Synthesis and characterization of [FeFe]-hydrogenase mimics appended with a 2-phenylazopyridine ligand

Article abstract of DOI:10.1080/17415993.2013.796553

Two new complexes in which 2-phenylazopyridine (pap) chelates iron in hydrogenase mimics, 1,2-(-benzenedithiolato)-2′- phenylazopyridinediirontetracarbonyl and 1,3-(-propanedithiolato)-2′- phenylazo- pyridinediirontetracarbonyl have been synthesized and fully characterized, including X-ray crystal structure determinations. The electronic structures of the two complexes are compared with the analogous 1,2-(-benzenedithiolato)diironhexacarbonyl and 1,3-(-propanedithiolato) diironhexacarbonyl complexes. Based on comparison of the crystal structures, the overall bonding in the 2Fe2S core of the molecules is little perturbed by replacing two carbonyl ligands with the pap ligand. Also, the coordinated pap ligand retains a similar structure and NN bond distance to that of the uncoordinated ligand. However, the charge asymmetry in the 2Fe2S core that results from chelating the pap ligand on one of the iron atoms induces substantial localization of the individual orbital characters in the 2Fe2S core. Most interesting, the pap-substituted complexes feature a novel strong long wavelength absorption in the visible region that imparts a deep blue color to the molecules. TDDFT calculations reveal the nature of this absorption as excitation to a low-lying empty orbital on the pap ligand mixed with filled primarily metal d orbitals of the 2Fe2S core.

Full text of DOI:10.1080/17415993.2013.796553

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Synthesis and characterization of
[FeFe]-hydrogenase mimics appended
with a 2-phenylazopyridine ligand
Raphael A. Seidela, Gabriel B. Halla, Matthew T. Swensona, Gary S.
Nichola, Dennis L. Lichtenbergera, Dennis H. Evansb & Richard S.
Glassa
a Department of Chemistry and Biochemistry, The University of
Arizona, Tucson, AZ 85721, USA
b Department of Chemistry, Purdue University, West Lafayette, IN
47907, USA
Published online: 03 Jun 2013.
To cite this article: Raphael A. Seidel, Gabriel B. Hall, Matthew T. Swenson, Gary S. Nichol,
Dennis L. Lichtenberger, Dennis H. Evans & Richard S. Glass (2013) Synthesis and characterization
of [FeFe]-hydrogenase mimics appended with a 2-phenylazopyridine ligand, Journal of Sulfur
Chemistry, 34:6, 566-579, DOI: 10.1080/17415993.2013.796553
To link to this article: http://dx.doi.org/10.1080/17415993.2013.796553
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Journal of Sulfur Chemistry, 2013
Vol. 34, No. 6, 566–579, http://dx.doi.org/10.1080/17415993.2013.796553
Synthesis and characterization of [FeFe]-hydrogenase mimics
appended with a 2-phenylazopyridine ligand
Raphael A. Seidela, Gabriel B. Halla, Matthew T. Swensona, Gary S. Nichola,
Dennis L. Lichtenbergera, Dennis H. Evansb and Richard S. Glassa*
aDepartment of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ 85721, USA;
bDepartment of Chemistry, Purdue University, West Lafayette, IN 47907, USA
(Received 1 March 2013; final version received 14 April 2013)
Two new complexes in which 2-phenylazopyridine (pap) chelates iron in hydrogenase mimics, 1,2-(μ-
benzenedithiolato)-2ꢀ-phenylazopyridinediirontetracarbonyl and 1,3-(μ-propanedithiolato)-2ꢀ-phenylazo-
pyridinediirontetracarbonylhavebeensynthesizedandfullycharacterized, includingX-raycrystalstructure
determinations. The electronic structures of the two complexes are compared with the analogous 1,2-
(μ-benzenedithiolato)diironhexacarbonyl and 1,3-(μ-propanedithiolato)diironhexacarbonyl complexes.
Based on comparison of the crystal structures, the overall bonding in the 2Fe2S core of the molecules
is little perturbed by replacing two carbonyl ligands with the pap ligand. Also, the coordinated pap ligand
=
retains a similar structure and N N bond distance to that of the uncoordinated ligand. However, the charge
asymmetry in the 2Fe2S core that results from chelating the pap ligand on one of the iron atoms induces
substantial localization of the individual orbital characters in the 2Fe2S core. Most interesting, the pap-
substituted complexes feature a novel strong long wavelength absorption in the visible region that imparts
a deep blue color to the molecules. TDDFT calculations reveal the nature of this absorption as excitation to
a low-lying empty orbital on the pap ligand mixed with filled primarily metal d orbitals of the 2Fe2S core.
Keywords: non-innocent; visible absorption spectra; 2Fe2S cores; metal-to-ligand charge transfer
*Corresponding author. Email: rglass@email.arizona.edu; rglass@u.arizona.edu
© 2013 Taylor & Francis

