Pre-catalyst resting states: A kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes

Article abstract of DOI:10.1039/b801951g

The treatment of cold (～3 °C) methanolic solutions of Li ₂PdCl₄ with two equivalents of 2-phenyl-2-oxazoline (Phox) results in the isolation of [PdCl₂(Phox)₂] (3). This complex undergoes remarkably slow isomerisation (CHCl₃-d) at room temperature to a corresponding thermodynamic form. In addition to a theoretical treatment (DFT), the isomerisation behaviour has been analysed both kinetically and thermodynamically. These investigations lead to the conclusion that the initially formed (i.e. kinetic) isomer of 3 is the cis-form which undergoes conversion to the corresponding thermodynamic trans-form via a dissociative (D) mechanism involving loss of a Phox ligand. The activation parameters ΔS^? and ΔH^? are found to be +304 (±3) J K^-1 mol^-1 and +176 (±1) kJ mol ^-1, respectively and indicate a high barrier to Pd-N bond cleavage under these conditions. The thermodynamic parameters show the expected endothermic nature of this process (+140 ± 17 kJ mol^-1) and a slight positive overall entropy (ΔS° = +17 ± 2 J K^-1 mol^-1); this latter parameter is presumably due to the formation of the lower dipole moment trans-product when compared to the cis-isomer. Calculated (DFT) values of ΔG^? and ΔH ^? are in excellent agreement to those found experimentally. Further theoretical investigation suggests that two 14-electron three-coordinate T-shaped transition states (i.e., [PdCl₂(Phox)]^?) are involved; the form pre-disposed to yield the thermodynamic trans-product following re-attachment of the released oxazoline is found to be energetically favoured. The analogous alkyloxazoline system [PdCl₂(Meox) ₂] (4: Meox = 2-methyl-2-oxazoline) has likewise been investigated. This material gives no indication of cis-trans isomerisation behaviour in solution (NMR) and is shown to exist (X-ray) in the trans-form in the solid-state (as do previously reported crystalline samples of 3). A DFT study of 4 reveals similar values of ΔS^? and ΔH ^? if a D type mechanism were operating to rapidly convert cis- to trans-4. However, a significantly higher thermodynamic stability of the trans-isomer relative to the cis-form is revealed versus similar calculations of the Phox derivative 3. This suggests the possibility that (i) reactions of Meox with Li₂PdCl₄ may lead directly to the trans-form of [PdCl₂(Meox)₂] or alternatively (ii) that alkyloxazoline complexes such as 4 may have a different, and presumably much more rapid, mechanism for isomerisation. The results are placed into the context that isomerisation behaviour, or lack thereof, could play a key preliminary role in later substrate modification. This is due to the fact that [PdX ₂(oxazoline)₂] compounds are well-known (pre-)catalysts for C-C bond forming chemistry. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b801951g

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Pre-catalyst resting states: a kinetic, thermodynamic and quantum
mechanical analyses of [PdCl2(2-oxazoline)2] complexes†‡§
Robert A. Gossage,*a Hilary A. Jenkins,b Nathan D. Jones,c Roderick C. Jonesd and Brian F. Yatesd
Received 4th February 2008, Accepted 28th March 2008
First published as an Advance Article on the web 7th May 2008
DOI: 10.1039/b801951g
The treatment of cold (∼3 ◦C) methanolic solutions of Li2PdCl4 with two equivalents of
2-phenyl-2-oxazoline (Phox) results in the isolation of [PdCl2(Phox)2] (3). This complex undergoes
remarkably slow isomerisation (CHCl3-d) at room temperature to a corresponding thermodynamic
form. In addition to a theoretical treatment (DFT), the isomerisation behaviour has been analysed both
kinetically and thermodynamically. These investigations lead to the conclusion that the initially formed
(i.e. kinetic) isomer of 3 is the cis-form which undergoes conversion to the corresponding
thermodynamic trans-form via a dissociative (D) mechanism involving loss of a Phox ligand. The
activation parameters DS‡ and DH‡ are found to be +304 ( 3) J K−1 mol−1 and +176 ( 1) kJ mol−1,
respectively and indicate a high barrier to Pd–N bond cleavage under these conditions. The
thermodynamic parameters show the expected endothermic nature of this process (+140 17 kJ mol−1)
and a slight positive overall entropy (DS◦ = +17 2 J K−1 mol−1); this latter parameter is presumably
due to the formation of the lower dipole moment trans-product when compared to the cis-isomer.
Calculated (DFT) values of DG‡ and DH‡ are in excellent agreement to those found experimentally.
