Synthesis and structural characterisation of new Re(III) complexes using aldimines of α-amino acids as coligands

Article abstract of DOI:10.1016/j.poly.2007.03.036

Complexes of the general formula [Re^III(L)Cl(PPh₃)₂] have been synthesised by reacting H₂L and [ReOCl₃(PPh₃)₂] in ethanol. Here H₂L represents imines of α-amino acids

Full text of DOI:10.1016/j.poly.2007.03.036

Polyhedron 26 (2007) 3465–3470
www.elsevier.com/locate/poly
Synthesis and structural characterisation of new Re(III)
complexes using aldimines of a-amino acids as coligands
a
a
b
a,
*
Sucharita Basak , Amrita Mondal , Deepak Chopra , Kajal Krishna Rajak
a
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata, 700 032, India
b
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560 012, India
Received 26 February 2007; accepted 21 March 2007
Available online 30 March 2007
Abstract
Complexes of the general formula [ReIII(L)Cl(PPh3)2] have been synthesised by reacting H2L and [ReOCl3(PPh3)2] in ethanol. Here
H2L represents imines of a-amino acids (glycine, L-alanine, L-valine, L-phenylalanine) derived from salicylaldehyde and naphthaldehyde.
The crystal structure of one complex has been determined. The complexes are mononuclear, paramagnetic and display paramagnetic 1H
NMR in CDCl3 solution. Their spectral and redox properties are scrutinised.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Rhenium(III); a-Amino acids; Schiff base
1. Introduction
L-valine, L-phenylalanine), starting from [ReOCl3(PPh3)2].
The X-ray structure of one representative case is reported.
Their spectral and redox behavior are discussed.
Considerable attention has been given to the coordina-
tion chemistry of rhenium due to its widespread applica-
tion in catalysis [1] as well as its b-emitting (Re186,Re188
)
2. Experimental
isotopes which are used as therapeutic agents in nuclear
medicine [2,3]. A sizable number of tridentate Schiff bases
[4] have been used to investigate the chemistry of rhenium.
However the chemistry of rhenium using aldimines of a-
amino acids is not known. This has motivated us to initiate
research of the chemistry of rhenium with the Schiff bases
of a-amino acid.
2.1. Materials
All the starting chemicals were analytically pure and
were used without further purification. The Schiff bases
were synthesized using the reported procedure [9].
The stability of a-amino acid Schiff base complexes
depends on various factors, such as the amino acid side
chain polarity [5], the metal, pH, solvent and temperature
[6]. Moreover racemic complexes are also observed even
in the case of optically active amino acid Schiff bases
[5,7,8]. Herein we describe a successful synthesis of a hith-
erto unknown family of mononuclear Re(III) species
chelated by aldimines of a-amino acids (glycine, L-alanine,
2.2. Physical measurements
UV–Vis spectra were recorded on a Perkin–Elmer
LAMBDA 25 spectrophotometer. IR spectra were mea-
sured with a Perkin–Elmer L-0100 spectrophotometer. H
1
NMR and 31P spectral measurements were carried out on
a Bruker FT 300 MHz spectrometer with TMS as an inter-
nal reference and Bruker FT 500 MHz spectrometer,
respectively. The atom-numbering scheme used for 1H
NMR is the same as that used in the crystallography and
in Chart 1. Electrochemical measurements were performed
(acetonitrile solution) on a CH620 electrochemical analyzer
*
Corresponding author.
E-mail address: kajalrajak@hotmail.com (K.K. Rajak).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.03.036

3466
S. Basak et al. / Polyhedron 26 (2007) 3465–3470
(ortho H of PPh3, d, J = 7.2 Hz); 8.34 (para H of PPh3, t,
1
4
OH
J = 7.2 Hz); 7.87 (meta H of PPh3, t, J = 7.5 Hz); À1.50
to 25.00 (ArH, 5H); À3.90 (H8, s, 2H); À23.95 (H2, d,
J = 8.7 Hz). E1/2 (ReIII/ReIV couple): 0.33 V (DEp,
80 mV); (ReIII/ReII couple): À1.40. MS (m/z): [M]+ Calcd.
973.16, 974.17, 975.17; Found: 972.89, 973.89, 974.90.
