Osazone anion radical complex of rhodium(III)

Article abstract of DOI:10.1021/ic101945d

One electron paramagnetic parent osazone complex of rhodium of type trans-Rh(L^NHPhH₂)(PPh₃)₂Cl ₂ (1), defined as an osazone anion radical complex of rhodium(III), trans-Rh^III(L^NHPh

Full text of DOI:10.1021/ic101945d

Inorg. Chem. 2011, 50, 1331–1338 1331
DOI: 10.1021/ic101945d
Osazone Anion Radical Complex of Rhodium(III)
Sarat Chandra Patra,† Manas Kumar Biswas,† Amarendra Nath Maity,‡ and Prasanta Ghosh*,†
†Department of Chemistry, R. K. Mission Residential College, Narendrapur, Kolkata-103, W.B., India, and
‡Department of Physics, National Dong Hwa University, Shou-Feng, Hualien 97401, Taiwan
Received September 23, 2010
One electron paramagnetic parent osazone complex of rhodium of type trans-Rh(LNHPhH2)(PPh3)2Cl2 (1), defined as
an osazone anion radical complex of rhodium(III), trans-RhIII(LNHPhH2•-)(PPh3)2Cl2, 1(t-RhL•), with a minor con-
tribution (∼2%) of rhodium(II) electromer, trans-RhII(LNHPhH2)(PPh3)2Cl2, 1(t-Rh•L), and their nonradical congener,
trans-[RhIII(LNHPhH2)(PPh3)2Cl2]I3 ([t-1]þI3-), have been isolated and are substantiated by spectra, bond para-
meters, and DFT calculations on equivalent soft complexes [Rh(LNHPhH2)(PMe3)2Cl2] (3) and [Rh(LNHPhH2)-
(PMe3)2Cl2]þ (3þ). 1 is not stable in solution and decomposes to [t-1]þ and a new rhodium(I) osazone complex,
[RhI(LNHPhH2)(PPh3)Cl] (2). 1 absorbs strongly at 351 nm due to MLCT and LLCT, while [t-1]þ and 2 absorb
moderately in the range of 300-450 nm, respectively, due to LMCT and MLCT elucidated by TD-DFT calculations on
3
(t-RhL•), [t-3]þ, and RhI(LNHPhH2)(PMe3)Cl (4). EPR spectra of solids at 295 and 77 K, and dichloromethane-
toluene frozen glass at 77 K of 1 are similar with g = 1.991, while g = 2.002 for the solid at 25 K. The EPR signal of 1 in
dichloromethane solution is weaker (g = 1.992). In cyclic voltammetry, 1 displays two irreversible one electron transfer
waves at þ0.13 and -1.22 V, with respect to Fcþ/Fc coupling, due to oxidation of 1(t-RhL•) to [t-1]þ at the anode and
reduction of rhodium(III) to rhodium(II), i.e., [t-1]þ to electromeric 1(t-Rh•L) at the cathode.
Introduction
trans-Rh(LNHPhH2)(PPh3)2Cl2 (1), its oxidized diamagnetic
analogue trans-[RhIII(LNHPhH2)(PPh3)2Cl2]I3 ([t-1]þI3-),
and a rhodium(I) osazone complex of type RhI(LNHPhH2)-
(PPh3)Cl (2) have been reported. A rhodium osazone com-
plex with molecular composition, Rh(LNHPhH2)(PPh3)2Cl2,
can have the following electromers (Scheme 1).
In this article, the existence of an osazone anion radical
complex, 1(t-RhL•) with 2% spin density, is localized on the
rhodium metal ion affording 1(t-Rh•L) in solids and solutions,
and frozen glasses of 1 stabilized by two trans bulky PPh3
ligands have been predicted.
Osazone-assimilating nitrogen-enriched conjugated link-
age (-HN-N=CH-CH=N-NH-) is a potential π-acidic
NN-chelating agent that can augment stabilization of fasci-
nating and reactive electronic states in coordination com-
plexes. Electronic and molecular structures of the first
transition metal complex of parent osazone (LNHPhH2) with
a ruthenium(II) ion along with triphenyl phosphine (PPh3)
and chloride (Cl) as coligands as in trans-RuII(LNHPhH2)
(PPh3)2Cl2 have recently been reported.1 Bond parameters of
the diimine fragment in conjunction with the density func-
tional theory (DFT) calculations have authenticated the
delocalization of significant electron density to the coordi-
nated diimine fragment of the osazone in this ruthenium(II)
complex. Ascomparedto ruthenium, rhodium isone electron
rich and has the possibility to localize the unpaired electron
on rhodium or on the coordinated diimine fragment gen-
erating electro-isomers called electromers.2 The electronic
structure of the rhodium osazone complex incorporating
PPh3 and Cl ligands has been a subject of investigation
here. In this article, the rhodium osazone complex of type
Experimental Section
Hard and Soft Materials. All the physiochemical data have
been collected on 1, [t-1]þI3-, and 2, which are defined as hard
materials. However, DFT or TD-DFT calculations at the
B3LYP level of theory to analyze the spin density distributions
(Figure 6), bond parameters (Chart 1), and transition probabil-
ities (Table 5) have been performed on the model cis and trans
isomers of Rh(LNHPhH2)(PMe3)2Cl2 (3), [Rh(LNHPhH2)(PMe3)2
Cl2]þ (3þ), and Rh(LNHPhH2)(PMe3)Cl (4), which are defined as
soft materials.
