Reduction chemistry of aryl- and alkyl-substituted bis(imino)pyridine iron dihalide compounds: Molecular and electronic structures of [(PDI)2Fe] derivatives

Article abstract of DOI:10.1021/ic801623m

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine iron dibromide complex, (EtPDI)FeBr2 (EtPDI = 2,6-(2,6-Et2-C6H3N=CMe)2C5H3N), under a dinitrogen atmosphere in pentane furnished the bis(chelate)iron compound, (EtPDI)2Fe. Characterizatio

Full text of DOI:10.1021/ic801623m

Inorg. Chem. 2009, 48, 4190-4200
Reduction Chemistry of Aryl- and Alkyl-Substituted Bis(imino)pyridine
Iron Dihalide Compounds: Molecular and Electronic Structures of
[(PDI)2Fe] Derivatives
Bradley M. Wile,† Ryan J. Trovitch,† Suzanne C. Bart,† Aaron M. Tondreau,† Emil Lobkovsky,†
Carsten Milsmann,‡ 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 August 26, 2008
Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine iron dibromide complex, (EtPDI)FeBr2 (EtPDI )
2,6-(2,6-Et2-C6H3NdCMe)2C5H3N), under a dinitrogen atmosphere in pentane furnished the bis(chelate)iron compound,
(EtPDI)2Fe. Characterization by X-ray crystallography established a distorted four-coordinate iron center with two
κ2-bis(imino)pyridine ligands. Reducing the steric demands of the imine substituent to either a less sterically
i
encumbered aryl ring (e.g., C6H4-4-OMe) or an alkyl group (e.g., Cy, Pr, cis-myrtanyl) also yielded bis(chelate)
compounds from sodium amalgam reduction of the corresponding dihalide. Characterization of the compounds
with smaller imine substituents by X-ray diffraction established six-coordinate, pseudo-octahedral compounds. In
one case, a neutral bis(chelate)iron compound was prepared by reduction of the corresponding iron dication,
[(PDI)2Fe]2+, providing chemical confirmation of electrochemically generated species that were previously reported
as too reducing to isolate. Magnetic measurements, metrical parameters from X-ray structures, Mo¨ssbauer
spectroscopy, and open-shell, broken symmetry DFT calculations were used to establish the electronic structure
of both types (four- and six-coordinate) of neutral bis(chelate) compounds. The experimentally observed S ) 1
compounds are best described as having high-spin ferrous (SFe ) 2) centers antiferromagnetically coupled to two
bis(imino)pyridine radical anions. Thus, the two-electron reduction of the diamagnetic, low-spin complex [(PDI)2Fe]2+
to [(PDI)2Fe] is ligand-based with a concomitant spin change at iron.
Introduction
regioregular, moderately isotactic polypropylene,2 whereas
those bearing a single methyl or 2,6-difluoro substituents are
selective for R-olefin production.3-6 In small-molecule
catalysis, the sterically protected bis(imino)pyridine iron
The discovery of ethylene and R-olefin polymerization by
methylaluminoxane-activated aryl-substituted bis(imino)py-
ridine iron dihalide compounds has spawned intense interest
in this class of compounds for both polymer and small
molecule catalysis.1 Modification of the imine aryl sub-
stituents is known to influence the outcome of the poly-
merization. For example, 2,6-disubstituted precatalysts yield
7
dinitrogen complex, (iPrPDI)Fe(N2)2 (iPrPDI ) 2,6-(2,6-
iPr2-C6H3NdCMe)2C5H3N), is effective for the hydrogena-
(2) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120.
(3) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish,
B. M.; Schiffhauer, M. F. Organometallics 2008, 27, 1147.
(4) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
Organometallics 2001, 20, 2321.
(5) Ionkin, A. S.; Marshall, W. J. Organometallics 2004, 23, 3276.
(6) Alt, H. G.; Licht, E. H.; Licht, A. I.; Schneider, K. J. Coord. Chem.
ReV. 2006, 250, 2.
(7) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
(8) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2006, 128, 13340.
(9) Trovitch, R. J.; Lobkovsky, E.; Bill, E; Chirik, P. J. Organometallics
2008, 27, 1470.
* Author to whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
†
Cornell University.
Max-Planck Institute of Bioinorganic Chemistry.
‡
(1) (a) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli,
A.; Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391. (b) Small,
B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998,
120, 4049. (c) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.;
Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams,
D. J. Chem. Commun. 1998, 849.
4190 Inorganic Chemistry, Vol. 48, No. 9, 2009
10.1021/ic801623m CCC: $40.75  2009 American Chemical Society
Published on Web 11/26/2008

Bis(imino)pyridine Iron Dihalide Compounds
Figure 1. Catalytically active bis(imino)pyridine iron dinitrogen complexes.
tion, hydrosilylation,7 and [2π + 2π] cycloisomerization of
olefins.8 For many examples, efficient turnover and broad
functional group tolerance were observed.9
Results and Discussion
Reduction of Aryl- and Alkyl-Substituted Bis(imino)py-
ridine Iron Dihalide Complexes. Because of the structural
similarity of the 2,6-diethyl-substituted bis(imino)pyridine
iron complex to the previously reported bis(dinitrogen)
compounds, the reduction of the corresponding ferrous
dibromide, (EtPDI)FeBr2, was initially examined. Mimicking
the conditions used to prepare (iPrPDI)Fe(N2)2,7 a pentane
slurry of (EtPDI)FeBr2 was stirred in the presence of excess
0.5% sodium amalgam under a dinitrogen atmosphere.
Filtration and recrystallization from pentane at -35 °C
furnished a brown solid that was not the desired bis(imi-
no)pyridine iron dinitrogen compound but rather the para-
magnetic iron bis(chelate) derivative, (EtPDI)2Fe (eq 1).
In addition to its catalytic performance, (iPrPDI)Fe(N2)2
also exhibits an interesting electronic structure. Spectro-
scopic, structural, and computational studies established an
intermediate spin ferrous center complexed by a doubly
reduced bis(imino)pyridine chelate dianion.10 In addition to
their well-established redox-activity,11,12 bis(imino)pyridines
are attractive ligands for iron chemistry due to their low cost,
air stability, and straightforward synthesis.13 Typically, these
compounds are prepared by the condensation of 2 equiv of
the appropriate amine with 2,6-diacetylpyridine. This method
has been exploited for the preparation of libraries of ligands
derived from anilines and aliphatic amines, as well as other
nonhydrocarbyl substituents.1a
Motivated by the rich chemistry of (iPrPDI)Fe(N2)2 in
catalytic processes and group transfer reactions,14,15 we
sought to prepare libraries of new bis(imino)pyridine iron
bis(dinitrogen) complexes to expand the scope, activity, and
selectivity of iron-catalyzed reactions (Figure 1). The only
other bis(imino)pyridine iron bis(dinitrogen) compound
reported to date is the case where the imine methyl groups
have been replaced with phenyl substituents.16 This com-
pound exhibits higher activity than (iPrPDI)Fe(N2)2 for the
catalytic hydrogenation and hydrosilylation of simple, un-
activated alkenes, although competing η6-arene coordination
was identified as a significant catalyst deactivation pathway.
