Synthesis and characterization of iron derivatives having a pyridine-linked bis(anilide) pincer ligand

Article abstract of DOI:10.1021/ic900083s

A pyridine-linked bis(aniline) pincer ligand, [2]H₂ ([2]H ₂ ) (2,6-NC₅H₃(2-(2,4,6-Me₃C ₆H₂)-NHC₆H₄)₂), has been synthesized in two steps. Deprotonation with Me₃SiCH₂Li followed by metalation with FeCl₂ yielded a LiCl adduct of [2]Fe. The complex is freed of LiCl with excess TlPF₆ or by crystallization from toluene/petroleum ether, giving [2]Fe(THF). [2]Fe(THF) reacts with I ₂ and O₂ to generate [2]FeI and ([2]Fe)₂O, respectively. The complexes have been characterized by ¹H NMR spectroscopy, elemental analysis, X-ray crystallography, and UV-vis spectroscopy. [2]Fe(THF) has been examined using cyclic voltammetry.

Full text of DOI:10.1021/ic900083s

Inorg. Chem. 2009, 48, 3808-3813
Synthesis and Characterization of Iron Derivatives Having a
Pyridine-Linked Bis(anilide) Pincer Ligand
Edward C. Weintrob, Daniel Tofan, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of
Technology, Pasadena, California 91125
Received January 15, 2009
A pyridine-linked bis(aniline) pincer ligand, [2]H2 ([2]H2 ) (2,6-NC5H3(2-(2,4,6-Me3C6H2)-NHC6H4)2), has been
synthesized in two steps. Deprotonation with Me3SiCH2Li followed by metalation with FeCl2 yielded a LiCl adduct
of [2]Fe. The complex is freed of LiCl with excess TlPF6 or by crystallization from toluene/petroleum ether, giving
[2]Fe(THF). [2]Fe(THF) reacts with I2 and O2 to generate [2]FeI and ([2]Fe)2O, respectively. The complexes have
been characterized by 1H NMR spectroscopy, elemental analysis, X-ray crystallography, and UV-vis spectroscopy.
[2]Fe(THF) has been examined using cyclic voltammetry.
Introduction
cross-coupling.15-17 Additionally, the well-studied area of
oxidation chemistry with iron complexes has made great
strides in mechanistic understanding, substrate scope, chemo-,
regio-, and enantioselectivity, and pertinence to industrial
applications.18-22
The new ligand investigated herein, [2]H2 ([2]H2 ) (2,6-
NC5H3(2-(2,4,6-Me3C6H2)-NHC6H4)2), possesses two fea-
tures which may engender unique properties upon complex-
ation with iron. First, the ligand forms two six-membered
chelate rings with the metal center. This ring size is less
common than the five-membered chelated rings found in
many iron chelates. Second, the deprotonated ligand [2]2-
is dianionic, unlike the neutral or monoanionic ligands more
common to iron chemistry.
Noble metals have been dominant in many areas of
catalytic organic transformations: a fact that may be partially
due to their readily accessible two electron redox couples,
as well as their oxygen, water, and functional group
tolerance.1 These abilities are counterbalanced, however, by
those same metals’ cost, and in some cases by environmental
or toxicological considerations. By comparison, iron is cheap,
relatively nontoxic, and environmentally benign.2 For these
reasons, this decade has seen an emergence of research in
areas not historically associated with iron,3-6 including
hydrogenation,7-13 hydrosilation,8 hydrodefluorination,14 and
* To whom correspondence should be addressed. E-mail: bercaw@
caltech.edu.
(1) Fu¨rstner, A.; Majima, K.; Martin, R.; Krause, H.; Kattnig, E.; Goddard,
R.; Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 1992–2004.
(2) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
3317–3321.
The relative rarity of the two aforementioned motifs is
surprising in light of the rich and diverse chemistry available
(12) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816–5817.
(13) Mikhailine, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2009,
131, 1394–1395.
(14) Vela, J.; Smith, J. M.; Yu, Y.; Ketterer, N. A.; Flaschenriem, C. J.;
Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2005, 127, 7857–
7870.
(3) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. ReV. 2004, 104,
6217–6254.
(4) Correa, A.; Manchen˜o, O.; Bolm, C. Chem. Soc. ReV. 2008, 37, 1108–
1117.
(5) Buschhorn, D.; Pink, M.; Fan, H.; Caulton, K. G. Inorg. Chem. 2008,
47, 5129–5135.
(6) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Rettig, S. J.; Young, V.
G, Jr. Organometallics 1998, 17, 2313–2323.
(7) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474–7485.
(8) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794–13807.
(15) Fu¨rstner, A.; Martin, R. Chem. Lett. 2005, 34, 624–629.
(16) Fu¨rstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.; Lehmann,
C. W. J. Am. Chem. Soc. 2008, 130, 8773–8787.
(17) Czaplik, W. M.; Mayer, M.; von Wangelin, A. J. Angew. Chem., Int.
