Synthesis of aryl-substituted bis(imino) pyridine iron dinitrogen complexes

Article abstract of DOI:10.1021/ic902162z

The synthesis and characterization of dimeric, aryl-substituted bis(imino)pyridine iron dinitrogen complexes is described. In contrast to reduction with sodium amalgam where bis(chelate) iron compounds were isolated, stirring (^ArPDI)FeBr₂ or (^MeBPDI)FeBr ₂ (PDI = 2,6-(ArN=CMe)₂C₅H₃N; Ar = 2,6-Et₂-C₆H₃N (^EtPDI), 2,6-Me ₂-C₆H₃N (^MePDI), 2- ⁱPr,6-Me-C₆H₃N (^Me,iPrPDI); ^MeBPDI = 2,6-(2,6-Me₂-C₆H₃N=CPh) ₂C₅H₃N) with sodium naphthalenide resulted in isolation of the desired iron dinitrogen compounds as diamagnetic solids. Two examples, [(^EtPDI)Fe(N₂)]₂(μ₂- N₂) and [(^MeBPDI)Fe(N₂)] ₂(μ₂-N₂), were characterized by X-ray diffraction. The solid state metrical parameters, in combination with infrared and Moessbauer spectroscopic data, establish ferrous compounds with doubly reduced chelates. Each new bis(imino)pyridine iron dinitrogen compound was screened for the catalytic hydrogenation of ethyl-3-methylbut-2-enoate, and the compound bearing the smallest aryl substituent, [(^MePDI)Fe(N ₂)]₂(μ₂-N₂), offers significant improvement over the original (^iPrPDI)Fe(N₂)₂ pre-catalyst and is one of the most active iron pre-catalysts known.

Full text of DOI:10.1021/ic902162z

2782 Inorg. Chem. 2010, 49, 2782–2792
DOI: 10.1021/ic902162z
Synthesis of Aryl-Substituted Bis(imino)pyridine Iron Dinitrogen Complexes
Sarah K. Russell, Jonathan M. Darmon, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853
Received November 2, 2009
The synthesis and characterization of dimeric, aryl-substituted bis(imino)pyridine iron dinitrogen complexes is
described. In contrast to reduction with sodium amalgam where bis(chelate) iron compounds were isolated, stirring
(ArPDI)FeBr2 or (MeBPDI)FeBr2 (PDI = 2,6-(ArNdCMe)2C5H3N; Ar = 2,6-Et2-C6H3N (EtPDI), 2,6-Me2-C6H3N
(MePDI), 2-iPr,6-Me-C6H3N (Me,iPrPDI); MeBPDI = 2,6-(2,6-Me2-C6H3NdCPh)2C5H3N) with sodium naphthalenide
resulted in isolation of the desired iron dinitrogen compounds as diamagnetic solids. Two examples,
[(EtPDI)Fe(N2)]2(μ2-N2) and [(MeBPDI)Fe(N2)]2(μ2-N2), were characterized by X-ray diffraction. The solid state
€
metrical parameters, in combination with infrared and Mossbauer spectroscopic data, establish ferrous compounds
with doubly reduced chelates. Each new bis(imino)pyridine iron dinitrogen compound was screened for the catalytic
hydrogenation of ethyl-3-methylbut-2-enoate, and the compound bearing the smallest aryl substituent,
[(MePDI)Fe(N2)]2(μ2-N2), offers significant improvement over the original (iPrPDI)Fe(N2)2 pre-catalyst and is one
of the most active iron pre-catalysts known.
Introduction
CMe)2C5H3N), exhibits rich catalytic chemistry including the
hydrogenation,5,6 hydrosilylation,5 and [2π þ 2π] cycloisome-
rization7 of olefins.8 (iPrPDI)Fe(N2)2 has also proven to be an
effective catalyst for the hydrogen-mediated reductive cycliza-
tion of enynes and diynes with substrate scope and turnover
frequencies comparable to rhodium catalysts.9 These results are
not surprising in light of the enormous success of aryl-sub-
stituted bis(imino)pyridine ligands in iron- and cobalt-catalyzed
olefin polymerization.10-13 In both polymerization and small
molecule catalysis, the redox-activity of the bis(imino)pyridine
chelate, defined as the ability to undergo reversible transfer of
1-3 electrons with the metal, appears to be a key ingredient for
high activity catalysts.14-17 In olefin polymerization chemistry,
studies into the interaction of the bis(imino)pyridine iron
The hydrogenation of olefins and ketones catalyzed by
homogeneous transition metal compounds is one of the most
well-studied and powerful transformations in organic synthesis.
Since Wilkinson’s discovery of (Ph3P)3RhCl over four decades
ago, precious metal catalysts based on rhodium, iridium, and
ruthenium have been widely studied and have resulted in
several commercial processes for the synthesis of pharmaceu-
ticals as well as fine and commodity chemicals. The environ-
mental and cost advantages of iron have renewed interest in
using earth abundant elements as alternatives to precious metal
catalysts for organic and commodity chemical synthesis.1-4
The aryl-substituted bis(imino)pyridine iron bis(dinitrogen)
complex, (iPrPDI)Fe(N2)2
(
iPrPDI = 2,6-(2,6-iPr2-C6H3Nd
(9) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772.
(10) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli,
A.; Segarra, A. M. Coord. Chem. Rev. 2006, 250, 1391.
(11) 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.
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4049.
(13) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberly, B. S.;
Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
*To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
(1) Bolm, C.; Legros, J.; Le-Paith, J.; Zani, L. Chem. Rev. 2004, 104, 6217.
(2) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
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~
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€
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M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40, 4719. (b) Knijnenburg, Q.;
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r
pubs.acs.org/IC
Published on Web 02/09/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2783
dihalide precursors with methylalumoxane (MAO) or trialk-
ylaluminums have demonstrated that a variety of processes
including chelate alkylation as well as metal and chelate redox
events accompany activation.18
Scheme 1
One of the attractive features of bis(imino)pyridine che-
lates is their modularity.19,20 Libraries of ligands are easily
accessed from condensation of the appropriate amine or
aniline with 2,6-diacetylpyridine. In principle, scores of new
iron dinitrogen compounds could be prepared with improved
properties as compared to (iPrPDI)Fe(N2)2. In addition to
iron, the aryl-substituted bis(imino)pyridine chelate has
been used to support dinitrogen complexes of vanadium,21
chromium,22 cobalt,23 and ruthenium.24 To date, the only
other neutral bis(imino)pyridine iron bis(dinitrogen) com-
plex reported is (iPrBPDI)Fe(N2)2 (iPrBPDI = 2,6-(2,6-iPr2-
C6H3NdCPh)2C5H3N) where the imine methyl substituents
have been replaced with phenyl rings.25 While improved
hydrogenation activity is observed with this catalyst for
simple unactivated olefins such as 1-hexene, η6-coordination
of both the imine phenyl ring and the diisopropyl aryl group
have been identified as catalyst deactivation pathways.
Gambarotta and co-workers26 have since reported the synthe-
sis of a family of anionic iron dinitrogen complexes follow-
ing treatment of (iPrPDI)FeCl2 with various amounts of
NaH. In certain instances, deprotonation of an imine methyl
group of the chelate accompanied dinitrogen complex for-
mation.
bis(imino)pyridine iron dinitrogen complexes. Each new iron
dinitrogen compound was evaluated for the catalytic hydro-
genation of ethyl-3-methylbut-2-enoate (ethyl 3,3-dimethyl-
acrylate), and iron pre-catalysts with vastly improved
hydrogenation activities over (iPrPDI)Fe(N2)2 have been
discovered.
Attempts to prepare bis(imino)pyridine iron dinitrogen
compounds with aryl substituents on the imine smaller than
2,6-diisopropyl or with alkyl groups have been unsuccessful.
For example, stirring the closely related precursor, (EtPDI)-
FeBr2 (EtPDI = 2,6-(2,6-Et2-C6H3NdCMe)2C5H3N) with
excess 0.5% Na(Hg) under a dinitrogen atmosphere, the
conditions used to prepare (iPrPDI)Fe(N2)2, did not yield the
desired N2 complex. The catalytically inactive bis(chelate)
iron derivative, (EtPDI)2Fe, was obtained instead (Scheme
1).27 Similar outcomes were observed for alkyl-substituted
bis(imino)pyridines and pyridine bis(oxazoline) complexes as
sodium amalgam reduction of (RAPDI)FeBr2 (RAPDI =
Results and Discussion
Synthesis of Aryl-Substituted Bis(imino)pyridine Iron
Dinitrogen Complexes. Different reduction conditions
were screened in an attempt to prepare bis(imino)pyridine
iron complexes bearing N-aryl groups smaller than 2,6-
diisopropyl phenyl. Initial experiments focused on the
reduction of (EtPDI)FeBr2 with excess 0.5% sodium amal-
gam in tetrahydrofuran (THF). The motivation for these
studies was the observation of reversible THF coordina-
tion to (iPrPDI)Fe(N2)2 to form (iPrPDI)Fe(THF)2. A
similar outcome was observed upon addition of THF to
i
2,6-(R-NdCMe)2C5H3N; R=Cy, Pr) or (iPrPybox)FeCl2
also yielded the bis(chelate) iron complexes, (RAPDI)2Fe
or (iPrPybox)2Fe,28 respectively. In this contribution, we
describe the exploration of alternative reductants and reac-
tion conditions for the preparation of new aryl-substituted
(
iPrBPDI)Fe(N2)2.25 Wereasonedthat if(EtPDI)Fe(THF)2
could be generated as the kinetic product of the reduction
in THF perhaps bis(chelate) iron complex formation could
be suppressed. Stirring a THF solution of (EtPDI)FeBr2
with 0.5% Na(Hg) under a dinitrogen atmosphere furnished
a mixture of products, one of which was likely the desired N2
compound. Notably, the amount of (EtPDI)2Fe observed by
1H NMR spectroscopy was diminished compared to reduc-
tions carried out in either toluene or pentane where the
bis(chelate) iron complex was the exclusive product.27
Inspired by the improved results observed in THF,
alternative reduction conditions were explored in an
attempt to obtain the desired iron dinitrogen compound.
