Selective ortho C-H activation of haloarenes by an Ir(I) system

Article abstract of DOI:10.1021/ja028362p

The cationic PNP-Ir(I)(cyclooctene) complex 1 (PNP = 2,6-bis-(di-tert-butyl phosphino methyl)pyridine) reacts with benzene at 25 °C to quantitatively yield the crystallographically characterized, square pyramidal, iridium phenyl hydride complex cis-(PNP)Ir(Ph)(H), 2, in which the hydride is trans to the vacant coordination site. The cationic complex 2 is stable to heating at 100 °C, in sharp contrast to the previously reported unstable neutral, isoelectronic (PCP)Ir(H)(Ph) (PCP = η³-2,6-(^tBu₂PCH₂)₂C₆H₃). Heating of 2 at 50 °C with other arenes results in arene exchange. Complex 1 activates C-H bonds of chloro- and bromobenzene with no C-halide oxidative addition being observed. Selective ortho C-H activation takes place, the process being directed by halogen coordination and being thermodynamically and kinetically favorable. The meta- and para-C-H activation products are formed at a slower rate than the ortho isomer and are converted to it. NMR data and an X-ray crystallographic study of the ortho-activated chlorobenzene complex, which was obtained as the only product upon heating of 1 with chlorobenzene at 60 °C, show that the chloro substituent is coordinated to the metal center. Copyright

Full text of DOI:10.1021/ja028362p

Published on Web 03/27/2003
Selective Ortho C-H Activation of Haloarenes by an Ir(I) System
Eyal Ben-Ari,† Mark Gandelman,† Haim Rozenberg,‡ Linda J. W. Shimon,‡ and David Milstein*,†
Departments of Organic Chemistry and Chemical SerVices, The Weizmann Institute of Science,
RehoVot 76100, Israel
Received September 1, 2002; E-mail: david.milstein@weizmann.ac.il
The selective activation of strong C-H bonds in the presence
of substitutionally reactiVe groups, such as halo-substituents, is
important for synthetic applications. Recent examples of C-H
activation in haloarenes by soluble transition metal complexes were
reported.1 Because of steric reasons, activation of the less hindered
meta or para C-H bonds in haloarenes was observed. Activation
of the ortho C-H bond is desirable since it may lead to the
functionalization of haloarenes in the most sterically hindered
position.2 Here we report on an electron-rich cationic Ir(I) system
which undergoes facile arene C-H bond activation, leading with
benzene to a crystallographically characterized, unsaturated, stable
Ir(ΙΙΙ) hydridophenyl complex. Remarkably, this system exhibits
unprecedented regioselectiVity toward ortho-C-H bond activation
in chloro- and bromobenzene, utilizing the halogen as a directing
group.
Figure 1. ORTEP drawings of 2 (left) and 4a. Hydrogen atoms (except
hydride) and hexafluorophosphate counterion were omitted for clarity.
Selected bond lengths (Å) and angles (deg): 2: Ir(1)-N(5) 2.142(5), Ir(1)-
C(1) 2.043(7), Ir(1)-H(1) 1.66(7), C(1)-Ir(1)-H(1) 88, N(5)-Ir(1)-H(1)
94. 4a: Ir(1)-C(21) 2.045, Ir(1)-Cl(1) 2.816, C(26)-Cl(1) 1.78, Ir(1)-
Cl(1)-C(26) 72, Cl(1)-C(26)-C(21) 63.
We have recently reported electron-rich, neutral iridium com-
plexes of 2,6-bis-(di-tert-butyl phosphino methyl)pyridine (PNP).3
When the cationic PNP-Ir(I) complex 1 4 was dissolved in benzene,
quantitative C-H activation took place, yielding the iridium phen-
yl hydride complex 2 within 48 h at 25 °C or 1 h at 60 °C
Scheme 1
4
(Scheme 1). 31P{1H} NMR of 2 in CD2Cl2 exhibits a doublet at
54 ppm and 1H NMR reveals a highfield shifted triplet at -44 ppm,
characteristic of a hydride trans to a vacant coordination site,5
indicating a square pyramidal geometry.
