Synthesis, spectroscopy, catalysis and crystal structure of [Rh(η4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato- κ2N,O}] (Ar = C6H5, 3-/4-MeOC 6H4, and 4-BrC6H4)

Article abstract of DOI:10.1016/j.ica.2012.01.013

Condensation of 2-hydroxy-1-naphthaldehyde with (R)-(Ar)ethylamine yields the enantiopure Schiff bases, (R)-N-(Ar)ethyl-2-hydroxy-1-naphthaldimine (Ar = C₆H₅, 3-/4-MeOC₆H₄, 4-BrC ₆H₄). Thes

Full text of DOI:10.1016/j.ica.2012.01.013

Inorganica Chimica Acta 387 (2012) 173–180
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Inorganica Chimica Acta
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Synthesis, spectroscopy, catalysis and crystal structure of
[Rh(g j
4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato- 2N,O}]
(Ar = C6H5, 3-/4-MeOC6H4, and 4-BrC6H4)
a
b
c
Mohammed Enamullah a, , A.K.M. Royhan Uddin , Graeme Hogarth , Christoph Janiak
⇑
a Department of Chemistry, Jahangirnagar University, Dhaka 1342, Bangladesh
b Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK
c Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Condensation of 2-hydroxy-1-naphthaldehyde with (R)-(Ar)ethylamine yields the enantiopure Schiff
bases, (R)-N-(Ar)ethyl-2-hydroxy-1-naphthaldimine (Ar = C6H5, 3-/4-MeOC6H4, 4-BrC6H4). These Schiff
Received 4 November 2011
Received in revised form 2 January 2012
Accepted 5 January 2012
bases readily react with the dinuclear complex [Rh(
g l-O2CMe)]2 to afford the mononuclear com-
4-cod)(
plexes [Rh(
g
4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-j2N,O}] (Ar = C6H5 (I); 3-MeOC6H4 (II); 4-
Available online 14 January 2012
BrC6H4 (III)), respectively in C6H6/MeOH (5:1, v/v). The Schiff bases and complexes are characterized by
IR, UV–Vis, 1H/13C NMR and mass spectrometry, polarimetry and HPLC. The polarimetric measurements
show the enantiopurity of the Schiff bases as well as the complexes. The X-ray structure determination
for III demonstrates that the deprotonated Schiff bases, (R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminate, co-
Keywords:
Chiral Schiff bases (R)-N-(Ar)ethyl-2-
hydroxy-1-naphthaldimine
ordinate to the [Rh(
nar geometry at the rhodium metal atom. Reaction of III with O2 leads to the formation of the oxidative
aduct [Rh( -O)]2 (IIIa). Compound I or [Rh(
4-cod)( 4-cod){(S or R)-N-(phenyl)ethyl-salicylaldiminato}]
g
4-cod)]-fragment as a six-membered N,O-chelate ligand with distorted square pla-
(g
4-cod)Rh(I)-complexes
Crystal structure
Enantiopurity
Catalysis
g
l
g
were used for reduction of acetophenone with diphenylsilane into ( )-1-phenyl-ethanol, and conversions
up to 93–97% have been achieved.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
(Scheme 1) [15–17]. The X-ray studies show five-membered
N,O-chelation of amino-carboxylate or amino-alcohol to the
Reactions of bidentate N,O-chelate (HSB) and tetradentate
N,O,N,O-chelate (H2SB0) Schiff base ligands with dinuclear [Rh(g4
cod)( -X)]2 (X = Cl, OMe, O2CMe; cod = 1,5-cyclooctadiene) give
mononuclear [Rh(
4-cod)(SB)] and dinuclear [{Rh(g4-cod)}2(SB0)],
respectively [1–7]. Similar reactions with the chiral bidentate
N,N-chelate Schiff bases afford the analogous Rh(
4-cod)(imine)-
complexes [8–14]. The in situ systems composed of dinuclear
[Rh(
4-cod)Cl]2 and chiral N,N-chelates have successfully been
Rh(
[Rh(
g
4-cod)-fragment in distorted square planar symmetry. The
-
g
4-cod)(AA)] complexes readily react with di-/tri-phosphine li-
l
gands (diphos/triphos) to synthesise the mononuclear [Rh(diphos/
triphos)(AA)] complexes [18]. The chiral bidentate N,O-chelate
Schiff base ligands (R)-N-(Ar)ethyl-salicylaldimine (Ar = phenyl,
o/m/p-methoxphenyl, p-bromophenyl and 1-naphthyl) (HSB)
(Scheme 1) [19], (R)-N-(Ar)ethyl-naphthaldimine (HSB1) [20], (R)-
2-(X-benzaldimine)-2-phenylethanol (X = H or 2,4-dimethoxy)
(HL) [21] react with the dinuclear [Rh(
mononuclear [Rh(
4-cod)(SB)] [19], [Rh(
[Rh(
4-cod)(HL)](acetate) [21], respectively. Similar reaction with
g
g
g
used for asymmetric reduction of ketone derivatives into the corre-
g
4-cod)(
4-cod)(SB1)] [6,20], and
l-O2CCH3)]2 to give
sponding chiral secondary alcohol up to 65% ee.
g
g
We have given attention to synthesise the analogous Rh(
(chiral amino acids or amino alcohols) complexes starting from the
dinuclear [Rh( -O2CMe)]2. We have reported the synthe-
4-cod)(
ses, spectroscopy and crystal structures of mononuclear [Rh(g4
cod)(AA)] (AA = amino acetate: -alaninato, S/R-phenylglycinato,
N-methyl-/-phenyl-glycinato, o-amino-benzoato/-phenolato) and
g
4-cod)
g
achiral bidentate N,O-chelate Schiff base ligands, N-(Ar)ethyl-
naphthaldimine (Ar = phenyl, o-tolyl) (HSB0) or with tetradentate
N,O,N,O-chelate, N,N0-R1-bis(salicylaldimine) {R1 = ethylene (H2salen)
or 1,2-phenylene (H2salophen)} give mononuclear [Rh(g4-cod)(SB0)]
g
l
-
L/D
or dinuclear [{Rh(g g
4-cod)}2(salen)] or [{Rh( 4-cod)}2(salophen)]
[Rh(g
4-cod)(AOH)](O2CMe) (AOH = amino alcohol: S-phenylglycinol)
complexes, respectively (Scheme 1) [6,20]. The X-ray results show
six-membered N,O-chelation of salicylaldiminate (SB or salen or
salophen) or naphthaldiminate (SB1 or SB0) to the Rh( 4-cod)-
g
⇑
Corresponding author.
