Cationic Ir(III) alkylidenes are key intermediates in C-H bond activation and C-C bond-forming reactions

Article abstract of DOI:10.1021/ja301759m

This work describes the chemical reactivity of a cationic (η⁵-C₅Me₅)Ir(III) complex that contains a bis(aryl) phosphine ligand, whose metalation determines its unusual coordination in a κ⁴-P,C,C′,C″ fashion. The complex (1 ⁺ in this paper) undergoes very facile intramolecular C-H bond activation of all benzylic sites, in all likelihood through an Ir(V) hydride intermediate. But most importantly, it transforms into a hydride phosphepine species 4⁺ by means of an also facile, base-catalyzed, intramolecular dehydrogenative C-C coupling reaction. Mechanistic studies demonstrate the participation as a key intermediate of an electrophilic cationic Ir(III) alkylidene, which has been characterized by low-temperature NMR spectroscopy and by isolation of its trimethylphosphonium ylide. DFT calculations provide theoretical support for these results.

Full text of DOI:10.1021/ja301759m

Article
pubs.acs.org/JACS
Cationic Ir(III) Alkylidenes Are Key Intermediates in C−H Bond
Activation and C−C Bond-Forming Reactions
́
Jesus Campos, Joaquín Lopez−Serrano, Eleuterio Alvarez, and Ernesto Carmona*
́
́
Departamento de Química Inorgan
Investigaciones Científicas, Avda. Amer
́
ica, Instituto de Investigaciones Químicas (IIQ), Universidad de Sevilla, Consejo Superior de
ico Vespucio 49, 41092 Sevilla, Spain
́
S
* Supporting Information
ABSTRACT: This work describes the chemical reactivity of a
cationic (η5-C5Me5)Ir(III) complex that contains a bis(aryl)
phosphine ligand, whose metalation determines its unusual
coordination in a κ4-P,C,C′,C″ fashion. The complex (1+ in this
paper) undergoes very facile intramolecular C−H bond
activation of all benzylic sites, in all likelihood through an
Ir(V) hydride intermediate. But most importantly, it trans-
forms into a hydride phosphepine species 4+ by means of an also facile, base-catalyzed, intramolecular dehydrogenative C−C
coupling reaction. Mechanistic studies demonstrate the participation as a key intermediate of an electrophilic cationic Ir(III)
alkylidene, which has been characterized by low-temperature NMR spectroscopy and by isolation of its trimethylphosphonium
ylide. DFT calculations provide theoretical support for these results.
structure.11 For the sake of simplicity and to prevent
INTRODUCTION
■
undesirable β-H elimination reactions, the bis(aryl)phosphine
PMeXyl2 (Xyl = 2,6-Me2C6H3) and other similar phosphines
have been chosen for this study.
The efficient functionalization of the C−H bonds of alkanes
and other common organic molecules remains a difficult and
elusive research objective despite its important practical
implications in many industrial and laboratory syntheses.1
Transition metal compounds that are able to promote C−H
bond activation with subsequent C−C (or C−X) bond
formation (X = O, N, S or other element) hold promise for
converting simple and generally available raw materials into
complex, elaborated molecules such as pharmaceutical and
natural products. Indeed, today many selective methods that
combine C−H bond activation with C−C or C−X bond
formation are available.2,3
Following the pioneering work of Bergman and co-workers
on electrophilic Ir(III) complexes such as [(η5-C5Me5)Ir(Me)-
(PMe3)(ClCH2Cl)]+ and related species,4 many studies based
on neutral or cationic Rh and Ir complexes that are constructed
around Cp*M fragments have been performed, under both
stoichiometric and catalytic conditions.5−9 Somewhat surpris-
ingly, the analogous rhodium cation [(η5-C5Me5)Rh(Me)-
(PMe3)(ClCH2Cl)]+ is less reactive toward C−H bonds,8c and
similarly, the closely related iridium molecule that contains
P(OMe)3 instead of PMe3, i.e., [(η5-C5Me5)Ir(Me)(P(OMe)3)-
(ClCH2Cl)]+, also shows diminished reactivity, as a con-
sequence of lower electron density at the metal center.8a The
carbonyl derivative, [Cp*Ir(Me)(CO)(ClCH2Cl)]+, was like-
wise found to be unreactive toward the activation of the C−H
bonds of benzene.10
Transition metal cyclometalated complexes12,13 have re-
ceived a great deal of attention in recent decades due to their
applications in many catalytic syntheses,14 in bioorganometallic
chemistry, including the development of anticancer drugs,15
and in material science.16
Following the straightforward synthesis17 of the chloride
complex precursor 1-Cl, we describe herein the generation of
+
the cationic dichloromethane adduct 1-CH2Cl2 (Scheme 1),
along with the study of its chemical reactivity. At variance with
Bergman’s complex [(Cp*)Ir(Me)(PMe3)(ClCH2Cl)]+, the
+
metalated benzylic unit of 1-CH2Cl2 permits stabilization of
a solvent-free cation (1+ in Scheme 1), in which the metalated
phosphine behaves as a tridentate, five-electron donor ligand
(using the covalent formalism), by means of an unusual κ4-
P,C,C′,C″ coordination mode, that consists of phosphine and
pseudoallylic η3-benzylic binding motifs. Whereas no inter-
molecular C−H bond activation chemistry has been disclosed
for 1+, the complex undergoes a very facile κ4-to-κ2 rearrange-
ment of the metalated ligand, most probably through species A
of Scheme 1 that, according to DFT calculations, features a very
weak C−H···Ir interaction involving one of the Me groups of
the non-metalated phosphine xylyl substituents. This provides
the molecules of 1+ with a unique chemical reactivity that in a
base-catalyzed process allows access to a highly electrophilic
cationic Ir(III) alkylidene (6+ in Scheme 1), which undergoes
C−C coupling with formation of the hydride phosphepine
With the above results in mind, we have recently undertaken
a study of Cp*M compounds of the above kind in which the
steric and electronic properties of the metal center are modified
by the use of a phosphine ligand that is prone to
cyclometalation to yield a five-membered metallacyclic
Received: February 22, 2012
Published: April 5, 2012
© 2012 American Chemical Society
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Information (SI), the reaction proceeds as represented in
Scheme 2. The first species that forms is a dinuclear chloride-
bridged cation 2+ that results, most probably, from the
interaction of still unreacted 1-Cl with the already formed 1-
Scheme 1. Some of the Cationic Complexes Investigated in
This Work
a
+
CH2Cl2 . Related complexes that contain a halide ligand
bridging two metal units have been produced in similar halide
abstraction reactions. Heinekey et al. recently reported10 the
formation of the binuclear cation [{Cp*Ir(Me)(CO)}2(μ-
Cl)]BArF, as the product of the reaction of Cp*Ir(Me)(Cl)-
(CO) with [Li(OEt2)2.5]BArF. Unlike this system that does not
progress further to give the desired mononuclear dichloro-
methane adduct, [Cp*Ir(Me)(CO)(CH2Cl2)]+, even if 2.5
equiv of the abstracting lithium salt is utilized, complex 2+
+
converts cleanly into 1-CH2Cl2 upon reaction with additional
amounts of NaBArF. The 1-CH2Cl2+ cation is clearly analogous
to Bergman’s complex, [Cp*Ir(Me)(PMe3)(CH2Cl2)]+, that is
known to undergo very facile C−H activation reactivity at low
temperatures but decomposes at room temperature in the
absence of a trapping reagent.4 In contrast, 1-CH2Cl2+ evolves,
even at low temperatures, to form the stable cationic complex
1+, in which the coordinated molecule of CH2Cl2 has been
replaced by the metalated phosphine, thanks to a change in
coordination from κ2-P,C to κ4-P,C,C′,C″. The chelating nature
of the phosphine−benzyl ligand of 1+ and the accessibility of its
κ4-coordination mode are probably the reasons for its reduced
reactivity toward C−H bond activation of different substrates
(C6H6, tetrahydrofuran, Et2O, etc.).
