Synthetic and computational studies of the palladium(iv) system Pd(alkyl)(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C-C reductive elimination

Article abstract of DOI:10.1039/c2dt31086d

Synthetic routes to methyl(aryl)alkynylpalladium(iv) motifs are presented, together with studies of selectivity in carbon-carbon coupling by reductive elimination from Pd^IV centres. The iodonium reagents IPh(CCR)(OTf) (R = SiMe₃, Bu<

Full text of DOI:10.1039/c2dt31086d

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PAPER
Synthetic and computational studies of the palladium(IV) system Pd(alkyl)-
(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C–C reductive
elimination†
Manab Sharma,a Alireza Ariafard,b Allan J. Canty,*a Brian F. Yates,*a Michael G. Gardinera and
Roderick C. Jonesa
Received 21st May 2012, Accepted 7th August 2012
DOI: 10.1039/c2dt31086d
Synthetic routes to methyl(aryl)alkynylpalladium(IV) motifs are presented, together with studies of
selectivity in carbon–carbon coupling by reductive elimination from PdIV centres. The iodonium reagents
IPh(CuCR)(OTf) (R = SiMe3, But, OTf = O3SCF3) oxidise PdIIMe(p-Tol)(L2) (1–3) [L2 = 1,2-bis-
(dimethylphosphino)ethane (dmpe) (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3)] in
acetone-d6 or toluene-d9 at −80 °C to form complexes PdIV(OTf)Me(p-Tol)(CuCR)(L2) [R = SiMe3,
L2 = dmpe (4), bpy (5), phen (6); R = But, L2 = dmpe (7), bpy (8), phen (9)] which reductively eliminate
predominantly (>90%) p-Tol-CuCR above ∼−50 °C. NMR spectra show that isomeric mixtures are
present for the PdIV complexes: three for dmpe complexes (4, 7), and two for bpy and phen complexes
(5, 6, 8, 9), with reversible reduction in the number of isomers to two occurring between −80 °C and
−60 °C observed for the dmpe complex 4 in toluene-d8. Kinetic data for reductive elimination from
PdIV(OTf)Me-(p-Tol)(CuCSiMe3)(dmpe) (4) yield similar activation parameters in acetone-d6
(66 2 kJ mol−1, ΔH‡ 64 2 kJ mol−1, ΔS‡ −67 2 J K−1 mol−1) and toluene-d8 (Ea 68 3 kJ mol−1
,
ΔH‡ 66 3 kJ mol−1, ΔS‡ −74 3 J K−1 mol−1). The reaction rate in acetone-d6 is unaffected by addition
of sodium triflate, indicative of reductive elimination without prior dissociation of triflate. DFT
computational studies at the B97-D level show that the energy difference between the three isomers of 4
is small (12.6 kJ mol−1), and is similar to the energy difference encompassing the six potential transition
state structures from these isomers leading to three feasible C–C coupling products (13.0 kJ mol−1). The
calculations are supportive of reductive elimination occurring directly from two of the three NMR
observed isomers of 4, involving lower activation energies to form p-TolCuCSiMe3 and earlier transition
states than for other products, and involving coupling of carbon atoms with higher s character of σ-bonds
(sp2 for p-Tol, sp for CuC–SiMe3) to form the product with the strongest C–C bond energy of the
potential coupling products. Reductive elimination occurs predominantly from the isomer with
Me3SiCuC trans to OTf. Crystal structure analyses are presented for PdIIMe(p-Tol)(dmpe) (1),
PdIIMe(p-Tol)(bpy) (2), and the acetonyl complex PdIIMe(CH2COMe)(bpy) (11).
but lacking spectroscopic evidence,1 has been followed by electro-
chemical studies2 indicating irreversible oxidation (PdII/PdIII) for
trans-[PdII(CuCR)2(PBu3)2]2 trans-[PdIICl(CuCR)-
and
Introduction
Early compelling evidence for the existence of bis(phosphine)-
triorganopalladium(IV) complexes of configuration ‘PdXC3P2’,
(PPh3)2] species,2b isolation of binuclear PdIII complexes,3 and
characterisation of several mono(phosphine)palladium(IV)
species ‘[PdC3N2P]+’4 exemplified by the X-ray structural analy-
sis for [PdIVMe3(phen)(PMe2Ph)](OTf) (phen = 1,10-phenan-
throline, OTf = triflate),4a and isolation of a ‘[PdC2N3P]2+’
complex [PdIV(CH2CMe2-o-C6H4-C,C′)(Tp-N,N′,N′′)(PMe3)]-
(NO3)2 [Tp = tris(pyrazol-1-yl)borate].5 The complex trans-[Pd-
(CuC-o-Tol)2(PMe2Ph)2]6 reacts with the iodonium reagent
IPh2(OTf) to form (o-TolCuC)2 and o-TolCuCPh), with IPh-
(CuCR)(OTf) (R = But, SiMe3) to form (o-TolCuC)2 and
o-TolCuC–CuCR), and with IPhCl2 to form o-TolCuCCl; for
these reactions spectroscopic evidence for intermediates is
aSchool of Chemistry, University of Tasmania, Private Bag 75, Hobart,
Tasmania 7001, Australia. E-mail: allan.canty@utas.edu.au,
brian.yates@utas.edu.au; Fax: +(61) 3-6226-2858;
Tel: +(61) 3-6226-2167
bDepartment of Chemistry, Faculty of Science, Central Tehran Branch,
Islamic Azad University, Shahrak Gharb, Iran
†Electronic supplementary information (ESI) available: Cartesian co-
ordinates for all computed structures, geometrical and NBO analyses,
molecular orbitals, details of augmented basis set. CCDC
825045–825047 for 1, 2 and 11. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt31086d
11820 | Dalton Trans., 2012, 41, 11820–11828
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Oxidation studies
present but is considered insufficient to definitively characterize
possible higher oxidation state intermediates.
