A model iridium hydroformylation system with the large bite angle ligand xantphos: Reactivity with parahydrogen and implications for hydroformylation catalysis

Article abstract of DOI:10.1021/ic060731l

Iridium complexes containing the large bite angle bisphosphine ligand xantphos have been synthesized and their reactivity studied. Several of these complexes are the first reported Ir(xantphos) systems to be characterized by X-ray diffraction. Variable-temperature NMR spectroscopic studies of IrI(CO)₂(xantphos) (1-I) and Ir(COEt)(CO)₂-(xantphos) (8) show two separate dynamic processes in which the phosphorus donors and the backbone methyl groups of the xantphos ligand are exchanged. The addition of parahydrogen (p-H₂) to 1-I leads to the formation of two dihydride isomers including one in which both hydride ligands are trans to the phosphorus donors, suggestive of an Ir(I) xantphos intermediate with the ligand chelated in a trans-spanning fashion (2b). The bromide and chloride Ir(I) analogues (1-Br and 1-Cl) also form this isomer upon reaction with parahydrogen, with 1-Cl yielding only this dihydride species. The trihydride complex IrH ₃(CO)(xantphos) (7) has been prepared, and its exchange with free hydrogen at elevated temperature is confirmed by reaction with p-H₂. The hydride complexes IrH(CO)₂(xantphos) (6) and IrH ₃(CO)(xantphos) (7), as well as the propionyl complex 8, are modest catalysts for the hydroformylation of 1-hexene and styrene under mild conditions. The addition of p-H₂ to 8 permits direct observation of the propionyl dihydride species IrH₂(COEt)(CO)(xantphos) (9) under both thermal and photolytic conditions, as well as unusual but weak polarization of the aldehydic proton of the propanal product that forms upon reductive elimination from 9.

Full text of DOI:10.1021/ic060731l

Inorg. Chem. 2006, 45, 7197−7209
A Model Iridium Hydroformylation System with the Large Bite Angle
Ligand Xantphos: Reactivity with Parahydrogen and Implications for
Hydroformylation Catalysis
Daniel J. Fox,† Simon B. Duckett,‡ Christine Flaschenriem,† William W. Brennessel,†
Jacob Schneider,† Ahmet Gunay,† and Richard Eisenberg*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and
Department of Chemistry, UniVersity of York, York YO10 5DD, U.K.
Received April 28, 2006
Iridium complexes containing the large bite angle bisphosphine ligand xantphos have been synthesized and their
reactivity studied. Several of these complexes are the first reported Ir(xantphos) systems to be characterized by
X-ray diffraction. Variable-temperature NMR spectroscopic studies of IrI(CO)2(xantphos) (1-I) and Ir(COEt)(CO)2-
(xantphos) (8) show two separate dynamic processes in which the phosphorus donors and the backbone methyl
groups of the xantphos ligand are exchanged. The addition of parahydrogen (p-H2) to 1-I leads to the formation of
two dihydride isomers including one in which both hydride ligands are trans to the phosphorus donors, suggestive
of an Ir(I) xantphos intermediate with the ligand chelated in a trans-spanning fashion (2b). The bromide and chloride
Ir(I) analogues (1-Br and 1-Cl) also form this isomer upon reaction with parahydrogen, with 1-Cl yielding only this
dihydride species. The trihydride complex IrH3(CO)(xantphos) (7) has been prepared, and its exchange with free
hydrogen at elevated temperature is confirmed by reaction with p-H2. The hydride complexes IrH(CO)2(xantphos)
(6) and IrH3(CO)(xantphos) (7), as well as the propionyl complex 8, are modest catalysts for the hydroformylation
of 1-hexene and styrene under mild conditions. The addition of p-H2 to 8 permits direct observation of the propionyl
dihydride species IrH2(COEt)(CO)(xantphos) (9) under both thermal and photolytic conditions, as well as unusual
but weak polarization of the aldehydic proton of the propanal product that forms upon reductive elimination from
9.
Introduction
RhH(CO)(PPh3)3 in the late 1960s, the accepted mechanism
for hydroformylation using metal phosphine homogeneous
catalysts involves an acyl dihydride species as the final
intermediate before aldehyde formation.10-12 While relatively
few hydroformylation studies employ iridium complexes as
active catalysts,13-15 such systems can be of value as stable
Hydroformylation is one of the largest-scale homogeneous
catalytic industrial processes involving dihydrogen as a
reactant.1-9 As proposed by Wilkinson and co-workers for
* To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu.
† University of Rochester.
(7) Buisman, G. J. H.; Martin, M. E.; Vos, E. J.; Klootwijk, A.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1995,
6, 719-738.
‡ University of York.
(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis. The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes,
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1.
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John Wiley & Sons: New York, 1988.
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Approach; van Santen, R. A., van Leeuwen, P. W. N. M., Moulijn, J.
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10.1021/ic060731l CCC: $33.50
Published on Web 08/03/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 18, 2006 7197

Fox et al.
models for reaction intermediates, and one example has
permitted characterization of the putative acyl dihydride
species in the form of IrH2(COEt)(CO)(dppe) (dppe ) 1,2-
bis(diphenylphosphino)ethane).16 One measure of success in
hydroformylation catalysis is the ratio of linear-to-branched
(l/b) aldehyde products, and both Casey and van Leeuwen
have shown that the use of large natural bite angle bidentate
ligands in the catalysts yield higher l/b product ratios,17-21
as well as more active catalysts,18,19,22 than those containing
chelating ligands with smaller natural bite angles. Xant-
phos (9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene; bite
angle ) 111.7°)23 is one of those large natural bite angle
ligands used in rhodium hydroformylation catalysts,19-21,24-26
and it has been examined in platinum hydroformylation
catalysts as well.27,28
In this report, the syntheses of a series of iridium xantphos
complexes, including propionyl species that model hydro-
formylation intermediates, are presented, and some of them
are shown to be catalytically active for hydroformylation
under relatively mild conditions. The ability of the xantphos
ligand to chelate in a trans fashion is highlighted by the
addition of parahydrogen (p-H2) to IrX(CO)2(xantphos) (1-
X; X ) I, Cl, Br), in which an unusual hydrogen addition
product with both hydride ligands trans to the phosphine
donors is formed. The reaction of a propionyl complex with
p-H2 also makes the observation of an acyl-dihydride species
through parahydrogen-induced polarization (PHIP) and the
detection of an unusual but weak one-hydrogen polarization
(oneH-PHIP) in the product aldehyde possible. PHIP has
been employed by us and others as a useful mechanistic
probe for solution-phase reactions involving molecular
hydrogen29-34 including hydroformylation.35-37
synthesized, in most cases, using the same general procedures
as those employed for the preparation of analogous Ir(dppe)
complexes.16,38,39 As shown in eq 1, the addition of a solution
of xantphos in THF to a solution of [Bu4N][IrI2(CO)2]38
results in the formation of IrI(CO)2(xantphos) (1-I). The
distorted trigonal bipyramidal geometry of 1-I is unambigu-
ously established by an X-ray diffraction study that shows a
structure in which one phosphine donor is in an axial position
trans to a carbonyl ligand and the other phosphine is in the
equatorial plane with iodide and the second carbonyl (Figure
1). The bromide analogue of 1-I, IrBr(CO)2(xantphos) (1-
Br), was synthesized in the same manner as 1-I from [Bu4N]-
[IrBr2(CO)2], while the chloride analogue, IrCl(CO)2-
(xantphos) (1-Cl), was prepared from [Ir(COD)Cl]240 via the
addition of xantphos and CO. X-ray diffraction studies of
1-Br and 1-Cl confirm their trigonal bipyramidal geometries,
but their structures differ significantly from that of 1-I. In
both 1-Br and 1-Cl, the halide and one carbonyl ligand are
in axial positions, while the two phosphine donors and the
second carbonyl are equatorial. Disorder exists between the
axial carbonyl and halide ligands in the two structures. In
1-Br, with an 84:16 disorder, the dominant form has the
halide on the same side of the molecule as the xantphos
backbone (Figure 2), while in 1-Cl, with an 85:15 disorder,
the dominant form has the axial carbonyl on the same side
as the xantphos backbone (Figure 3). In the structure of 1-Cl,
two of the xantphos phenyl groups are perfectly stacked with
a separation of 3.6 Å, suggestive of π-stacking stabilization
in the solid state. To our knowledge, these complexes are
the first examples of Ir(xantphos) complexes to be character-
ized by X-ray diffraction. See Tables 1-4 for the crystal-
lographic details of the three structures. The IR spectra of
complexes 1-X show two CO stretches (1-I νCO ) 2029,
1948 cm-1; 1-Br νCO ) 2023, 1950 cm-1; 1-Cl νCO ) 2017,
1944 cm-1), consistent with the X-ray structures.
