Carbonyl rhodium(I) complexes containing a hydrazonic tridentate HNN′O ligand. Synthesis, X-ray structure and reactivity toward methyl iodide

Article abstract of DOI:10.1016/S0020-1693(03)00121-X

The ligand (2-pyridinecarboxaldehyde) benzoylhydrazone (HNN′O) has been reacted with [Rh₂(μ-Cl)₂(CO)₄] in diethyl ether, isolating the carbonyl complex [Rh(κ²-HNN′)(CO)Cl] (1), where the neutral ligand coordinates the metal through the nitrogen atoms, the amidic oxygen been excluded by the coordination sphere. Repeated attempts aimed to force the ligand to an anionic tridentate coordination, both by prior deprotonation of the free ligand or deprotonation of 1, have resulted in extensive deposition of metallic rhodium. The reaction between HNN′O and [Rh₂(μ-Cl)₂(CO)₄] in basic media and in the presence of PPh₃, has led to the isolation of the complex [Rh(κ²-N′O)(PPh₃)(CO)]·1/2CH ₂Cl₂ (2), where the anionic ligand is N′O bidentate. Compound 1 has been reacted with an excess of MeI in CH₂Cl₂ or THF, isolating [Rh(κ²-HNN′)(CH₃CO)ClI] (4) and [Rh(κ²-HNN′)(CH₃CO)(THF)ClI] (5), respectively. Although the oxidative addition step is practically instantaneous in both cases, the migratory insertion step results faster in THF, as established by liquid film IR spectroscopy. Compound 2 has been reacted with MeI in THF isolating the complex [Rh(κ²-N′O)(PPh₃)(CO)(Me)I] (6), which, however, does not transform into the corresponding acyl complex. The crystal structure of complex 2·1/4CH₂Cl₂ has been solved.

Full text of DOI:10.1016/S0020-1693(03)00121-X

Inorganica Chimica Acta 351 (2003) 235ꢁ241
/
www.elsevier.com/locate/ica
Carbonyl rhodium(I) complexes containing a hydrazonic tridentate
HNN?O ligand. Synthesis, X-ray structure and reactivity toward
methyl iodide
Paolo Pelagatti a,*, Alberto Vergnani b, Mauro Carcelli a, Mirco Costa b,
Dominga Rogolino a, Alessia Bacchi a, Corrado Pelizzi a
a Dipartimento di Chimica Generale ed Inorganica, Chimica Fisica, Chimica Analitica, Universita` degli Studi di Parma, Parco Area delle Scienze 17/A,
43100 Parma, Italy
b Dipartimento di Chimica Organica ed Industriale, Universita` degli Studi di Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy
Received 30 January 2003; accepted 22 February 2003
Abstract
The ligand (2-pyridinecarboxaldehyde) benzoylhydrazone (HNN?O) has been reacted with [Rh2(m-Cl)2(CO)4] in diethyl ether,
isolating the carbonyl complex [Rh(k2-HNN?)(CO)Cl] (1), where the neutral ligand coordinates the metal through the nitrogen
atoms, the amidic oxygen been excluded by the coordination sphere. Repeated attempts aimed to force the ligand to an anionic
tridentate coordination, both by prior deprotonation of the free ligand or deprotonation of 1, have resulted in extensive deposition
of metallic rhodium. The reaction between HNN?O and [Rh2(m-Cl)2(CO)4] in basic media and in the presence of PPh3, has led to the
isolation of the complex [Rh(k2-N?O)(PPh3)(CO)]×
1/2CH2Cl2 (2), where the anionic ligand is N?O bidentate. Compound 1 has been
/
reacted with an excess of MeI in CH2Cl2 or THF, isolating [Rh(k2-HNN?)(CH3CO)ClI] (4) and [Rh(k2-HNN?)(CH3CO)(THF)ClI]
(5), respectively. Although the oxidative addition step is practically instantaneous in both cases, the migratory insertion step results
faster in THF, as established by liquid film IR spectroscopy. Compound 2 has been reacted with MeI in THF isolating the complex
[Rh(k2-N?O)(PPh3)(CO)(Me)I] (6), which, however, does not transform into the corresponding acyl complex. The crystal structure
of complex 2×
/
1/4CH2Cl2 has been solved.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium; Insertion; Tridentate ligands; Hydrazones; N ligands
1. Introduction
ligands, due to a donor atom which can reversibly bind a
metal, is often invoked as responsible of the observed
catalytic activities of the corresponding complexes
[2a,3]. Moreover, when the ligand carries an acidic
proton, an additional feature is represented by the
possibility of varying the electron density on the metal
nucleus, depending on the protic or anionic nature of
the ligand itself, and this can significantly affect the
reactivity of the complexes. As a part of our ongoing
interest towards the reactivity of d8 metal ion complexes
containing protic tridentate hydrazonic ligands [4], we
have recently shown as the neutral or anionic nature of
the HPNO ligand 2-(diphenylphosphino)benzaldehyde
benzoylhydrazone strongly influences the reactivity of
some carbonyl rhodium(I) complexes toward MeI [5].
