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Reactions of Iridium Hydride Pincer Complexes with Dioxygen:
New Peroxo Complexes and Reversible O2 Binding
D. Bridget Williams, Werner Kaminsky, James M. Mayer,* and Karen I. Goldberg*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington, 98195-1700
RECEIVED DATE (automatically inserted by publisher); [email protected]; [email protected]
Organometallic reactivity is utilized in a range of organic
transformations, but organometallic oxidation chemistry remains
relatively under-developed.i Molecular oxygen is the most
attractive terminal oxidant for chemical processes, in terms of
cost, atom economy, and the potential for few byproducts.i Recent
studies have described elegant palladium catalysts for the aerobic
oxidation of alcohols and other substrates, and have implicated
mechanisms where O2 serves only to re-oxidize the palladium and
does not interact with the substrates.ia In contrast, insertion of O2
into a palladium-hydride bond has been suggested in an
alternative mechanism and recently this reactivity has been
directly observed in model systems.ii Understanding the various
modes of reactivity of oxygen with organometallic complexes will
be key to developing effective homogeneous catalysis with this
oxidant.
Reported here are reactions of O2 with the iridium pincer
hydride complexes (tBuPCP)Ir(H)(X) [X = Cl (1)iiia, Ph (2)iiib, H
(3)iiic, OH (4)iiid and CCPh (5)iiie] [tBuPCP = 3-C6H3(CH2PtBu2)2].
Our long-term goal is to couple O2-reactivity with the ability of
such iridium-pincer complexes to catalytically activate C–H
bonds in saturated hydrocarbons.iv The chemistry described here
involves loss of H–X and reversible binding of O2 to form monoand bis-peroxo complexes. Reactions of iridium compounds with
O2 to form Ir(O2) species are well known, starting with Vaska’s
complex.v The products reported here, however, include the first
unsaturated iridium peroxide and the first late-transition metal
bis-peroxo complex.
The hydride-chloride complex, (tBuPCP)Ir(H)(Cl) (1), is
unreactive with 1 atm of O2 (research grade, < 0.5 ppm H2O) in
benzene-d6 over 12 h at 343 K. This was surprising given the
unsaturated nature of complex 1. In contrast, the phenyl-hydride
complex 2 reacts rapidly with 1 atm O2 in benzene solution at 298
K to give two species, 6 and 7, in addition to small amounts of
unidentified products (1H NMR: 10.7 (s), 15.8 (s)).
Decomposition is observed after ca. 40 min; however, isolated 7
can be stored in the solid state for several months. Both 6 and 7
have 1H and 31P{1H} NMR spectra that indicate C2v symmetry,
with equivalent phosphorus nuclei, methylene protons and tBu
groups.vi
The ratio of 6 to 7 depends on the concentration of oxygen in
solution. Under 5 atm O2 at 25°C, 1 is converted completely to 6,
and X-ray quality crystals of 6 were grown from pentane under
these conditions. Structure solution was accomplished (to R =
2.7%, Rw = 6.6%) using a disorder model for the twinned crystals
–
in which molecules stack in one direction along the 4 axis in some
–
domains of the I4 crystals but in the opposite direction in other
domains. The structure of the (pincer)iridium-bis(peroxo),
(tBuPCP)Ir(2-O2)2 (6) is shown in Figure 1. Due to the disorder,
the bond lengths are not very well defined, but they appear to fall
in normal ranges. For instance, the O–O distance of 1.54 (3) Å is
similar to that in other iridium peroxo complexes,vg,vii implying
that 6 is a bis-(O22–) complex with a formal 5+ Ir oxidation state.
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Figure 1. Molecular structure of (tBuPCP)Ir(2-O2)2 (7), with thermal
ellipsoids shown at 50% probability. Selected bond lengths (Å) and
angles (deg): Ir(1)-O(1) 2.008(10); Ir(1)-O(2) 2.040(9); O(1)-O(2)
1.54(3); Ir(1)-C(1) 2.023(12); Ir(1)-P(1) 2.380(4); O(1)-Ir(1)-C(1)
87.3(8); O(1)-Ir(1)-O(2) 44.7(9); O(2)-O(1)-Ir(1) 68.7(7); O(1)-O(2)-Ir(1)
66.6(7).
