Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity toward Alkynes and Hydroamination Catalysis

Article abstract of DOI:10.1021/acs.organomet.8b00365

The pincer ligand HN(CH₂CH₂PPh₂)₂ (1; PNHP) reacted with [{Ir(μ-X)(cod)}₂] (X = Cl, OMe), affording complexes [fac-(PNHP)Ir(cod)]Cl (2) and [fac-(PNP)Ir(cod)] (3), respectively. The X-ray molecular structure of 2 showed that the PNP ligand coordinates in a facial fashion, with the N atom in an axial site and both P atoms coordinated in the equatorial plane. Compound 1 is able to protonate the hydroxo bridges in the complex [{Ir(μ-OH)(coe)₂}₂] forming the new amido complex [mer-(PNP)Ir(coe)] (4). Complex 4 is an extremely air sensitive compound, as confirmed by the isolation of the oxo complex [mer-(PNP)Ir(σ²-O₂)] (8) from its interaction with air. Protonation of 4 with HBF₄ afforded the corresponding amino complex [mer-(PNHP)Ir(coe)]BF₄ (5), whose molecular structure enlightened by X-ray crystallography confirmed the PNP ligand to be coordinated in a meridional fashion. The coe ligand in 4 is tightly bonded to iridium; however, under an atmosphere of ethylene at 60 °C or with acrylonitrile at 70 °C complex 4 exchanges the olefin, affording compounds [mer-(PNP)Ir(σ²-C₂H₄)] (6) and [mer-(PNP)Ir(σ²-C₂H₃CN)] (7), respectively. Interaction of 4 with alkynes depends on the nature of the substrate; therefore, methyl phenylpropiolate reacted with 4, affording the adduct [mer-(PNP)Ir(σ²-PhCCC(O)OMe)] (9), while the parent acetylene undergoes a double C-H activation, affording the Ir(III) complex [fac-(PNHP)IrH(Ca‰?CH)₂] (10). A DFT theoretical analysis of this transformation supports a metal-ligand cooperation mechanism. The reaction starts by deprotonation of an alkyne moiety by the PNP ligand followed by oxidative addition of the C-H bond to the metal of a second alkyne molecule. Additionally, we have tested complex 4 as a catalyst for the addition of gaseous ammonia to activated unsaturated substrates. A DFT theoretical analysis disclosed the operative mechanism on these organic transformations, which starts with a nucleophilic attack of ammonia to the bound alkyne, hydrogen migration to the metal, and reductive elimination steps.

Full text of DOI:10.1021/acs.organomet.8b00365

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity
toward Alkynes and Hydroamination Catalysis
‡
‡
‡
‡
Pablo Hermosilla, Pablo Lopez, Pilar García-Orduna, Fernando J. Lahoz, Víctor Polo,§
́
̃
and Miguel A. Casado*,‡
‡
́
́
́
Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), Departamento de Química Inorganica, CSIC-Universidad de
Zaragoza, c/Pedro Cerbuna 12 50009, Zaragoza, Spain
§
́
Departamento de Química Física and Instituto de Biocomputacion y Física de los Sistemas Complejos (BIFI), Universidad de
Zaragoza, c/Pedro Cerbuna 12 50009, Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The pincer ligand HN(CH2CH2PPh2)2 (1;
PNHP) reacted with [{Ir(μ-X)(cod)}2] (X = Cl, OMe),
affording complexes [fac-(PNHP)Ir(cod)]Cl (2) and [fac-
(PNP)Ir(cod)] (3), respectively. The X-ray molecular
structure of 2 showed that the PNP ligand coordinates in a
facial fashion, with the N atom in an axial site and both P
atoms coordinated in the equatorial plane. Compound 1 is able to protonate the hydroxo bridges in the complex [{Ir(μ-
OH)(coe)2}2] forming the new amido complex [mer-(PNP)Ir(coe)] (4). Complex 4 is an extremely air sensitive compound, as
confirmed by the isolation of the oxo complex [mer-(PNP)Ir(η2-O2)] (8) from its interaction with air. Protonation of 4 with
HBF4 afforded the corresponding amino complex [mer-(PNHP)Ir(coe)]BF4 (5), whose molecular structure enlightened by X-
ray crystallography confirmed the PNP ligand to be coordinated in a meridional fashion. The coe ligand in 4 is tightly bonded to
iridium; however, under an atmosphere of ethylene at 60 °C or with acrylonitrile at 70 °C complex 4 exchanges the olefin,
affording compounds [mer-(PNP)Ir(η2-C2H4)] (6) and [mer-(PNP)Ir(η2-C2H3CN)] (7), respectively. Interaction of 4 with
alkynes depends on the nature of the substrate; therefore, methyl phenylpropiolate reacted with 4, affording the adduct [mer-
(PNP)Ir(η2-PhCCC(O)OMe)] (9), while the parent acetylene undergoes a double C−H activation, affording the Ir(III)
complex [fac-(PNHP)IrH(CCH)2] (10). A DFT theoretical analysis of this transformation supports a metal−ligand
cooperation mechanism. The reaction starts by deprotonation of an alkyne moiety by the PNP ligand followed by oxidative
addition of the C−H bond to the metal of a second alkyne molecule. Additionally, we have tested complex 4 as a catalyst for the
addition of gaseous ammonia to activated unsaturated substrates. A DFT theoretical analysis disclosed the operative mechanism
on these organic transformations, which starts with a nucleophilic attack of ammonia to the bound alkyne, hydrogen migration
to the metal, and reductive elimination steps.
last few years in the context of organometallic chemistry,
partially due to their strong coordination to metals that renders
reactive metallic fragments which have been found to be
operative in a number of catalytic organic transformations,8
especially those pincer systems in combination with
ruthenium.9 The intrinsic structure of these pincer ligands is
a key factor that has allowed the development of the late-metal
amido organometallic chemistry, precisely those systems that
share a common amido center (Figure 1).10 In this context,
PNP ligands based on a pyridine moiety have become quite
popular because of their participation in metal−ligand
cooperative catalysis via aromatization/dearomatization of
the pyridinic ring, which in turn involves a change concerning
the electronic distribution at the N atom (amine versus amido
coordination).11 The application of PNP ligands embedded
into an aliphatic array in the context of bond activation and
homogeneous catalysis was first pioneered by Fryzuk et al.,
INTRODUCTION
■
In contrast to early-transition-metal amides,1 late-transition-
metal amido complexes are less common. However, late-
transition-metal amido complexes still attract a great deal of
interest partially because of their potential participation in
catalytic hydroamination processes,2 C−N coupling reactions,3
and transfer hydrogenation to unsaturated substrates,4 among
other organic transformations.5 The main difference between
early- and late-transition-metal amide species relies on the
nature of the M−N bond. In the former cases favorable M−N
π bonding explains the abundance of such species, while in the
case of late metals repulsive filled pπ−dπ electronic repulsions
are expected, a situation that accounts for the relative
scarceness of such late-transition-metal amido species.6
Furthermore, amido ligands with C−H bonds in the α
position are potentially able to undergo β-hydride elimination
processes leading to imine formation.7
Pincer constructs constitute a very important family of
polyfunctional ligands whose architectures can be tailored by
ligand design; they have attracted enormous interest during the
Received: May 30, 2018
© XXXX American Chemical Society
A
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Figure 1. Different types of amido pincer ligands.
who synthesized a disilylamido ligand that allowed the
isolation of stable amido complexes of Ir, Ni, and Pd by the
assistance of the coordination of two phosphine fragments to
the respective metals.12 The use of alkylic PNP n,ionic ligands
of the type [N(CH2CH2PR2)2]− (R = alkyl, aryl) in
combination with late transition metals usually leads to
systems with a reactivity centered at the nitrogen atom in
terms of basicity and nucleophilicity.13 These species are
excellent candidates for exhibiting metal−ligand cooperativity.
