Non-Prussian Blue Structures and Magnetic Ordering of Na2MnII[MnII(CN)6] and Na2MnII[MnII(CN)6]·2H2O

The aqueous reaction of MnII and NaCN leads to the isolation of the 3-D Prussian blue analogue (PBA) Na2Mn­[Mn­(CN)6]·2H2O (1·H2O), which under careful dehydration forms 1. 1·H2O is monoclinic [P21/n, a = 10.66744(32) Å, b = 7.60223(23) Å, c = 7.40713(22) Å, β = 92.4379(28)°], while 1 is rhombohedral [R3̅, a = 6.6166(2) Å, c = 19.2585(6) Å], and both structures are atypical for PBAs, which are typically face centered cubic. Most notably, the average ∠Mn–N–C angles are 165.3(3)° and 142.4(4)° for 1·H2O and 1, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high-spin MnII accommodating a reduced ∠Mn–N–C to minimize void space. Both 1 and 1·H2O magnetically order as ferrimagnets below their ordering temperature, Tc, of 58 and 30 K, respectively, as determined from the average of several independent methods. 1 and 1·H2O are hard magnets with 5 K coercive fields of 15 300 and 850 Oe, and remnant magnetizations of 9075 and 102 emu·Oe/mol, respectively. These data along with previous Tc’s reported for related materials reveal that Tc increases as the ∠Mn–N–C deviates further from linearity. Hence, the bent cyanide bridges play a crucial role in the superexchange mechanism by increasing the coupling via shorter MnII···MnII separations, and perhaps an enhanced overlap.