Synthesis and characterisation of Pd(II) complexes with a derivative of aminoazobenzene. Dynamic 1H-NMR study of cyclopalladation reactions in DMF

Article abstract of DOI:10.1016/j.jorganchem.2003.07.013

Three new Pd(II) complexes, i.e. [PdCl₂L]₂ (A), PdCl₂L₂ (B) and [Pd(μ-Cl)(L-H)]₂ (C), each with two diethyl [α-(4-benzenazoanilino)-2-hydroxybenzyl] phosphonates (L) bound to either one or two palladium atoms, are synthesized and characterized by elemental analysis, by IR, UV-vis and solid-state ¹³C-NMR spectra. Complexes B and C are additionally characterized by ¹H-,¹³C- and ³¹P-NMR and electrospray mass spectrometry (ESMS) studies using dimethylformamide (DMF) as a solvent. In DMF solution adducts A and B undergo spontaneous rearrangement into the cyclopalladated complex C. Dynamic ¹H-NMR study of this rearrangement as well as of the reactions of L with PdCl₂ and Na₂PdCl₄ revealed a complex equilibrium in DMF solutions and enabled the formation mechanism of all involved species to be resolved. The complex A is immediately solvolyzed producing two molecules of intermediate M [PdCl₂(L)(DMF)]. Complex M was also the first intermediate in the reaction of L with PdCl₂. Once present in concentration above 10^-5 mol dm^-3 M dimerizes very fast into chloro-bridged dimer [PdCl(μ-Cl)(L ]₂ (D) which undergoes cyclopalladation and converts into the complex C. The formation of C from the intermediate D is clearly demonstrated by the concentration dependence of the cyclopalladation reaction which has order greater than one. Chloride ions, released by cyclopalladation, react with D by splitting chloro-bridge and binding to metal atoms to produce byproduct [PdCl₃(L)]^- (T). The same species T are formed in the reaction of L with Na₂PdCl₄ whereby a chloride ion is replaced by the ligand L. The complex B undergoes similar, but slower, solvolytic reaction producing M and L while further reaction steps are identical as in the solvolysis of A.

Full text of DOI:10.1016/j.jorganchem.2003.07.013

Journal of Organometallic Chemistry 687 (2003) 85ꢀ99
/
www.elsevier.com/locate/jorganchem
Synthesis and characterisation of Pd(II) complexes with a derivative
of aminoazobenzene
1
Dynamic H-NMR study of cyclopalladation reactions in DMF
a,
Manda Curic´ *, Darko Babic´ , Zeljko Marinic´ , Ljiljana Pasˇa-Tolic´ ,
a
a
b
´
ˇ
Vjera Butkovic´ a, Janez Plavec c, Ljerka Tusˇek-Bozˇic´ a
a Ruder Bosˇkovic Institute, Bijenicka 54, HR-10002 Zagreb, Croatia
b Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, USA
c NMR Center, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia
Received 26 March 2003; received in revised form 21 July 2003; accepted 21 July 2003
Abstract
Three new Pd(II) complexes, i.e. [PdCl2L]2 (A), PdCl2L2 (B) and [Pd(m-Cl)(L-H)]2 (C), each with two diethyl [a-(4-
benzenazoanilino)-2-hydroxybenzyl]phosphonates (L) bound to either one or two palladium atoms, are synthesized and
characterized by elemental analysis, by IR, UVꢀ
vis and solid-state 13C-NMR spectra. Complexes B and C are additionally
/
characterized by 1H-, 13C- and 31P-NMR and electrospray mass spectrometry (ESMS) studies using dimethylformamide (DMF) as a
solvent. In DMF solution adducts A and B undergo spontaneous rearrangement into the cyclopalladated complex C. Dynamic 1H-
NMR study of this rearrangement as well as of the reactions of L with PdCl2 and Na2PdCl4 revealed a complex equilibrium in DMF
solutions and enabled the formation mechanism of all involved species to be resolved. The complex A is immediately solvolyzed
producing two molecules of intermediate M [PdCl2(L)(DMF)]. Complex M was also the first intermediate in the reaction of L with
PdCl2. Once present in concentration above 10ꢁ5 mol dmꢁ3 M dimerizes very fast into chloro-bridged dimer [PdCl(m-Cl)(L)]2 (D)
which undergoes cyclopalladation and converts into the complex C. The formation of C from the intermediate D is clearly
demonstrated by the concentration dependence of the cyclopalladation reaction which has order greater than one. Chloride ions,
released by cyclopalladation, react with D by splitting chloro-bridge and binding to metal atoms to produce byproduct [PdCl3(L)]ꢁ
(T). The same species T are formed in the reaction of L with Na2PdCl4 whereby a chloride ion is replaced by the ligand L. The
complex B undergoes similar, but slower, solvolytic reaction producing M and L while further reaction steps are identical as in the
solvolysis of A.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Palladium; Azobenzenes; Cyclopalladation; 1H-NMR study; Dynamics
1. Introduction
due to their easy orthometallation by Pd(II) salts [7].
One of the first examples of cyclopalladation reaction
was reported for azobenzene [8]. Later it was found that
Pd(II) complexes may undergo cyclopalladation reac-
tions with other N-donor ligands [7] too.
Reaction mechanisms and kinetics of cyclopallada-
tion reactions have been recently resolved with N-
benzylamines [9], N-benzyltriamines [10], N-benzylide-
neamines [11] and 2-phenylpyridines [12] as N-donor
ligands. Still, a very little is known about the mechanism
and reaction dynamics of cyclopalladation reaction with
azobenzenes [13], although the first complexes have been
prepared more than 30 years ago. Bruce et al. [14] have
Preparation of cyclopalladated complexes has at-
tracted considerable attention by a number of research
groups due to their potential application in organic
synthesis [1], homogenous catalysis [2], photochemistry
[3], optical resolution [4], and also in the design of new
metallomesogenes [5] and antitumor drugs [6]. Particu-
larly interesting are complexes with N-donor ligands,
* Corresponding author. Fax: ꢂ385-1-4680084.
´
E-mail address: curic@rudjer.irb.hr (M. Curic´).
/
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.07.013

