Optically active transition metal complexes. Part 133. Preparation, epimerization and crystallization of chiral-at-metal rhodium(III) half-sandwich complexes

Article abstract of DOI:10.1016/S0277-5387(03)00346-2

The complexes Cp*Rh(beapy)Cl (5) and Cp*Ir(beapy)Cl (6) were synthesized by reaction of [Cp*MCl₂]₂ (M = Rh, Ir) and the deprotonated ligand Hbeapy, the condensation product of benzylamine and 2-pyrrolcarbaldehyde. In each case two enantiomers (S_M) and (R_M arise, differing only in the metal chirality. The CH₂ group of the benzyl substituent in the ligand forms an AB spectrum which was used for ¹H NMR coalescence measurements (T_c=115.1 °C). The half-life for the racemization of Cp*Rh(beapy)Cl (5) was 46 ms at 115.1 °C. Substitution of the chloride ligand in Cp*Rh(pepy)Cl (3), pepy anion of Schiff base (+)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine, afforded the complexes Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8). Single crystals of the two compounds contained only one of the two diastereomers. In solution the compounds epimerized via a change of the metal configuration, the half-life for (R_Rh, S_C)-Cp*Rh(NN*)I (8) being 40.8 min at -40 °C in CD₂Cl₂ solution (obtained by time dependent integration of suitable ¹H NMR signals). Reaction of [CpRhCl₂]₂ with (+)-N-[(S)-1-phenylethyl]salicylaldimine gave CpRh(pesa)Cl (9). Surprisingly, single crystals of 9 contained both diastereomers (R_Rh,S_C) and (S_Rh,S_C) in a 1:1 ratio following the molecular recognition motif of the inverted pianostools. The half-lifes for the epimerization of 9 with respect to the metal center, derived from ¹H NMR coalescence experiments, were 31 and 9.2 ms at 11.3 °C (CD₂Cl₂) for the forward and back reactions, respectively.

Full text of DOI:10.1016/S0277-5387(03)00346-2

Polyhedron 22 (2003) 2639Á2646
/
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Optically active transition metal complexes. Part 1331.
Preparation, epimerization and crystallization of chiral-at-metal
rhodium(III) half-sandwich complexes
Henri Brunner a,*, A. Kollnberger a, Manfred Zabel b
¨
a Institut fur Anorganische Chemie, Universitat Regensburg, D-93040 Regensburg, Germany
¨
¨
b Zentrale Analytik, Rontgenstrukturanalyse, Universitat Regensburg, D-93040 Regensburg, Germany
¨
¨
Received 13 March 2003; accepted 12 May 2003
Abstract
The complexes Cp*Rh(beapy)Cl (5) and Cp*Ir(beapy)Cl (6) were synthesized by reaction of [Cp*MCl2]2 (Mꢀ
deprotonated ligand Hbeapy, the condensation product of benzylamine and 2-pyrrolcarbaldehyde. In each case two enantiomers
/Rh, Ir) and the
(SM) and (RM) arise, differing only in the metal chirality. The CH2 group of the benzyl substituent in the ligand forms an AB
spectrum which was used for 1H NMR coalescence measurements (Tcꢀ
/
115.1 8C). The half-life for the racemization of
Cp*Rh(beapy)Cl (5) was 46 ms at 115.1 8C. Substitution of the chloride ligand in Cp*Rh(pepy)Cl (3), pepy anion of Schiff base
(ꢁ)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine, afforded the complexes Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8). Single crystals
of the two compounds contained only one of the two diastereomers. In solution the compounds epimerized via a change of the metal
configuration, the half-life for (RRh,SC)-Cp*Rh(NN*)I (8) being 40.8 min at ꢂ40 8C in CD2Cl2 solution (obtained by time
dependent integration of suitable H NMR signals). Reaction of [CpRhCl2]2 with (ꢁ)-N-[(S)-1-phenylethyl]salicylaldimine gave
/
/
1
/
CpRh(pesa)Cl (9). Surprisingly, single crystals of 9 contained both diastereomers (RRh,SC) and (SRh,SC) in a 1:1 ratio following the
molecular recognition motif of the inverted pianostools. The half-lifes for the epimerization of 9 with respect to the metal center,
derived from 1H NMR coalescence experiments, were 31 and 9.2 ms at 11.3 8C (CD2Cl2) for the forward and back reactions,
respectively.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Chiral-at-metal complexes; Rhodium; Configurational stability; Epimerization; X-ray structure analyses
1. Introduction
and (SM,SC) which differ only in the metal configura-
tion. In these Rh and Ir compounds the configuration at
Most of the organometallic chiral-at-metal com-
the metal center often is labile [4Á9]. Therefore, such
/
pounds are half-sandwich complexes [1Á
/
3]. A frequent
complexes epimerize in solution via a change of the
metal configuration. Occasionally, this aspect has been
overlooked and wrong conclusions have been drawn. In
1997 Loza et al. published the rhodium complex
ligand combination of this type in the chemistry of
rhodium and iridium is a cyclopentadienyl or penta-
methylcyclopentadienyl ligand, a halide ligand and an
unsymmetrical anionic chelate ligand with a chiral
carbon atom, e.g. of (S)-configuration. Such a ligand
combination gives rise to two diastereomers (RM,SC)
Cp*Rh(pesa)Cl (1) formed in the reaction of (ꢁ
[(S)-1-phenylethyl]salicylaldimine (Hpesa)
/
)-N-
and
[Cp*RhCl2]2 [10]. In the crystal of Cp*Rh(pesa)Cl
only the (RRh,SC)-diastereomer was present. As in the
1H NMR spectrum at RT only one set of broad signals
was observed, it was assigned to the (RRh,SC)-diaster-
eomer claiming a high diastereomeric excess [10]. In a
VT NMR study of Cp*Rh(pesa)Cl (1) we could show
that at RT the signals of the diastereomers (RRh,SC) and
(SRh,SC) coalesced, whereas at low temperatures they
* Corresponding author. Tel.: ꢁ
943-4439.
