Tetrameric iridium hydride-rich clusters formed under hydrogenation conditions

Article abstract of DOI:10.1002/anie.200804484

(Chemical Equation Presented) Butterfly clusters: Hydrogenation of a cationic iridium complex with the sterically bulky N-ligand tetrahydroquinoline (thq) leads to the formation of two distinct tetrameric polyhydrides (see scheme; Ir green, N blue, P purp

Full text of DOI:10.1002/anie.200804484

Communications
DOI: 10.1002/anie.200804484
Polyhydride Clusters
Tetrameric Iridium Hydride-Rich Clusters Formed under
Hydrogenation Conditions**
Yingjian Xu, Mehmet A. Celik, Amber L. Thompson, Hairong Cai, Mine Yurtsever,
Barbara Odell, Jennifer C. Green, D. Michael P. Mingos, and John M. Brown*
Asymmetric hydrogenation using iridium catalysts has been
recognized as an important alternative to rhodium catalysis in
recent years, because it enables the creation of a new
stereogenic center without reliance on polar functionality in
[
1]
the substrate. Previous study of Ir hydrogenations provided
[2]
significant insights into the mechanism, and underscored
that an important deactivation process involves the formation
[
3]
of catalytically inactive trimers. The formation of such
species may be limited by a judicious choice of nitrogen and
phosphorus ligands, which leads to a new family of active
catalysts. Herein, we highlight a quite different aspect of these
studies. Hydrogenation of a specific iridium procatalyst under
substrate free conditions leads not only to trimers but also
higher nuclearity hydride-rich cluster compounds of iridium.
Weller and McIndoe demonstrated that hydride-rich cluster
compounds of rhodium have interesting structural and
chemical features, but the corresponding compounds of
iridium have not previously been studied. Herein, we
describe the unique structures of two tetrameric hydride-
rich iridium clusters which are formed irreversibly under
substrate-free conditions, in addition to a conventional trimer,
as previously described (Scheme 1). The research suggests an
important general route to this class of cluster compounds,
because the nuclearities of the clusters formed under these
conditions may in future be fine-tuned by varying the steric
and electronic properties of the ligands.
Scheme 1. Hydrogenation of complex 1 under ambient substrate-free
conditions.
1
complete. At this stage, the H NMR spectrum showed a
complex iridium hydride region, in which the species 2 (an
[
3a]
analogue of the dicationic Crabtree trimer ) was detected,
although it provided ꢀ 40% of the total signal intensity. The
high resolution electrospray (ES) mass spectrum of solutions
analyzed after complete reduction also revealed the presence
[
4]
2+
of the standard trimer [Ir H (PCy ) (C H N) ] (2), and an
3
7
3
3
9
11
3
ion formed by loss of one N ligand from 2. In addition, two
2
+
strong peaks corresponding to [Ir H (PCy ) (C H N) ]
4
10
3
4
9
11
2
+
(3a) and [Ir H (PCy ) (C H N)] (3b) were observed.
4
11
3
4
9
11
Both of these gave characteristic patterns based on the
1
91
193
stable isotope ratio ( Ir/ Ir= 37.3:62.7), providing unam-
biguous confirmation of the cation formulae (Figure 1).
The structure obtained by single-crystal X-ray diffraction,
after very slow crystallization of the hydrogenation product
from dichloromethane/pentane, corresponds to the complex
Hydrogenation
of
complex
1
([Ir(PCy )(cod)-
3
1
2+
À
[5]
(
C H N)]PF ) in CD Cl was monitored by H NMR spec-
3a, [Ir H (Cy P) (C H N) ] (PF ) . The hydrogen atoms
9
11
6
2
2
4 10 3 4 9 11 2 6 2
troscopy, until the reduction of 1,5-cyclooctadiene (cod) was
could not be located in the difference map, owing to the close
proximity of heavy atoms and the associated termination
[*] Dr. Y. Xu, Dr. A. L. Thompson, H. Cai, Dr. B. Odell,
Prof. D. M. P. Mingos, Dr. J. M. Brown
Chemistry Research Laboratory, Oxford University
12 Mansfield Rd., Oxford OX1 3TA (UK)
Fax: (+44)1865-285-002
E-mail: john.brown@chem.ox.ac.uk
Homepage: http://www.chem.ox.ac.uk/researchguide/jbrown.html
Prof. J. C. Green
Inorganic Chemistry Laboratory, Oxford University (UK)
M. A. Celik, Prof. M. Yurtsever
Istanbul Technical University, Faculty of Arts & Science (Turkey)
[**] We thank Merck for an unrestricted grant (Y.X.) and Oxford and
Istanbul Technical Universities (ITU-HPCC) for computing facilities.
