N-H activation of hydrazines by iridium(I). Double N-H activation to form iridium aminonitrene complexes

Article abstract of DOI:10.1021/ja1053835

Iridium(I) complexes of aromatic (PCP) and aliphatic (D^tBPP) pincer ligands undergo single cleavage of the N-H bonds of hydrazine derivatives to form hydrazido complexes and geminal double cleavage to form unusual late transition metal aminonit

Full text of DOI:10.1021/ja1053835

Published on Web 07/30/2010
N-H Activation of Hydrazines by Iridium(I). Double N-H Activation To Form
Iridium Aminonitrene Complexes
Zheng Huang, Jianrong (Steve) Zhou, and John F. Hartwig*
Department of Chemistry, UniVersity of Illinois, 600 South Mathews AVenue, Urbana, Illinois 61801
Received June 19, 2010; E-mail: jhartwig@illinois.edu
The oxidative addition of hydrazine derivatives to iridium(I)
Abstract: Iridium(I) complexes of aromatic (PCP) and aliphatic
complexes of aromatic and aliphatic pincer ligands to form
(DtBPP) pincer ligands undergo single cleavage of the N-H
hydridoiridium hydrazido complexes 2a, 2b, 3a, and 3b is shown
bonds of hydrazine derivatives to form hydrazido complexes and
in Scheme 1. Addition of 1.1 equiv of benzophenone hydrazone to
geminal double cleavage to form unusual late transition metal
alkene complex 1a containing an aliphatic pincer ligand, or
aminonitrene complexes. In some cases, the cleavage of the N-N
phenyliridium hydride complex 1b containing an aromatic pincer
bond in the hydrazine is also observed. Oxidative additions of
ligand, occurred within 5 min at room temperature. This NsH
the N-H bonds of benzophenone hydrazone and 1-aminopip-
activation reaction occurred exclusively over potential competing
eridine to iridium(I) complexes give the corresponding hydridoiri-
CsH activation of the hydrazone aryl groups or NsN bond
dium(III) hydrazido complexes within minutes. The complex con-
cleavage. The products of this addition, (DtBPP)Ir(H)(NHsNdCPh2)
taining an aromatic pincer ligand, (PCP)Ir(H)(NHNC5H10), slowly
(2a) and (PCP)Ir(H)(NHsNdCPh2) (2b), were stable at room tem-
undergoes a second N-H bond cleavage at the R-N-H bond and
perature and fully characterized. Moreover, addition of 10 equiv
elimination of hydrogen to generate an aminonitrene complex and
of 1-pentene to 2a or dissolution of 2b in benzene did not regenerate
1a or 1b.
dihydrogen in high yield. The reactions of the (PCP)Ir(I) fragment
containing an aromatic pincer ligand with methyl-substituted hydra-
The structures of 2a and 2b were determined by X-ray diffraction
(Figure 1). The two complexes have similar distorted square-
pyramidal structures with an apical hydride. In both cases, the lone
pair of the amido nitrogen is oriented to allow π-donation into the
empty d-orbital that lies in the plane of the iridium(III) centers
containing the ipso carbon, the hydride, and the Ir-bound nitrogen.
The Ir-N bond distances in 2a and 2b are short (2.018(7) and
2.022(3) Å, respectively), reflecting a partial double-bond character
between Ir and N.2b,6
zines form a mixture of aminonitrene complexes, isocyanide irid-
ium(III) dihydride complexes, and ammonia. The latter two products
are likely formed by initial oxidative addition of the methyl C-H bond
and the subsequent N-N bond cleavage. Reactions of the amino-
nitrene complex with CO or reagents that undergo oxidative addition
(MeI and PhOH) lead to release of the “isodiazine” fragment to give
tetrazene and tetrazine derivatives.
