Syntheses, spectroscopy, and X-ray structures of 3-{(R)-(Ar)-ethylimino}-1, 3-dihydro-indol-2-one (Ar = Ph, MeOC6H4, BrC 6H4, 1-naphthyl) and [Rh(η4-cod){3-((R)- (Ar)-ethylimino)-3H-indol-2-olato}]

Article abstract of DOI:10.1080/00958972.2012.738293

Condensation of 1H-indole-2,3-dione (isatin) with (R)-(Ar)-ethylamines gives enantiopure Schiff bases, 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one (HL) {Ar=Ph (HL1), 2-MeOC₆H₄ (HL2), 4-MeOC ₆H₄ (HL3), 4-BrC<

Full text of DOI:10.1080/00958972.2012.738293

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Syntheses, spectroscopy, and
X-ray structures of 3-{(R)-(Ar)-
ethylimino}-1,3-dihydro-indol-2-
one (Arꢀ=ꢀPh, MeOC6H4, BrC6H4, 1-
naphthyl) and [Rh(η4-cod){3-((R)-(Ar)-
ethylimino)-3H-indol-2-olato}]
Mohammed Enamullah a , A. K. M Royhan Uddin a & Graeme
Hogarth b
a Department of Chemistry , Jahangirnagar University ,
Dhaka-1342 , Bangladesh
b Department of Chemistry , University College London , 20
Gordon Street, London , WC1H 0AJ , UK
Accepted author version posted online: 09 Oct 2012.Published
online: 26 Oct 2012.
To cite this article: Mohammed Enamullah , A. K. M Royhan Uddin & Graeme Hogarth (2012)
Syntheses, spectroscopy, and X-ray structures of 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one
(Arꢀ=ꢀPh, MeOC6H4, BrC6H4, 1-naphthyl) and [Rh(η4-cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-olato}],
Journal of Coordination Chemistry, 65:24, 4263-4276, DOI: 10.1080/00958972.2012.738293
To link to this article: http://dx.doi.org/10.1080/00958972.2012.738293
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Journal of Coordination Chemistry
Vol. 65, No. 24, 20 December 2012, 4263–4276
Syntheses, spectroscopy, and X-ray structures of 3-{(R)-(Ar)-
ethylimino}-1,3-dihydro-indol-2-one (Ar ^ Ph, MeOC6H4,
BrC6H4, 1-naphthyl) and [Rh(g4-cod){3-((R)-(Ar)-ethylimino)-
3H-indol-2-olato}]
MOHAMMED ENAMULLAH*y, A. K. M ROYHAN UDDINy
and GRAEME HOGARTHz
yDepartment of Chemistry, Jahangirnagar University, Dhaka-1342, Bangladesh
zDepartment of Chemistry, University College London, 20 Gordon Street, London,
WC1H 0AJ, UK
(Received 2 November 2011; in final form 10 September 2012)
Condensation of 1H-indole-2,3-dione (isatin) with (R)-(Ar)-ethylamines gives enantiopure
Schiff bases, 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one (HL) {Ar ¼ Ph (HL1),
2-MeOC6H4 (HL2), 4-MeOC6H4 (HL3), 4-BrC6H4 (HL4), and 1-naphthyl (HL5)}. The
Schiff bases readily coordinate to [Rh(ꢀ-O2CMe)(ꢁ4-cod)]2 (cod ¼ 1,5-cyclooctadiene) to give
mononuclear
[Rh(ꢁ4-cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-olato}]
{Ar ¼ Ph
(1),
4-MeOC6H4 (2), and 4-BrC6H4 (3)}, respectively. The Schiff bases and complexes have been
fully characterized by IR, UV-Vis, 1H-NMR, mass, and circular dichroism (CD) spectrometry.
Polarimetry and CD measurements show the enantiopurity of the Schiff bases as well as the
complexes. 1H NMR measurements reveal slow conversion of the lactam to the enol form of the
Schiff bases in solution. In the solid state the lactam form dominates as shown by crystal
structures of HL1 and HL4. While gross structural features of both are similar, the molecules
differ significantly in the relative orientations of the aryl and lactam rings. The difference is
mostly rotation about the N2–C9 bond with different C8–N2–C9–C11 torsion angle of
þ89.77(12)ꢀ for HL1 and C2–N2–C9–C11 of þ106.8(3)ꢀ for HL4.
Keywords: Chiral isatin Schiff bases; Rh(ꢁ4-cod)(chiral isatin Schiff bases); Lactam–enol
isomerization; Hydrogen-bond; X-ray structure
1. Introduction
1H-Indole-2,3-dione (isatin) is a commercially available and cheap indole derivative and
is an attractive starting material for drug syntheses. Schiff bases of isatin are
particularly noteworthy as they display a wide range of pharmacological properties
including acting as antibacterial, antifungal, antiviral, antileukemic, anticancer, and
antiprotozoal agents [1–3]. Consequently, syntheses of many achiral isatin Schiff bases
and their derivatives, derived by condensation of isatin with arylamine [4], hydrazine [5]
*Corresponding author. Email: enamullahju@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.738293