Under vacuum, 6 cleanly forms 7 which is assigned as the 5coordinate mono-peroxo IrIII complex (tBuPCP)Ir(2-O2) (eq 1).
Treatment of 7 with O2 gives 6 by 1H and 31P{1H} NMR,
establishing the reversible equilibrium in eq 1. The IR spectrum
of 7 in a KBr pellet shows a Ir(O2) stretch at 893.5 cm-1 which is
absent in the 18O-labeled complex, apparently shifting to ca. 811.1
cm-1.viii Characteristic of late-metal peroxo complexes,ix complex 7
reacts with SO2 to give the iridium sulfato complex,
(tBuPCP)Ir(2-SO4) (8), based on IR, 1H, and 31P{1H} NMR
spectroscopies. [Mass spec?]
P(tBu)2
O
Ir
O
O
P(tBu)2
O
- O2
vacuum
P(tBu)2
O2
O
P(tBu)2
Ir
O
(1)
7
6
(tBuPCP)Ir(H)2
The dihydride complex
(3) reacts with 5 atm O2
to give a mixture of 6, a new species 9 (ratio ~2:1), and trace
amounts of 7 and (tBuPCP)Ir(H)(OH) (4) (eq 2). Over time the
amount of 9, 7, and 4 decreases and 6 is the sole product.
Complex 4 was identified by comparison with NMR spectra of an
authentic sample, particularly the hydride signal at -31.0 ppm.iiid
Independently synthesized 4 is converted by 5 atm of O2 to
cleanly give 6 in ca. 30 minutes. Monitoring the reaction of 3
with 1 atm O2 by 1H NMR shows the appearance and growth of a
peak for water,which gradually shifts under the tert-butyl signals.
This is supported by the observation of D2O via 2H NMR in the
analogous reaction between the dideuteride complex,
(tBuPCP)Ir(D)2, and 5 atm O2.
P(tBu)2
H
Ir
H
P(tBu)2
5 atm O2
P(tBu)2
O
Ir
+
O
O
P(tBu)2
O
3
P(tBu)2
H
Ir O
H O
P(tBu)2
6
(2)
9
O2
-HCCPh
P(tBu)2
HCCPh
O
P(tBu)2
Ir
O2
O
P(tBu)2
O
Ir
O
O
P(tBu)2
O
7
5
(3)
6
The most likely pathway for these transformations is O2induced reductive elimination from (tBuPCP)Ir(H)(X) to generate
7 directly (eq 4). O2 promoted reductive elimination of C-O and
C-N bonds from NiII has been reported.xi The dihydride and
acetylide complexes, 3 and 5, do not readily reductively eliminate
at 298 K in the absence of O2,xii ruling out pre-equilibrium loss of
HX followed by O2 binding to IrI. The tentative identification of
9 as a dihydride-peroxo complex, in essence an O2-adduct of 3,
supports this pathway, although a cis isomer would appear more
likely to lead to reductive elimination. In an attempt to observe a
similar adduct, (tBuPCP)Ir(H)(tolyl) (2a) was reacted with 5 atm of
O2 at 253 K in toluene, but the sole product was 6, similar to the
298 K reaction between 2 and O2. The hydrido-chloride complex
1 is inert presumably because loss of HCl from the metal center is
unfavorable. The displacement of O2 from 7 by HCCPh (vide
infra) may proceed by a similar associative mechanism.
P(tBu)2
Ir
X
H
P(tBu)2
O2
P(tBu)2
X
Ir H
O2
P(tBu)2
P(tBu)2
-HX
Ir
O
O
P(tBu)2
Ir
(4)
The peroxo complexes are not especially reactive. For
instance, they are inert to ethylene and cyclohexene. Complex 6
is a slow catalyst for the oxidation of PPh3 to OPPh3 by O2, giving
3.4 turnovers over three days under 5 atm of O2 in C6D6 (by 1H
NMR and GC/MS). Surprisingly, the mono-peroxo complex 7
does not oxidize PPh3, perhaps because of the instability of the
intermediate oxo complex that would likely be formed. Treatment
of isolated 7 with 5 atm H2 gives water and the tetrahydride 10
(eq 5), which is readily formed by addition of H2 to 3. Similarly,
7 + D2 yields (tBuPCP)Ir(D)4 (10-d4) and D2O by 2H NMR.
