Syntheses, spectral and structural studies of Schiff base complexes of η5-pentamethylcyclopentadienyl rhodium and iridium

Article abstract of DOI:10.1016/j.poly.2005.04.034

The compounds [(η⁵-C₅Me₅)M(μ-Cl)Cl} ₂] where M = Rh (1) and Ir (2) react with N,N′-donor chelating ligands, viz., para-substituted N-(pyrid-2-ylmethylene)phenylamines (2-PP) in methanol-acetone to afford the pen

Full text of DOI:10.1016/j.poly.2005.04.034

Polyhedron 24 (2005) 1710–1716
www.elsevier.com/locate/poly
Syntheses, spectral and structural studies of Schiff base complexes of
g5-pentamethylcyclopentadienyl rhodium and iridium
a
Padavattan Govindaswamy , Yurij A. Mozharivskyj , Mohan Rao Kollipara
b
a,
*
a
Department of Chemistry, North-Eastern Hill University, Shillong 793 022, India
b
Ames Laboratory, Iowa State University of Science and Technology, Ames, Iowa, IA 50011, USA
Received 6 April 2005; accepted 27 April 2005
Available online 14 July 2005
Abstract
The compounds [(g5-C5Me5)M(l-Cl)Cl}2] where M = Rh (1) and Ir (2) react with N,N0-donor chelating ligands, viz., para-substi-
tuted N-(pyrid-2-ylmethylene)phenylamines (2-PP) in methanol–acetone to afford the pentamethylcyclopentadienyl rhodium (III)
and iridium (III) derivatives of the type [(g5-C5Me5)MCl(C5H4N-2-CH@N–C6H4-p-X]+ (M = Rh, 3; M = Ir, 4), where
C5Me5 = pentamethylcyclopentadienyl; X = H (3a, 4a), CH3 (3b, 4b), OCH3 (3c, 4c), Cl (3d, 4d), NO2 (3e, 4e), respectively. These
compounds were characterized by FT-IR and FT-NMR spectroscopy as well as analytical data. The molecular structures of the
tetrafluoroborate salt of [(g5-C5Me5)RhCl(C5H4N-2-CH@N–C6H4-p-Cl)]+ (3d) and hexafluorophosphate salt of [(g5-
C5Me5)IrCl(C5H4N-2-CH@N–C6H4-p-NO2)]+ (4e) were established by single crystal X-ray diffraction studies.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Pentamethylcyclopentadienyl; N,N0-chelating ligands; Pyridine-2-carboxaldehyde; Rhodium and iridium complexes
1. Introduction
We had reported previously the syntheses of arene
and cyclopentadienyl ruthenium complexes containing
N,N0-donor Schiff base ligands derived from pyridine-
2-carboxaldehyde [8]. We had also communicated
syntheses of cyclopentadienyl–ruthenium (II) [9], inde-
nyl–ruthenium (II) [10], cyclopentadienyl–osmium (II)
and arene–ruthenium (II) [11] complexes with a variety
of nitrogen-based ligands. However, the analogous
pentamethylcyclopentadienyl–rhodium (III) and iridium
(III) Schiff base complexes have not been explored as
much as the corresponding cyclopentadienyl and arene
ruthenium (II) complexes [12]. As a part of our contin-
uing study, we would like to report herein the syntheses
and characterization of new cationic pentamethylcyclo-
pentadienyl rhodium (III) and iridium (III) complexes
with N,N0-donor Schiff base ligands, viz., various
para-substituted N-(pyrid-2-ylmethylene)phenylamines
(2-PP). The molecular structures of representative
compounds are solved by X-ray crystallography and
are reported as well.
Half-sandwich complexes have proved to be
extremely useful in stoichiometric and catalytic asym-
metric synthesis and have therefore attracted much
study [1–5]. In addition, their four-coordinate pseudo-
tetrahedral geometry makes them particularly suitable
for investigation of the stereochemistry of reactions at
the metal centre [6]. Many studies of cyclopentadienyl
and arene ruthenium (II) complexes with bidentate li-
gands have shown that substitution reactions occur pre-
dominantly with retention of configuration at the metal
[7]. Until recently far fewer studies had been carried out
on pentamethylcyclopentadienyl rhodium (III) and irid-
ium (III) complexes with chelating N,N0-donor bases.
