Nature of Magnetic Interactions in 3D {[M^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n (M = Mn, Fe, Co, Ni) Molecular Magnets

Abstract

The self-assembly of [Nb(IV)(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)].4H(2)O}(n), where M(II) = Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same I4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M(II) centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M(II)-NC-Nb(IV) linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M(II) metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic J(AF) = -21.6 cm(-1) originating from the d(xy), d(xz), and d(yz) orbitals of M(II) and ferromagnetic J(F) = +15.4 cm(-1) originating from d(z(2)) and d(x(2)-y(2)) orbitals of M(II). Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t(2g) level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.

Studies on magnetic properties of unique molecular magnet {[FeII(pyrazole)4]2[NbIV(CN)8]∙4H2O}n

“…Although 55 kOe magnetic field was applied, saturation was not observed. To extract the saturation magnetization (M S ), a polynomial formula: (1) was fitted to isothermal magnetization. The obtained value was about M S ≈8.0 μ B .…”

Neutral Low-Dimensional Assemblies of a Mn(III) Schiff Base Complex and Octacyanotungstate(V): Synthesis, Characterization, and Magnetic Properties

“…Some of them are 1D polymers [53][54][55][56]; others possess layered [57][58][59][60][61][62][63] or 3D network structures [64][65][66][67][68]. The paramagnetic cyanotungstate(V) tectones have been used to build discrete molecules [69][70][71], chains [53][54][55][56], 2D [57] and 3D networks [64][65][66][67][68], and some of them have displayed variable magnetic properties such as high ordering temperatures [72], photo-induced magnetism [73,74], SMMs [75], and SCMs [17,54,55]. However, the assemblies of the paramagnetic [W(CN) 8 ] 3− precursor and manganese(III) Schiff base complex, [Mn(SB)] + , are fairly limited and poorly studied in comparison with the large family of bimetallic compounds comprised of the [Mn(SB)] + cation and the 3d metal hexacyanides that are extensively investigated both structurally and magnetically [76].…”

Neutral Low-Dimensional Assemblies of a Mn(III) Schiff Base Complex and Octacyanotungstate(V): Synthesis, Characterization and Magnetic Properties

“…Some of them are 1D polymers [53][54][55][56]; others possess layered [57][58][59][60][61][62][63] or 3D network structure [64][65][66][67][68]. The paramagnetic cyanotungstate(V) tectones have been used to build discrete molecules [69][70][71], chains [53][54][55][56], 2D [57] and 3D networks [64][65][66][67][68] and some of them have displayed variable magnetic properties such as high ordering temperatures [72], photo-induced magnetism [73,74], SMMs [75], and SCMs [17,54,55].…”