E-mail address: henri.brunner@chemie.uni-regensburg.de (H.
Brunner).
/
49-941-943-4440; fax: ꢁ49-941-
/
1
Part 132. H. Brunner, M. Weber, M. Zabel, T. Zwack, Angew.
Chem., in press; Anbew. Chem. Int. Ed. in press.
0277-5387/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00346-2

2640
H. Brunner et al. / Polyhedron 22 (2003) 2639Á2646
/
were well resolved indicating a high configurational
instability at the metal atom (half-lifes for the inter-
conversion of the two diastereomers 28 and 6.7 ms at
21 8C for Cp*Rh(pesa)Cl (1) and 50 and 8 ms at
mine (Hpesa) [14] and the precursors [Cp*MCl2]2 and
[CpMCl2]2, MꢀRh, Ir [15] were synthesized by pub-
lished methods.
/
ꢂ
/
1.8 8C for Cp*Ir(pesa)Cl (2) [4,11] (Scheme 1).
2.3. Cp*Rh(beapy)Cl (5) and Cp*Ir(beapy)Cl (6)
In the complexes Cp*Rh(pepy)Cl (3) and Cp*Ir(pe-
py)Cl (4), derived from ligand (ꢁ
/
)-2-N-[(S)-1-phenyl-
Hbeapy (180 mg, 0.97 mmol) was deprotonated with
109 mg (1.36 mmol) of KOtBu in 30 ml of CH2Cl2 under
N2. After stirring for 1 h at RT 0.97 mmol of
[Cp*RhCl2]2 (300 mg) or [Cp*IrCl2]2 (386 mg) were
added. After stirring for 20 h at RT the solvent was
removed. The residue was dissolved in 20 ml of toluene
and filtered over celite†. For crystallization petroleum
ether was added to the filtrate.
ethyl]pyrrolcarbaldimine (Hpepy), the configuration at
the metal center proved to be labile, too, although the
half-lifes for approach to equilibrium were much longer
(19.4 min for Cp*Rh(pepy) (3) and 30.6 min for
Cp*Ir(pepy)Cl (4) at ꢂ
In the present paper we describe (i) the synthesis of
the complexes Cp*M(beapy)Cl, MꢀRh (5) and Ir (6),
/
50 8C [4]).
/
derived from the achiral ligand Hbeapy, the diastereo-
topic benzyl protons of which provided information on
the stability of the metal configuration in coalescence
experiments, (ii) the synthesis of the complexes
2.3.1. Cp*Rh(beapy)Cl (5)
Orange powder. Yield: 367 mg, 82%. M.p. (dec.)
1
ꢀ
7.41Á
6.55 (dd, Jꢀ
/
200 8C. H NMR (250 MHz, 297 K, CDCl3): dꢀ
/
Cp*Rh(pepy)Hal, Halꢀ/Br (7), I (8) to investigate the
/
7.34 (m, 6H, Ph, NCH), 7.13 (s, 1H, pyrr-H5),
3
4
influence of the halide ligand on the half-lifes of the
epimerization reactions and the diastereomer concentra-
tions at equilibrium, and (iii) the synthesis of CpRh(pe-
sa)Cl (9), the Cp analogue of 1, one of the rare cases in
which two diastereomers co-crystallized in a 1:1 ratio in
the same single crystal [11].
