Ruthenium(II), rhodium(III) and iridium(III) based effective catalysts for hydrogenation under aerobic conditions

Article abstract of DOI:10.1016/j.poly.2008.06.015

The new cationic mononuclear complexes [(η⁶-arene)Ru(Ph-BIAN)Cl]BF₄ [η⁶-arene = benzene (1), p-cymene (2)], [(η⁵-C₅H₅)Ru(Ph-BIAN)PPh₃]BF₄ (3) and [(η⁵-C

Full text of DOI:10.1016/j.poly.2008.06.015

Polyhedron 27 (2008) 2877–2882
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ruthenium(II), rhodium(III) and iridium(III) based effective catalysts
for hydrogenation under aerobic conditions
Sanjay Kumar Singh a, Santosh Kumar Dubey a, Rampal Pandey a, Lallan Mishra a, Ru-Qiang Zou b,
Qiang Xu b, Daya Shankar Pandey a,
*
a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, UP, India
b National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
a r t i c l e i n f o
a b s t r a c t
6-arene)Ru(Ph-BIAN)Cl]BF4
[g
6-arene = benzene (1), p-
Article history:
The new cationic mononuclear complexes [(
cymene (2)], [(g g
g
5-C5H5)Ru(Ph-BIAN)PPh3]BF4 (3) and [( 5-C5Me5)M(Ph-BIAN)Cl]BF4 [M = Rh (4), Ir (5)]
Received 9 May 2008
Accepted 3 June 2008
Available online 28 July 2008
incorporating 1,2-bis(phenylimino)acenaphthene (Ph-BIAN) are reported. The complexes have been fully
characterized by analytical and spectral (IR, NMR, FAB-MS, electronic and emission) studies. The molec-
ular structure of the representative iridium complex [(g
5-C5Me5)Ir(Ph-BIAN)Cl]BF4 has been determined
Keywords:
crystallographically. Complexes 1–5 effectively catalyze the reduction of terephthalaldehyde in the
(g
6-Arene)Ru
presence of HCOOH/CH3COONa in water under aerobic conditions and, among these complexes the rho-
(C5Me5)Rh
(C5Me5)Ir
Ph-BIAN
dium complex [(
g
5-C5Me5)Rh(Ph-BIAN)Cl]BF4 (4) displays the most effective catalytic activity.
Ó 2008 Elsevier Ltd. All rights reserved.
X-ray
Hydrogenation
1. Introduction
anhydrous reaction conditions. The use of organometallic com-
plexes as catalysts under aqueous and aerobic conditions in the
There has been intense research interest amongst the scientific
community in the chemistry of transition metal complexes contain-
ing mono/bi/polydentate ligands [1]. In this regard, highly conju-
gated diimine ligands based on acenaphthene quinone (bian)
have drawn considerable recent attention [2]. The extensive p-elec-
tron system of acenaphthene and two conjugated imine functions
hydrogenation of aldehyde and ketones has received considerable
current interest [7]. It has been observed that by using water as
the solvent, the catalytic reactions can be considerably accelerated
[8,9]. Encouraged by recent reports on hydrogenation reactions by
organometallic complexes, we have evaluated the catalytic poten-
tial of the piano-stool complexes 1–5 in the reduction of tereph-
thalaldehyde in air under aqueous conditions.
enables these substituted acenaphthene quinone based ligands to
act as a better
r
-donor and
p
-acceptor, comparable to the com-
We describe herein the synthetic and spectral features of the
monly used diimine ligands viz., bipy, phen, etc. [3]. These com-
bined properties and rigidity of the acenaphthene ring makes
them a better chelating ligand to coordinate with metal centres.
