Solvatomagnetic Studies on Cyano‐Bridged Bimetallic Chains Based on [Mn(cyclam)]³⁺ and Hexacyanometallates

Abstract

Structures and solvatomagnetic properties of two 1D CN‐bridged assemblies based on the complex [Mn(cyclam)]3+ (cyclam = 1,4,8,11‐tetraazacyclotetradecane) were characterised. The isostructural {[Mn(cyclam)][Cr(CN)6]·6H2O}n (1·6H2O) and {[Mn(cyclam)][Fe(CN)6]·6H2O}n (2·6H2O) chains crystallise in the space group C2/m. When dried in air at a slightly elevated temperature the hexahydrates undergo irreversible partial dehydration to stable dihydrates 1·2H2O and 2·2H2O. The structure model of 1·2H2O proposed from the powder XRD data shows lower symmetry of the network, described by the space group P21, and a markedly different relative arrangement of the chains. The Mn–Cr chains are characterised by antiferromagnetic (AFM) exchange through the CN bridges (J = –2.8 K for 1·6H2O and –6.0 K for 1·2H2O), which was modelled by quantum Monte Carlo simulations. Together with weaker AFM Mn–Cr interactions between the chains, it leads to ferrimagnetic ordering. FM intrachain interactions (J = 4.5 K for 2·6H2O and 7.1 K for 2·2H2O) and AFM 3D ordering are observed for the Mn–Fe chains. The dehydration process in both compounds causes an increase in the strength of magnetic exchange, which reflects changes in the CN bridge geometry.

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

“…Besides, interpretation of magnetic properties of 1 with the isotropic Heisenberg single J model may be rather ambiguous, because there is an alternative fit with antiferromagnetic Mn‐Fe spin coupling, J =−17 cm −1 (at g Fe =2.20, g Mn =2.00, and TIP(Fe)=0), which better reproduces the fast increase in χT below 10 K, but displays somewhat worse agreement at higher temperature (Figure S5 in Supporting Information). Similar AF fit in terms of the Seiden model was also found in a {[Mn III (cyclam)][Fe III (CN) 6 ] ⋅ 6 H 2 O} n chain compound …”

“…At first glance this would indicate the presence of significant intra‐chain Mn II −Fe III ferromagnetic coupling, mediated by cyanide bridges, as may be concluded from the analysis of the magnetic susceptibility in terms of the isotropic spin coupling approach ( J =+3.3 cm −1 , using the Seiden model, see below), Figure . Ferromagnetic exchange interactions in Fe(CN) 5 ‐CN‐Mn pairs were also found in many other related compounds –. However, below we show that the actual character of intra‐chain exchange interactions in the Mn II ‐NC‐Fe III moieties in 1 is much more complicated, due to the presence of unquenched orbital momentum on [Fe(CN) 6 ] 3− complexes; actually, their analysis goes far beyond the isotropic spin model.…”

“…(3)]. In this regard, it is important to note that, with few exceptions, most of the known cyano‐bridged Fe‐CN‐Mn bimetallic complexes invariably exhibit ferromagnetic exchange interactions –. This fact seems to be contrary to the general condition of strict orbital orthogonality for the occurrence of ferromagnetic spin coupling.…”

“…Assuming a single ferromagnetic J value for all Fe‐Mn pairs in the chain and using g Fe =2.20, g Mn =2.00, the best fit was obtained at J = +3.3 cm −1 (Figure ); in these calculations, the temperature‐independent paramagnetism (TIP) of [Fe III (CN) 6 ] 3− complexes was also applied, χ TIP =1.0×10 −3 mol −1 (see below). Ferromagnetic exchange interactions were also found in many other compounds containing a [Fe(CN) 5 ‐CN‐Mn] moiety, both in extended molecular structures and in finite clusters –. In this regard, however, it is relevant to note that the origin of ferromagnetism in these systems is rather difficult to substantiate from the viewpoint of a microscopic theory, according to which ferromagnetic interactions can only occur when the interacting half‐filled magnetic orbitals on two magnetic centers are all orthogonal; in fact, such a condition is difficult to meet for the actual low‐symmetry geometry of Fe‐CN‐Mn cyano‐bridged groups in most of these compounds .…”

Structural Diversity in Ni Chain Coordination Polymers: Synthesis, Structures, Isomerism and Magnetism

“…One‐dimensional coordination polymers have attracted much interest in recent years and many compounds with different physical properties and different structural motifs like linear, corrugated, ladder‐like or helical chains have been reported . This also includes magnetic coordination polymers that can show a variety of magnetic phenomena, which to a large extend depend on the strength of the magnetic exchange along the chains , .…”