NIOBIUM ALKOXIDES

Niobium pentakis(methoxide) Nb(OMe)5

Niobium pentakis(methoxide) Nb(OMe)5 has dimeric structure (with units connected by bridging methoxide groups). Nb(OMe)5 has relatively low volatility and was rarely used for MOCVD of Nb-contaning compounds.

Niobium pentakis(ethoxide) Nb(OEt)5

Fig. Vapor pressure of [Nb(OEt)5]2 vs. precursor vessel temperature (vapor pressure equation log10P(Torr)=(9.8±1.2)-(4350±350)/T(K) )

Niobium pentakis(ethoxide) Nb(OEt)5 has dimeric structure, similar to Nb methoxide (with units connected by bridging ethoxide groups). Nb(OEt)5 has sufficient volatility, and several reports of its application as MOCVD precursors are available.

Vapor pressure of [Nb(OEt)5]2 was determined as a function of precursor vessel temperature (in the range 25-130°C) and fitted (see Fig. ) to the following vapor pressure equation: log10P(Torr)=(9.8±1.2)-(4350±350)/T(K) [i]

[i] D. Saulys, V. Joshkin, M.Khoudiakov, T.F.Kuech, A.B.Ellis, S.R.Oktyabrsky, L. McCaughan, J. Crystal Growth, 2000, Vol. 217, Iss. 3, p.287-301, « An examination of the surface decomposition chemistry of lithium niobate precursors under high vacuum conditions », https://doi.org/10.1016/S0022-0248(00)00412-7, https://www.sciencedirect.com/science/article/pii/S0022024800004127

[Nb(OEt)5]2 surface decomposition chemistry study (by mass spectrometry during LiNbO3 growth by chemical beam epitaxy)

Surface decomposition chemistry of [Nb(OEt)5]2 precursor (as well as (for comparison) of other common CVD precursors for LiNbO3 MOCVD growth, Nb(thd)4 Li(thd), [Li(OBut)]6), was investigated during LiNbO3 film growth and its constituent metal oxides on sapphire and Si (0 0 1) by a combination of high vacuum (chemical beam) epitaxy and in situ mass spectrometry. Metal alkoxides (including [Nb(OEt)5]2) have higher thermodynamic stability compared to diketonates, but are prone to autocatalytic processes that can inhibit the film growth (as volatile metal-containing moieties are generated that subsequently desorb from the surface), whereas metal β-diketonates were found to be unstable at low pressures and temperatures, adversely impacting both storage and use. [Nb(OEt)5]2 produced Nb2O5 films whose degree of crystallinity depended on the presence of H2O (one of the autocatalytic elements). Li/Nb precursor ratio strongly influenced the growth rate of LiNbO3 films, suggesting a chemical interaction between the two precursors.[i]

Nb(OEt)5 for LiNbO3 by MOCVD

Nb(OEt)5 combined with Li(OBut), was applied as Nb precursor for the growth of LiNbO3 thin films on SiO2Si substrates by low-pressure MOCVD at 450–630°C temperatures. XRD revealed that the deposited films were crystalline LiNbO3. [i]

[i] A. Tanaka, K. Miyashita, T. Tashiro, M. Kimura, T. Sukegawa, J. Cryst. Growth, 1995, Vol.148, Iss. 3, p.324-326, "Preparation of lithium niobate films by metalorganic chemical vapor deposition with a lithium alkoxide source", https://www.sciencedirect.com/science/article/pii/002202489401003X

Niobium pentakis-neopentoxide pyridine adduct Nb(OCH2tBu)5(py) and Niobium pentakis-neopentoxide 4-dimethylaminopyridine adduct Nb(OCH2tBu)5(py-4-NMe2)

Monomeric octahedral niobium pentakis-neopentoxide adducts with pyridine-type ligands: Nb(OCH2tBu)5(py) and Nb(OCH2tBu)5(py-4-NMe2) (py-4-NMe2 = 4-dimethylaminopyridine) were synthesized in high yields (>60%) by metathesis reactions of Nb2Cl10 with neopentyl alcohol tBuCH2OH and pyridine (py) or 4-dimethylaminopyridine (DMAP). Both complexes were characterized by microanalysis, mass spectrometry, variable temperature multinuclear NMR spectroscopy, thermal analysis (TGA). Both adducts are solids at RT and demonstrate enhanced thermal and chemical stability compared to their non-adducted alkoxide parent compound [Nb(OCH2tBu)5]2, what show the stabilizing effect of pyridine ligands (also demonstrated by NMR and TG/DTA). The complexes are monomeric with metal center coordinated by four or five alkoxide groups and one N-donating or chelating ligand in distorted octahedrons in the solid and dissolved state; the monomeric nature contributes to the high volatility and good solubility of these adducts making them promising precursors for the growth of Nb-contaning layers (f.e Nb2O5) by MOCVD.[i]

