Ni(I) and Ni(II) Bis(trimethylsilyl)amides Obtained in Pursuit of the Elusive Structure of Ni{N(SiMe3)2}2

Salt metathesis routes to five new -N(SiMe3)2 nickel derivatives were studied to illuminate their mode of formation, structures, and spectroscopy. The reaction between NiI2 and K{N(SiMe3)2} afforded the Ni(II) and Ni(I) complexes [K][Ni{N(SiMe3)2}3] (1) and [K][Ni{N(SiMe3)2}2] (2). Dissolving 1 in tetrahydrofuran (THF) gave the Ni(II) species [K(THF)2][Ni{N(SiMe3)2}3] (3). The Ni(I) salt [K(DME)][Ni2{N(SiMe3)2}3] (4) was obtained by using NiCl2(DME) (DME = 1,2-dimethoxyethane) as the nickel source rather than NiI2. The isolation of the Ni(I) complexes 2 and 4 highlights the tendency for K{N(SiMe3)2} to function as a reducing agent. Introduction of adventitious O2 to solutions of [K][Ni{N(SiMe3)2}2] (2) gave the nickel inverse crown ether (ICE) species [K2][O(Ni{N(SiMe3)2}2)2] (5). Complex 5 is the first ICE complex of nickel and is one of four known ICE complexes for the 3d metals. The experimental results indicate that the reduced Ni(I) bis(trimethylsilyl)amides are relatively easily generated, whereas Ni(III) derivatives that might be expected from a disproportionation of a Ni(II) derivative are apparently not yet isolable by the above routes. Overall, the new species crystallize readily from the reaction mixtures, but under ambient conditions, they begin to decompose as solids within ca. 24 h, which hinders their characterization.

Hydrides, Halides, and Polymers: Some Unexpected Intermediates on the Routes to First-Row Transition Metal M{N(SiMe3)2}n (n = 2, 3) Complexes

The reaction of 2 equiv of LiN(SiMe₃)₂·Et₂O with TiCl₃(NMe₃)₂ or VCl₃(NMe₃)₂ afforded the dimeric, halide bridged complexes [Ti(μ-Cl){N(SiMe₃)₂}₂]₂ (1) or [V(μ-Cl){N(SiMe₃)₂}₂]₂ (2) in moderate yields. The reduction of titanium complex 1 with 3 equiv of 5% (wt) Na/NaCl gave the mixed metal titanium/sodium hydride cluster Ti₂(μ-H)₂{N(SiMe₃)₂}₃{N(SiMe₃)(SiMe₂CH)}(Na) (3), which was formed by activation of two C-H bonds at a single methyl group of one of the bis(trimethylsilyl)amide ligands. Attempts to form the analogous vanadium complex by reduction of 2 gave only intractable products. Treatment of Co{N(SiMe₃)₂}₂ with 1 equiv of BrN(SiMe₃)₂ (which was previously shown to give the unique three-coordinate cobalt(III) trisamide Co{N(SiMe₃)₂}₃) afforded the polymeric [(μ-Br)Co{μ-N(SiMe₃)(SiMe₂CH₂CH₂Me₂Si)(Me₃Si)μ-N}Co(μ-Br)]_∞ (4) as a second product, which was shown by a structural analysis to possess a carbon-carbon bond formed between the two ligands. Attempts to isolate manganese and iron complexes analogous to 4 were unsuccessful. The role of bromine in these reactions was further studied by examining the reaction of 0.5 equiv of elemental bromine with [Mn{N(SiMe₃)₂}₂]₂ or [Co{N(SiMe₃)₂}₂]₂, which for manganese was shown to give the previously reported manganese trisamide Mn{N(SiMe₃)₂}₃ but for cobalt gives the dimeric amide-bridged [Co(Br){μ-N(SiMe₃)₂}]₂ (5).

Low-Coordinate Iron Chalcogenolates and Their Complexes with Diethyl Ether and Ammonia

Treatment of Fe{N(SiMe₃)₂}₂ with 2 equiv of the appropriate phenol or thiol affords the dimers {Fe(OC₆H₂-2,6-Bu^t₂-4-Me)₂}₂ (1) and {Fe(OC₆H₃-2,6-Bu^t₂)₂}₂ (2) or the monomeric Fe{SC₆H₃-2,6-(C₆H₃-2,6-Prⁱ₂)₂}₂ (3) in moderate to excellent yields. Recrystallization of 1 and 2 from diethyl ether gives the corresponding three-coordinate ether complexes Fe(OC₆H₃-2,6-Bu^t₂-4-Me)₂(OEt₂) (4) and Fe(OC₆H₃-2,6-Bu^t₂)₂(OEt₂) (5). In contrast, no diethyl ether complex is formed by the dithiolate 3. The ¹H NMR spectra of 4 and 5 show equilibria between the ether complexes and the base-free dimers. A comparison of these spectra with those of the dimeric 1 and 2 allows an unambiguous assignment of the paramagnetically shifted signals. Treatment of 1 with excess ammonia gives the tetrahedral diammine Fe(OC₆H₂-2,6-Bu^t₂-4-Me)₂(NH₃)₂ (6). Ammonia is strongly coordinated in 6, with no apparent loss of ammine ligand either in solution or upon heating under low pressure. In contrast, significantly weaker ammonia coordination is observed when dithiolate 3 is treated with excess ammonia, which gives the diammine Fe{SC₆H₃-2,6-(2,6-Prⁱ₂-C₆H₃)₂}₂(NH₃)₂ (7). Complex 7 readily loses ammonia either in solution or under reduced pressure to give the monoammine complex Fe{SC₆H₃-2,6-(2,6-Prⁱ₂-C₆H₃)₂}₂(NH₃) (8). The weak binding of ammonia by iron thiolate 7 reflects the likely behavior of the proposed iron-sulfur active site in nitrogenases, where release of ammonia is required to close the catalytic cycle.