Article pubs.acs.org/Organometallics
Synthesis and Characterization of Novel Volatile ImidoAminoalkoxide Tantalum Compounds Bo Keun Park,† Hyo-Suk Kim,† Su Jung Shin,† Jae Ki Min,† Kang Mun Lee,‡ Youngkyu Do,‡ Chang Gyoun Kim,*,† and Taek-Mo Chung*,† †
Thin Film Materials Research Group, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong, Daejeon 305-600, Republic of Korea ‡ Department of Chemistry, KAIST, 291 Daehak-ro, Yuseong, Daejeon 305-701, Republic of Korea S Supporting Information *
ABSTRACT: A novel Ta(V) tBu-imido/aminoalkoxide complex, Ta(NtBu)(dmamp)2Cl (1), was synthesized by metathesis reaction between Ta(NtBu)Cl3(py)2 and 2 equiv of Na(dmamp), and subsequent reaction of 1 with 1 equiv of MeLi gave a new tantalum complex, Ta(NtBu)(dmamp)2Me (2). Compounds 1 and 2 have been characterized by IR, 1H and 13C NMR spectroscopy, and microanalytical data. The molecular structure of 1, determined by X-ray single crystallography, revealed distorted octahedral geometry. The behavior of compound 1 in solution was studied by variable-temperature 1 H NMR spectra. Thermogravimetric analysis revealed superior thermal properties of 2 as compared to those of 1.
M
etal imido complexes including those of tantalum have been extensively studied as reagents or catalysts for oxidation, olefin metathesis, and polymerization reactions.1 A major application of tantalum imido compounds has been as precursors to TaN thin films by chemical vapor deposition (CVD) and atomic layer deposition (ALD) including Ta(NtBu)(NEt2)3,2 Ta(NtBu)(amidinate)2X (X = halogen, NR1R2),3 Ta(NtBu)(guanidinate)2X (X = halogen, NR1R2),4 and Ta(NtBu)(hydrazido)2−nXn (n = 0, 1; X = halogen, NR1R2).4c,5 TaN thin films have been applied as diffusion barriers to prevent the formation of copper-doped silicon and copper silicides at the substrate interface.6 Ta2O5 thin films have usually been deposited by CVD and ALD with imido compounds (e.g., Ta(NtBu)(NEt2)3,2b Ta(NtBu)(tBu2pz)3,7 and Ta(NtBu)(iPrNC(Me)NiPr)2(NMe2)8), alkoxides (e.g., Ta(OEt)59 and Ta(OR)4(β-diketonate) (R = Me, Et)10), and amido compounds (e.g., Ta(NEt2)511). Ta2O5 has been studied for applications in dynamic random access memory,12 antireflective coatings,13 optical waveguides,14 and electrochromic devices.15 N-Doped tantalum oxide or tantalum oxynitride (TaOxNy) has been investigated as a photocatalyst in the solar production of hydrogen gas.16 Our research has focused on the development of novel precursors using aminoalkoxide ligands, which are alkoxides with donor-functionalized amino groups saturating the metal coordination sphere. Moreover, the two organic alkyl substituents on the alkoxide carbon atom effectively prevent intermolecular interactions, rendering the metal complexes monomeric in structure, improving the volatility characteristics toward use as MOCVD and ALD precursors.17 © 2012 American Chemical Society
Herein, we report the synthesis and characterization of two novel Ta(V) tBu-imido/aminoalkoxide complexes, Ta(NtBu)(dmamp)2Cl (1) and Ta(NtBu)(dmamp)2Me (2) [dmamp = (CH3)2NCH2C(CH3)2O, 1-(dimethylamino)-2-methyl-2-propoxide], for Ta-based thin films. Reaction of Ta(NtBu)Cl3(py)2 with 2 equiv of Na(dmamp) in toluene at room temperature afforded a pale yellow liquid compound containing two aminoalkoxide ligands, Ta(NtBu)(dmamp)2Cl (1), in 88% yield (Scheme 1). Treatment of 1 with 1 equiv of MeLi in hexane at 0 °C followed by stirring at room temperature gave a colorless