Design of Volatile Mixed-Ligand Tantalum(V) Compounds as

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Design of Volatile Mixed-Ligand Tantalum (V) Compounds as Precursors to Ta2O5 Films Sanjay Mathur, Linus Appel, Raquel Fiz, Wieland Tyrra, and Ingo Pantenburg Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501438k • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Crystal Growth & Design

Design of Volatile Mixed-Ligand Tantalum (V) Compounds as Precursors to Ta2O5 Films Linus Appel, Raquel Fiz, Wieland Tyrra, Ingo Pantenburg and Sanjay Mathur*# *

Inorganic and Material Chemistry, University of Cologne, Greinstraße 6, 50939, Cologne, Germany

#

International Research Center for Renewable Energy, School of Energy & Power Engineering, Xian Jiaotong University, Xian (Shaanxi) 710049, PR China

KEYWORDS: Tantalum Oxide, β-Heteroarylalkenolates, Alkoxide, Thin films, MOCVD ABSTRACT Synthesis and structural characterization of six monomeric, heteroleptic tantalum(V) complexes of the general

formula

Ta(OiPr)4(ArTFP),

where

Ar = pyridine

(1),

4,5- dimethyloxazole

(2),

4,5-dimethylthiazole (3), benzimidazole (4), benzoxazole (5), benzthiazole (6), and TFP = trifluoropropenol, are described. Introduction of a donor functionalized β-heteroarylalkenolate in the coordination sphere of Ta in the dimeric Ta2(OiPr)10 increases significantly the stability and volatility of these precursors, simplifying the depositions of Ta2O5. The molecular structures of (1-6) exhibited a distorted octahedral coordination around the tantalum center by four iso-propoxide groups and one β-heteroarylalkenolate. Thermal decomposition studies (TG/DTA) and analysis of by-products by NMR spectroscopy confirmed the decomposition mechanism and gas phase stability of the heteroleptic compounds necessary for Ta2O5 depositions. Chemical vapor deposition studies with (1) and (2) demonstrated their suitability as efficient precursors for the growth of Ta2O5 thin films, whose properties were compared with Ta2O5 thin films obtained from homoleptic alkoxides.

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1. INTRODUCTION Tantalum oxide (Ta2O5) thin films are receiving increasing attention in the fields of catalysis[1, 2], capacitors and resistors[3-6], optical devices[7-9] and even in biomedicine applications[11, 12] due to their high permittivity, high breakdown current, superior refractive index and superior chemical and thermal stability in comparison to other transition metal oxides.[13, 14] In fact, the interesting dielectric properties of Ta2O5 thin films suggest a usability as a replacement material for SiO2 with the purpose of scaling down microelectronics.[15-17] Among the methods used to fabricate Ta2O5 thin films, chemical vapor deposition (CVD) is a standard procedure chosen in silicon-based integrated circuit technology, with tantalum pentaethoxide as precursor.[15,

18-21]

However, this compound is highly sensitive towards moisture, which makes its

handling challenging. Simple modifications of the coordination sphere of tantalum alkoxides by introducing chelating ligands can alter the nuclearity of the compound, which often dismisses the vapor pressure and enhances the stability of the precursors. Reactions of Ta2(OEt)10 with β-diketones have been reported to stabilize the compound to an extent where gas phase depositions were only possible with

liquid

injection

methods.[22-26]

Other

modifications

include

β-ketoesters,[27]

1,2-dihydroxybenzene,[28] ethyl benzoylacetate,[29] N-aryl alkyl carbamates,[30] dibenzoyl methanate,[31] N-alkoxo-β-ketoiminates,[32] 4,4’-di-methoxy-2,2’-diol-benzophenone[33] and salicylaldoxime.[34] The

bidentate

chelating

ligands

used

in

this

study

namely

3,3,3-trifluoro-1-(heteroar-2-yl)propen-2-oles ArTFP (where Ar = pyridine [Py], 4,5-dimethyloxazole [DMO], 4,5-dimethylthiazole [DMT], benzimidazole [BI], benzoxazole [BO] and benzthiazole [BT]) offer a customized conjugated system to improve significantly the physico-chemical properties of the parent metal alkoxides as already demonstrated for niobium(V) derivatives.[35] Compared with “classical” β-diketonates, substituted alkenol ligands offer a positive inductive effect of different heteroaryl moieties and a negative inductive effect of the CF3-group (pseudo push-pull effect) to the metal center,[36] which together with the formation of a stable six-membered metallacycle offer


