Ambidentate thiocyanate and cyanate ligands in dinitrosyl iron complexes

Article abstract of DOI:10.1021/ic3025149

To explore the effect of delocalization in the Fe(NO)₂ unit on possible linkage isomerism of ambidentate ECN^- ligands, E = S and O, anionic DNICs, dinitrosyl iron complexes, (SCN)₂Fe(NO) ₂^- (1) and (O

Full text of DOI:10.1021/ic3025149

Article
pubs.acs.org/IC
Ambidentate Thiocyanate and Cyanate Ligands in Dinitrosyl Iron
Complexes
Chung-Hung Hsieh,† Scott M. Brothers,† Joseph H. Reibenspies,† Michael B. Hall,*,†
Codrina V. Popescu,*,‡ and Marcetta Y. Darensbourg*,†
†Department of Chemistry, Texas A & M University, College Station, Texas 77843, United States
‡Department of Chemistry, Ursinus College, Collegeville, Pennsylvania 19426, United States
S
* Supporting Information
ABSTRACT: To explore the effect of delocalization in the Fe(NO)2 unit on possible
linkage isomerism of ambidentate ECN− ligands, E = S and O, anionic DNICs, dinitrosyl
iron complexes, (SCN)2Fe(NO)2− (1) and (OCN)2Fe(NO)2− (2) were synthesized by the
reaction of in situ-generated [Fe(CO)2(NO)2]+ and PPN+ECN−. Other {Fe(NO)2}9
−
−
(Enemark−Feltham notation) complexes, (N3)2Fe(NO)2 and (PhS)2Fe(NO)2 , were
prepared for comparison. The X-ray diffraction analysis of 1 and 2 yielded the typical
tetrahedral structures of DNICs with two slightly bent Fe−N−O oriented toward each
other, and linear FeNCE units. The ν(NO) IR values shift to lower values for 1 > 2 >
−
(N3)2Fe(NO)2− > (PhS)2Fe(NO)2 , reflecting the increasing donor ability of the ancillary
ligands and consistent with the redox potentials of the complexes, and the small trends in
Mossbauer isomer shifts. Computational studies corroborate that the {Fe(NO) }9 motif
̈
2
prefers N-bound rather than E-bound isomers. The calculated energy differences between
the linkage isomers of 1 (Fe-NCS preferred over Fe-SCN by about 6 kcal/mol) are smaller
than those of 2 (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol), a difference that is justified by the frontier molecular
orbitals of the ligands themselves.
are challenged by the delocalization in the Fe-NO bonds.14,15
INTRODUCTION
■
Two redox levels are prevalent for DNICs, represented in the
Enemark−Feltham (E-F) notation16 as {Fe(NO)2}9 and
{Fe(NO)2}10 for oxidized and reduced levels respectively,
each of which have extensive examples in the form of
[X2Fe(NO)2]−, [L2Fe(NO)2]0, and so forth.17,18 Dimeric and
higher aggregates also exist when X− is a competent bridging
ligand such as thiolate sulfur, sulfide, and imidazolate.19−21 The
SCN− and OCN− ligands can likewise bind in both
monodentate and bridging bonding modes;1,2 however, to the
best of our knowledge, a systematic study of DNICs containing
SCN− and OCN− ligands, including full characterization and X-
ray analysis, is first reported herein.22
Ambidentate ligands such as thiocyanate, SCN−, have played
important roles in the development of bonding models relating
to factors that control linkage isomerism in transition metal
complexes.1 Intensive analyses over decades led to the prospect
that ambidentate ligands might serve as a chemical probe for
the electronic environment about the metal center, arising both
from the oxidation state of the metal as well as the effect of
ancillary ligands on the composition of the highest occupied
molecular orbital (HOMO) or lowest unoccupied molecular
orbital (LUMO).2 Some early reviews related metal−ligand
connectivities to hard (Mhard-NCS) and soft (Msoft-SCN)
bonding patterns, however, and with prescience, these authors
noted that intricacies of a steric nature and the influence of
supporting ligands might account for deviations from the
simple hard/soft interaction expectations.3,4 In fact, a search of
the Cambridge Crystal Structure Data Base from 1990 to
current indicates the M-NCS form greatly predominates in
almost 500 deposited structures.1−8 The M-SCN isomer is
typical of extreme thiophilic metals such as Hg or Cd, but the
M-NCS form is prevalent even in acceptably soft, zerovalent
metal carbonyls.8−10 Interest in the thiocyanates remains high
for both fundamental properties and isomerization mecha-
nisms.1−10 A special application as a photosensitizer, cis-
RuL2(NCS)2 (L = 2,2′-bipyridyl-4,4′-dicarboxylic acid), in solar
cells has seen continued development.11−13
The E-F notation has circumvented impasses in oxidation
state assignments, permitted convenient electron-bookkeeping
for redox processes of DNICs, and allowed for a cohesive
terminology in the extensive literature regarding DNICs.