/
3.8 Hz, Jꢀ
/
1.0 Hz, 1H, pyrr-H3/4), 6.28
3
4
(dd, Jꢀ
2Jꢀ
17.5 Hz, Jꢀ
17.5 Hz, 4Jꢀ
C5Me5). FAB-PI-LSIMS (CH2Cl2): m/zꢀ
Cl, 100). C22H26ClN2Rh (456.8): Calc. C 57.84,
/
3.8 Hz, Jꢀ
/
1.7 Hz, 1H, pyrr-H3/4), 5.08 (dd,
0.9 Hz, 1H, CHAHB), 5.04 (dd, Jꢀ
4
2
/
/
/
/
0.9 Hz, 1H, CHAHB), 1.74 (s, 15H,
/456 (M, 40),
421 (MÃ
/
H 5.74, N 6.13; Found: C 58.06, H 6.21, N 5.94%.
2.3.2. Cp*Ir(beapy)Cl (6)
2. Experimental
RedÁ
/
orange powder. Yield: 420 mg, 79%. M.p. (dec.)
1
ꢀ
7.44Á
/
200 8C. H NMR (250 MHz, 297 K, CDCl3): dꢀ
/
2.1. Physical measurements
3
/
7.12 (m, 7H, Ph, pyrr-H5, NCH), 6.58 (dd, Jꢀ
/
4
3
1.1 Hz, 1H, pyrr-H3/4), 6.30 (dd, Jꢀ
/
3.8
1H NMR spectra: Bruker AC 250 (250 MHz) and
ARX 400 (400 MHz); TMS as internal standard. MS:
Finnigan MAT 95 and Finnigan MAT 311A. The most
intense peak was specified. Elemental analyses: Elemen-
3.8 Hz, Jꢀ
/
4
2
Hz, Jꢀ
4Jꢀ
1.4 Hz, 1H, CHAHB), 5.02 (dd, Jꢀ
1.4 Hz, 1H, CHAHB), 1.72 (s, 15H, C5Me5). FAB-PI-
LSIMS (CH2Cl2): m/zꢀ Cl, 100).
/
1.7 Hz, 1H, pyrr-H3/4), 5.10 (dd, Jꢀ
/
15.7 Hz,
15.7 Hz, Jꢀ
2
4
/
/
/
/
546 (M, 68), 411 (MÃ
/
tar Vario EL III. Melting points: Buchi SMP 20.
¨
C22H26ClN2Ir (546.1): Calc. C 48.38, H 4.80, N 5.13;
Found: C 48.65, H 5.03, N 4.89%.
Polarimetric measurements: PerkinÁElmer polarimeter
/
241.
2.4. Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8)
2.2. Ligands and precursors
Cp*Rh(pepy)Cl (3) (70.6 mg, 0.15 mmol) was dis-
solved in 15 ml of absolute methanol under N2. A
tenfold amount of NaBr (154.4 mg, 1.5 mmol) or NaI
(224.8 mg, 1.5 mmol) was added. After stirring for 1 h at
RT, the solvent was removed. The residue was dissolved
in 10 ml of CH2Cl2 and the salts were removed by
filtration over celite†. To complete the halide exchange,
this procedure was repeated another two times. After
the last exchange process, the solvent was removed to
give the product as a red powder.
The ligands 2-N-benzylpyrrolcarbaldimine (Hbeapy)
[12], (ꢁ)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine
(Hpepy) [13], (ꢁ)-2-N-[(S)-1-phenylethyl]salicylaldi-
/
/
2.4.1. Cp*Rh(pepy)Br (7)
Red powder. Yield: 75.6 mg, 98%. M.p. (dec.)
ꢀ
170 8C. 1H NMR (400 MHz, CDCl3) of the
(RRh,SC)-diastereomer, (SRh,SC)-diastereomer in brack-
/
Scheme 1. Complexes 1Á/4 (only the (RM,SC)-diastereomers shown).

H. Brunner et al. / Polyhedron 22 (2003) 2639Á
/
2646
2641
ets: dꢀ
/
7.66Á
/
7.09 (m, 7H, NÄ
/
CH, pyrr-H5, Ph), 6.55
1.1 Hz, 1H, pyrr-H4), 6.26
1.8 Hz, Jꢀ
0.3 Hz, 1H, pyrr-H3),
7.0 Hz, 1H, CH3CHPh), 1.77 [1.83] (d,
7.0 Hz, 3H, CH3CHPh), 1.66 [1.71] (s, 15H,
C5Me5). FAB-PI-LSIMS (CH2Cl2): m/zꢀ516 (M, 8),
Br, 100). C23H28BrN2Rh (514.3): Calc. C 53.61,
a riding model was used during the refinement process.
For the ‘pesa-compound’ all H-atoms were located by
difference Fourier synthesis and refined isotropically,
and for the ‘pepy-compounds’ a mixture of both
methods was used.
