Activation of a coordinated alkyne by electron transfer: Crystal structures of [Pd{PPh2CH=C(But)NN=C(But)CH 2PPh2}{C(CO2Me)=CH(CO2Me)}] and [Pd{(Z,Z)PPh2CH2

Article abstract of DOI:10.1016/S0022-328X(99)00730-5

A new zerovalent complex [Pd{PPh₂CH₂C(Bu^t)=NN=C(Bu^t)CH ₂PPh₂}{C(CO₂Me)C(CO₂Me)}] (1) was prepared and its crystal structure was determined by X-ray diffraction analysi

Full text of DOI:10.1016/S0022-328X(99)00730-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 598 (2000) 318–328
Activation of a coordinated alkyne by electron transfer: crystal
¸¹¹¹¹¹¹¹º¹¹¹¹¹¹º
structures of [Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}-
{C(CO2Me)ꢀCH(CO2Me)}] and
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Pd{(Z,Z)PPh2CH2C(But)ꢀNNꢀC(But)CH2PPh2}-
{C(CO2Me)ꢁC(CO2Me)}]
M. Fernanda N.N. Carvalho a,*, Adelino M. Galva˜o a, Armando J.L. Pombeiro a,
b
b
c
c
&
&
Jan Cerma´k , S. Sabata , Pavel Vojt´ısˇek , Jaroslav Podlaha
a Centro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico, A6. Ro6isco Pais, P-1049-001 Lisbon, Portugal
b Institute of Chemical Process Fundamentals, CZ-16502 Prague 6-Suchdol, Czech Republic
c Department of Inorganic Chemistry, Uni6ersita Karlo6a, Hla6o6a 2030, CZ-12840 Prague, Czech Republic
Received 25 November 1998; accepted 9 November 1999
Abstract
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
A new zerovalent complex [Pd{PPh2CH2C(But)ꢀNNꢀC(But)CH2PPh2}{C(CO2Me)ꢁC(CO2Me)}] (1) was prepared and its
crystal structure was determined by X-ray diffraction analysis, which shows it crystallises in the C2/C monoclinic space group with
,
,
,
a=10.940(1) A, b=22.086(1) A, c=19.042(2) A and i=92.692(9)°. The azine diphosphine ligand chelates the metal in a (Z,Z)
configuration with a PꢂPdꢂP bite angle of 114.63(4)°. The alkyne carbon atoms lie essentially in the PꢂPdꢂP plane. The
electrochemical reduction of 1 promoted the conversion of the alkyne ligand to a vinyl species, and of the azine diphosphine to
ene-hydrazone diphosphine, with formation of [Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}{C(CO2Me)ꢀCH(CO2Me)}] (2). X-ray
diffraction analysis of complex 2 shows it crystallises in the triclinic space group P1 with a=11.302(1) A, b=12.528(1) A,
c=16.028(2) A, h=107.64(2)°, i=92.27(1)° and k=111.79(2)°. It displays a square-planar geometry with the two phosphorus
atoms in trans position. Extended Hu¨ckel MO calculations were performed in order to elucidate the redox process. © 2000
¸¹¹¹¹¹¹¹¹º¹¹¹¹¹¹º
(
,
,
,
Elsevier Science S.A. All rights reserved.
Keywords: Electron transfer; Zerovalent complexes; Redox process
1. Introduction
tion-state metal sites, e.g. Au(I), Ag(I) or Cu(I), the
Z,Z-PSP form is kept upon coordination [4].
The azine diphosphine, Z,Z-Ph2PCH2C(But)ꢀ
NꢂNꢀC(But)CH2PPh2 (Z,Z-PSP), was reported [1] to
act as a bridging ligand at palladium and platinum
complexes of general formula [{(MX2)(m-Z,Z-PSP)}n]
(n=2, Pd or n=6, Pt; X=Cl [2] or 1/2 alkyne [3]).
However, it acts as a chelating ligand for most of the
transition metals forming mononuclear compounds by
isomerisation to the (E, Z) form. In some low-oxida-
The electronic and coordinative versatility of the
azine diphosphine ligand makes it a convenient ligand
for most of the transition metals, since it can act as a
two-, four- or six-electron donor and occupy one, two
or three coordination positions. The possibility of hap-
ticity change is an important feature in the activation of
organic substrates through coordination to metal sites.
Here we describe the reaction of the azine diphos-
phine (Z,Z-PSP) with a palladium(0) complex leading
to the isolation and characterisation of a zerovalent
complex in which the ligand displays the (Z,Z) configu-
* Corresponding author. Tel.: +351-846-237; fax: +351-846-4455.
E-mail address: pombeiro@alfa.ist.utl.pt (A.J.L. Pombeiro)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00730-5

M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
319
ration. This result suggests that the (Z,Z) configuration
is more common than thought previously.
The redox properties of 1 were studied by cyclic
voltammetry and controlled potential electrolysis within
our interest in the study of the electrochemical proper-
ties of coordination compounds.
