C-H bond activation reactions with ligand adducts of a β-diiminate iridium dihydride

Article abstract of DOI:10.1021/om050301n

Addition of neutral ligands to the trigonal prismatic iridium tetrahydride (^iprBDI)IrH₄ (BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6- ⁱPr₂C₆H₃) induces reductive elimination of dihydrogen to afford (^iprBDI)IrH₂(L) (L = phosphine, tetrahydrothiophene, cyclohexene) compounds. A combination of solution NMR data and infrared spectroscopic studies have established that the phosphine dihydride compounds and the cyclohexene adduct are best described as classical iridium(III) dihydride or "stretched dihydrogen" complexes. The corresponding tetrahydrothiophene adduct exhibits spectroscopic features and reactivity patterns consistent with increased iridium(I) dihydrogen character. In complexes containing large cone angle phosphines such as PCy₃ and PⁱPr₃, ligand dissociation is facile and isotopic exchange in arene substrates is observed. The experimental data support an Ir(III)-Ir(V) oxidative addition-reductive elimination sequence for C-H bond activation, in contrast to more traditional coordinatively saturated precursors, where Ir(I)-Ir(III) couples are preferred.

Full text of DOI:10.1021/om050301n

Organometallics 2005, 24, 4367-4373
4367
C-H Bond Activation Reactions with Ligand Adducts of
a â-Diiminate Iridium Dihydride
Wesley H. Bernskoetter, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, New York 14853
Received April 18, 2005
Addition of neutral ligands to the trigonal prismatic iridium tetrahydride (iPrBDI)IrH4
(BDI ) ArNC(Me)CH(Me)CNAr, Ar ) 2,6-iPr2C6H3) induces reductive elimination of
dihydrogen to afford (iPrBDI)IrH2(L) (L ) phosphine, tetrahydrothiophene, cyclohexene)
compounds. A combination of solution NMR data and infrared spectroscopic studies have
established that the phosphine dihydride compounds and the cyclohexene adduct are best
described as classical iridium(III) dihydride or “stretched dihydrogen” complexes. The
corresponding tetrahydrothiophene adduct exhibits spectroscopic features and reactivity
patterns consistent with increased iridium(I) dihydrogen character. In complexes containing
large cone angle phosphines such as PCy3 and PiPr3, ligand dissociation is facile and isotopic
exchange in arene substrates is observed. The experimental data support an Ir(III)-Ir(V)
oxidative addition-reductive elimination sequence for C-H bond activation, in contrast to
more traditional coordinatively saturated precursors, where Ir(I)-Ir(III) couples are
preferred.
Introduction
hydroarylation of olefins.10 With respect to the latter
class of reactions, computational studies support C-H
activation through an unusual “oxidative hydrogen
migration” mechanism.11 Substituted â-diiminate an-
ions of the general formula ArNC(Me)CH(Me)CNAr (Ar
) aryl group) are another class of LX ligand that has
found widespread application in transition-metal chem-
istry owing to their ease of synthesis and steric and
electronic modularity.12 With respect to C-H activation
chemistry, Fekl and Goldberg have reported â-diiminate
platinum compounds that promote H/D exchange in
pentane13 as well as stoichiometric alkane dehydroge-
nation.14 We recently reported the synthesis and crys-
tallographic characterization of the 16-electron iridium-
(V) tetrahydride (iPrBDI)IrH4 (1-H4; iPrBDI ) ArNC(Me)-
CH(Me)CNAr, Ar ) 2,6-iPr2C6H3).15 At 23 °C, this
Organometallic and coordination complexes of iridium
have attracted considerable interest, due to their role
in hydrocarbon activation.1,2 Traditionally this chemis-
try has been dominated by iridium(III) complexes with
“L2X-type” ancillary ligands such as cyclopentadienyl,
tris(pyrazolyl)borate,3 and pincer-type4-6 ligands. Pro-
totypical examples include Cp*Ir(PMe3)(R)H7 and (PCP)-
IrH2 (Cp* ) η5-C5Me5, R ) alkyl, PCP ) κ3-2,6-
(R2PCH2)2C6H3),8 which undergo reductive elimination
of alkane or dihydrogen to afford formally 16-electron
iridium(I) intermediates for oxidative addition of a C-H
bond.
Introduction of lower electron count “LX-type” ligand
architectures into iridium chemistry allows the synthe-
sis of coordinatively unsaturated precursors that may
open unique pathways for hydrocarbon activation. Pe-
riana has developed a family of iridium(III) acetyl-
acetonate and related complexes that promote C-H
activation in saturated alkanes9 and are active for the
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Acc. Chem. Res. 1995, 28, 154.