Complexes based on substituted acenaphthene quinone derivative
ligands with late transition metals are being used as efficient cata-
lysts for various important reactions [4]. A number of complexes of
Pd, Pt, Zn, Ni and Cu containing acenaphthene quinone ligands have
been prepared and well studied, however the chemistry of these li-
gands with arene ruthenium or rhodium complexes have scarcely
been explored [2,5].
piano-stool arene ruthenium(II) complexes [(
BIAN)Cl]BF4
6-arene = benzene (1), p-cymene (2)], the cyclopen-
tadienyl complex [(
5-C5H5)Ru(Ph-BIAN)PPh3]BF4 (3) and the
pentamethylcylcopentadienyl rhodium(III) and iridium(III) com-
plexes [(
5-C5Me5)M(Ph-BIAN)Cl]BF4 [M = Rh (4), Ir (5)]. Also, we
g
6-arene)Ru(Ph-
[
g
g
g
describe herein the reduction of terephthalaldehyde in the pres-
ence of the complexes 1–5.
2. Experimental
Furthermore, remarkable efforts have been devoted towards the
development of catalysts based on organometallic complexes for
hydrogenation reactions [6]. One of the major problems associated
with the practical application of theses catalysts is the necessity of
2.1. General
All the synthetic manipulations were performed under a nitrogen
atmosphere. Analar grade chemicals were used throughout.
Solvents were dried and distilled prior to use [10]. Hydrated ruthe-
nium(III) chloride, ammonium tetrafluoroborate, acenaphthenequi-
none, aniline and hydroxylamine hydrochloride were procured from
* Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368174.
E-mail address: dspbhu@bhu.ac.in (D.S. Pandey).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.06.015

2878
S.K. Singh et al. / Polyhedron 27 (2008) 2877–2882
Aldrich Chemical Company, Inc. USA and were used as received. The
precursor complexes [{( -Cl)}2], [(
6-arene)RuCl( 6-arene)RuCl2-
(PyCN)] (arene = benzene and p-cymene), [(
5-C5H5)RuCl(PPh3)2]
and [( -Cl)}2] (M = Rh and Ir) and the ligand (1,2-
5-C5Me5)MCl(
(0.332 g, 1.0 mmol) and the solution was refluxed for ꢀ10.0 h. The
resulting dark purple-black solution was filtered through Celite,
and a saturated methanolic solution of NH4BF4 was added to the
filtrate and left for slow crystallization. The dark purple-black
product thus obtained was filtered and washed with diethyl ether
and dried under vacuum. The product was further recrystallized
from dichloromethane/diethyl ether. Yield: 0.584 g (69 %). Anal.
Calc. (%) for BC47F4H36N2PRu: C, 66.58; H, 4.25; N, 3.30. Found: C,
65.98; H, 4.11; N, 3.87%. FAB-MS: m/z = 763 (760) [M+]; 500
(498) [MÀPPh3+]. 1H NMR (d ppm, 300 MHz, CDCl3, 298 K): 8.13
(d, J = 8.1 Hz), 8.01 (d, J = 7.0 Hz), 7.91 (t, J = 7.1 Hz), 7.71
(t, J = 6.9 Hz), 7.60 (m), 4.65 (s). 31P NMR (d ppm, 120 MHz, CDCl3):
g
l
g
g
g
l
diphenylimino)acenaphthene (Ph-BIAN) were prepared and puri-
fied according to the published procedures [11,5f]. Microanalysis
and FAB mass spectra were acquired from the Sophisticated Analyt-
ical Instrument Facility, Central Drug Research Institute, Lucknow.
Electronic and luminescence spectra were recorded on a Shimadzu
UV-1601 and Perkin–Elmer LS-45 spectrofluorophotometers,
respectively. NMR spectra were acquired on a JEOL AL 300 MHz
spectrometer, where chemical shifts were referenced to internal tet-
ramethylsilane for 1H NMR and phosphorous trichloride for 31P NMR
at room temperature.
43.29 (s). UV–Vis: kmax
(e
, dm3 molÀ1 cmÀ1) = 561 (1.9 Â 104), 323
(2.6 Â 104), 244 (4.1 Â 104) nm. Emission: kem (kex
, U
/10À6) = 617
(561, 0.9) nm.
2.2. Synthesis and characterization of [(g
6-C6H6)RuCl(Ph-BIAN)]BF4
(1)
2.5. Synthesis and characterization of [(g
5-C5(CH3)5)RhCl(Ph-BIAN)]
BF4 (4)
This was prepared following the above synthetic procedure for
1 using [{( -Cl)}2] (0.618 g, 1.0 mmol) in place of
5-C5(CH3)5)RhCl(
[{( -Cl)}2]. Red brown, Yield: 0.499 g (72 %). Anal.
