New bidentate cationic and zwitterionic relatives of Crabtree's hydrogenation catalyst

Article abstract of DOI:10.1039/b510253g

Inspired by the monodentate P,N ligation strategy featured in Crabtree's catalyst [(COD)Ir(PCy₃)(Py)]⁺PF₆ ^-([1]⁺PF₆^-), a new class of bidentate cationic ([2]₊X^-

Full text of DOI:10.1039/b510253g

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COMMUNICATION
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New bidentate cationic and zwitterionic relatives of Crabtree’s
hydrogenation catalyst{
Judy Cipot,a Robert McDonaldb and Mark Stradiotto*a
Received (in Berkeley, CA, USA) 20th July 2005, Accepted 17th August 2005
First published as an Advance Article on the web 7th September 2005
DOI: 10.1039/b510253g
activity of more traditional [(COD)Ir(k2-P,N)]+X2 catalysts
Inspired by the monodentate P,N ligation strategy featured in
2
Crabtree’s catalyst [(COD)Ir(PCy3)(Py)]+PF6 ([1]+PF62), a
with the desirable solubility properties associated with neutral
new class of bidentate cationic ([2]+X2) and zwitterionic (3) Ir
complexes, while avoiding undesirable counteranion effects. The
development of hydrocarbon-soluble zwitterionic Ir catalysts of
this type is of particular interest, both in terms of providing a
general means of circumventing the use of environmentally
harmful chlorocarbon solvents for Ir-mediated alkene hydrogena-
tions, and in light of the improved catalyst lifetime, reaction rate,
and selectivity that might be achieved by employing a relatively
inert, low-coordinating hydrocarbon as the reaction medium for
such hydrogenations. Moreover, head-to-head reactivity studies
involving [2]+X2 and the isostructural zwitterion 3 provide a rare
opportunity to assess the impact on catalytic performance of
adjusting only the electronic characteristics of an electrophilic Ir
center on going from [2]+X2 to 3, without substantially modifying
the steric profile of the reactive metal coordination sphere.7,8
Herein we describe the synthesis, structural characterization, and
alkene hydrogenation capabilities of the new cationic complexes
[2]+X2 (X 5 PF6, BF4 and SO3CF3), as well as the hydrocarbon-
soluble complex 3 – the first formally zwitterionic analogue of
Crabtree’s catalyst.
complexes have been developed, which are capable of mediat-
ing the hydrogenation of alkenes under mild conditions and in
a wider range of solvents than is possible for [1]+PF6
2
.
The metal-mediated hydrogenation of an alkene-containing
substrate represents an important synthetic step in a wide range
of prominent bench-top and industrial processes.1 In this context,
Crabtree’s catalyst, [(COD)Ir(PCy3)(Py)]+PF62 ([1]+PF62; COD 5
g4-1,5-cyclooctadiene, Cy 5 cyclohexyl, Py 5 pyridine), has
emerged as one of the most active homogeneous catalysts for
alkene hydrogenation; in contrast to Ru- and Rh-based catalysts,
[1]+PF62 is capable of reducing a range of substituted olefins under
mild conditions (y1 atm H2, 0 uC).2 In evaluating the design
features of [1]+PF62, Crabtree and co-workers noted that a
formally cationic Ir center and a mixed P,N ancillary ligand set
were needed to achieve optimal catalytic performance.2 Building
on this pioneering work, emphasis has shifted to the preparation of
more rigid bidentate k2-P,N variants of [1]+PF62, in anticipation
that these may be less susceptible to thermal decomposition or
bimolecular deactivation.2 Notably, chiral catalysts of this type
have been developed that mediate the rather challenging
The cationic complexes [2]+X2 were prepared from
[(COD)IrCl]2 and 1-PiPr2-2-NMe2-indene by use of AgX as the
halide abstraction agent (X 5 PF6, 78%; BF4, 37%; and SO3CF3,
47%),{ while the structurally analogous k2-P,N zwitterion (3) was
prepared via lithiation of 1-PiPr2-2-NMe2-indene followed by
treatment with [(COD)IrCl]2 (93%). The structural formulations
presented in Scheme 1 are based in part on data obtained
from solution NMR spectroscopic studies, and in the case of
asymmetric reduction of prochiral styrenes lacking directing
2
functionalities.3 In keeping with [1]+PF6
, most bidentate
[(COD)Ir(k2-P,N)]+X2 catalysts require the use of high-dielectric
chlorinated solvents both to provide solubility and to maximize
catalytic turnover.4 As well, the nature of the counteranion (X) in
such complexes has been shown to have a profound influence on
the performance of the catalyst, often in a manner that cannot be
predicted easily.5
2
[2]+SO3CF3 and 3, X-ray diffraction data (Fig. 1).§ The overall
structural features of the Ir coordination spheres in [2]+SO3CF3
2
˚
and 3 are strikingly similar, with the Ir–P (2.299(1) A) and Ir–N
In an effort to expand the scope and utility of Ir-mediated
hydrogenation catalysis we became interested in comparing the
structural and catalytic properties of cations of the type [2]+X2
with less-conventional zwitterions that feature a formally cationic
Ir center counterbalanced by a 10p-electron indenide anion built
into the backbone of a supporting k2-P,N ligand, as in the neutral
complex 3 (Scheme 1).6 We anticipated that formally charge-
separated species such as 3 might combine the appealing catalytic
˚
(2.192(4) A) distances in the former differing only modestly from
˚
those found in the latter (2.3171(6) and 2.216(2) A, respectively).