Physical Measurements. Reagents or analytical grade materi-
als were obtained from commercial suppliers and used without
further purification. Spectroscopic grade solvents were used for
spectroscopic and electrochemical measurements. The C, H,
and N content of the compounds were obtained from a Perkin-
Elmer 2400 series II elemental analyzer. Infrared spectra of the
samples were measured from 4000 to 400 cm-1 with the KBr
*To whom correspondence should be addressed. E-mail: ghoshp_chem@
yahoo.co.in. Phone: þ91-33-2428-7017. Fax: þ91-33-2477-3597.
(1) Roy, A. S.; Tuononen, H. M.; Rath, S. P.; Ghosh, P. Inorg. Chem.
2007, 46, 5942.
€
(2) Puschmann, F. F.; Harmer, J.; Stein, D.; Ruegger, H.; Bruin, B. D.;
€
Grutzmacher, H. Angew. Chem., Int. Ed. 2010, 49, 385–389 and relevant
references there in.
r
2011 American Chemical Society
Published on Web 01/24/2011
pubs.acs.org/IC

1332 Inorganic Chemistry, Vol. 50, No. 4, 2011
Patra et al.
Scheme 1. Possible Electromers of Rh(LNHPhH2)(PPh3)2Cl2
Table 1. Crystallographic Data for [t-1]þI3
-
chemical formula
fw
space group
C50 H44 Cl2 I3 N4 P2 Rh
1317.34
P2/c
12.0948(8)
13.6017(7)
15.2096(8)
100.467(2)
2460.5(2)
2
T (K)
296(2)
1.778
30578/52
4330/3149
281/0
0.71073/2.441
0.0663/1.137
0.1743
F calcd (g cm-3
)
reflns collected (2Θmax
)
˚
a (A)
unique reflns [I > 2σ (Ι)]
no. of params/restr.
˚
b (A)
-1
˚
c (A)
˚
λ (A/μ) (Mo, KR) (cm
R1a (GOF)b
)
β (deg)
V (A)
Z
wR2c [I > 2σ (Ι)]
residual density (eA
˚
-3
˚
)
2.108
a Observation criterion: R1 = Σ||Fo| - |Fc||/Σ|Fo|. b Observation criterion: GOF = {Σ[w(Fo2 - Fc2)2]/(n - p)}1/2
.
c Observation criterion: wR2 =
{Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2, where w = 1/[σ2(Fo2) þ (aP)2 þ bP], P = (Fo þ 2Fc2)/3.
2
pellet at room temperature on a Perkin-Elmer Spectrum RX 1,
FT-IR spectrophotometer. 1H NMR spectra in DMSO-d6 and
CDCl3 solvents were carried out on a Bruker DPX-300 MHz
spectrometer. ESI mass spectra were recorded on a micro mass
Q-TOF mass spectrometer. Electronic absorption spectra in
solution at 298 K were carried out on a Perkin-Elmer Lambda
25 spectrophotometer in the range of 1100-200 nm. The
X-band electron paramagnetic resonance (EPR) spectra were
measured on a Bruker EMX spectrometer, where the microwave
frequency was measured with a Hewlett-Packard 5246 L elec-
tronic counter. Magnetic susceptibility at 298 K has been
measured on a Sherwood magnetic susceptibility balance. The
electro-analytical instrument, BASi Epsilon-EC, has been used
for cyclic voltammetric experiments and spectroelectrochemis-
try measurements.
(ppm) = 8.33 (s, 1H, N=CH), 7.77 (m, 9H, N=CH, Ph), 7.58
(m, 12H, Ph, and N=CH), 7.37 (m, 14H, Ph), 7.13 (t, 3H, Ph),
7.10 (d, 1H, Ph), 6.05 (d, 2H, Ph). IR (KBr): ν = 3289 (m),
1600(m), 1492(m), 1437(s), 1190(s), 1120 (vs), 722(vs), 694(vs),
and 542(vs) cm-1
.
RhI(LNHPhH2)(PPh3)Cl (2). Sample 1 (200 mg, 0.2 mmol) was
stirred in dichloromethane for 10 h at 295 K. The solution was
then filtered off ,and the filtrate was evaporated under vacuum.
A red solid was obtained, which was thoroughly washed with
boiling hexane to remove PPh3 as one of the products. The
desired product, 2, was isolated after purification of the crude
mass on a basic alumina column and using CHCl3 as the eluent.