Stirring a THF solution of (EtPDI)FeBr2 with 2 equiv of
sodium naphthalenide for 1 h under a dinitrogen atmo-
sphere furnished [(EtPDI)Fe(N2)]2(μ2-N2) as a dark
brown solid. The stoichiometric naphthalene byproduct
proved difficult to remove completely without decompo-
sition of the iron dinitrogen compound. Because naph-
thalene is required both to synthesize the reductant,
sodium naphthalenide, and is a byproduct of the synthesis
of the iron dinitrogen compound, we reasoned it could be
(17) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H.
M. Eur. J. Inorg. Chem. 2004, 1204.
(18) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A.
Organometallics 2009, 28, 3225.
(19) Gibson, V C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745.
(20) Matsugi, T.; Fujita, T. Chem. Soc. Rev. 2008, 37, 1264.
(21) Vidyaratne, I.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
Inorg. Chem. 2005, 44, 1187.
(22) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Budzelaar, P. H. M. Inorg.
Chem. 2007, 46, 7040.
(23) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252.
(24) Gallagher, M.; Wieder, N. L.; Dioumaev, V. K.; Carroll, P. J.; Berry,
D. H. Organometallics 2010, ASAP.
(25) Archer, A. M.; Bouwkamp, M. W.; Cortez, M. -P.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2006, 25, 4269.
(26) Scott, J.; Vidyaratne, I.; Korobkov, I.; Gambarotta, S.; Budzelaar, P.
H. M. Inorg. Chem. 2008, 47, 896.
(27) Wile, B. M.; Trovitch, R. J.; Bart, S. C.; Tondreau, A. M.;
Lobkovsky, E.; Milsmann, C.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg.
Chem. 2009, 48, 4190.
(28) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 3928.

2784 Inorganic Chemistry, Vol. 49, No. 6, 2010
Russell et al.
used in catalytic rather than stoichiometric amounts. Stirring
a THF solution of (EtPDI)FeBr2 with 2 equiv of sodium
metal in the presence of 5 mol % of naphthalene for 3-4 h
followed by solvent removal, extraction into diethyl ether,
and recrystallization at -35 °C furnished pure [(EtPDI)-
Fe(N2)]2(μ2-N2), free of naphthalene contamination.
with a phenyl substituent resulted in a more active iron
catalyst for the hydrogenation of simple unactivated
olefins such as 1-hexene.25 This iron dinitrogen complex,
(
iPrBPDI)Fe(N2)2, also underwent competitive intra-
molecular reactions to form catalytically inactive η6-aryl
and -phenyl compounds thereby reducing the overall hydro-
genation activity with more hindered olefins. With the
application of the new sodium naphthalenide reduction
protocols, we sought to prepare an example of an iron
dinitrogen complex with a phenylated bis(imino)pyridine
chelate bearing a smaller 2,6-disubstituted imine aryl
ring. Stirring a THF solution of (MeBPDI)FeCl2 with
2 equiv of sodium naphthalenide for 30 min at ambient
temperature followed by extraction and repeated recrys-
tallizations from diethyl ether furnished the desired iron
dinitrogen compound, [(MeBPDI)Fe(N2)]2(μ2-N2), as an
intensely dark brown solid. The recrystallized material also
contained significant quantities of (MeBPDI)Fe(THF)2
With a new reduction protocol in hand, the synthesis of
other bis(imino)pyridine iron dinitrogen complexes was
explored. Both (MePDI)FeBr2 and rac/meso-(Me,iPrPDI)-
FeBr2 underwent smooth conversion to the correspond-
ing dinitrogen complexes, [(MePDI)Fe(N2)]2(μ2-N2) and
[(Me,iPrPDI)Fe(N2)]2(μ2-N2), respectively (eq 1). Because
(
Me,iPrPDI)FeBr2 was prepared as a 9: 1 mixture of
rac and meso diastereomers,29 the dinitrogen complex,
[(Me,iPrPDI)Fe(N2)]2(μ2-N2), was also likely isolated as a
mixture of isomers. However, the benzene-d6 1H NMR
spectrum of the compound at 23 °C is broadened because
of dynamics (vide infra), and both infrared and
andthecorresponding η6-aryl compound, (κ2-MeBPDI)Fe-
€
Mossbauer spectroscopies exhibit the number of peaks
6
consistent with formation of only one compound. It is
possible that both techniques are insensitive to stereo-
chemistry and that multiple diastereomers are actually
present. To address this issue, a benzene-d6 solution of
[(Me,iPrPDI)Fe(N2)]2(μ2-N2) was treated with excess car-
bon monoxide, and a 4:1 mixture of two isomers of
the corresponding, monomeric bis(imino)pyridine iron di-
carbonyl compound, (Me,iPrPDI)Fe(CO)2, was obtained.5,28
Because we are unable to differentiate rac and meso diaster-
eomers based on NMR spectroscopy,29 it is not known
whether the rac isomer predominates and if epimerization
occurred during the synthesis of the dinitrogen compound or
upon carbonylation. The major isomer of (Me,iPrPDI)Fe(CO)2
was isolated free of the minor following recrystallization.
The same dinitrogen compound, [(Me,iPrPDI)Fe(N2)]2-
€
(η -aryl) (vide infra) as judged by Mossbauer spectroscopy
(see Supporting Information for details). Unfortunately,
the [(MeBPDI)Fe(N2)]2(μ2-N2)/(MeBPDI)Fe(THF)2 mix-
ture obtained from this method could not be isolated
free of residual naphthalene. In a typical batch approxi-
mately 25% (w/w) naphthalene remained. Attempts to
remove this byproduct by sublimation resulted in de-
composition of the iron dinitrogen compound while
attempts at selective recrystallization failed to achieve
separation. The THF compound, (MeBPDI)Fe(THF)2,
was independently prepared by reduction of (MeBPDI)-
FeCl2 under vacuum but was not isolated in pure
form because of competing formation of the η6-aryl
compound, (κ2-MeBPDI)Fe(η6-aryl), as judged by
€
Mossbauer spectroscopy (see Supporting Information).
2
(μ -N2), was also obtained from 0.5% sodium amalgam
reduction of (Me,iPrPDI)FeBr2 in toluene under a dinitro-
gen atmosphere, the only new iron dinitrogen compound
reported in this study that could be prepared in this
manner. Inthe sodium amalgam reductions, no resonances
for the bis(chelate) iron complex, (Me,iPrPDI)2Fe, were
detected in the benzene-d6 1H NMR spectrum. Treat-
ment of the resulting iron dinitrogen complex with car-
bon monoxide furnished the same 4:1 ratio of carbonyl
compounds as material prepared with sodium naphthale-
nide. For preparation of the dinitrogen compound,
[(Me,iPrPDI)Fe(N2)]2(μ2-N2), the sodium with catalytic
naphthalenide method was preferred because of its relative
operational simplicity over the sodium amalgam route.
Attempts to obviate contamination by this compound
by using shorter reduction times resulted in mixtures of
(
MeBPDI)Fe(THF)2 and (MeBPDI)FeCl.
To avoid the complications of contamination with
free naphthalene, the reduction of (MeBPDI)FeCl2 was
repeated using 2 equiv of sodium with 5 mol % of naphtha-
lene (eq 2). This method provided [(MeBPDI)Fe(N2)]2-
(μ2-N2) along with (MeBPDI)Fe(THF)2 as judged by
€
Mossbauer spectroscopy (see Supporting Information)
and degradation experiments. In a typical batch, approxi-
mately 10-20% of the mixture was (MeBPDI)Fe(THF)2.
The benzene-d6 1H NMR spectrum of this mixture was
broad and featureless. However, simply adding 1-2
drops of THF allowed observation of the diamagnetic
C2v symmetric (because of rapid interchange of the
THF ligands) (MeBPDI)Fe(THF)2 with sharp signals.
Because of the inability to reproducibly separately
these compounds on a preparative scale and the sensi-
tivity of (MeBPDI)Fe(THF)2 to dinitrogen, meaningful
combustion analysis was not obtained. It should also
be noted that the reduction using sodium with catalytic
naphthalene was best accomplished with concentrated
(∼0.24 M) iron solutions and short reaction times
(∼1 h) to prevent formation of significant quantitites
of η6-arene compounds.
Previous studies from our laboratory have demon-
strated that replacing the backbone imine methyl group
ꢀ
ꢀ
(29) Campora, J.; Cartes, M. A.; Rodrı
´
guez-Delgado, A.; Naz, A. M.;
ꢀ
Palma, P.; Perez, C. M. Inorg. Chem. 2009, 48, 3679.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2785
Figure 1. Aryl- and alkyl-substituted bis(imino)pyridine iron com-
pounds that failed to yield a dinitrogen compound following sodium
naphthalenide reduction.