Further theoretical investigation suggests that two 14-electron three-coordinate T-shaped transition
states (i.e., [PdCl2(Phox)]‡) are involved; the form pre-disposed to yield the thermodynamic
trans-product following re-attachment of the released oxazoline is found to be energetically favoured.
The analogous alkyloxazoline system [PdCl2(Meox)2] (4: Meox = 2-methyl-2-oxazoline) has likewise
been investigated. This material gives no indication of cis–trans isomerisation behaviour in solution
(NMR) and is shown to exist (X-ray) in the trans-form in the solid-state (as do previously reported
crystalline samples of 3). A DFT study of 4 reveals similar values of DS‡ and DH‡ if a D type
mechanism were operating to rapidly convert cis- to trans-4. However, a significantly higher
thermodynamic stability of the trans-isomer relative to the cis-form is revealed versus similar
calculations of the Phox derivative 3. This suggests the possibility that (i) reactions of Meox with
Li2PdCl4 may lead directly to the trans-form of [PdCl2(Meox)2] or alternatively (ii) that alkyloxazoline
complexes such as 4 may have a different, and presumably much more rapid, mechanism for
isomerisation. The results are placed into the context that isomerisation behaviour, or lack thereof,
could play a key preliminary role in later substrate modification. This is due to the fact that
[PdX2(oxazoline)2] compounds are well-known (pre-)catalysts for C–C bond forming chemistry.
Introduction
aThe David Upton Hill Laboratories of Inorganic Chemistry, Department
During the last 25 years, the use of palladium complexes in transi-
tion metal-mediated catalysis has become one of the cornerstones
of synthetic chemistry.1 Palladium-based materials are now widely
of Chemistry, Acadia University, Wolfville, NS, B4P 2R6, Canada.
E-mail: rob.gossage@acadiau.ca; Fax: +1-902-5851114; Tel: +1-902-
5851109
bDepartment of Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3,
used in both the regio- and enantio-selective synthesis of C–C,
Canada. E-mail: haj@smu.ca
C–H and C–N bonds, in addition to a wide variety of other
cDepartment of Chemistry, The University of Western Ontario, 1151
classes of bond-forming reactions.1 Despite these advances, the
Richmond Street, London, ON, N6A 5B7, Canada
nature, role and structure of the active Pd species have often
remained elusive. Indeed considerable effort, both experimental
and theoretical, has been applied to elucidate the nature of the
reactive intermediates during substrate modification.2 In contrast,
much less attention has been paid to the “resting” state(s) of
the pre-catalytic complexes that appear during, before or after
catalysis has been initiated.3
dThe School of Chemistry, The University of Tasmania, Private Bag 50,
Hobart, Tasmania, 7001, Australia
† Oxazoline chemistry. Part XVI. For Part XV, see ref. 7f .
‡ This article is dedicated to the memory of Mr. Charles R. “Chuck” Eisnor
(1954–2006).
§ Electronic supplementary information (ESI) available: details of the
characterization of 3 and 4, a crystallographic information (.cif) file for
complex 4 (CCDC# 657356), details and graphical representations of the
kinetic plots used to characterize the isomerisation of 3 and details of all
DFT calculations described herein. CCDC reference number 657356. For
ESI or crystallographic data in CIF or other electronic format see DOI:
10.1039/b801951g
We recently reported that a simple palladium(II) chloride com-
plex containing a ligated monodentate oxazoline (viz. [PdCl2(Etox-
j1N)2]: 1; Etox = 2-ethyl-2-oxazoline: Fig. 1) can be used as a
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the trans ligand arrangement being likewise confirmed for the
solid material by an X-ray diffraction study. As substituted aryl-
oxazolines are readily obtained,12 complex 3 and its analogues
presented to us a useful starting point to investigate possible
electronic (or other) effects on this isomerisation behaviour.9 Thus,
our initial intentions were to attempt to map out the electronic
(or steric) nature of such isomer distributions. We began by re-
investigating the parent material (3) and have found that a less
stable form can be observed if accessed via low temperature
synthesis. This material exhibits remarkably slow isomerisation
behaviour (cf. 2a) at room temperature (RT) but reaches an
equilibrium almost completely dominated by the (solid-state)
isolable trans isomer as reported.11 Herein, we report a full
kinetic, thermodynamic and DFT investigation of this system
and include a comparative synthetic, structural and DFT study
of the simplest alkyl analogue of 3, namely [PdCl2(Meox-j1N)2]
(4: Meox = 2-methyl-2-oxazoline). These data support the notion
that the observed isomerisation behaviour of 3 takes place via the
dissociation of an oxazoline ligand, which is a less common route
(cf. associative mechanism) for formally d8 metal systems.13 The
relative energies of the isomers, as observed spectroscopically and
calculated by DFT, support the observed general characteristics of
the [PdCl2(oxazoline)2] system, namely that both 3 and the alkyl-
oxazoline complex 4 are thermodynamically favoured to exist in
the trans isomeric form. The observed rate of stereomutation of cis-
to trans-3 is one of the slowest ever observed for the isomerisation
of a square planar (SQP) formally Pd2+ halide complex. These
results are further discussed in terms of the use of compounds such
as 1 as catalyst precursors. In addition, the possible significance
of such (low concentration) cis forms as important intermediates
and/or resting states in subsequent substrate modification is put
forward.
