Neutral-ligand complexes of bis(imino)pyridine iron: Synthesis, structure, and spectroscopy

Article abstract of DOI:10.1021/ic700869h

A family of bis(imino)pyridine iron neutral-ligand derivatives, ( ^iPrPDI)FeL_n (^iPrPDI = 2,6-(2,6- ⁱPr₂-C₆H₃N=CMe)₂C ₆H₃N), has been synthesized from the corresponding bis(dinitrogen) complex, (^iPrPDI)Fe(N₂)₂. When L is a strong-field ligand such as ^tBuNC or a chelating alkyl diphosphine such as DEPE (DEPE = 1,2-bis(diethylphosphino)ethane), a five-coordinate, diamagnetic compound results with no spectroscopic evidence for mixing of paramagnetic states. Reducing the field strength of the neutral donor to principally σ-type ligands such as ^tBuNH₂ or THT (THT = tetrahydrothiophene) also yielded diamagnetic compounds. However, the ¹H NMR chemical shifts of the in-plane bis(imino)pyridine hydrogens exhibit a large chemical shift dispersion indicative of temperature-independent paramagnetism (TIP) arising from mixing of an S = 1 excited state via spin-orbit coupling. Metrical data from X-ray diffraction establish bis(imino)pyridine chelate reduction for each structural type, while Moessbauer parameters and NMR spectroscopic data differentiate the spin states of the iron and identify contributions from paramagnetic excited states.

Full text of DOI:10.1021/ic700869h

Inorg. Chem. 2007, 46, 7055−7063
Neutral-Ligand Complexes of Bis(imino)pyridine Iron: Synthesis,
Structure, and Spectroscopy
Suzanne C. Bart,† Emil Lobkovsky,† Eckhard Bill,‡ Karl Wieghardt,‡ and Paul J. Chirik*,†
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca,
New York 14853, and Max-Planck Institute of Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470
Mu¨lheim an der Ruhr, Germany
Received May 5, 2007
A family of bis(imino)pyridine iron neutral-ligand derivatives, (iPrPDI)FeLn (iPrPDI
has been synthesized from the corresponding bis(dinitrogen) complex, (iPrPDI)Fe(N2)2. When L is a strong-field
ligand such as tBuNC or a chelating alkyl diphosphine such as DEPE (DEPE
1,2-bis(diethylphosphino)ethane),
) −C6H3NdCMe)2C6H3N),
2,6-(2,6-iPr2
)
a five-coordinate, diamagnetic compound results with no spectroscopic evidence for mixing of paramagnetic states.
t
Reducing the field strength of the neutral donor to principally
σ
-type ligands such as BuNH2 or THT (THT
)
tetrahydrothiophene) also yielded diamagnetic compounds. However, the 1H NMR chemical shifts of the in-plane
bis(imino)pyridine hydrogens exhibit a large chemical shift dispersion indicative of temperature-independent
paramagnetism (TIP) arising from mixing of an S
) 1 excited state via spin−orbit coupling. Metrical data from
X-ray diffraction establish bis(imino)pyridine chelate reduction for each structural type, while Mo¨ssbauer parameters
and NMR spectroscopic data differentiate the spin states of the iron and identify contributions from paramagnetic
excited states.
Introduction
planar iron chloride and methyl complexes, (iPrPDI)FeX-
(iPrPDI ) 2,6-(2,6-iPr2C6H3NCMe)2C5H3N; X ) Cl, CH3),5
exhibit Mo¨ssbauer isomer shifts, X-ray metrical parameters,
and magnetic properties consistent with a high spin-ferrous
ion antiferromagnetically coupled to a monoreduced chelate,
[iPrPDI]- (Figure 1).6 This description of the electronic
structure has been corroborated by DFT calculations where
the computed optimized structures successfully reproduce
both the X-ray data and the Mo¨ssbauer isomer shifts.
Continued reduction in the presence of neutral donors such
as dinitrogen, carbon monoxide, or 4-(dimethylamino)-
pyridine (DMAP) furnished the corresponding (iPrPDI)FeLn
derivatives.6,7 For these compounds, computational, spec-
troscopic, and structural data support an intermediate-spin
ferrous ion complexed by a dianionic [iPrPDI]2- chelate
(Figure 1). A similar electronic structure assignment has been
suggested by Gambarotta and co-workers for the square
planar bis(imino)pyridine iron methyl anion, [(iPrPDI)FeMe]-.8
Bis(imino)pyridine ligands have attracted considerable
attention due to their ease of synthesis, steric and electronic
modularity, and well-documented ability to support a range
of catalytically active metal centers and other interesting
structural types.1 One notable feature of this ligand class is
its redox activity,2 the ability to accept three electrons in
the π-system of the terdentate chelate.3 As noted by
Gambarotta, Budzelaar, and co-workers, this phenomenon
stabilizes reduced compounds whose low oxidation state
assignment is often deceiving.4
Ligand-centered redox activity is particularly important
in reduced bis(imino)pyridine iron compounds. The square-
* To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
† Cornell University.
‡ Max-Planck Institute of Bioinorganic Chemistry.
(1) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.;
Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391.
(2) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936.
(5) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406.
(6) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901.
(3) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew.
Chem., Int. Ed. 2002, 41, 3873.
(4) Scott, J.; Gambarotta, S.; Korobkov, I.; Knijnenburg, Q.; de Bruin,
B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204.
(7) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton Trans.
2006, 46, 5442.
10.1021/ic700869h CCC: $37.00
Published on Web 07/26/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 17, 2007 7055

Bart et al.
Figure 1. Ligand-centered reductions in bis(imino)pyridine iron compounds.
the resulting iron compounds. The bis(dinitrogen) precursor,
(iPrPDI)Fe(N2)2 (1-(N2)2),12 has been shown to be an effective
pre-catalyst for the intramolecular [2π + 2π] cyclization of
R,ω-dienes to yield the corresponding bicycloheptane deriva-
tives as a single isomer.13 Preliminary mechanistic studies
involving model compounds support a pathway whereby the
ferrous oxidation state is maintained throughout the catalytic
cycle, obviating complications from metal deposition arising
from formal reductive elimination and generation of M(0)
species (Figure 3).14
Figure 2. Electronic structure and the origin of temperature-independent
paramagnetism in 1-DMAP.
