Comparison of coordination mode of some biphosphine ligands in cyclometalated organoplatinum(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2014.01.008

The reaction of the cyclometalated complexes [PtR(?N)(SMe ₂)], 1, in which R is Me or 4-MeC₆H₄, and ?N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo-h-quinoline), with 1,2-bis[bis(pentafluorophen

Full text of DOI:10.1016/j.jorganchem.2014.01.008

Journal of Organometallic Chemistry 755 (2014) 93e100
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Comparison of coordination mode of some biphosphine ligands in
cyclometalated organoplatinum(II) complexes
*
Ahmad R. Esmaeilbeig , Mohsen Golbon Haghighi, Sahebeh Nikahd, Sara Hashemi,
*
Mohammad Mosarezaee, Mehdi Rashidi, S. Masoud Nabavizadeh
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the cyclometalated complexes [PtR(C^N)(SMe2)], 1, in which R is Me or 4-MeC6H4, and C^N
is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo-h-quinoline), with 1,2-bis
[bis(pentafluorophenyl)phosphino]ethane (dfppe), were studied. When 0.5 equiv of dfppe was added,
Received 11 November 2013
Received in revised form
7 January 2014
the binuclear cyclometalated Pt(II) complexes [Pt2R2(C^N)2(m-dfppe)], 2iel, were formed, which are the
Accepted 10 January 2014
first examples of diplatinum complexes with bridging dfppe ligands. When the ligand dfppe was reacted
with complex 1 in the ratio of 1:1, only in the case of R ¼ Me and C^N ¼ ppy, the complex [Pt(Me)(k1
-
Keywords:
ppy)(k
2-dfppe)], 4d, was obtained, in which the ppy ligand is monodentate and dfppe is chelating, and in
Organoplatinum
Phosphine ligands
DFT calculations
other cases the dimers 2jel were formed. The results were compared, using density functional theory
(DFT), with other diphosphine ligands, 1,1-bis(diphenylphosphino)methane (dppm) and 1,2-
bis(diphenylphosphino)ethane (dppe) to clarify the electronic and steric effects of diphosphine ligands
on their coordination mode ability.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
dimeric complexes with bridging dfppe [12e17]. However, to the
best of our knowledge, diplatinum complexes containing bridging
2-Phenypyridine or benzo[h]quinoline (abbreviated as HC^N)
and biphosphines (PP) are two of the most common and important
classes of ligands employed in organometallic chemistry. The
coupling of these two ligand types is of current interest, since the
resulting hybrid C^NePP ligands are expected to affect complex
properties. The cyclometalated platinum(II) complexes containing
deprotonated 2-phenylpyridine (ppy) or benzo[h]quinoline (bhq)
have already been reported as interesting complexes [1e11].
In this paper we report the effect of the presence of electron-
withdrawing substituents near the phosphorus atoms of the PP
ligand on its coordination mode when it is reacted with cyclo-
platinated(II) complexes [PtR(C^N)(SMe2)], in which R ¼ Me or 4-
MeC6H4 and C^N ¼ ppy or bhq. To this purpose, an electron-poor
diphosphine formally derived from substitution of phenyl hydro-
gens of dppe (1,2-bis(diphenylphosphino)ethane) with electro-
negative fluorine atoms (i. e. 1,2-bis[bis(pentafluorophenyl)
phosphino]ethane ¼ dfppe) is employed. The majority of the re-
ported transition metal complexes with dfppe are mononuclear
complexes bearing a single or two chelating dfppe ligand and
dfppe are not known. We have recently reported some mono- and
binuclear platinum(II) complexes, in each of which biphosphine
ligands such as 1,1-bis(diphenylphosphino)methane (dppm), dppe
and 1,10-bis(diphenylphosphino)ferrocene (dppf) act as mono-
dentate, chelating or bridging ligands [1,18e20].
