Syntheses, structures, and reactivities of {Fe-NO}6 nitrosyls derived from polypyridine-carboxamide ligands: Photoactive NO-donors and reagents for S-nitrosylation of alkyl thiols

Article abstract of DOI:10.1021/ic040057c

Two new iron nitrosyls derived from two designed pentadentate ligands N,N-bis(2-pyridylmethyl)-amine-N′-(2-pyridylmethyl)acetamide and N,N-bis(2-pyridylmethyl)-amine-N′-[1-(2-pyridinyl)ethyl]acetamide (PcPy₃H and MePcPy₃H, respectively, where H is the dissociable amide proton) have been structurally characterized. These complexes are similar to a previously reported {Fe-NO}⁶ complex, [(PaPy ₃)Fe(NO)](ClO₄)₂ (1) that releases NO under mild conditions. The present nitrosyls, namely [(PcPy₃)Fe(NO)] (ClO₄)₂ (2) and [(MePcPy₃)Fe(NO)](ClO ₄)₂ (3), belong to the same {Fe-NO}⁶ family and exhibit (a) clean ¹H NMR spectra in CD₃CN indicating S = 0 ground state, (b) almost linear Fe-N-O angles (177.3(5)° and 177.6(4)° for 2 and 3, respectively), and (c) N-O stretching frequencies (ν_NO) in the range 1900-1925 cm^-1. The binding of NO at the non-heme iron centers of 1-3 is completely reversible and all three nitrosyls rapidly release NO when exposed to light (50 W tungsten bulb). In addition to acting as photoactive NO-donors, these complexes also nitrosylate thiols such as N-acetylpenicillamine, 3-mercaptopropionic acid, and N-acetyl-cysteine-methyl-ester in yields that range from 30 to 90% in the absence of light. The addition of alkyl or aryl thiolate (RS^-) to the {Fe-NO}⁶ complexes in the absence of dioxygen results in the reduction of the iron metal center to afford the corresponding {Fe-NO} ⁷ species.

Full text of DOI:10.1021/ic040057c

Inorg. Chem. 2004, 43, 5736−5743
Syntheses, Structures, and Reactivities of {Fe−NO}6 Nitrosyls Derived
from Polypyridine-Carboxamide Ligands: Photoactive NO-Donors and
Reagents for S-Nitrosylation of Alkyl Thiols
,†
Raman K. Afshar,† Apurba K. Patra,† Marilyn M. Olmstead,‡ and Pradip K. Mascharak*
Department of Chemistry and Biochemistry, UniVersity of California,
Santa Cruz, California 95064, and Department of Chemistry, UniVersity of California,
DaVis, California 95616
Received April 12, 2004
Two new iron nitrosyls derived from two designed pentadentate ligands N,N-bis(2-pyridylmethyl)-amine-N′-(2-
pyridylmethyl)acetamide and N,N-bis(2-pyridylmethyl)-amine-N′-[1-(2-pyridinyl)ethyl]acetamide (PcPy3H and MePcPy3H,
respectively, where H is the dissociable amide proton) have been structurally characterized. These complexes are
similar to a previously reported {Fe−NO}6 complex, [(PaPy3)Fe(NO)](ClO ) (1) that releases NO under mild
4 2
conditions. The present nitrosyls, namely [(PcPy3)Fe(NO)](ClO ) (2) and [(MePcPy3)Fe(NO)](ClO ) (3), belong to
4 2
4 2
the same {Fe−NO}6 family and exhibit (a) clean 1H NMR spectra in CD CN indicating S ) 0 ground state, (b)
3
almost linear Fe−N−O angles (177.3(5)° and 177.6(4)° for 2 and 3, respectively), and (c) N−O stretching frequencies
-1
(νNO) in the range 1900−1925 cm . The binding of NO at the non-heme iron centers of 1−3 is completely reversible
and all three nitrosyls rapidly release NO when exposed to light (50 W tungsten bulb). In addition to acting as
photoactive NO-donors, these complexes also nitrosylate thiols such as N-acetylpenicillamine, 3-mercaptopropionic
acid, and N-acetyl-cysteine-methyl-ester in yields that range from 30 to 90% in the absence of light. The addition
of alkyl or aryl thiolate (RS-) to the {Fe−NO}6 complexes in the absence of dioxygen results in the reduction of
the iron metal center to afford the corresponding {Fe−NO}7 species.
Introduction
suppression. Indeed attempts have been made in recent years
to deliver NO to hypoxic cancerous cells via the use of
organic nitrites (such as GTN), S-nitrosothiols (such as
SNAP), or metal complexes (such as SNP, Figure 1).6,7 The
majority of these NO-drugs require heat, light, or metabolic
activation to release NO. Among the NO-donors, compounds
that release NO upon illumination deserve more attention
since they could be utilized in photodynamic therapy against
cancer.6 A close scrutiny of the literature, however, reveals
that very few non-heme iron NO complexes have been used
to deliver NO. To date, only Na2[Fe(CN)5NO] (SNP),7b Na2-
[Fe2S2(NO)4] (Roussin’s Red Salt, RRS),8 Na[Fe(EDTA)-
Nitric oxide (NO) has been shown to participate in a
variety of physiological functions that include regulation of
blood pressure, modulation of neurotransmission, inhibition
of platelet aggregation, cell-mediated immune response, and
antimicrobial activity.1-5 NO, a secondary messenger in
many biochemical processes, targets metal-heme centers and
thiol (-SH) residues in proteins. Since a sudden increase in
free NO concentration in cells triggers apoptosis, selective
delivery of NO to cancerous cells could result in tumor
* Author to whom correspondence should be addressed. Tel: 831-459-
4251. Fax: 831-459-2935. E-mail: mascharak@chemistry ucsc.edu.
† University of California, Santa Cruz.
‡ University of California, Davis.
(6) Ackroyd, R.; Kelty, C.; Brown, N.; Reed, M. Photochem. Photobiol.
2001, 74, 656-669. (b) Pandey, R. K. J. Porphyrins Phthalocyanines
2000, 4, 368-373.
(1) Nitric Oxide: Biology and Pathobiology; Ignarro, L. J., Ed.; Academic
Press: San Diego, CA, 2000.
(2) Methods in Nitric Oxide Research; Feelisch, M., Stamler, J. S., Eds.;
J. Wiley and Sons: Chichester, England, 1996.
(3) Schmidt, H. H. H. W.; Walter, U. Cell 1994, 78, 919-925.
(4) Zhang, J.; Snyder, S. H. Annu. ReV. Pharmacol. Toxicol. 1995, 35,
213-233.
(5) Wink, D. A.; Vodovotz, Y.; Laval, J.; Laval, F.; Dewhirst, M. W.;
Mitchell, J. B. Carcinogenesis 1998, 19, 711-721.
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Janczuk, A. J. Chem. ReV. 2002, 102, 1091-1134. (b) Kudo, S.;
Bourassa, J. L.; Boggs, S. E.; Sato, Y.; Ford, P. C. Anal. Biochem.
1997, 247, 193-202.
