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)(xapproximate to2) 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)A. 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=87 N/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 400 K associated with samples made by both the CVD and CH2Cl2 solution methods.