Rhodium and Iridium Complexes of Bulky Tertiary Phosphine Ligands. Searching for Isolable Cationic MIII Alkylidenes

Article abstract of DOI:10.1021/om500910t

Cyclometalated chloride complexes of rhodium and iridium based on (η⁵-C₅Me₅)M^III fragments that result from the metalation of the xylyl substituent of a coordinated PR₂(Xyl) phosphine (Xyl = 2,6-Me₂C₆H₃) have been prepared by reaction of the appropriate metal precursor with the corresponding phosphine. For iridium, the four complexes 1a-d, derived from the phosphines PⁱPr₂(Xyl), PCy₂(Xyl), PMe₂(Xyl), and PPh₂(Xyl), respectively, have been prepared, whereas for rhodium only the complexes 2a,d, derived from PⁱPr₂(Xyl) and PMe₂(Xyl), respectively, have been studied. Chloride abstraction from compounds 1 and 2 by NaBAr_F (BAr_F = B(3,5-C₆H₃(CF₃)₂)₄) leads to either cationic dichloromethane adducts or to cationic hydride alkylidene structures resulting from α-H elimination. The rhodium complexes investigated yield only dichloromethane adducts. However, in the iridium system the less sterically demanding phosphines PMe₂(Xyl) and PPh₂(Xyl) also provide dichloromethane adducts as the only observable products, whereas for the bulkier PⁱPr₂(Xyl) and PCy₂(Xyl) ligands the hydride alkylidene formulation prevails. Nonetheless, variable-temperature NMR studies reveal that in solution each of these two structures exists in equilibrium with undetectable concentrations of the other by means of facile reversible α-H elimination and migratory insertion reactions. Reactivity studies on the cationic hydride alkylidene complexes of iridium are reported as well.

Full text of DOI:10.1021/om500910t

Article
pubs.acs.org/Organometallics
Rhodium and Iridium Complexes of Bulky Tertiary Phosphine
Ligands. Searching for Isolable Cationic MIII Alkylidenes
Jesus Campos† and Ernesto Carmona*
́
́
́
Instituto de Investigaciones Quımicas-Departamento de Quımica Inorgan
́
ica, Universidad de Sevilla-Consejo Superior de
ico Vespucio 49, 41092 Sevilla, Spain
́
Investigaciones Cientıficas, Avda. Amer
́
S
* Supporting Information
ABSTRACT: Cyclometalated chloride complexes of rhodium and
iridium based on (η5-C5Me5)MIII fragments that result from the
metalation of the xylyl substituent of a coordinated PR2(Xyl)
phosphine (Xyl = 2,6-Me2C6H3) have been prepared by reaction of
the appropriate metal precursor with the corresponding phosphine.
For iridium, the four complexes 1a−d, derived from the phosphines
PiPr2(Xyl), PCy2(Xyl), PMe2(Xyl), and PPh2(Xyl), respectively, have
been prepared, whereas for rhodium only the complexes 2a,d, derived
from PiPr2(Xyl) and PMe2(Xyl), respectively, have been studied.
Chloride abstraction from compounds 1 and 2 by NaBArF (BArF =
B(3,5-C6H3(CF3)2)4) leads to either cationic dichloromethane adducts
or to cationic hydride alkylidene structures resulting from α-H
elimination. The rhodium complexes investigated yield only dichloro-
methane adducts. However, in the iridium system the less sterically demanding phosphines PMe2(Xyl) and PPh2(Xyl) also
provide dichloromethane adducts as the only observable products, whereas for the bulkier PiPr2(Xyl) and PCy2(Xyl) ligands the
hydride alkylidene formulation prevails. Nonetheless, variable-temperature NMR studies reveal that in solution each of these two
structures exists in equilibrium with undetectable concentrations of the other by means of facile reversible α-H elimination and
migratory insertion reactions. Reactivity studies on the cationic hydride alkylidene complexes of iridium are reported as well.
Not unexpectedly, cationic Ir(III) alkylidenes are rather
fleeting species, although examples have been known for many
years. A transient hydride ethylidene complex (A in Figure 1)
was generated by our group at −100 °C by protonation of
an Ir−CHCH2 unit at the β carbon atom, but at −50 °C it
rearranged irreversibly to the thermodynamically more stable
hydride ethylene isomer.14a,b Incorporation of the IrC
functionality into a metallacyclic structure hindered β-H
elimination and allowed for the isolation of a stable hydride
alkylidene and observation of reversible carbene migratory
insertion and α-H elimination.14c However, analogous 1,2-C
shifts are usually irreversible processes. We have reported
recently that the cationic bis(iridacycle) B (Figure 1), which
contains Ir−CH2R and IrCHR termini, participates actively
in C−H activation and C−C bond forming reactions that
ultimately lead to unusual hydride phosphepine structures.15
Intermediate B was in fact generated in situ by α-hydride
elimination from the neutral dialkyl C (Figure 1), which was
derived formally from double metalation of a bis(xylyl)
phosphine ligand, PMe(Xyl)2, for Xyl = 2,6-Me2C6H3.15
INTRODUCTION
■
Transition-metal alkyl and carbene complexes are renowned
families of organometallic complexes that have been intensively
investigated in the past decades. Metal alkyls1,2 are at the heart
of organometallic chemistry, as their reactive M−C σ bonds
participate in many useful reactions. In turn, the chemistry of
metal carbenes has become one the most successful areas of
research in organometallic catalysis, due to the rich reactivity of
the MC bond.3−5 Even though effort has focused on carbo-
cyclic6 and heteroatom-stabilized carbenes,7,8 encompassing
N-heterocyclic carbenes,4a,9 metal alkylidenes, that is MCR2
complexes where R is hydrogen or a hydrocarbyl fragment,
continue to attract widespread attention.