Yield: 50 mg (∼36% with respect to 1). ESI (positive ion)-MS in
CH3CN; m/z: 637.20 (2, calcd 638.95). Anal. Calcd for
C32H29ClN4PRh: C, 60.15; H, 4.57; N, 8.77. Found: C, 60.95;
H, 4.25; N, 8.32. 1H NMR (CDCl3, 300 MH, 295 K) δ (ppm) =
8.28 (s, 2H, NH), 7.99 (s, 2H, N=CH), 7.79 (d, 6H, PPh3), 7.67
(m, 4H, Ph, PPh3), 7.5 (m, 7H, PPh3), 7.09 (t, 4H, Ph), 6.96 (t,
2H, Ph), 6.01 (d, 2H, Ph). IR (KBr): ν = 3277(m), 1599(m),
1559(m), 1540(m), 1490(m), 1436(m), 1260(m), 1088(s), 744(s),
Syntheses. Glyoxalbis(N-phenyl)osazone (LNHPhH2). This
was prepared by the reported procedure.1
trans-Rh(LNHPhH2)(PPh3)2Cl2 (1). To a hot solution of
glyoxalbis-(N-phenyl)osazone (LNHPhH2) ligand (0.6 mmol) in
absolute ethanol (30 mL), RhCl3 (0.5 mmol) and PPh3 (1.2
mmol) were added successively, and the reaction mixture was
refluxed for 40 min (78 °C) under argon atmosphere. A red solid
separated out. The solution mixture was cooled at 20 °C and
filtered. The residue was dried in air and collected. Yield: 430 mg
(∼ 92% with respect to rhodium). ESI (positive ion)-MS in
CH3CN; m/z: 936.83(1, calcd 936.68): Anal. Calcd for C50-
H44Cl2N4P2Rh: C, 64.11; H, 4.73; N, 5.98. Found: C, 63.33;
H, 4.44; N, 5.41. IR (KBr): ν = 3289 (m), 1599(s), 1493(vs),
692(vs), and 518(vs) cm-1
.
X-ray Crystallographic Data Collection and Refinement of the
-
Structure. A dark brown single crystal of [t-1]þI3 was picked
and mounted on a Bruker APEX-II CCD diffractometer
equipped with a Mo-target rotating-anode X-ray source and a
˚
graphite monochromator (Mo-KR, λ = 0.71073 A). Final cell
constants were obtained from least-squares fits of all measured
reflections. The structure was readily solved by direct method
and subsequent difference Fourier techniques. The crystallo-
graphic data of [t-1]þI3- is listed in Table 1. ShelX973 was used
for the structure solution and refinement. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
placed at calculated positions and refined as riding atoms with
isotropic displacement parameters.
1434(s), 1286(s), 1089(m), 744(s), 694(vs), and 519(vs) cm-1
.
trans-[RhIII(LNHPhH2)(PPh3)2Cl2]I3 ([t-1]þI3-). To a clear
solution of 1 (100 mg, 0.1 mmol) in dichloromethane (30 mL),
iodine (20 mg) in DCM (10 mL) was added slowly under argon,
and the resulting solution mixture was kept at 25 °C. Red
-
crystals of [t-1]þI3 separated out from the reaction solution
after 2-3 days, which were collected and used for single-crystal
X-ray diffraction studies and other physiochemical measure-
ments. Yield: 15 mg (12% with respect to 1). ESI (positive ion)-
MS in CH3CN; m/z: 934.93 (1þ, calcd 936.68). Anal. Calcd for
C50H44Cl2N4P2RhþI3-: C, 45.58; H, 3.37; N, 4.25. Found: C,
45.34; H, 3.45; N, 4.11. 1H NMR (DMSO-d6, 300 MH, 295 K) δ
Density Functional Theory (DFT) Calculations. All calcula-
tions reported in this article were done with the Gaussian 03W4
€
€
€
(3) (a) Sheldrick, G. M. ShelXS97; Universitat Gottingen: Gottingen,
€
€
Germany, 1997. (b) Sheldrick, G. M. ShelXL97; Universitat Gottingen:
€
Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1333
program package supported by GaussView 4.1. The DFT5 and
TD-DFT6 calculations have been performed at the level of
Becke three parameter hybrid functional with the nonlocal
correlation functional of Lee-Yang-Parr (B3LYP).7 The gas
phase geometries of cis and trans isomers of Rh(LNHPhH2)-
(PMe3)2Cl2 (3), [Rh(LNHPhH2)(PMe3)2Cl2]þ (3þ), and Rh(LNHPh
-
H2)(PMe3)Cl (4) have been optimized on theoretical coordinates
using Pualy’s Direct Inversion8 in the Iterative Subspace (DIIS),
using the “tight” convergent SCF procedure9 ignoring symme-
try. In all calculations, a LANL2DZ basis set, along with the
corresponding effective core potential (ECP), was used for
rhodium metal.10-12 Valence double-ζ basis set, 6-31G13 for H
was used. For C, N, P, and Cl non-hydrogen atoms valence
double-ζ plus diffuse and polarization functions, 6-31þþG**14
as basis set were employed for the calculations. The percentage
contribution of ligand and metal to the frontier orbitals of 3, 3þ,
and 4 have been calculated using the GaussSum program pack-
age.15 The 60 lowest singlet excitation energies on the optimized
geometries of 3, 3þ, and 4 in dichloromethane using the CPCM
model16 have been elucidated with the TD-DFT method.