4264
M. Enamullah et al.
semicarbazide [6], thiosemicarbazide [7–10], and dithiocarbazate [11], respectively, or
with N-substituted derivatives of these compounds, and transition metal-isatin Schiff-
base complexes, have been reported. Somewhat surprisingly given the importance of
chirality in drug efficiency, there are no reports of chiral isatin Schiff bases and their
complexes with transition metals.
We recently reported syntheses of chiral Schiff bases (R)-N-(Ar)-ethyl-salicylaldimine
[12] or (R)-N-(Ar)-ethyl-2-oxo-1-naphthyldimine [13, 14] derived from condensation of
(R)-(Ar)-ethylamine with salicylaldehyde or 2-hydroxy-1-naphthaldehyde. The Schiff
bases readily react with [Rh(ꢀ-O2CMe)(ꢁ4-cod)]2 (cod ¼ 1,5-cyclooctadiene) to give
[Rh(SB)(ꢁ4-cod)] {SBꢁ ¼ deprotonated Schiff bases ¼ (R)-N-(Ar)-ethyl-salicyldeiminate
or (R)-N-(Ar)-ethyl-2-oxo-1-naphthyldiminate} [12–14]. The X-ray results show that
SBꢁ coordinate to Rh(ꢁ4-cod) as six-membered N,O-chelates with distorted
square-planar geometry at rhodium. Syntheses, stereochemistry, and spectroscopic
characterizations of [Rh(ꢁ4-cod){(R)-2-(X-benzaldimine)-2-phenylethanol-ꢂ2N,O}]
(acetate) [15], [Rh(ꢁ4-cod){(R or S)-2-(salicylaldiminato)-2-phenylethanol-ꢂ2N,O}] [16]
and [Rh(ꢁ4-cod){(rac)-2-(salicylaldiminato)-1-phenylethanol-ꢂ2N,O}] [16] have been
reported. Similar reactions with chiral amino acids (HXY) or amino alcohols (AA)
give [Rh(ꢁ4-cod)(XY)] or [Rh(ꢁ4-cod)(AA)](acetate) [17–19], where deprotonated amino
acids or amino alcohols coordinate to Rh(ꢁ4-cod) as five-membered N,O-chelates as
shown by X-ray measurements. However, phosphine ligands (dppe or dppp or triphos)
react with the [Rh(ꢁ4-cod)(XY)] to give the cationic [Rh(dppe)2](XY) or neutral
[Rh(dppp or triphos)(XY)] [20].
Herein, we report the syntheses, spectroscopic characterizations, and molecular
structures of enantiopure isatin Schiff bases, 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-
indol-2-one (Ar ¼ Ph, 2-/4-MeOC6H4, 4-BrC6H4, and 1-naphthyl) and their complexes,
[Rh(ꢁ4-cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-olato}].
2. Experimental
2.1. Materials and methods
Methanol was distilled over CaO. UV-Vis spectra were obtained with a Shimadzu UV
3150 spectrophotometer in CHCl3 at 20ꢀC. IR spectra were recorded on a Bruker Optik
IFS 25 spectrometer as KBr discs at ambient temperature. Elemental analyses were
done on a VarioEL from Elementaranalysensysteme GmbH. NMR spectra were run on
a Bruker Avance DPX-300 or GEOL-400 or GEOL-500 spectrometer, operating at
300 MHz (1H), 75 MHz (13C), or at 400 MHz (1H) or at 500 MHz (1H) at 20ꢀC with
1
calibration against the residual protonated solvent signal (CDCl3: 7.25 ppm for H,
1
1
77.0 ppm for 13C; acetone-d6: 2.05 ppm for H; DMSO-d6: 2.50 ppm for H). ESI-MS
experiments were carried out on a QStar Elite quadrupole time-of-flight (Q-TOF)
instrument (MDS Analytical Technologies, Concord, ON, Canada), equipped with a
‘‘turbo ion spray’’ ion source. Polarimetric measurements were carried out with JASCO
DIP-181 and Rudolph Research Analytical AUTOPOL II Instruments in CHCl3 at
25ꢀC; values of [ꢃ]25 were determined according to the literature [12]. Circular dichroism
(CD) spectra were obtained with a JASCO J-170 Spectropolarimeter in CHCl3 at 20ꢀC.

Rhodium isatin Schiff base
4265
1H-Indole-2,3-dione (isatin) was used as received from Merck. The enantiopure amines,
(R)-(Ar)-ethylamine, were used as received from the BASF, Germany.
2.2. General procedure for syntheses of the Schiff bases 3-{(R)-(Ar)-ethylimino}-1,
3-dihydro-indol-2-one (HL1–HL5)
To 1H-Indole-2,3-dione (isatin) (1.50 g, 10.20 mmol) in methanol (10 mL) was added
2–3 drops of conc. H2SO4 and stirred for 10 min. To this was added an equimolar amount
of enantiopure (R)-(phenyl)-ethylamine (1.23 g, 10.20 mmol), resulting in a color change
to orange-yellow, and the mixture was refluxed for 4–5 h. Then, the solvent was
evaporated to 50% (v/v) and left standing at room temperature for crystallization.
Orange-yellow crystals were formed after 3–4 days by slow evaporation of the solvent.
The crystals were filtered off and washed three times with methanol (5 mL each). Drying
the crystals in air gave enantiopure 3-{(R)-(phenyl)-ethylimino}-1,3-dihydro-indol-2-one
(HL1). The same procedure was followed to synthesize 3-{(R)-(2-methoxyphenyl)-
ethylimino}-1,3-dihydro-indol-2-one (HL2), 3-{(R)-(4-methoxyphenyl)-ethylimino}-1,
3-dihydro-indol-2-one (HL3), 3-{(R)-(4-bromophenyl)-ethylimino}-1,3-dihydro-indol-
2-one (HL4), and 3-{(R)-(1-naphthyl)-ethylimino}-1,3-dihydro-indol-2-one (HL5) using
(R)-(2-methoxyphenyl)-ethylamine, (R)-(4-methoxyphenyl)-ethylamine, (R)-(4-bromo-
phenyl)-ethylamine and (R)-(1-naphthayl)-ethylamine, respectively.
2.2.1. 3-{(R)-(phenyl)-ethylimino}-1,3-dihydro-indol-2-one (HL1). Yield: 2.15 g (84%)
(from isatin). [ꢃ]25 (c ¼ 0.28, CHCl3): þ 50.17ꢀ (598 nm). UV-Vis (4.88 ꢂ 10ꢁ4 mol dmꢁ3
,
CHCl3):
ꢄ
max/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 381 (1454) and 290 (4080). CD
(4.88 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (Dꢅmax/dm molꢁ1 cmꢁ1) ¼ 308 (ꢁ0.89) and
377 (ꢁ1.42). IR (KBr): 3300sb (ꢆN–H), 3097, 3027, 2967, 2926m (ꢆC–H), 1747, 1720vs
(ꢆC¼Oasy), 1640, 1612, 1591vs (ꢆC¼N), and 1464vs (ꢆC¼Osy) cmꢁ1. MS (ESI): m/z
(%) 539 (50) [M2 þ K]þ, 523 (25) [M2 þ Na]þ, 501 (10) [M2 þ H]þ, 289 (100) [M þ K]þ,
273 (20) [M þ Na]þ, and 251 (15) [M þ H]þ. 1H NMR (300 MHz, CDCl3): ꢇ 1.61
(d, JHH ¼ 6.7 Hz, 3H, CH3–E), 1.74 (d, JHH ¼ 7.2 Hz, 3H, CH3–L), 5.53
(q, JHH ¼ 6.5 Hz, 1H, CH–L), 6.58 (q, JHH ¼ 6.5 Hz, 1H, CH–E), 6.81
(d, JHH ¼ 7.8 Hz, 1H, HAr–E), 6.97 (d, JHH ¼ 7.8 Hz, 1H, HAr–L), 7.04
(ddd, JHH ¼ 7.6 Hz, 2.4 Hz, 2H, HAr–E, L), 7.22–7.29 (m, 2H, HAr–E, L), 7.30–7.37
(m, 6H, HAr–E, L), 7.49 (d, JHH ¼ 7.4 Hz, 2H, HAr–E), 7.56 (d, 2H, JHH ¼ 7.4 Hz,
HAr–L), 7.67 (d, JHH ¼ 7.5 Hz, 1H, HAr–E), 7.71 (d, JHH ¼ 7.7 Hz, 1H, HAr–L),
8.70 (b, 1H, NH–L), and 9.88 (b, 1H, OH–E) ppm (L and E stand for lactam and enol
forms, respectively, and lactam : enol ca 1 : 1). 13C{1H} NMR (75 MHz, CDCl3): ꢇ 24.4,
(CH3–L), 24.7 (CH3–E), 58.3 (CH–L), 61.6 (CH–E), 110.4 (C–Ar), 112.0 (C–Ar), 116.7
(C–Ar), 122.6 (C–Ar), 123.0 (C–Ar), 126.6 (2C–Ar), 126.9 (C–Ar), 127.0 (3C–Ar), 127.2
(2C–Ar), 128.4 (3C–Ar), 128.7 (3C–Ar), 132.7 (C–Ar), 133.5 (C–Ar), 142.8 (C–Ar),
144.0 (C–Ar), 145.3 (C–Ar), 151.3 (C ¼ N–L), 153.4 (C¼N–E), 160.2 (C¼O–L), and
1
165.9 (C–OH–E) ppm. H NMR (400 MHz, acetone-d6): ꢇ 1.52 (d, JHH ¼ 6.4 Hz, 3H,
CH3–E), 1.62 (d, JHH ¼ 6.4 Hz, 3H, CH3–L), 5.64 (q, JHH ¼ 6.4 Hz, 1H, CH–L), 6.61
(q, JHH ¼ 6.4 Hz, 1H, CH–E), 6.91 (d, JHH ¼ 8.0 Hz, 1H, HAr–E), 6.99
(dd, JHH ¼ 6.8 Hz, 0.8 Hz, 1H, HAr–L), 7.05 (ddd, JHH ¼ 6.8 Hz, 0.8 Hz, 2H, HAr–E,
L), 7.21–7.24 (m, 2H, HAr–E, L), 7.30–7.37 (m, 4H, HAr–E, L), 7.38–7.43 (m, 2H,
HAr–E & 1H, HAr–L), 7.50–7.55 (m, 2H, HAr–E), 7.55–7.60 (m, 2H, HAr–L), 7.91