As with monomeric (iPrBPDI)Fe(N2)2, [(MeBPDI)Fe-
(N2)]2(μ2-N2) is prone to N2 dissociation and irreversible
formation of intramolecular η6-arene compounds. Allo-
wing a benzene-d6 solution of the compound to stand at
23 °C for 48 h resulted in smooth conversion to a 1.4:1
mixture of (κ2-MeBPDI)Fe(η6-phenyl) and (κ2-MeBPDI)-
Fe(η6-aryl), respectively (eq 3). Each of the η6-arene
compounds is diamagnetic and was readily identified by
Table 1. Infrared Stretching Frequencies of [(ArPDI)Fe(N2)]2(μ2-N2) and
[(MeBPDI)Fe(N2)]2(μ2-N2) Compounds Recorded in Toluene Solution
compound
iPrPDI)Fe(N2)2
ν(NtN) (cm-1)
(
2122, 2058
2099, 2084
2101, 2086
2102, 2085
2104, 2094a
2120, 2109b
[(Me,iPrPDI)Fe(N2)]2(μ2-N2)
[(EtPDI)Fe(N2)]2(μ2-N2)
[(MePDI)Fe(N2)]2(μ2-N2)
[(MeBPDI)Fe(N2)]2(μ2-N2)
1
1H and 13C NMR spectroscopy. The H NMR reso-
nances for the hydrogens of the coordinated arene ring
shift upfield and are diagnostic of coordination to iron
(see Experimental Section for details). Because the for-
mation of η6-arene compounds is irreversible,25 the ratio
of the phenyl to the aryl compound is kinetically con-
trolled and did not change over time
a Values recorded in KBr. b Toluene solution values observed by
React-IR spectroscopy at 10 °C.
subunits in a 9:1 ratio. When taking this mixture into
account, 93% of the total amount of the expected N2 was
obtained. It should be noted that depending on the
specific handling and recrystallizations conditions,
[(EtPDI)Fe(N2)]2(μ2-N2) can be isolated exclusively. In
the case of [(MePDI)Fe(N2)]2(μ2-N2), the combination of
€
Mossbauer spectroscopy and Toepler pump experiments
yielded 2.9 equiv of dinitrogen per two iron centers (97%
of the expected total) and demonstrated little contamina-
tion from the dimeric structure composed of two four-co-
ordinate iron monomers lacking terminal N2 ligands.24,30
The dimeric structures with two terminal dinitrogen
ligands were readily identified by solution infrared spec-
troscopy. The values of the symmetric and asymmetric
NtN bands recorded in toluene solution are reported in
Table 1. One exception is [(MeBPDI)Fe(N2)]2(μ2-N2)
where the reported values are from a solid state (KBr)
spectrum because of the complications of competing
THF coordination at 23 °C. Toluene solution values were
obtained by in situ infrared spectroscopy below 10 °C in
the presence of 1 atm of N2. In all cases no stretches were
identified that could be assigned to the bridging N2
ligand, which may be a result of a centrosymmetric
structure in solution. Recently, Berry reported the obser-
vation of the bridging N2 stretch in [(ArPDI)Ru]2(μ2-N2)
compounds.24
The relatively high frequencies of the NtN bands
suggest little activation of the dinitrogen ligands by the
iron center. Within the series, there is little variation, the
most notable being the values for [(MeBPDI)Fe(N2)]2-
(μ2-N2) compared to the other compounds. As has been
established previously,25,31 introduction of phenyl sub-
stituents into the imine backbone results in a more
electron poor iron center. As a result, higher frequency
NtN bands are observed. It is therefore not surprising
that [(MeBPDI)Fe(N2)]2(μ2-N2) has the highest affinity
Although the sodium naphthalenide reduction meth-
ods were successful for the preparation of new dinitrogen
compounds, it has not proven completely general. Iron
compounds with bis(imino)pyridine ligands bearing aryl
rings with a single ortho-substituent (R = iPr, tBu), 3,5-
dimethyl or an N-alkyl (R1 =cyclohexyl) group pro-
duced a mixture of unidentified products using either
stoichiometric sodium naphthalenide or 2 equiv of so-
dium with catalytic naphthalene. The specific iron com-
pounds explored are presented in Figure 1. In cases where
rac and meso diastereomers are possible, a mixture of
isomers was used in the reduction reaction.
In contrast to monomeric
(
iPrPDI)Fe(N2)2 and
(
iPrBPDI)Fe(N2)2, all of the compounds prepared in this
study were isolated as dimers with two terminal and one
bridging N2 ligand. The dinitrogen ligands are labile as
evidenced by exchange with 15N2 and detection of com-
pounds where the two metal centers are four-coordinate
and bridged with a terminal dinitrogen ligand. This struc-
tural type has been observed by Berry and co-workers in
related bis(imino)pyridine ruthenium chemistry.24,30
Toepler pump experiments were conducted to quanti-
tate the amount of bulk dinitrogen per sample. Addition
of PbCl2 to [(EtPDI)Fe(N2)]2(μ2-N2) resulted in collection
of 2.6 equiv of dinitrogen per dimer. Analysis of this
€
specific sample by Mossbauer spectroscopy (vide infra)
prior to addition of the PbCl2 established a mixture
of dimers with five- and four-coordinate monomeric
€
€
(31) Kleigrewe, N.; Steffen, W.; Blomker, T.; Kehr, G.; Frohlich, R.;
Wibbeling, B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. C. J. Am. Chem.
Soc. 2005, 127, 13955.
(30) Yoo, H.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 2006, 128,
6038.

2786 Inorganic Chemistry, Vol. 49, No. 6, 2010
Russell et al.
Figure 2. Representative zero-field 57Fe Mossbauer spectra of the bis(imino)pyridine iron dinitrogen compounds described in this work. The data were
collected at 80 K.
€
for THF, likely because of the increased electrophilicity of
the iron center from the electron withdrawing bis-
(imino)pyridine.
spectra were observed if the data were collected under
vacuum, suggesting an intramolecular exchange process
occurs both between bridging and terminal dinitrogen
ligands.
Each of the diamagnetic, dimeric dinitrogen com-
pounds was also studied by NMR spectroscopy. The
benzene-d6 1H NMR spectra of [(EtPDI)Fe(N2)]2(μ2-
N2), [(Me,iPrPDI)Fe(N2)]2(μ2-N2), and [(MeBPDI)Fe-
(N2)]2(μ2-N2) are broad and featureless at 23 °C. This
behavior precluded assignment of the number of isomers
present for [(Me,iPrPDI)Fe(N2)]2(μ2-N2). The ethylated
derivative, [(EtPDI)Fe(N2)]2(μ2-N2), was studied by vari-
able temperature NMR spectroscopy in benzene-d6 (20 to
80 °C) or toluene-d8 (-80 to 20 °C). Upon warming, many
of the ligand resonances sharpened, although not to the
point where coupling was observed. At low temperatures,
the number of peaks increased, particularly the number of
bis(imino)pyridine methyl groups, indicating inequiva-
lency within the dimer; however, the spectra remained
quite broad and otherwise difficult to assign. The 15N
NMR spectrum of [(EtPDI)Fe(15N2)]2(μ2-15N2), prepared
by addition of 15N2 gas to the natural abundance com-
pound, was recorded in toluene-d8 at -80 °C. Three broad
peaks centered at -324.1, -331.5, and -356.7 ppm were
observed, consistent with two terminal and one bridging
dinitrogen ligand. The peak broadness observed at this
temperature suggests that exchange processes are still
operative, and the lack of coupling information precludes
assignments of the peaks. This phenomenon has been
observed previously in phosphine-ligated iron dinitrogen
compounds.32 No improvements in the quality of the
For the methylated complex, [(MePDI)Fe(N2)]2(μ2-N2),
a sharper, more informative 1H NMR spectrum was
obtained at 20 °C. The number of peaks consistent with
a C2v symmetric molecule was observed, again indicating
a rapid fluxional process and interchange of the terminal
and bridging N2 ligands on the NMR time scale. Notably,
the imine methyl group was observed at 1.78 ppm, in
contrast to the downfield value of 13.61 ppm observed for
(
iPrPDI)Fe(N2).5
The bis(imino)pyridine iron dinitrogen complexes pre-
pared in this study were also characterized by zero-field
57
€
Fe Mossbauer spectroscopy at 80 K. Representative
spectra are presented in Figure 2, and the experimentally
determined parameters are reported in Table 2. The
previously reported isomer shift and quadrupole split-
ting5 for (iPrPDI)Fe(N2)2 as well as for the THF com-
pounds, (EtPDI)Fe(THF) and (MeBPDI)Fe(THF)2, are
also included for comparison. The spectrum of (EtPDI)-
Fe(THF) is presented in Figure 3 as a representative
example while the spectrum of (MeBPDI)Fe(THF)2 is
reported in the Supporting Information. During the
€
course of this investigation, the zero-field Mossbauer
spectrum of (iPrBPDI)Fe(N2)2 was also recorded, and
the experimentally determined parameters are reported
in Table 2
Et
€
The Mossbauer data for [( PDI)Fe(N2)]2(μ2-N2) and
(EtPDI)Fe(THF) clearly establish that the former com-
pound is not contaminated with the latter and also demon-
strates the reduced affinity of the methyl-substituted
(32) Field, L. D.; Hazari, N.; Li, H. L.; Luck, I. J. Magn. Reson. Chem.
