Self-assembly of organometallic polymers through hydrogen bonding: A diplatinum(IV) complex with a single bridging halide ligand

Article abstract of DOI:10.1016/j.poly.2005.08.020

The reaction of [PtMe₂(bu₂bipy)], bu₂bipy = 4,4′-di-t-butyl-2,2′-bipyridine (1) with RCH₂Br occurs by trans oxidative addition to give the platinum(IV) complexes [PtBrMe ₂(CH₂R)(bu₂bipy)], R = C₆H ₄CO₂H (2); R = C₆H₄CH ₂CO₂H (3) and these reacted with half an equivalent of Ag[PF₆] to give the unusual diplatinum(IV) complexes [{PtMe ₂(CH₂R)(bu₂bipy)}₂(μ-Br)][PF ₆], R = C₆H₄CO₂H (4); R = C ₆H₄CH₂CO₂H (5). These diplatinum(IV) complexes, each of which contains a hydrogen bonding carboxylic acid group, are connected only by an unsupported bridging bromide ligand. In the solid state, complex 4 forms a self-assembled zig-zag polymer through intermolecular hydrogen bonding between pairs of carboxylic acid groups.

Full text of DOI:10.1016/j.poly.2005.08.020

Polyhedron 25 (2006) 266–270
www.elsevier.com/locate/poly
Self-assembly of organometallic polymers through hydrogen bonding:
A diplatinum(IV) complex with a single bridging halide ligand
*
Christopher S.A. Fraser, Dana J. Eisler, Richard J. Puddephatt
Department of Chemistry, University of Western Ontario, Richmond Street, London, Canada N6A 5B7
Received 8 April 2005; accepted 15 August 2005
Available online 7 October 2005
Dedicated to Professor Malcolm Chisholm, master of molecular architecture, in recognition of his outstanding contributions to inorganic chemistry.
Abstract
The reaction of [PtMe2(bu2bipy)], bu2bipy = 4,40-di-t-butyl-2,20-bipyridine (1) with RCH2Br occurs by trans oxidative addition to
give the platinum(IV) complexes [PtBrMe2(CH2R)(bu2bipy)], R = C6H4CO2H (2); R = C6H4CH2CO2H (3) and these reacted with half
an equivalent of Ag[PF6] to give the unusual diplatinum(IV) complexes [{PtMe2(CH2R)(bu2bipy)}2(l-Br)][PF6], R = C6H4CO2H (4);
R = C6H4CH2CO2H (5). These diplatinum(IV) complexes, each of which contains a hydrogen bonding carboxylic acid group, are con-
nected only by an unsupported bridging bromide ligand. In the solid state, complex 4 forms a self-assembled zig-zag polymer through
intermolecular hydrogen bonding between pairs of carboxylic acid groups.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Platinum; Self-assembly; Polymer; Hydrogen bond
1. Introduction
tetramethylplatinum(IV) compounds [9], the organoplati-
num(IV) products, 2 and 3 of Scheme 1, that contain a car-
boxylic acid functional group are stable compounds. In the
solid state, they form dimers by self-assembly as illustrated
for complex 2 in Scheme 1 [6].
In order to form polymers by self-assembly, it is neces-
sary to have at least two hydrogen-bonding donor/acceptor
pairs per molecule. One way to prepare such ‘‘building
block’’ molecules is to abstract the bromide ligand from
2 or 3 in the presence of an assembling ligand such as
4,40-bipyridine, and the diplatinum(IV) complexes formed
in this way have been shown to self-assemble to give unu-
sual supramolecular polymers [6–8]. This paper reports a
simpler way to prepare diplatinum(IV) complexes by for-
mation of a bridging halide linking group, and shows that
self-assembly can then give a supramolecular polymer.
Hydrogen bonding is widely used in supramolecular
chemistry, such as in crystal engineering and in the design
of molecular materials [1–3]. However, this approach has
natural limitations in organometallic chemistry, since many
alkyl-metal bonds react with the functional groups (e.g.
OH or NH bonds) typically used as hydrogen bond donors
[4]. Although methylplatinum(II) bonds can be cleaved by
carboxylic acids [5], the reactions of [PtMe2(bu2bipy)],
bu2bipy = 4,40-di-t-butyl-2,20-bipyridine (1) with BrCH2-
C6H4CO2H and BrCH2C6H4CH2CO2H occur selectively
by trans oxidative addition of the C–Br bond to give the
platinum(IV) complexes 2 and 3, rather than by protonol-
ysis of a methylplatinum bond, as shown in Scheme 1 [6–8].
Since methyl-platinum(IV) bonds are typically inert to-
wards protonolysis [4,5], with the notable exception of
2. Experimental
*
Corresponding author. Tel.: +1 519 661 2111/679 2111; fax: +1 519
All reactions were performed under a nitrogen atmo-
sphere by using standard Schlenk techniques. NMR and
661 3022.
E-mail address: pudd@uwo.ca (R.J. Puddephatt).
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.08.020

