N-benzoylimido complexes of palladium. Synthesis, structural characterisation and structure-reactivity relationship

Article abstract of DOI:10.1039/b403200d

Benzoyl azides, ArC(O)N₃, 2, (Ar = phenyl or substituted phenyl), react with [Pd₂Cl₂(dppm)₂], 1, [dppm = bis(diphenylphosphino)methane] with the formation of novel [Pd ₂Cl₂(μ-NC(O)Ar)(dppm)₂], 3, benzoylnitrene complexes that were structurally characterised by multinuclear magnetic resonance and IR spectroscopy and, in several instances, by single crystal X-ray diffraction. As shown by crystallographie studies, the C₂P ₄Pd₂ rings adopt extended twist-boat conformations with methylene groups bending towards the bridging benzoylimido moieties. X-ray diffraction studies have revealed the chiral nature of the imido complexes, the chiral element being the propeller-like C₂P₄Pd₂ ring. Structural data accumulated on complexes 3 such as short C-N distances (1.32 A), elongated C=O bonds (1.30 A) as well as the outstandingly high barrier to internal rotation around the N-C(O) linkage (88.3 kJ mol ^-1) are in line with extensive pπ-pπ interaction between the bridging nitrogen and the carbonyl carbon atoms. Theoretical calculations indicate an electron shift from the dimer towards the apical nitrogen atom, which, in turn, facilitates the donation of electrons towards the carbonyl moiety. To elucidate the structure-reactivity relationship of benzoyl azides towards 1, crystallographic and solution IR spectroscopic studies were carried out on a series of para-substituted benzoyl azides. The reaction obeys the Hammett equation. The large positive value of the reaction constant indicates that the azides act as electrophiles in the reaction studied. The enhanced reactivity of 2-nitrobenzoyl azide has been attributed to a decreased conjugation of the phenyl and carbonyl moieties in this reagent.

Full text of DOI:10.1039/b403200d

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N-benzoylimido complexes of palladium. Synthesis, structural
characterisation and structure–reactivity relationship
Gábor Besenyei,*a László Párkányi,a Gábor Szalontai,b Sándor Holly,a Imre Pápai,a
Gábor Kereszturya and Andrea Nagya
a Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences,
1525 Budapest, Pf. 17, Hungary
b NMR Laboratory, University of Veszprém, 8201 Veszprém, Pf. 158, Hungary.
E-mail: besenyei@chemres.hu; Fax: 36 1 325 7554; Tel: 36 1 438 4141
Received 2nd March 2004, Accepted 22nd April 2004
First published as an Advance Article on the web 7th May 2004
Benzoyl azides, ArC(O)N3, 2, (Ar = phenyl or substituted phenyl), react with [Pd2Cl2(dppm)2], 1, [dppm =
bis(diphenylphosphino)methane] with the formation of novel [Pd2Cl2(µ-NC(O)Ar)(dppm)2], 3, benzoylnitrene
complexes that were structurally characterised by multinuclear magnetic resonance and IR spectroscopy and, in
several instances, by single crystal X-ray diffraction. As shown by crystallographic studies, the C2P4Pd2 rings adopt
extended twist–boat conformations with methylene groups bending towards the bridging benzoylimido moieties.
X-ray diffraction studies have revealed the chiral nature of the imido complexes, the chiral element being the
propeller-like C2P4Pd2 ring. Structural data accumulated on complexes 3 such as short C–N distances (1.32 Å),
elongated C᎐O bonds (1.30 Å) as well as the outstandingly high barrier to internal rotation around the N–C(O)
᎐
linkage (88.3 kJ molϪ1) are in line with extensive pπ–pπ interaction between the bridging nitrogen and the carbonyl
carbon atoms. Theoretical calculations indicate an electron shift from the dimer towards the apical nitrogen atom,
which, in turn, facilitates the donation of electrons towards the carbonyl moiety. To elucidate the structure–reactivity
relationship of benzoyl azides towards 1, crystallographic and solution IR spectroscopic studies were carried out on a
series of para-substituted benzoyl azides. The reaction obeys the Hammett equation. The large positive value of the
reaction constant indicates that the azides act as electrophiles in the reaction studied. The enhanced reactivity of
2-nitrobenzoyl azide has been attributed to a decreased conjugation of the phenyl and carbonyl moieties in this
reagent.
platinum hydrides with benzoyl azide was reported to yield
Introduction
air stable N-benzoylamido complexes rather than imides.10
N-benzoylnitrene adducts have shown up sometimes, however,
although in low yield. The photochemical decomposition of
benzoyl azide in the presence of [CpMo(CO)3CH3] produced
traces (2 %) of a dimolybdenum complex incorporating a
bridging N-benzoylimido moiety.11a Rather unexpectedly, the
interaction of the tungsten complex [WCl2(NC6H3Pri2-
2,6)(PMe3)3] with unpurified p-toluonitrile gave reproducibly
small amounts of the mononuclear benzoylnitrene complex
[WCl2(NC6H3Pri2-2,6)(NC(O)C6H4Me-4)(OPMe3)(PMe3)].12 It
might be due to the inefficiency of synthetic procedures that
structural data related to benzoylimido adducts are rare and
there seems to be no systematic studies aimed at their synthesis
and characterisation.
The chemical literature reflects an unceasing interest in the
chemical and structural properties of transition metal imido
(nitrene) complexes.1 The attractiveness of imido complexes can
partly be attributed to the diversity of coordination modes of
the imido moieties attached to metal centres.1e Another reason
for initiating studies on nitrene complexes has been the expect-
ation that such species may emerge as intermediates in catalytic
transformations.2 Nowadays, the observation that imido
complexes possess remarkable catalytic activity expands the
scope of both the synthetic and the catalytic investigations
carried out with coordination compounds incorporating
nitrene species.3
The relatively easy access to a wide range of organic azides
and their readiness to liberate dinitrogen in the coordination
sphere of metal ions have made these compounds promising
substrates for the preparation of imido complexes. A summary
of the earlier results has appeared4 and, utilising the reactivity
of organic azides toward metal complexes, numerous novel
nitrene adducts have been prepared since then.5 In the majority
of publications, aryl azides and, to a lesser extent, arenesulfonyl
azides were employed as reagents. Aroyl azides, however, have
played so far only an episodic role. Apart from the early
observations of Kaska and coworkers,6 the thermal reaction of
aroyl azides with coordination compounds did not result in
the formation of isolable nitrene complexes. Collman and
coworkers demonstrated that the reaction of iridium complexes
with aroyl azides gives dinitrogen complexes in protic solvents
while the same reaction results in acyl isocyanate complexes in
the absence of acidic hydrogen atoms.7 A similar reaction
course was suggested for the interaction of a rhodium dimer
with benzoyl azide.8 The formation of acyl isocyanate com-
plexes were also noted in the reaction of monomeric Ru(0)
complexes with benzoyl azide.9 On the contrary, the reaction of
In the course of our search for viable routes to nitrene
complexes of palladium, we have observed that the interaction
of the dimer [Pd2Cl2(dppm)2], 1, with arenesulfonyl azides
affords dinuclear nitrene adducts in high yield.13 Moreover, this
reaction has allowed the isolation of azide complexes if the
sulfonyl azide carries at least one ortho-substituent.13,14 The
structure–reactivity relationship was also investigated for a
series of para-substituted benzenesulfonyl azides.15
The scarcity of structural data of N-aroylimido complexes,
the lack of systematic investigations on the interaction of
aroyl azides with metal complexes and also the special atten-
tion devoted to the imido complexes of the late transition
metals1a,b,16 have prompted us to extend our studies to the inter-
action of the dimeric palladium complex 1 with benzoyl azides.