Fig. 1 Schematic representations of the compounds discussed herein.
catalyst precursor for a number of C–C bond forming reactions.4
This material operates in open air and the addition of excess “free”
oxazoline ligand is not required for good catalytic activity. In
fact, this has been shown to retard the production of organic
products.4b,5 This observation suggests the presence of a key
coordinatively unsaturated intermediate, either within the catalytic
cycle itself and/or as an important pre-catalyst leading to the
active form(s). We surmised4b that one such species could be the
formally fourteen-electron metal complex [PdCl2(Etox)]‡ formed
via dissociation of one of the Etox ligands. There is currently
little mechanistic or structural information on exactly how this4
or related systems5 operate and indeed there are few details on the
properties of the pre-catalyst forms (i.e., the ‘resting states’).4a,5b An
important background observation to the present discussion ap-
peared in the literature over a dozen years ago.6 In 1994, van Koten
and co-workers reported a study detailing the cyclopalladation of
the naphthalene-derived oxazoline referred to hereafter as napox
(i.e., 4,4-dimethyl-2-{2ꢀ-naphthyl}-2-oxazoline: Fig. 1). A minor
component of this work described the synthesis and isolation of
the mononuclear coordination complex [PdCl2(napox-j1N)2] (2a:
Fig. 1), which was viewed as a possible intermediate in subsequent
cyclometalation chemistry. Complex 2a is an obvious relative
of our catalyst precursor 1. Crystalline samples of the isolated
form of 2a exist as the trans-isomer only (X-ray diffraction)
with the coordination of napox to the Pd metal centre through
the N-donor atom, as expected.4,7 However, crystalline samples
of 2a, upon dissolution in CDCl3, invariably give 1H NMR
spectra consistent with the presence of two structural forms.6,8
This observation was attributed to the rapid formation of an
equilibrium mixture of the cis and trans isomers of 2a, although
at the time detailed experiments to confirm this notion were not
performed.6 Curiously, the analogous complex devoid of methyl
groups (2b: Fig. 1) did not give any indication (NMR) of mixed
(solution cis ꢀ trans) isomer behaviour.6
Experimental section
General
All reagents and solvents used in this work were of reagent grade
and obtained from commercial sources; synthetic procedures were
carried out using standard bench-top techniques. A stock solution
(0.23 M) of Li2PdCl4 was obtained by dissolving an appropriate
quantity of PdCl2 (2.0 g: 11 mmol) in MeOH (∼40 mL) in the
presence of 2 equiv. (0.95 g) of LiCl. The mixture was then
stirred for approximately 30 min at RT and filtered directly into
a 50.00 mL volumetric flask which was subsequently filled with
additional MeOH to give the required total solution volume.
Complex 3 was made by an identical route to that of 4 (below)
and the identity of the product confirmed by comparison to the
data previously reported.11 NMR spectra (CDCl3 solution) were
recorded at RT using Bruker AC-250 or Advance 300 NMR
spectrometers operating at 250 MHz or 300 MHz, respectively.
Chemical shift values are reported in ppm using TMS (and/or
residual CHCl3 solvent resonance) as internal standard. Coupling
constants are reported in Hertz. Elemental analysis was performed
by L.U.C.A.S., Thunder Bay, Ontario, Canada.