*
Corresponding author. Tel.: +91 364 272 2620; fax: +91 364 255
0076.
E-mail addresses: kmrao@nehu.ac.in, mrkollipara@yahoo.com
(M.R. Kollipara).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.04.034

P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
1711
2. Experimental
was concentrated to 2 ml, when addition of excess
hexane gave the orange-yellow complex, which was
separated and dried under vacuum.
2.1. Physical measurements
Method 2:
A
mixture of the complex [{(g5-
Elemental analyses were performed on a Perkin–
Elmer-2400 CHN/O analyzer. Infrared spectra were
recorded on a Perkin–Elmer Model 983 spectrophotom-
eter with the sample prepared as KBr pellets. Electronic
spectra were recorded on a Hitachi-300 spectrophotom-
C5Me5)Rh(l-Cl)Cl}2] (1) (100 mg, 0.110 mmol), the li-
gand 2-PP (0.242 mmol) and NH4PF6 (0.242 mmol)
was stirred in methanol (20 ml) for a few hours, where-
upon the orange-yellow product separated out. The or-
ange-yellow product was centrifuged, washed with
diethyl ether and dried under vacuum.
1
eter. The H NMR and 13C{1H} NMR spectra were
recorded in acetone-d6 and CDCl3 solvents with TMS
as internal standard and recorded on a Bruker ACF-
300 (300 MHz) spectrometer, the coupling constants J
being given in Hz. Electro-chemical studies (in acetoni-
trile) were performed with a PARC electrochemistry
system as described elsewhere [13]. In cyclic volta-
metry (CV) the following parameters were used, namely,
the scan rate 100 mV sꢀ1 and the formal potential
E0 = 0.5(Epa + Epc), where Epa and Epc are the anodic
and cathodic peak potentials, respectively; and DEp is
the peak-to-peak separation.
3a. Yield: 152 mg, (78%). IR (KBr pellets, cmꢀ1):
m(C@N) 1593 (s), m(P–F) 844 (s).
1H NMR (CDCl3 and acetone-d6, d): 1.56 (s, 15H,
C5Me5), 7.32–7.56 (m, 3H, Ph), 7.64 (d, 2H, JH–H
=
6.53 Hz, Ph), 8.03–8.55 (m, 3H, py), 8.97 (d, 1H,
JH–H = 4.86 Hz, py), 9.24 (s, 1H, imine C–H).
13C NMR (acetone-d6, d): 7.60 (CH3, C5Me5), 97.53
(C, C5Me5), 122.44, 129.43, 129.79, 131.36 (Ph),
140.34, 148.80, 152.66, 154.04, 155.32 (py), 166.97
(C–H).
Elemental Anal. Calc. for C22H25RhN2ClPF6: C,
43.98; H, 4.19; N, 4.66. Found: C, 43.65; H, 4.02; N,
4.71%.
2.2. Materials and methods
UV–Vis (acetone): kmax = 359.6 nm.
All chemicals used were of reagent grade. All reac-
tions were carried out in distilled and dried solvents.
Rhodium chloride and iridium chloride were purchased
from Arora Matthey Ltd., and used as received. Pyri-
dine 2-carboxaldehyde (Fluka) was used as received.
All liquid aromatic amines were reagent grade and were
distilled prior to use, while solid aromatic amines were
used as such. The ligands C5H4N-2-CH@NC6H4-p-X
(where X = H, CH3, OCH3, Cl, NO2) [14], and the pre-
cursor complexes [{(g5-C5Me5)M(l-Cl)Cl}2] (where
M = Rh (1) and Ir (2)) [15] were prepared by known
literature methods. The purification of acetonitrile and
the preparation of tetrabutylammonium perchlorate
3b. Yield: 159 mg, (80%). IR (KBr pellets, cmꢀ1):
m(C@N) 1596 (m), m(P–F) 844 (s).
1H NMR (CDCl3 and acetone-d6, d): 1.52 (s, 15H,
C5Me5), 2.44 (s, 3H, CH3), 7.35 (d, 2H, JH–H = 2.31
Hz), 7.67 (d, 2H, JH–H = 8.22 Hz), 7.83–7.87 (dd, 1H,
JH–H = 3.96 Hz), 8.20 (d, 2H, JH–H = 4.22 Hz), 8.57 (d,
1H, JH–H = 2.81 Hz), 8.89 (d, 2H, JH–H = 5.36 Hz).