Along this line, there are recent examples of late-transition-
metal/PNP systems active in the catalytic dehydrogenation of
alcohols,14 acceptorless dehydrogenative coupling of alcohols
to esters,15 hydrogenation of amides16 and esters,17 N-
alkylation of amines with alcohols,18 and α-alkylation of
ketones with primary alcohol,s19 among other interesting
catalytic transformations.20 Most of the aforementioned
catalytic processes have been found to proceed through
metal−ligand cooperation.21
Figure 2. Molecular structure of the cation of complex 2, [fac-
(PNHP)Ir(cod)]+. Hydrogen atoms (except that bound to an N
atom) have been omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complex 2
a
Ir−P1
Ir−P2
Ir−N
P1−Ir−P2
P1−Ir−N
P1−Ir−Ct1
P1−Ir−Ct2
P2−Ir−N
2.3316(5)
2.3251(5)
2.1554(15)
104.017(16)
81.90(4)
131.85(5)
100.55(5)
82.74(4)
Ir−Ct1
Ir−Ct2
2.0290(18)
2.0702(20)
a
P2−Ir−Ct1
P2−Ir−Ct2
N−Ir−Ct1
N−Ir−Ct2
Ct1-Ir-Ct2
121.57(6)
101.60(6)
88.83(7)
174.24(7)
85.66(8)
In this paper we report on the ability of the known PNHP
pincer ligand (PNHP = HN(CH2CH2PPh2)2 (1))22 to
stabilize reactive iridium amine and amido complexes, some
of which readily activate C−H bonds of unsaturated substrates.
Additionally, we disclose the catalytic activity of an Ir(I) PNP-
based complex in the hydroamination of activated substrates
with ammonia.
a
Ct1 and Ct2 represent the midpoints of the C(1)C(2) and
C(5)C(6) bonds, respectively.
RESULTS AND DISCUSSION
C23−N, Q = 0.4434(18) Å, ϕ = 108.78(16)°, E4
conformation).24
■
The known P2,N-donor pincer ligand HN(CH2CH2Ph2)2 (1;
PNHP) was prepared by in high yield following a modified
established procedure and isolated as a viscous oil.23
Treatment of the chloro-bridged complex [{Ir(μ-Cl)(cod)}2]
(cod = 1,5-cyclooctadiene) with 1 in a 1:2 molar ratio in
toluene produced a white crystalline solid in good yield (69%).
This has been fully characterized as the cationic complex [fac-
(PNHP)Ir(cod)]Cl (2). The slow diffusion of diethyl ether
into a concentrated solution of 2 in acetone produced white
single crystals, which allowed the study of its molecular
structure by X-ray methods.
The amine N atom exhibits bond angles larger than the
109.5° ideal value for a perfect tetrahedron (Ir−N−C23
112.72(11)°, Ir−N−C22 116.01(11)°, and C22−N−C23
111.17(14)°). The sum of these bond angles (339.9(2)°)
evidenced the pyramidalization of the nitrogen atom, located at
2.1554(15) Å from the metal atom. The larger trans influence
exerted by N atom, in comparison to that of the P1 atom,
induce Ir−olefin bond distances to be dissimilar, with Ir−Ct2
(2.0702(20) Å) slightly longer than the Ir−Ct1 bond
(2.0290(18) Å).
The molecular structure of the cation of complex 2 is shown
in Figure 2, and Table 1 collects selected geometrical
parameters. Complex 2 is cationic, with the iridium atom in
a slightly distorted trigonal bipyramidal geometry, with the
apical sites occupied by the N atom and a double bond of the
diolefin and the axial positions fulfilled by the phosphorus
atoms of the pincer ligand and the other alkene arm of the cod
fragment. The tridentate ligand is coordinated in a facial
manner to iridium, with a P−Ir−P angle of 104.017(16)°, the
smallest angle in the equatorial plane, most probably due to the
intrinsic architecture of the ligand which prevents further
opening. The tridentate coordination of PNHP ligands leads to
the formation of two highly puckered five-membered Ir−P−
C−C−N iridacycles (Ir−P1−C21−C22−N, Q = 0.5270(16)
The hydrogen atom of the amine establishes a hydrogen
bond with the chloride anion, characterized by a 3.186(2) Å
N···Cl interatomic distance and a 165(2)° N−H···Cl angle
(see Figure S30). On the whole, the molecular structure of 2
exhibits features in common with those of the related complex
[HN(2-PPh2-4-Me-C6H3)2Ir(cod)]Cl.25
Complex 2 was characterized in solution by means of NMR
spectroscopy and mass spectrometry. The 31P{1H} NMR
spectrum of 2 at room temperature showed a sole singlet at δ
1
14.4 ppm, which indicated Cs symmetry in solution. The H
NMR spectrum of 2 in deuterated acetone at room
temperature showed the olefinic protons of the cod ligand as
a broad resonance centered at δ 3.93 ppm, signaling that the
complex undergoes a dynamic process in solution. Therefore,
we performed a VT NMR study, finding out that at 233 K the
2
Å, ϕ = −117.74(17)°, T3/E3 conformation; Ir−P2−C24−
B
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fluxional movement was almost frozen (see Figure S1). At this
temperature, the olefinic protons were observed as two
separated broad signals, a situation that fits with the bpt
geometry observed in the solid state. The 13C{1H}-APT NMR
spectrum acquired at 233 K showed the CH2P (δ 32.4 ppm)
and CH2N (δ 57.4 ppm) carbons of each arm chemically
equivalent, respectively, which confirms the presence of the Cs
symmetry plane contained in the Ir−N axis. Additionally, the
mass ESI+ spectrum of 2 showed a peak at m/z 742.2333 that
corresponded to the calculated value. We took advantage of
the presence of the reactive NH proton in 1 to attempt its
deprotonation with the complex [{Ir(μ-OMe)(cod)}2], which
bears highly basic ligands that are able to deprotonate weakly
acidic protons in a number of cases.26 In fact, the complex
[{Ir(μ-OMe)(cod)}2] was reacted with 1 in a 1:2 molar ratio,
affording a dark red solid characterized as the complex [fac-
(PNP)Ir(cod)] (3), where the NH proton of 1 readily
protonated the methoxo bridging ligands (Scheme 1).
Monitoring of the reaction in deuterated toluene by NMR
techniques showed the clean formation of 3 along with
released methanol.
P-containing complexes, as confirmed upon inspection of the
31P{1H} NMR spectra.
Due to these complications, we decided to shift to the
additive deprotonation approach employed in the synthesis of
complex 3. For that purpose we reacted the known hydroxo-
bridged complex [{Ir(μ−OH)(coe)2}2]28 with 1 in a 1:2 molar
ratio in toluene, which led to the isolation of an orange solid
characterized as the compound [mer-(PNP)Ir(coe)] (4) with a
64% yield (Scheme 1). On the whole, the structural
information gathered indicates that complex 4 is square planar,
where the PNP ligand is κ2P,κN-coordinated in a meridional
fashion and the coe ligand is coordinated trans to N.
The ESI+ mass spectrum of 4 gave a peak at m/z 744.2498
that corresponded to the molecular ion [(PNP)Ir(coe)]+,
suggesting that there is only one coe molecule coordinated to
iridium. VT NMR studies on 4 in toluene-d8 showed that, as
opposed to diolefin complexes 2 and 3, it was static within the
range of temperature studied (193−293 K). The static
behavior of 4 does not involve steric factors, since molecular
modeling studies clearly indicated that rotation of the coe
ligand around the N−Ir axis is not blocked by the steric
pressure of the phenyl groups of the pincer ligand (vide infra).
1
An inspection of the H−13C HSQC NMR spectrum of 4
Scheme 1. Synthesis of Complexes 3 and 4
showed the carbons of the CH2N fragments as two distinct
multiplets at δ(13C) 59.3 and 60.2 ppm, which correlated with
complex signals at δ(1H) 3.05 and 3.22 ppm. The same
situation was observed for the CH2P fragments, information
which revealed the lack of a symmetry plane that would relate
both alkylic arms in 4. In accord with this, the 31P{1H} NMR
spectrum of 4 showed an AB system centered at δ 43.6 ppm
(2JP−P = 390 Hz). The olefinic protons of the coe olefin were at
δ(1H) 2.18 ppm, a resonance that correlated to a sole multiplet
1
at δ(13C) 37.7 ppm in the H−13C HSQC NMR spectrum,
signaling the presence of a symmetry plane. The low chemical
shift of the olefinic protons (and carbons) can be tentatively
explained in terms of a strong π back-donation from the filled d
orbitals of the metal to the olefin, which involves a reduction of
the CC bond order. This electron donation is in part due to
the presence of an amidic N atom, which on the NMR time
scale displays a planar geometry in which the lone electron pair
should be involved in the Ir−N bond.29
The solubility of 3 in aromatic solvents suggested a neutral
nature and allowed its spectroscopic characterization by NMR
techniques, which in general showed a fluxionality pattern at
room temperature similar to that observed for complex 2. The
1H NMR spectrum of 3 showed broad bands at room
temperature for all the protons of the molecule. However, at
253 K the spectrum dramatically changed, showing a new
pattern of resonances that reflected a trigonal-bipyramidal
geometry around iridium and a Cs symmetry in solution (see
Figure S2). An alternative synthetic route toward compound 3
relies on the relative acidity of the NH proton in 2, which in
the presence of triethylamine cleanly afforded complex 3 upon
removal of HNEt3Cl.
