Cyclometalated platinum(II) complexes of 2,2′-bipyridine N-oxide containing a 1,1′-bis(diphenylphosphino)ferrocene ligand: structural, computational and electrochemical studies

Article abstract of DOI:10.1039/c6dt04085c

The preparation and characterization of new heteronuclear-platinum(ii) complexes containing a 1,1'-bis(diphenylphosphino)ferrocene (dppf) ligand are described. The reaction of the known starting complex [PtMe(κ²N,C-bipyO-H)(SMe₂)], A, in which bipyO-H is a cyclometalated rollover 2,2'-bipyridine N-oxide, with the dppf ligand in a 2:1 ratio or an equimolar ratio led to the formation of the corresponding binuclear complex [Pt₂Me₂(κ²N,C-bipyO-H)₂(μ-dppf)], 1, or the mononuclear complex [PtMe(κ¹C-bipyO-H)(dppf)], 2, respectively. According to the reaction conditions, the dppf ligand in 1 and 2 behaves as either a bridging or chelating ligand. All complexes were characterized by NMR spectroscopy. The solid-state structure of 2 was determined by the single-crystal X-ray diffraction method and it was shown that the chelating dppf ligand in this complex was arranged in a “synclinal-staggered” conformation. Also, the occurrence of intermolecular C-H_Cp?O_bipyO-H interactions in the solid-state gave rise to an extended 1-D network. The electronic absorption spectra and the electrochemical behavior of these complexes are discussed. Density functional theory (DFT) was used for geometry optimization of the singlet states in solution and for electronic structure calculations. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in 2 was performed.

Full text of DOI:10.1039/c6dt04085c

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Cyclometalated platinum(II) complexes of
2,2’-bipyridine N-oxide containing a
1,1’-bis(diphenylphosphino)ferrocene ligand:
structural, computational and electrochemical
studies†‡
Cite this: DOI: 10.1039/c6dt04085c
Hamid R. Shahsavari,*a Masood Fereidoonnezhad,*b,e Maryam Niazi,a
S. Talaat Mosavi,a Sayed Habib Kazemi,a Reza Kia,*c Shima Shirkhan,b
Siamak Abdollahi Aghdama and Paul R. Raithbyd
The preparation and characterization of new heteronuclear-platinum(II) complexes containing a 1,1’-bis
(diphenylphosphino)ferrocene (dppf) ligand are described. The reaction of the known starting complex
[PtMe(κ2N,C-bipyO-H)(SMe2)], A, in which bipyO-H is a cyclometalated rollover 2,2’-bipyridine N-oxide,
with the dppf ligand in a 2 : 1 ratio or an equimolar ratio led to the formation of the corresponding bi-
nuclear complex [Pt2Me2(κ2N,C-bipyO-H)2(µ-dppf)], 1, or the mononuclear complex [PtMe(κ1C-bipyO-H)
(dppf)], 2, respectively. According to the reaction conditions, the dppf ligand in 1 and 2 behaves as either
a bridging or chelating ligand. All complexes were characterized by NMR spectroscopy. The solid-state
structure of 2 was determined by the single-crystal X-ray diffraction method and it was shown that the
chelating dppf ligand in this complex was arranged in a “synclinal–staggered” conformation. Also, the
occurrence of intermolecular C–HCp⋯ObipyO-H interactions in the solid-state gave rise to an extended
1-D network. The electronic absorption spectra and the electrochemical behavior of these complexes are
discussed. Density functional theory (DFT) was used for geometry optimization of the singlet states in
solution and for electronic structure calculations. The analysis of the molecular orbital (MO) compositions
in terms of occupied and unoccupied fragment orbitals in 2 was performed.
Received 24th October 2016,
Accepted 12th January 2017
DOI: 10.1039/c6dt04085c
www.rsc.org/dalton
coordination chemistry.1–3 One of the most important ferro-
cenyl phosphine compounds is 1,1′-bis(diphenylphosphino)
Introduction
ferrocene (dppf).1,2,4 The dppf is a biphosphine ligand and
adopts numerous coordination modes in the complexes of a
range of transition metals.1–18 These dppf complexes have
been utilized in several areas such as catalysis, materials
science, electrochemistry, and biology.1,2,7,9,10,19–21
Ever since the serendipitous synthesis of ferrocene, numerous
derivatives have been prepared.1,2 In this regard, ferrocenyl
phosphine compounds are a well-known family of ferrocene
derivatives that are extensively used in organometallic and
On the other hand, the chemistry of cyclometalated com-
plexes is of great interest because of their use in a wide range
of applications.22–26 For example, one of the most fascinating
classes of cyclometalated complexes is the rollover category.27–30
In this family, the cyclometalated moieties are bidentate hetero-
cyclic donors (2,2′-bipyridine (bipy),29 2,2′-bipyridine N-oxide
(bipyO)31) rather than classical monodentate ligands (2-phenyl-
pyridine (ppy),24 2-vinylpyridine (vpy)32,33). The bipyO ligand
has been by far less explored than the bipy ligand in rollover
cyclometalation31 or in coordination chemistry.34 The 2,2′-bi-
pyridine N-oxide ligand has the two most important possible
binding modes in coordination to different metals (Chart 1)
which are related to the nature of the central metal.31,34
aDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Yousef Sobouti Blvd., Zanjan 45137-6731, Iran. E-mail: shahsavari@iasbs.ac.ir
bDepartment of Medicinal Chemistry, School of Pharmacy, Ahvaz Jundishapur
University of Medical Sciences, Ahvaz, Iran. E-mail: fereidoonnezhad-m@ajums.ac.ir
cChemistry Department, Sharif University of Technology, P.O. Box 11155-3516,
Tehran, Iran. E-mail: rkia@sharif.edu
dDepartment of Chemistry, University of Bath, Claverton Down, Bath, Avon BA2 7AY,
UK
eCancer, Environmental and Petroleum Pollutants Research Center,
Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
†This paper is dedicated to Professor Georg Süss-Fink.
