Coordination chemistry of platinum and palladium in the solid-state: Synthesis of imidazole and pyrazole complexes

Article abstract of DOI:10.1039/b919665j

Solid-state reactions of palladium(ii) and platinum(ii) chloride complexes with imidazole (Him) and pyrazole (Hpz) or their hydrochloride salts are shown to produce metal complex salts and coordination compounds. Thus, K ₂[MCl₄] or MCl₂ can be ground with imidazolium chloride ([H₂im]Cl) to produce the salts [H₂im] ₂[MCl₄] (M = Pd, 1; Pt, 5), which can then be dehydrochlorinated in the solid state to produce the coordination compounds trans-[PdCl₂(Him)₂] 3 or cis-[PtCl₂(Him) ₂] 6. The complex cis-[PdCl₂(Him)₂] 2 is produced when Pd(OAc)₂ is ground with [H₂im]Cl. Reaction of platinum chloride reagents with imidazole (Him) also produces cis-[PtCl ₂(Him)₂] 6, but reaction of imidazole with analogous palladium chloride reagents first produces [Pd(Him)₄]Cl₂4 which then slowly converts to trans-[PdCl₂(Him)₂] 3. Grinding pyrazolium chloride with K₂[MCl₄] produces [H₂pz]₂[MCl₄] (M = Pd, 7; Pt, 10), which may also be dehydrochlorinated in the solid state to produce the coordination compounds trans-[PdCl₂(Hpz)₂] 8 or cis-[PtCl ₂(Hpz)₂] 11. Grinding K₂[PdCl₄] or PdCl₂ with pyrazole gives [Pd(Hpz)₄]Cl₂9, which is then slowly converted into trans-[PdCl₂(Hpz)₂] 8. Grinding PtCl₂ with Hpz generates [Pt(Hpz)₄]Cl ₂12, but using K₂PtCl₄ as the metal source does not generate the same product. The single-crystal structures of 8, a new polymorph of 11 and [H₂pz]₂[PtCl₆] ·2H₂O (isolated as a decomposition product) are reported for the first time, and the structures of 5 and 10 have been solved ab ibitio from XRPD data.

Full text of DOI:10.1039/b919665j

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Coordination chemistry of platinum and palladium in the solid-state:
Synthesis of imidazole and pyrazole complexes†
Christopher J. Adams,* Mairi F. Haddow, Robert J. I. Hughes, Mukhtar A. Kurawa and A. Guy Orpen
Received 21st September 2009, Accepted 19th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b919665j
Solid-state reactions of palladium(II) and platinum(II) chloride complexes with imidazole (Him) and
pyrazole (Hpz) or their hydrochloride salts are shown to produce metal complex salts and coordination
compounds. Thus, K2[MCl4] or MCl2 can be ground with imidazolium chloride ([H2im]Cl) to produce
the salts [H2im]2[MCl4] (M = Pd, 1; Pt, 5), which can then be dehydrochlorinated in the solid state to
produce the coordination compounds trans-[PdCl2(Him)2] 3 or cis-[PtCl2(Him)2] 6. The complex
cis-[PdCl2(Him)2] 2 is produced when Pd(OAc)2 is ground with [H2im]Cl. Reaction of platinum chloride
reagents with imidazole (Him) also produces cis-[PtCl2(Him)2] 6, but reaction of imidazole with
analogous palladium chloride reagents first produces [Pd(Him)4]Cl2 4 which then slowly converts to
trans-[PdCl2(Him)2] 3. Grinding pyrazolium chloride with K2[MCl4] produces [H2pz]2[MCl4] (M = Pd,
7; Pt, 10), which may also be dehydrochlorinated in the solid state to produce the coordination
compounds trans-[PdCl2(Hpz)2] 8 or cis-[PtCl2(Hpz)2] 11. Grinding K2[PdCl4] or PdCl2 with pyrazole
gives [Pd(Hpz)4]Cl2 9, which is then slowly converted into trans-[PdCl2(Hpz)2] 8. Grinding PtCl2 with
Hpz generates [Pt(Hpz)4]Cl2 12, but using K2PtCl4 as the metal source does not generate the same
product. The single-crystal structures of 8, a new polymorph of 11 and [H2pz]2[PtCl6]·2H2O (isolated as
a decomposition product) are reported for the first time, and the structures of 5 and 10 have been solved
ab ibitio from XRPD data.
with the metal halide source (as an anion) through the formation
Introduction
of NH ◊ ◊ ◊ Cl hydrogen bonded supramolecular synthons that lead
It has long been recognised that salts of the general form
to “crystal engineered” arrangements of the ions. This amounts
[AH]2[MXn], where AH is a protonated Lewis base (of, for
to a view that crystal engineering, in the classic sense expounded
by Schmidt (the topochemical postulate),6 is important in gaining
control over the solid state reactivity of the molecular components
of these salts. In this paper we will challenge that orthodoxy and
show that, at least in some cases, the transport of reactants in
the solid state can be relatively easily achieved, even at room
temperature, through mechanical means. This work therefore tests
whether these systems have the molecular mobility required for
reactivity, in a manner that relates to that cited by Kaupp as
being important in his studies of reactivity in the molecular solid
state.7
Reports have recently begun to appear concerning the solid-
state mechanochemical (i.e. by grinding) synthesis of compounds
of palladium and platinum that have traditionally been synthesised
by solution methods, despite the fact that substitution at these
metals (especially platinum) is generally slower than at divalent
first row metals; in many cases these solid-state methods are more
efficient than the traditional ones, or give better selectivity, or
both. Examples include the grinding of PPh3 with PtCl2 in a
ball mill to produce cis-[PtCl2(PPh3)2] in 98% yield, which can
subsequently react with solid K2CO3 to give [Pt(CO3)(PPh3)2],8
and the solvent-free preparation of polymetallic rings and cages by
manually grinding 4,4¢-bipyridine and [Pt(NO3)2(en)] (en = ethan-
1,2-diamine), forming the corresponding square tetraplatinum
complex in only 10 min.9 We therefore have set out to explore the
viability of such methods in the controlled preparation of some
archetypal metal complexes of platinum and palladium.
