Page 3 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

FIGURE 2. Single crystal X-ray structures of dearomatized PNP complexes 2 and 5.

SCHEME 1

SCHEME 2

SCHEME 3

Upon substitution of the COE ligand by acetone, CHbond arm. Indeed, complex 23, generated and fully characterized 8
cleavage of acetone forms intermediate 21 followed by at 70 °C by deprotonation of 3, reacts with H 2 to form the
proton migration to the side arm with concomitant aromati- trans-dihydride complex 22. Thus, what seems to be H 2
zation to form the aromatic complex 19 (Scheme 5). 13 oxidative addition to Ir(I) is actually H 2 activation by Ir(III)
(Scheme 6). The Ir(III) intermediate can be trapped by treat-
Reaction of H 2 with (PNP)Ir(I): Activation of H 2
ing the Ir(I) phenyl complex 3 with CO, forming complex 25.
Is by a Dearomatized Ir(III) Intermediate
Involvement of the dearomatized Ir(III) intermediate 23 in
Interestingly, complex 3 reacts with H 2 to provide an Ir(III) the stereoselective activation of H 2 to exclusively form the
trans-dihydride complex exclusively. Use of D 2 reveals, trans-dihydride complex is supported by density functional
surprisingly, formation of the DIrH complex 22 with theory (DFT) calculations, 14,15 which suggest that a catalytic
one D atom attached to the pyridinylmethylenic carbon, amount (relative to 3) of adventitious water assists the
suggesting that H 2 is actually activated by the dearomatized proton-transfer step by bridging the gap the proton has to
Ir(III) complex 23, formed by proton migration from the side travel (in 23 , the hydrogen atom is 3.6 Å away from
0

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