Semiconductor properties in an iodine-doped platinum(II) dinuclear complex

Article abstract of DOI:10.1021/ic001351h

The complex [{Pt(trpy)}₂(μ-Me₂dicyd)] [PF₆]₂, where trpy is 2,2′:6′,2″-terpyridine and Me₂dicyd^2- is 2,5-dimethyl-1,4-dicyanamidebenzene dianion, has been synthesized. Upon doping with iodi

Full text of DOI:10.1021/ic001351h

1
406
Inorg. Chem. 2001, 40, 1406-1407
Semiconductor Properties in an Iodine-Doped Platinum(II) Dinuclear Complex
Maria C. DeRosa, Fahad Al-mutlaq, and Robert J. Crutchley*
Ottawa-Carleton Chemistry Institute, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6
ReceiVed NoVember 30, 2000
The conducting properties of organic charge transfer salts have
been studied for many years and arise from the orientation of
conjugated planar organic molecules on top of one another to
1
form “pancake” stack structures within the crystal lattice. The π
interactions between molecules making up the stack create a
continuum of molecular orbitals (a band structure) and, under
the right conditions, can result in conductivity in the direction of
the stack. Attempts to combine metal ion chemistry with organic
charge transfer salts has mainly focused on metal phthalocyanins
and porphyrins, as these complexes naturally form π-stacking
2
architectures in their crystal lattices. These materials all show
weak conductivity, but upon doping by either chemical or
electrochemical means significant improvement in conductivity
Supporting Information) is entirely consistent with a dinuclear
-
5
-1
-1 2
is observed (σRT ) 10 -10 S cm ). Another important class
of materials are the metal N,N′-dicyanoquinonediimine com-
complex, showing chemical shifts of a single terpyridine and a
symmetric bridging Me
2
dicyd2 ligand with appropriate proton
-
3
-
integration.6 In addition, the C NMR spectrum of 1 (see
13
2 2 n 2
pounds. For example, [Cu(Me dicyd) ] , where Me dicyd is the
6
radical anion of 2,5-dimethyl-N,N′-dicyanoquinonediimine, ex-
hibited metallic conductivity down to 3.5 K with σ ) 500 000 S
Supporting Information) shows the expected 13 line pattern.
The UV-vis spectrum of 1 in DMF (see Supporting Informa-
tion) shows a terpyridine π f π* band at 283 nm (ꢀmax ) 44 300
-1 4
cm . The crystal structure of this material showed that the Me
dicyd molecules form π stacks with copper ions interconnected
to these stacks by bridging Me dicyd. Supramolecular methods
2
-
-
1
-1
M
cm ) and Pt(II) f terpyridine metal-to-ligand charge transfer
-
1
-1
transitions at 334 (28 800 M cm ) and 386 nm (shoulder ꢀ )
1
2
-
-1 7,8
have also proved successful with the synthesis of tetracyano-
quinodimethanide stacks in nickel and copper tetraazamacrocyclic
10 500 M cm ). In addition, a broad band at 588 nm (ꢀmax)
-
1
-1
2800 M cm ) is assigned to a π f σ* ligand-to-metal charge
transfer transition arising from the Pt(II)-cyanamide chro-
mophore. This band is too low in energy and too intense to be a
metal-centered transition, and similar absorption bands have been
5
systems. These materials are good semiconductors with powder
-
3
-2
-1 5b
conductivities of approximately 10 -10 S cm .
An alternative way to form novel conducting materials is to
coordinate a donor or acceptor conjugated planar molecule to a
metal ion in a square planar coordination sphere. Provided that
the complex is planar, it should be possible to form π stacks in
the crystal lattice and, with appropriate doping, create the
conditions for conductivity. To demonstrate that this can be a
