Two-step solid-state synthesis of PEPPSI-type compounds

Article abstract of DOI:10.1039/c5cc02924d

The two-step mechanochemical preparation of carbene-pyridine complexes of palladium and platinum is reported. The organometallic products, which represent a class of commercially available catalysts, are rapidly formed in excellent yield proving solvent-free synthesis to be a viable synthetic alternative even in the case of NHC-containing compounds.

Full text of DOI:10.1039/c5cc02924d

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Two-step solid-state synthesis of PEPPSI – type compounds
a
a,
a
a
Christopher J. Adams, Matteo Lusi, † * Emily M. Mutambi and A. Guy Orpen
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The two-step mechanochemical preparation of carbene-pyridine requires that the excess pyridine be distilled away at the end of the
5
complexes of palladium and platinum is reported. The procedure.
organometallic products, which represent a class of commercially Our group has previously reported solid-state syntheses of
availabe catalysts, are rapidly formed in excellent yield proving transition-metal coordination complexes in which target species can
solvent-free synthesis to be a viable synthetic alternative even in be formed rapidly and quantitatively by grinding together
the case of NHC-containing compounds.
2 4
appropriate starting materials. Thus, K MCl (M = Pd, Pt) will react
with two equivalents of chloride salts of a range of protonated
nitrogen ligands to eliminate KCl and form the tetrachlorometallate
Solvent-free reactions represent an opportunity to tackle difficult
synthetic problems, to understand and control the behaviour of
chemical species in the solid state, and to reduce the economic and
1
0
salt of the cation in a simple ion-metathesis reaction (Scheme 1).
We have also shown that it is then possible to derive coordination
complexes by solid-state dehydrochlorination of the metal-chloride
salts of protonated amines and heterocyclic nitrogen bases such as
pyridine, imidazole and pyrazole in the solid state with an external
base such as KOH. On reaction with the base, the cation is
deprotonated and coordinates to the metal, displacing a chloride
1
environmental costs of solution-phase synthesis. The growing
importance of the topic is highlighted in recent journal special
2
issues. Most of the recent publications on the topic have as their
object supramolecular (hydrogen and halogen bonded) and
metallorganic complexes. Although solid state synthesis of
3
10-13
organometallic adducts dates back almost 50 years, little progress
ion which is eliminated as KCl (Scheme 1).
This KCl is easily
4
has been made in this field since then. In order to exploit the full
removed, if necessary, by extracting the metal complex product
into an organic solvent. Notably, if the palladium species were to be
used in catalysis (for example) then this step may not be needed:
many coupling reactions eliminate KCl or KBr, so its presence as an
inert contaminant added with the catalyst may be benign.
potential of solid state reactions, an effort is required to widen the
range of chemical species produced in the solid state. This work
describes the mechanochemical preparation of a commercially
important class of organometallic compounds known as PEPPSIs
(
pyridine-enhanced precatalyst preparation stabilization and
Like the salts of the protonated nitrogen bases mentioned above,
those of N,N′-disubstitutedimidazolium (protonated NHC)
complexes form hydrogen bonds between the tetrachlorometallate
5
initiation).
PEPPSI catalysts comprise a palladium(II) centre bonded to two
chloride, one N-heterocyclic carbene (NHC) and one pyridine ligand.
They are active in a range of organic reactions including Negishi,
14
anions and the cations. On the basis of this similarity, we
conjectured that the formation of N,N′-disubstitutedimidazolium
metal-chloride salts might serve to pre-arrange the anion and
cation into a geometry in which the hydrogen and chlorine atoms to
be eliminated on treatment with KOH are adjacent to each other. In
testing this conjecture we are mindful of the difference in acidity of
6
-9
Buchwald-Hartwig, Kumada and Suzuki-Miyaura couplings. As
well as being excellent catalysts for these reactions they possess
extremely good stability, being indefinitely stable to air and
surviving prolonged heating in DMSO. The synthesis of these
compounds usually involves refluxing a mixture of PdCl , K CO and
2
2
3
the hydrochloride salt of the NHC in neat 3-chloropyridine at
elevated temperatures for 16 h, which gives excellent yields but
Scheme 1. Solid-state route to coordination complexes of
=
heterocyclic nitrogen ligands (L
pyrazole).
pyridine, imidazole,
[7]
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Scheme 2. Synthesis of 1 – 6.
the hydrogen atoms in question (in, say, pyridinium [pK
a
~ 5] vs.