O
O
P(tBu)2
7
Complex 9 is tentatively assigned as the trans-dihydride peroxo
complex trans-(tBuPCP)Ir(H)2(O2) based on a hydride signal at
-6.8 ppm in the 1H NMR which integrates to two protons relative
to the methylene protons of the ligand. This complex appears to
be analogous to trans-dihydride species (iPrPCP)Ir(CO)(H)2,xa
[(tBuPNP)Ir(CO)(H)2]PF6,xb and (tBuPNP)Ir(Ph)(H)2,xc all of which
have hydride signals between -6 and -10 ppm.
The acetylide (tBuPCP)Ir(H)(CCPh) (5) also reacts with 5 atm
O2 to very rapidly form 6. To our surprise, removal of the O2 by
freeze-pump-thaw degassing yields not only 7, but also
regenerates 5 (22%) (eq 3). This reaction indicates that not only
can 6 convert to 7 under vacuum, but that the dioxygen ligand of
7 can also apparently be displaced.
P(tBu)2
C Ph
Ir C
H
P(tBu)2
P(tBu)2
P(tBu)2
5 atm H2
H
H
+ H2 O
H
H
t
P( Bu)2
Ir
(5)
10
The only oxygenated products formed in any of these reactions
are water and trace amounts of the hydroxy-hydride, 4, from 3 +
O2 (eq 2), and water from 7 + H2 (eq 5). In none of the other
(tBuPCP)Ir(H)(X) reactions is there any evidence for O2 insertion
into an Ir–H or Ir–C bond. This contrasts recent reports of O2
insertion into metal-hydride bonds.ii,xiii In the reaction of 2 with
O2, neither hydroxy-phenyl nor hydrido-phenoxy complexes were
observed by 1H NMR spectroscopy, and no phenol was seen by
GC [GC experiment@]. Further work is needed to better define
the basic pathways for reactions of O2 with organometallic
compounds.
In summary, a number of iridium pincer hydride compounds
(tBuPCP)Ir(H)(X) react with O2 to give IrIII and IrV peroxo
complexes (tBuPCP)Ir(O2)n, n = 1 (7), or 2 (6). Dioxygen in most
cases appears to promote reductive elimination of HX rather than
inserting into an Ir–H or Ir–C bond. Yet the peroxo complex 7
reacts with hydrogen to form water and the tetrahydride complex
10. This type of reaction could be a potential alternative to more
traditional hydrogen scavengers such as t-butylethylene. Studies
are continuing in order to more fully develop the organometallic
chemistry of O2 and incorporate it into new oxidation catalysis.
Acknowledgements. Support from the NSF Center for the
Activation and Transformation of Chemical Bonds (CATSB) is
gratefully acknowledged. We thank A. S. Goldman for helpful
discussions, and Mira Kanzelberger.
Supporting Information Available: Full experimental details
and crystallographic data for 6.
(i) (a) Stahl, S. S. Angew. Chem. Int. Ed. 2004, 43, 3400-3420. (b) Sheldon,
R. A.; Kochi, J. K. Metal Catalysed Oxidations of Organic Compounds;
Academic Press: NewYork, 1981. (c) Dioxygen Activation and
Homogeneous Catalytic Oxidation; Simándi, L. I., Ed.; Elsevier Science
Publishing Company Inc.: New York, 1991. (d) Dible, B. R.; Sigman, M.
S. J. Am. Chem. Soc. 2003, 125, 872-873. (e) Stahl, S. S. Science 2005,
309, 1824-1826.
(ii) (a) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 2508-2509. (b) Konnick, M. M.;
Gandhi, B. A.; Guzei, I. A.; Stahl, S. S. Angew. Chem. Int. Ed. 2006, 45,
2904-2907.