C.S.A. Fraser et al. / Polyhedron 25 (2006) 266–270
267
t-Bu
N
Me
Pt
N
Me
t-Bu
(i)
1
(ii)
OH
O
O
OH
t-Bu
t-Bu
N
N
N
Me
Me
Me
Pt
Br
Pt
Br
N
Me
t-Bu
t-Bu
3
2
t-Bu
t-Bu
O
O
H O
O
N
N
t-Bu
t-Bu
H
N
Pt Me
Me
Me Pt N
Me
Br
Br
Scheme 1. Reagents are: (i) BrCH2C6H4CO2H and (ii) BrCH2C6H4CH2CO2H.
IR spectra were recorded by using a Varian Gemini 400
NMR and a Perkin-Elmer FT-IR 2000 spectrometer,
respectively. Compound 1 was prepared using a method re-
ported in the literature [10].
O4PPt2: C, 46.13; H, 5.20; N, 3.71. Found: C, 45.71; H,
5.21; N, 3.58%. NMR in acetone-d6: d(1H) = 1.02 [s,
2
12H, J(PtH) = 70 Hz, PtMe]; 1.48 [s, 36H, bu]; 2.75 [s,
4H, 2J(PtH) = 90 Hz, PtCH2]; 6.15 [m, 4H, C6H4 H2];
7.45 [m, 4H, C6H4 H3]; 7.70 [m, 4H, bipy H5]; 8.3–8.6
[m, br, 8H, bipy H3 + bipy H6]. IR (Nujol mull):
2.1. [{PtMe2(bu2bipy)(CH2C6H4CO2H)}2(l-Br)][PF6]-
(4[PF6])
m(CO) = 1696 cmꢀ1, m(OH) = 2652, 2516 cmꢀ1
.
To a solution of [PtMe2(bu2bipy)] (100 mg) in acetone
(30 mL) was added bromotoluic acid (44 mg), and the solu-
tion was stirred for 30 min., after which time a solution of
AgPF6 (26 mg) in acetone (5 mL) was added. After stirring
for further 30 min, the mixture was filtered through Celite
and evaporated under vacuum to give a white solid. Yield:
86%. Anal. Calc. for C56H74BrF6N4O4PPt2: C, 45.38; H,
5.03; N, 3.78. Found: C, 45.07; H, 5.21; N, 3.55%. NMR
in acetone-d6: d(1H) = 1.04 [s, 12H, 2J(PtH) = 70 Hz,
PtMe]; 1.44 [s, 36H, bu]; 2.82 [s, 4H, 2J(PtH) = 90 Hz,
PtCH2]; 6.35 [m, 4H, C6H4 H2]; 7.25 [m, 4H, C6H4 H3];
7.70 [m, 4H, bipy H5]; 8.45 [m, 4H, bipy H3]; 8.60 [m,
2.3. X-ray structure determination
Crystals of 4[PF6] Æ 4C6H6 were grown by slow diffu-
sion of benzene into an acetonitrile solution. A colorless
plate was mounted on a glass fiber. Data were collected
at 200 K using a Nonius Kappa CCD diffractometer
with COLLECT (Nonius, B.V., 1998) software. The unit cell
parameters were calculated and refined from the full data
set. Crystal cell refinement and data reduction were car-
ried out using the Nonius DENZO package. After data
reduction, the data were scaled based on equivalent
reflections using SCALEPACK (Nonius, B.V. 1998). The
crystal data and refinement parameters are listed in
Table 1. The SHELX-TL 5.1 (Sheldrick, G.M., Madison,
WI) program package was used to solve the structure
by direct methods. There was some disorder in the hexa-
fluorophosphate anion, which was modeled as a mixture
of two 50:50 occupancy groups, with the phosphorus
fluorine bond lengths restrained to be equal and allowed
to refine. Several of the solvent molecules were disor-
dered and were modeled as isotropic 50:50 mixtures.
4H, bipy H6]. IR (Nujol mull): m(CO) = 1697 cmꢀ1
m(OH) = 2624, 2548 cmꢀ1
,
.
2.2. [{PtMe2(bu2bipy)(CH2C6H4CH2CO2H)}2(l-Br)]-
[PF6] (5[PF6])
This was prepared similarly from [PtMe2(bu2bipy)]
(100 mg), bromomethylphenylacetic acid (46 mg) and
AgPF6 (26 mg). Yield: 87%. Anal. Calc. for C58H78BrF6N4-

