Enthalpy of ligand substitution in cis organopalladium complexes with monodentate ligands

Article abstract of DOI:10.1039/b910055e

The enthalpy for the substitution reaction cis-[PdRf₂(THF) ₂] + 2 L →cis-[PdRf₂L₂] + 2THF (THF = tetrahydrofuran) has been measured in THF by calorimetric methods for Rf = 3,5-dichloro-2,4,6-trifluorophenyl, L =

Full text of DOI:10.1039/b910055e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Enthalpy of ligand substitution in cis organopalladium complexes with
monodentate ligands†
Gorka Salas, Juan A. Casares* and Pablo Espinet*
Received 22nd May 2009, Accepted 29th July 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b910055e
The enthalpy for the substitution reaction cis-[PdRf2(THF)2] + 2 L → cis-[PdRf2L2] + 2THF
(THF = tetrahydrofuran) has been measured in THF by calorimetric methods for Rf = 3,5-dichloro-
2,4,6-trifluorophenyl, L = PPh3, AsPh3, SbPh3, PMePh2, PCyPh2, PMe3, AsMePh2, or L2 = dppe
(1,2-bis(diphenylphosphino)ethane), dppf (1,1¢-bis(diphenylphosphino)ferrocene). The values
determined show that the substitution enthalpy has a strong dependence on the electronic and steric
properties of the ligand. The study of the consecutive substitution reactions cis-[PdRf2(THF)2] + L →
cis-[PdRf2L(THF)] + THF, and cis-[PdRf2L(THF)] + L → cis-[PdRf2L2] + THF has been carried our
for L = PPh3 and L = PCyPh2. The first substitution is clearly more favorable for the bulkier leaving
ligand, but the second gives practically the same DH value for both cases, indicating that the differences
in steric hindrance happen to compensate the electronic differences for both ligands. The X-ray
structures of cis-[PdRf2(PMePh2)2], cis-[PdRf2(dppe)] and cis-[PdRf2(dppf)] are reported.
Introduction
The success of a metal-catalyzed cycle often depends on the ease
of ligand dissociation or ligand substitution in steps involving
isomerization, insertion, or reductive elimination. Thus, ligand
dissociation or ligand substitution enthalpy data are interesting
to estimate the feasibility of a process, or to understand the
catalytic activity observed. For organometallic compounds there
are direct calorimetric determinations on Fe,1 Cr and W,2 Ni,3
Mo,4 Ru,5 Rh,4c,5l,6 Os,7 and Pt8 complexes. Surprisingly, although
Scheme 1
palladium complexes, often bearing phosphines or arsines as
not interfered by competitive coupling. Moreover, the reaction
products are easily characterized by 19F NMR.11,12,13 Finally, THF
is such a weak ligand for Pd that substitution can be measured
even with weak ligands such as arsines and stibines.
ancillary ligands (L), are among the most versatile and useful
catalysts in organic synthesis,9 the only data available for Pd refer
to the substitution of benzonitrile in [PdCl2(C6H5CN)2] by PPh3
and by dppe (dppe = 1,2-bis(diphenylphosphino)ethane).10 No
data are available for ligand dissociation or ligand substitution
enthalpy in Pd organometallics.
Results and discussion
In this work we present a calorimetric study of the substitution
of THF, in the aryl complex cis-[PdRf2(THF)2] (1) (Rf = 3,5-
dichloro-2,4,6-trifluorophenyl; THF = tetrahydrofuran), by lig-
ands often used in homogeneous catalysis (PPh3, PMePh2,
PCyPh2, PMe3, dppe (1,2-bis(diphenylphosphino)ethane), dppf
(1,1¢-bis(diphenylphosphino)ferrocene, AsPh3, AsMePh2, and
SbPh3) (Scheme 1). Complex 1 was chosen as a model of the
type of intermediate from which reductive elimination takes place,
either directly via L substitution or via L dissociation, because of
the remarkable stability of the fluoroaryl palladium(II) derivatives.
In fact this complex does not undergo reductive elimination during
the calorimetric study and the measurement of L substitutions is
For preparative purposes [PdRf2(COD)] (COD
=
1,5-
cyclooctadiene), more stable and easier to store and handle than
1, was used to prepare complexes 2–10 by substitution of COD
in THF solution. The new compounds 3, 4, and 6–10 were fully
characterized by elemental analysis and spectroscopic methods.
The spectroscopic data of the complexes were used for their
identification in solution. Crystals suitable for X-ray diffraction
were obtained for 4, 6 and 7 (see Fig. 1 for ORTEP diagrams, and
Table 1 for selected data) and confirmed that the three complexes
are mononuclear cis isomers. Full crystallographic data are given
in the ESI (Tables SI4, SI5 and S16).†
¯
Complex 4 crystallizes in the P1 group with one molecule in the
asymmetric unit. The Pd–C lengths and C–Pd–C angle are very
IU CINQUIMA/Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de
Valladolid, E-47071, Valladolid, Spain. E-mail: espinet@qi.uva.es; Fax: +34
983423013; Tel: +34 983423231
† Electronic supplementary information (ESI) available: Tables of calori-
metric determinations, and X-ray crystal data (6 pages). CCDC reference
numbers 732755 (4), 732756 (6 ) and 732757 (7). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b910055e
˚
similar to those reported for cis-[Pd(C6F5)2(PPh3)2] (2.053(6) A,
◦
14
˚
2.060(6) A and 84.7(3) ). The P–Pd lengths and P–Pd–P and
C–Pd–C angles are also close to those found in other complexes
with two cis PPh2Me ligands.15,16 Complex 6 crystallizes in the
P21/n group, with one molecule in the asymmetric unit. The
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 8413–8420 | 8413
©

View Article Online
Table 1 Selected bond lengths and angles for complexes 4, 6 and 7
lengths for 4 are very different from those found in chloro com-
plexes, but fairly close to those found for cis-[PdMe2(PMePh2)2]
(2.321(1), 2.326(1) A), suggesting comparable trans influences
Bond angles/◦
˚
15
Bond lengths/A
˚
for Pf and Me.
