Structure and properties of the volatile isomers of bis(1,1,1-trifluoro-5,5-dimethylhexane-2,4-dionato) of platinum(II)

Article abstract of DOI:10.1134/S0022476616060196

In the work, isomeric complexes of platinum(II) with the (ptac)^–1 pivaloyltrifluoroacetonate ion (Pt((CH₃)₃–CO–CH–CO–CF₃)₂) are studied. The synthesis and chromatographic separation of Pt(ptac)₂ isomers are described, TGA data for the separated isomers are given, and the crystal structures of the solid phases are studied. The cis-Pt(ptac)₂ complex crystallizes in the space group P-1, a = 10.7091(4) ?, b = 12.7787(6) ?, c = 16.0154(8) ?, α = 92.389(2)°, β = 90.868(2)°, γ = 112.1260(10)°, V = 2027.39(16) ?³, Z = 4, d_calc = 1.918 g/cm³. The trans-Pt(ptac)₂ complex crystallizes in the space group C2/m, a = 13.3235(5) ?, b = 8.5515(3) ?, c = 9.6694(3) ?, β = 118.5880(10)°, V = 967.38(6) ?3, Z = 2, d_calc = 2.010 g/cm3. The structures of the complexes are molecular, the Pt atom has a square coordination of four oxygen atoms of two ligands; for cis-Pt(ptac)2, the Pt–Oav distance is 1.968 ?, for trans-Pt(ptac)2 it is 1.980 ?.

Full text of DOI:10.1134/S0022476616060196

Journal of Structural Chemistry. Vol. 57, No. 6, pp. 1188-1194, 2016.
Original Russian Text © 2016 I. A. Baidina, G. I. Zharkova, and D. A. Piryazev.
STRUCTURE AND PROPERTIES OF THE VOLATILE
ISOMERS OF BIS(1,1,1-TRIFLUORO-5,5-
DIMETHYLHEXANE-2,4-DIONATO) OF PLATINUM(II)
I. A. Baidina, G. I. Zharkova*, and D. A. Piryazev
UDC 546.924:547.442
–1
In the work, isomeric complexes of platinum(II) with the (ptac) pivaloyltrifluoroacetonate ion
(Pt((CH3)3–CO–CH–CO–CF3)2) are studied. The synthesis and chromatographic separation of Pt(ptac)2
isomers are described, TGA data for the separated isomers are given, and the crystal structures of the solid
phases are studied. The cis-Pt(ptac)2 complex crystallizes in the space group P-1, a = 10.7091(4) Å,
b = 12.7787(6) Å,
c = 16.0154(8) Å,
α = 92.389(2)°,
β = 90.868(2)°,
γ = 112.1260(10)°,
3 3
V = 2027.39(16) Å , Z = 4, dcalc = 1.918 g/cm . The trans-Pt(ptac)2 complex crystallizes in the space group
3
C2/m, a = 13.3235(5) Å, b = 8.5515(3) Å, c = 9.6694(3) Å, β = 118.5880(10)°, V = 967.38(6) Å , Z = 2,
3
dcalc = 2.010 g/cm . The structures of the complexes are molecular, the Pt atom has a square coordination of
four oxygen atoms of two ligands; for cis-Pt(ptac)2, the Pt–Oav distance is 1.968 Å, for trans-Pt(ptac)2 it is
1.980 Å.
DOI: 10.1134/S0022476616060196
Keywords: β-diketonates of platinum (II), isomers, structure, volatility.
Modern high-tech trends are inextricably linked to constantly rising requirements to the composition and structure of
new functional materials. Nanostructures and nanolayers based on platinum metals are promising materials for electrodes in
medicine and microelectronics, integrated circuits, microheaters, temperature detectors, catalysts, and so on [1-5]. One of the
most widespread methods to obtain platinum films is metal-organic chemical vapor deposition (MOCVD) using volatile
β-diketonates of platinum [6-8]. The formation of functional layers occurs by the decomposition of the initial compound
(precursor) vapor on the support surface. The precursor type affects the film characteristics and the temperature modes of its
deposition. The important characteristics of these compounds such as the volatility and thermal stability depend on the
composition and structure of initial β-diketone. Therefore, the study of the composition and structural features of volatile
platinum β-diketonates based on the asymmetric β-diketones is relevant. Our previous experiment on the direct measurement
of the pressure of saturated vapor for the cis- and trans-isomers of the platinum(II) complex with the Pt(tfac)2
trifluoroacetylacetonate ion revealed a significant difference in the volatility of the cis- and trans-isomers [9], which is
probably associated with different packing of isomer molecules in the crystal lattice. It is natural to suppose that the
symmetric trans-isomer has a closer molecular packing as compared to the cis-form giving an increased lattice energy and,
consequently, a lower volatility, which we later confirmed by the results of our single crystal XRD study of the cis- and
trans-isomers of Pt(tfac)2 [10].
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia;
*zharkova@niic.nsc.ru. Translated from Zhurnal Strukturnoi Khimii, Vol. 57, No. 6, pp. 1248-1254, July-August, 2016.
Original article submitted December 7, 2015; revised March 17, 2016.
1188
0022-4766/16/5706-1188 © 2016 by Pleiades Publishing, Ltd.