9
In contrast to the alternating bond lengths observed in the
indene backbone of [2]+SO3CF32, the carbocyclic framework of
the k2-P,N ligand in 3 exhibits a much more delocalized structure
aDepartment of Chemistry, Dalhousie University, Halifax, Canada
B3H 4J3. E-mail: mark.stradiotto@dal.ca; Fax: 1 902 494 1310;
Tel: 1 902 494 7190
bX-Ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Canada T6G 2G2
{ Electronic supplementary information (ESI) available: Experimental
procedures and characterization data. See http://dx.doi.org/10.1039/
b510253g
Scheme 1 Synthesis of new cationic and zwitterionic iridium catalyst
complexes. Reagents: (i) 0.5 [(COD)IrCl]2, AgX (X 5 PF6, BF4, or
SO3CF3), then NEt3 for X 5 PF6 or SO3CF3; (ii) n-BuLi, then
0.5 [(COD)IrCl]2 (COD 5 g4-1,5-cyclooctadiene).
4932 | Chem. Commun., 2005, 4932–4934
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Fig. 1 ORTEP diagrams for [2]+SO3CF32 and 3 shown with 50% displacement ellipsoids and with the atomic numbering scheme depicted; the anion in
2
+
[2]+SO3CF3 and selected hydrogen atoms have been omitted for clarity. Selected bond lengths (A) for [2] SO3CF32: Ir–P 2.299(1), Ir–N 2.192(4), P–C3
˚
1.807(5), N–C2 1.475(6), C1–C2 1.500(6), C2–C3 1.339(6), C3–C3a 1.491(6), C3a–C4 1.380(7), C4–C5 1.396(7), C5–C6 1.381(7), C6–C7 1.383(7), C7–C7a
…
˚
˚
1.390(6), C7a–C1 1.498(6), C3a–C7a 1.404(7); shortest Ir SO3CF3 contact y6.3 A. Selected bond lengths (A) for 3: Ir–P 2.3171(6), Ir–N 2.216(2), P–C3
1.755(2), N–C2 1.478(3), C1–C2 1.381(3), C2–C3 1.413(3), C3–C3a 1.435(3), C3a–C4 1.406(3), C4–C5 1.374(4), C5–C6 1.411(4), C6–C7 1.370(4), C7–C7a
1.412(4), C7a–C1 1.424(4), C3a–C7a 1.440(3).
consistent with a 10p-electron indenide anion, as was observed in
the corresponding Rh zwitterion.6 However, while the formally
zwitterionic 3 lacks a conventional resonance structure that places
the anionic charge onto either of the N- or P-donor fragments, the
complete consumption of styrene was not observed for hydro-
genations mediated by [2]+SO3CF32 even after 4 h (entry 1-4). The
2
dependence of the counteranion (i.e. PF6 . BF4 . SO3CF3
2
2
)
on catalytic activity in this series of complexes mirrors that noted
in some other [(COD)Ir(k2-P,N)]+X2 catalyst systems.5 In stark
contrast to the negligible catalytic hydrogenation activity typically
observed for neutral square-planar Ir complexes such as
(PPh3)3IrCl,12 the charge-neutral zwitterion 3 proved capable of
shortening of the P–C3 distance on going from [2]+SO3CF3
2
˚
˚
(1.807(5) A) to 3 (1.755(2) A) suggests that a less-conventional (and
non-zwitterionic) resonance structure featuring a PLC3 bond and a
negative charge on phosphorus may also contribute in 3.10
The hydrogenation of styrene in CH2Cl2 (y1 atm H2, 22 uC,
1.0 mol%) was examined in an initial effort to benchmark
reducing styrene to a significant extent (54% over 0.5 h, entry 1-5),
and with a faster initial rate than was found for [2]+SO3CF3
2
.