Figure 1. Molecular structure of the [t-1]þ in crystals of [t-1]þI3 (I3
ion is omitted for clarity).
-
-
Results and Discussion
Syntheses. The dark red compound, trans-Rh(LNHPhH2)-
(PPh3)2Cl2 (1), stable in solid state, has been synthesized
in high yield (90% with respect to Rh) by reacting RhCl3
with PPh3 and the osazone ligand (LNHPhH2) in boiling
ethanol under anaerobic condition. 1 is one electron
paramagnetic (μeff = 1.70 μB), and the spin is mostly
localized on the osazone ligand forming the 1(t-RhL•)
electromer. 1 is not stable and even under argon slowly
produces trans-[RhIII(LNHPhH2)(PPh3)2Cl2]þ, [t-1]þ, and a
rhodium(I) osazone complex, RhI(LNHPhH2)(PPh3)Cl (2).
Formation of products, [t-1]þ and 2 from 1 indicates in
solution the contribution of 1(t-Rh•L) electromer, which is
a rhodium(II) osazone complex that undergoes facile
disproportionation reaction17 to a rhodium(III) osazone
complex, [t-1]þ and 2, a new square planar rhodium(I)
osazone complex as eq 1. Upon chromatographic sepa-
ration, 2 has been isolated as a pure product. As the
electromers, 1(t-RhL•) and 1(t-Rh•L) are very reactive in
solution, and they undergo oxidation affording a [t-1]þ
cation in the presence of any kind of oxidizing agent as in
eq 2. Oxidation of 1 by I2 in dichloromethane under argon
affords the crystals of [t-1]þI3- in low yield (∼12% with
respect to 1).
(4) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
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Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
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Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
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2 1ðt-Rh•LÞ f ½t-1ꢀþ þ 2 þ PPh3 þ Cl-
1ðt-RhL•Þ f ½t-1ꢀþ þ e
ð1Þ
ð2Þ
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characterized by elemental analyses and various spectra
including the single crystal X-ray structure determination
of [t-1]þI3-. In IR spectra, the NH stretching frequency
appears at 3289, 3289, and 3277 cm-1 for 1, [t-1]þI3-, and 2,
respectively.
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1334 Inorganic Chemistry, Vol. 50, No. 4, 2011
Patra et al.
Chart 1. (a) Experimental Bond Lengths of Osazone in [t-1]þI3-, with
Corresponding Calculated Lengths of [t-3]þ in Parentheses. (b) Experi-
mental Bond Lengths of Osazone in trans-RuII(LNHPhH2)(PPh3)2
Cl2 (ref 1). (c) Calculated Lengths of Osazone in 3(t-RhL•) Electromer.
(d) Calculated Lengths of Osazone in 4
þ
-
˚
Table 2. Selected Bond Distances (A) and Angles (deg) of [t-1] I3
Rh(1)-Cl(1)
Rh(1)-P(1)
2.356(19)
2.409(18)
2.033(6)
1.339(8)
1.404(9)
1.319(10)
1.403(16)
127.4(7)
123.4(6)
122.8(7)
116.9(4)
Rh(1)-N(2)
N(1)-N(2)
N(1)-C(1)
N(2)-C(7)
C(7)-C(7A)
N(2)-N(1)-C(1)
C(7)-N(2)-N(1)
C(6)-C(1)-N(1)
N(2)-C(7)-C(7A)
with the atom numbering scheme is depicted in Figure 1,
and the relevant bond parameters are summarized in
Table 2. In crystals of [t-1]þI3-, two bulky triphenyl pho-
sphine ligands are trans to each other. The average
Rh(1)-P(1) and Rh(1)-Cl(1) distances are 2.4090(18)
˚
and 2.3556(19) A, respectively. The corresponding
RuII-P and RuII-Cl distances in trans-RuII(LNHPhH2)-
(PPh3)2Cl2 are 2.3718(8) and 2.4421(8) A, respectively.
˚
6
Both Rh(III) and Ru(II) are t2g metal ions but the
effective nuclear charge (Zeff*) of Rh(III) ion is higher
than that of Ru(II) ion. The metal ion with the higher
Zeff*, will expectedly result in shorter M-Cl bonds
(σ bonding) but longer M-PPh3 bonds (π bonding) as
observed in Rh(III)-Cl and Rh(III)-PPh3 length trends
(Rh(III)-Cl < Ru(II)-Cl and Rh(III)-PPh3 > Ru-
(II)-PPh3).
