# Synthesis and Crystal Structure of Alkynylplatinum(IV) Complex Containing the Terpyridine Ligand

Authors: Tsugiko Takase; Dai Oyama
Journal: Journal of Crystallography
 (2014)

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## Abstract

Reaction of square planar [PtII(C≡CPh)(tpy)]+ (tpy = 2,2′:6′,2′′-terpyridine) with bromine at low temperature provides a general route for the synthesis of octahedral alkynyl(terpyridine)platinum(IV) complex. In this first example of alkynyl(terpyridine)platinum(IV) complex, the alkynyl group is  situated in trans position relative to the central nitrogen atom of the terpyridine ligand, and the two bromido ligands are situated in trans positions; an X-ray structural analysis has been completed for trans(Br)-[PtIVBr2(C≡CPh)(tpy)]+.


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## Body

## 1. Introduction

Platinum(II) terpyridyl complexes with alkynyl ligands have attracted great interest in recent years due to their unique photophysics [1, 2] and their potential applications as photocatalysts for hydrogen evolution [3, 4] and molecular frameworks for light-to-chemical energy conversion [5]. On the other hand, there is a limited range of reported alkynylplatinum(IV) complexes [6–8]: there is no report on alkynylplatinum(IV) complex with terpyridine ligands. Although general oxidizing agents such as halogens and hydrogen peroxide are utilized for oxidation reactions of PtII to PtIV centers, halogens are particularly the most useful reagents [9, 10]. However, only one example for oxidation of alkynylplatinum(II) by halogens has been known in [PtII(Me2bpy)(C≡C-4-tol)2] by iodine to form [PtIVI2(Me2bpy)(C≡C-4-tol)2] [6], due to instability of the PtII-alkynyl bond to halogen-containing oxidants [11]. Therefore, other procedures using 4-nitrophenyl azide and alkynyliodine(III) reagents as oxidants have been explored for oxidation of PtII to PtIV [7, 8, 12]. In order to generalize about routes to alkynylplatinum(IV) compounds by halogen-oxidation reaction without Pt-alkynyl bond breaking, we have optimized reaction conditions (the kind of oxidants and temperature) using alkynylplatinum(II) containing the terpyridine ligand as a prototype complex.


## 2. Materials and Methods

All chemicals employed here were used without further purification unless otherwise stated. All solvents purchased for organic synthesis were anhydrous and used without further purification. A Br2–CHCl3 solution was prepared by addition of 12 drops of Br2 to 10 mL of CHCl3 [10]. A Cl2–CHCl3 solution was prepared by a saturated Cl2 gas through 10 mL of CHCl3 for approximately 15 sec [9]. The platinum(II) precursor ([PtII(C≡CPh)(tpy)]PF6) was prepared in accordance with the published method [2]. 1H NMR spectra were recorded on a JEOL JMN-AL300 spectrometer (25°C) operating at 1H frequency of 300 MHz. ESI-MS data were measured on a Bruker Daltonics micrOTOF equipped with electrospray ionization (ESI) source and CH3CN was used as the solvent. The instrument was operated at positive ion mode usingm/z range of 100–1000. IR spectra were obtained using the KBr pellet method with a JASCO FT-IR 4100 spectrometer.


