Iridium Bis(phosphinite) p-XPCP Pincer Complexes: Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes

Article abstract of DOI:10.1021/ja0385235

A series of new bis(phosphinite) p-XPCPlrHCl pincer complexes {[PCP = η³-5-X-C₆H₂[OP-(tBu)₂] ₂-1,3], X = MeO (4a), Me (4b), H (4c), F (4d), C₆F ₅ (4e), and Ar^F [=3,5-bis(trifluoromethyl)phenyl] (4f)} have been synthesized. Treatment of compounds 4a-f with NatOBu in cyclooctane (COA)/tert-butylethylene (TBE) mixtures generates species with unprecedented catalytic activity for the catalyzed transfer dehydrogenation of COA with TBE as acceptor to form cyclooctene (COE) and tert-butylethane (TBA). With substrate: precatalyst ratios of 3030COA:3030TBE:1 p-XPCPlrHCl (4): 1.1NaOtBu, turnover numbers (TONs) between 1400 and 2200 (up to 72% conversion in TBE) and initial turnover frequencies (TOFs) between 1.6 and 2.4 s^-1 have been observed at 200 °C.

Full text of DOI:10.1021/ja0385235

Published on Web 01/24/2004
Iridium Bis(phosphinite) p-XPCP Pincer Complexes: Highly
Active Catalysts for the Transfer Dehydrogenation of Alkanes
Inigo Go¨ttker-Schnetmann, Peter White, and Maurice Brookhart*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received September 15, 2003; E-mail: mbrookhart@unc.edu
Abstract: A series of new bis(phosphinite) p-XPCPIrHCl pincer complexes {[PCP ) η3-5-X-C6H2[OP-
(tBu)2]2-1,3], X ) MeO (4a), Me (4b), H (4c), F (4d), C6F5 (4e), and ArF [)3,5-bis(trifluoromethyl)phenyl]
(4f)} have been synthesized. Treatment of compounds 4a-f with NatOBu in cyclooctane (COA)/tert-
butylethylene (TBE) mixtures generates species with unprecedented catalytic activity for the catalyzed
transfer dehydrogenation of COA with TBE as acceptor to form cyclooctene (COE) and tert-butylethane
(TBA). With substrate:precatalyst ratios of 3030COA:3030TBE:1p-XPCPIrHCl (4):1.1NaOtBu, turnover
numbers (TONs) between 1400 and 2200 (up to 72% conversion in TBE) and initial turnover frequencies
(TOFs) between 1.6 and 2.4 s-1 have been observed at 200 °C.
Chart 1. Catalysts 1-3
Introduction
The selective, catalytic functionalization of alkanes remains
a challenging and significant goal due to the prospects of
transforming inexpensive hydrocarbon feedstocks into more
useful, value-added products by low energy processes.1 In this
respect, catalytic alkane dehydrogenation is a potentially
significant reaction in that alkanes are converted to olefins which
are valuable synthetic intermediates and are used in a variety
of industrial processes.
Following early examples of homogeneous dehydrogenations
that are substoichiometric in transition metal,2 a major break-
through was achieved by Kaska, Jensen et al. who reported that
the iridium pincer complex {C6H3-2,6-[CH2P(tBu)2]2}IrH2 (1)
(Chart 1) catalyzes the transfer dehydrogenation reaction of
cyclooctane (COA) with tert-butylethylene (TBE) to form
cyclooctene (COE) and tert-butylethane (TBA) at 200 °C. While
turnover numbers up to 1000 can be achieved, catalyst inhibition
by TBE (as well as the COE product) requires periodic addition
of TBE to the reaction to achieve such TONs.1c,3 Subsequently,
mechanistic details of these transfer dehydrogenations by means
of experimental and computational studies have been disclosed.4
Transfer dehydrogenation overcomes the thermodynamic (ca.
30-35 kcal/mol endothermic)5 and kinetic difficulties of
hydrogen release but requires an acceptor molecule. To carry
out “acceptorless” dehydrogenation, an efficient removal of
hydrogen from the reaction mixture is required. Acceptorless
dehydrogenation has been achieved using both complex 24j,6
and the even more thermostable anthraphos pincer complex 3
(Chart 1).7 Turnover numbers (TONs) up to 1000 have been
(1) For recent reviews on this topic: (a) Labinger, J. A.; Bercaw, J. E. Nature
2002, 417, 507-514. (b) Crabtree, R. H. J. Chem. Soc., Dalton Trans.
2001, 2437-2450. (c) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res.
2001, 34, 633-639. (d) Jensen, C. M. Chem. Commun. 1999, 2443-2449.
(e) Sen, A. Acc. Chem. Res. 1998, 31, 550-557. (f) Shilov, A. E.; Shul’pin,
G. B. Chem. ReV. 1997, 97, 2879-2932. (g) Arndtsen, B. A.; Bergman,
R.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154-162.
For leading reviews on the functionalization of sp and sp2 C-H bonds:
(h) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731-1769.
(i) Guari, Y.; Sabo-Etienne, S. Eur. J. Inorg. Chem. 1999, 1047-1055. (j)
Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1699-1712.
(2) (a) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1979,
101, 7738-7740. (b) Baudry, M. J.; Ephritikine, M.; Felkin, H.; Holmes-
Smith, R. J. Chem. Soc., Chem. Commun. 1983, 788-789. (c) Baudry, M.
J.; Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics 1984, 3,
816-817. For some further catalytic examples: (d) Burk, M. W.; Crabtree,
R. H. J. Am. Chem. Soc. 1987, 109, 8025-8032. (e) Maguire, J. A.;
Goldman, A. S. J. Am. Chem. Soc. 1991, 113, 6706-6708. (f) Maguire, J.
A.; Petrillo, A.; Goldman, A. S. J. Am. Chem. Soc. 1992, 114, 9492-
9498. (g) Belli, J.; Jensen, C. M. Organometallics 1996, 15, 1532-1534.
(3) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem.
Commun. 1996, 2083-2084.
(4) (a) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc.
2003, 125, 7770-7771. (b) Krogh-Jespersen, K.; Czerw, M.; Summa, N.;
Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002,
124, 11404-11416. (c) Krogh-Jespersen, K.; Czerw, M.; Zhu, K.; Singh,
B.; Kanzelberger, M.; Darji, N.; Achord, P. D.; Renkema, K. B.; Goldman,
A. S. J. Am. Chem. Soc. 2002, 124, 10797-10809. (d) Li, S.; Hall, M. B.
Organometallics 2001, 20, 2153-2160. (e) Morales-Morales, D.; Lee, D.
W.; Wang, Z.; Jensen, C. M. Organometallics 2001, 20, 1144-1147. (f)
Krogh-Jespersen, K.; Czerw, M.; Kanzelberger, M.; Goldman, A. S. J.
Chem. Inf. Comput. Sci. 2001, 41, 56-63. (g) Kanzelberger, M.; Singh,
B.; Czerw, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc.
2000, 122, 11017-11018. (h) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C.
M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087. (i) Niu, S.;
Hall, M. B. J. Am. Chem. Soc. 1999, 121, 3992-3999. (j) Liu, F.; Goldman,
A. S. Chem. Commun. 1999, 655-656. (k) Lee, D. W.; Kaska, W. C.;
Jensen, C. M. Organometallics 1998, 17, 1-3. (l) Gupta, M.; Hagen, C.;
Kaska, C. W.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119,
840-841.
(5) NIST Standard Reference Database Number 69 - March, 2003 Release,
http://webbook.nist.gov/chemistry/.
(6) Xu, W.-W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; Krogh-
Jespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273-2274.
9
1804
J. AM. CHEM. SOC. 2004, 126, 1804-1811
10.1021/ja0385235 CCC: $27.50 © 2004 American Chemical Society

Iridium Bis(phosphinite) p-XPCP Pincer Complexes
A R T I C L E S
bis(trifluoromethyl)bromobenzene, respectively (Scheme 2).11
Reaction conditions developed by Nolan et al. for the Suzuki
coupling of 3,5-bis(trifluoromethyl)bromobenzene using Cs2-
CO3 as base and diimine ligands proved to be useful for the
high yield synthesis of 7f,12 but gave unsatisfactory yields of
compound 7e.13 However, excellent yields of compound 7e
could be obtained using KF as base and triphenylphosphine as
ligand (Scheme 2).