Redox-active bis(imino)pyridines have also been shown
to impart unique reactivity into iron-nitrogen bonds.15
Treatment of 1-(N2)2 with 1 equiv of an aryl azide results in
a net two-electron oxidation to yield a family of (iPrPDI)-
FedNAr compounds (Ar ) substituted aryl).16 A combina-
tion of X-ray diffraction, zero-field Mo¨ssbauer experiments,
and SQUID magnetization studies established an overall S
) 1 complex with an intermediate-spin ferric ion (SFe ) 3/2)
antiferromagnetically coupled to a bis(imino)pyridine monoan-
ion, [iPrPDI]- (SL ) 1/2). This electronic structure translates
onto the reactivity of this family of compounds, as complete
hydrogenation of the iron-nitrogen linkage to yield the
corresponding free aniline was observed. Similar hydroge-
native N-N bond cleavage has also been observed with
related diazoalkane derivatives.17
In this contribution, we further explore the role of TIP in
bis(imino)pyridine iron compounds bearing neutral ligands
as a function of the donor. Studies of this type may ultimately
prove essential in understanding the electronic structure of
key intermediates during catalytic turnover. The results of
this combined synthetic and spectroscopic study establish a
correlation between the field strength of the neutral donor,
spin state of the iron, and the degree of contribution from
the excited state.
1
The H NMR spectrum of the bis(imino)pyridine iron
DMAP derivative, (iPrPDI)Fe(DMAP) (1-DMAP), is similar
to isoelectronic (iPrPDI)CoMe9 and provides additional insight
into the electronic structure of the compound. While the
hydrogens on the orthogonal aryl substituents appear close
to their diamagnetic reference values (e.g., those in the free
iPrPDI ligand), the protons in the iron-chelate plane are
shifted dramatically. The origin of this behavior is temper-
ature-independent paramagnetism (TIP), whereby an ener-
getically similar S ) 1 excited-state mixes into the S ) 0
ground state via spin-orbit coupling or antisymmetric
exchange.7 Results of DFT calculations establish intermedi-
ate-spin (SFe ) 1) ferrous ions in both the ground and excited
states.6 The only difference in the electronic configuration
lies in the spin state of the bis(imino)pyridine dianion. In
the ground state, a triplet diradical dianion (SL ) 1) is
antiferromagnetically coupled to the ferrous ion, while in
the excited state, the ligand is a singlet diradical (SL ) 0)
(Figure 2). Coupling of the two electrons occurs through a
metal d orbital, in agreement with predictions by the
Goodenough-Kanamori rules.10 This description of the
electronic structure is distinct from the iron bis(dinitrogen)
complex reported by Danopoulos and co-workers where the
imines have been replaced by N-heterocyclic carbenes and
there is no evidence for chelate reduction.11
Results and Discussion
Synthesis and Solution Characterization. The iron bis-
(dinitrogen) complex, 1-(N2)2, was treated with the appropri-
The redox activity of the bis(imino)pyridine chelate
appears to play an important role in the catalytic activity of
(12) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
(13) Bouwkamp, M. W.; Bowman, A. W.; Lobkovsky, E.; Chirik, P. J. J.
Am. Chem. Soc. 2006, 128, 13340.
(8) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. Orga-
nometallics 2005, 24, 6298.
(9) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H.
M. Eur. J. Inorg. Chem. 2004, 1204.
(10) (a) Kahn, O. Molecular Magnetism; Wiley-VCH: New York, 1993.
(b) Kanamori, J. J. Phys. Chem. Solids 1959, 10, 87. (c) Goodenough,
J. B J. Phys. Chem. Solids 1958, 6, 287. (d) Goodenough, J. B. Phys.
ReV. 1955, 100, 564.
(11) Danopoulos, A. A.; Wright, J. A.; Motherwell, W. B. Chem. Commun.
2005, 784.
(14) Campora, J.; Palma, P.; Carmona, E. Coord. Chem. ReV. 1999, 193-
95, 207.
(15) Mehn, M. P.; Peters, J. C. J. Inorg. Biochem. 2006, 100, 634.
(16) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 5302.
(17) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2007, 129, 7212.
7056 Inorganic Chemistry, Vol. 46, No. 17, 2007

Neutral-Ligand Complexes of Bis(imino)pyridine Iron
Figure 3. Catalytic [2π + 2π] cycloaddition promoted by bis(imino)pyridine iron.
Figure 4. Bis(imino)pyridine iron neutral-ligand compounds prepared in this study.
ate donor to yield the desired neutral-ligand derivative. As
will be described below, coordination of either one or two
new donors was observed and, in some cases, retention of
one of the N2 ligands occurred.
77.01 ppm for the inequivalent phosphines. Again, each of
the [iPrPDI] resonances appear close to their diamagnetic
reference values, demonstrating a pure S ) 0 ground state
with little contribution from S ) 1 excited states.
Addition of 2 equiv of tert-butyl isonitrile to 1-(N2)2
furnished purple crystals identified as the five-coordinate,
diamagnetic bis(isonitrile) complex, (iPrPDI)Fe(CNtBu)2 (1-
(CNtBu)2) (Figure 4). The benzene-d6 1H NMR spectrum
of 1-(CNtBu)2 at 23 °C exhibits the number of peaks for a
Cs-symmetric molecule with distinct resonances for the tert-
butyl groups for the inequivalent apical and basal positions.
Accordingly, four inequivalent isopropyl methyl and two
methine groups on the bis(imino)pyridine chelate are also
observed. This behavior is in contrast to other five-coordinate
complexes, 1-(CO)2 and 1-(N2)2, where rapid exchange
between the apical and basal positions occurs on the NMR
time scale. The observation of distinct apical and basal sites
of 1-(CNtBu)2 is likely a consequence of the large tert-butyl
groups which raise the barrier for site interchange. Notably,
all of the 1H NMR resonances appear close to their
diamagnetic reference values, establishing little to no con-
tribution from paramagnetic excited states.