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improved precursors with respect to their handling. Furthermore, the volatility of the molecules can be enhanced by replacing hydrogen atoms by fluorine atoms. Sievers et al. have examined this phenomenon by measuring the vapor pressure of several metal β-diketonates, where the CH3-group of acetylacetone had been exchanged to CF3- and C3F7-groups.[37] The measured vapor pressures along with thermogravimetric data confirmed a direct dependence of volatility to the degree of fluorination of the examined metal β-diketonates. The ligand PyTFP was first reported in 1955 and was used as a chelate with copper, iron, cobalt, and zinc in 1971[38, 39]; however, their potential as volatile air-stable precursors for material synthesis was explored only lately.[40-42] Recently, we have shown the suitability of the homoleptic Sn(DMOTFP)2 complex as a precursor in the chemical vapor deposition of SnO2 nanowires.[40] Further investigations produced air-stable complexes of platinum (II) and palladium (II) where the side chain extension of a similar ligand PyTFP with longer fluoroalkyl groups was directly correlated to an increased volatility of these complexes.[36] Furthermore, air stable complexes of aluminum (III) with these ligands were prepared and the structure of the tetragonal {AlL2}+ ions in the gas phase have been elucidated by mass spectrometry and computational methods.[41] Recently, monomeric, volatile and more stable alkoxide complexes of niobium (V) with these ligands have been reported, including their use in CVD processes to

obtain

Nb2O5

thin

films.[43]

Herein,

syntheses

and

characterizations

of

six

tantalum (V) iso-propoxides with ligands of the general formula Ta(OiPr)4(ArTFP) are reported. A detailed study of their volatility and decomposition has been undertaken to get a deeper insight in their behavior during deposition processes with formation of Ta2O5.

2. MATERIALS AND METHODS 2.1. Chemicals and Methods. All preparations were performed in a modified all-glass Stock-type assembly under an atmosphere of nitrogen. The aliphatic solvents were dried over sodium with benzophenone as indicator with subsequent storage over sodium wire to ensure dryness. Iso-propanol was dried by refluxing it over sodium (approx. 3 g/l) until complete dissolution and stored over molar ACS Paragon Plus Environment


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sieves (3 Å). Tantalum(V)chloride was obtained from Alfa Aeser in a purity of 98.5% and was used without further purification. Tantalum(V)-iso-propoxide was synthesized following a literature procedure.[44] The organic fluorine containing ligands (eg. PyTFP) were synthesized according to modified literature procedures.[40, 45] Elemental analysis were performed on a HEKAtech CHNS Euro EA 3000. MS spectra were recorded with a Finnigan MAT 95 (20 eV) in m/z (relative percent) using electron impact ionization. NMR spectra were measured with a Bruker Avance II 300 spectrometer (1H at 300.13 MHz,

13

C at 75.02 MHz and

19

F at 282.45 MHz; BBI probe with Z-gradient) or, for the 2D

H,F-correlation NMR spectra, with a Bruker AV 400 (1H at 400.13 MHz, 13C at 100.61 MHz and 19F at 376.50 MHz; H, F and X TBI probe with Z-gradient). Chemical shifts of 1H and 13C signals are given relative to tetramethylsilane (TMS) as external standard.

19

F chemical shifts are relative to external

CCl3F. TG/DTA measurements were performed with a TGA/DSC 1 STARe system by Mettler (Mettler Toledo GmbH, Giessen, Germany) with a gas controller GC 100. Samples were measured in a sealed aluminum cartridge (HEKAtech GmbH, 12x3 mm). Each measurement was performed under a N2 flow between 40 and 630 °C with a heating rate of 5 °C/min. Visible melting and decomposition points were determined on a HWS Mainz Laboratoriumstechnik SG 2000 apparatus in sealed 4 mm NMR tubes in N2 atmosphere. Single crystals were placed in highly viscous oil and mounted in a sling on the goniometer head directly before the measurement. Diffraction data for crystals of the discussed compounds were collected on an imaging plate diffractometer (IPDS 2T, STOE & CIE) equipped with a fine focus sealed tube X-ray source (Mo-Kα, λ = 71.07 pm) at 170 K. Further crystallographic and refinement data are summarized in Table 1. Structure solution and refinement (based on full-matrix least-squares on F2) were carried out using STOE’s X-Area[46] and the WINGX suite of programs[47] including SIR92[48] and SHELXL97.[49] H-atoms were calculated geometrically and a riding model was applied during the refinement process. The thermal parameters for H-atoms were taken as Uiso = 1.5 Ueq(CH3) and Uiso = 1.2 Ueq(CH, CAr), where Ueq(C) was the equivalent parameter for the carbon atom to which the hydrogen atom is attached. Final agreement factors are listed in Table 1.