### 2.1. X-Ray Crystal Structure Determination

Single crystals of1Br were obtained from a solution of the complex in DMF/diethyl ether. A brown crystal of1Br with dimensions 0.15×0.15×0.03 mm was mounted on a glass fiber. All measurements were performed on a Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo Kα radiation (λ=0.71075Å). All calculations were conducted using theCrystalStructure program package [13] except for refinement, which was performed usingSHELXL97 [14]. The structure was solved by direct methods usingSIR92 program [15]. A numerical absorption correction (ABSCOR) [16] was applied to data. Aromatic hydrogen atoms were fixed at C–H lengths of 0.95 Å refined as riding, with Uiso(H)=1.2Ueq(C). Both the highest residual electron peak and the deepest hole are located within 1 Å from atom Pt1. The crystallographic data are summarized in Table 1 and the geometrical parameters are summarized in Table 2. The crystal data for1Br has been deposited with Cambridge Crystallographic Data Centre as supplementary publication number CCDC-1014039.Table 1
Crystallographic details fortrans(Br)-[PtBr2(C≡CPh)(tpy)]Br.
Empirical formula
C23H16Br3N3Pt
Fw
769.20
Temperature (K)
173(2)
λ (Å)
0.71075
Crystal system
Triclinic
Space group
P-1
a (Å)
7.2090(4)
b (Å)
12.9921(6)
c (Å)
13.1231(6)
α (deg)
67.0477(14)
β (deg)
75.8967(15)
γ (deg)
78.1381(19)
V (Å3)
1089.25(9)
Z
2
ρ
calc (Mg/m3)
2.345
μ (Mo Kα) (mm−1)
11.950
Number of reflections
10839
Unique reflections
4955
GOF (F2)
1.095
R
int
⁡
0.0902
R1a (I>2σ)
0.0486
w
R2b (all data)
0.1210
a
R
1
=
∑
|
|
F
o
|
-
|
F
c
|
|
/
∑
|
F
o
|. bwR2={∑w(Fo2-Fc2)2/∑w(Fc2)2}1/2.Table 2
Geometrical parameters.
(a)
Selected bond lengths and angles (Å, °)
Pt1–Br1
2.4498(12)
C1–C2
1.059(17)
Pt1–Br2
2.4644(12)
N1–Pt1–N2
80.8(4)
Pt1–N1
2.041(10)
N2–Pt1–N3
80.2(4)
Pt1–N2
1.976(9)
Pt1–C1–C2
175.3(12)
Pt1–N3
2.030(11)
C1–C2–C3
174.4(14)
Pt1–C1
2.054(10)
Br1–Pt1–Br2
178.46(5)
(b)
Interatomic distances (Å) for intermolecularπ-π interactions
C2⋯C22i
3.299(15)
C3⋯C21i
3.373(15)
C3⋯C22i
3.302(15)
C4⋯C23ii
3.366(17)
C5⋯C20i
3.290(17)
C5⋯C22ii
3.362(17)
C5⋯C23ii
3.256(16)
C8⋯C21i
3.374(18)
Symmetry operators:i-x+1, -y+1, -z+1; ii-x+2, -y+1, -z+1.
(c)
Hydrogen-bond geometry (Å, °)
D–H⋯A
D–H
H⋯A
D
⋯
A
D–H⋯A
C4–H1⋯Br1i
0.95
2.84
3.688(14)
149
C11–H8⋯Br1ii
0.95
2.86
3.761(13)
160
C22–H15⋯Br1i
0.95
3.34
3.836(14)
115
C23–H16⋯Br1i
0.95
2.80
3.576(13)
140
C4–H1⋯Br2iii
0.95
3.49
4.072(13)
122
C5–H2⋯Br2iii
0.95
3.17
3.922(14)
137
C16–H11⋯Br2iv
0.95
2.87
3.702(14)
147
C10–H7⋯Br2v
0.95
2.94
3.733(13)
142
C8–H5⋯Br3ii
0.95
3.12
3.758(13)
126
C9–H6⋯Br3ii
0.95
3.14
3.738(11)
122
C22–H15⋯Br3vi
0.95
3.20
3.872(12)
129
C10–H7⋯Br3ii
0.95
3.12
3.718(13)
122
C12–H9⋯Br3
0.95
3.19
3.945(13)
138
C15–H10⋯Br3
0.95
2.89
3.597(13)
132
C6–H3⋯Br3vii
0.95
3.32
3.943(13)
125
C17–H12⋯Br3viii
0.95
2.89
3.819(15)
166
C21–H14⋯Br3vi
0.95
3.28
3.909(13)
126
Symmetry operators:i-x+1, -y+1, -z+1; ii-x, -y+1, -z+2; iii-x+2, -y+1, -z+1; iv-x+1, -y, -z+2; v-x+1, -y+1, -z+2; vix+1, y, z-1; viix+1, y+1, z-1; viii-x, -y, -z+2.


### 2.2. Preparation oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br (1Br)

An orange suspension of [Pt(C≡CPh)(tpy)]PF6 (30 mg, 0.044 mmol) in acetonitrile (10 mL) was stirred at −20°C for 30 min. Addition of Br2–CHCl3 (70 drops) to the suspension resulted in a homogeneous solution. The mixture was further stirred at −20°C for 2 h; during this time some light brown solids gradually appeared out of the solution. The product was collected by filtration, washed with diethyl ether, and then dried in vacuo (25 mg, 73%). ESI+-MS:m/z = 690 ([M]+), 529 ([M–2Br]+). 1H NMR (DMSO-d6): δ 9.29 (d with broad 195Pt satellites, 2H), 9.13–8.88 (m, 5H), 8.70 (t, 2H), 8.20 (t, 2H), 7.65 (d, 2H), 7.47–7.35 (m, 3H). IR: ν(C≡C) 2165 cm−1.