Previously, we had found that the reactivity with CO of
some methyl Pd(II) complexes containing different
The use of tridentate ligands in organometallic
chemistry and catalysis has received increased attention
in the last years [1]. However, the use of protic tridentate
ligands in the synthesis of d8 metal ion complexes does
not count a great number of examples [2]. One of the
more interesting chemical aspects of this type of ligands,
lies in the fact that they can adopt different coordination
modes, i.e., mono-, bi- or tridentate, thus giving rise to
different structures and steric encumbrances around the
metal centre. The hemilability of the bi- and tridentate
* Corresponding author. Tel.: ꢀ
905557.
E-mail address: paolo.pelagatti@unipr.it (P. Pelagatti).
/
39-0521-905426; fax: ꢀ39-0521-
/
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00121-X

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P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ241
/
HNN?O ligands, was strongly dependent on the anionic
or neutral nature of these [6]. In this paper, we show the
preparation of carbonyl Rh(I) complexes containing this
latter type of ligand and their reactivity in the oxidative
addition of MeI, highlighting how the coordination
mode and the electronic properties of HNN?O rule the
reaction course and the nature of the final products.
for 10 min. The solid was filtered off, washed with
diethyl ether and dried in vacuum. Yield: 0.066 g (79%).
The characterization was in agreement with the litera-
ture data [13].
2.2.3. [Rh(k2-N?O)(PPh3)(CO)]×
/
1/2CH2Cl2 (2)
HNN?O (0.050 g, 0.222 mmol) was dissolved in
dichloromethane (25 ml). To the obtained solution
were subsequently added MTBD (0.034 g, 0.04 ml,
0.222 mmol), PPh3 (0.080 g, 0.222 mmol) and [Rh(m-
Cl)2(CO)4] (0.060 g, 0.111 mmol). The resulting blood-
red solution was stirred at r.t. for 2 h during which time
it turned to deep orange. The solvent was partially
removed under vacuum and n-hexane was added.
2. Experimental
2.1. General
All reactions were performed under an atmosphere of
dry nitrogen employing standard Schlenk techniques.
Solvents were dried prior to use and stored under
nitrogen. Elemental analysis (C, H, N) were performed
by using a Carlo Erba Mod. EA 1108 apparatus.
Infrared spectra were recorded with a Nicolet 5PCFT-
Refrigeration at ꢂ
Yield: 0.129 g (84%). Anal. Calc. for C32H25N3O2PRh×
1/2CH2Cl2: C, 59.15; H, 3.97; N, 6.37. Found: C, 59.42;
H, 4.03; N, 6.47%. 31P NMR (CDCl3): dꢃ
29 (d,
127 Hz). H NMR (CDCl3, 253 K): dꢃ9.38
(d, 1H, H4, 3JH,H 8.0 Hz), 8.74 (d, 1H, H1, 3JH,H
4.4
Hz), 8.24 (d, 1H, C(H)Ä 3.3 Hz), 7.92 (t, 1H,
N, 3JRh,H
H3, 3JH,H 7.6 Hz), 7.89 (d, 2H, Ph o-C(O), 3JH,H
7.4
Hz), 7.72ꢁ7.26 (m, 19H, H2 and Ph), 5.31 (s, 1H,
CH2Cl2). IR (KBr, n, cmꢂ1): 1975vs (CÅ
O). Red
crystals of 2×1/4CH2Cl2 suitable for X-ray analysis
were collected after repeated recrystallization of 2 in a
CH2Cl2/n-hexane mixture at ꢂ18 8C.
/
18 8C led to a deep orange powder.
/
/
1
1JRh,P
ꢃ
/
/
IR spectrophotometer in the 4000ꢁ
400 cmꢂ1 range by
/
ꢃ
/
ꢃ
/
1
using KBr disks or a NaCl cell. H NMR spectra were
obtained on a Bruker 300 FT spectrometer using SiMe4
as internal standard, while 31P {1H} NMR spectra were
recorded on a Bruker CPX 200 FT using H3PO4 85% as
external standard. All spectra were collected at 300 K,
unless otherwise reported. The ligand HNN?O was
synthesised as previously described [6]. [Rh2(m-
Cl)2(CO)4] and MeI were purchased by Aldrich, while
7-methyl-1,5,7-triazabicyclo-[4.4.0]dec-5-ene (MTBD)
was purchased by Fluka.