For comparative purposes, the free azine diphos-
phine and the dimethylacetylene dicarboxylate were
also studied.
other singlets at 26.9 and 31.4 ppm, and a nine-line
pattern centred at 13 ppm). The appearance of the
spectrum does not change on treatment of the mixture
with phenylacetylene, propadiene, tert-butyl isocyanide,
or carbon monoxide. However, upon addition of a
stoichiometric amount of dimethylacetylene dicarboxy-
late, a new compound displaying a singlet at 36.0 ppm
in the 31P{1H}-NMR spectrum is formed.
1
In the H- and 13C-NMR spectra, tert-butyl carbon
atoms and protons are equivalent as well as the carbons
and protons of ester groups. Methylene carbon atoms
next to phosphorus appear as doublets but coordinated
alkyne carbon atoms appear as a signal split into a
doublet of doublets due to interaction with trans- and
cis-phosphorus atoms. Methylene protons are not
equivalent with relatively large differences in chemical
shifts and are coupled only to the neighbouring phos-
phorus atoms, forming thereby two doublets of dou-
blets. The corresponding methylene proton signal in the
gold(I) analogue was found [4] to be a multiplet. We
ascribed the different appearance of this signal in our
spectrum to a bite angle smaller than in the gold
complex and therefore lower coupling constant (B1 Hz
as shown by NMR spectrum simulation) between the
phosphorus atoms.
2. Results and discussion
The reaction of [Pd2(DBA)3] (DBA=dibenzylide-
neacetone) with the azine diphosphine (Z,Z-PSP)
(Scheme 1), in aromatic solvents or THF leads to a
mixture of three different compounds according to the
phosphorus NMR spectrum (a singlet at 36.9 ppm, two
Suitable crystals for X-ray analysis were grown from
benzene–pentane as a benzene solvate.
2.1. Crystal structure of
¸¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Pd{PPh2CH2C(But)ꢀNNꢀC(But)CH2PPh2}{C-
(CO2Me)ꢁC(CO2Me)}]
The complex displays a pseudotrigonal planar coor-
dination sphere with the alkyne and diphosphine lig-
ands oriented to give an overall C2 symmetry for the
molecule. The twofold axis is coincident with the crys-
tallographic binary axis as a consequence of the special
position of the heavy atom. The alkyne coordinates the
metal as a h2 ligand and the CꢂC bond is at the (P, Pd,
Scheme 1. Reagents and conditions: (i) Pd2(DBA)3 in toluene, then
DMAD; (ii) controlled potential reduction at Erped= −2.2 V.
Table 1
a
,
,
Bond lengths (A) for complex 1
P) plane (average deviation 0.02 A). The geometry of
the phosphorus atoms is tetrahedral and the PdꢂP and
PꢂC bonds lengths (see Table 1) are within the usual
range [5,6]. The bite angle of the diphosphine is
114.63(4)° and the NꢂC double bonds are planar (the
average deviation of the atoms C(1), C(2), C(22), N(3),
PdꢂC(4)
PdꢂP(1)
2.060(3)
2.3229(8)
1.830(3)
1.841(3)
1.866(3)
1.504(5)
1.280(4)
1.409(5)
1.317(5)
1.430(5)
1.533(5)
1.258(7)
1.451(5)
1.189(5)
P(1)ꢂC(31)
P(1)ꢂC(21)
P(1)ꢂC(1)
C(1)ꢂC(2)
N(3)ꢂC(2)
N(3)ꢂN(3)c1
O(6)ꢂC(5)
O(6)ꢂC(7)
C(2)ꢂC(22)
C(4)ꢂC(4)c1
C(4)ꢂC(5)
C(5)ꢂO(7)
,
is 0.06 A) and oriented nearly perpendicular (87.00(8)°)
to the coordination plane.
The molecule co-crystallises in a 1:1 ratio with dis-
crete molecules of benzene. Selected bond lengths and
angles are listed in Tables 1 and 2. Fig. 1 displays a
view of the structure of complex with the atom number-
ing scheme.
Although nine-member rings are generally rare, they
are often found [4] in systems with the azine diphos-
phine (Z,Z-PSP) ligand. The conformation of this ring
a Symmetry transformations used to generate equivalent atoms:
c1, −x, y, −z+1/2; c2, −x+1, y, −z+1/2.

320
M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
Table 2
the atoms directly bound to it, due to the (Z,Z)
configuration of both double bonds mentioned above.
No bonding contacts exist between palladium and the
Bond angles (°) for complex 1 a
C(4)c1ꢂPdꢂC(4)
C(4)c1ꢂPdꢂP(1)
C(4)ꢂPdꢂP(1)
P(1)ꢂPdꢂP(1)c1
C(4)c1ꢂPdꢂN(3)
C(4)ꢂPdꢂN(3)
35.6(2)
140.42(10)
104.93(10)
114.63(4)
158.73(11)
160.47(10)
59.44(5)
56.33(5)
101.6(2)
104.2(2)
99.3(2)
111.42(12)
112.45(11)
124.98(11)
115.8(2)
114.4(3)
77.1(2)
,
nitrogen atoms (PdꢂN(3)=3.971(3) A).
The palladium atom is localised at the special posi-
tion 2, which makes the tert-butyl, phenyl and methoxy
carbonyl groups occupy alternate positions towards the
ring plane.