(8) (a) Bourassa, J. L.; Ford, P. C. Coord. Chem. ReV. 2000, 200, 887-
900. (b) Bourassa, J.; Degraff, W.; Kudo, S.; Wink, D. A.; Mitchell,
J. B.; Ford, P. C. J. Am. Chem. Soc. 1997, 119, 2853-2860.
5736 Inorganic Chemistry, Vol. 43, No. 18, 2004
10.1021/ic040057c CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/13/2004

{Fe-NO}6 Nitrosyls from Polypyridine-Carboxamide Ligands
ethanamide (MePcPy3H). In both these ligands, the carboxa-
mide group is not conjugated to the pyridine ring although
the overall binding characteristics are identical to that of
PaPy3H. In this paper we report the syntheses and structures
of the two nitrosyls [(PcPy3)Fe(NO)](ClO4)2 (2) and
[(MePcPy3)Fe(NO)](ClO4)2 (3). The spectroscopic properties
of all three nitrosyls 1-3 and rates of the NO release from
these three nitrosyls are also reported. In biological systems,
nitrosothiols derived from NO donors and compounds with
SH groups such as glutathione often act as potential NO
storage.7a,14 We were therefore curious to find out whether
the metal nitrosyls 1-3 could transfer NO to compounds
with free thiol groups to generate nitrosothiols. Herein we
report that all three nitrosyls transfer NO to alkylthiols such
as N-acetylpenicillamine, N-acetylcysteine, 3-mercaptopro-
pionic acid, and glutathione.
Figure 1. Examples of organic and inorganic NO donors.
(NO)],9 (Na2[Fe2(SR)2(NO)4] (Roussin’s Red Ester, RRE),10
and NH4[Fe4S3(NO)7] (Roussin’s Black Salt, RBS)8b,11 have
found some use as NO donor. Use of these inorganic NO
donors is often limited due to serious side reactions. For
example, release of CN- from SNP in biological matrix
severely limits its use as a NO donor in the treatment of
hypertensive emergencies. The photoinduced release of NO
from the inorganic NO donors such as SNP and RBS has
been employed in photodynamic therapy. Release of NO
from these complexes, however, requires high-intensity light
in the near UV region (340-440 nm) and the mechanism of
NO release is still unknown.8b
In our previous work, we have synthesized an Fe(III)
nitrosyl complex, [(PaPy3)Fe(NO)](ClO4)2 (1, where PaPy3H
is N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-car-
boxamide) that contains a deprotonated carboxamido nitrogen
trans to the bound NO.12 This {Fe-NO}6 nitrosyl (according
to the notation of Enemark and Feltham13) is photolabile and
rapidly releases NO under mild illumination (50 W tungsten
lamp). In MeCN, loss of NO from [(PaPy3)Fe(NO)](ClO4)2
(1) affords the solvent-bound [(PaPy3)Fe(MeCN)](ClO4)2
species. The photoreaction is clean (no other secondary
byproduct) and quantitative. Since passage of NO through a
solution of [(PaPy3)Fe(MeCN)](ClO4)2 in MeCN generates
1, the reaction is also reversible.
To probe the effect(s) of the pyridine-2-carboxamido
moiety on the release of NO from 1 upon illumination,
we have now synthesized two additional derivatives of
the designed ligand PaPy3H, namely N-(2-pyridylmethyl)-
2-(bis(2-pyridylmethyl)-amine)-ethanamide (PcPy3H) and
N-(1-(2-pyridylethyl)-2-(bis(2-pyridylmethyl)-amine)-
Experimental Section
2-(Aminomethyl)-pyridine, bromoacetate, p-chlorobenzene thiol,
sodium nitroprusside, and bromoacetylbromide were purchased
from Aldrich Chemical Co. and used without further purification.
The Fe(III) starting material [Fe(DMF)6](ClO4)3,15 1-(2-pyridyl)-
ethylamine,16 bis(2-pyridylmethyl)amine (BPA),17 and PaPy3H18
were synthesized by following the published procedures. NH4[Fe4S3-
(NO)7] and (Et4N)[S-p-C6H4Cl] were synthesized by literature
methods.19,20 S-Nitroso-mercaptopropanoic acid and S-nitroso-N-
acetyl-cysteine methyl ester were synthesized by the method
described by Hart21 while S-nitroso-N-acetyl penicillamine was
synthesized by the method of Field and co-workers.22 S-nitroso-
glutathione was synthesized as previously reported.23 Acetonitrile
(MeCN), ethanol (EtOH), methanol (MeOH), triethylamine (Et3N),
and diethyl ether (Et2O) were obtained from Fischer Chemical Co.
and were distilled from CaH2, Mg(OEt)2, Mg(OMe)2, sodium, and
(14) (a) Williams, D. L. H. Acc. Chem. Res. 1999, 32, 869-876. (b) Butler,
A. R.; Rhodes, P. Anal. Biochem. 1997, 249, 1-9.
(15) Hogkinson, J.; Jordan, R. B.; J. Am. Chem. Soc. 1973, 95, 763-768.
(16) Canary, J. W.; Wang, Y.; Roy, R., Jr. Inorg. Synth. 1998, 32, 70-75.
(17) Sams, C. K.; Somoza, F.; Bernal, I.; Toftlund, H. Inorg. Chim. Acta
2001, 318, 45-52.
(9) Schneppensieper, T.; Wanat, A.; Stochel, G.; Goldstein, S.; Meyerstein,
D.; van Eldik, R. Eur. J. Inorg. Chem. 2001, 9, 2318-2325.
(10) Ford, P. C.; Bourassa, J.; Miranda, K.; Lee, B.; Lorkovic, I.; Boggs,
S.; Kudo, S.; Laverman, L. Coord. Chem. ReV. 1998, 171, 185-202.
(11) Bourassa, J.; Lee, B.; Bernard, S.; Schoonover, J.; Ford, P. C. Inorg.
Chem. 1999, 38, 2947-2952.
(12) Patra, A. K.; Afshar, R.; Olmstead, M. M.; Mascharak, P. K. Angew.
Chem., Int. Ed. 2002, 41, 2512-2515.
(13) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339-
406.
(18) Rowland, J. M.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
2001, 40, 2810-2817.
(19) Seyferth, D.; Gallagher, M. Organometallics 1986, 5, 539-548.
(20) Rosenfield, S. G.; Armstrong, W. H.; Mascharak, P. K. Inorg. Chem.
1986, 25, 3014-3018.
(21) Hart, T. W. Tetrahedron Lett. 1985, 26, 2013-2016.
(22) Field, L.; Dilts, R. V.; Ravichandran, R.; Lenhert, P. G.; Carnahan,
G. E. Chem. Commun. 1978, 249-250.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5737

Afshar et al.
sodium/benzophenone, respectively, prior to use. Nitric oxide gas
was purchased from Johnson Mathew Chemical Co. and was
purified from higher oxides by passage through a long KOH column
before use in reactions.