2003, 41, 709.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2787
Table 2. Zero-Field 57Fe Mossbauer Parameters for the Bis(imino)pyridine Iron
€
Dinitrogen and THF Complexes
compound
iPrPDI)Fe(N2)2
δ (mm/s)
ΔEQ (mm/s)
(
(
0.39
0.38
0.53
1.72
iPrPDI)Fe(N2)
[(EtPDI)Fe(N2)]2(μ2-N2)
[(EtPDI)Fe]2(μ2-N2)
0.37
0.33
0.39
0.51
1.50
2.04
(
EtPDI)Fe(THF)a
[(MePDI)Fe(N2)]2(μ2-N2)
0.37
0.38
0.49
0.49
[(Me,iPrPDI)Fe(N2)]2(μ2-N2)
Figure 3. Zero-field 57Fe Mossbauer spectrum of (EtPDI)Fe(THF).
iPrBPDI)Fe(N2)2
iPrBPDI)Fe(N2)
0.43
0.43
0.41
1.91
€
(
(
[(MeBPDI)Fe(N2)]2(μ2-N2)b
(
0.37
0.35
0.34
2.17
reported in Tables 3 and 4. The data for the monomeric
analogues, (iPrPDI)Fe(N2)2 and (iPrBPDI)Fe(N2)2, are in-
cluded in the appropriate table for comparison.
MeBPDI)Fe(THF)2
a
a Both samples of (EtPDI)Fe(THF) and (MeBPDI)Fe(THF)2 were
The molecular geometry of each iron monomer in
[(EtPDI)Fe(N2)]2(μ2-N2) is best described as an idealized
square pyramid with the bridging dinitrogen ligand occu-
pying the apical position on one iron center and a basal
position on the other. The monomeric subunits in
[(MeBPDI)Fe(N2)2](μ2-N2) are also idealized square pyr-
amidal; however, in this case the bridging N2 ligand is
apical for both iron centers. These different arrangements
change the relative orientation of the planes of the bis-
(imino)pyridine chelates with respect to each other. In
[(EtPDI)Fe(N2)]2(μ2-N2) an “edge to face” arrangement is
present while in [(MeBPDI)Fe(N2)]2(μ2-N2), a “face to
face” structure with S4 molecular symmetry is observed.
The metrical parameters of bis(imino)pyridines are
well-known to be diagnostic of redox-activity.15 While
the Nimine-Cimine distances, with one exception, are
statistically indistinguishable between [(EtPDI)Fe(N2)]2-
(μ2-N2) and monomeric (iPrPDI)Fe(N2)2,5 the Cimine-Cpyridine
distances are more contracted in the dimeric compound.
Overall the distortions are within the range for a two
electron reduced chelate and consistent with the
independently synthesized for analysis. b Contains 88% dinitrogen
compound and 12% (MeBPDI)Fe(THF)2.
bis(imino)pyridine-ligated compound for THF as com-
pared to [(MeBPDI)Fe(N2)]2(μ2-N2). Spectroscopic studies
and degradation experiments with carbon monoxide also
support these conclusions. It should also be recognized
that (EtPDI)Fe(THF) coordinates one equivalent of
THF while (MeBPDI)Fe(THF)2 is isolated with two. Both
[(MePDI)Fe(N2)]2(μ2-N2) and [(Me,iPrPDI)Fe(N2)]2(μ2-N2)
were isolated following recrystallization solely as dimers
with two terminal dinitrogen ligands
€
The Mossbauer parameters of the iron dinitrogen
complexes fall into two distinct classes that are diagnostic
for the two different types of N2 compounds that are
present. Five-coordinate, (ArPDI) iron bis(dinitrogen)
complexes have isomer shifts (δ) between 0.37 and 0.39
mm/sec and small quadrupole splittings (ΔEQ) between
0.49 and 0.53 mm/sec. The corresponding four-coordi-
nate mono(dinitrogen) compounds have slightly lower
isomer shifts and larger quadrupole splittings. In both
classes, the isomer shifts are consistent with intermediate
spin iron(II) complexes with doubly reduced, bis-
(imino)pyridine chelates as described previously.15b,33
The differences in the values of the quadrupole splittings
are likely due to a change in electric field gradient arising
from different ordering, and hence population, of the
cloverleaf d-orbitals and dz2 between the two coordina-
tion numbers. It should be recognized that additional
spectroscopic and computational studies are required to
€
Mossbauer isomer shift, which established an intermedi-
ate spin ferrous center.33 Examining the metrical para-
meters for [(MeBPDI)Fe(N2)]2(μ2-N2) and the previously
reported (iPrBPDI)Fe(N2)225 also establishes two electron
ligand reduction. For the dimer, elongated Nimine-Cimine
distances of 1.341(2) and 1.340(2) A are well within
the accepted range for an intermediate spin bis(imino)-
pyridine ferrous compound. One of the Cimine-Cpyridine
distances is significantly contracted to 1.419(3) A and
is consistent with two electron reduction. The other
Cimine-Cpyridine distance is slightly longer at 1.437(2) A
but is also indicative of a doubly reduced, dianionic
bis(imino)pyridine.
€
support this assertion. The Mossbauer data for the two
THF compounds, (EtPDI)Fe(THF) and (MeBPDI)Fe-
(THF)2, are also consistent with intermediate spin ferrous
compounds with dianionic bis(imino)pyridine chelates as
observed with other neutral ligand compounds of this type.33
Catalytic Hydrogenation of Ethyl-3-Methylbut-2-Eno-
ate. Each of the new bis(imino)pyridine iron dinitrogen
complexes synthesized in this study was evaluated for the
catalytic hydrogenation of ethyl-3-methylbut-2-enoate.
Standard conditions employed a 0.92 M solution of sub-
strate in benzene-d6, 5 mol % of iron (2.5% of the dimer),
and four atmospheres of dihydrogen. When possible, each
Two
compounds,
[(EtPDI)Fe(N2)]2(μ2-N2)
and
[(MeBPDI)Fe(N2)2](μ2-N2), were also characterized by
X-ray diffraction and obtained from pentane solution
-35 °C and contain two terminal dinitrogen ligands. The
molecular structure of [(EtPDI)Fe(N2)]2(μ2-N2) is presen-
ted in Figure 4 while that of [(MeBPDI)Fe(N2)]2(μ2-N2)
is shown in Figure 5. Selected metrical parameters are
1
reaction was run to >95% conversion as judged by H
NMR spectroscopy. As presented in Table 5, the imino
aryl substituents on the bis(imino)pyridine chelate have
a significant influence on the catalytic hydrogenation
(33) Bart, S. C.; Lobkovsky, E.; Bill, E.; Wieghardt, K.; Chirik, P. J.
Inorg. Chem. 2007, 46, 7055.

2788 Inorganic Chemistry, Vol. 49, No. 6, 2010
Russell et al.
Figure 4. Solid statestructure of[(EtPDI)Fe(N2)]2(μ2-N2) at 30% probability ellipsoids. A truncated versionofthe dimer is presented onthe left, a fullview
of the monomeric unit on the right.
Figure 5. Solid state structure of [(MeBPDI)Fe(N2)]2(μ2-N2) at 30% probability ellipsoids. A truncated version of the dimer is presented on the left, a full
view of the monomeric unit on the right.
activity of the iron dinitrogen complex and widen the
substrate scope for this class of molecules. The originally
reported compound, (iPrPDI)Fe(N2)2, reached only 60%
conversion to product after 72 h. At longer reaction times
no appreciable additional conversion was observed as the
iron complex also undergoes competitive deactivation by
irreversible C-O bond cleavage.6,34 In contrast, replacing
one of the isopropyl groups with a methyl substituent as in
the case of rac/meso-[(Me,iPrPDI)Fe(N2)]2(μ2-N2), resulted
in complete conversion in just 5 h. Similar improvements
were observed when [(EtPDI)Fe(N2)]2(μ2-N2) and
[(MePDI)Fe(N2)]2(μ2-N2) were used as the catalyst precur-
sors, and the time to reach >95% was 10 and 1.5 h,
respectively. The turnover frequencies observed for
[(MePDI)Fe(N2)]2(μ2-N2) establish this precursor as one
of the most active iron hydrogenation catalysts known.
The phenylated iron pre-catalyst, [(MeBPDI)Fe-
(N2)]2(μ2-N2), was also active for the hydrogenation of
ethyl-3-methylbut-2-enoate, albeit with reduced turnover
frequency and number as compared to the methylated
(imine backbone) series of dinitrogen compounds. Ana-
lysis of the iron compound by H NMR spectroscopy
1
following hydrogenation established the formation of the
unsaturated and saturated bis(imino)pyridine iron ester
complexes (vide infra) with little evidence for forma-
tion of η6-arene complexes. However, a significant
amount of unidentified paramagnetic compounds were
also observed and are likely derived from C-O bond
cleavage chemistry.34 Because this material is often con-
taminated with the THF compound, the catalytic hydro-
genations were repeated in the presence of 2-5 equiv of
THF, and no significant change in the hydrogenation
performance was observed. The isopropyl-substituted
variant, (iPrBPDI)Fe(N2)2, was also screened for catalytic
hydrogenation activity. No turnover was observed after
stirring ethyl-3-methylbut-2-enoate with 4 atm of dihy-
drogen for days at 23 °C.
