N-H Activation by Rh(I) via Metal-Ligand Cooperation
Moran Feller,
†
Yael Diskin-Posner,
‡
Linda J. W. Shimon,
‡
Eyal Ben-Ari,
†
and David Milstein*
,†
†
Department of Organic Chemistry and
‡
Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot,
76100, Israel
* S Supporting Information
ABSTRACT: In continuation of our studies on bond
activation and catalysis by pincer complexes, based on
metal-ligand cooperation, we present here a rare example of
amine N-H activation by Rh(I) complexes. The novel
dearomatized pincer complexes [(PNN*)RhL′] (PNN = 2-
(CH
2
-P
t
Bu
2
)-6-(CH
2
-NEt
2
)C
5
H
3
N, PNN* = deprotonated
PNN, L′ =N
2
(5), C
2
H
4
(6)) and [(
i
PrPNP*)RhL′](
i
PrPNP
= 2,6-(CH
2
-P
i
Pr
2
)
2
C
5
H
3
N,
i
PrPNP* = deprotonated
i
PrPNP,
L′ =C
2
H
4
(7), cyclooctene (9)) were prepared and fully characterized by NMR and X-ray analysis. Complexes 5-7 and 9
undergo facile N-H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation
state of the metal center to form stable anilide complexes [(PNN)Rh(NHAr)] and [(
i
PrPNP)Rh(NHAr)] (Ar = C
6
H
5
, o-Br-
C
6
H
4
, m-Cl-p-Cl-C
6
H
3
, p-NO
2
-C
6
H
4
). Anilines possessing electron-withdrawing groups accelerate the N-H activation and yield
more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative
mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the
NH- protons of anilines prior to and after the N-H activation. The reverse N-H formation by metal-ligand cooperation from
the anilide complexes was observed to give free anilines and dearomatized Rh(I) complexes upon addition of CO or PEt
3
.
Deprotonation of complexes [(PNL)Rh(p-NO
2
-NH
2
C
6
H
4
)] (13,P=P
t
Bu
2
, L = NEt
2
; 15,P=L=P
i
Pr
2
) yields the
dearomatized anionic complexes [(PNL*)Rh(p-NO
2
-NH
2
C
6
H
4
)]. An associative mechanism, involving N-H activation of an
apically coordinated aniline in a pentacoordinated Rh(I) complex, is suggested.
■
INTRODUCTION
Our group has discovered a new mode of metal-ligand
cooperation, involving aromatization-dearomatization of pyr-
idine-based
R
PNP and PNN (R =
t
Bu;
i
Pr) pincer ligands
(Scheme 1), as well as bipyridine-based PNN pincer ligands.
Metal-ligand cooperation based on acridine pincer complexes
has also been demonstrated.
1
Metal-ligand cooperation by aromatization-dearomatiza-
tion is involved in several new, environmentally benign
reactions developed by us, catalyzed by pincer-type PNN-
Ru,
2,3
PNP-Ru,
2a,h,4
acridine-PNP-Ru,
5
and PNP-Fe
6
com-
plexes, including (a) dehydrogenative coupling of primary
alcohols to form esters,
2a,d,h,4a
(b) dehydrogenation of
secondary alcohols to ketones,
2b,h,4a
(c) hydrogenation of
esters to alcohols
2c,h,7
and cyclic diesters to diols,
3c
(d) coupling
of alcohols with amines to form amides with liberation of H
2
,
2d
(e) formation of polyamides by dehydrogenative coupling of
diamines and diols,
2i,8
(f) hydrogenation of amides to amines
and alcohols,
3a
(g) dehydrogenative acylation of alcohols with
esters,
2e
(h) dehydrogenative amidation of esters with amines,
2f
(i) hydrogenation of organic carbonates, carbamates, and
formate esters, indicating a mild, two-step hydrogenation of
CO
2
to methanol,
2g
(j) hydrogenation of urea derivatives to
form methanol and amines,
3b
(k) selective formation of primary
amines from alcohols and ammonia, with liberation of water,
5c
(l) dehydrogenative coupling of primary alcohols to form
acetals,
5b
(m) coupling of β-amino alcohols to form peptides by
PNN-Ru, or pyrazines by PNP-Ru, with H
2
liberation,
9
(n) Fe-
catalyzed hydrogenation of ketones at low pressure and
ambient temperature,
6a,c
(o) Fe-catalyzed hydrogenation of
CO
2
to formate salts at low pressure
6b
and (p) direct synthesis
of imines from alcohols and amines with liberation of H
2
and
water.
4b
Polymerization of diols to form polyesters with H
2
liberation catalyzed by the PNN-Ru complex was also
reported.
10
Several computational studies (DFT) of these
catalytic reactions have appeared.
11
Other examples of bond activation and catalysis that involve
(or may involve) metal ligand cooperation by aromatization-
dearomatization of pyridine-based pincer complexes were
recently reported.
12
The activation of X-H (X = H, C, O, N) bonds via metal-
ligand cooperation by aromatization-dearomatization of the
Received: May 7, 2012
Published: May 18, 2012
Scheme 1
Article
pubs.acs.org/Organometallics
© 2012 American Chemical Society 4083 dx.doi.org/10.1021/om300248r | Organometallics 2012, 31, 4083-4101