Metal-ligand cooperation in C-H and H2 activation by an electron-rich PNP Ir(I) system: Facile ligand dearomatization-aromatization as key steps

Article abstract of DOI:10.1021/ja066411i

Unusual reactions are reported, in which the aromatic PNP ligand (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) acts in concert with the metal in the activation of H₂ and benzene, via facile aromatization/dearomatization processes of the ligand. A new, dearomatized electron-rich (PNP*)Ir(I) complex 2 (PNP* = deprotonated PNP) activates benzene to form the aromatic (PNP)Ir(I)Ph 4, which upon treatment with CO undergoes a surprising oxidation process to form (PNP*)Ir(III)(H)CO 6, involving proton migration from the ligand "arm" to the metal, with concomitant dearomatization. 4 undergoes stereoselective activation of H₂ to exclusively form the trans-dihydride 7, rather than the expected cis-dihydride complex. Our evidence, including D-labeling, suggests the possibility that the Ir(I)-Ph complex is transformed to the dearomatized Ir(III)(Ph)(H) (independently prepared at low temperature), which may be the actual intermediate undergoing H₂ activation. Copyright

Full text of DOI:10.1021/ja066411i

Published on Web 11/11/2006
Metal-Ligand Cooperation in C-H and H2 Activation by an Electron-Rich PNP
Ir(I) System: Facile Ligand Dearomatization-Aromatization as Key Steps
Eyal Ben-Ari,† Gregory Leitus,‡ Linda J. W. Shimon,‡ and David Milstein*,†
Department of Organic Chemistry and Unit of Chemical Research Support, Weizmann Institute of Science,
RehoVot, 76100 Israel
Received September 5, 2006; E-mail: david.milstein@weizmann.ac.il
Scheme 1
Bond activation by metal complexes, in which both the metal
center and the ligand play active, synergistic roles, can provide
new opportunities in organometallic chemistry and catalysis.1 An
example is the Ru(II)-catalyzed hydrogenation of unsaturated polar
bonds, in which H2 activation involves both the metal center and
a polar (N- or O-based) ligand, followed by a concerted transfer of
acidic and hydridic hydrogens to an unsaturated substrate.1 Here
we report on a (PNP)Ir(I) (PNP) 2,6-bis-(di-tert-butylphosphi-
nomethyl)pyridine) system in which the PNP ligand and the metal
act in concert in H2 and C-H activation via dearomatization/
aromatization processes of the ligand. Moreover, our work raises
the question whether a seemingly simple H2 oxidative addition to
Ir(I) might actually involve H2 activation by an Ir(III) intermediate.
Upon reaction of our previously reported electron-rich cationic
Ir(I) complex 12 with tBuOK in THF at room temperature, a rapid
color change from pink-red to violet-red was observed with
formation of the novel dearomatized complex 2 (Scheme 1).
The 31P NMR spectrum of 2 exhibits an AB quartet centered at
33 ppm, indicating nonequivalent phosphorus atoms. Signals at 5.4,
1
6.35, and 6.43 ppm in the H NMR spectrum, corresponding to
three protons, indicate dearomatization of the pyridine ring. An
X-ray crystallographic study of complex 2 shows unambiguously
that the pyridine ring underwent dearomatization. Thus, the
C(1)-C(2) bond (1.35 Å) is much shorter than C(6)-C(7) (1.505
Å) (Scheme 1). Such dearomatization following deprotonation of
the benzylic protons is known3 and was shown by us to be involved
in Ru-catalyzed dehydrogenation of alcohols to form esters3a and
in hydrogenation of esters.3b
NMR showed that 3 was quantitatively converted to 4 after 10 h
When a benzene solution of complex (PNP*)Ir(I) 2 (PNP* )
dearomatized PNP) was mildly heated at 60 °C for 2 h, quantitative
C-H activation to form the complex (PNP)Ir(I)(C6H6), 4 (with no
overall change in oxidation state), took place (Scheme 1).