In order to facilitate the interpretation of our experimental
findings, we performed density functional calculations for a
series of benzoyl azides as well as for some of the N-benzoyl-
imido complexes. The results of our theoretical studies are
also reported here. An extensive study on the crystal structures
of the benzoyl azide reagents has been published recently.17
T h i s j o u r n a l i s © 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 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
2041

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See http://www.rsc.org/suppdata/dt/b4/b403200d/ for crystal-
lographic data in CIF or other electronic format.
Experimental
The work outlined below required benzoyl azides that are
stable as isolated compounds and can be purified efficiently for
kinetic and IR investigations. It was also our aim that the
substituents on the phenyl ring cover a wide range of
Hammett’s σ constants. An overview of literature data18
allowed a choice of a series of azides satisfying our require-
ments. Although we did not experience any dangerous
situations with the compounds selected, one must keep in mind
that working with isolated benzoyl azides can be hazardous
especially when ortho substituents are present.19
Kinetic studies
All kinetic experiments were carried out in dichloromethane at
25 ЊC. The rate constants k (MϪ1 Ϫ1) were determined for the
s
most reactive azides, 2a, 2b and 2h. In these kinetic runs, the
reaction of [Pd2Cl2(dppm)2], 1, with benzoyl azides was
monitored by recording the UV-vis spectra as a function of
time on a Hewlett–Packard 8453 diode-array spectrophoto-
meter (method 1). In the case of the less reactive azides,
determination of the ratio of rate constants for a selected pair
of azides was based on experiments in which 1 was allowed to
react simultaneously with two benzoyl azides added in the
same (25-fold) molar excess to the reaction mixture. Integration
of selected regions of the 1H NMR spectra of the isolated
products allowed us to calculate the molar ratio of the imido
complexes formed that equals the ratio of the rate constants
as well (method 2).25 A typical experiment was carried out
as follows: to a solution of 20.6 mg (0.0196 mmol) of
[Pd2Cl2(dppm)2] in 0.4 ml of dichloromethane was added a
solution of 43.2 mg (0.225 mmol) of 4-nitrobenzoyl azide and
43.0 mg (0.224 mmol) of 2-nitrobenzoyl azide in 0.6 ml of
dichloromethane. The capped vial was kept for 30 min in a
Experimental methods
NMR spectra were recorded in CDCl3 or CD2Cl2 solutions at
ambient temperature on Varian Unity Inova and Varian Unity
300 spectrometers. The resonance frequencies were 400 and 300
MHz for 1H, 101 and 75.4 MHz for 13C, 121.4 MHz for 31P and
282.2 MHz for 19F. Chemical shifts are reported with reference
to TMS (1H, 13C), 85 % H3PO4 (31P) and CFCl3 (19F). VT NMR
experiments were carried out in DMSO-d6 solutions. Infrared
spectra of the benzoyl azides were recorded in 0.02–0.03 M
CCl4 solutions using a Nicolet Magna 750 FT IR spectrometer.
IR spectra of complexes 3 were measured in CH2Cl2 solutions
on Nicolet Avatar 320 FT IR equipment (l = 0.0127 cm, reso-
lution 4 cmϪ1). Because of the poor solubility of the complexes
(1.2–3.0 mM), 2048 scans were accumulated to achieve a good
S/N ratio. UV-vis spectra were taken on a HP 8453 diode array
spectrophotometer using CH2Cl2 as solvent. Mass spectra were
obtained on samples suspended in m-nitrobenzyl alcohol
using a VG-ZAB-2SEQ spectrometer. Elemental analyses were
carried out on vacuum dried samples (methanol reflux, ca. 0.1
Torr).
water bath thermostated at 25
0.1 ЊC. TLC indicated full
conversion of the dimer. n-Hexane (6 ml) was added to the
reaction mixture and the solvents were removed under vacuum.
The yellow precipitate was suspended in hexane, transferred
onto a glass filter and washed repeatedly with hexanes to
remove the unreacted azides. Integration of the resonances
emerging at 3.95 and 3.21 ppm in the 1H NMR spectrum
allowed the calculation of the molar ratio of the imido com-
plexes formed from the two competing azides. Because of their
low solubilities, special care should be taken with mixtures
containing imido complexes derived from 4-fluorobenzoyl azide
or benzoyl azide to ensure complete dissolution of the sample.
X-ray diffraction
Intensity data were collected on an Enraf-Nonius CAD4
diffractometer at 293(2) K (graphite monochromator; Mo-Kα
radiation, λ = 0.71073 Å [3b, 3f] and Cu-Kα radiation,
λ = 1.54184 Å [3c, 3e]) using ω/2θ scans. Empirical absorption
corrections20 (psi-scans) were applied in all cases. The
structures were solved by direct methods21 and Fourier tech-
niques and refined by anisotropic least-squares22 on F 2 for all
non-hydrogen atoms. Hydrogen atomic positions were calcu-
lated from assumed geometries and were included in structure
factor calculations but were not refined. The structures of
3b–c were also determined in space groups P43/P41. The
C(O)C6H4CN moieties turned out to be disordered. The site
occupation factor for this moiety in 3b refined to 0.5 was within
2σ. Atomic coordinates for all non-hydrogen atoms indicated
the presence of an additional symmetry element, a two-fold axis
passing through the nitrene nitrogen and the carbonyl carbon
atoms (space group P43212, with half a molecule in the
asymmetric unit). In order to test which space group gives a
more adequate description of the crystal structure, the refine-
ment in both space groups was repeated starting with isotropic
atoms. Every tenth reflections (1997 in P43 and 999 in P43212)
were put aside for R(free) testing. The R(free) values were
always lower for the higher symmetry model (the final R(free)
values were 0.0604 for 1119 Fo > 4σ(Fo) and 0.1266 for all data
for P43 and 0.0476 for 573 Fo > 4σ(Fo) and 0.0925 for all data
for P43212). A similar procedure was followed for 3c verifying
that the space group was indeed P41212. This type of
disorder is not unprecedented at all,23 and analogous cases are
described23f,g for two A-frame complexes of palladium (space
group P41212). All phenyl rings were treated as regular
hexagons (C–C = 1.390 Å) for 3c because further positional
disorder was detected. Neutral atomic scattering factors and
anomalous scattering factors were taken from ref. 24.
Computational details
The structure of the investigated species has been optimized
at the B3LYP/SDDP level of density functional theory,
where B3LYP refers to the hybrid-type exhange-correlation
functional,26–28 and SDDP denotes a basis set that involves the
Stuttgart–Dresden quasi-relativistic ECP basis set for Pd29 and
the Dunning/Huzinaga DZP all electron basis set for the lighter
atoms.30–32 The harmonic vibrational frequencies have been
obtained from analytical second derivatives at the same level of
theory.
A simplified structural model was used to represent the
[Pd2Cl2(µ-NC(O)Ar)(dppm)2] complexes, where the phenyl
groups of the dppm ligands were replaced by hydrogens. As a
consequence of the dppm
H2PCH2PH2 approximation,
which was introduced to keep the computations within reason-
able bounds, the steric interactions between the bulky dppm
ligands and the aryl group of the benzoylimido moieties are
neglected, however, we expect that the electronic effects govern-
ing the structure of the Pd2(µ-NC(O)Ar) units are described
reasonably well with the [Pd2Cl2(µ-NC(O)Ar)(H2PCH2PH2)2]
model. All our calculations were performed using the Gaussian
98 program package.33
Preparations
[Pd2Cl2(dppm)2] was prepared according to an established
procedure.34 The known benzoyl azides 2a–h were synthesised
by literature methods.35 Purification of the crude products was
accomplished either by column chromatography or by sublim-
ation at ca. 0.1 Torr. To our knowledge, compound 2i is new18
and was obtained via the following procedure. A solution of
0.44 g (6.8 mmol) of NaN3 in 4 ml of water was added drop-
CCDC reference numbers: 232756–232759.