cis-[PdRf2(PMePh2)2] (4)
Pd(1)–C(1)
Pd(1)–C(7)
Pd(1)–P(1)
Pd(1)–P(2)
2.048(3)
C(1)–Pd(1)–C(7)
C(7)–Pd(1)–P(1)
C(1)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
84.98(10)
90.16(7)
89.44(7)
95.48(3)
2.060(2)
2.3282(9)
2.3317(9)
Behavior of complex 1 in THF solution
The existence of equilibria between aquo-complexes and com-
plexes containing weak ligands is fairly common in group 10
cationic complexes,23,24 but has been scarcely studied in neutral
complexes.12,13,25 When complex 1 is dissolved in THF, the residual
water contained in the solvent competes with the coordinated
THF for the palladium center. This substitution reaction is
not observable by NMR spectroscopy at room temperature
because the fast exchange between coordinated water and THF
gives averaged signals, but at 223 K the signals of 1 and cis-
[PdRf2(THF)(OH2)] (11) are clearly separated (Fig. 2).12,13,26 The
content in water of the THF solvent handled in the calorimetry
experiments, determined by Karl-Fischer titration, is 2.70 ¥
10-3 M. The experiment shown in Fig. 2 was carried out with a
sample that, in the absence of substitution rearrangement, should
be 7.4 ¥ 10-3 M in 1. The equilibrium concentrations at 223 K,
(calculated from the integrated signals) were 6.4 ¥ 10-3 M for 1 and
1.0 ¥ 10-3 M for 11, which gives an estimation of Keq = 1.1 ¥ 103,
and DGeq = -13 kJ mol-1.27,28 From now on, we will use cis-
[PdRf2(solv)2] to refer to this mixture of cis-[PdRf2(THF)2] and
cis-[PdRf2(THF)(OH2)], formed when pure cis-[PdRf2(THF)2] is
dissolved in THF under working conditions.
[PdRf2(dppe)] (6)
Pd(1)–C(1)
Pd(1)–C(7)
Pd(1)–P(1)
Pd(1)–P(2)
2.016(9)
2.057(8)
2.304(2)
2.323(2)
C(1)–Pd(1)–C(7)
C(1)–Pd(1)–P(1)
C(7)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
87.8(3)
91.4(2)
96.4(2)
84.72(8)
[PdRf2(dppf)] (7)
Pd(1)–C(1)
Pd(1)–C(7)
Pd(1)–P(1)
2.058(2)
2.066(2)
2.3397(8)
2.3685(9)
C(1)–Pd(1)–C(7)
C(1)–Pd(1)–P(1)
C(7)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
85.20(9)
88.41(6)
87.16(7)
99.07(2)
Pd(1)–P(2)
Fig. 1 ORTEP diagram of the complexes cis-[PdRf2(PMePh2)2] (4),
[PdRf2(dppe)] (6), and [PdRf2(dppf)] (7). H atoms have been omitted for
clarity.
Pd–C and Pd–P lengths are similar to the reported values for
other palladium complexes with dppe,17,18,19,20 with two cis PMePh2
ligands. It is also very common that the P–Pd–P angles are larger
for complexes with two PMePh2 ligands than with dppe, while the
contrary holds for the C–Pd–C angle.
Complex 7 crystallizes in the P21/n group, with one molecule
in the asymmetric unit. The Pd–C and Pd–P lengths found in
[PdRf2(dppf)] are close to the values reported in the two structures
of cis-organometallic complexes with a Pd(dppf) moiety and two
Pd–C bonds have been previously reported,21 while the P–Pd–P
angle is in between the two values reported.
Fig. 2 19F NMR spectrum of 1 in THF at -50 ◦C, showing minor signals
of 11.
Calorimetric measures of substitution reactions
Ligand substitution reactions (eqn (1)) were carried out in THF at
271.3 K, under atmospheric pressure of dry nitrogen, using a 7.42 ¥
10-3 M concentration of cis-[PdRf2(solv)2], and a L : Pd = 10 : 1
ratio of added ligand L to ensure a fast and essentially complete
shift of the equilibrium to cis-[PdRf2L2]. In order to compensate
for the dilution heat, an identical solution of ligand was added
to pure THF in the reference cell. After each experiment was
calorimetrically complete, 19F and 31P NMR of the mixture were
registered at the working temperature, without dilution of the
sample, to verify that the reaction had finished.
A
comparison of the Pd–P lengths reported for cis-
˚
[PdCl2(PMePh2)2] (2.267(2), 2.262(2) A), [PdCl2(dppe)] (2.264(3),
˚
˚
2.284(3) A), and [PdCl2(dppf)] (2.301(1), 2.283(1) A), with those
found for 4, 6, and 7 (Table 1) shows that the trans influence of the
aryl ligand C6Cl2F3 is higher than that of Cl.22 Moreover, the Pd–P
8414 | Dalton Trans., 2009, 8413–8420
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Table 2 Enthalpies of substitution (kJ mol-1) in the reactions of eqn (1), corrected data for cis-[PdRf2(THF)2], and NMR data of the products
Entry
L
-DH/kJ mol-1 (standard deviation)a
-DH/kJ mol-1 correctedb
19F NMR (ppm)c
31P NMR (ppm)c
1
2
3
4
5
6
7
8
9
PPh3
109 (4)
126 (6)
160 (8)
187 (2)
154 (2)
143 (1)
72 (2)
110
127
160
187
154
143
72
-89.1, -120.9
-87.6, -120.3
-88.8, -121.3
-88.7, -120.3
-88.2, -120.9
-89.1, -120.8
-88.5, -120.4
-88.2, -120.6
-86.8, -119.8
19.4
19.8
4.3
-18.0
49.2
17.0
—
PCyPh2
PMePh2
PMe3
12 dppe
12 dppf
AsPh3
AsMePh2
SbPh3
96 (2)
60 (3)
96
60
—
—
a Substitution on cis-[PdRf2(solv)2]. Each data point is the average of three experiments. b Substitution starting on cis-[PdRf2(THF)2] (calculated data).
c The NMR of the reaction solution was registered at 273.3 K, using a coaxial tube containing deuterated acetone for deuterium-lock.
cis-[PdRf(solv)2] + 2L ꢀ cis-[PdRf2L2] + 2solv
(1)
The ligand–metal orbital overlap also depends on the coordi-
nation angles, which in cis complexes are often different from
the ideal 90◦. This can be particularly important when bulky
ligands force wider angles (probably the case of entry 2), and
when chelating ligands impose a constrained bite angle, whether
larger or smaller. Thus, comparing the electronically similar dppe
and PMePh2 ligands, both P–Pd–P coordination angles are about
5◦ from 90◦ (Table 1). Consistently, their enthalpies for the
substitution of two THF ligands (Table 2, entries 3 and 5) are quite
similar (just 6 kJ mol-1 lower for dppe). The ◦higher geometrical
constraint for dppf32 leads to a bite angle of 99 in 7 (Table 1), and
a further enthalpy decrease of 10.5 kJ mol-1 compared to dppe.33
Interestingly, DH for dppf is still almost 33 kJ mol-1 larger than for
PPh3, suggesting that the ferrocenyl substituent on phosphorus is
more similar to an alkyl than to an aryl.
