Low temperature structures and magnetic interactions in the organic-based ferromagnetic and metamagnetic polymorphs of decamethylferrocenium 7,7,8,8-tetracyano-p-quinodimethanide, [FeCp*₂]˙⁺[TCNQ]˙

To identify the genesis of the differing magnetic behaviors for the ferro- (FO) and metamagnetic (MM) polymorphs of [FeCp*₂][TCNQ] (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) the low temperature (18 ± 1 K) structures of each polymorph were determined from high-resolution synchrotron powder diffraction data. Each polymorph possesses chains of alternating S = 1/2 [FeCp*₂]˙⁺ cations and S = 1/2 [TCNQ]˙⁺, but with differing relative orientations. These as well as an additional paramagnetic polymorph do not thermally interconvert. In addition, the room and low (<70 ± 10 K) temperature structures of the MM polymorph, MM_RT and MM_LT, respectively, differ from that previously reported at 167 K (-106 °C) MM structure, and no evidence of either phase transition was previously noted even from the magnetic data. This transition temperature and enthalpy of this phase transition for MM_RT⇌MM was determined to be 226.5 ± 0.4 K (-46.7 ± 0.4 °C) and 0.68 ± 0.04 kJ mol^-1 upon warming, respectively, from differential calorimetry studies (DSC). All three MM phases are triclinic (P1[combining macron]) with the room temperature phase having a doubled unit cell relative to the other two. The lower temperature phase transition involves a small rearrangement of the molecular ions and shift in lattice parameters. These three MM and FO polymorphs have been characterized and form extended 1-D chains with alternating S = 1/2 [FeCp*₂]˙⁺ cations, and S = 1/2 [TCNQ]˙^- anions, whereas the fifth, paramagnetic (P) polymorph possesses S = 0 π-[TCNQ]₂^2- dimers. At 18 ± 1 K the intrachain FeFe separations are 10.738(2) and 10.439(3) Å for the FO and MM_LT polymorphs, respectively. The key structural differences between FO and MM_LT at 18 ± 1 K are the 10% shorter interchain NN and the 2.8% shorter intrachain FeFe separation present for MM_LT. Computational analysis of all nearest-neighbor spin couplings for the 18 K structures of FO and MM_LT indicates that the intrachain [FeCp*₂]˙⁺[TCNQ]˙^- spin couplings (H = -2S_i·S_j) are the strongest (4.95 and 6.5 cm^-1 for FO and MM_LT, respectively), as previously hypothesized, and are ferromagnetic due to their S = 1/2 spins residing in orthogonal orbitals. The change in relative [TCNQ]˙^-[TCNQ]˙^- orientations leads to a computed change from the ferromagnetic interaction (0.2 cm^-1) for FO to an antiferromagnetic interaction (-0.1 cm^-1) for MM_LT in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.