Ed. 2009, 48, 607–610.
(18) White, M. C.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2001,
123, 7194–7195.
(9) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006, 45,
7252–7260.
(19) Chen, J.; Woo, L. K. J. Organomet. Chem. 2000, 601, 57–68.
(20) Chen, M. S.; White, M. C. Science 2007, 318, 783–787.
(21) Suzuki, K.; Oldenburg, P. D., Jr. Angew. Chem., Int. Ed. 2008, 47,
1887–1889.
(22) Gedalcha, F. G.; Bitterlich, B.; Anilkumar, G.; Tse, M. K.; Beller, M.
Angew. Chem., Int. Ed. 2007, 46, 7293–7296.
(10) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organome-
tallics 2005, 24, 5518–5527.
(11) Enthaler, S.; Hagemann, B.; Erre, G.; Junge, K.; Beller,
M
Chem.sAsian. J. 2006, 1, 598–604.
3808 Inorganic Chemistry, Vol. 48, No. 8, 2009
10.1021/ic900083s CCC: $40.75  2009 American Chemical Society
Published on Web 03/20/2009

Pyridine-Linked Bis(anilide) Pincer Ligand
Scheme 1. Synthesis of [2]H2
Figure 1. Structure of [2]Fe(THF) with displacement ellipsoids at the 50%
probability level. Hydrogen atoms were omitted for clarity. Selected bond
lengths (Å) and angles(deg): Fe-N3, 1.928(1); Fe-N1, 1.932(1); Fe-N2,
2.036(1); Fe-O, 2.113(1); N3-Fe-N1, 139.52(4); N3-Fe-N2, 94.97(4);
N1-Fe-N2, 96.44(4); N3-Fe-O, 110.45(4); N1-Fe-O, 106.54(4);
N2-Fe-O, 96.85(4); Sum of angles N3-Fe-N1, N3-Fe-O, and
N1-Fe-O ) 356.5°.
Scheme 2. Synthesis of [2]Fe(THF)
is close to the spin-only value of 4.9 µB for a quintet ground
state. X-ray quality crystals were obtained by cooling a
saturated toluene/petroleum ether solution at -30 °C over-
night. [2]Fe(THF) (Figure 1) does not closely resemble any
regular polyhedron, but can best be described as having a
distorted trigonal monopyramidal geometry, with the THF
and the anilide nitrogens forming the basal plane and the
pyridine nitrogen at the apex. Tetrahedral, and to a lesser
extent square planar, are the dominant geometries for four-
coordinate iron. In contrast, there are only a handful of
structurally characterized iron complexes with a trigonal
monopyramidal geometry.25-28 Although the sum of the
angles in the basal plane is close to 360°, the individual
angles are all quite different from 120° (approximately 140°,
110°, 107°).
The binding pocket of the ligand is too small to accom-
modate a C2V-type ligand geometry, which is partially a result
of the two six-membered chelate rings that are formed upon
metalation. Chirik has reported several iron compounds from
a bis(enamide)pyridine ligand with two five-membered
chelate rings;29 these complexes comprise the only other
crystallographically characterized iron pincer compounds
with anilide arms. The most useful comparison (Figure 2)
may be made with his pyridinebis(anilide) iron (II) amine
adduct 3. In contrast to [2]Fe(THF), 3 supports a slightly
to iron porphyrins,23 which are dianionic and form six-
membered rings upon chelation. With respect to that goal,
we report herein a chelating pyridine-linked bis(aniline)
ligand and iron complexes derived therefrom.
Results and Discussion
The synthesis of the ligand precursor 1 was ac-
complished via in situ borylation of 2-bromoaniline,
followed by a 2-fold Suzuki coupling with 2,6-dibro-
mopyridine. The arylation of 1 with an excess of mesityl
bromide was accomplished with Buchwald-Hartwig
coupling, yielding the ligand [2]H2 (Scheme 1). X-ray
quality crystals of the ligand were obtained by slow
evaporation of a diethyl ether solution (see Supporting
Information). [2]H2 was deprotonated with a slight excess
of trimethylsilylmethyllithium in toluene, giving the
dilithium salt [2]Li2 as a bright yellow solid (Scheme 2).
[2]Li2 was subsequently allowed to react with anhydrous
ferrous chloride in tetrahydrofuran (THF) for 1 day, yielding
a lithium chloride adduct of [2]Fe. Lithium chloride-free
[2]Fe(THF) may be obtained by addition of thallium
hexafluorophosphate or by crystallization from a toluene-
petroleum ether mixture. Reaction of [2]Fe(THF) with
lithium chloride in THF regenerates the lithium chloride
adduct.
(25) Hung, C.-H.; Chang, F.-C.; Lin, C.-Y.; Rachlewicz, K.; Steˆpien˜, M.;
Latos-Graz˙yn˜ski, L.; Lee, G.-S.; Peng, S.-M. Inorg. Chem. 2004, 43,
4118–4120.