(d, 1H); 7.38 (d, 1H); 7.47 (d, 2H); 7.60 (m, 3H), 8.51 (d, 2H);
8.57 (d, 1H), 9.16 (d, 1H). Selected IR frequency (NaCl plate) νCO
) 1666 cm-1
.
[(PaPy3)Fe(MeOH)](ClO4)2. A slurry of 0.50 g (0.63 mmol)
of [Fe(DMF)6](ClO4)3 in 10 mL of MeOH was added to a solution
of 0.22 g (0.63 mmol) of PaPy3H and 0.018 g (0.76 mmol) of NaH
in 10 mL of MeOH. The mixture became homogeneous and turned
red within 20 min of stirring. The desired complex was obtained
as a dark red microcrystalline precipitate from this solution upon
further stirring (3 h). It was filtered and dried under vacuum (yield
0.27 g, 67%). Anal. Calcd for C21H24N5O10Cl2Fe: C, 39.83, H,
3.82, N, 11.06. Found: C, 39.80, H, 3.97, N, 11.11. Selected IR
bands (KBr pellet, cm-1): 3554 (w, νOH); 3075 (w); 3029 (w); 2947
(w); 1645 (s, νCO); 1634 (w); 1567 (m); 1488 (w); 1471 (m); 1434
(s); 1385 (m); 1367 (m); 1286 (m); 1089 (s, νClO4); 1026 (m); 788
(m); 761 (s); 626 (s, νClO4).
[(PcPy3)Fe(NO)](ClO4)2‚MeCN (2‚MeCN). A slurry of 0.67 g
(0.84 mmol) of [Fe(DMF)6](ClO4)3 in 10 mL of MeOH was added
to a solution of 0.30 g (0.84 mmol) of PcPy3H (0.30 g, 0.84) and
0.023 g (0.96 mmol) of NaH in 10 mL of MeOH. The mixture
became homogeneous within 20 min of stirring and the color turned
to deep red. Upon further stirring, [(PcPy3)Fe(MeOH)](ClO4)2
precipitated out of the solution as a light tan precipitate which was
collected on a sintered glass funnel (yield 0.28 g, 50%). [Selected
IR bands (KBr pellet, cm-1): 3557 (w, νOH); 3080 (m); 2960 (w);
1634 (s, νCO); 1609 (w); 1486 (m); 1445 (s); 1360 (s); 1089 (s,
Synthesis Safety Note. Transition metal perchlorates should be
handled with great caution and be prepared in small quantities as
metal perchlorates are hazardous and may explode upon heating.
Methyl(bis(2-pyridylmethyl)amino)acetate. This intermediate
was synthesized according to a modified literature procedure.24
A
mixture of 8.10 g (58.6 mmol) of potassium carbonate and 2.75 g
(15 mmol) of methyl bromoacetate was added to a solution of 3.00
g (15 mmol) of bis(2-pyridylmethyl)amine (BPA) in 20 mL of
anhydrous DMF under dinitrogen. The resulting suspension was
protected from light and stirred at 35 °C for 72 h. The solvent was
then removed under vacuum and the residue was dissolved in 50
mL of chloroform (CHCl3). Following sequential washes with
aqueous HCl (pH 5), saturated brine solution, and concentrated
NaOH solution, the CHCl3 layer was finally dried with anhydrous
MgSO4 and filtered. Removal of the solvent afforded the desired
compound as an oil (yield 2.92 g, 72%). 1H NMR (303 K, CDCl3,
500 MHz), δ (ppm from TMS): 3.44 (s, 2H); 3.65 (s, 3H); 3.96
(s, 4H); 7.10 (m, 2H); 7.52 (m, 2H); 7.60 (m, 2H); 8.48 (m, 2H).
N,N-Bis(2-pyridylmethyl)-amine-N′-(2-pyridylmethyl)aceta-
mide (PcPy3H). A mixture of 1.0 g (3.7 mmol) of methyl(bis(2-
pyridylmethyl)amino)acetate, 1.60 g (14.8 mmol) of 2-(aminomethyl)-
pyridine, and 0.4 mL of pyridine was refluxed for 4 h. The thick
brown solution was then taken up in 100 mL of CHCl3 and washed
successively with an aqueous HCl solution (pH 5, 5 × 100 mL),
saturated brine solution (5 × 100 mL), and concentrated NaOH
solution (5 × 100 mL). Next, the CHCl3 solution was dried with
anhydrous MgSO4 and filtered. Removal of the solvent afforded
νClO4); 1026 (m); 861 (s); 761 (s); 626 (s, νClO4).] A batch of 0.23
g (0.37 mmol) of this [(PcPy3)Fe(MeOH)](ClO4)2 complex was
added to 12 mL of MeCN and the mixture was stirred for 20 h.
The solution slowly turned purple (a strong band with λmax at 535
nm) during this time. Next, it was degassed and a stream of purified
NO gas was slowly passed through it for 10 min. The solution was
then stored at -20 °C for 8 h. The dark red microcrystalline
precipitate of complex 2 thus obtained was collected on a sintered
glass funnel and dried (yield 0.16 g, 67%). Slow diffusion of Et2O
into a solution of 2 in MeCN in the dark afforded crystals of 2‚
MeCN, which were suitable for crystallographic analysis. 1H NMR
(303 K, CD3CN, 500 MHz) δ (ppm from TMS): 4.19 (s, 2H);
4.89 (d, 2H); 5.00 (d, 2H); 5.26 (s, 2H); 6.78 (d, 2H); 7.45 (t, 2H);
7.73 (d, 2H); 7.84 (t, 1H); 7.94 (d, 1H); 8.13 (m, 2H), 8.35 (m,
1H); 8.87 (d, 1H). Selected IR bands (KBr pellet, cm-1) 3085 (w),
2947 (w), 2285 (w, νCN), 1897 (s, νNO), 1622 (s, νCO), 1486 (w),
1466 (w), 1447 (w), 1400 (s), 1297 (w), 1227 (w), 1089 (s, νClO4),
907 (m), 819 (w), 769 (s), 624 (s). Electronic absorption in MeCN,
1
the ligand as brown oil (yield 3.77 g, 98%). H NMR (303 K,
CDCl3, 500 MHz), δ (ppm from TMS): 3.38 (s, 2H); 3.86 (s, 4H);
4.59 (d, 2H); 7.10 (m, 2H); 7.14 (t, 1H); 7.20 (d, 1H); 7.37 (d,
2H); 7.58 (m, 3H); 8.44 (m, 2H), 8.52 (d, 1H); 9.19 (t, 1H). Selected
IR frequency (NaCl plate) νCO ) 1660 cm-1
.