example, a nitrogen-donor ligand A) and [MXn] is a metal-
halide anion, can be thermally decomposed to yield HCl and
the corresponding coordination compound; the decomposition
of [Hpy]2[PtCl6] to [PtCl4(py)2] (py = pyridine) in solution was
first reported by Anderson in 1855,1 and has been dubbed the
‘Anderson rearrangement’.2 The reaction also occurs in the solid
state; an early demonstration was by Balbiano, who showed
in 1892 that heating of [H2im]2[PtCl6] to 192 ◦C generated
[PtCl4(Him)2] (Him = imidazole).3 This area has been extensively
studied by Kukushkin and co-workers, who have demonstrated
that a variety of compounds of the form [AH]2[MCl4] (M =
Pd, Pt) can be thermally decomposed to give cis-[MCl2A2], and
that these compounds often then isomerise to the trans isomers.2
More recently, we have demonstrated, inter alia, that [Hpic]2[PtCl4]
(pic = 4-picoline) can be thermally decomposed in two steps to
yield firstly [Hpic][PtCl3(pic)] and then trans-[PtCl2(pic)2],4 and
that [4,4¢-H2bipy][PtCl4] gives the coordination polymer trans-
{PtCl2(4,4¢-bipy)}n upon heating.5 In these cases we have placed
emphasis on the pre-organisation of the protonated nitrogen base
School of Chemistry, University of Bristol, Bristol, BS8 1TS. E-mail:
chcja@bris.ac.uk
† Electronic supplementary information (ESI) available: Complete powder
X-ray diffraction patterns for all compounds, TGA traces, and tables
of hydrogen bonds. CCDC reference numbers 748472, 748473, 748474,
748475, and 748476. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b919665j
3714 | Dalton Trans., 2010, 39, 3714–3724
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Scheme 1 Reaction chemistry explored in this work.
Herein, we report our studies upon imidazole (Him) and
Results and discussion
pyrazole (Hpz) complexes of platinum and palladium, demon-
strating that combining both thermal and mechanochemical
methods can produce a wide array of coordination compounds
without recourse to solution chemistry. Scheme 1 summarises
the chemistry we set out to explore. In choosing to focus on
this chemistry we draw upon previous experience in which some
analogous reactions have been achieve using nitrogen ligands such
as 4,4¢-bipyridine or 4-picoline,4,5 and on related chemistry of
cobalt and zinc.10
In this study we compare the behaviour of pyrazole and
imidazole in terms of their basicity (as isosteric but rather
different bases), and of their nitrogen atom location (and therefore
supramolecular chemistry). We also report the use of a wider range
of bases to cause dehydrochlorination (beyond KOH as previously
used), and of metal complex precursors to the coordination
compounds (beyond hydroxides and carbonates). Finally, the
variations in metal (Pd vs. Pt), precursor (MCl2 vs. K2MCl4])
and ligand (Hpz vs. Him) allow for insight into the reaction
mechanism, at least to the level of providing data on the reaction
outcomes (composition and isomer) that result.
Imidazole compounds
The synthesis of the imidazolium salt [H2im]2[PdCl4] 1 (in 54%
yield) has been reported from reaction of K2PdCl4 and imidazole
in hydrochloric acid.11 Its crystal structure has also been reported12
(CSD refcode: RORVOJ), and shown to consist of square planar
[PdCl4]2- anions and imidazolium cations which are connected
into a hydrogen-bonded ribbon along the crystallographic c-axis
by NH ◊ ◊ ◊ Cl interactions (see Fig. 1).
We have found that grinding of PdCl2 and two molar equivalents
of imidazolium chloride [H2im]Cl results in their conversion
to 1, as a reddish-brown powder of the known structure (as
judged by X-ray powder diffraction (XRPD) analysis). Simi-
larly, grinding K2PdCl4 and [H2im]Cl affords 1 as an orange
polycrystalline powder, whose XRPD pattern confirms that it
contains only phase 1 (by comparison with that of RORVOJ)
and KCl. Furthermore, exposure of the coordination compounds
cis-[PdCl2(Him)2] (2) or trans-[PdCl2(Him)2] (3) (vide infra) to
vapour from concentrated hydrochloric acid solution or to
Fig. 1 The hydrogen-bonded ribbon in [H2im]2[PdCl4] 1.
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Scheme 2 Synthesis and interconversion of [PdCl2(Him)2] and [H2im]2[PdCl4].
dry HCl gas in a sealed vessel leads to complete conversion
to 1.
It was not possible to readily synthesise 2 by grinding of
palladium-containing starting materials with imidazole or by
deprotonation of the salt 1 with a base. In these cases the reaction
mixture always contained [Pd(Him)4]Cl2 4, along with various
other by-products depending upon the stoichiometry used in the
reaction (see Scheme 3). Salt 4 is a known compound,13 and may be
deliberately synthesised in the solid state by grinding either PdCl2
or K2PdCl4 with four molar equivalents of imidazole, although the
reaction is rather slow and needs to be left for several days to reach
completion. The crystal structure of 4 is not known. Interestingly,
after standing for 6 months the powder pattern of the 1 : 1 mixture
of [Pd(Him)4]Cl2 4 and PdCl2 generated by the 1 : 2 reaction of
PdCl2 with imidazole showed the presence of 3, indicating that it
is possible to form 3 in this manner but that the reaction is rather
slow in the solid state.
The coordination compounds cis-[PdCl2(Him)2] 2 and trans-
[PdCl2(Him)2] 3 are also known,13 the former synthesised through
a (solution) route employing [Pd(C2O4)(Him)2] and the latter
directly from reaction of PdCl2 and imidazole in DMF. Complex
2 is reported to rapidly isomerise to 3 in DMF solution at
room temperature,13 and also in the solid state at between 170
◦
and 230 C.14 The crystal structure of 3 has been reported15
(CSD refcode: TCDIMP01) and consists of molecules linked by
NH ◊ ◊ ◊ ·Cl hydrogen bonds into a sheet. The crystal structure of 2
has not been reported, although an analysis of its powder pattern
has appeared.16
We found that it was possible to selectively synthesise 2 or 3 in
the solid state (see Scheme 2). Grinding together palladium acetate
and two molar equivalents of imidazolium chloride caused the
elimination of acetic acid and formation of 2. The cis configuration
and the phase crystallised was identified by comparison of the
powder pattern for 2 prepared by this method with that of
its platinum analogue (Fig. 2); the pattern did not match that
previously reported for cis-[PdCl2(Him)2].16 The trans-isomer 3
could be synthesised by heating the salt 1 to 170 ◦C for 4 h, as
previously reported.14
Scheme 3 Synthetic routes to [Pd(Him)4]Cl2 4. All products and crys-
talline by-products have been identified in the XRPD patterns of the
appropriate reactions.
Solution synthesis (in 70-75% yield) of the imidazolium tetra-
chloroplatinate salt [H2im]2[PtCl4] (5) has been reported,17 al-
though the crystal structure is not known. We found that grinding
of PtCl2 or K2PtCl4 with imidazolium chloride formed 5 (with KCl
in the latter case) as a polycrystalline powder. However, attempts
to synthesize 5 by exposing cis-[PtCl2(Him)2] 6 (below) to vapours
from concentrated aqueous HCl solution or dry HCl gas were
unsuccessful, and no reaction was observed.
Comparison of the experimental XRPD pattern for the
mechanochemically synthesized 5 with the pattern calculated from
the crystal structure of its palladium analogue [H2im]2[PdCl4] 1
Fig. 2 The XRPD pattern for cis-[PdCl2(Him)2] 2 (bottom) compared
with that of cis-[PtCl2(Him)2] 6 prepared by thermal decomposition of 5
(top).