Journal of Sulfur Chemistry 567
1. Introduction
[FeFe]-Hydrogenase is an exceptionally active catalyst for the reduction of water to generate
molecular hydrogen (1). The active site of this enzyme is shown in 1 with X = NH (2).
The 4Fe4S cluster is part of the redox clusters that are believed to provide a conduit for electrons
to the active site to effect reduction of protons (3). Could reducible organic ligands provide
this conduit? Alternatively viewed would addition of an electron to a complex with a reducible
organic ligand annulated to a 2Fe2S core occur on the ligand, the core or be delocalized over
both (4)? That is, what is the interplay between such redox centers? Much interest has been
focussed on “noninnocent” ligands (5) for which these questions are pertinent. One such ligand
is 2-phenylazopyridine (2a). This ligand is readily reduced by two one-electron processes – first
to the radical anion and then to the dianion (6). Metal complexes of this ligand are known (7) in
which the ligand is in the neutral, radical anion or dianion states. Furthermore, a ruthenium (II)
complex with this ligand as the radical anion has been reported (8) to reduce protons to form molec-
ular hydrogen. A diiron complex has been reported (9) in which three 2-p-tolylazopyridine 2b
moieties are coordinated. Two of these ligands bridge both Fe atoms and are assigned as dianionic
6e−donors, and the nonbridging ligand is assigned as a neutral 4e− donor. Here, we report the syn-
thesis and characterization of two 2Fe2S complexes each with a redox active 2-phenylazopyridine
ligand which are analogues of the [FeFe]-hydrogenase active site. The results suggest that the
special nature of the interactions between the sulfur atoms and the iron atoms in the 2Fe2S core
lead to fluid orbital characters that adjust to the ligand perturbations and electronic excitations of
these complexes.
2. Results and discussion
1,2-(μ-Benzenedithiolato)diironhexacarbonyl 3 (10) on treatment with trimethylamine-N-oxide
and 2-phenylazopyridine 2a afforded 4 in low yield (6.5%) but irradiation of a solution of 3 and

568 R.A. Seidel et al.
Figure 1. Molecular structure of 4 with anisotropic displacement ellipsoids at the 30% probability level and hydrogen
atoms omitted.
2a in THF at low temperature provided 4 in higher yield (38.5%).
Interestingly, 4 is a dark blue solid, whereas 3 is a light orange-red solid. The structure of 4
was unequivocally established by an X-ray crystallographic structure study (Table 1). An ORTEP
=
drawing of the molecule is shown in Figure 1. The N N bond length in this complex of 1.311(3)Å
is only slightly longer than the mean distance of 1.25 0.02Å of representative free molecules of
=
type 2 with various substituents R (11) and consistent with the known range for an N N double
=
bond distance (7, 9) providing evidence for a primarily neutral ligand. In addition, a strong N N
stretching band in the IR is found close to that of the uncomplexed ligand (1461 cm−1). The
structure of the 2Fe2S core is very little perturbed by replacing the two basal carbonyl ligands on
one iron center of 3 with the ligand 2a to form 4. The Fe–Fe distance in 4 is 2.49Å compared
with 2.48Å in 3.
The UV–visible absorption spectrum of 4 is much different from that of 3 as seen in Figure 2.
Most significantly, a longer wavelength (∼600 nm) absorption is observed in 4 with a substantial
extinction coefficient. The extinction coefficient suggests that this new absorption is due to a
charge-transfer band. Since the energies of charge-transfer bands are often sensitive to solvent,
the spectrum was measured in solvents of increasing polarity (THF, CH2Cl2, EtOH, CH3CN, and
DMSO). The wavelength maximum varied over a modest range (ca. 7 nm) but did not vary in

