The surprisingly facile formation of Pd(i)-phosphido complexes from: Ortho -biphenylphosphines and palladium acetate

Article abstract of DOI:10.1039/c8dt04926b

The widely-used ortho-biphenylphosphine ligands SPhos and RuPhos not only undergo facile orthometallation with palladium acetate, yielding strained, four-membered dimeric palladacycles but more surprisingly, in the presence of alcoholic solvents, along with the less encumbered analogue MePhos, yield unusual dinuclear Pd(i) complexes, in which the Pd-centers are bridged by both a phosphide ligand and by the arene of a coordinated phosphine donor.

Full text of DOI:10.1039/c8dt04926b

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DOI: 10.1039/C8DT04926B
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The surprisingly facile formation of Pd(I)-phosphido complexes
from ortho-biphenylphosphines and palladium acetate†
Michelle Montgomery, Harry M. O’Brien, Carolina Méndez-Gálvez, Caitlin R. Bromfield, Jack P. M.
Roberts, Anna M. Winnicka, Andrew Horner, David Elorriaga, Hazel A. Sparkes and Robin B.
Bedford*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The widely-used ortho-biphenylphosphine ligands SPhos and conditions, but that in the presence of alcoholic solvents these
RuPhos not only undergo facile orthometallation with palladium ligands, and the related but less encumbered o-BPP ligand
acetate, yielding strained, four-membered dimeric palladacycles MePhos, undergo facile and unexpected P-C cleavage to
but more surprisingly, in the presence of alcoholic solvents, along generate dimeric palladium(I) phosphide-bridged complexes.
with the less encumbered analogue MePhos, yield unusual
Warming Pd(OAc)2 with 1 equivalent of SPhos or RuPhos in
dinuclear Pd(I) complexes, in which the Pd-centers are bridged by toluene at 60 C for two hours gave the palladacyclic complexes,
both a phosphide ligand and by the arene of a coordinated 1a and b (Scheme 1);9 the single crystal X-ray structure of
phosphine donor.
complex 1a is shown in Figure 1(a). The 31P NMR spectrum of
complex 1a shows a singlet at  -47.7 ppm, consistent with the
formation of a strained, 4-membered palladacycle. To the best
of our knowledge, no 4-membered palladacycles of SPhos has
been structurally characterised, although structures with other
o-BPP ligands with bulkier tert-butyl residues on the
phosphorus are known.10 Interestingly, when a 1:1 mixture of
Pd(OAc)2 and SPhos in a mixture of dichloromethane and
methanol (2:1) was left standing at room temperature for
several weeks a small number of crystals of the chloride-bridged
analogue 2 were obtained, the crystal structure of which is
shown in Figure 1(b) suggesting reasonably facile halide
abstraction from the solvent. This reaction proved highly
capricious and as yet we have been unable to produce 2 in
useful amounts.
SPhos was one of the first of a class of highly successful ortho-
biphenylphosphines, o-BPPs, introduced by Buchwald and co-
workers.1 To date, over 1100 publications include the use of
SPhos as a ligand, while Buchwald’s original report on the
preparation and use of SPhos in palladium-catalysed Suzuki
cross-coupling has been cited nearly 700 times. Simple mixtures
of SPhos-ligated palladium pre-catalysts have been exploited in
a wide range of catalytic transformations, including the Suzuki
reaction,2,3 Kumada-Corriu cross-coupling,4 Negishi coupling,5
Buchwald-Hartwig amination,6 the borylation of aryl chlorides,7
and (hetero)aromatic C-H arylation reactions.8
Given the very high utility of palladium-SPhos mixtures as
pre-catalysts in a wide variety of transformations it is important
to understand the precise roles and reactivities of the ligand.
Herein we report that not only do SPhos and the related ligand
RuPhos give strained, four-membered palladacycles under mild
a. School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK. E-
mail: r.bedford@bristol.ac.uk
† Dedicated to Professor Geoff Cloke on occasion of his 65th birthday, many happy
returns Geoff.
Electronic Supplementary Information (ESI) available: Crystallographic data for
compounds 1a, 2, 4a and 4b, CCDC 1884584-1884587. Experimental details,
Scheme 1. Syntheses of palladacycles.
spectroscopic
data,
crystallographic
data
and
CIF
files,
see
DOI: 10.1039/x0xx00000x
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Reaction of the palladacycle 1a with SPhos in toluene (2.6246(9) and 2.6076(4) Å respectively) fall in the range of
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16,17
yielded the mononuclear adduct 3. The 31P NMR spectrum of those reported for one-atom bridged Pd(DI)OdI:im10e.1r0s3.9/C8DT04926B
the complex 3 revealed an AB system with a high-field doublet
for the orthometallated SPhos ligand at  -49.2 and a low field
doublet at 49.7 for the non-orthometallated SPhos ligand. The
large (394.7 Hz) mutual coupling indicates a trans-disposition of
the two inequivalent P-donors.
(a)
Scheme 2. Synthesis of Pd(I)-phosphido dimers.
(a)
(b)
(b)
Figure 1. Crystal structures of the palladacycles of SPhos. (a) Complex 1a with
hydrogen atoms and toluene solvent omitted for clarity. Symmetry code i ½-x,
+y, ½-z. (b) Complex 2 Crystal structure of 2, with hydrogen atoms omitted for
clarity. Symmetry code ii 1-x, -y, -z.
In contrast with the reactions in toluene, heating palladium
acetate with SPhos, RuPhos or the smaller analogue MePhos in
methanol followed by salt metathesis with Na[PF6]11 and
purification by column chromatography on silica, yielded the
unusual dinuclear phosphido- and arene-bridged Pd(I) cationic