the catalytic performance of [2]+X2 and 3 against that of [1]+PF6
2
These observations indicate that the lower total productivity of 3
relative to [1]+PF6 and [2]+X2 may reflect the instability of 3
2
2
2
(Table 1). Complexes [1]+PF6 and [2]+PF6 proved equally
effective, with clean conversion to ethylbenzene observed after
15 min (entries 1-1 and 1-2). Whereas quantitative reduction was
under these reaction conditions, rather than the inherent activity
of this zwitterionic catalyst. Encouraged by these preliminary
catalytic results, the hydrogenation of styrene in various solvents
and at a lower catalyst loading (0.5 mol%) was also examined.
2
also achieved after 30 min by employing [2]+BF4 (entry 1-3),11
Reactions employing [1]+PF6 and [2]+PF6 in CH2Cl2 were
quantitative, although [2]+PF62 exhibited both a faster initial rate
and a reduced total reaction time (entries 1-6 and 1-7). Clean
reductions were also observed for reactions employing [2]+PF62 in
THF rather than CH2Cl2, albeit at the cost of increased reaction
time; in contrast, [1]+PF62 is only partially soluble in THF and the
complete consumption of styrene was not achieved (entries 1-8 and
1-9). Interestingly, complex 3 performed similarly well in both
CH2Cl2 and THF (entries 1-10 and 1-11), providing comparable
yields and even faster initial rates at the 0.5 mol% catalyst loading
level than were observed at the 1.0 mol% level (entry 1-5). Whereas
2
2
Table 1 Styrene hydrogenation resultsa
Entry Catalyst
Solvent
Yield (%) tc/h TON/TOFd
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9
1-10
1-11
1-12
1-13
a
[1]+PF62
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THFb
.99
.99
.99
97
0.25 100/400
0.25 100/400
2
[2]+PF62
[2]+BF4
[2]+SO3CF3
3
0.5
4
100/280
97/165
55/210
200/630
200/740
183/415
200/400
113/355
116/305
75/165
34/35
2
54
0.5
4
2
[1]+PF62
.99
.99
91
.99
56
58
37
17
[2]+PF62
0.5
4
[1]+PF62
[2]+PF6
THF
4
4
4
4
3
3
3
3
CH2Cl2
THF
C6H6
2
[1]+PF6 and [2]+X2 are insoluble in all hydrocarbons, the
benzene-soluble zwitterion (3) exhibited moderate catalytic
activity both in this solvent (entry 1-12), as well as in aliphatic
hydrocarbons (entry 1-13). By comparison, negligible styrene
conversion (,3%) was achieved in benzene in the presence
of catalytic amounts of the non-zwitterionic neutral species
(COD)Ir(Cl)(k1-1-PiPr2-2-NMe2-indene)," which supports the
Hexanesb
4
Conditions: 22 uC and y1 atm H2; entries 1–5: 1.0 mol% and
b
entries 6–13: 0.5 mol%. Catalyst was not completely soluble in this
c
solvent. Time at which the conversion (yield) was achieved.
d
TON 5 moles alkane produced/moles catalyst and turnover
frequency (TOF) at 0.25 h.
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a
Innovation, the Nova Scotia Research and Innovation Trust
Fund, and Dalhousie University for their generous support of this
work. We also thank Drs Bob Berno and Michael Lumsden
(Atlantic Region Magnetic Resonance Center, Dalhousie) for
assistance in the acquisition of NMR data.