Results and Discussion
Synthesis and Characterization of New Iridium(xant-
phos) Complexes. The new Ir(xantphos) complexes are
(16) Deutsch, P. P.; Eisenberg, R. Organometallics 1990, 9, 709-718.
(17) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J. A.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535-
5543.
(18) van der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
Organometallics 1999, 18, 4765-4777.
(19) Freixa, Z.; van Leeuwen, P. W. N. M. Dalton Trans. 2003, 1890-
1901.
(20) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Acc. Chem.
Res. 2001, 34, 895-904.
(21) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes,
P. Chem. ReV. 2000, 100, 2741-2769.
1
For complex 1-I, the 31P{1H} and H NMR spectra at
(22) Bronger, R. P. J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
Organometallics 2003, 22, 5358-5369.
-70 °C display, respectively, one sharp singlet at δ -13.5
(23) Kraneneburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics 1995,
14, 3081-3089.
(32) Natterer, J.; Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 1997,
31, 293-315.
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D. M., Harris, R. K., Eds.; John Wiley & Sons: Chichester, U.K.,
2002; Vol. 9 (Advances in NMR), pp 750-770.
(34) Duckett, S. B.; Blazina, D. Eur. J. Inorg. Chem. 2003, 2901-2912.
(35) Permin, A. B.; Eisenberg, R. J. Am. Chem. Soc. 2002, 124, 12406-
12407.
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192, 883-900.
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742.
(27) van der Veen, L. A.; Keeven, P. K.; Kamer, P. C. J.; van Leeuwen,
P. W. N. M. J. Chem. Soc., Dalton Trans. 2000, 2105-2112.
(28) Petocz, G.; Berente, Z.; Kegl, T.; Kollar, L. J. Organomet. Chem.
2004, 689, 1188-1193.
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A. C. Chem. Commun. 2004, 1826-1827.
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Am. Chem. Soc. 2005, 127, 4994-4995.
(29) Duckett, S. B.; Sleigh, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1999,
23, 71-92.
(38) Albietz, P. J. PhD Thesis, University of Rochester, Rochester, NY,
2001.
(39) Fisher, B. J.; Eisenberg, R. Organometallics 1983, 2, 764-767.
(40) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18-20.
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Chem. Ser. 1992, 230, 47-74.
7198 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
Figure 1. Two ORTEP views of 1-I showing a partial labeling scheme; ellipsoids are at the 50% probability level. Hydrogen atoms have been removed
for clarity.
Figure 2. Two ORTEP views of 1-Br showing a partial labeling scheme; ellipsoids are at the 50% probability level. Hydrogen atoms have been removed
for clarity.
Figure 3. Two ORTEP views of 1-Cl showing a partial labeling scheme; ellipsoids are at the 50% probability level. Hydrogen atoms have been removed
for clarity.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7199

Fox et al.
Table 1. Crystallographic Data for IrI(CO)2(xantphos) (1-I), IrBr(CO)2(xantphos) (1-Br), IrCl(CO)2(xantphos) (1-Cl), IrH2Cl(CO)(xantphos) (4-Cl),
and Ir(COEt)(CO)2(xantphos) (8)
1-I
1-Br
1-Cl
4-Cl
8
empirical formula
fw
C41H32IIrO3P2
953.71
C41H32BrIrO3P2
906.72
C41H32ClIrO3P2
862.26
C40H34ClIrO2P2
836.26
C
54.5H48IrO4P2
1021.07
T (K)
100.0(1)
100.0(1)
100.0(1)
100.0(1)
100(2)
λ (Å)
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
0.71073
triclinic
P1h
2
0.71073
triclinic
P1h
2
cryst syst
space group
Z
4
4
4
a (Å)
b (Å)
c (Å)
17.5313(8)
10.9903(5)
18.3293(8)
92.401(1)
3528.5(3)
1.795
17.034(2)
11.2558(13)
20.251(2)
90.457(2)
3882.7(7)
1.551
17.058(2)
9.8230(12)
20.468(2)
97.126(2)
3403.1(7)
1.683
9.8245(10)
10.9194(11)
17.7056(18)
75.507(1)
1713.9(3)
1.620
10.207(2)
12.798(3)
18.194(4)
101.94(3)
2300.2(9)
1.474
â (deg)
V (Å3)
F
calcd (Mg/m3)
µ (mm-1
)
4.788
4.585
4.136
4.101
3.018
abs correction
transm range
F(000)
SADABS
0.1571-0.2674
1848
SADABS
0.4207-0.5261
1776
SADABS
0.3868-0.5652
1704
SADABS
0.3131-0.6189
828
SADABS
0.3496-0.7893
1028
2θ range (deg)
2.16-30.03
-24 e h e 24
-15 e k e 15
-25 e l e 25
52 260
2.01-32.58
-25 e h e 25
-16 e k e 17
-30 e l e 30
68 408
2.01-30.51
-24 e h e 24
-13 e k e 14
-28 e l e 29
52 021
2.00-30.51
-14 e h e 14
-15 e k e 15
-25 e l e 25
28 003
1.15-29.57
-24 e h e 24
-17 e k e 17
-24 e l e 25
34 426
limiting indices
no. of reflns collected
no. of data/restraints/params
GOF
10 254/0/435
1.040
13 968/0/439
1.056
10 237/3/439
1.059
10 410/15/424
1.097
12 770/50/601
1.066
R1, wR2 (I > 2σ)
R1, wR2 (all data)
0.0148, 0.0351
0.0164, 0.0355
0.0279, 0.0631
0.0374, 0.0653
0.0188, 0.0444
0.0228, 0.0454
0.0282, 0.0604
0.0301, 0.0611
0.0365, 0.0995
0.0403, 0.0988
Scheme 1
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-I
Ir(1)-C(2)
Ir(1)-C(1)
Ir(1)-P(2)
Ir(1)-P(1)
1.8611(17)
1.8942(17)
2.3875(4)
2.4221(4)
Ir(1)-I(1)
C(1)-O(1)
C(2)-O(2)
2.79885(16)
1.137(2)
1.154(2)
C(2)-Ir(1)-C(1)
C(2)-Ir(1)-P(2)
C(1)-Ir(1)-P(2)
C(2)-Ir(1)-P(1)
C(1)-Ir(1)-P(1)
P(2)-Ir(1)-P(1)
87.42(7)
84.35(5)
168.35(5)
136.27(5)
91.36(5)
C(2)-Ir(1)-I(1)
C(1)-Ir(1)-I(1)
P(2)-Ir(1)-I(1)
P(1)-Ir(1)-I(1)
O(1)-C(1)-Ir(1)
O(2)-C(2)-Ir(1)
132.27(5)
87.97(5)
91.471(9)
91.307(9)
175.04(15)
177.34(16)
100.287(13)
(a) One Berry pseudorotation using iodide as pivot to equilibrate the
phosphine donors with intermediate shown. (b) Two Berry pseudorotations
using the two CO ligands as consecutive pivots to equilibrate Me and Me.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the Major
Form of 1-Br
Ir(1)-C(1)
Ir(1)-C(2)
Ir(1)-P(2)
Ir(1)-P(1)
1.830(3)
1.897(2)
2.4063(6)
2.4230(6)
Ir(1)-Br(1)
C(1)-O(1)
C(2)-O(2)
2.5253(4)
1.163(4)
1.141(3)
In light of the structure determined for 1-I by crystallography
and the variable-temperature NMR experiments, the fluxional
behavior of 1-I can be explained by consideration of the two
separable dynamic processes corresponding to Berry pseu-
dorotation and intermolecular exchange. At low temperature,
Berry pseudorotation using the Ir-I bond as the “pivot”
exchanges the two phosphine donors (Scheme 1a), while
successive pseudorotations using the two carbonyl ligands
as “pivots” exchange the two inequivalent methyl groups
on the xantphos backbone (Scheme 1b). A second, higher
activation barrier process also takes place that involves the
loss of CO followed by re-coordination of CO and accounts
for the broad single 31P resonance seen between 0 and 30
°C and its sharpening as the temperature is increased. The
feasibility of CO loss and re-coordination is confirmed by
exchange with 13CO at ambient temperature: as shown in
the inset of Figure 4, the 31P resonance of 1 shows coupling
to two 13C nuclei at -50 °C. A similar combination of low-
temperature pseudorotations and higher-temperature inter-
molecular exchange has been used to rationalize the dynamic
behavior of the related 5-coordinate Ir(I) complex Ir(CO)-
(η2-C2H4)2(dppe)+.41 An alternative mechanism can also be
C(1)-Ir(1)-C(2)
C(1)-Ir(1)-P(2)
C(2)-Ir(1)-P(2)
C(1)-Ir(1)-P(1)
C(2)-Ir(1)-P(1)
88.42(14)
93.21(11)
130.01(8)
94.43(12)