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molecule promotes H/D exchange in deuterated arene
solvents such as benzene-d6 and toluene-d8. Preliminary
mechanistic investigations support the intermediacy of
an iridium(III) dihydride, 1-H2, consistent with an Ir-
(III)-Ir(V) oxidative addition-reductive elimination
sequence for C-H activation and isotopic exchange. This
pathway has been observed with several iridium(V)
(6) Weng, W.; Guo, C.; Moura, C.; Yang, L.; Foxman, B. M.; Ozerov,
O. V. Organometallics 2005, 24, 3487.
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Chem. Soc. 2003, 125, 14292.
10.1021/om050301n CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/29/2005

4368 Organometallics, Vol. 24, No. 18, 2005
Bernskoetter et al.
Figure 1. Possible pathways for C-H activation with â-diiminate iridium dihydride complexes.
tetrahydride precursors supported by monoanionic
ligands.16-19
was isolated in low yield, owing to the high lipophilicity
of the compound. Attempts to prepare a similar adduct
by addition of PtBu3 to 1-H4 produced no reaction.
Sulfur donors such as tetrahydrothiophene (THT)
were also found to promote reductive elimination. Thus,
In this report, we describe the synthesis of a family
of (iPrBDI)IrH2L complexes and examine their ground-
state structures (e.g. classical iridium(III) dihydrides
versus iridium(I) dihydrogen complexes) by NMR and
IR spectroscopy and, in some cases, X-ray diffraction.
These molecules were prepared to examine whether
C-H activation occurs via an Ir(I)-Ir(III) couple ac-
cessed by reductive elimination or by an Ir(III)-Ir(V)
pathway stemming from ligand dissociation (Figure 1).
The relative rate of H/D exchange as a function of donor
ligand has been determined, and preliminary mecha-
nistic investigations into the preferred pathway for C-H
activation have been explored.
(
iPrBDI)IrH2(THT) (1-H2(THT)) was isolated as a bright
yellow powder in excellent yield (eq 2). Performing a
similar procedure with THF produced no reaction.
Addition of cyclohexene to 1-H4 induced H2 loss to afford
the olefin dihydride adduct (iPrBDI)IrH2(η2-C6H10) (1-
H2(Cy)). Cyclohexane has been observed as a byproduct
of excess olefin addition. Regeneration of 1-H4 can be
achieved by addition of H2. Consistent with this obser-
vation, both 1-H2(Cy) and 1-H4 serve as hydrogenation
catalysts for cyclohexene. While 1-H2(Cy) is structurally
related to (MeBDI)IrH2(COE) (MeBDI ) ArNC(Me)CH-
(Me)CNAr, Ar ) 2,6-Me2C6H3, COE ) cyclooctene),20 the
latter compound contains smaller â-diiminate aryl sub-
stituents and does not participate in hydrogenation
chemistry. These results highlight the importance of the
size of the aryl substituents in destabilizing the ground
state of the olefin hydride complexes to impart catalytic
activity.
Results and Discussion
As reported previously,15 addition of PMe3 to 1-H4
results in loss of H2 to yield (iPrBDI)IrH2(PMe3) (1-H2-
(PMe3)). However, 1-H2(PMe3) shows no propensity to
participate in arene C-H activation reactions at ambi-
ent temperature, possibly due to the high affinity of the
small, electron-rich phosphine for the iridium center.
On the basis of these results, larger phosphines were
explored with the goal of destabilizing the 1-H2(PR3)
ground state to facilitate generation of 1-H2 in solution.
Addition of PPh3, PiPr3, and PCy3 to 1-H4 in pentane
at 23 °C induced reductive elimination of H2 and
furnished a series of (iPrBDI)IrH2(PR3) complexes (eq 2).
Each of the â-diiminate iridium dihydride ligand
adducts prepared in this study has been characterized
by multinuclear NMR spectroscopy. Selected spectro-
scopic features are presented in Table 1. Also included
are the Tolman cone angle and electronic parameters
for the free phosphines.21 In each case except for 1-H2-
1
(THT), C2v-symmetric compounds are observed by H
and 13C NMR spectroscopy with equivalent â-diiminate
substituents and iridium hydrides. The THT adduct is
Cs symmetric in benzene-d6 solution, exhibiting in-
equivalent methyl peaks along the ligand backbone as
well as inequivalent aryl groups. All of the compounds
display Ir-H resonances which are shifted substantially
upfield of SiMe4, appearing between -20 and -30 ppm.
For the phosphine adducts, the hydrides appear as
(13) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 6804.
(14) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286.
(15) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem.
Commun. 2004, 764.
Each phosphine adduct was obtained as an orange
powder in moderate yield. One exception, 1-H2(PCy3),
(16) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
(b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000,
122, 1816.
(10) (a) Bhalla, G.; Oxgaard, J.; Goddard, W. A.; Periana, R. A.
Organometallics 2005, 24, 3229. (b) Periana, R. A.; Lui, X. Y.; Bhalla,
G. Chem. Commun. 2002, 3000. (c) Matsumoto, T.; Taube, D. J.;
Periana, R. A.; Taube, H.; Yoshida, H. J. Am. Chem. Soc. 2000, 122,
7414.
