Synthesis of a β-diiminate iridium tetrahydride for arene C-H bond activationt

Article abstract of DOI:10.1039/b315817a

The preparation of the iridium tetrahydride (^iPrBDI)IrH ₄ {BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-ⁱPr ₂C₆H₃} and its activity in the catalytic C-H activation of arenes is described.

Full text of DOI:10.1039/b315817a

Synthesis of a b-diiminate iridium tetrahydride for arene C–H bond
activation†
Wesley H. Bernskoetter, Emil Lobkovsky and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY, USA.
E-mail: pc92@cornell.edu
Received (in West Lafayette, IN, USA) 4th December 2003, Accepted 4th February 2004
First published as an Advance Article on the web 27th February 2004
The preparation of the iridium tetrahydride (iPrBDI)IrH4 {BDI
ArNC(Me)CH(Me)CNAr, Ar
2,6-iPr2C6H3} and its
activity in the catalytic C–H activation of arenes is described.
cm21 in benzene. Both the 1H and 13C NMR spectra of 1 in
benzene-d6 indicate Cs symmetry, consistent with the solid-state
structure.
=
=
Organometallic complexes of iridium have shown considerable
promise for the selective activation of carbon–hydrogen bonds.1
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3
Iridium ‘pincer’ complexes, (PCP)IrH2 {PCP
=
h -2,6-
(R2PCH2)2C6H3}, are particularly noteworthy given their high
thermal stability and efficiency in catalytic alkane2 and amine3
dehydrogenation. As part of our program aimed at developing
synthetic methods involving both C–C and C–H cleavage,4 we are
developing new platforms for iridium hydride chemistry with the
goal of discovering efficient and selective catalysts.
Treatment of a pentane solution of 1 with 4 atm of H2 resulted in
rapid hydrogenation of the COE ligand and allowed isolation of the
iridium tetrahydride complex, (iPrBDI)IrH4 (2) as bright yellow
blocks (eqn. 2). In toluene-d8, 2 displays C2v symmetry and a single
Monoanionic b-diiminate ligands are attractive ancillary ligands
for catalytic C–H bond activation given their ease of synthesis,
steric and electronic tunability and conceptual relationship to the
well-studied pincer ligands.5 Fekl and Goldberg have recently
demonstrated the utility of b-diiminate platinum compounds for
promoting facile H/D exchange in pentane,6 as well as alkane
dehydrogenation.7 b-Diiminate iridium chemistry has been pio-
neered by Budzelaar et al., who reported the synthesis of
1
hydride resonance centered at 216.99 ppm is observed in its H
NMR spectrum. This peak is unchanged upon cooling the solution
to 280 °C. Whereas the smaller (MeBDI)Ir(COE) complex
oxidatively adds H2 to form a relatively unreactive olefin dihydride
complex,8 incorporation of larger isopropyl aryl substituents allows
synthesis of 2, demonstrating the profound impact of subtle
changes in the ligand architecture on the reactivity of iridium olefin
complexes.
(
MeBDI)Ir(COE) {BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-
Me2C6H3, COE = cyclooctene}, a molecule that adopts an Ir(III
)
allyl hydride ground-state structure.8 Addition of dihydrogen to this
complex yields a stable iridium(III) olefin dihydride that is
unreactive toward excess alkene or H2.9
We hypothesized that coordination of larger b-diiminate ligands
to iridium may facilitate olefin dissociation, engendering more
reactive iridium hydride species. The addition of one such example,
Li[iPrBDI]·Et2O,10 to [Ir(COE)2Cl]2 in the presence of N2 afforded
bright yellow crystals identified as (iPrBDI)Ir(COE)N2 (1; eqn. 1).11
X-Ray diffraction studies‡ (Fig. 1) revealed a square-planar iridium
center with the b-diiminate aryl groups and the disordered COE
ligand oriented perpendicular to the square plane. The N–N bond
length of 1.107(4) Å and the strong N–N stretch at 2126 cm21 in the
solid-state infrared spectrum are consistent with a weakly activated
N2 ligand. Curiously, no N–N stretch is observed in the solution IR
spectrum in pentane, although a strong peak is observed at 2122
Fig. 2 Molecular structure of (iPrBDI)IrH4 (2) plotted with 30% probability
ellipsoids.
