Ligand control on molecular oxygen activation by rhodium quinone complexes

Article abstract of DOI:10.1021/ic990999v

Reaction of 9,10-phenanthrenequinone with [Rh(PPh₃)₃Cl] afforded a peroxysemiquinonate complex of rhodium(III) (1), presumably via the formation of a reactive rhodium(II)-semiquinone intermediate followed by reaction with molecular oxygen. Similar reaction of [Rh(PPh₃)₃Cl] with a more oxidizing quinone, viz. 3,4,5,6-tetrachloro-1,2-benzoquine, however, yielded a stable catecholate complex of rhodium(III) (2).

Full text of DOI:10.1021/ic990999v

Inorg. Chem. 2000, 39, 2231-2234
2231
activate molecular oxygen. We have tested this hypothesis using
Wilkinson’s catalyst, [Rh(PPh3)3Cl], as the source of rhodium-
(I) and two symmetric orthoquinones with significant difference
in their oxidizing capacities, viz. 9,10-phenanthrenequinone
(PBQ, 1) and 3,4,5,6-tetrachloro-1,2-benzoquinone (Cl4BQ, 2).
Ligand Control on Molecular Oxygen Activation
by Rhodium Quinone Complexes
Swati Dutta,† Shie-Ming Peng,‡ and
Samaresh Bhattacharya*,†
Department of Chemistry, Inorganic Chemistry Section,
Jadavpur University, Calcutta 700032, India, and
Department of Chemistry, National Taiwan University,
Taipei, Taiwan, ROC
ReceiVed August 18, 1999
Introduction
9,10-Phenanthrenequinone shows the two reductions at -0.70
and -1.21 V vs SCE, while 3,4,5,6-tetrachloro-1,2-benzo-
quinone undergoes two similar reductions at 0.08 and -0.59 V
vs SCE.14 Wilkinson’s catalyst has been chosen as the source
of rhodium because this tetracoordinated [Rh(PPh3)3Cl] complex
is known to release one PPh3 in solution15 and thus it offers
three available coordination sites of the metal, two of which
may be occupied by the quinone ligand leaving the pentacoor-
dinated rhodium approachable by a monodentate ligand. Reac-
tion of the above two quinones with [Rh(PPh3)3Cl] indeed
afforded two different complexes with remarkably different
properties, and a brief account of this chemistry is described
here.
Activation of molecular oxygen has been an attractive area
of chemical research1-11 with reference to utilization of the
cheapest oxidant for bringing about useful redox transformations.
Synthesis of transition metal complexes with potential ability
to interact with molecular oxygen has thus been of significant
importance. Herein we wish to disclose some interesting results
of our studies in this area involving quinone complexes of
rhodium. Rhodium offers two stable oxidation states, viz. +1
and +3, while the intermediate +2 state is very unstable.12 In
the present work we have studied the reaction between rhodium-
(I) and 1,2-benzoquinones. 1,2-Benzoquinones (BQ) are known
to undergo two successive one-electron reductions (eq 1)
Experimental Section
Materials. Rhodium trichloride was obtained from Johnson Matthey,
and triphenylphosphine was purchased from Loba, Mumbai, India. [Rh-
(PPh3)3Cl] was synthesized by following a reported procedure.16 3,4,5,6-
Tetrachloro-1,2-benzoquinone was purchased from Aldrich, and 9,10-
phenanthrenequinone was purchased from Merck, Mumbai, India.
Purification of dichloromethane and acetonitrile and preparation of
tetrabutylammonium perchlorate (TBAP) for electrochemical work were
performed as before.17,18
Preparation of [Rh(PPh3)2(O2-PSQ)Cl]. 9,10-Phenanthrenequinone
(23 mg, 0.11 mmol) was dissolved in dry benzene (40 cm3), and to it
was added [Rh(PPh3)3Cl] (100 mg, 0.11 mmol). The mixture was then
stirred for 4 h to produce a red solution. On partial evaporation of the
solution [Rh(PPh3)2(O2-PSQ)Cl] separated as a reddish-orange crystal-
line solid, which was then collected by filtration, washed thoroughly
with hexane, and dried in air. Yield: 75%. Anal. Calcd for C50H38O4-
ClP2Rh: C, 66.50; H, 4.21. Found: C, 66.56; H, 4.25.
