Halobenzenes and Ir(I): Kinetic C-H oxidative addition and thermodynamic C-Hal oxidative addition

Article abstract of DOI:10.1021/ja0557637

A (PNP)Ir fragment undergoes facile, room-temperature oxidative addition of C-H bonds in arenes and haloarenes in preference to aromatic carbon-halogen bonds. This preference, however, is determined to be kinetic in nature. Oxidative addition of C-Cl and C-Br is preferred thermodynamically. The products of the C-Cl or C-Br oxidative addition are separated from the C-H oxidative addition products by a high activation barrier and are only accessible at >100 °C. Of the C-H oxidative addition products of chlorobenzene, the isomer with the o-ClC₆H₄ ligand has the lowest energy. Copyright

Full text of DOI:10.1021/ja0557637

Published on Web 11/10/2005
Halobenzenes and Ir(I): Kinetic C-H Oxidative Addition and Thermodynamic
C-Hal Oxidative Addition
Lei Fan,† Sean Parkin,‡ and Oleg V. Ozerov*,†
Department of Chemistry, Brandeis UniVersity, MS015, 415 South Street, Waltham, Massachusetts 02454, and
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506
Received August 22, 2005; E-mail: ozerov@brandeis.edu
Haloarenes are among the most commonly used organic sub-
strates. Their involvement in transition-metal-catalyzed processes
often entails oxidative addition (OA) reactions.1 In this context,
the metal has a choice of one or more different C-H bonds and a
C-Hal bond (Hal ) halide). An important question is why some
metal centers prefer C-Hal bonds while others prefer C-H bonds.
OA of aromatic C-Cl, C-Br, and C-I bonds has been studied
almost exclusively in the context of the chemistry of zero-valent
group 10 metals (especially Pd).2,3 OA of aromatic C-F received
more attention for group 9 metals.4 Aromatic C-H OA reactions
are much more common across the periodic table, particularly for
d8 metal centers.1,5 Selective C-H activation in the presence of
aromatic C-Cl or C-Br bonds has been reported in some Pd, Ir,
and Au systems.6
7 (Scheme 3). When PhF was used as solvent, a mixture of four
isomers of 6 was produced (four new 19F NMR resonances and
1
four hydridic H NMR resonances observed).9 Upon thermolysis
(100 °C, 20 h), this mixture evolves into a mixture of only two
isomers. No products arising from OA of C-F were observed.
Scheme 3
Recent work by Milstein et al. described the activation of only
C-H bonds in halobenzenes via OA to the cationic (PNP*)IrI
fragment (Scheme 1).6d Mild thermolysis of the mixture of isomers
of 2 (and their Br analogues) gave isomerization to the thermody-
namically most stable haloaryl/hydride product 2a.6d In the present
work, we show that, in a closely related Ir system, all C-H OA
products are kinetic, and the global minimum for an OA reaction
is the product of the C-Hal OA.
The reaction between 5 and NBE in PhCl produced a mixture
of four C-H OA products (9)9 along with a small (5%) fraction of
the C-Cl OA product 8a (Scheme 3). Thermolysis of this mixture
at 70 °C led to slow isomerization of the other isomers of 9 into
9a (71% after 72 h) without change in the fraction of 8a present.
9a was isolated in 44% yield by recrystallization. Thermolysis of
9a (or of the mixture in Scheme 3) at >100 °C for 24-48 h led to
the >80% isomerization to the C-Cl OA product 8a. Thermolysis
of isolated, pure 9a does not lead to the formation of other isomers
of 9, consistent with 9a being the lowest energy C-H OA isomer.
The reaction of 5 with NBE in PhBr at 22 °C produced a mixture
of two isomers of (PNP)Ir(H)(C6H4Br) (10) and (PNP)Ir(Ph)(Br)
(8b). Upon thermolysis at 100 °C, the isomers of 10 slowly evolved
into 8b (80% after 48 h). 8a and 8b were synthesized independently
from 7 (Scheme 3). No isomerization of 8a or 8b was detected
after thermolysis of pure samples at 120 °C in C6D6. 8a and 8b are
thus global OA minima in the [(PNP)Ir + PhHal] systems.
The structure of 8b was elucidated by an X-ray diffraction study
(Figure 1). The geometry about Ir in 8b is approximately square
Scheme 1
We recently reported the preparation of 4 and other work
exploring the group 9 chemistry of the anionic PNP pincer ligands.7
The dihydride 5 can be easily synthesized from 4 (Scheme 2). We
were generally interested in the OA reactivity of the (PNP)IrI
fragment, especially in the context of the (PCP)Ir chemistry8 and
the work with (PNP*)IrI.6d
Scheme 2
On the basis of the (PCP)Ir methodology by Goldman et al.,8b
we opted to use norbornene (NBE) to strip the hydrides from the
Ir center in 5. The reaction of 5 with NBE in C6H6 cleanly produced
Figure 1. POV-Ray rendered ORTEP plots of 8b (left) and 9a (right),
50% thermal ellipsoids.13 H atoms and CH3 groups are omitted for clarity.
pyramidal. The P-Ir-P angle is 167.14(3)°, typical for PNP
complexes,7 and the N-Ir-Br angle is 162.57(7)°. Square pyra-
midal geometry is expected for five-coordinate d6 compounds with
† Brandeis University.