Addition of 1 equiv of monodentate σ-donors to 1-(N2)2
resulted in five-coordinate complexes where one of the N2
ligands is retained. For example, treatment of 1-(N2)2 with
1 equiv of PEt3 produced a bright green solid identified as
(iPrPDI)Fe(PEt3)N2 (1-(PEt3)N2). Coordinated dinitrogen was
confirmed by X-ray diffraction (vide infra), infrared spec-
troscopy, and 15N NMR studies. A strong NtN stretch
centered at 2028 cm-1 was observed in the KBr infrared
spectrum and shifted slightly to 2051 cm-1 upon dissolution
in pentane. Additionally, two 15N NMR resonances were
observed at 330.23 and 354.43 ppm for the inequivalent
nitrogen atoms of the N2 ligand.
Danopoulos and co-workers11 have reported the synthesis
and characterization of related pyridine bis(N-heterocyclic
carbene) iron phosphine dinitrogen complexes with PMe3
and PCy3. Both compounds are diamagnetic with no NMR
spectroscopic evidence for contributions from paramagnetic
excited states. The PMe3 compound exhibits an NtN stretch
at 2032 cm-1 in Nujol, similar to the value obtained in the
solid state for 1-(PEt3)N2.
The ambient temperature 1H NMR spectrum of 1-(PEt3)-
N2 in toluene-d8 exhibits five broadened peaks in the
diamagnetic region, suggesting a dynamic process on the
NMR time scale. Cooling the solution (Figure 5) resulted in
a gradual sharpening of the peaks as a function of temper-
ature. At -80 °C, the number of peaks expected for a Cs-
symmetric compound was observed. The dynamic process
is most likely dissociation and recoordination of either the
dinitrogen or the PEt3 ligand on the NMR time scale.
Treatment of 1-(N2)2 with a stoichiometric quantity of the
bis(phosphine), Et2PCH2CH2PEt2 (DEPE), furnished black
crystals identified as (iPrPDI)Fe(DEPE) (1-DEPE) (Figure
4). As with 1-(CNtBu)2, the benzene-d6 1H NMR spectrum
of 1-(DEPE) recorded at 23 °C exhibits the number of peaks
for a Cs-symmetric compound with unique apical and basal
phosphine coordination sites, indicating slow site exchange
on the NMR time scale. Inequivalent protons are observed
1
by H NMR spectroscopy for the methylene groups of the
backbone of the DEPE ligand. Accordingly, the 31P{1H}
NMR spectrum exhibits two peaks centered at -45.46 and
Inorganic Chemistry, Vol. 46, No. 17, 2007 7057

Bart et al.
observed upon cooling the sample to -80 °C, as would be
expected for a temperature-dependent paramagnet obeying
the Curie-Weiss Law.
The amine complex, 1-(NH2tBu), was prepared by addition
of tert-butylamine to 1-(N2)2 and was isolated as a brown
solid. Integration of the benzene-d6 1H NMR spectrum
established coordination of only 1 equiv of amine. Both
infrared spectroscopy and combustion analysis are also
consistent with a four-coordinate iron compound without a
dinitrogen ligand. Evidence for an intact amine ligand, rather
than an N-H oxidative addition product, is derived from
observation of a broad peak centered at 5.55 ppm integrating
to two protons (1H NMR) and an N-H stretch centered at
3322 cm-1 in the solid-state (KBr) infrared spectrum. The
1H NMR chemical shifts of the in-plane hydrogens indicate
TIP.7 An upfield-shifted imine methyl group, NdCMe, is
observed at -7.02 ppm, while the p- and m-pyridine peaks
appear downfield at 8.87 and 11.50 ppm, respectively. As
shown in Figure 6, these peaks do not shift upon cooling a
toluene-d8 solution to -80 °C. The broadening observed at
-10 °C is likely due to exchange between free and
coordinated amine on the NMR time scale.
One final compound, 1-(NCPh)2, was prepared by addition
of 2 equiv of benzonitrile to 1-(N2)2. This compound exhibits
spectroscopic features between the pure S ) 0 compounds
(e.g., 1-(CNtBu)2, 1-DEPE) and those that have contributions
from S ) 1 excited states. The imine methyl resonance is
shifted to 0.09 ppm, upfield of the diamagnetic reference
value of 2.27 ppm for free iPrPDI. However, this deviation
is much smaller than those observed with 1-DMAP or
Figure 5. Variable-temperature 1H NMR spectra of 1-(PEt3)N2 in toluene-
d8.
Importantly, all of the resonances appear close to their
diamagnetic reference values, suggesting little or no contri-
bution from TIP.
Addition of a sulfur rather than phosphorus donor changes
1
t
the H NMR spectroscopic properties of the resulting iron
1-(NH2 Bu), suggesting a diminished contribution from S
compound dramatically. The tetrahydrothiophene (THT)
complex, 1-(THT)N2, was isolated as a brown-red powder
following addition of 1 equiv of THT to 1-(N2)2 and
recrystallization from pentane at -35 °C. Confirmation of
the dinitrogen ligand was provided by infrared spectroscopy.
An intense NtN band was identified at 2045 cm-1 in the
solid-state (KBr) spectrum, a slightly higher frequency than
) 1 excited states upon introduction of the modest π-ac-
ceptor.
Compiled in Table 1 are the 1H NMR chemical shifts (23
°C) of the in-plane bis(imino)pyridine chelate hydrogens for
each iron derivative recorded in benzene-d6. The absolute
value of the difference in the imine methyl group chemical
shift of the compound of interest and that in the free iPrPDI
ligand provides a measure of the contribution from para-
magnetic excited states. For complexes with the strongest
field ligands in the series, 1-(CO)2 and 1-(CNtBu)2, this
difference is near zero and indicates no detectable contribu-
tion from TIP. Slightly reducing the field strength to
phosphine ligands slightly increases the chemical shift
difference.
that for 1-(PEt3)N2(νN ) 2028 cm-1). This observation is
2
likely a consequence of THT being a weaker-field ligand
than PEt3.
