Synthesis of bis(imino)pyridine iron amide and ammonia compounds from an N-H transfer agent

Article abstract of DOI:10.1021/ic9003017

Addition of the [NH] transfer reagent Hdbabh (dbabh = 2,3:5,6-dibenzo-7- azabicyelo[2.2.1]hepta-2,5-diene) to the bis(imino)pyr-idine iron bis(dinitrogen) complex, (^lPrPDI)Fe(N₂)₂ ( ^PrPDI = 2,6-(2,6^-1

Full text of DOI:10.1021/ic9003017

Inorg. Chem. 2009, 48, 5587–5589 5587
DOI: 10.1021/ic9003017
Synthesis of Bis(imino)pyridine Iron Amide and Ammonia Compounds from an
N-H Transfer Agent
Amanda C. Bowman,† Suzanne C. Bart,†,‡ Frank W. Heinemann,‡ Karsten Meyer,*,‡ and Paul J. Chirik*,†
†Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York
::
14853 and ‡Department of Chemistry and Pharmacy, Inorganic Chemistry, University of Erlangen-Nurnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany
Received February 17, 2009
Addition of the [NH] transfer reagent Hdbabh (dbabh = 2,3:5,6-
dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to the bis(imino)pyr-
idine iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-
(2,6-iPr2C6H3NdCMe)2C5H3N), furnished the corresponding iron
amide and ammonia compounds, resulting from cleavage of the
strained amine. Isotopic labeling studies support N-H bond
activation by a transient, parent imide, [(iPrPDI)FeNH].
corresponding iron arylimide complexes, (iPrPDI)FedNAr
(1dNAr).5 The metrical parameters from X-ray diffraction,
::
in combination with the zero-field 57Fe Mossbauer (MB)
parameters and magnetic susceptibility data, establish an
iron(III) complex with a redox-active, one-electron-reduced
bis(imino)pyridine chelate. Notably, these compounds un-
dergo complete hydrogenation of the Fe-N bond to yield the
corresponding iron dihydrogen complex and the free aniline.
In fact, catalytic azide hydrogenation to amines was observed
in cases where the imide substituents were sufficiently large to
inhibit aniline coordination.5
Inspired by these observations, we sought to prepare other
examples of bis(imino)pyridine iron imides and explore their
reactivity in hydrogenation and related processes. The parent
iron imide, (iPrPDI)FeNH, is an attractive target given the
paucity of such compounds intheliterature as wellaspossible
relevance to biological and industrial ammonia synthesis.
Because of the dangers associated with the preparation
and handling of HN3, we chose to study the chemistry of
Hdbabh (dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hep-
ta-2,5-diene)6 as an [NH] group transfer agent (Scheme 1).7
The addition of 1 equiv of Hdbabh to a diethyl ether
solution of 1-(N2)2 yielded two new iron products identified
as the paramagnetic iron amide compound, 1-dbabh, and the
diamagnetic iron ammonia derivative, 1-NH3. Free anthra-
cene, equal to the amount of 1-NH3, was also detected by 1H
NMR spectroscopy. 1-NH3 was previously reported and
independently prepared by the treatment of 1-(N2)2 with
anhydrous ammonia.8
Iron imido and nitrido compounds LnFedNR and
LnFetN, respectively, continue to attract attention because
of their potential intermediacy in nitrogen fixation and cata-
lytic group and atom transfer reactions1.,2 Structurally char-
acterized, terminal iron imides are now known for oxidation
states 2+ to 5+.3,4 One notable recent addition is Power’s
iron(V) bis(imide), Ar*Fe(N1Ad)2 (Ar* = C6H-2,6-(C6H2-
2,4,6-iPr3)2-3,5-iPr2), prepared by addition of 1-adamantyl
azide to Ar*Fe(η6-C6H6).3 Our laboratory reported that
the addition of aryl azides to the bis(imino)pyridine iron
bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-
(2,6-iPr2C6H3NdCMe)2C5H3N, 1-(N2)2), furnished the
*To whom correspondence should be addressed. E-mail: Karsten.
Meyer@chemie.uni-erlangen.de (K.M.), pc92@cornell.edu (P.J.C.).
(1) Mayer, A. C.; Bolm, C. In Iron Catalysts in Organic Chemistry,
Reactions and Applications; Plietker, B., Ed.; Wiley-VCH: Weinheim,
Germany, 2008; pp 87-89.
(2) (a) Berry, J. F.; Bill, E.; Bothe, E.; George, S. D.; Mienert, B.; Neese,
F.; Wieghardt, K. Science 2006, 312, 1937. (b) Rohde, J.; Betley, T. A.;
Jackson, T. A.; Saouma, C. T.; Peters, J. C.; Que, L. Inorg. Chem. 2007, 46,
5720. (c) Vogel, C.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer, K.