268
C.S.A. Fraser et al. / Polyhedron 25 (2006) 266–270
Table 1
All other non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms were cal-
culated geometrically and were riding on their respective
carbon atoms.
Crystal data and structure refinement for complex 4[PF6] Æ 4C6H6
Empirical formula
Formula weight
Temperature (K)
C80H98BrF6N4O4PPt2
1794.68
200(2)
˚
Wavelength (A)
0.71073
Crystal system
Space group
Unit cell dimensions
monoclinic
P2(1)/n
3. Results and discussion
˚
a (A)
14.57770(10)
32.5272(3)
19.4740(2)
101.894(1)
9035.8(4)
4
1.319
3.608
3592
51358
20503 [Rint = 0.048]
99.0
integration
The synthesis of the building blocks for self-assembly
was achieved according to Scheme 2. The organoplati-
num(IV) complexes 2 and 3, each of which contains a single
carboxylic acid substituent, were prepared in situ by reac-
tion of [PtMe2(bu2bipy)] with the corresponding benzyl
bromide derivative according to Scheme 1. Reaction of
these complexes with silver hexafluorophosphate in a 2:1
molar ratio occurred by bromide abstraction, to give insol-
uble AgBr, and then by formation of a bridging bromide
group to give the cationic diplatinum(IV) complexes 4
and 5, which were isolated as the hexafluorophosphate
salts. In these reactions, it can be considered that the bromo
ligand of one platinum(IV) center acts as a ligand at the
˚
b (A)
˚
c (A)
b (°)
Volume (A )
3
˚
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
Reflections collected
Independent reflections
Completeness to h = 27.48° (%)
Absorption correction
Final R indices [I > 2r(I)]
R indices (all data)
)
R1 = 0.0707, wR2 = 0.2073
R1 = 0.1083, wR2 = 0.2361
+
R
R
R
Ag+
Me
Me
Me
2
Me Me
Br
Me
Pt
Pt
N
N
Pt
N
- AgBr
N
t-Bu
t-Bu
t-Bu
N
N
Br
t-Bu
t-Bu
t-Bu
2, R = CH2C6H4CO2H
3, R = CH2C6H4CH2CO2H
4, R = CH2C6H4CO2H
5, R = CH2C6H4CH2CO2H
Br
N
Br
n+
N
N
N
N
N
Pt
N
N
Pt
Pt
Pt
Me Me
Me Me
Me
Me
Me
Me
O
H
O
O
O
O
O
O
H
O
H
H
H
O
H
O
O
O
Me
Pt
Me
Me Me
Br
Pt
N
N
N
N
Scheme 2. In the self-assembled polymer only the core atoms of the bu2bipy ligands are shown.