A combination of X-ray diffraction, computational and solution
spectroscopic studies were carried out to elucidate the ground-state
structure of 2. Possibilities include a classical iridium(
V
) tetra-
2
hydride or rapidly equilibrating Ir(III) or Ir(
I) h -dihydrogen
complexes.12 Single crystal X-ray diffraction‡ on bright yellow
crystals of 2 revealed a trigonal prismatic iridium tetrahydride in
the solid state (Fig. 2). The data were of sufficient quality that each
hydrogen atom, including the iridium hydrides, could be located in
the difference map and isotropically refined. The iridium hydride
distances are statistically invariant and range between 1.45–1.48 Å,
slightly shorter, as expected, than typical Ir–H distances of
1.58–1.59 Å determined by neutron diffraction.13 The closest
H…H contacts are 1.25 and 1.30 Å, suggesting a classical iridium
tetrahydride in the solid state.13
Fig. 1 Molecular structure of (iPrBDI)Ir(COE)N2 (1) plotted with 30%
probability ellipsoids.
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† Electronic supplementary information (ESI) available: full experimental
and computational details, plus X-ray crystallographic data for 1-N2 and 2.
See http://www.rsc.org/suppdata/cc/b3/b315817a/
764
C h e m . C o m m u n . , 2 0 0 4 , 7 6 4 – 7 6 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Geometry optimizations on 2 were carried out using the ADF
program suite (TZ2P/ZORA).† The computational results are in
good agreement with the solid-state structure, predicting a trigonal
prismatic iridium tetrahydride with isopropyl aryl groups flanking
the iridium–BDI plane. The computed Ir–H distances are slightly
longer ( ~ 0.1 Å) than the solid-state structure, highlighting the
underestimation of the Ir–H distances by the X-ray diffraction
experiment.
methyl group (2 : 1 statistically corrected), consistent with
precoordination of the arene.17 Within the aromatic positions, a
slight preference for the meta over the ortho/para sites is observed.
This lack of selectivity contrasts with previously observed rhodium
arene activitations, where metal–carbon bond strengths dictate the
position of C–H activation,17 although deviation from this trend
with iridium complexes has been reported.18
The increased reactivity of formally 16-electron 2 toward
exogeneous ligands and arene activation contrasts with the forcing
conditions required for ligand-induced reductive elimination re-
ported for the 18-electron piano stool complexes Cp*IrH419 (Cp* =
A combination of NMR and IR spectroscopic studies were used
to elucidate the structure of 2 in solution. The isotopomers
(
iPrBDI)IrD4 (2-d4) and (iPrBDI)IrD2H2 (2-d2) were prepared by
5
addition of D2 and HD, respectively, to pentane solutions of 1. In
general, the iridium hydride (deuteride) resonance is unaffected by
isotopic substitution (216.86 ppm for 2-d4 and 216.96 ppm for
h -C5Me5) and Tp*IrH4.20 These initial investigations demonstrate
the rich and potentially unique chemistry associated with b-
diiminate iridium hydride complexes. Access to these molecules
was made possible by subtle manipulation of the ligand archi-
tecture, leading to facile olefin hydrogenation. Further exploration
of these molecules in carbon–hydrogen bond activation processes
and other catalytic reactions is currently under investigation in our
laboratory.
