Functional group tolerance and substrate scope in bis(imino)pyridine iron catalyzed alkene hydrogenation

Article abstract of DOI:10.1021/om701091z

The functional group tolerance and substrate scope for bis(imino)pyridine iron catalyzed olefin hydrogenation have been examined. Allyl amines, R ₂NCH₂CH=CH₂ (R = H, Me), are hydrogenated in the presence of (^iPrPDI)Fe(N₂)₂ ( ^iPrPDI = 2,6-(2,6-ⁱPr₂-C₆H ₃N=CMe)₂C₅H₃N, 1-(N ₂)₂) and the observed turnover frequencies increase with steric protection of the nitrogen donor. Likewise, ether-substituted olefins such as ethyl vinyl ether, ethyl allyl ether, and allyl ether are hydrogenated with turnover frequencies indistinguishable from the analogous α-olefins. For carbonyl-substituted alkenes, the hydrogenation activity is exquisitely sensitive to the position and type of the functional group. Esters such as dimethyl itaconate and iraas-methyl cinnamate are effectively hydrogenated at 23 °C while ketones such as 5-hexen-2-one require heating to 65 °C for efficient turnover. In contrast, conjugated α,β-unsaturated ketones induce decomposition of the iron compound. Stoichiometric experiments were conducted to probe the iron substrate interaction and to gauge the coordination affinity of the functional group relative to the alkene. In addition, several new bis(irnino)pyridine iron amine and ketone complexes were synthesized and the molecular and electronic structures probed by NMR spectroscopy, Moessbauer spectroscopy, and in one case, X-ray diffraction.

Full text of DOI:10.1021/om701091z

1470
Organometallics 2008, 27, 1470–1478
Functional Group Tolerance and Substrate Scope in
Bis(imino)pyridine Iron Catalyzed Alkene Hydrogenation
Ryan J. Trovitch, Emil Lobkovsky, Eckhard Bill,† and Paul J. Chirik*
Max-Planck Institute of Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr,
Germany, and Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell UniVersity, Ithaca, New York 14853
ReceiVed October 29, 2007
The functional group tolerance and substrate scope for bis(imino)pyridine iron catalyzed olefin
hydrogenation have been examined. Allyl amines, R2NCH2CHdCH2 (R ) H, Me), are hydrogenated in
the presence of (iPrPDI)Fe(N2)2 (iPrPDI ) 2,6-(2,6-iPr2-C6H3NdCMe)2C5H3N, 1-(N2)2) and the observed
turnover frequencies increase with steric protection of the nitrogen donor. Likewise, ether-substituted
olefins such as ethyl vinyl ether, ethyl allyl ether, and allyl ether are hydrogenated with turnover frequencies
indistinguishable from the analogous R-olefins. For carbonyl-substituted alkenes, the hydrogenation activity
is exquisitely sensitive to the position and type of the functional group. Esters such as dimethyl itaconate
and trans-methyl cinnamate are effectively hydrogenated at 23 °C while ketones such as 5-hexen-2-one
require heating to 65 °C for efficient turnover. In contrast, conjugated R,ꢀ-unsaturated ketones induce
decomposition of the iron compound. Stoichiometric experiments were conducted to probe the iron
substrate interaction and to gauge the coordination affinity of the functional group relative to the alkene.
In addition, several new bis(imino)pyridine iron amine and ketone complexes were synthesized and the
molecular and electronic structures probed by NMR spectroscopy, Mössbauer spectroscopy, and in one
case, X-ray diffraction.
Seminal studies in iron-catalyzed olefin hydrogenation utilized
Fe(CO)5 as the precursor to the active species. The Fe(CO)5-
promoted hydrogenation of methyl linoleate occurred under
thermal conditions, although temperatures in excess of 180 °C
and dihydrogen pressures of 400 psi were required for effective
turnover.6 Subsequently, Wrighton and co-workers described a
photocatalytic method for the hydrogenation of simple, unac-
tivated olefins such as 1-hexene at ambient temperature and 1
atm of dihydrogen pressure.7 Seeking to mimic the proposed
[Fe(CO)3] active species8 under mild thermal conditions, our
laboratory has been exploring reduced bis(imino)pyridine iron
compounds.9 Concurrent with our efforts, Daida and Peters
reported a family of tris(phosphino)borate iron(II) alkyl com-
plexes that serve as catalyst precursors for the catalytic
hydrogenation of simple unactivated olefins with modest
turnover frequencies at 50 °C.10 In more recent work, Casey
and Guan have reported a bifunctional, Shvo-type11 iron catalyst
for the hydrogenation of ketones.12
Introduction
Interest continues to grow in developing inexpensive and
environmentally responsible iron catalysts as alternatives to more
widely used precious metals.1 Given the indispensability of
olefin and alkyne hydrogenation in fine and commodity chemical
synthesis,2 these transformations are important targets for more
environmentally benign catalytic methods. While impressive
levels of activity, specificity, and selectivity have been achieved
with ruthenium, rhodium, and iridium hydrogenation catalysts,3
the high cost of the metal–ligand combinations as well as
stringent requirements on acceptable levels of trace metal
impurities in the resulting alkane inspire the search for alterna-
tives. Effective, highly enantioselective, metal-free organocata-
lytic transfer hydrogenations have recently been described,4
although these reactions often rely on carbonyl-containing
substrates for interaction with the iminium catalyst.5 Thus,
general methods for olefin and alkyne hydrogenation, especially
for unactivated substrates, are likely to continue to rely on
transition metal catalysts.
The aryl-substituted bis(imino)pyridine iron bis(dinitrogen)
complex, (iPrPDI)Fe(N2)2 (iPrPDI ) 2,6-(2,6-iPr2-C6H3NdCMe)2-
C5H3N, 1-(N2)2), has been shown to be an effective precursor
* To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
(6) Frankel, E. N.; Emken, E. A.; Peters, H. M.; Davison, V. L.;
Butterfield, R. O. J. Org. Chem. 1964, 29, 3292.
(7) (a) Schroeder, M. A.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98,
551. (b) Schroeder, M. A.; Wrighton, M. S. J. Organomet. Chem. 1977,
128, 345.
† Max-Planck Institute of Bioinorganic Chemistry.
(1) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. ReV. 2004, 104,
6217.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(8) Kismartoni, L. C.; Weitz, E.; Cedeno, D. L. Organometallics 2005,
24, 4714.
(3) (a) Knowles, W. S. AdV. Synth. Catal. 2003, 345, 3. (b) Noyori, R.
(9) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
Angew. Chem., Int. Ed. 2002, 41, 2008.
(4) (a) Tuttle, J. B.; Ouellet, S. G.; MacMillan, D. W. C. J. Am. Chem.
Soc. 2006, 128, 12662. (b) Ouellet, S. G.; Tuttle, J. B.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2005, 127, 32. (c) Mayer, S.; List, B. Angew. Chem.,
Int. Ed. 2006, 45, 4193.
(10) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474.
(11) (a) Casey, C. P.; Johnson, J. B. J. Am. Chem. Soc. 2005, 127, 1883.
(b) Casey, C. P.; Bikhanova, G. A.; Guzei, I. A. J. Am. Chem. Soc. 2006,
128, 2286. (c) Shvo, Y.; Czarkie, D.; Rahamim, Y. J. Am. Chem. Soc. 1986,
108, 7400.
(5) For an example of imine reduction see: Rueping, M.; Antonchick,
A. P.; Theismann, T. Angew. Chem., Int. Ed. 2006, 45, 3683.
(12) Guan, H.; Casey, C. P. J. Am. Chem. Soc. 2007, 129, 5816.