126.68(8)
P(2)-Ir(1)-P(1)
C(1)-Ir(1)-Br(1)
C(2)-Ir(1)-Br(1)
P(2)-Ir(1)-Br(1)
P(1)-Ir(1)-Br(1)
103.03(2)
172.05(12)
83.84(7)
90.429(17)
91.631(16)
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the Major
Form of 1-Cl
Ir(1)-C(1)
Ir(1)-C(2)
Ir(1)-Cl(1)
Ir(1)-P(2)
1.865(3)
1.881(2)
2.3900(6)
2.4131(5)
Ir(1)-P(1)
C(1)-O(1)
C(2)-O(2)
2.4174(5)
1.152(3)
1.145(2)
C(1)-Ir(1)-C(2)
C(1)-Ir(1)-Cl(1)
C(2)-Ir(1)-Cl(1)
C(1)-Ir(1)-P(2)
C(2)-Ir(1)-P(2)
87.73(10)
172.29(8)
85.09(6)
92.87(8)
122.56(7)
Cl(1)-Ir(1)-P(2)
C(1)-Ir(1)-P(1)
C(2)-Ir(1)-P(1)
Cl(1)-Ir(1)-P(1)
P(2)-Ir(1)-P(1)
93.364(18)
92.52(8)
137.38(7)
90.862(18)
100.008(18)
(Figure 4) and a substantially broadened (or coalesced)
xantphos methyl resonance, indicative of dynamic behavior
of the complex. At ambient temperature, a single broad
resonance is seen in the 31P{1H} NMR spectrum at δ -13.5.
7200 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
Figure 4. Variable-temperature 31P NMR spectra of complex 1 in toluene-d8. The inset shows the 31P NMR spectrum at -50 °C after exchange with 13CO.
1
1
Figure 5. Hydride region of the H (top) and H{31P} NMR spectra upon addition of p-H2 to 1 at 50 °C in C6D6.
invoked to explain the dynamic behavior of 1-I and the
exchange of 13CO for 12CO. The dynamic behavior can be
explained by reversible dissociation of one arm of the
xantphos ligand, while CO exchange occurs by phosphine
dissociation followed by 13CO coordination, loss of 12CO,
and rechelation of the xantphos ligand. However, on the basis
of the similarity of the dynamic behavior of 1-I with that of
Ir(CO)(η2-C2H4)2(dppe)+41 and the fact that dppe shows a
greatly reduced tendency to dissociate, we favor the pseu-
dorotation mechanism at low temperature.
Reaction of 1-X with Hydrogen. In benzene-d6 at 50 °C,
the reaction of 1-I with para-enriched hydrogen results in
the detection of two oxidative addition products showing
PHIP effects (Figure 5). One product (3-I) exhibits a hydride
signal at δ -11.91 that shows a coupling indicative of a
hydride ligand trans to a phosphine donor, while the other
hydride ligand is trans to carbonyl and provides a signal at
δ -9.39. These observations are in agreement with previ-
ously reported observations for the kinetic dihydride product
formed using IrI(CO)(dppe) and related systems.30,31,34,42,43
The second dihydride product (4-I) exhibits a broad doublet
at δ -10.35 in the 1H NMR spectrum, with a separation of
161 Hz between the peak centers, although only the
downfield component possesses clearly discernible emission/
absorption (E/A) polarization. The acquisition of a 1H{31P}
NMR spectrum of the reaction solution reveals that the major
doublet splitting in 4-I results from coupling to the 31P nuclei
(Figure 3, bottom) and quenches the antiphase character. We
therefore conclude that 4-I, which forms to an appreciable
extent in solution, corresponds to an isomer of IrH2I(CO)-
(xantphos) in which both hydride ligands are trans to the
two xantphos phosphorus donors (Scheme 2). On the basis
of the concerted nature of the H2 oxidative addition to d8
square planar complexes, the observation of both 3-I and
4-I means that on CO elimination from 1-I, two isomers of
(42) Duckett, S. B.; Field, L. D.; Messerle, B. A.; Shaw, W. J.; Soler, L.
P. J. Chem. Soc., Dalton Trans. 2000, 2251-2253.
(43) Eisenschmid, T. C.; McDonald, J.; Eisenberg, R.; Lawler, R. G. J.
Am. Chem. Soc. 1989, 111, 7267-7269.
(41) Albietz, P. J.; Cleary, B. P.; Paw, W.; Eisenberg, R. Inorg. Chem.
2002, 41, 2095-2108.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7201

Fox et al.
Figure 6. Hydride region of 1H NMR spectra showing loss of polarization in the initially formed products and formation of the thermodynamically favored
product over time upon addition of p-H2 to 1 at 50 °C in C6D6.
Scheme 2
2-I form, one with the chelating phosphine donors cis to each
other and the other with the xantphos phosphines mutually
trans (2b-I). Oxidative addition to the latter isomer with the
H2 molecular axis parallel to the P-Ir-P axis thus yields
4-I. In contrast, the addition of H2 to 2b-I with H2 parallel
to I-Ir-CO is precluded by unfavorable steric interactions
between iodide, CO, and xantphos that would develop as
iodide and CO attempt to move into mutually cis coordina-
tion positions during the concerted oxidative addition reac-
tion. While the trans-spanning mode of the xantphos ligand,
as written, suggests requisite coordination of the xanthene
oxygen,44-46 which in the present case would lead to a
coordinatively saturated system, the structural results for 1-I,
1-Br, and 1-Cl show that the Ir xantphos chelate ring is
distinctly nonplanar, having an envelope conformation, and
that the xanthene O atom is not required to bind to the Ir(I)
center.
With regard to the polarized resonances of 4-I, the
separation between the two components in the polarized
doublet at δ -10.35 is not simply due to a JPH coupling
because the absorption maxima of the resonances when
polarized are separated by 176 Hz (Figure 6, bottom),
whereas when the system is fully relaxed (Figure 6, middle),
the splitting is 161 Hz. The results suggest that the PHIP
intensities come from a second-order effect caused by the
creation of an [AA′XX′] spin system47 for the pair of
chemically equivalent protons. While the configuration seen
in 4-I has not been observed previously in H2 oxidative
addition to IrX(CO)L2 complexes where L2 is a bidentate
ligand, it has been seen before in a minor product through
p-H2 addition to trans-IrCl(CO)(PPh3)2.48,49
The reaction of 1-Br with hydrogen is similar to that of
1-I and requires modest heating for the dissociation of CO
to generate the coordinatively unsaturated species IrBr(CO)-
(xantphos) (2-Br). On the other hand, the reaction of 1-Cl
with para hydrogen yields initially only the product in which
both hydride ligands are trans to phosphorus (4-Cl), while
3-Cl is never observed. Complex 4-Cl is quite stable, and a
preparative-scale synthesis was carried out, allowing for the
isolation of crystals for an X-ray diffraction study. The X-ray
diffraction study confirms the geometry of 4-Cl (see Figure
7) and shows disorder (72:28) between the mutually trans
chloride and carbonyl ligands. (Since the xantphos ligand
dominates the packing in the solid state, the Ir position
exhibits a concomitant disorder as well.) Tables 1 and 5 show
the crystallographic details. The dominant packing form has
the chloride ligand on the same side of the molecule as the
xantphos backbone, in contrast with the structure of 1-Cl
where the chloride is on the opposite side. Consistent with
the two packing forms in the X-ray structure, low-temper-
1
ature H and 31P{1H} NMR spectroscopy of the isolated
chloro dihydride complex 4-Cl show the presence of two
isomeric dihydrides in a 5.6:1 ratio at -40 °C, both of which
(44) Sandee, A. J.; van der Veen, L. A.; Reek, J. N. H.; Kamer, P. C. J.;
Lutz, M.; Spek, A. L.; van Leeuwen, P. W. N. M. Angew. Chem., Int.