1
In contrast to the nature of 2a, our H NMR examination of
1
3,9 revealed no spectroscopic evidence for isomeric solution equi-
librium (cf. 2b) and a single crystal X-ray diffraction study gave,
as above, only a single isomer (i.e., trans-[PdCl2(Etox-j1N)2]).4a
We hypothesized that perhaps the van Koten system 2a operates
differently, possibly due to the unique electronic nature of napox
when compared with that of Etox or its 4,4-dihydro congener. This
idea is based on the notion that napox is the larger of the three
ligands and hence would intuitively pre-dispose a resulting PdCl2
complex with a structure containing trans-disposed oxazolines
as opposed to a more sterically encumbered cis-form.10 These
results are further supported by a more recent study of a simpler
aryl-oxazoline complex reported by Smoliakova et al., viz. trans-
[PdCl2(Phox-j1N)2] (3: Phox = 2-phenyl-2-oxazoline).11 As with
our Etox system (1) and 2b, the authors reported only a single
isomer of 3 in solution after re-crystallization and isolation;
Synthesis of 4
A quantity of Li2PdCl4 (6.1 mL; 0.23 M in MeOH: 1.4 mmol)
was added to further MeOH (∼10 mL) and the mixture cooled to
3116 | Dalton Trans., 2008, 3115–3122
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◦
∼3 C. Meox (0.25 g: 2.9 mmol) was then added by syringe and
Results and discussion
the dark brown mixture was then stirred for approximately 1 h.
During this period, the colour of the solution changed to light
yellow and an orange precipitate formed. This solid was isolated
by filtration, washed with MeOH (3 × 5 mL) and thrice with Et2O
(5 mL) and then allowed to dry in air. The yield of 4 was 0.27 g
(56%) in the form of an orange solid; mp: 230–232 ◦C (decomp.).
1H NMR (250 MHz; CDCl3): d = 4.52 (t, 4H, J = 9.8, CH2O),
3.99 (t, 4H, J = 9.8, CH2N), 2.52 (s, 6H, CH3). Anal. calcd for
C8H14N2O2Cl2Pd: C, 27.65; H, 4.06; N, 8.06; found: C, 27.47; H,
3.73; N, 8.04%.
Synthesis and characterisation of 3 and 4
We employed a modified synthesis11 of 3 and 4 which involves
protocols similar to those used previously in the production of
1 and 2a/b.4a,6 This involves treatment of Li2PdCl4 in MeOH
solution with the ligand in question at a temperature below
ambient. Complexes 3 and 4 are air- and moisture-stable materials
in both the solid-state and in solution (CDCl3, CH2Cl2, benzene).
The low temperature synthesis of 3 allows one to arrest the
stereomutation process which is an inherent characteristic of this
compound. Two distinct sets (Fig. 2 and ESI§) of signals are
evident upon examination of the 1H NMR spectrum (CDCl3
solution) of 3 isolated in this manner. The methylene region
(d = 5.0–3.0 ppm: Fig. 2) of the 1H NMR spectrum is the
most provocative indicator of the presence of the two isomeric
forms. The chemical shift values of the two downfield AAꢀBBꢀ
(i.e., pseudo-triplet pattern) signals (Fig. 2) match those reported
for isolated trans-311 while the other set of upfield signals are
previously unreported. A solution of this mixture was thereafter
left at RT for approximately 24 h during which time most (>70%)
of the initially more intense resonances had disappeared whilst
those of known trans-3 grew in intensity. No Pd “black” was
observed to form under these conditions nor were any other NMR
X-Ray crystallographic study of 4
Orange crystals of 4, suitable for X-ray diffraction, were grown
from a CH2Cl2 solution of 4 which had been layered with
MeOH, sealed (septum) and thereafter had been left standing
for several days at −22 ◦C. Further details of the crystallographic
investigation of 4 can be found in the ESI§ including an ORTEP
diagram of one of the molecules of 4.
Crystal data. Empirical formula C8H14Cl2N2O2Pd, M 347.51,
¯
T 223(2) K, crystal system triclinic, space group P1, a = 7.5416(8)
◦
◦
˚
˚
A, a = 84.997(2) , b = 8.4479(9) A, b = 81.775(2) , c = 9.6688(10)
◦
◦
3
˚
˚
signals noted. Heating of the NMR tube in an oil bath at 60 C
A, c = 85.269(2) , V 605.81(11) A , Z 2, final R indices [I>2r(I)],
R1 = 0.0278, wR2 = 0.0798, R indices (all data), R1 = 0.0306,
wR2 = 0.0813.
for 8 h lead to virtually complete conversion (NMR) of the sample
to trans-3; again there was no evidence for sample decomposition.
This result confirms the observations previously reported,11 as
the minor isomer is present in less than 3% at equilibrium (RT).
