N-O bond cleavage and various bonding modes of the nitrato ligand in rhodium complexes

Article abstract of DOI:10.1016/S0020-1693(02)01466-4

The addition of 2 equiv. of AgNO₃ to a dinuclear chloro-bridged Rh(III) compound, [Cp*RhCl₂]₂, resulted in the replacement of the terminal chloro ligand to give [Cp*Rh(η¹-NO₃)(μ-Cl)]₂ (1). On the other hand, the reaction of [Cp*RhCl₂]₂ with 4 equiv. of AgNO₃ led to the formation of a mononuclear bis(nitrato)-rhodium compound, [Cp*Rh(η¹-NO₃)(η²-NO ₃)] (2). Compound 1 reacted with PPh₃ and cyclohexyl isocyanide (CN-C₆H₁₁) to give a chiral compound, {[Cp*RhCl(PPh₃)(CN-C₆H₁₁)] ₂(NO₂)(NO₃)}·2H₂O (3.2H₂O), which shows an unusual example of the nitrate-to-nitrite reduction in the rhodium coordination sphere. The reaction of compound 2 with either NaN₃ or N₃SiMe₃ gave a dinuclear (η¹-nitrato)-rhodium compound, [Cp*Rh(η¹-NO₃)(η¹,μ-N ₃)]₂ (4). On the other hand, on addition of silver triflate (AgOTf), compound 2 underwent replacement to give a mononuclear (η²-nitrato)-rhodium compound, [Cp*Rh(η²-NO₃)(OTf)] (5). In addition, compound 2 reacted with a mixture of PPh₃ and 2,6-dimethylphenyl isocyanide (CN-2,6-Me₂C₆H₃) to give trans-{[Rh(PPh₃)₂(CN-2,6-Me₂C ₆H₃)₂](NO₃)}·H ₂O (6·H₂O), which exhibits the complete elimination of all ligands in compound 2 and indicates the formal reduction of the rhodium metal from +3 to +1. The nitro-rhodium compound, {[Cp*₂Rh₂(η²,μ-NO ₂-N,O)₂(η¹,μ-N₃)](PF ₆)}·0.5H₂O (7·0.5H₂O), was prepared by the reaction of [Cp*RhCl(dppf)](PF₆) (dppf=Fe(η⁵-C₅H₄PPh₂) ₂) with a mixture of N₃SiMe₃ and AgNO₃, which also displays the N-O bond cleavage of nitrate (NO₃^-) to nitrite (NO₂^-). The structures of 1, 3·2H₂O, 4, 5, 6·H₂O, and 7·0.5H₂O were determined by X-ray diffraction.

Full text of DOI:10.1016/S0020-1693(02)01466-4

Inorganica Chimica Acta 348 (2003) 15ꢄ24
/
www.elsevier.com/locate/ica
Nꢀ
/
O bond cleavage and various bonding modes of the nitrato ligand
in rhodium complexes: syntheses and structures of [Cp*Rh(h1-
NO3)(m-Cl)]2, {[Cp*RhCl(PPh3)(CNꢀ
/
C6H11)]2(NO2)(NO3)}×
/
2H2O,
[Cp*Rh(h1-NO3)(h1,m-N3)]2, [Cp*Rh(h2-NO3)(O3SCF3)], trans-
2
ꢀ
{[Rh(PPh3)2(CNꢀ
/
2,6-Me2C6H3)2](NO3)}×
/
H2O, and {[Cp2 Rh2(h ,m-
NO2-N,O)2(h1,m-N3)](PF6)}×
/
0.5H2O
Won Seok Han, Soon W. Lee *
Department of Chemistry (BK21), Institute of Basic Science, Sungkyunkwan University, Natural Science Campus, Suwon 440-746, South Korea
Received 27 May 2002; accepted 11 November 2002
Abstract
The addition of 2 equiv. of AgNO3 to a dinuclear chloro-bridged Rh(III) compound, [Cp*RhCl2]2, resulted in the replacement of
the terminal chloro ligand to give [Cp*Rh(h1-NO3)(m-Cl)]2 (1). On the other hand, the reaction of [Cp*RhCl2]2 with 4 equiv. of
AgNO3 led to the formation of a mononuclear bis(nitrato)-rhodium compound, [Cp*Rh(h1-NO3)(h2-NO3)] (2). Compound 1
reacted with PPh3 and cyclohexyl isocyanide (CNꢀ
/
C6H11) to give
a
chiral compound, {[Cp*RhCl(PPh3)(CNꢀ
/
C6H11)]2(NO2)(NO3)}×
/
2H2O (3×
/
2H2O), which shows an unusual example of the nitrate-to-nitrite reduction in the rhodium
coordination sphere. The reaction of compound 2 with either NaN3 or N3SiMe3 gave a dinuclear (h1-nitrato)-rhodium compound,
[Cp*Rh(h1-NO3)(h1,m-N3)]2 (4). On the other hand, on addition of silver triflate (AgOTf), compound 2 underwent replacement to
give a mononuclear (h2-nitrato)-rhodium compound, [Cp*Rh(h2-NO3)(OTf)] (5). In addition, compound 2 reacted with a mixture
of PPh3 and 2,6-dimethylphenyl isocyanide (CNꢀ
/
2,6-Me2C6H3) to give trans-{[Rh(PPh3)2(CNꢀ
/
2,6-Me2C6H3)2](NO3)}×
/
H2O (6×
/
H2O), which exhibits the complete elimination of all ligands in compound 2 and indicates the formal reduction of the rhodium metal
2
1
ꢀ
from ꢁ
the reaction of [Cp*RhCl(dppf)](PF6) (dppfꢂ
Nꢀ
O bond cleavage of nitrate (NO3ꢃ) to nitrite (NO2ꢃ). The structures of 1, 3×
by X-ray diffraction.