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2.2. General synthetic procedure: Ta2(OiPr)10 was dissolved in an appropriate anhydrous solvent in an inert atmosphere. Afterwards, 2 eq. of the corresponding chelating ligand was added and the reaction mixture was stirred at room temperature for six hours or put in an ultrasonic bath for 0.5 h. After removal of solvent the residue was washed with n-heptane. The raw material was purified by sublimation (10-3 mbar) or crystallization at 4 °C from nearly saturated solutions. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(pyridine-2-yl)propen-2-olato)-tantalum(V) (1): The reaction of Ta2(OiPr)10 (0.665 g, 0.698 mmol) with PyTFP (0.276 g, 1.460 mmol) in toluene (25 ml) resulted in a slightly yellow oily raw product. The reaction time was 6 h, the raw product was washed with n-heptane (2x5 ml). Colorless crystals were grown from 2-propanol at 4 °C. Yield: 98% (0.829 g, 1.369 mmol). Sublimation temperature: 70 °C/10-3 mbar. Melting point: 76 °C. 1H NMR (300 MHz, C6D6, RT): δ [ppm] = 9.07 (d, 3JH-H = 6.0 Hz, 1 H, 1-H), 6.90 (t, 3JH-H = 7.6 Hz, 1 H, 3-H), 6.52 (t, 3JH-H = 6.7 Hz, 1 H, 2-H), 6.50 (d, 3JH-H = 7.7 Hz, 1 H, 4-H), 5.96 (s, 1 H, 6-H), 5.23 (sep, 3JH-H = 6.1 Hz, 1 H, 9-H), 5.09 (sep, 3JH-H = 6.1 Hz, 1 H, 10-H), 4.65 (sep, 3JH-H = 6.0 Hz, 2 H, 11-H), 1.55 (d, 3JH-H = 6.2 Hz, 6 H, 12-H), 1.46 (d, 3JH-H = 6.2 Hz, 6 H, 13-H), 1.27 (d, 3JH-H = 6.4 Hz, 6 H, 14-H), 1.07 (d, 3JH-H = 6.0 Hz, 6 H, 15-H).

13

C NMR (300 MHz, C6D6, RT): δ [ppm] = 154.3 (5-C), 153.5 (7-C), 149.0 (1-C), 137.7

(3-C), 124.1 (4-C), 120.9 (8-C), 119.6 (2-C), 98.9 (6-C), 76.4 (9-C), 74.6 (10-C), 72.3 (11-C), 25.7 (15-C), 25.6 (14-C), 25.5 (12-C), 25.0 (13-C). 1

JF,C = 287 Hz,

2

JF,C = 33 Hz).

MS:

19

F NMR (300 MHz, C6D6, RT): δ [ppm] = -73.3 (s,

m/z (T~48 °C) = 605

(1%,

[M]+),

546

(34%,

[Ta(OiPr)3(C8H5ONF3)]+), 504 (4%, [Ta(OiPr)2(OH)(C8H5ONF3)]+), 487 (16%, [TaC13H15NO4F3]+), 417 (100%, [Ta(OiPr)4]+), 375 (12%, [Ta(OiPr)3(OH)]+), 333 (8%, [Ta(OiPr)2(OH)2]+), 291 (4%, [Ta(OiPr)(OH)3]+), 151 (12%, [C8H6FNO]+). Anal Calcd. for TaC20H33NO5F3 [%]: C 39.68; H 5.49; N 2.31. Found: C 39.72; H 5.88, N 1.58. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(4,5-dimethyloxazole-2-yl)propen-2-olato)-tantalum(V)

(2):

The reaction of Ta2(OiPr)10 (1.006 g, 1.056 mmol) in toluene (25 ml) with DMOTFP (0.447 g, 2.158 mmol) resulted in a colorless oily raw product. The reaction time was 6 h and the raw product was washed with n-heptane (2x5 ml). Colorless crystals were grown from 2-propanol at 4 °C. Yield:



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98% (1.224 g, 1.963 mmol). Sublimation temperature: 60 °C/10-3 mbar. Melting point: 55 °C. 1H NMR (300 MHz, C6D6, RT): δ [ppm] = 5.97 (s, 1 H, 6-H), 5.19 (sep, 3JH-H = 6.1 Hz, 1 H, 9-H), 4.96 (sep, 3

JH-H = 6.2 Hz, 1 H, 10-H), 4.51 (sep, 3JH-H = 6.1 Hz, 2 H, 11-H), 2.21 (s, 3 H, 4-H), 1.85 (s, 3 H, 5-H),

1.47 (d, 3JH-H = 6.2 Hz, 6 H, 12-H), 1.43 (d, 3JH-H = 6.2 Hz, 6 H, 13-H), 1.21 (d, 3JH-H = 6.0 Hz, 6 H, 15-H), 1.07 (d, 3JH-H = 6.2 Hz, 6 H, 14-H).

13

C NMR (300 MHz, C6D6, RT): δ [ppm] = 160.5 (1-C),

157.7 (7-C), 141.7 (2-C), 128.1 (3-C), 120.5 (8-C), 85.5 (6-C), 75.7 (9-C), 74.8 (10-C), 72.1 (11-C), 25.6 (14-C, 15-C), 25.3 (13-C), 24.8 (12-C), 11.0 (4-C), 8.8 (5-C).