NHC imidazolium [pK ~ 23]). We note, however, that the acidity of
a
1
5
the cation may be very different at the solid state interface, and
that a concerted reaction may be facilitated by agostic interactions
between the metal and the carbon atom yielding the target
Figure 1. Supramolecular imidazolium – chlorine H-bonds for
the structures a) 3c; b) 4b; c) 4c; and the organometallic
complexes for d) 5a; e) 5b; f) 5c.
16
organometallic adducts.
Herein we report the solid-state synthesis of PEPPSI-type
compounds. The target materials can be obtained in a two-step
strategy by manually grinding together the reactants in an agate
mortar. In the first step a three-component mechanochemical
[
HL]+[PtCl
pyridinium cation and the concomitant displacement of a chloride
ligand at the metal. The crystal structures of [HIPr]+[PdCl py]- (3c),
HIMes]+[PtCl py]- (4b) and [HIPr]+[PtCl py]- (4c) were determined
3
py]- (4), which are formed by deprotonation of the
3
[
3
3
reaction involving K
one of the imidazolium hydrochloride salts
dibenzylimidazolium chloride (IBz·HCl), 1,3-bis(2,4,6-
2
MCl
4
, pyridinium hydrochloride (py·HCl) and
from crystals grown in this way (Fig. 1). Often, two other differently
coloured crystalline products were also produced in these
recrystallisations; these were identified by X-ray diffraction as the
-
1,3-
trimethylphenyl)imidazole chloride (IMes·HCl), or 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (IPr·HCl) - in a 1:1:1 ratio
bis(imidazolium) salts [Him]
2
[MCl
4
]
(as red crystals) and
bis(pyridine) complexes [MCl py
2
2
] (colourless crystals) probably
+
+
2-
quantitatively affords salts of general formula [HL] [Hpy] [MCl
M = Pd, 1; Pt, 2; L = IBz, a; IMes, b; IPr c; py = pyridine) in a few
minutes (Scheme 2). Powder X-ray diffraction (PXRD) patterns of
the product mixtures reveals no evidence of the K MCl , pyridinium
4
]
resulting from ligand redistribution reactions which will be reported
in full elsewhere.
(
Deprotonation of the pyridinium fragment, as required to form 3
and 4 does not take place in the solid state, as evidenced by
elemental analysis, which is not consistent with loss of HCl after the
grinding and prior to crystallization (see Experimental Section).
Furthermore, the powder patterns obtained following grinding do
not match those calculated from the crystal structures of 3c, 4b and
2
4
chloride or imidazolium chloride salts being still present, but all
contain a peak at 2θ = 28.3° due to crystalline KCl, indicating the
reaction has progressed (Fig. S1 and S2).
Attempts to grow single crystals of these salts by recrystallization
from common solvents were unsuccessful, giving instead (in some
4
c (and nor do they contain patterns from any of the three starting
cases)
3
a second series of salts: [HL]+[PdCl py]- (3) and
materials). Therefore, it appears that the pyridinium cation is
2
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=
28.3°, PXRD measurements show that the palladium and platinum
organometallic complexes are isomorphous in pairs (Fig. 2). In some
cases single crystals of the complexes could be grown from solution
and allow comparison with the powder patterns obtained from
solid-state reaction. Unsolvated crystal structures of 5a, 5b, 5c and
6
5
a were determined (Fig. 1), as were the structures of 5c·CH
2 2
Cl ,
c·toluene and 6b·CH Cl The individual structures are
2
2
.
unremarkable and will not be discussed in detail here; full details
are available as supporting information.
For 5a, 5b and 6a X-ray powder diffraction patterns of the products
match those calculated from the crystal structures; in the case of 6b
an unsolvated crystal structure is not available, but the powder
pattern of the reaction product resembles that of the palladium
analogue 5b.
The only case in which the PXRD pattern obtained from the as-
synthesised powder does not match that calculated from the
unsolvated single crystal structure is 5c suggesting that, in this case,
the solid-state synthesis produces an as yet unidentified
polymorph. The powder pattern for the as-synthesised 6c shows a
marked similarity to that of 5c, suggesting that they are
isomorphous (Fig. 2).
Conclusions
In conclusions this work shows that:
1)
Grinding together equimolar amounts of K
pyridinium hydrochloride and imidazolium chloride salt
generates the mixed salts [HIm][Hpy][MCl ] 1 and 2, rather
2 4
MCl (M = Pd, Pt),
4
than the known bis(pyridinium) or bis(imidazolium)
tetrachlorometallates.
2
)
)
The pyridinium counterions in 1 and 2 spontaneously
deprotonate in solution, leading to formation of the salts 3
and 4, containing imidazolium cations hydrogen-bonded to
-
[
MCl
3
py] anions.