The oxidative addition of N-H bonds can lead to new catalytic
chemistry,1 but this reaction has been studied less than the oxidative
addition of C-H or O-H bonds. The oxidative addition of amines
and ammonia to complexes of late transition metals is rare, particular
additions to a single metal center.2 We recently showed that the
iridium(I) complex of an electron-rich DtBPP-pincer ligand undergoes
oxidative addition of ammonia to form a monomeric amido hydride
complex.2c Subsequent studies have led to the oxidative addition of
amides and aromatic amines to related pincer complexes of iridium(I),2d,e
and several groups have recently reported oxidative additions of
ammonia to other late-metal systems.2e,f,3
Scheme 1. Formation of Hydridoiridium Hydrazido Complexes via
N-H Bond Addition
R-Elimination of amido complexes by internal or external base
is a second type of N-H bond cleavage, and this reaction is a
common route by which amido complexes of the early and middle
transition elements convert to metal-nitrene complexes.4 In
contrast, analogous R-eliminations of amido complexes of the late
transtition metals to form nitrene complexes are rare5 and have
been limited to examples triggered by addition of an external base.5
Because initial studies on the reactivity of (PCP)Ir(H)(NH2)
complexes were dominated by reductive elimination of ammonia,2b
we began to investigate the oxidative addition of ammonia
equivalents that could form more stable products. Our initial studies
focused on the reactions of iridium(I) complexes with hydrazines. We
show that the oxidative addition of hydrazines to the iridium(I) pincer
complex occurs readily and that several products of this oxidative
addition undergo a second N-H activation in the form of an
R-elimination or abstraction to generate rare low valent, late transition
metal aminonitrene or “isodiazene” complexes of the third row.
Figure 1. ORTEP diagrams of (DtBPP)Ir(H)(NHsNdCPh2) (2a, left, IrsN
) 2.018(7) Å, NsN ) 1.397(9) Å, <HsIrsC ) 83.8(14)°, <CsIrsN )
163.2(4)°, <IrsNsN ) 132.6(6)°) and (PCP)Ir(H)(NHsNdCPh2) (2b,
right, IrsN ) 2.022(3) Å, NsN ) 1.356(4) Å, <HsIrsC ) 75.1(11)°,
<CsIrsN ) 164.79(13)°, <IrsNsN ) 130.2(2)°) (tert-butyl groups and
most H atoms omitted for clarity).
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C O M M U N I C A T I O N S
sten complexes [Cp2WH(NNHC6H4F)] PF6 (W-N ) 1.837(7) Å,
N-N ) 1.315(9) Å, W-N-N ) 146.4(5)°)10b and fac-
(CO)3(DPPE)WNNMe2 (W-N ) 2.12(1) Å, N-N ) 1.21(2) Å,
W-N-N ) 139(1)°)8a have bent M-N-N angles but very
different M-N and N-N distances. The high-valent tungsten
complex contains a shorter W-N bond and a longer N-N bond than
the low-valent compound, suggesting that it adopts a structure more
like resonance structure B1, whereas the low-valent tungsten complex
adopts a structure more like B2. The short N-N distance of our iridium
complex 4 suggests that resonance form B2 contributes most to its
structure, and after correcting for the covalent radii of Ir (1.41 Å) and
W (1.62 Å),11 the M-N bond distance in 4 (2.008(11) Å) is
comparable to that in the W(0) species fac-(CO)3(DPPE)-WNNMe2.
Finally, the Ir-N distance is much longer than that in the closest
iridium analog, imido complex Cp*IrNtBu containing an Ir-N distance
of 1.712(7) Å and Ir-N triple bond character.5a
DFT calculations of the structure and bonding of 4 were conducted.
The geometry of the calculated structure (see Supporting Information)
resembles that of the experimental one. The C-Ir-N angle in the
computed structure is bent, but this angle in a derivative containing methyl
groups at phosphorus was computed to be more linear, implying that steric
effects control this feature.12 The computed natural bond order13 implies
that the N-N bond has multiple bond character (bond order: 1.76). In
this case, the dominant Ir-N interaction results from donation of the
nitrogen electron pair to iridium with little backbonding. Thus, both
experimental and computational results imply that 4 is a 16-electron
iridium(I) species containing a neutral aminonitrene ligand like that in
structure B2.