[ReIII(L2)Cl(PPh3)2] 1b. Yield: 81 mg (62%). Anal. Calc.
for C50H41NClO3P2Re: C, 60.79; H, 4.15; N, 1.41. Found:
C, 60.71; H, 4.02; N, 1.38%. UV–Vis (kmax/nm (e/
R1
7
C
8
COOH
N
C
H
R
H
1
R1
4,5-Benzo
4,5-Benzo
R
R
R
,
,
H
H2L
;
;
2
R1
R1
R1
CH3
CH2Ph
H2L
3
,
4,5-Benzo
H
H2L
;
4
M
À1 cmÀ1) CH2Cl2 solution): 267(25464), 338(11680),
363(9585), 431(10474), 492(3494), 1440(128). CD (kmax
,
H2L
R
R
R
H
;
;
;
,
,
H2L5
R1
R1
H
H
CH3
CH2Ph
/
6
nm (De/MÀ1 cmÀ1) CH2Cl2 solution): 265(6.39), 295
(À7.65), 331(1.96), 355(À6.82), 390(1.52), 430(À19.58),
458(À10.25), 475(À10.75). IR (KBr, cmÀ1): m(Re–Cl) 325;
m(C@N) 1672; m(CO2 sym) 1312; m(CO2 asym) 1665, 1627;
m(Re–P) 747. 1H NMR (CDCl3): d 47.90 (H7, s); 9.34 (ortho
H of PPh3, d, J = 7.8 Hz); 8.70 (para H of PPh3, t,
J = 7.4 Hz); 7.85 (meta H of PPh3, t, J = 7.2 Hz); À1.00 to
20.00 (ArH, 5H); À5.34 (H8, d, J = 8.4 Hz); À22.22 (H2, q,
J = 8.8 Hz); 1.30 (CH3 of amino acid, d, J = 7.2 Hz, 3H).
E1/2 (ReIII/ReIV couple): 0.44 V (DEp, 80 mV); (ReIII/ReII
couple): À1.10 V. MS (m/z): [M]+ Calcd. 987.46, 988.17,
985.65. Found: 987.10, 988.34, 985.70.
H2L
[Re(L1)Cl(PPh ) ]
[Re(L2)Cl(PPh3)2]
[Re(L3)Cl(PPh3)2] ,
[Re(L4)Cl(PPh ) ]
,
1a
1b
1c
3
2
PPh3
,
O
N
Cl
Re
2a
,
3
2
O
[Re(L5)Cl(PPh ) ]
[Re(L6)Cl(PPh ) ]
,
,
2b
2c
3
2
PPh3
3
2
Chart 1.
using a platinum electrode under a dinitrogen atmosphere.
Tetraethylammonium perchlorate (TEAP) was used as a
supporting electrolyte and the potentials are referenced to
the standard calomel electrode (SCE) without junction cor-
rection. Magnetic susceptibilities were measured on a
PAR-155 vibrating-sample magnetometer. Microanalyses
(C, H, N) were obtained from a Perkin–Elmer 2400 Series
II elemental analyzer. CD spectra were recorded on a
JASCO-715 polarimeter and mass spectra (70 eV) were
recorded on a Qtof Micro YA263 spectrometer.
[ReIII(L3)Cl(PPh3)2] 1c. Yield: 80 mg (63%). Anal. Calc.
for C56H46NClO3P2Re: C, 63.19; H, 4.32; N, 1.31. Found:
C, 62.89; H, 4.02; N, 1.29%. UV–Vis (kmax/nm (e/
M
À1 cmÀ1) CH2Cl2 solution): 269(21537); 332(10752),
360(8840), 432(16148), 489(3299), 1423(133). CD (kmax
/
nm (De/MÀ1 cmÀ1) CH2Cl2): 268(10.8), 295(À14.65),
331(À3.09), 357(À9.58), 395(5.16), 427 (À22.30),
460(À17.54). IR (KBr, cmÀ1): m(Re–Cl) 315; m(C@N)
1675; m(CO2 sym) 1315; m(CO2 asym) 1662, 1625; m(Re–P)
1
748. H NMR (CDCl3): d 49.42 (H7, s); 9.43 (ortho H of
2.3. Synthesis of the complexes
PPh3, d, J = 8.4 Hz); 8.41 (para
H
of PPh3, t,
J = 8.7 Hz); 7.86 (meta H of PPh3, t, J = 7.5 Hz); À1.20
to 20.00 (ArH, 10H); À5.34 (H8, d, J = 8.6 Hz); À22.25
(H2, t, J = 8.7 Hz); 0.84 (CH2Ph, d, 2H). E1/2 (ReIII/ReIV
couple): 0.42 V (DEp, 70 mV); (ReIII/ReII couple):
À1.25 V. MS (m/z): [M]+Calcd. 1063.40, 1064.20; Found:
1063.30, 1064.50.
