SODIUM ALKOXIDES

    Sodium has only two non-fluorinated sublimable alkoxides: tert-butoxide and tert-butyl-bis(isopropoxymethyl)carbinolate, while fluorinated alkoxides of sodium are generally very volatile (Table ).

Sodium tert-butoxide Na(OtBu)

     Sodium tert-butoxide Na(OtBu) is one of two non-fluorinated sublimable sodium alkoxides. Na(OtBu) (M = 96.11) is white to off-white powder or crystals (d = 1.104), has melting point 180°C (by other data 101-103°C), decomposes at 186°C (by other data 250°C or 263-270°C). Sodium tert-butoxide was characterised by IR, TGA, thermochem., single crystal XRD.

     Na(OtBu) was found to be safe to handle in air for few minutes, but is hygroscopic for longer periods: it turns completely into hydroxide NaOH + carbonate Na2CO3 after couple of hours of air exposure.

      Na(OtBu) is soluble in THF (16%; 330 g/l), boiling alcohols (7.8%), toluene (5 g/l), diglyme, hexane, cyclohexane, α = 8 (in benzene), 4 (in THF or pyridine), 1 (in ROH), ΔHform = -114. 

Molecular structure of NaOtBu

Fig. Molecular structure of NaOtBu

   The crystal structure/ molecular structure of NaOtBu was determined by single crystal XRD (solved and refined at 1.0 Å resolution). It was determined that NaOtBu crystallizes in hexamer units with highly disordered tert-butyl groups (Fig. ).[i] the structure of NaOtBu was reported to partly match previously reported structure consisting of similar hexamers and also nonamers.[ii]

[i] E. Østreng, H. H. Sønsteby, S. Øien, O. Nilsen, H. Fjellvåg, Dalton Trans., 2014, 43, 16666-16672, « Atomic layer deposition of sodium and potassium oxides: evaluation of precursors and deposition of thin films », DOI: 10.1039/C4DT01930J, https://pubs.rsc.org/en/content/articlehtml/2014/dt/c4dt01930j

[ii] T. Greiser and E. Weiss, Chem. Ber., 1977, 110, 3388–3396

Thermal properties of NaOtBu

Na(OtBu) sublimes at 140°C/10-3 or 180°C/ 1 Torr,

The thermal properties of NaOtBu (evaporation characteristics, possible sublimation temperature) were determined by TGA  (Fig. 2). NaOtBu appeared to be promising enough to attempt Na-containing layer depositions.

Sodium tert-butyl-bis(isopropoxymethyl)carbinolate

       Sodium tert-butyl-bis(isopropoxymethyl)carbinolate is the second non-fluorinated sublimable sodium alkoxide (together with Na(OtBu)) (subl. temp. Subl. 155°/10-3 Torr ) and is potentially applicable as Na precursor for MOCVD.

Fluorinated sodium alkoxides

       Fluorinated sodium alkoxides with general formula Na(OCR(CF3)2) (R = H, CH3, CF3) were studied as precursors for NaF films preparation by MOCVD at temperatures 255-400°C; the lowest growth temperature was observed in case of R =  H. [4 and refs. therein]

Sodium hexafluorisopropoxide Na(OCH(CF3)2) , or Na(hfip)

    Sodium hexafluoroisopropoxide Na(hfip) is solid (crystals with melting point 114°C), subliming at 80°C/10-2 (with decomposition) or 140°C (atm. press.), soluble in diethyl ether. Na(hfip)  was characterised by IR, 1H, 19F NMR, TGA, X-R.

     Sodium hexafluoroisopropoxide Na(hfip) was tested as Na CVD precursor. In particular, it was sublimed to provide vapours for CVD deposition of sodium fluoride NaF layers [[i]]

[i] L. J. Lingg, A. D. Berry, A. P. Purdy, K. J. Ewing, Thin Solid Films, 1992, 209, 9-16.

Sodium methyl-bis(trifluoromethyl)carbinolate NaOCMe(CF3)2

     Sodium methyl-bis(trifluoromethyl)carbinolate NaOCMe(CF3)2 is crystalline solid, subliming at 100°C/10-2 Torr, decomposing at 162°C, soluble in diethyl ether Et2O and perfluorobenzene C6F6. NaOCMe(CF3)2 was characterized by IR, 1H NMR, 19F NMR, TGA, single crystal XRD.

NaOCMe(CF3)2  has been applied as sodium precursor for the growth of NaF films by MOCVD at deposition temperature 255-400°C.

Sodium perfluoro(tert-butoxide) NaOC(CF3)3, or Na(perfluoro-OtBu)

    Sodium perfluoro(tert-butoxide) Na(OC(CF3)3) (or Na(perfluoro-OtBu)) is crystalline solid with melting point 143°C (with decomposition starting from 119°C). NaOC(CF3)3 sublimes at 30°C/10-2, is soluble in diethyl ether (Et2O). NaOC(CF3)3 was characterised by 1H NMR, 19F NMR, mass-spectrometry, TGA. According to single crystal XRD, it has tetrameric structure in solid :   [NaOC(CF3)3]4   (having cube [M4(μ3-O)4] as main structural feature).

     Sodium perfluoro(tert-butoxide) [NaOC(CF3)3]4  has been applied as precursor for the MOCVD of NaF thin films by MOCVD at deposition temperature 255-400°C []

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