Anomalous Non-Prussian Blue Structures and Magnetic Ordering of K₂Mn^II[Mn^II(CN)₆] and Rb₂Mn^II[Mn^II(CN)₆]

The reaction of MnII 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)o]; 2 [a = 10.4101(1) Å, b = 7.4492(1) Å, c = 7.2132(1) Å, β = 90.072(1)o]}, 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 MnII accommodating a reduced M−CN−M′ angle and minimizing void space. Compounds 1a,b have a sharp, strong νOH band at 3628 cm−1, while 1e lacks a νOH absorption. The νOH absorption in 1a,b is attributed to surface water, as use of D2O shifts the νOH absorption to 2677 cm−1, and that 1a−e are isostructural. Also, fcc Prussian blue-structured Cs2Mn[Mn(CN)6] (3) has been structurally [Fm3̅m: 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 Å (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)···Mn(II) separations, and perhaps enhanced overlap. In addition, the temperature dependent magnetic behavior of K4[MnII(CN)6]·3H2O is reported.