P(1)ꢂPdꢂN(3)
At the alkyne ligand the bond distance (CꢂC=
P(1)c1ꢂPdꢂN(3)
C(31)ꢂP(1)ꢂC(21)
C(31)ꢂP(1)ꢂC(1)
C(21)ꢂP(1)ꢂC(1)
C(31)ꢂP(1)ꢂPd
C(21)ꢂP(1)ꢂPd
C(1)ꢂP(1)ꢂPd
C(2)ꢂC(1)ꢂP(1)
C(2)ꢂN(3)ꢂN(3)c1
C(2)ꢂN(3)ꢂPd
,
1.258(7) A) is longer than that expected for a typical
,
triple bond (1.20 A) and the angle C(5)ꢂC(4)ꢂC(4)=
145.9(2)° is considerably bent, this being consistent with
a decrease in the CꢂC bond order. The CꢂCꢀCꢂC
moiety is nearly planar (7°).
2.2. Electrochemical studies
N(3)c1ꢂN(3)ꢂPd
C(5)ꢂO(6)ꢂC(7)
N(3)ꢂC(2)ꢂC(1)
N(3)ꢂC(2)ꢂC(22)
C(1)ꢂC(2)ꢂC(22)
C(4)c1ꢂC(4)ꢂC(5)
C(4)c1ꢂC(4)ꢂPd
C(5)ꢂC(4)ꢂPd
79.78(4)
116.0(4)
122.0(3)
117.8(3)
120.1(3)
145.9(2)
72.22(10)
141.8(3)
123.6(4)
124.1(4)
112.4(3)
The palladium(0) complex [Pd{C(CO2Me)ꢁC(CO2-
Me)}(m-Z,Z-PSP)] (1) displays by cyclic voltammetry,
at a Pt wire electrode, in 0.2 M [NBu4][BF4]–THF, at
scan rates lower than 200 mV s−1, one irreversible
cathodic wave at Erped= −2.2 V. However, at higher
scan rates some degree of reversibility is observed,
which increases with the scan rate. No original anodic
process is detected. Upon reduction, on the reverse
O(7)ꢂC(5)ꢂO(6)
O(7)ꢂC(5)ꢂC(4)
O(6)ꢂC(5)ꢂC(4)
scan, one irreversible anodic wave is observed at Epox
=
a Symmetry transformations used to generate equivalent atoms:
c1, −x, y, −z+1/2; c2, −x+1, y, −z+1/2.
0.5 V. This behaviour is in agreement with the forma-
tion of a new species upon reduction, which is oxidised
at that anodic wave (Fig. 2).
To have an insight into the cathodic process, we
performed a controlled potential electrolysis in a THF
solution of 1, at the potential of its cathodic wave,
using as working electrode a platinum gauze. After one
electron per molecule was transferred, the work up of
the electrolyte resulted in the isolation of trans-
¸¹¹¹¹¹¹¹º¹¹¹¹¹¹¹º
[Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}{C(CO2Me)ꢀ
CH(CO2Me)}] (2) (for its full characterisation see be-
low), as an orange crystalline solid (Scheme 1). The
31P-NMR of the solution measured immediately after
electrolysis confirms that no other product formed.
The study by cyclic voltammetry of complex 2 shows
it displays one irreversible anodic and one reversible
cathodic process, which occur at Eopx=0.5 V and
red
1/2
E
= −2.03 V, respectively. The value measured for
Fig. 1. Molecular structure of complex 1 displaying atom labelling
scheme, ellipsoids drawn at 25% probability.
the anodic process agrees with that observed on the
reverse anodic scan performed on the solution of com-
plex 1.
is strongly determined by the geometry of the two
conjugated double bonds, which are both in the (Z)-
configuration as confirmed by the torsion angle value
(N, N, C, C=7.4(4)°). In complex 1 the NꢂN bond
In order to gather information on the site where
reduction occurs on complex 1, we also studied by
cyclic voltammetry the behaviour of the azine diphos-
phine (Z,Z-PSP) and the dimethylacetylene dicarboxy-
late. The former was found to oxidise at Eopx=1.02 V
(in CH3CN) but no cathodic wave was detected,
whereas the latter compound is reduced at Erped= −
1.90 V (in THF) and the cyclic voltammogram displays,
,
distance (1.409(5) A) is within the expected value for
two sp2 nitrogen atoms connected by a single bond
,
(average value: 1.401 A. [5,6]) and this bond is nearly
perpendicular to the plane containing palladium and

M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
321
Fig. 2. Cyclic voltammogram of complex 1, in 0.2 M [NBu4][BF4]–THF. Scan rate 200 mV s−1. Potentials quoted versus SCE. (*) Anodic wave
generated by reduction.
