Rhodium(III) and rhodium(II) complexes of novel bis(oxazoline) pincer ligands

Article abstract of DOI:10.1021/om0207562

A series of novel C₂-symmetric bisoxazoline ligands (benbox and benboxMe₂) have been prepared. A variety of coordinatively unsaturated rhodium(III) complexes (3a-g, 4a-g) have been isolated. Trapping reactions of the agostically stabilized complex [RhCl₂{(S,S)-ib-benbox(Me₂)}] (4d) are described. Metal-to-aryl methyl migration reactions are described for the complex [RhMe{(S,S)-ip-benbox(Me₂)}][BAr_f] (11c). Full characterization details and reactivity studies of air-stable, isolable, mononuclear, paramagnetic rhodium(II) complexes [RhCl₂{(S,S)-tb-benbox(Me₂)}(H)] (15e) and [RhCl₂{dm-benbox(Me₂)H}] (15g) are reported.

Full text of DOI:10.1021/om0207562

Organometallics 2003, 22, 47-58
47
Rh od iu m (III) a n d Rh od iu m (II) Com p lexes of Novel
Bis(oxa zolin e) P in cer Liga n d s
Michael Gerisch, J ennifer 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 September 11, 2002
A series of novel C2-symmetric bisoxazoline ligands (benbox and benboxMe2) have been
prepared. A variety of coordinatively unsaturated rhodium(III) complexes (3a -g, 4a -g) have
been isolated. Trapping reactions of the agostically stabilized complex [RhCl2{(S,S)-ib-benbox-
(Me2)}] (4d ) are described. Metal-to-aryl methyl migration reactions are described for the
complex [RhMe{(S,S)-ip-benbox(Me2)}][BArf] (11c). Full characterization details and reactiv-
ity studies of air-stable, isolable, mononuclear, paramagnetic rhodium(II) complexes [RhCl2-
{(S,S)-tb-benbox(Me2)}(H)] (15e) and [RhCl2{dm-benbox(Me2)H}] (15g) are reported.
In tr od u ction
In recent years, NCN pincer ligands that resemble
pybox have been prepared.10 The metal complexes of
these 2,6-bis(oxazolyl)phenyl (phebox) ligands have been
used as catalysts for the enantioselective allylation of
aldehydes,11 Diels-Alder reactions,10b alkylation of al-
dimines,12 and cyclopropanation.10c The C2-symmetric
phebox ligands feature the ease of preparation and
chirality of pybox, as well as the robust binding motif
found in pincer ligands.
C2-symmetric bisoxazoline ligands are easily prepared
from commercially available chiral starting materials,
and their metal complexes display remarkable efficacy
in a wide variety of important organic transformations.1
In one example, Nishiyama and co-workers prepared a
variety of metal complexes of the trischelating C2-
symmetric bisoxazoline ligand 2,6-bis(2-oxazolin-2-yl)-
pyridine (pybox).2 Pybox ligand complexes have proven
to be effective catalysts for a variety of enantioselective
transformations, including hydrosilylation of ketones2b
and cyclopropanation of olefins.3 However, these pro-
cesses are somewhat limited by the lability of the
datively bound pybox ligand, which may result in low
enantiomeric excesses and makes these catalysts un-
suitable for applications at elevated temperatures.2a,4
Monoanionic tridentate ligands, such as the “PCP
pincer” (alkyl or aryl bis(phosphino)) and “NCN pincer”
(alkyl or aryl bis(amino)) ligands bind more tightly to
metal centers than trischelate ligands that can form
only dative bonds. Transition metal complexes of pincer
ligands have been applied to carbon-carbon bond
cleavage,5 catalytic aliphatic dehydrogenation,6 Khar-
asch reactions,7 methylene transfers,8 and various
catalytic enantioselective transformations.9 Pincer ligand
complexes are quite thermally robust, presumably due
to the strong covalent bond between a monoanionic
tridentate ligand and the metal center.
We are interested in developing new C2-symmetric
pincer ligands for use in metal-catalyzed enantioselec-
tive transformations. Our ligand design was guided by
preliminary molecular mechanics modeling, which sug-
gested that benzylic homologation of phebox would
widen the ligand N-M-N bite angle. Bite angle has
been shown to have a dramatic effect on the reactivity
of a variety of metal-ligand complexes.13 Bite angle
controls the steric environment around the metal center
and also directly affects the shape and energy of the
metal frontier molecular orbitals. Additionally, our
preliminary molecular mechanics models predicted that
(6) (a) Liu, F. C.; Pak, E. B.; Singh, B.; J ensen, C. M.; Goldman, A.
S. J . Am. Chem. Soc. 1999, 121, 4086. (b) J ensen, C. M. Chem Commun.
1999, 2443. (c) Lee, D. W.; Kaska, W. C.; J ensen, C. M. Organometallics
1998, 17, 1. (d) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.;
J ensen, C. M. J . Am. Chem. Soc. 1997, 119, 840. (e) Gupta, M.; Kaska,
W. C.; J ensen, C. M. Chem. Commun. 1997, 461. (f) Gupta, M.; Hagen,
C.; Flesher, R. J .; Kaska, W. C.; J ensen, C. M. Chem. Commun. 1996,
2083.
(7) (a) van de Kuil, L. A.; Grove, D. M.; Gossage, R. A.; Zwikker, J .
W.; J enneskens, L. W.; Drenth, W.; van Koten, G. Organometallics
1997, 16, 4985. (b) Gossage, R. A.; van de Kuil, L. A.; van Koten, G.
Acc. Chem. Res. 1998, 31, 423.
(8) Gozin, M.; Aizenberg, M.; Liou, S. Y.; Weisman, A.; Ben-David,
Y.; Milstein, D. Nature 1994, 42.
(9) For examples, see: (a) Seligson, A. L.; Trogler, W. C. Organo-
metallics 1993, 12, 744. (b) Gorla, F.; Venanzi, L. M. Organometallics
1994, 13, 43. (c) Ohff, A.; Ohff, M.; Milstein, D. J . Am. Chem. Soc.
1997, 119, 11687.
* To whom correspondence should be addressed. E-mail: bergman@
cchem.berkeley.edu (R.G.B.); tdtilley@socrates.berkeley.edu (T.D.T.).
(1) For reviews, see: (a) Pfaltz, A. Acc. Chem. Res. 1993, 26, 339.
(b) Ghosh, A. K.; Mathivanan, P.; Cappiello, J . Tetrahedron: Asym-
metry 1998, 9, 1. (c) Ojima, I. Catalytic Asymmetric Synthesis, 2nd ed.;
Wiley-VCH: New York, 2000. (d) Fache, F.; Schulz, E.; Tommasino,
M. L.; Lemaire, M. Chem. Rev. 2000, 100, 2159.
(2) (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.
(3) Nishiyama, H.; Itoh, Y.; Matsumoto, H.; Park, B.; Itoh, K. J . Am.
Chem. Soc. 1994, 116, 2223.
(10) (a) Motoyama, Y.; Makihara, N.; Mikami, Y.; Aoki, K.; Nish-
iyama, H. Chem. Lett. 1997, 951. (b) Stark, M. A.; Richards, C. J .
Tetrahedron Lett. 1997, 38, 5881. (c) Denmark, S. E.; Stavanger, R.
A.; Faucher, A. M.; Edwards, J . P. J . Org. Chem. 1997, 62, 3375.
(11) Motoyama, Y.; Narusawa, H.; Nishiyama, H. Chem. Commun.
1999, 131.
(4) Nesper, R.; Pregosin, P. S.; Pu¨ntener, K.; Wo¨rle, M.; Albinati,
A. J . Organomet. Chem. 1996, 507, 85.
(5) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. Engl. 1999,
38, 871.
(12) Motoyama, Y.; Mikami, Y.; Kawakami, H.; Aoki, K.; Nishiyama,
H. Organometallics 1999, 18, 3584.
10.1021/om0207562 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 12/07/2002

48 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
Sch em e 1. Liga n d Syn th esis
zene in the presence of catalytic Cd(OAc)2‚2H2O. Col-
umn chromatography was used to purify the ligands,
which were obtained in good to moderate yields as white
or yellow solids (melting points ) 56-86 °C) and were
fully characterized. Enantiomerically pure bis(oxazo-
lines) were additionally characterized by optical rota-
tion.
Several methods of spectroscopic characterization
were used to establish the C2-symmetry of ligands 1a -g
and 2a -g. Only one C-N stretching vibration appears
in the IR spectra of these ligands, in the range 1659-
1
1670 cm-1. The H NMR spectra of the ligands are also
consistent with a C2-symmetrical environment; a dou-
blet of doublet splitting for the oxazoline CH2’s is
observed. The oxazolyl-carbon atom resonances in the
13C{1H} NMR spectra were observed between 163.3 and
167.0 ppm.
Syn th esis of Rh od iu m (III) Com p lexes. Syntheses
of the rhodium(III) benbox and benboxMe2 complexes
were based on previously reported procedures for cy-
clometalated rhodium pincer ligand compounds.10a,19
Addition of RhCl3‚3H2O to refluxing ethanolic solutions
of 1a -g or 2a -g led in all cases to formation of the
corresponding rhodium(III) complexes [RhCl2(benbox)]
(3a -g) and [RhCl2(benboxMe2)] (4a -g) in 8-59% yield
(eq 1). In contrast to related 18-valence-electron “NCN
widening the N-M-N bite angles of phebox-type com-
plexes would draw the oxazolyl alkyl groups closer to
the metal center than previously observed. The resulting
complexes should possess a more sterically congested
ligand environment, which might, in turn, lead to higher
selectivities in catalysis.
We prepared two series of new NCN pincer ligands,
the parent bis(oxazolylmethyl)benzene ((S,S)-benbox)
ligands 1a -g and the methylated (S,S)-bis(oxazolylm-
ethyl)-4,6-dimethylbenzene ((S,S)-benboxMe2) ligands
2a -g (Scheme 1), which were expected to resist meta-
lation in the 4- and 6-positions of the aromatic ring.14
We have recently reported the synthesis of monomeric
rhodium(II) and coordinatively unsaturated rhodium-
(III) metal complexes of the benbox and benboxMe2
ligands.15 In this report we fully describe the synthesis
and reaction chemistry of these complexes.
Resu lts a n d Discu ssion
pincer” rhodium(III) complexes, which were obtained as
H2O adducts, the benbox and benboxMe2 complexes are
coordinatively unsaturated.10a,19b This effect is presum-
ably a result of steric shielding of the open coordination
site by the pincer ligand (vide infra).
Liga n d Syn th esis. All amino alcohols employed in
the ligand syntheses are commercially available as both
enantiomers (with the exception of tert-leucinol, which
is only available as the (S) antipode), as is the dinitrile
used in the synthesis of ligands 1a -g. The meta-
dimethyl dinitrile (for ligands 2a -g) was synthesized
from 2,4-dimethyldi-1,5-(chloromethyl)benzene, which
was obtained by chloromethylation of m-xylene.16,17 The
new benbox and benboxMe2 ligands 1a -g and 2a -g
were obtained using the procedure reported by Witte
and Bolm for the synthesis of the corresponding phebox
ligands (Scheme 1).18 Each benbox ligand was prepared
by refluxing a solution of the appropriate dinitrile with
an excess of the appropriate amino alcohol in chloroben-
Rhodium(III) benbox and benboxMe2 complexes 3a -g
and 4a -g were isolated as orange, air-stable micro-
crystalline compounds which are highly soluble in meth-
ylene chloride and tetrahydrofuran and moderately
soluble in benzene and acetone. Coordination of the bis-
(oxazoline) ligand to the metal center results (except in
the case of of 3e) in an increase in the frequency for
the C-N stretching vibration relative to those of the
free ligand, giving rise to νCN values between 1668 and
1683 cm-1. These results differ from those reported for
the (phebox)-rhodium(III) complexes, in which a de-
crease of the frequency is observed upon coordination.10a
This difference may be related to a bite-angle-induced
change in the metal-ligand orbital overlap.
(13) (a) Casey, C. P.; Whiteker, G. T. Isr. J . Chem. 1990, 30, 299.
(b) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J . A.; Powell, D. R. J . Am. Chem. Soc. 1992, 114, 5535. (c)
Hansen, S. M.; Rominger, F.; Metz, M.; Hofmann, P. Chem.-Eur. J .
1999, 5, 557. (d) van Leeuwen, P.; Kamer, P. C. J .; Reek, J . N. H.;
Dierkes, P. Chem. Rev. 2000, 100, 2741. (e) Kamer, P. C. J .; van
Leeuwen, P. W. N.; Reek, J . N. H. Acc. Chem. Res. 2001, 34, 895.
(14) (a) Steenwinkel, P.; J ames, S. L.; Grove, D. M.; Kooijman, H.;
Spek, A. L.; van Koten, G. Organometallics 1997, 16, 513. (b) Steen-
winkel, P.; J ames, S. L.; Gossage, R. A.; Grove, D. M.; Kooijman, H.;
Smeets, W. J . J .; Spek, A. L.; van Koten, G. Organometallics 1998,
17, 4680. (c) Steel, P. J .; Hartshorn, C. M. Organometallics 1998, 17,
3487.
The two hydrogens attached to each bridging meth-
ylene carbon are diastereotopic in all rhodium(III)
complexes of benbox and benboxMe2 ligands containing
a stereogenic center and give rise to two doublets in the
1H NMR spectra between 5.25 and 3.28 ppm (J HH
)
16.8-19.0 Hz). The resonances due to the metal-bound
ipso-carbons in 3a -g and 4a -g appear between 129.5
and 135.5 ppm in the 13C{1H} spectrum, revealing
(15) Gerisch, M.; Krumper, J . R.; Bergman, R. G.; Tilley, T. D. J .
Am. Chem. Soc. 2001, 123, 5818.
(16) van der Made, A. W.; van der Made, R. H. J . Org. Chem. 1993,
58, 1262.
(17) Ciba Corp. Patent, US 3483209, 1969.
(19) (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.
(18) (a) Bolm, C.; Weickhardt, K.; Zehnder, M.; Rauff, T. Chem. Ber.
1991, 124, 1173. (b) Witte, H.; Seeliger, W. Liebigs Ann. Chem. 1976,
996.