The X-ray structure of 24 reveals that the iridium atom is located
in the center of a slightly distorted square pyramid with the hydride
positioned trans to the vacant site (Figure 1).
Complex 2 does not decompose upon heating to 100 °C for
24 h in the solid state or in a benzene solution, representing a rare
example of a thermally stable coordinatively unsaturated M(III) d6
hydridoaryl complex. Unsaturated hydridoaryl complexes were
postulated as key intermediates in catalytic processes such as
hydroarylation of alkenes2 and decarbonylation of aldehydes6
although not observed in either case. Interestingly, Goldman
reported recently the neutral, isoelectronic, thermally unstable five-
coordinate (PCP)Ir-hydridophenyl complex7 (PCP ) η3-2,6-(tBu2-
PCH2)2C6H3) which was characterized in solution at low temper-
ature. Werner reported the thermally stable Ir(PiPr3)2(H)(Ph)(Cl),8
which is the only crystallographically characterized unsaturated
Ir(H)(R) (R ) alkyl, aryl) complex. This trigonal bipyramidal
complex is stabilized by strong π-donation of the chloride ligand
to an empty d-orbital of the metal. Complex 2 lacks such
stabilization. We believe that it is the first crystallographically
characterized square pyramidal iridium hydrido-aryl (or -alkyl)
complex.9 Square pyramidal d6 PCP complexes are common.10
Although stable, complex 2 exhibits arene exchange reactivity.
When a slurry of 2 in C6D6 was heated at 50 °C, disappearance of
the hydride and phenyl signals was observed in the 1H NMR, while
the 31P NMR remained unchanged.
Formation of (PNP)Ir(D)(C6D5) was confirmed by 2H NMR. As
shown recently by Goldman, such an intermolecular arene exchange
in the unstable PCP-based Ir(H)(Ph) system takes place rapidly at
low temperature and follows a dissociative mechanism.7 It is likely
that the arene exchange in the case of 2 requires heating to overcome
the barrier of C-H reductive elimination.
2 and (PCP)Ir(H)(Ph) represent a rare case of a differently
charged, isoelectronic couple of the same metal that undergoes C-H
reductive elimination/oxidative addition. A comparison of the CO
absorption frequency of (PCP)Ir(H)(Ph)CO (1973 cm-1)7 and that
of 2-CO (2005 cm-1; H trans to CO in both cases)4 indicates that
the electron density at the metal center in the cationic Ir(III) complex
is lower, as expected. The reasons for the higher stability of 2,
which seems counterintuitive, are being explored.11
Complex 1 activates C-H bonds of aryl halides with not even
traces of C-halide oxidative addition products, even with bromoben-
zene (vide infra). When 1 was heated in fluorobenzene at 50 °C,
three Ir-hydridoaryl complexes were formed and characterized by
† Department of Organic Chemistry.
‡ Department of Chemical Services.
9
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C O M M U N I C A T I O N S
NMR as ortho-, meta-, and para-activated (PNP)Ir(H)(C6H4F) (3a-
c, respectively), in a ratio of 3a:3b:3c, 2:2:1, indicating lack of
any selectivity in C-H activation.
The observed statistical C-H activation with C6H5F and the
selective C-H cleavage with C6H5Cl and C6H5Br, directed by
halogen coordination, is consistent with the ligating ability F ,
Cl < Br. To our knowledge, this is the first example of selectiVe
ortho C-H bond actiVation of haloarenes in solution.17 Usually,
C-H bond activation in haloarenes takes place at the less hindered
positions, resulting predominantly in a mixture of meta- and para-
activated complexes.
In conclusion, a PNP-based cationic iridium(I) system exhibits
high activity and selectivity in aromatic C-H activation. A stable
square pyramidal Ir-hydridophenyl complex was crystallographi-
cally characterized. Unprecedented selective ortho C-H activation
of haloarenes was achieved, the process being directed by halogen
coordination, and being thermodynamically and kinetically favor-
able.