19

F NMR (300 MHz, C6D6, RT): δ

[ppm] = -74.2 (s, 1JF,C = 278 Hz, 2JF,C = 34 Hz). MS: m/z (T~36 °C) = 623 (1%, [M]+), 564 (20%, [Ta(OiPr)3(C8H7O2NF3)]+), 505 (1%, [Ta(OiPr)2(C8H7O2NF3)]+), 480 (1%, [TaC15H25NO5]+), 438 (1%, [TaC12H19NO5]+),

417

(100%,

[Ta(OiPr)4]+),

375

(12%,

[Ta(OiPr)3(OH)]+),

333

(4%,

[Ta(OiPr)2(OH)2]+), 291 (2%, [Ta(OiPr)(OH)3]+). Anal Calcd. for TaC20H35NO6F3 [%]:C 38.53; H 5.66; N 2.25. Found: C 37.69; H 6.47; N 2.42. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(4,5-dimethylthiazole-2-yl)propen-2-olato)-tantalum(V)

(3):

The reaction of Ta2(OiPr)10 (0.841 g, 0.883 mmol) in 2-propanol (20 ml) with DMTTFP (0.406 g, 1.819 mmol) gave a light brown raw product. The reaction time was 6 h and the raw product was washed with n-heptane (1x5 ml). Brownish crystals were grown from 2-propanol at 4 °C. Yield: 98% (1.110 g, 1.736 mmol). Sublimation temperature: 90 °C/10-3 mbar. Melting point: 80 °C. 1H NMR (300 MHz, C6D6, RT): δ [ppm] = 6.17 (s, 1 H, 6-H), 5.30 (sep, 3JH-H = 6.1 Hz, 1 H, 9-H), 5.01 (sep, 3

JH-H = 6.1 Hz, 1 H, 10-H), 4.64 (sep, 3JH-H = 6.1 Hz, 2 H, 11-H), 2.55 (s, 3 H, 5-H), 1.75 (s, 3 H, 4-H),

1.54 (d, 3JH-H = 6.0 Hz, 6 H, 13-H), 1.45 (d, 3JH-H = 6.0 Hz, 6 H, 12-H), 1.29 (d, 3JH-H = 6.2 Hz, 6 H, 15-H), 1.12 (d, 3JH-H = 6.1 Hz, 6 H, 14-H).

13

C NMR (300 MHz, C6D6, RT): δ [ppm] = 164.0 (1-C),

153.2 (7-C), 146.4 (2-C), 122.4 (3-C), 120.9 (8-C), 94.1 (6-C), 75.9 (9-C), 74.8 (10-C), 72.6 (11-C), 25.7 (14-C), 25.6 (15-C), 25.1 (12-C), 25.0 (13-C), 15.2 (5-C), 10.8 (4-C). 19F NMR (300 MHz, C6D6, RT):

δ

[ppm] = -73.2

(s,

1

JF,C = 289 Hz,

2

JF,C = 33 Hz).

MS:

m/z (T~40 °C) = 580

(16%,

[Ta(OiPr)3(C8H7ONSF3)]+), 521 (6%, [Ta(OiPr)2(C8H7ONSF3)]+), 417 (100%, [Ta(OiPr)4]+), 375 (12%,


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[Ta(OiPr)3(OH)]+), 333 (8%, [Ta(OiPr)2(OH)2]+), 291 (4%, [Ta(OiPr)(OH)3]+). Anal Calcd. for TaC20H35NO5SF3 [%]: C 37.56; H 5.52; N 2.19. Found: C 37.64; H 5.58; N 2.45. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(benzimidiazole-2-yl)propen-2-olato)-tantalum(V) (4): The reaction of Ta2(OiPr)10 (0.913 g, 0.958 mmol) in 2-propanol (25 ml) with BITFP (0.448 g, 1.936 mmol) gave a slightly yellow raw product. The reaction time was 0.5 h in an ultrasonic bath. Colorless crystals were grown from 2-propanol at 4 °C. Yield: 84% (1.035 g, 1.606 mmol). Sublimation temperature: 160 °C/10-3 mbar. Melting point: not observed.

1

H NMR (300 MHz,

acetone-d6, RT): δ [ppm] = 11.90 (s, 1 H, NH), 8.26 (m, 1 H, 3-H), 7.53 (m, 1 H, 6-H), 7.31 (m, 2 H, 4-H, 5-H), 6.11 (s, 1 H, 8-H), 5.02 (sep, 3JH-H = 6.0 Hz, 1 H, 12-H), 4.96 (sep, 3JH-H = 6.1 Hz, 1 H, 11-H), 4.30 (sep, 3JH-H = 6.1 Hz, 2 H, 13-H), 1.33 (d, 3JH-H = 4.5 Hz, 6 H, 14-H), 1.31 (d, 3JH-H = 4.5 Hz, 6 H, 15-H), 0.98 (d, 3JH-H = 6.0 Hz, 6 H, 16-H), 0.80 (d, 3JH-H = 6.0 Hz, 6 H, 17-H).