Figure 2. ORTEP diagrams of (DtBPP)Ir(H)(NHNC5H10) (3a, left, IrsN
) 1.9875(18) Å, NsN ) 1.435(2) Å, <HsIrsC ) 73.9(8)°, <CsIrsN )
154.47(9)°, <IrsNsN ) 132.53(14)°, <NsNsC ) 108.83(17)° and
108.61(17)°, <CsNsC ) 109.37(17)°) and (PCP)IrNdNC5H10 (4, right,
IrsN ) 2.008(11) Å, NsN ) 1.244(16) Å, <CsIrsN ) 154.9(4)°,
<IrsNsN ) 135.1(12)°, <NsNsC ) 128.4(16)° and 118.8(15)°, <CsNsC
) 112.8(14)°) (tert-butyl groups and most H atoms omitted for clarity).
1-Aminopiperidine (C5H10NNH2, 1.1 equiv) also reacted at room
temperature with 1a and 1b to give the products of oxidative addition
(DtBPP)Ir(H)(NHNC5H10) (3a) and (PCP)Ir(H)(NHNC5H10) (3b)
(Scheme 1). Structural characterization (Figure 2) of 3a showed that
this complex also adopts a structure that is consistent with an interaction
of the nitrogen electron pair with the metal center, resulting in a short
Ir-N bond distance of 1.9875(18) Å.2b,6 The C-Ir-N angle in 3a
(154.47(9)°) is more acute than those in 2a and 2b (163.2(4)° and
164.79(13)°, respectively), and the structure of 3a is best described as
a distorted trigonal bipyramid.
Pincer-ligated iridium complexes are well-known to add the C-H
bonds of aromatic groups14 and methyl groups R-to heteroatoms.15
To determine the relative rates for addition of N-H bonds versus
addition of these types of C-H bonds, we studied the reactions of
PCP-iridium complex 1b with 1-methyl-1-phenylhydrazine (Me-
PhNNH2) and 1,1-dimethylhydrazine (Scheme 2). These reactions
of 1b with MePhNNH2 and Me2NNH2 indicate that both double
N-H activation and a combination of R-methyl C-H activation
and subsequent N-N bond cleavage occur readily.
Complex 3a was stable at room temperature, but complex 3b
underwent a second N-H bond cleaVage at room temperature over
several days or at 60 °C over 8 h to form an aminonitrene complex,
4 (eq 1). The identity of this species was determined by NMR
spectroscopy and X-ray diffraction. The reaction formed dihydrogen
as the byproduct, and the H2 was observed as a broad signal in the 1H
NMR spectrum between 4.0 and 4.4 ppm due to a reversible interaction
with iridium.7 The conversion of hydrido hydrazido complex 3b to
nitrene 4 was cleanly first-order (kobs ) 8.3 × 10-6 s-1 at 23 °C; ∆G‡
) 24.2 kcal/mol; see Supporting Information for the raw data).
The solid-state structure of complex 4 is shown in Figure 2. This
complex contains a distorted square planar geometry with a
C-Ir-N angle of 154.9(4)°. The N-N bond (1.244(16) Å) in 4 is
considerably shorter than the N-N bonds in complexes 2a, 2b,
and 3a (1.356(4)-1.435(2) Å), and this difference indicates that 4
contains N-N multiple bond character. The ꢀ-N atom (N2) in the
hydrazido hydride complex 3a is pyramidal, whereas the ꢀ-N atom
in 4 is nearly planar, more akin to the geometry of aminonitrene
or isodiazene complexes.8 The Ir-N bond distance (2.008(11) Å)
in 4 is similar to those in complexes 2a, 2b, and 3a. The NNC5H10
ligand in 4 is bent with an Ir-N-N angle of 135.1(12)°.
Scheme 2. Reactions of 1b with MePhNNH2 and Me2NNH2
The reaction of 1b with 1.1 equiv of MePhNNH2 at room
temperature rapidly formed a mixture of two products in a ca. 2:1
ratio. The major product was (PCP)IrNdNPhMe (5) from double
NsH activation;16 the minor product was the phenyl isocyanide
trans-dihydride complex (PCP)Ir(H)2CtNPh (6) resulting from
NsN bond cleavage. A broad singlet at 0.2 ppm for NH3 was
observed in the 1H NMR spectrum of the reaction mixture.17
Aminonitrene complex 5 was isolated and fully characterized. This
complex is thermally unstable at 23 °C over days and decayed
quantitatively to a metallacyclic aminonitrene complex 7, the crystal
structure of which is provided in the Supporting Information.