Journal of Sulfur Chemistry 569
Table 1. Crystal data and structure refinement for 4.
Chemical formula
Formula weight
C21H13Fe2N3O4S2
547.16
Temperature
298(2) K
Radiation, wavelength
Crystal system, space group
Unit cell parameters
MoKα, 0.71073Å
triclinic, P1
¯
a = 7.739(3) Å
b = 12.670(4)Å
c = 13.072(5)Å
1085.6(7)Å3
2
α = 116.361(5)◦
β = 98.608(6)◦
γ = 101.158(6)◦
Cell volume
Z
Calculated density
Absorption coefficient μ
F(000)
Crystal color and size
Reflections for cell refinement
Data collection method
1.674 g/cm3
1.563 mm−1
552
blue, 0.51 × 0.21 × 0.18 mm3
3149 (θ range 2.7–26.5◦)
Bruker SMART 1000 CCD diffractometer
Thin-slice ω scans
1.8–25.0◦
θ range for data collection
Index ranges
h = 9–9, k = 15–15, l = 15–15
Completeness to θ = 25.0◦
99.0%
Reflections collected
7722
Independent reflections
Reflections with F2 > 2σ
Absorption correction
Min. and max. transmission
Structure solution
3776 (Rint = 0.0238)
2893
Numerical
0.5029 and 0.7662
Direct methods
Refinement method
Full-matrix least-squares on F2
0.0378, 0.0897
Weighting parameters a, b
Data/restraints/parameters
Final R indices (F2 > 2σ)
R indices (all data)
3776/0/289
R1 = 0.0321, wR2 = 0.0717
R1 = 0.0483, wR2 = 0.0795
1.065
Goodness-of-fit on F2
Largest and mean shift/su
Largest diff. peak and hole
0.000 and 0.000
0.39 and −0.23 eÅ−3
a systematic way with solvent polarity (the solvent dielectric constants are 8, 9, 25, 36, and 37,
respectively) (12).
DFT computations were performed to gain further insight into this absorption and the electronic
structure of this complex. The DFT-optimized geometry was found to agree well with the crystal
structure with most calculated bond distances within 0.01Å of the crystal structure values (see
=
Supporting Information). The slight increase of ∼0.05Å in the N N distance from the free to the
complexed ligand 2a is also reproduced by the computations. In addition, the calculated metal-CO
stretching vibrations account well for both the relative frequencies and intensities observed in the
IR spectrum (Figure 3). The calculated HOMO and LUMO of 3 and 4 are shown in Figure 4.
As can be seen in Figure 4, the HOMO is almost entirely localized on the substituted Fe with
contribution from this Fe totalling 75% of the orbital character, and only 9% orbital character
on the other Fe. Only 4% of the orbital character is distributed between the two S atoms. This is
in contrast to the higher symmetry molecule 3 in which the HOMO contains considerable Fe–S
mixing with 39% distributed equally among the two Fe atoms, and 18% distributed equally among
the two S atoms. This preferential contribution of orbital character from one half of the molecule,
and minimal 2a π character, is seen in all occupied orbitals from the HOMO to HOMO-6, which
comprise primarily the three occupied d orbitals on each metal and the metal–metal bond. The
HOMO energy of 4 is raised relative to 3 due to the loss of π-backbonding to the two carbonyl
ligands that 2a replaces (Figure 5) and removal of delocalization to the S atoms. The ligand 2a
has some slight π-backbonding ability such that the σ-donation of charge is compensated and the