Bond parameters of the osazone ligand in [t-1]þI3-, as
given in Chart 1a are of importance to analyze the
electronic structure of the complex. The =CH-CH=
bond of the diimine fragment is shorter than the expected
2
˚
length of two sp hybridized carbon atoms (1.47 A), but
two -NdC- bonds are longer than a normal -NdC- bond
(1.28 A). In [t-1] I3-, the average -NdC- and =CH-
þ
˚
˚
CH= bond lengths are 1.319(6) and 1.403(16) A, respec-
No signal has been detected at 310 K in dichloro-
methane solution. The g parameter for the measurements
in frozen glasses at 77 K and solid (room 295 and 77 K) is
1.991, which is 2.002 at 25 K for solids. In dichloro-
methane solution at 295 K, although the paramagnetic
signal is weaker, it appears at g = 1.992 (Figure 2). All
these data are consistent with the presence of a single
paramagnetic center, i.e., one pure electromer in the bulk
sample of 1. The deviation from the g = 2.004 may be due
to localization of some amount of spin density on the
metal center having the 1(t-Rh•L) electromer. But at 25 K,
the g value is 2.002, prompting that the electron is more
localized on the osazone ligand.
tively. These bond parameters of the -HN-N=CH-
CH=N-NH- fragment correlate well to those reported
in the first osazone complex, RuII(LNHPhH2)(PPh3)2Cl2
1
(Chart 1b), and are consistent with the neutral diimine
description as established in other neutral diimine species
reported by Raston et al.18 and Wieghardt et al.19 Thþe
N-N distance in [t-1]þI3 is 1.339(8) A. Thus, the [t-1]
-
˚
ion has been described as an osazone complex of
rhodium(III) ion, and the trend of such bond parameters
has been investigated by DFT calculations.
EPR Spectra. The solid state EPR spectra of 1 at 295,
77, and 25 K have been recorded. The EPR spectra of 1 in
dichloromethane at 295 and 310 K and dichloromethane-
toluene frozen glass at 77 K have also been recorded. All
the spectra are shown in Figures 2 and 3.
The high energy difference between 3(t-RhL•) and
3(c-RhL•) electromers does not support the presence of
equilibrium mixture of trans and cis electromers localiz-
ing 2% and 4% spin density on the rhodium ion as shown
in Figure 6. But the mixing of 1(t-RhL•) with the excited
state (LUMO), where the electron is localized more on the
rhodium metal ion can broaden the spectrum more. The
possibility of transition has been noted in cyclic voltam-
metry study and LMCT bands in the electronic spectra
of 1. The EPR spectral features and instabilities of 1 in
solution at room temperature thus can be well explained by
the presence of a low percentage of 1(t-Rh•L) with the elec-
tromer, 1(t-RhL•). 1(t-Rh•L) is a rhodium(II) compound
and disproportionates spontaneously to [t-1]þ and 2 as
observed experimentally and in other reported complexes.17
(18) (a) Cloke, F. G. N.; Dalby, C. I.; Henderson, M. J.; Hitchcock, P. B.;
Kennard, C. H. L.; Lamb, R. N.; Raston, C. L. J. Chem. Soc., Chem.
Commun. 1990, 1394. (b) Cloke, F. G. N.; Hanson, G. R.; Henderson, M. J.;
Hitchcock, P. B.; Kennard, C. H. L.; Raston, C. L. J. Chem. Soc., Chem.
Commun. 1989, 1002. (c) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee,
F. C.; Raston, C. L. Inorg. Chem. 1994, 33, 2456.
€
(19) (a) Ghosh, P.; Bill, E.; Bothe, E.; Weyhermuller, T.; Neese, F.;
Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 1293. (b) Muresan, N.; Chlopek,
K.; Weyherm€uller, T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2007, 46, 5327.
(c) Muresan, N.; Lu, C. C.; Ghosh, M.; Peters, J. C.; Abe, M.; Henling, L. M.;
Weyherm€uller, T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2008, 47, 4579.
(d) Ghosh, M.; Sproules, S.; Weyhermueller, T.; Wieghardt, K. Inorg. Chem.
2008, 47, 5963. (e) Ghosh, M.; Weyherm€uller, T.; Wieghardt, K. Dalton Trans.
2008, 5149.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1335
Table 3. Calculated Bond Parameters of the Coordination Sphere for 3(t-RhL•)
[t-3]þ, and 4 at the B3LYP Level of the Theory
,
average
[t-3]þ
4
3(t-RhL•)
Rh-Cl
Rh-P
Rh-N
2.405
2.432
2.085
2.411
2.304
2.076
2.459
2.417
2.073
of 3þ have also been optimized with singlet spin state to
compare the stabilities (Figure 4). The calculated bond
parameters of significant isomers have been summarized
in Table 3. The calculation shows that the trans isomer of
3þ, abbreviated as [t-3]þ, is stabilized by 35.6 KJ/mol
compared to the cis isomer, [c-3]þ. The closed shell singlet
(CSS) solution of trans-isomer of [t-3]þ satisfactorily
reproduces the experimental bond parameters of the
osazone ligand of [t-1]þI3-. Because of the replacement
of PPh3 by PMe3, the calculated bond parameters of the
coordination sphere of [t-3]þ are comparatively higher
Figure 2. X-band EPR spectrum of 1 in dichloromethane at 295 K (red
line denotes the average plot; frequency, 9.446 GHz; modulation ampli-
tude, 5.0 G).
