Synthesis and hydrogenation of bis(imino)pyridine iron imides

Article abstract of DOI:10.1021/ja057165y

Treatment of the iron bis(dinitrogen) complex, (^iPrPDI)Fe(N₂)₂ (^iPrPDI = (2,6-ⁱPr₂C₆H₃N=CMe)₂C₅H₃N), with a series of aryl azides resulted in loss of 3 equiv of N₂ and formation of the corresponding four-coordinate iron imide compounds, (^iPrPDI)Fe(NAr). These complexes, two of which (Ar = 2,6-ⁱPr₂-C₆H₃ and 2,4,6-Me₃-C₆H₂) have been characterized by X-ray diffraction, are significantly distorted from planarity. The metrical parameters in combination with Moessbauer spectroscopic and SQUID magnetic data suggest an intermediate spin iron(III) center antiferromagnetically coupled to a ligand-centered radical. Nitrene group transfer has been accomplished by addition of 1 atm of CO, yielding aryl isocyanates, ArNCO, and (^iPrPDI)Fe(CO)₂. Hydrogenation of the more sterically hindered members of the series furnished free aniline and the previously reported iron dihydrogen complex. Catalytic aryl azide hydrogenation has also been achieved, and the observed relative rates are consistent with N-H bond formation as the rate-determining step in aniline formation. Copyright