570 R.A. Seidel et al.
Table 2. Crystal data and structure refinement for 6.
Chemical formula
Formula weight
C18H15Fe2N3O4S2
513.15
Temperature
220(2) K
Radiation, wavelength
Crystal system, space group
Unit cell parameters
MoKα, 0.71073Å
monoclinic, P21/c
a = 8.5466(6)Å
b = 10.8673(8)Å
c = 22.1286(17)Å
2018.5(3)Å3
4
α = 90◦
β = 100.8600(10)◦
γ = 90◦
Cell volume
Z
Calculated density
Absorption coefficient μ
F(000)
Crystal color and size
Reflections for cell refinement
Data collection method
1.689 g/cm3
1.674 mm−1
1040
Dark blue, 0.31 × 0.30 × 0.30 mm3
6823 (θ range 2.4–26.4◦)
Bruker SMART 1000 CCD diffractometer thin-slice ω scans
Thin-slice ω scans
θ range for data collection
Index ranges
1.9–26.4◦
h = 10–10, k = 13–13, l = 27–27
Completeness to θ = 26.4◦
99.8%
Reflections collected
15,741
Independent reflections
Reflections with F2 > 2σ
Absorption correction
Min. and max. transmission
Structure solution
4136 (Rint = 0.0233)
3459
Numerical
0.6249 and 0.6335
Patterson synthesis
Full-matrix least-squares on F2
0.0267, 1.2296
Refinement method
Weighting parameters a, b
Data/restraints/parameters
Final R indices (F2 > 2σ)
R indices (all data)
4136/0/322
R1 = 0.0271, wR2 = 0.0608
R1 = 0.0354, wR2 = 0.0652
1.076
Goodness-of-fit on F2
Largest and mean shift/su
Largest diff. peak and hole
0.002 and 0.000
0.33 and −0.28 eÅ−3
Figure 2. Visible absorption spectra in dichloromethane of 2a (red), 3 (purple), and 4 (blue).
overall ligand is near neutral. The 2a acceptor orbital has a π antibonding symmetry interaction
between the two azo N atoms and the weak π back donation from the metal is consistent with the
=
observed ∼0.05Å increase in the N N distance from the free ligand molecule mentioned earlier.

Journal of Sulfur Chemistry 571
Figure 3. Comparison of the experimental IR spectrum of 4 (black) and calculated spectrum (blue) in the metal-carbonyl
stretching frequency region.
Figure 4. LUMO (above) and HOMO (below) calculated for 3 and 4.

572 R.A. Seidel et al.
Figure 5. Orbital energy correlation diagram. (a) Orbital energies of molecule 3 (the “2Fe2S” is a block of seven orbitals
that derive from the two d6 Fe centers and the Fe–Fe bond). (b) The “2Fe2S” fragment orbital energies in the electron
distribution of molecule 4. (c) Orbital energies of molecule 4, note the large amount of 2a LUMO contributing to the
LUMO of 4. (d) Orbital energy of 2a LUMO in the electron distribution of molecule 4.
The LUMO of 3, like its HOMO, has a distribution of orbital character between the Fe atoms
and the S atoms with 44% Fe character and 18% S character. Incorporation of the ligand 2a
raises the energy of the LUMO of 3 just as it raised the HOMO so that the LUMO of 3 now
forms the LUMO + 1 of 4, retaining a strong resemblance to the LUMO of 3. The new low-lying
empty orbital introduced by 2a dominates the LUMO of molecule 4 (66% character from 2a).
The antibonding interaction between the azo N atoms alone accounts for 30% of the LUMO of 4.
The LUMO of 4 includes some mixing and character traced to the HOMO and LUMO + 1 of 3,
resulting in the Fe atoms contributing 16%, and 9% and almost eliminating the contribution of
the S atoms (3%) in the LUMO. This mixing and increased electron richness in the 2Fe2S core
when two carbonyls are substituted by 2a results coincidently in the LUMO’s of 3 and 4 having
nearly the same energy.
Time-dependent DFT (TDDFT) computations predict a strong absorption at 578 nm in good
agreement with the observed transition at 598 3 nm for 4. The shift of this absorption to longer
wavelength from the first significantly allowed transition of 3 is calculated to be 135 nm in good
agreement with the observed shift of 145 5 nm. The TDDFT computations describe this absorp-
tion of 4 as excitation into the LUMO from 0.64 HOMO-2, 0.14 HOMO-1, and 0.10 HOMO-4.
The longer wavelength of the first absorption for 4 compared with 3 is ascribed to a closing of the
gap between the LUMO and the 2Fe2S occupied orbitals in 4 due to raising of the 2Fe2S occupied
orbital energies (Figure 5) compared with 3. Because the highest occupied orbitals are primarily
metal based and the LUMO is primarily 2a based, this absorption is defined as a metal-to-ligand
charge transfer transition. The calculated oscillator strength for excitation to the LUMO is five
times greater for 4 than for 3, which is reasonable in comparison to the increased extinction
coefficient for this transition observed in the spectrum. The assignment of the absorption to a

