Rh and Ir complexes containing multidentate, C2-symmetry ligands. Structural and catalytic properties in asymmetric hydrogenation

Article abstract of DOI:10.1016/S0022-328X(00)00083-8

A comparative investigation of the interaction of Rh(I) and Ir(I) with a series of mixed-donor multidentate, C₂-symmetry ligands has been carried out. The complexes have been prepared by the reaction of [MCl(cod)]₂ (cod=1,5-cyclooctadiene) with AgPF₆ and further treatment with the ligand. All ligands form 1:1 (metal:ligand) species with the above metal ions although, in a few instances, species of type [M₂L₂]²⁺ were also detected. The structures of these complexes were elucidated by analytical and spectroscopic data (elemental analysis, mass spectroscopy, IR, ¹H- and ¹³C-NMR). Complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%.

Full text of DOI:10.1016/S0022-328X(00)00083-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 601 (2000) 284–292
Rh and Ir complexes containing multidentate, C2-symmetry
ligands. Structural and catalytic properties in asymmetric
hydrogenation
M.J. Alco´n a, M. Iglesias* a,1, F. Sa´nchez* b,2, I. Viani b
a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain
b Instituto de Qu´ımica Orga´nica, CSIC, Juan de la Cier6a, 3. E-28006 Madrid, Spain
Received 9 December 1999; received in revised form 23 January 2000
Abstract
A comparative investigation of the interaction of Rh(I) and Ir(I) with a series of mixed-donor multidentate, C2-symmetry
ligands has been carried out. The complexes have been prepared by the reaction of [MCl(cod)]2 (cod=1,5-cyclooctadiene) with
AgPF6 and further treatment with the ligand. All ligands form 1:1 (metal:ligand) species with the above metal ions although, in
a few instances, species of type [M2L2]2+ were also detected. The structures of these complexes were elucidated by analytical and
spectroscopic data (elemental analysis, mass spectroscopy, IR, 1H- and 13C-NMR). Complexes were tested in the asymmetric
hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Rhodium; Iridium; Asymmetric hydrogenation; Multidentate ligands
1. Introduction
many more possibilities than phosphorus compounds.
Serious attempts are now being made to evaluate the
possibilities of these ligands in both catalytic systems
where the efficiency of phosphines has already been
demonstrated and for new specific catalytic reactions.
This last point, especially when non-precious transition
metals are used, offers a great potential interest.
C2-symmetric diamines could be used in carbonyl
reduction by hydride transfer with more than 99% ee
[2]. It is noticeable that even groups known for their
work on the use of chiral phosphines have turned their
attention to nitrogen-containing ligands. Thus, in 1995,
Noyori and his group used a synergetic effect of di-
amines and phosphines in carbonyl reduction by hydro-
genation with molecular H2 [3]. Chiral diureas with the
C2-diamine basic structure gave also promising results
[4]. Nitrogen-containing ligands may be used in asym-
metric catalysis with transition metals less expensive
than noble metals. Thus, semicorrins were used in
cobalt complexes by Pfaltz and co-workers [5] for the
reduction of a,b-unsaturated molecules with NaBH4.
With these facts in mind (nitrogen ligands as efficient
alternatives to phosphorus ligands and the importance
of the C2-symmetry of the ligands), we decided to
Nitrogen-containing ligands are being used more and
more in asymmetric catalysis [1]. They turn out to be
suitable for any type of catalysis and especially for
heterogeneous catalysis, which is one of their main
advantages over phosphines. If we consider the latest
new methodologies developed to obtain easy separation
of the catalyst from the reaction products (i.e. heteroge-
neous catalysts with modifiers, homogeneous supported
catalysts, liquid polyphasic catalysts, molecular im-
printing…), nitrogen-containing ligands appear to offer
Fig. 1. C2-symmetry ligands.
1 *Corresponding author. Tel.: +34-91-3349032; fax: +34-91-
3720623; e-mail: marta.iglesias@icmm.csic.es
2 *Corresponding author.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00083-8

M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
285
investigate the preparation and use, as catalytic precur-
2.1. Synthesis of ligands
sors in some enantioselective reactions, of rhodium(I)
and iridium(I) cationic complexes containing C2-sym-
metry ligands. In previous papers we have conducted
an extensive study of the synthesis of new chiral ligands
in order to apply them in asymmetric synthesis. For
that purpose, special efforts have been devoted toward
the synthesis of easily accessible ligands based on natu-
ral aminoacids [6]. These ligands have been applied
with success in asymmetric catalysis like addition of
diethylzinc to enones [7], hydrogenation [8], cyclo-
propanation [9] and oxidation of olefins [10]. Here, we
report on the preparation of some chiral multidentate,
C2-symmetry ligands (Fig. 1) and their Rh(I) and Ir(I)
complexes, structural data for the ligands and com-
plexes, and catalytic properties of the new metal com-
plexes. The complexes have proven to be active in the
asymmetric hydrogenation of prochiral olefins at 4 bar
and 313 K. A motivation for the present study was to
investigate the effect on complex formation of the
structural variation that occurs along this series of
ligands.
