Chemical bonding and magnetic exchange in two-dimensional [M(TCNE)(NCMe)2]X (M = Fe, Mn; X = FeCl4, SbF6) magnets: A pressure study

Abstract

Pressure-dependent X-ray diffraction studies reveal the bulk modulus and compression anisotropy of the 2D magnet [Mn(TCNE)(NCMe)(2)]SbF(6). The Raman response of this and the similar [Fe(TCNE)(NCMe)(2)]FeCl(4) layered magnet, shows that the evolution of the a(g) ν(C=C) frequency correlates well with the magnetic exchange and T(c) variations of these materials under pressure. There is a significantly more complex correlation between the a(g) ν(C≡N) frequency and T(c) despite the fact that some unpaired π* electron density (~0.125 e) is localized on each of TCNE nitrile N≡C group. The shortening of the M-NC bond with pressure (<0.5 GPa) does not result in a T(c) increase, which suggests a more complex bond length magnetic exchange relationship.

Magnetic transitions and spin-glass reentrance in two-dimensional [MnII(TCNE)(NCMe)2]X (X = PF6,AsF6,SbF6) molecular magnets

“…Recently we have shown that the unit cell volume of 3 varies continuously with pressure up to 2 GPa with a bulk modulus K = −V(dP/dV) = 10.4 GPa. The main change in the unit cell volume occurs interlayer distance while Mn-µ 4 -TCNE layers remain relatively rigid reflecting the two-dimensional character of the network [24]. The observed differences in the intra-and interlayer structural parameters of 1 and 3 (3% volume and 2.3% interlayer distance decrease, respectively) due to the anion size change are consistent with the 'chemical pressure' effect corresponding to ∼0.5-0.6 GPa hydrostatic pressure application [24].…”

“…The main change in the unit cell volume occurs interlayer distance while Mn-µ 4 -TCNE layers remain relatively rigid reflecting the two-dimensional character of the network [24]. The observed differences in the intra-and interlayer structural parameters of 1 and 3 (3% volume and 2.3% interlayer distance decrease, respectively) due to the anion size change are consistent with the 'chemical pressure' effect corresponding to ∼0.5-0.6 GPa hydrostatic pressure application [24]. However, the magnetic studies of 3 under pressure have shown a sizable (up to 5%) increase of T c at 0.6 GPa.…”