2. Experimental section
The 1H NMR spectra were recorded by using a Bruker Avance
DPX 250 or DRX 500 spectrometer with TMS as reference. The 31
P
NMR spectra were recorded using a Bruker Avance DRX 500
spectrometer with 85% H3PO4 as reference. The microanalyses were
performed using a Thermofinigan Flash EAe1112 CHNO rapid
elemental analyzer. The complexes [PtR(C^N)(SMe2)], R ¼ Me or 4-
MeC6H4 and C^N
¼
deprotonated 2-phenylpyridine (ppy) or
deprotonated benzo[h]quinoline (bhq) were made by the known
methods [18]. 1,2-Bis[bis(pentafluorophenyl)phosphino]ethane
(dfppe) and dppm were purchased from commercial sources.
2.1. [Pt(Me)(k k
2-ppy)( 1-dppm)], 3a
* Corresponding authors. Tel.: þ98 711 613 7110; fax: þ98 711 646 0788.
E-mail addresses: esmaeilbeig@chem.susc.ac.ir (A.R. Esmaeilbeig), nabavi@
chem.susc.ac.ir, nabavizadeh@yahoo.com (S.M. Nabavizadeh).
To a solution of [Pt(Me)(ppy)(SMe2)], 1a, (43 mg, 0.1 mmol) in
acetone (20 mL) was added dppm (40 mg, 0.1 mmol). The mixture
0022-328X/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2014.01.008

94
A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
was stirred at room temperature for 1 h. After removal of the sol-
vent by evaporation, a residue was obtained which was washed
several times with hexane and dried under vacuum. It was isolated
as a yellow powder. Yield 60%; mp ¼ 115 ꢀC (decomp). Anal. Calcd.
3. Computational details
Density functional calculations were performed with the pro-
gram suite Gaussian03 [21] using the B3LYP level of theory [22e
24]. The LANL2DZ basis set [25e28] was chosen to describe Pt.
The 6-31G(d) basis set was used for other atoms. The solvent effects
were calculated by the CPCM model in acetone. The geometries of
complexes were fully optimized by employing the density func-
tional theory without imposing any symmetry constraints. To
evaluate and ensure the optimized structures of the molecules,
frequency calculations were carried out using analytical second
derivatives. In all cases only real frequencies were obtained for the
optimized structures.
for C37H33NP2Pt: C, 59.4; H, 4.4; N, 1.9; Found: C, 59.6; H, 4.3; N, 2.1.
3
Selected NMR in CDCl3:
d
(1H) ¼ 0.86 [d, 3H, JPH ¼ 8 Hz,
2JPtH ¼ 83 Hz, MePt], 3.37 [d, 2H, 2JPH ¼ 8 Hz, 3JPtH ¼ 19 Hz, CH2P2];
6.52 [ddd,1H, 3JH5 H6 ¼ 6 Hz, 3JH5 H4 ¼ 7 Hz, 4JH5 H3 ¼ 1 Hz, H5];
d
(
31P) ¼ 19.9
2
2
[d, JPP ¼ 72 Hz, 1JPtP ¼ 2082 Hz, 1 P],
d
¼ ꢁ25.2 [d, JPP ¼ 72 Hz,
3JPtP ¼ 64 Hz, 1 P].
2.2. [Pt(Me)(
k k
1-ppy)( 2-dfppe)], 4d
Dfppe (76 mg, 0.1 mmol) was added to
a
solution of
[Pt(Me)(ppy)(SMe2)], 1a, (43 mg, 0.1 mmol) in acetone (20 mL). The
mixture was stirred at room temperature for 20 h. After removal of
the solvent by evaporation, a residue was obtained which was
further purified by washing with cold ether. The product, as a white
solid, was dried under vacuum. Yield 65%, mp ¼ 240 ꢀC (decomp).