Table 2 Hydrogenation of substituted alkenes in CH2Cl2
Yield
(%)
Entry Catalyst Substrate
tb/h TON/TOFc
2-1
2-2
2-3
2-4
2-5
a
[1]+PF62 Cyclohexene
.99 0.25 200/800
.99 0.5 196/755
2
[2]+PF6
Cyclohexene
Cyclohexene
3
26
97
83
4
4
4
b
52/170
194/490
167/120
[1]+PF62 1-Methylcyclohexene
1-Methylcyclohexene
2
Notes and references
[2]+PF6
{ Attempts to prepare [2]+B(C6F5)42 by employing Li(Et2O)2.5B(C6F5)4 as
the halide abstracting reagent generated intractable dark solids that gave
rise to numerous 31P NMR resonances. All isolated [2]+X2 complexes
exhibited identical 1H, 13C{1H}, and 31P{1H} NMR features for [2]+.
§ Both structures are space group 5 P21/n (monoclinic) and Z 5 4;
D/R/P 5 data/restraints/parameters and GOF 5 goodness of fit. Crystal
Conditions: 22 uC, y1 atm H2, and 0.5 mol%. Time at which the
c
conversion (yield) was achieved. TON 5 moles alkane produced/
moles catalyst and turnover frequency (TOF) at 0.25 h.
view that the catalytic activity displayed by 3 can be attributed
to the charge-separated nature of this neutral complex. In
surveying the ability of these complexes to mediate the
reduction of more substituted alkenes in CH2Cl2 under analogous
data for [2]+SO3CF3 : a 5 9.9608(6), b 5 20.271(1), c 5 14.0988(9) A,
2
˚
3
˚
b 5 103.575(1)u, V 5 2767.2(3) A , D/R/P 5 5676/0/325, GOF 5 1.002,
R1 5 0.0315, wR2 5 0.0654. Crystal data for 3: a 5 9.6714(5),
b 5 16.9783(9), c 5 14.5080(7) A, b 5 109.4177(7)u, V 5 2246.8(2) A ,
D/R/P 5 4579/0/253, GOF 5 1.086, R1 5 0.0174, wR2 5 0.0426. CCDC
270594 and 270595. See http://dx.doi.org/10.1039/b510253g for crystal-
lographic data in CIF or other electronic format.
3
˚
˚
conditions (Table 2), [2]+PF6 proved capable of quantitatively
2
hydrogenating cyclohexene at a rate only slightly below that of
2
[1]+PF6 (entries 2-1 and 2-2), and partial conversion was also
" This compound was prepared in-situ based on methods established for
the synthesis of the Rh analogue.6
noted for reactions employing 3 as a catalyst (26%, entry 2-3).
2
However, while [2]+PF6 remained somewhat competitive with
1 C. Pettinari, F. Marchetti and D. Martini, in Comprehensive
Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer,
Elsevier Ltd., Oxford UK, 2004, ch. 9, p. 75.
[1]+PF6 with respect to the reduction of 1-methylcyclohexene
2
(entries 2-4 and 2-5), the zwitterion 3 proved incapable of reducing
this trisubstituted alkene to any extent, either in CH2Cl2 or
benzene.
2 R. H. Crabtree, H. Felkin and G. E. Morris, J. Organomet. Chem.,
1977, 141, 205; R. Crabtree, Acc. Chem. Res., 1979, 12, 331;
R. H. Crabtree, P. C. Demou, D. Eden, J. M. Mihelcic, C. A. Parnell,
J. M. Quirk and G. E. Morris, J. Am. Chem. Soc., 1982, 104, 6994.
3 For example: D.-R. Hou, J. Reibenspies, T. J. Colacot and K. Burgess,
Chem. Eur. J., 2001, 7, 5391; L. B. Schenkel and J. A. Ellman, J. Org.
Chem., 2004, 69, 1800; D. Liu, W. Tang and X. Zhang, Org. Lett., 2004,
6, 513; W. J. Drury III, N. Zimmermann, M. Keenan, M. Hayashi,
S. Kaiser, R. Goddard and A. Pfaltz, Angew. Chem., Int. Ed., 2004,
43, 70.
4 In select cases toluene has been successfully employed as an alternative
to chlorinated solvents: T. Bunlaksananusorn, K. Polborn and
P. Knochel, Angew. Chem., Int. Ed., 2003, 42, 3941; A. Pfaltz,
J. Blankenstein, R. Hilgraf, E. Ho¨rmann, S. McIntyre, F. Menges,
M. Scho¨nleber, S. P. Smidt, B. Wu¨stenberg and N. Zimmermann, Adv.
Synth. Catal., 2003, 345, 33.
