Platinum (II) phosphine complexes with acetylene ligands containing 1,4,5,8-naphthalenediimide: Synthesis, crystal structure and electrochemistry

Article abstract of DOI:10.1016/j.jorganchem.2006.10.060

A new acetylene ligand modified with the electron-accepting 1,4,5,8-naphthalenediimide group and its Pt(II) phosphine complexes are reported. The Pt(II) complexes incorporate optical properties of the imide group, and display two reversible imide-centred

Full text of DOI:10.1016/j.jorganchem.2006.10.060

Journal of Organometallic Chemistry 692 (2007) 921–925
www.elsevier.com/locate/jorganchem
Note
Platinum (II) phosphine complexes with acetylene ligands
containing 1,4,5,8-naphthalenediimide: Synthesis, crystal structure
and electrochemistry
*
Nail M. Shavaleev, Harry Adams, Jonathan Best, Julia A. Weinstein
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, England, UK
Received 6 October 2006; received in revised form 30 October 2006; accepted 30 October 2006
Available online 7 November 2006
Abstract
A new acetylene ligand modified with the electron-accepting 1,4,5,8-naphthalenediimide group and its Pt(II) phosphine complexes are
reported. The Pt(II) complexes incorporate optical properties of the imide group, and display two reversible imide-centred reduction
processes.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Platinum; Acetylene; Imide; Ligand; Complex; Electrochemistry; Phosphine; Ferrocene
1. Introduction
It should be noted that McAdam et al. and Robinson
et al. recently reported 1,8-naphthalimide substituted acet-
ylene and its metal complexes; however these authors pre-
pared the acetylene ligand by a synthetic approach [6]
different to the one reported here.
The aromatic acid imides are chemically- and photo-sta-
ble compounds that combine efficient absorption and fluo-
rescence [1] with strong electron-accepting properties [2].
These compounds can be used in the synthesis of metal
complexes by attachment of the imide group to a suitable
neutral [3–5] or anionic [6,7] ligand to give materials with
tuneable optical and electrochemical properties.
2. Results and discussion
The acetylene ligand HC2-NDI was obtained by the reac-
tion of 4-ethynylaniline with N-octyl-1,4,5,8-naphthalene-
tetracarboxylic monoanhydride in DMF at reflux (Scheme
1). The Pt(PP)Cl2 precursor complexes (PP = dppe or dppf)
were prepared in quantitative yield by the reaction of the PP
ligand with PtCl2(DMSO)2 in dichloromethane. The target
acetylide complexes Pt(PP)(C2-NDI)2 were synthesised by
the reaction of HC2-NDI with Pt(PP)Cl2 in dichlorometh-
ane in the presence of i-Pr2NH as a base and CuI as a
catalyst [18]. All compounds were purified by column
chromatography and were identified by EI mass-spectrom-
etry, elemental analysis and 1H and 31P NMR spectroscopy.
The acetylene proton C„CH was observed as a singlet
at 3.16 ppm in the 1H NMR spectra of the HC2-NDI
ligand. This signal was not present in the 1H NMR spectra
Herein, we report a simple approach to imide-modified
acetylene ligands and illustrate it by the synthesis of acety-
lene with attached 1,4,5,8-naphthalenediimide (NDI)
group and its Pt(II) phosphine complexes. Acetylene
ligands containing various functional groups, such as
metallocene [8], metal-arene [9], metal-carbonyl [10], por-
phyrin [11], organic radical [12] or imide [6] have been used
previously to prepare Pt(II) complexes. The Pt(II) phos-
phine acetylides have been developed for application as
molecular wires [13], luminescent [14], NLO [15] or
charge-transport [16] materials, and solar cell dyes [17].
*
Corresponding author. Tel.: +44 114 222 9408; fax: +44 114 222 9346.
