The Instability of Ni{N(SiMe₃)₂}₂: A Fifty Year Old Transition Metal Silylamide Mystery

Abstract

The characterization of the unstable NiII bis(silylamide) Ni{N(SiMe3)2}2 (1), its THF complex Ni{N(SiMe3)2}2(THF) (2), and the stable bis(pyridine) derivative trans‐Ni{N(SiMe3)2}2(py)2 (3), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric NiI species, [Ni{N(SiMe3)2}]4 (4), also obtainable from LiN(SiMe3)2 and NiCl2(DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of NiII to NiI and the stabilization of 4 through dispersion forces.

Beyond Ni{N(SiMe3)2}2: Synthesis of a Stable Solvated Sodium Tris-Amido Nickelate

“…1.PMDETA is poorly soluble in nondonor solvents such as toluene, benzene, and hexane and was found to decompose to Ni{N(SiMe 3 ) 2 } 2 and (PMDETA)Na{N(SiMe 3 ) 2 }, giving a characteristic deep red solution with 1 H NMR resonances at δ 10.7 and 0.4 ppm, respectively, for the monometallic amides. 13 On this basis, and the earlier observations of rapid desolvation, we propose the equilibrium shown in Scheme 1.…”

“…The three Ni−N distances and N−Ni−N angles are nearly identical within error, resulting in an almost perfectly trigonal planar geometry (∑°= 359.9°). The average Ni−N distance (1.9286 Å) is considerably longer in comparison to those in II 3 and (THF)Ni{N(SiMe 3 ) 2 } 2 13 (1.887 and 1.861 Å, respectively) due to increased steric crowding and the anionic constitution. In the pseudo-octahedral cation, the six N−Na contacts range from 2.493(4) to 2.999(4) Å, which is similar to previous reports.…”

“…1 H NMR (300.13 MHz, THF-d 8 ): δ 1.17 (br, 54H, N{Si(CH 3 ) 3 } 2 ). 13 C{ 1 H} NMR (75.47 MHz, THFd 8 ): no signal observed for N{Si(CH 3 ) 3 } 2 . Evans method: Values between 3.4 and 3.6 μ B were consistently measured on crystalline samples, which is significantly higher than the expected value of 2.83 μ B for a Ni II center with two unpaired electrons.…”

“…While the Mn, − Fe, , and Co ,, species are thermally stable, the Ni analogue, which was first reported in 1963, rapidly decomposes above −20 °C, hindering its further use as a Ni II synthon. Later work revealed that the originally reported Mn, , Co, and Ni complexes were in fact their THF adducts. Recent reinvestigations into Ni­{N­(SiMe 3 ) 2 } 2 ( III ) by Power in 2015 has shown that it in fact decomposes to the tetrameric Ni I amide [Ni­{N­(SiMe 3 ) 2 }] 4 .…”

“…Later work revealed that the originally reported Mn, 5,11 Co, 12 and Ni 13 complexes were in fact their THF adducts. Recent reinvestigations into Ni{N(SiMe 3 ) 2 } 2 (III) by Power in 2015 13 has shown that it in fact decomposes to the tetrameric Ni I amide [Ni{N-(SiMe 3 ) 2 }] 4 . The THF and bis-pyridine adducts of Ni{N-(SiMe 3 ) 2 } 2 could also be isolated and characterized by solution and solid-state methods, but the reactivity of Ni{N(SiMe 3 ) 2 } 2 remains unexplored.…”