13C NMR (acetone-d6, d): 7.63 (CH3, C5Me5), 18.34
(CH3), 98.37 (C, C5Me5), 112.24, 115.62, 116.96,
122.32 (Ph), 123.37, 126.21, 128.56, 131.34, 145.36
(py), 158.56 (C–H).
Elemental Anal. Calc. for C23H27RhN2ClPF6: C,
44.93; H, 4.42; N, 4.55. Found: C, 45.56; H, 4.38; N,
4.56%.
½Bun Nꢁ½ClO4ꢁ, used as solvent and supporting electro-
4
lyte, respectively, in the electrochemical experiments,
were performed following a literature method [16].
UV–Vis (acetone): kmax = 344.6 nm.
3c. Yield: 146 mg, (72%). IR (KBr pellets, cmꢀ1):
m(C@N) 1596 (s), m(P–F) 840 (s).
2.3. Synthesis of [(g5-C5Me5)RhCl(C5H5N-2-CH@
NC6H4-p-X)]+ (3a-e), (X = H (3a), CH3 (3b), OCH3
(3c), Cl (3d), NO2 (3e))
1H NMR (acetone-d6, d): 1.55 (s, 15H, C5Me5), 3.89
(s, 3H, CH3), 7.17 (d, 2H, JH–H = 4.55 Hz), 7.79 (d,
2H, JH–H = 4.48 Hz), 7.99 (td, 2H, JH–H = 2.14 Hz,
5.22 Hz), 8.37 (d, 1H, JH–H = 7.36 Hz), 8.66 (d, 1H,
JH–H = 2.79 Hz), 9.15 (d, 1H, JH–H = 5.29 Hz).
13C NMR (acetone-d6, d): 8.80 (CH3, C5Me5), 56.08
(CH3), 96.35 (C, C5Me5), 115.35, 118.38, 122.46,
124.96 (Ph), 126.24, 127.37, 130.44, 141.27, 153.56
(py), 166.36 (C–H).
The following general procedure was used for the
preparation of these five complexes:
Method 1: A mixture of the starting complex [{(g5-
C5Me5)Rh(l-Cl)Cl}2] (1) (100 mg, 0.110 mmol), the
appropriate Schiff base ligand 2-PP (0.242 mmol)
and NH4PF6 (0.242 mmol) was stirred in methanol–
acetone (20/10 v/v mixture) for 6 h until the color
of the solution changed from red to orange. The sol-
vent was removed in vacuo, the residue dissolved in
dichloromethane (5 ml), and the solution filtered to
remove ammonium chloride. The orange solution
Elemental Anal. Calc. for C23H27RhN2OClPF6: C,
43.79; H, 4.31; N, 4.44. Found: C, 43.36; H, 4.36; N,
4.53%.
UV–Vis (acetone): kmax = 363.6 nm.
3d. Yield: 161 mg, (79%). IR (KBr pellets, cmꢀ1):
m(C@N) 1600 (s), m(B–F) 1056 (s).

1712
P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
1H NMR (CDCl3, d): 1.82 (s, 15H, C5Me5), 7.52 (d,
131.26 (Ph), 141.50, 148.23, 152.54, 153.20, 155.21
(py), 166.76 (imine C–H).
2H, JH–H = 8.68 Hz, Ph), 7.89 (d, 2H, JH–H = 8.64 Hz,
Ph), 8.14 (td, 1H, JH–H = 9.04 Hz, 7.68 = Hz, py), 8.24
(d, 2H, JH–H = 7.28 Hz, py), 8.58 (d, 1H, JH–H = 2.76
Hz, imine C–H), 8.74 (d, 1H, JH–H = 5.76 Hz, py).
13C NMR (CDCl3, d): 8.57 (CH3, C5Me5), 90.17 (C,
C5Me5), 121.32, 126.26, 130.38, 132.47 (Ph), 135.58,
139.24, 145.31, 149.27, 155.51 (py), 164.73 (C–H).
Elemental Anal. Calc. for C22H24RhN2Cl2PF6: C,
41.66; H, 3.81; N, 4.41. Found: C, 41.34; H, 3.47; N,
4.26%.
Elemental Anal. Calc. for C23H27IrN2ClPF6: C, 39.23;
H, 3.86; N, 3.97. Found: C, 40.32; H, 3.83; N, 4.04%.