Recent computations have sought a better understanding of
the electronic structure of DNICs isolated in oxidized and
reduced redox levels and containing identical supporting Ar-
nacnac ligands (Ar-nacnac = anion of [(2,6-diisopropylphenyl)-
NC(Me)]2CH).18,23 While the Fe-NO bond is highly covalent
in both, Ye and Neese conclude that oxidized DNIC,
{Fe(NO)2}9, is best explained by resonance structures that
III
II
include high spin ferric (SFe = 5/2) and ferrous (SFe = 2)
The case of dinitrosyl iron complexes (DNICs) presented
herein is particularly intriguing as oxidation state assignments
Received: November 19, 2012
Published: February 1, 2013
© 2013 American Chemical Society
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antiferromagnetically coupled to triplet NO− ligands. Reduced
X-ray Crystal Structure Analyses. A Bausch and Lomb 10×
microscope was used to identify suitable crystals of the same habit.
Each crystal was coated in paratone, affixed to a Nylon loop and placed
under streaming nitrogen (110 K) in a SMART Apex CCD
diffractometer (See details in .cif files). The space groups were
determined on the basis of systematic absences and intensity statistics.
The structures were solved by direct methods and refined by full-
matrix least-squares on F2. Anisotropic displacement parameters were
determined for all nonhydrogen atoms. Hydrogen atoms were placed
at idealized positions and refined with fixed isotropic displacement
parameters. The following is a list of programs used: data collection
and cell refinement, APEX2;32 data reductions, SAINTPLUS Version
6.63;33 absorption correction, SADABAS;34 structural solutions,
SHELXS-97;35 structural refinement, SHELXL-97;36 graphics and
publication materials, Mercury Version 2.3.37
{Fe(NO)2}10 was more clearly ferrous, with SFeII = 2, again spin
14
3
coupled to the NO− ligands. The four electrons involved in
the three-center FeNO π bonding are generally polarized
toward the Fe, so an alternative view would have these four
electrons on the Fe but strongly delocalized into the π* orbitals
of NO+. Still the covalent character of the Fe−N bonds in both
redox levels begs the question of whether the iron is to be
considered a soft or hard center in terms of ligand bonding in
DNICs.
Mononitrosyl iron complexes, especially when porphyrin-
based, are known to be of great significance to human
physiology. However a similarly important role for DNICs,
known to be formed from the degradation of iron−sulfur
clusters by excess NO, is controversial.24−26 Extensive synthetic
and reactivity studies have been reported that are likely of
import to in vivo mobilization by displacement from protein-
̈
Mossbauer Measurements. Low-field (0.04 T), variable temper-
ature (4.5−300 K) Mossbauer spectra were recorded on a closed-cycle
̈
refrigerator spectrometer, model CCR4K, equipped with a 0.04 T
permanent magnet, maintaining temperatures between 4.5−300 K.