In addition to the alkyne to vinyl conversion, in-
duced by electrochemical reduction, the formal oxida-
tion of Pd(0) to Pd(II) also occurs.
on the reverse anodic scan, following reduction, one
oxidation wave at E =0.16 V; no original anodic
wave was observed for the acetylene. These observa-
tions suggest that the reduction process of complex 1 is
formally localised at the coordinated alkyne. As a
result, a highly unstable intermediate (conceivably of
ox
1/2
A pronounced geometric rearrangement accompanies
the alkyne to vinyl conversion, since at complex 2 (see
below) the vinyl ligand occupies a plane perpendicular
to the metal coordination plane. Such a geometrical
change brings one of the nitrogen atoms close enough
to the metal to interact (see below) and is expected to
enhance the palladium contribution to the HOMO,
thus being a driving force to the oxidation of the metal.
This rationalisation is corroborated by some theoret-
ical calculations by the extended Hu¨ckel method (see
below).
the type of a6 ) would be formed in which proton transfer
from the CH2 group of the azine diphosphine to the
carbonion moiety would generate the vinyl ligand. At
related complexes, the base deprotonation of the CH2
group of the five-member ring of the tert-dentate azine
diphosphine, with azine to ene-hydrazone conversion,
was reported by Shaw and co-workers [3].
1
Complex 2 was characterised by IR, H-, 31P{1H}-
and 13C-NMR spectroscopic techniques, and FAB MS
and the structure confirmed by X-ray diffraction
analysis.
In the IR spectrum of 2 no separate bands at-
tributable to wCꢀC or wCꢀN are clearly observed. How-
ever, in the stretching region of the carbonyl of the
ester groups, a band is observed at 1700 cm−1, a value
considerably lower than that known (wCO=1800 cm−1
)
for the complex 1 precursor. This behaviour suggests
the coupling of the wCꢀN and wCꢀO IR bands, which then
occur at intermediate values as observed before [8].
The 31P{1H}-NMR spectrum consists of an AB spin
system (2J(PP)=361 Hz) observed at l 25.5 and
44.8 ppm, relative to H3PO4, for the resonances of the
two phosphorous atoms of the azine diphosphine
ligand. The chemical shifts are comparable with
An alternative route, which cannot be completely
ruled out, to explain the electrochemical behaviour
observed, would involve the reduction of one of the
imine groups of the azine backbone. Such a process
would induce stereochemical changes at the azine
diphosphine ligand and possibly hydrogen migration to
the metal affording a hydride-type complex or a direct
attack [7] of the methylene group to the CꢂC multiple
bond. The hydride intermediate could then undergo
alkyne insertion to give the final product. In the ab-
sence of any observable reduction process at the free
azine diphosphine ligand, we favour the former process
for which the initial electron input occurs at the alkyne
ligand.
those reported for the related complexes trans-
¸¹¹¹¹¹¹¹º¹¹¹¹¹¹¹º
[Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}X] (X=Cl,
Br or Me) [3], and although the coupling constant is
considerably lower than those observed for the latter
compounds (457, 459 or 409 Hz for X=Cl, Br or Me,
respectively), it is well above the values (ca. 40 Hz) [9]

322
M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
6.04 ppm, dd; 4J(PH)=2.0, 4J(PH)=0.5 Hz), was
accomplished on the basis of the COSY(H,C)
correlation.
In the 13C-NMR spectrum, the resonances corre-
sponding to the CO carbon atoms (l 186.6 and 189.4
ppm) of the ester groups are within the expected range.
The resonances of the enamine (ꢂCꢀCꢂNꢂ) and imine
(ꢂNꢀCꢂ) carbons occur at l 167.3 and 174.3 ppm,
respectively. At the vinyl ligand, the signal for the
resonance of the coordinated carbon atom is observed
at l 145.3 ppm, while that of the ꢀCꢂH carbon atom, it
occurs at 129.0 ppm. Moreover, the doublet (J(PC)=
54.9 Hz) observed at l 72.3 ppm is assigned to the
carbon atom of the ene-hydrazone of the diphosphine
ligand. The COSY(C,H) correlation confirms this
assignment.
The FAB MS spectrum clearly displays the molecular
ion (m/z 815 [M+2]), that upon fragmentation loses
the vinyl ligand.
Fig. 3. Molecular structure of complex 2 displaying atoms labelling
scheme.