Similar η6-aryl coordination was observed for (iPrPDI)Fe(N2)2,
but much higher temperatures were required.17
Here, we describe our efforts to synthesize analogs of
(iPrPDI)Fe(N2)2 with both aryl- and alkyl-substituted bis(imi-
no)pyridines. Reduction of the corresponding dibromide
complexes furnished a family of paramagnetic, bis(che-
late)iron complexes. The electronic structures of these
compounds were elucidated by a combination of X-ray
diffraction, SQUID magnetometry, Mo¨ssbauer spectroscopy,
NMR spectroscopy, and DFT calculations.
After the reduction product of (EtPDI)FeBr2 was identified
as the bis(chelate)iron compound, (EtPDI)2Fe, the synthetic
procedure was modified to include an additional equivalent
of free bis(imino)pyridine ligand. This alteration improved
the isolated yield to 55% following recrystallization. The
benzene-d6 1H NMR spectrum of paramagnetic (EtPDI)2Fe
exhibits four diagnostic, broad peaks that are not readily
assigned but which are useful as a means of confirming the
presence of the compound in solution.
The isolation of (EtPDI)2Fe from sodium amalgam reduc-
tion of the corresponding bis(imino)pyridine iron dibromide
is reminiscent of the electrochemically generated bis(che-
late)iron complexes previously reported by Toma et al.18,19
and subsequently by Wieghardt and co-workers.11 In these
studies, the [(PDI)2Fe] complexes were the least well-
characterized, owing to their “extraordinarily strong reducing
power”.11 Specifically, solutions of [(PDI)2Fe]2+ were elec-
trochemically reduced and the resulting monocationic and
neutral species identified by electronic absorption spectros-
copy. These studies are seminal examples of the redox
activity of bis(imino)pyridine ligands in reduced transition
metal chemistry.10,12
(10) 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.
(11) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936.
(12) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton Trans.
2006, 5442.
(13) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. ReV. 2007, 107, 1745.
(14) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 5302.
Inspired by these observations, the preparation and sub-
(15) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2007, 129, 7212.
sequent chemical reduction of [(EtPDI)2Fe]2+ was targeted.
(16) Archer, A. M.; Bouwkamp, M. W.; Cortez, M.-P.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2006, 25, 4269.
(17) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Organometallics 2008,
ASAP (DOI: 10.1021/om8005596).
(18) Kuwabara, I. H.; Comninos, F. C. M.; Pardini, V. L.; Viertler, H.;
Toma, H. E. Electrochim. Acta 1994, 39, 2401.
(19) Toma, H. E.; Chavez-Gil, T. E. Inorg. Chim. Acta 1997, 257, 197.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4191

Wile et al.
Figure 2. Molecular structure of (EtPDI)2Fe at 30% probability ellipsoids. Hydrogen atoms and aryl groups on one bis(imino)pyridine ligand omitted for
clarity. A view of the core of the molecule is presented on the right.
Treatment of (EtPDI)FeBr2 with NH4PF6 in methanol11 was
unsuccessful, yielding a mixture of unidentified products.
Attempts to prepare [(iPrPDI)2Fe](PF6)2 using a similar
method were also unsuccessful, highlighting the ability of
sterically demanding aryl substituents to prevent the forma-
tion of dicationic bis(chelate)iron compounds. Because such
compounds could not be isolated, attention was devoted to
bis(imino)pyridine iron complexes with smaller aryl substituents.
Previously, Wieghardt and co-workers reported the syn-
thesis and electrochemical reduction of [(4-OMePDI)2Fe]-
(PF6)2.11 In the course of these studies, (4-OMePDI)2Fe was
spectroscopically observed but not isolated or crystallo-
graphically characterized. Sodium amalgam reduction of a
toluene slurry of either [(4-OMePDI)2Fe](PF6)2 or (4-OMePDI)-
FeBr220 furnished an olive green crystalline solid identified
as the desired neutral bis(chelate) compound, (4-OMePDI)2Fe
(eq 2).
the two compounds are indeed the same. Representative
spectra for (4-OMePDI)2Fe and (iPrAPDI)2Fe along with peak
maxima and extinction coefficients are reported in the
Supporting Information. Importantly, chemical reduction of
[(PDI)2Fe]2+ compounds can be used to prepare the corre-
sponding neutral bis(chelate)iron derivatives, [(PDI)2Fe].
Performing the reduction in the presence of excess free
ligand did not increase the yield of (4-OMePDI)2Fe, resulting
principally in the recovery of the added bis(imino)pyridine.
The 1H NMR spectrum to (EtPDI)2Fe, the 1H NMR spectrum
of (4-OMePDI)2Fe in benzene-d6 at 23 °C exhibits seven
paramagnetically shifted and broadened peaks.
Reduction of alkyl-substituted bis(imino)pyridine iron
dibromide compounds was also explored. Interest in these
compounds is derived from their ease of synthesis using a
broad spectrum of commercially available (including chiral)
amines as well as the superior performance of (CyAPDI)Fe-
(CH2SiMe3)2 (CyAPDI ) 2,6-(C6H11NdCMe)2C5H3N) in
catalytic aldehyde and ketone hydrosilylation reactions.21
Sodium amalgam reduction of toluene or pentane slurries
of (CyAPDI)FeBr2, (iPrAPDI)FeBr2, and (MyrAPDI)FeBr2
followed by filtration and recrystallization furnished dark
green solids identified as the corresponding bis(chelate)iron
compounds (eq 3). The benzene-d6 1H NMR spectra of
paramagnetic (CyAPDI)2Fe, (iPrAPDI)2Fe, and (MyrAPDI)2Fe
are similar to those observed for (4-OMePDI)2Fe and exhibit
more features than (EtPDI)2Fe.
To confirm that the isolated material, (4-OMePDI)2Fe, was
identical to the electrochemically generated species, the
electronic absorption spectrum of the green product was
recorded in a toluene solution at 23 °C. Peaks identical to
those reported previously were observed,11 confirming that
X-Ray Crystallographic Studies. Several of the neutral
bis(chelate)iron compounds prepared in this work were
4192 Inorganic Chemistry, Vol. 48, No. 9, 2009

Bis(imino)pyridine Iron Dihalide Compounds
Table 1. Selected Bond Distances (Å) and Angles (deg) for (EtPDI)2Fe
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(5)
Fe(1)-N(6)
N(1)-C(2)
2.1246(16)
1.9853(15)
2.5579(16)
2.1098(16)
1.9864(15)
2.6096(16)
1.332(2)
1.390(2)
1.367(2)
1.291(2)
1.435(3)
1.478(3)
1.334(3)
1.387(2)
1.369(2)
1.291(2)
1.435(3)
1.481(3)
78.99(6)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
N(4)-C(31)
N(5)-C(32)
N(5)-C(36)
N(6)-C(37)
C(31)-C(32)
C(36)-C(37)
N(1)-Fe(1)-N(2)
N(4)-Fe(1)-N(5)
N(1)-Fe(1)-N(4)
N(2)-Fe(1)-N(5)
Figure 3. Molecular structure of (4-OMePDI)2Fe at 30% probability ellipsoids.
Hydrogen atoms and aryl rings on one bis(imino)pyridine chelate removed
for clarity.