Angew. Chem., Int. Ed. 2008, 47, 2681. (d) Scepaniak, J. J.; Fulton, M. D.;
Bontchev, R. P.; Duesler, E. N.; Kirk, M. L.; Smith, J. M. J. Am. Chem. Soc.
2008, 130, 10515. (e) Aliaga-Alcalde, N.; George, S. D.; Mienert, B.; Bill, E.;
Wieghardt, K.; Neese, F. Inorg. Chem. 2005, 44, 2908 and references cited
therein.
(3) (a) Verna, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc.
2000, 122, 11013. (b) Duncan, J. S.; Zdilla, M. J.; Lee, S. C. Inorg. Chem. 2007,
46, 1071. (c) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 322. (d) Brown, S. D.; Peter, J. C. J. Am. Chem. Soc. 2005, 127, 1913. (e)
Thomas, C. M.;Mankad, N. P.;Peters, J. C. J. Am. Chem. Soc. 2006, 128, 4956.
(f) Eckert, N. A.; Vaddadi, S.; Stoian, S.; Lachicotte, R. J.; Cundari, T. R.;
Holland, P. L. Angew. Chem., Int. Ed. 2006, 45, 6868. (g) Ni, C. B.; Fettinger, J.
C.; Long, G. J.; Brynda, M.; Power, P. P. Chem. Commun. 2008, 6045.
(4) Mehn, M. P.; Peters, J. C. J. Inorg. Biochem. 2006, 100, 634.
(5) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 5302.
(6) Carpino, L. A.; Padykula, R. E.; Barr, D. E.; Hall, F. H.; Krause, J. G.;
Dufresne, R. F.; Thoman, C. J. J. Org. Chem. 1988, 53, 2565.
(7) For use of Li[dbabh] in metal nitride synthesis: (a) Mindiola, D. J.;
Cummins, C. C. Angew. Chem., Int. Ed. 1998, 37, 945. (b) Betley, T. A.;
Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. (c) Meyer, K.; Mindiola, D.
J.; Baker, T. A.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int. Ed. 2000,
39, 3063. (d) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg.
Chem. 2004, 43, 4645. (e) Lu, C. C.; Peters, J. C. Inorg. Chem. 2006, 45, 8597.
(f) Adhikari, D.; Basuli, F.; Fan, H.; Huffman, J. C.; Pink, M.; Mindiola, D.
J. Inorg. Chem. 2008, 47, 4439.
(8) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2007, 129, 7212.
r
2009 American Chemical Society
Published on Web 04/10/2009
pubs.acs.org/IC

5588 Inorganic Chemistry, Vol. 48, No. 13, 2009
As is well-established in bis(imino)pyridine chemistry,9-11
distortions to the chelate are diagnostic of ligand reduction
and redox activity. The metrical parameters of the bis(imino)
pyridine chelate in 1-NH3 are consistent with two-electron
reduction.9,11 Elongated CdN imine bond distances of 1.358
˚
(3) and 1.371(3) A and contracted Cimine-Cipso lengths 1.420
2- 9-11
˚
(3) and 1.436(3) A are typical of [PDI]
.
The bond
distance changes for 1-dbabh are less pronounced than those
for 1-NH3 and are consistent with those of [PDI]-.9,11 The
˚
CdN distances of 1.318(3) and 1.301(2) A are only slightly
Figure 1. Zero-field 57Fe MB spectrum of the products from the addi-
tion of Hdbabh to 1-(N2)2 recorded at 80 K.
elongated from the free ligand values, while the C-C dis-
tances of 1.441(3) and 1.457(3) A show a minor contraction.
˚
Variable-temperature SQUID measurements on solid
samples of 1-dbabh over the temperature range of 5-300 K
(Figure S2 in the Supporting Information) established that
the effective magnetic moment, μeff, undergoes a steady
decline as the temperature is lowered, decreasing from
3.90 μB at 300 K to 2.90 μB at 5 K. These data are consistent
with an S = 3/2 ground state and are similar to those reported
for 1-Cl9 and 1-Br.12 The zero-field 57Fe MB isomer shift, δ,
of 0.75 mm s-1 and the quadrupole splitting, ΔEQ, of 1.22
mm s-1 of 1-dbabh are similar to the values reported for 1-Cl9
and are consistent with high-spin iron(II). Thus, the ground
state for 1-dbabh is best described as an S = 3/2 compound
with a high-spin ferrous center (SFe = 2) antiferromagneti-
cally coupled to a chelate radical anion (SPDI = 1/2).