Isocyanide complex 6 was characterized by NMR spectrscopy and
Figure 3. Two limiting resonance structures of bent aminonitrene complexes.
Limiting structures of M-N-NR2 species are shown in Figure
3. Most complexes of :NNR2 ligands have linear M-N-N linkages
(structure A),8b,9 but a few contain bent M-N-NR2 units (B) that
are resonance hybrids of structures B1 and B2.8a,10 The two tung-
1
X-ray diffraction. The H NMR spectrum of 6 contained a triplet at
-9.85 ppm (2JP-H ) 14.0 Hz) for two hydrogens, and the infrared
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C O M M U N I C A T I O N S
spectrum contained a νCtN band at 2073 cm-1. The structure of 6
determined by X-ray diffraction (see Supporting Information) consists
of an octahedral geometry with two apical hydrides. The length of the
C-N bond (1.172(5) Å) corresponds to that of a C-N triple bond.
Treatment of 1b with 1.1 equiv of Me2NNH2 formed an
analogous mixture of two products (PCP)IrNdNMe2 (9) and
(PCP)Ir(H)2CtNMe (10). The formation of ammonia again ac-
companied the formation of the isocyanide complex. At the early
stage of this reaction (5 min), the hydrido hydrazido complex 8
that had formed from the first N-H bond addition of Me2NNH2
was observed in 94% yield. Like complex 3b, monohydride 8
underwent a second N-H activation to form the aminonitrene
complex (PCP)IrNdNMe2 (9). The reaction was complete after 9 h,
and a mixture of complex 9 and the isocyanide trans-dihydride
complex 10 was formed in a ca. 2:3 ratio.
The proposed mechanism for the reactions of the phenyliridium
hydride complex 1b with MePhNNH2 and Me2NNH2 is shown in
Scheme 3. Like the aminonitrene complex 4, the nitrene complexes 5
and 9 result from double N-H bond cleavages and elimination of H2.
The formation of the isocyanide complexes likely proceeds by initial
oxidative addition of the methyl C-H bonds, followed by N-N bond
cleavage via ꢀ-NH2 elimination to generate ammonia and the imine.
Addition of the C-H bond of the resulting imine and the subsequent
deinsertion of the isocyanide unit18 would form the final iridium
product. Because the reaction of 1b with Me2NNH2 initially formed
the monohydride 8 in >90% yield, but the nitrene complex 9 derived
from 8 was formed in 40% yield with a 57% yield of isocyanide
complex 10, the first N-H bond activation is likely reversible.
Scheme 4. Preliminary Reactivity of Complex 4
Acknowledgment. We thank the Department of Energy for
support and Johnson-Matthey for iridium.
Supporting Information Available: Experimental procedures,
spectra data for all compounds, and crystallographic data for 2a, 2b,
3a, 4, 6, and 7 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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summarized in Scheme 4. In the presence of excess CO, the iridium(I)
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CO, followed by release of the free isodiazene, which is known to
undergo dimerization to form tetrazene 11.19 Treatment of 4 with MeI
immediately formed the iridium(III) methyl iodide and the tetrazene
11. The reaction of 4 with phenol formed the hydrido iridium(III)
phenoxide complex after 7 days at room temperature. In this reaction
dipyridododecahydro-s-tetrazine (12) was formed exclusively over the
azopiperidine 11. Tetrazine 12 is known to form over 11 in protic
media.20 Finally, addition of 4 atm of H2 to the aminonitrene led to
the formation of the iridium tetrahydride and 1-aminopiperidine,
indicating that both of the N-H bond activation steps are reversible.
In summary, we have shown that one or two N-H bond
cleavages of hydrazines occur to form a hydrido hydrazido or an
aminonitrene complex, depending on the ligand at iridium and the
substituents on the hydrazine. The reactions with methyl-substituted
hydrazines formed a mixture of aminonitrene complexes and the
combination of isocyanide iridium(III) dihydrides and ammonia by
initial oxidative addition of the methyl C-H bond and subsequent
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