4266
M. Enamullah et al.
(d, JHH ¼ 7.2 Hz, 1H, HAr–L), 9.73 (b, 1H, NH–L), and 9.84 (b, 1H, OH–E) ppm
(lactam : enol ca 4.5 : 1, after 15 min of solution preparation). 1H NMR (500 MHz,
DMSO-d6): ꢇ 1.48 (d, JHH ¼ 6.5 Hz, 3H, CH3–E), 1.54 (d, JHH ¼ 6.5 Hz, 3H, CH3–L),
5.55 (q, JHH ¼ 6.4 Hz, 1H, CH–L), 6.54 (q, JHH ¼ 6.9 Hz, 1H, CH–E), 6.84
(d, JHH ¼ 7.5 Hz, 1H, HAr–E), 6.90 (d, JHH ¼ 7.5 Hz, 1H, HAr–L), 7.01–7.05 (m, 2H,
HAr–E, L), 7.21–7.26 (m, 2H, HAr–E, L), 7.32–7.36 (m, 6H, HAr–E, L), 7.37–7.39
(m, 1H, HAr–E), 7.39–7.42 (m, 2H, HAr–E), 7.47–7.52 (m, 2H, HAr–L), 7.87
(d, JHH ¼ 7.5 Hz, 1H, HAr–L), 10.84 (s, 1H, NH–L), and 10.94 (s, 1H, OH–E) ppm
(lactam : enol ca 1.4 : 1). Anal. Calcd for C16H14N2O (250.30): C, 76.78; H, 5.64; N,
11.19. Found: C, 76.18; H, 5.42; N, 10.89.
2.2.2. 3-{(R)-(2-methoxyphenyl)-ethylimino}-1,3-dihydro-indol-2-one
(HL2). Isatin
(1.50 g, 10.20 mmol) and (R)-(2-methoxyphenyl)ethylamine (1.54 g, 10.20 mmol).
Yield: 2.41 g (84% from isatin). [ꢃ]25 (c ¼ 0.30, CHCl3): þ 70.32ꢀ (589 nm). UV-Vis
(4.22 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 381 (1456), and 291
(4070). IR (KBr): 3270sb (ꢆN–H), 3094, 3028, 2968, 2923m (ꢆC–H), 1749, 1724vs
(ꢆC¼Oasy), 1640, 1619, 1593vs (ꢆC¼N), 1517vs (ꢆC–Oasy), and 1464vs (ꢆC¼Osy)
cmꢁ1. MS (ESI): m/z (%) 599 (70) [M2 þ K]þ, 583 (40) [M2 þ Na]þ, 561 (7) [M2 þ H]þ,
1
319 (100) [M þ K]þ, 303 (15) [M þ Na]þ, and 281 (30) [M þ H]þ. H NMR (300 MHz,
CDCl3): ꢇ 1.59 (d, JHH ¼ 6.5 Hz, 3H, CH3–E), 1.70 (d, JHH ¼ 6.5 Hz, 3H, CH3–L), 3.75
(s, OCH3–E), 3.81 (s, OCH3–L), 5.52 (q, JHH ¼ 6.5 Hz, 1H, CH–L), 6.56 (q,
JHH ¼ 6.5 Hz, 1H, CH–E), 6.75 (d, JHH ¼ 7.8 Hz, 1H, HAr–E), 6.88–6.91 (m, 4H,
HAr–L, E), 6.92 (d, JHH ¼ 7.8 Hz, 1H, HAr–L), 7.05 (t, JHH ¼ 7.5 Hz, 2H, HAr–L, E),
7.36 (ddd, JHH ¼ 7.6 Hz, 2H, HAr–L, E), 7.40 (d, JHH ¼ 8.6 Hz, 2H, HAr–E), 7.45 (d, 2H,
JHH ¼ 5.6, HAr–L), 7.68 (d, JHH ¼ 7.5, 1H, HAr, E), 7.73 (d, JHH ¼ 7.6, 1H, HAr, L), 8.10
(b, 1H, NH–L), and 9.19 (b, 1H, OH–E) ppm (lactam : enol ca 1 : 1.2).
2.2.3. 3-{(R)-(4-methoxyphenyl)-ethylimino}-1,3-dihydro-indol-2-one
(HL3). Isatin
(1.50 g, 10.20 mmol) and (R)-(4-methoxyphenyl)ethylamine (1.54 g, 10.20 mmol).
Yield: 2.35 g (82% from isatin). [ꢃ]25 (c ¼ 0.24, CHCl3): þ 75.00ꢀ (589 nm). UV-Vis
(4.20 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 382 (1400), and 291
(4130). IR (KBr): 3238sb (ꢆN–H), 3090, 3008, 2981, 2920m (ꢆC–H), 1751, 1722vs
(ꢆC¼Oasy), 1639, 1612, 1590vs (ꢆC¼N), 1514vs (ꢆC–Oasy), and 1466vs (ꢆC¼Osy)
cmꢁ1. MS (ESI): m/z (%) 599 (90) [M2 þ K]þ, 583 (50) [M2 þ Na]þ, 561 (5) [M2 þ H]þ,
1
319 (100) [M þ K]þ, 303 (12) [M þ Na]þ, and 281 (30) [M þ H]þ. H NMR (300 MHz,
CDCl3): ꢇ 1.57 (d, JHH ¼ 6.5 Hz, 3H, CH3–E), 1.72 (d, JHH ¼ 6.5 Hz, 3H, CH3–L), 3.78
(s, OCH3–E), 3.79 (s, OCH3–L), 5.50 (q, JHH ¼ 6.5 Hz, 1H, CH–L), 6.54
(q, JHH ¼ 6.5 Hz, 1H, CH–E), 6.79 (d, JHH ¼ 7.8 Hz, 1H, HAr–E), 6.86–6.89 (m, 4H,
HAr–L, E), 6.94 (d, JHH ¼ 7.8 Hz, 1H, HAr–L), 7.04 (t, JHH ¼ 7.5 Hz, 2H, HAr–L, E),
7.34 (ddd, JHH ¼ 7.6 Hz, 2H, HAr–L, E), 7.41 (d, JHH ¼ 8.6 Hz, 2H, HAr–E), 7.47 (d, 2H,
JHH ¼ 5.6, HAr–L), 7.66 (d, JHH ¼ 7.5, 1H, HAr, E), 7.72 (d, JHH ¼ 7.6, 1H, HAr, L), 8.14
(b, 1H, NH–L), and 9.17 (b, 1H, OH–E) ppm (lactam : enol ca 1 : 1.2). Anal. Calcd for
C17H16N2O2 (280.32): C, 72.84; H, 5.75; N, 9.99. Found: C, 71.90; H, 5.50; N, 9.59.
2.2.4. 3-{(R)-(4-bromophenyl)-ethylimino}-1,3-dihydro-indol-2-one (HL4). Isatin
(1.50 g, 10.20 mmol) and (R)-(4-bromophenyl)ethylamine (2.04 g, 10.20 mmol).