Rh Complexes of Bis(oxazoline) Pincer Ligands
Organometallics, Vol. 22, No. 1, 2003 49
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for 4c, 9c, a n d 15g
4c
9c
15g
Crystal Data
empirical formula
fw
C
22H31Cl2N2O2Rh‚CH2Cl2
C22H30F3N2O5SRh
594.45
C20H28Cl2N2O2Rh
502.27
614.24
cryst color, habit
cryst dimens (mm)
cryst syst
orange, columnar
0.33 × 0.12 × 0.12
orthorhombic
primitive
yellow, blade
0.30 × 0.17 × 0.02
monoclinic
primitive
orange, dendritic
0.15 × 0.20 × 0.30
monoclinic
primitive
lattice type
lattice params
a ) 8.3012(2) Å
b ) 16.1358(4) Å
c ) 19.2670(5) Å
a ) 9.4600(3) Å
b ) 53.891(2) Å
c ) 9.7688(3) Å
â ) 92.453(1)°
4975.7(3)
P21 (#4)
8
1.587
a ) 9.0415(4) Å
b ) 12.3993(5) Å
c ) 10.0072(4) Å
â ) 95.135(1)°
1117.39(8)
P21 (#4)
V (Å3)
2580.75(10)
P212121 (#19)
4
space group
Z value
2
D
calc (g/cm3)
1.581
1.493
F000
1256.00
10.97
2432.00
8.26
514.00
10.18
µ(Mo KR) (cm-1
)
Intensity Measurements
temperature (°C)
2θ range (deg)
2θmax (deg)
-127 ( 1
-102 ( 1
-102 ( 1
3.00-46.00
3.50-45.00
3.00-45.00
52.1
46.6
52.0
total: 5412
unique: 2269 (Rint ) 0.026)
no. of reflns measd
total: 12 541
unique: 2725 (Rint ) 0.035)
total: 21 101
unique: 7428 (Rint ) 0.091)
Structure Solution and Refinement
p-factor
0.0300
0.0300
0.0300
no. of observations (I>3.00σ(I))
no. of variables
3686
289
6812
749
3022
239
refln/param ratio
12.75
0.025, 0.026, 0.037
0.98
0.00
9.09
12.64
0.029, 0.044, 0.030
1.92
0.01
residuals:a R, Rw, Rall
goodness of fit indicator
max. shift/error in final cycle
0.068, 0.068, 0.106
1.93
0.01
a
2
R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/∑wFo
]
1/2, where w ) 1/σ2(Fo).
almost no change in chemical shift relative to the
starting ligand, and an upfield shift of roughly 50 ppm
compared to those in the (phebox)-rhodium(III) com-
1
plexes reported by Nishiyama et al.10a The J Rh-C
coupling constants range from 34.5 to 39.4 Hz.
Orange crystals of 4c suitable for X-ray analysis were
obtained by layering a CH2Cl2 solution of 4c with
pentane at room temperature. The molecular structure
is shown in Figure 1, crystal data and structure refine-
ment information is presented in Table 1, and selected
bond distances and angles are given in Table 2. The
rhodium atom adopts a square pyramidal coordination
environment, with the aryl ring of the pincer ligand in
the apical position. The structural parameters for 4c
can be compared to those of the related five-membered
chelate complex [RhCl2(t-BuNC)((S,S)-ip-phebox))].10a
The six-membered metal-ligand chelate ring results in
an N-Rh-N angle for 4c (178.01(1)°) that is much
larger than that in the phebox complex (157.3(2)°).
Consequently, the ligand isopropyl groups are quite
close to the metal center in complex 4c. The Rh‚‚‚C(21)
(3.11 Å) and Rh‚‚‚H(C(21)) (2.38 Å) distances and the
Rh-H-C(21) angle (133.8°) are consistent with a weak
Rh-H agostic interaction between the isopropyl group
and rhodium. The Rh-C(9) distance of 2.004(3) Å is
slightly longer than the corresponding value in the
(phebox)-rhodium(III) complex (1.941(7) Å).10a The Rh-
Cl distances (2.355(2), 2.365(1) Å) are equivalent within
3σ, as are the Rh-N distances (2.074(4), 2.072(4) Å).
The Rh-N bond lengths in the benbox complex are
similar to those reported for the (phebox)-rhodium(III)
complex (2.053(5), 2.055(5) Å).10a
F igu r e 1. Molecular structure of 4c. Methylene chloride
molecule is omitted for clarity. Thermal elipsoids are shown
at the 50% probability level.
rhodium center, no IR band could be assigned to an
agostic C-H vibration for this complex. However, the
complexes 3d and 4d , bearing isobutyl substitutents at
the oxazoline rings, exhibit two low-frequency bands of
medium strength (3d : νRhCH ) 2762, 2752 cm-1, 4d :
νRhCH ) 2764, 2756 cm-1) assigned to the agostic C-H
Although the X-ray structure of 4c reveals a weak
Rh-H interaction involving one isopropyl group and the

50 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
Sch em e 3. Syn th esis of Meth ylr h od iu m
Ta ble 2. Selected Dista n ces (Å) a n d Bon d An gles
(d eg) for [Rh Cl2{(S,S)-ip -ben boxMe2}] (4c)
Com p lexes
Rh(1)-Cl(1)
Rh(1)-Cl(2)
Rh(1)-N(1)
Rh(1)-N(2)
Rh(1)-C(9)
Rh(1)‚‚‚C(21)
2.336(1)
2.336(1)
2.032(3)
2.026(3)
2.004(3)
3.11
Cl(1)-Rh(1)-Cl(2)
N(1)-Rh(1)-N(2)
Cl(1)-Rh(1)-C(9)
Cl(2)-Rh(1)-C(9)
N(1)-Rh(1)-C(9)
N(2)-Rh(1)-C(9)
170.9(4)
178.0 (1)
93.8 (1)
95.3(1)
90.1(1)
91.5 (1)
Sch em e 2. Tr a p p in g th e Un sa tu r a ted
Rh od iu m (III) Com p lexes
Com p lexes. Inspired by the results of Milstein et al.5,21
and van Koten et al.,22 we explored the synthesis and
reactivity of methylrhodium(III) complexes bearing ben-
box ligands. Although treatment of the dichlororhodium-
(III) complex 3c or 4c with methyllithium or methyl
Grignard reagents led to decomposition, dimethylzinc
was found to be an effective methylating agent. When
3c or 4c was treated with dimethylzinc in tetrahydro-
furan, a color change from orange to yellow was ob-
served. After treatment with alumina(III), the methyl-
(chloro)rhodium(III) complexes 7c (37%) and 8c (36%)
were obtained (Scheme 3).
stretching frequencies.20 No upfield resonances for these
agostic interactions were observed in the 1H NMR
spectrum from room temperature to -88 °C.
Coor d in a tively Sa tu r a ted Rh od iu m (III) Com -
p lexes. Reaction of 4d with excess CO led to the
formation of [RhCl2{(S,S)-ib-benbox(Me2)}(CO)] (5d )
(90%) after 3 days at 75 °C (Scheme 2). Over the course
of the reaction, the two IR bands assigned to the weak
Rh-C-H interactions in 4d disappeared and were
replaced by a strong C-O stretching vibration at 2070
cm-1. Furthermore, the C-N stretching vibration of the
oxazoline ring was observed at a lower wavenumber (νCN
) 1666 cm-1) than that in 4d (νCN )1683 cm-1). The
appearance of four different resonances for the methyl
Little variation is observed when comparing the
ligand C-N stretching vibrations of the dichloror-
hodium complexes 3c (νCN ) 1674 cm-1) and 4c (νCN
)
1680 cm-1) with those of the methylrhodium complexes
7c (νCN ) 1673 cm-1) and 8c (νCN ) 1672 cm-1).
Convincing evidence for the structural assignments of
1
7c and 8c is found in the H and 13C NMR spectra of
these complexes. The most diagnostic features in the
1H NMR spectra of 7c and 8c are the doublet resonances
of the rhodium-bound methyl groups at 1.16 (2J Rh-H
)
1
protons of the isobutyl groups in the H NMR spectrum
2.0 Hz) and 1.18 (2J Rh-H ) 2.0 Hz), respectively. The
13C{1H} resonances of these rhodium-bound methyl
groups appear at -5.6 (1J Rh-C ) 28.0 Hz) and -5.5
(1J Rh-C ) 28.7 Hz) ppm in 7c and 8c, respectively. The
appearance of four distinct diastereotopic resonances for
the methyl protons of the isopropyl groups in the 1H
NMR spectra of 7c and 8c and the inequivalence of
these carbon atoms in the 13C{1H} NMR spectra are
consistent with the reduced symmetry of these chlo-
romethylrhodium complexes compared to the dichloro-
rhodium complexes 3c and 4c.
of 5d and the lack of chemically equivalent isobutyl
carbon atoms in the 13C{1H} NMR spectrum of this
complex are indicative of a cis-arrangement of the
chloride ligands.
Reaction of 4d with tert-butylisonitrile at room tem-
perature led to formation of [RhCl2{(S,S)-ib-benbox-
(Me2)}(tBuNC)] (6d ) (82%) within seconds (Scheme 2).
As in the reaction with carbon monoxide, the two IR
bands for the agostic interactions in 4d disappeared and
were replaced by a strong C-N stretching vibration at
t
2177 cm-1 for the coordinated BuNC in 6d . As with
Treatment of the chloromethylrhodium complex 7c or
8c with silver triflate led to quantitative formation of
triflate complexes 9c and 10c as yellow, microcrystalline
solids. For these triflate complexes, the methyl protons
of the ligand isopropyl groups appear as four distinct
diastereotopic resonances in the 1H NMR spectra,
suggesting that exchange of the triflate ligand is slow
relative to the NMR time scale at room temperature.
Yellow crystals of 9c suitable for X-ray analysis were
carbonyl complex 5d , the C-N stretching vibration of
the oxazoline ring for 6d (νCN )1669 cm-1) is observed
at a lower wavenumber than that in 4d (νCN )1683
1
cm-1). The H NMR spectrum of 6d is consistent with
a C2-symmetric molecule, implying a trans arrangement
of the chloride ligands. The difference in stereochemistry
between complexes 5d and 6d may be caused by the
need for avoidance of unfavorable steric interactions
between the oxazoyl alkyl groups and the isonitrile tert-
butyl group in 6d .
(21) (a) van der Boom, M. E.; Higgit, C. L.; Milstein, D. Organome-
tallics 1999, 18, 2413. (b) Vigalok, A.; Uzan, O.; Shimon, L. J . W.; Ben-
David, Y.; Martin, J . M. L.; Milstein, D. J . Am. Chem. Soc. 1998, 120,
12539.
Syn th esis a n d Rea ctivity of Meth ylr h od iu m (III)
(20) (a) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg.
Chem. 1988, 36, 1. (b) Crabtree, R.; Hamilton, D. G. Adv. Organomet.
Chem. 1988, 28, 299.
(22) van der Zeijden, A. A. H.; van Koten, G.; Ernsting, J . M.;
Elsevier, C. J .; Krijnen, B.; Stam, C. H. J . Chem. Soc., Dalton Trans.
1989, 317.

