Electronic perturbations of iron dipyrrinato complexes via ligand β-halogenation and meso-fluoroarylation

Article abstract of DOI:10.1021/ic2009539

Systematic electronic variations were introduced into the monoanionic dipyrrinato ligand scaffold via halogenation of the pyrrolic β-positions and/or via the use of fluorinated aryl substituents in the ligand bridgehead position in order to synthesize proligands of the type 1,9-dimesityl-β- R₄-5-Ar-dipyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F ₃C)₂C₆H₃, C₆F₅ in ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated ( ^β,ArL)H]. The electronic perturbations were probed using standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes. The free-ligand variations cause modest shifts in the electronic absorption maxima (λ_max: 464-499 nm) and more pronounced shifts in the electrochemical redox potentials for one-electron proligand reductions (E_1/2: -1.25 to -1.99 V) and oxidations (E_1/2: +0.52 to +1.14 V vs [Cp₂Fe] ^+/0). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands followed by treatment with FeCl₂ and excess pyridine in tetrahydrofuran to afford complexes of the type (^β,ArL)FeCl(py) (py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox potential of the fully reversible Fe^III/II couple spans more than 400 mV (E_1/2: -0.34 to +0.50 V vs [Cp ₂Fe]^+/0); λ_max spans more than 40 nm (506-548 nm); and the ⁵⁷Fe Moessbauer quadrupole splitting (|ΔE_Q|) spans nearly 2.0 mm/s while the isomer shift (δ) remains essentially constant (0.86-0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.

Full text of DOI:10.1021/ic2009539

ARTICLE
pubs.acs.org/IC
Electronic Perturbations of Iron Dipyrrinato Complexes via Ligand
β-Halogenation and meso-Fluoroarylation
Austin B. Scharf and Theodore A. Betley*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
S Supporting Information
b
ABSTRACT: Systematic electronic variations were introduced
into the monoanionic dipyrrinato ligand scaffold via halogena-
tion of the pyrrolic β-positions and/or via the use of fluorinated
aryl substituents in the ligand bridgehead position in order to
synthesize proligands of the type 1,9-dimesityl-β-R4-5-Ar-di-
pyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F3C)2C6H3, C6F5 in
ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated
(
β,ArL)H]. The electronic perturbations were probed using
standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes.
The free-ligand variations cause modest shifts in the electronic absorption maxima (λmax: 464À499 nm) and more pronounced
shifts in the electrochemical redox potentials for one-electron proligand reductions (E1/2: À1.25 to À1.99 V) and oxidations (E1/2
:
+0.52 to +1.14 V vs [Cp2Fe]+/0). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands
followed by treatment with FeCl2 and excess pyridine in tetrahydrofuran to afford complexes of the type (β,ArL)FeCl(py)
(py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox
potential of the fully reversible FeIII/II couple spans more than 400 mV (E1/2: À0.34 to +0.50 V vs [Cp2Fe]+/0); λmax spans more
than 40 nm (506À548 nm); and the 57Fe M€ossbauer quadrupole splitting (|ΔEQ|) spans nearly 2.0 mm/s while the isomer shift (δ)
remains essentially constant (0.86À0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the
dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.
coordination complexes revealed little variation upon metal ion
substitution, suggesting that the observed redox chemistry was
ligand-dominated with minimal influence from the bound transi-
tion metal ion. As a result, outer-sphere electron transfer
originated from the ligand pyrrole subunits, giving rise to
deleterious oxidation patterns wherein the metal ion was
expelled from the oxidized ligand scaffold. We therefore
shifted our attention to the two-electron oxidized dipyrrin
platforms, in which the conjugation of the two pyrrole sub-
units stabilizes the scaffold, permitting both ligand- and metal-
based redox chemistry to occur without problematic decom-
position.7a Despite both one-electron reduction and oxidation
being observed for the dipyrrin scaffold itself, metal-based
redox activity can also be cleanly observed. The use of the
dipyrrin platform should thus permit us to investigate how
systematic changes in the electronic properties of the ligand
affect the bound transition metal ion. We herein report the
synthesis of a series of nine dipyrrins varying at the pyrrole
backbone (2, 3, 7, and 8; or, collectively, β) positions and the
bridgehead methine (5- or meso-) position (see Figure 1) and
their FeII complexes; we also report the spectroscopic and
electrochemical characterization of both the free ligands and
the iron complexes.