The 31P NMR spectrum of 4 exhibits a singlet at 55 ppm,
indicating C2V symmetry. The benzylic protons give rise to one
(Scheme 2). No intermediates were observed. When the isotopomer
t
complex 5a was reacted with BuOK at -78 °C, an Ir-D signal
was observed at -47 ppm in the 2H NMR spectrum, and no hydride
was observed, indicating that no direct Ir-D deprotonation took
place. Moreover, upon warming 3a to room temperature, complex
4 incorporating a D-labeled benzyl “arm” was formed, as a result
1
signal at 2.8 ppm in the H NMR spectrum, implying rearomati-
zation of the PNP ligand. The X-ray structure of 4 (Figure 1) reveals
that the iridium atom is located in the center of a slightly distorted
square-planar geometry, with the phenyl group located trans to the
pyridine ring, perpendicular to the PNP plane.
Upon reaction of complex 2 with C6D6, deuterium incorporation
into a benzylic group of complex 4a was observed (Scheme 1),
the 1H NMR spectrum revealing only three benzylic protons.4 While
no intermediates were directly observed in the C-H activation
process, indirect evidence suggests that intermediacy of the Ir(III)
complex 3 is a viable possibility.
Thus, deprotonation of the cationic complex [(PNP)Ir(H)(C6H5)]-
Figure 1. ORTEP drawings of 4 (left) and 7 at 50% probability level.
Hydrogen atoms (except hydrides) are omitted for clarity. Selected bond
lengths (Å) 4: Ir(1)-N(1) 2.087(4), Ir(1)-C(24) 2.057(5), Ir(1)-P(1)
2.272(2), Ir(1)-P(2) 2.267(2). 7: Ir(1)-N(1) 2.119(4), Ir(1)-C(24)
2.075(4), Ir(1)-H(1) 1.64(5), Ir(1)-H(2) 1.73(6), Ir(1)-P(1) 2.287(1),
Ir(1)-P(2) 2.287(1).
t
PF6, 5,2 in THF at -78 °C with BuOK resulted in formation of
the neutral Ir(III) complex 3.5 Reaction follow-up at -50 °C by
† Department of Organic Chemistry.
‡ Unit of Chemical Research Support.
9
15390
J. AM. CHEM. SOC. 2006, 128, 15390-15391
10.1021/ja066411i CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
(-78 °C) to give complex 7. Coordination of H2 to complex 3
followed by its deprotonation by the amide nitrogen11 or directly
by the ligand “arm” may lead to formation of complex 7 (Scheme
3). Intriguingly, regardless of the exact mechanism, our eVidence
raises the possibility of actiVation of H2 by Ir(III) in an oVerall
apparent oxidatiVe addition of H2 to Ir(I).
In conclusion, a number of unusual reactions are reported, in
which the PNP ligand takes an active role in the activation of H2
and benzene, via facile aromatization/dearomatization processes of
the ligand. The new, dearomatized electron-rich PNP-based Ir(I)
complex 2 activates benzene to form an Ir(I)-Ph complex, which
upon treatment with CO undergoes a surprising oxidation process
to form the Ir(III) complex 6, involving proton migration from the
ligand “arm” to the metal, with concomitant dearomatization.
Another interesting transformation of 4 is the stereoselective
activation of molecular hydrogen to exclusively form the trans-
dihydride 7. Our evidence, including D-labeling, suggests that the
16e Ir(I) complex 4 may be transformed to the (independently
prepared) 16e Ir(III) 3 which could be the actual complex
undergoing H2 activation. Further experimental and theoretical
investigations on this system are in progress.
Scheme 3
Acknowledgment. This research was supported by the German
Federal Ministry of Education and Research (BMBF) under the
framework of the German Israeli Cooperation, by the Israel Science
Foundation (Grant 412/04) and by the Kimmel Center for Molecular
design. D.M. is the Israel Matz Professor.
Supporting Information Available: Experimental procedures and
characterization of complexes 1-7, and X-ray data for 2, 4 and 7 (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
of migration of the deuteride ligand from the metal to the “arm”.
