Ni^IV cubanes: synthesis and characterization of the [Ni₄Se₄(Se₃)₅(Se₄)]^4- and [Ni₄Te₄(Te₂)₂(Te₃)₄]^4- anions

The compounds [NEt₄]₄[Ni₄Se₄(Se₃)₅(Se₄)]·xNEt₄Cl (x=0, 1) and [NEt₄]₄[Ni₄Te₄(Te₂)₂(Te₃)₄] have been prepared from the reaction in DMF of Ni(S₂COEt)₂ with Li₂Se and Se or Li₂Te and Te, respectively. These compounds result from spontaneous assembly reactions in which Ni^II centers are oxidized to Ni^IV with concomitant reduction of Se- or Te-containing species. The anion [Ni₄Se₄(Se₃)₅(Se₄)]^4- possesses a Ni₄Se₄ cubane core and has five Se₃^2- chains and one Se₄^2- chain bridging the Ni atoms on the cubane faces. The anion [Ni₄Te₄(Te₂)₂(Te₃)₄]^4- has a Ni₄Te₄ cubane core and has two Te₂^2- chains bridging Ni atoms on opposite faces of the cubane core and four Te₃^2- chains bridging the Ni^IV atoms on the remaining cubane faces. The presence of Ni^IV is necessitated by charge balance. As expected for such d⁶ systems the compounds are diamagnetic. The [Ni₄Se₄(Se₃)₅(Se₄)]^4- anion exhibits a sharp, albeit complex, ⁷⁷Se NMR spectrum with fifteen resonances at δ 103, 134, 289, 361, 387, 401, 411, 470, 660, 680, 726, 749, 773, 789 and 804 ppm. The [Ni₄Te₄(Te₂)₂(Te₃)₄]^4- anion exhibits a simpler ¹²⁵Te NMR spectrum with five resonances at δ -632, -324, 65, 325 and 766 ppm. In the cubane anions each Ni atom is in a distorted octahedral environment, being coordinated by three Se or Te atoms in the cubane framework and by three Se or Te atoms in chains that bridge to the other three Ni atoms. While the bridging Se₃^2- chains appear to produce little strain on the Ni₄Se₄ cubane core, the Se₄^2- chain distorts it, as evidenced from the NiNi distances, the Se_cubSe_cub distances and the cubane bond angles. In contrast, the bridging Te₃^2- chains distort the Ni₄Te₄ cubane core as evidenced from the NiNi distances, the Te_cubTe_cub distances and the cubane bond angles; the Te₂^2- chains produce less strain. [NEt₄]₄[Ni₄Se₄(Se₃)₅(Se₄)]·NEt₄Cl crystallizes in the monoclinic space group C⁵_2h-P2₁/n with a=21.817(4), b=14.319(1), c=26.421(5) Å, β=112.69(2)°, V=7615 ų and Z=4 (-120 °C). [NEt₄]₄[Ni₄Te₄(Te₂)₂(Te₃)₄] crystallizes in the triclinic space group C_i¹-P1 with a=14.399(2), b=15.153(2), c=18.069(3) Å, α=70.97(1), β=79.05(1), γ=74.20(1)°, V=3564 ų and Z=2 (-167 °C).