successful strategy, we have synthesized and characterized the
7
9
observed for the mononuclear palladium(II) and platinum(II)
terpyridine complexes of phenylcyanamide ligands in which
crystal structures clearly show the coordination of the cyanamide
group to the metal ion.
The cyclic voltammogram of 1 in DMF solution showed two
-
/2-
reduction couples at positive potentials, Me
2
/-
dicyd
) 0.694
V vs NHE (quasi-reversible) and Me
2
dicyd0 ) 1.51 V vs NHE
dinuclear complex [{Pt(trpy)}
where trpy is 2,2′:6′,2′′-terpyridine and Me dicyd is 2,5-
2
2
2 6 2
(µ-Me dicyd)][PF ] (1),
2
-
(anodic peak position). The corresponding quasi-reversible free
ligand reduction couples are -0.348 and 0.390 V versus NHE in
DMF. This approximately 1 V positive shift in ligand reduction
potentials upon coordination has been observed before in polyam-
mineruthenium(III) complexes of this ligand.10 Irreversible waves
and fouling of the working electrode were observed when
scanning at negative potentials.
dimethyl-1,4-dicyanamidebenzene dianion.
dicyd2-
1
was prepared by reacting the thallium salt of Me
2
6
with [Pt(trpy)Cl][PF ] in N,N′-dimethylformamide (DMF) under
argon for 1 week. The orange reaction mixture slowly turned a
dark purple and precipitated white TlCl. Recrystallization by ether
diffusion into a filtered DMF solution of the complex gave deep
6
Chemical doping of 1 was achieved by exposing a powder
purple microcrystalline powder in 84% yield. The complex is
sample of 1 to iodine in a developing chamber. This results in
soluble in only strongly donating solvents, and various attempts
to recrystallize the complex has so far yielded only powders.
-
the formation of radical anion Me
2
dicyd as was shown by the
1
Nevertheless, the H NMR spectrum of the complex (see
(
6) Anal. Calcd for [{Pt(trpy)}
2
(µ-Me
2
dicyd)][PF
6
]
2
‚H
2 32 10 2
O (C40H N F12OP -
Pt ): C, 35.62; H, 2.39; N, 10.38. Found: C, 35.56; H, 2.33; N, 10.48.
2
-1 1
(
1) Ferraro, J. R.; Williams, J. M. Introduction to Synthetic Electrical
Conductors; Academic Press Inc.: New York, 1987.
IR (KBr disk): ν(NCN) ) 2114 cm . H NMR (400 MHz): 7.94-
8.63 (terpyridine-H, multiplets, 22H), 6.73 (phenyl-H, singlet, 2H), and
2.11 ppm (methyl-H, singlet, 6H). C NMR (100 MHz): 157.8, 154.5,
150.9, 142.9, 141.9, 137.3, 130.8, 129.4, 127.4, 126.0, 124.4, 123.3, and
17.9 ppm.
1
3
(
(
2) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48, 123.
3) Kato, R.; Kobayashi, H.; Kobayashi, A. J. Am. Chem. Soc. 1989, 111,
5
224.
(
4) Aum u¨ ller, A.; Erk, P.; Klebe, G.; H u¨ nig, S.; von Sch u¨ tz, J. U.; Werner,
J.-P. Angew Chem., Int. Ed. Engl. 1986, 25, 740.
(7) Zhang, W.; Bensimon, C.; Crutchley, R. J. Inorg. Chem. 1993, 32, 5808.
(8) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Publishing Co.: Amsterdam, 1984.
(9) (a) Al-mutlaq, F. M. Sc. Thesis, Carleton University, 1999. (b) Al-mutlaq,
F.; Crutchley, R. J. Manuscript in preparation.
(5) (a) Ballester, L.; Gil, A. M.; Guti e´ rrez, A.; Perpi n˜ a´ n, M. F.; Azcondo,
M. T.; S a´ nchez, A. E.; Amador, U.; Campo, J.; Palacio, F. Inorg. Chem.
1
997, 36, 5291. (b) Ballester, L.; Gil, A. M.; Guti e´ rrez, A.; Perpi n˜ a´ n,
M. F.; Azcondo, M. T.; S a´ nchez, A. E.; Coronado, E.; G o´ mez-Garci a´ ,
C. J. Inorg. Chem. 2000, 39, 2837.
(10) Rezvani, A. R.; Evans, C. E. B.; Crutchley, R. J. Inorg. Chem. 1995,
34, 4600.
1
0.1021/ic001351h CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/02/2001