Rh Complexes of Bis(oxazoline) Pincer Ligands
Organometallics, Vol. 22, No. 1, 2003 51
Sch em e 4. Red u ctive Elim in a tion Rea ction s
ring at 2.72 ppm. Two multiplet resonances at 3.50 and
3.27 ppm are assigned to the metal-bound ethylene
ligand. The H NMR spectrum of complex 12c reveals
F igu r e 2. Molecular structure of one independent rhod-
ium center from the crystal structure of 9c. Hydrogen
atoms are omitted for clarity. Thermal elipsoids are shown
at the 50% probability level.
1
four different doublet resonances for the ligand isopropyl
groups, which is consistent with a decrease in symmetry
of the rhodium cation caused by methyl migration. The
reductive elimination reaction described for the forma-
tion of ethylene complex 12c is similar to methyl
migrations described by Milstein and van Koten for
related PCP pincer complexes.21b,23
In a related reaction, methylrhodium cation 11c
rapidly underwent methyl-arene reductive elimination
when treated with carbon monoxide to give the mono-
carbonyl complex 13c (Scheme 4). The spectroscopic
features of 13c are similar to those of the ethylene
complex 12c, with the proton resonance for the methyl
group bound to the aryl ipso-carbon appearing at 2.86
ppm in 13c. Due to rapid association of a second
equivalent of CO, the monocarbonyl complex 13c could
not be isolated (vide infra).
Ta ble 3. Selected Dista n ces (Å) a n d Bon d An gles
(d eg) for [Rh (OTf)Me{(S,S)-ip -ben box] (9c)
Rh(2)-N(3)
Rh(2)-N(4)
Rh(2)-O(8)
Rh(2)-C(44)
Rh(2)-C(32)
2.08(1)
2.08(1)
2.28(1)
1.97(2)
1.90(2)
N(3)-Rh(2)-N(4)
O(8)-Rh(2)-C(44)
C(32)-Rh(2)-C(44)
O(8)-Rh(2)-C(32)
S(2)-O(8)-Rh(2)
179.4(6)
172.3(8)
92.8(9)
94.6(7)
140.4(8)
obtained by layering a concentrated CH2Cl2 solution of
9c with pentane at room temperature. The compound
crystallizes with four independent molecules of 9c in
the asymmetric unit, and the molecular structure of one
of these is shown in Figure 2. Crystal data and structure
refinement information is presented in Table 1, and
representative bond lengths and angles for 9c are given
in Table 3. The molecular structure may be described
as a square-based pyramid, with the methyl and triflate
groups in a trans relationship. As was observed in 4c,
the Rh-N distances (2.08(1), 2.08(1) Å) are equivalent,
and the N-Rh-N axis is effectively linear (N(1)-Rh-
(1)-N(2) ) 179.4(6)°).
When treated with NaBArf (BArf ) tetrakis{(3,5-
trifluoromethyl)phenyl}borate), complex 10c was con-
verted to the tetraarylborate salt [RhMe{(S,S)-ip-
benboxMe2}]BArf (11c). Complex 11c was isolated in
97% yield as an air-sensitive, bright yellow microcrys-
talline solid. Decomposition was observed when meth-
ylene chloride solutions of 11c were heated to temper-
atures above 72 °C. The IR spectrum of 11c contains a
weak band at 2809 cm-1, which is assigned to an agostic
interaction of an isopropyl group with the cationic metal
center. Much like the methylrhodium complexes 7c-
10c, the rhodium cation 11c exhibits diastereotopic
resonances for the isopropyl groups in the 1H NMR
spectrum, indicating that the compound is not C2-
symmetric and, hence, that the methyl group resides
in the apical coordination site.
The reaction between 11c and carbon monoxide was
allowed to proceed for 14 h at room temperature, and
over this time period the monocarbonyl complex 13c was
completely converted to the dicarbonylrhodium(I) com-
plex 14c. The proton resonance for the ipso-carbon-
bound methyl group appears at 2.40 ppm in the 1H
NMR spectrum of 14c. On the basis of the IR stretching
frequencies of the carbonyl ligands (νCO ) 2094, 2029
cm-1), 14c is formulated as a dicarbonyl and not an acyl
complex (the product of insertion of CO into the Rh-
CH3 bond). To confirm this assignment, 14c was syn-
thesized with isotopically enriched 13CO. The IR spec-
trum of 14c(13C) displays 13C-O stretching vibrations
13CO
with the expected isotopic shifts (ν
) 2052, 1993).
These data definitively confirm that 14c is not a
rhodium-acyl complex. Furthermore, in the 13C{1H}
NMR spectrum of 14c(13C), two resonances at 180.3 and
1
2
179.9 ppm (dd, J Rh-C ) 70 Hz, J C-C ) 6.3 Hz) are
assigned to the carbonyl carbon atoms.
The formation of monocarbonyl complex 13c is similar
to processes described by Milstein for PCP pincer
complexes.21b,23c An important difference is the coordi-
nation of an additional equivalent of CO to form
dicarbonyl complex 14c. The monocarbonylrhodium
PCP pincer compounds described by Milstein are sta-
Exposing methylene chloride solutions of 11c to
ethylene yielded the cationic (ethylene)rhodium(I) com-
plex 12c, apparently as a result of reductive elimination
(Scheme 4). The formulation of complex 12c shown in
Scheme 4 is supported by the disappearance of the
resonance of the rhodium-bound methyl group in the
1H NMR spectrum and appearance of a resonance for a
methyl group bound to the ipso-carbon of the ligand aryl
(23) (a) Grove, D. M.; van Koten, G.; Louwen, J . N.; Noltes, J . G.;
Spek, A. L.; Ubbels, H. J . C. J . Am. Chem. Soc. 1982, 104, 6609. (b)
Terheijden, J .; van Koten, G.; Vrieze, K.; Vinke, I. C.; Spek, A. L. J .
Am. Chem. Soc. 1985, 107, 2891. (c) Vigalok, A.; Milstein, D. Orga-
nometallics 2000, 19, 2341.

52 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
bilized by strong interactions between the rhodium and
arene, leading to localization of the cationic charge on
the aromatic system. These compounds do not associate
additional carbon monoxide to form dicarbonyl com-
plexes analogous to 14c. Although a weak rhodium-
arenium interaction may be present in 13c, it does not
prevent association of a second equivalent of CO. This
difference may be related to the lesser steric demand
of the isopropyl benboxMe2 ligand of 13c relative to
Milstein’s tert-butyl phosphine-substituted pincer ligand
or to the increased conformational flexibility offered by
the six-membered chelate ring of 13c relative to the five-
membered chelate ring of Milstein’s complex.
Although the rhodium(III) pincer complexes 3 and 4
undergo a wide variety of interesting stiochiometric
reactions, these compounds were found to be ineffective
as catalysts for asymmetric transformations. Nishiya-
ma’s rhodium(III) pybox complexes catalyze hydrosila-
tion, and we had hoped to find analogous reactivity with
the benbox rhodium(III) complexes.2b Unfortunately,
appreciable yields of hydrosilation products were not
observed in attempted catalytic reactions using the
benbox rhodium complexes 4 with 1-hexene or aceto-
phenone and phenylsilane or diphenylsilane. Similarly,
attempts to catalyze allylation of benzaldehyde (a reac-
tion achieved by the phebox complexes of rhodium(III))
with complex 4c were unsuccessful.11 We attribute this
catalytic inactivity to the greater steric demands of the
benbox ligands relative to the pybox and phebox lig-
ands. Although the steric bulk of the ligand seems to
limit the catalytic activity of the rhodium(III) com-
plexes, it has allowed for the isolation of several unique
and catalytically active rhodium(II) complexes (vide
infra).
F igu r e 3. Molecular structure of 15g. Hydrogen atoms
are omitted for clarity. Thermal elipsoids are shown at the
50% probability level.
Ta ble 4. Selected Dista n ces (Å) a n d Bon d An gles
(d eg) for [Rh Cl2{d m -ben box(Me2)}] (15g)
Rh(1)-Cl(1)
Rh(1)-Cl(2)
Rh(1)-N(1)
Rh(1)-N(2)
Rh(1)‚‚‚C(1)
2.336(2)
2.364(1)
2.074(4)
2.072(4)
2.575(5)
Cl(1)-Rh(1)-Cl(2)
N(1)-Rh(1)-N(2)
Cl(1)-Rh(1)-N(1)
Cl(1)-Rh(1)-N(2)
173.1 (5)
176.1(1)
92.6 (1)
90.5 (1)
distances (2.074(4)/2.072(4) Å). The phenyl group of the
ligand defined by the carbon atoms C(1)-C(6) is roughly
parallel (17°) to the coordination plane of the rhodium.
The distance between the rhodium center and the
nearest aryl carbon (C(1)) is 2.575(5) Å, indicating that
a weak agostic interaction may exist between these two
atoms. This rhodium-carbon distance is significantly
longer than that in a related “PCP pincer” rhodium(I)
complex containing an η2 agostic Rh‚‚‚C-H bond (2.273-
(5) Å).21b The carbon-carbon bond distances in the
phenyl ring range from 1.362(8) to 1.4223(8) Å, showing
a slight distortion from aromaticity.
The paramagnetism of complexes 15e,g was estab-
lished by several spectroscopic and magnetic techniques.
The 1H NMR spectra of the complexes contain only
broad resonances, which range between 37 and -33
ppm for each compound. In the infrared spectra, the
C-N stretching vibrations appear at lower frequencies
(15e: 1647, 1637 cm-1; 15g: 1654, 1641 cm-1) than
those in the corresponding rhodium(III) complexes 4e
(1670 cm-1) and 4g (1680 cm-1) and those in the
uncoordinated bis(oxazolines) 2e (1665 cm-1) and 2g
(1659 cm-1). The observed shift in the infrared spectra
is consistent with the increased electron-richness of the
rhodium(II) centers over rhodium(III) centers.
The rhodium(II) complexes 15e,g exhibit nearly iden-
tical cyclic voltammograms. Cyclic voltammetry was
performed with a platinum electrode over the scan rate
range of 10-1000 mV‚s-1 at room temperature in
dichloromethane solution with 0.1 M [NBu4]PF6. One
irreversible oxidation wave (Rh(II)/Rh(III)) was observed
at +0.68 V (100 mV/s) for 15e and at +0.80 V (100 mV/
s) for 15g. In addition, irreversible reduction processes
(Rh(II)/Rh(I) were observed at -1.43 V for 15e and at
-1.44 V for 15g.
Syn th esis a n d Ch a r a cter iza tion of Rh od iu m (II)
Com p lexes. When the sterically demanding ligands 2e
and 2g were heated with RhCl3‚3H2O in ethanol, the
monomeric rhodium(II) complexes 15e (19%) and 15g
(49%) were formed in addition to the expected cyclo-
metalated rhodium(III) complexes 4e and 4g (eq 2).
These complexes were cleanly separated by column
chromatography. The yield of the tert-butyl-substituted
rhodium(II) complex 15e was improved by running the
reaction at high dilution, giving an optimized yield of
57%.
X-ray quality crystals were obtained by cooling a
concentrated toluene/pentane solution of 15g. The mo-
lecular structure of 15g is shown in Figure 3. Crystal
data and structure refinement information is presented
in Table 1, and selected interatomic distances and bond
angles are given in Table 4. The complex crystallizes
as discrete molecules with no noticeable interatomic
interactions. The geometry at the rhodium center is
approximately square-planar (maximum deviation of
0.137(2) Å by Cl(2) for the Rh(1)Cl(1)Cl(2)N(1)N(2) least
squares plane). The Rh-Cl distances (2.356(2)/2.365-
(1) Å) are equivalent within 3σ, as are the Rh-N
X-band EPR spectra for 15e,g were obtained in glassy
frozen 2-methyltetrahydrofuran, yielding g-values of g1
) 2.864, g2 ) 2.320, and g3 ) 1.903 for 15e (spectrum
obtained at 57 K) (Figure 4). Doublet splitting from the

