Non-Prussian Blue Structures and Magnetic Ordering of Na₂Mn^II[Mn^II(CN)₆] and Na₂Mn^II[Mn^II(CN)₆]·2H₂O

The aqueous reaction of Mn^II and NaCN leads to the isolation of the 3-D Prussian blue analogue (PBA) Na₂Mn­[Mn­(CN)₆]·2H₂O (<b>1·H</b>_<b>2</b><b>O</b>), which under careful dehydration forms <b>1</b>. <b>1·H</b>_<b>2</b><b>O</b> is monoclinic [<i>P</i>2₁/<i>n</i>, <i>a</i> = 10.66744(32) Å, <i>b</i> = 7.60223(23) Å, <i>c</i> = 7.40713(22) Å, β = 92.4379(28)°], while <b>1</b> is rhombohedral [<i>R</i>3̅, <i>a</i> = 6.6166(2) Å, <i>c</i> = 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 <b>1·H</b>_<b>2</b><b>O</b> and <b>1</b>, 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 <b>1</b> and <b>1·H</b>_<b>2</b><b>O</b> magnetically order as ferrimagnets below their ordering temperature, <i>T</i>_c, of 58 and 30 K, respectively, as determined from the average of several independent methods. <b>1</b> and <b>1·H</b>_<b>2</b><b>O</b> 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 <i>T</i>_c’s reported for related materials reveal that <i>T</i>_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