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Table 1 Crystal data for 5, 8, 10 b-11 and [H2pz]2[PtCl6]
Crystal Data
5a
8
10a
b-11
[H2pz]2[PtCl6]·2H2O
T/K
293(2)
[C3H5N2]2[PtCl4]
475.07
Brown
Fine-powder
2.598
100(2)
[PdCl2(C3H4N2)]
313.46
Yellow
Block
2.150
2.424
293(2)
[C3H5N2]2[PtCl4]
475.07
Dark brown
Powder
2.471
100(2)
[(C3H4N2)2PtCl2]
402.14
Yellow
Needle
2.550
13.865
100(2)
Empirical formula
Formula weight
Colour
[C3H5N2]2[PtCl6]·2H2O
582
Yellow
Block
2.366
9.634
Triclinic
Morphology
Dc/Mg m-3
m/mm-1
Crystal system
Space group
Triclinic
¯
P1
Triclinic
¯
P1
Triclinic
¯
P1
Triclinic
¯
P1
¯
P1
˚
a/A
7.0225(3)
7.6375(3)
6.7148(3)
110.2813(19)
96.859(2)
64.110(2)
303.69(2)
1.5406
4.0628(5)
7.2328(7)
8.3361(8)
93.805(7)
99.073(6)
91.601(7)
242.05(4)
0.71073
1
3731
1116
0.0347
6.35, 2.59
8.5229(2)
7.0222(3)
7.3333(3)
66.635(3)
127.557(3)
105.595(3)
319.18(2)
1.5406
11.0643(15)
13.1094(17)
14.912(2)
94.583(8)
98.669(8)
99.749(8)
2095.0(5)
0.71073
8
6.8406(2)
11.1912(4)
11.2366(4)
84.332(2)
76.887(2)
75.824(2)
811.41(5)
0.71073
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
˚
l/A
Z
1
1
Reflections collected
Unique reflections
R(int)
9564
9564
0
5527
5527
0
wR2, R1 (%)
22.56, 7.45
8.13, 3.04
a Structure determined from powder diffraction data.
showed the compounds to be isomorphous. This was exploited
to refine the crystal structure of 5 against the XRPD data, with
starting geometry based on the structure of 1. The unit cell of
5 is smaller than that of 1 (see Table 1), and the Pt–Cl bonds
being formed. The XRPD pattern of 6 closely resembled that of
2, suggesting them to be isomorphous (Fig. 2).
Pyrazole compounds
˚
(2.264(14) and 2.276(15) A) of 5 marginally shorter than the Pd–
˚
The palladium-pyrazolium salt [H2pz]2[PdCl4] 7 has not previously
been reported, but we found that grinding of K2PdCl4 with two
molar equivalents of pyrazolium chloride affords 7, with two
equivalents of KCl as by-product in the former case. The XRPD
pattern of 7 synthesised in this manner closely resembles that of
its platinum analogue 10, suggesting them to be isostructural. 7
appears to be relatively unstable at room temperature, undergoing
spontaneous dehydrochlorination to trans-[PdCl2(Hpz)2] 8, which
makes its isolation as a pure phase difficult; attempting its
formation by grinding PdCl2 with pyrazolium chloride always
afforded mixtures containing both 7 and 8. Allowing a sample
of 7 to stand at room temperature for a period of weeks leads to its
complete conversion to 8, a process which can be accelerated by
heating 7 at 125 ◦C for 4 h, or by grinding 7 with two molar
equivalents of potassium tert-butoxide. 8 can also be formed
by direct mechanochemical 1 : 2 reaction of either K2PdCl4 and
pyrazole (though the reaction requires ball-milling for 30 min in
order to reach completion) or Pd(OAc)2 and pyrazolium chloride
(see Scheme 5).
Cl bonds (2.3142(15) and 2.3134(15) A) in 1. Table 1 compares
cell parameters for 1 and 5.
Solution preparation of cis-[PtCl2(Him)2] 6 has been reported,
but was not straightforward and could not be achieved starting
from K2PtCl4.13 The solid-state synthesis of 6 was the least
satisfactory of all the syntheses reported herein; although grinding
of PtCl2 with imidazole led to the formation of the coordination
complex 6 (Scheme 4), an unidentified by-product was also
formed. Equally, whilst mechanochemical dehydrochlorination of
5 by grinding with Ag2O resulted in the formation of 6 + AgCl,
use of KOH generated the same unknown by-products as before.
Salt ◦5 could also be converted into 6 by heating for 5 days at
125 C, contrary to expectations based on the literature,17 and
this proved the cleanest method of synthesis, although heating at
higher temperatures also led to small amounts of the trans isomer
Although 8 is a known compound,18,19 its crystal structure has
not previously been determined. Single crystals suitable for X-ray
diffraction were obtained by the extremely slow evaporation of a
dry dichloromethane solution of a sample of 8 prepared by litera-
ture solution methods.19 The structure of the compound consists of
discrete centrosymmetric trans-[PdCl2(Hpz)2] molecules, in which
the N–Pd–N and Cl–Pd–Cl angles are necessarily 180.0◦, and
the N(1)-Pd(1)-Cl(1) angle is 89.92(6)◦. The palladium–chlorine
Scheme
cis-[PtCl2(Him)2] 6.
4
Synthesis and interconversion of [H2im]2[PtCl4] 5 and
˚
bond length is 2.3079(6) A, and the palladium-nitrogen distance is
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Scheme 5 Synthesis and interconversion of the palladium pyrazole species [H2pz]2[PdCl4] (7), trans-[PdCl2(Hpz)2] (8) and [Pd(Hpz)4]Cl2 (9).
˚
2.028(6) A. Crystal and structure refinement data are presented
˚
in Table 1. The Pd–N distance is close to that of 2.011(2) A
found for a related complex trans-[PdCl2(HEP)2] (HEP = bis-(1-
(2-hydroxyethyl)pyrazole).20 The [PdCl2(Hpz)2] molecules in 8 are
˚
connected into a chain by N–H ◊ ◊ ◊ Cl interactions (2.62 A) between
the chloride ligands and the pyrazole NH donors, and there
˚
are also shorter intramolecular N–H ◊ ◊ ◊ Cl interactions (2.46 A)
(Fig. 3).
Van Kralingen et al.18 observed that 8 exists in more than
one polymorphic form, but in our hands preparation of 8 by
a wide range of solid state methods (Scheme 5) all led to the
formation of just one crystalline form as deduced from XRPD
patterns (Fig. 4). However, it is evident from the Fig. 4 that the
calculated powder pattern differs from the experimental powder
patterns (which were measured at room temperature), especially
at higher values of 2q, leading us to conclude that there may be
some crystallographic changes occurring between the temperature
at which the crystal structure and the powder patterns were de-
termined. We have previously seen similar behaviour in imidazole
chemistry.10
Fig. 4 XRPD patterns for trans-[PdCl2(Hpz)2] 8 (see Scheme 5). Dark
blue = calculated from the single crystal structure; green = solution
synthesis; brown = thermal elimination from 7; turquoise = Pd(OAc)2 +
2[H2pz]Cl; orange = K2PdCl4 + 2Hpz (ball milling); pink = 7 + 2ButOK.
temperature effects in play, and adventitious water has been
acquired.