10.1021/om701091z CCC: $40.75
2008 American Chemical Society
Publication on Web 03/13/2008

Bis(imino)pyridine Iron-Catalyzed Alkene Hydrogenation
Organometallics, Vol. 27, No. 7, 2008 1471
Table 1. Catalytic hydrogenation of amino-substituted olefins and
4-methyl-1-pentene with 1-(N2)2
Figure 1. Synthesis and electronic structure of 1-(N2)2.
for the catalytic hydrogenation and hydrosilylation of olefins
and alkynes9 and for the [2π + 2π] cycloisomerization of R,ω-
dienes.13 In catalytic hydrogenation reactions, efficient turnover
was observed with unactivated olefins at low catalyst loadings,
typically 0.3 mol %, and levels as low as 40 ppm were also
effective. More detailed investigations into the electronic
structure of 1-(N2)2 and related neutral ligand derivatives14 by
Mössbauer, infrared and NMR spectroscopy, and DFT calcula-
tions established a two-electron-reduced bis(imino)pyridine
chelate and a ferrous center (Figure 1).15
Relatively weak field, σ-donating ligands such as amines,
dinitrogen, and tetrahydrothiophene produce intermediate spin
(SFe ) 1) ferrous centers that are antiferromagnetically coupled
to triplet (SL ) 1) chelate dianions.14,15 For amines and N2, a
triplet excited state arising from the singlet (SL ) 0) chelate
diradical mixes with the ground state resulting in temperature
independent paramagnetism.14,15 These contributions are mini-
mized with stronger field ligands such as phosphines.14 The
strongest field donors such as carbon monoxide and tert-
butylisocyanide produce low-spin iron(II), d6 compounds also
with doubly reduced bis(imino)pyridines.14,15 Both Mössbauer
spectroscopy and computational studies demonstrate that con-
tributions from an iron(0) canonical form with a high degree
of covalency are also important.
If iron-catalyzed olefin hydrogenation is to enjoy the utility
of well-established rhodium and ruthenium compounds, pre-
catalysts that exhibit good functional group tolerance and a wide
substrate scope must be developed.16 Inspired by recent targets
identified by the pharmaceutical industry,16,17 the hydrogenation
of a range of substituted alkenes catalyzed by 1-(N2)2 was
studied. Here we describe the initial results of those efforts,
report a broad substrate scope for iron-catalyzed olefin hydro-
genation, and probe iron-substrate interactions by NMR and
Mössbauer spectroscopy.
a Determined by 1H NMR spectroscopy. b Estimated from 1H NMR
spectroscopy at 23 °C.
atmospheres of dihydrogen were used for each catalytic experi-
ment.19 Efficient turnover was also observed in toluene and
pentane solution. The progress of the reaction was monitored
1
by H NMR spectroscopy and the experimentally determined
turnover frequencies and conversions are reported in Table 1.
In general, the turnover frequencies for allyl group hydro-
genation increased with increased nitrogen substitution. The
slowest member of the series was allyl amine, which reached
only 20% conversion after 24 h. Increased methylation of the
nitrogen also increased the turnover frequency such that N,N-
dimethylallylamine was hydrogenated at a rate indistinguishable
from that of the analogous hydrocarbon, 4-methyl-1-pentene
(Table 1).
Catalytic deuteration experiments were also conducted to
gauge competition from chain running processes. Addition of
four atmospheres of deuterium gas to N,N-dimethylallylamine
in the presence of 1.5 mol % of 1-(N2)2 yielded the correspond-
ing propyl amine with deuterium incorporation exclusively in
the 2 and 3 positions, providing no evidence for chain running
at this D2 pressure (eq 1). This experiment was conducted at
65 °C to ensure facile conversion to the desired product. No
deuterium incorporation was observed in the N-H positions,
suggesting that oxidative addition of the amine is not competitive
during catalytic turnover.
Results and Discussion
Amino Olefins. The first class of substrates examined were
substituted allyl amines. Interest in these molecules derives from
the importance of amines in numerous pharmaceutical processes
and final products.18 Our studies commenced with the catalytic
hydrogenation of a series of commercially available allyl amines,
R2NCH2CHdCH2 (R ) H, Me), as representative examples.
Standard conditions employing 0.3 mol % of 1-(N2)2 in a 0.915
M solution of the desired substrate in benzene-d6 with four
(1)
A series of stoichiometric reactions between 1-(N2)2 and the
amines presented in Table 1 was carried out to gain additional
insight into the interaction between the substrate and the iron
center. Addition of 1 equiv of allylamine to 1-(N2)2 resulted in
liberation of dinitrogen gas and yielded a brown solid identi-
fied as 1-NH2CH2CHdCH2 (eq 2). Analogous products,
1-NH2CH2CH2CH3 and 1-NH(Me)CH2CHdCH2, were iso-
lated by using a similar procedure with propylamine and
N-methylallylamine, respectively. Experiments with N,N-di-
methylallylamine and 4-methyl-1-pentene produced no change
in the benzene-d6 1H NMR spectra of 1-(N2)2. These observa-
(13) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2006, 128, 13340.
(14) Bart, S. C.; Bill, E.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J.
Inorg. Chem. 2007, 46, 7055.
(15) 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.
(16) Shimizu, H.; Nagasaki, I.; Matsumura, K.; Sayo, N.; Saito, T. Acc.
Chem. Res. 2007, 40, 1385.
(19) All catalytic hydrogenation reactions were conducted in benzene-
d6 or toluene as the solvent. Pentane may also be used and in some cases
has been shown to produce higher activity. See: Archer, A. M.; Bouwkamp,
M. W.; Cortez, M. P.; Lobkovsky, E.; Chirik, P. J. Organometallics 2006,
25, 4269.
(17) Johnson, N. B.; Lennon, I. C.; Moran, P. H.; Ramsden, J. A. Acc.
Chem. Res. 2007, 40, 1291.
(18) Janvier, P.; Sun, X.; Bienaymé, H.; Zhu, J. J. Am. Chem. Soc. 2002,
124, 2560.

1472 Organometallics, Vol. 27, No. 7, 2008
TroVitch et al.
Table 2. Catalytic Hydrogenation of Oxygen-Substituted and
tions suggest that both amine and olefin coordination are
minimal for these substrates at 23 °C and 1 atm of N2.
Halogenated Olefins with 1-(N2)2
(2)
Equilibrium constants (Table 1) for amine coordination to
1-(N2)2 were also examined as a function of nitrogen substituent
and alkyl chain. Addition of 1 equiv of propylamine to
1-NH2CH2CHdCH2 furnished a 2:1 mixture of the saturated
and unsaturated amine complexes. An identical ratio of products
was obtained when the converse experiment was conducted,
whereby allyl amine was added to 1-NH2CH2CH2CH3. Finally,
addition of 1 equiv of both propyl and allyl amine to 1-(N2)2
yielded the same product distribution. This series of experiments
established that the coordination of the saturated amine is
slightly favored (Keq ) 2.0) at 23 °C.
To gauge the preferences of 1-NH2CH2CHdCH2 versus
1-NH(Me)CH2CHdCH2 coordination, 1-(N2)2 was treated with
an excess of each olefin (in equimolar quantities) and an 85:15
ratio, favoring the allyl amine compound, was formed. From
these data, the equilibrium constant for coordination of N-
methylallylamine to 1-(N2)2 was estimated as 15 M-1 at 23 °C.
Exchange between free and coordinated allylamine was also
observed by EXSY spectroscopy at 23 °C with a mixing time
of 200 ms, demonstrating that H2NCH2CHdCH2 is a labile
ligand.
a All catalytic reactions carried out with 5 mol % 1-(N2)2. Progress of
the reaction and turnover determined by 1H NMR spectroscopy.
b Catalytic reaction conducted at 65 °C. c Data from ref 9; trans-ꢀ-
methylstyrene is included for comparison.
benzene-d6 1H NMR spectrum. The 13C resonance of the former
substituent was observed as a broadened peak at 39.17 ppm.
