-MeIm), 1.22 (dd (apparent t), J = 1.2, 1.2 Hz, 1H of 1-
MeIm). LR-FAB-MS m/z (%): 903 ([(TPP)Ru(PhNO)(1-
MeIm)]+, 18), 822 ([(TPP)Ru(PhNO) + H]+, 19), 796
([(TPP)Ru(1-MeIm)]+, 51), 714 ([(TPP)Ru]+, 100).
The (TTP)Ru(PhNO)(1-MeIm) compound was obtained
similarly in 57% isolated yield after recrystallization from a
CH2Cl2–hexane solution (1:5) at –20°C. UV–vis (CH2Cl2) ꢃ
(nm): 310 (8), 414 (100), 533 (4). IR (KBr pellet) (cm–1):
ꢀNO 1321 (s); also 3127 (vw), 3020 (w), 2953 (vw), 2921
(w), 2869 (vw), 1530 (m), 1506 (w), 1449 (w), 1348 (m),
1287 (w), 1237 (w), 1212 (w), 1181 (m), 1108 (m), 1092
(w), 1071 (m), 1007 (vs), 947 (vw), 886 (w), 847 (vw), 798
(TPP)Ru(CO) (0.100 g, 0.135 mmol) and excess PhNO
(0.040 g, 0.362 mmol) were dissolved in CH2Cl2 (20 mL).
The reaction mixture was stirred at room temperature for
30 min, during which time it turned from red to brown. The
solvent was removed in vacuo. The residue was redissolved
in CH2Cl2 (10 mL), and the product solution was transferred
to the top of a neutral alumina column (2 × 20 cm), and
eluted with CH2Cl2 in air. The brown product was collected
and dried in vacuo. The residue was dissolved in CH2Cl2
(10 mL). Slow evaporation of the solvent in air gave crystal-
line (TPP)Ru(PhNO)2 (0.086 g, 0.093 mmol, 69% isolated
yield). UV–vis (5.18 × 10–6 M in CH2Cl2) ꢃ (nm) (ꢂ (mM–1,
cm–1)): 307 (37), 411 (184), 525 (17). IR (KBr pellet) (cm–1):
ꢀNO 1348 (s) (overlapping with the porphyrin band); also
3055 (vw), 3024 (vw), 1597 (m), 1587 (vw), 1529 (w), 1487
(w), 1454 (w), 1441 (w), 1306 (m), 1208 (w), 1177 (w),
© 2002 NRC Canada

1254
Can. J. Chem. Vol. 80, 2002
Table 1. Crystal data and structure refinement.
Compound
(TPP)Ru(PhNO)2
(TPP)Ru(PhNO)(1-MeIm)·0.25CH2Cl2
Empirical formula
Formula weight
T (K)
C56H38N6O2Ru
927.99
203(2)
C54.25H39.5N7O1Cl0.5Ru
924.22
202(2)
Crystal system
Space group
a (Å), ꢅ (°)
b (Å), ꢇ (°)
c (Å), ꢈ (°)
Triclinic
P1
Monoclinic
P2(1)/c
11.3020(14), 82.49(2)
11.4637(14), 77.93(2)
18.207(2), 73.26(2)
2202.8(5), 2
1.399
13.246(2), 90
22.622(4), 123.36(1)
17.608(2), 90
4406.9(11), 4
1.393
V (Å3), Z
Dcalcd. (g cm–3)
Absorption coefficient ( ) (mm–1)
F(000)
0.408
952
0.435
1898
Crystal size (mm)
0.35 × 0.15 × 0.15
1.15–27.61
98.6 (27.61°)
–14 ? h ? 14, –14 ? k ? 14, –23 ? l ? 23
23 276
0.21 × 0.18 × 0.08
1.84ꢊ25.00
99.9 (25.00°)
–15 ? h ? 15, –18 ? k ? 26, –20 ? l ? 16
31 193
ꢉ range for data collection (°)
Completeness to ꢉ (%)
Index ranges
Reflections collected
Independent reflections
Absorption correction
Max and min transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2ꢋ(I)]
R indices (all data)
10 077 (Rint = 0.0467)
Empirical
0.9414 and 0.8705
10 077/0/586
7753 (Rint = 0.0776)
Empirical
0.9660 and 0.9141
7753/6/576
1.016
1.126
R1 = 0.0447, wR2 = 0.0882
R1 = 0.0767, wR2 = 0.0971
0.470 and ꢊ0.370
R1 = 0.0754, wR2 = 0.1483
R1 = 0.1170, wR2 = 0.1623
1.325 and –1.241
Largest diff. peak and hole (e Å–3)
(s), 764 (w), 716 (m), 694 (m), 667 (vw), 644 (vw), 524 (m).