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/
demonstrated that the palladium atom acts as an
electrophile since the favoured site of attack in asymme-
trically substituted azobenzenes is the aromatic ring with
an electron-releasing group. It is commonly assumed
that the first step in the cyclopalladation reaction is
coordinative bonding of the azo-nitrogen to the palla-
dium atom [13a,14,15]. Though this assumption is quite
logical, a direct experimental proof is still lacking.
In this paper we report on the synthesis and char-
acterization of new mononuclear and binuclear Pd(II)
complexes, i.e. PdCl2L2 (B), [PdCl2L]2 (A) and [Pd(m-
Cl)(L-H)]2 (C) where ligand L is diethyl [a-(4-benzena-
zoanilino)-2-hydroxybenzyl]phosphonate (Fig. 1) and L-
H is produced by release of proton from L (Scheme 1).
Synthesis and characterization of new Pd(II) complexes
is a part of our long-term efforts to prepare a new class
of potential antitumor and antiviral agents [16]. An
unexpected spontaneous conversion of the binuclear A
as well as mononuclear B complex into the chloro-
bridged binuclear cyclopalladated complex C was ob-
served during their characterization in dimethylforma-
recorded by using 85% H3PO4 as external standard (dꢃ
0.00 ppm). The 1H-1H COSY and 1H-13C-HSQC
spectra were recorded by using Bruker Avance DRX
500 NMR spectrometer. The 1H-1H COSY spectra were
recorded using 1 K in F2 dimension and 256 increments
in F1 dimension. The latter were zero filled to 1 K. Each
increment was obtained with eight transients using 6000
Hz spectral width. The 1H-13C HSQC spectra were
/
measured with one-bond CÃ/H coupling value set to 140
Hz, using 2 K in F2 dimensions and 256 increments in
F1 dimension. The latter were zero-filled to 1 K.
Increments were obtained using 256 transients. Spectral
widths were 31,250 Hz in F2 and 6000 Hz in F1
dimension. Solid-state 13C-NMR experiments were
carried out at 76.19 MHz on Varian Unity plus spectro-
meter operating at a field of 7T using cross-polarization
(CP) and magic angle spinning (MAS) with high power
1H decoupling. The 13C CP-MAS spectra were obtained
with a 4 ms CP contact time and 1 s recycle time. The
spinning rate using 7 mm Doty Scientific probe was ca. 4
kHz. The 7 tesla ESI Fourier Transform ion cyclotron
resonance mass spectrometer used for the present study
has been described in detail elsewhere [17]. An atmo-
spheric ESI source comprises a stainless steel heated
capillary inlet to the mass spectrometer that incorpo-
rates an electrodynamic ion funnel assembly to improve
ion transmission through interface followed by a colli-
sional focusing quadrupole. The compounds were dis-
solved in DMF and this solution was injected directly
into the mass spectrometer at a flow rate of 0.3 ml minꢁ1
in a pulled glass capillary. A source potential of ca. 2 kV
was applied to produce a stable ion current, while the
mide (DMF).
A detailed investigation has been
performed to resolve the mechanism of these transfor-
mations and results are reported in this paper. The new
insight into the transformation of adducts into the
binuclear cyclopalladated complex and particularly
into the mechanism of cyclopalladation reaction of 4-
aminoazobenzene derivative will enable the rational
design and synthesis of a new generation of Pd(II)
complexes with N-donor ligands.
2. Experimental
heated capillary was maintained at 200 V and 60ꢀ
/
2.1. General methods
120 8C. Ions were transferred from the ESI interface to
the ICR trap using an rf-quadrupole ion guide for
collisional focusing at 200 mtorr, followed by two sets of
rf-only quadrupole ion guides in the higher vacuum
regions of the mass spectrometer. Ion accumulation was
accomplished by introducing N2 into the FTICR trap at
the pressure of 10ꢁ5 torr via a piezoelectric valve
(Lasertechniques Inc., Albuquerque, NM). Background
pressure in the ICR trap was maintained at 10ꢁ9 torr
using a custom cryopumping assembly. Mass spectra
were acquired using a standard experimental sequence
for ion injection and accumulation, pump-down, excita-
tion, and detection. The analysis of the FTICR spectra
was performed using ICR-2LS software [18]. X-ray
Infrared spectra were recorded on a Perkinꢀ
/
Elmer
580 B and on a FT Perkinꢀ
/
Elmer 2000 spectrophot-
ometers using KBr pellets (4000-250 cmꢁ1) and Nujol
mulls in polyethylene (400-200 cmꢁ1). Visible and
ultraviolet spectra were taken on a Cary 219 spectro-
photometer with thermostated cell compartment and a
1
Hewlettꢀ
/
Packard 8452A spectrophotometer. H-, 13C-
and 31P-NMR data were recorded in DMF-d7 solvent by
using Varian Gemini 300 FT NMR spectrometer
operating at 300.08, 75.54 and 121.44 MHz for 1H,
13C and 31P nuclei, respectively. 31P-NMR spectra were
diffraction powder data were obtained using Cuꢀ
/
Ka
1.5406 A) on a Philips PW 3710 diffract-
ometer. Data were collected in the 3B20B658 range, in
the u ꢀ2u step scan mode with D20ꢃ0.028 and tꢃ12 s.
Magnetic susceptibilities were determined by the stan-
dard Gouy method at 23 8C using CuSO4×5H2O for
calibration.
˚
radiation (lꢃ
/
/
/
/
/
/
/
Fig. 1. Diethyl[a-(4-benzenazoanilino)-2-hydroxybenzyl)]phosphonate
(L).

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M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ
/
99
87
Scheme 1.
2.2. Materials
C46H52Cl4N6O8P2Pd2: C, 44.78 (44.39); H, 4.26 (4.58);
N, 6.81 (7.10); Pd, 17.25 (17.41). IR (KBr pellet, cmꢁ1):
3070 (w-m, nNÃH), 1601 (m, dNH and n(CÄC)ar), 1229 (s,
nPÄO); (Nujol mull in polyethylene): 341 (w, nPdÃCl). 31P-
NMR of D, 25.46 ppm (2P). 13C CP-MAS NMR, 16.2,
18.7 ppm (CH3, 4C), 63.6, 64.8 ppm (OCH2, 4C), these
Diethyl
[a-(4-benzenazoanilino)-2-hydroxyben-
zyl]phosphonate [19] and dichlorobis (acetonitrile) pa-
ladium(II) [20] were prepared according to a literature
procedures. All other chemicals and solvents were of
reagent grade and used without further purification. IR
of L (KBr pellet, cmꢁ1): 3294 (ms, nNÃH), 1601 (s, dNH
and n(CÃC)ar), 1233 (s, nPÄO). 31P-NMR, 25.65 ppm (1P).
13C CP-MAS NMR, 18.2 ppm (CH3, 2C), 48.6 ppm
two signals overlapped with PCH signal, 115.0ꢀ
/138.0
ppm (CarÃ
/
H and C-1, 28C), 142.5ꢀ159.0 ppm (C-
/
2,7,10,13, 8C).
(PCH, 1C), 63.6 ppm (OCH2, 2C), 109.5ꢀ
(CarÃH and C-1, 14C), 140.5ꢀ158.0 ppm (C-2,7,10,13,
4C).
/
134.5 ppm
2.2.2. Preparation of PdCl2L2 (B)
/
/
Trans-PdCl2(CH3CN)2 (92 mg, 0.35 mmol) was dis-
solved in warm dichloromethane (10 ml) and added to
the solution of L (300 mg, 0.70 mmol) in dichloro-
methane (5 ml). The reaction mixture was refluxed for 5
h. The red precipitate was filtered off, washed with
dichloromethane and dried under vacuum. Yield 50%.
Anal. Calc. (Found) for C46H52Cl2N6O8P2Pd: C, 52.30
(52.35); H, 4.97 (4.72); N, 7.96 (7.90); Pd 10.07 (9.92).
IR (KBr, cmꢁ1): 3320 (m, nNÃH), 1601 (s, dNH and
n(CÄC)ar), 1235 (s, nPÄO); (Nujol mull in polyethylene):
340 (w, nPdÃCl). 31P-NMR, 25.63 ppm (2P). 13C CP-
2.2.1. Preparation of [PdCl2L]2 (A)
Trans-PdCl2(CH3CN)2 (90 mg, 0.34 mmol) was dis-
solved in warm dichloromethane (10 m;) and added to
the solution of 150 mg (0.34 mmol) of L in dichlor-
omethane (5 ml). The reaction mixture was refluxed for
3 h. The yellow precipitate was filtered off, washed with
dichloromethane and dried under vacuum at room
temperature (r.t.). Yield 31%. Anal. Calc. (Found) for