## 2.1. X-Ray Crystal Structure Determination

Single crystals of1Br were obtained from a solution of the complex in DMF/diethyl ether. A brown crystal of1Br with dimensions 0.15×0.15×0.03 mm was mounted on a glass fiber. All measurements were performed on a Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo Kα radiation (λ=0.71075Å). All calculations were conducted using theCrystalStructure program package [13] except for refinement, which was performed usingSHELXL97 [14]. The structure was solved by direct methods usingSIR92 program [15]. A numerical absorption correction (ABSCOR) [16] was applied to data. Aromatic hydrogen atoms were fixed at C–H lengths of 0.95 Å refined as riding, with Uiso(H)=1.2Ueq(C). Both the highest residual electron peak and the deepest hole are located within 1 Å from atom Pt1. The crystallographic data are summarized in Table 1 and the geometrical parameters are summarized in Table 2. The crystal data for1Br has been deposited with Cambridge Crystallographic Data Centre as supplementary publication number CCDC-1014039.Table 1
Crystallographic details fortrans(Br)-[PtBr2(C≡CPh)(tpy)]Br.
Empirical formula
C23H16Br3N3Pt
Fw
769.20
Temperature (K)
173(2)
λ (Å)
0.71075
Crystal system
Triclinic
Space group
P-1
a (Å)
7.2090(4)
b (Å)
12.9921(6)
c (Å)
13.1231(6)
α (deg)
67.0477(14)
β (deg)
75.8967(15)
γ (deg)
78.1381(19)
V (Å3)
1089.25(9)
Z
2
ρ
calc (Mg/m3)
2.345
μ (Mo Kα) (mm−1)
11.950
Number of reflections
10839
Unique reflections
4955
GOF (F2)
1.095
R
int
⁡
0.0902
R1a (I>2σ)
0.0486
w
R2b (all data)
0.1210
a
R
1
=
∑
|
|
F
o
|
-
|
F
c
|
|
/
∑
|
F
o
|. bwR2={∑w(Fo2-Fc2)2/∑w(Fc2)2}1/2.Table 2
Geometrical parameters.
(a)
Selected bond lengths and angles (Å, °)
Pt1–Br1
2.4498(12)
C1–C2
1.059(17)
Pt1–Br2
2.4644(12)
N1–Pt1–N2
80.8(4)
Pt1–N1
2.041(10)
N2–Pt1–N3
80.2(4)
Pt1–N2
1.976(9)
Pt1–C1–C2
175.3(12)
Pt1–N3
2.030(11)
C1–C2–C3
174.4(14)
Pt1–C1
2.054(10)
Br1–Pt1–Br2
178.46(5)
(b)
Interatomic distances (Å) for intermolecularπ-π interactions
C2⋯C22i
3.299(15)
C3⋯C21i
3.373(15)
C3⋯C22i
3.302(15)
C4⋯C23ii
3.366(17)
C5⋯C20i
3.290(17)
C5⋯C22ii
3.362(17)
C5⋯C23ii
3.256(16)
C8⋯C21i
3.374(18)
Symmetry operators:i-x+1, -y+1, -z+1; ii-x+2, -y+1, -z+1.
(c)
Hydrogen-bond geometry (Å, °)
D–H⋯A
D–H
H⋯A
D
⋯
A
D–H⋯A
C4–H1⋯Br1i
0.95
2.84
3.688(14)
149
C11–H8⋯Br1ii
0.95
2.86
3.761(13)
160
C22–H15⋯Br1i
0.95
3.34
3.836(14)
115
C23–H16⋯Br1i
0.95
2.80
3.576(13)
140
C4–H1⋯Br2iii
0.95
3.49
4.072(13)
122
C5–H2⋯Br2iii
0.95
3.17
3.922(14)
137
C16–H11⋯Br2iv
0.95
2.87
3.702(14)
147
C10–H7⋯Br2v
0.95
2.94
3.733(13)
142
C8–H5⋯Br3ii
0.95
3.12
3.758(13)
126
C9–H6⋯Br3ii
0.95
3.14
3.738(11)
122
C22–H15⋯Br3vi
0.95
3.20
3.872(12)
129
C10–H7⋯Br3ii
0.95
3.12
3.718(13)
122
C12–H9⋯Br3
0.95
3.19
3.945(13)
138
C15–H10⋯Br3
0.95
2.89
3.597(13)
132
C6–H3⋯Br3vii
0.95
3.32
3.943(13)
125
C17–H12⋯Br3viii
0.95
2.89
3.819(15)
166
C21–H14⋯Br3vi
0.95
3.28
3.909(13)
126
Symmetry operators:i-x+1, -y+1, -z+1; ii-x, -y+1, -z+2; iii-x+2, -y+1, -z+1; iv-x+1, -y, -z+2; v-x+1, -y+1, -z+2; vix+1, y, z-1; viix+1, y+1, z-1; viii-x, -y, -z+2.