We have recently reported that H NMR studies in acetone-d6
1
A solution of IPh(CuCR)(OTf) (4.17 × 10−2 mmol) in acetone-
d6 or toluene-d8 (0.5 mL) at −80 °C was added to a solution of
PdMe(p-Tol)(L2) (L2 = dmpe, bpy, phen) (4.17 × 10−2 mmol) in
acetone-d6 or toluene-d8 (1.0 mL) in a round-bottomed flask at
−80 °C in an acetone slush bath, and an aliquot immediately
transferred to a pre-cooled NMR tube and quickly inserted into
the NMR probe equilibrated at −80 °C. As all spectra were
obtained at −80 °C some resonances are broad and not well
resolved, in particular for 31P spectra.
of the reaction of PdIIMe2(dmpe) [dmpe = 1,2-bis(dimethylphos-
phino)ethane] with the iodine(III) oxidant IPh(CuCSiMe3)(OTf)
at −50 °C, followed by addition of sodium iodide at −50 °C,
indicate the formation of unstable PdIVIMe2(CuCSiMe3)(dmpe)
containing the ‘fac-[PdIC3P2]’ configuration7a related to the
early reports by Stille and coworkers where spectroscopic evi-
dence was not accessible.1 The platinum(IV) analogue was iso-
lated and, following subsequent studies of the detection of
PtIV(OTf)Me(p-Tol)(CuCSiMe3)(dmpe) in solution and X-ray
structural characterisation of fac-[PtIVIMe(p-Tol)(CuCSiMe3)-
(dmpe)],7b in this report we have explored the reactivity of
PdIIMe(p-Tol)(L2) (L2 = dmpe, bpy, phen) (bpy = 2,2′-bipyri-
dine) toward IPh(CuCR)(OTf) (R = But, SiMe3) at −80 °C.
Pd(OTf)Me(p-Tol)(CuCSiMe3)(dmpe)
(4). 1H
NMR
(400 MHz, acetone-d6): δ 8.51 (d, 2H, H(2,6)Tol ), 8.44 (d, 2H, H
(2,6)Tol), 8.34 (d, 2H, H(2,6)Tol), 7.81 (t, 2H, H(3,5)Tol ), 7.78
(m, 2H, H(3,5)Tol ), 7.63 (m, 2H, H(3,5)Tol ), 2.37 (s, 3H,
(CH3)Tol, 2.21 (s, 3H, (CH3)Tol, 2.17 (s, 3H, (CH3)Tol, 1.82–1.46
(m, overlapping for PCH2, PCH3), 0.40 (bs, 3H, PdCH3), 0.35
(bs, 3H, PdCH3), 0.32 (bs, 3H, PdCH3), 0.16 (s, 9H, Si(CH3)3)
0.12 (bs, 18H, Si(CH3)3). 31P NMR (162 MHz, acetone-d6): δ
41.25, 34.78 (d, JPP = 27), 30.70 (d, JPP = 21 Hz), 26.15 (d, JPP
1
Unstable PdIV complexes are detected by H and 31P NMR, and
the selectivity in reductive elimination at −60 °C examined from
the d6 ‘PdOC3L2’ centres containing three different organyl
groups (sp3, sp2, sp) that are unrestrained by, for example, being
part of chelating groups. This situation represents, apparently, a
unique opportunity to examine reductive elimination selectivity
under very mild and unrestrained conditions, and we have com-
plemented the NMR studies with DFT calculations to further
probe the selectivity in reductive elimination and the nature of
transition states.