13

C NMR

(300 MHz, acetone-d6, RT): δ [ppm] = 156.4 (9-C), 150.7 (1-C), 139.3 (2-C), 132.0 (7-C), 123.0 (4-C, 5-C), 120.3 (10-C), 119.1 (3-C), 110.7 (6-C), 87.2 (8-C), 75.0 (11-C, 12-C), 71.8 (13-C), 25.1 (17-C), 24.9 (16-C), 24.8 (14-C, 15-C). 1

19

F NMR (300 MHz, acetone-d6, RT): δ [ppm] = -74.6 (s,

JF,C = 282 Hz, 2JF,C = 33 Hz). MS: m/z (T~134 °C) = 585 (34%, [Ta(OiPr)3(C10H6ON2F3)]+), 566 (6%,

[Ta(OiPr)3(C10H6ON2F2)]+),

526

(40%,

[Ta(OiPr)2(C10H6ON2F3)]+),

483

(12%,

[Ta(OiPr)(OH)(C10H6ON2F3)]+), 441 (6%, [Ta(OH)2(C10H6ON2F3)]+), 417 (100%, [Ta(OiPr)4]+), 375 (12%, [Ta(OiPr)3(OH)]+), 333 (8%, [Ta(OiPr)2(OH)2]+). Anal Calcd. for TaC20H34N2O5F3 [%]: C 38.53; H 5.66; N 2.25. Found: C 40.38; H 5.89; N 4.33. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(benzoxazole-2-yl)propen-2-olato)-tantalum(V)

(5):

The

reaction of Ta2(OiPr)10 (0.965 g, 1.013 mmol) in toluene (25 ml) with BOTFP (0,474 g, 2.025 mmol) resulted in a yellow oily raw product. The reaction time was 6 h and the raw product was washed with n-heptane (2x3 ml). Colorless crystals were grown from 2-propanol at 4 °C. Yield: 93% (1.218 g, 1.887 mmol). Sublimation temperature: 70 °C/10-3 mbar. Melting point: 52 °C. 1H NMR (300 MHz, C6D6, RT): δ [ppm] = 8.30 (d, 3JH-H = 8.9 Hz, 1 H, 3-H), 7.11 (t, 3JH-H = 8.1 Hz, 1 H, 4-H), 7.06 (d, 3

JH-H = 8.1 Hz, 1 H, 6-H), 6.94 (t, 3JH-H = 7.8 Hz, 1 H, 5-H), 6.01 (s, 1H, 8-H), 5.11 (sep, 3JH-H = 6.5 Hz,



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1 H, 11-H), 5.00 (sep, 3JH-H = 6.5 Hz, 1 H, 12-H), 4.43 (sep, 3JH-H = 6.1 Hz, 2 H, 13-H), 1.39 (d, 3

JH-H = 6.0 Hz, 6 H, 14-H), 1.36 (d, 3JH-H = 6.2 Hz, 6 H, 15-H), 1.07 (d, 3JH-H = 6.2 Hz, 6 H, 16-H), 0.87

(d, 3JH-H = 6.4 Hz, 6 H, 17-H).

13

C NMR (300 MHz, C6D6, RT): δ [ppm] = 165.1 (1-C), 162.7 (9-C),

148.8 (7-C), 138.2 (2-C), 125.4 (4-C), 120.7 (10-C), 119.9 (3-C), 110.7 (5-C, 6-C), 84.8 (8-C), 76.9 (12-C), 76.1 (11-C), 73.1 (13-C), 25.8 (15-C), 25.4 (14-C), 26.0 (16-C, 17-C). 1

C6D6, RT): δ [ppm] = -74.3 (s, [Ta(OiPr)3(C10H5O2NF3)]+),

JF,C = 280 Hz,

544

(4%,

2

19

F NMR (300 MHz,

JF,C = 35 Hz). MS: m/z (T~40 °C) = 586 (18%,

[Ta(OiPr)2(OH)(C10H5O2NF3)]+),

527

(12%,

[Ta(C15H15O2NF3)]+), 460 (4%, [Ta(OH)3(C10H5O2NF3)]+), 417 (100%, [Ta(OiPr)4]+), 375 (12%, [Ta(OiPr)3(OH)]+), 333 (8%, [Ta(OiPr)2(OH)2]+). Anal Calcd. for TaC22H33NO6F3 [%]: C 40.94; H 5.15; N 2.17. Found: C 40.73; H 5.76; N 2.34. Tetra-iso-propoxy-(3,3,3-trifluoro-1-(benzthiazole-2-yl)propen-2-olato)-tantalum(V)

(6):

The

reaction of Ta2(OiPr)10 (0.852 g, 0.894 mmol) in 2-propanol (25 ml) with BTTFP (0.450 g, 1.835 mmol) gave a brown raw product. The reaction time was 0.5 h in an ultrasonic bath. Brownish crystals were grown from 2-propanol at 4 °C. Yield: 76% (0.902 g, 1.364 mmol). Sublimation point: 65 °C/10-3 mbar. Melting point: 55 °C. 1H NMR (300 MHz, acetone-d6, RT): δ [ppm] = 8.75 (d, 3JH-H = 8.5 Hz, 1 H, 3-H), 7.97 (d, 3JH-H = 8.9 Hz, 1 H, 6-H), 7.59 (t, 3JH-H = 8.9 Hz, 1 H, 4-H), 7.44 (t, 3JH-H = 7.0 Hz, 1 H, 5-H), 6.44 (s, 1 H, 8-H), 5.07 (sep, 3JH-H = 6.1 Hz, 1 H, 11-H), 4.89 (sep, 3JH-H = 6.1 Hz, 1 H, 12-H), 4.42 (sep, 3JH-H = 6.1 Hz, 2 H, 13-H), 1.36 (d, 3JH-H = 6.4 Hz, 6 H, 14-H), 1.26 (d, 3JH-H = 6.2 Hz, 6 H, 15-H), 1.04 (d, 3JH-H = 6.2 Hz, 6 H, 16-H), 0.87 (d, 3JH-H = 6.4 Hz, 6 H, 17-H).