1H NMR (CDCl3) ꢄ: 8.38 (s, 8H, pyrrole-H of TTP), 7.95
(dd, J = 7.6, 1.6 Hz, 4H, o-H of TTP), 7.89 (dd, J = 7.6,
1.6 Hz, 4H, o>-H of TTP), 7.47 (br d, J = 7.6 Hz, 4H, m-H
of TTP), 7.42 (br d, J = 7.6 Hz, 4H, m>-H of TTP), 6.36 (br
t, J = 8 Hz, 1H, p-H of PhNO), 5.93 (br t, J = 8 Hz, 2H, m-
H of PhNO), 4.71 (br dd (apparent br t), 1H of 1-MeIm),
2.65 (s, 12H, CH3 of TTP), 2.62 (br d, J = 8 Hz, 2H, o-H of
PhNO), 2.16 (s, 3H, CH3 of 1-MeIm), 1.56 (br, 1H of 1-
MeIm), 1.20 (br dd (apparent br t), 1H of 1-MeIm). LR-
FAB-MS m/z (%): 959 ([(TTP)Ru(PhNO)(1-MeIm)]+, 23),
877 ([(TTP)Ru(PhNO)]+, 20), 852 ([(TTP)Ru(1-MeIm)]+,
48), 770 ([(TTP)Ru]+, 100).
ing SAINTPlus (11) on all observed reflections. Data reduc-
tion and correction for Lp and decay was performed using
the SAINTPlus software. Absorption corrections were ap-
plied using SADABS (12). The structures were solved by di-
rect methods and refined by the least-squares method on F2
using the SHELXTL v.5.10 program package (13). The
structure was solved in the respective space groups by anal-
yses of systematic absences. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in their
geometrically generated positions and refined using a riding
model. No decomposition was observed during data collec-
tion. Details of the data collection and refinement are given
in Table 1. Further details are provided in the Supporting in-
formation.2 Displacement ellipsoids in Figs. 1 and 2 were
drawn at the 50% probability level.
X-ray crystallographic studies
Crystals were removed from the flasks and covered with a
layer of hydrocarbon oil. Suitable crystals were selected, at-
tached to a glass fiber and placed in the low-temperature ni-
trogen stream. Data were collected at low temperature using
a Siemens (Bruker) SMART 1K instrument (Mo Kꢅ radia-
tion, ꢃ = 0.71073 Å) equipped with a Siemens (Bruker) LT-
2A low-temperature device. Data were measured using ꢆ
scans of 0.3° per frame for 30 s, and a full sphere of data
was collected. The first 50 frames were recollected at the
end of data collection to monitor for decay. Cell parameters
were retrieved using SMART (10) software and refined us-
(TPP)Ru(PhNO)2 (1)
Purple needles of the compound were grown by
recrystallization from a CH2Cl2–hexane (1:10) mixture left
for 1 week at –20°C. A total of 2132 frames were collected
with a final resolution of 0.77 Å.
(TPP)Ru(PhNO)(1-MeIm)·(0.25CH2Cl2) (2)
Suitable crystals for X-ray crystallography were grown
from a CH2Cl2–hexane solution (1:3) left for 2 weeks at
2 Supplementary data may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). CCDC
190593 and 190594 contain the supplementary data for this paper. These data can be obtained, free of charge, via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, U.K.; fax
+44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2002 NRC Canada

Lee et al.