Preparation of [Rh(PPh3)2(Cl4Cat)Cl]. This was prepared by
following the same above procedure using 3,4,5,6-tetrachloro-1,2-
benzoquinone instead of 9,10-phenanthrenequinone. Yield: 70%. Anal.
Calcd for C42H30O2Cl5P2Rh: C, 55.48; H, 3.30. Found: C, 55.47; H,
3.31.
Peparation of [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]. [Rh(PPh3)2(Cl4-
Cat)Cl] (50 mg, 0.05 mmol) was dissolved in acetonitrile. Partial
evaporation of the solvent afforded [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl] as
a violet crystalline solid, which was collected by decanting the excess
solvent and dried in vacuo. Yield: 80%. Anal. Calcd for C44H33O2-
Cl5P2NRh: C, 55.61; H, 3.48; N, 1.47. Found: C, 55.62; H, 3.47; N,
1.48.
producing respectively the semiquinone radical anion (SQ) and
the catecholate ion (Cat).13 Similarly rhodium(I) is capable of
undergoing two one-electron oxidations to afford respectively
rhodium(II) and rhodium(III). Hence reaction of the quinones
with rhodium(I) may result in either a reactive RhII-SQ
intermediate or a stable RhIII-Cat complex. The fate of such
redox reaction is expected to be dictated solely by the quinone
ligand. Quinones with strong oxidizing power are likely to end
up giving stable RhIII-Cat complexes, whereas quinones that
are weaker oxidants may reach only up to the RhII-SQ stage.
The RhII-SQ complexes are expected to be susceptible to attack
by molecular oxygen, provided the metal is approachable by
dioxygen in these RhII-SQ complexes, and thus they may
† Jadavpur University.
‡ National Taiwan University.
(1) Chem. ReV. 1994, 94.
(2) Oxygen Metabolism. Chem. ReV. 1996, 96, pp 2541-2927.
(3) Tollman, W. Acc. Chem. Res. 1997, 30, 227.
(4) Hamstra, B. J.; Colpas, G. J.; Pecoraro, V. L. Inorg. Chem. 1998, 37,
949.
(5) Liu, X. Y.; Palacios, A. A.; Novoa, J. J.; Alvarez, S. Inorg. Chem.
1998, 37, 1202.
(6) Zauche, T. H.; Espenson, J. H. Inorg. Chem. 1998, 37, 6827.
(7) Butler, A. Coord. Chem. ReV. 1999, 187, 17.
(8) Kachadourian, R.; Batiuilc-Haberle, I.; Fridovich, I. Inorg. Chem. 1999,
38, 391.
(9) McGrady, J. E.; Stranger, R. Inorg. Chem. 1999, 38, 550.
(10) Tetzlaff, H. R.; Espenson, J. H. Inorg. Chem. 1999, 38, 881.
(11) Thapper, A.; Death, R. J.; Norlander, E. Inorg. Chem. 1999, 38, 1015.
(12) ComprehensiVe Coordination Chemistry; Pergamon Press: New York,
1987; Vol. 4, p 930.
(14) Potentials obtained from cyclic voltametry carried out by us.
(15) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; Wiley
Eastern: New Delhi, 1985; p 788.
(16) Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1967, 10, 67.
(17) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry for
Chemists; Wiley: New York, 1974; pp 167-215.
(13) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. ReV. 1981, 38, 45.
(18) Walter, M.; Ramaley, L. Anal. Chem. 1973, 45, 165.
10.1021/ic990999v CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/01/2000

2232 Inorganic Chemistry, Vol. 39, No. 10, 2000
Notes
Table 1. Crystallographic Data for [Rh(PPh3)2(O2-PSQ)Cl]‚C6H6
and [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]
empirical formula
fw
C56H44ClO4P2Rh
981.21
C44H33Cl5NO2P2Rh
949.81
space group
a, Å
b, Å
monoclinic, P21/C
10.5316(2)
19.9374(1)
22.4162(5)
90
96.712(1)
90
4674.53(14)
4
1.394
orthorhombic, Pbca
16.2358(7)
18.3175(7)
27.0680(8)
90
90
90
8050.0(5)
8
1.567
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcd, g cm-3
F
obsd, g cm-3
1.397
1.565
λ, Å
0.71073
0.40 × 0.10 × 0.10
22
0.538
0.0767
0.1317
1.024
0.71073
0.20 × 0.04 × 0.02
22
cryst size, mm
T, °C
µ, mm-1
R1a
0.876
0.1146
0.1246
1.379
wR2b
GOF on F2 c
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2.