In order to correct the measured heat for the presence of
complex 11 in the initial solution, we tried to measure the
enthalpy of substitution of THF by water, applying eqn (1) for
OH2 as ligand. However, the heat released in the reaction of cis-
[PdRf2(solv)2] with OH2 was too small for accurate integration
of the curve obtained in the calorimeter. Alternatively, the heat
of the substitution reaction of cis-[PdRf2(OH2)2] prepared in situ
(see Experimental) with PPh3 was measured (93(3) kJ mol-1).
This value, together with the enthalpy of substitution on cis-
[PdRf2(solv)2] with PPh3 (109(4) kJ mol-1), allows calculation of
the reaction heat of the substitution of the two coordinated THF
ligands by two OH2 molecules (DH = -17 kJ mol-1). Since the
concentrations of cis-[PdRf2(THF)2] and cis-[PdRf2(THF)(OH2)]
in the initial mixture are known from NMR, it was possible to
calculate values of the reaction heats corrected for the percentage
of substitution of one THF for one OH2 for all the ligands
employed in this work (Table 2).29 It turned out that the differences
between the experimental data measured on cis-[PdRf2(solv)2]
and the corrected data for cis-[PdRf2(THF)2] are equal within
experimental error, so that in fact the corrections can be neglected.
Comparing the results collected in Table 2, a large decrease in
reaction heat is observed for analogous ligands differing only in
the donor atom (Table 2, entries 1, 7, 9 and entries 3, 8). Relative
to PPh3, AsPh3 releases 66% of heat, and SbPh3 only 55%. This
effect, qualitatively well known but not previously quantified, is
explained by the loss of bond directionality in group 15 ligands
when going down in the group, due to the increasing s orbital
participation of the donor pair.30 This loss of s bond energy is
not compensated by a gain in p backbonding, due to the high
stability of the filled d orbitals in Pd(II).31 The closest system
previously studied, the substitution of THF in the cation trans-
[PtMe(THF)(PMePh2)2]+, shows a similar but more steep descent
in bond energy, probably because the cationic Pt(II) is harder than
the neutral Pd(II): the energy involved in the substitution by SbPh3
in the Pt complex is only a 31% of that for PPh3.8a The reaction
heats for simple phosphines PPh3 < PMePh2 < PMe3 (Table 2,
entries 1, 3, 4) or arsines AsPh3 < AsMePh2 (entries 7, 8) follow
the trend imposed mostly by the ligand basicity, according to the
different substituents, whereas the sequence PCyPh2 < PMePh2
(entries 2 and 3) reflects mostly the unfavorable effect of steric
encumbrance associated to the bulk Cy compared to the small Me
(see, however, Discussion below).
Analysis of consecutive reactions
The heats collected in Table 1, measured for different L ligands
according to reaction (1), are in fact the result of two consecutive
substitution reactions (Scheme 2)
Scheme 2
A stepwise direct experimental determination of each of the
two consecutive enthalpies involved was not possible because the
addition of only one equivalent of L per palladium leads to a
mixture in equilibrium of cis-[PdRf2L(solv)], cis-[PdRf2L2], and
starting material cis-[PdRf2(solv)2]. The addition of more than one
equivalent of L leads to increasing proportions of cis-[PdRf2L2].
Fortunately the mono- and disubstituted products give separated
signals in 19F NMR, allowing for independent integration.
With these data in hand, the heat measured in any substitution
reaction can be correlated with the enthalpies of the two consecu-
tive reactions and the concentrations of products in equilibrium.34
The enthalpies of the consecutive substitutions DH1 and DH2 can
be obtained from the slope and the y-intercept of the linear plot
Qi/n1I versus n2I /n1I . For the experiment i, Qi is the observed heat
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 8413–8420 | 8415
©

View Article Online
released, n1I is the number of moles of cis-[PdRf2L(solv)] formed
during the reaction, and n2I is the number of moles of cis-[PdRf2L2]
in the final equilibrium (see Experimental for more details). This
procedure was applied to experiments performed with PPh3 and
PCyPh2. The values calculated for the two consecutive reactions,
DH1 and DH2, obtained from the linear plots in Fig. 3, are
consistent with the data directly measured DHtotal. In effect, the
sum DH1 + DH2 (-109 kJ mol-1 for PPh3 and -125 kJ mol-1
for PCyPh2) is almost identical to the experimental value DHtotal
(-109(4) and -126(8) kJ mol-1, respectively; Table 2).
Fig. 4 Plot of first and second substitution enthalpies (DH1 and DH2) for
PPh3 and PCyPh2.
PPh3. In simple terms one could say that, although PCyPh2 is
a stronger ligand and the formation of cis-[PdRf2L2] is, overall,
more exothermic for L = PCyPh2, the replacement of one L in cis-
[PdRf2L2] to give cis-[PdRf2L(solv)], releasing one L, is thermally
less costly for L = PCyPh2, in spite of the commonly accepted idea
that it is a “better donor ligand” than PPh3. The structural data
of similar complexes (particularly the P–Pd–P angles) suggest that
there should be very little crowding in complex 2,14 so the small
decrease in DH from the first to the second substitution (DH2 is
9% smaller than DH1) is probably caused mostly by the lowering
in Pd electrophilicity after one PPh3 has been coordinated.
For PCyPh2, however, the effect observed (33%) is much larger
than one would probably expect on electronic grounds, indicating
that much of this reduction must be due to the steric crowding
effect. It is interesting to note that the increase in cone angle is
not that big (PPh3 = 145◦; PCyPh2 = 153◦), but produces a large
effect in cis-[PdR2L2] complexes. In fact, further increase of steric
hindrance of the ligand greatly disfavor its coordination ability,
and cis-[PdRf2(PCy3)2] could not be prepared.
Fig. 3 Linear plot Qi/n1 versus n2/n1 from values obtained by partial
substitution experiments with PPh3 and PCyPh2.
Influence of steric hindrance on the bond energy
Two parameters proposed by Tolman are used to classify ligand
properties:35 ligand hindrance is estimated by the Tolman “cone
angle”, and electron richness by the Tolman electronic param-
eter (TEP). The sequence of enthalpy values observed for the
substitution reactions with monodentate phosphines (Table 2) is
PMe3 > PMePh2 > PCyPh2 > PPh3, and must be the overall
reflection of electronic and steric influences. The cone angles and
TEP values for these phosphines are: PMe3 = 118◦, 2064.1 cm-1;
PMePh2 = 136◦, 2067.0 cm-1; PCyPh2 = 153◦, 2064.8 cm-1;
PPh3 = 145◦, 2068.9 cm-1.
Kinetic study of ligand substitution
According to the results discussed above, and assuming that the
ligand exchange equilibrium between cis-[PdRf2(PCyPh2)2] and
cis-[PdRf2(PPh3)2] is dominated by enthalpy, a preference for
the formation of the mixed complex cis-[PdRf2(PPh3)(PCyPh2)]
might be expected. For the same reason, since the reaction cis-
[PdRf2(solv)2] + 2 L does not lead to complete solvent substitution,
one could expect that on dissolving any of the two phosphine
complexes above in THF, the equilibrium containing some cis-
[PdRf2(solv)2] and some cis-[PdRf2L(solv)] should be established.