1
= 21 Hz), 23.61 (d, JPP = 21 Hz), 23.22 (d, JPP = 26 Hz). H
NMR (400 MHz, toluene-d8): δ): δ 7.97 (m, 2H, H(2,6)Tol), 9.81
(m, 2H, H(2,6)Tol ), 7.31(d, 2H, H(3,5)Tol ), 7.18 (d, 2H, H
(3,5)Tol), 2.40 (s, 3H, (CH3)Tol ), 2.32 (s, 3H, (CH3)Tol ),
0.84–0.68 (m, overlapping for PCH2, PCH3, PdCH3), 0.64 (bs,
18H, Si(CH3)3). 31P NMR (162 MHz, toluene) δ 23.40 (d, JPP
10 Hz) 20.83 (bs), 20.16 (d, JPP = 10 Hz).
=
Pd(OTf)Me(p-Tol)(CuCSiMe3)(bpy)
(5). 1H
NMR
Experimental
(400 MHz, acetone-d6): δ 9.19 (d, 2H, bpy), 8.97 (d, 2H, bpy),
8.96–8.91 (m, 8H, overlapping bpy and tolyl), 8.44–8.36 (m,
8H, overlapping bpy and tolyl), 8.00–7.90 (m, 4H, bpy), 2.31 (s,
3H, (CH3)Tol, 2.24 (s, 3H, (CH3)Tol ), 1.87 and 1.65 (bs, 6H,
PdCH3), 0.10 and 0.07 (s, 18H, Si(CH3)3).
Air-sensitive materials were handled under an argon atmosphere
using standard Schlenk techniques, and all solvents were dried
and distilled by conventional methods prior to use. The com-
plexes PdIIMe(p-Tol)(L2) (L2 = dmpe (1),8a bpy (2), phen (3)8b)
and IPh(CuCR)(OTf) (R = SiMe3, But)9 were prepared as pre-
viously reported. NMR spectra were recorded on a Varian Unity
Pd(OTf)Me(p-Tol)(CuCSiMe3)(phen)
(6). 1H
NMR
1
(400 MHz, acetone-d6): 9.71 (d, 2H, phen), 9.46 (d, 2H, phen),
9.15 (d, 2H, phen), 9.01–8.94 (m, 8H, phen and tolyl), 8.86 (d,
2H, phen), 8.31–8.25 (m, 6H, phen and tolyl), 8.21 (d, 2H,
tolyl), 2.30 and 2.10 (s, 3H, (CH3)Tol ), 1.80 (s, 3H, PdCH3),
1.66 (s, 3H, PdCH3), 0.08 and 0.03 (bs, 18H, Si(CH3)3).
Inova 400 WB spectrometer. H NMR spectra were secondary-
referenced to residual solvent signals; assignments of all reson-
ances were supported by two-dimensional NMR spectroscopy
(COSY, HMBC). 31P NMR spectra were externally referenced to
85% H3PO4.
GCMS analyses were carried out on a Varian 3800 GC
coupled to a Varian 1200 triple quadrupole mass spectrometer in
single quadrupole mode. AVarian ‘Factor Four’ VF-5 ms (30 m
× 0.25 mm internal diameter and 0.25 μm film) column was
used. Injections of 1 μL were made using a Varian CP-8400
autosampler and a Varian 1177 split/splitless injector at 220 °C
with a split ratio of 30 : 1. The ion source was at 220 °C, and the
transfer line at 290 °C. The carrier gas was helium at 1.2 mL
min−1 using constant flow mode. The column oven was held at
50 °C for 2 min then ramped to 290 °C at 8° min−1. The range
from m/z 35 to 550 was scanned 3 times per second. GC-FID
was carried out on a Varian 450-GC with 1177 split/splitless
injector using similar conditions to the above, except that the
carrier gas was nitrogen. The FID response factors for trimethyl-
silylacetylene, and n-heptadecane were determined from a solu-
tion of known concentration, employing where necessary the
‘effective carbon number’ concept of known negative effects on
FID response.10,11
Pd(OTf)Me(p-Tol)(CuCBut)(dmpe)
(7). 1H
NMR
(400 MHz, acetone-d6): 8.48–8.44 (dd, 4H, tolyl), 8.36 (d, J = 8
Hz, 2H, tolyl), 7.65–7.61 (m, 4H,), 7.46–7.40 (m, 2H), 2.19,
2.15, 2.13 (s, 9H, (CH3)Tol), 1.80–1.55 (m, 48H, PCH2, PCH3),
1.47–1.40 and 1.34–1.30 (m, 27H, C(CH3)3, 1.07, 1.03, 1.01 (s,
9H, PdCH3); 31P NMR (162 MHz, acetone-d6) δ; 32.46 (d, JPP
= 26), 29.36 (d, JPP = 26 Hz), 26.61 (d, JPP = 26 Hz), 23.42 (d,
J
PP = 21 Hz), 21.05 (d, JPP = 19 Hz), 19.85 (d, JPP = 21 Hz).
Pd(OTf)Me(p-Tol)(CuCBut)(bpy) (8). 1H NMR (400 MHz,
acetone-d6): 9.37 (d, 2H, bpy), 9.04 (d, 2H, bpy), 8.88 (t, 4H,
bpy), 8.75–8.68 (m, 4H, bpy) 8.48–8.39 (m, 4H, bpy and tolyl),
8.04–7.99 (m, 4H, bpy and tolyl), 7.90–7.86 (m, 4H, bpy and
tolyl), 2.30 (s, 3H, (CH3)Tol ), 2.01 (s, 3H, (CH3)Tol )), 1.83 and
1.62 (bs, 6H, PdCH3) 1.14 (bs, 18H, C(CH3)3).
