Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Article abstract of DOI:10.1021/ic102300d

Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6- diisopropylphenyl)NC(Me)]₂CH). The dinitrosyliron complexes [Fe(NO)₂(Ar-nacnac)] (1) and (Bu₄N)[Fe(NO) ₂(Ar-nacnac)] (2) react with [Fe^III(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [Fe^II(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO) ₂(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported.

Full text of DOI:10.1021/ic102300d

1570 Inorg. Chem. 2011, 50, 1570–1579
DOI: 10.1021/ic102300d
Chemistry of Nitrosyliron Complexes Supported by a β-Diketiminate Ligand
ꢀ
Zachary J. Tonzetich, Florent Heroguel, Loi H. Do, and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
United States
Received November 17, 2010
Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-
nacnac = anion of [(2,6-diisopropylphenyl)NC(Me)]2CH). The dinitrosyliron complexes [Fe(NO)2(Ar-nacnac)] (1)
and (Bu4N)[Fe(NO)2(Ar-nacnac)] (2) react with [FeIII(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate
[FeII(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with
dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron
complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO)2-
(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these
compounds as determined by X-ray crystallography are reported.
Introduction
little work has been devoted to understanding the mechanism
of NO transfer from DNICs.15-18 In a biological context, the
transfer of NO can be tedious to study since the nature of a
DNIC can be difficult to ascertain. Broadly defined, DNICs
are any member of a class of compounds containing the
{Fe(NO)2} unit,19 although the moniker DNIC is typically
reserved for thiolate-bound, mononuclear complexes of the
type [Fe(NO)2(SR)2]-.20,21 Biologically, the -SR anion typi-
cally derives from cysteinate residues in proteins or mobile
units such as glutathione, although examples of biological
DNICs with nonthiolate ligands are also known.22,23
The potential for dinitrosyliron complexes (DNICs) to
serve as storage and transfer units for nitric oxide (NO) in
vivo has stimulated interest in biomimetic chemistry that
reveals the principles underlying such reactivity.1 Much of
this interest stems from the demonstration that DNICs can
participate in a host of physiological processes normally
mediated by NO gas.2-8 DNICs also hold promise as
therapeutic delivery agents of NO.9-14 The release of free
NO by iron nitrosyls has been examined in detail, yet very
Formation of DNICs in vivo occurs through interaction
of NO with chelatable pools of ferrous iron24 or by attack of
NO on protein-based iron centers such as iron-sulfur
clusters.25-31 Mononuclear DNICs that result are denoted
{Fe(NO)2}9 in the Enemark-Feltham notation32 and dis-
play a characteristic gavg = 2.03 EPR signal arising from an
*To whom correspondence should be addressed. E-mail: lippard@
mit.edu.
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pubs.acs.org/IC
Published on Web 01/18/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1571
Scheme 1. Different Scenarios for Transfer of NO between Donor
(LnM) and Acceptor (A)
Chart 1. Chemical Structures of Compounds 1 and 2
1
S = /2 ground state.33 The precise electronic structure of
{Fe(NO)2}9 DNICs remains a point of contention, and
descriptions ranging from {HS-Fe(I)}-{2(NO )}2, to {HS-
3
been isolated and characterized.42 With thiolate (RS-) li-
gands, however, chemical reduction of the {Fe(NO)2}9
DNIC leads to dissociation of disulfide (RSSR) and dimer-
ization to form a valence-delocalized {Fe(NO)2}9-{Fe-
(NO)2}10 species.43,44
Recently, we reported the synthesis of a pair of homo-
logous DNIC redox partners, {Fe(NO)2}9/10, containing a
sterically hindered β-diketiminate ligand (Chart 1, com-
pounds 1 and 2).45 The identical ligand sets in these DNICs
permit an evaluation of the effect that iron redox state plays
on their structures and reactivity. One very interesting obser-
vation from this study is that the isomer shifts (δ) in the
Fe(III)}-{3(NO-)}2, to {Fe(-I)}-{1(NOþ)}2, where HS de-
notes “high-spin”, have been proffered.34-39 A more detailed
knowledge of DNIC electronic structure is important for
understanding both the nature of the metal-nitrosyl bond
and the mechanism of NO transfer. Such information would
also facilitate comparison with the chemistry of nitrosothiols,
which are generally agreed to play an important role in NO-
mediated processes.40,41
Implicit to a discussion of NO-transfer reactivity is the
question of NO redox state. Three different scenarios are
possible for transfer of the nitrosyl group from donor to
acceptor, formally involving NOþ, NO•, and NO- and
resulting in oxidative nitrosylation (nitrosation), nitrosyla-
tion, or reductive nitrosylation of the acceptor moiety, respec-
tively (Scheme 1). With nitrosothiols, reductive nitrosylation
is unlikely because the reaction would require formation of
RSþ. With metal nitrosyls, however, transfer of each redox
form of NO must be considered. An added layer of complex-
ity to the chemistry of DNICs is their ability to participate in
redox chemistry prior to NO transfer. DNICs containing the
{Fe(NO)2}9 unit display an electrochemically reversible one-
electron reduction to the diamagnetic {Fe(NO)2}10 state.
With π-acidic ligands such as carbonyls, phosphines, and
certain nitrogen-based ligands, such reduced species have
€
Mossbauer spectra of the two DNICs are nearly identical,
δ 0.19(2) and 0.23(2) mm/s for 1 and 2, respectively, which is
unexpected for a metal-centered reduction.34 This result
clearly reveals the intricate nature of the electronic structure
of {Fe(NO)2} units and indicates that redox reactions of
DNICs are more complex than anticipated from simple
metal-based processes.
In the present work, we have used these well-defined
DNICs to investigate the mechanism of NO transfer. The
β-diketiminate ligand, Ar-nacnac (Ar = 2,6-diisopropyl-
phenyl),46,47 is critical to this study because it helps stabilize
both the DNIC starting materials and the products of NO
transfer. Moreover, the nitrogen-rich coordination environ-
ment afforded by the Ar-nancac ligand is a reasonable approx-
imation of the histidine residue nitrogen-donor atoms.48,49
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Results
Dinitrosyl Complexes. The synthesis and characteriza-
tion of DNICs 1 and 2 were described previously.45 Each
compound has pseudotetrahedral coordination geome-
try, with the {Fe(NO)2} unit residing in a pocket created
by the sterically demanding diisopropylphenyl substitu-
ents. Compounds 1 and 2 are thermally robust and dis-
play sharp melting points at temperatures above 150 °C.
Compound 1 is stable in air for short periods of time,
whereas compound 2 oxidizes rapidly in the presence of
dioxygen to generate 1 (Figure S1 in the Supporting
Information, SI). Both compounds display an electro-
chemically reversible one-electron couple at -1.34 V vs
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1572 Inorganic Chemistry, Vol. 50, No. 4, 2011
Tonzetich et al.