Compound 1-BArF was obtained as a yellow microcrystalline
solid by crystallization from CH2Cl2:C5H12 solvent mixtures
and was characterized by microanalysis and variable-temper-
ature 1D and 2D NMR spectroscopy. Crystals suitable for X-
ray studies could not be obtained. However, at −60 °C, its 1H,
13C{1H}, and 31P{1H} NMR spectra (CD2Cl2 solutions)
resemble closely those of the X-ray-characterized rhodium
analogue.18 Thus, the Ir−CH2 protons appear as the AB
portion of an ABX spin system (X = 31P), with δ 2.98 (HA, t;
2JHH = 3JHP = 4.8 Hz) and 1.15 (HB, dd, 2JHH = 4.8, 3JHP = 15.6
Hz) ppm (see Experimental Section for additional data and
Figure 1 for the atom labeling scheme employed). The
a
Species A is a proposed, readily accessible intermediate (according to
DFT calculations).
complex 4+ (vide infra). Alkylidene 6+ has been fully
characterized by solution NMR studies and by isolation of its
PMe3 phosphonium ylide. The rhodium analogue of 1+ is a
stable molecule that features interesting catalytic properties.18
RESULTS AND DISCUSSION
■
1. Cationic Ir(III) Complexes Generated by Chloride
Abstraction from 1-Cl. Reaction of the neutral compound 1-
Cl with 1 equiv of NaBArF (BArF = [B(3,5-C6H3(CF3)2)4]), in
CH2Cl2 as the solvent, led to complex 1+ isolated as the BArF
salt (see Scheme 2). However, low-temperature 1H and
31P{1H} NMR monitoring of the reaction permitted observa-
tion of two intermediates, 2+ and 1-CH2Cl2 , which at room
temperature converted cleanly into 1-BArF. The interaction of
1-Cl with NaBArF is slow at −80 °C. In accordance with
observations that are discussed in detail in the Supporting
+
Scheme 2. Chloride Abstraction from 1-Cl by NaBArF,
Leading to 1+ Isolated as 1-BArF
Figure 1. Labeling scheme for species 1+, 4+, 5, and 6+ used for the
assignment of 1H and 13C NMR signals. The labeling criteria for 1+ are
also employed for the cationic derivatives 1-L+, 3+, 6+, and 7+.
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corresponding 13C resonance appears at δ 27.0 and exhibits
activation that could occur in a concerted manner, by a σ-
complex-assisted metathesis mechanism,19 or alternatively
through a cationic doubly metalated Ir(V) hydride intermediate
(C), resulting from oxidative cleavage of a C−H bond. This
species would give back 1+ by reductive coupling of the hydride
with either of the Ir−CH2 bonds, accompanied by κ4-
coordination of the metalated ligand fragment. In either case,
the roles of the two xylyl rings are exchanged, the metalated
becoming non-metalated and vice versa. Theoretical calculations
(see SI, Figure S10) support the latter reaction pathway and
reveal that access to the Ir(V) intermediate, which lies only 7.2
kcal mol−1 above 1+, needs to overcome a barrier of ca. ΔG⧧ =
14 kcal mol−1, in good agreement with the NMR measurements
described above.
2. Reactivity Studies of Complex 1-BArF. 2.1. Reac-
tions with Classical Lewis Bases. As expected, addition of a
Lewis base (NCMe, C5H5N, NH3, CO, and PMe3) to CH2Cl2
solutions of 1-BArF resulted in the instantaneous formation of
the corresponding adducts 1-L+, which were isolated in nearly
quantitative yields (eq 1). 31P{1H} NMR monitoring of the
1
negligible coupling to 31P and a one-bond, JCH coupling
constant of 163 Hz. A distinct, strongly shielded 31P{1H}
resonance for 1+ is recorded at −44.9 ppm. However, at room
1
temperature, all H NMR signals are broad with the exception
of those due to the C5Me5 and P−Me protons, which appear as
two doublets, with δ 1.63 (4JHP = 1.8 Hz) and 2.20 (2JHP = 12.8
1
Hz) ppm, respectively. The low-temperature (−60 °C) H
NMR resonances of the methyl groups of the phosphine xylyl
substituents detected at 2.50, 2.44, and 2.09 ppm and those of
the iridium-bonded methylene protons coalesce to a broad
signal centered at ca. 2.4 ppm. Resonances due to the aromatic
protons are also broad and can be found at 7.4 (2H) and 2.3
(4H) ppm. Clearly, these observations are indicative of a
dynamic behavior in solution of 1+ that implies exchange of its
metalated and non-metalated xylyl units, thereby including all
methylene and methyl protons of the phosphine ligand.
To clarify the mechanism of this exchange, a detailed NMR
analysis, complemented by computational studies, was carried
out. 2D EXSY NMR experiments demonstrated the absence of
magnetization transfer at −60 °C, whereas at −45 °C transfer
from the methylene protons (δ 1.14 and 2.98) to Meβ (δ 2.09;
see Figure S1) was seen. Weak magnetization effects between
Meγ (2.50 ppm) and Meβ were detected too, denoting rotation
of the non-metalated xylyl ring around the P−C bond.
Moreover, NOE cross peaks connected the two methylene
protons at this temperature. At −35 °C, EXSY spectroscopy
demonstrated involvement of all CH3 and CH2 protons in the
chemical exchange. Rate constants for the site exchange
between the CH2 and CH3 positions were estimated by full
relaxation matrix analysis of the intensities of the EXSY peaks.
A rate constant of 0.9 s−1 was measured at this temperature for
the exchange of Meβ and Hα. An Eyring study in the
temperature interval from −45 to −25 °C yielded values of
reactions revealed a marked shift of the resonance from the
characteristic −44.9 ppm value of 1+ to positive chemical shifts
for 1-L+, in the interval 1.0−11.4 ppm. The NMR data
collected for these compounds in the Experimental Section and
the SI are in agreement with the proposed formulation. The
solid-state structures of the BArF salts of 1-NCMe+ and 1-
−
+
PMe3 were determined by X-ray crystallography (Figures S4
the activation parameters ΔH⧧ = 19.2
0.8 kcal mol−1 and
and S5).
ΔS⧧ = 20 3 cal mol−1 K−1, with ΔG⧧298K = 13 2 kcal mol−1.
Evaluation of the other CH2/CH3 exchanges gave similar
results.
2.2. Reaction with H2. Exposure of a dichloromethane
solution of 1+ to 1 bar of H2 at 20 °C allowed the formation of
the cationic bis(hydride) complex 3+ in quantitative yield by 1H
NMR (Scheme 4). Formation of 3+ was found to be reversible,
and removal of H2 under vacuum regenerated cleanly the
Scheme 3 presents a plausible mechanism for this exchange.
The approach of one of the methyl C−H bonds of the non-
metalated xylyl group to the iridium center, to form
intermediate A, causes a change in the coordination of the
metalated benzylic unit from η3 to η1. This is followed by C−H
Scheme 4. Reversible Reaction of 1+ with H2 To Yield 3+,
and Possible Intermediates, B1 and B2, for the Dynamic
Behavior of 3+
Scheme 3. Proposed Mechanism To Account for the
Solution Dynamic Behavior of 1+
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starting compound 1+. A classical dihydride structure is
proposed for 3+, in agreement with the observation of two
different solvents, etc.), small amounts of a new hydride
1
complex that featured H and 31P{1H} NMR resonances at δ
1
2
deshielded H NMR resonances (500 MHz, CD2Cl2, −80 °C)
−13.03 (d, JHP = 25.5 Hz) and 8.9 ppm, respectively, were
at δ −12.79 (s, 1H) and −13.37 (d, JHP = 17.5 Hz, 1H) ppm,
observed. Small quantities of bis(hydride) 3+ were detected too.