Communications
Inorganic Chemistry, Vol. 40, No. 7, 2001 1407
Figure 3. Schematic to explain the mechanism of conduction in π-stacked
orbitals depending on orbital occupancy.
Figure 1. Conductivity versus I -
dicyd)][PF [I . The data from two different experiments are plotted.
3
2 2
equivalences for [{Pt(trpy)} (µ-Me -
6 2 3 x
] ]
for Me
Me dicyd in the molecular metal [Cu (Me
and low-temperature X-ray diffraction indicate an average oxida-
2
dicyd. This is approximately the same charge found for
2
2
dicyd) , where XPS
2 n
]
4
2
3,13
tion state of /
3
3 2
for Cu and hence - / for Me dicyd.
14
While we do not have a crystal structure of 1 and our attempts
to grow crystals of doped 1 proved unsuccessful, it is probable
that the architecture responsible for conductivity in these materials
2
is π stacking of Me dicyd molecules. Assuming π stacking, Figure
3
gives our interpretation of the mechanisms of conductivity for
the data shown in Figures 1 and 2. Clearly, electron movement
between filled orbitals is not possible, and if the orbitals making
up a π stack each possess one electron, there will be a significant
Coulombic barrier to electron movement. This can be overcome
if partial oxidation occurs (less than or more than 1 equiv, x )
1
5
0
.8 or x ) 1.3, as shown in Figures 1 and 2, respectively). In
the case of x ) 1.0, conductivity should probably be much lower
than observed but is not because of the heterogeneity of doping.
It is likely that the surface of a particle of 1 is more strongly
Figure 2. Conductivity versus I -
3
2 2
equivalences for [{Pt(trpy)} (µ-Me -
dicyd)][PF
]
6 2
[I ]
3 x
. The data from two different experiments are plotted.
doped than the interior. Indeed, it is possible that polyiodides
-
such as I
5
may form under conditions of excess iodine in the
-
16
growth of absorption bands characteristic of the radical anion in
the doped complex’s DMF solution spectra.10 At different time
intervals, portions were taken for elemental analysis of iodine
and conductivity measurements,11 and the combined results of
two separate experiments are plotted in Figures 1 and 2. We have
assumed that doping results mainly in the formation of I
analogy with crystallographic and spectroscopic studies performed
on iodine-doped metal phthalocyanin complexes. Accordingly,
the value of x in Figures 1 and 2 represents the number of
3
presence of I .
Powder conductivity is generally 100 times smaller than that
of a crystal,11 but this would still classify these doped materials
as weak semiconductors. Nevertheless, the results of this study
open up a new family of materials and strongly suggest that
supramolecular assembly of π-stacked architectures can lead to
conducting materials.
-
3
by
2
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada (NSERC) for
financial support. M.C.D. thanks NSERC for a postgraduate
scholarship.
oxidizing equivalents (i.e., 1‚[I
A powder sample of [AsPh
3
]
x
).
4
]
2
[Me
2
dicyd] was also doped with
iodine. However, conductivity measurements of doped samples
taken during the doping process showed no evidence of conduc-
1
Supporting Information Available: Four figures showing H and
C NMR spectra, a quantitative electronic absorption spectrum, and
tivity.11 In the crystal structure of [AsPh
13
4
2
] [Me
2
dicyd], the
-
Me
2
dicyd2 anions are not π stacked and are separated from each
conductivity versus time plots. This material is available free of charge
via the Internet at http://pubs.acs.org.
12
other by the tetraphenylarsonium cations.
The novel observation of double maxima in conductivity with
IC001351H
-
1
doping that is seen in Figures 1 and 2, x ) 0.8 (σ ) 8 µS cm ),
-1
and x ) 1.3 (σ ) 39 µS cm ), respectively, has been reproduced
in two additional conductivity versus doping experiments with
very little scatter in data points (see Supporting Information).
Indeed, the maximum in Figure 2 corresponds to a charge of -0.7
(13) Kobayashi, A.; Kato, R.; Kobayashi, H.; Mori, T.; Inokuchi, H. Solid
State Commun. 1987, 64, 45.
(
14) A crystal structure of the mononuclear complex, [Pd(trpy)(2,6-dichlo-
6
rophenylcyanamide)][PF ] (see ref 7), shows that the cation is planar.
2-
In addition, Me
2
dicyd is also planar. See ref 12. It is therefore
reasonable to suggest that 1 will be planar as well.
(
11) Conductivity was measured by using the two-probe method for com-
(15) A referee has pointed out that, in order to confirm n-type conductivity,
thermopower or Hall-effect measurements should be made. Unfortu-
nately, these measurements would be open to interpretation because of
the low conductivity and lack of stability of 1‚[I ] .
3 x
(16) Hotta, S.; Shimotsuma, W.; Taketani, M.; Kohiki, S. Synth. Met. 1985,
11, 139.
pressed powders. See: Wudl, F.; Bryce, M. R. J. Chem. Educ. 1990,
-3
6
cm
7, 717. The lower limit for our conductivity measurements is 10 µS
-1
.
(12) Aquino, M. A. S.; Crutchley, R. J.; Lee, F. L.; Gabe, E. J.; Bensimon,
C. Acta Crystallogr. 1993, C49, 1543.