Rh Complexes of Bis(oxazoline) Pincer Ligands
Organometallics, Vol. 22, No. 1, 2003 53
Sch em e 5. P r op osed Mech a n ism for
Disp r op or tion a tion
F igu r e 4. X-band EPR spectrum of 15g obtained at 57 K
in glassy frozen THF.
hyperfine interaction with 103Rh (I ) 1/2) was observed
for g2 (A2 ) 14.5 G) and g3 (A3 ) 28.3 G). Complex 15g
also shows three g values: g1 ) 2.926, g2 ) 2.307, and
g3 ) 1.881 (spectrum obtained at 1.2 K). For 15g,
hyperfine coupling to 103Rh of only g3 is observed
(doublet; A3 ) 23.7 Hz; Figure 4). The EPR spectra
acquired for 15e,g are rhombic, consistent with the
asymmetric coordination environments around the metal
centers in these complexes.
The magnetic properties of 15g were investigated
using SQUID magnetometry. Complex 15g exhibits
clean 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, deter-
mined by the Evans method, yielded µeff values of 1.87
µB (15e) and 1.98 µB (15g).24 The solid and solution
phase magnetic measurements are both consistent with
an S ) 1/2 ground state, as expected for a monomeric
low-spin d7 metal complex. The electronic spectra of
The reactions that form 15e,g are heterogeneous (an
orange precipitate forms as the reaction progresses),
which prohibits the use of kinetic tools to study the
mechanisms in detail. Complex 15e is isolated in higher
yield under dilute synthesis conditions, suggesting that
a dinuclear decomposition pathway is competitive with
formation of 15e,g (vide infra).
Stoich iom etr ic Rea ctivity of Rh od iu m (II) Com -
p lexes. When concentrated solutions of 15g are heated
to 75 °C in benzene solution, disproportionation to cyclo-
metalated rhodium(III) dichloride 4g and hydridorhod-
ium(III) dichloride 16g is observed (Scheme 5). The
identity of the hydridorhodium(III) complex 16g was
confirmed by independent synthesis (eq 3). When a
complexes 15e,g display three major bands at λmax
)
410, 309, and 229 nm for 15e and at λmax ) 405, 303,
and 231 nm for 15g. Bands similar to those observed
for 15e,g have been noted for other four-coordinate
monomeric rhodium(II) complexes. In these complexes,
bands near 300 nm have been assigned to ligand-based
π-π* transitions, and those near 400 nm have been
assigned to a ligand-to-metal charge transfer.25 No
bands indicative of d-d transitions were observed in the
visible region of the spectrum.25,26
The chemistry of rhodium is generally associated with
the oxidation states +1 and +3. Although many rhod-
ium(II) dimers have been characterized,27 far fewer
monomeric rhodium(II) complexes have been isola-
ted.25,26,28 In contrast to the vast majority of compounds
containing mononuclear rhodium(II) centers, complexes
15e,g are air-stable and melt cleanly at temperatures
above 180 °C. The stability of complexes 15e and 15g
is presumably due to their sterically congested ligand
environments.
benzene solution of 4g was treated with 35 atm H2,
complete conversion to 16g was observed after 14 h at
75 °C. The hydridorhodium(III) compound was cleanly
isolated in 75% yield as an analytically pure yellow
powder. The chemistry and full characterization of 16g
and a series of related hydridorhodium complexes will
be fully described in a future publication.
(28) (a) DeWitt, D. G. Coord. Chem. Rev. 1996, 147, 209. (b) Billig,
E.; Shupack, S. I.; Waters, J . H.; Williams, R.; Gray, H. B. J . Am. Chem.
Soc. 1964, 86, 926. (c) Bennett, M. A.; Longstaff, P. A. J . Am. Chem.
Soc. 1969, 91, 6266. (d) Masters, C.; Shaw, B. L. J . Chem. Soc. A 1971,
3679. (e) Dunbar, K. R.; Haefner, S. C.; Pence, L. E. J . Am. Chem.
Soc. 1989, 111, 5504. (f) Haefner, S. C.; Dunbar, K. R.; Bender, C. J .
Am. Chem. Soc. 1991, 113, 9540. (g) Wayland, B. B.; Ba, S.; Sherry,
A. E. J . Am. Chem. Soc. 1991, 113, 5305. (h) Zhang, X. X.; Wayland,
B. B. J . Am. Chem. Soc. 1994, 116, 7897. (i) Bunn, A. G.; Wayland, B.
B. J . Am. Chem. Soc. 1992, 114, 6917. (j) Bunn, A. G.; Wei, M. L.;
Wayland, B. B. Organometallics 1994, 13, 3390. (k) Connelly, N. G.;
Emslie, D. J . H.; Metz, B.; Orpen, A. G.; Quayle, M. J . Chem. Commun.
1996, 2289. (l) Zhang, X. X.; Parks, G. F.; Wayland, B. B. J . Am. Chem.
Soc. 1997, 119, 7938. (m) Parimal, P.; Tyagi, B.; Bilakhiya, A. K.;
Bhadbhade, M. M.; Suresh, E. J . Chem. Soc., Dalton Trans. 1999, 2009.
(n) Collman, J . P.; Boulatov, R. J . Am. Chem. Soc. 2000, 122, 11812.
(o) Connelly, N. G.; Emslie, D. J . H.; Geiger, W. E.; Hayward, O. D.;
Linehan, E. B.; Orpen, A. G.; Quayle, M. J .; Rieger, P. H. J . Chem.
Soc., Dalton Trans. 2001, 670. (p) Willems, S. T. H.; Russcher, J . C.;
Budzelaar, P. H. M.; de Bruin, B.; de Gelder, R.; Smits, J . M. M.; Gal,
A. W. Chem. Commun. 2002, 148. (q) Dixon, F. M.; Farrell, J . R.; Doan,
P. E.; Williamson, A.; Weinberger, D. A.; Mirkin, C. A.; Stern, C.;
Incarvito, C. D.; Liable-Sands, L. M.; Zakharov, L. N.; Rheingold, A.
L. Organometallics 2002, 21, 3091.
The formation of rhodium(II) products from ethanolic
solutions of RhCl3(H2O)3 and bulky ligands has been ob-
served previously. These reactions are assumed to occur
via oxidation of the ethanol solvent or excess ligand,
although the mechanisms have not been studied.25,28c
(24) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Evans, D. A.;
Miller, S. J .; Lectka, T. J . Am. Chem. Soc. 1993, 115, 6460.
(25) Pandey, K. K. Coord. Chem. Rev. 1992, 121, 1.
(26) Dunbar, K. R.; Haefner, S. C. Organometallics 1992, 11, 1431.
(27) Doyle, M. P.; Forbes, D. C. Chem. Rev. 1998, 98, 911.

54 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
Sch em e 6. Rea ction s of Rh od iu m (II) Com p lex 15g
access to chiral metal complexes. Although the rhodium-
(III) complexes were found to be inactive for catalysis,
a variety of rare and interesting new compounds were
isolated and fully characterized. The 16-electron rhod-
ium(III) dichloride complexes are stabilized by agostic
interactions with the ligand alkyl groups and can be
trapped by several nucleophilic ligands to give coordi-
natively saturated complexes. Methylrhodium(III) cat-
ionic complexes were synthesized and were shown to
undergo 1,2-methyl migration reactions similar to those
described by Milstein and van Koten. Importantly, the
most sterically demanding benbox ligands allowed for
isolation of mononuclear rhodium(II) complexes, which
were extensively characterized in the solid and solution
phases. Further studies of the catalytic reactivity of the
rhodium(II) compounds are currently underway in our
laboratories.
The decomposition of 15g to 4g and 16g presumably
occurs via a bimolecular hydrogen transfer reaction. A
suggested mechanism for the decomposition reaction is
illustrated in Scheme 5. A rapid, reversible C-H activa-
tion of the proximal ligand aryl C-H bond is proposed
as the first step. C-H activation reactions have been
observed previously with rhodium(II) porphyrin com-
plexes, although these reactions are typically thought
to involve concerted action of two rhodium(II) centers.28g
The proposed decomposition mechanism invokes a di-
nuclear step to accomplish the one-electron oxidation
process. Unfortunately, this reaction was not amenable
to kinetic study due to the slow precipitation of rhodium
metal that is observed during its course.
Exp er im en ta l Section
Gen er a l P r oced u r es. Unless otherwise noted, reactions
and manipulations were performed at 23 °C in an inert
atmosphere (N2) glovebox, or using standard Schlenk and high-
vacuum line techniques. Glassware was dried overnight at 150
°C before use. All NMR spectra were obtained using Bruker
AMX-300, AM-400, or DRX-500 MHz spectrometers. Except
where noted, all NMR spectra were acquired at room temper-
ature. Infrared (IR) spectra were recorded using samples
prepared as KBr pellets, and spectral data are reported in
wavenumbers. Reactions with gases and low-boiling liquids
involved condensation of a calculated pressure of gas from a
bulb of known volume into the reaction vessel at -196 °C.
Known-volume bulb vacuum transfers were accomplished with
a digital MKS Baratron gauge attached to a high-vacuum line.
Mass spectrometric (MS) analyses were obtained at the
University of California, Berkeley Mass Spectrometry Facility,
on VT ProSpec, ZAB2-EQ, and 70-FE mass spectrometers.
Unless otherwise noted, all FABMS data were acquired from
samples in a methylene chloride/3-nitrobenzyl alcohol matrix.
Elemental analyses were performed at the University of
California, Berkeley Microanalytical Facility, on a Perkin-
Elmer 2400 Series II CHNO/S analyzer. Optical rotation
measurements were performed on a Perkin-Elmer polarimeter
(sodium D line). X-band EPR spectra of samples in glassy
2-methyltetrahydrofuran were recorded in deoxygenated EPR
tubes at both 57 and 1.2 K on a Bruker EMX-300 EPR.
All electrochemical experiments were performed with a
Bioanalytical Systems (BAS) CV-50W voltammetric analyzer
at room temperature using a Pt working electrode (area ) 0.03
cm2), a Pt-wire auxiliary electrode, and a BAS nonaqueous
Ag/AgNO3 reference electrode containing 0.01 M AgNO3 in a
0.1 M tetra-n-butylammonium hexafluorophosphate (TBAH)
methylene chloride solution. A normal three-electrode config-
uration was used, and the cell contained a 0.3 M solution of
supporting electrolyte (TBAH) in THF. All potentials are
reported versus Ag/AgNO3 and were not corrected for liquid
junction potential. The standard current convention is used
(anodic currents are negative). Potentials measured versus the
ferrocene/ferrocenium couple, Fc0/+, were obtained by external
calibration, due to the reactivity of the compounds with
ferrocene. Controlled potential coulometry experiments were
carried out on stirred solutions using a vitrious carbon elec-
trode, a Pt auxiliary electrode, and a nonaqueous Ag/AgNO3
reference electrode. The auxiliary electrode was separated from
the working compartment by a fritted glass salt bridge.
Ma ter ia ls. Unless otherwise noted, reagents were pur-
chased from commercial suppliers and used without further
purification. Pentane, hexanes, methylene chloride, and ben-
zene (Fisher) were passed through a column of activated
Reactions of complex 15g with several reducing
agents (trimethylphosphine, sodium, magnesium, tert-
butylisonitrile, and pyridine or Bu3SnH) led to clean
formation of free ligand 2g and unidentifiable rhodium
products (Scheme 6). The rhodium(III) complex 4g was
obtained in 56% yield by exposing a benzene-d6 solution
of 15g to 800 mbar of O2 and heating to 45 °C for 2 days,
or in 92% yield by heating a benzene-d6 solution of 15g
with CCl4 at 75 °C for 6 days, or in 80% yield by reaction
of 15g with CuCl2 in methylene chloride at room
temperature for 2 days (Scheme 6). These reactions
demonstrate that it is possible to oxidize the hindered
rhodium(II) complex by chemical means, despite the
compound’s stability to air. No reaction was observed
when complex 15g was treated with benzyl chloride,
triethylamine, methane, methyl iodide, acetonitrile, tert-
butylnitrile, or lead dichloride.
A few monomeric 15e- rhodium(II) complexes have
been described previously (either as isolable compounds
or as transiently generated species).26,28h,n,29 Several of
these compounds react readily with donor ligands (CO,
nitriles, phosphines) to form 17e- complexes or undergo
redox processes.28h,n,29a Given this precedent, the lack
of similar trapping reactions (vide supra) with 15g is
somewhat surprising. This result is similar to the obser-
vation that the rhodium(III) complexes (4a -g) in this
ligand series are isolated as water-free 16e- complexes.
The steric bulk of the ligand presumably inhibits coor-
dination of additional ligands in the open coordination
sites of both the rhodium(II) and rhodium(III) com-
plexes.
Con clu sion
We have prepared a series of new NCN bis(oxazoline)
pincer (“benbox”) ligands and used these ligands to
prepare unique, coordinatively unsaturated rhodium-
(III) compounds and mononuclear rhodium(II) com-
plexes. The ligands are readily synthesized from com-
mercially available amino alcohols, providing easy
(29) (a) Poszmik, G.; Carroll, P. J .; Wayland, B. B. Organometallics
1993, 12, 3410. (b) Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J . Am.
Chem. Soc. 1993, 115, 7675.