(26) Cotton, F. A.; Daniels, L. M.; Falvello, L. R.; Matonic, J. H.; Murillo,
C. A. Inorg. Chim. Acta 1997, 256, 269–275.
(27) Govindaswamy, N.; Quarless Jr., D. A.; Koch, S. A. J. Am. Chem.
Soc. 1995, 117, 8468–8469.
(28) Ray, M.; Golombek, A. P.; Hendrich, M. P.; Young, V. G., Jr.;
Borovik, A. S. J. Am. Chem. Soc. 1996, 118, 6084–6085.
(29) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006,
45, 2–4.
(30) The peak current is inversely proportional to the root of the scan rate
(see Supporting Information), which is consistent with the Cottrell
equation for diffusion controlled waves, Bard, A. J.; Faulkner, L. R.
Electrochemical Methods. Fundamentals and Applications, 2nd ed.;
Wiley: New York, 2001.
The magnetic moment of [2]Fe(THF) was measured via
Evans method in benzene-d6,24 giving µeff ) 4.8 µB, which
(23) Simonneaux, G.; Tagliatesta, P. J. Porphyrins Phthalocyanines 2004,
8, 1166–1171.
(24) Evans, D. F. J. Chem. Soc. 1959, 2003, 2005.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3809

Weintrob et al.
Figure 2. Comparison between five and six-membered chelate rings in
pyridinebis(anilide) iron complexes.
Figure 4. Structure of [2]FeI with displacement ellipsoids at the 50%
probability level. Two virtually identical [2]FeI molecules were present in
the asymmetric unit. Hydrogen atoms, solvent molecules, and the other
[2]FeI molecule were omitted for clarity. Selected bond lengths (Å) and
angles(deg): Fe-N1, 1.8993(1); Fe-N2, 1.8834(1); Fe-N3, 2.0274(1);
Fe-I, 2.5784(1); N1-Fe-N2, 118.297(5); N1-Fe-N3, 93.436(4);
N2-Fe-N3, 94.660(4); N1-Fe-I, 115.771(4); N2-Fe-I, 118.771(4);
N3-Fe-I, 108.740(4).
Figure 3. Voltammogram of 0.003 M [2]Fe(THF) in THF with 0.3 M
nBu4NBF4 as supporting electrolyte. Data were recorded at 100 mV/s, and
peaks were referenced to the ferrocene/ferrocenium couple.
Scheme 3. Synthesis of [2]FeI
distorted square planar configuration (the sum of the four
angles about iron is ∼361°).
Cyclic voltammetry studies were performed on [2]Fe(THF)
(Figure 3). A quasireversible, diffusion controlled30 reduction
occurs at E1/2 ) -0.96 V (versus the ferrocene/ferrocenium
couple), and an irreversible oxidation is also observed at
-0.37 V. Two new reduction waves (-0.92 V, -1.14 V)
are coupled to the irreversible oxidation event, which are
observed in addition to the original reversible reduction wave.
The electrochemical data suggest that the [2]Fe framework
is capable of supporting either reduced or oxidized species.
In light of the electrochemical data, chemical oxidation
of [2]Fe(THF) was attempted. Oxidation using molecular
iodine generates [2]FeI, which resembles a chelated version
of a previously reported complex (Scheme 3).31
Figure 5. Structure of ([2]Fe)2O (left) and iron coordination sphere (right)
with displacement ellipsoids at the 50% probability level. Hydrogen atoms
and solvent were omitted for clarity. Selected bond lengths (Å) and
angles(deg): Fe1-N1, 1.9174(1); Fe1-N2, 2.0161(1); Fe1-N3, 1.9262(1);
Fe1-O, 1.7802(1); Fe2-N4, 1.9263(1); Fe2-N5, 2.0154(1); Fe2-N6,
1.9234(1); Fe2-O, 1.7761(1); Fe1-O-Fe2, 156.601(3); N1-Fe1-N3,
117.501(2); N1-Fe1-N2, 92.864(2); N2-Fe1-N3, 95.140(2); N1-Fe1-O,
120.084(3); N3-Fe1-O, 116.843(3); N2-Fe1-O, 104.570; N6-Fe2-N4,
116.394(2); N6-Fe2-N5, 95.056(2); N5-Fe2-N4, 93.669; N6-Fe2-O,
117.267(3); N4-Fe2-O, 119.968(3); N5-Fe2-O, 106.367(2).
Scheme 4. Synthesis of ([2]Fe)2O
[2]FeI exhibits a solution magnetic moment of 5.8 µB in
benzene-d6, consistent with a sextet ground state. The solid
state structure of [2]FeI (Figure 4) reveals a more tetrahedral,
but still quite distorted geometry about the iron center, in
contrast to the more trigonal monopyramidal geometry
observed for [2]Fe(THF). Oxidation of [2]Fe(THF) via
dioxygen generates a bridging oxo dimer, ([2]Fe)2O (Scheme
4, Figure 5).
iron anilide nitrogen angles (118.13° and 117.50°, respec-
tively) than does [2]Fe(THF) (139.52°); the reasons for this
distortion are not immediately apparent.
