Thermal solid state synthesis of coordination complexes from hydrogen bonded precursors

Article abstract of DOI:10.1039/b501555c

Thermal dehydrochlorination of crystalline 4-picolinium salts of [PtCl ₄]^2- and [PdCl₄]² leads to formation of trans-[MCl₂(4-picoline)₂] (M = Pt, Pd). The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501555c

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Thermal solid state synthesis of coordination complexes from hydrogen
bonded precursors{
Christopher J. Adams, Paul C. Crawford, A. Guy Orpen,* Thomas J. Podesta and Benjamin Salt
Received (in Cambridge, UK) 3rd February 2005, Accepted 8th March 2005
First published as an Advance Article on the web 18th March 2005
DOI: 10.1039/b501555c
resembles the structure of [4,49-H2bipy][MCl4] (M 5 Pt, Pd)1 in
Thermal dehydrochlorination of crystalline 4-picolinium salts
of [PtCl4]22 and [PdCl4]22 leads to formation of trans-[MCl2(4-
that it consists of (close-packed) neutral, linear ribbons composed
of anions which act as chelating hydrogen bond acceptors
(through participation in supramolecular synthon4 A) to two
pyridinium groups on trans edges of the square planar anion
(see Fig. 2). In the ribbons in 2 the pyridinium groups interact
through Me…Me contacts between 4-picolinium ions rather than
through the covalent central C–C bond of the bipyridinium ion
seen in the structure of [4,49-H2bipy][PtCl4].1 In contrast, in 1,
synthon A is formed on two cis edges of the dianion (see Fig. 1).
picoline)2] (M 5 Pt, Pd).
We have developed and applied a strategy for synthesis of novel
crystal structures based on two components.1 In this approach, the
molecular components of the product crystalline salts are complex
ions – typically anionic metal complexes and organic cations. The
peripheral functional groups of these components are chosen to
control their supramolecular interactions; in particular the metal
complexes are typically functionalised with hydrogen bond
acceptor groups such as halides and the cations with protonated
nitrogen bases such as pyridines. The resultant pyridinium
perchlorometallate crystal structures therefore contain NH…Cl–
M interactions in well defined (and in favourable cases pre-
planned) arrangements. Here we report the solid state reactivity of
such phases. Reactivity studies in crystals are at the heart of the
history of crystal engineering2 and have received sustained and
focused attention from a relatively small number of groups,
primarily in solid state organic and organometallic chemistry.3
Whilst there is a wide variety of methods for the preparation of
metal complexes of, for example, pyridine ligands, the vast
majority require solution phase chemistry and as a result are at the
mercy of the essentially isotropic environment that this provides
for molecular reaction chemistry. We have sought to explore the
possibility of eliminating HCl from crystalline solids such as
pyridinium salts of chlorometallates, thereby obtaining access to
coordination complexes, by heating precursor hydrogen bonded
systems as shown in Scheme 1. Herein we report such a study
in which this concept is demonstrated by its application to
the 4-picolinium salts of the platinum(II) and palladium(II)
tetrachloride dianions.
Thermogravimetric analysis of the [4-picolinium]2[MCl4] (1,
M 5 Pt; 2, M 5 Pd) salts showed loss of two equivalents of HCl in
both cases." The product of this thermal process was identified by
heating 1 and 2 under N2 for ca. 90 minutes at 160 uC.{ In the
case of 1, the main product of heating for a shorter period has
a stoichiometry resulting from loss of just one equivalent of HCl,
[4-picolinium][PtCl3(4-picoline)] (3).
The crystal structure of 3 was determined§ from a sample
recrystallised from dichloromethane/hexane solution. As shown in
Scheme 1 Dehydrochlorination of a chlorometallate pyridinium salt to
afford a metal pyridine complex.
The 4-picolinium salts of [PtCl4]22 and [PdCl4]22 (1 and 2
respectively) were prepared by treatment of benzyltriphenylphos-
phonium salts of the anions with 4-picolinium tetrafluoroborate in
dichloromethane solution.{ The crystal structures of the salts
(Figs. 1 and 2) show that they have different geometries.§ That of 2
Fig. 1 Structure of [4-picolinium]2[PtCl4] (1).
{ Electronic supplementary information (ESI) available: details of
syntheses of 1–5. See http://www.rsc.org/suppdata/cc/b5/b501555c/
Fig. 2 Structure of [4-picolinium]2[PdCl4] (2).
Chem. Commun., 2005, 2457–2458 | 2457
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(ii) The (apparent) simplicity of this observation implies that
this may be a general phenomenon.
(iii) The availability of a wide range of ‘‘crystal engineered’’ salts
of anionic metal halide complexes of pyridinium and other
protonated nitrogen bases offers the prospect of applying this and
related reactions under the control of crystalline environments.
Christopher J. Adams, Paul C. Crawford, A. Guy Orpen,*
Thomas J. Podesta and Benjamin Salt
