Synthesis, structure and redox properties of some 2-(arylazo)phenolate complexes of rhodium(in) f

Article abstract of DOI:10.1039/b005902l

Reaction of 2-(arylazo)phenols 2-(4′-RC₆H₄N=N)C₆H₃OH(Me-4) (H2ap-R, where H₂ stands for the two dissociable protons and R for the substituent in the phenyl ring of the arylazo fragment) with [Rh(PPh

Full text of DOI:10.1039/b005902l

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Synthesis, structure and redox properties of some 2-(arylazo)-
phenolate complexes of rhodium(III)†
Swati Dutta,a Shie-Ming Pengb and Samaresh Bhattacharya*a
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University,
Calcutta 700032, India
b Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
Received 21st July 2000, Accepted 16th October 2000
First published as an Advance Article on the web 1st December 2000
Reaction of 2-(arylazo)phenols 2-(4Ј-RC H N᎐N)C H OH(Me-4) (H ap-R, where H stands for the two dissociable
᎐
6
4
6
3
2
2
protons and R for the substituent in the phenyl ring of the arylazo fragment) with [Rh(PPh3)3Cl] afforded a family
of organometallic complexes of rhodium() of type [Rh(PPh3)2(ap-R)Cl]. The crystal structure of [Rh(PPh3)2(ap-
NO2)Cl] has been determined. The 2-(arylazo)phenols are coordinated via dissociation of the phenolic proton and
the phenyl proton at the ortho position of the phenyl ring in the arylazo fragment, as dianionic tridentate C,N,O-
donors forming two five-membered chelate rings. 1H and 13C NMR spectra of the complexes are in excellent agree-
ment with their composition and stereochemistry. The complexes are diamagnetic (low-spin d6, S = 0) and show
intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry for all the complexes show a quasi-
reversible oxidation within 0.65 to 1.10 V vs. SCE and an irreversible reduction within Ϫ1.34 to Ϫ1.50 V vs. SCE.
The potential of the oxidation is found to be sensitive to the nature of the substituent R in the 2-(arylazo)phenolate
ligand.
Introduction
Transition metal-mediated organic transformations via acti-
vation of C–H bonds have been an active area of chemical
research.1 Most of these reactions proceed via an organometal-
lic intermediate. Hence synthesis of new organometallic com-
plexes is of significant importance, particularly with regard to
reaction of the metal–carbon bonds. Herein we report the
results of our studies on a group of organorhodium complexes
obtained from the reaction of 2-(arylazo)phenols 1 with
Wilkinson’s catalyst, viz. [Rh(PPh3)3Cl]. Rhodium complexes
in general and organorhodium complexes in particular are
known to exhibit antitumor activities and hence they have wide
application as chemotherapeutic agents.2 Wilkinson’s catalyst
has indeed turned out to be very successful in affording a
has been chosen as the rhodium starting material because of
its well known efficiency in bringing about catalytic trans-
formations of organic substrates.3 The reason behind the choice
of 2-(arylazo)phenols as the main ligand is manifold. Simple
azobenzenes are well known to bind to soft metal centers as
monoanionic bidentate C,N-donor ligands forming five-
membered metallacycles of type 2.4 However, [Rh(PPh3)3Cl]
undergoes dissociation in solution producing free PPh3 and
a formally three-coordinated RhI(PPh3)2Cl fragment. For
oxidative addition of organic ligands to this fragment, the
incoming organic ligand is desired to be capable of satisfying
the residual positive charge of rhodium() and the remaining
three coordination sites. As phenolate oxygen is a recognized
hard donor and a familiar stabilizer of the higher oxidation
states of transition metals,5 we have chosen a modified azo-
benzene with a potential third donor site, viz. the 2-(arylazo)-
phenols 1, as our test ligand in this reaction. This strategy
family of rhodium() cyclometallates of type 3. While
there are numerous examples of organorhodium complexes
in the literature,6 cyclometallates of rhodium() appear to be
relatively less common.7 The chemistry of the new family
of cyclometallated rhodium() complexes is described here
with special reference to synthesis, characterization and redox
properties.
Experimental
Materials
Rhodium trichloride was obtained from Johnson Matthey
and triphenylphosphine was purchased from Loba, India.