In conclusion, the syntheses of iron complexes based on
the new ligand [2]2- has been accomplished. Investigations
of the reactivity of this class of compounds are currently
underway.
Bridging-oxo diiron complexes are well-known.32,33 [2]FeI
and ([2]Fe)2O possess significantly smaller anilide nitrogen
Experimental Section
(31) Stokes, S. L; Davis, W. M.; Odom, A. L.; Cummins, C. C.
Organometallics 1996, 15, 4521–4530.
(32) Kurtz, D. M, Jr. Chem. ReV. 1990, 90, 585–606.
(33) Fontecave, M.; Me´nage, S.; Duboc-Toia, C. Coord. Chem. ReV. 1998,
178-180, 1555–1572.
General Methods. Unless otherwise specified, air exposed solids
were dried under vacuum prior to use, liquids were degassed or
bubbled with argon, protio solvents were dried via Grubbs’
3810 Inorganic Chemistry, Vol. 48, No. 8, 2009

Pyridine-Linked Bis(anilide) Pincer Ligand
method,34 reagents were used as received from the supplier, and
reactions were performed under an inert atmosphere or vacuum.
All air and moisture sensitive compounds were handled using
standard glovebox, Schlenk, and high-vacuum line techniques.
Deuterated chloroform, benzene, and THF were obtained from
Cambridge Isotope Laboratories. Deuterated chloroform was used
as received and not stored under inert atmosphere. Deuterated
benzene and THF were dried with disodium benzophenone.
Deuterated benzene was subsequently dried using titanocene
dihydride. 2-Bromoaniline was obtained from Avocado. 2-(Dicy-
clohexylphosphino)biphenyl, tris(dibenzylideneacetone) dipalladium
(0), and thallium hexafluorophosphate were obtained from Strem.
Palladium (II) acetate, racemic 2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl (BINAP), sodium tert-butoxide, mesityl bromide, tri-
ethylamine, 2,6-dibromopyridine, 3 Å Linde type molecular sieves,
and calcium hydride were obtained from Aldrich. Sodium and
benzophenone were obtained from Lancaster and MCB reagents,
respectively. Mesityl bromide was dried on 3 Å Linde type
molecular sieves for 6 days prior to use. Triethylamine was stirred
on calcium hydride for several days, then vacuum transferred onto
3 Å Linde type molecular sieves prior to use. 1,4-Dioxane was
obtained from EMD, dried sequentially with 3 Å Linde type
molecular sieves and disodium benzophenone, then vacuum trans-
ferred before use. Pinacolborane was obtained from Aldrich or Alfa
Aesar, and stored at -30 °C. Barium hydroxide octahydrate was
obtained from Mallinckrodt, and stored open to the atmosphere.
Trimethylsilylmethyllithium was sublimed before use. Ferrous
chloride 99.99% was obtained from Aldrich as anhydrous beads.
NMR spectra were recorded on Varian Mercury 300 Megahertz
NMR spectrometers, and referenced according to the solvent
residual peak.35 Solution magnetic moments were determined via
Evans Method.24 The paramagnetism of the iron complexes
precluded assignment of peaks in their 1H NMR spectra. The
paramagnetism also implies that the integrations must only be
regarded as rough estimations. Electrochemical measurements were
performed using a glassy carbon rod as a working electrode, a
platinum wire as auxiliary electrode, and partitioned Ag/AgCl wire
as a pseudoreference electrode. Data were obtained using BAS100W
software, on a BAS100A Electrochemical Analyzer. 0.3 M
nBu4NBF4 THF solutions were employed, with 0.003 M concentra-
tion for the analyte. Ferrocene was employed as the internal
standard, and all potentials were referenced to the ferrocene/
ferrocenium couple. All data were obtained in an inert atmosphere
glovebox. X-ray diffraction data were obtained on a Bruker SMART
1000 or Bruker KAPPA APEXII. UV-vis spectra were recorded
on an Agilent 8453 UV-vis spectrometer. High resolution mass
spectra (HRMS) were obtained at the California Institute of
Technology Mass Spectral Facility using a JEOL JMS-600H
magnetic sector mass spectrometer. Elemental analyses were carried
rane (88 mL, 610 mmol) was cannulated into the addition funnel
and added dropwise to the stirring solution over 43 min. Upon
addition, vigorous bubbling occurred, and the solution turned olive
green. After addition was complete, the reaction mixture was heated
to 80 °C for 2.5 h, then allowed to cool to room temperature. Under
an argon purge, solid barium hydroxide octahydrate (195.2 g, 618.7
mmol) was slowly added until the bubbling ceased, then the
remainder was added. Subsequently a solution of 2,6-dibromopy-
ridine (21.58 g, 91.11 mmol) in 140 mL of dioxane was added via
cannula. Finally, 97 mL of deionized water was bubbled with argon
for approximately 10 min, then cannulated into the reaction mixture.