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99
MAS NMR, 14.7 ppm (CH3, 4C), 47.3 ppm (PCH, 2C),
62.4 ppm (OCH2, 4C), 107.5ꢀ137.5 ppm (CarÃH and C-
1, 28C), 142.5ꢀ159.5 ppm (C-2,7,10,13, 8C).
thicknesses were 0.1 and 1 cm depending on the Pd(II)
compounds and L concentrations.
/
/
/
3. Results and discussion
2.2.3. Preparation of [Pd(m-Cl)(L-H)]2 (C)
A methanolic solution (10 ml) of L (300 mg, 0.70
mmol) was added dropwise to a methanolic solution (15
ml) of Na2PdCl4 (200 mg, 0.68 mmol). The resulting
mixture was stirred continuously at r.t. for 5 days. The
brown-red precipitate was filtered off, washed first with
water and then with cold methanol and dried under
vacuum. Yield 55%. Anal. Calc. (Found) for
C46H50Cl2N6O8P2Pd2: C, 47.60 (47.30); H, 4.35 (4.48);
N, 7.96 (7.90); Pd, 18.33 (17.98). IR (KBr, cmꢁ1): 3360
(m, nNÃH); 1579 (vs, dNH and n(CÃC)ar), 1229 (s, nPÄO);
(Nujol mull in polyethylene): 314 and 295 (vw, nPdÃCl).
31P-NMR, 25.66 ppm (2P). 13C CP-MAS NMR, 16.1
ppm (CH3, 4C), 46.2 ppm (PCH, 2C), 64.2 ppm (OCH2,
3.1. Synthesis and characterization
[PdCl2L]2 (A) and PdCl2L2 (B) complexes were
L
obtained in CH2Cl2 by reaction of
with
PdCl2(CH3CN)2 in 1:1 and 1:2 metal:ligand ratio,
respectively. The yield of A (31%) is low due to
concurrent reactions that end up with the complex C
which is more soluble in CH2Cl2 than A and B. The
dimeric complex [Pd(m-Cl)(L-H)]2 (C) was prepared in
methanol by the reaction of equimolar amounts of L
and Na2PdCl4.
The structural assignments of complexes are based on
their elemental analysis, spectroscopic studies (IR, UVꢀ
/
4C), 105.0ꢀ
/
132.0 ppm (CarÃ
/
H and C-1, 28C), 144.5ꢀ
/
vis, electrospray mass spectrometry (ESMS), solid-state
1
160.0 ppm (C-2,7,10,13, 8C).
13C-NMR as well as H-, 13C- and 31P-NMR in DMF).
In comparison with B and C, A is either insoluble or
when soluble, undergoes rapid decomposition which
prevents common structural characterization (1H-, 13C-
and 31P-NMR and ESMS. From the whole body of
collected data, presented and discussed here, we pro-
posed structures of complexes A, B and C (Scheme 1).
All complexes are stable in air and only slightly
soluble in common organic solvents (CHCl3, CH3OH,
CH3CN, etc.). The complexes A and B rearrange into C
in the solvents of poor co-ordinating ability, i.e. CHCl3
or CH3OH while in dimethyl sulfoxide (DMSO) and
CH3CN, a strong S- and N-donor towards palladium,
ligands are replaced by solvent molecules. Low solubi-
lity of A and B in CHCl3 and CH3OH prevent more
detailed study of the rearrangement reactions in these
solvents. In DMF, the results of dissolving A and B
strongly depend on their concentrations. At concentra-
tions close to 10ꢁ5 mol dmꢁ3 the reaction of ligand
replacement by solvent molecules is dominant while at
concentrations close to 10ꢁ4 mol dmꢁ3 rearrangement
into C and decomposition to L are concurrent processes.
At even higher concentrations of A or B the formation
of C is preferred and consequently we tried to synthesize
C directly from A and B in DMF. Unfortunately, it was
impossible to isolate pure C due to simultaneous
formation of byproduct in both reactions as well as
ligand in the reaction from B.
2.3. Dynamic measurements
The rearrangement dynamics of dissolved complexes
A and B, and of reactions of L with Pd(II) compounds,
were monitored by 1H-NMR spectroscopy on the
1
Varian Gemini 300 FT NMR spectrometer. H spectra
were collected by using a spectral width of 4700 Hz, a
number of transients 12, a pulse width of 5 ms (308), an
acquisition time of 2.54 s and no time delay. To follow
the rearrangement dynamics of complexes A and B, the
complexes (4.9 mg, 4.0ꢄ
/
10ꢁ3 mmol of A and 14.8 mg,
1.4ꢄ
/
10ꢁ2 mmol of B) were dissolved in 1.0 ml DMF-
d7. Spectra were were accumulated at various time
intervals (1, 2, 4, 8 and 16 min). To monitor the reaction
of the ligand L with Pd(II) compounds, the following
three reaction mixtures were prepared in DMF-d7 (1.0
ml): (a) L (11.2 mg, 2.6ꢄ
mg, 2.7ꢄ
10ꢁ2 mmol); (b) L (8.5 mg, 1.9ꢄ
and trans-PdCl2(Me2SO)2, (6.0 mg, 1.8ꢄ
10ꢁ2 mmol);
and (c) L (4.3 mg, 9.8ꢄ
10ꢁ3 mmol) with Na2PdCl4
(9.38 mg, 2.7ꢄ
10ꢁ2 mmol). Spectra were collected at
either 1, 2, 4, 8 and 16 min intervals. All reactions were
monitored at 1891 8C in 5 mm NMR tube.
The former reactions were also monitored by UVꢀ
spectroscopy on a HP 8452A spectrophotometer in the
range 260ꢀ750 nm at 25 8C. The rearrangement dy-
namics of A and B were followed by using two
concentrations of each complex (9.7ꢄ
10ꢁ6 and 9.8ꢄ
10ꢁ5 mol dmꢁ3 of A; 1.2ꢄ10ꢁ5 and 1.18ꢄ10ꢁ4 mol
dmꢁ3 of B). To follow the interaction of L with Pd(II)
compounds, two reaction mixtures were prepared (1ꢄ
10ꢁ5 and 1ꢄ10ꢁ4 mol dmꢁ3 of L and PdCl2). Spectra
/
10ꢁ2 mmol) with PdCl2 (4.7
10ꢁ2mmol)
/
/
/
/
/
/
/
vis
/
The
intermediates
[PdCl(m-Cl)L]2
(D),
/
/
[PdCl2(L)(DMF)] (M), [PdCl2(L)(DMSO)] (M*) and
byproduct [PdCl3(L)]ꢁ (T) were observed in the re-
arrangement reactions of A and B into C and in the
reactions of L with Pd (II) compounds in DMF. Their
/
/
/
1
/
structures, resolved by the combination of H- and 13C-
of all reactions were accumulated for 15 h at various
time intervals (0.25, 0.5, 1, and 10 min). The cell
NMR, UVꢀ
/
vis and ESMS studies, are given in the
rearrangement (Scheme 1). All attempts to isolate

