Dihydrogen complexes of iridium and rhodium

Article abstract of DOI:10.1021/ic202630x

A series of iridium and rhodium pincer complexes have been synthesized and characterized: [(POCOP)Ir(H)(H₂)] [BAr^f₄] (1-H₃), (POCOP)Rh(H₂) (2-H₂), [(PONOP)Ir(C ₂H₄)] [BAr^f₄] (3-C₂H ₄), [(PONOP)Ir(H)₂)] [BAr^f₄] (3-H₂), [(PONOP)Rh(C₂H₄)] [BAr^f ₄] (4-C₂H₄) and [(PONOP)Rh(H₂)] [BAr^f₄] (4-H₂) (POCOP = I³-C ₆H₃-2,6-[OP(tBu)₂]₂; PONOP = 2,6-(tBu₂PO)₂C₅H₃N; BAr ^f₄ = tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydrogen-metal interaction was probed using NMR spectroscopic studies. Complexes 1-H₃, 2-H₂, and 4-H₂ retain the H-H bond and are classified as η²-dihydrogen adducts. In contrast, complex 3-H₂ is best described as a classical dihydride system. The presence of bound dihydrogen was determined using both T₁ and ¹J_HD coupling values: T₁ = 14 ms, ¹J_HD = 33 Hz for the dihydrogen ligand in 1-H₃, T₁(min) = 23 ms, ¹J_HD = 32 Hz for 2-H ₂, T₁(min) = 873 ms for 3-H₂, T ₁(min) = 33 ms, ¹J_HD = 30.1 Hz for 4-H ₂.

Full text of DOI:10.1021/ic202630x

Article
pubs.acs.org/IC
Dihydrogen Complexes of Iridium and Rhodium
Michael Findlater,† Katherine M. Schultz,‡ Wesley H. Bernskoetter,† Alison Cartwright-Sykes,†
D. Michael Heinekey,*,‡ and Maurice Brookhart*,†
†Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
‡Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
S
* Supporting Information
ABSTRACT: A series of iridium and rhodium pincer complexes have
been synthesized and characterized: [(POCOP)Ir(H)(H2)] [BArf4]
(1-H3), (POCOP)Rh(H2) (2-H2), [(PONOP)Ir(C2H4)] [BArf4]
(3-C2H4), [(PONOP)Ir(H)2)] [BArf4] (3-H2), [(PONOP)Rh(C2H4)]
[BArf4] (4-C2H4) and [(PONOP)Rh(H2)] [BArf4] (4-H2) (POCOP =
κ3-C6H3-2,6-[OP(tBu)2]2; PONOP = 2,6-(tBu2PO)2C5H3N; BArf4
=
tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydro-
gen−metal interaction was probed using NMR spectroscopic studies.
Complexes 1-H3, 2-H2, and 4-H2 retain the H−H bond and are
classified as η2-dihydrogen adducts. In contrast, complex 3-H2 is best
described as a classical dihydride system. The presence of bound
1
dihydrogen was determined using both T1 and JHD coupling values:
T1 = 14 ms, 1JHD = 33 Hz for the dihydrogen ligand in 1-H3, T1(min) =
23 ms, JHD = 32 Hz for 2-H2, T1(min) = 873 ms for 3-H2, T1(min) = 33 ms, JHD = 30.1 Hz for 4-H2.
1
1
The POCOP and PCP ligands (Chart 1) have been
employed by several groups for the preparation of iridium
INTRODUCTION
■
Since the seminal work of Kubas1 and co-workers, the co-
ordination chemistry of the dihydrogen ligand has been studied
extensively and expanded rapidly.2 The bonding features in
such complexes have now been well-described, with electron-
deficient metal centers favoring the support of dihydrogen
ligands since there is a decrease in the electron-density, back-
bonding into the σ* orbital is reduced and prevents rupture of
the H−H bond. Thus, it is unsurprising that the vast majority of
dihydrogen complexes reported to date are cationic and that
neutral dihydrogen complexes remain rare.
