Functional phosphines. Part XIV. Cationic (P,N)2-coordinated hydrides of iridium(III): Catalysts for C=O hydrogenation or transfer hydrogenation?

Article abstract of DOI:10.1016/j.ica.2003.12.027

Treatment of trans-[IrCl(CO)(PPh₃)₂] with Ph ₂PCH₂CH₂NH₂ in refluxing para-xylene gave (OC-6-43)-[Ir(H)(Cl)(Ph₂PCH₂CH₂NH ₂)₂]Cl (1) which interacted with K[BH(s-Bu₃)] to produce a mixture of (OC-6-22)-[IrH₂(Ph₂PCH ₂CH₂NH₂)₂]Cl (2a) and (OC-6-32)-[Ir(H)(Cl)(Ph₂PCH₂CH₂NH ₂)₂]Cl (2b). The trans-dihydride 2a was isolated in pure form from the reaction between 1 and KOH/i-PrOH. Different from its isoelectronic (P,N)₂-coordinated Ru^II analogues, the cationic chloro hydrido complex 1 does not act as a catalyst for the direct hydrogenation of acetophenone by molecular H₂, if activated by strong alkoxide base, but rather catalyzes the transfer hydrogenation of the C=O bond with methanol or isopropanol as proton/hydride sources. Dihydrido complex 2a is ascribed the role of the actual catalyst as it supports the transfer hydrogenation reaction even in the absence of base. The crystal structure of the addition compound 1·2EtOH has been determined.

Full text of DOI:10.1016/j.ica.2003.12.027

Inorganica Chimica Acta 357 (2004) 2875–2880
www.elsevier.com/locate/ica
Functional phosphines. Part XIV. Cationic (P,N)2-coordinated
hydrides of iridium(III): catalysts for ˜C@O hydrogenation
or transfer hydrogenation? q
*
€
Lutz Dahlenburg , Rainer Gotz
Institut fu€r Anorganische Chemie der Universit€at Erlangen-N€urnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany
Received 18 December 2003; accepted 26 December 2003
Available online 4 February 2004
Abstract
Treatment of trans-[IrCl(CO)(PPh3)2] with Ph2PCH2CH2NH2 in refluxing para-xylene gave (OC-6-43)-[Ir(H)(Cl)
(Ph2PCH2CH2NH2)2]Cl (1) which interacted with K[BH(s-Bu3)] to produce a mixture of (OC-6-22)-[IrH2(Ph2PCH2CH2NH2)2]Cl
(2a) and (OC-6-32)-[Ir(H)(Cl)(Ph2PCH2CH2NH2)2]Cl (2b). The trans-dihydride 2a was isolated in pure form from the reaction
between 1 and KOH/i-PrOH. Different from its isoelectronic (P; N)2-coordinated RuII analogues, the cationic chloro hydrido
complex 1 does not act as a catalyst for the direct hydrogenation of acetophenone by molecular H2, if activated by strong alkoxide
base, but rather catalyzes the transfer hydrogenation of the ˜C@O bond with methanol or isopropanol as proton/hydride sources.