‡ University of Kentucky.
9
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10.1021/ja0557637 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
only one distinctly strong trans-influence ligand (Ph here).10 The
C6H5 ring is “sandwiched” between the two PPri2 groups. The
rotation about the Ir-C axis in solution is inhibited as evidenced
Supporting Information Available: Experimental details, crystal-
lographic information in the form of the CIF files, characterization data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
1
by the observation of five inequivalent H NMR resonances for
the C6H5 group in 8a or 8b.
References
We propose, by analogy with the (PCP)Ir systems, that the OA
reactions proceed via a 14-electron (PNP)Ir species 12 (Scheme
4).11 It is thought to arise from 11 via C-H reductive elimination.
Notably, 13 is not an intermediate in these reactions. 13 can be
prepared independently and does not react with halobenzenes at
22 °C. Neither does 5 react with haloarenes. 12 can also be accessed
by dehydrohalogenation of 4.8a Treatment of 4 with KOBut in C6H6
or C6H5Cl leads to the same products as from 5 and NBE.12
(1) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; Wiley-Interscience: New York, 2001; pp 149-173. (b) van
Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding the Art;
Kluwer Academic Publishers: Dordrecht, Boston, London, 2004; pp 271-
298 and 387-402.
(2) (a) Negishi, E. I. Handbook of Organopalladium Chemistry for Organic
Synthesis; Wiley: New York, 2002. (b) Barrios-Landeros, F.; Hartwig, J.
F. J. Am. Chem. Soc. 2005, 127, 6944. (c) Lewis, A. K. de K.; Caddick,
S.; Cloke, F. G. N.; Billingham, N. C.; Hitchcock, P. B.; Leonard, J. J.
Am. Chem. Soc. 2003, 125, 10066. (d) Portnoy, M.; Milstein, D.
Organometallics 1993, 12, 1665. (e) Strawser, D.; Karton, A.; Zenkina,
O. V.; Iron, M. A.; Shimon, L. J. W.; Martin, J. M. L.; van der Boom, M.
E. J. Am. Chem. Soc. 2005, 127, 9322.
Scheme 4
(3) Group 9 examples: (a) Wang, X.; Lane, B. S.; Sames, D. J. Am. Chem.
Soc. 2005, 127, 4996. (b) Willems, S. T. H.; Budzelaar, P. H. M.; Moonen,
N. N. P.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Chem.sEur. J.
2002, 8, 1310. (c) Grushin, V. V.; Alper, H. Chem. ReV. 1994, 94,
1047.
(4) (a) Aizenberg, M.; Milstein, D. Science 1994, 265, 359. (b) Aizenberg,
M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 8674. (c) Young, R. J.,
Jr.; Grushin, V. V. Organometallics 1999, 18, 294. (d) Yang, H.; Gao,
H.; Angelici, R. Organometallics 1999, 18, 2285.
(5) (a) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471. (b) Jones, W. D.
Acc. Chem. Res. 2003, 36, 140. (c) van der Boom, M. E.; Milstein, D.
Chem. ReV. 2003, 103, 1759. (d) Labinger, J. A.; Bercaw, J. E. Nature
2002, 417, 507. (e) 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.
(6) (a) Zaitsev, V. G.; Daugulis, O. J. Am. Chem. Soc. 2005, 127, 4156. (b)
Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am. Chem.
Soc. 2005, 127, 7330. (c) Chotana, G. A.; Rak, M. A.; Smith, M. R., III.
J. Am. Chem. Soc. 2005, 127, 10539. (d) Ben-Ari, E.; Gandelman, M.;
Rozenberg, H.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2003,
125, 4714. (e) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson,
D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1400-1410. (f)
Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig,
J. F. J. Am. Chem. Soc. 2002, 124, 390-391. (g) Cho, J.-Y.; Tse, M. K.;
Holmes, D.; Maleczka, R. E., Jr.; Smith, M. R., III. Science 2002, 295,
305. (h) Fuchita, Y.; Utsunomiya, Y.; Yasutake, M. J. Chem. Soc., Dalton
Trans. 2001, 2330.
(7) (a) Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman, B. M. J. Am. Chem.
Soc. 2004, 126, 4792. (b) Weng, W.; Guo, C.; Moura, C.; Yang, L.;
Foxman, B. M.; Ozerov, O. V. Organometallics 2005, 24, 3487.
(8) (a) Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126,
9330. (b) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J.
Am. Chem. Soc. 2004, 126, 13192. (c) Krogh-Jespersen, K.; Czerw, M.;
Summa, N.; Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem.