E-mail addresses: enamullahju@yahoo.com (M. Enamullah), g.hogarth@ucl.ac.uk
(G. Hogarth), janiak@uni-duesseldorf.de (C. Janiak).
fragment in these complexes. The structure of chiral enantiopure
[Rh(g
4-cod)((R)-N-(4-methoxphenyl)ethyl-2-oxo-1-naphthaldiminato)]
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.013

174
M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
2-oxo-1-naphthaldiminato-j2N,O}] (I–III) (cf. Scheme 2). The sin-
gle-crystal structure was determined for III. The in situ formed
compound from [Rh(
4-cod)Cl]2 and (R)-N-(phenyl)ethyl-2-hydro-
g
xy-1-naphthaldimine (HSB1) has been used as catalyst for reduc-
tion of acetophenone into ( )-1-phenyl-ethanol.
2. Experimental
The syntheses of Rh(I)(g
4-cod)-(R)-Schiff bases complexes were
carried out under an atmosphere of dry nitrogen using standard
Schlenk techniques. Solvents used were dried and distilled under
nitrogen prior to use: benzene, diethyl ether, dichloromethane
over Na metal and methanol over CaO. UV–Vis spectra were ob-
tained with Shimadzu UV 3150 spectrophotometer in CH2Cl2 at
25 °C. IR-spectra were recorded on a Bruker Optik IFS 25 spectrom-
eter as KBr disks at ambient temperature. Elemental analysis were
done on a VarioEL from Elementaranalysensysteme GmbH. NMR-
spectra were run on a Bruker Avance DPX 200 spectrometer oper-
ating at 200 MHz (1H) or on a Bruker AC DPX 400 at 400 MHz (1H)
and 100 MHz (13C) at 20 °C with calibration against the residual
protonated solvent signal (CDCl3: 1H NMR 7.25 ppm, 13C NMR
77.0 ppm). NMR grade solvent CDCl3 was deoxygenated prior to
use. EI-MS: Thermo-Finnigan TSQ 700. ESI-MS: QStar Elite quadru-
pole time-of-flight (Q-TOF) instrument (MDS Analytical Technolo-
gies, Concord, ON, Canada). Polarimetric measurements were
Fig. 1. Molecular structure of III (50% thermal ellipsoids, H atoms with arbitrary
radii). Selected bond distances [Å] and angles [°]: RhAO1 2.028(2), RhAN 2.073(3),
RhACcod 2.111(3)–2.146(3), O1AC 1.302(4) and O1ARhAN 88.87(10).
carried with a Perkin–Elmer 241 instrument in CHCl3 at 25 °C
displays a herring-bone arrangement and achiral [Rh(g
4-cod)(N-
25
and values of [
a]
were determined according to the literature
(o-tolyl)-2-oxo-1-naphthaldiminato)] crystallizes in the non-
centrosymmetric polar space group Cc where all molecules show
the same orientation.
[19,22]. Analytical HPLC4: LaChrom Elite, equipped with OB-H chi-
ral column was used for ee (%) value determination. The dinuclear
[Rh(g g
4-cod)(O2CMe)]2 was synthesized from [Rh( 4-cod)Cl]2
The present paper, in continuation, reports the synthesis,
stereochemistry and enantiopurity of (R)-N-(Ar)ethyl-2-hydroxy-
according to the literature [15]. (S or R)-N-(1-phenyl)ethylsalicyl-
aldimine}] (S- or R-HSB) was synthesized according to literature
[19]. Enantiopure (R)-(phenyl)ethylamine, (R)-(3-methoxyphenyl)
1-naphthaldimine (HSB1–HSB4), and [Rh(g
4-cod){(R)-N-(Ar)ethyl-
Fig. 2. Packing diagram of III, viewed along a (H atoms omitted for clarity).

M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
175
amino acid (AAH)
LRh
HO
NH2
NH2
(S)
O
*
R
*
O
R = Me, Ph
(S)
R
O
–CH3COOH
[Rh(η4-cod)(AA)] (AA = amino acetate)
chiral
amino alcohol (AOH)
+
LRh
HO
NH2
HO
NH2
–
O2CMe
*
Ph
(S)
*
Ph
(S)
[Rh(η4-cod)(AOH)](O2CMe)
chiral Schiff base (HSB)
Rh
(R)
O
O
LRh
( )
(R)
*
*
Me
N
O
N
2
OH
X
X
[Rh(η4-cod)(O2CMe)]2
–CH3COOH
[Rh(η4-cod)(SB)]
(X = o/m/p-MeO-, p-Br-)
achiral Schiff base (HSB')
LRh
LRh =
Ar
N
Ar
O
N
Rh
OH
[Rh(η4-cod)(SB')]
(Ar = Ph or o-tolyl)
–CH3COOH
L
Rh
O
N
O
N
Rh
L
H2salen
or
[{Rh(η4-cod)}2(salen)]
H2salophen
–2CH3COOH
L
Rh
O
N
N
O
LRh
[{Rh(η4-cod)}2(salophen)]
Scheme 1. Rh(g
4-cod) (chiral/achiral ligand) complexes with chiral ligand = AA, chiral amino acetate or =AOH, chiral amino alcohol or =HSB, chiral Schiff base; and with
achiral ligand = HSB0 or H2salen or H2salophen, achiral Schiff base.
ethylamine, (R)-(4-methoxyphenyl)ethylamine and (R)-(4-bromo-
phenyl)ethylamine were used as received from the BASF AG,
Ludwigshafen, Germany.
needle-shaped crystals were obtained. The crystals were washed
three times with MeOH (5 ml each) and dried in vacuo at 40 °C for
5–6 h to give (R)-N-(phenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB1) as a bright-yellow compound. The other (R)-N-(Ar)ethyl-
2-hydroxy-1-naphthaldimine derivatives were obtained by using
(R)-(3-methoxyphenyl)ethylamine (for HSB2), (R)-(4-methoxy-
phenyl)ethylamine (for HSB3), and (R)-(4-bromophenyl)ethyla-
mine (for HSB4).
2.1. General procedure to synthesise the(R)-N-(Ar)ethyl-2-hydroxy-1-
naphthaldimine (HSB1–HSB4)
2-Hydroxy-1-naphthaldehyde (5.0 g, 29 mmol) was dissolved
in 10 ml of methanol, 2–3 drops of conc. H2SO4 added into this
solution and the solution stirred for 10 min at room temperature.
An equimolar amount of (R)-phenylethylamine (3.7 ml, 29 mmol)
was added into this solution, the color changed to bright yellow
and the solution was refluxed for 5–6 h. Afterward the solvent
was evaporated in vacuo to about 50% and the yellow solution
was then left standing for crystallization through slow solvent
evaporation at room temperature. After 2–3 days bright-yellow
2.1.1. (R)-N-(phenyl)ethyl-2-hydroxy-1-naphthaldimine (HSB1)
Yield: 6.5 g (81%). [
a
]
25 (c = 0.54, CHCl3): ꢀ176° (598 nm). Anal.