Chart 1
The importance of transition metal hydrides in catalysis has
long been recognized.3 More specifically, and in the context of
pincer chemistry, it has been shown that the complex (POCOP)
IrH2 (POCOP = 2,6-bis(di-tert-butylphosphinito) benzene) is
a highly effective catalyst for dehydrogenation of ammonia
borane.4 When this reaction was carried out at ambient
temperatures, the major Ir-containing species in solution was a
tetrahydride comprised of both dihydrogen and dihydride
ligands. A combined experimental and computational study by
Goldman and Heinekey probed the structure of pincer iridium
polyhydride species.5 Recently we have also explored the nature
of iridium and rhodium hydrido−olefin pincer complexes6 as
models of key catalytic intermediates. These studies elucidated
the remarkable influence of coordination geometry upon the
migratory aptitude of the hydride ligand in such systems, which
displayed a significant departure from earlier work in cyclo-
pentadienyl-supported iridium and rhodium hydrido−olefin
complexes.7
complexes which were mainly applied as alkane transfer dehydro-
genation catalysts.8 The dihydride complexes (POCOP)IrH2 and
(PCP)IrH2 are key intermediates in these dehydrogenation
reactions and the synthesis, chemistry, and structures of these
complexes and closely related derivatives have been thoroughly
investigated.4,5,9
Milstein and co-workers have reported a family of iridium
complexes supported by the neutral pincer ligand 2,6-bis
(di-tert-butylphosphinomethyl)pyridine (PNP) (Chart 1).10
Received: December 6, 2011
Published: February 24, 2012
© 2012 American Chemical Society
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These species show a range of intriguing reactivity and thorough
mechanistic investigations indicated that the methylene groups
on the PNP ligand play an important role in the reactivity of the
metal complex. These CH2 groups are susceptible to
deprotonation by external base or the transition metal itself
resulting in dearomatization of the pyridine ligand. More
recently, Nozaki has reported the synthesis of an iridium
trihydride complex supported by the PNP ligand.11 Intuitively,
modification of the PNP scaffold in a vein similar to the PCP
system (replacement of the methylene bridges with O atoms)
may result in different reactivity patterns. Thus, an alteration of
the PNP ligand would yield the pincer ligand, 2,6-bis(di-tert-
butylphosphinito)pyridine (PONOP, Chart 1). This ligand
framework offers the advantage of exchanging the reactive
methylene bridges with an -O- bridging unit which precludes
ligand deprotonation. We have recently reported the use of the
PONOP ligand to support a variety of iridium and rhodium
complexes including an observable methane complex.12
Rhodium(I) dihydrogen complexes supported by the PCP
ligand framework have been prepared previously.15 The
corresponding POCOP-based rhodium complex, 2-H2, can be
accessed directly from the hydrido−chloride complex
(POCOP)Rh(H)(Cl).16 Thus, treatment of (POCOP)Rh(H)-
(Cl) with LiN(SiMe3)2 in benzene under a hydrogen
atmosphere affords the desired complex (eq 2), which may be
isolated as a red−brown powder via filtration and subsequent
solvent evaporation. Complex 2-H2 undergoes hydrogen−
deuterium exchange in C6D6 and thus was characterized in
deuterated cyclohexane to preclude deuterium scrambling. The
solubility of this complex is greatly diminished in cyclohexane.
Initial attempts to prepare cationic (PONOP)IrH2, 3-H2, via
simple chloride abstraction from 3-Cl12a using [Na][BArf4] in
methylene chloride under an H2 atmosphere were frustrated
by generation of unidentified hydride species, as indicated by
1H NMR spectroscopy. However, the cationic 3-C2H4 could be
cleanly generated using similar halide abstraction under an
atmosphere of ethylene. Subsequent hydrogenation of 3-C2H4
in methylene chloride solution under ambient conditions
allowed access to the desired 3-H2 in good yield (Scheme 1)
with concomitant formation of one equivalent of ethane. Both
3-C2H4 and 3-H2 were isolated as orange−brown powders and
characterized by 1H, 13C, and 31P NMR spectroscopy. The
analogous cationic PONOP rhodium dihydride, 4-H2, can be
generated directly from halide abstraction of 4-Cl12b in methylene
chloride solution under an H2 atmosphere, without the need to
generate the rhodium-ethylene complex as an intermediate. How-
ever, the ethylene complex 4-C2H4 can be easily accessed in a
manner analogous to 3-C2H4 and also affords 4-H2 upon treatment
of solutions of 4-C2H4 with an atmosphere of H2. Both routes
afford 4-H2 in near quantitative yield as a brown powder. This com-
plex was also characterized using multinuclear NMR spectroscopy.
Characterization of POCOP/PONOP Iridium and
Rhodium Hydride Complexes by NMR Spectroscopy.
[(POCOP)Ir(H)(H2)][BArf4] (1-H3). At room temperature the
proton NMR spectrum of 1-H3 shows a very broad iridium
hydride resonance centered at −14 ppm integrating for three
protons. At temperatures below ca. 200 K, this resonance
separates into two broad signals at 0.3 and −41.9 ppm in a 2:1
integral ratio, respectively. Attempts to measure the minimum
spin−lattice relaxation time (T1(min)) for each resonance were
unsuccessful, since at the lowest accessible temperatures the
relaxation times were still decreasing. At 150 K in CDCl2F at
Herein we report the synthesis of several new POCOP and
PONOP iridium and rhodium hydride complexes. The nature
of the hydride ligands (classical vs nonclassical) was determined
using established NMR techniques.13,14 The dynamic behavior
of the [(POCOP)−Ir(H)(H2)][BArf4] complex was studied by
1
variable temperature H NMR spectroscopy. These studies
further demonstrate the role of ligand electronic effects, charge,
and metal oxidation state in dictating the structure of metal
hydrogenation products.
RESULTS
■
Syntheses of POCOP/PONOP Iridium and Rhodium
Hydride Complexes. Complex 1-H3 was obtained by
hydrogenation of the cationic propylene complex [(POCOP)-
Ir(H)(C3H6)][BArf4]6 in methylene chloride (eq 1). The
cationic trihydride complex, 1-H3, is cleanly generated with
concomitant production of one equivalent of propane, which
does not interfere with the NMR characterization. While
complex 1-H3 can be quantitatively generated in situ in methy-
lene chloride, fluorobenzene, and CHCl2F, the coordinated
dihydrogen is labile and attempts to isolate this complex have so
far been unsuccessful. Complex 1-H3 has been observed to be
quite stable in fluorobenzene, while slow decomposition occurs
in the chlorinated solvents. Additionally, 1-H3 is inert to oxygen
and stable to mild heating in fluorobenzene.
