Structure and Magnetic Interactions in the Organic-Based Ferrornagnet Decamethylferrocen i urn Tetracyanoethenide, [FeCp*2]•+[TCNE]•-.

The structures of three temperature-dependent polymorphs of solvent-free decamethylferrocenium tetracyanoethenide, [FeCp*[sub2]][TCNE], are determined from high-resolution synchrotron powder diffraction data. [FeCp*[sub2]][TCNE] is the first organic-based ferromagnetic material to be synthesized and is known to have two structural phase transitions at 249 and 282 K. The low-temperature phase, which exhibits spontaneous ferromagnetic order below 4.8 K, was determined at 12 K. At that temperature, it has monoclinic space group P2[sub1]/c [a = 9.6637(4) Å, b = 14.1217(5) Å, c = 18.6256(7) A, β = 113.231 (2)°, Z = 4] and consists of parallel chains of alternating [Fe(C[sub5]Me[sub5])[sub2]][sup∙+] and [TCNE][sup∙-] ions, with an intrachain Fe⋯Fe distance of 10.45 Å. Structures of the intermediate and ambient temperature phases, also studied here, are characterized by increasing disorder. At 250 K, the unit cell space group is F2[sub1]/m [a = 9.7100(3) Å, b = 14.4926(4) Å, c = 9.4997(3) Å, β = 113.153(1)°, Z= 2]. At ambient temperature, the lattice, albeit quite disordered, belongs to the orthorhombic space group Cmcm [a = 10.629(1) Å, b = 16.128(1) Å, c 14.593(1) Å, Z = 4]. Nearest-neighbor magnetic interactions were evaluated for the 12 K structure by CASSCF and CASSCF/MCQDPT calculations (a methodology similar to the CASPT2 method). Similar trends are observed in computations with and without inclusion of spin-orbit coupling. The strongest are two intrachain [FeCp*[sub2]][sup&3x2219;-]⋯[TCNEI interactions (ferromagnetic with values of ∼45 and ∼29 cm[sup1]), although weaker, nonnegligible, ferro- or antiferromagnetic interchain interactions of less than ±0.2 cm[sup1] are also present. Magnetic interactions that lead to ordering are therefore three-dimensional, despite the vastly different intra- and interchain coupling strengths. [ABSTRACT FROM AUTHOR]