Calc. for C19H17NO (275.36): C 82.88; H 6.22; N 5.09. Found: C
82.26; H 6.30; N 5.03%. IR (KBr, cm–1): 3050w, 2993m, 2934w
(mHAAr), 1628vs, 1611sh (mC@N), and 1597s (mC@C). MS (EI,
70 eV): m/z 275 (100) [M]+, 260 (5) [MꢀCH3]+, 170 (60)
[MꢀCH(CH3)(C6H5)]+, 105 (50) [CH(CH3)(C6H5)]+, and 77 (10)

176
M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
Table 1
Table 2
Crystal data and structure refinement for [Rh(
g
4-cod)(SB4)] (III).
C27H27BrNORh
Results for reduction of acetophenone with diphenylsilane (DPS) into ( )-1-phenyl-
ethanol using [Rh( -Cl)]2 (0.02 mmol) with chiral Schiff bases (L) at (0–5 °C).
g
4-cod)(
l
Chemical formula
Entry Schiff bases (L)
Rh/ Rh/L Rh/DPS Time Conversion
Formula weight
T (K)
Crystal system, space group
564.32
150(2)
monoclinic, P21
acetophenone (molar (molar (h)
(molar ratio) ratio) ratio)
(%)
Unit cell parameters
a (Å)
b (Å)
c (Å)
1
2
3
4
–
1:210
1:176
1:211
1:196
1:0
1:200 48
1:167 48
1:201 48
58
93
97
85
91
95
10.2218(14)
10.4813(15)
11.1563(16)
107.771(2)
1138.2(3)
(S)-HSB
(R)-HSB
(R)-HSB1
1:4.5
1:5.0
1:5.4
1:186
1
b (°)
6
24
V (Å3)
Z
2
Dcalc. (g/cm3)
1.647
2.525
HSB = (S or R)-N-(phenyl)ethyl-salicylaldimine; HSB1 = (R)-N-(phenyl)ethyl-1-
naphthaldimine.
Absorption coefficient
l
(mmꢀ1
)
F(000)
568
Crystal color and size (mm)
yellow,
77.25; H 6.27; N 4.47%. IR (KBr, cm–1): 3052m (
C@N), and 1589vs (
C@C). MS (EI, 70 eV): m/z 305 (100) [M]+,
mHAAr), 1620vs
0.28 ꢁ 0.24 ꢁ 0.18
2.73–28.27
13, 13, 14
99.3%
9391
4990 (Rint = 0.0308)
4600
0.5383 and 0.6593
0.0000, 0.0000
4990/1/280
R1 = 0.0300, wR2 = 0.0609
R1 = 0.0337, wR2 = 0.0619
0.948
h range for data collection (°)
Index ranges, h, k, l
Completeness to h = 26.00°
Reflections collected
(m
m
290 (5) [MꢀCH3]+, 170 (40) [MꢀCH(CH3)(C6H4OMe)]+, 135 (50)
[CH(CH3)(C6H4OMe)]+, 105 (10) [CH(CH3)(C6H5)]+, and 77 (8)
[C6H5]+. 1H NMR (200 MHz, CDCl3): d = 1.73 (d, JHH = 6.8 Hz,
3H, H13), 3.81 (s, 3H, H20), 4.75 (q, JHH = 6.8 Hz, 1H, H12), 6.95
(m, 4H, H15, 17, 18–19), 7.35 (m, 3H, H3, 6–7), 7.62 (dd,
JHH = 7.2 Hz, JHH = 0.8 Hz, 1H, H8), 7.71 (d, JHH = 9.0 Hz, 1H,
H5), 7.82 (d, JHH = 8.4 Hz, 1H, H4), 8.85 (s, 1H, H11), and 14.85
(br, 1H, OH).
Independent reflections
Reflections with F2 > 2
r
Minimum and maximum transmission
Weighting parameters a, b
Data/restraints/parameters
Final R indices [F2 > 2
R indices (all data)
r]
Goodness-of-fit (GOF) on F2
Absolute structure parameter, Flack value [30–
32]
0.012(8)
Largest difference in peak and hole (e Åꢀ3
)
0.893 and ꢀ0.763
2.1.3. (R)-N-(4-methoxyphenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB3)
[C6H5]+. 1H NMR (200 MHz, CDCl3): d = 1.75 (d, JHH = 6.8 Hz,
3H, H13), 4.80 (q, JHH = 6.8 Hz, 1H, H12), 7.05 (d, JHH = 9.2 Hz, 1H,
Yield: 7.5 g (85%). [
a]
25 (c = 0.52, CHCl3): ꢀ222° (598 nm). Anal.
Calc. for C20H19NO2 (305.39): C 78.66; H 6.27; N 4.59. Found: C
77.02; H 6.30; N 4.56%. IR (KBr, cm–1): 3051w, 2970m (
1624vs, 1600sh ( C@N), 1590sh ( C@C), and 1251s ( CAO). MS
mHAAr),
H17), 7.35 (m, 7H, H3, 6–7, 15–16, 18–19), 7.63 (dd, JHH
=
7.8 Hz, JHH = 0.8 Hz, 1H, H8), 7.71 (d, JHH = 9.2 Hz, 1H, H5),
7.82 (d, JHH = 8.4 Hz, 1H, H4), 8.87 (s, 1H, H11), and 14.80 (br, 1H,
OH).
m
m
m
(EI, 70 eV): m/z 305 (50) [M]+, 170 (5) [MꢀCH(CH3)(C6H4OMe)]+,
135 (100) [CH(CH3)(C6H4OMe)]+, and 105 (7) [CH(CH3)(C6H5)]+.
1H NMR (200 MHz, CDCl3): d = 1.71 (d, JHH = 6.4 Hz, 3H, H13),
3.80 (s, 3H, H20), 4.73 (q, JHH = 6.4 Hz, 1H, H12), 6.95 (m, 3H,
H15–16,18), 7.35 (m, 4H, H3, 6–7, 19), 7.60 (d, JHH = 7.7 Hz, 1H,
H8), 7.68 (d, JHH = 9.3 Hz, 1H, H5), 7.78 (d, JHH = 8.0 Hz, 1H, H4),
8.78 (s, 1H, H11), and 14.72 (br, 1H, OH).
2.1.2. (R)-N-(3-methoxyphenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB2)
Yield: 7.4 g (83%). [
Calc. for C20H19NO2 (305.39): C 78.66; H 6.27; N 4.59. Found: C
a
]
25 (c = 0.76, CHCl3): ꢀ132° (598 nm). Anal.