1255
Fig. 1. Molecular structure of (TPP)Ru(PhNO)2. Hydrogen atoms
Fig. 2. Molecular structure of (TPP)Ru(PhNO)(1-MeIm). Hydro-
have been omitted for clarity.
gen atoms have been omitted for clarity.
–20°C. A total of 1471 frames were collected with a final
resolution of 0.84 Å.
1
The H NMR spectra of the (por)Ru(PhNO)2 complexes
in CDCl3 show, as expected, peaks for both the porphyrin
macrocycle and the PhNO ligands in these diamagnetic, and
formally d6, Ru(II) complexes. For example, the pyrrole-
protons of the porphyrin macrocycle are at 8.54 ppm for
both the (por)Ru(PhNO)2 complexes (por = TPP, TTP). The
o- and m-protons of the tetraaryl groups in (TTP)Ru(PhNO)2
are resolved, and are at 7.99 and 7.50 ppm, respectively.
Results and discussion
Synthesis of the bis-nitrosobenzene complexes
James and co-workers (14) previously reported the syn-
thesis of (OEP)Ru(PhNO)2 from the reaction of
(OEP)Ru(CO) with excess PhNO in dichloromethane. Using
similar methodology, we successfully synthesized and re-
ported the related compound (OEP)Ru(ONC6H4NMe2)2 (6).
We have now extended this synthetic methodology to the
Synthesis of the mono-nitrosobenzene complexes
Many heme-containing biomolecules contain histidine as
the axial proximal ligand. Thus, we were interested in syn-
thesizing the axially unsymmetrical (por)Ru(PhNO)(1-
MeIm) complexes to examine their spectroscopic properties
and to determine the effect of the axial imidazole ligand on
the binding of PhNO in these complexes.
The mono-nitrosobenzene complexes, (por)Ru(PhNO)(1-
MeIm) (por = TPP, TTP), are readily prepared in 80% (for
TPP) and 57% (for TTP) isolated yields, from the reaction
of the precursor, (por)Ru(PhNO)2, with an excess of 1-MeIm
(eq. [2]).
preparation
of
bis-nitrosobenzene
complexes
of
tetraarylporphyrin complexes of Ru.
The reaction of excess PhNO with the (por)Ru(CO) com-
pounds (por = TPP, TTP) generates, after work-up, the bis-
nitrosobenzene complexes (por)Ru(PhNO)2 in 69% (TPP)
and 75% (TTP) isolated yields (eq. [1]).
[1]
(por)Ru(CO) + excess PhNO
J (por)Ru(PhNO)2 + CO
These brown bis-nitrosobenzene complexes are moder-
ately air-stable in solution and can be stored in air in the
solid state for several months without noticeable decomposi-
tion. They are soluble in CH2Cl2 and toluene, but are insolu-
ble in hexane. We were not able to detect the proposed
intermediate (por)Ru(CO)(PhNO) compounds by IR spec-
troscopy, and this differs from the case of the previously re-
ported (and similar) reaction of (OEP)Ru(CO) with N,N-
dimethyl-p-nitrosoaniline to give (OEP)Ru(ONC6H4NMe2)2
via a detectable intermediate, (OEP)Ru(CO)(ONC6H4NMe2)
(6).
The IR spectra of the new (por)Ru(PhNO)2 complexes (as
KBr pellets) show new bands at -1348 cm–1 assignable to
ꢀNO. The marked drop in ꢀNO from free PhNO at 1506 cm–1
is consistent with the view that the ꢁ* orbitals of the PhNO
ligands withdraw electron density from the “(por)Ru” frag-
ment in the (por)Ru(PhNO)2 complexes (1). Further evi-
dence of the PhNO ligands behaving as ꢁ-acids comes from
the X-ray crystal structure of (TPP)Ru(PhNO)2 (see later).