The continuation of our work on the study of volatile platinum(II) β-diketonates based on asymmetric fluorinated β-
diketones is the investigation of the structure of the platinum complex with the pivaloyltrifluoroacetonate ion: Pt[C(CH3)3–
CO–CH–CO–CF3]2 (hereinafter, Pt(ptac)2). The preparation of this complex and its physicochemical and thermal
characteristics we described in [11]. Here, we describe the synthesis of Pt(ptac)2, the chromatographic separation of the cis-
and trans-complexes, report the TGA data for the separated isomeric phases and the results of their crystal chemical study,
which was performed for the first time.
EXPERIMENTAL
Synthesis of the Pt(ptac) complex. K2PtF6 salt was used as a precursor. In the first stage, Pt(IV) reduces to Pt(II)
2
in ethanol in a perchloric acid solution with the formation of the Pt(II) aqua ion [11]. The addition of a slight excess of
pivaloyltrifluoroacetone (Hptac) to the obtained solution with a subsequent increase in pH to 5 by the KOH solution with
stirring leads to the formation of a yellow Pt(ptac)2 precipitate from the solution. The filtered precipitate was dried and
–2
purified by sublimation in vacuum (P = 10 Torr, T = 200 °C); the yield of the sublimated product was 85%. The crystals are
yellow, Tmelt = 164 °C. Found, %: C 33.9, H 3.5, F 19.6. For C16 H20F6O4Pt calculated, %: C 33.7, H 3.4, F 19.5.
–1
IR study of the sublimated product (carried out on a Scimitar FTS-2000 spectrometer in a range 400-4000 cm ,
КBr pellet): ν(C–H) 2979, 2939, 2878; ν(C=O+C=C) 1580, 1536, 1512, 1435; ν(H–C–H) 1396, 1369; 1312; ν(C–F) 1256,
1196, 1160; ν(Pt–O) 861, 806.730, 623, 500, 398. Complication of the structure of the complex due to the asymmetry of the
ligand used results in the appearance of additional bands in the Pt–O bond vibration region [12], since in this case the
spectrum is the sum of the spectra for a mixture of the cis- and trans-isomers.
Powder XRD study was carried out on a DRON-3M diffractometer (R = 192 mm, CuK radiation, Ni filter,
α
scintillation detector with an amplitude discrimination in the 2θ angle range from 5° to 50° at room temperature). The
experimental diffraction pattern of the sublimated Pt(ptac)2 product given in Fig. 1 evidences that it is not a single phase.
Separation of the isomeric phases of the sublimated Pt(ptac)2 complex was performed on a chromatographic
column, SiO2 support, a hexane/benzene solvent mixture in the 3:1 ratio used as the eluent. cis-Pt(ptac)2 elutes first from the
column and then trans-Pt(ptac)2. The isolated isomeric forms of the complex are very soluble in chloroform, benzene, hexane
and less soluble in alcohol.
Single crystals of the cis- and trans-isomers for the XRD analysis were grown by slow crystallization from an
alcohol solution at room temperature; they crystallize as yellow transparent prisms; the crystals of the cis-isomer are more
Fig. 1. Diffraction pattern of the sublimated product
Pt(ptac)2 (1) in comparison with the calculated
diffraction patterns of three phases: cis (2), trans (3),
mix (4).
1189