UV–Vis (acetone): kmax = 385.75 nm.
4c. Yield: 133 mg, (73%). IR (KBr pellets, cmꢀ1):
m(C@N) 1606 (s), m(P–F) 844 (s).
1H NMR (acetone-d6, d): 1.57 (s, 15H, C5Me5), 3.76
(s, 3H, CH3), 7.23 (d, 2H, JH–H = 6.23 Hz, Ph), 7.62
(d, 2H, JH–H = 6.23 Hz, Ph), 7.86–8.34 (m, 3H, py),
8.58 (d, 1H, JH–H = 3.06 Hz, a proton of py), 9.23
(1H, JH–H = 4.66 Hz, imine C–H).
UV–Vis (acetone): kmax = 372.5 nm.
3e. Yield 153 mg, (73%). IR (KBr pellets, cmꢀ1):
m(C@N) 1593 (s), m(NO , asymmetric) 1527 (s), m(NO , symmetric)
13C NMR (acetone-d6, d): 7.57 (CH3, C5Me5), 55.15
(CH3), 90.12 (C, C5Me5), 114.35, 114.60, 124.14,
129.43 (Ph), 130.10, 140.45, 148.32, 149.69, 152.15
(py), 166.64 (imine C–H).
Elemental Anal. Calc. for C23H27IrN2OClPF6: C,
38.36; H, 3.77; N, 3.89. Found: C, 38.06; H, 3.46; N,
3.95%.
2
2
1348 (s), m(P–F) 837 (s).
1H NMR (acetone-d6, d): 1.47 (s, 15H, C5Me5), 8.06
(m, 3H, py), 8.43 (d, 2H, JH–H = 8.63 Hz, Ph), 8.54 (d,
2H, JH–H = 8.91 Hz), 9.07 (d, 1H, JH–H = 2.80 Hz,
CH), 9.24 (d, 1H, JH–H = 5.36 Hz, a proton of py).
13C NMR (acetone-d6, d): 8.84 (CH3, C5Me5), 90.34
(C, C5Me5), 98.46, 106.54, 115.68, 124.55 (Ph), 125.92,
131.16, 131.62, 141.29, 153.57 (py), 169.84 (C–H).
Elemental Anal. Calc. for C22H24RhN3O2ClPF6: C,
40.92; H, 3.74; N, 6.50. Found: C, 39.48; H, 3.88; N,
6.08%.
UV–Vis (acetone): kmax = 368.23 nm.
4d. Yield: 128 mg, (71%). IR (KBr pellets, cmꢀ1):
m(C@N) 1633 (s), m(P–F) 844 (s).
1H NMR (acetone-d6, d): 1.56 (s, 15H, C5Me5), 7.73
(d, 2H, JH–H = 8.55 Hz, Ph), 7.65 (d, 2H, JH–H = 8.81
Hz, Ph), 8.01–8.06 (m, 3H, py), 8.95 (d, 1H, JH–H
=
UV–Vis (acetone): kmax = 369.4 nm.
2.74 Hz, imine C–H), 9.21 (d, 1H, JH–H = 5.39 Hz, a
proton-py).
2.4. Synthesis of [(g5-C5Me5)IrCl(C5H5N-2-CH@
N–C6H4-p-X)]+ (4a-e), (X = H (4a), CH3 (4b), OCH3
(4c), Cl (4d), NO2 (4e)
13C NMR (acetone-d6, d): 7.57 (CH3, C5Me5), 90.28
(C, C5Me5), 124.35, 129.56, 130.12, 130.58 (Ph),
140.56, 142.63, 147.34, 149.52, 152.28 (py), 169.08
(imine C–H).
These complexes were prepared by the same method
given in Section 2.3 using [{(g5-C5Me5)Ir(l-Cl)Cl}2]
(2) instead of complex [{(g5-C5Me5)Rh(l-Cl)Cl}2] (1).
4a. Yield: 136 mg, (79%). IR (KBr pellets, cmꢀ1):
m(C@N) 1600 (s), m(P–F) 844 (s).
Elemental Anal. Calc. for C22H24IrN2Cl2PF6: C,
36.52; H, 3.34; N, 3.87. Found: C, 36.61; H, 3.08; N,
3.66%.
UV–Vis (acetone): kmax = 385.75 nm.