Figure 1. View of the [Rh(PPh3)2(O2-PSQ)Cl] molecule.
Table 2. Selected Bond Distances and Bond Angles for
c GOF ) [∑[w(Fo - Fc2)2]/(M - N)]1/2, where M is the number of
2
reflections and N is the number of parameters refined.
[Rh(PPh3)2(O2-PSQ)Cl]‚C6H6 and [Rh(PPh3)2(Cl4Cat)(CH3CN)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 were recorded on a Shimadzu UV-1601
spectrophotometer. Magnetic susceptibilities were measured using a
[Rh(PPh3)2(O2-PSQ)Cl]‚C6H6
Bond Distances (Å)
Rh-O(1)
Rh-O(2)
Rh-O(3)
Rh-P(1)
2.015(4)
2.179(4)
2.040(4)
2.267(2)
2.336(2)
2.365(2)
1.475(8)
C(1)-O(1)
C(1)-O(4)
C(14)-O(2)
O(3)-O(4)
C(1)-C(2)
C(1)-C(14)
1.362(7)
1.472(7)
1.247(7)
1.493(5)
1.528(8)
1.497(9)
1
PAR 155 vibrating-sample magnetometer. H and 31P NMR spectra
were obtained on a Brucker drx500 NMR spectrometer using TMS as
the internal standard. Solid-state thermal investigations were carried
out with the help of a Shimadzu DT-30 thermal analyzer. Electrochemi-
cal measurements were made using a PAR model 273 potentiostat. A
platinum-disk 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. Electrochemical measurements were made
under a dinitrogen atmosphere. All electrochemical data were collected
at 298 K and are uncorrected for junction potentials.
Crystallography of [Rh(PPh3)2(O2-PSQ)Cl]‚C6H6. Single crystals
were grown by slow evaporation of a benzene solution of the complex.
Selected crystal data and data collection parameters are given in Table
1. Data were collected on a SMART CCD diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å) by ω scans. X-ray
data reduction, structure solution, and refinement were done using the
SHELXTL-PLUS package. The structure was solved by direct methods.
Crystallography of [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]. Single crys-
tals were grown by slow evaporation of an acetonitrile solution of [Rh-
(PPh3)(Cl4Cat)(CH3CN)Cl]. Selected crystal data and data collection
parameters are given in Table 1. Data collection and structure solution
and refinement were done similarly as above.
Rh-P(2)
Rh-Cl(1)
C(13)-C(14)
Bond Angles (deg)
O(3)-Rh-Cl(1)
O(2)-Rh-P(1)
O(1)-Rh-P(2)
166.83(12)
165.80(12)
169.43(12)
O(3)-Rh-O(1)
O(1)-Rh-O(2)
O(1)-Rh-P(1)
O(3)-Rh-P(2)
O(2)-Rh-P(2)
O(2)-Rh-Cl(1)
P(2)-Rh-Cl(1)
O(3)-Rh-O(2)
O(3)-Rh-P(1)
P(1)-Rh-P(2)
O(1)-Rh-Cl(1)
P(1)-Rh-Cl(1)
82.7(2)
77.6(2)
O(1)-C(1)-C(14)
O(1)-C(1)-C(2)
C(14)-C(1)-C(2)
O(1)-C(1)-O(4)
O(4)-C(1)-C(14)
O(4)-C(1)-C(2)
O(2)-C(14)-C(1)
O(2)-C(14)-C(13)
C(13)-C(14)-C(1)
O(4)-O(3)-Rh
110.1(5)
113.1(5)
114.8(5)
110.5(5)
102.6(5)
105.0(5)
117.8(6)
121.4(6)
120.7(6)
108.6(2)
106.8(4)
88.19(12)
89.40(12)
94.01(12)
88.24(12)
92.93(6)
78.7(2)
98.76(12)
99.94(6)
93.28(12)
93.60(6)
C(1)-O(4)-O(3)
[Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]
Bond Distances (Å)
Rh-O(1)
Rh-O(2)
Rh-N(1)
Rh-P(1)
Rh-P(2)
Rh-Cl(1)
C(1)-O(1)
C(1)-C(2)
1.995(6)
2.055(6)
2.009(8)
2.349(3)
2.328(3)
2.408(3)
1.344(10)
1.375(13)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(1)
C(6)-O(2)
C(7)-N(1)
1.384(13)
1.398(14)
1.403(14)
1.399(12)
1.418(13)
1.333(11)
1.117(11)
Results and Discussion
Reaction of 9,10-phenanthrenequinone with [Rh(PPh3)3Cl] at
ambient temperature afforded the [Rh(PPh3)2(O2-PSQ)Cl] com-
plex whose identity has been revealed by its molecular structure
determination by X-ray crystallography. A view of the complex
molecule is shown in Figure 1, and selected bond parameters
are listed in Table 2. The quinone ligand has undergone
interesting chemical transformation. One oxygen molecule has
been occluded in this complex which is present in the peroxo
form bridging the metal and the tetrahedral carbon of the
quinone ligand. The two PPh3 ligands have occupied cis
positions. The O3P2Cl coordination sphere around rhodium is
significantly distorted from ideal octahedral geometry, which
is reflected in the bond parameters, particularly in the O(2)-
Rh-P(2), O(1)-Rh-P(1), and O(3)-Rh-Cl bond angles.