Rhodium isatin Schiff base
4267
Yield: 2.85 (85% from isatin). [ꢃ]25 (c ¼ 0.19, CHCl3): þ 42.00ꢀ (589 nm). UV-Vis
(4.50 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 382 (1430), and 290
(4100). IR (KBr): 3278sb (ꢆN–H), 3090, 3058, 2980, 2920m (ꢆC–H), 1751, 1720vs
(ꢆC¼Oasy), 1641, 1610, 1589vs (ꢆC¼N) and 1464vs (ꢆC¼Osy) cmꢁ1. MS (ESI): m/z
(%) 351 (100) [M þ Na]þ, and 329 (15) [M þ H]þ
(
79/81Br isotopic pattern is clearly
visible following these peaks). 1H NMR (300 MHz, CDCl3): ꢇ 1.55 (d, JHH ¼ 6.4 Hz, 3H,
CH3–E), 1.71 (d, JHH ¼ 6.6 Hz, 3H, CH3–L), 5.49 (q, JHH ¼ 6.6 Hz, 1H, CH–L), 6.51 (q,
JHH ¼ 6.6 Hz, 1H, CH–E), 6.81 (d, JHH ¼ 7.8 Hz, 1H, HAr–E), 6.97 (d, JHH ¼ 7.8 Hz,
1H, HAr–L), 7.06 (dt, JHH ¼ 6.9 Hz, 1.8 Hz, 2H, HAr–E, –L), 7.33–7.35 (m, 1H, HAr–E),
7.36–7.38 (m, 1H, HAr–L), 7.39–7.40 (m, 2H, HAr–E, –L), 7.41–7.44 (m, 3H,
HAr–E þ 2H, HAr–L), 7.46–7.48 (m, 1H, HAr–L), 7.66 (d, JHH ¼ 7.4 Hz, 1H, HAr–E),
7.68 (d, JHH ¼ 7.6 Hz, 1H, HAr–L), 8.45 (b, 1H, NH–L), and 9.54 (b, 1H, OH–E) ppm
(lactam : enol ca 1 : 1.1). 13C{1H} NMR (75 MHz, CDCl3): ꢇ 24.1, (CH3–L), 24.7
(CH3–E), 57.7 (CH–L), 60.9 (CH–E), 110.4 (C–Ar), 112.0 (C–Ar), 116.6 (C–Ar), 120.6
(BrC–Ar), 121.0 (BrC–Ar), 122.4 (C–Ar), 122.7 (C–Ar), 123.2 (C–Ar), 126.9 (C–Ar),
128.4 (2C–Ar), 128.7 (3C–Ar), 131.5 (3C–Ar), 131.7 (2C–Ar), 132.9 (C–Ar), 133.7 (C–
Ar), 143.1 (C–Ar), 144.4 (C–Ar), 145.2 (C–Ar), 151.5 (C¼N–L), 153.6 (C¼N–E), 160.1
(C¼O–L), and 165.5 (C–OH–E) ppm. Anal. Calcd for C16H13BrN2O (329.19): C, 58.38;
H, 3.98; N, 8.51. Found: C, 58.12; H, 3.73; N, 8.33.
2.2.5. 3-{(R)-(1-naphthyl)-ethylimino}-1,3-dihydro-indol-2-one (HL5). Isatin (1.50 g,
10.20 mmol) and (R)-(1-naphthyl)ethylamine (1.75 g, 10.22 mmol). Yield: 2.50 g (82%
from isatin). [ꢃ]25 (c ¼ 0.23, CHCl3): þ 21.74ꢀ (589 nm). UV-Vis (4.70 ꢂ 10ꢁ4 mol dmꢁ3
,
CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 381 (1397), and 291 (4023). IR (KBr): 3253sb
(ꢆN–H), 3088, 3052, 2975, 2929m (ꢆC–H), 1755, 1720vs (ꢆC¼O), 1645, 1612, 1578vs
(ꢆC¼N) and 1464vs (ꢆC¼Osy) cmꢁ1. MS (ESI): m/z (%) 639 (10) [M2 þ K]þ, 623 (25)
[M2 þ Na]þ, 601 (12) [M2 þ H]þ, 339 (15) [M þ K]þ, 323 (20) [M þ Na]þ, and 301 (100)
1
[M þ H]þ. H NMR (300 MHz, CDCl3): ꢇ 1.68 (d, JHH ¼ 6.5 Hz, 3H, CH3–E), 1.82 (d,
JHH ¼ 6.6 Hz, 3H, CH3–L), 5.68 (q, JHH ¼ 6.6 Hz, 1H, CH–L), 6.73 (q, JHH ¼ 6.5 Hz,
1H, CH–E), 6.80 (d, JHH ¼ 7.7 Hz, 1H, HAr–E), 6.95 (d, JHH ¼ 7.8 Hz, 1H, HAr–L),
7.00–7.11 (m, 2H, HAr–E, L), 7.37–7.40 (m, 2H, HAr–E, L), 7.41–7.46 (m, 4H, HAr–E,
L), 7.65 (dd, JHH ¼ 6.7 Hz, 1.7 Hz, 1H, HAr–L), 7.71–7.75 (m, 2H, HAr–E & 1H, HAr–
L), 7.80–7.86 (m, 6H, HAr–E, L), 7.92 (s, 1H, HAr–E), 7.97 (s, 1H, HAr–L), 8.12 (b, 1H,
NH–L), and 9.22 (b, 1H, OH-E) ppm (lactam : enol ca 1 : 1.3). 13C{1H} NMR (75 MHz,
CDCl3): ꢇ 24.5, (CH3–L), 24.7 (CH3–E), 58.5 (CH–L), 61.7 (CH–E), 110.3 (C–Ar),
111.8 (C–Ar), 122.6 (C–Ar), 122.7 (C–Ar), 123.1 (2C–Ar), 124.9 (C–Ar), 125.0 (C–Ar),
125.3 (2C–Ar), 125.5 (2C–Ar), 125.7 (2C–Ar), 125.9 (C–Ar), 126.1 (C–Ar), 127.1 (2C–
Ar), 127.6 (2C–Ar), 127.9 (C–Ar), 128.1 (2C–Ar), 128.5 (C–Ar), 132.7 (2C–Ar), 133.5
(2C–Ar), 141.5 (C–Ar), 142.6 (C–Ar), 142.8 (C–Ar), 145.1 (C–Ar), 151.8 (C¼N–L),
153.2 (C¼N–E), 160.8 (C¼O–L), and 166.1 (C–OH–E) ppm. Anal. Calcd for
C20H16N2O (300.35): C, 79.98; H, 5.37; N, 9.33. Found: C, 79.29; H, 5.13; N, 9.03.
2.3. General procedure to synthesize the [Rh(ꢁ4-cod){3-((R)-(Ar)-ethylimino)-3H-
indol-2-olato}] (1–3)
Two equivalents of 3-{(R)-(phenyl)-ethylimino}-1,3-dihydro-indol-2-one (HL1) and
one equivalent of [Rh(ꢁ4-cod)(ꢀ-O2CCH3)]2 (cod ¼ 1,5-cyclooctadiene) were dissolved