6-C6H6)RuCl(
(a) To a suspension of the chloro-bridged dimeric ruthenium
complex [{( -Cl)}2] (0.502 g, 1.0 mmol) in meth-
6-C6H6)RuCl(
g
l
g
l
g
l
anol (25 mL), Ph-BIAN (0.664 g, 2.0 mmol) was added and the
suspension was stirred at room temperature for ꢀ10.0 h. Slowly
it dissolved and gave a clear brown-black solution. It was fil-
tered through Celite to remove any solid impurities. The filtrate
was treated with NH4BF4 dissolved in methanol and left for
slow crystallization. The dark brown product thus obtained
was separated by filtration, washed with diethyl ether and
dried under vacuum. It was further recrystallized from dichloro-
methane/diethyl ether. Yield: 0.471 g (68 %). Anal. Calc. (%) for
Calc. (%) for BC34ClF4H31N2Rh: C, 58.87; H, 4.47; N, 4.04. Found:
C, 58.68; H, 4.82; N, 4.37%. 1H NMR (d ppm, 300 MHz, CDCl3,
298 K): 7.98 (d, 2H, J = 8.1 Hz), 7.95 (d, 2H, J = 7.8 Hz), 7.83 (d,
2H, J = 7.5 Hz), 7.71–7.52 (m, 6H), 7.41 (t, 1H, J = 8.1Hz), 6.91 (d,
2H, J = 6.9 Hz), 1.33 (s, 15H). UV–Vis: kmax (e
, dm3 molÀ1 cmÀ1) =
375 (3.5 Â 104), 314 (3.9 Â 103), 257 (4.1 Â 104) nm.
2.6. Synthesis and characterization of [(g
5-C5(CH3)5)IrCl(Ph-BIAN)]BF4
(5)
C
30ClF4H22N2BRu: C, 52.02; H, 3.18; N, 4.05. Found: C, 51.94;
H, 2.83; N, 4.24%. FAB-MS: m/z = 547 (547) [M+]; 512 (512)
[MÀCl+]. 1H NMR (d ppm, 300 MHz, CDCl3, 298 K): 8.05 (d,
2H, J = 7.5 Hz), 7.99 (d, 2H, J = 7.2 Hz), 7.85 (d, 2H, J = 6.9 Hz),
7.73 (t, 2H, J = 6.6 Hz), 7.61 (m, 4H, J = 8.1 Hz), 7.44 (t, 2H,
J = 7.5 Hz), 6.92 (d, 2H, J = 7.2 Hz), 5.58 (s, 6H). UV–Vis: kmax
This was prepared following the above synthetic procedure for
1 using [{( -Cl)}2] (0.796 g, 1.0 mmol) in place of
5-C5(CH3)5)IrCl(
[{( -Cl)}2]. Red brown, Yield: 0.547 g (70 %). Anal.
6-C6H6)RuCl(
g
l
g
l
Calc. (%) for BC34ClF4H31N2Ir: C, 52.17; H, 3.96; N, 3.58. Found: C,
52.31; H, 3.82; N, 3.77%. 1H NMR (d ppm, 300 MHz, CDCl3, 298K):
8.02 (d, 2H, J = 8.1 Hz), 7.99 (d, 2H, J = 6.9 Hz), 7.78 (d, 2H,
J = 7.8 Hz), 7.71–7.52 (m, 6H), 7.43 (t, 2H, J = 7.8 Hz), 6.99 (d, 2H,
(e
, dm3 molÀ1 cmÀ1) = 706 (5.8 Â 103), 446 (1.9 Â 104), 300
(3.9 Â 104), 256 (4.1 Â 104) nm. Emission: kem (kex
, U
/10À6) =
577 (446, 1.9) nm.
J = 7.5 Hz), 1.31 (s, 15H). UV–Vis: kmax (e
, dm3 molÀ1 cmÀ1) = 435
(b) Alternately, complex 1 can be synthesized by the reaction of
[(
6-C6H6)RuCl2(PyCN)] with Ph-BIAN in methanol under stir-
ring conditions at room temperature.
