Chlorometallate and palladium cluster complexes of wide-span diimine and diamine ligands

Article abstract of DOI:10.1039/c1dt10861a

Diimine and diamine ligands that are unable to coordinate to a single metal favour the formation of unusual, high-nuclearity Zn chlorometallate and palladium chloride complexes.

Full text of DOI:10.1039/c1dt10861a

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COMMUNICATION
Chlorometallate and palladium cluster complexes of wide-span diimine and
diamine ligands†
John S. Hart, Simon Parsons and Jason B. Love*
Received 8th May 2011, Accepted 10th June 2011
DOI: 10.1039/c1dt10861a
Diimine and diamine ligands that are unable to coordinate
to a single metal favour the formation of unusual, high-
nuclearity Zn chlorometallate and palladium chloride com-
plexes.
Ligands that can promote the assembly of multiple metal cations
into precisely-structured arrangements are highly desirable as
these multimetallic motifs impact on a diverse range of chemical
topics such as catalysis,1 bio-inorganic chemistry,2 medicine,3 and
single molecule magnetism.4 In particular, ligand designs incorpo-
rating 1,4-disubstituted arenes have been intensively investigated
as platforms that favour bi- and multinuclear complex formation
Scheme 1 Synthesis of wide-span diimine (LIm) and diamine (LAm) ligands
and their chlorometallate and cluster complexes. Reagents and conditions:
due to the inability of the two donors to chelate to a single
metal cation.5 For example, imine and amine acyclic ligands,6
marocycles,7 and cryptands8 derived from 1,4-disubstituted arenes
have been developed and show a propensity to form polynuclear
complexes, although in simple systems cyclometallation reactions
at the central aryl ring are observed, particularly with late
transition metals.9
We are interested in developing straightforwardly-synthesised
and inexpensive ligands for the assembly of multinuclear com-
plexes of metals from across the Periodic Table,10 in particular to
access new chemistry that can occur within well-defined cavities
such as found in Pacman complexes,11 and related supramolecular
assemblies.12 We report here on the use of new wide-span diimine
and diamine ligands as outer and inner-sphere coordination plat-
forms for the generation of new, high-nuclearity chlorometallate
and palladium chloride complexes. The use of durene in the
ligand design ensures that undesired cyclometallation reactions
are minimised.
The pro-ligands LIm and LAm were synthesised in high yield
from the readily available precursors 1 and 2 through straightfor-
ward bromide substitution and Schiff-base condensation routes,
respectively (Scheme 1). Borohydride reduction of diimine LImR
proved to be the most reproducible route to diamine LAmR. The
2,6-diisopropylaryl-substituted diimine LImAr and diamine LAmAr
compounds were crystallised from saturated solutions of MeCN
t
(i) H2NR/Ar, MeCN; (ii) H2NR/Ar, MeCN; (iii) NaBH4, MeOH, R = Bu;
(iv) 2 HCl, Et2O, CoCl2; (v) 2 HCl, Et2O, 4 ZnCl2; (vi) 3 PdCl2(MeCN)2,
CH2Cl2.
and their X-ray crystal structures determined (Fig. 1). In the
solid state, both LImAr and LAmAr form extended structures through
intermolecular hydrogen-bonding interactions and display no
solvent incorporation. In LImAr (Fig. 1, top), the imine N1–
˚
C13 bonds (1.236(2) A) adopt an anti-conformation (torsion
angle = 180.0(2)◦) and are offset by 29.8(3)◦ with respect to the
i
central aryl ring. The sterically-demanding Pr substituents on
the terminal aryl groups result in an approximately orthogonal
orientation (72.7(2)◦) of this aryl ring to the imine N C group.
This orientation results in an intermolecular interaction between
the C10 isopropyl methyl hydrogens and the central aryl ring
˚
(C10 ◊ ◊ ◊ C16 = 3.745(4) A), so forming a chain motif in the
extended structure. Reduction of the N C double bond in LImAr
to form LAmAr allows free rotation around the N1–C13 single
˚
bond (1.489(2) A) and results in a different structure in the
solid state (Fig. 1, bottom) in which the N1–C13 bonds adopt
a syn-conformation (torsion angles = 79.9(2)/84.0(2)◦). As with
i
LImAr, the steric demand of the Pr2(C6H3) substituent results in
an approximately orthogonal orientation of this group in relation
to the amine N1–C13 bond (76.1(2)/68.2(2)◦), but in this case
the presence of the amino hydrogen results in a hydrogen bond
˚
between the two amines (N1 ◊ ◊ ◊ N1¢ = 3.264(2) A); p-H interactions
are also seen in the extended structure.
EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black
Building, The King’s Buildings, West Mains Road, Edinburgh, U. K., EH9
3JJ. E-mail: jason.love@ed.ac.uk; Fax: (+) 44 131 6504743
† Electronic supplementary information (ESI) available: Full experimental
data and crystal data. CCDC reference numbers 824566–824571. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10861a
The coordination chemistry of these ligands was investigated
(Scheme 1). Reactions between LIm or LAm and either CoCl2 or
ZnCl2 in THF showed little change in the resonances for L in
1
the H NMR spectra. However, the use of CDCl3 as the NMR
solvent resulted in the crystallisation of the chlorometallate salts
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12025–12027 | 12025
©