Scheme 1
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1
500 MHz, the resonance at 0.3 ppm had a T1 of 14 ms, while
the resonance at −41.9 ppm had a T1 of 175 ms.
[(PONOP)IrH2][BArf4] (3-H2). In the room temperature H
NMR spectrum of 3-H2, a single hydridic resonance integrating
to two H atoms was observed as a triplet at −25.07 ppm (JP−H
A value of the one-bond HD coupling (1JHD) was obtained
by examining a partially deuterated sample. To observe HD
coupling arising primarily from the 1-HD2 isotopologue, a
deuterated sample of 1-H3 in fluorobenzene-d5 was prepared by
establishing a 90% D2/10% H2 atmosphere in the headspace of a
J. Young NMR tube. This generated a sample that contained a
mixture of 1-HD2 and 1-H2D in a 12:1 ratio. Large temperature-
independent downfield isotope shifts are noted for the hydride
resonances of the deuterated complexes; these isotopologues are
=
14.6 Hz). The T1(min) for 3-H2 was observed to be 873 ms
(264 K, methylene chloride-d2, 500 MHz). Partially deuterated
3-HD displays a single hydridic resonance at −25.29 ppm in the
1H NMR spectrum with no evidence of HD coupling.
[(PONOP)Rh(H2)][BArf4] (4-H2). A single hydridic resonance
1
was observed in the room temperature H NMR spectrum of
4-H2 as a doublet of triplets (JRh−H = 28 Hz, JP−H = 4.5 Hz), at
−8.26 ppm corresponding to two hydrogen atoms. The T1(min)
for 4-H2 was observed to be 33 ms (methylene chloride-d2,
500 MHz, 264 K). Labeling experiments conducted with
HD generated a complex splitting pattern in the hydride region
(Figure 2) confirming the presence of HD coupling (JH‑D = 30.1 Hz).
1
observable in the H NMR spectrum as two distinct signals
separated by 0.6 ppm. The 1H{31P} NMR signal of 1-HD2, even
when observed at a lower field (200 MHz) and elevated
temperatures (up to 328 K), was too broad to allow for
resolution of HD coupling
In a separate experiment, complex 1-H3 was partially
deuterated by purging both D2 and H2 through the solution.
This generated a sample that contained a mixture of 1-H3,
1-H2D, 1-HD2, and 1-D3 complexes. The 1H NMR signal at 0.3
ppm was too broad and complex to determine a JHD coupling
constant. In the normal 2H NMR spectrum at 173 K, the peaks
representing HD and D−D coordinated to the iridium could be
seen in approximately a 1:1 ratio. Although the HD coupling
could be observed (Figure 1), it was difficult to obtain an
accurate measurement of the HD coupling constant.
1
Figure 2. Partial (hydride region) H NMR spectrum of 4-HD in
CD2Cl2.
The observed splitting pattern arises from a combination of the
hydride coupling with rhodium (JRh−H = 27.5 Hz), deuterium
(JH‑D = 30.1 Hz), and with phosphorus (JP−H = 4.5 Hz).
DISCUSSION
■
POCOP Complexes. (a). [(POCOP)Ir(H)(H2)][BArf4] (1-H3).
Formation of transition-metal dihydrogen complexes and their
structures have been of interest since the first report of a stable
M−(η2-H2) complex by Kubas.1 For complexes with three H
atoms in the coordination sphere of the metal, there are two
possible structures: a classical trihydride complex and a
nonclassical dihydrogen/hydride complex. T1(min) values and
HD coupling constants have been employed to distinguish these
two structures. The structural assignment is made more difficult
by the generally low barriers to site exchange usually observed
when hydride and dihydrogen ligands are adjacent in high
symmetry complexes.17 For example, Oldham and co-workers
reported a cis-dihydrogen/hydride structure with a very low
barrier to site exchange in the case of an iridium tris-
pyrazolylborate complex.18 A similar rapid permutation of
dihydrogen and hydride ligands was described by Parkin and
co-workers in an ansa-molybdenum species.19
2
Figure 1. Overlay of the H NMR spectrum of the dihydrogen region
of a partially deuterated sample of 1-H3 in CH2Cl2. The top spectrum is
a deuterium observed 1D-HMQC. The bottom spectrum is a 2H NMR.
To circumvent this problem and determine the coupling
constant, a one-dimensional HMQC NMR experiment was
performed in which the observed nucleus is deuterium. This
experiment allowed only signals that contain HD coupling to be
1
visible, suppressing all signals that do not have coupling to H
(i.e., the D2 signal). This HMQC experiment is depicted in
Figure 1 where the bottom spectrum shows the normal 2H NMR
spectrum, and the top spectrum shows the result of the one-
dimensional 2H-HMQC experiment, where the doublet
resonance for the partially deuterated complex is clearly
observed. The measured JHD coupling constant was 33 Hz.