# 2002 Elsevier Science B.V. All rights reserved.
/
3 to ꢁ
/
1. The nitro-rhodium compound, {[Cp2Rh2(h ,m-NO2-N,O)2(h ,m-N3)](PF6)}×
Fe(h5-C5H4PPh2)2) with a mixture of N3SiMe3 and AgNO3, which also displays the
2H2O, 4, 5, 6×H2O, and 7×0.5H2O were determined
/
0.5H2O (7×
/
0.5H2O), was prepared by
/
/
/
/
/
Keywords: NꢀO bond cleavage; Rhodium; Nitrate-to-nitrite reduction
/
1. Introduction
the nitrato ligand correlate with the activities of metal-
substituted carbonic anhydrases [2]. However, the
reactivity of nitrato complexes has been relatively
unexplored.
Chemical conversions of nitrogen oxides to nitrogen
dioxide have long been investigated. These reactions
proceed through the processes illustrated in Scheme 1:
(1) the oxidation of nitrogen monoxide, (2) the reduc-
tion of nitrogen trioxide, (3) the disproportionation of
nitrogen monoxide, and (4) the dissociation of dinitro-
Coordination chemistry of a nitrate anion (NO3ꢃ
)
has received considerable attraction because of inter-
esting linkage isomerisms and various bonding modes in
its compounds [1]. For example, Parkin and co-workers
very recently reported that the coordination modes of
* Corresponding author. Tel.: ꢁ
7075.
E-mail address: swlee@chem.skku.ac.kr (S.W. Lee).
/
82-31-290 7066; fax: ꢁ82-31-290
/
gen tetraoxide [3ꢄ/6]. Recently, Shapley and co-workers
reported the synthesis of Cp2Rh2(NO)(NO2) from
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01466-4

16
W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ24
/
spectrophotometer. Elemental analyses were performed
by the Korea Basic Science Institute.
2.1. [Cp*Rh(h1-NO3)(m-Cl)]2 (1)
Scheme 1.
[Cp*RhCl2]2 (100 mg, 0.12 mmol) and AgNO3 (41
mg, 0.24 mmol) in a mixed solvent (30 ml) of acetone
and dichloromethane (1:1) were stirred for 3 h, and then
the solvent was removed under vacuum. The resulting
CpRh(CO)2 by a two-step reaction (NO bubbling and
1
air oxidation). On the basis of H NMR, IR, and mass
spectra, they proposed the structure of this compound
to contain a bridging NO and a N,O-bridging NO2ꢃ
ligand [7].
solids were extracted with dichloromethane (30 ml ꢅ2),
and then the solvent was removed. The solid product
was washed with hexane (20 ml) and diethyl ether (20 ml
/
Several bonding types of the nitrato ligand are now
known [1]: symmetrical bidentate (A) [8], asymmetrical
bidentate (B) [2,9], monodentate (C) [10], long-bonded
symmetrical (D) [11], uncoordinate (E) [12], h1,m3(F)
[13], and h1,m4(G) [14]. The addition of two-electron,
neutral ligands converts A- and C-type compounds to
the C- and E-type ones, respectively, through the
modification of the hapticity of the nitrato ligand.
ꢅ
/
2), and then dried under vacuum (red crystals, 105
mg, 96.4%). M.p.ꢂ211ꢄ213 8C (dec.). IR (KBr): 1458
(NO). 1382 (NO) cmꢃ1. H NMR: d 1.61 (s, C5Me5).
13C{1H} NMR: d 94.58 (d, JRhꢀC
7.4 Hz, C5Me5),
/
/
1
ꢂ
/
9.11 (s, C5Me5). Anal. Calc. for C20H30Cl2N2O6Rh2: C,
35.79; H, 4.51; N, 4.17. Found: C, 35.56; H, 4.42; N,
4.13%.
2.2. [Cp*Rh(h1-NO3)(h2-NO3)] (2)
This compound was prepared analogously to com-
pound 1. [Cp*RhCl2]2 (100 mg, 0.12 mmol) was treated
with AgNO3 (82 mg, 0.48 mmol) to give orange crystals
(109 mg, 93.1%). Compound 2 was identified by the
comparison with literature data [17,18].
2.3. {[Cp*RhCl(PPh3)(CNꢀ
/
C6H11)]2(NO2)(NO3)}×
/
Because the Rh-nitrato compounds are somewhat
rare and because their reactivity has been relatively
unexplored, we set out to prepare this type of rhodium
compounds and investigate their reactivity. Herein, we
report the preparation and structures of rhodium-
nitrato or -nitro complexes, together with some exam-
2H2O (3×2H2O)
/
Compound 1 (100 mg, 0.28 mmol) and PPh3 (145 mg,
0.55 mmol) in dichloromethane (30 ml) were stirred for
3 h. Then, cyclohexyl isocyanide (68 ml, 0.55 mmol) in
dichloromethane (15 ml) was added. The reaction
mixture was further stirred for 3 h, and the solvent
was removed. The resulting yellow solids were washed
ples of their reactivity, including an unusual Nꢀ
/
O bond
cleavage of nitrato (NO3ꢃ) to nitro (NO2ꢃ).