Anal. Calcd. for C38H15F20NP2Pt: C, 40.7; H, 1.4; N, 1.3; Found: C,
4. Results
4.1. Synthesis and characterization of new complexes
We have recently reported some mono- and binuclear plati-
num(II) complexes, containing biphosphine ligands such as dppm,
dppe and dppf. In these complexes diphosphine ligands act as
monodentate, chelating or bridging ligands [1,18e20] (see Scheme
1S in Supporting Information for summery of these reactions). The
reactions studied in the present work are summarized in Scheme 1.
The starting complexes [PtR(C^N)(SMe2)], 1, in which R ¼ Me or
4-MeC6H4 and C^N ¼ ppy or bhq, were prepared as reported pre-
viously [18,20]. As is depicted in Scheme 1 and Scheme 1S, the
reaction of a solution of the dimethylsulfide complexes 1 with 0.5
equiv of PP ligands (PP is dppm, dppe or dfppe), followed by
replacement of the labile ligand SMe2 with the P ligating atoms of
PP, at room temperature afforded the bridging PP complexes
41.0; H, 1.7; N, 1.4. Selected NMR in CDCl3:
d
(1H) ¼ 0.78 [br. s, 3H,
2JPtH ¼ 76 Hz, MePt]; 2.70 [br, 4H, CH2P]; 6.75e8.75 [aromatic
hydrogens];
d(
31P) ¼ 9.9 [br. s, 1JPtP ¼ 1693 Hz, P trans to ppy]; 16.7
[br. s, 1JPtP ¼ 1615 Hz, P trans to Me].
2.3. [Pt2(Me)2(ppy)2(m-dfppe)], 2i
Dfppe (38 mg, 0.05 mmol) was added to a solution of
[Pt(Me)(ppy)(SMe2)], 1a, (43 mg, 0.1 mmol) in acetone (20 mL). The
mixture was stirred at room temperature for 20 h. After removal of
the solvent by evaporation, a residue was obtained which was
washed with cold ether to give the product as a white solid. Yield
76%, mp ¼ 177 ꢀC (decomp). Anal. Calcd. for C50H26F20N2P2Pt2: C,
40.4; H, 1.8; N, 1.9; Found: C, 40.7; H, 1.7; N, 1.7. Selected NMR in
[Pt2R2(C^N)2(m-PP)], 2, in good yields. The complexes 2aeh (see
Scheme 1S) containing bridging dppm and dppe were also pre-
pared as described previously [18,20,29]. In the present work, the
CDCl3:
d
(1H) ¼ 0.47 [d, 3JPH ¼ 10 Hz, 2JPtH ¼ 80 Hz, 6H, MePt]; 3.39
analogous complexes [Pt2R2(C^N)2(m-dfppe)], 2iel, were synthe-
[br. s, 4H, CH2P]; 6.39e8.20 [aromatic hydrogens];
3JPP ¼ 69 Hz, 1JPtP ¼ 1898 Hz, PtP].
d(
31P) ¼ 9.0 [s,
sized and characterized by 1H and 31P NMR spectroscopy (see
Supporting Information for the detailed discussion about the
spectroscopic data for complex 2j). The 31P NMR data of
The following complexes were made similarly by using the
appropriate starting complexes 1 and dfppe:
[Pt2R2(C^N)2(
[Pt2R2(C^N)2(
m
m
-dfppe)]dimers, 2iel, along with those known for
-dppe)], 2eeh, are reported in Table 1S (Supporting
2.4. [Pt2(Me)2(bhq)2(
m
-dfppe)], 2j
Information). As can be seen, there is a steady increase in the values
of the PteP coupling constants, which shifts, for example, from
1898 Hz for 2i to 2085 Hz for 2e. This behavior can be associated
with the increasing electron-withdrawing character of the dfppe
ligand.
Yield 80%, mp
¼
179 ꢀC (decomp). Anal. Calcd. for
C
54H26F20N2P2Pt2: C, 42.3; H,1.7; N,1.8; Found: C, 42.7; H,1.6; N, 2.1.