4268
M. Enamullah et al.
in 10 mL of C6H6/MeOH (4 : 1, v/v) and stirred at room temperature. The color
changed from red-orange to red-brown after 10 min stirring. The solvent was
evaporated in vacuo at 40ꢀC after 5–6 h stirring. The products were dissolved in
10 mL of C6H6/MeOH (4 : 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 40ꢀC to give the red-brown [Rh(ꢁ4-cod){3-((R)-(phenyl)-
ethylimino)-3H-indol-2-olato}] (1). The same procedure was followed for syntheses of 2
and 3 by using 3-{(R)-(4-methoxyphenyl)-ethylimino}-1,3-dihydro-indol-2-one (HL3)
and 3-{(R)-(4-bromophenyl)-ethylimino}-1,3-dihydro-indol-2-one (HL4), respectively.
2.3.1. [Rh(g4-cod){3-((R)-(phenyl)-ethylimino)-3H-indol-2-olato}] (1). HL1 (65 mg,
0.26 mmol) and [Rh(ꢁ4-cod)(ꢀ-O2CCH3)]2 (70 mg, 0.13 mmol). Yield: 85 mg (71%).
UV-Vis (1.66 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 344 (2675)
and
506sh
(308).
CD
(5.80 ꢂ 10ꢁ4 dmꢁ3 mol,
CHCl3):
ꢄmax/nm
(Dꢅmax/dm molꢁ1 cmꢁ1) ¼ 322 (þ 0.53), 412 (þ 0.22), and 584 (ꢁ0.19). IR (KBr):
3064, 3050, 2925w (ꢆC–H), and 1734, 1618, 1577vs (ꢆC¼N) cmꢁ1. MS (ESI): m/z (%)
460 (100) [M þ H]þ. H NMR (500 MHz, CDCl3): ꢇ 1.74 (d, JHH ¼ 7.5 Hz, 3H, CH3),
1
1.90 (m, 4H, CH2codexo), 2.20, 2.37 (m, 4H, CH2codendo), 4.10, 4.20 (m, 4H, CHcod),
4.58 (m, 1H, CH), 6.55 (d, JHH ¼ 6.5 Hz, 1H, HAr), 6.80 (d, JHH ¼ 7.5 Hz, 1H, HAr),
7.07 (d, JHH ¼ 7.5 Hz, 1H, HAr), 7.26–7.40 (m, 5H, HAr), and 7.55 (d, JHH ¼ 7.5 Hz,
1H, HAr).
2.3.2. [Rh(g4-cod){3-((R)-(4-methoxyphenyl)-ethylimino)-3H-indol-2-olato}] (2). HL3
(68 mg, 0.24 mmol) and [Rh(ꢁ4-cod)(ꢀ-O2CCH3)]2 (65 mg, 0.12 mmol). Yield: 80 mg
(68%). UV-Vis (1.79 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 346
(2755) and 506 (358). IR (KBr): 3060, 3055, 2930w (ꢆC–H), and 1735, 1620, 1570vs
1
(ꢆC ¼ N) cmꢁ1. MS (ESI): m/z (%) 490 (100) [M þ H]þ. H NMR (500 MHz, CDCl3):
ꢇ 1.73 (d, JHH ¼ 7.0 Hz, 3H, CH3), 1.91 (m, 4H, CH2codexo), 2.27, 2.42 (m, 4H,
CH2codendo), 3.71 (s, 3H, OCH3), 4.08, 4.17 (m, 4H, CHcod), 4.55 (m, 1H, CH), 6.53 (d,
JHH ¼ 6.5 Hz, 1H, HAr), 6.80 (d, JHH ¼ 7.5 Hz, 1H, HAr), 7.03 (d, JHH ¼ 7.5 Hz, 1H,
HAr), 7.20–7.41 (m, 4H, HAr), and 7.62 (d, JHH ¼ 7.5 Hz, 1H, HAr).
2.3.3. [Rh(g4-cod){3-((R)-(4-bromophenyl)-ethylimino)-3H-indol-2-olato}]
(3). HL4
(79 mg, 0.24 mmol) and [Rh(ꢁ4-cod)(ꢀ-O2CCH3)]2 (65 mg, 0.12 mmol). Yield: 85 mg
(66%). UV-Vis (1.53 ꢂ 10ꢁ4 mol dmꢁ3, CHCl3): ꢄmax/nm (ꢅmax/dm molꢁ1 cmꢁ1) ¼ 348
(2695) and 506sh (348). IR (KBr): 3065, 3052, 2928w (ꢆC–H), and 1738, 1622, 1572vs
(ꢆC ¼ N) cmꢁ1. MS (ESI): m/z (%) 539 (20) [M þ H]þ, 329 (15) [HL4 þ H]þ, and 211
(30) [Rh(ꢁ4-cod)]þ 79/81Br isotopic pattern is clearly visible following the peaks at 539
(
and 329). 1H NMR (500 MHz, CDCl3): ꢇ 1.71 (d, JHH ¼ 7.0 Hz, 3H, CH3), 1.95 (m, 4H,
CH2codexo), 2.25, 2.40 (m, 4H, CH2codendo), 4.05, 4.20 (m, 4H, CHcod), 4.52 (m, 1H,
CH), 6.58 (d, JHH ¼ 6.5 Hz, 1H, HAr), 6.83 (d, JHH ¼ 7.5 Hz, 1H, HAr), 7.02 (d,
JHH ¼ 7.5 Hz, 1H, HAr), 7.20–7.48 (m, 4H, HAr), and 7.71 (d, JHH ¼ 7.5 Hz, 1H, HAr).