Scheme 1. Synthesis of Bis(phosphinite) PCP Pincer Ligands
5a-f
Complexes 4a-f were obtained in 87-93% isolated yield
from reaction of [(COD)IrCl]2 with the respective PCP ligands
5a-f in toluene at 150 °C (Scheme 3). As was confirmed by
NMR experiments, formation of complexes 4e,f is complete
after 1-2 h at 150 °C in toluene-d8, whereas formation of 4a-d
takes 6-12 h under the same conditions. Unlike the methylene-
bridged {4-MeO-C6H2-[CH2P(tBu)2]2-2,6}IrHCl pincer com-
plex 9, the oxygen-bridged compound 4a does not undergo C-H
oxidative addition of one of the tert-butyl methyl groups and
subsequent elimination of hydrogen to form a stable product.14
The poor solubility of the products 4a-d in alkane solvents
allows a straightforward separation of these complexes by
filtration from their pentane suspensions. The more soluble
complexes 4e,f were isolated in combined yields by filtration
of the solid product and flash chromatography of the remaining
pentane filtrate.
achieved for the acceptorless dehydrogenation of cyclodecane
at 200 °C with 2 and at 250 °C with 3, respectively.
Driven by our interest in employing C-H activation reactions
in catalytic transformations8 and the possible advantages of using
more electron-deficient iridium pincer complexes, we report here
our initial studies of the catalytic properties of resorcinol-derived
phosphinite pincer complexes conveniently generated from
readily accessible hydridochloride precursors, {p-XC6H2-2,6-
[OP(tBu)2]2}IrHCl (4a-f). These species exhibit unprecedented
initial turnover frequencies (TOFs) and are less subject to TBE
and product inhibition relative to complexes 1-3.
Typical spectroscopic features of p-XPCPIrHCl (4) show two
overlapping virtual triplets of the diastereotopic tert-butyl groups
in the narrow range from 1.3 to 1.4 ppm as well the hydridic
2
IrH resonances appearing as a triplet with JPH ) 13.0-13.2
1
Results and Discussion
Hz in the range of -40.96 (4f) to -41.87 ppm (4a) in the H
NMR spectra. The aromatic CH and hydridic resonances of
compound 4d exhibit further splitting due to the 19F nucleus
para to iridium. The 31P{1H} NMR spectra of compounds 4a-f
exhibit singlets between 175.8 (4c) and 179.1 ppm (4d).
Acquisition of 13C{1H) NMR spectra requires concentrated
solutions of compounds 4a-f due to the splitting into virtual
triplets of the tert-butyl-, C2, C3, C5, and C6 resonances caused
by the two equivalent 31P nuclei. In the case of compound 4d,
(a) Ligand and Complex Synthesis. In analogy to the
reported synthesis of {C6H3-P(OiPr2)2-1,3},9 the bis(phosphin-
ite) PCP ligands [5-xC6H2(OPtBu2)2-1,3] (5a-f) {x ) MeO
(5a), Me (5b), H (5c), F (5d), C6F5 (5e), ArF [)3,5-bis-
(trifluoromethyl)phenyl] (5f)} under investigation are obtained
by diphosphorylation of the respective 5-substituted resorcinols
(6a-f) with tert-butyl-chlorophosphine and sodium hydride as
a base (Scheme 1). The yields of compounds 5a-f are generally
high (82-96%), and the crude products can be used without
further purification. 5-Fluororesorcinol (6d) was obtained from
1-fluoro-3,5-dimethoxybenzene (7d) by deprotection with BCl3/
n-Bu4I according to a literature procedure.10 Finally, the 5-aryl
substituted resorcinols (6e,f) were obtained in good overall
yields (6e, 78%; 6f, 85%) in a Suzuki coupling/deprotection
sequence (BCl3/n-Bu4I-mediated) starting from 3,5-dimethoxy-
benzene boronic acid (8) and pentafluorobromobenzene, or 3,5-
additional splitting due to 19F is observed for each aromatic 13
C
nucleus. The Ir-C1 resonances appear as broad multiplets in
the range of 108.1 (4a) to 121.5 ppm (4f) (rather than a triplet),
due to incomplete {1H} decoupling of the hydride signal (ca.
-41 ppm). Two sets of diastereotopic tert-butyl 13C signals are
observed at ca. 43.4 and 39.8 ppm for the C(CH3)3 and ca. 27.8
and 27.6 ppm for the C(CH3)3 groups for compounds 4a-f.
On the basis of these observations and of the reported X-ray
structure of the related {4-NO2-C6H2-[CH2P(tBu)2]2-2,6}IrHCl
pincer complex 10,15 we consider complexes 4 as having a
square pyramidal geometry with the apical site occupied by the
hydride and a mirror plane perpendicular to the plane of the
(7) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan, H.-J.; Hall,
M. B. Angew. Chem., Int. Ed. 2001, 40, 3596-3600.
(8) (a) Diaz-Requejo, M. M.; DiSalvo, D.; Brookhart, M. J. Am. Chem. Soc.
2003, 125, 2038-2039. (b) Reinartz, S.; White, P. S.; Brookhart, M.;
Templeton, J. J. Am. Chem. Soc. 2001, 123, 12724-12725. (c) Reinartz,
S.; White, P. S.; Brookhart, M.; Templeton, J. Organometallics 2001, 20,
1709-1712. (d) Lenges, C. P.; Brookhart, M. J. Am. Chem. Soc. 1999,
121, 6616-6623. (e) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am.
Chem. Soc. 1999, 121, 4385-4396. (f) Lenges, C. P.; Brookhart, M.; Grant,
B. J. Organomet. Chem. 1997, 528, 199-203.
(11) 3,5-Dimethoxybenzene boronic acid is commercially available at Asymchem
of Durham, NC, or can be easily synthesized according to: Dol, G. C.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M. Eur. J. Org. Chem. 1998, 359,
9-364.
(9) The synthesis of the related {C6H4-1,3-[OP(iPr)2]2} and catalytic application
of its palladium complex has been reported by: (a) Morales-Morales, D.;
Grause, C.; Kasaoka, K.; Redo´n, R.; Cramer, R. E.; Jensen, C. M. Inorg.
Chim. Acta 2000, 300-302, 958-963. (b) Morales-Morales, D.; Redo´n,
R.; Yung, C.; Jensen, C. M. Chem. Commun. 2000, 1619-1620. (c)
Eberhard, M. R.; Wang, Z.; Jensen, C. M. Chem. Commun. 2002, 818-
819. Synthesis of {C6H4-1,3-[OP(Ph)2]2} and its application has been
reported by: (d) Hunks, W. J.; Jennings, M. C.; Puddephatt, R. J. Inorg.
Chem. 2000, 39, 2699-2702. (e) Bedford, R. B.; Draper, S. M.; Noelle
Scully, P.; Welch, S. L. New J. Chem. 2000, 24, 745-747.
(12) The Suzuki coupling of 3,5-bis(trifluoromethyl)bromobenzene has been
previously reported by: Grasa, G. A.; Hiller, A. C.; Nolan, S. P. Org. Lett.
2001, 3, 1077-1080.
(13) The Suzuki coupling of pentafluorobromobenzene has been previously
reported: Irngartinger, H.; Escher, T. Tetrahedron 1999, 55(35), 10753-
10760. However, the protocol used herein as well as that used by Nolan
(ref 12) gave only poor yields.
(14) Mohammad, H. A. Y.; Grimm, J. C.; Eichele, K.; Mack, H.-G.; Speiser,
B.; Novak, F.; Quintan˜illa, M. G.; Kaska, W. C.; Mayer, H. A. Organo-
metallics 2002, 21, 5775-5784.
(15) Grimm, J. C.; Nachtigal, C.; Mack, H.-G.; Kaska, W. C.; Mayer, H. A.
Inorg. Chem. Commun. 2000, 3, 511-514.
(10) Brooks, P. R.; Wirtz, M. C.; Vetelino, M. G.; Rescek, D. M.; Woodworth,
G. F.; Morgan, B. P.; Coe, J. W. J. Org. Chem. 1999, 64, 9719-9721.
9
J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1805

A R T I C L E S
Go¨ttker-Schnetmann et al.
Scheme 2. Synthesis of 5-Arylresorcinols 6e,f
Scheme 3. Synthesis of p-XPCPIrHCl Complexes (4a-f)
p-XPCP ligand along the C-Ir-Cl axis. The proposed geometry
was confirmed by an X-ray crystal structure analysis of
compound 4a (Figure 1). The most striking feature of this
structure as compared to the related methylene-bridged complex
10 is the nearly identical geometry around the Ir center despite
the much shorter P1-O1 [1.654(4) Å], P2-O2 [1.664(4)], O1-
C2 [1.397(6)], and O8-C6 [1.400(7)] distance. However, as
compared to compound 10, compound 4a exhibits the shorter
Figure 1. ORTEP plot of compound 4a with thermal ellipsoids at a 50%
probability level. Selected bond lengths (Å) and angles (deg): Ir1-C7 2.004-
(5), Ir1-P1 2.2949(16), Ir1-P2 2.2889(15), Ir1-Cl 2.4044(14), P1-O1
1.654(4), O1-C2 1.397(6), P2-O8 1.664(4); P1-Ir-P2 159.79(5), P1-
Ir-C7 79.91(16), Ir1-P1-O1 104.68(14), P2-Ir-C7 80.07(16), Ir1-P2-
O8 104.65(15), Cl1-Ir1-C7 174.75(15).
removal of dihydrogen from the formed tetrahydride {C6H3-
2,6[CH2P(tBu)2]2}IrH4 at 130 °C.4l Unfortunately, as was
explicitly noted, the catalytic activity of 1 is very sensitive to
boron containing impurities, and thus an in situ generation of
catalytic active species was precluded due to the use of
LiBHEt3.1d The advantage of an in situ generation of the
catalytic active species from precursor complex 11 or 4a-f,
however, is obvious, because 11 and 4a-f are air stable for at
least several months in the solid state and for several weeks in
solution.