with hexane (20 ml) and diethyl ether (20 ml ꢅ
then dried (yellow crystals, 132 mg, 90.4%). M.p.ꢂ
159ꢄ161 8C. IR (KBr): 2205 (CN), 1437 (NO), 1382
(NO) cmꢃ1. H NMR: d 1.62 (s, C5Me5), 1.98ꢄ
(10H, m, Cy), 4.04 (1H, br, ipso-H(Cy)), 7.69ꢄ
(15H, m, Ph). 13C{1H} NMR: d 105.50 (d, JRhꢀC
Hz, C5Me5), 9.45 (s, C5Me5), 57.37ꢄ
134.41ꢄ
128.68 (m, Ph). 31P{1H} NMR: d 35.23 (d,
JPꢀRh 122 Hz). Anal. Calc. for C70H84Cl2N4O7P2Rh2:
/
2), and
/
/
1
2. Experimental
/1.44
/7.47
All reactions have been performed under argon at
room temperature (r.t.). Rhodium(III) chloride hydrate,
silver nitrate, silver trifluoromethanesulfonate, triphe-
ꢂ/6.6
/
22.72 (m, Cy),
/
nylphosphine,
1,2,3,4,5-pentamethylcyclopentadiene
ꢂ
/
(Cp*), sodium azide, trimethylsilyl azide, 2,6-dimethyl-
phenyl isocyanide, and cyclohexyl isocyanide were
purchased. [Cp*RhCl2]2 [15] and [Cp*RhCl(dppf)](PF6)
C, 58.71; H, 5.91; N, 3.91. Found: C, 58.74; H, 5.81; N,
3.85%.
(dppfꢂ
literature methods. All products were recrystallized from
either dichloromethaneꢀpentane or dichloromethaneꢀ
hexane.
/
Fe(h5-C5H4PPh2)2) [16] were prepared by the
2.4. [Cp*Rh(h1-NO3)(h1,m-N3)]2 (4)
/
/
2.4.1. Method A
A dichloromethane solution (30 ml) containing com-
pound 2 (100 mg, 0.28 mmol) and NaN3 (54 mg, 0.83
mmol) was stirred for 24 h and filtered, and the solvent
was removed. The resulting solids were washed with
1H, 13C{1H}, and 31P{1H} NMR spectra were re-
corded with a Bruker AMX 500 MHz spectrometer. IR
spectra were recorded with a Nicolet Avatar 320 FTIR

W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ
/24
17
2
1
ꢀ
hexane (20 ml) and diethyl ether (20 ml ꢅ
/2), and then
2.7. {[Cp2Rh2(h ,m-NO2-N,O)2(h ,m-N3)](PF6)}×
/
0.5H2O (7×0.5H2O)
/
dried (orangeꢄred crystals, 84 mg, 89.2%).
/
[Cp*RhCl(dppf)](PF6) (100 mg, 0.12 mmol) and
N3SiMe3 (16 ml, 0.12 mmol) in a mixed solvent (30 ml)
of acetone and dichloromethane (1:1) were stirred for 24
h, and then AgNO3 (62 mg, 0.36 mmol) was added. The
reaction mixture was stirred for 12 h, and the solvent
was removed. The remaining solids were extracted with
2.4.2. Method B
A dichloromethane solution (30 ml) containing com-
pound 2 (100 mg, 0.28 mmol) and N3SiMe3 (109 ml, 0.83
mmol) was stirred for 24 h, and the solvent was
removed. The resulting solids were washed with hexane
dichloromethane (30 ml ꢅ
removed again. The resulting solids were washed with
hexane (20 ml) and diethyl ether (20 ml ꢅ2), and then
dried (orange crystals, 11 mg, 25.9%). M.p.ꢂ221ꢄ
/2), and the solvent was
(20 ml) and diethyl ether (20 ml ꢅ
mg, 84.7%). M.p.: 219ꢄ221 8C. IR (KBr): 2058 (N3),
1464 (NO), 1382 (NO) cmꢃ1 1H NMR: d 1.61 (s,
C5Me5). 13C{1H} NMR: d 94.14 (d, JRhꢀC
9.3 Hz,
C5Me5), 8.75 (s, C5Me5). Anal. Calc. for
C20H30N8O6Rh2: C, 35.10; H, 4.42; N, 16.38. Found:
C, 35.49; H, 4.50; N, 16.49%.
/
2), and then dried (80
/
/
.
/
/
ꢂ
/
223 8C. IR (KBr): 2057 (N3), 1459 (NO), 1378 (NO),
838 (PF) cmꢃ1. 1H NMR: d 1.72, 1.65, 1.63, 1.53 (all s,
C5Me5). 13C{1H} NMR: d 98.79 (d, JRhꢀC
ꢂ
/
8.8 Hz,
C5Me5), 8.96 (s, C5Me5). 31P{1H} NMR: d ꢃ
144.22
(sep, JPꢀF 695.9 Hz, PF6). Anal. Calc. for
/
ꢂ
/
C20H31F6N5O4.5PRh2: C, 31.43; H, 4.09; N, 9.17.
Found: C, 30.98; H, 4.15; N, 9.07%.
2.5. [Cp*Rh(h2-NO3)(OTf)] (5)
2.8. X-ray structure determination
Compound 1 (100 mg, 0.28 mmol) and AgOTf (71
mg, 0.28 mmol) in a mixed solvent (30 ml) of acetone
and dichloromethane (1:1) were stirred for 3 h, and then
the solvent was removed. The remaining solids were
All X-ray data were collected with the use of a
Siemens P4 diffractometer equipped with a Mo X-ray
tube and a graphite monochromator. Details on crystal
and intensity data are given in Table 1. The orientation
matrix and unit-cell parameters were determined by the
least-squares analyses of the setting angles of 23 (for 1),
extracted with dichloromethane (30 ml ꢅ
solvent was removed. The resulting solids were washed
with hexane (20 ml) and diethyl ether (20 ml ꢅ2), and
then dried (orange crystals, 105 mg, 84.7%). M.p.ꢂ
173ꢄ
175 8C. IR (KBr): 1471 (NO), 1384 (NO) cmꢃ1
1H NMR: d 1.80 (s, C5Me5). 13C{1H} NMR: d 93.99 (d,
JRhꢀC 10.1 Hz, C5Me5), 9.33 (s, C5Me5). Anal. Calc.