2.1.1. N,N%-Bis[(S)-N-benzyloxycarbonyl-
prolyl]ethylenediamine (2)
To a solution of (S)-N-benzyloxycarbonylproline
(1a) (3.7 g, 14.8 mmol) and triethylamine (1.5 g, 14.8
mmol) in THF (80 ml), ice-cooled ethyl chloroformate
(1.6 g, 14.8 mmol) was added dropwise with vigorous
stirring. When the addition was finished the pasty reac-
tion mixture was stirred for 30 min (temperature 5–
10°C), and a very reactive mixed anhydride was
obtained. To the anhydride solution, ethylenediamine
(0.5 ml, 7.4 mmol) in THF (5 ml) was added dropwise
for 15 min and the mixture was stirred at 0°C for 1 h,
and filtered. The solvent was evaporated and the
residue extracted with ethyl acetate and washed succes-
sively with water, aqueous NaHCO3 and brine. The
organic layer was dried over magnesium sulphate and
evaporated in vacuo to give 3.6 g (94%) of 2. M.p.
163–164°C. [h]2D0 −42.4° (c 1, CH2Cl2). C28H34N4O6
(522): calc. C, 64.4; H, 6.6; N, 10.7. Found: C, 64.6; H,
6.8; N, 11.0%. IR (cm−1): w=3380, 3340 (NH); 1710
1
(CꢀOCbz); 1670 (amide I); 1530 (amide II). H-NMR
(CDCl3, 50°C): l 7.35–7.26 (m, 10H, Ph); 7.26–6.53 (s,
br, 2H, NꢁH); 5.18 (d, 2H, 12.5 Hz, OCH2Ph); 5.10 (d,
2H, 12.5 Hz, OCH2Ph); 4.24–4.21 (m, 2H, H2); 3.57–
3.34 (m, 4H, H5); 3.20–3.19 (m, 4H, CH2ꢁNH); 2.17–
2.15 (m, 2H, H3); 2.07–1.96 (m, 4H, H3%, H4); 1.89–1.83
(m, 2H, H4%). 13C-NMR (CDCl3, 50°C): l 172.90
(CꢀOamide); 155.37 (CꢀOester); 136.49 (CPhꢁR); 128.38–
127.68 (CPhꢁH); 67.06 (OCH2Ph); 60.79 (C2); 47.07
(C5); 39.22 (CH2ꢁNH); 29.22 (C3); 24.43 (C4). MS (m/z,
%): 522 [M+, 0.5]; 306 (43); 204 (3); 108 (77); 79 (100);
77 (63); 70 (37).
2. Experimental
All preparations of organometallic complexes were
carried out under dinitrogen by conventional Schlenk-
tube techniques. The starting complexes [Rh(cod)Cl]2
and [Ir(cod)Cl]2 were prepared according to the litera-
ture methods [11,12]. All solvents were carefully de-
gassed before use. C, H and N analysis was carried out
by the analytical department of the Institute of Materi-
als Science (CSIC) with a Perkin–Elmer 240 apparatus.
Metal contents were analysed by atomic absorption
using a Unicam Philips SP9 apparatus and a plasma
ICP Perkin–Elmer 40. Mass spectra were performed on
a Hewlett–Packard 1100 MSD mass spectrometer
(ESI-MS, APCI-MS) with positive mode and VG-Au-
tospec (FAB+). IR spectra were recorded with a Nico-
let XR60 spectrophotometer (range 4000–200 cm−1) in
2.1.2. N,N%-Bis[(S)-prolyl]ethylenediamine (3)
A mixture of N,N%-bis[(S)-N-benzyloxycarbonylpro-
lyl]ethylenediamine (2) (1 g, 1.9 mmol), cyclohexene
(0.31 g, 3.8 mmol) and 180 mg of commercial Pd–C
(10%) in 20 ml of ethanol was heated under reflux for 1
h in argon, cooled and filtered over Celite. The catalyst
was washed with ethanol, and the filtrate and wash
liquids were evaporated under reduced pressure to give
405 mg of 3. Yield: 0.46 g (94%). M.p. 92–93°C. [h]D20
−76.8° (c 1, CH2Cl2). C12H22N4O2 (254): calc. C, 56.7;
H, 8.7; N, 22.0. Found: C, 56.30; H, 8.6; N, 22.3%. IR
(KBr, cm−1): w=3278 (NH); 1646 (amide I); 1552
1
KBr pellets; H and 13C spectra were taken on Varian
1
XR300 and Bruker 200 spectrometers, repectively. H-
NMR chemical shifts are given in ppm using te-
tramethylsilane as an internal standard. High-resolution
13C-MAS or CP/MAS-NMR spectra of powdered sam-
ples, in some cases also with a Toss sequence, in order
to eliminate the spinning side bands, were recorded at
100.63 MHz, 6 ms 90° pulse width, 2 ms contact time
and 5–10 ms recycle delay, using a Bruker MSL 400
spectrometer equipped with an FT unit. The spinning
frequency at the magic angle (54°44%) was 4 KHz.
Optical rotation values were measured at the sodium-D
line (589 nm) with a Perkin–Elmer 241 MC polarime-
ter. Gas chromatography analysis was performed using
a Hewlett–Packard 5890 II with a flame ionisation
detector in a cross-linked methylsilicone column [13].