‡Electronic supplementary information (ESI) available: Full NMR spectra, crys-
tallographic, electrochemistry, computational and molecular simulation details.
CCDC 1511315. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6dt04085c
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the binuclear complex [Pt2Me2(κ2N,C-bipyO-H)2(µ-dppf)], 1,
which formed as a yellow precipitate. In this complex the dppf
ligand adopts an “open bridge” conformation.2,5–10 However,
when complex A was treated with one equivalent of the dppf
ligand the Pt–NbipyO-H bond easily dissociated, and the mono-
nuclear complex [PtMe(κ1C-bipyO-H)(dppf)], 2, was formed.
This bond breaking is related to the potent chelating ability of
the dppf ligand due to its large P^P bite angle.2 A similar bond
cleavage reaction has been observed in analogous cycloplati-
nated(II) complexes.6,10,31,39–41 Complexes 1 and 2 are air-stable
and obtained in high yields. They are soluble in halogenated
solvents such as CH2Cl2 or CHCl3 and stable for several days at
room temperature in solution.
Chart 1 The potential binding modes of the 2,2’-bipyridine N-oxide
ligand: left, bipyO-H mode and right, bipyO mode.
Complexes 1 and 2 were characterized by elemental ana-
lyses and their integrity in solution was confirmed by multi-
nuclear NMR spectroscopy (1H, 31P and 195Pt (CDCl3)), more
details are collected in the Experimental section. The sche-
matic labeling for complexes is shown in Scheme 2.
The structure of binuclear complex 1 was established firstly
by 1H NMR spectroscopy (Fig. S1‡). In complex 1 the equi-
valent methyl signal (δ = 0.90 ppm) is split into a doublet by
As part of our ongoing research into the cyclometalation
chemistry of platinum,32,35–37 we now present the preparation
and characterization of new neutral platinum(II) rollover
complexes bearing the dppf as a spacer or chelating ligand.
Furthermore the optical and electrochemical properties of
these complexes are described, the results being supported by
theoretical calculations.
Results and discussion
Synthesis and characterization
The new chemistry to prepare cycloplatinated(II) rollover com-
plexes is depicted in Scheme 1.
The precursor complex [PtMe(κ2N,C-bipyO-H)(SMe2)], A,31
in which bipyO-H is a cyclometalated rollover 2,2′-bipyridine
N-oxide, was prepared by a published method, by reaction of
the known dimeric dimethylplatinum(II) complex cis,cis-
[PtMe2(μ-SMe2)2PtMe2]38 with 2 equivalents of 2,2′-bipyridine
N-oxide in acetone. Complex A was reacted with 0.5 equivalents
of the 1,1′-bis(diphenylphosphino)ferrocene (dppf) ligand by
replacement of the labile dimethyl sulfide ligand with the phos-
phorus donor atoms of the dppf ligand. This reaction afforded
Scheme 2 Representative ligands with position labeling.
Scheme 1 The mononuclear and binuclear cycloplatinated(II) rollover complexes containing a dppf ligand.