It was soon realized that generation of these complexes was due
to the presence of adventitious water that performed a
2
with a spin−lattice relaxation time T1 = 261 ms (average value
for the two hydride resonances at the above temperature).
Generation of 3+ in the reaction of 1+ with H2 contrasts with
results reported for the similar reaction of the analogous
rhodium compound.18a In the latter case, an agostic hydride
like B1 in Scheme 4, detected only at low temperatures
(around −70 to −90 °C), was observed as the minor
constituent of an equilibrium dominated by the κ4-rhodium
cation. These differences are most likely dictated by the higher
propensity of Ir, in comparison with Rh, to attain the higher
oxidation state, M(V). Many Ir(V) hydrides have been reported
in the literature and their role as catalysts for different
transformations demonstrated.20−22
Bronsted−Lowry base catalytic role. Trace amounts of water
̈
acting as an acid catalyst have been reported recently by the
groups of Milstein and Brookhart to facilitate the hydro-
genation step of reactions of H2 with Ir complexes of pincer
ligands.24
Following the screening of different base reagents and
solvents, optimal experimental conditions for the formation of
4+ were determined to involve the reaction of 1-Cl with
NaBArF in dichloromethane, in the presence of catalytic
amounts of a saturated aqueous solution of NaHCO3 (ca. 30
mol %, 20 °C, 16 h). A buffer solution of piperidinium/
piperidine can also be used. The already discussed cationic
bis(hydride) complex 3+ was observed as a side product of this
rearrangement. In fact, when the above reaction was effected in
a sealed NMR tube, a 1:1 mixture of 4+ and 3+ was cleanly
generated (Scheme 5). In order to isolate 4-BArF in high yields
At −80 °C, 3-BArF is stable in solution toward loss of H2. In
addition to the two 1H NMR hydride signals, cation 3+ features
two doublets at 3.93 and 3.38 ppm (2JHH = 14.2 Hz), each with
a relative intensity corresponding to one hydrogen atom,
attributable to the diastereotopic Ir−CH2 protons. Upon
warming to −60 °C, the two Ir−H and the two Ir−CH2
resonances broaden and coalesce into the baseline. Exchange
between the two hydride hydrogen atoms can occur through an
Ir(η2-H2) species. However, for the additional exchange that
implicates also the Ir−CH2 hydrogen atoms, the situation is not
clear-cut, and two possible pathways involving intermediates B1
and B2 of Scheme 4 may be foreseen. Present data do not
permit to distinguish unambiguously between these two routes.
Accordingly, we defer a definite proposal on this matter until
the results of work being presently developed in our laboratory
on related complexes become available. At room temperature,
the corresponding four low-temperature resonances are
replaced by a broad singlet at −4.6 ppm that integrates for
four hydrogen atoms. This chemical shift matches closely the
average of the resonances due to the four individual protons
(ca. −4.7 ppm). Line-shape analysis of these resonances at
different temperatures, in conjunction with an Eyring study of
the observed rate constants, yields the following activation
Scheme 5. Base-Catalyzed Dehydrogenative C−C Coupling
of 1+ Leading to the Hydride Phosphepine Cation 4+
(ca. 90%, see Experimental Section), the above catalyzed
reaction was performed for a period of about 16 h, with
intermittent vacuum−argon cycles to remove the H2 generated
in the C−C coupling reaction. In accord with these
observations, strong base catalysts like NaOH also gave rise
to the neutral hydride 1-H as a result of fast deprotonation of
3+ by the base.
Compound 4-BArF was isolated as a crystalline solid with
low reactivity toward oxygen and water. Its Ir−H linkage
features an IR absorption at 2135 cm−1 and a 1H NMR
resonance at δ −13.03 (d, 2JHP = 28.5 Hz), whereas the alkene
part of the chelating ligand is characterized by 1H NMR
resonances at 4.97 (dd, 3JHH = 8.6 Hz, 2JHP = 2.8 Hz) and 4.51
parameters for the overall process: ΔH⧧ = 12.6
0.4 kcal
mol−1, ΔS⧧ = 6 2 cal mol−1 K−1, and ΔG⧧300K = 11 1 kcal
mol−1.
The Bronsted−Lowry acidity of the cationic Ir(V) bis-
̈
(hydride) complex 3+ finds ample literature precedent, not only
for electrophilic transition metal hydrides but also for related
dihydrogen complexes.23 Indeed, treatment of 3+ with NEt3 or
with 2,2,6,6-tetramethylpiperidine (TMPP) afforded the
known17b neutral hydride 1-H (eq 2).
3
(d, JHH = 8.6 Hz) ppm. Corresponding 13C{1H} signals are
found at δ 56.1 (d, 2JCP = 8 Hz) and 63.3 (d, 2JCP = 4 Hz) ppm
and exhibit one-bond 13C−1H couplings of 168 and 160 Hz,
respectively. Cation 4+ was also characterized by a single-crystal
−
X-ray study of its BArF salt, with the results represented in
Figure 2 (see SI for details). The structure clearly shows that
the new ligand is coordinated to iridium in a bidentate fashion
through the phosphorus atom and the CC bond, with an
Ir1−P1 bond distance of 2.268(3) Å and Ir1−C17 and Ir1−
C25 bond distances of 2.186(13) and 2.161(13) Å, respectively.
The coordinated carbon−carbon double bond has a length of
1.45(2) Å and forms an iridium-centered angle with the P1
atom of Ccentr.−Ir1−P1 = 77.06°. These bond parameters are
comparable to those found in the literature for somewhat
related complexes.13a,c,26c,g,h The two aromatic rings that are
part of the chelating phosphepine ligand feature an almost
perpendicular distribution, as they form a dihedral angle of
81.3°, while the plane that contains the coordinated atoms P1,
2.3. Base-Catalyzed Intramolecular Dehydrogenative C−C
Coupling in 1+ Leading to the Hydride Phosphepine Cation
4+. Although compound 1-BArF seems to be stable in solution
and in the solid state for long periods of time, in some of its
preparations, and also during unsuccessful attempts to grow
single crystals (which required several manipulations, use of
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from 5 promoted by HB+, could produce the cationic benzyl
benzylidene structure 6+ plus H2. Fast reaction between the
latter two molecules would lead back to C, in all likelihood in
an irreversible manner, thereby making the process unpro-
ductive until irreversible migratory insertion of the Ir−CH2
bond of 6+ onto the IrCH− unit becomes competitive with
hydrogenation. The hydrido phosphepine cation 4+ would thus
form, along with equimolar quantities of H2 that could combine
with 1+ (or A) to yield 3+, the other product of this
rearrangement. Employing a concentration of 5 mol % of
TMPP as the base catalyst, a half-life t1/2 = 47 min was
measured at 298 K for the overall conversion of 1+ into a
mixture of the final products, 3+ and 4+. As discussed later, the
bis(hydride) 3+ can be obtained by reaction of 6+ with an
excess of H2 under appropriate conditions.
Figure 2. X-ray molecular structure of 4+, with H atoms and BArF
anion omitted for clarity.
3.1. Synthesis and Characterization of Neutral, Doubly
Metalated Complex 5. Treatment of 1-Cl with an excess of
NaMeO (generated in situ from NaOH and MeOH) yielded
hydride 1-H as the main reaction product, accompanied by
minor amounts of the desired dimetallacycle 5 (ca. 9:1 ratio).
