I ... π halogen-bonding in the structures of platinum-alkynyl compounds: An overlooked supramolecular interaction

Article abstract of DOI:10.1039/b506025g

The crystal structures of both a symmetric and an unsymmetric platinum-alkynyl compound with terminal iodophenyl functionalities have been determined, and found to exhibit an attractive interaction between the iodine atom and the carbon-carbon triple bond of the alkynyl ligand. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506025g

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C O M M U N I C A T I O N
I · · · p halogen-bonding in the structures of platinum–alkynyl
compounds: an overlooked supramolecular interaction†
Christopher J. Adams* and Lucy E. Bowen
School of Chemistry, University of Bristol, Bristol, UK BS8 4QQ. E-mail: chcja@bris.ac.uk
Received 29th April 2005, Accepted 27th May 2005
First published as an Advance Article on the web 9th June 2005
The crystal structures of both a symmetric and an unsym-
metric platinum–alkynyl compound with terminal iodo-
phenyl functionalities have been determined, and found to
exhibit an attractive interaction between the iodine atom
and the carbon–carbon triple bond of the alkynyl ligand.
such a non-centrosymmetric motif. The compounds chosen for
investigation were platinum–alkynyl systems of the form trans-
Pt(PEt3)2(C≡C-p-C6H4–I)(C≡C-p-C6H4–R), where the donor
groups R are NO2 (3) or CN (4), and in which the donor and
acceptor ends of the desired synthon are located at 180◦ to each
other on the ends of arylalkynyl ligands (Fig. 2). These systems,
although unknown, were expected to be stable and synthetically
accessible, and have a linear, relatively two-dimensional shape
which thus mimics 4-iodonitrobenzene and 4-iodobenzonitrile,
respectively. They also contain no other groups that seemed
likely to become involved in strong intermolecular interactions
that could prevent the formation of the one-dimensional pattern
demonstrated by their organic analogues.
In recent years there has been a surge of interest in the use
of intermolecular interactions to control crystal packing, the
field of crystal engineering. With small organic molecules it is
possible to do this by placing donor and acceptor groups at
opposite ends of a molecule, the resulting crystal structure often
displaying an interaction between the two. The orientation
of the molecule in the solid state is such that this interaction
is maximised, and as a result the packing of the molecule is
influenced. An example of this is shown in Fig. 1, which is part of
the crystal structure of 4-iodobenzonitrile.1 There is a lone pair
of electrons on the nitrogen atom, and a relatively large, low-
energy C–I r* orbital protruding from the iodine atom, opposite
the carbon–iodine bond. A donor–acceptor interaction between
the two results in the formation of a (partial) dative bond,
causing the molecules to form a chain in the solid state. Pyridine
groups also commonly act as the donor moiety.2 A second type
of interaction may also be present, of the polarisation induced
dipole–dipole type. Here, the electron density of the donor
species causes polarisation of the electron density on the iodine
atom, thus inducing a temporary positive dipole there which is
attracted to the negative dipole of the donor.3 This mechanism
is responsible for the iodo–nitro interaction seen, for example,
in the structure of 4-iodonitrobenzene, another case where the
intermolecular donor–acceptor interaction creates linear chains
of molecules.4 The relative magnitudes of the two interactions
depend upon the precise system in question, but the two
complement each other and are often discussed together under
the name of ‘halogen bonding’.
Fig. 2 Target bis-alkynyl compounds: R = NO2 3, CN 4 or I 5.