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS
Fig. 3 Structure of [4-picolinium][PtCl3(4-picoline)] (3).
Notes and references
{ Syntheses of 1–5. Details are given in Electronic Supplementary
Information. Salts 1 and 2 were prepared by treatment of the
benzyltriphenylphosphonium salt of the anion with two equivalents of
4-picolinium tetrafluoroborate in dichloromethane solution. Thermolysis
reactions were carried out under N2 at 160 uC for 40 minutes for 3 and
90 minutes for 4 and 5. All compounds gave satisfactory elemental analyses
except crude 3 as recovered from the thermolysis reaction (because of
contamination with 1 and 4).
Fig. 3, 3 has one picoline which is coordinated directly to the Pt(II)
centre and a second which is protonated and forms an interaction
of form A with the resultant [PtCl3(4-picoline)]2 anion. The
powder diffraction pattern of the reaction product prior to
recrystallisation indicates that phase 3 is formed during the
thermal treatment of 1 and is the main component of the crude
product (which contains some starting material 1 as well as 4). The
formation of highly crystalline 3 in this reaction is interesting and
implies phase reconstruction after the first dehydrochlorination.
The mechanism of this reaction deserves investigation.
§ Crystal structure analyses of 1, 2, 3 and 5: Crystal data: [4-
picolinium]2[PtCl4] (1), C12H16Cl4N2Pt, M 5 525.16, triclinic, space group
¯
P1 (no. 2), a 5 7.430(6), b 5 8.401(6), c 5 14.686(11) A, a 5 102.459(11),
˚
˚
3
b 5 91.078(11), c 5 114.901(11)u, U 5 805.7(10) A , Z 5 2, m 5
9.358 mm21
picolinium]2[PdCl4] (2), C12H16Cl4N2Pd, M 5 436.47, monoclinic, space
,
T
5
173, 3653 unique data, R1
5 0.0255. [4-
Complete dehydrochlorination of 1 and 2 is readily achieved in
bulk by more prolonged treatment under N2 at 160 uC. The
products, trans-[MCl2(4-picoline)2] (M 5 Pt, 4, M 5 Pd, 5), were
characterised by elemental analysis and powder and single crystal
X-ray diffraction§ (the latter on a sample of 5 recrystallised from
dichloromethane/hexane solution). While the molecular structures
of 4 and 5 are very similar (see Fig. 4), the crystal structures of 4
(which is known5) and 5 are substantially different. They crystallise
in the same space group and have the long molecular axis
approximately parallel to the unit cell c axis, which is itself
the longest of the cell dimensions and approximately equal to the
length of the molecules (from methyl to methyl). However the
packing is significantly different as reflected in the cell dimensions.§
˚
group C2/m (no. 12), a 5 10.592(3), b 5 11.311(3), c 5 7.4277(17) A,
b 5 112.81(2)u, U 5 820.3(4) A , Z 5 2, m 5 1.770 mm21, T 5 173, 986
3
˚
unique data, R1 5 0.0245. [4-picolinium][PdCl3(4-picoline)] (3),
C12H15Cl3N2Pd, M 5 488.70, monoclinic, space group P21/n (no. 14),
˚
a 5 9.5223(19), b 5 16.206(3), c 5 9.6271(19) A, b 5 102.73(3)u,
U 5 1449.1(5) A , Z 5 4, m 5 10.219 mm21, T 5 100, 3330 unique data,
3
˚
R1 5 0.0250. trans-[PdCl2(4-picoline)2] (5), C12H14Cl2N2Pd, M 5 363.55,
monoclinic, space group P21/n (no. 14), a 5 10.461(2), b 5 3.9775(8),
3
21
˚
˚
c 5 15.911(3) A, b 5 98.73(3)u, U 5 654.4(2) A , Z 5 2, m 5 1.804 mm
,
T 5 100, 1503 unique data, R1 5 0.0252. All hydrogen atoms were located
in difference maps and included in idealised positions. Bulk samples of 1–5
gave powder diffraction patterns consistent with the structures determined
by single crystal methods. CCDC 263081–263084. See http://www.rsc.org/
suppdata/cc/b5/b501555c/ for crystallographic data in CIF or other
electronic format.
" Thermogravimetric analysis of the [4-picolinium]2[MCl4] salts (1, M 5 Pt;
2, M 5 Pd) was carried out on a Netzsch STA 409EP DSC/TGA
instrument under N2 flow for temperature range 20–400 uC at a heating
rate of 5.0 K/min. Salt 1 showed a mass loss of 14% (cf. 13.9% calculated
for loss of 2 HCl) between 140 and 240 uC while 2 showed a mass loss of
16% between 120 and 190 uC (cf. 16.7% calculated for loss of 2 HCl).
1 G. R. Lewis and A. G. Orpen, Chem. Commun., 1998, 1873; A. L. Gillon,
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Fig. 4 Structure of trans-[PtCl2(4-picoline)2] (4).
Complexes 4 and 5 are coordination complexes of platinum(II)
and palladium(II) that are accessible by conventional solution
phase routes4 but which are here prepared by apparently direct
routes in the solid phase. The different course of the reactions for 1
and 2 implies that their crystal structures affect their reactivity, as is
required if solid state structure is to confer selectivity on such
reactions.
In conclusion we note that:
(i) The hydrogen-bonded complex salts 1 and 2 can be
readily transformed into coordination complexes by thermal
dehydrochlorination.
4 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311;
G. R. Desiraju, Chem. Commun., 1997, 1475.
5 C. Tessier and F. D. Rochon, Inorg. Chim. Acta, 1999, 295, 25.
2458 | Chem. Commun., 2005, 2457–2458
This journal is ß The Royal Society of Chemistry 2005