Ar =
(R)
H2N
Ar
CHO
OH
N
Ar
MeOH / H+
4-5 h
OH
-phenyl (HSB1, I)
OMe
–H2O
(R)-N-(Ar)ethyl-2-hydroxy-
1-naphthaldimine
HSB1-HSB4
-3-methoxyphenyl (HSB2, II)
(R)
OMe
Rh
C6H6/MeOH
O
N
Ar
1/2 [Rh(η4-cod)(O2CCH3)]2 + HSB
-4-methoxyphenyl (HSB3)
5-6 h
–CH 3COOH
Br
-4-bromophenyl (HSB4, III)
[Rh(η4-cod)(SB1-SB4)]
I - III
Scheme 2. Synthetic route to the formation of (R)-N-(Ar)ethyl-1-naphthaldimine (HSB1–HSB4) and [Rh(g
4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-j2N,O}] (I–III).

M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
177
2.1.4. (R)-N-(4-bromophenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB4)
CHCl3): +65° (578 nm). Anal. Calc. for C28H30NO2Rh (515.46): C
65.24; H 5.87; N 2.72. Found: C 64.91; H 5.96; N 2.50%. UV–Vis
Yield: 8.7 g (85%). [
a]
25 (c = 0.48, CHCl3): ꢀ104° (598 nm). Anal.
(5.149 ꢁ 10ꢀ4 mol dmꢀ3
,
CHCl3,
IR (KBr, cm–1): 3060
C@C). MS (EI, 70 eV): m/z 515
25 °C):
kmax = 396 nm;
Calc. for C19H16NOBr (354.26): C 64.42; H 4.55; N 3.95. Found: C
e
max = 4835 dm3 molꢀ1 cmꢀ1
.
(mHAAr),
63.58; H 4.55; N 3.86%. IR (KBr, cm–1): 3049m, 2984m (
1628vs, 1600sh ( C@N), and 1590s ( C@C). MS (EI, 70 eV): m/z
m
HAAr),
1615vs (mC@N), and 1578vs (
m
m
m
(100) [M]+, 407 (55) [Mꢀcod]+, 305 (5) [HSB3]+, and 218 (10)
[Rh(C6H5CH3CN)]+. 1H NMR (200 MHz, CDCl3): d = 1.76 (d,
JHH = 6.8 Hz, 3H, H13), 2.02 (m, 4H, CH2codexo), 2.51 (m, 4H, CH2co-
353 (100) [M]+, 338 (10) [MꢀCH3]+, 183 (40) [CH(CH3)(C6H4Br)]+,
170 (80) [MꢀCH(CH3)(C6H4Br)]+, 104 (55) [CH(CH3)(C6H5)ꢀH]+,
and 77 (10) [C6H5]+ (the 79/81Br isotopic pattern is clearly visible
for patterns following the 353, 338, and 183 peaks, with masses gi-
ven for the slightly more abundant 79Br-containing fragment). 1H
NMR (200 MHz, CDCl3): d = 1.71 (d, JHH = 6.6 Hz, 3H, H13), 4.74
(q, JHH = 6.6 Hz, 1H, H12), 7.05 (d, JHH = 9.2 Hz, 1H, H15), 7.28 (m,
3H, H3, 6, 19), 7.45 (m, 3H, H7, 16, 18), 7.65 (d, JHH = 7.8 Hz, 1H,
H8), 7.73 (d, JHH = 9.0 Hz, 1H, H5), 7.87 (d, JHH = 8.4 Hz, 1H, H4),
8.93 (s, 1H, H11), and 14.80 (br, 1H, OH).
dendo), 3.85 (m, 3H, H20), 3.95 (m, 2H, CHcod), 4.45 (q, JHH = 6.8 Hz,
1H, H12), 4.64 (m, 2H, CHcod), 7.02 (m, 2H, HAr), 7.21 (m, 6H, HAr),
7.62 (m, 2H, HAr), and 8.83 (d, JHH = 1.8 Hz, 1H, H11). 13C NMR
(100 MHz, CDCl3): d = 22.5 (C13), 28.2, 28.8, 31.1, 31.7 (CH2cod),
54.8 (C12), 60.2 (C20), 71.0 (d, JCRh = 14.3 Hz, CHcod), 73.2 (d,
JCRh = 14.2 Hz, CHcod), 84.2 (d, JCRh = 11.65 Hz, CHcod), 84.6 (d,
JCRh = 11.75 Hz, CHcod), 113.5 (C3, 17, 18), 118.1 (C1), 121.4 (C6),
124.5 (5), 126.1 (C7), 126.8 (C8), 128.5 (C15, 19), 128.5 (C10),
134.6 (C4), 134.7 (C14), 134.8 (C9), 157.6 (C2), 158.5 (C16), and
165.4 (C11).
7
13
6
5
8
9
2.2.3. [Rh(
naphthaldiminato-
(R)-N-(4-bromophenyl)ethyl-2-hydroxy-1-naphthaldimine (HSB4)
(135 mg, 0.38 mmol). Yield: 160 mg (75%). [a 25
(c = 0.27, CHCl3):
+52° (589 nm). Anal. Calc. for C27H27BrNORh (564.33): C 57.47;
g
4-cod){(R)-N-(4-bromophenyl)ethyl-2-oxo-1-
2N,O}], [Rh( 4-cod)(SB4)] (III)
11
19
16
14
18
17
O
j
g
N
12
10
4
1
20
15
2
OH
]
3
H
4.82;
N
2.48. Found:
C
57.21;
CHCl3, 25 °C): kmax = 392 nm;
HAAr), 1610vs,
1601vs (mC@N), and 1584vs (m
C@C). MS (ESI): m/z 602 (8) [M+K]+,
H
5.07;
N
2.55%. UV–Vis
(5.316 ꢁ 10ꢀ4 mol dmꢀ3
,
emax = 5
175 dm3 molꢀ1 cmꢀ1). IR (KBr, cm–1): 3057w
(m
2.2. General procedure to synthesise [Rh(
oxo-1-naphthaldiminato-
2N,O}] (I–III)
g
4-cod){(R)-N-(Ar)ethyl-2-
586 (20) [M+Na]+, 376 (5) [HSB4+Na]+, and 183 (40) [CH3CHC6H4Br]+
(the 79/81Br isotopic pattern is clearly visible for patterns following
the 602, 586, 376 and 183 peaks, with masses given for the slightly
more abundant 79Br-containing fragment). 1H NMR (400 MHz, CDCl3):
d = 1.74 (d, JHH = 6.8 Hz, 3H, H13), 1.95, 2.01 (m, 4H, CH2codexo), 2.48,
2.62 (m, 4H, CH2codendo), 3.86 (m, 2H, CHcod), 4.47 (q, JHH = 6.6 Hz,
1H, H12), 4.66 (m, 2H, CHcod), 7.04 (d, JHH = 9.1 Hz, 1H, H3), 7.18
(t, JHH = 7.3 Hz, 1H, H6), 7.32 (m, 3H, H7,15,19), 7.46 (d, JHH = 8.4 Hz,
1H, H5), 7.54 (d, JHH = 7.8 Hz, 2H, H16, 18), 7.62 (d, JHH = 7.8 Hz, 1H,
H8), 7.67 (d, JHH = 9.2 Hz, 1H, H4), and 8.86 (s, 1H, H11). 13C NMR
(100 MHz, CDCl3): d = 22.6 (C13), 28.7, 29.1, 31.5, 31.9 (CH2cod), 60.2
(C12), 71.7 (d, JCRh = 14.3 Hz, CHcod), 73.4 (d, JCRh = 14.2 Hz, CHcod),
84.6 (d, JCRh = 11.6 Hz, CHcod), 85.1 (d, JCRh = 11.5 Hz, CHcod), 109.1
(C1), 118.4 (C3), 121.3 (C8), 121.8 (C17), 124.9 (C6), 126.6 (C7),
127.1 (C5), 128.8 (C10), 129.2 (C15,19), 131.7 (C16,18), 134.9 (C4),
135.1 (C9), 142.5 (C14), 158.1 (C2), and 166.0 (C11).