Selected NMR in CDCl3:
d
(1H) ¼ 0.46 [d, 3JPH ¼ 10 Hz, 2JPtH ¼ 82 Hz,
6H, MePt]; 3.74 [br. s, 4H, CH2P]; 8.02 [br. d, 3JH5 H6 ¼ 5 Hz, 2H, H6 of
The monomeric complexes [Pt(4-MeC6H4)(C^N)(
C^N ¼ ppy, 3b, or bhq, 3c, were also prepared as described previ-
ously [20]. The analogous complex [PtMe(ppy)(
1-dppm)], 3a, was
k
1-dppm)],
bhq];
d(
31P) ¼ 10.6 [s, 3JPP ¼ 74 Hz, 1JPtP ¼ 2037 Hz, PtP].
k
2.5. [Pt2(4-MeC6H4)2(ppy)2(
m
-dfppe)], 2k
synthesized by the reaction of [PtMe(ppy)(SMe2)],1a, with dppm in
a 1:1 ratio according to Scheme 1. The free phosphorus atom of the
dppm ligand in complex 3a did not displace the nitrogen-donor
Yield 70%, mp
62H34F20N2P2Pt2: C, 45.4; H, 2.1; N, 1.7; Found: C, 45.9; H, 2.5; N,
¼
250 ꢀC (decomp). Anal. Calcd. for
C
from metal to give complex [PtMe(k k
1-C-ppy)( 2-dppm)], analo-
1.7. Selected NMR in CDCl3:
d
(1H) ¼ 2.10 [s, 6H, MeC]; 3.33 [br. s, 4H,
gous to complex 4d (see Scheme 1). The structure of complex 3a
was readily deduced from its 1H and 31P NMR spectra (see Sup-
porting Information for the detailed discussion about the spectro-
scopic data for complex 3a).
3 m
3 m
CH2P]; 6.53 [d,
J
o ¼ 7 Hz, 4H, Hm]; 6.90 [d,
J
o ¼ 7 Hz,
H H
H H
3JPtH ¼ 56 Hz, 4H, Ho]; 8.07 [br. d,
J
HH
¼ 5 Hz, 2H, H6 of ppy];
3 5 6
d(
31P) ¼ 9.4 [br. s, 1JPtP ¼ 1774 Hz, PtP].
The complexes [PtR(
k
1-C^N)(
k
2-dppe)], (4a, R ¼ Me, C^N ¼ ppy;
2.6. [Pt2(4-MeC6H4)2(bhq)2(
m
-dfppe)], 2l
4b, R ¼ 4-MeC6H4, C^N ¼ ppy; 4c, R ¼ 4-MeC6H4, C^N ¼ bhq), have
been made by the reaction of [Pt(R)(C^N)(SMe2)], 1, with dppe in a
1:1 ratio (Scheme 1S) [18]. Surprisingly, in the related reactions of
complexes 1 with dfppe, in the 1:1 ratio, diplatinum complexes 2je
l and 0.5 equiv of unreacted dfppe were obtained (see Fig. 1S-b),
Yield 75%, mp
66H34F20N2P2Pt2: C, 47.0; H, 2.0; N,1.7; Found: C, 47.5; H, 2.0; N,1.7.
¼
240 ꢀC (decomp). Anal. Calcd. for
C
Selected NMR in CDCl3:
d
(1H) ¼ 2.1 [s, 6H, MeC]; 3.75 [br. s, 4H,
3 m
3 m
CH2P]; 6.54 [d,
J
o ¼ 7 Hz, 4H, Hm]; 6.98 [d,
J
o ¼ 7 Hz,
except in the case of complex 1a which only complex [Pt(Me)(k1
-
HH
HH
3 5 6
3JPtH ¼ 57 Hz, 4H, Ho]; 8.27 [br. d,
J
HH
¼ 5 Hz, 2H, H6 of bhq];
ppy)(
Fig. 1S-c). The latter was isolated as a stable yellow solid which was
k
2-dfppe)], 4d, with chelating biphosphine was formed (see
3
d(
31P) ¼ 9 [s, JPP ¼ 75 Hz, 1JPtP ¼ 2045 Hz, PtP].