[i] L. Appel, R. Fiz, W. Tyrra, S. Mathur, Dalton Trans., 2012, 41, 1981, « New iso-propoxides, tert-butoxides and neo-pentoxides of niobium(V): Synthesis, structure, characterization and stabilization by trifluoroheteroarylalkenolates and pyridine ligands », https://pubs.rsc.org/en/content/articlelanding/2012/dt/c1dt11668a/unauth#!divAbstract, https://www.researchgate.net/profile/Wieland_Tyrra/publication/51899393_New_iso-propoxides_tert-butoxides_and_neo-pentoxides_of_niobiumV_Synthesis_structure_characterization_and_stabilization_by_trifluoroheteroarylalkenolates_and_pyridine_ligands/links/00b49530cbc3aa030b000000/New-iso-propoxides-tert-butoxides-and-neo-pentoxides-of-niobiumV-Synthesis-structure-characterization-and-stabilization-by-trifluoroheteroarylalkenolates-and-pyridine-ligands.pdf

Synthesis of Nb(OCH2tBu)5(py) and Nb(OCH2tBu)5(py-4-NMe2))

Both pyridine adducts were synthesized by metathesis of niobium pentachloride and neopentyl alcohol in the presence of pyridine (or py-4-NMe2) ligand and excess of NH3 which removes Cl in the form of NH4Cl:

Synthesis and characterization of Nb(OCH2tBu)5(py)

Fig. Nb(ONep)5(py) molecular structure

Synthesis of Nb(OCH2tBu)5(py)

Niobium pentakis-neopentoxide pyridine adduct Nb(OCH2tBu)5(py) was synthesized by adding a mixture of tBuCH2OH (126 mmol, ca. 40% excess) and pyridine (50ml) to Nb2Cl10 (9 mmol) at 0 ◦C via a dropping funnel; the reaction mixture stirred for 2 h until a brown solution formed. Then excess of NH3 was added (the precipitated green solid was removed by filtration), the remaining yellow filtrate was dried under reduced pressure forming brown solid which was suspended and washed in n-heptane (3 x 25 ml). Then all volatile components were removed at low pressure, and the raw product (brown solid) was purified by sublimation (110 ◦C/ 10-3 mbar), to form pure Nb(OCH2tBu)5(py) as colorless solid (yield of 22% (3.87 mmol)) having melting point 205 ◦C.

Characterization of Nb(OCH2tBu)5(py)

1H NMR(-70 ◦C, Toluene-d8): d [ppm] = 8.88 (d, H(2) H(6) Py, J = 4.8 Hz, Int. = 2.00), 6.72 (t, H(4) Py, J = 7.4 Hz, Int. = 1.01), 6.56 (t, H(3) H(5) Py, J = 6.4 Hz, Int. = 2.03), 4.61 (s, H[CH2(ax)], Int. = 2.02), 4.15 (s, H[CH2(eq)], Int. = 7.89), 1.30 (s, H[CH3(ax)], Int. = 9.88), 1.27 (s, H[CH3(eq)], Int. = 33.74), 1.21 (s, Int. = 14.88), 1.15 (s, Int. = 6.44).

13C NMR (-70 ◦C, Toluene-d8): d [ppm] = 149.0 (C(2) C(6) Py), 137.6 (C(4) Py), 123.8 (C(3) C(5) Py), 84.9 (C[CH2(ax)]), 81.9 (C[CH2(eq)]), 34.0 (C[Cq(ax)] C[Cq(eq)]), 26.3 (C[CH3(ax)] C[CH3(eq)]).

Mass-spectroscopy (MS): m/z (T ~ 95 ◦C) = 441 (100%, [Nb(OCH2 tBu)4]+), 371 (6%, [Nb(OCH2tBu)3(OH)]+), 301 (5%, [Nb(OCH2tBu)2(OH)2]+), 231 (5%, [Nb(OCH2tBu)(OH)3]+), 161 (3%, [Nb(OH)4]+), 79 (8%, [C5H5N]+Σ).

Anal. Calcd. for C30H60NbO5N [%]: N 2.30, C 59.29, H 9.95. Found: N 2.07, C 58.12, H 10.34.