In general, alkylidene complexes MCR2 of the late transition
elements contain the metal in a low oxidation state.10 With
reference to iridium, many Ir(I) alkylidenes have been reported11
comprising examples of the IrCH2 parent unit, which
interestingly may exhibit either electrophilic or nucleophilic
carbene reactivity.11b−d In contrast, their Ir(III) counterparts are
somewhat elusive species that are often proposed as reactive
intermediates for many relevant transformations.12 Recently, the
oxidative addition of CH3F to a pincer-ligated Ir(I) complex has
been proposed to involve the intermediacy of an IrIIICH2
species that results from C−H activation of the fluorocarbon
followed by α-fluorine migration.13
In view of these results, we planned the synthesis of isolable
cationic alkylidene complexes of Rh(III) and Ir(III) from
Special Issue: Mike Lappert Memorial Issue
Received: September 2, 2014
© XXXX American Chemical Society
A
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Figure 1. Previous examples of cationic Ir(III) alkylidene structures [IrIIIC(H)(R)]+ (A and B) and the bimetallacycle precursor for B (C).14a,15
Scheme 1: i.e., cationic M·CH2Cl2 adducts (D) and hydride
metallacyclic M−P−CH2 structures akin to those represented
alkylidene structures (E).
for iridium in Figure 1. To avoid the irreversible C−C coupling
reaction that prevented the isolation of B, mono(xylyl)
RESULTS AND DISCUSSION
Synthesis of Iridium and Rhodium Precursors. As
■
phosphines PR2(Xyl), rather than bis(xylyl) phosphines,
PR(Xyl)2,15 were chosen for this work. We envisaged that
cationic cyclometalated16 rhodium and iridium solvento
stated implicitly, the four phosphine ligands selected are
prone to cyclometalation.16 They were prepared by conven-
complexes of type D in Scheme 1 might experience reversible
tional procedures (see the Supporting Information for details).
The corresponding iridium complexes (1a−d, Scheme 2)
resulted from the straightforward reaction of the Ir(III) dimer
[{(η5-C5Me5)IrCl2}2] with the appropriate phosphine in the pre-
sence of the noncoordinating base 2,2,6,6-tetramethylpiperidine
(TMPP in Scheme 2). Under the specified conditions, com-
plexes 1a,b were obtained as the exclusive reaction products in
yields of around 90%. Nevertheless, compounds 1c,d formed
together with the nonmetalated dichlorides (η5-C5Me5)Ir-
(Cl)2PR2(Xyl) in ca. 1:5 (PMe2(Xyl)) and 2:1 (PPh2(Xyl))
ratios. Mild heating of dichloromethane solutions of the non-
metalated compounds (45 °C) in a sealed flask in the presence
of TMPP resulted in their quantitative conversion to the desired
complexes 1c,d.
Scheme 1. Presumed Reversible 1,2-Shifts in the Cationic
Rh(III) and Ir(III) Complexes Investigated in This Work
For rhodium, only the PMe2(Xyl) and PiPr2(Xyl) derivatives
were investigated. In contrast to the iridium analogues, com-
plexes 2a,c could be obtained in yields of <50% only after
heating in refluxing toluene for 4 days and were accompanied
by other unidentified compounds. Accordingly, the synthetic
route of Scheme 3, which takes advantage of the capacity of
Zn(C5Me5)2 to act as a mild Cp* transfer reagent,19 was utilized.
Once again, the PiPr2(Xyl) complex 2a was the exclusive product
of this transformation, while for the PMe2(Xyl) reaction, 2c was
the minor product, as it was accompanied by (η5-C5Me5)Rh-
(Cl)2PMe2(Xyl) in ca. double molar quantities. Conversion of
the former into the latter required treatment with 1 equiv of
LinBu at −20 °C and gave 2c in moderate yields (ca. 60%).
α-H elimination leading to the desired hydride alkylidene
complexes (E, in Scheme 1).17 As the pioneering work of
Schrock on the somewhat related α-H abstraction reaction,
which converts a M(CH2R)2 fragment into MC(H)R with
elimination of RCH3, demonstrated the importance of increased
steric pressure at the metal coordination sphere,18 four tertiary
phosphines PR2(Xyl) with different steric properties were
assayed. As indicated also in Scheme 1, the R substituents
range from the small methyl group to the very bulky cyclohexyl,
with phenyl and isopropyl having intermediate, increasing
size. In the following sections we provide details of this work
that has led to stable complexes of the two kinds depicted in
a
Scheme 2. Synthesis of Cyclometalated Iridium Chloride Complexes 1a−d
a
Lower-case letters a−d will be used throughout this paper to denote complexes of the phosphines as specified in this scheme.
B
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Scheme 3. Synthesis of Rhodium Chloride Complexes 2a,c
The new complexes 1 and 2 feature characteristic 31P{1H}
resonances which for the rhodium complexes appear as the
expected doublet with 1JPRh values of ca. 160 Hz. In the 1H NMR
spectra, the C5Me5 ring gives a doublet (4JHP ≈ 1.5−2.5 Hz) at
roughly 1.7 ppm, while the diastereotopic M−CH2 protons
Scheme 4. Generation of Cationic Hydride Alkylidene and
a
+
CH2Cl2 Adducts of Iridium 3+ and 4·CH2Cl2 , Respectively
1
resonate between 3 and 4 ppm and exhibit a two-bond H−1H
coupling constant of around 12−14 Hz. In the iridium com-
pounds, only one of these two proton signals exhibits coupling
to the phosphorus nucleus (3JHP ≈ 4−5 Hz). Corresponding
13C{1H} resonances appear at around 17 (Ir) and 30 (Rh) ppm
1
(2JCP ≈ 3 (Ir), 8 (Rh) and JCRh ≈ 23 Hz). Slow diffusion of
pentane into concentrated dichloromethane solutions of
compounds 1a,c and 2a provided crystals suitable for X-ray
studies (Figure 2 and Figure S5 (Supporting Information)),
a
i
For R = Pr and Cy the 3+ ↔ 4+ equilibria favor 3 (i.e. 3a+ or 3b+),
+
while for R = Me, Ph only compounds of type 4·CH2Cl2 have been
isolated ([4·CH2Cl2]BArF). The rhodium analogues of compounds 1
(viz. 2a,c, Scheme 3) form exclusively dichloromethane adducts
+
+
(5a·CH2Cl2 and 5c·CH2Cl2 , respectively).
PMe2(Xyl) and PPh2(Xyl) gave the corresponding cationic
adducts 4c·CH2Cl2+ and 4d·CH2Cl2+ in quantitative yields (by
NMR). In a similar manner, the rhodium chloride precursors
Figure 2. ORTEP diagrams for compounds 1a and 2a. Thermal
ellipsoids are drawn at the 50% probability level, and most hydrogen
atoms have been omitted for clarity.
+
2a,c led also to the dichloromethane adducts 5a·CH2Cl2 and
+
5c·CH2Cl2 , once more as the only observable products. These
complexes are not represented in Scheme 4 but possess
+
which further confirmed the cyclometalation of the xylyl-
substituted phosphine. In the three structures, the M−C,
M−P, and M−Cl bond distances are similar to those found
for related compounds previously reported by our group.15,20,21
Bond angles for the three-legged piano stool are close to the
ideal 90° value.
