Formation of stable rhodium I·6-arene complexes with aniline derivatives

Article abstract of DOI:10.1002/zaac.201200008

Three new, hitherto unknown, rhodium complexes with aniline and two of its derivatives (2, 6-dimethylaniline and N-methylaniline) are presented. All complexes have been characterized by X-ray analysis. Copyright

Full text of DOI:10.1002/zaac.201200008

SHORT COMMUNICATION
DOI: 10.1002/zaac.201200008
Formation of Stable Rhodium η6-Arene Complexes with Aniline Derivatives
Christian Fischer,[a] Anja König,[a] Hans-Joachim Drexler,[a] and Detlef Heller*[a]
Keywords: η6-Arene complexes; Rhodium; DPPF; Asymmetric catalysis; Catalyst inhibition
Abstract. Three new, hitherto unknown, rhodium complexes with ani-
line and two of its derivatives (2,6-dimethylaniline and N-methylani-
line) are presented. All complexes have been characterized by X-ray
analysis.
ether red crystals suitable for X-ray analysis are grown. The
solid-state structure (Figure 1) surprisingly shows that neither
the formation of a multinuclear complex takes place nor does
the 2,6-dimethylaniline coordinate to rhodium via the nitrogen.
Instead 2,6-dimethylaniline exclusively binds to rhodium
through its phenyl ring. Similar rhodium-arene complexes
with, among others, benzene and toluene, have been long
known and, in some cases, characterized by X-ray analy-
sis.[1a,3]
Introduction
The use of Lewis bases like triethylamine as additives in
combination with rhodium diphosphine complexes in asym-
metric catalysis leads to inactive rhodium trinuclear species.
These complexes are well known: they are highly stable and
their formation probably takes place in a reversible consecutive
reaction from the solvate complex [Rh(PP*)(solvent)2]Anion
(PP* = chelating diphosphine) and a Lewis base to give a μ2-
bridged dimer intermediate such as [Rh2(PP*)2(μ2-Base)2]
which then evolves into a μ3-bridged trimer [Rh3(PP*)3(μ3-
Base)2]BF4.[1,2] In a preceding equilibrium, the weaker Lewis
base, e.g. MeO–, is formed from the Lewis base triethylamine
and the solvent MeOH which then acts as the bridging ligand.
Aniline and its derivatives are Lewis bases and therefore
suitable candidates to promote the formation of such multinu-
clear rhodium complexes. However the interaction with solv-
ate complexes could also lead to different species in which
aniline coordinates to rhodium via the free electron pair on
nitrogen or the π-electrons of the phenyl ring.
The aim of this work was to investigate the reaction of ani-
line, N-methylaniline and 2,6-dimethylaniline with solvate
complexes of the type [Rh(PP*)(solvent)2]BF4 exemplified by
the ligand DPPF (1,1Ј-Bis(diphenylphosphino)ferrocene) and
their coordination mode to rhodium.
Figure 1. Molecular structure of the cation [Rh(DPPF)(2,6-dimethyl-
η6-aniline)]BF4; ORTEP, 30% probability ellipsoids. Hydrogen atoms
are omitted for clarity. Selected distances and bond angles are summa-
rized in Table 1. CCDC-861119.
Results and Discussion
When a tenfold excess of 2,6-dimethylaniline is added to a
solution of [Rh(DPPF)(MeOH)2]BF4 (0.01 mmol) in methanol
(10 mL), a color change from yellow to light red takes place.
31P NMR spectroscopy shows that only one species is present
in solution (supp. info). By layering the solution with diethyl
The enantioselective hydrogenation of prochiral dehy-
droamino acid derivatives has been investigated for a long time
both in academia and in industry. The amino group of this
kind of substrates needs to be protected to prevent nitrogen
coordination to rhodium and catalyst deactivation.[4] To rule
out the possibility that in the case of 2,6-dimethylaniline coor-
dination through the nitrogen was hampered by the steric hin-
drance of the two methyl groups thus favoring binding of the
arene moiety, other substrates with a less hindered nitrogen,
namely N-methylaniline and aniline, were tested. Indeed, even