/2), and the
/
/
24 (for 3×
and 26 (for 7×
2u B25.08. Three check-reflections were measured every
/
2H2O), 29 (for 4), 28 (for 5), 23 (for 6×
/
H2O)
/
.
/
0.5H2O) reflections in the range of 15.0B
/
/
ꢂ
/
100 reflections throughout data collection. Intensity
data were corrected for Lp. Decay corrections were
also made. The intensity data were empirically corrected
for absorption with c-scan data. All calculations were
carried out with the use of SHELXTL programs [19].
All crystal structures were solved by direct methods.
All non-hydrogen atoms were refined anisotropically
for C11H15F3NO6SRh: C, 29.41; H, 3.37; N, 3.12; S,
7.14. Found: C, 28.96; H, 3.50; N, 3.09; S, 7.19%.
2.6. trans-{[Rh(PPh3)2(CNꢀ
/
2,6-Me2C6H3)2](NO3)}×
/
H2O (6×H2O)
/
except the fluorine atoms in compound 7×
/
0.5H2O, in
which the counter-anion (PF6ꢃ) exhibited an extreme
structural disorder and, therefore, the fluorine atoms
were refined isotropically. All hydrogen atoms were
generated in ideal positions and refined in a riding
model.
Compound 1 (100 mg, 0.28 mmol) and PPh3 (145 mg,
0.55 mmol) in dichloromethane (30 ml) were stirred for
3 h. 2,6-Dimethylphenyl isocyanide (72 mg, 0.55 mmol)
in dichloromethane (15 ml) was then slowly added. The
reaction mixture was further stirred for 3 h, and the
solvent was removed. The resulting solids were washed
with hexane (20 ml) and diethyl ether (20 ml ꢅ
then dried (yellow crystals, 114 mg, 42.6%). M.p.ꢂ
157ꢄ159 8C. IR (KBr): 2122 (CN), 1434 (NO), 1384
(NO) cmꢃ1. H NMR: d 1.64 (s, Me2C6H3), 1.62 (s,
Me2C6H3), 7.03ꢄ6.84 (m, Me2C6H3), 7.67ꢄ7.33 (30H,
m, Ph). 13C{1H} NMR: d 134.86ꢄ
128.35 (m, Me2C6H3
and Ph), 18.59 (s, Me2C6H3). 31P{1H} NMR: d 31.55
(d, JPꢀRh 122 Hz). Anal. Calc. for C54H50N3O4P2Rh:
/
2), and
/
3. Results and discussion
/
1
3.1. Preparation
/
/
/
All compounds (1ꢄ7) in this study are air-stable both
/
in solution and in the solid state. An 18-electron,
dinuclear compound [Cp*RhCl2]2 reacts with the stoi-
chiometric amount of AgNO3 (2 equiv. for 1 and 4
equiv. for 2) in a mixed solvent of acetone and
ꢂ
/
C, 66.87; H, 5.20; N, 4.33. Found: C, 66.99; H, 5.28; N,
4.29%.

18
W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ24
/
Table 1
X-ray data collection and structure refinement
1
3×
/
2H2O
4
5
6×
/
H2O
7×
/
0.5H2O
Formula
fw
C
20H30Cl2N2O6Rh2 C70H84Cl2N4O7P2Rh2 C20H30N8O6Rh2 C11H15F3NO6SRh C54H50N3O4P2Rh C20H31F6N5O4.5PRh2
671.18
296(2)
1432.07
296(2)
684.34
295(2)
449.21
295(2)
969.82
296(2)
triclinic
P1
764.29
296(2)
triclinic
Temperature (K)
Crystal system
Space group
monoclinic
P21/n
triclinic
P1
othorhombic
Pbca
monoclinic
P21/n
¯
P1
˚
a (A)
8.3874(5)
10.4549(6)
14.8585(9)
10.319(1)
10.558(1)
17.371(1)
97.011(8)
94.348(11)
112.154(6)
1724.5(3)
1
14.454(3)
12.472(3)
14.767(3)
7.4528(6)
13.4580(9)
16.9124(14)
9.359(2)
11.098(2)
12.374(2)
73.71(1)
71.41(1)
80.67(1)
1165.5(3)
1
12.9148(9)
14.7295(9)
15.9260(15)
74.367(6)
81.960(5)
88.816(5)
2888.4(4)
4
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
104.385(5)
102.344(7)
3
˚
V (A )
Z
1262.1(1)
2
2662(1)
4
1657.1(2)
4
dcalc (g cmꢃ3
)
1.766
1.556
672
1.379
0.656
1.707
1.289
1376
1.801
1.213
896
1.382
0.485
1.758
1.273
m (mmꢃ1
F(000)
Tmin
)
742
0.5742
0.8705
502
0.4171
0.4356
1524
0.3664
0.7780
0.6937
0.9126
0.2299
0.2865
0.4048
0.7977
Tmax
2u range (8)
Scan type
3.5ꢄ
/
50
3.5ꢄ
/
50
3.5ꢄ
/
50
3.5ꢄ
/
50
3.5ꢄ
/
50
3.5ꢄ50
/
v
variable
2364
v
variable
6401
v
Variable
2315
v
variable
3122
v
variable
4325
v
variable
10 458
Scan speed
Number of reflec-
tions measured
Number of reflec-
tions unique
2204
2062
146
6401
5715
797
2315
2074
164
2887
2725
209
4325
3912
562
9990
8552
703
Number of reflec-
tions with I ꢀ2s(I)
Number of para-
meters refined
/
ꢃ3
˚
Max., in Dr (e A
)
)
0.298
0.603
0.373
0.372
0.289
1.449
ꢃ3
˚
Min., in Dr (e A
ꢃ
/
0.300
ꢃ
/
0.739
ꢃ
/
0.431
ꢃ
/
0.572
ꢃ
/
0.389
ꢃ0.603
/
GOF on F2
1.027
0.0298
0.0623
1.050
0.0347
0.0897
1.047
0.0272
0.0733
1.066
0.0273
0.0730
1.030
0.0297
0.0722
1.041
0.0431
0.1129
a
R1
b
WR2
a
R1ꢂ
wR2ꢂ
/
S½½Fo½ꢃ
/
½Fc½½S½Fo½.
