Thermodynamic study of sublimation, melting and vaporization of scandium(III) dipivaloylmethanate derivatives

J. Chem. Thermodynamics 101 (2016) 162–167

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Thermodynamic study of sublimation, melting and vaporization of scandium(III) dipivaloylmethanate derivatives Kseniya V. Zherikova a,⇑, Ludmila N. Zelenina a,b, Tamara P. Chusova a, Nikolay V. Gelfond a, Natalia B. Morozova a a b

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, Ac. Lavrentyev Ave. 3, 630090 Novosibirsk, Russia Novosibirsk State University, Pirogova Street 2, 630090 Novosibirsk, Russia

a r t i c l e

i n f o

Article history: Received 23 July 2015 Received in revised form 25 April 2016 Accepted 23 May 2016 Available online 24 May 2016 Keywords: Scandium beta-diketonates DSC Static method with glass membrane gaugemanometer Vapor pressure Thermodynamic characteristics Melting and evaporation processes

a b s t r a c t The present work deals with the investigation of thermal properties of two volatile scandium(III) beta-diketonates with 2,2,6,6-tetramethyl-4-fluoro-3,5-heptanedione and 1,1,1-trifluoro-5,5-dimethyl2,4-hexanedione which have been synthesized and purified. Using the static method with glass membrane gauge-manometer the temperature dependencies of saturated and unsaturated vapor pressure were measured for the first time. The temperatures and enthalpies of melting were measured for these compounds by differential scanning calorimetry. The standard thermodynamic characteristics of enthalpy and entropy for sublimation, vaporization and melting processes were derived. Ó 2016 Elsevier Ltd.

1. Introduction The functional coatings on the base of scandium oxide have been found to be interesting materials mainly for optical applications. Scandium oxide films have been used in luminescence displays, optical amplifiers, and high-power UV lasers [1–7]. Besides scandium oxide possess a set of properties that makes it perspective as dielectric layers in CMOS technology [8,9] and also enable their use as scandia-stabilized ZrO2 electrolyte layers in solid oxide fuel cells [10,11]. One of the methods widely applied for fabricating films and coatings (including Sc2O3 ones) with assigned structure and functional properties is Metal-Organic Chemical Vapor Deposition (MOCVD) [12–14]. An important factor of successful MOCVD experiments is the usage of the precursors with certain physicochemical properties satisfying the following basic requirements of this method: high vapor pressure at low temperatures and thermal stability at vaporization temperatures. The above features of metal beta-diketonates make these compounds are very useful in the MOCVD process [15–18]. However, to choose optimal deposition conditions it is necessary to possess information about thermodynamic characteristics of the MOCVD precursor, its saturated ⇑ Corresponding author. E-mail address: [email protected] (K.V. Zherikova). http://dx.doi.org/10.1016/j.jct.2016.05.020 0021-9614/Ó 2016 Elsevier Ltd.

vapor pressure values and thermal behavior in condensed and gaseous phases. Earlier, we have published the data on investigation of three processes (sublimation, evaporation, and melting) for the precursor widely used in MOCVD of Sc2O3 films – scandium dipivaloylmethanate, Sc(thd)3 (Fig. 1) [19]. We have measured saturated vapor pressure over crystalline and liquid Sc(thd)3 using static method with glass membrane gauge-manometer with the following calculations of the corresponding values of enthalpy and entropy. The thermodynamic characteristics for melting were obtained using differential scanning calorimetry (DSC). The necessity to create another high purity volatile precursors has provoked us to synthesize the ‘‘analogues” of Sc(thd)3 where the replacements of hydrogen atom at c-C by fluorine (Sc(tfhd)3) and one tert-butyl group by CF3 (Sc(ptac)3) were made (see Fig. 1). This work devotes to the same comprehensive thermochemical investigation of these two compounds which was done to establish the effect of terminal and c-F substitutions on volatility and other thermal properties of the complexes. 2. Experimental 2.1. Synthesis and identification of Sc compounds All chemicals used in the synthesis were commercially available products of reagent grade and applied without further purification

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(H3C)3C

(H3C)3C

O

O O (H3C)3C

C(CH3)3

O

F

Sc O

(H3C)3C

(H3C)3C

O

O

O

(H3C)3C

CF3

O O

O

Sc

Sc O

O

C(CH3)3 (H3C)3C

Sc(thd)3

F 3C

O

C(CH3)3 (H3C)3C

(H3C)3C

C(CH3)3

O O O

O

F

C(CH3)3 F3C

F

Sc(tfhd)3

Sc(ptac)3

Fig. 1. Compounds described in this work: Sc(thd)3 – scandium 2,2,6,6-tetramethyl-3,5-heptanedionate/scandium dipivaloylmethanate, Sc(tfhd)3 – scandium 2,2,6,6tetramethyl-4-fluoro-3,5-heptanedionate, Sc(ptac)3 – scandium 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionate/scandium pivaloyltrifluoroacetonate.