Scheme 1
Isolation and characterization of 1-dbabh from Hdbabh
cleavage is striking because previous attempts to synthesize
1-NMe2 or 1-N(SiMe3)2 by salt metathesis with 1-Cl resulted
in unidentified mixtures of products. The treatment of 1-Cl2
with 2 equiv of LiNMe2 resulted in chelate deprotonation
and formation of the iron amine complex, (iPrPDEA)
FeNHMe2.13 In contrast, the treatment of 1-Cl with 1 equiv
of Li[dbabh] in diethyl ether cleanly furnished 1-dbabh with
no evidence for competing chelate deprotonation. Attempts
to prepare 1-dbabh by the addition of 2 equiv of Li[dbabh] to
1-Cl2 resulted in a complex mixture of products. Similarly,
attempts to synthesize 1-Cl by the addition of Li[dbabh] to
1-Cl2 were unsuccessful, demonstrating the inability of the
amide reagent to serve as a reductant and establishing this
step as the origin of decomposition in the 2 equiv procedure.
Inspired by isolation of 1-dbabh from salt metathesis, 1-Cl
was treated with lithium pyrrolidide, Li[NC4H8]. Unfortu-
nately, a mixture of paramagnetic iron products was ob-
Because of the complications of quantifying paramagnetic
versus diamagnetic compounds by H NMR spectroscopy,
1
the product mixture from the addition of Hdbabh to 1-(N2)2
was analyzed by zero-field MB spectroscopy. This spectro-
scopic technique was also used to ensure that no other
NMR-silent iron species were formed. A representative
zero-field 57Fe MB spectrum, recorded at 80 K, is presented
in Figure 1 and establishes clean and reproducible formation
of only two iron products in an approximately 80:20 ratio.
This ratio deviates from the idealized 2:1 likely due to
recrystallization of thesample prior to spectroscopic analysis.
The independent preparation of the ammonia compound
established 1-NH3 (δ = 0.33 mm s-1; ΔEQ = 2.22 mm s-1) as
the minor product in the MB spectrum (Figure S1).8
Attempts to induce a loss of anthracene from 1-dbabh were
unsuccessful. No change was observed upon letting the
compound stand at 23 °C in benzene-d6 for 1 week. Warming
benzene-d6 solutions of 1-dbabh to 45 °C resulted in decom-
position after 24 h. No anthracene was observed from the
decomposition. Changing the order of the addition, Hdbabh
to 1-(N2)2 or vice versa, has no measurable effect on the ratio
of the products.
1
served by H NMR spectroscopy. As was observed with
LiNMe2,13 the addition of 2 equiv of Li[NC4H8] to 1-Cl2
resulted in clean chelate deprotonation and formation of the
iron pyrrolidine complex (iPrPDEA)FeNHC4H8.
Additional experiments were conducted to gain insight
into the scope and mechanism of Hdbabh cleavage. The
generality of the reaction was explored with other cyclic
amines and aziridines. The addition of 1 equiv of pyrrolidine
(HNC4H8) to 1-(N2)2 resulted in the loss of N2 and formation
of the corresponding bis(imino)pyridine iron pyrrolidine
compound, 1-(NHC4H8). The absence of significant ring
Both 1-dbabh and 1-NH3 were characterized by X-ray
diffraction (Figure 2). In both cases, essentially square-planar
molecules are observed, with the sum of the angles around
iron equal to 362.17(13)° (1-dbabh) and 360.01(16)° (1-NH3).
The larger deviation observed for 1-dbabh is a result of the
slight lifting of the iron-amide bond out of the idealized
square plane [N2-Fe-N4 = 169.92(7)°]. The iron is also
˚
raised by 0.3880(17) A out of the idealized plane of the
(10) Bart, S. C.; Lobkovsky, E.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg.
Chem. 2007, 46, 7055.
(11) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton Trans.
2006, 5442.
chelate. The amide substituents lie above and below the plane
of the iron and the chelate, likely to avoid steric interactions
with the large aryl groups.
(12) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008,
130, 11631.
(9) 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.
(13) (a) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem.
i
2006, 45, 2. (b) PrPDEA = (2,6-iPr2C6H3NCdCH2)2C5H3N.

Communication
Inorganic Chemistry, Vol. 48, No. 13, 2009 5589
Figure 2. Molecular structures of 1-dbabh (left and center) and 1-NH3 (right). Each structure is shown at 30% probability ellipsoids. H atoms, except those
on the ammonia in 1-NH3, are omitted for clarity. Aryl groups are omitted on the central structure of 1-dbabh.