79.44(6)
101.15(6)
138.95(6)
Table 2. Selected Bond Distances (Å) and Angles (deg) for
(
4-OMePDI)2Fe
characterized by single-crystal X-ray diffraction. High-quality
metrical parameters allow an accurate determination of ligand
distortions that are diagnostic of redox activity10-12 and
provide interesting comparisons to structurally characterized
[(4-OMePDI)2Fe](PF6)2.11 The solid-state structure of
(EtPDI)2Fe is presented in Figure 2, and selected bond
distances and angles are reported in Table 1. Two indepen-
dent iron molecules were located in the asymmetric unit as
well as pentane and diethyl ether molecules from recrystal-
lization. Because the two molecules differ only slightly, only
one is presented and discussed in detail.
(
4-OMePDI)2Fe
[(4-OMePDI)2Fe]2+ (ref 10)
Fe(1)-N(1)
2.1481(19)
2.0166(18)
2.1278(19)
1.306(3)
1.357(3)
1.370(3)
1.313(3)
1.444(3)
1.432(3)
1.974(2), 1.999(2)
1.8707(14), 1.8655(14)
1.9817(14), 1.9922(14)
1.306(2), 1.309(2)
1.356(2), 1.351(2)
1.355(2), 1.351(2)
1.311(2), 1.311(2)
1.466(2), 1.466(2)
1.466(2), 1.470(2)
159.09(6), 159.69(6)
99.70(6)
Fe(1)-N(2)
Fe(1)-N(3)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
N(1)-Fe(1)-N(3)
N(1)-Fe(1)-N(1A)
N(2)-Fe(1)-N(2A)
148.17(7)
100.44(7)
171.76(11)
One salient feature of the solid-state structure of (EtPDI)2Fe
is the deviation from a hexacoordinate, idealized octahedral
geometry to a distorted tetrahedral environment with two
κ2-bis(imino)pyridine ligands. Each ligand has an imine
“arm” at a long distance (dFe-N ) 2.6096(16) and 2.5579(16)
Å), well outside the sum of the covalent radii for iron (1.25
Å) and nitrogen (0.75 Å). The κ2- versus κ3-bis(imino)py-
ridine is likely steric in origin; the relatively large 2,6-diethyl
aryl substituents prevent formation of a bona fide six-
coordinate compound. The nonbonding imine on each chelate
allows for an internal comparison of ligand distortions
diagnostic of redox activity. The coordinated Cimine-Nimine
distances of 1.332(2) and 1.334(3) Å are elongated from the
value of 1.291(2) Å for both free imines. Likewise, the
Cimine-Cpyridine distances of 1.435(3) Å of the coordinated
portion of the chelate are contracted compared to the 1.478(3)
and 1.481(3) Å bond lengths found in the free “arms”. These
distortions are diagnostic of single-electron reduction on each
chelate,10,12 analogous to other reduced iron compounds with
bidentate nitrogen donor ligands.22
178.80(6)
overall molecular symmetry is idealized D2d with an S4 axis
that relates C(1) and C(1A), N(1) and N(1A), and so forth.
Selected bond distances and angles for (4-OMePDI)2Fe are
reported in Table 2. Also contained in Table 2 are the
previously reported11 bond distances for [(4-OMePDI)2Fe]2+,
where the chelate is undoubtedly in its neutral form and the
ferrous ion is low-spin. The metrical parameters of both
bis(imino)pyridines in the neutral compound are consistent
with one-electron reduction, as previously proposed by
Wieghardt and co-workers for the electrochemically gener-
ated species.11 Most pronounced is the contraction of the
Cimine-Cpyridine bond lengths. In the reduced compounds, these
values are 1.444(3) and 1.432(3) Å, similar to those reported
(Cimine-Cpyridine ) 1.442(2), 1.441(2), 1.440(2), and 1.443(2)
Å) for the related manganese cation, [(4-OMePDI)2MnIII]+,
where one-electron reduced chelates are proposed.11 One
other notable feature of the metrical parameters are the
changes in iron-nitrogen bond lengths upon the reduction
of [(4-OMePDI)2Fe]2+ to (4-OMePDI)2Fe. Statistically significant
lengthening of each of these bonds is observed with the
addition of two electrons, suggesting a spin change from low
to high spin at iron. More evidence for this effect will be
presented in a later section of the article.
The solid-state structure of (4-OMePDI)2Fe, presented in
Figure 3, established an octahedral geometry for the complex
with less sterically protected imine aryl substituents. The
(20) Bluhm, M. E.; Folli, C.; Do¨ring, M. J. Mol. Catal. A 2004, 212, 13.
(21) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett. 2008, 10,
2789.
(22) Muresan, N.; Lu, C. C.; Ghosh, M.; Peters, J. C.; Abe, M.; Henling,
L. M.; Weyhermu¨ller, T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2008,
47, 4579.
Two other bis(chelate)iron compounds, (CyAPDI)2Fe and
(iPrAPDI)2Fe, were also characterized by X-ray diffraction.
Representations of the molecular structures are presented in
Figure 4; selected bond distances and angles are reported in
Inorganic Chemistry, Vol. 48, No. 9, 2009 4193

Wile et al.
Figure 4. Molecular structures of (a) (CyAPDI)2Fe and (b) (iPrAPDI)2Fe at 30% probability ellipsoids. Hydrogen atoms and selected imine substituents
removed for clarity. View of the cores of the molecules are presented below each structure.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
(
CyAPDI)2Fe and (iPrAPDI)2Fe
(
CyAPDI)2Fe
(
iPrAPDI)2Fe
Fe(1)-N(1)
2.243(2)
2.005(2)
2.221(2)
2.209(2)
2.005(2)
2.277(3)
1.293(4)
1.384(4)
1.375(3)
1.310(3)
1.437(4)
1.442(4)
1.303(3)
1.380(4)
1.386(4)
1.282(4)
1.443(4)
1.445(5)
149.64(8)
149.25(9)
93.01(8)
176.46(10)
2.252(2)
2.0180(17)
2.2506(19)
2.259(2)
2.0236(18)
2.1846(18)
1.316(3)
1.399(3)
1.362(3)
1.299(3)
1.431(3)
1.457(4)
1.312(3)
1.387(3)
1.358(3)
1.304(3)
1.426(3)
1.450(4)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(5)
Fe(1)-N(6)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
N(3)-C(8)
C(2)-C(3)
Figure 5. Zero-field Mo¨ssbauer spectrum of (EtPDI)2Fe at 80 K.