13

C NMR (300 MHz,

acetone-d6, RT): δ [ppm] = 168.7 (1-C), 156.5 (9-C), 149.9 (2-C), 130.9 (7-C), 126.3 (4-C), 125.3 (5-C), 123.9 (3-C), 121.4 (6-C), 120.0 (10-C), 93.9 (8-C), 76.0 (11-C), 75.6 (12-C), 72.4 (13-C), 25.2 (17-C), 25.0 (16-C), 24.8 (15-C), 24.6 (14-C). 19F NMR (300 MHz, acetone-d6, RT): δ [ppm] = -74.3 (s, 1

JF,C = 280 Hz, 2JF,C = 33 Hz). MS: m/z (T~80 °C) = 602 (20%, [Ta(OiPr)3(C10H5ONSF3)]+), 543 (16%,

[Ta(OiPr)2(C10H5ONSF3)]+), 417 (100%, [Ta(OiPr)4]+), 375 (12%, [Ta(OiPr)3(OH)]+), 333 (8%, [Ta(OiPr)2(OH)2]+), 291 (4%, [Ta(OiPr)(OH)3]+). Anal Calcd. for TaC22H33NO5SF3 [%]: C 39.94; H 5.03; N 2.12, S 4.85. Found: C 39.77; H 5.32; N 2.14; S 4.62.


Page 9 of 27

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2.3. Materials Characterization. The synthesized compounds were used as single source precursors in a low pressure horizontal cold wall CVD reactor described in previous works,[40] where silicon wafer (Si) and alumina (polycrystalline Al2O3) were used as substrates. The tantalum oxide deposits were grown in the range of temperatures (500-1000 °C). The morphology of the deposits was investigated in a FE-SEM FEI 430 Nova NanoSEM system; Atomic Force microscopy was performed in contact mode in a XE-100 ParkSystem equipped with 910ACTA cantilever. The crystal structure of thin films was studied by means of X-ray diffraction (XRD), operating in Bragg−Brentano mode (XRD Stoe Stadi MP vertical diffractometer with Cu Kα; source (λ = 154.18 pm)). 3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of tantalum(V) compounds (1-6) The acid-base reaction of dimeric Ta2(OiPr)10 with six different trifluoromethylheteroarylalkenoles (cf. Figure 1) in toluene or iso-propanol at room temperature in a 1:2 stoichiometric ratio yielded monomeric

complexes

of

the

general

formula

Ta(OiPr)4(ArTFP)

(Ar = pyridine

(1),

4,5-dimethyloxazole (2), 4,5-dimethylthiazole (3), benzimidazole (4), benzoxazole (5), benzthiazole (6), TFP = trifluoropropenol), whose molecular structures are displayed in Figure 2.

Figure 1 Synthetic procedure for compounds (1-6). Attempts to substitute Ta(OiPr)4(ArTFP) with 2 or 3 ligands by changing the stoichiometric ratio exclusively and selectively delivered the monosubstituted product, possibly due to the coordinative saturation and favorable octahedral coordination around Ta. All compounds were isolated in high yields (>76%) and high-purity which was confirmed by microanalysis, except for compound (4) where the



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Page 10 of 27

values derived considerably from the theoretical carbon and nitrogen values, possibly due to partial hydrolysis.

15

14 13

11

O

10

O

13

2

N O

9

O O

12

11 14

11 10

1

Ta 12

3 4

13

6 12

9

CF3 8

O

15

5

4

2

14

15

1

8

11

5

O

12

X N 1 Ta 6 O 7 O O CF3

12

17 13 4

3

O

5 7

16

15

14 13

15

2, 3 X = O, S

15

O

2

3

N O

Ta 14

11

O O

14

Y 1

8 9

CF3 10

13 16

6 7

17

4, 5, 6 Y = O, S, NH

Figure 2. Schematic drawing of the molecular structures for compounds (1-6) with arbitrary atomic numbering. In contrast to highly fluxional Ta2(OiPr)10,[50, 51] NMR analysis of complexes (1-6) revealed that all compounds held a nearly rigid structure in solution. The spectra exhibited sharp signals with shifts and characteristic multiplicities for iso-propoxide groups (doublets and septets) and the attached chelating ligand (higher order multiplets for the aromatic protons and a singlet for the vinylic proton). A combination of 19F-13C, 1H-13C, 1H-1H and