˚
1.16 A) consistent with the poorer π-back-bonding ability
of Co(I) versus Fe(I) and Fe(0).
Compound 3 was examined by cyclic voltammetry
(CV) to determine whether the {Co(NO)2}9 state could
be accessed electrochemically. The cyclic voltammogram
displayed no reversible oxidation in THF but did exhibit a
quasi-reversible cathodic process at -1.80 V (Figure 2).
Reduction of the {Co(NO)2}10 core is surprising consid-
ering that a putative {Co(NO)2}11 species would have
19 electrons. Chemical reduction of 3 was attempted
with KC8 in benzene. IR, UV-vis, and EPR spectra of
the resulting reduced species are all consistent with a
{Co(NO)2}11 complex (see the SI). A new optical band
appeared at 607 nm upon reduction. Addition of dry air
to this reduced species resulted in disappearance of the
band at 607 nm and regeneration of 3, as judged by
UV-vis spectroscopy (Figure S5). The IR bands of 3 dis-
appear upon reduction, being replaced with lower energy
bands <1600 cm-1. Unfortunately, the {Co(NO)2}11
complex could not be isolated. The Kþ ions appear to
be important in stabilizing the reduced species, because
attempts to replace them by treatment with Bu4NCl and
PPNCl [PPN = cation of μ-nitridobis(triphenylphos-
phine)] failed, resulting in decomposition.
Mononitrosyl Complexes. The success of the β-diketi-
minate in stabilizing compounds containing both {Fe(NO)2}9
and {Fe(NO)2}10 units prompted us to evaluate its po-
tential as a ligand for the {Fe-NO}7 unit. Furthermore,
mononitrosyliron complexes (MNICs) are relevant to
NO-transfer chemistry because they represent logical
byproducts of nitrosyl loss from DNICs. The synthesis
of a compound containing the {Fe-NO}7 unit was accom-
plished by salt metathesis of the lithium or sodium dike-
timinate with the mononitrosyl precursor, (M)[Fe(NO)Br3]
(eq 1, M = Et4N or PPN). Upon addition of the dike-
timinate salt to [Fe(NO)Br3]-, an immediate color change
from yellow-brown to dark green occurred, indicating
formation on a new species. The precise nature of this
initial complex is unknown but most likely corresponds to
a dibromo complex having the formula (M)[Fe(NO)Br2-
(Ar-nacnac)] (40). After several hours at 25 °C, Et4NCl or
PPNCl precipitated and the supernatant changed from
green to dark brown, indicating formation of the desired
compound, [Fe(NO)Br(Ar-nacnac)] (4). Prolonged reac-
tion times were detrimental to both the yield and purity of
4 due to competing disproportionation to form 1 (vide
infra).
Figure 1. Thermal ellipsoid (50%) rendering of one of the two crystal-
lographically independent molecules of 3 in the asymmetric unit. Hydro-
˚
gen atoms omitted for clarity. Selected bond distances (A) and angles
(deg): Co(1)-N(1) = 1.955(4); Co(1)-N(2) = 1.957(4); Co(1)-N(3) =
1.633(3); Co(1)-N(4) = 1.695(4); N(3)-O(1) = 1.165(5); N(4)-O(2) =
1.166(5); Co(1)-N(3)-O(1) = 173.0(4); Co(1)-N(4)-O(2) = 150.1(4);
N(3)-Co(1)-N(4) = 110.5(2).
Fc/Fcþ in tetrahydrofuran (Figure S2). This potential is
higher than that observed for DNICs containing thiolate
ligands, reflecting the neutral charge of 1. Other exam-
ples of neutral DNICs with mixed histidine/thiolate
ligands have comparable potentials for the {Fe(NO)2}9/10
couple.50
As a means of comparison to compounds 1 and 2, the
cobalt analogue, [Co(NO)2(Ar-nacnac)], was prepared. A
variety of {Co(NO)2}10 species have been reported pre-
viously, and these compounds are generally similar to the
corresponding {Fe(NO)2}9 species.51,52 The dicobalt pre-
cursor, [Co(NO)2(μ-Cl)]2,53 was chosen as a starting
material for the synthesis of [Co(NO)2(Ar-nacnac)]. Re-
action of Na(Ar-nacnac) with [Co(NO)2(μ-Cl)]2 in THF
or diethyl ether afforded the desired {Co(NO)2}10 species,
3, in moderate yield after recrystallization from pentane.
The compound is diamagnetic as expected for a {M(NO)2}10
species, giving a sharp, well-resolved 1H NMR spectrum
at 25 °C in benzene-d6. Much like compounds 1 and 2,
compound 3 displays two intense IR peaks arising from
the two nitrosyl ligands. These peaks appear at 1801 and
1706 cm-1 in benzene-d6 and should be compared with
values of 1761 and 1709 cm-1 for 1 and 1627 and 1567
cm-1 for 2. The solid-state structure of 3 is displayed in
Figure 1 (see Table 1 for refinement details). Compound 3
crystallizes with two chemically equivalent but crystal-
lographically independent molecules in the asymmetric
unit. Like 1 and 2, compound 3 displays pseudotetrahe-
dral geometry at the metal center, with the flanking
diisopropylphenyl groups creating a pocket for the
{Co(NO)2}10 unit. The geometric parameters about
the nitrosyl ligands compare well with those in 1 and 2
but with slightly shorter N-O bond lengths (average of
ðMÞ½FeðNOÞBr3ꢀ þ NaðAr-nacnacÞ f
½FeðNOÞBrðAr-nacnacÞꢀ þ MBr þ NaBr
ð1Þ
7
4, fFe-NOg
MNIC 4 is a dark-brown crystalline solid that is soluble
in common organic solvents including alkanes. Its IR
spectrum displays an intense peak at 1777 cm-1 (benzene-
d6) corresponding to the νNO fundamental (see the SI).
Substitution with 15NO leads to a 34 cm-1 red shift,
consistent with a simple harmonic oscillator (calculated
ΔνNO = 32 cm-1). As with other mononuclear {Fe-NO}7
species, 4 is paramagnetic. Its EPR spectrum at 77 K in
(50) Tsai, M.-L.; Liaw, W.-F. Inorg. Chem. 2006, 45, 6583–6585.
(51) Haymore, B.; Feltham, R. D. Inorg. Synth. 1973, 14, 81–89.
(52) Tennyson, A. G.; Dhar, S.; Lippard, S. J. J. Am. Chem. Soc. 2008,
130, 15087–15098.