N,N-Bis(2-pyridylmethyl)-amine-N′-[1-(2-pyridinyl)ethyl]ac-
etamide (MePcPy3H). This ligand was synthesized according to a
modified literature procedure.25 A solution of 2.00 g (16.4 mmol)
of 1-(2-pyridyl)ethylamine and 1.66 g (16.4 mmol) of Et3N in 25
mL THF was added dropwise to a solution of 3.30 g (16.4 mmol)
of bromoacetylbromide in 20 mL of THF at 0 °C over a period of
20 min. The white precipitate of Et3NHBr was filtered off and the
filtrate was added to a solution of 3.27 g (16.4 mmol) of bis(2-
pyridylmethyl)amine and 1.66 g (16.4 mmol) of Et3N in 20 mL of
THF. The solution was then refluxed for 20 h. Next, it was filtered
to remove the Et3NHBr (white precipitate) and the solvent was
removed. The resulting red-brown oil was then taken up in 100
mL of CHCl3 and washed successively with an aqueous HCl
solution (pH 5, 5 × 100 mL), saturated brine solution (5 × 100
mL), and concentrated NaOH solution (5 × 100 mL). The CHCl3
solution was dried with anhydrous MgSO4 and filtered. Removal
of the solvent afforded the ligand as brown oil (yield 3.67 g, 62%).
1H NMR (303 K, CDCl3, 500 MHz), δ (ppm from TMS): 1.50 (d,
3H); 3.32 (q, 2H); 3.88 (s, 4H); 5.16 (p, 1H); 7.14 (m, 3H); 7.25
λ
max, nm (ꢀ ) M-1 cm-1): 363 (2040), 500 (925).
[(MePcPy3)Fe(NO)](ClO4)2‚1.75MeCN (3‚1.75MeCN). A slurry
of 0.44 g (0.56 mmol) of [Fe(DMF)6](ClO4)3 in 15 mL of EtOH
was added to a solution of 0.20 g (0.56 mmol) of MePcPy3H and
0.015 g (0.62 mmol) of NaH in 15 mL of EtOH, and the mixture
was stirred for 1 h. The mixture eventually turned light green and
the [(MePcPy3)Fe(EtOH)](ClO4)2 complex separated out as a tan
precipitate which was collected on a sintered glass funnel (yield
0.23 g, 62%). Selected IR bands (KBr pellet, cm-1): 3560 (w, νOH);
3067 (w); 2933 (w); 1634 (s, νCO); 1608 (w); 1481 (w); 1445 (m);
1388 (w); 1290 (w); 1090 (s, νClO4); 1026 (m); 861 (m); 770 (m);
627 (s, νClO4). A batch of 0.15 g (0.23 mmol) of this [(MePcPy3)-
Fe(EtOH)](ClO4)2 complex was added to 16 mL of MeCN and
the mixture was stirred for 20 h. The solution slowly turned purple
(a strong band with λmax at 535 nm) during this time. Next, the
solution was degassed and NO was passed through it for 5 min. It
was then stirred in the dark for 2 h. Finally, a batch of 20 mL of
Et2O was layered onto the solution and the reaction flask was stored
at -20 °C. The dark red microcrystals of 3 thus obtained
(23) (a) Shishido, S. M.; Seabra, A. B.; Loh, W.; de Oliveira, M. G.
Biomaterials 2003, 24, 3543-3553. (b) Askew, S. C.; Butler, A. R.;
Flitney, F. W.; Kemp, G. D.; Megson, I. L. Bioorg. Med. Chem. 1995,
3, 1-9.
(24) Banerjee, S. R.; Wei, L.; Levadala, M. K.; Lazarova, N.; Golub, V.
O.; O’Connor, C. J.; Stephenson, K. A.; Valliant, J. F.; Babich, J.
W.; Zubieta, J. Inorg. Chem. 2002, 41, 5795-5802.
(25) Niklas, N.; Walter, O.; Alsfasser, R. Eur. J. Inorg. Chem. 2000, 8,
1723-1731.
5738 Inorganic Chemistry, Vol. 43, No. 18, 2004

{Fe-NO}6 Nitrosyls from Polypyridine-Carboxamide Ligands
Table 1. Summary of Crystal Data and Intensity Collection and
Structural Refinement Parameters for [(PcPy3)Fe(NO)](ClO4)2‚MeCN
(2‚MeCN) and [(MePcPy3)Fe(NO)](ClO4)2‚1.75MeCN (3‚1.75MeCN)
were collected on a sintered glass funnel and dried in the absence
of light (yield 0.07 g, 47%). Crystals of 3‚1.75MeCN, suitable for
X-ray studies, were grown in the absence of light by diffusing Et2O
1
formula
mol wt
crystal color, habit
T, K
C22H23Cl2FeN7O10
672.22
violet plate
93 (2)
C24.50H27.25Cl2FeN7.75O10
into a dilute solution of 3 in MeCN. H NMR (303 K, CD3CN,
717.04
red plate
93 (2)
500 MHz) δ (ppm from TMS): 8.88 (d, 1H); 8.38 (t, 1H); 8.13 (q,
2H); 7.91 (d, 1H); 7.84 (t, 1H); 7.75 (d, 2H); 7.48 (d, 2H); 7.11 (d,
1H); 6.55 (d, 1H); 5.65 (d, 1H); 5.00 (m, 2H); 4.90 (d, 2H); 4.36
(d, 1H); 3.42 (q, 1H); 1.60 (d, 3H). Selected IR bands (KBr pellet,
cm-1): 3079 (w); 2962 (w); 2284 (w, νCN); 1918 (s); 1628 (s, νCO);
1613 (s); 1489 (w); 1470 (w); 1452 (w); 1386 (m); 1299 (w); 1090
(s, νClO4); 908 (w); 806 (w); 771 (m); 624 (s, νClO4); 539(w).
Electronic absorption in MeCN λmax, nm (ꢀ ) M-1 cm-1): 360
(2012), 505 (735).
Transnitrosylation of RSH by [(L)Fe(NO)](ClO4)2. In a typical
experiment, a 1:1 mixture of the solid [(L)Fe(NO)](ClO4)2 and RSH
(RSH ) N-acetyl penicillamine, 3-mercaptopropionic acid, glu-
tathione, or N-acetyl cysteine methyl ester)was placed in a 20-mL
Schlenk flask in the absence of light. Next, 10 mL of MeCN was
added and the solution was stirred for 15 min (only in case of GSH,
10 mL of water was used). A 100- µL portion of the reaction
mixture was then removed and analyzed on a Spectra-Physics UV
2000 HPLC (Grace VYDAC reverse-phase C18 column, 5-µm
particle size, 250 × 4.6 mm, 10-µL loop volume) using an isocratic
elution with a mixture of MeCN and H2O. All wavelength readings
were taken at 220 and 340 nm. The retention times were as
follows: S-nitroso-N-acetyl penicillamine (SNAP), 4.36 min at 65:
35 MeCN/H2O; S-nitroso-mercaptopropionic acid (SNMP), 9.30
min at 20:80 MeCN/H2O; S-nitroso-N-acetyl cysteine methyl ester
(SNACE), 8.00 min at 80:20 MeCN/H2O; and S-nitroso-glutathione
(GSNO), 8.38 min at 6:94 MeOH/H2O. The areas under the peaks
were determined by using authentic samples of known concentration
run under identical conditions. The eluting samples from each run
were collected from the HPLC and their identities were confirmed
by electrospray ionization mass spectroscopy.