˙
TꢀBITAK provided a six-month scholarship that enabled M.A.C. to
work in Oxford in 2007. We gratefully acknowledge help from Dr.
T. D. W. Claridge (NMR); Dr. N. Oldham and Dr. J. McCullagh (MS).
Johnson-Matthey kindly provided a loan of iridium salts. Prof.
Andrew Weller provided very helpful comments on this manuscript.
Figure 1. a) Experimental and b) calculated ES mass spectra for the
2
+
Supporting Information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804484.
[Ir H (PCy ) (C H N) ] dication 3a. c) Experimental and d) calcu-
4 10 3 4 9 11 2
+
lated ES mass spectra for the [Ir H (Cy P) (C H N)] cation 3b.
4
11
3
4
9
11
5
82
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Angewandte
Chemie
1
errors, but the non-hydrogen atom positions are clearly
defined. The central iridium core of the cation consists of a
The H NMR spectrum of the hydrogenation product
revealed signals in the IrÀH region corresponding to 3a,
butterfly motif with a C axis perpendicular to the Ir2ÀIr2’
albeit as the minor component (Figure 3, italicized H). The
2
hinge. Each Ir center is coordinated by PCy (Cy = cyclohex-
3
yl) ligands, with tetrahydroquinoline (THQ) ligands bound to
the two terminal Ir centers. The location of the hydrogen
atoms was computed by density functional (DFT) calcula-
tions, using PMe and pyridine as model ligands. Starting from
3
the coordinates of the heavy atom core, which were allowed
to relax, trial structures with differing hydrogen positions
[
6a]
were optimized.
1
Three low-energy structures within
À1
kJmol of one another were located; they had similar
core structures, with different rotameric forms of PMe . The
3
structure that resembled the X-ray conformation most closely
(
Figure 2b) retained symmetry, with the C2 axis passing
1
Figure 3. H NMR spectrum at 283 K of a typical reaction mixture,
derived from hydrogenation of complex 2 in CD Cl in the region
2
2
d=À5 to À35 ppm. The trimer signals are shown as underlined H,
the dication 3a as italic H, and the monocation 3b as plain H.
two central bridging hydrides H1 and H2 are apparent at d =
À6.46 and À8.48 ppm, and the other two pairs of bridging
hydrides H3 and H5 at d = À13.46 and À22.59 ppm. The
terminal hydrides H4 and H6 resonate at d = À16.44 and
À34.74 ppm respectively. The signals all correlate with the
expected phosphorus resonances [P1: d = 18.0 ppm; P2: d =
Figure 2. a) Molecular structure of complex 3a as its CH Cl solvate,
2
2
[
C H N P Ir ](PF ) ·2CH Cl ) determined by single-crystal X-ray dif-
90 164 2 4 4 6 2 2 2
fraction. Counterions, solvent molecules, carbon atoms, and hydrogen
atoms are omitted for clarity (see the Supporting Information for full
details). b) The core structure arising from DFT calculations on [Ir H -
4
10
6
0.5 ppm]. These results provide support for the proposed
structure.
The same spectrum demonstrated that the major species
was monocation with eleven inequivalent hydrides
2+
(
Me P) (C H N) ] .