Rhodium isatin Schiff base
2.4. X-ray data collection and solution
4269
Single crystals were mounted on glass fibers and all geometric and intensity data were
taken from these samples using Bruker Apex-II CCD (for HL1) or Bruker SMART
APEX CCD (for HL4) diffractometers using graphite-monochromated Mo-Ka
˚
radiation (ꢄ ¼ 0.71073 A) at 172 ꢃ 2 K (HL1) or 150 ꢃ 2 K (HL4). Data reductions
were carried out with SAINT PLUS and absorption corrections applied using
SADABS. Structures were solved by direct methods and developed using alternating
cycles of least-squares refinement and difference Fourier synthesis. All atoms were
located by difference-Fourier synthesis and non-hydrogen atoms were refined aniso-
tropically and hydrogen atoms isotropically. Structure solution used SHELXTL PLUS
V6.10 program package.
2.4.1. Crystallographic
data
for
HL1. Yellow
needle,
C16H14N2O,
0.50 ꢂ 0.28 ꢂ 0.20 mm3, orthorhombic, P212121, a ¼ 9.898(2) A, b ¼ 11.024(2) A,
˚
˚
c ¼ 12.001(2) A, V ¼ 1309.5(3) A , Z ¼ 4, dcalc ¼ 1.270 g cmꢁ3
,
ꢀ ¼ 0.081 mmꢁ1
,
3
˚
˚
F(000) ¼ 528, final R indices [F2 > 2ꢈ] R1 ¼ 0.0389, wR2 ¼ 0.0999; R indices (all data):
R1 ¼ 0.0432, wR2 ¼ 0.1040, and Flack parameter ¼ 0.3(11).
2.4.2. Crystallographic
0.28 ꢂ 0.20 ꢂ 0.12 mm3,
data for
monoclinic,
HL4. Yellow
P21,
block,
a ¼ 5.019(1) A,
C16H13BrN2O,
b ¼ 9.515(2) A,
˚
˚
3
ꢉ ¼ 91.057(4)ꢀ,
V ¼ 681.9(3) A ,
Z ¼ 2,
dcalc ¼ 1.603 g cmꢁ3
,
˚
˚
c ¼ 14.281(3) A,
ꢀ ¼ 3.011 mmꢁ1, F(000) ¼ 332, final R indices [F2 > 2ꢈ] R1 ¼ 0.0339, wR2 ¼ 0.0704; R
indices (all data): R1 ¼ 0.0409, wR2 ¼ 0.0721, and Flack parameter ¼ 0.003(10).
3. Results and discussion
Condensation of isatin with primary amines, (R)-(Ar)-ethylamine gives enantiopure
isatin Schiff bases, 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one (HL) {Ar ¼ Ph
(HL1), 2-MeOC6H4 (HL2), 4-MeOC6H4 (HL3) 4-BrC6H4 (HL4), and 1-naphthyl
(HL5)} in high yields (82–85%) (scheme 1). The Schiff bases, upon conversion to the
enol form, coordinate to [Rh(ꢀ-O2CMe)(ꢁ4-cod)]2 (cod ¼ 1,5-cyclooctadiene) to give
[Rh(ꢁ4-cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-olato}], [Rh(ꢁ4-cod)(L)] {Ar ¼ Ph (1),
4-MeOC6H4 (2) and 4-BrC6H4 (3)} in C6H6/MeOH (4 : 1, v/v) (scheme 1).
3.1. Mass spectra
ESI-MS spectral data of the Schiff bases show the parent ion peak [M þ H]þ at
m/z ¼ 251 (HL1), 281 (HL2 or HL3), 329 (HL4), and 301 (HL5), respectively. The
spectra are further dominated by several ion peaks for [M þ Na/K]þ, [M2 þ H/Na/K]þ,
and [CH(CH3)(Ar)]þ species, respectively. Mass spectra of the complexes show the
parent ion peak [M þ H]þ at m/z ¼ 460 (1), 490 (2), and 539 (3), respectively. Further,
79/81Br isotopic pattern is clearly visible from the mass spectra at m/z ¼ 351, 329 in HL4
and at m/z ¼ 539, 329 in 3.

4270
M. Enamullah et al.
Ar
Ar
H2N
N
1/2[Rh(η4-cod)(µ-
O2CCH3)]2
O
N
Rh
Ar
O
O
O
C6H6/MeOH
4-5 h, rt
- CH3CO2H
MeOH / H+
4-5 h, reflux
N
N
H
N
H
[Rh(η4-cod)(L)] {Ar = Ph (1); 4-
MeOC6H4 (2); 4-BrC6H4 (3)}
3-{(R)-(Ar)-ethylimino}-1,3-
dihydro-indol-2-one (HL) {Ar =
Ph (HL1); 2-MeOC6H4 (HL2); 4-
MeOC6H4 (HL3); 4-BrC6H4
(HL4); 1-naphthyl (HL5)}
Scheme 1. Synthetic route to the formulation of 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one (HL) and
[Rh(ꢁ4-cod)(L)].
Ar
O
Ar
N
N
OH
N
H
N
3-{(R)-(Ar)-ethylimino}-1,3-
3-{(R)-(Ar)-ethylimino}-3 H-
dihydro-indol-2-one
indol-2-ol
(Lactam, L)
(Enol, E)
Scheme 2. Lactam (L) Ð enol (E) isomerization in solution.
3.2. IR spectra
IR spectral data (Supplementary material) are mainly characteristic of the
Schiff bases [9–15]. Thus each shows a strong broad band at 3300–3238 cmꢁ1 assigned
to ꢆ(N–H). Two very strong bands at 1755–1720 cmꢁ1 and 1466–1464 cmꢁ1 are assigned
to the ꢆ(C¼O) asymmetric stretch and symmetric stretch, respectively, in the Schiff
bases. Several strong bands at 1645–1640, 1620–1612, and 1598–1590 cmꢁ1 are assigned
to ꢆ(C¼N) in the Schiff bases and complexes. The Schiff bases undergo lactam (L) Ð
enol (E) isomerization (scheme 2) in solution, and subsequently, the enol
form coordinates to Rh(ꢁ4-cod) to form the complexes. The O–H bond (enol form) is
deprotonated while forming the ionic bond between the Rhþ and Oꢁ in the
complexes (scheme 1). As a result, any bands associated with ꢆ(N–H) and/or ꢆ(O–H)
are absent in the complexes. Similarly, two very strong ꢆ(C¼O) bands of the
Schiff bases are absent in the complexes. Several weaker bands at 3097–3008 and
2980–2925 cmꢁ1 are assigned to ꢆ(H–C) for aromatic and aliphatic hydrogen atoms,
respectively.