(2.2 Â 104), 319 (3.9 Â 104), 244 (4.0 Â 104) nm.
g
3. Results and discussion
2.3. Synthesis and characterization of [(g
6-C10H14)RuCl(Ph-BIAN)]BF4
(2)
3.1. Synthesis and characterization of the complexes 1–5
The cationic mononuclear complexes [(
BIAN)Cl]+ ( 6-arene = benzene, 1 and p-cymene, 2) (Scheme 1) have
been synthesized by the reactions of the arene ruthenium precursor
complexes [{( -Cl)}2] with Ph-BIAN in methanol at
6-arene)RuCl(
room temperature. Complexes 1 and 2 were also prepared by the
reaction of the complexes [(
6-arene)RuCl2(PyCN)] ( 6-arene =
g
6-arene)Ru(Ph-
This was prepared following the above synthetic procedure for
1 using [{( -Cl)}2] (0.612 g, 1.0 mmol) in place of
6-C10H14)RuCl(
[{( -Cl)}2]. Red brown, Yield: 0.531 g (71 %). Anal.
6-C6H6)RuCl(
g
g
l
g
l
g
l
Calc. (%) for BC34ClF4H30N2Ru: C, 54.55; H, 4.01; N, 3.74. Found:
C, 53.88; H, 3.82; N, 3.57%. FAB-MS: m/z = 603 (603) [M+]; 568
(568) [MÀCl+]. 1H NMR (d ppm, 300 MHz, CDCl3, 298 K): 7.99 (d,
2H, J = 8.1 Hz), 7.93 (d, 2H, J = 6.6 Hz), 7.82 (d, 2H, J = 6.3 Hz),
7.74 (t, 2H, J = 6.9 Hz), 7.62 (m, 4H), 7.43 (t, 2H, J = 8.1 Hz), 6.89
(d, 2H, J = 6.9 Hz), 5.33 (s, 4H), 2.51 (sep., 1H, J = 6.9 Hz), 1.99 (s,
g
g
benzene 1, p-cymene, 2; PyCN = 4-cyanopyridine) with Ph-BIAN in
methanol. It was observed that in the reactions of the arene
ruthenium complexes containing 4-cyanopyridine [(g
6-arene)-
RuCl2(PyCN)] with Ph-BIAN, the coordinated 4-cyanopyridine gets
substituted by Ph-BIAN to afford the complexes 1 and 2. This obser-
vation is consistent with our earlier findings on the analogous arene
3H), 1.09 (d, 6H, J = 6.9Hz). UV–Vis: kmax (e
, dm3 molÀ1 cmÀ1) = 462
(2.3 Â 104), 310 (3.9 Â 104), 255 (4.1 Â 104) nm. Emission: kem (kex
,
U
/10À6) = 547 (462, 1.8) nm.
ruthenium [(
complexes [(g3
pyridine, where the respective bases (EPh3, E = P, As, Sb; bipy, phen)
displaced the metal bound 4-cyanopyridine [12]. Treatment of [(g5
C5H5)RuCl(PPh3)2] with Ph-BIAN in refluxing methanol led to the
formation of the cationic ruthenium complex [(
5-C5H5)Ru
(Ph-BIAN)(PPh3)]+ (Scheme 1), which was isolated as the BF4À salt.
g
6-arene)RuCl2(PyCN)] and 3:3-bis allyl ruthenium
:g
3-C10H16)RuCl2(PyCN)] incorporating 4-cyano-
2.4. Synthesis and characterization of [(g
5-C5H5)Ru(PPh3)(Ph-BIAN)]
BF4 (3)
-
A suspension of the ruthenium complex [(
(0.728 g, 1.0 mmol) in methanol (25 mL) was treated with Ph-BIAN
g
5-C5H5)RuCl(PPh3)2]
g

S.K. Singh et al. / Polyhedron 27 (2008) 2877–2882
2879
Scheme 1.