Table 3
2.3. Crystal structure of
trans-[Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}{C-
(CO2Me)ꢀCH(CO2Me)}] (2)
,
Selected bond lengths (A) for complex 2
¸¹¹¹¹¹¹¹º¹¹¹¹¹¹¹º
Pd(1)ꢂC(4)
Pd(1)ꢂN(3)
Pd(1)ꢂP(1)
Pd(1)ꢂP(2)
P(1)ꢂC(12)
P(1)ꢂC(11)
P(1)ꢂC(13)
P(2)ꢂC(21)
P(2)ꢂC(22)
2.00(2)
2.087(11)
2.257(4)
2.288(4)
1.754(14)
1.817(13)
1.82(2)
1.783(13)
1.81(2)
1.820(14)
1.38(2)
1.388(14)
1.31(2)
1.48(2)
1.36(2)
1.31(2)
1.49(2)
1.55(2)
1.57(2)
1.19(2)
1.44(2)
1.31(2)
1.45(2)
1.20(2)
1.34(2)
1.41(2)
The complex displays a square-planar geometry with
the two phosphorus atoms of the diphosphine ligand in
trans position to each other, as shown in Fig. 3. The
relevant bond lengths and geometrical parameters are
summarised in Tables 3 and 4, respectively. The palla-
dium, nitrogen and vinyl carbon atoms lie 0.05 A below
the average plane containing the metal centre and the
four coordinated atoms while the two phosphorus
,
P(2)ꢂC(23)
N(3)ꢂC(5)
N(3)ꢂN(31)
C(4)ꢂC(41A)
C(4)ꢂC(41)
C(12)ꢂC(5)
C(6)ꢂN(31)
C(6)ꢂC(23)
C(6)ꢂC(61)
C(51)ꢂC(5)
O(42)ꢂC(41)
C(41A)ꢂC(42A)
C(41)ꢂO(43)
O(43)ꢂC(45)
O(43A)ꢂC(42A)
O(44A)ꢂC(42A)
O(44A)ꢂC(45A)
,
atoms lie 0.08 A above it. The plane containing the
vinyl ligand is almost perpendicular to the (Pd, C, P, P,
N) coordination plane and bisects the azine–diphos-
phine ligand at the N(3) atom.
The atoms of the two cycles of the azine-diphosphine
ligand display considerably different behaviour: the
atoms in the five member ring are all below the (Pd, C,
P, P, N) plane, as observed for the unbound nitrogen
atom at the six member ring; at this ring the two
carbon atoms are significantly above that plane, e.g.
,
C(23) is 1 A above it.
The ester group bound to the uncoordinated carbon
of the vinyl is coplanar with CꢀCꢂPd, while the other
ester group makes an angle of 53° with it. Both the
ester groups are planar with typical bond lengths and
angles.
that have been reported for the related complexes with
the cis geometry.
In the H-NMR spectrum, the doublet of doublets
1
A projection of the position of the carbon atoms
bound to the phosphorus atoms in a plane perpendicu-
lar to the PꢂPdꢂP direction shows that the two halves
of the diphosphine have alternate conformations.
The PdꢂP and PꢂC bond lengths are within the
normal range but the PdꢂP(1) and P(1)ꢂC(12) are
(2J(PH)=5.5, 4J(PH)=1.5 Hz) observed at l 4.09
ppm is attributed to the CꢂH resonance of the enamine
moiety of the diphosphine ligand, since these values are
within the expected ranges [3]. The resonance of the
methylene protons is observed as a multiplet at l
2.97–3.06 ppm. The definitive assignment of these sig-
nals as well as the resonance of the vinyl CH proton (l
,
slightly shorter (0.03 and 0.06 A, respectively) than the
PdꢂP(2) and P(2)ꢂC(23) bond lengths (Table 3).

M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
323
Excluding the C(23)ꢂC(6) bond length, which is
In order to get further information on the process
that converts complex 1 into complex 2, as well as on
the structural characteristics of the intermediate species,
we carried out extended Hu¨ckel MO calculations [10]
using several model complexes.
In a first approach we tried to understand the elec-
tronic and steric factors that control the structural
trends of the alkyne moiety.
The structure of complex 2 obtained by X-rays shows
that the vinyl ligand is almost perpendicular to the
square-planar coordination plane, while in most of the
alkyne ligands the CꢂC bond is coplanar with all the
other atoms in the coordination sphere. In order to
elucidate this aspect [Pd{C2(CO2Me)2}(PH3)2] [11] was
used as a model to study the bonding of the coplanar
alkyne ligand. Fig. 4 shows an FMO diagram depicting
the interaction between the Pd(PH3)2 and {C2-
(CO2Me)2} fragments.
clearly within the range commonly assigned to a single
,
bond (1.49 A), all the CꢂC, CꢂN or NꢂN bond lengths
in the azine–diphosphine ligand display values between
,
single- and double-bond character (1.31–1.39 A). This
feature is possibly a consequence of a considerable
electron delocalisation through the PꢂNꢂP frame. The
,
PdꢂN distance of 2.09 A is within the expected range [6]
for palladium complexes with tert-dentated ligands,
although stretched to the longer side of this range.
,
,
The C(4)ꢂC(41) A bond length (1.31 A) at the vinyl
ligand is longer than a typical CꢂC double bond, this
pointing to a p-back donation effect from the filled
metal dxz orbital to the p* orbital of the ligand. This
feature is reinforced by the perpendicular position of
the vinyl ligand to the coordination plane (see Section
2.4).
The five occupied orbitals of the metallic fragment
are essentially xz, x2−y2, yz, xy and z2, while the three
occupied orbitals of the alkyne ligand are the CꢂC s
bond, the CꢂC out-of-plane p-bond and the CꢂC in-
plane p-bond. Although these three orbitals have ap-
propriate symmetry to interact with z2, yz and z2 metal
orbitals, respectively, the result for each of them is a
repulsive four-electron interaction, which does not con-
tribute to the bonding. The only orbital with a positive
overlap population (0.2 electrons) corresponds to a p
donation from the xz orbital to the in-plane CꢂC p*
orbital of the alkyne ligand.