b
/
S[w(Fo 2ꢃ
/
Fc 2)2]/S[w(Fo 2)2]1/2
.
dichloromethane at room temperature to give a di-
nuclear Rh(III) compound, [Cp*Rh(h1-NO3)(m-Cl)]2
The Rh metal in compounds 1 and 2 has a formal
electron count of 18. Compound 2 was previously
prepared by stirring [Cp*Rh(N3)2]2 in nitrogen tetra-
oxide (N2O4) [17], and its crystal structure was reported
by Hursthouse and co-workers [18]. Therefore, the
structure of compound 2 could be confirmed by
comparing its spectral and crystallographic data with
(1), and
a
mononuclear Rh(III) compound,
[Cp*Rh(h1-NO3)(h2-NO3)] (2), in high isolation yields
(96.4 and 93.1%, respectively) (Scheme 2).
1
those for the genuine compound. In H NMR spectra,
the Cp* methyl protons of compound 1 appear as a
singlet at d 1.61 ppm, which suggests a high molecular
symmetry. In IR spectra of 1, the bands at 1458 and
1382 cmꢃ1 can be assigned to the Nꢀ
/O stretching.
Compound 3 is prepared by the stepwise addition of
two ligands to compound 1. In the first step, PPh3 is
added to compound 1 in dichloromethane, and the
solution is stirred for 3 h. In the second step, cyclohexyl
isocyanide is added, and the resultant solution is stirred
further for
[Cp*RhCl(PPh3)(CNꢀ
90.4% yield (Eq. (1)).
3
h
to give
a
C6H11)]2(NO2)(NO3)
chiral compound,
(3) in
/
Scheme 2.

W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ
/24
19
NO3) transformation. In 1H NMR spectra of 4, the Cp*
methyl protons appear as a singlet at d 1.61 ppm,
probably due to the symmetrically bridging mode of the
azido ligands. As expected, the strong IR band at 2058
cmꢃ1, assigned to the N3 bond, is consistent with those
found for azido-bridged dinuclear Rh(III) complexes
[17].
ð1Þ
We speculate that compound 1 converts from a
dinuclear species to a mononuclear intermediate due to
the coordination of the incoming PPh3 in the first step.
In the second step, the cyclohexyl isocyanide replaces
the nitrato ligand, which now acts as a counterion.
Interestingly, the X-ray structure of compound 3 (see
below) reveals that it has two different counter-anions,
nitrate (NO3ꢃ) and nitrite (NO2ꢃ) ions. The formation
Compound 2 reacts with AgOTf in a mixed solvent of
acetone and dichloromethane to give a bidentate
nitratoꢀ
Rh(III) compound, [Cp*Rh(h2-NO3)(OTf)]
/
(5), in 84.7% yield (Scheme 3). Apparently, the mono-
dentate nitrato ligand has been replaced by the incoming
triflate during the reaction.
An ionic compound, trans-{[Rh(PPh3)2(CNꢀ
/
2,6-
of the nitrite counterion indicates the Nꢀ
/
O bond
Me2C6H3)2](NO3)}×H2O (6×H2O), has been prepared
/
/
cleavage of the nitrato ligand during the reaction.
Unfortunately, we cannot explain this unusual nitrate-
to-nitrite reduction. However, this phenomenon has also
been observed in the preparation of compound 7 (see
below). Related to the chiral compound 3, Jones and co-
workers previously reported achiral Rh-isocyanide com-
by the two-step addition of PPh3 and 2,6-dimethylphe-
nyl isocyanide to compound 2 (Scheme 3). The mole-
cular geometry has been changed from a pseudo-
octahedron to a square plane during the reaction.
Both nitrato ligands in compound 2 have transformed
to nitrate counterions in compound 6. In this reaction,
pounds of the type Cp*Rh(CNR)Cl2 (Rꢂt-Bu or 2,6-
xylyl) from [Cp*RhCl2]2 and CNR in tetrahydrofuran
[20]. In IR spectra of 3, the band at 2205 cmꢃ1 can be
/
the Rh metal has been formally reduced from ꢁ
/
3 to ꢁ1.
/
It is worth noting that the strongly bound Cp* ligand
has been eliminated during the reaction. Unfortunately,
we cannot give an explanation for the unusual stoichio-
metry and reactivity (Cp* group elimination and Rh
metal reduction). 1H NMR spectra of 6 display two
singlets at d 1.64 and 1.62 ppm, which indicates
diastereotopic methyl protons of 2,6-dimethylphenyl
isocyanide. In 31P{1H} NMR spectra of 6, PPh3 ligands
assigned to the CÅ/N stretching.