Physical Measurements. Absorption spectra were recorded on
a Varian Cary 50 spectrophotometer. A Perkin-Elmer 1600 FTIR
spectrophotometer was employed to monitor the infrared spectra.
1H NMR spectra were recorded at 298 K on a Varian 500 MHz
instrument. The HPLC analysis was performed on a Spectra-Physics
UV 2000 high-pressure liquid chromatograph system with an UV
detector. Electrospray mass spectra were recorded on an ESI-ZMD
(Micromass 4000) mass spectrometer. Positive ion spectra were
recorded by spraying sample mixtures from metal-coated borosili-
cate capillaries using a Harvard syringe pump at 10 µL/min.
Photolysis Experiments. Kinetic measurements on the photo-
induced NO release from 1-3 were carried out with a Cary 50
Varian spectrophotometer equipped with a fiber optics probe to
detect the absorbance values at a fixed wavelength. The lens fitted
on the top of the fiber was dipped into the solution of the [(L)Fe-
(NO)](ClO4)2 complex (prepared in dark) in MeCN. Tungsten lamps
of different light intensity (25, 60, and 100 W) were used. The
lamps were fixed at a distance of 5 cm from the sample solution.
Continuous scanning of the absorbance values at 535 nm was
performed in case of 2 and 3 since both nitrosyls exhibit maximum
spectral changes at this wavelength. The pseudo first-order condi-
tions were maintained by the use of dilute solutions (∼0.3 × 10-3
M, solvent in excess). Data collection began as soon as the light
crystal system
space group
a, Å
b, Å
c, Å
orthorhombic
Pbca
7.6984 (13)
22.015 (4)
30.754 (5)
90
monoclinic
I2/a
14.665 (5)
18.429 (6)
21.460 (7)
90
93.221 (7)
90
5791 (3)
8
R, deg
â, deg
90
90
γ, deg
V, Å3
5212.2 (15)
8
1.713
0.856
0.796
Z
d
calcd, g cm-3
1.645
0.777
1.118
6.83
abs coeff, µ, mm-1
GOFa on F2
R1,b %
5.18
10.31
wR2,c %
19.17
2
a GOF ) [∑w(Fo2 - Fc )2]/(M - N)]1/2 (M ) number of reflections, N
) number of parameters refined). b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 )
2
2
[∑w(Fo - Fc )2]/[∑w(Fo)2]]1/2
.
Table 2. Selected Bond Distances [Å] and Bond Angles [deg]
complex 2
complex 3
Fe-N1
1.968(6)
1.868(5)
1.987(5)
1.984(5)
1.971(6)
1.680(6)
1.147(7)
1.247(8)
1.329(8)
81.7(2)
166.2(2)
94.6(2)
97.1(2)
98.1(3)
84.4(2)
87.6(2)
87.8(2)
178.4(3)
84.3(2)
82.8(2)
95.8(2)
166.7(2)
93.9(2)
90.7(2)
177.3(5)
1.961(4)
1.888(4)
1.985(4)
1.983(4)
1.988(4)
1.678(4)
1.147(5)
1.234(6)
1.337(6)
82.06(17)
166.82(17)
97.42(17)
94.55(16)
97.30(19)
84.83(17)
88.51(17)
87.40(17)
178.49(19)
83.61(16)
83.41(16)
95.77(18)
166.69(17)
92.94(18)
91.29(18)
177.6(4)
Fe-N2
Fe-N3
Fe-N4
Fe-N5
Fe-N6
N6-O2
C7-O1
C7-N2
N1-Fe-N2
N1-Fe-N3
N1-Fe-N4
N1-Fe-N5
N1-Fe-N6
N2-Fe-N3
N2-Fe-N4
N2-Fe-N5
N2-Fe-N6
N3-Fe-N4
N3-Fe-N5
N3-Fe-N6
N4-Fe-N5
N4-Fe-N6
N5-Fe-N6
Fe-N6-O2
solution of the complex in MeCN via slow diffusion of Et2O at 0
°C. Red plates of 3‚1.75MeCN were grown from a dilute MeCN
solution of the complex via diffusion of Et2O at 0 °C. Diffraction
data for both complexes were collected at 91 K on a Bruker
SMART 1000 system. Mo KR (0.710 73 Å) radiation was used,
and the data were corrected for absorption. The structures were
solved by direct methods (standard SHELXS-97 package). Machine
parameters, crystal data, and data collection parameters for all the
complexes are summarized in Table 1 , while selected bond
distances and angles are listed in Table 2. Complete crystallographic
data for [(PcPy3)Fe(NO)](ClO4)2‚MeCN (2‚MeCN) and [(MePcPy3)-
Fe(NO)](ClO4)2‚1.75MeCN (3‚1.75MeCN) are available in the
Supporting Information.
source was turned on. The observed NO-off rate constant (KNO
)
values were calculated by fitting the kinetic traces to the equation
A(t) ) A + (A0 - A ){exp(-KNOt)}, where A0 and A are the initial
and final absorbances, respectively, at the fixed wavelength.
X-ray Data Collection and Structure Solution and Refine-
ment. Violet plates of 2‚MeCN were obtained from a saturated
Results and Discussion
Synthesis and Characterization of [(L)Fe(NO)](ClO4)2
Complexes. As described in a previous paper,12 complex 1
Inorganic Chemistry, Vol. 43, No. 18, 2004 5739

Afshar et al.
can be conveniently synthesized by passing NO gas through
a methanolic solution of [(PaPy3)Fe(MeOH)](ClO4)2 (formed
in situ by mixing PaPy3H, [Fe(DMF)6](ClO4)3, and NEt3) at
45 °C. This method, however, did not work in the case of
PcPy3H and MePcPy3H ligands presumably due to slow
“loading” of the nonconjugated carboxamide moieties. It
appears that the complexation of the carboxamido nitrogen
to the Fe(III) center is facilitated by the conjugation present
in the pyridine-2-carboxamide portion of PaPy3H. Since the
carboxamide group in both PcPy3H and MePcPy3H ligands
is not conjugated with the pyridine ring, coordination of these
ligands to the Fe(III) center is somewhat more intricate.
Tuftlund and co-workers have reported a similar N5 ligand,
namely N-(2-pyridylmethyl)-3-bis(2-pyridylmethyl)amine)-
propanamide, that also contains a nonconjugated carboxa-
mide moiety.17 Interestingly, Fe(II) or Fe(III) complexes of
this ligand have not been structurally characterized. The
researchers concluded that “the coordination of the carboxa-
mide moiety is unfavorable” with this ligand in case of iron;
only the two pyridine nitrogens and the tertiary amine
coordinate to the metal center.17
Figure 2. Thermal ellipsoid (probability level 50%) plot of [(PcPy3)Fe-
(NO)]2+ (cation 2) with the atom-labeling scheme. H atoms are omitted
for the sake of clarity.