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M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ
/
99
89
intermediate D were unsuccessful since we always
obtained the mixture of D and C species, as confirmed
1
by H-NMR spectroscopy.
Table 1
UVꢀvis spectral data
/
Compound lmax, nm (o, Mꢁ1 cmꢁ1
a
)
Solvent
3.1.1. IR spectra
L
400 (2.9ꢄ
383 (2.7ꢄ
455 (1.5ꢄ
104), 280 (2.2ꢄ
/
104), 272 (1.1ꢄ
104), 263 (1.0ꢄ
/
104)
104)
DMF
CHCl3
CHCl3
The infrared spectra of A, B and C strongly indicate
different modes of ligand bonding to palladium. The
most significant differences were observed for the
/
/
A
/
104) b, 413 (1.5ꢄ
104) c
104), 310 (1.5ꢄ
/
104) b, 318 (2.9ꢄ
/
/
B d
480 (6.9ꢄ
/
/
104) c, 272 (2.9ꢄ
/
DMF
following vibration modes: (i) NÃ
(3000ꢀ
3300 cmꢁ1); (ii) NH bending vibration and
benzene ring stretching vibrations (1500ꢀ
1610 cmꢁ1);
and (iii) PdÃCl stretching vibrations (200ꢀ
400 cmꢁ1).
The presence of PdÃN(amino) bond in A is confirmed
by the considerably reduced frequency and intensity of
H stretching vibration (from 3294 in free ligand
/
H stretching vibration
104)
444, 268
/
CHCl3
104), DMF
/
C
520 (5.7ꢄ
286 (2.5ꢄ
500 (4.0ꢄ
298 (2.3ꢄ
464 (3.5ꢄ
451 (3.4ꢄ
/
104), 385 (2.0ꢄ
104)
104), 393 (1.9ꢄ
/
104), 310 (2.4ꢄ
/
/
/
/
104), 332 (1.8ꢄ
/
104), CHCl3
/
/
/
/
104), 277 (2.2ꢄ
104), 277 (1.8ꢄ
104), 279 (1.9ꢄ
/
104)
104)
104)
M e
T
/
/
DMF
DMF
the NÃ
/
to 3070 cmꢁ1 in A). In addition, the reduced intensity of
NH bending vibration relative to the free ligand is also
detected. The same was observed in Pd(II) complexes
with anilinobenzylphosphonates as ligands in which
metal atom is coordinated by amino-nitrogen [16c]. In
complexes B and C these NH frequencies are similar to
those in the free ligand excluding the coordination
through the amino nitrogen in these complexes.
/
/
a
The spectra of A and B were recorded with the freshly prepared
solutions.
b
This band probably corresponds to the ligand transitions of other
species present in the solution.
c
Shoulder.
Since B is very weakly soluble in CH3Cl it was impossible to
d
determine the extinction coefficients.
e
The spectrum obtained by dissolving A at initial concentration
10ꢁ5 mol dmꢁ3 corresponds to the first intermediate M.
A single n(PdÃ
(at about 340 cmꢁ1) of A and B indicates trans position
of chlorine atoms. The presence of only one n(PdÃCl)
vibration band in the spectrum of A indicates that A and
D are structurally different species, with the same
elemental composition. The spectrum of intermediate
D (not isolated as pure compound) must have three
/
Cl) vibration in the far infrared region
nꢀ
A prominent band observed at 272 nm is assigned to the
second pꢀp* transition [23] (Fig. 2a). The same absorp-
tion bands also occur in CHCl3 (383 and 263 nm).
An additional support for the presence of PdÃ
N(amino) bond in A is given by its spectrum in
CHCl3, which possesses a strong band at 318 nm,
/
p* transition which is expected in the same region [23]
/
/
/
n(PdÃ/Cl) vibration bands [21]. Additional arguments
for different structures of A and D are obtained from the
solid-state 13C-NMR spectra of complexes and from the
reaction dynamics of complex A. The chloro-bridged
complex C exhibits two absorptions at 314 and 295
assigned to the pꢀp* transition of the bound ligands
/
(Fig. 2b). This transition is considerably shifted to
higher energy with respect to the corresponding transi-
tion in the free ligand L (383 nm) and B (444 nm). The
cmꢁ1 assigned to the bridging n(PdÃ
other vibrational frequencies of B and C are in agree-
ment with those observed for structurally similar com-
plexes of azobenzenes [16c,16e,22] (see Section 2).
/Cl) vibrations. The
position and intensity of the pꢀp* transition suggest
/
that there is no conjugation of the phenylazo group with
amino group when palladium atom is bound to it. Since,
in CHCl3 A also rearranges into C, the two overlapping
bands at 413 and 455 nm, with complex temporal
dynamics, probably correspond to the ligand transitions
of other species present in the solution (Fig. 2b).
For all complexes the position of PÄ
/O stretching
vibration is almost identical as in free ligand excluding
the possibility of coordination through the phosphoryl
oxygen (see Section 2).
The spectrum of A in DMF could not be obtained due
to its immediate solvolysis into M. The initial spectrum
obtained by dissolving A in DMF is characterized by a
very strong absorption at 464 nm which is assigned to
3.1.2. UVꢀvis spectra
/
The electronic absorption spectra of L, A, B and C
recorded in DMF and/or CHCl3 are summarized in
Table 1. The spectra of A and B were always recorded
with freshly prepared solutions since they undergo
decomposition and/or rearrangement. The spectrum of
T in DMF, obtained by recording 15 h old reaction
solution of L and Na2PdCl4, is also given in Table 1.
The spectrum of L in DMF exhibits a strong and
rather broad absorption band at 400 nm which is
transitions of the bound ligand: spin-allowed pꢀ
nꢀp*. The position of this pꢀp* transition (red shifted
in comparison with the corresponding maximum of L)
suggests immediate breaking of two PdÃN(amino)
/
p* and
/
/
/
bonds in A in DMF and its decomposition into the first
intermediate M which dimerize to D once present in
sufficient concentration. If free electron pair at amino
nitrogen were engaged in a bond to Pd atom (present in
A), one would expect blue shift instead of the red shift
assigned to the spin-allowed pꢀ
strong and broad band overlaps a weak, spin-allowed
/
p* transition. This

´
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M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ99
/
Fig. 2. Electronic absorption spectra of L, M, B, C in DMF (a) and A in CHCl3 (b).
observed here. The intensity of this band at initial
concentration of A 10ꢁ5 mol dmꢁ3 is decreasing
monotonically during the reaction indicating the forma-
tion of only one species whose absorption occurs at 464
nm. Thus, the band at 464 nm corresponds to M. At
initial concentration of A 10ꢁ4 mol dmꢁ3, the two-
phase dynamics have been observed for this band during
the whole course of reaction suggesting the formation of
two species, most probably M and D, having strong
absorption at very close positions.
3.1.3. Mass spectra
Positive and negative ion electrospray mass spectra
were recorded by using DMF as a solvent in order to
identify products and intermediates in the rearrange-
ment reactions of A and B into C. Such analysis is
possible since the ESMS technique enables transfer of
preexisting ions from solution into the gas phase [25].
All electrospray mass spectra measurements were made
with freshly prepared solutions as well as with the
solutions that were left to stand for several hours having
initial concentrations close to 10ꢁ4 and 10ꢁ5 mol
dmꢁ3. In both cases (freshly prepared and old solutions)
spectra were recorded continuously for 25 min. Peaks in
the ES mass spectra were identified by the most intense
m/z value within the isotopic mass distribution. There is
a very good agreement between experimental and
calculated m/z values.
The spectrum of B in DMF shows a very strong
absorption band at 480 nm which probably includes
contributions from metal to ligand charge transfer
(MLCT) and two ligand transitions: spin-allowed pꢀ
p* and nꢀp*. The second pꢀp* transition in the
spectrum of B is located at the same position as in the
free ligand (272 nm) suggesting that only the first pꢀp*
/
/
/
/
transition of the ligand is affected by binding to
The additional evidence for rearrangement of A into
H]ꢂ at m/z
palladium (Fig. 2a).
C is the presence of peaks due to ions of [Cꢂ
/
In the reaction of L and Na2PdCl4 isomers Ta and Tb
were formed, where a and b denote which aza-nitrogen
is bound to palladium. The presence of a and b species
1161 and [Cꢁ
/
H]ꢁ at m/z 1159 in the positive and
negative electrospray mass spectra of A, respectively
(Fig. 3). At concentration close to 10ꢁ4 mol dmꢁ3 the
spectra of A are characterized by the very fast changes in
time and by the presence of peaks at higher m/z values
(above 1000). In contrast, at concentrations close to
10ꢁ5 mol dmꢁ3 dominant peaks have m/z values under
1000 and spectral changes are much slower in time. The
peak at m/z 1235 (1233) corresponds either to proto-
1
have been determined by H-NMR spectra and this is
described in the later sections ‘reaction dynamics of
ligand L with PdCl2’ and ‘reaction dynamics of ligand L
with Na2PdCl4’. Since a isomer is four time more
abundant than b isomer (Fig. 9), a prominent band
observed at 451 nm in the spectrum of this reaction is
assigned to pꢀ
ligand in Ta. A band corresponding to the second pꢀ
transition occurs in this spectrum at 279 nm.
The most intense band in the spectrum of C in DMF
(520 nm) corresponds to the pꢀp* transition localized
/
p* and nꢀ
/
p* transitions of the bound
/
p*
/
on the orthometallated ligand [3a,24]. Its intensity
suggests that this transition is a charge-transfer in
character. The band at 385 nm is interpreted as a
ligand-field transition [3a]. The literature data on similar
orthometallated Pd complexes [3a] suggest that one of
the bands at 286 and 310 nm corresponds to the second
pꢀ
/
p* transition localized on the orthometallated ligand
Fig. 3. Negative ion electrospray mass spectra of A recorded 10 min
after dissolving in DMF.
(Fig. 2a).