Complex 8 could not be made by grinding pyrazole with
PdCl2 or with K2PdCl4 in a mortar and pestle. Such reactions
instead produced [Pd(Hpz)4]Cl2 9, together with a variety of
palladium-containing by-products unless the Hpz:Pd ratio used in
the reaction was 4 : 1. The crystal structure of 9 as a monohydrate
has been reported21 (CSD refcode: IHONAU), and the powder
pattern calculated from this closely matches that produced by
our solid-state syntheses, though again there apparently are some
The platinum pyrazolium salt [H2pz]2[PtCl4] 10 does not appear
to have been reported before, but was readily made by grinding
of pyrazolium chloride with PtCl2 or K2PtCl4 (with elimination
of KCl once again in the latter case). The crystal structure of
10 is unknown, but comparisons of the X-ray powder diffraction
patterns of the samples obtained from both synthetic routes with
˚
Fig. 3 Inter and intramolecular N–H ◊ ◊ ◊ Cl distances in 8 viewed down the a-axis. Selected bond lengths (A): Pd–N, 2.0128(2); Pd–Cl 2.3079(6).
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Fig. 5 The structure of 10, determined from powder XRD.
◦
that of its palladium analogue 7 showed the salts 7 and 10 to be
isomorphous (Fig. 6). Attempts to afford a suitable single crystal
of 10 by recrystallization from concentrated HCl solution led
to oxidation of the platinum(II) centre to platinum(IV), forming
a [PtCl6]2- anion which crystallised as a hydrated pyrazolium
salt {the crystal structure determination of this compound,
[H2pz]2[PtCl6]·2H2O, is reported in the Supporting Information
(see Table 1)}, so the structure of 10 was solved ab initio from the
powder pattern.
Heating a sample of 10 at 125 C for 4 h in air resulted in the
elimination of two molar equivalents of HCl and the subsequent
formation of the coordination compound cis-[PtCl2(Hpz)2] 11.
The solid state conversion of 11 into its trans isomer by heating
◦
at 110 C for 8 h has been reported,22 but in our hands the
thermal elimination reaction cleanly generated the cis-isomer.
Mechanochemical dehydrochlorination of 10 with Ag2O also
leads to the formation of 11 (and AgCl). The published crystal
structure of 11, previously determined at both room temperature23
(CSD refcode: VAZWEY01) and low temperature22 (CSD refcode:
VAZWEY) consists of a central platinum(II) cation bonded to two
chloride anions and two pyrazole molecules in a slightly distorted
square planar cis coordination geometry. Comparison of the
XRPD patterns of the products of our solid-state syntheses with
a calculated pattern from the room temperature determination23
showed 11 as prepared by both routes to be the VAZWEY phase.
Mechanochemical dehydrochlorination by grinding of a hydro-
gen bonded salt of a chlorometallate anions with KOH has been
used several times to afford the related coordination compound,
and usually produces KCl as a by-product.5,10 However, this
method did not work well for salt [H2pz]2[PtCl4] 10, which on
grinding with nominally dry KOH resulted in the formation of
K2PtCl4, trace amounts of the desired product 11 and some
uncoordinated pyrazole (pyrazole and K2PtCl4 react slowly;
vide infra). A similar observation was made when ButOK was used
as external base, which method produces KCl and t-butanol as the
by-products We therefore utilised silver oxide as the external base
to eliminate 2 molecules of HCl from 10, forming cis-[PtCl2(Hpz)2]
11 with AgCl and H2O as by-products, and avoiding the formation
of KCl and the risk of subsequent transformation of the target
compound into K2PtCl4 by (formal) reaction of 2 KCl with 11
with elimination of pyrazole. Attempts to convert the coordination
compound cis-[PtCl2(Hpz)2] 11 back into [H2pz]2[PtCl4] 10 by
exposure to aqueous HCl vapour or dry HCl gas were unsuccessful
(see Scheme 6).
The powder patterns obtained for 10 (see Table 1) were readily
3
˚
indexed as triclinic, with the unit cell volume of 319 A implying
that there was only half of the [H2pz]2[PtCl4] formula unit in the
asymmetric unit, and therefore that the [PtCl4]2- anion must lie
on an inversion centre. From this starting point the structure was
refined with the [H2pz]+ cation as a freely moving rigid body and
the platinum atom constrained to the origin, and the resultant
crystal structure is shown in Fig. 5. The pyrazole ring makes
hydrogen bonds from both NH groups to chloride atoms, and there
are also short CH ◊ ◊ ◊ Cl contacts. (The assignment of NH and CH
groups within the pyrazolium ring was done on the basis that the
shortest H ◊ ◊ ◊ Cl interaction should be from a NH group, but it
is entirely possible that the ring shows some rotational disorder
and that all the NH and CH contacts are mixed. However, the
arrangement settled upon produces sensible hydrogen-bonding
patterns and geometries). The pyrazolium rings are p-stacked,
and the overall packing is of ribbons of alternating cations and
anions that run along the a axis (Fig. 5). These ribbons are then
linked to neighbouring ones by intra-strand NH ◊ ◊ ◊ Cl hydrogen
bonds.
A single crystal of 11 was serendipitously grown from a
solution of 10 + 2 KCl in hydrochloric acid that had been left
to evaporate, and X-ray structure determination on this revealed
a new polymorph of 11 (hereafter b-11), which contains 4 unique
molecules in its asymmetric unit. These form two pairs which
are arranged in a stacked fashion, with the coordination planes
about the platinum atom approximately parallel and the platinum
˚
centres only 3.6 A apart (Fig. 7). As noted above, this is not the
Fig. 6 Measured powder patterns for [H2pz]2[PdCl4] 7 and [H2pz]2[PtCl4]
phase prepared by the solid state methods employed here. The
structure of b-11 is notably less dense than that of the VAZWEY
phase (by ca. 5%).
10. Green = 10 from PtCl2 + 2[H2pz]Cl; brown = 10 from K2PtCl4
+
2[H2pz]Cl; blue = 10 from calculated structure; red = 7 from K2PdCl4 +
2[H2pz]Cl.
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Scheme 6 Synthesis and interconversion of platinum pyrazole species [H2pz]2[PtCl4] 10, cis-[PtCl2(Hpz)2] 11, and [Pt(Hpz)4]Cl2·H2O 12.
Fig. 7 The asymmetric unit of b-11, showing the four molecules of [PtCl2(Hpz)2] contained therein and their arrangement into pairs.