Rh Complexes of Bis(oxazoline) Pincer Ligands
Organometallics, Vol. 22, No. 1, 2003 55
alumina (type A2, size 12 × 32, Purifry Co.) under nitrogen
pressure and sparged with N2 prior to use. Diethyl ether and
tetrahydrofuran (Fisher) were distilled from purple sodium/
benzophenone ketyl under N2 prior to use. Deuterated solvents
(Cambridge Isotope Laboratories) were either purified by
vacuum transfer from the appropriate drying agent (Na/Ph2-
CO or CaH2) prior to use (C6D6, CD2Cl2) or used as received
(CDCl3). Chlorobenzene and dimethyl sulfoxide were used as
received under nitrogen. Trimethylphosphine (Aldrich) was
vacuum-transferred from sodium metal prior to use. Tetra-n-
butylammonium hexafluorophosphate (Aldrich) was dried
under vacuum for 24 h and stored in a glovebox. Spectral and
analytical data for compounds 1b-g, 2b-g, 3d -g, and 4e-g
are provided in the Supporting Information.
N, 9.32. Found: C, 71.81; H, 8.05; N, 9.32. IR (KBr, cm-1):
1663 (νCN). 1H NMR (300 MHz, CDCl3): δ 7.08 (s, 1H, CH),
6.97 (s, 1H, CH), 4.30 (dd, 2H, ox-CH2), 4.14 (m, 2H, ox-CH),
3.76 (dd, 2H, ox-CH2), 3.56 (s, 4H, CH2), 2.27 (s, 6H, CH3),
1.23 (d, 6H, ox-CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 165.5,
135.7, 132.5, 131.4, 131.2, 74.0, 61.4, 32.3, 21.4, 19.1.
Gen er a l P r oced u r e for P r ep a r a tion of Rh od iu m Com -
p lexes 3 a n d 4. To a solution of the bis(oxazoline) 1 or 2 (1.3
mmol) in 10 mL of degassed ethanol at 70 °C was added RhCl3‚
3H2O (263 mg, 1.0 mmol). The resulting orange-red suspension
was heated at reflux for 5-30 min. After evaporation to
dryness at room temperature the product was isolated by
column chromatography on silica (CH2Cl2) as an air-stable
orange-red microcrystalline solid.
1,3-Bis(ch lor om eth yl)-4,6-dim eth ylben zen e.16 A l L three-
necked flask was loaded with m-xylene (20 mL, 164 mmol),
paraformaldehyde (11.6 g, 363 mmol), 160 mL of concentrated
hydrochloric acid, and 40 mL of glacial acetic acid. The mixture
was heated at 69-70 °C for 2 days, over which time the
solution turned colorless and a white precipitate formed. The
solution was filtered and extracted with methylene chloride
(5 × 50 mL). The organic phases were combined and rinsed
with aqueous 10% NaHCO3 solution (3 × 50 mL) and water
(3 × 50 mL). The organic phase was dried with sodium sulfate,
filtered, and evaporated to dryness in vacuo to give a white
solid. The product was purified by crystallization from hexane.
[Rh Cl2{(S,S)-m e-ben box}] (3a ): yield 84 mg, 19% (30 min
reflux). Mp: >260 °C. Rf ) <0.01 (methylene chloride). Anal.
Calcd for
C16H19Cl2N2O2Rh: C, 43.17; H, 4.30; N, 6.29.
Found: C, 42.91; H, 4.31; N, 5.98. IR (KBr, cm-1): 1668 (νCN).
1H NMR (300 MHz, C6D6): δ 6.80 (t, 1H, CH), 6.56 (d, 2H,
CH), 4.78 (d, 2H, CH2, 2J H-H ) 17.7 Hz), 4.60 (m, 2H, ox-CH),
3.69 (dd, 2H, ox-CH2), 3.44 (d, 2H, CH2, 2J H-H ) 17.7 Hz), 3.35
(t, 2H, ox-CH2), 1.38 (d, 6H, ox-CH3). 13C{1H} NMR (126 MHz,
1
CDCl3): δ 170.5, 139.0, 131.4 (d, J Rh-C ) 34.5 Hz), 127.6,
124.8, 76.2, 59.9, 36.3, 20.8.
[Rh Cl2{(S,S)-et-ben box}] (3b): yield 180 mg, 38% (30 min
reflux). Mp: >260 °C. Rf ) 0.27 (methylene chloride). Anal.
1
Yield: 21.1 g, 63%. Mp: 95 °C. H NMR (300 MHz, C6D6): δ
Calcd for
C18H23Cl2N2O2Rh: C, 45.69; H, 4.90; N, 5.92.
Found: C, 45.53; H, 4.71; N, 5.83. IR (KBr, cm-1): 1677 (νCN).
1H NMR (300 MHz, C6D6): δ 6.76 (t, 1H, CH), 6.53 (d, 2H,
CH), 4.98 (d, 2H, CH2, 2J H-H ) 17.6 Hz), 4.47 (m, 2H, ox-CH),
3.59 (dd, 2H, ox-CH2), 3.47 (t, 2H, ox-CH2), 3.28 (d, 2H, CH2,
2J H-H ) 17.6 Hz), 1.73 (sept, 2H, Et-CH2), 1.56 (sept, 2H, Et-
CH2), 1.08 (d, 6H, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ
6.76 (s, 1H, CH), 6.59 (s, 1H, CH), 4.07 (s, 4H, CH2), 2.03 (s,
6H, CH3). 13C{1H} NMR (101 MHz, C6D6): δ 138.2, 134.1,
133.6, 131.9, 44.7, 18.5.
(4,6-Dim eth yl-m -p h en ylen e)d ia ceton itr ile.17 In a 500
mL round-bottom flask, 10.8 g (220 mmol) of sodium cyanide
was dried under vacuum at 100 °C for 30 min. The flask was
cooled to 10-15 °C, and dry dimethyl sulfoxide (200 mL) was
added carefully over 5 min. After this time, 20.3 g (100 mmol)
of 1,3-bis(chloromethyl)-4,6-dimethylbenzene was added over
10 min, producing a light yellow suspension. The mixture was
heated with stirring at 40-50 °C for 2 h. After cooling to 25
°C, this suspension was poured onto 1 L of water, then
extracted with benzene (6 × 80 mL). The combined organic
extracts were dried over sodium sulfate. After filtration, the
solvent was removed by rotary evaporation to give a yellow
oil. Methanol (50 mL) was added to the oil, and the mixture
was cooled to 0 °C. Colorless crystals precipitated and were
2
1
170.5, 140.1 (d, J Rh-C ) 2.1 Hz), 134.6 (d, J Rh-C ) 34.5 Hz),
127.7, 124.7, 73.8, 65.3, 36.9, 27.8, 11.4.
[Rh Cl2{(S,S)-ip -ben box}] (3c): yield 295 mg, 59% (30 min
reflux). Mp: >260 °C. Rf ) 0.42 (methylene chloride). Anal.
Calcd for
C20H27Cl2N2O2Rh: C, 47.92; H, 5.43; N, 5.59.
Found: C, 47.91; H, 5.51; N, 5.70. IR (KBr, cm-1): 1674 (νCN).
1H NMR (300 MHz, C6D6): δ 6.76 (t, 1H, CH), 6.54 (d, 2H,
CH), 5.05 (d, 2H, CH2, 2J H-H ) 16.8 Hz), 4.52 (m, 2H, ox-CH),
3.69 (t, 2H, ox-CH2), 3.57 (t, 2H, ox-CH2), 3.29 (d, 2H, CH2,
2J H-H ) 16.8 Hz), 2.10 (sept, 2H, i-Pr-CH), 1.03 (d, 6H, CH3),
0.97 (d, 6H, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 170.6,
1
2
1
collected by filtration. Yield: 13.8 g, 75%. Mp: 88-90 °C. H
139.5 (d, J Rh-C ) 1.6 Hz), 133.3 (d, J Rh-C ) 36.9 Hz), 127.9,
125.1, 71.7, 69.4, 36.4, 31.3, 20.1, 17.4.
NMR (300 MHz, C6D6): δ 7.27 (s, 1H, CH), 7.07 (s, 1H, CH),
3.64 (s, 4H, CH2), 2.32 (s, 6H, CH3). 13C{1H} NMR (101 MHz,
C6D6): δ 136.3, 133.2, 129.3, 127.8, 21.1, 18.7.
[Rh Cl2{(S,S)-m e-ben box(Me2)}] (4a ): yield 227 mg, 48%
(30 min reflux). Mp: >260 °C. Rf ) 0.05 (methylene chloride).
Anal. Calcd for C18H23Cl2N2O2Rh: C, 45.69; H, 4.90; N, 5.92.
Found: C, 45.67; H, 5.10; N, 5.74. IR (KBr, cm-1): 1676 (νCN).
1H NMR (300 MHz, C6D6): δ 6.52 (s, 1H, CH), 4.63 (d, 2H,
CH2, 2J H-H ) 17.6 Hz), 4.53 (m, 2H, ox-CH), 3.64 (d, 2H, CH2,
2J H-H ) 17.6 Hz), 3.61 (t, 2H, ox-CH2), 3.33 (t, 2H, ox-CH2),
1.88 (s, 6H, CH3), 1.39 (d, 6H, ox-CH3). 13C{1H} NMR (126
MHz, CDCl3): δ 170.5, 135.0 (d, 2J Rh-C ) 2.1 Hz), 133.7, 130.5,
Gen er a l P r oced u r e for th e P r ep a r a tion of Liga n d s 1
a n d 2. A 250 mL three-necked flask was charged with Cd-
(OAc)2‚2H2O (266 mg, 1.0 mmol), 20 mmol of the appropriate
dinitrile, 75 mmol of the appropriate amino alcohol, and 80
mL of chlorobenzene. The mixture was heated at reflux for
2-6 days under nitrogen. The solvent was removed under
reduced pressure, giving an oily residue. The residue was
purified by flash chromatography on silica gel (gradient 50%-
80% ethyl acetate/hexane) to afford the desired bis(2-oxazoline)
as either an off-white solid or a light yellow oily solid.
1,3-Bis[{4′-(S)-m et h yloxa zolin -2′-yl}m et h yl]b en zen e,
(S,S)-m e-ben box (1a ): off-white solid, yield 3.54 g, 65%. Mp:
56-58 °C. Rf ) 0.03 (ethyl acetate). [R]D ) -39.6°. Anal. Calcd
for C16H20N2O2: C, 70.58; H, 7.40; N, 10.29. Found: C, 70.88;
H, 7.39; N, 10.17. IR (KBr, cm-1): 1661 (νCN). 1H NMR (300
MHz, CDCl3): δ 7.21-7.09 (m, 4H, CH), 4.18 (dd, 2H, ox-CH2),
4.06 (m, 2H, ox-CH), 3.62 (dd, 2H, ox-CH2), 3.46 (s, 4H, CH2),
1.13 (d, 6H, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 164.9,
134.9, 128.9, 128.1, 126.9, 73.5, 60.8, 34.0, 20.8.
1
130.3 (d, J Rh-C ) 36.8 Hz), 76.1, 59.7, 31.4, 21.3, 20.8. FAB-
MS (m/z): 437.1 [M - HCl]+, 402.1 [M - HCl - Cl]+.
[Rh Cl2{(S,S)-et-ben box(Me2)}] (4b): yield 250 mg, 50%
(30 min reflux). Mp: >260 °C. Rf ) 0.25 (methylene chloride).
Anal. Calcd for C20H27Cl2N2O2Rh: C, 47.92; H, 5.43; N, 5.59.
Found: C, 47.62; H, 5.65; N, 5.53. IR (KBr, cm-1): 1676 (νCN).
1H NMR (300 MHz, C6D6): δ 6.55 (s, 1H, CH), 4.79 (d, 2H,
CH2, 2J H-H ) 17.6 Hz), 4.53 (m, 2H, ox-CH), 3.65 (d, 2H, CH2,
2J H-H ) 17.6 Hz), 3.61 (t, 2H, ox-CH2), 3.51 (t, 2H, ox-CH2),
1.92 (s, 6H, CH3), 1.76 (sept, 2H, Et-CH2), 1.61 (sept, 2H, Et-
CH2), 1.11 (t, 6H, Et-CH3). 13C{1H} NMR (126 MHz, C6D6): δ
2
1
171.5, 136.4 (d, J Rh-C ) 2.1 Hz), 135.6 (d, J Rh-C ) 36.8 Hz),
134.2, 130.3, 74.6, 66.1, 32.9, 28.7, 22.0, 12.1. FABMS (m/z):
500.1 [M]+, 465.1 [M - Cl]+, 429.2 [M - Cl - HCl]+.
1,3-Bis[{4′-(S)-m eth yloxazolin -2′-yl}m eth yl]-4,6-dim eth -
ylben zen e, (S,S)-m e-ben box(Me2) (2a ): off-white solid,
yield 3.27 g, 63%. Mp: 58-59 °C. Rf ) 0.12 (ethyl acetate).
[R]D ) -50.7°. Anal. Calcd for C18H24N2O2: C, 71.97; H, 8.05;
[Rh Cl2{(S,S)-ip -ben box(Me2)}] (4c): yield 319 mg, 52%
(30 min reflux). Mp: >260 °C. Rf ) 0.49 (methylene chloride).