Broadening of this peak and the chemical shifts of the other
resonances are consistent with diradical character for the
delocalized π-system in the plane of the iron center and are
also consistent with the results of DFT calculations and
spectroscopic data reported for 1-DMAP.15
Previously, our laboratory has reported the synthesis and
t
characterization of 1-DMAP15 and 1-NH2 Bu14 and established
diamagnetic ground states for both compounds with contribu-
tions from temperature independent paramagnetism arising from
mixing of an S ) 1 excited state via spin–orbit coupling. The
1H NMR spectroscopic and magnetic data for amine com-
pounds prepared in this work, 1-NH2CH2CHdCH2 and
1-NH(Me)CH2CHdCH2, are consistent with a similar elec-
tronic structure. For 1-NH(Me)CH2CHdCH2, the number of
1H and 13C resonances observed is consistent with a molecule
of C2V symmetry, establishing fast iron-nitrogen (and possibly
N-C) bond rotation on the time scale of the experiment.
Oxygen-Substituted Alkenes. Alkenes bearing various oxygen-
containing functional groups were also explored in catalytic
hydrogenation reactions promoted by 1-(N2)2. The substrates
examined, conversions, and corresponding turnover frequencies
are reported in Table 2. Each catalytic reaction was conducted
with 5 mol % 1-(N2)2 in benzene-d6 at 23 °C with 4 atm of H2
mimicking the conditions used previously for the hydrogenation
of dimethyl itaconate.9 To prevent side reactions,20 solutions
containing the iron compound were frozen at liquid nitrogen
temperature and the substrate and H2 gas were added before
thawing. Conversions were determined by 1H NMR spectroscopy.
1
Variable-temperature H NMR experiments in toluene-d8 es-
tablished a coalescence temperature of -25 °C for the imine
methyl groups corresponding to a barrier of rotation of 11.2
kcal/mol. At 23 °C, the in-plane methyl groups appear at -6.55
ppm and the m-pyridine peak is located at 12.62 ppm in the
(20) At low hydrogen pressures, many oxygenated substrates undergo
competing C–O bond cleavage: Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J.
to be published.

Bis(imino)pyridine Iron-Catalyzed Alkene Hydrogenation
Organometallics, Vol. 27, No. 7, 2008 1473
electronic structure of the resulting complexes. Addition of 1
equiv of acetophenone to 1-(N2)2 furnished a dark green,
diamagnetic powder identified as the iron acetophenone com-
plex, (iPrPDI)FeOC(Ph)Me (1-OC(Ph)Me). By using a similar
procedure, bis(imino)pyridine iron compounds of benzophenone,
5-hexen-2-one, 2-hexanone, benzylacetone, and (+)-dihydro-
carvone were also isolated (eq 4).
Figure 2. R,ꢀ-Unsaturated ketones that induce decomposition of
1-(N2)2 under catalytic hydrogenation conditions.
The catalytic hydrogenation reactions exhibited remarkable
sensitivity to the nature and position of the oxygenated
functional group. Introduction of ether functionality has little
impact on the hydrogenation turnover frequencies. Ethyl vinyl
ether, allyl ethyl ether, and allyl ether hydrogenated at rates
indistinguishable from pure hydrocarbon R-olefins.9 In contrast,
carbonyl substitution had a dramatic impact on catalytic activity.
The ketone, 5-hexen-2-one, required heating to 65 °C for
reasonable turnover while (+)-dihydrocarvone was not hydro-
genated even after heating for 15 h (65 °C). The lack of
hydrogenation of the latter substrate is likely due to the inability
of the gem-disubstituted olefin to effectively compete with
ketone coordination.
Carboxylated alkenes produced varied hydrogenation activi-
ties. Specifically, trans-methyl cinnamate exhibited a turnover
frequency indistinguishable from that of trans-ꢀ-methylstyrene,
while vinyl acetate and allyl acetate produced no turnover.
Previous work from our laboratory has also demonstrated that
dimethyl itaconate was effectively hydrogenated in the presence
of 1-(N2)2.9 Importantly, internal and trisubstituted olefins were
rapidly hydrogenated upon introduction of an ethyl-substituted
ester. In contrast, attempts to hydrogenate related sterically
hindered, unactivated tri- and tetrasubstituted olefins such as
2-methyl-2-butene and 2,3-dimethyl-2-butene produced only
minimal turnover under similar conditions.
(4)
Each ketone studied formed an isolable bis(imino)pyridine
iron carbonyl complex that exhibits spectroscopic features
consistent with temperature independent paramagnetism.15 The
5-hexen-2-one compound, 1-OC(Me)(3-butenyl), exhibits no
evidence for coordination of the alkene at -60 °C as judged
Flourinated olefins are also well tolerated by the iron catalyst.
Both 4-flourostyrene and pentaflurostyrene were hydrogenated
at rates similar to those observed with styrene9 and trans-ꢀ-
methylstyrene, demonstrating little interaction or decomposition
of the iron from introduction of the halogen substituents.
The possibility of chain running during iron-catalyzed olefin
hydrogenation was probed by the addition of deuterium gas to
1
by H NMR spectrscopy. As with the iron amine compounds,
exchange studies were conducted to assay when carbonyl
coordination was labile and reversible. Addition of 2-hexanone-
1,1,1,3,3-d5 to a benzene-d6 solution of 1-OC(Me)(nBu) resulted
in slow exchange over the course of 1 h at 23 °C (eq 5),
contrasting amine exchange that occurs on the NMR time scale.
A similar rate of exchange was observed upon addition of the
partially deuteratated 2-hexanone to 1-OC(Me)(3-butenyl).
These studies establish that the relative coordination affinity of
the carbonyl group compared to the olefin is greater than that
in the corresponding amine cases where rapid hydrogenation
was observed at 23 °C and lower catalyst loadings.
2
5-hexen-2-one. Monitoring the experiment by H NMR spec-
troscopy established incorporation of the isotopic label in only
the 5 and 6 positions of product, demonstrating that chain
running was not operative under the conditions employed.
The lack of catalytic turnover observed upon attempted
hydrogenation of R,ꢀ-unsaturated ketones was also probed in a
series of stoichiometric experiments. Treatment of 1-(N2)2 with
each of the enones presented in Figure 2 resulted in almost
immediate decomposition of the iron compound and formation
of free iPrPDI ligand. Identical results were obtained when the
addition reaction was conducted under a dihydrogen atmosphere.
One exception is (-)-(R)-carvone, where a bis(imino)pyridine
iron ketone compound, 1-Car, exhibiting features consistent
with temperature independent paramagnetism was observed by
1H NMR spectroscopy (eq 6). However, 1-Car decomposed to
a complex mixture of paramagnetic products over the course
of approximately 5 min at 23 °C. These rapid decomposition
reactions account for the inability to hydrogenate this class of
substrate and are likely a result of the relatively low one-
electron-reduction potential of R,ꢀ-unsaturated ketones as
(3)
Attempts to extend the scope of bis(imino)pyridine iron-
catalyzed olefin hydrogenation to R,ꢀ-unsaturated ketones have
been unsuccessful (Figure 2). No hydrogenation of the alkene
in trans-chalcone, trans-4-hexen-3-one, trans-4-phenyl-3-buten-
2-one, 2-cyclohexenone, or carvone was observed with 10 mol
% of 1-(N2)2 and 4 atm of dihydrogen. These substrates induced
rapid decomposition of the iron compound at 23 °C, precluding
examination of the hydrogenation reactions.
Stoichiometric Experiments. The remarkable sensitivity
exhibited by 1-(N2)2 to the type and position of oxygen-
containing functional groups prompted a more thorough inves-
tigation into the iron-substrate interaction. Initial studies were
conducted with commercially available ketones to gain insight
into their coordination chemistry, and the molecular and

1474 Organometallics, Vol. 27, No. 7, 2008
TroVitch et al.