A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
95
N
SMe2
R
R
P
P
R
N
+0.5 equiv dfppe
Pt
Pt
Pt
N
R
R
CN
CN
Me
Me
4-MC6H4
4-MC6H4
ppy
bhq
ppy
bhq
Me
Me
4-MC6H4
4-MC6H4
ppy
bhq
ppy
bhq
1a
1b
1c
1d
2i
2j
2k
2l
+ 1 equiv dfppe
+ 1equiv dppm
N
dfppe =
H2
C
N
P
P
P
(C6F5)2P
P(C6F5)2
Pt
Pt
Me
P
Me
4d
3a
from A
form B
Scheme 1.
characterized by its 1H and 31P NMR spectra (see Supporting In-
formation for spectroscopic data).
functional theory and are shown in Fig. 1; the selected bond lengths
and bond angles are listed in Table 1. The geometries are best
described as distorted square-planar and the bond angles around
the Pt center range from 79ꢀ to 101.5ꢀ. The chelating C^N bite angles,
i.e. (C^N)C-Pt-N(C^N), amount to 79.0ꢀe79.5ꢀ, which are significantly
smaller than 90ꢀ, implying that the chelate is probably under strain.
The complexes have the dihedral angle PCCP ¼ 162ꢀe165ꢀ.
4.2. DFT e computed geometries for the dimers 2iej
The structures of new diplatinum complexes containing dfppe
as bridging ligand, 2iej, were further investigated by density
Fig. 1. The DFT-optimized structures of dimers 2. The H atoms are omitted for more clarity.

96
A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
Table 1
ꢀ
ꢀ
The selected DFT-optimized bond lengths (A) and angles ( ) for the dimers 2.
Complex
Bond distance
Angle
PteC (R)
PteC (C^N)
PteN (C^N)
PteP
(R) CePteC (C^N)
(C^N) CePteN (C^N)
(C^N) NePteP
PePteC (R)
[Pt2Me2(ppy)2(
[Pt2Me2(bhq)2(
m
m
-dfppe)], 2i
-dfppe)], 2j
[Pt2(4-MeC6H4)2(ppy)2( -dfppe)], 2k
-dfppe)], 2l
2.075
2.074
2.032
2.030
2.039
2.045
2.040
2.044
2.213
2.226
2.205
2.225
2.392
2.388
2.419
2.412
91.7
91.6
91.8
91.0
79.0
79.5
79.0
79.5
101.5
100.7
100.0
99.8
88.2
89.0
89.5
90.1
m
m
[Pt2(4-MeC6H4)2(bhq)2(
5. Discussion
to ꢁ19.8 kJ molꢁ1. The lowest energy optimized structures of these
complexes are shown in Fig. 2. More negative value of DE obtained
We were interested in assembling the cyclometalated com-
plexes to form more complex structures for potential applications
in molecular materials. In this regard, the reaction of the complexes
1 with general formula [PtR(C^N)(SMe2)] (see Scheme 1 and Scheme
1S; R ¼ Me or 4-MeC6H4; C^N ¼ ppy or bhq) with dppm, dppe or
dfppe ligands in 1:0.5 M ratio gave the symmetrical bridged
diplatinum complexes 2. When the ratio of 1:1 was used, the short
bite angle diphosphine ligand, dppm, does not displace the pyri-
dine donor and forms only a monodentate dppm complex, A. The
chelate ring in complex 1 can be opened by reaction with a
chelating diphosphine, dppe, to give the monodentate 2-
pyridilphenyl platinum complex B. As shown in Scheme 1, com-
plex A has a free phosphine donor while B has a free pyridyl group.