The reaction was heated to 101 °C for approximately 24 h, then
allowed to cool to room temperature. The solvent was removed in
vacuo. The remaining solid was repeatedly pulverized then washed
with methylene chloride (1.7 L) to remove trapped product from
insoluble barium hydroxide, as indicated by the washings no longer
darkening. The solution was extracted with an equal amount of
water, then concentrated in vacuo. The crude black sludge was
purified by column chromatography using methylene chloride and
ethyl acetate. Because the separation is somewhat nontrivial, it is
reported in detail in the Supporting Information. After chromatog-
raphy, the resulting yellow solid was washed repeatedly with large
amounts of diethyl ether until the ether lost most of its yellow color
(though the pure compound is very faintly yellow). After the
washings, the solid was dried in vacuo, giving 7.91 g of 1 as an
1
off-white solid in 33% yield. H NMR (CDCl3): δ 5.38 (s(broad),
2H, NH), 6.76 (dd, JH-H ) 8 Hz, 1 Hz, 2H, CH), 6.82 (td, JH-H
)
8 Hz, 1 Hz, 2H, CH), 7.19 (dd, JH-H ) 12 Hz, 7 Hz, CH), 7.52
(s(broad), 2H, CH), 7.54 (s(broad), 2H, CH), 7.86 (t, JH-H ) 8.0
Hz, 1H, p-NC5H3). 13C NMR (CDCl3): δ 117.1, 117.8, 120.2, 123.0,
129.9, 130.0, 138.1, 146.1, 157.7. HRMS (FAB+) m/z calcd for
C17H16N3: 262.1344. Found: 262.1348 (M + H), 245.0991 (M -
NH2).
37
Synthesis of Ligand [2]H2.
In a glovebox, 1 (7.91 g, 30.3
mmol), tris(dibenzylideneacetone)-dipalladium (0) (1.39 g, 1.51
mmol), rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (2.34 g,
3.75 mmol), sodium tert-butoxide (8.82 g, 91.7 mmol), 300 mL of
toluene, and mesityl bromide (56 mL, 360 mmol) were combined
in a 3-neck, 1 L round-bottom flask. The solution was refluxed
under an argon atmosphere for approximately 41 h. Under an argon
purge, the reaction was quenched with 20 mL of water, and then
concentrated in vacuo. Excess mesityl bromide was distilled away
by heating the solid at 70 °C under high vacuum. The solid was
washed with 50 mL of petroleum ether, then dissolved in 250 mL
of methylene chloride. The solution was extracted with an equal
amount of water, filtered, and concentrated in vacuo, giving a yellow
solid. Any palladium containing species were removed by passing
a methylene chloride solution of the product through 1 L of silica
gel packed with methylene chloride. The solution was concentrated
to a yellow solid. The solid was washed repeatedly with methanol
until the solid appeared off-white. [2]H2 was obtained in 46% yield
out by Desert Analytics, Tucson, AZ 85714.
36
Synthesis of Bis(aniline) 1.
In a glovebox, 2-bromoaniline
1
(34.48 g, 200.5 mmol), palladium (II) acetate (2.33 g, 10.4 mmol),
and 2-(dicyclohexylphosphino) biphenyl (13.80, 19.36 mmol) were
added to a 2 L, 3 neck round-bottom flask, and the flask was sealed.
Under an argon purge, the flask was equipped with a reflux
condenser and an addition funnel. Triethylamine (112 mL, 803
mmol) and 425 mL of dioxane were added via cannula. Pinacolbo-
(6.93 g). H NMR (C6D6): δ 2.01 (s, 12H, o-CH3), 2.15 (s, 6H,
p-CH3), 6.50 (d, JH-H ) 8 Hz, 2H, CH), 6.74 (s, 4H, mesityl-CH,
t, 2H, JH-H ) 7 Hz, CH), 7.03 (t, JH-H ) 8 Hz, 2H, CH), 7.2-7.4
(m, 3H, CH), 7.55 (d, JH-H ) 8 Hz, 2H, CH), 8.53 (s, 2H, NH).
1H NMR (d8-THF): δ 1.97 (s, 12H, o-CH3), 2.23 (s, 6H, p-CH3),
6.20 (d, JH-H ) 8 Hz, 2H, CH), 6.69 (t, JH-H ) 8 Hz, 2H, CH),
6.82 (s, 4H, CH), 7.02 (t, JH-H ) 8 Hz, 2H, CH), 7.59 (d, JH-H
)
(34) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(35) Gottlieb, H. E.; Vadim Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997,
62, 7512–7515.
8 Hz, 2H, CH), 7.76 (d, JH-H ) 8 Hz, 2H, CH), 7.97 (t, JH-H ) 8
Hz, 1H, p-NC5H3), 8.42 (s, 2H, NH). 1H NMR (CDCl3): δ 2.01 (s,
(36) The borylation and Suzuki coupling were based on a similar procedure
found in Rebstock, A. S.; Mongin, F.; Tre´court, F.; Que´guiner, G.