(11) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; Periana, R. A. J.
Am. Chem. Soc. 2004, 126, 352.
(12) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev.
2002, 102, 3031.
(17) Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1996, 118, 6068.
(18) Niu, S. Q.; Hall, M. B. J. Am. Chem. Soc. 1999, 121, 3992.
(19) Su, M.-D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119, 5939.
(20) (a) Budzelaar, P. H. M.; de Gelder, R.; Gal, A. Organometallics
1998, 17, 4121. (b) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder,
R.; Smits, J. M. M.; Gal, A. Eur. J. Inorg. Chem. 2000, 753.
(21) Tolman, C. A. Chem. Rev. 1977, 77, 313.

C-H Bond Activation Reactions
Organometallics, Vol. 24, No. 18, 2005 4369
Table 1. Spectroscopic Characterization of 1-H2(L)
Complexes
cone
2
angle ν(CO) δ(Ir-H)b δ(Ir-P)b JP-H ν(Ir-H)c
a
compd
(deg)a (cm-1
)
(ppm)
(ppm)
(Hz)
(cm-1
)
1-H2(PMe3)
1-H2(PPh3)
1-H2(PiPr3)
118
145
160
2064.1 -28.19
2068.1 -26.38
2059.2 -27.28
2056.4 -27.65
-25.46
-48.33
1.81
22.79
14.00
40
33
32
31
2147
2183
2171
2178
2168
2255
1-H2(PCy3)d 170
1-H2(THT)
1-H2(Cy)
-23.09
a Values taken from ref 21. b Spectra recorded in toluene-d8.
c Spectra recorded in KBr. d Spectra recorded in cyclohexane-d12
.
doublets with 2JP-H coupling constants ranging between
31 and 40 Hz. These observations are consistent with
iridium(III) dihydride complexes rather than iridium-
(I) η2-dihydrogen compounds.22 Mixtures of deuterated
isotopologs have also been prepared, but reliable H-D
coupling constants could not be discerned from the
iridium hydride peaks, indicating that the coupling
constants may be below the 0.6 Hz line width resolution.
Further support for the iridium(III) dihydride descriptor
was also provided by infrared spectroscopy, where
strong Ir-H bands are observed between 2147 and 2255
Figure 2. Partially labeled view of the molecular structure
of 1-H2(PiPr3) with ellipsoids at the 30% probability level.
cm-1 22
.
T1(min) values of the iridium dihydrides were also
determined at 500 MHz for several representative
adducts. For the tricyclohexylphosphine derivative 1-H2-
(PCy3), a value of 366 ms was measured at -20 °C,
yielding a computed H-H distance of 1.85 Å,23 consis-
tent with the value of 1.83 Å measured from the solid-
state structure (vide infra). A T1(min) value of 487 ms
was determined at -50 °C for 1-H2(PiPr3), producing
an approximate H-H distance of 2.14 Å, also in agree-
ment with an iridium(III) dihydride structure. A shorter
T1(min) value of 117 ms was determined for 1-H2(Cy)
at -35 °C, but this value is still within the range
typically ascribed to metal hydride rather than η2-
dihydrogen character,21 although classification as a
“stretched dihydrogen complex” may be appropriate.24
For 1-H2(THT), the lowest measured T1 value was 248
ms at -20 °C. However, this value is not a true T1(min),
as broadening of the iridium hydride at temperatures
below -20 °C has made quantitation unreliable.
Figure 3. Partially labeled view of the molecular structure
of 1-H2(PCy3) with ellipsoids at the 30% probability level.
Cyclohexyl substituents and hydrogens, except for the
iridium hydrides, are omitted for clarity.
tures are presented in Figures 2 and 3, respectively,
while selected metrical parameters are reported in Table
2. The data for 1-H2(PCy3) were of sufficient quality
such that the iridium hydrides were located and refined.
The crystals of 1-H2(PiPr3) suffered from twinning and
yielded weaker data, preventing definitive location of
the iridium hydrides. In both structures, the isopropyl
aryl groups are oriented perpendicular to the metal-
ligand plane. The geometry of 1-H2(PCy3) is distorted
between the idealized trigonal-bipyramidal and square-
pyramidal extremes. Unlike 1-H4, where statistically
invariant iridium-nitrogen distances of 2.049(2) and
2.051(2) Å are observed, 1-H2(PCy3) contains a long Ir-
(1)-N(1) distance of 2.159(2) Å and a shorter value of
2.067(2) Å for Ir(1)-N(2). These data are consistent with
the known strong trans influence of the hydride ligand,
producing an elongated iridium-nitrogen bond. This
bond alternation is also present in 1-H2(PiPr3), where
iridium-nitrogen bond distances of 2.151(7) and 2.055-
(7) Å are observed. Although the hydride ligands were
not found in the difference maps from the diffraction
data for 1-H2(PiPr3), the elongated Ir(1)-N(1) distance
offers strong indirect evidence for its location trans to
1-H2(THT) also displays unique dynamic behavior
with free dihydrogen. Exposure of a benzene-d6 solution
of 1-H2(THT) to even trace amounts of H2 resulted in
disappearance of the iridium hydride resonance, con-
sistent with rapid exchange between the hydride ligands
and free dihydrogen. Removal of the free gas regener-
ates the signal. To further investigate this behavior, a
solution of 1-H2(THT) was exposed to 0.5 atm of D2 for
1
2
30 min and the gas removed. Analysis by H and H
NMR spectroscopy revealed complete conversion to 1-D2-
(THT), consistent with rapid substitution. These ob-
servations, in conjunction with the observed Cs symme-
try in solution, suggest increased η2-dihydrogen character
in the ground state of 1-H2(THT).
