The first salt of an isolated pentachloroplumbate(II) trianion

Article abstract of DOI:10.1107/S010827010600429X

The title compound, tris(cyclohexane-1,2-diamine-κ²N, N′)-cobalt pentachloroplumbate sesquihydrate, [Co(C₆H ₁₄N₂)₃]-[PbCl₅]·1.5H ₂O, crystallizes in the monoclinic space group C2/c, with a tricationic cobalt complex, a pentachloroplumbate trianion, one water molecule in a general position and a second water molecule on a crystallographic twofold axis. The compound is the first example of an isolated [PbCl₅] ^3- moiety; the Pb atom is coordinated in a square-pyramidal fashion, with four longer bonds to Cl atoms in the basal plane and a shorter distance to the apex. The ionic constituents and the solvent molecules form a three-dimensional network of hydrogen bonds.

Full text of DOI:10.1107/S010827010600429X

metal-organic compounds
isolated [PbCl5]3 (Fig. 2) anions. To the best of our knowl-
edge, the latter moiety has not yet been documented in the
solid. Earlier reports on `pentachloroplumbates' do not refer
to structures with isolated [PbCl5]3 building blocks but rather
to compounds with chains of corner-sharing octahedra
(Mousdis et al., 1998; Ng, 2000). We note that the formation of
(I) necessarily competes with reprecipitation of lead(II)
chloride, which is only sparingly soluble in cold water.
Acta Crystallographica Section C
Crystal Structure
Communications
ISSN 0108-2701
The first salt of an isolated penta-
chloroplumbate(II) trianion
Irmgard Kalf and Ulli Englert*
Institut fur Anorganische Chemie, RWTH Aachen, Landoltweg 1, 52074 Aachen,
È
Germany
Correspondence e-mail: ullrich.englert@ac.rwth-aachen.de
Received 5 December 2005
Accepted 3 February 2006
Online 11 March 2006
The title compound, tris(cyclohexane-1,2-diamine-ꢀ2N,N0)-
cobalt pentachloroplumbate sesquihydrate, [Co(C6H14N2)3]-
[PbCl5]Á1.5H2O, crystallizes in the monoclinic space group
C2/c, with a tricationic cobalt complex, a pentachloroplumbate
trianion, one water molecule in a general position and a
second water molecule on a crystallographic twofold axis. The
compound is the ®rst example of an isolated [PbCl5]3 moiety;
the Pb atom is coordinated in a square-pyramidal fashion, with
four longer bonds to Cl atoms in the basal plane and a shorter
distance to the apex. The ionic constituents and the solvent
molecules form a three-dimensional network of hydrogen
bonds.
Fig. 2 shows that our pentachloroplumbate anion adopts an
essentially square-pyramidal geometry, with a shorter apical
bond and four longer coordination distances to the chloro
substituents in the tetragonal plane (Table 1). The metal atom
Ê
lies 0.0368 (4) A above this plane, in the direction of the apical
ligand. Building blocks of an extended solid cannot be
considered `isolated' in a strict sense. In the present case,
however, this description of the anion as being `isolated'
rather than being part of an extended moiety is justi®ed;
additional neighbors of the central Pb atom are located at
much longer distances and involve two C-bonded H atoms at
Ê
3.287 (3) and 3.485 (3) A, as well as a water O atom at
Comment
Pentachlorometallates containing isolated [MCl5]n anions
have been documented for both main group (M = In, Tl, Ge,
Sn and Sb) and transition metals (M = V, Mn, Fe, Cu, Pt and
Hg). For these elements, the Cambridge Structural Database
(CSD; Version of August 2005; Allen, 2002) contains at least
one error-free example, without disorder, for which three-
dimensional atomic coordinates are available. The prototypic
[PbCl5]3 anion, presumably a simple building block, has not
yet been structurally characterized; we communicate here the
synthesis and ®rst X-ray diffraction study of such a penta-
chloroplumbate. In the context of a systematic comparison of
homo- and heterochiral crystals, we performed a cocrystalli-
zation experiment on tris(trans-cyclohexane-1,2-diamine)-
cobalt trichloride, [Co(chxn)3]Cl3, and PbCl2 in aqueous
solution. The tricationic cobalt species represented the
enantiomeric mixture of the thermodynamically stable
isomers, namely the Á con®gured complex of the (R,R)-
diamine and its Ã-(S,S) mirror image (Kalf et al., 2002). From
a solution containing both starting materials, a red crystalline
product precipitates according to equation (1) below.
Figure 1
A displacement ellipsoid plot [PLATON (Spek, 2003); 50% probability,
with H atoms of arbitrary radii] of the cation and the solvent water
molecules in (I). The solvent molecule associated with atom O2 is located
on a crystallographic twofold axis in space group C2/c.
Single-crystal X-ray diffraction analysis showed that the
solid (I) contains the expected complex cations (Fig. 1) and
Acta Cryst. (2006). C62, m129±m131
DOI: 10.1107/S010827010600429X
# 2006 International Union of Crystallography m129