[2]
(por)Ru(PhNO)2 + 1-MeIm
J (por)Ru(PhNO)(1-MeIm) + PhNO
1
The H NMR spectra of the crude product mixtures in
CDCl3 (after a 30 min reaction time in CH2Cl2 at room tem-
perature, followed by solvent removal in vacuo, and
redissolution of the product in CDCl3) showed the presence
of the desired mono-nitrosobenzene complexes and
unreacted organic 1-MeIm as the only H-containing species.
Despite the use of an excess of 1-MeIm in these reactions,
the formation of the mono-nitrosobenzene complexes as the
only Ru-containing products indicates that only one PhNO
ligand in the (por)Ru(PhNO)2 complexes is susceptible to
substitution reaction under our reaction conditions. Interest-
ingly, a similar observation has been made by James and co-
workers (14) who reported that (OEP)Ru(PhNO)2 reacts
with 2 equiv of pyridine at room temperature to generate the
mono-substituted product (OEP)Ru(PhNO)(py) within
1
10 min. We have also obtained H NMR spectroscopic
© 2002 NRC Canada

1256
Can. J. Chem. Vol. 80, 2002
evidence for the formation of (OEP)Os(PhNO)(1-MeIm)
from the reaction of the previously reported
(OEP)Os(PhNO)2 with an excess of 1-MeIm.3 These red-
purple mono-nitrosobenzene complexes of Ru are moder-
ately air-stable, showing no signs of decomposition in air af-
ter 1 month in the solid state and at least 8 h in solution. As
with their bis-nitrosobenzene precursors, they are soluble in
CH2Cl2 and toluene, but are insoluble in hexane.
planarity around the nitroso N-atoms. The axial N-Ru-N
bond angle is 171.67(9)°, and this deviates only slightly
from strict linearity. The axial ligand orientations in
(TPP)Ru(PhNO)2 are shown in Fig. 3(a). The PhNO ligands
in (TPP)Ru(PhNO)2 are oriented perpendicular to each
other, and their C-NO groups essentially bisect the
porphyrin nitrogens.
The molecular structure of (TPP)Ru(PhNO)(1-MeIm) was
The IR spectra of the (por)Ru(PhNO)(1-MeIm) complexes
show bands at -1321 cm–1 assignable to ꢀNO. These bands
are shifted by -27 cm–1 to lower wavenumbers relative to
the bis-nitrosobenzene precursors (ꢀNO 1348 cm–1 (TPP) or
1346 cm–1 (TTP)). This shifting of ꢀNO to lower
wavenumbers is consistent with the replacement of one of
the ꢁ-acid PhNO ligands (in the bis-nitrosobenzene precur-
sor) with the more basic 1-MeIm ligand, which results in
more RuJN(O)Ph back-donation in the mono-
nitrosobenzene complexes. The 1H NMR spectra of the
mono-nitrosobenzene complexes in CDCl3 also reveal, in ad-
dition to porphyrin signals, peaks due to both the coordi-
nated PhNO and 1-MeIm ligands. The chemical shifts of the
ꢄpyrrole protons of the mono-nitrosobenzene complexes (8.37–
8.38 ppm) are slightly upfield shifted relative to those of the
bis-nitrosobenzene complexes (8.54 ppm), indicative of a
slight increase in electron-density of the porphyrin
macrocycles in the mono-nitrosobenzene complexes relative
to the bis-nitrosobenzene complexes. Importantly, the 1H
NMR spectrum of (TTP)Ru(PhNO)(1-MeIm) reveals that
the ortho-protons of the meso-aryl substituent on the
porphyrin rings are not equivalent, and the meta-protons are
also nonequivalent; this is a common feature for p-substi-
tuted tetraphenylporphyrin complexes of the form
(por)M(X)(Y) containing different ligands on each face of
also obtained and shown in Figs. 2 and 3(b). As with
(TPP)Ru(PhNO)2,
the
nitrosobenzene
ligand
in
(TPP)Ru(PhNO)(1-MeIm) is bound to the Ru(II) center via
an ꢌ1-N bonding mode. The Ru—N(por) distances are in the
2.039(4) Å to 2.062(5) Å range. Perpendicular atom dis-
placements of the atoms in the porphyrin skeleton from the
24-atom mean porphyrin plane are shown in the middle of
Fig. 3(b), and unlike (TPP)Ru(PhNO)2, the porphyrin
macrocycle of (TPP)Ru(PhNO)(1-MeIm) exhibits a ruffled
distortion. The axial Ru—N(PhNO) and O—N(Ph) bond
lengths are 1.884(6) Å and 1.267(7) Å, respectively. The ax-
ial ligand orientations in (TPP)Ru(PhNO)(1-MeIm) are also
shown in Fig. 3(b). Interestingly, and unlike in the case of
(TPP)Ru(PhNO)2, the PhNO and 1-MeIm ligands in
(TPP)Ru(PhNO)(1-MeIm) are oriented essentially parallel to
each other, and their C-NO (for PhNO) and C-N-C (for 1-
MeIm) groups essentially bisect the porphyrin nitrogens.