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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Table 1 Selected spectroscopic data of the imido complexes [Pd2Cl2(µ-NC(O)Ar)(dppm)2], 3a–i
1H NMR
31P NMRb
13C NMRb
IR datae
Aliphatic H atoms
δH/ppm (JHH; JHP)/Hz
CO group
δC/ppm
Other aliphatic
carbon atoms δC/ppm
δP/ppm
ν(CO)/cmϪ1
ν(CN)/cmϪ1
f
g
3a
4-NO2
3b
4-CN
3c
4-Br
3d
4-F
3e
4-H
3f
2.56 (12.9; 3.4)a
3.21 (12.9; 5.3)
2.59(13.0; 3.4)b
3.15 (13.0; 5.4)
2.56 (12.9; 3.4)b
3.19 (12.9; 5.5)
2.56 (12.9; 3.3)b
3.23 (12.9; 5.5)
2.56 (12.8; 3.3)b
3.26 (12.8; 5.5)
2.56 (12.9; 3.3)b
3.24 (12.9; 5.5)
3.56 (3H, s)
10.8
12.7
10.9
12.7
11.1
13.0
10.1
11.9
11.3
12.9
11.5
13.3
171.2
171.4
119.3 (s, CN)
1505
1357
172.5
1496
1356
c
1494
1362
c
1500, 1489
1488
1361
173.4
55.4 (s, CH3O)
1362, 1352
4-CH3O
3g
2.48 (13.0; 3.4)b
3.28 (13.0; 5.3)
2.75 (6H, s)
11.8
13.3
174.3
40.4 (s, (CH3)2N)
1482
1357
4-(CH3)2N
3h
2.66 (12.9; 3.3)a
3.95 (12.9; 5.5)
2.54 (12.8; 3.3)a
3.24 (12.8; 5.4)
10.5
11.2
10.0
12.0
167.4
169.9
(1499)f
1521
(1351)g
(1321)
2-NO2
3i
123.1 (qua, CF3, JCF = 273 Hz)d
3,5-CF3
a CDCl3. b CD2Cl2. c Not determined. d 19F NMR: 65.2 ppm (s). e In CH2Cl2 solutions. f Masked by νas(NO2) band. g Masked by νs(NO2) band;
m—multiplet; qua—quartet; s—singlet.
by-drop to 1.52 g (5.5 mmol) of 3,5-bis(trifluoromethyl)benzoyl
chloride dissolved in 12 ml of acetone at 0 ЊC. After 30 min of
stirring at this temperature, 30 ml of water was added and the
reaction mixture was extracted with CH2Cl2 (3 × 15 ml). Drying
over anhydrous MgSO4 and evaporation under vacuum
afforded a slightly yellow oil that was chromatographed on a
silica gel column using hexane, and mixtures of dichloro-
methane and hexane as eluents. Yield after purification: 0.9 g
C59H54Cl2N2OP4Pd2 requires 1214.1. 3h (1 h, 96 %), Found: C,
56.3; H, 4.0; N, 2.3. C57H48Cl2N2O3P4Pd2 requires C, 56.27; H,
3.98; N, 2.30%. 3i (30 h, 69 %), Found: C, 51.7; H, 3.5; N, 1.0.
C59H47Cl2F6NOP4Pd2ؒCH2Cl2 requires C, 51.75; H, 3.55; N,
1.01%. FAB-MS: m/z (Mϩ) 1307.1; C59H47Cl2F6NOP4Pd2
requires 1307.0.
Results and discussion
(58%). NMR data (CDCl3), 1H NMR: 8.47 (2H), 8.09 (1H); 13
C
Benzoyl azides ArC(O)N3, 2, react with the dimer [Pd2Cl2-
(dppm)2], 1, affording novel N-benzoylimido complexes, 3 (eqn.
(1)) (Ar = 3a—4-NO2C6H4, 3b—4-CNC6H4, 3c—4-BrC6H4,
3d—4-FC6H4, 3e—C6H5, 3f—4-CH3OC6H4, 3g—4-(CH3)2-
NC6H4, 3h—2-NO2C6H4, 3i—3,5-(CF3)2C6H3).
NMR: 170.2 (CO), 132.9, 132.8 (qua, JCF = 34 Hz), 129.6,
127.6, 122.9 (CF3, qua, JCF = 274 Hz); IR data (CCl4, cmϪ1):
2217 and 2147 (νas(N3)), 1706 (ν(CO)), 1280, 1221, 1187, 1149.
Preparation of the imido complexes 3a–i. The following recipe
describes a typical synthesis. 210 mg (0.93 mmol) of 4-BrC6H4-
C(O)N3 dissolved in 2 ml CH2Cl2 was added to a solution of
158 mg (0.15 mmol) of [Pd2Cl2(dppm)2] in 4 ml of CH2Cl2. The
solution was kept in the dark for 24 h. TLC (Merck Kieselgel
60 F254, dichloromethane/ethyl acetate 1:1) indicated full con-
version of the dimer. Adding methanol completed precipitation
of the product, which was collected on a glass filter and washed
repeatedly with small portions of hexane and ethyl ether. The
isolated yellow crystals (0.17 g, 91%) were dissolved in CH2Cl2
and reprecipitated with hexanes to remove traces of unreacted
azide. Other N-benzoylimido complexes were prepared
analogously but the time necessary to complete the reaction
was strongly influenced by the substituent of the benzoyl azide.
Approximate reaction times, yields and elemental analytical
data: 3a (4 h, 94 %), Found: C, 56.2; H, 3.8; N, 2.4.
C57H48Cl2N2O3P4Pd2 requires C, 56.27; H, 3.98; N, 2.30%. 3b
(6 h, 98 %), Found: C, 57.9; H, 4.1; N, 2.4. C58H48Cl2N2OP4Pd2
requires C, 58.21; H, 4.04; N, 2.34%. 3c (20 h, 91 %), Found: C,
54.7; H, 3.7; N, 1.2. C57H48BrCl2NOP4Pd2 requires C, 54.75; H,
3.87; N, 1.12%. 3d (2 d, 88 %), Found: C, 57.3; H, 4.1; N, 1.1.
C57H48Cl2FNOP4Pd2 requires C, 57.55; H, 4.07; N, 1.18%. 3e (4
d, 88 %), Found: C, 57.8; H, 4.3; N, 1.2. C57H49Cl2NOP4Pd2ؒ
H2O requires C, 57.55; H, 4.32; N, 1.18%. FAB-MS: m/z (Mϩ)
1171.1; C57H49Cl2NOP4Pd2 requires 1171.0. 3f (5 d, 70 %),
Found: C, 57.8; H, 4.2; N, 1.2. C58H51Cl2NO2P4Pd2 requires C,
57.97; H, 4.28; N, 1.17%. 3g (10 d, 60 %), Found: C, 56.5;
H, 4.2; N, 2.1. C59H54Cl2N2OP4Pd2ؒ1/2CH2Cl2 requires C,
56.84; H, 4.41; N, 2.23%. FAB-MS: m/z (Mϩ) 1214.4;
[Pd2Cl2(dppm)2] (1) ϩ ArC(O)N3 (2)
[Pd2Cl2(µ-NC(O)Ar)(dppm)2] (3) ϩ N2 (1)
Reaction (1) takes place smoothly at ambient temperature
(20–25 ЊC) in the dark. Benzoyl azides are known to rearrange
thermally to amines or carbamic acid derivatives with the
involvement of free nitrene intermediates. Although a small
amount of N,NЈ-bis(2-nitrophenyl)urea was observed in the
reaction of 1 with 2h, the rest of the azides did not produce
noticeable amounts of Curtius-type by-products and the
unreacted azides could be recovered and identified (IR, TLC) at
the end of the reaction. The good selectivity for the imido
adducts allows the conclusion that reaction (1) is not associated
with the appearance of free nitrene species. Most nitrenes are
stated to be extremely reactive and one cannot expect them
to react particularly selectively with a single constituent of a
reaction mixture. Although a dimolybdenum benzoylimido
complex was generated under photolytic conditions with
the potential participation of free nitrene intermediates,11a in
reaction (1) decomposition of the azides certainly takes place
in the coordination sphere of the palladium ions.