Fig. 2. Thermogravimetric curves of the compounds:
mix-Pt(ptac)2 (1), cis-Pt(ptac)2 (2), trans-Pt(ptac)2 (3).
isometric. From a heptane solution, the crystals of a new phase, mix-Pt(ptac)2, were obtained, which is a solid solution of the
isomers in the cis–trans ratio of 2:1. Single crystals of all phases melt without decomposition: for cis-Pt(ptac)2 Tmelt = 163 °C,
for trans-Pt(ptac)2 Tmelt = 182 °C, for mix-Pt(ptac)2 Tmelt = 168 °C.
Thermal study of the Pt(ptac)2 isomeric phases was performed on a NETZSCH TG 209 F1 device: a temperature
range 20-350 °C, helium atmosphere, gas flow rate 30 ml/min, heating rate 10 deg/min, a 5-6 mg portion, a standard open
crucible. Nearly quantitative weight loss of all compounds under the measurement conditions occurs in a range 150-210 °C
(Fig. 2). In the thermogravimetric experiment, the cis-Pt(ptac)2 complex passes to the gas phase with partial decomposition:
its weight loss is ∼90%. Cis-Pt(ptac)2 and mix-Pt(ptac)2 pass to the gas phase almost in the same temperature range and their
volatility is somewhat higher than that of trans-Pt(ptac)2.
Single crystal XRD study. The unit cell parameters and experimental intensities to determine the crystal structures
were measured at room temperature on an automated Bruker-Nonius X8 Apex four-circle diffractometer equipped with
a two-dimensional detector (MoK radiation, graphite monochromator). The structures were solved by the standard heavy
α
atom method and refined in the anisotropic/isotropic (for H) approximation; the positions of the hydrogen atoms of the CH3
groups were calculated geometrically. The crystallographic characteristics of the studied compounds, the parameters of the
experiment and structure refinement are given in Table 1; interatomic distances and bond angles are listed in Table 2. All
calculations were performed using the SHELX-97 software [13]. The coordinates of the basic atoms have been deposited
with the Cambridge Structural Database (CCDC) under numbers: 1424 107 for the cis-Pt(ptac)2 phase, 1424 105 for the
trans-Pt(ptac)2 phase, 1424 106 for the mix-Pt(ptac)2 phase; see deposit@ccde.cam.ac.uk.
DESCRIPTION OF CRYSTAL STRUCTURES. CONCLUSIONS
Structure of cis-Pt(ptаc) is molecular formed of isolated complexes whose structure with atomic numbering is
2
given in Fig. 3. The crystals are triclinic; the structure contains two crystallographically independent complexes having
different orientations in the unit cell. The Pt atom has a square coordination of four oxygen atoms of two ligands; the Pt–O
distances and the chelate O–Pt–O bond angles are in the ranges of 1.940-1.985 Å and 94.8-95.6°; their average values are
1.968 Å and 95.0° respectively. The maximum difference in the O–C bond lengths is only 0.03 Å; the average C–Cγ bond
lengths from the side of different substituents are 1.42 (CH3) Å and 1.36 (CF3) Å. In the CF3 groups, the average C–F bond
length is 1.30 Å. The molecules of the complexes (excluding terminal substituents) are almost planar; the metal ring bend
angles along the O…O line do not exceed 2.4°; the intramolecular F…Hγ contacts are estimated as 2.32 Å. The angle
between the normals to the coordination square planes of different complexes is 68.5°. The packing of molecules along the Х
axis is shown in Fig. 4 a. The Pt(1) and Pt(2) complexes are packed in infinite stacks with the Pt…Pt distance of 5.018-
5.534 Å (Fig. 4 b); the other Pt…Pt distances in the structure are >9.890 Å. The slope angles of the coordination squares to
1190