CarylIr distance Ir1-C7 2.004(5) Å [2.015(3) (10)] as well as
the smaller P-Ir-P angle 159.79(5)° [166.80(3) (10)] and
Caryl-Ir-Cl angle 174.75(15)° [179.33(14) (10)].
(b) Catalytic Activity of Species Generated from Com-
plexes 4a-f. Kaska and Jensen et al. and Goldman et al. have
shown that the iridium PCP pincer complex {C6H3-2,6-[CH2P-
(tBu)2]2}IrH2 (1) is an active dehydrogenation catalyst for
several alkanes in both the presence and the absence of a
hydrogen acceptor.3,4j,l,6,16 For the commonly used benchmark
reaction of cyclooctane (COA) and tert-butylethylene (TBE)
to form cyclooctene COE and tert-butylethane (TBA) (Scheme
4), turnover numbers (TONs) as high as 720-1000, turnover
frequencies (TOFs) as high as 12 min-1, and conversions of
10-25% COA have been reported for complex 1 at 200 °C.
However, inhibition of 1 by TBE:1 ratios >350 is reported, a
feature which requires periodic addition of TBE to obtain higher
TONs.1d Complex 1 is an oxygen- and nitrogen-sensitive
compound with high thermal stability obtained in 85% yield
after treatment of the {C6H3-2,6[CH2P(tBu)2]2}IrHCl (11)
precursor with LiBHEt3 under an atmosphere of hydrogen and
We have succeeded in generating catalytically active species
for the transfer dehydrogenation of COA with TBE starting from
either 11 or complexes 4a-f. Thus, simple addition of NaOtBu
to mixtures of COA, TBE, and either complexes 4a-f or 11
initiates the generation of TBA and COE according to Scheme
4 upon heating the resulting mixtures to 70-200 °C. Remark-
ably, we find that precursors 4a-f upon reaction with NaOtBu
generate species which are much less subject to inhibition by
TBE or product COE than the catalytically active species
generated from complex 11 (or catalyst 1). In an exemplified
series of experiments, reaction of COA, TBE, NaOtBu, and
4a-f in a 3030:3030:1.1:1 ratio results in 1480-2070 TONs
(16) Gupta, M.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1997, 461-462.
9
1806 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Iridium Bis(phosphinite) p-XPCP Pincer Complexes
A R T I C L E S
Scheme 4. Transfer Dehydrogenation Catalyzed by 1 or by Hydridochloride Precursors 4a-f or 11 and NaOtBu
Table 1. TONs for the Transfer Dehydrogenation of COA and
(48.8-68.3% conversion of TBE) after 40 h at 200 °C, while
230 TONs are obtained with precatalyst 11 under identical
conditions.
TBE Catalyzed by 4a-f and 11 Plus NaOtBu Obtained at 200 °C
and the COE:1,3-COD Product Ratioa
4 (p-X))
More than 50% of the overall TONs for a given precatalyst
4b-f (not 4a) already occur after 8 min at 200 °C, while >90%
of all TONs take place within 40 h (see Table 1). The TONs
after 8 min translate into initial average TOFs between 1.6 and
2.4 s-1, which compares to 0.2 s-1 for the activity of isolated
{C6H3-2,6-[CH2P(tBu)2]2}IrH2 (1).1d,3 In contrast to the reported
catalytic activity of 1, precatalysts 4a-f give rise to the
generation of substantial amounts of 1,3-cylooctadiene (1,3-
a
MeO
b
Me
c
H
d
F
e
f
11
H
C F
6
ArF
5
8 min
(COE/COD)b (100/0) (100/0) (99/1) (99/1) (94/6) (94/6) (100/0)
31 min 1226 1087 1194 1108 1401 1424 198
(COE/COD)b (93/7) (95/5) (93/7) (93/7) (90/10) (89/11) (100/0)
178 min 1564 1356 1514 1380 1699 1735 216
(COE/COD)b (86/14) (87/13) (86/14) (85/15) (83/17) (82/18) (100/0)
918 min 1674 1413 1512 1465 1863 1893 212
(COE/COD)b (83/17) (87/13) (86/14) (85/15) (80/20) (79/21) (100/0)
2398 min 1904 1484 1583 1530 2041 2070 227
(COE/COD)b (81/19) (86/14) (84/16) (84/16) (78/22) (76/24) (100/0)
6170 min 2017 1488 1609 1605 2175 2186 230
(COE/COD)b (78/22) (86/14) (83/17) (83/17) (75/25) (75/25) (100/0)
806
811
922
840 1150 1162
156
1
COD),17 as was determined by H NMR, with more 1,3-COD
produced by the more active systems (Table 1, entries 4a,e,f as
compared to 4b,c,d).18
While a more quantitative analysis of the electronic influence
of the p-X-substituent in precatalysts 4a-f is underway,19 σp
Hammett constants of these p-X-substituents identify 4a [σp-
(MeO) ) -0.27] as the most electron-rich and 4e [σp(C6F5) )
+0.27] [σp(Me) ) -0.17, σp(H) ) 0.00, σp(F) ) +0.06, σp-
(ArF) not reported] as the most electron-deficient systems under
investigation.20 Based on these Hammett constants, a correlation
between increasing initial TOFs and increasing electron defi-
ciency of precatalysts 4a-f exists. With respect to the overall
TONs, however, this correlation does not persist, because
catalysis with 4a results in nearly as many TOs as with 4e,f
and significantly more than with 4b,c,d (Table 1). We tentatively
ascribe this behavior to the counterbalancing effects of activity
and inhibition of active species generated from 4a-f, both
increasing with electron deficiency. As was already observed
for 1,3 the catalytic activity in the reactions described here, for
example, with 4c, is fully restored after evaporation of all
volatiles and addition of fresh COA/TBE solutions.
20 305 min
2047 1485 1603 1633 2170 2210
230
(COE/COD)b (78/22) (85/15) (83/17) (82/18) (75/25) (75/25) (100/0)
a Average of three runs, based on conversion of TBE determined by 1H
NMR, 3030 TO ) 100% conversion, all reactions performed under an argon
atmosphere. b Determined by 1H NMR, the sum of COE and COD double
bonds equals TON of TBE within 2% difference.
COD when catalysts derived from complexes 4a-f are used in
the transfer dehydrogenation of COA with TBE. It remained
unclear, however, whether TBE serves exclusively as the
hydrogen acceptor in these reactions. Therefore, in independent
experiments, the possible formation of 1,3-COD and COA via
disproportionation COE was probed. No formation of 1,3-COD,
however, was observed in neat COE solutions with COE:Ir ratios
>1000:1 at 200 °C if precatalyst 4c was used, thus indicating
that TBE likely is the hydrogen acceptor for COE dehydroge-
nation in COA/TBE mixtures (vide supra). By lowering the
COE:Ir ratio to ca. 450:1, however, we could monitor a slow
buildup of 1,3-COD and of o-xylene and ethylbenzene as higher
(c) COE Disproportionation into COA and 1,3-COD.
Production of Alkylated Benzenes by Further Dehydroge-
nation of 1,3-COD. As already mentioned, and in striking
contrast to catalytically active species derived from {C6H3-2,6-
[CH2P(tBu)2]2}IrHCl (11), we find substantial amounts of 1,3-
1
dehydrogenation products by H NMR (cyclooctatriene is not
observed). After 192 h at 200 °C, nearly all COE is consumed,
and a 4:1-mixture of o-xylene:ethylbenzene, as well as COA
and 1,2-dimethylcyclohexane (as hydrogenation products), is
observed by H and 13C NMR, besides traces of 1,3-COD and
further unidentified (cyclo)alkanes (ca. 300 TO in COE, 150
COE serve as hydrogen donor, 300 COE serve as hydrogen
acceptor). A selective formation of 1,3-COD in nearly quantita-
tive yield (ca. 240 TO in COE) without formation of even traces
of o-xylene or ethylbenzene, however, was accomplished by
reacting a 500:500:1:1.1 mixture of COE, tert-butyl alcohol (or
cyclohexane), 4c, and NaOtBu for 48 h at 200 °C. Even after
1
(17) We have not observed formation of 1,4- or 1,5-cyclooctadiene in these
reactions. However, while monitoring the synthesis of precatalysts 4a-f
1
from [(COD)IrCl]2 and ligands 5a-f by H NMR, we have observed that
the 1,5-cyclooctadiene liberated in these reactions isomerizes quantitatively
to 1,3-cyclooctadiene even at 150 °C.