1. INTRODUCTION
The majority of research involving dipyrrins (or dipyrro-
methenes) has been in relation to boron difluoride dipyrrin
(BODIPY) complexes, which are efficient fluorophores and have
found a wide range of applications.1 Transition metal complexes
of dipyrrinato ligands have been known at least since the time of
Fischer, who first developed dipyrrinato chemistry in the 1930s,2
but have only recently received a resurgence of attention for their
potential applications in metalÀorganic frameworks,3 fluores-
cence labeling,4 light-harvesting arrays,5 coordination polymers,6
and, as reported by our lab, CÀH activation and functionaliza-
tion chemistry.7 While there is a vast body of literature on the
effects of ligand variation on the optical properties of BODIPY
dyes,1 as well as the experimental properties of free-base and
metallo-porphyrins8 and other macrocyclic polypyrrole species,9
there has been little systematic exploration of how peripheral
ligand variations affect the chemistry of transition-metal dipyrri-
nato complexes.10
We have pursued pyrrole-based ligand platforms as truncated
analogues of porphyrin and porphyrinogen ligands to examine
the utility of their transition metal complexes to perform redox
chemistry. The nonconjugated dipyrromethane11 and tris-
(pyrrolyl)ethane platforms12 were found to suitably bind diva-
lent metals across the 3d series. The high-lying pyrrole-based
π-electrons proved to be a liability during the examination of
these complexes' redox properties. Cyclic voltammetry on the
Received: May 6, 2011
Published: June 21, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Figure 1. β-Halogenation and meso-fluoroarylation of porphyrins and dipyrrins.
Scheme 1. Ligand Synthesis
a For Ar = mesityl, acetal used; Ar = fluoroaryl, aldehyde used. b Solvent = hexanes, acetone, or CH2Cl2. c Chlorination: 6.0 equiv of N-chlorosuccinimide,
tetrahydrofuran, 70 °C, 48 h. Bromination: N-bromosuccinimide, tetrahydrofuran, room temperature, 1À16 h. Iodination: excess I2 and excess KOH,
dimethylformamide, 70 °C, 48 h.
Table 1. Dipyrrin Abbreviations and Yields
2. RESULTS AND DISCUSSION
2.1. Synthesis of Substituted Dipyrrins (β,ArL)H. Symme-
trically substituted dipyrrins are synthesized via the acid-cata-
lyzed condensation of 2 equiv of pyrrole with an aldehyde or
acetal followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ).13 Our typical synthetic protocol involves
reacting 2 equiv of 2-mesityl pyrrole14 [mesityl (Mes) = 2,4,6-
trimethylphenyl] with an aromatic aldehyde or acetal using
pyridinium p-toluenesulfonate (PPTS) catalyst (10 mol %) in
dichloromethane at room temperature for 12À24 h (Scheme 1).
For electron-deficient aromatic aldehydes such as perfluoro-
benzaldehyde or 3,5-bis(trifluoromethyl)-benzaldehyde, the pyr-
role condensation could be performed directly with the aldehyde
in the presence of molecular sieves, whereas the reaction with
mesitaldehyde dimethyl acetal was faster and higher-yielding
than that with the corresponding aldehyde. The dipyrromethane
products were isolated by eluting the crude reaction mixture
through a silica plug to remove residual acid catalyst and highly
colored byproducts and by concentrating the filtrate in vacuo.
Oxidation of the dipyrromethanes was effected with DDQ (1.1
equiv, Scheme 1). Upon addition of the oxidant to the reaction
solution, a color change from light yellow—characteristic of the
dipyrromethanes—to dark reddish-purple was immediately ob-
yield of last synthetic
stepa (%)
Ar
R1/R2
abbrev
H ,MesL)H
97
60
81
98
68
68
31
58
58
4
1
2
3
4
5
6
7
8
9
Mes
Mes
H/H
Cl/Cl
Br/H
Br/Br
I/I
(
(
(
(
(
(
(
(
(
Cl ,MesL)H
4
Br ,MesL)H
2
Mes
Br ,MesL)H
4
Mes
I ,MesL)H
4
Mes
H4,CF3
3,5-(CF3)2C6H3
C6F5
H/H
H/H
Cl/Cl
Br/Br
L)H
H4,C6F5
L)H
Cl4,C6F5
C6F5
L)H
Br4,C6F5
C6F5
L)H
a For compounds 1, 6, and 7, the reported yield is for DDQ oxidation;
for all others, the reported yield is for halogenation.
for 24h, overthecourse of which thesolution turned darkred. The
bright orange tetrachlorodipyrrin (Cl ,MesL)H (2) was obtained
4
afteraqueous workup in 60% yield. Bromination could be achieved
at room temperature with N-bromosuccinimide (NBS) in THF
and was significantly faster than chlorination, occurring within
minutes after addition of the NBS. Interestingly, dibromination of
H ,MesL)H could be performed regioselectively at the 2- and
4
served, though the reaction was not complete for several hours.