## 2.2. Preparation oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br (1Br)

An orange suspension of [Pt(C≡CPh)(tpy)]PF6 (30 mg, 0.044 mmol) in acetonitrile (10 mL) was stirred at −20°C for 30 min. Addition of Br2–CHCl3 (70 drops) to the suspension resulted in a homogeneous solution. The mixture was further stirred at −20°C for 2 h; during this time some light brown solids gradually appeared out of the solution. The product was collected by filtration, washed with diethyl ether, and then dried in vacuo (25 mg, 73%). ESI+-MS:m/z = 690 ([M]+), 529 ([M–2Br]+). 1H NMR (DMSO-d6): δ 9.29 (d with broad 195Pt satellites, 2H), 9.13–8.88 (m, 5H), 8.70 (t, 2H), 8.20 (t, 2H), 7.65 (d, 2H), 7.47–7.35 (m, 3H). IR: ν(C≡C) 2165 cm−1.


## 3. Results and Discussion

### 3.1. Synthesis oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br

The synthetic route for the formation of platinum(IV) terpyridyl complexes is summarized in Scheme1. Addition of Br2–CHCl3 solution to [PtII(C≡CPh)(tpy)]+ in acetonitrile at −20°C gave [PtIVBr2(C≡CPh)(tpy)]+ (1+) as light brown solid in 73% yield. When this reaction was performed at room temperature,1+ was produced but a small quantity of the alkynyl dissociated species was detected. In the same condition, however, addition of Cl2–CHCl3 solution gave some impurities in addition to [PtIVCl3(tpy)]+ (m/z = 534, major product) and [PtIVCl2(C≡CPh)(tpy)]+ (m/z = 600, minor product) (Scheme 1). These results indicate that the Pt–C bond in [Pt(C≡CPh)(tpy)]+ is influenced by the strength of oxidants and reaction temperatures: milder conditions (Br2 as an oxidant and low temperature) are required for bond retention of the Pt-alkynyl moiety.Scheme 1
Synthetic route for platinum(IV) complexes in this study.


### 3.2. Characterization oftrans(Br)-[PtBr2(C≡CPh)(tpy)]+

The identities of the complex1+ have been confirmed by IR spectroscopy, 1H NMR spectroscopy, and ESI-mass spectroscopy. The molecular structure of1Br has also been determined. To the best of our knowledge, this study demonstrates the first crystal structure of an alkynylplatinum(IV) with the terpyridine ligand.The IR spectrum of1Br exhibited ν(C≡C) absorption at 2165 cm−1, which is 40 cm−1 higher than that of [PtII(C≡CPh)(tpy)]+ [17]. The 1H NMR spectra of1+ showed resonances at more downfield regions compared to the corresponding PtII complex [17]. For example, the doublet signals of 6,6′′-positions in terpyridine ligand of1+ appeared at 9.29 ppm, whereas those of [PtII(C≡CPh)(tpy)]+ appeared at 9.10 ppm, indicating a lowering of the electron density on a Pt center.The molecular structure of1Br is shown in Figure 1 with atom-labeling scheme. The complex consists of a distorted octahedral geometry around the PtIV center with three nitrogen atoms of the terpyridine ligand, two bromido ligands intrans positions, and an alkynyl group intrans position relative to the central nitrogen atom (1′-position) of the terpyridine ligand. The terpyridine ligand is coordinated in a planar tridentate fashion with the central nitrogen atom closest to the PtIV atom. As shown in Table 2(a), the bond lengths of Pt–N are comparable to the precursor platinum(II) terpyridine complex [2]. Additionally, the Pt1–C1–C2 and C1–C2–C3 fragments are nearly linear with bond angles of 175.3(12)° and 174.4(14)°, respectively. However, both the Pt–C and the C≡C bond lengths (2.054(10) and 1.059(17) Å, resp.) are different from those of the corresponding PtII complex (1.98(1) and 1.19(1) Å, resp.) [2]. In the crystal, the cation of1Br features intermolecular π-π stacking between the phenyl group of the alkynyl ligand and the terpyridine ligand, as represented by the shortest contact of 3.256(16) Å for C5⋯C23 (Table 2(b)). The phenyl ring of the alkynyl ligand is almost parallel to the plane of Pt-terpyridine with a dihedral angle of 5.438°, indicating the formation of a face-to-face π-π interaction (Figure 2). In addition, there are a number of intermolecular weak C–H⋯Br interactions in the crystal as illustrated in Figure 2. These interactions help to stabilize the structure.Figure 1
The molecular structure oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br (1Br), showing the atom-labeling scheme and displacement ellipsoids drawn at the 50% probability level.Figure 2
Packing diagram. The green lines denote intermolecular hydrogen bonds.