1
The benzene-d6 H NMR spectrum of 1-(THT)N2 recorded
at 23 °C exhibits the number of peaks expected for a
molecule of C2V symmetry, suggesting a rapid dynamic
process, most likely N2 or THT dissociation and recoordi-
nation, on the NMR time scale. Such a process renders
equivalent the aryl substituents above and below the iron
chelate plane. Notably, the in-plane hydrogens on the
[iPrPDI] chelate are shifted from their diamagnetic reference
values. The imine methyl group, NdCMe, appears as a
singlet centered at -4.62 ppm, significantly upfield of the
shift observed at 2.08 ppm for 1-(CO)2 and 2.31 ppm for
1-(CNtBu)2. Both the p- and m-pyridine hydrogens are
shifted downfield to 9.14 and 10.97 ppm, respectively, from
free iPrPDI. Chemical shifts with this dispersion are indicative
of TIP,6 suggesting that coordination of weaker, σ-type
ligands results in a more energetically accessible S ) 1
excited state.6 Importantly, no change in chemical shifts was
At the other extreme are compounds bearing relatively
t
weak-field, purely σ-donating amines18 such as 1-(NH2 Bu)
and 1-(NHC6H10), which exhibit the largest chemical-shift
displacements in the series corresponding to the largest
contributions from TIP. As was observed with 1-DMAP,6
the diamagnetic ground state of each iron compound
establishes that the lowest energy state is the SFe ) 1 ferrous
ion and is antiferromagnetically coupled to the triplet (SL )
1) [PDI]2- anion. Contribution from the low-lying excited
state, where the bis(imino)pyridine ligand is a spin singlet
(18) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. Organometallics
2007, 26, 2637.
7058 Inorganic Chemistry, Vol. 46, No. 17, 2007

Neutral-Ligand Complexes of Bis(imino)pyridine Iron
1
t
Figure 6. Variable-temperature H NMR spectra of 1-(NH2 Bu) in toluene-d8 highlighting the m-pyridine and imine methyl resonances.
Chart 1
Table 1. 1H NMR Chemical Shifts of the In-Plane Bis(imino)pyridine
Hydrogens for Spectra Recorded in Benzene-d6 at 23 °C
structures of 1-(CNtBu)2 and 1-DEPE are presented in Figure
7, while those of 1-(PEt3)N2 and 1-(THT)N2 are shown in
Figure 8. The overall molecular geometry of each compound
is best described as distorted square pyramidal where the
bis(imino)pyridine chelate defines the three sites of the
idealized basal plane. For both 1-(PEt3)N2 and 1-(THT)N2,
the more π-acidic dinitrogen ligand occupies the basal
position while the σ-ligand is coordinated in the apical site.
1-(PEt3)N2 is structurally similar to a related pyridine bis-
(N-heterocyclic carbene) iron trimethylphosphine dinitrogen
compound reported by Danopoulos.11
δ NdCMe
(ppm)
δ m-pyridine
δ p-pyridine
|∆δ|
compound
(ppm)
(ppm)
(ppm)c
iPrPDI
2.27
2.31
2.08
1.90
1.71
8.50
7.11
7.63
7.75
8.27
10.83
10.97
12.42
11.50
12.89
7.28
7.20
7.20
7.28
7.39
8.87
9.14
?.04
8.87
8.87
0
1-(CNtBu)2
0.04
0.19
0.37
0.56
2.18
6.89
8.12
9.29
9.17
a
1-(CO)2
1-(PEt3)N2
1-(DEPE)
1-(NCPh)2
1-(THT)N2
1-DMAPa
0.09
-4.62
-5.85
-7.02
-6.90
t
1-(NH2 Bu)
1-(NHC6H10)b
The metrical parameters for the bis(imino)pyridine chelates
of the iron derivatives are presented in Table 2. Data for the
free ligand, as well as other compounds crystallographically
characterized in our laboratory,6,12,13 are also reported for
comparison. All of the iron compounds presented in Table
2 exhibit elongated Cimine-Nimine and Cipso-Npyridine bonds
in addition to contracted Cimine-Cipso bonds with respect to
free iPrPDI, consistent with two-electron reduction of the
chelate to form [iPrPDI]2-.6 The two new dinitrogen com-
plexes, 1-(PEt3)N2 and 1-(THT)N2, have short N-N bonds
of 1.0853(16) and 1.117(3) Å, indicating little reduction by
the ferrous center.
a Data taken from ref 6. b Data taken from ref 12. c Difference in chemical
shift between the imine methyl group in the compound of interest and the
free PrPDI ligand.
i
1
(SL ) 0), is the origin of TIP and the unusual H NMR
chemical shifts. Introduction of a ligand of intermediate field
strength, benzonitrile, resulted in a chemical shift dispersion
between the two extremes with an NdCMe group appearing
at 0.09 ppm. Chart 1 ranks the chemical shift dispersion
observed in the bis(imino)pyridine iron complexes as a
function of ligand.
Solid-State Structures. Four compounds reported in this
study were characterized by X-ray diffraction. The molecular
Inorganic Chemistry, Vol. 46, No. 17, 2007 7059

Bart et al.
Figure 7. Molecular structures of 1-(CNtBu)2 and 1-DEPE at 30% probability ellipsoids. Hydrogen atoms omitted for clarity.
Figure 8. Molecular structures of 1-(PEt3)N2 and 1-(THT)N2 at 30% probability ellipsoids. Hydrogen atoms omitted for clarity.