Since it is known that 1-H rapidly regenerates 1-Cl in the
presence of CHCl3, an efficient, high-yield synthesis of 5 was
developed by performing the above reaction in a 1:1 solvent
mixture of MeOH and CHCl3 (Scheme 7).
C17, and C25 forms a dihedral angle of ca. 23.7° with the Cp*
ring.
Hybrid phosphine−olefin ligands find ample use in
homogeneous catalysis. Transition metal compounds of
tridentate diphosphine−olefin ligands generated by intra-
molecular C−C coupling reactions analogous to that described
in this paper were reported previously by Bennet25 and later by
Baratta.13a,c In addition, different research groups have
employed chiral and non-chiral phosphine−olefin ligands for
different catalytic applications.26 Clearly, a mechanistic under-
standing of the reaction pathway leading to cation 4+ was
desirable. Accordingly, some experimental studies toward this
aim were performed and are described in the following
paragraphs.
Scheme 7. Synthesis of the Doubly Metalated Complex 5
3. Isolation of Key Intermediates along the Reaction
Pathway Leading to 4+. On the basis of the chemical
reactivity already discussed for complex 1+ and taking
additionally into consideration the molecular complexity of
the phosphepine ligand of 4+, the mechanistic hypothesis
presented in Scheme 6 can be suggested for the base-catalyzed
conversion of the former into the latter. As argued previously,
the low-energy intermediates A and C are responsible for the
facile exchange of the metalated and non-metalated xylyl groups
of 1+. Similarly to the cationic bis(hydride) 3+, species C,
resulting from oxidative cleavage of a C−H bond of A, should
be acidic. Hence, its deprotonation by the base catalyst B would
give the neutral dimetallacycle complex 5, together with HB+,
which in a subsequent step, that consists of hydride abstraction
Although in the solid state the two metalated xylyl groups of
5 are distinctly different (see below), in solution average
resonances for two equivalent rings are observed down to −80
°C, due to a dynamic process that exchanges the conformation
of the two rings. Thus, only one set of resonances (two
2
doublets at 3.19 and 2.79 ppm, with JHH = 14.6 Hz) is
observed for the Ir−CH2 protons and only one signal (2.35
ppm) for the Me groups of the xylyl substituents. In the
13C{1H} NMR spectrum, the Ir−CH2 groups give rise to a
Scheme 6. Proposed Mechanism for the Dehydrogenative C−C Bond Formation That Leads to Complex 4+
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1
doublet at 11.7 ppm (2JCP = 1.8 Hz). As a consequence of the
metalation of the second Xyl group, the 31P{1H} resonance of
5, with δ 34.5, appears deshielded by more than 20 ppm from
those of 1-Cl and 1-H.
Single crystals of 5 suitable for X-ray studies were collected
by crystallization from its concentrated solutions in pentane at
−20 °C. As shown in Figure 3, the phosphepine ligand of this
the 13C{1H} NMR spectrum at δ 10.8 (d, JCP = 39 Hz) and
258.3 ppm.
Only a few species of this type have been reported for
iridium. They are rarely isolable or even observable,27 in
particular those of Ir(III). Indeed, upon warming to room
temperature a freshly prepared solution of 6+ in CD2Cl2 at −60
°C, the hydride phosphepine 4+ was formed in essentially
quantitative yield (by NMR) in less than 1 h (Scheme 9). The
Scheme 9. C−C Bond Coupling from Alkylidene 6+
rate of formation of 4+ according to Scheme 9 was measured by
31P{1H} NMR spectroscopy at 0 °C, using CD2Cl2 solutions of
4+ generated in situ at −60 °C from 5 and [Ph3C]+[PF6]−. A
first-order dependence on the concentration of 4+ was disclosed
(Figure S9), with a calculated rate constant of 2.2 × 10−4 s−1
and an associated energy barrier, ΔG⧧ = 20.5 kcal mol−1. The
alkyl/alkylidene C−C coupling reaction that leads to 4+ may be
viewed as a model for the key step of several catalytic processes,
including C1 polymerization of diazo compounds.28
The hydride abstraction reaction that converts compound 5
into the metallacyclic alkylidene 6+ can actually be reversed. As
depicted in Scheme 10, treatment of CD2Cl2 solutions of 6+
with LiAlH4 at −40 °C produces cleanly the neutral
dimetallacycle 5.
Figure 3. X-ray molecular structure of 5, with H atoms omitted for
clarity.
complex is acting as tridentate and is coordinated to iridium
through the phosphorus atom P1 and the metalated carbon
atoms C17 and C25, one from each of the original xylyl groups,
making the overall geometry a distorted octahedron. In the
solid-state structure these rings have a mutual spatial
distribution that approaches perpendicularity (dihedral angle
76.2°), and since one of them is almost perpendicular to the
C5Me5 ring (80.4°), the other is nearly parallel (dihedral angle
17.8°). The two Ir−C bond distances are almost identical
(2.093(7) and 2.123(8) Å), and the Ir−P bond has a normal
length of 2.205(2) Å. The double metalation undergone by the
ligand to become tridentate does not appear to introduce much
coordination strain, as the iridium-centered angles C17−Ir1−
C25, C17−Ir1−P1, and C25−Ir1−P1 have values of 86.3(3),
83.1(2), and 75.8(2)°, respectively, close to the ideal 90° value
corresponding to octahedral coordination.
Scheme 10. Hydride Addition to Alkylidene Complex 6+
Additional support for the structure proposed for cation 6+
was obtained with the generation of the phosphonium ylide 7+
that resulted when solutions of PMe3 were added to 6+ at −60
°C. A dramatic shift of the methylidene proton from 15.91 ppm
3.2. Synthesis and Characterization of the Cationic
Alkylidene 6+. The cationic intermediate 6+ responsible for
the C−C coupling reaction was successfully generated (Scheme
8) by the low-temperature (−60 °C) reaction of 5 and
2
in 6+ to 3.72 ppm (d, JHP = 21 Hz) in 7+ was observed,
Scheme 8. Synthesis of Iridium Alkylidene 6+ and Its
Phosphonium Ylide 7+
accompanied by the appearance of a 13C{1H} doublet at 2.5
ppm (1JCP = 26 Hz). Two doublets were detected in the
31P{1H} NMR spectrum at 33.3 (Ir−P) and 26.8 ppm (PMe3,
3JPP = 3 Hz).
3.3. Some Reactivity Studies of 5 and 6+. To gain
additional information on key steps of Scheme 6 that involves
species 5 and 6+, some additional experiments were performed,
involving hydrogenation of the latter and protonation of the
former.
As discussed above, hydride abstraction from dimetallacycle
5 by HB+ yields equimolar amounts of 6+ and H2. The highly
electrophilic character of the benzylidene terminus of 6+ is
evidently responsible for the migratory insertion reaction that
produces ultimately the hydride phosphepine cation 4+.
Nonetheless, interaction of 6+ with H2 to afford 1+ (through
intermediates C and A), and subsequently 3+ if sufficient H2 is
available, is also expected to be a facile process.