Rhodium isatin Schiff base
4271
Figure
(—:
1. Electronic
4.88 ꢂ 10ꢁ4 mol dmꢁ3
spectra
and
of
3-((R)-(1-phenyl)-ethylimino)-1,3-dihydro-indol-2-one
[Rh(ꢁ4-cod){3-((R)-(phenyl)-ethylimino)-3H-indol-2-olato}]
(HL1)
(1)
)
(- - -: 1.66 ꢂ 10ꢁ4 mol dmꢁ3) in CHCl3 at 20ꢀC.
3.3. Polarimetry
Polarimetric measurements in CHCl3 exhibit rotations to the right at þ50.17ꢀ (HL1),
þ70.32ꢀ (HL2), þ75.00ꢀ (HL3), þ42.00ꢀ (HL4), and þ21.74ꢀ (HL5) at 589 nm and 25ꢀC
for the enantiopure Schiff bases of 3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one.
3.4. Electronic spectra
Electronic spectra (figure 1) show (i) a strong broad band at 325–450 nm with
absorption maxima at ꢄmax ¼ 381–382 nm (ꢅmax ¼ 1397–1456 dm3 molꢁ1 cmꢁ1), associ-
ated with intraligand n!ꢊ* transitions of imine, and (ii) a very strong band at higher
energy (<325 nm) with absorption maxima at ꢄmax ¼ 290–291 nm (ꢅmax ¼ 4070–
4130 dm3 molꢁ1 cmꢁ1), associated with intraligand ꢊ ! ꢊ* transitions in the Schiff
bases [12–20]. The same bands are also found in the complexes (figure 1). The
complexes further show a weak broad band at lower energy (450–700 nm) due to
charge-transfer (CT) transitions based on the formation of [Rh(ꢁ4-cod)]þ and [Rh(L1)]
(L1 ¼ deprotonated Schiff base) species [12–20].
3.5. CD spectra
CD spectrum of HL1 (figure 2) shows the presence of two strong bands (below 320 nm
and 320–450 nm) with two negative
ellipticity maxima
at 308 nm
(Dꢅ ¼ ꢁ0.89 dm3 molꢁ1 cmꢁ1) and 377 nm (Dꢅ ¼ ꢁ1.42 dm3 molꢁ1 cmꢁ1) for intraligand
ꢊ ! ꢊ* and n ! ꢊ* transitions, respectively, confirming the enantiopurity of the Schiff
base. Complex 1 shows the same bands (figure 2) with positive ellipticity maxima at
322 nm (Dꢅ ¼ þ0.53 dm3 molꢁ1 cmꢁ1) and 412 nm (Dꢅ ¼ þ0.22 dm3 molꢁ1 cmꢁ1), respec-
tively, confirming its enantiopurity. A weak broad band at 450–700 nm with negative
ellipticity maximum at 584 nm (Dꢅ ¼ ꢁ0.19 dm3 molꢁ1 cmꢁ1) is due to CT transitions of
[Rh(ꢁ4-cod)]þ and [Rh(L1)] (L1 ¼ deprotonated Schiff base) species [12–20].

4272
M. Enamullah et al.
Figure
(4.88 ꢂ 10ꢁ4 mol dm
2. CD ꢁ3 spectra
(5.80 ꢂ 10ꢁ4 mol dmꢁ3) in CHCl3 at 20ꢀC.
of
3-((R)-(1-phenyl)-ethylimino)-1,3-dihydro-indol-2-one
[Rh(ꢁ4-cod){3-((R)-(phenyl)-ethylimino)-3H-indol-2-olato}]
(HL1)
(1)
)
and
3.6. NMR spectra
3.6.1. 1H NMR spectra and lactam–enol interconversion. 1H NMR spectra of HL1–
HL5 in CDCl3 are shown in ‘‘Supplementary material,’’ and spectral data are
summarized in section 2. The spectra correspond well with those of related compounds
[8–15]. The Schiff bases undergo slow isomerization from one tautomer to another,
lactam (L) Ð enol (E) (scheme 2), and both forms appear simultaneously in solution as
shown from the 1H NMR spectrum of HL1 (Supplementary material). The methyls are
two doublets of equal intensities at ꢇ 1.6–1.7 and 1.7–1.8 ppm (J ¼ 6.5 Hz) for the enol
and lactam forms, respectively. The methine protons are quartets at ꢇ 5.6 and 6.6 ppm
(J ¼ 6.5 Hz), the enol form being shifted to lower field by ca 1.0 ppm. The N–H (lactam)
and O–H (enol) protons show broad signals between ꢇ 8.1–8.7 and 9.2–9.8 ppm,
respectively. Methoxy protons are singlets at ꢇ 3.75–3.77 and ꢇ 3.79–3.81 ppm for the
enol and lactam forms, respectively, in HL2 and HL3. Several doublets and multiplets
are observed at the range of ꢇ 6.6–7.8 ppm for the aromatic protons (section 2).
In order to further probe the lactam and enol forms, time-dependent 1H NMR
spectra of HL1 were taken in acetone-d6 (figure 3). The spectra show that immediately
upon dissolution (ca 15 min) the major tautomer is the lactam form with only a small
amount of the enol form present (lactam : enol ca 4.5 : 1). This ratio becomes 1 : 1 after
approximately 2 h, and after 1 day the enol form is the major tautomer (lactam:enol ca
1 : 4.5). No further significant changes were noted upon further standing, suggesting
that equilibrium has been established. The peak assignments for the lactam and enol
1
forms are made based on the time-dependent H NMR spectra of HL1 in acetone-d6
(figure 3).
3.6.2. Solvent effects. To study solvent effects the 1H NMR spectrum of HL1 was run
in acetone-d6 and DMSO-d6. The chemical shifts of CH3, CH, and aromatic protons for
both the enol and lactam forms remain almost unchanged in CDCl3, acetone-d6, and
DMSO-d6 solution (see section 2). The N–H (lactam) signal shifts to lower field in
acetone-d6 (ꢇ 9.73 ppm) in contrast to that in CDCl3 (ꢇ 8.70 ppm), whereas the O–H
(enol) signal remains unchanged (ꢇ 9.88 ppm in CDCl3 and 9.84 ppm in acetone-d6).