1
(amide II). H-NMR (CDCl3, 50°C): l 7.81–7.63 (s, br,
2H, NꢁH); 3.67 (dd, 2H, 5.1, 8.9 Hz, H2); 3.36–3.34
(m, 4H, CH2ꢁNH); 2.97 (ddd, 2H, 10.2, 6.6, 7.0 Hz,
H5); 2.88 (ddd, 2H, 10.2, 5.4, 7.3 Hz, H5%); 2.6–2.3 (s,
br, 2H, NꢁH); 2.16–2.02 (m, 2H, H3); 1.94–1.82 (m,
2H, H3%); 1.72–1.63 (m, 4H, H4). 13C-NMR (CDCl3,
50°C): l 175.81 (CꢀO); 60.61 (C2); 47.05 (C5); 39.05
(CH2ꢁNH); 30.56 (C3); 25.96 (C4). MS (m/z, %): 255
([M++1], 5.62); 185 (15); 157 (6); 142 (5); 115 (2); 98
(5); 70 (100).

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2.1.3. N,N%-Bis{[(S)-pyrrolidin-2-yl]methyl}-
ethylenediamine (4)
C, 76.2; H, 9.4; N, 13.6%. IR (film, cm−1): w=3360
(NꢁH). H-NMR (CDCl3, 50°C): l 7.58–7.12 (m, 10H,
1
To a solution of N,N%-bis[(S)-prolyl]ethylenediamine
(3) (1 g, 4 mmol) in THF (25 ml) ice-cooled Li[AlH4]
(1.4 g, 36 mmol) was added dropwise. When the addi-
tion was finished the reaction mixture was stirred for 24
h under reflux. The reaction products were analysed by
GC–MS. The product was isolated as an air-sensitive
yellow oil. Yield: 0.62 g (70%). [h]2D0 +19.3° (c 5.5,
CH2Cl2). C12H26N4 (226): calc. C, 63.7; H, 11.5; N,
24.8. Found: C, 63.8; H, 11.2; N, 24.4%. IR (film,
cm−1): w=3288 (NꢁH). 1H-NMR (CDCl3, 50°C, D2O):
l 3.18–3.11 (m, 2H, H2); 2.93–2.71 (m, 4H, H5);
2.70–2.65 (m, 4H, CH2ꢁNH); 2.57 (dd, 2H, 4.9 Hz,
11.6 Hz, H6); 2.48 (dd, 2H, 7.9 Hz, 11.6 Hz, H6%);
1.87–1.73 (m, 4H, H3); 1.73–1.59 (m, 4H, H4%). 13C-
NMR (CDCl3, 50°C): l 58.17 (C2); 54.72 (C6); 49.36
(NHꢁCH2ꢁCH2ꢁNH); 46.11 (C5); 29.43 (C3); 25.35
(C4). MS (m/z): 227 [M++1], 156, 113, 84, 70.
Ph); 3.93 (d, 12.9 Hz, 2H, CH2Ph); 3.23 (d, 12.9 Hz,
2H, CH2Ph); 2.93–2.78 (m, 2H, H5); 2.20–2.02 (m, 2H,
H5%); 2.73–2.42 (m, 10H, HNꢁCH2ꢁCH2ꢁNH, H6, H2);
2.20–2.02 (m, 2H, H5%); 2.02–1.75 (m, 4H, H3); 1.75–
1.56 (m, 4H, H4). 13C-NMR (CDCl3, 50°C): l 139.9
(CPhꢁR); 128.9–126.7 (CPhꢁH); 63.8 (C2); 59.4 (CH2-
Ph); 54.5 (C5); 53.1 (C6); 49.7 (HNꢁCH2ꢁCH2ꢁNH);
29.2 (C3); 22.8 (C4). MS (m/z): 407 [M++1], 316, 225,
91.
2.2. Preparation of {[M(C2-ligand)(THF)]PF6}n
complexes (M=Rh, Ir; n=1, 2) (I–VIII)
Only the preparation of {[Rh(3)(THF)2]PF6}2·THF
(I) is described in detail, the experimental procedure
being the same for all the other complexes. Silver
hexafluorophosphate (0.4 mmol) in THF (40 ml) was
added to [Rh(cod)Cl]2 (0.2 mmol) in THF (10 ml) and
the mixture was stirred vigorously at room temperature
for 30 min. Precipitated silver chloride was filtered off
and the yellow solution was treated with the ligand (0.4
mmol) in THF. The mixture was stirred for 24 h under
reflux. The solvent was evaporated under reduced pres-
sure to 2 ml. Careful addition of diethyl ether caused
the precipitation of a yellow–orange solid which was
collected by filtration, washed with diethyl ether and
dried under vacuum (10–3 mmHg) to give the yellow
2.1.4. N,N%-Bis[(S)-N-benzylprolyl]ethylenediamine (5)
To a solution of N,N%-bis[(S)-prolyl]ethylenediamine
(3) (1 g, 4 mmol) and triethylamine (0.8 g, 1.12 ml, 8
mmol) in ice-cooled THF (25 ml), benzyl bromide (1.37
g, 0.95 ml, 8 mmol) was added dropwise with vigorous
stirring. When the addition was finished the reaction
mixture was stirred for 2 h at 0°C. The solvent was
evaporated and the residue extracted with ethyl acetate
and washed successively with water, aqueous NaHCO3
and brine. The organic layer was dried over magnesium
sulphate and evaporated in vacuo to give 5 as a white
solid. The product was purified by flash chromatogra-
phy with 1:1 THF–hexane. Yield: 1.63 g (97%); m.p.
103–104°C. [h]2D0 −67.2° (c 1, CH2Cl2). C26H34N4O2
(434): calc. C, 71.9; H, 7.9; N, 12.9. Found: C, 72.1; H,
7.7; N, 12.7%. IR (KBr, cm−1): w=3312 (NH); 1656
cationic
complex.