Why then are both [(MeBPDI)Fe(N2)]2(μ2-N2) and
(
iPrBPDI)Fe(N2)2 such poor pre-catalysts for catalytic
ethyl-3-methylbut-2-enoate hydrogenation as compared
to [(MePDI)Fe(N2)]2(μ2-N2)? To answer this question, a
series of stoichiometric experiments and NMR studies
were carried out. For (iPrBPDI)Fe(N2)2, recovering the
(34) Trovitch, R. J.; Lobkovsky, E.; Bouwkamp, M. W.; Chirik, P. J.
Organometallics 2008, 27, 6264.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2789
Table 3. Metrical Parameters for [(EtPDI)Fe(N2)]2(μ2-N2)a
[(EtPDI)Fe(N2)]2(μ2-N2)
(OC(OEt)(CHdCMe2)), was cleanly observed. This com-
pound, along with (MeBPDI)Fe(OC(OEt)(CHdCMe2)),
was independently prepared by addition of ethyl-3-
methylbut-2-enoate to the corresponding dinitrogen
compounds. Both ester complexes exhibit typical NMR
spectral features for neutral ligand derivatives of bis-
(imino)pyridine iron (see Experimental Section).33,34
Because formation of η6-arene complexes is not the
origin of catalyst deactivation in either case, both iron
ester complexes were studied in stoichiometric hydroge-
nation reactions (eq 4). Exposure of a benzene-d6 solution
of (MeBPDI)Fe(OC(OEt)(CHdCMe2)) to 4 atm of H2 at
23 °C resulted in smooth and complete (>95%) conver-
sion to the corresponding saturated ester complex over
the course of 24 h. No other paramagnetic products were
observed during the reaction. Repeating this procedure
with (iPrBPDI)Fe(OC(OEt)(CHdCMe2)) produced no
change over the course of hours demonstrating that the
iron ester complex is not an initiator of hydrogenation
catalysis.
(
iPrPDI)Fe(N2)2
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(7)
Fe(1)-N(9)
Fe(2)-N(4)
Fe(2)-N(5)
Fe(2)-N(6)
Fe(2)-N(8)
Fe(2)-N(11)
1.929(3)
1.818(3)
1.926(3)
1.873(3)
1.849(3)
1.923(3)
1.834(2)
1.932(3)
1.879(3)
1.837(3)
1.9473(16)
1.8362(14)
1.9452(16)
1.8800(19)
1.8341(16)
N(1)-C(2)
N(3)-C(8)
N(4)-C(31)
N(6)-C(37)
1.328(4)
1.318(4)
1.328(4)
1.331(4)
1.333(2)
1.332(2)
C(2)-C(3)
1.410(5)
1.406(4)
1.423(4)
1.400(4)
1.427(2)
1.428(3)
C(7)-C(8)
C(31)-C(32)
C(36)-C(37)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-N(7)
N(2)-Fe(1)-N(3)
N(3)-Fe(1)-N(7)
N(2)-Fe(1)-N(7)
N(2)-Fe(1)-N(9)
N(7)-Fe(1)-N(9)
79.64(12)
98.86(12)
79.41(11)
97.15(11)
135.96(12)
130.21(13)
93.82(13)
79.90(6)
97.41(7)
79.49(6)
96.65(7)
159.09(8)
102.89(7)
98.02(8)
N(4)-Fe(2)-N(5)
N(4)-Fe(2)-N(11)
N(5)-Fe(2)-N(6)
N(6)-Fe(2)-N(11)
N(5)-Fe(2)-N(11)
N(5)-Fe(2)-N(8)
N(8)-Fe(2)-N(11)
79.52(11)
95.28(11)
79.62(11)
95.96(12)
149.01(12)
114.04(11)
96.95(12)
The differences in hydrogenation activity of [(MePDI)-
Fe(N2)]2(μ2-N2) and [(MeBPDI)Fe(N2)]2(μ2-N2) and
(
iPrBPDI)Fe(N2)2 may be traced to the affinity of the latter
a The values for (iPrPDI)Fe(N2)2 are included for comparison. Data
taken from reference 5.
compounds for coordination of the carbonyl oxygen of the
ester substrates. This behavior arises from a more electro-
philic iron center imparted by the electron withdrawing
phenylated bis(imino)pyridine chelates. The more con-
gested isopropyl-substituted compound, [(iPrBPDI)Fe],
has a more sterically protected iron center that prohibits
coordination of the π-face of the hindered trisubstituted
alkene, favoring formation of the carbonyl adduct and
preventing catalytic turnover. Making the iron center more
accessible, as in the case of [(MeBPDI)Fe], allows compe-
titive coordination of the alkene relative to the O-bound
ester compound. However, hydrogenation is still suffi-
ciently slow such that C-O bond cleavage pathways that
result in catalyst decomposition become competitive. It is
possible that higher dihydrogen pressures would minimize
or completely suppress this pathway.
Table 4. Metrical Parameters for (RBPDI) Iron Dinitrogen Complexes (R =
iPr, Me)a
[(MeBPDI)Fe(N2)]2(μ2-N2)
(
iPrBPDI)Fe(N2)2
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(5)
N(4)-N(4A)
N(5)-N(6)
N(1)-C(7)
N(2)-C(8)
N(2)-C(12)
N(3)-C(13)
C(7)-C(8)
C(12)-C(13)
1.9320(13)
1.8385(13)
1.9284(13)
1.8800(13)
1.8536(14)
1.123(3)
1.102(2)
1.340(2)
1.369(2)
1.927(4)
1.842(4)
1.935(5)
1.865(5)
1.841(5)
1.107(5)
1.106(6)
1.344(7)
1.376(7)
1.379(6)
1.355(7)
1.429(8)
1.430(7)
1.3811(18)
1.341(2)
1.437(2)
1.419(3)
Concluding Remarks
Fe(1)-N(4)-N(4A) 174.96(12)
170.4(5)
178.5(5)
99.0(2)
96.6(2)
97.2(2)
79.3(2)
80.9(2)
The synthesis of new, dimeric aryl-substituted bis-
(imino)pyridine iron dinitrogen complexes has been accom-
plished by reduction of the corresponding iron dihalide
precursor with in situ generated sodium naphthalenide. This
reduction protocol contrasts the outcome with sodium amal-
gam, where often catalytically inactive iron bis(chelate) com-
plexes were isolated. The metrical parameters determined by
Fe(1)-N(5)-N(6)
N(4)-Fe(1)-N(5)
N(1)-Fe(1)-N(5)
N(3)-Fe(1)-N(5)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
179.16(15)
93.85(6)
97.80(6)
95.78(6)
79.84(5)
80.11(6)
a Data for R = iPr taken from reference 26.
X-ray diffraction, infrared stretching frequencies, the zero-
57
€
iron species following attempted catalytic hydrogenation
and analysis by H NMR spectroscopy again indicated
field Fe Mossbauer parameters, and observed diamagnet-
1
ism are consistent with a ferrous center with a bis-
(imino)pyridine dianion. Evaluation of the performance of
each of the new compounds for the catalytic hydrogenation of
little deactivation by formation of η6-arene compounds.
Instead, the diamagnetic ester complex, (iPrBPDI)Fe-

2790 Inorganic Chemistry, Vol. 49, No. 6, 2010
Russell et al.
Table 5. Comparison of Various Aryl-Substituted Bis(imino)pyridine Iron
Dinitrogen Complexes in the Catalytic Hydrogenation of Ethyl-3-methylbut-2-
enoate
tube (λ = 0.71073 A). Preliminary data revealed the crystal
system. A hemisphere routine was used for data collection and
determination of lattice constants. The space group was identi-
fied, and the data were processed using the Bruker SAINTþ
program and corrected for absorption using SADABS. The
structures were solved using direct methods (SHELXS) com-
pleted by subsequent Fourier synthesis and refined by full-
matrix least-squares procedures.
€
57
Zero-field Fe Mossbauer spectra were recorded on a SEE
compound
time (hrs)a
€
Co. Mossbauer spectrometer (MS4) at 80 K in constant accel-
eration mode. 57Co/Rh was used as the radiation source.
WMOSS software was used for the quantitative evaluation of
the spectral parameters (least-squares fitting to Lorentzian
peaks). The minimum experimental line widths were 0.23 mm
s-1. The temperature of the sample was controlled by a Janis
Research Co. CCS-850 He/N2 cryostat within an accuracy of
(0.3 K. Isomer shifts were determined relative to R-iron at
298 K.
In situ solution infrared spectra were recorded with a Mettler
Toledo ReactIR iC10 equipped with a DS Series 9.5 mm ꢀ 1.5 m
AgX fiber conduit with a silicon probe tip on a 0.005 M solution
of the compound in toluene. The spectra were acquired at 30 s
intervals with a resolution of eight using a toluene background
taken at 22 °C. The solution was cooled using an acetone/dry ice
bath, and the temperature was monitored using an external,
low-temperature thermometer. Data was analyzed using the iC
IR 4.0 software.