[Rh(PPh3)3Cl] was synthesized following a reported procedure.8
The para-substituted anilines and p-cresol were obtained
from S.D., India. The 2-(arylazo)phenol ligands were prepared
by coupling diazotized p-substituted anilines with p-cresol.
All other chemicals and solvents were reagent grade com-
mercial materials used as received. Purification of dichloro-
methane, acetonitrile and preparation of tetrabutylammonium
perchlorate for electrochemical work were as reported.9
† Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra for [Rh(PPh3)2(ap-R)Cl]. See http://www.rsc.org/suppdata/
dt/b0/b005902l/
DOI: 10.1039/b005902l
J. Chem. Soc., Dalton Trans., 2000, 4623–4627
This journal is © The Royal Society of Chemistry 2000
4623

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Preparations
given in Table 1. Data were collected on a SMART CCD dif-
fractometer using graphite monochromated Mo-Kα radiation
by ω scan. X-Ray data reduction, structure solution and refine-
ment were done using the SHELXS-97 and SHELXL-97
programs.10 The structure was solved by direct methods.
CCDC reference number 186/2240.
[Rh(PPh3)2(ap-NO2)Cl]. H2ap-NO2 (29 mg, 0.11 mmol) was
dissolved in toluene (40 mL) and to it were added [Rh(PPh3)3Cl]
(100 mg, 0.11 mmol) and triethylamine (58 mg, 0.24 mmol).
The mixture was then refluxed under a dinitrogen atmosphere
for 3 h, when a green solution was obtained. Evaporation
of this solution afforded a dark solid which was purified by
thin layer chromatography on a silica plate with toluene as the
eluent. A green band separated and the complex was extracted
from it with acetonitrile. The green solid, obtained upon
evaporation of the solvent, was recrystallized from dichloro-
methane–hexane to afford [Rh(PPh3)2(ap-NO2)Cl] as a crystal-
line green solid. Yield: 55%. Calc.: C, 61.70; H, 3.96; N, 4.50.
Found: C, 61.69; H, 3.97; N, 4.53%.
See http://www.rsc.org/suppdata/dt/b0/b005902l/ for crystal-
lographic files in .cif format.
Results and discussion
Five 2-(arylazo)phenols 4 have been used in the present study.
The ligands are abbreviated in general as H2ap-R, where H2
stands for two hydrogens (one phenolic and one phenyl (at
2Ј position)) that undergo dissociation upon complexation
(see below) and R is the substituent at 4Ј position of the phenyl
ring in the arylazo fragment. Reaction of these ligands with
[Rh(PPh3)3Cl] proceeds smoothly in refluxing toluene in the
presence of triethylamine to afford a family of organorhodium
complexes of type [Rh(PPh3)2(ap-R)Cl]. Elemental (C, H, N)
analytical data are consistent with these compositions. Hence
it appears that rhodium exists in the ϩ3 oxidation state in these
complexes and the 2-(arylazo)phenol ligands are coordinated as
dianionic tridentate C,N,O-donors. Magnetic susceptibility
measurements show that the complexes are diamagnetic, which
supports the trivalent state of rhodium (low-spin d6, S = 0).
To find out the actual coordination mode of the 2-(arylazo)-
phenolate ligands and stereochemistry of the complexes the
molecular structure of a representative member of the family,
viz. [Rh(PPh3)2(ap-NO2)Cl], has been determined by X-ray
crystallography.
[Rh(PPh3)2(ap-Cl)Cl]. This complex was prepared and puri-
fied by following the above procedure using H2ap-Cl instead
of H2ap-NO2 and the reflux time was extended to 5 h. Yield
50%. Calc.: C, 64.84; H, 4.30; N, 3.09. Found: C, 65.09; H, 4.38;
N, 3.11%.
[Rh(PPh3)2(ap-H)Cl]. This complex was prepared and puri-
fied following the same procedure using H2ap-H instead of
H2ap-Cl and the reflux time was extended to 8 h. Yield
46%. Calc.: C, 67.32; H, 4.58; N, 3.21. Found: C, 67.35; H, 4.60;
N, 3.22%.