The complexes [ReIII(L)Cl(PPh3)2] were prepared by
using a general method with [ReOCl3(PPh3)2] [10] as the
starting material. Details are given below for one represen-
tative case (1a).
[ReIII(L1)Cl(PPh3)2] 1a. 42 mg (0.18 mmol) of H2L1
were added to a solution of 100 mg (0.13 mmol) of
[ReOCl3(PPh3)2] in 20 ml dry ethanol in the presence of
excess PPh3. The reaction mixture was refluxed for 6 h,
producing a red precipitate. This was filtered and the solid
mass was then dissolved in a minimum volume of dichloro-
methane and subjected to column chromatography on a sil-
ica gel column (12 · 1 cm, 60–120 mesh). A red-orange
band was eluted using benzene–acetonitrile (10:1) mixture.
A red-orange solid was obtained after removal of solvent
from the eluate under reduced pressure. Yield: 88 mg
(68%). Anal. Calc. for C49H39NClO3P2Re: C, 60.40; H,
5.04; N, 1.44. Found: C, 60.32; H, 4.98; N, 1.39%. UV–
[ReIII(L4)Cl(PPh3)2] 2a. Yield: 78 mg (64%). Anal. Calc.
for C45H35NClO3P2Re: C, 58.6; H, 3.79; N, 1.51. Found:
C, 58.49; H, 3.72; N, 1.49%. UV–Vis (kmax/nm (e/
M
À1 cmÀ1) CH2Cl2 solution): 275(13585); 325(10894),
358(8960), 410(18067), 468(4150), 1445(125). IR (KBr,
cmÀ1): m(Re–Cl) 332; m(C@N) 1665; m(CO2 sym) 1315;
1
m(CO2 asym) 1658, 1618; m(Re–P) 746. H NMR (CDCl3):
d 45.50 (H7, s); 9.90 (ortho H of PPh3, d, J = 7.5 Hz); 8.42
(para H of PPh3, t, J = 7.3 Hz); 7.86 (meta H of PPh3, t,
J = 7.7 Hz); 1.00–20.00 (ArH, 3H); À8.40 (H8, s, 2H);
À14.05 (H2, d, J = 7.5 Hz). E1/2 (ReIII/ReIV couple):
0.41 V (DEp, 70 mV); (ReIII/ReII couple): À1.15 V. MS
(m/z): [M]+ Calcd. 921.60, 923.20. Found: 920.80, 922.50.
[ReIII(L5)Cl(PPh3)2] 2b. Yield: 79 mg (65%). Anal. Calc.
for C46H37NClO3P2Re: C, 59.0; H, 3.95; N, 1.49. Found:
C, 58.6; H, 3.88; N, 1.45%. UV–Vis (kmax/nm (e/MÀ1
cmÀ1) CH2Cl2 solution): 274(20247), 334(7457), 360
Vis (kmax/nm (e/MÀ1 cmÀ1
281(20780), 327(12480), 365(10695),
)
CH2Cl2 solution):
430(13300),
494(4281), 1456(134). IR (KBr, cmÀ1): m(Re–Cl) 320;
m(C@N) 1670; m(CO2 sym) 1310; m(CO2 asym) 1660, 1625;
m(Re–P) 742. 1H NMR (CDCl3): d 48.60 (H7, s); 9.80

S. Basak et al. / Polyhedron 26 (2007) 3465–3470
3467
for
Table 1
(9035), 437(13635), 467(2742), 1453(124). CD (kmax/nm
(De/MÀ1 cmÀ1) CH2Cl2): 255(8.28), 292(À7.10), 325(0.92),
345(À4.25), 385(6.08), 415(À13.02). IR (KBr, cmÀ1):
m(Re–Cl) 335; m(C@N) 1669; m(CO2 sym) 1308; m(CO2
asym) 1661, 1618; m(Re–P) 745. 1H NMR (CDCl3): d
48.3(H7, s); 9.60 (ortho H of PPh3, d, J = 6.8 Hz); 8.44
(para H of PPh3, t, J = 7.2 Hz); 7.84 (meta H of PPh3, t,
J = 7.5 Hz); 1.20–20.00 (ArH, 3H); À9.80 (H8, d,
J = 8.8 Hz); À11.85 (H2, q, J = 7.9 Hz), 1.42 (CH3 of
amino acid, d, J = 6.9 Hz, 3H). E1/2 (ReIII/ReIV couple):
0.42 V (DEp, 80 mV); (ReIII/ReII couple): À1.10 V. MS
(m/z): [M]+Calcd. 935.40, 937.80; Found: 935.60, 936.20.