Ed. 1999, 38, 3231-3235.
(47) Emsley, J. W.; Feeney, J.; Sutcliffe, L. H. High-Resolution Nuclear
Magnetic Resonance Spectroscopy; Pergamon Press: London, 1965.
(48) Hasnip, S. K.; Colebrooke, S. A.; Sleigh, C. J.; Duckett, S. B.; Taylor,
D. R.; Barlow, G. K.; Taylor, M. J. J. Chem. Soc., Dalton Trans.
2002, 743-751.
(49) Hasnip, S. K.; Duckett, S. B.; Sleigh, C. J.; Taylor, D. R.; Barlow, G.
K.; Tayor, M. J. Chem. Commun. 1999, 1717-1718.
(45) Zuideveld, M. A.; Swennenhuis, B. H. G.; Boele, M. D. K.; Guari,
Y.; van Strijdonek, G. P. F.; Reek, J. N. H.; Kamer, P. C. J.; Goubitz,
K.; Fraanje, J.; Lutz, M.; Spek, A. L.; van Leeuwen, P. W. N. M. J.
Chem. Soc., Dalton Trans. 2002, 2308-2317.
(46) Nieczypor, P.; van Leeuwen, P. W. N. M.; Mol, J. C.; Lutz, M.; Spek,
A. L. J. Organomet. Chem. 2001, 625, 58-66.
7202 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
Figure 7. Two ORTEP views of 4-Cl showing a partial labeling scheme; ellipsoids are at the 50% probability level. Hydrogen atoms other than the hydride
ligands have been removed for clarity.
Figure 8. 1H NMR spectra (left) and 31P NMR spectra (right) of 4-Cl at 25 °C (top) and -40 °C (bottom) in CD2Cl2.
Table 5. Selected Bond Lengths (Å) and Angles (deg) for the Major
Form of 4-Cl
that if this mechanism were operative, it would rule against
dechelation of xantphos as the mechanism to explain the
Ir(1)-C(1)
Ir(1)-Cl(1)
Ir(1)-P(1)
Ir(1)-P(2)
1.877(8)
Ir(1)-H(1)
Ir(1)-H(2)
C(1)-O(1)
1.56
1.56
1.078(8)
fluxional behavior of 1-X since this would mean that the H2
addition reaction should occur readily at ambient temperature,
which it does not.
2.3980(16)
2.4231(7)
2.4250(7)
1
HMQC experiments confirm the H-31P and 31P-31P
C(1)-Ir(1)-Cl(1)
C(1)-Ir(1)-P(1)
Cl(1)-Ir(1)-P(1)
C(1)-Ir(1)-P(2)
Cl(1)-Ir(1)-P(2)
P(1)-Ir(1)-P(2)
C(1)-Ir(1)-H(1)
Cl(1)-Ir(1)-H(1)
167.8(4)
98.0(4)
91.59(6)
94.9(4)
90.34(6)
102.27(2)
88.2
P(1)-Ir(1)-H(1)
P(2)-Ir(1)-H(1)
C(1)-Ir(1)-H(2)
Cl(1)-Ir(1)-H(2)
P(1)-Ir(1)-H(2)
P(2)-Ir(1)-H(2)
H(1)-Ir(1)-H(2)
166.5
89.0
86.5
86.8
85.2
172.1
83.3
couplings of 3-X and 4-X (X ) I, Br) in this reaction system.
It is notable that over longer time periods at 50 °C, both
dihydride isomers, 3-X and 4-X (X ) I, Br), convert cleanly
to a more stable isomer (5-X), in which the hydride ligands
are trans to phosphorus and iodide, in approximately 2 h
(Scheme 2, Figure 6). Analogous thermodynamic product
isomers in H2 oxidative addition have been described
previously for reactions involving IrX(CO)(dppe) and related
systems.50,51 The chloride complex 4-Cl behaves somewhat
differently, with conversion from 4-Cl to 5-Cl requiring
heating at 90 °C for 1 day.
80.9
possess two hydrides that are trans to phosphorus (Figure
8). As with complex 1-Cl, there is an apparent π-stacking
interaction between two phenyl groups of the xantphos
ligand, with a separation of 3.6 Å.
An alternative mechanism to rationalize the formation of
4-X is illustrated in Scheme 3 and involves dechelation of
the xantphos ligand in 1-X to give a four-coordinate 16-
electron species with a monodentate xantphos ligand, which
then adds hydrogen, followed by rechelation of the xantphos
ligand with loss of CO to generate 4-X (as well as 3-X).
This mechanism cannot be ruled out without additional
experiments including determination of the rate law for the
reaction and its dependence on H2 pressure. It is noteworthy
Synthesis and Characterization of Trihydride and Acyl
39
Complexes. The reaction of complex 1-I with NaBH4 at
ambient temperature yields a mixture of IrH(CO)2(xantphos)
(6) and IrH3(CO)(xantphos) (7) in a 1.4:1 ratio, and pho-
(50) Johnson, C. E.; Fisher, B. J.; Eisenberg, R. J. Am. Chem. Soc. 1983,
105, 7772-7774.
(51) Johnson, C. E.; Eisenberg, R. J. Am. Chem. Soc. 1985, 107, 3148-
3160.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7203

Fox et al.
Scheme 3
Scheme 4
Table 6. Selected Bond Lengths and Angles for 8
tolysis of the product mixture under 2 atm of H2 gives
trihydride 7 cleanly (eq 2). The resonance in the H NMR
spectrum corresponding to the hydride ligands trans to the
two phosphorus donors of the xantphos ligand consists of a
second-order pattern similar to that of the dppe analogue
(Figure 9, top).39 When the reaction of a sample of 7 with
1
Ir(1)-C(1)
Ir(1)-C(2)
Ir(1)-C(3)
Ir(1)-P(1)
1.864(4)
1.945(4)
2.104(4)
2.4172(12)
Ir(1)-P(2)
C(1)-O(1)
C(2)-O(2)
C(3)-O(3)
2.4191(12)
1.189(5)
1.131(5)
1.227(6)
C(1)-Ir(1)-C(2)
C(1)-Ir(1)-C(3)
C(2)-Ir(1)-C(3)
C(1)-Ir(1)-P(1)
C(2)-Ir(1)-P(1)
C(3)-Ir(1)-P(1)
C(1)-Ir(1)-P(2)
C(2)-Ir(1)-P(2)
C(3)-Ir(1)-P(2)
132.37(17)
87.81(19)
87.03(17)
87.01(14)
89.98(12)
169.80(12)
118.42(13)
108.81(12)
89.14(13)
P(1)-Ir(1)-P(2)
C(36)-P(1)-Ir(1)
C(12)-P(1)-Ir(1)
C(6)-P(1)-Ir(1)
C(24)-P(2)-Ir(1)
C(18)-P(2)-Ir(1)
C(30)-P(2)-Ir(1)
O(1)-C(1)-Ir(1)
O(2)-C(2)-Ir(1)
101.05(4)
115.46(11)
118.79(12)
113.08(12)
112.96(12)
116.60(12)
119.63(11)
174.4(4)
171.3(4)
involves an ethyl ethylene complex.16 Crystals of 8 suitable
for X-ray diffraction were grown by vapor diffusion of
pentane into a toluene solution at -35 °C. The geometry of
8, which is shown in Figure 10, is a distorted trigonal
bipyramid, as expected based on the related dppe complex,16
with one phosphorus donor in an equatorial position, the
other phosphorus donor axial, and the propionyl group in
the other axial position. Tables 1 and 6 list the crystal-
lographic details. The most striking difference between the
structure of 8 and that of Ir(COEt)(CO)2(dppe) is the P-Ir-P
bond angle, which is 101.06° in 8 and 83.71° in the dppe
complex.16 In other respects, the structures are very similar.