j
(R)-N-(phenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB1)
(0.38 mmol) and [Rh(
g
4-cod)(O2CMe)]2 (102 mg, 0.19 mmol) were
dissolved in 10 ml of C6H6/MeOH (5:1, v/v) and the solution stirred
for 5–6 h at room temperature. The color changed from red-orange
to bright-yellow. Then the solvent was evaporated in vacuo at
40 °C. The products were again dissolved in 10 ml of C6H6/MeOH
(5:1, v/v), stirred for 30 min and the solvent evaporated in vacuo.
This procedure was repeated three times, and finally the products
were dried in vacuo (0.1–0.2 mbar) at 40 °C to give the yellow com-
plex of [Rh(g
4-cod){(R)-N-(phenyl)ethyl-2-oxo-1-naphthaldimina-
to-j2N,O}] (I). The same procedure was followed for syntheses of II
and III by using the Schiff bases of HSB2, and HSB4, respectively.
2.2.1. [Rh(
N,O}] [Rh(
g
g
4-cod){(R)-N-(phenyl)ethyl-2-oxo-1-naphthaldiminato-j2
4-cod)(SB1)] (I)
2.2.4. Reaction of [Rh(
g
4-cod)(SB4)] (III) with O2
4-cod)(SB4)] (III) in CHCl3
(R)-N-(phenyl)ethyl-2-hydroxy-1-naphthaldimine
(HSB1)
The orange-yellow solution of [Rh(
g
(104 mg, 0.38 mmol). Yield: 150 mg (82%) (based on [Rh(g4
-
was left standing for 2 weeks in air, and the color changed to
25
cod)(O2CMe)]2). [
a]
(c = 0.85, CHCl3): +88° (578 nm). Anal. Calc.
red-orange. The products were dried, and a products mixture of
[Rh(g g l-O)]2 (IIIa), and (HSB4) was
4-cod)(SB4)] (III), [Rh( 4-cod)(
for C27H28NORh (485.43): C 66.81; H 5.81; N 2.89. Found: C
67.91; H 6.35; N 2.46%. UV–Vis (6.339 ꢁ 10ꢀ4 mol dmꢀ3, CHCl3,
obtained. 1H NMR (400 MHz, CDCl3): d = 1.71 (d, JHH = 6.7 Hz, 3H,
H13 HSB4), 1.73 (d, JHH = 6.9 Hz, 3H, H13 III), 1.75, 1.92 (m, 4H,
CH2codexo IIIa), 1.95, 2.03 (m, 4H, CH2codexo III), 2.57 (m, 4H,
CH2codendo III), 2.49 (m, 4H, CH2codendo IIIa), 3.83 (m, 2H, CHcod
III), 4.24 (m, 4H, CHcod IIIa), 4.46 (q, JHH = 6.6 Hz, 1H, H12 III),
4.64 (m, 2H, CHcod III), 4.74 (q, J = 6.8 Hz, 1H, H12 HSB4), 8.84
(d, JHH = 1.8 Hz, 1H, H11 III), 8.91 (s, 1H, H11 HSB4), and 14.95
(br, 1H, OH HSB4). 13C NMR (100 MHz, CDCl3): d = 22.6 (C13 III),
24.2 (C13 HSB4), 28.7, 29.1, 31.5, 31.9 (CH2cod III), 30.8 (CH2cod
IIIa), 60.2 (C12 III), 63.4 (C12 HSB4), 71.7 (d, JCRh = 14.3 Hz, CHcod
III), 73.4 (d, JCRh = 14.2 Hz, CHcod III), 78.6 (d, JCRh = 13.9 Hz, CHcod
IIIa), 84.6 (d, JCRh = 11.6 Hz, CHcod III), 85.1 (d, JCRh = 11.5 Hz,
CHcod III), 157.3 (C11 HSB4), 158.1 (C2 III), and 166.0 (C11 III)
(aromatic protons and carbons are not given).
25 °C): kmax = 394 nm;
e
max = 4985 dm3 molꢀ1 cmꢀ1
.
IR (KBr,
cm–1): 3056w (
mHAAr), 1618vs (
mC@N), and 1577vs (
m
C@C). MS
(EI, 70 eV): m/z 485 (100) [M]+, 377 (78) [Mꢀcod]+, 275 (5)
[HSB1]+, 218 (15) [Rh(C6H5CH3CN)]+, 211 (5) [Rh(cod)]+, 208 (12)
[Rh(cod)ꢀH2ꢀH]+, and 105 (12) [CH(CH3)(C6H5)]+. 1H NMR
(200 MHz, CDCl3): d = 1.74 (d, JHH = 6.8 Hz, 3H, H13), 1.96 (m, 4H,
CH2codexo), 2.49 (m, 4H, CH2codendo), 3.94 (m, 2H, CHcod), 4.45
(q, JHH = 6.8 Hz, 1H, H12), 4.58 (m, 2H, CHcod), 6.98–7.03 (m, 2H,
HAr), 7.24–7.42 (m, 6H, HAr), 7.51–7.69 (m, 3H, HAr), and 8.85 (d,
JHH = 2.0 Hz, 1H, H11).