located in the difference Fourier map and refined with riding
thermal parameters and bond distance restraints.
In the solid state structure of [H2LAmR][CoCl4] (Fig. 2., top)
the amine appendages adopt a syn-conformation (torsion angle
= 6.5(3)◦) that results in a hydrogen-bonded cavity suited for
chlorometallate recognition (over chloride), with N–H ◊ ◊ ◊ Cl and
C–H ◊ ◊ ◊ Cl hydrogen-bonding interactions ranging in distance
˚
from 3.165(3) to 3.357(3) A. The chelation of the cobaltate anion
by the wide-span ligand causes some strain, as evidenced by the
10◦ curvature of the central diaminoarene and the inclusion of a
molecule of water that interacts both with the protonated amine
˚
˚
(N1 ◊ ◊ ◊ O1 2.817(5) A) and the cobaltate (Cl4 ◊ ◊ ◊ O1 3.165(3) A); as
with the free ligands above, other intra- and intermolecular close
contacts (to CHCl3 solvent of crystallisation) are present in the
extended structure.
In the solid state structure of [H2LImAr][Zn4Cl10] (Fig. 2, bottom),
the protonated imine arms adopt an anti-conformation (torsion
angle = 176.0(2)◦) and the terminal aryl groups, and to a lesser
extent the central arene, are oriented-orthogonally to the N
C
bond (torsion angles = 88.1(3) and 53.4(4)◦, respectively). As
with the cobaltate above, a variety of weak interactions exist
between the doubly-protonated diimine and the zincate, with N–
H ◊ ◊ ◊ Cl and C–H ◊ ◊ ◊ Cl hydrogen bonding interactings ranging
˚
Fig. 1 Ball and stick representations of the X-ray crystal structures of
diimine LImAr (top) and diamine LAmAr (bottom). For clarity, all hydrogen
atoms except those involved in hydrogen bonding are omitted.
in distance from 3.152(3) to 3.759(3) A. However, in this case,
the wide-span ligand framework has favoured the isolation of
an unusual Zn4Cl10 anionic cluster. In this cluster, each zinc
2-
cation is tetrahedral with one terminal chloride and three chlorides
bridging to adjacent zinc cations. This results in a rectangular
arrangement of zinc cations in which two opposing sides of the
rectangle comprise either a single bridging chloride or two bridging
[H2LAmR][CoCl4] and [H2LImAr][Zn4Cl10], presumably as a result of
the presence of adventitious HCl in CDCl3. Scaled-up reactions
were carried out in which the ligands were protonated by HCl
(2 eq) in Et2O prior to the addition of CoCl2 (1 eq) or ZnCl2
(4 eq) in Et2O. This latter procedure resulted in the formation
of analytically pure materials. The X-ray crystal structures of
both [H2LAmR][CoCl4] and [H2LImAr][Zn4Cl10] were determined
(Fig. 2) and show that the protonated ligands interact with
the chlorometallate salts through a series of hydrogen-bonding
interactions; all hydrogens involved in hydrogen-bonding were
˚
chlorides, with Zn ◊ ◊ ◊ Zn distances of 3.8345(5) A and 3.1405(5)
◦
˚
A and Zn–Cl–Zn angles of 114.25(3) and 85.30(2) . Also, the
anti-conformation of the imines allows for an intermolecular
˚
N1 ◊ ◊ ◊ Cl3 hydrogen bond (3.152(3) A) from which a linear chain
of alternately sandwiched ligand and metallate clusters grows.
2-
While Zn4Cl10 anionic clusters have been observed before, to
our knowledge they adopt adamantane or open-chain structural
motifs and not the new rectangular arrangement of Zn cations seen
here.13 Furthermore, metal chloride clusters that adopt a similar
structural motif are rare, and limited primarily to o-phenylene-
3-
metallated Pt4 chlorides,14 Mo4Cl12 clusters,15 and the chlorobis-
muthate and antimonates M4Cl18 (M = Bi, Sb).16 Significantly,
6-
the recognition of high-nuclearity chlorometallates by LIm and LAm
may impact on the high-efficiency, hydrometallurgical extraction
of base and precious metals.17
The reaction between PdCl2(MeCN)2 and either LImAr or LImR
in CH2Cl2 led to the sole formation of the trinuclear palladium
chloride cluster [Pd3Cl6(L)]; this formulation was supported by
elemental analysis. The X-ray crystal structures of [Pd3Cl6(LImAr)]
and [Pd3Cl6(LImR)] were determined (Fig. 3) and were found to be
isostructural; as such, only the former structure will be discussed
(Fig. 3, top). In a similar manner to [H2LAmR][CoCl4], the imine
appendages adopt a syn-conformation (torsion angle = 6(2)◦) that
allows the ligand to chelate to the single Pd3Cl6 cluster through
the imine nitrogens N1 and N2. Also, the terminal and central
aryl rings are similarly tilted with respect to the N C bond by
67(4) and 73(4)◦, respectively. Each Pd cation is square planar but
the Pd3Cl6 cluster is not linear due to the constraints imposed
by the chelating diimine ligand. This feature results in a cradle
Fig. 2 Ball and stick representations of the X-ray crystal structures of
[H2LAmR][CoCl4](H2O)(CHCl3) (top) and [H2LImAr][Zn4Cl10] (bottom). For
clarity, CHCl3 solvent of crystallisation and all hydrogen atoms except
those involved in hydrogen bonding are omitted.
12026 | Dalton Trans., 2011, 40, 12025–12027
This journal is
The Royal Society of Chemistry 2011
©

