Novel bis(oxazoline) pincer ligands: Formation of mononuclear rhodium(II) complexes [3]

Full text of DOI:10.1021/ja005857a

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J. Am. Chem. Soc. 2001, 123, 5818-5819
benzene ((S,S)-benboxMe2) ligands (1a-h) (eq 1) which were
expected to resist metalation in the 4- and 6-positions of the
aromatic ring.9 These ligands were synthesized in a three-step
sequence from m-xylene, using an amino alcohol-nitrile con-
densation similar to that reported by Witte and Bolm for the
phebox ligands.10 Ligands 1a-h were obtained as off-white or
yellow solids (41-96%).
Novel Bis(oxazoline) Pincer Ligands: Formation of
Mononuclear Rhodium(II) Complexes
Michael Gerisch, Jennifer R. Krumper,
Robert G. Bergman,* and T. Don Tilley*
Department of Chemistry and Center for
New Directions in Organic Synthesis (CNDOS)
UniVersity of California and DiVision of
Chemical Sciences, Lawrence Berkeley National Laboratory
Berkeley, California 94702
ReceiVed December 7, 2000
The chemistry of rhodium is generally associated with the Rh-
(I) and Rh(III) oxidation states. Although many rhodium(II)
dimers have been characterized,1 few monomeric rhodium(II)
complexes have been isolated.2,3 Herein we report the synthesis
and characterization of monomeric rhodium(II) bis(oxazoline)
complexes bearing new C2-symmetric ligands. These new ligands
also enforce unsaturated coordination spheres for rhodium(III),
giving rise to square pyramidal complexes.
Transition metal complexes with C2-symmetry, such as those
formed with bis(oxazoline) ligands, have proven to be versatile
catalysts for asymmetric transformations.4,5 In recent years, several
groups have explored the chemistry of ligands such as phebox
(phebox ) bis(oxazolyl)phenyl),6 which feature both C2-symmetry
and the robust “NCN pincer” binding mode.7,8 Our ligand design
was guided by preliminary molecular mechanics modeling, which
suggested that homologation of phebox would widen the ligand’s
N-Rh-N bite-angle. We predicted that a widening of this angle
would draw the oxazolyl alkyl groups closer to the metal center,
providing a well-defined, sterically congested chiral environment.
Thus, we prepared the (S,S)-bis(oxazolyl-methyl)-4,6-dimethyl-
Addition of RhCl3(H2O)3 to refluxing ethanolic solutions of
ligands 1a-g led to the new cyclometalated rhodium(III) pincer
complexes [trans-RhCl2((S,S)-benbox(Me2))] (2a-g), and in some
cases to the unexpected rhodium(II) complexes [trans-RhCl2((S,S)-
benbox(Me2)H)] (3e, 3g) (Scheme 1). The rhodium(III) complexes
were isolated as orange, air-stable microcrystalline compounds
(8-59%). In contrast to previously reported “NCN pincer”
rhodium(III) complexes (which were obtained as 18 valence-
electron H2O adducts6c,8b), compounds 2a-h are coordinatively
unsaturated (vide infra). This unsaturation is presumably a result
of steric shielding of the open coordination site by the pincer
ligand.
Orange crystals of the bis-iso-propyl-substituted complex were
obtained by layering a CH2Cl2 solution of 2c with pentane at room
temperature. The molecular structure is shown in Scheme 1. The
rhodium atom occupies a square pyramidal coordination environ-
ment, with the aryl ring of the pincer ligand in the apical position.
The six-membered metal-ligand chelate ring results in an
N-Rh-N angle (178.01(13)°) much larger than that in the related
five-membered chelate complex, [RhCl2(t-BuNC)((S,S)-ip-
phebox))] (157.3(2)°).6c Consequentially, the ligand iso-propyl
groups are placed quite close to the metal center in complex 2c.
The Rh‚‚‚C(21) (3.11 Å) and Rh‚‚‚H(C(21)) (2.38 Å) distances
are consistent with a weak Rh-H interaction between the iso-
propyl group and rhodium.
(1) (a) Doyle, M. P.; Forbes, D. C. Chem. ReV. 1998, 98, 911. (b) Crawford,
C. A.; Matonic, J. H.; Huffman, J. C.; Folting, K.; Dunbar, K. R.; Christou,
G. Inorg. Chem. 1997, 36, 2361. (c) Prater, M. E.; Pence, L. E.; Clerac, R.;
Finniss, G. M.; Campana, C.; Auban-Senzier, P.; Jerome, D.; Canadell, E.;
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D. G. Coord. Chem. ReV. 1996, 147, 209. (b) Pandey, K. K. Coord. Chem.
ReV. 1992, 121, 1.