Synthesis and characterization of (Nb(OCH2tBu)5(py-4-NMe2)):

Fig. Nb(ONep)5(DMAP) molecular structure

Synthesis (Nb(OCH2tBu)5(py-4-NMe2)):

Niobium pentakis(neopentoxide) (4-dimethylaminopyridine) Nb(OCH2tBu)5(py-NMe2 ) was synthesized by slow adding a mixture of tBuCH2OH (62.5 mmol) in n-heptane (50 ml) at 0 ◦C to Nb2Cl10 (3.1 m mol) suspended in n-heptane (25 ml), then after stirring for 24h adding 4-dimethylaminopyridine (DMAP, or py-4-NMe2) (8.27 mmol), and after 2 hours introducing excess of NH3 to the milky solution. The filtrate was dried under low pressure (after removing precipitated NH4Cl), the excess py-4-NMe2 was removed by moderate heating at low pressure (70 ◦C, 10-3 mbar). The pure Nb(OCH2tBu)5 (py-4-NMe2) (melting point 190 ◦C) was obtained in high yield (76%, 4.69 mmol) by subliming the colorless raw product at 110 ◦C/ 10-3 mbar.

Characterisation (Nb(OCH2tBu)5(py-4-NMe2)):

1H NMR (-73 ◦C, Toluene-d8): d [ppm] = 8.59 (d, H(2) H(6) [py-4-NMe2], J = 6.5 Hz, Int. = 2.00), 5.85 (d, H(3) H(5) [py-4-NMe2], J = 6.5 Hz, Int. = 1.89), 4.67 (s, H[CH2(ax)], Int. = 1.88), 4.31 (s, H[CH2(eq)], Int. = 7.70), 1.92 (s, H[CH3] [py-4-NMe2], Int. = 5.94), 1.35 (s, H[CH3(ax)] , H[CH3(eq)], Int. = 46.62).

13C NMR (-73 ◦C, Toluene-d8): d [ppm] = 153.7 (Cq), 148.6 (C(2) C(6) [py-4-NMe2]), 105.6 (C(3) C(5) [py-4-NMe2]), 84.3 (C[CH2(ax)]), 81.7 (C[CH2(eq)]), 37.9 (C[CH3] [py-4-NMe2]), 34.2 (C[Cq(ax)] C[Cq(eq)]), 26.9 (C[CH3(ax)], C[CH3(eq)]).

Mas-spectroscopy (MS): m/z (T ~ 120 ◦C) = 441 (100%, [Nb(OCH2 tBu)4]+), 371 (6%, [Nb(OCH2 tBu)3(OH)]+), 301 (5%, [Nb(OCH2 tBu)2(OH)2]+), 231 (5%, [Nb(OCH2 tBu)(OH)3]+), 161 (3%, [Nb(OH)4]+), 122 (8%, [C7H10N2]+Σ).

Anal. Calcd. for C32H65NbO5N2 [%]: N 4.30, C 59.06, H 10.07. Found: N 3.28, C 56.48, H 10.67.

Thermal analysis (TGA/DTA) for Nb(OCH2tBu)5(py), Nb(OCH2tBu)5(py-4-NMe2) and Nb2(OCH2tBu)10

Fig. TGA of Nb(ONep)5 and adducts with py and py-4-NMe2

TGA/DTA spectra were recorded under N2 in sealed Al cartridges to prevent hydrolysis and to investigate pure decomposition process without additional O2. The TGA/DTA spectra of neopentoxides (Fig.) show actual (theoretical) weight losses of 74.6 (78.1) wt% for Nb(OCH2tBu)5(py), 77.3 (79.6) wt% for Nb(OCH2tBu)5(py-4-NMe2)), compared to 52.8 (75.5) wt% for Nb2(OCH2tBu)10.

Small endotherms (melting points of complexes as confirmed visually) were present at 210 ◦C for Nb(OCH2tBu)5(py) and 180 ◦C for Nb(OCH2tBu)5(py-4-NMe2)). All remaining TGA events were endothermic with an appropriate weight loss in the DTA (except for Nb2(OCH2tBu)10 ). For Nb(OCH2tBu)5(py) a continuous weight loss, was observed until 280 ◦C, where the decomposition point was indicated by an endothermic peak signals. Nb(OCH2tBu)5(py-4-NMe2) demonstrated relatively stable weight until 210 ◦C where a first decomposition step of the molecule was indicated by a small endothermic peak. Above 300 ◦C further endothermic signals were obtained. The temperatures for complete combustion were 310 ◦C for Nb(OCH2tBu)5(py) and 330 ◦C for Nb(OCH2tBu)5(py-4-NMe2) (indicating higher stability for more salt-like having py-4-NMe2 ligand).

For comparison, the parent unadducted alkoxide Nb2(OCH2tBu)10 (or Nb2(OnPn)10 ) presented incomplete decomposition shown by the inappropriate mass loss indicating massive contamination of carbon.

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