Journal of Sulfur Chemistry 573
charge-transfer band is consistent with the large extinction coefficient as observed, but in appar-
ent contradiction to the lack of λmax shift with solvent polarity. However, the large size of these
molecules and the fluid electron density provided by the 2Fe2S core and carbonyl ligands may
minimize the ability of the solvent to differentially stabilize the ground and excited electronic
states. As a test of this possibility, the difference in solvation energy stabilization between the
ground state and various contributing excited states was calculated via DFT for a variety of sol-
vents with a range of dielectric constants. The solvation energy differences were found to produce
shifts of no more than 3 nm, which is within the certainty of the experiment (Tables S-2 and S-3).
Thus, the computations show a strong charge-transfer absorption at long wavelength that does
not shift significantly with solvent polarity, in agreement with the experiment. Apparently, the
absorption wavelength dependence on polarity is a poor test for a charge transfer transition for
this class of molecules.
Thermal reaction of 1,3-(μ-propanedithiolato)diironhexacarbonyl, 5, and 2-phenylazopyridine
2a gave complex 6 in low yield (5%) but irradiation in THF afforded complex 6 in 44% yield as
a blue solid.
ThestructureofthiscomplexwasunequivocallydeterminedbyanX-raycrystallographicstructure
study (Table 2) and its molecular structure, shown in Figure 6, is analogous to that of 4. The
=
=
N N distance in this complex is 1.315(2)Å which is again consistent with a N N bond slightly
=
weakened by backbonding from the metals and further supported by the N N stretching frequency
seen in the IR spectrum. The experimental IR spectrum of this complex in the metal-carbonyl
stretching frequency region and the calculated spectrum are compared in Figure 7. Interestingly,
the 1H NMR spectrum of 6 clearly shows that the C1–C3 bridge of the propanedithiolato ligand
does not invert at room temperature, in contrast to the parent 5. As seen in Figure 6, the C1–C3
bridge is directed away from the apical CO group of the substituted Fe. Inversion would engender
Figure 6. (a) Molecular structure of 6 with anisotropic displacement parameters at the 50% probability level and
hydrogen atoms omitted. (b) Overlay of the crystal structures of 5 and 6 (black bonds).

574 R.A. Seidel et al.
Figure 7. Comparison of the experimental IR spectrum of 6 (black) and calculated spectrum (blue) in the metal-carbonyl
stretching frequency region.
Figure 8. Visible absorption spectra in dichloromethane of 2a (red), 5 (purple), and 6 (blue).
more steric interaction between C2 and the apical CO on the substituted Fe because this CO is
closer to the bridge than that of the Fe(CO)3 moiety, or as seen in the overlay in Figure 6(b),
than the apical CO ligands in 5 (13). The detailed NMR spectroscopic assignments are given in
Section 4.
The UV–visible absorption spectrum for 6 showed a new band at long wavelength (∼605 nm),
shifted from that seen in 5, analogous to that seen in 4 (Figure 8). This absorption band shifted only
modestly (605, 608, 602, 605, and 609 nm) in different solvents (CH2Cl2, THF, EtOH, CH3CN,
and DMSO), respectively, and as with 4 did not correlate with solvent polarity. DFT calculations
again suggest that the shift in absorption for 6 compared with 5 is attributable primarily to a
raising of the HOMO as can be seen in Figure 9. Figure 10 shows that the composition of the
HOMO of 6 is very similar to that of 4 with 75% of the orbital character coming from the Fe
ligated with 2a. The high energy occupied orbitals remain primarily 2Fe2S in character, and the
2a ligand LUMO comprises 64% of the LUMO of 6. According to TDDFT calculations, the long
wavelength absorption in 6 is from the HOMO-2 (0.36), HOMO-1 (0.26), and HOMO-3 (0.22)
to the LUMO. The key orbital compositions are illustrated in Figure 10.

