Identification of [MII(arene)2]2+ (M = V, Cr) as the key intermediate in the formation of V[TCNE]x•ysolvent magnets and Cr[TCNE]x•solvent

To elucidate the mechanism of the reaction between V-0(C6H6)(2) and tetracyanoethylene (TCNE) that leads to the room-temperature magnet V[TCNE]](x). yCH(2)Cl(2) (x similar to 2: y similar to 1/2), reactions between bis(arene) vanadium (arene = 1,3,5-trimethylbenzene, 1,3,5-tri-tert-butylbenzene) and its cations and TCNE and its anion were studied. The reaction of V-0(arene)(2) with TCNE yields magnets with critical temperatures ranging from 28 to 400 K. Products from the reaction of [V-1(anne)(2)](+) with [TCNE](.-) were not magnets; however, reaction of [V-1(arene)(2)](+) with [TCNE](.-) in the presence of TCNE forms a magnetically ordered material. The reaction of V-0(1,3,5-C6H3Me3)(2) with 2 equiv of one-electron oxidant [Fe-III(C5H5)(2)](+), and subsequently with [TCNE](.-), also leads to a magnetic material. The results of these investigations suggest that V-0(arene)(2) undergoes two one-electron oxidation reactions with TCNE to form sequentially [V-1(anne)(2)](+) and "[V-II(arene)(2)]2(+)", the latter being the key intermediate that reacts with [TCNE](.-) to produce the magnetic product. [V-1(arene)(2)](+) has been isolated, whereas and "[V-II(arene)(2)](2+)" has not. The one-electron oxidation of V-0[C6H3(t-Bu)(3)](2) with Ag-[BPh4] produces {V-1[C6H3(t-Bu)(3)](2)}(+)[BPh4](-). The stoichiometric reaction of V-0[C6H3(t-Bu)(3)](2) with TCNE leads to paramagnetic {V-1[1,3,5-C6H3(tBu)(3)](2)}(+)[TCNE](.-). {V-1[C6H3(t-Bu)(3)](2)}(+)[BPh4](-) and {V-1[1,3,5-C6H3-(tBu)(3)](2)}(+)[TCNE](.-) have been structurally characterized. When [V-II(NCMe)(6)]{B[3,5-C6H3(CF3)(2)](4)}(2) is reacted with [TCNE](.-), a route that does not use bis(arene)vanadium complexes, a magnetic precipitate is also produced. As established earlier, the reaction of Cr-0(arene)(2) with TCNE forms nonmagnetically ordered [Cr-1(arene)(2)](.+)-[TCNE](.-). Reaction of lower oxidation potential (CrNp2)-Np-0 (Np = naphthalene) with TCNE putatively forms "[(CrNp2)-Np-II](2+)" which reacts with [TCNE](.-) to form Cr[TCNE](x). yS, which was isolated and, unexpectedly, does not magnetically order. Similar results are obtained when [Cr-II(NCMe)(4)][BF4](2) or [Cr-II(NCMe)(6)][B(3,5-C6H3(CF3)(2))(4)](2) is reacted with [n-Bu4N][TCNE].