4e. Yield: 132 mg, (72%). IR (KBr pellets, cmꢀ1):
m(C@N) 1593 (s), m(NO , asymmetric) 1527 (s), m(NO , symmetric)
1H NMR (acetone-d6, d): 1.55 (s, 15H, C5Me5), 7.55–
7.61 (m, 3H, Ph), 7.80 (d, 2H, JH–H = 7.95 Hz, Ph),
7.99–8.40 (m, 3H, py), 9.19 (d, 1H, JH–H = 5.05 Hz, a
proton of py), 9.35 (s, 1H, CH).
2
2
1348 (s), m(P–F) 844 (s).
1H NMR (acetone-d6, d): 1.59 (s, 15H, C5Me5), 8.05–
8.10 (m, 2H, py), 8.41 (d, 2H, JH–H = 1.41 Hz, Ph), 8.54–
8.61 (m, 1H, py), 9.26 (d, 1H, JH–H = 5.5 Hz, a proton-
py), 9.54 (s, 1H, imine C–H).
13C NMR (acetone-d6, d): 8.51 (CH3, C5Me5), 91.22
(C, C5Me5), 123.57, 130.47, 130.90, 131.42 (Ph), 136.32,
138.47, 141.51,153.24, 157.83 (py), 169.43 (imine C–H).
Elemental Anal. Calc. for C22H25IrN2ClPF6: C,
38.29; H, 3.65; N, 4.06. Found: C, 38.47; H, 3.32; N,
4.12%.
13C NMR (acetone-d6, d): 8.39 (CH3, C5Me5), 91.25
(C, C5Me5), 124.77, 125.91, 131.55, 131.77 (Ph),
141.45, 149.36, 153.20, 154.26, 156.54 (py), 171.26
(imine C–H).
UV–Vis (acetone): kmax = 385.7 nm.
Elemental Anal. Calc. for C22H24IrN3O2ClPF6: C,
35.94; H, 3.29; N, 5.71. Found: C, 35.76; H, 3.43; N,
5.92%.
4b. Yield: 112 mg, (63%). IR (KBr pellets, cmꢀ1):
m(C@N) 1633 (s), m(P–F) 844 (s).
1H NMR (acetone-d6, d): 1.55 (s, 15H, C5Me5), 2.47 (s,
3H, CH3), 7.48 (d, 2H, JH–H = 8.14 Hz, Ph), 7.70 (d, 2H,
JH–H = 8.31 Hz, Ph), 8.00–8.03 (m, 1H, py), 8.35–8.40 (m,
2H, py), 8.51 (d 1H, JH–H = 7.39 Hz, imine C–H), 9.18 (d,
1H, JH–H = 5.39 Hz, a proton of py).
UV–Vis (acetone): kmax = 387.13 nm.
3. Crystallographic investigations
13C NMR (acetone-d6, d): 8.54 (CH3, C5Me5), 21.16
(CH3), 91.17 (C, C5Me5), 123.49, 128.36, 130.71,
Crystals suitable for X-ray diffraction study for com-
pound 3d[BF4] were grown by slow diffusion of hexane

P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
1713
into an acetonitrile solution of 3d[BF4], and for com-
4. Results and discussion
pound 4e[PF6] by slow diffusion of hexane into a dichlo-
romethane solution of 4e[PF6]. The orange crystals of
compounds 3d[BF4] and 4e[PF6] were mounted on a Bru-
ker Apex CCD diffractometer in a full reciprocal sphere
equipped with a CCD detector, and were used for data
collection. X-ray intensity data were collected with
graphite monochromated Mo Ka radiation at 293(2)
K, with 0.3ꢁ x scan mode and 10 s per frame. The inten-
sity data were corrected for Lorentz and polarization ef-
fects. Absorption corrections were done using the SAINT
program [17]. A summary of the crystal data, data collec-
tion parameters and convergence results is compiled in
Table 1. An empirical absorption correction was made
by modelling a transmission surface by spherical har-
monics employing equivalent reflections with I > 2r(I)
(program SADABS) [18]. The structures were solved by di-
rect methods [19]. All the non-hydrogen atoms were re-
fined anisotropically using the full-matrix, least-squares
technique on F2 using the SHELXL-97 software [20]. All
the hydrogen atoms were found from difference Fourier
synthesis after four cycles of an isotropic refinement and
as per the ‘‘riding’’ model. Figs. 3 and 4 display the OR-
TEP [21] representations of the molecules with 50%
probability thermal ellipsoids. Refinement converged at
final R1 values of 0.0366 and 0.0313 (for observed data
F) for compounds 3d[BF4] and 4e[PF6], respectively.