(POCOP)Rh(H2) (2-H2). At room temperature a single
1
hydridic resonance is observed in the H NMR spectrum of
Examination of the HD coupling constants in a partially
2-H2 at −3.00 ppm, corresponding to two hydrogen atoms.
The T1(min) for 2-H2 was observed at 222 K as 23 ms in
methylene chloride-d2 (500 MHz). Above 283 K this resonance
appeared as a broad doublet. From 283 K down to 223 K,
coupling to phosphorus atoms was resolved; below 223 K this
coupling was no longer resolved. Partially deuterated 2-HD was
generated in a manner similar to 2-H2 using HD gas instead of
deuterated sample provides a way to differentiate the structures.
1
The JH‑D coupling constant for free HD is 43 Hz. When
dihydrogen coordinates to a metal, the 1JH‑D coupling constant is
diminished, usually to 25−35 Hz (e.g., 33.5 Hz in Kubas’
complex).1,20 The 2JH‑D coupling constant between two terminal
hydrides is much lower and tends to be ca. 1 Hz or less. For
example, the classical anionic trihydride [(POCOP)IrH3][Na]
exhibits 2JH−H = 4.3 Hz, so 2JH‑D will be 0.7 Hz for this species.9m
1
H2. The observed JHD coupling constant was 32 Hz.
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For comparison to this anionic species, we were interested
in generating the cationic iridium “trihydride” complex
[(POCOP)Ir(H3)][BArf4], 1-H3, and determining if the
structure is a classical or nonclassical trihydride (Figure 3).
Scheme 2
Figure 3. Possible structures for 1-H3.
The three hydrogen atoms in 1-H3 undergo rapid intra-
molecular scrambling, leading to a single broad resonance at
ambient temperature. At low temperature, the 1H NMR
spectrum decoalesces to two resonances with intensity ratios
of 2:1 at 0.3 and −41.9 ppm. The very high field signal at
−41.9 ppm is consistent with a hydride ligand trans to a vacant
H atom site exchange in complex 1-H3 is significantly greater
than in [TpIr(PMe3)(H2)(H)]+, which is a closely related cis-
dihydrogen/hydride complex of iridium.18
2
site. Low-temperature H NMR spectra of deuterium-labeled
1
samples reveals JH‑D = 33 Hz in the low field resonance,
consistent with the presence of a dihydrogen ligand.
(b). (POCOP)Rh(H2) (2-H2). Rather surprisingly, given the
important role that rhodium hydrides play in homogeneous
catalysis, relatively few rhodium dihydrogen complexes have been
documented.15,22 In contrast to the iridium POCOP complex,
(POCOP)IrH2, the rhodium complex 2-H2 displays both
To access a temperature cold enough to restrict hydrogen site
exchange and the resulting relaxation averaging, it was necessary
to employ a Freon solvent (CDCl2F). At the lowest accessible
temperature (150 K, 500 MHz) the extremely short value of
T1 = 14 ms supports the assignment of the downfield resonance
at 0.3 ppm as a dihydrogen ligand. The value of T1 = 175 ms for
the upfield resonance at −41.9 ppm is in the expected range for
a hydride ligand. Though a direct calculation of the HH distance
is not possible without a minimum T1, the estimated values are
consistent with formulation of 1-H3 as a cationic dihydrogen/
hydride complex.
1
T1(min) and JH‑D coupling values consistent with a dihydrogen
structure. Using the analysis of Halpern,13 the minimum
relaxation time of 23 ms corresponds to an H−H distance of
0.87 Å assuming fast rotation of the bound dihydrogen ligand.
Further confirmation of the dihydrogen formulation of complex
1
2-H2 was provided by the presence of a JH‑D coupling in the
HD isotopomer of 32 Hz. Using the previously developed
correlation between 1JH‑D and H−H distance, the H−H distance
is calculated to be 0.88 Å.14 This observation confirms the
assumption that the H2 ligand is rotating rapidly compared to
the rate of molecular tumbling. These observations suggest
that the structure of 2-H2 is likely very similar to the previously
reported dihydrogen complex (PCP)Rh(H2).15
The exchange rate for the dihydrogen/hydride complex was
measured for both 1-D3 and 1-H3 complexes employing line-
broadening techniques. For 1-D3, the line-width of the η2-D2
resonance was measured in the temperature range of 184−208
K. In the case of 1-H3, the rate was calculated by measuring the
line-broadening of the hydride signal from 175 to 197 K. The
rate of exchange in the triproteo-complex corresponded to
ΔG‡ = 9.1 kcal/mol, while the rate in the trideuterated-complex
corresponded to ΔG‡ = 9.8 kcal/mol (see Supporting
Information). Based on these data, an isotope effect of kH/kD ≈
6 was estimated. The existence of an isotope effect, in which the
energy barrier for the deuterated-complex is higher than the
triproteo-complex, was expected since the breaking of the H−H
(or D−D) bond in the η2-H2 (η2-D2) is probably involved in the
transition state for site exchange.