Both 1-H2(PiPr3) and 1-H2(PCy3) were characterized
by single-crystal X-ray diffraction. The solid-state struc-
(22) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120.
(23) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J.
Am. Chem. Soc. 1991, 113, 4173.
(24) Heinekey, D. M.; Lledo´s, A.; Lluch, J. M. Chem. Soc. Rev. 2004,
33, 175.

4370 Organometallics, Vol. 24, No. 18, 2005
Bernskoetter et al.
Figure 4. Optimized geometries for the isomers of 1-H2(PMe3).
Figure 5. Relative activity of â-diiminate iridium dihydride ligand adducts for H/D exchange in benzene-d6.
Table 2. Selected Bond Distances (Å) and Angles
this position. Significantly, the solid-state structures of
(deg) for 1-H2(PiPr3) and 1-H2(PCy3)
both 1-H2(PCy3) and 1-H2(PiPr3) do not match the high
1-H2(PiPr3)
1-H2(PCy3)
symmetry observed in solution, suggesting a dynamic
process that renders both the ligand substituents and
the iridium hydrides equivalent on the time scale of the
NMR experiment.
Ir(1)-H(1)
Ir(1)-H(2)
Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-P(1)
1.71(4)
1.50(3)
2.159(2)
2.067(2)
2.2706(7)
2.151(7)
2.055(7)
2.189(2)
To gain further insight into the relative energies of
the different isomers of these five-coordinate structures,
full-molecule computational studies were conducted on
1-H2(PMe3). Three idealized geometries were optimized
and are presented in Figure 4. The lowest energy
structure, A, where the â-diiminate nitrogens are es-
sentially trans to a phosphine and a hydride, is in
excellent agreement with the solid-state structure of
1-H2(PCy3). Interchanging the phosphine with one of
the hydrides affords isomer B, with an energy ap-
proximately 2 kcal/mol higher than that of isomer A.
This structure is in good agreement with the observed
solid-state structure of 1-H2(PiPr3). While the kinetic
barriers for isomer interconversion have not been
computed, a fluxional process where A and B exchange
through a turnstile-type mechanism seems likely. Sup-
port for such a dynamic process has been previously
proposed by Budzelaar20 with a related â-diiminate
dihydride olefin complex. The computed kinetic barriers
were in good agreement with a fluxional process on the
H(1)-Ir(1)-P(1)
H(2)-Ir(1)-P(1)
H(2)-Ir(1)-N(1)
N(1)-Ir(1)-P(1)
N(2)-Ir(1)-P(1)
N(2)-Ir(1)-N(1)
78.1(14)
74.5(12)
170.9(120
109.83(6)
162.49(6)
87.47(8)
112.9(2)
136.7(2)
89.8(3)
ligands was also considered and found to be significantly
higher in energy (∼40 kcal/mol) than A and is not
believed to be accessible in solution.
With a series of â-diiminate iridium dihydride ligand
adducts in hand, each complex was assayed for C-H
activation and isotopic exchange reactions with deuter-
ated arenes. In a typical experiment, a 2.0 mM solution
of the desired iridium complex was prepared in benzene-
d6 and the disappearance of the iridium hydride reso-
nance monitored as a function of time by 1H NMR
spectroscopy. The relative rates of benzene H/D ex-
change (statistically corrected) as a function of the donor
are presented in Figure 5. Relative rates were measured
in parallel NMR experiments at 65 °C. The complex
containing the largest cone angle and most reducing
phosphine, 1-H2(PCy3), promotes arene H/D exchange
over the course of hours at 23 °C. Decreasing the size
and nucleophilicity of the phosphine, as in the case of
1-H2(PiPr3), reduces the rate of C-H activation, as only
1
time scale of ambient-temperature H NMR spectros-
copy.