[Rh(PPh3)2(ap-Me)Cl]. This complex was prepared by
following the same procedure using H2ap-Me instead of
H2ap-H and the reflux time was extended to 10 h. Purification
was achieved by thin layer chromatography on a silica plate
with toluene–acetonitrile (5:1) as the eluent. A greenish blue
band separated and the complex extracted from it with aceto-
nitrile. The greenish blue solid, obtained upon evaporation of
the solvent, was recrystallized from dichloromethane–hexane
to afford [Rh(PPh3)2(ap-Me)Cl] as a crystalline greenish blue
solid. Yield 34%. Calc.: C, 67.61; H, 4.73; N, 3.15. Found: C,
67.63; H, 4.77; N, 3.16%.
[Rh(PPh3)2(ap-OMe)Cl]. This complex was prepared and
purified by following the same procedure using H2ap-OMe
instead of H2ap-Me and the reflux time was extended to 23 h.
Yield 33%. Calc.: C, 66.42; H, 4.65; N, 3.09. Found: C, 66.44;
H, 4.66; N, 3.10%.
A view of the complex molecule is shown in Fig. 1 and
selected bond parameters are listed in Table 2. The 2-(arylazo)-
phenol ligand is indeed coordinated as a tridentate C,N,O-
donor forming two five-membered chelate rings with bite
angles of 80.57(9) (O–Rh–N) and 79.69(10)Њ (N–Rh–C). The
2-(arylazo)phenol ligand, rhodium and chloride constitute the
equatorial plane with the latter in trans position with respect to
the azo-nitrogen, while the two PPh3 ligands occupy mutually
trans positions. The CNOP2Cl coordination sphere around
rhodium is distorted octahedral in nature, which is reflected
in all the bond parameters around rhodium. Cyclometallates of
rhodium() of the observed type appear to be unprecedented.
The Rh–C, Rh–O, Rh–P and Rh–Cl distances are all quite
normal, according to structurally characterized complexes of
rhodium() containing these bonds.11 While the Rh–N1 dis-
tance is rather short relative to that found in complexes where
the Rh–N interaction can only be of σ type,12 the observed azo
N1–N2 distance is longer than when uncoordinated azo.13 The
decrease in Rh–N distance and increase in N–N bond length
may be attributed to back bonding from the filled t2 orbitals of
rhodium to the vacant π* orbital localized on the azo function.
As all the [Rh(PPh3)2(ap-R)Cl] complexes display similar
spectral and electrochemical properties (see below), the other
four complexes are assumed to have a similar structure to that
of [Rh(PPh3)2(ap-NO2)Cl].
Physical measurements
Microanalyses (C,H,N) were performed using a Perkin-Elmer
240C elemental analyzer. IR spectra were obtained on a
Perkin-Elmer 783 spectrometer with samples prepared as
KBr pellets, electronic spectra on a Shimadzu UV-1601
spectrophotometer. Magnetic susceptibilities were measured
1
using a PAR 155 vibrating sample magnetometer. 13C and H
NMR spectra were obtained on a Brucker drx 500 NMR
spectrometer using TMS as the internal standard. Electro-
chemical measurements were made under
a dinotrogen
atmosphere using a PAR model 273 potentiostat. A platinum-
disc working electrode, a platinum wire auxiliary electrode
and an aqueous saturated calomel reference electrode (SCE)
were used in a three electrode configuration. A RE 0089 X-Y
recorder was used to trace the voltammograms. All electro-
chemical data were collected at 298 K and are uncorrected for
junction potentials.
Crystallography
Single crystals were grown by slow diffusion of hexane into a
dichloromethane solution of the complex [Rh(PPh3)2(ap-NO2)-
Cl]. Selected crystal data and data collection parameters are
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J. Chem. Soc., Dalton Trans., 2000, 4623–4627

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Table 1 Crystallographic data of [Rh(PPh3)2(ap-NO2)Cl]
Empirical formula
M
C49H39ClN3O3P2Rh
918.13
Space group
a/Å
b/Å
c/Å
Monoclinic, P21/n
11.9111(2)
16.9049(3)
21.4067(3)
96.744(1)
4280.54(12)
4
β/Њ
V/Å3
Z
λ/Å
0.71073
22
0.582
Fig. 2 Electronic spectrum of [Rh(PPh3)2(ap-OMe)Cl] in dichloro-
methane solution.