Two possible mechanisms for this hydrogen exchange are
depicted in Scheme 2. In the first mechanism, A, the dihydrogen
oxidatively adds to the iridium center forming a trihydride
intermediate or transition state (formally IrV). Subsequent
reductive coupling of H2 can lead to the starting complex, where
no site exchange has occurred, or the complex in which Ha,
originally the terminal hydride, has been incorporated into the
η2-H2 site. Mechanism B involves a proton transfer from the
dihydrogen to the hydride through a transition state in which
there is partial and symmetrical bonding of the migrating
hydrogen to both remaining hydrogens.21 No trihydride
intermediate is required. Both of these mechanisms require
breaking of the H−H bond, so would be expected to lead to
a KIE. It is interesting to note that the activation energy for
PONOP Complexes. (a). [(PONOP)Ir(H)2][BArf4] (3-H2).
As stated previously, the majority of late transition metal
dihydrogen complexes are cationic. Thus, we chose to explore
cationic iridium centers (see 1-H3 and discussion thereof) in
the hopes of suppressing oxidative addition of hydrogen at the
group 9 element center. Utilizing the neutral PONOP ligand,
we prepared in a two-step procedure the cationic iridium
complex 3-H2. This complex was subjected to analogous NMR
1
studies as described above. The T1(min) as determined by H
NMR spectroscopy was found to be 873 ms (264 K) and no
1JH‑D coupling could be detected when HD gas was used in
place of dihydrogen in the hydrogenation of 3-C2H4. These
observations lead us to conclude that, even with a formal
positive charge, the iridium center undergoes oxidation to the
+3 state and a classical dihydride complex results.
(b). [(PONOP)Rh(H2)][BArf4] (4-H2). Based upon the η2-
binding of dihydrogen in the neutral 2-H2, we assumed that a
similar η2-binding motif would be observed in the cationic 4-H2
complex. On the basis of both T1(min) and 1JH‑D coupling values,
we attribute the hydride signals in 4-H2 to an η2-dihydrogen
ligand in a formally Rh(I) metal complex. The T1(min) of 4-H2
1
was determined to be 33 ms (264K) and the JH‑D coupling
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value 30.1 Hz, which correspond to H−H bond lengths of 0.93
and 0.91 Å, respectively. Despite the formal positive charge at the
rhodium center of 4-H2 an elongation, rather than the expected
shortening, of the calculated H−H bond in the dihydrogen ligand
is observed in comparison with neutral 2-H2. This likely arises
due to the trans-influence of the nitrogen of the tightly bound
pyridine of the PONOP ligand. Complexes 2-H2 and 4-H2 are
novel, as very few rhodium dihydrogen complexes are known,
especially in a four-coordinate geometry. 4-H2 is distinguished
again by its T1(min) value which indicates fast rotation of the
η2‑H2 ligand.
EXPERIMENTAL SECTION
■
All manipulations were carried out using standard Schlenk, high-
vacuum, and glovebox techniques. Argon was purified by passage
through columns of BASF R3-11 (chemalog) and 4 Å molecular sieves.
Pentane and methylene chloride were passed through columns of
activated alumina and deoxygenated by purging with N2. Benzene was
dried over 4 Å molecular sieves and degassed to remove both oxygen
and nitrogen. NMR spectra were recorded on Bruker DRX 400, AMX
300 and 500 MHz instruments and are referenced to residual protio
solvent peaks. 31P chemical shifts are referenced to an external H3PO4
standard. Since there is a strong 31P−31P coupling in the pincer
complexes, many of the 1H and 13C signals exhibit virtual coupling and
appear as triplets. These are specified as vt with the apparent coupling
simply noted as J. All reagents, unless otherwise noted, were purchased
from Sigma-Aldrich and used without further purification. Propylene
was used as received from National Specialty Gases of Durham, NC.
Ethylene was purchased from Matheson. The syntheses of the
phosphinite complexes, (POCOP)Ir(H)(Cl), (POCOP)Rh(H)(Cl),
[(POCOP)Ir(H)(C3H6)][BArf4], (PONOP)Ir(Cl), and (PONOP)Rh-
(Cl) have been previously described in the literature. The abbreviation
of BArf4 is used to represent the counterion tetrakis(3,5-trifluorome-
CONCLUSION
■
A series of iridium and rhodium hydride complexes, supported
by both anionic and neutral tridentate pincer ligands, have been
developed as a platform for the examination of the reaction of
dihydrogen with d8 Ir(I) and Rh(I) complexes. The character of
the metal-bound hydrides (classical vs η2-ligation) was
established using T1(min) and 1JH‑D coupling values. Preparation
of the cationic [(POCOP)Ir(H)(H2)][BArf4] (1-H3), which was
found to exhibit a dihydrogen-hydride structure, affirmed the
important role electron density plays in the rupture of the H−H
bond in a coordinated dihydrogen ligand. As expected, based
upon the stability of the Rh(I) oxidation state, (POCOP)Rh(H2)
(2-H2) is an η2-dihydrogen complex, as determined by NMR
spectroscopy. Surprisingly, given the important role of rhodium
in homogeneous catalysis, complex 2-H2 represents a rare
example of a neutral rhodium dihydrogen complex.