(53) Sacco, A.; Rossi, M.; Nobile, C. F. Ann. Chim. (Rome) 1967, 57, 499–
507.
3
a 2-MeTHF glass is consistent with an S = /2 ground
state (see the SI). Such a spin state for a four-coordinate

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1573
Table 1. X-ray Data Collection and Refinement Parameters for Compounds 3-6a
3
4
5
6
empirical formula
formula weight
(g/mol)
C29H41CoN4O2
536.59
C29H41BrFeN3O
583.41
C33H49BrFeN2O
625.50
C29H41FeN2Br2
633.31
temperature (K)
crystal system,
space group
100(2)
triclinic, P1
110(2)
monoclinic, P21/n
110(2)
monoclinic, P21/n
100(2)
monoclinic, P21/n
˚
˚
˚
˚
unit cell dimensions
a = 12.590(2) A
a = 12.442(3) A
a = 10.1792(6) A
a = 12.5714(10) A
˚
b = 16.240(3) A
˚
b = 20.007(5) A
˚
b = 14.7884(8) A
˚
b = 19.9181(16) A
˚
c = 16.649(3) A
˚
c = 13.102(3) A
˚
c = 21.7305(13) A
˚
c = 13.2247(11) A
R = 111.071(3)°
β = 99.370(3)°
γ = 109.899(3)°
2824.8(8)
β = 115.805(4)°
β = 93.3570(10)°
β = 116.9340(10)°
3
volume (A )
Z, calculated
density (g/cm3)
˚
2936.2(12)
4, 1.320
3265.6(3)
4, 1.272
2952.2(4)
4, 1.425
4, 1.262
absorption coefficient 0.639
(mm-1
F(000)
1.899
1.711
3.237
)
1144
1220
0.28 ꢁ 0.17 ꢁ 0.13
1320
0.40 ꢁ 0.24 ꢁ 0.20
1300
0.24 ꢁ 0.18 ꢁ 0.14
crystal size (mm)
θ range (deg)
limiting indices
0.13 ꢁ 0.06 ꢁ 0.05
1.39-24.71
1.89-29.57
1.67-25.03
1.86-26.33
-14 e h e 14, -19 e k e
19, -19 e l e 19
42 216/9575
-17 e h e 17, -27 e
k e 27, -18 e l e 18
64 961/8235
-12 e h e 12, -17 e k e 17, -15 e h e 15, -24 e
-25 e l e 25
52 989/5786
[R(int) = 0.0290]
99.9
k e 24, -16 e l e 16
52 094/5987
[R(int) = 0.0302]
99.6
reflections collected/
unique
completeness to
[R(int) = 0.1065]
99.5
[R(int) = 0.0628]
100
θ (%)
absorption correction empirical
empirical
0.7904 and 0.6184
empirical
0.7259 and 0.5476
empirical
0.6600 and 0.5105
max and min
transmission
data/restraints/
parameters
0.9688 and 0.9215
9575/0/653
1.018
R1 = 0.0599, wR2 = 0.1235 R1 = 0.0554, wR2 = 0.1103 R1 = 0.0431, wR2 = 0.0992 R1 = 0.0203, wR2 = 0.0506
8235/62/354
1.294
5786/0/346
1.265
5987/0/317
1.043
GOF on F2
final R indices
[I > 2σ(I)]
R indices (all data)
largest diff peak
R1 = 0.1132, wR2 = 0.1465 R1 = 0.0691, wR2 = 0.1138 R1 = 0.0446, wR2 = 0.0999 R1 = 0.0236, wR2 = 0.0520
0.852 and -0.524
0.466 and -0.523
0.936 and -0.574
0.609 and -0.292
3
and hole (e/A )
˚
P
P
P
a
2
2
Refinement method was full-matrix least squares on F ; wavelength = 0.710 73 A. R1 = ||Fo| - |Fc||/ |Fo|; wR2 = { [w(Fo - Fc2)2]/
˚
P
[w(Fo2)2]}1/2
.
N-Fe-N bite angle of 96.49(10)° for the Ar-nacnac
ligand. The Br and NO groups are disordered in the
crystal lattice but were modeled satisfactorily with oc-
cupancies of 67% and 33% for the major and minor
components, respectively. In addition, the thermal ellip-
soid for the oxygen atom of the NO group is elongated in
a plane perpendicular to the N-O bond vector. This
elongation may be a consequence of slight nonlinearity of
the Fe-N-O bond angle that could not be effectively
modeled due to the proximity of the disordered bromine
atom. The geometric parameters about the iron nitrosyl
unit (see Figure 3 caption) are similar to those reported
for other {Fe-NO}7 species.54,56 The Mossbauer spectrum
€
of 4 (see the SI) displays a single quadrupole doublet with
an isomer shift of 0.33(2) mm/s and a quadrupole splitting
of 0.92(2) mm/s. The isomer shift compares moderately
well with that of 0.26(2) mm/s reported for [Fe(NO)(S-t-
Bu)3]- 54 but is substantially lower than values for five-
and six-coordinate S = 3/2 {Fe-NO}7 species, which typi-
cally fall between 0.62 and 0.77 mm/s.57
Figure 2. Cyclic voltammogram of compound 3 in THF at a glassy
carbon electrode (0.1 M Bu4BArF4 supporting electrolyte; 50 mV/s scan
rate). Fc/Fcþ indicates the ferrocene reference couple.
{Fe-NO}7 species is not uncommon and has been observed
for both [Fe(NO)Br3]- and [Fe(NO)(S-t-Bu)3]-.54,55
The solid-state structure of 4 is displayed in Figure 3
(see Table 1 for refinement details). Much like com-
pounds 1 and 2, compound 4 is pseudotetrahedral with a
(54) Harrop, T. C.; Song, D.; Lippard, S. J. J. Am. Chem. Soc. 2006, 128,
3528–3529.
(55) Connelly, N. G.; Gardner, C. J. Chem. Soc., Dalton Trans. 1976,
1525–1527.
(56) Lu, T.-T.; Chiou, S.-J.; Chen, C.-Y.; Liaw, W.-F. Inorg. Chem. 2006,
45, 8799–8806.

1574 Inorganic Chemistry, Vol. 50, No. 4, 2011
Tonzetich et al.
Figure 5. Thermal ellipsoid (50%) rendering of the solid-state structure
˚
of 5. Hydrogen atoms omitted for clarity. Selected bond distances (A) and
anlges (deg): Fe(1)-N(1) 2.010(3); Fe(1)-N(2) 1.999(3); Fe(1)-O(1)
2.062(3); Fe(1)-Br(1) 2.4114(6); Br(1)-Fe(1)-O(1) = 98.50(7).
Figure 3. Thermal ellipsoid (50%) rendering of the solid-state structure
of 4. The major component of the disordered structure is pictured.