We prepared an azido-bridged dinuclear Rh(III)
compound, [Cp*Rh(h1-NO3)(h1,m-N3)]2 (4), in 89.2%
yield, by treating compound 2 with excess NaN3 (3
equiv.). Compound 4 could also be prepared from
compound 2 and excess N3SiMe3 (3 equiv.) in 84.7%
yield (Scheme 3). Compound 4 contains bridging azido
and terminal h1-NO3 ligands. In this reaction, the
incoming N3ꢃ has appeared to replace the h2-NO3
ligand, although we cannot rule out the possibility of the
appear as a doublet at d 31.55 ppm (JPꢀRh
due to the coupling of the Rh metal with the equivalent
ꢂ122 Hz),
/
P atoms. In IR spectra of 6, the band at 2122 cmꢃ1 can
replacement of h1-NO3 followed by the (h2-NO3)0(h1-
/
be assigned to the CÅ
/
N stretching.
Rh(III)
{[Cp2 Rh2(h ,m-NO2-N,O)2(h ,m-N3)](PF6)}×
(7×0.5H2O), has been prepared by the stepwise addition
An ionic, dinuclear
compound,
0.5H2O
2
1
ꢀ
/
/
of two ligands. An 18-electron, ionic compound
[Cp*RhCl(dppf)](PF6) reacts with N3SiMe3 in a mixed
solvent of acetone and dichloromethane for 24 h, and
the resultant solution is further treated with 3 equiv. of
AgNO3 to give compound 7 in 25.9% yield (Eq. (2)).
During the reaction, the dppf ligand has been elimi-
nated, and the mononuclear complex has been con-
verted to the dinuclear one. On the contrary, Yamamoto
and co-workers reported that [Cp*RhCl(dppf)](PF6)
reacted with excess NaPF6 (5 equiv.) in the presence of
excess AgNO3 (10 equiv.) in acetonitrile (MeCN) to give
an
ionic,
solvent-coordinated
compound
[Cp*Rh(dppf)(MeCN)][PF6]2 [16]. In other words, the
NO3ꢃ ion has not been incorporated into the Rh
1
coordination sphere in their reaction. H NMR spectra
of 7 display four singlets at d 1.72, 1.65, 1.63, and 1.53
ppm, which clearly indicates the Cp* groups to be
inequivalent.
Scheme 3.

20
W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ24
/
The molecular structure of compound 4 is shown in
Fig. 3. Compound 4 has two bridging, end-on azido and
two terminal nitrato ligands. The coordination sphere of
the Rh metal can be described as a distorted octahedron.
The two Cp* ligands are trans to each other and
staggered. The end-on azido ligands are essentially
linear with the bond angle of 177.8(4)8. The Rhꢀ ꢀ ꢀRh
ð2Þ
˚
distance in compound 4 is 3.455(6) A.
The molecular structure of compound 5 is shown in
Fig. 4. The coordination sphere of Rh can be described
as either a pseudo-octahedron or a three-legged piano
stool. Two Rhꢀ
/
O bond lengths are essentially the same
˚
3.2. Structure
˚
(Rh1ꢀO1ꢂ2.183(2) A and Rh1ꢀ
/
/
/
O2ꢂ2.179(2) A).
/
Therefore, the nitrate ligand can be described as a
symmetric, bidentate ligand.
The molecular structure of compound 1 is shown in
Fig. 1. Compound 1 has two bridging chloro and two
terminal nitrato ligands. The coordination sphere of the
Rh metal can be described as a distorted octahedron, if
the coordination number of the Cp* ligand is taken to
be three. Compound 1 has a center of symmetry, which
probably explains the singlet for Cp* methyl protons in
its 1H NMR spectrum. The two Cp* ligands are trans to
each other and staggered. The Rhꢀ ꢀ ꢀRh distance in
The molecular structure of compound 6 is shown in
Fig. 5. The coordination sphere of Rh can be described
as a square plane, with two trans isocyanide and two
trans triphenylphosphine ligands. The coordination
plane, defined by C37, C46, P1, P2, and Rh1 atoms, is
essentially planar with the average atomic displacement
ꢃ
˚
of 0.0026 A. The NO3 acts as a counterion and forms
˚
an ionic intramolecular hydrogen bond of the Oꢀ
Hꢀ ꢀ ꢀOꢀ
NO2ꢃ type with the co-crystallized water: O4ꢀ
H4Bꢂ
1.06 A, O3ꢀ ꢀ ꢀO4ꢂ
and O4ꢀH4BꢀO3ꢂ124.88. The Cꢀ
/
compound 1 is 3.590(4) A.
/
/
The structure of one of the two cations in compound 3
is shown in Fig. 2. The coordination sphere of Rh can
also be described as either a pseudo-octahedron or a
three-legged piano stool. Interestingly, an asymmetric
˚
˚
˚
/
/
2.599 A, H4Bꢀ ꢀ ꢀO3ꢂ
/
1.801 A
2,6-
/
/
/
/
NꢀR? (R?ꢂ
/
/
Me2C6H3) ligands are nearly linear with the bond angles
of 174(1) and 175(1)8.
unit
[Cp*RhCl(PPh3)(CNꢀ
ferent counterions, nitrite (NO2ꢃ) and nitrate (NO3ꢃ).