Bond Angles (deg)
O(1)-Rh-N(1)
O(2)-Rh-P(1)
P(2)-Rh-Cl(1)
177.9(3)
169.7(2)
165.20(10)
O(1)-Rh-O(2)
O(1)-Rh-P(1)
O(1)-Rh-P(2)
O(1)-Rh-Cl(1)
O(2)-Rh-P(2)
O(2)-Rh-Cl(1)
N(1)-Rh-O(2)
83.6(3)
90.1(2)
93.5(2)
89.8(2)
79.4(2)
86.6(2)
94.8(3)
N(1)-Rh-P(1)
P(1)-Rh-Cl(1)
N(1)-Rh-P(2)
P(2)-Rh-P(2)
N(1)-Rh-Cl(1)
C(7)-N(1)-Rh)
N(1)-C(7)-C(8)
91.4(2)
85.30(9)
87.4(2)
109.10(10)
88.9(2)
177.8(9)
177.1(12)

Notes
Inorganic Chemistry, Vol. 39, No. 10, 2000 2233
Table 3. Electronic Spectral and Cyclic Voltammetric Data
electronic spectral data:
max, nm (ꢀ, M-1 cm-1
cyclic voltammetric data:
compd
solvent
λ
)
E, V vs SCE
[Rh(PPh3)2(O2-PSQ)Cl]
[Rh(PPh3)2(Cl4Cat)Cl]
CH2Cl2
CH2Cl2
400 (3500), 310 (11 000), 255 (43 000), 250 (42 000), 225 (39 300)
700 (3800), 470 (2200), 380 (2000), 280 (32 500), 250 (44 400),
215 (85 000)
0.90,a -1.00b
0.85c (300),d -0.90b
[Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]
CH3CN
510 (641), 315 (9800), 250 (30 000), 220 (53 000), 205 (47 000)
0.65c (60),d -1.04b
b
c
a Epa value. Epc value. E1/2 ) 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potential, respectively. d ∆Ep ) Epa - Epc in mV.
Formation of this [Rh(PPh3)2(O2-PSQ)Cl] complex may be
rationalized as shown in Scheme 1. 9,10-Phenanthrenequinone,
spectrum of this complex, recorded in CDCl3 solution, shows a
number of overlapping resonances in the aromatic (7.0-8.0
ppm) region due to signals from the O2-PSQ ligand as well as
the two nonequivalent PPh3 ligands. However, the 31P NMR
spectrum of the same complex shows only two distinct
resonances at 43.22 and 43.87 ppm which is in accordance with
the observed structure of this complex. The electronic spectrum
of [Rh(PPh3)2(O2-PSQ)Cl], recorded in dichloromethane solu-
tion, shows three very intense absorptions in the UV region
together with two intense absorptions in the visible region (Table
3). The absorptions in the UV region are assignable to transitions
within the ligand orbitals while those in the visible region are
probably due to metal-to-ligand charge-transfer transitions.
Besides small differences in band positions and intensities, the
electronic spectrum of [Rh(PPh3)2(O2-PSQ)Cl] in acetonitrile
solution is qualitatively similar to that observed in the dichlo-
romethane solution.