Pd(OTf)Me(p-Tol)(CuCBut)(phen) (9). 1H NMR (400 MHz,
acetone-d6): 9.75 (d, 2H, phen), 9.45 (d, 2H, phen), 9.08–8.97
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(m, 8H, phen and tolyl), 8.84 (bd, 4H, tolyl), 8.41 (bs, 2H,
phen), 8.31 (bs, 2H, phen), 8.19 (d, 2H, tolyl), 7.87 (d, 2H,
tolyl), 2.30 (s, 3H, (CH3)Tol ),1.90 (s, 3H, (CH3)Tol), 1.68, 1.46
(bs, 6H, PdCH3), 1.11 (bs, 18H, C(CH3)3).
b = 9.1480(7), c = 9.4860(6) Å, α = 84.0070(10), β = 80.678(5),
3
ˉ
γ = 79.390(4)°, V = 643.99(9) Å , T = 100 K, space group P1
(no. 2), Z = 2, 6554 reflections measured, 1750 unique (Rint
=
0.0786), 1694 > 4σ(F), R = 0.0385 (observed), Rw = 0.0978
(all data).
Decomposition studies
Computational details
The reaction mixtures (2.48 M) in acetone-d6 or toluene-d8,
obtained as above, were transferred to an NMR tube and
immediately inserted into the NMR probe which was maintained
at −80 °C. The decomposition of PdIV complexes was monitored
by the disappearance of the Hortho resonances of the tolyl group
which were better resolved from other resonances, and the rate
of decomposition was calculated from the combined integration
of these resonances. First order rate constants kobs (sec−1) were
obtained from plots of ln(Ao/At) versus time at specific tempera-
tures. The overall reaction order was determined using different
concentrations of the complex (3.71 × 10−2 M, 2.48 × 10−2 M,
1.65 × 10−2 M and 1.35 × 10−2 M); plots of kobs against concen-
tration of the complexes gave a straight line through the origin
indicating first order behaviour.
The influence of temperature on the rate of decomposition was
investigated by monitoring the reaction at different temperature
(−40°, −35°, −30°, −25°, −20° in acetone-d6 and −35°, −25°,
−20°, −15° in toluene-d8) with a constant concentration of the
complex (2.48 × 10−2 M). Energies of activation were calculated
from Arrhenius plots (ln kobs = ln Ao − Ea/RT, R = 8.314 J mol−1
K); and enthalpies and entropies of activation were obtained
from Eyring plots (ln(kobs/T) = ln(κ/h) − ΔH‡/RT + ΔS‡/R
(κ = 1.38 × 10−23 J K−1, h = 6.626 × 10−34 J s).
The equilibrium molecular structures as well as transition state
geometries were optimized using the hybrid density functional
B97-D which includes dispersion energy corrections.15 For the
geometry optimisations, the effective core potentials of Hay and
Wadt with double-ζ valence basis sets (LANL2DZ)16 were
chosen to describe Pd, and the 6-31G(d) basis set was used for
other atoms.17 Frequency calculations were performed to
confirm that optimized structures were minima or saddle points.
All transition structures contained exactly one imaginary fre-
quency and were linked to reactants, products, or intermediates
using the intrinsic reaction coordinate (IRC) algorithm.18 To test
the effect of polarization and diffusion functions, single point
energy calculations were carried out for all structures with a
larger basis set, LANL2TZ+(3f) basis set on Pd (including
diffuse functions and polarisation functions16 (see ESI†), and the
6-311+G(2d,p) basis set on other atoms. All thermodynamic data
were calculated at the standard state (298.15 K and 1 atm). Non-
explicit solvent effects were also tested with the IEFPCM
(SCRF) model19 using acetone and toluene as solvents. All final
energies reported in this manuscript are with the larger basis set
and include acetone solvent corrections. All calculations were
carried out with the Gaussian09 suite of codes.20 All Mulliken,
NBO electron densities and bonding analyses were analysed
using the natural bond order (NBO) technique.21 The NBO
analysis describes donor–acceptor interactions according to a
second-order perturbation theory.
Structural determinations
Crystals of 1 and 2 were obtained directly from the preparations,
and complex 11 was obtained as a component of decomposition
of 5 after an acetone-d6 solution was left for several days at
−40 °C. Data for 1, 2 and 11 were collected at −173 °C for crys-
tals mounted on a Hampton Scientific cryoloop at the MX1
beamline of the Australian Synchrotron.12 The structures were
solved by direct methods with SHELXS-97,13 and visualized
using X-SEED.14 All non-hydrogen atoms were refined anisotro-
pically; hydrogen atoms were placed in calculated positions and
refined using a riding model with fixed C–H distances of 0.95
(sp2C–H), 0.99 (CH2), 0.98 Å (CH3). The thermal parameters of
all hydrogen atoms were estimated as Uiso(H) = 1.2Ueq(C)
except for CH3 where Uiso(H) = 1.5Ueq(C). A summary of crys-
tallographic data given below.