Table 3. Benzene-d6 1H NMR Chemical Shifts of the Imine Methyl
Groups and m- and p-Pyridine Resonances for Bis(imino)pyridine
Iron Ketone Compounds Exhibiting Temperature Independent
Paramagnetism
δ (ppm)
compd
NdCMe
m-pyridine
p-pyridine
iPrPDI
1-OC(Ph)Me
1-OCPh2
1-OC(Me)(3-butenyl)
1-OC(Me)(nBu)
1-OC(Me)(PhEt)
1-DHC
2.27
-2.23
-18.05
-2.87
-3.07
-3.04
-2.88, -3.39
8.50
10.05
13.83
10.49
10.59
10.53
10.84, 10.47
7.28
8.68
36.94
8.51
8.51
8.53
a
8.56, 8.46
a Values at 23 °C.
complex prepared from (+)-dihydrocarvone, 1-DHC, was
isolated as a mixture of isomers. Subsequent recrystallization
from pentane resulted in isolation of predominantly (∼6:1) one
diastereomer. For 1-OC(Me)(3-butenyl), it is possible that the
pendant olefin coordinates to the iron center to form a chelate;
however, the 1H NMR shifts of the alkene protons are not
significantly perturbed from those of the free olefin and suggest
little or no interaction with the metal center.22
(5)
compared to the corresponding carbonyl-substituted alkenes
lacking conjugation.21 Importantly, R,ꢀ-unsaturated esters are
well tolerated by the iron catalyst and are also sufficiently
reducing to prevent catalyst decomposition.
One notable compound is 1-OCPh2. The 1H NMR spectrum
exhibits features suggestive of a somewhat different electronic
structure than those of the other ketone compounds. In benzene-
d6 at 23 °C, resonances are observed over a 55 ppm chemical
shift window, much broader than the usual 15–20 ppm range.
The imine methyl groups appear substantially upfield at -18.05
ppm while the p-pyridine appears downfield at 36.94 ppm.
Notably, the resonances for the orthogonal aryl substituents are
broadened and are not sufficiently resolved to exhibit typical
3JH-H couplings. All of the peaks are temperature dependent and
move toward the diamagnetic region of the spectrum upon
cooling in toluene-d8 to -80 °C. The peaks remain broad and
featureless at low temperatures. This behavior contrasts typical
bis(imino)pyridine iron neutral ligand compounds which exhibit
3
1H NMR spectra with expected JH-H couplings and chemical
shifts that are temperature independent.14,15
The solid state structure of 1-OCPh2 was determined by
X-ray diffraction and is depicted in Figure 3. Selected bond
distances and angles are presented in Table 4. The geometry of
the iron is best described as idealized square planar with the
angles around the metal summing to 360°. The phenyl rings of
the benzophenone are nearly orthogonal to the iron chelate plane
but are canted with respect to each other. This arrangement is
likely to avoid unfavorable interactions with the sterically
demanding imine aryl groups.
The metrical parameters for the bis(imino)pyridine ligand in
1-OCPh2 (Table 4) are consistent with two-electron chelate
reduction.14,15 The imine NdC bonds are elongated to 1.342(2)
Å while the C(2)-C(3) and C(7)-C(8) bonds are contracted
to 1.414(3) and 1.411(3) Å, respectively. Distortions of this type
are consistent with population of the π-system that is antibond-
ing with respect to the imine and bonding with respect to the
Cpyr-Cimine bonds.15 The carbonyl bond distance, O(1)-C(34),
of 1.262(4) Å is slightly elongated compared to the value of
1.23(1) Å determined for free benzophenone.23
(6)
In all but one case, benzophenone (vide infra), the benzene-
d6 1H NMR spectra of the bis(imino)pyridine iron ketone
complexes exhibit features consistent with temperature inde-
pendent paramagnetism, arising from mixing of an S ) 1 excited
state with the diamagnetic ground state via spin–orbit coupling.14
Diagnostic chemical shifts for each compound are reported in
Table 3. The imine methyl resonances are shifted upfield of
the diamagnetic reference values (free iPrPDI ligand) and appear
upfield of 0 ppm. Likewise, the m- and p-pyridine peaks are
shifted downfield relative to those of the free ligand (Table 3).
Asymmetrically substituted ketones exhibited the number of
bis(imino)pyridine resonances at 23 °C consistent with C2V
symmetry, likely arising from rapid rotation about the Fe-O
bond on the NMR time scale. Cooling a toluene-d8 solution of
1-OC(Ph)Me to -70 °C slowed the dynamic process and
broadened the peaks although the static limit and hence
estimation of the kinetic barrier was not achieved. The iron
One notable feature of the structure is the short Fe-O bond
length of 1.7924(3) Å. This value is considerably contracted
(22) (a) Casey, C. P.; Lee, T. Y.; Tunge, J. A.; Carpenetti, D. W. J. Am.
Chem. Soc. 2001, 123, 10762. (b) Carpentier, J. F.; Wu, Z.; Lee, C. W.;
Stromberg, S.; Christopher, J. N.; Jordan, R. F. J. Am. Chem. Soc. 2000,
122, 7750.
(21) For example, the one electron reduction potential of trans-4-phenyl-
3-buten-2-one in acetonitrile is -2.01 V while that of the corresponding
saturated compound, methyl-3-phenylpropionate, does not occur before
reduction of the solvent. Value referenced to ferrocene/ferrocenium.
(23) Fleischer, E. B.; Sung, N.; Hawkinson, S. J. Phys. Chem. 1968,
72, 4311.

Bis(imino)pyridine Iron-Catalyzed Alkene Hydrogenation
Organometallics, Vol. 27, No. 7, 2008 1475
Figure 4. Zero-field Mössbauer spectra of 1-OCPh2 and
1-OC(Ph)Me recorded at 80 K.
Figure 3. Molecular structure of 1-OCPh2 at 30% probability
Mössbauer Spectroscopy. The electronic structures of
representative bis(imino)pyridine iron ketone compounds were
also studied by Mössbauer spectroscopy. Zero-field spectra of
1-OC(Ph)Me and 1-OCPh2 were recorded at 80 K (Figure 4).
The experimentally determined isomer shift (δ) of 0.31 mm · s-1
for 1-OC(Ph)Me is consistent with an intermediate spin iron(II)
center and similar to values observed with other four-coordinate
bis(imino)pyridine iron compounds with neutral ligands.14,15
Despite different NMR spectroscopic properties, the iron
benzophenone compound, 1-OCPh2, exhibits a similar isomer
shift of 0.28 mm · s-1, also consistent with an intermediate spin,
d6 ferrous ion. On the basis of the X-ray diffraction and
Mössbauer data, 1-OCPh2 appears to have a similar electronic
structure to other bis(imino)pyridine iron ketone compounds.
As stated previously, the unique 1H NMR spectrum of the
compound is likely a result of radical character on the carbonyl
carbon and a contribution from an iron(III) center rather than a
gross change in electronic structure.
Therefore, the electronic structures for the amine and ketone
compounds prepared in this work are similar to those of other
bis(imino)pyridine neutral ligand derivatives.14 The observed
S ) 0 ground states are readily accommodated by an intermedi-
ate spin ferrous ion antiferromagnetically coupled to a chelate
dianion. Mixing via spin–orbit coupling of an energetically
similar S ) 1 excited state accounts for the unusual NMR
chemical shifts reported in Table 3.
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 4. Selected bond distances (Å) and angles (deg) for 1-OCPh2
N(1)-C(2)
1.342(2)
1.342(2)
1.414(3)
1.411(3)
1.262(2)
1.9113(14)
1.8226(15)
1.9225(14)
1.7924(3)
167.05(11)
80.72(6)
179.15(6)
360
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
O(1)-C(34)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-O(1)
Fe(1)-O(1)-C(34)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-O(1)
sum of angles about Fe(1)
from the distances found in related iron alkoxide (dFe-O ∼ 1.85
Å) and carboxylate complexes (dFe-O ∼ 2.10 Å) and is suggestive
of multiple bond character. By way of comparison, Holland and
co-workers reported an Fe-O distance of 1.8076(16) Å in the
iron(II) ꢀ-diiminate iron alkoxide complex, LMeFeOCHPh2,24
while Aldridge and co-workers reported an Fe-O distance of
1.982(3) Å and a carbonyl C-O distance of 1.256(5) Å in (η5-
C5Me5)Fe(CO)2(OCPh2)]+.25 In fact, the short Fe-O distance
observed in 1-OCPh2 is reminiscent of iron-oxygen bond
distances of 1.77 Å in µ-oxo diferrous26 and ferric compounds27
and suggests significant contribution from an iron(III) center
with a C(34)-centered radical. This view of the electronic
structure also accounts for the peak broadening observed by
1H NMR spectroscopy.