Solution characterisation of complexes 3
The 1H NMR data reported in Table 1 readily prove that
complexes possessing the expected A-frame structures are
formed. The doublet/quintet splitting pattern of the methylene
hydrogen atoms supports the apical position of the imido
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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fragments. The range of the chemical shifts and the extent of
the JHH and JHP coupling constants correspond to those
observed with structurally closely related complexes.13,36 It
seems worth mentioning that the high and low frequency
resonances of the methylene hydrogens show up in narrow
ranges of 2.48–2.66 and 3.15–3.28 ppm, respectively, with
the only exception of the high frequency multiplet of the
2-nitro-derivative, 3h, that appears at 3.95 ppm. The DFT
calculations carried out for the [Pd2Cl2(µ-NC(O)C6H4NO2-2)-
(H2PCH2PH2)2] model compound reveals the existence of an
appreciable intramolecular interaction between the nitro group
and the axial methylene hydrogens (d(O ؒ ؒ ؒ H) = 2.26 Å, see
position of the apical moiety with respect to the P nuclei. As
proved by structural investigations (see below), the non-equiv-
alence of the P nuclei can be attributed to hindered rotation
about the N–C(O) bonds. This behaviour of the benzoylimido
complexes is characteristically different from that of the sulfo-
nylnitrene congeners which all but the sterically overcrowded
[Pd2Cl2(µ-NSO2Tip)(dppm)2] adduct displayed a sharp singlet
in their solution 31P spectra thereby supporting free rotation
around the N–S axes (Tip
= 2,4,6-tris(isopropyl)phenyl
group).13,14 No hindered rotation was observed to emerge in the
peripheral regions of molecules 3. Resonances ascribed to the
hydrogen and carbon atoms residing in the ortho positions of
the dppm phenyl groups support free rotation around the
P–Caryl bonds. Likewise, there is no sign of a barrier to internal
rotation for the phenyl ring of the para- and meta-substituted
benzoyl fragments at ambient temperature.
Fig. 1). As the benzoyl phenyl group twists about the Cipso
–
C(O) bond, the ortho nitro group can significantly alter the
environment of the axial methylene hydrogen atoms compared
to that experienced in the case of the para- and meta-substi-
tuted derivatives. We believe that it is the influence of the
ortho nitro substituent that is reflected by the remarkable value
of this chemical shift. A similar hydrogen bond for the para-
substituted derivative is not feasible.
Crystallographic description of complexes 3
In order to shed light on the fine structural details of the imido
complexes, X-ray diffraction studies were carried out on single
crystals of 3b–c and 3e–f. The molecular structures are shown
in Figs. 2–3. Crystal data and data collection parameters
are summarised in Table 2. Selected bond lengths and angles are
presented in Table 3.
The crystal structures of 3b–c and 3e–f are composed of
discrete molecules that all crystallise in chiral space groups. The
comparison of the enantiomorphic 3b–c and 3e–f reveals the
chiral nature of these complexes. The chiral element is the
propeller-like C2P4Pd2 ring. A search of the Cambridge Crystal-
lographic Database37 for [M2Cl2(µ-X)(dppm)2], (M = Pt, Pd,
X = any atom or group) analogues crystallising in tetragonal
space groups retrieved 16 hits23 (7 P41; 6 P43; 2 P41212, 1
P43212) showing that the specimens subjected to investigation
were selected randomly from spontaneously resolved crystal
samples.
Fig. 1 Calculated structures for 2-NO2- (a) and 4-NO2-substituted
(b) [Pd2Cl2(µ-NC(O)Ar)(H2PCH2PH2)2] model complexes (H atoms
belonging to the aryl and PH2 groups have been omitted for clarity).
The palladium centres are bridged by two mutually trans
dppm ligands and by the imido nitrogen atoms, which, in turn,
are trans to the chloro ligands. In general, these molecules
can be described as typical A-frame structures in which the
methylene carbon atoms, as expected, are bent toward the
apical nitrogen atoms. This arrangement lends an extended
boat conformation to the eight-membered C2P4Pd2 rings. The
Pd–P distances (2.303–2.342 Å) and the Pd–Cl bond lengths
(2.336–2.350 Å) lie in the expected ranges and are essentially
the same as those observed for the sulfonylnitrene complexes.13
The analysis of the aromatic region of the 1H and 13C NMR
spectra revealed four different sets of dppm phenyl groups. The
non-equivalence of the phenyl groups located on the same side
of the C2P4Pd2 ring indicates a reduced symmetry compared to
the previously described sulfonylnitrene complexes13 and is
supported by the 31P{H} NMR spectra which invariably show
up as AAЈBBЈ-type multiplets for all the imido complexes 3.
This splitting pattern is typical for unsymmetrical A-frame
complexes and is generally induced by an asymmetric dis-
Fig. 2 The molecular structures of 3c (left) and 3b. Only one disordered fragment is shown. Hydrogen atoms are omitted for clarity.
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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Table 2 Crystal data, data collection and refinement parameters of N-benzoylnitrene complexes 3b–c and 3e–f
3b
3c
3e
3f
Empirical formula
Formula weight
Crystal system
C58H50Cl2N2OP4Pd2
1198.58
Tetragonal
P43212
C57H48BrCl2NOP4Pd2
1250.45
Tetragonal
P41212
C57H49Cl2NOP4Pd2
1171.55
Tetragonal
P41
C58H51Cl2NO2P4Pd2
1201.58
Tetragonal
P43
Space group
Unit cell dimensions/Å
a = 15.011(7)
c = 25.622(5)
5773(4)
a = 15.079(1)
c = 25.970(3)
5905.0(9)
4
1.407
7.846
a = 21.247(3)
c = 14.214(2)
6416.7(13)
4
1.213
6.485
a = 21.224(5)
c = 14.230(2)
6410(2)
Volume/Å3
Z
4
4
Density (calculated)/Mg mϪ3
Absorption coefficient, µ/mmϪ1
F(000)
1.379
0.865
2424
1.245
0.780
2432
2504
2368
Crystal size/mm
0.50 × 0.45 × 0.25
1.0000/0.8656
2.74–31.98
22041
0.25 × 0.20 × 0.12
1.0000/0.6526
4.48–75.49
13245
0.25 × 0.10 × 0.10
1.0000/0.8267
2.94–75.04
14586
0.40 × 0.16 × 0.16
1.0000/0.9938
2.58–31.97
24367
Maximum/minimum transmission
θ-range for data collection/Њ
Reflections collected
Decay (%)
21%
2%
2%
6%
Independent reflections/R(int)
Reflections I > 2σ(I )
Data/parameters
9952/0.0731
5652
9952/ 356
0.908
Ϫ0.01(3)
R1 0.0440
wR2 0.0898
R1 0.0926
wR2 0.1036
0.470/Ϫ0.949
6031/0.0888
5118
6031/ 284
1.041
12907/0.0290
8537
12907/ 607
0.912
Ϫ0.02(1)
R1 0.0513
wR2 0.1079
R1 0.0933
wR2 0.1211
0.402/Ϫ0.763
22018/0.0178
12729
22018/ 623
0.903
Ϫ0.07(2)
R1 0.0542
wR2 0.1176
R1 0.1150
wR2 0.1318
1.237/Ϫ1.552
Goodness-of-fit on F 2
Absolute structure parameter
Final R indices [I > 2σ(I )]
0.00(2)
R1 0.0610
wR2 0.1501
R1 0.0690
wR2 0.1564
1.330/Ϫ1.147
R indices (all data)
Largest diff. peak/hole/e ÅϪ3
Fig. 3 The molecular structures of 3e (left) and 3f. Hydrogen atoms are omitted for clarity.