Crystal data for 1: C14H26P2Pd, M = 362.69, monoclinic, a =
8.782(2), b = 18.624(3), c = 10.664(2) Å, β = 107.991(16)°, V =
1658.9(6) Å3, T = 100 K, space group P21/n (no. 14), Z = 4,
17 001 reflections measured, 2520 unique (Rint = 0.0926), 2152
> 4σ(F), R = 0.0449 (observed), Rw = 0.1131 (all data). Crystal
data for 2: C18H18N2Pd, M = 368.74, monoclinic, a = 47.366(2),
b = 8.4950(9), c = 25.0650(15) Å, β = 115.3600(10)°, V =
9113.6(12) Å3, T = 100 K, space group C2/c (no. 2), Z = 24,
46 306 reflections measured, 6953 unique (Rint = 0.0324), 6758
> 4σ(F), R = 0.0458 (observed), Rw = 0.1075 (all data). Crystal
data for 11: C14H16N2OPd, M = 334.69, triclinic, a = 7.6740(7),
Boltzmann population distributions, at −80, −35, and
25.15 °C, were obtained by normalizing the individual popu-
lation distribution calculated by using the equation Ni
=
exp(−ΔG/RT); where ΔG is the relative free energy difference
from the most stable isomer. Individual rate constants were
calculated by using the k(T) = (kBT/hco) exp(−ΔG/RT) from
transition state theory.
Results
Synthesis and characterisation of PdIV complexes
1H NMR studies at −80 °C indicated that complexes are formed
according to Scheme 1, where PdIV complexes are present as a
mixture of isomers, and, for the dmpe complex 4 in toluene-d8,
Scheme 1
11822 | Dalton Trans., 2012, 41, 11820–11828
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the ratio of isomers is altered reversibly between −80 and 60 °C
(see below).
solutions in acetone-d6 or toluene-d8 decomposed above
∼−50 °C to form light brown solutions containing p-Tol-
CuCSiMe3 as the dominant organic product (>90%) and minor
quantities of p-Tol-Me and Me-CuCSiMe3 (Scheme 1).
Complex 1H NMR spectra have not permitted characterisation of
the initial palladium containing products of reductive elimin-
ation, apparently owing to subsequent decomposition of the
assumed products ‘PdR(L2)(OTf)’. Partial decomposition to Pd0
species is anticipated to lead to oxidative addition reactions with
iodobenzene (byproduct of reaction of IPh(CuCR)(OTf) with
1–3) and transfer of organyl groups between PdII species to gen-
erate several palladium containing products. This process would
also account for the observed presence of minor organic products
(Ph–Ph, p-Tol-Ph). Supportive of this interpretation, the amount
of these organic products increases overnight.
An acetonylpalladium(II) complex, PdMe(CH2COMe)(bpy)
(11) was detected amongst other products by X-ray crystallo-
graphic analysis from the decomposition of 4 in acetone-d6
(Fig. 1(c)). Acetonylpalladium(II) complexes are known to be
readily formed on the reaction of chloropalladium(II) complexes
with acetone in the presence of hydroxide23 or on direct reaction
of hydroxopalladium(II) complexes with acetone.24 Platinum(II)
complexes may be similarly formed,23,25 PtCl2(NCMe)2 reacts
with bpy in acetone under reflux in air to form PtCl(CH2COMe)-
(bpy),26 and the cyclohexyne complex Pt0(η2-C6H8)(dppe) (dppe
= 1,2-bis(diphenylphosphino)ethane) reacts with acetone/water
to form the cyclohexenyl complex PtII(CH2COMe)(η1-C6H9)-
(dppe).27
1H and 31P{1H} NMR spectra at −80 °C in acetone-d6 indi-
cate the presence of three isomers for the dmpe complexes and
two isomers for bpy and phen complexes, in approximately
equal ratio based upon 1H NMR assignments for all PdMe,
p-Tol, SiMe3, and some bpy and phen resonances, e.g. Fig. 2
illustrates the presence of three isomers for 4 (three Hortho and
Hmeta environments for the p-Tol group (confirmed by COSY
spectra)) and 7 (six phosphorus environments). There is a
1
general feature of downfield shifts for H NMR resonances of
bpy (∼0.5–1 ppm), phen (∼0.5 ppm), Hortho,meta of p-Tol
(∼0.6–1.5 ppm), PMe (0.2–0.8 ppm), and PdMe
(∼0.3–1.5 ppm), similar to those reported on oxidative addition
of benzyl bromide to PdMePh(bpy)22 and PdMe(p-Tol)(phen).8b
Complex 4 was found to exhibit 1H NMR spectra that are
more easily interpreted and monitored during reactions, in
particular the upfield trimethylsilyl resonance, well resolved
Hortho,meta resonances (Fig. 2(a)), and the availability of 31P
NMR resonances, and thus this complex was selected for further
decomposition studies. In acetone-d6 and toluene-d8, the initial
ratio of isomers at −80 °C is ∼1 : 1 : 1, but the isomer with the
most downfield Hortho diminished to give two isomers in ∼1 : 1
ratio. In toluene-d8, at −80 °C the process is very slow, and only
proceeds rapidly almost to completion at −60 °C. This process is
reversible in toluene-d8 indicating facile conversion of one
isomer to (i) both of the other isomers, or (ii) one of the other
isomers and interconversion between these, or (iii) occurrence of
both (i) and (ii). The Hmeta resonances of the isomers of 4 in
toluene-d8 are coincident with the solvent resonances, unlike the
well resolved spectrum in acetone-d6 (Fig. 2a). The t-butylethy-
nyl analogue (7) lacks sufficient resolution of Hortho and Hmeta
resonances to study the assumed occurrence of equilibria.