2.2.2. [Rh(
naphthaldiminato-
(R)-N-(3-methoxyphenyl)ethyl-2-hydroxy-1-naphthaldimine
g
4-cod){(R)-N-(3-methoxphenyl)ethyl-2-oxo-1-
2N,O}] [Rh( 4-cod)(SB2)] (II)
j
g
The above product mixture was dissolved again in CHCl3, and
left standing for a further 2 weeks in air. The products were dried,
25
(HSB2) (116 mg, 0.38 mmol). Yield: 165 mg (85%). [
a]
(c = 0.75,

178
M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
and a mixture of (IIIa), and (HSB4) was obtained (that is, III com-
pletely disappeared and was no longer seen by 1H NMR). 1H
NMR (400 MHz, CDCl3): d = 1.76 (d, JHH = 6.7 Hz, 3H, H13 HSB4),
1.77, 1.92 (m, 4H, CH2codexo IIIa), 2.50 (m, 4H, CH2codendo IIIa),
and 4.25 (m, 4H, CHcod IIIa), and 9.02 (s, 1H, H11 HSB4). 13C
NMR (100 MHz, CDCl3): d = 23.9 (C13 HSB4), 30.8 (CH2cod IIIa),
and 78.6 (d, JCRh = 13.9 Hz, CHcod IIIa). MS (ESI): m/z 455 (35)
Uiso(H) = 1.2Ueq(CH, CH2) and Uiso(H) = 1.5Ueq(CH3). Details of the
X-ray structure determinations and refinements are provided in
Table 1. Graphics were drawn with DIAMOND (Version 3.2) [25]. Com-
putations on the supramolecular interactions were carried out
with PLATON for Windows [26].
3. Results and discussion
[{Rh(cod)(
[{Rh(cod)(
l
l
-O)}2+H]+,
354
(10)
[HSB4+H]+,
252
(100)
-O)}+H2+Na]+, and 239 (35) [Rh(cod)(CO)]+ (79/81Br iso-
Condensation of 2-hydroxy-1-naphthaldehyde with enantiopure
(R)-(Ar)ethylamines yields the enantiopure Schiff bases, (R)-N-
(Ar)ethyl-2-hydroxy-1-naphthaldimine (HSB; Ar = phenyl (HSB1);
3-methoxphenyl (HSB2); 4-methoxphenyl (HSB3); 4-bromophenyl
(HSB4)) (Scheme 2). These Schiff bases readily react with the
topic pattern is clearly visible following m/z 354).
Isolation of [Rh( -O)]2 (IIIa): the product mixture of
g
4-cod)(
l
(IIIa) and (HSB4) was partly dissolved in C6H6:diethyl ether
(50:50 vol.%) to yield a yellow suspension. The insoluble part was
filtered off, the red-orange filtrate was collected and dried to give
only IIIa according to 1H NMR (400 MHz, CDCl3): d = 1.75, 1.94
(m, 4H, CH2codexo), 2.48 (m, 4H, CH2codendo), and 4.25 (m, 4H,
dinuclear [Rh(
g l-O2CMe)]2 to afford the mononuclear
4-cod)(
[Rh(g
4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-j2N,O}] (Ar =
phenyl (I); 3-methoxphenyl (II); 4-bromophenyl (III)) complexes,
respectively in C6H6/MeOH (5:1, v/v) (Scheme 2).
CHcod). MS (ESI): m/z 455 (25) [{Rh(cod)(l
-O)}2+H]+.
2.3. Reduction of acetophenone
3.1. Spectroscopy and analyzes
Acetophenone (0.470 g, 3.92 mmol), [Rh(cod)Cl]2 (10.8 mg,
0.02 mmol, for [Rh]/[acetophenone] = 1:196) and (R)-N-(phenyl)-
ethyl-2-hydroxy-1-naphthaldimine (HSB1) (29.7 mg, 0.11 mmol,
for [Rh]/[HSB1] = 1:5.4) were combined into a 50 mL Schlenk tube.
The mixture was degassed three times by evacuation and refilling
with N2, and the Schlenk tube was placed in an ice bath (0–5 °C).
After 10 min diphenylsilane (DPS) (0.685 g, 3.72 mmol, for [Rh]/
[DPS] = 1:186) was very slowly added into the mixture with a syr-
inge, and stirring was continued until the reaction was completed.
The progress of the catalytic reaction was monitored by taking 1H
NMR spectra of the reaction mixture after 1, 6 and 24 h in CDCl3. In
the course of the reaction, the singlet for CH3 of acetophenone dis-
appeared, and simultaneously, a doublet for the same group of ( )-
1-phenyl-ethanol appeared. Comparison of integration values for
these two peaks give the conversion (%) of acetophenone into
( )-1-phenyl-ethanol. Eventually the mixture was diluted in ace-
tone (5 ml) and hydrochloric acid solution (2.5 ml 37% HCl + 5 ml
H2O) and vigorous stirring continued for 2 h more in the ice bath.
The mixture was extracted with diethylether, and the organic layer
dried over K2CO3. The ether was then removed and the crude prod-
uct was purified by bulb-to-bulb distillation. The fraction from 100
to 110 °C (at 0.8–1.0 mbar) was collected as pure product and used
for chiral HPLC measurement to determine a potential enantio-
meric excess. The same procedure was followed using the
Schiff bases (S or R)-N-(phenyl)ethyl-salicylaldimine [19] with
[Rh(cod)Cl]2.
IR spectra show a very strong band at 1600–1628 cm–1 for
C@N and characteristic for the imine group in a Schiff base as well
m
as in its complexes [1–3,6,7,19–22]. EI mass spectra show the par-
ent ion peaks ([M]+) at m/z 275 (HSB1), 305 (HSB2 or HSB3), 353
(HSB4), 485 (I), and 515 (II), respectively (Section 2). ESI mass
shows the parent ion peaks as ([M+Na/K]+) at m/z 586/602 for III.
The spectra are further dominated by several ion peaks for
[MꢀCH3]+, [MꢀCH(CH3)(C6H4-H/OMe/Br)]+, [CH(CH3)(C6H4-H/
OMe/Br)]+, [CH(CH3)(C6H5)]+ in Schiff bases, and for [Mꢀcod]+,
[Rh(g
4-cod)]+ in the complexes (Section 2). The polarimetric mea-
surements show the rotations to the left at ꢀ176° (HSB1), ꢀ132°
(HSB2), ꢀ222° (HSB3), and ꢀ176° (HSB4) for the enantiopure R-
Schiff bases, and to the right at +88° (I), +65° (II), and +52° (III)
for the Rh(g
4-cod)-(R)-Schiff base complexes in CHCl3 [6,7,19–22].
1H-/13C NMR spectral data of the Schiff bases (HSB1–HSB4) and
complexes I–III are summarized in the Section 2, and their assign-
ments are made based on the related literature [1–6,19–22,33–45].