3 4 5 5 2
a
through the two hydrogen atoms, which bridged the hinge
iridium atoms Ir2 and Ir2’. It also confirmed the key
experimental features (see Table 1), although IrÀIr bond
(Figure 3, plain H). A combination of X-ray crystallographic
analysis, detailed NMR spectroscopic studies, and DFT
calculations proved necessary before this structure could be
1
lengths were 0.06–0.1 ꢀ greater than the values given by X-
ray diffraction (Table 1). The octahedral and square-pyrami-
dal environments about the iridium centers indicated local-
ized 18e (Ir2, Ir2’) and 16e (Ir1, Ir1’) iridium(III) centers. For
comparison, the DFT-optimized structure of the model trimer
fully defined. The ambient temperature H NMR spectrum
shows four of the hydride resonances are broad and exchang-
ing, namely H1, H3, H4, and H6. Only H2 and H6 make NOE
contacts with aromatic protons of the thq ligand. H2
resembles H1 of the dication 3a, and H11 likewise resembles
H6 of the dication. With the exception of H4 (see below), all
2
+
[
Ir H (PMe ) py ] is included in the Supporting Informa-
3 7 3 3 3
3
1
tion. Aside from the triple bridge, the hydrogen positions
were not revealed by X-ray diffraction.
of the protons can be assigned to an iridium partner by { P–
1
H} correlation spectroscopy and, in the case of H2, H5, H7,
and H10, further bridging was evident. When a hydrogenated
sample was diluted with MeOH, a yellow precipitate was
formed, affording a cleaner H NMR spectrum on redissolu-
Table 1: Comparison of bond lengths and angles afforded by single-
crystal X-ray diffraction (XRD) and density functional calculations (DFT)
for 3a.
1
tion in CD Cl . X-ray diffraction of the sample, recrystallized
2
2
from CD Cl /pentane, gave a new structure, confirmed to be
2
2
Distance [ꢁ]
XRD
DFT
Angle [8]
XRD
DFT
the
(C H N)] PF .
6
monocationic
complex
3b,
[Ir H (Cy P) -
4 11 3 4
+
À
[5]
Ir1ÀIr2
Ir2ÀIr2’
Ir1ÀP1
Ir1ÀN1
Ir2ÀP2
2.7703(6)
2.6241(4)
2.284(3)
2.099(6)
2.275(3)
2.879
2.686
2.299
2.119
2.299
N1-Ir1-P1
N1-Ir1-Ir2
P1-Ir1-Ir2
P2-Ir2-Ir2’
Ir2-Ir1-Ir2’
94.7(3)
93.9
Again, structural refinement located
9
11
92.1(3)
109.4(1)
140.3(1)
56.17(2)
98.0
104.0
137.2
55.9
heavy atoms and the counterion, but not the hydrogen
atoms. In the derived structure, the four iridium atoms are
close to coplanarity (Figure 4a). This new structure indicates
the formal replacement of one thq ligand (in the structure of
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Communications
Figure 4. a) Molecular structure of complex 3b [C H NP Ir ]PF
6
8
1
154
4
4
determined by single-crystal X-ray diffraction. Counterions, carbon
atoms, and hydrogen atoms are omitted for clarity (see the Supporting
Information for full details). b) Core structure from DFT calculations
Figure 5. Results of the four EXSY exchange experiments at 288 K in
CD Cl (see text for details). Each nucleus, H , H , H , and H , is
2
2
1
3
4
6
irradiated in turn, and the three remaining in the exchanging set
detected over the mixing time range t=10–500 ms (shown here to
+
on [Ir H (Me P) (C H N)] .
4
11
3
4
5
5
t=100 ms). The superimposed lines represent simulation of the
À1
process illustrated, with kdiss =7.5 s . t (the proton spin-lattice
1
3
a) by a hydride ion. Initial attempts to simulate the
relaxation time) for the four exchanging hydrides was (310Æ25) ms.
The molecular structure (right) shows trans pairs of exchanging
protons in black and gray.
experimental structure by DFT failed. Having established
the X-ray crystal structure, modeling was attempted based on
acceptable IrÀH and CÀH contact distances, utilizing the full
range of NMR spectroscopic data. DFT calculations based on
this structure converged to an acceptable minimum (Fig-
rotation about the P3-Ir3-H2 axis. Each dissociation step
(kdiss = 7.5 s ) permits only one rotation, but equally in both
[
6a]
À1
ure 4b, Table 2). The model predicts that H4, one of the
exchanging set, is bound to Ir4. In the heteronuclear
correlation spectrum, H4 exhibits weak coupling to P1. This
apparent contradiction is resolved by calculations that predict
the detected long-range coupling between H4 and P1. The
structure is based on localized octahedral geometries at Ir2,
Ir3, and Ir4, and a square pyramidal geometry at Ir1, that is, a
canting of the H2-Ir2-H1-Ir3 moiety so that H1 forms a third
bridge to Ir4.
directions.