Presumably, the resulting solid-state crystalline phase of 3 exists,
as in the case of van Koten’s 2a and our 1, solely in the reported
trans form.11
Kinetic and thermodynamic investigations of 3
The details of the kinetic and thermodynamic investigation
(including concentration versus time plots) of complex 3 and the
DFT studies of 3 and 4 and the intermediates derived thereof can
be found in the ESI.§
Computational methods
Full geometry optimisations were carried out with the use of the
B3LYP density functional level of theory14–16 combined with the
LANL2DZ basis set (which incorporates the Hay and Wadt17
small-core relativistic effective core potential and double-zeta
valence basis set) on Pd, and the 6-31G(d) basis set18–20 on all
other atoms. Sets of five d-functions were used in the basis sets
throughout these calculations. For the optimised geometries, har-
monic vibrational frequencies were calculated at the B3LYP level.
All transition structures possessed one and only one imaginary
frequency, and they were further characterised by following the
corresponding normal mode towards each product and reactant.
Single-point energies on the B3LYP/LANL2DZ:6-31G(d) opti-
mised geometries were calculated at the B3LYP level with the
LANL2 augmented:6-311+G(2d,p) basis set,21 which incorporates
the LANL2 effective core potential and augmented basis set on
Pd (the large f-polarised valence basis set of Bauschlicher and co-
workers22 was used) together with the 6-311+G(2d,p) basis set23–25
on all other atoms. Energies from these single-point calculations
were combined with the zero-point vibrational energy corrections
from the lower level of theory to yield DH◦ and DG◦ numbers. All
calculations were carried out with the Gaussian 03 program.26
1
Fig. 2 A typical H NMR spectrum in the region d = 5.0–3.0 ppm of
compound 3 at approximately a 50 : 50 cis/trans isomer ratio.
Compound 4 (Fig. 1) was isolated as described above for 3,
however no evidence (NMR) was observed for the presence of
more than one isomer in solution. A determination of the solid-
state structure of 4 was therefore carried out by single crystal
X-ray diffraction (Table 3 and ESI§). Two different molecules
of 4 are found within the unit cell; both forms have a trans-
disposition of the two halide and two oxazoline ligands around
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the metal center. The primary structural difference between the
two lies in the tilting of the oxazoline ring planes relative to the
Pd coordination plane, which is more pronounced in one of the
forms than in the other (torsion angle for molecule 1: ∠C3–N1–
Pd1–Cl1 = ∼111◦; molecule 2: ∠C7–N2–Pd2–Cl2 = ∼97◦; see
ESI§). A molecular representation of one of the molecules of 4
within the unit cell can be found in Fig. 3. The bond lengths
and angles of both molecules of 4 are otherwise unsurprising and
are comparable with that of 1,4a 2a,6 311 and other Pd–oxazoline
complexes (vide infra and Table 3, below).5b,27 Note that there is
little variation within this series in terms of the Pd–Cl and Pd–N
bond lengths and the angles between the atoms ligating the Pd
centre (Table 3). Although the solid-state structures of free Phox,
Etox or Meox are not known (these compounds all have melting
points well below RT), X-ray diffraction (and DFT) data of the
related compound 2-(4ꢀ-hydroxyphenyl)-2-oxazoline (5: Fig. 1) has
Kinetic and thermodynamic study of 3
The initial observation of slow isomerisation of 3 (vide supra)
recommended this as an ideal system for a kinetic study of
the stereomutation process by NMR spectroscopy. The rate of
disappearance of the initially major isomeric component and
the complementary appearance of the known compound trans-
3 was found to be first-order in Pd over all concentration and
temperature ranges investigated. This result effectively eliminates
the possibility that the precursor to trans-3 is a dimeric species or
a higher order aggregate.3a Thus, the initial kinetic product in the
synthesis of trans-3 is assumed to be the cisoidal isomer (hereafter
referred to as cis-3). This result lends support to van Koten’s earlier
hypothesis6 that the analogous complex 2a exists in solution as a
cis/trans isomeric mixture.
The observed kinetic and thermodynamic data for the isomeri-
sation of cis-3 are given in Table 1 and Table 2. The retarding of
the rate of isomerisation upon added Phox (Table 1; entry 5), in
addition to the large calculated DS‡ value (Table 2), strongly sug-
gests a dissociative (D) type mechanism for this inter-conversion
(Scheme 1). The first order nature of the data imply that cis-3 is a
mononuclear complex and thus presumably an initial (unsaturated
14-electron) intermediate I (i.e. [PdCl2(Phox)]‡) results from the
rate determining step. One assumes that I isomerizes rapidly to a
second form II, which is trapped by the previously released Phox
in another fast step to give the thermodynamically more stable
trans-3 (Scheme 1). These aspects will be further discussed in the
theoretical examination below.
been reported.12f Of note is the almost identical N C (oxazoline)
=
˚
bond length (1.2775(14) A) between this “free” ligand and the
coordinated oxazolines found in 1, 3 and/or 4. This suggests little
disturbance of the N–C p-system upon coordination of the Etox,
Phox or Meox to Pd, as might be expected.