Next we turned our attention to iridium complexes bearing
simple olefins, looking for the formation of 16 e− unsaturated
species from 1, which we expected to be more reactive.
Specifically, we carried out reactions of 1 with the known
chloro-bridged complex [{Ir(μ-Cl)(coe)2}2] and the com-
pound [Ir(acetone)2(coe)2]PF6 (coe = cyclooctene) in several
solvents, all of which led invariably to mixtures, as revealed by
NMR spectroscopy of the isolated solids, which showed
hydrido signals most probably as a consequence of C−H
activation processes from the released coe, as has already been
observed in related systems.27 Along this line, deprotonation of
1 with strong bases (tBuOK, MeLi) and further addition of
[{Ir(μ-Cl)(coe)2}2] led again to the formation of mixtures of
This situation explains the lack of reactivity of 4 with
electrophiles such as methyl triflate and trifluoroacetic acid,
which we expected to methylate (or protonate) the amidic
nitrogen, as has already been observed in a related system.30
However, the reaction of 4 with a powerful electrophile such as
tetrafluoroboric acid proceeded cleanly through protonation at
the nitrogen atom (instead of protonation at the metal) and
allowed the isolation of the cationic complex [mer-(PNHP)-
Ir(coe)]BF4 (5) in good yield. Deep red single crystals of 5
were grown by slow diffusion of hexanes into a concentrated
solution of 5 in CH2Cl2, which were subjected to an X-ray
structural analysis; the molecular structure of the cation 5+ is
depicted in Figure 3, and bond lengths and angles character-
izing the metal coordination sphere are reported in Table 2.
Deviations from an ideal square-planar geometry in cation 5+
may be related to geometrical restraints of the tridentate
coordination of the PNHP ligand, whose P−Ir−N angles
values (83.1(3) and 82.2(3)°) are similar to those found in
complex 2. The pincer ligand in 5+ is found to be mer-
coordinated to iridium, with the phosphine groups trans to
each other, while the coe ligand fills the fourth coordination
site, trans to the nitrogen atom. The Ir−P−C−C−N iridacycle
C
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However, the pattern of resonances from the coe ligand did
not fit with the apparent Cs symmetry of 5 in solution. Only
one resonance was observed for the CH protons at δ 3.33
ppm, which correlated to a sole signal at δ 60.0 ppm in the
13C{1H}-APT NMR spectrum, reflecting some degree of
symmetry that seems to relate both sides of the olefin. One
could argue that flexibility of the ligand backbone could give
rise to this phenomenon; however, protonation of the nitrogen
atom avoids inversion at nitrogen, and subsequently the pucker
chirality of the tridentate ligand is fixed. Therefore, the
observed situation is probably the consequence of fluxional
processes in solution, most probably related to a fast coe
rotation around the Ir−N axis (as indicated by the solid-state
structure), which did not freeze even at 183 K in CD2Cl2.31
Along this line, from an electronic perspective the drastic
downfield shift of the resonances of the CH fragments of the
coe molecule in 5 in comparison to those for amido complex 4
(Δ(δ(1H)) = 1.15 ppm; Δ(δ(13C)) = 22.3 ppm) is indicative
of a much stronger π back-donation from the N atom to the
metal in 4 in comparison to amine complex 5. This electronic
arrangement in 4 enhances the binding of the coe olefin in
comparison to 5.
Figure 3. Molecular structure of the cation [mer-(PNHP)Ir(coe)]+
(5+). For clarity, only the H atom bound to N and the major
component of the disordered coe fragment have been included.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complex 5
Ir−P1
Ir−P2
P1−Ir−P2
P1−Ir−N
P2−Ir−N
2.278(3)
2.301(3)
163.21(12)
83.1(3)
Ir−N
Ir−Ct1
2.144(9)
2.0619(1)
105.3(4)
82.2(3)
a
a
a
P1−Ir−Ct
P2−Ir−Ct
In order to explore the reactivity of olefin complex 4, we
carried out a series of reactions with distinct substrates. For
instance, addition of phosphanes to solutions of 4 did not
induce any transformations, which is surprising since it is well-
known that π-coordinated olefins easily exchange with strongly
electron donating phosphines, a chemical behavior that
reinforces the existence of a strong Ir−olefin bond. Along
this line, a number of amines (including ammonia) did not
react with 4, even under harsh conditions. Stirring toluene
solutions of 4 under an atmosphere of ethylene (1 bar) at 60
°C for 3 h in a J. Young pressure tube or with acrylonitrile at
70 °C afforded cleanly the corresponding olefin adducts [mer-
(PNP)Ir(η2-olefin)] (olefin = ethylene (6), acrylonitrile (7)).
Complex 6 reflected a C2h symmetry at all temperature ranges
(δ(31P) 42.5 ppm), where the η2-bound ethylene was observed
as a virtual triplet at δ(1H) 1.99 ppm. Multinuclear NMR
spectra of 7 in C6D6 at variable temperature is coherent with a
square-planar geometry, in which the PNP ligand is mer-
coordinated. As in complexes 4 and 6, the symmetry plane
containing both P and N atoms suggests some degree of
planarization at nitrogen; additionally, the 31P{1H} NMR
spectrum of 7 showed an AB system with a large coupling
constant (2JP−P = 385 Hz). The loss of the symmetry plane in
the molecule clearly indicates that the olefin is static at all
ranges of temperature, most probably due to the strong
binding to the metal, as suggested by the low chemical shifts
a
82.2(3)
Ct −Ir-N
169.6(4)
a
Ct1 is the centroid of the olefinic bond of the coe ligand (main
component).
ring puckering amplitudes (Ir−P1−C21−C22−N, Q =
0.491(3) Å, ϕ = −87.9(6)°, T3 conformation; Ir−P2−C24−
24
3
C23−N, Q = 0.490(3) Å, ϕ = 93.0(6)°, T4 conformation)
are similar to those observed in complex 2. Pyramidalization of
the nitrogen atom in 5+ is analogous to that observed in 2, as
evidenced by the sum of the bonding angles (342.1(13)°). The
observed Ir−N distance (2.144(9) Å) is comparable to that of
complex 2 and similar to other bond distances reported in
related Ir-based PNHP pincer systems. For example, 2.176(3),
2.131(4), and 2.121(6) Å Ir−N bond lengths have been
reported in [Ir(PMe3)(PNHPiPr)]+, [Ir(C2H4)(PNHPiPr)]+
and [Ir(CO(PNHPiPr)]+ cations. Slight differences among
these values may be attributed to different trans influences,
exerted by phosphine,30 ethylene,27a and CO27a ligands,
respectively. These large distances (in comparison to those
observed in Ir(PNP) fragments) have been related to the weak
coordinative M−N bonding.27a Interestingly, the coe fragment
in 5+ has been found to be disordered. The structural model
includes this ligand in two sets of positions (see Figure S30)
roughly related by a 180° rotation about the Ir−N axis. This
feature may indicate that steric requirements resulting from the
tridentate ligand coordination are not restrictive enough to fix
the coe position in this complex.
1
observed for the olefin in both the H and 13C{1H} NMR
spectra.
For preparative purposes, the synthesis of adducts 6 and 7
must be carried out in an oxygen-free solvent. For example,
when the solvent is not deoxygenated by means of three freeze
and thaw cycles, stirring of 4 under an atmosphere of ethylene
at 60 °C afforded a dark greenish solid, which was found to be
a 1:1 mixture of complex 6 and a new species. This were
further characterized as the oxo complex [mer-(PNP)Ir(η2-
O2)] (8),32 a clear indication that olefin complex 4 is highly
sensitive toward oxygen, an observation that led to synthesiz-
ing oxo compound 8 simply by stirring solutions of 4 in air.