The strategy adopted for the synthesis of trans-
Pt(PEt3)2(C≡C-p-C6H4–I)(C≡C-p-C6H4–R) (R = NO2 3, CN
4) was based upon the known methodology for the synthesis
of similar symmetric bis-alkynyl compounds. Starting from
trans-Pt(PEt3)2I2, a copper(I) iodide catalysed reaction with
1.05 equivalents of H–C≡C-p-C6H4–R (R = NO2 or CN) in
the presence of base generates the mono-alkynyl compounds
trans-Pt(PEt3)2(C≡C-p-C6H4–R)I (R = NO2 1, CN 2) in good
yield, as well as a small amount of the bis(alkynyl) compounds
trans-Pt(PEt3)2(C≡C-p-C6H4–R)2, and some unreacted diiodo
starting material; the product distribution shows that replace-
ment of the first iodide at the platinum centre is faster than
the second. The three species are readily separated by column
chromatography, and synthetic, spectroscopic and analytical
details for all compounds are given as ESI.† Further reaction
of 1 or 2 with H–C≡C-p-C6H4–I under the same conditions
generates 3 or 4, respectively. This represents the first synthesis of
unsymmetrical platinum bis-alkynyl compounds from the parent
alkynes; at no point has any evidence for any exchange of alkynyl
ligands been observed. The crystal structure of 2 was determined
by X-ray diffraction, and is reported in the ESI.†
Fig. 1 Chains of molecules within the structure of 4-iodobenzonitrile;
iodine atoms are purple and nitrogen atoms are blue.
Structural analysis‡ of crystals of 3 revealed that there
were significant intermolecular interactions within the crystal
structure, but not of the envisaged I · · · NO2 variety. Rather, the
major supramolecular interaction seen is between the iodine
atom of one molecule and a carbon–carbon triple bond within a
neighbouring molecule. This has the effect of making these two
molecules perpendicular to each other, so the overall effect is to
create a zigzag chain (Fig. 3).
These polar chains lie antiparallel to each other, to give a
centrosymmetric two-dimensional sheet that lies parallel to the
b axis of the cell. There are no short contacts involving the
nitro group within the structure. There is a small amount of
disorder present, with the nitro group being replaced 14% of the
time by an iodine atom as the molecule is reversed within the
lattice. It was not possible to grow crystals of 4 suitable for X-ray
diffraction.
As the interactions outlined above can be used to generate
polar chains of molecules (as in Fig. 1), there has recently been
interest in using them to create non-centrosymmetric crystals.
Many compounds have physical properties (such as NLO
activity) which are enhanced in such an environment, and this
is as true for inorganic as organic species. However, whilst there
have been studies towards the use of halogen-bonding in purely
organic systems,5 there has been only one attempt6 to apply the
effect to metal-containing molecules. Therefore, the aim of the
work herein was to determine whether the principles outlined
above could be applied to organometallic systems to produce
† Electronic supplementary information (ESI) available: Synthetic and
characterisation details for all compounds. See http://www.rsc.org/
suppdata/dt/b5/b506025g/
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 2 3 9 – 2 2 4 0
2 2 3 9
©

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Table 1 Similarities between halogen bonding motifs of terminal
alkynes (left) and aryl iodides (right), showing how the I · · · p interaction
(bottom right) might have been predicted by consideration of the similar
alkynyl · · · p motif
Fig. 3 Solid-state packing of 3: hydrogen atoms and the ethyl groups
of the PEt3 ligands (orange) have been removed for clarity.
In order to establish whether the I · · · p interaction seen
above is a reproducible phenomenon, we next synthesised
Pt(PEt3)2(C≡C-p-C6H4–I)2 5. Crystallographic analysis reveals
that this structure also contains the same type of interaction,
but now occurring symmetrically at both ends of the molecule
(Fig. 4). The sheet of molecules thus formed have their platinum
atoms lying in the (–2 0 4) plane of the cell.
of 5.11 This is emphasised in Table 1, which illustrates known
similarities between the recognition properties of iodo and
alkyne groups and shows how the I · · · p interaction continues
this trend.
To summarise, we report:
(i) the first straightforward synthesis of mono- and unsym-
metric bis-alkynyl platinum compounds.
(ii) Identification of the I · · · p synthon as a halogen-bonding
interaction which mimics the ≡CH · · · p hydrogen bond.
(iii) That the influence of halogen-bonding on organic com-
pounds may be reproduced in the structures of inorganic mimics.
Notes and references
‡ Crystal structure data for 2, 3 and 5: 2: C21H34INP2Pt, M = 684.42,
¯
triclinic, space group P1 (no. 2), a = 8.609(2), b = 14.534(6), c =
◦
˚
19.817(9) A, a = 95.63(3), b = 90.51(3), c = 90.93(4) , U = 2467.1(16)
A , Z = 4, Dc = 1.843 g cm−3, l = 7.074 mm−1, T = 100, 27899
3
˚
reflections collected, 11193 unique data (Rint = 0.0314), R1 = 0.0363.