However, none of these rearrangements was observed with the
complexes cis-[PdRf2(PCyPh2)2] or cis-[PdRf2(PPh3)2]. In effect,
1 : 1 mixtures of cis-[PdRf2(PCyPh2)2] and cis-[PdRf2(PPh3)2] did
not rearrange perceptibly even after long periods of time, and
any of these complexes remained unaffected after a long time in
THF. This inertness towards ligand substitution precluded assess-
ing experimentally the relative thermodynamic ease of substitution
predicted for each of the two phosphines. The very different
rates of the ligand exchange for the fast forward reaction on cis-
[PdRf2L(solv)], and slow reverse reaction on cis-[PdRf2(PRPh2)2]
It is very likely that the influence of the steric hindrance con-
tribution will be relatively larger in the square-planar complexes
studied here (L–Pd–L, angle about 90◦, two non-small non-linear
ligands) than in the less crowded tetrahedral Ni(CO)3L (L–Ni–
CO, angle of about 109◦, and one small linear ligand, CO), which
are the kind of complexes used as reference to define TEP.36 The
values of the first and second substitution enthalpies found for
PCyPh2 and PPh3 give a nice illustration of the complex influence
of steric and electronic properties on bond energy. In the absence
of steric factors, the bond energy should depend on the electronic
properties of the ligand. The order of DHtotal values found for the
double substitution reaction is PCyPh2 > PPh3 (Table 2) would
be consistent with the expectations from the electronic properties.
An inspection of the consecutive substitution enthalpies affords a
richer view of the process. Fig. 4 (lower plot) shows graphically
the enthalpy data for the two ligands. The ratio DH2/DH1 is
about 0.91 for PPh3, but only about 0.66 for PCyPh2. In fact
DH1 is considerably larger for PCyPh2, but DH2 is larger for
8416 | Dalton Trans., 2009, 8413–8420
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
(R = Ph, Cy) suggests that, whether the mechanism is associative
or dissociative, the elongation of the bond to the leaving ligand
must be important in the transition state. This is why the reaction
is fast when the leaving group is a weak THF ligand, but slow
when the leaving ligand is a phosphine.
distilled under nitrogen; benzophenone was used as a moisture
1
indicator. H NMR (300.16 MHz) and 19F NMR (282.4 MHz)
spectra were recorded on a Bruker ARX 300 instrument equipped
with a VT-100 variable temperature probe. Chemical shifts are
reported in ppm from tetramethylsilane (1H), or CCl3F (19F), with
positive shifts downfield, at ambient probe temperature unless
otherwise stated. In the 19F NMR spectra registered in non-
deuterated solvents, a coaxial tube containing acetone-d6 was used
to maintain the lock 2H signal, and the chemical shifts are reported
from the CCl3F signal in deuterated acetone. Combustion CHN
analyses were made on a Perkin-Elmer 2400 CHN microanalyzer.
Infrared spectra were recorded on a Perkin-Elmer 843 apparatus
(range 4000–200 cm-1) with Nujol mulls between polyethylene
sheets or in dichloromethane solution between NaCl plates.
The ligand substitution reactions were examined then for the
ligand exchange cis-[PdRf2L2] + 2 L¢, L and L¢ being the two phos-
phines. In both cases the formation of cis-[PdRf2LL¢] and a minute
amount of cis-[PdRf2L¢2] was observed. The reaction (in THF at
room temperature) was very slow, and after three days the reaction
was still far from equilibrium, while some decomposition started
to interfere with the experiment. This behavior precluded the direct
determination of the equilibrium constants, but at least the kinetics
of these slow substitutions could be measured at 323 K. It turned
out that the substitution of PPh3 by PCyPh2 in cis-[PdRf2(PPh3)2]
to give cis-[PdRf2(PPh3)(PCyPh2)], and that of PCyPh2 by PPh3 in
cis-[PdRf2(PCyPh2)2], take place with almost the same rate (0.022
and 0.037 min-1 respectively), and with similar activation free
energies (100.6 and 99.1 kJ mol-1). The difference between these
DG‡ (1.5 kJ mol-1) values is essentially identical to the difference
in substitution enthalpy of the first ligand substitution by THF
determined calorimetrically (2 kJ mol-1) supporting the idea that
both substitutions have the same entropic influence. This does
not help to suggest any substitution mechanism over another.
However, it is not unreasonable that a dissociative mechanism
could be preferred, as the extreme crowding in the pentacoor-
dinated intermediate of an associative substitution mechanism
would make it even higher in energy. Dissociative mechanisms in
related Pt complexes were proposed and supported by Romeo,37
and have been suggested for substitution reactions in hindered
cis-[Pd(Fmes)2(SR2)2] (Fmes = 2,4,6-tris(trifluoromethyl)phenyl)
complexes.38
39
Ligand AsMePh2 and complexes cis-[Pd(C6Cl2F3)2(THF)2]
(1), cis-[Pd(C6Cl2F3)2(COD)2] (COD = 1,5-cyclooctadiene), cis-
[Pd(C6Cl2F3)2(PPh3)2] (2), and cis-[Pd(C6Cl2F3)2(PMe3)2] (5) were
prepared by published methods.12 Complex (1), used repeatedly
for the calorimetric studies, was prepared on a large scale in order
to use the same material for all the experiments.
Characterization of the mixture cis-[PdRf2(solv)2]
A solution of complex cis-[PdRf2(THF)2] (4.8 mg, 7.4 mmol) in
THF (1.0 mL) was prepared. Two singlets are observed in 19F
NMR at 271.3 K, which are due respectively to Fortho and Fpara of cis-
[PdRf2(THF)2] and cis-[PdRf2(THF)(OH2)] in fast exchange equi-
librium. 19F NMR (THF, 271.3 K): d = -89.8 (s, 4F, Fortho), -119.1
(s, 2F, Fpara). At 223 K both complexes are observed, with concen-
trations of 6.4 ¥ 10-3 M and 1.0 ¥ 10-3 M for cis-[PdRf2(THF)2]
and cis-[PdRf2(THF)(OH2)], respectively. 19F NMR (THF, 223
K): d = -89.1 (s, 2F, Fortho of cis-[PdRf2(THF)(OH2)]), -89.6 (s,
2F, Fortho of cis-[PdRf2(THF)(OH2)]), -89.9 (s, 4F, Fortho of cis-
[PdRf2(THF)2]), -119.2 (s, 2F, Fpara of cis-[PdRf2(THF)2]), -119.8
(s, 1F, Fpara of cis-[PdRf2(THF)(OH2)]), -120.1 (s, 1F, Fpara of cis-
[PdRf2(THF)(OH2)]).