Kinetic studies of decomposition of PdIV complexes
To examine the mechanism of decomposition and the potential
role of triflate dissociation from palladium(IV) complexes of the
type formed here, kinetic studies of the decomposition of
Pd(OTf)Me(p-Tol)(CuCSiMe3)(dmpe) (4) at −25 °C were
undertaken in the absence and presence of sodium triflate.
Decomposition of PdIV complexes
Reductive elimination above ∼−50 °C was examined by 1H
NMR spectroscopy and GC-MS. The complexes as pale yellow
Identical results were obtained on H NMR monitoring the loss
1
of combined Hortho,meta resonances in the absence or presence of
Fig. 1 ORTEP representations of the structures of (a) PdMe(p-Tol)(dmpe) (1), PdMe(p-Tol)(bpy) (2) (one of three crystallographically independent
molecules), and (c) PdMe(CH2COMe)(bpy) (11). Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°) for PdMe(p-Tol)(dmpe) (1): Pd(1)–C(1, 2) 2.093(6), 2.058(5); Pd(1)–P(1, 2) 2.3000(14), 2.903(14);
C(1)–Pd(1)–C(2) 86.8(2), C(1)–Pd(1)–P(1, 2) 175.49(19), 92.64(16); C(2)–Pd(1)-P(1, 2) 94.41(14), 177.77(17); P(1)–Pd(1)P(2) 85.99(5). (b) PdMe
(p-Tol)(bpy) (2): Pd(1n)–C(1n) 2.051(6), 2.044(6), 2.039(6); Pd(1n)–C(2n) 1.994(5), 2.010(5), 2.003(5); Pd(1n)–N(1n) 2.135(4), 2.139(5), 2.140(5);
Pd(1n)–N(2n) 2.121(4), 2.138(5), 2.138(4); C(1n)–Pd(1n)–C(2n) 87.6(2), 87.7(2), 88.4(2); C(1n)–Pd(1n)–N(1n) 174.68(19), 172.8(2), 172.6(2);
C(1n)–Pd(1n)–N(2n) 98.07(19), 95.7(2), 96.2(2); C(2n)–Pd(1n)–N(1n) 97.30(19), 99.4(2), 98.56(19); C(2n)–Pd(1n)–N(2n) 172.03(19), 173.66(19),
171.01(19); N(1n)–Pd(1n)–N(2n) 77.28(17), 77.32(17), 77.30(17). (c) PdMe(CH2COMe)(bpy) (11): Pd(1)–C(1, 2) 2.036(5), 2.082(5); Pd(1)–N(1, 2)
2.137(4), 2.111(4); C(1)–Pd(1)–C(2) 88.5(2), C(3)–C(2, 4) 1.462(7), 1.520(7); C(3)–O(1) 1.237(5), C(1)–Pd(1)–N(1, 2) 172.98(15), 95.63(18);
C(2)–Pd(1)–N(1, 2) 98.24(17), 175.22(14); N(1)–Pd(1)–N(2) 77.77(14), Pd(1)–C(2)–C(3) 101.4(3), C(2)–C(3)–C(4) 118.5(4), C(2)–C(3)–O(1)
123.9(4), C(4)–C(3)–O(1) 117.6(4).
This journal is © The Royal Society of Chemistry 2012
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NaOTf (×3, ×5 excess) at −25 °C (kobs 2.5 × 10−3 s−1 at 2.48 ×
10−2 M [PdIV]). If triflate dissociates prior to reductive elimin-
ation, reaction would be anticipated to be retarded by the pres-
ence of additional triflate. The concentration dependence of kobs
indicates first-order behavior, and Arrhenius and Eyring plots
(ESI†) allowed determination of Ea, ΔH‡, and ΔS‡, based on
dated collected at −40 to −15 °C for 4 in acetone-d6 (Ea 66
2 kJ mol−1, ΔH‡ 64 2 kJ mol−1, ΔS‡ −67 2 J K−1 mol−1
)
and toluene-d8 (Ea 68 3 kJ mol−1, ΔH‡ 66 3 kJ mol−1, ΔS‡
−74 3 J K−1 mol−1). Similar values for activation parameters
in acetone-d6 and toluene-d8 are supportive of a non-dissociative
pathway. Under identical conditions of concentration (2.48 ×
10−3 M) and temperature (−35 °C), reaction is ∼7 times faster in
acetone-d6 than toluene-d8, most likely attributable to formation
of a polar transition state implied by negative values for ΔS‡.