In the lowest energy structure A, an H-H distance
of 1.86 Å is computed and is in excellent agreement with
both the value determined from the T1(min) measure-
ment (1.85 Å) and the solid-state structure (1.83 Å) of
1-H2(PCy3). A third isomer, C, with trans hydride

C-H Bond Activation Reactions
Organometallics, Vol. 24, No. 18, 2005 4371
Figure 6. Proposed mechanism for C-H activation and H/D exchange with 1-H2(L) complexes.
minimal isotopic exchange is observed after several days
at ambient temperature. Rates comparable to those for
1-H2(PCy3) can be achieved by heating the reaction
mixture to 50 °C. All of the other adducts prepared in
this study are inactive for H/D exchange, even after
heating for several days at 50 °C.
Stirring a 15 mM solution of 1-H4 in mesitylene-d12 for
several days at 23 °C induced isotopic exchange in both
the methyl and aryl C-D bonds (eq 4). An approximate
Analysis of 1-H4, 1-H2(PCy3), and 1-H2(PiPr3) by 2H
NMR spectroscopy in C6H6 after isotopic exchange
revealed deuterium incorporation into the methyl sub-
stituents on the isopropyl aryl groups. Trace amounts
of deuterium were also observed in the methine posi-
tions after prolonged reaction times. Evidence for re-
versible cyclometalation of the isopropyl aryl substitu-
ents has been observed previously during isotopic
exchange reactions with 1-H4 and by Goldberg with
related platinum compounds.13 In the present cases,
experiments with more concentrated (∼30 mmol) samples
resulted in deuterium incorporation in the backbone of
the â-diiminate ligand, arising from a bimetallic C-H
activation event (eq 3).25
5:1 preference (statistically corrected) for sp3 methyl
activation over sp2 aryl activation is observed and is
most likely a result of the relative steric accessibility of
the two sites.
Several possible mechanistic pathways for isotopic
exchange involving arene C-H bond activation were
considered. The preferred mechanism that accounts for
the experimental observations is presented in Figure
6. Dissociation of the neutral donor generates a formally
14-electron iridium(III) dihydride, which promotes the
oxidative addition of the aromatic C-H bond. This
pathway is analogous to that proposed for the isotopic
exchange reactions involving 1-H4.15 Reductive elimina-
tion of an arene C-H bond followed by coordination of
the neutral donor (or oxidative addition of H2 in the case
of 1-H4) furnishes the iridium deuteride product.
Competitive with the oxidative addition of the arene
is cyclometalation of an isopropyl aryl substituent. As
in the arene cases, C-H bond reductive elimination
followed by ligand complexation affords the experimen-
tally observed isotopologue. Other pathways involving
initial ligand dissociation followed by σ-bond metathesis
as well as reductive elimination of dihydrogen followed
by oxidative addition of the arene were also considered
but are believed to be less likely, on the basis of the
experimental data presented below.
The observation of H/D exchange along the â-diimi-
nate backbone suggested that 1-H4 may effect C-H
activation in sterically hindered aromatic substrates.
If ligand dissociation to afford 1-H2 were operative,
the rate of isotopic exchange should be inhibited by
added phosphine. Experimentally, these studies have
been complicated by secondary reactions between 1-H2-
(25) Deuteration of the ligand backbone could also occur by transfer
of an iridium deuteride, forming a tautomer of the â-diiminate ligand.
For examples of such behavior, see ref 12. The observed concentration
dependence seems to argue against this possibility.

4372 Organometallics, Vol. 24, No. 18, 2005
Bernskoetter et al.
nitrogen. The M. Braun drybox was equipped with a cold well
designed for freezing samples in liquid nitrogen. Solvents for
air- and moisture-sensitive manipulations were dried and
deoxygenated using literature procedures.26 Deuterated sol-
vents for NMR spectroscopy were purchased from Cambridge
Isotope Laboratories and were distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular
sieves. Hydrogen and argon gas were purchased from Airgas
Inc. and passed through a column containing manganese oxide
supported on vermiculite and 4 Å molecular sieves before
admission to the high-vacuum line. 1-H4 and 1-H2(PMe3) were
prepared as described previously.15 PPh3, PCy3, and PiPr3 were
purchased from Acros and used as received. THT and cyclo-
hexene were purchased from Acros and distilled from CaH2
before use. Mesitylene-d12 was purchased from Acros and
passed over alumina before use.
1H spectra were recorded on Varian Mercury 300 and Inova
400 and 500 spectrometers operating at 299.763, 399.780, and
500.62 MHz, respectively. 13C NMR spectra were obtained on
Varian Mercury 300 and Inova 500 spectrometers operating
at 75.37 and 100.52 MHz, respectively. All chemical shifts are
reported relative to SiMe4 using 1H (residual) chemical shifts
of the solvent as a secondary standard. 2H NMR spectra were
recorded on a Varian Inova 500 spectrometer operating at
76.851 MHz, and the spectra were referenced using an internal
benzene-d6 standard. 31P NMR spectroscopy was carried out
on a Varian Inova 400 spectrometer operating at 161.83 MHz,
and spectra were referenced to an external H3PO4 standard.