´
M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ
/
99
91
nated (deprotonated) molecular ion of [PdCl2L]2 (A) or
to intermediate complex [PdCl(m-Cl)L]2 (D).
The possibility that this peak is due to [A9
excluded since in DMF solution the UVꢀvis and NMR
results indicated immediate breaking of two PdÃN(ami-
no) bonds and formation of intermediate
3.1.4. 1H-NMR spectra
The 1H-NMR spectra of L, B, C, intermediates D and
M* and byproducts T, recorded in DMF, are given in
Tables 2 and 3. The signal assignment was based on the
/
H]9 is
/
chemical shifts and spinꢀspin couplings, two-dimen-
/
/
1
sional experiments and on the comparison with the H
spectra of the related compounds [26]. 1H-NMR spectra
of A could not be obtained, not even at low temperature
M
[PdCl2(L)(DMF] which dimerize to D once present in
sufficient concentration. So, the peak at m/z 1235 (1233)
is due to protonated (deprotonated) molecular ion of D
(Fig. 3). It should be pointed out that only this peak and
peak at m/z 1199, formed by the loss of HCl from D, are
present in the first spectrum of freshly prepared reaction
recording (ꢁ
/
40 8C) due to its fast rearrangement into
C.
Coordination of the metal atom by a azo-nitrogen in
Da and B and Ta results in a large downfield shift of
ortho protons H-9,11 and H-14,18. This shift is larger
for protons H-9,11 although, in terms of bonds, protons
H-14,18 are closer to the coordinating azo nitrogen
(Na). This effect has been earlier observed and inter-
preted as paramagnetic anisotropy of the metal [21].
The reactions of L with Pd(II) compounds in DMF
produce both a and b coordinated intermediates and
byproducts. In both species H-9,11 were shifted stronger
than H-14,18 and this shift was more pronounced in a-
coordinated species (Tables 2 and 3). However, the
spectrum of Pd(II) compounds with 4-methylazoben-
zene displays a reverse order of the two ortho-proton
pairs [27]. Thus, it seems more plausible that the relative
shifts of ortho-proton pairs are determined by the nature
of para-substituent on the aromatic ring.
solutions of A at concentration close to 10ꢁ4 mol dmꢁ3
.
The formation of byproduct [PdCl3(L)]ꢁ (T) during
the cyclopalladation reactions is also supported by the
negative ion mass spectra of A which contain peak at
652 m/z corresponding to T (Fig. 4).
The spectra of freshly prepared solution of B change
very slow in time if compared with the spectra of A.
They contain only a weak peak at m/z 1057 correspond-
ing to the protonated molecular ion of B and more
intense peaks at m/z 1079 and 1095 due to [Bꢂ
and [Bꢂ
K]ꢂ ions, but not the peak at m/z 1161
corresponding to protonated molecular ion of C or
peaks due to [Cꢂ
Na]ꢂ or [CꢂK]ꢂ ions. The peak at
/
Na]ꢂ
/
/
/
m/z 1161 occurs in the spectra of several hours old
solution of B. In the same spectra are also visible very
weak peak at m/z 1235 corresponding to protonated
molecular ion of D. The presence of the weak peak due
The positions of H-9,11 and H-14,18 signals of Da are
very close to those of Ta (Tables 2 and 3). The same was
also observed for H-9,11 and H-14,18 signals of Db and
Tb. This suggests similar environment of H-9,11 and H-
14,18 protons in Da and Ta as well as in Db and Tb
(Scheme 1).
All protons in b-phenyl rings of the ortho-palladated
complex C are non-equivalent due to the covalent bond
of palladium with the ortho-carbon of b-phenyl ring.
This bond causes a downfield shift of H-12 signal by
0.36 ppm and an upfield shift of H-8 and H-9 signals by
0.38 and 0.13 ppm, respectively, relative to the free
ligand signals.
to [Cꢂ
/
H]ꢂ only in the spectra of several hours old
vis
solution of B is in accordance with results from UVꢀ
/
and NMR studies which indicate that the rearrangement
of B into C proceeds at much slower rate than the
rearrangement of A.
Complex C was identified by the presence of proto-
nated molecular ion in the positive ion electrospray mass
spectra at m/z 1061 and also by more intense peak at m/
z 1183 assigned to ion [Cꢂ
Na]ꢂ.
/
Electrospray mass spectra gave reliable information
on the species present in the solution. They support the
proposed structures of complexes B and C as well as of
the intermediates M and D and byproduct T.
3.1.5. 13C-NMR spectra in solution and in the solid state
13C-NMR data of L, B, C and byproducts T in DMF
are summarized in Table 4. The signal assignment was
ascertained by heteronuclear experiments. As complex A
decomposes very fast in DMF solution, we have also
recorded the solid-state 13C cross-polarization-magic-
angle spinning (CP-MAS) spectra of A, L, B and C (see
Section 2).
The ortho-carbons C-9,11 and C-14,18 of all com-
plexed species, except C, show the same shifts pattern as
the analogous protons (Table 4). Signals of C-9,11 are
always shifted downfield stronger than C-14,18 and this
shift is also larger for a-coordinated species than for b-
coordinated species. Except this, the most significant
differences between a and b species were observed for
Fig. 4. Comparison experimental (a) and calculated (b) isotopic mass
distribution for the [PdCl3L]ꢁ (T) ion.