Conclusions
The enthalpies of substitution in cis-diarylpalladium complexes
follow the expected trends considering the electronic and steric
parameters used to describe the behavior of ligands. However, for
the bulkiest ligands there are large differences between the energy
involved in the first and the second substitution, due to the steric
hindrance imposed by the first ligand over the second. Therefore,
the second substitution enthalpy may differ very significantly from
the values one could guess from parameters determined on simpler
systems, such as Tolman’s parameters. The results also show that
there is no straightforward relationship between the substitution
enthalpy and the kinetics of the substitution reaction: although
enthalpy would favor the release of bulky ligands, hindrance
can induce inertness towards associative substitutions, leading to
dissociative or interchange dissociative mechanisms that, when
involving strong ligands, may require high reaction temperatures
for the kinetics to be appreciable.
Calorimetric measurements
Calorimetric measurements were performed using an Omnical
reaction calorimeter. The instrument contains a calorimeter that
compares the heat released or consumed in a sample vessel to
an empty reference vessel, and an internal magnetic stirrer. The
vessels were 16 mL 21 mm ¥ 70 mm borosilicate screw-thread
vials fit with open top screw caps and PTFE septa, and charged
with Teflon stir bars. The temperature of the calorimeter was held
constant using a Julabo F25 circulator, ensuring that the reaction
would proceed under isothermal conditions. The temperature of
the circulating system was set at 268 K. After 12 h, the temperature
in the calorimeter was constant and stabilized in the final value
271.3 K. In a typical experiment, the reaction vessel was charged
with 19.30 mg of cis-[PdRf2(THF)2] (29.67 mmol) and 2 mL of
THF, and the reference vessel with 2 mL of THF. In the sample
injection ports of both vessels were placed syringes containing
2 mL of solution of the ligand (296.7 mmol for monodentate
ligands, 148.35 mmol for bidentate ligands) in THF. The system
was allowed to reach thermal equilibrium for at least one hour.
Then, the solution of ligand was simultaneously injected in both
vessels. When the reaction finished, an aliquot of the solution was
Experimental
General comments
All reagents were purchased from commercial sources and used as
received. Solvents were dried by known procedures and distilled
under nitrogen prior to use. THF was refluxed over sodium and
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 8413–8420 | 8417
©

View Article Online
placed in a NMR tube containing a coaxial capillary tube with
deuterated acetone, and 19F (and 31P NMR) spectra were recorded
to verify the absence of side reactions.
After 30 min the solution was concentrated to 10 mL and
n-hexane (10 mL) was added. Further evaporation of the solvent
yielded the product as white crystals. Yield 569 mg (90%). Anal.
calcd for C38H26Cl4F6P2Pd (%): C 50.33, H 2.94. Found: C 50.15,
H 2.94. 1H NMR (CDCl3, 293 K): d = 7.4–7.2 (m, 20H, Ph), 1.51
(m, 3H, Me). 19F NMR (CDCl3/THF, 293 K): d = -88.9/-88.7
(t, J = 7.6/7.6 Hz, 4Fortho), -120.7/-121.2 (s, 2Fpara). 31P NMR
(CDCl3/THF, 293 K): d = 3.6/4.3 (m).
To measure the heat released in the substitution reaction of
PPh3 with cis-[PdRf2(OH2)2] the experiment was similar, but used
a 2.0 M solution of water in THF instead of pure THF as solvent.
Prior to this experiment, a solution of cis-[PdRf2(THF)2] (29.67
mmol) in a 2.0 M solution of water in THF was checked by NMR to
make sure that cis-[PdRf2(OH2)2] was the only complex in solution:
19F NMR (THF, 271.3 K): d = -88.7 (s, 4F, Fortho), -121.0 (s, 2F,
Fpara).
[PdRf2(dppe)] (6). Prepared as described for 4 but using dppe
(302 mg, 0.758 mmol) instead of PMePh2. Yield 595 mg (94%).
Anal. calcd for C38H24Cl4F6P2Pd (%): C 50.45, H 2.67. Found:
C 50.49, H 3.03. 1H NMR (CDCl3, 293 K): d = 7.6–7.3 (m,
20H, Ph), 2.36 (s, 2H, CH2–CH2), 2.29 (s, 2H, CH2–CH2). 19F
NMR (CDCl3/THF, 293 K): d = -89.4/-88.0 (t, J = 7.6/7.6 Hz,
2Fortho), -120.3/-120.8 (s, 1Fpara). 31P NMR (CDCl3/THF, 293 K):
d = 48.0/49.1 (m).
Evaluation of consecutive reaction enthalpies
The calorimetric experiments were performed as described above.
In a double substitution process of the type shown in Scheme 2,
the reaction of the starting complex cis-PdRf2(solv)2 with more
than one equivalent of ligand L lead to the formation of n1 mol of
(cis-[PdRf2L(solv)] + cis-[PdRf2L2]) and n2 mol of [PdRf2L2]. Note
that n1 is the sum of the mol of the complexes cis-[PdRf2L(solv)]
and cis-[PdRf2L2] because both have suffered the first substitution,
while n2 is just the amount of cis-[PdRf2L2].
[PdRf2(dpp(f)] (7). To a stirred solution of [PdRf2(COD)]
(180 mg, 0.294 mmol) in THF (10 mL), was added dppf (180.3 mg,
0.758 mmol). After 30 min the solution was concentrated to 5 mL
and n-hexane (10 mL) was added. Further evaporation of the
solvent yielded the product as yellow crystals. Yield 275 mg (88%).
Anal. calcd for C46H28Cl4F6FeP2Pd (%): C 52.09, H 2.66. Found:
cis-[PdRf2(solv)2] + L → cis-[PdRf2L(solv)], DH1
cis-[PdRf2L(solv)] + L → cis-[PdRf2L2], DH2
1
C 52.10, H 2.94. H NMR (CDCl3, 293 K): d = 7.57 (m, 8H,
Ph), 7.46 (m, 4H, Ph), 7.34 (m, 8H, Ph), 4.34 (m, 8H, Cp). 19F
NMR (CDCl3/THF, 293 K): d = -90.1/-88.9 (t, J = 7.6/7.6 Hz,
4Fortho), -120.5/-120.7 (s, 2Fpara). 31P NMR (CDCl3/THF, 293 K):
d = 16.4/16.9 (m).
cis-[PdRf2(solv)2] + 2L → cis-[PdRf2L2], DH = DH1 + DH2
The concentrations of cis-[PdRf2L(solv)] and cis-[PdRf2L2] were
obtained from their integrated signals in the 19F NMR spectra,
registered at 271.3 K from a aliquot of the solution employed
in the calorimetric experiment. The overall heat released by the
reaction Q, measured as the integral of the calorimeter signal, is
the sum of the heat released in steps 1 and 2 (q1 and q2).
cis-[PdRf2(AsPh3)2] (8). To a stirred solution of [PdRf2(COD)]
(210 mg, 0.342 mmol) in THF (20 mL), was added AsPh3
(231.8 mg, 0.757 mmol). After 30 min the solution was con-
centrated to 5 mL and n-hexane (10 mL) was added. Further
evaporation of the solvent yielded the product as white crystals.