The angles formed by the neighbouring ligands of the metal
centres (85–95Њ) reflect a slight distortion from the square
planar configuration around the palladium atoms. Accordingly,
the P–Pd–P and N–Pd–Cl axes are slightly bent, the deviation
from the ideal 180Њ is more pronounced in the former case. The
Pd ؒ ؒ ؒ Pd distances cover the range of 3.127(1)–3.247(1) Å and
are consistent with the lack of metal–metal bonding inter-
action. These values, however, are noticeably shorter than those
found in the sulfonylnitrene congeners, in which the bridging
nitrogen atoms span Pd ؒ ؒ ؒ Pd distances of 3.231(1)–3.350(1)
Å. According to a recent crystallographic study, the palladium–
palladium bond length in the parent [Pd2Cl2(dppm)2], 1, is
2.661(1) Å.38 The increase of the Pd ؒ ؒ ؒ Pd separation by
0.5–0.6 Å is in conformity with the structural changes observed
at the formation of other A-frames incorporating one-
membered apical bridges.36 The bridging imido nitrogen atoms
reside symmetrically between the palladium centres. The Pd–N
bond lengths do not seem to be influenced by the electron
withdrawing or electron-donating nature of the substituent
attached to the benzoyl group and fall into the narrow range of
1.972(5)–2.001(4) Å. The mean value of the corresponding
bond lengths is slightly longer in the sulfonylnitrene adducts
(d(Pd–N,av) = 2.015 Å)
The most remarkable structural features of the benzoyl-
nitrene complexes are associated with the imido moiety. The
short N(1)–C(3) and the long O(1)–C(3) bond distances are
common properties of the compounds 3b–c and 3e–f and
suggest electron donation from the bridging imido nitrogen
atom towards the carbonyl group. The multiple-bond nature of
the N–C(O) bonds is substantiated by the N(1)–C(3) distances
of about 1.32 Å. For comparison, C(sp3)–N and C(sp2)–N bond
lengths have mean values of 1.47 and 1.38 Å, while C᎐N double
᎐
bonds are typically 1.28 Å long.39 Further supporting evidence
for the strengthening of the N–C bonds in the imido complexes
comes from the inspection of the data presented in Table 4. As
shown in line 3, the respective bond lengths, C(7)–N(1), in the
parent azides are by far longer (1.41–1.42 Å) and correspond
rather to a carbon–nitrogen single bond.
As expected, the shrinkage of the N–C bonds is accompanied
by the elongation of the O(1)–C(3) distances. Unfortunately,
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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Table
3
Selected bond lengths and angles of the structurally
characterised N-benzoylimido complexes 3b–c and 3e–f
documented and have been attributed to the contribution of
the polar resonance form B to the distribution of the valence
electrons along the N–C(O) fragment (Chart 1).40
3b
3c
3e
3f
Pd(1) ؒ ؒ ؒ Pd(2)
Pd(1)–N(1)
Pd(2)–N(1)
C(3)–N(1)
C(3)–O(1)
C(3)–C(4)
Pd(1)–Cl(1)
Pd(2)–Cl(2)
Pd(1)–P(1)
Pd(1)–P(2)
Pd(2)–P(3)
Pd(2)–P(4)
C(1)–P(1)
3.127(1)
1.988(3)
3.151(1)
1.972(5)
3.247(1)
1.986(6)
1.994(5)
1.296(9)
1.31(1)
3.246(1)
1.999(4)
2.001(4)
1.319(7)
1.295(7)
1.479(7)
2.336(1)
2.350(1)
2.319(1)
2.336(1)
2.314(1)
2.326(1)
1.837(5)
1.840(5)
1.830(5)
1.849(5)
1.321(6)
1.319(4)
1.45(2)
1.310(7)
1.321(1)
1.416(7)
2.339(2)
1.47(1)
2.337(1)
2.342(2)
2.342(2)
2.303(2)
2.320(2)
2.324(2)
2.335(2)
1.837(7)
1.826(7)
1.832(7)
1.835(7)
2.309(1)
2.342(1)
2.335(2)
2.313(2)
Chart 1
The structural peculiarities of the N–C(O) moiety in complexes 3
1.831(4)
1.835(4)
1.817(5)
1.837(8)
C(1)–P(4)
C(2)–P(2)
C(2)–P(3)
Benzoyl amides have been repeatedly subjected to spectroscopic
and crystallographic studies and the bonding properties of the
amide moiety as well as the energetics of the rotation about the
N–C(O) axis have been investigated in detail using various
model compounds. In their comprehensive study, Brown and
co-workers have demonstrated that the physicochemical data of
various anilides and toluamides are basically consistent with
the resonance model.40a Where resonance is favoured, short
N–C(O) bonds (1.34–1.37 Å) are formed and the rotational
barriers about the N–C(O) axis are high (61–69 kJ molϪ1).
Disfavoured resonance is, however, accompanied by the
elongation of the N–C(O) bond distance (1.40–1.42 Å) and
reduced barriers to rotation (25–34 kJ molϪ1). The ν(CO)
stretching band located at lower frequencies in the former and
at higher frequencies in the latter case has been found to follow
the structural changes. Prompted by the indisputable structural
relationship of complexes 3 and the organic amides, we decided
to compare some physicochemical data of these two groups of
compounds.
Pd(1)–N(1)–Pd(2)
Pd(1)–N(1)–C(3)
Pd(2)–N(1)–C(3)
N(1)–C(3)–C(4)
N(1)–C(3)–O(1)
O(1)–C(3)–C(4)
Cl(1)–Pd(1)–N(1)
Cl(2)–Pd(2)–N(1)
P(1)–Pd(1)–P(2)
P(3)–Pd(2)–P(4)
P(1)–C(1)–P(4)
P(2)–C(2)–P(3)
103.7(2)
128.1(1)
106.0(4)
127.0(2)
109.3(3)
117.8(5)
131.6(5)
126.3(8)
119.5(8)
114.2(8)
176.7(2)
178.8(2)
173.3(1)
174.0(1)
115.2(4)
115.0(4)
108.5(2)
134.8(3)
115.6(3)
124.8(5)
119.4(5)
115.8(5)
178.3(1)
176.5(1)
175.0(1)
173.6(1)
114.2(2)
114.4(2)
130.0(5)
115.5(4)
113.9(6)
176.6(1)
139.0(7)
115.1(7)
105(1)
176.5(1)
170.1(1)
114.5(2)
169.3(1)
115.4(4)
the benzoyl moieties in compounds 3b and 3c are disordered
and this circumstance does not allow determination of some
important bond distances with sufficient accuracy. The respect-
ive bond length in 3f, however, clearly shows that the C(3)–O(1)
distance is longer by ca. 0.08 Å than the corresponding C(7)–
O(1) bond in the parent azide. The intermediate value of the
The dynamic behaviour of the imido complexes was studied
by observing their 31P NMR spectra at elevated temperatures.
The spectral lines recorded in a DMSO-d6 solution of 3a
showed broadening and merging upon stepwise increase of the
temperature but coalescence was not achieved even at 448 K.
Darkening of the solution indicated partial decomposition of
the sample. Spectral changes of a solution of 3f carrying the
electron-donating Me2N group could be traced up to 423 K
without approaching coalescence but severe disintegration was
observed at higher temperatures and the sample was rapidly
destroyed at 443 K. Coalescence was observed, however, at 438
K in the case of the 2-NO2 derivative, 3h, which allowed us to
C(3)–O(1) bond (1.295(7) Å) between a C(sp2)᎐O bond distance
᎐
(1.21 Å) and a C(sp3)–O single bond length (1.41 Å) unequivo-
cally indicates that the C᎐O bond has weakened compared to its
᎐
strength in the free azide and has a fractional bond order
between 1 and 2. It seems reasonable to suppose that a similar
statement holds true for the rest of the imido complexes as
well.