than those found in [t-1]þI3 experimentally (Tables 1
-
and 3). The CSS solution has no instability due to triplet
perturbations rejecting the possibility of having diradical
singlet (S = 0) as the ground state. The features of
unexpected longer -CdN and shorter = CH-CH=
distances as given in Chart 1(a) are due to the mixing of
the πdiimine* of the osazone ligand with the filled d-orbital
of the Rh(III) ion. One such case is shown in Figure 5,
which is defined as d f πdiimine* back bonding that results
in transfer of a metal electron to the LUMO of osazone as
observed in trans-RuII(LNHPhH2)(PPh3)2Cl2.1
DFT calculations on trans and cis isomers of 3 show
that in both cases, the electron density is localized on the
osazone ligand generating 3(t-RhL•) and 3(c-RhL•) types of
electromers, respectively, as depicted in Figure 6. Similar
to the order of higher stabilities of trans isomers [t-3]þ
compared to the cis isomer [c-3]þ of [3]þ ion, the trans
electromer, 3(t-RhL•) is more stable than the cis electromer,
3(c-RhL•) by 64 KJ/mol, excluding the possibility of exis-
tence of 3(c-RhL•) in solid or solution.
The calculated bond parameters of the coordination
sphere of 3(t-RhL•) are listed in Table 3. The bond para-
meters of the osazone fragment (Chart 1c) are remarkably
different from those obtained from single crystal X-ray
structure determinations and DFT calculations on
-
[t-1]þI3 and trans-RuII(LNHPhH2)(PPh3)2Cl2 as given
in Chart 1b. The comparatively much longer -CdN-
˚
˚
(1.348 A), N-N (1.389 A), and shorter =CH-CH=
˚
(1.394 A) bond length trends of the osazone moiety of the
optimized structure of 3(t-RhL•) correlate well to those
found in the diimine anion radical complexes.18-20 Mul-
liken spin population analyses have shown that spin
density is mostly localized on the diimine fragment of
the osazone ligand with around 2% only on the rhodium
ion (Figure 6), correlating well with the EPR spectra .
Thus, compound 1 here has been defined as the first
osazone anion radical complex of type 1(t-RhL•) with
around 2% contribution of rhodium(II) osazone electro-
mer, 1(t-Rh•L), as shown in Scheme 1. The calculation has
shown that formation of osazone anion radical lengthens
the Rh-Cl bond (Table 3) trans to an imine nitrogen,
where a significant amount of spin density is localized.
Figure 3. X-band EPR spectra of 1 in frozen glass (dichloromethane) at
77 K and solid at 295, 77, and 25 K (frequency, 9.132-9.433 GHz).
Gas Phase Geometries and Electronic Structures of 1,
1þ, and 2. Gas phase geometries and bond parameters of 1
and 1þ have been investigated by density functional theory
(DFT) calculations on model compounds Rh(LNHPhH2)-
(PMe3)2Cl2 (3) and [Rh(LNHPhH2)(PMe3)2Cl2]þ (3þ).
Geometries of both cis and trans isomers of 3 have been
optimized with doublet spin state to identify the most
stable electromer. Geometries of both cis and trans isomers
(20) Bailey, P. J.; Dick, C. M.; Fabre, S.; Parsons, S.; Yellowlees, L. J.
Dalton Trans. 2006, 1602.

1336 Inorganic Chemistry, Vol. 50, No. 4, 2011
Patra et al.
Figure 4. Optimized geometries of [t-3]þ, [c-3]þ, and 4.
Figure 7. Cyclic voltammogram of 1 in CH2Cl2 at 295 K (conditions:
0.1(M) [N(n-Bu)4]PF6 supporting electrolyte; scan rates 50 (black), 100
(red), 200 (green), and400(blue) mVs-1; glassy carbon working electrode).
Figure 5. Mixing of filled dRh with πdiimine* of LNHPhH2.
