Spectroscopic study of (two-dimensional) molecule-based magnets: [MII(TCNE)(NCMe)2][SbF6] (M = Fe, Mn, Ni)

Abstract

The M-[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) system is one of the most interesting classes of molecule-based magnets, exhibiting a plethora of compositions and structures (inorganic polymer chains, 2D layers, 3D networks, and amorphous solids) with a wide range of magnetic ordering temperatures (up to 400 K). A systematic study of vibrational (both infrared and, for the first time, Raman) properties of the family of new TCNE-based magnets of M(II)(TCNE) (NCMe)(2)[SbF(6)] [M = Mn, Fe, Ni] composition is discussed in conjunction with their magnetic behavior and newly reso-lved crystal structures. The vibrational properties of the isolated TCNE(●-) anion in the paramagnetic Bu(4)N [TCNE(●-)] salt and recently characterized 2D layered magnet Fe(II)(TCNE)(NCMe)(2)[FeCl(4)] are also reported for comparison. Additionally, a linear correlation between ν(C=C) (a(g)) frequency of the TCNE ligand and its formal charge Z (the spin density on the π* orbital), Z = [1571 - ν(C=C) (a(g))]/154.5 [e], is presented. It is shown that monitoring Z by Raman spectroscopy is of great use in providing information that allows understanding the peculiarity of the superexchange interaction in M-[TCNE] magnets and establishing the structure-magnetic properties correlations in this class of magnetic material.

Magnetic Exchange in MnII[TCNE] (TCNE = Tetracyanoethylene) Molecule-Based Magnets with Two- and Three-Dimensional Magnetic Networks

“…These results support the hypothesized stabilization of AFM ordering between localized Mn II -3d and π* [TCNE] − electrons, in accord with the low temperature saturated magnetization studies. 9,35 Thus, our DFT simulations confirm the AFM ground state for both the 2D and 3D Mn- [TCNE] magnets.…”

“…9,35 Compound 3D is the recently discovered molecular magnet [Mn II (TCNE) 1.5 ](I 3 ) 0.5 , 11 in which each Mn II ion is octahedrally coordinated to six (NC−) nitrile groups of TCNE anions (Figure 1c). This structure maintains the similar Mn II -μ 4 -[TCNE] − corrugated plane motif, but additionally The experimental structural parameters for the studied 2D and 3D compounds are compared to the ones after geometry optimization in Table S1 (Supporting Information).…”

Mean Field Analysis of the Exchange Coupling (J) for Two- and Three-Dimensional Structured Tetracyanoethenide (TCNE•–)-Based Magnets

“…This agrees with previously reported experimental data. 11,12 The intralayer, J, and interlayer, K, coupling constants for 2 of structure-type B (M = Mn II , L S = μ 4 -[TCNE] •− , Figure 6. Generalized bonding schemes for M(L S1 )(L S2 ) 1/2 of structure-type C 1 , and M(LS) 3/2 of structure-type C 2 .…”

“…8,11,12,16,20 To be selfconsistent with our MF analysis, the same method, namely, the temperature at which the maxima in the frequency independent χ′(T) is used as T c . These values are listed in Table 1. MF theory is useful for predicting trends and for qualitative estimates of exchange constants.…”

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

“…16 Recently, it was shown that monitoring the frequencies of C=C and C≡N bonds by Raman spectroscopy can provide quantitative information about the degree of ligand-tometal charge transfer in [M(TCNE)(NCMe) 2 ]SbF 6 (M = Fe, Mn, Ni) layered magnets. 17 The application of high pressure shortens all TCNE bonds, as well as the metal-to-nitrogen and nitrile group bonds, thus altering total orbital overlap. In addition, it causes the reduction of the interlayer distances affecting the dipole-dipole, inter-layer coupling.…”