Surprisingly, when the ligand dfppe was reacted with complex 1 in
the ratio of 1:1, the complex B was only obtained when R ¼ Me and
C^N ¼ ppy, and in other cases the dimers 2 were formed (see Scheme
1S). It seems that there is a competition between chelation by the
diphosphine ligand and by the C^N-donor ligand. We have per-
formed a series of calculations in order to investigate the relative
stabilities of the isomers A and B as a function of R (¼Me or Ph), C^N
(¼ppy or bhq) and PP (¼dppm, dppe or dfppe) ligands. Here, we
for Ph shows that the electron-withdrawing ability of the Ph ligand
and more importantly its steric hindrance favor more isomer A over
isomer B.
5.2. Effect of C^N ligand
The C^N ligands presented in this study are deprotonated 2-
phenylpyridine (ppy) or deprotonated benzo[h]quinoline (bhq)
and as is clear from their structures, they are flexible and rigid li-
gands, respectively. Considering the energy differences between
forms A and B (
D
E), the complexes [PtR(bhq)(PP)] have more
negative
example
DE values compare to those for [PtR(ppy)(PP)]. For
D
E values for complexes [PtMe(bhq)(dppe)] and
[PtMe(ppy)(dppe)] are ꢁ14.8 and ꢁ2.8 kJ molꢁ1, respectively. The
lowest energy structures of the complexes [PtMe(bhq)(dppe)] and
[PtMe(ppy)(dppe)], (for both isomers A and B) as calculated by DFT
calculations, are shown in Fig. 3. More negative calculated values
for
k
D
E in the case of C^N ¼ bhq predicts a greater preference for the
2-C-isomer (form A) for the bhq compared to the ppy complex. So
the chelate Pt(bhq) is expected to be relatively favored compared to
the chelate Pt(ppy) ring, based on the fact that bhq is more rigid
than ppy. The calculated results are in agreement with experi-
have used the naming form A (general formula [PtR(
PP)]), and B (general formula [PtR(
1-C^N)( 2-PP)]) for monodentate
and chelating PP, respectively, for ease of discussion (see Scheme 1).
The lowest energy structures of the complexes [PtR(
2-C^N)(k1
PP)] (isomer A) and [PtR(
1-C^N)( 2-PP)] (isomer B), as calculated by
k -
2-C^N)(k1
mental finding [18]. Attempts to prepare the complex [PtMe(k1
-
k
k
bhq)(dppe)], were unsuccessful indicating that the rigid ligand bhq
is less favored than the more flexible ppy when bound in the k1
form.
k
-
k
k
DFT calculations, are shown in Figs. 2e4 and their selected bond
lengths and angles are reported in Table 2S (see Supporting
Information).
5.3. Effect of PP ligand
The DFT-calculated structures of isomers A and B for the com-
5.1. Effect of R ligand
plexes [PtMe(ppy)(dppm)] and [PtMe(ppy)(dfppe)] are shown in
Fig. 4. The energy differences between isomers A and B (or
DE)
Energy differences between forms A and B (or
the complexes [PtMe(C^N)(PP)] are lower than the
D
E ¼ EA ꢁ EB) of
of
E for com-
when PP ¼ dppm is more negative than that when PP ¼ dfppe or
D
E
dppe. For example, the DE values for complexes [PtMe(p-
[PtPh(C^N)(PP)] in acetone as solvent. As an example,
D
py)(dppm)] and [PtMe(ppy)(dfppe)] are ꢁ33.9 and ꢁ9.9 kJ molꢁ1
,
plex [PtMe(ppy)(dppe)] is ꢁ2.8 kJ molꢁ1, while this value for
respectively. It means that the isomer A is the favored isomer in the
case of dppm while the formation of isomer B is likely when PP is
complex including Ph group, [PtPh(ppy)(dppe)], is equal
Fig. 2. DFT-calculated structures of (a) [PtMe(
dppe)] (isomer B).
k k k k k k k -
2-ppy)( 1-dppe)] (isomer A), (b) [PtMe( 1-ppy)( 2-dppe)] (isomer B), (c) [PtPh( 2-ppy)( 1-dppe)] (isomer A), (d) [PtPh( 1-ppy)(k2

A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
97
Fig. 3. DFT-calculated structures of (a) [PtMe(
dppe)] (isomer B).