Mechanochemical grinding of K2PtCl4 or PtCl2 with pyrazole
using a mortar and pestle or ball mill does not lead to 11.
Instead, analogously to the palladium system, grinding PtCl2
with pyrazole (even in a 1 : 2 ratio) gives [Pt(Hpz)4]Cl2·H2O 12,
which is isomorphous with the palladium compound 9. The
reaction is slow, and only reaches a conclusion after standing for
about two weeks at room temperature following grinding (XRPD
patterns recorded over the course of a fortnight are given in
the supplementary information). Unlike the palladium system,
however, grinding pyrazole with K2PtCl4 gives a different product
to the reaction with PtCl2, in a reaction which is even slower (see
supplementary information for XRPD patterns). The structure
of the product is unknown, but it appears to be neither form of
the cis coordination compound 11 (CSD refcode: VAZWEY01)23
or b-11, nor trans-[PtCl2(Hpz)2] (CSD refcode: NEFXAY),22 or
the constitutional isomer [PtCl4][Pt(Hpz)4][cis-PtCl2(Hpz)2] (CSD
refcode: XALKOK).24
M = Pd 1, Pt 5; H2L = Hpz; M = Pd 7, Pt 10) and coordination
compounds [MCl2(HL)2] (HL = Him; M = Pd 2 (cis), 3 (trans);
Pt 6; HL = Hpz; M = Pd 8, Pt 11) without recourse to solvents.
2. Mechanochemical grinding of the hydrochlorides of the
ligands (imidazole or pyrazole) with K2PtCl4 or PtCl2 readily
gives the tetrachloroplatinate salts 5 and 10. The X-ray powder
diffraction data of these compounds were used to identify
their single crystal structures (not known until now). Similarly,
reactions of the ligand hydrochloride salts with K2PdCl4 afford
the crystalline salts 1 and 7, which are isostructural to 5 and 10
respectively, again with elimination of KCl. The reactions with
the tetrachlorometallate salts are interesting, as the position of the
salt redistribution reaction K2MCl4 + 2[H2L]Cl → [H2L]2[MCl4] +
2 KCl appears to lie well to the right, presumably driven by the
formation of KCl.
3. Grinding palladium(II) acetate withligand hydrochloridesalts
affords the crystalline [PdCl2(HL)2] coordination compounds with
concomitant elimination of acetic acid, without the addition of
base, solvent or heating. This proved to be the most convenient
way to synthesise cis-[PdCl2(Him)2] 2 in the solid state.
Discussion and conclusions
It has been shown that:
4. The preparation of cis-[PtCl2(Him)2] 6 by reacting K2PtCl4
and imidazole in solution could not be achieved, but solid-state
grinding or ball milling of K2PtCl4 or PtCl2 with imidazole
1. A wide range of solid-state techniques can be used to readily
access the hydrogen bonded complexes [H2L]2[MCl4] (H2L = Him;
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affords 6, though not cleanly. 6 was also obtained by heating
of 5 (the best method of preparation) or by mechanochemical
grinding of 5 with Ag2O. Similarly, thermal and mechanochemical
dehydrochlorination of 10 affords cis-[PtCl2(Hpz)2] 11.
5. [PdCl2(Him)2] can be isolated as both cis (2) and trans (3)
isomers, whilst in contrast [PdCl2(Hpz)2] is always formed as the
trans isomer 8, irrespective of the synthetic route used.
6. Exposure of the platinum coordination compounds cis-
[PtCl2(HL)2] (HL = Him 6, Hpz 11) to vapour from concentrated
aqueous HCl or dry HCl gas does not convert them to the
salts 5 and 10 even after long periods of exposure. Exposure of
trans-[PdCl2(Hpz)2] 8 to HCl did not afford the hydrogen-bonded
salt 7 either, but exposure of either cis-[PdCl2(Him)2] 2 or trans-
[PdCl2(Him)2] 3 to HCl converts them into 1.
7. Silver oxide (Ag2O) can be used as an external base instead
of KOH or K2CO3 to avoid the formation of KCl and the
potential for subsequent formation of K2MCl4 (as observed in the
mechanochemical dehydrochlorination reactions of the platinum
salts). We have also demonstrated that it is possible to use KOtBu
as a base in solid-state grinding reactions.
8. Thermal dehydrochlorination from [H2L]2[MCl4] (M = Pd,
Pt; H2L = Him, Hpz) takes place smoothly and is a simple
and convenient method for the synthesis of the coordination
compounds. The process involves the transfer of a proton from
the amine to chloride leading to the breaking of the M–Cl bond
and the formation of HCl gas. The ligand then occupies the
coordination site vacated by the chloride resulting in the formation
of M–N bonds.2
9. Solid-state preparative methods often seem to produce only
one polymorph of a given compound, even when more than one
are known. Thus, producing 11 by thermal or mechanochemical
dehydrochlorination of 10 gives the VAZWEY phase of 11
only.
The reactions of [PtCl4]2- in aqueous solution with nitrogen-
donor ligands such as pyridine and amines have been thoroughly
studied over many years. When the ligand L has a low trans-effect,
as is the case for imidazole and pyrazole, then the first stable
product is the cis-[PtCl2L2] compound; following replacement of
the first chloride of [PtCl4]2- to form the [PtCl3L]- ion; the next
chloride to be replaced is one opposite another chloride (owing
to the greater trans-effect of chloride than L). In the presence of
free L, the next compound formed is the tetrakis salt [PtL4]2+,
which then reacts slowly with chloride to form the trans-[PtCl2L2]
compound; following replacement of the first L to form the
[PtL3Cl]- ion, the next L to be replaced is the one opposite the
chloride (again owing to the greater trans-effect of chloride than
L) (Scheme 7).2
greater than that of platinum, under given reaction conditions
the palladium compounds may be expected to progress more
readily through the reaction sequence in Scheme 7 than their
platinum analogues. Thus, grinding K2PdCl4 and pyrazole gives
[Pd(Hpz)4]Cl2 9 (presumably via unobserved cis-[Pd(Hpz)2Cl2]),
but further reaction can be obtained by ball-milling to give trans-
[Pd(Hpz)2Cl2] 8. The reaction of PtCl2 with pyrazole affords
[Pt(Hpz)4]Cl2 12, but this does not then appear to react further to
afford trans-[Pt(Hpz)2Cl2]–it is notable that the solution reaction
requires 12 to be refluxed in concentrated hydrochloric acid in
order to yield the trans isomer,18 indicating that in the solid state
at room temperature and without a large excess of chloride ions
the reaction is likely to be exceedingly slow. Similar behaviour
was observed for the imidazole compounds - in the reaction of
PdCl2 with imidazole, [Pd(Him)4]Cl2 4 was the product observed
initially; though in this case prolonged ball-milling did not cause
further reaction, after 6 months peaks due to the product trans-
[Pd(Him)2Cl2] 3 were visible in the powder pattern. The slowness
of this reaction compared to the analogous pyrazole chemistry
may be attributed to the greater basicity of imidazole compared
to pyrazole, and that as a consequence the palladium-imidazole
bond is more difficult to break than the palladium-pyrazole bond.