(18) We have not been able to resolve COE from 1,3-COD by GC analysis
using a Agilent HP-1 column (30 m, 0.32 mm widebore, 0.25 µm film). A
1,3-COD shoulder on the COE signal was resolved at isothermic 70 °C,
flow rate 0.2 mL/min.
(19) In preparation.
(20) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
9
J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1807

A R T I C L E S
Go¨ttker-Schnetmann et al.
procedure.22 3,5-Dimethoxybenzene boronic acid11 and 5-fluororesor-
cinol10 were synthesized by known procedures. NMR spectra were
recorded on Bruker DRX 400 and AMX 300 MHz instruments and
are referenced either to residual protio solvent or to TMS as internal
standard. 31P chemical shifts are referenced to an external H3PO4
standard. 19F chemical shifts have not been referenced. IR spectra were
recorded on an ASI ReactIR 1000 spectrometer. Elemental analyses
were carried out by Atlantic Microlab, Inc. of Norcross, GA.
Scheme 5. Proposed Pathway for the Formation of Alkylated
Benzenes from COE
General Procedure for the Synthesis of 5-XPCP Pincer Ligands
5. A suspension of 2.1 mmol of NaH in 5 mL of THF was slowly
added via syringe to a solution of 1 mmol of the 5-substituted resorcinol
in 15 mL of THF (caution: hydrogen evolution). The mixture was
heated to reflux for 1 h, a solution of 2.1 mmol of di-tert-butyl-
chlorophosphine in 5 mL of THF was then added via syringe, and the
mixture was refluxed for additional 1 h. After evaporation of the solvent
in high vacuum (10-3 mbar, 1 h), the residue was extracted with 40
mL of pentane, and the extract was cannula transferred and filtered
through a pad of Celite. After removal of pentane under vacuum, the
flask was heated to 55 °C for 2 h at high vacuum which removed
residual amounts of di-tert-butyl-chlorophosphine. The crude products
were obtained as colorless viscous oils which occasionally solidified
upon standing and generally exhibited ca. 95% purity by NMR. They
were used without further purification.
additional heating to 200 °C (ca. 12-24 h), no buildup of
alkylated benzenes was observed. While it remains unclear so
far how tert-butyl alcohol or cyclohexane are involved in
suppressing a further dehydrogenation of 1,3-COD, the forma-
tion of o-xylene and ethylbenzene in neat COE solvent likely
proceeds through the equilibrium disrotatory ring closure of
cyclooctatriene to form [4.2.0]octa-2,4-diene 12,21 rather than
through a Ir-mediated carbon carbon bond cleavage of the eight-
membered ring (Scheme 5).
Summary
New bis(phosphinite) p-XPCPIrHCl complexes 4a-f have
been synthesized. In situ generation of catalytically active
species for the transfer dehydrogenation of cyclooctane with
tert-butylethylene to form cyclooctene and tert-butylethane is
very easily accomplished by reaction of 4a-f with NaOtBu in
the reaction mixture. Under comparable conditions, these new
catalysts are roughly 1 order of magnitude more active in terms
of TOFs, TONs, and substrate conversion than the benchmark
catalyst {C6H3-2,6-[CH2P(tBu)2]2}IrH2 (1), providing TOFs up
to 2.4 s-1 at 200 °C. A further dehydrogenation of COE to form
1,3-COD is accomplished by transfer dehydrogenation in the
presence of TBE at high COE concentration. In the absence of
another hydrogen acceptor, COE itself serves as hydrogen
acceptor and gives rise to a disproportionation of COE into COA
and 1,3-COD, which is further transformed into o-xylene and
ethylbenzene at temperatures as low as 200 °C. However,
disproportionation of COE into 1,3-COD and COA at 200 °C
is only operative at relatively low COE to catalyst ratios (ca.
450:1).
5-MeOPCP (5a). From 140 mg (1 mmol) of 5-methoxyresorcinol
(6a), 53 mg (2.1 mmol) of NaH, and 395 mg (2.1 mmol) of di-tert-
butyl-chlorophosphine was obtained 340 mg (0.82 mmol, 82%) of
5-MeOPCP (5a) as a colorless oil. 1H NMR (300 MHz, 23 °C, C6D6):
δ 7.15 (m, 1H, 2-H), 6.71 (m, 2H, 4- and 6-H), 3.33 (s, 3H, OCH3),
3
1.12 [d, JP-H ) 11.7 Hz, 36H, 2 × P(tBu)2]. 31P{1H} NMR (121.5
MHz, 23 °C, C6D6): δ 152.0. 13C{1H} NMR (75.5 MHz, 23 °C,
2
C6D6): δ 162.1 (Cq, d, JP-C ) 9.4 Hz, C1 and C3), 162.0 (Cq, C5),
101.5 (CH, t, 3JP-C ) 11.9 Hz, C2), 98.8 (CH, d, 3JP-C ) 11.5 Hz, C4
1
and C6), 54.8 (OCH3), 35.6 [Cq, d, JP-C ) 26.7 Hz, 2 × P(tBu)2],
2
27.5 [CH3, d, JP-C ) 15.8 Hz, 2 × P(tBu)2].
5-MePCP (5b). From 124 mg (1 mmol) of 3,5-dihydroxytoluene
(6b), 53 mg (2.1 mmol) of NaH, and 395 mg (2.1 mmol) of di-tert-
butyl-chlorophosphine was obtained 346 mg (0.87 mmol, 87%) of
5-MePCP (5b) as an off-white solid. 1H NMR (300 MHz, 23 °C,
C6D6): δ 6.70 (m, 1H, 2-H), 6.52 (m, 2H, 4- and 6-H), 2.18 (s, 3H,
3
4-CH3), 1.08 [d, JP-H ) 11.8 Hz, 36H, 2 × P(tBu)2]. 31P{1H} NMR
(121.5 MHz, 23 °C, C6D6): δ 153.1. 13C{1H} NMR (75.5 MHz, 23
2
°C, C6D6): δ 160.5 (Cq, d, JP-C ) 9.0 Hz, C1 and C3), 139.7 (Cq, s,
3
3
C5), 112.0 (CH, d, JP-C ) 11.0 Hz, C4 and C6), 105.9 (CH, t, JP-C
Experimental Section
) 10.0 Hz, C2), 35.5 [Cq, d, 1JP-C ) 25.3 Hz, 2 × P(tBu)2], 27.3 [CH3,
General Considerations. All manipulations were carried out using
standard Schlenk, high vacuum, and glovebox techniques. Argon was
purified by passage through columns of BASF R3-11 (Chemaolg) and
4 Å molecular sieves. THF was distilled from benzophenone ketyl prior
to use. Hexanes, pentane, toluene, diethyl ether, and dichloromethane
were passed through columns of activated alumina. Solvents and
reagents used in the generation of catalytically active Ir-complexes were
thoroughly freed from nitrogen (and oxygen) by several freeze-pump-
thaw cycles and stored in an argon atmosphere glovebox. Benzene-d6
and toluene-d8 were dried over sodium, degassed, and vacuum
transferred prior to use. CD2Cl2 was dried over P2O5, degassed, and
vacuum transferred prior to use. Cyclooctane (COA) was stirred over
concentrated H2SO4 for several hours until olefin- and arene-free (GC
and NMR analysis), then distilled under vacuum, and stored in an argon
atmosphere glovebox. tert-Butylethylene (TBE) (95%, Aldrich) was
degassed, vacuum transferred, and stored in the glovebox prior to use.
Resorcinol, 3,5-dihydroxytoluene, 1-fluoro-3,5-dimethoxybenzene, pen-
tafluorobromobenzene, sodium hydride (95%), and di-tert-butyl-chlo-
rophosphine (96%) were used as purchased from Aldrich. [(COD)IrCl]2
was used as purchased from Strem or synthesized by a known
2
d, JP-C ) 15.4 Hz, 2 × P(tBu)2], 21.5 (CH3, s, C4-Me).