However, while the Ir-C(O) bond lengths are virtually
identical in the dppe complex (1.869 and 1.870 Å), they are
significantly different in 8 (1.864 and 1.944 Å). While the
structures of 1-I and 8 are both trigonal bipyramidal with
one phosphine ligand equatorial and the other axial, the major
difference between them is the fact that in 1-I the iodide
ligand occupies an equatorial position, while in 8 the anionic
propionyl ligand is axial. The P-Ir-P angles are similar in
the two structures (100.29° for 1-I and 101.06° for 8), and
the sum of the three equatorial bond angles in each is within
one degree of 360°, demonstrating the coplanarity of the
equatorial ligands.
Figure 9. (top) Hydride region of the 1H NMR spectrum of 7 at room
temperature in C6D6. (bottom) Hydride region of the 1H NMR spectrum of
7 upon addition of p-H2 at 80 °C in C6D6.
1
p-H2 is monitored by H NMR spectroscopy at 80 °C, the
hydride signals show polarization (Figure 9, bottom), indicat-
ing exchange with free p-H2: no polarization is seen at
temperatures lower than 70 °C, consistent with the view that
a thermally driven reductive elimination of H2 is necessary
for exchange between the trihydride and free dihydrogen.
The NMR spectra of 8 indicate fluxional behavior in
1
solution: the propionyl ethyl resonances in the H NMR
spectrum are broad, and only one broad resonance is seen
in the 31P{1H} spectrum. A variable-temperature NMR study
shows that 8 undergoes fluxional behavior similar to that of
iodide complex 1-I (vide supra). When the temperature is
The acyl complex Ir(COEt)(CO)2(xantphos) (8) is syn-
thesized from trihydride 7 via the sequence shown in Scheme
4 that, based on previous work with the dppe analogue of 7,
7204 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
Figure 10. Two ORTEP views of 8 showing a partial labeling scheme; ellipsoids are at the 50% probability level. Hydrogen atoms have been removed
for clarity.
lowered from 25 to 0 °C, the single phosphorus resonance
of 8 sharpens, and coupling in the broadened propionyl ethyl
resonances (JHH ) 7.1 Hz) becomes observable in the H
higher yields of aldehyde (in part because of the harsher
conditions) but with lower linear-to-branched selectivity.14
The Ir(xantphos) complexes are also catalysts for styrene
hydroformylation, although the activity of these complexes
is far lower than that of RhH(CO)2(xantphos) generated in
situ from Rh(CO)2(acac) (1.4 vs 77% conversion after 20 h
at 80 °C).52 However, the linear-to-branched selectivity is
higher using the iridium systems than rhodium (3.3:1 vs 1:1.6
after 20 h at 80 °C). The linear-to-branched selectivity
observed with the iridium complexes is unusual because most
hydroformylation catalysts yield more branched than linear
products in the hydroformylation of styrene.19-21,23 As with
1-hexene, hydroformylation catalysis by the iridium com-
plexes is inhibited in the presence of the free xantphos ligand.
The much lower rate of reaction when using an Ir(xantphos)
catalyst rather than a rhodium analogue is consistent with
the ability to observe acyl complexes 8 and IrH2(COEt)-
(CO)(xantphos) (9) under noncatalytic conditions (vide infra).
Under catalytic conditions, only monohydride 6 (or its dppe
analog) is observed by NMR spectroscopy.
The addition of p-H2 to acyl complex 8 at room temper-
ature leads to polarized hydride resonances in the 1H NMR
spectrum corresponding to propionyl dihydride complex 9
(Figure 11a). Acyl dihydride species are intermediates in
hydroformylation catalysis, and although these species have
not been observed directly in catalytically active systems,
they have been seen in model systems.35,36 Over the course
of 5 min, the polarization in the hydride resonances of 9
diminishes until the hydride signals are only just visible.
Shaking of the NMR tube to dissolve more unreacted p-H2
does not cause the polarized signals to return (Figure 11b),
which is not unexpected because of the coordinative satura-
tion of complex 8 which remains largely unreacted. However,
upon heating of the sample to 70 °C, a return of polarization
in the hydride resonances of 9 occurs along with conversion
1
NMR spectrum. At -30 °C, the 31P{1H} spectrum once again
displays only a broad signal, and no clear resonances are
seen until -70 °C when two broad resonances appear.
Further cooling to -80 °C gives better-resolved peaks at
-15 and -24 ppm, consistent with the trigonal bipyramidal
structure determined crystallographically (see Supporting
Information). Additionally, the resonance corresponding to
the two methyl groups on the xantphos backbone broadens
as the temperature is lowered and then splits into two separate
resonances at -50 °C. When the sample is cooled below
-30 °C, the propionyl ethyl resonances rebroaden, although
that of the CH2 group becomes broad at higher temperature
than does that of the CH3 group. As with complex 1-I, the
fluxional behavior of 8 can be explained in terms of two
processes: first, Berry pseudorotations with a lower-energy
barrier and coalescence at -40 °C, and second, intermo-
lecular CO ligand exchange with a higher-energy barrier seen
by the broadening of resonances at 25 °C (vide supra).
Use of Ir(xantphos) Complexes as Hydroformylation
Catalysts and Hydroformylation Model Systems. Com-
plexes 6, 7, and 8 are catalysts for the hydroformylation of
1-hexene under 3 atm of a 2:1 mixture of H2 and CO at 75
°C. The aldehyde is produced in about a 10% NMR yield
after 24 h with a modest linear-to-branched ratio of 4 to 1.
Over the same time period, alkene isomerization is competi-
tive with hydroformylation such that >50% of the 1-hexene
substrate is converted to internal isomers. The Ir(COEt)(CO)2-
(dppe) complex also catalyzes 1-hexene hydroformylation
with similar activity and linear-to-branched selectivity.
Complete catalyst inhibition occurs in the presence of added
xantphos (2-fold excess relative to iridium), such that no
hydroformylation or alkene isomerization is observed. The
catalyst inhibition suggests the possibility that xantphos
dissociation may be necessary for catalysis to occur. A
previous study of 1-hexene hydroformylation using Ir4(CO)12
as catalyst, albeit at higher temperature and pressure, showed
(52) Note that there is 5% catalyst loading in the system, and therefore,
1.4% conversion is not strictly catalytic. Even if only small quantities
of active catalyst are present, a very low number of turnovers is
observed.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7205

Fox et al.
Figure 12. 1H NMR spectrum showing polarization in the hydride
resonances of acyl dihydride 9 and trihydride 7, as well as the oneH-
PHIP in the propanal product upon addition of p-H2 to 8 at 75 °C in C6D6
(9 ) 9, 2 ) 7, and b ) 6).
overlapping hydride resonances of the intermediate acyl
dihydride complex, which was proposed to be necessary in
the earlier report.35 The reaction of 8 with p-H2 clearly shows
that the hydride resonances of 9 are well separated such that
there are no second-order effects (see Figures 11 and 12).
The oneH-PHIP effect is observed in both benzene-d6 and
acetone-d6 at temperatures ranging from 75 to 85 °C in the
present system.
Figure 11. Hydride region of 1H NMR spectra upon addition of p-H2 to
complex 8 in C6D6: (a) initial spectrum at 25 °C, (b) spectrum after shaking
to redissolve p-H2 once polarization had disappeared, and (c) spectrum after
heating to 70 °C (9 ) 9, 2 ) 7, and b ) 6).