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the 31P nucleus (3JPH = 7.2 Hz) and there is also splitting of the trans coupling constants to the P atoms.6,18 The platinum
signal from the 195Pt nucleus (2JPtH = 82.2 Hz). The platinum coupling constant value obtained for 2 was significantly lower
coupling constant value confirms that the Me groups are than the observed value for 1 and it was confirmed that the
located trans to the nitrogen atoms rather than trans to the methyl ligand in complex 2 adopts the trans C–Pt–P arrange-
phosphorus atoms.6,8 Also, the observation of a single methyl- ment. The α and β protons of the dppf ligand appeared as
platinum signal confirms that the two PtMe(bipyO-H) moieties eight broad singlet signals between δ = 3.47–4.97, confirming
are equivalent.6,8 Another useful piece of evidence for confirm- that all hydrogen atoms of the Cp rings are in different
ing this equivalency was the observation of only one low field environments, i.e. the two Cp moieties are inequivalent. This
signal for each of the two H3′ (δ = 9.91 ppm) or H6 (δ = pattern indicates that the dppf chelate is fairly rigid and
8.11 ppm) protons of the pyridyl ring of bipyO-H ligands.6–8,31 adopts the staggered conformation.2,6 These data are consist-
Moreover, the uncommon chemical shift observed for the H3′ ent with the replacement of the nitrogen atom of the cyclo-
proton is dependent on the orientation of this proton toward metalated ligand by means of the dppf ligand and confirm the
the N-oxide (N–O) group.31,42 The two broad singlet resonances high chelating ability of this ligand.2 The 31P {1H} NMR spec-
at δ = 4.39 and 4.30 can be assigned to the four α and β trum of complex 2 is shown in Fig. 1, and it confirmed the che-
protons of the cyclopentadienyl moieties (Cp) of the dppf lated coordination of the dppf ligand. It displays two distinc-
ligand, respectively. This observation confirmed the equi- tive doublets at δ = 19.1 ppm and 23.6 ppm, assigned to the
valency of the two Cp rings in the dppf ligand.6–10 The 31P {1H} chemically different phosphorus atom donors, with platinum
NMR spectrum of complex 1 (Fig. S2‡) revealed only a singlet satellites of 1JPt–P = 2191 Hz (Pa atom trans to the κ1C-bipyO-H)
1
1
signal along with 195Pt satellites at δ = 23.3 ppm with JPtP
=
and JPt–P = 1959 Hz (Pb atom trans to the Me), respectively.
2307 Hz, which supports the assignment that both P atoms This result indicates that the methyl ligand exerted a higher
were equivalent and positioned trans to
a coordinated trans influence than the coordinated carbon atom of the
carbon atom of the cyclometalated ligand. The 195Pt {1H} NMR bipyO-H ligand.6 As will be shown in the Structural determi-
spectrum of complex 1 (Fig. S3‡) displayed a doublet at nation section, the Pt–Pb bond distance is somewhat longer
1
δ = −4167 ppm with JPtP = 2316 Hz, which was close to than that of the Pt–Pa bond length. It is interesting to note
the obtained value from the 31P {1H} NMR spectrum. that perhaps owing to the dppf ligand exerting more strain in
Additionally, these observations confirmed that the dppf its chelating form, the 195Pt coupling constant value (P atom
ligand was symmetrically bridged between the two platinum trans to the C atom of the cyclometalated ligand) in 2 is notice-
centers.6–10
ably lower than in 1 (acting as a bridging ligand).6 Also, in 2 a
1
a b
The H NMR spectrum of complex 2 was particularly infor- cis-P–P coupling (2JP P = 17.2 Hz) was detected, clearly con-
mative and in accordance with the suggested formulation firming that the two inequivalent phosphorus atoms couple to
(Fig. S4‡). This spectrum exhibited a doublet of doublet reso- each other through the platinum center.6,18 Consistent with
nance for Pt–Me protons at δ = 0.03 ppm which coupled to the the 31P {1H} NMR spectrum, in the 195Pt {1H} NMR spectrum
platinum center (2JPtH = 65.6 Hz) and to two different phos- (Fig. S5‡) of 2, a doublet of doublets was observed at δ =
phorus atoms (3JPH = 8.7 and 6.6 Hz) representing the cis and −4578 ppm with 1JPtP = 2193 Hz and 1968 Hz.
Fig. 1 31P {1H} NMR spectrum of complex 2 in CDCl3 at room temperature.
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Fig. 2 Two orientations of the unique molecule of 2, as determined crystallographically, with thermal ellipsoids at the 40% probability level.
Hydrogen atoms have been omitted for clarity. Pt(1)–C(100) 2.075(10), Pt(1)–C(38) 2.127(10), Pt(1)–P(3) 2.301(3), Pt(1)–P(4) 2.295(3), P(3)–C(21)
1.803(10), P(4)–C(37) 1.821(11), P(4)–Pt(1)–P(3) 97.96(10), C(100)–Pt(1)–C(38) 85.5(4), C(100)–Pt(1)–P(3) 89.7(3), C(100)–Pt(1)–P(4) 172.0(3), C(38)–
Pt(1)–P(3) 175.2(3), C(38)–Pt(1)–P(4) 86.8(3).
Structural determination
The structure of 2 was determined by X-ray crystallography and
two views of the molecule are shown in Fig. 2. Crystallographic
data are summarized in Table S1.‡ Complex 2 contains a
square planar platinum(II) center with the methyl group, the
ortho C of the bipyO-H ligand, and two P atoms of the dppf
ligand acting as the coordinating atoms. The angles around
the Pt center are rather close to the ideal angle of 90°, and this
could be contrasted with the strain observed in, for example,
organoplatinum complexes containing bis(diphenylphos-
phino)amine, dppa, as a chelating ligand.43
The imine N is not coordinated and is positioned opposite
to the platinum center, and this is in contrast to the usual pre-
ference of the bipyO-H ligand to form cyclometalated com-
plexes.31 The dppf ligand is arranged in the usually preferred
synclinal–staggered conformation.4 The Pt(1)–P(3) distance of
2.301(2) Å is longer than the Pt(1)–P(4) distance of 2.295(3) Å,
indicating that the methyl ligand probably exerts a higher trans
influence than the bipyO-κ1C ligand.31 This observation is in
Fig. 3 The crystal packing of 2 viewed down the b-axis showing a 1-D
agreement with NMR spectroscopic data. On the other hand, extended chain along the a-axis by the intermolecular C–H⋯O
interactions.