Org. Biomol. Chem. 2003, 1, 3064–3068.
(37) The Buchwald Hartwig coupling was based on a similar procedure
found in Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald,
S. L. J. Org. Chem. 2000, 65, 1158–1174.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3811

Weintrob et al.
12H, o-CH3), 2.27 (s, 6H, p-CH3), 6.27 (d, JH-H ) 8 Hz, 2H, CH),
6.76 (t, JH-H ) 7 Hz, 2H, CH), 6.85 (s, 4H, mesityl-CH), 7.09 (t,
JH-H ) 8 Hz, 2H, CH), 7.57 (d, JH-H ) 8 Hz, 2H, CH), 7.66 (d,
JH-H ) 8 Hz, 2H, CH), 7.91 (t, JH-H ) 8 Hz, 1H, p-NC5H3), 8.14
(s, 2H, NH).13C NMR (CDCl3): δ 18.5, 21.0, 113.2, 117.1, 120.8,
123.3, 129.1, 130.0, 130.1, 135.1, 136.0, 136.1, 137.9, 145.4, 158.1.
HRMS (FAB+) m/z calcd. for C35H35N3: 497.2831. Found:
497.2849 (M+), 482.2596 (M-CH3). UV-vis (THF, nm (M-1
cm-1)): 275 (sh), 354 (1.3 × 104). X-ray quality crystals were
obtained by slow evaporation of a saturated diethyl ether solution.
The crystal data are summarized as follows: formula, C35H35N3;
formula weight, 497.66; lattice system, monoclinic; space group
P21/n (No. 14); temperature 100 K; lattice parameters a )
12.1774(12) Å, b ) 8.3901(8) Å, c ) 27.039 (3) Å, ꢀ ) 93.803(2)°;
unit cell volume V ) 2756.5(5) Å3; calculated density Dcalc ) 1.199
g/cm3; number of molecules in the unit cell Z ) 4; linear absorption
coefficient µ ) 0.070 mm-1; no empirical absorption correction;
Mo KR radiation recorded on a Bruker SMART 1000 diffracto-
meter; 34841 reflections collected, 6476 unique reflections (4122
with I > 2σ(I)); θmax ) 28.41°; 349 parameters; 0 restraints; H atoms
were placed in calculated positions; all other atoms were refined
anisotropically, full matrix least-squares on F2 refinement method;
reliability factor R for all data ) 0.0900 (for data I > 2σ(I) )
0.0554), weighted reliability factor Rw ) 0.0939 (for data I > 2σ(I)
) 0.0907), goodness-of-fit on F2, 1.579. Crystallographic data have
been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. Copies can be obtained on request, free of charge, by quoting
the publication citation and the deposition number 65353 or by
visiting http://www.ccdc.cam.ac.uk/data_request/cif.
overnight. A 531 mg quantity of black crystalline [2]Fe(THF) was
obtained after drying in vacuo in 54% yield. H NMR (C6D6): δ
1
-52.03 (s, 2H, CH), -27.83 (s, 2H, CH), 2.85 (s, 4H), 33.41 (s,
4H), 47.42 (s, 2H, CH), 52.78 (s, 6H, p-CH3), 54.28 (s, 2H, CH),
55.40 (s, 2H, CH), 63.46 (s, 1H, p-NC5H3). 1H NMR (d8-THF): δ
-52.39 (s, 2H, CH), -31.47 (s, 2H, CH), 33.04 (s, 4H), 44.15 (s,
10H), 46.66 (s, 2H, CH), 52.35 (s, 2H, CH), 52.78 (s, 6H, CH3),
56.97 (s, 2H, CH), 66.25 (s, 1H, p-NC5H3). Anal. Calcd for
C39H41FeN3O: C, 75.11; H, 6.63; N, 6.74. Found1: C, 74.90; H,
6.74; N, 6.67. Found2: C, 73.72; H, 6.50; N, 6.64. CV (THF): E1/2
,
V versus Ferrocene at 100 mV/s (∆Ep, ipa/ipc): -0.96 V (90 mV,
0.88) and -0.37 V (irreversible). UV-vis (THF, nm (M-1 cm-1)):
275 (sh), 371 (1.1 × 104), 410 (sh).
