Non-Prussian Blue Structures and Magnetic Ordering of Na_2Mn~Ⅱ[Mn~Ⅱ(CN)_6] and Na_2Mn~Ⅱ[Mn~Ⅱ(CN)_6]·2H_2O

摘要

The aqueous reaction of Mn and NaCN leads to the isolation of the 3-D Prussian blue analogue (PBA) Na_2Mn[Mn(CN)_6]·2H_2O (1·H_2O), which under careful dehydration forms 1. 1·H_2O is monoclinic [P2_1, a = 10,66744(32) A, h = 7.60223(23)_A, c = 7.40713(22) A,β = 92.4379(28)°], while 1 is rhombohedral [R3, a = 6.6166(2) A, c = 19.2585(6) A], 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·H_2O and 1, respectively, which are significantly reduced from linearity. This is attributed to the ionic nature of high-spin Mn~Ⅱ accommodating a reduced ∠Mn-N-C to minimize void space. Both 1 and 1·H_2O magnetically order as ferrimagnets below their ordering temperature, T_c of 58 and 30 K, respectively, as determined from the average of several independent methods. 1 and 1·H_2O 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 T_c's reported for related materials reveal that T_c 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 Mn~Ⅱ-Mn~Ⅱ separations, and perhaps an enhanced overlap.