[Ph3C][BF4] in CH2Cl2 solution, as demonstrated by NMR
spectroscopy. Hydride abstraction was instantaneous at this
temperature. In agreement with the proposed benzyl (Ir−
CH2−)/benzylidene (IrCH−) formulation, widely separated
resonances were recorded for the Ir−CH2 (two doublets with δ
2
3.70 and 2.70 ppm and JHH = 14.2 Hz) and IrCH protons
(singlet at δ 15.91 ppm). Corresponding signals were found in
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In agreement with this assumption, when a solution of 6+
(generated by the procedure of Scheme 8) was treated with a
large excess of H2 (1.5 bar) and the mixture allowed to reach
slowly room temperature with intense stirring, clean
quantitative transformation into 3+ was achieved (Scheme
11). Hence, under these conditions, alkylidene hydrogenation is
Scheme 12. Protonation Reactions of 5 (See Text for
Details)
Scheme 11. Reaction of Alkylidene 6+ with an Excess of H2
−40 °C the reaction of 6+ with the H2 liberated in the
conversion of 5 into 6+ is slow, these results may be taken as
indicative that protonation of 5 occurs in both the 5 → C and
the 5 → 6+ directions in Scheme 6, with the former being
seemingly kinetically favored over the latter.
much faster than benzyl−benzylidene coupling and subsequent
β-H elimination. Not unexpectedly, an analogous NMR tube
reaction afforded slightly different results, most probably due to
slow diffusion of H2. At −80 °C, ca. 5% of 3+ was observed, but
its concentration remained unchanged until −40 °C, suggesting
that inaccurate temperature control during the mixing of
reagents permitted its formation. At −40 °C, both hydro-
genation and C−C coupling are slow processes, as ca. 67% of
the original alkylidene remained unreacted after 40 min. After
this time the mixture of 1+ and 3+ accounted for ca. 29% of the
product mixture and was complemented by 4% of hydrido
phosphepine 4+. The NMR tube was then allowed to reach
room temperature for a period of about 15 min, with frequent
shaking to facilitate mixing of the reagents. Following this
operation, a 1:4 mixture of 4+ and 3+ was ascertained by NMR.
Clearly, the low solution concentration of H2 retarded
hydrogenation and allowed partial formation of the C−C
coupling product.
With regard to protonation reactions, 1 equiv of [H-
(OEt2)2][BArF] was added to a solution of 5 cooled at −80 °C.
Under these conditions only minor amounts of 1+ were
detected by NMR spectroscopy (ca. 7%, but varying between 3
and 8% from one experiment to another; once again we believe
that this is due to a local temperature increase during the
reagent mixing and solution process). Upon warming at −40
°C, the relative concentration of 1+ increased to ca. 15% and
alkylidene 6+ also became detectable (ca. 15%), along with an
unknown species (ca. 7%) characterized by a hydride signal at
−17.4 ppm (2JHP = 34.5 Hz; corresponding 31P{1H} resonance
at 7 ppm). Even if the exact nature of this hydride remains
undisclosed, it is tempting to formulate it as the cationic Ir(V)
hydride C of Scheme 6. Regardless of this particular detail,
upon further warming to room temperature all these species
converted cleanly into a ca. 2.5:1:3 mixture of complexes 1+, 3+,
and 4+ (Scheme 12A). A similar experiment employing 3 equiv
of [H(OEt2)2][BArF] gave analogous results. At −60 °C the
protonation of 5 is slow (95% of 5 and 5% of the κ4 complex
1+), but upon warming to −40 °C conversion into 1+, 3+, and
alkylidene 6+ was readily observed. After 40 min at this
temperature, the neutral dimetalated compound 5 reacted
completely with the acid to furnish the κ4 complex 1+ (65%),
the bis(hydrido) 3+ (15%), and the cationic alkylidene 6+
(20%). Further warming to room temperature permitted
conversion of alkylidene 6+ into the hydridophosphepine 4+,
and a final 1+:3+:4+ product ratio of ca. 3:1:1 was identified. If
one keeps in mind that under these acid conditions alkylidene
6+ is the only source of hydridophosphepine 4+, and that at
As a further test for this hypothesis, one last acid-promoted
hydride abstraction reaction from 5 was effected utilizing 1
equiv of [HTMPP][BArF] (HTMPP = 2,2,6,6-tetramethylpi-
peridinium). In terms of reactivity, this is similar to the base-
catalyzed conversion of 1+ into a 1:1 mixture of 3+ and 4+.
However, in this case 1 equiv of the base, rather than a catalytic
amount, would be produced. Hence, a 1:1 mixture of the
hydride−phosphepine compound 4+ and of the neutral hydride
1-H should be expected (the latter resulting from deprotona-
tion of 3+ according to Scheme 5), which is in excellent
agreement with the experimental results shown in Scheme 12B.
4. Computational Studies on the Formation of 4+.
DFT calculations at the M06/6-31G(d,p) and SDD level were
carried out to gain insight into the mechanism for the
formation of the hydride phosphepine 4+. The calculations
were carried out using the real system in the gas phase and
corrected for bulk solvent effects. Two mechanistic pathways
were considered to account for the formation of the hydride
phosphepine 4+. In the first route (Scheme 13A), α-elimination
Scheme 13. Initial Steps in the Two Mechanisms Considered
for the C−C Bond Formation Reaction Leading to Hydride
Phosphepine 4+
in A yields the carbene hydride intermediate B1, and then C−C
coupling followed by dihydrogen elimination gives the hydride
phosphepine. The second route (Scheme 12B) requires
elimination of two hydrogen atoms from the Ir(V) intermediate
C prior to C−C coupling.
Both routes start from the cationic species 1+. Calculations
support a κ4-P,C,C′,C″ structure for 1+ in equilibrium with a κ2-
P,C structure (A). This result is similar to that previously
reported for the analogous Rh system.18a However, while the
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Rh analogue of A features an agostic interaction with a CH3 of
the non-metalated xylyl fragment (dRh−H = 2.07 Å), in this case
the shortest Ir−CH3 distance is 2.90 Å. Interestingly, the
calculations predict that the formation of A in dichloromethane
is endothermic by just 0.9 kcal mol−1, with an energy barrier of
6.4 kcal mol−1, but vibrational analysis yields a free energy
variation of −4.0 kcal mol−1 (i.e., the step is predicted to be
exergonic). This result, which contrasts with the experimental
data, suggests overestimation of the solvation effects (ΔE = 4.3
kcal mol−1 in gas phase) or the entropic effects. As already
discussed, intermediate A is instrumental in the dynamic
behavior of 1+ in solution, and it also plays a key role in the
formation of the hydride phosphepine 4+.
Route A in Scheme 13 begins with α-elimination from A to
yield an 18-electron hydride carbene (B1). This step is
endothermic by 10.9 kcal mol−1 and needs to overcome a
moderate potential energy barrier of 16.8 kcal mol−1. Reversible
α-elimination is well documented for related metal systems,29
and formation of B1 may compete with formation of C
according to calculations (Figure 4). Even though formation of
that described for 1+. Conformational changes in F yield two
intermediates G and H, which are connected through low
energy barriers. In the first of these conformers, G, the
secondary interaction with the metalated xylyl is lost. However,
this results only in a small destabilization of G relative to F by
3.1 kcal mol−1 when the calculations are carried out in the gas
phase. When the solvent effects are taken into account, G is
stabilized by 1.2 kcal mol−1 relative to F. The second
conformer, H, is marginally more stable than F by 0.5 kcal
mol−1, likely due to the existence of an agostic interaction
between the metal and one of the C−H bonds of the β-CH2
carbon. β-Elimination from H is nearly barrier-less and yields
the final product in an exoergic step with an energy return of
11.8 kcal mol−1. The overall reaction from 6+ and 1+ is exoergic
by 12.9 and 2.2 kcal mol−1, respectively. These results are
outlined in Figure 5.
Figure 5. Free energy profile in dichloromethane (in kcal mol−1) for
the formation of the hydride phosphepine 4+ from the alkyl alkylidene
derivative 6+ (note 1+ is the origin of free energies). The inset shows
the calculated geometry for TS6→F, indicating the forming C−C bond
with a dotted line.
Figure 4. Potential energy profiles in dichloromethane (in kcal mol−1)
for the formation of the Ir(V) hydride C (blue solid line) and hydride
carbene B1 (red dashed line).
the hydride phosphepine 4+ from the C−C coupling product E
(see Figure S11) is facile, this route must be discarded since the
energy barrier for the preceding C−C coupling step (i.e., from
B1 to E) exceeds 55 kcal mol−1. Further details of this pathway
are given in the SI.