(
iPrPDI)Fe(N2)2
50% (24 h)b
5
10
1.5
0% (24 h)
21% (24 h)c
[(Me,iPr PDI)Fe(N2)]2(μ2-N2)
[(EtPDI)Fe(N2)]2(μ2-N2)
[(MePDI)Fe(N2)]2(μ2-N2)
(
iPrBPDI)Fe(N2)2
[(MeBPDI)Fe(N2)]2(μ2-N2)
a Time required to reach >95% conversion as judged by H NMR
spectroscopy. Conditions: 5.0 mol % [Fe], 0.92 M substrate in benzene-
d6 solution, 4 atm H2, 23 °C. b Maximum 60% conversion due to catalyst
deactivation. c 45% observed after 72 h.
1
ethyl-3-methylbut-2-enoate demonstrates the importance of
the 2,6-disubstituted aryl groups; the compound bearing 2,6-
dimethyl aryl rings is one of the most active iron olefin
hydrogenation catalysts reported to date. However, bis-
(imino)pyridines with phenyl substituents in the imine back-
bone create more electrophilic iron centers which in turn are
more subject to contamination by THF complexes and
exhibit a greater affinity for carbonyl coordination thereby
reducing hydrogenation efficiency of carboxylated alkenes.
Preparation of [(MePDI)Fe(N2)]2(μ2-N2). A 100 mL round-
bottom flask was charged with 1.00 g (MePDI)FeBr2 (1.71
mmol), 0.081 g sodium metal (3.50 mmol, 2.1 equiv) and 0.011
g of naphthalene (0.085 mmol, 0.05 equiv). To the flask was
added approximately 20 mL of THF, and the resulting reaction
mixture was stirred for 3-4 h. During this time, the solution
changed color from blue to green to red-brown. After reduction
was complete (indicated by the formation of the red solution),
the THF was removed in vacuo. The resulting solid was
dissolved in diethyl ether and filtered through Celite. The filtrate
was collected and concentrated and stored at -35 °C yielding
Experimental Section
General Considerations. All air- and moisture-sensitive
manipulations were carried out using standard vacuum line,
Schlenk, and cannula techniques or in an MBraun inert atmo-
sphere drybox containing an atmosphere of purified nitrogen.
Solvents for air- and moisture-sensitive manipulations were
initially dried and deoxygenated using literature procedures.35
Benzene-d6 was purchased from Cambridge Isotope Labora-
tories, distilled from Na metal, and dried over 4 A molecular
0.550
g (69%) of a dark brown solid identified as
[(MePDI)Fe(N2)]2(μ2-N2). Analysis for C50H54N12Fe: Calcd C,
64.25; H, 5.82; N, 17.98. Found C, 63.87; H, 6.19; N, 17.80. 1H
NMR (benzene-d6, 20 °C): δ = 1.35 (bs, 24H, Ar CH3), 1.78 (bs,
12H, C(CH3)), 6.92 (bs, 12H, m- and p-Ar), 7.52 (bs, 2H, p-py),
8.10 (bs, 4H, m-py). 13C NMR (benzene-d6, 20 °C): δ = 16.17
(Ar CH3), 19.50 (C(CH3)), 115.92 (m- or p- py), 125.35
(m- or p- Ar), 130.93 (m- or p- Ar), 148.76 (m- or p- py),
153.05, 3 peaks not located. 15N NMR (toluene-d8, -80 °C)
δ=-355.5 (bs), -334.2 (bs), -322.4 (bs). IR(toluene): ν(N2) =
sieves. The complexes: (EtPDI)FeBr2,27 Me,iPrPDI)FeBr2,36 and
(
(
MeBPDI)FeCl237 were prepared according to literature proce-
dures. (MePDI)FeBr2 was prepared according to the published
procedure for the corresponding iron dichloride compound
using FeBr2.12b
1H NMR spectra were recorded on Varian Mercury 300,
Inova 400, 500, and 600 spectrometers operating at 299.76,
399.78, 500.62, and 599.78 MHz, respectively. 13C NMR spectra
were recorded on an Inova 500 spectrometer operating at
125.893 MHz. All 1H and 13C NMR chemical shifts are reported
relative to SiMe4 using the 1H (residual) and 13C chemical shifts
of the solvent as a secondary standard. 15N NMR spectra were
recorded on a Varian 500 spectrometer operating at 50.663
MHz, and chemical shifts were externally referenced to liquid
ammonia. Infrared spectra were collected on a Thermo Nicolet
spectrometer. Elemental analyses were performed at Robertson
Microlit Laboratories, Inc., in Madison, NJ.
2102, 2085 cm-1
.
Preparation of [(EtPDI)Fe(N2)]2(μ2-N2). The compound was
prepared in a similar manner to [(MePDI)Fe(N2)]2(μ2-N2) with
1.00 g (1.56 mmol) (EtPDI)FeBr2, 0.073 g (3.20 mmol) sodium
metal, and 0.010 g (0.078 mmol) of naphthalene. Following
recrystallization from diethyl ether at -35 °C, 0.500 g (61%
yield) of [(EtPDI)Fe(N2)]2(μ2-N2) were isolated as dark green
needles. Crystals suitable for X-ray analysis were grown from a
pentane/ether solution. Analysis for C58H70N12Fe2: Calcd C,
66.54; H, 6.74; N, 16.05. Found C, 66.27; H, 7.10; N, 15.65. 1H
NMR (benzene-d6, 20 °C): δ = 0.72 (bs, 24H, CH2CH3), 1.65
(bs, 28H, C(CH3)) and CH2CH3), 7.04 (bm, 12H, m- and p-Ar),
7.64 (bs, 2H, p-py), 8.16 (bs, 4H, m-py). 13C NMR (benzene-d6,
20 °C): δ = 14.79 (CH2CH3), 18.07 (C(CH3)), 24.68 (CH2CH3),
117.04 (m- or p- py), 125.77 (m- or p- Ar), 126.19 (m- or p- Ar),
135.94 (m- or p- py), 4 peaks not located. 15N NMR (toluene-d8,
-80 °C) δ = -356.7 (bs), -331.5 (bs), -324.1 (bs). IR (toluene):
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
(35) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
ν(N2) = 2101, 2086 cm-1
.
Timmers, F. J. Organometallics 1996, 15, 1518.
ꢀ
ꢀ
(36) Campora, J.; Cartes, M. A.; Rodrıguez-Delgado, A.; Naz, A. M.;
´
Preparation of (EtPDI)Fe(THF). A 20 mL scintillation vial
was charged with 0.100 g (0.096 mmol) of [(EtPDI)Fe(N2)]2(μ2-
N2), approximately 7 mL of pentane, and approximately 3 mL
ꢀ
Palma, P.; Perez, C. M. Inorg. Chem. 2009, 48, 3679.
(37) Schmidt, R.; Welch, M. B.; Palackal, S. J.; Alt, H. G. J. Mol. Catal.
A: Chem. 2002, 179, 155.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2791
of THF. The resulting red-brown solution was stirred for 10
min. The solution was filtered through Celite, and all volatiles
removed to yield 0.100 g (95%) of a red-brown powder identi-
fied as (EtPDI)Fe(THF). Analysis for C33H41N3O1Fe: Calcd C,
(C(CH3) or Ar CH3), 18.98 (C(CH3) or Ar CH3), 24.36
(CH(CH3)2), 25.45 (CH(CH3)2), 27.97 (CH(CH3)2), 117.48 (p-
py), 121.42 (m-py), 124.58 (m-Ar), 126.65 (p-Ar), 129.84 (m-Ar),
129.61, 140.75, 145.78, 151.88, 155.49, 214.58 (quaternary
1
71.86; H, 7.49; N, 7.62. Found C, 71.58; H, 7.48; N, 7.51. H
carbons). IR(pentane, 23 °C) ν(CO) = 1896, 1967 cm-1
.
Preparation of [(MeBPDI)Fe(N2)]2(μ2-N2). A scintillation vial
was charged with 0.750 g of (MeBPDI)FeCl2 (1.21 mmol), 0.058
g of sodium metal (2.5 mmol, 2.1 equiv), and 0.008 g of
naphthalene (0.06 mmol, 0.05 equiv). To the vial was added
5 mL THF, and the resulting mixture was stirred for 1.5 h.
During this time, the solution turned from blue-green to brown.
After reduction was complete (signaled by the formation of the
brown solution), the THF was removed in vacuo. The dark
residue was taken up in diethyl ether to yield a dark orange-
brown mixture and was filtered through a plug of Celite.
Removal of the solvent in vacuo and recrystallization of the
residue from ether at -35 °C afforded 0.321 g (45%)of a brown
solid identified as [(MeBPDI)Fe(N2)]2(μ-N2). Crystals suitable
for X-ray diffraction were grown from a dilute diethyl ether
solution. 1H NMR (benzene-d6, 20 °C): δ = 1.17 (bs, 12H, aryl
CH3), 6.57-7.48 (bm, 19H, aryl-, phenyl-, and py-H). 13C NMR
(benzene-d6, 20 °C): δ = 23.2 (b, CH3), 108.2, 112.9, 124.9,
126.1, 128.6, 131.9, 134.1, 134.7, 136.7, 147.9 (b), 151.1 (b), 158.2
(b). IR (KBr): ν(N2) = 2104, 2094 cm-1. IR (toluene, 10 °C):
NMR (benzene-d6, 23 °C) δ, -6.32 (s, 6H, C(CH3)), 0.70 (t, 7.6
Hz, 12H, CH2CH3), 1.02 (bs, 4H, THF CH2), 1.52 (q, 7.6 Hz,
4H, CH2CH3), 1.90 (q, 7.6 Hz, 4H, CH2CH3), 2.40 (bs, 4H, THF
OCH2), 7.12 (d, 7.6 Hz, 2H, m-Ar), 7.61 (t, 7.6 Hz, 2H, p-Ar),
8.89 (t, 7.6 Hz, 1H, p-py), 12.36 (d, 7.6 Hz, 2H, m-py). 13C NMR
(benzene-d6, 23 °C) δ = 13.87 (CH2CH3), 24.01 (CH2CH3),
25.83 (CH2CH3), 26.94 (THF CH2), 68.85 (THF OCH2), 102.80
(m-py), 124.33 (p-Ar), 125.97 (m-Ar), 131.60 (p-py), quaternary
carbons not located.