Analysis of 8 by NMR spectroscopy showed only resonances
assignable to the PNP ligand skeleton, two multiplets due to
the fragments CH2N and CH2P, which correlated with two
1
The H NMR spectrum of 5 in CD2Cl2 clearly showed a
static compound at all ranges of temperature studied (see
Figure S5); the NH proton was observed as a broad triplet at δ
5.80 ppm, which correlated with a resonance at 57.2 ppm in
the 1H−15N HMBC spectrum. The 31P{1H} NMR spectrum of
5 showed a singlet at δ 34.1 ppm, which indicated the existence
of a plane of symmetry that relates both alkylic arms of the
PNP ligand, as confirmed by the 1H−13C HSQC NMR
spectrum of 5, which reflected this structural arrangement.
Pyramidalization at the N atom upon protonation becomes
evident, since the original symmetry plane observed in 4
(which contains both P and N atoms) is not present in 5.
D
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triplets at δ 66.2 and 32.7 ppm in the 13C{1H} NMR spectrum,
respectively. The pattern of the multinuclear NMR spectra of 8
indicated a C2h symmetry in solution; along this line, the
31P{1H} NMR spectrum of 8 showed a sole singlet at δ 19.2
ppm. The presence of the coordinated dioxygen ligand was
detected clearly in the mass spectrum of 8, which showed a
peak at m/z 666.1316 that corresponded to the molecular ion
[(PNP)Ir(O2)]+; additionally, microanalytical data obtained
on 8 agreed with the proposed formulation.
The efficiency of olefin coordination with deprotonated
ligand 1 led us to explore the reactivity of 4 with alkynes. For
that purpose, we reacted complex 4 with methyl phenyl-
propiolate to give an orange solid in moderate yield,
characterized as the complex [(PNP)Ir(η2-PhCCC(O)-
OMe)] (9). In contrast, treatment of a solution of 4 in
toluene under an atmosphere of acetylene (1 bar) in a Kontes
pressure tube for 2 h at room temperature afforded a light
yellow solid in 58% yield, further characterized as the Ir(III)
complex [fac-(PNHP)IrH(CCH)2] (10) (Scheme 2).
enlightened its static nature in solution. A hydrido ligand
was observed at δ −10.15 ppm as a doublet of doublets (2JH−P1
= 21 Hz, 2JH−P2 = 161 Hz), which located it trans to P2 and cis
to P1 (Scheme 2), while the NH proton was observed at δ 5.26
ppm and the alkynyl protons in 10 were easily distinguished by
their different multiplicity. The 31P{1H} NMR spectrum of 10
showed two doublets with a low coupling constant which
indicated that they were mutually cis and therefore signaled a
fac coordination of the PNHP ligand. The off-resonance 31P
NMR spectrum of 10 split both signals into a well-resolved
doublet of doublets (2JP1−H = 20 Hz; 2JP2−H = 149 Hz), which
allowed us to unambiguously establish the stereochemistry of
10. The 13C{1H}-APT NMR spectrum of 10 showed four well-
separated signals for the carbons of the alkynyl fragments, in
which the respective magnitude of the coupling constants
allowed their correct assignment (see the Experimental Section
and Figure S17).
The spectroscopic information gathered for adduct 9
indicated a structure different from that assigned to complex
10. The 31P{1H} NMR spectrum of 9 showed a sole singlet,
indicating some degree of symmetry which related both alkylic
arms of the ligand, a situation confirmed by the observation of
sole signals for the CH2N and CH2P carbons in the 13C{1H}-
APT NMR spectrum, respectively. Additionally, we observed
the C(sp) carbons of the coordinated alkyne as two distinct
multiplets at δ 106.9 and 67.9 ppm, with small coupling
constants (2JC−P = 2−3 Hz). On the whole, the Cs symmetry in
solution observed for 9 is coherent with a square-planar
geometry, in which the PNP ligand is coordinated in a
meridional fashion to iridium and the alkyne π-coordinated out
of plane.33 However, we cannot exclude some distortion of this
planar geometry, since there are several related iridium
molecular structures with a distorted-tetrahedral environ-
ment.34
Scheme 2. Synthesis of Complexes 6−10
1
The pattern of the H NMR spectrum of 10 in CD2Cl2 did
In order to understand the reaction of complex 4 with
terminal alkynes, theoretical calculations at the DFT level were
carried out on a model system, replacing the phenyl ligands by
methyl groups. The Gibbs energetic profile for the reaction is
not change with the temperature and showed eight different
resonances assigned to the protons from the alkylic chains of
the ligand in the range of δ 2.28−3.47 ppm, which indicated
that 10 did not have any elements of symmetry and
Figure 4. Gibbs energy profile (in kcal mol−1, relative to A and isolated molecules) for the formation of 10.
E
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shown in Figure 4. Starting from structure A, direct oxidative
addition of the C−H bond to the Ir(I) center (green path),
upon replacement of coe by the alkyne, requires an activation
energy of 35.6 kcal mol−1, which is not affordable under the
experimental conditions. Alternatively, coordination of a
second alkyne molecule to the metal may take place through
TSBD, leading to the planar-bipyramidal structure D.
Table 3. Hydroamination of Activated Unsaturated
Substrates with NH3 Catalyzed by 4
a
The transformation of D into F requires the consecutive C−
H bond activation of two acetylene molecules by the metallic
center and by assistance of the N atom of the pincer ligand.
The two possible reaction pathways, namely (i) oxidative
addition first followed by a ligand-assisted process (red path in
Figure 4) and (ii) a ligand-assisted process followed by
oxidative addition (blue path in Figure 4) have been calculated.
Path i is characterized by the affordable structure TSDE
showing an overall energetic barrier of 23.6 kcal mol−1. The
hydride intermediate E can deprotonate the coordinated
alkyne through TSEF, forming the observed product F. The
process is strongly exergonic, −29.6 kcal mol−1. The alternative
path ii starts by activation of the C−H bond by the N atom of
the ligand as shown by TSDE′, presenting an overall energetic
barrier of 20.8 kcal mol−1. The formed alkynyl intermediate E′
may undergo oxidative addition of the Ir atom through TSEF′,
showing an energetic barrier of 13.3 kcal mol−1, to yield the
final product F.
Catalytic Activity of Complex 4 in the Hydro-
amination of Activated Substrates with Ammonia.
The catalytic activity of the complex [mer-(PNP)Ir(coe)]
(4) for the addition of ammonia to unsaturated substrates was
evaluated. In general, catalytic reactions were first monitored in
J. Young pressure NMR tubes with a catalyst loading of 5 mol
% and the specific substrate, and these mixtures were then
stirred at 60 °C in C6D6 under an ammonia atmosphere.
Nonactivated alkenes (styrene, 1-hexene) or alkynes (phenyl-
acetylene, 3-hexyne) were checked, and no hydroamination
products were obtained under the catalytic conditions
employed. In the case of terminal activated alkenes
(acrylonitrile and methyl acrylate), the transformations were
complete within 6 h. The analysis of pure samples of the
products of the catalytic reactions by multinuclear NMR
spectroscopy showed the formation of mixtures of the
corresponding secondary and tertiary amines, while no traces
of the primary amines were observed (see Table 3).
HA product
b
(mol %)
c
entry
substrate
n = 1
n = 2
yield (%)
1
2
3
4
CH2CHCN
CH2CHCO2Me
DMAD
70
57
30
33
83
88
93
91
PhCCCO2Me
a
Catalytic reactions were carried out in toluene (3 mL) at 60 °C for
20 h using 5 mol % of 4 and gaseous ammonia (3 atm). Calculated
by NMR integration. Yield after workup.
b
c
3-phenylacrylate selectively. No further addition of ammonia
across the double bond of the enamines was observed.
DFT calculations were performed on the catalytic cycle to
shed light into the reaction mechanism. As in the previous
DFT calculations, phenyl groups attached to the organo-
metallic complex were replaced by methyl substituents for
computational efficiency and two ammonia molecules were
employed in the calculations instead of one in order to get a
more realistic description of charged species and proton
transfer processes. The Gibbs energetic profile is shown in
Figure 5. Hence, the catalytic cycle starts by nucleophilic attack
of ammonia to the coordinated alkyne as shown by TSGH,
leading to intermediate H. Then, proton transfer from the
protonated amine to the metal occurs through intermediate
TSHI. This transition state determines the overall energetic
barrier for the catalytic cycle, 28.5 kcal mol−1, relative to G.