There are several notes of interest regarding the molecular
structures of (TPP)Ru(PhNO)2 and (TPP)Ru(PhNO)(1-
MeIm). For example, the axial Ru—N(PhNO) bond length
(1.953(2) to 2.049(2) Å) in (TPP)Ru(PhNO)2 is longer than
the related axial Ru—N(PhNO) distance of 1.884(6) Å seen
in (TPP)Ru(PhNO)(1-MeIm), and this observation is sugges-
tive of decreased RuJN(O)Ph back-bonding in
(TPP)Ru(PhNO)2 relative to that in (TPP)Ru(PhNO)(1-
1
the porphyrin plane (15, 16). The H NMR spectral peak
patterns for the coordinated 1-MeIm ligands are assigned ac-
cording to Leyden and Cox (17).
1
MeIm). This view is consistent with the IR and H NMR
spectroscopic results discussed in the previous section. The
O—N(Ph) bond lengths are not significantly different within
these bis- and mono-nitrosobenzene complexes.
X-ray crystallographic characterization
The most chemically interesting features of the structures
of these (TPP)Ru(PhNO)-containing complexes are the axial
ligand orientations. Indeed, the examination of the axial
ligand orientations in heme model complexes is of impor-
tance due partly to the fact that the axial ligand orientations
are affected by the electronic structure of the metal center.
We have previously reported the single-crystal X-ray struc-
ture of the (TPP)Fe(PhNO)2 complex containing two axial
ꢁ-acidic PhNO ligands, and proposed that the presence of
two axial ꢁ-acid ligands results in the perpendicular orienta-
tion of the axial ligands, and that this orientation maximizes
the ꢁ-back-bonding from the HOMOs of the low-spin
(TPP)Fe(II) fragment (namely the filled dxz and dyz orbitals)
into the empty ꢁ* orbitals of the axial PhNO ligands (3). We
have observed similar perpendicular axial ArNO ligand ori-
entations in (por)Os(ArNO)2 complexes (7). Indeed, and as
expected, the PhNO ligands in (TPP)Ru(PhNO)2 are ori-
ented perpendicular to each other, indicating that the PhNO
ligands act as ꢁ-acids in this complex.
The molecular structures of (TPP)Ru(PhNO)2 and
(TPP)Ru(PhNO)(1-MeIm) are shown in Figs. 1 and 2, re-
spectively. Selected bond lengths, angles, and axial ligand
orientations are shown in Fig. 3.
As can be seen in Figs. 1 and 2, the nitrosobenzene lig-
ands are attached to the formally Ru(II) centers via an ꢌ1-N
bonding mode. The Ru—N(por) distances for (TPP)Ru(PhNO)2
fall in the range of 2.047(2)–2.060(2) Å. Perpendicular atom
displacements of the atoms in the porphyrin skeleton from
the 24-atom mean porphyrin plane are shown in the middle
of Fig. 3(a). The porphyrin macrocycle of (TPP)Ru(PhNO)2
exhibits a saddle distortion, and the Ru atom resides in the
plane of the porphyrin (a calculated displacement from the
24-atom mean porphyrin plane of only 0.03 Å).