The dinuclear complexes [{(g5-C5Me5)M(l-Cl)Cl}2]
{M = Rh (1) and Ir (2)} undergo a bridge cleavage reac-
tion with N,N0-donor Schiff base ligands (2-PP) in meth-
anol to yield cationic complexes 3 and 4 (Scheme 1),
respectively. These complexes are isolated as the hexa-
fluorophosphate salts. The orange-yellow complexes
are air-stable microcrystalline solids, partially soluble
in chloroform, methanol and benzene, soluble in dichlo-
romethane, acetone and insoluble in hexane, petroleum
ether and diethyl ether.
The infrared spectra of these complexes show a
strong band in the range 1593–1633 cmꢀ1 due to the
m
C@N group of the Schiff base ligand, and a strong band
in the range 837–844 cmꢀ1 due to the mP–F mode of the
PF6 group. The complexes 3e and 4e exhibit the char-
ꢀ
acteristic bands for mNO (asymmetric stretch at 1527
2
cmꢀ1 and symmetric stretch at 1346 cmꢀ1). The 1H
NMR spectral data of these complexes along with their
assignments are recorded in the experimental section. In
1
the H NMR spectra of these complexes, the methyl
group of the pentamethylcyclopentadienyl protons ap-
pears as a singlet in the range 1.47–1.82 ppm, which
are shifted downfield as compared to that in the precur-
sor complexes. A downfield shift in the position of the
Cp* protons might result from the change in electron
density on the metal center due to chelation of the Schiff
base ligands through their nitrogen atoms. The multiplet
observed in the range 7.32–8.61 ppm is due to the phenyl
protons of the amine group and the N-heterocyclic ring
of the ligands (2-PP), these resonance values being con-
sistent with the reported values [8]. The 13C {1H} NMR
spectra of complexes 3 and 4 exhibit resonances for the
methyl carbons of the pentamethylcyclopentadienyl
group at around 8.00 ppm and for the ring carbons at
around 90.12–98.37 ppm. The resonance observed in
the range 158.56–171.26 ppm is assigned to the imine
group of the ligand (2-PP). The spectra also show reso-
nances in the range 98.46–157.83 ppm for the aromatic
carbon of the phenyl group and pyridyl group carbons
(see Fig. 1).
Table 1
Crystal data and structure refinement parameters for complexes
3d[BF4] and 4e[PF6]
Formula
Mr
C22H24BCl2F4N2Rh C22H24ClF6IrN3O2P
735.06
293(2)
577.05
293(2)
T (K)
˚
Wavelength (A)
Crystal system
Space group
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/n
˚
a (A)
10.1944(6)
11.6110(6)
20.4223(11)
96.334(10)
2402.6(2)
4
10.4116(6)
11.5786(6)
21.7809(12)
98.097(10)
2599.5(2)
4
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Crystal size (mm3)
Dcalc (g cmꢀ3
F(000)
0.45 · 0.27 · 0.25
1.595
1160
0.15 · 0.28 · 0.35
1.878
1424
)
h (ꢁ)
Reflections collected
2.01–28.30
20524
5645 [0.0180]
1.89–28.30
22158
6119 [0.0285]
PF
6
Cl
Cl
Cl
Cl
2-PP, NH PF
4
6
M
M
M
Independent
reflections [Rint
acetone-MeOH
]
N
Cl
N
Absorption
coefficient (mmꢀ1
0.977
5.368
)
X
Completeness to h (ꢁ)
Data/restraints/
28.30
5645/0/255
28.30
6119/0/297
M = Rh(1) Ir(2)
M = Rh; X = H(3a)
2-PP = N-(pyrid-2-ylmethelene)phenylamine
M = Ir
X = H(4a)
parameters
Goodness-of-fit on F2
R1 (I > 2r(I)), wR2
R1, R2 (all data)
Largest difference in
1.046
0.0366, 0.1062
0.0410, 0.1100
1.059
0.0313, 0.0812
0.0417, 0.0863
CH (4b)
CH (3b)
3
3
OCH (4c)
3
OCH (3c)
3
Cl(3d)
NO (3e)
2
Cl(4d)
NO (4e)
2
+0.804 and ꢀ0.617 +2.159 and ꢀ0.616
ꢀ3
˚
peak and hole (e A
)
Scheme 1.