The proton is transferred to the metal, forming intermediate I,
which can be described as an Ir(III) metal displaying trigonal-
bipyramidal arrangement. Finally, reductive elimination of the
alkene yields the observed product and regenerates the catalyst.
An alternative proposal, starting by the oxidative addition of a
molecule of ammonia by the metal has also been calculated
(see the Supporting Information) but the energetic barrier is
excessively high.
These organic compounds are the result of the addition of
ammonia to the alkenes. Additionally, the absence of the
primary amines (H2NCH2CH2R; R = CN, CO2Me) shows
that they are more nucleophilic than ammonia itself, and
therefore they react more quickly with olefins than gaseous
NH3.35 However, internal alkenes (crotonitrile, methyl
crotonate) did not undergo any transformations under the
above catalytic conditions. We performed some catalytic
addition of ammonia on activated alkynes, such as dimethyl
acetylenedicarboxylate (DMAD) and methyl phenylpropiolate.
First, we ran blank experiments (without catalyst 4), finding
that while no reaction between ammonia and methyl
phenylpropiolate was observed at 60 °C upon 24 h, DMAD
reacted with ammonia at 60 °C for 20 h to yield pure (Z)-
dimethyl 2-aminofumarate quantitatively. When the reaction
was carried out with catalyst 4 the transformation was
complete upon 20 h, affording pure enamine (E)-dimethyl 2-
aminofumarate selectively.36 In the case of methyl phenyl-
propiolate, the catalytic reaction afforded (Z)-methyl 3 amino-
CONCLUSIONS
■
In this contribution we have utilized a known PNP pincer
ligand to stabilize new Ir(I) amido olefin complexes, in which
the ligand may adopt both mer or fac coordination depending
on the nature of the olefin. In general, the spectroscopic
information gathered for the amido complexes characterized
indicates a strong binding of the corresponding olefin (coe,
ethylene, acrylonitrile), favored by a strong π back-donation
from the metal induced by the amidic nitrogen atom of the
ligand, which on the NMR time scale seems to be planar, most
probably as a consequence of the participation of the free
electron pair in the Ir−N bond. This electronic arrangement
situation changes upon protonation at nitrogen with a strong
electrophile, affording the corresponding amine organometallic
salt. In these species the geometry of the nitrogen atom is
tetrahedral and there is no inversion in solution; in this case
the olefin (coe) is more labile and the olefin seems to undergo
a fast rotation in solution. The amido complex exchanges the
F
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Article
Figure 5. Gibbs energy profile (in kcal mol−1, relative to A and isolated molecules) for the catalytic cycle.
[{Ir(μ−OH)(coe)2}2]28 were prepared according to published
procedures.
coe olefin with alkynes, where the internal substrate
coordinates out of plane and the parent acetylene undergoes
two consecutive C−H activation processes, affording a hydrido
Ir(III) complex where the PNHP ligand coordinates in a mer
fashion, following a mechanism based on the concept of
metal−ligand cooperation. The reaction starts by deprotona-
tion of a second alkyne molecule by the PNP ligand and it is
followed by oxidative addition of the C−H bond to the metal.
Additionally, we have shown that an amido coe complex is
active in the catalytic hydroamination of activated substrates.
The operative mechanism studied by theoretical methods
shows that these organic transformations follow a nucleophilic
attack of ammonia to the bound alkyne, hydrogen migration to
the metal, and reductive elimination steps.
Synthesis of PNHP (1). To a suspension of potassium tert-
butoxide (6.71 g, 59.8 mmol) in THF (30 mL) was added pure
diphenylphosphane (4 mL, 23.0 mmol) via syringe, giving a deep red
solution which was stirred for 20 min. Then, solid bis(2-chloroethyl)-
amine hydrochloride (2.05 g, 11.45 mmol) was added to the mixture
and the resulting solution was stirred under reflux for 10 h. Upon
reaching room temperature, the reaction mixture was quenched with a
solution of 1 M hydrochloric acid (10 mL, 10 mmol), leading to a
two-phase system. The organic layer was separated, dried over
anhydrous MgSO4, and then dried under vacuum, affording a whitish,
viscous oil. Yield: 4.83 g (88%). 1H NMR (400 MHz, C6D6, 298 K): δ
7.33−7.46 (set of m, 20 H; PPh2), 2.75 (dd, 4H, 3JH−H = 7 Hz, 3JH−H
= 16 Hz; CH2N), 2.24 (m, 4H; CH2P), 1.39 (br s, 1H; NH). 31P{1H}
NMR (162 MHz, C6D6, 298 K): δ −20.6 (s).
Synthesis of [fac-(PNHP)Ir(cod)]Cl (2).
EXPERIMENTAL SECTION
■
Scientific Equipment. C, H, and N analyses were carried out in a
PerkinElmer 2400 Series II CHNS/O analyzer. IR spectra of solid
samples were recorded with a PerkinElmer 100 FT-IR spectrometer
(4000−400 cm−1) equipped with attenuated total reflectance. 1H,
31P{1H}, and 13C{1H} NMR spectra were recorded on Bruker Avance
300 (300.13, 121.42, and 75.48 MHz) and Bruker Avance 400
(400.16, 161.99, and 100.61 MHz) spectrometers. NMR chemical
shifts are reported in ppm relative to tetramethylsilane and are
referenced to partially deuterated solvent resonances. Coupling
constants (J) are given in hertz. Spectral assignments were achieved
To a solution of 1 (51 mg, 0.11 mmol) in toluene (5 mL) was added
solid [{Ir(μ-Cl)(cod)}2] (35 mg, 0.05 mmol). After the mixture was
stirred for 1 h, a white microcrystalline solid crystallized out of the
solution, which was isolated by filtration with a cannula, washed with
diethyl ether, and dried under vacuum. Yield: 56 mg (69%). Anal.
Calcd for C36H41ClIrNP2: C, 55.62; H, 5.32; N, 1.80. Found: C,
55.65; H, 5.25; N, 1.74. IR (solid, cm−1): ν(N−H), 3049 (s), 3217
1
1
by a combination of H−1H COSY, 13C-APT, H−13C HSQC, and
1H−15N HMBC NMR experiments. ESI+ mass spectra were obtained
on a Bruker MICROFLEX spectrometer using 1,8-dihydroxy-9,10-
dihydroanthracen-9-one (DIT, dithranol), as matrix. GC-MS analyses
were recorded on an Agilent 5973 mass selective detector interfaced
to an Agilent 6890 series gas chromatograph system, using a HP-5MS
5% phenyl methyl siloxane column (30 m × 250 mm with a 0.25 mm
film thickness).
1
(s), 3356 (s). H NMR (400 MHz, (CD3)2CO, 298 K): δ 8.08 (br,
1H; NH), 7.13−7.70 (set of m, 20H; PPh2), 3.93 (br, 4H; = CH
cod), 3.33 (m, 2H; CH2N), 2.86 (m, 4H; CH2N + CH2P), 2.56 (m,
2H; CH2P), 1.63 (m, 4H), 1.44 (m, 4H) (CH2 cod). 1H NMR (400
MHz, (CD3)2CO, 233 K): δ 8.19 (br, 1H; NH), 7.13−7.63 (set of m,
20H; PPh2), 4.16 (m, 2H), 3.53 (m, 2H) (CH cod), 3.28 (m, 2H),
2.97 (m, 2H) (CH2N), 2.86 (m, 2H), 2.56 (m, 2H) (CH2P), 1.83 (m,
2H), 1.51 (m, 2H), 1.33 (m, 4H) (CH2 cod). 31P{1H} NMR (162
MHz, (CD3)2CO, 233 K): δ 14.4 (s). 13C{1H} NMR (100 MHz,
Synthesis. All experiments were carried out under an atmosphere
of argon using Schlenk techniques. Solvents were distilled
immediately prior to use from the appropriate drying agents or
obtained from a Solvent Purification System (Innovative Technolo-
gies). Oxygen-free solvents were employed throughout. CD2Cl2 and
(CD3)2CO were dried using activated molecular sieves, while C6D6
and toluene-d8 were dried over a solid Na/K amalgam. Gaseous
ethylene, acetylene, and ammonia were purchased from Air Liquide.