-
-
To ensure that the PcPy3 and MePcPy3 ligands bind
Fe(III) in pentadentate fashion in the present work, the
methanol and ethanol adducts, [(PcPy3)Fe(MeOH)](ClO4)2
and [(MePcPy3)Fe(EtOH)](ClO4)2, were first synthesized and
their spectral parameters were compared to the [(PaPy3)Fe-
(MeOH)](ClO4)2 precursor. The two alcohol adducts exhibit
their carbonyl stretching frequency (νCO) at 1634 cm-1. Since
this value is close to the νCO value of [(PaPy3)Fe(MeOH)]-
(ClO4)2 (1645 cm-1), we were confident of the fact that
both PcPy3- and MePcPy3- are properly coordinated to the
Fe(III) centers of the two precursors [(PcPy3)Fe(MeOH)]-
(ClO4)2 and [(MePcPy3)Fe(EtOH)](ClO4)2. These precursors
have been valuable in the syntheses of the target nitrosyls 2
and 3.
Figure 3. Thermal ellipsoid (probability level 50%) plot of [(MePcPy3)-
Fe(NO)]2+ (cation 3) with the atom-labeling scheme. H atoms are omitted
for the sake of clarity.
The Fe-N(O) bond distance (1.680(6) Å) of 2 lies within
the range of other structurally characterized {Fe-NO}6
complexes (1.6-1.7 Å) including 1 (Fe-N(O) distance )
1.677(2) Å). This short Fe-N(O) distance and the almost
linear Fe-N-O angle (177.3(5)°) are typical of {Fe-NO}6
nitrosyls. Although the average Fe-Npy (1.974(5) Å) and
Fe-Namine (1.987(5) Å) distances of 2 are very similar to
The limited solubility of [(PcPy3)Fe(MeOH)](ClO4)2 and
[(MePcPy3)Fe(EtOH)](ClO4)2 in alcoholic media requires
their conversion into the corresponding MeCN adducts before
the reaction with NO. Interestingly, the rate of solvent (the
sixth ligand) exchange is much faster with [(PaPy3)Fe-
(MeOH)](ClO4)2 than that with either [(PcPy3)Fe(MeOH)]-
(ClO4)2 or [(MePcPy3)Fe(EtOH)](ClO4)2 (5 min compared
to 20 h). However, once the MeCN adducts are formed (as
indicated by the appearance of the strong absorption band
at 535 nm), NO reacts rapidly with both [(PcPy3)Fe(MeCN)]-
(ClO4)2 and [(MePcPy3)Fe(MeCN)](ClO4)2 to afford the
desired NO complexes [(PcPy3)Fe(NO)](ClO4)2 (2) and
[(MePcPy3)Fe(NO)](ClO4)2 (3) in good yields (50-60%).
Structures of the Complexes. [(PcPy3)Fe(NO)](ClO4)2‚
MeCN (2‚MeCN). The structure of [(PcPy3)Fe(NO)]2+ (the
cation of 2) is shown in Figure 2, and the selected bond
distances and angles are listed in Table 2. The mode of
those noted for 1 (Fe-Npy ) 1.981(2) Å and Fe-Namine
)
1.972(2) Å), the Fe-Namido distance (1.868(5) Å) of 2 is
noticeably shorter than that of 1 (Fe-Namido ) 1.9009(19)
Å). The shortening of the Namido bond (∼0.033 Å) results
from the loss of conjugation of the carboxamido nitrogen
with the pyridine ring.
[(MePcPy3)Fe(NO)](ClO4)2‚1.75MeCN (3‚1.75MeCN).
The structure of [(MePcPy3)Fe(NO)]2+, the cation of complex
3, is shown in Figure 3 and the selected bond distances and
angles are displayed in Table 2. Structurally, 3 is similar to
2, with the exception of the elongation of the C7-N2 bond
(1.338(6) Å in 3 versus 1.329(8) Å in 2), and the shortening
of the C7-O1 bond (1.234(6) Å in 3 versus 1.247(8) Å in
2). The extra methyl group at position C6 of 3 thus gives
rise to a shorter CdO bond (more double bond character)
and a longer C-N bond (more single bond character) when
compared to 2. The Fe-N(O) (1.678(4) Å) bond distance is
-
coordination of the pentadentate monoanionic ligand PcPy3
to the Fe(III) center is very similar to that noted in [(PaPy3)-
Fe(NO)]2+ (cation of 1) and related complexes;12,18,26 the
three pyridine nitrogens and the tertiary amine nitrogen reside
in the equatorial plane while the carboxamido nitrogen
occupies an axial position that is trans to the bound NO.
(26) (a) Patra, A. K.; Afshar, R. K.; Rowland, J. M.; Olmstead, M. M.;
Mascharak, P. K. Angew. Chem., Int. Ed. 2003, 42, 4517-4521. (b)
Patra, A. K.; Rowland, J. M.; Marlin, D. S.; Bill, E.; Olmstead, M.
M.; Mascharak, P. K. Inorg. Chem. 2003, 42, 6812-6823.
5740 Inorganic Chemistry, Vol. 43, No. 18, 2004

{Fe-NO}6 Nitrosyls from Polypyridine-Carboxamide Ligands
Figure 5. Conversion of [(MePcPy3)Fe(NO)](ClO4)2 (3), λmax ) 505 nm,
to [(MePcPy3)Fe(MeCN)](ClO4)2, λmax ) 535 nm, in MeCN under
illumination with a 50-W tungsten lamp (t1/2 ) 90 s). Concentration of 3:
0.15 mM. The scans were recorded at intervals of 30 s.
Figure 4. 1H NMR (500 MHz) spectrum of [(MePcPy3)Fe(NO)](ClO4)2
(3) in CD3CN at 295 K. The two peaks around 2 ppm are due to solvent
and water.
Table 3. Values of KNO of Complexes 1-3 in MeCN with Varying
Light Intensities
similar to 2 (1.6880(6) Å) while the Fe-Namido (1.888(4) Å)
bond length lies between that of 1 (1.9009(19) Å) and 2
(1.868(5) Å).
light intensity
(W)
KNO × 103
complex
(s-1
)
[(PaPy3)Fe(NO)]2+ (1)
25
60
100
25
60
100
25
1.73 ( 0.01
5.60 ( 0.01
10.20 ( 0.02
2.10 ( 0.03
7.90 ( 0.01
11.60 ( 0.02
2.43 ( 0.02
7.00 ( 0.02
10.70 ( 0.03
Spectroscopic Properties. The two nitrosyls of the present
work display strong NO band (νNO) at 1897 cm-1 (value for
2) and 1918 cm-1 (value for 3). These νNO values are within
the range (1822-1937 cm-1) expected for {Fe-NO}6 type
complexes.12,26,27 Both complexes also exhibit strong carbonyl
stretching frequency (νCO) at 1622 cm-1 (value for 2) and
1628 cm-1 (value for 3) The red shift of νCO of the
corresponding free ligands (1660 cm-1 for PcPy3H and 1666
cm-1 for MePcPy3H) confirm coordination of the deproto-
nated carboxamido nitrogen to the Fe(III) center in these
[(PcPy3)Fe(NO)]2+ (2)
[(MePcPy3)Fe(NO)]2+ (3)
60
100
report,12,26b [(MePcPy3)Fe(NO)]2+ (cation of 3) is converted
into [(MePcPy3)Fe(MeCN)]2+ upon illumination. The isos-
bestic points at 323, 463, and 644 nm illustrate the clean
conversion of the NO-bound complex into the MeCN-bound
species. The MeCN adduct is quite stable and a back reaction
does not occur to reform 3 in the dark. However, 3 can be
synthesized by bubbling excess NO through the solution of
[(MePcPy3)Fe(MeCN)]2+ in MeCN. Thus, binding of NO
to the Fe(III) center is reversible. Very similar behavior has
been noted with 2. In MeCN, 2 displays isosbestic points at
320, 462 nm, and 650 nm (Figure S2, Supporting Informa-
tion) during the [(PcPy3)Fe(NO)]2+ f [(PcPy3)Fe(MeCN)]2+
transformation.