Chloride Abstraction from Rh and Ir Chloride
Complexes. As anticipated in the Introduction, we foresaw
that chloride abstraction from complexes of types 1 and 2
would generate cationic cyclometalated dichloromethane
adducts that could further rearrange by α-H elimination
and formation of the desired hydride alkylidene derivatives.
As summarized in Scheme 4, treatment of dichloromethane
solutions of complexes 1a,b of the bulky PiPr2(Xyl) and
PCy2(Xyl) ligands with NaBArF (BArF = B(3,5-C6H3(CF3)2)4)
led to the targeted alkylidenes 3a+ and 3b+, which formed as
the only observable reaction products. Instead, the analogous
chloride complexes of the less sterically demanding phosphines
structures like that of the iridium analogues 4·CH2Cl2 . It is
rather intriguing that NMR investigations on the new com-
pounds provided no indication for the existence of a structure
in which the cyclometalated R2P(Xyl) unit binds to the metal
center in a κ4P,C,C′,C″ fashion, similarly to our previous findings
for iridium15 and rhodium21 compounds constructed around the
PMe(Xyl)2 ligand. It seems clear that two Xyl units are required
to stabilize such an unusual κ4 binding mode.
Variable-temperature NMR studies for the cationic iridium
complexes of Scheme 4 evince their interesting solution dynamic
1
behavior. Thus, the room-temperature H NMR spectra of
i
hydride alkylidenes 3+ show equivalent Pr (3a+) and Cy (3b+)
phosphine substituents and no signals attributable to their key
Ir−H and IrCH functionalities. However, when the temper-
ature is lowered to −60 °C, 1H NMR resonances at ca. 15.5 and
−15.2 ppm due to IrCH and Ir−H protons of 3a+ (Figure S1,
Supporting Information) and 3b+ become discernible. The low-
frequency peaks correspond to the hydride ligands and exhibit a
C
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two-bond coupling to the phosphorus nucleus of around 25 Hz,
whereas signals due to the carbenic protons exhibit no resolvable
nuclear coupling. At these low temperatures (below −60 °C),
Other geometrical parameters are comparable to those in related
structures already considered and need no further discussion.
Despite the lability of the coordinated molecule of CH2Cl2
i
the phosphine Pr substituents of 3a+ become inequivalent and
in the cationic solvento complexes of iridium 4c·CH2Cl2 and
+
+
give rise to four somewhat broad, albeit clearly distinct,
resonances. In almost all probability this solution dynamic
behavior results from a rapid and reversible 1,2-H shift between
Ir and the alkylidene carbon (vide infra). We analyzed the
exchange of these protons by 1D-EXSY studies. Rate constants
of 14.7 and 45.5 s−1 were measured at −80 °C for compounds
3a+ and 3b+, respectively. An Eyring analysis (see the Supporting
Information, Figure S3) in the temperature interval from −70 to
4d·CH2Cl2 (Scheme 4), α-H elimination was not observed in
any case. Evidently, for these complexes derived from the less
bulky PMe2(Xyl) and PPh2(Xyl) ligands, the dichloromethane-
solvated Ir−CH2 metallacyclic structure is preferred relative to
the nonsolvated hydride alkylidene formulation (D and E,
respectively, in Scheme 1). However, the dynamic behavior
exhibited by these complexes in solution may be suggestive
+
+
of a fast equilibration of 4c·CH2Cl2 and 4d·CH2Cl2 with
undetectable concentrations of the corresponding hydride
alkylidene structures. Thus, the two cations present broad
−90 °C yielded values of the activation parameters ΔH⧧
=
7.3 0.6 kcal mol−1 and ΔS⧧ = −12 1 cal mol−1 K−1, with
ΔG⧧ = 11 1 kcal mol−1 for compound 3a+ (and similar
+
31P{1H} NMR signals centered at 8.3 (4c·CH2Cl2 ) and
298 K
values for compound 3b+: ΔH⧧ = 9 2 kcal mol−1 and ΔS⧧ =
+
35.5 ppm (4d·CH2Cl2 ). Furthermore, the room-temperature
−6 1 cal mol−1 K−1, with ΔG⧧
= 11 2 kcal mol−1).
We thought it of interest to demonstrate further the existence
of an alkylidene unit in 3a+ by converting its cis hydride ligand
to bromide by the action of N-bromosuccinimide (Scheme 5).
+
298 K
1H NMR spectrum of 4d·CH2Cl2 contains signals similar
to those of the neutral chloride precursor 1d, except for
resonances due to the Ir−CH2 protons, which are missing.
Cooling to −20 °C results in the appearance of two broad
doublets centered at 3.82 and 3.42 ppm, with a 2JHH coupling of
Scheme 5. Reaction of 3a+ with N-Bromosuccinimide
+
ca. 16 Hz. For the cation 4c·CH2Cl2 with the least bulky
phosphine substituents, cooling to −90 °C is needed for the
observation of two broad signals at δ 3.52 and 3.20 ppm. At
20 °C, they convert into a broad peak shifted to ca. 2.90 ppm.
These observations may be indicative of the attainment in
solution of the equilibria shown in Scheme 6, implying fast
reversible CH2Cl2 dissociation also accompanied by reversible
α-H elimination.22
As summarized in Scheme 7, adducts 4c·py+ and 4d·NCMe+
were instantly generated upon addition of a slight excess of the
+
Lewis base to solutions of the appropriate 4·CH2Cl2 cationic
species. They display sharp 31P{1H} NMR resonances at −0.5
(4c·py+) and 2.84 ppm (4d·NCMe+) with chemical shifts close
to those of the neutral chloride precursors (3.3 and 28.6 ppm
for 1c,d, respectively). In addition, the superior binding pro-
perties of C5H5N and NCMe in comparison with CH2Cl2 result
in sharp IrCH2 resonances. The molecular structures of the two
complexes were ascertained by X-ray crystallography (see the
Supporting Information, Figure S7).
The new alkylidene, 6a+, is dark green and has carbenic NMR
resonances observable at room temperature at δ 16.1 (1H) and
1
262.2 (13C) ppm, the latter with JCH = 152 Hz. As expected,
complex 6a+ displays no dynamic behavior in solution.
The molecular structures of the cationic alkylidenes 3a+ and
6a+ were confirmed by X-ray diffraction studies (Figure 3).
Characteristic IrC bond lengths of 1.896(5) (3a+) and
1.907(2) Å (6a+), which are appreciably shorter than the
Ir−CH2 bond distances of ca. 2.10 Å found in other
related complexes discussed in this paper or elsewhere,15,20
are in accordance with the proposed alkylidene formulation.