Yield 326.4 mg (85%). Anal. calcd for C48H30As2Cl4F6Pd (%): C
Q = q1 + q2 = -n1DH1 - n2DH2
1
51.53, H 2.70. Found: C 51.18, H 2.67. H NMR (CDCl3, 293
K): d = 7.4-7.2 (m, 20H, Ph). 19F NMR (CDCl3/THF, 293 K):
Q
n2
= −DH1 − DH2
n1
d = -89.4/-88.3 (s, 4Fortho), -120.2/-120.3 (s, 2Fpara).
n1
cis-[PdRf2(AsMePh2)2] (9). To
a
stirred solution of
[PdRf2(COD)] (139 mg, 0.226 mmol) in THF (20 mL) was
added AsMePh2 (122.0 mg, 0.500 mmol). After 30 min, the
solution was concentrated to 5 mL and Et2O (10 mL) was added.
Further evaporation of the solvent yielded the product as white
crystals. Yield: 162 mg (86%). Anal. calcd for C38H26As2Cl4F6Pd
(%): C 45.89, H 2.63. Found: C 45.72, H 2.73. 1H NMR (CDCl3,
293 K): d = 7.4–7.2 (m, 20H, Ph), 1.38 (s, 6H, Me). 19F NMR
(CDCl3/THF, 293 K): d = -89.3/-88.1 (s, 4Fortho), -119.9/-120.4
(s, 2Fpara).
A plot of Q/n1 versus n2/n1 gives a straight line with slope -DH2
and y-intercept -DH1.
Synthesis and NMR data of the palladium complexes
cis-[PdRf2(PCyPh2)2] (3). To
a
stirred solution of
[PdRf2(COD)] (111 mg, 0.181 mmol) in THF (20 mL), was
added PCyPh2 (110 mg, 0.409 mmol). After 30 min the solution
was concentrated to 5 mL and 10 mL of n-hexane were added.
Further evaporation of the solvent yielded the product as white
crystals. Yield 179 mg (80%). Anal. calcd for C48H42Cl4F6P2Pd
(%): C 55.28, H 4.06. Found: C 54.87, H 3.91. 1H NMR (CDCl3,
293 K): d = 7.38 (m, 4H, Ph), 7.19 (m, 8H, Ph), 7.00 (m, 8H, Ph),
1.58 (m, 10H, Cy), 0.91 (m, 8H, Cy), 0.54 (m, 4H, Cy). 19F NMR
(CDCl3/THF, 293 K): d = -88.5/-87.3 (s, 4Fortho), -120.0/-120.2
(s, 2Fpara). 31P NMR (CDCl3/THF, 293 K): d = 19.6/19.8.
cis-[PdRf2(SbPh3)2] (10). A stirred solution of [PdRf2(COD)]
(257 mg, 0.418 mmol) in THF (20 mL), was cooled to 0 ◦C. Then
SbPh3 (326.3 mg, 0.924 mmol) was added. After 45 min the solu-
tion was concentrated to 10 mL and n-hexane (15 mL) was added.
Further evaporation of the solvent yielded the product as white
crystals. Yield 456 mg (90%). Anal. calcd for C48H30Cl4F6PdSb2
(%): C 47.55, H 2.49. Found: C 47.23, H 2.76. 1H NMR (CDCl3,
293 K): d = 7.33 (m, 12H, Ph), 7.21 (m, 18H, Ph). 19F NMR
(CDCl3/THF, 293 K): d = -87.8/-86.7 (s, 4Fortho), -119.5/-119.7
(s, 2Fpara).
cis-[PdRf2(PMePh2)2] (4). To
a
stirred solution of
[PdRf2(COD)] (428 mg, 0.697 mmol) in THF (30 mL), was
added PMePh2 (5.2 mL of a 0.3 M solution in THF, 1.6 mmol).
8418 | Dalton Trans., 2009, 8413–8420
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
11 (a) A. L. Casado, J. A. Casares and P. Espinet, Organometallics, 1997,
16, 5730; (b) J. A. Casares, P. Espinet, K. Soulantica, I. Pascual
and A. G. Orpen, Inorg. Chem., 1997, 36, 5251; (c) M. A. Alonso,
J. A. Casares, P. Espinet, J. M. Mart´ınez-Ilarduya and C. Pe´rez-Briso,
Eur. J. Inorg. Chem., 1998, 1745; (d) P. Espinet, A. M. Gallego, J. M.
Mart´ınez-Ilarduya and E. Pastor, Inorg. Chem., 2000, 39, 975; (e) R. de
la Cruz, P. Espinet, A. M. Gallego, J. M. Mart´ın-Alvarez and J. M.
Mart´ınez-Ilarduya, J. Organomet. Chem., 2002, 663, 108; (f) J. A.
Casares, P. Espinet, J. M. Mart´ın-Alvarez, J. M. Mart´ınez-Ilarduya
and G. Salas, Eur. J. Inorg. Chem., 2005, 3825.
12 P. Espinet, J. M. Mart´ınez-Ilarduya, C. Pe´rez-Briso, A. L. Casado and
M. A. Alonso, J. Organomet. Chem., 1998, 551, 9.
13 J. A. Casares, P. Espinet, J. M. Mart´ınez-Ilarduya, J. J. Mucientes and
G. Salas, Inorg. Chem., 2007, 46, 1027.
Acknowledgements
We thank the Ministerio de Ciencia y Tecnolog´ıa (CTQ2007-
67411/BQU), Consolider Ingenio 2010 (CSD2006-0003), and the
Junta de Castilla y Leo´n (VA044A07 and GR169) for financial
support.
Notes and references
1 (a) L. Luo and S. P. Nolan, Organometallics, 1992, 11, 3483; (b) L. Luo
and S. P. Nolan, Inorg. Chem., 1993, 32, 2410; (c) C. Li and S. P. Nolan,
Organometallics, 1995, 14, 1327; (d) C. Li, E. D. Stevens and S. P. Nolan,
Organometallics, 1995, 14, 3791; (e) S. A. Serron and S. P. Nolan, Inorg.
Chim. Acta, 1996, 252, 107; (f) C. M. Haar and S. P. Nolan, Inorg.
Chim. Acta, 1999, 291, 32; (g) D. C. Smith, A. L. Klaman, J. Cadoret
and S. P. Nolan, Inorg. Chim. Acta, 2000, 300–302, 987.