E-mail address: julia.weinstein@sheffield.ac.uk (J.A. Weinstein).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.060

922
N.M. Shavaleev et al. / Journal of Organometallic Chemistry 692 (2007) 921–925
O
N
O
O
N
O
O
N
O
O
O
O
The solutions of Pt(PP)(C2-NDI)2 in freshly distilled
and degassed dry CH2Cl2 or CH3CN underwent a facile
photochemical decomposition under laser irradiation in
the flash-photolysis experiments (laser wavelength
355 nm, pulse duration 9 ns, pulse energy 5 mJ), which pre-
cluded investigations of excited state processes.
a
C8H17
C8H17
HC2-NDI
b
The Pt(II) complexes show several redox processes in the
potential window between 1.5 and ꢀ2 V (all potentials are
quoted vs. Fc/Fc+). The relevant cyclic voltammograms
and electrochemical data are presented in Fig. 1 and Table
2, respectively.
PP =
PPh2
Pt(PP)
Ph2P
dppe
Ph2P
O
N
O
A single electrochemically irreversible oxidation was
observed for the Pt(dppe)(C2-NDI)2 complex at ꢁ1.1. V
and is likely to be associated with a Pt(II)/Pt(III) couple
[19]. The first oxidation of the Pt(dppf)(C2-NDI)2 complex
at ꢁ0.6 V is electrochemically irreversible and is followed
by a second, electrochemically reversible, oxidation at
0.83 V. A similar redox behaviour was reported for
Pt(dppf)C„CPh2 complex in which an irreversible oxida-
tion at 0.52 V was followed by a reversible one at 0.83 V
[20]. These processes might be assigned either as an irre-
versible Pt(II)-centred oxidation, followed by a reversible
oxidation of Pt(dppf) unit (for comparison, the reversible
oxidation of ferrocene in Pt(dppf)Cl2 is observed at
Fe
N
PPh2
O
O
dppf
O
O
N
N
C8H17
O
O
C8H17
cis-Pt(PP)(C2-NDI)2
Scheme 1. Synthesis and structures of the ligand HC2-NDI and
Pt(PP)(C2-NDI)2 complexes: (a) 4-ethynylaniline, DMF, reflux, N2, 24 h;
(b) Pt(PP)Cl2, CuI (catalyst), i-Pr2NH (excess), CH2Cl2, N2, r.t., 48 h.
of the corresponding Pt(II) acetylide complexes, confirming
coordination of the ligand to the metal centre.
The absorption spectra of the new compounds are
shown in Figs. 1S–3S and are summarized in Table 1.
The acetylene HC2-NDI was obtained as a pale yellow
solid; its solutions in organic solvents do not have absorp-
tion bands above 400 nm. The complexes Pt(PP)(C2-NDI)2
were obtained as pale pink or brown solids. Their colour in
solution was determined by the broad and weak unresolved
absorption shoulder e < 103 Mꢀ1 cmꢀ1 in the visible region
that stretched to 500 nm. The absorption of the ferrocene
chromophore at 426 nm was observed in the spectra of
both the precursor Pt(dppf)Cl2 and the acetylide
Pt(dppf)(C2-NDI)2 complexes. The absorption spectra of
the new acetylene and its Pt(II) phosphine complexes in
the UV region (300–400 nm) were dominated by the intense
structured bands of the 1,4,5,8-naphthalenediimide chro-
mophore [2].
dppf
A
/
I
dppe
-2.5
-2
-1.5
-1
-0.5
0
1
1.5
0.5
The HC2-NDI ligand and Pt(PP)(C2-NDI)2 complexes
do not show luminescence in degassed CH2Cl2 solutions
at r.t. (the upper limit of the luminescence quantum yield
would be 0.01%).
E
/ V vs. Fc/Fc+
Fig. 1. Cyclic voltamograms of Pt(dppe)(C2-NDI)2 (200 mV/s) and
Pt(dppf)(C2-NDI)2 (50 mV/s) in CH2Cl2 containing 0.1 M (NBu4)PF6.