Concluding Remarks
In summary, the bis(imino)pyridine iron bis(dinitrogen)
complex, 1-(N2)2, is a precatalyst for the hydrogenation of olefins
in the presence of unprotected amines, various carbonyls, and
fluorinated hydrocarbons. More detailed investigations into the
iron-substrate interaction established formation of bis(imino)-
pyridine iron-substrate complexes where the coordination
affinity of the functional group (e.g., amines, carbonyls) was
inversely proportional to the turnover frequency. These studies
also demonstrated that R,ꢀ-unsaturated ketones induce rapid
decomposition of the iron compound, likely due to a relatively
low one-electron redox potential. In contrast, R,ꢀ-unsaturated
esters are readily tolerated by the iron catalyst resulting in
efficient hydrogenation turnover.
Both the reduction potential and infrared carbonyl stretching
frequency of benzophenone compared to acetophenone support
a more thermodynamically favorable one-electron reduction of
the diaryl ketone. In acetonitrile solution, the reduction potential
of benzophenone is -2.24 V while the value for acetophenone
is shifted to -2.46 V (versus Cp2Fe+/Cp2Fe). Similarly, the
pentane solution CdO stretch for benzophenone is 1671 cm-1
while that for acetophenone is 1696 cm-1
.
(24) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.;
Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L. Organometallics 2004,
23, 5226.
(25) Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones, C.; Willock, D. J.
Organometallics 2004, 23, 2911.
Experimental Section
(26) Eckert, N. A.; Stoian, S.; Smith, J. M.; Bominaar, E. L.; Münck,
E.; Holland, P. L. J. Am. Chem. Soc. 2005, 127, 9344.
(27) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987.
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out with standard vacuum line, Schlenk,

1476 Organometallics, Vol. 27, No. 7, 2008
TroVitch et al.
and cannula techniques or in an MBraun inert atmosphere dry box
containing an atmosphere of purified nitrogen. Solvents for air- and
moisture-sensitive manipulations were initially dried and deoxy-
genated by using literature procedures.28 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 or titanocene, respectively. 1-(N2)2 and 1-Br were prepared
according to literature procedures.9,29 N-Allylmethylamine, N,N-
dimethylallylamine, propylamine, 2-hexanone, 5-hexen-2-one, 4-hex-
en-3-one, acetophenone, vinyl acetate, allyl acetate, (+)-dihydro-
carvone as a mixture of isomers, trans-ethyl crotonate, ethyl
3,3-dimethylacrylate, and 4-fluorostyrene were all purchased from
Aldrich and dried over calcium hydride for at least 24 h before
being used. Allylamine and (-)-carvone were purchased from Acros
and purified in a similar manner. 2-Cyclohexen-1-one was pur-
chased from Fisher Scientific and dried as described above.
Ethyl vinyl ether and allyl ether were purchased from Aldrich
and dried over calcium hydride for 24 h. Allyl ethyl ether was
purchased from Acros and dried over calcium hydride for 24 h
before use. 2,3,4,5,6-Pentafluorostyrene was purchased from Matrix
Scientific and was dried in a similar fashion. 4-Methyl-1-pentene
was vacuum transferred from lithium aluminum hydride after drying
overnight. Benzylacetone and trans-ꢀ-methylstyrene were purchased
from Aldrich and dried over molecular sieves before use. trans-
Methyl cinnamate and trans-chalcone were purchased from Aldrich
and dried under vacuum for 16 h. After drying, trans-chalcone was
recrystallized from a concentrated ethereal solution at -35 °C.
Benzophenone was purchased from Fisher Scientific and dried under
vacuum for 24 h. trans-4-Phenyl-3-buten-2-one was purchased from
Acros and dried in vacuo for approximately 1 h before use.
2-Hexanone-1,1,1,3,3-d5 was prepared by stirring a solution of
2-hexanone in D2O in the presence of D2SO4 and dried with MgSO4
following extraction with ether.
by using direct methods (SHELXS) completed by subsequent
Fourier synthesis and refined by full-matrix least-squares procedures.
Mössbauer data were collected 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 Mössbauer-Spectromag 2000 cyrostat.
Reported isomer shifts (δ) are referenced to iron metal at 293 K.
General Procedure for the Catalytic Hydrogenation of
Amino-Substituted Olefins. A stock solution containing 0.026 g
(0.044 mmol) of 1-(N2)2 and 15.00 g (178.3 mmol) of benzene-d6
was prepared and stored at -35 °C. Upon thawing to ambient
temperature, 0.65 g of the resulting solution was charged into a
thick-walled glass vessel. With a microsyringe, 0.633 mmol of the
desired substrate was added and the vessels were quickly sealed.
After being submerged in liquid nitrogen, the frozen solutions were
treated with 4 atm of dihydrogen. The timer was started when the
solutions began to stir upon thawing. The catalytic reactions were
quenched by again submerging the bomb in liquid nitrogen,
evacuating the remaining dihydrogen, and transferring the volatile
components of the resulting solution to a J. Young tube for analysis
by 1H NMR spectroscopy. Conversions were measured by integrat-
ing the residual olefin resonances against their saturated counterparts.
Catalytic Deuteration of Allylamine. A J. Young tube was
charged with 0.012 g (0.020 mmol) of 1-(N2)2 and approximately
0.65 g of benzene. With a microsyringe, 0.076 g (100 µL, 1.33
mmol) of allylamine was added to the tube. This solution was then
submerged in liquid nitrogen, the tube was evacuated, and 4 atm
of deuterium was added. The tube was placed in a 65 °C bath for
approximately 1 week before the solution was analyzed by 2H NMR
spectroscopy.
Catalytic Hydrogenation of Oxygen-Substituted Olefins. For
each independent trial, a thick-walled glass vessel was charged with
a solution containing 0.019 g (0.032 mmol) of 1-(N2)2 in 0.65 g
(7.72 mmol) of benzene-d6. After standing in a liquid nitrogen
chilled cold well for approximately 20 min, 0.633 mmol of the
desired substrate was added to the vessel. Immediately after
addition, the vessel was submerged in liquid nitrogen to prevent
reaction of the substrate and catalyst. On the high-vacuum line, 4
atm of dihydrogen was added and the solution was warmed to
ambient temperature or 65 °C. At the desired reaction time, the
vessel was opened to air and the catalyst solution was filtered
through a glass frit into an NMR tube. Conversions were determined
by integrating the remaining 1H NMR olefin resonances against
their saturated analogues.
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.76, 399.78, and
500.62 MHz, respectively. All spectra were obtained at 23 °C unless
2
stated otherwise. H NMR spectra were recorded at 20 °C on the
Inova 400 and 500 spectrometers operating at 61.37 and 76.85 MHz,
respectively. 13C NMR spectra were recorded on the same
spectrometers operating at 101.535 or 125.893 MHz, respectively.
All 1H and 13C NMR chemical shifts are reported relative to SiMe4
with 1H (residual) and 13C chemical shifts of the solvent as a
secondary standard. For complexes exhibiting temperature inde-
pendent magnetism, many assignments were made based on COSY,
HSQC, and HMBC NMR experiments. Solution magnetic moments
were determined by Evans method30 with a ferrocene standard and
are the average value of at least two independent measurements.