Dihydrido complex 2a is ascribed the role of the actual catalyst as it supports the transfer hydrogenation reaction even in the
absence of base. The crystal structure of the addition compound 1 ꢀ 2EtOH has been determined.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Iridium; Hydrido complexes; Aminophosphine ligands; Transfer hydrogenation; X-ray structure analysis
1. Introduction
catalyst systems (P\P ¼ (R)-BINAP or 2 PPh3; H2N\
NH2 ¼ H2NCMe2CMe2NH2; base ¼ KOBu-t or K[BH
(s-Bu)3]) [3] has provided convincing evidence that the
addition of dihydrogen to the vacant coordination site
of initially formed amine–amido hydrido complexes,
[RuH(ꢀ)(P\P)(HN\NH2)], with subsequent rate-de-
termining deprotonation of the g2-H2 ligand by the
amide nitrogen atom is the turn-over limiting step of the
catalysis. From the resulting diamine dihydrides,
[RuH2(P\P)(H2N\NH2)], Hdþ=Hdꢁ equivalents are
rapidly transferred to the ketonic substrate, once the
carbonyl group has come into close contact with the
second coordination sphere of the catalyst through
an unconventional Ru–Hdꢁ ꢀ ꢀ ꢀ ˜Cdþ@Odꢁ ꢀ ꢀ ꢀ Hdþ–N
‘‘metal–ligand bifunctional’’ interaction [3–8]. Hart-
mann and Chen have shown that the activation of the
Ru(II) precursor [RuCl2{(S)-BINAP}{(S; S)-H2NCH
(Ph)CH(Ph)NH2}] for catalytic ˜C@O hydrogenation
not only needs a strong base for dehydrohalogenation
but also requires the presence of alkali metal cations,
Kþ being particularly efficacious. They postulated that
Metal complexes containing both P and N donor
ligands have been dominating the field of homogeneous
hydrogenation of organic carbonyl compounds for
several years. NoyoriÕs ruthenium complexes [RuCl2
{bis(phosphine)}(1,2-diamine)] in particular have been
found to be excellent catalysts for the reduction of
simple ketones by H2, if activated by an excess of strong
base in isopropanol solution. These compounds give
consistently high enantioselectivities if appropriate chi-
ral bis(phosphines) and diamines – especially the
enantiomers of the BINAP ligand in combination with
optically active 1,2-diphenylethylenediamine – are used
as steering ligands [2]. Seminal work of the Morris
group with two [RuH(Cl)(P\P)(H2N\NH2)]/base/H2
q Part XIII: [1].
* Corresponding author. Tel.: +49-9131-8527353; fax: +49-9131-
8527387.
E-mail address: dahlenburg@chemie.uni-erlangen.de (L. Dahlen-
burg).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.12.027

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L. Dahlenburg, R. G€otz / Inorganica Chimica Acta 357 (2004) 2875–2880
the role of the Lewis acidic potassium ion is to coor-
dinate to the amido nitrogen atom, thereby making the
(g2-H2)–Ru-amide intermediates cationic, i.e., suffi-
ciently electron-poor for enhanced heterolysis of the
dihydrogen ligand. As a result, the rate-determining
cleavage of H2 into Hþ and Hꢁ is accelerated to such a
degree that transfer hydrogenation from the solvent is
no longer competitive [9].
Against this background, it seemed worthwhile
investigating whether IrIII complexes with P,N-domi-
nated coordination environments, which a priori are
cationic by themselves, would act as catalysts for
the direct hydrogenation of ketones by molecular H2
or would rather catalyze the ˜C@O hydrogena-
tion by transfer of Hþ and Hꢁequivalents from the
solvent.
2.2.2. (OC-6-22)-[IrH2(Ph2PCH2CH2NH2)2]Cl (2a)
and (OC-6-32)-[IrH2(Ph2PCH2CH2NH2)2]Cl (2b)
A solution of 614 mg (0.85 mmol) of the chloro hy-
drido complex 1 in 20 mL of isopropanol was treated
with 50 mg (0.89 mmol) of KOH powder. The mixture
was stirred for 1 h at ambient conditions and then
evaporated to dryness. Crystallization of the residue
from diethyl ether gave 540 mg (92%) of 2a as a bright
yellow solid. 1H NMR ([D8]THF): d ¼ ꢁ8:03 (t, cis-
2J(P,H) ¼ 13.75 Hz, 2H, IrH2), 2.52, 2.94, 3.01, (all m; 4,
2, 2H; all CH2), 6.52 [s (br), 4H, NH2], 7.1–7.4 (m, 20 H,
C6H5). 13C{1H} NMR ([D4]methanol): d ¼ 38:09 (‘‘fil-
led-in AA0 X-d00 , RJ(P,C) ¼ 38.51 Hz, PCH2), 47.92 (s,
NCH2), 128.6–134.5 (C6H5). 31P{1H} NMR ([D8]THF):
d ¼ 30:56. Anal. Calc. for C28H34ClIrN2P2 (688.2): C,
48.87; H, 4.98; N, 4.07. Found: C, 48.75; H, 4.78; N,
3.95%.