The unit on the vertical axis is 5 lA.
Table 1
Table 2
Absorption spectra of acetylene HC2-NDI and Pt(II) phosphine com-
plexes in CH2Cl2
Electrochemical data (in V vs. Fc/Fc+) for Pt(II) complexes in CH2Cl2
a
containing 0.1 M (NBu4)PF6
Complex
k )
max/nm (10ꢀ3 e/Mꢀ1 cmꢀ1
Complex
Reduction
Oxidation
Pt(dppe)Cl2
Pt(dppf)Cl2
HC2-NDI
Pt(dppe)(C2-NDI)2
298 (2.4), 256 (18)
426 (0.27)
380 (30), 359 (25), 342 (15)
380 (74), 359 (61), 340 (43), 322 (41),
274 (42), 257 (49)
Pt(dppe)(C2-NDI)2
Pt(dppf)(C2-NDI)2
ꢀ1.07, ꢀ1.50
ꢀ1.07, ꢀ1.55
ꢁ1.1b
ꢁ0.6b, 0.83
a
Anodic/cathodic peak separations for reversible processes were in the
range 80–115 mV at scan rate 50 mV/s; for Fc/Fc+ couple used as an
internal standard these values were in the range 80–100 mV.
Pt(dppf)(C2-NDI)2
426 (0.47, sh), 380 (73), 359 (61), 340 (42),
316 (43), 271 (41)
b
Irreversible process; the anodic peak potential is reported.

N.M. Shavaleev et al. / Journal of Organometallic Chemistry 692 (2007) 921–925
923
0.57 V [21a]), [20,21] or as an irreversible oxidation of
3. Experimental
Pt(dppf) unit followed by a reversible oxidation of the
chemical product formed in the first step [22].
All reactions were carried out in the dark under N2,
using general grade solvents that were degassed by bub-
bling N2 for 10 min. Purification, crystal growth and han-
dling of all compounds were carried out under air with a
minimum exposure to light. All products were air- and
moisture-stable solids which were stored in the dark when
not used.
Both complexes undergo two reversible reductions, at
ꢀ1.07 V and ꢁꢀ1.5 V. Comparison with the literature data
[2] allows us to assign these processes as two consecutive
one-electron reductions of the NDI group. The two NDI
groups present in the complex are being reduced at the
same potentials, indicating a lack of electronic communica-
tion between them. The reduction potentials of the NDI
group are not sensitive to the change of the Pt(PP) group,
indicating that these two units are decoupled.
3.1. Synthesis of N-octyl-1,4,5,8-naphthalenetetracarboxylic
monoanhydride
The crystal structure of Pt(dppe)(C2-NDI)2 is shown in
Fig. 2. The Pt(II) centre is in a distorted square planar
coordination environment. The Pt–C„C fragment is not
linear, but bent, with angles of 162–165ꢁ. The shortest
The synthesis is based on the reported procedure
[1a] which was significantly modified. n-Octylamine
(2 ml, 1.56 g, 12 mmol, excess) was added to a suspension
of 1,4,5,8-naphthalenetetracarboxylic dianhydride (1 g,
3.73 mmol) in a mixture of water (10 ml) and n-propanol
(10 ml) at r.t. under N2. The suspension dissolved in
10 min, and the resulting solution was heated at 50 ꢁC
(important) for 24 h to give a white precipitate. The reac-
tion mixture was cooled down to r.t., acidified to pH 1 with
HCl to give more precipitate, and was then further stirred
for 1 h at r.t. The solid was filtered off and stirred in acetic
acid (50 ml or more) at reflux for 1 h or longer, producing
first a suspension and then a clear yellow solution. It was
cooled to r.t. (a precipitate is formed on cooling), diluted
with CH2Cl2 (200 ml) and extracted with water (200 ml)
to give clear organic and aqueous layers (the formation
of an inseparable emulsion at this stage can be avoided
by addition of more acetic acid). The organic layer was
dried if necessary (MgSO4) and then reduced in volume.