+
At variance with the case for the iridium cations 4·CH2Cl2+,
the analogous rhodium complexes, 5a·CH2Cl2+ and 5c·CH2Cl2 ,
exhibited no dynamic behavior in solution, in agreement with
Figure 3. ORTEP diagrams for compounds 3a+ and 6a+. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms and
counterions have been omitted for clarity.
D
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+
+
Scheme 6. Proposed Dynamic Behavior for 4c·CH2Cl2 and 4d·CH2Cl2
+
Scheme 7. Reaction of Cationic Adducts 4c·CH2Cl2 and
coordinated molecule of the dichloromethane and the metalated
xylyl ring (Figure 4) might account for this remarkable stability.
This weak interaction is characterized by CH(25b)···C(12) and
CH(25b)···C(13) bond distances of ca. 2.64 and 2.73 Å,
whereas the C−H(25b)···Centr angle has a value of ca. 157.6°
(Centr = metalated xylyl ring centroid).
+
4d·CH2Cl2 with Lewis Bases
Reactivity Studies of Hydride Alkylidene Complex
3a+. The solution dynamic behavior of hydride alkylidene
complexes 3+ (Scheme 4), which was ascribed to very facile
migratory insertion of the hydride ligand into the cationic
IrCH− functionality, was a clear sign of the otherwise
expected electrophilicity of these complexes. To gain additional
insight into this chemical functioning, we performed supple-
mentary reactivity studies employing 3a+ as a representative
complex. As shown in Scheme 8, its reaction with PMe3
resulted in an immediate color change, due to the generation
of the phosphonium ylide 7a+ as the major reaction product
their failure to undergo α-H elimination reactions. For instance,
+
for the cation 5a·CH2Cl2 , the 31P{1H} NMR spectrum consists
of a doublet with δ 77.3 ppm and 1JRhP = 158 Hz. The rhodium
benzylic linkage, Rh−CH2−, gives rise to a proton multiplet
centered at 3.49 ppm and to a 13C{1H} doublet at δ 33.9 ppm
with a one-bond 13C−103Rh coupling of 23 Hz. It is worth
noting that all Rh and Ir dichloromethane adducts studied in
this work feature good thermal stability at room temperature
and remain unaltered after prolonged periods of time when kept
under an inert atmosphere. This is in contrast with reports on
related complexes.15,23 We therefore performed X-ray studies on
[5a·CH2Cl2]BArF, with the results shown in Figure 4. Only a
few rhodium complexes with coordinated CH2Cl2 have been
authenticated by X-ray crystallography,23b,c,24 two of which are
+
(85%). The cationic adduct 4a·PMe3 was formed as well,
+
albeit in low proportions (ca. 15%). The compound 4a·PMe3
exhibits a 31P{1H} NMR signal at 48.1 ppm (very close to
+
corresponding signals due to 1a, at 49.6 ppm, and 4a·C2H4 , at
42.7 ppm; vide infra), which appears as a doublet (2Jpp= 21 Hz)
due to coupling to the PMe3 phosphorus nucleus. The latter
also features a doublet at −47.3 ppm. In contrast, the 31P{1H}
NMR signal due to the metalated phosphine of compound 7a+
is found at 59.3 ppm, while its phosphonium PMe3 group yields
a resonance at 33.1 ppm, highly deshielded with respect to the
+
closely related to 5a·CH2Cl2 : namely, [(η5-C5Me5)Rh(PMe3)-
+
Me(CH2Cl2)]+ and [(η5-C5Me5)Rh(PMe3)Ph(CH2Cl2)]+.
The latter are air-sensitive, thermally unstable solids that
readily decompose at room temperature even under an inert
corresponding resonance in 4a·PMe3 (−47.3 ppm), without
observable coupling to the phosphorus center of the cyclo-
metalated phosphine. A characteristic doublet due to the Ir−H
1
+
unit appears in the H NMR spectrum at −17.7 ppm (dd,
atmosphere. In contrast, 5a·CH2Cl2 is stable in solution for
3
2JHP = 33.3, JHP = 10.6 Hz). In the 13C{1H} NMR spectrum,
many days, when stored under argon, and remains unaltered
after several months in the solid state. We propose that the
existence of an intramolecular CH−π interaction between the
the signal due to the IrCHPMe3 is observed at 6.3 ppm (d,
1JCP = 32 Hz), widely shifted with respect to the resonance due
+
Figure 4. (left) ORTEP diagram for the complex 5a·CH2Cl2 . Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms and
+
the counterion have been omitted for clarity. (right) Intramolecular CH−π contacts in 5a·CH2Cl2 (Centr = xylyl ring centroid).
E
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Scheme 8. Reaction of 3a+ with PMe3
to the carbenic carbon of 3a+ (263.8 ppm). 4a·PMe3 was
further characterized by X-ray diffraction studies, and details
can be found in the Supporting Information (Figure S6).
Once again, formation of the phosphonium ylide 7a+ in the
above reaction was an obvious sign of the electrophilicity of the
carbene linkage of complex 3a+. However, 7a+ was the kinetic
Information for details). We also studied the reaction of 3a+
with H2, which resulted in quantitative formation of the cationic
alkyl dihydride species 8a+. This reaction might proceed either
by direct addition of H2 to the IrCH unit or more likely
by dihydrogen-induced migratory insertion chemistry, which
would involve coordination of H2 to a species analogous to F in
Scheme 6. Interestingly, reaction of 3a+ with D2 led to the
+
+
product of the reaction, as heating a mixture of 4a·PMe3
and 7a+ at 40 °C for 16 h caused quantitative conversion of
the ylide species into the phosphine adduct (Scheme 8).