/
ꢃ
/
ꢃ
/
ꢃ
/
/
/
/
/
2.3. Preparation of the Rh(III) complexes
2.2. Preparation of the Rh(I) complexes
2.3.1. General method
The carbonyl Rh(I) complex (0.050 g) (0.122 mmol
for 1, in 5 ml of CH2Cl2 or THF, 0.076 mmol for 2, in 5
ml of THF) was treated with a fortyfold excess (0.32 ml
for 1, 0.20 ml for 2) of MeI at r.t. The reactions were
monitored by liquid film IR until no changes were
observed. The final products were isolated by partial
removal of the solvent under reduced pressure and
addition of n-hexane, followed by drying under vacuum
for several hours. The reported yields refer to the
products recovered as solids.
2.2.1. [Rh(k2-HNN?)(CO)Cl] (1)
[Rh2(m-Cl)2(CO)4] (0.043 g, 0.111 mmol) was dis-
solved in diethyl ether (20 ml) and HNN?O (0.050 g,
0.222 mmol) was added at room temperature (r.t.). The
resulting purple solution was stirred at r.t. for 3 h,
during which time a deep purple solid was released;
consequently the solution became almost uncoloured.
The solid was filtered off and dried under vacuum.
Yield: 0.076 g (84%). Anal. Calc. for C14H11ClN3O2Rh×
/
H2O: C, 40.91; H, 2.79; N, 10.09. Found: C, 41.05; H,
1
3.20; N, 10.26%. H NMR (CDCl3): dꢃ
NꢁH), 9.91 (s, 1H, C(H)ÄN), 8.68 (dbr, 1H, H1), 7.94
(d, 2H, Ph o-C(O), JH,H
H3ꢁH4ꢁPh), 7.26 (t, 1H, H2, JH,H
n, cmꢂ1): 3220m (Nꢁ
CH2Cl2), 1675s (CÄO).
/
11.73 (s, 1H,
/
/
2.3.2. [Rh(k2-HNN?)(MeCO)ClI] (4)
The reaction was carried out in dichloromethane.
Green solid. Yield: 0.068 g (45%). Anal. Calc. for
C15H14ClIN3O2Rh: C, 33.77; H, 2.64; N, 7.88. Found:
3
ꢃ
/
7.3 Hz), 7.63ꢁ
/
7.48 (m, 5H,
6.3 Hz). IR (KBr,
3
/
/
ꢃ
/
/
H), 1974vs (CÅ
/
O) (1994 in
1
/
C, 33.93; H, 2.70; N, 7.76%. H NMR (CDCl3): dꢃ
/
3
11.80 (s, 1H, Nꢁ
Hz), 9.47 (d, 1H, H1, JH,H
3JH,H
7.3 Hz), 7.97 (d, 2H, Ph o-C(O), JH.H
Hz), 7.84 (d, 1H, H4, 3JH,H
7.7 Hz), 7.54 (m, 2H, H2ꢀ
Ph p-C(O)), 7.42 (t, 2H, Ph m-C(O), JH,H
2.82 (s, 3H, MeC(O)ꢁ
Rh). IR (KBr, n, cmꢂ1): 3215w
(NꢁH), 1713vs (MeCOꢀCÄO).
/
H), 9.99 (d, 1H, C(H)Ä
/
N, JRh,H
ꢃ
/
2.7
3
2.2.2. [trans-(PPh3)2(CO)Cl]
ꢃ
/
5.4 Hz), 8.03 (t, 1H, H3,
3
Compound 1 (0.050 g, 0.122 mmol) was dissolved in
dichloromethane (20 ml). PPh3 (0.032g, 0.122 mmol)
was added, obtaining the instantaneous colour change
from purple to yellow. Within 2 min a yellow solid
precipitated and the resulting mixture was stirred at r.t.
ꢃ
/
ꢃ7.5
/
ꢃ
/
/
3
ꢃ7.6 Hz),
/
/
/
/
/

P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ
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237
2.3.3. [Rh(k2-HNN?)(MeCO)(THF)ClI] (5)
Table 1
Crystal data and structure refinement for compound 2×
/
1/4CH2Cl2
The reaction was carried out in THF. Brown solid.