The complexes [{Ir(μ-Cl)(cod)}2],37 [{Ir(μ-OMe)(cod)}2],38 and
2
2
(CD3)2CO, 233 K): δ 132.3 (d, JC−P = 5 Hz), 132.2 (d, JC−P = 5
Hz), 131.3 (d, JC−P = 5 Hz), 131.2 (d, JC−P = 5 Hz) (Co PPh2),
130.0 (s), 129.7 (s) (Cp PPh2), 128.7 (m; Cm PPh2), 65.0 (m), 53.1
(m) (CH cod), 57.4 (m; CH2N), 32.4 (m; CH2P), 31.1−31.5 (set
2
2
G
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Article
of s; CH2 cod). Mass Calcd for C36H41IrNP2: 741.8823. MS (ESI+):
m/z 742.2333 (100%; M+ − 1H).
removed by vacuum, affording an intense orange powder. Yield: 0.07
1
g (78%). H NMR (300 MHz, CD2Cl2, 298 K): δ 8.19 (m, 4H; Ho
Synthesis of [fac-(PNP)Ir(cod)] (3).
PPh2), 7.50−7.64 (set of m, 16H; Ho + Hm + Hp PPh2), 5.60 (m, 1H;
NH), 3.51 (m, 2H; CH2N), 3.33 (m, 2H; CH coe), 2.85 (m, 2H),
2.50 (m, 2H) (CH2P), 2.24 (m, 2H; CH2N), 1.85 (m, 2H), 1.18 (m,
4H), 0.90 (m, 6H) (CH2 coe). 31P{1H} NMR (121 MHz, CD2Cl2,
298 K): δ 34.1 (s). 13C{1H}-APT NMR (75 MHz, CD2Cl2, 298 K): δ
135.1, 132.0 (m; Co PPh2), 131.5, 130.5 (s; Cp PPh2), 128.7−128−9
(set of signals, Cp + Cm PPh2), 60.0 (s; CH coe), 52.5 (m; CH2N),
34.8 (m; CH2P), 32.0 (m; CH2CH coe), 31.4, 26.0 (s; CH2 coe).
1H−15N HMBC NMR (41 MHz, CD2Cl2, 298 K): δ 57.2. Mass Calcd
for C36H43IrNP2: 743.8903. MS (ESI+): m/z 744.2495 (100%; M+ +
1H).
To a solution of 1 (0.18 g, 0.41 mmol) in toluene (10 mL) was added
yellow solid [{Ir(μ-OMe)(cod)}2] (0.13 g, 0.20 mmol), giving a red
solution within 45 min. The volume of the solvent was reduced to ca.
2 mL by vacuum, and the addition of hexanes induced the
precipitation of a dark red solid. This was collected via filtration
with a cannula, washed with hexanes, and then vacuum-dried. Yield:
0.24 g (82%). Anal. Calcd for C36H40IrNP2: C, 58.36; H, 5.44; N,
1.89. Found: C, 58.29; H, 5.31; N, 1.84. 1H NMR (400 MHz,
CD2Cl2, 298 K): δ 7.09−7.57 (set of m, 20H; PPh2), 3.90 (br, 4H; =
CH cod), 3.35 (m, 2H), 2.84 (m, 2H) (CH2N), 2.76 (m, 2H), 2.31
Synthesis of [mer-(PNP)Ir(η2-C2H4)] (6).
A solution of 4 (65 mg, 0.08 mmol) in oxygen-free toluene (5 mL)
was transferred to a pressure Kontes tube, and then it was bubbled
with gaseous ethylene (1 bar) for 2 min. After the pressure reactor
was sealed, the solution was stirred at 60 °C, for 3 h leading to an
orange solution. This was filtered to eliminate some solid impurities,
and then the solvent was removed by vacuum, yielding an orange
powder. Yield: 40 mg (76%). Anal. Calcd for C30H32IrNP2: C, 54.53;
1
(m, 2H) (CH2P), 1.46 (br, 8H; CH2 cod). H NMR (400 MHz,
CD2Cl2, 253 K): δ 7.56−6.91 (set of m, 20H; PPh2), 3.93 (br, 2H),
3.65 (br, 2H) (CH cod), 3.26 (br, 2H), 2.84 (br, 2H) (CH2N),
2.76 (m, 2H), 2.29 (m, 2H) (CH2P), 1.92 (br, 2H), 1.57 (br, 2H),
1.27 (br, 4H) (CH2 cod). 31P{1H} NMR (162 MHz, CD2Cl2, 253 K):
δ 14.7 (s). 13C{1H} NMR (100 MHz, CD2Cl2, 253 K): δ 138.3 (m),
1
H, 4.88; N, 2.12. Found: C, 54.46; H, 4.91; N, 2.23. H NMR (300
2
2
134.1 (m) (Cipso PPh2), 132.3 (d, JC−P = 5 Hz), 132.2 (d, JC−P = 5
MHz, C6D6, 298 K): δ 7.11−8.23 (set of m, 20H; PPh2), 3.57 (m,
Hz), 131.2 (d, JC−P = 5 Hz), 131.2 (d, JC−P = 5 Hz) (Co PPh2),
130.0 (s), 129.7 (s) (Cp PPh2), 128.6 (m; Cm PPh2), 65.3 (m), 54.0
(m) (CH cod), 56.9 (m; CH2N), 33.1 (m; CH2P), 31.3 (br; CH2
cod). Mass Calcd for C36H40IrNP2: 740.8744. MS (ESI+): m/z
742.2369 (100%; M+ + 1H).
3
2
2
4H; CH2N), 2.58 (m, 4H; CH2P), 1.99 (t, JH−P = 4 Hz, 4H; CH
η2-C2H4). 31P{1H} NMR (121 MHz, C6D6, 298 K): δ 42.5 (s).
13C{1H} NMR (75 MHz, C6D6, 298 K): δ 128.3−136.6 (set of m;
PPh2), 61.3 (m; CH2N), 36.9 (m; CH2P), 18.0 (m; η2-C2H4). Mass
Calcd for C30H32IrNP2: 660.745. MS (ESI+): m/z 660.789 (100%;
M+).
Synthesis of [mer-(PNP)Ir(coe)] (4).
Synthesis of [mer-(PNP)Ir(η2-C2H3CN)] (7).
To a solution of 1 (0.51 g, 1.15 mmol) in toluene (10 mL) was added
yellow solid [{Ir(μ−OH)(coe)2}2] (0.36 g, 0.42 mmol), and the
resulting mixture was stirred for 1 h at room temperature, affording a
colorless solution. Removal of the solvent to ca. 1 mL by vacuum and
further addition of hexanes gave a whitish precipitate, which was
subsequently filtered, washed with hexanes, and then dried under
vacuum. Yield: 0.55 g (64%). Anal. Calcd for C36H42IrNP2: C, 58.20;
A solution of 4 (49 mg, 0.067 mmol) in toluene (5 mL) was treated
with neat acrylonitrile (12 mg, 14 μL, 0.206 mmol), and the resulting
solution was stirred at 70 °C for 2 h. The volatiles were removed by
vacuum, leading to the isolation of an orange solid. Yield: 38 mg
(82%). 1H NMR (400 MHz, C6D6, 298 K): δ 8.29 (m, 2H), 7.88 (m,
4H), 7.43 (m, 2H) (Ho PPh2), 7.07−7.36 (set of m, 12H; Hm + Hp
PPh2), 3.34 (m, 4H; CH2N), 2.65 (m, 1H), 2.36 (m, 3H) (CH2P),
2.00 (m, 1H; CHCN), 1.69 (m, 1H), 1.47 (m, 1H) (CH2)
(acrylonitrile). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 43.7
(AB system, 2JP−P = 385 Hz). 13C{1H} NMR (100 MHz, CD2Cl2, 298
K): δ 135.7 (dd, 2JC−P = 9 Hz, 4JC−P = 6 Hz), 133.9 (dd, 2JC−P = 8 Hz,
1
H, 5.70; N, 1.89. Found: C, 58.15; H, 5.55; N, 1.81. H NMR (300
MHz, toluene-d8, 253 K): δ 7.90 (m, 2H), 7.68 (m, 2H) (Ho PPh2),
6.93−7.09 (set of m, 16H; Ho + Hm + Hp PPh2), 3.22 (m, 2H), 3.05
(m, 2H) (CH2N), 2.38 (m, 2H), 2.29 (m, 2H) (CH2P), 2.18 (m, 2H;
CH coe), 1.38 (m, 12H; CH2 coe). 31P{1H} NMR (121 MHz,
2
toluene-d8, 298 K): δ 43.6 (AB system, JP−P = 390 Hz). 13C{1H}
2
4
4JC−P = 5 Hz), 132.8 (dd, JC−P = 6 Hz, JC−P = 4 Hz), 132.5 (dd,
NMR (75 MHz, toluene-d8, 253 K): δ 135.3 (d, 1JC−P = 20 Hz), 135.0
4
2JC−P = 7 Hz, JC−P = 5 Hz) (Co PPh2), 128.2−130.4 (set of signals,
(d, 1JC−P = 20 Hz), 132.4 (d, 1JC−P = 20 Hz), 132.0 (d, 1JC−P = 20 Hz)
Cm + Cp PPh2), 126.5 (t, 3JC−P = 3 Hz; CN), 61.7, 61.2 (m; CH2N),
2
2
(Cipso PPh2), 134.1 (d, JC−P = 5 Hz), 134.0 (d, JC−P = 5 Hz), 133.4
1
3
1
38.4 (dd, JC−P = 20 Hz; JC−P = 11 Hz), 36.1 (dd, JC−P = 20 Hz;
3JC−P = 11 Hz) (CH2P), 17.4 (m; CH2), −4.6 (t, 2JC−P = 1 Hz; 
CHCN). Mass Calcd for C31H31IrN2P2: 685.7560. MS (ESI+): m/z
685.8754.