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M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ99
/
Table 2
1H-NMR data in DMF (d ppm, J Hz)
H
Compound
a
L
B
Ca
7.14 d J(HH)ꢃ
Da
H-3
H-4
7.14 d J(HH)ꢃ
/
8.0
7.24 d J(HH)ꢃ
/
8.1
/
8.0
7.18 d J(HH)ꢃ7.7
7.35 t J(HH)ꢃ
7.04 t J(HH)ꢃ
/
7.30 t J(HH)ꢃ
6.98 t J(HH)ꢃ
7.74 d J(HH)ꢃ
7.20 d J(HH)ꢃ
7.91 d J(HH)ꢃ
7.96 d J(HH)ꢃ
/
7.6
7.38 t b
7.31 t J(HH)ꢃ
7.00 t J(HH)ꢃ
7.76 d J(HH)ꢃ
6.88 d, br, 7.56 s d J(HH)ꢃ
7.78 d J(HH)ꢃ8.9
8.2 d J(HH)ꢃ8.0
7.63 t J(HH)ꢃ7.6
7.58 d J(HH)ꢃ7.2
/
7.7
/
7.7
H-5
H-6
/
7.5
7.06 t J(HH)ꢃ
/
7.5
/
7.5
/
7.5
c
c
/
7.5
8.9
8.6
8.0
/
7.5
H-8,12
H-9,11
H-14,18
H-15,17
H-16
OH
/
7.40 d J(HH)ꢃ
9.52 d J(HH)ꢃ
/9.1
/
8.3
7.49 d, br J(HH)ꢃ
9.98 e
8.88 d J(HH)ꢃ
/
8.3
/
/9.1
/
/
8.47 br, d J(HH)ꢃ
/
7.5
/
/7.9
c
c
7.70 t J(HH)ꢃ
7.61 t J(HH)ꢃ
10.46 s
7.43 dd J(HH)ꢃ
3J(PH)ꢃ
6.6
5.70 dd J(HH)ꢃ
2J(PH)ꢃ
23.7
/
7.7
/
7.73 t J(HH)ꢃ
/
8.0
c
/
7.2
/
10.56 s
10.47 s
8.18 f
10.61 s
NH
/9.3
8.61 dd J(HH)ꢃ
3J(PH)ꢃ
6.6
5.89 dd J(HH)ꢃ
2J(PH)ꢃ
23.0
4.22 m g
7.0 1.31, 1.45 t J(HH)ꢃ
/
8.9
8.67 dd J(HH)ꢃ
3J(PH)ꢃ
6.5
5.87 dd J(HH)ꢃ
2J(PH)ꢃ
22.8
4.16 m g
1.28, 1.44 t J(HH)ꢃ7.0
/
8.8
/
/
/
PCH
/9.3
/
8.9
5.81 dd, (5.73 dd) a J(HH)ꢃ
2J(PH)ꢃ22.8, 2J(PH)ꢃ
22.8
4.15 m g
7.0 1.28, 1.42 t J(HH)ꢃ
/
9.3, J(HH)ꢃ
/9.3
/
9.0
/
/
/
/
/
OCH2CH3 4.13 m g
OCH2CH3 1.25, 1.41 t J(HH)ꢃ
/
/
/7.0
/
a
b
c
d
e
f
The spectrum obtained by dissolving of A in DMF.
Partly overlapped with signal of H-8,12.
The signals of H-6, 15, 16, 17 are together overlapped in the range from 7.72 to 7.85 ppm.
H-8,12 are not equivalent in C.
Unresolved and broad doublet.
Overlapped with signal of DMF.
Centar of complex multiplet.
g
the carbons C-13 and C-10 which are the closest to the
coordinating azo nitrogen atoms Na or Nb (Table 4).
The coordination of the metal atom by a azo-nitrogen
atoms in B and Ta causes downfield shift of C-13 (1.5
and 1.8 ppm, respectively) and upfield shift of C-10 (3.9
and 3.5 ppm, respectively) relative to the ligand. In the
spectrum of byproduct Tb, upfield shift of C-13 carbon
is 3 ppm while the signal of C-10 carbon shifts downfield
for 1.6 ppm.
All carbons in b-phenyl rings of the ortho-palladated
complex C show the similar shifts pattern as the
analogous protons (Table 4). Only C-9 signal displays
reverse shift (downfield by 8.3 ppm) in comparison with
H-9. The largest downfield shift was observed for the
ortho-carbon C-11 (33.3 ppm) which is bound to
palladium. This is in agreement with results reported
for different cyclopalladated compounds [16c,16e].
It should be also pointed out that in all considered
species the position of proton and carbon signals
corresponding to the CH and (OCH2CH3)2 groups are
almost on the same place as in the free ligand. This
excludes the possibility of coordination through the
phosphoryl oxygen or amino nitrogen.
13C CP-MAS spectra confirmed solid-state structure
of A. The significant shift of benzylic carbon signal (CH)
was observed only in the spectrum of A. This signal,
which is overlapped with two very close signals of CH2
groups, moved downfield by ca. 16 ppm relative to the
Table 3
1H-NMR data in DMF (d ppm, J Hz)
H
Compound
a
Db
Ta
Tb
Cb
/
Maꢅ
H-9,11
H-14,18
PCH
9.07 d J(HH)ꢃ
8.88 dd J(HH)ꢃ
5.75 d J(HH)ꢃ9.3
2J(PH)ꢃ
23.4
10.57 s
/
9.0
10.08 d J(HH)ꢃ
8.98 d J(HH)ꢃ7.8
5.83 dd J(HH)ꢃ9.2
2J(PH)ꢃ
23.4
10.63 s
8.28 dd
/
9.0
9.16 d J(HH)ꢃ
/
9.0
9.42 d J(HH)ꢃ
/
8.4
/
7.9
/
9.06 d J(HH)ꢃ
/
7.8
8.62 d J(HH)ꢃ
/
7.8
/
/
5.73 dd J(HH)ꢃ
2J(PH)ꢃ
23.2
10.56 s
/9.4
5.66 dd J(HH)ꢃ
/9.2
5.86 dd J(HH)ꢃ
2J(PH)ꢃ
22.4
10.86 s
/9.0
/
/
/
2J(PH)ꢃ
/
23.2
/
OH
NH
b
b
b
CH3SOCH3
3.59, 3.60 s
a
All signals of Cb except of signal PCH proton overlapped with the signals of the other species present in solution.
Overlapped with aromatic protons.
b

