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.