cyclic voltammetry, which displays two irreversible one
electron transfer waves at þ0.13 and -1.22 V with respect
to the Fcþ/Fc couple as shown in Figure 7. The anodic
peak at 0.13 V has been assigned to the oxidation of the
osazone anion radical to osazone, i.e., 1(t-RhL•) to [t-1]þ,
while the cathodic peak at -1.22 V has been assigned to
the reduction of rhodium(III) to rhodium(II), i.e., [t-1]þ
to 1(t-Rh•L). The RhIII/RhII reduction potential (-1.22 V
with respect to the Fcþ/Fc couple) of 1 is comparable
to those reported in other rhodium(III)-heterocyclic
diimine or pyridyl complexes.17e,f,21,22
The event is noteworthy and is a phenomenon of
electro-isomerism.2 It explains the stability of the cation
in solution that has been isolated successfully by oxidizing
1(t-RhL•) with an I2 solution. The large shift of the potential
of this redox couple (1.35 V, 130 KJ/mol for one electron
transfer) is due to the transfer of the electron from the
anion ligand to the metal, transforming rhodium(III) to
rhodium(II) in a similar geometrical feature. The trans-
formation of 1(t-RhL•) to 1(t-Rh•L) is costly and correlates
well with the HOMO to LUMO excitation energies in 3
(in dichloromethane). HOMO is composed of LNHPhH2
(100%), while the LUMO is composed of RhþCl orbitals
(75%) and LNHPhH2 (25%). The calculated HOMO
to LUMO excitation energy is 97 kJ/mol, but for a
LUMO with no contribution of LNHPhH2, the excitation
energy is expected to be 129 kJ/mol that is very similar to
the experimental voltage shift of 1.35 V during the
reduction process of [t-1]þ/1 couple in dichloromethane
solution.
Figure 6. Mulliken spin density plot of 3(t-RhL•) (bottom) and 3(c-RhL•)
(top) as derived from UB3LYP/DFT.
To elucidate the trend of bond parameters of the
osazone in 2, the gas-phase geometry of Rh(LNHPhH2)-
(PMe3)Cl (4) has been optimized (Figure 4), and relevant
bond parameters are listed in Table 3 and Chart 1d. The
calculated bond parameters of 4 are very similar to those
found in [t-1]þI3-, [t-3]þ, and trans-RuII(LNHPhH2)-
(PPh3)2Cl2 (Chart 1) and are consistent with the presence
of a neutral osazone ligand coordinated to rhodium(I)
in 2.
(21) Gonzalez, V.; Adams, H.; Thomas, J. A. Dalton Trans. 2005, 110–
115.
(22) Amarante, D.; Cherian, C.; Catapano, A.; Adams, R.; Wang, M. H.;
Megehee, E. G. Inorg. Chem. 2005, 44, 8804–8809.
Cyclic Voltammetry Study and Electronic Spectra. The
electron transfer events of 1 in dichloromethane solution
under a completely N2-atmosphere have been studied by

Article
The electronic spectra of 1, [t-1]þ, and 2 in dichloro-
Inorganic Chemistry, Vol. 50, No. 4, 2011 1337
continuum (CPCM) model. Calculated energies, signifi-
cant contributions, transitions types, and dominant con-
tributions of these three calculations are listed in Table 5.
Calculations have shown that for the rhodium(III)
osazone complex, [t-3]þ, all the significant absorptions
above 300 nm are due to ligand-to-metal charge transfers
(LMCT), while for the rhodium(I) osazone complex, 4,
the significant absorptions are due to metal-to-ligand
charge transfer (MLCT). The radical complex 3 has
mixed origins, i.e., LMCT, LLCT, and MLCT. The
high-intensity transition at 351.2 nm for compound 3
involves following electrons transfers: alpha HOMO
(based on the singly occupied πdiimine* orbital of the
osazone) f high lying ligand based R LUMOþ8 and
LUMOþ10; β HOMO-3 (based on the rhodium(III)
methane solutions are characteristic and are shown in
Figure 8. Data have been summarized in Table 4. Com-
pound 1 absorbs strongly at 352 nm with a shoulder at
443 nm. The absorption features of [t-1]þ and 2 at 300-
440 nm are different. To elucidate the origins of absorp-
tions of all these complexes, TD-DFT calculations have
been carried out on 3, [t-3]þ, and Rh(LNHPhH2)(PMe3)Cl
(4). For each compound, 60 lowest singlet excitation
energies on the optimized geometries in dichloromethane
have been elucidated using a conductor-like polarizable
d-orbital) f LUMO (based on the unoccupied πdiimine
*
orbital of the osazone); and HOMO-2 (based on the
rhodium(III) d-orbital) f LUMO (based on the unoccu-
pied πdiimine* orbital of the osazone). Thus, the strong
absorption band at 352 nm of 1 has been assigned to
LLCT and MLCT origins.
Calculations have further shown the origins of the low-
energy absorption band of 1 and [t-1]þ at higher than 440
nm. Calculations for 3 find an absorption band at 553.9
nm because of the transition of the electron from the beta
HOMO (based on the πdiimine of the osazone) to LUMO
(based on the πdiimine* of the osazone), while the band at
456.7 nm is due to the transitions from alpha HOMO-1
(πdiimine of the osazone) to the LUMO (based on the
rhodium(III) d-orbital) and beta HOMO (πdiimine of
the osazone) to LUMOþ1 (based on the rhodium(III)
d-orbital). Experimentally, no significant absorption
band above 500 nm has been observed in the complex
of 1, but the low-energy band at around 440 nm of 1 has
been assigned to a LMCT band. Similarly for [t-3]þ, the
Figure 8. Electronic spectra of 1 (blue), [t-1]þI3- (red), and 2 (green) in
dichloromethane at 295 K.