3
[6.57] (dd, Jꢀ
/
3.7 Hz, Jꢀ
3.7 Hz, Jꢀ
5.27 (q, 3Jꢀ
3Jꢀ
/
3
(ddd, Jꢀ
/
/
/
/
/
/
435 (MÃ
/
H 5.48, N 5.46; Found: C 51.85, H 5.48, N 5.37%.
3. Cp*Rh(beapy)Cl (5) and Cp*Ir(beapy)Cl (6)
2.4.2. Cp*Rh(pepy)I (8)
Red powder. Yield: 81.7 mg, 97%. M.p. (dec.)
3.1. Synthesis and X-ray structure analyses
ꢀ
145 8C. 1H NMR (400 MHz, CDCl3) of the
/
Condensation of benzylamine and 2-pyrrolcarbalde-
hyde in methanol gave ligand Hbeapy. Hbeapy was
deprotonated with KOtBu in CH2Cl2 and reacted with
the organometallic precursors [Cp*RhCl2]2 and
[Cp*IrCl2]2 (Scheme 2). The two enantiomers of the
neutral complexes (RRh)-Cp*Rh(beapy)Cl (RRh)-(5) and
(SRh)-Cp*Rh(beapy)Cl (SRh)-(5) as well as (RIr)-
Cp*Ir(beapy)Cl (RIr)-(6) and (SIr)-Cp*Ir(beapy)Cl
(SIr)-(6) were formed differing only in the metal config-
uration.
(RRh,SC)-diastereomer, (SRh,SC)-diastereomer in brack-
ets: dꢀ7.57Á7.10 (m, 7H, NÄ
CH, pyrr-H5, Ph), 6.62
(dd, Jꢀ3.8 Hz, Jꢀ
1.1 Hz, 1H, pyrr-H4), 6.27 [6.25]
(ddd, Jꢀ3.8 Hz, Jꢀ1.8 Hz, Jꢀ
0.4 Hz, 1H, pyrr-H3),
5.22 [5.13] (q, Jꢀ
/
/
/
3
/
/
3
/
/
/
3
/
7.0 Hz, 1H, CH3CHPh), 1.79 [1.83]
3
(d, Jꢀ
/
7.0 Hz, 3H, CH3CHPh), 1.72 [1.81] (s, 15 H,
C5Me5). FAB-PI-LSIMS (CH2Cl2): m/zꢀ/563 (M, 10),
435 (MÃ
/
I, 100). C23H28IN2Rh (561.3): Calc. C 49.13, H
5.02, N 4.98; Found: C 48.35, H 4.98, N 4.74%.
Single crystals of Cp*Rh(beapy)Cl (5) and Cp*Ir(bea-
py)Cl (6) were obtained by slow diffusion of petroleum
ether into a toluene solution of the complexes at RT.
Fig. 1 shows the crystal structure of Cp*Rh(beapy)Cl
(5). For the specification of the metal configuration the
2.5. CpRh(pesa)Cl (9)
(S)-Hpesa (225 mg, 1.0 mmol) was deprotonated with
112 mg (1.0 mmol) of KOtBu in 10 ml of abs. CH2Cl2.
After 1 h [CpRhCl2]2 (100 mg, 0.27 mmol) was added at
0 8C. The solution was stirred for 4 h at RT and then
filtered over celite†. After removing the solvent the
residue was washed with pentane. For crystallization the
residue was dissolved in toluene with petroleum ether
slowly diffusing into the toluene solution.
ligand priority sequence Cpꢀ
/
Clꢀ
/
N(imine)ꢀN(pyr-
/
role) was used [18,19]. There is a center of inversion
between the two enantiomers (RRh)-Cp*Rh(beapy)Cl
and (SRh)-Cp*Rh(beapy)Cl.
The iridium compound Cp*Ir(beapy)Cl (6) crystal-
lized analogously. Table 1 contains the crystallographic
data and Table 2 the most important bond lengths, bond
angles and dihedral angles of 5 and 6.
Red powder. Yield: 137 mg, 60%. M.p. (dec.) ꢀ
/
1
200 8C. H NMR (400 MHz, 213 K, CD2Cl2) of the
major diastereomer, minor diastereomer in brackets:
dꢀ
/
8.09 [7.81] (s, 1H, NCH), 7.55Á
/
7.08 (m, 7H, Ph, sal-
3
H), 6.89 [6.80] (d, Jꢀ
/
8.0 Hz, 1H, sal-H), 6.55 [6.44]
3.2. Enantiomer interconversion
3
3
4
(ddd, Jꢀ
5.80 [5.33] (q, Jꢀ
(s, 5H, C5H5); 1.74 [1.89] (d, 3Jꢀ
CH3CHPh). FAB-PI-LSIMS (CH2Cl2): m/zꢀ
Cl, 100). C20H19ClNOP (427.7): Calc. C
/
8.0 Hz, Jꢀ
/
8.0 Hz, Jꢀ
/
1.1 Hz, 1H, sal-H),
3
/
7.0 Hz, 1H, CH3CHPh), 5.00 [5.27]
7.0 Hz, 3H,
427 (M,
At 297 K the CH2 group of the benzyl substituent in
Cp*Rh(beapy)Cl (5) showed two strongly coupling
doublets at 5.08 and 5.04 ppm indicating an AB
spectrum of two geminal protons. Increasing the tem-
perature the signals of the two protons coalesced
forming a horizontal connection at the coalescence
/
/
32), 392 (MÃ
/
56.16, H 4.48, N 3.27; Found: C 57.96, H 4.90, N 3.30%.