5-HPCP (5c). From 110 mg (1 mmol) of resorcinol (6a), 53 mg
(2.1 mmol) of NaH, and 395 mg (2.1 mmol) of di-tert-butyl-
chlorophosphine was obtained 371 mg (0.93 mmol, 93%) of 5-HPCP
1
(5c) as a colorless oil which solidified upon standing. H NMR (300
MHz, 23 °C, C6D6): δ 7.48 (m, 1H, 2-H), 7.00 (m, 3H, 4-6-H), 1.11
(d, 3JP-H ) 11.6 Hz, 2 × P(tBu)2. 31P{1H} NMR (121.5 MHz, 23 °C,
C6D6): δ 153.1. 13C{1H} NMR (75.5 MHz, 23 °C, C6D6): δ 161.5
2
(Cq, d, JP-C ) 9.5 Hz, C1 and C3), 130.1 (CH, C5), 111.8 (CH, d,
3
3JP-C ) 11.3 Hz, C4 and C6), 109.0 (CH, t, JP-C ) 11.4 Hz, C2),
35.6 [Cq, d, 1JP-C ) 26.7 Hz, 2 × P(tBu)2], 27.5 [CH3, d, 3JP-C ) 15.8
Hz, 2 × P(tBu)2].
5-FPCP (5d). From 128 mg (1 mmol) of 5-fluororesorcinol (6d),
53 mg (2.1 mmol) of NaH, and 395 mg (2.1 mmol) of di-tert-butyl-
chlorophosphine was obtained 401 mg (0.96 mmol, 96%) of 5-FPCP
1
(5d) as a colorless oil. H NMR (300 MHz, 23 °C, CDCl3): δ 6.78
3
3
(m, 1H, 2-H), 6.44 (dm, JF-H ) 10.5 Hz, 4- and 6-H), 1.07 [d, JP-H
) 12.0 Hz, 36H, 2 × P(tBu)2]. 31P{1H} NMR (121.5 MHz, 23 °C,
CDCl3): δ 155.4. 19F{1H} NMR (376.5 MHz, 23 °C, CDCl3): δ
-111.9. 13C{1H} NMR (75.5 MHz, 23 °C, CDCl3): δ 163.5 (Cq, d,
(21) Huisgen, R.; Dahmen, A.; Huber, H. J. Am. Chem. Soc. 1967, 89, 7130-
7131.
(22) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18-19.
9
1808 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Iridium Bis(phosphinite) p-XPCP Pincer Complexes
A R T I C L E S
2
1JF-C ) 243.5 Hz, C5), 161.4 (Cq, dd, 2JP-C and 3JF-C ) 10.2 and 14.1
Hz, C1 and C3), 104.3 (CH, dt, 4JF-C ) 3.3 Hz, 3JP-C ) 10.2 Hz, C2),
99.0 (CH, dd, 2JF-C ) 25.0 Hz and 3JP-C ) 12.2 Hz, C4 and C6), 35.6
[Cq, d, 1JP-C ) 25.8 Hz, 2 × P(tBu)2], 27.3 [CH3, d, 2JP-C ) 15.5 Hz,
2 × P(tBu)2].
Tol-d8): δ 161.6 (Cq, d, JP-C ) 10.0 Hz, C1 and C3), 144.4, 140.5,
1
and 138.0 (Cq each, dm, JF-C ) 247.1, 253.0, and 251.7 Hz, C2′-
C6′), C5 not detected, 115.8 (Cq, m, C1′), 113.7 (CH, d, 3JP-C ) 12.5
Hz, C4 and C6), 110.0 (CH, t, 3JP-C ) 10.7 Hz, C2), 36.0 [Cq, d, 1JP-C
) 26.9 Hz, 2 × P(tBu)2], 27.5 [CH3, d, 2JP-C ) 15.6 Hz, 2 × P(tBu)2].
5-ArFPh(OMe)2 (7f). This compound was synthesized in analogy
to a known procedure:11 A septum-capped Schlenk flask was charged
with 1.365 mg (7.5 mmol) of 3,5-dimethoxyphenylboronic acid (8),
6.517 g (20.0 mmol) of Cs2CO3, 11 mg (0.05 mmol) of Pd(OAc)2, and
42 mg (0.11 mmol) of N,N′-1,2-acenaphthylidene-bis(2,6-dimethyl)-
benzenamine. After evacuation and refill with argon, a solution of 1.465
g (5 mmol) of 3,5-bis(trifluoromethyl)bromobenzene in 10 mL of 1,4-
dioxane was added, and the slurry was vigorously stirred for 8 h at 50
°C (TLC control indicated consumption of 3,5-bis(trifluoromethyl)-
bromobenzene). The resulting slurry was added to a separatory funnel
containing 20 mL of 0.05 M NaOH and extracted with 3 × 25 mL of
diethyl ether. The combined organic layers were dried over MgSO4
and filtered. After removal of the solvent, the resulting solid was
separated by column chromotography on silica gel [15 × 1 cm, hexanes:
diethyl ether 10:1, Rf ) 0.6 (hexanes:diethyl ether 5:1)] to give 1.613
g (4.61 mmol, 92%) of 3,5-dimethoxy-1-[3,5-bis(trifluoromethyl)-
phenyl]benzene (7f) as a white crystalline solid.
5-(C6F5)-1,3-(OMe)2Ph (7e). First attempts to synthesize compound
7e in analogy to compound 7f resulted in very low yields. An improved
synthesis used KF as base: A septum-capped Schlenk flask was charged
with 1.365 mg (7.5 mmol) of 3,5-dimethoxyphenylboronic acid (8),
1.740 g (30.0 mmol) of KF, 11 mg (0.05 mmol) of Pd(OAc)2, and 55
mg (0.21 mmol) of triphenylphosphine. After evacuation and refill with
argon, a solution of 1.235 g (5 mmol) of pentafluorobromobenzene in
5 mL of THF was added, and the slurry was vigorously stirred for 24
h at 23 °C (TLC indicated consumption of pentafluorobromobenzene).
The resulting slurry was added to a separatory funnel containing 20
mL of 0.05 M NaOH and extracted with 3 × 25 mL of diethyl ether.
The combined organic layers were dried with MgSO4 and filtrated.
After removal of the solvent, the resulting solid was separated by
column chromotography on silica gel [15 × 1 cm, hexanes:diethyl ether
10:1, Rf ) 0.5 (hexanes:diethyl ether 10:1)] to give 1.386 g (4.56 mmol,
91%) of 3,5-dimethoxy-1-(pentafluorophenyl)benzene (7e) as a white
crystalline solid. 1H NMR (300 MHz, 23 °C, CDCl3): δ 6.42 (m, 3H,
2-, 4-, and 6-H), 3.70 (s, 6H, 2 × OMe). 19F{1H} NMR (376.5 MHz,
23 °C, CDCl3): δ -142.96 (m, 2F, 2′- and 6′-F), -156.25 (t, 3JF-F
)
20.9 Hz, 1F, 4′-F), -162.89 (m, 2F, 3′- and 5′-F). 13C{1H} NMR (75.5
MHz, 23 °C, CDCl3): δ 160.8 (Cq, C1 and C3), 144.2, 140.4, and
1
137.8 (Cq each, dm each, JF-C ) 247.5, 252.5, and 253.0 Hz, C2′-
C6′), 127.8 (Cq, s, C5), 115.9 (Cq, m, C1′), 108.2 (CH, s, C4 and C6),
101.2 (CH, s, C2), 55.3 (CH3, s, 2 × OMe).
5-C6F5Ph(OH)2 (6e). Compound 6e was synthesized in analogy to
a known procedure:10 To a solution of 1.217 g (4.0 mmol) of 3,5-
dimethoxy-1-(pentafluorophenyl)benzene (7e) and 3.990 g (10.8 mmol)
of tetrabutylammoniumiodide in 35 mL of dichloromethane at -78
°C was added 10.5 mL (10.5 mmol) of a 1.0 M solution of boron
trichloride in dichloromethane under stirring in 20 min. The solution
was allowed to warm to 23 °C and was stirred for 18 h. Water (50
mL) was added, and the biphasic system was vigorously stirred for 30
min. Dichloromethane was evaporated under reduced pressure (200
mbar, 40 °C), 100 mL of diethyl ether was added to the residue, and
the organic layer was extracted with 4 × 50 mL of 1 M HCl (discarded)
and finally with 20 mL of 0.5 M NaOH. Dropwise addition of 10 mL
of 3 M HCl to the aqueous phase caused precipitation of the crude
product which was extracted into 4 × 15 mL of diethyl ether. After
the mixture was dried over MgSO4, the solvent was removed under
reduced pressure (400 mbar, 40 °C, then 10-3 mbar, 23 °C) to yield
948 mg (3.43 mmol, 86%) of a white powder which contained ca. 95%
of 5-(pentafluorophenyl)resorcinol (6e) by NMR and was used without
further purification in the next step. Depending on the solvent used,
compound 6e exhibits tautomerism between the â-ketohydroxy and the
1H NMR (300 MHz, 23 °C, C6D6): δ 7.77 (s, 2H, 2′- and 6′-H),
7.73 (s, 1H, 4′-H), 6.50 (s, 2H, 4- and 6-H), 3.29 (s, 6H, 2 × OMe).