Yield: 0.029 g (40%). Irreproducible amounts of THF
prevented a correct elemental analysis. 1H NMR
(CDCl3): the spectrum is identical to that found for 4,
with the exception of two multiplets corresponding to
uncoordinated THF (3.74 and 1.84 ppm). IR (KBr, n,
Empirical formula
Formula weight
Temperature (K)
(C32H25N3O2PRh)×
638.66
293(2)
/1/4CH2Cl2
˚
Wavelength (A)
0.71073
monoclinic
C2/c
Crystal system
Space group
Unit cell dimensions
cmꢂ1): 3217w (Nꢁ
/
H), 1702vs (MeCOꢀ
/
CÄ/O).
˚
a (A)
31.650(5)
8.8630(10)
25.725(4)
124.688(2)
5933.6(15)
8
2.3.4. [Rh(k2-N?O)(PPh3)(CO)(Me)I] (6)
The reaction was carried out in dichloromethane.
Orange solid. Yield: 0.025 g (41%). Anal. Calc. for
˚
b (A)
˚
c (A)
b (8)
3
V (A )
˚
C33H28IN3O2PRh×
/
1/2CH2Cl2: C, 50.17; H, 3.65; N,
5.24. Found: C, 50.16; H, 3.55; N, 5.25%. 31P NMR
Z
Dcalc (Mg mꢂ3
)
1.430
0.708
2596
1
30.3d, JRh,P
ꢃ
134 Hz. 1H NMR
/
Absorption coefficient (mmꢂ1
F(000)
)
([D6] DMSO): dꢃ
/
(CD2Cl2): dꢃ
1H, CHÄ
2H, Ph o-C(O), JH,H
matics), 7.58ꢁ
7.36 (m, aromatics), 1.34 (t, 3H, 2JRh,H
2 Hz, Meꢁ O).
Rh). IR (KBr, n, cmꢂ1): 2066s (CÅ
/
9.32 (d, 1H, H4, 3JH,H
ꢃ
/
8.1 Hz), 8.73 (m,
5.4 Hz), 8.15 (d,
7.92 (m, aro-
Crystal size (mm3)
0.3ꢄ
/
0.4ꢄ
24.72
375h 5
305l 530
26561
5070 [Rint
/
0.7
37, ꢂ
3
/
N), 8.18 (d, 1H, H1, JH,H
ꢃ
/
u Range for data collection (8) 1.56ꢁ
Index ranges
/
3
ꢃ
/
9.5 Hz), 7.99ꢁ
/
ꢂ/
/
/
/
105
/
k 5
/
10, ꢂ
/
/
ꢃ
/
/
/
/
/
Reflections collected
Independent reflections
Refinement method
ꢃ0.0413]
/
full-matrix least-squares on F2
2.4. Crystal structure determination of 2×/1/4CH2Cl2
Data/restraints/parameters
Goodness-of-fit on F2
5070/0/366
1.124
Single crystals of [Rh(k2-N?O)(PPh3)(CO)]×
4CH2Cl2 suitable for X-ray structure analysis were
obtained from dichloromethane/n-hexane. X-ray dif-
/1/
Final R indices [I ꢀ
R indices (all data)
Largest DF maximum/minimum 1.728/ꢂ
/
2s(I)]
R1ꢃ
/
0.0475, wR2ꢃ
0.0570, wR2ꢃ
0.745
/
0.1500
R1ꢃ
/
/0.1620
/
ꢂ3
˚
(e A
)
fraction data were collected on a Brukerꢁ
SMART AXS 1000 equipped with CCD detector, using
graphite monochromated Mo Ka radiation (lꢃ0.71069
A). Data collection details are: crystal to detector
distanceꢃ5.0 cm, 2424 frames collected (complete
sphere mode), time per frameꢃ
/
Siemens
in diethyl ether at room temperature, the fast precipita-
tion of the chloro-carbonyl Rh(I) complex [Rh(k2-
HNN?)(CO)Cl] (1) occurs in high yields. The ligand is
neutral and NN? coordinated, the carbonyl oxygen
being excluded by the coordination. The square planar
geometry is completed by a chlorine atom and a
carbonyl group. The proposed structure of 1 is depicted
in Scheme 1.