´
M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ
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93
Table 4
13C-NMR data in DMF (d ppm, J Hz)
C
Compound
L
B
Ca
Ta (Tb)
C-1
C-2
124.8
123.9
124.4
124.2 (124.5)
157.1 3J(PC)ꢃ
116.8
130.6
/
6.7
157.0 3J(PC)ꢃ
116.7
130.7
/6.4
156.9 3J(PC)ꢃ
116.8
130.5
/7.0
157.5 (157.6) 3J(PC)ꢃ
116.8 (116.6)
130.7 a
/
6.7 (6.8)
C-3
C-4
C-5
C-6
121.0
130.7 3J(PC)ꢃ
121.0
130.8 3J(PC)ꢃ
120.8
120.9 (120.8)
130.6 a,b
/
5.2
12.7
/
3.8
130.8 3J(PC)ꢃ
/
3.4
C-7
153.0 3J(PC)ꢃ
114.6
126.5
/
156.1 3J(PC)ꢃ
114.9
129.9
/
12.4
151.9 3J(PC)ꢃ
114.3, 121.2 c
133.4, 159.8 c
156.2
/
10.3
155.5 (153.8) 3J(PC)ꢃ
114.6 (113.5)
129.8 (129.9)
142.3 (147.4)
156.5 (151.7)
126.5 (124.0)
130.0 a
/
12.2
C-8,12
C-9,11
C-10
145.8
154.7
141.9
156.2
C-13
152.9
125.8
C-14,18
C-15,17
C-16
123.5
130.9
131.4
125.7
130.4
131.5
129.8
130.0
131.3 (132.2)
PCH
CH2CH3
CH2CH3
48.8 1J(PC)ꢃ
/
156.7
64.1, 64.3 2J(PC)ꢃ
17.3, 17.5 J(PC)ꢃ5.4
48.9 1J(PC)ꢃ
/
155.8
65.3, 64.5 2J(PC)ꢃ
17.4, 17.6 J(PC)ꢃ5.2
48.7 1J(PC)ꢃ
/
155.2
64.1, 64.2 2J(PC)ꢃ
17.3, 17.6 J(PC)ꢃ5.1
48.8 1J(PC)ꢃ
/
156.4
64.17, 64.4 2J(PC)ꢃ
17.3, 17.5 J(PC)ꢃ5.0
/
6.9
/
7.0
/7.0
/
7.0
/
/
/
/
a
The signals of Tb corresponding to C-4, 6, 15,17 are overlapped with the same signals of Ta and L in the range from 130.0 to 131.3 ppm.
Partly overlapped with the same signals of Tb and L and 3J(PC) could not be determined.
These carbons are not equivalent in C.
b
c
free ligand signal (48.6 ppm). Such a large shift can be
and bound DMSO (3.20ꢀ3.70 ppm). All results support
/
only a consequence of the vicinity of metal ligation site
and not the influence of surrounding molecules in the
solid state. This is supported by the fact that the same
signal in spectra of B and C moved upfield only for 1.5
and 2.6 ppm, respectively, relative to the free ligand
signal. In B and C the metal ligation sites are not close to
benzylic carbon and consequently changes in shift of
benzylic carbon signal are much smaller than in A, in
which each metal atom is coordinated by amino-
nitrogen of one ligand and by azo-nitrogen (Na) of
another ligand (Scheme 1).
Solid-state 13C spectra also supported the proposed
structure of B and C. Although it is not possible to
assign each signal in aromatic region the solid-state
spectra of L, B and C are comparable with those
recorded in DMF (see Section 2 and Table 4).
postulated solution dynamics presented in Scheme 1.
3.2.1. Reaction dynamics of complex A
UVꢀvis studies suggested that A is immediately
/
solvolyzed in DMF producing two molecules of inter-
mediate M [PdCl2(L)(DMF)]. The monomer M is also
susceptible to further solvolysis to L and at low
concentration (ca. 10ꢁ5 mol dmꢁ3) this becomes a
dominant process (Fig. 5). At higher concentration of A
(ca. 10ꢁ4 mol dmꢁ3) monomer M either dimerizes to D
or decomposes to L. Those two concurrent processes
3.2. Mechanistic study
Upon dissolving in DMF, A and B rearrange into C,
and the reactions reach equilibrium in 35 min or in a
day, respectively. These reactions, and those between 1
mol equivalent of L and Pd(II) compounds, were
studied by UVꢀvis spectroscopy monitoring the bands
/
at 400, 464, 480 and 520 nm which correspond to the
most intense transitions of L, M, B and C species,
respectively. The same reactions were also followed by
1H-NMR spectroscopy measuring intensities for several
type of signals: (i) aromatic ortho-protons H-9,11 and
Fig. 5. Electronic spectra obtained by dissolving A in DMF with
different starting concentrations: (*
(9.7ꢄ
10ꢁ5 mol dmꢁ3); (. . .) the same solution 9.5 h later (in 0.1 cm
cell); (---) the solution of A (9.7ꢄ
10ꢁ6 mol dmꢁ3) 9.5 h after
dissolution (in 1 cm cell).
/) freshly prepared solution of A
/
H-14,18 (8.00ꢀ/12.00 ppm); (ii) proton of CH group
/
(5.60ꢀ5.80 ppm); and (iii) protons of CH3 groups of free
/

´
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M. Curic´ et al. / Journal of Organometallic Chemistry 687 (2003) 85ꢀ
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99
occurre at comparable rates (Fig. 5). Further increase in
concentration promotes the formation of C.
two reactions, we have also observed signals of mono-
mer [PdCl2(L)(DMSO)] (M*) in addition to signals of D
(see Section 3.2.4).
The intensity of signals of dimer D decreases very fast
in time. This coincidences with the dynamic of appear-
ance of signal at 6.88 ppm corresponding to H-8 of C
and signals at 10.08 and 8.98 ppm attributed to H-9,11
Hence, the formation of C is (at least) a bimolecular
process and most probably proceeds via: (i) immediate
solvolysis of A producing the monomer M; (ii) sub-
sequent dimerization of M into a chloro-bridged dimer
D; which (iii) undergoes cyclopalladation and produces
complex C. The presence of intermediate D is supported
by the non-linear concentration dependence of the
formation of C as well as by the electrospray mass
spectrum obtained by dissolving A in DMF. The dimeric
type intermediates have been already postulated in
cyclopalladation reactions with o-tolylphosphane [28]
and benzylamine [29] ligands.
and H-14,18, respectively, of the monomer
[PdCl3(L)]ꢁ.
T
Since signals of T are getting stronger as the signals of
D got weaker, it was logical to conclude that byproduct
was also formed from D. Our assumption is that
chloride ions, released by cyclopalladation, react with
D by splitting chloride-bridge and binding to palladium
atoms to produce T. Thus, for each C molecule
produced, two molecules of HCl are released and two
molecules of T are formed. This is supported by the
higher intensity of T signals if compared with the signals
of C. The other possibility for the formation of T is
replacement of DMF in M with the chloride ion. We
have concluded that T is mainly produced by the
reaction of D and chloride ions for two reasons: (i) D
is present in much higher concentration and (ii) the rates
of reactions of the bridged complexes are about two to
three orders of magnitude faster than those for corre-
sponding monomeric species [30]. The formation of
monomeric species type [PdCl3(L)]ꢁ from dimeric
complexes has been reported [30,31].
1
The same reaction was studied by H-NMR spectro-
scopy by recording spectra at regular intervals after
dissolution of A in DMF. The most prominent features
are presented in Fig. 6.
The two doublets (9.98 and 8.88 ppm) observed at the
early stage of this reaction were attributed to H-9,11 and
H-14,18 of dimer D since UVꢀvis and ESMS studies
/
suggested that the equilibrium between intermediates M
and D is shifted towards D if initial concentration of A
was higher than 10ꢁ4 mol dmꢁ3. Thus, it seems that the
concentration of M is too low to be visible in NMR
spectra. Such assignment is also supported by: (i)
temporal profile of these signals (Fig. 6); (ii) their
position in comparison with the position of analogous
signals in B, T and M* species; and (iii) their presence in
the reactions of PdCl2 and PdCl2(DMSO)2 with L as
well as in the reaction of A in DMF with addition a
small amounts DMSO-h6. At the early stage of the last
The presence of species T [PdCl3(L)]ꢁ in the reaction
solution of A is confirmed by its negative ion electro-
spray mass spectra and occurrence in the other specific
reactions (see below).
Fig. 6. 1H-NMR spectra of A recorded immediately after dissolving in DMF.