1H NMR multiplicity and coupling constants are reported where
applicable. The peak width at half-height is given for paramag-
netically broadened resonances. Elemental analyses were performed
at Robertson Microlit Laboratories, Inc., in Madison, NJ.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and
then quickly transferred to the goniometer head of a Bruker X8
APEX2 diffractometer equipped with a molybdenum X-ray tube
(λ ) 0.71073 Å). Preliminary data revealed the crystal system. A
hemisphere routine was used for data collection and determination
of lattice constants. The space group was identified and the data
were processed by using the Bruker SAINT+ program and
corrected for absorption with SADABS. The structures were solved
Catalytic Deuteration of 5-Hexen-2-one. This reaction was
conducted in a manner similar to that of the oxygenated olefins
with an identical catalyst loading in benzene solution with 4 atm
of deuterium gas. The reaction was opened to air after 90 min at
65 °C and the solution was filtered through a glass frit into an NMR
tube. Deuterium incorporation was determined by 2H NMR
spectroscopy.
Preparation of (iPrPDI)Fe(NH2CH2CHdCH2) (1-NH2CH2-
CHdCH2). A 20-mL scintillation vial was charged with 0.100 g
(0.168 mmol) of 1-(N2)2 and approximately 10 mL of pentane. With
a microsyringe, 0.010 g (13 µL, 0.168 mmol) of allylamine was
added to the stirring solution. Upon addition of the amine, dinitrogen
evolution was observed and a reddish-brown solution formed. After
20 min, the solution was filtered though a frit and the solvent was
removed in vacuo to yield 0.077 g (77%) of a dark brown solid
identified as 1-NH2CH2CHdCH2. Anal. Calcd for C36H50FeN4:
1
C, 72.71; H, 8.48; N, 9.42. Found: C, 72.63; H, 8.77; N, 8.99. H
NMR (benzene-d6) δ 12.05 (d, 7.5 Hz, 2H, m-pyr), 8.77 (t, 7.5 Hz,
1H, p-pyr), 7.63 (t, 8.0 Hz, 2H, p-aryl), 7.24 (d, 8.0 Hz, 4H, m-aryl),
5.35 (m, 1H, CH2CHdCH2), 4.90 (t, 7.5 Hz, 2H, NH2CH2), 4.76
(d, 10.5 Hz, 1H, CH2CHdCH2), 4.70 (d, 17.0 Hz, 1H,
CH2CHdCH2), 2.78 (sept., 7.0 Hz, 4H, CH(CH3)2), 1.63 (m, 2H,
(28) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(29) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406.
(30) Sur, S. K. J. Magn. Reson. 1989, 82, 169.

Bis(imino)pyridine Iron-Catalyzed Alkene Hydrogenation
Organometallics, Vol. 27, No. 7, 2008 1477
1
NH2CH2), 1.24 (d, 7.0 Hz, 12H, CH(CH3)2), -0.34 (d, 7.0 Hz,
12H, CH(CH3)2), -6.09 (s, 6H, C(CH3)). 13C{1H} NMR (benzene-
d6) δ 189.45, 165.16, 164.61 (quaternary carbons), 140.00 (p-pyr),
137.26 (CH2CHdCH2), 136.57, 125.25 (p-aryl), 124.43 (m-aryl),
116.86 103.09 (m-pyr), (CH2CHdCH2), 44.81 (NH2CH2), 39.17
(br s C(CH3)), 28.53 (CH(CH3)2), 24.23 (CH(CH3)2), 23.15
(CH(CH3)2).
Preparation of (iPrPDI)Fe(NH(Me)CH2CHdCH2) (1-NH(Me)-
CH2CHdCH2)). This molecule was prepared in a manner similar
to 1-NH2CH2CHdCH2 with 0.042 g (0.071 mmol) of 1-(N2)2 and
0.005 g (7 µL, 0.071 mmol) of N-allylmethylamine to yield 0.023
g (53%) of a dark brown solid identified as 1-NH(Me)CH2-
CHdCH2. Anal. Calcd for C37H52N4Fe: C, 73.01; H, 8.61; N, 9.20.
Found: C, 72.79; H, 8.25; N, 8.96. 1H NMR (benzene-d6) δ 12.62
(d, 8.0 Hz, 2H, m-pyr), 8.82 (t, 8.0 Hz, 1H, p-pyr), 8.28 (m, 1H,
NH(CH3)), 7.68 (t, 8.0 Hz, 2H, p-aryl), 7.24 (d, 8.0 Hz, 4H, m-aryl),
4.99 (m, 1H, CH2CHdCH2), 4.92 (d, 10.5 Hz, 1H, CH2CHdCH2),
4.75 (d, 17.0 Hz, 1H, CH2CHdCH2), 2.16 (m, 2H, NH(CH2)), 1.22
(d, 7.0 Hz, 12H, CH(CH3)2), 1.02 (d, 7.0 Hz, 3H, NH(CH3)), 0.25
(d, 7.0 Hz, 12H, CH(CH3)2), -6.55 (s, 6H, C(CH3)), one peak not
located. 13C{1H} NMR (benzene-d6) δ 192.79, 165.77, 142.46,
137.04, 134.06, 125.50 (p-aryl), 124.54 (m-aryl), 123.91, 121.22,
103.07 (m-pyr), 52.36 (NHCH2), 40.93 (br s C(CH3)), 34.31
(NHCH3), 28.55 (CH(CH3)2), 24.76 (CH(CH3)2), 23.22 (CH(CH3)2).
73.43; H, 8.43; N, 6.41. H NMR (toluene-d8, 20 °C) δ 10.49 (br
s, 7.5 Hz, 2H, m-pyr), 7.42 (t, 8.0 Hz, 2H, p-aryl), 7.14 (d, 8.0 Hz,
4H, m-aryl), 5.29 (br s, 1H, CH2CHdCH2), 4.83 (br s 2H,
CH2CHdCH2), 2.78 (sept., 7.0 Hz, 4H, CH(CH3)2), 1.44 (br s, 2H,
CH2), 1.20 (d, 7.0 Hz, 12H, CH(CH3)2), 0.98 (br s, 2H, CH2), 0.20
1
(d, 7.0 Hz, 12H, CH(CH3)2), -2.87 (br s, 6H, C(CH3)). H NMR
(toluene-d8, -60 °C) δ 10.45 (d, 7.5 Hz, 2H, m-pyr), 8.55 (t, 7.5
Hz, 1H, p-pyr), 7.44 (t, 8.0 Hz, 2H, p-aryl), 7.12 (d, 8.0 Hz, 4H,
m-aryl), 5.30 (m, 1H, CH2CHdCH2), 4.89 (d, 11.0 Hz, 1H,
CH2CHdCH2), 4.85 (d, 17.5 Hz, 1H, CH2CHdCH2), 2.74 (sept.,
7.0 Hz, 4H, CH(CH3)2), 1.32 (m, 2H, CH2), 1.22 (d, 7.0 Hz, 12H,
CH(CH3)2), 0.97 (m, 2H, CH2), 0.22 (d, 7.0 Hz, 12H, CH(CH3)2),
0.18 (s, 3H, CO(CH3)), -2.78 (s, 6H, C(CH3)). 13C{1H} NMR
(toluene-d8, -60 °C) δ 205.11 (CdO), 171.01, 160.02, 155.51,
139.06, 137.62 (CH2CHdCH2), 137.12, 131.25 (p-pyr), 124.90 (p-
aryl), 123.53 (m-aryl), 115.97 (CH2CHdCH2), 105.09 (m-pyr),
46.96 (CH2CHdCH2), 31.62 (COCH3), 30.78 (br s C(CH3)), 28.36
(CH(CH3)2), 27.51 (COCH2), 24.24 (CH(CH3)2), 23.30 (CH(CH3)2).