The CꢀNꢀR (RꢂC6H11) ligand is essentially linear
with the bond angle of 179(1)8.
has
two
chemically
equal
cations
/
C6H11)]ꢁ, together with two dif-
The molecular structure of compound 7 is shown in
Fig. 6. The coordination sphere of Rh can be described
as a pseudo-octahedron. The local geometry of the Rh
metal can also be described as a three-legged piano
/
/
/
˚
Fig. 1. ORTEP drawing of 1, showing the atom-labeling scheme and 50% probability thermal ellipsoids. Selected bond lengths (A) and bond angles
(8): Rh1ꢀ
N1ꢀO1 120.2(3), O3ꢀ
/
O1 2.132(2), Rh1ꢀ
/
Cl1 2.4548(7), O1ꢀ
/
N1 1.285(3), O2ꢀ
/
N1 1.226(4), O3ꢀ
/
N1 1.234(4); O1ꢀ
/
Rh1ꢀ
/
Cl1 80.30(6), N1ꢀ
/
O1ꢀ
/
Rh1 122.4(2), O2ꢀ
/
/
/
N1ꢀO1 116.3(3), O2ꢀ
/
/
N1ꢀ
/O3 123.5(3).

W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ
/24
21
˚
Fig. 2. ORTEP drawing of the cationic part of 3×
2.400(3), C29ꢀN1 1.18(2), N1ꢀC30 1.44(2); C29ꢀ
C30 179(1).
/
2H2O. Selected bond lengths (A) and bond angles (8): Rh1ꢀ
/
C29 1.93(1), Rh1ꢀ
/
P1 2.346(3), Rh1ꢀ
/
Cl1
/
/
/
Rh1ꢀP1 87.6(4), C29ꢀRh1ꢀCl1 94.6(4), P1ꢀRh1ꢀCl1 88.0(1), N1ꢀ
/
/
/
/
/
/
C29ꢀRh1 176(1), C29ꢀ
/
/
N1ꢀ
/
˚
2.113(4) A
lengths are essentially the same (Rh1ꢀ
/
N1ꢂ
2.116(4) A). The end-on azido ligand is
almost linear with the NꢀNꢀN bond angle of 178(1)8,
and the difference between the two NꢀN bond lengths is
/
˚
and Rh2ꢀ
/
N1ꢂ
/
/
/
/
˚
0.039 A. The Rhꢀ ꢀ ꢀRh distance in compound 7 is
The NO2ꢃ ion can behave as a nitrogen-donor ligand
˚
3.478(1) A.
to form nitro compounds (LnMꢀ
/
NO2) or as an oxygen-
ONO).
donor ligand to form nitrito compounds (LnMꢀ
/
Several bonding modes of the NO2ꢃ ligand are shown in
Chart 1 [21]. The NO2ꢃ ligand in compound 7 belongs
to the bridging type E.
3.3. Nꢀ
/
O bond cleavage of NO3ꢃ to NO2ꢃ
˚
Fig. 3. ORTEP drawing of 4. Selected bond lengths (A) and bond angles
(8): Rh1ꢀN1 2.166(3), Rh1ꢀO1 2.153(2), N1ꢀN2 1.134(5), N2ꢀN3
1.181(5), O1ꢀN4 1.298(4), O2ꢀN4 1.225(4), O3ꢀN4 1.240(4); O1ꢀ
Rh1ꢀN1 85.0(1), N4ꢀO1ꢀRh1 119.0(2), N1ꢀN2ꢀN3 177.8(4), O1ꢀ
N4ꢀO2 119.8(3), O1ꢀN4ꢀO3 116.6(4), O2ꢀN4ꢀO3 123.6(3).
The reduction of a nitrate ion (NO3ꢃ) is interesting in
probing reduction processes of nitrogen oxides in nature
[22]. Compounds 3 and 7 show a formal reduction of the
nitrato ligand. The molecular structure of compound 3
suggests that one of the nitrato ligands has been
converted to the nitrite ion (NO2ꢃ) by oxygen dissocia-
tion. Although we cannot explain how this conversion
occurs, the X-ray structure of compound 3 definitely
shows the nitrate-to-nitrite reduction. The molecular
structure of [Cp2Rh2(h ,m-NO2-N,O)2(h ,m-N3)](PF6)
(7) also suggests the formal reduction of the nitrato
ligand to the nitro ligand. Both NO2ꢃ ligands in
compound 7 exhibit an asymmetrically bridging mode
of (h2,m-NO2-N,O).
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
stool. There are two crystallographically independent
molecules in an asymmetric unit, which explains that
this crystal has the Z value of 4 instead of 2. These two
molecules are chemically equal. The Rh2(NO2)2 core
adopts a boat form, in which the dihedral angle between
Plane 1 (O1, N4, O3, and N5) and Plane 2 (Rh1, N4,
and N5) is 46.4(2)8, and that between Plane 1 and 3
(Rh2, O1, and O3) is 44.9(2)8, together with the dihedral
2
1
ꢀ
angle of 88.8(2)8 between Planes 2 and 3. As expected,
Because various oxidation states of nitrogen are
available, the reduction of NO3ꢃ generates several
products, including NO2ꢃ, NO, NH3, and NH4ꢁ
depending on reaction conditions [23,24]. In particular,
˚
O bond distances (1.285(5) and 1.289(5) A) in the
the Nꢀ
/
boat framework are longer than the exo Nꢀ
lengths (1.215(6) and 1.219(6) A). Two Rhꢀ
/O bond
˚
/N (N3) bond

22
W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ24
/
˚
Fig. 4. ORTEP drawing of 5. Selected bond lengths (A) and bond angles (8): Rh1ꢀ
/
O1 2.183(2), Rh1ꢀ
/
O2 2.179(2), Rh1ꢀ
/
O4 2.153(2), O1ꢀ
/N1 1.264(4),
O2ꢀN1 1.282(4), O3ꢀN1 1.208(4); S1ꢀO4ꢀRh1 128.35(13), O2ꢀRh1ꢀO1 58.84(9), N1ꢀ
/
/
/
/
/
/
/
O1ꢀRh1 93.28(17), N1ꢀ
/
/
O2ꢀ
/Rh1 92.94(17).