the geometry around the metal center is also affected by the
intramolecular π⋯π [3.670(6) Å] interaction of the pyridine-N-
oxide segment and one of the phenyl rings of the dppf ligand.
tion spectral data for the dppf ligand3,19 are included for
The interesting feature of the crystal packing of complex 2
is connection of the neighboring molecules into a 1-D
extended chain along the a-axis by the intermolecular
C–H⋯O [C14–H14A⋯O1i, (i) 1 − x, − 1/2 + y, 9/2 − z and
C22–H22A⋯O1ii, (ii) − 1 + x, y, z] hydrogen bonding (Fig. 3).
comparison.
Complex A shows high energy and more intense transitions
between 280–320 nm (Table S2‡). These transitions are attribut-
able to the intraligand charge transfer transitions (1ILCT, π →
π*) located on the bipyO-H cyclometalated ligand which are
slightly perturbed by coordination to the metal center.10,44 The
less intense bands at 361 nm and 388 nm are assignable to
Optical properties
The UV-vis absorption spectra of all complexes A, 1 and 2 were mixed spin-allowed 1IL and 1MLCT transitions (metal to ligand
obtained in CH2Cl2 at ambient temperature (Fig. 4) and the charge transfer).32,44 The spectrum of the dppf ligand displays
data are summarized in Table S2.‡ Also, the electronic absorp- two main absorption bands (Table S2‡). The more intense
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the bipyO-H cyclometalating ligand and the dppf ligand. Each
part of the percentage contributions is the sum of the atomic
orbital coefficient squares. The frontier molecular orbitals (5 occu-
pied MOs and 5 unoccupied MOs) are listed according to their
energy and segment contribution. The assignment of each MO
was made on the basis of its composition from the segments.
The composition of the LUMO is localized on the dppf ligand
(π* character) but the HOMO orbital is mainly localized on the
bipyO-H ligand. The HOMO−1 and HOMO−2 orbitals have
significant contribution from the Pt metal centre (Table S3‡).
Analysis of chemical bonding (extent of donation and back-
donation by charge decomposition analysis). The chemical
bonding between the platinum center and the ligands in
complex 2 can be explained on the basis of donation from the
ligands to the metal and π-back-donation from the metal
segment to the ligand(s) (σ-donation & π-back-donation) in
terms of charge decomposition analysis (CDA) and extended
charge decomposition analysis (ECDA).47,48 The CDA and
Mulliken population analysis (MPA) studies were used to
provide better qualitative and quantitative understanding of
the chemical bonding in this complex. The procedure for
calculating the contributions for each occupied molecular
orbital of complex 2: (i) charge donation through the com-
bination of the occupied orbitals of the ligands and the un-
occupied orbitals of the metal segment which can be further
split into σ and π donation, (ii) charge π-back-bonding through
the combination of the occupied orbitals of the metal segment
and the unoccupied orbitals (mostly π*) of the ligand and (iii)
electronic polarization of the metal segment and the ligand.
The electronic polarization of fragments is due to the distor-
tion of the electron distribution of the fragments on each
other and includes the interactions between all permanent
charges and charge multipoles and induced multipoles. The
CDA gives reasonable estimates of donation and π-back-donation
between the molecular segments only if there is no electronic
polarization or it is significantly small. If it is not the case, the
difference between the amount of the electron donation and
back-donation will not be the same as the net charge transfer
between the segments, as calculated from the sum of the atomic
charges. Table 1 summarizes some of the properties derived
from the charge decomposition analysis of complex 2. It can be
Fig. 4 Electronic absorption spectra of complexes A, 1, 2 and the dppf
ligand in CH2Cl2 solution (5 × 10−5 M) at room temperature (inset: low
energy region).
band at higher energy (in the UV region) is due to the π → π*
transition and the less intense visible absorption band is
mostly due to the 1MLCT transition (d(FeII) → π*).19,45
Complexes 1 and 2 reveal two main bands with different
extinction coefficients (Table S2‡). The high energy band in
the UV region is remarkably intense and red shifted relative to
that of the reported band for complex A. This profile can be
ascribed to a the π → π* (1ILCT) transition which is localized
on the bipyO-H cyclometalated and the dppf ligands.10,19
While the low intensity bands (ε = 8.6–9.2 M−1 cm−1) in the
visible region for 1 are similar to complex A with a slight red
shift, probably this shift is associated with increasing of contri-
bution of the 1MLCT transition and the dppf ligand has hardly
any influence on these lower energy profiles (Fig. 4).10
However, in contrast, in 2 the less intense peaks (ε = 3.4–6.1
M−1 cm−1) in this region are admixture transitions and are
assignable to 1L′LCT (ligand to ligand charge transfer,
π bipyO-H → π* dppf) with a minor contribution of 1MLCT
(d(PtII) → bipyO-H), an assignment which is supported by
time-dependent DFT (TD-DFT) calculations (see below).