(
H4,C6F5
The dipyrrins (H ,MesL)H (1), (H ,CF L)H (6), and (
L)H
8-positions with strictly 2 mol equiv of NBS, affording the orange,
4
4
3
dibrominated dipyrrin (Br ,MesL)H (3) in 81% yield following
2
(7) were obtained as solids after aqueous workup. Both
H4,CF3
(
L)H and (H4,C6F5L)H required purification by chromatog-
workup. Crystals of 3 were grown from saturated chloroform-d
(CDCl3) solutions, permitting the determination of the dibromi-
nation regiochemistry by X-ray diffraction analysis (Figure 2).
raphy on a neutral alumina column with hexanes as the eluent
and were isolated as bright orange solids in 68 and 31% yields,
respectively (see Table 1).
Tetrabromination of (H ,MesL)H was also facile at room tempera-
4
Tetrachlorination15 of the orange-brown 5-mesityl dipyrrin
ture in THF with excess NBS (4.4 equivalents), affording bright
H ,MesL)H proceeded cleanly in the presence of excess N-chloro-
red (Br ,MesL)H (4) in near quantitative yield under conditions
4
4
(
succinimide (NCS, 6.6 equiv) at 70 °C in tetrahydrofuran (THF)
identical to those employed for dibromination.16 Iodination with
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Inorganic Chemistry
ARTICLE
Figure 2. Regioselectivity of dibromination reaction. Solid-state mo-
lecular structure of (Br ,MesL)H (3) with ellipsoids drawn at 50%
2
probability level (C, black; H, white; N, blue; Br, brown).
Figure 3. Representative UV/vis spectra obtained for proligands 1, 3,
and 5À9. Spectra obtained at room temperature in dichloromethane;
average ε values calculated from a minimum of four concentrations.
N-iodosuccinimide gave a mixture of products, whereas reaction
with I2 under basic conditions (excess KOH) in dimethylforma-
mide (DMF)17 at 70 °C for 48 h gave the dark red tetraiododi-
pyrrin (I ,MesL)H (5) in good yield (67%) after workup. Unfor-
4
tunately, attempts at electrophilic fluorination of the dipyrrin
fluoroaryl groups20) could be obtained from the 13C and 19F
18
4
3
backbone with Selectfluor-(BF4)2 (which generated the
NMR spectra of both (H ,CF L)H and its precursor dipyrro-
methane.
boron-difluoride complex as observed by mass spectrometry),
fluorination with Selectfluor-(PF6)2, or nucleophilic fluorination
with AgF in dichloromethane at elevated temperatures19 were
As π-conjugated organic molecules, dipyrrins display promi-
nent π f π* transitions in the visible region of the UV/visible
spectrum and are consequently highly colored molecules.21 Of
unsuccessful.
H4,C6F5
The 5-fluoroaryl dipyrrins (H ,CF L)H and (
L)H were,
the nine dipyrrins described here, the colors range from light
4
3
H4,C6F5
4
4
3
as anticipated, less reactive toward electrophilic halogenation.
orange [(H ,MesL)H, (H ,CF L)H, and (
L)H] to maroon
Chlorination and bromination of (H ,CF L)H gave competitive
[(I ,MesL)H and (Br ,C F L)H], with absorption maxima (λmax
)
4
3
4
4
6 5
4
4
decomposition of the compounds by an unidentified pathway
ranging from 464 nm [(H ,MesL)H] to 499 nm [(I ,MesL)H].
Halogenation red-shifts λmax proportionally with the atomic
radius of the halogen; shifts of ∼15, 20, and 35 nm are seen
for tetrachlorination, bromination, and iodination, respectively
(atomic radii 100, 115, and 140 pm22). Dibromination in 3 red-
shifts λmax by 7 nm, roughly half the shift seen in the case of
tetrabromination in 4. Incorporation of the fluoroaryl substitu-
ents in the 5-position has a less pronounced effect on the
1
(as determined by H NMR). Halogenation of (H ,C F L)H,
4
6 5
however, was successful, albeit with longer reaction times and
lower yields than those for the 5-mesityl dipyrrin, affording red
Cl4,C6F5
(
L)H (8) and maroon (Br4,C6F5L)H (9) in 58% yield in
both cases. The 5-fluoroaryl compounds (6À9) exhibited greater
solubility in nonpolar organic solvents than the 5-mesityl variants
(1À5), and as such, the aqueous workup was modified (see the
Supporting Information for details). After halogenation was
complete (as monitored by LC-MS), the reaction solvent was
removed by rotary evaporation, and the residue was dissolved in
hexanes and/or ethyl acetate, washed with saturated aqueous
sodium bicarbonate, water, and brine, dried over anhydrous
absorption maxima, where red-shifts of less than 10 nm are seen
H4,C6F5
for both (H ,CF L)H and (
L)H relative to (H ,MesL)H.