Soc. 2002, 124, 11404. (d) Xu, W.; Rosini, G. P.; Gupta, M.; Jensen, C.
M.; Kaska, W. C.; Krogh-Jespersen, K.; Goldman, A. S. Chem. Commun.
1997, 2273.
(9) (a) Ostensibly, some of the observed isomers are rotamers.8b,9b Activation
of p-xylene produced two rotamers of (PNP)Ir(xylyl)(H) (S1). Activation
of mesitylene produced only one isomer of (PNP)Ir(mesityl)(H) (S2) in
which the rotation about the Ir-C bond is slow on the NMR time scale
(see Supporting Information). This is in contrast to the unobstructed
rotation about the Ir-C bond in 7. It is likely that for (PNP)Ir(aryl)(H)
the rotation about the Ir-C bond is restricted only for the ortho-substituted
aryls. (b) Renkema, K. B.; Bosque, R.; Streib, W. E.; Maseras, F.;
Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 10895.
(10) For analysis of the structural preferences of five-coordinate d6 complexes,
see the following. (a) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z.;
Marder, T. B.; Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre,
G. J. Organometallics 2003, 22, 4557. (b) Rachidi, I. E.-I.; Eisenstein,
O.; Jean, Y. New J. Chem. 1990, 14, 671. (c) Riehl, J.-F.; Jean, Y.;
Eisenstein, O.; Pelissier, M. Organometallics 1992, 11, 729. (d) Olivan,
M.; Eisenstein, O.; Caulton, K. G. Organometallics 1997, 16, 2227.
The structure of 9a (Figure 1) can be described as square
pyramidal with the hydride ligand occupying the apical site. The
orientation of the aryl ligand in the crystal of 9a is disordered by
a 180° rotation of the chlorophenyl ring (Cl above or below the
PNP-Ir plane) and the presumed concomitant disorder of the
hydride. The hydride ligand was not located in the XRD study. Its
presence is unambiguously inferred from the characteristic 1H NMR
resonance in solution (δ -40.3 ppm). The ortho-Cl is oriented
appropriately for additional Cl f Ir donation, but the Ir-Cl distance
is quite long (2.96 or 3.00 Å). This is ca. 0.2 Å longer than the
Ir-Cl distance in 2a. It is likely that the positive charge and the
inability of the PNP* ligand to stabilize unsaturation via π-donation
in 2a translate into higher Lewis acidity for the Ir center in 2a
compared with 9a. In solution NMR spectra, the hydrides of 2a
and 6a resonate at δ -33 and -40.3 ppm, respectively. In square
pyramidal five-coordinate IrIII compounds, hydrides trans to an
empty site resonate at ca. -45 ppm.6d,7,8 A downfield shift is
reflective of coordination of a sixth ligand trans to H as is the case
in 2a but much more weakly (if at all) in 6a. It is tempting to
stipulate that the preference for 9a (or 2a) among the C-H OA
products arises from the stabilizing Ir-Cl interaction. However,
given that this interaction is at best very weak in 9a, it is also
possible that o-ClC6H4 simply forms the strongest σ-Ir-C bond
among the isomeric ClC6H4 ligands.8b
In summary, we report that, for PhCl/PhBr and (PNP)IrI, the
product of C-Hal OA is thermodynamically preferred over the
products of C-H OA but is separated by a high activation barrier
and is only kinetically accessible at >100 °C. The intramolecular
isomerization among the C-H OA isomers proceeds at 60-70 °C,
well below the temperature required for detectable isomerization
to the C-Hal OA product. Among the C-H OA isomers, the one
with the ortho-halophenyl ligand is of the lowest energy. We
anticipate that a similar energetic picture may apply to a number
of other transition metal systems capable of OA reactions. Although
related findings have been reported for fluoroarenes,14 this is the
first such demonstration for the more synthetically relevant heavier
haloarenes.
(11) (a) 13 may be either the “naked” three-coordinate (PNP)Ir or its kinetic
equivalent. (b) Peterson, T. H.; Golden, J. T.; Bergman, R. G. J. Am.
Chem. Soc. 2001, 123, 455.
(12) A small amount of 5 is also observed in these reactions. Its origin is
uncertain, but γ-H elimination from a putative Ir-OBut species is one
possibility.
(13) (a) POV-Ray, available at http://www.povray.org/. (b) Ortep-3 for
Windows. Farugia, L. J. Appl. Crystallogr. 1997, 30, 565.
(14) Bosque, R.; Clot, E.; Fantacci, S.; Maseras, F.; Eisenstein, O.; Perutz, R.
N.; Renkema, K. B.; Caulton, K. G. J. Am. Chem. Soc. 1998, 120,
12634.
Acknowledgment. We gratefully acknowledge Research Cor-
poration, Brandeis University, and NSF (Grant Nos. MRI-0319176
to S.P. and CHE-0517798 to O.V.O.) for support of this research.
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