2.4. MO calculations
The alkyne to vinyl conversion is accompanied by a
major geometrical change at the ligand, which brings it
from in-plane to perpendicular-to-coordination plane.
This rearrangement increases the contribution of the
metal to the HOMO and possibly is the driving force
for the oxidation of the metal site.
Table 4
Selected bond angles (°) for complex 2
C(4)ꢂPd(1)ꢂN(3)
C(4)ꢂPd(1)ꢂP(1)
N(3)ꢂPd(1)ꢂP(1)
C(4)ꢂPd(1)ꢂP(2)
N(3)ꢂPd(1)ꢂP(2)
P(1)ꢂPd(1)ꢂP(2)
175.3(5)
91.7(4)
83.6(3)
92.0(4)
92.7(3)
Neverthless, the second higher occupied molecular
orbital (SHOMO) that results from one of the four
electron interactions is responsible for the in-plane con-
formational preference, as can be seen from the Walsh
diagram corresponding to a 90° rotation of the alkyne
ligand (Fig. 5(a)).
172.2(2)
111.8(11)
116.8(9)
129.5(8)
115(2)
127.5(13)
117.5(11)
118.1(11)
128.7(12)
115.1(13)
116.1(12)
118.4(11)
119.9(12)
117.4(12)
122.5(11)
112.6(10)
122(2)
121(2)
127(2)
112.3(14)
117(2)
114(2)
121(2)
127(2)
112(2)
C(5)ꢂN(3)ꢂN(31)
C(5)ꢂN(3)ꢂPd(1)
N(31)ꢂN(3)ꢂPd(1)
C(41A)ꢂC(4)ꢂC(41)
C(41A)ꢂC(4)ꢂPd(1)
C(41)ꢂC(4)ꢂPd(1)
C(5)ꢂC(12)ꢂP(1)
N(31)ꢂC(6)ꢂC(23)
N(31)ꢂC(6)ꢂC(61)
C(23)ꢂC(6)ꢂC(61)
C(6)ꢂN(31)ꢂN(3)
C(12)ꢂC(5)ꢂN(3)
C(12)ꢂC(5)ꢂC(51)
N(3)ꢂC(5)ꢂC(51)
C(6)ꢂC(23)ꢂP(2)
C(4)ꢂC(41A)ꢂC(42A)
O(42)ꢂC(41)ꢂO(43)
O(42)ꢂC(41)ꢂC(4)
O(43)ꢂC(41)ꢂC(4)
C(41)ꢂO(43)ꢂC(45)
C(42A)ꢂO(44A)ꢂC(45A)
O(43A)ꢂC(42A)ꢂO(44A)
O(43A)ꢂC(42A)ꢂC(41A)
O(44A)ꢂC(42A)ꢂC(41A)
The results displayed on Fig. 5(a) allow us to con-
clude that the energy barrier for the rotation of the
alkyne is about 0.8 eV, which makes the rotation
almost impossible. In fact, from Fig. 6, which depicts
the SHOMO orbital (interaction with the out-of-plane
CꢂC p-bond) for the in-plane (a), or perpendicular-to-
plane (b) conformations of the alkyne ligand, we can
conclude that the orbital in the perpendicular configu-
ration (b) has anti-bonding character in what concerns
the phosphine ligand, this being in agreement with the
high energy for the rotation of the alkyne ligand.
In contrast, for the LUMO orbital (interaction with
in-plane p* orbital), the perpendicular conformation
(Fig. 7(b)) is more stable due to the PdꢂP anti-bonding
character in the in-plane conformation (Fig. 7(a)).
By electrochemical reduction, one electron is placed
at the LUMO, which has a PꢂPdꢂP anti-bonding char-
acter for the in-plane conformation (see Fig. 4), thus
reducing the energy barrier for the rotation of the
alkyne ligand. If we consider that the electron trans-

324
M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
Fig. 4. FMO interaction diagram for the [Pd{C2(CO2Me)2}(PH3)2] complex.
Fig. 5. Walsh diagram for the 90° rotation of the alkyne (a) or the vinyl-type ligand (b). The dashed line corresponds to the total energy. At the
x-axis, 0° corresponds to the in-plane and 90° to perpendicular to the in-plane conformation. (*) Corresponds to the former LUMO for the
in-plane conformation (a).

M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
325
Fig. 6. SHOMO orbital for the two limiting conformations of the alkyne ligand.
Fig. 7. LUMO orbital for the in-plane (a) or perpendicular-to-plane (b) conformations of the alkyne ligand.
ferred to the LUMO decreases the metalꢂligand bond
order to 0.5, then the transfer of one electron from the
HOMO to the LUMO (more localised at the organic lig-
and) will allow the LUMO to fill up, thus corresponding
to the oxidation of the metal, in the process that converts
the organic fragment into a vinyl-type ligand. The elec-
tron transfer from the HOMO, with a significant metal
character, induced by the geometry change, from in-
plane to perpendicular-to-plane, will then formally oxi-
dise the metal from Pd(0) to Pd(I) as discussed before.
The Walsh diagram (Fig. 5(b)) allows us to conclude
that the perpendicular conformation will become fa-
vourable by about 0.8 eV.