Computational studies
Electronic structure. The singlet state geometry of complex 2
was optimized in the gas phase using the B3LYP functional of
the Gaussian 09 program package.46 The molecular orbitals
(MO) and singlet excited states of this complex were also
studied. The data for the composition of the most important
occupied and unoccupied molecular orbitals and the atomic
orbital compositions for each complex (%) are listed in
Table S3.‡ The contour plots of the selected important frontier
molecular orbitals of 2 are depicted in Fig. S6.‡ Because of the
important role of the frontier molecular orbitals in the elec-
tronic excitations and the electronic transition characteristics,
it will be useful to map the highest occupied (HOMO) and
lowest unoccupied (LUMO) orbitals of 2 on the framework for the
excited-state TD-DFT calculations. Therefore, 2 was divided into
four segments: the platinum centre (Pt), the methyl group (Me),
Table 1 Summary of the computed properties of complex 2
Property
%LUFOaPt in OMOsc
89.16 (1.78e)
35.51 (0.71e)
21.39 (0.43e)
0.71e
0.43e
0.12e
%HOFObbipyO-H in UMOsd
%HOFOdppf in UMOs
Donation (au); Me → Pt
Donation (au); bipyO-H → Pt
Donation (au); dppf → Pt
π-CTe (au); Pt → bipyO-H
π-CT (au); Pt → dppf
0.05e
0.24e
a LUFO = lowest unoccupied fragment orbital. b HOFO = highest occu-
pied fragment orbital. c OMOs = occupied molecular orbitals in the
complex. d UMOs = unoccupied molecular orbitals in the complex.
e π-Back-donation from the Pt fragment to L.
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Table 2 Selected TD-DFT calculated excitation energies and compo-
sitions of the lowest-lying singlet excited states for complex 2
ations, wherein a linear combination of several occupied-to-
unoccupied MO excitations comprises a given optical tran-
sition. Assignment of the character of each excited state was
based on the compositions of the occupied and unoccupied
MOs of the dominant configuration(s) for that excited state.
The highest occupied molecular orbital (HOMO) of complex 2
contains contributions from the bipyO-H ligand (91.6% of p
orbitals). The HOMO−1 and HOMO−2 are mainly located on
the Pt and bipyO-H ligand. On the other hand, the LUMO
and LUMO+1 have significant contribution from the dppf
ligand. The low-energy calculated absorption band (437 nm) in
complex 2 corresponds to excited state S7. It is attributed to
E (eV)/λ
States (nm)
f
λexp Type
Assignment
7
3.92/437 0.05 449 HOMO → LUMO (71%)
HOMO−1 → LUMO (9%)
4.94/251 0.06 248 HOMO−5 → LUMO+1
(33%)
LL′CT, MLCT
39
π → π*(IL)
H
and L refer to the highest-occupied and lowest-unoccupied
molecular orbitals, respectively.
noted from Table 1 that based on the quantitative data resulting the charge transfer from the bipyO-H ligand to the dppf ligand
from CDA calculations by AOMix 6.88 software the σ-donation and minor charge transfer from the Pt to the bipyO-H ligand
character of the methyl group is greater than the bipyO-H ligand (HOMO−1 → LUMO contribution). The high-energy absorp-
and this is in line with the stronger trans influence of the methyl tion band (251 nm) corresponds to excited state 39 and is
group in lengthening of the opposite bond.
attributed to the charge transfer from molecular orbitals
Calculated electronic absorption spectra. TD-DFT calcu- which are mainly localized on the bipyO-H and the dppf
lations were employed to examine the 50 low-lying singlet ligands. Electron density difference maps (EDDMs) derived
excited states of complex 2. The overlay of the experimental from the TD-DFT calculations were used to show the electron
and simulated UV-vis spectra of complex 2 is depicted in density changes between the ground and excited states upon
Fig. S7.‡ It can be seen that the simulated optical absorption different electronic excitations. It represents a way for visuali-
spectra from the TD-DFT calculations reproduce the main zing the electronic distribution, for which one can subtract the
features of the experimental spectra. Selected low-lying singlet ground-state electron density (S0) from the Franck–Condon
excited states together with their vertical excitation energies, electron density of the excited state, thereby providing a
oscillator strengths and assignment of the transitions for picture of the redistribution of the electron density after the
complex 2 are summarized in Table 2. The energy of each vertical transition from the ground-state to any of the Franck–
excited state is the vertical excitation energy in electron volts Condon excited states. Visualization of these difference
from the ground state. The spectral assignment is on the basis density plots can provide an insight into the subsequent geo-
of the comparison of experimental band maxima with the cal- metric changes occurring on the excited-state potential energy
culated energies of the transitions with reasonable oscillator surface, and help to determine what type of excitation is occur-
strengths. The excitation of an electron from an occupied to an ring.49,50 The electron density difference maps (EDDMs) of
unoccupied molecular orbital is an experimental model for the complex 2 are shown in Fig. 5, confirming the assignment of
excited state (i.e., a one-electron picture). However, the excited the calculated electronic transitions in Table 2.
states calculated herein demonstrate that excited-state elec-
Electrochemistry. The electrochemical behavior of the free
tronic structures are best described in terms of multi-configur- dppf ligand and complexes 1, 2, [Pt2Me2(κ2N,C-ppy)2(µ-dppf)],
Fig. 5 Electron density difference maps (EDDMs) for the main transitions in complex 2. Red indicates a decrease in charge transfer, while green
indicates an increase.