Table 2. Selected Bond Distances (Å) for Crystallographically
ticularly in the Cimine-Cipso distances, are observed in
1-DEPE and 1-(PEt3)N2, and are likely a consequence of
the strong σ-donation of the phosphines which in turn
engenders an electron-rich iron center capable of engaging
in electron transfer with the bis(imino)pyridine. Accordingly,
complexes bearing the nitrogen donors, 1-DMAP and
1-(NHC6H10), exhibit similar distortions. Compounds con-
taining π-acids, 1-(CO)2, 1-(CNtBu)2, and 1-(N2)2, have the
smallest distortions in the series. Notably, the metrical
parameters of 1-(CO)2 and 1-(CNtBu)2 are statistically
indistinguishable from those for 1-(N2)2, despite the differ-
ence in electronic structure. This observation demonstrates
that the spin state of the ligand has little influence on the
metrical parameters, as predicted by DFT calculations.6
Characterized Bis(imino)pyridine Chelates
compound
iPrPDIa
Nimine-Cimine
Cimine-Cipso
Cipso-Npyridine
1.274(3)
1.274(3)
1.3408(16)
1.3370(17)
1.330(2)
1.335(5)
1.333(2)
1.332(2)
1.351(3)
1.358(3)
1.3533(16)
1.3471(14)
1.335(3)
1.352(3)
1.358(5)
1.350(5)
1.360(5)
1.372(5)
1.489(3)
1.487(3)
1.4177(19)
1.4220(18)
1.425(2)
1.423(2)
1.427(2)
1.428(3)
1.412(3)
1.405(3)
1.4167(17)
1.4147(17)
1.431(4)
1.428(3)
1.405(5)
1.414(5)
1.412(6)
1.389(6)
1.342(3)
1.345(3)
1.3844(16)
1.3788(17)
1.376(2)
1.372(2)
1.376(2)
1.379(2)
1.376(3)
1.380(3)
1.3750(15)
1.3749(15)
1.379(3)
1.372(3)
1.387(5)
1.389(5)
1.360(5)
1.386(5)
1-(CNtBu)2
a
1-(CO)2
b
1-(N2)2
1-DEPE
1-(PEt3)N2
1-(THT)N2
1-DMAPa
1-(HNC6H10)c
Mo1ssbauer Spectroscopy. Although X-ray diffraction has
proven effective for identifying bis(imino)pyridine chelate
reduction, no statistical difference in bond lengths was
observed between pure S ) 0 compounds and those exhibit-
ing TIP.6 In contrast, NMR spectroscopy does differentiate
the compounds by virtue of the chemical shift dispersion of
the in-plane chelate hydrogens. Zero-field Mo¨ssbauer spec-
troscopy was performed at 80 K to further explore and
distinguish the electronic structure of the compounds as a
function of neutral donor.
a Data taken from ref 6. b Data taken from ref 12. c Data taken from
ref 13.
While distortions within the series are relatively small, a
general trend in the structural data may be established, where
better σ-donors produce the most electron-rich iron center
and hence the greatest backbonding into the bis(imino)-
pyridine chelate. Relatively large chelate distortions, par-
7060 Inorganic Chemistry, Vol. 46, No. 17, 2007

Neutral-Ligand Complexes of Bis(imino)pyridine Iron
t
Figure 9. Zero-field Mo¨ssbauer spectra for 1-(CNtBu)2 (left), 1-DEPE (center left), 1-(NH2 Bu) (center right), and 1-(PEt3)N2(right) recorded at 80 K.
The ordinate shows relative transmission, and the solid lines are the result of fits by using Lorentzian doublets. The dotted line in the right panel is a minor
contribution from an iron(II) or iron(III) high-spin contaminant accounting for 4% of the observed iron, with δ ) 1.21 mm/s and ∆EQ ) 2.53 mm/s.
Table 3. Zero-Field Mo¨ssbauer Parameters for a Family of
Bis(imino)pyridine Iron Neutral-Ligand Derivatives
The Mo¨ssbauer spectrum for 1-(PEt3)N2 (Figure 9)
deserves additional comment. Recording the data at zero-
compound
δ (mm/s)
∆EQ (mm/s)
%
field at 80 K reveals a major compound accounting for 96%
of the iron with the expected 0.30 mm/s isomer shift. A minor
compound, accounting for 4% of the observed iron, was also
observed with δ ) 1.21 mm/s and ∆EQ ) 2.53 mm/s,
consistent with a high-spin iron(II) or iron(III) compound.
Regardless, the spectroscopic parameters for 1-(PEt3)N2
clearly establish an intermediate-spin ferrous ion with a
dianionic bis(imino)pyridine chelate.
1-(CNtBu)2
1-(CO)2
1-(N2)2
1-N2
1-(DEPE)
1-(PEt3)N2
0.06
0.03
0.39
0.38
0.33
0.30
1.21
0.39
0.31
1.18
1.17
0.53
1.76
1.43
1.04
2.53
1.78
1.94
100
ref 6
ref 6
ref 6
100
96
4
t
1-(NH2 Bu)
100
ref 6
1-DMAP
Thus, Mo¨ssbauer spectroscopy has proven useful in
differentiating the spin state of the iron center of the various
neutral-ligand derivatives of bis(imino)pyridine iron. The
compounds bearing the strongest field ligands are low-spin
iron(II) with isomer shifts around 0 mm/s. Those with
weaker-field ligands have higher isomer shifts consistent with
intermediate-spin iron centers and reduced covalency, indica-
tive of more classical coordination compounds. However,
Mo¨ssbauer spectroscopy is ineffective in distinguishing
contributions from TIP within this series of molecules.
Representative spectra for 1-(CNtBu)2, 1-(DEPE), and
1-(NH2 Bu) are presented in Figure 9. The resulting isomer
t
shifts (δ) and quadrupole splittings (∆EQ) are reported in
Table 3. Also included in Table 3 are the values for 1-(CO)2,
1-DMAP, 1-(N2)2, and 1-(N2) for comparison.6 The experi-
mental isomer shifts demonstrate that the Mo¨ssbauer pa-
rameters are sensitive to the field strength of the neutral
donor. In general, two classes of compounds have been
identified.