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Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305. (g) Zhang, X. X.;
Wayland, B. B. J. Am. Chem. Soc. 1994, 116, 7897. (h) Bunn, A. G.; Wei,
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P.; Boulatov, R. J. Am. Chem. Soc. 2000, 122, 11812. (m) Connelly, N. G.;
Emslie, D. J. H.; Geiger, W. E.; Hayward, O. D.; Linehan, E. B.; Orpen, A.
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(4) For reviews, see: (a) Ojima, I. Catalytic Asymmetric Synthesis, 2nd
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Cappiello, J. Tetrahedron: Asymmetry 1998, 9, 1.
Further evidence for interactions between the oxazoyl alkyl
groups and the rhodium center was obtained by infrared spec-
troscopy. Although no bands for a rhodium-hydrogen interaction
were identified for complex 2c, the complex bearing an iso-butyl
substitutent at the oxazoline ring (2d) shows two bands of medium
(7) For reviews of NCN pincer complexes, see: (a) Gossage, R. A.; van
de Kuil, L. A.; van Koten, G. Acc. Chem. Res. 1998, 31, 423. (b) Mayer, H.
A.; Kaska, W. C. Chem. ReV. 1994, 94, 1239. (c) Bianchini, C.; Meli, A.;
Peruzzini, M.; Vizza, F.; Zanobini, F. Coord. Chem. ReV. 1992, 120, 193. (d)
Cotton, F. A.; Hong, B. Prog. Inorg. Chem. 1992, 40, 179.
(5) For examples, see: (a) Nishiyama, H.; Sakaguchi, H.; Nakamura, T.;
Horihata, M.; Kondo, M.; Itoh, K. Organometallics 1989, 8, 846. (b)
Nishiyama, H.; Kondo, M.; Nakamura, T.; Itoh, K. Organometallics 1991,
10, 500. (c) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993,
115, 6460. (d) Nishiyama, H.; Itoh, Y.; Matsumoto, H.; Park, B.; Itoh, K. J.
Am. Chem. Soc. 1994, 116, 2223. (e) Nesper, R.; Pregosin, P. S.; Pu¨ntener,
K.; Wo¨rle, M.; Albinati, A. J. Organomet. Chem. 1996, 507, 85.
(6) For C2-symmetric NCN pincer complexes, see: (a) Stark, M. A.;
Richards, C. J. Tetrahedron Lett. 1997, 38, 5881. (b) Denmark, S. E.;
Stavanger, R. A.; Faucher, A. M.; Edwards, J. P. J. Org. Chem. 1997, 62,
3375. (c) Motoyama, Y.; Makihara, N.; Mikami, Y.; Aoki, K.; Nishiyama,
H. Chem. Lett. 1997, 951. (d) Motoyama, Y.; Narusawa, H.; Nishiyama, H.
Chem. Commun. 1999, 131. (e) Motoyama, Y.; Mikami, Y.; Kawakami, H.;
Aoki, K.; Nishiyama, H. Organometallics 1999, 18, 3584.
(8) (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020. (b) van der Zeijden, A. A. H.; van Koten, G.; Luijk, R.; Vrieze, K.;
Slob, C.; Krabbendam, H.; Spek, A. L. Inorg. Chem. 1988, 27, 1014. (c) van
Koten, G. Pure Appl. Chem. 1989, 61, 1681. (d) Steenwinkel, P.; James, S.
L.; Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G. Chem. Eur. J.
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Chem. 1997, 21, 751.
(9) (a) Steenwinkel, P.; James, S. L.; Grove, D. M.; Kooijman, H.; Spek,
A. L.; van Koten, G. Organometallics 1997, 16, 513. (b) Steenwinkel, P.;
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10.1021/ja005857a CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/24/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5819
Scheme 1. Structures and Reactions of Rhodium(II) and
Rhodium(III) Pincer Complexes
plexes exhibit unusually high thermal stability (mp: 191-193
°C (3e); 190-192 °C (3g)), and are stable to air at room
temperature. The yield of the rhodium(II) complex 3e is improved
by running the reaction at low concentration, resulting in an
optimized yield of 57%.
The paramagnetism and mononuclearity of complexes 3e and
3g were established using several spectroscopic and magnetic
techniques. X-band EPR spectra for 3e and 3g were obtained in
glassy frozen 2-methyl-tetrahydrofuran, yielding g-values of g1
) 2.864, g2 ) 2.320, and g3 ) 1.903 for 3g (spectrum obtained
at 57 K). Doublet splitting from the hyperfine interaction with
1
103Rh (I ) /2) was observed for g2 (A2 ) 14.5 G) and g3 (A3 )
28.3 G). Complex 3e also shows three g values: g1 ) 2.926, g2
) 2.307, and g3 ) 1.881 (spectrum obtained at 1.2 K). For 3e,
hyperfine coupling (doublet) to 103Rh of only g3 is observed (A3
) 23.7 Hz). Consistent with the asymmetrical coordination
environments around the rhodium centers in these complexes, the
EPR spectra for both 3e and 3g are rhombic.3f
Complex 3g exhibits Curie-Weiss behavior between 5 and
289 K, with an average magnetic moment of 2.00(2) µb. The
solution paramagnetism of the rhodium(II) complexes was
determined by the Evans method, yielding µeff values of 1.87 µB
(3e) and 1.98 µB (3g).14 The solid- and solution-phase magnetic
1
measurements are both consistent with an S ) /2 ground state,
as expected for a monomeric low-spin d7 metal complex.