Yield:
68%;
m.p.\230°C.
C24H44F12N8O4P2Rh2·5THF (1365): calc. C, 38.7; H,
6.2; N, 8.2; Rh, 15.1. Found: C, 38.5; H, 6.3; N, 8.6;
Rh, 14.6%. IR (KBr, cm−1): w=3400, 3200 (NꢁH);
1670, 1600 (amide I); 1550 (amide II); 840 (PꢁF).
13C-NMR (solid): l 183.95, 168.66 (CꢀO); 76.33 (C2);
65.29 (C5); 30.47 (CH2ꢁNH, C3, C4). UV–vis (DMF,
10−3 M): umax (log ‡) 378 nm (3.37), 292 (3.93). \M
(V−1 cm2 mol−1, 10−3 M, CH3CN) 142–158. MS+
(m/z): 821 ({[Rh(3)]PF6}2−PF8); 675 ({[Rh(3)]PF6}2−
3−4F); 465 ({[Rh(3)]PF6}2−3−2PF6); 255 (3+1).
Complexes II–VIII were prepared through a proce-
dure similar to that given for (I) using 0.2 equivalents
of [Rh(cod)Cl]2 or [Ir(cod)Cl)]2 and 0.4 equivalents of
the appropriate ligand.
1
(amide I); 1524 (amide II). H-NMR (CDCl3, 50°C): l
7.62–7.49 (s, br, 2H, NꢁH); 7.38–7.15 (m, 10H, Ph);
3.79 (d, 12.9 Hz, 2H, CH2Ph); 3.51 (d, 12.9 Hz, 2H,
CH2Ph); 3.37–3.28 (m, 4H, HNꢁCH2ꢁCH2ꢁNH); 3.22–
3.13 (m, 2H, H2); 3.08–2.99 (m, 2H, H5); 2.42–2.32 (m,
2H, H5%); 2.26–2.11 (m, 2H, H3); 1.91–1.78 (m, 2H,
H3%); 1.78–1.61 (m, 4H, H4). 13C-NMR (CDCl3, 50°C):
l
175.18 (CꢀO); 138.42 (CPhꢁR); 128.70–127.21
(CPhꢁH); 67.15 (C2); 59.88 (CH2Ph); 54.01 (C5); 38.81
(HNꢁCH2ꢁCH2ꢁNH); 30.75 (C3); 24.05 (C4). MS (m/
z): 435 [M++1]; 252, 91.
2.2.1. [Rh(4)(THF)2]PF6 (II)
Yellow. Yield: 61%; m.p.\230°C. C12H26F6N4-
PRh·2THF (618): calc. C, 38.8; H, 6.9; N, 9.1; Rh, 16.6.
Found: C, 38.7; H, 6.3; N, 8.7; Rh, 16.3%. IR (KBr,
cm−1): w=3400, 3260 (NꢁH); 840 (PꢁF). 13C-NMR
(solid): l 59.33 (C2); 50.12 (br, NHꢁCH2ꢁCH2ꢁNH, C6,
C5); 27.34 (br, C3, C4). UV–vis (DMF, 10−3 M): umax
(log ‡) 377.5 (2.36); 288.5 nm (3.39), 292 (3.93). \M
(V−1 cm2 mol−1, 10−3 M, CH3CN) 132–143. MS+
(m/z): 455 ([Rh(4)]PF6−F); 329 ([Rh(4)]+).
2.1.5. N,N%-Bis{[(S)-N-benzyl-pyrrolidin-
2-yl]methyl}ethylenediamine (6)
The diamine 6 was prepared by a procedure similar
to that given for 4 starting from N,N%-bis[(S)-N-benzyl-
prolyl]ethylenediamine (5) (1.74 g, 4 mmol). The
product was isolated as an air-sensitive yellow oil.
Yield: 1.42 g (87%). [h]2D0 +23.6° (c 1, CH2Cl2).
C26H38N4 (406): calc. C, 76.8; H, 9.4; N, 13.8. Found:

M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
287
2.2.2. {[Rh(5)(THF)]PF6}2 (III)
(CH2Ph), 49.4 (C5); 44.1 (CH2ꢁNH); 34.7 (C3); 25.0
(C4). UV–vis (DMF, 10−3 M): umax (log m) 399.5 nm
(2.17); 289.0 (3.44). \M (V−1 cm2 mol−1, 10−3 M,
CH3CN) 110. MS+ (m/z): 627 ([Ir(5)]+), 435 (5).
Yellow. Yield: 68%; m.p. 123–126°C. [h]2D0 −60° (c
0.5, EtOH); C52H68F12N8O4P2Rh2·2THF (1504): calc.
C, 47.9; H, 5.6; N, 7.4; Rh, 13.7. Found: C, 48.4; H,
5.3; N, 7.2; Rh, 13.3%. IR (KBr, cm−1): w=3376, 3260
(NꢁH); 1660, 1612 (amide I); 1526 (amide II); 834
(PꢁF). 13C-NMR (solid): l 183.7, 177.04 (CꢀO); 129.47
(Ph); 66.62 (C2, CH2Ph, C5); 28.28 (CH2ꢁNH, C3, C4).