−
bound (cysteinyl-S)2Fe(NO)2 (high-molecular weight
Mossbauer spectra were analyzed using the software WMOSS
̈
(Thomas Kent, SeeCo.us, Edina, Minnesota). The samples were
polycrystalline powders, suspended in nujol, placed in Delrin 1.00 mL
cups and frozen in liquid nitrogen.
DNICs) by free cysteine or glutathione (producing low
molecular weight DNICs).27 A study of DNICs derived from
the intracellular iron pool (CIP) in the presence of glutathione
and exogeneous NO conclude such DNICs to have the largest
concentration and longest lifetime of all NO-derived cellular
adducts.28,29
Synthesis of Complex [PPN][(SCN)2Fe(NO)2] (1). The [PPN]-
[Fe(CO)3(NO)] (0.28 g, 0.40 mmol) [NO]BF4 (0.096g, 0.80 mmol)
and KSCN (0.078 g, 0.80 mmol) were loaded in a septum-sealed 50
mL Schlenk flask, and 20 mL of THF solvent was added by cannula.
The reaction mixture was stirred for overnight at room temperature;
its IR spectrum (THF solution) found ν(NO) bands at 1786 (s), 1718
(vs) cm−1. The solution was filtered through Celite to remove
insoluble solid. Addition of pentane to the THF solution yielded a
brown precipitate, which was washed successively by 1:1 pentane-
diethyl ether (3 × 20 mL) to remove impurities (Yield: 0.24 g, 78%).
Recrystallization in THF/pentane at −35 °C afforded crystals of
complex 1 suitable for X-ray crystallographic study. IR (THF), cm−1:
ν(CN) 2076 (sh), 2056 (vs) (SCN); ν(NO) 1786 (s), 1718 (vs). vis−
UV, THF solution, nm: 705 (vw), 516 (sh), 399 (m), 292 (sh on
intense CT absorption). Elem. anal., found (calc’d for
C38H30FeN5O2P2S2) %: C, 59.59 (59.23); H, 3.76 (3.92), N, 8.90
(9.09).
Thiocyanate iron nitrosyl species have been proposed in
studies that mimicked conditions resulting from human
consumption of iron supplements. The thiocyanate was
encountered as a component of saliva while NO was derived
from Fe-promoted NO2− degradation in the stomach, resulting
in ulceration.30 Despite the paucity of data confirming the
nature of such a formulation, and in view of the fundamental
inorganic chemistry associated with small molecules as ligands
to iron, we report below the synthesis and characterization of
bis-triphenylphosphineiminium (PPN+) salts of (SCN)2Fe-
−
−
(NO)2 (1) and (OCN)2Fe(NO)2 (2), as well as a unique
−
octahedral ferric thiocyanate, trans-(SCN)4Fe(THF)2 (3).
Characterization by spectroscopies, density functional theory
(DFT) analysis, and X-ray diffraction define the nature of the
Fe-NCX bonds in the three complexes and compare to related
Synthesis of Complex [PPN][(OCN)2Fe(NO)2] (2). Following
similar procedures as used with complex 1, [PPN][Fe(CO)3(NO)]
(0.283 g, 0.40 mmol) [NO]BF4 (0.096g, 0.80 mmol) and NaOCN
(0.052 g, 0.80 mmol) were loaded in a septum-sealed 50 mL Schlenk
flask, and 20 mL of THF solvent was added by cannula and stirred
overnight. The solution was filtered through Celite to remove
insoluble solid. Complex 2 was isolated as a greenish brown solid
after addition of pentane to the THF solution of 2. (Yield: 0.22g, 75%)
Recrystallization in THF/pentane at −35 °C afforded crystals of
complex 2 suitable for X-ray crystallographic study. IR (THF), cm−1:
ν(CN) 2223 (s), 2197 (vs) (OCN); ν(NO) 1766 (s), 1698 (vs). vis−
UV, THF solution, nm: 657 (vw), 508 (m), 375 (sh on intense CT
absorption). Elem. anal., found (calc’d for C38H30FeN5O2P2S2) %: C,
62.11 (61.80); H, 4.04 (4.09), N, 9.80 (9.48).