As before, in the platinum case there is less progress through the
reaction pathway, and the first product cis-[PtCl2(Him)2] 6 is the
only one seen.
One unexpected result is that reactions of pyrazole with PtCl2
and K2PtCl4 do not produce the same product; in the former
case the product is [Pt(Hpz)4]Cl2 12, but in the latter an uniden-
tified phase is slowly formed. This also explains why grinding
[H2pz]2[PtCl4] 10 with KOH does not yield cis-[Pt(Hpz)2Cl2] 11,
and actually gives an insight into the mechanism of the reaction.
The fact that K2PtCl4 and free pyrazole are observed indicates
that the first step of the reaction is deprotonation of the [H2pz]+
rather than abstraction of a chloride ion from the [PtCl4]2- ion.
The reaction only proceeds if a base that is also a potent chloride
abstractor (Ag2O) is used; presumably, pyrazole is not a strong
enough Lewis base to displace chloride from [PtCl4]2-.
We see no reason why the reactions reported above should not be
extended to related systems. Substituted imidazole and pyrazole
compounds of palladium and platinum are often of interest in
polymerisation studies or as anticancer agents,25-27 and it should be
possible to synthesise them using the solid-state methods reported
above. It is normally the case that the cis isomers exhibit much
higher antitumour activity than the trans, and so methods that
allow synthesis of particular isomers are especially useful.28 We
also note that compound [H2pz]2[PtCl4] 10 has recently been
implicated as an active species in the Shilov-type activation of
methane in ionic liquids, the hydrogen bonding between anion
and cation being particularly important in preventing catalyst
degradation,29 and that bis(imidazolium) tetrachloropalladate
species are active catalyst precursors for Suzuki-Miyaura cross-
coupling reactions.30
The solid-state reactions of imidazole and pyrazole with MCl2
and [MCl4]2- that we have reported herein appear to obey
the same reaction sequence, and depending upon the relative
lability of the compounds various members of the series may
be isolated. Given that the lability of palladium is substantially
Scheme 7 The sequence that leads to the formation of trans compounds from the reaction of metal-chloride precursors with ligands of low trans-effect.
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Our aim in reporting this work is to try and draw the attention
of the chemistry community to the fact that many coordination
compounds can be synthesised simply by grinding together
appropriate starting materials. Many of the synthetic pathways
reported herein generate the platinum or palladium-containing
products as mixtures with potassium chloride (or in some cases,
silver chloride). We have not attempted further purification of
these compounds as we have deliberately limited the scope of this
paper to solid-state chemistry and methods of analysis, but in many
cases simply washing the mixture with water will remove the KCl
by-product. If feasible, the best option if the procedures outlined
above are to prove useful synthetic pathways to starting materials
for further synthetic transformations might be to simply ignore the
presence of the AgCl or KCl by-products and use the compounds
as mixtures, removing the by-product at a later stage. Nevertheless,
every one of the compounds presented in this paper (apart from 8,
which spontaneously decomposes) can be synthesised as a single-
phase solid (with no by-products) by solid-state methods in one
or two steps from commercially available starting materials (this
requires the use of the MCl2 or M(OAc)2 species as the metal
source, as using K2MCl4 necessarily leaves KCl behind).
of imidazolium chloride in an agate mortar, forming 2 as a
yellow polycrystalline solid. This was dried in vacuo to evap-
orate any remaining acetic acid. Microanalytical data (%) for
[PdCl2(C3H4N2)2], Calculated: C, 22.99; H, 2.57; N, 17.87. Found
C, 23.19; H, 2.81; N, 17.57.
trans-[PdCl2(Him)2] (3). Thermal elimination: 3 was syn-
thesized from 1 as reported.14 Microanalytical data (%) for
[PdCl2(C3H4N2)2]: Calculated: C, 22.99; H, 2.57; N, 17.87 Found
C, 22.58; H, 2.53; N, 17.75.
[Pd(Him)4]Cl2 (4). Mechanochemical synthesis: Using K2PdCl4
and imidazole (1:4): 163 mg (0.5 mmol) of K2PdCl4 was forcefully
ground with 136 mg (2 mmol) of imidazole forming 4 + 2
KCl as bright white polycrystalline powder. It takes 48 h after
grinding for the reaction to reach completion. Microanalytical
data (%), Calculated for [Pd(C3H4N2)4]Cl2 + 2 KCl: C, 24.07;
H, 2.69; N, 18.71. Found C, 23.83; H, 2.78; N, 17.36. Using
PdCl2 and imidazole (1:4): 89 mg (0.5 mmol) of PdCl2 was
forcefully ground with 136 mg (2 mmol) of imidazole forming
4 as a white polycrystalline powder. The reaction takes about
5 days after grinding to reach completion. Microanalytical data
(%), Calculated for [Pd(C3H4N2)4]Cl2: C, 32.05; H, 3.59; N, 24.92.
Found C, 32.30; H, 3.62; N, 24.83.
Experimental
[H2im]2[PtCl4] (5). Mechanochemical synthesis: Using PtCl2:
266 mg (1 mmol) of PtCl2 and 209 mg (2 mmol) of imidazolium
chloride were ground in an agate mortar. The resulting ochre poly-
crystalline powder of 5 was analysed using XRPD. Microanalytical
data (%) for [C3H5N2]2[PtCl4], Calculated: C, 15.17; H, 2.12; N,
11.79. Found C, 15.14; H, 2.19; N, 11.35. Using K2PtCl4: 208 mg
(0.5 mmol) of K2PtCl4 and 105 mg (1 mmol) of imidazolium
chloride were forcefully ground in an agate mortar. This resulted
in the formation of an orange polycrystalline powder, and X-ray
diffraction pattern on the powder revealed the presence of KCl
in the product. Microanalytical data (%) for [C3H5N2]2[PtCl4] + 2
KCl, Calculated: C, 11.55; H, 1.61; N, 8.98. Found C, 11.93; H,
2.00; N, 8.53.
Synthesis
Samples were ground by hand using an agate mortar and pestle
in air. Ball milling was performed in a Retsch MM 200 ball mill
(mixer mill) for the specified times at 30 Hz. All reagents were
purchased from Aldrich, Strem or Lancaster and used without
further purification. Product samples were dried in vacuo or in the
oven at 50 C. The PtCl2 used was purchased from Aldrich and
shown by XRPD to be of the b-phase.31
n.b. The reactions reported below did not all produce pure
samples of the desired product; the results section above should
be consulted for further details.