C.S.A. Fraser et al. / Polyhedron 25 (2006) 266–270
269
second platinum(IV) center by occupying the coordination
site which is opened up by the bromide abstraction step.
The complexes 4 and 5 are air-stable complexes which
are soluble in organic solvents such as acetone and dichlo-
1
romethane. In acetone-d6 solution, the H NMR spectrum
of complex 4 contained a single methylplatinum resonance
2
at d(1H) = 1.04, with coupling constant J(PtH) = 70 Hz,
showing that all methylplatinum(IV) groups are equivalent.
The benzylic protons also occurred as a singlet resonance at
d = 2.82, with coupling constant 2J(PtH) = 90 Hz. The
infrared spectrum of complex 4 as a Nujol mull contained
a band due to m(C@O) at 1697 cmꢀ1 and bands due to
m(OH) at 2624 and 2548 cmꢀ1. The NMR data are fully con-
sistent with the proposed structure of 4 in solution (Scheme
2), while the IR data indicate the presence of hydrogen-
bonded carboxylic acid groups in the solid state [8,9]. Com-
plex 5 was characterized spectroscopically in a similar way.
The molecular structure of complex 4 is shown in Fig. 1. It
confirms the overall stereochemistry deduced from the spec-
troscopic data, with the methylplatinum groups trans to the
bipyridine ligand and the substituted benzyl group trans to
bromide. The bromide ligand bridges the two platinum
atoms with roughly equal distances Pt(1)–Br(1) =
Fig. 2. Part of the zig-zag polymeric chain structure formed by self-
assembly through hydrogen bonding between units of complex 4.
substituents, as shown in Fig. 2. The intermolecular hydro-
gen bonding between the V-shaped building blocks 4 natu-
rally leads to formation of a zig-zag polymeric structure.
˚
The Oꢁ ꢁ ꢁO distances O(1Q)ꢁ ꢁ ꢁO(1R) = 2.62(2) A and
˚
O(2Q)ꢁ ꢁ ꢁO(2R) = 2.71(2) A are in the expected range for
hydrogen bonds in carboxylic acids [5–7,9].
There are very few diplatinum complexes that have been
characterized to contain a single unsupported halide
bridge, and particularly few organoplatinum(IV) examples
[11–14], so the characterization of the molecular structure
of complex 4 is significant. More noteworthy is the obser-
vation that this is the first time that a complex of this type
has been used in the self-assembly of supramolecular poly-
meric structures. Since the oxidative addition of alkyl
halides to platinum(II) is a versatile method for the synthe-
sis of alkyl(halogeno)platinum(IV) complexes containing
functionally substituted alkyl groups [5–7], the finding that
these monoplatinum(IV) complexes can easily be converted
to the diplatinum(IV) complexes with bridging halide pro-
vides a valuable new method for the synthesis of bifunc-
tional organodiplatinum(IV) complexes for use in the
self-assembly of molecular materials.
˚
˚
2.560(2) A and Pt(2)–Br(1) = 2.565(2) A. The large angle
Pt(1)–Br(1)–Pt(2) = 127.88(8)° and correspondingly long
˚
distance Ptꢁ ꢁ ꢁPt = 4.60 A are clear indications that there is
no platinum-platinum bonding interaction [10]. Thus, the
natural bond angle M–X–M is close to 90° for complexes
containing a bridging halide and no other bridging halides
[10]. An M–X–M angle much less than 90° then indicates
the presence of a metal–metal bonding interaction, while
an M–X–M angle much greater than 90° indicates both the
lack of an attractive force between the metals and the pres-
ence of steric interactions around the bridging halide [10–15].
The molecules of complex 4 associate through comple-
mentary hydrogen bonding between carboxylic acid
Acknowledgments
We thank the NSERC (Canada) for financial support.
R.J.P. thanks the Government of Canada for a Canada
Research Chair.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.
2005.08.020.
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Fig. 1. A view of the structure of the cationic diplatinum(IV) complex 4.
Selected bond parameters: Pt(1)–C(2Q) 2.044(9) A, Pt(1)–C(11Q)
˚
˚
˚
˚
˚
˚
2.074(9) A, Pt(1)–C(1Q) 2.075(10) A, Pt(1)–N(2) 2.145(7) A, Pt(1)–N(1)
˚
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˚
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175.0(3)°, C(11R)–Pt(2)–Br(1) 174.3(3)°, Pt(1)–Br(1)–Pt(2) 127.88(8)°.

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