T/ЊC
µ/mmϪ1
Reflections collected
Independent reflections
R1
29308
8742 (Rint = 0.0449)
0.0348
respectively, which are due to the methyl and methoxy groups
in the arylazo fragment of the respective ligand. The aromatic
region of the spectra (δ 5.0–8.0) appears a bit complex due to
overlap of some signals and hence assignment of all the signals
in this region to specific protons has not been possible. How-
ever, intensity measurement of the signals corresponds to the
total number of aromatic protons present in the respective
complexes. 13C NMR spectra of the [Rh(PPh3)2(ap-R)Cl] com-
plexes were also recorded in CDCl3 solution. Each complex
displays the expected number of signals. The PPh3 ligands show
four intense signals near δ 128, 129, 130 and 135 in an intensity
ratio of 1:2:1:2. The methyl carbon in the p-cresol fragment
of the 2-(arylazo)phenolate ligands shows an isolated signal
near δ 20. The aromatic carbons of the 2-(arylazo)phenolate
ligands are observed within δ 110–180, of which the most
deshielded one (near δ 175) is assigned to the metallated carbon.
The NMR spectral data are therefore in excellent agreement
with the composition and stereochemistry of the complexes.
Infrared spectra of the complexes show many sharp and
strong vibrations within 1600–200 cmϪ1. Assignment of all
these vibrations has not been attempted. However, a sharp
vibration observed near 325 cmϪ1 for all the complexes is
assigned to the ν(Rh–Cl) stretch.14 Each complex displays
strong bands near 515, 690 and 740 cmϪ1 which may be attri-
buted to vibrations arising from the trans-Rh(PPh3)2 moiety as
similar trans-M(PPh3)2 fragments are known to display such
vibrations.15 Comparison of the spectra of [Rh(PPh3)2(ap-R)Cl]
complexes with the spectrum of [Rh(PPh3)3Cl] shows the
presence of some new bands (e.g. vibrations near 810, 1250,
1300, 1360, 1425 and 1470 cmϪ1) in the former complexes,
which must be due to the coordinated 2-(arylazo)phenolate
ligand.
wR2
0.0823
Table 2 Selected bond parameters (distances in Å, angles in Њ) for
[Rh(PPh3)2(ap-NO2)Cl]
Rh–N(1)
Rh–O(1)
Rh–P(1)
Rh–P(2)
Rh–Cl
1.951(2)
2.181(7)
2.3734(7)
2.3686(7)
2.3805(8)
1.987(3)
C(7)–N(2)
N(1)–N(2)
N(1)–C(6)
C(6)–C(1)
C(1)–O(1)
1.408(4)
1.290(3)
1.386(3)
1.420(4)
1.307(4)
Rh–C(12)
N(1)–Rh–C(12)
N(1)–Rh–O(1)
N(1)–Rh–P(2)
O(1)–Rh–P(2)
C(12)–Rh–P(1)
C(12)–Rh–P(2)
O(1)–Rh–P(1)
N(1)–Rh–P(1)
O(1)–Rh–Cl
79.69(10)
80.57(9)
91.87(6)
92.62(6)
92.52(8)
87.64(8)
88.50(6)
91.84(6)
100.54(6)
99.20(8)
88.30(3)
87.98(3)
C(12)–Rh–O(1)
N(1)–Rh–Cl
160.25(10)
178.86(7)
176.25(3)
110(2)
119.2(2)
109.4(2)
121.7(2)
116.2(2)
113.8(3)
122.1(3)
107.3(2)
P(1)–Rh–P(2)
C(7)–C(12)–Rh
N(2)–C(7)–C(12)
N(1)–N(2)–C(7)
N(2)–N(1)–Rh
C(6)–N(1)–Rh
N(1)–C(6)–C(1)
O(1)–C(1)–C(6)
C(1)–O(1)–Rh
C(12)–Rh–Cl
P(2)–Rh–Cl
P(1)–Rh–Cl
The [Rh(PPh3)2(ap-R)Cl] complexes are soluble in common
polar organic solvents such as acetonitrile, dichloromethane,
acetone, etc., producing intense greenish blue solutions except
for [Rh(PPh3)2(ap-NO2)Cl] which yields a green solution.
