1,4–Diazadiene Imido Complexes of Group V and VI Elements and Their Application in Chemical Vapour Deposition of TaN Films
The present work describes new perspectives in chemistry of 1,4-diaza-1,3-diene (R,R’–DAD) ligands. From the wide spectrum of described R,R’–DAD ligands 1,4-di-tert-butyl-1,4-diazabuta-1,3-diene (tBu,H–DAD or simply DAD) was chosen. This derivative has several advantages, namely, it is very simple t...
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|Summary:||The present work describes new perspectives in chemistry of 1,4-diaza-1,3-diene (R,R’–DAD) ligands. From the wide spectrum of described R,R’–DAD ligands 1,4-di-tert-butyl-1,4-diazabuta-1,3-diene (tBu,H–DAD or simply DAD) was chosen. This derivative has several advantages, namely, it is very simple to synthesise on large scale and it possesses a high vapour pressure needed for the design of covalent complexes of high volatility carrying this ligand. DAD is a redox active ligand, which can be easily introduced into complexes in its fully reduced ene-diamido form DAD2–. A fruitful liaison of the DAD ligand and p-basic imido ligands uncovered a new class of volatile DAD2– imido complexes of niobium and tantalum, which were tested in Chemical Vapour Deposition (CVD) process as possible precursors of metal nitride M1.0N1.0 films – barrier materials between copper strip conductors and silicon wafers in microelectronics. Several different synthetic methods to the key compounds [(DAD)M(NtBu)(m-Cl)]2 (A) and [(DAD)M(NtBu)(NHtBu)], M = Nb and Ta, were developed. Reaction conditions were optimised for large scale syntheses using MCl5 as starting materials. The chemical reactivity of the dimeric chloro complexes A and its monomeric pyridine analogues [(DAD)M(NtBu)(Cl)(py)], M = Nb and Ta, in nucleophilic substitution, reduction and metathesis reactions with amines was investigated. Amido imido complexes [(DAD)M(NtBu)(NHtBu)], M = Nb and Ta, were found to be active in metathesis reactions with amines, acidic organic molecules and lithium alkyls. The complete set of complexes for the elements VI group with the common formula [(DAD)M(NR)2], M = Cr, Mo, W; R = tBu, Mes; was synthesised. The target compounds of molybdenum and tungsten were obtained by reaction of [M(NR)2Cl2L2], L = DME and pyridine, with DAD in the presence of magnesium. Under the same conditions the chromium compound [Cr(NtBu)2Cl2] formed a dinuclear d1–d1 complex [Cr2(NtBu)2(m-NtBu)]2(m-DAD)] with an uncommon coordinated bridging DAD2– ligand. Complex [(DAD)Cr(NtBu)2] was isolated in the reaction of [Cr(NtBu)2Cl2] with Li2DAD. Interestingly, no reduction was observed in the reaction of compounds of the type [W(NR)Cl4(L)] with Li2DAD, complexes [(DAD)W(NR)Cl2], R = Ph and Cy, were obtained in good yields. According to X-ray analysis, tetrahedral and tetragonal pyramidal coordination polyhedra were observed for the newly synthesised imido DAD2– complexes of group V and VI elements. The DAD2–M metallacycle has folded conformation: the metal centre is not located in the plane formed by N2C2 atoms of the DAD2– moiety. All complexes characterised in this work by X-ray analysis exhibit supine conformation with respect to the imido group with one exception: a disorder of the double bond carbon atoms in [(DAD)Nb(NtBu)(Cl)(py)] with the site occupation factor of 18% leads to prone conformation. The tetrahedral complexes and tetragonal pyramidal complexes, in which dissociation of a ligand is impossible, do not show temperature dependence of their NMR spectra in solution. This is consistent with either the rapid supine–prone rearrangement, which cannot be localised on the NMR time scale or with the presence of only one isomer supine or prone. The dynamic behaviour observed for pyridine containing complexes [(DAD)M(NtBu)(X)(py)] and dimeric complexes [(DAD)M(NtBu)(m-Cl)]2, M = Nb and Ta, X = Cl and Bz, proceeds via an intramolecular dissociation/association mechanism, for which thermodynamic parameters of the dissociation barrier were calculated on the basis of NMR spectroscopy. In the tetrahedral complexes of the type [(DAD)M(NR)2], M = Cr, Mo, W; R = tBu, Mes, as well as in the isoelectronic unit [(DAD)M(NtBu)2]–, M = Nb and Ta, of complexes [(DAD)M(NtBu)2Li]2 and [(DAD)M(NtBu)2Li(dad)], ring inversion was observed in solution. Based on NMR experiments (2D-EXSY and 1D Band Shape Analysis) calculations of the thermodynamic parameter for the DAD2– ring inversion (‘flipping’ of the DAD2– double bond) reveal activation entropy values close to zero for all complexes. The activation enthalpy has the most prominent contribution in the formation of the ‘flat’ transition state. This transition state seems to be significantly stabilised in the case of chromium compared to molybdenum and tungsten.
Ceramic films with ~ 50 atom % of tantalum were obtained in cooperation with the workgroup of PD Dr. W. Stolz (WZMW, University of Marburg) from [(DAD)Ta(NtBu)(NHtBu)], [(DAD)Ta(NtBu)(h3-BH4)] and [(DAD)Ta(NtBu)(h3-C3H5)] using the CVD technique. The films contain surface oxygen originated from air, which seems to react with the unsaturated metal-containing surface. Unfortunately, incorporation of carbon atoms into films was detected by EDX analysis. Actually, this tendency is common for all known amido or imido amido tantalum compounds containing nitrogen–carbon bonds.|