Journal of Sulfur Chemistry 575
Figure 9. Orbital energy correlation diagram. (a) Orbital energies of molecule 5 (the “2Fe2S” is a block of seven orbitals
that derive from the two d6 Fe centers and the Fe-Fe bond). (b) The “2Fe2S” fragment orbital energies in the electron
distribution of molecule 6. (c) Orbital energies of molecule 6 showing the contribution to the LUMO of ligand 2a to the
LUMO of 6. (d) LUMO of 2a in the electron distribution of molecule 6.
Figure 10. Plot of LUMO (above) and HOMO (below) calculated for 5 and 6.

576 R.A. Seidel et al.
3. Conclusions
Two new diiron complexes of 2-phenylazopyridine, 2a, have been prepared and fully character-
ized. Introduction of 2a as a chelate ligand on one Fe in place of two CO ligands results in charge
asymmetry in the 2Fe2S core and localization of HOMO orbital character on the substituted
Fe. In addition, the LUMO of the substituted complexes shows extensive contribution from the
2-phenylazopyridine ligand π orbital. A consequence of these changes is that an absorption band
is observed at long wavelength (ca. 600 nm) with a substantial increase in extinction coefficient in
the visible absorption spectrum of these complexes. TDDFT calculations show that this absorp-
tion results from excitation from the filled Fe orbitals to the low-lying empty 2-phenylazopyridine
orbital. Polar solvents are not able to provide significant additional stabilization of the charge-
transfer excited state than provided within the electronic structure of the molecule itself. The
overall picture that emerges is one of fluid orbital distributions in the 2Fe2S core aided by the
nature of sulfur orbital interactions with iron atoms and the backbonding capabilities of carbonyls.
The mixed character and energy of the LUMO in the 2-phenylazopyridine complexes indicates
that this ligand will be noninnocent in the reduction chemistry of these complexes, and the deep
blue colors suggest the possibility of interesting excited state chemistry.
4. Experimental
Low-resolution mass spectra were measured with a Finnigan LCQ classic instrument with ESI-
ion trap detection; high-resolution mass spectroscopy was done with a Bruker 9. 4T FT-ICR
instrument with ESI. Melting points were taken on a Thomas-Hoover Uni-Melt capillary melting
point apparatus. IR spectra were recorded on a Nicolet Impact 410 IR spectrometer. NMR spectra
were obtained on Bruker AM-250, Varian Unity-300, or Bruker DRX-500 MHz instruments.
4.1. Crystallography
Single crystal X-ray diffraction intensity was measured on a Bruker Kappa Apex II Duo diffrac-
tometer using a graphite-monochromated Mo Kα radiation at a temperature of 298 K (6) and 220 K
(4). A data collection strategy was determined using SMART, integration and data reduction was
performed using SAINT (14), absorption corrections were performed using SADABS (15), data
were refined by full-matrix least-squares on F2 using SHELXTL (16). Crystal structure graph-
ics were generated using ORTEP (thermal ellipsoid plots) or OLEX2 (structure overlay) (17).
Structures were deposited in the CCDC with deposition numbers of 927678 (4) and 927679 (6).
4.2. Preparation of 1,2-(μ-benzenedithiolato)-2ꢀ-phenylazo pyridinediirontetracarbonyl (4)
Photochemical reaction. To a Schlenk flask, 1,2-(μ-benzenedithiolato)diironhexacarbonyl
(100 mg, 0.238 mmol) and 2-phenylazopyridine (90 mg, 0.49 mmol) were added. The Schlenk
flask was then purged with N2 and freshly distilled THF (20 mL) was added. The resulting red
solution was cooled to −78◦C with an acetone/dry ice bath and irradiated for 75 min inside a
Rayonet Srinivasan–Griffith photochemical reactor for 3.5 h. After removing the solvent under
reduced pressure the greenish black residue was purified by column chromatography on silica
gel, eluting with 20:80 ethylacetate/hexanes. This resulted in the isolation of 4 as a dark blue
powder (50.2 mg, 0.092 mmol, 38.5 % yield). Recrystallization from pentane gave small, dark
blue cubic crystals (30.8 mg, 0.056 mmol; 23.7%): λm(CH2Cl2) 338 (20,000) 600 (8700) nm IR
1
(hexanes) 2026, 1974, 1964, 1947 (CO), 1461 (C C), 1377 (N N) cm−1; H NMR (CDCl3,
=
=