C(7)-C(8)
N(4)-C(23)a
N(5)-C(24)
N(5)-C(28)
N(6)-C(29)
C(23)-C(24)
C(28)-C(29)
N(1)-Fe(1)-N(3)
N(4)-Fe(1)-N(6)
N(1)-Fe(1)-N(4)
N(2)-Fe(1)-N(5)
Cimine-Cpyridine contractions are consistent with one-electron
reduction (Table 3).10-12
Magnetochemistry and Mo¨ssbauer Spectroscopy. The
electronic structures of the neutral bis(imino)pyridine bis-
(chelate) compounds were further investigated with magne-
tochemistry and Mo¨ssbauer spectroscopy. Because of the
gross structural difference between (EtPDI)2Fe and the other
bis(chelate)iron compounds prepared in this work, (EtPDI)2Fe
will be treated separately. The zero-field Mo¨ssbauer spectrum
of (EtPDI)2Fe was recorded at 80 K and is presented in Figure
5. Two subspectra with similar isomer shifts, δ ) 0.91 and
0.92 mm s-1, and quadrupole splittings, ∆EQ ) 1.53 and
1.95 mm s-1, were obtained reproducibly from independently
prepared, analytically pure samples and are likely a result
of the two independent forms of the molecule found in the
149.37(7)
149.06(7)
94.99(7)
176.90(8)
a For purposes of the table, the numbering scheme for (CyAPDI)2Fe is
used for (iPrAPDI)2Fe.
Table 3. Both molecules have hexacoordinate geometries
similar to that observed with (4-OMePDI)2Fe. For (CyAPDI)2Fe,
the cyclohexyl substituents on the imine ligands are disor-
dered. The disorder was successfully modeled, and reliable
metrical parameters for the core of the molecule were
obtained. However, the Cimine-Nimine expansions and the
4194 Inorganic Chemistry, Vol. 48, No. 9, 2009

Bis(imino)pyridine Iron Dihalide Compounds
Table 4. Computed and Experimental Bond Distances (Å) and Angles
(deg) for (EtPDI)2Fe and (iPrAPDI)2Fe
(EtPDI)2Fe
(
iPrAPDI)2Fe
experimental calculated experimental calculated
Fe(1)-N(1)
2.1246(16)
1.9853(15)
2.5579(16)
2.1098(16)
1.9864(15)
2.6096(16)
1.332(2)
1.390(2)
1.367(2)
1.291(2)
1.435(2)
2.157
1.984
2.517
2.124
1.991
2.589
1.340
1.386
1.381
1.317
1.453
1.467
1.341
1.379
1.387
1.314
1.469
1.453
2.252(2)
2.0180(17)
2.2506(19)
2.259(2)
2.0236(18)
2.1846(18)
1.316(3)
1.399(3)
1.362(3)
1.299(3)
1.431(3)
1.457(4)
1.312(3)
1.387(3)
1.358(3)
1.304(3)
1.426(3)
1.450(4)
2.232
2.041
2.275
2.266
2.042
2.234
1.326
1.383
1.382
1.324
1.462
1.464
1.325
1.382
1.383
1.325
1.463
1.463
150.01
150.04
93.39
177.42
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(5)
Fe(1)-N(6)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
Figure 6. Variable-temperature SQUID magnetic data for (EtPDI)2Fe
measured at 1, 3, and 5 T. The solid lines are simulations for St ) 1 with
the parameters given in the text.
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
1.478(2)
1.334(3)
1.369(2)
1.387(2)
N(4)-C(31)/C(17)
N(5)-C(36)/C(22)
N(5)-C(32)/C(18)
N(6)-C(37)/C(23)
1.291(2)
C(36)/C(22)-C(37)/C(23) 1.481(3)
C(31)/C(17)-C(32)/C(18) 1.435(3)
N(1)-Fe(1)-N(3)
N(4)-Fe(1)-N(6)
N(1)-Fe(1)-N(4)
N(2)-Fe(1)-N(5)
78.99(6)
79.44(6)
101.15(6)
138.95(6)
78.9
79.8
97.5
149.37(7)
149.06(7)
94.99(7)
176.90(8)
145.6
nucleus. A smaller ∆EQ value of 0.073 mm s-1 was measured
for CsFe(H2O)6PO4 and was interpreted as having deviations
from cubic symmetry that are minimal.23 The isomer shifts
for (EtPDI)2Fe and (iPrAPDI)2Fe are significantly higher than
the values of 0.235 and 0.238 mm s-1 reported for the low-
Figure 7. Zero-field Mo¨ssbauer spectrum of (iPrAPDI)2Fe at 80 K.
11
spin, diamagnetic [(4-OMePDI)2Fe](PF6)2 and [(terpy)2Fe]-
(ClO4)2 (terpy ) terpyridine).24
crystal lattice. The Mo¨ssbauer parameters clearly characterize
(EtPDI)2Fe as a high-spin iron(II) complex.
Computational Studies. Broken symmetry (BS) density
functional theory (DFT) calculations were performed to
further elucidate the electronic structure of the neutral
bis(chelate)iron compounds. Two representative examples,
four-coordinate (EtPDI)2Fe and six-coordinate (iPrAPDI)2Fe,
were examined as these compounds were crystallographically
characterized and studied by Mo¨ssbauer spectroscopy. Ground-
state geometries were optimized using experimentally de-
termined S ) 1 spin states applying the BP86 functional.
The experimental and calculated geometric structures are in
excellent agreement. Selected bond distances and angles are
reported in Table 4. Single-point calculations on the opti-
mized structures were performed at the B3LYP level of DFT.
For both S ) 1 complexes, the robustness of the BS solutions
was checked by complementary calculations without broken
symmetry. In both cases, the latter calculations resulted in a
significantly (>10 kcal mol-1) higher energy state.
Temperature-dependent solid-state magnetic susceptibility
measurements on (EtPDI)2Fe (Figure 6) established a plateau
at 2.83 µB. This value is similar to the benzene-d6 solution
value of 2.7 µB measured at 23 °C. It reveals a total spin
triplet for the ground state of the compound, which appar-
ently owes its origin to the antiferromagnetic exchange
coupling of iron(II), SFe ) 2, and two ligand radicals (Srad
)
1/2). The ground state is well separated from excited states
by 1000 cm-1 or more, since no thermal population of excited
states with higher spin is observed up to T ) 300 K. This
yields a lower limit for the exchange coupling constant of
about -240 cm-1 (radical-radical coupling neglected). The
temperature dependence of the solid-state magnetic moment
exhibits a downward slope in the linear portion of the curve
between approximately 10 and 300 K. A spin-Hamiltonian
simulation with St ) 1 yielded the following parameters: gt
) 2.04 and |Dt| ) 8.8 cm-1 (rhombicity, E/D was neglected).
From these numbers, the local values for iron can be obtained
from the relation DFe ) 10/21Dt, gFe ) 2/3gt + 1/3grad,
derived from spin projection. The result, DFe ) 4.2 cm-1
and gFe ) 2.03 (assuming grad ) 2), is consistent with a high-
spin ferrous ion in a tetrahedral ligand field.
The zero-field Mo¨ssbauer spectrum of a six-coordinate
bis(chelate) complex was also determined. The data recorded
at 80 K for (iPrAPDI)2Fe are presented in Figure 7 and
produced an isomer shift (δ) of 0.93 mm s-1 and quadrupole
splitting (∆EQ) of 0.24 mm s-1. Whereas the isomer shift
clearly reveals high-spin iron(II), the small quadrupole
splitting is relatively rare for an iron(II) compound and
demonstrates a minimal electric field gradient at the 57Fe
The obtained broken symmetry (4,2) minimum is consis-
tent with a high-spin ferrous (SFe ) 2) center antiferromag-
netically coupled to two ligand centered radicals one on
s
each chelate. Qualitative molecular orbital diagrams for both
compounds are presented in Figure 8. For both compounds,
five principally iron-based d orbitals were identified. One
cloverleaf d orbital was found in both the spin-up and spin-
down manifolds and is therefore doubly occupied. The other
four were located in the spin-up manifold only and are singly
occupied. Additionally, two ligand-centered orbitals with
(23) Carver, G.; Dobe, C.; Jensen, T. B.; Tregenna-Piggott, P. L. W.;
Janssen, S.; Bill, E.; McIntyre, G. J.; Barra, A.-L. Inorg. Chem. 2006,
45, 4695.