˚
Hydrogen atoms are omitted for clarity. Selected bond distances (A)
and angles (deg): Fe(1)-N(1) = 1.644(5); Fe(1)-N(2) = 1.971(2);
Fe(1)-N(3) = 1.938(2); N(1)-O(1) 1.217(6); Fe(1)-Br(1) = 2.4136(9);
O(1)-N(1)-Fe(1) = 175.5(6); N(1)-Fe(1)-Br(1) = 115.22(15).
from dark brown to yellow-brown was observed. NMR
and IR spectra of the reaction mixtures confirmed the
presence of 2 along with a new paramagnetic Fe species.
Simple stoichiometry requires the formation of a new
Fe(II) species, as indicated in eq 2.
1
2
-
-
½FeðNOÞBrðAr-nacnacÞꢀ
f
½FeðNOÞ2ðAr-nacnacÞꢀ
8
10
fFe-NOg
2, fFeðNOÞ2g
1
-
þ ½FeIIBr2ðAr-nacnacÞꢀ
ð2Þ
2
An iron(II) bromide complex could be prepared inde-
pendently by reaction of the sodium salt of the β-diketi-
minate ligand with anhydrous FeBr2 in THF. This new
compound, 5, was isolated as a yellow THF adduct after
crystallization from THF/pentane (Figure 5). Divalent
iron halide species similar to 5 have been prepared pre-
viously as dimers in the absence of a coordinating
solvent.58 The 1H NMR features of 5 are nearly identical
to those observed for the Fe(II) species formed by reduc-
tion of 4, suggesting that 5, or a related complex such as
the dibromide anion shown in eq 2, is generated conco-
mitant with 2 during disproportionation of the putatative
{Fe-NO}8 species. Compound 5 loses THF in vacuo to
form an orange species, which is most likely the dimeric
[Fe2(μ-Br)2(Ar-nacnac)2] (50). Dimeric 50 has poor solu-
bility in noncoordinating solvents but dissolves readily in
THF to regenerate 5. Compound 5 also serves as a con-
venient synthon for MNIC 4, which forms by simple
addition of NO gas (eq 3). In this manner, the 15N ana-
logue of 4 can be prepared directly from 15NO(g).
Figure 4. Cyclic voltammogram (100 mV/s) of compound 4 indicating
disproportionation of the {Fe-NO}8 species to {Fe(NO)2}10 and “Fe(II)”.
The traces were recorded at a glassy carbon electrode in THF with 0.1 M
Bu4NPF6 as supporting electrolyte.
The cyclic voltammogram of 4 in THF displays a quasi-
reversible one-electron reduction at -1.23 V (vs Fc/Fcþ)
corresponding to the {Fe-NO}7/8 couple. Upon cycling of
this couple at 100 mV/s, a new species formed having a
midpoint potential of -1.34 V (Figure 4). This value
corresponds to the {Fe(NO)2}9/10 couple previously ob-
served for compounds 1 and 2, suggesting that the {Fe-
NO}8 species is unstable with respect to transformation to
a DNIC.
The electrochemical results with 4 were corroborated
chemically using Cp*2Co (Cp* = pentamethylcyclopen-
tadiene) as a reductant. Upon addition of Cp*2Co to
solutions of 4 in benzene-d6, an immediate change in color
½FeIIðTHFÞBrðAr-nacnacÞꢀ þ NOðgÞ f
5
½FeðNOÞBrðAr-nacnacÞꢀ þ THF
ð3Þ
7
4, fFe-NOg
Disproportionation of the {Fe-NO}8 unit accounts for
the observation that samples of recrystallized 4 contain
(57) Hauser, C.; Glaser, T.; Bill, E.; Weyhermuller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2000, 122, 4352–4365.
(58) Eckert, N. A.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Inorg.
Chem. 2004, 43, 3306–3321.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1575
Figure 6. Thermal ellipsoid (50%) rendering of the solid-state structure
˚
of 6. Hydrogen atoms omitted for clarity. Selected bond distances (A) and
angles (deg): Fe(1)-N(1) = 1.9459(13); Fe(1)-N(2) = 1.9730(13); Fe-
(1)-Br(1) = 2.3446(3); Fe(1)-Br(2) = 2.3188(3); Br(1)-Fe(1)-Br(2) =
116.068(11).
Figure 7. UV-vis spectral changes associated with the reaction of
[Fe(NO)2(Ar-nacnac)] with [FeIII(TPP)Cl] in THF with exposure to
ordinary room light.
over several hours in the absence of light. Upon heating
(from 50 to 80 °C) and/or irradiation with room light,
however, a new Fe(TPP) species formed within 1 h. When
the reaction was followed by optical spectroscopy, a clean
conversion to the new iron porphyrin species was de-
tected, with several isosbestic points indicating an A f B
type process (Figure 7). UV-vis and IR spectral studies
confirmed the identity of the new species as the nitrosy-
lated {Fe-NO}7 porphyrin, [FeII(NO)(TPP)].62 IR spec-
troscopy also identified a new NO-containing compound
with a νNO stretch near 1770 cm-1. This value agrees well
with that found for compound 4 (vida supra) and most
likely corresponds to the related MNIC, [Fe(NO)Cl(Ar-
nacnac)]. These results indicate that NO transfer from 1
to [FeIII(TPP)Cl] formally involves transfer of NO- with
reductive nitrosylation of the ferric porphyrin. Upon trans-
fer of NO- from DNIC 1, the corresponding {Fe-NO}7
MNIC is generated, a process that can be described by the
stoichiometry displayed in eq 5.
small amounts of 1, since the {Fe-NO}7 complex may be
reasonably supposed to disproportionate to 1 and an
Fe(III) species in an analogous fashion to its reduced
counterpart, albeit at a much slower rate. To test this
hypothesis, the Fe(III) complex [FeBr2(Ar-nacnac)] (6)
was prepared.59 Isolation and purification of [FeIIIBr2-
(Ar-nacnac)] was complicated by competing reduction of
Fe(III) by the Ar-nacnac ligand.60 Nevertheless, small
quantities of 6 could be isolated, and the solid-state
structure is displayed in Figure 6 (see also Table 1). Upon
mixing solutions of compounds 1 and 6 at ambient
temperature, partial conversion to MNIC 4 was observed
by IR spectroscopy over 24 h. This observation suggests
that all three species, 1, 4, and 6, can interconvert (eq 4),
with the equilibrium lying predominantly toward 4. Upon
reduction, a similar situation exists involving compounds
2 and 5 and the {Fe-NO}8 complex, but in this instance
the equilibrium lies entirely toward 2 and 5 (eq 2).