Computational studies
The reductive elimination pathways were investigated with
density functional theory (DFT). For this study we chose to use
the B97-D functional because this includes the effects of dis-
persion energy and should give a more accurate picture of the
energetics when steric repulsion is involved, such as in the
systems studied here.28 We have also compared the results
obtained with a number of other density functionals and these
comparisons (ESI†) emphasise the importance of including the
effects of dispersion energy.
We considered various reductive elimination reactions starting
from the three observed isomers of the PdIV reactant (Fig. 3).
Reductive elimination of equatorial substituents trans to the
bidentate phosphine donors is unlikely,29 and calculations for
elimination by this process indicated that these processes would
be very slow, having a high activation energy (ESI†).
Fig. 2 (a) 1H NMR showing Hortho and Hmeta resonances of the p-tolyl
group for Pd(OTf)Me(p-Tol)(CuCSiMe3)(dmpe) (4) in acetone-d6 at
−80 °C, and (b) 31P NMR spectra for Pd(OTf)Me(p-Tol)(CuCBut)-
(dmpe) (7) in acetone-d6 at −80 °C.
Fig. 3 Relative energy surfaces for B97-D concerted process.
11824 | Dalton Trans., 2012, 41, 11820–11828
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We have calculated three pairs of reductive elimination pro-
ducts, corresponding to the observed organic species and the
assumed PdII(OTf)R(dmpe) products. There is some uncertainty
in the structures of the PdII products since they could not be
detected spectroscopically, and are potentially unstable with
respect to decomposition. As the decomposition of 4 does not
involve triflate dissociation, ΔE values are considered appropriate
for estimation of relative energies of species.
structures as corresponding to early transition states.30 In turn,
the early transition states correspond to the preferred reaction
pathways. In a similar manner, examination of the HOMO’s
most directly related to formation of the new C–C bond, e.g. via
4b_TS1, indicate that there is a net gain in the binding energy
(ΔΔE = −0.079 eV) compared with other possibilities, e.g. ΔΔE
= +0.382 for 4b_TS2 for formation of Me-CuC-SiMe3 (Fig. 4).
These observations also clearly demonstrate the role of s char-
acter and directionality30,31 of the component orbitals during
transition state formation. Thus, the coupling of the sp2 and sp
carbons requires only a small amount of structural rearrangement
to form a bonding interaction, whereas the coupling of the sp3
and sp or sp2 and sp3 carbons requires a greater rearrangement in
order to achieve a bonding overlap of molecular orbitals (ESI†).
The amount of geometric deformation also corresponds with the
strength of the C–C bond formed in the reductive elimination
reaction, where a lesser deformation occurs for formation of
the strongest C–C bond, 569.2 kJ mol−1 for p-TolCuC-SiMe3
compared with 518.1 kJ mol−1 for Me-CuCSiMe3 and
425.1 kJ mol−1 for p-Tol-Me (Table S3†).
Although kinetic data indicate a pathway in which triflate dis-
sociation does not occur, calculations for a dissociative pathway
involving triflate dissociation were undertaken. These calcu-
lations show that the dissociative transition structures leading to
the observed product are 32–50 kJ mol−1 (ΔE‡) higher than for
the non-dissociative pathway (Fig. 5, and ESI†).
We have also carried out a Boltzmann population analysis of
isomers and effective rate constant for the various reductive
elimination processes (Table 1). Irrespective of temperature, 4a
has the highest population, followed by 4b and with a negligible
amount of 4c, which is consistent with observed two dominant
pseudo-stable isomers. The observed ∼1 : 1 ratio of two isomers
is most likely due to a dynamic equilibrium between these
isomers, as the process of interconversion of isomers was found
The geometries of the six transition states shown in Fig. 3 are
as expected, with the extra interest of an α-agostic Pd–H reaction
as shown for 4c_TS3. There are two transition structures leading
to each pair of products: 4b_TS1 and 4c_TS1 lead to the for-
mation of p-Tol-CuC-SiMe3, 4a_TS2 and 4b_TS2 lead to
Me-CuC-SiMe3, and 4a_TS3 and 4c_TS3 lead to p-Tol-Me.