Rhodium isatin Schiff base
4273
Figure 3. Time-dependent 1H NMR spectrum of 3-{(R)-(phenyl)-ethylimino}-1,3-dihydro-indol-2-one
(HL1) in acetone-d6 at 20ꢀC.
In DMSO-d6, both the N–H and O–H signals shift to lower field by ca 1–2 ppm (ꢇ 10.84
and 10.94 ppm, respectively) in contrast to those in CDCl3 (ꢇ 8.73 and 9.85 ppm). These
results suggest that both the N–H and O–H protons are involved in intermolecular
hydrogen-bonding and/or hydrogen-bonding to the solvent molecules [8, 14].
3.6.3. 13C NMR spectra. 13C NMR spectra of the chiral Schiff bases in CDCl3 (section
2) also show the existence of the enol and lactam forms simultaneously, as shown in
Supplementary material for HL4. Thus each Schiff base shows two methyl singlets at
ꢇ 24.1–24.5 ppm and ꢇ 24.5–24.7 ppm for the enol and lactam forms, respectively. The
methine carbon appears as two singlets at ꢇ 57.7–58.5 ppm and ꢇ 60.9–61.7 ppm for the

4274
M. Enamullah et al.
lactam and enol forms, respectively. The imide carbon (HC¼N) shows singlets at ꢇ 151.3
(HL1), 151.5 (HL4), and 151.8 ppm (HL5) for lactam forms, and at ꢇ 153.4 (HL1),
153.6 ppm (HL4), and 153.2 ppm (HL5) for enol forms. The ketonic carbon (C¼O)
shows singlets at ꢇ 160.2 (HL1), 160.1 (HL4), and 160.8 ppm (HL5) and the enolic carbon
(C–OH) at ꢇ 165.9 (HL1), 165.5 (HL4), and 166.1 ppm (HL5). The aromatic carbons
show several singlets for both the enol and lactam forms at ꢇ 110.0–145.0 ppm (section 2).
3.6.4. 1H NMR spectra of complexes. The exo-methylene protons of the coordinated
1,5-cyclooctadiene to Rh(I) show a multiplet at ꢇ 1.90–1.95 ppm, whereas the endo-
methylene protons show two multiplets at ꢇ 2.20–2.27 ppm and 2.37–2.42 ppm in 1–3.
The olefin protons show two multiplets, the downfield one at ꢇ 4.17–4.20 ppm isassigned
to ‘‘H trans to N’’ and the upfield one at 4.05–4.10 ppm to ‘‘H trans to O’’ [12–17]. The
methyl protons of coordinated Schiff base show doublets at ꢇ 1.71–1.74 ppm (J ¼ 7.0 Hz),
while the methine proton is a multiplet at ꢇ 4.52–4.58 ppm. The methoxy protons are a
singlet at ꢇ 3.71 ppm in 2. However, the N–H (lactam) and/or O–H (enol) protons are
absent in the complexes (broad peaks at ꢇ 8.1–8.7 and 9.2–9.8 ppm, respectively, in the
free Schiff bases), suggesting formation of an ionic bond between Rhþ and Oꢁ via
deprotonation due to complexation. Several doublets and multiplets are observed at
ꢇ 6.5–7.7 ppm for aromatic protons (section 2) in the complexes.
3.7. Solid-state structures of the Schiff bases
In order to fully elucidate the structures of the chiral Schiff bases in the solid state,
single-crystal X-ray studies were carried out on both HL1 and HL4. The molecular
structures are shown in figure 4 together with a summary of key bond lengths and
angles. Both molecules crystallize in chiral space groups; HL1 in orthorhombic P212121
and HL4 in the monoclinic P21 space group and the R-handed chirality is clear
(absolute structure parameter 0.3(11) for HL1, 0.003(10) for HL4). Both compounds
exist in the lactam form observed for the related achiral benzylamine-derived Schiff
bases of isatin [4–6]. Further discussion of these structures is presented in
‘‘Supplementary material.’’
(a)
(b)
Figure 4. Molecular structures of (a) HL1 and (b) HL4 showing 50% probability displacement ellipsoids
ꢀ
˚
and the atom-numbering schemes. Selected bond lengths (A) and angles ( ); HL1: C(7)–O(1) 1.216(1),
C(7)–C(8) 1.535(2), C(7)–N(1) 1.365(2), N(2)–C(8) 1.273(1), N(2)–C(9) 1.479(2), C(8)–N(2)–C(9) 119.8(1);
HL4: C(1)–O(1) 1.203(3), C(1)–C(2) 1.545(4), C(1)–N(1) 1.373(4), N(2)–C(2) 1.270(4), N(2)–C(9) 1.481(4),
C(14)–Br(1) 1.904(3), C(2)–N(2)–C(9) 120.8(3).

Rhodium isatin Schiff base
4275
4. Conclusion
We have shown that the synthesis and purification of enantiopure Schiff bases of isatin,
3-{(R)-(Ar)-ethylimino}-1,3-dihydro-indol-2-one is straightforward, suggesting that
such species are readily available for biological testing. In solution the Schiff bases
show similar behavior with slow conversion of the lactam to the enol form, both
observed. In the solid state the lactam form dominates as shown by the crystal
structures. The Schiff bases, upon conversion to the enol form, coordinate to [Rh(ꢀ-
O2CMe)(ꢁ4-cod)]2 giving enantiopure [Rh(ꢁ4-cod){3-((R)-(Ar)-ethylimino)-3H-indol-2-
olato}]. Unlike previously reported Rh(ꢁ4-cod)-chiral Schiff bases (where six-membered
N,O-chelation occurs), the isatin Schiff bases coordinate to Rh(ꢁ4-cod) as a five-
membered N,O-chelate. Further investigations to elucidate the molecular structures of
the complexes are in progress.
Supplementary material
Supplementary data contain figures S1–S6. CIF files provide details of the X-ray
crystallographic structure determinations. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystal data files are also available at http://
www.ccdc.cam.ac.uk/conts/retrieving.html as supplementary publications CCDC Nos
792 122 and 792123, CCDC, 12 Union Road, Cambridge, CB2 1FZ, UK; Fax: þ44-
1223-336033; E-mail: deposit@ccdc.cam.ac.uk
Acknowledgments
We acknowledge the financial support from the Ministry of Science and Information &
Communication Technology, Dhaka, Bangladesh under MSICT project 2009/10. We
thank Professor K.S. Hagen, Department of Chemistry, Emory University, Atlanta,
Georgia, for measuring the X-ray structure for HL1. We also thank Professor Y.
Fukuda and Dr Y. Mori, Department of Chemistry, Ochanomizu University, Tokyo,
for recording CD and time-dependent 1H NMR spectra. Our sincere thanks to
Professor A.B.P. Lever and Tamanna Rob, Department of Chemistry, York
University, Toronto, for recording mass spectra. We also thank Professor C. Janiak,
Institute of Inorganic and Structural Chemistry, University of Freiburg, Germany, for
providing the elemental and mass data.
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