4
3
4
The molar absorptivities (ε) of compounds 1À9 range from
(2.5 to 4.3) Â 104 MÀ1 cmÀ1 with no discernible pattern related
3
to halogenation or 5-substitution (Figure 3).
sodium sulfate, filtered, and concentrated in vacuo to afford
A number of reports describe the absorption spectra of
chlorinated and brominated (though not, to our knowledge,
iodinated) free-base porphyrins, all of which display bathochro-
mic shifts of their absorption maxima upon halogenation,
analogous to the shifts seen in our study.8,23 Electronegative
substituents such as halogens have a stabilizing effect on the
energy of both the HOMO and the LUMO of conjugated
systems, and when a differential stabilization of these two orbitals
occurs, a shift is observed. Calculations performed by Gray and
co-workers24 and separately by Nguyen and co-workers25 have
suggested that halogenation has an approximately equivalent
effect on both the HOMO and the LUMO of porphyrins, and
therefore, electronic effects seem to play a minor role in the
changes in the absorption spectra of these species upon halo-
genation. Steric effects, however, are very pronounced in halo-
genated porphyrins, and the introduction of chlorine or bromine
Cl4,C6F5
(
L)H (8) and (Br4,C6F5L)H (9). All ligands (1À9) were
characterized by 1H NMR, 13C NMR, 19F NMR (where
applicable), UV/visible spectroscopy, and high-resolution elec-
trospray ionization mass spectrometry (HR-ESI-MS), in which
all parent molecular ions [M + H]+ were detected. Full experi-
mental details and analytical data can be found in the Supporting
Information.
2.2. Characterization of Substituted Dipyrrins (β,ArL)H
(1À9). Multinuclear NMR spectra (1H, 13C, and 19F where
applicable) of all the dipyrrins show C2 symmetry on the NMR
time scale, indicative of rapid tautomerization and symmetrical
halogenation. The dipyrrin NÀH proton is characteristically
shifted downfield, ranging from 12.2 to 13.9 ppm in CDCl3.
CarbonÀfluorine coupling constants (1JCÀF = 272, 2JCÀF = 32,
3
and JCÀF = 3.8 Hz, in accord with previous reports of similar
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Inorganic Chemistry
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Figure 4. Effect of halogenation on dipyrrin oxidation potential. Cyclic
voltammograms obtained in CH2Cl2 at 25 °C, with 0.1 M (nBu4N)(PF6) as
the supporting electrolyte and a scan rate of e250 mV/s.
Figure 5. Correlation between electronegativity of pyrrole backbone
substituents and oxidation potential. Electronegativity values taken from
the ACS online periodic table22 (H = 2.20; Cl = 3.16; Br = 2.96; I = 2.66).
For reversible processes, E1/2 is reported; for quasi- or irreversible
processes, the potential at peak half-maximum is reported. Values of Eox
for 7À9 are normalized to account for the effect of 5-position variation.
substituents on the periphery leads to significant saddling
distortions of the porphyrin macrocycle,26 causing major devia-
tions from planarity. The amplitude of these deviations correlates
with the size and number of the halogen substituents, and it is to
this geometric distortion that Gray and co-workers largely
attribute the bathochromic shifts in the absorbance spectra.24
It is unclear at this time whether this explanation holds for
halogenated dipyrrins, which have more degrees of geometric
freedom than the macrocyclic porphyrins.
E1/2 = À2.0 V vs [Cp2Fe]+/0. Introduction of electron-with-
drawing substituents in the β- and/or 5-position made reduction
more facile. 5-Fluoroarylation shifted the reduction potentials
by +100 to +200 mV, and tetrahalogenation shifted the reduction
potential by roughly +450 mV. It is interesting to note that most
of these reduction events are reversible, indicating that stable
halide anions are not expelled on the time-scale of the voltam-
metric experiment. Cyclic voltammograms can be found in the
Supporting Information (Figures S19ÀS27). The redox activity
of the nonmetalated proligands indicates the potential for ligand
participation in redox processes of the derivative iron complexes.
Explorations of this reactivity are currently underway in our lab.
Studies of the redox properties of dipyrrins are numerous in
the early literature, where both oxidation and reduction events
have been recorded for the vast majority of species,21,27 but there
have been, to our knowledge, no reports on the effect of halo-
genation on these redox properties. There are, however, several
studies of the redox properties of halogenated porphyrins,24,28
and the trends in such systems parallel those observed for
halogenated dipyrrins, where the β-incorporation of halogens
anodically shifts both ligand oxidation and reduction processes
where a correlation can be drawn to the electronegativity of the
halogen. Similar studies have shown the effects of meso-fluoro-
arylation of porphyrins,29 again paralleling the trends observed
for the dipyrrins in this study.