Synthesis of Complex [PPN][(THF)2Fe(NCS)4] (3). Method A.
Complex 1 (0.077g, 0.10 mmol) was dissolved in 20 mL of THF
solution in a 100 mL Schlenk flask. Dry air was bubbled into the
solution resulting in a color change from brown to red-purple within
20 min. Infrared spectroscopy confirmed reaction completion. The
solution was filtered through Celite to remove insoluble solid. After
the reaction solution was concentrated to 5 mL, 30 mL of pentane was
added to precipitate the product (Yield: 0.030 g, 31%). Layering of
THF solution of 3 with a 2:1 mixture of pentane and diethyl ether
afforded dark red-purple single crystals after three weeks at −35 °C.
Method B. To a 200 mL Schlenk flask loaded with FeCl3·6H2O
(0.27 g, 1.00 mmol) and PPN+SCN− salt (2.33 g, 4.00 mmol), was
added 100 mL of THF. The solution was stirred overnight under N2
and filtered through Celite to remove insoluble solid. Addition of 1:1
pentane-diethyl ether to the THF solution of 3 yielded a dark red-
purple precipitate (Yield: 0.86 g, 89%). Elem. anal., found (calc’d for
−
−
complexes (N3)2Fe(NO)2 and (PhS)2Fe(NO)2 .
EXPERIMENTAL SECTION
■
General Materials and Techniques. All reactions and operations
were carried out on a double-manifold Schlenk vacuum line under N2
or Ar atmosphere. Tetrahydrofuran (THF), pentane, and diethyl ether
were freshly purified by an MBraun Manual Solvent Purification
System packed with Alcoa F200 activated alumina desiccant. The
purified THF, CH2Cl2, pentane, and diethyl ether were stored with
molecular sieves under N2 before experiments. The known complexes
[PPN][Fe(CO)3(NO)],31 [PPN][(N3)2Fe(NO)2,]19 and [PPN]-
[(PhS)2Fe(NO)2] were synthesized by published procedures.19 The
following materials were of reagent grade and were used as purchased
from Sigma-Aldrich: sodium thiophenolate, potassium thiocyanate
sodium cyanate, and nitrosyl tetrafluoroborate.
Physical Measurements. Infared spectrometry was performed on
a Bruker Tensor 27 FTIR spectrometer using 0.1 mm CaF2 sealed
cells. X-band electron paramagnetic resonance (EPR) measurements
were obtained on a Bruker EMX spectrometer equipped with
ER4102ST cavity and the Oxford Instruments ESR900 cryostat. The
microwave frequency was measured with a Hewlett-Packard 5352B
electronic counter. Voltammograms were obtained using a standard
three-electrode cell under an argon atmosphere at room temperature.
Samples in THF were run at a concentration of 2 mM with [n-
Bu4N]PF6 as the supporting electrolyte (100 mM). Elemental analyses
were performed by Atlantic Microlab, inc., Norcross, Georgia, U.S.A.
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C48H46FeN5O2P2S4) %: C, 59.21 (59.38); H, 4.44 (4.78), N, 7.43
(7.21).