SUMMARY
■
The Ir(III) cation 1+ that contains a monometalated xylyl
phosphine coordinated in a κ4-P,C,C′,C″ fashion through the
phosphorus and three benzylic carbon atoms experiences a
base-catalyzed C−C dehydrogenative coupling that results in
equimolar mixtures of hydride phosphepine complex 4+ and the
cationic bis(hydride) Ir(V) derivative 3+. The latter results from
a fast reaction of the H2 liberated in the C−C coupling leading
to 4+ with unconverted 1+ (or A in Scheme 6), whereas 4+
derives from a cationic alkyl−alkylidene intermediate 6+, which
experiences migration of the iridium−alkyl onto the iridium−
alkylidene, accompanied by β-H elimination.
The second route considered is based on the mechanism
which describes the dynamic behavior of 1+ in solution (from
this point the data discussed correspond to free energy in
dichloromethane relative to 1+ unless stated otherwise). The
first elemental step is CH3 oxidative addition in A to yield the
Ir(V) intermediate C. Next, according to experimental data,
elimination of two hydrogen atoms from C gives species 6+.
The calculations indicate that 6+ + H2 is less stable than the
starting material by 10.7 kcal mol−1, while C is predicted to lie
8.1 kcal mol−1 higher in energy than 1+. In this mechanism, C−
C coupling from 6+ remains the rate-limiting step, but the
barrier to form the hydride phosphepine 4+ from 6+ is
moderate (17.2 kcal mol−1; 27.9 kcal mol−1 when calculated
from 1+). This value is only ca. 3 kcal mol−1 below that
calculated from kinetic experiments (20.5 kcal mol−1, see
above), which is an acceptable difference. Interestingly, the
resulting species (F), which is formally an unsaturated 16-
electron complex, features a κ4-P,C,C′,C″ structure, showing a
secondary interaction with the metalated xylyl ring analogous to
EXPERIMENTAL SECTION
General. All operations were performed under an argon
atmosphere using standard Schlenk techniques, employing dry
solvents and glassware. Microanalyses were performed by the
■
́
Microanalytical Service of the Instituto de Investigaciones Quimicas
(Sevilla, Spain). Infrared spectra were recorded on a Bruker Vector 22
spectrometer. The NMR instruments were Bruker DRX-500, DRX-
400, and DRX-300 spectrometers. Spectra were referenced to external
SiMe4 (δ 0 ppm) using the residual proton solvent peaks (1H NMR
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experiments) or the characteristic resonances of the solvent nuclei
(13C NMR experiments) as internal standards, while 31P NMR was
referenced to external H3PO4. Spectral assignments were made by
routine one- and two-dimensional NMR experiments where
appropriate. The crystal structures were determined in a Bruker-
Nonius, X8Kappa diffractometer. Dimer [(η5-C5Me5)IrCl2]2,30
NaBArF,31 and [H(OEt2)2][BArF]32 were prepared according to
1-NCMe+: Anal. Calcd for C61H50BF24IrNP: C, 49.27; H, 3.39; N,
0.94. Found: C, 49.4; H, 3.6; N, 1.4. IR (Nujol): ν(CN) 2285 cm−1.
1H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.44 (d, 1 H, Ha), 7.26, (m, 3
4
H each, Hb, Hd/f, He), 6.98 (m, 1 H, Hd/f), 6.94 (dd, 1 H, JHP = 3.5
2
Hz, Hc) 3.52 (d, 1 H, JHH = 15.0 Hz, IrCHH), 3.32 (dd, 1 H, 2JHH
=
15.0, 2JHP = 3.5 Hz, IrCHH), 2.65, 1.44 (s, 3 H each, Meβ, Meγ), 2.55
(s, 3H, NCMe), 2.26 (d, 3 H, 2JHP = 9.9 Hz, PMe), 2.03 (s, 3H, Mea),
1
1.55 (d, 15 H, JHP = 1.8 Hz, C5Me5). 13C{1H} NMR (100 MHz,
4
literature procedures. In the H NMR spectra all aromatic couplings
CD2Cl2, 25 °C): δ 155.9 (d, 2JCP = 30 Hz, C1), 142.0, 140.9 (d, 2JCP
=
were of ca. 7.5 Hz, and signals corresponding to the BArF counterion
were invariant for different complexes (CD2Cl2; δ 7.74 (s, 8H, Ho),
7.57 (s, 4H, Hp)).
1
8 Hz, C4, C6), 140.8 (C3), 139.4 (d, JCP = 63 Hz, C2), 131.0, 130.3
(CHb, CHe), 130.9, 130.5 (d, 3JCP = 9 Hz, CHd, CHf), 127.4 (d, 1JCP
=
51 Hz, C5), 128.6 (d, 3JCP = 7 Hz, CHc), 126.8 (d, 3JCP = 15 Hz, CHa),
Computational Details. DFT calculations were performed with
the Gaussian 09 package33 with Truhlar’s hybrid meta-GGA functional
M06.34 The Ir atom was represented by the Stuttgart/Dresden
effective core potential and the associated basis set35 as implemented
in Gaussian 09 (SDDALL). The remaining H, C, P, and Si atoms were
represented by means of the 6-31G(d,p) basis set.36−38 The
geometries for all species described were optimized in the gas phase
without symmetry restrictions. Frequency calculations were performed
on the optimized structures at the same level of theory to characterize
the stationary points, as well as for the calculation of gas-phase
enthalpies (H), entropies (S), and Gibbs energies (G) at 298.15 K.
The nature of the intermediates connected was determined by intrinsic
reaction coordinate calculations or by perturbing the transition states
along the TS coordinate and optimizing to a minimum. The solvent
effects (dichloromethane ε = 8.93) were modeled with the SMD
continuum model39 by single-point calculations on gas-phase-
optimized geometries (test free optimizations in solution of some
species gave analogous results). The relative free energies in solution
were calculated according to ΔGsolution = ΔEsolution + (ΔGgas − ΔEgas),
where ΔEsolution is the electronic energy plus the solvent entropy.40
Synthesis of Compound 1+. To a solid mixture of 1-Cl (100 mg,
0.16 mmol) and NaBArF (143 mg, 0.16 mmol) was added 5 mL of
CH2Cl2 under argon. The reaction mixture was stirred for 10 min at
room temperature, after which the solution turned from orange to
yellow. The mixture was filtered and the solvent evaporated under
reduced pressure to obtain a yellow solid (220 mg, 95%). For further
purification, the complex can be crystallized from a 1:2 mixture of
CH2Cl2:pentane. Anal. Calcd for C59H47BF24IrP: C, 49.01; H, 3.28.
2
3
118.6 (NCMe), 95.0 (d, JCP = 2 Hz, C5Me5), 25.6, 23.2 (d, JCP = 6
3
3
Hz, JCP = 8 Hz, respectively, Meβ, Meγ), 20.5 (d, JCP = 3 Hz, Meα),
19.4 (d, 1JCP = 39 Hz, PMe), 17.3 (IrCH2), 8.2 (C5Me5), 4.4 (NCMe).
31P{1H} NMR (160 MHz, CD2Cl2, 25 °C): δ 8.9.
The minor diastereomer of compound Ir-NCMe+ is characterized
1
by H NMR multiplets with δ 3.55 and 3.29 ppm, due to the IrCH2
protons, which are partly overlapped with the signals corresponding to
the IrCH2 group of the major diastereomer. A doublet at 1.76 ppm is
associated with the C5Me5 ligand. In the 31P{1H} NMR spectrum a
singlet is recorded with δ 5.8 ppm. Analytical and spectroscopic data
for the other Lewis base adducts are reported in the SI.