Scheme 1
To test whether two-electron oxidation at the metal center
can be brought about by the quinone ligand alone and thus
participation of molecular oxygen in the synthetic reaction (as
shown in Scheme 1) can be chemically controlled, a reaction
was carried out between 3,4,5,6-tetrachloro-1,2-benzoquinone
and [Rh(PPh3)3Cl] under identical experimental conditions as
before. The product obtained from this reaction was indeed
found to be much different. Microanalytical data indicated that
the complex molecule contains two PPh3 ligands, one chloride,
and the quinone ligand coordinated to rhodium. The infrared
spectrum of this complex shows the ν(Rh-Cl) stretch at 325
cm-1 and strong vibrations due to the Rh(PPh3)2 fragment within
500-750 cm-1 as before. However the IR spectrum did not
show any ν(CdO) or ν(-O-O-) stretch. This indicates that
the quinone ligand is coordinated in the catecholate form. The
complex molecule may therefore be represented as [RhIII-
(PPh3)2(Cl4Cat)Cl]. This complex is diamagnetic as expected,
and its 1H NMR spectrum in CDCl3 solution shows the aromatic
proton resonances of the PPh3 ligands within 7.1-7.5 ppm. The
31P NMR spectrum shows a single sharp resonance at 46.87
ppm, which indicates that the two PPh3 ligands are magnetically
equivalent in this complex, and hence there are three possible
geometries for this complex molecule, one square pyramidal
(3) and two trigonal bipyramidal (4) and (5). Single crystals of
being a mild oxidant, oxidizes rhodium(I) to rhodium(II) and
itself gets coordinated in the semiquinone form. This pentaco-
ordinated and unstable [RhII(PPh3)2(PSQ)Cl] complex then
undergoes further oxidation by aerial oxygen to generate the
superoxo complex which is followed by obvious C-O bond
formation between two close radical centers to afford the [Rh-
(PPh3)2(O2-PSQ)Cl] complex. It may be noted here that there
are very few reports of peroxysemiquinone complexes in the
literature.19 The Rh-O, Rh-P, and Rh-Cl distances are quite
normal.20 The C14-O2 distance indicates that it is a localized
double bond, while the C1-O1 and C1-O4 distances are much
longer which confirms their single bond character. The O-O
distance compares well with the O-O distances in structurally
characterized peroxo complexes.20 The C1-C2, C1-C14, and
C14-C13 lengths indicate their single bond character, as
expected.
Infrared spectrum of the [Rh(PPh3)2(O2-PSQ)Cl] complex
shows a strong ν(CdO) vibration at 1580 cm-1 and a distinct
vibration at 850 cm-1 due to the bridging peroxo fragment.21
Strong vibrations are observed at 508, 519, 534, 678, 690, and
740 cm-1, and these may be attributed to the cis-Rh(PPh3)2
fragment as similar cis-M(PPh3)2 (M ) Ru, Os) fragments are
known to display such vibrations.22-24 The ν(Rh-Cl) stretch
is observed at 330 cm-1 25
. The [Rh(PPh3)2(O2-PSQ)Cl] complex
is diamagnetic, which corresponds to the trivalent state of
1
rhodium (low-spin d6, S ) 0) in this complex. The H NMR
(19) Barbaro, P.; Bianchini, C.; Linn, K.; Mealli, C.; Meli, A.; Vizza, F.
Inorg. Chim. Acta 1992, 198-200, 31.
(20) Benett, M. J.; Donaldson, P. B. Inorg. Chem. 1977, 99, 1581.
(21) Chaudhuri, M. K.; Ghosh, S. K. Inorg. Chem. 1982, 60, 4020.
(22) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 1511.
(23) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 2906.
(24) Pramanik, N. C.; Bhattacharya, S. Transition Met. Chem. 1999, 24,
95.
this complex could not be grown, and hence, this problem of
stereochemistry could not be sorted out. However, in view of
the structure of [Rh(PPh3)2(O2-PSQ)Cl], the square pyramidal
geometry (3) may be assumed for this [Rh(PPh3)2(Cl4Cat)Cl]
complex.
(25) Deb, A. K.; Goswami, S. J. Chem. Soc., Dalton. Trans. 1989, 1635.