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3,6 and [PtMe(κ1C-ppy)(dppf)], 4,6 in which ppy = 2-phenylpyri- that the dppf group in the structure of complex 2 does not
dinyl, between −2.0 and +2.0 V vs. reference electrode at a scan have insignificant changes in its electrochemical behavior.
rate of 5 mV s−1 is presented in Fig. S8‡ (complexes 3 and 4 Therefore, for 2 the electrochemistry is relatively similar to the
were selected for comparison purpose). Oxidation and Pt electrochemistry; and may indicate a lower stability of this
reduction potentials of these complexes are also provided in complex and possible destruction of complex 2 under poten-
Table S4.‡
tial cycling during electrochemical studies. Of course, both
The dppf ligand oxidation onset is around +0.5 V and the complexes 1 and 2 show a broad peak of dppf but in the case
peak potential is approximately located at 1.25 V vs. Ag/AgCl.19 of 1 redox peaks of the ferrocene part (Fc) are more dis-
Also, the cyclic voltammogram of the dppf ligand in aceto- tinguishable. Perhaps, in complex 1, the dppf ligand (particu-
nitrile shows almost irreversible kinetics in which a reduction larly, the Fc part) becomes more available for electron transfer.
peak was observed. The complexes containing the dppf ligand On the other hand, the redox behavior of complex 3 is similar
in their structure were also investigated by cyclic voltammetry. to complex 1, especially as seen in Fig. 6 in the potential range
As shown in Fig. S8,‡ both complexes 1 and 2 show a pair of of −1.0 V to 1.0 V. The redox behavior of complexes 1–4 seems
quasireversible oxidation and reduction peaks which can be to be nearly independent of the bipyO-H or ppy groups on
assigned to the Pt redox activity. Of course, the oxidation their structure.
peaks of the dppf ligand and Pt are overlapped but the
reduction peaks appearing at approximately −1.25 V originated
from the reduction of Pt. It was observed that the oxidation of
Pt and the dppf ligand occurs at more positive potentials com-
Conclusion
pared to free Pt or the dppf ligand. Moreover, cyclic voltammo- Dppf is an excellent ligand for the development of a molecular
grams of complexes 3 and 4 revealed that the oxidation behav- architecture with transition metals. The cycloplatinated
ior of the dppf ligand occurred at more positive potentials with complex A was smoothly reacted with the dppf ligand (in
lower peak currents compared to the free dppf ligand behavior. varying molar ratios) to produce different products due to the
Additionally, a broad reduction wave was observed for both various coordinating abilities of this ligand. When 0.5 equi-
complexes around −1.0 V which can be attributed to the valents of the dppf ligand were used it acted as a spacer ligand
reduction of Pt. Detailed electrochemical studies of the dppf and the dinuclear complex 1 was formed. In contrast, when
containing species in the narrower potential range were one molar equivalent of this ligand was applied, the dppf
carried out and it was found that only complexes 1 and 3 easily displaced the bipyO-H nitrogen to provide the P^P che-
display significant electrochemical activity in the range of −1.0 lated mononuclear complex 2; this behavior is related to a
to +1.0 V. Fig. 6 depicts the voltammograms of these two large bite angle of dppf. In the crystal structure of 2 the dppf
complexes.
ligand was found to have a synclinal–staggered conformation
As seen in Fig. S8,‡ the voltammogram of complex 2 is for the Cp rings. Moreover, this complex showed an interesting
similar to the electrochemical behavior of the Pt electrode, hydrogen bond interaction between C–HCp⋯ObipyO-H (Fig. 3)
which may indicate the lower interaction of Pt with the dppf generating a one-dimensional network structure in the solid-
ligand in this complex compared to complex 1. It may suggest state. The platinum–phosphorus bond distance for the phos-
phorus atom trans to the coordinated carbon atom of the
bipyO-H ligand was slightly shorter than the phosphorus atom
trans to the methyl ligand. This difference suggests that the
κ1C-bipyO-H ligand perhaps exerted a lower trans influence
than the Me ligand. Also, the charge decomposition analyses
and molecular orbital diagrams revealed the nature of the
compositions of the molecular orbitals of complex 2 based on
the fragment analysis and the charge donation and back-
donation character of the coordinated ligand. These calcu-
lations confirmed the greater trans influence of the Me ligand
than the C atom of the cyclometalating ligand in complex 2.
The UV-vis and electrochemical properties of these com-
plexes have been investigated. The lower energy bands in 1
and 2 were assignable to mixed spin-allowed IL/1MLCT or L′
LCT/1MLCT (which is supported by TD-DFT calculations),
respectively. Complexes 1 and 2 displayed a pair of quasirever-
sible oxidation and reduction peaks. They could be assigned to
the platinum center and the dppf unit. These platinum com-
plexes presented here could thus also potentially be bio-
logically active and the screening of their anticancer behavior
is currently under investigation.