Preparation of [(Me,iPrPDI)Fe(N2)]2(μ2-N2). The compound
was prepared in a similar manner to [(MePDI)Fe(N2)]2(μ2-N2)
with 1.00 g (1.56 mmol) of (Me,iPrPDI)FeBr2, 0.073 g (3.20
mmol) sodium metal, and 0.10 g (0.078 mmol) of naphthalene.
Following recrystallization from diethyl ether at -35 °C, 0.304 g
(37%) of [(Me,iPrPDI)Fe(N2)]2(μ-N2) was isolated as a dark
1
solid. H NMR (benzene-d6, 20 °C): δ = 0.71 (bs, 24H, CH-
(CH3)2), 0.80 - 2.50 (bm, 28H, C(CH3) and Ar-CH3 and CH-
(CH3)2), 6.50 - 7.30 (bm, 12H, m- and p- Ar), 7.50 (bs, 2H, p-py),
8.00 (bs, 4H, m-py). 13C NMR (benzene-d6, 20 °C): δ = 23.48,
25.61, 27.98, 30.58, 118.19, 123.84, 125.93, 129.61, 141.75,
ν(N2) = 2120, 2109 cm-1
.
151.21. IR (toluene): ν(N2) = 2099, 2084 cm-1
.
Allowing a benzene solution of [(MeBPDI)Fe(N2)]2(μ2-N2) to
stand at 22 °C for 48 h furnished a 1.4:1 mixture of
[(K2-MeBPDI)Fe(η6-Phenyl) and [(K2-MeBPDI)Fe(η6-Aryl), res-
pectively.
Alternate Preparation of [(Me,iPrPDI)Fe(N2)]2(μ2-N2). A 250
mL round-bottom was charged with 43.02 g (214.4 mmol) of
mercury and approximately 70 mL of toluene. With stirring,
0.215 g (9.36 mmol) of sodium metal was added to the flask, and
the resulting mixture stirred for 10 min. To this was added 1.00 g
(1.56 mmol) of (MeiPrPDI)FeBr2, and the resulting reaction
mixture was stirred for 16 h. After this time, the dark solution
was filtered through Celite, and the toluene removed in vacuo.
Recrystallization from diethyl ether at -35 °C afforded 0.350 g
(43%) of a dark red solid identified as [(Me,iPrPDI)Fe(N2)]2-
(μ-N2).
Preparation of (MeBPDI)Fe(THF)2. A 100 mL round-bottom
flask was charged with 0.100 g of [(MeBPDI)Fe(N2)]2(μ2-N2)
(0.090 mmol) and a needle valve attached. On a high vacuum
line, the vessel was evacuated, and 20 mL of THF was added by
vacuum transfer. After stirring for 20 min, the solvent was
removed in vacuo to afford 0.072 g (61%) of a dark solid
identified as (MeBPDI)Fe(THF)2. 1H NMR (benzene-d6, 20 °C):
δ = 0.90 (11 Hz, 12H, CH3), 1.45 (41 Hz, 8H, THF), 3.26 (24 Hz,
8H, THF), 6.77 (21 Hz, 4H, m-phenyl), 6.89 (19 Hz, 4H, m-aryl),
7.44 (19 Hz, 2H, p-Ar), 8.67 (26 Hz, 3H, p-pyr and p-aryl), 9.88
(18 Hz, 4H, o-phenyl), 15.30 (18 Hz, 2H, m-pyr). 1H NMR
(THF-d8, 20 °C): δ = 0.95 (41 Hz, 12H, CH3), 6.78 (60 Hz, 4H,
m-phenyl), 6.98 (62 Hz, 4H, m-aryl), 7.45 (23 Hz, 2H, p-aryl),
8.55 (21 Hz, 1H, p-pyr), 8.84 (49 Hz, 2H, p-phenyl), 9.71 (47 Hz,
4H, o-phenyl), 15.24 (48 Hz, 2H, m-pyr). 13C NMR THF-d8,
20 °C): δ = 22.7 (b, CH3), 107.1, 111.8, 118.0 (b), 124.4 (b),
125.8 (b), 128.4 (b), 131.9 (b), 132.9 (b), 136.7.
Characterization of [(K2-MeBPDI)Fe](η6-Phenyl). 1H NMR
(benzene-d6, 20 °C): δ = 2.25 (s, 6H, CH3), 2.32 (s, 6H, CH3),
3.69 (t, 6 Hz, 1H, p-Ph), 5.01 (dd, 2 Hz and 6 Hz, 2H, m-Ph), 5.59
(d, 4.1 Hz, 2H, o-Ph), 6.45 (dd, 8 Hz and 8 Hz, 1H, p-py),
6.83-7.02 (m, 11H, m- and p-aryl, and o-, m-, p-phenyl), 7.05 (d,
8 Hz, 1H, m-py), 7.60 (d, 8 Hz, 1H, m-py). 13C NMR (benzene-
d6, 20 °C): δ = 18.9 (CH3), 19.2 (CH3), 19.5 (CH3), 80.8 (p-Ph),
83.4 (o-Ph), 84.7 (m-Ph), 89.5 (quaternary-Ph), 108.9, 122.2,
123.2, 123.5, 124.0, 124.5, 126.1, 126.5, 129.3, 129.7, 130.8,
131.1, 134.1, 136.4, 143.5, 148.3, 149.4, 165.7, 168.5.
Characterization of [(K2-MeBPDI)Fe](η6-Aryl). 1H NMR
(benzene-d6, 20 °C): δ = 2.12 (s, 6H, CH3), 2.22 (s, 6H, CH3),
4.09 (t, 6 Hz, 1H, p-Ar), 5.21 (d, 6 Hz, 2H, m-Ar), 6.43 (dd, 8 Hz
and 8 Hz, 1H, p-pyr), 6.79-6.94 (m, 10H, p-py and phenyl),
7.28-7.34 (m, 2H, p-Ar and p- Ph), 7.41 (d, 8 Hz, 1H, m-pyr),
7.81 (d, 7 Hz, 2H, m-Ar). 13C NMR (benzene-d6, 20 °C): δ =
17.1 (CH3), 17.3 (CH3), 17.5 (CH3), 19.2 (CH3), 81.1, 86.3, 87.8,
108.2, 112.1, 124.9, 126.1, 127.0, 128.1, 129.0, 129.2, 129.3,
130.7, 131.9, 136.8, 162.6.
Characterization of (MeBPDI)Fe(ethyl 3,3-dimethylacrylate).
A scintillation vial was charged with 0.10 g of [(MeBPDI)Fe-
(N2)]2(μ2-N2) (0.085 mmol) and 5 mL of diethyl ether. To
this mixture was added 0.022 g of ethyl 3,3-dimethylacrylate
Preparation of rac/meso-(Me,iPrPDI)Fe(CO)2. A thick walled
glass vessel was charged with 0.100 g (0.096 mmol) of [(Me,
iPrPDI)Fe(N2)]2(μ2-N2) and approximately 20 mL of diethyl
ether. The vessel was brought out of the drybox, submerged in
liquid nitrogen, and evacuated on the high vacuum line. Four
atmospheres of carbon monoxide were added, and the vessel
and the contents warmed to room temperature after which the
solution turned from brown to green. The reaction mixture was
stirred for 1 h. The vessel was then degassed and brought back
into the drybox. The solution was filtered through Celite,
concentrated and stored at -35 °C yielding 0.42 (41%) of a
green solid identified as the major isomer of (Me,iPrPDI)Fe-
2
(CO) . Analysis for C31H35N3O2Fe: Calcd C, 69.28; H, 6.56;
N, 7.82. Found C, 69.10; H, 6.65; N, 7.95. 1H NMR (benzene-d6,
23 °C) δ = 0.87 (d, 6.8 Hz, 6H, CH(CH3)2), 1.41 (d, 6.8 Hz, 6H,
CH(CH3)2), 1.94 (s, 6H, C(CH3) or Ar CH3), 2.02 (s, 6H,
C(CH3) or Ar CH3), 2.98 (sept, 6.8 Hz, 2H, CH(CH3)2), 6.94
(d, 7.6 Hz, 2H, m-Ar), 7.03 (t, 7.6 Hz, 2H, p-Ar), 7.13 (d, 7.6 Hz,
2H, m-Ar), 7.15 (t, 7.6 Hz, 1H, p-py), 7.68 (d, 7.6 Hz, 2H, m-py).