Synthesis of Bis(Hydride) 3+. To a solid mixture of 1-Cl (100
mg, 0.16 mmol) and NaBArF (143 mg, 0.16 mmol) placed in a thick-
wall vessel was added 3 mL of CH2Cl2. The reaction mixture was
stirred for 30 min at room temperature under 1 bar of H2, after which
the original solution with intense yellow color became almost
colorless. Addition of pentane under hydrogen atmosphere caused
precipitation of complex 3+ as a fine, pale yellow powder in
quantitative yield. Workup of the complex in the absence of hydrogen
led to release of H2 and formation of 1+. 1H NMR (500 MHz, CD2Cl2,
25 °C, H2 atmosphere): δ 7.39 (d, 1 H, Ha), 7.33−7.25 (m, 3 H, Hb,
Hd/f, He), 7.05 (dd, 1 H, 4JHP = 3.8 Hz, Hd/f), 6.98 (dd, 1 H, 4JHP = 4.7
2
Hz, Hc), 2.59, 1.50 (s, 3 H each, Meβ, Meγ), 2.55 (d, 3H, JHP = 10.5
4
Hz, PMe), 2.00 (s, 3 H, Meα), 1.79 (d, 15 H, JHP = 1.1 Hz, C5Me5),
1
−4.63 (br. s, 4 H, IrCH2, Ir−H2). H NMR (400 MHz, CD2Cl2, −80
°C, H2 atmosphere): δ 7.31 (d, 1 H, Ha), 7.22−7.10 (m, 3 H, Hb, Hd/f
,
2
He), 6.88 (m, 2 H, Hc, Hd/f), 3.93 (d, 1 H, JHH = 14.2 Hz, IrCHH),
3.38 (d, 1 H, 2JHH = 14.2, IrCHH), 2.45 (m, 6 H, Meβ/γ, PMe), 1.92 (s,
3 H, Meα), 1.60 (s, 15 H, C5Me5), 1.31 (s, Meβ/γ), −12.79 (s, 1 H,
1
Found: C, 49.1; H, 3.7. H NMR (500 MHz, CD2Cl2, −60 °C): δ
7.36 (m, 1 H, Hc), 7.28 (t, 1 H, Hb), 7.24 (t, 1 H, He), 7.04, 6.94 (m, 1
H each, Hd, Hf), 6.73 (dd, 1 H, 4JHP = 3.7 Hz, Ha), 2.98 (t, 1 H, 2JHH
=
2
IrHH), −13.37 (d, 1 H, JHP = 17.5 Hz, IrHH). 13C {1H} NMR (125
3JHP = 4.8 Hz, IrCHα), 2.50 (s, 3 H, Meγ), 2.44 (s, 3 H, Meα), 2.20 (d,
3 H, 2JHP = 12.8 Hz, PMe), 2.09 (s, 3 H, Meβ),1.63 (s, 15 H, C5Me5),
MHz, CD2Cl2, 25 °C, H2 atmosphere): δ 152.4 (d, 2JCP = 29 Hz, C1),
141.6, 140.4 (d, 2JCP = 9 Hz, 8 Hz, C4, C6), 140.1 (d, 2JCP = 3 Hz, C3),
136.4 (d, 1JCP = 62 Hz, C2), 132.4, 132.1 (CHb, CHe), 131.3, 130.8 (d,
2
3
1
1.15 (dd, 1 H, JHH = 4.8, JHP = 15.6 Hz, IrCHβ). H NMR (300
MHz, CD2Cl2, 25 °C): δ 7.36, (m, 2 H, Ha/b/c/d/e/f), 7.12 (br. s, 4 H,
Ha/b/c/d/e/f), 2.44 (br. s, 11 H, Meα, Meβ, Meγ, IrCH2), 2.28 (d, 3 H,
2JHP = 12.8 Hz, PMe), 1.63 (d, 15 H, 4JHP = 1.8 Hz, C5Me5). 13C{1H}
NMR (125 MHz, CD2Cl2, −60 °C): δ 142.2, 141.4 (s, d, 2JCP = 16 Hz,
C4, C6), 136.7 (C3), 132.7 (CHb), 132.1 (CHe), 131.2 (d, 3JCP = 4 Hz,
CHc), 129.2, 128.7 (d, 3JCP = 10 Hz, CHd, CHf), 126.8 (d, 3JCP = 6 Hz,
3JCP = 9 Hz, CHd, CHf), 129.9 (d, 3JCP = 8 Hz, CHc), 126.4 (d, 3JCP
=
1
17 Hz, CHa), 124.9 (br. s, C5), 104.2 (C5Me5), 29.6 (br. d, JCP = 57
3
Hz, PMe), 24.9, 23.4 (d, JCP = 5 Hz, 8 Hz, Meβ, Meγ), 20.3 (Meα),
11.7 (br. s, IrCH2), 8.8 (C5Me5). 31P{1H} NMR (200 MHz, CD2Cl2,
25 °C, H2 atmosphere): δ 3.7 (br. s).
Synthesis of Hydride−Phosphepine 4+. A solid mixture of 1-Cl
(100 mg, 0.16 mmol) and NaBArF (145 mg, 0.16 mmol) placed in a
Schlenk flask was suspended in CH2Cl2 (5 mL) under argon. A
saturated aqueous solution of NaHCO3 (5 μL) was added to the
reaction mixture, which was stirred for 16 h with intermittent
vacuum−argon cycles to pump out the produced molecular hydrogen.
The yellow solution was filtered and the solvent evaporated under
reduced pressure to obtain a pale yellow solid, which was washed with
pentane to yield compound 4+ (210 mg, 90%). For further purification
4+ can be recrystallized from a 1:1 mixture of CH2Cl2:pentane. Anal.
Calcd for C59H45BF24IrP: C, 49.08; H, 3.14. Found: C, 48.7; H, 3.1. IR
(Nujol): ν(Ir−H) 2135 cm−1. EM (ES) m/z calcd for M+: 581.19.
1
2
CHa), 116.8 (d, JCP = 65 Hz, C5), 95.6 (d, JCP = 15 Hz, C1), 92.7
1
(C5Me5), 67.5 (d, JCP = 28 Hz, C2), 27.0 (IrCH2), 22.7 (Meα), 22.1,
21.9 (d, s, 3JCP = 16 Hz, 7 Hz, Meβ, Meγ), 12.8 (d, 1JCP = 41 Hz, PMe),
8.5 (C5Me5). 31P{1H} NMR (200 MHz, CD2Cl2, −60 °C): δ −44.9.
Synthesis of Cationic Adducts 1-L+ (L = NCMe, C5H5N, NH3,
CO, PMe3). To a solid mixture of 1-Cl (50 mg, 0.08 mmol) and
NaBArF (72 mg, 0.08 mmol) placed in a Schlenk flask was added 5 mL
of CH2Cl2. The reaction mixture was stirred for 5 min at room
temperature under 1.5 bar of CO or NH3 (a thick-wall vessel instead
of a conventional Schlenk flask was employed in these cases), or
alternatively treated with 100 μL of NCMe or pyridine (for
corresponding NCMe or pyridine adducts, respectively) or with a
toluene solution of PMe3 (0.4 mL, 1 M) (for corresponding cationic
PMe3 complex). The solution was filtered, and the solvent was then
evaporated under reduced pressure to obtain pale white (Ir-NCMe+,
1
Expt.: 581.2. H NMR (500 MHz, CD2Cl2, 25 °C): δ 7.34 (m, 2 H,
Ha, Hd), 7.16 (m, 2 H, Hb, He), 6.95, 6.92 (dd, 1H, 4JHP = 3.9 Hz Hc,
Hf), 4.97 (dd, 1 H, 3JHH = 8.6, 4JHP = 2.8 Hz, CHα), 4.51 (d, 1 H, 3JHH
= 8.6 Hz, CHβ), 2.58 (d, 3 H, 2JHP4 = 13.2 Hz, PMe), 2.52, 2.50 (s, 3 H
each, Meα, Meβ), 1.97 (d, 15 H, JHP = 1.0 Hz, C5Me5), −13.03 (d, 1
H, 2JHP = 28.5 Hz, IrH). 13C{1H} NMR (100 MHz, CD2Cl2, 25 °C): δ
148.2 (d, 2JCP = 17 Hz, C4), 143.6 (d, 2JCP = 23 Hz, C1), 139.2, 138.8,
137.1, 136.0 (C2, C3, C5, C6), 131.3, 131.2, 131.1, 130.5 (CHb, CHc,
+
+
+
Ir-PMe3 , Ir-CO+) or yellow powders (Ir-NC5H5 , Ir-NH3 ) in ca.
