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 V0(C6H6)2 and tetracyanoethylene (TCNE) that leads to the room-temperature magnet V[TCNE]]x·yCH2Cl2 (x ∼2; y ∼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 Vo(arene)2 with TCNE yields magnets with critical temperatures ranging from 28 to 400 K. Products from the reaction of [VI(arene)2]+ with [TCNE]•- were not magnets; however, reaction of [VI(arene)2]+ with [TCNE]•- in the presence of TCNE forms a magnetically ordered material. The reaction of V0(1,3,5-C6H3Me3)2 with 2 equiv of one-electron oxidant [FeIII(C5H5)2]+, and subsequently with [TCNE]•-, also leads to a magnetic material. The results of these investigations suggest that V0(arene)2 undergoes two one-electron oxidation reactions with TCNE to form sequentially [VI(arene)2]+ and "[VII(arene)2]2+", the latter being the key intermediate that reacts with [TCNE]•- to produce the magnetic product. [VI(arene)2]+ has been isolated, whereas and "[VII(arene)2]2+" has not. The one-electron oxidation of V0[C6H3(t-Bu)3]2 with Ag[BPh4] produces {VI[C6H3(t-Bu)3]2}+[BPh4]-. The stoichiometric reaction of V0[C6H3(t-Bu)3]2 with TCNE leads to paramagnetic {VI[1,3,5-C6H3(tBu)3]2}+[TCNE]•-. {VI[C6H3(t-Bu)3]2}+[BPh4]- and {VI[1,3,5-C6H3(tBu)3]2}+[TCNE]•- have been structurally characterized. When [VII(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 Cr0(arene)2 with TCNE forms nonmagnetically ordered [CrI(arene)2]•+[TCNE]•-. Reaction of lower oxidation potential Cr0Np2 (Np = naphthalene) with TCNE putatively forms "[CrIINp2]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 [CrII(NCMe)4][BF4]2 or [CrII(NCMe)6][B(3,5-C6H3(CF3)2)4]2 is reacted with [n-Bu4N][TCNE].