[ReIII(L6)Cl(PPh3)2] 2c. Yield: 74 mg (61%). Anal. Calc.
for C51H42NClO3P2Re: C, 60.5; H, 4.15; N, 1.38. Found:
C, 60.32; H, 4.08; N, 1.36%. UV–Vis (kmax/nm (e/
Crystal
data
and
structural
refinement
parameters
[ReIII(L1)Cl(PPh3)2] Æ CH2Cl2 Æ H2O
[ReIII(L1)Cl(PPh3)2].CH2Cl2 Æ H2O
Formula
C50H41Cl2O0.5NO4P2Re
1056.60
orthorhombic
F2dd
17.444(3)
32.079(6)
33.483(6)
18737(5)
16
1.498
2.849
1.76–27.49
36191
9472 (0.0290)
293(2)
Formula weight
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
V (A )
Z
Dcalc (Mg mÀ3
)
l (mmÀ1
h (ꢁ)
)
Measured reflections
Unique refection (Rint
Temperature (K)
À1 cmÀ1) CH2Cl2 solution): 271(18575), 337(11220),
365(9670), 439(10230), 474(3982), 1437(132). CD (kmax
nm (De/MÀ1 cmÀ1
)
M
/
R1,a wRb2[I > 2r(I)]
0.0306, 0.0719
1.060
Goodness-of-Fit on F2
)
CH2Cl2): 265(6.80), 295(À7.42),
P
P
322(0.62), 346(À6.49), 380(6.18), 418(À10.52). IR (KBr,
a
R1= |Fo| À |Fc|/ |Fo|.
cmÀ1): m(Re–Cl) 327; m(C@N) 1676; m(CO2 sym) 1316;
P
P
b
2
wR2=[ w(Fo À Fc2)2/ w(Fo2)2]1/2
.
1
m(CO2 asym) 1659, 1622; m(Re–P) 746. H NMR (CDCl3):
d 49.80 (H7, s); 9.50 (ortho H of PPh3, d, J = 6.6 Hz); 8.42
(para H of PPh3, t, J = 7.2 Hz); 7.84 (meta H of PPh3, d,
J = 7.5 Hz); 1.20–20.00 (ArH, 8H); À9.60 (H8, d,
J = 8.5 Hz); À12.30 (H2, t, J = 7.8 Hz); 0.88 (CH2Ph, d,
work. The aldimine ligands will be abbreviated as H2L
(Chart 1), the two H atoms (phenolic and carboxylic) being
potentially dissociable. The abbreviations for specific Schiff
bases are given in Chart 1.
The stoichiometric reaction of [ReOCl3(PPh3)2] with the
appropriate ligand in boiling ethanol afforded orange-red
coloured complexes of the general formula [ReIII(L)Cl-
PPh3)2]. It is noted that when the reaction is carried out in
the presence of excess PPh3 there is an increase in yield of
[ReIII(L)Cl(PPh3)2]. This observation indicates that the
reduction from Re(V) to Re(III) takes place via the abstrac-
tion of the Re=O oxygen atom by PPh3 [4e].
The characterization data of the complexes are given in
the Section 2. A strong C@N stretch is observed at ꢀ1670
cmÀ1. Carboxylate monocoordination [15,16] is consistent
with the presence of one symmetric (ꢀ1310 cmÀ1) and
two asymmetric (ꢀ1620 and 1660 cmÀ1) stretches. The
Re–Cl and Re–P stretches occur at ꢀ325 and ꢀ745 cmÀ1
respectively.
Electro spray ionization mass spectrometry (ESI-MS) of
compound 1a displayed the highest m/z peak at 972.90,
unambiguously assigned to the [ReIII(L1)Cl(PPh3)2] parent
ion on the basis of the isotopic distribution of 35,37Cl and
185,187Re. The pattern for (M–H)+ is also observed.
2H). E1/2(ReIII/ReIV couple): 0.45 V (DEp, 80 mV); (ReIII
/
ReII couple): À1.30 V. MS (m/z): [M]+Calcd. 1011.80,
1009.30; Found: 1010.60, 1008.90.