These observations support the intermediacy of 3 in the C-H
activation of benzene by the neutral Ir(I) complex 2.6
Surprisingly, when the Ir(I) phenyl complex 4 was reacted with
one equivalent of CO in benzene at 25 °C, oxidation of the metal
center took place, quantitatively forming the Ir(III) phenyl hydride
complex 6. Complex 6 was synthesized independently by depro-
tonation of [(PNP)Ir(C6H6)(H)(CO)]PF6 2 (Scheme 2). This process
might take place by reversible proton migration from the benzylic
position to the metal center, followed by trapping of the neutral
phenyl hydride complex 3 by CO.7 Indeed, complex 6 was formed
upon reaction of 3 with CO at -78 °C. Complex 6 was thermally
stable (up to 80 °C) in solution and did not eliminate benzene,
similar to its cationic aromatic analogue.2
Remarkably, upon reaction of complex 4 with 1 equiv of H2 at
25 °C in benzene (Scheme 3), only the trans-dihydride complex
(PNP)Ir(H)2(C6H5), 7, was observed. The two equivalent hydride
ligands give rise to a triplet at -8.3 ppm in the 1H NMR spectrum,
indicative of a trans-dihydride arrangement. There is no evidence
for the formation of the cis-dihydride complex.8 While cis-to-trans
isomerizations of octahedral Ir dihydrides are known,9 the barrier
is high,9b and the process requires heating.9a Thus, 7 is likely to be
the kinetic product. X-ray structure analysis of 7 (Figure 1) shows
a slightly distorted octahedral geometry with the trans-dihydride
ligands perpendicular to the PNP plane.
References
(1) (a) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
1466. (b) Noyori, R.; Koizumi, M.; Ishii, D.; Ohkuma, T. Pure Appl.
Chem. 2001, 73, 227. (c) Abbel, R.; Abdur-Rashid, K.; Faatz, M.;
Hadzovic, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2005, 127,
1870. (d) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem.
ReV. 2004, 248, 2201. (e) Casey, C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei,
I. L J. Am. Chem. Soc. 2005, 127, 14062.
(2) (a) Ben-Ari, E.; Cohen, R.; Gandelman, M.; Shimon, L. J. W.; Martin, J.
M. L.; Milstein, D. Organometallics 2006, 25, 3190. (b) Ben-Ari, E.;
Cohen, R.; Gandelman, M.; Shimon, L. J. W.; Martin, J. M. L.; Milstein,
D. J. Am. Chem. Soc. 2003, 125, 4714.
(3) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc.
2005, 127, 10840. (b) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D.
Angew., Chem. Int. Ed. 2006, 45, 1113. (c) Sacco, A.; Vasapollo, G.;
Nobile, C. F.; Piergiovanni, A.; Pellinghelli, M. A.; Lanfranchi, M. J.
Organomet. Chem. 1988, 356, 397.
(4) The protons were integrated with a time delay of 20 s, calibrated to the
pyridinic aromatic protons.
(5) The dearomatized pyridine protons of 3 give rise to three signals in the
1H NMR spectrum: one characteristic signal at 5.67 ppm as a broad
doublet and two multiplets at 6.45-6.85 ppm. The Ir-H appears at -47
ppm. For detailed assignments see Supporting Information.
(6) Migration of the hydride ligand might be either intra- or intermolecular.
Experimental and theoretical studies of the mechanism are in progress.
(7) While CO coordination to Ir(I) followed by proton migration to the metal
cannot be excluded at this stage, we think that it is less likely because of
the lower electron density at the Ir(CO) center. The mechanism of this
process is being studied.
Significantly, reaction of the Ir(I) complex 4 with D2 under
the same conditions did not yield the Ir dideuteride. Rather, the
Ir(H)(D) complex 7a was obtained, with incorporation of one deu-
terium atom into the benzylic “arm”. This indicates the involvement
of the benzylic carbons in the activation of H2. It might be suggested
that complex 3 is present in solution in low concentration in
equilibrium with 4 and is responsible for the unusual reactivity with
H2.10 Indeed, complex 3 reacts with H2 even at low temperature
(8) Preliminary DFT calculations showed that the cis-isomer is more stable
then the trans-isomer and should have been observed had it been formed.
(9) (a) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997,
16, 3786. (b) Li, S. H.; Hall, M. B. Organometallics 1999, 18, 5682.
(10) However, no NMR evidence for an equilibrium involving 3 was obtained
upon cooling solutions of 4 to -80 °C.
(11) H2 addition across an Ir(III)-amide bond was reported: Fryzuk, M. D.;
Montgomery, C. D.; Rettig, S. J. Organometallics 1991, 10, 467.
JA066411I
9
J. AM. CHEM. SOC. VOL. 128, NO. 48, 2006 15391