2-d2), consistent with a classical Ir( ) tetrahydride structure. The
V
relatively low H–D coupling constant of 3.4 Hz measured for 2-d2
is also in accord with this structural assignment.13 A T1(min) value
of 74 ms (500 MHz) was measured in toluene-d8 for the hydride
signal of 2, consistent with the short H…H distances observed in
the solid state.14 Infrared spectra of 2 in both the solid state and in
pentane solution display a single broad band centered at 2221 and
2236 cm21, respectively. These observations are similar to those
recently described for the seven-coordinate Tp*IrH4 (Tp* =
We would like to thank Cornell University and the US National
Science Foundation for financial support, as well as Dr Ivan
Keresztes for assistance with NMR spectroscopy.
HB(3,5-Me2pyrazol-1-yl)3, which adopts a classical Ir( ) tetra-
V
hydride structure similar in energy to a kinetically accessible Ir(III
)
dihydrogen complex.15
Notes and references
‡ Crystal data for 1: C40H62IrN4, M = 791.14, triclinic, a = 8.8311(3), b
= 12.1110(4), c = 18.4635(5) Å, a = 84.6470(10), b = 84.4850(10), g =
84.6470(10)°, V = 1893.95(10) Å3, T = 173(2) K, space group P1, Z = 2,
m(Mo-Ka) = 3.556 mm21, 40 861 reflections measured, 13 735 unique
(Rint = 0.0519), which were used in all calculations; the final R1 was
0.0394. For 2: C29H45IrN2, M = 613.87, monoclinic, a = 12.6062(7), b =
15.9992(9), c = 14.2536(8) Å, b = 105.309(2)°, V = 2772.8(3) Å3, T =
173(2) K, space group P21n, Z = 4, m(Mo-Ka) = 4.833 mm21, 47 442
reflections measured, 9822 unique (Rint = 0.0516), which were used in all
calculations; the final R1 was 0.0332. CCDC 225948 and 225949. See
http://www.rsc.org/suppdata/cc/b3/b315817a/ for crystallographic data in
CIF or other electronic format.
The reactivity of 2 with s-donor and aromatic hydrocarbons has
been explored. Addition of one equivalent of PMe3 to 2 resulted in
loss of one equivalent of dihydrogen, forming (iPrBDI)IrH2(PMe3)
(3) as an orange solid. Performing the reaction under four
atmospheres of H2 has no effect on the rate of substitution. A single
iridium hydride resonance is observed in benzene-d6 solution as a
doublet (2JP–H = 40 Hz) centered at 228.19 ppm. Likewise, a
singlet centered at 248.33 ppm is also observed by {1H}31P NMR
spectroscopy. This peak expands to a triplet, arising from coupling
to two equivalent iridium hydrides, upon selective decoupling of
the PMe3 ligand. The equivalent hydride resonances, along with a
¯
C
2v symmetric BDI ligand environment, are consistent with either
a fluxional five-coordinate molecule or a trigonal bipyramidal
iridium dihydride.
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Monitoring benzene-d6 solutions of 2 over the course of 3 days
at ambient temperature revealed complete disappearance of the
iridium hydride signal with concomitant growth of the C6D5H
resonance. Analysis of 2 by 2H NMR spectroscopy after complete
isotopic exchange demonstrated deuterium incorporation into the
iridium–hydride (deuteride) position as well as into the isopropyl
methyl groups of the BDI ligand (eqn. 3). Presumably, isotopic
exchange in the iridium hydride arises from oxidative addition of an
aromatic C–D bond, followed by reductive elimination of benzene-
d5.16 Consistent with this hypothesis, the rate of the isotopic
exchange reaction is inhibited by approximately 50% by excess
dihydrogen, suggesting an Ir(III) dihydride is the catalytically active
species.
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The deuterium present in the b-diiminate ligand may be
accounted for by reversible cyclometalation, similar to that
previously observed in the corresponding platinum system.6 In the
present case, no deuterium incorporation is observed in the
isopropyl methine position, suggesting cyclometalation only occurs
with the more accessible methyl groups.
Isotopic exchange reactions were also conducted with toluene-
d8. Proton incorporation is observed in all positions, with a
preference for activation of the aromatic C–D bonds over the
C h e m . C o m m u n . , 2 0 0 4 , 7 6 4 – 7 6 5
765