The molecular structure of 3g is shown in Scheme 1. No
unusually close intermolecular contacts were observed, further
confirming that the rhodium(II) center is mononuclear in the solid
state. The geometry at the rhodium atom is approximately square-
planar (maximum deviation of 0.137(2) Å from the plane defined
by rhodium’s four ligands). The aryl group of the ligand is roughly
parallel (17°) to the coordination plane of rhodium. The relatively
short rhodium-ipso-carbon distance of 2.575(5) Å indicates an
agostic interation between these two atoms. Notably, this rhodium-
carbon distance is somewhat longer than that in a related cationic
“PCP pincer” rhodium(I) complex containing an η2 agostic
Rh‚‚‚C-H bond (2.273(5) Å).15
In conclusion, we have prepared a series of new NCN bis-
(oxazoline) pincer ligands, and applied these ligands to give
unique, coordinatively unsaturated rhodium(III) compounds and
robust mononuclear rhodium(II) complexes. We have found both
the rhodium(II) and rhodium(III) complexes to be competent for
asymmetric catalysis and will describe these results in forthcoming
publications.16
strength (2764 and 2756 cm-1) in the agostic C-H region.11
Reaction of 2d with excess CO at 75 °C leads to the formation
of [RhCl2{(S,S)-ib-benbox(Me2)}(CO)] (4d) in 81% yield. The
corresponding reaction with t-BuNC at room temperature leads
to [RhCl2{(S,S)-ib-benbox(Me2)}(t-BuNC)] (5d) in 82% yield
(Scheme 1). In both reactions, displacement of the Rh-H
interaction by the incoming ligand is indicated in the IR (for 4d:
νCO ) 2070 cm-1; for 5d νCN ) 2177 cm-1).
Treatment of benzene solutions of 2b and 2c with H2 results
in formation of the hydride complexes 6b (85%) and 6c (45%)
(Scheme 1), in reactions analogous to those reported by Milstein
and Fryzuk.12 A distinctive feature of the products 6b and 6c is
1
that their H NMR spectra display coupling from the rhodium
hydride to the proton on the ipso carbon of the aromatic ring
(JH-H ) 2.5-3.1 Hz; confirmed by spin saturation transfer and
TOCSY experiments). The molecular structure of 6c is shown in
Scheme 1. The rhodium hydride was located and refined in the
difference map, and the rhodium-hydride bond distance was found
to be 1.47(5) Å. Most notable in the structure of 6c is the 2.632-
(6) Å distance from the ipso carbon to the rhodium center,
suggesting a weak interaction between these atoms.13
When the sterically demanding ligands 1e and 1g are heated
with RhCl3(H2O)3 in ethanol, the monomeric rhodium(II) com-
plexes 3e (19%) and 3g (49%) are formed in addition to the
expected cyclometalated rhodium(III) complexes 2e and 2g
(Scheme 1). These complexes were cleanly separated from each
other by column chromatography. These new rhodium(II) com-
Acknowledgment. This work was carried out under the auspices of
a CRADA project, administered by the Lawrence Berkeley National
Laboratory under contract no. DE-AC03-76SF00098, in cooperation with
the E.I. DuPont Co, and funded under the Initiatives for Proliferation
Prevention Program of the U.S. Department of Energy. The Center for
New Directions in Organic Synthesis is supported by Bristol-Myers
Squibb as Sponsoring Member. M.G. thanks Deutche Forschungsge-
meinschaft for a Postdoctoral Fellowship. J.R.K. thanks the National
Science Foundation for a Predoctoral Fellowship.
Supporting Information Available: Preparation procedures, spec-
troscopic and analytical data for compounds 1-6; ORTEP diagrams and
crystallographic data sets for 2c, 6c, and 3g (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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1988, 36, 1.
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JA005857A
(14) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Deutch, J. L.; Poling,
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M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539.
(16) Krumper, J. R.; Gerisch, M.; Bergman, R. G.; Tilley, T. D. Manuscript
in preparation.
(13) (a) Gusev, D. G.; Madott, M.; Dolgushin, F. M.; Lyssenko, K. A.;
Antipin, M. Y. Organometallics 2000, 19, 1734. (b) Pregosin, P. S.;
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