The first are those with isomer shifts around 0 mm/s. These
compounds have π-acids and pure S ) 0 ground states, as
judged by 1H NMR spectroscopy. The isomer shifts of 0.03
and 0.06 mm/s for 1-(CO)2 and 1-(CNtBu)2, respectively,
are consistent with low-spin iron(II) and are similar to the
archetypal low-spin iron(II) dication, [Fe(CO)6]2+ with δ )
0 mm/s.19 Covalency and backbonding may yield some
contribution from an iron(0) canonical form as established
by previously reported DFT calculations.6,12
Concluding Remarks
In summary, a family of neutral-ligand derivatives of bis-
(imino)pyridine iron have been prepared and in many cases
characterized by X-ray diffraction and Mo¨ssbauer spectros-
copy. In each example, the metrical parameters determined
by X-ray crystallography establish a doubly reduced bis-
(imino)pyridine chelate. The combination of Mo¨ssbauer
1
The second class of compounds have isomer shifts in the
range of 0.30-0.40 mm/s. Specific examples include the
mono- and bis(dinitrogen) complexes 1-N2 and 1-(N2)2 with
isomer shifts of 0.38 and 0.39 mm/s, as well as complexes
isomer shift and H NMR chemical shifts of the in-plane
hydrogens of the chelate provide further information about
the electronic structure of the compounds. For those with
the strongest field ligands in the series, carbon monoxide
and tert-butylisonitrile, the Mo¨ssbauer isomer shift estab-
lishes low-spin iron(II) centers while 1H NMR spectroscopy
demonstrates little contribution from TIP. As the field
strength of the ligand is reduced to principally σ-donors, the
Mo¨ssbauer isomer shift increases, consistent with intermedi-
ate-spin iron(II). Within this spin state, 1H NMR spectroscopy
is particularly valuable for identifying contributions from
paramagnetic excited states arising from an S ) 0 form of
the chelate. In the case of strong σ-donors, these contributions
are minimal, as evidenced by the relatively small chemical
shift dispersions of the imine methyl and pyridine hydrogens.
In the case of weaker-field amine and sulfur donors, the
contributions from the paramagnetic excited states increase
and the chemical shift dispersion increases accordingly.
t
bearing weaker-field ligands such as 1-(NH2 Bu) with a δ
value of 0.39 mm/s. Values in this range are consistent with
an intermediate-spin iron center (SFe ) 1) with a doubly
reduced bis(imino)pyridine chelate. These compounds, along
with 1-DMAP, 1-DEPE, and 1-(PEt3)N2, are more classical
Werner-type compounds as compared to 1-(CO)2 and
1-(CNtBu)2 and as a consequence have a lower degree of
covalency. Notably, all of these compounds have isomer
shifts lower than those reported for 1-Cl and 1-Cl2,6
supporting a higher degree of chelate reduction for the
neutral-ligand derivatives.
(19) Bernhardt, E.; Bley, B.; Wartchow, R.; Willner, H.; Bill, E.; Kuhn,
P.; Sham, I. H. T.; Bodenbinder, M.; Brochler, R.; Aubke, F. J. Am.
Chem. Soc. 1999, 121, 7188.
Inorganic Chemistry, Vol. 46, No. 17, 2007 7061

Bart et al.
Table 4. UV-Visible Data for a Family of Bis(imino)pyridine Iron
Neutral-Ligand Compounds
Overall, this work demonstrates that the spin state of the
ligand and hence contributions from paramagnetic excited
states can be readily manipulated by judicious choice of
ligand within the spectrochemical series.
compound
λ (nm)
ꢀ (M-1 cm-1
)
1-N2
444
544
700.5
910
7500
1922
2680
848
Experimental Section
1206
337
480
453
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard vacuum line, Schlenk,
and cannula techniques or in an MBraun inert-atmosphere dry box
containing an atmosphere of purified nitrogen. The dry box was
equipped with a cold well designed for freezing samples in liquid
nitrogen. Solvents for air- and moisture-sensitive manipulations
were initially dried and deoxygenated using literature procedures.20
Argon and hydrogen gas were purchased from Airgas Incorporated
and passed through a column containing manganese oxide supported
on vermiculite and 4 Å molecular sieves before admission to the
high-vacuum line. Benzene-d6 was purchased from Cambridge
Isotope Laboratories and distilled from sodium metal under an
atmosphere of argon and stored over 4 Å molecular sieves and
sodium metal. Benzonitrile, tert-butylamine, triethylphosphine, and
diethylphosphinoethane were dried over molecular sieves. Tetrahy-
drothiophene and tert-butylisonitrile were dried over calcium
hydride and vacuum-transferred before use. 1-(N2)2, 1-DMAP, and
1-(CO)2 were prepared according to literature procedures.6,12
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.763, 399.780,
and 500.62 MHz, respectively. All chemical shifts are reported
relative to SiMe4 using 1H (residual) chemical shifts of the solvent
as a secondary standard. 15N NMR spectra were recorded on a
Varian Inova 500 spectrometer operating at 50.663 MHz, and
chemical shifts were externally referenced to liquid ammonia. 31P-
{1H} NMR chemical shifts are reported downfield from H3PO4 and
referenced to an external 85% H3PO4 solution. Elemental analyses
were performed at Robertson Microlit Laboratories, Inc., Madison,
NJ. Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and
then quickly transferred to the goniometer head of a Bruker X8
APEX2 diffractometer equipped with a molybdenum X-ray tube
(λ ) 0.71073 Å). Preliminary data revealed the crystal system. A
hemisphere routine was used for data collection and determination
of lattice constants. The space group was identified and the data
were processed using the Bruker SAINT+ program and corrected
for absorption using SADABS. The structures were solved using
direct methods (SHELXS) completed by subsequent Fourier
synthesis and refined by full-matrix least-squares procedures.
Infrared data were collected either in the solid state (KBr pellet)
or in pentane solution.