Journal of Sulfur Chemistry 577
500 MHz) δ 8.49 (C14-H, d, J = 6 Hz, 1H), 8.28 (C17-H, d, J = 8 Hz, 1H), 7.93 (C9,11-H, m, 2H),
7.69 (C15-H, t, J = 7.5 Hz, 1H), 7.44 (C8,10,12-H, m, 3H), 7.26 and 7.05 (C2,5-H, d, J = 7 Hz,
1H), 7.10 (C16-H, t, J = 7.5 Hz, 1H), 6.63 and 5.89 (C3,4-H, t, J = 7 Hz, 1H and 1H); 13C NMR
(CDCl3, 23◦C, 125 MHz) δ 217.78 (C18), 208.24 (C19, C20, and C21), 164.66 (C13), 157.40 (C7),
150.47 and 149.93 (C1 and C6), 148.98 (C14), 133.81 (C16), 128.88 (C15), 128.05 (C9 and C11),
127.10 and 127.02 (C2 and C5), 125.71 (C3 and C4), 125.18 (C8 and C12), 124.65 (C10), 118.39
(C17); HRMS [MH- CO]+ Calcd m/z 519.9176; Found: m/z 519.9171.
Oxidative reaction of 1,2-(μ-benzenedithiolato)diironhexacarbonyl 3 with 2-phenylazo-
pyridine 2a and trimethylamine-N-oxide. In a Schlenk flask flushed with nitrogen, 1,2-(μ-
benzenedithiolato)diironhexacarbonyl (95 mg, 0.226 mmol) and 2-phenylazopyridine (44 mg,
0.240 mmol) were added to dried THF (18 mL). After addition of trimethylamine-N-oxide dihy-
drate (44 mg, 0.389 mmol), the color of the stirred red solution turned to dark brown. TLC analysis
(30% ethylacetate/hexanes) after 1 h showed the starting materials as well as a blue spot and a
greenish black residue. Purification on a silica gel preparative column (20% ethylacetate/hexanes)
gave 4 as a blue solid (8 mg, 0.015 mmol; 6.5%). Recrystallization from dichloromethane/pentane
gave 4.2 mg dark blue crystals (4.2 mg, 0.008 mmol; 3.4%).
4.3. Preparation of 1,3-(μ-propanedithiolato)-2ꢀ-phenylazo pyridinediirontetracarbonyl (6)
Photochemical reaction. 1,3-μ-(Propanedithiolato) diironhexacarbonyl 5 (100 mg, 0.259 mmol)
was added to dried THF (20 mL) in a 100 mL Schlenk flask flushed with nitrogen. To the stirred
solution 2-phenylazopyridine (47 mg, 0.259 mmol) was added under nitrogen. The Schlenk flask
was sealed with a septum and placed in an acetone/dry ice bath inside a Rayonet Srinivasan–
Griffith photochemical reactor. The solution was magnetically stirred and the reaction was
followed by TLC on silica gel plates after 25 and 75 min. To keep the temperature at −78◦C
more dry ice was added to the cooling bath. After 25 min TLC analysis (50% CH2Cl2/pentane)
showed the starting materials, a green residue at the origin and an intense blue spot. After 75 min
TLC analysis did not show any 2-phenylazopyridine but still unreacted 5. Therefore, additional
2-phenylazopyridine (47 mg, 0.259 mmol) was added under nitrogen.After a further 30 min under
the same conditions, the solvent was removed under reduced pressure. Purification on a silica gel
preparative column eluting with 50% CH2Cl2/pentane gave recovered 5 (43.9 mg, 0.114 mmol)
and 6 (58.5 mg, 0.114 mmol; 44%). Recrystallization of 6 from pentane gave small, dark blue
cubic crystals (36.6 mg, 0.071 mmol; 27.5%): λm(CH2Cl2) 335 (16,000), 603 (7100) nm; IR
(mineral oil) 2024, 1972, 1957, 1946 (CO), 1461 (C C), 1377 (N N) cm−1; 1H NMR (CDCl3,
500 MHz) δ 8.77 (C15-H, d, J = 6 Hz, 1H), 8.23 (C18-H, d, J = 8.5 Hz, 1H), 7.85 (C10,12-H, m,
2H), 7.68 (C17-H, t, J = 8 Hz, 1H), 7.45 (C9,11.13-H, m, 3H), 7.12 (C16-H, t, J = 6.5 Hz, 1H),
2.54 (C1,3-Heq, dt, J = 13, 4 Hz 1H), 2.15 (C1,3-Heq and C2-Heq, dt, J = 12.5, 5 Hz, 2H), 1.93
(C1,3-Hax, dt, J = 3, 13 Hz, 1H), 1.57 (C1,3-Hax, dt, J = 1.5, 12.5 Hz, 1H), 0.96 (C2-Hax, m,
1H); 13C NMR (CDCl3, 125 MHz) δ 217.34 (C4), 208.66 (C5,6,7), 164.66 (C14), 157.06 (C8),
147.53 (C18), 133.59 (C16), 128.59 (C17), 127.89 (C10,12), 125.12 (C9,13), 124.70 (C11), 117.62
(C15), 30.22 (C2), 24.67 and 24.83 (C1,3) HRMS: [MH]+ Calcd: m/z 513.9281 Found: m/z
513.9274.
=
=
Thermal reaction of 1,3-(μ-propanedithiolato)diironhexacarbonyl 5 with 2-phenylazopyridine
2a. To a nitrogen flushed Schlenk flask containing 5 (100 mg, 0.259 mmol) and 2a (95 mg,
0.519 mmol) was added 50 mL of dried toluene. The resulting red solution was refluxed for
2.5 h. The orange-red color of the solution slowly turned to dark brownish red. After removing the
solvent under reduced pressure, the product was purified by silica gel chromatography, eluting
with 40:60 chloroform/pentane. This gave recovered starting material (94.5 mg, 0.245 mmol) and
6 (6.9 mg, 0.013 mmol; 5.2%) as a blue powder.