Cyclic voltammetry revealed reversible or quasi-reversible
oxidation events for each dipyrrin (0.1 mM analyte concentra-
tion in dichloromethane, 0.1 M (nBu4N)(PF6), scan rate =
250 mV/s, glassy carbon working electrode, nonaqueous Ag+/Ag
reference electrode, and platinum wire counterelectrode; under a
nitrogen atmosphere at ambient temperature). The onset of
dipyrrin oxidation was seen at 470 mV versus [Cp2Fe]+/0 for the
parent proligand (H ,MesL)H, and was shifted +100 mV by the
4
introduction of 5-fluoroaryl groups. Halogenation had a more
pronounced effect on the oxidation potential of the proligand,
anodically shifting the oxidation by as much as +500 mV for the
most electronegative chlorine substituents (Figure 4). There was
a correlation between the sum of the Pauling electronegativities
of the pyrrole backbone (β position) substituents and the
oxidation potential of the dipyrrin (Figure 5, Table 2); as more
electronegative substituents are added to the dipyrrin backbone,
the proligand becomes significantly more difficult to oxidize. The
effects of 5-substitution and β-halogenation are additive; the
greatest anodic shift is observed for the 5-perfluorophenyl
tetrachlorodipyrrin (Cl ,C F L)H, shifting the proligand oxidation
2.3. Synthesis of Fe Complexes (β,ArL)FeCl(py). Initially,
tetrahydrofuran adducts of iron dipyrrinato complexes were
targeted,7a but these species did not exhibit the stability desired
to acquire a full suite of spectroscopic data, as significant decom-
position of the complexes (even in the solid state) was apparent
after several days. As such, the less labile pyridine adducts were
targeted to record all the metrical data presented herein. Under a
nitrogen atmosphere, deprotonation of the proligands (1À9)
(see Scheme 2) with 1 equiv of lithium hexamethyldisilazide in
thawing THF at 77 K afforded the lithium salts, which have been
4
6 5
nearly +600 mV from (H ,MesL)H. In the cases of the nonhalo-
4
H4,C6F5
genated compounds [(H ,MesL)H, (H ,CF L)H, and (
L)H],
4
4
3
a second one-electron, quasi-reversible oxidation event could be
observed (see Figures S19, S24, and S25 in the Supporting
Information).
Electrochemical reduction of the ligands was also reversible
or quasi-reversible across the series, with the exception of
Cl4,C6F5
(
L)H, for which reduction was irreversible. The parent
proligand (H ,MesL)H was the most difficult to reduce, with
4
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Table 2. Characterization Data of Dipyrrin Proligands
Ar
R1/R2
abbrev
λmaxa (nm)
εb (MÀ1 cmÀ1
)
E1/2(ox)c (V)
E1/2(red)c (V)
∑ENe (Pauling)
3
H ,MesL)H
464.0
480.0
471.0
485.0
499.4
472.0
473.0
490.0
497.0
31,000
36,000
38,000
26,000
43,000
27,000
32,000
27,000
31,000
0.520d
1.018
0.842
0.994
0.925
0.636d
0.676d
1.144
1.139
À1.999
À1.558
À1.763
À1.544
À1.547
À1.720
À1.707
À1.264d
À1.251
8.80
12.64
10.32
11.84
10.64
8.80
4
1
2
3
4
5
6
7
8
9
Mes
Mes
H/H
Cl/Cl
Br/H
Br/Br
I/I
(
(
(
(
(
(
(
(
(
Cl ,MesL)H
4
Br ,MesL)H
2
Mes
Br ,MesL)H
4
Mes
Mes
I ,MesL)H
H4,CF3
4
3,5-(CF3)2C6H3
C6F5
H/H
H/H
Cl/Cl
Br/Br
L)H
H4,C6F5
L)H
8.80
Cl4,C6F5
C6F5
L)H
12.64
11.84
Br4,C6F5
C6F5
L)H
a UV/vis spectra obtained in dichloromethane at 25 °C with a maximum scan rate of 600 nm/min. b Average extinction coefficients calculated from a
minimum of four concentrations. c Cyclic voltammetry performed in dichloromethane containing 0.1 M (nBu4N)(PF6) at 25 °C and a maximum scan
rate of 250 mV/s; referenced vs [Cp2Fe]+/0 d Irreversible; values reported are the potentials at peak half-maximum. e The sum of the Pauling
.
electronegativity values of the β position substituents, taken from the ACS online periodic table; H = 2.20, Cl = 3.16, Br = 2.96, I = 2.66.22
Scheme 2. Deprotonation and Metalation
shown to be effective precursors for transmetalation to transition
metals7,30 (1H NMR spectral data are presented in the Support-
ing Information). After concentration in vacuo, the salts were
isolated as dark red THF adducts with 1À2 molecules of THF
1
coordinating the Li, as determined by H NMR spectroscopy.