2234 Inorganic Chemistry, Vol. 39, No. 10, 2000
Notes
that removal of acetonitrile from the coordination sphere of [Rh-
(PPh3)2(Cl4Cat)(CH3CN)Cl] should result in formation of the
pentacoordinated [Rh(PPh3)2(Cl4Cat)Cl] complex associated
with a stereochemical change from octahedral to square
pyramidal. This was examined by thermal analysis of the [Rh-
(PPh3)2(Cl4Cat)(CH3CN)Cl] complex. At 210 °C an endothermic
change occurs associated with the loss of ∼40 g mass/mol of
the complex, which points to loss of the coordinated acetonitrile.
The product obtained after this heat treatment has indeed been
found to be [Rh(PPh3)2(Cl4Cat)Cl], identified by microanalysis
and spectroscopic properties. This shows that chemical inter-
conversion between [Rh(PPh3)2(Cl4Cat)Cl] and [Rh(PPh3)2-
(Cl4Cat)(CH3CN)Cl] can be performed without any diffi-
culty.
The electronic spectrum of [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]
in acetonitrile solution shows rhodium-to-catechol charge-
transfer transition in the visible region and usual intense
transitions in the UV region (Table 3). The 1H NMR spectrum
of this diamagnetic [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl] complex
shows the lack of symmetry in the complex molecule. The
aromatic protons of the PPh3 ligands show several resonances
within 7.1-7.8 ppm. The methyl signal of coordinated aceto-
nitrile is observed at 1.24 ppm. The 31P NMR spectrum shows
a relatively broad signal centered at 46.92 ppm due to overlap
of the two signals arising from the two PPh3 ligands.
Electron-transfer properties of all three complexes have been
studied by cyclic voltammetry. Each complex shows an oxida-
tive response on the positive side of SCE and a reductive
response on the negative side (Table 3). The reduction may be
assigned to rhodium(III)-rhodium(II) reduction, while the oxida-
tion is believed to be ligand (O2-PSQ or Cl4Cat) centered.
Generation of other RhII-SQ complexes using different types
of quinone ligands and their reactivity with molecular oxygen
are currently under investigation. Utilization of [Rh(PPh3)2(O2-
PSQ)Cl] and similar complexes as catalysts to bring about useful
oxo-transfer reactions is also being explored.
Figure 2. View of the [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl] molecule.
The electronic spectrum of [Rh(PPh3)2(Cl4Cat)Cl] in dichlo-
romethane solution shows intense metal-to-ligand (Cl4Cat)
charge-transfer transitions in the visible region together with
usual intense absorptions in the UV region. The spectrum of
[Rh(PPh3)2(Cl4Cat)Cl] in acetonitrile solution is observed to be
noticeably different, and this indicates coordination of the
solvent. This was scrutinized further by analyzing the crystalline
solid obtained from evaporation of a solution of [Rh(PPh3)2(Cl4-
Cat)Cl] complex in acetonitrile. The new complex was indeed
found to contain coordinated acetonitrile and is represented as
[Rh(PPh3)2(Cl4Cat)(CH3CN)Cl]. Formation of this complex has
been authenticated by its structure determination by X-ray
crystallography. The structure is shown in Figure 2, and selected
bond parameters are listed in Table 2. Rhodium is sitting in a
distorted octahedral NO2P2Cl coordination sphere with the two
PPh3 ligands in cis positions. The C-O bond lengths clearly
indicate catecholate nature of the Cl4Cat ligand.13 Acetonitrile
is coordinated to rhodium in the usual manner26 with slight
deviation from linearity around N(1) and C(7). This structure
determination thus proves that oxidation of Rh(I) to Rh(III) has
been done solely by the Cl4BQ ligand and therefore participation
of dioxygen was not necessary.
Acknowledgment. Financial assistance received from the
Council of Scientific and Industrial Research, New Delhi [Grant
No. 01(1408)/96/EMR-II], 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 700054, India, for NMR
spectral measurements. Thanks are also due to Professor N. Ray
Chaudhuri of the Indian Association for the Cultivation of
Science, Calcutta, India, for helping with the thermal analysis.
S.D. thanks the CSIR, New Delhi, for her fellowship.
Comparison of the compositions and geometries of [Rh-
(PPh3)2(Cl4Cat)Cl] and [Rh(PPh3)2(Cl4Cat)(CH3CN)Cl] suggests
(26) Pramanik, N. C.; Pramanik, K.; Ghosh, P.; Bhattacharya, S. Polyhedron
1998, 17, 1525.
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