Combination of 1 (340 mg, 0.47 mmol) with K[BH(s-
Bu)3] (0.70 mL of a 1 M THF solution) in THF (20 mL)
2. Experimental
for 2 h at room temperature, followed by removal of all
volatile material and crystallization of the remaining
solid from diethyl ether, gave the cationic dihydro
complex 2 as 3:1 mixture of its OC-6-22 and OC-6-32
stereoisomers, 2a and 2b, respectively. 1H NMR
([D8]THF): d ¼ ꢁ18:78 (‘‘t’’, Rcis-2J(P,H) ¼ 28.54 Hz;
2b: IrH trans-N), )9.34 (ddd, trans-2J(P,H) ¼ 147.06,
cis-2J (P,H) ¼ 20.32, cis-2J (H,H) ffi 3 Hz; 2b: IrH trans-
2.1. General remarks
All manipulations were performed under nitrogen
using standard Schlenk techniques. Solvents were dis-
tilled from the appropriate drying agents prior to use.
IR: Mattson Polaris. NMR: Bruker DPX 300 (300.1
MHz for 1H, 75.5 MHz for 13C, and 121.5 MHz for 31P)
at 20 ꢂ 2 °C with SiMe4 (or the solvent) as internal or
H3PO4 as external standards (downfield positive; ‘‘m’’:
deceptively simple multiplets). (2-Aminoethyl)diphenyl-
phosphine was used as obtained commercially from
Fluka. Trans-[IrCl(CO)(PPh3)2] was prepared as previ-
ously described [10].
1
P), 8.03 (t, cis-2J(P,H) ¼ 13.75 Hz; 2a: IrH2). H{31P}
NMR ([D8]THF): d ¼ ꢁ18:78 (d, cis-2J(H,H) ¼ 2.52
Hz), )9.34 (d), )8.03 (s). 31P{1H} NMR ([D8]THF):
d ¼ 23:13, 31.89 (both d, cis-2J(P,P) ¼ 5.5 Hz each; 2b),
30.56 (s; 2a).
2.3. X-ray structure determination
2.2. Metal complexes
Single crystals of the addition compound 1 ꢀ 2EtOH
(size 0.49 Â 0.21 Â 0.04 mm) were grown from ethanol.
Diffraction measurements were made at )80 ꢂ 2 °C on an
Enraf-Nonius CAD-4 MACH 3 diffractometer using
graphite-monochromated Mo Ka radiation (k ¼ 0:71073
2.2.1. (OC-6-43)-[Ir(H)(Cl)(Ph2PCH2CH2NH2)2]Cl
(1)
A solution of 2.214 g (2.84 mmol) of trans-
[IrCl(CO)(PPh3)2] and 1.366 g (5.96 mmol) of (2-ami-
noethyl)diphenylphosphine in para-xylene (50 mL) was
heated at reflux temperature for 3 h. The pale yellow
solid which precipitated from solution was collected by
filtration, washed with diethyl ether, and dried under
vacuum; yield 830 mg (41%). IR (KBr): 3435 (NH),
ꢀ
A): orientation matrices and unit cell parameters from the
setting angles of 25 centered medium-angle reflections;
collection of the diffraction intensities by x scans (data
corrected for absorption empirically by w scans [11];
Tmin ¼ 0:366, Tmax ¼ 0:841). The structure was solved by
direct methods and subsequently refined by full-matrix
least-squares procedures on F 2 with allowance for an-
isotropic thermal motion of all non-hydrogen atoms
employing the WINGX package [12a] with the relevant
programs (SIR 97 [13], SHELXL 97 [14], ORTEP 3 [12b])
implemented therein. C28H33ClIrN2P2, Cl, 2(C2H6O)
2202 (IrH) cmꢁ1
.