Full text of DOI:10.1021/ja057165y

Published on Web 03/30/2006
Synthesis and Hydrogenation of Bis(imino)pyridine Iron Imides
Suzanne C. Bart, Emil Lobkovsky, Eckhard Bill,† and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853,
and Max-Plank Institute of Bioinorganic Chemistry, Stiftstrasse 34-36, D-45470 Mu¨lheim an der Ruhr, Germany
Received October 20, 2005; E-mail: pc92@cornell.edu
Spectroscopically well-characterized or isolable complexes con-
taining iron-nitrogen multiple bonds are of interest given their role
in enzymatic transformations and potential to mediate atom- and
group-transfer processes.1 Iron amides (Fe-NR2),2 imides (FedNR),
and nitrides (FetN)3 have special relevance given their potential
intermediacy in nitrogen fixation schemes.4 While rare, terminal
iron imides in mixed-valent, 3Fe(III), Fe(IV) cubane structures,5
monomeric Fe(III) compounds,6 and a diamagnetic, four-coordinate
anion7 are known. Addition of H2 to the tris(phosphino)borate
iron(III) imido yields a ferrous iron amido, Fe-NHR.8 Heterolytic
dihydrogen activation to form diiron imide hydride products has
also been observed from a related bridging nitride.9 More recently,
Borovik has reported the synthesis of iron amido compounds from
H-atom abstraction reactions involving putative iron(IV) imido
compounds.10 In this communication, we describe the synthesis,
structural elucidation, and spectral features of bis(imino)pyridine
iron imides and their reactivity toward CO and H2. These
compounds have been identified as key intermediates in the catalytic
hydrogenation of aryl azides to the corresponding anilines.11
The bis(imino)pyridine iron bis(dinitrogen) complex, (iPrPDI)-
Fe(N2)2 (1-(N2)2; iPrPDI ) (2,6-iPr2C6H3NdCMe)2C5H3N) contains
one labile N2 ligand, which readily dissociates in solution at ambient
temperature.12 Inspired by this observation, we reasoned that N2
displacement may serve as an effective strategy for the synthesis
of iron-nitrogen multiple bonds. Treatment of 1-(N2)2 with a
stoichiometric quantity of a series of aryl azides induced loss of 3
equiv of N2, affording the deep-blue or purple iron imides, 2a-
2d, in excellent yields (eq 1).
Figure 1. Variable-temperature SQUID magnetic data and zero-field
Mo¨ssbauer spectrum (inset) of 2a at 80 K.
Table 1. Selected Bond Lengths (Å) and Angles (deg)
2a
2d
(
iPrPDI)FeCl2
Fe(1)-N(4)
1.7048(16)
1.8403(15)
2.0343(16)
1.320(2)
1.437(3)
1.346(2)
1.7165(15)
1.8718(14)
2.0105(14)
1.321(2)
1.436(2)
1.348(2)
Fe(1)-N(2)
2.091(4)
2.222(4)
1.301(7)
1.482(8)
Fe(1)-N(1)
N(1)-C(2)
C(2)-C(3)
N(4)-C(10)
N(2)-C(3)
1.377(3)
1.373(2)
2.091(4)
Fe(1)-N(4)-C(10)
N(1)-Fe(1)-N(3)
N(2)-Fe(1)-N(4)
Fe(dev) N(1),N(2),N(3) plane
165.68(15)
149.86(6)
138.79(7)
0.4456
159.00(13)
152.37(6)
154.75(7)
0.3386
140.23(16)
147.90(13)
0.5585
the FedNAr linkage being the most severely distorted. This is most
pronounced in 2a, where a contracted N(2)-Fe(1)-N(4) bond angle
of 138.79(7)° is observed, compared to the more open value of
154.75(7)° found in 2d. The distortion from planarity14,15 is similar
to that observed in Ru(0) and Ru(II) carbonyl complexes16 and is
2
likely a result of alleviation of σ* character in dz arising from
interaction of the taurus with an sp hybrid of the imido nitrogen.
Reduction of a four-electron repulsion between dxy and N px and
steric effects also contribute to the distortion.
The Fe(1)-N(4) bond distances of 1.7048(16) and 1.7165(15)
Å are elongated relative to the values of 1.651(3) and 1.6578(2) Å
found in Peters’ tris(phosphino)borate iron(II) and (III) imides,
respectively.7 The metrical parameters of the bis(imino)pyridine
chelate in 2a and 2d indicate one-electron ligand reduction.17 The
C(2)-C(3), Fe(1)-N(1), and Fe(1)-N(2) bond distances are
contracted with respect to (iPrPDI)FeCl2,18 where the terdentate
chelate is clearly a neutral, L3-donor.17 Accordingly, the N(1)-
C(2) distances are slightly elongated. To account for the overall S
) 1 magnetic ground state, the solid state and Mo¨ssbauer data are
most consistent with an intermediate spin iron(III) center (S ) 3/2),
antiferromagnetically coupled to a PDI-ligand centered radical (S
) -1/2). The small, negative zero field splitting determined from
the SQUID experiment also supports a ferric rather than ferrous
ion. DFT calculations are in progress to support this view of the
electronic structure.
The 1H NMR spectra of 2b and 2d exhibit the number of peaks
expected for C2V symmetric molecules over a 400 ppm chemical
shift range. For 2a, only four broadened resonances are observed
at 23 °C; cooling the sample in toluene-d8 to -80 °C resulted in
observation of additional peaks. The benzene-d6 1H NMR spectrum
of 2c displays the number of peaks for a Cs symmetric compound,
a result of the asymmetry imparted by the imido aryl substituents.
The electronic structure of 2a was further interrogated by SQUID
magnetometry and Mo¨ssbauer spectroscopy (Figure 1). The mag-
netic moment of 2.8 µB from 20 to 300 K clearly establishes an S
) 1 ground state with D ) -8.7(5) cm-1. The zero-field Mo¨ssbauer
parameters (δ ) 0.302 mm/s; ∆E ) 1.083 mm/s, 80 K) are
consistent with either an intermediate spin ferrous or ferric ion.13
Two of the bis(imino)pyridine iron imides have been further
characterized by X-ray diffraction (Figure 2, Table 1). In both cases,
the molecular geometry is significantly deviated from planarity with
The reactivity of the four-coordinate iron imides has also been
evaluated. Addition of 1 atm of CO to 2a-2d induced facile nitrene
group transfer, forming the aryl isocyanate, ArNCO, and the iron
† Max-Plank Institute of Bioinorganic Chemistry.
9
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J. AM. CHEM. SOC. 2006, 128, 5302-5303
10.1021/ja057165y CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Molecular structure of 2a (left) and 2d (middle) at 30% probability ellipsoids. Overlay (right) of the cores of 2a (dashed) and 2d (solid).
dicarbonyl complex, 1-(CO)2.12 Similar observations have been
reported for other ferric,7 cobalt,19 and nickel imides.20 In more
unique chemistry, exposure of benzene-d6 solutions of 2a-2c to 1
atm of H2 at 23 °C resulted in hydrogenation of the FedNAr linkage
to yield the iron dihydrogen complex, 1-H2, along with free aniline
(eq 2).
2,4,6-Me3-C6H2N3, suggests N-H reductive coupling as the rate-
determining step21 although additional kinetic and mechanistic
experiments must be performed to support this conclusion. The
combined synthetic, spectroscopic, and structural studies described
here once again highlight the “redox non-innocence”17 of the bis-
(imino)pyridine ligand and its importance in stabilizing catalytically
active iron centers.12
Acknowledgment. We thank the Packard Foundation for
financial support. P.J.C. is a Cottrell Scholar supported by the
Research Corporation. Professor Karl Wieghardt is also acknowl-
edged for helpful discussions.
Supporting Information Available: Experimental procedures and
crystallographic data for 2a and 2d. This material is available free of
charge via the Internet at http://pubs.acs.org.
Because 1-H2 rapidly converts to 1-(N2)2 upon exposure to N2,
1-(N2)2 seemed an ideal candidate for the catalytic hydrogenation
of aryl azides to the corresponding anilines. To further explore this
possibility, addition of each of the anilines to 1-(N2)2 under either
N2 or vacuum at 23 °C produced no reaction, demonstrating that
product inhibition would not be a limitation for catalytic turnover.
Gratifyingly, hydrogenation of the series of aryl azides used to
prepare 2a-2c at 23 °C and 1 atm of H2 in the presence of 10 mol
% (unoptimized) of 1-(N2)2 yielded the desired anilines in quantita-
tive yield (eq 3). Notably, the relative rates of catalytic hydrogena-
tion increased as the size of the aryl azide substituents closest to
the iron increased. Thus, 2,6-iPr2-C6H3N3 was the fastest in the
series, reaching completion in 6 h at 23 °C, while 2,6-Et2-C6H3N3
was the slowest, hydrogenating over the course of 96 h at 65 °C.
2,5-tBu2-C6H3N3 proceeded at an intermediate rate, requiring 16 h
for complete conversion at 65 °C. No catalytic hydrogenation of
2,4,6-Me3-C6H2N3 was observed, even upon heating to 65 °C for
24 h.
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To gain additional insight into the course of the catalytic aryl
azide hydrogenation, deuteration studies were performed. Treatment
of 2a-2c with 1 atm of D2 gas afforded the N-deuterated aniline,
ArND2, and the iron dideuterium complex, 1-D2. Analysis of the
product mixture by 2H NMR spectroscopy revealed isotopic
incorporation into the isopropyl aryl methyl groups in the terdentate
ligand and the aniline, arising from methyl group cyclometalation.
Competition experiments (H2 vs D2) with 2a at 65 °C established
a normal, primary kinetic isotope effect of 1.7(2) for hydrogenation
(deuteration).
Based on these findings, the mechanism for catalytic aryl azide
hydrogenation most likely involves 1,2-addition of H2 across the
iron-nitrogen bond ultimately resulting in reductive elimination
of aniline. The lack of catalytic turnover with the smallest aryl azide,
JA057165Y
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