At this stage the ligand can reorganise to improve
the overlap population with the z2 orbital as shown in
Fig. 8.
The one-electron-oxidised metal has now a strong ten-
dency to open the PꢂPdꢂP angle, provided that one elec-
tron will be removed from the still half-filled orbital (see
above), as shown in the Walsh diagram depicted in Fig.
9 (PꢂPdꢂP$170°). This will give the phosphines an al-
most trans geometry.
Fig. 8. Scheme of the mixing of the p and p* orbitals to produce a vinyl ligand by shifting the alkyne ligand in the y-direction.

326
M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
respectively, using hexamethyldisilane as an internal
standard. IR spectra were recorded in KBr pellets on a
Perkin–Elmer 683 infrared spectrophotometer. FAB
mass spectra was recorded in 1,4 butanediol matrix
using a Carlo Erba Instruments Auto/HRGC/Trio 2000
MS spectrometer at room temperature.
3.1. Synthesis of
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Pd{PPh2CH2C(But)ꢀNNꢀC(But)CH2PPh2}{C-
(CO2Me)CꢁC(CO2Me)} (1)
Azine diphosphine (PPh2CH2C(But)ꢀNNꢀC(But)-
CH2PPh2) (316 mg, 0.56 mmol) and tris(dibenzylide-
neacetone)dipalladium (257 mg, 0.28 mmol) in toluene
(20 cm3) were mixed and stirred overnight at room
temperature, then DBA (77 mg, 0.54 mmol) in toluene
(1 cm3) was added and the mixture was stirred again
overnight. The solution was concentrated to ca. 5 cm3
and upon addition of isooctane (5 cm3), yellow crystals
precipitated (after several days), which were identified
as dibenzylideneacetone. Off-white crystals of the
product separated from the solution after filtration,
yield 212 mg (48%). The product was recrystallised
from benzene–pentane.
Fig. 9. Walsh diagram for the vinyl complex displaying the effect of
the opening of the PꢂPdꢂP angle, from cis to trans.
NMR data for 1: lP (C6D6, ref. to 85% H3PO4) 36.0;
lH (C6D6, hexamethyldisilane) 0.67 [9H, s], 2.77 [1H,
2
2
dd, J(HH)=12.6 Hz, J(PH)=12.6 Hz], 2.80 [3H, s],
In complex 1 the widening of the PꢂPdꢂP angle will
favour the approach of the nitrogen atom of the azine
moiety to palladium and, thus, bond formation. More-
over, the molecule will then have a convenient geometry
to promote proton abstraction, from the five-membered
ring CH2 group of the azine diphosphine ligand, by the
anionic vinyl-type ligand.
2
2
4.45 [1H, dd, J(HH)=12.6 Hz, J(PH)=12.6 Hz]; lC
(C6D6, hexamethyldisilane) 28.8 [s, C(CH3)3], 33.9 [d,
CH2, 1J(PC)=6.5 Hz], 38.9 [s, C(CH3)3], 51.7 [s,
OCH3], 113.3 [dd, Cꢁ, 2J(PC)=7.2 Hz, 2J(PC)=28
Hz], 163.8 [t, CꢀN, ꢀ J(PC)+4J(PC)ꢀ=12.7 Hz], 170.6
2
[s, CꢀO].
The migration of such a proton from the diphosphine
ligand will complete the alkyne to vinyl conversion and
the azine to ene-hydrazone transformation.
At this stage electron transfer to a suitable oxidising
agent such as the solvent or adventitious oxygen will
afford the final neutral complex.
3.2. Electrochemical studies
The electrochemical studies were done in an EG&G
PAR 173 potentiostat/galvanostat and an EG&G
PARC 175 universal programmer, using a 0.2 M
[NBu4][BF4]–THF or CH3CN solution. THF was pre-
dried over sodium wire and distilled immediately before
use. All the manipulations were done under an inert
atmosphere. Pt wire and Pt gauze were used as working
electrodes for cyclic voltammetry and controlled poten-
tial electrolysis, respectively. The potential values are
quoted versus SCE and were measured using [Fe(h5-
C5H5)2]0/+ (E=0.54 V, vs. SCE) as an internal
standard.
3. Experimental
All the manipulations were done under nitrogen or
argon. The solvents were purchased from LabScan,
pre-dried and distilled immediately before use.
[Pd2(DBA)3] [12] and PPh2CH2C(But)ꢀNNꢀC(But)-
CH2PPh2 [1] were synthesised according to reported
procedures. Dimethylacetylene dicarboxylate (DBA)
was used as received from Aldrich.