pared and solution EPR, IR, CV, and solid state Mossbauer
̈
spectroscopic studies were carried out. For all, isotropic EPR
signals (22 °C in THF solvent), at g ∼ 2.03, the typical
signature for paramagnetic {Fe(NO)2}9 DNICs, were observed
with no detectable N-superhyperfine coupling (see Supporting
Information, Figure S7). Lippard and co-workers observed that
the room temperature EPR spectrum of oxidized N-bound
DNIC, (Ar-nacnac)Fe(NO)2 displays an isotropic signal
centered at 2.06. The reason(s) for the lack of resolved
hyperfine couplings in these complexes is unclear.18
RESULTS AND DISCUSSION
■
As outlined in Scheme 1, the Fe(CO)3(NO)− anion is a
−
convenient precursor to X2Fe(NO)2 complexes which makes
Scheme 1. Synthesis of [PPN][(ECN)2Fe(NO)2] (E = S and
O)
The two-band IR spectra in the ν(NO) region are sensitive
to ligand,39 shifting to lower values for 1 > 2 > (N3)2Fe(NO)2
−
−
> (PhS)2Fe(NO)2 , Table 1. Note that the ν(NO)avg. of
Table 1. Mossbauer Parameters: Isomer Shift (δ) and
̈
Quadrupole Splitting (ΔEQ), CV (E1/2) and IR Stretching
−
Frequencies of Complexes 1, 2, (N3)2Fe(NO)2 , and
(SPh)2Fe(NO)2 and Two Neutral {Fe(NO)2}9 Derivatives
−
for Comparison
a.
b.
c.
c.
ν(NO)
cm−1
E1/2
(v)
δ
ΔEQ
)
(mm/s)
(mm/s)
further use of NO+ as reactant and oxidant converting to an
unstable cationic dicarbonyl through the intermediacy of
neutral reduced DNIC.38,39 Anionic ligands readily replace
CO and stabilize the {Fe(NO)2}9 unit, as was observed here for
SCN− and OCN−, complexes 1 and 2, respectively. This route
Anionic
−
(SCN)2Fe(NO)2
1786, 1718 −1.21
1766, 1698 f.
1755, 1698 −1.51
1737, 1693 −1.68
Neutral
0.35
0.36
0.30
0.23
0.75
0.70
0.57
0.60
−
(OCN)2Fe(NO)2
−
(N3)2Fe(NO)2
−
(PhS)2Fe(NO)2
19
−
is also appropriate for formation of (N3)2Fe(NO)2
and
−
d.
(Ar-nacnac)Fe(NO)2
(PhS)2Fe(NO)2
.
1761,1709
−1.34
0.19
0.79
0.51
e.
Complexes 1 and 2 were isolated from 1:1 THF/pentane as
brown and greenish brown crystals, respectively. The X-ray
diffraction analysis yielded the typical tetrahedral structures of
DNICs shown in Figure 1; the NCS or NCO ligands are N-
(NHC)(PhS)Fe(NO)2
1757, 1712 −1.33
0.15
a.
b.
c.
THF solution. THF solution, reference to Fc+/Fc. Isomer shifts
are quoted at 7 K with respect to Fe metal standard at room
temperature. Line-widths are specified in the text. Uncertainties are
d.
e.
f.
0.05 mm/s. See reference 18. See reference 41. Irreversible
reduction in THF solution.
(PhS)2Fe(NO)2− is 40 cm−1 lower than that of 1. The trend of
increasing electronic donation of X− (X− = SCN−, N3 , PhS−)
−
to the {Fe(NO)2}9 motif is also reflected in the redox potentials
of this series of complexes.
The cyclic voltammetry scans of compounds 1, (N3)2Fe-
−
−
(NO)2 , and (PhS)2Fe(NO)2 , measured on 2 mM THF
solutions of PPN+ salts, display one highly reversible electro-
chemical response within the THF solvent window, Figure 2.
The E1/2 values are consistent with the ν(NO) IR data, that is,
better electron donating ligands render the {Fe(NO)2}9/10
couple less accessible.17,40,41 The CV of the cyanate complex
Figure 1. Selected metric data are listed below the molecular
structures of anionic complexes 1 (E = S) and 2 (E = O), derived from
X-ray diffraction analysis of their PPN+ salts, as thermal ellipsoid plots
(50% probability level) with alternate view in capped stick renditions.
bound in both. Differences in the selected metric data given
under each structure are largely insignificant, with the largest
discrepancy found within the average of Fe−N−C angles
(177.2(2)° for 1 and 169.6(3)° for 2. In both structures the
Fe−N−O units are slightly bent, avg. = 162°, and oriented in
toward each other. Full metric data for complexes 1 and 2 are
provided in Supporting Information, Tables S1 and S2.