Electronic spectra of all the complexes have been recorded
in dichloromethane solution. Spectral data are presented in
Table 3 and a selected spectrum is shown in Fig. 2. Each com-
plex shows several intense absorptions in the visible region
and two very intense absorptions in the ultraviolet region. The
former absorptions are likely to be due to intra-ligand charge-
transfer transitions taking place in the three-coordinated
2-(arylazo)phenolate ligand. Such intense absorptions are a
familiar property of azo molecules, and are responsible for their
usefulness in the dye industry. The absorptions in the ultra-
violet region may be attributed to usual n → π*, π → π*
transitions occurring within ligand orbitals.
Fig. 1 View of the [Rh(PPh3)2(ap-NO2)Cl] molecule.
Electron-transfer properties of the [Rh(PPh3)2(ap-R)Cl]
complexes have been studied in dichloromethane solution
(0.1 M NBu4ClO4) by cyclic voltammetry. All the complexes
show an oxidative response to more positive potentials with
respect to the SCE and a reductive response to negative
ones. A representative voltammogram is shown in Fig. 3 and
voltammetric data are presented in Table 3. Both the responses
are believed to be centered on the coordinated 2-(arylazo)-
1H NMR spectra of all the complexes have been recorded in
CDCl3 solution. A distinct methyl resonance is displayed by all
the five complexes near δ 1.7 which is assigned to the methyl
group in the p-cresol fragment of the 2-(arylazo)phenolate
ligands. Additional methyl signals are observed for [Rh(PPh3)2-
(ap-Me)Cl] and [Rh(PPh3)2(ap-OMe)Cl] at δ 1.86 and 3.45
J. Chem. Soc., Dalton Trans., 2000, 4623–4627
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Table 3 Electronic spectral and cyclic voltammetric data
a
Compound
λmax/nm (ε/MϪ1 cmϪ1
)
E/V vs. SCEa,b
[Rh(PPh3)2(ap-OMe)Cl]
630 (10500), 585 (9000), 535c (4300), 420c (3600), 380 (7000), 355 (17000), 290 (47300),
230 (60000)
0.65d (100),e Ϫ1.48f
[Rh(PPh3)2(ap-Me)Cl]
[Rh(PPh3)2(ap-H)Cl]
[Rh(PPh3)2(ap-Cl)Cl]
630 (8700), 610 (7600), 550c (4200), 385c (11500), 360c (18200), 290 (44500), 230 (59000)
630 (6600), 590 (2300), 355c (19700), 545c (3100), 290 (48500), 230 (6300)
650 (9500), 610 (8300), 560c (4000), 430c (2000), 390c (15000), 365c (23800), 300 (50700),
230 (84400)
0.76d (117),e Ϫ1.50f
0.81d (100),e Ϫ1.48f
0.84d (280),e Ϫ1.44f
[Rh(PPh3)2(ap-NO2)Cl]
710 (11300), 650 (10200), 600c (5200), 420c (15200), 380c (25000), 305 (53800), 240
(102700)
1.10d (120),e Ϫ1.34f
d
a In dichloromethane. b Supporting electrolyte, NBu4ClO4. c Shoulder. E1/2 = 0.5 (Epa ϩ Epc) where Epa and Epc are anodic and cathodic peak
f
potentials respectively. e ∆Ep = Epa Ϫ Epc in mV. Epc value.
the Rh–C bond and such possibilities are currently under
investigation.
Acknowledgements
Financial assistance received from the Department of Science
and Technology, New Delhi [Grant No. SP/S1/F33/98] is
gratefully acknowledged. The authors thank the Third World
Academy of Sciences for financial support for the purchase of
an electrochemical cell system and the Bose Institute, Calcutta,
for NMR spectral measurements. Sincere thanks are due to the
referees for their constructive criticisms which have been very
helpful during the revision. S. D. thanks the CSIR, New Delhi,
for her fellowship.
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Fig. 3 Cyclic voltammogram of [Rh(PPh3)2(ap-H)Cl] in dichloro-
methane solution (0.1 M NBu4ClO4) at scan rate 50 mV sϪ1. A least-
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Conclusion
The present study shows that organometallic complexes of
rhodium() can be synthesized without much difficulty by
appropriate choice of the reactants, viz. the rhodium starting
material and organic ligand. Generation of other organo-
rhodium systems is in progress. These organorhodium
complexes may be expected to exhibit interesting reactivities of
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