/
˚
/
/
30 s, oscillation Dvꢃ
/
0.3008. Crystal decay resulted negligible. Data were
processed by the SAINT package [7] and corrected for
absorption effects by the SADABS [8] procedure (Tmax
ꢃ
/
1.000, Tmin 0.662). Data reduction was performed up
ꢃ
/
˚
The neutral character of the ligand is well evidenced
1
by the H NMR signal of the hydrazonic proton, which
to dꢃ0.85 A. The phase problem was solved by direct
/
methods [9] and refined by full-matrix least-squares on
all F2 [10]. Anisotropic displacement parameters were
refined for all non hydrogen atoms, while hydrogen
atoms were introduced in idealized positions. A partially
occupied (50%) CH2Cl2 molecule was found on a
inversion position in the asymmetric unit. The final
map was featureless, except for residual density around
the partially occupied solvent site. Details of data
collection and structure refinement are in Table 1. For
the discussion use was made of the Cambridge Struc-
tural database facilities [11].
resonates as a singlet at 11.73 ppm. A further confirma-
tion of the protic nature of the ligand, derives from the
IR stretching band of the Nꢁ
band centred at 3220 cmꢂ1. The uncoordinated CÄ
group originates an intense band centred at 1675 cmꢂ1
/H bond, which is a weak
/
O
.
The high deshielding observed for the imine proton with
respect to the free ligand (9.91 vs. 7.27 ppm), indicates
that the metal-imine back donation is negligible. More
important is the metalꢁ
denced by the somewhat low n(CÅ
/
CÅ
/
O back donation, as evi-
/O) values (1974
3. Results and discussion
3.1. Carbonyl Rh(I) complexes
When the ligand (2-pyridincarboxaldehyde) benzoyl-
hydrazone, HNN?O, is reacted with [Rh2(m-Cl)2(CO)4]
Scheme 1.

238
P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ241
/
cmꢂ1). As usually observed for pyridylꢁ
complexes, also in this case the NN? bidentate coordina-
tion causes a change in the sequence of the pyridine H
NMR signals, with respect to the free ligand [6,12]. In
the last in fact, the sequence is, from low fields to high
/
imine metal
From a refrigerated dichloromethane/n-hexane mix-
ture, crystals suitable for X-ray analysis of 2×1/4CH2Cl2
/
1
have been collected (see Section 2.4); the structure is
depicted in Fig. 1. The ligand shows an anionic N?O
character, coordinating the metal through the imine
nitrogen and the oxygen atoms; the square planar
geometry is completed by a CO molecule and a PPh3
molecule, trans to the oxygen and to the nitrogen
donors, respectively. The ligand adopts a Z configura-
ones, H1ꢀ
H3ꢀH4ꢀH2. This change is due to the rotation around
the pyꢁC(imine) bond imposed by the coordination,
with consequent removal of the anisotropic effect that
N bond exerts on the H4 proton in the free
/
H4ꢀ
/
H3ꢀ
/
H2, whereas in 1 it becomes H1ꢀ
/
/
/
/
tion around the CÄ/N bond, with the pyridine ring
the C(H)Ä
/
excluded by the coordination sphere. The anionic
character of the ligand is indicated by the disappearance
of the NH spectroscopic signals, while the presence of a
ligand. Two possible isomers can be drawn for 1, where
the carbonyl ligand displays a cis or a trans disposition
with respect to the C(H)Ä/N bond. Based on the
Rhꢁ
disappearance of the n(CÄ
Moreover, the carbonyl ligand originates a strong
n(Cꢁ
O) band at 1975 cmꢂ1, while the phosphine gives
rise, in the 31P NMR spectrum, to a doublet centred at
29 ppm, with a J(RhꢁP)ꢃ127 Hz, typical values for a
/O coordinating bond is clearly pointed out by the
observation that the chemical shift of H1 in the complex
is similar to that observed in the free ligand (8.68 vs. 8.54
ppm), we assume that the arrangement around the metal
centre is that shown in Scheme 1, i.e. the carbonyl ligand
/O) band in the IR spectrum.