TABLE 1. Crystallographic Data and Conditions of the Diffraction Experiment for cis-Pt(ptac)2, trans-Pt(pta)2,
and mix-Pt(ptac)2 Crystalline Phases
Parameter
cis-Pt(ptac)2
trans-Pt(ptac)2
mix-Pt(ptac)2
Chemical formula
Formula weight
Temperature, K
Crystal symmetry
Space group
a, Å
C16 H20 F6 O4 Pt
585.41
C16 H20 F6 O4 Pt
585.41
C16 H20 F6 O4 Pt
585.41
296(2)
296(2)
296(2)
Triclinic
P-1
Monoclinic
C2/m
Triclinic
P-1
10.7091(4)
12.7787(6)
16.0154(8)
92.389(2)
90.868(2)
112.1260(10)
2027.39(16)
4
13.3235(5)
8.5515(3)
9.6694(3)
9.9249(8)
12.2193(9)
12.8075(9)
74.966(3)
84.223(3)
85.916(3)
1490.81(19)
3
b, Å
c, Å
α, deg
118.5880(10)
β, deg
γ, deg
3
Volume, Å
967.38(6)
2
Z
3
dcalc, g/cm
1.918
2.010
1.956
–1
Absorption coefficient, mm
F(000)
6.991
7.326
7.131
1120
560
840
Crystal size, mm
0.05×0.03×0.01
1.27-31.71
0.10×0.08×0.05
4.30-33.89
0.20×0.15×0.15
3.46-34.63
θ range, deg
h, k, l limits
–10 ≤ h ≤ 15, –17 ≤ k ≤ 18,
–23 ≤ l ≤ 22
–19 ≤ h ≤ 19, –9 ≤ k ≤ 13,
–12 ≤ l ≤ 14
–14 ≤ h ≤ 15, –16 ≤ k ≤ 17,
–17 ≤ l ≤ 12
I
hkl
meas. / I indep.
hkl
24384 / 11130
6458 / 1830
22229 / 8700
[R(int) = 0.0360]
[R(int) = 0.0232]
[R(int) = 0.0362]
2
1.274
1.082
1.130
GOOF on
F
hkl
R1 = 0.0573,
wR2 = 0.1380
R1 = 0.0173,
wR2 = 0.0430
R1 = 0.0529,
wR2 = 0.1384
R (I > 2σ )
I
R (Imeas
)
R1 = 0.0803,
wR2 = 0.1433
R1 = 0.0173,
wR2 = 0.0430
R1 = 0.0675,
wR2 = 0.1441
Residual electron density
3
(max / min), e/Å
2.767 / –1.539
0.993 / –0.637
6.158 / –1.461
TABLE 2. Main Interatomic Distances d (Å) and Bond Angles ω (deg) for the Studied Pt(pta)2 Complexes
mix-Pt(pta)2
Complex
cis-Pt(pta)2
trans-Pt(pta)2
cis-Pt(pta)2
trans-Pt(pta)2
Pt–O(CF3)
1.979
1.981
1.274
1.269
1.972
1.980
1.266
1.265
1.940-1.985 〈1.972〉
1.952-1.974 〈1.965〉
1.272-1.292 〈1.281〉
1.273-1.301 〈1.287〉
1.328-1.376 〈1.356〉
1.396-1.435 〈1.419〉
〈1.29〉
〈1.988〉
〈1.981〉
〈1.274〉
〈1.248〉
〈1.368〉
〈1.425〉
〈1.29〉
t
Pt–O(Bu )
O–C(CF3)
t
O–C(Bu )
1.369
1.415
1.375
1.407
C–Cγ(CF3)
t
C–Cγ(Bu )
C–F
〈1.282〉
〈1.28〉
O–Pt–O
95.0
94.6
94.8-95.6 〈95.1〉
〈95.2〉
1191