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIIBr2ðAr-nacnacÞꢀ a
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIIðTPPÞClꢀ f
9
6
9
1, fFeðNOÞ2g
1, fFeðNOÞ2g
½FeðNOÞClðAr-nacnacÞꢀ þ ½FeIIðNOÞðTPPÞꢀ ð5Þ
2 ½FeðNOÞBrðAr-nacnacÞꢀ
ð4Þ
7
7
7
4, fFe-NOg
fFe-NOg
fFe-NOg
NO-Transfer Reactions. NO-transfer experiments were
undertaken with DNICs 1 and 2 using [FeIII(TPP)Cl]
(TPP = meso-tetraphenylporphine) as the NO acceptor.
This porphyrin complex was chosen because of the docu-
mented importance of iron heme compounds to the
biological chemistry of NO and the precedent for transfer
of NO from DNICs to iron porphyrins.16,17,61 Further-
more, porphyrin compounds have intense optical bands
that facilitate reaction monitoring by UV-vis spectros-
copy. Addition of 1 to THF or benzene solutions of
[FeIII(TPP)Cl] resulted in little to no reaction at 25 °C
Initial kinetic studies of the reaction employing excess 1
at several temperatures >50 °C displayed complex beha-
vior that could not be fit to a pseudo-first-order process
(e.g., Figure 8). Although inconclusive, the data suggest
that NO transfer from 1 is unimolecular in DNIC con-
centration and independent of [FeIII(TPP)Cl]. For exam-
ple, halving the DNIC concentration led to an ∼50%
decrease in reaction rate, whereas halving the [FeIII
-
(TPP)Cl] concentration led to no discernible change.
These observations are consistent with a mechanism that
involves rate-limiting dissociation of NO• or NO- from 1
followed by rapid capture of the nitrosyl (or nitroxyl)
fragment by [FeIII(TPP)Cl] and immediate transfer of Cl•
or Cl- to the resulting {Fe-NO}8 or {Fe-NO}7 species,
(59) Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.; Klavins, P.;
Power, P. P. Inorg. Chem. 2002, 41, 3909–3916.
(60) Shimokawa, C.; Itoh, S. Inorg. Chem. 2005, 44, 3010–3012.
(61) Harrop, T. C.; Song, D.; Lippard, S. J. J. Inorg. Biochem. 2007, 101,
1730–1738.
(62) Scheidt, W. R.; Ellison, M. K. Acc. Chem. Res. 1999, 32, 350–359.

1576 Inorganic Chemistry, Vol. 50, No. 4, 2011
Tonzetich et al.
[FeII(NO)(TPP)] (1676 cm-1) and 2 (1627 and 1569 cm-1).
The appearance of 2 differs from the results for 1, which
revealed formation of 4 as a byproduct of NO transfer
(vide supra). In the reaction with an iron(II) porphyrin,
however, 1 must formally transfer 1 equiv of NO• (not
NO-) to yield [FeII(NO)(TPP)]. Transfer of NO• from 1
generates an {Fe-NO}8 species (eq 7) that immediately
disproportionates to give 2 (eq 8). Such a reaction se-
quence explains the observed IR spectral changes in
Figure 9 with the net stoichiometry displayed in eq 9.
-
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIIðTPPÞClꢀ f
10
2, fFeðNOÞ2g
-
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIðTPPÞClꢀ
ð6Þ
9
1, fFe-ðNOÞ2g
Figure 8. Representative kinetic trace (single wavelength, 419 nm) for
the reaction of 95 μM 1 with 4.7 μM [FeIII(TPP)Cl] in toluene at 60 °C.
-
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIðTPPÞClꢀ
f
9
1, fFeðNOÞ2g
-
½FeðNOÞClðAr-nacnacÞꢀ þ ½FeIIðNOÞðTPPÞꢀ ð7Þ
8
7
fFe-NOg
fFe-NOg
1
2
-
-
½FeðNOÞClðAr-nacnacÞꢀ
f
½FeðNOÞ2ðAr-nacnacÞꢀ
8
10
fFe-NOg
2, fFeðNOÞ2g
1
-
þ ½FeIICl2ðAr-nacnacÞꢀ
ð8Þ
2
-
½FeðNOÞ2ðAr-nacnacÞꢀ þ ½FeIIIðTPPÞClꢀ f ½FeIIðNOÞðTPPÞꢀ
7
10
fFe-NOg
2, fFeðNOÞ2g
1
1
2
-
-
þ
½FeðNOÞ2ðAr-nacnacÞꢀ
þ
½FeIICl2ðAr-nacnacÞꢀ
2
10
2, fFe-ðNOÞ2g
ð9Þ
Figure 9. IR spectral changes associated with the reaction of
(Bu4N)[Fe(NO)2(Ar-nacnac)] with [Fe(TPP)Cl] in C6D6 at 25 °C. The
decrease in intensity of the peak at 1676 cm-1 after 300 min is due to
precipitation of [Fe(NO)(TPP)].
NO transfer from 1 to independently prepared [FeII-
(TPP)] did not occur under the reactions conditions em-
ployed above for 2 and [FeIII(TPP)Cl] (eqs 7 and 8). When
Bu4NCl was added to the reaction mixture, however, NO
transfer proceeded as before over 4 h at ambient tem-
perature. This result suggests that chloride may facilitate
transfer of NO• from 1 to [FeII(TPP)]. The role of chloride
is not yet known, and kinetic experiments employing 2
and [Fe(TPP)Cl] could not be followed by UV-vis spectro-
scopy because the reaction proceeded too sluggishly at
the concentrations required for optical spectroscopy,
∼5-10 μM in porphyrin. This fact suggests that, unlike
the reaction of 1 with [FeIII(TPP)Cl], the rate-limiting
step for reaction of 1 with [FeII(TPP)] and Cl- is overall
bimolecular. Whether the reaction depends on the con-
centration of Cl- or [FeII(TPP)] is unknown at this time.
A transition state or intermediate involving a chloride
bridge is an attractive hypothesis, given the possibility for
the DNIC/MNIC to adopt a five-coordinate geometry
(vide supra, compound 40).
respectively. Other mechanistic scenarios involving sub-
sequent fast electron-transfer steps are also consistent
with the data. On the basis of the observed decomposition
route of the {Fe-NO}8 species, however, we favor a
pathway involving direct formation of {Fe-NO}7 from 1.
In order to compare the effect of DNIC redox state on
NO-transfer reactivity, the reaction of 2 with [FeIII
-
(TPP)Cl] was also examined. As with DNIC 1, reaction
of the ferric porphyrin with DNIC 2 cleanly afforded
[FeII(NO)(TPP)]. In contrast to reactions with 1, how-
ever, NO transfer with 2 proceeded in the absence of light
and/or heat over the course of 4 h in benzene-d6. Com-
pound 2 is consumed immediately upon reaction with
[FeIII(TPP)Cl], as judged by IR spectroscopy (Figure 9).
