Page 4 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

SCHEME 4 SCHEME 6

SCHEME 5
position. Mechanistically, H 2 and D 2 activation proceed via
intermediacy of the Ir(III) complex 21. DFT studies reveal
similar reactivity patterns of phenyl and acetonyl complexes
3 and 19.
Cooperative H 2 activation and catalytic ammonia borane
dehydrogenation by aliphatic pincer PNP-Ru complexes
were recently reported. 16

OH and NH Activation Based on
MetalLigand Cooperation

a. Water Splitting. The dearomatized PNN complex 11
readily cleaves the OH bond of water. Upon reaction with
water at room temperature, aromatization takes place to
methylene carbon). Deprotonation of the arm methylene by quantitatively form the trans-hydridohydroxo complex 28
Ir (3 23 )is endergonic (ΔG 298 = 11.1 kcal mol 1 ). Transition (Scheme 8). 17 This compound is probably formed by a
0
0
‡
states involved in the direct deprotonation by Ir (ΔG 298 = mechanism involving water coordination at the vacant site
‡
35.9 kcal mol 1 ) involving one water molecule (ΔG 298 = trans to the hydride, followed by proton migration to the
‡
26.5 kcal mol 1 ) and two water molecules (ΔG 298 =20.7 side arm, as indicated by DFT studies. 18
1
kcal mol ) were identified as depicted in Figure 3. The Interestingly, further reaction of complex 28 with water at
smaller barrier involving two water molecules is reasonable 100 °C releases H 2 and forms the cis-dihydroxo complex 29.
for a room temperature reaction. DFT studies, independently carried out by Yoshizawa et
Like the Ir(I) phenyl complex 3, the Ir(I) acetonyl complex al. 18a and Hall and Yang, 18b indicate that this process
19 also reacts with H 2 (1 equiv) in benzene-d 6 to provide involves H 2 liberation from 28 by coupling of the hydride
exclusively the trans-dihydride Ir(III) complex 26, whose ligand with a proton from the side arm, followed by addition
structure was corroborated by an X-ray diffraction study of H 2 O to the generated dearomatized intermediate. Sig-
(Scheme 7). 13 Further, reaction of 19 with D 2 in benzene nificantly, irradiation of complex 29 in the 320420 nm
leads to formation of the trans-HIrD complex 27, with range results in O 2 liberation with regeneration of complex
incorporation of one deuterium atom in the benzylic 28 (Scheme 8). Combining these separate stoichiometric

Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 591