19F{1H} NMR (376.5 MHz, 23 °C, C6D6): δ -62.88. 13C{1H} NMR
(75.5 MHz, 23 °C, C6D6): δ 161.9 (Cq, C1 and C3), 144.0 and 140.0
2
(Cq, each, C5 and C1′), 132.2 (Cq, q, JF-C ) 33.1 Hz, C3′ and C5′),
1
127.5 (CH, m, C2′ and C6′), 123.9 (Cq, q, JF-C ) 273.0 Hz, 3′- and
5′-CF3), 121.1 (CH, m, C4′), 105.9 (CH, C4 and C6), 100.9 (CH, C2),
54.9 (2 × OCH3).
5-ArFPh(OH)2 (6f). Synthesis of compound 6f was accomplished
in analogy to a published procedure:10 To a solution of 1.401 g (4.0
mmol) of 3,5-dimethoxy-1-[3,5-bis(trifluoromethyl)phenyl]benzene (7f)
and 3.990 g (10.8 mmol) of tetrabutylammoniumiodide in 35 mL of
dichloromethane at -78 °C was added 10.5 mL (10.5 mmol) of a 1.0
M solution of borontrichloride in dichloromethane under stirring in 20
min. The solution was allowed to warm to 23 °C and stirred for 18 h.
Water (50 mL) was added, and the biphasic system was vigorously
stirred for 30 min. Dichloromethane was evaporated under reduced
pressure (200 mbar, 40 °C), 100 mL of diethyl ether was added to the
residue, and the organic layer was extracted with 4 × 50 mL of 1 M
HCl (discarded) and finally with 20 mL of 0.5 M NaOH. Dropwise
addition of 10 mL of 3 M HCl to the aqueous phase caused precipitation
of the crude product which was extracted into 4 × 15 mL of diethyl
ether. Removal of the solvent under reduced pressure (400 mbar, 40
°C, then 10-3 mbar, 23 °C) yielded 1.189 g (3.69 mmol, 92%) of a
white powder which contained ca. 95% of 5-[3,5-bis(trifluoromethyl)-
phenyl]resorcinol (6f) by NMR and was used without further purifica-
tion in the next step. Depending on the solvent used, compound 6f
exhibits tautomerism between the â-ketohydroxy- and the dihydroxy
form.
1
dihydroxy form. H NMR (400 MHz, 23 °C, acetone-d6): δ 8.56 (s,
2H, 2 × OH), 6.50 (m, 1H, 2-H), 6.46 (m, 2H, 4- and 6-H). 19F{1H}
NMR (376.5 MHz, 23 °C, acetone-d6): δ -144.67 (m, 2F, 2′- and
6′-F), -159.21 (t, 3JF-F ) 20.7 Hz, 1F, 4′-F), -165.29 (m, 2F, 3′- and
5′-F). 13C{1H} NMR (100.6 MHz, 23 °C, acetone-d6): δ 159.7 (Cq,
C1 and C3), 145.2, 141.2, and 138.7 (Cq, each, dm each, 1JF-C ) 253.0,
250.9, and 245.8 Hz, C2′-C6′), 128.8 (Cq, C5), 117.3 (Cq, m, C1′),
109.7 (CH, C4 and C6), 104.6 (CH, C2).
5-C6F5PCP (5e). From 276 mg (1 mmol) of 5-(pentafluorophenyl)-
resorcinol (6e), 53 mg (2.1 mmol) of NaH, and 395 mg (2.1 mmol) of
di-tert-butylchloro-phosphine was obtained 513 mg (0.91 mmol, 91%)
1
of 5-(C6F5)PCP (5e) as a colorless oil. H NMR (400 MHz, 23 °C,
Tol-d8): δ 7.41 (m, 1H, 2-H), 7.08 (m br, 2H, 4- and 6-H), 1.11 [d,
3JP-H ) 11.8 Hz, 36H, 2 × P(tBu)2]. 31P{1H} NMR (162 MHz, 23 °C,
Tol-d8): δ 154.9. 19F{1H} NMR (376.5 MHz, 23 °C, Tol-d8):
δ
-143.17 (m, 2F, 2′- and 6′-F), -157.00 (t, 3JF-F ) 21.4 Hz, 1F, 4′-F),
-163.20 (m, 2F, 3′- and 5′-F). 13C{1H} NMR (100.6 MHz, 23 °C,
9
J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1809

A R T I C L E S
Go¨ttker-Schnetmann et al.
1H NMR [400 MHz, 23 °C, acetone-d6, dynamic mixture of two
(656.21): C, 42.09; H, 6.45. Found: C, 41.90; H, 6.33. X-ray structure
analysis of compound 4a (-100 °C): space group and cell dimensions,
orthorhombic Pbcn, a 34.8215(7), b 10.49720(20), c 14.3434(3) Å,
volume 5242.92(18) Å3, empirical formula, C23H41O3P2IrCl. Cell
dimensions were obtained from 7667 reflections with 2θ angle in the
range 5.00-56.00°. Crystal dimensions: 0.35 × 0.15 × 0.05 mm, FW
isomers, second isomer in {}]: δ 8.54 (s, 1H, OH), 8.14 {8.14} (s,
4
2H, 2′- and 6′-H), 7.98 {7.98} (s, 1H, 4′-H), 6.73 {6.71} (d, JH-H
)
4
2.1 Hz, 2H, 4- and 6-H), 6.49 {3.92} (t, JH-H ) 2.1 Hz, 1H, 2-H {s,
2H, 2-H2}). 1H NMR (300 MHz, 23 °C, CDCl3, only dihydroxy
form): δ 7.96 (s, 2H, 4′- and 6′-H), 7.86 (s, 1H, 2′-H), 6.67 (s, 2H, 4-
and 6-H), 6.45 (s, 1H, 2-H), 5.56 (s, 2H, 2 × OH). 19F{1H} NMR
(376.5 MHz, 23 °C, acetone-d6): δ -63.72. 13C{1H} NMR [100.6 MHz,
23 °C, acetone-d6, second isomer in {}]: δ 160.1 {160.2 and 210.2}
(Cq, C1 and C3), 144.6 and 140.7 {144.6 and 140.7} (Cq each, C5 and
) 655.19, Z ) 8, F(000) ) 2611.51, Fcalc 1.660 Mg/m3, µ 5.35 mm-1
,
λ 0.71073 Å, 2θ (max) 56.0°. The intensity data were collected on a
Bruker SMART 1K diffractometer, using the ω scan mode. The h,k,l
ranges used during structure solution and refinement are Hmin,max 0,
46; Kmin,max 0, 13; Lmin,max 0, 18. No. of reflections measured 25 346,
no. of unique reflections 6337, no. of reflections with Inet > 2.5σ (Inet)
4687. Merging R-value on intensities 0.041. Correction was made for
absorption using SADABS. Details of the last least squares cycle: 71
atoms, 272 parameters full-matrix on Fo counter wts (k 0.000050). The
residuals are as follows: significant reflections, 4682 RF 0.036, RW 0.037;
all reflections, 6337 RF 0.058, RW 0.041; included reflections, 4682 RF
2
C1′), 132.5 {132.5} (Cq, q, JF-C ) 33.1 Hz, C3′ and C5′), 128.1
{128.1} (CH, m br, C2′ and C6′), 124.5 {124.5} (Cq, q, 1JF-C ) 272.1
Hz, 3′- and 5′-CF3), 121.6 (CH, m, C4′), 106.7 {106.8 and 104.1} (CH,
C4 and C6), 104.0 {69.5 br} (CH {CH2}, C2).
5-ArFPCP (5f). From 322 mg (1 mmol) of 5-[3,5-bis(trifluoro-
methyl)phenyl]resorcinol (6f), 53 mg (2.1 mmol) of NaH, and 395 mg
(2.1 mmol) of di-tert-butyl-chlorophosphine was obtained 550 mg (0.90
mmol, 90%) of 5-(ArF)PCP (5f) as a colorless oil.
0.036, RW 0.037; GOF 1.6181, where RF ) ∑(Fo - Fc)/∑(Fo), RW
)
[∑(w(Fo - Fc)2)/∑(wFo )]1/2 and GOF ) [∑(w(Fo - Fc)2)/(no. of reflns
- no. of params.)]1/2. The maximum shift/sigma ratio was 0.000. Last
D-map: minimum density -2.540 e/Å3, maximum density 1.440 e/Å3.
Secondary ext. coeff. 0.2050 µm sigma 0.0463.
2
p-MePCPIrHCl (4b). Using the general procedure, we heated 168
mg (0.25 mmol) of [(COD)IrCl]2 and 227 mg (0.55 mmol) of 5-MePCP
(5b) for 12 h at 150 °C in 3 mL of toluene to yield 279 mg (0.44
mmol, 87%) of compound 4b as a red brown microcrystalline solid
1H NMR (300 MHz, 23 °C, C6D6): δ 7.78 (s, 2H, 2′- and 6′-H),
7.65 (s, 1H, 4′-H), 7.17 (s, 2H, 4- and 6-H), 7.08 (s, 1H, 2-H), 1.13 [d,
3JP-H ) 12.0 Hz, 2 × P(tBu)2]. 31P{1H} NMR (121.5 MHz, 23 °C,
C6D6): δ 154.8. 19F{1H} NMR (376.5 MHz, 23 °C, CDCl3): δ -62.82.