+
95% yield. Complexes Ir-NCMe+ and Ir-NH3 were obtained as
mixtures of diastereomers (88:12, L = NCMe; 94:6, L = NH3). These
complexes can be recrystallized from a 1:2 mixture of CH2Cl2:pentane.
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3
CHe, CHf), 126.6, 124.8 (d, JCP = 15, 10 Hz, CHa, CHd), 100.2 (s,
ASSOCIATED CONTENT
* Supporting Information
Experimental details for the reaction of 1-Cl with NaBArF;
analytical and spectroscopic data for adducts 1-L+; dynamic
NMR studies; T1 measurements; X-ray and computational
studies. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
C5Me5), 63.3 (d, 2JCP = 4, CHβ), 56.1 (d, 2JCP = 8 Hz, CHα), 22.1, 21.6
S
3
1
(d, JCP = 3 Hz, Meα, Meβ), 12.6 (d, JCP = 36 Hz, PMe), 8.9 (s,
C5Me5). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C): δ 8.9.
Synthesis of Compound 5. A solution of NaOH in MeOH (6
mL, 0.5M) was added to a solution of 1-Cl (100 mg, 0.16 mmol) in
CHCl3 (6 mL) under nitrogen. The reaction mixture was stirred at
room temperature for 15 h, after which the suspension turned bright
yellow. The solution was filtered and the solvent removed under
vacuum. The residue was extracted with Et2O, filtered through a pad of
Celite and the solution evaporated to dryness. Complex 5 was
obtained as a fine yellow powder (78 mg, 0.13 mmol, 84%). Anal.
AUTHOR INFORMATION
Corresponding Author
guzman@us.es
■
1
Calcd for C27H34IrP: C, 55.74; H, 5.89. Found: C, 55.3; H, 5.4. H
Notes
NMR (400 MHz, C6D6, 25 °C): δ 7.29 (d, 2 H, Ha), 6.90 (td, 2 H,
The authors declare no competing financial interest.
5JHP = 1.4 Hz, Hb), 6.61 (dd, 2 H, JHP = 2.4 Hz, Hc), 3.19 (d, 2 H,
4
2JHH = 14.6 Hz, IrCHH), 2.79 (d, 2 H, 2JHH = 14.6, IrCHH), 2.35 (s, 6
H, Meα), 1.94 (d, 3 H, 2JHP = 10.9 Hz, PMe), 1.67 (d, 15 H, 4JHP = 1.3
Hz, C5Me5). 13C {1H} NMR (100 MHz, C6D6, 25 °C): δ 162.1 (d,
2JCP = 32 Hz, C1), 140.1 (d, 1JCP = 54 Hz, C2), 137.7 (d, 2JCP = 3 Hz,
C3), 128.9 (d, 4JCP = 2.2 Hz, CHb), 126.8 (d, 3JCP = 6 Hz, CHc), 126.7
ACKNOWLEDGMENTS
Financial support (FEDER support) from the Spanish
Ministerio de Ciencia e Innovacion
17476) and Consolider-Ingenio2010 (No. CSD2007-0000),
and the Junta de Andalucia (Projects Nos. FQM-119 and P09-
FQM-4832) is gratefully acknowledged. J.C. thanks the
■
́
(Project No. CTQ2010-
3
3
(d, JCP = 14 Hz, CHa), 90.5 (C5Me5), 21.4 (d, JCP = 2.4 Hz, Meα),
1
2
14.0 (d, JCP = 29 Hz, PMe), 11.7 (d, JCP = 1.8 Hz, IrCH2), 8.7
(C5Me5). 31P{1H} NMR (160 MHz, C6D6, 25 °C): δ 34.5.
Ministerio de Educacio
AP20080256). J.L.-S. is thankful to the Spanish Ministry of
Science and Innovation (MICINN) for a Ramon y Cajal
́
n for a research grant (ref.
Synthesis of Alkylidene 6+. To a solid mixture of 1-Cl (30 mg,
0.05 mmol) and [Ph3C]+[PF6]− (18 mg, 0.05 mmol) placed in a
screw-cap NMR tube was added 0.6 mL of CD2Cl2 at −60 °C under
argon. The tube was rapidly shaken and the resulting orange solution
analyzed by 1H, 31P and 13C NMR spectroscopy at −60 °C. 1H NMR
(400 MHz, CD2Cl2, −60 °C): δ 15.91 (s, 1 H, IrCH−), 7.82 (d, 1
́
Contract. The use of computational facilities of the Consejo
Superior de Investigaciones Cientificas (CSIC, Cluster Trueno)
and the Supercomputing Centre of Galicia (CESGA) is also
acknowledged.
́
H, Ha), 7.61 (m, 1 H, Hc), 7.41 (t, 1 H, Hb), 7.30 (m, 1 H, Hd/e/f
,
2
overlapped with Ph3CH), 6.94 (m, 2 H, Hd/e/f), 3.70 (d, 1 H, JHH
=
REFERENCES
■
2
14.2 Hz, IrCHH), 2.70 (d, 1 H, JHH = 14.2, IrCHH), 2.61 (s, 3 H,
Meα), 2.55 (s, 3 H, Meβ), 2.24 (d, 3 H, 2JHP = 11.7 Hz, PMe), 1.87 (s,
15 H, C5Me5). 13C {1H} NMR (100 MHz, CD2Cl2, −60 °C): δ 258.3
(IrCH−), 162.5 (d, 2JCP = 29 Hz, C1), 157.0 (d, 2JCP = 27 Hz, C4),
144.5 (C3), 144.0 (C5), 140.8 (C2), 140.5 (C6), 135.2, 135.1 (CHb,
CHc), 130.4, 126.2, 128.1 (CHd, CHe, CHf), 128.5 (CHa), 104.2
(C5Me5), 22.5 (Meα), 20.5 (Meβ), 11.3 (IrCH2), 10.8 (d, 1JCP = 39 Hz,
PMe), 9.3 (C5Me5). 31P{1H} NMR (160 MHz, CD2Cl2, −60 °C): δ
47.3.
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2
2
1 H, JHP = 21 Hz, IrCHP), 3.54 (d, 1 H, JHH = 16.0 Hz, IrCHH),
2.88 (dd, 1 H, 2JHH = 16.0, 3JHP = 4.2 Hz, IrCHH), 2.72 (s, 3 H, Meα),
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3
3
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1
PMe3), 8.4 (C5Me5), 7.0 (IrCH2), 2.5 (d, JCP = 26 Hz, IrCHPMe3).
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́
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3
31P{1H} NMR (200 MHz, CD2Cl2, 25 °C): δ 33.3 (d, JPP = 3 Hz,
3
IrP), 26.8 (d, JPP = 3 Hz, PMe3).
7174
dx.doi.org/10.1021/ja301759m | J. Am. Chem. Soc. 2012, 134, 7165−7175

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