1H NMR (CDCl3): d ¼ ꢁ21:79
(t, cis-2J(P,H) ¼ 16.66 Hz, 1H, IrH), 2.53, 2.72, 3.43,
3.55 (all m; 4, 2, 1, 1H; all CH2), 4.11 [s (br), 2H,
NH], 7.0–7.5 (m, 20 H, C6H5), 7.77 [s (br), 2 H,
NHꢀ ꢀ ꢀCl]. 13C{1H} NMR (CDCl3): d ¼ 35:26 (‘‘filled-
in AA0 X-d00 , RJ(P,C) ¼ 38.51 Hz, PCH2), 44.75 (s,
NCH2), 127.8–134.5 (C6H5). 31P{1H} NMR (CDCl3):
d ¼ 18:99. Anal. Calc. for C28H33Cl2IrN2P2(722.7): C,
46.54; H, 4.60; N, 3.88. Found: C, 46.22; H, 4.87; N,
3.60%.
ꢁ
ꢀ
ꢀ
(814.7); triclinic, P1, a ¼ 9:706ð2Þ A, b ¼ 12:982ð5Þ A,
ꢀ
c ¼ 15:103ð5ÞA, a ¼ 102:90ð2Þ°, b ¼ 94:96ð2Þ°, c ¼
111:72°, V ¼ 1693:0ð9Þ A , Z ¼ 2, dcalc ¼ 1:598g cmꢁ3
,
3
ꢀ
l(Mo Ka) ¼ 4.226 mmꢁ1; 2:27° 6 H 6 25:07°, 6263
reflections collected (ꢁ11 6 h 6 þ 11, ꢁ15 6 k 6 þ 15,

L. Dahlenburg, R. G€otz / Inorganica Chimica Acta 357 (2004) 2875–2880
2877
Table 1
Homogeneous hydrogenation of acetophenone catalyzed by base-modified hydrido chloro complex 1 or by dihydride 2aa
No.
Complex/base (equivalent relative to cIr)
Solvent
p(H2) (bar)
T (°C)
t (h)
Conversion (%)
ꢃTONb
[Ir(H)(Cl)(Ph2PCH2CH2NH2)2]þ (1)
1/none
1
2
3
4
5
6
MeOH
THF
25
25
25
25
25
25
25
25
25
25
25
50
17
17
17
17
3
1/KOH (5)
1/KOH (5)
1/KOH (5)
MeOH
MeOH
i-PrOH
i-PrOH
36
37
55
45
20
20
1/KOBu-t (5)
1/KOBu-t (5)
[IrH2(Ph2PCH2CH2NH2)2]þ (2a)
2a/none
180
150
100
3
7
8
i-PrOH
i-PrOH
3
3
55
75
180
250
2a/none
a 20.0 mmol of acetophenone together with 0.02 mmol of 1 (plus added base) or 2a in 20 mL of solvent.
b mol Ph(Me)CHOH molꢁ1[Ir] hꢁ1
.
0 6 l 6 þ 17), 6015 unique; wR ¼ 0:1040 for all data and
487 parameters, R ¼ 0:0395 for 5375 structure factors
Fo > 4rFo.
aminophosphines R2P\NH2 [15,16], it was surprising to
see that a search of the CAS databases [17] gave no
indication of the existence of any isostructural cationic
iridium(III) complex [IrX2(R2P\PR2)(H2N\NH2)]þ.
Moreover, only two dichloro species, (OC-6-13)-[IrCl2-
(Me2PCH2CH2NH2)2]þ and (OC-6-13)-[IrCl2(Ph2PCH2-
CH2NH2)2]þ, were retrieved as examples of previously
2.4. General procedures for catalytic ˜C@O hydrogena-
tion
ꢁ
A Schlenk tube equipped with a small magnetic stir-
ring bar was charged with 20 mmol of acetophenone and
0.02 mmol of catalyst complex 1 or 2a dissolved in
methanol or isopropanol (typically 20 mL). Transfer
hydrogenations catalyzed by 2a were carried out in the
absence of base under nitrogen at 25 or 50 °C for 3 h. In
hydrogenations catalyzed by 1, the required equivalent of
activating base (cf. Table 1) was added, the mixtures were
stirred for 10 min at ambient conditions, and the tube
was inserted into an argon-filled stainless steel autoclave.