liquid, Ta(N t Bu)(dmamp) 2 Me (2), in high yield (95%) (Scheme 1). Compounds 1 and 2 were purified by vacuum distillation (10−1 Torr) at 120 and 115 °C, respectively. The assynthesized new tantalum compounds are soluble in common organic solvents, such as benzene, toluene, hexane, ether, and THF. Compounds 1 and 2 are liquid at room temperature, but crystals of 1 were obtained from toluene solution at −30 °C. Compound 2, unfortunately, did not crystallize under similar conditions. Single-crystal X-ray analysis of 1 reveals a distorted octahedral geometry about the central Ta atom with two bidentate dmamp ligands, one tBu imido group, and one chloride atom saturating the coordination sphere (Figure 1). In this structure, the tBu imido group and one nitrogen atom (N2) of dmamp are located in linear positions, while the Received: May 21, 2012 Published: November 20, 2012 8109
dx.doi.org/10.1021/om300436p | Organometallics 2012, 31, 8109−8113
Organometallics
Article
N1−C2−C1−O1 and Ta−N2−C8−C7−O2 with the bite angles N1−Ta−O1 and N2−Ta−O2 being 72.1(3)° and 70.4(3)°, respectively. The Ta−N3−C13 unit of the imido ligand is nearly linear because the angle (164.8(7)°) is 180− 160°.18 The Ta−N3 bond length (1.772(8) Å) is within the range (1.745−1.797 Å) of known Ta-tBu imido compounds.3−5,7,8,19 Ta−O and Ta−N bond distances between the central Ta atom and two dmamp ligands are 1.936(6) (Ta− O1), 1.941(6) (Ta−O2), 2.395(8) (Ta−N1), and 2.588(9) Å (Ta−N2), respectively. The coordination bond of Ta−N2 is longer than that of Ta−N1, and this is attributable to a strong trans influence of the tBu imido ligand, as previously reported for Ta(NtBu)(py)2Cl3 (2.452(4) Å),19a TpMs*TaCl2(NtBu) (2.451(3) Å),19e TpMs*TaCl2(N-2,6-iPr2-C6H3) (2.428(4) Å),19e Ta(NtBu)(amidinate)2X (X = halogens) (2.362−2.370 Å),3 and Ta(NtBu)(guanidinate)2NR2 (2.396−2.437 Å).4 Since Cl is substituted with a methyl group in 2, the molecular geometry of the latter is not expected to differ from that of compound 1. The variable-temperature (VT) 1H NMR spectra of 1 are shown in Figure 2. The spectrum at 193 K exhibits four singlet resonances (δ = 1.54 (3H), 1.40 (3H), 1.33 (3H), and 1.11 (3H) ppm) for the alkoxide carbon methyl groups, one singlet resonance (δ = 1.57 (9H) ppm) from the imido tBu group, four singlet resonances (δ = 2.61 (3H), 2.18 (3H), 2.10 (3H), and 1.84 (3H) ppm) from the amino methyl groups, and four doublet resonances (δ = 3.08 (1H, J = 11.7 Hz), 2.80 (1H, J =
Scheme 1. Synthesis of Compounds 1 and 2
Figure 1. Molecular structure of 1. Selected bond lengths (Å) and angles (deg) are as follows. Ta−N(1) = 2.395(8), Ta−N(2) = 2.588(9), Ta−N(3) = 1.772(8), Ta−O(1) = 1.936(6), Ta−O(2) = 1.941(6), Ta−Cl = 2.402(3), N(1)−C(2) = 1.475(17), O(1)−C(1) = 1.401(12), C(1)−C(2) = 1.55(2), N(2)−C(8) = 1.477(15), O(2)− C(7) = 1.418(11), C(7)−C(8) = 1.533(16), N(3)−C(13) = 1.418(13); N(1)−Ta−N(2) = 103.8(3), N(1)−Ta−N(3) = 89.6(4), N(1)−Ta−O(1) = 72.1(3), N(1)−Ta−O(2) = 173.8(3), N(1)−Ta− Cl = 82.5(3), N(2)−Ta−N(3) = 166.5(3), N(2)−Ta−O(1) = 79.0(3), N(2)−Ta−O(2) = 70.4(3), N(2)−Ta−Cl = 81.5(3), N(3)−Ta−O(1) = 105.4(3), N(3)−Ta−O(2) = 96.1(3), N(3)− Ta−Cl = 100.8(3), O(1)−Ta−O(2) = 104.2(3), O(1)−Ta−Cl = 142.9(2), O(2)−Ta−Cl = 98.5(2), Ta−N(1)−C(2) = 101.5(6), Ta− O(1)−C(1) = 127.5(7), N(1)−C(2)−C(1) = 113.0(9), C(2)−C(1)− O(1) = 106.5(9), Ta−N(2)−C(8) = 100.4(7), Ta−O(2)−C(7) = 132.4(6), N(2)−C(8)−C(7) = 114.5(9), C(8)−C(7)−O(2) = 107.9(8), Ta−N(3)−C(13) = 164.8(7).