k k k k k k k -
2-bhq)( 1-dppe)] (isomer A), (b) [PtMe( 1-bhq)( 2-dppe)] (isomer B), (c) [PtMe( 2-ppy)( 1-dppe)] (isomer A), (d) [PtMe( 1-ppy)(k2
dfppe. This is in agreement with experimental findings. For the
complexes studied in the present work, there is a dependence on
the ligand PP ¼ dppm, dppe, or dfppe. Only with dppm is the iso-
mer A favored, while both dppe and dfppe favor isomer B. It is well
known that the four-membered Pt(dppm) chelate ring is strained
whereas the five-membered Pt(dfppe) chelate ring is not [30,31],
and so this can rationalize the observation of isomer A when
PP ¼ dppm and isomer B when PP ¼ dfppe or dppe.
progress. The complex 1a contains a labile SMe2 ligand and two Pte
C bonds. The SMe2 dissociation step is facilitated by the strong trans
influence of the metalated C atom of ppy, which weakens the Pte
SMe2 bond. In the first step the PP (¼dppm or dfppe) ligand dis-
places the labile ligand SMe2 to give the intermediate complex
[PtMe(ppy)(PP-k k
1P)], A, in which PP- 1P is monodentate PP ligand
and 0.5 equiv of the starting complex 1a is remained unreacted. The
energy barriers for this step are 25.9 and 80.7 kJ molꢁ1 for dppm
and dfppe, respectively (see Fig. 5). These values are in agreement
with experimental findings showing the formation of dangling
dppm is easier than dfppe complexes. The complex A forms an
equilibrium with the chelating PP isomer complex [PtMe(PP)(ppy-
In general, the isomeric form B of dfppe ligand can only be
formed when R ¼ Me and C^N ¼ ppy, by the reaction of
[Pt(Me)(ppy)(SMe2)] with dfppe in a 1:1 ratio. When R is Ph or C^N is
bhq, the dfppe acts as bridging ligand and a dimer is formed, while
in the same condition with similar starting complexes 1, dppe
forms monomeric complexes B. It seems that when R ¼ Ph, coor-
dination around the dfppe chelating B is more sterically hindered
than that of Me, probably responsible for the formation of A. This
complex is also unstable and can react with more platinum com-
plex precursor (having labile SMe2 ligand) to give the unusual
diphosphine bridged binuclear complexes, even when PP ¼ dfppe.
In the case of C^N ¼ bhq, the rigidity of this ligand compared to ppy
and also lower Lewis basicity of dfppe compared to dppe (due to
the presence of electron-withdrawing fluoroaryl ligands) makes
the formation of isomer B difficult, responsible for the formation of
dimer for the case in which PP ¼ dfppe.
k
1C)], B, in which ppy- 1C is the deprotonated ppy ligand that is C-
k
ligated with the dangling N atom. The intermediates B are higher in
energy than isomers A (33.9 and 10.0 kJ molꢁ1 for dppm and dfppe,
respectively), so the equilibrium shifts to isomers A. In the second
step, the complex A is reacted with remaining second half of the
starting complex 1a to produce the final diplatinum complex
[Pt2Me2(ppy)2(m-PP)], 2a or 2j.
5.5. Frontier molecular orbitals
Qualitative representations of the highest occupied and lowest
unoccupied molecular orbitals in the newly synthesized cyclo-
platinated complexes 3a, 4d and 2i are presented in Fig. 6. The
energies of the relevant frontier orbitals of complexes 2ie2l are
also shown in Table 2. The HOMOeLUMO gap of 3a is equal to
4.069 eV. The highest occupied MO of 3a is mainly Pt d2z orbital with
small contribution of the pp orbitals of the ppy ligand. The LUMO is
predominately localized on the ppy orbitals. The value of the en-
ergy separations between the HOMO and LUMO of 4d and 2i are
5.4. DFT investigation of product formation
In order to gain further insight into the dimer formation, DFT
calculations were carried out for the complexes and intermediates,
using parameters for the solvent acetone and a mechanism
described in Scheme 2 is suggested to occur during the reaction
Fig. 4. DFT-calculated structures of (a) [PtMe(
dfppe)] (isomer B).