1714
P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
Fig. 1. 1H NMR spectrum of complex [(g5-C5Me5)RhCl(C5H4N-2-CH@N–C6H4-p-OMe)]+ (3c) in acetone-d6.
½ðg5 ꢀ C5Me5ÞRhClð2 ꢀ PPÞꢁþ þ eꢀ
¡ ½ðg5 ꢀ C5Me5ÞRhClð2 ꢀ PPÞꢁ
The electronic absorption spectra for rhodium (III)
and iridium (III) Schiff base complexes typically con-
tains p–p* and n–p* bands in the ultraviolet region
The interaction of the filled dp (t2g) orbitals on rhodium
(III) and iridium (III) with low lying p* orbitals on the
Schiff base imine (C@N) ligands should provide a me-
tal-to-ligand charge transfer (MLCT) transition (t2g-
p*) in the electronic spectra of these complexes [22–24]
with the transition energy of these bands varying with
the nature of the ligands acting as p-acceptors. The elec-
tronic spectra of these complexes in acetone exhibit
bands in the range of 344–387 nm. This low-energy
absorption is assigned to a Rh dp (or Ir dp) to ligand
p* metal-to-ligand charge transfer (MLCT) transition
(see Fig. 2).
ð1Þ
In general, ligand effects appear in the Ru(IV/III) and
Ru(III/II) redox potentials. Ligand effects on the Ru(III/
II) couple are well known, where Ru(III/II) potentials
are influenced along with the ligand. Ru(II) is stabilized
by the contribution of dp–p* (L) back bonding in the
presence of the ligands. The replacement of a Schiff-base
ligand by the strongly electron-donating pentamethylcy-
clopentadienyl group destabilizes the lower rhodium
oxidation states, making reduction more difficult. In-
deed, in our observation the reduction potential for
the +3/+1 couple of [Cp*RhCl(2-PP)]+ falling may arise
from the intermediate [Rh(Cp*)2]+ and [Rh(2-PP)3]3+
.
Electron transfer properties of the representative
compound 3a[PF6] are studied in acetonitrile solvent
by cyclic voltametry. The first reduction wave of this
rhodium (III) complex occurs at around ꢀ0.544 V and
the corresponding oxidation wave at ꢀ0.113 V; it shows
a quasi-reversible plot (DE = ꢀ431 mV). This is assigned
to the Rh(III)/Rh(II) couple [Eq. (1)]. The second reduc-
tion wave occurs at ꢀ1.199 V and the corresponding oxi-
dation wave ꢀ0.824 V; this also shows a quasi-reversible
plot (DE = ꢀ375 mV), which is a two-electron transfer
Rh(III)/Rh(I) couple. We have also observed here the
Rh(III)/Rh(II) and Rh(II)/Rh(I) couple.
5. Molecular structures
Single crystal X-ray structure determinations were
carried out for compounds 3d and 4e for confirmation
of their formulae and structure. The rhodium atom
compound 3d and the iridium atom compound 4e are
coordinated to two nitrogen atoms of the Schiff base
(2-PP) ligand, one chloride ion and the pentamethyl-
cyclopentadienyl molecule in a g5-fashion leading to
the usual Ôthree-legged piano stoolÕ structures. The
geometry around the metal atom can be regarded as dis-
torted octahedral if the g5-pentamethylcyclopentadienyl
moieties are assumed to occupy three facial-coordinated
positions. A summary of the single-crystal X-ray struc-
ture analyses is shown in Table 1. Selected bond lengths
and bond angles are given in Tables 2 and 3. The OR-
TEP drawings of compounds 3d and 4e are shown in
Figs. 3 and 4, respectively.
