Low temperature structures and magnetic interactions in the organic-based ferromagnetic and metamagnetic potymorphs of decamethytferrocenium 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-pquinodimethane) 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])center dot+ cations and S = 1/2 [TCNQ](center dot+) 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, MMRT and MMLT , respectively, differ from that previously reported at 167 K (-106 degrees 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 MMRT reversible arrow MM was determined to be 226.5 +/- 0.4 K (-46.7 +/- 0.4 degrees C) and 0.68 +/- 0.04 kJ mol(-1) upon warming, respectively, from differential calorimetry studies (DSC). All three MM phases are triclinic (P (1) over bar) 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 5 = 1/2 [FeCp*(2)](center dot+) - cations, and S = 1/2 [TCNQ](center dot-) - anions, whereas the fifth, paramagnetic (P) polymorph possesses S = 0 pi-[TCNQ](2-) dimers. At 18 +/- 1 K the intrachain Fe...Fe separations are 10.738(2) and 10.439(3) angstrom for the FO and MMLT polymorphs, respectively. The key structural differences between FO and MMLT at 18 +/- 1 K are the 10% shorter interchain N-N and the 2.8% shorter intrachain Fe-Fe separation present for MMLT. Computational analysis of all nearest-neighbor spin couplings for the 18 K structures of FO and MMLT indicates that the intrachain [FeCp*(2)](center dot+)center dot center dot center dot(TCNOr - spin couplings (H = -25,-Si) 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](center dot-)center dot center dot center dot(TCNQ](center dot-) - 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 MMLT in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.

Automated summary

The genesis of magnetic behaviors for different polymorphs of [FeCp*(2)][TCNQ] was identified through determination of low temperature structures using high-resolution synchrotron powder diffraction data. Analysis revealed that the polymorphs do not thermally interconvert, and structural differences between them were characterized. Computational analysis of spin couplings indicated strong intrachain magnetic interactions, with changes leading to different magnetic ground states.