2.4. X-ray structure determination
X-ray quality single crystals for the complex
[ReIII(L1)Cl(PPh3)2] were grown by slow diffusion of
diethyl ether into a dichloromethane solution. The X-ray
intensity data were measured at 293 K on a Bruker AXS
SMART APEX CCD diffractometer (Mo Ka, k =
˚
0.71073 A). The detector was placed at a distance of
6.03 cm from the crystal. A total of 606 frames were col-
lected with a scan width of 0.3ꢁ with different settings of
u. The data were reduced in SAINTPLUS [11] and an empiri-
cal absorption correction was applied using the SADABS
package [12]. The metal atom was located by direct meth-
ods using SIR92 [12]. The structure was refined by the full
matrix least-square method using SHELXL97 [13], that was
present in the program suite WINGX (Version 1.63.04a)
[14]. All non-hydrogen atoms were refined anisotropically.
The azomethine hydrogen atom was directly located in dif-
ference Fourier maps and the remaining hydrogen atoms
were included in calculated positions. Significant crystal
data are given in Table 1.
3.2. Crystal structure
The crystal structure of [ReIII(L1)Cl(PPh3)2] has been
determined. The molecular view is shown in Fig. 1 and
selected bond parameters are given in Table 2.
3. Result and discussion
The tridentate meridionally disposed [L1]2À contains
two excellently planar segments (OC10H6CHN and
3.1. Synthesis
˚
CCO2; mean deviation <0.02 A) and the dihedral angle
˚
between them is 5.8 A. In the distorted ReO2NClP2 octahe-
Aldimines of four a-amino acids (glycine, L-alanine,
dral environment two oxygen atoms, one chlorine atom
L-valine and L-phenylalanine) have been used in the present

3468
S. Basak et al. / Polyhedron 26 (2007) 3465–3470
Fig. 2. Cyclic voltammogram of ca. 10À3 M solution of [ReIII(L2)Cl-
(PPh3)2] in acetonitrile.
The anodic signal (ꢀ0.50 V) at a more positive potential
is assigned to the oxidation couple ReIV/III while the couple
at ꢀÀ1.10 V is believed to be the ReIII/II reduction. This
redox behavior is consistent with previously known Re(III)
complexes [17,19,20].
Fig. 1. Perspective view and atom-labelling scheme for the molecule
[ReIII(L1)Cl(PPh3)2]. All non-hydrogen atoms are represented by their
30% thermal probability ellipsoids.
and one nitrogen atom lie in the equatorial plane, whereas
the axial positions are occupied by two phosphine ligands.
The two Re–O bond distances are not similar and span the
3.4. Magnetism and NMR spectra
˚
range 1.89–1.98 A. The Re–P bond distances are slightly
The complexes show a magnetic moment value of ꢀ1.94
BM, corresponding to mononuclear octahedral rhe-
nium(III) d4 species. The low value of the magnetic
moment can be attributed to the large spin–orbit coupling
of the rhenium(III) ion [21,22]. The complexes display well-
resolved paramagnetic 1H NMR resonances in CDCl3
solution and the chemical shifts are observed within the
range ꢀÀ25.00 to +50.00 ppm. The resonances are
assigned on the basis of spin–spin structure and spin inten-
sity of the respective rhenium(III) complexes.
larger than for previously reported rhenium(III) mixed
ligand phosphine Schiff base complexes [17]. The Re–Cl
and Re–N bond distances are usual [18] (Table 2).
3.3. Electrochemical studies
The redox properties of the Re(III) a-amino acid Schiff
base complexes have been examined by cyclic voltammetry
using platinum as the working electrode and SCE as the
reference electrode. The cyclic voltammogram was
recorded at a scan rate of 50 mV/s. The relevant data are
given in Section 2 and a representative voltammogram is
shown in Fig. 2.
In the present case, the absence of phosphorous-proton
coupling is probably due to a rapid relaxation of the phos-
phorous nuclei through an electron-nuclear mechanism to
the paramagnetic Re(III) center [23–28]. The paramagneti-
All the complexes exhibit one quasi-reversible and one
irreversible one-electron wave in acetonitrile solution.
