Distinct Thermodynamics for the Formation and Cleavage of N-H Bonds in Aniline and Ammonia. Directly-Observed Reductive Elimination of Ammonia from an Isolated Amido Hydride Complex

Article abstract of DOI:10.1021/ja037363u

The reactions of aryl and alkylamines with the (PCP)Ir fragment (PCP = 1,3-di-tert-butylphosphinobenzene) were studied to determine the reactivities and stabilities of amine and amido hydride complexes relative to C?H activation products. Reaction of anil

Full text of DOI:10.1021/ja037363u

Published on Web 10/21/2003
Distinct Thermodynamics for the Formation and Cleavage of N-H Bonds in
Aniline and Ammonia. Directly-Observed Reductive Elimination of Ammonia
from an Isolated Amido Hydride Complex
Mira Kanzelberger,† Xiawei Zhang,† Thomas J. Emge,† Alan S. Goldman,*,†
Jing Zhao,‡ Christopher Incarvito,‡ and John F. Hartwig*,‡
Department of Chemistry, Rutgers UniVersity, 610 Taylor Road, Piscataway, New Jersey 08854-8087, and
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107
Received July 17, 2003; E-mail: agoldman@rutchem.rutgers.edu; john.hartwig@yale.edu
Metal-catalyzed reactions of ammonia or alkylamines that occur
by oxidative addition or reductive elimination of N-H bonds are
rare.1 Even examples of stoichiometric oxidative addition or
reductive elimination of ammonia or alkylamines are rare.2-5
Recently, anti-Markovnikov hydroamination and oxidative amina-
tion of vinylarenes with group 9 metal complexes have been
reported,6 and these reactions could occur by N-H oxidative
Figure 1. ORTEP diagrams of the products of reactions with “(PCP)Ir”
addition of an amine. To evaluate the potential of N-H activation
(a) oxidative addition of the aniline N-H bond 1a (left), (b) addition of
ammonia 2 (right). (For clarity, tert-butyl groups are omitted).
and elimination to occur during catalytic processes, the relative
thermodynamic stabilities and the kinetic barriers to interconversions
of late metal amido hydride complexes and late metal amine
complexes must be established experimentally.7
Scheme 1
During the past 20 years, many examples of the formation and
cleavage of C-H bonds by late metals have been discovered.8,9
Studies on the analogous reactions of N-H bonds with the same
metal fragments would provide a context to interpret data on the
reactivity of ammonia and amines. (PCP)Ir complexes (PCP ) η3-
C6H3-2,6-(CH2PR2)2) readily cleave hydrocarbon C-H bonds,10
activate the OH bond in water,11 and catalytically dehydrogenate
alkanes,12 alcohols,13 amines,14,15 ethers, and alkylarenes.12 Herein
we report studies on N-H bond formation and cleavage that have
revealed clear differences between the thermodynamics for addition
and elimination of ammonia and aniline and that have allowed the
first direct observation of reductive elimination of ammonia from
a monomeric amido hydride complex.
The reaction of (PCP)IrH2 with norbornene or tert-butylethylene
in benzene solvent is known to generate (PCP)Ir, which oxidatively
adds C-H bonds to form alkyl and aryl hydride complexes.10
Reaction of (PCP)IrH2 and norbornene with 1.2 equiv of aniline in
p-xylene solvent at room temperature gave the anilide hydride
(PCP)IrH(NHPh) (1a) in 95% yield by N-H oxidative addition.
This product was characterized by standard NMR techniques and
X-ray diffraction, as shown in Figure 1. In the 1H NMR spectrum
the hydride was detected as a triplet at -38.21 ppm, and the N-H
proton, as a singlet at 4.90 ppm.
(NH2) (1b), as shown in Scheme 1. Treatment of (PCP)Ir(H)(Cl)
with ammonia generated the ammonia complex (PCP)Ir(H)(Cl)-
(NH3) (3). Reaction of this species with 2.0 equiv of KN(SiMe3)2
at -78 °C in THF formed 1b in 100% yield, along with HN(SiMe3)2
and KCl (Scheme 1). Amido hydride 1b was stable below -10 °C.
It was, therefore, characterized in THF-d8 at -30 °C. A triplet
resonance at δ -26.66 ppm was observed for the hydride in the
1H NMR spectrum, along with a broad singlet at δ 4.66 ppm for
the amide. 15N NMR DEPT spectroscopy showed a triplet of doublet
2
of triplet resonance at 48.9 ppm (1JNH ) 70.9 Hz, JNH ) 8.0 Hz,
In contrast, reaction of (PCP)IrH2 with norbornene and 2.0 equiv
of ammonia at room temperature formed the ammonia complex
2JNP ) 4.0 Hz).
1
(PCP)Ir(NH3) (2) in 29% yield by H NMR spectroscopy with an
Warming of amido hydride 1b to room temperature generated
the ammonia complex 2 in 91% yield within 10 min at room
temperature by a rare N-H bond-forming reductive elimination of
an amido hydride complex. Thus, the formation of 2 instead of 1b
upon reaction of ammonia with the (PCP)Ir intermediate most likely
results from a thermodynamic stability of the ammonia complex
that is greater than that of amide hydride 1b. The contrast in stability
of the products from the reactions of aniline and ammonia16 can
be rationalized by two factors. The dative bond in an aniline
complex should be weaker than that in an ammonia complex
because of lower basicity, and an M-N bond in an anilide complex
internal standard. The ammonia complex was also characterized
by standard NMR methods and X-ray diffraction (Figure 1). In
addition, the presence of an ammonia ligand was confirmed by 15