2 S. L. Mukerjee, R. F. Lang, T. Ju, G. Kiss and C. D. Hoff, Inorg. Chem.,
1992, 31, 4885.
3 C. A. Tolman, D. W. Reutter and W. C. Seidel, J. Organomet. Chem.,
1976, 117, C30.
4 (a) S. P. Nolan and C. D. Hoff, J. Organomet. Chem., 1985, 290, 365;
(b) S. P. Nolan, R. Lo´pez, de la Vega and C. D. Hoff, Organometallics,
1986, 5, 2529; (c) S. L. Makerjee, S. P. Nolan, C. D. Hoff and R. de
la Vega, Inorg. Chem., 1988, 27, 81; (d) A. Huang, J. E. Marcone,
K. L. Mason, W. J. Marshall, K. G. Moloy, S. Serron and S. P. Nolan,
Organometallics, 1997, 16, 3377.
14 K. Miki, N. Kasai and H. Kurosawa, Acta Crystallogr., Sect. C: Cryst.
Struct. Commun., 1988, 44, 1132.
15 J. M. Wisner, T. J. Bartczak and J. A. Ibers, Organometallics, 1986, 5,
2044.
16 N. W. Alcock and J. H. Nelson, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 1985, 41, 1748.
17 T. Spaniel, H. Schmidt, C. Wagner, K. Merzweiler and D. Steinborn,
Eur. J. Inorg. Chem., 2002, 2868.
18 P. R. Alburquerque, A. R. Pinhas and J. A. Krause Bauer, Inorg. Chim.
Acta, 2000, 298, 239.
19 V. V. Grushin and W. J. Marshall, J. Am. Chem. Soc., 2006, 128,
4632.
20 S. Singh, N. K. Jha, P. Narula and T. P. Singh, Acta Crystallogr., Sect.
C: Cryst. Struct. Commun., 1995, 51, 593.
21 (a) A. S. K. Hashmi, F. Naumann, R. Probst and J. W. Bats, Angew.
Chem., Int. Ed. Engl., 1997, 36, 104–106; (b) H. S. Huh, Y. K. Lee and
S. W. J. Lee, J. Mol. Struct., 2006, 789, 209–219.
5 (a) L. Luo, S. P. Nolan and P. J. Fagan, Organometallics, 1993, 12,
4305; (b) L. Luo, N. Zhu, N.-J. Zhu, E. D. Stevens, S. P. Nolan and
P. J. Fagan, Organometallics, 1994, 13, 669; (c) C. Li, M. E. Cucullu,
R. A. McIntyre, E. D. Stevens and S. P. Nolan, Organometallics, 1994,
13, 3621; (d) L. Luo and S. P. Nolan, Organometallics, 1994, 13, 4781;
(e) M. E. Cucullu, L. Luo, S. P. Nolan, P. J. Fagan, N. L. Jones and
J. C. Calabrese, Organometallics, 1995, 14, 289; (f) L. Luo, C. Li, M. E.
Cucullu and S. P. Nolan, Organometallics, 1995, 14, 1333; (g) S. A.
Serron and S. P. Nolan, Organometallics, 1995, 14, 4611; (h) S. A.
Serron, L. Luo, C. Li, M. E. Cucullu and S. P. Nolan, Organometallics,
1995, 14, 5290; (i) C. L. Li Luo, S. P. Nolan, W. Marshall and P. J.
Fagan, Organometallics, 1996, 15, 3456; (j) C. Li, S. A. Serron, S. P.
Nolan and J. L. Petersen, Organometallics, 1996, 15, 4020; (k) S. A.
Serron, L. Luo, E. D. Stevens, S. P. Nolan, N. L. Jones and P. J. Fagan,
Organometallics, 1996, 15, 5209; (l) C. Li, S. P. Nolan and I. T. Horva´th,
Organometallics, 1998, 17, 452; (m) J. Huang, H. J. Schanz, E. D. Stevens
and S. P. Nolan, Organometallics, 1999, 18, 2370; (n) J. Huang, E. D.
Stevens, S. P. Nolan and J. L. Petersen, J. Am. Chem. Soc., 1999, 121,
2674; (o) M. S. Balakrishna, R. Panda, D. C. Smith, A. Klaman and
S. P. Nolan, J. Organomet. Chem., 2000, 599, 159; (p) N. M. Scott, H.
Clavier, P. Mahjoor, E. D. Stevens and S. P. Nolan, Organometallics,
2008, 27, 3181; (q) D. C. Smith, J. Cadoret, L. Jafarpour, E. D. Stevens
and S. P. Nolan, Can. J. Chem., 2001, 79, 626; (r) M. E. Cucullu, C. Li,
S. P. Nolan, S. T. Nguyen and R. H. Grubbs, Organometallics, 1998,
17, 5565; (s) J. Shen, E. D. Stevens and S. P. Nolan, Organometallics,
1998, 17, 3000.
6 (a) S. A. Serron, S. P. Nolan and K. G. Moloy, Organometallics, 1996,
15, 4301; (b) S. Serron, J. Huang and S. P. Nolan, Organometallics,
1998, 17, 534; (c) C. M. Haar, J. Huang, S. P. Nolan and J. L. Petersen,
Organometallics, 1998, 17, 5018; (d) J. Huang, C. M. Haar, S. P. Nolan,
W. J. Marshall and K. G. Moloy, J. Am. Chem. Soc., 1998, 120,
7806.
7 J. Huang, S. Serron and S. P. Nolan, Organometallics, 1998, 17, 4004.
8 (a) L. E. Manzer and C. A. Tolman, J. Am. Chem. Soc., 1975, 97,
1955–1986; (b) C. M. Haar, S. P. Nolan, W. J. Marshall, K. G. Moloy,
A. Prock and W. P. Giering, Organometallics, 1999, 18, 474; (c) D. C.
Smith, C. M. Haar, E. D. Stevens, S. P. Nolan, W. J. Marshall and
K. G. Moloy, Organometallics, 2000, 19, 1427; (d) C. Laporte, G.
Frison, H. Gru¨tzmacher, A. C. Hillier, W. Sommer and S. P. Nolan,
Organometallics, 2003, 22, 2202–2208; (e) M. S. Balakrishna, S. Priya,
W. Sommer and S. P. Nolan, Inorg. Chim. Acta, 2005, 358, 2817–2820.
9 (a) Applied Homogeneous Catalysis with Organometallic Compounds,
ed. B. Cornils and W. A. Hermann, Wiley-VCH, Weinheim, 2002;
(b) Organometallics in Synthesis, ed. M. Schlosser, John Wiley & Sons,
West Sussex, UK, 2002.