The methyl protons appear as a doublet at d = 1.70–1.75 ppm
(J = 6.8 Hz) both in the Schiff bases and complexes. The methine
proton appears as a quartet at d = 4.73–4.80 ppm (J = 6.8 Hz) in
Schiff bases which shifts upfield by d = 0.30 ppm in the complexes
(d = 4.43–4.47 ppm). The imine proton appears as a singlet at
d = 8.85–8.93 ppm in Schiff bases, and as a doublet (J = 2.0 Hz) in
the complexes due to 103Rh–1H coupling [1,4–8,19–22]. The phe-
nolic proton appears as a broad signal at d = 14.72–14.85 ppm in
Schiff bases due to strong intermolecular hydrogen bonding
[6,19,20], which is obviously absent in the complexes. Further,
the methoxy protons appear as singlet at d = 3.81–3.82 ppm in
HSB2, HSB3, and II. The exo- and endo-methylene protons of the
co-ordinated cod to the Rh(I) appear as multiplets at d = 1.95–
2.00 and 2.49–2.55 ppm in complexes I–III, respectively (Fig. S1
and Table S1). The olefin protons show two multiplets, the down-
field one (d = 4.58–4.66 ppm) is assigned to ‘H trans to N’, and the
upfield one (d = 3.85–3.95 ppm) to ‘H trans to O’ [1,3,6–8,19–22,40]
(Fig. S1 and Table S1). The difference in chemical shifts (0.7 ppm)
between these two multiplets reflects a different trans effect of
N,O-chelation on the olefinic protons resonances.
2.4. X-ray crystallography
Single crystals suitable for X-ray diffraction were grown from
slow diffusion of diethyl ether into a concentrated chloroform
solution of III over 3–4 days at room temperature. A single-crystal
was mounted on a glass fiber and all geometric and intensity data
were taken from this sample using a Bruker SMART APEX CCD dif-
fractometer with graphite-monochromated MoK
a radiation
(k = 0.71073 Å) at 150 2 K. Data reduction was carried out with
SAINT PLUS absorption correction with SADABS [23]. The structure
was solved by direct methods (SHELXS), refinement was done by
full-matrix least squares on F2 using the SHELXL program suite
[24]; all non-hydrogen positions refined with anisotropic displace-
ment parameters. Hydrogen atoms for aromatic CH, aliphatic or
olefinic CH, CH2 and methyl groups were positioned geometrically
(CAH@0.95 Å for aromatic CH, CAH = 1.00 Å for aliphatic and
olefinic CH, CAH = 0.99 Å for CH2, CAH = 0.98 Å for CH3) and
refined using a riding model (AFIX 43 for aromatic CH, AFIX 13
for aliphatic CH, AFIX 23 for CH2, AFIX 33 or 137 for CH3), with
In 13C NMR spectra of I–III, the four methylene carbon atoms of
co-ordinated cod give four singlets of equal intensity at d = 28.3–
31.9 ppm (Fig. S2 and Table S2) in contrast to only one singlet in
the dinuclear [Rh(
(amino-carboxylato)], and [Rh(
g
4-cod)(Cl)]2 [15], mononuclear [Rh(
g
4-cod)
g
4-cod)(amino-alcohol)](acetate)
[15,16]. The four olefin carbon atoms give four doublets, two of
them downfield (d = 84.0–85.1 ppm), assigned to ‘C trans to N’,
and two upfield (d = 71.0–73.3 ppm), assigned to ‘C trans to O’

M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
179
[1,3,6,19,20,40,43] (Fig. S2 and Table S2). However, the doublets
are due to coupling of olefinic carbon atoms with the Rh(I), giving
different 103Rh–13C(olefin) spin–spin coupling constants values
(Table S2), and thus reflecting fully asymmetric nature of each ole-
fin carbon atom. The observed J-values for ‘C trans to N’ (11.6–
11.8 Hz) and ‘C trans to O’ (14.2–14.3 Hz) agree well with those
isolated in C6H6:diethyl ether (50%). The exo- and endo-methylene
protons show multiplets at d = 1.75, 1.94 and 2.48 ppm (Table S1),
respectively. The olefinic protons exhibit
a
multiplet at
d = 4.25 ppm. ESI mass shows the parent ion peaks ([IIIa+H]+) at
m/z 455. However, 1H/13C NMR spectra of products mixture after
2 weeks show the proton and carbon peaks correspond well to
the (III), (IIIa), and (HSB4) in solution, respectively (Figs. S1, S2
found for the related Rh(g
4-cod)–imine complexes [1,3,6–8,19–
22,40]. However, the observation of four singlets and four doublets
for methylene and olefin carbons, respectively, has been explained
by steric and magnetic anisotropy effects in addition to the trans
influences of the N,O-chelate ligand on carbon resonances
[19,20,40]. The observed chemical shift differences between the
‘left’ and ‘right’ carbon atoms trans to the same donor atom are lar-
and Tables S1, S2). The spectrum shows
d = 14.95 ppm for phenolic proton (OH) for HSB4.
a broad peak at
3.3. Catalytic reduction of acetophenone into ( )-1-phenyl-ethanol
4-cod)(
l-Cl)]2 and
The in situ formed compounds from [Rh(
g
ger for ‘trans to O’
(Dd = 2.0 ppm) than for ‘trans to N’
(R)-N-(phenyl)ethyl-1-naphthaldimine (R-HSB1) or (S or R)-N-(1-
phenyl)ethylsalicylaldimine}] (S- or R-HSB) have been used for
the reduction of acetophenone with diphenylsilane into ( )-1-phe-
nyl-ethanol [7–10]. The conversion of acetophenone into ( )-1-
phenyl-ethanol is found to be 95% (R-HSB1), 93% (S-HSB), and
97% (R-HSB), respectively (Table 2) at 0–5 °C. The progress of cata-
lytic reaction using R-HSB1 has been monitored by taking 1H NMR
spectra of the reaction mixture after 1, 6, 24 h, and conversion is
85%, 91%, and 95%, respectively (Table 2). To our knowledge, this
(
D
d = 0.5 ppm) (‘left’ and ‘right’ is an arbitrary assignment for the
olefinic carbons to either side of a plane bisecting the C@C bond).