The availability of a good orbital overlap between the bridg-
ing N atom and the carbonyl group imposes a planar geometry
around the apical nitrogen. Accordingly, the sum of the angles
about the imido nitrogen approaches the ideal 360Њ (͚ꢀ(N) =
358.7–360.1Њ) and the displacement of the bridging nitrogen
atoms from the planes determined by the Pd(1), Pd(2) and C(3)
atoms does not exceed 0.1 Å. These structural peculiarities
of the benzoylimido complexes recall the most outstanding
structural features of carboxamides. The partial double bond
character of the N–C(O) bonds of carboxamides and the
hindered rotation about the N–C(O) axes have been well
calculate a ∆G‡ of 88.3 kJ molϪ1. Based on the assumption
rot
that the experimentally available highest temperature was at
least lower by 10 K than that of coalescence, a ∆G‡ ≥91 kJ
rot
molϪ1 was derived for 3a. Both values are much higher than the
rotational barriers observed for benzoyl amides. A collection of
selected physical properties associated with the dynamic
behaviour of the imido complexes 3 and their organic counter-
parts are presented in Table 5.
Table 4 Selected bond lengths and angles of the structurally characterised benzoyl azides 2a–b and 2f–h
2a
2b
2f
2g
2h
Molecule 1
Molecule 2
Molecule 1
Molecule 2
N(1)–N(2)
N(2)–N(3)
C(7)–N(1)
C(7)–O(1)
C(7)–C(1)
1.255(1)
1.111(1)
1.417(1)
1.202(1)
1.489(1)
1.257(2)
1.113(2)
1.410(2)
1.200(1)
1.485(2)
1.264(2)
1.110(2)
1.405(2)
1.201(1)
1.483(2)
1.247(2)
1.117(2)
1.419(2)
1.211(2)
1.467(2)
1.252(3)
1.121(3)
1.422(3)
1.218(2)
1.460(3)
1.261(3)
1.110(3)
1.411(3)
1.201(2)
1.484(3)
1.266(3)
1.103(3)
1.410(3)
1.210(2)
1.482(3)
N(1)–N(2)–N(3)
C(7)–N(1)–N(2)
C(1)–C(7)–N(1)
172.6(1)
113.6(1)
112.3(1)
175.1(1)
111.8(1)
112.5(1)
175.0(2)
111.7(1)
113.1(1)
174.1(2)
112.8(1)
111.8(1)
175.0(3)
113.2(2)
112.2(2)
173.7(2)
112.1(2)
112.3(2)
174.2(2)
111.6(2)
112.3(2)
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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Table 5 Selected data associated with the dynamic behaviour of para-
substituted dimethyl benzamides and para-substituted benzoyl imido
complexes 3
shows a notable amount of electron flow (Ϫ0.55 e) from the
Pd2Cl2(H2PCH2PH2)2 frame towards the imido group in 3eЈ,
which affects mostly the N atom (the net N atomic charges in
3eЈ and (CH3)2NC(O)(C6H5) are Ϫ0.51 and Ϫ0.06, respect-
ively). The charge transfer thus enhances the ability of the N
atom to donate its lone pair into the N–C(O) linkage, which
4-XC6H4C(O)NMe2
Imido complexes 3
d(C–N)/Å
1.35141a and 1.36741b
60.9–68.940b
1.30–1.32
≥88.3
∆G‡rot/kJ molϪ1
T c/K
strengthens the C–N bond and weakens C᎐O.
᎐
277–31740g
≥438
Although the strength of the N–C(O) bond is thought to be
the primary reason of the asymmetric disposition of the
benzoyl moiety, some other factors, e.g. the potential inter-
action of the carbonyl oxygen atom with the near-by palladium
center, were also considered. Due to the shift of electrons from
the bridging nitrogen atom toward the carbonyl group, there is
a partial negative charge on the carbonyl oxygen atom and
δ(13CO) (ppm)
ν(CO)/cmϪ1
169.2–171.540c
1642–165240e
171.2–174.3
1488–1521
Table 6 N–C(O) and C᎐O bond distances calculated at the B3LYP/
᎐
SDDP level of DFT for [Pd2Cl2(µ-NC(O)Ar)(H2PCH2PH2)2] and the
corresponding dimethyl benzamides
a weak C᎐O ؒ ؒ ؒ Pd bonding interaction may be formed. The
᎐
d(C–N)/Å
d(C–O)/Å
C᎐O ؒ ؒ ؒ Pd distances in the four crystallographically char-
᎐
N,N-dimethyl
benzamides
N,N-dimethyl
benzamides
acterised complexes range between 2.898 (3f ) and 3.153 (3b) Å
and do not differ significantly from the sum of the van der
Waals radii (3.03 Å). Moreover, such interaction would
probably cause the lengthening of the Pd–N and Pd–Cl bonds
of the palladium center involved but this expectation is not
supported by the experimental observations. Therefore,
Pd ؒ ؒ ؒ O attractions do not seem to play any role in deter-
mining the rotational barrier in complexes 3. Consequently, the
high values of the barrier to rotation in complexes 3 should be
attributed to the enhanced strength of the N–C(O) bonds
rather than to secondary effects.
We note, that ortho substituents decreasing the coplanarity of
the carbonyl and aryl moieties increase the barrier to rotation in
the benzoyl amide series.40b,d As a reverse statement seems to be
true for the ∆G‡ values obtained for 3a and 3h, it seems reason-
able to suppose that intramolecular interactions in 3h not only
result in a twist about the C(O)–Cipso bond but also twist the
C(O) group about the N–C(O) axis thereby weakening the
pπ–pπ interaction within the imidocarbonyl moiety. Unfortun-
ately, our efforts to grow single crystals of 3h have not been
successful and this conclusion could not be checked experi-
mentally. DFT calculations carried out on the model com-
pounds 3aЈ and 3hЈ did not reveal any significant differences
either for the N–C(O) or the C–Cipso bond lengths (see Fig. 1).
The different dynamic behaviour of these complexes might be
associated with intramolecular steric interactions between the
dppm phenyls and the apical moieties.
3Ј
3Ј
a
e
f
1.345
1.349
1.352
1.335
1.375
1.380
1.383
1.367
1.259
1.259
1.260
1.254
1.233
1.234
1.235
1.231
h
A comparison of the N–C(O) bond lengths makes obvious
that the main structural difference between the para-substituted
benzoyl amides and complexes 3 is the remarkably short C–N
distances in the latter. Besides the various pieces of crystallo-
graphic and NMR spectroscopic evidence, this is supported by
the infrared absorption spectra of the complexes providing
independent information on the unusual bonding properties
of the imido moiety. After interactive subtraction of the
IR absorption features of the dichloromethane solution
of [Pd2Cl2(dppm)2] and the solvent (CH2Cl2) from the spectra
of the complexes, the appearance of two broader spectral
features can be consistently observed: a strong one (showing
some additional structure) around 1500 cmϪ1 and a medium
intensity band near 1350 cmϪ1 (see Table 1), that should involve
the stretching vibrations of the C(3)–O(1) carbonyl bond and
the N(1)–C(3) bond, respectively. The assignments are fully
supported by our DFT frequency and normal mode calcu-
lations. The fact that these frequencies (especially the former
one) are outside the usual regions of carbonyl (C᎐O double
᎐
bond, above 1600 cmϪ1) and C–N single bond vibrations
(around 1100 cmϪ1) indicate once again that the CO bond is
significantly weaker (longer) and the CN bond is stronger
(shorter) than those in the corresponding amides which have no
notable absorptions in the above mentioned regions.