UV–vis (DMF, 10−3 M): umax (log ‡) 348.0 nm (3.27),
289.0 (3.70). \M (V−1 cm2 mol−1, 10−3 M, CH3CN)
144–157. MS+ (m/z): 855 ({[Rh(5)]PF6]2}−PF6−
4Bz); 663 ([Rh(5)]PF6−F); 435 ((5)+1).
2.2.7. [Ir(6)(THF)]PF6 (VIII)
Light brown. Yield: 61%; m.p. 135–140°C.
C26H38F6IrN4P·THF (816): calc.: C, 44.2; H, 5.8; N,
6.9; Ir, 23.6. Found: C, 44.0; H, 5.7; N, 7.0; Ir, 23.1%.
IR (KBr, cm−1): w=3450 (NꢁH); 845 (PꢁF). 13C-NMR
(solid): l 138.7 (CPhꢁR); 130–125 (CPhꢁH); 65–45 (C2,
CH2Ph, C5, C6, HNꢁCH2ꢁCH2ꢁNH); 30.0–20.0 (C3,
C4). UV–vis (DMF, 10−3 M): umax (log ‡) 352.3 nm
(2.47); 288.0 (3.32). \M (V−1 cm2 mol−1, 10−3 M,
CH3CN) 115. MS+ (m/z): 725 ([Ir(6)]PF6−F); 706
([Ir(6)]PF6−2F); 407 (6).
2.2.3. [Rh(6)]PF6 (IV)
Yellow. Yield: 67%; m.p. 165–170°C (dec.).
C26H38F6N4PRh (654.5): calc. C, 47.7; H, 5.9; N, 8.6;
Rh, 15.7. Found: C, 47.3; H, 5.6; N, 8.1; Rh, 15.2%. IR
(KBr, cm−1): w=3420 (NꢁH); 850 (PꢁF). 13C-NMR
(solid): l 140–125 (CPhꢁR, CPhꢁH); 65.1 (C2); 60–45
(CH2Ph, C5, C6, HNꢁCH2ꢁCH2ꢁNH); 29.0 (C3); 22.0
(C4). UV–vis (DMF, 10−3 M): umax (log ‡) 375.9 nm
(2.64); 288.0 (3.32). \M (V−1 cm2 mol−1, 10−3 M,
CH3CN) 146–159. MS+ (m/z): 635 ([Rh(6)]PF6ꢁF);
404 ((6)−2).
2.3. Preparation of {[Rh(CO)2(ligand)]PF6}n (n=1, 2)
Carbon monoxide (3 ml min−1
) was bubbled
through a thoroughly stirred dichloromethane solution
of the corresponding Rh complex for 3 h, during which
period colour intensification occurred. The precipitated
complexes were filtered off, washed with diethyl ether
and vacuum dried (10−3 mmHg). Yield: \95%.
2.2.4. [Ir(3)(THF)]PF6 (V)
Brick-red. Yield: 61%; m.p.\230°C. C12H22F6-
IrN4O2P·THF (664): calc.: C, 29.0; H, 4.6; N, 8.4; Ir,
29.0. Found: C, 28.9; H, 4.6; N, 8.9; Ir, 28.6%. IR
(KBr, cm−1): w=3360 (NꢁH); 1670 (amide I); 1560
(amide I, II); 850 (PꢁF). 13C-NMR (solid): l 168.7
(CꢀO); 60.7 (C2); 46.3 (C5); 39.4 (CH2ꢁNH); 30.1 (C3);
25.2 (C4). UV–vis (DMF, 10−3 M): umax (log ‡) 410.5
nm (3.13), 287.5 (3.46). \M (V−1 cm2 mol−1, 10−3 M,
DMF) 145. MS+ (m/z): 664 ([Ir(3)(THF)]PF6); 591
([Ir(3)]PF6); 572 ([Ir(3)]PF6ꢁF); 553 ([Ir(3)]PF6ꢁ2F); 255
(3+1).
2.4. Catalytic experiments
The catalytic properties, in hydrogenation reactions,
of the above Rh and Ir complexes were examined under
conventional conditions for batch reactions in a reactor
(Autoclave Engineers) of 100 ml capacity at 313 K
temperature,
4 atm dihydrogen pressure and a
metal:substrate molar ratio of 1:500. The results were
monitored by GLC. The kinetics results are shown in
Table 1.
2.2.5. [Ir(4)(THF)]PF6 (VI)
3. Results and discussion
Brick-red. Yield: 65%; m.p.\230°C. C12H26F6Ir-
N4P·THF (636): calc. C, 30.2; H, 5.4; N, 8.8; Ir, 30.2.
Found: C, 30.3; H, 5.0; N, 8.9; Ir, 29.5%. IR (KBr,
cm−1): w=3440 (NꢁH); 850 (PꢁF). 13C-NMR (solid): l
60.0–40.0 (C2, NHꢁCH2ꢁCH2ꢁNH, C6, C5); 27.7 (C3,
C4). UV–vis (DMF, 10−3 M): umax (log ‡) 356 nm
(2.97), 289 (3.21). \M (V−1 cm2 mol−1, 10−3 M,
DMF) 170. MS+ (m/z): 655 ([Ir(4)(THF)]PF6+H2O);
583 ([Ir(4)(H2O)]PF6); 543 ([Ir(4)]PF6ꢁF); 418 ([Ir(4)]+).