(24) Reif, W. M., Jr.; Erickson, N. E. J. Am. Chem. Soc. 1968, 90, 4794.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4195

Wile et al.
Figure 8. Qualitative molecular orbital diagram of the magnetic orbitals derived from BS(4,2) calculations of (a) (EtPDI)2Fe and (b) (iPrAPDI)2Fe. The
spatial overlap (S) of corresponding R and ꢀ orbitals is given. Orbital energies are not to scale.
Figure 9. Spin density plots for (a) (EtPDI)2Fe and (b) (iPrAPDI)2Fe derived from the BS(4,2) calculation (based on a Mulliken spin population).
minimal metal character were identified in the spin-down
manifold and correspond to two symmetry-adapted linear
combinations of the semioccupied molecular orbitals (SO-
MOs) of the two ligand radicals. Antiferromagnetic coupling
of these two electrons with iron d orbitals of appropriate
symmetry and energy accounts for the overall S ) 1 magnetic
state. The value of “S” denoted in both molecular orbital
diagrams is the spatial overlap of the two SOMOs. A value
of S ) 1 indicates a standard, doubly populated molecular
orbital with little spin polarization, while values of S , 1
indicate nonorthogonal magnetic orbital pairs.25
d-orbital splitting diagram, as expected from ligand field
theory. Three cloverleaf d orbitals, nominally dxz, dyz, and
2
2
2
dxy, comprise a low energy “t2g” set, while dx -y and dz
define an “eg” set and are found higher in energy. Using the
coordinate axis defined by the highest-energy d orbital (e.g.
2
dz ), antiferromagnetic coupling occurs with bis(imino)py-
ridine SOMOs and dxy and dyz. Notably, the population of
essentially σ* molecular orbitals accounts for the observed
elongation of the iron-nitrogen bonds in the neutral bis-
(chelate)iron compounds in the solid state, relative to their
low-spin dicationic counterparts.
The computed spin density plots for both compounds
(Figure 9) highlight the antiparallel spin alignment between
the high-spin ferrous center and the π radical chelates.
For pseudo-octahedral (iPrAPDI)2Fe, the frontier molecular
orbital representations shown in Figure 8 comprise a classic
(25) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
4196 Inorganic Chemistry, Vol. 48, No. 9, 2009

Bis(imino)pyridine Iron Dihalide Compounds
Table 5. Experimental and Computed (in Parentheses) Zero-Field
Mo¨ssbauer Parameters for (EtPDI)2Fe and (iPrAPDI)2Fe
(EtPDI)2Fe
(
iPrAPDI)2Fe
δ (mm s-1
∆EQ (mm s-1
)
0.91, 0.92 (0.72)
1.52, 1.95 (4.10)
0.93 (0.82)
0.24 (1.22)
)
complex yields the diamagnetic [(MeOPDI-)2MnIII]+ com-
pound. Thus, one-electron reduction of each chelate induces
a spin change from high-spin Mn(II) to low-spin Mn(III).
The origin of the apparently divergent effects in Fe and Mn
chemistry is likely a result of the electronic configuration of
the metal and its ability to engage in different π interactions
with each oxidation state of the bis(imino)pyridine. The d6
configuration of low-spin iron(II) is well-suited for π-back-
bonding with the empty π* orbitals of a neutral bis(imi-
no)pyridine. Reduction of the ligand to the monoanionic
form, [PDI]-, reduces its π-acidity and hence increases its
π-basicity. High-spin Fe(II) is a relatively weak π donor
(compared to low-spin Fe(II)) and a stronger π acceptor and
is well matched with [PDI]-.
In manganese chemistry, reduction of the bis(imino)py-
ridine in the bis(chelate) compound again converts the ligand
from its neutral π-accepting [PDI]0 form to a more π-donat-
ing [PDI]- configuration. Low-spin Mn(III) favors interaction
with a π-donating ligand and hence exhibits the opposite
effect of the iron chemistry; namely, a spin state change from
high-spin Mn(II) to low-spin Mn(III) accompanies ligand
reduction. Thus, the appropriate match of ligand and metal
oxidation state determines the influence of ligand reduction
on the spin state of the resulting metal ion.
Concluding Remarks. Attempts to prepare analogs of the
iron precatalyst, (iPrPDI)Fe(N2)2, by sodium amalgam reduc-
tion of the corresponding iron dihalide bearing smaller aryl
or alkyl imine substituents resulted in isolation of the neutral
bis(chelate)iron compounds. These species, previously gener-
ated electrochemically, were described as too reducing to
isolate. Characterization of the compound with 2,6-diethyl
phenyl substituents by X-ray crystallography established a
distorted tetrahedral compound with two κ2-bis(imino)py-
ridines. Compounds with 4-OCH3-substituted phenyl or alkyl
imine substituents were also structurally characterized and
found to have hexacoordinate geometries. For both the four-
and six-coordinate compounds, the metrical parameters,
Mo¨ssbauer isomer shifts, magnetic data, and open-shell,
broken-symmetry DFT calculations support high-spin ferrous
ions antiferromagnetically coupled to two chelate radical
anions. Thus, the previously reported electrochemical reduc-
tion of diamagnetic [(PDI)2Fe]2+ to [(PDI)2Fe] occurs at the
ligand, while the ferrous oxidation state is preserved. Notably,
the neutral [(PDI)2Fe] compounds undergo a spin change
from low- to high-spin iron(II), a result of a weaker-field,
less-π-acidic bis(imino)pyridine chelate upon single-electron
reduction.
Figure 10. Change in spin state upon reduction of [(PDI)2Fe]2+ compounds.
Analysis of the spin density in terms of individual atomic
contributions supports four unpaired electrons on the iron
(3.66 for (EtPDI)2Fe and 3.78 for (iPrAPDI)2Fe) and one
electron on each ligand. For the ligand-centered π radicals,
the spin density is concentrated on the imine arms, pyridine
nitrogen, and ortho- and para-pyridine carbons. Due to the
lower symmetry in four-coordinate (EtPDI)2Fe, the spin
density distribution on the ligand is asymmetric. The majority
of the spin density is localized on the pyridine and the
coordinated imine “arm” of the ligand, in agreement with
the differences in the observed bond lengths for the two
“arms”.
Mo¨ssbauer parameters were also computed for (EtPDI)2Fe
and (iPrAPDI)2Fe to calibrate the computational results.26,27
As reported in Table 5, the calculations on both compounds
successfully reproduce the experimentally observed isomer
shifts and are fully consistent with high-spin iron(II).