UV-vis spectroscopy confirmed that reduction of
[FeIII(TPP)Cl] takes place prior to NO transfer to gen-
erate 1 (Figure 9: 1761 and 1708 cm-1) and the ferrous
porphyrin [FeII(TPP)] (eq 6). This result is not surprising
considering the redox potential of 2 (E1/2 = -1.34 V).
After initial electron transfer, peaks corresponding to 1
diminish, with concomitant growth of features due to
Discussion
The transfer of NO ligands from DNICs to various
acceptors has been demonstrated previously with several
synthetic DNICs.15,16,61 In many of these systems, the form

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1577
of NO (NO•, NOþ, or NO-) formally being transferred from
the DNIC to the acceptor is dictated by the stability of the
resulting nitrosylated or nitrosated product. Such depen-
dence was noted previously in a study of NO transfer from
a cobalt nitrosyl to a variety of different metal complexes.63
The reactivity suggests that NO-transfer reactions are dic-
tated by the thermodynamic requirements of the acceptor
and that different forms of NO may be released or transferred
from the DNIC depending on the nature of the acceptor.
Biologically, this characteristic of DNICs may be important
because the NO acceptor may require NO• (heme sites), NO-
(oxidized heme sites), or NOþ (thiols, amines, and alcohols).
In the present work, we demonstrate a net formal transfer
of both NO-, {Fe(NO)2}9 to [FeIII(TPP)Cl], and NO•, {Fe-
(NO)2}9 to [FeII(TPP)]. In each case, the mechanism is
distinct and the qualitative reaction rates under similar
conditions are quite different. Formal transfer of NO- from
1 to [FeIII(TPP)Cl] operates by a unimolecular process with
rate-limiting loss of NO• or NO-. This mechanism is con-
sistent with the observation that light irradiation increases
the NO-transfer rate because excitation of electrons to M-N
antibonding orbitals should accelerate loss of a nitrosyl or
nitroxyl ligand. In contrast, transfer of NO• from 1 to
[FeII(TPP)] appears to be a bimolecular process. Further-
more, transfer of NO• does not occur in the absence of Cl-.
This fact suggests that coordination of a Lewis base to the
DNIC may be required for the transfer of NO• or perhaps
that ligands capable of bridging two metal centers are
important in mediating transfer of NO• between a DNIC
and its metal targets.
In addition to NO-transfer reactions between DNICs and
porphyrins, the related chemistry of MNICs was examined in
the present work and deserves comment. Both four-coordi-
nate {Fe-NO}7 and {Fe-NO}8 compounds disproportionate,
forming DNICs and Fe(III) ({Fe-NO}7) or Fe(II) ({Fe-
NO}8) compounds. In the case of the former, {Fe-NO}7
compound, this reaction appears to be reversible judging by
the fact that DNIC 1 and [FeIIIBr2(Ar-nacnac)] (6) compro-
portionate in solution at 25 °C. Similar reactivity also occurs
with four-coordinate MNICs containing thiolate ligands.
These species can disproportionate, leading to DNICs and
iron(III) thiolates.19 Such reactivity contrasts with that of
five- and six-coordinate MNICs, which are stable with
respect to disproportionation.
that sNOR activity could occur through the intermediacy
of an {Fe-NO}8 species.65,66
Conclusions
The NO-transfer chemistry of two DNICs supported by a
sterically demanding β-diketiminate ligand has been explored
using iron porphyrins as NO acceptors. The {Fe(NO)2}9
DNIC, 1, is capable of reductively nitrosylating [FeIII
-
(TPP)Cl)] at elevated temperatures or in the presence of light
to afford [FeII(NO)(TPP)] and the corresponding {Fe-NO}7
complex, 4. Preliminary kinetic measurements of this reac-
tion point to a rate-limiting step involving dissociation of
NO• or NO- from 1. The analogous reaction of the {Fe-
(NO)2}10 DNIC, 2, also affords [FeII(NO)(TPP)], but in this
instance, rapid electron transfer from 2 to [FeIII(TPP)Cl]
precedes NO transfer. The resulting {Fe(NO)2}9 DNIC then
nitrosylates the iron(II) porphyrin, generating a {Fe-NO}8
species that immediately disproportionates to 2 and an NO-
free Fe(II) complex, 5. This chemistry was verified by the
synthesis of MNIC 4 and examination of its redox chemistry.
In addition to iron, a four-coordinate {Co(NO)2}10 species, 3,
was isolated and fully characterized. The structural and
spectroscopic properties of 3 are similar to those of the iron
analogues, 1 and 2. Compound 3 undergoes a quasi-reversible
one-electron reduction to the putative {Co(NO)2}11 species.
Experimental Section
General Comments. Manipulations of air- and moisture-
sensitive materials were performed under an atmosphere of
nitrogen gas using standard Schlenk techniques or in an
MBraun glovebox under an atmosphere of purified nitrogen.
Tetrahydrofuran (THF), diethyl ether, pentane, benzene, and
toluene were purified by passage through activated alumina67
˚
and then stored over 4-A molecular sieves prior to use. Benzene-
d6 and THF-d8 were dried over sodium ketyl and vacuum-
distilled prior to use.
Materials. Compounds 1 and 2,45 (M)[Fe(NO)Br3],55 [FeII-
(TPP)],68 [Co(μ-Cl)(NO)2]2,53 KC8, anhydrous FeIIBr2, and
Cp*2Co were synthesized according to published procedures.
Na(Ar-nacnac) 2THF was prepared by deprotonation of
3
H(Ar-nacnac) with NaN(SiMe3)2 in THF followed by crystal-
lization from pentane. Li(Ar-nacnac) was prepared by deprotona-
tion of H(Ar-nacnac) with nBuLi followed by crystallization
from pentane at -30 °C. [FeIII(TPP)Cl] and anhydrous FeIIIBr3
were purchased from Strem Chemicals and used as received.
Nitric oxide (NO; Matheson, 99%) was purified by passage
through an Ascarite column (NaOH fused on silica gel) and a
6-ft coil filled with silica gel cooled to -78 °C.69 Purified NO gas
was stored and transferred under an inert atmosphere using
standard gas storage bulbs and gastight syringes, respectively.
For NO-transfer reactions, care was taken to prevent light
exposure by covering reaction glassware in aluminum foil or
by performing experiments in a darkened glovebox.