Reaction of the lithium salts with 1.1 equiv of FeCl2 in thawing
THF solutions in the presence of excess pyridine effected
transmetalation to iron. The mixtures were concentrated to
dryness in vacuo, dissolved in benzene, filtered through a plug
of diatomaceous earth to remove insoluble material, and again
concentrated to dryness. Washing the complexes with a minimal
amount of cold (À35 °C) hexanes or hexamethyldisiloxane
afforded the clean iron complexes in the following good yields
(51À95%): (H ,MesL)FeCl(py) (10, 95%); (Cl ,MesL)FeCl(py)
4
4
(11, 77%); (Br ,MesL)FeCl(py) (12, 81%); (Br ,MesL)FeCl(py)
2
4
4
4
3
Figure 6. Solid-state molecular structure of (H ,CF L)FeCl(py) (15)
with ellipsoids drawn at the 50% probability level (H atoms omitted for
clarity; C, black; N, blue; Fe, orange; F, yellow-green; Cl, green). Bond
lengths (Å) for 15: FeÀN1, 2.026(2); FeÀN2, 2.030(2); FeÀN3,
2.098(2); FeÀCl, 2.2304(8).
4
3
(13, 67%); (I ,MesL)FeCl(py) (14, 51%); (H ,CF L)FeCl(py) (15,
89%, the structure of which is shown in Figure 6); (H ,C F L)FeCl-
4
6 5
Br4,C6F5
(py) (16, 87%); (Cl ,C F L)FeCl(py) (17, 81%); (
L)FeCl-
4
6 5
(py) (18, 76%). In some cases, trace amounts (<5%) of free ligand
remained. The iron complexes ranged from dark orange—
H4,C6F5
(
H ,MesL)FeCl(py), (H ,CF L)FeCl(py), and (
L)FeCl(py)—
4
4
3
to very dark pink—(I ,MesL)FeCl(py) and (Br ,C F L)FeCl(py)—in
solution and as amorphous solids, but the were lustrous green to
green-brown solids in the microcrystalline state. All complexes were
significant paramagnetic broadening due to the presence of high-
spin FeII (S = 2);31 typical spectra were observed in a spectral
window of À40 to +60 ppm. Complexes with 5-fluoroaryl
substituents (15À18) gave useful 19F NMR spectra as well. In
4
4
6
5
characterized by H NMR, 19F NMR (where applicable), UV/
1
the case of (H ,CF L)FeCl(py) (15), the two trifluoromethyl
groups gave rise to two distinct signals, indicating an unexpected
lack of free rotation of the bis(trifluoromethyl)phenyl substitu-
ent. The corresponding 1H NMR spectrum also displayed more
paramagnetically shifted peaks than anticipated, but with the
visible spectroscopy, cyclic voltammetry, 57Fe M€ossbauer spectros-
copy, and combustion analysis (see the Supporting Information for
details).
4
3
2.4. Multinuclear NMR Spectra of (β,ArL)FeCl(py) Com-
plexes. 1H NMR spectra of all complexes in the series showed
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Inorganic Chemistry
ARTICLE
Table 3. Yields of Iron Complexes
abbrev
Ar
R1/R2
yield (%)
4
10
11
12
13
14
15
16
17
18
(
(
(
(
(
(
(
(
(
H ,MesL)FeCl(py)
Mes
Mes
H/H
Cl/Cl
Br/H
Br/Br
I/I
95
77
81
67
51
89
87
81
76
Cl ,MesL)FeCl(py)
4
Br ,MesL)FeCl(py)
Mes
2
Br ,MesL)FeCl(py)
Mes
4
I ,MesL)FeCl(py)
H4,CF3
Mes
4
L)FeCl(py)
3,5-(CF3)2C6H3
C6F5
H/H
H/H
Cl/Cl
Br/Br
H4,C6F5
L)FeCl(py)
Cl4,C6F5
L)FeCl(py)
C6F5
Br4,C6F5
L)FeCl(py)
C6F5
Figure 7. Representative UV/vis spectra for complexes 10, 13, 16, and
18. Spectra obtained at room temperature in dichloromethane; average
ε values calculated from a minimum of four concentrations.
Figure 8. FeIII/II couples of representative complexes. Cyclic voltam-
mograms obtained in THF with 0.3 M (nBu4N)(PF6) as supporting
electrolyte with a scan rate of 100 mV/s.
larger number of peaks and spectral broadening, the inequivalent
Figure 9. Zero-field 57Fe M€ossbauer spectra obtained at 110 K for the
ortho-hydrogen peaks could not be unambiguously identified.
following compounds: (Br ,MesL)FeCl(py), 12 (a); (Cl ,C F L)FeCl(py),
The 5-perfluorophenyl dipyrrinato complexes 16À18 (β,C F L)-
FeCl(py) uniformly showed three peaks for the aryl fluorine
nuclei, indicating that rotation in these complexes is not hindered
as in 15, although there is precedent for lack of free rotation
observed in octahalogenated meso-tetrakis(perfluorophenyl)
porphyrins.32 The peak for the para-fluorine was consistently
sharp, but the meta- and ortho-fluorine peaks were affected by
paramagnetic broadening due to their closer proximity to the FeII
center.33
2
4
6 5
6
5
H4,C6F5
17 (b); (H ,CF L)FeCl(py), 15 (c); (
L)FeCl(py), 16 (d);
4
3
4
4
4
(
I ,MesL)FeCl(py), 14 (e); (H ,MesL)FeCl(py), 10 (f); (Cl ,MesL)FeCl-
(py), 11 (g); and (Br ,MesL)FeCl(py), 13 (h).