5062
4610
1956
521
1-DMAP
577
667.5
1067
448
651
985.5
409
534
918
448.5
708.5
1292
470
550
656
765
1088
284
344
458
568
730
660
9223
3421
1570
5575
3461
1871
8812
5057
1347
9375
3229
1477
1154
1033
14797
11340
5880
1962
1015
1182
1-PEt3(N2)
1-(CNtBu)2
1-DEPE
1-(THT)(N2)
t
1-(NH2 Bu)
1040
to purple. The resulting reaction mixture was warmed to ambient
temperature and stirred for 1 h. The volatile components were
removed in vacuo to yield 0.134 g (98%) of a purple solid identified
as 1-(CNtBu)2. Anal. Calcd for C43H61N5Fe: C, 73.38; H, 8.74;
N, 9.95. Found C, 72.97; H, 8.98; N, 9.60. 1H NMR (benzene-d6):
δ 0.53 (s, 9H, C(CH3)3), 0.80 (s, 9H, C(CH3)3), 0.90 (d, 6.2 Hz,
6H, CHMe2), 1.01 (d, 6.2 Hz, 6H, CHMe2), 1.34 (d, 6.6 Hz, 6H,
CHMe2), 1.63 (d, 5.9 Hz, 6H, CHMe2), 2.23 (q, 6.0 Hz, 2H,
CHMe2), 2.31 (s, 6H, C(Me)), 3.26 (q, 7.0 Hz, 2H, CHMe2), 7.11
(d, 7.3 Hz, 2H, m-pyridine or p-aryl), 7.19-7.29 (m, 5H, p-pyridine
and m-aryl), 8.01 (d, 7.4 Hz, 2H, m-pyridine or p-aryl). 13C NMR
(benzene-d6): δ 16.23 (C(Me)), 23.84 (CHMe2), 24.67 (CHMe2),
26.74 (CHMe2), 27.49 (CHMe2), 28.65 (CHMe2), 31.01 (CMe3),
115.00 (p-pyridine), 116.71 (m-pyridine or p-aryl), 123.03 (m-aryl,
m-pyridine or p-aryl), 124.62 (m-aryl, m-pyridine or p-aryl), 139.82,
142.04, 145.38, 148.77, 153.24. IR (KBr): νCN ) 1976, 2056 cm-1
.
Preparation of (iPrPDI)Fe(Et2PCH2CH2PEt2) (1-DEPE). A 20-
mL scintillation vial was charged with 0.057 g (0.096 mmol) 1-(N2)2
and ∼5 mL of pentane. To the vial was added 0.022 mL (0.096
mmol) of diethylphosphinoethane by microsyringe with stirring.
The addition resulted in an immediate color change from olive green
to red-brown. After the reaction mixture was stirred for 1 h, all of
the volatiles were removed in vacuo to yield 0.069 g (97%) of a
black solid identified as 1-DEPE. Anal. Calcd for C43H67N3P2Fe:
Mo¨ssbauer data were collected on an alternating constant-
acceleration spectrometer. The minimum experimental line width
was 0.24 mm/s (full width at half-height). A constant sample
temperature was maintained with an Oxford Instruments Variox
or an Oxford Instruments Mo¨ssbauer-Spectromag 2000 cyrostat.
Reported isomer shifts (δ) are referenced to iron metal at 293 K.
Preparation of (iPrPDI)Fe(CNtBu)2 (1-(CNtBu)2). A 50-mL
round-bottomed flask was charged with 0.115 g (0.194 mmol)
1-(N2)2 and ∼20 mL of pentane. The flask was fitted with a 180°
needle valve, removed from the glovebox, and placed in a -78 °C
bath. On the high-vacuum line, the flask was degassed. Using a
calibrated gas bulb, 2 equiv of tert-butylisonitrile were added to
the flask, immediately producing a color change from olive green
1
C, 69.43; H, 9.08; N, 5.65. Found C, 69.29; H, 9.54; N, 5.28. H
NMR (benzene-d6): δ 0.32 (m, 8H, PCH2CH3), 0.42 (m, 12H,
PCH2CH3), 0.66 (d, 6.1 Hz, 6H, CHMe2), 0.81 (sept, 6.1 Hz, 1H,
P(CH2)2P),1.01 (d, 6.1 Hz, 6H, CHMe2), 1.17 (sept, 7.6 Hz, 1H,
P(CH2)2P), 1.27 (d, 6.1 Hz, 12H, CHMe2), 1.51 (m, 9.2 Hz, 1H,
P(CH2)2P), 1.72 (s, 6H, C(Me)), 1.90 (sept, 7.6 Hz, 1H, P(CH2)2P),
2.73 (q, 7.6 Hz, 2H, CHMe2), 3.09 (q, 7.6 Hz, 2H, CHMe2), 7.06
(d, 7.6 Hz, 2H, m-pyridine or p-aryl), 7.16-7.23 (m, 4H, m-aryl),
7.39 (t, 7.6 Hz, 1H, p-pyridine), 8.27 (d, 7.6 Hz, 2H, m-pyridine
or p-aryl). 13C NMR (benzene-d6): δ 7.21-7.53 (dd, 17 Hz, 10.5
Hz, 135 Hz, PCH2CH3), 13.57-14.17 (dd, 57.5 Hz, 45.5 Hz, 197
(20) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
7062 Inorganic Chemistry, Vol. 46, No. 17, 2007

Neutral-Ligand Complexes of Bis(imino)pyridine Iron
Hz, PCH2CH3), 20.35 (C(Me)), 22.72 (m, PCH2CH2P), 24.07
(CHMe2), 24.62 (CHMe2), 24.98 (CHMe2), 25.52 (CHMe2), 26.86
(CHMe2), 29.13 (CHMe2), 30.22 (m, PCH2CH2P), 113.26 (m-
pyridine or p-aryl), 118.27 (p-pyridine), 123.42 (m-aryl), 124.11
(m-pyridine or p-aryl), 125.22 (m-aryl), 140.11, 143.71, 149.61,
151.52, 157.66. 31P NMR (benzene-d6): δ 45.46, 77.01.
1-(N2)2 and ∼30 mL of pentane. The flask was fitted with a 180°
needle valve, removed from the glovebox, and cooled to -78 °C.
On the high-vacuum line, the contents of the flask were degassed.