1
thylphenyl)borate. The H and 13C spectral data for BArf4 in CD2Cl2
are the same for all complexes listed and therefore not reported in the
characterizations below. BArf4: 1H NMR (CD2Cl2): δ 7.72 (s, 8H, Ho),
7.56 (s, 4H, Hp). 13C{1H} NMR (CD2Cl2): δ 162.2 (q, JC−B = 37.4 Hz,
Cispo), 135.2 (Co), 129.3 (q, JC−F = 31.3 Hz, Cm), 125.0 (q, JC−F = 272.5,
CF3), 117.9 (Cp). Elemental analyses were performed by Robertson
Microlit Laboratories Inc. in Madison, NJ or Atlantic Microlab Inc. in
Norcross, GA.25
In Situ Generation. [(POCOP)IrH(H2)][BArf4] (1-H3). A solution
of [(POCOP)Ir(H)(C3H6)][BArf4] (10 mg, 0.0066 mmol) in
methylene chloride-d2 (0.5 mL) was added to a medium-walled
screw cap NMR tube. H2 was purged through solution at
Cationic iridium (3-H2) and rhodium (4-H2) complexes of
the neutral PONOP ligand were prepared to examine the effect
of electron density on the oxidative addition of dihydrogen. Our
results are consistent with our previous studies of related
iridium12a and rhodium12b PONOP complexes. Thus, the
iridium complex (3-H2) was found to undergo oxidative addition
where the rhodium congener (4-H2) did not. We attribute this
behavior to the greater ease of reduction of rhodium relative to
iridium.
1
−78 °C, shaking the tube occasionally for 2 min. H NMR
(500 MHz, CD2Cl2, 173 K): δ 7.16 (bs, 1H, 1-H), 6.82
(d, JH−H = 6.0 Hz, 2H, 2-H), 1.17 (m, 36H, P(tBu)2), 0.3
3
(b, 2H, H2), −41.88 (t, JP−H = 12.1 Hz, 1H, IrH). At 295 K, the
hydride resonance is a broad singlet at −14 ppm. 31P{1H} NMR
(202 MHz, CD2Cl2, 173 K): δ 194.7.
[(POCOP)Ir(D)(D2)][BArf4] 1-D3. [(POCOP)Ir(D)(D2)] [BArf4] was
generated using the same procedure as 1-H3 with the addition of
deuterium gas instead of hydrogen gas.
[(POCOP)Ir(H/D)(H2/HD/D2)][BArf4]. The partially deuterated sam-
ple was generated as above, with appropriate mixtures of H2 and D2.
(POCOP)Rh(H2) (2-H2). The dihydride can be generated directly
from the hydrido-chloride16 by reaction with (Et2O)Li−N(SiMe3)2
under an atmosphere of hydrogen. 1H NMR (500 MHz, C6D12):
δ 6.65 (t, 3JH−H = 8.0 Hz, 1H, 1-H), 6.35 (d, 3JH−H = 8.0 Hz, 2H, 2-H),
1.21 (vt, J = 6.8 Hz, 36H, P(tBu)2), −3.27 (br d, JRh−H = 18.5 Hz, 2H,
It is tempting to compare the dihydrogen σ-complex 4-H2
with the recently reported methane σ-complex [PONOP-
Rh(CH4)][BArf4].12b The strong, nonpolar, C−H σ bond is a
weak donor and steric interactions between an alkyl group and
the metal center also hamper efforts to isolate metal−alkane
complexes. Thus, dihydrogen σ-complexes are significantly more
well-studied.1,2,20,23 An indication of the strength of interaction
of the dihydrogen and methane moieties may be garnered by
examination of the relative extent of Rh−H coupling in these
Rh(H2)). 31P{1H} NMR (162 MHz, CD2Cl2): δ 219.10 (d, JRh−P
=
167 Hz). 13C{1H} NMR (125.8 MHz, C6D12): δ 168.80 (Ar), 126.34
(Ar), 104.6 (Ar), 39.10 (C(CH3)3), 29.05 (C(CH3)3). One Ar−C
resonance not detected.
1
complexes. The observed JRh−H coupling constant is 27.6 and
[(PONOP)Ir(C2H4)][BArf4] (3-C2H4). A heavy-walled glass reaction
vessel was charged with 0.020 g (0.032 mmol) of (PONOP)Ir−Cl,
0.026 g (0.029 mmol) of NaBArf4, and 10 mL of methylene chloride.
On a vacuum line, 1 atm of ethylene was added at ambient temp-
erature, and the orange solution was stirred for 3 days. The volatiles
were removed in vacuo, the residue was extracted in a minimal amount
of methylene chloride (ca. 1 mL), filtered through Celite, and layered
with pentane at −35 °C to afford 0.031 g (66%) of 3-C2H4 as orange
needles. Calcd for C55H55IrNP2O2BF24: C, 44.55; H, 3.74; N, 0.94.