The axial Ru—N (of PhNO) bond lengths in
(TPP)Ru(PhNO)2 are 1.953(2) and 2.049(2) Å, and the N—O
bond lengths are 1.243(3) and 1.248(3) Å. The sum of an-
gles around the nitroso nitrogens are -360°, indicative of the
3 Data for (OEP)Os(PhNO)(1-MeIm). 1H NMR (CDCl3) ꢄ: 9.18 (s, 4H, meso-H of OEP), 6.17 (br tt, 1H, p-H of PhNO), 5.74 (m, 2H, m-H of
PhNO), 4.61 (dd (apparent t), J = 1.2, 1.2 Hz, 1H of 1-MeIm), 3.75 (m, 16H, CH2CH3 of OEP), 2.99 (m, 2H, o-H of PhNO), 2.08 (s, 3H,
CH3 of 1-MeIm), 1.75 (t, J = 8 Hz, 24H, CH2CH3 of OEP), 1.33 (br, 1H of 1-MeIm), 1.05 (dd (apparent t), J = 1.2, 1.2 Hz, 1H of 1-MeIm).
© 2002 NRC Canada

Lee et al.
1257
Fig. 3. Structural data for (a) (TPP)Ru(PhNO)2 and (b) (TPP)Ru(PhNO)(1-MeIm). Selected bond lengths and angles are shown at the
top. Perpendicular atom displacements from the 24-atom porphyrin plane (in 0.01 Å units) are shown at the middle. Also shown are
the two axial ligand orientations at the middle and the bottom: ꢅ is the torsion angle involving O-N-Ru-N(por), and ꢇ is the torsion
angle involving O-N-Ru-N(por) (for (TPP)Ru(PhNO)2) or C-N-Ru-N(por) (for (TPP)Ru(PhNO)(1-MeIm)). The solid dot represents the
nitroso oxygen atom, and the solid line represents the O-N-C unit of the PhNO ligand situated above the porphyrin plane. The dashed
line represents the O-N-C unit of the second PhNO ligand situated below the porphyrin plane (for (TPP)Ru(PhNO)2), or the C-N-C
unit of the 1-MeIm ligand (for (TPP)Ru(PhNO)(1-MeIm)) situated below the porphyrin plane.
The axial ligand orientation observed for the solid-state
structure of (TPP)Ru(PhNO)(1-MeIm) (Fig. 3(b)) provides
an interesting comparison with that seen for
(TPP)Ru(PhNO)2 (Fig. 3(a)). Thus, the PhNO and 1-MeIm
ligands in (TPP)Ru(PhNO)(1-MeIm) are nearly parallel to
each other. This axial ligand orientation, observed for
(TPP)Ru(PhNO)(1-MeIm), suggests that the 1-MeIm ligand
could act as a ꢁ donor in this case. Indeed, the parallel orien-
tation seen for (TPP)Ru(PhNO)(1-MeIm) might be expected
to maximize the ꢁ-back-bonding from one of two filled dꢁ
© 2002 NRC Canada

1258
Can. J. Chem. Vol. 80, 2002
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ligand.
Conclusion
5. S.J. Fox, L. Chen, M.A. Khan, and G.B. Richter-Addo. Inorg.
Chem. 36, 6465 (1997).
We have successfully obtained isolable bis-nitrosobenzene
and mono-nitrosobenzene complexes of ruthenium
tetraarylporphyrins. The comparative spectroscopic and sin-
gle-crystal X-ray crystallographic information obtained from
this work reveals differences in the ꢁ-acidic and basic char-
acter of the PhNO and 1-MeIm ligands when situated trans
to PhNO. This result is of great interest to model heme
chemistry involving nitroso-based ligands, and poses addi-
tional questions that need to be addressed. For example, how
important is axial ligand orientation for the dissociation of
trans ligands? Such questions form the focus of future stud-
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Acknowledgements
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We are grateful to the National Science Foundation (CHE-
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© 2002 NRC Canada