Figure 2. Cyclic voltammograms of complexes 1 (blue; E1/2 = −1.21
V), [PPN][(N3)2Fe(NO)2] (red; E1/2 = −1.51 V), and [PPN]-
[(PhS)2Fe(NO)2] (black; E1/2 = −1.68 V) at scan rates 100 mV/s in 2
mM THF solution, referenced to Fc/Fc+.
For comparison of spectroscopic properties to 1 and 2, the
−
−
(N3)2Fe(NO)2 and (PhS)2Fe(NO)2 complexes were pre-
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2 shows irreversibility in the analogous redox event, see
Supporting Information, Figure S8.
approximately 6 kcal/mol per SCN− ligand and from N to O
requires approximately 16 kcal/mol per OCN− ligand. The
unpaired spin density plot, Supporting Information, Figure S11
and TOC graphic, shows that the unpaired density is confined
to the Fe(NO)2 unit.
The preference for N-binding over O or S is found in the
ligands themselves. The frontier molecular orbitals of OCN−,
specifically the π (HOMO) and σ (HOMO-1) orbitals, both
have primarily N-character, see Figure 4. The SCN− ligand also
The Mossbauer spectra of the four complexes in an applied
̈
field of 0.04 T exhibit broad quadrupole doublets (Supporting
Information, Figure S9, fwhm 0.45 mm/s) with no resolved
magnetic hyperfine structure, and temperature independent
between 5 and 100 K. The 7 K isomer shifts displayed in Table
1 are within the range of high spin Fe(III) complexes.42 The
−
observed isomer shift for (PhS)2Fe(NO)2 is consistent with
published parameters: δ = 0.18 mm/s and ΔEQ = 0.69 mm/
s.18,20b For our series of anionic, tetrahedral DNICs the overall
trend of δ to smaller values roughly correlates with increasing
electron-donating ability of the ancilliary ligand quite
reasonably represented by ν(NO) and E1/2 values, Table 1.
Also in Table 1 are neutral {Fe(NO)2}9 complexes, that is,
with only one anionic ligand, or in case of Ar-nacnac, a
monoanionic bidentate ligand, whose IR and E1/2 values readily
fit into the trend seen for the anionic DNICs. Note however
that the Mossbauer isomer shifts do not correlate with the
̈
trends in ν(NO) and E1/2 values. The overall effect of charge
appears to be a factor in the isomer shift data.
Upon exposure to air, the brown color of the THF solution
of 1 turns to red-purple, from which X-ray quality crystals were
produced on layering with pentane. The molecular structure
shown in Figure 3 is that of a monoanionic iron complex of
Figure 4. Selected Frontier Molecular Orbitals (FMOs), orbital
character, and orbital energies (eV) of the triatomic ligands OCN−,
−
N3 , and SCN−. Molecular orbital contours plotted at an isosurface
value of 0.02 au. Internal distances given for triatomic anions are from
computer optimized structures.
donates through the π (HOMO) and σ (HOMO-1) orbitals;
however, the former has significant S-character, while the latter
retains substantial N-character. Both OCN− and SCN− are
expected to have contributions from both the ECN− and
−
the E−CN resonance structures, with the latter structure
contributing more in SCN−. That this resonance structure is
more important in SCN− leads to the greater linearity of the
Fe−NC angle of complex 1 vs 2, see Supporting
Information.