56 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
Anal. Calcd for C22H31Cl2N2O2Rh‚CH2Cl2: C, 44.97; H, 5.42;
N, 4.56. Found: C, 45.06; H, 5.68; N, 4.48. IR (KBr, cm-1):
1680 (νCN). 1H NMR (300 MHz, C6D6): δ 6.54 (s, 1H, CH), 4.92
the alumina was washed with methylene chloride (2 × 5 mL).
A bright yellow microcrystalline solid was obtained after the
solvent had been removed from the filtrate under vacuum.
[Rh Me(Cl){(S,S)-ip -ben box)}] (7c): yield 89 mg, 37%.
Mp: 92-94 °C (dec). Anal. Calcd for C21H30ClN2O2Rh: C,
52.45; H, 6.29; N, 5.83. Found: C, 52.21; H, 6.51; N, 5.47. IR
2
(d, 2H, CH2, J H-H ) 17.5 Hz), 4.60 (m, 2H, ox-CH), 3.74 (t,
2
2H, ox-CH2), 3.69 (d, 2H, CH2, J H-H ) 17.5 Hz), 3.61 (t, 2H,
ox-CH2), 2.17 (sept, 2H, i-Pr-CH), 1.94 (s, 6H, CH3), 1.08 (d,
6H, i-Pr-CH3), 1.01 (d, 6H, i-Pr-CH3). 13C{1H} NMR (126 MHz,
1
(KBr, cm-1): 1673 (νCN). H NMR (300 MHz, C6D6): δ 6.79 (t,
2
1
C6D6): δ 171.0, 136.4 (d, J Rh-C ) 2.0 Hz), 135.5 (d, J Rh-C
)
1H, CH), 6.61 (‘t’, 2H, CH), 5.12 (m, 2H), 3.95 (m, 2H), 3.72-
2
36.9 Hz), 133.4, 130.3, 71.3, 69.4, 32.0, 31.6, 21.2, 19.8, 17.8.
3.55 (m, 4H), 3.45 (d, 2H, CH2, J H-H ) 16.8 Hz), 3.32 (d, 1H,
FABMS (m/z): 493.1 [M - Cl]+, 457.1 [M - 2HCl]+.
2
CH2, J H-H ) 18.8 Hz), 2.06 (sept, 1H, i-Pr-CH), 1.89 (sept,
2
1H, i-Pr-CH), 1.16 (d, 3H, Rh-CH3, J Rh-H ) 2.0 Hz), 0.82 (d,
[Rh Cl2{(S,S)-ib-ben box(Me2)}] (4d ): yield 228 mg, 41%
(30 min reflux). Mp: >260 °C. Rf ) 0.45 (methylene chloride).
Anal. Calcd for C24H35Cl2N2O2Rh: C, 51.72; H, 6.33; N, 5.03.
Found: C, 51.46; H, 6.49; N, 4.92. IR (KBr, cm-1): 2764, 2756
(νCH), 1683 (νCN). 1H NMR (300 MHz, C6D6): δ 6.54 (s, 1H,
CH), 4.82 (d, 2H, CH2, 2J H-H ) 17.0 Hz), 4.58 (m, 2H, ox-CH),
3H, CH3), 0.76 (d, 3H, CH3), 0.72 (d, 3H, CH3), 0.66 (d, 3H,
CH3). 13C{1H} NMR (126 MHz, C6D6): δ 169.8, 168.8, 146.6
1
(d, J Rh-C ) 41.6 Hz), 139.3, 138.8, 127.0, 126.7, 123.0, 71.0,
70.6, 69.9, 69.4, 37.6, 36.2, 31.6, 30.5, 19.9, 19.6, 17.1, 16.4,
1
-5.6 (d, J Rh-C ) 28.0 Hz).
2
3.70 (d, 2H, CH2, J H-H ) 17.0 Hz), 3.67 (t, 2H, ox-CH2), 3.50
[Rh Me(Cl){(S,S)-ip -ben box(Me2)}] (8c): yield 91 mg,
36%. Mp: 136-140 °C (dec). Anal. Calcd for C23H34ClN2O2-
Rh: C, 54.29; H, 6.73; N, 5.50. Found: C, 54.36; H, 6.65; N,
(t, 2H, ox-CH2), 2.02 (sept, 2H, i-Bu-CH), 1.96 (s, 6H, CH3),
1.54 (sept, 2H, i-Bu-CH2), 1.20 (dt, 2H, i-Bu-CH2), 1.08 (d, 6H,
i-Bu-CH3), 1.04 (d, 6H, i-Bu-CH3). 13C{1H} NMR (126 MHz,
1
5.18. IR (KBr, cm-1): 1672 (νCN). H NMR (300 MHz, C6D6):
2
1
C6D6): δ 170.4, 136.1 (d, J Rh-C ) 2.2 Hz), 134.8 (d, J Rh-C
)
δ 6.54 (s, 1H, CH), 5.17 (m, 2H), 3.90 (m, 1H, ox-CH), 3.81-
3.50 (m, 7H), 2.09 (sept, 1H, i-Pr-CH), 2.00 (s, 3H, CH3), 1.99
(s, 3H, CH3), 1.88 (sept, 1H, i-Pr-CH), 1.18 (d, 3H, Rh-CH3,
2J Rh-H ) 2.0 Hz), 0.88 (d, 3H, i-Pr-CH3), 0.80 (d, 3H, i-Pr-CH3),
0.74 (d, 3H, i-Pr-CH3), 0.72 (d, 3H, i-Pr-CH3). 13C{1H} NMR
37.5 Hz), 133.3, 130.2, 74.6, 62.8, 44.8, 32.1, 27.9, 22.7 (2×),
21.2. FABMS (m/z): 520.2 [M - HCl]+, 485.2 [M - Cl - HCl]+.
[Rh Cl2{(S,S)-ib-ben box(Me2)}(CO)] (5d ). An orange solu-
tion of [RhCl2){(S,S)-ib-benbox(Me2)}] (4d ) (55.7 mg, 0.10
mmol) in benzene (10 mL) was exposed to 1 atm of CO at
ambient temperature. This solution was heated at 75 °C,
producing a light yellow suspension within 3 days. The volatile
materials were removed under reduced pressure, and the
resulting pale orange solid was washed with pentane (3 × 10
mL). Anal. Calcd for C25H35Cl2N2O2Rh: C, 51.30; H, 6.03; N,
4.79. Found: C, 51.68; H, 6.35; N, 4.43. IR (KBr, cm-1): 2070
(νCO), 1666 (νCN). 1H NMR (300 MHz, C6D6): δ 6.61 (s, 1H, CH),
5.21 (m, 2H, ox-CH), 4.99 (d, 1H, CH2, 2J H-H ) 14.4 Hz), 3.85-
3.54 (m, 6H, CH2, ox-CH2), 3.37 (d, 1H, CH2, 2J H-H ) 14.7 Hz),
2.38 (dt, 1H, i-Bu-CH), 1.93 (s, 3H, CH3), 1.90 (s, 3H, CH3),
1.33 (dt, 1H, i-Bu-CH), 1.25-1.11 (m, 4H, i-Bu-CH2), 1.07 (d,
3H, i-Bu-CH3), 1.01 (d, 3H, i-Bu-CH3), 0.83 (d, 3H, i-Bu-CH3),
0.77 (d, 3H, i-Bu-CH3). 13C{1H} NMR (126 MHz, C6D6): δ 180.1
1
(126 MHz, C6D6): δ 170.2, 168.5, 145.7 (d, J Rh-C ) 43.6 Hz),
135.3, 135.0, 132.3, 132.1, 128.5, 71.0, 70.5, 69.2, 67.8, 32.3,
31.7, 31.6, 30.7, 21.2, 21.1, 19.9, 19.7, 17.2, 16.5, -5.5 (d, 1J Rh-C
) 28.7 Hz). EIMS (m/z): 508 [M]+, 472 [M - HCl]+, 370 [M -
HCl - Rh]+.
Gen er a l P r oced u r e for Syn th esis of Rh od iu m Com -
p lexes 9c a n d 10c. To a yellow methylene chloride (5 mL)
solution of [RhMe(Cl){(S,S)-ip-benbox}] (7c) (48 mg, 0.10
mmol) or [RhMe(Cl){(S,S)-ip-benbox(Me2)}] (8c) (51 mg, 0.10
mmol) was added AgOTf (25.7 mg, 0.10 mmol), producing a
yellow suspension. After stirring overnight, the bright orange-
yellow suspension was filtered. The methylene chloride was
removed under vacuum, leaving a yellow microrystalline solid.
Crystals suitable for X-ray diffraction were obtained by layer-
ing a concentrated methylene chloride solution of 9c with
pentane.
1
(d, J Rh-C ) 61.2 Hz), 171.6, 170.1, 142.0 (br d, ipso-C), 134.6,
133.2, 133.1, 130.8, 130.7, 75.1, 73.6, 66.3, 64.5, 43.4, 42.8, 33.6,
33.1, 26.3, 25.5, 24.2, 24.1, 22.6, 21.4, 21.3, 21.2. EI-MS (m/z):
556 [M - CO]+, 520 [M - CO - HCl]+, 484 [M - CO - 2HCl]+.
[Rh Cl2{(S,S)-ib-ben box(Me2)}(tBu NC)] (6d ). To an or-
ange solution of [RhCl2){(S,S)-ib-benbox(Me2)}] (4d ) (55.7 mg,
0.10 mmol) in methylene chloride (10 mL) at ambient tem-
[Rh Me(OTf){(S,S)-ip -ben box}] (9c): yield 55 mg, 93%.
Mp: 128-132 °C (dec). Anal. Calcd for C22H30F3N2O5RhS: C,
44.45; H, 5.09; N, 4.71. Found: C, 44.51; H, 5.11; N, 4.67. IR
1
(KBr, cm-1): 1669 (νCN). H NMR (300 MHz, C6D6): δ 6.74 (t,
1H, CH), 6.53 (d, 1H, CH), 6.49 (d, 1H, CH), 4.68 (m, 1H, ox-
t
2
perature was added BuNC (8.3 mg, 0.10 mmol), producing an
CH), 4.58 (d, 1H, CH2, J H-H ) 18.4 Hz), 3.73-3.62 (m, 6H),
2
2
immediate yellow suspension. After stirring overnight the
mixture was filtered. A yellow microcrystalline solid was
obtained after removing the solvent. The compound was
purified by column chromatography with methylene chloride
as eluent. Yield: 52.5 mg, 82%. Mp: >260 °C. Anal. Calcd for
3.48 (d, 1H, CH2, J H-H ) 18.4 Hz), 3.32 (d, 1H, CH2, J H-H
)
19.2 Hz), 2.14 (sept, 1H, i-Pr-CH), 1.91 (sept, 1H, i-Pr-CH),
2
1.07 (d, 3H, Rh-CH3, J Rh-H ) 2.4 Hz), 0.72 (d, 3H, CH3), 0.68
(d, 3H, CH3), 0.55 (d, 3H, CH3), 0.53 (d, 3H, CH3). 13C{1H}
2
NMR (126 MHz, C6D6): δ 170.5, 170.3 (d, J Rh-C ) 1.7 Hz),
1
2
C
29H44Cl2N3O2Rh: C, 54.38; H, 6.92; N, 6.56. Found: C, 54.10;
141.2 (d, J Rh-C ) 45.6 Hz), 139.0 (d, J Rh-C ) 2.0 Hz), 138.8,
128.3, 127.3, 123.9, 71.1, 70.3, 70.2, 69.7, 36.5 (d, 3J Rh-C ) 1.4
Hz), 36.1 (d, 3J Rh-C ) 1.4 Hz), 31.5, 29.7, 22.2, 19.2, 16.7, 14.9,
-8.3 (d, 1J Rh-C ) 31.6 Hz); the 13C{1H} NMR resonance of the
triflate ligand was not observed. 19F NMR (376.1 MHz, C6D6):
δ -77.7. EIMS (m/z): 594 [M]+, 579 [M - CH3]+, 444 [M -
HOTf]+.
H, 6.98; N, 6.28. IR (KBr, cm-1): 2177, 1669 (νCN). 1H NMR
(300 MHz, C6D6): δ 6.76 (s, 1H, CH), 5.0-4.3 (br, 4H), 4.41
(m, 2H), 3.82 (m, 2H), 3.64 (t, 2H), 2.64 (br, 2H, i-Bu-CH), 2.16
(s, 6H, CH3), 1.37 (m, 2H, i-Bu-CH2), 1.30 (m, 2H, i-Bu-CH2),
1.21 (s, 9H, t-BuNC-CH3), 0.86 (d, 6H, i-Bu-CH3), 0.72 (d, 6H,
i-Bu-CH3). EI-MS (m/z): 556 [M - t-BuNC]+, 520 [M - t-BuNC
- HCl]+, 484 [M - t-BuNC - 2HCl]+.
[Rh Me(OTf){(S,S)-ip -ben box(Me2)}] (10c): yield 59 mg,
95%. Mp: 108-110 °C (dec). Anal. Calcd for C24H34F3N2O5-
RhS: C, 46.31; H, 5.51; N, 4.50. Found: C, 46.54; H, 5.24; N,
Gen er a l P r oced u r e for Syn th esis of Rh od iu m Com -
p lexes 7c a n d 8c. A 2.0 M solution of dimethylzinc in toluene
(0.5 mL, 1.0 mmol) was added dropwise at -30 °C to an orange
solution of [RhCl2{(S,S)-ip-benbox}] (3c) (250 mg, 0.5 mmol)
or [RhCl2{(S,S)-ip-benbox(Me2)}] (4c) (265 mg, 0.5 mmol) in
10 mL of tetrahydrofuran. The mixture was warmed to room
temperature and was then stirred for 2 h, giving a yellow
cloudy solution. The solution was concentrated to dryness in
vacuo. The yellow-brown residue was dissolved in methylene
chloride (10 mL), and 1 g of alumina(III) was added. After
stirring for 2 h, the bright yellow suspension was filtered and
1
4.23. IR (KBr, cm-1): 1678 (νCN). H NMR (300 MHz, C6D6):
δ 6.49 (s, 1H, CH), 4.76 (m, 1H, ox-CH), 4.56 (d, 1H, CH2, 2J H-H
) 18.0 Hz), 3.81-3.55 (m, 7H), 3.44 (d, 1H, CH2, 2J H-H ) 18.8
Hz), 2.21 (sept, 1H, i-Pr-CH), 1.99 (sept, 1H, i-Pr-CH), 1.92
(s, 3H, CH3), 1.89 (s, 3H, CH3), 1.13 (d, 3H, Rh-CH3, 2J Rh-H
)
2.8 Hz), 0.75 (d, 3H, i-Pr-CH3), 0.72 (d, 3H, i-Pr-CH3), 0.61 (d,
3H, i-Pr-CH3), 0.55 (d, 3H, i-Pr-CH3). 13C{1H} NMR (126 MHz,
1
C6D6): δ 171.0, 170.2, 141.6 (d, J Rh-C ) 45.0 Hz), 135.1 (d,
2J Rh-C ) 1.6 Hz), 134.8, 133.1, 132.9, 129.5, 72.4, 70.2 (2×),