3.3. Electrosynthesis of
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
trans-[Pd{PPh2CHꢀC(But)NNꢀC(But)CH2PPh2}{C-
31P{H}-NMR spectra were recorded on a Varian
Unity 300 MHz instrument at 121.4 Hz and chemical
shifts were measured relatively to H3PO4. 1H-NMR,
13C-NMR and COSY correlation were made in a
Varian Unity 500 instrument at 499.9 or 125.7 Hz,
(CO2Me)ꢀCH(CO2Me)}] (1)
Complex 1 (180 mg, 0.22 mmol) was dissolved in the
electrolyte solution (15 cm3) contained in the cell. The

M.F.N.N. Car6alho et al. / Journal of Organometallic Chemistry 598 (2000) 318–328
327
potential was set at a value ca. 60 mV more negative
than that of the peak of the cathodic wave. When the
current intensity dropped to a constant value, the elec-
trolysis was stopped and the solution, which changed
from pale yellow to orange, was transferred to a
Schlenk tube and the solvent evaporated under vac-
uum. Recrystallisation from n-pentane–methanol al-
lowed the separation of the electrolyte ([NBu4][BF4], 3.0
mmol) and afforded (55 mg) of complex 2 (31% yield).
is less then 0.02% h−1). Using the CAD4 software data
were corrected for Lorentz and polarisation effects and
empirically for absorption (minimum transmission fac-
tor 83.5%, average transmission factor 94.9%). 6947
unique reflections with F]0 were used in structure
solution and refinement of 460 parameters. The posi-
tion of the Pd atom was obtained by a tridimensional
Patterson synthesis. All the other non-hydrogen atoms
were located in subsequent difference Fourier maps and
refined with anisotropic thermal motion parameters.
The hydrogen atoms were inserted in calculated posi-
tions and refined isotropically with fixed distances to
the parent carbon atom. Final refinement converged at
4. Crystal-structure determination of complexes 1 and 2
4.1. Crystal data for complex 1
R1=0.106 (I\3|(I)). The largest-peak in the final
−3
,
difference Fourier synthesis was 1.2 e A
and was
,
C42H48N2O4P2Pd.C6H6, M=969.4, crystallises at the
monoclinic space group C2/C with a=10.940(1), b=
located at 1.3 A of the Pd atom. The molecular struc-
ture is shown in Fig. 4. Selected crystallographic and
other relevant data, lists of observed and calculated
structure factors, tables of atomic coordinates, an-
isotropic thermal parameters, hydrogen atomic coordi-
nates, bond lengths and angles and inter- and
intra-molecular contact distances are available as sup-
plementary material. The structure solution and refine-
ment were done with SHELX-86 [13] and SHELX-93 [14],
respectively and the illustrations were drawn with OR-
TEP [15]. The atomic scattering factors and anomalous
scattering terms were taken from the international table
of X-ray crystallography [16].
,
22.086(1), c=19.042(2) A, i=92.692(9)°, U=4596
,
3
A , Dcalc. =1.29 g cm−3, T=293 K, Z=4, v(Mo–
Ka)=4.5 cm−1. The unit cell and orientation matrix
were obtained by least-squares refinement of 25 centred
reflections with 15BqB16° using CAD4 centring rou-
tines. 4376 reflections, from which 4042 independent
(Rint=0.014), were collected through a CAD4 diffrac-
tometer using variable scan speed to assure constant
statistical precision. Three standard reflections were
measured every hour, which were used to check the
stability of the crystal and of the experimental condi-
tions. The orientation of the crystal was checked by
measuring five standards every 100 reflections.
The position of the Pd atom was obtained by a
tridimensional Patterson synthesis and it was found to
be located at the special position 2. All the other
non-hydrogen atoms were located in subsequent differ-
ent Fourier maps and refined with anisotropic thermal
motion parameters. The hydrogen atoms were inserted
in calculated positions and refined isotropically with
fixed distances to the parent carbon atom. The benzene
molecule was refined isotropically. Final refinement
converged at R1=0.048 (R1=0.036 I\3|(I)).
5. Supplementary material
Tables with X-ray data for complexes 1 and 2 are
available on request from the author.
Acknowledgements
This work was partially supported by the Junta
Nacional de Investigac¸a˜o Cient´ıfica e Tecnolo´gica
(JNICT), the PRAXIS XXI programme, the Fundac¸a˜o
Calouste Gulbenkian (through a travel grant), the Cul-
tural Protocol between Portugal and the Czech Repub-
4.2. Crystal data for complex 2
C42H48N2O4P2Pd, M=891.4 crystallises at the tri-
lic, the Instituto de Cooperac¸a˜o Cient´ıfica
e
(
,
clinic space group P1 with a=11.302(1) A, b=
Tecnolo´gica Internacional (ICCTI)/Academy of Sci-
ences of the Czech Republic (ASCR) Protocol and the
Grant Agency of the Czech Republic (no. 203/97/1157).
,
,
12.528(1)
b=92.27(1)° and k=111.79(2)°, U=1978 A , Z=2,
calc. =1.37 g cm−3, v=(Mo–Ka)=5.2 cm−1. The
A,
c=16.028(2)
A,
h=107.64(2)°,
3
,
D
unit cell and orientation matrix were obtained by least-
squares refinement of 25 centred reflections with 15B
qB18°. 7225 reflections with 1.5BqB25° were
collected by the ꢀ−2q scan mode, in an Enraf–Non-
ius CAD4 diffractometer using graphite monochro-
mated radiation. Three standard reflections were
monitored during data collection but no decay or in-
strumental instability was detected (the loss of intensity
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