N
DEPT NMR spectroscopy. This spectrum contained a quartet at
75.1 ppm (1JNH ) 67.2 Hz).
To determine whether the distinct reactivity of aniline and
ammonia resulted from kinetic or thermodynamic factors, we
independently synthesized the parent amido hydride (PCP)Ir(H)-
† Rutgers University.
‡ Yale University.
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J. AM. CHEM. SOC. 2003, 125, 13644-13645
10.1021/ja037363u CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
from (PCP)IrPhH. The elimination of benzene from (PCP)IrPhH
occurs on the NMR time scale, even at -20 °C.10 Formation of
the ammonia complex from the 16-electron amido hydride 1b
required at least 2 h at -10 °C. The anilide complex 1a showed
no evidence of exchange with free aniline on the NMR time scale
(1H NMR, hydride resonance), even at 85 °C, although exchange
of the anilide in 1a with p-trifluoromethylaniline occurred at room
temperature within several minutes after mixing.
Figure 2. ORTEP diagrams of (PCP)Ir(H)(NHPh)(CO) (4a, left) and
(PCP)Ir(H)(NH2)(CN tBu) (4b, right) (tert-butyl groups omitted for clarity).
In conclusion, a monomeric iridium terminal amido complex has
been prepared, and the reductive elimination of ammonia from this
complex has been observed directly. In contrast, aniline adds to
the same metal fragment from which ammonia eliminated. The
products from N-H addition, especially from addition of aniline,
are more stable kinetically and thermodynamically than products
of C-H addition to the same (PCP)Ir fragment. Studies designed
to elucidate the reactivity, mechanism of addition and elimination,
and factors that control the thermodynamics of N-coordination and
N-H addition are in progress.
should be stronger (relative to the respective H-N bonds) than the
M-N bond in an amido complex because of a greater ionic
interaction.17
Addition of CO to the 16-electron anilide hydride 1a and amido
hydride 1b gave 18-electron adducts. The CO adduct of 1b was
unstable at room temperature, but addition of tBuNC to 1b formed
the isolable 18-electron amido hydride 4b (Scheme 1) in 56% yield.
The hydride of 4b was observed as a triplet at δ -10.82, and the
protons of the NH2 were observed as a broad singlet at δ -2.39
(-15 °C in THF-d8). The hydride of anilide carbonyl 4a appeared
as a triplet of doublets at δ -7.75, and the NHPh proton appeared
as a singlet at δ 1.95 (p-xylene-d10, 22 °C).
Isocyanide complex 4b underwent reductive elimination of NH3
at room temperature, in this case to generate the Ir(I) isocyanide
complex 5b (t1/2 ≈ 80 min) and free ammonia in 90% yield by 1H
NMR spectroscopy (Scheme 1). The anilide hydride 4a likewise
underwent reductive elimination to give the Ir(I) carbonyl complex,
5a. Elimination of aniline from 4a was much slower (t1/2 ≈ 4 h at
90 °C) than elimination of ammonia from 4b.
Acknowledgment. We thank the Department of Energy for
support (DE-FG02-96ER14678 to J.F.H. and DE-FG02-93ER14353
to A.S.G.) and Professor Patrick Loria and Roger Cole for assistance
in obtaining 15N DEPT spectra.
Supporting Information Available: Preparative procedures and
spectroscopic data for new compounds (PDF); crystallographic data
for 1a, 2, 4a, 4b (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
References
The structures of complexes 4a and 4b were determined by X-ray
crystallography (Figure 2). A comparison of the solid-state structures
of ammonia complex 2 and amide 4b shows little difference
between the length of the dative Ir-N bond in 2 (2.215(5) Å) and
that of the covalent Ir-N bond of amide 4b (2.193(4) Å). The Ir-N
bond in the six-coordinate anilide CO adduct 4a (2.142(2) Å) is
slightly shorter than that in 4b, despite the larger size of the anilide
group. The Ir-N bond distance of the five-coordinate anilide 1a,
however, is significantly shorter than the Ir-N distances in the other
complexes; two disordered components (88:12) were found with
Ir-N bond lengths of 2.082(2) and 2.000(3) Å, respectively. These
very short distances may reflect pπ-dπ donation18 in the Ir-N
bonding of the 16-electron complex 1a.
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The thermodynamics of addition of the aniline N-H bond to
the (PCP)Ir fragment are more favorable than those of addition of
arene C-H bonds. Monitoring of the equilibrium in eq 1 by 31P
NMR spectroscopy at 22 °C showed the value of Keq to be 105.
Thus, addition of the aniline N-H bond is much more favorable
than addition of the benzene C-H bond. The difference between
addition of the N-H bond of aniline and the more closely analogous
benzylic C-H bond of toluene is even greater; addition of toluene
occurs exclusively at the aryl C-H bonds, and reaction of a 0.01
M solution of benzene in mesitylene occurs only at the benzene
C-H bond.10 The preference for N-H over C-H addition to
(PCP)Ir is likely due to a stronger ionic component in the Ir-N
bond and to pπ-dπ bonding.
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The rates of the reductive eliminations of aniline from 1a, and
of ammonia from 1b are much slower than elimination of benzene
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