13C NMR (benzene-d6, 23 °C) δ = 15.82 (C(CH3) or Ar CH3),
18.91 (C(CH3) or Ar CH3), 23.94 (CH(CH3)2), 25.64
(CH(CH3)2), 28.10 (CH(CH3)2), 117.36 (p-py), 121.41 (m-py),
124.45 (m-Ar), 126.69 (p-Ar), 128.51 (m-Ar), 129.61, 141.07,
145.75, 151.88, 155.53, 212.96, 215.99 (quaternary carbons).
IR(pentane, 23 °C) ν(CO) = 1915, 1973 cm-1
.
Minor isomer (obtained only as a mixture with the major): 1H
NMR (benzene-d6, 23 °C) δ = 0.89 (d, 6.8 Hz, 6H, CH(CH3)2),
1.39 (d, 6.8 Hz, 6H, CH(CH3)2), 1.94 (s, 6H, C(CH3) or Ar CH3),
2.02 (s, 6H, C(CH3) or Ar CH3), 2.91 (sept, 6.8 Hz, 2H,
CH(CH3)2), 6.96 (d, 7.6 Hz, 2H, m-Ar), 7.07 (t, 7.6 Hz, 2H,
p-Ar), 7.13 (d, 7.6 Hz, 2H, m-Ar), 7.15 (t, 7.6 Hz, 1H, p-py), 7.67
(d, 7.6 Hz, 2H, m-py). 13C NMR (benzene-d6, 23 °C) δ = 15.85

2792 Inorganic Chemistry, Vol. 49, No. 6, 2010
Russell et al.
(0.17 mmol, 2 equiv), which elicited a color change from brown
to black. After 0.5 h, the solvent was removed in vacuo, and the
crude solid was recrystallized from pentane to afford 0.088 g
(77%) of a black solid identified as (MeBPDI)Fe(ethyl 3,3-
pentane to afford 0.058 g (88%) of a dark purple solid identified
as (iPrBPDI)Fe(ethyl 3,3-dimethylacrylate). 1H NMR (benzene-
d6, 20 °C): δ = -0.22 (d, 6 Hz, 12H, CH(CH3)2), 0.97 (t, 7 Hz,
3H, OCH2CH3), 1.01 (d, 7 Hz, 12H, CH(CH3)2), 1.12 (s, 3H,
olefin-CH3), 1.60 (s, 3H, olefin-CH3), 2.07 (sep, 7 Hz, 4 H,
CH(CH3)2), 3.34 (q, 7 Hz, 2H, OCH2CH3), 3.92 (s, 1H, olefin
CH), 6.86 (t, 8 Hz, 4H, m-phenyl), 7.07 (d, 8 Hz, 4H, m-aryl),
7.48 (t, 8 Hz, 2H, p-aryl), 8.31 (t, 8 Hz, 1H, p-pyr), 8.47 (t, 8 Hz,
2H, p-phenyl), 9.53 (d, 8 Hz, 4H, o-phenyl), 12.70 (d, 8 Hz, 2H,
m-pyr). 13C NMR (benzene-d6, 20 °C): δ = 13.6, 23.3, 24.1, 30.1,
63.6, 104.6, 108.6, 116.5, 120.6, 121.1, 123.1, 123.9, 124.2, 124.4,
128.1, 129.0, 131.5, 136.6, 160.6, 167.8, 168.1, 188.0.
1
dimethylacrylate). H NMR (benzene-d6, 20 °C): δ = 0.43 (s,
3H, olefin-CH3), 0.73 (s, 1H, olefin-CH), 0.78 (t, 7 Hz, 3H,
OCH2CH3), 1.10 (s, 3H, olefin-CH3), 1.51 (s, 12H, aryl-CH3),
3.31 (q, 7 Hz, 2H, OCH2CH3), 6.83 (d, 7 Hz, 4H, m-aryl), 6.94 (t, 8
Hz, 4H, m-phenyl), 7.27 (t, 7 Hz, 2H, p-aryl), 8.18 (t, 8 Hz, 1H, p-
pyr), 8.20 (t, 8 Hz, 2H, p-phenyl), 9.43 (d, 7 Hz, 4H, o-phenyl),
11.75 (d, 8 Hz, 2H m-pyr). 13C NMR (benzene-d6, 20 °C): δ =
21.3, 27.8, 32.3, 62.3, 76.5, 104.6, 108.1, 121.2, 122.8, 124.0, 126.7,
131.0, 131.9, 135.5, 140.3, 158.9, 167.3, 167.4, 169.5, 185.2.
Characterization of (MeBPDI)Fe(ethyl 3-methylbutanoate).
A thick walled vessel was charged with a solution of 0.040 g
(0.059 mmol) of (MeBPDI)Fe(ethyl 3,3-dimethylacrylate) in
5 mL of toluene. On the high-vacuum line, 4 atm of H2 was
added, and the resulting solution was allowed to stir for 24 h
eliciting a color change from black to brown. Removal of excess
H2 and solvent yielded 0.029 g (72%) of a brown solid identified
General Procedure for Catalytic Olefin Hydrogenation. In a
typical experiment, a thick walled glass vessel was charged with a
solution containing 0.016 mmol of the desired iron compound in
0.65 g (7.72 mmol) of benzene-d6 and a magnetic stir bar. The
vessel was placed in a liquid nitrogen cooled cold well for 20 min.
Once the solution was frozen, 0.081 g (0.63 mmol) of ethyl 3,3-
dimethylacrylate was layered on the reaction mixture. The vessel
was taken out of the drybox and transferred to a high vacuum
line while continuously submerged in liquid nitrogen. Following
evacuation of the dinitrogen atmosphere, 1 atm of dihydrogen
was admitted at 80 K. The solution was then thawed and stirred
and ambient temperature. After the desired reaction time, the
vessel was vented of dihydrogen and exposed to air. Decom-
posed iron compound was removed by filtration through Celite,
and the filtrate collected into an NMR tube. Conversions were
determined by integrating the olefinic methyl resonances of the
substrate against the isopropyl methyl groups of the product.
1
as (MeBPDI)Fe(ethyl 3-methylbutanoate). H NMR (benzene-
d6, 20 °C): δ = 0.05 (d, 7 Hz, 2H, CH2), 0.24 (d, 6 Hz, 3H,
CH(CH3)2), 0.86 (m, 1H, CH(CH3)2), 1.12 (t, 7 Hz, 3H,
OCH2CH3), 1.50 (s, 12H, aryl-CH3), 2.71 (q, 7 Hz, 2H,
OCH2CH3), 6.84 (d, 7 Hz, 4H, m-aryl), 6.93 (t, 8 Hz, 4H, m-
phenyl), 7.33 (t, 7 Hz, 2H, p-aryl), 8.15 (t, 8 Hz, 1H, p-pyr), 8.20
(t, 8 Hz, 2H, p-phenyl), 9.37 (d, 8 Hz, 4H, o-phenyl), 11.86 (d, 8
Hz, 2H, m-pyr). 13C NMR (benzene-d6, 20 °C): δ = 21.4, 32.4,
40.9, 47.8, 55.5, 56.7, 104.3, 107.2, 120.9, 123.0, 126.7, 128.7,
131.0, 141.0, 159.5, 163.2, 167.5, 167.8, 186.3.1H NMR
(benzene-d6, 20 °C): δ = 0.05 (d, 7 Hz, 2H, CH2), 0.24 (d, 6
Hz, 3H, CH(CH3)2), 0.86 (m, 1H, CH(CH3)2), 1.12 (t, 7 Hz, 3H,
OCH2CH3), 1.50 (s, 12H, aryl-CH3), 2.71 (q, 7 Hz, 2H,
OCH2CH3), 6.84 (d, 7 Hz, 4H, m-aryl), 6.93 (t, 8 Hz, 4H,
m-phenyl), 7.33 (t, 7 Hz, 2H, p-aryl), 8.15 (t, 8 Hz, 1H, p-pyr),
8.20 (t, 8 Hz, 2H, p-phenyl), 9.37 (d, 8 Hz, 4H, o-phenyl), 11.86
(d, 8 Hz, 2H, m-pyr).
Acknowledgment. We thank the U.S. National Science
Foundation and Deutsche Forschungsgemeinschaft for a
Cooperative Activities in Chemistry between U.S. and German
Investigators grant. Partial financial support from the Packard
Foundation is also acknowledged. Dr. Bradley Wile and Aaron
Tondreau are acknowledged for performing preliminary experi-
ments. We also thank Dr. Eckhard Bill (Max-Planck Institute of
Characterization of (iPrBPDI)Fe(ethyl 3,3-dimethylacrylate).
Bioinorganic Chemistry, Mulheim an der Ruhr, Germany) for
€
€
assistance with the collection and the analysis of the Mossbauer data.
A
(
scintillation vial was charged with 0.060
g
of
iPrBPDI)Fe(N2)2 (0.083 mmol) and approximately 5 mL of
€
diethyl ether. To this mixture was added 0.013 g (0.10 mmol, 1.2
equiv) of ethyl 3,3-dimethylacrylate resulting in a color change
from brown to dark purple. After 0.5 h, the solvent was removed
in vacuo, and the remaining solid was recrystallized from
Supporting Information Available: Additional Mossbauer
and NMR data and crystallographic details for (EtPDI)FeN2
and [(MeBPDI)Fe(N2)]2(μ2-N2) in cif format. This material is
available free of charge via the Internet at http://pubs.acs.org.