Table 4. Electronic Spectral Data of 1, [t-1]þI3-, and 2 in Dichloromethane
complexes
λ
max, nm (ε, 104 M-1cm-1
)
1
443 (3.1), 352 (9.3)
446 (1.2), 350 (3.2), 291(4.4)
448(0.9), 347(2.4)
[t-1]þI3
-
2
Table 5. Calculated Excitation Energies with Oscillator Strength and Significant Transitions of 3, [t-3]þ, and 4
λ/nm (f)
exp. λ
significant contributions
transition type
dominant contribution
3
553.9 (0.088)
456.7 (0.037)
β HOMO f LUMO (93%)
R HOMO-1 f LUMO (46%)
β HOMO f LUMO þ1 (25%)
R HOMO-2 f LUMO (41%)
R HOMO f LUMOþ6 (10%)
β HOMO-1 f LUMOþ1 (26%)
R HOMO f LUMOþ8 (17%)
R HOMO f LUMOþ10 (14%)
β HOMO-3 f LUMO (12%)
β HOMO-2 f LUMO (22%)
R HOMO f LUMOþ8 (28%)
β HOMO-2 f LUMO (21%)
π
π
π
π
π
π
π
π
L f π*L
LLCT
LMCT
LMCT
LMCT
LLCT
LMCT
LLCT
LLCT
MLCT
MLCT
LMCT
MLCT
443
L f dRh(III) þ π*L
L f dRh(III) þ π*L
L f dRh(III) þ π*L
L f π*L
372.0 (0.127)
351.2 (0.145)
L f dRh(III) þ π*L
352
L f π*L
L f π*L þ pP
dRh(III) þ πL þ pCl f π*L
dRh(III) þ pCl f π*L
335.9 (0.022)
πL f dRh(III)
dRh(III) þ pCl f π*L
[t-3]þ
446.8 (0.133)
446
350
HOMO-6 f LUMO (48%)
HOMO f LUMOþ1 (26%)
HOMO f LUMOþ2 (72%)
HOMO f LUMOþ1 (48%)
HOMO-2 f LUMOþ1 (62%)
HOMO-1 f LUMOþ1 (36%)
HOMO-5 f LUMOþ1 (80%)
π
π
π
π
L f π*L
LLCT
L f dRh(III) þ π*L þ pCl
L f dRh(III) þ pP
L f dRh(III) þ π*L þ pCl
LMCT
LMCT
LMCT
LMCT
LMCT
LMCT
438.9 (0.033)
427.6 (0.426)
342.1 (0.039)
πL þ pCl þ pP f dRh(III) þ π*L þ pCl
πL þ pCl þ pP f dRh(III) þ π*L þ pCl
317.5 (0.047)
π
L f dRh(III) þ π*L þ pCl
4
434.5 (0.243)
403.66 (0.532)
448
347
HOMO-3 f LUMO (41%)
HOMO-2 f LUMO (33%)
HOMO-3 f LUMO (33%)
HOMO-2 f LUMO (47%)
dRh f π*L
dRh f π*L
dRh f π*L
dRh f π*L
MLCT
MLCT
MLCT
MLCT

1338 Inorganic Chemistry, Vol. 50, No. 4, 2011
Patra et al.
absorption band at 446.8 nm has origins of LLCT and
LMCT, while the band at 438.9 is purely a LMCT band.
Thus, the calculations have established that LMCT is the
origin of the low-energy absorption bands of 1 and [t-1]þ
around 440 nm.
decomposed nonradical analogues, [trans-RhIII(LNHPhH2)-
(PPh3)2Cl2], [t-1]þ, and RhI(LNHPhH2)(PPh3)Cl (2). In cyclic
voltammetry, 1(t-RhL•) to [t-1]þ oxidation at the anode and [t-
1]þ to 1(t-Rh•L) reduction at the cathode generating electro-
meric species have been assigned. The study has established
that osazone is redox non-innocent, and with transition metal
ions, osazone can generate electromeric species. Elucidation
of molecular and electronic structures of those electron
transfer states appears to be worth investigation.
Conclusion
The first osazone anion radical (LNHPhH2•-) coordinated to
a rhodium(III) ion as in the trans-RhIII(LNHPhH2•-)(PPh3)2Cl2,
1(t-RhL•) electromer, a major component (∼98%) of the
paramagnetic trans-Rh(LNHPhH2)(PPh3)2Cl2 (1) complex,
has been authenticated by experimental and theoretical
studies. EPR parameters and DFT calculations have pre-
dicted a minor contribution (∼2%) of the rhodium(II)
electromer, trans-RhII(LNHPhH2)(PPh3)2Cl2, 1(t-Rh•L), in 1.
The bond parameters of the osazone anion radical and
spectral features of 1(t-RhL•) have been compared with its
Acknowledgment. We thank UGC (F. No. 34-315/2008
(SR) and DST (SR/S1/IC- 10/2008), New Delhi for
financial support.
Supporting Information Available: Crystallographic data in
CIF format for [t-1]þI3-. These materials are available free of
charge via the Internet at http://pubs.acs.org.