Electronic spectra (Fig. S3) of I–III mainly feature: (a) a very
strong band below 350 nm (<300 nm for IIIa), associated with
the intra-ligand
4-cod, (b) a strong broad band at 350–450 nm with absorption
maxima at kmax = 392–394 nm
max = 4800–5200 dm3 mol–1
cm–1), attributed to a charge transfer (ct) transition based on the
formation of [Rh(
4-cod)]+ and [Rh(SB)] [6,15,19,21,46]. However,
the ct transition shows two separate bands at 300–380 nm (kmax
326 nm) and 380–470 nm for [Rh( -O)], respec-
4-cod)]+ and [Rh(
tively in IIIa (Fig. S3). The spectrum of IIIa is very similar to the
analogous dinuclear bridged [Rh(
4-cod)( -Cl)]2 and [Rh(g4
cod)( -O2CCH3)]2 [6,15] (Fig. S3). The ct bands are found at 320–
370 nm (kmax/350 nm) and 370–430 nm for [Rh(
4-cod)]+ and
[Rh( -Cl)], respectively in [Rh( -Cl)]2. Similarly, the same
4-cod)(
bands are found at 330–370 nm (kmax/355 nm) and 380–480 nm
(kmax/421 nm) for [Rh( -O2CCH3)], respectively
4-cod)]+ and [Rh(
in [Rh( -O2CCH3)]2.
4-cod)(
p ?
p⁄ transition of the azomethine group and
g
(
e
g
is the first catalytic system composed of [Rh(g l-Cl)]2 and
4-cod)(
/
chiral N,O-chelate ligands (S- or R-HSB, R-HSB1) showing the
highest conversions in comparison to related systems in literature
[7–10]. In fact, in the absence of the chiral N,O-chelate ligand, the
conversion is 58% (Table 2). However, the ee (%) measurement
shows the formation of the racemic ( )-1-phenyl-ethanol in equal
amounts.
g
l
g
l
-
l
g
l
g
l
g
l
3.4. Crystal structure
g
l
3.2. Reaction of [Rh(g g l-O)]2
4-cod)(SB4)] (III) with O2 to [Rh( 4-cod)(
(IIIa)
The molecular structure of complex III ascertains the six-mem-
bered Rh-N,O-chelate ring formation of the 2-oxo-1-naphthaldim-
inato ligand and rhodium-bound
intermolecular packing in III shows no significant
g
4-cod fragment (Fig. 1). The
The solution of [Rh(g
4-cod)(SB4)] (III) in CHCl3 was left stand-
pꢂ ꢂ ꢂ
p
or CAHꢂ ꢂ ꢂ
p
contacts (see Table S3) despite the presence of an extended aro-
matic system in the naphthyl ring [53–58]. Only one CAHꢂ ꢂ ꢂBr con-
tact [27–29] can be noted (see Table S3 in Supporting information).
The crystal packing in III is a herring-bone pattern (Fig. 2), akin to
ing in air for 2 and 4 weeks, and color changed from orange-yellow
to red-orange. The UV–Vis/1H/13C NMR- and mass-spectral results
suggest that the complex reacts with molecular oxygen (from air)
and leads to the formation of dinuclear oxidative aduct of [Rh(g4
-
the packing in [Rh(g
4-cod)((R)-N-(4-methoxphenyl)ethyl-2-oxo-
cod)(l-O)]2 (IIIa), resulting from oxidation of Rh(I) to Rh(II) [47–
1-naphthaldiminato)] [6,20].
52]. A products mixture of (III), (IIIa), and (HSB4) was obtained
after 2 weeks, whereas, a mixture of (IIIa), and (HSB4) was ob-
tained after 4 weeks. The proton integration values show that
about 50% of reaction occurs after 2 weeks (i.e., solution contains
III, IIIa and HSB4), and 100% occurs after 4 weeks (i.e., solution
contains IIIa, and HSB4). In 1H NMR spectrum of products mixture
of IIIa and HSB4, the exo- and endo-methylene protons show mul-
tiplets at d = 1.77, 1.92 and 2.50 ppm (Fig. S1 and Table S1), respec-
tively. The olefin protons show one multiplet at d = 4.25 ppm
instead of two multiplets separated by 0.80 ppm in III or in II, as
mentioned in the above section. The methylene and olefin carbon
atoms of co-ordinated cod to Rh(I) give a singlet at d = 30.8 ppm,
4. Conclusions
Enantiopure
diminato-
2N,O}] complexes with Ar = C6H5 (I); 3-MeOC6H4 (II);
4-BrC6H4 (III) are easily obtained from the reaction of dinuclear
[Rh( -O2CMe)]2 and the enantiopure Schiff base naphtha-
4-cod)(
ldimine ligand. The deprotonated Schiff bases co-ordinate as
expected to the [Rh(
4-cod)]-fragment as a six-membered N,O-
[Rh(g
4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthal-
j
g
l
g
chelate ligand. The Schiff base ligand is more labile than the
and
a
doublet at d = 78.6 ppm (JCRh = 13.9 Hz), respectively
cod-ligand. Reaction of III with O2 replaces the Schiff base with
(Fig. S2 and Table S2). In contrary, the methylene and olefin carbon
atoms give four singlets and four doublets, respectively in III or in
II (Fig. S2 and Table S2). The spectrum, further, shows different
peaks for proton and carbon atoms associated to the HSB4
(Fig. S1 and Tables S1, S2). In fact, the proton and carbon peaks
assignment for IIIa corresponds well to the analoguos dinuclear
the formation of the oxidation product [Rh(
g l-O)]2 (IIIa).
4-cod)(
Enantiopure compounds I and [Rh(
g
4-cod){(S or R)-N-(phenyl)-
ethyl-salicylaldiminato}] were used for the attempted enantiose-
lective reduction of acetophenone with diphenylsilane into
( )-1-phenyl-ethanol. Yet, with no enantiomeric excess found it
can be concluded that the labile chiral Schiff base ligand was
replaced upon catalyst activation. Thus, introduction of chirality
in a rhodium(I) complex through a labile Schiff base ligand in the
presence of an inert cod-ligand does not allow to retain the
chirality for subsequent stoichiometric or catalytic reactions.
bridged [Rh(g l-Cl)]2 and [Rh(g l-O2CCH3)]2 [15]
4-cod)( 4-cod)(
(Tables S1, S2). ESI mass spectrum of products mixture of IIIa
and HSB4 shows the parent ion peaks ([IIIa or HSB4+H]+) at m/z
455 (IIIa) and 354 (HSB4), respectively. The red-orange (IIIa) was

180
M. Enamullah et al. / Inorganica Chimica Acta 387 (2012) 173–180
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We acknowledge the financial support from MSICT Project 2005
under the Ministry of Science and Information & Communication
Technology, Dhaka, Bangladesh. Our sincere thank to Professor
A.B.P. Lever, Department of Chemistry, York University, Toronto,
Canada for taking mass spectra. We thank the Alexander-
von-Humboldt Foundation (AvH), Bonn, for
M. Enamullah and Dr. Klaus Ditrich (ChiPros) at BASF, Ludwigshafen
for providing the enantiopure (R)-(Ar)ethylamines.
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