19

F-1H correlation experiments allowed unambiguously the

identification of the absolute molecular structure for (1-6) in solution. The detailed analysis of NMR singals and assignment of resonances can be found in the supporting information. Electron impact mass spectrometry of compounds (1-6) indicated a disaggregation into two fragments which were detected with highest intensities. A tantalum radical cation which was coordinated to four iso-propoxide groups (100%, m/z = 417) and an ion of higher masses that was coordinated by three iso-propoxide groups and a ligand moiety (16%-34%). This suggested that the initial ionization appeared to be most likely by loss of the chelating ligand or one iso-propoxy fragment. Both primary major fragments underwent further decay by β-hydride elimination accompanied by successive loss of propene resulting in hydroxides, which is a general feature of metal iso-propoxides during mass spectrometric measurements. A noticeable change of vaporization temperature of ligand modified compounds (1-6) compared to Ta2(OiPr)10 (~50 °C) was observed which suggested an enhanced ACS Paragon Plus Environment

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volatility of compounds (1) (~48 °C), (2) (~36 °C), (3) (~40 °C) and (5) (~40 °C). Compounds (4) (~134 °C) and (6) (~80 °C) required higher volatilization temperatures than Ta2(OiPr)10, which confirms a higher stability therefore making them less suitable for CVD processes. The molecular structures of compounds (1-6) (cf. Figures 3-8) are very similar to each other due to similar bite angles and steric demands of the ligands employed. All derivatives form distorted octahedral molecules with the tantalum center coordinated by four oxygen atoms of the iso-propoxy groups and the N-O moiety of the chelating ligands. Depending on the steric demand and electronic nature of the conjugated π-system of the ligands, octahedra occur more or less distorted. Another factor for the respective distortion is the fixed bite angle of the ligands which varies between 78.62(12)° (compound (4)) 79.16(8)° (compound (3)). This strain is compensated by the more flexible iso-propoxy groups. For example, the angle of the axial iso-propoxy groups were found to be in a range between 162.14(15)° (compound (2)) to 167.89(14)° (compound (4)) which is still typical for Oh coordination. The Ta-OiPr bond lengths ranged from 1.866(3) Ǻ (compound 2) to 1.957(3) Ǻ (compound (4)), whereas the bond lengths to O1 of the trifluoropropenol moiety were found to vary between 2.053(2) Ǻ (compound (1)) and 2.1168(18) Ǻ (compound 5). The Ta-N bond length diverged from 2.3197(19) Ǻ (compound (5)) to 2.354(2) Ǻ (compound (1)). In comparison with previously published data of comparable tantalum compounds, no peculiarities could be found. For example, terminal iso-propoxide groups were reported in Ta2O(OiPr)9 ranging from 1.871 Ǻ to 2.086 Ǻ[52] and in Ta2(OiPr)8(OMe)2 from 1.815 Ǻ to 1.894 Ǻ[53] (unfortunately only cell parameters for Ta2(OiPr)10 have been published). For the bonds to the chelating ligands similar values have also been reported for the niobium compound (Nb-N1 2.387(4) Ǻ, Nb-O1 2.074(4) Ǻ).[35]



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Page 12 of 27

Figure 3. Molecular structure of Ta(OiPr)4PyTFP (1). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.053(2), Ta1-O2 1.875(2), Ta1-O3 1.905(2), Ta1-O4 1.901(2), Ta1-O5 1.883(2), Ta1-N1 2.354(2). Selected bond angles [°]: O2-Ta1-O5 95.21(10), O2-Ta1-O4 97.67(10), O5-Ta1-O4 165.76(9), O2-Ta1-O3 100.59(10), O5-Ta1-O3 92.71(9), O4-Ta1-O3 90.90(9), O2-Ta1-O1 91.64(11), O5-Ta1-O1 87.66(9), O4-Ta1-O1 85.92(9), O3-Ta1-O1 167.68(10), O2-Ta1-N1 170.33(10), O5-Ta1-N1 82.41(9), O4-Ta1-N1 83.89(9), O3-Ta1-N1 88.90(9), O1-Ta1-N1 78.93(10).

Figure 4. Molecular structure of Ta(OiPr)4DMOTFP (2). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.105(3), Ta1-O3 1.894(3), Ta1-O4 1.866(3), Ta1-O5 1.887(3), Ta1-O6 1.896(3), Ta1-N1 2.327(3). Selected bond angles [°]:O4-Ta1-O5 98.35(14), O4-Ta1-O3 99.25(13), O5-Ta1-O3 92.44(13), O4-Ta1-O6 96.84(13), O5-Ta1-O6 92.85(13), O3-Ta1-O6 162.14(15), O4-Ta1-O1 86.30(13), O5-Ta1-O1 175.32(12), O3-Ta1-O1 87.25(13), O6-Ta1-O1 86.09(13), O4-Ta1-N1 165.23(12), O5-Ta1-N1 96.40(13), O3-Ta1-N1 80.88(12), O6-Ta1-N1 81.59(12), O1-Ta1-N1 78.94(12).