Monitoring the reaction by 31P{1H} NMR spectroscopy
provided a first-order kinetic rate constant of 3.4 × 10−5 s−1,
which corresponds to a ΔG⧧ value of 24.8 0.3 kcal mol−1 at
40 °C (see the Supporting Information, Figure S4). The latter
compound is thermally stable and does not revert to 7a+, even
after its solution in ClCH2CH2Cl is heated to over 80 °C for
several hours. It seems probable that whereas the ylide complex
7a+ could result from direct nucleophilic attack by PMe3 at the
+
formation of [D5]-8a-D2 , in which all benzylic positions of the
xylyl ring became deuterated. We recently reported an analogous
C−H/D scrambling for the parent compound bearing the
PMe(Xyl)2 ligand.15
Compounds 4a-C2H4+ and 9a feature characteristic 1H NMR
signals due to diastereotopic Ir−CH2 protons at 3.16 (d, 2JHH
=
14.2 Hz) and 2.05 (dd, 2JHH = 14.2, 3JHP = 3.4 Hz) ppm (9a) at
and 3.67 (d, 2JHH = 12.5 Hz) and 3.39 (dd, 2JHH = 12.5, 3JHP
=
+
3.3 Hz) ppm (4a-C2H4 ). The Ir−Me unit of 9a leads to a
shielded doublet at −0.20 ppm, with a three-bond scalar
coupling to phosphorus of 3.4 Hz. The ethylene ligand of
+
carbene carbon, adduct 4a·PMe3 must have been formed by
PMe3 coordination to an unsaturated intermediate such as F in
+
4a-C2H4 provides two broad resonances (2H each) at 2.77
Scheme 6.7d
and 2.29 ppm, with corresponding 13C{1H} NMR resonances
(also broad) at 46.8 and 42.5 ppm. The cationic dihydride 8a+
exhibits a remarkable dynamic behavior in solution (Figure 5),
consisting of the exchange of the two hydride ligands and the
two methylene protons. This exchange probably proceeds by
coupling of one of the hydrides with the Ir−CH2 bond and
formation of a monohydride agostic intermediate.15a,25 At room
Treatment of a dichloromethane solution of 3a+ with C2H4
caused an immediate color change, from the characteristic
intense red of the alkylidene to pale yellow, due to forma-
+
tion of the cationic ethylene adduct 4a·C2H4 (Scheme 9).
Scheme 9. Additional Reactivity Studies Performed with
Complex 3a+
1
temperature, the H NMR spectrum does not contain any
observable resonances due to these four protons, but at −20 °C
two broad signals become barely visible at 2.45 and 2.93 ppm
(Ir−CH2), along with a broad resonance at −13.62 ppm due
to the hydride ligands. Coalescence is attained at ca. −10 °C,
and further cooling to −40 °C allowed identifying a two-bond
coupling constant of 13.0 Hz between the diastereotopic
methylene protons. At −50 °C the signal for the two hydride
ligands splits into two different peaks, one of them displaying
coupling to the phosphorus center (2JHP = 16.1 Hz). Heating
the solution to 45 °C resulted in the appearance of a broad
signal attributable to four protons at −4.98 ppm (not shown in
Figure 5). Line-shape analysis of the corresponding resonances
at various temperatures in conjunction with Eyring analysis of
the observed rate constants yields the following activation para-
meters for the overall process: ΔH⧧ = 20.3 0.5 kcal mol−1,
ΔS⧧ = 29 2 cal mol−1 K−1 and ΔG⧧300 K = 12 2 kcal mol−1
(more details are given in the Supporting Information).
Concluding Remarks. Cationic hydride alkylidene com-
plexes of the (η5-C5Me5)IrIII fragment in which the alkylidene
functionality is part of a five-membered iridacycle that also
contains a phosphine terminus (complexes 3a+ and 3b+) have
been prepared and characterized using bulky PR2(Xyl)
phosphines which are prone to cyclometalation. Their pivotal
Ir−H and IrCH− units derive from a metalated benzylic
The reaction with LiMe led to the formation of the neutral
methyl complex 9a as the major product (ca. 70% yield), along
with other minor unidentified species. Use of the milder
alkylating reagent ZnMe2 led cleanly and quantitatively to 9a.
The molecular structure of this complex was confirmed by
X-ray diffraction studies (see Figure S6 in the Supporting
F
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Article
1
Figure 5. Solution dynamic behavior of complex 8a+ by H NMR.
Ir−CH2− linkage by reversible α-H elimination. In accordance
with previous findings, increased steric pressure at the metal
coordination sphere favors α-H elimination, thereby stabilizing
the hydride alkylidene structure. Indeed, this structure has
only been observed for the two bulkiest phosphines employed:
namely, PiPr2(Xyl) and PCy2(Xyl). At variance with these
observations, the cyclometalated Ir−CH2− binding motif of
complexes of the less sterically demanding PMe2(Xyl) and
PPh2(Xyl) ligands is favored over the Ir(H)(CH−) structure,
leading to stable, cationic dichloromethane adducts (complexes
+
+
4c·CH2Cl2 and 4d·CH2Cl2 , respectively). Only CH2Cl2
adducts analogous to the latter compounds can be isolated for
the similar (η5-C5Me5)RhIII−PR2(Xyl) reaction system.
Figure 6. Labeling scheme used for 1H and 13C{1H} NMR
assignments.
EXPERIMENTAL SECTION
■
General Synthesis of Iridium Chloride Compounds 1a−d.
[Cp*IrCl2]2 (0.30 g, ca. 0.38 mmol) dissolved in dry CH2Cl2 (5 mL)
and cooled to 0 °C was reacted with a dichloromethane solution of the
phosphine (0.76 mmol), in the presence of 2,2,6,6-tetramethylpiper-
idine (TMPP, 130 μL, 0.76 mmol). The reaction mixture was warmed
to room temperature and additionally stirred for 2 h (16 h at 45 °C in
the case of PMe2(Xyl) and 4 h at this temperature when the phosphine
was PPh2(Xyl)). The solvent was removed under vacuum and the
product extracted with toluene. The solution was evaporated to dryness,
providing a bright yellow powder, which was washed with pentane
to yield the desired chloride complexes in yields of ca. 90%. Variable
amounts of the nonmetalated (η5-C5Me5)IrCl2(PR2(Xyl)) compounds
were identified when performing the reactions with PMe2(Xyl) and
PPh2(Xyl) at room temperature. The labeling scheme used for NMR
assignments is given in Figure 6.
133.6 (d, 1JCP = 48 Hz, C2), 129.9 (CHb), 128.0 (d, 3JCP = 7 Hz, CHc),
3
1
127.5 (d, JCP = 12 Hz, CHa), 91.9 (C5Me5), 30.9 (d, JCP = 30 Hz,
CH(iPr)), 27.4 (d, 1JCP = 29 Hz, CH(iPr)), 22.8 (Meα), 20.9 (d, 2JCP
=
7 Hz, Me(iPr)), 20.2 Me(iPr)), 19.2 (d, JCP = 5 Hz, Me(iPr)), 18.5
2
(Me(iPr)), 16.9 (d, JCP = 3 Hz, IrCH2), 9.0 (C5Me5). 31P{1H} NMR
2
(202 MHz, CD2Cl2, 25 °C): δ 49.6. Anal. Calcd for C24H37ClIrP: C,
49.34; H, 6.38. Found: C, 49.3; H, 6.6. Corresponding data for the
remaining compounds of this type can be found in the Supporting
Information.