The pentamethylcyclopentadienyl complexes [(
BIAN)Cl]+ [M = Rh (4), Ir (5)] were obtained by the reactions of the
chloro-bridged dimeric complexes [{( -Cl)}2]
5-C5Me5)MCl(
g
5-C5Me5)M(Ph-
formulation of respective complexes. In the 31P NMR spectrum of
complex 3, the 31P nuclei of the coordinated triphenylphosphine res-
onated as a singlet in the normal range at 43.29 ppm.
g
l
[M = Rh and Ir] with two equivalents of Ph-BIAN in methanol and
these were isolated as BF4 salts.
Electronic and emission spectral data for all the complexes are
recorded in the experimental section. Comparative electronic spec-
tra for the mononuclear complexes 1–5 are shown in Fig. 2. The
electronic spectra of the ruthenium complexes 1 and 2 displayed
bands at ꢀ446, 310 and 255 nm. On the basis of its intensity and
position, the lower energy absorption band at 446 nm has been as-
signed to a [Ru(II) ? p* Ph-BIAN] metal-to-ligand charge transfer
(MLCT), where the high-lying filled orbital is localized on the metal
centre and the low-lying empty orbital is localized on Ph-BIAN.
À
Spectral and analytical data of complexes 1–5 corroborate well
to the proposed formulations. FAB-MS spectra of the complexes
displayed prominent peaks corresponding to the molecular ion
fragment. Representative spectra of 1–3 are shown in Fig. 1. Com-
plexes 1 and 2 displayed the loss of chloride in the first step,
whereas 3 displayed the loss of the coordinated triphenylphos-
phine. In these complexes the metal to Ph-BIAN bond is stronger
and remains intact. The FAB-MS spectral results corroborate well
to the proposed formulation of the complexes.
Bands at around 300 nm could be attributed to intra-ligand
transitions. Similar trends of absorptions have been observed in
p–
p*
1H NMR spectral data of the complexes, along with their assign-
ments, are recorded in the experimental section. In the 1H NMR
spectra of complexes 1 and 2, the protons associated with the arene
moiety and Ph-BIAN displayed downfield shifts as compared to that
in the precursor complexes and uncoordinated ligand, respectively.
The 1H NMR spectrum of complex 1 exhibited resonances at d 8.05
(d, 2H), 7.99 (d, 2H), 7.85 (d, 2H), 7.73 (t, 2H), 7.62 (t, 4H), 7.44 (t,
2H) and 6.92 (d, 2H) ppm corresponding to Ph-BIAN protons and
the electronic spectra of complexes 3–5.
The luminescence properties of complexes 1–3 were examined at
room temperature in dichloromethane, Fig. 3. Upon excitation at
their respective excitation bands (in parentheses), the complexes
exhibited moderate emissions [577 nm (446 nm) for 1, 547 nm
(462 nm) for 2 and 617 nm (561 nm) for 3]. The emission spectra
were found to be parallel to their respective absorption bands, there-
fore these bands may be assigned to the 3MLCT energy state of
at d 5.58 (s) ppm corresponding to the coordinated
g
6-C6H6 ring pro-
Rud -BIANp . The observed results could be attributed to the fact that
Ã
p
tons. Similarly, in the 1H NMR spectrum of complex 2 Ph-BIAN pro-
tons resonated at d 7.99 (d, 2H, 8.1 Hz), 7.93 (d, 2H, 6.6 Hz), 7.82 (d,
2H, 6.3 Hz), 7.74 (t, 2H, 6.9 Hz), 7.62 (m, 4H), 7.43 (t, 2H, 8.1 Hz) and
6.89 (d, 2H, 6.9 Hz) ppm, while the protons associated with the p-
cymene ring resonated at d 5.33 (s, 4H), 2.51 (sep, 1H, 6.9 Hz), 1.99
(s, 3H) and 1.09 (d, 6H, 6.9 Hz) ppm. It was observed that the protons
corresponding to Ph-BIAN resonated at almost same position in all
the excited state energy can be fine tuned with different substitu-
ents on the ligands that can be either electron donating or electron
withdrawing [1c,1d,1e,13]. Emission from arene ruthenium com-
plexes is often quenched following a non-radiative deactivation
pathway [14]. Further, the non-radiative decay could be hindered
by a judicious choice of ligands that controls the energy gap between
the lowest excited energy state and the ground state (Table 1).