Table 1 Characterization of the chemical samples used in this study. Chemical name

Source/supplier

State

Mass Fractional Purity

Sc2O3 Hthd Hptac Htfhd NaOH Chloroform EtOH Sc(tfhd)3 Sc(ptac)3

Dalchem Dalchem SIA ‘‘P&M-invest’’ Air Products Inc., USA ‘‘Component-reaktiv” Co., Ltd ‘‘Component-reaktiv” Co., Ltd Acros organics Synthesis (purification – double fractional sublimation) Synthesis (purification – double fractional sublimation)

Solid Liquid Liquid Liquid Solid Liquid Liquid Solid Solid

P0.99 P0.99 P0.99 P0.99 P0.98 P0.9995 P0.96 P0.99 (elemental CH analysis) P0.99 (elemental CHF analysis)

Table 2 Melting points (m.p.), enthalpies (DmeltH°m.p.) and entropies (DmeltS°m.p.)* obtained by DSC under saturated vapor pressure of the compounds (the values given in brackets corresponds to melting points determined visually).

* **

Compound

m.p./K

DmeltH°m.p./(kJ mol
1)

DmeltS°m.p./(J mol
1 K
1)

Ref

Sc(tfhd)3 Sc(ptac)3

423.0 ± 0.3 (423–425) 331.6 ± 0.5 (331–333) 329.7–330.2**

21.6 ± 1.3 25.2 ± 0.7 –

51.1 ± 1.3 76.6 ± 1.4 –

this work this work [26]

Combined expanded uncertainties Uc (0.95 level of confidence) are presented in the table for m.p., DmeltH°m.p. and DmeltS°m.p. Results are given as they were reported in the original sources.

(see Table 1). Only 2,2,6,6-tetramethyl-4-fluoro-3,5-heptanedione has been kindly given by Dr. J. Norman, Air Products Inc., USA. The synthesis of scandium(III) beta-diketonate derivatives was carried out according to the literature procedures [20]. Fullstrength nitric acid (10 ml) was added to scandium oxide (2.0 g, 14.5 mmol) and mixture obtained was evaporated up to wet salts. Then the procedure was repeated twice by adding full-strength hydrochloric acid (10 ml). For the last time mixture was evaporated up to almost dry salts. Scandium salt obtained was dissolved in 40 ml ethanol and then fourfold excess of corresponding ligand (116.0 mmol) was added at stirring and refluxing. Thereafter saturated ethanol solution of NaOH was surged to reaction mixture drop by drop up to pH = 5–6. The precipitation generated was filtered and dried in air. The filtrate was extracted with chloroform and the organic portion was evaporated. Then these dried precipitates were combined and purified by sublimation in a vacuum gradient furnace at a pressure 1 Pa and the temperatures 373 K (Sc(tfhd)3) and 353 K (Sc(ptac)3). The elemental analyses were carried out in Nikolaev Institute of Inorganic Chemistry SB RAS using Euro EA 3000 CHN-elemental analyzer for Sc(tfhd)3 and in Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS using Carlo-Erba-1106 (Italy) rapid elemental analyzer [21a,b] for Sc(ptac)3. The uncertainties in elemental determinations did not exceed 0.5 per cent.

Elemental analyses for C33H54F3O6Sc (%): Calcd: C 61.1, H 8.3; Found: C 60.9, H 8.6; for C24H30F9O6Sc (%): Calcd: C 45.7, H 4.8, F 27.1; Found: C 45.5, H 4.8, F 26.9. Melting points of complexes were determined visually using the Kofler’s table. The crystals of the compound were placed between two glasses at atmospheric pressure and observed during heating (see Table 2, column 2, values in brackets). 2.2. Calorimetric investigation Calorimetric measurements were performed using Setaram DSC 111; heating rate was (0.5–1.0) K min
1, sample weight – (8–18) mg. During the measurements the investigated substance was contained in evacuated glass ampoule. Six calorimetric experiments were carried out for each investigated compounds. The uncertainties (u) in the heat effect measurements estimated from calibration experiments (C6H5COOH, In) were less than 1 per cent. 2.3. Vapor pressure measurement The vapor pressures of the compounds were measured by the static method with glass membrane gauge-manometer [22]. The sensitivity of the membrane gauge-manometers used in the present study varied from 5 for experiments with Sc(tfhd)3 to 20 Pa