k k k k k k k -
2-ppy)( 1-dppm)] (isomer A), (b) [PtMe( 1-ppy)( 2-dppm)] (isomer B), (c) [PtMe( 2-ppy)( 1-dfppe)] (isomer A), (d) [PtMe( 1-ppy)(k2

98
A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
+0.5 equiv PP
N
N
N
SMe2
P
-0.5 equiv SMe2
PP=dppm, dfppe
P
SMe2
Me
Pt
0.5
+
0.5
Pt
Pt
Me
Me
1a
1a
A-dppm
A-dfppe
Me
P
P
N
P
Pt
Pt
Pt
N
N
Me
Me
P
2a or 2i
B-dppm
B-dfppe
Scheme 2. Suggested mechanism for formation of diplatinum(II) complexes.
smaller than that of 3a and equal to 3.813 and 3.830 eV, respec-
tively. The highest occupied molecular orbital of 4d is essentially
the d2z orbital of Pt atom with small contribution of ppy ligand and
the LUMO of this complex is manly localized on dfppe ligand. The
HOMO and LUMO of complex 2i can also be ascribed as combina-
tion of Pt d2z þ ppy and dfppe ligand, respectively.
electron density at the platinum center, is due to the presence of
electron-withdrawing fluoroaryl ligands.
Also, the Pt charge in isomers B (i.e. B-dppm and B-dfppe) is
negative in comparison with the positive charge on Pt atom of
isomers A (i.e. A-dppm and A-dfppe), which is probably the result
of the coordination of the more electronegative nitrogen atom of
C^N ligands to Pt center of isomers A.
Table 3 shows the atomic charges of the platinum center for the
complexes shown in Fig. 5, obtained from the natural bond orbital
(NBO) analysis [32]. The calculated charge on the Pt atom of A-
dppm and A-dfppe complexes are less positive than that for com-
plex 1a, which is the result of charge transfer from platinum to P
atom in forming complexes A-dppm and A-dfppe (Table 3). As is
clear from Table 3, the positive charge on the platinum atom in the
complex A-dfppe (0.113) is higher than that on the platinum atom
in the complex A-dppm (0.096). This higher positive charge on the
platinum atom in the complex A-dfppe, which means lower
6. Conclusion
We have now extended the cyclometalated platinum(II) com-
plexes containing deprotonated 2-phenylpyridine or benzo[h]
quinoline to synthesis unknown examples of square planar plati-
num(II) complexes containing bridging dfppe ligands as illustrated
by the structures of complexes 2iel (Scheme 1, Fig. 1). These
Fig. 5. Calculated structures and energies (kJ molꢁ1) of intermediates and compounds in the reaction of 1a with dppm or dfppe in acetone solution. Energies are with respect to 1a
and free molecules of SMe2 and PP are included in the related compounds.

A.R. Esmaeilbeig et al. / Journal of Organometallic Chemistry 755 (2014) 93e100
99
Fig. 6. Qualitative frontier molecular orbitals for complexes 3a, 4d and 2i.
Table 2
Energies (eV) of the relevant frontier orbitals of selected complexes.
Acknowledgments
The support of the Shiraz University Research Councils is
acknowledged. We are grateful to Dr. F. Niroomand Hosseini for
valuable assistance and many good suggestions.
Orbital
E (eV)
3a
4d
2i
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
ꢁ5.276
ꢁ1.207
ꢁ5.617
ꢁ1.804
ꢁ5.597
ꢁ1.767
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.01.008.
dimeric complexes are synthesized according to Scheme 1 in a
1:0.5 M ratio of Pt precursor and dfppe ligand.
References
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