Ogo and co-workers very recently reported a unique
pH-dependent H2-activation cycle that is coupled to the
NO3ꢃ (NO3ꢃ 0
best knowledge, compounds 3 and 7 show unprece-
dented examples of the nitrato-to-nitro reduction in the
rhodium coordination sphere. It is worth noting that the
/
NO2ꢃ 0
/
NO) (Eq. (3)) [25]. To our
1
reduction of the nitrate ion. On the basis of H NMR,
NOx gas analysis, and mass spectra, they proposed that,
at pHꢂ
/
ꢃ
/
1, the oxidation of the hydrido ligands of
Nꢀ
/
O bond cleavage of the nitrato ligand appears to
[(Cp*Ir)2(m-H)3](NO3) couples to the reduction of
occur only in the reactions that involve phosphine (PPh3
˚
Fig. 5. ORTEP drawing of the cationic part of compound 6×
Rh1ꢀP1 2.274(3), Rh1ꢀP2 2.309(3), N1ꢀC46 1.12(1), N2ꢀC47 1.42(1); C46ꢀ
Rh1ꢀP1 90.9(4), C37ꢀRh1ꢀP2 88.9(4), P1ꢀRh1ꢀP2 179.8(2) C37ꢀN1ꢀC38 174(1), C46ꢀ
178(1).
/
H2O. Selected bond lengths (A) and bond angles (8): Rh1ꢀ
/
C37 1.93(1), Rh1ꢀ
P1 88.6(4), C46ꢀRh1ꢀP2 91.6(3), C37ꢀ
C47 175(1), N1ꢀC37ꢀRh1 178(1), N2ꢀC46ꢀRh1
/
C46 1.94(1),
/
/
/
/
/
Rh1ꢀ
/
C37 178.8(7), C46ꢀ
/
Rh1ꢀ
/
/
/
/
/
/
/
/
/
/
/
/
N2ꢀ
/
/
/
/
/

W.S. Han, S.W. Lee / Inorganica Chimica Acta 348 (2003) 15ꢄ
/24
23
˚
Fig. 6. ORTEP drawing of the cationic part of compound 7×
2.117(4), Rh1ꢀN5 2.107(4), Rh2ꢀN1 2.116(4), Rh2ꢀO1 2.124(4), Rh2ꢀ
1.219(6), N1ꢀN2 1.186(7), N2ꢀN3 1.147(9); Rh1ꢀN1ꢀRh2 110.6(2), N1ꢀ
Rh1 121.7(3), O3ꢀN5ꢀRh1 122.0(0) N1ꢀRh2ꢀO1 82.8(2), N1ꢀRh2ꢀO3 82.5(2), O1ꢀ
120.4(3), N1ꢀN2ꢀN3 178(1).
/
0.5H2O. Selected bond lengths (A) and bond angles (8): Rh1ꢀ
O3 2.130(4), O1ꢀN4 1.289(5), O2ꢀN4 1.215(6), O3ꢀ
Rh1ꢀN4 83.1(2), N1ꢀRh1ꢀN5 82.6(2), N4ꢀRh1ꢀN5 84.4(2), O1ꢀ
Rh2ꢀO3 85.5(2), N4ꢀO1ꢀRh2 120.3(3), N5ꢀO3ꢀ
/
N1 2.113(4), Rh1ꢀ
/
N4
/
/
/
/
/
/
/
N5 1.285(5), O4ꢀ
/N5
/
/
/
/
/
/
/
/
/
/
/
N4ꢀ
/
/
/
/
/
/
/
/
/
/
/
/
/Rh2
/
/
NO3ꢃ. Compounds 1, 2, 4, and 5 show h1 or h2 bonding
modes of the nitrato ligand.
5. Supporting material
Crystallographic data for the structural analysis have
been deposited at the Cambridge Crystallographic Data
Center: CCDC No. 185913 for 1, 185914 for 3×
185915 for 4, 185916 for 5, 185917 for 6×H2O, and
185918 for compound 7×0.5H2O. Copies of this infor-
/
2H2O,
Chart 1.
/
/
mation may be obtained free of charge from: The
Director, CCDC, 12 Union Road, Cambridge, CB2
or dppf). Therefore, we speculate that the phosphine
plays a certain role (for example, an oxygen abstractor)
in this type of reduction.
1EZ, UK (fax: ꢁ44-1223-336-033; e-mail: deposit@
/
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
[(CpꢀIr)2(m-H)3](NO3)ꢁ3NO3ꢃ
0 2[CpꢀIr(h1-NO3)(h2-NO3)]ꢁH2 ꢁHꢁ ꢁ4eꢃ (3)
Acknowledgements
This work was supported by the 63 Research Fund of
Sungkyunkwan University (2001ꢄ2002).
/
4. Conclusions
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compounds and determined their structures by X-ray
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treating [Cp*RhCl2]2 with AgNO3 in different mole
ratios. In particular, X-ray structures of the ionic
compounds 3 and 7 reveals an unusual nitrate-to-nitrite
conversion through the formal oxygen dissociation of
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