In reactions run under hydrogen, the autoclave was
pressurized and vented several times with H2 (Messer–
Griesheim; 99.999%), and finally pressurized to 25 or 100
bar and kept at 25 °C for the times indicated in Table 1.
At the end of all catalytic runs, the residues remaining
after evaporation of the solvent were diluted with n-
pentane to precipitate the catalyst as a red oil. The
pentane solutions were decanted and chromatographed
on a silical gel column using diethyl ether/n-pentane
(1:1) as the eluent. Volatile material was distilled off and
described [IrX2(R2P\NH2)2]þ cations. Using PF6 as
the counter-ion, the former was isolated in poor yield
(5–9%) from the mixture of products formed upon
treatment of K3[IrCl6] with an excess of the amino-
phosphine. Moderate yields of the latter (<50%) were
recovered from the reaction between [Ir2(l-Cl2)(g2-
C8H14)4] and Ph2CH2CH2NH2, followed by chlorina-
tion of the initially produced iridium(I) intermediates
cis- and trans-[Ir(Ph2CH2CH2NH2)2]Cl and separation
of the desired complex from the oxidation products as a
ꢁ
BF4 salt [18].
In a less systematic but – from a preparative point of
view – more straightforward way, we obtained the clo-
sely related chloro hydrido compound (OC-6-43)-
[Ir(H)(Cl)(Ph2PCH2CH2NH2)2]Cl (1) by combining
trans-[IrCl(CO)(PPh3)2] with slightly more than two
equivalents of the P,N chelate ligand in refluxing para-
xylene for several hours (Scheme 1). The complex sep-
arated directly from solution and was readily isolated on
a multi-100mg scale. Hydrogen abstraction from the
solvent (or from the excess aminophosphine) presents
the most likely pathway for the formation of the Ir–H
bond, which has much precedence in the literature and is
even observed as a competing side reaction in the syn-
thesis of the Vaska complex trans-[IrCl(CO)(PPh3)2]
itself [10a].
1
the mixtures of products were analyzed by H NMR.
Conversions and product compositions were determined
on the basis of the integrations of the PhC(O)CH3 and
PhCH(OH)CH3 signals. Entries in Table 1 represent the
average values of, at least, duplicate runs.
Crystallization of 1 from ethanol furnished the addi-
tion compound 1ꢀ2EtOH as single crystals, the triclinic
unit cell of which contains two formula units connected to
a centrosymmetric dimer by multiple NHꢀ ꢀ ꢀCl, OHꢀ ꢀ ꢀN,
and NHꢀ ꢀ ꢀClꢀ ꢀ ꢀHO hydrogen bonding (Fig. 1). In the
cation itself, the two P,N ligands span the four equatorial
sites of a distorted octahedron such that the phosphino
and amino residues are in mutual trans-position. The two
3. Results and discussion
In view of the large number of well-characterized
(P,N)2-coordinated ruthenium(II) complexes [Ru(X)(Y)
(P,N)2] (X, Y: halide or hydride) bearing either a
bis(phosphine) R2P\PR2 ligand in addition to a che-
lating diamine H2N\NH2 [2,3] or, less frequently, two

2878
L. Dahlenburg, R. G€otz / Inorganica Chimica Acta 357 (2004) 2875–2880
ꢀ
Ir–N bond lengths average out at 2.137(5) A, which
differs only slightly from the mean metal-amine dis-
H2
N
H2
N
Cl−
H
Ir
Cl
PPh3
CO
Ph2PCH2CH2NH2
ꢀ
tance of 2.147(6) A that was previously determined for
. . .
Ir
refl. p-xylene
Ph3P
P
P
(OC-6-13)-[IrCl2(Ph2PCH2CH2NH2)2] BF4 [18]. In
contrast, the average value of the Ir–P separa-
ꢀ
tions measured as 2.268(2)A for 1 is considerably below
Cl
Ph2
Ph2
(1)
ꢀ
the 2.303(1) A found in the dichloro complex. As ex-
Scheme 1. Formation of chloro hydrido complex 1.
pected, the length of the Ir–Cl linkage opposite the
strong trans-bond-weakening hydride ligand, 2.505(2)
ꢀ
A, is much longer than the two trans-located Ir–Cl
bonds of the [IrCl2(Ph2PCH2CH2NH2)2]þ cation
ꢀ
(2.365(1) A [18]).