1
2
Scheme 2
compound by H and H NMR spectroscopy as well as by
solid-state (KBr) IR spectroscopy established the exclusive
formation of 1-ND3, identifying Ddbabh (or Hdbabh) as the
sole source of the H atoms in ammonia formation.
On the basis of these observations, Hdbabh cleavage by
1-(N2)2 likely proceeds via initial [NH] group transfer to form
the desired parent iron imide [1dNH] and free anthracene.
This transient species likely promotes H-atom abstraction
from the remaining Hdbabh to yield the observed ammonia
complex 1-NH3. Capture of the amide radical [dbabh] by
1-(N2)2 yields the amide complex 1-dbabh. Because of the
potential intermediacy of [1dNH], attempts were made to
observe or trap it with other reagents. The addition of
Hdbabh to 1-(N2)2 under an H2 or D2 atmosphere produced
no change to the product or isotopic distribution. The
cleavage reaction was also conducted in the presence of
styrene, 1,4-cyclohexadiene, 9,10-dihydroanthracene, fluor-
ene, and mesitylaniline, all with no effect. Performing the
addition of Hdbabh to 1-(N2)2 in diethyl ether at -78 °C for
12 h increased the amount of anthracene produced from 33 to
46% and yielded a new, unidentified paramagnetic iron
product (in addition to 1-dbabh and 1-NH3). Hydrolysis of
this mixture yielded a modified bis(imino)pyridine ligand,
suggestive of C-H activation by putative [1dNH]. Studies
are ongoing to gain additional information about this
process.
strain and hence a potential driving force for pyrrolidine to
act as an [NH] transfer agent prompted exploration of the
chemistry of aziridines. The addition of 1 equiv of cyclohex-
ene imine14 to a benzene-d6 solution of 1-(N2)2 cleanly yielded
the corresponding bis(imino)pyridine iron aziridine com-
plex, 1-(NHC6H10), with no evidence for [NH] group transfer
(Scheme 2). Heating a benzene-d6 solution of 1-(NHC6H10) at
65 °C for 24 h resulted in a mixture of iron compounds;
however, analysis of the volatile products of the reac-
tion mixture established the formation of cyclohexene and
ammonia in a 1:0.75 ratio. These results indicate that clea-
vage chemistry similar to that of Hdbabh may be possible at
elevated temperatures, but the resulting iron products are not
thermally stable.
The coordination of pyrrolidine and aziridine to 1-(N2)2
over [NH] group transfer highlights the unique reactivity of
Hdbabh. Assuming strain similar to that of cyclohexene
imine, the observation of group transfer from Hdbabh is
likely a consequence of the gain in aromaticity arising from
the elimination of anthracene, an effect noted by Cummins
and others7 in the application of the dbabh anion in the
synthesis of metal nitrides.
In summary, the addition of the [NH] group transfer agent,
Hdbabh, to a bis(imino)pyridine iron bis(dinitrogen) com-
pound formed the corresponding amide and ammonia deri-
vatives, likely through a transient parent imide. Additional
reactivity of the iron amide complex 1-dbabh, particularly
toward anthracene loss and nitride formation, is currently
under investigation in our laboratory.
Isotopic labeling studies were also conducted to determine
the course of the H atoms in the formation of 1-NH3 from
Hdbabh. Potential involvement of the imine methyl groups
of the bis(imino)pyridine ligand was assayed by treatment of
the deuterated dinitrogen complex 1**-(N2)2 (** indicates
deuterated imine methyl groups) with Hdbabh. Analysis of
Acknowledgment. P.J.C. thanks the Packard Founda-
tion and K.M. thanks the University of Erlangen-
::
Nurnberg and DFG (SFB 583) for financial support.
S.C.B. thanks the Alexander von Humboldt Foundation
1
2
::
the ammonia product by H and H NMR spectroscopy
established the exclusive formation of 1**-NH3, eliminating
imine methyl group participation. In a complementary
for a postdoctoral fellowship. We also thank Jorg Sutter
for collecting MB data.
2
experiment, Ddbabh (the H isotopologue of Hdbabh) was
Supporting Information Available: Experimental procedures,
the zero-field MB spectrum of 1-NH3, SQUID data for 1-dbabh,
and crystallographic data for 1-dbabh and 1-NH3 in CIF format.
This material is available free of charge via the Internet at http://
pubs.acs.org.
added to 1-(N2)2. Analysis of the resulting iron ammonia
(14) (a) Fang, Y.; Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 5660.
(b) Cyclohexene imine = 7-azabicyclo[4.1.0]heptane.