3.1. Synthesis and characterisation of ligands
The ligands were prepared and well characterised by
modifying the synthesis described previously [14]. All
reaction steps were fine tuned for high yield and selec-
tivity. The preparation of the diamides N,N%-bis[(S)-
prolyl]ethylenediamine (3) and N,N%-bis[(S)-N-ben-
zylprolyl]ethylenediamine (5) was achieved starting
from the easily available
L-proline, protected as the
2.2.6. [Ir(5)(THF)]PF6 (VII)
N-carbobenzyloxy (1a) or N-benzyl (1b) derivative, ac-
cording to Scheme 1. The corresponding amines N,N%-
bis{[(S)-pyrrolidin-2-yl]methyl}ethylenediamine (4) and
N,N%-bis{[(S)-N-benzyl-pyrrolidin-2-yl]methyl}ethylen-
ediamine (6) were obtained by reduction of the respec-
tive amide with lithium aluminium hydride. The ligands
have two optically active centres; both have the S
Light brown. Yield: 60%; m.p. 138–140°C.
C26H34F6IrN4O2P·THF (844): calc.: C, 42.7; H, 5.0; N,
6.6; Ir, 22.8. Found: C, 42.7; H, 4.6; N, 6.3; Ir, 22.4%.
IR (KBr, cm−1): w=3372 (NꢁH); 1670 (amide I); 1560
(amide I, II); 850 (PꢁF). 13C-NMR (solid): l 167.0
(CꢀO); 142.2 (CPhꢁR); 130.0–120.0 (Ph); 64.0 (C2); 57.5

288
M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
Table 1
Asymmetric hydrogenation of diethyl 2-methylbut-2-enedioate (cat./
subs.=1/500, T=313 K; pH2=4 atm)
Catalyst
Induction
time (min)
Conversion
(%)
TOR a Optical yield b
(% ee)
Scheme 2.
I
II
30
15
30
5
3
0
78
80
82
94
100
85
304
405 20.0
800 4.2
1467 21.5
6800 36.0
5000 35.4
5.5
III
IV
V
VI
VII
VIII
3.2. Synthesis of metal complexes
The complexes {[M(ligand)(solvent)]PF6}n (M=Rh,
Ir; n=1, 2) I–VIII, have been prepared as shown in
Scheme 2 following a general procedure.
0
0
78
95
5000
9.0
6333 19.5
The dimeric [M(cod)Cl]2 and two equivalents of
AgPF6 were reacted in THF for 2 h and the precipi-
tated AgCl was filtered off. The ligand was added to
the filtrate containing the cationic mononuclear
[M(cod)(THF)x]PF6 complex. The isolated complexes,
I–VIII, were characterised by elemental analysis, mass
spectrometry (FAB+, ESI-MS and APCI-MS with pos-
itive mode), IR and 13C-NMR spectroscopy, which
present values in accordance with the proposed struc-
tures depicted in Fig. 2.
Thus, the mononuclear complexes [M(L)(THF)]PF6
(M=Rh, Ir) (II, IV–VIII) were prepared in good yield
(\60%) by ligand exchange of L with [M(cod)-
(THF)2]PF6 in THF. The complexes were isolated as
yellow, microcrystaline, and air-stable solids. On the
other hand, the dinuclear yellow complexes {[Rh(L)-
(THF)]PF6}2, (I, III), were obtained in 68% yield by
reaction of [Rh(cod)(THF)2]PF6 and one equivalent of
ligand (3, 5) in THF. Mass spectrometry and elemental
analysis of these products indicated a dimeric formula
a mmol substrate mmol cat.−1 min–1
.
b Optical yield was determined by GLC using capillary a chiral
column based on alkylated cyclodextrins [13].
absolute configuration. All intermediates and final
products has been obtained with a total yield of
\80%. Diamides 3 and 5 were isolated as white solids
and amines 4 and 6 were isolated as colourless oils that
are stable at low temperature in an inert atmosphere.
The ligands were characterised unequivocally by mass
1
spectrometry, IR, and H- and 13C-NMR spectroscopy
and gave satisfactory elemental analyses. Mass spec-
trometry shows the highest ions at m/z 254, 226, 434
and 406, which corresponds to the molecular weights of
compounds 3–6, respectively. The H- and 13C-NMR
1
spectra obtained are in agreement with those expected
for these C2-symmetrical ligands.
Scheme 1.

M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
289
(C24H44F12N8O4P2Rh2 (I), C52H68F12N8O4P2Rh2 (III)),
suggesting that two rhodium atoms were bound to two
ligands in accordance with the results of the IR and
NMR studies. These microcrystaline products are very
stable but crystals suitable for X-ray analysis could not
be obtained.
I displays two separate resonances at 184.0 and 168.7
ppm indicating the presence of two inequivalent CꢀO
moieties which is in agreement with the structural pro-
posal show in Fig. 2. The signal at 184.0 ppm is
downfield shifted as corresponds to CꢀO coordination
and the signal at 168.7 shifted at high field due to
N-amide coordination and the existence of intermolecu-
lar ꢁCꢀO···HNꢁ interaction (as we have determined by
X-ray analysis in the analogous Cu complex [15]). This
shift to high field indicates a higher electron density in
this complex. The 13C-NMR spectrum of complex III
also shows two carbonyl signals at 183.7 and 177.0
ppm. The signal downfield shifted (+8.5 ppm) corre-
sponds to the Rh’OꢀCꢁ bond and the resonance at
177.0 ppm with a low high field shift (−1.9) corre-
sponds to amideꢁN coordination.