Unfortunately, the experimental quadrupole splittings were
not successfully reproduced by the calculations.28
The combined magnetic, structural, Mo¨ssbauer, and com-
putational data obtained for both (EtPDI)2Fe and (iPrAPDI)2Fe
firmly establish the electronic structure of the compounds
as containing a high-spin ferrous ion antiferromagnetically
coupled to two bis(imino)pyridine radical anions. Thus, the
(electro)chemical reduction of [(PDI)2Fe]2+ compounds to
neutral [(PDI)2Fe]0 derivatives is ligand-based; the ferrous
oxidation state is preserved throughout the reaction. How-
ever, the reduction of the bis(imino)pyridine from its neutral
to monoanionic form decreases its π-acceptor ability and
hence its ligand field strength, inducing a spin change from
low to high spin at the iron center (Figure 10).
The effect of the ligand reduction in iron chemistry,
namely, changing the spin state from low-spin iron(II) to
high-spin, is opposite that of previous reports for related
bis(chelate)manganese compounds.11 For Mn, two-electron
reduction of the high-spin (S ) 5/2) [(4-OMePDI)2MnII]2+
(26) Neese, F. Inorg. Chim. Acta 2002, 337, 181.
(27) Sinnecker, S.; Slep, L. D.; Bill, E.; Neese, F. Inorg. Chem. 2005, 44,
2245.
(28) Calculations on (iPrAPDI)2Fe were also run with the COSMO package.
Geometry optimization yielded δ ) 0.83 mm/s and ∆E ) 1.222 mm/
s. Repeating the calculation with fixed coordinates from the experi-
mentally determined X-ray structure, δ ) 0.78 mm/s and ∆E ) 1.072
mm/s. While the calculations do not successfully reproduce the
quadrupole splittings, we believe this has little impact on conclusions
about the electronic structure of the bis(chelate) complexes.
Experimental Section
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. Solvents for air- and
Inorganic Chemistry, Vol. 48, No. 9, 2009 4197

Wile et al.
Table 6. Crystallographic Parameters for (EtPDI)2Fe, (4-OMePDI)2Fe, (CyAPDI)2Fe, and (iPrAPDI)2Fe
(EtPDI)2Fe 4-OMePDI)2Fe
C126H164N12Fe2O2
(
(
CyAPDI)2Fe
(
iPrAPDI)2Fe
empirical formula
fw
C46H46FeN6O4
802.74
C42H62N6Fe
706.83
C30H46N6Fe
546.58
1990.39
cryst dimensions
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
0.30 × 0.20 × 0.10
0.30 × 0.25 × 0.20
monoclinic
P2/c
18.0838(14)
15.9439(19)
15.4641(16)
90
91.886(4)
90
4456.3(8)
4
0.40 × 0.25 × 0.10
monoclinic
P2(1)/n
16.3610(9)
13.2578(8)
18.1805(9)
90
96.083(2)
90
3921.3 (4)
4
0.40 × 0.15 × 0.10
monoclinic
P2(1)/c
9.7390(5)
16.4698(8)
20.7204(10)
90
119.165 (2)
90
2902.2 (2)
4
triclinic
j
P1
12.136(2)
19.040(3)
23.774(4)
92.268(9)
102.270(8)
90.946 (8)
5362.3 (16)
2
Z
Fcalcd (g cm-3
)
1.233
0.329
2144
1.196
0.386
1688
1.197
0.421
1528
1.251
0.549
1176
µ (mm-1
)
F(000)
µ range (deg)
1.72-28.28
-16 e h e 15
-25 e k e 25
-31 e l e 31
137395
1.70 to 24.71
-21 e h e 21
-18 e k e 18
-18 e l e 18
30244
1.77-20.82
-16 e h e 15
-13 e k e 12
-15 e l e 18
17487
1.67-26.37
-11 e h e 12
-16 e k e 20
-25 e l e 25
26087
total data collected
ind reflns
26277
7585
4096
5867
Rint
0.0388
0.0710
0.0583
0.0651
abs correction
range of transmission
refinement method
data/restraints/params
semiempirical by SADABS
0.9678-0.9076
semiempirical from equivalents semiempirical from equivalents semiempirical from equivalents
0.9269-0.8931
0.9591-0.8496
full-matrix least squares on F2
4096/1146/611
0.0598
0.9472-0.8104
full-matrix least squares on F2
5867/1/372
full-matrix least squares on F2 full-matrix least squares on F2
26277/0/1530
0.0472
0.1356
1.038
1.171, -0.824
7585/0/515
0.0500
2
2
R1 [Fo g 2σ(Fo )]
wR2 [Fo g -3σ(Fo )]
goodness-of-fit
0.0491
2
2
0.1187
0.1173
0.1074
0.972
1.071
1.012
largest peak, hole (eÅ-3
)
0.313, -0.509
0.214, -0.247
0.366, -0.355
moisture-sensitive manipulations were initially dried and deoxy-
genated using literature procedures.29 Hydrogen and deuterium gas
were passed through a column containing manganese oxide
supported on vermiculite and 4 Å molecular sieves before admission
to the high-vacuum line. Benzene-d6 and toluene-d8 were purchased
from Cambridge Isotope Laboratories and dried over 4 Å molecular
sieves and titanocene, respectively. The iron dihalide compounds,
Samples used for magnetization measurements were recrystallized
multiple times and checked for chemical composition and purity
by elemental analysis (C, H, and N) and H NMR spectroscopy.
Reproducibility was checked by collecting data on three indepen-
dently synthesized samples. The experimental data were corrected
for underlying diamagnetism by use of tabulated Pascal’s con-
stants34,35 as well as for temperature-independent paramagnetism.
The susceptibility and magnetization data were simulated with our
simulation julX for exchange-coupled systems.36
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 the 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.
Mo¨ssbauer data were collected at 80 K on an alternating constant-
acceleration spectrometer. The minimum experimental line width
was 0.24 mm s-1 (full width at half-height). A constant sample
temperature was maintained with an Oxford Instruments Variox
or an Oxford Instruments Mo¨ssbauer-Spectromag 2000 cryostat.
Reported isomer shifts (δ) are referenced to iron metal at 293 K.
1
(EtPDI)FeBr2,30
(
4-OMePDI)FeBr2,20 (CyAPDI)FeBr2,31 (iPrAP-
DI)FeBr2,31 and (MyrAPDI)FeBr2,32 were prepared according to
literature procedures.
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.76, 399.78, and
2
500.62 MHz, respectively. H NMR spectra were recorded at 20
°C on the Inova 400 and 500 spectrometers operating at 61.37 and
76.85 MHz, respectively. All 1H NMR chemical shifts are reported
relative to SiMe4 using residual chemical shifts of the solvent as a
secondary standard. Solution magnetic moments were determined
by the Evans method33 using a ferrocene standard and are the
average value of at least two independent measurements. 1H NMR
multiplicity and coupling constants are reported where applicable.
Peak width at half-height is given for paramagnetically broadened
resonances. Elemental analyses were performed at Robertson
Microlit Laboratories, Inc., in Madison, New Jersey. All electronic
absorption data were collected in toluene solution on a Shimadzu
UV-2101 PC scanning spectrophotometer. Solid-state magnetic
susceptibilities were determined at 23 °C using a Johnson Mathey
magnetic susceptibility balance calibrated with HgCo(SCN)4.
