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, MMRT and MMLT, 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 MMRT⇌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]22- dimers. At 18 ± 1 K the intrachain FeFe separations are 10.738(2) and 10.439(3) Å for the FO and MMLT polymorphs, respectively. The key structural differences between FO and MMLT at 18 ± 1 K are the 10% shorter interchain NN and the 2.8% shorter intrachain FeFe 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]˙+[TCNQ]˙- spin couplings (H = -2Si·Sj) are the strongest (4.95 and 6.5 cm-1 for FO and MMLT, 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 MMLT in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.

Thermodynamic investigation by heat capacity measurements of ferrimagnetic A2Mn[Mn(CN)6] (A=K, Rb, Cs) Prussian blue compounds.

Heat capacity measurements of a new series of Prussian blue analogs of A2Mn[Mn(CN)6] (A=K, Rb, Cs) composition were performed using thermal relaxation calorimetry. The Cs compound has a face-centered cubic structure with a linear Mn-C≡N-Mn linkage, while the monoclinic Rb and K compounds have nonlinear Mn-C≡N-Mn linkages. For all of the compounds, large broad thermal anomalies associated with magnetic transitions were observed in the temperature dependence of the heat capacity. The systematic changes in the heat capacity for the three compounds under magnetic fields of up to 7 T were found to be consistent with ferrimagnetic ordering with large spontaneous magnetization. Although the peak temperatures were slightly lower than reported values obtained by magnetic susceptibility measurements, the magnetic entropy was evaluated to be 22.0 ± 2.5 J K(-1) mol(-1). This value is consistent with an entropy of Rln12 corresponding to full entropy of one low-spin and one high-spin Mn(II) ion in the formula unit, though some ambiguity remains in lattice estimation. Broadening of the peak width of the magnetic heat capacity divided by the temperature was observed as the size of the alkali ions decreased from Cs to K. This behavior is consistent with an increase in the lattice distortion produced by the bending of the C≡N-Mn angles.

Pressure induced crossover between a ferromagnetic and a canted antiferromagnetic state for [bis(pentamethylcyclopentadienyl)-iron(III)][tetracyanoethenide], [FeCp2*][TCNE].

The reversible hydrostatic pressure dependent DC magnetic behavior of the ferromagnetically ordered electron transfer salt [Fe(III)Cp2*](•+)[TCNE](•-) (Cp* = pentamethylcyclopentadienide; TCNE = tetracyanoethylene) was studied up to 12.2 kbar. A significant departure from the ambient pressure ferromagnetic behavior was observed under pressure. The temperature dependent magnetization data were typical of a ferromagnet at ambient pressure but exhibited an extreme reduction with increasing applied pressure, while metamagnetic-like behavior was evident in the field dependent magnetization data at 4.2 kbar and above. Hence, the decrease of the intermolecular separations due to increasing pressure enhances the nearest neighbor couplings, leading to an increase in magnetic ordering temperature, T(c). Furthermore, the presence of a metamagnetic-like behavior suggests an increase of the antiferromagnetic contribution to the interchain interactions. The low field magnetization data indicate that spin canting is induced by pressure, leading to a canted antiferromagnetic phase with a much lower magnetization than the low-pressure ferromagnetic state. This unprecedented magnetic behavior is consistent with the field, temperature, and pressure dependences of the magnetization below 20 K.

Pressure induced transition from spin glass-like behavior to a metamagnet exhibiting weak ferromagnetism observed for decamethylferrocenium hexacyanobutadienide, [FeCp*2]˙+ [HCBD]˙-.

The magnetic properties of [Fe(III)Cp*2]˙(+)[HCBD]˙(-) (Cp* = pentamethylcyclopentadienide; HCBD = hexacyanobutadienide, C4(CN)6) and [Fe(III)Cp*2]˙(+)[DDQ]˙(-) (DDQ = 2,3-dichloro-5,6-dicyanoquinonide) were measured at ambient and applied hydrostatic pressures up to 11.4 and 9.2 kbar, respectively. At ambient pressure [FeCp*2][HCBD] exhibits spin glass-like behavior with a freezing temperature, Tf, of 2.93 K from the peak in χ'(T) at 10 Hz, but magnetic ordering is not evident due to the lack of a remnant magnetization, bifurcation temperature, and hysteresis. Above 3.1 kbar, [FeCp*2][HCBD] magnetically orders as a metamagnet with an antiferromagnetic ground state with an ordering temperature, Tc, of 2.46 K determined from the Fisher specific heat, which increases linearly to 4.80 K at 11.4 kbar at a rate of 0.28 K kbar(-1). Upon application of pressure metamagnetic-like behavior with hysteresis indicative of a weak ferromagnet (canted antiferromagnet) was observed at and above 3.1 kbar, with a coercive field, Hcr, of 65 Oe, which increases exponentially to 795 Oe at 11.4 kbar. [FeCp*2]˙(+)[DDQ]˙(-) did not magnetically order above 2 K, and magnetic order was not observed up to 9.2 kbar. The pressure dependencies are reversible.

