Syntheses, properties, and reactions of transition metal complexes of di(tert-butyl)amide and 2,2,6,6-tetramethylpiperidide

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. For example, for a high-spin, two-coordinate d2 compound of D∞h symmetry (where the d-orbitals order as (xy, x2-y2) < (xz, yz) < z2), the ground state does not have the expected (xy, x2-y2)2 configuration, but rather an (xy, x2-y2)1(xz, yz)1 configuration. The (xy, x2-y2)2 configuration confines the two electrons to a single plane, which incurs an energetic penalty larger than the cost of promoting an electron to the (xz, yz) orbitals. 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. Instead, the magnetic moment of 5.91 μB suggests the ground state of Fe(tmp)2 is more degenerate than that of Fe[N(t-Bu)2]2 (μeff = 5.55 μB). This conclusion is corroborated by zero-field hyperfine splitting of the Mössbauer spectrum of Fe(tmp)2 at 4.2 K. The internal hyperfine field, HINT ≈ 136 T, is the second-largest field reported to date for any iron compound, corresponding to an orbital field HL of ≈ 187 T. This orbital field exceeds that of Fe[N(t-Bu)2]2 by ca. 30 T. AOM calculations suggest that the splitting between the non-bonding xy and x2-y2 d-orbitals is controlled by weak interactions (δ-bonding or Fe•••H-C interactions) instead of by the bend angle; the smaller dihedral angle and compressed C-N-C angle in Fe(tmp)2 weaken these interactions, increasing the degeneracy. 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. IR spectroscopic studies demonstrate that the tmp ligand undergoes ring-opening dealkylation at early transition metal centers including Ti(III), Ti(IV), Hf(IV), V(III), Nb(III), Nb(V), and Mo(III), but not Cr(III). The evidence is consistent with a γ-deprotonation mechanism in which the first two equivalents of Li(tmp) install two tmp groups on the metal center, but a third equivalent of Li(tmp) results in deprotonation of one of these two now-coordinated groups. The resulting carbanion undergoes a rearrangement to afford an imido ligand and an olefin (the latter remaining bound to the imide through the tmp methylene units). 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 γ-deprotonation