22 T. Hayashi, M. Konishi, Y. Kobori, M. Kumada, T. Higuchi and K.
Hirotsu, J. Am. Chem. Soc., 1984, 106, 158–163.
23 J. Vicente and A. Arcas, Coord. Chem. Rev., 2005, 249, 1135.
24 For palladium aquo-complexes see: (a) A. J. Deeming, I. P. Rothwell,
M. B. Hursthouse and L. New, J. Chem. Soc., Dalton Trans., 1978, 1490;
(b) P. Castan, P. Jaud, S. Wimmer and F. L Wimmer, J. Chem. Soc.,
Dalton Trans., 1991, 1155; (c) J. Vicente, A. Arcas, M. V. Borrachero,
E. Mol´ıns and C. Miratvilles, J. Organomet. Chem., 1992, 441, 487;
(d) J. P. Sheehan, T. R. Spalding, G. Ferguson, J. F. Gallagher, B.
Kaitner and J. D. Kennedy, J. Chem. Soc., Dalton Trans., 1993, 35;
(e) A. L. Seligson and W. C. Trogler, Organometallics, 1993, 12, 738;
(f) F. Maassarani, M. F. Davidson, I. C. M. Wehman-Ooyevaar, D. M.
Grove, M. A. van Koten, W. J. J. Smeets, A. L. Spek and G. van
Koten, Inorg. Chim. Acta, 1995, 235, 327; (g) J. Vicente, A. Arcas,
D. Bautista and P. G. Jones, Organometallics, 1997, 16, 2127; (h) J.
Vicente, A. Arcas, M. A. Blasco, J. Lozano and M. C. Ram´ırez, de
Arellano, Organometallics, 1998, 17, 5374; (i) J. Ruiz, F. Florenciano,
C. Vicente, M. C. Ramı´ırez de Arellano and G. Lo´pez, Inorg. Chem.
Commun., 2000, 3, 73; (j) I. C. Barco, L. R. Falvello, S. Ferna´ndez, R.
Navarro and E. P. Urriolabeitia, J. Chem. Soc., Dalton Trans., 1998,
1699; (k) J.-F. Ma, Y. Kojima and Y. Yamamoto, J. Organomet.
Chem., 2000, 616, 149. Other cationic palladium organometallic aquo-
complexes reported but not characterized by diffraction methods:
(l) A. D. Ryabov, G. M. Kazankov, A. K. Yatsimirsky, L. G. Kuz’mina,
O. Y. Burtseva, N. V. Dvortsova and V. A. Polyakov, Inorg. Chem., 1992,
31, 3083. Crystal structures of cationic coordination palladium aquo-
complexes: (m) P. Leoni, M. Sommovigo, M. Pasquali, S. Midollini, D.
Braga and P. Sabatino, Organometallics, 1991, 10, 1038; (n) P. J. Stang,
D. H. Cao, G. T. Poulter and A. T. Arif, Organometallics, 1995, 14,
1110. Other cationic palladium coordination aquo-complexes reported
but not characterized by diffraction methods: (o) M. Fuss, H.-U. Siehl,
B. Olenyuk and P. J. Stang, Organometallics, 1999, 18, 758.
25 C. Bartolome´, P. Espinet, L. Vicente, F. Villafane, J. P. H. Charmant
and A. G. Orpen, Organometallics, 2002, 21, 3536.
26 The solvent was dried by standard procedures: see experimental. The
presence of some water is extremely difficult to avoid. It goes unnoticed
in normal circumstances, but it is detected here because of the formation
of complexes and their detection by highly sensitive 19F NMR.
27 For the calculation the uncorrected density of THF = 0.887 g mL-1 has
been used.
28 In order to use these data, measured at 223 K, in studies on substitution
reactions on cis-[PdRf2(solv)2] (solv = THF or OH2) carried out at
271.3 K, we assume that the variation in DS can be neglected; then
DG223 K ª DG271.3 K. The equilibrium constant and concentrations of 1
10 (a) W. Partenheimer, Inorg. Chem., 1972, 11, 743; (b) W. Partenheimer
and E. F. Hoy, Inorg. Chem., 1973, 12, 2805.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 8413–8420 | 8419
©

View Article Online
and 2 at 271.3 K calculated from DG271.3 K are: Keq(271.3) = 3.2 ¥ 102,
[1] = 7.0 ¥ 10-3 M, [2] = 0.4 ¥ 10-3 M.
32 G. Bandoli and A. Dolmella, Coord. Chem. Rev., 2000, 209, 161.
33 For a direct comparison of dppe and dppf ligands in palladium
complexes see: R. J. van Haaren, K. Goubitz, J. Fraanje, G. P. F. van
Strijdonck, H. Oevering, B. Coussens, J. N. H. Reek, P. C. J. Kamer
and P. W. N. M. van Leeuwen, Inorg. Chem., 2001, 40, 3363.
34 It is assumed that the differences in non-covalent solvation for the
different molecules are negligible.
29 The correction has been done using the concentrations of
[PdRf2(solv)2] = [PdRf2(THF)2] + [PdRf2(THF)(OH2)] found in the
spectrum at 223 K. We need to assume that the first substitution of a
THF on cis-[PdRf2(THF)2] (which yields cis-[PdRf2(THF)(OH2)]), has
an enthalpy not significantly different to the second substitution (which
yields cis-[PdRf2(OH2)2]). Since the 19F NMR spectrum (Fig. 2) shows
that only one THF has been substituted by one OH2, a correction of
8.5 kJ mol-1 was made.
35 C. A. Tolman, Chem. Rev., 1977, 77, 313.
36 The value of the Tolman Electronic Parameter (TEP) for a ligand L is
the value of the A1 n(CO) IR band for the complex [Ni(CO)3L], and is
related to the electronic properties of the ligand. See ref. 35.
37 R. Romeo, Comments Inorg. Chem., 1990, 11, 21, and references therein.
30 W. Levason and G. Reid, Acyclic Arsine, Stibine and Bismuthine Lig-
ands, in Comprehensive Coordination Chemistry II, ed. J. A. McCleverty
and T. J. Meyer, Elsevier-Pergamon, Oxford, UK, 2004, vol. 1, 383–385.
31 In carbonyl complexes of Pd(II) the IR data show that the p-retro-
donation is very small. See for instance: R. Uso´n, J. Fornie´s and
F. Mart´ınez, J. Organomet. Chem., 1976, 112, 105.
´
38 C. Bartolome´, P. Espinet, J. M. Mart´ın-Alvarez and F. Villafan˜e,
Eur. J. Inorg. Chem., 2004, 2326.
39 C. A. Bayse, R. L. Luck and E. J. Schelter, Inorg. Chem., 2001, 40,
3463.
8420 | Dalton Trans., 2009, 8413–8420
This journal is
The Royal Society of Chemistry 2009
©