13C{1H} NMR (75.5 MHz, 23 °C, C6D6): δ 162.1 (Cq, d, 2JP-C ) 10.2
Hz, C1 and C3), 143.8 and 140.9 (Cq each, C5 and C1′), 132.1 (Cq, q,
2JF-C ) 33.1 Hz, C3′ and C5′), 127.7 (CH, C2′ and C6′), 123.8 (Cq, q,
1JF-C ) 272.7 Hz, 3′- and 5′-CF3), 121.1 (CH, m, C4′), 111.1 (CH, d,
3JP-C ) 9.5 Hz, C4 and C6), 108.8 (CH, t, 3JP-C ) 14.1 Hz, C2), 35.8
[Cq, d, 1JP-C ) 26.7 Hz, 2 × P(tBu)2], 27.4 [CH3, d, 2JP-C ) 15.7 Hz,
2 × P(tBu)2].
1
slightly soluble in pentane. H NMR (400 MHz, 23 °C, CD2Cl2): δ
6.42 (s, 2H, 3- and 5-H), 2.26 (s, 3H, 4-CH3), 1.35 (m, 36H, 4 × tBu),
2
-41.63 (t, JP-H ) 13.1 Hz, 1H, IrH). 31P{1H} NMR (162 MHz, 23
°C, CD2Cl2): δ 176.1. 13C{1H} NMR (100.6 MHz, 23 °C, CD2Cl2): δ
1167.6 (Cq, vt, JP-C ) 5.9 Hz, C2 and C6), 136.3 (Cq, C4), 114.4 (Cq,
m br, C1), 106.0 (CH, vt, JP-C ) 5.3 Hz, C3 and C5), 43.3 and 39.7
[Cq each, vt each, JP-C ) 11.5 and 12.7 Hz, 2 × P(tBu)2], 27.8 and
27.6 [CH3 each, vt each, JP-C ) 3.1 and 3.1 Hz, 2 × P(tBu)2], 21.5
(4-CH3). IR (CH2Cl2, cm-1): 2945, 2904, 2871, 1596, 1557, 1475, 1392.
Elemental analysis calcd for C23H42O2P2ClIr (640.21): C, 43.15; H,
6.61. Found: C, 43.89; H, 6.70.
General Procedure for the Synthesis of Complexes p-XPCPIrHCl
(4a-f). A rubber septum-capped Schlenk flask was charged with 0.5
equiv of [(COD)IrCl]2 and 1.1 equiv of the respective 5-XPCP ligand
(5a-f). The flask was evacuated and refilled with argon. A small
amount of toluene was added via syringe, and the solution was stirred
in an oil bath for 1-16 h at 150 °C while the products 4a-d started
to precipitate. The reaction mixture was rapidly cooled to 30 °C. The
solvent and free (1,3)-COD were removed in a vacuum (10-3 mbar,
30 °C), and the residue was extracted with a small amount of pentane
under ultrasound (10 min). After filtration (no inert gas required), the
solid was washed with small amounts of pentane and dried under high
vacuum (10-3 mbar) to yield analytically pure samples. For higher
solubility, it is desirable to obtain the products as microcrystalline solids
when used for further transformations. In the case of compounds 4e,f,
the remaining mother liquors were flash chromatographed on neutral
alumina to yield an additional crop of compounds 4e,f.
p-HPCPIrHCl (4c). Using the general procedure, we heated 336
mg (0.5 mmol) of [(COD)IrCl]2 and 438 mg (1.1 mmol) of 5-HPCP
(5c) for 12 h at 150 °C in 3 mL of toluene to yield 585 mg (0.93
mmol, 93%) of compound 4c as a red brown microcrystalline solid
virtually insoluble in pentane. 1H NMR (400 MHz, 23 °C, CD2Cl2): δ
6.76 (t, 3JH-H ) 8.0 Hz, 1H, 4-H), 6.54 (d, 3JH-H ) 8.0 Hz, 2H, 3- and
2
5-H), 1.35 (m, 36H, 4 × tBu), -41.39 (t, JP-H ) 13.1 Hz, 1H, IrH).
31P{1H} NMR (162 MHz, 23 °C, CD2Cl2): δ 175.8. 13C{1H} NMR
(100.6 MHz, 23 °C, CD2Cl2): δ 167.7 (Cq, vt, JP-C ) 5.9 Hz, C2 and
C6), 125.7 (CH, C4), 118.5 (Cq, m br, C1), 105.0 (CH, C3 and C5),
43.3 and 39.7 [Cq, each, vt each, JP-C ) 11.5 and 12.7 Hz, 2 × P(tBu)2],
27.8 and 27.6 [CH3 each, vt each, JP-C ) 3.1 and 3.1 Hz, 2 × P(tBu)2].
IR (CH2Cl2, cm-1): 2945, 2904, 2871, 1579, 1562, 1475, 1371.
Elemental analysis calcd for C22H40O2P2ClIr (626.18): C, 42.19; H,
6.44. Found: C, 42.40; H, 6.42.
p-MeOPCPIrHCl (4a). Using the general procedure, we heated 168
mg (0.25 mmol) of [(COD)IrCl]2 and 236 mg (0.55 mmol) of
5-MeOPCP (5a) for 16 h at 150 °C in 3 mL of toluene to yield 293
mg (0.45 mmol, 89%) of compound 4a as a dark red microcrystalline
solid that is virtually insoluble in pentane. 1H NMR (300 MHz, 23 °C,
CDCl3): δ 6.27 (s, 2H, 3- and 5-H), 3.78 (s, 3H, OCH3), 1.37 [m,
p-FPCPIrHCl (4d). Using the general procedure, we heated 168
mg (0.25 mmol) of [(COD)IrCl]2 and 229 mg (0.55 mmol) of 5-FPCP
(5d) for 8 h at 150 °C in 3 mL of toluene to yield 300 mg (0.47 mmol,
93%) of compound 4d as a red brown microcrystalline solid virtually
insoluble in pentane. 1H NMR (400 MHz, 23 °C, CD2Cl2): δ 6.37 (d,
3JF-H ) 9.5 Hz, 2H, 3- and 5-H), 1.34 (m, 36H, 4 × tBu), -41.61 (t,
2JP-H ) 13.1 Hz, 1H, IrH). 31P{1H} NMR (162 MHz, 23 °C, CD2Cl2):
2
36H, 2 × P(tBu)2], -41.87 (t, JP-H ) 13.2 Hz, IrH). 31P{1H} NMR
(121.5 MHz, 23 °C, CDCl3): δ 177.6. 13C{1H} NMR (75.5 MHz, 23
°C, CDCl3): δ 167.3 (Cq, vt, JP-C ) 6.0 Hz, C2 and C6), 159.6 (Cq,
δ 179.1. 19F{1H} NMR (376.5 MHz, 23 °C, CD2Cl2): δ -119.01. 13C-
3
{1H} NMR (75.5 MHz, 23 °C, CD2Cl2): δ 167.0 (Cq, dvt, JF-C
)
C4), 108.1 (Cq, m br, C1), 91.9 (CH, t, JP-C ) 5.1 Hz, C3 and C5),
14.8 Hz, JP-C ) 5.8 Hz, C2 and C6), 162.7 (Cq, d, 1JF-C ) 237.0 Hz,
C4), 112.5 (Cq, m br, C1), 93.4 (CH, dvt, 2JF-C ) 26.0 Hz, JP-C ) 5.6
Hz, C3 and C5), 43.4 and 39.8 [Cq each, vt each, JP-C ) 11.3 and 12.5
Hz, 2 × P(tBu)2], 27.7 and 27.6 [CH3 each, vt each, JP-C ) 3.1 and
3.1 Hz, 2 × P(tBu)2]. IR (CH2Cl2, cm-1): 2946, 2904, 2871, 1594,
2
55.4 (OCH3), 43.0 and 39.5 [Cq each, vt each, JP-C ) 22.6 and 25.3
Hz, 2 × P(tBu)2], 27.7 and 27.5 [CH3, each vt, each, JP-C ) 3.2 and
3.1 Hz, 2 × P(tBu)2]. IR (CH2Cl2, cm-1): 2944, 2904, 2871, 1595,
1563, 1475, 1370. Elemental analysis calcd for C23H42O3P2ClIr
9
1810 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Iridium Bis(phosphinite) p-XPCP Pincer Complexes
A R T I C L E S
1581, 1475, 1371. Elemental analysis calcd for C22H39O2P2FClIr
(644.17): C, 41.02; H, 6.10. Found: C, 40.44; H, 5.92.