Notes and references
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Fig. 3 Ball and stick representations of the X-ray crystal structures
of [Pd3Cl6(LImAr)] and [Pd3Cl6(LImR)]. For clarity, disorder components,
solvent of crystallisation, and all hydrogen atoms are omitted.
arrangement in which the Pd1–Pd2–Pd3 angle is 127.4(4)◦ with
Pd(m-Cl)Pd angles of approximately 90◦.
The solid state structure of [Pd3Cl6(LImAr)] is retained in solution.
In the 1H NMR spectrum (Fig. S1, Supplementary information),
only one imine resonance is seen at 8.38 ppm, whereas two distinct
CH3 environments are seen at 3.75 and 2.09 ppm for the central
arene, which reflects the asymmetry of binding to the Pd3Cl6
cluster. Furthermore, free rotation about the N–C bond for the
terminal 2,6-diisopropyl aryl substituent is hindered, as reflected
by the presence of two isopropyl CH resonances at 3.56 and 3.49
ppm and four resonances for the CH3 environments. The 1H NMR
spectrum of [Pd3Cl6(LImR)] is similar (Fig. S2†), indicating that the
solid state structure of this complex is also retained in solution,
but shows only one resonance for the tBu substituent at 1.22 ppm.
Furthermore, the ESI mass spectrum of [Pd3Cl6(LImR)] shows an
ion at m/z 682 for the partially-intact cluster [Pd3Cl5(LImR)]+, and
indicates structural retention in solution.
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High nuclearity complexes of Pd chloride are rare and the
[Pd3Cl6(L)] complexes can be viewed as short fragments of
polymeric (PdCl2)n. While postulated through indirect evidence,18
only a single example of the crystallographic characterisation of a
PdCl2 trimer end-capped by organic ligands has been described;19
in this example, the unusual Pd3Cl6 motif was stabilised through
the use of a large cone-angled, bowl-shaped triarylphosphine.
Trinuclear PdCl structures are also known for allyl-complexes,
although in these cases one chloride has been substituted by
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the allyl ligand.20 The related dianion Pd3Cl8 has also been
2-
characterised structurally as its Bu4N ammonium salt.21 In all of
these previous structures, the Pd3Cl6 motif is linear, in contrast
to the cradle structure seen by us in [Pd3Cl6(L)]. Presumably,
this new bent geometry for the Pd3Cl6 cluster is necessary to
allow a good fit to the wide-span diimine ligands. Furthermore,
it is evident that methyl substitution of the central arene ligand
has ensured that the formation of cyclometallated products is
inhibited.
We thank the University of Edinburgh and the UK EPSRC for
financial support.
21 S. Schwarz, J. Stra¨hle and U. Weisser, Z. Anorg. Allg. Chem., 2002, 628,
2495.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12025–12027 | 12027
©