1
1
Fig. 6 The cyclic voltammograms of complexes 1 and 3 in 0.05 M
LiClO4–acetonitrile at a scan rate of 5 mV s−1
.
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CH2Cl2 solution of this complex. X-ray intensity data were col-
lected using the full sphere routine by φ and ω scans strategy
Experimental
All NMR spectra (1H, 31P {1H} and 195Pt {1H}) were recorded on a on the Agilent SuperNova dual wavelength EoS S2 diffrac-
Bruker Avance DPX 400 MHz instrument. References were TMS tometer with mirror monochromated Mo Kα radiation (λ =
or the residual peak of the solvent, i.e. CDCl3 (1H), 85% H3PO4 0.71073 Å). The crystal was cooled to 150 K using an Oxford
(31P), and aqueous Na2PtCl6 (195Pt). The chemical shifts (δ) were Diffraction Cryojet low-temperature attachment. The data
reported as ppm and coupling constants (J) expressed in Hz. The reduction, including an empirical absorption correction using
microanalyses were performed using a vario EL CHNS elemental spherical harmonics, implemented in the SCALE3 ABSPACK
analyzer. All solvents were purified and dried according to stan- scaling algorithm,52 was performed using the CrysAlisPro soft-
dard procedures.51 2,2′-Bipyridine N-oxide and 1,1′-bis(diphenyl- ware package.53 The crystal structure was solved by direct
phosphino)ferrocene were purchased from Aldrich. The precur- methods using the online version of AutoChem 2.0 in conjunc-
sor complex [PtMe(κ2N,C-bipyO-H)(SMe2)], A,31 was prepared by tion with the OLEX2 suite of programs implemented in the
the literature method. The NMR labeling for all ligands is shown CrysAlis software.54,55 The non-hydrogen atoms were refined
in Scheme 2 for clarifying the chemical shift assignments.
anisotropically. All of the hydrogen atoms were positioned geo-
metrically in idealized positions and refined with the riding
model approximation, with Uiso(H) = 1.2 or 1.5Ueq(C). For the
Synthesis of complexes
[Pt2Me2(κ2N,C-bipyO-H)2(µ-dppf)], 1. To
a
solution of molecular graphics the program SHELXTL was used.56 All
complex A (100 mg, 0.22 mmol) in acetone (10 mL) was added geometric calculations were carried out using the PLATON
0.5 equivalents of dppf (62.5 mg, 0.11 mmol) and the solution software.57
was stirred for 2 h at room temperature. A yellow solid was preci-
pitated which was separated and dried under vacuum. Yield:
68%, mp = 253 °C. Anal. Calcd for C56H48FeN4O2P2Pt2 (1317):
Computational details
C, 51.07; H, 3.67; N, 4.25. Found: C, 50.35; H, 3.51; N, 4.09. NMR Density functional theory (DFT) calculations have been per-
2
3
data in CDCl3: δ(1H) 0.90 (d, JPtH = 82.2 Hz, JPH = 7.2 Hz, 6H, formed using the Gaussian 09 package to perform geometry
2 Me groups), 4.30 (br s 4H, β and β′ of Cp protons), 4.39 (br s, 4H, optimizations, the vibrational frequencies and the electronic
3
α and α′ of Cp protons), 6.64 (t, JHH = 6.4 Hz, 2H, H5′), 7.18 (t, structures of complex 2.46 A frequency calculation after each
3JHH = 7.3 Hz, 2H, H5), 7.33–7.40 (m, 8H, overlapping multiplets), geometry optimization ensured that the calculated structures
3
7.41–7.47 (m, 4H, overlapping multiplets), 7.60 (t, JHH = 7.4 Hz, are real minima in the potential energy surface of the mole-
2H, H4′), 7.63–7.75 (m, 8H, overlapping multiplets), 7.76–7.86 (m, cules. The structure of complex 2 was optimized using the
3
4H, overlapping multiplets), 8.11 (d, JHH = 6.2 Hz, 2H, H6), 9.91 B3LYP exchange–correlation functionals with the quasirelati-