X-ray quality crystals were obtained from a toluene/petroleum
ether solution at -30 °C. The crystal data are summarized as
follows: formula, C39H41N3OFe; formula weight, 623.60; lattice
system, monoclinic; space group P21/c (No. 14); temperature 100
K; lattice parameters a ) 14.5396(6) Å, b ) 13.5644(6) Å, c )
16.6014(7) Å, ꢀ ) 98.247(2)°; unit cell volume V ) 3240.3(2)
Å3; calculated density Dcalc ) 1.278 g/cm3; number of molecules
in the unit cell Z ) 4; linear absorption coefficient µ ) 0.501 mm-1
;
no empirical absorption correction; Mo KR radiation recorded on
a Bruker KAPPA APEX II diffractometer; 97719 reflections
collected, 14879 unique reflections (10317 with I > 2σ(I)); θmax
)
36.53°; 403 parameters; 0 restraints; H atoms were placed in
calculated positions; all other atoms were refined anisotropically,
full matrix least-squares on F2 refinement method; reliability factor
R for all data ) 0.0792 (for data I > 2σ(I) ) 0.0501), weighted
reliability factor Rw ) 0.0860 (for data I > 2σ(I) ) 0.0848),
goodness-of-fit on F2, 2.823. Crystallographic data have been
deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
Copies can be obtained on request, free of charge, by quoting the
publication citation and the deposition number 678268 or by visiting
http://www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of [2]Li2. In a glovebox, [2]H2 (2.222 g, 4.465 mmol)
and trimethylsilylmethyllithium (0.943 g, 10.01 mmol) were
combined as solids. On the high vacuum line, ∼100 mL of toluene
were vacuum transferred from a titanocene dihydride pot onto the
solids at -78 °C. After transfer was complete, the mixture was
stirred and allowed to warm to room temperature. Upon warming,
the solids dissolved. After 2 h, the orange solution was concentrated
in vacuo. In the glovebox, the orange solid was suspended in 30
mL of pentane (obtained by vacuum transfer from a disodium
benzophenone/tetraglyme pot). The orange suspension was cooled
via a liquid nitrogen cooled cold well, then filtered. The aforemen-
tioned procedure was repeated with an additional 30 mL of pentane.
The remaining solid was dried in vacuo, giving 2.256 g of [2]Li2
Synthesis of [2]FeI. [2]Fe(THF) (116.8 mg, 187.3 µmol) and
iodine (23.8 mg, 93.6 µmol) were combined as solids, dissolved in
10 mL of toluene, and left stirring for 15 min. The solution was
then concentrated in vacuo. A 126 mg quantity of [2]FeI was
isolated in 77% yield. X-ray quality crystals were obtained by
dissolution with benzene, dilution in an equal quantity of petroleum
ether, and cooling to -30 °C. The crystal data are summarized as
follows: formula, 2(C35H33N3FeI) 1.5(C6H6); formula weight,
1
j
as a bright yellow solid in 99% yield. H NMR (d8-THF): δ 1.65
1473.98; lattice system, triclinic; space group P1(No. 2); temper-
(s, 12H, o-CH3), 2.20 (s, 6H, p-CH3), 5.99 (d, JH-H )8 Hz, 2H,
o-anilide CH), 6.14 (t, JH-H )7 Hz, 2H, p-anilide CH), 6.58 (s,
4H, mesityl aryl-CH), 6.70 (t, JH-H )7 Hz, 2H, m-anilide CH para
to pyridine ring), 7.54 (d, JH-H ) 8 Hz, 2H, m-anilide CH ortho to
pyridine ring), 7.64 (d, JH-H ) 8 Hz, 2H, m-NC5H3), 7.84 (t, JH-H
) 8 Hz, p-NC5H3). 13C NMR (C6D6) δ 18.54, 20.91, 112.34, 116.36,
119.56, 120.74, 129.52, 130.53, 131.44, 131.92, 131.97, 139.62,
147.51, 156.84, 160.77. Anal. Calcd. for C35H33Li2N3: C, 82.50;
H, 6.53; N, 8.25. Found1: C, 81.25; H, 6.14; N, 7.34. Found2: C,
80.85; H, 5.93; N, 7.33.