The IR spectra of Rhꢁamine complexes (II, IV) show
w(NꢁH) bands shifted at higher frequencies (3400
cm−1) and medium bands at 840 cm−1 characteristic of
the non-coordinated PF−6 anion. The 13C-NMR spectra
of these complexes show only small chemical shift
differences compared with the free ligands.
The IR spectra of the mononuclear Ir complexes
show the expected w(NꢁH) bands shifted to higher
frequencies. There is also no doubt that in the Irꢁamide
complexes (V, VII) the CꢀO unit of the amide groups is
not involved in the coordination to the metal, since in
the IR spectra only one CꢀO stretching frequency at
1670 cm−1 appears. The band at 1570 cm−1 is due to
amideꢁN coordination. The 13C-NMR spectra of V,
VII exhibit only one signal for the amide carbons at l
168.7, 167.0 ppm shifted at higher field (−7.1, −8.8
ppm) that indicates a high electron density in these
complexes. In the Irꢁamine complexes (VI, VIII) the
13C-NMR signals show only small chemical shift differ-
ences compared with the free ligands.
The IR spectra of the free ligands exhibit the charac-
teristic bands of amides and amines. The amide I band
appears at about 1650–1670 cm−1 and amide II at
1530–1550 cm−1. The IR spectrum of dinuclear com-
plex I show a w(NꢁH) band at 3400 cm−1, shifted to
higher frequencies, due to Rh-coordinated NH of
pyrrolidine ring and a broad band at 3200 cm−1 shifted
to lower frequencies that corresponds to the w(NꢁH)
amide band. The amide I band was split at 1664 and
1600 cm−1; the band at 1664 cm−1 was attributed to
the uncoordinated keto function and the band at 1600
cm−1 shifted to lower frequencies and is due to the
CꢀO group coordinated to the Rh atom. The amide II
band appears in the same position as the free ligand.
The w(PꢁF) frequency appears at 840 cm−1
.
The IR spectrum of complex III shows w(NꢁH)
bands at 3376–3260 cm−1. The amide I band splits in
two, the band at higher frequencies (1660 cm−1) corre-
sponds to uncoordinated CꢀO group and the band
shifted to lower frequencies (1612 cm−1) indicates that
exits Rh’O bond. The band at 1526 is attributed to
amideꢁN coordination.
These complexes are almost insoluble in organic sol-
vents and we have obtained the 13C-MAS and CP/
MAS-NMR spectra of powdered samples. The
13C-NMR spectra of free ligands exhibits a single signal
for the CꢀO at about 174–175 and in the spectra of
Rhꢁamide complexes (I, III) two peaks are found for
the amide carbons. The spectrum of dinuclear complex
Electronic absorption spectra of the rhodium and
iridium complexes, measured between 800 and 200 nm,
show a clear maximum at ca. 300 nm (‡ 6000–8000)
and one shoulder at ca. 400 nm (‡ 1500–2000). Taking
into account their energy position and intensity the
bands at 400 nm could be assigned [16] to d–d transi-
tions localised on the metal ion. At shorter wave-
lengths, the bands at 300 nm may be assigned to the
Rh“ligand charge-transfer transition and intra-ligand
transitions. The conductivity data suggest 1:1 elec-
trolytes for all cationic complexes.
In order to confirm the nuclearity, the mass spectra
of new complexes were examined. In the case of com-
plex {[Rh(3)(THF)2]PF6}2·THF (I) the spectrum shows
a peak at m/z=821 (2) corresponding to a dinuclear
cationic species; two peaks at m/z=675 (98), 465 (100)
correspond to the loss of a ligand and the PF6 anion in
the dinuclear complex. For complex [Rh(4)(THF)2]PF6
(II) the spectrum shows a peak at m/z=453 (37) that
corresponds with the molecular formula ([Rh(4)]PF6); a
Fig. 2. Proposed structures for the rhodium and iridium complexes
I–VIII.

290
M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
peak at 329 (41) corresponds to the mononuclear
cationic species [Rh(4)]+. In the case of complex
{[Rh(5)(THF)]PF6}2 (III) the mass spectrum shows
peaks at m/z=855 and 661 that correspond, respec-
tively, to the dinuclear species with loss of the PF6
anion and the benzyl groups and the mononuclear
complex. The spectrum of [Rh(6)]PF6 (IV) shows a
peak at m/z=635, corresponding to the mononuclear
cationic species; for all complexes solvent molecules are
also lost. The iridium complexes are mononuclear spe-
cies. Their mass spectra show peaks at m/z 664 for
[Ir(3)(THF)]PF6, 655 for [Ir(4)(THF)]PF6·H2O, 627 for
[Ir(5)]+ and 725 for [Ir(6)]PF6−F.
complexes do not present chelating keto groups in the
IR spectra. The carbonyl stretching frequency is inde-
pendent of the nature of the ligand. The value for the
carbonyl stretch (2070, 2000 cm−1) indicates a low
1
electron density on the rhodium centre. The H- and
13C-NMR spectra show the signals corresponding to
pyrrolidine protons and carbons slightly downfield
shifted as well as broadened signals for the atoms
spatially close to the metal. The two carbonyl ligands
appear as a sharp single resonance at l 190.6 ppm.
Rhꢁamine complexes and all Ir complexes do not
react with CO, probably due to the stability of the
tetracoordinated mononuclear structure of these com-
pounds, which present ligands coordinated through the
NꢁHꢁRh bonds.