The two tetramers 3a and 3b have distinct geometries,
which may be understood by considering the bridging
hydrides that form the structural links between pairs of
iridium nuclei. The elevation view of dication 3a, along the
axis of the butterfly hinge (Figure 6), shows that the two
terminal Ir and four bridging hydrides exhibit a boat
conformation. For the monocation 3b, the same view
demonstrates a chair conformation , explaining why the Ir
atoms in this complex are close to coplanar (XRD hinge
angles 40.8(1)8 for 3a and 9.4(1)8 for 3b). Both 3a and 3b
have 56 valence electrons, consistent with the cluster electron-
counting rules for Pt-metal clusters with hydrido ligands.
Planar and hinged butterfly structures are not distinguished
[
6b]
In this model, the four exchanging hydrogen atoms
(
identified by exchange spectroscopy, EXSY) are arranged
in a square around Ir3. Detailed analysis of the exchange
[
7]
process, exciting each of the four hydrogen atoms in turn,
revealed a defined exchange pathway. Each proton undergoes
primary exchange with its two neighbors, and only indirectly
with the trans proton. The data for the four EXSY experi-
ments (Figure 5) show a set of points representing exchange
with the two cis protons (a), and a second set representing the
secondary exchange process with the trans proton (b), and
demonstrates the operation of a single mechanism involving
[
8]
by these rules.
The distinction between precursor complex 1 and the
[
3a]
+
À
6
original Crabtree catalyst 4, [(Cy P)(C H N)Ir(cod)] PF
3
5
5
is simply the replacement of pyridine by tetrahydroquinoline,
and the differences are likely to arise from steric rather than
electronic factors. We considered the possibility that tetram-
ers could also have been formed in the original work that led
to trimer characterization. On repeating the hydrogenation
procedure using complex 4, the ES mass spectrum of the
resulting solution clearly shows the presence of such a species.
Table 2: Comparison of bond lengths and angles afforded by single-
crystal X-ray diffraction (XRD) and density functional calculations (DFT)
for 3b.
Distance [ꢁ]
XRD
DFT
Angle [8]
XRD
DFT
Only the N-dissociated tetrameric dication based on a
NP Ir } core is detected, identified by its distinctive half-
4 4
Ir1ÀIr2
Ir1ÀIr3
Ir2ÀIr3
Ir2ÀIr4
Ir3ÀIr4
Ir1ÀP1
Ir2ÀP2
Ir3ÀP3
Ir4ÀP4
Ir1ÀN1
2.7785(4)
2.7421(4)
2.6758(4)
2.7748(4)
2.7281(4)
2.300(2)
2.269(2)
2.285(2)
2.200(2)
2.129(8)
2.823
2.817
2.744
2.821
2.798
2.314
2.298
2.291
2.249
2.130
N1-Ir1-P1
P1-Ir1-Ir2
P1-Ir1-Ir3
Ir2-Ir4-Ir3
Ir2-Ir1-Ir3
Ir4-Ir3-Ir1
Ir1-Ir2-Ir4
P3-Ir3-Ir2
P2-Ir2-Ir3
P4-Ir4-Ir2
93.9(2)
92.8
2+
{
106.33(6)
152.87(6)
58.18(1)
99.8
148.0
58.5
57.98(1)
58.2
122.77(1)
119.70(1)
144.90(6)
140.62(5)
132.03(6)
121.1
121.0
143.0
140.7
133.5
Figure 6. Elevation views of the two tetrameric complexes: a) 3a and
b) 3b, emphasizing the origins of their geometric differences.