3
3
1
(d, JHH = 8.1 Hz, JPtH = 54.7 Hz, 2H, H3′); δ(31P) 23.3 (s, JPtP
=
vistic Stuttgart–Dresden (SDD) effective core pseudopotential
(ECP) and the corresponding set of basic functions for the Pt
2307 Hz, 2P of dppf); δ(195Pt) −4167 (d, 1JPtP = 2316, 2Pt).
[PtMe(κ1C-bipyO-H)(dppf)], 2. To a solution of complex A atom and 6-31G* (five pure d functions) for C, H, N, O, and P.58
(100 mg, 0.22 mmol) in acetone (10 mL) was added 1 equivalent Molecular orbital (MO) compositions and the overlap popu-
of dppf (125 mg, 0.22 mmol) and the solution was stirred for lations were calculated using the AOMix 6.88 program.59,60
2 h at room temperature, which produced a yellow solution. Atomic charges were calculated using the Mulliken population
After removing of the solvent under reduced pressure, the analysis (MPA) as implemented in Gaussian 09. The analysis of
residue was treated with diethyl ether (2 × 3 mL). The precipitate the molecular orbital (MO) compositions in terms of the
as a yellow solid was dried under vacuum. Yield: 78%, mp = highest occupied orbitals and lowest unoccupied orbitals of the
234 °C. Anal. Calcd for C45H38FeN2OP2Pt (936.7); C, 57.76; fragment species (HOFOs and LUFOs, respectively) and con-
H, 4.09; N, 2.99. Found: C, 56.84; H, 3.69; N, 3.08. NMR data in struction of orbital interaction diagrams were performed by the
2
3
CDCl3: δ(1H) 0.03 (dd, JPtH = 65.6 Hz, JPH = 8.7, 6.6 Hz, 3H, 1 AOMix 6.88 program. Charge decomposition analysis (CDA)
Me group), 3.47, 3.53, 3.99 and 4.06 (each br s, 4H, β and β′ of implemented in the AOMix 6.88 program was used to provide
Cp protons), 4.19, 4.31, 4.60 and 4.97 (each br s, 4H, α and α′ of better qualitative and quantitative understanding of the nature
3
Cp protons), 6.46 (t, JHH = 6.9 Hz, 1H, H5′), 7.01–7.21 (m, 6H, of the chemical bonding in the complex based on the electron
overlapping multiplets), 7.27–7.42 (m, 11H, overlapping multi- donation and back-donation between the metal and other
plets), 7.45–7.52 (m, 3H, overlapping multiplets), 7.56–7.69 (m, fragments. Excited singlet states (50 states) were calculated by
3H, overlapping multiplets), 7.71–7.79 (m, 2H, H6, H6′), 8.39 (d, time-dependent DFT (TD-DFT) in CH2Cl2 solvent which was
1
2
3JHH = 4.4 Hz, 1H, H3′); δ(31P) 19.1 (d, JPtP = 2191 Hz, JPP
=
described by using a conductor-like polarizable continuum
17.2 Hz, 1P, Pa atom trans to κ1C-bipyO-H), 23.6 (d, JPtP = 1959 model (CPCM).61 The TD-DFT output contained information for
1
Hz, JPP = 17.2 Hz, 1P, Pb atom trans to Me); δ(195Pt) −4578 (dd, the excited-state energies and oscillator strengths ( f ) and a list
2
1JPtP = 2193 and 1968 Hz, 1Pt).
of the excitations that gave rise to each excited state, the orbitals
involved as well as the wavefunction coefficients of the exci-
tations. From the TD-DFT calculations the electronic distri-
X-ray crystallography
Single crystals of 2 were suitable for X-ray diffraction analysis bution and the localization of the singlet excited states were
and were grown by slow vapor diffusion of n-hexane into visualized using electron density difference maps (EDDMs).62
Dalton Trans.
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Paper
GaussSum3 was used for EDDM calculations and for the elec-
tronic spectrum simulation.63
7 S. M. Nabavizadeh, H. Amini, H. R. Shahsavari,
M. Namdar, M. Rashidi, R. Kia, B. Hemmateenejad,
M. Nekoeinia, A. Ariafard, F. Niroomand Hosseini,
A. Gharavi, A. Khalafi-Nezhad, M. T. Sharbati and
F. Panahi, Organometallics, 2011, 30, 1466–1477.
Electrochemistry
Cyclic voltammetry (CV) measurements were performed in a
three-electrode cell using an Autolab101 potentiostat (Eco-
Chemie, The Netherlands). In this system, the platinum elec-
trode and Ag/AgCl electrodes were employed as counter and
reference electrodes, respectively. All complexes 1–4 and the
dppf ligand were introduced into the structure of a carbon
paste electrode for electrochemical investigations. Cyclic vol-
tammograms of the complexes were recorded at room temp-
erature in acetonitrile containing 0.05 M lithium perchlorate
(LiClO4) as a supporting electrolyte under an inert nitrogen
atmosphere. The electrochemistry of complexes 1–4 and the
dppf ligand was studied in different potential windows. At
first, cyclic voltammograms were recorded between +2.0 and
−2.0 V at different scan rates. Then, the electrochemical pro-
perties were studied in a narrower potential window (−1.0 V to
+1.0 V).
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Acknowledgements
This work was supported by the Institute for Advanced Studies
in Basic Sciences (IASBS) Research Council and the Iran
National Science Foundation (Grant No. 95822524).
Collaboration of Medicinal Chemistry, Faculty of Pharmacy,
Ahvaz Jundishapur University of Medical Sciences, in provid-
ing the required facilities for this work is greatly acknowl-
edged. RK is thankful to the Sharif University of Technology
research Council for the research facility. RK also thanks Prof.
Paul R. Raithby and Bath University for their support. PRR is
grateful to the Engineering and Physical Sciences Research
Council (EPSRC) for continued funding (EP/K004956/1).
Thanks are also due to Mr A. Biglari, the operator of the Bruker
NMR instrument at IASBS, for recording the NMR spectra.
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