In contrast, complex 1 reacts with chlorobenzene at 50 °C with
preference to the ortho position, yielding the ortho-, meta-, and
para-C-H activated complexes 4a-c in a ratio of 4.6:2:1,
respectively, at the point when 1 has completely disappeared.
Monitoring the reaction by NMR revealed that at 10% conversion
the ratio o/m/p was 4:2:1.4 Remarkably, continued heating of this
mixture at 60 °C after consumption of 1 results in clean conversion
of 4b and 4c to the ortho-activated 4a as the only product. Most
likely, competitive reversible formation of 4b and 4c and irreversible
formation of 4a are involved. These results clearly indicate that 4a
is thermodynamically and kinetically favored despite the steric
hindrance imposed by the chlorine atom.
Interestingly, whereas the hydride ligands of 4b and 4c exhibit
in the 1H NMR spectrum signals at -41 ppm and -42 ppm,
respectively, indicating a vacant site trans to the hydride, the hydride
of 4a appears at -33 ppm, which is characteristic of M-H trans
to an occupied coordination site. This indicates that the chlorine
atom of 4a is coordinated to the Ir center and is located trans- to
the hydride, as confirmed by X-ray (vide infra). The intramolecular
coordination of Cl explains the higher thermodynamic stability of
4a relative to 4b,c where such coordination is obviously impossible.
It is known that the ortho positions in haloarenes are the most
acidic ones, especially in fluoroarenes.12 Indeed, Caulton showed
that an Os(II) complex activates preferentially the o-C-H bonds
in fluoroarenes although no interaction between fluorine and the
metal was found.5b As explained, this is a result of the preference
of Os for interaction with the π-cloud ortho to the C-F bond,
making the ortho activation kinetically favored. Such arguments
are unlikely to be important in the case of the PNP-Ir system, as
the reaction of 1 with fluorobenzene results in a statistical mixture
of C-H cleaved products. Therefore, we conclude that the ortho
C-H activation in chlorobenzene is directed by pre-coordination
of the chlorine atom to iridium. Halocarbon coordination to
transition metals is well documented,13 although examples of chloro-
and bromoarene coordination are not common.13,14 As shown
experimentally13,15 and theoretically,15 due to the significant p-
character of the halide donor orbital, the M-X-C (X ) halide)
angles in halocarbon complexes must be close to 90-100°. Such
coordination of the haloarene to the PNP-Ir system will place the
ortho C-H bond in proximity of the metal, resulting in kinetically
favored insertion of the iridium center into this bond.
A single-crystal X-ray analysis of 4a4 reveals that the Ir atom is
located in the center of a distorted octahedron (Figure 1). The
chlorine atom is coordinated to the metal and, as predicted from
the NMR data, it is located trans to the hydride. Although the Ir-
Cl bond length of 2.816 Å is significantly shorter than the sum of
van der Waals radii of Ir and Cl atoms, it is longer than other
reported Ir-Cl bonds in coordinated chloroalkanes.13 This elonga-
tion is probably due to some strain in the four-membered ring, as
indicated by the 72° angle of Ir-Cl-C. Noteworthy, the C-Cl
bond (1.781 Å) is significantly elongated compared to the one in
free C6H5Cl (1.700 Å),16 suggesting π-back-donation from a filled
metal d-orbital to the empty C-Cl σ*-orbital. Cl Coordination
makes 4a the thermodynamically favored product.
Acknowledgment. This work was supported by the Israel
Science Foundation, by the MINERVA Foundation, Munich,
Germany and by the Israel Ministry of Science. D.M. is the Israel
Matz Professor of Organic Chemistry.
Supporting Information Available: Experimental procedures and
characterization of complexes 2-6 (PDF) and X-ray data for 3 and 5a
(CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
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Reaction of 1 with C6H5Br is similar to that of C6H5Cl, yielding
the ortho-, meta-, and para-C-H activated complexes 5a-c at 50
°C in a ratio of 7:2:1, respectively, at the point when 1 has
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is kinetically and thermodynamically favored.
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