SCHEDULED MAINTENANCE
Syntheses, structures and magnetism of α-Mn(dca)2, [Mn(dca)2(H2O)2]·H2O, [Mn(dca)2(C2H5OH)2]·(CH3)2CO, [Fe(dca)2(CH3OH)2] and [Mn(dca)2(L)2], where L = pyridine, CH3OH or DMF and dca– = dicyanamide, N(CN)2–
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Abstract
The dicyanamide anion has been observed to adopt two bridging co-ordination modes (µ and µ3) in α-Mn(dca)2, [Mn(dca)2(H2O)2]·H2O, [Mn(dca)2(C2H5OH)2]·(CH3)2CO, [Fe(dca)2(CH3OH)2] and [Mn(dca)2(L)2] [L = py, CH3OH or DMF; dca = dycanamide N(CN)2–], and generates weak ligand fields thus stabilising high spin configurations. The N- or O-bonded ligands L play an important role in the stabilisation of both the molecular structures and the three dimensional structure, via hydrogen bonding. The unsolvated α-Mn(dca)2 adopts a rutile-like single network structure, based on the near orthogonal packing of ‘ribbons’ of . . . Mn(N![[triple bond, length half m-dash]](https://www.rsc.org/images/entities/char_e007.gif)
C–N–CN)2Mn . . ., similar to that found for the isomorphous analogues of Co, Ni, Fe and Cu. Magnetisation measurements confirmed a high spin manganese d5 system displaying antiferromagnetic coupling (θ = –25 K) above 25 K and undergoing long range magnetic ordering (TN = 16 K) to a spin-canted antiferromagnet (weak ferromagnet). Magnetisation and heat capacity measurements on some samples of α-Mn(dca)2 indicated a possible second transition at ≈6 K, the nature of which is under investigation. From the hysteresis data at 2 K (remnant magnetisation of 29 cm3 Oe mol–1 and coercive field of 406 Oe) a canting angle of 0.05° is estimated for this soft magnet. Other samples gave a higher value for the coercive field. The α-M(dca)2 series has a diverse range of ground states; CuII (d9) is a paramagnet, NiII (d8) and CoII (d7) are ferromagnets and FeII (d6) and MnII (d5) are canted antiferromagnets. Reasons for this diversity are given on the basis of the nature of exchange coupling pathways within the rutile structure and a mechanism for the long range magnetic ordering is proposed. A range of 1-D chain complexes of type [Mn(dca)2(L)2], containing ‘ribbons’ of doubly bridged Mn(NC–N–CN)2Mn have been structurally characterised. The complex [Fe(dca)2(CH3OH)2] is isostructural with the manganese analogue. 2-D Square grids are found in crystals of [Mn(dca)2(C2H5OH)2]·(CH3)2CO and in [Mn(dca)2(H2O)2]·H2O, the latter displaying, in addition, penetration of ribbons of trans-Mn(dca)2(H2O)2 through the grids. Dehydration or desolvation results in formation of the α-Mn(dca)2 phase. The Lewis-base adducts all display very weak antiferromagnetic coupling (J ≈ –0.12 cm–1) and no magnetic long-range order. Dissolution of the compounds in protic solvents leads to complete dissociation of the dicyanamide, and the axially co-ordinated ligands, L, can readily be exchanged by reaction or recrystallisation in different co-ordinating solvents.
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