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for experiments with Sc(ptac)3. The compensating pressure is measured by use of mercury manometer (inner diameter 25 mm) and cathetometer, with an error of less than 5 Pa. The other instrumental errors (for temperature range (374–512) K) consisted from two parts: (1) a compensation error (dp = 30 Pa), (2) an amendment on temperature drift of zero position of a membrane owing to a difference in coefficients of thermal expansion of separate parts of the manometer (dp = 50 Pa for experiments with Sc(tfhd)3 and dp = 70 Pa for experiments with Sc(ptac)3). The main characteristics of the experimental set-up and the experimental procedure were described in detail in [23–25]. The standard uncertainties (u) in values of pressure and temperature were 50–70 Pa and 0.5 K, accordingly. The investigated compound was loaded into the inner chamber of membrane-gauge manometer and after evacuation it was sealed. Pressure measurements were recorded after reaching the equilibrium at a given temperature. The measurements were realized in wide intervals of temperature (374 6 T/K 6 512), pressure (6 6 p/Pa 6 3700) and concentration (4.791 6 m/V/g dm
3 6 0.249). The pressures measured from low to high temperatures and backwards were identical at the same temperature. This procedure guaranteed the achievement of equilibrium. The time of equilibrium establishment was 15–20 min. 3. Results and discussion 3.1. DSC investigation DSC was used for definition of thermodynamic characteristics of melting processes (m.p., DmeltH°m.p. DmeltS°m.p.). The compounds are thermally stable and exhibit only one phase transition – melting over the studied temperature range (300–440) K. The results obtained are presented in Table 2 (experimental data are listed in Table A.1, melting points are referred to the onset extrapolated from the corresponding DSC peaks given in Fig. A.1). The uncertainties in this table refer to a 95 per cent confidence limit. Melting points determined by means of DSC agrees with those observed visually. The value of melting point for Sc(ptac)3 conforms with that presented in [26].

Fig. 2. Temperature dependence of experimental pressure for Sc(ptac)3 (a) and Sc (tfhd)3 (b). Sample mass and manometer volume ratio (m/V, g dm
3) are presented in the legend of this figure for each experiment.

3.2. Investigation of temperature dependencies of vapor pressure

In that case the temperature dependence of saturated vapor pressure may be described by the Clapeyron–Clausius equation:

Two series of measurements were performed by static method with the samples of Sc(ptac)3 ((m/V)1 = 4.791 g dm
3, (m/V)2 = 0.301 g dm
3) and three series of measurements with the samples of Sc(tfhd)3 ((m/V)1 = 1.894 g dm
3, (m/V)2 = 0.249 g dm
3, (m/V)3 = 1.455 g dm
3). Experimental data are shown in Fig. 2a, b and listed in Table A.2. Data analysis on saturated vapor pressure showed that the pressure did not depend on initial concentrations under the examined conditions, thus, the equilibria observed are monovariant. The average molecular weight of gas calculated using ideal gas law from the experimental data on unsaturated vapor was close to the molecular weight of monomer for both compounds (Fig. 3a, b), that was evidence of the absence of other molecular forms in the gas phase within the accuracy specified above. Decrease of the molecular weight of gas above 460 K (for Sc(ptac)3) and 510 K (for Sc(tfhd)3) coincides with the beginning of compound decomposition. The values of these temperatures denote greater thermal stability of Sc(tfhd)3 compare with Sc(ptac)3. The presence of only one molecular form in the gas phase allowed us to process data using the simplest physicochemical model of process,

pcalc ¼ p expð
Dpr HT =RT þ Dpr S T =RÞ;

AðsÞ ¼ AðgÞ

ð1Þ

AðlÞ ¼ AðgÞ

ð2Þ

ð3Þ

where DprHT and DprS°T are the enthalpy and entropy of vaporization (sublimation) at temperature T, p° is 101,325 Pa, and R is the gas constant. Processing experimental data on the saturated Sc(ptac)3 vapor pressure was performed by using the objective function recommended in [27]. This function allowed us to use the principle of maximum likelihood in the least square method to estimate the thermodynamic parameters of a process. In the case of Sc(tfhd)3 we had experimental data on three processes obtained by two independent methods: sublimation and vaporization (static method) and melting (DSC). This advantage enabled us to apply the joint processing of experimental data on melting, vaporization and sublimation processes described in detail in our work [19]. This technique of data processing used the condition of equality of vapor pressure over the solid and liquid phases at the melting temperature, while the enthalpy of melting is regarded as a known parameter. Due to this technique we could estimate the thermodynamic characteristics of sublimation with reasonable accuracy (see Table 3) in spite of low (for static method) vapor pressure values. Both procedures of data processing included the calculation of required parameters (DprHT and DprS°T) as well as their expanded uncertainties U (0.95 level of confidence). Since experimental heat capacity data of the substances under consideration were lacking

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Fig. 4. Difference between the values of experimental pressure (pexp) and pressure calculated via the corresponding equation in Table 3 (pcalc) for vaporization of Sc (ptac)3 (a) and for sublimation and vaporization of Sc(tfhd)3 (b).