2.6. X-ray crystallography
temperature Tcꢀ115.1 8C. At temperatures of 413 K
/
there was only one sharp signal for the CH2 protons.
The process underlying the coalescence is a rapid
interconversion of the enantiomers of 5 which make
the diastereotopic protons of the benzyl group identical
on the NMR time scale. An evaluation of the coales-
cence measurement by published methods [21] gave a
half-life of 46 ms at 115.1 8C for Cp*Rh(beapy)Cl (5).
The parameters of the measurement are summarized in
Table 4. Coalescence studies with Cp*Ir(beapy)Cl (6)
were not possible because decomposition started at
temperatures above 50 8C.
Diffraction data were collected on a STOE-IPDS
˚
0.7107 A). Crystal data and
details of the structure determination are shown in
Table 1. The structures were solved by direct methods
SIR-97 [16]). They were refined on F2 by the full-matrix
least-squares technique (SHELXL-97 [17]). For all com-
pounds, where the absorption coefficient was greater
than 1.0 mmꢂ1, numerical absorption correction from
crystal shape was applied. All hydrogen atoms for
‘beapy-compounds’ were calculated geometrically and
using Mo Ka radiation (lꢀ
/
(

2642
H. Brunner et al. / Polyhedron 22 (2003) 2639Á2646
/
Table 1
Crystallographic data of complexes 5Á9
/
Compound
Cp*Rh(beapy)Cl (5)
Cp*Ir(beapy)Cl (6) Cp*Rh(pepy)Br (7) Cp*Rh(pepy)I (8)
CpRh(pesa)Cl (9)
Empirical formula
Formula weight
C
22H26ClN2Rh
456.81
0.58ꢃ
red rod
C22H26ClIrN2
546.12
C23H28BrN2Rh
515.28
C23H28IN2Rh
562.28
C20H19ClNORh
427.72
Crystal size (mmꢃ
Crystal color, habit
Crystal system
/
mmꢃ
/
mm)
/
0.14ꢃ
/0.06
0.60ꢃ
/
0.38ꢃ
/
0.36
0.24ꢃ
/
0.10ꢃ
/
0.06
0.60ꢃ
/
0.16ꢃ
/
0.10
0.31ꢃ
/
0.12ꢃ0.06
/
orange prism
orthorhombic
Pccn
red prism
orthorhombic
P212121
red rod
orthorhombic
P212121
brown prism
monoclinic
P21
orthorhombic
Pccn
Space group
Unit cell dimensions
˚
a (A)
7.7632(4)
21.8151(13)
26.6522(13)
90
7.8444(3)
21.7956(11)
26.3499(17)
90
7.6165(4)
11.2156(6)
24.9561(15)
90
7.6843(4)
11.1852(6)
25.650(2)
90
12.3696(13)
10.5226(9)
14.7016(17)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
90
90
90
90
90
90
90
90
111.677(12)
90
3
˚
V (A )
4513.7(4)
8
4505.1(4)
8
2131.84(17)
4
1.605
2204.6(2)
4
1778.2(3)
4
1.598
Z
Dcalc (g cmꢂ3
F (0 0 0)
Absorption coefficient (mmꢂ1
Data collection
T (K)
)
1.344
1872
0.88
1.610
2128
6.05
1.694
1112
2.18
1040
2.69
864
1.12
)
173(1)
173(1)
173(1)
173(1)
297(2)
u Range for data collection (8)
Index ranges
2.48Á
95
265
325
/
25.83
h 58,
k 526,
l 532
2.43Á
85
265
325
/
25.79
h 58,
k 526,
l 532
2.44Á
95
135
305
/
25.77
h 59,
k 513,
l 530
2.42Á
95
135
315
/
25.73
h 58,
k 513,
l 531
1.85Á
155
125
185
/
25.84
h 5
k 5
l 517
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
15,
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
ꢂ
/
/
/
12,
ꢂ/
/
/
ꢂ/
/
/
ꢂ/
/
/
ꢂ/
/
/
ꢂ/
/
/
Reflections collected
Independent reflections
34 559
4333
59 960
4122
27 987
4068
17 360
4170
21 761
6770
Observed reflections [I ꢀ
Rint
/2s(I)]
2824
0.0807
3871
0.0579
3733
0.0579
3998
0.0403
4866
0.0714
Refinement
Hydrogen treatment
Data, restraints, parameters
Goodness-of-fit on F2
constr.