Infrared spectroscopy was conducted on a Mattson RS-10500
Research Series FT-IR spectrometer calibrated with a poly-
styrene standard.
Single crystals suitable for X-ray diffraction were coated
with polyisobutylene oil in a drybox and were quickly trans-
ferred to the goniometer head of a Siemens SMART CCD area
detector system equipped with a molybdenum X-ray tube (λ
) 0.710 73 Å). Preliminary data revealed the crystal system.
A hemisphere routine was used for data collection and deter-
mination of lattice constants. The space group was identified,
and the data were processed using the Bruker SAINT program
and corrected for absorption using SADABS. The structures
were solved using direct methods (SHELXS) completed by
subsequent Fourier synthesis and refined by full-matrix least-
squares procedures. Elemental analyses were performed at
Robertson Microlit Laboratories, Inc., in Madison, NJ.
All calculations were performed with the Amsterdam Den-
sity Functional Theory (ADF2003.01) suite of programs.27-29
The calculations included scalar relativistic effects (ZORA)30
for all atoms. The Vosko, Wilk, and Nusair (VWM) local
density approximation,31 Becke’s exchange,32 and Perdew’s
correlation33 were used. The cores of the atoms were frozen
(PCy3) and excess PCy3. Over the course of hours at
23 °C, approximately half of the 1-H2(PCy3) in solution
converts to a new iridium species that exhibits no
iridium hydride resonance and a new singlet in the 31
P
NMR spectrum. Complete conversion to this new com-
pound is observed after several days. While definitive
characterization has not been accomplished, we tenta-
tively assign this species as the iridium(I) bis(phos-
phine) complex 1-(PCy3)2. Despite these limitations,
ligand exchange studies could be performed at low
concentrations of added phosphine. Addition of small
quantities of PCy3 to 1-H2(PiPr3) resulted in phosphine
exchange over the course of days at 23 °C, demonstrat-
ing the lability of the phosphine ligands.
Additional support for the pathway proposed in
Figure 5 is derived from the relative rates of H/D
exchange as a function of donor ligand. Larger and more
basic phosphines such as PCy3 and PiPr3 produce
iridium complexes that are more active for isotopic
exchange than their smaller counterparts such as PMe3.
These data, in combination with the isolability of 1-H4
and computational studies by Hall17 that favor oxidative
addition to Ir(V) over concerted Ir(III) pathways, also
support the proposed mechanism.
In addition, the effect of excess dihydrogen on the rate
of isotopic exchange with benzene-d6 was also explored.
A side-by-side experiment was conducted whereby solu-
tions of 1-H2(PiPr3) were heated to 50 °C in benzene-
d6 with and without excess hydrogen. The rates of
isotopic exchange were nearly identical in both tubes,
suggesting that reductive elimination of dihydrogen to
form the transient, 14-electron iridium(I) species 1-
(PiPr3) is not a productive pathway for C-H activation
and isotopic exchange.
Because 1-H4 was the most active species for H/D
exchange in arene solvents, this compound was also
investigated for isotopic exchange with saturated hy-
drocarbons. Unfortunately, heating 1-H4 to 85 °C in
cyclohexane-d12 for 36 h produced no reaction. Likewise,
thermolysis of 1-D4 at 65 °C for 60 h in the presence of
a large excess (∼10 equiv) of either methane or butane
in benzene-d6 solution produced no observable change.
Extending the reaction time in all cases leads to
decomposition of the iridium complex.
In summary, a series of â-diiminate iridium(III)
dihydride ligand adducts have been prepared and
characterized. These molecules are best characterized
as classical iridium(III) dihydrides or as “stretched
dihydrogen” complexes. Preliminary findings suggest
that ligand dissociation provides access to a reactive
iridium(III) dihydride species that promotes oxidative
addition of arene C-H bonds, even in hindered sub-
strates such as mesitylene and the â-diiminate ligand
backbone. This proposed mechanistic pathway is in
contrast to observations in coordinatively saturated
iridium(III) dihydride precursors supported by L2X-type
ligands, where reductive elimination of dihydrogen to
form an Ir(I) species is preferred.
(26) Pangborn, A.; Giardello, M.; Grubbs, R. H.; Rosen, R.; Timmers,
F. Organometallics 1996, 15, 1518.
(27) te Velde, G.; Bickelahaupt, F. M.; van Gisbergen, S. J. A.;
Fonseca Guerra, C.; Barends, E. J.; Snijders, J. G.; Zielger, T. J.
Comput. Chem. 2001, 22, 931.
(28) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerend, E.