/
trans to the C(H)Ä/N moiety. In the cis arrangement the
/
/
chlorine ligand would be adjacent to the pyridine ring,
and a stronger deshielding of the H1 proton would be
expected [12]. Furthermore, in the trans isomer an
phosphine trans to an imine nitrogen and cis to a CO
1
group [14]. The H NMR spectrum recorded at 300 K
shows only broad signals which does not allow any
interpretation. The spectrum at 253 K shows sharper
signals: H1 generates a doublet centred at 8.74 ppm
intramolecular Rhꢁ
feasible, as structurally observed for a chlorideꢁ
/
ClÁ Á ÁHꢁ
/
N hydrogen bond becomes
/methyl
Pd(II) complex containing the same ligand [6]. Complex
1 is fairly stable in the solid state (rhodium release
occurs only after several months at room temperature),
while in solution at room temperature it decomposes to
metallic rhodium within 1 day. The same decomposition
takes place after one week at 255 K. Repeated attempts
aimed to isolate a carbonyl Rh(I) complex of the type
[Rh(k3-NN?O)(CO)], i.e. containing the anionic ligand
behaving as tridentate, were unsuccessful and resulted in
extensive decomposition to metallic rhodium and un-
characterizable products.1 It is interesting to note that
the carbonyl rhodium(I) complex [Rh(k3-PNO)(CO)],
i.e. containing an anionic tridentate ligand, has been
isolated and structurally characterized [5]; evidently, the
soft phosphorous atom is necessary to remove the excess
of negative charge placed on the metal. Complex 1 has
been reacted with an equimolar amount of PPh3 in
dichloromethane at room temperature. The reaction
leads to the fast precipitation of [trans-
Rh(PPh3)2(CO)Cl], as evidenced by the spectroscopic
characterization of the product [13]. However, when a
dichloromethane solution of HNN?O is reacted with
[Rh2(m-Cl)2(CO)4] in the presence of PPh3 and MTBD in
a 1:0.5:1:1 molar ratio, the carbonyl phosphine Rh(I)
(JHH
(JHH
doublet centred at 8.24 ppm (JRhꢁH
ꢃ
/
4.4 Hz), H4 a doublet centred at 9.38 ppm
ꢃ
/
8.0 Hz), while the imine proton generates a
ꢃ3.3 Hz). The high
/
chemical shift of H4 can be ascribed to the vicinal imine
moiety (see Section 2.4); the shielding found for the
C(H)Ä
/
N proton can be instead ascribed to the trans
influence of the phosphine ligand (see Fig. 1). Complex
2 results stable both in the solid state and in solution.
3.2. Structure of complex 2×
/
1/4CH2Cl2
The molecular structure of 2×/1/4CH2Cl2 is shown in
Fig. 1, while a list of the most relevant bond distances
and angles is in Table 2. The rhodium adopts a square
planar geometry, with the deprotonated ligand behaving
as a N?O bidentate donor via the imine nitrogen and the
amide carbonyl. The CO ligand is placed in cis to the
imine nitrogen N?, replacing the pyridinic N coordina-
tion site adopted by the HNN? ligand in complex 1. The
phosphine ligand therefore occupies the coordination
site trans to the N? donor, and this geometry differ-
entiates complex 2×
/
1/4CH2Cl2 from the analogue com-
plex
[Rh(k3-PNO)(CO)]
(diphenylphosphino)benzaldehyde benzoylhydrazone)
[5]. The only relevant difference between the two
(PNOꢃ2-
/
complex [Rh(k2-N?O)(PPh3)(CO)]×
(Scheme 2).
/1/2CH2Cl2 (2) forms
coordination modes concerns the RhꢁP bond length,
/
˚
1/4CH2Cl2 is remarkably longer (2.264(1) A)
which in 2×
/
1
3
˚
These reactions were conducted following two different
procedures: method (a) a free ligand solution was treated with a
stoichiometric amount of a base (MTBD, NaOMe or Et3N) and
subsequently with [Rh2(m-Cl)2(CO)4] at room temperature; method (b)
a solution of 1 was treated with a stoichiometric amount of a base
(MTBD, NaOMe or Et3N) at room temperature.
than in [Rh(k -PNO)(CO)] (2.236(1) A). This can be
ascribed to the trans influence of the iminic N? donor,
which is larger than the one of the amidic O donor. The
difference in the trans influence of N? and O on the Rhꢁ
/
CO bond in the two compounds is not equally evident

P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ
/241
239
Scheme 2.
The ligand N?O coordination defines a penta-atomic
chelation ring, with geometry comparable to the one
observed in the above mentioned PNO Rh(I) complex
[5] (Cꢁ
NꢁC(O)ꢃ
torted due to the nature of the ligands: the chelation bite
angle is N2ꢁRhꢁO1ꢃ77.3(1)8, while the exocyclic Pꢁ
RhꢁO1 coordination angle (ꢃ96.9(1)8) is widened. The
/
Oꢃ
/
1.23(2), Nꢁ
/
Nꢃ
/
1.40(2), Nꢁ
/
CHꢃ
/
1.23(4),
˚
/
/1.32(4) A). The square coordination is dis-
/
/
/
/
/
/
phosphine donor is slightly displaced from the coordi-
˚
nation plane (0.11 A). The entire N?O ligand is planar
˚
within 0.22 A, the largest deviations being due to the
terminal phenyl ring. The terminal pyridinic ring is
oriented away from the coordination centre, (N1ꢁ
C8ꢁC9ꢃ 1.2(8)8), while the N atom is facing the CO
ligand, probably due to a favourable intramolecular
interaction between C13ꢁ
/
N2ꢁ
/
/
/
ꢂ
/
/
H and N1 (H13Á Á ÁN1ꢃ
/
˚
2.296(5) A), accounting also for the chemical shift
1
observed for H(C13) in the H NMR spectrum.