We note that the lowest energy isomer of the PdIV precursor (4a)
does not lead to the major product, and that the lowest energy
transition structure (4b_TS1) does not lead to the lowest energy
pair of products [1,4-dimethylbenzene and Pd(OTf)-
(CuCSiMe3)(dmpe)]. However, this may be explained by inter-
conversion between isomers as detected by NMR spectroscopy
(see above) demonstrating that kinetic studies above −50 °C
involve decomposition from one or two of the three isomers. In
addition, the lowest activation barriers leading to products from
4b and 4c lead to the observed product (p-TolCuCSiMe3), via
transition states that have characteristics suggestive of ‘early tran-
sition states’. As shown in ESI (Table S4†), the geometries of
the TS1 structures show much less change compared to the reac-
tants (smaller changes in bond lengths) than the TS2 and TS3
structures. In addition, NBO analyses indicate that the population
of the dx2−y2 orbital (corresponding to the reductive elimination
process) increases by only 0.15 and 0.14 electrons for formation
of 4b_TS1 and 4c_TS1, compared to increases of 0.21, 0.20,
0.28 and 0.23 electrons for 4a_TS2, 4b_TS2, 4a_TS3 and
4b_TS3 (Table S5†). This supports the interpretation of the TS1
Fig. 4 HOMO’s identified as significant in forming C–C bonds from 4a to give (a) p-Tol-CuC-SiMe3, and (b) Me-CuC-SiMe3.
This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11820–11828 | 11825

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Fig. 5 Relative energy surfaces (B97-D) for dissociative (and associative) pathways.
Table 1 Boltzmann population and rate constant analysis at various temperatures
Temperature = 25.15 °C Temperature = −35.00 °C
Temperature = −80.00 °C
Normalized
population
Effective rate
Normalized
population
Effective rate
Normalized
population
Effective rate
Isomers
constant (s−1
)
constant (s−1
)
constant (s−1
)
4a
9.36 × 10−1
6.27 × 10−2
9.5 × 10−4
9.69 × 10−1
3.11 × 10−2
2.69 × 10−4
9.87 × 10−1
1.34 × 10−2
5.91 × 10−5
4a_TS1
4a_TS2
4a_TS3
4b
4b_TS1
4b_TS2
4b_TS3
4c
2.38
3.68 × 10−3
2.44 × 10−1
8.63 × 10−1
3.11 × 10−6
6.22 × 10−4
3.35 × 10−3
6.51 × 101
1.46 × 102
1.45 × 103
7.94 × 101
7.48 × 10−5
1.32 × 101
3.93
8.55 × 10−2
1.24 × 10−2
9.91 × 10−14
2.15 × 10−9
4c_TS1
4c_TS2
4c_TS3
5.30 × 101
5.03 × 10−7
7.37
3.21
8.41 × 10−7
3.36 × 10−21
1.21 × 10−9
9.64 × 10−12
1.51 × 10−2
to be reversible (see above). Most interestingly, irrespective of
temperature, the calculated effective rate for 4b_TS1 (Table 1) is
the highest, which is the transition structure leading to the for-
mation of the observed major product. Again, the highest reac-
tion rate is via 4b_TS1, further supporting the interpretation of
an early transition state for TS1. Thus, from the NMR study and
theoretical investigation, it has become clear that there might be
an equilibrium between isomers, and final reductive elimination
predominantly takes place through one of the isomers. This situ-
ation can be represented by Scheme 2 showing the dominant
pathway for product formation.
Scheme 2
Discussion
The synthetic results delineate a route to methyl(aryl)alkynyl-
palladium(IV) systems exhibiting the ‘PdIVOC3P2’ and ‘PdIVOC3N2’
11826 | Dalton Trans., 2012, 41, 11820–11828
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kernels, and examples of selectivity in reductive elimination
from d6 MCsp3Csp2Csp species where the organyl groups are not
constrained, e.g. the sp3 centre is a methyl group, the aryl group
is unsubstituted at the ortho-positions, and palladacycles or
palladocycles involving the organyl groups are not present. The
observed decomposition pathway under mild conditions in solu-
tion for all of the complexes involves preferential formation of
Acknowledgements
We thank the Australian Research Council for financial support,
Dr James Horne of the Central Science Laboratory for assistance
with analysis of organic products, and the National Compu-
tational Infrastructure for high performance computing resources.
We thank Professor Michelle Coote (Australian National Univer-
sity) for valuable suggestions and discussion. Aspects of this
research were undertaken at the MX1 beamline at the Australian
Synchrotron, Victoria, Australia.
C
sp2–Csp bonds, in this case p-Tol-CuCR (R = But, SiMe3).
Reports to date of C–C reductive elimination at PdIV and
related PtIV centres indicate either direct elimination from octa-
hedral species or following dissociation of one donor to give a
five-coordinate intermediate. Kinetic32a–c and related studies32d,e
indicate dissociation of iodide from PdIVIMe3(L2) (L2 = biden-
tate nitrogen donor) in reductive elimination of ethane,32a,d,e
although this reaction proceeds without dissociation in the pres-
ence of high excess iodide concentration.32a In the present
system, where direct reaction from octahedral PdIV(OTf)Me-
(p-Tol)(CuCR)(dmpe) is indicated, and has been examined at
much lower temperature than other studies, the activation par-
ameters are similar to those for ethane elimination from PdIV-
IMe3(bpy) when conducted at high iodide concentration in
acetone-d6, when reaction is non-dissociative (Ea 78 11 kJ mol−1,
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