J. Theor. Chem. Acc. 1998, 99, 391.
(29) Baerends, E. J.; Autschbach, J. A.; Berces, A.; Bo, C.; Boerrigter,
P. M.; Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D. E.;
Fan, L.; Fischer, T. H.; Guerra Fonseca, C.; van Gisbergen, S. J. A.;
Groeneveld, J. A.; Gritsenko, O. V.; Gru¨ning, M.; Harris, F. E.; van
den Hoek, P.; Jacobsen, H.; van Kessel, G.; Koostra, F.; van Lenthe,
E.; Osinga, V. P.; Patchkovshii, S.; Philipsen, P. H. T.; Post, D.; Pye,
C. C.; Ravenek, W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.;
Snijders, J. G.; Sola, M.; Swart, M.; Swerhone, D.; te Velde, G.;
Vernooijs, P.; Versluis, L.; Visser, O.; van Wezenbeek, E.; Wiesenekker,
G.; Wolff, S. K.; Woo, T. K.; Ziegler, T. ADF 2002.03; SCM Theoretical
Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, 2002;
http://www.scm.com/.
Experimental Section
(30) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999,
110, 8943.
General Considerations. All air- and moisture-sensitive
manipulations were carried out using standard high-vacuum-
line, Schlenk, or cannula techniques or in an M. Braun inert-
atmosphere drybox containing an atmosphere of purified
(31) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1990, 58, 1200.
(32) Becke, A. D. Phys. Rev. 1988, A38, 2398.
(33) Perdew, J. P. Phys. Rev. 1986, B33, 8822.

C-H Bond Activation Reactions
Organometallics, Vol. 24, No. 18, 2005 4373
up to 1s for C and N, 2p for P, and 4s for Ir. Uncontracted
Slater-type orbitals (STOs) of triple-ú quality with one polar-
ization were employed. This basis set is denoted as TZP in
the ADF program. Each geometry optimization was carried
out without symmetry constraints.
-27.65 (d, 2H, 32 Hz, Ir-H). 31P{1H} NMR (benzene-d6): δ
14.00 (s, Ir-PCy3). 13C NMR (cyclohexane-d12): δ 24.97, 27.90,
28.24 (PCy3), 25.17, (CHMe2), 28.62 (CMe), 31.38 (CHMe2),
101.25 (CH), 123.65 (Ar p-C), 125.49 (Ar m-C), 140.74 (Ar o-C).
IR (KBr): νIr-H 2178 cm-1
.
Preparation of (iPrBDI)IrH2(PPh3) (1-H2(PPh3)). A 20
mL scintillation vial was charged with 0.012 g (0.020 mmol)
of 1-H4 and approximately 10 mL of pentane. To the vial was
added 0.005 g (0.019 mmol) of PPh3, resulting in effervescence
of dihydrogen and an immediate color change from yellow to
orange. The resulting reaction mixture was stirred at ambient
temperature for 16 h, after which time the solution was
concentrated and chilled to -35 °C to yield 0.008 (48%) of 1-H2-
(PPh3) as an orange powder. Anal. Calcd for C47H58IrN2P: C,
Preparation of (iPrBDI)IrH2(η2-C6H10) (1-H2(Cy)). A 25
mL round-bottom flask was charged with 0.035 g (0.057 mmol)
of 1-H4 and approximately 10 mL of pentane. A 180° needle
valve was attached, and 1.1 equiv of dry cyclohexene was
vacuum-transferred onto the solution on the high-vacuum line
using a calibrated gas bulb. The yellow reaction solution was
stirred overnight, and pentane was removed in vacuo to yield
0.024 g (40%) of 1-H2(Cy) as a dark yellow solid. Anal. Calcd
for C35H53IrN2: C, 60.57; H, 7.70; N, 4.04. Found: C, 60.86;
H, 7.53; N, 3.75. 1H NMR (benzene-d6): δ 1.12 (d, 12H, 6 Hz,
CHMe2), 1.49 (br d, 12H, 6 Hz, CHMe2), 1.79 (s, 6H, CH3), 2.23
(m, 4H, Cy H), 3.35 (br, 4H, CHMe2), 3.55 (br, 2H, Cy H), 5.40
(s, 1H, CH), 7.07-7.19 (Ar H), -23.09 (s, 2H, Ir-H). 13C NMR
(benzene-d6): δ 21.69, 24.74, 59.64 (cyclohexene), 24.14, 24.58
(CHMe2), 28.16 (CMe), 32.54 (CHMe2), 101.14 (CH), 123.93 (Ar
p-C), 126.56 (Ar m-C), 141.98 (Ar o-C). IR (KBr): νIr-H 2255
1
64.58; H, 6.69; N, 3.20. Found: C, 64.37; H, 6.42; N, 2.93. H
NMR (toluene-d8): δ 1.01 (d, 12H, 7 Hz, CHMe2), 1.16 (d, 12H,
7 Hz, CHMe2), 1.75 (s, 6H, CH3), 3.45 (sept, 4H, 7 Hz, CHMe2),
5.43 (s, 1H, CH), 6.84-7.21 (Ar H), -26.30 (d, 2H, 30 Hz, Ir-
H). 31P{1H} NMR (toluene-d8): δ 1.81 (s, Ir-PPh3). 31P NMR
(toluene-d8): δ 1.87 (t, 32 Hz). 13C NMR (benzene-d6): δ 24.34,
24.90 (CHMe2), 28.83 (CMe), 102.30 (CH), 123.94 (Ar p-C),
125.82 (Ar m-C), 128.92, 129.24, 134.35 (PPh3), 140.72 (Ar o-C).
cm-1
.