6.3 Hz in 4-H2 and [PONOP-Rh(CH4)][BArf4], respectively.
1
However, the JRh−H observed in the methane complex is an
average of coupling to the “coordinated” H and the remaining
“terminal” H’s since methane tumbles rapidly in the coordination
sphere. If the 3 terminal H’s show weak coupling (ca. 0) then the
coupling of the coordinated H is 4 × 6.3, or about 25 Hz, close
to the value of 28 Hz in 4-H2. A closely related PNP-Rh(H-SiR3)
has been reported by Ozerov and co-workers (PNP = bis
1
Found: C, 44.28; H, 3.60; N, 0.90. H NMR (500 MHz, CD2Cl2):
(o-diisopropylphosphinophenyl)amide).24 The JRh−H coupling
1
3
3
δ 7.98 (t, JH−H = 8 Hz, 1H), 6.97 (d, JH−H = 8.0 Hz, 2H), 3.42 (vt,
J = 4 Hz, 4H, C2H4), 1.38 (vt, 36H, J = 6.5 Hz, P(tBu)2). 31P{1H}
NMR (162 MHz, CD2Cl2): δ 188.8. 13C{1H} NMR (125.8 MHz,
CD2Cl2): δ 28.1 (vt, 3.3 Hz, P−C(CH3)3), 39.1 (C2H4), 42.9 (vt, 9.0
Hz, P-C(CH3)3), 103.1, 146.2, 161.5 (C6H3N).
constant in this silane σ-complex was found to be 24 Hz (room
temperature in C6D6) and is consistent with the values of both 4-
H2 and [PONOP-Rh(CH4)][BArf4], indicating similar
σ-interactions are present in all three σ-complexes.
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Inorganic Chemistry
Article
[(PONOP)Ir(H)2][BArf4] (3-H2). A heavy-walled glass reaction vessel
was charged with 0.050 g (0.034 mmol) of 3-C2H4 and 10 mL of
methylene chloride. On a vacuum line, the solution was frozen at
−196 °C and 1 atm of dihydrogen was admitted to the vessel. The
solution was warmed to ambient temperature and stirred for 8 h. The
volatiles were removed in vacuo, the residue extracted in a minimal
amount of methylene chloride (ca. 1 mL), filtered through Celite, and
layered with pentane at −35 °C to afford 0.035 g (71%) of 3-H2 as
(4) (a) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.;
Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048−12049.
(b) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.;
Linehan, J. C. Inorg. Chem. 2008, 47, 8583−8585.
(5) Hebden, T. J.; Goldberg, K. I.; Heinekey, D. M.; Zhang, X.;
Emge, T. J.; Goldman, A. S.; Krogh-Jesperon, K. Inorg. Chem. 2010, 49,
1733−1742.
(6) Findlater, M.; Cartwright-Sykes, A.; White, P. S.; Schauer, C. K.;
Brookhart, M. J. Am. Chem. Soc. 2011, 133, 12274−12284.
(7) (a) Brookhart, M.; Lincoln, D. M. J. Am. Chem. Soc. 1988, 110,
8719−8720. (b) Brookhart, M.; Hauptmann, E.; Lincoln, D. M. J. Am.
Chem. Soc. 1992, 114, 10394−10401.
1
3
orange blocks. H NMR (400 MHz, C6D5Cl): δ 8.00 (t, JH−H = 8 Hz,
3
1H), 7.09 (d, JH−H = 8.4 Hz, 2H), 1.34 (vt, 36H, J = 8 Hz, P(tBu)2),
−25.07 (vt, J = 12 Hz, 2H, IrH). 31P{1H} NMR (162 MHz, C6D5Cl):
δ 206.8. 13C{1H} NMR (125.8 MHz, C6D5Cl): δ 26.8 (vt, 3.1 Hz, P−
C(CH3)3), 41.2 (vt, 10.7 Hz, P-C(CH3)3), 103.3, 146.1, 163.3 (C6H3N).
[(PONOP)Rh(C2H4)][BArf4] (4-C2H4). A heavy-walled glass reaction
vessel was charged with 0.050 g (0.093 mmol) of (PONOP)Rh−Cl,
0.083 g (0.093 mmol) of NaBArf4, and 10 mL of methylene chloride.
On a vacuum line, 1 atm of ethylene was added at ambient temp-
erature, and the yellow solution was stirred for 8 h. The volatiles were
removed in vacuo, the residue was extracted in a minimal amount of
methylene chloride (ca. 1 mL), filtered through Celite, and layered
with pentane at −35 °C to afford 0.059 g (45%) of 4-C2H4 as yellow
needles. Calcd for C55H55RhNP2O2BF24: C, 47.40; H, 3.98; N, 1.01.
Found: C, 47.17; H, 3.75; N, 1.01. 1H NMR (23 °C, CD2Cl2): δ 1.36
(vt, 7.6 Hz, 36H, P−C(CH3)3), 3.71 (m, 4H, C2H4), 6.88 (d, 8.4 Hz,
2H, m-C6H3N), 7.88 (t, 8.4 Hz, 1H, p-C6H3N). 31P{1H} NMR (23 °C,
CD2Cl2): δ 211.8 (d, 124 Hz). 13C{1H} NMR (23 °C, CD2Cl2):
δ 27.9 (vt, 4.0 Hz, P−C(CH3)3), 42.0 (m, P-C(CH3)3), 52.2 (C2H4),
103.4, 145.6, 163.9 (C6H3N).
(8) (a) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. J. Am.
̈
Chem. Soc. 2004, 126, 1804−1811. (b) Gottker-Schnetmann, I.; White,
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P. S.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330−9338.