◦
[H2im]2[PdCl4] (1). Mechanochemical synthesis: Using PdCl2:
177 mg (1 mmol) of PdCl2 and 209 mg (2 mmol) of imidazolium
chloride were ground in an agate mortar. This resulted in the
formation of a hygroscopic reddish-brown powder. Microan-
alytical data (%) for [C3H5N2]2[PdCl4], Calculated: C, 18.65;
H, 2.61; N, 14.50. Found C, 18.88; H, 2.69; N, 14.65. Using
K2PdCl4: 163 mg (0.5 mmol) of K2PdCl4 and 105 mg (1 mmol)
of imidazolium chloride were ground in an agate mortar. This
resulted in the formation of an orange polycrystalline powder,
which XRPD revealed to contain KCl. Microanalytical data (%)
for [C3H5N2]2[PdCl4] + 2 KCl, Calculated: C, 13.46; H, 1.88; N,
10.46. Found C, 13.82; H, 2.17; N, 10.90. Vapour absorption: A
vial containing 25 mg of cis-[PdCl2(Him)2] 2 was placed in a jar
containing concentrated aqueous HCl. The yellow powder became
orange after a few minutes and finally reddish-brown. The reaction
took 48 h to reach completeness. Microanalytical data (%) for
[C3H5N2]2[PdCl4], Calculated: C, 18.65; H, 2.61; N, 14.50. Found
C, 18.71; H, 2.54; N, 14.27. Similar conversion occurred when
trans-[PdCl2(Him)2] 3 was exposed to dry HCl gas, but there was
no colour change observed.
cis-[PtCl2(Him)2] (6). Mechanochemical reaction: Using PtCl2
and imidazole: 266 mg (1 mmol) of PtCl2 and 136 mg (2 mmol) of
imidazole were forcefully ground in an agate mortar, resulting in
the formation an olive-green polycrystalline powder. Microanalyt-
ical data (%) for [PtCl2(C3H4N2)2], Calculated: C, 17.92; H, 2.01;
N, 13.93. Found C, 17.87; H, 2.15; N, 13.95. Thermal elimination:
48 mg (0.1 mmol) of 5 was heated under vacuum at 125 ◦C for
5 days. This resulted in a colour change from brown to light
grey. Microanalytical data (%) for [PtCl2(C3H4N2)2]: Calculated:
C, 17.92; H, 2.01; N, 13.93. Found C, 18.04; H, 2.17; N, 13.67.
Mechanochemical elimination (Ag2O): 48 mg (0.1 mmol) of 5 was
forcefully ground with 23 mg (0.1 mmol) of Ag2O. The reaction
proceeded with a visible colour change from dark brown to grey,
and water was removed in vacuo. Microanalytical data (%) for
[PtCl2(C3H4N2)2] + 2AgCl, Calculated: C, 10.46; H, 1.17; N, 8.13.
Found C, 10.48; H, 1.18; N, 8.47.
[H2pz]2[PdCl4] (7). Mechanochemical synthesis. Using PdCl2:
177 mg (1 mmol) of PdCl2 and 209 mg (2 mmol) of pyrazolium
chloride were ground in an agate mortar. This resulted in
the formation of 7 as a reddish-brown polycrystalline powder.
Microanalytical data (%) for [C3H5N2]2[PdCl4], Calculated: C,
cis-[PdCl2(Him)2] (2). Mechanochemical synthesis: 225 mg
(1 mmol) of Pd(OAc)2 was ground with 209 mg (2 mmol)
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18.65; H, 2.61; N, 14.50. Found C, 18.35; H, 2.61; N, 14.63. Using
K2PdCl4: 163 mg (0.5 mmol) of K2PdCl4 and 105 mg (1 mmol)
of pyrazolium chloride were ground in an agate mortar. This
resulted in the formation of an orange polycrystalline powder,
which XRPD revealed to contain KCl. Microanalytical data (%)
for [C3H5N2]2[PdCl4] + 2 KCl, Calculated: C, 13.46; H, 1.88; N,
10.46. Found C, 14.07; H, 2.03; N, 10.67.
cis-[PtCl2(Hpz)2] (11). Thermal elimination: ◦47.5 mg
(0.1 mmol) of [C3H5N2]2[PtCl4] 10 was heated to 125 C for 4 h,
the dark brown powder becoming light grey. The XRPD pattern
measured for the product matches with that calculated from the
computed crystal structure of cis-[PtCl2(Hpz)2]. Microanalytical
data (%) for [PtCl2(C3H4N2)2]: Calculated: C, 17.92; H, 2.01;
N, 13.93. Found C, 17.41; H, 1.97; N, 13.53. Mechanochemical
elimination (Ag2O): 48 mg (0.1 mmol) of [C3H5N2]2[PtCl4] 10 was
forcefully ground with 23 mg (0.2 mmol) of Ag2O. The reaction
proceeded with a visible colour change from dark brown to grey
and excess water was removed in vacuo. Microanalytical data (%)
for [PtCl2(C3H4N2)2] + 2AgCl, Calculated: C, 10.46; H, 1.17; N,
8.13. Found C, 10.48; H, 1.18; N, 8.47.
trans-[PdCl2(Hpz)2] (8). Mechanochemical synthesis: Using
K2PdCl4 and pyrazole: 163 mg (0.5 mmol) of K2PdCl4 and 68 mg
(1 mmol) of pyrazole were ball milled for 30 min forming an
orange polycrystalline powder of 8. Microanalytical data (%) for
[PdCl2(C3H4N2)2] + 2 KCl, Calculated: C, 15.58; H, 1.74; N,
12.11. Found C, 15.99; H, 1.96; N, 11.96. Using Pd(OAc)2 and
[H2pz]Cl: 225 mg (1 mmol) of Pd(OAc)2 was ground with 209 mg
(2 mmol) of pyrazolium chloride in an agate mortar forming 8 as
a yellow-orange polycrystalline powder. This was dried in vacuo to
evaporate any remaining acetic acid. Microanalytical data (%) for
[PdCl2(C3H4N2)2], Calculated: C, 22.99; H, 2.57; N, 17.87. Found
C, 23.79; H, 2.81; N, 17.65. Solution synthesis: The title compound
was prepared as previously reported.19 Microanalytical data (%)
for [PdCl2(C3H4N2)2]: Calculated: C, 22.99; H, 2.57; N, 17.87
Found C, 23.08; H, 2.75; N, 18.24. Thermal elimination: 39 mg
(0.1 mmol) of 7 was heated to 125 ◦C for 4 h. The elimination
of 2HCl was accomplished with a colour change from dark
orange to yellow. Microanalytical data (%) for [PdCl2(C3H4N2)2]:
Calculated: C, 22.99; H, 2.57; N, 17.87. Found C, 23.44; H, 2.69;