ature 100 K; lattice parameters a ) 14.8740(7) Å, b ) 14.9489(7)
Å, c ) 16.9750(8) Å, R ) 69.528(3)°, ꢀ ) 72.061(3)°, γ )
76.207(3)°; unit cell volume V ) 3327.7(3) Å3; calculated density
Dcalc ) 1.471 g/cm3; number of molecules in the unit cell Z ) 4;
linear absorption coefficient µ ) 1.413 mm-1; semiempirical
absorption correction from equivalents; Mo KR radiation recorded
on a Bruker KAPPA APEX II diffractometer; 85468 reflections
collected, 22141 unique reflections (13476 with I > 2σ(I)); θmax
)
32.13°; 1102 parameters; 0 restraints; H atoms were located via a
Difference Fourier map; all other atoms were refined anisotropically,
full matrix least-squares on F2 refinement method; reliability factor
R for all data ) 0.1132 (for data I > 2σ(I) ) 0.0507), weighted
reliability factor Rw ) 0.0647 (for data I > 2σ(I) ) 0.0602),
goodness-of-fit on F2, 1.524. Crystallographic data have been
deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
Copies can be obtained on request, free of charge, by quoting the
publication citation and the deposition number 695390 or by visiting
http://www.ccdc.cam.ac.uk/data_request/cif. 1H NMR (d8-THF): δ
-208.67 (s, 2H), -187.22 (s, 2H), 38.38 (s, 1H), 64.06 (s, 4H),
88.58 (2 overlapping peaks, s, 6H) 96.40 (s, 2H), 109.64 (s, 2H),
Synthesis of [2]Fe(THF). Ferrous chloride (199 mg, 1.57 mmol)
and [2]Li2 (800 mg, 1.57 mmol) were combined as solids, then
dissolved in 20 mL of THF. The reaction mixture was stirred for
25 h, then concentrated in vacuo to give the lithium chloride adduct
1
[2]Fe(THF)x(LiCl)y. H NMR (d8-THF): δ -51.32 (s, 2H, CH),
-38.59 (s, 2H, CH), 18.25 (s, 4H), 19.82 (s, 2H, CH), 28.43 (s,
2H, CH), 41.07 (s, 6H), 44.42 (s, 2H, CH), 47.59 (s, 2H, CH),
66.06 (s, 2H, CH), 73.51 (s, 1H, p-NC5H3), 77.72 (s, 4H). The
lithium chloride adduct was dissolved in 55 mL of toluene, filtered,
diluted with 100 mL of petroleum ether, and left at -30 °C
3812 Inorganic Chemistry, Vol. 48, No. 8, 2009

Pyridine-Linked Bis(anilide) Pincer Ligand
115.71 (s, 6H), 121.91 (s, 2H), 137.74 (s, 2H). Anal. Calcd for
C35H3FeN3I: C, 61.97; H, 4.90; N, 6.19. Found1: C, 62.25; H, 4.46;
N, 6.15. Found2: C, 62.42; H, 4.47; N, 5.95. UV-vis (THF, nm
(M-1 cm-1)): 371 (1.8 × 104), 456 (6.2 × 103), 510 (sh), 741 (7.3
× 103).
0.0416), weighted reliability factor Rw ) 0.0567 (for data I > 2σ(I)
) 0.0558), goodness-of-fit on F2, 1.792. Crystallographic data have
been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. Copies can be obtained on request, free of charge, by quoting
the publication citation and the deposition number 697910 or by
visiting http://www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of ([2]Fe)2O. In the glovebox, [2]Fe(THF) (167.9 mg,
269 µmol) was dissolved in 25 mL of toluene. On the high vacuum
line, the solution was degassed. ∼1 atm of dioxygen was dried via
a dry ice/acetone trap for 1 h, then exposed to the solution. The
solution immediately turned from dark red to dark green. After 30
min, the solution was concentrated in vacuo to an intractable sludge.
The sludge was treated with petroleum ether, stirred briefly, then
concentrated in vacuo to a green powder and isolated in 85% yield.
X-ray quality crystals were obtained by vapor diffusion of petroleum
ether into a saturated toluene solution. The crystal data are
summarized as follows: formula, C70H66N6OFe2 ·0.5(C7H8); formula
weight, 1165.05; lattice system, orthorhombic; space group Iba2
(No. 45); temperature 100 K; lattice parameters a ) 19.8707(9)
Å, b ) 39.4031(19) Å, c ) 15.1038(6) Å; unit cell volume V )
11825.8(9) Å3; calculated density Dcalc ) 1.309 g/cm3; number of
molecules in the unit cell Z ) 8; linear absorption coefficient µ )
0.542 mm-1; no empirical absorption correction; Mo KR radiation
recorded on a Bruker KAPPA APEX II diffractometer; 121688
reflections collected, 18216 unique reflections (14462 with I >
2σ(I)); θmax ) 32.23°; 757 parameters; 10 restraints; H atoms were
placed in calculated positions; all other atoms were refined
anisotropically, full matrix least-squares on F2 refinement method;
reliability factor R for all data ) 0.0600 (for data I > 2σ(I) )
1H NMR (C6D6): δ -10.59 (s), δ -8.66 (s), δ 10.35 (s), δ 11.24
(s), δ 12.86 (s), δ 13.64 (s), δ 15.99 (s), δ 16.67 (s) (the overlap
of peaks precluded integration). Anal. Calcd for C70H66N6OFe2 (and
0.5 equiv C7H8): C, 75.77; H, 6.06; N, 7.21. Found1: C, 75.40; H,
5.81; N, 5.71. Found2: C, 75.32; H, 5.90; N, 6.32.
Acknowledgment. We thank Larry Henling and Dr. Mike
Day for their assistance with crystallographic characteriza-
tion. The Bruker KAPPA APEXII X-ray diffractometer was
purchased via an NSF CRIF:MU award to the California
Institute of Technology, CHE-0639094. We also thank Dr.
Aaron Wilson for his help with electrochemical methods.
We are grateful to the USDOE Office of Basic Energy
Sciences (Grant DE-FG03-85ER13431) and the Moore
Foundation for financial support.
Supporting Information Available: UV-vis spectra, diffraction
data, and voltammograms. Crystallographic data in CIF file format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC900083S
Inorganic Chemistry, Vol. 48, No. 8, 2009 3813