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 (C 6 H 6 ) 2 and tetracyanoethylene (TCNE) that leads to the room-temperature magnet V[TCNE]] x · y CH 2 Cl 2 ( 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 V o (arene) 2 with TCNE yields magnets with critical temperatures ranging from 28 to 400 K. Products from the reaction of [V I (arene) 2 ] + with [TCNE] • - were not magnets; however, reaction of [V I (arene) 2 ] + with [TCNE] • - in the presence of TCNE forms a magnetically ordered material. The reaction of V 0 (1,3,5-C 6 H 3 Me 3 ) 2 with 2 equiv of one-electron oxidant [Fe III (C 5 H 5 ) 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 I (arene) 2 ] + and “[V II (arene) 2 ] 2+ ”, the latter being the key intermediate that reacts with [TCNE] • - to produce the magnetic product. [V I (arene) 2 ] + has been isolated, whereas and “[V II (arene) 2 ] 2+ ” has not. The one-electron oxidation of V 0 [C 6 H 3 ( t- Bu) 3 ] 2 with Ag[BPh 4 ] produces {V I [C 6 H 3 ( t- Bu) 3 ] 2 } + [BPh 4 ] - . The stoichiometric reaction of V 0 [C 6 H 3 ( t- Bu) 3 ] 2 with TCNE leads to paramagnetic {V I [1,3,5-C 6 H 3 ( t Bu) 3 ] 2 } + [TCNE] • - . {V I [C 6 H 3 ( t- Bu) 3 ] 2 } + [BPh 4 ] - and {V I [1,3,5-C 6 H 3 ( t Bu) 3 ] 2 } + [TCNE] • - have been structurally characterized. When [V II (NCMe) 6 ]{B[3,5-C 6 H 3 (CF 3 ) 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 I (arene) 2 ] • + [TCNE] • - . Reaction of lower oxidation potential Cr 0 Np 2 (Np = naphthalene) with TCNE putatively forms “[Cr II Np 2 ] 2+ ” which reacts with [TCNE] • - to form Cr[TCNE] x · y S, which was isolated and, unexpectedly, does not magnetically order. Similar results are obtained when [Cr II (NCMe) 4 ][BF 4 ] 2 or [Cr II (NCMe) 6 ][B(3,5-C 6 H 3 (CF 3 ) 2 ) 4 ] 2 is reacted with [ n -Bu 4 N][TCNE].