SQUID magnetization data of crystalline powdered samples were
recorded with a SQUID magnetometer (Quantum Design) at 1 T
between 5 and 300 K and at 1, 3, and 5 T in the range 2-30 K.
(32) Abu-Surrah, A. S.; Lappalainen, K.; Piironen, U.; Lehmus, P.; Repo,
T.; Leskela¨, M. J. Organomet. Chem. 2002, 648, 55.
(33) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(34) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203.
(35) Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics;
CRC Press Inc.: Boca Raton, FL, 1979.
(29) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(30) Schmidt, R.; Welsh, M. B.; Palackal, S. J.; Alt, H. G. J. Mol. Catal.
A 2002, 179, 155.
(31) Castro, P. M.; Lappalainen, K.; Ahlgre´n, M.; Leskela¨, M.; Repo, T.
J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1380.
(36) Bill, E. julX, available from http://ewww.mpi-muelheim.mpg.de/bac/
logins/bill/julX_en.php (accessed Nov. 2008).
4198 Inorganic Chemistry, Vol. 48, No. 9, 2009

Bis(imino)pyridine Iron Dihalide Compounds
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.37 The geometry
optimizations of the complexes were carried out at the BP8638-40
level of DFT. Single-point calculations on the optimized geometries
were carried out using the B3LYP38,41,42 functional. This hybrid
functional often gives better results for transition metal compounds
than pure gradient-corrected functionals, especially with regard to
metal-ligand covalency.43 The all-electron Gaussian basis sets were
those developed by the Ahlrichs group.44,45 Triple-ꢁ-quality basis
sets TZV(P) with one set of polarization functions on the metals
and the atoms directly coordinated to the metal center were used.45
For the carbon and hydrogen atoms, slightly smaller polarized split-
valence SV(P) basis sets were used, which were of double-ꢁ quality
in the valence region and contained a polarizing set of d functions
on the non-hydrogen atoms.44 Auxiliary basis sets used to expand
the electron density in the resolution-of-the-identity approach were
chosen46-48 where applicable, to match the orbital basis.
The self-consistent field calculations were tightly converged (1
× 10-8 Eh in energy, 1 × 10-7 Eh in the density change, and 1 ×
10-7 in maximum element of the DIIS error vector). The geometry
optimizations for all complexes were carried out in redundant
internal coordinates without imposing symmetry constraints. In all
cases, the geometries were considered converged after the energy
change was less than 5 × 10-6 Eh, the gradient norm and maximum
gradient element were smaller than 1 × 10-4 Eh Bohr-1 and 3 ×
10-4 Eh Bohr-1, respectively, and the root-mean square and
maximum displacements of all atoms were smaller than 2 × 10-3
b and 4 × 10-3 b, respectively.
shifts were calculated from the computed electron densities at the
iron centers as previously described.53
Preparation of (EtPDI)2Fe. A 250 mL round-bottomed flask was
charged with mercury (100.0 g, 498.5 mmol) and approximately
100 mL of pentane. Sodium metal (0.502 g, 21.8 mmol) was added
slowly in small pieces to the flask with stirring, and the resulting
amalgam was stirred for 20 min. A slurry of (EtPDI)FeBr2 (2.00 g,
3.12 mmol) in pentane (10 mL) was added, and the resulting
reaction mixture was stirred for 24 h at ambient temperature. The
solution was filtered through Celite, and pentane and other volatiles
were removed in vacuo. The resulting brown solid was recrystallized
from pentane and yielded 0.717 g (44%) of a dark brown solid
identified as (EtPDI)2Fe. The addition of 1 equiv of EtPDI increased
the yield to 55%. Anal. calcd for C58H70N6Fe: C, 76.80; H, 7.78;
N, 9.27. Found: C, 76.49; H, 7.65; N, 8.73. 1H NMR (benzene-d6,
293 K): δ 166.3 (7602 Hz), 58.7 (5291 Hz), 1.42 (359 Hz), -0.49
(186 Hz). Magnetic susceptibility: µeff ) 2.7µB (benzene-d6,
293 K).
Preparation of (4-OMePDI)2Fe. A 100 mL round-bottomed flask
was charged with mercury (24.8 g, 124 mmol) and approximately
40 mL of toluene. Sodium metal (0.122 g, 5.31 mmol) was added
slowly, in small pieces, to the mercury with stirring, and the
resulting amalgam was stirred for 20 min to completely dissolve
the metal. A slurry of (4-OMePDI)FeBr2 (0.610 g, 1.04 mmol) in
toluene (10 mL) was added, and the capped solution was stirred
for 24 h at room temperature. The solution was filtered through
Celite, and toluene and other volatiles were removed in vacuo. The
resulting brown solid was rinsed twice with pentane and recrystal-
lized from toluene to yield olive-green/brown crystals identified as
Throughout this paper, we describe our computational results
by using the BS approach by Ginsberg49 and Noodleman et al.50
Because several broken-symmetry solutions to the spin-unrestricted
Kohn-Sham equations may be obtained, the general notation
BS(m,n)51 has been adopted, where m (n) denotes the number of
spin-up (spin-down) electrons at the two interacting fragments.
Canonical and corresponding orbitals,25 as well as spin density plots,
were generated with the program Molekel.52
Nonrelativistic single-point calculations on the optimized ge-
ometry were carried out to predict Mo¨ssbauer spectral parameters
(isomer shifts and quadrupole splittings). These calculations
employed the CP(PPP) basis set26 for iron. The Mo¨ssbauer isomer
(
4-OMePDI)2Fe (0.072 g, 0.090 mmol, 18% based on (4-OMeP-
DI)FeBr2). Anal. calcd for C46H46N6FeO4: C, 68.83; H, 5.78; N,
1
10.47. Found: C, 68.70; H, 5.79; N, 10.29. H NMR (benzene-d6,
293 K): δ 161.3 (3235 Hz), 113.9 (2302 Hz), 5.58 (112 Hz), 3.45
(20 Hz), 1.62 (18 Hz), 1.28 (21 Hz), -201.1 (4236 Hz). Magnetic
susceptibility: µeff ) 2.4µB (benzene-d6, 293 K).
Preparation of (CyAPDI)2Fe. This molecule was prepared in a
similar manner to (EtPDI)2Fe with 0.438 g (0.81 mmol) of
(CyAPDI)FeBr2 and 0.098 g (4.26 mmol) of sodium metal in 19.60
g (97.8 mmol) of mercury and 40 mL of toluene. Recrystallization
from pentane at -35 °C yielded 0.189 g (66%) of a dark green
solid identified as (CyAPDI)2Fe. Anal. calcd for C42H62N6Fe: C,
1
71.37; H, 8.84; N, 11.89. Found: C, 71.08; H, 8.74; N, 11.49. H
(37) Neese, F. Orca, version 2.6, revision 4; Institut fu¨r Physikalische und
Theoretische Chemie, Universita¨t Bonn: Bonn, Germany, 2007.
(38) Becke, A. D. J. Chem. Phys. 1986, 84, 4524.
(39) Perdew, J. P.; Yue, W. Phys. ReV. B: Condens. Matter Mater. Phys.
1986, 33, 8800.