IR (KBr): νIr-H 2183 cm-1
.
Preparation of (iPrBDI)IrH2(SC4H4) (1-H2(THT)). This
molecule was prepared in a manner similar to that for 1-H2-
(Cy) with 0.037 g (0.060 mmol) of 1-H4 and 1.2 equiv of dry
THT, yielding 0.041 g (97%) of 1-H2(THT) as a bright yellow
powder. Anal. Calcd for C33H51IrN2S: C, 56.62; H, 7.34; N,
4.00. Found: C, 56.28; H, 6.97; N, 3.97. 1H NMR (benzene-
d6): δ 1.17 (dd, 12H, 7 Hz, CHMe2), 1.21 (m, 4H, R-H THT),
1.35 (d, 6H, 7 Hz, CHMe2), 1.57 (d, 6H, 7 Hz, CHMe2), 1.74 (s,
3H, CH3), 1.91 (s, 3H, CH3), 2.19 (br t, 4H, â-H THT), 3.48 (m,
4H, CHMe2), 5.38 (s, 1H, CH), 7.09 (br s, 2H, Ar m-H), 7.11
(br s, 2H, Ar m-H), 7.18 (br s, 1H, Ar p-H), 7.20 (br s, 1H, Ar
p-H), -25.46 (s, 2H, Ir-H). 13C NMR (benzene-d8): δ 24.00
(â-C THT), 24.45, 24.48, 24.70, 24.77 (CHMe2), 24.90, 28.16
(CMe), 29.28 (CHMe2), 47.13 (R-C THT), 101.01 (CH), 123.48,
123.74 (Ar p-C), 125.81, 126.13 (Ar m-C), 140.91, 141.90 (Ar
Preparation of (iPrBDI)IrH2(PiPr3) (1-H2(PiPr3)). A 20
mL scintillation vial was charged with 0.065 g (0.105 mmol)
of 1-H4 and approximately 10 mL of pentane. Via microsyringe,
22 µL (0.115 mmol) of PiPr3 was added and an immediate color
change from yellow to orange was observed. The resulting
orange reaction mixture was stirred at ambient temperature
for 16 h, after which time the solution was concentrated and
chilled to -35 °C to yield 0.056 g (69%) of 1-H2(PiPr3) as
orange crystals. Anal. Calcd for C38H64IrN2P: C, 59.11; H, 8.35;
N, 3.63. Found: C, 58.91; H, 8.53; N, 3.38. 1H NMR (benzene-
d6): δ 0.77 (m, 21H, PiPr3), 1.95 (d, 12H, 7 Hz, CHMe2), 1.41
(d, 12H, 7 Hz, CHMe2), 1.75 (s, 6H, CH3), 3.56 (sept, 4H, 7 Hz,
CHMe2), 5.37 (s, 1H, CH), 7.09 (br s, 4H, Ar m-H), 7.16 (br s,
2H, Ar p-H), -27.28 (d, 2H, 32 Hz, Ir-H). 31P{1H} NMR
(benzene-d6): δ 22.79 (s, Ir-PMe3). 13C NMR (benzene-d6): δ
20.19, 28.71 (PCHMe2), 24.83, 25.15 (CHMe2), 25.45 (CHMe2),
27.96 (PCHMe2) 101.66 (CH), 123.87 (p-Ar), 125.58 (m-Ar),
o-C). IR (KBr): νIr-H 2168 cm-1
.
Acknowledgment. We thank Cornell University
and the National Science Foundation (CAREER Award
to P.J.C.) for financial support.
140.72 (o-Ar). IR (KBr): νIr-H 2171 cm-1
.
Preparation of (iPrBDI)IrH2(PCy3) (1-H2(PCy3)). This
molecule was prepared in a manner similar to that for
1-H2(PPh3) with 0.030 g (0.049 mmol) of 1-H4 and 0.016 g
(0.057 mmol) of PCy3, yielding 0.008 g (18%) of 1-H2(PCy3)
Supporting Information Available: Crystal structure
data for 1-H2(PiPr3) and 1-H2(PCy3) as CIF files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
as red crystals. H NMR (cyclohexane-d12): δ 0.99 (m, PCy3),
1.13 (d, 12H, 7 Hz, CHMe2), 1.28 (d, 12H, 7 Hz, CHMe2), 1.56
(m, PCy3) 1.66 (s, 6H, CH3), 3.38 (sept, 4H, 7 Hz, CHMe2), 5.21
(s, 1H, CH), 7.03 (br s, 4H, Ar m-H), 7.21 (br s, 2H, Ar p-H),
OM050301N