4
primarily ligand-based π f π* transitions.34 The less intense of
the two absorptions is red-shifted from the free ligand absorption
bands and appears as a shoulder under the more prominent,
slightly lower-energy π f π* transition. The more intense
π f π* absorptions (ε = 5.4 Â 104 to 1.3 Â 105 MÀ1 cmÀ1
)
2.5. Electronic Absorption Spectra of (β,ArL)FeCl(py) Com-
plexes. Each of the iron complexes displayed two prominent
absorption bands in the UV/visible region, both attributable to
occur with the absorption maxima ranging from 506 nm for
(
H ,MesL)FeCl(py) (10) to 548 nm for (Br ,C F L)FeCl(py) (18)
4
4
6 5
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Inorganic Chemistry
ARTICLE
Table 4. Characterization Data of Iron Dipyrrinato Complexes (10À18)
abbrev
Ar
R1/R2
E1/2 FeIII/II a (mV)
λmaxb (nm)
εc (MÀ1 cmÀ1
)
δd (mm/s)
|ΔEQ| (mm/s)
3
4
10
11
12
13
14
15
16
17
18
(
(
(
(
(
(
(
(
(
H ,MesL)FeCl(py)
Mes
Mes
H/H
Cl/Cl
Br/H
Br/Br
I/I
À336
À79
506.0
524.5
528.0
530.5
545.6
508.5
516.7
541.5
547.5
73,000
83,000
97,000
130,000
83,000
62,000
84,000
54,000
95,000
0.87
0.87
0.86e
0.88e
0.89e
0.89
0.89
2.19
2.9
Cl ,MesL)FeCl(py)
4
Br ,MesL)FeCl(py)
Mes
À154
À95
1.46e
3.24e
1.89e
1.74
1.92
2
Br ,MesL)FeCl(py)
Mes
4
I ,MesL)FeCl(py)
H4,CF3
Mes
À170
À238
À227
+50
4
L)FeCl(py)
3,5-(CF3)2C6H3
C6F5
H/H
H/H
Cl/Cl
Br/Br
H4,C6F5
L)FeCl(py)
Cl4,C6F5
L)FeCl(py)
C6F5
0.87
f
1.35
f
Br4,C6F5
L)FeCl(py)
C6F5
+20
a Cyclic voltammetry performed in THF containing 0.3 M (nBu4N)(PF6) at 25 °C. b UV/vis spectra obtained in CH2Cl2 at 25 °C with a scan rate of
300 nm/min. c Average extinction coefficients calculated from a minimum of four concentrations. d M€ossbauer spectra obtained on samples suspended in
Paratone at 110 K and fit with Lorentzian functions in IGOR Pro. e Bromine- and iodine-containing samples showed poor M€ossbauer absorbance at 110
K and a variety of concentrations, so signal-to-noise ratios were poor for these samples; however, the fits obtained were reproducible. f The signal-to-
noise ratio of 18 was too poor to obtain reproducible M€ossbauer parameters.
(Figure 7) and follow patterns analogous to those described for
the proligands (vide supra).
the β-protio and β-chlorinated samples could be obtained much
more rapidly and with smaller samples (40À50 mg). The
spectrum for the parent complex (H ,MesL)FeCl(py) (10) con-
4
The optical properties of dipyrrinato-metal complexes are of
particular importance, as these complexes are receiving attention
as fluorophores4a,c,35 and phosphorophores.36 While our iron com-
plexes do not display any appreciable fluorescence or phosphor-
escence, the ability to tune absorption maxima by simple
transformations such as β-halogenation or 5-fluoroaryl substitu-
tion may have implications for the analogous systems. Addition-
ally, incorporation of bromine or iodine provides a functional
group handle for the synthesis of other dipyrrinato derivatives
(by metal-catalyzed cross-coupling reactions), which may allow
for a wider range of wavelengths, absorptivities, and emissivities
to be accessed.