Synthesis of Rhodium Chloride Compounds 2a,c. A solution
of the phosphine (P(iPr)2(Xyl), 111 mg, 0.5 mmol; PMe2(Xyl), 83 mg,
0.5 mmol) in 2 mL of THF was added, at −40 °C, to a solution of
[RhCl(C2H4)2]2 (100 mg, 0.25 mmol) in 3 mL of THF. The reaction
mixture was stirred for 3 h at this temperature. Then, a solution of
ZnCp2* (84 mg, 0.25 mmol) in 1 mL of THF was added and the
mixture was stirred for 5 h, with slow warming to −25 °C. The solvent
was removed under vacuum, the residue was extracted with diethyl
ether, and the extract was then evaporated to dryness. The solid was
dissolved in 5 mL of CHCl3 and stirred for 3 h at room temperature.
The solvent was removed under vacuum, and the crude product was
washed with pentane to yield chloride complexes 2a,c as orange solids,
which were purified by column chromatography from Et2O/pentane
(2a, 175 mg, 70%; 2c, 110 mg, 83%). Reaction with PMe2(Xyl) gave a
Data for compound 1a are as follows. 1H NMR (500 MHz, CD2Cl2,
5
25 °C): δ 7.22 (d, 1 H, Ha), 7.06 (td, 1 H, JHP = 1.6 Hz, Hb), 6.85
2
(d, 1 H, Hc), 3,67 (d, 1 H, JHH = 13.9 Hz, IrCHH), 3.24 (m, 1 H,
CH(iPr)), 3.07 (dd, 1 H, 2JHH = 13.9, 3JHP = 4.4 Hz, IrCHH), 2,50 (s,
3 H, Meα), 2.27 (m, 1 H, CH(iPr)), 1.71 (d, 15 H, JHP = 1.3 Hz,
4
C5Me5), 1.30 (dd, 3 H, 3JHP = 18.6, 3JHH = 6.8 Hz, Me(iPr)}), 1.24 (dd,
3
3
3
3 H, JHP = 12.8, JHH = 7.2 Hz, Me(iPr)), 1.18 (dd, 3 H, JHP = 14.6,
3JHH = 7.1 Hz, Me(iPr)), 0.73 (dd, 3 H, JHP = 15.7, JHH = 7.1 Hz,
3
3
Me(iPr)). All aromatic couplings are ca. 7.5 Hz. 13C{1H} NMR (125
2
MHz, CD2Cl2, 25 °C): δ 162.9 (d, JCP = 26 Hz, C1), 140.5 (C3),
G
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Article
mixture of metalated 2c and (η5-C5Me5)RhCl2(PMe3(Xyl)) complexes
in a ratio of ca. 30:70. Both compounds were separated by column
chromatography from Et2O/pentane to give pure samples of 2c
(42 mg, 19%) and the nonmetalated dichloride (130 mg, 55%) as
crystalline orange solids. In order to increase the amount of 2c, a THF
solution of the latter product (50 mg, 0.105 mmol) was reacted with
a solution of LinBu (3 M in hexanes, 45 μL) at −20 °C. After 30 min
of stirring at this temperature, the reaction was quenched with MeOH
(10 μL) and the volatiles were removed under vacuum. The product
was extracted with Et2O, the solvent was evaporated to dryness, and
the orange solid was washed with pentane to give complex 2c in 62%
yield (29 mg).
1.98 (m, 1 H, CH(iPr)), 1.94 (t, 15 H, 4JHP = 1.1 Hz, C5Me5), 1.66 (dd,
3
3
3 H, JHP = 15.8, JHH = 6.9 Hz, Me(iPr)), 1.30 (m, 6 H, 2 Me(iPr)),
3
3
0.24 (dd, 3 H, JHP = 17.3, JHH = 7.0 Hz, Me(iPr)). All aromatic
couplings are ca. 7.5 Hz. 13C{1H} NMR (125 MHz, CD2Cl2, 25 °C): δ
262.2 (1JCH = 152 Hz, IrCH), 167.2 (d, JCP = 25 Hz, C1), 145.5
2
3
1
(C3), 139.9 (d, JCP = 8 Hz, CHa), 139.1 (d, JCP = 47 Hz, C2), 134.2
(CHb), 131.1 (d, 3JCP = 12 Hz, CHc), 107.6 (C5Me5), 29.7 (Meα), 28.1
(d, JCP = 28 Hz, CH(iPr)), 27.2 (d, JCP = 31 Hz, CH(iPr), 22.4
1
1
(Me(iPr)), 19.5 (d, JCP = 4 Hz, Me(iPr)), 18.6 (d, JCP = 6 Hz,
Me(iPr)), 18.5 (Me(iPr)), 9.4 (C5Me5). 31P{1H} NMR (200 MHz,
CD2Cl2, 25 °C): δ 60.3. Anal. Calcd for C56H47BBrF24IrP: C, 45.15; H,
3.18. Found: C, 45.1; H, 3.2.
2
2
Data for compound 2a are as follows. 1H NMR (500 MHz, CDCl3,
Reaction of 3a+ with PMe3. A solution of PMe3 in toluene
(33 μL, 1.1 M, 0.033 mmol) was added under argon over a CH2Cl2
5
25 °C): δ 7.14 (d, 1 H, Ha), 7.01 (td, 1 H, JHP = 2.2 Hz, Hb), 6.81
4
2
(dd, 1 H, JHP = 2.4 Hz, Hc), 3.42 (br. d, 1 H, JHH = 12.1 Hz,
(1 mL) solution of alkylidene 3a+ (BArF salt; 40 mg, 0.028 mmol)
−
2
RhCHH), 3.24 (br. d, 1 H, JHH = 12.1 Hz, RhCHH), 2.95 (septet,
2 H, JHH = 7.0 Hz, CH(iPr)), 2.42 (s, 3 H, Meα), 2.29 (m, 2 H,
3
placed in a Schlenk flask. The solution rapidly cleared up, and then it
was heated to 40 °C for 16 h. The volatiles were removed under
vacuum, and the residue was washed with pentane to give complex
4a·PMe3+ as a pale orange powder (35 mg, 84%). Crystals suitable for
X-ray analysis were obtained by slow diffusion from CH2Cl2/pentane.