(iii) (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton, 1976, 1020-1024.
(b) Göttker-Schnetmann, I.; White, P. S.; Brookhart, M. Organometallics
2004, 23, 1766-1776. (c) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R.
E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840-841. (d) MoralesMorales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M. Organometallics 2001,
20, 1144-1147. (e) Personal communication with A. S. Goldman.
(4) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M.
Chem. Commun. 1996, 2083-2084. (b) Liu, F.; Pak, E. B.; Singh, B.;
Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087.
(c)
(v) (a) Vaska, L. Science 1963, 140, 809-810. (b) Roundhill, D. M.; Allen, G.
H.; Bechtold, R. A.; Beaulieu, W. B. Inorg. Chim. Acta. 1981, 54, L99L100. (c) Gopinathan, S.; Joseph, K.; Gopinathan, C. Polyhedron 1985, 4,
955-958. (d) Lawson, H. J.; Atwood, J. D. J. Am. Chem. Soc. 1989, 111,
6223-6227. (e) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F. J. Am.
Chem. Soc. 1990, 112, 6726-6728. (f) Barbaro, P.; Bianchini, C.; Laschi,
F.; Midollini, S.; Moneti, S.; Scapacci, G.; Zanello, P. Inorg. Chem. 1994,
33, 1622-1630.
(vi) See Supporting Information for full experimental and structural details.
Caution should be taken when adding pressures of O 2 to organic
solvents.
(vii)
(a) Nolte, M. J.; Singleton, E.; Laing, M. J. Am. Chem. Soc. 1975,
97, 6396-6400. (b) Mezzetti, A.; Novelli, V.; Del Zotto, A.; Rigo, P. Acta
Cryst. 1987, C43, 425-428. (c) Wang, J.-C.; Chou, L.-Y.; Hsien, W.-Y.
Acta Cryst. 1994, C50, 879-882.
(8) Using the simple harmonic oscillator equation the calculated shift is 842
cm-1. Ligand stretches are also located in this region.
(ix) Valentine, J. S. Chem. Rev. 1973, 73, 235-245.
(x) (a) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997,
16, 3786-3793. (b) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I.
Organometallics 2006, 25, 3007-3011. (c) Personal communication with
S. M. Kloek.
(xi) (a) Han, R.; Hillhouse, G. L. J. Am. Chem. Soc. 1997, 119, 8135-8136. (b)
Lin, B. L.; Clough, C. R.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124,
2890-2891.
(xii) (a) Krogh-Jespersen, K.; Czerw, M.; Summa, N.; Renkema, K. B.;
Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124, 1140411416. (b) Personal communication with A. S. Goldman.
(xiii) Other M-H plus O2 reactions
Table of Contents Graphic:
P(tBu)2
P(tBu)2
Ir
X
O2
-HX
H
P(tBu)2
Ir
O
O2
O
P(tBu)2
Abstract:

Iridium pincer hydride complexes (tBuPCP)Ir(H)(X) [X = Ph (2), H (3), OH (4) and CCPh (5)] react with O2 in benzene
solution to give the peroxo complexes ( tBuPCP)Ir(O2)n [tBuPCP = 3-C6H3(CH2PtBu2)2; n = 1 (6), 2 (7)]. The bis(peroxo)
complex 7 has been characterized by an X-ray crystal structure. Despite significant disorder, the structure reveals a
pentagonal bipyramidal molecule with both peroxo ligands and the aryl carbon of the pincer ligand in the pentagonal plane.
Under vacuum, complex 7 is converted to the mono-peroxo complex 6, which is reversed under O2. The reaction of the
hydride-acetylide 5 with O2 is also reversible. The O2 reactions of (tBuPCP)Ir(H)(X) typically appear to proceed by dioxygeninduced reductive elimination of HX, rather than by insertion of O 2 into an Ir–H or Ir–C bond. (tBuPCP)Ir(H)(Cl) is unreactive
with dioxygen, possibly because elimination of HCl does not occur. Complex 6 reacts with H2 (5 atm) to give (tBuPCP)Ir(H)4
and water, and 7 (but not 6) oxidizes PPh3.