578 R.A. Seidel et al.
4.4. Computational methods
All geometry optimizations were performed with ADF2009.01, and all TDDFT and excited state
energies were performed withADF 2012.01 (18, 19). Geometry optimizations and frequency cal-
culations were carried out using the VWN functional with the Stoll correction implemented (20).
Recent comparisons of OPBE to other common functionals found it to be the best for the predic-
tion of nuclear magnetic constants (21) and the only functional to correctly predict the spin states
of seven different iron complexes (22). Our comparisons with other common functionals in the
ADF package have also shown it to be among the best at predicting the oxidation and reduction
potentials of several iron complexes and the pKa values of the acids. Figures of the optimized
geometries were created with the program Visual Molecular Dynamics 1.9 (23). A triple-ζSTO
basis set with one polarization function (TZP), available in the ADF package, was used in all cal-
culations. Relativistic effects were taken into account by using the scalar ZORA formalism (24),
implemented as part of the ADF2009.01 and ADF2012.01 programs. For orbital energy correla-
tion diagrams, the wave function was transformed to a basis of the neutral (H4C6S2Fe2(CO)4),
(H6C3S2Fe2(CO)4) and 2a fragment orbitals to evaluate the molecular orbitals of 4 and 6 in terms
of the fragment orbitals.
Acknowledgements
The support of the National Science Foundation (Grant No. 1111570 and Grant No. 1111718) is gratefully acknowledged.
R.A.S. thanks the Konrad Adenauer Stiftung and German National Academic Foundation (Studienstiftung des deutschen
Volkes) for support.
Supporting information available
Supplementary content may be viewed online at http://dx.doi.org/10.1080/17415993.2013.796553.
The supporting information contains expanded UV–visible spectra of 2a, 3, 4, 5, and 6 plotted against absorbance
rather than ∈, a comparison of select experimental and computational bond distances for 4 and 6, a comparison of solvated
energies for the ground and selected excited states of 4 and 6, and the optimized Cartesian coordinates for 2a, 3, 4, 5, and 6.
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