3
Grinding 1 or 2 with two equivalents of KOH cleanly and
quickly generates the PEPPSI-type mixed ligand complexes 5
and 6 in essentially quantitative yield.
Figure 2. PXRD: a) From bottom to top calculated for 5a,
measured for 5a (reaction product), calculated for 6a and
measured for 6a (reaction product). b) From bottom to top
calculated for 5b, measured for 5b (reaction product) and 6b
(reaction product); c) From bottom to top calculated for 5c,
measured for 5c (reaction product) and 6c (reaction product).
Observation 1 is interesting in that three components generate a
single mixed-cation product, rather than the statistical 1:2:1
distribution of bis(imidazolium) : mixed salt : bis(pyridinium)
compound that might be expected from a random distribution of
ions. This implies that the stability of the mixed salts 1 and 2 is
greater than that of the bis(imidazolium) and bis(pyridinium) salts
and that the kinetics of the redistribution reactions are readily
accessed by room temperature grinding. As we have found on
previous occasions in related reactions, KCl is formed and it seems
likely that this is a powerful driving force. It is clear that the
redistribution reaction proceeds promptly in these solid state
reactions without addition of solvents and thermodynamic control
over the products formed is readily achieved.
deprotonated during the dissolution/crystallization process.
However the NMR spectra obtained from freshly prepared solutions
of 1 and 2 (Fig. S3 to S8) show resonances typical of the pyridinium
moiety (Fig. S15).
Notably, when the materials 1 + 2KCl and 2 + 2KCl prepared in the
first step of each reaction are ground with KOH, crystalline
coordination compounds trans-PdCl
2
(py)(NHC) (5) and trans-
PtCl (py)(NHC) (6) (NHC = N-heterocyclic carbene: IBz, a; IMes, b;
2
Our central conjecture is that the charge-assisted hydrogen-
bonding between the anions and cations is an important factor in
generating the complexes, pre-organising an acidic proton near to a
negatively charged chloride ion so HCl elimination is readily
achieved. Although we do not have crystal structures for any of the
mixed-salts 1 and 2 it seems likely (given the known predisposition
of imidazolium cations to hydrogen bond to chlorometallate anions
IPr, c) are produced (Scheme 2). The products were characterized
by spectroscopic techniques, elemental analysis and X-ray powder
diffraction.
The microcrystalline products of mechanochemical reactions have
microanalytical data consistent with those expected for 5 (or 6) and
four equivalents of KCl. Apart from the characteristic KCl peak at 2θ
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through the central CH) that the same kind of pre-organisation 4. N. J. Coville and L. Cheng, J. Organomet. Chem., 1998, 571, 149-
1
69.
occurrs in those species. Whether in the ground state crystal
structure or readily accessed from it, this would presumably
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ligand and the metal fragment in proximity and so lead them to
form the coordination complex when deprotonation occurs.
5
.
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6
7
8
.
.
.
The complexes reported are all rapidly formed in excellent yield
(practically quantitative), and can be readily separated from the KCl
1
449.
by-product by extraction into an organic solvent. This solvent-free
synthesis is therefore a viable synthetic route to useful NHC-
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1
1
1
1
1
1
1
1
1
1
Notes and references
All reagents were purchased from Aldrich and used without
further purification. The1 ligands 1,3-dibenzylimidazolium
7,
18
chloride
diisopropylphenyl)imidazolinium chloride (IPr·HCl), and 1,3-
(IBz·HCl),
1,3-bis(2,6-
1
9
2
0
bis(2,4,6-trimethylphenyl)imidazolinium chloride (IMesHCl)
were prepared according to the literature methods, and dried in
vacuo in the oven at 60 ˚C (IBz·HCl) and 40 ˚C (IMes·HCl) and
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2
011, 100, 1607-1617.
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1
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1 13
residual protons of the solvent ( H) or to the C signals of the
1
3
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5
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2
1
(
combined with measurements at different azimuthal angles.
SADABS)
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2
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2
2
using the SHELXTL suite of programs. Non-hydrogen atoms
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calculated positions refined using idealized geometries (riding
model) and assigned fixed isotropic displacement parameters.
The Crystallographic Information Files are available from the
CCDC (deposition numbers 1058574-1058584).
All crystalline phases were analyzed at room temperature by
powder X-ray diffraction on a Bruker D8 diffractometer using Cu-
K
α
X-radiation.
1
.
.
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4438; ChemComm Mechanochemistry: fundamentals and
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