with these substrates the corresponding η6-arene complexes
were isolated (Figure 2 and Figure 3).
* Prof. Dr. D. Heller
Fax: +49-381-1281-50183
E-Mail: detlef.heller@catalysis.de
[a] Leibniz-Institut für Katalyse e.V.
Universität Rostock
Albert-Einstein Straße 29a
18059 Rostock, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200008.
Z. Anorg. Allg. Chem. 2012, 638, (᭿), 1–3
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Fischer, A. König, H.-J. Drexler, D. Heller
SHORT COMMUNICATION
Data in Table 1 show that significant distances and angles Table 1. Selected distances /Å and bond angles /° of the isolated com-
plexes with aniline and its derivatives.
in the three complexes are very similar.
Arene
Distance /Å
Rh–P
Distance /Å
Angle /°
P–Rh–P
Rh–η6C
aniline
2,230–
2,280–
95,48(5)
2,254(2)
2,238–
2,485(6)
2,260–
N-methylaniline
95,69;
2,254(2)
2,6-dimethylaniline 2,227–
2,242(1)
2,522(4)
2,263–
2,552(3)
97,22(4)
95,34(3)
MEA in the prochiral substrate load lead to a drop in the cata-
lyst activity,[6,7] probably because of the formation of inactive
(hydride ?) arene complexes.
Conclusions
In summary it has been shown that cationic rhodium diphos-
phine solvate complexes react with aniline and its derivatives
to give stable arene complexes. X-ray structures of three such
complexes containing the ligand DPPF and respectively ani-
line, N-methylaniline and 2,6-dimethylaniline are provided.
Figure 2. Molecular structure of the cation [Rh(DPPF)(N-methyl-η6-
aniline)]BF4; ORTEP, 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity. Selected distances and bond angles are summarized
in Table 1. CCDC-861121.
Supporting Information (see footnote on the first page of this article):
31P NMR spectra of the rhodium diphosphine solvate complexes, crys-
tallographic data, specification of data collection and structural refine-
ment of the prepared X-Ray structures.
References
[1] a) J. Halpern, D. P. Riley, A. S. C. Chan, J. J. Pluth, J. Am. Chem.
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[2] T. Yamagata, K. Tani, Y. Tatsuno, T. Saito, J. Chem. Soc., Chem.
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[3] D. Heller, H.-J. Drexler, A. Spannenberg, B. Heller, J. You, W.
Baumann, Angew. Chem. Int. Ed. 2002, 41, 777–780.
[4] Up to now there are very less examples in literature describing the
hydrogenation of unprotected amino acids: Y. Hsiao, N. R. Rivera,
T. Rosner, S. W. Krska, E. Njolito, F. Wang, Y. Sun, J. D. Arm-
strong III, E. J. J. Grabowski, R. D. Tillyer, F. Spindler, C. Malan,
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[6] H.-U. Blaser, R. Hanreich, H.-D. Schneider, F. Spindler, B. Stein-
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Figure 3. Molecular structure of the cation [Rh(DPPF)(aniline)]BF4;
ORTEP, 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity. Selected distances and bond angles are summarized in Table 1.
CCDC-861120.
The apparent high stability of such rhodium-arene com-
plexes and their iridium analogues might lead to deactivation
phenomena in catalysis.[3,5] An example is provided by the
industrial synthesis of Metolachlor. In the key step the so-
called “MEA imine” ((E)-2-ethyl-N-(1-methoxypropan-2-ylid-
ene)-6-methylaniline) is hydrogenated stereoselectively with a
chiral iridium complex. The substrate is prepared by condensa-
tion of 2-ethyl-6-methylaniline (MEA) with methoxyacetone.
During process optimization it has been shown that traces of
Received: January 11, 2012
Published Online: ᭿
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Z. Anorg. Allg. Chem. 2012, 1–3

Formation of Stable Rhodium η6-Arene Complexes with Aniline Derivatives
C. Fischer, A. König, H.-J. Drexler, D. Heller* .............. 1–3
Formation of Stable Rhodium η6-Arene Complexes with Ani-
line Derivatives
Z. Anorg. Allg. Chem. 2012, 1–3
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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