Structure and Magnetic Ordering of the Anomalous Layered (2D) Ferrimagnet [NEt₄]₂Mn^II₃(CN)₈ and 3D Bridged‐Layered Antiferromagnet [NEt₄]Mn^II₃(CN)₇ Prussian Blue Analogues

Abstract

The reaction of Mn(II) and [NEt(4)]CN leads to the isolation of solvated [NEt(4)]Mn(3)(CN)(7) (1) and [NEt(4)](2) Mn(3)(CN)(8) (2), which have hexagonal unit cells [1: R3m, a = 8.0738(1), c = 29.086(1) Å; 2: P3m1, a = 7.9992(3), c = 14.014(1) Å] rather than the face centered cubic lattice that is typical of Prussian blue structured materials. The formula units of both 1 and 2 are composed of one low- and two high-spin Mn(II) ions. Each low-spin, octahedral [Mn(II)(CN)(6)](4-) bonds to six high-spin tetrahedral Mn(II) ions through the N atoms, and each of the tetrahedral Mn(II) ions are bound to three low-spin octahedral [Mn(II)(CN)(6)](4-) moieties. For 2, the fourth cyanide on the tetrahedral Mn(II) site is C bound and is terminal. In contrast, it is orientationally disordered and bridges two tetrahedral Mn(II) centers for 1 forming an extended 3D network structure. The layers of octahedra are separated by 14.01 Å (c axis) for 2, and 9.70 Å (c/3) for 1. The [NEt(4)](+) cations and solvent are disordered and reside between the layers. Both 1 and 2 possess antiferromagnetic superexchange coupling between each low-spin (S = 1/2) octahedral Mn(II) site and two high-spin (S = 5/2) tetrahedral Mn(II) sites within a layer. Analogue 2 orders as a ferrimagnet at 27(±1) K with a coercive field and remanent magnetization of 1140 Oe and 22,800 emuOe mol(-1), respectively, and the magnetization approaches saturation of 49,800 emuOe mol(-1) at 90,000 Oe. In contrast, the bonding via bridging cyanides between the ferrimagnetic layers leads to antiferromagnetic coupling, and 3D structured 1 has a different magnetic behavior to 2. Thus, 1 is a Prussian blue analogue with an antiferromagnetic ground state [T(c) = 27 K from d(χT)/dT].

Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13(A = NMe4, NEtMe3). A Cation Adaptive Structure

“…The interlayer Mn···Mn separation, c , is 5.78 Å for 1·H 2 O and 5.94 for 2·H 2 O , and these are >0.4 Å longer than those reported for Cs 2 Mn­[Mn­(CN) 6 ] (5.303 Å), [NEt 4 ] 2 Mn II 3 (CN) 8 (5.162 Å), and [NEt 4 ]­Mn II 3 (CN) 7 (5.115 Å) . This is attributed to the long axial Mn–(C/N) bond in the trigonal bipyramidal sites in both materials and/or the stacking of the square pyramidal sites, whether ordered in 2·H 2 O (Figure a) or disordered in 1· H 2 O (Figure b).…”

“…The antiferromagnetic ground states arise from antiferromagnetic coupling via the layer of nonmagnetic cyanides that bridge between adjacent ferrimagnetic layers. Each layer is ferrimagnetic due to the antiferromagnetic coupling between adjacent high- ( S = 5/2) and low-spin ( S = 1/2) Mn II sites, as occurs for antiferromagnet [NEt 4 ]­Mn II 3 (CN) 7 and ferrimagnet [NEt 4 ] 2 Mn II 3 (CN) 8 . Hence, A 3 Mn 5 (CN) 13 (A = NMe 4 , NEtMe 3 ) are rare examples of intrinsic artificial antiferromagnets (AAF), which are increasingly important for the development of spin valves (albeit using extrinsic, deposition-prepared AAFs) .…”

“…A 2 Mn II [Mn II (CN) 6 ] (A = Na, K, Rb 6 ), however, has the same framework topology, but a distorted lattice with nonlinear M′NC linkages with an average ∠M′NC ≪ 180°. Use of the larger NEt 4 + forms the layered (2-D) ferrimagnet [NEt 4 ] 2 Mn II 3 (CN) 8 and 3-D bridged-layered antiferromagnet [NEt 4 ]­Mn II 3 (CN) 7 that can be thermally decomposed to Mn­(CN) 2 with an interpenetrating diamondoid structure . To study the effect of cation size in Prussian blue related magnets and to identify new magnetic materials and structural motifs, we investigated NMe 4 + and NEtMe 3 + to target A 2 Mn II [Mn II (CN) 6 ] (A = NMe 4 , NEtMe 3 ).…”

Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3Me)2MnII5(CN)12 and (NEt2Me2)2MnII5(CN)12: A Cation‐Adaptive Structure

“…There are four distinct CN locations in general positions of the unit cell, leading to an Mn:CN ratio of 5:12. Mn1 (8 sites) and Mn2 (24 sites) are both octahedrally coordinated by Catoms at distances ranging from 1.93(1) to 1.99 (1) .M n3 (48 sites) is tetrahedrally coordinated by Na toms at distances ranging from 2.05(1)t o2 .09 (1) .T he ff C-Mn-C angles range from 82.9 to 99.78 for both Mn1 [14a,b] K 2 Mn[Mn(CN) 6 ], [5] (NEt 4 ) 2 Mn II 3 (CN) 8 ,a nd (NEt 4 )Mn II 3 (CN) 7 [8] where the octahedral Mn II sites are C-bonded, and the tetrahedral Mn II sites are N-bonded. Due to the reduced crystal field stabilization energy for tetrahedral Mn II sites, these sites are expected to be high spin, as occurs for [Mn II (CN) 4 ] 2À [10,15] and [Cr II (CN) 5 ] 3À .…”

“…To study the effect of cation size in Prussian blue‐related magnets, and to identify new magnetic materials and structural motifs, we previously investigated larger cations, NMe 4 + , NEtMe 3 + and NEt 4 + to target A 2 Mn II [Mn II (CN) 6 ] [7, 8] . However, these larger cations unexpectedly formed trigonal and hexagonal lattices with completely different topologies, that is, 3D bridged‐layered synthetic/artificial antiferromagnets of (NMe 4 ) 3 Mn II 5 (CN) 13 , [7] (NEtMe 3 )Mn II 5 (CN) 13 , [7] and (NEt 4 )Mn II 3 (CN) 7 , [8] composition, and the layered (2D) (NEt 4 ) 2 Mn II 3 (CN) 8 ferrimagnet [8, 9] . These compounds typically possess adjacent low‐spin ( S= 1/2) and high‐spin ( S= 5/2) Mn II sites that antiferromagnetically couple.…”