1
complexes. The H NMR spectra of complexes 2 and 3
(prepared and run in the dark) indicate an S ) 0 ground
state (Figure 4). Interestingly, the C8 protons (protons of
the CH2 unit between the tertiary amine and carbonyl group)
1
appear as a singlet in the H NMR spectrum of 2 (Figure
S1, Supporting Information), while in case of 3, these protons
give rise to doublet of doublets (peak marked with HB in
Figure 4, J ) 170.5 Hz).
Photolability of the Bound NO. In the solid state, both
2 and 3 are stable and quite resistant to damage by light.
Solutions of these nitrosyls in MeCN can be stored in the
A pseudo-first-order behavior has been noted in the
photolysis of 2 and 3 in MeCN. The value of the NO off
rate constant, KNO, increases linearly with the intensity of
light (Figure 5). Complexes 2 and 3 release NO slightly faster
than 1 (Table 3). However, the stability of [(PaPy3)Fe(NO)]-
(ClO4)2 (1) in solvents such as DMF or water is greater than
that of either 2 or 3. The latter two nitrosyls are rapidly
converted into the corresponding solvated species even
in the absence of light. It is therefore evident that the
conjugation in the pyridine-2-carboxamido moiety of PaPy3-
provides extra stability to 1 toward solvolysis. This is
supported by the fact that while [(PaPy3)Fe(MeCN)](ClO4)2
has been isolated in crystalline state,18 the MeCN-bound
species derived from 2 or 3 have not been isolated in pure
1
dark for weeks (as confirmed by their H NMR spectra).
However, when such solutions are exposed to visible light
(50-W tungsten lamp), rapid color changes are observed. For
example, when a solution of 3 in MeCN is exposed to light,
the color of the solution changes from red to purple. The
changes in the absorption spectrum are shown in Figure 5.
Much like the behavior of 1 described in our earlier
(27) (a) Grapperhaus, C. A.; Patra, A. K.; Mashuta, M. S. Inorg. Chem.
2002, 41, 1039-1041. (b) Chatel, S.; Chauvin, A.-S.; Tuchagues, J.-
P.; Leduc, P.; Bill, E.; Chottard, J.-C.; Mansuy, D.; Artaud, I. Inorg.
Chim. Acta 2002, 336, 19-28. (c) Schweitzer, D.; Ellison, J. J.; Shoner,
S. C.; Lovell, S.; Kovacs, J. A. J. Am. Chem. Soc. 1998, 120, 10996-
10997. (d) Scheidt, W. R.; Lee, Y. J.; Hatano, K. J. Am. Chem. Soc.
1984, 106, 3191-3198. (e) Nappier, T. E.; Feltham, R. D.; Enemark,
J. H.; Kruse, A.; Cooke, M. Inorg. Chem. 1975, 14, 806-812.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5741

Afshar et al.
and solid form so far. Solutions of [(PcPy3)Fe(MeCN)]2+
and [(MePcPy3)Fe(MeCN)]2+ in MeCN also change color
slowly in air. For example, exposure to air causes the purple
color of these two MeCN-adducts to turn yellow (in case of
[(PcPy3)Fe(MeCN)]2+) and red (in case of [(MePcPy3)Fe-
(MeCN)]2+) within 24 h. The nature of the species in such
solution is unknown at this time.
Table 4. Yields of Nitrosothiols Obtained in S-Nitrosylation of Thiols
with 1-3 in MeCN or Water as Determined by High- Pressure Liquid
Chromatography (HPLC)
complex
thiola
% yield (SNO)
[(PaPy3)Fe(NO)](ClO4)2 (1)
NAP
MP
NACE
GSHb
NAP
MP
NACE
GSHb
NAP
MP
91.4
92.8
80.7
90.0
34.0
65.1
83.8
81.7
30.2
62.2
83.6
73.4
[(PcPy3)Fe(NO)](ClO4)2 (2)
S-Nitrosylation of Thiols using 1-3. S-Nitrosylation of
thiol (Cys) group(s) in proteins activate several biochemical
processes such as bronchodilation in human airways, va-
sorelaxation of blood vessels, and regulation of blood flow.1,2
The nitrosylated proteins also serve as compartments for
NO storage or as a transporter of NO.28,29 Recently, studies
have also shown that nitrosylation of specific thiols by NO
can modulate the enzymatic activity of several cellular
enzymes.30-32 These in vitro studies utilized NO donors that
consisted of S-nitrosylated compounds such as S-nitroso-
glutathione (GSNO) and S-nitroso-N-acetyl-penicillamine
(SNAP), which transfer NO to available vicinal thiol(s)
present in the protein. The addition of the S-nitrosylated
thiols (RSNOs) to proteins (P-SH) afford the corresponding
nitrosylated proteins (P-SNO) as well as the S-thiolate
adduct (P-SSR).33 To find out whether the new complexes
1-3 can transfer NO to compounds containing -SH group,
we have examined the reactions of these nitrosyls with four
such compounds, namely N-acetyl-penicillamine (NAP),
N-acetyl-cysteine-methyl ester (NACE), 3-mercaptopropionic
acid (MP), and glutathione (GSH). The addition of any of
these {FeNO}6 complexes to these compounds with -SH
group in MeCN resulted in rapid formation of the corre-
sponding SNOs, which were identified by mass spectrometry
following separation by HPLC (Table 4). This S-nitrosylation
reaction is rapid (complete within 5 min) at room temperature
in the absence of light. To our knowledge, there is no
example of an iron nitrosyl that transfers NO to a -SH group
at ambient temperature. Compounds such as RBS, RRS,
RRE, and SNP all require heat or intense light to release
NO. For example, in a control reaction, prolonged heating
(24 h) of a solution of RBS and GSH in water at 45 °C
afforded only 6.0% of GSNO. Likewise, SNP yielded only
3.4% of GSNO under the same condition. In contrast,
complex 1 affords 90% GSNO at 25 °C within 5 min (Table
4). Among the three nitrosyls, 3 exhibits the lowest capacity
to transfer NO to all the substrates presumably due to its
instability in solution. Nevertheless, significant extent of
S-nitrosylation is observed with this nitrosyl and the products
can be easily identified by HPLC and mass spectrometry
[(MePcPy3)Fe(NO)](ClO4)2 (3)
NACE
GSHb
a NAP ) N-acetylpenicillamine, MP ) 3-mercaptopropionic acid, NACE
) N-acetyl-cysteine methyl ester, and GSH ) glutathione. b Reaction
performed in water.