complexes 4a – c respectively (Scheme 2) and the crystal
structures of the cations 4a and b are shown in Figure 2. The
Figure 2. X-ray crystal structures of the cationic complexes 4a and b, [PF6]- and H-atoms
omitted for clarity.
yields of the products suggest that the formation of the
phosphido-bridged products is favoured by increased steric bulk
The 31P NMR spectra of the complexes 4a and b recorded at
in the ortho-position of the substituted arene of the ligands.
room temperature show distinct environments for the terminal
Clearly, these dinuclear palladium (I) complexes are related to
and arene-bridging SPhos donors, along with a low-field doublet
of doublets for the phosphide (280.1 and 276.9 respectively for
4a and b) with mutual couplings between all of the P-donors.9
By contrast, while a doublet of doublets is apparent at 290.9
ppm for the phosphide ligand in 4c at room temperature, a
single, very broad peak for the phosphine environments
centered at 43 ppm is suggestive of a fluxional process, most
the previously reported species 5 and 6,12 with the exception
that they contain a phosphido-bridge, formed by P-C bond
scission.13 Similarly, phosphide-bridged Pd(I) dimers such as 7
are known,14 but the phosphide-bridged, arene-bridged motif
displayed by the complexes 4 is very rare with, to the best of
our knowledge, only one structurally-characterised example (8)
reported previously.15 The Pd-Pd bond-lengths in 4a and b
likely the interchange of coordinating and non-coordinating
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arene groups on the two phosphines. Variable temperature 31
COMMUNICATION
P
reaction, Kumada coupling, Negishi coupling and Buchwald
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NMR spectroscopy of complexes 4a and b indicate that Hartwig amination.20-27 Accordingly, weDbOrIi:e1f0l.y10e39x/aCm8DinTe0d49t2h6Be
coalescence occurs in the former above 90 C, whereas in the amination of 4-bromotoluene with morpholine, under
latter the two phosphine environments coalesce to a doublet by conditions reported recently using complex 6 as a pre-catalyst27
90 C.9
and the results are summarised in Scheme 3. As can be seen,
complex 4a is a viable pre-catalyst for the reaction, rather than
a decomposition product, giving similar performance to a 1:1
mixture of palladium acetate and SPhos.
a
Scheme 3. Buchwald-Hartwig amination. Yield determined by GC, dodecane
internal standard.
In conclusion, the o-BPP ligands SPhos, RuPhos and MePhos
do not simply behave as innocent P-donor ligands on reaction
with palladium acetate in either toluene or alcohols at elevated
temperatures. In the former solvent, SPhos and RuPhos give
strained 4-membered palladacycles, while in alcoholic solvents
that possess -hydrogens all three ligands give Pd(I) dimers
bridged by an arene from one of the coordinated phosphines
and a phosphide derived by P-C bond-cleavage. We are
currently fully exploring the implications of this non-innocent
behaviour to catalysis and these results will be published in due
course.
In order to briefly probe the mechanism of formation of 4a,
we recorded the 31P NMR spectra of crude reaction mixtures
obtained on heating SPhos with palladium acetate in a variety
of alcohols (85 C, 2 h)9 or 5% water in 1,4-dioxane. From this it
was established that the formation of 4a fell in the order MeOH
> EtOH > MeOCH2CH2OH > n-butanol while no product was
observed with t-amyl alcohol or water. We have recently shown
that palladium acetate reacts very readily with water or alcohols
to give palladium trimers with a 2-OR or -OH ligand or a
hexametallic complex with a 4-oxo,18 but it seems we can rule
out such species acting simply as intramolecular bases here, due
to the lack of reactivity in water or t-amyl alcohol. Simple
protonation of an intermediate by the alcohol can also be
excluded, again due to the lack of formation of 4a in either t-
amyl alcohol or water. It seems likely, therefore, that the
function of the alcohol is to act as a -hydride donor via the
formation of a palladium alkoxide.
Acknowledgements
We thank the EPSRC for the provision of a studentship through
the Bristol Chemical Synthesis Centre for Doctoral Training
(M.M.), for postdoctoral support (D.E.) and CONICYT for the
provision of a Becas Chile postdoctoral fellowship (to C.M.G.).
Conflicts of interest
There are no conflicts to declare.
All the 31P NMR spectra recorded for the crude reaction
mixtures obtained in ethanol, n-butanol, 2-methoxyethanol and
t-amyl alcohol also showed the presence of the palladacyclic
SPhos-adduct 3.9 However, heating solutions of the palladacycle
1a or the adduct 3 in methanol did not give any of the
phosphide complex 4a, as determined by 31P NMR spectroscopy
of crude reaction mixtures. This suggests that while
palladacycles can form on heating in alcohols, they do not
appear to be intermediates in the formation of the phosphide-
bridged dimers 4. Indeed, we have previously shown that the
arene motif in a palladacycle can itself act as a bridging group in
a Pd(I) dimer.19
Notes and references
1
2
3
4
5
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the Pd(I)-dimers, complex 4a, in a representative catalytic
reaction. Pd(I) dimers have been exploited as pre-catalysts in a
range of catalytic reactions, including Suzuki coupling, the Heck
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DOI: 10.1039/C8DT04926B
Text for graphical abstract:
Heating palladium acetate with ortho-biphenylphosphine ligands in alcoholic solvents gives
facile P-C bond-cleavage, yielding Pd(I)-dimers bridged by phosphide ligands.