4333, 0, 235
0.943
constr.
4122, 0, 235
1.224
mixed
4068, 0, 264
0.997
mixed
4289, 0, 378
1.042
constr.
6770, 1, 433
0.894
Final R indices [I ꢀ
/
2s(I)]
R1ꢀ
wR2ꢀ
R1ꢀ0.0838,
wR2ꢀ0.0744
/
0.0349,
R1ꢀ
wR2ꢀ
R1ꢀ0.0350,
wR2ꢀ0.0789
/
0.0310,
R1ꢀ
wR2ꢀ
R1ꢀ0.0258,
wR2ꢀ0.0488
0.01(1)
0.529 and ꢂ
202465
/
0.0220,
R1ꢀ
wR2ꢀ
R1ꢀ0.0195,
wR2ꢀ0.0461
0.03(2)
0.719 and ꢂ
202464
/
0.0184,
R1ꢀ
wR2ꢀ
R1ꢀ0.0631,
wR2ꢀ0.0812
0.01(4)
0.326 0.795 and ꢂ
202466
/
0.0392,
/
0.0622
/0.0769
/0.0480
/0.0457
/
0.0761
R indices (all data)
/
/
/
/
/
/
/
/
/
/
Absolute structure parameter
ꢂ3
ꢂ/
ꢂ/
˚
Largest peak, hole (e A
CCDC number
)
0.755 and ꢂ
/
0.334
1.233 and ꢂ
/
0.522
/
0.240
/
/0.276
202467
202468
4. Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8)
these substitutions Cp*Rh(pepy)Cl (3) was dissolved in
methanol and a tenfold excess of NaBr or NaI was
added. After a short time the reaction mixture was at
equilibrium. By removing the solvent, dissolving the
residue in CH2Cl2 and filtering over celite† the product
was isolated. For a halide exchange of 99.9% the process
4.1. Synthesis and X-ray structure analyses
The complexes Cp*Rh(pepy)Br (7) and Cp*Rh(pe-
py)I (8) were synthesized by halogen exchange reactions
starting from Cp*Rh(pepy)Cl (3) [4] (Scheme 3). For
1
must be repeated another two times. As the H NMR
signals of the products differ from those of the starting
material, the progress of the reaction can be monitored
1
by H NMR spectroscopy.
Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8) form two
diastereomers (RRh,SC)- and (SRh,SC)-Cp*Rh(pepy)Br
and (RRh,SC)- and (SRh,SC)-Cp*Rh(pepy)I differing
only in the metal configuration. In CD2Cl2 solution at
room temperature the equilibrium concentrations are
(RRh,SC):(SRh,SC)ꢀ
/
87:13 both for Cp*Rh(pepy)Br (7)
Scheme 2. Complexes 5 and 6.
and Cp*Rh(pepy)I (8).

H. Brunner et al. / Polyhedron 22 (2003) 2639Á
/
2646
2643
Scheme 4 shows the crystal structure of (RRh,SC)-
Cp*Rh(pepy)Br (7). The structure of (RRh,SC)-
Cp*Rh(pepy)I (8) is very similar.
Table 1 contains the crystallographic data and Table 2
representative molecular parameters of the crystal
structures of (RRh,SC)-Cp*Rh(pepy)Br (7) and
(RRh,SC)-Cp*Rh(pepy)I (8).
4.2. Epimerization studies
Powdered crystals of (RRh,SC)-Cp*Rh(pepy)I (8) were
dissolved in CD2Cl2 at ꢂ
/
80 8C. Due to the time
Fig. 1. Crystal structure of Cp*Rh(beapy)Cl (5).
necessary for dissolution and preparing the first mea-
1
surement at ꢂ
tꢀ0 min showed not only the signals of the (RRh,SC)-
diastereomer but also those of the (SRh,SC)-diastereo-
mer. During the measurement at ꢂ40 8C the signals of
/
40 8C, the H NMR spectrum taken at
Slow diffusion of petroleum ether into toluene solu-
tions of Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8)
afforded single crystals suitable for X-ray structure
analyses. Only the (RRh,SC)-diastereomers of
Cp*Rh(pepy)Br (7) and Cp*Rh(pepy)I (8) crystallized.