Reduction of 4 to give the unstable {Fe-NO}8 species is
intriguing, given its possible relevance to the mechanism of
NO-scavenging reductases (sNORs).64 These enzymes con-
tain adjacent carboxylate-bridged {Fe-NO}7 centers in their
NO-bound state. In one proposed mechanism, reduction of
both iron centers leads to formation of the corresponding
{Fe-NO}8 species. These reduced nitrosyls then revert back
to diiron(II) species in the presence of protons with conco-
mitant release of N2O and H2O. In the case of compound 4,
reduction to {Fe-NO}8 leads to formation of a DNIC and not
to chemistry at the NO ligands. The fact that Fe(II) is gen-
erated in the process, however, lends credence to the proposal
Physical Measurements. 1H NMR spectra were recorded on a
Varian INOVA spectrometer operating at 500 MHz. FT-IR
spectra were recorded with a ThermoNicolet Avatar 360 spec-
trophotometer running the OMNIC software; solid samples
were pressed into KBr disks, and solution samples were pre-
pared in THF or benzene-d6 in an airtight Graseby-Specac
(63) Ungermann, C. B.; Caulton, K. G. J. Am. Chem. Soc. 1976, 98, 3862–
3868.
(64) Kurtz, D. M., Jr. Dalton Trans. 2007, 4115–4121.
(65) Hayashi, T.; Caranto, J. D.; Wampler, D. A.; Kurtz, D. M., Jr.;
(67) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(68) Hu, C.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2007,
46, 619–621.
€
Moenne-Loccoz, P. Biochemistry 2010, 49, 7040–7049.
ꢀ
(66) Silaghi-Dumitrescu, R.; Ng, K. Y.; Viswanathan, R.; Kurtz, D. M.,
(69) Lim, M. D.; Lorkovic, I. M.; Ford, P. C. Methods in Enzymology;
Jr. Biochemistry 2005, 44, 3572–3579.
Academic Press: New York, 2005; Vol. 396, pp 3-17.

1578 Inorganic Chemistry, Vol. 50, No. 4, 2011
Tonzetich et al.
solution cell with CaF2 windows and 0.1 mm spacers. UV-vis
spectra were recorded on a Cary-50 spectrophotometer in air-
filtration through a plug of Celite, the pentane solution was
concentrated in vacuo to 3 mL and allowed to stand at -30 °C
for 24 h. Compound 5 precipitated as 0.527 g (0.75%) of brown
cubes in two crops. 1H NMR (benzene-d6): δ 7.07 (m, 6 ArH),
5.15 (s, 1 CH), 3.16 (sep, 4 CHMe2), 1.73 (s, 6 Me), 1.23 (d, 12
CHMe2), 1.15 (d, 12 CHMe2). IR (benzene-d6, cm-1): 3057,
2963, 2928, 2869, 1801 (νNO), 1706 (νNO), 1554, 1522, 1458,
57
tight Teflon-capped quartz cells. Samples for Fe Mossbauer
€
studies were prepared by grinding solids with Apiezon-N grease.
Samples were placed in a 90 K cryostat during measurement.
All isomer shift (δ) and quadrupole splitting (ΔEQ) values are
reported with respect to 57Fe-enriched metallic iron foil that
was used for velocity calibration. The displayed spectrum was
folded to enhance the signal-to-noise ratio. Fits of the data were
calculated by the WMOSS version 2.5 plot and fit program.70
X-band EPR spectra were recorded on a Bruker EMX EPR
spectrometer. Temperature control was maintained with a
quartz finger dewar (77 K). Spectra were recorded in 4-mm-
o.d. quartz EPR tubes capped with a tight-fitting rubber sep-
tum. Electrochemical measurements were performed at 25 °C on
a VersaSTAT3 Princeton Applied Research potentiostat run-
ning the V3-Studio electrochemical analysis software. A three-
electrode setup was employed comprising a glassy carbon work-
ing electrode, a platinum wire auxiliary electrode, and a 0.1 M
Ag/Agþ reference electrode. Triply recrystallized Bu4NPF6 or
1438, 1399, 1316, 1177. CV (THF): E1/2
= -1.80 V
{Co(NO)2}10/11; Eox = þ0.53 V. UV-vis (THF) [λmax, nm
(ε, M-1 cm-1)]: 322 (6700), 360 (5700), 415 (sh), 570 (sh). Anal.
Calcd for C29H41CoN4O2: C, 64.91; H, 7.70; N, 10.44. Found:
C, 64.09; H, 7.58; N, 10.17.
[Fe(NO)Br(Ar-nacnac)] (4). Procedure A: To 0.114 g
(0.250 mmol) of (Et4N)[Fe(NO)Br3] dissolved in 10 mL of
THF was added a solution of 0.106 g (0.250 mmol) of Li(Ar-
nacnac) in 5 mL of THF. The solution immediately changed
from yellow-brown to dark forest green upon addition of the
diketiminate salt. The reaction was allowed to stir for an
additional 2 h at ambient temperature, during which time the
forest-green solution changed to a dark-brown mixture. All
volatiles were removed in vacuo, and the residue was extracted
into 15 mL of pentane. The extract was filtered through a plug of
Celite to remove Et4NBr and LiCl. The resulting pentane
solution was concentrated in vacuo to 5 mL and allowed to
stand at -30 °C for 24 h. During this time, compound 3
precipitated in two crops as 0.0886 g (61%) of brown cubes.
See below for spectral data.
Bu4NBArF (ArF = 3,5-bis(trifluoromethyl)phenyl) was used
4
as the supporting electrolyte. All electrochemical data were
referenced internally to the ferrocene/ferrocenium couple at
0.00 V. Elemental analyses were performed by Midwest Micro-
lab, LLC, Indianapolis, IN.
X-ray Data Collection and Structure Solution Refinement.
Crystals of 3-6 suitable for X-ray diffraction were mounted
in Paratone N oil using 30 μm aperture MiTeGen MicroMounts
(Ithaca, NY) and frozen under a nitrogen cold stream main-
tained by a KRYO-FLEX low-temperature apparatus. Data
were collected on a Bruker SMART APEX CCD X-ray dif-
Procedure B: To 0.0530 g (0.099 mmol) of compound 4 in
2 mL of THF was added 2.5 mL (∼0.10 mmol) of NO gas via
syringe. The solution immediately changed from bright yellow
to dark brown. The solution was allowed to stir at ambient
temperature for 20 min. All volatiles were removed in vacuo,
and the residue was extracted into 5 mL of pentane. After filtra-
tion through glass filter paper, the pentane solution was con-
centrated in vacuo to 2 mL and allowed to stand at -30 °C for
24 h. During this time, compound 3 precipitated as 0.0247 g
˚
fractometer with Mo KR radiation (λ = 0.710 73 A) controlled
by the APEX2 software package (version 2010.1-2). Data
reduction was performed with SAINT.71 Empirical absorption
corrections were applied with SADABS,72 and the structure was
checked for higher symmetry by the PLATON software.73 The
structures were solved by direct methods with refinement by full-
matrix least squares based on F2 using SHELXTL-97.74 All non-
hydrogen atoms were located and their positions refined aniso-
tropically. Hydrogen atoms were assigned to idealized positions
and given thermal parameters equal to either 1.5 (methyl hydro-
gen atoms) or 1.2 (non-methyl hydrogen atoms) times the
thermal parameters of the atoms to which they were attached.