metal-organic compounds
Ê
3.5487 (17) A. Solid (I) contains a cation, an anion, a water
short report with the negative outcome of a closely related
experiment. Our new pentachloroplumbate (I) exists in the
form of racemic crystals. Their formation is obviously
preferred over the statistically less popular (Jacques et al.,
1994) crystallization alternative, namely a conglomerate,
whereas the nitrate of the same cationic complex shows
spontaneous resolution (Morooka et al., 1991). We therefore
expected that the enantiomerically pure cobalt complex as a
starting material would enforce a chiral space group and hence
an independent second example of a solid of the chemical
composition [Co(chxn)3]PbCl5, with the pentachloroplumbate
in a different packing environment. However, when aqueous
solutions of enantiomerically pure [Co(chxn)3]Cl3 and PbCl2
were mixed and evaporated under the same conditions, no
precipitation of an analogous, necessarily homochiral,
compound was observed; after prolonged evaporation, PbCl2
reprecipitated. We conclude that the competition between
(I) and lead chloride is a close one, and that the formation
of further pentachloroplumbate derivatives will not be
trivial.
molecule in a general position of the space group C2/c and an
additional water molecule on the twofold crystallographic
axis; the compound is a sesquihydrate.
All potential hydrogen-bond donors, viz. six NH and three
symmetrically independent OH groups, are involved in
moderate-to-weak hydrogen bonding, the acceptor atoms
being either the plumbate Cl or the hydrate O atoms. Table 2
summarizes the hydrogen-bond geometry.
To which structures should the new [PbCl5]3 anion be
compared? The apical PbIIÐCl distance is almost as short as
those in chloro complexes of PbIV (Olafsson et al., 2000;
Cashin et al., 2002), whereas the longer bonds in the base
closely match the mean PbÐCl distance for more than 200
structures in the CSD; for 202 error-free observations of PbÐ
Ê
Cl distances, the mean value is 2.873 A and the median is
Ê
2.864 A. The distances and angles in the structurally char-
acterized pentachloroantimonates cover a wide range. Several
[SbCl5]2 groups in the literature (Zaleski & Pietraszko, 1995;
Li et al., 1995; Derwahl et al., 1996; Ohta & Yamashita, 1997)
not only adopt a square-pyramidal geometry but also match
the metal±halide distance pattern of one shorter apical and
four longer bonds. However, in these antimonates, the metal
atom and the apical ligand are situated on opposite faces of
the square plane, whereas in our pentachloroplumbate, the
metal is slightly displaced towards the apex.
In crystalline (I), loss of crystal water occurs at 443 K and
decomposition starts at ca 553 K. We have no direct evidence
about the species present in solution, but 207Pb NMR data
provide some insight; an aqueous solution of tris(cyclo-
hexanediamine)cobalt pentachloroplumbate shows a signal at
2224 p.p.m. This single resonance shifts to 1902 p.p.m.
upon addition of LiCl (5 equivalents), i.e. when the Cl/Pb ratio
is increased. Under the same conditions, a saturated solution
of PbCl2, with a lower Cl/Pb ratio, shows a resonance at
2383 p.p.m. The `hydrated Pb2+ cation' is found at ca
2800 p.p.m. (Harrison et al., 1983). These numbers, which
will surely depend on concentration or supersaturation and
should not be overinterpreted, are in agreement with the
presence of Pb2+ cations with variable numbers of solvent
water molecules and chloride ions in the coordination envir-
onment.
Experimental
From a solution containing [Co(chxn)3]Cl3Á2H2O and PbCl2, the red
crystalline product precipitates according to equation (1) (see
Comment). [Co(chxn)3]Cl3Á2H2O (0.5 mmol, 0.272 g) [for the synth-
esis and structure of this compound, see Kalf et al. (2002)] and PbCl2
(0.5 mmol, 0.139 g) were dissolved separately in water (10 ml each) at
318 K. The orange-coloured mixture was stirred for 30 min at 318 K
and then allowed to cool to room temperature; after 2 d, precipitation
of large red crystals of [Co(chxn)3]PbCl5Á1.5H2O began (yield
89%). The saturated solution should not be exposed to low
temperatures; cooling of a saturated solution of (I) to 279 K resulted
in the formation of PbCl2. Analysis calculated for C36H90Cl10Co2-
N12O3Pb2: C 26.59, H 5.58, N 10.34%; found: C 26.23, H 5.98, N
10.17%.
Crystal data
[Co(C6H14N2)3][PbCl5]Á1.5H2O
Mo Kꢂ radiation
Cell parameters from 6549
re¯ections
Mr = 812.97
Monoclinic, C2=c
ꢃ = 2.5±29.8ꢀ
Ê
a = 27.066 (2) A
b = 12.4657 (11) A
1
Ê
ꢄ = 6.83 mm
T = 110 (2) K
Ê
c = 21.3066 (19) A
ꢁ = 125.889 (1)ꢀ
Ê
V = 5824.0 (8) A
Fragment, red
0.39 Â 0.28 Â 0.17 mm
3
Originally, our interest in homo- and heterochiral crystals
had induced us to perform reaction (1), and we conclude our
Z = 8
Dx = 1.854 Mg m
3
Data collection
Bruker SMART CCD area-detector
diffractometer
! scans
Absorption correction: analytical
(PLATON; Spek, 2003)
Tmin = 0.084, Tmax = 0.357
42820 measured re¯ections
8402 independent re¯ections
6901 re¯ections with I > 2ꢅ(I)
Rint = 0.066
ꢃmax = 30.0ꢀ
h = 37 ! 38
k = 17 ! 17
l = 29 ! 29
Re®nement
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.026
wR(F2) = 0.048
S = 1.01
8402 re¯ections
294 parameters
w = 1/[ꢅ2(F2o) + (0.007P)2]
where P = (F2o + 2Fc2)/3
(Á/ꢅ)max = 0.001
3
Ê
Figure 2
A displacement ellipsoid plot [PLATON (Spek, 2003); 50% probability]
of the pentachloroplumbate anion in (I).
Áꢆmax = 1.46 e A
3
Ê
1.19 e A
Áꢆmin
=
ꢁ
m130 Kalf and Englert [Co(C6H14N2)3][PbCl5]Á1.5H2O
Acta Cryst. (2006). C62, m129±m131