Fig. 1. Perspective view and labelling scheme of the molecular
structure of compound 2×1/4CH2Cl2, with thermal ellipsoids at the
3.3. Oxidative addition reactions
/
The carbonyl complexes 1 and 2 have been reacted
with a forty fold excess of MeI at room temperature; the
oxidative additions have been monitored by liquid film
IR spectroscopy. The reaction with 1 has been at first
performed in dichloromethane: the immediate disap-
Table 2
˚
Relevant bond lengths (A) and bond angles (8) for compound 2×
4CH2Cl2
/1/
Bond lengths
pearance of the n(CÅ
/
O) band of the starting complex
Rhꢁ
Rhꢁ
Rhꢁ
Rhꢁ
N1ꢁ
N1ꢁ
/
C14
O1
N2
P
1.803(6)
2.046(3)
2.085(4)
2.264(1)
1.318(6)
1.396(5)
N2ꢁ
O1ꢁ
O2ꢁ
C1ꢁ
C8ꢁ
C9ꢁ
/
C8
C1
1.283(6)
1.290(5)
1.150(6)
1.476(6)
1.464(6)
1.362(6)
(1994 cmꢂ1 in CH2Cl2) in favour of a new intense band
centred at 2093 cmꢂ1 has been observed. This is
indicative of the formation of the carbonyl Rh(III)
complex [Rh(k2-HNN?)(CO)(CH3)ClI] (3 in Scheme 3).
/
/
/
/
C14
/
/
C2
C9
N3
/
C1
N2
/
/
/
Well visible are the bands of the n(CÄ
/
O) and n(NꢁH)
/
Bond angles
(1685 and 3247 cmꢂ1, respectively), which indicate that
the coordination mode of the ligand does not vary
during the oxidative addition. With time, the carbonyl
band of 3 vanishes in favour of a band centred at 1705
cmꢂ1, indicating the formation of the pentacoordinated
C14ꢁ
C14ꢁ
O1ꢁRhꢁ
C14ꢁRhꢁ
O1ꢁRhꢁ
N2ꢁRhꢁ
C1ꢁN1ꢁ
/Rhꢁ
/
O1
N2
175.0(2)
97.7(2)
77.3(1)
88.1(2)
96.88(9)
173.6(1)
111.2(4)
C8ꢁ
C8ꢁ
N1ꢁ
C1ꢁO1ꢁ
O1ꢁC1ꢁ
N2ꢁC8ꢁ
O2ꢁC14ꢁ
/
N2ꢁ
N2ꢁ
N2ꢁ
/
N1
118.1(4)
128.0(3)
113.8(3)
112.2(3)
125.3(4)
131.0(4)
178.1(5)
/Rhꢁ
/
/
/Rh
/
/
N2
/
/
Rh
Rh
N1
/
/
P
/
/
/
/
P
/
/
/
/
P
/
/C9
acetyl
rhodium(III)
complex
[Rh(k2-
/
/
N2
/
/
Rh
HNN?)(CH3CO)ClI] (4); the migratory insertion results
complete within 24 h, although a weak residual band
centred at 2072 cmꢂ1 persists after 30 h of reaction.
Once isolated complex 4 shows an intense IR stretching
3
˚
(Rhꢁ
/
Cꢃ
/
1.77(2) in [Rh(k -PNO)(CO)] and 1.803(6) A
in 2), nevertheless the labilization of the Rhꢁ
/
CO bond
1
band at 1713 cmꢂ1 and a singlet at 2.82 ppm in the H
when it is in trans to the iminic N? may help in
understanding the reason of the different reactivity of
1 and 2 toward the migratory insertion reaction.
NMR spectrum, which testify to the presence of an
acetyl group bonded to rhodium; a weak band at 3215

240
P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ241
/
the following formula for complex 5: [Rh(k2-
HNN?)(CH3CO)(THF)ClI]. The coordinated THF mo-
lecule results easily displaceable: 4 can in fact be
recovered from a chloroform solution of 5 by addition
of an excess of diethyl ether, as depicted in Scheme 3.
