Anomalous non-prussian blue structures and magnetic ordering of K ₂Mn^II[Mn^II(CN)₆] and Rb ₂Mn^II[Mn^II(CN)₆]

The reaction of Mn^II and KCN in aqueous and non-aqueous media leads to the isolation of three-dimensional (3-D) Prussian blue analogues, K₂Mn[Mn(CN)₆] (1a-d, 1e, respectively). Use of RbCN forms Rb₂Mn[Mn(CN)₆] (2). 1 and 2 are isomorphic {monoclinic, P2₁/n: 1 [a = 10.1786(1) Å, b = 7.4124(1) Å, c = 6.9758(1) Å, β = 90.206(1)°]; 2 [a = 10.4101(1) Å, b = 7.4492(1) Å, c = 7.2132(1) Å, β = 90.072(1)°]}, with a small monoclinic distortion from the face centered cubic (fcc) structure that is typical of Prussian blue structured materials that was previously reported for K2Mn[Mn(CN)₆]. Most notably the average Mn-N-C angles are 148.8° and 153.3° for 1 and 2, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high spin Mn^II accommodating a reduced M-CN-M0 angle and minimizing void space. Compounds 1a, b have a sharp, strong.OH band at 3628 cm^-1, while 1e lacks a v _OH absorption. The v_OH absorption in 1a, b is attributed to surface water, as use of D₂O shifts the.OH absorption to 2677 cm^-1, and that 1a-e are isostructural. Also, fcc Prussian blue-structured Cs₂Mn[Mn(CN)₆] (3) has been structurally [Fm3hm: a = 10.6061(1) Å] and magnetically characterized. The magnetic ordering temperature, Tc, increases as K⁺ (41 K) < Rb⁺ (34.6 K) < Cs⁺ (21 K) for A₂Mn[Mn(CN)₆] in accord with the increasing deviation for linearity of the Mn-N-C linkages [148.8 (K⁺) < 153.3 (Rb+) < 180° (Cs⁺)], decreasing Mn(II) · · · Mn(II) separations [5.09 (K+) > 5.19 (Rb+) > 5.30 A° (Cs+)], and decreasing size of the cation (increasing electrostatic interactions). Hence, the bent cyanide bridges play a crucial role in the superexchange mechanism by increasing the coupling via shorter Mn(II) 3 3 3 Mn(II) separations, and perhaps enhanced overlap. In addition, the temperature dependent magnetic behavior of K₄[Mn^II(CN) ₆] · 3H₂O is reported.