p-XPCPIrHCl (4a-f), and 1.1 mg of NaOtBu was occasionally shaken
and aged for at least 2 h in the glovebox. Next, 1.5 mL aliquots of
these stock solutions were transferred into 4 mL thick walled Kontes
reactors closed with Teflon screw caps, and the reactors were placed
in the cavities of a heated aluminum block at 200 °C. After the desired
reaction time, the Kontes reactors were removed from the aluminum
block and cooled by a stream of air. GC analysis (Agilent, HP-1, 30
m, 0.32 mm widebore, 0.25 µm film) of the reaction mixtures even
under isothermic low temperature (70 °C) and low flow rate (0.2 mL/
min) conditions was hampered due to identical retention times of COE
and 1,3-COD. Therefore, aliquots of the reaction mixtures were taken
p-C6F5PCPIrHCl (4e). Using the general procedure, we heated 168
mg (0.25 mmol) of [(COD)IrCl]2 and 311 mg (0.55 mmol) of 5-(C6F5)-
PCP (5e) for 2 h at 150 °C in 3 mL of toluene to yield 313 mg (0.40
mmol, 79%) of compound 4e as an orange microcrystalline solid
moderately soluble in pentane. An additional 34 mg (0.04 mmol, 9%)
of compound 4e was obtained by column chromatography (8 × 0.3
cm, hexanes:diethyl ether 20:1, Rf ) 0.6 hexanes:diethyl ether 10:1)
1
of the collected filtrates (88% overall yield). H NMR (400 MHz, 23
°C, CDCl3): δ 6.63 (s, 2H, 3- and 5-H), 1.40 (m, 36H, 4 × tBu),
1
2
-41.06 (t, JP-H ) 13.0 Hz, 1H, IrH). 31P{1H} NMR (162 MHz, 23
in the glovebox and analyzed by H NMR. An average of three runs
°C, CDCl3): δ 177.4. 19F{1H} NMR (376.5 MHz, 23 °C, CDCl3): δ
-143.02 (m, 2F, 2′- and 6′-F), -157.13 (t, 3JF-F ) 42.2 Hz, 1F, 4′-F),
-163.22 (m, 2H, 3′- and 5′-F). 13C{1H} NMR (100.6 MHz, 23 °C,
CDCl3): δ 167.7 (Cq, vt, JP-C ) 6.0 Hz, C2 and C6), 144.7, 140.3,
(all from the same stock solution) was taken to determine the TONs at
a given time. TONs were extracted from the ratio of both the integrals
of the olefinic COE, 1,3-COD, and TBE signals and the integrals of
the tert-butyl resonance of TBE (s, 9H) and the overlapping methyl
resonances of TBA (s and t, 12H). The ratio of COE and 1,3-COD
was extracted from the two overlapping olefinic signals of COE and
1,3-COD (2,3-H) and the isolated olefinic 1,3-COD signal (1,4-H).
Except for the 5% impurity in the commercially available TBE, which
was proven to be unreactive with a mesitylene capillary standard in a
deuterium free NMR tube experiment at 200 °C (and which therefore
can be used as an internal standard), no signals other than COA, COE,
1,3-COD, TBE, and TBA were detected in these reaction mixtures.
The number of COE and 1,3-COD double bonds equaled the number
of produced TBA molecules within <2% deviation in each experiment.
COE Disproportionation into COA and 1,3-COD. 1,3-COD
Dehydrogenation To Form Alkylbenzenes. To 6.3 mg (10 µmol) of
precatalyst 4c and 1.1 mg (11 µmol) of NaOtBu in a J. Young tube
was added 496 mg (4.5 mmol) of COE under argon. The tube was
1
and 138.3 (Cq each, dm each, JF-C ) 246.1, 252.5, and 249.9 Hz,
C2′-C6′), 123.2 and 121.9 (Cq each, C4 and C1′), 116.9 (Cq, m, C1),
106.9 (CH, m br, C3 and C5), 43.4 and 39.8 [Cq each, vt each, JP-C
)
11.3 Hz, 2 × P(tBu)2], 27.8 and 27.7 [CH3 each, vt each, 3.0 and 3.0
Hz, 2 × P(tBu)2]. IR (CH2Cl2, cm-1): 2926, 2906, 2871, 1545, 1523,
1493, 1475. Elemental analysis calcd for C28H39O2P2F5ClIr (792.23):
C, 42.45; H, 4.96. Found: C, 42.92; H, 5.04.
p-ArFPCPIrHCl (4f). Using the general procedure, we heated 336
mg (0.5 mmol) of [(COD)IrCl]2 and 672 mg (1.1 mmol) of 5-ArFPCP
(5f) for 2 h at 150 °C in 3 mL of toluene to yield 764 mg (0.91 mmol,
91%) of compound 4f as an orange red microcrystalline solid
moderately soluble in pentane. An additional 20 mg (0.02 mmol, 2%)
of compound 4f was obtained by column chromatography (8 × 0.3
cm, hexanes:diethyl ether 20:1, Rf ) 0.5 hexanes:diethyl ether 10:1)
of the collected filtrates (93% overall yield).
1
then immersed in a sand bath at 200 °C for 192 h. Periodically, H
NMR spectra were recorded which indicated the formation of 1,3-COD,
and subsequently (but already at an early stage of the reaction) the
formation of o-xylene and ethylbenzene besides COA, 1,2-dimethyl-
cyclohexane, and trace amounts of further unidentified (cyclo)alkanes.
The formation of o-xylene, ethylbenzene, COA, 1,2-dimethylcyclo-
1
hexane, and 1,3-COD was unambiguously proven by comparing H
and 13C NMR spectra of the final product mixture after 192 h at 200
°C with authentic samples (1,2-dimethylcyclohexane as received from
Aldrich was a mixture of the cis- and trans-isomer). When stock
solutions prepared from 6.3 mg (10 µmol) of precatalyst 4c, 1.1 mg
(11 µmol) of NaOtBu, 550 mg (5.0 mmol) of COE, and 421 mg (5.0
mmol) of cyclohexane [or 370 mg (5.0 mmol) of tert-butyl alcohol]
were heated in a J. Young tube for 48-72 h at 200 °C, exclusive
formation of 1,3-COD and COA in nearly quantitative yield as based
on the 1H NMR spectra was observed, while cyclohexane (or tert-butyl
alcohol) remained unreacted.
1H NMR (400 MHz, 23 °C, CD2Cl2): δ 8.10 (s, 2H, 2′- and 6′-H),
7.83 (s, 1H, 4′-H), 6.91 (s, 2H, 3- and 5-H), 1.39 (m, 36H, 4 × tBu),
-40.96 (t, 2JP-H ) 13.0 Hz, 1H, IrH). 31P{1H} NMR (162.5 MHz, 23
°C, CD2Cl2): δ 177.8. 19F{1H} NMR (376.5 MHz, 23 °C, CD2Cl2): δ
-63.58. 13C{1H} NMR (100.6 MHz, 23 °C, CD2Cl2): δ 168.5 (Cq, vt,
JP-C ) 5.9 Hz, C2 and C6), 143.8 and 136.0 (Cq each, C4 and C1′),
2
132.2 (Cq, q, JF-C ) 33.0 Hz, C3′ and C5′), 127.1 (CH, m C2′ and
1
C6′), 124.0 (Cq, q, JF-C ) 272.6 Hz, 2 × CF3), 121.5 (Cq, m, C1),
Acknowledgment. We gratefully acknowledge funding by
the National Institutes of Health (grant no. GM 28938) and the
Deutsche Akademie der Naturforscher Leopoldina (grant no.
BMBF-LPD 9901/8-60 to I.G.-S.). We thank Alan Goldman
for fruitful discussions.
120.6 (CH, m, C4′), 103.9 (CH, vt, JP-C ) 5.3 Hz, C3 and C5), 43.5
and 40.0 [Cq, each, vt each, JP-C ) 11.3 Hz, 2 × P(tBu)2], 27.8 and
27.7 [CH3 each, vt each, JP-C ) 3.1 and 3.0 Hz]. IR (CH2Cl2, cm-1):
2945, 2904, 2871, 1619, 1593, 1545, 1476, 1373. Elemental analysis
calcd for C30H42O2P2F6ClIr (838.27): C, 42.98; H, 5.05. Found: C,
43.02; H, 5.14.
Supporting Information Available: Crystallographic data of
compound 4a. This material is available free of charge via the
Internet at http://pubs.acs.org.
General Procedure for the Transfer Dehydrogenation of Cy-
clooctane with tert-Butylethylene. A stock solution prepared from
3.400 g of cyclooctane (30.31 mmol), 2.690 g of tert-butylethylene
(30.36 mmol based on 95% purity), 10 µmol of the respective
JA0385235
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J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1811