2
2
2
(d, JC−P = 5 Hz), 133.3 (d, JC−P = 5 Hz), 133.1 (d, JC−P = 6 Hz),
133.0 (d, JC−P = 6 Hz) (Co PPh2), 130.0 (s), 129.9 (s), 129.4 (s),
2
129.2 (s) (Cp PPh2), 127.8−128.3 (set of m; Cm PPh2), 60.2 (m),
59.3 (m) (CH2N), 39.4 (m), 35.7 (m) (CH2P), 37.7 (CH coe),
25.3−30.1 (set of s, CH2 coe). Mass Calcd for C36H42IrNP2:
742.8903. MS (ESI+): m/z 744.2498 (100%; M+ + 1H).
Synthesis of [mer-(PNHP)Ir(coe)]BF4 (5).
Synthesis of [mer-(PNP)Ir(η2-O2)] (8).
A solution of 4 (0.10 g, 0.13 mmol) in toluene (5 mL) was stirred in
air. After 30 min, a green solid began crystallizing out of the solution,
which was isolated by filtration, washed with hexanes, and then dried
under vacuum. Yield: 39 mg (45%). Anal. Calcd for C28H28IrNO2P2:
To a solution of 4 (0.08 g, 0.1 mmol) in toluene (5 mL) was added
HBF4·Et2O (16 μL, 0.12 mmol) at 213 K, and the resulting mixture
was stirred for 10 min, affording an orange solution which was
allowed to reach room temperature. The solution was filtered via
cannula to remove some solid impurities, and then the solvent was
1
C, 50.60; H, 4.25; N, 2.11. Found: C, 50.45; H, 4.11; N, 2.01. H
NMR (300 MHz, C6D6, 298 K): δ 7.11−8.23 (set of m, 20H; PPh2),
2.89 (m, 4H; CH2N), 2.29 (m, 4H; CH2P). 31P{1H} NMR (121
MHz, C6D6, 298 K): δ 19.2 (s). 13C{1H} NMR (75 MHz, C6D6, 298
H
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3
K): δ 128.3−136.6 (set of m; PPh2), 66.2 (t, JH−P = 4 Hz; CH2N),
For preparative purposes, we performed the experiments as follows.
A medium Fisher−Porter pressure reactor was charged with the
corresponding substrate (2.15 mmol), the catalyst 4 (80 mg, 0.1
mmol) ,and toluene (3 mL), and then the mixture was cooled to −95
°C. The reactor was evacuated and then charged with anhydrous
ammonia. The resulting mixture was stirred for 20 h at 60 °C, where
the pressure of ammonia was 3 atm. When the mixture was cooled to
room temperature, the volatiles were removed by vacuum, and the
crude oils isolated were transferred to a short glass column packed
with silica gel and then eluted with CH2Cl2; upon evaporation of the
solvent heavy oils were isolated, which were further analyzed by GC-
3
32.7 (t, JH−P = 15 Hz; CH2P). Mass Calcd for C28H28IrNO2P2:
664.6920. MS (ESI+): m/z 666.1316 (100%; M+ + 1H).
Synthesis of [mer-(PNP)Ir(η2-PhCCC(O)OMe)] (9).
A solution of 4 (0.10 g, 0.13 mmol) in oxygen-free toluene (5 mL)
was treated with neat methyl phenylpropiolate (26 mg, 0.16 mmol)
via syringe, and the resulting orange solution was stirred at 60 °C for 1
h. This was filtered to eliminate some solid impurities, and the solvent
was removed by vacuum, leading to the isolation of an orange solid.
Yield: 79 mg (77%). 1H NMR (300 MHz, C6D6, 298 K): δ 6.77−8.14
(set of m, 25H; PPh2 + Ph), 3.55 (s, 3H; OMe), 3.41(m, 4H; CH2N),
2.48 (m, 2H) 2.33 (m, 2H) (CH2P). 31P{1H} NMR (121 MHz,
C6D6, 298 K): δ 32.5 (s). 13C{1H} NMR (75 MHz, C6D6, 298 K): δ
155.7 (s; CO), 134.7 (m; Co), 133.9 (d, 1JC−P = 46 Hz), 133.5 (d,
1JC−P = 53 Hz) (Cipso), 130.8 (s), 132.0 (m; Co), 130.2, 129.9, 129.0,
1
MS and 1H, 13C{1H}-APT, and H−13C HSQC NMR techniques
(see the Supporting Information).
Crystal Structure Determination of Complexes 2 and 5. X-
ray diffraction data were collected on a Smart APEX Bruker
diffractometer at 100(2) K, using a narrow oscillation frame (Δω =
0.3°) with graphite-monochromated Mo Kα radiation (λ = 0.71073
Å). Intensities were integrated and corrected for absorption correction
with SAINT39 and SADABS40 programs, integrated in the APEX2
package. The structures were solved by direct methods with the
SHELXS41 program and refined by full-matrix least-squares refine-
ments in F2 with SHELXL42 included in the WinGX43 package.
Special details about the refinement are listed below.
128.4, 127.3 (set of s; Co + Cp Ph), 106.9 (t, JC−P = 3 Hz; 
2
2
CC(O)OMe), 67.9 (t, JC−P = 2 Hz; ≡CPh), 63.4 (m; CH2N), 51.9
(s; OMe), 36.6 (m; CH2P). Mass Calcd for C38H36IrNO2P2:
Crystal data for 2: C36H41ClIrNP2·2CH2Cl2; Mr = 947.14; white
prism, 0.240 × 0.300 × 0.310 mm; triclinic, P1; a = 10.8841(5) Å, b =
̅
792.8625. MS (ESI+): m/z 793.2345.
Synthesis of [fac-(PNHP)IrH(CCH)2] (10).
11.4719(5) Å, c = 16.4525(8) Å; α = 86.6180(5)°, β = 76.3103(4)°, γ
= 72.2091(4)°; V = 1900.28(15) Å3; Z = 2; ρcalc = 1.655 g cm−3; μ =
3.978 cm−1; minimum and maximum transmission factors 0.2958 and
0.4523; 2θmax = 57.368°; 46713 reflections collected; 9200 unique
reflections (Rint = 0.0264); number of data/restraints/parameters
9200/1/588; final GOF = 1.040; R1 = 0.0172 (8974 reflections, I >
2σ(I)), wR2 = 0.0433 for all data; largest difference peak 1.758 e Å−3.
Hydrogen atoms of the solvent were included in the model in
calculated positions and refined with a riding model. The rest of the
hydrogen atoms were included in observed positions and refined with
a restraint in a C−H bond length.
A solution of 4 (80 mg, 0.10 mmol) in toluene (5 mL) was
transferred to a pressure Kontes tube, and then it was bubbled with
gaseous acetylene (1 bar) for 10 min. The pressure tube was sealed,
and the solution was stirred for 2 h at room temperature. A light
yellow solid crystallized out of the solution, and then it was collected
by filtration via cannula, washed with hexanes, and vacuum-dried.