NMR (benzene-d6, 293 K): δ 227.5 (1407 Hz), 92.19 (747 Hz),
79.50 (943 Hz), 1.36 (m, 86 Hz), 0.58 (81 Hz), -3.24 (78 Hz),
-154.9 (1477 Hz). Magnetic susceptibility: µeff ) 2.5µB (benzene-
d6, 293 K).
(40) Perdew, J. P. Phys. ReV. B: Condens. Matter Mater. Phys. 1986, 33,
Preparation of (iPrAPDI)2Fe. This molecule was prepared in a
similar manner to (EtPDI)2Fe with 0.636 g (1.38 mmol) of
(iPrAPDI)FeBr2 and 0.159 g (6.92 mmol) of sodium metal in 31.80
g (159 mmol) of mercury and 100 mL of toluene. Recrystallization
from pentane at -35 °C yielded 0.276 g (73%) of a dark green
crystalline solid identified as (iPrAPDI)2Fe. Anal. calcd for
C30H46N6Fe: C, 65.92; H, 8.48; N, 15.38. Found: C, 57.26; H, 7.36;
N, 11.89. 1H NMR (benzene-d6, 293 K): δ 225.4 (1040 Hz), 87.57
(392 Hz), 81.25 (368 Hz), -18.17 (177 Hz), -158.4 (1394 Hz).
Magnetic susceptibility: µeff ) 3.3µB (Gouy balance, 293 K).
Preparation of (MyrAPDI)2Fe. This molecule was prepared in
a similar manner to (EtPDI)2Fe with 0.268 g (0.42 mmol) of
8822.
(41) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(42) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B: Condens. Matter
Mater. Phys. 1988, 37, 785.
(43) Neese, F; Solomon, E. I. In Magnetoscience: From Molecules to
Materials; Miller, J. S., Drillon, M., Eds.; Wiley: New York, 2002;
Vol. 4, p 345.
(44) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(45) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(46) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem.
Acc. 1997, 97, 119.
¨
(47) Eichkorn, K.; Treutler, O.; Ohm, H.; Ha¨ser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 240, 283.
¨
(48) Eichkorn, K.; Treutler, O.; Ohm, H.; Ha¨ser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 242, 652.
(
MyrPDI)FeBr2 and 0.053 g (2.30 mmol) of sodium metal in 9.54 g
(49) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111.
(50) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J. M. Coord.
Chem. ReV. 1995, 144, 199.
(47.6 mmol) of mercury and 40 mL of toluene. Recrystallization
from pentane at -35 °C yielded 0.120 g (63%) of a dark green
(51) Kirchner, B; Wennmohs, F.; Ye, S.; Neese, F. Curr. Opin. Chem. Biol
2007, 11, 134.
(52) Molekel, Advanced Interactive 3D-Graphics for Molecular Sciences,
available under http://www.cscs.ch/molekel/ (accessed Nov. 2008).
(53) Sinnecker, S.; Slep, L. D.; Bill, E.; Neese, F. Inorg. Chem. 2005, 44,
2245.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4199

Wile et al.
1
crystalline solid identified as (MyrAPDI)2Fe. Anal. calcd for
C58H84N6Fe: C, 75.62; H, 9.19; N, 9.12. Found: C, 75.53; H, 9.54;
N, 8.71. H NMR (benzene-d6, 293 K): δ 189.8 (3115 Hz), 110.0
Characterization of (iPrAPDI)FeBr2. H NMR (CD2Cl2, 293
K): δ 176.04 (458 Hz, 2H, iPrCH), 74.38 (194 Hz, 2H, m-py), 32.46
(521 Hz, 1H, p-py), 0.40 (840 Hz, 12H, iPrCH3), -11.49 (148 Hz,
6H, NdCCH3).
1
(2455 Hz), 72.3 (2823 Hz), 1.6 (52 Hz), 1.18 (m, 91 Hz), 0.75 (38
Hz), 0.10 (146 Hz), -55.4 (3777 Hz), -176.9 (5055 Hz). Magnetic
susceptibility: µeff ) 1.8µB (benzene-d6, 293 K).
Characterization of (MyrAPDI)FeBr2. 1H NMR (DMSO-d6, 293
3
3
K): δ 8.90 (d, JHH ) 8.0 Hz, 2H, m-py), 8.77 (t, JHH ) 8.0 Hz,
p-py), 3.12 (t, 3JHH ) 12.4 Hz, 2H, CH), 2.72 (s, 6H, N ) CCH3),
2.06 (s, 2H, CH), 0.93 (s, 2H, CH), 0.81 (s, 6H, CH3), 0.77-0.64
Crystallographic parameters for (EtPDI)2Fe, (4-OMePDI)2Fe,
(CyAPDI)2Fe, and (iPrAPDI)2Fe are given in Table 6.
3
(m, 6H), 0.50 (s, 2H, CH), 0.39 (d, JHH ) 9.2 Hz, 2H, CH).
Spectroscopic Data for Bis(imino)pyridine Iron Dihalide
Compounds.
Characterization of (EtPDI)FeBr2. 1H NMR (CD2Cl2): δ 77.39
(41 Hz, 2H, m-py), 61.07 (30 Hz, 1H, p-py), 15.90 (20 Hz, 4H,
m-Ar), 9.69 (204 Hz, 4H, CH2CH3), 8.84 (151 Hz, 4H, CH2CH3),
-2.60 (34 Hz, 12H, CH2CH3), -11.51 (20 Hz, 2H, p-Ar), -24.30
(34 Hz, 6H, NdCCH3).
Supporting Information Available: Crystallographic data for
(EtPDI)2Fe, (4-OMePDI)2Fe, (CyAPDI)2Fe, and (iPrAPDI)2Fe in CIF
format. Representative electronic absorption spectra and a Curie
plot. This material is available free of charge via the internet at
http://pubs.acs.org.
Characterization of (4-OMePDI)FeBr2. 1H NMR (DMSO-d6, 293
3
3
K): δ 8.44 (d, JHH ) 8.0 Hz, 2H, m-py), 8.28 (t, JHH ) 8.0 Hz,
1H, p-py), 6.67 (d, 3JHH ) 9.2 Hz, 4H, Ar-H), 6.16 (d, 3JHH ) 9.2
Hz, 4H, Ar-H), 3.63 (s, 6H, OCH3), 2.59 (s, 6H, NdCCH3).
Acknowledgment. We thank the Packard Foundation
(Fellowship in Science and Engineering to P.J.C.) and the
U.S. National Science Foundation and Deutsche Forschungs-
gemeinschaft for a Cooperative Activities in Chemistry
between U.S. and German Investigators grant.
Characterization of (CyAPDI)FeBr2. H NMR (CD2Cl2, 293
1
K): δ 175.05 (126 Hz, 2H, CyCH), 72.59 (23 Hz, 2H, m-py), 37.52
(17 Hz, 1H, p-py), 7.27 (229 Hz, 4H, CyCH2), 2.44 (32 Hz, 4H,
CyCH2), 0.65 (24 Hz, 4H, CyCH2), 0.33 (15 Hz, 4H, CyCH2),
-11.25 (57 Hz, 6H, N ) CCH3), -18.48 (100 Hz, 4H, CyCH2).
IC801623M
4200 Inorganic Chemistry, Vol. 48, No. 9, 2009