In turn, characterization of the related ylide 7a+ was achieved by the
following procedure. A screw-capped NMR tube was charged with 3a+
(BArF− salt; 30 mg, 0.021 mmol) and CD2Cl2 (0.6 mL). The tube was
shaken and cooled to −40 °C, and PMe3 (2.5 μL, 0.026 mmol) was
added at this temperature. 31P{1H} NMR monitoring of the reaction
showed immediate conversion of the alkylidene to a mixture of
CH(iPr)), 1.63 (d, 15 H, 4JHP = 2.4 Hz, C5Me5), 1.36 (dd, 6 H, 3JHP
=
19.1, JHH = 6.8, Me(iPr)), 1.24 (dd, 6 H, JHP = 12.3, JHH = 7.1,
3
3
3
Me(iPr)), 1.19 (dd, 6 H, JHP = 14.5, JHH = 7.1, Me(iPr)), 0.80 (dd,
3
3
6 H, JHP = 15.5, JHH = 7.1, Me(iPr)). All aromatic couplings are ca.
3
3
7.5 Hz. 13C{1H} NMR (160 MHz, CDCl3, 25 °C): δ 160.7 (d, 2JCP
=
=
1
4
28 Hz, C1), 139.5 (C3), 131.2 (d, JCP = 42 Hz, C2), 129.6 (d, JCP
4
4
2 Hz, CHb), 127.7 (d, JCP = 6 Hz, CHc), 126.6 (d, JCP = 15 Hz,
CHa), 98.1 (t, 1JCRh = 2JCP = 4 Hz, C5Me5), 30.5 (dd, 1JCRh = 23, 2JCP
=
8 Hz, RhCH2), 30.1 (d, 1JCP = 21 Hz, CH(iPr)), 27.5 (d, 1JCP = 22 Hz,
CH(iPr)), 22.8 (Meα), 20.7 (d, 2JCP = 8 Hz, Me(iPr)), 20.2 (Me(iPr)),
19.4 (d, JCP = 5 Hz, Me(iPr)), 19.1 (Me(iPr)), 9.6 (C5Me5).31P{1H}
2
+
the ylide 7a+ and the cationic adduct 4a·PMe3 in a ratio of ca. 97:3.
1
NMR (200 MHz, CDCl3, 25 °C): δ 83.0 (d, JRhP = 159 Hz). Anal.
Spectroscopic data were obtained at −40 °C without further purifi-
Calcd for C24H37ClPRh: C, 58.25; H, 7.54. Found: C, 58.4; H, 7.4.
Corresponding data for the remaining compounds of this type can be
found in the Supporting Information.
+
cation in order to avoid isomerization to 4a·PMe3 , which is slow at
this temperature.
+
1
Synthesis of Cationic Hydride Alkylidenes 3a+ and 3b+. To a
solid mixture of 1a or 1b (0.08 mmol) and NaBArF (72 mg, 0.08 mmol)
placed in a Schlenk flask was added 5 mL of CH2Cl2. The resulting
intensely red solution was stirred for 15 min at room temperature and
then filtered and the solvent removed under vacuum. The red solid was
washed with pentane to give the BArF− salts of alkylidene 3a+ or 3b+ in
ca. 95% yield. These complexes can be recrystallized from a 1/2 mixture
of CH2Cl2 and pentane.
Data for compound 4a·PMe3 are as follows. H NMR (400 MHz,
CD2Cl2, 25 °C): δ 7.27 (d, 1 H, Ha), 7.20 (td, 1 H, 5JHP = 2.7 Hz, Hb),
5
6.98 (dd, 1 H, JHP = 3.1 Hz, Hc), 3.43 (m, 1 H, IrCHH), 3.29 (dd,
2
3
2
1 H, JHH = 7.6, JHP = 3.1 Hz, IrCHH), 3.28 (dseptet, 1 H, JHP
=
3
10.9, JHH = 7.1 Hz, CH(iPr)), 2.51 (s, 3 H, Meα), 2.33 (m, 1 H,
CH(iPr)), 1.77 (t, 15 H, 4JHP = 1.9 Hz, C5Me5), 1.28 (dd, 3 H, 3JHP
=
3
2
13.2, JHH = 7.2 Hz, Me(iPr)), 1.24 (d, 9 H, JHP = 10.3 Hz, PMe3),
3
3
1.15 (dd, 3 H, JHP = 18.9, JHH = 7.1 Hz, Me(iPr)), 1.03 (dd, 3 H,
Data for compound 3a+ are as follows. 1H NMR (500 MHz,
CD25Cl2, 25 °C): δ 7.69 (d, 1 H, Ha), 7.62 (dd, 4JHP = 2.6 Hz, Hc), 7.34
(td, JHP = 2.2 Hz, 1 H, Hb), 2.73 (m, 2 H, 2 CH(iPr)), 2,69 (s, 3 H,
Me), 2.17 (s, 15 H, C5Me5), 1.06, (dd, 6 H, 3JHP = 17.2, 3JHH = 6.9 Hz,
3JHP = 16.5, 3JHH = 6.7 Hz, Me(iPr)), 0.59 (dd, 3 H, 3JHP = 16.3, 3JHH
=
7.1 Hz, Me(iPr)). All aromatic couplings are ca. 7.5 Hz. 13C{1H} NMR
(100 MHz, CD2Cl2, 25 °C): δ 158.5 (d, 2JCP = 24 Hz, C1), 140.1 (C3),
1
4
132.4 (d, JCP = 47 Hz, C2), 131.9 (d, JCP = 3 Hz, CHb), 130.4 (d,
3
3
Me(iPr)), 0.89 (dd, 3 H, JHP = 18.5, JHH = 6.9 Hz, Me(iPr)). All
3JCP = 7 Hz, CHc), 126.9 (d, JCP = 12 Hz, CHa), 98.5 (C5Me5), 31.2
3
1
aromatic couplings are of ca. 7.5 Hz; H NMR (500 MHz, CD2Cl2,
(dd, 1JCP = 29, 3JCP = 2 Hz, CH(iPr)), 26.3 (d, 1JCP = 30 Hz, CH(iPr),
−80 °C): δ 15.51 (s, 1 H, IrCH), −15.21 (d, 1 H, 2JHP = 24.7 Hz, Ir−
H). Hydride and carbene signals are only detectable at temperatures
below −50 °C. 13C{1H} NMR (125 MHz, CD2Cl2, 25 °C): δ 263.8
22.6 (Meα), 20.7 (Me(iPr)), 20.4 (d, JCP = 5 Hz, Me(iPr)), 20.0 (d,
2
2JCP = 6 Hz, Me(iPr)), 18.6 (Me(iPr)), 18.4 (d, JCP = 39 Hz, PMe3),
1
9.8 (C5Me5), 6.4 (dd, JCP = 8, JCP = 3 Hz, IrCH2). 31P{1H} NMR
2
2
2
3
(IrCH), 166.4 (d, JCP = 27 Hz, C1), 144.2 (C3) 137.2 (d, JCP
=
(160 MHz, CD2Cl2, 25 °C): δ 48.1 (d, JPP = 21 Hz, P(iPr)2Xyl),
2
7 Hz, CHc), 135.2 (C2, overlapped with BArF), 134.1 (CHb), 128.7 (d,
3JCP = 12 Hz, CHa), 104.5 (C5Me5), 25.5 (d, 1JCP = 32 Hz, CH(iPr)),
22.0 (Me), 18.7, 18.3 (Me(iPr)), 10.3 (C5Me5). 31P{1H} NMR
(202 MHz, CD2Cl2, 25 °C): δ 73.1. IR (Nujol): ν(IrH) 2165 cm−1.