On the basis of these observations, the reaction of
Ir(COEt)(CO)2(dppe) with p-H2 at 75 °C was re-examined
on a 500 MHz NMR spectrometer to remove the second-
order effects seen previously at 400 MHz, but oneH-PHIP
was still seen to occur. The 1H NMR spectrum clearly shows
that the overlap observed in the hydride signals of IrH2-
(COEt)(CO)(dppe) at 400 MHz is nonexistent in the 500
MHz spectrum (see Supporting Information). Additionally,
the experiment done under the same conditions, but with
the acquisition of a 31P-decoupled spectrum in which the two
hydride resonances of IrH2(COEt)(CO)(dppe) are well
separated, also shows oneH-PHIP in the aldehyde product.
of 9 to form trihydride 7 and propanal (Figure 11c). The
fact that polarization does not return at room temperature
upon introduction of more p-H2 into solution shows that the
addition of p-H2 to form dihydride 9 is not reversible at that
temperature, indicating the likelihood that a small quantity
of the four-coordinate Ir(COEt)(CO)(xantphos) is present
initially to react with p-H2 and form the dihydride species.
As with complexes 1-X, it is possible that a mechanism
involving dechelation of xantphos prior to oxidative addition
of dihydrogen is active in this system, but there is no direct
evidence for it.
It is worth noting that, as 8 is consumed, the concentration
of free CO increases and will compete for any open
coordination sites on the Ir center, thus slowing the reaction
with dihydrogen. Consistent with this hypothesis, the addition
of p-H2 to 8 in the presence of 1 atm CO does not lead to
the polarized resonances corresponding to 9 until the sample
is heated to 50 °C, at which temperature CO loss presumably
occurs. Additionally, the dissociation of one of the two
carbonyl ligands is effected by photolysis of 8 for 10 s
outside the NMR spectrometer at room temperature in the
presence of p-H2, supporting a mechanism involving initial
dissociation of CO; strongly polarized signals corresponding
to 9 are immediately apparent when the 1H NMR spectrum
is recorded at ambient temperature. Both 8 and 9 are
consumed completely after photolysis for longer periods to
give trihydride 7 and propanal cleanly.
Conclusions
A new series of iridium complexes bearing the large
natural bite angle xantphos ligand has been synthesized and
studied, and five of the compounds have been characterized
using X-ray diffraction. The trans-chelating ability of the
xantphos ligand results in the formation of an unusual H2
oxidative addition product upon addition of p-H2 to 1-X
(X ) I, Br, Cl). Variable-temperature NMR spectroscopy
was used to examine the high- and low-temperature dynamic
processes in complexes 1-I and 8 that cause the intercon-
version of the two phosphine donors and the two methyl
groups of the xantphos ligand. Parahydrogen-induced po-
larization was used to great effect to allow observation of
propionyl dihydride complex 9, and it also made the
characterization of dihydrides 3-X, 4-X, and 5-X by 1H and
31P NMR spectroscopy, including the use of two-dimensional
techniques, possible. Additionally, polarization in the alde-
hyde product upon the reaction of para hydrogen with 8 and
with Ir(COEt)(CO)2(dppe) at different magnetic field strengths
has shown that the unusual oneH-PHIP observation reported
earlier is not limited to cases in which second-order effects
exist in the hydride resonances of the model acyl dihydride
intermediates. Finally, while complex 8 serves as a model
Interestingly, the 1H NMR spectrum recorded upon reac-
tion of 8 with p-H2 at 80 °C shows the aldehydic proton of
the propanal product in weak net emission (Figure 12). This
effect (called oneH-PHIP because only one proton in the
product aldehyde originates from a p-H2 molecule) was
observed previously in an analogous reaction with Ir(COEt)-
(CO)2(dppe).35 The current result demonstrates that the
oneH-PHIP effect might be more common and not limited
to the specific conditions of a second-order effect in the
7206 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
(t, JHH ) 7.6 Hz, aryl, 2H), 6.62 (br m, aryl, 2H), 1.37 (br, CH3,
6H). 31P{1H} NMR (C6D6, 162 MHz): δ -13.5 (br). IR (KBr):
2029, 1948 cm-1. Anal. Calcd for C41H32IIrO3P2: C, 51.64; H, 3.38.
Found: C, 51.28; H, 3.52.
system for a key hydroformylation intermediate, 8, 6, and
7, as well as Ir(COEt)(CO)2(dppe), have been found to
catalyze the hydroformylation of 1-hexene and styrene under
mild conditions, but only modestly.
IrBr(CO)2(xantphos) (1-Br). Complex 1-Br was synthesized
as described above for compex 1-I using [Bu4N][IrBr2(CO)2] (500
mg, 0.77 mmol) as the starting material. Yield: 650 mg (93%).
Crystals suitable for X-ray diffraction were obtained from a
concentrated CH2Cl2 solution layered with pentane at -35 °C. The
X-ray and NMR data were consistent with complex 1-Br plus 0.5
equiv of dichloromethane, as was the elemental analysis. 1H NMR
(CDCl3, 500 MHz, 25 °C): δ 7.60-7.00 (br m, 20H, phenyl), 7.50
(d, JHH ) 7.7 Hz, 2H, aryl), 7.11 (d, JHH ) 7.7 Hz, 2H, aryl), 6.38
(br, 2H, aryl), 1.58 (s, 6H, CH3). 31P{1H} NMR (CD2Cl2, 203 MHz,
50 °C): δ -7.27 (s). IR (KBr): 2023, 1950 cm-1. Anal. Calcd for
C41.5H33BrClIrO3P2: C, 52.51; H, 3.50. Found: C, 52.48; H, 3.15.
Experimental Section
General Considerations. Unless otherwise stated, all reactions
and manipulations were performed in dry glassware under a nitrogen
atmosphere using either standard Schlenk techniques or an inert-
atmosphere glovebox. Styrene and 1-hexene were purchased from
Aldrich and used without further purification. Benzene-d6, acetone-
d6, dichloromethane-d2, and toluene-d8 were purchased from
Cambridge Isotope Laboratories. THF, CH2Cl2, and toluene were
purified as described by Grubbs.53 All NMR spectra were recorded
on either Bruker Avance 400 or 500 MHz spectrometers. 1H NMR
chemical shifts (in ppm) are relative to tetramethylsilane and
IrCl(CO)2(xantphos) (1-Cl). In a glovebox, a solution of
xantphos (920 mg, 1.6 mmol) in CH2Cl2 (20 mL) was added to a
stirred solution of [Ir(COD)Cl]2 (540 mg, 0.80 mmol) also in CH2-
Cl2 (30 mL). CO was bubbled through the resulting yellow solution
for 15 h. Volatile materials were then allowed to evaporate from
the (now light orange) solution under an increased flow of CO.
The resulting orange solid was crystallized from CH2Cl2 layered
with pentane at -35 °C to give dark orange crystals, which were
washed with pentane. The crystals were dried in vacuo to give 1-Cl
as an orange powder. Yield: 420 mg (30%). 1H NMR (C6D6, 500
MHz, 25 °C): δ 8.01 (br m, 8H, phenyl), 7.22 (dd, JHH ) 7.7, 1.4
Hz, 2H, aryl), 7.04 (br m, 12H, phenyl), 6.91 (t, JHH ) 7.55 Hz,
2H, aryl), 6.78 (br, 2H, aryl), 1.51 (br s, 6H, CH3). 31P{1H} NMR
referenced using chemical shifts of residual solvent resonances. 31
P
NMR chemical shifts (in ppm) are relative to an external 85%
solution of phosphoric acid in the appropriate solvent. The cis and
trans designations in the assignments of phosphorus NMR signals
are relative to the hydride ligands. [Bu4N][IrI2(CO)2],38 [Ir(COD)-
Cl]2,40 Ir(COEt)(CO)2(dppe),16 Rh(CO)2(acac),54 and xantphos23
were prepared as described previously. [Bu4N][IrBr2(CO)2] was
prepared by heating to reflux a solution of K3IrBr6 in concentrated
HBr and formic acid, followed by the addition of [Bu4N]Br under
inert atmosphere. Hydrogen enriched in the para spin state was
prepared by the cooling of hydrogen over FeCl3 adsorbed onto silica
at 77 K.55 Elemental analyses were performed by Desert Analytics,
Inc.