1
cally shifted H NMR spectrum of a representative case is
shown in Fig. 3 and the chemical shift values of the
Table 2
1
Selected structural parameters for [ReIII(L )Cl(PPh3)2] (distances in A and
˚
angles in ꢁ)
Distances
Re(1)–O(1)
Re(1)–O(2)
Re(1)–N(1)
1.896(3)
1.980(3)
2.013(4)
Re(1)–P(1)
Re (1)–P(2)
Re(1)–Cl(1)
2.5769(12)
2.5807(12)
2.4101(14)
Angles
O(1)–Re(1)–O(2)
O(1)–Re(1)–N(1)
O(2)–Re(1)–N(1)
O(1)–Re(1)–Cl(1)
O(2)–Re(1)–Cl(1)
O(1)–Re(1)–P(1)
O(2)–Re(1)–P(1)
O(1)–Re(1)–P(2)
168.07(12)
88.80(14)
79.38(15)
94.64(10)
97.18(11)
87.58(12)
91.17(10)
89.60(12)
O(2)–Re(1)–P(2)
N(1)–Re(1)–P(2)
N(1)–Re(1)–P(1)
N(1)–Re(1)–Cl(1)
Cl(1)–Re(1)–P(1)
Cl(1)–Re(1)–P(2)
P(1)–Re(1)–P(2)
92.24(10)
91.54(12)
91.60(12)
176.56(12)
88.40(5)
88.65(5)
175.74(5)
Fig. 3. 1H NMR spectrum of [ReIII(L1)Cl(PPh3)2] in CDCl3 at room
temperature.

S. Basak et al. / Polyhedron 26 (2007) 3465–3470
3469
complexes are given in Section 2. The numbering scheme is
4. Conclusion
as in Chart 1 and Fig. 1. In solution, the azomethine hydro-
gen occurs as a singlet at ꢀ49.00 ppm. The ortho protons
of the coordinated phosphine appear as a doublet at
ꢀ9.50 ppm (J = 7–8 Hz). The meta and para protons are
observed as triplets at ꢀ7.80 and 8.40 ppm respectively,
with a coupling constant J = 7–9 Hz. All other aromatic
H-atoms fall in the range +25.00 to À25.00 ppm. The a-
hydrogen atom of the amino acid residue appears as sin-
glet/multiplet in the negative region.
The Schiff bases of salicyladehyde/naphthaldehyde and
a-amino acids (glycine, L-alanine, L-valine, L-phenylala-
nine) react with [ReOCl3(PPh3)2] toafford Re(III) com-
plexes of the general formula [ReIII(L)Cl(PPh3)2]. The
species [ReIII(L)Cl(PPh3)2] represent the first example of
structurally characterized Re(III) complexes incorporating
a-amino acid Schiff bases as a coligand. The CD spectra
confirm the L-absolute configuration of the amino acids
in the complexes. Our search for new rhenium complexes
of Schiff bases of different amino acids and peptides are
in progress.
The 31P NMR signals for the phosphines coordinated to
the Re(III) center were not observed and the result is con-
sistent with previously reported paramagnetic Re(III) com-
plexes [26,29,30].
Acknowledgements
3.5. Electronic and CD spectra
Financial support from the University Grant Commis-
sion, New Delhi, India, Department of Science and Tech-
nology, New Delhi, India and from the Council of
Scientific and Industrial Research, New Delhi, India is
greatly acknowledged. We are also thankful to the DST
for data collection on the CCD facility setup (Indian Insti-
tute of Science, Bangalore, India) under the IRHPA-DST
program.
The electronic and CD spectra were recorded in dichloro-
methane solution. The electronic and CD spectral data are
given in Section 2. The CD spectra of the Re(III) chelates
of amino acid Schiff bases below 550 nm are given in Fig. 4.
The low energy d–d absorption of the complexes is
found near 1000 and 1470 nm. A moderately intense
absorption band is observed at ꢀ360 nm and this can be
attributed to the p–p* transition originating mainly from
the azomethine chromophore. The bathochromic shift of
this band compared to the free amino acid Schiff bases
[31]is due to the increase of conjugation in the molecule
upon coordination. The intense absorption band at higher
energy, ꢀ270 nm, is presumably from the p–p* transition of
the benzene/naphthalene ring of the Schiff bases. The CD
spectra are more informative than the corresponding
absorption spectra. A careful comparison of the CD spec-
tra of the Re(III) amino acid Schiff base complexes reveal
that the sign and pattern of three bands or shoulder in
the UV region as well as the Cotton effect is same within
the entire family. This observation clearly indicates that
the coordinating ligands adopt a common conformation.
It has been recognized that a negative azomethine CD band
corresponds to the L-absolute configuration of the amino
acid residue. The present results are consistent with previ-
ously reported complexes using aldimines of a-amino acids
[6,32].
Appendix A. Supplementary material
CCDC 285571 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.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.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.poly.2007.03.036.
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