Addition of ethanol (50 ml) and evaporation of CH2Cl2
gave a suspension of the product in ethanol that was fil-
tered off, washed with ethanol, hexane and small amount
of ether.
˚
Pt. . .Pt distance is 8.054 A indicating the absence of
metal–metal interaction. The bond lengths and angles
around the Pt(II) centre, as well as the non-linearity of
the Pt–C„C fragment in Pt(dppe)(C2-NDI)2 are similar
to the previously reported structures of cis-Pt(dppe) acety-
lide complexes [14c].
The NDI groups are planar and are almost orthogonal
to the attached phenylacetylene group with dihedral angles
of ꢁ80ꢁ. The molecule lacks elements of symmetry in the
solid state structure. As a result, each NDI group in the
molecule is unique, and is involved in one intermolecular
aromatic p–p interaction with the NDI group of another
˚
molecule, with the inter-planar distances of 3.356 A and
˚
3.359 A, respectively. The net result is the formation of
an infinite chain of molecules connected through p–p inter-
actions and arranged in a head-to-head fashion.
In conclusion, we have shown that an acetylene ligand
can be easily modified with an electron-accepting 1,4,5,8-
naphthalenediimide group and consequently used to pre-
pare stable metal complexes. The Pt(dppf)(C2-NDI)2 com-
plex which combines ferrocene and imide groups can be
considered as a Pt(II)-based donor–acceptor system.
The synthetic approach to modified acetylenes described
in this ‘Note’ could potentially be extended to incorporate
any imide group. A simple variation of the attached imide
is expected to fine-tune the optical and electrochemical
properties of the acetylene ligand and its metal complexes.
The product was obtained as a 1:12 molar mixture of the
bis- and mono-imide and was used without further purifi-
cation (the bis-imide is an inert impurity which is easily
removed in the next reaction step). It should be noted that
the amount of bis-imide impurity increases if the reaction is
carried out at temperatures higher than 50 ꢁC. Crude yield
– 0.99 g, contains 0.90 g (2.38 mmol, 64%) of mono-imide.
˚
Fig. 2. Molecular structure of Pt(dppe)(C2-NDI)2 (50% probability ellipsoids, H atoms omitted). Selected bond lengths (A): Pt(1)–C(27) 2.024(11), Pt(1)–
C(57) 2.042(9), Pt(1)–P(3) 2.262(2), Pt(1)–P(4) 2.263(2). Selected angles (ꢁ): C(27)–Pt(1)–C(57) 97.9(3), P(3)–Pt(1)–P(4) 86.49(9), C(57)–Pt(1)–P(3) 88.2(2),
C(27)–Pt(1)–P(4) 87.4(3), C(27)–Pt(1)–P(3) 173.9(3), C(57)–Pt(1)–P(4) 174.6(3), C(28)–C(27)–Pt(1) 164.5(8), C(58)–C(57)–Pt(1) 162.0(8). The dihedral
angle between the planes defined by P(3) Pt(1) P(4) and C(27) Pt(1) C(57) atoms is 1.05ꢁ.

924
N.M. Shavaleev et al. / Journal of Organometallic Chemistry 692 (2007) 921–925
The product was obtained as a white or pale pink solid;
C22H21NO5 (MW 379.41); soluble in CH2Cl2 and acetone,
insoluble in ethanol and hexane. Characteristic H NMR
(CDCl3) signals were observed in aromatic region for the
mono-imide at 8.81 ppm (s) and for the bis-imide at
8.75 ppm (s).
3.4. X-ray crystallography
1
Single crystals of the Pt(dppe)(C2-NDI)2 complex were
obtained by slow evaporation of a CH2Cl2-heptane
solution.