the complexes. Further, the protons associated with the
g
5-C5H5
ring in complex 3 resonated at 4.65 ppm and the methyl protonscor-
responding to the pentamethylcyclopentadienyl rings in complexes
4 and 5 appeared as a singlet at ꢀ1.30 ppm. The position and inte-
grated intensity of the various signals corresponded well to the
3.2. Solid-state characterization of complex 5
Suitable crystals with distinct morphology were grown from a
dichloromethane solution and diethylether by slow diffusion. The

2880
S.K. Singh et al. / Polyhedron 27 (2008) 2877–2882
Fig. 3. Emission spectra of 1–3 in dichloromethane solution.
Table 1
Crystallographic data for 5
Empirical formula
Formula weight
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C6.18H5.64B0.18Cl0.18F0.73Ir0.18N0.36
142.19
0.71073
orthorhombic
Pnma
12.073(2)
b (Å)
16.432(3)
c (Å)
15.775(3)
a
(°)
90
b (°)
90
c
(°)
90
Volume (Å3)
3129.4(11)
Z
22
1.660
Dcalc (Mg/m3)
Fig. 1. FAB mass spectra of 1–3.
Reflections collected/unique (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
28948/3706 (0.0730)
3706/0/211
0.998
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0431, wR2 = 0.1164
R1 = 0.0488, wR2 = 0.1210
Fig. 2. Electronic spectra of 1–5 in dichloromethane.
ORTEP [15] depiction, including the atom-numbering scheme for
the complex cation of 5, is shown in Fig. 4, and the selected crystal-
lographic data and bond parameters are listed in Tables 2 and 3,
respectively. Complex 5 crystallizes in the orthorhombic crystal
system with the Pnma space group. The coordination geometry
about the metal centre Ir(1) is completed by nitrogen N(1) of Ph-
BIAN, chloride ligand Cl(1) and C5Me5(Cp*) in
mode. Considering C5Me5 as a single coordination site, the overall
g
5-coordination
Fig. 4. ORTEP view of 5 with 30% ellipsoid probability.

S.K. Singh et al. / Polyhedron 27 (2008) 2877–2882
2881
Table 2
same reduction and RuCP-BIAN (3) is least effective in this regard.
Among the ruthenium p-cymeme and benzene complexes, the
ruthenium p-cymene complex (2) is more effective than its ben-
zene (1) analogue. This may be due to the inductive effect of the
isopropyl and methyl substituents on the capped arene ligand. Fur-
ther, Rh-BIAN was also found to catalyze the hydrogenation of 2-
acetyl-naphthalene, however only 20% conversion was achieved
even after 10 h.
Important bond parameters (Å, °) for 5
Ir(1)–N(1)
2.101(4)
2.133(8)
2.145(7)
2.166(6)
2.151
2.151
2.4047(18)
1.368
Ir(1)–C(14)
Ir(1)–C(16)
Ir(1)–C(15)
Ir–Ccp*
Ir–Cct
Ir(1)–Cl(1)
C–C(C5Me5)
C(C5Me5)–C(C5Me5)
C(1)–N(1)
C(1)–C(2)
C(1)–C(1)#1
N(1)–Ir(1)–N(1)
1.473
1.299(7)
1.467(7)
1.473(10)
76.9
4. Conclusion
In conclusion, in this work a new series of mononuclear piano-
stool complexes [(
(1), C10H14 (2)], [(
g [g
6-arene)Ru(Ph-BIAN)Cl]BF4 6-arene = C6H6
g
5-C5H5)Ru(Ph-BIAN)PPh3]BF4 (3) and [(g5
-
C5Me5)M(Ph-BIAN)Cl]BF4 [M = Rh (4), Ir (5)] have been synthesized
using a highly conjugated diimine ligand with a rigid acenaphthene
skeleton. Apart from the non-radiative nature of arene ruthenium
complexes, the reported complexes were found to be luminescent
at room temperature in dichloromethane. Furthermore, it has been
shown that complexes 1–5 effectively catalyze the reduction of
Table 3
Hydrogenation of terephthalaldehyde with M-BIAN in H2O at a S/C ratio of 100
H
O
O
H
H
O
M-BIAN
OH
H
H2O (THF in trace)
HCOOH+CH3COONa, 40oC
terephthalaldehyde, and among these the rhodium complex [(g5
-
H
C5Me5)Rh(Ph-BIAN)Cl]BF4 (4) is an effective hydrogenating catalyst
for use in water and air, and delivers faster rates without requiring
inert gas protection or substrate solubility in water.