Page 13 of 27

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Figure 5. Molecular structure of Ta(OiPr)4DMTTFP (3). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.0761(19), Ta1-O2 1.9080(18), Ta1-O3 1.8867(18), Ta1-O4 1.8954(19), Ta1-O5 1.8890(19), Ta1-N1 2.340(2). Selected bond angles [°]:O3-Ta1-O5 95.37(8), O3-Ta1-O4 96.48(8), O5-Ta1-O4 93.41(9), O3-Ta1-O2 99.65(8), O5-Ta1-O2 163.88(8), O4-Ta1-O2 90.69(8), O3-Ta1-O1 85.51(8), O5-Ta1-O1 88.12(9), O4-Ta1-O1 177.36(8), O2-Ta1-O1 87.26(8), O3-Ta1-N1 164.39(8), O5-Ta1-N1 81.32(8), O4-Ta1-N1 98.93(8), O2-Ta1-N1 82.64(8), O1-Ta1-N1 79.16(8).

Figure 6. Molecular structure of Ta(OiPr)4BITFP (4). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.094(3), Ta1-O2 1.882(3), Ta1-O3 1.896(3), Ta1-O4 1.957(3), Ta1-O5 1.872(3), Ta1-N1 2.263(3). Selected bond angles [°]: O5-Ta1-O2 96.03(15), O5-Ta1-O3 94.26(16), O2-Ta1-O3 99.69(15), O5-Ta1-O4 167.88(15), O2-Ta1-O4 94.96(13), O3-Ta1-O4 88.91(13), O5-Ta1-O1 86.77(15), O2-Ta1-O1 86.44(14), O3-Ta1-O1 173.63(13), O4-Ta1-O1 88.83(12), O5-Ta1-N1 84.70(15), O2-Ta1-N1 164.98(14), O3-Ta1-N1 95.21(14), O4-Ta1-N1 83.36(12), O1-Ta1-N1 78.61(12).



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Page 14 of 27

Figure 7. Molecular structure of Ta(OiPr)4BOTFP (5). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.1168(18), Ta1-O2 1.8924(17), Ta1-O3 1.8771(17), Ta1-O4 1.8891(17), Ta1-O5 1.8916(18), Ta1-N1 2.3197(19). Selected bond angles [°]:O3-Ta1-O4 100.76(7), O3-Ta1-O5 95.00(8), O4-Ta1-O5 93.67(8), O3-Ta1-O2 95.68(8), O4-Ta1-O2 93.35(7), O5-Ta1-O2 165.90(8), O3-Ta1-O1 89.91(7), O4-Ta1-O1 169.32(7), O5-Ta1-O1 85.41(8), O2-Ta1-O1 85.43(7), O3-Ta1-N1 167.87(8), O4-Ta1-N1 91.37(7), O5-Ta1-N1 84.01(7), O2-Ta1-N1 83.62(7), O1-Ta1-N1 77.96(7).

Figure 8. Molecular structure of Ta(OiPr)4BTTFP (6). The displacement ellipsoids are drawn with 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths [Ǻ]: Ta1-O1 2.1000(17), Ta1-O2 1.8819(17), Ta1-O3 1.9006(17), Ta1-O4 1.8837(17), Ta1-O5 1.9021(16), Ta1-N1 2.3294(19). Selected bond angles [°]:O2-Ta1-O4 95.93(7), O2-Ta1-O3 97.93(8), O2-Ta1-O5 99.71(7), O2-Ta1-O1 85.72(7), O2-Ta1-N1 164.31(7), O4-Ta1-O3 93.15(8), O4-Ta1-O5 163.33(7), O4-Ta1-O1 87.30(7), O4-Ta1-N1 80.93(7), O3-Ta1-O5 90.39(7), O3-Ta1-O1 176.25(7), O3-Ta1-N1 97.59(7), O5-Ta1-O1 88.14(7), O5-Ta1-N1 82.46(7), O1-Ta1-N1 78.80(7)

Table 1. Crystal Data and experimental details for Ta(OiPr)4PyTFP (1), Ta(OiPr)4DMOTFP (2), Ta(OiPr)4DMTTFP (3), Ta(OiPr)4BITFP (4), Ta(OiPr)4BOTFP (5), Ta(OiPr)4BTTFP (6). Complex Empirical formula Formula weight [g/mol] Crystal system Space group Unit cell dimenstions [Ǻ] or [°]

(1) C20H33F3NO5Ta 605.42 monoclinic P21/n (no. 14) a = 8.1622(3) b = 18.7684(4) c = 16.1059(5) β = 90.814(3)

3

Cell volume [Ǻ ] Formula unit Calculated density Absorption coefficient [mm-1] F(000) Theta range for data collection Limiting indices

2467.04(13) 4 1.630 4.505 1200 2.51

Design of Volatile Mixed-Ligand Tantalum(V) Compounds as

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