Combination of the cationic chloro hydrido complex
1 with K[BH(s-Bu)3] in THF cleanly produced the di-
hydride [IrH2(Ph2PCH2CH2NH2)2]Cl as a 3:1 mixture
of its OC-6-22 and OC-6-32 stereoisomers, 2a and 2b,
respectively. Coordination geometries and product
compositions were readily inferred from 1H NMR
spectroscopy (Section 2). Potassium isopropoxide
(KOH/i-PrOH) interacted with 1 to selectively form the
trans-dihydride 2a (Scheme 2). This latter transforma-
tion resembles the base-induced conversion of the d6
low-spin and, hence, substitutionally inert Noyori-type
precatalysts [Ru(Cl)(X){bis(phosphine)}(1,2-diamine)]
(X ¼ H, Cl) into the actually active dihydrido species
[RuH2{bis(phosphine)}(1,2-diamine)] [3c,3d,19] and
prompted us to examine whether the cationic bis(am-
inophosphine)iridium(III) complex 2a parallels its iso-
electronic {bis(phosphine)(1,2-diamine)} ruthenium(II)
analogues in the ability to catalyze the hydrogenation of
ketones.
Fig. 1. Perspective view of the multiply hydrogen-bridged centrosym-
metric dimer of [Ir(H)(Cl)(Ph2PCH2CH2NH2)2]Clꢀ2EtOH; selected
For that purpose, both the chloro hydrido compound
1 and the trans-dihydride 2a were probed for their be-
havior as ˜C@O hydrogenation catalysts under the
conditions summarized in Table 1. Using acetophenone
as a standard test substrate, virtually no conversion to 1-
phenylethanol was observed under 25 bar of H2 on at-
tempted catalysis by 1 in the absence of an activating
base in a protic solvent such as methanol or, vice versa,
in an aprotic THF medium containing such a basic
additive (entries 1 and 2). Only if combined with po-
ꢀ
bond lengths [A] and angles [°]: Ir1–Cl1, 2.5048(17); Ir1–P1,
2.2717(17); Ir1–P2, 2.2632(18); Ir1–N1, 2.136(5); Ir1–N2, 2.138(5);
Ir1–H, 1.59(2); N1–Hꢀ ꢀ ꢀ Cl10, 3.421(6); N1–Hꢀ ꢀ ꢀ O1, 2.992(9); N2–
Hꢀ ꢀ ꢀ Cl10, 3.419(6); N2–Hꢀ ꢀ ꢀCl2, 3.425(6); O2–Hꢀ ꢀ ꢀ Cl2, 3.098(7).
Cl1Ir1–P1, 94.14(6); Cl1–Ir1–P2, 99.44(6); Cl1–Ir1–N1, 85.96(16);
Cl1–Ir1–N2, 85.24(16); Cl1–Ir1–H 173(3); P1–Ir1–P2, 105.25(6); P1–
Ir1–N1, 84.17(16); P1–Ir1–N2, 170.96(15); P1–Ir1–H, 88(3); P2–Ir1–
N1, 168.64(16); P2Ir1–N2, 83.74(16); P2–Ir1–H, 87(3); N1–Ir–N2,
86.8(2); N1–Ir1–H, 87(3); N2–Ir1–H, 92(3); N1–Hꢀ ꢀ ꢀCl10, 134.5;
N1Hꢀ ꢀ ꢀO1, 166.3; N2–Hꢀ ꢀ ꢀCl10, 169.4; N2–Hꢀ ꢀ ꢀCl2, 159.1.
