Origin of Dissimilar Single-Molecule Magnet Behavior of Three Mn^II₂Mo^III Complexes Based on [Mo^III(CN)₇]^4– Heptacyanomolybdate: Interplay of Mo^III–CN–Mn^II Anisotropic Exchange Interactions

The origin of contrasting single-molecule magnet (SMM) behavior of three Mn^II₂Mo^III complexes based on [Mo^III(CN)₇]^4– heptacyanomolybdate is analyzed; only the apical Mn₂Mo isomer exhibits SMM properties with <i>U</i>_eff = 40.5 cm^–1 and <i>T</i>_B = 3.2 K, while the two equatorial isomers are simple paramagnets [Qian, K.; J. Am. Chem. Soc. 2013, 135, 13302]. A microscopic theory of anisotropic spin coupling between orbitally degenerate [Mo^III(CN)₇]^4– complexes (pentagonal bipyramid) and bound Mn^II ions is developed. It is shown that the [Mo^III(CN)₇]^4– complex has a unique property of uniaxial anisotropic spin coupling in the apical and equatorial Mo^III–CN–Mn^II pairs, <i>H̑</i>_eff = −<i>J</i>_<i>xy</i>(<i>S</i>_Mo^<i>x</i><i>S</i>_Mn^<i>x</i> + <i>S</i>_Mo^<i>y</i><i>S</i>_Mn^<i>y</i>) – <i>J</i>_<i>z</i><i>S</i>_Mo^<i>z</i><i>S</i>_Mn^<i>z</i>, regardless of their actual low symmetry. The difference in the SMM behavior originates from a different ratio between the anisotropic exchange parameters <i>J</i>_<i>z</i> and <i>J</i>_<i>xy</i> for the apical and equatorial Mo–CN–Mn groups. In the apical Mn₂Mo isomer, an Ising-type anisotropic spin coupling (<i>J</i>_<i>z</i> = −34, <i>J</i>_<i>xy</i> = −11 cm^–1) produces a double-well potential of spin states resulting in SMM behavior. Exchange anisotropy of an <i>xy</i>-type (|<i>J</i>_<i>z</i>| < |<i>J</i>_<i>xy</i>|) in the equatorial Mn₂Mo isomers results in a single-well potential with no SMM properties. The prospects of anisotropic uniaxial spin coupling in engineering of high <i>U</i>_eff and <i>T</i>_B values are discussed

[Mn^III(Schiff Base)]₃[Re^IV(CN)₇], Highly Anisotropic 3D Coordination Framework: Synthesis, Crystal Structure, Magnetic Investigations, and Theoretical Analysis

A new highly anisotropic coordination heterobimetallic polymer [Mn^III(Schiff-base)]₃­[Re^IV(CN)₇] was synthesized and characterized structurally and magnetically. The single crystal X-ray analysis has revealed that this is the first framework among the complexes composed of homoleptic cyanometallate and Mn^III complex of the tetradentate Schiff base ligand. A formation of 3D assembly is possible due to both the pentagonal bipyrimidal geometry of the cyanometallate unit and suitable size of constituents: [Re­(CN)₇]^3– and [Mn^III(acacen)]⁺, where acacen = <i>N</i>,<i>N</i>′-ethylenebis­(acetylacetoneiminato). The powder and crystal magnetic studies show that the compound undergoes an antiferromagnetic ordering of a complicated character below Neel temperature of 13 K, and exhibits a metamagnetic behavior and strong magnetic anisotropy similar to those observed in related 3D Mn^II–[Mo­(CN)₇]^4– systems. Unusual magnetic properties of [Mn^III(acacen)]₃­[Re^IV(CN)₇] (<b>1</b>) originate from an interplay of Re–Mn anisotropic spin coupling and ZFS effect of Mn^III ions with a noncollinear orientation of the local magnetic axes in the cyano-bridged 3D network. A theoretical model of anisotropic spin coupling between orbitally degenerate [Re^IV(CN)₇]^3– complexes and Mn^III ions is developed, and specific microscopic mechanisms of highly anisotropic spin coupling in Re^IV–CN–Mn^III linkages in complex <b>1</b> are analyzed in detail

Synthesis, Structure, and Magnetic Properties of 1D {[Mn^III(CN)₆][Mn^II(dapsc)]}_<i>n</i> Coordination Polymers: Origin of Unconventional Single-Chain Magnet Behavior

Two one-dimensional cyano-bridged coordination polymers, namely, {[Mn^II(dapsc)]­[Mn^III(CN)₆]­[K­(H₂O)_2.75(MeOH)_0.5]}_<i>n</i>·0.5<i>n</i>(H₂O) (<b>I</b>) and {[Mn^II(dapsc)]­[Mn^III(CN)₆]­[K­(H₂O)₂(MeOH)₂]}_<i>n</i> (<b>II</b>), based on alternating high-spin Mn^II(dapsc) (dapsc = 2,6-diacetylpyridine bis­(semicarbazone)) complexes and low-spin orbitally degenerate hexacyanomanganate­(III) complexes were synthesized and characterized structurally and magnetically. Static and dynamic magnetic measurements reveal a single-chain magnet (SCM) behavior of <b>I</b> with an energy barrier of <i>U</i>_eff ≈ 40 K. Magnetic properties of <b>I</b> are analyzed in detail in terms of a microscopic theory. It is shown that compound <b>I</b> refers to a peculiar case of SCM that does not fall into the usual Ising and Heisenberg limits due to unconventional character of the Mn^III–CN–Mn^II spin coupling resulting from a nonmagnetic singlet ground state of orbitally degenerate complexes [Mn^III(CN)₆]^3–. The prospects of [Mn^III(CN)₆]^3– complex as magnetically anisotropic molecular building block for engineering molecular magnets are critically analyzed