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

Abstract

Two one-dimensional cyano-bridged coordination polymers, namely, {[Mn(dapsc)][Mn(CN)][K(HO)(MeOH)]}·0.5n(HO) (I) and {[Mn(dapsc)][Mn(CN)][K(HO)(MeOH)]} (II), based on alternating high-spin Mn(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 I with an energy barrier of U ≈ 40 K. Magnetic properties of I are analyzed in detail in terms of a microscopic theory. It is shown that compound I 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-CN-Mn spin coupling resulting from a nonmagnetic singlet ground state of orbitally degenerate complexes [Mn(CN)]. The prospects of [Mn(CN)] complex as magnetically anisotropic molecular building block for engineering molecular magnets are critically analyzed.

Slow Magnetic Relaxation, Antiferromagnetic Ordering, and Metamagnetism in MnII(H2dapsc)‐FeIII(CN)6Chain Complex with Highly Anisotropic Fe‐CN‐Mn Spin Coupling

“…In this case, spin coupling between Fe III and Mn II is described by an orbitally‐dependent isotropic spin Hamiltonian H orb , which is written as [Eq. ]: …”

“…(2) was calculated in terms of a many‐electron superexchange model, which was applied for the two nonequivalent [Fe III (CN) 6 ] 3− —Mn II (H 2 dapsc) exchange‐coupled pairs involved in the Fe‐Mn‐Fe asymmetric unit in the chains of 1 (Figure S7, pairs 1 and 2; see the Supporting Information for more computational detail). Similar calculations were previously performed for Mn III ‐Mn II pairs in closely related {[Mn III (CN) 6 ]‐Mn II (H 2 dapsc)} n chain compound . These calculations reveal a non‐diagonal structure of the 3×3 matrix R for the two Fe III ‐Mn II pairs in the chains of 1 [Eq.…”

“…The Mn II ion has a distorted pentagonal bipyramidal coordination, formed by two oxygen and three nitrogen atoms of the equatorial H 2 dapsc ligand and two axial nitrogen atoms from cyanide bridges. The Mn−O, N bond lengths in the Mn II (H 2 dapsc) moiety (Table S1) are somewhat longer in 1 as compared with relevant bonds in the structures of previously published chain complexes with Mn(H 2 dapsc) . The Mn(H 2 dapsc) fragment is also less flat: the dihedral angle between the two semicarbazone planes defined by atoms O(1), C(1), N(3), N(5), C(4), C(5), N(7) and O(2), C(2), N(4), N(6), C(10), C(9), N(7) is 12.78(9)°.…”

“…The AF ordering of 1 below 6 K is confirmed by the frequency‐independent peaks at 5.8 K in the χ′ and no signal in the χ ” at this temperature (Figure ). Below 3.2 K, both components of the ac susceptibility exhibit frequency‐dependent signals, indicating the slow magnetic relaxation (Figure ) as observed in the related SCM, {[Mn II (H 2 dapsc)][Mn III (CN) 6 ][K(H 2 O) 2.75 (MeOH) 0.5 ]} n ⋅ 0.5 n H 2 O ( 3 ) . Although complexes 1 and 3 have similar structures and both are SCMs, the antiferromagnetic ground state and, accordingly, the metamagnetic transition are not realized in complex 3 , most likely due to a fundamental difference between the ground‐state magnetic characteristics of [Mn III (CN) 6 ] 3− and [Fe III (CN) 6 ] 3− hexacyanometallates: in fact, [Mn III (CN) 6 ] 3− has a non‐magnetic ground state (a spin‐orbit singlet) resulting in a peculiar second‐order Mn III ‐CN‐Mn II spin coupling and unconventional SCM behavior .…”

“…However, several important 1D compounds have been synthesized, such as bimetallic chain complexes based on [M(H 2 dapsc)Cl 2 ] 0 (M=Mn II and Fe II ) units in combination with [Fe(CN) 5 NO] 2− , [Mn(CN) 6 ] 3− (M=Mn II ) and [Cr(dapbh)(CN) 2 ] (M=Fe II ). The former compound exhibits photochromism stemming from the nitroprusside anion and the latter are SCMs . The chain complex was also obtained in the reaction between [Fe(H 2 dapbh)Cl 2 ] and [Ni(CN) 4 ] 2− , which showed field‐induced slow magnetic relaxation .…”