Pressure-dependent reversible increase in T(c) for the ferrimagnetic 2-D Mn(II)(TCNE)I(OH2) and 3-D Mn(II)(TCNE)(3/2)(I3)(1/2)·zTHF organic-based magnets.

The pressure dependence of the magnetic properties of ferrimagnetic Mn(II)(TCNE)I(OH2) up to 14.05 kbar and Mn(II)(TCNE)(3/2)(I3)(1/2)·zTHF up to 14.32 kbar were studied. For Mn(II)(TCNE)I(OH2), two distinct pressure regions separated at ∼1 kbar were evident in both the temperature and the field-dependent magnetic measurements. No increase of the magnetic properties was observed in the low-pressure region, while significant increases to the magnetic ordering temperature, T(c), bifurcation temperature, T(b), coercive field, H(cr), and remnant magnetization, M(r), were evident in the high-pressure region. The T(c), T(b), H(cr), M(r), and M(5 T) reversibly increased from ambient pressure values of 169 K, 169 K, 690 Oe, 620 emuOe/mol, and 13,800 emuOe/mol to 257 K, 261 K, 1460 Oe, 2300 emuOe/mol, and 17,100 emuOe/mol at 14.05 kbar, respectively. For Mn(II)(TCNE)(3/2)(I3)(1/2)·zTHF, the T(c) and T(b) were nearly coincident and increased linearly from 173 and 173 K, respectively, at ambient pressure to 273 and 272 K, respectively, at 14.32 kbar. Thus, the T(c) increased at an average rate of 6.25 and 7.18 K/kbar for Mn(II)(TCNE)I(OH2) and Mn(II)(TCNE)(3/2)(I3)(1/2)·zTHF, respectively. For Mn(TCNE)(1/2)(I3)(1/2)·zTHF remnant magnetization and saturation magnetization did not significantly change with applied pressure. The H(cr) exhibited a linear increase from ambient pressure to 5.00 kbar, reaching 860 Oe, but only achieving 880 Oe at 14.32 kbar.

Pressure-dependent increase in T(c) and magnetic behavior of [Ru2(O2CBu(t))4]3[M(CN)6]·2H2O (M = Cr, Fe).

Magnetization as a function of applied pressure up to 10.16 kbar and magnetic field were obtained for layered [Ru(2)(O(2)CBu(t))(4)](3)[M(CN)(6)]·2H(2)O (M = Cr, Fe). For M = Fe, the T(c) increased by 13% from 6.1 to 6.9 K with a significant increase in the coercive field, H(cr), from 5 to 65 Oe, followed by a sharp decrease to less than 10 Oe at further applied pressure. A 32% increase in T(c) from 37.8 to 50.0 K was observed for M = Cr as well as a linear decrease of H(cr) upon increasing pressure from 6380 to 2380 Oe.

Pressure-dependent enhanced T(c) and magnetic behavior of the metamagnetic and ferromagnetic polymorphs of [Fe(III)Cp*2]•+ [TCNQ]•- (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane).

The magnetic behaviors of the metamagnetic and ferromagnetic polymorphs of [Fe(III)Cp(2)*](•+)[TCNQ](•-) (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) were studied as a function of hydrostatic pressure. Both polymorphs exhibit a reversible enhancement of magnetic properties with increasing pressure. The T(c) for the ferromagnetic polymorph increased by 70% from 2.95 to 5.01 K at 10.3 kbar at a rate of 0.21 K/kbar, which is similar to the 0.22 K/kbar reported for [FeCp(2)*](+)[TCNE](-). The coercive field and remnant magnetization exhibit exponential-like growth upon application of external pressure, increasing from zero at ambient pressure to 550 Oe and 8880 emu·Oe/mol at 10.3 kbar, respectively. The T(c) for the metamagnetic polymorph was determined to be 2.10 K from the maximum in the Fisher specific heat data, that is, d(χT)/dT, and it increases by 38% to 2.90 K at 2.9 kbar at a rate of 0.28 K/kbar, before vanishing, in accord with a transition to a paramagnetic state. The metamagnetic critical field, H(c), determined from dM/dH increases linearly from 1300 Oe at ambient pressure to 1800 Oe at 2.9 kbar, but is not evident at and above 3.9 kbar, also in accord with a transition to a paramagnetic state.