3.3. Reacti6ity towards H2 and CO
Transition-metal hydride complexes have been the
focus of considerable research because of their promi-
nent role in many catalytic hydrogenation processes.
The oxidative addition of hydrogen is, usually, the
rate-determining step for the asymmetric hydrogenation
of olefins using rhodium complexes, which contain
chiral diphosphines and diphosphites. For this reason
we believe that is interesting to study the reactivity of
the diamide, amine complexes synthesised with the H2.
The corresponding cis-dihydrido complexes were
formed at lower temperature, but they are only stable
in solution under a H2 atmosphere, to the effect that
attempts at their isolation in crystalline form rapidly
restored the starting product as the H2-loss product.
This behaviour has also been observed for related com-
plexes; cis-[IrH2(cod)L2]PF6 (L=PMePh2, PPh3, 0.5-
dppe, PBun3 or 0.5DIOP) also behave similarly [17].
On the other hand, we have also studied the reactiv-
ity of above described complexes towards CO. Thus,
bubbling carbon monoxide through dichloromethane
solutions of Rhꢁamide complexes leads to the displace-
ment of the THF molecules and to the formation of the
carbonyl derivatives. The {[Rh(3)(THF)2]PF6}2 com-
plex maintains its dimer structure in the carbonyl
derivative; in effect, the mass spectra of this compound
shows a peak at m/z=1116 that corresponds to
{[Rh(3)(CO)2]PF6}2, and peaks at 767, 639 corresponds
to the loss of fluorine and anion PF6, also, a peak at
413 is due to the mononuclear cationic species
([Rh(3)(CO)2]+). Also, the dimer {[Rh(5)(THF)]PF6}2
complex yields the carbonyl complex [Rh(5)(CO)2]PF6,
which shows in their mass spectra peaks at m/z 891,
that (corresponding to the loss of benzyl groups, anion
PF6 and four fluorine atoms) 662 that corresponds to
the loss of four fluorine anions. Also, a peak at 593 is
due to the mononuclear cationic species.
3.4. Catalytic acti6ity
In the first stage, metal complexes were evaluated as
catalysts in the hydrogenation of cyclohexene. In the
presence of 1 mol% of catalysts in ethanol at 313 K
under a pressure of 4 bar H2, the substrate was fully
converted to the alkane in 1 h. All complexes gave
similar results.
Multidentate ligands 3–6 have been used in the
rhodium, iridium asymmetric hydrogenation of diethyl
2-methylbut-2-enedioate at 313 K under 4 bar of H2.
Conversion and enantioselectivity results are shown in
Table 1.
Investigation of enantioselective hydrogenation of di-
ethyl 2-methylbut-2-enedioate was carried out to opti-
mise parameters. The precision of gas chromatographic
estimation of enantiomers allowed us to observe the
influence of the temperature on the ee with high accu-
racy. We observed an increase in ee with decreasing
temperature (8–9% ee at 293 K and 5–6% ee at 313–
323 K). A second effect is seen in the influence of H2
pressure. While several authors reported a pronounced
decrease or even a reversal [18] of the enantioselectivity
with increasing pressure, we found almost no depen-
dence of ee on pH2 between 1 and 10 atm.
In all experiments with these homogeneous catalysts
we observed an induction period, depending on the
catalyst, from 5 min for amine derivatives to 30 min for
Rhꢁamide complexes I, III (Fig. 3). The reactivity and
enantioselectivity of the hydrogenation of diethyl
mesaconate turned out to be dependent on the type of
chelate ring formed by the ligand with the rhodium
metal. Chelating ligands forming a five-membered ring
with N,O-coordination such as 3 and 5 gave only poor
enantioselectivities of 5 and 4% ee, and low chemical
yields (78 and 82%). With the ligands 4 and 6, consti-
tuting a five-membered ring with N,N-coordination, an
enantioselectivity of 20 and 21.5% ee and chemical
yields of \80% were obtained.
These carbonyl complexes exhibit in their IR spectra
two symmetrical bands at 2070, 2000 cm−1 that corre-
spond to two cis-carbonyls in the molecule and bands
for uncoordinated keto group at 1674 cm−1; these

M.J. Alco´n et al. / Journal of Organometallic Chemistry 601 (2000) 284–292
291
Fig. 3. Catalytic activity of the complexes I–VIII in the asymmetric hydrogenation of diethyl 2-methylbut-2-enedioate (cat./subs.=1/500, T=313
K; pH2=4 atm).
Iridium complexes are more efficient than rhodium
complexes. The hydrogenation with the iridium com-
plexes is completed in 30 min with higher ee (36%). The
higher ee obtained with iridium complexes could be due
to the Ir-N,N-coordination (absence of the IrꢁO bond
for amide complexes).
Acknowledgements
The authors are grateful for the financial support
from Direccio´n General de Investigacio´n Cient´ıfica y
Te´cnica of Spain (Project MAT97-1016-C02-02).
On the other hand, some complexes of ruthenium
with chiral nitrogen ligands were shown to be excellent
catalysts for enantioselective transfer hydrogenation
and the synthesis of Ru complexes with the multiden-
tate ligands described here would be interesting.
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A series of new microcrystalline air-stable rhodium
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ligands has been synthesised and characterised by spec-
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Rh(I) and Ir(I) complexes are efficient catalysts for the
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