Crystallographic data: Crystals suitable for single crystal X-ray
diffraction analyses for 5 were grown from dichloromethane and
diethyl ether at room temperature using the diffusion technique.
Preliminary data on the space group and unit cell dimensions as
well as intensity data were collected on a R-AXIS RAPID II diffrac-
Catalyst
% Con
t (h)
TOFa (hÀ1
)
RuB-BIAN (1)
RuPC-BIAN (2)
RuCP-BIAN (3)
Rh-BIAN (4)
Ir-BIAN (5)
>99
>99
>99
>99
>99
7
3
10
0.75
2
14
33
10
133
50
tometer using graphite monochromatized Mo K
a radiation. The
a
Based on >99 % conversion.
structure was solved by direct methods and refined using SHELX
97 [20]. The non-hydrogen atoms were refined with anisotropic
thermal parameters. The H-atoms attached to carbon atoms were
included as a fixed contribution and were geometrically calculated
and refined using the SHELX riding model.
coordination geometry about the metal centre is a typical piano-
stool geometry. The average Ir—Ccp distance is 2.151 Å (range
Ã
2.133–2.166 Å; Ir–Cct 1.810 Å) [16,17a]. The Ir–Cl and Ir–N dis-
tances are normal and have values of 2.4047 and 2.101 Å, respec-
tively [16a–18]. The chelating N–Ir–N bite for the coordination of
the imino nitrogen of Ph-BIAN is 76.9°. The arrangement of phenyl
rings with respect to the acenaphthene ring is almost orthogonal,
with a dihedral angle of 71.97°. The acenaphthene ring is planar
with a deviation of 0.76°. The C–C(C5Me5) bond lengths in C5Me5
are in the range 1.25–1.472 Å, with an average value of 1.368 Å,
and the exterior C(C5Me5)–C(C5Me5) average bond length is
1.473 Å. The C–C bond lengths within the Cp* ring and the C–CH3
bond lengths are normal [19].
5. Supplementary data
CCDC 670670 contains the supplementary crystallographic data
for complex 5. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
Thanks are due to the Council for Scientific and Industrial Re-
search, New Delhi for providing financial assistance through the
scheme HRDG, 01(1784)/02/EMR-II and for a Senior Research Fel-
lowship to S.K.S. Thanks are also due to the Head, Department of
Chemistry, Banaras Hindu University, Varanasi (UP) and the Na-
tional Institute of Advanced Industrial Science and Technology
(AIST), Ikeda, Osaka, Japan for extending facilities. We also thank
Mr. Atish Chandra (Department of Chemistry, Banaras Hindu Uni-
versity, Varanasi 221 005) for his help in monitoring the catalytic
reactions.
3.3. Catalytic application of complexes 1–5
To evaluate the selectivity and efficiency of complexes 1–5 as
catalysts towards the reduction of the mono/di-formyl group,
terephthalaldehyde was used as a model substrate. Hydrogenation
was initiated by introducing formic acid–sodium acetate and
terephthalaldehyde (1.0 mmol) in water (a few drops of freshly
distilled THF was used to dissolve the catalyst) and air at 40 °C
with 1% catalyst. All the complexes were found to catalyze the
hydrogenation of terephthalaldehyde to produce 4-hydroxymethy-
benzaldehyde in aqueous solution (Table 3). It was observed that in
all cases only one formyl group of terephthalaldehyde was selec-
tively reduced. Further, we found that the rhodium complex Rh-
BIAN (4) led to almost complete conversion of terephthalaldehyde
into 4-hydroxymethybenzaldehyde in 45 min. The analogous irid-
ium complex Ir-BIAN (5) was less active as compared to the rho-
dium complex Rh-BIAN (4) and complete conversion of the
substrate took place in 2 h. RuPC-BIAN (2) required 3 h for the
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