Preparation of (iPrPDI)Fe(OC(Ph)Me) (1-OC(Ph)Me). This
compound was prepared in a similar manner to 1-OC(Me)(nBu)
with 0.100 g (0.168 mmol) of 1-(N2)2 and 0.020 g (20 µL, 0.168
mmol) of acetophenone to yield 0.092 g (83%) of a dark green
solid identified as 1-OC(Ph)Me. Anal. Calcd for C41H51FeN3O:
1
C, 74.87; H, 7.82; N, 6.39. Found: C, 75.02; H, 7.41; N, 6.46. H
Preparation of (iPrPDI)Fe(NH2CH2CH2CH3) (1-NH2CH2CH2CH3).
This molecule was prepared in a manner similar to 1-NH2-
CH2CHdCH2 with 0.100 g (0.168 mmol) of 1-(N2)2 and 0.010 g
(14 µL, 0.168 mmol) of allylamine to yield 0.075 g (75%) of a
dark orange solid identified as 1-NH2CH2CH2CH3. Anal. Calcd
for C36H52N4Fe: C, 72.47; H, 8.78; N, 9.39. Found: C, 72.25; H,
8.35; N, 9.00. 1H NMR (benzene-d6) δ 12.12 (d, 7.5 Hz, 2H, m-pyr),
8.75 (t, 7.5 Hz, 1H, p-pyr), 7.63 (t, 8.0 Hz, 2H, p-aryl), 7.24 (d,
8.0 Hz, 4H, m-aryl), 4.83 (t, 7.5 Hz, 2H, NH2CH2), 2.72 (sept., 7.0
Hz, 4H, CH(CH3)2), 1.25 (d, 7.0 Hz, 12H, CH(CH3)2), 0.91 (m,
2H, NH2CH2), 0.83 (m, 2H, CH2CH3), 0.41 (t, 7.0 Hz, 3H,
CH2CH3), -0.26 (d, 7.0 Hz, 12H, CH(CH3)2), -6.09 (s, 6H,
C(CH3)). 13C{1H} NMR (benzene-d6) δ 189.77, 165.29, 164.41,
140.20, 136.38 (p-pyr), 125.15 (p-aryl), 124.31 (m-aryl), 102.95
(m-pyr), 43.68 (NH2CH2), 38.82 (br s C(CH3)), 28.48 (CH(CH3)2),
26.53 (CH2CH3), 24.30 (CH(CH3)2), 23.14 (CH(CH3)2), 10.80
(CH2CH3).
NMR (benzene-d6) δ 10.05 (d, 7.5 Hz, 2H, m-pyr), 8.80 (t, 7.5 Hz,
1H, p-pyr), 7.44 (br s, 1H, p-phenyl), 7.42 (t, 8.0 Hz, 2H, p-aryl),
7.15 (d, 8.0 Hz, 4H, m-aryl), 7.13 (br s, 2H, o-phenyl), 6.69 (br s,
2H, m-phenyl), 2.70 (sept., 7.0 Hz, 4H, CH(CH3)2), 1.19 (d, 7.0
Hz, 12H, CH(CH3)2), 0.15 (d, 7.0 Hz, 12H, CH(CH3)2), -0.52 (br
s, 3H, COCH3), -2.33 (s, 6H, C(CH3)). 1H NMR (toluene-d8, -80
°C) δ 9.96 (d, 7.5 Hz, 2H, m-pyr), 8.68 (t, 7.5 Hz, 1H, p-pyr), 7.43
(t, 7.5 Hz, 2H, p-aryl), 7.26 (t, 7.5 Hz, 1H, p-phenyl), 6.83 (d, 7.0
Hz, 2H, o-phenyl), 6.69 (m, 2H, m-phenyl), 3.09 (br s, 2H,
CH(CH3)2), 2.27 (br s, 2H, CH(CH3)2), 1.28 (br m, 6H, CH(CH3)2),
1.18 (br m, 6H, CH(CH3)2), 0.33 (br s, 6H, CH(CH3)2), -0.04 (br
s, 6H, CH(CH3)2), -0.11 (s, 3H, COCH3), -2.01 (s, 6H, C(CH3)),
m-aryl peak not located. 13C{1H} NMR (benzene-d6) δ 166.42,
159.33, 154.74, 139.82, 125.56 (p-aryl), 124.01 (m-aryl), 107.43
(m-pyr), 30.37 (br s C(CH3)), 28.25 (CH(CH3)2), 24.47 (CH(CH3)2),
23.14 (CH(CH3)2), p-pyr and acetophenone peaks not located.
Preparation of (iPrPDI)Fe(OCPh2) (1-OCPh2). This compound was
prepared in a manner similar to 1-OC(Me)(nBu) with 0.100 g
(0.168 mmol) of 1-(N2)2 and 0.031 g (0.168 mmol) of benzophenone
to yield 0.082 g (68%) of a dark green solid identified as 1-OCPh2.
Anal. Calcd for C41H51FeN3O: C, 76.76; H, 7.42; N, 5.84. Found:
C, 76.52; H, 7.69; N, 5.62. Magnetic susceptibility: µeff ) 1.2 µB
Preparation of (iPrPDI)Fe(OC(Me)(nBu)) (1-OC(Me)(nBu)). A 20-
mL scintillation vial was charged with 0.100 g (0.168 mmol) of
1-(N2)2 and approximately 10 mL of pentane. While stirring, 0.017
g (21 µL, 0.168 mmol) of 2-hexanone was added by microsyringe
and the solution immediately began to evolve N2 and turn brown.
After 20 min, the solution was filtered though a frit and the solvent
was removed in vacuo to yield 0.085 g (79%) of a dark brown
solid identified as 1-OC(Me)(n-Bu). Anal. Calcd for C39H55FeN3O:
1
(benzene-d6). H NMR (benzene-d6) δ 36.94 (37 Hz, 1H, p-pyr),
29.23 (239 Hz, 1H, phenyl), 26.66 (290 Hz, 2H, phenyl), 13.83
(14 Hz, 2H, m-pyr), 5.95 (d, 7.5 Hz, 4H, m-aryl), 5.52 (t, 7.5 Hz,
2H, p-aryl), -0.65 (7 Hz, 12H, CH(CH3)2), -1.95 (11 Hz, 12H,
CH(CH3)2), -5.22 (49 Hz, 4H, CH(CH3)2), -18.05 (25 Hz, 6H,
C(CH3)), one phenyl peak not located.
1
C, 73.45; H, 8.69; N, 6.59. Found: C, 73.25; H, 8.30; N, 6.46. H
NMR (benzene-d6) δ 10.59 (d, 7.5 Hz, 2H, m-pyr), 8.51 (t, 7.5 Hz,
1H, p-pyr), 7.46 (t, 8.0 Hz, 2H, p-aryl), 7.16 (d, 8.0 Hz, 4H, m-aryl),
2.82 (sept., 7.0 Hz, 4H, CH(CH3)2), 1.21 (d, 7.0 Hz, 12H,
CH(CH3)2), 1.14 (m, 2H, butyl), 0.93 (m, 2H, butyl), 0.85 (m, 2H,
butyl), 0.71 (m, 3H, CH2CH3), 0.29 (s, 3H, CO(CH3)), 0.22 (d, 7.0
Hz, 12H, CH(CH3)2), -3.07 (s, 6H, C(CH3)). 13C{1H} NMR
(benzene-d6) δ 172.82, 160.82, 155.87, 139.11, 132.07 (p-pyr),
124.83 (p-aryl), 123.51 (m-aryl), 104.68 (m-pyr), 47.98, 31.84
(COCH3), 31.41 (br s C(CH3)), 28.43 (CH(CH3)2), 25.87 (butyl),
24.32 (CH(CH3)2), 23.37 (CH(CH3)2), 23.35 (butyl), 14.16
(CH2CH3), one peak not located.
Preparation of (iPrPDI)Fe(OC(Me)(3-butenyl)) (1-OC(Me)(3-bute-
nyl)). This compound was prepared in a similar manner to
1-OC(Me)(nBu) with 0.100 g (0.168 mmol) of 1-(N2)2 and 0.016
g (20 µL, 0.168 mmol) of 5-hexen-2-one yielding 0.041 g (38%)
of a dark brown solid identified as 1-OC(Me)(3-butenyl). Anal.