N, 17.16. Mechanochemical elimination: 39 mg (0.1 mmol) of
[H2pz]2[PdCl4] 7 was forcefully ground with 22 mg (0.2 mmol) of
t-BuOK forming a grey polycrystalline powder, which was dried
in vacuo to remove excess t-butanol. Microanalytical data (%),
Calculated for [PdCl2(C3H4N2)2] + 2 KCl: C, 15.58; H, 1.74; N,
12.11. Found C, 15.29; H, 1.95; N, 11.98.
[Pt(Hpz)4]Cl2 (12). Using K2PtCl4 and pyrazole (1:4): 208 mg
(0.5 mmol) of K2PtCl4 were forcefully ground with 136 mg
(2 mmol) of pyrazole, forming an unknown phase + 2 KCl as a
pink polycrystalline powder. Microanalytical data (%), Calculated
for [Pt(C3H4N2)4]Cl2 + 2 KCl: C, 20.97; H, 2.35; N, 16.30. Found
C, 21.04; H, 2.76; N, 16.28. Using PtCl2 and pyrazole (1:4): 133 mg
(0.5 mmol) of PtCl2 and 136 mg (2 mmol) of pyrazole were
forcefully ground in an agate mortar, resulting in the formation
of 12 as a beige polycrystalline powder. The reaction is slow and
takes about a week to reach completion whilst standing at room
temperature. Microanalytical data (%) for [(C3H4N2)4PtCl2]·H2O,
Calculated: C, 25.91; H, 3.26; N, 20.14. Found C, 25.69; H, 3.07;
N, 19.63.
[H2pz]2[PtCl6]·2H2O. A small amount of dark brown 10 was
dissolved in 5 ml of concentrated HCl solution. This was allowed
to evaporate slowly at room temperature and after a few days
yellow prismatic crystals were obtained.
[Pd(Hpz)4]Cl2 (9). Using K2PdCl4 and pyrazole (1:4): 163 mg
(0.5 mmol) of K2PdCl4 was forcefully ground with 136 mg
(2 mmol) of pyrazole forming 9 + 2 KCl as off-white poly-
crystalline powder. Microanalytical data (%), Calculated for
[Pd(C3H4N2)4]Cl2 + 2 KCl: C, 24.07; H, 2.69; N, 18.71. Found
C, 23.87; H, 2.78; N, 18.18. Using PdCl2 and pyrazole (1:4):
89 mg (0.5 mmol) of PdCl2 and 136 mg (2 mmol) of pyrazole were
forcefully ground in an agate mortar, resulting in the formation
of 9 as a grey polycrystalline powder. The reaction is slow
and takes a few weeks to reach completion whilst standing at
room temperature. Microanalytical data (%) for [Pd(C3H4N2)4]Cl2
Calculated: C, 32.05; H, 3.59; N, 24.92. Found C, 32.24; H, 3.64;
N, 25.23.
X-ray powder diffraction analysis
All crystalline phases were analysed at room temperature by
powder X-ray diffraction on a Bruker D8 diffractometer using
Cu-Ka X-radiation. In all the cases the experimental pattern
matches that calculated on the basis of the single crystal structure
determination at room temperature (if known). The powder
patterns of the products of mechanochemical eliminations show
an extra peak at 2q = 28◦ due to the presence of KCl.
A model of [H2im]2[PtCl4] 5, was created from the crystal struc-
ture of [H2im]2[PdCl4] 1 by replacing palladium with platinum.
The positions of the chlorine atoms were allowed to vary, but
those of the carbon, nitrogen and hydrogen atoms relative to
each other were fixed (the position of platinum is constrained
by symmetry). The isotropic displacement parameter was fixed at
reasonable values for all atoms. Refinement was carried out using
the FullProf suite of programmes32 using Gaussian peak profiles
in the range from 10-40◦ in 2q.
[H2pz]2[PtCl4] (10). Mechanochemical synthesis using PtCl2:
266 mg (1 mmol) of PtCl2 and 209 mg (2 mmol) of pyrazolium
chloride were ground in an agate mortar leading to the formation
of a dark brown polycrystalline powder. Microanalytical data (%)
for [C3H5N2]2[PtCl4], Calculated: C, 15.17; H, 2.12; N, 11.79.
Found C, 15.15; H, 2.14; N, 11.34. Mechanochemical synthesis
using K2PtCl4: 208 mg (0.5 mmol) of K2PtCl4 and 105 mg
(1 mmol) of pyrazolium chloride were ground in an agate mortar.
This resulted in the formation of an orange polycrystalline
powder. Microanalytical data (%) for [C3H5N2]2[PtCl4] + 2 KCl,
Calculated: C, 11.55; H, 1.61; N, 8.98. Found C, 11.28; H, 1.47;
N, 8.53.
The powder pattern of 10 was indexed using the program
DASH.33 Rigid models of the [PtCl4]2- anion (with a geometry
taken from the structure of [H2pz]2[PtCl6]·2H2O by removal of two
chloride atoms from the anion) and a pyrazolium cation (from
the same source) were then placed in the unit cell by simulated
annealing using the same program. Final refinement was carried
out using FullProf as above; the position of the platinum atom
is again constrained by symmetry, but the chloride atoms were
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allowed to move freely. The location of the pyrazolium cation was
fixed.
6 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647–678.
7 G. Kaupp, CrystEngComm, 2003, 5, 117–133.
8 V. P. Balema, J. W. Wiench, M. Pruski and V. K. Pecharsky, Chem.
Commun., 2002, 1606–1607.
9 A. Orita, L. S. Jiang, T. Nakano, N. C. Ma and J. Otera, Chem.
Commun., 2002, 1362–1363.
X-ray single crystal analysis
X-ray data for 8, b-11 and [H2pz]2[PtCl6]·2H2O were collected
at 100 K on a Bruker APEX diffractometer using Mo-Ka
X-radiation. Data for 8 were corrected for absorption using
empirical methods (SADABS)34 based upon symmetry-equivalent
reflections combined with measurements at different azimuthal
angles. The crystals used for the structural determinations of b-11
and [H2pz]2[PtCl6]·2H2O were both twinned; the components were
identified using the program “cell now” and integrated and merged
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solved and refined against all F2 values using the SHELXTL suite
of programs.35 Non-hydrogen atoms were refined anisotropically
and hydrogen atoms were placed in calculated positions refined
using idealized geometries (riding model) and assigned fixed
isotropic displacement parameters.
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Thermogravimetric analysis
Thermal analysis of the samples was carried out on a Perkin-
Elm◦er STA 6000 simultaneous thermal analyzer between 50 and
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Pyris thermal analysis software. TGA traces are shown in the
Supporting Information.†
Acknowledgements
27 E. Ciesielska, A. Szulawska, K. Studzian, J. Ochocki, K. Malinowska,
K. Kik and L. Szmigiero, J. Inorg. Biochem., 2006, 100, 1579–1585.
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We thank the Universities of Bristol and Bayero for support
and Professor Dario Braga of the Institute of Advanced Studies,
University of Bologna for hospitality (to AGO).
29 Z. Xu, J. Oxgaard and W. Goddard, Organometallics, 2008, 27, 3770–
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30 W. Zawartka, A. Gniewek, A. M. Trzeciak, J. J. Zio´lkowski and J.
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