Anal. Calcd for C56H48BF24IrP: C, 47.67 ; H, 3.43. Found: C, 47.4; H,
3.5. See the Supporting Information for corresponding data for 3b+.
Synthesis of Cationic Bromide Alkylidene 6a+. Chloride
complex 1a (100 mg, 0.171 mmol) and NaBArF (152 mg, 0.171 mmol)
were placed in a Schlenk flask and dissolved in CH2Cl2 (5 mL) under
argon. After the mixture was stirred at room temperature for 15 min,
the red solution of the resulting of complex 3a+ was filtered over
a Schlenk flask containing N-bromosuccinimide (30 mg, 0.171 mmol),
with a change in color to dark green. The reaction mixture was stirred
for 15 min and then the succinimide was removed by extraction
with deoxygenated water. Compound [6a]BArF was obtained as dark
green crystals (215 mg, 84%) by slow diffusion of pentane into a
dichloromethane solution of the alkylidene. 1H NMR (500 MHz,
CD2Cl2, 25 °C): δ 16.1 (s, 1 H, IrCH), 8.10 (d, 1 H, Ha), 6.98 (dd,
2
−47.3 (d, JPP = 21 Hz, PMe3). Anal. Calcd for C59H58BF24IrP2: C,
47.62; H, 3.93. Found: C, 47.9; H, 3.8.
Data for compound 7a+ are as follows. 1H NMR (500 MHz,
CD2Cl2, 25 °C): δ 7.19 (m, 2 H, Ha, Hb), 7.10 (m, 1 H, Hc), 3.60 (d,
2
1 H, JHP = 11.0 Hz, IrCHPMe3), 2.94 (m, 1 H, CH(iPr)), 2.56 (s,
3 H, Meα), 2.31 (m, 1 H, CH(iPr)), 1.90 (t, 15 H, JHP = 1.8 Hz,
4
2
3
C5Me5), 1.33 (d, 9 H, JHP = 11.9 Hz, PMe3), 1.23 (dd, 3 H, JHP
=
=
3
3
3
12.8, JHH = 7.4 Hz, Me(iPr)), 1.13 (dd, 3 H, JHP = 18.3, JHH
6.8 Hz, Me(iPr)), 1.03 (dd, 3 H, 3JHP = 12.0, 3JHH = 7.2 Hz, Me(iPr)),
3
3
0.16 (dd, 3 H, JHP = 16.0, JHH = 7.1 Hz, Me(iPr)), −17.7 (dd, 1 H,
2JHP = 33.3, 3JHP = 10.6 Hz, IrH). All aromatic couplings are ca. 7.5 Hz.
13C{1H} NMR (125 MHz, CD2Cl2, −40 °C): δ 150.6 (d, JCP = 27
2
Hz, C1), 141.8 (C3), 133.4 (C2, overlapped with BArF), 130.2 (CHb),
1
129.3 (CHc), 125.4 (CHa), 92.1 (C5Me5), 27.6 (d, JCP = 28 Hz,
CH(iPr)), 24.7 (d, 1JCP = 36 Hz, CH(iPr), 22.0 (Meα), 18.5 (Me(iPr)),
18.3 (Me(iPr)), 17.8 (Me(iPr)), 16.5 (Me(iPr)), 10.6 (d, 1JCP = 56 Hz,
1
PMe3), 9.8 (C5Me5), 6.3 (dd, JCP = 32 Hz, IrCHPMe3). 31P{1H}
5
1 H, JHP = 2.9 Hz, Hc), 7.56 (Hb, overlapped with NaBArF),
2
3
3.74 (dseptet, 1 H, JHP = 11.9, JHH = 6.8 Hz), 2.79 (s, 3 H, Meα),
NMR (200 MHz, CD2Cl2, 25 °C): δ 59.3 (P(iPr)2Xyl), 33.1 (PMe3).
H
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving general experimental
■
S
details, synthesis and characterization of phosphine ligands and
1
rhodium and iridium complexes, solution dynamic H NMR
spectroscopy studies, kinetic studies on the conversion of 7a+
+
into 4a·PMe3 , and X-ray crystallographic studies. This material
is available free of charge via the Internet at http://pubs.acs.org.
(9) Selected reviews: (a) Schuster, O.; Yang, L.; Raubenheimer, H.
G.; Albrecht, M. Chem. Rev. 2009, 109, 3445−3478. (b) Schaper, L.-A.;
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.S.: guzman@us.es.
■
Hock, S. J.; Herrmann, W. A.; Kuhn, F. E. Angew. Chem., Int. Ed. 2013,
̈
52, 270−289. (c) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.;
Cavallo, L. Coord. Chem. Rev. 2009, 253, 687−703.
(10) (a) Vidavsky, Y.; Anaby, A.; Lemcoff, N. G. Dalton Trans. 2012,
41, 32−43. (b) Adams, R. D.; Mindiola, D. J. J. Organomet. Chem.
2011, 696, 3909 (special issue dedicated to the activation of small
molecules).
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P. A.; Massey, R. L. J. Am. Chem. Soc. 1993, 115, 10581−10590.
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Present Address
†Inorganic Chemistry Laboratory, Department of Chemistry,
University of Oxford, South Parks Road, Oxford OX1 3QR,
U.K.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Spanish Ministry of Science and
Innovation (Projects CTQ2010-17476 and Consolider-Ingenio
■
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́
2010 CSD2007-00006) and the Junta de Andalucıa (Projects
FQM-119 and P09-FQM-4832) is gratefully acknowledged. J.C.
thanks the Spanish Ministry of Education for a research grant
(AP-20080256).
DEDICATION
■
This paper is dedicated to the memory of Professor M. F.
Lappert, in recognition of an outstanding career and of his most
valuable contributions to the development of Inorganic and
Organometallic Chemistry.
(13) Choi, J.; Wang, D. Y.; Kundu, S.; Choliy, Y.; Emge, T. J.; Krogh-
Jespersen, K.; Goldman, A. S. Science 2011, 332, 1545−1548.
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