Page 7 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

FIGURE 4. (left) Calculated free energies (ΔG 298 , kcal mol 1 ) for (I) the unbound starting materials and (II) coordinated and (III) NH-activated amine
complexes. (right) Calculated structure of TS(IIIII), the transition state for activation of NH 3 (H's on methyl groups are omitted for clarity). Figure
adapted with permission from ref 19. Copyright 2010 American Chemical Society.
Long-Range MetalLigand Cooperation
SCHEME 12
We have developed a unique, “long-range” mode of metal
ligand cooperativity involving an acridine-based pincer
system. 20 The X-ray structure of the acridine PNP complex
34 exhibits an unusually long RuN bond (2.479 Å), 21
suggesting that N-coordination of the acridine ligand might
be hemilabile. The acridine-PNP ligand affords a flexible
ligand framework, as it forms six-membered rings (versus
five membered rings of the pyridine-based complexes).
Reaction of 34 with H 2 /KOH is conceptually different from
6
that of pyridine-based PNP complexes, and it results in
dearomatization of the central acridine ring in 34 as a result
of heterolytic splitting of H 2 (Scheme 12). 22 A similar reaction
1
2
with D 2 yields complex 36 with D 2 splitting. The H and H
NMR spectra of 36 indicate the presence of a major amount
of RuD (broad singlet at 20.75 ppm) and a very minor
amount of RuH (triplet at 20.84 ppm), respectively. The
2
H NMR spectrum also exhibits a broad multiplet corre-
sponding to the CDH group of the middle acridine ring
(C9). These observations confirm the dearomatization as a
result of D 2 splitting. Dearomatization in 35 is further corro-
borated by an X-ray structure, which exhibits a trigonal-
bipyramidal geometry. The RuN bond (2.171 Å) is drama-
tically shorter (by 0.308Å) thanthat in34. Thecentral acridine
ring has a boat-type conformation, and the acridine moiety is
by DFT studies, 18 followed by β-H elimination and H 2 tilted toward the Ru center with a C(11)N(1)C(10)Ru(1)
readdition. 19 torsion angle of 145.16°. DFT calculations indicate that this
The trend observed in theexperimentaldataofNHactiva- process involves formation of a Ru dihydride intermediate
tion reactions is also reflected in DFT studies (Figure 4). The bearing a decoordinated, bent acridine ligand in which C9 is
barriersfortheexchangebetweencoordinated(II) andactivated in close proximity to a hydride, followed by through-space
(III) states are low and accessible at room temperature. hydride transfer.

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