It has been previously established that there are three distinct
mechanisms by which Pd2+/Pt2+ systems carry out cis–trans iso-
merisation chemistry. The most common is the classical associative
(A) mechanism,10a,28–31 a somewhat more rare non-dissociative
stereomutation has been established (i.e. an intramolecular re-
arrangement via a pseudo-tetrahedral transition state without
bond rupture)32,33 in addition to the D mechanism.34,35 Obviously,
only this latter process is characterized by a large positive DS‡
value. The D process is now well-recognized, but certainly less
common in SQP Pd2+/Pt2+ ligand exchange chemistry than the A
case.10,13,28–30,36,37
DFT characterization of cis/trans-3 and 4
Fig. 3 A molecular representation of one of the molecules of 4 found
within the unit cell.
The kinetic aspects of the system, as examined above, yields a high
energy barrier and a dissociative process for the conversion of cis-
to trans-3 (Table 2) but we have no experimental comparison for
the analogous chemistry with alkyl-oxazoline derivatives such as
Table 1 Kinetic and thermodynamic data for the cis-to-trans isomerization of 3
Entry
Ta/K
105k1b/s−1
105k−1b/s−1
Keq
DG◦/kJ mol−1
1
2
3
4
5
6
294
298
303
308
308d
313
0.753 ( 0.025)
nmc
7.10 ( 0.33)
24.4 ( 1.9)
3.2
0.036 ( 0.002)
21
34
140 20
236 22
nm
8
4
−7.4 1.1
−8.7 0.4
−12.4 0.4
−14.0 0.3
0.051 ( 0.01)
0.10 ( 0.03)
54.9 ( 2.0)
nm
a Temperature 1 K. b k1 = rate constant for the isomerization of the cis-to-trans isomer, k−1 = rate constant for the isomerization from the trans-to-cis
isomer; k−1 calculated from the expression Keq = k1/k−1; values of the equilibrium constants (Keq) were determined by integration of the methylene 1H
NMR signals at t∞. c nm = not measured. d Contains additional “free” Phox; [Phox] : [3]0 = 1 : 3; single dataset determination.
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Table 2 Calculated and observed properties of 3
The DFT calculations involving complex 3 parallel the kinetic
and thermodynamic observations discussed above. A representa-
tion of the lowest energy cis-form of 3 appears in Fig. 4 along with
the complementary trans-form. This latter molecular geometry
is very close in general molecular characteristics to the solid-
state form reported previously.11 A comparison of the important
bond lengths and angles (observed and calculated) appears in
Table 3. Generally, these calculated values are quite close to the
observed values found in the solid-state, although the calculated
gas phase values suggest slightly longer bond lengths (notably
the Pd–L interactions) than those experimentally determined in
the solid phase. The energy difference calculated between the cis
and trans gas phase forms (DE = ∼29 kJ mol−1) implies that
the trans-form is thermodynamically favoured,10b,28 as observed.11
Calculations of the 14-electron transition state complexes are also
quite informative. The two calculated structural isomers of the
[PdCl2(Phox)]‡ species, which could be classified as independent
T-shaped forms (I and II) are both found to be plausible structural
isomers (Fig. 5). The loss of Phox from cis-3 leads directly to
intermediate I which has an approximate right angle between the
two ◦chloride ligands (calculated value of the Cl–Pd–Cl angle is
97.6 ). This isomer is calculated to be approximately +3.2 kJ mol−1
higher in energy t◦han isomeric form II (Fig. 5: calculated Cl–Pd–
Cl angle of 172.4 ).40 Hence, the initial 14-electron intermediate
I is energetically pre-disposed to rearrange to form a second
structural motif (i.e., II) that upon re-addition of released Phox
would lead to trans-3 (Scheme 1). We have further applied DFT
methods to mimic the empirical observations above (Table 3).
The calculated DG‡ value (+86.8) is in excellent agreement to
that found from the experimental determination. Although our
calculations do admittedly underestimate the magnitude of both
Quantity
Valuea,b
Quantity
Valuea
DG‡
DH‡
DS‡
+85 3 (+86.8)
+176 1 (+137.9)
+304 3 (+171.4)
DH◦
DS◦
+140 17
+17
2
a DG and DH are reported in units of kJ mol−1; DS values are given units
of J K−1 mol−1 b Calculated values (DFT) in parentheses.
