Versatile Tautomerization of EH₂-Substituted Silylenes (E = N, P, As) in the Coordination Sphere of Nickel

The synthesis and tautomerization of a “half-parent” aminosilylene and its heavy P- and As-analogues (ᵀᴹSLSi-EH₂; E = N, P, As; ᵀᴹSL = N(SiMe₃)(2,6-ⁱPr₂C₆H₃)) in the coordination sphere of nickel(0) to give the corresponding side-on η²-RSi(H)═EH and RH₂Si-E (“silylpnictinidene”) nickel complexes are reported. These complexes can be accessed through salt metathesis reactions of the lithium dihydropnictides LiEH₂ with the acyclic chlorosilylene nickel(0) complex 1, [ᵀᴹSL(Cl)Si → Ni(NHC)₂; NHC = :C[(ⁱPr)NC(Me)]₂). In addition, we report the facile E–H bond activation reactions of EH₃ with 1, which furnished a silyl nickel(II) complex through NH₃ activation, but phosphido and arsenido complexes in the activation of PH₃ and AsH₃, respectively. Notably, reaction of 1 with LiNH₂ leads to the acyclic bis(amido)silylene complex [ᵀᴹSL(H₂N)Si → Ni(NHC)₂] 5, which does not undergo N–H proton migration to silicon(II) under ambient conditions. The transformation of the P- and As-analogues of 1 furnishes directly the respective side-on Si═E Ni complexes (nickelacycles), [η²-{ᵀᴹSL(H)Si═E(H)}Ni(NHC)₂] (E = P, 6; E = As, 9). These nickelacycles show a vastly different stability in solutions. While 6 is stable for several days at ambient temperature, 9 undergoes further rearrangement processes within minutes of its formation. Given the high acidity of the As–H proton in 9, however, this moiety can be trapped as a highly charge separated metalated-η²-silaarsene nickel complex 12 that is best described as an [AsSiNi] nickelacycle with Si–As multiple bond character. Taken as a whole, these results give, for the first time, insights into the relative stability of the tautomeric forms of side-on silaldimine transition metal complexes. The electronic nature and the rearrangement processes of these compounds were also investigated by quantum chemical calculations.