H2
N
H2
N
H2
N
Cl−
Cl−
H
Ir
H
Ir
H
K[BH(s-Bu)3]
+
Ph2P
P
P
P
H
N
H2
N
Ph2
Ph2
H2
N
Cl−
Ph2
H2
H
Ir
(2a)
(2b)
P
P
Cl
Ph2
Ph2
H2
N
H2
N
Cl−
H
Ir
(1)
KOH/i-PrOH
P
P
H
Ph2
Ph2
(2a)
Scheme 2. Syntheses of P,N-coordinated dihydrido complexes 2a and 2b.

L. Dahlenburg, R. G€otz / Inorganica Chimica Acta 357 (2004) 2875–2880
2879
tassium hydroxide (5 equiv.) in methanol, complex 1
was seen to act as a (slow) catalyst for the hydrogena-
tion of the ketone – in fact, irrespective of whether the
reaction was carried out under H2 pressure or under an
inert atmosphere of nitrogen or argon (entries 3 and 4).
These observations clearly demonstrate (i) that the
chloro hydrido species 1 as such does not behave as a
catalyst for homogeneous ˜C@O reduction and (ii) that
the combined catalytic system 1–KOH/MeOH facilitates
the transfer of Hþ and Hꢁ equivalents from the solvent
relative to the direct addition of dihydrogen to the
substrate.
As anticipated [20], a marked acceleration of the re-
action was noted if precatalyst 1 was activated by strong
base in isopropanol which, as a secondary alcohol, is a
better hydrogen donor than methanol and, hence, is
the reducing agent of choice employed for transfer hy-
drogenations. Thus, acetophenone was converted to
1-phenylethanol in 55% yield, if kept in Me2CHOH
under argon at 25 °C for 3 h in the presence of 0.1 mol%
of added 1–KOBu-t (1:5); entry 5. If such reaction
mixtures were pressurized to 100 bar of H2, the degree of
substrate transformation dropped to only 45%, showing
that the catalytic system looses part of its hydrogenation
activity in the presence of hydrogen gas (entry 6).
Hydrogen transfer from the solvent to the substrate
in the presence of 0.1 mol% of dihydrido complex 2a
occurs even in the absence of base, giving the alcohol
in exactly the same yield as produced in reactions that
were carried out under equal conditions with 1–
KOBu-t (1:5) as hydrogenation catalyst (entry 7).
Raising the temperature from 25 to 50 °C resulted in
an increased conversion of the substrate as expected
(entry 8). Since the trans-dihydride 2a is the sole
product formed in the reaction of the precursor chloro
hydrido complex 1 with alkoxide base in isopropyl
alcohol, it is evident that 2a also operates as the true
catalyst of the transfer hydrogenations supported by
the combined system 1-KOBu-t.
(3–10 equiv.) and combined with ketones (neat or dis-
solved in C6D6) at substrate-to-catalyst ratios between
400:1 and 2500:1 at ambient conditions under an at-
mosphere of hydrogen gas (ꢃ3.5 bar). In isopropanol at
elevated temperature and H2 pressure (60 °C, 40 bar), in
the presence of a large excess of KOBu-t (>1000 equiv.),
the bis(aminophosphine) ruthenium complex even dis-
plays the extraordinarily high hydrogenation activity
characteristic of all [Ru(X)(Y){bis(phosphine)}(1,2-di-
amine)] catalysts, quantitatively transforming the sub-
strate within a few hours at s/c ratios exceeding
100 000:1. As a further difference between cationic
[Ir(H)(Cl)(Ph2PCH2CH2NH2)2]þ and neutral [Ru(H)
(Cl)(Ph2PCH2CH2NH2)2], we note that the latter quite
surprisingly also behaves as a rather efficient hydroge-
nation catalyst in the absence of any promoting base
[16]. No such catalytic behavior is exhibited by the
chloro hydrido species 1, which requires transformation
into dihydride 2a in order to become catalytically active.