Reactions with p-H2. In a typical experiment, approximately
1-2 mg of sample was added to a J. Young NMR tube after which
the desired solvent (∼0.6 mL) was added to the tube. The tube
was degassed using three freeze-pump-thaw cycles and p-H2 (ca.
3 atm at 298 K) was added to the tube while it was immersed in
liquid N2. The sample was thawed and shaken vigorously im-
mediately before being inserted into the NMR probe heated to the
desired temperature.
IrI(CO)2(xantphos) (1-I). Complex 1-I was synthesized using
a procedure similar to that previously described for the synthesis
of IrI(CO)(dppe).38 Under an N2 atmosphere, a solution of xantphos
(777 mg, 1.34 mmol) in THF (25 mL) was added dropwise to a
stirred solution of [Bu4N][IrI2(CO)2] (1.00 g, 1.34 mmol) in THF
(50 mL), cooled to -78 °C. The yellow solution turned orange
immediately upon the ligand addition. The mixture was stirred at
-78 °C for 1 h, then allowed to warm to ambient temperature, and
stirred an additional 2 h. The volume of the solution was reduced
to 20 mL under vacuum, and 20 mL of degassed ethanol was added.
The volume of the solution was reduced to 10 mL, resulting in the
precipitation of an orange solid. The mixture was cooled to -35
°C in a freezer for several hours. The orange product was collected
on a frit in air, washed with cold ethanol, hexanes, and diethyl
ether (15 mL each), and dried in vacuo. Yield: 1.1 g (84%).
Crystallization of 1-I from dichloromethane layered with diethyl
ether at -35 °C yielded orange blocks suitable for an X-ray
diffraction study. 1H NMR (C6D6, 400 MHz): δ 7.67 (br, phenyl,
8H), 7.09 (d, JHH ) 7.6 Hz, aryl, 2H), 6.92 (br, phenyl, 12H), 6.77
(C6D6, 203 MHz, 50 °C): δ 0.51 (s). IR (KBr): 2017, 1944 cm-1
.
Anal. Calcd for C41H32ClIrO3P2: C, 57.11; H, 3.74. Found: C,
56.77; H, 3.49.
IrH2Cl(CO)(xantphos) (4-Cl). IrCl(CO)2(xantphos) (200 mg,
0.23 mmol) was dissolved in toluene (25 mL) and added to a glass
vessel equipped with a Teflon stopcock and magnetic stir bar. The
vessel was degassed and pressurized with 2 atm of H2. The reaction
mixture was heated at 70 °C for 1.5 h, resulting in a colorless
solution and a yellow precipitate. The mixture was allowed to cool,
and the supernatant was decanted from the yellow precipitate. The
product was dissolved in CH2Cl2 and crystallized by layering the
solution with diethyl ether at ambient temperature. The isolated
golden crystals of 4-Cl were washed with pentane and dried in
vacuo. Yield: 120 mg (59%). 1H NMR (CD2Cl2, 500 MHz,
25 °C): δ 7.7-7.5 (br, 4H, phenyl), 7.60 (br d, JHH ) 7.7 Hz, 2H,
aryl), 7.5-7.2 (m, 14H, phenyl), 7.14 (td, JHH ) 7.7, 1.3 Hz, 2H,
aryl), 6.34 (br, 2H, aryl), 1.88 (br s, 3H, CH3), 1.60 (br s, 3H,
CH3), -9.39 (br d, 2H IrH2). 31P{1H} NMR (CD2Cl2, 203 MHz,
50 °C): δ -6.49 (s). IR (KBr): 2123, 2083, 2017 cm-1. Anal.
Calcd for C40H34ClIrO2P2: C, 57.45; H, 4.10. Found: C, 58.07;
H, 4.17.
IrH2I(CO)(xantphos) (3-I). 1H NMR (C6D6, 500 MHz,
50 °C): δ -9.39 (dd, JHP ) 13, 22 Hz, IrH), -11.91 (dd, JHP
)
150, 18 Hz, IrH). 31P{1H} NMR (C6D6, 203 MHz, 50 °C): δ -10.9
(br, IrPcis/cis), -26.1 (br, IrPtrans/cis).
IrH2I(CO)(xantphos) (4-I). 1H NMR (C6D6, 500 MHz,
50 °C): δ -10.35 (br d, JHP ) 165 Hz). 31P{1H} NMR (C6D6, 203
MHz, 50 °C): δ -22.1 (s).
(53) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(54) Hernandez-Gruel, M. A. F.; Perez-Torrente, J. J.; Ciriano, M. A.; Oro,
L. A. Inorg. Synth. 2004, 34, 127-132.
(55) Millar, S. P.; Jang, M.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chim.
Acta 1998, 270, 363-375.
IrH2I(CO)(xantphos) (5-I). 1H NMR (C6D6, 500 MHz,
50 °C): δ -11.09 (dd, JHP ) 149, 25 Hz, IrH), -14.73 (m, IrH).
31P{1H} NMR (C6D6, 203 MHz, 50 °C): δ -15.4 (d, JPP
23 Hz, IrPcis/cis), -21.5 (d, JPP ) 23 Hz, IrPtrans/cis).
)
Inorganic Chemistry, Vol. 45, No. 18, 2006 7207

Fox et al.
IrH2Br(CO)(xantphos) (3-Br). 1H NMR (C6D6, 500 MHz,
(br, phenyl, 8H), 7.00 (br, phenyl, 12H), 6.84 (m, aryl, 6H), 2.82
(br, C(O)CH2CH3, 2H), 1.46 (br, CH3, 6H), 0.84 (br, C(O)CH2CH3,
3H). 31P{1H} NMR (C6D6, 203 MHz): δ -20.24 (s).
70 °C): δ -8.00 (dd, JHP ) 21, 14 Hz, IrH), -10.91 (dd, JHP
)
159, 20 Hz, IrH). 31P{1H} NMR (C6D6, 203 MHz, 80 °C): δ -19.6
(br, IrPtrans/cis), -25.3 (br, IrPcis/cis).
1
IrH2(COEt)(CO)(xantphos) (9). H NMR (C6D6, 500 MHz):
IrH2Br(CO)(xantphos) (4-Br). 1H NMR (C6D6, 500 MHz,
70 °C): δ -9.22 (dd, JHP ) 188, 12 Hz, IrH). 31P{1H} NMR (C6D6,
203 MHz, 70 °C): δ -15.8 (s).
δ -7.44 (t, JHP ) 18.80 Hz, IrH, 1H), -9.12 (dd, JHP ) 128.79,
19.87 Hz, IrH, 2H). 31P{1H} NMR (C6D6, 203 MHz): δ -11.8 (d,
JPP ) 20.1 Hz, IrPtrans/cis), -21.9 (d, JPP ) 22.6 Hz, IrPcis/cis).
Hydroformylation Reactions. In a typical experiment, a J.
Young NMR tube was charged with IrH3(CO)(xantphos) (3.1 mg,
0.0039 mmol), styrene (8.9 µL, 0.078 mmol) or 1-hexene (9.7 µL,
0.078 mmol), ferrocene as internal standard (1.0 mg, 0.0054 mmol),
and benzene-d6 (0.6 mL). The tube was degassed using three
freeze-pump-thaw cycles, and CO (1 atm) was added. The tube
was frozen in liquid N2, and H2 (∼2 atm) was added. The reaction
was heated in an oil bath at the desired temperature and monitored
regularly by NMR spectroscopy.
IrH2Br(CO)(xantphos) (5-Br). 1H NMR (C6D6, 500 MHz,
80 °C): δ -10.71 (ddd, JHP ) 153, 26 Hz, JHH ) 5.4 Hz, IrH),
-16.61 (m, IrH). 31P{1H} NMR (C6D6, 203 MHz, 80 °C): δ -9.57
(d, JPP ) 33 Hz, IrPcis/cis), -14.9 (d, JPP ) 31 Hz, IrPtrans/cis).
IrH2Cl(CO)(xantphos) (5-Cl). 1H NMR (C6D6, 500 MHz,
80 °C): δ -10.13 (dd, JHP ) 157, 25 Hz, IrH), -17.3 (m, IrH).
31P{1H} NMR (C6D6, 203 MHz, 80 °C): δ -4.97 (br, IrPcis/cis),
-9.92 (br, IrPtrans/cis).
