Local structural order in the disordered vanadium tetracyanoethylene room-temperature molecule-based magnet

We determined the vanadium oxidation state and local coordination environment in disordered samples of magnetic V[TCNE]ₓ(x≈2) prepared by chemical vapor deposition (CVD). Systematic studies of the x-ray absorption near-edge structure (XANES) in this material and reference compounds show that V ions have a valence state near $2+$. Extended x-ray absorption fine structure (EXAFS) analysis shows that vanadium ions are coordinated by 6.04±0.25 nitrogen atoms at a room-temperature average distance of 2.084(5). The local environment is well defined with a distribution of V-N bond lengths comparable to that commonly found in ordered compounds. This distribution is mostly vibrational in origin, with static contributions being at least four fold smaller. The small disorder in V-N distances is a consequence of strong binding between V and TCNE, with an effective local force constant of k=870.3em0exN∕m. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of V[TCNE]ₓ with six N nearest neighbors results in magnetic ordering above room temperature. The strong V-N bonding explains in part the insoluble nature of this compared to other molecule-based magnets. The room-temperature XANES and EXAFS results for the CVD-prepared samples are compared to those for V[TCNE]ₓ prepared as a powder from CH₂Cl₂ solvent, which has a similar magnetic ordering temperature but a magnetization that is more strongly temperature dependent. This comparison suggests that coordination of 6 nitrogens around each V(II) with little variation in the V-N distances is important for achieving the high magnetic ordering temperature of 4000.3em0exK associated with samples made by both the CVD and CH₂Cl₂ solution methods.