oxygen (O2) atom of the dmamp group, one nitrogen (N1) and one oxygen atom (O1) of the other dmamp group, and one Cl atom combine the four remaining binding sites of the tantalum center. The bond angles of N3−Ta−N2, N1−Ta− O2, and O1−Ta−Cl are 166.5(3)°, 173.8(3)°, and 142.9(2)°, respectively, which show deviation from an ideal angle (180°) of an octahedral geometry. Two dmamp ligands and the tantalum atom form two five-membered metallacycles, Ta−
Figure 2. 1H NMR spectra of 1 in toluene-d8 at different temperatures; signal arising from toluene and impurities marked with * and •. 8110
dx.doi.org/10.1021/om300436p | Organometallics 2012, 31, 8109−8113
Organometallics
Article
12.2 Hz), 1.68 (1H, J = 11.7 Hz), and 1.64 (1H, J = 12.2 Hz) ppm) from the methylene groups, respectively. This spectrum corresponds exactly to what is expected for a crystal structure such as that of compound 1.3 As the temperature increases, two doublet resonances (δ = 1.68 and 1.64 ppm at 193 K) from one methylene group coalesce (Tc = 228 ± 3 K) and sharpen into a single doublet resonance (J = 12.0 Hz) at δ = 1.82 ppm at 233 K. Coalescence temperature of two singlet resonances (δ = 2.21 (3H) and 2.15 (3H) ppm at 233 K) from the amino methyl groups is 248 ± 3 K, merging one singlet at 2.20 (6H) ppm at 253 K. The two broad doublet resonances from the remaining methylene group (δ = 3.15 (1H) and 2.74 (1H) ppm at 253 K), three singlet resonances (δ = 2.62 (3H), 2.20 (6H), and 1.96 (3H) ppm at 253 K) from the amino methyl groups, and four singlet resonances (δ = 1.50 (3H), 1.31 (3H), 1.27 (3H), and 1.11 (3H) ppm at 253 K) from the alkoxide carbon methyl groups coalesce (Tc = 268 ± 3 K) and then sharpen at 323 K into a one doublet resonance (δ = 2.97 (2H) ppm, J = 11.9 Hz), one singlet resonance (δ = 2.35 (12H) ppm), and two singlet resonances (δ = 1.39 (6H) and 1.21 (6H) ppm), respectively. These results suggest that compound 1 presents in a crystal structure form at low temperatures in solution. At high temperatures, a square pyramidal intermediate is formed by dissociation of the Cl ligand, and then the Cl ligand is recoordinated at the Ta atom; consequently, dmamp ligand rearrangement takes place (Figures S2 and S3). This mechanism is already described with Ta(NtBu)(amidinate)2X (X = halogen, NR1R2) compounds.3 VT 1H NMR spectra of compound 2 did not show splitting of the peaks from the two dmamp ligands. Thermogravimetric analyses (TGA) of 1 and 2 were carried out from room temperature to 900 °C under a constant flow of nitrogen gas. Weight loss of 1 happens two times; the first weight loss of about 5% at 100−130 °C appears as loss of the Cl ligand (calcd loss of Cl: about 5% (exposure to air, Cl/OH exchange); about 7% (no exposure)), and then about 50% weight loss at 180−300 °C is observed corresponding to the loss of the other decomposed ligands (Figure 3). The total weight loss of 1 is about 60%. On the other hand, TGA of 2 exhibits that a single weight loss (68%) begins at about 200 °C and is complete by about 320 °C (Figure 3). The weight loss was 10−12% greater than the calculated value due to
sublimation of 2 before decomposition. Consequently, the thermal property of 2 is slightly better than that of 1 because 2 has a single step weight loss and is likely to sublime before decomposition. TGA data of Ta(OEt)5 and Ta(OEt)4(βdiketonate) showed weight losses at 160−250 and 90−370 °C, respectively, with residues of
