| Nitrides of the transition metals of groups 7-11 possess desirable properties, such as higher hardness and saturation magnetization than the corresponding metals. These nitrides have realized and potential applications in tool coatings and magnetic recording media. In order to develop new chemical vapor deposition (CVD) precursors for these late transition metal nitrides, we have explored the synthesis, characterization, and CVD of late transition metal complexes of the sterically demanding ligands di(tert-butyl)amide and 2,2,6,6-tetramethylpiperidide.;Treatment of MnBr2(thf)2, FeBr2(dme), CoBr2(dme), and NiBr2(dme) with two equivalents of LiN(t-Bu)2 in pentane, followed by sublimation in static vacuum, affords the two-coordinate compounds M[N(t-Bu)2]2 (M = Mn, Fe, Co, Ni) previously reported by our group. Previous work established that the Mn and Fe compounds have linear N-M-N angles, whereas the Co and Ni compounds are bent. In addition, the Fe and Co compounds have large orbital contributions to their magnetic moments, whereas the Mn and Ni compounds do not. In order to understand these properties, the electronic structures of the M[N(t-Bu)2]2 compounds have been described using the Angular Overlap Model (AOM). Two conclusions help rationalize the previously observed molecular properties of these and other two-coordinate compounds: (1) The potential energy surfaces for two-coordinate compounds are nearly flat, varying only a few kcal/mol even with 30° changes in the N-M-N angle, and (2) the ground state configurations for two-coordinate d2 and d3 (and therefore also d7 and d8) compounds experience strong inter-electron repulsion and extensive state mixing.;Low-temperature CVD from the reaction between these two-coordinate M[N(t-Bu)2]2 compounds and ammonia affords manganese, iron, cobalt, and nickel nitride thin films. Deposition rates as high as 18 nm/min are observed for cobalt nitride, and deposition temperatures as low as 25 °C are observed for iron nitride. The XPS binding energies confirm that the nitrogen is present as nitride in all cases. The M:N ratio in the deposited films decreases from Mn (2.5) to Fe (4) to Co (4.6-6) to Ni (9). Carbon contamination in the films is minimal for the manganese, iron, and cobalt nitride films, but similar to the nitrogen content in the nickel nitride films. Comparing the growth onset temperatures with the decomposition temperatures of the compounds M(NH2)2 supports the intermediacy in the film growth of the latter species, which are the expected products of the transamination of M[N(t-Bu)2]2 compounds with NH3.;Treatment of MnBr2(thf)2, FeBr2(dme), and CoBr2(dme) with two equivalents of lithium 2,2,6,6-tetramethylpiperidide, Li(tmp), in pentane, followed by sublimation in static vacuum, affords the new compounds M(tmp)2 (M = Mn, Fe, Co). The heteroleptic, dinuclear three-coordinate compounds Fe2(tmp)3(OEt) and [Li(dme)][CoBr(tmp)2] have also been isolated. Crystallographic studies of Fe(tmp)2 demonstrate that the tmp ligand, which is the cyclic analogue of di(tert-butyl)amide with a slightly compressed C-N-C angle, can also enforce two-coordination. The Fe-N bonds are similar to those in the linear compound Fe[N(t-Bu)2]2, although the N-Fe-N angle of 173° and ligand dihedral angle of 73° are smaller. Fe(tmp)2 might therefore be expected to have a less degenerate ground state.;Treatment of TiCl4 with two equivalents of H(tmp) affords the new compound Ti2Cl6(tmp)2. In contrast, treatment of TiCl4 or TiCl4(thf)2 with three equivalents of Li(tmp) induces ring-opening dealkylation of one tmp ligand and formation of a 1,1,5-trimethyl-5-hexenylimido ligand. A similar reaction is known for the di(tert-butyl)amido ligand.We conclude that the synthesis of early transition metal tmp and di(tert-butyl)amide compounds is hampered by the high electropositivity of these metals, which activates the methyl protons towards gamma-deprotonation. (Abstract shortened by UMI.). |