3: C28H38I1.14N0.86O1.72P2Pt, M = 815.71, monoclinic, space group P21/c
Fig. 4 Solid-state packing of 5: hydrogen atoms and the ethyl groups
of the PEt3 ligands (orange) have been removed for clarity.
◦
˚
(no. 14), a = 9.897(2), b = 28.205(6), c = 10.995(2) A, b = 91.19(3) ,
U = 3068.5(11) A , Z = 4, Dc = 1.765 g cm−3, l = 5.844 mm−1
,
3
˚
Within 3 and 5 the distance between the carbon atoms of the
T = 100, 29713 reflections collected, 7040 unique data (Rint = 0.0306),
R1 = 0.0385. 5: C28H38I2P2Pt, M = 885.41, monoclinic, space group
P21/c (no. 14), a = 9.4839(10), b = 15.0660(14), c = 11.1185(17)
˚
triple bond and the iodo atom is in the order of 3.4–3.7 A,
less than or equal to the sum of their van der Waals’ radii
◦
A, b = 97.685(9) , U = 1574.4(3) A , Z = 2, Dc = 1.868 g cm−3
,
3
˚
˚
˚
(3.68 A). The distance from the centre of the triple bond to
l = 6.533 mm−1, T = 173, 17917 reflections collected, 3618 unique
data (Rint = 0.0209), R1 = 0.0163. CCDC reference numbers 262633–
262635. See http://www.rsc.org/suppdata/dt/b5/b506025g/ for crys-
tallographic data in CIF or other electronic format.
˚
the iodine atom is 3.60 and 3.43 A, respectively, for 3 and
5. Clearly there is a halogen-bonding interaction between the
two, in which the p-system is the electron-donor and polarising
species.
1 CSD refcode IOBNIT01: G. R. Desiraju and R. L. Harlow, J. Am.
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A search of the Cambridge Structural Database7 reveals
that it contains 39 structures containing both a carbon–carbon
triple bond and a terminal carbon–iodine bond. The iodo–p
interaction is present in three of these structures, and is thus not
newly discovered but has merely been overlooked in discussions
of halogen-bonding. Packing within the crystal structure of
1,2-diiodoacetylene8 adopts a similar motif to that seen for
5, reflecting the disposition of iodine atoms and triple-bonds
in both species; equally, the packing of 1-(ethynyl)-4-iodo-3-
(methylaminocarbonyl)benzene9 creates a zigzag similar to 3, as
both compounds contain a single iodo group and a carbon–
carbon triple bond at opposite ends of an approximately linear
molecule. The third example, 1,4-bis(4-tert-butylphenyl)-1,2-
diiodobut-1-en-3-yne,10 has a different arrangement of groups,
which creates a dimeric unit within the crystal.
Similarities between the recognition properties of organic
iodides and terminal alkynes have been reported before,11
and in this respect the iodo · · · p interaction discussed herein
is equivalent to the ≡CH · · · p synthon. As a result of this
equivalence, the structures of the 4-halogenoethynylbenzenes
(apart from the fluoro analogue) display the same motif as 3,12
and the packing of 1,4-diethynylbenzene is the same as that
10 CSD refcode RETRIR: J. Barluenga, I. Llorente, L. J. Alvarez-
Garcia, J. M. Gonzalez, P. J. Campos, M. R. Diaz and S. Garcia-
Granda, J. Am. Chem. Soc., 1997, 119, 6933.
11 J. M. A. Robinson, B. M. Kariuki, K. D. M. Harris and D. Philp,
J. Chem. Soc., Perkin Trans. 2, 1998, 2459.
12 H. C. Weiss, R. Boese, H. L. Smith and M. M. Haley, Chem.
Commun., 1997, 2403.
2 2 4 0
D a l t o n T r a n s . , 2 0 0 5 , 2 2 3 9 – 2 2 4 0