/
/
diastereomer (SRh,SC)-Cp*Rh(pepy)I increased up to
13% (Scheme 5). The equilibrium concentration was
Table 2
Selected bond lengths (A), bond angles (8) and dihedral angles (8) of complexes 5Á
˚
/8
rac-Cp*Rh(beapy)Cl (5)
rac-Cp*Ir(beapy)Cl (6)
Cp*Rh(pepy)Br (7)
Cp*Rh(pepy)I (8)
Bond lengths
M1Ã
M1Ã
M1Ã
M1Ã
/
Cp
1.779
1.779
1.795
1.795
/N1
2.064(3)
2.111(3)
2.4083(11)
2.060(4)
2.100(4)
2.4083(13)
2.075(2)
2.151(2)
2.5396(4)
2.079(2)
2.139(2)
2.7016(4)
/N2
/Hal1
Bond angles
N1Ã
N2Ã
Hal1Ã
/
M1Ã
/
N2
Hal1
77.25(13)
85.23(9)
89.88(9)
76.29(14)
83.34(11)
87.77(11)
77.44(9)
86.19(6)
90.04(6)
77.61(9)
87.77(6)
89.92(6)
/
M1Ã
/
/
M1ÃN1
/
Dihedral angles
M1ÃN1ÃC14Ã
N1ÃC14ÃC15Ã
C14ÃC15ÃN2Ã
C15ÃN2ÃM1Ã
/
/
/
C15
1.0(5)
1.9(5)
8.8(3)
6.3(3)
/
/
/N2
ꢂ
/
2.1(6)
2.1(5)
1.2(3)
ꢂ
/
2.0(6)
1.1(6)
0.0(3)
ꢂ
/
6.1(4)
0.3(4)
3.3(2)
ꢂ
/
4.3(4)
0.1(4)
2.5(2)
/
/
/M1
/
/
/N1
ꢂ
/
Scheme 3. Halide exchange in Cp*Rh(pepy)Cl (3) (only one diastereomer shown).

2644
H. Brunner et al. / Polyhedron 22 (2003) 2639Á2646
/
determined after thermostating the sample at ꢂ
/
40 8C
for 24 h.
Time dependent integration of the Cp*-signals at 1.68
and 1.72 ppm and treatment of the data according to
first-order led to the half-life for the approach to the
epimerization equilibrium (Scheme 6). This half-life for
(RRh,SC)-Cp*Rh(pepy)I (8) at 233 K in CD2Cl2 is 40.8
min compared to 2.7 min for (RRh,SC)-Cp*Rh(pepy)Cl
(1) at 237 K in CD2Cl2 [4]. Thus, in going from the
chloro complex 1 to the corresponding iodo complex 8
the configurational stability of the metal atom increases.
Scheme 4. Crystal structure of (RRh,SC)-Cp*Rh(pepy)Br (7).
5. CpRh(pesa)Cl (9)
5.1. Synthesis and X-ray structure analysis
For the synthesis of CpRh(pesa)Cl (9), the Cp
analogue of Cp*Rh(pesa)Cl (1) [4,10], [CpRhCl2]2 was
added to the deprotonated pesa ligand in CH2Cl2 at 0 8C
(Scheme 7). Again, two diastereomers (RRh,SC)- and
(SRh,SC)-CpRh(pesa)Cl (9) arise. However, the 1H
NMR spectrum at RT showed only one set of broad
signals (see below).
Slow diffusion of petroleum ether into a toluene
solution of CpRh(pesa)Cl (9) at RT gave crystals
suitable for X-ray structure analysis. Surprisingly, the
two diastereomers (RRh,SC)- and (SRh,SC)-CpRh(pe-
sa)Cl (9) were present in a 1:1 ratio in the same single
crystal. Thus, the two diastereomers of compound 9
follow the molecular recognition motif of the inverted
pianostools [20]. There is an ‘almost inversion center’
between the two diastereomers (RRh,SC) and (SRh,SC),
Scheme 5. Time dependent 1H NMR spectra of Cp*Rh(pepy)I (8)
(233 K, 400 MHz, CD2Cl2).
Scheme 6. Kinetics of the epimerization of (RRh,SC)-Cp*Rh(pepy)I (8) (233 K, CD2Cl2). Ordinate:integral minor (SRh,SC)-diastereomer.

H. Brunner et al. / Polyhedron 22 (2003) 2639Á
/
2646
2645
Scheme 7. Synthesis of CpRh(pesa)Cl (9).
˚
the outside of the inversion pairs. With 5.487 A the
distance between the two rhodium atoms is typical for
the molecular recognition motif of the inverted pianos-
tools. The crystal lattice of CpRh(pesa)Cl (9) is formed
by translation of the inversion pairs of Scheme 8.
Table 1 contains the crystallographic data and Table 3
the most important distances and angles of the two
diastereomers of CpRh(pesa)Cl (9).