Calcd for C39H53FeN3O: C, 73.69; H, 8.40; N, 6.61. Found: C,
Preparation of (iPrPDI)Fe(OC(Me)(PhEt)) (1-OC(Me)(PhEt)).
This compound was prepared in a manner similar to 1-OC(Me)
(nBu) with 0.100 g (0.168 mmol) of 1-(N2)2 and 0.025 g (26 µL,
0.168 mmol) of 4-phenyl-2-butanone. The resulting residual solid
was recrystallized from pentane at -35 °C to yield 0.030 g (26%)
of a dark brown solid identified as 1-OC(Me)(PhEt). Anal. Calcd
for C43H55FeN3O: C, 75.31; H, 8.08; N, 6.13. Found: C, 74.99; H,
7.86; N, 5.81. 1H NMR (benzene-d6) δ 10.53 (d, 7.5 Hz, 2H, m-pyr),
8.52 (t, 7.5 Hz, 1H, p-pyr), 7.46 (t, 8.0 Hz, 2H, p-aryl), 7.17 (d,
8.0 Hz, 4H, m-aryl), 7.11 (d, 7.5 Hz, 2H, phenyl), 7.02 (t, 7.5 Hz,
1H, phenyl), 6.90 (d, 7.5 Hz, 2H, phenyl), 2.82 (sept., 7.0 Hz, 4H,
CH(CH3)2), 2.07 (m, 2H, CH2), 1.33 (m, 2H, CH2), 1.20 (d, 7.0
Hz, 12H, CH(CH3)2), 0.21 (d, 7.0 Hz, 12H, CH(CH3)2), 0.17 (s,
3H, CO(CH3)), -3.04 (s, 6H, C(CH3)). 13C{1H} NMR (benzene-
d6) δ 204.77 (CdO), 172.32, 160.75, 155.98, 139.29, 131.94,

1478 Organometallics, Vol. 27, No. 7, 2008
TroVitch et al.
129.14, 129.03, 127.01, 126.63, 124.98 (p-aryl), 123.64 (m-aryl),
104.87 (m-pyr), 49.32 (CH2), 32.58 (COCH3), 31.31 (br s C(CH3)),
29.17 (CH2), 28.44 (CH(CH3)2), 24.30 (CH(CH3)2), 23.39
(CH(CH3)2).
(DHC), 43.66 (DHC), 30.39 (br s C(CH3)), 28.38 (CH(CH3)2), 27.99
(CH(CH3)2), 24.29 (CH(CH3)2), 23.69 (CH(CH3)2), 23.39
(CH(CH3)2), 22.48 (CH(CH3)2), 20.08 (DHC CCH3), 13.53 (DHC
CHCH3), one peak not located.
Preparation of (iPrPDI)Fe(DHC) (1-DHC). This compound was
prepared in a manner similar to 1-OC(Me)(nBu) with 0.100 g
(0.168 mmol) of 1-(N2)2 and 0.025 g (28 µL, 0.168 mmol) of (+)-
dihydrocarvone as a mixture of isomers. After filtration, the solvent
was removed in vacuo and recrystallized from pentane to yield
0.087 g (75%) of a dark purple solid identified as 1-DHC as a
mixture of two diastereomers. Anal. Calcd for C43H59FeN3O: C,
74.87; H, 8.62; N, 6.09. Found: C, 74.80; H, 8.58; N, 5.83. 1H
NMR (major diastereomer, benzene-d6) δ 10.47 (d, 7.5 Hz, 2H,
m-pyr), 8.46 (t, 7.5 Hz, 1H, p-pyr), 7.42 (t, 8.0 Hz, 2H, p-aryl),
7.19 (d, 8.0 Hz, 4H, m-aryl), 4.63 (s, 1H, CdCH2), 4.52 (s, 1H,
CdCH2), 3.18 (sept., 7.0 Hz, 2H, CH(CH3)2), 2.59 (sept., 7.0 Hz,
2H, CH(CH3)2), 1.83 (m, 2H, DHC), 1.50 (s, 3H, DHC CCH3),
1.21 (d, 7.0 Hz, 6H, CH(CH3)2), 1.15 (d, 7.0 Hz, 6H, CH(CH3)2),
0.90–1.07 (m, 6H, DHC), 0.56 (d, 7.0 Hz, 6H, CH(CH3)2), 0.40
(d, 7.0 Hz, DHC CHCH3), 0.13 (d, 7.0 Hz, 6H, CH(CH3)2), -2.89
(s, 6H, C(CH3)). 1H NMR (minor diastereomer, benzene-d6) δ 10.84
(d, 7.5 Hz, 2H, m-pyr), 8.56 (t, 7.5 Hz, 1H, p-pyr), 7.50 (t, 8.0 Hz,
2H, p-aryl), 7.20 (d, 8.0 Hz, 4H, m-aryl), 4.56 (s, 1H, CdCH2),
4.45 (s, 1H, CdCH2), 2.82 (sept., 7.0 Hz, 2H, CH(CH3)2), 2.50
(m, 2H, DHC), 1.43 (s, 3H, DHC CCH3), 1.22 (d, 7.0 Hz, 6H,
CH(CH3)2), 1.19 (d, 7.0 Hz, 6H, CH(CH3)2), 0.90–1.07 (m, 6H,
DHC), 0.36 (d, 7.0 Hz, 6H, CH(CH3)2), 0.29 (d, 7.0 Hz, DHC
CHCH3), 0.18 (d, 7.0 Hz, 6H, CH(CH3)2), -3.39 (s, 6H, C(CH3)),
one peak not located. 13C{1H} NMR (major diastereomer, benzene-
d6) δ 210.52 (CdO), 171.21, 161.20, 155.18, 147.94, 139.70,
139.10, 130.96 (p-pyr), 124.54 (p-aryl), 123.74 (m-aryl), 123.47
(m-aryl), 110.38 (CdCH2), 104.31 (m-pyr), 46.96 (DHC), 46.16
Observation of (iPrPDI)Fe(Car) (1-Car). With a microsyringe,
0.005 g (5.25 µL, 0.034 mmol) of (-)-carvone was added to a
solution of 1-(N2)2 in approximately 0.70 g of benzene-d6. The
resulting bright green solution was quickly filtered through Celite
into a J. Young tube and submerged in liquid nitrogen to prevent
decomposition. The 1H NMR spectrum of this complex was
recorded immediately after thawing. The solution turned dark brown
during the course of spectral acquisition. 1H NMR (benzene-d6) δ
10.37 (d, 7.5 Hz, 2H, m-pyr), 8.62 (t, 7.5 Hz, 1H, p-pyr), 7.47 (t,
8.0 Hz, 2H, p-aryl), 7.26 (d, 8.0 Hz, 4H, m-aryl), 6.41 (s, 1H,
CdCHCH2), 4.64 (s, 1H, CdCH2), 4.56 (s, 1H, CdCH2), 2.93
(sept., 7.0 Hz, 2H, CH(CH3)2), 2.61 (m, 2H, CH(CH3)2 or carvone
CH2), 2.22 (m, 1H, carVone), 1.41 (s, 3H, carvone CH3), 1.30 (s,
3H, carvone CH3), 1.16–1.25 (m, 12H, CH(CH3)2), 0.48 (d, 7.0
Hz, 6H, CH(CH3)2), 0.09 (d, 7.0 Hz, 6H, CH(CH3)2), -0.78 (dd,
16 Hz, 1H, COCH2), -2.59 (s, 6H, C(CH3)), two peaks not located.
Acknowledgment. We thank the Packard Foundation for
support of this research. P.J.C. is a Cottrell Scholar supported
by the Research Corporation and a Camille Dreyfus Teacher-
Scholar.
Supporting Information Available: Crystallographic data for
1-OCPh2 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM701091Z