The results detailed herein confirm that the k2-P,N ancillary
ligand in [2]+X2 supports a coordination environment that is
conducive to the Ir-mediated hydrogenation of olefins; notably,
the catalytic performance of [2]+PF62 was found to be competitive
with, and in some cases superior to, that of Crabtree’s catalyst,
[1]+PF62. The formally zwitterionic 3 has also proven to be an
active catalyst for olefin hydrogenation under very mild conditions
(y1 atm H2, 22 uC). While the catalytic abilities of 3 were found to
be inferior to those of the more traditional catalyst salts [2]+X2 and
[1]+PF62 under the conditions examined, the neutral character of 3
enabled hydrogenations to be carried out in a range of solvents,
including hydrocarbons in which the cationic species are insoluble.
In light of the nearly isostructural relationship that exists between
the Ir coordination spheres in [2]+X2 and 3, their disparate
catalytic abilities can in principle be ascribed to differences
associated with the electronic characteristics of the Ir centers in
5 S. P. Smidt, N. Zimmermann, M. Studer and A. Pfaltz, Chem. Eur. J.,
2004, 10, 4685, and references cited therein.
6 For studies involving related platinum-group metal zwitterions, see: (a)
(Rh): M. Stradiotto, J. Cipot and R. McDonald, J. Am. Chem. Soc.,
2003, 125, 5618; (b) (Ru): M. A. Rankin, R. McDonald, M. J. Ferguson
and M. Stradiotto, Angew. Chem., Int. Ed., 2005, 44, 3603.
7 While square-planar, zwitterionic Rh complexes have been shown to
catalyze a range of E–H additions to alkenes (E 5 H, B, C, Si),
analogous Ir complexes have yet to appear in the literature, see:
T. A. Betley and J. C. Peters, Angew. Chem., Int. Ed., 2003, 42, 2385 and
ref. 6.
8 Zwitterionic complexes featuring formally cationic Ir centers are rare,
none feature k2-P,N ligation, and their catalytic properties have not been
systematically explored. For selected examples, see: R. R. Schrock and
J. A. Osborn, Inorg. Chem., 1970, 9, 2339; F. Torres, E. Sola, M. Mart´ın,
C. Ochs, G. Picazo, J. A. Lo´pez, F. J. Lahoz and L. A. Oro,
Organometallics, 2001, 20, 2716; L. Turculet, J. D. Feldman and
T. D. Tilley, Organometallics, 2004, 23, 2488.
9 For some other crystallographically characterized [(COD)Ir(k2-
P,N)]+X2 complexes, see: L. Dahlenburg and R. Go¨tz, Eur. J. Inorg.
Chem., 2004, 888 and refs. 3–5.
2
these complexes. Whereas [2]+PF6 is a highly active hydrogena-
tion catalyst that has proven capable of reducing even trisub-
stituted alkenes, the electrophilicity of the formally cationic Ir
center in the structurally analogous zwitterion 3 is apparently
attenuated, possibly due to partial delocalization of the ancillary
ligand anionic charge onto the P-donor fragment (vide supra).
Based on these observations, we are currently developing more
lipophilic analogues of 3 with the specific aim to augment the
electrophilic character of the Ir center by discouraging charge
delocalization onto the pnictogen donor fragments. Our progress
in this regard, and in the study of chiral derivatives of [2]+X2 and
3, will be the subject of future reports.
10 K. Izod, Coord. Chem. Rev., 2002, 227, 153.
Acknowledgment is made to the Natural Sciences and
Engineering Research Council (NSERC) of Canada (including a
Discovery Grant for M. S. and a Postgraduate Scholarship for
J. C.), the Killam Trust (Dalhousie University; including a
Research Prize for M. S.), the Canada Foundation for
11 While the hydrogenation of alkenes catalyzed by [(COD)-
Ir(k2-iPr2PCH2CH2NMe2)]+BF42 has been reported, the data provided
do not allow for a direct comparison with [2]+BF42: M. A. Esteruelas,
A. M. Lo´pez, L. A. Oro, A. Pe´rez, M. Schulz and H. Werner,
Organometallics, 1993, 12, 1823.
12 M. A. Bennett and D. L. Milner, J. Am. Chem. Soc., 1969, 91, 6983.
4934 | Chem. Commun., 2005, 4932–4934
This journal is ß The Royal Society of Chemistry 2005