Since the NO ligands in the iron dinitrosyl unit are very
strong π-acceptors, the ancillary ligands should be expected to
bind in a manner that would yield the better σ donations. Thus,
the preferred SCN− and OCN− derivatives are bound through
the N. The competition between the S π vs N σ donation is
responsible for the ambidenticity of the SCN− ligand and for
the relatively small isomerism energy in 1, while the OCN−
ligand with both σ and π donor orbitals on N shows a much
larger isomerism energy in 2. Further, the π-donation when S-
bound results in a strongly bent SCN− linkage (average Fe−S−
C angle of 107.2°), while the σ-donation resulting from N-
binding produces a nearly linear linkage (average Fe−NC
angle of 169.4°), which is the structural form found in our
studies.
The influence of the other ligands with respect to the linkage
isomerism of SCN− has been further explored by investigation
of the carbonyl derivatives of the Fe0 and FeII compounds:
Fe(CO)4(NCS)− and (η5-C5H5)Fe(CO)2(NCS), see Figure 5.
These complexes have ΔE between N- and S-bound SCN−
ligands of 2.82 and 1.08 kcal/mol respectively, consistent with
the decrease in the π-acceptor strength of the other ligands, that
is, CO and Cp. The known experimental isomers provide
confirmation of this conclusion, as Fe(CO)4(NCS)− has been
crystallized in only the N-bound form,51 whereas both N- and
S-bound isomers of (η-C5H5)Fe(CO)2(NCS) have been
separated by chromatography and isolated.52
Figure 3. Molecular structure of complex [PPN][(THF)2Fe(NCS)4]
(3) as thermal ellipsoid plot (50% probability level). Selected bond
distances (Å) and angles (degree): Fe-NCSavg. 2.023(3); Fe−
O(THF)avg. 2.089(5); ∠Fe−N−Cavg. 171.6.(3); ∠N−C−Savg
178.5(4).. (see Supporting Information, Tables S4 for details of
crystallization).
−
trans-(THF)2Fe(NCS)4 (3). This complex is air stable and
43
−
analogous to the reported trans-(pyridine)2Fe(NCS)4 . The
Gouy balance magnetic susceptibility measurement indicated 5
unpaired electrons, consistent with a high-spin FeIII complex.
Complex 3 can also be obtained via reaction of FeCl3·6H2O
and 4 equiv of PPN+SCN− salt in THF solution.
Computational Studies. The molecular structures of the
ligands N3 , OCN−, and SCN−, as well as the corresponding
−
−
DNICs, (N3)2Fe(NO)2−, (NCS)2Fe(NO)2 (1), and
−
(NCO)2Fe(NO)2 (2), have been optimized and analyzed by
DFT computations using a methodology previously determined
to be accurate for DNICs: the BP86 functional44,45 and a mixed
basis set of SDD ECP on Fe and 6-311++G(d,p)46−49 on all
other atoms.50 See Supporting Information for details. The O-
bound and S-bound linkage isomers of the latter two complexes
have also been investigated. In all cases, the N-bound
derivatives were found as the ground state, consistent with
experiment; linkage isomerism from N to S requires
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−
(NCS)2Fe (NO)2 . This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: marcetta@mail.chem.tamu.edu (M.Y.D.), hall@chem.
tamu.edu (M.B.H.), cpopescu@ursinus.edu (C.V.P.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Science
Foundation (CHE-0910679 to MYD, CHE-0956779 to CVP,
and CHE-0910552 to MBH) and the R. A. Welch Foundation
(A-0924 to MYD and A-0648 to MBH).
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data (CIF) from the structure determi-
nations, ORTEPs, and full lists of metric parameters for
complexes 1−3. EPR spectra of complexes 1 and 2. Cyclic
voltammogram of complex 2. Computational metric data and
APT population analysis for complexes 1, 2, [PPN][(N3)2Fe-
(NO)2], and [PPN][(PhS)2Fe(NO)2]. Calculated isomers and
electronic energy differences of 1 and 2. Unpaired spin density
plots for the {Fe(NO)2}9 isomers of 1 and 2. Frontier
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+
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(NO)2−, (NCO)2Fe(NO)2−, (SCN)2Fe(NO)2−, and
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