tains a single quadrupole doublet that can be modeled with
parameters typical of high-spin, tetrahedral FeII complexes:
δ = 0.87 mm/s and |ΔEQ| = 2.19 mm/s. The isomer shifts (δ)
of the other complexes in this series were nearly constant and
varied by no more than 0.02 mm/s from those for 10 (see Table 3
for spectral parameters), which is consistent with an FeII
formulation despite the potential for ligand-based redox activity
as demonstrated by cyclic voltammetry (vide supra). Though
there was little perturbation of the isomer shift for complexes
10À18, the quadrupole splitting parameters (|ΔEQ|) for the
series span a range of values, from a minimum of 1.35 mm/s for
Cl4,C6F5
2.6. Cyclic Voltammetry of (β,ArL)FeCl(py) Complexes. The
iron complexes in this study (10À18) showed fully reversible
FeIII/II couples by cyclic voltammetry (0.1 mM analyte concen-
tration in THF, 0.3 M (nBu4N)(PF6), scan rate = 100 mV/s,
glassy carbon working electrode, nonaqueous Ag+/Ag reference
electrode, and platinum wire counterelectrode; under a nitrogen
atmosphere at ambient temperature; Figures S28ÀS36 in the
Supporting Information). Both β-halogenation and 5-fluoroaryl
substitution anodically shift the oxidation potential of the com-
4
(
L)FeCl(py) to a maximum of 3.24 mm/s for (Br ,MesL)-
FeCl(py) (see Figure 9 and Table 4). Although both the absorp-
tion maxima and cyclic voltammetry data varied as a function of
β-halogen size and/or electronegativity, no correlation for the
quadrupole splitting based on these considerations was imme-
diately forthcoming. Quadrupole splitting reflects the electric
field gradient at the iron nucleus and is therefore a function of the
asymmetry around the iron center and the geometry of the
complex.39 We surmise that the β-halogenation gives rise to
significant geometric variation within the series, accounting for
the large range of quadrupole splitting parameters observed.
plexes relative to that of the parent complex (H ,MesL)FeCl(py).
4
Representative complexes from the series 10À18 are shown
overlaid in Figure 8. The introduction of a 5-fluoroaryl group
raised the oxidation potential by roughly +100 mV, while β-
halogenation raised the potential by +257 mV in (Cl ,MesL)FeCl-
3. CONCLUSIONS
4
(py) (11), +182 mV in (Br ,MesL)FeCl(py) (12), +241 mV in
Systematic electronic variations were introduced into the
monoanionic dipyrrin ligand scaffold via halogenation of the
pyrrolic β-positions and/or via the use of fluorinated aryl sub-
stituents in the ligand bridgehead position yielding proligands of
the type (β,ArL)H. The electronic perturbations were probed using
standard spectroscopic and electrochemical techniques on the
different proligand variations and their divalent iron complexes.
The proligand variations display modest shifts in the electronic
absorption maxima and a more pronounced effect in the
electrochemical redox potentials for the one-electron ligand
reductions and oxidations. The electrochemical and spectro-
scopic behavior of the iron-bound complexes varies significantly
across the series; most notably the redox potential of the fully
reversible FeIII/II couple spans more than 400 mV as electron-
withdrawing substituents are appended to the dipyrrinato platform.
4
Br4,Mes
L)FeCl(py) (13), and +166 mV in (I ,MesL)FeCl(py)
4
(
(14), where less electronegative halogens impart a smaller anodic
shift. As in the nonmetalated dipyrrin proligands, these effects
were additive, and the most difficult species to oxidize was
Cl4,C6F5
(
L)FeCl(py) (17), which displays a +386 mV anodic shift
from the parent compound (H ,MesL)FeCl(py) (10).
4
2.7. Zero-Field Fe Mossbauer Spectroscopy of (β,ArL)-
57
€
FeCl(py) Complexes. Acquisition of zero-field 57Fe M€ossbauer
spectra was slow because of the poor absorbance of several
samples due to the relatively low percent by mass of iron in the
brominated and iodinated complexes, as well as the low Debye
temperature of tetrahedral FeII complexes.37 However, accepta-
ble spectra could be obtained with 80À100 mg of these com-
plexes at 110 K38 and acquisition times of several days. Spectra of
6843
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Inorganic Chemistry
ARTICLE
These effects demonstrate how the peripheral variation of the
dipyrrinato ligand scaffold can allow the systematic variation of
the chemical and physical properties of iron dipyrrinato com-
plexes. This last observation is in stark contrast to the nonconju-
gated dipyrromethane and tris(pyrrolyl)ethane metal complexes
which show no variation in the ligand-based redox behavior
regardless of the complex metal-ion inclusion, complex spin-
state, or geometry.
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’ ACKNOWLEDGMENT
The authors thank the NSF (CHE-0955885) and Harvard
University for financial support; Guy Edouard, Danielle Raad,
Sam Goldstein, and Evan King for the synthesis of starting
materials; Graham Sazama, Dr. Alison Fout, and Dr. Shao-Liang
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