## 3.1. Synthesis oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br

The synthetic route for the formation of platinum(IV) terpyridyl complexes is summarized in Scheme1. Addition of Br2–CHCl3 solution to [PtII(C≡CPh)(tpy)]+ in acetonitrile at −20°C gave [PtIVBr2(C≡CPh)(tpy)]+ (1+) as light brown solid in 73% yield. When this reaction was performed at room temperature,1+ was produced but a small quantity of the alkynyl dissociated species was detected. In the same condition, however, addition of Cl2–CHCl3 solution gave some impurities in addition to [PtIVCl3(tpy)]+ (m/z = 534, major product) and [PtIVCl2(C≡CPh)(tpy)]+ (m/z = 600, minor product) (Scheme 1). These results indicate that the Pt–C bond in [Pt(C≡CPh)(tpy)]+ is influenced by the strength of oxidants and reaction temperatures: milder conditions (Br2 as an oxidant and low temperature) are required for bond retention of the Pt-alkynyl moiety.Scheme 1
Synthetic route for platinum(IV) complexes in this study.


## 3.2. Characterization oftrans(Br)-[PtBr2(C≡CPh)(tpy)]+

The identities of the complex1+ have been confirmed by IR spectroscopy, 1H NMR spectroscopy, and ESI-mass spectroscopy. The molecular structure of1Br has also been determined. To the best of our knowledge, this study demonstrates the first crystal structure of an alkynylplatinum(IV) with the terpyridine ligand.The IR spectrum of1Br exhibited ν(C≡C) absorption at 2165 cm−1, which is 40 cm−1 higher than that of [PtII(C≡CPh)(tpy)]+ [17]. The 1H NMR spectra of1+ showed resonances at more downfield regions compared to the corresponding PtII complex [17]. For example, the doublet signals of 6,6′′-positions in terpyridine ligand of1+ appeared at 9.29 ppm, whereas those of [PtII(C≡CPh)(tpy)]+ appeared at 9.10 ppm, indicating a lowering of the electron density on a Pt center.The molecular structure of1Br is shown in Figure 1 with atom-labeling scheme. The complex consists of a distorted octahedral geometry around the PtIV center with three nitrogen atoms of the terpyridine ligand, two bromido ligands intrans positions, and an alkynyl group intrans position relative to the central nitrogen atom (1′-position) of the terpyridine ligand. The terpyridine ligand is coordinated in a planar tridentate fashion with the central nitrogen atom closest to the PtIV atom. As shown in Table 2(a), the bond lengths of Pt–N are comparable to the precursor platinum(II) terpyridine complex [2]. Additionally, the Pt1–C1–C2 and C1–C2–C3 fragments are nearly linear with bond angles of 175.3(12)° and 174.4(14)°, respectively. However, both the Pt–C and the C≡C bond lengths (2.054(10) and 1.059(17) Å, resp.) are different from those of the corresponding PtII complex (1.98(1) and 1.19(1) Å, resp.) [2]. In the crystal, the cation of1Br features intermolecular π-π stacking between the phenyl group of the alkynyl ligand and the terpyridine ligand, as represented by the shortest contact of 3.256(16) Å for C5⋯C23 (Table 2(b)). The phenyl ring of the alkynyl ligand is almost parallel to the plane of Pt-terpyridine with a dihedral angle of 5.438°, indicating the formation of a face-to-face π-π interaction (Figure 2). In addition, there are a number of intermolecular weak C–H⋯Br interactions in the crystal as illustrated in Figure 2. These interactions help to stabilize the structure.Figure 1
The molecular structure oftrans(Br)-[PtBr2(C≡CPh)(tpy)]Br (1Br), showing the atom-labeling scheme and displacement ellipsoids drawn at the 50% probability level.Figure 2
Packing diagram. The green lines denote intermolecular hydrogen bonds.


## 4. Conclusions

In summary, we have prepared an alkynylplatinum(IV) complex by simple oxidation of the corresponding alkynylplatinum(II) one. It is anticipated that the successful route described here provides a useful methodology to obtain a variety of platinum(IV) complexes with a wide range of alkynyl ligands.



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*Source: 280247-2014-09-03.xml*