Using a calibrated gas bulb, 2 equiv (excess) of tBuNH2 were added
to the flask. The resulting reaction mixture was warmed to room
temperature and stirred for 1 h, during which time the olive green
solution turned red-brown. The volatiles were removed in vacuo
to yield 0.106 g (88%) of a red-brown solid identified as
Preparation of (iPrPDI)Fe(THT)(N2) (1-(THT)N2). A 50-mL
round-bottomed flask was charged with 0.130 g (0.219 mmol)
1-(N2)2 and ∼20 mL of pentane. The flask was fitted with a 180°
needle valve, removed from the glovebox, and cooled to -78 °C.
On the high-vacuum line, the contents of the flask were degassed.
Two equivalents (excess) of tetrahydrothiophene were added to the
flask by calibrated gas bulb. The resulting reaction mixture was
warmed to room temperature and stirred for 1 h, during which time
the olive green solution turned brownish-red. All of the volatiles
were removed in vacuo to yield 0.137 g (95%) of a red-brown solid
identified as 1-(THT)N2. Anal. Calcd for C37H51N3SFeN2: C,
71.02; H, 8.22; N, 6.72. Found C, 70.83; H, 8.08; N, 6.45. 1H NMR
(benzene-d6): δ -4.62 (s, 6H, C(Me)), 0.01 (d, 12H, CHMe2), 1.00
(s, 4H, THT), 1.28 (d, 12H, CHMe2), 2.28 (s, 4H, THT), 3.46 (q,
4H, CHMe2), 7.37 (d, 4H, m-aryl), 7.72 (t, 2H, p-aryl), 9.14 (t,
1H, p-pyridine), 10.97 (d, 2H, m-pyridine). IR (KBr): νNN ) 2045
t
1-(NH2 Bu). 1H NMR (benzene-d6): δ -7.02 (s, 6H, C(Me)), 0.19
t
(s, 9H, BuNH2), 0.25 (d, 6.1 Hz, 12H, CHMe2), 1.22 (d, 6.1 Hz,
12H, CHMe2), 2.53 (q, 7.6 Hz, 4H, CHMe2), 5.55 (s, 2H, tBuNH2),
7.31 (d, 7.6,4H, m-aryl), 7.72 (t, 7.6 Hz, 2H, p-aryl), 8.87 (t, 7.6
Hz, 1H, p-pyridine), 11.50 (d, 6.1 Hz, 2H, m-pyridine). 13C NMR
(benzene-d6): δ 23.40 (CHMe2), 24.87 (CHMe2), 28.36 (CHMe2),
32.61 (tBuNH2), 41.83 (C(Me)), 61.26, 102.82 (m- pyridine or p-
aryl), 124.98 (m- aryl), 125.80 (m-pyridine or p-aryl), 136.47,
143.57 (p-pyridine), 166.85, 167.71, 194.04. IR (KBr): νNH ) 3322
cm-1
.
Preparation of (iPrPDI)Fe(NCPh)2 (1-(NCPh)2). This molecule
was prepared in a similar manner to 1-(DEPE) with 0.020 g (0.034
mmol) of 1-(N2)2 and 4.0 µL of benzonitrile and yielded 0.019 g
1
cm-1
.
(71%) of a blue-green solid identified as 1-(NCPh)2. H NMR
Preparation of (iPrPDI)Fe(PEt3)(N2) (1-(PEt3)N2). A 20-mL
scintillation vial was charged with 0.152 g (0.256 mmol) 1-(N2)2
and ∼10 mL of pentane. With stirring, 0.037 g (0.256 mmol) of
triethylphosphine (PEt3) was added to the vial via microsyringe.
The addition induced a color change from olive green to bright
grass green. The resulting reaction mixture was stirred for 1 h, after
which time the volatiles were removed in vacuo to yield 0.165 g
(98%) of 1-(PEt3)N2. Anal. Calcd for C59H67N3P2Fe: C, 68.51; H,
8.55; N, 10.24. Found C, 68.87; H, 9.01; N, 9.92. 1H NMR
(benzene-d6): δ 0.75 (53.78), 1.06 (84.38), 2.07 (47.02), 7.89
(27.49). 13C NMR (benzene-d6): δ 9.42 (PCH2CH3), 18.23 (PCH2-
CH3), 25.24 (CHMe2), 25.90 (CHMe2), 28.14 (CHMe2), 113.58 (m-
pyridine or p-aryl), 120.57 (p-pyridine), 124.35 m-aryl), 126.26
(m-pyridine or p-aryl), 149.30, 150.69. 31P NMR (benzene-d6): δ
27.91 (-80 °C). 15N NMR (benzene-d6): δ 330.23, 354.43. IR:
νNN ) 2051 (pentane), 2028 cm-1 (KBr).
(benzene-d6): δ -0.42 (d, 6.5 Hz, 12H, CH(CH3)2), 0.09 (s, 6H,
C(CH3)), 1.43 (d, 6.5 Hz, 12H, CH(CH3)2), 2.49 (sept., 6.5 Hz,
4H, CH(CH3)2), 6.37 (br s, 1H, p-phenyl), 6.45 (br s, 2H, o- or
m-phenyl), 6.86 (br s, 2H, o- or m-phenyl), 7.60 (d, 7.5 Hz, 4H,
m-aryl), 7.89 (t, 7.5 Hz, 2H, p-aryl), 8.87 (t, 8.5 Hz, 1H, p-pyr),
10.83 (d, 8.5 Hz, 2H, m-pyr).
Acknowledgment. We thank the Packard Foundation for
support of this research. P.J.C. is a Cottrell Scholar supported
by the Research Corporation and a Camille Dreyfus Teacher-
Scholar.
Supporting Information Available: Crystallographic data for
1-(CNtBu)2, 1-DEPE, 1-(PEt3)N2, and 1-(THT)N2 in cif format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
t
t
Preparation of (iPrPDI)Fe(NH2 Bu) (1-(NH2 Bu)). A 50-mL
round-bottomed flask was charged with 0.117 g (0.197 mmol) of
IC700869H
Inorganic Chemistry, Vol. 46, No. 17, 2007 7063