Figure 6. (Top) HPLC trace of a reaction mixture containing 3 and
3-mercaptopropionic acid (1:1) in MeCN in the absence of light at 25 °C.
The iron-containing species give rise to the first peak in the chromatogram.
(Bottom) MS trace of the nitrosothiol (retention time of 9.298 min) that
shows a peak at 134, which corresponds to mass of the parent ion.
(28) Moncada, S.; Palmer, M. J.; Higgs, E. A. Pharm. ReV. 1991, 43, 109-
142.
(29) Butler, A. R.; Williams, D. L. H. Chem. Soc. ReV. 1993, 233-241.
(30) Padgett, C. M.; Whorton, A. R. Cell Biochem. Biophys. 1995, 27, 157-
177.
(31) Mannick, J. B.; Schonhoff, C. M. Arch. Biochem. Biophys. 2002, 408,
1-6.
(32) Foster, M. W.; McMahon, T. J.; Stamler, J. S. Trends Mol. Med. 2003,
9, 160-168.
(33) (a) Srivastava, S.; Dixit, B. L.; Ramana, K. V.; Chandra, A.; Chandra,
D.; Zacarias, A.; Petrash, J. M.; Bhatnagar, A.; Srivastava, S. K.
Biochem. J. 2001, 358, 111-118. (b) Chandra, A.; Srivastava, S.;
Petrash, J. M.; Bhatnagar, A.; Srivastava, S. K. Biochemistry 1997,
36, 15801-15809.
(Figure 6). The high NO donor capacity of 1-3 at ambient
temperature is noteworthy. At this time, we are studying their
utility as nitrosylating agents for proteins.
Since all the nitrosyls of the present study (1-3) show
greater stability in MeCN solutions, we have further explored
the nitrosylation reactions in MeCN. These nitrosyls ef-
5742 Inorganic Chemistry, Vol. 43, No. 18, 2004

{Fe-NO}6 Nitrosyls from Polypyridine-Carboxamide Ligands
ficiently transfer NO to substrates containing -SH group
(Table 4). However, the reaction takes a very different course
when the corresponding thiolates are used as the substrates.
For example, the reaction of 1 and NACE gives rise to the
corresponding nitrosothiol SNACE in ∼80% yield. In
contrast, when deprotonated NACE (a thiolate) is used, one
does not obtain any nitrosothiol; instead, the {Fe-NO}6
complex 1 is quantitatively converted to the corresponding
{Fe-NO}7 species [(PaPy3)Fe(NO)](ClO4). The same result
is obtained with aromatic thiolates. The addition of (Et4N+)-
(p-ClC6H4S-) to a solution of 1 in MeCN in the absence of
light and oxygen affords the {Fe-NO}7 species [(PaPy3)-
Fe(NO)](ClO4) within minutes (as evidenced by the appear-
ance of a band at 476 nm in the absorption spectrum).26b
The thiolate substrate in each case causes reduction of the
iron center according to eq 1
MeCN when the corresponding RSNOs are obtained in
∼90% yield. Only a trace of disulfide is obtained in these
reactions.36 Since the other product in these reactions is the
corresponding [(PaPy3)Fe(MeCN)]2+ (an Fe(III) species), it
is evident that a bimolecular nucleophilic attack by RSH
occurs at the coordinated NO moiety.37 We are currently
performing more experiments to elucidate the mechanism
of this NO transfer reaction by 1-3.
Conclusion
The three {Fe-NO}6 nitrosyls, namely [(PaPy3)Fe(NO)]-
(ClO4)2 (1), [(PcPy3)Fe(NO)](ClO4)2 (2), and [(MePcPy3)-
Fe(NO)](ClO4)2 (3), are photolabile and rapidly release NO
upon illumination with visible light. The rates of NO release
from these nitrosyls show minor changes upon alteration of
the pyridine/carboxamide portion of the ligand frame al-
though removal of conjugation in this portion of the ligand
results in reduced stability of the metal complex. These
nitrosyls are efficient NO donors to compounds with -SH
group and the S-nitrosylation reaction occurs rapidly at room
temperature in the absence of light. When thiolates are used
as the substrates, the {Fe-NO}6 nitrosyls are reduced to the
corresponding {Fe-NO}7 species and no nitrosothiols are
obtained.
{Fe-NO}6 + (Et4N+)(p-ClC6H4S-) f {Fe-NO}7 +
p-ClC6H4S-SC6H4Cl-p (1)
and no NO transfer is observed. This result demonstrates
that the {Fe-NO}7 species are not NO transfer agents. The
reduction of Fe(III) complexes by thiols has been studied
by various groups.34 The reaction 1 however is the first
example of a thiolate reducing a designed non-heme {Fe-
NO}6 complex to the corresponding {Fe-NO}7 species. The
other product of this reaction is the disulfide, which results
from the coupling of the thiyl radicals.35
The reactions of 1-3 with thiols in MeCN deserve further
discussion. Since the NO transfer occurs in the absence of
light, it is evident that these reactions do not proceed simply
via loss of NO from the nitrosyls followed by addition of
NO to the RSH molecules. Indeed, reactions of purified NO
gas with thiols such as NACE in MeCN afford mostly
disulfide even in the absence of dioxygen. This behavior is
in sharp contrast to the reaction of 1 with NACE and MP in
Acknowledgment. Financial support from NIH (GM
61636) is gratefully acknowledged. The Bruker SMART
1000 diffractometer was funded in part by an NSF Instru-
mentation Grant CHE-9808259.
Supporting Information Available: 1H NMR spectrum of 2
(Figure S1) and changes in the absorption spectrum of 2 in MeCN
upon illumination (Figure S2) (PDF); and X-ray crystallographic
data (in CIF format) and tables for the structure determination of
complexes [(PcPy3)Fe(NO)](ClO4)2‚MeCN (2‚MeCN) and
[(MePcPy3)Fe(NO)](ClO4)2‚1.75MeCN (3‚1.75MeCN). This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
IC040057C
(36) (a) Butler, A. R.; Calsy-Harrison, A. M.; Glidewell, C. Polyhedron
1988, 7, 1197-1202. (b) Johnson, M. D.; Wilkins, R. G. Inorg. Chem.
1984, 23, 231-235.
(37) The same trend is observed in the reaction of 1 and GSH in water. In
the absence of light and dioxygen, one obtains ∼90% GSNO and only
a trace of disulfide.
(34) For example, see Ellis, K. J.; Lappin, A. G.; McAuley, A. J. Chem.
Soc., Dalton Trans. 1975, 1930-1934 and references therein.
(35) Singh, R. J.; Hogg, N.; Joseph, J.; Kalyanaraman, B. J. Biol. Chem.
1996, 271, 18596-18603.
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