Pt(dppe)(C2-NDI)2: C86H74N4O8P2Pt: MW 1548.52; pink
plates; size (mm): 0.18 · 0.07 · 0.03; T = 150(2)K; k =
1
˚
ꢀ
˚
3.2. Synthesis of the acetylene HC2-NDI
0.71073 A; triclinic; space group P1 ðCi ; no:2Þ; a/b/c (A)=
8.0541(7)/18.5909(19)/24.538(2); a/b/c (ꢁ) = 98.279(8)/
3
˚
4-Ethynylaniline (131 mg, 1.12 mmol) and N-octyl-
1,4,5,8-naphthalenetetracarboxylic
90.059(5)/102.383(6); V = 3549.3(6) A ; Z = 2; qcalc
=
monoanhydride
1.449 g cmꢀ3; l = 2.085 mmꢀ1; F(000) = 1580; h range
(ꢁ) = 0.84–25.00; min/max transmission = 0.7054/0.9401;
reflections collected/Rint = 48203/0.1002; data/restraints/
parameters = 12051/0/910; GOF on F2 = 1.052; R(I > 2rI):
R1 = 0.0588, wR2 = 0.1206; R(all data): R1 = 0.1264, wR2 =
(424 mg, 1.12 mmol) were refluxed in dry degassed DMF
(5 ml) for 24 h. Cooling the reaction mixture to r.t. fol-
lowed by addition of a mixture of hexane and ether precip-
itated the crude product that was filtered and purified by
column chromatography (SiO2, 0–0.2% CH3OH in
CH2Cl2). Yellow solid – 183 mg (0.38 mmol, 34%). Anal.
Calc. for C30H26N2O4 (MW 478.54): C, 75.30; H, 5.48;
N, 5.85. Found: C, 75.50; H, 5.54; N, 5.80%. EI MS m/z:
0.1563; Dqmin/max (e Aꢀ3) = ꢀ2.717/2.123.
˚
Acknowledgements
1
478 (100%, M+). H NMR (CDCl3): 0.82–0.92 (m, 3H),
We thank the EPSRC, the EPSRC/RAL laser loan pool
scheme, and the Royal Society for support. J.B. thanks the
CMSD CCLRC research network and the EPSRC CAN
award for funding.
1.2–1.5 (m, 10H), 1.68–1.82 (m, 2H), 3.16 (s, 1H), 4.20 (t,
J 7.6, 2H), 7.29 (d, J 8.9, 2H), 7.69 (d, J 8.9, 2H), 8.80
(s, 4H).
3.3. General synthesis of Pt(PP)(C2-NDI)2 complexes
Appendix A. Supplementary material
Pt(PP)Cl2 (PP = dppe or dppf), HC2-NDI (excess) and
CuI (catalyst) in a degassed mixture of dry CH2Cl2
(10 ml) and dry i-Pr2NH (1 ml) under N2 were sonicated
for 1 h and stirred for 48 h at r.t. to give yellow or orange
solution. The reaction mixture was diluted with CH2Cl2
and extracted with water. The organic phase was separated,
dried (MgSO4) and evaporated to dryness. Purification by
column chromatography (SiO2, 0.5% CH3OH in CH2Cl2)
gave the product as a first major yellow fraction. It was
reduced in volume and the product was precipitated with
ether. The complexes are highly soluble in CH2Cl2.
CCDC 623168 contain the supplementary crystallo-
graphic data for Pt(dppe)(C2-NDI)2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article (additional experimental details and absorption
spectra) can be found, in the online version, at doi:10.
1016/j.jorganchem.2006.10.060.
Pt(dppe)(C2-NDI)2: Pt(dppe)Cl2 Æ (CH2Cl2)0.5 (49 mg,
0.069 mmol), HC2-NDI (75 mg, 0.157 mmol, excess) and
CuI (5 mg, catalyst) gave 65 mg (0.042 mmol, 61%) of the
product as a pink solid. Anal. Calc. for C86H74N4O8P2Pt
(MW 1548.56): C, 66.70; H, 4.82; N, 3.62. Found: C,
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