Crystals of 3-6 were grown by slow cooling of a saturated
solution of each complex in pentane. No incorporation of
solvent occurred in any of the crystal lattices. The asymmetric
unit of complex 3 contains two independent molecules having
the same molecular structure. Complex 4 was modeled with a
positional disorder between the NO and Br- ligands. The NO
and Br- ligands were observed to be positionally disordered in a
difference Fourier map and the occupancy factors of the two
disordered components refined to 67% and 33%. Further
information is provided in the Supporting Information. Table 1
reports crystallographic data and additional refinement details.
[Co(NO)2(Ar-nacnac)] (3). To 0.200 g (0.650 mmol) of [Co-
(μ-Cl)(NO)2]2 in 5 mL Et2O was added a solution of 0.555 g
1
(50%) of brown cubes. Mp: 190-195 °C. H NMR (benzene-
d6): δ 33.7 (2 H), 27.1 (2 H), 13.5 (v br, 1H), 4.5 (2 H), 3.7 (12 H),
2.8 (12 H), 1.4 (2 H), -21.2 (2 H), -33.2 (6 H). IR (benzene-d6,
cm-1): 3060, 2965, 2927, 2869, 1777 (νNO), 1519, 1463, 1437,
1371, 1317, 1174, 1106, 1022; 1743 (ν15NO). EPR (X-band,
2-MeTHF): 77 K g1 = 4.62, g2 = 3.41, g3 = 1.96. CV (THF):
E1/2=-1.23 V {Fe-NO}7/8. UV-vis (THF) [λmax, nm (ε, M-1
cm-1)]: 313 (19 000), 417 (sh), 552 (1800). 57Fe Mossbauer: δ =
€
0.33(2) mm/s, ΔEQ = 0.92(2) mm/s, Γ = 0.44(2) mm/s. Anal.
Calcd for C29H41BrFeN3O: C, 59.70; H, 7.08; N, 7.20. Found:
C, 60.19; H, 6.98; N, 7.41.
[Fe(THF)Br(Ar-nacnac)] (5). To 0.167 g (0.774 mmol) of
anhydrous FeIIBr2 in 20 mL of THF was added 0.454 g (0.776
mmol) of Na(Ar-nacnac) 2THF. The mixture was allowed to
3
stir at ambient temperature for 2 h, during which time the color
changed from orange to brown-yellow. The mixture was filtered
through a plug of Celite to remove NaBr. All volatiles were
removed in vacuo. The resulting residue was washed with
pentane, causing formation of a yellow precipitate. The solid
was isolated by filtration and washed with pentane to give 0.377
g (85%) of 5 as a yellow powder. Storing the filtrate at -30 °C
for 24 h afforded an additional 0.030 g of yellow needles
that were used for X-ray diffraction. 1H NMR (THF-d8):
δ 19.0, 7.0, 5.1, 1.1, 0.8, -8.8, -38.9, -68.9, -80.2. UV-vis
(THF) [λmax, nm (ε, M-1 cm-1)]: 331 (4200), 428 (sh). UV-vis
(toluene) [λmax, nm (ε, M-1 cm-1)]: 325 (11 500), 389 (3500), 520
(670). CV (THF): E1/2 = -0.61 V. Anal. Calcd for C33H49BrFe-
N2O: C, 63.37; H, 7.90; N, 4.48. Found: C, 63.31; H, 7.68; N, 4.51.
[FeIIIBr2(Ar-nacnac)] (6). This compound could only be pre-
pared in small quantities by reaction of equimolar amounts of
(1.30 mmol) of Na(Ar-nacnac) 2THF in 10 mL of Et2O. The
3
resulting brown-yellow solution was allowed to stir at ambient
temperature for 2 h. All volatiles were removed in vacuo, and the
resulting residue was extracted into 10 mL of pentane. After
€
(70) Kent, T. A. WMOSS, version 2.5; Mossbauer Spectral Analysis
Software, WEB Research Co.; Minneapolis, MN, 1998.
(71) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, A64, 112–122.
(72) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
€
€
University of Gottingen: Gottingen, Germany, 2001.
(73) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Na(Ar-nacnac) 2THF and anhydrous FeBr3 in toluene. After
stirring for 2 h at ambient temperature, all volatiles were removed
in vacuo, affording a dark-forest-green residue. Extraction of
3
Utrecht University: Utrecht, The Netherlands, 2000.
(74) Sheldrick, G. M. SHELXTL97: Program for Refinement of Crystal
€
€
Structures; University of Gottingen: Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1579
this residue into pentane generated a dark-green solution and a
large quantity of brown insoluble material, Fe(II) byproducts
formed by reduction of Fe(III) by the diketiminate salt. The
pentane solution was filtered through a plug of Celite, concen-
trated in vacuo, and allowed to stand at -35 °C for 24 h. After
this time, small green cubes of 6 appeared (typically <10 mg).
These cubes were suitable for X-ray diffraction, but repeated
elemental analysis and solution magnetic susceptibility mea-
surements indicated that 6 contained diamagnetic impurities,
most likely the coupled Ar-nacnac ligand. EPR (X-band,
2-MeTHF): 77 K g = 9.02, 5.24, 3.55, 2.60, 2.04, 1.30 (see the
SI). CV (THF): E1/2 = -0.61 V.
solution of the desired iron porphyrin compound in a vial or
quartz cuvette. Reactions were allowed to proceed in the absence
of light as much as possible. For UV-vis reactions conducted in
the presence of light irradiation, the cuvette was exposed to
fluorescent room light for 3 min intervals between spectra.
Acknowledgment. This work was supported by Grant CHE-
0907905 from the National Science Foundation. Z.J.T.
acknowledges NIGMS for a postdoctoral fellowship, F32
GM082031-03.
Supporting Information Available: Additional figures, com-
plete spectra, crystallographic data, and fully labeled thermal
ellipsoid diagrams for compounds 3-6, as well as the corre-
sponding CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
General Procedure for NO-Transfer Reactions. Solutions of 1
or 2 in THF (UV-vis) or benzene-d6 (IR) were prepared in the
glovebox at appropriate concentrations (∼100 μ M for UV and
∼20 mM for IR). These solutions were then combined with a