Fig. 3. Temperature dependence of average weight of gas for Sc(ptac)3 (a) and Sc (tfhd)3 (b).

the sought values were calculated at the average temperature of measured interval and the melting temperature for Sc(ptac)3 and Sc(tfhd)3 respectively. The results of processing are summarized in Table 3. The deviations of experimental pressure values from those calculated by the equations given in Table 3 did not exceed standard uncertainties in temperature u(T) and pressure u(p) measurements (Fig. 4). They were random in character and it was evidence of the absence of significant systematic errors in our results. Fig. 5 illustrates graphically the data on the temperature dependencies of saturated vapor pressure of the complexes under investigation. The data for Sc(thd)3 published previously in [19] are also presented in this figure. It should be noted that the vapor pressure of Sc(ptac)3 is significantly higher than those of two other compounds. A substitution of one tert-butyl group for –CF3 causes an increase in vapor pressure (approximately by 2 orders of magnitude) and dramatically decreases the value of the melting

point (almost 100 K). On the contrary the introduction of fluorine atom into the c-position of 2,2,6,6-tetramethyl-3,5-heptanedione has no substantial effect on the volatility of the compound. Sc (tfhd)3 and Sc(thd)3 are similar in thermal properties; they have close values of vapor pressures and melting points (for Sc(thd)3: m.p. = 425.6 ± 0.5 K) as thermodynamic characteristics (for Sc(thd)3: DsubHm.p. = 103.5 ± 5.6 kJ mol
1, DsubS°m.p. =
1 184.9 ± 4.3 J K mol
1 in range of T = (387–425) K; DvapHm.p. = 76.8 ± 5.8 kJ mol
1, DvapS°m.p. = 122.2 ± 4.9 J K
1 mol
1 in range of T = (426–492) K; DmeltH°m.p. = 26.7 ± 0.2 kJ mol
1, DmeltS°m.p. =
1
1 62.7 ± 0.6 J mol K ). Such increase in vapor pressures of metal complexes with beta-diketones when a fluorine-containing terminal substituent is introduced into the molecule ligand and the absence of any influence on thermal properties when a hydrogen atom at gamma-carbon of ligand is replaced by fluorine was observed earlier [28,29].

Table 3 Temperature dependences of saturated vapor pressure ln(p/p°) ± 2r = A
B/T, where p° is the standard pressure of 101.325 kPa, r2 = f(T); enthalpies (DprHT) and entropies (DprS°T) of evaporation processes at the average temperature (417.0 K) of measured interval (DT) for Sc(ptac)3 and the melting point (423.0 K) for Sc(tfhd)3.* Compound Sc(ptac)3 Sc(tfhd)3 *

Process vap. sub. vap.

DT/K 382–452 374–420 426–511

ln(p/p°) = A
B/T, r2 2

14.01–8145/T, 9176.6/T –41.700/T + 0.047438 20.09–11434/T, 63991/T2–96.204/T + 0.036158 13.95–8836/T, 63991/T2–96.204/T + 0.036158

Combined expanded uncertainties Uc (0.95 level of confidence) are presented in the table for DprH.T and DprS°T.

DprHT/(kJ mol
1)

DprS°T/(J K
1 mol
1)

67.7 ± 0.8 95.1 ± 2.1 73.5 ± 2.1

116.5 ± 1.8 167.1 ± 1.6 116.0 ± 1.6

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4. Conclusion Experimental studies of two fluorinated scandium(III) dipivaloylmethanate derivatives have allowed us to establish the effect of fluorine containing at the terminal substituent and gammaposition in ligand on thermal properties of the complexes. We have applied static method with glass membrane gauge-manometer and DSC to measure temperature dependencies of saturated and unsaturated vapor pressures and heat effects for the first time. Sets of enthalpies and entropies of sublimation, vaporization and melting processes have been derived from these data. A comparison of these complexes with non-fluorinated scandium(III) dipivaloylmethanate has been carried out. The substitution of one tertbutyl group for CF3 leads to substantial vapor pressure growth and melting point lowering as well as a decrease of thermal stability of the compound. The replacement of hydrogen atom at c-C by fluorine has influence neither volatility nor thermal stability of the compound. Fig. 5. Temperature dependencies of saturated vapor pressure for Sc(thd)3 [19], Sc (tfhd)3, and Sc(ptac)3.