5.2. Epimerization studies
1
The H NMR spectrum of CpRh(pesa)Cl (9) at RT
Scheme 8. Crystal structure of CpRh(pesa)Cl (9).
showed only broad signals (Scheme 9). On cooling the
sample down to ꢂ60 8C in CD2H2 solution, the
/
which approach each other with their ‘racemic sides’
spectrum became sharp and the signals for the two
diastereomers could be observed separately. The coales-
cence temperature Tc proved to be 283 K. The ratio at
213 K was 77:23, an assignment to the diastereomers
(RRh,SC) and (SRh,SC) not being possible. The kinetic
forming hydrogen bridges between CÃ/H bonds of the
Cp ring and the oxygen and chlorine atoms of the
neighbor molecule. The two (SC)-configurated N-sub-
stituents which disturb centrosymmetry are oriented to
parameters
for
the
epimerization
reaction
(RRh; SC)-CpRh(pesa)ClX(SRh; SC)-CpRh(pesa)Cl
were calculated on the basis of the data in Table 4 [4,21].
The half-lifes at 11.3 8C in CD2Cl2 are 31 ms for the
forward reaction and 9.2 ms for the back reaction of the
epimerization of the two diastereomers A and B.
However, an assignment of A/B to (RRh,SC)/(SRh,SC)
Table 3
˚
Selected bond lengths (A), bond angles (8) and dihedral angles (8) of
complex 9
(RRh,SC)-CpRh- (SRh,SC)-CpRh-
(pesa)Cl (9)
(pesa)Cl (9)
cannot be made. The reaction is faster by a factor of 2Á3
/
Bond lengths
than the epimerization of Cp*Rh(pesa)Cl (1) in CD2Cl2
(28 ms/6.7 ms at 21 8C) [4].
Rh1/2Ã
Rh1/2Ã
Rh1/2Ã
Rh1/2Ã
/
Cp1/Cp2
N1/2
O1/2
1.795
1.802
/
2.083(5)
2.046(5)
2.409(2)
2.103(6)
2.046(5)
2.406(9)
/
/
Cl1/2
Bond angles
N1/2ÃRh1/2Ã
O1/2ÃRh1/2Ã
Cl1/2ÃRh1/2Ã
/
/
O1/2
Cl1/2
N1/2
89.9(2)
88.01(14)
84.20(16)
89.6(2)
87.01(16)
87.97(16)
6. Supplementary material
/
/
/
/
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre. For CCDC numbers see Table 1. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2
Dihedral angles
Rh1/2ÃN1/2ÃC12/32Ã
N1/2ÃC12/32ÃC11/31Ã
C12/32ÃC11/31ÃC6/26Ã
C11/31ÃC6/26ÃO1/2ÃRh1/2
C6/26ÃO1/2ÃRh1/2ÃN1/2
O1/2ÃRh1/2ÃN1/2ÃC12/32
/
/
/
C11/31
C6/26
O1/2
5.2(12)
0.2(11)
3.1(12)
/
/
/
ꢂ
ꢂ/
/
0.9(13)
3.5(12)
2.6(10)
0.9(6)
/
/
/
ꢂ
ꢂ/
/
0.9(11)
4.4(10)
5.6(6)
/
/
/
/
/
/
/
/
/
ꢂ
/
4.5(6)
ꢂ
/
3.4(5)
1EZ, UK (fax: ꢁ44-1223-336033; e-mail: deposit@
/
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

2646
H. Brunner et al. / Polyhedron 22 (2003) 2639Á2646
/
Scheme 9. 1H NMR spectrum of CpRh(pesa)Cl (9) (400 MHz, CD2Cl2) at variable temperatures.
Table 4
Results of the coalescence measurements of Cp*Rh(beapy)Cl (5), CpRh(pesa)Cl (9), Cp*Rh(pesa)Cl (1) and Cp*Ir(pesa)Cl (2)
Compound
Cp*Rh(beapy)Cl (5)
CpRh(pesa)Cl (9)
Cp*Rh(pesa)Cl (1) (Ref. [4])
Cp*Ir(pesa)Cl (2) (Ref. [4])
Coalescence temperature Tc (8C)
Dn at Tc (extrapolated) (Hz)
Equilibrium constant (K)
115:1(93)
16.84
1.00
/
11.3(93)
/
21(93)
/
ꢂ
/
1.8(93)
/
39.30
0.30
22.4
31
52.60
0.24
24.7
28
39.04
0.18
13.7
50
Rate constant kA (sꢂ1
)
15.0
46
Half-life tA (ms)
Rate constant kB (sꢂ1
Half-life tB (ms)
)
15.0
46
75.0
9.2
127.5
6.7
127.5
90.6
8.0
Rate constant kꢀ
/
kAꢁ
/
kB (sꢂ1
)
30.0
400 MHz, DMSO-D6
97.4
90.6
400 MHz, CDCl3
1H NMR parameters
400 MHz, CD2Cl2 400 MHz, CD2Cl2
[11] A. Kollnberger, Dissertation, University of Regensburg, Ger-
¨
many, 2002.
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