The tetrafluoroborate salt of complex [(g5-C5Me5)-
RhCl(C5H4N-2-CH@N–C6H4-p-Cl)]+3d and hexaflu-
orophosphate salt of complex [(g5-C5Me5)IrCl(C5H4N-
2-CH@N–C6H4-p-NO2)]+ 4e crystallize in the monoclinic
space group P21/n (Figs. 3 and 4). The two metal-
Fig. 2. Cyclic voltammogram of complex [(g5-C5Me5)RhCl(C5H4N-2-
CH@N–C6H5)]+ (3a) in acetonitrile using a Pt-disk milli electrode at
298 K, SCE reference and TBAP (0.1 M) supporting electrolyte.
˚
centroid distances are indistinguishable (1.791 A). The
bond distances between the rhodium and iridium metal

P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
1715
Table 2
Selected bond lengths (A) and bond angles (ꢁ) for [(g5-
˚
C5Me5)RhCl(C5H4N–2CH@N–C6H4-p-Cl)]+ (3d)
˚
Bond lengths (A)
Rh–N
2.123(2)
1.422(2)
1.3900
N–C(21)
N(32)–C(31)
Rh–N(32)
N–C
2.0949(11)
1.271(4)
1.791
Rh–Ca
Rh–Cl(1)
N(32)–C(33)
2.3756(7)
1.3900
Bond angles (ꢁ)
N(32)–Rh–N
N(32)–Rh–Cl(1)
N–Rh–Cl(1)
76.17(7)
85.13(4)
86.76(6)
Fig. 4. ORTEP representation of the complex 4e with 50% probability
ꢀ
a
Rhodium to centroid of Cp*.
thermal ellipsoids. Hydrogens and PF6 are omitted for clarity.
Table 3
Selected bond lengths (A) and bond angles (ꢁ) for [(g5-
˚
˚
A and pointing to p-electron delocalization in the ring.
C5Me5)IrCl(C5H4N-2-CH@N–C6H4-p-NO2)]+ (4e)
Furthermore, the five membered ring is planar as evi-
dent in the nearly equal bond distances between metal
atom and the ring carbons. The Rh–Cl and Ir–Cl bond
˚
Bond lengths (A)
Ir–N(1)
2.099(4)
1.3900
˚
N(21)–C(26)
N(1)–C(1)
Ir–Ca
distances are 2.3756(7) and 2.3726(11) A, respectively,
1.285(5)
1.792
which are closely similar to another related two-coordi-
nated chelating polypyridyl rhodium complex [Cp*Rh-
Cl(Phterpy)]+ (2.3984(1)) [25].
Ir–Cl
2.3726(11)
1.3900
N(21)–C(22)
N(2)–O(1)
Ir–N(21)
N(1)–C(31)
N(2)–O(2)
1.198(8)
2.0862(18)
1.408(4)
1.171(8)
6. Concluding remarks
Bond angles (ꢁ)
N(21)–Ir–Cl
N(1)–Ir–Cl
We have observed interesting phenomena in the case
of the pentamethylcyclopentadienyl compounds of 1
and 2, which distinguish them from the analogous
pentamethylcyclopentadienyl or indenyl ruthenium
(II) systems, even though these are all isoelectronic.
Unlike in the case of the compounds 1 and 2, the reac-
tions of Schiff bases with the [(g5-Cp*)Ru(PPh3)2Cl]
and [(g5-ind.)Ru(PPh3)2Cl] systems yielded complexes
without any organic group. In order to obtain similar
complexes for these cases, one has to start from the
acetonitrile complexes.
84.45(8)
85.17(9)
75.98(11)
N(21)–Ir–N(1)
a
Iridium to centroid of Cp*.
Acknowledgements
We thank Professor R.H. Duncan Lyngdoh for his
help in preparing the manuscript and Dr. A. Lemtur
for helpful discussions in the electrochemistry part.
Appendix A. Supplementary data
Fig. 3. ORTEP representation of the complex 3d with 50% probability
ꢀ
thermal ellipsoids. Hydrogens and BF4 are omitted for clarity.
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication No. CCDC
265789 for complex 3d[BF4] and CCDC 265788 for
atoms and the chelated ligand nitrogen atoms are simi-
˚
lar to each other (ꢂ2.1 A). There is no significant differ-
ence
in
the
C–C
bond
lengths
in
pentamethylcyclopentadienyl ring, all being about 1.42
the

1716
P. Govindaswamy et al. / Polyhedron 24 (2005) 1710–1716
[13] S. Pal, T.K. Misra, C. Sinha, A.M.Z. Slawin, J.D. Woolins,
Polyhedron 19 (2000) 1925.
complex 4e[PF6]. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: Int. code +(44) 1223/
336033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk). Supplementary data associated
with this article can be found, in the online version at
doi:10.1016/j.poly.2005.04.034.
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