Due to the scarcity of structurally characterised N-benzoyl-
imido adducts, there is little room for a comparative discussion.
In a mononuclear tungsten complex incorporating the 4-CH3-
C6H4C(O)N fragment as ligand,12 the C᎐O bond is character-
᎐
istically shorter than that in complexes 3 and the W–N linkage
The structural peculiarities of complexes 3 and those of
dimethyl benzamides are reflected by the results of our DFT
calculations (Table 6), which reveal clear differences in the
structure of the N–C–O linkages in (CH3)2NC(O)Ar dimethyl
benzamides and the corresponding [Pd2Cl2(µ-NC(O)(Ar)(H2-
PCH2PH2)2] model compounds 3Ј (the meaning of the letters a,
e, f, and h is the same as for the unprimed series; 3). These
results indicate that the C–N bonds are systematically shorter
by about 0.03 Å in palladium complexes, whereas the C–O
bonds are elongated to the same extent with respect to those in
the benzamide analogues.
It seems reasonable to assume that the relatively rigid nature
of the N–C(O) linkage in 3 can be attributed to differences that
exist in the electronic structure of the Pd–N and CH3–N bonds.
It has been postulated that the importance of the resonance
form B depends on the ease with which a positive charge on the
nitrogen is stabilised in the molecule.40a As palladium is less
electronegative than carbon, the Pd–N bond is more polarised
than the CH3–N linkage, which induces an excess of electron
density on the imido nitrogen as compared to that in dimethyl
benzamides. Indeed, the Mulliken population analysis carried
out for 3eЈ and the corresponding amide, C6H5C(O)N(CH3)2,
seems to be of a multiple-bond nature. In a tetranuclear
triruthenium cobalt complex, a C᎐O bond length of 1.22(1) Å
᎐
and a N–C(O) bond distance of 1.43(1) Å was observed42
within the N-benzoylimido µ4-nitrene moiety, indicating that
the nitrogen lone pair is involved in N–metal coordinative
bond(s). Likewise, undisturbed C᎐O double bonds were found
᎐
in the dimolybdenum complexes [CpMoO(µ-NC(O)OEt)2-
MoOCp]
and
[CpMoO(µ-NC(O)OEt)(µ-O)MoOCp]11b
embedding N-ethoxycarbonyl moieties. Again, the nitrogen
lone pair is shifted toward the high valent Mo(V) centers rather
than toward the carbonyl group. In all these cases, the nitrogen–
metal interaction is dominating in the accommodation of the
nitrogen lone pair. On the contrary, donation from the nitrogen
towards the neighbouring organic group has been well
documented and seems to be a general behaviour of the
bridging imido nitrogen in dinuclear late transition metal
complexes. The dπ-pπ interaction of the bridging imido
nitrogen atom with the sulfur atom resulted in short S–N bond
distances [1.541(3)–1.565(3) Å] and a nearly planar geometry
about the nitrogen atom in a series of dinuclear sulfonyl nitrene
complexes of palladium.13 Sharp and coworkers have observed
that in the arylnitrene complex [Rh2(CO)2(µ-NC6H4NO2-4)-
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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(dppm)2] the imido nitrogen donates its electron pair towards
the phenyl group and induces a quinoidal alternation of the
C–C bonds in the latter.43 Ongoing studies in our laboratory on
the interaction of 1 with phenyl azides have revealed that the
analogous [Pd2Cl2(µ-NC6H4NO2-4)(dppm)2] nitrene adduct
possesses a trigonal planar symmetry about the bridging
nitrogen atom and long (1.428(3), 1.398(4) Å) and short
(1.365(3) Å) C–C bonds alternate in the 4-nitrophenyl group
attached to it.44
An inspection of data presented in Table 4 demonstrates that
the bond distances and angles of the azidocarbonyl moieties
are only moderately affected by the electronic nature of the
substituents. The most characteristic structural data supporting
the expectation that substituents do influence the bondings
at the reactive end of the molecules are the differences between
the N(1)–N(2) and N(2)–N(3) bonds. The deviation of the two
N–N bond lengths (∆dN) is relatively large for the electron-
withdrawing nitro- (2h: 0.157 Å, 2a: 0.147 Å) and cyano-groups
(2b: 0.149 Å) and noticeably shorter for 2f (0.130 Å) and 2g
(0.131 Å), both carrying electron-releasing substituents. The
larger value of ∆dN for 2a, 2b, and 2h is in favour of an
increased contribution of the canonical form D (Chart 2) to
the resonance hybrid when electron-withdrawing substituents
are present and also suggests the azide to be more electrophilic.
We note that an analogous trend of the ∆dN differences has
been observed for sulfonyl azides.15,17
Structure–reactivity relationship
The structure–reactivity relationship in the reaction of arene-
sulfonyl azides with [Pd2Cl2(dppm)2] has been studied by
kinetic45 and structural15 investigations. It seemed of interest to
us to check if a similar relationship between the structural
parameters and the reactivity exists for the benzoyl azide
congeners. Due to the potential thermal instability of reagents
2, it was not our aim to conduct an extensive kinetic
investigation. Instead, our studies were confined to establishing
an order of reactivity at 25 ЊC.
Curve 1 in Fig. 4 demonstrates that the reactivity of azides 2
is significantly affected by the substituents of the phenyl moiety.
The linear dependence of the relative reactivities as a function
of σ constants46 suggests that in reaction (1) the Hammett rule
is obeyed. The higher reactivity of the substrates carrying
electron-withdrawing substituents indicates that azides 2 act as
electrophiles toward the palladium dimer. The large positive
value of the ρ reaction constant (1.80) serves as evidence for
the relatively larger impact of the substituents in determining
the electronic properties of the azide moiety in compounds 2
compared to that with sulfonyl azides (ρ = 1.3945).
Chart 2
Although the C(7)–O(1) and C(7)–C(1) distances show
changes that can be explained in terms of inductive and meso-
meric effects of R groups, other data presented in Table 4
demonstrate the insensitivity of the structural parameters to the
electronic properties of the substituent.
IR spectroscopy has been an efficient tool for the structural
characterisation of organic azides. In the case of benzoyl
derivatives, however, observation of the N3 stretching bands
does not offer an instant help in understanding how the
substituents influence the bonding properties of the azido
group because the νas(N3) bands (the corresponding normal
mode has a prevailing contribution from the stretching
᎐
vibration of the terminal N᎐N bond) observed in the 2200–
Fig. 4 Dependence of the log(k/kH) values (solid squares) and the
νasЊ(N3) frequencies (experimental: open triangles, DFT method, scaled
data: solid triangles) on the σ constants.
᎐
2100 cmϪ1 region show splitting of varying extent. The phen-
omenon has been explained in terms of Fermi resonance with
the combination mode of the νs(N3) vibration around 1180
cmϪ1 and the aromatic ring vibration at 990 cmϪ1 falling in the
same region.47 Based on the well known principles of Fermi
resonance48 (taking into account the anharmonicity of the
vibrations involved as well), we have made some efforts to
calculate the unperturbed frequency of the νas(N3) vibration,
i.e. locate the position νЊ of this band in the IR spectra of all
azides studied. Our efforts to find the νasЊ(N3) frequencies were
successful with six benzoyl azides but, unfortunately, gave
doubtful results with 3c and 3f where the picture was compli-
cated by the presence of more than two potentially interacting
bands. The positions of the pairs of observed components of
the νas(N3) doublet, the corrected νasЊ(N3) values obtained with
A comparison of the second-order rate constant, k, of the
4-nitrobenzoyl azide (3.9 × 10Ϫ3 MϪ1 Ϫ1) with that of 4-nitro-
s
benzenesulfonyl azide (96.0 × 10Ϫ3 MϪ1 Ϫ1) reveals the more
s
moderate reactivity of the benzoyl azide reagents. Moreover,
2a, one of the most reactive benzoyl azides, reacts even more
sluggishly than the 4-methoxybenzenesulfonyl azide, the least
reactive member of the sulfonyl azide series studied earlier
(k25 = 28.9 × 10Ϫ3 MϪ1 sϪ1).45 The significantly higher reactivity
of the sulfonyl azide can be rationalised in terms of the
more pronounced electrophilic nature of the sulfonyl group as
compared to that of the carbonyl group.
