Investigation of Transition Metal Alkyl, Allyl, Hydride, and Borohydride Complexes as Potential Precursors for Atomic Layer Deposition

Abstract

The bulk of this work focuses on the syntheses of various transition metal complexes bearing alkyl, allyl, hydride, and borohydride ligands, as well as the evaluation of these complexes as potential new chemical precursors for the ALD of mainly electropositive transition metals. The compounds bearing alkyl ligands which were investigated are [Cr(CH2SiMe3)4] (1), [Cr(CH2CMe3)4] (2), [V(CH2SiMe3)4] (3), [W2(CH2SiMe3)6] (5), [Mo2(CH2SiMe3)6] (6), and [Cr(dmpe)2(Me)2] (7). Syntheses for complexes 1–3 and 5–7 have previously been published, but this work will introduce alternate or slightly modified procedures, as well as the description of the solid-state structure of 3. Each complex was shown to sublime without decomposition. Thermal stability studies revealed that the complexes lacked the thermal stability on a long timescale to be effective as ALD precursors in a typical ALD reactor, assuming that a delivery temperature 40 °C higher than the sublimation temperature at 5 mTorr will be required. The vanadium containing complex [{V(CH2SiMe3)3O}2Mg(dme)2] (4) was isolated as an intermediate during the synthesis of 3 and structurally characterized by single crystal X-ray diffraction. Additionally, homoleptic allyl complexes [M{1,3-bis(trimethylsilyl)allyl}2] M = Ni (8), Cr (9), Co (10) were investigated. The solution state reactivity of each complex towards the potential co-reactants AlEt3, BEt3, and PhSiH3 was examined. Promising reactivity was observed between complex 8 and AlEt3, as well as complex 10 and AlEt3 or complex 10 and PhSiH3. However, subsequent study of the thermal stability of each complex revealed decomposition of each complex 40 °C above the sublimation temperature. Nickel complex 8 thermally decomposed to yield nickel metal (as analyzed by powder X-ray diffraction), and as such is suggested as a candidate for nickel metal chemical vapour deposition, contingent upon characterization of the minor unidentified reflection observed during powder X-ray diffraction. Borohydride complex [(dmpe)2Mn(μ-H)2BH2] (11) was obtained through the reaction of [(dmpe)MnH(C2H4)] with BH3(NMe3). The complex sublimes without decomposition at 80 °C, but was shown to undergo thermal decomposition after being heated to 120 °C, lacking the desired thermal stability for ALD use. Complexes [{(dmpe)2MnH}2(μ-dmpe)] (12) and [(dmpe)2MnH(κ1-dmpe)] (13) were identified in crude reaction mixtures during the synthesis of 11, and were subsequently synthesized independently. The synthetic scheme for 13 was extended to [(dmpe)2MnH(PMe3)] (14), and both complexes were crystallographically characterized. Compounds 12–14 exist in solution as isomers with cis- or trans- disposed hydride and κ1-PR3 ligands. For each complex, isomerization from trans- to cis- was shown to be photochemically induced, while the reverse isomerization will take place either slowly (days) in the absence of light, or quickly (hours) through heating. Manganese compound 14 was shown to be volatile at 60 °C and to only undergo moderate decomposition after being heated to 100 °C.