Yield: 42 mg (58%). Anal. Calcd for C32H32IrNP2: C, 56.13; H, 4.71;
Crystal data for 5: C36H43BF4IrNP2·2CH2Cl2; Mr = 1000.51;
orange needle, 0.056 × 0.090 × 0.120 mm; monoclinic, P21/n; a =
10.6822(8) Å, b = 20.5768(15) Å, c = 17.9348(13) Å; β =
106.708(2)°; V = 3775.7(5) Å3; Z = 4; ρcalc = 1.760 g cm−3; μ = 3.955
cm−1; minimum and maximum transmission factors 0.5060 and
0.7786; 2θmax = 56.688°; 39503 reflections collected; 9238 unique
reflections (Rint = 0.0876); number of data/restraints/parameters
9238/0/383; final GOF = 1.191; R1 = 0.0816 (6719 reflections, I >
2σ(I)), wR2 = 0.2083 for all data; largest difference peak 3.444 e Å−3.
The coe fragment of the metal complex was found to be
disordered. Two sets of positions and complementary occupancy
factors (0.53/0.47(4)) were used in the refinement. Two fluorine
atoms of the counterion were also disordered. Solvent was also found
to be disordered, and its refinement led to unrealistic parameters.
Therefore, the contribution of the solvent to the calculated structure
factors was calculated with the SQUEEZE program.44
1
N, 2.05. Found: C, 56.01; H, 4.89; N, 2.14. H NMR (400 MHz,
CD2Cl2, 298 K): δ 8.16 (m, 4H), 8.02 (m, 4H) (Ho PPh2), 6.81−7.42
(set of m, 12H; Hm + Hp PPh2), 5.26 (br, 1H; NH), 3.47 (m, 1H;
CH2N), 3.17 (m, 1H), 3.10 (m, 1H) (CH2N), 2.72 (m, 2H; CH2N1 +
CH2P1), 2.58 (m, 1H; CH2P1), 2.46 (m, 1H; CH2P2), 2.28 (m, 1H),
(CH2P2), 1.86 (t, JH2−P1 = 2 Hz, 1H; H2), 1.84 (dd, JH1−P1 = 5 Hz,
4
4
4JH1−P2 = 3 Hz; H1), −10.15 (dd, JH−P1 = 21 Hz, JH−P2 = 161 Hz;
2
2
Ir−H). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 6.1 (d, JP−P
=
2
11 Hz), 0.4 (d, JP−P = 11 Hz). 13C{1H} NMR (100 MHz, CD2Cl2,
2
298 K): δ 137.2 (dd, 1JC−P = 54 Hz, 3JC−P = 5 Hz; Cipso PPh2), 134.2
(d, JC−P = 10 Hz), 133.9 (d, JC−P = 10 Hz) (Co PPh2), 133.5 (m;
2
2
C
ipso PPh2), 127.5−130.4 (set of m, Cm + Cp PPh2), 91.2 (dd, 2JC−P1
=
2 2
30 Hz, JC−P2 = 3 Hz; C1), 89.6 (m; C3), 86.6 (dd, JC−P1 = 116 Hz,
2JC−P2 = 13 Hz; C2), 85.7 (t, 3JC−P = 3 Hz; C4), 54.6 (d, 2JC−P = 7 Hz;
CCDC 1846013−1846014 contains the supplementary crystallo-
graphic data for this paper. The data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/structures.
2
1
CH2N), 52.9 (d, JC1−P = 7 Hz; CH2N), 30.5 (d, JC−P = 29 Hz;
CH2P1), 30.3 (d, JC−P = 32 Hz; CH2P2). Mass Calcd for
C32H32IrNP2: 684.7679. MS (ESI+): m/z 684.7534 (100%; M+
+
Computational Details. All DFT theoretical calculations were
performed using the Gaussian09 program package (Gaussian 09,
Revision D.01, see the Supporting Information for full citation). The
B3LYP method45 was employed, including the D3 dispersion
correction scheme developed by Grimme46 using the Becke−Johnson
damping47 for both energies and gradient calculations in conjunction
with the “ultrafine” grid. The def2-SVP basis set48 was selected for all
atoms for geometry optimizations, single-point calculations being
performed with the def2-TZVP basis set to refine energy results. The
nature of the stationary points was confirmed by analytical frequency
analysis, and transition states were characterized by calculation of
reaction paths following the intrinsic reaction coordinate. All reported
energies are Gibbs free energies referred to a 1m standard state at
1H).
Procedure for the Ir-Catalyzed Addition of Ammonia to
Activated Unsaturated Substrates. The NMR monitoring of the
catalytic reactions was carried out as follows. An oxygen-free J. Young
pressure NMR tube (Wilmad) was charged with the olefin (0.67
mmol), solid 4 (25 mg, 0.033 mmol), and C6D6 (0.4 mL). Then the
tube was frozen, subjected to three freeze and thaw cycles, and
subsequently anhydrous ammonia was introduced. At this point, the
molar ratio of the olefin versus ammonia was deduced by integrating
the corresponding resonances in the 1H NMR spectra of each sample.
The reaction tube was sealed and then placed in an oil bath at 60 °C
for 20 h. After the reaction mixture was cooled, the products formed
1
were analyzed by H NMR spectroscopy.
I
DOI: 10.1021/acs.organomet.8b00365
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
298.15 K, with the contribution to the translational entropy removed,
as indicated by Morokuma et al.49
The Metal−Ligand Bifunctional Catalysis: A Theoretical Study on the
Ruthenium(II)-Catalyzed Hydrogen Transfer between Alcohols and
Carbonyl Compounds. J. Am. Chem. Soc. 2000, 122, 1466−1478.
(5) (a) Desnoyer, A. N.; Love, J. A. Recent Advances in Well-
Defined, Late Transition Metal Complexes that Make and/or Break
C−N, C−O and C−S Bonds. Chem. Soc. Rev. 2017, 46, 197−238.
(b) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G.
Formation, Reactivity, and Properties of Nondative Late Transition
Metal− Oxygen and−Nitrogen Bonds. Acc. Chem. Res. 2002, 35, 44−
56.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00365.
■
S
Multinuclear NMR spectra of complexes 2−10 and
amines, hydrogen bonding in complex 2, disorder of the
coe ligand in 5+, DFT absolute and relative energies, and
geometrical representation of all DFT calculated
structures (PDF).
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Dialkylamides and Disilylamides. Acc. Chem. Res. 1976, 9, 273−280.
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Organometallics 2017, 36, 97−108. (b) Yan, K. K.; Ellern, A.; Sadow,
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Accession Codes
CCDC 1846013−1846014 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.A.C.: mcasado@unizar.es.
■
(8) (a) The Chemistry of Pincer Compounds; Morales-Morales, D.,
ORCID
́
Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007. (b) Valdes, H.;
Fernando J. Lahoz: 0000-0001-8054-2237
Víctor Polo: 0000-0001-5823-7965
Miguel A. Casado: 0000-0003-1707-3022
Gonzalez-Sebastian, L.; Morales-Morales, D. Aromatic para-Function-
alized NCN Pincer Compounds. J. Organomet. Chem. 2017, 845,
229−257. (c) van der Vlugt, J. I.; Reek, J. N. H. Neutral Tridentate
PNP Ligands and Their Hybrid Analogues: Versatile Non-Innocent
Scaffolds for Homogeneous Catalysis. Angew. Chem., Int. Ed. 2009, 48,
8832−8846. (d) Werkmeister, S.; Neumann, J.; Junge, K.; Beller, M.
Pincer-Type Complexes for Catalytic (De)Hydrogenation and
Transfer (De)Hydrogenation Reactions: Recent Progress. Chem. -
Eur. J. 2015, 21, 12226−12250.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors express their appreciation for the financial support
of MINECO/FEDER project CTQ2015-67366-P and DGA/
FSE (group E42_17R). V.P. thankfully acknowledges the
resources from the supercomputers “Memento” and “Termi-
nus” and the technical expertise and assistance provided by the
Institute for Biocomputation and Physics of Complex Systems
(BIFI)−Universidad de Zaragoza. P.G.-O. acknowledges the
CSIC and European Social Fund for a PTA contract.
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Ruthenium Pincer Complexes: Synthesis and Catalytic Applications.
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