Rh Complexes of Bis(oxazoline) Pincer Ligands
Organometallics, Vol. 22, No. 1, 2003 57
69.8, 32.0, 31.4 (2×), 29.9, 22.8, 21.1, 20.9, 19.4, 17.0, 15.1,
-8.0 (d, 1J Rh-C ) 30.0 Hz); the 13C{1H} NMR resonance of the
triflate ligand was not observed. 19F NMR (376.1 MHz, C6D6):
δ -77.7. EIMS (m/z): 622 [M]+, 607 [M - CH3]+, 472 [M -
HOTf]+.
CH), 7.15 (s, 1H, CH), 4.44 (m, 4H, ox-CH2), 4.17 (d, 1H, CH2,
2
2J H-H ) 17.2 Hz), 4.06 (d, 1H, CH2, J H-H ) 18.8 Hz), 3.92 (d,
2
2
1H, CH2, J H-H ) 5.2 Hz), 3.89 (d, 1H, CH2, J H-H ) 2.8 Hz),
3.70 (m, 1H, ox-CH), 3.59 (m, 1H, ox-CH), 2.86 (s, 3H, CH3),
2.32 (m, 1H, i-Pr-CH), 2.28 (m, 1H, i-Pr-CH), 2.21 (s, 3H, CH3),
2.19 (s, 3H, CH3), 0.91 (d, 3H, i-Pr-CH3), 0.86 (d, 3H, i-Pr-
CH3), 0.71 (d, 3H, i-Pr-CH3), 0.56 (d, 3H, i-Pr-CH3). 19F NMR
(376.1 MHz, CD2Cl2): δ -61.0.
[Rh Me{(S,S)-ip -ben box(Me2)}][BAr f] (11c). A yellow so-
lution of [RhMe(OTf){(S,S)-ip-benbox(Me2)}] (10c) (62.3 mg,
0.10 mmol) in methylene chloride (10 mL) was prepared.
NaBArf (88.6 mg, 0.10 mmol) was added at ambient temper-
ature, producing a cloudy yellow mixture. After stirring for 4
h at room temperature, the mixture was filtered and the
solvent was removed under vacuum. The resulting oil was
triturated with pentane (3 × 5 mL), and a yellow microrys-
talline solid was obtained after the residul solvent was
removed under vacuum. Yield: 138 mg, 97%. Mp: 72-74 °C
(dec). Anal. Calcd for C55H46F24N2O2RhB: C, 49.42; H, 3.47;
N, 2.10. Found: C, 49.72; H, 3.47; N, 2.00. IR (KBr, cm-1):
[Rh (CO)2{(S,S)-ip -ben box(Me2)(2-Me)}][BAr f] (14c). In
the glovebox, a Teflon-stoppered sealable 50 mL reaction vessel
was charged with 11c (55.2 mg, 0.041 mmol) and 10 mL of
CH2Cl2. The solution was degassed with three freeze-pump-
thaw cycles, then exposed to approximately 700 mbar of CO,
and after 14 h at room temperature the volatile materials were
removed under vacuum. A yellow microcrystalline solid was
isolated. The same procedure was used to synthesize the 13C-
enriched isomer with isotopically enriched 13CO for identifica-
tion of the IR bands and 13C NMR shifts. Yield: 54.0 mg, 96%.
Anal. Calcd for C57H46F24N2O4RhB: C, 49.15; H, 3.33; N, 2.01.
Found: C, 49.55; H, 3.51; N, 1.71. IR (KBr): 2094, 2029 (ν12CO),
1
2809 (νCH), 1678, 1610 (νCN). H NMR (300 MHz, CD2Cl2): δ
7.74 (s, 8H, BArf-CH), 7.58 (s, 4H, BArf-CH), 6.83 (s, 1H, CH),
4.60 (t, 1H, ox-CH2), 4.44 (dd, 1H, ox-CH2), 4.39 (t, 1H, ox-
CH2), 4.34 (t, 1H, ox-CH2), 4.05 (m, 2H, ox-CH), 4.00 (d, 1H,
1
2052, 1993 (ν13CO), 1614 (νCN). H NMR (300 MHz, CD2Cl2): δ
2
7.72 (s, 8H, BArf-CH), 7.56 (s, 4H, BArf-CH), 7.24 (s, 1H, CH),
4.60-4.35 (m, 4H), 4.11-3.84 (m, 6H), 2.43 (s, 3H, CH3), 2.40
(s, 3H, CH3), 2.15 (m, 2H, i-Pr-CH), 1.91 (s, 3H, CH3), 0.96 (d,
3H, i-Pr-CH3), 0.91 (d, 3H, i-Pr-CH3), 0.83 (d, 3H, i-Pr-CH3),
CH2, J H-H ) 18.8 Hz), 3.92 (s, 2H, CH2), 3.88 (d, 1H, CH2,
2J H-H ) 18.8 Hz), 2.26 (s, 3H, CH3), 2.25 (s, 3H, CH3), 1.94
(sept, 1H, i-Pr-CH), 1.89 (sept, 1H, i-Pr-CH), 1.32 (d, 3H, Rh-
CH3, 2J Rh-H ) 2.0 Hz), 0.91 (d, 6H, i-Pr-CH3), 0.89 (d, 3H, i-Pr-
CH3), 0.82 (d, 3H, i-Pr-CH3). 13C{1H} NMR (126 MHz, CD2-
0.80 (d, 3H, i-Pr-CH3). 13C{1H} NMR (125 MHz, CD2Cl2): δ
2
180.3 (identified only in 13C-enriched spectrum, dd, J C-C
)
2
1
Cl2): δ 172.0 (d, J Rh-C ) 2.3 Hz), 171.4, 162.4 (q, J B-C ) 50
6.3 Hz, 1J Rh-C ) 70 Hz), 179.7 (identified only in 13C-enriched
1
Hz, ipso-C-BArf), 139.1 (d, J Rh-C ) 40.1 Hz), 137.6, 135.4 (s,
2
1
2
2
spectrum, dd, J C-C ) 6.3 Hz, J Rh-C ) 70 Hz), 176.7, 176.5,
163.6 (q, 1J B-C ) 50 Hz, ipso-C-BArf), 133.5 (s, o-C-BArf), 131.4,
131.2 (2×), 130.8 (qq, 2J F-C ) 30 Hz, 4J F-C ) 3 Hz, m-C-BArf),
o-C-BArf), 135.2, 135.1 (d, J Rh-C ) 2.0 Hz), 134.2 (d, J Rh-C
)
2
4
2.0 Hz), 131.3, 129.5 (qq, J F-C ) 31 Hz, J F-C ) 3 Hz, m-C-
BArf), 125.2 (q, 1J F-C ) 270 Hz, BArfCF3), 118.1 (sept, 3J F-C
)
1
3
129.4 (2×), 129.2, 126.3 (q, J F-C ) 271 Hz, BArfCF3), 119.3
4 Hz), 72.7, 72.5, 72.3, 70.4, 32.3, 32.2 (d, J Rh-C ) 2.1 Hz),
3
31.9 (d, 3J Rh-C ) 1.4 Hz), 31.8, 21.8, 21.7, 21.6 (2×), 16.9, 16.8,
(sept, J F-C ) 4 Hz), 74.6, 72.3, 70.8, 70.6, 32.3, 32.1, 31.5,
31.3, 21.6, 21.5, 20.9, 20.8, 19.4, 16.5, 15.1. 19F NMR (376.1
MHz, CD2Cl2): δ -61.0.
-3.3 (d, J Rh-C ) 32.7 Hz). 19F NMR (376.1 MHz, CD2Cl2): δ
1
-61.0. FABMS (m/z): 473 [M - BArf]+, 458 [M - BArf
-
CH3]+.
Gen er a l P r oced u r e for th e P r ep a r a tion of Rh od iu m -
(II) Com p lexes 15. RhCl3‚3H2O (263 mg, 1.0 mmol) was
added to a solution of the bisoxazoline 2e (500 mg, 1.3 mmol)
or 2g (426 mg, 1.3 mmol), in 10 mL of degassed ethanol at 70
°C. The resulting orange-red suspension was heated at reflux
for 30 min. After evaporation to dryness at room temperature,
column chromatography on silica was performed with meth-
ylene chloride as eluent, giving as the first fraction (orange)
the corresponding rhodium(III) complex. The second fraction
(yellow) yielded the rhodium(II) complex as an air-stable
yellow microcrystalline solid. The yield of the tert-butyl-
substituted complex 15e was dramatically increased by run-
ning the reaction at higher dilution (quantities listed above
in 100 mL of ethanol). Crystals suitable for elemental and
X-ray analysis were obtained by layering a concentrated
benzene solution of 15g with pentane.
[Rh Cl2{(S,S)-tb-ben box(Me2)}(H)] (15e): yield 106 mg,
19% (reaction run in 10 mL ethanol); 318 mg, 57% (reaction
run in 100 mL ethanol). Mp: ca. 180 °C. Rf ) 0.31 (methylene
chloride). Anal. Calcd for C24H36Cl2N2O2Rh: C, 51.63; H, 6.50;
N, 5.02. Found: C, 51.97; H, 6.54; N, 4.82. IR (KBr, cm-1):
1647, 1637 (νCN). 1H NMR (300 MHz, C6D6): δ 20.5, 18.6, 11.1,
8.5, -37.7. FABMS (m/z): 522.3 [M - Cl]+, 486.3 [M - Cl -
HCl]+. µeff ) 1.87 µB (solution).
[Rh Cl2{d m -ben box(Me2)H}] (15g): yield 246 mg, 49%.
Mp: 190-192 °C. Rf ) 0.06 (methylene chloride). Anal. Calcd
for C20H28Cl2N2O2Rh: C 47.83; H 5.62; N 5.58. Found: C 47.76;
H 5.90; N 5.44. IR (KBr, cm-1): 1654, 1641 (νCN). 1H NMR
(300 MHz, C6D6): δ 35.8, 20.6, 17.5, 10.9, -33.3. EIMS (m/z):
465 [M - HCl]+, 429 [M - Cl - HCl]+. µeff ) 1.98 µB (solution),
µeff ) 2.00(2) µB (solid state).
Th er m olysis of 15g. The rhodium(II) complex 15g (13 mg,
0.026 mmol) and 1,4-dimethoxybenzene (standard, 2.7 mg,
0.019 mmol) were dissolved in 300 µL of C6D6 and transferred
to a medium-walled NMR tube. The solution was degassed,
and the tube was flame-sealed under vacuum. The sample was
[Rh (C2H4){(S,S)-ip -ben box(Me2)(2-Me)}][BAr f](12c). In
the glovebox, a Teflon-stoppered sealable 50 mL reaction vessel
was charged with 11c (70.0 mg, 0.052 mmol) and 5 mL of CH2-
Cl2. The solution was degassed with three freeze-pump-thaw
cycles, then exposed to approximately 700 mbar of ethylene.
After 14 h at room temperature, the volatile materials were
removed under vacuum. The resulting yellow oil was triturated
with pentane (3 × 3 mL), and a yellow microcrystalline solid
was obtained after the residual solvent had been removed
under vacuum. Yield: 69.3 mg, 97%. Anal. Calcd for C57H50
-
F
24N2O2RhB: C, 50.16; H, 3.69; N, 2.05. Found: C, 50.04; H,
3.69; N, 1.89. IR (CD2Cl2 cm-1): 1609 (νCN). 1H NMR (300 MHz,
CD2Cl2): δ 7.72 (s, 8H, BArf-CH), 7.56 (s, 4H, BArf-CH), 7.31
(s, 1H, CH), 4.25 (m, 2H, ox-CH), 4.15 (t, 1H, ox-CH2), 4.08 (t,
1H, ox-CH2), 3.92-3.81 (m, 6H), 3.50 (m, 2H, C2H4), 3.27 (m,
2H, C2H4), 2.72 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.31 (s, 3H,
CH3), 2.04 (m, 1H, i-Pr-CH), 1.41 (m, 1H, i-Pr-CH), 0.86 (d,
3H, i-Pr-CH3), 0.75 (d, 3H, i-Pr-CH3), 0.73 (d, 3H, i-Pr-CH3),
0.48 (d, 3H, i-Pr-CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ
177.1, 175.1, 163.5 (q, 1J B-C ) 49 Hz, ipso-C-BArf), 138.5, 138.4,
2
137.8, 136.6 (s, o-C-BArf), 135.1 (2×), 134.4, 130.8 (qq, J F-C
4
1
) 30 Hz, J F-C ) 3 Hz, m-C-BArf), 126.4 (q, J F-C ) 271 Hz,
BArfCF3), 119.3 (sept, 3J F-C ) 4 Hz), 71.6, 70.9, 70.3, 69.9, 32.5,
31.2, 30.6, 30.3, 24.1, 22.0, 21.7, 20.6, 20.1, 15.9, 15.3. No
carbon peaks for the bound ethylene ligand were observed. 19
F
NMR (376.1 MHz, CD2Cl2): δ -61.0.
[Rh (CO){(S,S)-ip -ben box(Me2)(2-Me)}][BAr f] (13c). In
the glovebox, a J -Young NMR tube was charged with complex
11c (7.0 mg, 0.005 mmol) and 300 µL of CD2Cl2. The solution
was degassed with three freeze-pump-thaw cycles, and the
tube was exposed to 700 mbar of CO. The reaction progress
was monitored by 1H NMR spectroscopy. 1H NMR after 10 min
revealed the formation of 13c, which is fully converted to 14c
within 14 h and could not be isolated independently. 1H NMR
(300 MHz, CD2Cl2): δ 7.72 (s, 8H, BArf-CH), 7.56 (s, 4H, BArf-

