Table 1 Crystallographic data for [Rh(PPh3)2(HL2)(L2)]
Microanalyses (C, H, N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. IR spectra were obtained
on a Shimadzu FTIR-8300 spectrometer with samples pre-
pared as KBr pellets. Electronic spectra were recorded on a
JASCO V-570 spectrophotometer. Magnetic susceptibilities
were measured using a PAR 155 vibrating sample magneto-
meter fitted with a Walker Scientific L75FBAL magnet. 1H
NMR spectra were recorded in CDCl3 solution with a Bruker
Avance DPX 300 NMR spectrometer using TMS as the inter-
nal standard. Electrochemical measurements were done in 1:9
dichloromethane–acetonitrile solution (0.1 M TBAP) using a
CH Instruments model 600A electrochemical analyzer. The
addition of 10% dichloromethane was necessary to dissolve
the complexes and acetonitrile was necessary for detection of
the reductive response. A platinum disc working electrode, a
platinum wire auxiliary electrode and an aqueous saturated
calomel reference electrode (SCE) were used in a three-elec-
trode configuration. Electrochemical measurements were made
under a dinitrogen atmosphere. All electrochemical data were
collected at 298 K and are uncorrected for junction potentials.
Formula
C52H45N2O4P2Rh
Formula weight
Crystal system
Space group
926.75
Monoclinic
P21/n
11.168(3)
˚
a/A
˚
b/A
17.158(2)
23.129(6)
96.68(2)
4401.9(17)
4
˚
c/A
b/deg
3
˚
U/A
Z
T/K
100(2)
0.509
9565
7654
0.0908
m/mmꢀ1
Reflections collected
Independent reflections
Rint
R1 [I > 2s(I )]
wR2 [I > 2s(I )]
0.0578
0.1113
and refinement were done using the SHELXS-97 and
SHELXL-97 programs.11 Hydrogen atoms were fixed at
calculated positions and were refined using a riding mode.
The structure was solved by direct methods.y
Synthetic procedures
[Rh(PPh3)2(HL1)(L1)]. H2L1 (30 mg, 0.22 mmol) was taken
up in benzene (40 mL) and to it was added triethylamine (44
mg, 0.44 mmol); the flask was purged with a stream of nitrogen
for 5 min. Then [Rh(PPh3)3Cl] (100 mg, 0.11 mmol) was added
and the mixture was heated at reflux under a nitrogen atmo-
sphere for 12 h to yield an orange solution. Evaporation of this
solution gave a yellowish-orange solid, which was subjected to
purification by thin layer chromatography on a silica plate.
With benzene–acetonitrile (10:1) as the eluant, an orange band
separated, which was extracted with acetonitrile. Upon eva-
poration of the acetonitrile extract [Rh(PPh3)2(HL1)(L1)] was
obtained as a crystalline orange solid. Yield: 75%. Anal. calcd:
C, 66.82; H, 4.57; N, 3.12; found: C, 66.07; H, 4.53; N, 3.16%.
1H NMR (300 MHz, CDCl3): d 5.66 (d, J ¼ 7.6 Hz, 2H), 5.84
(t, J ¼ 7.0 Hz, 2H), 6.01 (s, 2H), 6.51 (d, J ¼ 8.2 Hz, 2H), 6.64
(t, J ¼ 6.9 Hz, 2H), 6.9–7.5 (2PPh3), 19.38 (s, 1H). UV-vis
(CH2Cl2): lmax/nm (e/Mꢀ1 cmꢀ1) 44210 (6200), 39810 (8200),
29610 (26 800).
Results and discussion
Reaction of the oxime ligands (H2L, 1) with [Rh(PPh3)3Cl]
proceeds smoothly in refluxing benzene in the presence of
triethylamine to afford the [Rh(PPh3)2(HL)(L)] complexes in
decent yields. Triethylamine has been used as a base to favor
the dissociation of the acidic protons. Elemental (C, H, N)
analytical data of the complexes are consistent with their com-
positions. The net synthetic reaction can be expressed as in
eqn. 1:
½RhiðPPh3Þ3Clꢁ þ 2H2L ! ½RhiiiðPPh3Þ2ðHLÞðLÞꢁ
þ PPh3 þ HCl þ 2Hþ þ 2eꢀ ð1Þ
It is interesting to note that rhodium has undergone a two-elec-
tron oxidation during the course of the synthetic reaction and
a trace of oxygen, present in the reaction vessel, might have
served as the oxidizing agent. It is also interesting to note that
the coordinated chloride in [Rh(PPh3)3Cl], which was found to
be retained in all of our earlier reactions,2 has been displaced
by the oxime ligands during the present reactions.
[Rh(PPh3)2(HL2)(L2)]. This complex was prepared by fol-
lowing the same procedure as above using H2L2 instead of
H2L1. Yield: 72%. Anal. calcd: C, 67.39; H, 4.86; N, 3.02;
found: C, 68.13; H 4.88; N, 3.07%. 1H NMR (300 MHz,
CDCl3): d 1.13 (s, 2CH3), 6.06–6.14 (4H), 6.69–6.82 (4H),
7.1–7.7 (2PPh3), 19.32 (s, 1H). UV-vis (CH2Cl2): lmax/nm
(e/Mꢀ1 cmꢀ1) 42210 (5400), 36410 (7800), 298 (39 300).
[Rh(PPh3)2(HL3)(L3)]. This complex was prepared by fol-
lowing the same procedure as for [Rh(PPh3)2(HL1)(L1)] using
H2L3 instead of H2L1. Yield: 75%. Anal. calcd: C, 69.75; H,
1
4.51; N, 2.81; found: C, 69.07; H, 4.57; N, 2.89%. H NMR
As the oxime ligands 1 are unsymmetric bidentate in nature,
these [Rh(PPh3)2(HL)(L)] complexes may exist, in principle, in
several geometric isomeric forms. To determine the stereo-
chemistry of these complexes, the structure of one representa-
tive member of this family, viz. [Rh(PPh3)2(HL2)(L2)], has been
determined by X-ray crystallography. A view of the complex
molecule is shown in Fig. 1 and selected bond parameters
are listed in Table 2. Both the partly deprotonated oxime
ligand (HL2) and the fully deprotonated oxime ligand (L2)
are coordinated to rhodium as bidentate N,O-donors, forming
six-membered chelate rings (2) with bite angles of ꢂ88ꢃ.
(300 MHz, CDCl3): d 6.89 (t, J ¼ 7.8 Hz, 2H), 6.99 (d,
J ¼ 9.0 Hz, 2H), 7.32 (d, J ¼ 8.3 Hz, 2H), 7.33 (s, 2H), 7.40
(d, J ¼ 6.9 Hz, 2H), 7.41–7.58 (4H), 7.0–7.8 (2PPh3), 19.41
(s, 1H). UV-vis (CH2Cl2): lmax/nm (e/Mꢀ1 cmꢀ1) 46810
(6900), 432 (9700), 33210 (17 250).
X-Ray crystallography
Single crystals of [Rh(PPh3)2(HL2)(L2)] were grown by slow
evaporation of an acetonitrile solution of the complex.
Selected crystal data and data collection parameters are given
in Table 1. Data were collected on a Bruker P4 diffractometer
using graphite-monochromated MoKa radiation (l ¼ 0.71069
suppdata/nj/b3/b309412j/ for crystallographic data in .cif or other
electronic format.
˚
A) by f and o scans. The data were corrected for empirical
absorption on the basis of psi scans. X-Ray structure solution
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
116
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 1 1 5 – 1 1 9