In order to gain a deeper insight into the structure–reactivity
relationship, we determined first the structures of the most and
the least reactive benzoyl azides by means of single crystal
X-ray diffraction. The common structural properties and the
individual peculiarities of azides 2a, 2b and 2g–h have been
discussed.17 We recall here only selected data (see Table 4) that
may have relevance to the structure–reactivity relationship of
azides 2.
reasonable certainty as well as the frequencies of the ν(C᎐O)
stretching bands are collected in Table 7. Two more columns
contain, for comparison, frequencies of ν Њ(N ) and ν(C᎐O)
vibrations calculated at the B3LYP/SDDP level of DFT. They
are available for all nine molecules, and are unaffected by
uncertainties connected to Fermi resonance or experimental
difficulties.
᎐
᎐
as
3
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
2048

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Fig. 5 Molecular structures and Newman projections of benzoyl azides 2a (left) and 2h (right).
Table 7 The experimental and the calculated positions of the νas(N3)
and ν(CO) bands and the relative reactivities of benzoyl azides 2
Table 8 Selected structural data of benzoyl azides 2a and 2h
2a 2h
Observed in IR spectra
DFT calculated data
0.147
Ϫ1.8(2)
2149.1
∆dN/Å
0.157
18.8(2), mean
2151.7
1713.1
νas(N3)
νasЊ(N3)
ν(C᎐O)
νasЊ(N3)a
ν(C᎐O)b
k/kH
᎐
᎐
ꢀ[C(2)–C(1)–C(7)–O(1)]/Њ
νasЊ(N3)/cmϪ1 (experimental)
ν(CO)/cmϪ1(experimental)
2a
2b
2c
2d
2e
2f
2178.6
2136.1c
2180.7
2136.4c
2178.1
2135.4c
2178.3
2136.2c
2173.8
2134.7c
2174.6
2134.7c
2173.0
2132.5c
2171.9
2132.9a
2216.8
2146.9c
2149.1
2152.1
1701.2
1701.1
1696.0
1698.5
1699.5
1693.5
1684.1
1713.1
1706.0
2150.8
(2259.2)
2149.7
(2258.1)
2146.7
(2254.9)
2146.2
(2254.4)
2144.7
(2252.8)
2142.5
(2250.5)
2138.9
(2246.7)
2149.0
(2257.4)
2153.5
1702.1
(1765.7)
1700.8
(1764.3)
1699.4
(1762.9)
1698.7
(1762.1)
1699.6
(1763.1)
1693.9
(1757.2)
1687.8
(1750.8)
1709.8
(1773.7)
1705.1
30.0
18.6
3.2
1701.2
Fig. 5 presents the molecular structures and the Newman
projections of 2a and 2h. Selected IR and crystallographic data
of the same compounds are given in Table 8. The drawings
clearly show that the steric interaction between the carbonyl
and nitro groups in 2h not only twists the latter about the C(2)–
N(4) bond but hampers the ideal coplanar arrangement of the
phenyl ring with the carbonyl moiety (see the respective
dihedral angles in Table 8). The weakened conjugation of the
carbonyl group towards the phenyl ring makes the CO unit
more electrophilic, which, in turn, results in a more efficient
polarisation of the azido group. Both the IR spectroscopic data
and the differences of the N–N bonds reflect the different
geometry of the two azides and predict 2h to be the most
reactive member of the series examined. We note that the
increased barrier to rotation observed with various ortho-
substituted benzamides has also been attributed to a decreased
conjugation of the carbonyl and phenyl groups.40b
2146.6
2144.5
1.8
1
0.61
0.08
2g
2h
2i
2135.2
2151.7
(2262.1)
(1768.8)
a The raw calculated values (in parentheses) scaled by 0.952. b The
raw calculated values (in parentheses) scaled by 0.964. c The major
component of the νas(N3) band involved in Fermi resonance.
We feel it important to mention at this point that structural
data predicted the enhanced reactivity of 2-nitrobenzene-
sulfonyl azide over the 4-nitro derivative but this expectation
was not proved experimentally.15 The question why a potentially
higher reactivity can prevail with benzoyl azides but cannot be
effective with the benzenesulfonyl azide congeners remains
unanswered at the moment. We think that a detailed analysis
of the reaction mechanism with special attention to intra-
molecular steric interactions could be of help in this matter.
From the data presented in Table 7 we can conclude that the
positions of the doublets ascribed to the N3 asymmetric stretch-
ing band vary only negligibly with the electronic nature of the
substituent. The corrected νasЊ(N3) frequencies do show, how-
ever, the expected dependence, viz. a shift of N3 stretching band
toward higher wave numbers on going from electron-releasing
to electron-withdrawing substituents.
The trend of the νЊ values is in line with the observed differ-
ence of the N–N bonds discussed above and also coincides with
the shift of the νas(N3) frequencies reported earlier for sulfonyl
azides.15 While the position of the νas(N3) band has proved to be
a most convenient parameter to estimate the relative reactivities
of sulfonyl azides, in the case of the benzoyl congeners, how-
ever, it is the position of the νasЊ(N3) band (obtained from the
observed IR spectra or from DFT calculation) that most closely
reflects the reactivity of the azide reagents (Fig. 4, curve 2).
Finally, we would like to draw attention to the differences
emerging between the structural parameters and the reactivity
of benzoyl azides carrying a nitro substituent in para (2a) or
ortho (2h) positions. As shown by kinetic investigations,
2-nitrobenzoyl azide is the most reactive member of azides
2a–2h. The more pronounced tendency of 2h to form imido
complexes was demonstrated by the rate constants (k2h = 23 ×
10Ϫ3 MϪ1 sϪ1; k2a = 3.9 × 10Ϫ3 MϪ1 sϪ1) and also by the product
distribution when the reaction was conducted under pseudo-
first-order conditions ([3h]/[3a] = 6, see Experimental section).
Conclusions
The reaction of benzoyl azides with [Pd2Cl2(dppm)2] has
proved to be an efficient route for the preparation of dinuclear
N-benzoylnitrene complexes of palladium. The novel com-
plexes possess surprisingly strong N–C(O) bonding as
demonstrated by the high values of ∆G‡ (≥ 88.3 kJ molϪ1
)
rot
and T c (≥ 438 K). The unusually strong double bond character
of the N–C(O) linkage in complexes 3 is attributed to pπ–pπ
interaction of the nitrogen and carbon atoms supported by the
electron rich palladium centers. It is believed that the Pd–N
bonds enhance the electron shift towards the carbonyl group by
stabilising the positive charge on the bridging nitrogen atom.
The steric interaction of the nitro and carbonyl groups in
2-nitrobenzoyl azide is seen as the primary reason of the
outstanding structural features and the enhanced reactivity of
this reagent.
D a l t o n T r a n s . , 2 0 0 4 , 2 0 4 1 – 2 0 5 0
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Organometallics, 1999, 18, 5420; (l ) S. R. Klopfenstein, C. Kluwe,
Acknowledgements
K. Firschbaum and J. A. Davies, Can. J. Chem., 1996, 74, 2331; (m)
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(p) R. A. Stockland, Jr., M. Janka, G. R. Hoel, H. P. Rath and
G. K. Anderson, Organometallics, 2001, 20, 5212.
This work was supported by OTKA (Grant No. T 37676).
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