metal-organic compounds
Table 1
Selected geometric parameters (A, ).
structure: SHELXL97 (Sheldrick, 1997); molecular graphics:
PLATON (Spek, 2003); software used to prepare material for
publication: SHELXL97.
ꢀ
Ê
Pb1ÐCl3
Pb1ÐCl1
Pb1ÐCl2
2.6211 (7)
2.8241 (7)
2.8677 (7)
Pb1ÐCl5
Pb1ÐCl4
2.8748 (8)
2.8863 (7)
This work was supported by Deutsche Forschungsgemein-
schaft (`Racemic Crystals and Conglomerates'). The authors
thank Professor Dr G. E. Herberich for helpful discussions.
Cl3ÐPb1ÐCl1
Cl3ÐPb1ÐCl2
Cl1ÐPb1ÐCl2
Cl3ÐPb1ÐCl5
Cl1ÐPb1ÐCl5
90.01 (2)
98.06 (2)
86.51 (2)
88.76 (2)
88.56 (2)
Cl2ÐPb1ÐCl5
Cl3ÐPb1ÐCl4
Cl1ÐPb1ÐCl4
Cl2ÐPb1ÐCl4
Cl5ÐPb1ÐCl4
171.57 (2)
87.25 (2)
166.97 (2)
81.27 (2)
104.11 (2)
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: AV1278). Services for accessing these data are
described at the back of the journal.
Table 2
Hydrogen-bond geometry (A, ).
ꢀ
Ê
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ꢁ
Acta Cryst. (2006). C62, m129±m131
Kalf and Englert
[Co(C6H14N2)3][PbCl5]Á1.5H2O m131