Molecular Routes to Metal−Carbides, −Nitrides, and −Oxides. 3.¹ Chemical Vapor Deposition Employing X₃W⋮*CCMe₃, Where X = CH₂CMe₃, O^tBu, and O^tBu-d₉ and *C = ¹²C or ¹³C, and a Comparison with the Chemistry of (^tBuO)₃W⋮N

Abstract

Low-pressure (10-3 Torr) chemical vapor deposition (LPCVD) employing a hot-walled quartz reactor (300−360 °C) and silica/quartz or silicon substrates have been carried out with X3W⋮*CtBu, where X = CH2CMe3, OtBu, and OBut-d 9 and *C = 12C and 13C. Thin films formed on SiO2 or Si have been examined by XPS, RBS, XRD, SEM, and SIMS. The volatile components evolved under the LPCVD conditions were trapped in an N2(l)-cooled U-tube packed with glass beads and examined by GC−MS and 1H and 13C NMR spectroscopy. Gray-black reflective films of amorphous tungsten carbide were formed (320−360°C), when X = Me3CCH2, contaminated with 5−9 atom % oxygen. The 12C:13C ratio in the film was ca. 7:1, and the volatile components were Me4C, Me3C13CH3 (54 mol % combined), Me2CCH2 (35 mol %), Me3C13C⋮CCMe3 (6 mol %), and Me3C13CHCHCMe3 (5 mol %) for the LPCVD employing the 13C-labeled alkylidyne carbon. Films grown from (tBuO)3W⋮CCMe3 were WO2 (determined by XRD after annealing) contaminated with some carbon. The volatile components were Me2CCH2 (45 mol %), tBuOH (38 mol %), and Me4C (5 mol %), together with Me3CC⋮CCMe3, Me3CCHCHCMe3, and Me3CCH2CH2CMe3 (totaling ca. 10 mol %). Reactions employing (tBu-d 9O)3W⋮CC(CH3)3 yielded perdeuterated isobutylene and tert-butyl alcohol together with (CH3)3CCD3, (CH3)3CCDCDC(CH3)3, and (CH3)3CCD2CD2C(CH3)3. Reactions employing (tBuO)3W⋮N were less clean because the precursor was less volatile and it decomposes with sublimation. Nevertheless, the films obtained were predominately WO2 contaminated with small amounts of C and N (<4%), and the volatile components revealed the presence of NH3, isobutylene, and tBuOH. Collectively these results show that the metal−ligand multiple bond is labile to hydrogen-transfer reactions, which may lead to loss of the W⋮X atom. In the case of WC formation, the elimination of Me3CC⋮CCMe3 and Me3CCHCHCMe3 implicate surface chemistry involving C−C coupling of alkylidyne and alkylidene ligands.

Transition metal alkylidene complexes. Pathways in their formation and tautomerization between bis-alkylidenes and alkyl alkylidynes

“…The bis-alkylidene intermediate 1c was not directly observed. A similar exchange was observed in 13 C-labeled alkyl alkylidyne (Me 3 CCH 2 ) 3 W^1 3 CCMe 3 (2a- 13 C, Scheme 1) [25] where the alkylidyne carbon atom was found to quickly exchange between the four a-C atoms with a nearly statistical distribution of 25% 13 C-labeling on each a-C atom.…”

“…In the 7a # 7b / 10 conversion at 333e358 K with the PMe 3 analog (Scheme 15), the activation parameters are: DH s ¼ 29(2) kcal/mol, DS s ¼ 4(1) eu, and DG 2 s 338 K ¼ 28(3) kcal/mol. These values are typical for cyclometalation/a-H abstraction [13,23] prior to the addition of a second (25) without evidence of intermediates [9]. In benzene-d 6 solution of 2a with PMe 3 at room temperature, a similar reaction to give 25 was observed.…”

“…The silyl alkylidyne is much closer in energy to its bisalkylidene tautomer and is therefore observed in the a-H exchange, whereas the proposed intermediate "(Me 3 CCH 2 ) 2 W(]CHSiMe 3 )(] CHCMe 3 ) (1c)" is not observed because it is much higher in energy than the ground state alkylidyne 1a [23,25].…”

“…The formation of high-oxidation-state alkyl alkylidyne complexes have led to many studies related to a-H migrations and abstractions as these processes are often involved in their formation [2e4, 10,11,23,25,32,38]. We were interested in silyl analogs of alkyl alkylidyne complexes [32,38].…”

“…Density functional theory (DFT) molecular orbital calculations indicate that the relative stabilities of the bis-alkylidene increases with the increasing p-accepting ability of the X ligand for the d 0 W and Mo complexes [23,25,33]. The increased stability can be explained in terms of the p interaction between ligand X and the electron density in the metal-alkylidyne/alkylidene bonds.…”