5
84
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Angewandte
Chemie
[
4] a) A. S. Weller, J. S. McIndoe, Eur. J. Inorg. Chem. 2007, 4411 –
4
6
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[
5] CCDC 692069 and 692070 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Crystallographic data: 3a:
C90.75H165.50Cl1.50F Ir N P ; M = 2493.48; crystal size 0.04 ꢁ
12
4
2
6
r
0
.11 ꢁ 0.13 mm; monoclinic (C ); a = 22.8325(3), b = 17.2007(3),
2
3
c = 16.6365(3) ꢀ; b = 118.473(1)8; V= 5743.42(16) ꢀ ; Z = 2;
m = 4.794 mm ; 1calcd = 1.457 gcm ; T= 150(2) K; 45368 reflec-
tions measured; 1296 independent reflections (Rint = 0.1119);
À1
À3
R1 = 0.0708; wR2 = 0.1730 [I > 2s(I)]; D1 = À2.841, D1
=
min
max
3
5
.995 eꢀ . 3b: C H Cl F Ir N O P ; M = 2316.83; crystal
8
2
160.5
2
6
4
1
3.25
5
r
¯
size 0.08 ꢁ 0.12 ꢁ 0.14 mm; triclinic (P1); a = 14.59410(10), b =
7.6600(2), c = 20.9926(2) ꢀ; a = 108.3094(5), b = 103.8296(4),
1
3
À1
g = 93.7822(5)8; V= 4928.91(8) ꢀ ; Z = 2; m = 5.570 mm
1calcd = 1.553 gcm ; T= 150(2) K; 37501 reflections measured;
,
À3
2
2323 independent reflections (Rint = 0.115); R1 = 0.0557;
Figure 7. ES mass spectrum of hydrogenated Crabtree’s catalyst,
(cod)Ir(PCy )(C H N)]PF (4, cod=1,5-cyclooctadiene), showing the
3
wR2 = 0.1450 [I > 2s(I)]; D1min = À3.69, D1 = 4.09 eꢀ .
max
[
3
5
5
6
[
6] a) Geometry optimizations of the Ir tetramers were carried out
by density functional theory (DFT) methods with the empirically
parameterized hybrid functional B3LYP as implemented in
Gaussian 03. The equilibrium structures have been characterized
by a lack of imaginary vibrations. The 6-31g(d) basis set was
applied to all elements except Ir which is described with the
LANL2DZ basis set; b) NMR spectroscopic calculations were
carried out using the ADF 2007.01 program suite: ADF2007.01,
SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands, http://www.scm.com. Further details are given
in the Supporting Information.
2
+
presence of tetrameric dication [Ir H (Cy P) (C H N)]
.
4
10
3
4
5
5
integer isotope pattern at 4% of the intensity of the main
trimer peak (Figure 7).
Complexes 3a and 3b represent novel additions both to
[
9]
iridium chemistry, and the emerging field of high hydride-
content clusters. Structures with similar core geometries are
[
10]
uncommon. Ligand-free iridium tetramers are known in
the gas-phase, and the most stable form of naked Ir is
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4
[11]
square. The closest condensed phase analogies, both in the
nature of the preparative routes and the high hydrogen
content of the products, arise extensive work by Weller and
[
8] D. M. P. Mingos, A. S. May in The Chemistry of Metal Cluster
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[
12,13]
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[9] For a tetrahedral tetrameric hydride, [Ir H (CO) (PPh ) ], see:
4
8
4
3 4
[
14]
uptake capabilities of mixed PtRh and PtOs clusters. The
sensitivity of our hydrogenation product structure to the
nature of the nitrogen ligand in this work suggests that a rich
iridium hydride cluster chemistry is accessible.
L. Garlaschelli, F. Greco, G. Peli, M. Manassero, M. Sansoni, R.
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[
10] a) For a butterfly Pt cationic heptahydride tetramer see: R. J.
4
Goodfellow, E. M. Hamon, J. A. K. Howard, J. L. Spencer, D. G.
Turner, J. Chem. Soc. Chem. Commun. 1984, 1604 – 1606; b) for a
planar rhomboidal Re Au tetrameric hexahydride see: B. R.
Received: September 11, 2008
Published online: December 9, 2008
2
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Keywords: cluster compounds · hydrogenation · iridium ·
.
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