As regards complex 2, owing to the low solubility in
THF, the oxidative addition of MeI has been performed
only in dichloromethane. After the addition of MeI,
besides the carbonyl stretching band of 2 (1974 cmꢂ1), a
new intense band appears at 2058 cmꢂ1, to indicate the
formation of a carbonyl Rh(III) species. After 30 min,
the band at 1974 cmꢂ1 disappears, and it remains an
intense band centred at 2064 cmꢂ1. The situation does
not change in the next 12 h. Thus, the oxidative addition
of MeI to 2 blocks to the formation of the methyl
carbonyl
rhodium(III)
complex
[Rh(k2-
N?O)(PPh3)(CO)(CH3)I] (6), and the migratory insertion
does not take place, as depicted in Scheme 4. Once
Scheme 3.
isolated as solid, complex 6 shows an intense n(CÅ
/
O)
band centred at 2066 cmꢂ1. In the H NMR spectrum
the methyl bonded to rhodium appears as a multiplet at
1.34 ppm, because of the coupling with the rhodium and
phosphorous nuclei. The P atom originates, in the 31P
NMR spectrum, a doublet centred at 30.3 ppm, with a
1
cmꢂ1 and a singlet at 11.80 ppm confirm the protic
nature of the ligand. The n(CÄO) band is obscured by
/
the intense acetyl stretching signal. The strong deshield-
ing of H1 with respect to the free ligand (9.47 vs. 8.54
ppm), indicates the presence of an halogen adjacent to
the pyridine, while the acetyl ligand probably occupies
an apical position [15]. Complex 4 results stable under
reflux in dichloromethane as well as under CO atmo-
sphere. The residual band at 2072 cmꢂ1 can be
tentatively attributed to an isomer of 3, in which the
methyl and the carbonyl are mutually trans, i.e. an
unfavourable disposition for the migratory insertion.
The same reaction has been repeated in THF: again the
disappearance of the carbonyl stretching band of 1 (1989
cmꢂ1 in THF) is immediate after the addition of MeI,
resulting in the appearance of an intense band centred at
2086 cmꢂ1 (3 in Scheme 3). The stretching bands of the
JRhꢁP
ꢃ
/
134 Hz. The NMR data of 6 agree with those
8-
reported for trans-[Rh(ox)(CO)(PPh3)(CH3)(I)] (oxꢃ
/
hydroxyquinolinato), whose X-ray structure shows the
methyl and iodide ligands mutually trans [17]; the same
strereochemistry is proposed for complex 6 (Scheme 4).
A clear explanation of the not occurred migratory
insertion is not easy at present. It should be considered
that in 2, because of the anionic character of the ligand,
the metal nucleus has a higher electron density than in 1;
this makes stronger the Rhꢁ/Me bond and favours the
metalꢁCO back donation, factors that are considered to
/
disfavour the migratory insertion [5]. However, a steric
effect of the phosphine cannot ruled out.
CÄ
/
O and NꢁH groups are still visible at 1689 and 3232
/
cmꢂ1, respectively. The band at 2086 cmꢂ1 diminishes
progressively with time, and it disappears completely
within 3 h and a half, leaving an intense acetyl band
centred at 1706 cmꢂ1 (5 in Scheme 3). As seen in the
reaction conducted in dichloromethane, a residual weak
band persists at 2071 cmꢂ1 after 5 h of reaction. The
different reaction rates observed with the two different
solvents are not surprising, since it is known that
coordinating solvents, like THF, may speed up the
migratory insertion step of the reaction [16]. In the solid
state 5 shows a n(CH3CO) band at 1702 cmꢂ1, while the
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 194780. Copies of this
information may be obtained free of charge from The
n(CÄ/O) is obscured by the intense stretching band of the
acetyl group. The differences observed in the IR
stretching frequencies of the acetyl group for the
1
complexes 4 and 5, together with the fact that the H
NMR spectrum of 5 recorded in CDCl3 is identical to
that of 4, with the only exception of two multiplets
characteristic of uncoordinated THF, allow to propose
Scheme 4.

P. Pelagatti et al. / Inorganica Chimica Acta 351 (2003) 235ꢁ
/241
241
[4] (a) A. Bacchi, M. Carcelli, M. Costa, A. Leporati, E. Leporati, P.
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107;
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: ꢀ44-1223-336-033; e-mail: depos-
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.a-
/
(b) P. Pelagatti, A. Bacchi, M. Carcelli, M. Costa, A. Fochi, P.
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facilities.
Fochi, H.-W. Fruhauf, K. Goubitz, K. Vrieze, Eur. J. Inorg.
¨
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