IrH3(CO)(xantphos) (7). The preparation of 7 was carried out
in a manner similar to the preparation of IrH3(CO)(dppe) from IrI-
(CO)(dppe).39 Under N2 (nearly identical results were obtained
under an H2 atmosphere), a solution of NaBH4 (280 mg, 7.3 mmol)
in ethanol (50 mL) was added to a stirred solution of 1 (700 mg,
0.73 mmol) in dichloromethane (10 mL). The orange solution faded
to colorless immediately upon addition of the NaBH4 solution. The
reaction mixture was stirred at ambient temperature for 1 h, after
which 2 g of silica gel was added. The mixture was stirred until
gas evolution ceased (∼1 h). The silica gel was removed from the
solution by filtration through a glass frit in air, and the product
was washed through the frit with additional dichloromethane (10
mL). The solution was concentrated on a rotary evaporator to 10
mL, and the very pale yellow precipitate that formed was collected
on a glass frit and washed with cold ethanol and diethyl ether (10
mL each). The crude product (a 1:0.7 mixture of 6 and 7) was
dissolved in 25 mL of toluene and added to a glass vessel with a
Kontes stopcock and a magnetic stir bar. The vessel was degassed,
and 2 atm of H2 were added. The reaction mixture was photolyzed
with an unfiltered 200 W mercury xenon lamp for 20 h to give
clean 7. Yield: 420 mg (71%). Crystallization from dichloro-
methane layered with diethyl ether at -35 °C yielded pure 7 as a
Crystal Structure Determinations. For each structure deter-
mination, a crystal was placed onto the tip of a 0.1 mm diameter
glass fiber and mounted on a Bruker SMART APEX II Platform
CCD diffractometer for a data collection at 100.0(1) K, except for
8 (100(2) K). A preliminary set of cell constants was calculated
from the reflections harvested from three sets of 12 (1-I, 1-Cl) or
20 (1-Br, 4-Cl, 8) frames. These initial sets of frames were oriented
such that orthogonal wedges of reciprocal space were surveyed.
This produced initial orientation matrixes determined from 165
reflections. The data collection for each determination was carried
out using Mo KR radiation (graphite monochromator). A randomly
oriented region of reciprocal space was surveyed to the extent of
one sphere and to a resolution of 0.70-0.72 Å. Four major sections
of frames were collected with 0.30° (1-I, 8) or 0.50° (1-Br, 1-Cl,
4-Cl) steps in ω at four different φ settings and a detector position
of -28° (1-I, 1-Cl, 8) or -33° (1-Br, 4-Cl) in 2θ. The intensity
data were corrected for absorption (SADABS).56 Final cell constants
were calculated from the xyz centroids of the strong reflections from
the actual data collection after integration (SAINT).
Each structure was solved using SHELXS-9757 and refined using
SHELXL-97.57 The space groups P21/c for 1-X (X ) I, Br, Cl)
and P1h for 4-Cl and 8 were determined. For each determination, a
direct-methods solution was calculated which provided most non-
hydrogen atoms from the E-map. Full-matrix least squares/
difference Fourier cycles were performed which located the
remaining non-hydrogen atoms. All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were placed in ideal positions and refined as riding atoms
with relative isotropic displacement parameters. Unit cell, space
group, data collection, and refinement parameters for all structures
are summarized in Table 1.
1
light tan solid. H NMR (C6D6, 500 MHz): δ 7.71 (br, phenyl,
4H), 7.60 (br, phenyl, 4H), 7.07 (d, JHH ) 7.95 Hz, aryl, 2H), 6.94
(br, phenyl, 6H), 6.9-6.7 (m, phenyl, 6H), 6.69 (td, JHH ) 7.74,
1.18 Hz, aryl, 2H), 6.49 (t, JHH ) 8.07 Hz, aryl, 2H), 1.43 (s, CH3,
3H), 1.37 (s, CH3, 3H), -9.37 (t, JHP ) 18.61 Hz, IrH, 1H), -10.35
(ddd, JHP ) 129.62, 15.70 Hz, JHH ) 2.58 Hz, IrH, 2H). 31P{1H}
NMR (C6D6, 203 MHz): δ -8.62. IR (KBr): 2060 (νIrH), 2008
(νIrH), 1954 (νCO), 1917 (νIrH) cm-1. Anal. Calcd for C40H35IrO2P2:
C, 59.92; H, 4.40. Found: C, 59.63; H, 4.23.
Ir(COEt)(CO)2(xantphos) (8). Complex 8 was synthesized
using the method previously reported for the preparation of
Ir(COEt)(CO)2(dppe).16 Trihydride 7 (600 mg, 0.75 mmol) was
dissolved in toluene (30 mL) and heated at 80 °C with ethylene
bubbling through the solution for 2 h. The golden solution was
allowed to cool to room temperature and was stirred with CO
bubbling through the solution for 20 h at ambient temperature.
Ethanol (30 mL) was added to the golden solution, and the entire
mixture was concentrated to ∼5 mL by rotary evaporation. The
tan powdery solid that precipitated from the solution was isolated
from the supernatant solution and washed with cold ethanol (10
mL). NMR spectroscopy revealed the solid to be a mixture of the
desired complex and 6 in a 1:1.5 ratio (525 mg). Crystals of
sufficient quality for an X-ray diffraction study were obtained by
vapor diffusion of pentane into a toluene solution of the product at
-35 °C, but samples of sufficient purity for acceptable elemental
analysis could not be obtained. 1H NMR (C6D6, 500 MHz): δ 7.67
Details for the Structure of 1-Br. All atoms lie in general
positions. The axial carbonyl and bromide ligands are modeled as
disordered with each other (84:16). Thermal constraints were
applied to the disorder model. Highly disordered solvent is present
in channels parallel to the a axis and appears to be dichloromethane.
The reflection contibutions from this solvent were removed using
the program PLATON, function SQUEEZE.58
Details for the Structure of 1-Cl. The chloride ligand is modeled
as disordered with the axial carbonyl ligand (85:15). π-Stacking
(56) The SADABS absorption correction program is based on the method
of Blessing; Blessing, R. H. Acta Crystallogr. A 1995, 51, 33-38.
(57) SHELXTL: Structure Analysis Program, version 5.04; Siemens
Industrial Automation Inc.: Madison, WI, 1995.
(58) Spek, A. L. PLATON, version 300106; Utrecht University: Utrecht,
The Netherlands, 2006; Spek, A. L. Acta Crystallogr. A 1990, 46,
C34.
7208 Inorganic Chemistry, Vol. 45, No. 18, 2006

Iridium Hydroformylation System
(∼3.6 Å) is likely present between two of the phenyl rings; the
angle between their planes is 2.46(7)°.
Acknowledgment. We gratefully acknowledge Mr.
Abdurrahman C. Atesin for helpful discussions and for
providing [Ir(COD)Cl]2, Dr. Mesfin Janka for helpful dis-
cussions, and Professor George G. Stanley for insightful
comments. We also thank the National Science Foundation
(Grant CHE-0092446) and the donors of the Petroleum
Research Fund for support of this work.
Details for the Structure of 4-Cl. The chloride and carbonyl
ligands are disordered with each other, such that the iridium atom
is also disordered over two positions (72:28). Thermal constraints
and geometrical restraints were applied to the disorder model.
π-Stacking may be present: two of the phenyl rings are eclipsed
with a distance of approximately 3.6 Å and an angle of 6.35(16)°.
Details for the Structure of 8. The asymmetric unit consists of
one iridium molecule and one and a half toluene molecules. Toluene
molecule C45-C51 is modeled as disordered over two positions
(60:40), and toluene molecule C52-C58 is modeled as disordered
over a crystallographic inversion center (50:50). Fixed geometries
were applied to the benzene portions of both toluene molecules
(AFIX 66).
Supporting Information Available: Crystal data, atomic
coordinates, bond distances, bond angles, and anisotropic displace-
ment parameters for 1-X (X ) I, Br, Cl), 4-Cl, and 8 (in CIF
format), as well as some NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC060731L
Inorganic Chemistry, Vol. 45, No. 18, 2006 7209