58 Organometallics, Vol. 22, No. 1, 2003
Gerisch et al.
warmed to room temperature, and an initial 1H NMR spectrum
was recorded. The tube was heated at 75 °C in an oil bath
and cooled to room temperature, and a H NMR spectrum was
CH2Cl2 cocrystallized with 4c. The data were collected using
10 s frames with an ω scan of 0.3°. Empirical absorption
corrections based on comparison of redundant and equivalent
1
recorded. The progress of the reaction was analyzed by
integrating the product signals against the internal standard.
After 48 h of heating at 75 °C, formation of 4g (42%) and 16g
(39%) was observed. The formation of a small amount of
rhodium metal at the NMR tube surface was also observed.
[Rh Cl2(H){(S,S)-d m -ben box(Me2)H}] (16g). A high-pres-
sure steel reaction vessel was charged with a dark orange
solution of the rhodium complex 4g (121 mg, 0.24 mmol) in
30 mL of benzene. The vessel was pressurized to 35 atm with
H2 and heated to 75 °C for 10 h. After this time, the pressure
was vented, and the resulting yellow solution was filtered,
pumped down, and then triturated with pentane (3 × 7 mL)
to yield a spectroscopically and analytically pure yellow
powder. Yield ) 94 mg, 75%. Anal. Calcd for C20H29Cl2N2O2-
Rh: C, 47.73; H, 5.81; N, 5.57. Found: C, 47.61; H, 5.85; N,
5.51. IR (KBr): 2197 (νRhH), 1642 (νCN). 1H NMR (300 MHz,
reflections were applied using SADABS35 (Tmax ) 0.93, Tmin
)
0.72). The maximum peak in the final difference map was 0.81
e-/Å3, and the minimum peak was -0.47 e-/Å3. The correct
enantiomorphs of the molecule and space group were deter-
mined by comparison of Friedel pairs.
(b) [Rh Me(OTf){(S,S)-ip -ben box}] (9c). The compound
crystallizes with four molecules in the asymmetric unit of the
acentric space group P21. The data were collected using 20 s
frames with an ω scan of 0.3°. Empirical absorption corrections
based on comparison of redundant and equivalent reflections
were applied using SADABS35 (Tmax ) 0.98, Tmin ) 0.60). The
maximum peak in the final difference map was 1.21 e-/Å3, and
the minimum peak was -1.74 e-/Å3. The correct enantiomor-
phs of the molecule and space group were determined by
comparison of Friedel pairs.
(c) [Rh Cl2{d m -ben box(Me2)H}] (15g). The data were
collected using 10 s frames with an ω scan of 0.3°. Empirical
absorption corrections based on comparison of redundant and
2
C6D6): δ 9.03 (d, 1H, CH, J H-H ) 3.3 Hz), 7.18 (s, 1H, CH),
3.69 (d, 2H, CH2, 2J H-H ) 16.6 Hz), 3.46 (m, 6H, ox-CH2, CH2),
2.01 (s, 6H, CH3), 1.49 (s, 6H, ox-CH3), 1.28 (s, 6H, ox-CH3),
equivalent reflections were applied using SADABS35 (Tmax
)
1
-19.70 (dd, 1H, Rh-H, J H-H ) 3.3 Hz, J Rh-H ) 18.8 Hz). 13C-
0.90, Tmin ) 0.71). The maximum peak in the final difference
map was 0.44 e-/Å3, and the minimum peak was -0.71 e-/Å3.
{1H} NMR (126 MHz, CDCl3): δ 170.7, 136.6, 134.1, 130.5,
121.6, 79.1, 70.0, 31.4, 29.1, 29.0, 18.7.
Ackn owledgm en t. Drs. Fred Hollander, Dana Caul-
der, and Allan Oliver at the UCB X-ray diffraction
facility (CHEXRAY) are gratefully acknowledged for
determination of the crystal structures of 4c, 9c, and
15g. 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 Mem-
ber. M.G. thanks Deutche Forschungsgemeinschaft for
a Postdoctoral Fellowship. J .R.K. thanks the National
Science Foundation for a Predoctoral Fellowship.
X-r a y Str u ctu r e Deter m in a tion s. Gen er a l Con sid er -
a tion s. X-ray crystal structures were obtained by Fred Hol-
lander, Dana Caulder, and Allan Oliver at the UCB X-ray
facility (CHEXRAY). Crystals were mounted on glass fibers
using Paratone N hydrocarbon oil. All measurements were
made on
a
SMART30 CCD area detector with graphite-
monochromated Mo KR radiation (λ ) 0.71069 Å). The data
were collected with a detector position of 60.00 mm. Data were
integrated by the program SAINT31 and were corrected for
Lorentz and polarization effects. Data were analyzed for
agreement and possible absorption using XPREP.32 The struc-
tures were solved by direct methods33 and expanded using
Fourier techniques.34
(a ) [Rh Cl2{(S,S)-ip -ben box(Me2)}] (4c). One molecule of
(30) SMART: Area-Detector Software Package; Siemens Industrial
Automation Inc.: Madison, WI, 1995.
(31) SAINT: SAX Area-Detector Integration Program, V4.024; Si-
emens Industrial Automation Inc.: Madison, WI, 1995.
(32) XPREP: (v. 5.03) Part of the SHELXTL Crystal Structure
Determination; Siemens Industrial Automation Inc.: Madison, WI,
1995.
Su p p or tin g In for m a tion Ava ila ble: Spectral and ana-
lytical data for compounds 1b-g, 2b-g, 3d -g, and 4e-g;
crystal structure data for 4c, 9c, and 15g, including additional
molecular structure diagrams, tables of positional and thermal
parameters, bond distances and angles.
(33) SIR92: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi,
A. J . Appl. Crystallogr. 1993, 26, 343.
(34) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J . M. M. The DIRDIF-
94 program system, Technical Report of the Crystallography Labora-
tory; University of Nijmegen: The Netherlands, 1994.
OM0207562
(35) SADABS: Siemens Area Detector Abssorption Correction
Program; Sheldrick, G. 1996. Advance copy, private communication.