Fig. 3. Molecular structure in the cis-Pt(ptac)2 phase with atomic numbering.
Fig. 4. Packing of molecules in the cis-Pt(ptac)2 phase along the X axis (a) and the
stack of Pt(1) complexes (b).
the stack axes are ∼42°. In a stack, the platinum coordination is completed to pyramidal by contacts with the CF3 group of the
neighboring complex at the Pt…F distance of 3.58 Å. The intermolecular F…F and F…H interactions in the crystal have the
shortest estimates of 3.05 Å and 2.48 Å.
Structure of trans-Pt(ptаc) is formed of isolated centrosymmetric complexes lying on the mirror plane m; the
2
structure of the molecule of the complex with atomic numbering is shown in Fig. 5 a. The coordination square is [PtO4]; the
Pt–O bond lengths from the side of different substituents are almost equal and are 1.980 Å; the chelate O–Pt–O angle is
95.0°. The maximum difference of 0.05 Å occurs in the O–C bonds; the average C–F and C–C bond lengths in the ligand
substituents are 1.282 Å and 1.526 Å. The packing of molecules in the crystal along the Y axis is given in Fig. 5 b. In the
structure, the complexes have the same orientation; each molecule is surrounded by six neighbors at the Pt…Pt distances of
7.916-8.552 Å. The shortest F…F, F…H intermolecular contacts have lower estimates of 2.86 Å and 2.50 Å. The crystals of
1192

Fig. 5. Structure of complexes in the trans-Pt(ptac)2 phase (a) and the
packing of molecules in the crystal along the Y axis (b).
Fig. 6. Packing of molecules of the cis- and trans-isomers in the mix-Pt(ptac)2 phase along the Z
axis (a); mutual orientation of the Pt(1) and Pt(2) complexes in the structure (b).
1193

the cis- and trans-isomers have different structural characters; the calculated density of the trans-isomer is larger than that of
3
the cis-isomer by 0.09 g/cm .
Structure of mix-Pt(ptаc) is a mixture of the cis–trans isomers in the 2:1 ratio. The compound crystallizes in the
2
triclinic crystal symmetry; the structure is molecular formed of monomeric complexes whose structure is shown in Fig. 6 a;
the trans-complex is centrosymmetric. The covalent Pt–O bond lengths vary in a range 1.972-1.991 Å; their average values
are 1.984 Å for the cis-isomer and 1.976 Å for the trans-isomer. The chelate O–Pt–O angles are 95.2° and 94.6° respectively.
The packing of the complexes along the Z axis is given in Fig. 6 b. The isomers are packed in infinite corrugated stacks with
the Pt…Pt distances of 5.32 Å and 5.50 Å.
From the single crystal data, we calculated the theoretical diffraction patterns for all crystalline phases of Pt(ptac)2
and compared them with the experimental diffraction pattern of the sublimated product in which all isolated phases of the
Pt(ptac)2 isomers are present (Fig. 1).
Therefore, this paper reports the single crystal XRD study of solid phases of the Pt(ptаc)2 isomeric complexes,
which was performed for the first time. The crystal structures of the cis- and trans-isomers and their mixed phase were
determined. From the TGA data it follows that the isomeric complexes have very similar thermal characteristics. The
observed small difference in the volatility of the complexes is likely to be due to the different packing of molecules of the
isomers in the crystal lattice. It can be supposed that the less volatile trans-isomer of Pt(ptac)2, which has a greater X-ray
density and the uniform intermolecular environment, is characterized by a greater energy of the crystal lattice.
The authors are grateful to I.V. Korolkov for carrying out the powder XRD study.
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