Table A.1 Experimental value of temperature, m.p.exp, and enthalpy, DmeltH°m.p.exp, of melting obtained by DSC.* Sc(ptac)3

*

Sc(tfhd)3

m/g

m.p./K

DmeltH°m.p.exp/(kJ mol
1)

m/g

m.p./K

DmeltH°m.p.exp/(kJ mol
1)

0.0144 0.0156 0.0185 0.0175 0.0151 0.0155

332.2 332.2 331.7 331.2 331.2 331.2

24.9 25.3 26.5 25.0 25.0 24.4

0.0087 0.0118 0.0163 0.0114 0.0112 0.0113

422.9 423.2 423.2 423.2 422.7 422.9

20.4 21.5 21.9 21.0 22.4 22.5

Standard uncertainties u are u(m) = 0.0001 g, u(m.p.) = 0.1 K, u(DmeltH°m.p.) = 10
2 * DmeltH°m.p.

Table A.2 Saturated and unsaturated vapor pressures over solid and liquid compounds: pexp, experimental value of vapor pressure; M, molecular weight of gas calculated using ideal gas law from the experimental unsaturated vapor pressures.* Sc(ptac)3 Over liquid

Unsaturated vapor

Nexp

T/K

pexp/Pa

Nexp

T/K

pexp/Pa

Nexp

T/K

pexp/Pa

Nexp

T/K

p/Pa

M/(g mol
1)

1

382.5 389.9 393.3 393.1 396.6 400.2 410.3 417.5

118.4 171.5 55.9 57.2 150.3 180.9 242.1 412.3

1

422.4 427.6 436.2 438.4 444.7 453.7 388.4 398.7

504.1 718.2 927.0 984.2 1306.1 1993.7 97.1 214.1

2

407.6 418.5 427.0 431.8 433.1 441.0 446.7 452.0

263.3 434.9 667.7 831.2 852.5 1146.5 1459.0 1786.2

2

452.0 455.4 456.9 458.8 459.1

1786.2 1799.5 1802.1 1814.6 1830.1

631.29 631.25 632.43 630.75 625.77

Nexp

T/K

pexp/Pa

Nexp

T/K

pexp/Pa

Nexp

T/K

pexp/Pa

Nexp

T/K

p/Pa

M/(g mol
1)

1

408.5 417.9 384.7 414.4 374.1 390.1 404.9 420.8 414.2 395.9 407.1 417.4

28.911 110.15 6.4299 127.51 8.9778 28.731 23.173 57.591 25.698 36.817 54.056 88.633

1

428.7 436.6 449.4 460.4 470.8 468.0 480.2 491.5 501.7 511.9 427.1 439.9 449.2 461.3

105.4 270.7 411.9 577.2 841.1 806.4 1210.3 1776.8 2564.6 3706.9 144.7 229.8 408.6 618.5

2

472.2 481.1 427.4 437.7 443.7 448.8 454.2 458.1 469.2 476.6 485.4 498.2 499.1 498.8

854.1 1214.8 163.5 236.0 270.4 320.1 414.8 543.4 806.7 1069.8 1495.9 2287.0 2336.3 2292.8

2

491.6 489.5 493.2 493.9 496.2 497.1 498.9 503.6 504.3 505.5 505.9 508.8 510.5

1565.2 1546.4 1609.9 1609.1 1595.7 1607.7 1611.9 1582.5 1581.1 1595.1 1594.2 1654.8 1658.1

649.33 654.44 633.37 634.52 642.87 639.23 639.92 657.90 659.44 655.13 656.09 635.60 636.49

2

Sc(tfhd)3 Over solid

2

3

*

Over liquid

2

Unsaturated vapor

3

Standard uncertainties u are u(T) = 0.5 K, u(p) = 70 Pa for Sc(ptac)3, u(p) = 50 Pa for Sc(tfhd)3, and u(M) = 0.01 g mol
1.

K.V. Zherikova et al. / J. Chem. Thermodynamics 101 (2016) 162–167

Fig. A.1. DSC curves for Sc(ptac)3 (a) and Sc(tfhd)3 (b).

Acknowledgement We gratefully acknowledge Dr. J. Norman for kindly given 2,2, 6,6-tetramethyl-4-fluoro-3,5-heptanedione.

Appendix A Tables A.1 and A.2 and Fig. A.1.

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JCT 15-506