.
Scheme 1 The proposed mechanism for the isomerisation of cis- to
trans-3.
4. It must be assumed that this latter compound reaches isomeric
equilibrium very rapidly or that the cis congener is not formed at all
by our synthetic protocol. Any subsequent equilibrium chemistry
is apparently completely (NMR) dominated by a single isomer,
which is presumably the crystallographically characterised trans-
form. The present NMR investigation likewise gives no indication
of the presence or structural nature of the surmised transition
states formed from 3 which we have referred to thus far simply as
“I” and “II”. We have therefore applied DFT38,39 to examine the
nature of these isomerisations and to confirm the thermodynamic
preference, at least in the gas phase, for both 3 and 4 to exist as the
crystallographically identified trans-isomers. We shall first discuss
the isomerisation behaviour of 3.
Fig. 5 Calculated (DFT) T-shaped structures of 3 following loss of a
Phox ligand: initial “I” form (left) and form “II” (right).
Fig. 4 Calculated (DFT) structures of trans- (left) and cis-3 (right).
Table 3 Calculated and observed bond lengths and angles of complexes 3 and 4a
Measurement
t-3b,c
t-3 (calc.)
t-4b,d
t-4b,e
t-4 (calc)d
c-3 (calc.)
c-4 (calc.)
Pd–N
Pd–Cl
=
2.0074(14)
2.300(2)
1.271(2)
180.0
180.0
2.053
2.3632.378
1.291
176.5
180.0
2.008(2)
2.3011(8)
1.261(4)
180.00(6)
180.00(4)
2.026(2)
2.2876(8)
1.272(4)
180.00(3)
180.00(3)
2.052
2.368
1.286
180.0
180.0
2.094
2.333
1.290
93.4
93.2
2.095
2.330
1.287
93.5
93.3
N
C
N–Pd–N
Cl–Pd–Cl
N–Pd–Cl
89.13(5)90.87(5)
88.391.7
89.15(7)90.85(7)
89.82(7)90.18(7)
88.991.1
86.7(c)178.9(t)
86.8(c)179.5(t)
◦
a
˚
Estimated standard deviations are in parentheses; bond lengths are reported in A units; bond angles are given in degrees ( ); t = trans; c = cis; calc. =
calculated (gas phase) values obtained from DFT. b Observed (solid-state) values obtained from X-ray diffraction data. c Data taken from reference 11.
d Molecule #1 in the unit cell; this work. e Molecule #2 in the unit cell; this work.
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DH‡ and DS‡, a distinctly large and positive enthalpy and entropy
is theoretically determined, as observed. Possible reasons for this
result are that there are solvation effects that are not accounted
for in the (gas phase) calculations. Certainly our large DS‡ value
suggests a considerable increase in solvent randomness following
ligand dissociation in addition to any possible influence of the
formation of intermediate I.
No related isomerisation observations have been found for
alkyloxazoline complex 4 although calculations of the activation
energies for a possible dissocative isomerisation pathway are found
to be similar to aryloxazoline complex 3. These parameters are
calculated as DG‡ = +89.4 kJ mol−1; DH‡ = +138.7 kJ mol−1 and a
DS‡ value of +165.4 J K−1 mol−1. Thus, both complexes 3 and 4 are
calculated to have a fairly high barrier to isomerisation but there
appears to be little difference energetically between the two species
in terms of a D mechanism for interconversion. However, the
ground state energy difference between cis-4 and trans-4 (Fig. 6)
is calculated to be much larger (DE = ∼45 kJ mol−1) than that of
3. These data strongly suggest that the observed solution (NMR)
form of 4 is most likely the structurally characterized solid-state
trans-form.
tion pathway are found to be similar to that of aryloxazoline
complex 3.
Significance and conclusions
The justification for a D mechanism for the isomerisation of 3 is
supported by the observed slower kinetic profile in the presence
of added Phox and the large positive value of DS‡. Both the
non-dissociative stereomutation and D mechanisms are usually
observed in SQP systems containing ligands of high trans-effect
(e.g., silyl, alkyl, hydrido, etc.).13,33–35 Thus, dissociative processes
can be expected to operate under catalysis conditions due to the
typical presence of intermediate complexes containing transition
metal (M) bonds such as M–H, M–SiR3 and/or M–C. In the case
described here, the relatively weak trans-effect chloride ligand is
located trans to the oxazoline. Dissociation of this oxazoline leads
to T-shaped species such as I and II; complexes such as 3 therefore
represent a rare dissociative system that does not contain ligands
of high trans-effect.10 This helps to explain the rather large barrier
to isomerisation (in addition to the obvious requirement to cleave a
Pd–N bond) and thus slow kinetic profile of the compound studied
here. Many systems which utilize Pd have been suggested to include
one or more key cis–trans isomerisation step(s) during catalysis.
