Anomalous Non-Prussian Blue Structures and Magnetic Ordering of K2MnII[MnII(CN)6] and Rb2MnII[MnII(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, P2(1)/n: 1 [a = 10.1786(1) angstrom, b = 7.4124(1) angstrom, c = 6.9758(1) angstrom, beta = 90.206(1)degrees]; 2 [a = 10.4101 (1) angstrom, b = 7.4492(1) angstrom, c= 7.2132(1) angstrom, beta = 90.072(1)degrees]}, 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 degrees and 153.3 degrees 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-M' angle and minimizing void space. Compounds 1a,b have a sharp, strong nu(OH) band at 3628 cm(-1), while 1e lacks a nu(OH) absorption. The nu(OH) absorption in 1a,b is attributed to surface water, as use of D2O shifts the nu(OH) absorption to 2677 cm(-1), and that la-e are isostructural. Also, fcc Prussian blue-structured Cs2Mn[Mn(CN)(6)] (3) has been structurally [Fm (3) over barm: a = 10.6061 (1) angstrom] and magnetically characterized. The magnetic ordering temperature, T-c, increases as K+ (41 K) > Rb+ (34.6 K) > Cs+ (21 K) for A(2)Mn[Mn(CN)(6)] in accord with the increasing deviation for linearity of the Mn-N-C linkages [148.8 (K+) > 153.3 (Rb+) > 180 degrees (Cs+)], decreasing Mn(II)center dot center dot center dot Mn(II) separations [5.09 (K+) < 5.19 (Rb+) < 5.30 angstrom (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)center dot center dot center dot Mn(II) separations, and perhaps enhanced overlap. In addition, the temperature dependent magnetic behavior of K-4[Mn-II(CN)(6)]center dot 3H(2)O is reported.