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3.2.2. Reaction dynamics of complex B
the temporal profile corresponding to D. The doublet at
8.88 ppm corresponding to H-14,18 of D was two times
greater than its analogous signal in A. These signals
were explained as arising from two different dimeric
species with Pd bound to Na and Nb azo-nitrogens,
respectively.
The formation rate of C from B is at least one order of
magnitude slower than that from A because the
coordinating bond between the metal and azo-nitrogen
is more resistant to solvent attack than the coordinating
bond with the amino-nitrogen.
The signals at 9.52 and 8.47 ppm correspond to the
protons H-9,11 and H-14,18, respectively of B (Fig. 7).
Their intensities slowly decrease in comparison to the
analogue signals in the reaction of A (Fig. 6). The signals
of T are visible and their temporal profile coincides with
that of C, in accordance with the action of chloride ions
released by cyclopalladation. Although signals of D are
not observed, the presence of T signals suggests that B
undergoes solvolytic reaction that produced M and L
with further steps identical as in the solvolysis of A.
(Scheme 1). The proposed mechanism is additionally
supported by: (i) the higher order concentration depen-
dence of the solvolysis of B; and (ii) the electrospray
mass spectrum of B which contains very weak peaks
corresponding to protonated molecular ions of M and D
intermediates.
The formation of both species is a consequence of
similar affinities of Na and Nb azo-nitrogens towards
palladium. This was additionally confirmed by the
reactions of PdCl2 with model azobenzene ligands.
Only one isomer has been detected by 1H-NMR
spectroscopy for azobenzene and two for 4-aminoazo-
benzene and 4-methylazobenzene [27].
Thus, the signals at 9.07 and 8.88 ppm were assigned
to H-9,11 and H-14,18, respectively, of the dimer with
Nb coordination. Equal positions of the signals of H-
14,18 in Da and Db explained the increase in intensity of
one of the two Da signals in comparison to those
observed with A (Tables 2 and 3).
In the same reaction additional signals with the
temporal profile of T were also observed, at 9.16 and
9.06 ppm. Logically, they were attributed to the Tb with
Nb azo-nitrogen coordinating the metal atom, and
specifically to the H-9,11 and H-14,18 protons, respec-
tively. The signals of Ta and Tb were also observed in the
reaction of L with Na2PdCl4 (see Fig. 9).
3.2.3. Reaction dynamics of ligand L with PdCl2
The higher order concentration dependence was also
observed in the reaction of L with PdCl2 in DMF. The
formation of complex C was detected by UVꢀ
/vis
spectroscopy only at reactants concentrations higher
3.2.4. Reaction dynamics of ligand L with
PdCl2(DMSO)2
than 10ꢁ5 mol dmꢁ3
.
At the early stage of this reaction (Fig. 8) the 1H
spectra contain two doublets (9.98 and 8.88 ppm) as the
spectra of A and one additional doublet (9.07 ppm) with
In order to confirm the formation of monomeric and
dimeric intermediates (M and D) on the path to C as
well as the formation of byproduct type [PdCl3L]ꢁ we
Fig. 7. 1H-NMR spectra of B recorded immediately after dissolving in DMF.

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/
Fig. 8. 1H-NMR spectra of the reaction of L with PdCl2 recorded immediately after dissolving in DMF.
1
performed the reaction of L with PdCl2(DMSO)2
complex in DMF.
The dissolution of PdCl2(DMSO)2 in DMF produced
a dimeric complex [PdCl(m-Cl)DMSO]2 which was
resolved and confirmed by its H-NMR spectrum. The
spectrum exhibits two separate peaks with equal in-
tensities at 2.76 and 3.66 ppm, corresponding to free and
bound DMSO molecules, respectively.
Fig. 9. 1H-NMR spectra of the reaction of L with Na2PdCl4 recorded immediately after dissolving in DMF.

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3.2.5. Reaction dynamics of ligand L with Na2PdCl4
In this reaction molecule of L displaces chloride ion in
Na2PdCl4 producing monomeric species T. Ta and Tb
are formed at similar rates at the beginning of reaction,
while at the later stage Ta becomes dominant (Fig. 9).
This suggests that formation of Ta and Tb involves
similar energy barriers and that Ta is thermodynami-
cally more stable. The very weak signal corresponding
to C was detected only after 2 days.
3.2.6. The formation of b-isomer of complex C
According to the proposed mechanism, whenever Da
and Db were present, one would expect formation of Ca
and Cb species, too. Indeed, this was confirmed in the
reaction of L with PdCl2, by analyzing the PCH signals
which occur isolated in the interval 5.60ꢀ5.90 ppm. At
/
Scheme 2.
the end of the reaction there are five different PCH
signals. They were interpreted by comparison with
spectra of other studied solutions (obtained from A, B
and from the reaction of L with Na2PdCl4). One of the
signals was assigned to unreacted L, two corresponded
to Ta and Tb, and another one was recognized as Ca,
since it was present also in the reactions of A and B
(where only a-species are present). The remaining signal
was attributed to Cb since it has similar temporal
dynamics as Ca. Significant overlap of signals corre-
sponding to aromatic protons of both C-species (except
H-8 of Ca) and all other species present in the solution
prevented further assignment of signals in the range
The formation of C in the reaction of L and
PdCl2(DMSO)2 proceeds via a similar mechanism as
in reaction of L and PdCl2 (Scheme 2). The initial dimer
[PdCl(m-Cl)DMSO]2 was cleaved by L producing mono-
meric complex [PdCl2(L)(DMSO)] (M*) that dimerizes
into D if present in sufficient concentration. Complex D
transforms rapidly into C releasing two HCl molecules.
The signals corresponding to monomeric species T are
hardly visible in this reaction because the released
chloride ions react with the initial dimer [PdCl(m-
Cl)DMSO]2 producing new monomeric species
[PdCl3(DMSO)]ꢁ. The formation of this species is a
result of the higher concentration of the initial complex
than D, which transforms very fast into C. The signal of
the bound DMSO in new species occur at 3.43 ppm and
it was getting stronger as complex C is being formed.
The formation of [PdCl3(DMSO)]ꢁ was checked by 1H-
NMR spectra of PdCl2(DMSO)2 in DMF after addition
of DCl. This produced the same signal at 3.43 ppm,
while the signal at 3.66 ppm, corresponding to bound
DMSO in the initial complex, has disappeared. The
presence of signal at 3.43 ppm was also detected if
DMSO-h6 was added to DMF solution of Na2PdCl4. In
this case molecule of DMSO displaces chloride ion in
Na2PdCl4 also producing [PdCl3(DMSO)]ꢁ species.
The presence of PdCl2(L)(DMSO) (M*) as one of the
intermediates on the path to C was indicated by
occurrence of new signals at 9.42 and 8.62 ppm,
attributed to H-9,11 and H-14,18, respectively with the
same temporal profile as new signal of DMSO at 3.59
ppm.
7.00ꢀ8.00 ppm.
/
1
We believe that comparative study of the H-NMR
spectra recorded during the reactions of A and B, on one
side, and during the reactions of L with PdCl2 and
Na2PdCl4, on the other side, convincingly indicate
formation of only a-species (Da, Ta and Ca) in the first
case, and formation of both a and b species (Dab, Tab
and Cab) in the second case.
3.2.7. The role of chlorides and protons in the reaction
dynamics
As we mentioned earlier, two HCl molecules are
released for each produced molecule of C. The released
chlorides inhibit cyclopalladation reaction since they
form stable monomer T by splitting the bridge in D or
by replacing DMF in M. The monomer T undergoes the
reactions of cyclopaladation and solvolysis but very
slow. In the reaction of PdCl2(DMSO)2 and L the
released chloride ions and initial dimer form stable
monomer [PdCl3(DMSO)]ꢁ which does not participate
in the cyclopalladation reaction.
In all reactions where C was formed, the water signal
is strongly shifted (Fig. 10) due to the interaction of
water molecules with protons released during the
cyclopalladation. In the reaction of L and Na2PdCl4
this signal remains almost at its initial position after 24
The stronger coordinating affinity of DMSO in
comparison with DMF was used to check the presence
of cyclopalladated monomer [PdCl(L-H)(DMF)] (C/2).
DMSO-h6 was added to a solution of C in DMF-d7
prepared for recording 1H-NMR spectra. During the
next 15 days the signals of C and free DMSO-h6 did not
change at all.

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/
Fig. 10. 1H-NMR spectra of the reaction of L with PdCl2 showing the shift of the water signal due to the release of protons by cyclopalladation
h. Such a specific behavior of water signal was very
helpful in the interpretation of reaction dynamics.
Acknowledgements
The Ministry of Science and Technology of the
Republic of Croatia (Grant no. 0098035) provided
financial support for this research.
4. Conclusion
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The dynamics presented in Scheme 1 should be
considered as a complex equilibrium rather than as a
set of individual reaction paths. Apart from two
predominantly irreversible steps (solvolysis of A and
cyclopalladation reaction) all other reactions are rever-
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of their intensities (approaching 10%) precludes accurate
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spectra can be obtained only at high concentrations,
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