This is particularly noteworthy in situations leading to reductive
elimination chemistry,1,2,42 as invariably the two coupled fragments
must be previously oriented in an intermediate cis-geometry.43
However, situations similar to those described above are rarely
considered in processes which occur before substrate activation.
What is currently unknown is how the presence (or absence) of
a mixture of catalytic precursors affects the subsequent product
profiles. We have shown that in some cases mixtures of pre-catalytic
cis/trans isomers can be observed but in other cases the cis form
appears to exist, at best, at very low concentrations relative to the
trans isomer. The relative rates at which two such isomers enter
a catalytic cycle(s) however, is currently not known nor are the
(ligand) steric and electronic factors that control Keq between these
forms. Situations such as those described above were discussed
some time ago in a seminal review by Anderson and Cross,10b but
the factors that control Pd isomeric mixtures, particularly in the
solution phase, are still far from well-established.43 As the general
class of [PdX2(oxazoline)n] complexes are active mediators of C–C
bond formation,4,5 such information may be vital in improving
and modifying subsequent catalysis. This investigation has further
presented the concept that detailed kinetic investigations whose
overall objective is to clarify aspects of catalytic cycle(s) may also
have to involve study of any pre-catalyst materials. Indeed, a re-
examination (DFT, Keq, etc.) of the original van Koten napox
system (2a/b) and substituted structural analogues of 35a,11,12 in
addition to the possible role of cis- and trans-isomers in catalysis
by these materials, are currently in progress. Results of these
investigations will be published in due course.
Fig. 6 Calculated (DFT) structures of cis- (left) and trans-4 (right).
The results described above lead one to suggest two possible
reasons for the different observations between 3 and 4. Firstly, it is
possible that reactions of Meox with Li2PdCl4 lead directly to only
the trans-form of [PdCl2(Meox)2], due perhaps to a strong thermo-
dynamic preference for this isomer. Alternatively, alkyloxazoline
complexes such as 4 may have a different, and presumably much
more rapid, mechanism for isomerisation and are thus kinetically
biased to yield the trans form. The alkyloxazoline ligand Meox
is certainly sterically smaller than Phox and presumably more
basic.41 The steric size of Meox could allow the Pd centre of
4 to be more accessible relative to that of 3 and thus perhaps
the small alkyloxazoline ligands open up rapid associative and/or
non-dissocative stereomutation pathways for isomerisation. These
hypotheses suggests that there may exist a fundmental difference
between aryl-, when compared to alkyl-, oxazoline PdX2 materials.
Overall, the data generated here create a picture in which
oxazoline ligand dissociation from 3 leads to a T-shaped 3-
coordinate intermediate form which is energetically favoured
to undergo re-arrangement to the thermodynamic trans-isomer
upon re-attachment of the dissociated oxazoline. Our kinetic
investigations of the properties of 3 have indicated a mechanistic
scheme as depicted in Scheme 1. Note that in the case of k−1k−2
<
Acknowledgements
k2k3 (as suggested by the observed data and DFT) and the steady-
state approximation of “free” [Phox] ≈ 0; then kobs collapses to
kct (i.e. k1). This very situation has been previously described
by Minniti.35a No related isomerisation observations have been
found for the alkyloxazoline complex 4 although calculations
of the activation energies for a potential dissocative isomerisa-
This research was supported by the generous contributions of
The Killam Trusts (NDJ), The Royal Society of Chemistry
(specifically in the form of a J.W.T. Jones Travelling Fellowship
to RAG), the Australian Research Council, Acadia University,
Saint Mary’s University, The University of Western Ontario and
3120 | Dalton Trans., 2008, 3115–3122
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by The University of Tasmania. The provision of further funding,
in the form of a “Discovery” Grant (RAG), from the Natural
Sciences and Engineering Research Council (NSERC) of Canada
is likewise acknowledged. Johnson-Matthey Inc. is thanked for a
generous loan of PdCl2 that was used in this work. The authors are
also indebted to the assistance of Australia’s Program of Advanced
Computing (Australian National University, Canberra).
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3122 | Dalton Trans., 2008, 3115–3122
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