In this respect, the iridium(III) precatalyst 1 is more
closely akin to, e.g., the chloro hydrido complexes [Ru
(H)(Cl)(PPh3)2{(R,R)-1,2-(H2N)2C6H10}] and [Ru(H)
(Cl){(R)-BINAP)(H2NCMe2CMe2NH2)}] which likewise
do not act as ˜C@O hydrogenation catalysts unless
converted into the corresponding dihydrido derivatives
[3]. These dihydrides of RuII, [RuH2(PPh3)2{(R,R)-1,2-
(H2N)2C6H10}] and [RuH2{(R-BINAP)(H2NCMe2CMe2
NH2)}], then behave as active catalysts for direct hy-
drogenations of ketones in the absence of extra base
as does iridium(III) dihydride 2a for ˜C@O transfer
hydrogenations.
Both the observation that the addition of excess po-
tassium tert-butoxide to cationic complex 1 does not
favor direct over transfer hydrogenation and the finding
that the catalysis is even slowed down under H2 pressure
show that there is neither a ‘‘potassium effect’’ [9] nor a
beneficial influence of positive charge in the cases stud-
ied in this work. This could be due to a mechanism
which does not occur via a concerted hydrogen transfer
with no direct metal–ketone binding along the pathway
as observed for ruthenium(II). The catalytic activity of
the coordinatively saturated dihydride 2a, which as low-
spin d6 system should be quite substitutionally inert,
does however suggest that such an unconventional
Ir–Hdꢁ ꢀ ꢀ ꢀ ˜Cdþ@Odꢁ ꢀ ꢀ ꢀ Hdþ ꢁ N ‘‘metal–ligand bi-
functional’’ interaction is actually operative along the
reaction path. It has been proposed by others that the
role of Kþ ions in Noyori–Morris systems could merely
be to precipitate chloride during catalyst activation ra-
ther than act as a Lewis acid which accelerates H–H
cleavage at a metal–amide bond bearing a partial posi-
tive charge [3d]. Further work aiming at the use of other
diamine-bis(phosphine) complexes of IrIII and RhIII as
catalysts for the homogeneous reduction of organic
carbonyl compounds by direct or transfer hydrogena-
tion is underway.
4. Concluding remarks
Although the use of IrI and IrIII complexes as transfer
hydrogenation catalysts is well documented [20,21], we
are not aware of any previously employed cationic
iridium(III) compound bearing a direct resemblance to
the famous Noyori-Morris [Ru(X)(Y){bis(phosphine)}
(1,2-diamine)] hydrogenation catalysts (X, Y ¼ H,Cl).
The exactly matching RuII analogue of the [Ir(H)(Cl)
(Ph2PCH2CH2NH2)2]þ cation, [Ru(H)(Cl)(Ph2PCH2
CH2NH2)2], has recently been described in a patent
application [16]. That ruthenium complex differs from
its isoelectronic iridium congener reported in this com-
munication in that it efficiently catalyzes the direct ad-
dition of H2 to the ˜C@O bond if activated by KOPr-i

2880
L. Dahlenburg, R. G€otz / Inorganica Chimica Acta 357 (2004) 2875–2880
[7] D.G.I. Petra, J.N.H. Reek, J.-W. Handgraaf, E.J. Meijer, P.
5. Supplementary material
Dierkes, P.C.J. Kamer, J. Brussee, H.E. Schoemaker, P.W.N.M.
van Leeuwen, Chem. Eur. J. 6 (2000) 2818.
Crystallographic data (excluding structure factors)
for the structure reported in this paper have been de-
posited at the Cambridge Crystallographic Data Centre
and allocated the deposition number CCDC-215606.
Copies of the data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or on appli-
cation to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK [fax: (internat.) +44-1223/336-033; e-mail: de-
posit@ccdc.cam.ac.uk].
[8] C.P. Casey, S.W. Singer, D.R. Powell, R.K. Hayashi, M. Kavana,
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Acknowledgements
€
€
[14] G.M. Sheldrick, SHELXL-97 (Release 97-2), Universitat Gottin-
Support of this work by the Deutsche Forschungs-
gemeinschaft (Bonn, SFB 583) and the Fonds der
Chemischen Industrie (Frankfurt/Main) is gratefully ac-
knowledged. We are also indebted to Mrs. S. Hoffmann
for her skillful assistance.
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