(c) Bailey, B. C.; Schrock, R. R.; Kundu, S.; Goldman, A. S.; Huang,
Z.; Brookhart, M. Organometallics 2009, 28, 355. (d) Goldman, A. S.;
Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science
2006, 312, 257. (e) Gutpa, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.;
Jensen, C. M. Chem. Commun. 1996, 2083. (f) Gupta, M.; Hagen, C.;
Kaska, C. W.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997,
119, 840. (g) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics
1998, 17, 1. (h) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman,
A. S. J. Am. Chem. Soc. 1999, 121, 4086. (i) Liu, F.; Goldman, A. S.
Chem. Commun. 1999, 655. (j) Morales-Morales, D.; Lee, D. W.;
Wang, Z.; Jensen, C. M. Organometallics 2001, 20, 1144. (k) Krogh-
Jespersen, K.; Czerw, M.; Kanzelberger, M.; Goldman, A. S. J. Chem.
Inf. Comput. Sci. 2001, 41, 56. (l) Krogh-Jespersen, K.; Czerw, M.;
Zhu, K.; Singh, B.; Kanzelberger, M.; Darji, N.; Achord, P. D.;
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M. B. Angew. Chem., Int. Ed. 2001, 40, 3596. (e) Kanzelberger, M.;
Singh, B.; Czerw, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am.
Chem. Soc. 2000, 122, 11017. (f) Wang, Z.; Tonks, I.; Belli, J.; Jensen,
C. M. J. Organomet. Chem. 2009, 694, 2854. (g) Staubitz, A.; Sloan, M.
E.; Robertson, A. P. M.; Friedrich, A.; Schneider, S.; Gates, P. J.;
Schmedt auf der Gunne, J.; Manners, I. J. Am. Chem. Soc. 2010, 132,
13332. (h) Yang, J.; Brookhart, M. Adv. Synth. Catal. 2009, 351, 175.
(i) Staubitz, A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed. 2008,
47, 6212. (j) Hebden, T. J.; Denney, M. C.; Pons, V.; Piccoli, P. M. B.;
Koetzle, T. F.; Schultz, A. J.; Kaminsky, W.; Goldberg, K. I.; Heinekey,
D. M. J. Am. Chem. Soc. 2008, 130, 10812. (k) Bernskoetter, W. H.;
Brookhart, M. Organometallics 2008, 27, 2036. (l) Yang, J.; Brookhart,
[(PONOP)Rh(H2)][BArf4] (4-H2). 4-H2 was prepared in a fashion
1
similar to 3-H2 in 65% yield. H NMR (400 MHz, CD2Cl2): δ 7.90
(t, 3JH−H = 8 Hz, 1H), 6.91 (d, 3JH−H = 8.4 Hz, 2H), 1.32 (vt, 36H, J =
8 Hz, P(tBu)2), −8.26 (dt, JRh−H = 27.6 Hz, JP−H = 4.4 Hz, 2H, RhH).
31P{1H} NMR (162 MHz, CD2Cl2): δ 224.6 (d, JRh−P = 126.4 Hz).
13C{1H} NMR (125.8 MHz, CD2Cl2): δ 27.8 (vt, 4.0 Hz, P−C(CH3)3),
40.4 (m, P-C(CH3)3), 103.9, 146.5 (C6H3N), one C6H3N signal not
located.
ASSOCIATED CONTENT
■
S
* Supporting Information
Calculations of exchange rates of D and H in 1-H3 and kH/kD.
Detailed calculation of H−H distances in dihydrogen
complexes. Sample NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
M. J. Am. Chem. Soc. 2007, 129, 12656. (m) Gottker-Schnetmann, I.;
̈
*E-mail: mbrookhart@unc.edu (M.B.), heinekey@chem.
washington.edu (D.M.H.).
Heinekey, D. M.; Brookhart, M. J. Am. Chem. Soc. 2006, 128, 17114.
(10) See for example: (a) Hermann, D.; Gandelman, M.; Rozenberg,
H.; Shimon, L. J. W.; Milstein, D. Organometallics 2002, 21, 812−818.
(b) Feller, M.; Karton, A.; Leitus, G.; Martin, J. M. L.; Milstein, D.
J. Am. Chem. Soc. 2006, 128, 12400−12401. (c) Iron, M. A.; Ben-Ari,
E.; Cohen, R.; Milstein, D. Dalton Trans. 2009, 9433−9439.
(11) Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14168−
14169.
Author Contributions
All authors have given approval to the final version of the
manuscript.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(12) (a) Bernskoetter, W. H.; Hanson, S. K.; Buzak, S. K.; Davis, Z.;
White, P. S.; Swartz, R.; Goldberg, K. I.; Brookhart, M. J. Am. Chem.
Soc. 2009, 131, 8603−8613. (b) Bernskoetter, W. H.; Schauer, C. K.;
Goldberg, K. I.; Brookhart, M. Science 2009, 326, 553−556.
(c) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem.
Soc. 2010, 132, 4534.
■
We gratefully acknowledge the financial support of the NSF as
part of the Center for Enabling New Technologies through
Catalysis (CENTC, Grant CHE-0650456).
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